MyD88-independent signaling pathway is shared by TLR3 and TLR4 cascades. TIR-domain-containing adapter-inducing interferon-beta (TRIF or TICAM1) is a key adapter molecule in transducing signals from TLR3 and TLR4 in a MyD88-independent manner (Yamamoto M et al. 2003a). TRIF is recruited to ligand-stimulated TLR3 or 4 complex via its TIR domain. TLR3 directly binds TRIF (Oshiumi H et al 2003). In contrast, TLR4-mediated signaling pathway requires two adapter molecules, TRAM (TRIF-related adapter molecule or TICAM2) and TRIF. TRAM(TICAM2) is thought to bridge between the activated TLR4 complex and TRIF (Yamamoto M et al. 2003b, Tanimura N et al. 2008, Kagan LC et al. 2008).
TRIF recruitment to TLR complex stimulates distinct pathways leading to production of type 1 interferons (IFNs), pro-inflammatory cytokines and induction of programmed cell death.
Kalai M, Van Loo G, Vanden Berghe T, Meeus A, Burm W, Saelens X, Vandenabeele P.; ''Tipping the balance between necrosis and apoptosis in human and murine cells treated with interferon and dsRNA.''; PubMedEurope PMCScholia
Wang C, Deng L, Hong M, Akkaraju GR, Inoue J, Chen ZJ.; ''TAK1 is a ubiquitin-dependent kinase of MKK and IKK.''; PubMedEurope PMCScholia
Fitzgerald KA, McWhirter SM, Faia KL, Rowe DC, Latz E, Golenbock DT, Coyle AJ, Liao SM, Maniatis T.; ''IKKepsilon and TBK1 are essential components of the IRF3 signaling pathway.''; PubMedEurope PMCScholia
Deng L, Wang C, Spencer E, Yang L, Braun A, You J, Slaughter C, Pickart C, Chen ZJ.; ''Activation of the IkappaB kinase complex by TRAF6 requires a dimeric ubiquitin-conjugating enzyme complex and a unique polyubiquitin chain.''; PubMedEurope PMCScholia
Keller N, Mares J, Zerbe O, Grütter MG.; ''Structural and biochemical studies on procaspase-8: new insights on initiator caspase activation.''; PubMedEurope PMCScholia
Bardwell AJ, Frankson E, Bardwell L.; ''Selectivity of docking sites in MAPK kinases.''; PubMedEurope PMCScholia
Blanchard H, Kodandapani L, Mittl PR, Marco SD, Krebs JF, Wu JC, Tomaselli KJ, Grütter MG.; ''The three-dimensional structure of caspase-8: an initiator enzyme in apoptosis.''; PubMedEurope PMCScholia
Yamamoto M, Sato S, Hemmi H, Uematsu S, Hoshino K, Kaisho T, Takeuchi O, Takeda K, Akira S.; ''TRAM is specifically involved in the Toll-like receptor 4-mediated MyD88-independent signaling pathway.''; PubMedEurope PMCScholia
Dong C, Davis RJ, Flavell RA.; ''MAP kinases in the immune response.''; PubMedEurope PMCScholia
Han KJ, Su X, Xu LG, Bin LH, Zhang J, Shu HB.; ''Mechanisms of the TRIF-induced interferon-stimulated response element and NF-kappaB activation and apoptosis pathways.''; PubMedEurope PMCScholia
Fitzgerald KA, Rowe DC, Golenbock DT.; ''Endotoxin recognition and signal transduction by the TLR4/MD2-complex.''; PubMedEurope PMCScholia
Thiefes A, Wolter S, Mushinski JF, Hoffmann E, Dittrich-Breiholz O, Graue N, Dörrie A, Schneider H, Wirth D, Luckow B, Resch K, Kracht M.; ''Simultaneous blockade of NFkappaB, JNK, and p38 MAPK by a kinase-inactive mutant of the protein kinase TAK1 sensitizes cells to apoptosis and affects a distinct spectrum of tumor necrosis factor [corrected] target genes.''; PubMedEurope PMCScholia
Larabi A, Devos JM, Ng SL, Nanao MH, Round A, Maniatis T, Panne D.; ''Crystal structure and mechanism of activation of TANK-binding kinase 1.''; PubMedEurope PMCScholia
Arch RH, Gedrich RW, Thompson CB.; ''Tumor necrosis factor receptor-associated factors (TRAFs)--a family of adapter proteins that regulates life and death.''; PubMedEurope PMCScholia
Sato S, Sanjo H, Takeda K, Ninomiya-Tsuji J, Yamamoto M, Kawai T, Matsumoto K, Takeuchi O, Akira S.; ''Essential function for the kinase TAK1 in innate and adaptive immune responses.''; PubMedEurope PMCScholia
Mori M, Yoneyama M, Ito T, Takahashi K, Inagaki F, Fujita T.; ''Identification of Ser-386 of interferon regulatory factor 3 as critical target for inducible phosphorylation that determines activation.''; PubMedEurope PMCScholia
Rothwarf DM, Zandi E, Natoli G, Karin M.; ''IKK-gamma is an essential regulatory subunit of the IkappaB kinase complex.''; PubMedEurope PMCScholia
Lin R, Heylbroeck C, Pitha PM, Hiscott J.; ''Virus-dependent phosphorylation of the IRF-3 transcription factor regulates nuclear translocation, transactivation potential, and proteasome-mediated degradation.''; PubMedEurope PMCScholia
Meylan E, Burns K, Hofmann K, Blancheteau V, Martinon F, Kelliher M, Tschopp J.; ''RIP1 is an essential mediator of Toll-like receptor 3-induced NF-kappa B activation.''; PubMedEurope PMCScholia
Adhikari A, Xu M, Chen ZJ.; ''Ubiquitin-mediated activation of TAK1 and IKK.''; PubMedEurope PMCScholia
Cory S, Huang DC, Adams JM.; ''The Bcl-2 family: roles in cell survival and oncogenesis.''; PubMedEurope PMCScholia
Ashkenazi A.; ''Targeting death and decoy receptors of the tumour-necrosis factor superfamily.''; PubMedEurope PMCScholia
Shim JH, Xiao C, Paschal AE, Bailey ST, Rao P, Hayden MS, Lee KY, Bussey C, Steckel M, Tanaka N, Yamada G, Akira S, Matsumoto K, Ghosh S.; ''TAK1, but not TAB1 or TAB2, plays an essential role in multiple signaling pathways in vivo.''; PubMedEurope PMCScholia
MacFarlane M, Williams AC.; ''Apoptosis and disease: a life or death decision.''; PubMedEurope PMCScholia
Sato S, Sugiyama M, Yamamoto M, Watanabe Y, Kawai T, Takeda K, Akira S.; ''Toll/IL-1 receptor domain-containing adaptor inducing IFN-beta (TRIF) associates with TNF receptor-associated factor 6 and TANK-binding kinase 1, and activates two distinct transcription factors, NF-kappa B and IFN-regulatory factor-3, in the Toll-like receptor signaling.''; PubMedEurope PMCScholia
Kerr JF.; ''History of the events leading to the formulation of the apoptosis concept.''; PubMedEurope PMCScholia
Häcker H, Karin M.; ''Regulation and function of IKK and IKK-related kinases.''; PubMedEurope PMCScholia
Cusson-Hermance N, Khurana S, Lee TH, Fitzgerald KA, Kelliher MA.; ''Rip1 mediates the Trif-dependent toll-like receptor 3- and 4-induced NF-{kappa}B activation but does not contribute to interferon regulatory factor 3 activation.''; PubMedEurope PMCScholia
Jiang Z, Mak TW, Sen G, Li X.; ''Toll-like receptor 3-mediated activation of NF-kappaB and IRF3 diverges at Toll-IL-1 receptor domain-containing adapter inducing IFN-beta.''; PubMedEurope PMCScholia
Takahasi K, Suzuki NN, Horiuchi M, Mori M, Suhara W, Okabe Y, Fukuhara Y, Terasawa H, Akira S, Fujita T, Inagaki F.; ''X-ray crystal structure of IRF-3 and its functional implications.''; PubMedEurope PMCScholia
Donepudi M, Mac Sweeney A, Briand C, Grütter MG.; ''Insights into the regulatory mechanism for caspase-8 activation.''; PubMedEurope PMCScholia
Piao W, Song C, Chen H, Diaz MA, Wahl LM, Fitzgerald KA, Li L, Medvedev AE.; ''Endotoxin tolerance dysregulates MyD88- and Toll/IL-1R domain-containing adapter inducing IFN-beta-dependent pathways and increases expression of negative regulators of TLR signaling.''; PubMedEurope PMCScholia
Häcker H, Redecke V, Blagoev B, Kratchmarova I, Hsu LC, Wang GG, Kamps MP, Raz E, Wagner H, Häcker G, Mann M, Karin M.; ''Specificity in Toll-like receptor signalling through distinct effector functions of TRAF3 and TRAF6.''; PubMedEurope PMCScholia
Krappmann D, Hatada EN, Tegethoff S, Li J, Klippel A, Giese K, Baeuerle PA, Scheidereit C.; ''The I kappa B kinase (IKK) complex is tripartite and contains IKK gamma but not IKAP as a regular component.''; PubMedEurope PMCScholia
Sasai M, Tatematsu M, Oshiumi H, Funami K, Matsumoto M, Hatakeyama S, Seya T.; ''Direct binding of TRAF2 and TRAF6 to TICAM-1/TRIF adaptor participates in activation of the Toll-like receptor 3/4 pathway.''; PubMedEurope PMCScholia
Oberst A, Pop C, Tremblay AG, Blais V, Denault JB, Salvesen GS, Green DR.; ''Inducible dimerization and inducible cleavage reveal a requirement for both processes in caspase-8 activation.''; PubMedEurope PMCScholia
Yamamoto M, Sato S, Hemmi H, Hoshino K, Kaisho T, Sanjo H, Takeuchi O, Sugiyama M, Okabe M, Takeda K, Akira S.; ''Role of adaptor TRIF in the MyD88-independent toll-like receptor signaling pathway.''; PubMedEurope PMCScholia
Takaesu G, Kishida S, Hiyama A, Yamaguchi K, Shibuya H, Irie K, Ninomiya-Tsuji J, Matsumoto K.; ''TAB2, a novel adaptor protein, mediates activation of TAK1 MAPKKK by linking TAK1 to TRAF6 in the IL-1 signal transduction pathway.''; PubMedEurope PMCScholia
Tenev T, Bianchi K, Darding M, Broemer M, Langlais C, Wallberg F, Zachariou A, Lopez J, MacFarlane M, Cain K, Meier P.; ''The Ripoptosome, a signaling platform that assembles in response to genotoxic stress and loss of IAPs.''; PubMedEurope PMCScholia
Fitzgerald KA, Rowe DC, Barnes BJ, Caffrey DR, Visintin A, Latz E, Monks B, Pitha PM, Golenbock DT.; ''LPS-TLR4 signaling to IRF-3/7 and NF-kappaB involves the toll adapters TRAM and TRIF.''; PubMedEurope PMCScholia
Xia ZP, Sun L, Chen X, Pineda G, Jiang X, Adhikari A, Zeng W, Chen ZJ.; ''Direct activation of protein kinases by unanchored polyubiquitin chains.''; PubMedEurope PMCScholia
Lamothe B, Besse A, Campos AD, Webster WK, Wu H, Darnay BG.; ''Site-specific Lys-63-linked tumor necrosis factor receptor-associated factor 6 auto-ubiquitination is a critical determinant of I kappa B kinase activation.''; PubMedEurope PMCScholia
Cui J, Zhu L, Xia X, Wang HY, Legras X, Hong J, Ji J, Shen P, Zheng S, Chen ZJ, Wang RF.; ''NLRC5 negatively regulates the NF-kappaB and type I interferon signaling pathways.''; PubMedEurope PMCScholia
Kaiser WJ, Offermann MK.; ''Apoptosis induced by the toll-like receptor adaptor TRIF is dependent on its receptor interacting protein homotypic interaction motif.''; PubMedEurope PMCScholia
Oganesyan G, Saha SK, Guo B, He JQ, Shahangian A, Zarnegar B, Perry A, Cheng G.; ''Critical role of TRAF3 in the Toll-like receptor-dependent and -independent antiviral response.''; PubMedEurope PMCScholia
Creagh EM, Conroy H, Martin SJ.; ''Caspase-activation pathways in apoptosis and immunity.''; PubMedEurope PMCScholia
Nikoletopoulou V, Markaki M, Palikaras K, Tavernarakis N.; ''Crosstalk between apoptosis, necrosis and autophagy.''; PubMedEurope PMCScholia
Gatot JS, Gioia R, Chau TL, Patrascu F, Warnier M, Close P, Chapelle JP, Muraille E, Brown K, Siebenlist U, Piette J, Dejardin E, Chariot A.; ''Lipopolysaccharide-mediated interferon regulatory factor activation involves TBK1-IKKepsilon-dependent Lys(63)-linked polyubiquitination and phosphorylation of TANK/I-TRAF.''; PubMedEurope PMCScholia
He S, Liang Y, Shao F, Wang X.; ''Toll-like receptors activate programmed necrosis in macrophages through a receptor-interacting kinase-3-mediated pathway.''; PubMedEurope PMCScholia
Cory S, Adams JM.; ''The Bcl2 family: regulators of the cellular life-or-death switch.''; PubMedEurope PMCScholia
Panne D, McWhirter SM, Maniatis T, Harrison SC.; ''Interferon regulatory factor 3 is regulated by a dual phosphorylation-dependent switch.''; PubMedEurope PMCScholia
Jiang Z, Ninomiya-Tsuji J, Qian Y, Matsumoto K, Li X.; ''Interleukin-1 (IL-1) receptor-associated kinase-dependent IL-1-induced signaling complexes phosphorylate TAK1 and TAB2 at the plasma membrane and activate TAK1 in the cytosol.''; PubMedEurope PMCScholia
Ea CK, Deng L, Xia ZP, Pineda G, Chen ZJ.; ''Activation of IKK by TNFalpha requires site-specific ubiquitination of RIP1 and polyubiquitin binding by NEMO.''; PubMedEurope PMCScholia
Jiang Z, Zamanian-Daryoush M, Nie H, Silva AM, Williams BR, Li X.; ''Poly(I-C)-induced Toll-like receptor 3 (TLR3)-mediated activation of NFkappa B and MAP kinase is through an interleukin-1 receptor-associated kinase (IRAK)-independent pathway employing the signaling components TLR3-TRAF6-TAK1-TAB2-PKR .''; PubMedEurope PMCScholia
Gangloff M, Gay NJ.; ''MD-2: the Toll 'gatekeeper' in endotoxin signalling.''; PubMedEurope PMCScholia
Wu CJ, Conze DB, Li T, Srinivasula SM, Ashwell JD.; ''Sensing of Lys 63-linked polyubiquitination by NEMO is a key event in NF-kappaB activation [corrected].''; PubMedEurope PMCScholia
Núñez Miguel R, Wong J, Westoll JF, Brooks HJ, O'Neill LA, Gay NJ, Bryant CE, Monie TP.; ''A dimer of the Toll-like receptor 4 cytoplasmic domain provides a specific scaffold for the recruitment of signalling adaptor proteins.''; PubMedEurope PMCScholia
Kishimoto K, Matsumoto K, Ninomiya-Tsuji J.; ''TAK1 mitogen-activated protein kinase kinase kinase is activated by autophosphorylation within its activation loop.''; PubMedEurope PMCScholia
Tseng PH, Matsuzawa A, Zhang W, Mino T, Vignali DA, Karin M.; ''Different modes of ubiquitination of the adaptor TRAF3 selectively activate the expression of type I interferons and proinflammatory cytokines.''; PubMedEurope PMCScholia
Watt W, Koeplinger KA, Mildner AM, Heinrikson RL, Tomasselli AG, Watenpaugh KD.; ''The atomic-resolution structure of human caspase-8, a key activator of apoptosis.''; PubMedEurope PMCScholia
Bertrand MJ, Lippens S, Staes A, Gilbert B, Roelandt R, De Medts J, Gevaert K, Declercq W, Vandenabeele P.; ''cIAP1/2 are direct E3 ligases conjugating diverse types of ubiquitin chains to receptor interacting proteins kinases 1 to 4 (RIP1-4).''; PubMedEurope PMCScholia
Banerjee A, Gerondakis S.; ''Coordinating TLR-activated signaling pathways in cells of the immune system.''; PubMedEurope PMCScholia
Chinnaiyan AM, O'Rourke K, Tewari M, Dixit VM.; ''FADD, a novel death domain-containing protein, interacts with the death domain of Fas and initiates apoptosis.''; PubMedEurope PMCScholia
Mocarski ES, Kaiser WJ, Livingston-Rosanoff D, Upton JW, Daley-Bauer LP.; ''True grit: programmed necrosis in antiviral host defense, inflammation, and immunogenicity.''; PubMedEurope PMCScholia
An H, Zhao W, Hou J, Zhang Y, Xie Y, Zheng Y, Xu H, Qian C, Zhou J, Yu Y, Liu S, Feng G, Cao X.; ''SHP-2 phosphatase negatively regulates the TRIF adaptor protein-dependent type I interferon and proinflammatory cytokine production.''; PubMedEurope PMCScholia
Kanayama A, Seth RB, Sun L, Ea CK, Hong M, Shaito A, Chiu YH, Deng L, Chen ZJ.; ''TAB2 and TAB3 activate the NF-kappaB pathway through binding to polyubiquitin chains.''; PubMedEurope PMCScholia
Carty M, Goodbody R, Schröder M, Stack J, Moynagh PN, Bowie AG.; ''The human adaptor SARM negatively regulates adaptor protein TRIF-dependent Toll-like receptor signaling.''; PubMedEurope PMCScholia
Feoktistova M, Geserick P, Kellert B, Dimitrova DP, Langlais C, Hupe M, Cain K, MacFarlane M, Häcker G, Leverkus M.; ''cIAPs block Ripoptosome formation, a RIP1/caspase-8 containing intracellular cell death complex differentially regulated by cFLIP isoforms.''; PubMedEurope PMCScholia
Hemmi H, Takeuchi O, Sato S, Yamamoto M, Kaisho T, Sanjo H, Kawai T, Hoshino K, Takeda K, Akira S.; ''The roles of two IkappaB kinase-related kinases in lipopolysaccharide and double stranded RNA signaling and viral infection.''; PubMedEurope PMCScholia
Kerr JF, Wyllie AH, Currie AR.; ''Apoptosis: a basic biological phenomenon with wide-ranging implications in tissue kinetics.''; PubMedEurope PMCScholia
Vanden Berghe T, Linkermann A, Jouan-Lanhouet S, Walczak H, Vandenabeele P.; ''Regulated necrosis: the expanding network of non-apoptotic cell death pathways.''; PubMedEurope PMCScholia
The mitogen activated protein kinase (MAPK) cascade, one of the most ancient and evolutionarily conserved signaling pathways, is involved in many processes of immune responses. The MAP kinases cascade transduces signals from the cell membrane to the nucleus in response to a wide range of stimuli (Chang and Karin, 2001; Johnson et al, 2002).
There are three major groups of MAP kinases
the extracellular signal-regulated protein kinases ERK1/2,
the p38 MAP kinase
and the c-Jun NH-terminal kinases JNK.
ERK1 and ERK2 are activated in response to growth stimuli. Both JNKs and p38-MAPK are activated in response to a variety of cellular and environmental stresses. The MAP kinases are activated by dual phosphorylation of Thr and Tyr within the tripeptide motif Thr-Xaa-Tyr. The sequence of this tripeptide motif is different in each group of MAP kinases: ERK (Thr-Glu-Tyr); p38 (Thr-Gly-Tyr); and JNK (Thr-Pro-Tyr).
MAPK activation is mediated by signal transduction in the conserved three-tiered kinase cascade: MAPKKKK (MAP4K or MKKKK or MAPKKK Kinase) activates the MAPKKK. The MAPKKKs then phosphorylates a dual-specificity protein kinase MAPKK, which in turn phosphorylates the MAPK.
The dual specificity MAP kinase kinases (MAPKK or MKK) differ for each group of MAPK. The ERK MAP kinases are activated by the MKK1 and MKK2; the p38 MAP kinases are activated by MKK3, MKK4, and MKK6; and the JNK pathway is activated by MKK4 and MKK7. The ability of MAP kinase kinases (MKKs, or MEKs) to recognize their cognate MAPKs is facilitated by a short docking motif (the D-site) in the MKK N-terminus, which binds to a complementary region on the MAPK. MAPKs then recognize many of their targets using the same strategy, because many MAPK substrates also contain D-sites.
The upstream signaling events in the TLR cascade that initiate and mediate the ERK signaling pathway remain unclear.
NF-kappaB is sequestered in the cytoplasm in a complex with inhibitor of NF-kappaB (IkB). Almost all NF-kappaB activation pathways are mediated by IkB kinase (IKK), which phosphorylates IkB resulting in dissociation of NF-kappaB from the complex. This allows translocation of NF-kappaB to the nucleus where it regulates gene expression.
TRIF signaling activates TRAF3 self-mediated polyubiquitination trough Lys-63 of ubiquitin. The ubiquitinated TRAF3 in turn activates the interferon response [Tseng PH et al 2010].
TRAF3 is a ubiquitin ligase recruited to both MYD88- and TRIF-assembled signalling complexes [Hacker H et al 2006]. However, TRAF3 controls the production of interferon and proinflammatory cytokines in different ways [Tseng PH et al 2010]. Positive or negative type of regulation is dictated by TRAF3 subcellular distribution and its mode of ubiquitination. Thus, TRIF-mediated signaling initiated on endosomes triggers TRAF3 self-ubiquitination through noncanonical (K63-linked) polyubiquitination, which is essential for activation of IRF3/7 and the interferon response. In contrast, during MyD88-dependent signaling initiated from plasma membrane TRAF3 functions as a negative regulator of inflammatory cytokines and mitogen-activated protein kinases (MAPKs), unless it undergoes degradative (K48-linked) polyubiquitination mediated by TRAF6 and a pair of the ubiquitin ligases cIAP1 and cIAP2. The degradation of TRAF3 is essential for MAPK activation via TAK1 and MEKK1 [Tseng PH et al 2010].
SARM (sterile alpha-and armadillo-motif-containing protein) is a TIR-domain-containing adaptor, which functions as a negative regulator of TRIF(TICAM1)-dependent Toll-like receptor signaling in humans. LPS treatment led to a rapid increase of the SARM expression in peripheral blood mononuclear cells (PBMCs) and as a result an increased association between SARM and TRIF. SARM expression was also shown to inhibit poly(I:C)-induced TRIF-dependent NF-kB activaion, RANTES production and IRF activation in human embryonic kidney HEK293 cells [Carty M et al 2006]. Moreover, suppression of endogenous SARM expression by siRNA led to enhanced TLR3- and TLR4-dependent gene induction in both transformed HEK293 and primary PBMC cells, while endotoxin-tolerant human monocytes showed increased expression of SARM and decreased activation of TRIF-dependent cytokines [Carty M et al 2006; Piao W et al 2009]. Thus, SARM inhibits TLR4 and TLR3 signaling by targeting TRIF. The complex of TRIF:SARM is thought to inhibit downstream TRIF signaling by preventing the recruitment of TRIF effector proteins [Carty M et al 2006].
TLR3 and TLR4 -directed programmed necrosis (necroptosis) is mediated by the TRIF-RIP3 pathway in mouse macrophages [He S e al 2011]. RIP3 was shown to be essential mediator in TLR3-induced necroptotic cell death in human epithelial cell lines. Knockdown of RIP3 in human keratinocyte HaCaT cells blocked TLR3-mediated necroptosis without affecting the apoptotic response. Moreover, overexpression of RIP3 in human epithelial carcinoma cell line HeLa led to increased caspase-independent TLR3-induced cell death in the absence of IAPs [Feoktistova M et al 2011]. In addition, in caspase-8- or FADD-deficient human Jurkat cells dsRNA induced programmed necrosis, instead of apoptosis [Kalai M et al 2002]. Thus, when caspase-dependent apoptosis is inhibited or absent, the alternative RIP3-mediated programmed cell death is induced.
TLR3/4 signaling component were shown to mediate apoptosis in various human cell lines in the FADD:caspasse-8-dependent manner [Kalai M et al 2002; Kaiser WJ and Offermann MK 2005; Estornes Y et al 2012]. Caspase-8 zymogens (procaspase-8) are present in the cells as inactive monomers, containing a large N-terminal prodomain with two death effector domains (DED), and a C-terminal catalytic subunit composed of small and a large domains separated by a smaller linker region [Donepudi M et al 2003; Keller N et al 2009]. Dimerization is required for caspase-8 activation [Donepudi M et al 2003]. The dimerization event occurs at the receptor signaling complex. Once dimerized, caspase-8 zymogen undergoes a series of autoproteolytic cleavage events at aspartic acid residues in their interdomain linker regions. A second cleavage event between the the N-terminal prodomain and the catalytic domain releases the active caspase from the activation complex into the cytosol. The resulting fully active enzyme is a homodimer of catalytic domains, where each domain is compsed of a large p18 and a small p10 subunit [Keller N et al 2009; Oberst A et al 2010].
TRIF was repored to efficiently induce apoptosis when overexpressed in human HEK293T cells. TRIF-induced apoptosis occurred through activation of the FADD-caspase-8 axis [Kaiser WJ and Offermann MK 2005; Kalai M et al 2002; Estornes Y et al 2012]. C-terminus of TRIF was shown to form complexes with both RIP1 and RIP3, and disruption of these interactions by mutating the RHIM eliminated the ability of TRIF to induce apoptosis [Kaiser WJ and Offermann MK 2005].
Prevention of RIP1 ubiquitination leads to a strong association of RIP1 and caspase-8 [Feoktistova M et al 2011, Tenev et al 2011].
The mechanism of TBK1 or IKKi{epsilon} activation by TICAM1 is unclear. It was suggested that these protein kinases undergo autophosphorylation. The most recent studies showed that phosphorylation and the activation of TBK1/ IKKi are catalyzed by a distinct protein kinase (Clark et al. 2009). Other studies demonstrated an essential role of TRAF3 in the activation of TBK1 (Hacker et al 2006).
It was also shown that the ubiquitin like domain (ULD) of TBK1 and IKK-i, a regulatory component, is involved in the control of kinase activation, substrate presentation and downstream signaling.
TAK1-binding protein 2 (TAB2) and/or TAB3, as part of a complex that also contains TAK1 and TAB1, binds polyubiquitinated TRAF6. The TAB2 and TAB3 regulatory subunits of the TAK1 complex contain C-terminal Npl4 zinc finger (NZF) motifs that recognize with Lys63-pUb chains (Kanayama et al. 2004). The recognition mechanism is specific for Lys63-linked ubiquitin chains [Kulathu Y et al 2009]. TAK1 can be activated by unattached Lys63-polyubiquitinated chains when TRAF6 has no detectable polyubiquitination (Xia et al. 2009) and thus the synthesis of these chains by TRAF6 may be the signal transduction mechanism.
The TAK1 complex consists of the transforming growth factor-? (TGF-beta)-activated kinase (TAK1) and the TAK1-binding proteins TAB1, TAB2 and TAB3. TAK1 requires TAB1 for its kinase activity (Sakurai H et al 2000; Shibuya H et al 2000). TAB1 promotes autophosphorylation of the TAK1 kinase activation lobe, likely through an allosteric mechanism (Sakurai H et al 2000 ; Kishimoyo K et al 2000). The TAK1 complex is regulated by polyubiquitination. The TAK1 complex consists of the transforming growth factor-? (TGF- ?)-activated kinase (TAK1) and the TAK1-binding proteins TAB1, TAB2 and TAB3. TAK1 requires TAB1 for its kinase activity (Shibuya H et al 1996; Sakurai H et al 2000). TAB1 promotes autophosphorylation of the TAK1 kinase activation lobe, likely through an allosteric mechanism (Brown K et al 2005; Ono K et al 2001). The TAK1 complex is regulated by polyubiquitination. Binding of TAB2 and TAB3 to Lys63-linked polyubiquitin chains leads to the activation of TAK1 by an uncertain mechanism. Binding of multiple TAK1 complexes onto the same polyubiquitin chain may promote oligomerization of TAK1, facilitating TAK1 autophosphorylation and subsequent activation of its kinase activity (Kishimoto et al. 2000). The binding of TAB2/3 to polyubiquitinated TRAF6 may facilitate polyubiquitination of TAB2/3 by TRAF6 (Ishitani et al. 2003), which might result in conformational changes within the TAK1 complex that leads to the activation of TAK1. Another possibility is that TAB2/3 may recruit the IKK complex by binding to ubiquitinated NEMO; polyubiquitin chains may function as a scaffold for higher order signaling complexes that allow interaction between TAK1 and IKK (Kanayama et al. 2004).
TRAF6 possesses ubiquitin ligase activity and undergoes K-63-linked auto-ubiquitination after its oligomerization. In the first step, ubiquitin is activated by an E1 ubiquitin activating enzyme. The activated ubiquitin is transferred to a E2 conjugating enzyme (a heterodimer of proteins Ubc13 and Uev1A) forming the E2-Ub thioester. Finally, in the presence of ubiquitin-protein ligase E3 (TRAF6, a RING-domain E3), ubiquitin is attached to the target protein (TRAF6 on residue Lysine 124) through an isopeptide bond between the C-terminus of ubiquitin and the epsilon-amino group of a lysine residue in the target protein. In contrast to K-48-linked ubiquitination that leads to the proteosomal degradation of the target protein, K-63-linked polyubiquitin chains act as a scaffold to assemble protein kinase complexes and mediate their activation through proteosome-independent mechanisms. This K63 polyubiquitinated TRAF6 activates the TAK1 kinase complex.
Polyubiquitinated TRAF6 (as E3 ubiquitin ligase) generates free K63 -linked polyubiquitin chains that non-covalently associate with ubiquitin receptors of TAB2/TAB3 regulatory proteins of the TAK1 complex, leading to the activation of the TAK1 kinase.
RIP1 is recruited to the activated TLR receptor by binding to TICAM1(TRIF) via its RHIM motif, followed by its polyubiquitination. Polyubiquitination is possibly mediated by TRAF6 that is also recruited to the TICAM1 [Cusson-Hermance N et al 2005]. Other E3-ubiquitin ligases - cIAP1 and cIAP2 - have been reported to promote polyubiquitination of RIP proteins [Bertrand MJM et al 2011].
RIP3 was shown to inhibit TRIF-induced NF-kB activation in dose-dependent manner when overexpressed in HEK293T cells by competing with TRIF to bind RIP1 [Meylan E et al 2004].
Human IRF-3 is activated through a two step phosphorylation in the C-terminal domain mediated by TBK1 and/or IKK-i. It requires Ser386 and/or Ser385 (site 1) and a cluster of serine/threonine residues between Ser396 and Ser405 (site 2) [Panne et al 2007]. Phosphorylated residues at site 2 alleviate autoinhibition to allow interaction with CBP (CREB-binding protein) and facilitate phosphorylation at site 1. Phosphorylation at site 1 is required for IRF-3 dimerization.
IRF-3 and IRF-7 transcription factors possess distinct structural characteristics; IRF-7 is phosphorylated on Ser477 and Ser479 residues [Lin R et al 2000]. TRAF6 mediated ubiquitination of IRF7 is also required and essential for IRF7 phosphorylation and activation. The K-63 linked ubiquitination occurs on the last three C-terminal lysine sites (positions 444, 446, and 452) of human IRF7 independently of its C-terminal functional phosphorylation sites.[Ning et al 2008].
RIP1 polyubiquitination was induced upon TNF- or poly(I-C) treatment of the macrophage cell line RAW264.7 and the U373 astrocytoma line (Cusson-Hermance et al 2005). These workers have suggested that RIP1 may use similar mechanisms to induce NF-kB in the TNFR1- and Trif-dependent TLR pathways.
RIP1 modification with Lys-63 polyubiquitin chains was shown to be essential for TNF-induced activation of NF-kB (Ea et al. 2006). It is thought that TRAF family members mediate this Lys63-linked ubiquitination of RIP1 (Wertz et al. 2004, Tada et al 2001, Vallabhapurapu and Karin 2009), which may facilitate recruitment of the TAK1 complex and thus activation of NF-kB. Binding of NEMO to Lys63-linked polyubiquitinated RIP1 is also required in the signaling cascade from the activated receptor to the IKK-mediated NF-kB activation (Wu et al. 2006).
Two members of the interferon regulatory factor (IRF) family IRF-3 and IRF-7 are the major modulators of IFN gene expression. Activation of IRF-3 and IRF-7, which is mediated by TBK1/IKK protein kinases, promotes IFN gene expression and the production of IFN developing an effective antiviral immune response.
Irf-3 deficient mice were found to be more vulnerable to virus infection. Mouse cells defective in IRF-3 and IRF-7 expression totally fail to induce IFN genes in response to viral infection. It was shown on mice and mouse cells that both IRF-3 and IRF-7 have non redundant and distinct roles. IRF-3 is expressed at a basal level in normally growing cells and is a major factor in the early induction phase of IFN-alpha/beta production, while the IRF-7 gene expression is induced upon IFNs stimulation and IRF-7 is involved in the late induction phase.
SH2-containing protein tyrosine phosphatase 2 (SHP-2) has been shown to inhibit the TRIF-dependent production of proin?ammatory cytokines and type I interferon in LPS or poly(I-C)-stimulated mouse peritoneal macrophages. SHP-2 overexpression also inhibited TRIF-induced IFN-? luciferase reporter gene expression in human embryonic kidney HEK293 cells. Experiments with truncated SHP-2 or truncated TBK1 mutants revealed that C-terminal domain of SHP-2 associates with N-terminal domain of TBK1 when coexpressed in HEK293 cells. Furthermore, SHP-2 is thought to prevent TBK1-mediated downstream substrate phosphorylation in tyrosine phosphatase activity independent manner by binding to kinase domain of TBK1 [An H et al 2006].
TIR-domain-containing adaptor inducing interferon-beta (TRIF or TICAM1) was shown to play an essential role in TLR3 signaling. All poly(I:C)-induced pathways leading to NFkB and IRF3 activation were abolished in TRIF-/- mice [Yamamoto et al. 2003].
TRIF recruitment to TLR complex stimulates distinct pathways leading to production of type 1 interferons (IFNs), pro-inflammatory cytokines and induction of programmed cell death.
Original Pathway at Reactome: http://www.reactome.org/PathwayBrowser/#DB=gk_current&FOCUS_SPECIES_ID=48887&FOCUS_PATHWAY_ID=166166
Try the New WikiPathways
View approved pathways at the new wikipathways.org.Quality Tags
Ontology Terms
Bibliography
History
External references
DataNodes
IKKB
NEMOTRIF
activated TLR3/TLR4There are three major groups of MAP kinases
ERK1 and ERK2 are activated in response to growth stimuli. Both JNKs and p38-MAPK are activated in response to a variety of cellular and environmental stresses. The MAP kinases are activated by dual phosphorylation of Thr and Tyr within the tripeptide motif Thr-Xaa-Tyr. The sequence of this tripeptide motif is different in each group of MAP kinases: ERK (Thr-Glu-Tyr); p38 (Thr-Gly-Tyr); and JNK (Thr-Pro-Tyr).
MAPK activation is mediated by signal transduction in the conserved three-tiered kinase cascade: MAPKKKK (MAP4K or MKKKK or MAPKKK Kinase) activates the MAPKKK. The MAPKKKs then phosphorylates a dual-specificity protein kinase MAPKK, which in turn phosphorylates the MAPK.
The dual specificity MAP kinase kinases (MAPKK or MKK) differ for each group of MAPK. The ERK MAP kinases are activated by the MKK1 and MKK2; the p38 MAP kinases are activated by MKK3, MKK4, and MKK6; and the JNK pathway is activated by MKK4 and MKK7. The ability of MAP kinase kinases (MKKs, or MEKs) to recognize their cognate MAPKs is facilitated by a short docking motif (the D-site) in the MKK N-terminus, which binds to a complementary region on the MAPK. MAPKs then recognize many of their targets using the same strategy, because many MAPK substrates also contain D-sites.
The upstream signaling events in the TLR cascade that initiate and mediate the ERK signaling pathway remain unclear.
TRIF
activated TLR3/TLR4TRIF
activated TLR3/TLR4TRIF
activated TLR3/TLR4TAB2/TAB3
TAK1TRIF
activated TLR3/TLR4TLR4 MD2 LPS
CD14TRAM TLR4 MD2 LPS
CD14TRIF K63-poly-Ub-TRAF3
p-TBK1/p-IKK epsilonTRIF K63-poly-Ub-TRAF3 p-TBK1/p-IKKE
IRF3/IRF7TRIF RIP1 FADD
pro-caspase-8TRIF RIP1
FADDTRIF K63-pUb-RIP1
IKKcomplexTRIF
K63-pUb-RIP1TRIF K63-pUb-TRAF6 free K63-linked pUb
TAK1complexTRIF K63-pUb-TRAF6 free K63-linked pUb
activated TAK1 complexTRIF
K63-pUb-TRAF6TRIF
TRAF6TAB1 TAB2/TAB3 free polyubiquitin chain
phospho-TAK1TLR3
TRIFAnnotated Interactions
IKKB
NEMOTRIF
activated TLR3/TLR4Prevention of RIP1 ubiquitination leads to a strong association of RIP1 and caspase-8 [Feoktistova M et al 2011, Tenev et al 2011].
It was also shown that the ubiquitin like domain (ULD) of TBK1 and IKK-i, a regulatory component, is involved in the control of kinase activation, substrate presentation and downstream signaling.
RIP3 was shown to inhibit TRIF-induced NF-kB activation in dose-dependent manner when overexpressed in HEK293T cells by competing with TRIF to bind RIP1 [Meylan E et al 2004].
IRF-3 and IRF-7 transcription factors possess distinct structural characteristics; IRF-7 is phosphorylated on Ser477 and Ser479 residues [Lin R et al 2000]. TRAF6 mediated ubiquitination of IRF7 is also required and essential for IRF7 phosphorylation and activation. The K-63 linked ubiquitination occurs on the last three C-terminal lysine sites (positions 444, 446, and 452) of human IRF7 independently of its C-terminal functional phosphorylation sites.[Ning et al 2008].
RIP1 modification with Lys-63 polyubiquitin chains was shown to be essential for TNF-induced activation of NF-kB (Ea et al. 2006). It is thought that TRAF family members mediate this Lys63-linked ubiquitination of RIP1 (Wertz et al. 2004, Tada et al 2001, Vallabhapurapu and Karin 2009), which may facilitate recruitment of the TAK1 complex and thus activation of NF-kB. Binding of NEMO to Lys63-linked polyubiquitinated RIP1 is also required in the signaling cascade from the activated receptor to the IKK-mediated NF-kB activation (Wu et al. 2006).
Irf-3 deficient mice were found to be more vulnerable to virus infection. Mouse cells defective in IRF-3 and IRF-7 expression totally fail to induce IFN genes in response to viral infection. It was shown on mice and mouse cells that both IRF-3 and IRF-7 have non redundant and distinct roles. IRF-3 is expressed at a basal level in normally growing cells and is a major factor in the early induction phase of IFN-alpha/beta production, while the IRF-7 gene expression is induced upon IFNs stimulation and IRF-7 is involved in the late induction phase.
TRIF
activated TLR3/TLR4TRIF
activated TLR3/TLR4TRIF
activated TLR3/TLR4TRIF
activated TLR3/TLR4TRIF
activated TLR3/TLR4TAB2/TAB3
TAK1TRIF
activated TLR3/TLR4TLR4 MD2 LPS
CD14TRIF K63-poly-Ub-TRAF3
p-TBK1/p-IKK epsilonTRIF K63-poly-Ub-TRAF3
p-TBK1/p-IKK epsilonTRIF K63-poly-Ub-TRAF3
p-TBK1/p-IKK epsilonTRIF K63-poly-Ub-TRAF3 p-TBK1/p-IKKE
IRF3/IRF7TRIF RIP1 FADD
pro-caspase-8TRIF RIP1
FADDTRIF
K63-pUb-RIP1TRIF K63-pUb-TRAF6 free K63-linked pUb
TAK1complexTRIF K63-pUb-TRAF6 free K63-linked pUb
activated TAK1 complexTRIF
K63-pUb-TRAF6TRIF
TRAF6TAB1 TAB2/TAB3 free polyubiquitin chain
phospho-TAK1