TAB1: an activator of the TAK1 MAPKKK in TGF-beta signal transduction

Inhibition of p38 MAP kinase as a therapeutic strategy

Since the discovery of p38 MAP kinase in 1994, our understanding of its biology has progressed dramatically. The key advances include (1) identification of p38 MAP kinase homologs and protein kinases that act upstream and downstream from p38 MAP kinase, (2) identification of interesting and potentially important substrates, (3) elucidation of the role of p38 MAP kinase in cellular processes and (4) the establishment of the mechanism by which the pyridinylimidazole p38 MAP kinase inhibitors inhibit enzyme activity. It is now known that there are four members of the p38 MAP kinase family. They differ in their tissue distribution, regulation of kinase activation and subsequent phosphorylation of downstream substrates. They also differ in terms of their sensitivities toward the p38 MAP kinase inhibitors. The best-studied isoform is p38 alpha, whose activation has been observed in many hematopoietic and non-hematopoietic cell types upon treatment with appropriate stimuli. The pyridinylimidazole compounds, exemplified by SB 203580, were originally prepared as inflammatory cytokine synthesis inhibitors that subsequently were found to be selective inhibitors of p38 MAP kinase. SB 203580 inhibits the catalytic activity of p38 MAP kinase by competitive binding in the ATP pocket. X-ray crystallographic studies of the target enzyme complexed with inhibitor reinforce the observations made from site-directed mutagenesis studies, thereby providing a molecular basis for understanding the kinase selectivity of these inhibitors. The p38 MAP kinase inhibitors are efficacious in several disease models, including inflammation, arthritis and other joint diseases, septic shock, and myocardial injury. In all cases, p38 activation in key cell types correlated with disease initiation and progression. Treatment with p38 MAP kinase inhibitors attenuated both p38 activation and disease severity. Structurally diverse p38 MAP kinase inhibitors have been tested extensively in preclinical studies.

TAK1 participates in c-Jun N-terminal kinase signaling during Drosophila development

Transforming growth factor beta (TGF-beta)-activated kinase 1 (TAK1) is a member of the MAPKKK superfamily and has been characterized as a component of the TGF-beta/bone morphogenetic protein signaling pathway. TAK1 function has been extensively studied in cultured cells, but its in vivo function is not fully understood. In this study, we isolated a Drosophila homolog of TAK1 (dTAK1) which contains an extensively conserved NH(2)-terminal kinase domain and a partially conserved COOH-terminal domain. To learn about possible endogenous roles of TAK1 during animal development, we generated transgenic flies which express dTAK1 or the mouse TAK1 (mTAK1) gene in the fly visual system. Ectopic activation of TAK1 signaling leads to a small eye phenotype, and genetic analysis reveals that this phenotype is a result of ectopically induced apoptosis. Genetic and biochemical analyses also indicate that the c-Jun amino-terminal kinase (JNK) signaling pathway is specifically activated by TAK1 signaling. Expression of a dominant negative form of dTAK during embryonic development resulted in various embryonic cuticle defects including dorsal open phenotypes. Our results strongly suggest that in Drosophila melanogaster, TAK1 functions as a MAPKKK in the JNK signaling pathway and participates in such diverse roles as control of cell shape and regulation of apoptosis.

TAB2, a novel adaptor protein, mediates activation of TAK1 MAPKKK by linking TAK1 to TRAF6 in the IL-1 signal transduction 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.

Cross-regulation of the Wnt signalling pathway: a role of MAP kinases

The Wnt signal transduction pathway regulates various aspects of embryonal development and is involved in cancer formation. Wnts induce the stabilisation of cytosolic (beta)-catenin, which then associates with TCF transcription factors to regulate expression of Wnt-target genes. At various levels the Wnt pathway is subject to cross-regulation by other components. Recent evidence suggests that a specific MAP kinase pathway involving the MAP kinase kinase kinase TAK1 and the MAP kinase NLK counteract Wnt signalling. In particular, homologues of TAK1 and NLK, MOM-4 and LIT-1, negatively regulate Wnt-controlled cell fate decision in the early Caenorhabditis elegans embryo. Moreover, TAK1 activates NLK, which phosphorylates TCFs bound to (beta)-catenin. This blocks nuclear localization and DNA binding of TCFs. Since TAK1 is activated by TGF-(beta) and various cytokines, it might provide an entry point for regulation of the Wnt system by other pathways. In addition, alterations in TAK1-NLK might play a role in cancer.

TAK1 mitogen-activated protein kinase kinase kinase is activated by autophosphorylation within its activation loop

TAK1, a member of the mitogen-activated kinase kinase kinase family, is activated in vivo by various cytokines, including interleukin-1 (IL-1), or when ectopically expressed together with the TAK1-binding protein TAB1. However, this molecular mechanism of activation is not yet understood. We show here that 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.

Molecular cloning of MINK, a novel member of mammalian GCK family kinases, which is up-regulated during postnatal mouse cerebral development

A new germinal center kinase (GCK) family kinase, Misshapen/NIKs-related kinase (MINK), has been cloned and its expression has been characterized in several tissues and various developmental stages of the mouse brain. MINK encodes a 1300 amino acid polypeptide, consisting of an N-terminal kinase domain, a proline-rich intermediate region, and a C-terminal GCK homology region. The expression of MINK is up-regulated during the postnatal development of the mouse brain. MINK activates the cJun N-terminal kinase and the p38 pathways.

TGF-beta1 activates MAP kinase in human mesangial cells: a possible role in collagen expression

BACKGROUND

Although the pathogenic relevance of transforming growth factor-beta (TGF-beta) to glomerular sclerosis has been established, the intracellular mechanisms by which TGF-beta induces extracellular matrix accumulation are not fully understood. We examined whether the mitogen-activated protein (MAP) kinase pathway is involved in TGF-beta1-induced collagen expression by cultured human mesangial cells.

METHODS

The activation of MAP kinase pathways by TGF-beta1 was assessed by immunoblot with anti-phospho-ERK or -JNK antibodies and by transfection of plasmids expressing pathway-specific transcription activators fused to the DNA-binding domain of GAL4, as well as a GAL4 response element-luciferase reporter gene. The role of MAP kinase was assessed using biochemical inhibitors and transiently expressed dominant negative mutant constructs. The effects on TGF-beta1-induced alpha1(I) collagen expression were evaluated by Northern blot and by activation of a transiently transfected alpha1(I) promoter-luciferase reporter construct.

RESULTS

ERK and JNK phosphorylation occurred 30 minutes and one hour, respectively, after TGF-beta1 treatment. A biochemical blockade of the ERK pathway inhibited TGF-beta1-induced alpha1(I) collagen expression. A dominant negative mutant of ERK1 but not of JNK decreased alpha1(I) gene promoter activation. Activation of the TGF-beta-responsive p3TP-Lux construct was partially inhibited by cotransfection of an ERK1 dominant negative mutant.

CONCLUSION

These data indicate that MAP kinase pathways can be activated by TGF-beta1 in mesangial cells and that the ERK MAP kinase plays a role in TGF-beta-stimulated collagen I expression. Because we have shown previously that SMADs mediate TGF-beta1-stimulated collagen I expression, our findings raise the possibility of interactions between the MAP kinase and the SMAD pathways.

Functional complementation of the Schizosaccharomyces pombe wis1 mutant by Arabidopsis MEK1 and non-catalytic enhancement by CTR1

Arabidopsis thaliana MEK1 encodes a MAPKK homolog whose role in plants is currently unknown. High (but not low) expression of MEK1 rescued the Deltawis1 (MAPKK) mutant of the Schizosaccharomyces pombe Win1/Wis4-Wis1-Sty1 stress-activated MAPK pathway. Rescue was dependent upon upstream and downstream components of the pathway, suggesting that MEK1 might function in a homologous MAPK pathway in plants. When MEK1 was expressed at a low level, rescue of Deltawis1 was achieved by co-expressing Arabidopsis CTR1 (a putative MAPKK kinase (MAPKKK)). CTR1 constructs alone did not rescue the pathway, indicating that CTR1 augmented MEK1 function. Further data indicated that this enhancement was not due to CTR1 kinase activity.

TGF-beta-activating kinase-1 inhibits cell cycle and expression of cyclin D1 and A in LLC-PK1 cells

BACKGROUND

Transforming growth factor-beta (TGF-beta) is known to play an important role in the pathophysiology of renal tubular disease. Researchers have recently identified a novel mitogen-activated protein kinase kinase kinase (MAPKKK), TAK (TGF-beta activated kinase)1, which stimulates the MKK3/6-p38K pathway. The purpose of our study was to investigate the functional role of the TAK1-MKK3/6-p38K pathway and classical MAPK cascades in the progression of the cell cycle in renal tubular cells.

METHODS

The constitutive active form and negative form of TAK1 (TAK1dN and TAK1K63W, respectively), and active and negative forms of the p42/44 MAPK-activator, MKK1 (S222E and S222A, respectively) were transfected to LLC-PK1 cells. Western blot analyses and promoter-luciferase assay of cyclins D1, D2, D3, E, and A were performed, and cell cycle progression was analyzed by FACS scan.

RESULTS

TAK1dN stimulated MKK6 and p38K activity and inhibited the percentage of the S and G2/M phases. TAK1K63 W inhibited TGF-beta-stimulated MKK6 and p38K activity. Cyclin D1 and cyclin A protein levels and promoter activities were negatively regulated by TAK1dN. In contrast, overexpression of the active form of p42/44 MAPK-activator, MKK1, increased cyclin D1 and A promoter activity and protein levels.

CONCLUSION

The growth-inhibitory effects of TGF-beta are at least partially mediated by the TAK1-MKK6-p38K pathway. Cyclin D1 and A promoter activity and cell cycle progression in renal tubular cells are negatively regulated by the TAK1-MKK6-p38K pathway and positively regulated by the MKK1-p42/44MAPK pathway.

Steps in integrin beta1-chain glycosylation mediated by TGFbeta1 signaling through Ras

Ras is activated by transforming growth factor beta (TGFbeta) in several cell types, but the biological consequences of this activation are largely unknown. We now show that ras mediates two stages in integrin beta1-chain maturation: 1) glycosylation of the 86-kD core peptide, which is a TGFbeta1-independent process, and 2) TGFbeta1-mediated conversion of the 115-kD beta1 integrin precursor into the mature 130-kD form. HD3 colon epithelial cells maintain elevated levels of integrin alpha2beta1 heterodimers, strong binding to collagen I, and autocrine regulation by TGFbeta1, which converts beta1 integrin into the mature cell surface form. Each of three HD3 cell clones that stably express dominant negative ras (N17ras) exhibited abnormal glycosylation of the integrin beta1-chain, decreased cell surface expression of the mature integrin beta1, and impaired binding to collagen and laminin. Autocrine levels of TGFbeta were not altered by expression of N17ras. The aberrant glycosylation of the integrin beta1-chain was reversed by antisense oligonucleotides specific to the DNA sequence encoding the rasS17N mutation. Glycosylation of the 86-kD core peptide was delayed in the N17ras transfectants, but was not altered by either the addition of TGFbeta1 or inhibition of autocrine TGFbeta1. In contrast, conversion of the partially glycosylated beta1 integrin precursor into the mature 130-kD isoform was accelerated by exogenous TGFbeta1 and blocked by neutralizing antibody to autocrine TGFbeta1 in control cell lines. Neither effect was seen in the N17ras transfectants, indicating that TGFbeta1 modulates integrin beta1-chain maturation by activating ras proteins. Cell fractionation studies demonstrated that this conversion takes place within the Golgi.

Involvement of the p38 mitogen-activated protein kinase pathway in transforming growth factor-beta-induced gene expression

Transforming growth factor-beta (TGF-beta)-activated kinase 1 (TAK1), a member of the mitogen-activated protein kinase kinase kinase family, is suggested to be involved in TGF-beta-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-beta. Expression of dominant negative MKK6 or dominant negative TAK1 inhibited the TGF-beta-induced transcriptional activation as well as the p38 activation. Constitutive activation of the p38 pathway in the absence of TGF-beta 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-beta, that ATF-2 forms a complex with Smad4, and that the complex formation is enhanced by TGF-beta. In addition, expression of a nonphosphorylatable form of ATF-2 inhibited the TGF-beta-induced transcriptional activation. These results show that the p38 pathway is activated by TGF-beta and is involved in the TGF-beta-induced transcriptional activation by regulating the Smad-mediated pathway.

Cloning, expression profile, and genomic organization of the mouse STAP/A170 gene

The preferential screening of cDNA libraries derived from the mouse osteoblastic cell line MC3T3-E1 has yielded a cDNA clone encoding a 442-amino-acid protein designated STAP (signal transduction and adaptor protein), which contains several motifs shared among transcription factors and adaptors such as a Zn-finger like motif, a proline-rich domain, and a PEST sequence. The amino acid sequence homology search also reveals that STAP is identical to a mouse oxidative stress protein, A170, and has 90% homology with a human p62 protein that binds to the tyrosine kinase p56(lck) SH2 domain. Northern blot analysis indicated a broad expression profile of STAP mRNA in various tissues and cell lines. In MC3T3-E1 cells, STAP mRNA was induced by treatment with TGF-beta, but not with BMP-2 or GDF-5. Analysis of the mouse STAP gene isolated from the genomic library revealed that the STAP gene spans a region of over 11 kb and comprises eight exons. The transcription start site was identified by primer extension analysis to be located 35 bp upstream from the translation initiation site. Sequencing analysis of the 5' flanking region of the STAP gene revealed multiple consensus motifs/sequences for several DNA binding transcription factors. The STAP gene had a TATA box, but no CCAAT box. Potential Sp1, AP-1, NF-E2, MyoD, and NF-kappaB binding sites were found in the 5' flanking region (1.4 kb) of the STAP gene.

p38 mitogen-activated protein kinase functionally contributes to chondrogenesis induced by growth/differentiation factor-5 in ATDC5 cells

Recent studies of intracellular signal transduction mechanisms for the transforming growth factor-beta (TGF-beta) superfamily have focused on Smad proteins, but have paid little attention to mitogen-activated protein (MAP) kinase cascades. Here we demonstrate that growth/differentiation factor-5 (GDF-5), but neither bone morphogenetic protein-2 (BMP-2) nor TGF-beta1, fully promotes the early phase of the chondrogenic response by inducing cellular condensation followed by cartilage nodule formation in a mouse chondrogenic cell line, ATDC5. We investigated which, if any, of the three major types of MAP kinase plays a functional role in the promotion of chondrogenesis induced by GDF-5. GDF-5 induced phosphorylation of p38 MAP kinase and extracellular signal-regulated kinase (ERK) but not that of c-Jun N-terminal kinase (JNK). The phosphorylation of p38 MAP kinase was also induced by BMP-2 and TGF-beta1. An inhibitor of p38 and p38 beta MAP kinase, SB202190, showed complete inhibition of cartilage nodule formation but failed to affect alkaline phosphatase (ALP) activity induced by GDF-5. Expression of the type II collagen gene, a hallmark of chondrogenesis in vertebrates, was also induced by GDF-5 treatment and strongly suppressed by SB202190. On the other hand, although an inhibitor of MAP/ERK kinase, PD98059, inhibited the rapid phosphorylation of ERK by GDF-5, it inhibited neither ALP activity nor cartilage nodule formation induced by GDF-5. These results strongly suggest that the p38 MAP kinase cascade is involved in GDF-5 signaling pathways and that a role of the p38 MAP kinase pathway is necessary over a longer period to promote chondrogenesis in ATDC5 cells.

The TAK1-NLK-MAPK-related pathway antagonizes signalling between beta-catenin and transcription factor TCF

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.

MAP kinase and Wnt pathways converge to downregulate an HMG-domain repressor in Caenorhabditis elegans

The signalling 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 signalling downregulates POP-1 activity in one posterior daughter cell called E. Here we show that the genes mom-4 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. We conclude that a MAPK-related pathway cooperates with Wnt signal transduction to downregulate POP-1 activity. These functions are likely to be conserved in vertebrates, as TAK1 and NLK can downregulate HMG-domain-containing proteins related to POP-1.

TGF-beta family signal transduction in Drosophila development: from Mad to Smads

The transforming growth factor-beta (TGF-beta) superfamily encompasses a large group of soluble extracellular proteins that are potent regulators of development in both vertebrates and invertebrates. Drosophila TGF-beta family members include three proteins with homology to vertebrate bone morphogenetic proteins (BMPs): Decapentaplegic (Dpp), Screw, and Glass bottom boat-60A. Genetic studies of Dpp signaling led to the identification of Smad proteins as central mediators of signal transduction by TGF-beta family members. Work in mammalian tissue culture has elucidated a biochemical model for signal transduction, in which activation of receptor serine-threonine kinase activity leads to phosphorylation of specific Smad proteins and translocation of heteromeric Smad protein complexes to the nucleus. Once in the nucleus Smad proteins interact with other DNA binding proteins to regulate transcription of specific target genes. Dissection of Dpp-response elements from genes expressed during embryonic mesoderm patterning and midgut morphogenesis provides important insights into the contributions of Smad proteins and tissue-specific transcription factors to spatial regulation of gene expression. Genetic studies in Drosophila are now expanding to include multiple BMP ligands and receptors and have uncovered activities not explained by the current signal transduction model. Identification of more ligand sequences and demonstration of a functional Drosophila activin-like signal transduction pathway suggest that all TGF-beta signal transduction pathways are present in flies.

Stimulation of Elk1 transcriptional activity by mitogen-activated protein kinases is negatively regulated by protein phosphatase 2B (calcineurin)

Cellular calcium (Ca2+) and the Ca2+-binding protein calmodulin (CaM) regulate the activities of Ca2+/CaM-dependent protein kinases and protein phosphatase 2B (calcineurin). Functional interactions between CaM kinases and mitogen-activated protein (MAP) kinases were described. In this report, we describe cross-talk between calcineurin and mitogen-activated protein kinase signaling. Calcineurin was found to specifically down-regulate the transcriptional activity of transcription factor Elk1, following stimulation of this activity by the ERK, Jun N-terminal kinase, or p38 MAP kinase pathways. Expression of constitutively activated calcineurin or activation of endogenous calcineurin by Ca2+ ionophore decreased the phosphorylation of Elk1 at sites that positively regulate its transcriptional activity. Calcineurin specifically dephosphorylates Elk1 at phosphoserine 383, a site whose phosphorylation by MAP kinases makes a critical contribution to the enhanced transcriptional activity of Elk1. The cross-talk between calcineurin and MAP kinases is of physiological significance as low doses of Ca2+ ionophore which by themselves are insufficient for c-fos induction can actually inhibit induction of c-fos expression by activators of MAP kinases. Thus through the effect of calcineurin on Elk1 phosphorylation, Ca2+ can have a negative effect on expression of Elk1 target genes. This mechanism explains why different levels of intracellular Ca2+ can result in very different effects on gene expression.

Inhibition of rat hepatocyte proliferation by transforming growth factor beta and glucagon is associated with inhibition of ERK2 and p70 S6 kinase

Stimulation of hepatocyte proliferation by epidermal growth factor (EGF) and insulin is inhibited by transforming growth factor beta (TGF-beta) and by glucagon. It is also suppressed by inhibitors of various protein kinases, including rapamycin, which blocks activation of p70 S6 kinase (p70(S6k)), PD98059, which inhibits the activation of extracellular-regulated kinase (ERK), and SB 203580, an inhibitor of the p38 mitogen-activated protein kinase (p38 MAPK). In this study, we investigated whether the inhibition of proliferation by TGF-beta involves these protein kinase cascades. Culture of hepatocytes with TGF-beta for 16 hours decreased the stimulation by EGF of ERK2 and p70(S6k) (by 50% and 35%, respectively), but did not affect the stimulation of either p38 MAPK, c-jun NH2-terminal kinase (JNK), or protein kinase B (PKB). Culture of hepatocytes with glucagon for 16 hours also inhibited the stimulation by EGF of activation of ERK2 and p70(S6k) (by approximately 50%). The inhibitory effects of glucagon were observed when the hormone was added either 10 minutes or 60 minutes before EGF addition, whereas no effects of TGF-beta were observed after 10-minute or 60-minute incubation. These results suggest that the inhibition of hepatocyte proliferation by TGF-beta may be in part mediated by inhibition of ERK2 and p70(S6k), but does not involve PKB, JNK, or p38 MAPK. Unlike glucagon, the effects of TGF-beta are not elicited in response to short-term treatment.

Functional interactions of transforming growth factor beta-activated kinase 1 with IkappaB kinases to stimulate NF-kappaB activation

Several mitogen-activated protein kinase kinase kinases play critical roles in nuclear factor-kappaB (NF-kappaB) activation. We recently reported that the 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. Here we investigated the molecular mechanism of TAK1-induced NF-kappaB activation. Dominant negative mutants of IkappaB kinase (IKK) alpha and IKKbeta inhibited TAK1-induced NF-kappaB activation. TAK1 activated IKKalpha and IKKbeta in the presence of TAB1. IKKalpha and IKKbeta were coimmunoprecipitated with TAK1 in the absence of TAB1. TAB1-induced TAK1 activation promoted the dissociation of active forms of IKKalpha and IKKbeta from active TAK1, whereas the IKK mutants remained to interact with active TAK1. Furthermore, tumor necrosis factor-alpha activated endogenous TAK1, and the kinase-negative TAK1 acted as a dominant negative inhibitor against tumor necrosis factor-alpha-induced NF-kappaB activation. These results demonstrated a novel signaling pathway to NF-kappaB activation through TAK1 in which TAK1 may act as a regulatory kinase of IKKs.

Association of TGF-beta signaling in angiotensin II-induced PAI-1 mRNA upregulation in mesangial cells: role of PKC

This study aimed to identify the intracellular signaling pathway in angiotensin II (Ang II)-induced upregulation of plasminogen activator inhibitor type 1 (PAI-1) mRNA expression in cultured rat glomerular mesangial cells, and to examine the interaction between Ang II and TGF-beta signaling. Ang II-induced upregulation of PAI-1 mRNA expression was prevented by a protein kinase C (PKC) inhibitor, bisindorylmaleimide I. While phorbol 12-myristate 13-acetate (PMA) upregulated the PAI-1 mRNA expression, a calcium ionophore, ionomycin, had little effect. Mesangial cells pretreated with PMA for 24 h to downregulate PKC demonstrated attenuated response to Ang II. A protein tyrosine kinase inhibitor, genistein, completely blocked both Ang II- and PMA-induced PAI-1 mRNA expression. Transforming growth factor-beta1 (TGF-beta1) alone induced the expression, and in the presence of Ang II, TGF-beta1 superinduced PAI-1 mRNA expression to a higher extent. Both bisindorylmaleimide I and genistein suppressed the Ang II plus TGF-beta1-induced PAI-1 mRNA upregulation to the basal level, while downregulation of PKC attenuated the synergistic upregulation of PAI-1 mRNA expression to the level comparable to TGF-beta1 alone. These data suggest that, in rat mesangial cells, (1) PKC and protein tyrosine kinase(s) are involved in the Ang II signaling cascade, (2) protein tyrosine kinase(s) works downstream from PKC in the cascade, and (3) there is an interaction between the Ang II and TGF-beta signal pathways downstream from PKC. In in vivo settings, local activation of renin-angiotensin and TGF-beta systems in the glomeruli may synergistically augment PAI-1 expression, promote mesangial matrix accumulation and progression of glomerular injury.

MEKK-1, a component of the stress (stress-activated protein kinase/c-Jun N-terminal kinase) pathway, can selectively activate Smad2-mediated transcriptional activation in endothelial cells

Smad proteins are essential components of the intracellular signaling pathways utilized by members of the transforming growth factor-beta (TGF-beta) superfamily of growth factors. Certain Smad proteins (e.g. Smad1, -2, and -3) can act as regulated transcriptional activators, a process that involves phosphorylation of these proteins by activated TGF-beta superfamily receptors. We demonstrate that the intracellular kinase mitogen-activated protein kinase kinase kinase-1 (MEKK-1), an upstream activator of the stress-activated protein kinase/c-Jun N-terminal kinase pathway, can participate in Smad2-dependent transcriptional events in cultured endothelial cells. A constitutively active form of MEKK-1 but not mitogen-activated protein kinase kinase-1 (MEK-1) or TGF-beta-activated kinase-1, two distinct intracellular kinases, can specifically activate a Gal4-Smad2 fusion protein, and this effect correlates with an increase in the phosphorylation state of the Smad2 protein. These effects do not require the presence of the C-terminal SSXS motif of Smad2 that is the site of TGF-beta type 1 receptor-mediated phosphorylation. Activation of Smad2 by active MEKK-1 results in enhanced Smad2-Smad4 interactions, nuclear localization of Smad2 and Smad4, and the stimulation of Smad protein-transcriptional coactivator interactions in endothelial cells. Overexpression of Smad7 can inhibit the MEKK-1-mediated stimulation of Smad2 transcriptional activity. A physiological level of fluid shear stress, a known activator of endogenous MEKK-1 activity in endothelial cells, can stimulate Smad2-mediated transcriptional activity. These data demonstrate a novel mechanism for activation of Smad protein-mediated signaling in endothelial cells and suggest that Smad2 may act as an integrator of diverse stimuli in these cells.

ATF-2 is a common nuclear target of Smad and TAK1 pathways in transforming growth factor-beta signaling

Upon transforming growth factor-beta (TGF-beta) binding to its cognate receptor, Smad3 and Smad4 form heterodimers and transduce the TGF-beta signal to the nucleus. In addition to the Smad pathway, another pathway involving a member of the mitogen-activated protein kinase kinase kinase family of kinases, TGF-beta-activated kinase-1 (TAK1), is required for TGF-beta signaling. However, it is unknown how these pathways function together to synergistically amplify TGF-beta signaling. Here we report that the transcription factor ATF-2 (also called CRE-BP1) is bound by a hetero-oligomer of Smad3 and Smad4 upon TGF-beta stimulation. ATF-2 is one member of the ATF/CREB family that binds to the cAMP response element, and its activity is enhanced after phosphorylation by stress-activated protein kinases such as c-Jun N-terminal kinase and p38. The binding between ATF-2 and Smad3/4 is mediated via the MH1 region of the Smad proteins and the basic leucine zipper region of ATF-2. TGF-beta signaling also induces the phosphorylation of ATF-2 via TAK1 and p38. Both of these actions are shown to be responsible for the synergistic stimulation of ATF-2 trans-activating capacity. These results indicate that ATF-2 plays a central role in TGF-beta signaling by acting as a common nuclear target of both Smad and TAK1 pathways.

A short loop on the ALK-2 and ALK-4 activin receptors regulates signaling specificity but cannot account for all their effects on early Xenopus development

Activin, a member of the transforming growth factor beta (TGF-beta) superfamily, signals through a heteromeric complex of type I and type II serine-threonine kinase receptors. The two activin type I receptors previously identified, ALK-2 (ActR-I) and ALK-4 (ActR-IB), have distinct effects on gene expression, differentiation and morphogenesis in the Xenopus animal cap assay. ALK-4 reproduces the effects of activin treatment including the dose-dependent induction of progressively more dorso-anterior mesodermal and endodermal markers, whereas ALK-2 induces only ventral mesodermal markers and counteracts the effects of ALK-4. To identify regions of the receptors that determine signaling specificity we have generated chimeras of the constitutively active ALK-2 and ALK-4 receptors (termed ALK-2* and ALK-4*). The effects of these chimeric receptors on gene expression and morphogenetic movements implicate the loop between kinase subdomains IV and V in mediating the strong dorsal gene-inducing properties of ALK-4*; when the seven amino acids comprising this loop are transferred from ALK-4* to ALK-2*, the resulting chimeric receptor is capable of inducing the expression of dorsal-specific genes. In contrast, when the equivalent region of ALK-2* is transferred to the ALK-4* backbone it cannot effectively counteract the dorsalizing effects of ALK-4*, suggesting that other regions of type I receptors are also involved in determining signal specificity.

The kinase TAK1 can activate the NIK-I kappaB as well as the MAP kinase cascade in the IL-1 signalling pathway

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.

Role of a complex containing Rad17, Mec3, and Ddc1 in the yeast DNA damage checkpoint pathway

Genetic analysis has suggested that RAD17, RAD24, MEC3, and DDC1 play similar roles in the DNA damage checkpoint control in budding yeast. These genes are required for DNA damage-induced Rad53 phosphorylation and considered to function upstream of RAD53 in the DNA damage checkpoint pathway. Here we identify Mec3 as a protein that associates with Rad17 in a two-hybrid screen and demonstrate that Rad17 and Mec3 interact physically in vivo. The amino terminus of Rad17 is required for its interaction with Mec3, and the protein encoded by the rad17-1 allele, containing a missense mutation at the amino terminus, is defective for its interaction with Mec3 in vivo. Ddc1 interacts physically and cosediments with both Rad17 and Mec3, indicating that these three proteins form a complex. On the other hand, Rad24 is not found to associate with Rad17, Mec3, and Ddc1. DDC1 overexpression can partially suppress the phenotypes of the rad24Delta mutation: sensitivity to DNA damage, defect in the DNA damage checkpoint and decrease in DNA damage-induced phosphorylation of Rad53. Taken together, our results suggest that Rad17, Mec3, and Ddc1 form a complex which functions downstream of Rad24 in the DNA damage checkpoint pathway.

XIAP, a cellular member of the inhibitor of apoptosis protein family, links the receptors to TAB1-TAK1 in the BMP signaling pathway

Signals elicited by transforming growth factor-beta (TGF-beta) 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. A yeast interaction screen was used to search for proteins that function in the pathway linking the receptors and TAB1-TAK1. The human X-chromosome-linked inhibitor of apoptosis protein (XIAP) was isolated as a TAB1-binding protein. XIAP associated not only with TAB1 but also with the BMP receptors in mammalian cells. Injection of XIAP mRNA into dorsal blastomeres enhanced the ventralization of Xenopus embryos in a TAB1-TAK1-dependent manner. Furthermore, a truncated form of XIAP lacking the TAB1-binding domain partially blocked the expression of ventral mesodermal marker genes induced by a constitutively active BMP type I receptor. These results suggest that XIAP participates in the BMP signaling pathway as a positive regulator linking the BMP receptors and TAB1-TAK1.

Genetic events and the role of TGF beta in epithelial tumour progression

The mouse skin model of chemical carcinogenesis has been very well characterized with respect to epigenetic changes, which occur during tumour cell initiation, promotion and progression. The use of transgenic and gene knock-out mice has contributed greatly to knowledge in this area. The H-ras genetic locus has been shown to undergo multiple genetic changes, including mutagenic activation, amplification of the mutant gene, and loss of the normal allele. These different genetic events lead to thresholds of ras activity which contribute to different stages along the pathway to neoplasia. The genetic and epigenetic events which lead to tumour invasion and metastasis have been less well characterized than studies on tumour initiation and promotion, despite the fact that it is metastases which ultimately kill the animal/patient. In the mouse skin model, loss of p53 contributes to malignant conversion. Gene deletion of the INK4 locus is associated with transformation to a highly invasive spindle cell tumor phenotype. This spindle cell transformation can also be induced in vitro or in vivo by TGF beta 1, possible by synergizing with mutant H-ras. TGF beta can have both positive and negative effects on tumourigenesis, acting early as a tumour suppresser, but later as a stimulator of tumour invasion. It is this latter effect which may be clinically more significant, since many human tumours overexpress TGF beta, yet the majority still retain the intracellular signaling systems necessary for the cell to respond to this growth factor.

Roles of pathway-specific and inhibitory Smads in activin receptor signaling

Activins and other members of the transforming growth factor-beta-like superfamily of growth factors transduce their signals by interacting with two types of receptor serine/threonine kinases. The Smad proteins, a new family of intracellular mediators are involved in the signaling pathways of these receptors, but the initial stages of their activation as well as their specific functions remain to be defined. We report here that the pathway-specific Smad2 and 3 can form a complex with the activin receptor in a ligand-dependent manner. This complex formation is rapid but also transient. Indeed, soon after their association with the activin receptor, Smad2 and Smad3 are released into the cytoplasm where they interact with the common partner Smad4. These Smad complexes then mediate activin-induced transcription. Finally, we show that the inhibitory Smad7 can prevent the association of the two pathway-specific Smads with the activin receptor complex, thereby blocking the activin signal.

Isolation of ATMEKK1 (a MAP kinase kinase kinase)-interacting proteins and analysis of a MAP kinase cascade in Arabidopsis

In plants, a number of MAP kinase (MAPK), MAPK kinase (MAPKK), and MAPKK kinase (MAPKKK) homologues have been reported. However, there have been no reports of protein-protein interactions between these kinases or molecular analysis of MAPK cascades in higher plants. To analyze a possible MAPK cascade in Arabidopsis thaliana, we took two molecular approaches. One is the two-hybrid screening of ATMEKK1 (a MAPKKK)-interacting proteins; the other is an analysis of physical and functional interactions among isolated MAPK, MAPKK, and MAPKKK homologues from Arabidopsis. In two-hybrid screening using ATMEKK1 as bait, we isolated a novel MAPKK homologue, ATMKK2, a MAPK homologue, ATMPK4, and an unknown protein. ATMKK2 has high sequence similarity with MEK1 (a MAPKK) in Arabidopsis. Based on yeast two-hybrid analysis, we detected protein-protein interactions between ATMEKK1 and ATMKK2/MEK1 (MAPKKs), between ATMKK2/MEK1 and ATMPK4 (a MAPK), and between ATMPK4 and ATMEKK1. ATMPK4 and ATMKK2/MEK1 interacted with two distinct regions of ATMEKK1, the N-terminal regulatory domain and the C-terminal kinase domain, respectively. Coexpression of ATMEKK1 increased the ability of two closely related MAPKKs, ATMKK2 and MEK1, to complement a growth defect of the yeast pbs2 mutant. Coexpression of ATMPK4 and MEK1 complemented a growth defect of the yeast mpk1 and bck1 mutants. By contrast, other combinations of MAPKs and MAPKKs did not suppress these yeast mutations. These results suggest that ATMEKK1, ATMKK2/MEK1, and ATMPK4 may constitute a MAP kinase cascade.

Fosfomycin, an antibiotic, possessed TGF-beta-like immunoregulatory activities

The regulatory effects of fosfomycin (FOM) were correlated closely with the multifunction of TGF-beta in the modulation of immune responses in vivo and in vitro. LPS-induced polyclonal IgM and IgG antibody responses were depressed at 3 days after the initial culture and subsequently enhanced at day 10 by FOM or TGF-beta. Neither FOM nor TGF-beta inhibited LPS-induced IgA antibody responses, whereas dexamethasone (DX) reduced polyclonal IgM, IgG and IgA antibody responses wholly. The suppression of antibody responses and Mv1Lu cell proliferation induced by FOM or TGF-beta was partly overcome with soluble TFG-beta receptors (sRIII). Oral, i.v. and i.p. administration of FOM exhibited similar enhanced SRBC-specific antibody responses to that seen after oral administration of TGF-beta. The addition of FOM and latent TGF-beta inhibited the proliferation of Mv1Lu cells, but FOM did not lead to an increase in plasmin activities, which convert latent to active TGF-beta, and further the expression of TGF-beta receptors on the cell surface. In addition, FOM failed to enhance TGF-beta secretion. These findings suggest that immunomodulation of FOM results in increased sensitivity of cells to TGF-beta.

Downstream factors in transforming growth factor-beta family signaling

The recently identified family of Smad proteins has given insight in the understanding of how members of the transforming growth factor-beta (TGF-beta) family relay their signal to the nucleus. Besides Smad proteins, G proteins and MAPKs are also involved in the downstream signaling of TGF-beta family members. The identification of elements that function downstream in the TGF-beta signaling pathway and the fact that these downstream players can interact with the signaling cascade of other growth factors, may give insight into the diverse biological responses evoked by the TGF-beta family members.

Extracellular matrix-associated transforming growth factor-beta: role in cancer cell growth and invasion

Growth factors of the transforming growth factor-beta (TGF-beta) family inhibit the proliferation of epithelial, endothelial, and hematopoietic cells, and stimulate the synthesis of extracellular matrix components. TGF-beta s are secreted from cells in high-molecular-mass protein complexes that are composed of three proteins, the mature TGF-beta-dimer, the TGF-beta propeptide dimer, or latency-associated protein (LAP), and the latent TGF-beta binding protein (LTBP). Mature TGF-beta is cleaved from its propeptide during secretion, but the proteins remain associated by noncovalent interactions. LTBP is required for efficient secretion and processing of latent TGF-beta and it binds to LAP via disulfide bond(s). LTBP is a component of extracellular matrix microfibrils, and it targets the latent TGF-beta complex to the extracellular matrix. TGF-beta signaling is initiated by proteolytic cleavage of LTBP that results in the release of the latent TGF-beta complex from the extracellular matrix. TGF-beta is activated by dissociation of LAP from the mature TGF-beta. Subsequent signaling involves binding of active TGF-beta to its type II cell surface receptors, which phosphorylate and activate type I TGF-beta receptors. Type I receptors, in turn, phosphorylate cytoplasmic transcriptional activator proteins Smad2 and Smad3, inducing their translocation to the nucleus. Recent evidence suggests that acquisition of resistance to TGF-beta growth inhibition plays a major role in the progression of epithelial and hematopoietic cell malignancies. The role of secretion of TGF-beta in tumorigenesis is more complex. The secretion of TGF-beta s by tumor cells may contribute to autocrine growth inhibition, but on the other hand, it may also promote invasion, metastasis, angiogenesis, and even immunosuppression. Tumor cells may also fail to deposit LTBP:TGF-beta complexes to the extracellular matrix. The elucidation of the mechanisms of the release of TGF-beta from the matrix and its subsequent activation aids the understanding of the pathophysiologic roles of TGF-beta in malignant growth, and allows the development of therapeutic agents that regulate the activity of TGF-beta.

Reactive oxygen species- and dimerization-induced activation of apoptosis signal-regulating kinase 1 in tumor necrosis factor-alpha signal transduction

Reactive oxygen species (ROS) have been implicated in the induction of apoptosis by tumor necrosis factor-alpha (TNFalpha) and other cytotoxic insults, although the molecule(s) regulated by ROS in TNFalpha signaling have not been identified. Apoptosis signal-regulating kinase 1 (ASK1) is a member of the mitogen-activated protein kinase kinase kinase (MAPKKK) superfamily that has been shown to be activated during TNFalpha-induced apoptosis. ASK1 increases apoptosis when overexpressed, but the mechanism of ASK1 activation and the mechanisms of ASK1-induced apoptosis are unclear. We now report that hydrogen peroxide induces the activation of ASK1 in 293 cells. TNFalpha-induced activation of ASK1 was inhibited by antioxidants. Hydrogen peroxide-induced apoptosis was markedly enhanced by the expression of ASK1. These results suggest that TNFalpha-induced activation of ASK1 is mediated by ROS. We also examined how ASK1 activity is regulated by ROS. We found that ASK1 formed dimers or higher order oligomers in 293 cells. TNFalpha or hydrogen peroxide treatment increased the dimeric form of ASK1, and pretreatment with N-acetylcysteine decreased it. Furthermore, synthetic dimerization of an ASK1-gyrase B fusion protein by coumermycin resulted in substantial activation of ASK1, suggesting that dimerization of ASK1 is sufficient for its activation. These results taken together suggest that TNFalpha causes ASK1 activation via ROS-mediated dimerization of ASK1.

Induction of apoptosis by SB202190 through inhibition of p38beta mitogen-activated protein kinase

p38, a subfamily of the mitogen-activated protein kinase, regulates gene expression in response to various extracellular stimuli. The pyridinyl imidazoles like SB202190 are specific inhibitors of p38alpha and p38beta and have been widely used in investigation of the biological functions of p38. Here we show that SB202190 by itself was sufficient to induce cell death, with typical apoptotic features such as nucleus condensation and intranucleosomal DNA fragmentation. SB202190 stimulated the activity of CPP32-like caspases, and its apoptotic effect was completely blocked by the protease inhibitor benzyloxycarbonyl-Val-Ala-Asp-fluoromethyl ketone and expression of bcl-2. In addition, SB202190 was able to potentiate apoptosis induced by Fas(APO-1) ligation or UV irradiation. Expression of p38beta attenuated the apoptotic effect of SB202190 and the cell death induced by Fas ligation and UV irradiation. In contrast, expression of p38alpha induced cell death mildly. These results indicate that SB202190 induces apoptosis through activation of CPP32-like caspases and suggest that distinct members of the p38 subfamily of mitogen-activated protein kinase have different functions in apoptosis.

Adenovirus-mediated ectopic expression of Msx2 in even-numbered rhombomeres induces apoptotic elimination of cranial neural crest cells in ovo

Distinct cranial neural crest-derived cell types (a number of neuronal as well as non-neuronal cell lineages) are generated at characteristic times and positions in the rhombomeres of the hindbrain in developing vertebrate embryos. To examine this developmental process, we developed a novel strategy designed to test the efficacy of gain-of-function Msx2 expression within rhombomeres in ovo prior to the emigration of cranial neural crest cells (CNCC). Previous studies indicate that CNCC from odd-numbered rhombomeres (r3 and r5) undergo apoptosis in response to exogenous BMP4. We provide evidence that targeted infection in ovo using adenovirus containing Msx2 and a reporter molecule indicative of translation can induce apoptosis in either even- or odd-numbered rhombomeres. Furthermore, infected lacZ-control explants indicated that CNCC emigrated, and that 20% of these cells were double positive for crest cell markers HNK-1 and beta-gal. In contrast, there were no HNK-1 and Msx2 double positive cells emigrating from Msx2 infected explants. These results support the hypothesis that apoptotic elimination of CNCC can be induced by ‘gain-of-function’ Msx2 expression in even-numbered rhombomeres. These inductive interactions involve qualitative, quantitative, positional and temporal differences in TGF-beta-related signals, Msx2 expression and other transcriptional control.

Expression of Smad1 and Smad2 during embryogenesis suggests a role in organ development

Smad proteins are intracellular signalling molecules and putative transcription factors that transduce signals elicited by members of the transforming growth factor beta (TGF-beta) superfamily. By comparing the expression of Smad1 and Smad2 during embryonic development, we show that mRNAs of both Smad isoforms are present in a variety of tissues. The major sites of expression of both Smads can be correlated with the expression domains of several members of the TGF-beta superfamily. Our expression data suggest that Smad proteins are involved in organ development, particularly that of organs arising from mesenchymal-epithelial interactions. A second site of strong expression is the central nervous system. Transcriptional control mediated by Smad1 and Smad2, therefore, may exert an important function in differentiation processes of embryonic development that are controlled by ligands of the TGF-beta superfamily.

BRAM1, a BMP receptor-associated molecule involved in BMP signalling

BACKGROUND

TGF-beta superfamily members elicit signals through the stimulation of serine/threonine-kinase receptors. Recently, molecules associated with several TGF-beta family receptors have been cloned. One such molecule, the immunophilin FKBP12, has been reported to interact with TGF-beta family type I receptors. However, the identity of signalling specific molecules interacting with the receptor was unknown.

RESULTS

To clarify the factors mediating bone morphogenetic protein (BMP) receptor signalling, a cytoplasmic molecule associated with the BMP type IA receptor (BMPR-IA) was isolated using the yeast two-hybrid system. We designated the molecule BMP receptor associated molecule 1 (BRAM1). BRAM1 is an alternatively spliced form of BS69, a factor previously identified as an adenovirus E1A-associated protein. BRAM1 was localized to the cytoplasmic region in mammalian cells, whereas BS69 is localized to the nucleus. BRAM1 bound specifically to BMPR-IA in mammalian cells. The C-terminal half of BRAM1 was found to be sufficient for binding to BMPR-IA.

CONCLUSIONS

BRAM1, a BMPR-IA associated molecule, was isolated using the yeast two-hybrid system, and found to associate specifically with BMPR-IA. BRAM1 may thus serve as an interacting protein in the BMP signal pathway.

Role of TAK1 and TAB1 in BMP signaling in early Xenopus development

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. Here, we have examined the role of TAK1 and TAB1 in the dorsoventral patterning of early Xenopus embryos. Ectopic expression of Xenopus TAK1 (xTAK1) in early embryos induced 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) which is known to inhibit endogenous signaling could partially rescue phenotypes generated by the expression of a constitutively active BMP-2/4 type IA receptor (BMPR-IA). Moreover, xTAK1KN could block the expression of ventral mesoderm marker genes induced by Smad1 or 5. These results thus suggest that xTAK1 and xTAB1 function in the BMP signal transduction pathway in Xenopus embryos in a cooperative manner.

TGF-beta-activated kinase 1 stimulates NF-kappa B activation by an NF-kappa B-inducing kinase-independent mechanism

Several mitogen-activated protein kinase kinase kinases (MAPKKKs), including NF-kappa B-inducing kinase (NIK), play critical roles in NF-kappa B activation. We isolated cDNA for human TGF-beta activated kinase 1 (TAK1), a member of the MAPKKK family, and evaluated its ability to stimulate NF-kappa B activation. 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.

Molecular cloning of human TAK1 and its mutational analysis in human lung cancer

In previous reports, we described that DPC4/Smad4 and Smad2 are mutated in a fraction of human lung cancers and suggested possible roles of the downstream mediators of transforming growth factor-beta (TGF-beta)-elicited signals in the pathogenesis of this most common cancer. In the present study, we investigated whether another downstream mediator, human TGF-beta-activated kinase 1 (hTAK1), also is altered in lung cancer. For this purpose, the hTAK1 gene was cloned with the aid of an expression sequence tag database search and cDNA library screening, and hTAK1 was found to be expressed ubiquitously in 2 distinct isoforms regulated in a tissue-specific manner in fetal and adult normal tissues. Interestingly, hTAK1 was assigned to the chromosome region 6q14-21, which is deleted frequently in various human malignancies, including lung cancer. Despite our extensive search for alterations in 39 lung cancer specimens as well as in 16 lung cancer cell lines, somatic mutations of hTAK1 were not identified, indicating that hTAK1 itself is not a frequent target for genetic alterations in lung cancer.

Selective activation of p38 mitogen-activated protein (MAP) kinase isoforms by the MAP kinase kinases MKK3 and MKK6

The cellular response to treatment with proinflammatory cytokines or exposure to environmental stress is mediated, in part, by the p38 group of mitogen-activated protein (MAP) kinases. We report the molecular cloning of a novel isoform of p38 MAP kinase, p38 beta 2. This p38 MAP kinase, like p38 alpha, is inhibited by the pyridinyl imidazole drug SB203580. The p38 MAP kinase kinase MKK6 is identified as a common activator of p38 alpha, p38 beta 2, and p38 gamma MAP kinase isoforms, while MKK3 activates only p38 alpha and p38 gamma MAP kinase isoforms. The MKK3 and MKK6 signal transduction pathways are therefore coupled to distinct, but overlapping, groups of p38 MAP kinases.

Cellular factors may enable squamous carcinoma cells to overcome TGF beta-mediated repression of CDK2 activity

Cell lines developed from head and neck squamous cell carcinomas exhibit variable responses to the negative regulatory effects of transforming growth factor beta (TGF beta) on cell growth. To analyse the effects of TGF beta on regulators of cell cycle progression, we characterised cell lines derived from head and neck squamous cell carcinoma (HNSCC) for their biological sensitivities to TGF beta, growth inhibition, then examined the effects of TGF beta treatment on the expression and activity of cyclin dependent kinases (CDKs) and inhibitors of these kinases. Western blot analysis of cell lysates from untreated or TGF beta-treated cultures showed no alterations in expression of CDK2, CDK4, CDK6 or cyclin E in cell lines which were either sensitive (HaCaT, HN6) or refractory (HN12, HN30) to the growth-inhibitory effects of TGF beta. However, treatment of cells with TGF beta resulted in a several fold increase in cellular levels of p21 (WAF1/Cip1), irrespective of biological response. Immune complex in vitro kinase assays demonstrated that the activity of CDK2 was inhibited by exposure to ligand in each case, confirming that a TGF beta signalling pathway which regulates kinase activity was intact in these cell lines. The data suggest that cellular factors expressed in HN12 and HN30 enable these cells to override TGF beta-mediated inhibition of CDK2 activity and allow cell cycle progression. This may represent an important mechanism which allows cells to evade growth arrest during malignant progression.

TGF-beta signal transduction

The transforming growth factor beta (TGF-beta) family of growth factors control the development and homeostasis of most tissues in metazoan organisms. Work over the past few years has led to the elucidation of a TGF-beta signal transduction network. This network involves receptor serine/threonine kinases at the cell surface and their substrates, the SMAD proteins, which move into the nucleus, where they activate target gene transcription in association with DNA-binding partners. Distinct repertoires of receptors, SMAD proteins, and DNA-binding partners seemingly underlie, in a cell-specific manner, the multifunctional nature of TGF-beta and related factors. Mutations in these pathways are the cause of various forms of human cancer and developmental disorders.