B4G2 Induces Mitochondrial Apoptosis by the ROS-Mediated Opening of Ca2+-Dependent Permeability Transition Pores

Abstract

Background/Aims: Hepatocellular carcinoma (HCC) is the most common type of liver cancer. At present, only sorafenib is approved to treat HCC. In this study, we found that a 23-hydroxybetulinic acid derivative, B4G2, exhibited potent antiproliferative activity in HCC cell lines. Methods: We used four HCC cell lines (HepG2, HepG2/ADM, Hep3B and Bel-7402) to evaluate the anti-tumour activity and explore underlying mechanisms by which B4G2 induces apoptosis. Results: Among these cell lines, HepG2 showed the highest sensitivity to B4G2. HepG2 cells treated with B4G2 showed a depolarized mitochondrial membrane potential, released cytochrome c, activated caspase-9 and caspase-3 and cleaved poly ADP-ribose polymerase (PARP). However, Z-VAD-FMK, a pan-caspase inhibitor, did not attenuate B4G2-induced apoptosis, implying that the induction of mitochondrial apoptosis by B4G2 may be independent of caspases. Moreover, pre-treatment with MgCl₂, a blocker of Ca²⁺-dependent permeability transition (PT) pores, attenuated the depolarization of the mitochondrial potential and decreased the population of apoptotic cells, indicating that B4G2-induced apoptosis was partly dependent on the opening of the Ca²⁺-dependent PT pores. B4G2 also increased the levels of intracellular calcium and reactive oxygen species (ROS). Furthermore, an ROS scavenger, N-acetyl-cysteine (NAC), markedly decreased the accumulation of intracellular calcium and apoptosis. Conclusion: This is the first demonstration that B4G2 inhibits the growth of HCC cells and induces mitochondrial apoptosis in hepatocellular carcinoma cells by the ROS-mediated opening of Ca²⁺-dependent permeability transition pores.