Lactate Enhances Non-Homologous End Joining Repair and Chemoresistance Through Facilitating XRCC4-LIG4 Complex Assembly in Ovarian Cancer.

Background: Non-homologous end joining (NHEJ) is a crucial pathway for repairing DNA double-strand breaks and a key contributor to chemoresistance in cancer. The assembly of the DNA Ligase IV (LIG4)-XRCC4 complex is essential for NHEJ fidelity, however, the regulatory mechanisms governing this complex in cancer remain poorly understood. This study aims to investigate whether and how lactate, a key metabolic byproduct of the Warburg effect, regulates the XRCC4-LIG4 complex and influences chemoresistance. Methods: The functional role of lactate in NHEJ was assessed using DNA repair reporter assays in ovarian cancer cells. Protein-protein interactions were examined through co-immunoprecipitation and pull-down assays. The molecular mechanism of lactate's action was delineated using a combination of site-directed mutagenesis, in vitro binding assays, and molecular docking. Finally, the physiological relevance of lactate-mediated NHEJ was validated in a preclinical ovarian cancer mouse model treated with cisplatin. Results: We demonstrated that lactate enhances NHEJ repair efficiency and confers resistance to DNA-damaging chemotherapeutics. Mechanistically, lactate directly binds to XRCC4 at key residues, including Y66, E55, and S110, thereby strengthening the XRCC4-LIG4 association. This interaction is independent of protein lactylation. In vivo studies confirmed that lactate-driven NHEJ promotes chemoresistance in ovarian cancer. Conclusions: Our findings reveal lactate as a novel metabolic regulator of the NHEJ pathway by directly allosterically modulating the XRCC4-LIG4 complex. This work establishes a direct molecular link between the Warburg effect and DNA repair-driven chemoresistance, offering new insights into potential therapeutic strategies for ovarian cancer.