Elevation of glutathione in RAW 264.7 cells by low-dose γ-ray irradiation and its responsibility for the appearance of radioresistance

We examined the relationship between the induction of glutathione (GSH) level in macrophage-like RAW264.7 cells by a low (adapting) dose γ-rays and the cell damage caused by a lethal dose of γ-rays at various intervals after the adapting dose. The reduced glutathione (GSH) level increased soon after exposure of the cells to 25 cGy of γ-rays, peaked between 3 hr and 6 hr, and returned almost to the zero time (0 hr) level by 24 hr post-irradiation. Cell damage was assessed by measuring the ³H-thymidine (³H-TdR) incorporation into cellular DNA. γ-Ray irradiation produced a dose-dependent cell damage in RAW cells, causing about 40% and 60% inhibition of ³H-TdR incorporation into DNA at 1.0 Gy and 2.0 Gy, respectively, as compared with non-irradiated cells. Treatment with the adapting dose of 25 cGy at 1 hr or 24 hr before the lethal irradiation was ineffective. However, pre-irradiation with 25 cGy at 3 hr or 6 hr prior to lethal irradiation inhibited the decrease of ³H-TdR incorporation into DNA, indicating a protective effect. GSH exogenously added to the medium also inhibited the cell damage induced by lethal doses of γ-Rays in a dose-dependent manner. These results indicate that the induction of endogenous GSH in living cells immediately following low-dose γ-Ray irradiation is at least partially responsible for the appearance of radioresistance to a subsequent lethal dose of radiation, and may make it possible to use higher doses of radiation in radiotherapy for tumor patients.