Apoptosis-mediated cytotoxicity of ouabain, digoxin and proscillaridin A in the estrogen independent MDA-MB-231 breast cancer cells

We examined the effects of three cardiac glycosides, ouabain, digoxin and proscillaridin A, on the proliferation of estrogen independent MDA-MB-231 breast cancer cells. In terms of reduction in cell viability, the compounds rank for both 24 h and 48 h of incubation in MDA-MB-231 cells in the order proscillaridin A > digoxin - ouabain. Digoxin for 24 h and 48 h of incubation in MDA-MB-231 cells proved to be only slightly more potent than ouabain, with IC50 values of 122 +/- 2 and 70 +/- 2 nM, respectively, compared to 150 +/- 2 and 90 +/- 2 nM for ouabain. In contrast, proscillaridin A, was much more active and showed a high level of cytotoxic potency, IC50 51 +/- 2 and 15 +/- 2 nM for 24 h and 48 h of incubation, respectively. The concentrations of digoxin, ouabain and proscillaridin A needed to inhibit [3 H]thymidine incorporation into DNA by 50% (IC50) in MDA-MB-231 cells for 24 h of incubation were found to be 124 +/- 2 nM, 142 2 nM, and 48 +/- 2 nM, respectively. In the present study, we demonstrated that ouabain, digoxin, and proscillaridin A induce apoptosis in MDA-MB-231 cells by increasing free calcium concentration and by activating caspase-3.