Oxidative DNA Damage in Human Cells Induced by Paraquat

Abstract

Oxidative DNA damage was studied after exposing two human transformed cell lines (HeLa and Hep G2) and freshly isolated human peripheral lymphocytes to the herbicide, paraquat. We used the alkaline comet assay, modified by incubating nucleoid DNA with endonuclease III to detect oxidised pyrimidines, and with formamidopyrimidine glycosylase to detect 8-oxo-guanine and ring-opened purines. Paraquat induces both strand breaks and oxidised bases, the amounts of each being dependent on the concentration of paraquat and the cell type exposed. Exposure to lower concentrations of paraquat for 1 hour induced dose-dependent DNA damage in Hep G2 cells and in human peripheral lymphocytes. DNA damage was reduced at higher concentrations. Our results support the finding that paraquat induces oxidative stress but, over a certain concentration range, also stimulates antioxidant protection. Reduction of DNA damage was not found in HeLa cells after exposure for 1 hour or 24 hours. Short-term exposure to paraquat induced a moderate amount of oxidative DNA damage (mainly oxidized pyrimidines) in HeLa cells. Exposure for 24 hours induced a high proportion of oxidised bases and strand breaks. Hep G2 cells showed the greatest number of DNA strand breaks, with no sign of base oxidation.