Mechanism of Apoptosis Induced by Diazoxide, a $K^{+}$ Channel Opener, in HepG2 Human Hepatoma Cells

The effect of diazoxide, a $K^{+}$channel opener, on apoptotic cell death was investigated in HepG2 human hepatoblastoma cells. Diazoxide induced apoptosis in a dose-dependent manner and this was evaluated by flow cytometric assays of annexin-V binding and hypodiploid nuclei stained with propidium iodide. Diazoxide did not alter intracellular $K^{+}$concentration, and various inhibitors of $K^{+}$channels had no influence on the diazoxide-induced apoptosis; this implies that $K^{+}$channels activated by diazoxide may be absent in the HepG2 cells. However, diazoxide induced a rapid and sustained increase in intracellular $Ca^{2+}$ concentration, and this was completely inhibited by the extracellular $Ca^{2+}$ chelation with EGTA, but not by blockers of intracellular $Ca^{2+}$ release (dantrolene and TMB-8). This result indicated that the diazoxide-induced increase of intracellular $Ca^{2+}$ might be due to the activation of a Ca2+ influx pathway. Diazoxide-induced $Ca^{2+}$ influx was not significantly inhibited by either voltage-operative $Ca^{2+}$ channel blockers (nifedipinen or verapamil), or by inhibitors of $Na^{+}$, $Ca^{2+}$-exchanger (bepridil and benzamil), but it was inhibited by flufenamic acid (FA), a $Ca^{2+}$-permeable nonselective cation channel blocker. A quantitative analysis of apoptosis by flow cytometry revealed that a treatment with either FA or BAPTA, an intracellular $Ca^{2+}$ chelator, significantly inhibited the diazoxide-induced apoptosis. Taken together, these results suggest that the observed diazoxide-induced apoptosis in the HepG2 cells may result from a $Ca^{2+}$ influx through the activation of $Ca^{2+}$-permeable non-selective cation channels. These results are very significant, and they lead us to further suggest that diazoxide may be valuable for the therapeutic intervention of human hepatomas.