Overexpression of bcl-2 Protects Prostate Cancer Cells from Apoptosis in Vitro and Confers Resistance to Androgen Depletion in Vivo1

Normal (nonneoplastic) human prostatic secretory epithelial cells do not express the hcl-2 protein. However, a recent immunohistochemical .survey of neoplastic human prostate tissues showed that a fraction of primary untreated prostate adenocarcinoma cells expressed this apoptosis-suppressing oncoprotein at significant levels (Colombe! el al.. Am. J. Pillimi., 143: 390-400, 1993). Additionally, a number of hormone-refrac tory prostatic adenocarcinomas obtained from hormonally-treated pa tients (subsequent to surgical or drug castration therapy) were examined and were found to be uniform in their elevated expression of bcl-2 oncoprotein. The results of this preliminary survey imply that bcl-2 expression distinguishes a subgroup of primary human prostate cancers and that the expression of this protein might be a factor enabling prostate cancer cells to survive in an androgen-deprived environment. The current study was undertaken to determine the degree to which overexpression of bcl-2 can protect human prostate cancer cells from apoptotic stimuli in vitro and in vivo. Human prostate cancer cells (LNCaP) were transfected with a neomycin-selectable eucaryotic expression vector containing cDNA encoding human bcl-2. Transfected clonal variants that express bcl-2 protein (LNCaP/bcl-2) were unaltered with regard to their basal growth rate in 10% serum-containing medium, or with regard to their expression of the differentiated human prostate cell gene products prostate-specific antigen or androgen receptor protein. The bcl-2-transfected clones were altered, however, with regard to their growth rate in charcoal-stripped serum lacking dihydrotestosterone. Additionally, in contrast to the paren tal or control-transfected cell lines, LNCaP/bcl-2 cells were highly resist ant to a variety of apoptotic stimuli in vitro including serum starvation and 10 MMphorbol ester (phorbol 12-myristate 13-acetate) supplementation of the medium. Lastly, the overexpression of bcl-2 by these prostate cancer cells altered their tumorigenic potential in a nude mouse assay, s.c. injections of Id1'LNCaP/bcl-2 cells into male nude mice resulted in earlier and larger tumor formation compared to an equivalent injection of pa rental or control-transfected LNCaP cells. When these variant cell lines were injected into castrated male nude mice, only the LNCaP/bcl-2transformed cells gave rise to tumors. Moreover, LNCaP/bcl-2 tumors grown in intact male nude mice were refractory to the growth-inhibiting effects of castration demonstrated by parental LNCaP cells. Data obtained in this study demonstrate that the bcl-2 oncoprotein can protect prostate cancer cells from apoptotic stimuli hi vitro and suggest that such protec tion correlates with the ability to form hormone-refractory prostate tu mors in vivo.