A ginseng saponin metabolite-induced apoptosis in HepG2 cells involves a mitochondria-mediated pathway and its downstream caspase-8 activation and Bid cleavage

Abstract

20-O-(β-d-Glucopyranosyl)-20(S)-protopanaxadiol (IH901), an intestinal bacterial metabolite of ginseng saponin formed from ginsenosides Rb1, Rb2, and Rc, is suggested to be a potential chemopreventive agent. Here, we show that IH901 induces apoptosis in human hepatoblastoma HepG2 cells. IH901 led to an early activation of procaspase-3 (12 h posttreatment), and the activation of caspase-8 became evident only later (18 h posttreatment). Caspase activation was a necessary requirement for apoptosis because caspase inhibitors significantly inhibited cell death by IH901. Treatment of HepG2 cells with IH901 also induced the cleavage of cytosolic factors such as Bid and Bax and translocation of truncated Bid (tBid) to mitochondria. A time-dependent release of cytochrome c from mitochondria was observed, which was accompanied by activation of caspase-9. A broad-spectrum caspase inhibitor, N-benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone (zVAD-fmk), and a specific inhibitor for caspase-8, N-benzyloxycarbonyl-Ile-Glu-Thr-Asp-fluoromethylketone (zIETD-fmk), abrogated Bid processing and translocation, and caspase-3 activation. Cytochrome c release was inhibited by zVAD-fmk, however, the inhibition by zIETD-fmk was not complete. The activation of caspase-8 was inhibited not only by zIETD-fmk but also by zVAD-fmk. The results, together with the kinetic change of caspase activation, indicate that activation of caspase-8 occurred downstream of caspase-3 and -9. Our data suggest that the activation of caspase-8 after early caspase-3 activation might act as an amplification loop necessary for successful apoptosis. Primary hepatocytes isolated from normal Sprague–Dawley rats were not affected by IH901 (0–60 μM). The very low toxicity in normal hepatocytes and high activity in hepatoblastoma HepG2 cells suggest that IH901 is a promising experimental cancer chemopreventive agent.