Isolation of Chinese Hamster Ovary Cell Mutants Deficient in Excision Repair and Mitomycin C Bioactivation

Mitomycin C (MMC), a bifunctional alkylating agent, requires meta bolic reduction to become biologically active. We have identified a series of genetically related Chinese hamster ovary cell lines which span ap proximately three orders of magnitude in the concentration of MMC required for cell killing. Many mechanisms, including drug transport, drug activation, drug detoxification, and the elimination, or repair, of drug-induced lesions, may contribute to the level of drug resistance in cells. By exploring each of the above mechanisms in the various Chinese hamster ovary cell lines, we have been able to classify these cell lines into four categories. Proceeding from least resistant to most resistant to MMC, the cell lines are: (a) proficient in the bioreduction of MMC and deficient in DNA excision repair; (b) deficient in some aspects of MMC bioreduction and deficient in repair; (c) bioreduction and repair proficient; and (aerobic conditions in CHO cells (10). It has been observed that dicumarol, an inhibitor of DT-diaphorase (11), increases MMC resistance under aerobic conditions (1, 12). Therefore, while the mechanisms of MMC bioreduction under aerobic conditions are not fully understood, they may involve DT-diaphorase, NADPH:cytochrome P-450 reductase, and xanthine oxidase. We report here the isolation of CHO cell lines, from both DNA excision repair-proficient and -deficient progenitors, that are deficient in some aspects of the bioreduction of MMC. The various combinations of excision repair ability and drug acti vation potential lead to a range of several orders of magnitude in MMC sensitivity.