Radiation-induced apoptosis of human prostate cancer cells is independent of mutant p53 overexpression

Background: Previous studies have demonstrated that androgen-independent prostate cancer cells undergo apoptosis response to ionizing irradiation. The p53 protein controls cell cycle arrest and apoptosis by acting as a checkpoint control that halts the cell cycle in G1, while DNA damage is present. In this study the effect of overexpression of mutant p53 protein, on radiation-induced apoptotic cell death of human prostate cancer cells PC-3 was investigated. Materials and Methods: PC-3 cells were transfected with the plasmid encoding the mutant p53 sequence, and the neomycin resistance gene. Selected transfectant clones, were characterized at the molecular level (gene integration, and level of mRNA and protein expression) and cloned transfectants expressing high levels of p53 protein were treated with increasing doses of ionizing irradiation. The cellular response to radiation was determined on the basis of: a) clonogenic survival (colony forming ability of irradiated cells); b) induction of apoptosis as determined by the terminal transferase assay; c) apoptotic DNA fragmentation; and d) induction of expression of genes associated with prostate-apoptosis. Results: Both mutant p53 transfectant and parental PC-3 cells underwent apoptosis in response to ionizing irradiation following similar kinetics of induction of DNA fragmentation. In addition, the magnitude of induction of expression of prostate apoptosis associated genes, SGP-2 and TGF-β, was similar in the mutant p53 overexpressing and parental PC-3 cells and coincidental with DNA fragmentation. Conclusions: These findings seriously challenge the involvement of p53 in radiation-induced apoptosis in human prostate cancer cells and suggest that p53 mutations provide no selective advantage in the development of radioresistance of prostate tumor cells within the context of p53-independent apoptotic pathway.