The role of Smad7 and TRAF6 in Prostate Cancer Cell Invasion, Migration and Survival

Sammanfattning: Transforming growth factor (TGF) ? is a tumor suppressor during early tumor development, by inhibiting proliferation and inducing apoptosis. At later stages of cancer, it becomes a tumor promoter, and promotes tumor cell migration and invasion. TGF? signals via its type II and type I receptors to several downstream signaling pathways. In the present work we have focused on the TRAF6 (tumor necrosis factor receptor-associated factor 6)/ TAK1 (TGF? activated kinase 1) signaling pathway and the Smad7-dependent activation of p38 in prostate carcinoma cells (PC3U). We found that TGF?-induced activation of the ubiquitin ligase TRAF6 was needed for cell invasion, by a mechanism that involves activation of the metalloproteinase TNF? converting enzyme (TACE), via protein kinase C? (PKC?). TACE cleaves the T?RI, whereafter the intracellular domain (ICD) translocates to the nucleus, where it binds to the transcriptional co-activator p300 and regulates gene expression, promoting invasion. Interestingly, the translocation of the T?RI ICD was observed in several cancer cell lines and in sections of primary tumors, but not in primary prostate epithelial cells. We also found that Smad7 and adenomatous polyposis coli (APC) are important for TGF?- and epidermal growth factor (EGF)-induced cell migration in PC3U cells. TGF? induces the formation of a complex consisting of Smad7, p38, glycogene synthase kinase 3? (GSK-3?), APC and ?-catenin, which localizes to the membrane ruffles in the leading edge of migrating cells. The complex links the T?RI to the microtubule system and promotes membrane ruffling and microtubule polarization, which are known to be important for cell migration. In the EGF signaling pathway, Smad7 was found to be important for phosphorylation of the EGF receptor at Tyr1068, for the activation of p38 and JNK, and for induction of membrane ruffles. Smad7 is required for TGF?-induced activation of p38 and apoptosis. We found that Smad7 forms a complex with p38 and ataxia telangiectasia mutated (ATM), which is important for activation of p53 mediated apoptosis. Many tumor cells including the PC3U cells lack a functional p53, which is one of the reasons to why cancer cells can avoid the tumor suppressor effects of TGF?.