Function and Regulation of a TGF-Β in the Prostate
My laboratory focuses on the role of transforming growth factor-beta (TGF-Β) as a tumor suppressor and regulator of growth, apoptosis and androgenic responses in the prostate. An important role for TGF-Β in androgenic control of the prostate has been previously implicated by a number of in vivo studies which showed androgens negatively regulate expression of TGF-Βs, TGF-Β receptors (TΒRI and TΒRII) and activation of rSmads (Smads 2 and 3) in the prostate. The normal cellular responses to TGF-Βs are lost or altered during prostatic carcinogenesis, concomitant with loss of TGF-Β receptor levels and loss of androgen dependence, implicating a role for TGF-Β signaling in regulation of androgen dependence and tumor suppression. To test this latter role, we disrupted the function of TGF-Β receptors in non-tumorigenic rat prostate epithelial cell lines, NRP-152 and DP-153, by over-expressing a truncated TΒRII that functions as a dominant-negative receptor (DNR). Overexpression of this DNR by retroviral transduction resulted in the malignant transformation of NRP-152 and DP-153 as assessed by s.c. tumor in athymic mice. These data fully support TGF-Β's role as a tumor suppression of the prostate.
Using primary prostate epithelial cell lines developed in my laboratory, we were first to report that TGF-Β can directly induce apoptosis of isolated prostatic cells in culture. Since then an important focus of our work has been to understand how TGF-Β induces apoptosis and how this mechanism is altered in prostate cancer. Using our cell culture system, we have demonstrated that TGF-Β induces the release of cytochrome c and the subsequent activation of caspases -9 and -3. We showed that TGF-Β may promote apoptosis by down-regulating the expression of the anti-apoptotic proteins Bcl-xl and survivin through distinct mechanisms. Survivin is an inhibitor of apoptosis protein (IAP) whose overexpression is tightly correlated with the aggressiveness of prostate cancer and is believed to play a role in chemo- and hormone-resistance in advanced prostate cancer. We have shown that an intact TGF-Β signaling pathway in pre-neoplastic prostate epithelial cells suppresses the activity of the survivin gene promoter through activating the retinoblastoma protein (Rb) and/or other pocket proteins (by a Smad-dependent mechanism), which then interact with CHR and CDE elements in the proximal region of the survivin promoter.
A second focus of this laboratory is to investigate the mechanisms governing loss of tumor suppression by TGF-Β during prostate cancer progression. Along these lines we have provided evidence that activation of the androgen receptor (AR) and the IGF-I/ PI3K/Akt/mTOR pathway, inactivation of the retinoblastoma protein (Rb) and induced expression of the LIM domain protein Hic-5 in prostate cancer may disrupt the tumor suppressive function of TGF-Β. Some of these pathways may also be important contributors to switching the function of TGF-Β from a tumor suppressor to that of an oncogene, a phenomenon associated with late-stage cancers. Understanding these mechanisms is likely to aid in the development of therapeutic strategies for the intervention of prostate cancer.
We recently made a novel discovery that Smad2 functions as a critical tumor suppressor in prostate epithelial cells. We showed that silencing the expression of Smad2 alone (by lentiviral-mediated shRNA) can promote the malignant transformation of NRP-152 cells. This has opened investigation of the role of Smad2 as a suppressor of the prostate, and we are now exploring potential changes in the posttranslational modification of Smad2 that may occur during prostate carcinogenesis.