Transforming growth factor-beta-activated kinase 1 (TAK1) has been linked to interleukin 1 receptor and tumor necrosis factor receptor signaling. Here we generated mouse strains with conditional expression of a Map3k7 allele encoding part of TAK1. TAK1-deficient embryonic fibroblasts demonstrated loss of responses to interleukin 1beta and tumor necrosis factor. Studies of mice with B cell-specific TAK1 deficiency showed that TAK1 was indispensable for cellular responses to Toll-like receptor ligands, CD40 and B cell receptor crosslinking. In addition, antigen-induced immune responses were considerably impaired in mice with B cell-specific TAK1 deficiency. TAK1-deficient cells failed to activate transcription factor NF-kappaB and mitogen-activated protein kinases in response to interleukin 1beta, tumor necrosis factor and Toll-like receptor ligands. However, TAK1-deficient B cells were able to activate NF-kappaB but not the kinase Jnk in response to B cell receptor stimulation. These results collectively indicate that TAK1 is key in the cellular response to a variety of stimuli.

Interleukin-1 (IL-1) is a proinflammatory cytokine that has several effects in the inflammation process. When it binds to its cell-surface receptor, IL-1 initiates a signalling cascade that leads to activation of the transcription factor NF-kappaB and is relayed through the protein TRAF6 and a succession of kinase enzymes, including NF-kappaB-inducing kinase (NIK) and I kappaB kinases (IKKs). However, the molecular mechanism by which NIK is activated is not understood. Here we show that the MAPKK kinase TAK1 acts upstream of NIK in the IL-1-activated signalling pathway and that TAK1 associates with TRAF6 during IL-1 signalling. Stimulation of TAK1 causes activation of NF-kappaB, which is blocked by dominant-negative mutants of NIK, and an inactive TAK1 mutant prevents activation of NF-kappaB that is mediated by IL-1 but not by NIK. Activated TAK1 phosphorylates NIK, which stimulates IKK-alpha activity. Our results indicate that TAK1 links TRAF6 to the NIK-IKK cascade in the IL-1 signalling pathway.

The TAK1 MAPKKK mediates activation of JNK and NF-KB in the IL-1-activated signaling pathway. Here we report the identification of TAB2, a novel intermediate in the IL-1 pathway that functionally links TAK1 to TRAF6. Expression of TAB2 induces JNK and NF-kappaB activation, whereas a dominant-negative mutant TAB2 impairs their activation by IL-1. IL-1 stimulates translocation of TAB2 from the membrane to the cytosol where it mediates the IL-1-dependent association of TAK1 with TRAF6. These results define TAB2 as an adaptor linking TAK1 and TRAF6 and as a mediator of TAK1 activation in the IL-1 signaling pathway.

The Wnt signalling pathway regulates many developmental processes through a complex of beta-catenin and the T-cell factor/lymphoid enhancer factor (TCF/LEF) family of high-mobility-group transcription factors. Wnt stabilizes cytosolic beta-catenin, which then binds to TCF and activates gene transcription. This signalling cascade is conserved in vertebrates, Drosophila and Caenorhabditis elegans. In C. elegans, the proteins MOM-4 and LIT-1 regulate Wnt signalling to polarize responding cells during embryogenesis. MOM-4 and LIT-1 are homologous to TAK1 (a kinase activated by transforming growth factor-beta) mitogen-activated protein-kinase-kinase kinase (MAP3K) and MAP kinase (MAPK)-related NEMO-like kinase (NLK), respectively, in mammalian cells. These results raise the possibility that TAK1 and NLK are also involved in Wnt signalling in mammalian cells. Here we show that TAK1 activation stimulates NLK activity and downregulates transcriptional activation mediated by beta-catenin and TCF. Injection of NLK suppresses the induction of axis duplication by microinjected beta-catenin in Xenopus embryos. NLK phosphorylates TCF/LEF factors and inhibits the interaction of the beta-catenin-TCF complex with DNA. Thus, the TAK1-NLK-MAPK-like pathway negatively regulates the Wnt signalling pathway.

Wnt signaling controls a variety of developmental processes. The canonical Wnt/β-catenin pathway functions to stabilize β-catenin, and the noncanonical Wnt/Ca2+ pathway activates Ca2+/calmodulin-dependent protein kinase II (CaMKII). In addition, the Wnt/Ca2+ pathway activated by Wnt-5a antagonizes the Wnt/β-catenin pathway via an unknown mechanism. The mitogen-activated protein kinase (MAPK) pathway composed of TAK1 MAPK kinase kinase and NLK MAPK also negatively regulates the canonical Wnt/β-catenin signaling pathway. Here we show that activation of CaMKII induces stimulation of the TAK1-NLK pathway. Overexpression of Wnt-5a in HEK293 cells activates NLK through TAK1. Furthermore, by using a chimeric receptor (β2AR-Rfz-2) containing the ligand-binding and transmembrane segments from the β2-adrenergic receptor (β2AR) and the cytoplasmic domains from rat Frizzled-2 (Rfz-2), stimulation with the β-adrenergic agonist isoproterenol activates activities of endogenous CaMKII, TAK1, and NLK and inhibits β-catenin-induced transcriptional activation. These results suggest that the TAK1-NLK MAPK cascade is activated by the noncanonical Wnt-5a/Ca2+ pathway and antagonizes canonical Wnt/β-catenin signaling.

TAK1, a member of the mitogen-activated kinase kinase kinase (MAPKKK) family, participates in proinflammatory cellular signaling pathways by activating JNK/p38 MAPKs and NF-B. To identify drugs that prevent inflammation, we screened inhibitors of TAK1 catalytic activity. We identified a natural resorcylic lactone of fungal origin, 5Z-7-oxozeaenol, as a highly potent inhibitor of TAK1. This compound did not effectively inhibit the catalytic activities of the MEKK1 or ASK1 MAPKKKs, suggesting that 5Z-7-oxozeaenol is a selective inhibitor of TAK1. In cell culture, 5Z-7-oxozeaenol blocked interleukin-1-induced activation of TAK1, JNK/ p38 MAPK, IB kinases, and NF-B, resulting in inhibition of cyclooxgenase-2 production. Furthermore, in vivo 5Z-7-oxozeaenol was able to inhibit picryl chlorideinduced ear swelling. Thus, 5Z-7-oxozeaenol blocks proinflammatory signaling by selectively inhibiting TAK1 MAPKKK.TAK1 is a member of the mitogen-activated protein kinase kinase kinase (MAPKKK) 1 family that phosphorylates and activates MKK3, MKK4, MKK6, and MKK7 MAPKKs, which in turn activate the c-Jun N-terminal kinase (JNK) and p38 MAPKs (1-3). We have recently demonstrated that TAK1 also activates IB kinases (IKKs), ultimately leading to activation of the transcription factor NF-B (4). TAK1 participates in proinflammatory cellular signaling pathways such as the interleukin-1 (IL-1) pathway by activating both JNK/p38 MAPKs and IKKs. Exposure of cells to IL-1 induces the interaction between endogenous TAK1 and TRAF6 (tumor necrosis factor (TNF) receptor-associated factor 6), a molecule essential for IL-1 activation of both JNK/p38 and NF-B. This interaction in turn leads to TAK1 activation. We have previously identified two TAK1-binding proteins, TAB1 and TAB2 (5, 6). When ectopically co-expressed, TAB1 augments the kinase activity of TAK1, indicating that TAB1 functions as an activator of TAK1 (5, 7). TAB2 functions as an adaptor linking TAK1 to TRAF6 by directly binding to both, thereby mediating TAK1 activation in the IL-1 signaling pathway (6,8).Several lines of evidence suggest that TAK1 is a key molecule in proinflammatory signaling pathways. Various proinflammatory cytokines and endotoxins activate the kinase activity of endogenous TAK1 (4, 9, 10). Overexpression of kinase-dead TAK1 inhibits IL-1-and TNF-induced activation of both JNK/p38 and NF-B (4, 10). The Drosophila homolog of TAK1 was recently identified as an essential molecule for host defense signaling in Drosophila (11). Furthermore, the TAK1 gene-silencing study using the small interfering RNA method defined that TAK1 is essential for both IL-1-and TNF-induced NF-B activation in mammalian cells (12). Therefore, it can be expected that inhibition of TAK1 activity may be effective in preventing inflammation and tissue destruction promoted by proinflammatory cytokines.In this study, we screened for compounds that can inhibit TAK1 kinase activity. This strategy resulted in the isolation of one natural compound 5Z-7-oxozeaenol, a resorcylic lactone of fungal ori...

Transforming growth factor-␤ (TGF-␤)-activated kinase 1 (TAK1), a member of the mitogen-activated protein kinase kinase kinase family, is suggested to be involved in TGF-␤-induced gene expression, but the signaling mechanism from TAK1 to the nucleus remains largely undefined. We have found that p38 mitogen-activated protein kinase, and its direct activator MKK6 are rapidly activated in response to TGF-␤. Expression of dominant negative MKK6 or dominant negative TAK1 inhibited the TGF-␤-induced transcriptional activation as well as the p38 activation. Constitutive activation of the p38 pathway in the absence of TGF-␤ induced the transcriptional activation, which was enhanced synergistically by coexpression of Smad2 and Smad4 and was inhibited by expression of the C-terminal truncated, dominant negative Smad4. Furthermore, we have found that activating transcription factor-2 (ATF-2), which is known as a nuclear target of p38, becomes phosphorylated in the N-terminal activation domain in response to TGF-␤, that ATF-2 forms a complex with Smad4, and that the complex formation is enhanced by TGF-␤. In addition, expression of a nonphosphorylatable form of ATF-2 inhibited the TGF-␤-induced transcriptional activation. These results show that the p38 pathway is activated by TGF-␤ and is involved in the TGF-␤-induced transcriptional activation by regulating the Smad-mediated pathway.

Signals elicited by transforming growth factor-β (TGF-β) superfamily ligands are generated following the formation of heteromeric receptor complexes consisting of type I and type II receptors. TAK1, a member of the MAP kinase kinase kinase family, and its activator, TAB1, participate in the bone morphogenetic protein (BMP) signaling pathway involved in mesoderm induction and patterning in early Xenopus embryos. However, the events leading from receptor activation to TAK1 activation remain to be identified.