Comparison of biological effects of DNA damage induced by ionizing radiation and hydrogen peroxide in CHO cells

Purpose: Free OH radicals are considered to be the common mediator of DNA d amage after ionizing radiation and oxidative stress. In particular, double- strand breaks (dsb) have a major impact on cell killing after irradiation, while the mechanism of cell killing is less clear for oxidative injury. The latter nor only affects DNA, but also equally other cell compartments, suc h as membranes and mitochondria, which may trigger cell death. This study i ntended to clarify the relationship between DNA damage induction, repair an d cell inactivation for hydrogen peroxide and ionizing radiation. Materials and methods: Chinese hamster ovary (CHO) cells were treated with H2O2 in serum-free medium in combination with/without X-irradiation. DNA da mage was measured using the alkaline unwinding method or neutral constant-f ield gel electrophoresis. Cell survival was recorded using the colony-forma tion assay. Results: Hydrogen peroxide induced a large number of single-strand breaks ( ssb>36 000/cell) without impairing cell survival. This number reached a max imum (36 Gy-equiv. at 3 X 10(-4) mol/dm(3)) without further increase after higher concentrations. Repair kinetics of ssb were similar to those after i rradiation. Dsb were found only after very high concentrations of H2O2 (>3 X 10(-2) mol/dm(3)), which is different from irradiation which generated ss b and dsb in the same dose range. A linear-quadratic increase of dsb was fo und with increasing concentrations of H2O2 suggesting a single or a pairwis e action of OH radicals to form a dsb. After either irradiation or peroxide treatment cell killing was observed only after doses which also allowed ds b detection. The number of dsb calculated per lethal event was in the same range but slightly higher after irradiation (1.7-fold) than after H2O2 trea tment. Conclusions: Cell killing after irradiation or hydrogen peroxide appears to be due to dsb, whereas cells withstand large numbers of single-strand lesi ons and other types of non-DNA damage occurring at lower concentrations of hydrogen peroxide. The number of ssb saturates at intermediate concentratio ns of H2O2 suggesting thai a limited amount of chromatin-bound metal ions i s available for OH radical generation.