The influence of Na+,K(+)-pump blockade on doxorubicin-mediated cytotoxicity and DNA strand breakage in human tumor cells.

We have previously shown that blockade of the Na+,K(+)-pump by the cardiac glycoside ouabain produces doxorubicin resistance and decreases topoisomerase II-mediated DNA strand breakage in hamster cells. To determine if this were a general phenomenon, the effect of pump blockade on doxorubicin resistance was assessed in three human tumor cell lines: A549 lung and HT29 colon adenocarcinomas and U1 melanoma. When cells were exposed to 1 microM ouabain prior to and during incubation with doxorubicin, cytotoxicity was markedly reduced. Ouabain had no effect on either the influx or the efflux of doxorubicin. However, all cell lines showed a ouabain-induced decrease in doxorubicin-induced topoisomerase-mediated DNA strand breakage (SSB). These data suggest that blockade of the Na+,K+ pump decreases doxorubicin cytotoxicity in human tumor cells by inhibiting topoisomerase-mediated SSB. Furthermore, they indicate that altered ionic gradients are a potential cause of resistance to drugs that use topoisomerase II as a target.