Background
Virtually all prostate cancer deaths occur due to obtaining the castration-resistant phenotype after prostate cancer cells escaped from apoptosis and/or growth suppression initially induced by androgen receptor blockade. TNF-related apoptosis-inducing ligand (TRAIL) was an attractive cancer therapeutic agent due to its minimal toxicity to normal cells and remarkable apoptotic activity in tumor cells. However, most localized cancers including prostate cancer are resistant to TRAIL-induced apoptosis, thereby creating a therapeutic challenge of inducing TRAIL sensitivity in cancer cells. Herein the effects of cyproterone acetate, an antiandrogen steroid, on the TRAIL-induced apoptosis of androgen receptor-negative prostate cancer cells are reported.
Conclusions
Our study shows a novel effect of cyproterone acetate on apoptosis pathways in prostate cancer cells and raises the possibility that a combination of TRAIL with cyproterone acetate could be a promising strategy for treating castration-resistant prostate cancer.
Methods
Cell apoptosis was assessed by both annexin V/propidium iodide labeling and poly (ADP-ribose) polymerase cleavage assays. Gene and protein expression changes were determined by quantitative real-time PCR and western blot assays. The effect of cyproterone acetate on gene promoter activity was determined by luciferase reporter assay.
Results
Cyproterone acetate but not AR antagonist bicalutamide dramatically increased the susceptibility of androgen receptor-negative human prostate cancer PC-3 and DU145 cells to TRAIL-induced apoptosis but no effects on immortalized human prostate stromal PS30 cells and human embryonic kidney HEK293 cells. Further investigation of the TRAIL-induced apoptosis pathway revealed that cyproterone acetate exerted its effect by selectively increasing death receptor 5 (DR5) mRNA and protein expression. Cyproterone acetate treatment also increased DR5 gene promoter activity, which could be abolished by mutation of a consensus binding domain of transcription factor CCAAT-enhancer-binding protein homologous protein (CHOP) in the DR5 gene promoter. Cyproterone acetate increases CHOP expression in a concentration and time-dependent manner and endoplasmic reticulum stress reducer 4-phenylbutyrate could block cyproterone acetate-induced CHOP and DR5 up-regulation. More importantly, siRNA silencing of CHOP significantly reduced cyproterone acetate-induced DR5 up-regulation and TRAIL sensitivity in prostate cancer cells. Conclusions: Our study shows a novel effect of cyproterone acetate on apoptosis pathways in prostate cancer cells and raises the possibility that a combination of TRAIL with cyproterone acetate could be a promising strategy for treating castration-resistant prostate cancer.