ERBB4, also known as HER4, belongs to the ERBB family of receptors, which also includes ERBB1 (EGFR i.e. HER1), ERBB2 (HER2 i.e. 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 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, but their downstream signaling and physiological significance have not been studied.
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 transmembrane receptor, 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. 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). In mammary cells, ERBB4 s80 recruits STAT5A transcription factor in the cytosol, shuttles it to the nucleus, and acts as the 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 (Williams et al. 2004, Muraoka-Cook et al. 2008). ERBB4 s80 was also shown to bind activated estrogen receptor in the nucleus and act as its transcriptional co-factor in promoting transcription of some estrogen-regulated genes, such as progesterone receptor gene NR3C3 and CXCL12 i.e. SDF1 (Zhu 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 may be a co-regulator of YAP1-mediated transcription (Komuro et al. 2003, Omerovic et al. 2004). 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). 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). 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).
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).
Goffin V, Binart N, Touraine P, Kelly PA.; ''Prolactin: the new biology of an old hormone.''; PubMedEurope PMCScholia
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Rio C, Buxbaum JD, Peschon JJ, Corfas G.; ''Tumor necrosis factor-alpha-converting enzyme is required for cleavage of erbB4/HER4.''; PubMedEurope PMCScholia
Aqeilan RI, Donati V, Palamarchuk A, Trapasso F, Kaou M, Pekarsky Y, Sudol M, Croce CM.; ''WW domain-containing proteins, WWOX and YAP, compete for interaction with ErbB-4 and modulate its transcriptional function.''; PubMedEurope PMCScholia
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Brown MD, Sacks DB.; ''Protein scaffolds in MAP kinase signalling.''; PubMedEurope PMCScholia
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Ni CY, Murphy MP, Golde TE, Carpenter G.; ''gamma -Secretase cleavage and nuclear localization of ErbB-4 receptor tyrosine kinase.''; PubMedEurope PMCScholia
Paoletti P, Bellone C, Zhou Q.; ''NMDA receptor subunit diversity: impact on receptor properties, synaptic plasticity and disease.''; 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
Williams CC, Allison JG, Vidal GA, Burow ME, Beckman BS, Marrero L, Jones FE.; ''The ERBB4/HER4 receptor tyrosine kinase regulates gene expression by functioning as a STAT5A nuclear chaperone.''; PubMedEurope PMCScholia
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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
Gilmore-Hebert M, Ramabhadran R, Stern DF.; ''Interactions of ErbB4 and Kap1 connect the growth factor and DNA damage response pathways.''; PubMedEurope PMCScholia
Haskins JW, Zhang S, Means RE, Kelleher JK, Cline GW, Canfrán-Duque A, Suárez Y, Stern DF.; ''Neuregulin-activated ERBB4 induces the SREBP-2 cholesterol biosynthetic pathway and increases low-density lipoprotein uptake.''; 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
Cohen S, Greenberg ME.; ''Communication between the synapse and the nucleus in neuronal development, plasticity, and disease.''; PubMedEurope PMCScholia
Kelly PA, Binart N, Freemark M, Lucas B, Goffin V, Bouchard B.; ''Prolactin receptor signal transduction pathways and actions determined in prolactin receptor knockout mice.''; PubMedEurope PMCScholia
Li Z, Mei Y, Liu X, Zhou M.; ''Neuregulin-1 only induces trans-phosphorylation between ErbB receptor heterodimer partners.''; PubMedEurope PMCScholia
Feng SM, Muraoka-Cook RS, Hunter D, Sandahl MA, Caskey LS, Miyazawa K, Atfi A, Earp HS.; ''The E3 ubiquitin ligase WWP1 selectively targets HER4 and its proteolytically derived signaling isoforms for degradation.''; PubMedEurope PMCScholia
Zhu Y, Sullivan LL, Nair SS, Williams CC, Pandey AK, Marrero L, Vadlamudi RK, Jones FE.; ''Coregulation of estrogen receptor by ERBB4/HER4 establishes a growth-promoting autocrine signal in breast tumor cells.''; 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
Omerovic J, Puggioni EM, Napoletano S, Visco V, Fraioli R, Frati L, Gulino A, Alimandi M.; ''Ligand-regulated association of ErbB-4 to the transcriptional co-activator YAP65 controls transcription at the nuclear level.''; PubMedEurope PMCScholia
Arasada RR, Carpenter G.; ''Secretase-dependent tyrosine phosphorylation of Mdm2 by the ErbB-4 intracellular domain fragment.''; PubMedEurope PMCScholia
Elenius K, Paul S, Allison G, Sun J, Klagsbrun M.; ''Activation of HER4 by heparin-binding EGF-like growth factor stimulates chemotaxis but not proliferation.''; PubMedEurope PMCScholia
Del Pino I, García-Frigola C, Dehorter N, Brotons-Mas JR, Alvarez-Salvado E, Martínez de Lagrán M, Ciceri G, Gabaldón MV, Moratal D, Dierssen M, Canals S, Marín O, Rico B.; ''Erbb4 deletion from fast-spiking interneurons causes schizophrenia-like phenotypes.''; PubMedEurope PMCScholia
Riese DJ, Komurasaki T, Plowman GD, Stern DF.; ''Activation of ErbB4 by the bifunctional epidermal growth factor family hormone epiregulin is regulated by ErbB2.''; PubMedEurope PMCScholia
Sardi SP, Murtie J, Koirala S, Patten BA, Corfas G.; ''Presenilin-dependent ErbB4 nuclear signaling regulates the timing of astrogenesis in the developing brain.''; PubMedEurope PMCScholia
Omerovic J, Santangelo L, Puggioni EM, Marrocco J, Dall'Armi C, Palumbo C, Belleudi F, Di Marcotullio L, Frati L, Torrisi MR, Cesareni G, Gulino A, Alimandi M.; ''The E3 ligase Aip4/Itch ubiquitinates and targets ErbB-4 for degradation.''; PubMedEurope PMCScholia
Cargnello M, Roux PP.; ''Activation and function of the MAPKs and their substrates, the MAPK-activated protein kinases.''; 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
Oh H, Irvine KD.; ''Yorkie: the final destination of Hippo signaling.''; PubMedEurope PMCScholia
Cantwell-Dorris ER, O'Leary JJ, Sheils OM.; ''BRAFV600E: implications for carcinogenesis and molecular therapy.''; PubMedEurope PMCScholia
Xiao L, Chen Y, Ji M, Dong J.; ''KIBRA regulates Hippo signaling activity via interactions with large tumor suppressor kinases.''; PubMedEurope PMCScholia
Remue E, Meerschaert K, Oka T, Boucherie C, Vandekerckhove J, Sudol M, Gettemans J.; ''TAZ interacts with zonula occludens-1 and -2 proteins in a PDZ-1 dependent manner.''; PubMedEurope PMCScholia
Cohen BD, Green JM, Foy L, Fell HP.; ''HER4-mediated biological and biochemical properties in NIH 3T3 cells. Evidence for HER1-HER4 heterodimers.''; PubMedEurope PMCScholia
Andersson ER, Lendahl U.; ''Therapeutic modulation of Notch signalling--are we there yet?''; PubMedEurope PMCScholia
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Woo RS, Lee JH, Kim HS, Baek CH, Song DY, Suh YH, Baik TK.; ''Neuregulin-1 protects against neurotoxicities induced by Swedish amyloid precursor protein via the ErbB4 receptor.''; PubMedEurope PMCScholia
Zhang D, Sliwkowski MX, Mark M, Frantz G, Akita R, Sun Y, Hillan K, Crowley C, Brush J, Godowski PJ.; ''Neuregulin-3 (NRG3): a novel neural tissue-enriched protein that binds and activates ErbB4.''; PubMedEurope PMCScholia
Paatero I, Jokilammi A, Heikkinen PT, Iljin K, Kallioniemi OP, Jones FE, Jaakkola PM, Elenius K.; ''Interaction with ErbB4 promotes hypoxia-inducible factor-1α signaling.''; PubMedEurope PMCScholia
Fukumoto T, Kubota Y, Kitanaka A, Yamaoka G, Ohara-Waki F, Imataki O, Ohnishi H, Ishida T, Tanaka T.; ''Gab1 transduces PI3K-mediated erythropoietin signals to the Erk pathway and regulates erythropoietin-dependent proliferation and survival of erythroid cells.''; PubMedEurope PMCScholia
Kyriakis JM, Avruch J.; ''Mammalian MAPK signal transduction pathways activated by stress and inflammation: a 10-year update.''; PubMedEurope PMCScholia
Guan YF, Wu CY, Fang YY, Zeng YN, Luo ZY, Li SJ, Li XW, Zhu XH, Mei L, Gao TM.; ''Neuregulin 1 protects against ischemic brain injury via ErbB4 receptors by increasing GABAergic transmission.''; PubMedEurope PMCScholia
Schulze WX, Deng L, Mann M.; ''Phosphotyrosine interactome of the ErbB-receptor kinase family.''; PubMedEurope PMCScholia
Bole-Feysot C, Goffin V, Edery M, Binart N, Kelly PA.; ''Prolactin (PRL) and its receptor: actions, signal transduction pathways and phenotypes observed in PRL receptor knockout mice.''; PubMedEurope PMCScholia
Zhao B, Li L, Lei Q, Guan KL.; ''The Hippo-YAP pathway in organ size control and tumorigenesis: an updated version.''; PubMedEurope PMCScholia
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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
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Sudol M, Harvey KF.; ''Modularity in the Hippo signaling pathway.''; PubMedEurope PMCScholia
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Pinkas-Kramarski R, Soussan L, Waterman H, Levkowitz G, Alroy I, Klapper L, Lavi S, Seger R, Ratzkin BJ, Sela M, Yarden Y.; ''Diversification of Neu differentiation factor and epidermal growth factor signaling by combinatorial receptor interactions.''; PubMedEurope PMCScholia
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Lu Y, Sun XD, Hou FQ, Bi LL, Yin DM, Liu F, Chen YJ, Bean JC, Jiao HF, Liu X, Li BM, Xiong WC, Gao TM, Mei L.; ''Maintenance of GABAergic activity by neuregulin 1-ErbB4 in amygdala for fear memory.''; PubMedEurope PMCScholia
Chan SW, Lim CJ, Chong YF, Pobbati AV, Huang C, Hong W.; ''Hippo pathway-independent restriction of TAZ and YAP by angiomotin.''; PubMedEurope PMCScholia
Murakami M, Nakagawa M, Olson EN, Nakagawa O.; ''A WW domain protein TAZ is a critical coactivator for TBX5, a transcription factor implicated in Holt-Oram syndrome.''; PubMedEurope PMCScholia
Hardingham GE, Bading H.; ''Synaptic versus extrasynaptic NMDA receptor signalling: implications for neurodegenerative disorders.''; PubMedEurope PMCScholia
Jones FE, Welte T, Fu XY, Stern DF.; ''ErbB4 signaling in the mammary gland is required for lobuloalveolar development and Stat5 activation during lactation.''; PubMedEurope PMCScholia
Penington DJ, Bryant I, Riese DJ.; ''Constitutively active ErbB4 and ErbB2 mutants exhibit distinct biological activities.''; PubMedEurope PMCScholia
Cseh B, Doma E, Baccarini M.; ''"RAF" neighborhood: protein-protein interaction in the Raf/Mek/Erk pathway.''; PubMedEurope PMCScholia
Roskoski R.; ''MEK1/2 dual-specificity protein kinases: structure and regulation.''; PubMedEurope PMCScholia
Cheng QC, Tikhomirov O, Zhou W, Carpenter G.; ''Ectodomain cleavage of ErbB-4: characterization of the cleavage site and m80 fragment.''; PubMedEurope PMCScholia
Wali VB, Haskins JW, Gilmore-Hebert M, Platt JT, Liu Z, Stern DF.; ''Convergent and divergent cellular responses by ErbB4 isoforms in mammary epithelial cells.''; PubMedEurope PMCScholia
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Komuro A, Nagai M, Navin NE, Sudol M.; ''WW domain-containing protein YAP associates with ErbB-4 and acts as a co-transcriptional activator for the carboxyl-terminal fragment of ErbB-4 that translocates to the nucleus.''; PubMedEurope PMCScholia
Hazan R, Margolis B, Dombalagian M, Ullrich A, Zilberstein A, Schlessinger J.; ''Identification of autophosphorylation sites of HER2/neu.''; PubMedEurope PMCScholia
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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.
Prolactin (PRL) is a hormone secreted mainly by the anterior pituitary gland. It was originally identified by its ability to stimulate the development of the mammary gland and lactation, but is now known to have numerous and varied functions (Bole-Feysot et al. 1998). Despite this, few pathologies have been associated with abnormalities in prolactin receptor (PRLR) signaling, though roles in various forms of cancer and certain autoimmune disorders have been suggested (Goffin et al. 2002). A vast body of literature suggests effects of PRL in immune cells (Matera 1996) but PRLR KO mice have unaltered immune system development and function (Bouchard et al. 1999). In addition to the pituitary, numerous other tissues produce PRL, including the decidua and myometrium, certain cells of the immune system, brain, skin and exocrine glands such as the mammary, sweat and lacrimal glands (Ben-Jonathan et al. 1996). Pituitary PRL secretion is negatively regulated by inhibitory factors originating from the hypothalamus, the most important of which is dopamine, acting through the D2 subclass of dopamine receptors present in lactotrophs (Freeman et al. 2000). PRL-binding sites or receptors have been identified in numerous cells and tissues of adult mammals. Various forms of PRLR, generated by alternative splicing, have been reported in several species including humans (Kelly et al. 1991, Clevenger et al. 2003).
PRLR is a member of the cytokine receptor superfamily. Like many other members of this family, the first step in receptor activation was generally believed to be ligand-induced dimerization whereby one molecule of PRL bound to two molecules of receptor (Elkins et al. 2000). Recent reports suggest that PRLR pre-assembles at the plasma membrane in the absence of ligand (Gadd & Clevenger 2006, Tallet et al. 2011), suggesting that ligand-induced activation involves conformational changes in preformed PRLR dimers (Broutin et al. 2010).
PRLR has no intrinsic kinase activity but associates (Lebrun et al. 1994, 1995) with Janus kinase 2 (JAK2) which is activated following receptor activation (Campbell et al. 1994, Rui et al. 1994, Carter-Su et al. 2000, Barua et al. 2009). JAK2-dependent activation of JAK1 has also been reported (Neilson et al. 2007). It is generally accepted that activation of JAK2 occurs by transphosphorylation upon ligand-induced receptor activation, based on JAK activation by chimeric receptors in which various extracellular domains of cytokine or tyrosine kinase receptors were fused to the IL-2 receptor beta chain (see Ihle et al. 1994). This activation step involves the tyrosine phosphorylation of JAK2, which in turn phosphorylates PRLR on specific intracellular tyrosine residues leading to STAT5 recruitment and signaling, considered to be the most important signaling cascade for PRLR. STAT1 and STAT3 activation have also been reported (DaSilva et al. 1996) as have many other signaling pathways; signaling through MAP kinases (Shc/SOS/Grb2/Ras/Raf/MAPK) has been reported as a consequence of PRL stimuilation in many different cellular systems (see Bole-Feysot et al. 1998) though it is not clear how this signal is propagated. Other cascades non exhaustively include Src kinases, Focal adhesion kinase, phospholipase C gamma, PI3 kinase/Akt and Nek3 (Clevenger et al. 2003, Miller et al. 2007). The protein tyrosine phosphatase SHP2 is recruited to the C terminal tyrosine of PRLR and may have a regulatory role (Ali & Ali 2000). PRLR phosphotyrosines can recruit insulin receptor substrates (IRS) and other adaptor proteins to the receptor complex (Bole-Feysot et al. 1998).
Female homozygous PRLR knockout mice are completely infertile and show a lack of mammary development (Ormandy et al. 1997). Hemizogotes are unable to lactate following their first pregnancy and depending on the genetic background, this phenotype can persist through subsequent pregnancies (Kelly et al. 2001).
The MAP kinase cascade describes a sequence of phosphorylation events involving serine/threonine-specific protein kinases. Used by various signal transduction pathways, this cascade constitutes a common 'module' in the transmission of an extracellular signal into the nucleus.
Cytosolic ERBB4s80 is able to translocate to mitochondria where its BH3 domain, characteristic of BCL2 family members, may enable it to act as a pro-apoptotic factor.
Phosphorylated ligand-bound homodimers of ERBB4 JM-A isoforms are cleaved by ADAM17 metalloproteinase to yield ligand-bound ERBB4 extracellular domain and membrane bound ERBB4 fragment of 80 kDa (ERBB4m80).
The complex of ERBB4s80 and activated estrogen receptor ESR1 promotes transcription of estrogen regulated genes NR3C3 (coding for progesterone receptor) and CXCL12 (SDF1).
WWOX binds to ERBB4s80 through WW-domain binding motifs in the C-tail of ERBB4. Formation of ERBB4s80:WWOX complex competes with the formation of ERBB4:YAP1 complex and prevents translocation of ERBB4s80 to the nucleus. Feng et al. established that WWOX binds with the same affinity to s80CYT1 and s80CYT2, and identified PY3 as the most important WW-domain binding motif for WWOX binding.
Transcription of GFAP and S100B genes, involved in astrocyte differentiation, is inhibited by ERBB4s80:TAB2:NCOR1 complex which binds promoters of GFAP and S100B.
p85 subunit of PI3K (PIK3R1) directly binds to phosphorylated ERBB4 CYT1 homodimers through docking tyrosine residues on either ERBB4 JM A CYT1 (tyrosine Y1056) or ERBB4 JM B CYT1 (tyrosine Y1046) isoform.
ERBB4s80 binds STAT5A through STAT5A SH2 domain. This interaction likely depends on STAT5A activation induced by prolactin and mediated by JAK2. Heterodimers of prolactin receptor (PRLR) and JAK2 are activated by prolactin binding, resulting in STAT5 recruitment and phosphorylation, and subsequent formation of phosphorylated STAT5 homodimers. There is evidence that ERBB4 may be part of the PRLR:JAK2 complex and that it may be activated by JAK2-mediated phosphorylation, in the absence of ERBB4 growth factors (Muraoka-Cook et al. 2008).
All three ERBB4 isoforms are activated by binding of neuregulins (NRG1, NRG2, NRG3 and NRG4) or EGF like growth factors (betacellulin, epiregulin, HB EGF) to their extracellular domain.
E3 ubiquitin ligase NEDD4 mediates ubiquitination of ERBB4 JM-A CYT-1 intracellular domain s80 (ERBB4jmAcyt1s80) produced by ERBB4 cleavage. This induces degradation of ERBB4jmAcyt1s80, and decreases the amount of ERBB4jmAcyt1s80 that reaches the nucleus.
After ERBB4 is cleaved by ADAM17, gamma-secretase complex performs additional cleavage in the transmembrane region of the m80 ERBB4 fragment, releasing the soluble ERBB4 intracellular domain of 80 kDa, known as s80 or E4ICD.
Formation of cytosolic complex of ERBB4s80 and STAT5A promotes translocation of STAT5A to the nucleus, with ERBB4s80 acting as a nuclear chaperone of STAT5A.
Ligand-stimulated ERBB4 was shown to form heterodimers with ligand-stimulated ERBB3 when human ERBB4 and ERBB3 were exogenously expressed in mouse pro-B-lymphocyte cell line. Heterodimers of ERBB4 and ERBB3 undergo trans-autophosphorylation, but the exact phosphorylation pattern, downstream signaling and physiological significance of these heterodimers have not been studied.
Homodimers of ERBB4 CYT 1 isoforms trans autophosphorylate on six tyrosine residues (three on each monomer) that serve as docking sites for SHC1 (tyrosines Y1188 and 1242 in the isoform ERBB4 JM-A CYT1; tyrosines Y1178 and Y1232 in the isoform ERBB4 JM-B CYT1) and the p85 subunit of PI3K (tyrosine Y1056 in the isoform ERBB4 JM-A CYT1; tyrosine Y1046 in the isoform ERBB4 JM-B CYT1), while ERBB4 CYT2 isoform homodimer trans-autophosphorylates on four SHC1 binding tyrosines (two on each monomer - tyrosines Y1172 and Y1226).
SOS1 in complex with GRB2 and p-Y349,350-SHC1:p-ERBB4 activates RAS by mediating guanyl nucleotide exchange, which results in the activation of RAF/MAP kinase cascade.
Intact ERBB4 isoforms and their membrane bound and cytosolic cleavage products, m80 and s80, bind NEDD4 family E3 ubiquitin ligases WWP1 and ITCH through WW-binding motifs in the C-tail. This interaction is independent of ligand binding and ERBB4 phosphorylation. CYT1 isoforms of ERBB4 have three WW-binding motifs: PY1, PY2 and PY3. PY2 motif is unique to CYT1 isoforms and overlaps with the PIK3R1 binding site. CYT2 isoform of ERBB4 has two WW-binding motifs: PY1 and PY3. While both CYT1 and CYT2 isoforms of ERBB4 all bind WWP1, CYT1 intracellular domain exhibits higher affinity for WWP1. Based on co-immunoprecipitation experiments in which individual WW-binding motifs of ERBB4 were mutated, Feng et al. established that PY2 had the highest affinity for WWP1, followed by PY3, while PY1 showed the lowest affinity.
Upon binding to ERBB4 or its cleavage products m80 and s80, NEDD4 family ligases WWP1 and ITCH ubiquitinate intact and cleaved ERBB4 and target it for degradation.
ERBB4s80 interacts with a co-transcriptional activator YAP1 through its WW-domain binding motifs in the C-tail. Feng et al. established that the PY2 motif, present in CYT1 isoforms of ERBB4 only, has the highest affinity for YAP1 binding. PY1 and PY3 motifs, shared between CYT1 and CYT2 isoforms, have lower binding affinity for YAP1, with PY1 motif being the least important for YAP1 interaction.
ERBB4s80:STAT5A complex binds to and stimulates transcription from the beta-casein (CSN2) promoter, and it probably regulates transcription of other lactation-related genes in mammary cells. By over-expressing either human ERBB4cyt1s80 or ERBB4cyt2s80 in mouse mammary cell line HC11 or transgenic mice, Muraoka-Cook et al. showed differential effects of CYT1 and CYT2 isoforms on mammary epithelium. CYT1s80 over-expression decreases cell proliferation, promotes STAT5A-mediated transcription of beta-casein (CSN2) and lactogenic differentiation. In contrast, CYT2s80 over-expression causes epithelial hyperplasia, increased levels of Wnt and beta-catenin, as well as elevated expression of c-myc and cyclin D1 (Muraoka-Cook et al. 2009).
Ligand-stimulated ERBB4 was shown to form heterodimers with ligand-stimulated EGFR when human ERBB4 and EGFR were exogenously expressed in mouse fibroblast cell line. Heterodimers of ERBB4 and EGFR undergo trans-autophosphorylation, but the exact phosphorylation pattern, downstream signaling and physiological significance of these heterodimers have not been studied.
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 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, but their downstream signaling and physiological significance have not been studied.
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 transmembrane receptor, 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. 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). In mammary cells, ERBB4 s80 recruits STAT5A transcription factor in the cytosol, shuttles it to the nucleus, and acts as the 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 (Williams et al. 2004, Muraoka-Cook et al. 2008). ERBB4 s80 was also shown to bind activated estrogen receptor in the nucleus and act as its transcriptional co-factor in promoting transcription of some estrogen-regulated genes, such as progesterone receptor gene NR3C3 and CXCL12 i.e. SDF1 (Zhu 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 may be a co-regulator of YAP1-mediated transcription (Komuro et al. 2003, Omerovic et al. 2004). 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). 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). 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).
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).
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NCOR1TAB2
NCOR1SOS1 p-Y349,350-SHC1
p-ERBB4PRLR is a member of the cytokine receptor superfamily. Like many other members of this family, the first step in receptor activation was generally believed to be ligand-induced dimerization whereby one molecule of PRL bound to two molecules of receptor (Elkins et al. 2000). Recent reports suggest that PRLR pre-assembles at the plasma membrane in the absence of ligand (Gadd & Clevenger 2006, Tallet et al. 2011), suggesting that ligand-induced activation involves conformational changes in preformed PRLR dimers (Broutin et al. 2010).
PRLR has no intrinsic kinase activity but associates (Lebrun et al. 1994, 1995) with Janus kinase 2 (JAK2) which is activated following receptor activation (Campbell et al. 1994, Rui et al. 1994, Carter-Su et al. 2000, Barua et al. 2009). JAK2-dependent activation of JAK1 has also been reported (Neilson et al. 2007). It is generally accepted that activation of JAK2 occurs by transphosphorylation upon ligand-induced receptor activation, based on JAK activation by chimeric receptors in which various extracellular domains of cytokine or tyrosine kinase receptors were fused to the IL-2 receptor beta chain (see Ihle et al. 1994). This activation step involves the tyrosine phosphorylation of JAK2, which in turn phosphorylates PRLR on specific intracellular tyrosine residues leading to STAT5 recruitment and signaling, considered to be the most important signaling cascade for PRLR. STAT1 and STAT3 activation have also been reported (DaSilva et al. 1996) as have many other signaling pathways; signaling through MAP kinases (Shc/SOS/Grb2/Ras/Raf/MAPK) has been reported as a consequence of PRL stimuilation in many different cellular systems (see Bole-Feysot et al. 1998) though it is not clear how this signal is propagated. Other cascades non exhaustively include Src kinases, Focal adhesion kinase, phospholipase C gamma, PI3 kinase/Akt and Nek3 (Clevenger et al. 2003, Miller et al. 2007). The protein tyrosine phosphatase SHP2 is recruited to the C terminal tyrosine of PRLR and may have a regulatory role (Ali & Ali 2000). PRLR phosphotyrosines can recruit insulin receptor substrates (IRS) and other adaptor proteins to the receptor complex (Bole-Feysot et al. 1998).
Female homozygous PRLR knockout mice are completely infertile and show a lack of mammary development (Ormandy et al. 1997). Hemizogotes are unable to lactate following their first pregnancy and depending on the genetic background, this phenotype can persist through subsequent pregnancies (Kelly et al. 2001).
Annotated Interactions
TAB2
NCOR1SOS1 p-Y349,350-SHC1
p-ERBB4