TGF-β signaling: A new route to p38 and JNK

TGF-β activates p38/JNK through TRAF6-mediated polyubiquitylation of TAK1.

Transforming growth factor-β (TGF-β) is a dimeric cytokine that induces the dimerization and cross-phosphorylation of its cellular receptors, TGF-β receptor Type I and II (TβRI and TβRII). Activated TβRI/TβRII then phosphorylates receptor-associated (R)-Smad proteins, promoting their nuclear translocation and gene regulation. TGF-β associated kinase (TAK1) activates p38 and c-Jun N-terminal protein kinase (JNK) in response to TGF-β, leading to apoptosis. The ubiquitin ligase TRAF6 (tumor necrosis factor (TNF) receptor-associated factor-6) has been implicated in the activation of TAK1 by interleukin-1, but the mechanism of TAK1 activation by TGF-β has remained elusive. Reporting in Nature Cell Biology, Sorrentino et al. now identify a kinase-independent pathway of TGF-β receptor signaling that connects auto-ubiquitylation of TRAF6 with the polyubiquitylation of TAK1 and the activation of p38.

The authors observed that TAK1 underwent lysine 63-linked polyubiquitylation in response to TGF-β, which correlated with the phosphorylation and activation of both TAK1 and p38. Chemical inhibitors of TβRI kinase activity and kinase-deficient TβRI mutants inhibited the activation of Smad proteins — but not of TAK1 or p38 — suggesting that the pathway does not require TβRI kinase activity. Consistent with previous studies, TGF-β induced TRAF6 auto-ubiquitylation and activation. Point mutations in the TRAF6 binding motif of TβRI abrogated TAK1 and p38 activation, suggesting that the association between TRAF6 and TβRI is necessary for TAK1 activation. TRAF6-deficient cells demonstrated defects in TAK1 and p38 activation, as well as in TGF-β-induced apoptosis. The authors found that TAK1 was directly ubiquitylated by TRAF6 in vitro, and that TAK1 activation required both TRAF6 E3-ligase activity and the presence of the TAK1 ubiquitin acceptor site.

The authors propose that TGF-β binding to TβRI induces the auto-ubiquitylation of TRAF6 and subsequent TRAF6-mediated polyubiquitylation of TAK1. Phosphorylation of TAK1 completes TAK1 activation and leads to p38 activation and apoptosis. Thus, TβRI has a dual role: its kinase activity is required for signaling and gene transcription via the Smad proteins, but is not required for TRAF6-mediated activation of TAK1 and apoptosis. Further dissection of the TGF-β pathway is necessary to identify the mechanism of TAK1 phosphorylation following ubiquitylation by TRAF6.

Anna S. Kushnir
Nature Publishing Group

Original Reference:
Sorrentino, A. et al.
The type I TGF-β receptor engages TRAF6 to activate TAK1 in a receptor kinase-independent manner
Nature Cell Biology 10, 1199-1207 (2008)
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Colon cancer: A β-catenin–CDK8–E2F1 ménage à trois

CDK8 and E2F1 have antagonistic roles in β-catenin-mediated transcription.

The WNT-β-catenin signaling pathway is aberrantly activated in most colorectal cancers. WNT signaling causes β-catenin to translocate into the nucleus, where it binds and activates the transcription factor TCF. Two reports in Nature now provide insight into the regulation of β-catenin/TCF-stimulated transcription in colon cancer. William Hahn and colleagues show that cyclin-dependent kinase-8 (CDK8) is required for β-catenin-mediated oncogenic transformation, whereas Nicholas Dyson and colleagues report that the transcription factor E2F1 downregulates β-catenin and that this repression is relieved by the tumor suppressor retinoblastoma (Rb) and CDK8.

Hahn and colleagues identified CDK8 in a screen for proteins that regulate β-catenin function in colon cancer. Depletion of CDK8 reduced β-catenin-dependent transcriptional activity, whereas overexpression of CDK8 induced oncogenic transformation. Furthermore, a kinase-dead CDK8 mutant blocked β-catenin-driven oncogenic transformation, indicating that kinase activity is necessary for β-catenin activation.

CDK8 facilitates transcription by linking transcription factors to RNA polymerase II. Chromatin immunoprecipitation experiments revealed that CDK8 bound to the cMYC proto-oncogene promoter — a known target for β-catenin/TCF. Depletion of CDK8 blocked the ability of β-catenin to bind to the cMYC promoter; however, dominant-negative TCF only partially suppressed CDK8 oncogenic transformation, suggesting that CDK8 has other targets in addition to β-catenin.

Dyson and colleagues uncovered a link between E2F1 and β-catenin signaling by showing that the Drosophila β-catenin and TCF orthologs blocked E2f1-induced apoptosis. In the Saos2 osteocarcinoma cell line, E2F1 — but not other E2F activators — blocked the transcriptional activity of a constitutively active β-catenin mutant. Furthermore, exogenous expression of E2F1 led to decreased cMYC mRNA and protein levels. Interestingly, E2F1 also upregulated the expression of proteins that mediate β-catenin degradation, such as AXIN1/2 and SIAH1.

The tumor suppressor Rb represses E2F and impedes cell cycle progression. Whereas Rb is inactivated in many human cancers, colorectal cancers instead frequently harbor copy number gains in the Rb locus, suggesting that Rb is required to repress E2F1 and permit β-catenin signaling in this setting. Indeed, depletion of Rb blocked β-catenin-dependent transcription and inhibited cell proliferation in a colon cancer cell line, which was rescued by exogenous expression of constitutively active β-catenin.

Intriguingly, CDK8 interacted with and phosphorylated E2F1 in vitro, and the CDK8/E2F1 complex bound to the cMYC promoter. CDK8 also blocked the inhibitory effect of E2F1 on β-catenin-stimulated transcription in a kinase-dependent manner. Together, these studies show that CDK8 and E2F1 have antagonistic roles in β-catenin-stimulated transcription, and provide a mechanism by which increased CDK8 levels impede E2F1-mediated downregulation of β-catenin in colon cancer. Thus, pharmacological agents that disable CDK8 and rescue E2F1 activity may be of therapeutic value in colon cancer.

Emily J. Chenette
Signaling Gateway

Original References:
Firestein, R. et al.
CDK8 is a colorectal cancer oncogene that regulates β-catenin activity
Nature 455, 547-551 (2008)
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Morris, E. J., Ji, J.-Y. et al.
E2F1 represses β-catenin transcription and is antagonized by both pRB and CDK8.
Nature 455, 552-556 (2008)
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