Antioxidant enzymes and the mechanism of the bystander effect induced by ultraviolet C irradiation of A375 human melanoma cells

摘要

Irradiated cells generate dynamic responses in non-irradiated cells; this signaling phenomenon is known as the bystander effect (BE). Factors secreted by the irradiated cells communicate some of these signals. Conditioned medium from UVC-irradiated A375 human melanoma cells was used to study the BE. Exposure of cells to conditioned medium induce cell-cycle arrest at the G2/M transition. Although conditioned medium treatment, by itself, did not alter cell viability, treated cells were more resistant to the lethal action of UVC or H_2O_2. This protective effect of conditioned medium was lost within 8 h. Apoptotic or autophagic cell death was not involved in this resistance. Exposure to conditioned medium did not influence the rate of DNA repair, as measured by NAD~+ depletion. The activities of catalase and superoxide dismutase were elevated in cells exposed to conditioned medium, but returned to normal levels by 8h post-treatment. These results indicate a close correlation between BE-stimulated antioxidant activity and cellular sensitivity. Cell-cycle arrest and stimulation of antioxidant activity may account for the resistance to killing that was observed in bystander cells exposed to UVC or H_2O_2 treatment and are consistent with the role of the BE as a natural defense function triggered by UVC irradiation.