(-)-Englerin A-evoked Cytotoxicity is Mediated by Na+ Influx and Counteracted by Na+/K+-ATPase

(-)-Englerin A ((-)-EA) has a rapid and potent cytotoxic effect on several types of cancer cell that is mediated by plasma membrane ion channels containing Transient Receptor Potential Canonical 4 protein (TRPC4). Since these channels are Ca2+ permeable it was initially thought that the cytotoxicity arose as a consequence of Ca2+ overload. Here we show that this is not the case and that the effect of (-)-EA is mediated by a heteromer of TRPC4 and TRPC1 proteins. Both TRPC4 and TRPC1 were required for (-)-EA cytotoxicity, however, although TRPC4 was necessary for the (-)-EA-evoked Ca2+-elevation, TRPC1 was not. TRPC1 either had no role or was a negative regulator of Ca2+ entry. By contrast, both TRPC4 and TRPC1 were necessary for monovalent cation entry evoked by (-)-EA and (-)-EA-evoked cell death was dependent upon entry of the monovalent cation Na+. We therefore hypothesized that Na+/K+-ATPase might act protectively by counteracting the Na+ load resulting from sustained Na+ entry. Indeed, inhibition of Na+/K+-ATPase by ouabain potently and strongly increased (-)-EA-evoked cytotoxicity. The data suggest that (-)-EA achieves cancer cell cytotoxicity by inducing sustained Na+ entry through heteromeric TRPC1/TRPC4 channels and that the cytotoxic effect of (-)-EA can be potentiated by Na+/K+-ATPase inhibition.