Abstract
Radiotherapy has been one of the most effective nonsurgical treatments for cancer. However, recurrence frequently occurs through tumor radioresistance and results in failures of clinical outcome. Therefore, the determination of underlying mechanisms of tumor radioresistance may provide novel therapeutic approaches to eradicate radioresistant tumor cells.
We have studied DNA damage response to fractionated radiation (FR), which is the common practice in radiotherapy. We have demonstrated that cells with long-term FR exposure to 0.5 Gy of X-rays twice per day for more than 31 days (31FR cells) conferred radioresistance of tumor cells with cyclinD1 overexpression. CyclinD1 overexpression was also observed in 31FR-31NR cells, 31FR cells cultured without FR for 31 days. Abundant amounts of cyclinD1 suppressed progression of S-phase, resulting in the accumulation of 31FR-31NR cells in S-phase. In addition, 31FR-31NR cells exhibited numerous gamma-H2AX foci and Rad51 foci in S-phase even on 31 days after the cessation of irradiation. Thus, DNA double strand breaks were induced in 31FR-31NR cells and homologous recombination repair (HRR) was activated. To elucidate the role of HRR on the acquired radioresistance of long-term FR cells, we used siRNA targeting Rad51 for suppression of HRR. Radioresistance of long-term FR cells was completely suppressed by down-regulation of Rad51. In conclusion, the radioresistant phenotype of tumor cells acquired by long-term radiation exposure can be controlled by suppressing HRR.
