Signaling by AKT is one of the key outcomes of receptor tyrosine kinase (RTK) activation. AKT is activated by the cellular second messenger PIP3, a phospholipid that is generated by PI3K. In ustimulated cells, PI3K class IA enzymes reside in the cytosol as inactive heterodimers composed of p85 regulatory subunit and p110 catalytic subunit. In this complex, p85 stabilizes p110 while inhibiting its catalytic activity. Upon binding of extracellular ligands to RTKs, receptors dimerize and undergo autophosphorylation. The regulatory subunit of PI3K, p85, is recruited to phosphorylated cytosolic RTK domains either directly or indirectly, through adaptor proteins, leading to a conformational change in the PI3K IA heterodimer that relieves inhibition of the p110 catalytic subunit. Activated PI3K IA phosphorylates PIP2, converting it to PIP3; this reaction is negatively regulated by PTEN phosphatase. PIP3 recruits AKT to the plasma membrane, allowing TORC2 to phosphorylate a conserved serine residue of AKT. Phosphorylation of this serine induces a conformation change in AKT, exposing a conserved threonine residue that is then phosphorylated by PDPK1 (PDK1). Phosphorylation of both the threonine and the serine residue is required to fully activate AKT. The active AKT then dissociates from PIP3 and phosphorylates a number of cytosolic and nuclear proteins that play important roles in cell survival and metabolism. For a recent review of AKT signaling, please refer to Manning and Cantley, 2007.
View original pathway at Reactome.
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Bagchi S, Weinmann R, Raychaudhuri P.; ''The retinoblastoma protein copurifies with E2F-I, an E1A-regulated inhibitor of the transcription factor E2F.''; PubMedEurope PMCScholia
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Wang X, Trotman LC, Koppie T, Alimonti A, Chen Z, Gao Z, Wang J, Erdjument-Bromage H, Tempst P, Cordon-Cardo C, Pandolfi PP, Jiang X.; ''NEDD4-1 is a proto-oncogenic ubiquitin ligase for PTEN.''; PubMedEurope PMCScholia
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Law CL, Sidorenko SP, Chandran KA, Draves KE, Chan AC, Weiss A, Edelhoff S, Disteche CM, Clark EA.; ''Molecular cloning of human Syk. A B cell protein-tyrosine kinase associated with the surface immunoglobulin M-B cell receptor complex.''; PubMedEurope PMCScholia
Schmidt L, Duh FM, Chen F, Kishida T, Glenn G, Choyke P, Scherer SW, Zhuang Z, Lubensky I, Dean M, Allikmets R, Chidambaram A, Bergerheim UR, Feltis JT, Casadevall C, Zamarron A, Bernues M, Richard S, Lips CJ, Walther MM, Tsui LC, Geil L, Orcutt ML, Stackhouse T, Lipan J, Slife L, Brauch H, Decker J, Niehans G, Hughson MD, Moch H, Storkel S, Lerman MI, Linehan WM, Zbar B.; ''Germline and somatic mutations in the tyrosine kinase domain of the MET proto-oncogene in papillary renal carcinomas.''; PubMedEurope PMCScholia
Torres J, Pulido R.; ''The tumor suppressor PTEN is phosphorylated by the protein kinase CK2 at its C terminus. Implications for PTEN stability to proteasome-mediated degradation.''; PubMedEurope PMCScholia
Shinohara H, Kurosaki T.; ''Comprehending the complex connection between PKCbeta, TAK1, and IKK in BCR signaling.''; PubMedEurope PMCScholia
Lamorte L, Royal I, Naujokas M, Park M.; ''Crk adapter proteins promote an epithelial-mesenchymal-like transition and are required for HGF-mediated cell spreading and breakdown of epithelial adherens junctions.''; PubMedEurope PMCScholia
Li N, Zhang Y, Han X, Liang K, Wang J, Feng L, Wang W, Songyang Z, Lin C, Yang L, Yu Y, Chen J.; ''Poly-ADP ribosylation of PTEN by tankyrases promotes PTEN degradation and tumor growth.''; PubMedEurope PMCScholia
Ikenoue T, Inoki K, Zhao B, Guan KL.; ''PTEN acetylation modulates its interaction with PDZ domain.''; PubMedEurope PMCScholia
Cargnello M, Roux PP.; ''Activation and function of the MAPKs and their substrates, the MAPK-activated protein kinases.''; PubMedEurope PMCScholia
Schmidt C, Bladt F, Goedecke S, Brinkmann V, Zschiesche W, Sharpe M, Gherardi E, Birchmeier C.; ''Scatter factor/hepatocyte growth factor is essential for liver development.''; PubMedEurope PMCScholia
Zhang X, Loijens JC, Boronenkov IV, Parker GJ, Norris FA, Chen J, Thum O, Prestwich GD, Majerus PW, Anderson RA.; ''Phosphatidylinositol-4-phosphate 5-kinase isozymes catalyze the synthesis of 3-phosphate-containing phosphatidylinositol signaling molecules.''; PubMedEurope PMCScholia
Marlin MC, Li G.; ''Biogenesis and function of the NGF/TrkA signaling endosome.''; PubMedEurope PMCScholia
Yang H, Kong W, He L, Zhao JJ, O'Donnell JD, Wang J, Wenham RM, Coppola D, Kruk PA, Nicosia SV, Cheng JQ.; ''MicroRNA expression profiling in human ovarian cancer: miR-214 induces cell survival and cisplatin resistance by targeting PTEN.''; PubMedEurope PMCScholia
Ozes ON, Mayo LD, Gustin JA, Pfeffer SR, Pfeffer LM, Donner DB.; ''NF-kappaB activation by tumour necrosis factor requires the Akt serine-threonine kinase.''; PubMedEurope PMCScholia
Tan PL, Shavlakadze T, Grounds MD, Arthur PG.; ''Differential thiol oxidation of the signaling proteins Akt, PTEN or PP2A determines whether Akt phosphorylation is enhanced or inhibited by oxidative stress in C2C12 myotubes derived from skeletal muscle.''; PubMedEurope PMCScholia
Ahmed SF, Deb S, Paul I, Chatterjee A, Mandal T, Chatterjee U, Ghosh MK.; ''The chaperone-assisted E3 ligase C terminus of Hsc70-interacting protein (CHIP) targets PTEN for proteasomal degradation.''; PubMedEurope PMCScholia
Biswas DK, Singh S, Shi Q, Pardee AB, Iglehart JD.; ''Crossroads of estrogen receptor and NF-kappaB signaling.''; PubMedEurope PMCScholia
Arasada RR, Carpenter G.; ''Secretase-dependent tyrosine phosphorylation of Mdm2 by the ErbB-4 intracellular domain fragment.''; PubMedEurope PMCScholia
Lennartsson J, Rönnstrand L.; ''The stem cell factor receptor/c-Kit as a drug target in cancer.''; PubMedEurope PMCScholia
Marino M, Ascenzi P, Acconcia F.; ''S-palmitoylation modulates estrogen receptor alpha localization and functions.''; PubMedEurope PMCScholia
La Rosa P, Pesiri V, Leclercq G, Marino M, Acconcia F.; ''Palmitoylation regulates 17β-estradiol-induced estrogen receptor-α degradation and transcriptional activity.''; PubMedEurope PMCScholia
Gao T, Furnari F, Newton AC.; ''PHLPP: a phosphatase that directly dephosphorylates Akt, promotes apoptosis, and suppresses tumor growth.''; PubMedEurope PMCScholia
Song LB, Li J, Liao WT, Feng Y, Yu CP, Hu LJ, Kong QL, Xu LH, Zhang X, Liu WL, Li MZ, Zhang L, Kang TB, Fu LW, Huang WL, Xia YF, Tsao SW, Li M, Band V, Band H, Shi QH, Zeng YX, Zeng MS.; ''The polycomb group protein Bmi-1 represses the tumor suppressor PTEN and induces epithelial-mesenchymal transition in human nasopharyngeal epithelial cells.''; PubMedEurope PMCScholia
Zeng F, Xu J, Harris RC.; ''Nedd4 mediates ErbB4 JM-a/CYT-1 ICD ubiquitination and degradation in MDCK II cells.''; PubMedEurope PMCScholia
Crosier PS, Ricciardi ST, Hall LR, Vitas MR, Clark SC, Crosier KE.; ''Expression of isoforms of the human receptor tyrosine kinase c-kit in leukemic cell lines and acute myeloid leukemia.''; PubMedEurope PMCScholia
Trotman LC, Wang X, Alimonti A, Chen Z, Teruya-Feldstein J, Yang H, Pavletich NP, Carver BS, Cordon-Cardo C, Erdjument-Bromage H, Tempst P, Chi SG, Kim HJ, Misteli T, Jiang X, Pandolfi PP.; ''Ubiquitination regulates PTEN nuclear import and tumor suppression.''; PubMedEurope PMCScholia
Schaeper U, Gehring NH, Fuchs KP, Sachs M, Kempkes B, Birchmeier W.; ''Coupling of Gab1 to c-Met, Grb2, and Shp2 mediates biological responses.''; PubMedEurope PMCScholia
Shenoy SS, Nanda H, Lösche M.; ''Membrane association of the PTEN tumor suppressor: electrostatic interaction with phosphatidylserine-containing bilayers and regulatory role of the C-terminal tail.''; PubMedEurope PMCScholia
Kim YJ, Sekiya F, Poulin B, Bae YS, Rhee SG.; ''Mechanism of B-cell receptor-induced phosphorylation and activation of phospholipase C-gamma2.''; PubMedEurope PMCScholia
Manning BD, Tee AR, Logsdon MN, Blenis J, Cantley LC.; ''Identification of the tuberous sclerosis complex-2 tumor suppressor gene product tuberin as a target of the phosphoinositide 3-kinase/akt pathway.''; PubMedEurope PMCScholia
Slavik JM, Hutchcroft JE, Bierer BE.; ''CD28/CTLA-4 and CD80/CD86 families: signaling and function.''; PubMedEurope PMCScholia
Das S, Dixon JE, Cho W.; ''Membrane-binding and activation mechanism of PTEN.''; PubMedEurope PMCScholia
Uygur B, Abramo K, Leikina E, Vary C, Liaw L, Wu WS.; ''SLUG is a direct transcriptional repressor of PTEN tumor suppressor.''; PubMedEurope PMCScholia
Sweeney C, Lai C, Riese DJ, Diamonti AJ, Cantley LC, Carraway KL.; ''Ligand discrimination in signaling through an ErbB4 receptor homodimer.''; PubMedEurope PMCScholia
Letourneux C, Rocher G, Porteu F.; ''B56-containing PP2A dephosphorylate ERK and their activity is controlled by the early gene IEX-1 and ERK.''; PubMedEurope PMCScholia
Smolen GA, Sordella R, Muir B, Mohapatra G, Barmettler A, Archibald H, Kim WJ, Okimoto RA, Bell DW, Sgroi DC, Christensen JG, Settleman J, Haber DA.; ''Amplification of MET may identify a subset of cancers with extreme sensitivity to the selective tyrosine kinase inhibitor PHA-665752.''; PubMedEurope PMCScholia
Voncken JW, Niessen H, Neufeld B, Rennefahrt U, Dahlmans V, Kubben N, Holzer B, Ludwig S, Rapp UR.; ''MAPKAP kinase 3pK phosphorylates and regulates chromatin association of the polycomb group protein Bmi1.''; PubMedEurope PMCScholia
Huh CG, Factor VM, Sánchez A, Uchida K, Conner EA, Thorgeirsson SS.; ''Hepatocyte growth factor/c-met signaling pathway is required for efficient liver regeneration and repair.''; PubMedEurope PMCScholia
Jacobs FM, van der Heide LP, Wijchers PJ, Burbach JP, Hoekman MF, Smidt MP.; ''FoxO6, a novel member of the FoxO class of transcription factors with distinct shuttling dynamics.''; PubMedEurope PMCScholia
Rudd CE, Schneider H.; ''Unifying concepts in CD28, ICOS and CTLA4 co-receptor signalling.''; PubMedEurope PMCScholia
Meng F, Henson R, Wehbe-Janek H, Ghoshal K, Jacob ST, Patel T.; ''MicroRNA-21 regulates expression of the PTEN tumor suppressor gene in human hepatocellular cancer.''; PubMedEurope PMCScholia
Kim H, Huang W, Jiang X, Pennicooke B, Park PJ, Johnson MD.; ''Integrative genome analysis reveals an oncomir/oncogene cluster regulating glioblastoma survivorship.''; PubMedEurope PMCScholia
Lang V, Aillet F, Da Silva-Ferrada E, Xolalpa W, Zabaleta L, Rivas C, Rodriguez MS.; ''Analysis of PTEN ubiquitylation and SUMOylation using molecular traps.''; PubMedEurope PMCScholia
Woo SY, Kim DH, Jun CB, Kim YM, Haar EV, Lee SI, Hegg JW, Bandhakavi S, Griffin TJ, Kim DH.; ''PRR5, a novel component of mTOR complex 2, regulates platelet-derived growth factor receptor beta expression and signaling.''; PubMedEurope PMCScholia
Rönnstrand L.; ''Signal transduction via the stem cell factor receptor/c-Kit.''; PubMedEurope PMCScholia
Meier R, Alessi DR, Cron P, Andjelković M, Hemmings BA.; ''Mitogenic activation, phosphorylation, and nuclear translocation of protein kinase Bbeta.''; PubMedEurope PMCScholia
Eswarakumar VP, Lax I, Schlessinger J.; ''Cellular signaling by fibroblast growth factor receptors.''; PubMedEurope PMCScholia
Maehama T, Dixon JE.; ''The tumor suppressor, PTEN/MMAC1, dephosphorylates the lipid second messenger, phosphatidylinositol 3,4,5-trisphosphate.''; PubMedEurope PMCScholia
Kaushansky A, Gordus A, Budnik BA, Lane WS, Rush J, MacBeath G.; ''System-wide investigation of ErbB4 reveals 19 sites of Tyr phosphorylation that are unusually selective in their recruitment properties.''; PubMedEurope PMCScholia
Zhang H.; ''Life without kinase: cyclin E promotes DNA replication licensing and beyond.''; PubMedEurope PMCScholia
Yamanashi Y, Kakiuchi T, Mizuguchi J, Yamamoto T, Toyoshima K.; ''Association of B cell antigen receptor with protein tyrosine kinase Lyn.''; PubMedEurope PMCScholia
Acuto O, Michel F.; ''CD28-mediated co-stimulation: a quantitative support for TCR signalling.''; PubMedEurope PMCScholia
Naldini L, Vigna E, Narsimhan RP, Gaudino G, Zarnegar R, Michalopoulos GK, Comoglio PM.; ''Hepatocyte growth factor (HGF) stimulates the tyrosine kinase activity of the receptor encoded by the proto-oncogene c-MET.''; PubMedEurope PMCScholia
Mandelker D, Gabelli SB, Schmidt-Kittler O, Zhu J, Cheong I, Huang CH, Kinzler KW, Vogelstein B, Amzel LM.; ''A frequent kinase domain mutation that changes the interaction between PI3Kalpha and the membrane.''; PubMedEurope PMCScholia
Di Paolo G, Pellegrini L, Letinic K, Cestra G, Zoncu R, Voronov S, Chang S, Guo J, Wenk MR, De Camilli P.; ''Recruitment and regulation of phosphatidylinositol phosphate kinase type 1 gamma by the FERM domain of talin.''; PubMedEurope PMCScholia
Hou XJ, Ni KM, Yang JM, Li XM.; ''Neuregulin 1/ErbB4 enhances synchronized oscillations of prefrontal cortex neurons via inhibitory synapses.''; PubMedEurope PMCScholia
Ornitz DM, Marie PJ.; ''FGF signaling pathways in endochondral and intramembranous bone development and human genetic disease.''; PubMedEurope PMCScholia
Roberts PJ, Der CJ.; ''Targeting the Raf-MEK-ERK mitogen-activated protein kinase cascade for the treatment of cancer.''; PubMedEurope PMCScholia
Besser D, Bardelli A, Didichenko S, Thelen M, Comoglio PM, Ponzetto C, Nagamine Y.; ''Regulation of the urokinase-type plasminogen activator gene by the oncogene Tpr-Met involves GRB2.''; PubMedEurope PMCScholia
Maina F, Hilton MC, Ponzetto C, Davies AM, Klein R.; ''Met receptor signaling is required for sensory nerve development and HGF promotes axonal growth and survival of sensory neurons.''; PubMedEurope PMCScholia
Hodakoski C, Hopkins BD, Barrows D, Mense SM, Keniry M, Anderson KE, Kern PA, Hawkins PT, Stephens LR, Parsons R.; ''Regulation of PTEN inhibition by the pleckstrin homology domain of P-REX2 during insulin signaling and glucose homeostasis.''; PubMedEurope PMCScholia
Kovacina KS, Park GY, Bae SS, Guzzetta AW, Schaefer E, Birnbaum MJ, Roth RA.; ''Identification of a proline-rich Akt substrate as a 14-3-3 binding partner.''; PubMedEurope PMCScholia
Pelicci G, Giordano S, Zhen Z, Salcini AE, Lanfrancone L, Bardelli A, Panayotou G, Waterfield MD, Ponzetto C, Pelicci PG.; ''The motogenic and mitogenic responses to HGF are amplified by the Shc adaptor protein.''; PubMedEurope PMCScholia
Lessmann V, Gottmann K, Malcangio M.; ''Neurotrophin secretion: current facts and future prospects.''; PubMedEurope PMCScholia
Alegre ML, Frauwirth KA, Thompson CB.; ''T-cell regulation by CD28 and CTLA-4.''; PubMedEurope PMCScholia
Hazan R, Margolis B, Dombalagian M, Ullrich A, Zilberstein A, Schlessinger J.; ''Identification of autophosphorylation sites of HER2/neu.''; PubMedEurope PMCScholia
Geng F, Zhang J, Wu JL, Zou WJ, Liang ZP, Bi LL, Liu JH, Kong Y, Huang CQ, Li XW, Yang JM, Gao TM.; ''Neuregulin 1-ErbB4 signaling in the bed nucleus of the stria terminalis regulates anxiety-like behavior.''; PubMedEurope PMCScholia
Tzahar E, Levkowitz G, Karunagaran D, Yi L, Peles E, Lavi S, Chang D, Liu N, Yayon A, Wen D.; ''ErbB-3 and ErbB-4 function as the respective low and high affinity receptors of all Neu differentiation factor/heregulin isoforms.''; PubMedEurope PMCScholia
Collaud S, Tischler V, Atanassoff A, Wiedl T, Komminoth P, Oehlschlegel C, Weder W, Soltermann A.; ''Lung neuroendocrine tumors: correlation of ubiquitinylation and sumoylation with nucleo-cytosolic partitioning of PTEN.''; PubMedEurope PMCScholia
Williams EJ, Furness J, Walsh FS, Doherty P.; ''Activation of the FGF receptor underlies neurite outgrowth stimulated by L1, N-CAM, and N-cadherin.''; PubMedEurope PMCScholia
Cheng Q, Cross B, Li B, Chen L, Li Z, Chen J.; ''Regulation of MDM2 E3 ligase activity by phosphorylation after DNA damage.''; PubMedEurope PMCScholia
Sun Y, Ikrar T, Davis MF, Gong N, Zheng X, Luo ZD, Lai C, Mei L, Holmes TC, Gandhi SP, Xu X.; ''Neuregulin-1/ErbB4 Signaling Regulates Visual Cortical Plasticity.''; PubMedEurope PMCScholia
Roskoski R.; ''RAF protein-serine/threonine kinases: structure and regulation.''; PubMedEurope PMCScholia
Zhang CL, Zou Y, Yu RT, Gage FH, Evans RM.; ''Nuclear receptor TLX prevents retinal dystrophy and recruits the corepressor atrophin1.''; PubMedEurope PMCScholia
Tay Y, Rinn J, Pandolfi PP.; ''The multilayered complexity of ceRNA crosstalk and competition.''; PubMedEurope PMCScholia
Muik M, Frischauf I, Derler I, Fahrner M, Bergsmann J, Eder P, Schindl R, Hesch C, Polzinger B, Fritsch R, Kahr H, Madl J, Gruber H, Groschner K, Romanin C.; ''Dynamic coupling of the putative coiled-coil domain of ORAI1 with STIM1 mediates ORAI1 channel activation.''; PubMedEurope PMCScholia
Salvesen GS, Duckett CS.; ''IAP proteins: blocking the road to death's door.''; PubMedEurope PMCScholia
Li Z, Mei Y, Liu X, Zhou M.; ''Neuregulin-1 only induces trans-phosphorylation between ErbB receptor heterodimer partners.''; PubMedEurope PMCScholia
Patel L, Pass I, Coxon P, Downes CP, Smith SA, Macphee CH.; ''Tumor suppressor and anti-inflammatory actions of PPARgamma agonists are mediated via upregulation of PTEN.''; PubMedEurope PMCScholia
Clarke JH, Wang M, Irvine RF.; ''Localization, regulation and function of type II phosphatidylinositol 5-phosphate 4-kinases.''; PubMedEurope PMCScholia
Carraway KL, Weber JL, Unger MJ, Ledesma J, Yu N, Gassmann M, Lai C.; ''Neuregulin-2, a new ligand of ErbB3/ErbB4-receptor tyrosine kinases.''; PubMedEurope PMCScholia
Sakkab D, Lewitzky M, Posern G, Schaeper U, Sachs M, Birchmeier W, Feller SM.; ''Signaling of hepatocyte growth factor/scatter factor (HGF) to the small GTPase Rap1 via the large docking protein Gab1 and the adapter protein CRKL.''; PubMedEurope PMCScholia
Ponzetto C, Bardelli A, Zhen Z, Maina F, dalla Zonca P, Giordano S, Graziani A, Panayotou G, Comoglio PM.; ''A multifunctional docking site mediates signaling and transformation by the hepatocyte growth factor/scatter factor receptor family.''; PubMedEurope PMCScholia
Ichim G, Genevois AL, Ménard M, Yu LY, Coelho-Aguiar JM, Llambi F, Jarrosson-Wuilleme L, Lefebvre J, Tulasne D, Dupin E, Le Douarin N, Arumäe U, Tauszig-Delamasure S, Mehlen P.; ''The dependence receptor TrkC triggers mitochondria-dependent apoptosis upon Cobra-1 recruitment.''; PubMedEurope PMCScholia
Kirchhofer D, Yao X, Peek M, Eigenbrot C, Lipari MT, Billeci KL, Maun HR, Moran P, Santell L, Wiesmann C, Lazarus RA.; ''Structural and functional basis of the serine protease-like hepatocyte growth factor beta-chain in Met binding and signaling.''; PubMedEurope PMCScholia
Myers MP, Pass I, Batty IH, Van der Kaay J, Stolarov JP, Hemmings BA, Wigler MH, Downes CP, Tonks NK.; ''The lipid phosphatase activity of PTEN is critical for its tumor supressor function.''; PubMedEurope PMCScholia
Pekarsky Y, Hallas C, Palamarchuk A, Koval A, Bullrich F, Hirata Y, Bichi R, Letofsky J, Croce CM.; ''Akt phosphorylates and regulates the orphan nuclear receptor Nur77.''; PubMedEurope PMCScholia
Muraoka-Cook RS, Sandahl M, Hunter D, Miraglia L, Earp HS.; ''Prolactin and ErbB4/HER4 signaling interact via Janus kinase 2 to induce mammary epithelial cell gene expression differentiation.''; PubMedEurope PMCScholia
Naresh A, Long W, Vidal GA, Wimley WC, Marrero L, Sartor CI, Tovey S, Cooke TG, Bartlett JM, Jones FE.; ''The ERBB4/HER4 intracellular domain 4ICD is a BH3-only protein promoting apoptosis of breast cancer cells.''; PubMedEurope PMCScholia
Gual P, Giordano S, Williams TA, Rocchi S, Van Obberghen E, Comoglio PM.; ''Sustained recruitment of phospholipase C-gamma to Gab1 is required for HGF-induced branching tubulogenesis.''; PubMedEurope PMCScholia
Shen K, Novak RF.; ''DDT stimulates c-erbB2, c-met, and STATS tyrosine phosphorylation, Grb2-Sos association, MAPK phosphorylation, and proliferation of human breast epithelial cells.''; PubMedEurope PMCScholia
Palamidessi A, Frittoli E, Garré M, Faretta M, Mione M, Testa I, Diaspro A, Lanzetti L, Scita G, Di Fiore PP.; ''Endocytic trafficking of Rac is required for the spatial restriction of signaling in cell migration.''; PubMedEurope PMCScholia
Serra V, Markman B, Scaltriti M, Eichhorn PJ, Valero V, Guzman M, Botero ML, Llonch E, Atzori F, Di Cosimo S, Maira M, Garcia-Echeverria C, Parra JL, Arribas J, Baselga J.; ''NVP-BEZ235, a dual PI3K/mTOR inhibitor, prevents PI3K signaling and inhibits the growth of cancer cells with activating PI3K mutations.''; PubMedEurope PMCScholia
Mayo LD, Donner DB.; ''A phosphatidylinositol 3-kinase/Akt pathway promotes translocation of Mdm2 from the cytoplasm to the nucleus.''; PubMedEurope PMCScholia
Cseh B, Doma E, Baccarini M.; ''"RAF" neighborhood: protein-protein interaction in the Raf/Mek/Erk pathway.''; PubMedEurope PMCScholia
This CandidateSet contains sequences identified by William Pearson's analysis of Reactome catalyst entities. Catalyst entity sequences were used to identify analagous sequences that shared overall homology and active site homology. Sequences in this Candidate set were identified in an April 24, 2012 analysis.
This CandidateSet contains sequences identified by William Pearson's analysis of Reactome catalyst entities. Catalyst entity sequences were used to identify analagous sequences that shared overall homology and active site homology. Sequences in this Candidate set were identified in an April 24, 2012 analysis.
Optimal activation of T-lymphocytes requires at least two signals. A primary one is delivered by the T-cell receptor (TCR) complex after antigen recognition and additional costimulatory signals are delivered by the engagement of costimulatory receptors such as CD28. The best-characterized costimulatory pathways are mediated by a set of cosignaling molecules belonging to the CD28 superfamily, including CD28, CTLA4, ICOS, PD1 and BTLA receptors. These proteins deliver both positive and negative second signals to T-cells by interacting with B7 family ligands expressed on antigen presenting cells. Different subsets of T-cells have very different requirements for costimulation. CD28 family mediated costimulation is not required for all T-cell responses in vivo, and alternative costimulatory pathways also exist. Different receptors of the CD28 family and their ligands have different regulation of expression. CD28 is constitutively expressed on naive T cells whereas CTLA4 expression is dependent on CD28/B7 engagement and the other receptor members ICOS, PD1 and BTLA are induced after initial T-cell stimulation. The positive signals induced by CD28 and ICOS molecules are counterbalanced by other members of the CD28 family, including cytotoxic T-lymphocyte associated antigen (CTLA)4, programmed cell death (PD)1, and B and T lymphocyte attenuator (BTLA), which dampen immune responses. The balance of stimulatory and inhibitory signals is crucial to maximize protective immune responses while maintaining immunological tolerance and preventing autoimmunity. The costimulatory receptors CD28, CTLA4, ICOS and PD1 are composed of single extracellular IgV-like domains, whereas BTLA has one IgC-like domain. Receptors CTLA4, CD28 and ICOS are covalent homodimers, due to an interchain disulphide linkage. The costimulatory ligands B71, B72, B7H2, B7H1 and B7DC, have a membrane proximal IgC-like domain and a membrane distal IgV-like domain that is responsible for receptor binding and dimerization. CD28 and CTLA4 have no known intrinsic enzymatic activity. Instead, engagement by their physiologic ligands B71 and B72 leads to the physical recruitment and activation of downstream T-cell effector molecules.
In addition to its well-characterized role in estrogen-dependent transcription, estrogen (beta-estradiol, also known as E2) also plays a rapid, non-genomic role through interaction with receptors localized at the plasma membrane by virtue of dynamic palmitoylation. Estrogen receptor palmitoylation is a prerequisite for the E2-dependent activation of extra-nuclear signaling both in vitro and in animal models (Acconcia et al, 2004; Acconcia et al, 2005; Marino et al, 2006; Marino and Ascenzi, 2006). Non-genomic signaling through the estrogen receptor ESR1 also depends on receptor arginine methylation by PMRT1 (Pedram et al, 2007; Pedram et al, 2012; Le Romancer et al, 2008; reviewed in Arnal, 2017; Le Romancer et al, 2011 ). E2-evoked extra-nuclear signaling is independent of the transcriptional activity of estrogen receptors and occurs within seconds to minutes following E2 administration to target cells. Extra-nuclear signaling consists of the activation of a plethora of signaling pathways including the RAF/MAP kinase cascade and the PI3K/AKT signaling cascade and governs processes such as apoptosis, cellular proliferation and metastasis (reviewed in Hammes et al, 2007; Handa et al, 2012; Lange et al, 2007; Losel et al, 2003; Arnal et al, 2017; Le Romancer et al, 2011). ESR-mediated signaling also cross-talks with receptor tyrosine kinase, NF- kappa beta and GPCR signaling pathways by modulating the post-translational modification of enzymes and other proteins and regulating second messengers (reviewed in Arnal et al, 2017; Schwartz et al, 2016; Boonyaratanakornkit, 2011; Biswas et al, 2005). In the nervous system, E2 affects neural functions such as cognition, behaviour, stress responses and reproduction in part by inducing such rapid extra-nuclear responses (Farach-Carson and Davis, 2003; Losel et al, 2003), while in endothelial cells, non-genomic ESR-dependent signaling also regulates vasodilation through the eNOS pathway (reviewed in Levin, 2011). Extra-nuclear signaling additionally cross-talks with nuclear estrogen receptor signaling and is required to control ER protein stability (La Rosa et al, 2012) Recent data have demonstrated that the membrane ESR1 can interact with various endocytic proteins to traffic and signal within the cytoplasm. This receptor intracellular trafficking appears to be dependent on the phyical interaction of ESR1 with specific trans-membrane receptors such as IGR-1R and beta 1-integrin (Sampayo et al, 2018)
The intrinsic (Bcl-2 inhibitable or mitochondrial) pathway of apoptosis functions in response to various types of intracellular stress including growth factor withdrawal, DNA damage, unfolding stresses in the endoplasmic reticulum and death receptor stimulation. Following the reception of stress signals, proapoptotic BCL-2 family proteins are activated and subsequently interact with and inactivate antiapoptotic BCL-2 proteins. This interaction leads to the destabilization of the mitochondrial membrane and release of apoptotic factors. These factors induce the caspase proteolytic cascade, chromatin condensation, and DNA fragmentation, ultimately leading to cell death. The key players in the Intrinsic pathway are the Bcl-2 family of proteins that are critical death regulators residing immediately upstream of mitochondria. The Bcl-2 family consists of both anti- and proapoptotic members that possess conserved alpha-helices with sequence conservation clustered in BCL-2 Homology (BH) domains. Proapoptotic members are organized as follows:
1. "Multidomain" BAX family proteins such as BAX, BAK etc. that display sequence conservation in their BH1-3 regions. These proteins act downstream in mitochondrial disruption.
2. "BH3-only" proteins such as BID,BAD, NOXA, PUMA,BIM, and BMF have only the short BH3 motif. These act upstream in the pathway, detecting developmental death cues or intracellular damage. Anti-apoptotic members like Bcl-2, Bcl-XL and their relatives exhibit homology in all segments BH1-4. One of the critical functions of BCL-2/BCL-XL proteins is to maintain the integrity of the mitochondrial outer membrane.
Target of rapamycin (mTOR) is a highly-conserved serine/threonine kinase that regulates cell growth and division in response to energy levels, growth signals, and nutrients (Zoncu et al. 2011). Control of mTOR activity is critical for the cell since its dysregulation leads to cancer, metabolic disease, and diabetes (Laplante & Sabatini 2012). In cells, mTOR exists as two structurally distinct complexes termed mTOR complex 1 (mTORC1) and mTOR complex 2 (mTORC2), each one with specificity for different sets of effectors. mTORC1 couples energy and nutrient abundance to cell growth and proliferation by balancing anabolic (protein synthesis and nutrient storage) and catabolic (autophagy and utilization of energy stores) processes.
Nitric oxide (NO), a multifunctional second messenger, is implicated in physiological processes in mammals that range from immune response and potentiation of synaptic transmission to dilation of blood vessels and muscle relaxation. NO is a highly active molecule that diffuses across cell membranes and cannot be stored inside the producing cell. Its signaling capacity is controlled at the levels of biosynthesis and local availability. Its production by NO synthases is under complex and tight control, being regulated at transcriptional and translational levels, through co- and posttranslational modifications, and by subcellular localization. NO is synthesized from L-arginine by a family of nitric oxide synthases (NOS). Three NOS isoforms have been characterized: neuronal NOS (nNOS, NOS1) primarily found in neuronal tissue and skeletal muscle; inducible NOS (iNOS, NOS2) originally isolated from macrophages and later discovered in many other cell types; and endothelial NOS (eNOS, NOS3) present in vascular endothelial cells, cardiac myocytes, and in blood platelets. The enzymatic activity of all three isoforms is dependent on calmodulin, which binds to nNOS and eNOS at elevated intracellular calcium levels, while it is tightly associated with iNOS even at basal calcium levels. As a result, the enzymatic activity of nNOS and eNOS is modulated by changes in intracellular calcium levels, leading to transient NO production, while iNOS continuously releases NO independent of fluctuations in intracellular calcium levels and is mainly regulated at the gene expression level (Pacher et al. 2007).
The NOS enzymes share a common basic structural organization and requirement for substrate cofactors for enzymatic activity. A central calmodulin-binding motif separates an NH2-terminal oxygenase domain from a COOH-terminal reductase domain. Binding sites for cofactors NADPH, FAD, and FMN are located within the reductase domain, while binding sites for tetrahydrobiopterin (BH4) and heme are located within the oxygenase domain. Once calmodulin binds, it facilitates electron transfer from the cofactors in the reductase domain to heme enabling nitric oxide production. Both nNOS and eNOS contain an additional insert (40-50 amino acids) in the middle of the FMN-binding subdomain that serves as autoinhibitory loop, destabilizing calmodulin binding at low calcium levels and inhibiting electron transfer from FMN to the heme in the absence of calmodulin. iNOS does not contain this insert.
In this Reactome pathway module, details of eNOS activation and regulation are annotated. Originally identified as endothelium-derived relaxing factor, eNOS derived NO is a critical signaling molecule in vascular homeostasis. It regulates blood pressure and vascular tone, and is involved in vascular smooth muscle cell proliferation, platelet aggregation, and leukocyte adhesion. Loss of endothelium derived NO is a key feature of endothelial dysfunction, implicated in the pathogenesis of hypertension and atherosclerosis. The endothelial isoform eNOS is unique among the nitric oxide synthase (NOS) family in that it is co-translationally modified at its amino terminus by myristoylation and is further acylated by palmitoylation (two residues next to the myristoylation site). These modifications target eNOS to the plasma membrane caveolae and lipid rafts.
Factors that stimulate eNOS activation and nitric oxide (NO) production include fluid shear stress generated by blood flow, vascular endothelial growth factor (VEGF), bradykinin, estrogen, insulin, and angiopoietin. The activity of eNOS is further regulated by numerous post-translational modifications, including protein-protein interactions, phosphorylation, and subcellular localization.
Following activation, eNOS shuttles between caveolae and other subcellular compartments such as the noncaveolar plasma membrane portions, Golgi apparatus, and perinuclear structures. This subcellular distribution is variable depending upon cell type and mode of activation.
Subcellular localization of eNOS has a profound effect on its ability to produce NO as the availability of its substrates and cofactors will vary with location. eNOS is primarily particulate, and depending on the cell type, eNOS can be found in several membrane compartments: plasma membrane caveolae, lipid rafts, and intracellular membranes such as the Golgi complex.
Mitotic G1-G1/S phase involves G1 phase of the mitotic interphase and G1/S transition, when a cell commits to DNA replication and divison genetic and cellular material to two daughter cells.
During early G1, cells can enter a quiescent G0 state. In quiescent cells, the evolutionarily conserved DREAM complex, consisting of the pocket protein family member p130 (RBL2), bound to E2F4 or E2F5, and the MuvB complex, represses transcription of cell cycle genes (reviewed by Sadasivam and DeCaprio 2013).
During early G1 phase in actively cycling cells, transcription of cell cycle genes is repressed by another pocket protein family member, p107 (RBL1), which forms a complex with E2F4 (Ferreira et al. 1998, Cobrinik 2005). RB1 tumor suppressor, the product of the retinoblastoma susceptibility gene, is the third member of the pocket protein family. RB1 binds to E2F transcription factors E2F1, E2F2 and E2F3 and inhibits their transcriptional activity, resulting in prevention of G1/S transition (Chellappan et al. 1991, Bagchi et al. 1991, Chittenden et al. 1991, Lees et al. 1993, Hiebert 1993, Wu et al. 2001). Once RB1 is phosphorylated on serine residue S795 by Cyclin D:CDK4/6 complexes, it can no longer associate with and inhibit E2F1-3. Thus, CDK4/6-mediated phosphorylation of RB1 leads to transcriptional activation of E2F1-3 target genes needed for the S phase of the cell cycle (Connell-Crowley et al. 1997). CDK2, in complex with cyclin E, contributes to RB1 inactivation and also activates proteins needed for the initiation of DNA replication (Zhang 2007). Expression of D type cyclins is regulated by extracellular mitogens (Cheng et al. 1998, Depoortere et al. 1998). Catalytic activities of CDK4/6 and CDK2 are controlled by CDK inhibitors of the INK4 family (Serrano et al. 1993, Hannon and Beach 1994, Guan et al. 1994, Guan et al. 1996, Parry et al. 1995) and the Cip/Kip family, respectively.
PTEN is regulated at the level of gene transcription, mRNA translation, localization and protein stability.
Transcription of the PTEN gene is regulated at multiple levels. Epigenetic repression involves the recruitment of Mi-2/NuRD upon SALL4 binding to the PTEN promoter (Yang et al. 2008, Lu et al. 2009) or EVI1-mediated recruitment of the polycomb repressor complex (PRC) to the PTEN promoter (Song et al. 2009, Yoshimi et al. 2011). Transcriptional regulation is also elicited by negative regulators, including NR2E1:ATN1 (atrophin-1) complex, JUN (c-Jun), SNAIL and SLUG (Zhang et al. 2006, Vasudevan et al. 2007, Escriva et al. 2008, Uygur et al. 2015) and positive regulators such as TP53 (p53), MAF1, ATF2, EGR1 or PPARG (Stambolic et al. 2001, Virolle et al. 2001, Patel et al. 2001, Shen et al. 2006, Li et al. 2016).
MicroRNAs miR-26A1, miR-26A2, miR-22, miR-25, miR-302, miR-214, miR-17-5p, miR-19 and miR-205 bind PTEN mRNA and inhibit its translation into protein. These microRNAs are altered in cancer and can account for changes in PTEN levels (Meng et al. 2007, Xiao et al. 2008, Yang et al. 2008, Huse et al. 2009, Kim et al. 2010, Poliseno, Salmena, Riccardi et al. 2010, Cai et al. 2013). In addition, coding and non-coding RNAs can prevent microRNAs from binding to PTEN mRNA. These RNAs are termed competing endogenous RNAs or ceRNAs. Transcripts of the pseudogene PTENP1 and mRNAs transcribed from SERINC1, VAPA and CNOT6L genes exhibit this activity (Poliseno, Salmena, Zhang et al. 2010, Tay et al. 2011, Tay et al. 2014).
PTEN can translocate from the cytosol to the nucleus after undergoing monoubiquitination. PTEN's ability to localize to the nucleus contributes to its tumor suppressive role (Trotman et al. 2007). The ubiquitin protease USP7 (HAUSP) targets monoubiquitinated PTEN in the nucleus, resulting in PTEN deubiquitination and nuclear exclusion. PML, via an unknown mechanism that involves USP7- and PML-interacting protein DAXX, inhibits USP7-mediated deubiquitination of PTEN, thus promoting PTEN nuclear localization. Disruption of PML function in acute promyelocytic leukemia, through a chromosomal translocation that results in expression of a fusion protein PML-RARA, leads to aberrant PTEN localization (Song et al. 2008).
Several ubiquitin ligases, including NEDD4, WWP2, STUB1 (CHIP), RNF146, XIAP and MKRN1, polyubiquitinate PTEN and target it for proteasome-mediated degradation (Wang et al. 2007, Van Themsche et al. 2009, Ahmed et al. 2011, Maddika et al. 2011, Lee et al. 2015, Li et al. 2015). The ubiquitin proteases USP13 and OTUD3, frequently down-regulated in breast cancer, remove polyubiquitin chains from PTEN, thus preventing its degradation and increasing its half-life (Zhang et al. 2013, Yuan et al. 2015). The catalytic activity of PTEN is negatively regulated by PREX2 binding (Fine et al. 2009, Hodakoski et al. 2014) and TRIM27-mediated ubiquitination (Lee et al. 2013), most likely through altered PTEN conformation.
In addition to ubiquitination, PTEN also undergoes SUMOylation (Gonzalez-Santamaria et al. 2012, Da Silva Ferrada et al. 2013, Lang et al. 2015, Leslie et al. 2016). SUMOylation of the C2 domain of PTEN may regulate PTEN association with the plasma membrane (Shenoy et al. 2012) as well as nuclear localization of PTEN (Bassi et al. 2013, Collaud et al. 2016). PIASx-alpha, a splicing isorom of E3 SUMO-protein ligase PIAS2 has been implicated in PTEN SUMOylation (Wang et al. 2014). SUMOylation of PTEN may be regulated by activated AKT (Lin et al. 2016). Reactions describing PTEN SUMOylation will be annotated when mechanistic details become available.
Phosphorylation affects the stability and activity of PTEN. FRK tyrosine kinase (RAK) phosphorylates PTEN on tyrosine residue Y336, which increases PTEN half-life by inhibiting NEDD4-mediated polyubiquitination and subsequent degradation of PTEN. FRK-mediated phosphorylation also increases PTEN enzymatic activity (Yim et al. 2009). Casein kinase II (CK2) constitutively phosphorylates the C-terminal tail of PTEN on serine and threonine residues S370, S380, T382, T383 and S385. CK2-mediated phosphorylation increases PTEN protein stability (Torres and Pulido 2001) but results in ~30% reduction in PTEN lipid phosphatase activity (Miller et al. 2002).
PTEN localization and activity are affected by acetylation of its lysine residues (Okumura et al. 2006, Ikenoue et al. 2008, Meng et al. 2016). PTEN can undergo oxidation, which affects its function, but the mechanism is poorly understood (Tan et al. 2015, Shen et al. 2015, Verrastro et al. 2016).
The RAS-RAF-MEK-ERK pathway regulates processes such as proliferation, differentiation, survival, senescence and cell motility in response to growth factors, hormones and cytokines, among others. Binding of these stimuli to receptors in the plasma membrane promotes the GEF-mediated activation of RAS at the plasma membrane and initiates the three-tiered kinase cascade of the conventional MAPK cascades. GTP-bound RAS recruits RAF (the MAPK kinase kinase), and promotes its dimerization and activation (reviewed in Cseh et al, 2014; Roskoski, 2010; McKay and Morrison, 2007; Wellbrock et al, 2004). Activated RAF phosphorylates the MAPK kinase proteins MEK1 and MEK2 (also known as MAP2K1 and MAP2K2), which in turn phophorylate the proline-directed kinases ERK1 and 2 (also known as MAPK3 and MAPK1) (reviewed in Roskoski, 2012a, b; Kryiakis and Avruch, 2012). Activated ERK proteins may undergo dimerization and have identified targets in both the nucleus and the cytosol; consistent with this, a proportion of activated ERK protein relocalizes to the nucleus in response to stimuli (reviewed in Roskoski 2012b; Turjanski et al, 2007; Plotnikov et al, 2010; Cargnello et al, 2011). Although initially seen as a linear cascade originating at the plasma membrane and culminating in the nucleus, the RAS/RAF MAPK cascade is now also known to be activated from various intracellular location. Temporal and spatial specificity of the cascade is achieved in part through the interaction of pathway components with numerous scaffolding proteins (reviewed in McKay and Morrison, 2007; Brown and Sacks, 2009). The importance of the RAS/RAF MAPK cascade is highlighted by the fact that components of this pathway are mutated with high frequency in a large number of human cancers. Activating mutations in RAS are found in approximately one third of human cancers, while ~8% of tumors express an activated form of BRAF (Roberts and Der, 2007; Davies et al, 2002; Cantwell-Dorris et al, 2011).
TP53 (p53) tumor suppressor protein is a transcription factor that functions as a homotetramer (Jeffrey et al. 1995). The protein levels of TP53 are low in unstressed cells due to MDM2-mediated ubiquitination that triggers proteasome-mediated degradation of TP53 (Wu et al. 1993). The E3 ubiquitin ligase MDM2 functions as a homodimer/homo-oligomer or a heterodimer/hetero-oligomer with MDM4 (MDMX) (Linares et al. 2003, Toledo and Wahl 2007, Cheng et al. 2011, Wade et al. 2013).
Activating phosphorylation of TP53 at serine residues S15 and S20 in response to genotoxic stress disrupts TP53 interaction with MDM2. In contrast to MDM2, E3 ubiquitin ligases RNF34 (CARP1) and RFFL (CARP2) can ubiquitinate phosphorylated TP53 (Yang et al. 2007). Binding of MDM2 to TP53 is also inhibited by the tumor suppressor p14-ARF, transcribed from the CDKN2A gene in response to oncogenic signaling or oxidative stress (Zhang et al. 1998, Parisi et al. 2002, Voncken et al. 2005). Ubiquitin-dependant degradation of TP53 can also be promoted by PIRH2 (Leng et al. 2003) and COP1 (Dornan et al. 2004) ubiquitin ligases. HAUSP (USP7) can deubiquitinate TP53, contributing to TP53 stabilization (Li et al. 2002).
While post-translational regulation plays a prominent role, TP53 activity is also controlled at the level of promoter function (reviewed in Saldana-Meyer and Recillas-Targa 2011), mRNA stability and translation efficiency (Mahmoudi et al. 2009, Le et al. 2009, Takagi et al. 2005).
The epidermal growth factor receptor (EGFR) is one member of the ERBB family of transmembrane glycoprotein tyrosine receptor kinases (RTK). Binding of EGFR to its ligands induces conformational change that unmasks the dimerization interface in the extracellular domain of EGFR, leading to receptor homo- or heterodimerization at the cell surface. Dimerization of the extracellular regions of EGFR triggers additional conformational change of the cytoplasmic EGFR regions, enabling the kinase domains of two EGFR molecules to achieve the catalytically active conformation. Ligand activated EGFR dimers trans-autophosphorylate on tyrosine residues in the cytoplasmic tail of the receptor. Phosphorylated tyrosines serve as binding sites for the recruitment of signal transducers and activators of intracellular substrates, which then stimulate intracellular signal transduction cascades that are involved in regulating cellular proliferation, differentiation, and survival. Recruitment of complexes containing GRB2 and SOS1 to phosphorylated EGFR dimers either directly, through phosphotyrosine residues that serve as GRB2 docking sites, or indirectly, through SHC1 recruitment, promotes GDP to GTP exchange on RAS, resulting in the activation of RAF/MAP kinase cascade. Binding of complexes of GRB2 and GAB1 to phosphorylated EGFR dimers leads to formation of the active PI3K complex, conversion of PIP2 into PIP3, and activation of AKT signaling. Phospholipase C-gamma1 (PLCG1) can also be recruited directly, through EGFR phosphotyrosine residues that serve as PLCG1 docking sites, which leads to PLCG1 phosphorylation by EGFR and activation of DAG and IP3 signaling. EGFR signaling is downregulated by the action of ubiquitin ligase CBL. CBL binds directly to the phosphorylated EGFR dimer through the phosphotyrosine Y1045 in the C-tail of EGFR, and after CBL is phosphorylated by EGFR, it becomes active and ubiquitinates phosphorylated EGFR dimers, targeting them for degradation. For a recent review of EGFR signaling, please refer to Avraham and Yarden, 2011.
ERBB2, also known as HER2 or NEU, is a receptor tyrosine kinase (RTK) belonging to the EGFR family. ERBB2 possesses an extracellular domain that does not bind any known ligand, contrary to other EGFR family members, a single transmembrane domain, and an intracellular domain consisting of an active kinase and a C-tail with multiple tyrosine phosphorylation sites. Inactive ERBB2 is associated with a chaperone heat shock protein 90 (HSP90) and its co-chaperone CDC37 (Xu et al. 2001, Citri et al. 2004, Xu et al. 2005). In addition, ERBB2 is associated with ERBB2IP (also known as ERBIN or LAP2), a protein responsible for proper localization of ERBB2. In epithelial cells, ERBB2IP restricts expression of ERBB2 to basolateral plasma membrane regions (Borg et al. 2000).
ERBB2 becomes activated by forming a heterodimer with another ligand-activated EGFR family member, either EGFR, ERBB3 or ERBB4, which is accompanied by dissociation of chaperoning proteins HSP90 and CDC37 (Citri et al. 2004), as well as ERBB2IP (Borg et al. 2000) from ERBB2. ERBB2 heterodimers function to promote cell proliferation, cell survival and differentiation, depending on the cellular context. ERBB2 can also be activated by homodimerization when it is overexpressed, in cancer for example.
In cells expressing both ERBB2 and EGFR, EGF stimulation of EGFR leads to formation of both ERBB2:EGFR heterodimers (Wada et al. 1990, Karunagaran et al. 1996) and EGFR homodimers. Heterodimers of ERBB2 and EGFR trans-autophosphorylate on twelve tyrosine residues, six in the C-tail of EGFR and six in the C-tail of ERBB2 - Y1023, Y1139, Y1196, Y1221, Y1222 and Y1248 (Margolis et al. 1989, Hazan et al. 1990,Walton et al. 1990, Helin et al. 1991, Ricci et al. 1995, Pinkas-Kramarski 1996). Phosphorylated tyrosine residues in the C-tail of EGFR and ERBB2 serve as docking sites for downstream signaling molecules. Three key signaling pathways activated by ERBB2:EGFR heterodimers are RAF/MAP kinase cascade, PI3K-induced AKT signaling, and signaling by phospholipase C gamma (PLCG1). Downregulation of EGFR signaling is mediated by ubiquitin ligase CBL, and is shown under Signaling by EGFR.
In cells expressing ERBB2 and ERBB3, ERBB3 activated by neuregulin NRG1 or NRG2 binding (Tzahar et al. 1994) forms a heterodimer with ERBB2 (Pinkas-Kramarski et al. 1996, Citri et al. 2004). ERBB3 is the only EGFR family member with no kinase activity, and can only function in heterodimers, with ERBB2 being its preferred heterodimerization partner. After heterodimerization, ERBB2 phosphorylates ten tyrosine residues in the C-tail of ERBB3, Y1054, Y1197, Y1199, Y1222, Y1224, Y1260, Y1262, Y1276, Y1289 and Y1328 (Prigent et al. 1994, Pinkas-Kramarski et al. 1996, Vijapurkar et al. 2003, Li et al. 2007) that subsequently serve as docking sites for downstream signaling molecules, resulting in activation of PI3K-induced AKT signaling and RAF/MAP kinase cascade. Signaling by ERBB3 is downregulated by the action of RNF41 ubiquitin ligase, also known as NRDP1.
In cells expressing ERBB2 and ERBB4, ligand stimulated ERBB4 can either homodimerize or form heterodimers with ERBB2 (Li et al. 2007), resulting in trans-autophosphorylation of ERBB2 and ERBB4 on C-tail tyrosine residues that will subsequently serve as docking sites for downstream signaling molecules, leading to activation of RAF/MAP kinase cascade and, in the case of ERBB4 CYT1 isoforms, PI3K-induced AKT signaling (Hazan et al. 1990, Cohen et al. 1996, Li et al. 2007, Kaushansky et al. 2008). Signaling by ERBB4 is downregulated by the action of WWP1 and ITCH ubiquitin ligases, and is shown under Signaling by ERBB4.
ERBB4, also known as HER4, belongs to the ERBB family of receptors, which also includes ERBB1 (EGFR/HER1), ERBB2 (HER2/NEU) and ERBB3 (HER3). Similar to EGFR, ERBB4 has an extracellular ligand binding domain, a single transmembrane domain and a cytoplasmic domain which contains an active tyrosine kinase and a C-tail with multiple phosphorylation sites. At least three and possibly four splicing isoforms of ERBB4 exist that differ in their C-tail and/or the extracellular juxtamembrane regions: ERBB4 JM-A CYT1, ERBB4 JM-A CYT2 and ERBB4 JM-B CYT1 (the existence of ERBB4 JM-B CYT2 has not been confirmed).
ERBB4 becomes activated by binding one of its seven ligands, three of which, HB-EGF, epiregulin EPR and betacellulin BTC, are EGF-like (Elenius et al. 1997, Riese et al. 1998), while four, NRG1, NRG2, NRG3 and NRG4, belong to the related neuregulin family (Tzahar et al. 1994, Carraway et al. 1997, Zhang et al. 1997, Hayes et al. 2007). Upon ligand binding, ERBB4 forms homodimers (Sweeney et al. 2000) or it heterodimerizes with ERBB2 (Li et al. 2007). Dimers of ERBB4 undergo trans-autophosphorylation on tyrosine residues in the C-tail (Cohen et al. 1996, Kaushansky et al. 2008, Hazan et al. 1990, Li et al. 2007), triggering downstream signaling cascades. The pathway Signaling by ERBB4 only shows signaling by ERBB4 homodimers. Signaling by heterodimers of ERBB4 and ERBB2 is shown in the pathway Signaling by ERBB2. Ligand-stimulated ERBB4 is also able to form heterodimers with ligand-stimulated EGFR (Cohen et al. 1996) and ligand-stimulated ERBB3 (Riese et al. 1995). Dimers of ERBB4 with EGFR and dimers of ERBB4 with ERBB3 were demonstrated in mouse cell lines in which human ERBB4 and EGFR or ERBB3 were exogenously expressed. These heterodimers undergo trans-autophosphorylation. The promiscuous heteromerization of ERBBs adds combinatorial diversity to ERBB signaling processes. As ERBB4 binds more ligands than other ERBBs, but has restricted expression, ERBB4 expression channels responses to ERBB ligands. The signaling capabilities of the four receptors have been compared (Schulze et al. 2005).
As for other receptor tyrosine kinases, ERBB4 signaling effectors are largely dictated through binding of effector proteins to ERBB4 peptides that are phosphorylated upon ligand binding. All splicing isoforms of ERBB4 possess two tyrosine residues in the C-tail that serve as docking sites for SHC1 (Kaushansky et al. 2008, Pinkas-Kramarski et al. 1996, Cohen et al. 1996). Once bound to ERBB4, SHC1 becomes phosphorylated on tyrosine residues by the tyrosine kinase activity of ERBB4, which enables it to recruit the complex of GRB2 and SOS1, resulting in the guanyl-nucleotide exchange on RAS and activation of RAF and MAP kinase cascade (Kainulainen et al. 2000).
The CYT1 isoforms of ERBB4 also possess a C-tail tyrosine residue that, upon trans-autophosphorylation, serves as a docking site for the p85 alpha subunit of PI3K (Kaushansky et al. 2008, Cohen et al. 1996), leading to assembly of an active PI3K complex that converts PIP2 to PIP3 and activates AKT signaling (Kainulainen et al. 2000).
Besides signaling as a conventional transmembrane receptor kinase, ERBB4 differs from other ERBBs in that JM-A isoforms signal through efficient release of a soluble intracellular domain. Ligand activated homodimers of ERBB4 JM-A isoforms (ERBB4 JM-A CYT1 and ERBB4 JM-A CYT2) undergo proteolytic cleavage by ADAM17 (TACE) in the juxtamembrane region, resulting in shedding of the extracellular domain and formation of an 80 kDa membrane bound ERBB4 fragment known as ERBB4 m80 (Rio et al. 2000, Cheng et al. 2003). ERBB4 m80 undergoes further proteolytic cleavage, mediated by the gamma-secretase complex, which releases the soluble 80 kDa ERBB4 intracellular domain, known as ERBB4 s80 or E4ICD, into the cytosol (Ni et al. 2001). ERBB4 s80 is able to translocate to the nucleus, promote nuclear translocation of various transcription factors, and act as a transcription co-factor. For example, in mammary cells, ERBB4 binds SH2 transcription factor STAT5A. ERBB4 s80 shuttles STAT5A to the nucleus, and actsa as a STAT5A co-factor in binding to and promoting transcription from the beta-casein (CSN2) promoter, and may be involved in the regulation of other lactation-related genes (Jones et al. 1999, Williams et al. 2004, Muraoka-Cook et al. 2008). ERBB4 s80 binds activated estrogen receptor in the nucleus and acts as a transcriptional co-factor in promoting transcription of some estrogen-regulated genes, including progesterone receptor gene NR3C3 and CXCL12 (SDF1) (Zhu et al. 2006). In neuronal precursors, ERBB4 s80 binds the complex of TAB and NCOR1, helps to move the complex into the nucleus, and is a co-factor of TAB:NCOR1-mediated inhibition of expression of astrocyte differentiation genes GFAP and S100B (Sardi et al. 2006).
The C-tail of ERBB4 possesses several WW-domain binding motifs (three in CYT1 isoform and two in CYT2 isoform), which enable interaction of ERBB4 with WW-domain containing proteins. ERBB4 s80, through WW-domain binding motifs, interacts with YAP1 transcription factor, a known proto-oncogene, and is a co-regulator of YAP1-mediated transcription in association with TEAD transcription factors (Komuro et al. 2003, Omerovic et al. 2004). Hence, the WW binding motif couples ERBB4 to the major effector arm of the HIPPO signaling pathway. The tumor suppressor WWOX, another WW-domain containing protein, competes with YAP1 in binding to ERBB4 s80 and prevents translocation of ERBB4 s80 to the nucleus (Aqeilan et al. 2005).
WW-domain binding motifs in the C-tail of ERBB4 play an important role in the downregulation of ERBB4 receptor signaling, enabling the interaction of intact ERBB4, ERBB4 m80 and ERBB4 s80 with NEDD4 family of E3 ubiquitin ligases WWP1 and ITCH. The interaction of WWP1 and ITCH with intact ERBB4 is independent of receptor activation and autophosphorylation. Binding of WWP1 and ITCH ubiquitin ligases leads to ubiquitination of ERBB4 and its cleavage products, and subsequent degradation through both proteasomal and lysosomal routes (Omerovic et al. 2007, Feng et al. 2009). In addition, the s80 cleavage product of ERBB4 JM-A CYT-1 isoform is the target of NEDD4 ubiquitin ligase. NEDD4 binds ERBB4 JM-A CYT-1 s80 (ERBB4jmAcyt1s80) through its PIK3R1 interaction site and mediates ERBB4jmAcyt1s80 ubiquitination, thereby decreasing the amount of ERBB4jmAcyt1s80 that reaches the nucleus (Zeng et al. 2009).
ERBB4 also binds the E3 ubiquitin ligase MDM2, and inhibitor of p53 (Arasada et al. 2005). Other proteins that bind to ERBB4 intracellular domain have been identified by co-immunoprecipitation and mass spectrometry (Gilmore-Hebert et al., 2010), and include transcriptional co-repressor TRIM28/KAP1, which promotes chromatin compaction. DNA damage signaling through ATM releases TRIM28-associated heterochromatinization. Interactions of ERBB4 with TRIM28 and MDM2 may be important for integration of growth factor responses and DNA damage responses.
In human breast cancer cell lines, ERBB4 activation enhances anchorage-independent colony formation in soft agar but inhibits cell growth in a monolayer culture. Different ERBB4 ligands induce different gene expression changes in breast cancer cell lines. Some of the genes induced in response to ERBB4 signaling in breast cancer cell lines are RAB2, EPS15R and GATA4. It is not known if these gene are direct transcriptional targets of ERBB4 (Amin et al. 2004).
Transcriptome and ChIP-seq comparisons of full-length and intracellular domain isoforms in isogenic MCF10A mammary cell background have revealed the diversification of ERBB4 signaling engendered by alternative splicing and cleavage (Wali et al., 2014). ERBB4 broadly affected protease expression, cholesterol biosynthesis, HIF1-alpha signaling, and HIPPO signaling pathways, and other pathways were differentially activated by CYT1 and CYT2 isoforms. For example, CYT1 promoted expression of transcription factors TWIST1 and SNAIL1 that promote epithelial-mesenchymal transition. HIF1-alpha and HIPPO signaling are mediated, respectively, by binding of ERBB4 to HIF1-alpha and to YAP (Paatero et al., 2012, Komuro et al., 2003). ERBB4 increases activity of the transcription factor SREBF2, resulting in increased expression of SREBF2-target genes involved in cholesterol biosynthesis. The mechanism is not known and may involve facilitation of SREBF2 cleavage through ERBB4-mediated PI3K signaling (Haskins et al. 2016).
In some contexts, ERBB4 promotes growth suppression or apoptosis (Penington et al., 2002). Activation of ERBB4 in breast cancer cell lines leads to JNK dependent increase in BRCA1 mRNA level and mitotic cell cycle delay, but the exact mechanism has not been elucidated (Muraoka Cook et al. 2006). The nature of growth responses may be connected with the spliced isoforms expressed. In comparisons of CYT1 vs CYT2 (full-length and ICD) expression in mammary cells, CYT1 was a weaker growth inducer, associated with attenuated MAPK signaling relative to CYT2 (Wali et al., 2014). ERBB4 s80 is also able to translocate to the mitochondrial matrix, presumably when its nuclear translocation is inhibited. Once in the mitochondrion, the BH3 domain of ERBB4, characteristic of BCL2 family members, may enable it to act as a pro apoptotic factor (Naresh et al. 2006).
ERBB4 plays important roles in the developing and adult nervous system. Erbb4 deficiency in somatostatin-expressing neurons of the thalamic reticular nucleus alters behaviors dependent on sensory selection (Ahrens et al. 2015). NRG1-activated ERBB4 signaling enhances AMPA receptor responses through PKC-dependent AMPA receptor exocytosis. This results in an increased excitatory input to parvalbumin-expressing inhibitory neurons in the visual cortex and regulates visual cortical plasticity (Sun et al. 2016). NRG1-activated ERBB4 signaling is involved in GABAergic activity in amygdala which mediates fear conditioning (fear memory) (Lu et al. 2014). Conditional Erbb4 deletion from fast-spiking interneurons, chandelier and basket cells of the cerebral cortex leads to synaptic defects associated with increased locomotor activity and abnormal emotional, social and cognitive function that can be linked to some of the schizophrenia features. The level of GAD1 (GAD67) protein is reduced in the cortex of conditional Erbb4 mutants. GAD1 is a GABA synthesizing enzyme. Cortical mRNA levels of GAD67 are consistently decreased in schizophrenia (Del Pino et al. 2014). Erbb4 is expressed in the GABAergic neurons of the bed nucleus stria terminalis, a part of the extended amygdala. Inhibition of NRG1-triggered ERBB4 signaling induces anxiety-like behavior, which depends on GABAergic neurotransmission. NRG1-ERBB4 signaling stimulates presynaptic GABA release, but the exact mechanism is not known (Geng et al. 2016). NRG1 protects cortical interneurons against ischemic brain injury through ERBB4-mediated increase in GABAergic transmission (Guan et al. 2015). NRG2-activated ERBB4 can reduce the duration of GABAergic transmission by binding to GABA receptors at the postsynaptic membrane via their GABRA1 subunit and promoting endocytosis of GABA receptors (Mitchell et al. 2013). NRG1 promotes synchronization of prefrontal cortex interneurons in an ERBB4 dependent manner (Hou et al. 2014). NRG1-ERBB4 signaling protects neurons from the cell death induced by a mutant form of the amyloid precursor protein (APP) (Woo et al. 2012).
Clinical relevance of ERBB4 has been identified in several contexts. In cancer, putative and validated gain-of-function mutations or gene amplification that may be drivers have been identified at modest frequencies, and may also contribute to resistance to EGFR and ERBB2-targeted therapies. This is noteworthy as ERBB4 kinase activity is inhibited by pan-ERBB tyrosine kinase inhibitors, including lapatinib, which is approved by the US FDA. The reduced prevalence relative to EGFR and ERBB2 in cancer may reflect more restricted expression of ERBB4, or differential signaling, as specific ERBB4 isoforms have been linked to growth inhibition or apoptosis in experimental systems. ERBB2/ERBB4 heterodimers protect cardiomyocytes, so reduced activity of ERBB4 in patients treated with the ERBB2-targeted therapeutic antibody trastuzumab may contribute to the cardiotoxicity of this agent when used in combination with (cardiotoxic) anthracyclines.
With the importance of ERBB4 in developing and adult nervous system, NRG1 and/or ERBB4 polymorphisms, splicing aberrations and mutations have been linked to nervous system disorders including schizophrenia and amyotrophic lateral sclerosis, although these findings are not yet definitive.
The 22 members of the fibroblast growth factor (FGF) family of growth factors mediate their cellular responses by binding to and activating the different isoforms encoded by the four receptor tyrosine kinases (RTKs) designated FGFR1, FGFR2, FGFR3 and FGFR4. These receptors are key regulators of several developmental processes in which cell fate and differentiation to various tissue lineages are determined. Unlike other growth factors, FGFs act in concert with heparin or heparan sulfate proteoglycan (HSPG) to activate FGFRs and to induce the pleiotropic responses that lead to the variety of cellular responses induced by this large family of growth factors. An alternative, FGF-independent, source of FGFR activation originates from the interaction with cell adhesion molecules, typically in the context of interactions on neural cell membranes and is crucial for neuronal survival and development.
Upon ligand binding, receptor dimers are formed and their intrinsic tyrosine kinase is activated causing phosphorylation of multiple tyrosine residues on the receptors. These then serve as docking sites for the recruitment of SH2 (src homology-2) or PTB (phosphotyrosine binding) domains of adaptors, docking proteins or signaling enzymes. Signaling complexes are assembled and recruited to the active receptors resulting in a cascade of phosphorylation events.
This leads to stimulation of intracellular signaling pathways that control cell proliferation, cell differentiation, cell migration, cell survival and cell shape, depending on the cell type or stage of maturation.
MET is a receptor tyrosine kinase (RTK) (Cooper et al. 1984, Park et al. 1984) activated by binding to its ligand, Hepatocyte growth factor/Scatter factor (HGF/SF) (Bottaro et al. 1991, Naldini et al. 1991). Similar to other related RTKs, such as EGFR, ligand binding induces MET dimerization and trans-autophosphorylation, resulting in the active MET receptor complex (Ferracini et al. 1991, Longati et al. 1994, Rodrigues and Park 1994, Kirchhofer et al. 2004, Stamos et al. 2004, Hays and Watowich 2004). Phosphorylated tyrosines in the cytoplasmic tail of MET serve as docking sites for binding of adapter proteins, such as GRB2, SHC1 and GAB1, which trigger signal transduction cascades that activate PI3K/AKT, RAS, STAT3, PTK2, RAC1 and RAP1 signaling (Ponzetto et al. 1994, Pelicci et al. 1995, Weidner et al. 1995, Besser et al. 1997, Shen and Novak 1997, Beviglia and Kramer 1999, Rodrigues et al. 2000, Sakkab et al. 2000, Schaeper et al. 2000, Lamorte et al. 2002, Wang et al. 2002, Chen and Chen 2006, Palamidessi et al. 2008, Chen et al. 2011, Murray et al. 2014). Activation of PLC gamma 1 (PLCG1) signaling by MET remains unclear. It has been reported that PLCG1 can bind to MET directly (Ponzetto et al. 1994) or be recruited by phosphorylated GAB1 (Gual et al. 2000). Tyrosine residue Y307 of GAB1 that serves as docking sites for PLCG1 may be phosphorylated either by activated MET (Watanabe et al. 2006) or SRC (Chan et al. 2010). Another PCLG1 docking site on GAB1, tyrosine residue Y373, was reported as the SRC target, while the kinase for the main PLCG1 docking site, Y407 of GAB1, is not known (Chan et al. 2010). Signaling by MET promotes cell growth, cell survival and motility, which are essential for embryonic development (Weidner et al. 1993, Schmidt et al. 1995, Uehara et al. 1995, Bladt et al. 1995, Maina et al. 1997, Maina et al. 2001, Helmbacher et al. 2003) and tissue regeneration (Huh et al. 2004, Borowiak et al. 2004, Liu 2004, Chmielowiec et al. 2007). MET signaling is frequently aberrantly activated in cancer, through MET overexpression or activating MET mutations (Schmidt et al. 1997, Pennacchietti et al. 2003, Smolen et al. 2006, Bertotti et al. 2009). Considerable progress has recently been made in the development of HGF-MET inhibitors in cancer therapy. These include inhibitors of HGF activators, HGF inhibitors and MET antagonists, which are protein therapeutics that act outside the cell. Kinase inhibitors function inside the cell and have constituted the largest effort towards MET-based therapeutics (Gherardi et al. 2012). Pathogenic bacteria of the species Listeria monocytogenes, exploit MET receptor as an entryway to host cells (Shen et al. 2000, Veiga and Cossart 2005, Neimann et al. 2007). For review of MET signaling, please refer to Birchmeier et al. 2003, Trusolino et al. 2010, Gherardi et al. 2012, Petrini 2015.
Neurotrophins (NGF, BDNF, NTF3 and NTF4) play pivotal roles in survival, differentiation, and plasticity of neurons in the peripheral and central nervous system. They are produced, and secreted in minute amounts, by a variety of tissues. They signal through two types of receptors: NTRK (TRK) tyrosine kinase receptors (TRKA, TRKB, TRKC), which differ in their preferred neurotrophin ligand, and p75NTR death receptor, which interacts with all neurotrophins. Besides the nervous system, TRK receptors and p75NTR are expressed in a variety of other tissues. For review, please refer to Bibel and Barde 2000, Poo 2001, Lu et al. 2005, Skaper 2012, Park and Poo 2013.
NTRK receptors, NTRK1 (TRKA), NTRK2 (TRKB) and NTRK3 (TRKC) are receptor tyrosine kinases activated by ligand binding to their extracellular domain. Ligand binding induces receptor dimerization, followed by trans-autophosphorylation of dimerized receptors on conserved tyrosine residues in the cytoplasmic region. Phosphorylated tyrosines in the intracellular domain of the receptor serve as docking sites for adapter proteins, triggering downstream signaling cascades.
NTRK1 (TRKA) is the receptor for the nerve growth factor (NGF). NGF is primarily secreted by tissues that are innervated by sensory and sympathetic neurons. NTRK1 signaling promotes growth and survival of neurons during embryonic development and maintenance of neuronal cell integrity in adulthood (reviewed by Marlin and Li 2015).
Brain-derived neurotrophic factor (BDNF) and neurotrophin-4 (NTF4, also known as NT-4) are two high affinity ligands for NTRK2 (TRKB). Neurotrophin-3 (NTF3, also known as NT-3) binds to NTRK2 with low affinity and may not be a physiologically relevant ligand. Nerve growth factor (NGF), a high affinity ligand for NTRK1, does not interact with NTRK2. NTRK2 signaling is implicated in neuronal development in both the peripheral (PNS) and central nervous system (CNS) and may play a role in long-term potentiation (LTP) and learning (reviewed by Minichiello 2009). NTRK2 may modify neuronal excitability and synaptic transmission by directly phosphorylating voltage gated channels (Rogalski et al. 2000).
NTF3 (NT-3) is the ligand for NTRK3 (TRKC). Signaling downstream of activated NTRK3, regulates cell survival, proliferation and motility. In the absence of its ligand, NTRK3 functions as a dependence receptor and triggers BAX and CASP9-dependent cell death (Tauszig-Delamasure et al. 2007, Ichim et al. 2013).
Platelet-derived Growth Factor (PDGF) is a potent stimulator of growth and motility of connective tissue cells such as fibroblasts and smooth muscle cells as well as other cells such as capillary endothelial cells and neurons.The PDGF family of growth factors is composed of four different polypeptide chains encoded by four different genes. The classical PDGF chains, PDGF-A and PDGF-B, and more recently discovered PDGF-C and PDGF-D. The four PDGF chains assemble into disulphide-bonded dimers via homo- or heterodimerization, and five different dimeric isoforms have been described so far; PDGF-AA, PDGF-AB, PDGF-BB, PDGF-CC and PDGF-DD. It is notable that no heterodimers involving PDGF-C and PDGF-D chains have been described. PDGF exerts its effects by binding to, and activating, two protein tyrosine kinase (PTK) receptors, alpha and beta. These receptors dimerize and undergo autophosphorylation. The phosphorylation sites then attract downstream effectors to transduct the signal into the cell.
Stem cell factor (SCF) is a growth factor with membrane bound and soluble forms. It is expressed by fibroblasts and endothelial cells throughout the body, promoting proliferation, migration, survival and differentiation of hematopoetic progenitors, melanocytes and germ cells.(Linnekin 1999, Ronnstrand 2004, Lennartsson and Ronnstrand 2006). The receptor for SCF is KIT, a tyrosine kinase receptor (RTK) closely related to the receptors for platelet derived growth factor receptor, colony stimulating factor 1 (Linnekin 1999) and Flt3 (Rosnet et al. 1991). Four isoforms of c-Kit have been identified in humans. Alternative splicing results in isoforms of KIT differing in the presence or absence of four residues (GNNK) in the extracellular region. This occurs due to the use of an alternate 5' splice donor site. These GNNK+ and GNNK- variants are co-expressed in most tissues; the GNNK- form predominates and was more strongly tyrosine-phosphorylated and more rapidly internalized (Ronnstrand 2004). There are also splice variants that arise from alternative usage of splice acceptor site resulting in the presence or absence of a serine residue (Crosier et al., 1993). Finally, there is an alternative shorter transcript of KIT expressed in postmeiotic germ cells in the testis which encodes a truncated KIT consisting only of the second part of the kinase domain and thus lackig the extracellular and transmembrane domains as well as the first part of the kinase domain (Rossi et al. 1991). Binding of SCF homodimers to KIT results in KIT homodimerization followed by activation of its intrinsic tyrosine kinase activity. KIT stimulation activates a wide array of signalling pathways including MAPK, PI3K and JAK/STAT (Reber et al. 2006, Ronnstrand 2004). Defects of KIT in humans are associated with different genetic diseases and also in several types of cancers like mast cell leukaemia, germ cell tumours, certain subtypes of malignant melanoma and gastrointestinal tumours.
Mature B cells express IgM and IgD immunoglobulins which are complexed at the plasma membrane with Ig-alpha (CD79A, MB-1) and Ig-beta (CD79B, B29) to form the B cell receptor (BCR) (Fu et al. 1974, Fu et al. 1975, Kunkel et al. 1975, Van Noesel et al. 1992, Sanchez et al. 1993, reviewed in Brezski and Monroe 2008). Binding of antigen to the immunoglobulin activates phosphorylation of immunoreceptor tyrosine-based activation motifs (ITAMs) in the cytoplasmic tails of Ig-alpha and Ig-beta by Src family tyrosine kinases, including LYN, FYN, and BLK (Nel et al. 1984, Yamanashi et al. 1991, Flaswinkel and Reth 1994, Saouaf et al. 1994, Hata et al. 1994, Saouaf et al. 1995, reviewed in Gauld and Cambier 2004, reviewed in Harwood and Batista 2010). The protein kinase SYK binds the phosphorylated immunoreceptor tyrosine-activated motifs (ITAMs) on the cytoplasmic tails of Ig-alpha (CD79A, MB-1) and Ig-beta (CD79B, B29) (Wienands et al. 1995, Rowley et al. 1995, Tsang et al. 2008). The binding causes the activation and autophosphorylation of SYK (Law et al. 1994, Baldock et al. 2000, Irish et al. 2006, Tsang et al. 2008, reviewed in Bradshaw 2010). Activated SYK and other kinases phosphorylate BLNK (SLP-65), BCAP, and CD19 which serve as scaffolds for the assembly of large complexes, the signalosomes, by recruiting phosphoinositol 3-kinase (PI3K), phospholipase C gamma (predominantly PLC-gamma2 in B cells, Coggeshall et al. 1992), NCK, BAM32, BTK, VAV1, and SHC. The effectors are phosphorylated by SYK and other kinases. PLC-gamma associated with BLNK hydrolyzes phosphatidylinositol-4,5-bisphosphate to yield inositol-1,4,5-trisphosphate (IP3) and diacylglycerol (Carter et al. 1991, Kim et al. 2004). IP3 binds receptors on the endoplasmic reticulum and causes release of calcium ions from the ER into the cytosol. The depletion of calcium from the ER in turn activates STIM1 to interact with ORAI and TRPC1 channels in the plasma membrane, resulting in an influx of extracellular calcium ions (Muik et al. 2008, Luik et al. 2008, Park et al. 2009, Mori et al. 2002). PI3K associated with BCAP and CD19 phosphorylates phosphatidylinositol 4,5-bisphosphate to yield phosphatidyinositol 3,4,5-trisphosphate. Second messengers (calcium, diacylglycerol, inositol 1,4,5-trisphosphate, and phosphatidylinositol 3,4,5-trisphosphate) trigger signaling pathways: NF-kappaB is activated via protein kinase C beta, RAS is activated via RasGRP proteins, NF-AT is activated via calcineurin, and AKT (PKB) is activated via PDK1 (reviewed in Shinohara and Kurosaki 2009, Stone 2006).
The TCR is a multisubunit complex that consists of clonotypic alpha/beta chains noncovalently associated with the invariant CD3 delta/epsilon/gamma and TCR zeta chains. T cell activation by antigen presenting cells (APCs) results in the activation of protein tyrosine kinases (PTKs) that associate with CD3 and TCR zeta subunits and the co-receptor CD4. Members of the Src kinases (Lck), Syk kinases (ZAP-70), Tec (Itk) and Csk families of nonreceptor PTKs play a crucial role in T cell activation. Activation of PTKs following TCR engagement results in the recruitment and tyrosine phosphorylation of enzymes such as phospholipase C gamma1 and Vav as well as critical adaptor proteins such as LAT, SLP-76 and Gads. These proximal activation leads to reorganization of the cytoskeleton as well as transcription activation of multiple genes leading to T lymphocyte proliferation, differentiation and/or effector function.
While the existence of a "b" isoform of fibroblast growth factor receptor 1 is well established and its biochemical and functional properties have been extensively characterized (e.g., Mohammadi et al. 2005; Zhang et al. 2006), its amino acid sequence is not represented in reference protein sequence databases, except as the 47-residue polypeptide (deposited in GenBank as accession AAB19502) first used by Johnson et al. (1991) to distinguish the "b" and "c" isoforms of the receptor.
PIP3 produced by PI3K activity is essential for receptor-driven stimulation of Rac activation, but PI3K also lies downstream of Rac, as Rac1 can form a complex with PI3K alpha leading to its activation.
At the plasma membrane, phosphatidylinositol-5-phosphate 4-kinase type-2 alpha (PIP4K2A), beta (PIP4K2B) and gamma (PIP4K2C) homodimers and heterodimers (Clarke et al. 2010, Clarke and Irvine 2013, Clarke et al. 2015) phosphorylate phosphatidylinositol 5-phosphate (PI5P) to phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2).
The following lists the above proteins with their corresponding literature references: PIP4K2A (Rameh et al. 1997, Clarke et al. 2008, Clarke and Irvine 2013), PIP4K2B (Rameh et al. 1997, Clarke and Irvine 2013) and PIP4K2C (Clarke and Irvine 2013, Clarke et al. 2015).
At the plasma membrane, phosphatidylinositol-4-phosphate 5-kinase type-1 alpha (PIP5K1A), beta (PIP5K1B), and gamma (PIP5K1C) phosphorylate phosphatidylinositol 4-phosphate (PI4P) to produce phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2).
The following lists the above proteins with their corresponding literature references: PIP5K1A (Halstead et al. 2006, Zhang et al. 1997), PIP5K1B (Zhang et al. 1997), and PIP5K1C (Di Paolo et al. 2002).
This reaction is of particular interest because its regulation by small GTPases of the RHO and ARF families, not yet annotated here, ties the process of phosphatidylinositol phosphate biosynthesis to regulation of the actin cytoskeleton and vesicular trafficking, and hence to diverse aspects of cell motility and signalling (Oude Weernink et al. 2004, 2007).
Once AKT is localized at the plasma membrane, it is phosphorylated at two critical residues for its full activation. These residues are a threonine (T308 in AKT1) in the activation loop within the catalytic domain, and a serine (S473 in AKT1), in a hydrophobic motif (HM) within the carboxy terminal, non-catalytic region. PDPK1 (PDK1) is the activation loop kinase; this kinase can also directly phosphorylate p70S6K. The HM kinase, previously termed PDK2, has been identified as the mammalian TOR (Target Of Rapamycin; Sarbassov et al., 2005) but several other kinases are also able to phosphorylate AKT at S473. Phosphorylation of AKT at S473 by TORC2 complex is a prerequisite for PDPK1-mediated phosphorylation of AKT threonine T308 (Scheid et al. 2002, Sarabassov et al. 2005).
AKT, phosphorylated at threonine (AKT1 308; AKT2 309; AKT3 305) and serine (AKT1 473; AKT2 474; AKT3 472) translocates to the nucleus, reaching a maximum after 15 min and returning to a basal level after 45 min of NGF stimulation. Control of the amount of nuclear AKT is achieved through the action of the phosphatase PP2A (Borgatti et al. 2003).
Activated AKT phosphorylates the BCL-2 family member BAD at serine 99 (corresponds to serine residue S136 of mouse Bad), blocking the BAD-induced cell death (Datta et al. 1997, del Peso et al. 1997, Khor et al. 2004).
GSK3 (glycogen synthase kinase-3) participates in the Wnt signaling pathway. It is implicated in the hormonal control of several regulatory proteins including glycogen synthase, and the transcription factors MYB and JUN. GSK3 phosphorylates JUN at sites proximal to its DNA-binding domain, thereby reducing its affinity for DNA. GSK3 is inhibited when phosphorylated by AKT1.
AKT phosphorylates MDM2 on two serine residues, at positions 166 and 188 (Mayo and Donner 2001, Feng et al. 2004, Milne et al. 2004). AKT-mediated phosphorylation of the E3 ubiquitin-protein ligase MDM2 promotes nuclear localization and interferes with the interaction between MDM2 and p14-ARF, thereby decreasing p53 stability. This leads to a decreased expression of p53 target genes, such as BAX, that promote apoptosis (Zhou et al. 2001, Mayo and Donner 2001).
AKT phosphorylates and inhibits TSC2 (tuberin), a suppressor of the TOR kinase pathway, which senses nutrient levels in the environment. TSC2 forms a protein complex with TSC1 and this complex acts as a GAP (GTPase activating protein) for the RHEB G-protein. RHEB, in turn, activates the TOR kinase. Thus, an active AKT1 activates the TOR kinase, both of which are positive signals for cell growth (an increase in cell mass) and division. The TOR kinase regulates two major processes: translation of selected mRNAs in the cell and autophagy. In the presence of high nutrient levels TOR is active and phosphorylates the 4EBP protein releasing the eukaryotic initiation factor 4E (eIF4E), which is essential for cap-dependent initiation of translation and promoting growth of the cell (PMID: 15314020). TOR also phosphorylates the S6 kinase, which is implicated in ribosome biogenesis as well as in the modification of the S6 ribosomal protein. AKT can also activate mTOR by another mechanism, involving phosphorylation of PRAS40, an inhibitor of mTOR activity.
AKT mediates IKKalpha (Inhibitor of nuclear factor kappa B kinase subunit alpha) phosphorylation at threonine 23, which is required for NF-kB activation. NF-kB promoted gene transcription enhances neuronal survival.
Phosphorylation of p27Kip1 at T157 and of p21Cip1 at T145 by AKT leads to their retention in the cytoplasm, segregating these cyclin-dependent kinase (CDK) inhibitors from cyclin-CDK complexes.
Under conditions of growth and mitogen stimulation S473 phosphorylation of AKT is carried out by mTOR (mammalian Target of Rapamycin). This kinase is found in two structurally and functionally distinct protein complexes, named TOR complex 1 (TORC1) and TOR complex 2 (TORC2). It is TORC2 complex, which is composed of mTOR, RICTOR, SIN1 (also named MAPKAP1) and LST8, that phosphorylates AKT at S473 (Sarbassov et al., 2005). This complex also regulates actin cytoskeletal reorganization (Jacinto et al., 2004; Sarbassov et al., 2004). TORC1, on the other hand, is a major regulator of ribosomal biogenesis and protein synthesis (Hay and Sonenberg, 2004). TORC1 regulates these processes largely by the phosphorylation/inactivation of the repressors of mRNA translation 4E binding proteins (4E BPs) and by the phosphorylation/activation of ribosomal S6 kinase (S6K1). TORC1 is also the principal regulator of autophagy. In other physiological conditions, other kinases may be responsible for AKT S473 phosphorylation.
Phosphorylation of AKT on S473 by TORC2 complex is a prerequisite for AKT phosphorylation on T308 by PDPK1 (Scheid et al. 2002, Sarabassov et al. 2005).
AKT-mediated phosphorylation of Forkhead box (FOX) transcription factors of the FOXO family, FOXO1 (FKHR), FOXO3 (FoxO3a, also known as FKHRL1) and FOXO4 (AFX) contributes to PI3K/AKT signaling-stimulated cell survival and growth. Activated AKT1 phosphorylates FOXO1 on threonine residue T24 and serine residues S256 and S319 (Rena et al. 1999), FOXO3 on threonine residue T32 and serine residues S253 and S315 (Brunet et al. 1999), and FOXO4 on threonine residue T32 and serine residues S197 and S262 (Kops et al. 1999). Based on studies with recombinant mouse Foxo6 expressed in the human embryonic kidney cell line HEK293, FOXO6 has two conserved AKT phosphorylation sites: T26 and S184. Mouse Foxo6 has a third predicted Akt phosphorylation site at the C-terminus, T338, which is not present in other Foxo family members and is not conserved in human FOXO6. T26 and S184 are phosphorylated in response to growth factors known to activate PI3K/AKT signaling, but AKT has not been explicitly identified as the responsible kinase. In contrast to other FOXO family members, FOXO6 remains predominantly nuclear irrespective of growth factor-induced signaling, and only a small portion of phosphorylated FOXO6 may shuttle to the cytosol. Phosphorylation of FOXO6 on putative AKT sites, however, may inhibit binding of FOXO6 to target DNA sites (Jacobs et al. 2003, van der Heide et al. 2005). Protein phosphatase DUSP6 (MKP3) may act to dephosphorylate FOXO1 after AKT-mediated phosphorylation (Rodrigues et al. 2017).
The PH domain leucine-rich repeat-containing protein phosphatases, PHLPP1 (Gao et al. 2005) and PHLPP2 (Brognard et al. 2007) can specifically dephosphorylate the serine residue and inactivate AKT.
The phosphorylation of membrane-recruited AKT at threonine and serine can be inhibited by direct binding of two different proteins, C-terminal modulator protein (THEM4 i.e. CTMP), which binds to the carboxy-terminal tail of AKT (Maira et al. 2001), or Tribbles homolog 3 (TRIB3), which binds to the catalytic domain of AKT (Du et al. 2003).
At the plasma membrane, phosphatidylinositol-3,4,5-trisphosphate 3-phosphatase and dual-specificity protein phosphatase aka phosphatase and tensin homolog (PTEN) dephosphorylates phosphatidylinositol 3,4,5-trisphosphate (PI(3,4,5)P3) to phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2) (Maehama & Dixon 1998, Myers et al. 1998, Das et al. 2003). The PI3K network is negatively regulated by phospholipid phosphatases that dephosphorylate PIP3, thus hampering AKT activation (Myers et al. 1998). The tumour suppressor PTEN is the primary phospholipid phosphatase. Early studies indicated that magnesium ion, Mg2+, was needed for the catalytic activity of PTEN isolated from bovine thymus (Kabuyama et al. 1996). Subsequent studies have shown that PTEN was catalytically active in buffers free of magnesium and magnesium was not detected as part of the PTEN crystal (Lee et al. 1999).
Ribosomal protein S6 kinase beta-2 (RSK) activation is a highly conserved mitogenic response, and the activities of RSK are stimulated by multiple serine/threonine phosphorylations by different upstream kinases, one of which is AKT.
AKT inhibits DNA binding of NUR77 and inhibits its pro-apoptotic function (PMID 11438550). However, the relevance of AKT for NUR77 phosphorylation has recently been questioned: according to recent work, NUR77 is phosphorylated by RSK (and MSK) rather than by AKT (PMID 16223362).
PRAS40 (proline-rich Akt/PKB substrate 40 kDa) is a substrate of AKT, the phosphorylation of which leads to the binding of this protein to 14-3-3. PRAS40 binds to mTOR complexes, mediating AKT signals to mTOR. Interaction of PRAS40 with the mTOR kinase domain is induced under conditions that inhibit mTOR signalling, such as growth factor deprivation. Binding of PRAS40 inhibits mTOR. PRAS40 phosphorylation by AKT and association with the cytosolic anchor protein 14-3-3, lead to mTOR stimulation (Vander Haar E, et al, 2007). Although it was originally identified in the context of insulin signalling, it was later shown that PRAS40 may also play a role in nerve growth factor-mediated neuroprotection (Saito A, et al, 2004).
PDPK1 (PDK1) possesses low affinity for PIP2, so small amounts of PDPK1 are always present at the membrane, in the absence of PI3K activity (Currie et al. 1999).
Substitution of glutamic acid with lysine at position 17 of AKT1 results in constitutive plasma membrane localization of AKT1, independent of PI3K activity and PIP3 generation (Carpten et al. 2007). This constitutive plasma membrane targeting of AKT1 E17K mutant is due to an increased affinity for PIP2 (Landgraf et al. 2008).
A portion of PDPK1 (PDK1) is anchored to the plasma membrane in the absence of PI3K activity through PIP2 binding (Currie et al. 1999). This PIP2-bound PDPK1 is able to bind and phosphorylate PIP2-bound AKT E17K mutants (Carpten et al. 2007, Landgraf et al. 2008) phosphorylated on serine residue S473.
PIP2-binding AKT1 E17K mutants are anchored to the plasma membrane in the absence of PI3K activity and are constitutively phosphorylated on serine S473, presumably by the TORC2 complex (Carpten et al. 2007, Landgraf et al. 2008).
PIP2-bound AKT1 E17K mutant is constitutively phosphorylated on threonine residue T308 (Carpten et al. 2007, Landgraf et al. 2008), presumably by PIP2-bound PDPK1 (Currie et al. 1999).
PIP3 generated by PI3K recruits phosphatidylinositide-dependent protein kinase 1 (PDPK1 i.e. PDK1) to the membrane, through its PH (pleckstrin-homology) domain. PDPK1 binds PIP3 with high affinity, and also shows low affinity for PIP2 (Currie et al. 1999).
A number of different extracellular signals converge on PI3K activation. PI3K can be activated downstream of receptor tyrosine kinases (RTKs) such as FGFR (Ong et al. 2001, Eswarakumar et al. 2005), KIT (Chian et al. 2001, Ronnstrand 2004, Reber et al. 2006), PDGF (Coughlin et al. 1989, Fantl et al. 1992, Heldin et al. 1998), insulin receptor IGF1R (Hadari et al. 1992, Kooijman et al. 1995), and EGFR and its family members (Rodrigues et al. 2000, Jackson et al. 2004, Kainulainen et al. 2000, Junttila et al. 2009). Other proteins, such as CD28 (Pages et al. 1996, Koyasu 2003, Kane and Weiss, 2003) and TRAT1 (Bruyns et al. 1998, Koyasu 2003, Kolsch et al. 2006), can also trigger PI3K activity.
In unstimulated cells, PI3K class IA exists as an inactive heterodimer of a p85 regulatory subunit (encoded by PIK3R1, PIK3R2 or PIK3R3) and a p110 catalytic subunit (encoded by PIK3CA, PIK3CB or PIK3CD). Binding of the iSH2 domain of the p85 regulatory subunit to the ABD and C2 domains of the p110 catalytic subunit both stabilizes p110 and inhibits its catalytic activity. This inhibition is relieved when the SH2 domains of p85 bind phosphorylated tyrosines on activated RTKs or their adaptor proteins. Binding to membrane-associated receptors brings activated PI3K in proximity to its membrane-localized substrate, PIP2 (Mandelker et al. 2009, Burke et al. 2011).
Once phosphorylated on serine residue S473, AKT bound to PIP3 forms a complex with PIP3-bound PDPK1 i.e. PDK1 (Scheid et al. 2002, Sarabassov et al. 2005)
PIP3 generated by PI3K recruits AKT (also known as protein kinase B) to the membrane, through its PH (pleckstrin-homology) domains. The binding of PIP3 to the PH domain of AKT is the rate-limiting step in AKT activation (Scheid et al. 2002). In mammals there are three AKT isoforms (AKT1-3) encoded by three separate genes. The three isoforms share a high degree of amino acid identity and have indistinguishable substrate specificity in vitro. However, isoform-preferred substrates in vivo cannot be ruled out. The relative expression of the three isoforms differs in different mammalian tissues: AKT1 is the predominant isoform in the majority of tissues, AKT2 is the predominant isoform in insulin-responsive tissues, and AKT3 is the predominant isoform in brain and testes. All 3 isoforms are expressed in human and mouse platelets (Yin et al. 2008; O'Brien et al. 2008). Note: all data in the pathway refer to AKT1, which is the most studied.
A variety of inhibitors capable of blocking the phosphoinositide kinase activity of PI3K have been developed. These inhibitors display differential selectivity and inhibit kinase activity of their substrates by distinct mechanisms. For example, the first-generation PI3K inhibitor wortmannin (Wymann et al. 1996) covalently and irreversibly binds all classes of PI3K enzymes, as well as other kinases including mTOR, at a residue critical for catalytic activity. Although wortmannin is precluded from in vivo and clinical use due to its toxicity, it has proven to be a useful tool for in vitro laboratory studies. Newer inhibitors, such as BEZ235, are currently being investigated in Phase I clinical trials. BEZ235 is a dual pan-class I PI3K/mTOR inhibitor that blocks kinase activity by binding competitively to the ATP-binding pocket of these enzymes (Serra et al. 2008, Maira et al. 2008). BGT226 (Chang et al. 2011) and XL765 (Prasad et al. 2011) also inhibits both PI3K class I enzymes and mTOR. Other inhibitors in clinical trials, such as BKM120 (Maira et al. 2012), GDC0941 (Folkes et al. 2008, Junttila et al. 2009) and XL147 (Chakrabarty et al. 2012), are specific for class I PI3Ks and exhibit no activity against mTOR. Current research aims to identify isoform-specific PI3K inhibitors. Small molecule inhibitors that selectively inhibit PIK3CA (p110alpha), e.g. PIK-75 and A66, were used to study the role of p110alpha in signaling and growth of tumor cells (Knight et al. 2006, Sun et al. 2010, Jamieson et al. 2011, Utermark et al. 2012). The PIK3CB (p110beta) specific inhibitor TGX221 has been used in in vitro models of vascular injury (Jackson et al. 2005), and the TGX221 derivative KIN-193 has been shown to block AKT activity and tumor growth in mice with p110beta activation or PTEN loss (Ni et al. 2012). CAL-101 is a PIK3CD (p110delta) specific inhibitor that is being clinically investigated as a therapeutic for lymphoid malignancies (Herman et al. 2010). It is hoped that, in the future, more specific inhibitors, such as those targeting selective PI3K isoforms, will provide optimum treatment while minimizing unwanted side effects. For a recent review, please refer to Liu et al. 2009.
AKT inhibitors bind AKT and prevent its association with the membrane, thereby blocking AKT activation (Kondapaka et al. 2003, Yap et al. 2011, Berndt et al. 2010). AKT inhibitors annotated here target all AKT isoforms (AKT1, AKT2 and AKT3). None of the annotated inhibitors are AKT E17K mutant specific and none of them have been approved for clinical use. For a recent review, please refer to Liu et al. 2009.
Activated MAPK1 (ERK2) or MAPK3 (ERK1), recruited to the PP2A complex through IER3 (IEX-1), phosphorylate the regulatory subunit PPP2R5B (B56-beta) or PPP2R5C (B56-gamma) of the PP2A complex on serine residue S368 or S337, respectively. ERK-mediated phosphorylation of the PP2A regulatory subunits causes dissociation of the PP2A complex and prevents PP2A-mediated dephosphorylation of AKT1 (Letourneux et al. 2006, Rocher et al. 2007).
IER3 (IEX-1) recruits both an activated MAPK (MAPK1 (ERK2) or MAPK3 (ERK1)) and the protein phosphatase 2A (PP2A) complex containing regulatory subunits B56-beta (PPP2R5B) or B56-gamma (PPP2R5C), through an interaction with the B56 subunit, forming a tripartite complex (Letourneux et al. 2006, Rocher et al. 2007).
The protein phosphatase 2A (PP2A) complex containing a regulatory subunit B56 beta (PPP2R5B) or B56 gamma (PPP2R5C) dephosphorylates activated AKT1 on threonine residue T308 and serine residue S473, thus halting PI3K/AKT signaling (Rocher et al. 2007). Phosphatidylinositol-5-phosphate (PI5P) negatively regulates PP2A-mediated dephosphorylation of AKT1 by promoting, through an unknown mechanism, an inhibitory phosphorylation on tyrosine residue Y307 (Chen et al. 1992) of the catalytic subunit of PP2A (Ramel et al. 2009).
SRC family tyrosine kinases, such as SRC and LCK, as well as receptor tyrosine kinases, such as EGFR and insulin receptor, can phosphorylate the catalytic subunit of serine/threonine protein phosphatase PP2A at tyrosine residue Y307. Phosphorylation at Y307 inhibits the catalytic activity of PP2A. Phosphatidylinositol-5-phosphate (PI5P) positively regulates phosphorylation of the catalytic subunit of PP2A at Y307.
AKT1 (and possibly AKT2 and AKT3), activated in response to EGF treatment, phosphorylates MKRN1, an E3 ubiquitin ligase, on serine residue S109. AKT-mediated phosphorylation results in stabilization of MKRN1, protecting it from ubiquitination and proteasome-mediated degradation (Lee et al. 2015).
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The positive signals induced by CD28 and ICOS molecules are counterbalanced by other members of the CD28 family, including cytotoxic T-lymphocyte associated antigen (CTLA)4, programmed cell death (PD)1, and B and T lymphocyte attenuator (BTLA), which dampen immune responses. The balance of stimulatory and inhibitory signals is crucial to maximize protective immune responses while maintaining immunological tolerance and preventing autoimmunity.
The costimulatory receptors CD28, CTLA4, ICOS and PD1 are composed of single extracellular IgV-like domains, whereas BTLA has one IgC-like domain. Receptors CTLA4, CD28 and ICOS are covalent homodimers, due to an interchain disulphide linkage. The costimulatory ligands B71, B72, B7H2, B7H1 and B7DC, have a membrane proximal IgC-like domain and a membrane distal IgV-like domain that is responsible for receptor binding and dimerization. CD28 and CTLA4 have no known intrinsic enzymatic activity. Instead, engagement by their physiologic ligands B71 and B72 leads to the physical recruitment and activation of downstream T-cell effector molecules.
E2-evoked extra-nuclear signaling is independent of the transcriptional activity of estrogen receptors and occurs within seconds to minutes following E2 administration to target cells. Extra-nuclear signaling consists of the activation of a plethora of signaling pathways including the RAF/MAP kinase cascade and the PI3K/AKT signaling cascade and governs processes such as apoptosis, cellular proliferation and metastasis (reviewed in Hammes et al, 2007; Handa et al, 2012; Lange et al, 2007; Losel et al, 2003; Arnal et al, 2017; Le Romancer et al, 2011). ESR-mediated signaling also cross-talks with receptor tyrosine kinase, NF- kappa beta and GPCR signaling pathways by modulating the post-translational modification of enzymes and other proteins and regulating second messengers (reviewed in Arnal et al, 2017; Schwartz et al, 2016; Boonyaratanakornkit, 2011; Biswas et al, 2005). In the nervous system, E2 affects neural functions such as cognition, behaviour, stress responses and reproduction in part by inducing such rapid extra-nuclear responses (Farach-Carson and Davis, 2003; Losel et al, 2003), while in endothelial cells, non-genomic ESR-dependent signaling also regulates vasodilation through the eNOS pathway (reviewed in Levin, 2011).
Extra-nuclear signaling additionally cross-talks with nuclear estrogen receptor signaling and is required to control ER protein stability (La Rosa et al, 2012)
Recent data have demonstrated that the membrane ESR1 can interact with various endocytic proteins to traffic and signal within the cytoplasm. This receptor intracellular trafficking appears to be dependent on the phyical interaction of ESR1 with specific trans-membrane receptors such as IGR-1R and beta 1-integrin (Sampayo et al, 2018)
1. "Multidomain" BAX family proteins such as BAX, BAK etc. that display sequence conservation in their BH1-3 regions. These proteins act downstream in mitochondrial disruption.
2. "BH3-only" proteins such as BID,BAD, NOXA, PUMA,BIM, and BMF have only the short BH3 motif. These act upstream in the pathway, detecting developmental death cues or intracellular damage. Anti-apoptotic members like Bcl-2, Bcl-XL and their relatives exhibit homology in all segments BH1-4. One of the critical functions of BCL-2/BCL-XL proteins is to maintain the integrity of the mitochondrial outer membrane.
oxide: NOS3 activation and
regulationThe NOS enzymes share a common basic structural organization and requirement for substrate cofactors for enzymatic activity. A central calmodulin-binding motif separates an NH2-terminal oxygenase domain from a COOH-terminal reductase domain. Binding sites for cofactors NADPH, FAD, and FMN are located within the reductase domain, while binding sites for tetrahydrobiopterin (BH4) and heme are located within the oxygenase domain. Once calmodulin binds, it facilitates electron transfer from the cofactors in the reductase domain to heme enabling nitric oxide production. Both nNOS and eNOS contain an additional insert (40-50 amino acids) in the middle of the FMN-binding subdomain that serves as autoinhibitory loop, destabilizing calmodulin binding at low calcium levels and inhibiting electron transfer from FMN to the heme in the absence of calmodulin. iNOS does not contain this insert.
In this Reactome pathway module, details of eNOS activation and regulation are annotated. Originally identified as endothelium-derived relaxing factor, eNOS derived NO is a critical signaling molecule in vascular homeostasis. It regulates blood pressure and vascular tone, and is involved in vascular smooth muscle cell proliferation, platelet aggregation, and leukocyte adhesion. Loss of endothelium derived NO is a key feature of endothelial dysfunction, implicated in the pathogenesis of hypertension and atherosclerosis. The endothelial isoform eNOS is unique among the nitric oxide synthase (NOS) family in that it is co-translationally modified at its amino terminus by myristoylation and is further acylated by palmitoylation (two residues next to the myristoylation site). These modifications target eNOS to the plasma membrane caveolae and lipid rafts.
Factors that stimulate eNOS activation and nitric oxide (NO) production include fluid shear stress generated by blood flow, vascular endothelial growth factor (VEGF), bradykinin, estrogen, insulin, and angiopoietin. The activity of eNOS is further regulated by numerous post-translational modifications, including protein-protein interactions, phosphorylation, and subcellular localization.
Following activation, eNOS shuttles between caveolae and other subcellular compartments such as the noncaveolar plasma membrane portions, Golgi apparatus, and perinuclear structures. This subcellular distribution is variable depending upon cell type and mode of activation.
Subcellular localization of eNOS has a profound effect on its ability to produce NO as the availability of its substrates and cofactors will vary with location. eNOS is primarily particulate, and depending on the cell type, eNOS can be found in several membrane compartments: plasma membrane caveolae, lipid rafts, and intracellular membranes such as the Golgi complex.
During early G1, cells can enter a quiescent G0 state. In quiescent cells, the evolutionarily conserved DREAM complex, consisting of the pocket protein family member p130 (RBL2), bound to E2F4 or E2F5, and the MuvB complex, represses transcription of cell cycle genes (reviewed by Sadasivam and DeCaprio 2013).
During early G1 phase in actively cycling cells, transcription of cell cycle genes is repressed by another pocket protein family member, p107 (RBL1), which forms a complex with E2F4 (Ferreira et al. 1998, Cobrinik 2005). RB1 tumor suppressor, the product of the retinoblastoma susceptibility gene, is the third member of the pocket protein family. RB1 binds to E2F transcription factors E2F1, E2F2 and E2F3 and inhibits their transcriptional activity, resulting in prevention of G1/S transition (Chellappan et al. 1991, Bagchi et al. 1991, Chittenden et al. 1991, Lees et al. 1993, Hiebert 1993, Wu et al. 2001). Once RB1 is phosphorylated on serine residue S795 by Cyclin D:CDK4/6 complexes, it can no longer associate with and inhibit E2F1-3. Thus, CDK4/6-mediated phosphorylation of RB1 leads to transcriptional activation of E2F1-3 target genes needed for the S phase of the cell cycle (Connell-Crowley et al. 1997). CDK2, in complex with cyclin E, contributes to RB1 inactivation and also activates proteins needed for the initiation of DNA replication (Zhang 2007). Expression of D type cyclins is regulated by extracellular mitogens (Cheng et al. 1998, Depoortere et al. 1998). Catalytic activities of CDK4/6 and CDK2 are controlled by CDK inhibitors of the INK4 family (Serrano et al. 1993, Hannon and Beach 1994, Guan et al. 1994, Guan et al. 1996, Parry et al. 1995) and the Cip/Kip family, respectively.
Transcription of the PTEN gene is regulated at multiple levels. Epigenetic repression involves the recruitment of Mi-2/NuRD upon SALL4 binding to the PTEN promoter (Yang et al. 2008, Lu et al. 2009) or EVI1-mediated recruitment of the polycomb repressor complex (PRC) to the PTEN promoter (Song et al. 2009, Yoshimi et al. 2011). Transcriptional regulation is also elicited by negative regulators, including NR2E1:ATN1 (atrophin-1) complex, JUN (c-Jun), SNAIL and SLUG (Zhang et al. 2006, Vasudevan et al. 2007, Escriva et al. 2008, Uygur et al. 2015) and positive regulators such as TP53 (p53), MAF1, ATF2, EGR1 or PPARG (Stambolic et al. 2001, Virolle et al. 2001, Patel et al. 2001, Shen et al. 2006, Li et al. 2016).
MicroRNAs miR-26A1, miR-26A2, miR-22, miR-25, miR-302, miR-214, miR-17-5p, miR-19 and miR-205 bind PTEN mRNA and inhibit its translation into protein. These microRNAs are altered in cancer and can account for changes in PTEN levels (Meng et al. 2007, Xiao et al. 2008, Yang et al. 2008, Huse et al. 2009, Kim et al. 2010, Poliseno, Salmena, Riccardi et al. 2010, Cai et al. 2013). In addition, coding and non-coding RNAs can prevent microRNAs from binding to PTEN mRNA. These RNAs are termed competing endogenous RNAs or ceRNAs. Transcripts of the pseudogene PTENP1 and mRNAs transcribed from SERINC1, VAPA and CNOT6L genes exhibit this activity (Poliseno, Salmena, Zhang et al. 2010, Tay et al. 2011, Tay et al. 2014).
PTEN can translocate from the cytosol to the nucleus after undergoing monoubiquitination. PTEN's ability to localize to the nucleus contributes to its tumor suppressive role (Trotman et al. 2007). The ubiquitin protease USP7 (HAUSP) targets monoubiquitinated PTEN in the nucleus, resulting in PTEN deubiquitination and nuclear exclusion. PML, via an unknown mechanism that involves USP7- and PML-interacting protein DAXX, inhibits USP7-mediated deubiquitination of PTEN, thus promoting PTEN nuclear localization. Disruption of PML function in acute promyelocytic leukemia, through a chromosomal translocation that results in expression of a fusion protein PML-RARA, leads to aberrant PTEN localization (Song et al. 2008).
Several ubiquitin ligases, including NEDD4, WWP2, STUB1 (CHIP), RNF146, XIAP and MKRN1, polyubiquitinate PTEN and target it for proteasome-mediated degradation (Wang et al. 2007, Van Themsche et al. 2009, Ahmed et al. 2011, Maddika et al. 2011, Lee et al. 2015, Li et al. 2015). The ubiquitin proteases USP13 and OTUD3, frequently down-regulated in breast cancer, remove polyubiquitin chains from PTEN, thus preventing its degradation and increasing its half-life (Zhang et al. 2013, Yuan et al. 2015). The catalytic activity of PTEN is negatively regulated by PREX2 binding (Fine et al. 2009, Hodakoski et al. 2014) and TRIM27-mediated ubiquitination (Lee et al. 2013), most likely through altered PTEN conformation.
In addition to ubiquitination, PTEN also undergoes SUMOylation (Gonzalez-Santamaria et al. 2012, Da Silva Ferrada et al. 2013, Lang et al. 2015, Leslie et al. 2016). SUMOylation of the C2 domain of PTEN may regulate PTEN association with the plasma membrane (Shenoy et al. 2012) as well as nuclear localization of PTEN (Bassi et al. 2013, Collaud et al. 2016). PIASx-alpha, a splicing isorom of E3 SUMO-protein ligase PIAS2 has been implicated in PTEN SUMOylation (Wang et al. 2014). SUMOylation of PTEN may be regulated by activated AKT (Lin et al. 2016). Reactions describing PTEN SUMOylation will be annotated when mechanistic details become available.
Phosphorylation affects the stability and activity of PTEN. FRK tyrosine kinase (RAK) phosphorylates PTEN on tyrosine residue Y336, which increases PTEN half-life by inhibiting NEDD4-mediated polyubiquitination and subsequent degradation of PTEN. FRK-mediated phosphorylation also increases PTEN enzymatic activity (Yim et al. 2009). Casein kinase II (CK2) constitutively phosphorylates the C-terminal tail of PTEN on serine and threonine residues S370, S380, T382, T383 and S385. CK2-mediated phosphorylation increases PTEN protein stability (Torres and Pulido 2001) but results in ~30% reduction in PTEN lipid phosphatase activity (Miller et al. 2002).
PTEN localization and activity are affected by acetylation of its lysine residues (Okumura et al. 2006, Ikenoue et al. 2008, Meng et al. 2016). PTEN can undergo oxidation, which affects its function, but the mechanism is poorly understood (Tan et al. 2015, Shen et al. 2015, Verrastro et al. 2016).
The importance of the RAS/RAF MAPK cascade is highlighted by the fact that components of this pathway are mutated with high frequency in a large number of human cancers. Activating mutations in RAS are found in approximately one third of human cancers, while ~8% of tumors express an activated form of BRAF (Roberts and Der, 2007; Davies et al, 2002; Cantwell-Dorris et al, 2011).
Expression and
DegradationActivating phosphorylation of TP53 at serine residues S15 and S20 in response to genotoxic stress disrupts TP53 interaction with MDM2. In contrast to MDM2, E3 ubiquitin ligases RNF34 (CARP1) and RFFL (CARP2) can ubiquitinate phosphorylated TP53 (Yang et al. 2007). Binding of MDM2 to TP53 is also inhibited by the tumor suppressor p14-ARF, transcribed from the CDKN2A gene in response to oncogenic signaling or oxidative stress (Zhang et al. 1998, Parisi et al. 2002, Voncken et al. 2005). Ubiquitin-dependant degradation of TP53 can also be promoted by PIRH2 (Leng et al. 2003) and COP1 (Dornan et al. 2004) ubiquitin ligases. HAUSP (USP7) can deubiquitinate TP53, contributing to TP53 stabilization (Li et al. 2002).
While post-translational regulation plays a prominent role, TP53 activity is also controlled at the level of promoter function (reviewed in Saldana-Meyer and Recillas-Targa 2011), mRNA stability and translation efficiency (Mahmoudi et al. 2009, Le et al. 2009, Takagi et al. 2005).
ERBB2 becomes activated by forming a heterodimer with another ligand-activated EGFR family member, either EGFR, ERBB3 or ERBB4, which is accompanied by dissociation of chaperoning proteins HSP90 and CDC37 (Citri et al. 2004), as well as ERBB2IP (Borg et al. 2000) from ERBB2. ERBB2 heterodimers function to promote cell proliferation, cell survival and differentiation, depending on the cellular context. ERBB2 can also be activated by homodimerization when it is overexpressed, in cancer for example.
In cells expressing ERBB2 and ERBB4, ligand stimulated ERBB4 can either homodimerize or form heterodimers with ERBB2 (Li et al. 2007), resulting in trans-autophosphorylation of ERBB2 and ERBB4 on C-tail tyrosine residues that will subsequently serve as docking sites for downstream signaling molecules, leading to activation of RAF/MAP kinase cascade and, in the case of ERBB4 CYT1 isoforms, PI3K-induced AKT signaling (Hazan et al. 1990, Cohen et al. 1996, Li et al. 2007, Kaushansky et al. 2008). Signaling by ERBB4 is downregulated by the action of WWP1 and ITCH ubiquitin ligases, and is shown under Signaling by ERBB4.In cells expressing both ERBB2 and EGFR, EGF stimulation of EGFR leads to formation of both ERBB2:EGFR heterodimers (Wada et al. 1990, Karunagaran et al. 1996) and EGFR homodimers. Heterodimers of ERBB2 and EGFR trans-autophosphorylate on twelve tyrosine residues, six in the C-tail of EGFR and six in the C-tail of ERBB2 - Y1023, Y1139, Y1196, Y1221, Y1222 and Y1248 (Margolis et al. 1989, Hazan et al. 1990,Walton et al. 1990, Helin et al. 1991, Ricci et al. 1995, Pinkas-Kramarski 1996). Phosphorylated tyrosine residues in the C-tail of EGFR and ERBB2 serve as docking sites for downstream signaling molecules. Three key signaling pathways activated by ERBB2:EGFR heterodimers are RAF/MAP kinase cascade, PI3K-induced AKT signaling, and signaling by phospholipase C gamma (PLCG1). Downregulation of EGFR signaling is mediated by ubiquitin ligase CBL, and is shown under Signaling by EGFR.
In cells expressing ERBB2 and ERBB3, ERBB3 activated by neuregulin NRG1 or NRG2 binding (Tzahar et al. 1994) forms a heterodimer with ERBB2 (Pinkas-Kramarski et al. 1996, Citri et al. 2004). ERBB3 is the only EGFR family member with no kinase activity, and can only function in heterodimers, with ERBB2 being its preferred heterodimerization partner. After heterodimerization, ERBB2 phosphorylates ten tyrosine residues in the C-tail of ERBB3, Y1054, Y1197, Y1199, Y1222, Y1224, Y1260, Y1262, Y1276, Y1289 and Y1328 (Prigent et al. 1994, Pinkas-Kramarski et al. 1996, Vijapurkar et al. 2003, Li et al. 2007) that subsequently serve as docking sites for downstream signaling molecules, resulting in activation of PI3K-induced AKT signaling and RAF/MAP kinase cascade. Signaling by ERBB3 is downregulated by the action of RNF41 ubiquitin ligase, also known as NRDP1.
ERBB4 becomes activated by binding one of its seven ligands, three of which, HB-EGF, epiregulin EPR and betacellulin BTC, are EGF-like (Elenius et al. 1997, Riese et al. 1998), while four, NRG1, NRG2, NRG3 and NRG4, belong to the related neuregulin family (Tzahar et al. 1994, Carraway et al. 1997, Zhang et al. 1997, Hayes et al. 2007). Upon ligand binding, ERBB4 forms homodimers (Sweeney et al. 2000) or it heterodimerizes with ERBB2 (Li et al. 2007). Dimers of ERBB4 undergo trans-autophosphorylation on tyrosine residues in the C-tail (Cohen et al. 1996, Kaushansky et al. 2008, Hazan et al. 1990, Li et al. 2007), triggering downstream signaling cascades. The pathway Signaling by ERBB4 only shows signaling by ERBB4 homodimers. Signaling by heterodimers of ERBB4 and ERBB2 is shown in the pathway Signaling by ERBB2. Ligand-stimulated ERBB4 is also able to form heterodimers with ligand-stimulated EGFR (Cohen et al. 1996) and ligand-stimulated ERBB3 (Riese et al. 1995). Dimers of ERBB4 with EGFR and dimers of ERBB4 with ERBB3 were demonstrated in mouse cell lines in which human ERBB4 and EGFR or ERBB3 were exogenously expressed. These heterodimers undergo trans-autophosphorylation. The promiscuous heteromerization of ERBBs adds combinatorial diversity to ERBB signaling processes. As ERBB4 binds more ligands than other ERBBs, but has restricted expression, ERBB4 expression channels responses to ERBB ligands. The signaling capabilities of the four receptors have been compared (Schulze et al. 2005).
As for other receptor tyrosine kinases, ERBB4 signaling effectors are largely dictated through binding of effector proteins to ERBB4 peptides that are phosphorylated upon ligand binding. All splicing isoforms of ERBB4 possess two tyrosine residues in the C-tail that serve as docking sites for SHC1 (Kaushansky et al. 2008, Pinkas-Kramarski et al. 1996, Cohen et al. 1996). Once bound to ERBB4, SHC1 becomes phosphorylated on tyrosine residues by the tyrosine kinase activity of ERBB4, which enables it to recruit the complex of GRB2 and SOS1, resulting in the guanyl-nucleotide exchange on RAS and activation of RAF and MAP kinase cascade (Kainulainen et al. 2000).
The CYT1 isoforms of ERBB4 also possess a C-tail tyrosine residue that, upon trans-autophosphorylation, serves as a docking site for the p85 alpha subunit of PI3K (Kaushansky et al. 2008, Cohen et al. 1996), leading to assembly of an active PI3K complex that converts PIP2 to PIP3 and activates AKT signaling (Kainulainen et al. 2000).
Besides signaling as a conventional transmembrane receptor kinase, ERBB4 differs from other ERBBs in that JM-A isoforms signal through efficient release of a soluble intracellular domain. Ligand activated homodimers of ERBB4 JM-A isoforms (ERBB4 JM-A CYT1 and ERBB4 JM-A CYT2) undergo proteolytic cleavage by ADAM17 (TACE) in the juxtamembrane region, resulting in shedding of the extracellular domain and formation of an 80 kDa membrane bound ERBB4 fragment known as ERBB4 m80 (Rio et al. 2000, Cheng et al. 2003). ERBB4 m80 undergoes further proteolytic cleavage, mediated by the gamma-secretase complex, which releases the soluble 80 kDa ERBB4 intracellular domain, known as ERBB4 s80 or E4ICD, into the cytosol (Ni et al. 2001). ERBB4 s80 is able to translocate to the nucleus, promote nuclear translocation of various transcription factors, and act as a transcription co-factor. For example, in mammary cells, ERBB4 binds SH2 transcription factor STAT5A. ERBB4 s80 shuttles STAT5A to the nucleus, and actsa as a STAT5A co-factor in binding to and promoting transcription from the beta-casein (CSN2) promoter, and may be involved in the regulation of other lactation-related genes (Jones et al. 1999, Williams et al. 2004, Muraoka-Cook et al. 2008). ERBB4 s80 binds activated estrogen receptor in the nucleus and acts as a transcriptional co-factor in promoting transcription of some estrogen-regulated genes, including progesterone receptor gene NR3C3 and CXCL12 (SDF1) (Zhu et al. 2006). In neuronal precursors, ERBB4 s80 binds the complex of TAB and NCOR1, helps to move the complex into the nucleus, and is a co-factor of TAB:NCOR1-mediated inhibition of expression of astrocyte differentiation genes GFAP and S100B (Sardi et al. 2006).
The C-tail of ERBB4 possesses several WW-domain binding motifs (three in CYT1 isoform and two in CYT2 isoform), which enable interaction of ERBB4 with WW-domain containing proteins. ERBB4 s80, through WW-domain binding motifs, interacts with YAP1 transcription factor, a known proto-oncogene, and is a co-regulator of YAP1-mediated transcription in association with TEAD transcription factors (Komuro et al. 2003, Omerovic et al. 2004). Hence, the WW binding motif couples ERBB4 to the major effector arm of the HIPPO signaling pathway. The tumor suppressor WWOX, another WW-domain containing protein, competes with YAP1 in binding to ERBB4 s80 and prevents translocation of ERBB4 s80 to the nucleus (Aqeilan et al. 2005).
WW-domain binding motifs in the C-tail of ERBB4 play an important role in the downregulation of ERBB4 receptor signaling, enabling the interaction of intact ERBB4, ERBB4 m80 and ERBB4 s80 with NEDD4 family of E3 ubiquitin ligases WWP1 and ITCH. The interaction of WWP1 and ITCH with intact ERBB4 is independent of receptor activation and autophosphorylation. Binding of WWP1 and ITCH ubiquitin ligases leads to ubiquitination of ERBB4 and its cleavage products, and subsequent degradation through both proteasomal and lysosomal routes (Omerovic et al. 2007, Feng et al. 2009). In addition, the s80 cleavage product of ERBB4 JM-A CYT-1 isoform is the target of NEDD4 ubiquitin ligase. NEDD4 binds ERBB4 JM-A CYT-1 s80 (ERBB4jmAcyt1s80) through its PIK3R1 interaction site and mediates ERBB4jmAcyt1s80 ubiquitination, thereby decreasing the amount of ERBB4jmAcyt1s80 that reaches the nucleus (Zeng et al. 2009).
ERBB4 also binds the E3 ubiquitin ligase MDM2, and inhibitor of p53 (Arasada et al. 2005). Other proteins that bind to ERBB4 intracellular domain have been identified by co-immunoprecipitation and mass spectrometry (Gilmore-Hebert et al., 2010), and include transcriptional co-repressor TRIM28/KAP1, which promotes chromatin compaction. DNA damage signaling through ATM releases TRIM28-associated heterochromatinization. Interactions of ERBB4 with TRIM28 and MDM2 may be important for integration of growth factor responses and DNA damage responses.
In human breast cancer cell lines, ERBB4 activation enhances anchorage-independent colony formation in soft agar but inhibits cell growth in a monolayer culture. Different ERBB4 ligands induce different gene expression changes in breast cancer cell lines. Some of the genes induced in response to ERBB4 signaling in breast cancer cell lines are RAB2, EPS15R and GATA4. It is not known if these gene are direct transcriptional targets of ERBB4 (Amin et al. 2004).
Transcriptome and ChIP-seq comparisons of full-length and intracellular domain isoforms in isogenic MCF10A mammary cell background have revealed the diversification of ERBB4 signaling engendered by alternative splicing and cleavage (Wali et al., 2014). ERBB4 broadly affected protease expression, cholesterol biosynthesis, HIF1-alpha signaling, and HIPPO signaling pathways, and other pathways were differentially activated by CYT1 and CYT2 isoforms. For example, CYT1 promoted expression of transcription factors TWIST1 and SNAIL1 that promote epithelial-mesenchymal transition. HIF1-alpha and HIPPO signaling are mediated, respectively, by binding of ERBB4 to HIF1-alpha and to YAP (Paatero et al., 2012, Komuro et al., 2003). ERBB4 increases activity of the transcription factor SREBF2, resulting in increased expression of SREBF2-target genes involved in cholesterol biosynthesis. The mechanism is not known and may involve facilitation of SREBF2 cleavage through ERBB4-mediated PI3K signaling (Haskins et al. 2016).
In some contexts, ERBB4 promotes growth suppression or apoptosis (Penington et al., 2002). Activation of ERBB4 in breast cancer cell lines leads to JNK dependent increase in BRCA1 mRNA level and mitotic cell cycle delay, but the exact mechanism has not been elucidated (Muraoka Cook et al. 2006). The nature of growth responses may be connected with the spliced isoforms expressed. In comparisons of CYT1 vs CYT2 (full-length and ICD) expression in mammary cells, CYT1 was a weaker growth inducer, associated with attenuated MAPK signaling relative to CYT2 (Wali et al., 2014). ERBB4 s80 is also able to translocate to the mitochondrial matrix, presumably when its nuclear translocation is inhibited. Once in the mitochondrion, the BH3 domain of ERBB4, characteristic of BCL2 family members, may enable it to act as a pro apoptotic factor (Naresh et al. 2006).
ERBB4 plays important roles in the developing and adult nervous system. Erbb4 deficiency in somatostatin-expressing neurons of the thalamic reticular nucleus alters behaviors dependent on sensory selection (Ahrens et al. 2015). NRG1-activated ERBB4 signaling enhances AMPA receptor responses through PKC-dependent AMPA receptor exocytosis. This results in an increased excitatory input to parvalbumin-expressing inhibitory neurons in the visual cortex and regulates visual cortical plasticity (Sun et al. 2016). NRG1-activated ERBB4 signaling is involved in GABAergic activity in amygdala which mediates fear conditioning (fear memory) (Lu et al. 2014). Conditional Erbb4 deletion from fast-spiking interneurons, chandelier and basket cells of the cerebral cortex leads to synaptic defects associated with increased locomotor activity and abnormal emotional, social and cognitive function that can be linked to some of the schizophrenia features. The level of GAD1 (GAD67) protein is reduced in the cortex of conditional Erbb4 mutants. GAD1 is a GABA synthesizing enzyme. Cortical mRNA levels of GAD67 are consistently decreased in schizophrenia (Del Pino et al. 2014). Erbb4 is expressed in the GABAergic neurons of the bed nucleus stria terminalis, a part of the extended amygdala. Inhibition of NRG1-triggered ERBB4 signaling induces anxiety-like behavior, which depends on GABAergic neurotransmission. NRG1-ERBB4 signaling stimulates presynaptic GABA release, but the exact mechanism is not known (Geng et al. 2016). NRG1 protects cortical interneurons against ischemic brain injury through ERBB4-mediated increase in GABAergic transmission (Guan et al. 2015). NRG2-activated ERBB4 can reduce the duration of GABAergic transmission by binding to GABA receptors at the postsynaptic membrane via their GABRA1 subunit and promoting endocytosis of GABA receptors (Mitchell et al. 2013). NRG1 promotes synchronization of prefrontal cortex interneurons in an ERBB4 dependent manner (Hou et al. 2014). NRG1-ERBB4 signaling protects neurons from the cell death induced by a mutant form of the amyloid precursor protein (APP) (Woo et al. 2012).
Clinical relevance of ERBB4 has been identified in several contexts. In cancer, putative and validated gain-of-function mutations or gene amplification that may be drivers have been identified at modest frequencies, and may also contribute to resistance to EGFR and ERBB2-targeted therapies. This is noteworthy as ERBB4 kinase activity is inhibited by pan-ERBB tyrosine kinase inhibitors, including lapatinib, which is approved by the US FDA. The reduced prevalence relative to EGFR and ERBB2 in cancer may reflect more restricted expression of ERBB4, or differential signaling, as specific ERBB4 isoforms have been linked to growth inhibition or apoptosis in experimental systems. ERBB2/ERBB4 heterodimers protect cardiomyocytes, so reduced activity of ERBB4 in patients treated with the ERBB2-targeted therapeutic antibody trastuzumab may contribute to the cardiotoxicity of this agent when used in combination with (cardiotoxic) anthracyclines.
With the importance of ERBB4 in developing and adult nervous system, NRG1 and/or ERBB4 polymorphisms, splicing aberrations and mutations have been linked to nervous system disorders including schizophrenia and amyotrophic lateral sclerosis, although these findings are not yet definitive.
Upon ligand binding, receptor dimers are formed and their intrinsic tyrosine kinase is activated causing phosphorylation of multiple tyrosine residues on the receptors. These then serve as docking sites for the recruitment of SH2 (src homology-2) or PTB (phosphotyrosine binding) domains of adaptors, docking proteins or signaling enzymes. Signaling complexes are assembled and recruited to the active receptors resulting in a cascade of phosphorylation events.
This leads to stimulation of intracellular signaling pathways that control cell proliferation, cell differentiation, cell migration, cell survival and cell shape, depending on the cell type or stage of maturation.
Activation of PLC gamma 1 (PLCG1) signaling by MET remains unclear. It has been reported that PLCG1 can bind to MET directly (Ponzetto et al. 1994) or be recruited by phosphorylated GAB1 (Gual et al. 2000). Tyrosine residue Y307 of GAB1 that serves as docking sites for PLCG1 may be phosphorylated either by activated MET (Watanabe et al. 2006) or SRC (Chan et al. 2010). Another PCLG1 docking site on GAB1, tyrosine residue Y373, was reported as the SRC target, while the kinase for the main PLCG1 docking site, Y407 of GAB1, is not known (Chan et al. 2010).
Signaling by MET promotes cell growth, cell survival and motility, which are essential for embryonic development (Weidner et al. 1993, Schmidt et al. 1995, Uehara et al. 1995, Bladt et al. 1995, Maina et al. 1997, Maina et al. 2001, Helmbacher et al. 2003) and tissue regeneration (Huh et al. 2004, Borowiak et al. 2004, Liu 2004, Chmielowiec et al. 2007). MET signaling is frequently aberrantly activated in cancer, through MET overexpression or activating MET mutations (Schmidt et al. 1997, Pennacchietti et al. 2003, Smolen et al. 2006, Bertotti et al. 2009).
Considerable progress has recently been made in the development of HGF-MET inhibitors in cancer therapy. These include inhibitors of HGF activators, HGF inhibitors and MET antagonists, which are protein therapeutics that act outside the cell. Kinase inhibitors function inside the cell and have constituted the largest effort towards MET-based therapeutics (Gherardi et al. 2012).
Pathogenic bacteria of the species Listeria monocytogenes, exploit MET receptor as an entryway to host cells (Shen et al. 2000, Veiga and Cossart 2005, Neimann et al. 2007).
For review of MET signaling, please refer to Birchmeier et al. 2003, Trusolino et al. 2010, Gherardi et al. 2012, Petrini 2015.
NTRK receptors, NTRK1 (TRKA), NTRK2 (TRKB) and NTRK3 (TRKC) are receptor tyrosine kinases activated by ligand binding to their extracellular domain. Ligand binding induces receptor dimerization, followed by trans-autophosphorylation of dimerized receptors on conserved tyrosine residues in the cytoplasmic region. Phosphorylated tyrosines in the intracellular domain of the receptor serve as docking sites for adapter proteins, triggering downstream signaling cascades.
NTRK1 (TRKA) is the receptor for the nerve growth factor (NGF). NGF is primarily secreted by tissues that are innervated by sensory and sympathetic neurons. NTRK1 signaling promotes growth and survival of neurons during embryonic development and maintenance of neuronal cell integrity in adulthood (reviewed by Marlin and Li 2015).
Brain-derived neurotrophic factor (BDNF) and neurotrophin-4 (NTF4, also known as NT-4) are two high affinity ligands for NTRK2 (TRKB). Neurotrophin-3 (NTF3, also known as NT-3) binds to NTRK2 with low affinity and may not be a physiologically relevant ligand. Nerve growth factor (NGF), a high affinity ligand for NTRK1, does not interact with NTRK2. NTRK2 signaling is implicated in neuronal development in both the peripheral (PNS) and central nervous system (CNS) and may play a role in long-term potentiation (LTP) and learning (reviewed by Minichiello 2009). NTRK2 may modify neuronal excitability and synaptic transmission by directly phosphorylating voltage gated channels (Rogalski et al. 2000).
NTF3 (NT-3) is the ligand for NTRK3 (TRKC). Signaling downstream of activated NTRK3, regulates cell survival, proliferation and motility. In the absence of its ligand, NTRK3 functions as a dependence receptor and triggers BAX and CASP9-dependent cell death (Tauszig-Delamasure et al. 2007, Ichim et al. 2013).
The protein kinase SYK binds the phosphorylated immunoreceptor tyrosine-activated motifs (ITAMs) on the cytoplasmic tails of Ig-alpha (CD79A, MB-1) and Ig-beta (CD79B, B29) (Wienands et al. 1995, Rowley et al. 1995, Tsang et al. 2008). The binding causes the activation and autophosphorylation of SYK (Law et al. 1994, Baldock et al. 2000, Irish et al. 2006, Tsang et al. 2008, reviewed in Bradshaw 2010).
Activated SYK and other kinases phosphorylate BLNK (SLP-65), BCAP, and CD19 which serve as scaffolds for the assembly of large complexes, the signalosomes, by recruiting phosphoinositol 3-kinase (PI3K), phospholipase C gamma (predominantly PLC-gamma2 in B cells, Coggeshall et al. 1992), NCK, BAM32, BTK, VAV1, and SHC. The effectors are phosphorylated by SYK and other kinases.
PLC-gamma associated with BLNK hydrolyzes phosphatidylinositol-4,5-bisphosphate to yield inositol-1,4,5-trisphosphate (IP3) and diacylglycerol (Carter et al. 1991, Kim et al. 2004). IP3 binds receptors on the endoplasmic reticulum and causes release of calcium ions from the ER into the cytosol. The depletion of calcium from the ER in turn activates STIM1 to interact with ORAI and TRPC1 channels in the plasma membrane, resulting in an influx of extracellular calcium ions (Muik et al. 2008, Luik et al. 2008, Park et al. 2009, Mori et al. 2002). PI3K associated with BCAP and CD19 phosphorylates phosphatidylinositol 4,5-bisphosphate to yield phosphatidyinositol 3,4,5-trisphosphate.
Second messengers (calcium, diacylglycerol, inositol 1,4,5-trisphosphate, and phosphatidylinositol 3,4,5-trisphosphate) trigger signaling pathways: NF-kappaB is activated via protein kinase C beta, RAS is activated via RasGRP proteins, NF-AT is activated via calcineurin, and AKT (PKB) is activated via PDK1 (reviewed in Shinohara and Kurosaki 2009, Stone 2006).
Annotated Interactions
The following lists the above proteins with their corresponding literature references: PIP4K2A (Rameh et al. 1997, Clarke et al. 2008, Clarke and Irvine 2013), PIP4K2B (Rameh et al. 1997, Clarke and Irvine 2013) and PIP4K2C (Clarke and Irvine 2013, Clarke et al. 2015).
The following lists the above proteins with their corresponding literature references: PIP5K1A (Halstead et al. 2006, Zhang et al. 1997), PIP5K1B (Zhang et al. 1997), and PIP5K1C (Di Paolo et al. 2002).
This reaction is of particular interest because its regulation by small GTPases of the RHO and ARF families, not yet annotated here, ties the process of phosphatidylinositol phosphate biosynthesis to regulation of the actin cytoskeleton and vesicular trafficking, and hence to diverse aspects of cell motility and signalling (Oude Weernink et al. 2004, 2007).
The TOR kinase regulates two major processes: translation of selected mRNAs in the cell and autophagy. In the presence of high nutrient levels TOR is active and phosphorylates the 4EBP protein releasing the eukaryotic initiation factor 4E (eIF4E), which is essential for cap-dependent initiation of translation and promoting growth of the cell (PMID: 15314020). TOR also phosphorylates the S6 kinase, which is implicated in ribosome biogenesis as well as in the modification of the S6 ribosomal protein. AKT can also activate mTOR by another mechanism, involving phosphorylation of PRAS40, an inhibitor of mTOR activity.
Phosphorylation of AKT on S473 by TORC2 complex is a prerequisite for AKT phosphorylation on T308 by PDPK1 (Scheid et al. 2002, Sarabassov et al. 2005).
Based on studies with recombinant mouse Foxo6 expressed in the human embryonic kidney cell line HEK293, FOXO6 has two conserved AKT phosphorylation sites: T26 and S184. Mouse Foxo6 has a third predicted Akt phosphorylation site at the C-terminus, T338, which is not present in other Foxo family members and is not conserved in human FOXO6. T26 and S184 are phosphorylated in response to growth factors known to activate PI3K/AKT signaling, but AKT has not been explicitly identified as the responsible kinase. In contrast to other FOXO family members, FOXO6 remains predominantly nuclear irrespective of growth factor-induced signaling, and only a small portion of phosphorylated FOXO6 may shuttle to the cytosol. Phosphorylation of FOXO6 on putative AKT sites, however, may inhibit binding of FOXO6 to target DNA sites (Jacobs et al. 2003, van der Heide et al. 2005).
Protein phosphatase DUSP6 (MKP3) may act to dephosphorylate FOXO1 after AKT-mediated phosphorylation (Rodrigues et al. 2017).
Early studies indicated that magnesium ion, Mg2+, was needed for the catalytic activity of PTEN isolated from bovine thymus (Kabuyama et al. 1996). Subsequent studies have shown that PTEN was catalytically active in buffers free of magnesium and magnesium was not detected as part of the PTEN crystal (Lee et al. 1999).
In unstimulated cells, PI3K class IA exists as an inactive heterodimer of a p85 regulatory subunit (encoded by PIK3R1, PIK3R2 or PIK3R3) and a p110 catalytic subunit (encoded by PIK3CA, PIK3CB or PIK3CD). Binding of the iSH2 domain of the p85 regulatory subunit to the ABD and C2 domains of the p110 catalytic subunit both stabilizes p110 and inhibits its catalytic activity. This inhibition is relieved when the SH2 domains of p85 bind phosphorylated tyrosines on activated RTKs or their adaptor proteins. Binding to membrane-associated receptors brings activated PI3K in proximity to its membrane-localized substrate, PIP2 (Mandelker et al. 2009, Burke et al. 2011).