Role of mitochondrial DNA in cells exposed to irradiation: Generation of reactive oxygen species (ROS) is required for G2 checkpoint upon irradiation

Mitochondria have their own genome encoding subunits of the electron transport chain. Using cells lacking mitochondrial DNA (mtDNA, p(0) cells), we studied the role of mtDNA in irradiation. Loss of mtDNA inhibited cell growth and reduced the level of reactive oxygen species (ROS) as compared to p(+) cells. (+) cells were more resistant to irradiation than p(0) cells. Upon irradiation, p(0) cells showed delayed G2 arrest and decreased ability of a cell to recover from the G2 checkpoint compared to p(+) cells. Irradiation increased the generation of ROS even more in p(+) cells. Irradiation markedly increased the levels of phosphorylated forms of extracellular-regulated kinases, p42 and p44 (ERK1/2) in p(+) cells, whereas phosphorylated levels of the kinases were affected slightly in p(0) cells. Furthermore, inhibition of the ERK pathway led to a delayed G2 arrest and a delayed recovery from the arrest in irradiated p(+) cells, and treatment with NAC also induced dysfunction of the G2 checkpoint in irradiated p(+) cells. These results suggest that the accumulation of ROS potentiated ERK1/2 kinases after irradiation in p(+) cells, leading to less sensitivity to irradiation. Thus, mtDNA is important for the generation of ROS that act as second messenger.