Hydrogen peroxide mediates the radiationinduced mutator phenotype in mammalian cells

Chronic oxidative stress has been associated with genomic instability following exposure to ionizing radiation. However, results showing direct causal linkages between specific ROS (re-active oxygen species) and the ionizing radiation-induced mutator phenotype are lacking. The present study demonstrates that ionizing radiation-induced genomically unstable cells (charac-terized by chromosomal instability and an increase in mutation and gene amplification frequencies) show a 3-fold increase in steady-state levels of hydrogen peroxide, but not superoxide. Furthermore, stable clones isolated from parallel studies showed significant increases in catalase and GPx (glutathione peroxidase) activity. Treatment of unstable cells with PEG-CAT (polyethylene glycol-conjugated catalase) reduced the mutation frequency and mutation rate in a dose-dependent fashion. In addition, inhibiting catalase activity in the stable clones using AT (3-amino-triazole) increased mutation frequency and rate. These results clearly demonstrate the causal relationship between chronic oxidative stress mediated by hydrogen peroxide and the mutator phenotype that persists for many generations following exposure of mammalian cells to ionizing radiation. Key words: catalase, genomic instability, ionizing radiation, mutation rate, oxidative stress