AbstractBACKGROUNDProstate cancer is a worldwide significant health care problem, due to its high incidence and mortality. In particular, androgen‐independent tumors have the worst prognosis, because they are refractory to almost all kinds of available therapy. Hence, there is the need of new treatment opportunities targeting androgen‐independent, growth factor‐mediated, tumor signaling. One of these new promising opportunities is vitamin D3 and its related analogues.METHODSWe investigated the effect of a vitamin D3 analogue, analogue (V), on proliferation of several human prostate cancer cells in basal condition and after treatment with KGF, one of the intraprostatic growth factors that might participate in the progression of prostate cancer. In addition, in the androgen‐independent cell line DU 145, we also studied the effect of analogue (V), KGF, and their mutual interaction on protein tyrosine phosphorylation, bcl‐2 expression and apoptosis.RESULTSOverall, we found that analogue (V) dose‐dependently decreased basal and KGF‐induced prostate cancer cell growth, although to a different extent. Maximal effect was obtained in DU 145 cells. In these cells, KGF stimulated tyrosine phosphorylation of a protein corresponding to its receptor, induced bcl‐2 expression, and prolonged cell survival. Analogue (V) not only counteracted all these KGF‐mediated events, but also decreased basal bcl‐2 expression, therefore, allowing DU 145 cells to undergo an apoptotic program.CONCLUSIONSOur results indicated that in prostate cancer cells analogue (V) decreased basal and KGF‐induced cell proliferation. This effect, at least in DU 145 cells, is in part mediated by negative interactions with cell survival and KGF signaling. Prostate 50: 15–26, 2002. © 2002 Wiley‐Liss, Inc.