2′-epi-2′-O-Acetylthevetin B extracted from seeds of Cerbera manghas L. induces cell cycle arrest and apoptosis in human hepatocellular carcinoma HepG2 cells

2′-epi-2′-O-Acetylthevetin B (GHSC-74) is a cardiac glycoside isolated from the seeds of Cerbera manghas L. We have demonstrated that GHSC-74 reduced the viability of HepG2 cells in a time- and dose-dependent manner. The present study was designed to explore cellular mechanisms whereby GHSC-74 led to cell cycle arrest and apoptosis in HepG2 cells. Cell cycle flow cytometry demonstrated that HepG2 cells treated with GHSC-74 (4 μM) resulted in S and G2 phase arrest in a time-dependent manner, as confirmed by mitotic index analysis. G2 phase arrest was accompanied with down-regulation of CDC2 and Cyclin B1 protein. Furthermore, GHSC-74-induced apoptotic killing, as demonstrated by DNA fragmentation, DAPI staining, and flow cytometric detection of sub-G1 DNA content in HepG2 cells. GHSC-74 treatment resulted in a significant increase in reactive oxygen species, activation of caspase-9, dissipation of mitochondrial membrane potential, and translocation of apoptosis-inducing factor (AIF) from the mitochondrion to the nucleus in HepG2 cells. Nevertheless, after GHSC-74 exposure, no significant Fas and FasL up-regulation was observed in HepG2 cells by flow cytometry. In addition, treatment with antioxidant N-acetyl-l-cysteine (NAC) and broad-spectrum caspase inhibitor z-VAD-fmk partially prevented apoptosis but did not abrogate GHSC-74-induced nuclear translocation of AIF. In conclusion, we have demonstrated that GHSC-74 inhibited growth of HepG2 cells by inducing S and G2 phase arrest of the cell cycle and by triggering apoptosis via mitochondrial disruption including both caspase-dependent and -independent pathways, and ROS generation.