The Defect of Ku70 Affects Sensitivity to X-Ray and Radiation-Induced Caspase-Dependent Apoptosis in Lung Cells

The DNA repair protein Ku70 is a key player in chemoresistance to anticancer agents (e.g., etoposide) or radioresistance. The responses of different organs to radiation vary widely and likely depend on the cell population in the organs. Previously, we established and characterized Ku70-deficient murine lung epithelial (Ku70 -/- MLE) cells and found that these cells are more sensitive than Ku70 +/- MLE cells (control cells) to X-irradiation, as determined by clonogenic survival assay; however, the mechanism underlying this sensitivity remains unclear. In this study, we examined the mechanism by which X-irradiation triggers the death of Ku70 -/- MLE cells. Our results showed that Ku70 -/- MLE cells were more sensitive to radiation-induced apoptosis than control cells, although X-irradiation activated caspase-3 and caspase-7, and cleaved PARP in both cell lines. We also examined the expression level of phosphorylated H2AX (γH2AX), which is a marker of DSB, and observed the phosphorylation of H2AX and the elimination of γH2AX in both cell lines after X-irradiation. The elimination in Ku70 -/- MLE cells was slower than that in control cells, suggesting that DSB repair activity in the Ku70 -/- MLE cells is lower than that in control cells. These findings suggest that Ku70 might play a key role in the inhibition of apoptosis through the DSB repair pathway in lung epithelial cells. Our findings also suggest that these cell lines might be useful for the study of Ku70 functions and the Ku70-dependent DSB repair pathway in lung epithelial cells.