OVEREXPRESSION OF BCL-2 PROTECTS PROSTATE-CANCER CELLS FROM APOPTOSISIN-VITRO AND CONFERS RESISTANCE TO ANDROGEN DEPLETION IN-VIVO

Normal (nonneoplastic) human prostatic secretory epithelial cells do n ot express the bcl-2 protein. However, a recent immunohistochemical su rvey of neoplastic human prostate tissues showed that a fraction of pr imary untreated prostate adenocarcinoma cells expressed this apoptosis -suppressing oncoprotein at significant levels (Colombel et al., Am. J . Pathol., 143: 390-400, 1993). Additionally, a number of hormone-refr actory prostatic adenocarcinomas obtained from hormonally-treated pati ents (subsequent to surgical or drug castration therapy) were examined and were found to be uniform in their elevated expression of bcl-2 on coprotein. The results of this preliminary survey imply that bcl-2 exp ression distinguishes a subgroup of primary human prostate cancers and that the expression of this protein might be a factor enabling prosta te cancer cells to survive in an androgen-deprived environment. The cu rrent study was undertaken to determine the degree to which overexpres sion of bcl-2 can protect human prostate cancer cells from apoptotic s timuli in vitro and in vivo. Human prostate cancer cells (LNCaP) were transfected with a neomycin-selectable eucaryotic expression vector co ntaining cDNA encoding human bcl-2. Transfected clonal variants that e xpress bcl-2 protein (LNCaP/bcl-2) were unaltered with regard to their basal growth rate in 10% serum-containing medium, or with regard to t heir expression of the differentiated human prostate cell gene product s prostate-specific antigen or androgen receptor protein, The bcl-2-tr ansfected clones were altered, however, with regard to their growth ra te in charcoal-stripped serum lacking dihydrotestosterone, Additionall y, in contrast to the parental or control-transfected cell lines, LNCa P/bcl-2 cells were highly resistant to a variety of apoptotic stimuli in vitro including serum starvation and 10 nM phorbol ester (phorbol 1 2-myristate 13-acetate) supplementation of the medium. Lastly, the ove rexpression of bcl-2 by these prostate cancer cells altered their tumo rigenic potential in a nude mouse assay, s.c. injections of 10(6) LNCa P/bcl-2 cells into male nude mice resulted in earlier and larger tumor formation compared to an equivalent injection of parental or control- transfected LNCaP cells, When these variant cell lines were injected i nto castrated male nude mice, only the LNCaP/bcl-2-transformed cells g ave rise to tumors, Moreover, LNCaP/bcl-2 tumors grown in intact male nude mice were refractory to the growth-inhibiting effects of castrati on demonstrated by parental LNCaP cells. Data obtained in this study d emonstrate that the bcl-2 oncoprotein can protect prostate cancer cell s from apoptotic stimuli in vitro and suggest that such protection cor relates with the ability to form hormone-refractory prostate tumors in vivo.