Abstract
Gypenosides (Gyps) are triterpenoid saponins contained in an extract from Gynostemma pentaphyllum Makino and reported to induce apoptosis in human hepatoma cells through Ca(2+)-implicated endoplasmic reticulum (ER) stress and mitochondria-dependent pathways. The mechanism underlying the Gyp-increased intracellular Ca(2+) concentration ([Ca(2+)]i) is unclear. Here, we examined Gyp-induced necrosis and apoptosis in human hepatoma HepG2 cells. Gyp-induced apoptotic cell death was accompanied by a sustained increase in [Ca(2+)]i level. Gyp-increased [Ca(2+)]i level was partly inhibited by removal of extracellular Ca(2+) by Ca(2+) chelator EGTA, store-operated Ca(2+) channel (SOC) inhibitor 2- aminoethoxydiphenyl borate (2-APB), and ER Ca(2+)-release-antagonist 3,4,5-trimethoxybenzoic acid 8-(diethylamino) octyl ester (TMB-8). The strongest inhibitory effect was observed with TMB-8. EGTA, 2-APB, and TMB-8 also protected against Gyp-induced apoptosis in HepG2 cells. The combination of 2-APB and TMB-8 almost completely abolished the Gyp-induced Ca(2+) response and apoptosis. In contrast, the sarco/endoplasmic-reticulum-Ca(2+)-ATPase (SERCA) inhibitor thapsigargin slightly elevated Gyp-induced [Ca(2+)]i increase and apoptosis in HepG2 cells. Exposure to 300 μg/mL Gyp for 24 hours upregulated protein levels of inositol 1,4,5-trisphosphate receptor and SOC and downregulated that of SERCA for at least 72 hours. Thus, Gyp-induced increase in [Ca(2+)]i level and consequent apoptosis in HepG2 cells may be mainly due to enhanced Ca(2+) release from ER stores and increased store-operated Ca(2+) entry.