2′-Epi-2′-O-acetylthevetin B induces apoptosis partly via Ca2+-mediated mitochondrial pathway in human hepatocellular carcinoma HepG2 cells

2′-Epi-2′-O-acetylthevetin B (GHSC-74), a cardiac glycoside, can be isolated from the seeds of Cerbera manghas L. We demonstrated that GHSC-74 reduced the viability of HepG2 cells in a time- and dose-dependent manner, and efficiently induced apoptosis without significantly decreasing the viability of Chang human liver cells and Swiss albino 3T3 fibroblasts, as indicated by annexin-V/PI binding assay and Hoechst 33342 staining. In addition, stimulation of HepG2 cells with GHSC-74 induced a series of intracellular events: (1) loss of mitochondrial membrane potential; (2) sustained elevation of cytosolic [Ca2+]; and (3) downregulation of Bcl-2. BAPTA-AM, a cytosolic Ca2+ chelator, partly suppressed cell death and prevented mitochondrial membrane potential from losing in GHSC-74-treated HepG2 cells. In contrast, EGTA, an extracellular Ca2+ chelator, exhibited a weaker effect as compared to that of BAPTA-AM. Taken together, the Ca2+-mediated mitochondrial pathway was found to be involved in GHSC-74-induced HepG2 cell apoptosis.