Cells enriched for catalase are sensitized to the toxicities of bleomycin, adriamycin, and paraquat

L cells were enriched about 100-fold for catalase by the transcription of the transfected cDNA for human catalase. In spite of substantial enrichment for catalase, the transfected cells (LFN7C/B3) were not resistant to the cytotoxicity of a variety of agents which reduce dioxygen, including paraquat, menadione, adriamycin, phenazine methosulfate, and bleomycin. Instead, they were more sensitive to paraquat, bleomycin, and adriamycin than the untransfected cells from which they derived. Desferrioxamine afforded the cells enriched for catalase modest protection from the toxicities of bleomycin, paraquat, or adriamycin, suggesting that enrichment for iron as a consequence of the increased cellular content of catalase accounted for some of the increased sensitivities. The increased sensitivity of the LFN7C/B3 cells to bleomycin and paraquat was also attributed to the ability of catalase in cells to prevent the drug-induced consumption of O2; by capturing H2O2 before it can escape the cell and converting it to O2, catalase can maintain the concentration of O2 either for repeated rounds of chemical reduction or for direct interaction with the toxin. As predicted by this formulation, the toxicities of paraquat and of bleomycin were increased in an atmosphere of 40% dioxygen in comparison with an atmosphere of 20% dioxygen.

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.

Superoxide mediates the toxicity of paraquat for Chinese hamster ovary cells

The roles of superoxide and H2O2 in the cytotoxicity of paraquat were assessed in Chinese hamster ovary cells. Neither catalase nor superoxide dismutase inhibited the loss of ability to form colonies when added to the medium. When introduced into the cells, superoxide dismutase but not catalase inhibited the toxicity of paraquat. That superoxide dismutase acted by its known catalytic action is shown by the loss of inhibition when the enzyme was inactivated by H2O2 before being introduced into the cells. The lack of inhibition by catalase, by dimethyl sulfoxide, and by desferoxamine suggests that the toxicity is not mediated by a reaction between H2O2 and superoxide to engender the hydroxyl radical. Exposure of Chinese hamster ovary cells to paraquat may be a suitable means to determine the effects of superoxide anion in cultured cells and the ways in which cells can resist this toxic action.