Restoration of transforming growth factor beta signaling pathway in human prostate cancer cells suppresses tumorigenicity via induction of caspase-1-mediated apoptosis

Previous studies (Y, Guo and N. Kyprianou, Cell Growth Diff,, 9: 185-193, 1 998) have demonstrated that overexpression of transforming growth factor (T GF) beta type II receptor (T beta RII) gene in human prostate cancer cells LNCaP, which are refractory to TGF-beta 1 and lack T beta RII receptor expr ession, can restore TGF-beta 1 sensitivity and suppress in vitro tumorigeni c growth by inhibiting cell proliferation. In the present study, we investi gated the effect of T beta RII receptor overexpression in LNCaP cells on ap optosis induction and tumorigenicity, The ability of LNCaP cells that overe xpress T beta RII to undergo apoptosis in response to TGF-beta 1 was examin ed by DNA fragmentation and terminal transferase-mediated dUTP-biotin end l abeling analysis, To explore the potential apoptotic nature of TGF-beta 1-m ediated antitumor effect against human prostate cancer cells, the expressio n of apoptotic proteins bcl-2 and bar was examined by Western blot analyses , The significance of caspase 1 in TGF-beta 1-mediated apoptosis was also d etermined by examining the expression and activation of caspase 1 by revers e transcription-PCR and Western blot analyses, respectively, Comparative an alysis of tumorigenicity of the parental LNCaP and T beta RII-overexpressin g clones in severely combined immunodeficient mice revealed a significant s uppression of tumor growth in T beta RII transfectant clones compared with parental LN-Cap cells and neomycin-control clones (P < 0.05). A significant ly higher incidence of endogenous apoptosis was observed in T beta RII clon e-61-derived tumor compared with the parental LNCaP tumors. This induction of apoptosis in the LNCaP tumors with restored TGF-beta 1 signaling was ass ociated with decreased bcl-2 expression, increased bar, and caspase-1 immun oreactivty. Moreover, an increased expression of the cyclin-dependent kinas e inhibitor p27(Kip1) was detected in T beta RII-overexpressing tumors comp ared with the parental tumors. LNCaP T beta RII transfectant cells exhibite d a marked induction of apoptosis, paralleled with a decreased bcl-2 expres sion In response to TGF-beta 1 treatment in vitro. This TGF-beta 1-mediated apoptosis induction in T beta RII transfectant cells was significantly pro tected by the caspase-1 inhibitor (zVAD-fmk) in a dose-dependent manner. Fu rthermore, a significant temporal induction of caspase-1 mRNA and protein e xpression was detected in T beta RII cells in response to TGF-beta 1 treatm ent. Our findings suggest that restoration of TGF-beta 1 signaling suppress es tumorigenicity of human prostate cancer cells by inducing apoptosis, pot entially via a caspase-1-mediated pathway.