TGFβ-induced invasion of prostate cancer cells is promoted by c-Jun-dependent transcriptional activation of Snail1

Abstract

High levels of transforming growth factor-β (TGFβ) correlate with poor prognosis for patients with prostate cancer and other cancers. TGFβ is a multifunctional cytokine and crucial regulator of cell fate, such as epithelial to mesenchymal transition (EMT), which is implicated in cancer invasion and progression. TGFβ conveys its signals upon binding to type I and type II serine/threonine kinase receptors (TβRI/II); phosphorylation of Smad2 and Smad3 promotes their association with Smad4, which regulates expression of targets genes, such as Smad7, p21, and c-Jun. TGFβ also activates the ubiquitin ligase tumor necrosis factor receptor-associated factor 6 (TRAF6), which associates with TβRI and activates the p38 mitogen-activated protein kinase (MAPK) pathway. Snail1 is a key transcription factor, induced by TGFβ that promotes migration and invasion of cancer cells. In this study, we have identified a novel binding site for c-Jun in the promoter of the Snail1 gene and report that the activation of the TGFβ–TRAF6–p38 MAPK pathway promotes both c-Jun expression and its activation via p38α-dependent phosphorylation of c-Jun at Ser63. The TRAF6-dependent activation of p38 also leads to increased stability of c-Jun, due to p38-dependent inactivation of glycogen synthase kinase (GSK) 3β by phosphorylation at Ser9. Thus, our findings elucidate a novel role for the p38 MAPK pathway in stimulated cells, leading to activation of c-Jun and its binding to the promoter of Snail1, thereby triggering motility and invasiveness of aggressive human prostate cancer cells.

TRAF6 promotes TGFβ-induced invasion and cell-cycle regulation via Lys63-linked polyubiquitination of Lys178 in TGFβ type I receptor

“…36,37 We have previously shown that TRAF6 in a noncanonical, Smad-independent pathway, regulates both the expression of c-Jun, a transcription factor implicated in stress responses, and p21, a classical target gene for TGFb and p53, in cells treated with TGFb. 25 As cyclin D1 has been reported to contain binding sites for AP-1 transcription factor (heterodimers of the Fos and Jun families), 38,39 c-Jun may regulate both the levels of both cyclin D1 and the pro-invasive protein Snail. 40,41 Here, we found that TRAF6, via its Lys63-linked polyubiquitination of K178 in TbRI, regulates both EMT-associated genes as well as cyclin D1 expression which might reflect that the cancer cells also become senescent due to conflicting signaling inputs.…”

“…[19][20][21][22] We recently reported a mechanism whereby TGFb promotes the invasiveness of cancer cells. [23][24][25] TGFb induces the activation of TRAF6, which ubiquitinates TbRI in a Lys63-dependent manner and promotes proteolytic cleavage of the receptor by TNF a-converting enzyme (TACE) and presenilin-1, resulting in the release of the intracellular domain (ICD) of TbRI. [23][24][25] The TbRI ICD is then translocated to the nucleus, where it associates with the transcriptional co-activator p300 and promotes tumor-cell invasion by inducing the expression of genes encoding TbRI, Snail1, and matrix metallopeptidase 2 (MMP2).…”

“…[23][24][25] TGFb induces the activation of TRAF6, which ubiquitinates TbRI in a Lys63-dependent manner and promotes proteolytic cleavage of the receptor by TNF a-converting enzyme (TACE) and presenilin-1, resulting in the release of the intracellular domain (ICD) of TbRI. [23][24][25] The TbRI ICD is then translocated to the nucleus, where it associates with the transcriptional co-activator p300 and promotes tumor-cell invasion by inducing the expression of genes encoding TbRI, Snail1, and matrix metallopeptidase 2 (MMP2). 23,24,26 In the present study, we identified the acceptor lysine in TbRI that is ubiquitinated by TRAF6, and determined the biological significance of this ubiquitination.…”

Invasive cells follow Snail’s slow and persistent pace

“…Actually, Snail1 is rapidly induced by TGF-β not only in epithelial but also in mesenchymal cells and is required for a full response to this cytokine including the activation of invasion-related genes 2 . The article by Thakur et al 3 in this issue of Cell Cycle has investigated the mechanism of activation of Snail1 transcription by TGF-β.…”

“…In parallel, TGF-β receptors activate various protein kinases by recruiting directly adaptor proteins such as Shc and ubiquitin ligases such as TRAF6. Among these protein kinases, TGF-β activates TAK1 and the downstream effector p38 MAPK, at focus in the paper by Thakur et al 3 This pathway is now shown to contribute via specific mechanisms to multiple physiological outcomes of TGF-β biology. Specifically, the TRAF6/TAK1 module provides transcriptional inputs to various gene targets, such as Snail1, that triggers cell invasion, and the cyclin-dependent kinase inhibitor p21 that feeds the cytostatic program and other pro-apoptotic genes 3 .…”