Published data indicate that antiprogestins and antiestrogens could inhibit prostate cancer cell growth in vitro and in vivo. The main objective of the present studies was to explore the role of bcl(2) and TGFbeta(1) for induction of apoptosis in LNCaP prostate cancer cells growing in culture as a treatment response to the antiprogestin, mifepristone, and the antiestrogen, 4-hydroxytamoxifen. In vitro cell viability (cytotoxicity), DNA fragmentation, and changes in the expression of bcl(2) and TGFbeta(1) proteins were assessed using the sulforhodamine B protein dye-binding assay, specific ELISA, and competitive inhibition assays. Both steroid antagonists induced a significant time- and dose-dependent cell growth inhibition (cytotoxicity). This inhibition of viable cells was associated with a significant increase in DNA fragmentation (apoptosis), downregulation of bcl(2), and induction of TGFbeta(1) protein. Abrogation of the mifepristone- and 4-hydroxytamoxifen-induced cytotoxicity by TGFbeta(1)-neutralizing antibody and by the addition of mannose-6-phosphate confirmed the correlation between induction of active TGFbeta(1) and subsequent prostate cancer cell death. The effect of mifepristone was not significantly reduced or prevented by occupying the progesterone or glucocorticoid receptors by their corresponding high-affinity native ligands. On the contrary, the effect of a combination of mifepristone with progesterone or hydrocortisone on the increase in DNA fragmentation, bcl(2) downregulation, and induction of TGFbeta(1) protein was additive and significantly different (P < 0.05) from the effect of mifepristone monotherapy. Our data suggest that mifepristone and tamoxifen are effective inducers of apoptosis and may represent nonandrogen-ablation, novel therapeutic approaches to overcome a potential intrinsic apoptosis resistance of androgen-independent prostate cancer cells. Copyright 2000 Wiley-Liss, Inc.