TRAF6, a key regulator of TGF?-induced oncogenesis in prostate cancer

Abstract: Prostate cancer is the most common cancer in men, with the incidence rapidly increasing in Europe over the past two decades. Reliable biomarkers for prostate cancer are currently unavailable. Thus, there is an urgent need for improved biomarkers to diagnose prostate cancer at an early stage and to determine the best treatment options. Higher expression of transforming growth factor-? (TGF?) has been reported in patients with aggressive cancer.TGF? is a multifunctional cytokine that acts as a tumor suppressor during early tumor development, and as a tumor promoter at later stages of cancer. TGF? signals through the canonical Smad or non-Smad cascade via TGF? type II and type I receptors. The TGF? signaling cascade is regulated by various post-translational modifications of its key components. The present investigation aimed to identify a potential function of TRAF6 in TGF?-induced responses in prostate cancer.The first two articles of this thesis unveil the proteolytic cleavage of TGF? type I receptor (T?RI), and the biological importance of the liberated T?RI intracellular domain (T?RI-ICD) in the nucleus. We found that tumor necrosis factor receptor-associated factor 6 (TRAF6) polyubiquitinates T?RI, which leads to cleavage of T?RI by tumor necrosis factor alpha converting enzyme (TACE) in a protein kinase C zeta (PKC?)-dependent manner. Following ectodomain shedding, T?RI undergoes a second cleavage by presenilin 1 (PS1), which liberates T?RI-ICD. T?RI-ICD translocates to the nucleus, where it regulates its own expression as well as expression of the pro-invasive gene Snail1, thereby promoting invasion. We further found that T?RI-ICD associates with Notch intracellular domain (NICD) to drive expression of the pro-invasive gene Snail1, as well as Notch1 ligand Jag1.The third article provides evidence that TRAF6 promotes Lys63-linked polyubiquitination of T?RI at Lys178 in a TGF?-dependent manner. T?RI polyubiquitination was found to be a prerequisite for T?RI nuclear translocation, and thus for regulation of the genes involved in cell cycle, differentiation, and invasion of prostate cancer cells.In the fourth article we investigated the role of the pro-invasive gene Snail1 in TGF?-induced epithelial-to-mesenchymal transition (EMT) in prostate cancer cells.