Abstract
DNA double strand break (DSB) is the main factor for radiation induced cell death. Cells repair those lethal DSBs with two major repair pathways; homologous recombination repair (HRR) and non-homologous end joining (NHEJ). HRR is the error free repair process and efficient in late S to G2-phases of cell cycle. Rad51 proteins, essential for the HRR, form nucleoprotein filaments on the single strand DNA induced by DSB to search homologous pairing for repair and strand exchange reactions between homologous sequences. HRR deficiency enhances cellular radiosensitivity.
The aim of this study is to evaluate the radiosensitization of Hsp90 inhibitor 17-Allylamino-17-demethoxygeldanamycin (17AAG), specifically the effect of 17AAG on the DNA DSB repair machinery. Our constant field gel electrophoresis studies indicated that pretreatment with 17AAG for 24 hours inhibited radiation induced DSB repair in two cancer cell lines (DU145 and SQ-5). The treatment of 17AAG alone leads to the reduction of Rad51 protein expression by western blotting, and the combined treatment with X-irradiation caused a delay in the formation of nuclear Rad51 foci by immuno-staining. These results suggest that 17AAG affects the key protein(s) for HRR, resulting in the radiosensitization of tumor cells. Our data show for the first time that 17AAG is a DNA DSB repair inhibitor, predominantly affecting the HRR pathway. The combination of ionizing radiation and 17AAG would be a useful method for the tumor radio-therapy.
