Carbon ion beams induce hepatoma cell death by NADPH oxidase-mediated mitochondrial damage

Mitochondria are a major source of reactive oxygen species (ROS) and are also the target of cellular ROS. ROS damage to mitochondria leads to dysfunction that further enhances the production of mitochondrial ROS. This feed-forward vicious cycle between mitochondria and ROS induces cell death. Within a few minutes of radiation exposure, NADPH oxidase is activated to elevate the ROS level. Activated NADPH oxidase might induce the feed-forward cycle of mitochondria and this is a possible mechanism for cancer cell death induced by heavy ion irradiation. We found that after 4 Gy of (12) C(6+) ion radiation of HepG2 cells, the NADPH oxidase membrane subunit gp91(phox) was not involved in enzyme activation through increased expression; however, the subunit p47(phox) was involved in activation by being translocated to the membrane. (12) C(6+) ion radiation clearly decreased the ΔΨm of HepG2 cells, increasing mitochondrial DNA damage and inducing cell death. Pretreatment with apocynin (APO, an NADPH oxidase inhibitor) effectively prevented the ΔΨm decrease, mitochondrial DNA damage, and cell death induced by radiation. However, these protective effects were not observed with APO treatment after irradiation exposure. These data demonstrated that NADPH oxidase activation was an initiator in mitochondrial damage. Once mitochondria entered the feed-forward cycle, cell fate was no longer controlled by NADPH oxidase. Only antioxidants that targeted mitochondria such as MitoQ could break the cycle and release cells from death.