Oleandrin-mediated inhibition of human tumor cell proliferation: importance of Na,K-ATPase α subunits as drug targets

Cardiac glycosides such as oleandrin are known to inhibit the Na,K-ATPase pump, resulting in a consequent increase in calcium influx in heart muscle. Here, we investigated the effect of oleandrin on the growth of human and mouse cancer cells in relation to Na,K-ATPase subunits. Oleandrin treatment resulted in selective inhibition of human cancer cell growth but not rodent cell proliferation, which corresponded to the relative level of Na,K-ATPase alpha 3 subunit protein expression. Human pancreatic cancer cell lines were found to differentially express varying levels of alpha 3 protein, but rodent cancer cells lacked discernable expression of this Na,K-ATPase isoform. A correlation was observed between the ratio of alpha 3 to alpha 1 isoforms and the level of oleandrin uptake during inhibition of cell growth and initiation of cell death; the higher the alpha 3 expression relative to alpha 1 expression, the more sensitive the cell was to treatment with oleandrin. Inhibition of proliferation of Panc-1 cells by oleandrin was significantly reduced when the relative expression of alpha 3 was decreased by knocking down the expression of alpha 3 isoform with alpha 3 siRNA or increasing expression of the alpha 1 isoform through transient transfection of alpha 1 cDNA to the cells. Our data suggest that the relative lack of alpha 3 (relative to alpha 1) in rodent and some human tumor cells may explain their unresponsiveness to cardiac glycosides. In conclusion, the relatively higher expression of alpha 3 with the limited expression of alpha 1 may help predict which human tumors are likely to be responsive to treatment with potent lipid-soluble cardiac glycosides such as oleandrin. [Mol Cancer Ther 2009;8(8):2319-28]