Superoxide mediates the toxicity of paraquat for cultured mammalian cells.

To establish some of the necessary steps in the toxic action of paraquat for cultured mammalian cells, we isolated paraquat-resistant HeLa cells after lethal increments in concentration of paraquat in the medium. The paraquat-resistant cells had increased the cellular content of both the Mn-containing and the CuZn-containing superoxide dismutases. The effect of paraquat on the consumption of oxygen by the wild-type and the resistance cells was similar; in both cases exposure to paraquat for 24 or more hours produced similar proportions of cyanide-resistant consumption of oxygen, suggesting that paraquat entered both cells, underwent reduction, and donated electrons to molecular oxygen. When cultivated for 5 months in the absence of paraquat the paraquat-resistant cells maintained the increased cellular content of superoxide dismutases and remained resistant to paraquat. This observation suggested that resistance to paraquat might be caused by enrichment for the two superoxide dismutases and, further, that the increased cellular content of the two enzymes was not a response to growth under stressful conditions. NIH/3T3 cells whose content of superoxide dismutase was increased by transcription of the transfected cDNA for the human CuZn superoxide dismutase were also resistant to paraquat, suggesting strongly that paraquat promotes the formation of O2- as a necessary part of its cytotoxic effects in two types of cultured mammalian cells.