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 (LY181984), 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.