Abstract
Reactive oxygen species (ROS) are generated within cells by ionizing radiation via primary ionizing events as well as through secondary amplification systems including metabolic synthesis. ROS not only cause dysfunction of the target molecules but also perturb intra- and inter-cellular signal transduction pathways. To elucidate the contribution of ROS to the generation of DNA damage, elicited by ionizing radiation, we compared cellular responses to hydrogen peroxide (H2O2) and X-rays in an XRCC4-deficient mutant cell line generated by gene targeting and its parental human colon tumor cell line HCT116. XRCC4-deficient cells exhibited lower survival rates and more frequent induction of chromosomal aberrations than XRCC4-proficient parental cells after exposure to either H2O2 or X-rays. Since XRCC4 is a key component of the non-homologous end-joining, a predominant repair pathway for DNA double-strand breaks (DSBs), the increase in susceptibility of XRCC4-deficient cells to these insults must be attributed to the induction of DSBs. Consistently, formation of γH2AX foci, a marker of DSBs, was observed in both cells exposed to H2O2 as well as X-rays. Curiously, most foci of γH2AX colocalized with ATM[pS1981] foci in X-irradiated cells, but did not in cells exposed to H2O2. In addition, sensitivity of cell cycle phases to the induction of chromosomal aberrations after the insults showed remarkable differences between X-irradiated and H2O2 treated cells. These results suggest the presence of different processes for DSB-induction by H2O2 and X-rays.
