The mitogen-activated protein kinase (MAPK) pathway is a conserved eukaryotic signaling module that converts receptor signals into various outputs. MAPK is activated through phosphorylation by MAPK kinase (MAPKK), which is first activated by MAPKK kinase (MAPKKK). A genetic selection based on a MAPK pathway in yeast was used to identify a mouse protein kinase (TAK1) distinct from other members of the MAPKKK family. TAK1 participates in regulation of transcription by transforming growth factor-beta (TGF-beta). Furthermore, kinase activity of TAK1 is stimulated in response to TGF-beta and bone morphogenetic protein. These results suggest that TAK1 functions as a mediator in the signaling pathway of TGF-beta superfamily members (Yamaguchi, 1995).
TAK1 has been linked to interleukin 1 receptor and tumor necrosis factor receptor signaling. This study generated mouse strains with conditional expression of a Map3k7 allele encoding part of TAK1. TAK1-deficient embryonic fibroblasts demonstrate loss of responses to interleukin 1beta and tumor necrosis factor. Studies of mice with B cell-specific TAK1 deficiency showed that TAK1 is indispensable for cellular responses to Toll-like receptor ligands, CD40 and B cell receptor crosslinking. In addition, antigen-induced immune responses are considerably impaired in mice with B cell-specific TAK1 deficiency. TAK1-deficient cells fail 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 are 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 (Sato, 2005).
TAK1, a member of the MAPKKK family, is thought to be a key modulator of the inducible transcription factors NF-kappaB and AP-1 and, therefore, plays a crucial role in regulating the genes that mediate inflammation. Although in vitro biochemical studies have revealed the existence of a TAK1 complex, which includes TAK1 and the adapter proteins TAB1 and TAB2, it remains unclear which members of this complex are essential for signaling. To analyze the function of TAK1 in vivo, the Tak1 gene was deleted in mice, with the resulting phenotype being early embryonic lethality. Using embryonic fibroblasts lacking TAK1, TAB1, or TAB2, it was found that TNFR1, IL-1R, TLR3, and TLR4-mediated NF-kappaB and AP-1 activation are severely impaired in Tak1m/m cells, but they are normal in Tab1-/- and Tab2-/- cells. In addition, Tak1m/m cells are highly sensitive to TNF-induced apoptosis. TAK1 mediates IKK activation in TNF-alpha and IL-1 signaling pathways, where it functions downstream of RIP1-TRAF2 and MyD88-IRAK1-TRAF6, respectively. However, TAK1 is not required for NF-kappaB activation through the alternative pathway following LT-beta signaling. In the TGF-β signaling pathway, TAK1 deletion leads to impaired NF-kappaB and c-Jun N-terminal kinase (JNK) activation without impacting Smad2 activation or TGF-beta-induced gene expression. Therefore, these studies suggests that TAK1 acts as an upstream activating kinase for IKKbeta and JNK, but not IKKalpha, revealing an unexpectedly specific role of TAK1 in inflammatory signaling pathways (Shim, 2005).
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 signaling 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. The MAPKK kinase TAK1 acts upstream of NIK in the IL-1-activated signaling pathway and TAK1 associates with TRAF6 during IL-1 signaling. 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. These results indicate that TAK1 links TRAF6 to the NIK-IKK cascade in the IL-1 signaling pathway (Ninomiya-Tsuji, 1999).
Stimulation of monocytes/macrophages with lipopolysaccharide (LPS) results in activation of nuclear factor-kappaB (NF-kappaB), which plays crucial roles in regulating expression of many genes involved in the subsequent inflammatory responses. Roles of transforming growth factor-beta activated kinase 1 (TGF-TAK1), a mitogen-activated protein kinase kinase kinase (MAPKKK), in the LPS-induced signaling cascade, have been investigated. A kinase-negative mutant of TAK1 inhibits LPS-induced NF-kappaB activation both in a macrophage-like cell line, RAW 264.7, and in human embryonic kidney 293 cells that express toll-like receptor 2 or 4. Furthermore, endogenous TAK1 is phosphorylated upon simulation of RAW 264.7 cells with LPS. These results indicate that TAK1 functions as a critical mediator in the LPS-induced signaling pathway (Irie, 2000).
Nontypeable Hemophilus influenzae (NTHi) is an important human pathogen in both children and adults. In children, it causes otitis media, the most common childhood infection and the leading cause of conductive hearing loss in the United States. In adults, it causes lower respiratory tract infections in the setting of chronic obstructive pulmonary disease, the fourth leading cause of death in the United States. The molecular mechanisms underlying the pathogenesis of NTHi-induced infections remain undefined, but they may involve activation of NF-kappa B, a transcriptional activator of multiple host defense genes involved in immune and inflammatory responses. NTHi strongly activates NF-kappa B in human epithelial cells via two distinct signaling pathways, NF-kappa B translocation-dependent and -independent pathways. The NF-kappa B translocation-dependent pathway involves activation of NF-kappa B inducing kinase (NIK)--IKK alpha/beta complex leading to I kappa B alpha phosphorylation and degradation, whereas the NF-kappa B translocation-independent pathway involves activation of MKK3/6--p38 mitogen-activated protein (MAP) kinase pathway. Bifurcation of NTHi-induced NIK-IKK alpha/beta-I kappa B alpha and MKK3/6--p38 MAP kinase pathways may occur at transforming growth factor-beta activated kinase 1 (TAK1). Toll-like receptor 2 (TLR2) is required for NTHi-induced NF-kappa B activation. In addition, several key inflammatory mediators including IL-1 beta, IL-8, and tumor necrosis factor-alpha are up-regulated by NTHi. Finally, P6, a 16-kDa lipoprotein highly conserved in the outer membrane of all NTHi and H. influenzae type b strains, appears to also activate NF-kappa B via similar signaling pathways. Taken together, these results demonstrate that NTHi activates NF-kappa B via TLR2-TAK1-dependent NIK--IKK alpha/beta-I kappa B alpha and MKK3/6--p38 MAP kinase signaling pathways. These studies may bring new insights into molecular pathogenesis of NTHi-induced infections and open up new therapeutic targets for these diseases (Shuto, 2001).
Mechanisms of fulminant gene induction during an inflammatory response were investigated using expression of the chemoattractant cytokine interleukin-8 (IL-8) as a model. Coordinate activation of NF-kappaB and c-Jun N-terminal protein kinase (JNK) is required for strong IL-8 transcription, whereas the p38 MAP kinase (MAPK) pathway stabilizes the IL-8 mRNA. It is unclear how these pathways are coupled to the receptor for IL-1, an important physiological inducer of IL-8. Expression of the MAP kinase kinase kinase (MAPKKK) TAK1 together with its coactivator TAB1 in HeLa cells activates all three pathways and is sufficient to induce IL-8 formation, NF-kappaB and JNK2-mediated transcription from a minimal IL-8 promoter, and p38 MAPK-mediated stabilization of a reporter mRNA containing IL-8-derived regulatory mRNA sequences. Expression of a kinase-inactive mutant of TAK1 largely blocks IL-1-induced transcription and mRNA stabilization, as well as formation of endogenous IL-8. Truncated TAB1, lacking the TAK1 binding domain, or a TAK1-derived peptide containing a TAK1 autoinhibitory domain are also efficient in inhibition. These data indicate that the previously described three-pathway model of IL-8 induction is operative in response to a physiological stimulus, IL-1, and that the MAPKKK TAK1 couples the IL-1 receptor to both transcriptional and RNA-targeted mechanisms mediated by the three pathways (Holtmann, 2001).
Ceramide has been proposed as a second messenger molecule implicated in a variety of biological processes. It has recently been reported that ceramide activates stress-activated protein kinase (SAPK, also known as c-Jun NH2-terminal kinase JNK), a subfamily member of mitogen-activated protein kinase superfamily molecules and that the ceramide/SAPK/JNK signaling pathway is required for stress-induced apoptosis. However, the molecular mechanism by which ceramide induces SAPK/JNK activation is unknown. TAK1, a member of the mitogen-activated protein kinase kinase kinase family, is activated by treatment of cells with agents and stresses that induce an increase in ceramide. Ceramide itself stimulates the kinase activity of TAK1. Expression of a constitutively active form of TAK1 results in activation of SAPK/JNK and SEK1/MKK4, a direct activator of SAPK/JNK. Furthermore, expression of a kinase-negative form of TAK1 interfers with the activation of SAPK/JNK induced by ceramide. These results indicate that TAK1 may function as a mediator of ceramide signaling to SAPK/JNK activation (Shirakabe, 1997).
Transforming growth factor-beta (TGF-beta) superfamily members elicit signals through stimulation of serine/threonine kinase receptors. Recent studies of this signaling pathway have identified two types of novel mediating molecules, the Smads and TGF-beta activated kinase 1 (TAK1). Smads were shown to mimic the effects of bone morphogenetic protein (BMP), activin and TGF-beta. TAK1 and TAB1 were identified as a MAPKKK and its activator, respectively, which might be involved in the up-regulation of TGF-beta superfamily-induced gene expression, but their biological role is poorly understood. The role of TAK1 and TAB1 in the dorsoventral patterning of early Xenopus embryos has been examined. Ectopic expression of Xenopus TAK1 (xTAK1) in early embryos induces cell death. Interestingly, however, concomitant overexpression of bcl-2 with the activated form of xTAK1 or both xTAK1 and xTAB1 in dorsal blastomeres not only rescued the cells but also caused the ventralization of the embryos. In addition, a kinase-negative form of xTAK1 (xTAK1KN), known to inhibit endogenous signaling, can partially rescue phenotypes generated by the expression of a constitutively active BMP-2/4 type IA receptor (BMPR-IA). Moreover, xTAK1KN can block the expression of ventral mesoderm marker genes induced by Smad1 or 5. These results thus suggest that xTAK1 and xTAB1 function in a cooperative manner in the BMP signal transduction pathway in Xenopus embryos (Shibuya, 1998).
Bone morphogenetic proteins (BMPs) have been shown to induce ectopic expression of cardiac transcription factors and beating cardiomyocytes in nonprecardiac mesodermal cells in chicks, suggesting that BMPs are inductive signaling molecules that participate in the development of the heart. However, the precise molecular mechanisms by which BMPs regulate cardiac development are largely unknown. In the present study, the molecular mechanisms by which BMPs induce cardiac differentiation was examined by using the P19CL6 in vitro cardiomyocyte differentiation system, a clonal derivative of P19 embryonic teratocarcinoma cells. A permanent P19CL6 cell line, P19CL6noggin, was established that constitutively overexpresses the BMP antagonist noggin. Although almost all parental P19CL6 cells differentiate into beating cardiomyocytes when treated with 1% dimethyl sulfoxide, P19CL6noggin cells do not differentiate into beating cardiomyocytes nor do they express cardiac transcription factors or contractile protein genes. The failure of differentiation is rescued by overexpression of BMP-2 or addition of BMP protein to the culture media, indicating that BMPs were indispensable for cardiomyocyte differentiation in this system. Overexpression of TAK1, a member of the mitogen-activated protein kinase kinase kinase superfamily that transduces BMP signaling, restores the ability of P19CL6noggin cells to differentiate into cardiomyocytes and concomitantly express cardiac genes, whereas overexpression of the dominant negative form of TAK1 in parental P19CL6 cells inhibits cardiomyocyte differentiation. Overexpression of both cardiac transcription factors Csx/Nkx-2.5 and GATA-4 but not of Csx/Nkx-2.5 or GATA-4 alone also induces differentiation of P19CL6noggin cells into cardiomyocytes. These results suggest that TAK1, Csx/Nkx-2.5, and GATA-4 play a pivotal role in the cardiogenic BMP signaling pathway (Monzen, 2000).
Bone morphogenetic proteins (BMPs) induce cardiomyocyte differentiation through the mitogen-activated protein kinase kinase kinase TAK1. Smads are transcription factors that mediate transforming growth factor-beta signaling and the ATF/CREB family transcription factor ATF-2 acts as a common target of the Smad and the TAK1 pathways. The role of Smads and ATF-2 in cardiomyocyte differentiation of P19CL6, a clonal derivative of murine P19 cells, has been examined. Although P19CL6 efficiently differentiates into cardiomyocytes when treated with dimethyl sulfoxide, P19CL6noggin, a P19CL6 cell line constitutively overexpressing the BMP antagonist noggin, does not differentiate into cardiomyocytes. Co-overexpression of Smad1, a ligand-specific Smad, and Smad4, a common Smad, restores the ability of P19CL6noggin to differentiate into cardiomyocytes, whereas stable overexpression of Smad6, an inhibitory Smad, completely blocks differentiation of P19CL6, suggesting that the Smad pathway is necessary for cardiomyocyte differentiation. ATF-2 stimulated the betaMHC promoter activity by the synergistic manner with Smad1/4 and TAK1 and promotes terminal cardiomyocyte differentiation of P19CL6noggin, whereas overexpression of the dominant negative form of ATF-2 reduces the promoter activities of several cardiac-specific genes and inhibited differentiation of P19CL6. These results suggest that Smads, TAK1, and their common target ATF-2, cooperatively play a critical role in cardiomyocyte differentiation (Monzen, 2001).
The molecular mechanism by which BMP2 induces apoptosis has not been fully elucidated. A BMP2 signaling pathway is proposed that mediates apoptosis in mouse hybridoma MH60 cells whose growth is interleukin-6 (IL-6)-dependent. BMP2 dose-dependently induces apoptosis in MH60 cells even in the presence of IL-6. BMP2 has no inhibitory effect on the IL-6-induced tyrosine phosphorylation of STAT3, and the bcl-2 gene expression which is known to be regulated by STAT3, suggesting that BMP2-induced apoptosis is not attributed to alteration of the IL-6-mediated bcl-2 pathway. BMP2 induces activation of TGF-beta-activated kinase (TAK1) and subsequent phosphorylation of p38 stress-activated protein kinase. In addition, forced expression of kinase-negative TAK1 in MH60 cells blocks BMP2-induced apoptosis. These results indicate that BMP2-induced apoptosis is mediated through the TAK1-p38 pathway in MH60 cells. MH60-derived transfectants expressing Smad6 are resistant to the apoptotic signal of BMP2. Interestingly, this ectopic expression of Smad6 blocks BMP2-induced TAK1 activation and p38 phosphorylation. Moreover, Smad6 can directly bind to TAK1. These findings suggest that Smad6 is likely to function as a negative regulator of the TAK1 pathway in the BMP2 signaling, in addition to the previously reported Smad pathway (Kimura, 2000).
Transforming growth factor-β regulates a variety of physiologic processes through essential intracellular mediators Smads. The SnoN oncoprotein is an inhibitor of TGF-β signaling. SnoN recruits transcriptional repressor complex to block Smad-dependent transcriptional activation of TGF-β-responsive genes. Following TGF-β stimulation, SnoN is rapidly degraded, thereby allowing the activation of TGF-β target genes. This study reports the role of TAK1 as a SnoN protein kinase. TAK1 interacts with and phosphorylates SnoN, and this phosphorylation regulates the stability of SnoN. Inactivation of TAK1 prevents TGF-β-induced SnoN degradation, and impairs induction of the TGF-β-responsive genes. These data suggest that TAK1 modulates TGF-β dependent cellular responses by targeting SnoN for degradation (Kajino, 2007).
The Wnt signaling 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 signaling cascade is conserved in vertebrates, Drosophila and Caenorhabditis elegans. In C. elegans, the proteins MOM-4 and LIT-1 regulate Wnt signaling to polarize responding cells during embryogenesis. In mammalian cells, 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: see Drosophila Nemo), respectively. These results raise the possibility that TAK1 and NLK are also involved in Wnt signaling in mammalian cells. 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 signaling pathway (Ishitani, 1999).
The signaling protein Wnt regulates transcription factors containing high-mobility-group (HMG) domains to direct decisions on cell fate during animal development. In Caenorhabditis elegans, the HMG-domain-containing repressor POP-1 distinguishes the fates of anterior daughter cells from their posterior sisters throughout development, and Wnt signaling downregulates POP-1 activity in one posterior daughter cell called E. The genes mom-4 (coding for a Tak1 homolog) and lit-1 are also required to downregulate POP-1, not only in E but also in other posterior daughter cells. Consistent with action in a common pathway, mom-4 and lit-1 exhibit similar mutant phenotypes and encode components of the mitogen-activated protein kinase (MAPK) pathway that are homologous to vertebrate transforming-growth-factor-beta-activated kinase (TAK1) and NEMO-like kinase (NLK), respectively. Furthermore, MOM-4 and TAK1 bind related proteins that promote their kinase activities. It is concluded that a MAPK-related pathway cooperates with Wnt signal transduction to downregulate POP-1 activity. These functions are likely to be conserved in vertebrates, since TAK1 and NLK can downregulate HMG-domain-containing proteins related to POP-1 (Meneghini, 1999).
During C. elegans embryonic development, many of the cell divisions in all regions of the embryo are oriented along the anterior/posterior (AP) axis: essentially all of these divisions result in AP daughter cells with different fates. Genetic studies of this polarity signaling process have defined a mechanism that involves several proteins with similarities to known Wnt/WG signaling components. For example, MOM-2 is related to the secreted protein Wnt/WG, and MOM-5 is related to the membrane protein Frizzled, a candidate Wnt/WG receptor. As in other Wnt/WG systems, these upstream factors appear to act through a beta-catenin-related protein, WRM-1. A key difference between polarity signaling in C. elegans and Wnt/WG signaling in vertebrates and Drosophila concerns the relationship of WRM-1 and a protein related to vertebrate TCF (T cell factor)/LEF (lymphoid enhancer factor) transcription factors, POP-1. In vertebrates and Drosophila, the WRM-1-related proteins, beta-catenin and Armadillo, are thought to enter the nucleus in response to signaling, where they bind to and activate TCF/LEF-related factors. In C. elegans, although WRM-1 is an effector of signaling, WRM-1 appears to have the opposite downstream activity, downregulating rather than activating POP-1. In studies on wild-type embryos, POP-1 exhibits a lower level of nuclear immunofluorescence staining in the posterior daughters of many AP divisions than in the anterior daughters. Genetic studies have shown that WRM-1 and other signaling components are required for this POP-1 asymmetry between AP sister cells. Thus, in C. elegans, Wnt/WG signaling through WRM-1 leads to downregulation of POP-1 (Shin, 1999).
Possible insights into POP-1 regulation by WRM-1 have come from analysis of the gene lit-1. Mutations in lit-1 result in a loss of AP cell fate asymmetries. Phenotypic and genetic analysis of lit-1 place this gene in both the MOM-2 and LIN-44 Wnt/WG signaling systems in C. elegans. In the embryo, LIT-1 appears to function along with WRM-1 in a process that reduces POP-1 levels or activity in posterior daughters of AP divisions. The LIT-1 protein is related to serine/threonine protein kinases and is most similar to the Drosophila tissue polarity protein, Nemo, and to the mouse protein Nlk. WRM-1 and LIT-1 appear to form a stable protein complex in vivo in C. elegans and in transfected vertebrate cells. In vertebrate cells, WRM-1 activates the LIT-1 protein kinase leading to phosphorylation of WRM-1, LIT-1, and POP-1. These observations support a model in which signaling activates the WRM-1/LIT-1 kinase complex. This complex then directly phosphorylates POP-1, leading to its downregulation in posterior daughters of AP divisions (Shin, 1999).
How upstream signaling events lead to activation of the WRM-1/LIT-1 kinase is not understood. LIT-1/Nemo/Nlk kinases make up a small subfamily of protein serine/threonine kinases distinct from, but closely related to, MAP kinases (MAPK). MAPK signaling pathways, which involve sequential activation of protein kinases called MAPK kinase kinases and MAPK kinases, are highly conserved from yeast to metazoans and regulate many developmental decisions in C. elegans, Drosophila, and vertebrates. In the present study, the cloning of the AP polarity gene, mom-4 is reported. The mom-4 locus was previously identified by a set of maternal mutations that cause defects in polarity signaling in the early embryo. This study shows that mom-4 activity is required for POP-1 asymmetries between anterior and posterior daughters of AP divisions. mom-4 encodes a C. elegans homolog of mammalian TAK1 (TGF-beta activated kinase), which is thought to function as a MAPK kinase kinase. When expressed in cultured mammalian cells, both MOM-4 and TAK1 are able to stimulate WRM-1/LIT-1 kinase activity leading to the increased phosphorylation of POP-1. This activation is dependent on the putative kinase activation loop of LIT-1 that serves as a target for activating phosphorylation in related kinases. The structural similarities of LIT-1 to MAPK and of MOM-4 to MAPK kinase kinase raise the possibility that a MAPK-like kinase cascade contributes to AP polarity signaling in C. elegans. Thus mom-4, a genetically defined effector of polarity signaling, encodes a MAP kinase kinase kinase-related protein that stimulates the WRM-1/LIT-1-dependent phosphorylation of POP-1. LIT-1 kinase activity requires a conserved residue analogous to an activating phosphorylation site in other kinases, including MAP kinases. These findings suggest that anterior/posterior polarity signaling in C. elegans may involve a MAP kinase-like signaling mechanism (Shin, 1999).
Genetic studies suggest that the signaling pathway is branched upstream of WRM-1 and LIT-1 and may have polarity inputs from sources other than the MOM-2/Wnt-related protein. For example, a large percentage of embryos from mutant strains carrying apparent null alleles of both mom-2(Wnt) and mom-5(Frizzled) nevertheless exhibit proper specification of posterior cell fates, strongly suggesting that alternative polarity signals must be able to activate the WRM-1/LIT-1 kinase. In the present study, mutations in mom-4 are shown to strongly synergize with mutations in mom-2 and mom-5, raising the possibility that mom-4(+) activity is required for Wnt/WG independent polarity signaling. Within its kinase domain, LIT-1 is approximately 45% identical (132 out of 292 residues) to human p38 MAP kinase and 43% identical (126 out of 292 residues) to human ERK1, respectively. Furthermore, LIT-1 activation appears to require a conserved motif analogous to a site required for activating phosphorylation by MAPK kinases. Thus, MOM-4 is similar to MAPK kinase kinase and LIT-1, which is similar to MAPK in amino acid sequence, is also similar to MAPK in its activation. These observations together with the genetic synergy between mom-4 mutants and Wnt/WG pathway mutants suggest that a MAP kinase-like cascade may function in parallel with Wnt/WG signaling to specify AP cell fate differences during C. elegans development. In the future, understanding how MOM-4 is activated and how MOM-4 in turn activates LIT-1 is likely to shed light on how Wnt/WG signals interact with other signaling pathways to control cell polarity and cell fate (Shin, 1999).
Wnt signaling controls a variety of developmental processes. The canonical Wnt/beta-catenin pathway functions to stabilize beta-catenin, and the noncanonical Wnt/Ca(2+) pathway activates Ca(2+)/calmodulin-dependent protein kinase II (CaMKII). In addition, the Wnt/Ca(2+) pathway activated by Wnt-5a antagonizes the Wnt/beta-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/beta-catenin signaling pathway. 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 [beta(2)AR-Rfz-2] containing the ligand-binding and transmembrane segments from the beta(2)-adrenergic receptor [beta(2)AR] and the cytoplasmic domains from rat Frizzled-2 (Rfz-2), stimulation with the beta-adrenergic agonist isoproterenol activates activities of endogenous CaMKII, TAK1, and NLK and inhibits beta-catenin-induced transcriptional activation. These results suggest that the TAK1-NLK MAPK cascade is activated by the noncanonical Wnt-5a/Ca(2+) pathway and antagonizes canonical Wnt/beta-catenin signaling (Ishitani, 2003a; full text of article).
The Wnt/beta-catenin signaling pathway regulates many developmental processes by modulating gene expression. Wnt signaling induces the stabilization of cytosolic beta-catenin, which then associates with lymphoid enhancer factor and T-cell factor (LEF-1/TCF) to form a transcription complex that activates Wnt target genes. A specific mitogen-activated protein (MAP) kinase pathway involving the MAP kinase kinase kinase TAK1 and MAP kinase-related Nemo-like kinase (NLK) suppresses Wnt signaling. This study investigated the relationships among NLK, beta-catenin, and LEF-1/TCF. It was found that NLK interacts directly with LEF-1/TCF and indirectly with beta-catenin via LEF-1/TCF to form a complex. NLK phosphorylates LEF-1/TCF on two serine/threonine residues located in its central region. Mutation of both residues to alanine enhanced LEF-1 transcriptional activity and rendered it resistant to inhibition by NLK. Phosphorylation of TCF-4 by NLK inhibited DNA binding by the beta-catenin-TCF-4 complex. However, this inhibition was abrogated when a mutant form of TCF-4 was used in which both threonines were replaced with valines. These results suggest that NLK phosphorylation on these sites contributes to the down-regulation of LEF-1/TCF transcriptional activity (Ishitani, 2003b; full text of article).
The c-myb proto-oncogene product (c-Myb) regulates both the proliferation and apoptosis of hematopoietic cells by inducing the transcription of a group of target genes. However, the biologically relevant molecular mechanisms that regulate c-Myb activity remain unclear. c-Myb protein is phosphorylated and degraded by Wnt-1 signal via the pathway involving TAK1 (TGF-beta-activated kinase), HIPK (homeodomain-interacting protein kinase), and NLK (Nemo-like kinase). Wnt-1 signal causes the nuclear entry of TAK1, which then activates HIPK and the mitogen-activated protein (MAP) kinase-like kinase NLK. NLK binds directly to c-Myb together with HIPK, which results in the phosphorylation of c-Myb at multiple sites, followed by its ubiquitination and proteasome-dependent degradation. Furthermore, overexpression of NLK in M1 cells abrogates the ability of c-Myb to maintain the undifferentiated state of these cells. The down-regulation of Myb by Wnt-1 signal may play an important role in a variety of developmental steps (Kanei-Oshii, 2004).
Genetic studies on endoderm-mesoderm specification in C. elegans have demonstrated a role for several Wnt cascade components as well as for a MAPK-like pathway in this process. The latter pathway includes the MAPK kinase kinase-like MOM-4/Tak1, its adaptor TAP-1/Tab1, and the MAPK-like LIT-1/Nemo-like kinase. A model has been proposed in which the Tak1 kinase cascade counteracts the Wnt cascade at the level of beta-catenin/TCF phosphorylation. In this model, the signal that activates the Tak1 kinase cascade is unknown. As an alternative explanation of these genetic data, whether Tak1 is directly activated by Wnt was explored. It was found that Wnt1 stimulation results in autophosphorylation and activation of MOM-4/Tak1 in a TAP-1/Tab1-dependent fashion. Wnt1-induced Tak1 stimulation activates Nemo-like kinase, resulting in the phosphorylation of TCF. These results combined with the genetic data from C. elegans imply a mechanism whereby Wnt directly activates the MOM-4/Tak1 kinase signaling pathway. Thus, Wnt signal transduction through the canonical pathway activates beta-catenin/TCF, whereas Wnt signal transduction through the Tak1 pathway phosphorylates and inhibits TCF, which might function as a feedback mechanism (Smit, 2004; full text of article).
Transforming growth factor-beta (TGF-beta) regulates many aspects of cellular function. A member of the mitogen-activated protein kinase kinase kinase (MAPKKK) family, TAK1, is a mediator in the signaling pathway of TGF-beta superfamily members. The yeast two-hybrid system has now revealed two human proteins, termed TAB1 and TAB2 (for TAK1 binding protein), that interact with TAK1. TAB1 and TAK1 are co-immunoprecipitated from mammalian cells. Overproduction of TAB1 enhances activity of the plasminogen activator inhibitor 1 gene promoter, which is regulated by TGF-beta, and increases the kinase activity of TAK1. TAB1 may function as an activator of the TAK1 MAPKKK in TGF-beta signal transduction (Shibuya, 1996).
The TAK1 MAPKKK mediates activation of JNK and NF-KB in the IL-1-activated signaling pathway. TAB2 is 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 (Takaesu, 2000).
Endogenous TAK1 is constitutively associated with TAB1 and phosphorylated following IL-1 stimulation. Furthermore, TAK1 is constitutively phosphorylated when ectopically overexpressed with TAB1. In both cases, dephosphorylation of TAK1 renders it inactive, but it can be reactivated by preincubation with ATP. A mutant of TAK1 that lacks kinase activity is not phosphorylated either following IL-1 treatment or when coexpressed with TAB1, indicating that TAK1 phosphorylation is due to autophosphorylation. Furthermore, mutation to alanine of a conserved serine residue (Ser-192) in the activation loop between kinase domains VII and VIII abolishes both phosphorylation and activation of TAK1. These results suggest that IL-1 and ectopic expression of TAB1 both activate TAK1 via autophosphorylation of Ser-192 (Kishimoto, 2000).
Apoptosis signal-regulating kinase 1 (ASK1) is a member of the MAPKKK family in the JNK and p38 mitogen-activated protein kinase cascades and critically involved in stress- and cytokine-induced apoptosis. The transcription factor nuclear factor-kappaB (NF-kappaB) is a pivotal regulator of immune and inflammatory responses and exerts anti-apoptotic roles in various cells. ASK1 directly interacts with transforming growth factor-beta-activated kinase 1 (TAK1), another MAPKKK that has been identified as a signaling intermediate in the interleukin 1 (IL-1)-induced NF-kappaB pathway as well as the transforming growth factor-beta superfamily-induced JNK/p38 pathway. Overexpression of ASK1 inhibits IL-1-, TRAF6-, or TAK1-induced, but not NF-kappaB-inducing kinase-induced, NF-kappaB activation. ASK1 dissociates TAK1 but not NF-kappaB-inducing kinase from TRAF6. Moreover, IL-1-induced complex formation of endogenous TAK1 and TRAF6 is blocked by ASK1 overexpression. It thus appears that the inhibition of NF-kappaB by ASK1 may result at least in part from the disruption of the TRAF6.TAK1 complex formation in the IL-1 signaling pathway. These results provide a new insight in the mode of action of MAPKKK family members; two distinct MAPKKKs in the same MAP kinase cascades directly interact and exert opposite effects in another signaling pathway, NF-kappaB (Mochida, 2000).
TAK1 is a mitogen-activated protein kinase kinase kinase (MAP3K) that is involved in the c-Jun N-terminal kinase/p38 MAPKs and NF-kappaB signaling pathways. The molecular mechanisms of TAK1 activation by its specific activator TAB1 have been investigated. Autophosphorylation of two threonine residues in the activation loop of TAK1 is necessary for TAK1 activation. Association with TAK1 and induction of TAK1 autophosphorylation requires the C-terminal 24 amino acids of TAB1, but full TAK1 activation required additional C-terminal Ser/Thr rich sequences. These results demonstrate that the association between the kinase domain of TAK1 and the C-terminal TAB1 triggers the phosphorylation-dependent TAK1 activation mechanism (Sakurai, 2000).
Exposure of endothelial and many other cell types to tumor necrosis factor alpha generates both apoptotic and anti-apoptotic signals. The anti-apoptotic pathway leads to activation of the transcription factor NF-kappaB that regulates the expression of genes such as A20 or members of the IAP gene family that protect cells from tumor necrosis factor alpha-mediated apoptosis. In turn, some anti-apoptotic genes have been shown to modulate NF-kappaB activity. XIAP, a NF-kappaB-dependent member of the IAP gene family, is a strong stimulator of NF-kappaB. Expression of XIAP leads to increased nuclear translocation of the p65 subunit of NF-kappaB via a novel signaling pathway that involves the mitogen-activated protein kinase kinase kinase TAK1. TAK1 physically interacts with NIK and with IKK2, and both XIAP or active TAK1 can stimulate IKK2 kinase activity. Thus, XIAP may be part of a system of regulatory loops that balance a cell's response to environmental stimuli (Hofer-Warbinek, 2000).
TAK1, a member of the MAPKKK family, is involved in the intracellular signaling pathways mediated by transforming growth factor beta, interleukin 1, and Wnt. TAK1 kinase activity is specifically activated by the TAK1-binding protein TAB1. The C-terminal 68-amino acid sequence of TAB1 (TAB1-C68) is sufficient for TAK1 interaction and activation. Analysis of various truncated versions of TAB1-C68 defined a C-terminal 30-amino acid sequence (TAB1-C30) necessary for TAK1 binding and activation. NMR studies reveal that the TAB1-C30 region has a unique alpha-helical structure. A conserved sequence motif, PYVDXA/TXF, has been identified in the C-terminal domain of mammalian TAB1, Xenopus TAB1, and its Caenorhabditis elegans homolog TAP-1, suggesting that this motif constitutes a specific TAK1 docking site. Alanine substitution mutagenesis has shown that TAB1 Phe-484, located in the conserved motif, is crucial for TAK1 binding and activation. The C. elegans homolog of TAB1, TAP-1, is able to interact with and activate the C. elegans homolog of TAK1, MOM-4. However, the site in TAP-1 corresponding to Phe-484 of TAB1 is an alanine residue (Ala-364), and changing this residue to Phe abrogates the ability of TAP-1 to interact with and activate MOM-4. These results suggest that the Phe or Ala residue within the conserved motif of the TAB1-related proteins is important for interaction with and activation of specific TAK1 MAPKKK family members in vivo (Ono, 2001).
Hgs/Hrs is a tyrosine-phosphorylated FYVE finger protein that is induced by stimulation with various cytokines and growth factors. Hgs plays critical roles in the signaling pathway for the interleukin-2-induced activation of the serum-response element and cyclic AMP-response element of the c-fos promoter. Hgs associates physically with transforming growth factor-beta-activated kinase 1 (TAK1) and p21-activated kinase 1 (Pak1), which mediates the activation of c-Jun N-terminal kinase and serum response factor, respectively, leading to transactivation via the serum-response element and cyclic AMP-response element. These results suggest that Hgs is involved in the TAK1-JNK and Pak1-serums response factor pathways for the c-fos induction that is initiated by cytokines (Sasaki, 2001).
Interleukin-1 (IL-1) is a proinflammatory cytokine that recognizes a surface receptor complex and generates multiple cellular responses. IL-1 stimulation activates the mitogen-activated protein kinase kinase kinase TAK1, which in turn mediates activation of c-Jun N-terminal kinase and NF-kappaB. TAB2 interacts with both TAK1 and TRAF6 and promotes their association, thereby triggering subsequent IL-1 signaling events. The serine/threonine kinase IL-1 receptor-associated kinase (IRAK) also plays a role in IL-1 signaling, being recruited to the IL-1 receptor complex early in the signal cascade. The role of IRAK in the activation of TAK1 has been investiged. Genetic analysis reveals that IRAK is required for IL-1-induced activation of TAK1. IL-1 stimulation induces the rapid but transient association of IRAK, TRAF6, TAB2, and TAK1. TAB2 is recruited to this complex following translocation from the membrane to the cytosol upon IL-1 stimulation. In IRAK-deficient cells, TAB2 translocation and its association with TRAF6 are abolished. These results suggest that IRAK regulates the redistribution of TAB2 upon IL-1 stimulation and facilitates the formation of a TRAF6-TAB2-TAK1 complex. Formation of this complex is an essential step in the activation of TAK1 in the IL-1 signaling pathway (Takaesu, 2001).
Protein phosphatase 2C (PP2C) is implicated in the negative regulation of stress-activated protein kinase cascades in yeast and mammalian cells. This study examines the role of PP2Cbeta-1, a major isoform of mammalian PP2C, in the TAK1 signaling pathway, a stress-activated protein kinase cascade that is activated by interleukin-1, transforming growth factor-beta, or stress. Ectopic expression of PP2Cbeta-1 inhibits the TAK1-mediated mitogen-activated protein kinase kinase 4-c-Jun amino-terminal kinase and mitogen-activated protein kinase kinase 6-p38 signaling pathways. In vitro, PP2Cbeta-1 dephosphorylates and inactivates TAK1. Coimmunoprecipitation experiments indicate that PP2Cbeta-1 associates with the central region of TAK1. A phosphatase-negative mutant of PP2Cbeta-1, PP2Cbeta-1 (R/G), acts as a dominant negative mutant, inhibiting dephosphorylation of TAK1 by wild-type PP2Cbeta-1 in vitro. In addition, ectopic expression of PP2Cbeta-1(R/G) enhances interleukin-1-induced activation of an AP-1 reporter gene. Collectively, these results indicate that PP2Cbeta negatively regulates the TAK1 signaling pathway by direct dephosphorylation of TAK1 (Hanada, 2001).
Defining the molecular mechanisms that integrate diverse signaling pathways at the level of gene transcription remains a central issue in biology. Interleukin-1ß (IL-1ß) causes nuclear export of a specific N-CoR corepressor complex, resulting in derepression of a specific subset of NF-kappaB-regulated genes, exemplified by the tetraspanin KAI1 that regulates membrane receptor function. Nuclear export of the N-CoR/TAB2/HDAC3 complex by IL-1ß is temporally linked to selective recruitment of a Tip60 coactivator complex. Surprisingly, KAI1 is also directly activated by a ternary complex, dependent on the acetyltransferase activity of Tip60, consisting of the presenilin-dependent C-terminal cleavage product of the amyloid ß precursor protein (APP: Drosophila homolog: ß amyloid protein precursor-like), Fe65, and Tip60, identifying a specific in vivo gene target of an APP-dependent transcription complex in the brain (Baek, 2002).
This work defines a molecular mechanism that links inflammation to derepression of a specific subset of NF-kappaB-regulated genes via control of a previously unknown stable N-CoR complex. This N-CoR/TAB2/HDAC3-containing complex binds to p50-regulated target genes and undergoes a nuclear to cytoplasmic translocation in response to IL-1ß signaling. TAB2 (Drosophila homolog: TGF-ß activated kinase 1) itself enhances N-CoR-dependent repression, but the apparently critical function of TAB2 is to regulate IL-1ß-mediated translocation of the N-CoR complex out of the nucleus. TAB2 thus seems to have dual roles upon activation of the NF-kappaB pathway, serving to both derepress p50-dependent transcription units (nuclear function) as well as to activate the p50/p65 targets (cytoplasmic function) (Baek, 2002).
Evidence is provided indicating that the molecular basis of IL-1ß-dependent nuclear export of the nuclear N-CoR likely represents a MEKK1-dependent phosphorylation of TAB2 in the nucleus, putatively causing an allosteric alteration that exposes the TAB2 nuclear export signal. Thus, MEKK1 might also serve to integrate signal transduction pathways, both in the nucleus and in the cytoplasm (Baek, 2002).
These data indicate that KAI1/CD82 is an IL-1ß-induced NF-kappaB target gene based on binding of p50 homodimer. Under unstimulated conditions, p50, but not p65, was detected on the KAI1 promoter, while after IL-1ß stimulation, the level of promoter-associated p50 remained constant, without any binding by p65. Bcl3 occupies the KAI1 promoter in the presence or absence of IL-1ß, and several independent studies have suggested that Bcl3 can act as a bridging factor linking NF-kappaB to nuclear coregulators. Tip60 has been suggested to be a binding partner of Bcl3, enhancing Bcl3/p50-activated transcription through a NF-kappaB binding site (Baek, 2002).
Thus, the recruitment of Tip60 to KAI1/CD82 promoter after IL-1ß treatment, possibly requiring Bcl3, appears to be of functional importance to activation of the gene, which is accompanied by acetylation of histones H3/H4. Tip60 has been identified as a component of a multimeric protein complex containing histone acetylase, ATPase, DNA helicase activity, and structural DNA binding activity, which links it to DNA repair function. In the case of KAI1/CD82 promoter, Tip60 appears to be recruited as a component of a TRRAP-containing complex, likely distinct from the purified repair complex, although the precise complement of corecruited factors remains to be defined. The Tip60 HAT function appears to be required, directly or indirectly, for effective gene activation, because Tip60 HATmut abolishes histone H3 and H4 acetylation and recruitment of Pol II. The observed acetylation of histones H3/H4 during IL-1ß-stimulated KAI1 transcription could reflect direct acetylation by Tip60 in a promoter-specific fashion, or it could reflect recruitment of an as yet unidentified histone acetyltransferase. In contrast to KAI1, examination of two other NF-kappaB-regulated genes that recruit p50/p65 heterodimers revealed no recruitment of the N-CoR/TAB2/HDAC3 complex. The identification of a large number of transcriptional coactivators and corepressors, capable of interacting with distinct DNA bound transcription factors, has raised questions regarding their potential specificity and complementarity in gene regulation events. The NF-kappaB-regulated genes examined appear to recruit distinct coactivator machinery during gene activation events in response to IL-1ß, which is in contrast to the apparently more uniform, ligand-dependent recruitment of many coactivator complexes in estrogen receptor-regulated genes (Baek, 2002).
gamma-Secretase cleavage of APP releases not only Aß from the membrane but also the intracellular fragment AICD, which was identified only very recently because of its instability. A number of investigators have speculated that by analogy to signaling by NICD derived from Notch 1 receptor, the corresponding AICD may also function in signal transduction. In this manuscript, it has been shown that transgenic mice overexpressing APP, which develop age-related amyloid deposits and associated pathologic changes, unexpectedly exhibit increased expression of both Fe65 and Tip60 in the CNS. All three components of the AICD/Fe65/Tip60 complex are unexpectedly induced, forming a complex binding to the KAI1/CD82 promoter. This complex is capable of displacing the N-CoR/TAB2/HDAC3 complex in the absence of an IL-1ß signal and causing target gene activation. KAI1 itself provides a potentially intriguing transcriptional target of APP overexpression. As many cell surface receptors and cell adhesion molecules that regulate cytoskeletal functions are impacted by the tetraspanin KAI1/CD82, it is tempting to speculate that expression of KAI1 might contribute to later pathological events (Baek, 2002).
Remarkably, the acetyltransferase function of Tip60 is required to form the ternary complex that can displace an N-CoR/TAB2/HDAC3 corepressor complex. In addition to autoacetylation of Tip60, increased levels of acetylated Fe65 are found in the trimeric complex, suggesting that acetylation of Fe65 might be a regulatory component of ternary complex formation. These data provide a striking example of acetylation as a critical regulatory aspect of coactivator complex assembly, required for specific gene activation events. Since the transcriptional activation of KAI1 by AICD/Fe65/Tip60 is abolished by both the NSAIDs ibuprofen and naproxen with restoration of the binding of the NCoR/TAB2/HDAC3 complex on the promoter, it is postulated that NSAIDs may act at some step(s) distal to generation of AICD by an as yet unknown mechanism (Baek, 2002).
Together, these data are consistent with a model in which IL-1ß acts physiologically to cause dismissal of a specific N-CoR corepressor complex and recruitment of a Tip60-containing coactivator complex resulting in activation of p50 target genes. The AICD/Fe65/Tip60 trimeric complex can similarly displace the N-CoR complex, derepressing gene targets such as KAI1/CD82, providing a potential transcriptional activation strategy that may underlie specific aspects of APP function, both in normal physiology and in Alzheimer's disease (Baek, 2002).
The earliest decision in vertebrate neural development is the acquisition of a neural identity by embryonic ectodermal cells. The default model for neural induction postulates that neural fate specification in the vertebrate embryo occurs by inhibition of epidermal inducing signals in the gastrula ectoderm. Bone morphogenetic proteins (BMPs) act as epidermal inducers, and all identified direct neural inducers block BMP signaling either intra- or extra-cellularly. Although the mechanism of action of the secreted neural inducers has been elucidated, the relevance of intracellular BMP inhibitors in neural induction is not clear. In order to address this issue and to identify downstream targets after BMP inhibition, the transcriptional changes have been monitored in ectodermal explants neuralized by Smad7 using a Xenopus laevis 5000-clone gastrula-stage cDNA microarray. 142 genes have been identified whose transcriptional profiles change in the neuralized explants. In order to address the potential involvement during neural induction of genes identified in the array, gain-of-function studies were performed in ectodermal explants. This approach lead to the identification of four genes that can function as neural inducers in Xenopus and three others that can synergize with known neural inducers in promoting neural fates. Based on these studies, a role is proposed for post-transcriptional control of gene expression during neural induction in vertebrates and a model is presented whereby sustained BMP inhibition is promoted partly through the regulation of TGFß activated kinase (TAK1) activity by a novel TAK1-binding protein (TAB3) (Muñoz-Sanjuán, 2002).
BMP inhibitors have been characterized mostly in the context of the organizer, which acts to impart dorsal/anterior fates in the surrounding germ layers, and neural fates in the overlaying ectoderm. However, the potential involvement of neural progenitors in promoting neuralization of the ectoderm has not been addressed. The phenomenon of homeogenetic induction (neural tissue induces neuralization in non-neural ectoderm in recombination experiments) can be explained if neural tissue itself produces inhibitors of BMP signaling. In the ectoderm, BMPs activate two biochemical pathways, one mediated by Smads and a second mediated by the p38/MAP kinase pathway downstream of TAK1. Preliminary analysis suggests that Smad7-mediated neural induction operates through an inhibition of both branches of BMP signaling. A secreted factor has been identified with weak homology to the cerberus and gremlin-families of BMP inhibitors, expressed in gastrulastage embryos, with an ability to inhibit BMP and TGFß ligands. This, together with the broad expression of Smad7 itself in the prospective neural plate suggests that prolonged BMP inhibition is a requirement for neural development (Muñoz-Sanjuán, 2002).
A novel TAK1-binding protein (TAB3) is upregulated after Smad7 expression and can induce neural marker expression in isolated explants. TAK1/TAB1 complexes have been shown to promote epidermal fates downstream of BMPs in Xenopus ectoderm and inhibition of TAK1 induces neural gene expression in animal caps. There is presently little known about how neuralizing molecules modulate TAK1 activity during neural induction, or whether any other TABs associated with TAK1 in the emerging neural plate. This remains an important point, since associated TABs might switch the specificity of TAK1/TAB1 complexes during neural induction, hence effectively blocking BMP inputs. For example, TAB2 has been identified as a key effector in the activation of the NFkB and JNK pathways. In this report, TAB3 has been shown to promote neural fates and a C-terminal dominant-negative form of TAB3 can inhibit neural induction downstream of BMP inhibition. Because a C-terminal dominant negative TAB2 has been shown to bind TAK1, it is likely that TAB3C also acts by binding TAK1. xTAB3C lacks the ubiquitin ligase-binding domain, and therefore it is likely that signaling through TAB3 is mediated by ubiquitination of TAK1. Therefore, it is proposed that the activation of TAB3 by neural inducers might be a mechanism for inhibiting epidermal fates mediated by TAK1/TAB1. This latter point is important because pathways other than BMP inhibition have been implicated in neural induction, namely the FGF and Wnt pathways. Whether BMP inhibition is sufficient for neuralization or whether it acts in synergy or in parallel with FGFs and Wnts remains a highly debated issue. Of particular importance is the modulation of p38 MAPK pathways by TAK1. Therefore, the regulation of TAK1 probably plays a role in signaling crosstalk between BMPs and FGFs, and might reconcile some findings in Xenopus and chick embryos about the involvement of both BMP inhibition and FGF signaling during neural induction (Muñoz-Sanjuán, 2002).
Whether TAB3 links TAK1 activity to NFkappaB or JNK instead of p38 pathways remains to be addressed. However, the activation of the JNK pathway downstream of Smad7 has been demonstrated in epithelial cells and NFkappaB homologs have been implicated in early dorsoventral patterning in Xenopus. Intriguingly, several genes associated with NFkB and interleukin-related pathways have also been identified in the array, such as IkB-epsilon, cyclophilin-binding protein, interleukin enhancer binding factor 2 and interferon-related regulator. Whether these genes will be implicated in neural fate acquisition downstream or in parallel with BMP inhibition, and whether JNK or NFkB pathways are regulated by TAK1/TAB3 complexes remains to be explored. However, the regulation of TAK1 activity by Smad7 and possibly other BMP inhibitors suggests that TAK1 might be at the center of signaling crosstalk between BMP inputs and other pathways that may play additional roles during early neural fate acquisition (Muñoz-Sanjuán, 2002).
Cytokine treatment stimulates the IkappaB kinases, IKKalpha and IKKbeta, which phosphorylate the IkappaB proteins, leading to their degradation and activation of NF-kappaB regulated genes. A clear definition of the specific roles of IKKalpha and IKKbeta in activating the NF-kappaB pathway and the upstream kinases that regulate IKK activity remain to be elucidated. This study has utilized small interfering RNAs (siRNAs) directed against IKKalpha, IKKbeta and the upstream regulatory kinase TAK1 in order to better define their roles in cytokine-induced activation of the NF-kappaB pathway. In contrast to previous results with mouse embryo fibroblasts lacking either IKKalpha or IKKbeta, which indicated that only IKKbeta is involved in cytokine-induced NF-kappaB activation, it was found that both IKKalpha and IKKbeta are important in activating the NF-kappaB pathway. Furthermore, it was found that the MAP3K TAK1, which has been implicated in IL-1-induced activation of the NF-kappaB pathway, is also critical for TNFalpha-induced activation of the NF-kappaB pathway. TNFalpha activation of the NF-kappaB pathway is associated with the inducible binding of TAK1 to TRAF2 and both IKKalpha and IKKbeta. This analysis further defines the distinct in vivo roles of IKKalpha, IKKbeta and TAK1 in cytokine-induced activation of the NF-kappaB pathway (Takaesu, 2003).
A cDNA encoding a novel member of the mitogen-activated protein kinase kinase (MAPKK) family, MAPKK6, was isolated and found to encode a protein of 334 amino acids, with a calculated molecular mass of 37 kDa that is 79% identical to MKK3. MAPKK6 was shown to phosphorylate and specifically activate the p38/MPK2 subgroup of the mitogen-activated protein kinase superfamily and is phosphorylated and activated in vitro by TAK1, a recently identified MAPKK kinase. MKK3 was also shown to be a good substrate for TAK1 in vitro. Furthermore, when co-expressed with TAK1 in cells in culture, both MAPKK6 and MKK3 were strongly activated. In addition, co-expression of TAK1 and p38/MPK2 in cells resulted in activation of p38/MPK2. These results indicate the existence of a novel kinase cascade consisting of TAK1, MAPKK6/MKK3, and p38/MPK2 (Moriguchi, 1996).
Transforming growth factor beta (TGF-beta)-activated kinase (TAK1) is known for its involvement in TGF-beta signaling and its ability to activate the p38-mitogen-activated protein kinase (MAPK) pathway. This report shows that TAK1 is also a strong activator of c-Jun N-terminal kinase (JNK). Both the wild-type and a constitutively active mutant of TAK1 stimulate JNK in transient transfection assays. Mitogen-activated protein kinase kinase 4 (MKK4)/stress-activated protein kinase/extracellular signal-regulated kinase (SEK1), a dual-specificity kinase that phosphorylates and activates JNK, synergizes with TAK1 in activating JNK. Conversely, a dominant-negative (MKK4/SEK1) mutant inhibits TAK1-induced JNK activation. A kinase defective mutant of TAK1 effectively suppresses hematopoietic progenitor kinase-1 (HPK1)-induced JNK activity but has little effect on germinal center kinase activation of JNK. There are two additional MAPK kinase kinases -- MEKK1 and mixed lineage kinase 3 (MLK3) -- that are also downstream of HPK1 and upstream of MKK4/SEK mutant. However, because the dominant-negative mutants of MEKK1 and MLK3 does not inhibit TAK1-induced JNK activity, it has been concluded that activation of JNK1 by TAK1 is independent of MEKK1 and MLK3. In addition to TAK1, TGF-beta also stimulates JNK activity. Taken together, these results identify TAK1 as a regulator in the HPK1 --> TAK1 --> MKK4/SEK1 --> JNK kinase cascade and indicate the involvement of JNK in the TGF-beta signaling pathway. These results also suggest the potential roles of TAK1 not only in the TGF-beta pathway but also in the other HPK1/JNK1-mediated pathways (Wang, 1997).
Several mitogen-activated protein kinase kinase kinases (MAPKKKs), including NF-kappa B-inducing kinase (NIK), play critical roles in NF-kappa B activation. cDNA for human TGF-beta activated kinase 1 (TAK1), a member of the MAPKKK family, was isolated, and its ability to stimulate NF-kappa B activation was evaluated. Overexpression of TAK1 together with its activator protein, TAK1 binding protein 1 (TAB1), induced the nuclear translocation of NF-kappa B p50/p65 heterodimer accompanied by the degradation of I kappa B alpha and I kappa B beta, and the expression of kappa B-dependent reporter gene. A dominant negative mutant of NIK did not inhibit TAK1-induced NF-kappa B activation. These results suggest that TAK1 induces NF-kappa B activation through a novel NIK-independent signaling pathway (Sakurai, 1998).
Several mitogen-activated protein kinase kinase kinases play critical roles in nuclear factor-kappaB (NF-kappaB) activation. Overexpression of transforming growth factor-beta-activated kinase 1 (TAK1), a member of the mitogen-activated protein kinase kinase kinase family, together with its activator TAK1-binding protein 1 (TAB1) stimulates NF-kappaB activation. The molecular mechanism of TAK1-induced NF-kappaB activation has been investigated. Dominant negative mutants of IkappaB kinase (IKK) alpha and IKKbeta inhibit TAK1-induced NF-kappaB activation. TAK1 activates IKKalpha and IKKbeta in the presence of TAB1. IKKalpha and IKKbeta are coimmunoprecipitated with TAK1 in the absence of TAB1. TAB1-induced TAK1 activation promotes the dissociation of active forms of IKKalpha and IKKbeta from active TAK1, whereas the IKK mutants remain to interact with active TAK1. Furthermore, tumor necrosis factor-alpha activated endogenous TAK1, and the kinase-negative TAK1 act as a dominant negative inhibitor against tumor necrosis factor-alpha-induced NF-kappaB activation. These results demonstrate a novel signaling pathway to NF-kappaB activation through TAK1 in which TAK1 may act as a regulatory kinase of IKKs (Sakurai, 1999).
TRAF6 is a signal transducer that activates IkappaB kinase (IKK) and Jun amino-terminal kinase (JNK) in response to pro-inflammatory mediators such as interleukin-1 (IL-1) and lipopolysaccharides (LPS). IKK activation by TRAF6 requires two intermediary factors, TRAF6-regulated IKK activator 1 (TRIKA1) and TRIKA2. TRIKA1 is a dimeric ubiquitin-conjugating enzyme complex composed of Ubc13 and Uev1A (or the functionally equivalent Mms2). This Ubc complex, together with TRAF6, catalyses the formation of a Lys 63 (K63)-linked polyubiquitin chain that mediates IKK activation through a unique proteasome-independent mechanism. This study reports the purification and identification of TRIKA2, which is composed of TAK1, TAB1 and TAB2, a protein kinase complex previously implicated in IKK activation through an unknown mechanism. The TAK1 kinase complex phosphorylates and activates IKK in a manner that depends on TRAF6 and Ubc13-Uev1A. Moreover, the activity of TAK1 to phosphorylate MKK6, which activates the JNK-p38 kinase pathway, is directly regulated by K63-linked polyubiquitination. Evidence is provided that TRAF6 is conjugated by the K63 polyubiquitin chains. These results indicate that ubiquitination has an important regulatory role in stress response pathways, including those of IKK and JNK (Wang, 2001).
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