The intracellular location of NADH:cytochrome b5 reductase modulates the cytotoxicity of the mitomycins to Chinese hamster ovary cells

NADH:cytochrome b(5) reductase activates the mitomycins to alkylating intermediates in vitro. To investigate the intracellular role of this enzyme in mitomycin bioactivation, Chinese hamster ovary cell transfectants overexpressing rat NADH:cytochrome b(5) reductase were generated. An NADH:cytochrome b(5) reductase-transfected clone expressed 9-fold more enzyme than did parental cells; the levels of other mitomycin-activating oxidoreductases were unchanged. Although this enzyme activates the mitomycins in vitro, its overexpression in living cells caused decreases in sensitivity to mitomycin C in air and decreases in sensitivity to porfiromycin under both air and hypoxia, Mitomycin C cytotoxicity under hypoxia was similar to parental cells. Because NADH:cytochrome b(5) reductase resides predominantly in the mitochondria of these cells, this enzyme may sequester these drugs in this compartment, thereby decreasing nuclear DNA alkylations and reducing cytotoxicity. A cytosolic form of NADH:cytochrome b(5) reductase was generated. Transfectants expressing the cytosolic enzyme were restored to parental line sensitivity to both mitomycin C and porfiromycin in air with marked increases in drug sensitivity under hypoxia. The results implicate NADH:cytochrome b(5) reductase in the differential bioactivation of the mitomycins and indicate that the subcellular site of drug activation can have complex effects on drug cytotoxicity.