ER stress suppresses DNA double-strand break repair and sensitizes tumor cells to ionizing radiation by stimulating proteasomal degradation of Rad51

•Tunicamycin selectively suppresses Rad51 protein levels.•Tunicamycin stimulates Rad51 degradation via the 26S proteasome.•Tunicamycin impairs DSB repair and enhances radiosensitivity.•Glucose deprivation down-regulates Rad51 expression and DSB repair. In this study, we provide evidence that endoplasmic reticulum (ER) stress suppresses DNA double-strand break (DSB) repair and increases radiosensitivity of tumor cells by altering Rad51 levels. We show that the ER stress inducer tunicamycin stimulates selective degradation of Rad51 via the 26S proteasome, impairing DSB repair and enhancing radiosensitivity in human lung cancer A549 cells. We also found that glucose deprivation, which is a physiological inducer of ER stress, triggered similar events. These findings suggest that ER stress caused by the intratumoral environment influences tumor radiosensitivity, and that it has potential as a novel target to improve cancer radiotherapy.