Superoxide-Mediated Cytotoxicity in Superoxide Dismutase-Deficient Fetal Fibroblasts

To investigate the roles of CuZn superoxide dismutase (CuZnSOD) and Mn superoxide dismutase (MnSOD) in oxygen radical-mediated cytotoxicity and to distinguish the actions of these two enzymes, fetal fibroblasts were derived from mouse fetuses that are either deficient in CuZnSOD (Sod1−/+ and −/−) or MnSOD (Sod2−/+ and −/−) forin vitrostudies. Whereas the phenotype of theSod1mutant animals did not differ from that of their normal littermates, the growth ofSod1−/− fetal fibroblasts was only 25% of that of the −/+ and +/+ cells. On the other hand, although almost all homozygousSod2mutant animals (−/−) died within 10 days after birth, cultivation ofSod2−/− fetal fibroblasts was possible and their growth was about 60% that of −/+ and +/+ cells. When cultured cells were subjected to treatment with paraquat to assess their ability to grow in the presence of high levels of superoxide radicals,Sod1−/− cells were 80 times more sensitive andSod2−/− cells were 12 times more sensitive to paraquat than wild-type cells. In addition, whereas the loss of 50% CuZnSOD renderedSod1−/+ cells almost twice more sensitive to paraquat than +/+ cells, loss of 50% MnSOD had no effect on paraquat sensitivity. Our results suggest that CuZnSOD-deficient cells are more sensitive to oxygen toxicity than are MnSOD-deficient cells, that paraquat causes free radical-induced damage in both the mitochondria and cytoplasm, and that SOD compartmentalized in the cytosol cannot compensate for the loss of SOD in the mitochondria and vice versa.