Based on the concept that activated oxygen species are causally involved in Adriamycin toxicity, endogenous antioxidant defenses are expected to be important determinants of cellular Adriamycin tolerance. We have tested this prediction by making use of an oxygen-resistant variant subline of Chinese hamster ovary cells (CHOr), which is characterized by increased levels of glutathione, copper- and zinc-containing superoxide dismutase, manganese-containing superoxide dismutase, catalase, and glutathione peroxidase. The levels of antioxidant defenses in wild-type CHO (CHOs) cells were within the range reported for human tumor cell lines, except for catalase, which was comparatively high. Oxygen-tolerant CHOr cells, which contained 4.3-fold more catalase activity than CHOs cells, were proportionally more resistant to H2O2, indicating that catalase activity in wild-type CHOs cells was still limiting H2O2 tolerance. The Adriamycin sensitivity of CHOs cells was compared to that of CHOr cells by clonogenic cell survival. After correcting for differential drug uptake in CHOs and CHOr cells, no significant difference in Adriamycin sensitivity could be detected. Furthermore, drug-induced cyanide-resistant oxygen consumption and electron spin resonance data indicated that both cell strains were equally efficient in reducing Adriamycin to its semiquinone radical and in generating activated oxygen species through oxidation-reduction cycling. These results indicate that Adriamycin tolerance of wild-type CHO cells, as determined by clonogenic cell survival, is not limited by endogenous glutathione, copper- and zinc-containing superoxide dismutase, manganese-containing superoxide dismutase, catalase, or glutathione peroxidase.
Financial support from The Netherlands Cancer Foundation (Queen Wilhelmina Fund). Part of these results was presented in preliminary form at a Conference on Superoxide and Superoxide Dismutase, Rome, 1985 (23).