Caspase-4 deficiency impairs nuclear actin-mediated DNA double-strand break repair and increases radiosensitivity in cancer cells

Radiotherapy exerts its cytotoxic effects primarily through DNA double-strand breaks (DSBs) and is a cornerstone in cancer treatment. The efficiency of DSB repair is an important determinant of radiosensitivity. Caspase-4 (CASP4) is frequently overexpressed in human malignancies. It has been implicated in actin cytoskeletal remodeling; however, its role in nuclear actin dynamics and DNA repair remains unclear. In this study, we demonstrated that CASP4 knockout (KO) sensitizes cells to ionizing radiation by disrupting nuclear actin-mediated DNA repair. CASP4 KO HepG2 and NCI-H292 cells exhibited reduced colony formation following irradiation, along with persistent 53BP1 foci, which are indicative of impaired DSB resolution. Although CASP4 KO cells showed increased cytoplasmic F-actin levels, nuclear actin translocation was decreased, suggesting a defect in actin shuttling. To visualize nuclear actin, we developed a new measurement system, which enables the quantitation of nuclear actin levels. CASP4-deficient cells exhibited reduced nuclear actin signals despite comparable actin expression. Forced nuclear actin expression rescued the DNA repair defect in CASP4 KO cells. These results indicate a novel function of CASP4 in facilitating DNA repair by regulating the balance of cytoplasmic and nuclear actin, thereby enhancing radioresistance. Overall, we uncover a novel role of CASP4 in modulating nuclear actin dynamics to promote DNA DSB repair and suggest attenuating CASP4 in tumor cells may block tumor cell resistance to ionizing radiation.