Several reports from various laboratories, including ours, find that transforming growth factor-β (TGF-β) functions as a tumor suppressor of the prostate. Tumor suppression mediated by TGF-β largely depends on its ability to induce growth arrest and/or apoptosis of prostate epithelial cells. Although TGF-β directly activates both Smads2 and 3, the relative importance of these Smads in the transcriptional control of TGF-β-regulated cell growth especially with respect to apoptosis is still unclear. In this light, we mainly used two rat prostate epithelial cell lines (NRP-152 and NRP-154) that are extremely sensitive to TGF-β-induced apoptosis. Here, we provided evidence that Smad2 is predominantly involved in TGF-β-mediated transcriptional and apoptotic responses in NRP-152 cells, whereas Smad3 contributes more than Smad2 to TGF-β responses in NRP-154 cells. Such a discrepancy may depend on the differentiation states of the epithelial cells. Furthermore, silencing Smad2 alone caused malignant transformation of NRP-152 cells, indicating that Smad2 is essential for TGF-β-mediated tumor suppression.
Upon examination of Smad2/3-regulated genes involved in TGF-β-induced apoptosis, we found survivin to be transcriptionally downregulated by the TGF-β/Smad pathway. Survivin is a well-known pro-survival and anti-apoptotic protein which is overexpressed in many types of human cancers. Here we demonstrated that TGF-β/Smad signaling triggers Rb activation and the subsequent binding of Rb and E2F4 proteins onto the survivin gene promoter, thus repressing survivin transcription. Further functional analysis revealed critical roles of survivin in suppressing TGF-β- and chemotherapeutic drug-induced apoptosis. Importantly, our data also suggest that regulation of survivin by TGF-β is lost during prostate tumor progression. We thus propose a novel TGF-β/Smad/Rb/survivin signaling pathway with a putative role in prostate homeostasis and tumor prevention.
Taken as a whole, TGF-β-activated Smad2 and Smad3 play diverse roles in mediating TGF-β’s transcriptional and apoptotic responses in prostate epithelial cells. We also identified survivin as a transcriptional target of TGF-β/Smad signaling in apoptosis. Considering the complexity of TGF-β signaling in carcinogenesis, our study to distinguish the roles of Smad2 versus Smad3 and future endeavors to uncover more Smad-regulated apoptosis-related genes, are likely to contribute to the rational design of therapeutic interventions that specifically restore TGF-β*#8217;s tumor suppressive functions.