Is the drug-responsive NADH oxidase of the cancer cell plasma membrane a molecular target for adriamycin?

Enhanced growth inhibition and antitumor responses to adriamycin have been observed repeatedly from several laboratories using impermeant forms of adriamycin where entry into the cell was greatly reduced or prevented. Our laboratory has described an NADH oxidase activity at the external surface of plasma membrane vesicles from tumor cells where inhibition by an antitumor sulfonylurea, N-(4-methylphenylsulfonyl)-N'-(4-chlorophenyl)urea (LY 181984), and by the vanilloid, capsaicin (8-methyl-N-vanillyl-6-noneamide) correlated with inhibition of growth. Here we report that the oxidation of NADH by isolated plasma membrane vesicles was inhibited, as well, by adriamycin. An external site of inhibition was indicated from studies where impermeant adriamycin conjugates were used. The EC50 for inhibition of the oxidase of rat hepatoma plasma membranes by adriamycin was several orders of magnitude less than that for rat liver. Adriamycin cross-linked to diferric transferrin and other impermeant supports also was effective in inhibition of NADH oxidation by isolated plasma membrane vesicles and in inhibition of growth of cultured cells. The findings suggest the NADH oxidase of the plasma membrane as a growth-related adriamycin target at the surface of cancer cells responsive to adriamycin. Whereas DNA intercalation remains clearly one of the principal bases for the cytotoxic action of free adriamycin, this second site, possibly related to a more specific antitumor action, may be helpful in understanding the enhanced efficacy reported previously for immobilized adriamycin forms compared to free adriamycin.