Impairment of homologous recombination directed DNA repair in prostate cancer cells exposed to anchorage independence

Abstract

9638 Background: The majority of patients with androgen dependant prostate cancer develop a transition to highly invasive metastatic disease, which is refractory to hormonal therapy. This complication relies on the acquisition of new properties, which allow cancer cells to invade other tissues. One mechanism via which rapid genetic changes could occur is a failure of systems controlling stability of the genome, including DNA repair. In parallel, metastatic cells are temporarily exposed to anchorage-independence -a condition in which cellular interactions with extracellular matrix proteins are restricted. The purpose of this study was to evaluate whether and how anchorage-independence could predispose prostate cancer cells to develop genomic instability. Methods: DNA repair mechanisms: non-homologous end joining (NHEJ) and homologous recombination directed DNA repair (HRR) were evaluated in human prostate cancer cell lines: DU145, LNCaP and PC-3. Cell survival was evaluated by trypan blue exclusion, cell cycle distribution by flow-cytometry, and DNA repair proteins by western blotting and immunocytofluorescence. Results: The sensitivity of prostate cancer cells to irradiation, cisplatin and mitomycin increased several fold in anchorage-independence. This enhanced drug sensitivity was associated with a severe impairment of HRR. The mechanism involved Rad51, which is the major enzymatic component of HRR. The protein level of Rad51 and its recruitment to DNA breaks were both attenuated. Rad51 deficiency in anchorage-independence was not associated with Rad51 promoter activity, and was not compensated by a constitutive overexpression of Rad51 cDNA. However, Rad51 protein was efficiently restored in the presence of the proteosom inhibitor epoxomycin, or when cells from suspension cultures were allowed to reattach. Conclusions: Anchorage-independence sensitizes prostate cancer cells to genotoxic agents; however, it also attenuates a faithful component of DNA repair by targeting the stability of Rad51. This may contribute to the development of the metastatic phenotype in cells, which survive genotoxic treatment. No significant financial relationships to disclose.