AbstractBACKGROUNDProstate tumors initially regress in response to androgen‐ablation therapy. However, most cancers eventually relapse with an androgen‐depletion‐independent (ADI) phenotype that is often more aggressive than the original androgen‐dependent (AD) tumor. Importantly, most relapsed tumors still rely upon androgen receptor (AR) activity for proliferation and survival. The cellular Fas/FasL‐associated death domain protein‐like inhibitory protein (FLIP) inhibits activation of procaspase‐8 by death receptor‐mediated signaling at the cell surface. In the current study, we examined the androgenic regulation of FLIP and its contribution to protecting prostate cancer cells from death receptor‐mediated apoptosis.METHODSFLIP expression in tissues from intact and castrated rats as well as androgen‐treated prostate cancer cell lines (LNCaP, C4‐2, LNCaP‐Rf, and DU‐145) was monitored via Real‐Time RT‐PCR and immunoblot. Induction of apoptosis by TRAIL, the death receptor ligand, was determined via microscopic observation and cell counting of fragmented nuclei following fixation and staining with Hoechst 33285.RESULTSFLIP mRNA and protein expression was reduced following castration in multiple rat tissues, including dorsolateral prostate and seminal vesicles. Androgenic induction of FLIP mRNA and protein was observed in isogenic AD LNCaP and ADI LNCaP‐Rf cells, but not the isogenic ADI C4‐2 cell line. Protection from TRAIL‐induced apoptosis by androgen was completely blocked when LNCaP‐Rf cells were depleted of endogenous FLIP via siRNA transfection.CONCLUSIONSAndrogenic protection from TRAIL‐induced apoptosis is predominantly via enhanced transcription of FLIP in prostate cancer cells. Loss of androgen‐sensitivity in ADI prostate cancer cells highlights this pathway as a potential target for future therapy of prostate cancer. Prostate 68: 1696–1706, 2008. © 2008 Wiley‐Liss, Inc.