Mitochondrial Dysfunction Predisposes Radioresistant Insect Cells to Ionizing Radiation by Impairing DNA Repair

Radioresistant insect cell lines, such as Spodoptera frugiperda Sf9 cells, are well-established eukaryotic model systems for investigating cellular responses to ionizing radiation (IR). Mitochondria are central to these responses due to their roles in reactive oxygen species (ROS) production and in regulating apoptotic signaling pathways. In this study, we compared the radiation responses of parental Sf9 cells with those of a mitochondrial dysfunction variant, Sf9N, to elucidate the contribution of mitochondria to the intrinsic radioresistance of lepidopteran insect cells. Both cell types were exposed to γ-radiation (1000 and 2000 Gy), and several endpoints were assessed, including apoptosis, oxidative stress, mitochondrial function, DNA damage and repair kinetics, and the involvement of the mitochondrial permeability transition pore (mPTP). Sf9N cells exhibited increased radiosensitivity, characterized by greater mitochondrial membrane potential (MMP) collapse and elevated cardiolipin oxidation compared with Sf9 cells. DNA fragmentation analysis revealed enhanced apoptotic features in Sf9N cells, including a higher frequency of Type-C comets. Kinetic analysis further demonstrated more efficient DNA repair in Sf9 cells, whereas Sf9N cells showed persistent DNA damage over time. Importantly, pretreatment with cyclosporin A (CsA), a known mPTP inhibitor, preserved MMP integrity and significantly improved post-irradiation viability in Sf9N cells, implicating mPTP opening as a critical mediator of IR-induced cytotoxicity. The protective effect of CsA highlights the key role of mitochondrial regulation in determining cellular radiosensitivity. Collectively, these findings reveal distinct mitochondrial and nuclear responses to radiation stress and provide new mechanistic insights into the determinants of radiosensitivity in insect cells.