Induction of radioresistance by a nitric oxide-mediated bystander effect

Abstract To elucidate whether nitric oxide excreted from irradiated cells affects cellular radiosensitivity, we examined (a) the accumulation of inducible nitric oxide synthase, p53 and Hsp70, (b) the concentration of nitrite in the medium of cells after irradiation with X-rays, and (c) cellular radiosensitivity using two human glioblastoma cell lines, A-172, having the wild type p53 gene, and transfectant of A-172 cells, A-172/mp53, bearing a mutated p53 gene. Accumulation of inducible nitric oxide synthase was caused by X-ray irradiation in the mutant p53 cells but not in the wild type p53 cells. Accumulation of p53 and Hsp70 was observed in the wild type p53 cells after exposure to the conditioned medium from X-irradiated mutant p53 cells, and the accumulation was abolished by the addition of a specific nitric oxide scavenger to the medium. The radiosensitivity of wild type p53 cells was reduced when the cells were cultured in the conditioned medium from X-irradiated mutant p53 cells, as compared with the conventional fresh growth medium. These findings indicate that one of the possible mechanisms of the radiation-induced bystander effect is an intercellular signal transduction initiated by nitric oxide radicals.