Reversal of mitomycin C resistance by overexpression of bioreductive enzymes in Chinese hamster ovary cells

The clinical utility of antineoplastic agents is limited by the development of drug resistance by tumors. Mitomycin C (MC) is a bacterial product that must be enzymatically reduced to exert anticancer activity. We have demons trated that expression of the bacterial MC resistance-associated (MCRA) pro tein in Chinese hamster ovary (CHO) cells confers profound resistance to th is antibiotic under aerobic conditions, but not under hypoxia. MCRA produce s resistance to MC by redox cycling of the activated hydroquinone intermedi ate back to the prodrug form. A CHO cell line developed by stepwise exposur e to increasing concentrations of MC likewise expressed high level resistan ce to MC in air, but not under hypoxia. The overexpression of DT-diaphorase and NADPH:cytochrome c (P-450) reductase, two enzymes known to activate MC , restored sensitivity to MC in both MCRA-transfected and drug-selected cel l lines. The level of sensitization was proportional to the quantity of enz yme activity expressed, supporting the concept that the levels of these two activating enzymes are important for sensitivity to MC. The findings of re sistance to MC in air but not under hypoxic conditions and of restoration o f sensitivity to MC by increasing levels of DT-diaphorase activity, propert ies not adequately explained by other resistance mechanisms (i.e., decrease s in MC activation, repair of DNA lesions, and/or drug efflux), support the hypothesis that a functional mammalian homologue of MCRA may be involved i n producing resistance to MC.