Redox regulation in radiation-induced cytochrome c release from mitochondria of human lung carcinoma A549 cells

Mitochondria in mammalian cells are well-known to play an important role in the intrinsic pathway of genotoxic-agent-induced apoptosis by releasing cytochrome c into cytosol and to be a major source of reactive oxygen species (ROS). The aim of this study was to examine whether mitochondrial ROS are involved in radiation-induced apoptotic signaling in A549 cells. Post-irradiation treatment with N-acetyl-l-cystein (NAC) inhibited cytochrome c release from mitochondria but did not affect expression levels of Bcl-2, Bcl-XL and Bax, suggesting that late production of ROS triggered cytochrome c release. Experiments using DCFDA (a classical ROS fluorescence probe) and MitoAR (a novel mitochondrial ROS probe) demonstrated that intracellular and mitochondrial ROS were enhanced 6 h after X irradiation. Furthermore, the O2- production ability of mitochondria isolated from A549 cells was evaluated by ESR spectroscopy combined with a spin-trapping reagent (CYPMPO). When isolated mitochondria were incubated with NADH, succinate and CYPMPO, an ESR spectrum due to CYPMPO–OOH was detected. This NADH/succinate-dependent O2- production from mitochondria of irradiated cells was significantly increased in comparison with that of unirradiated cells. These results indicate that ionizing radiation enhances O2- production from mitochondria to trigger cytochrome c release in A549 cells.