Equol induces apoptosis in human hepatocellular carcinoma SMMC-7721 cells through the intrinsic pathway and the endoplasmic reticulum stress pathway.

Equol, a microbial metabolite of the isoflavone daidzein, is currently receiving much attention because of its strong antiproliferative effect on hormone-related human breast cancer cells; however, in our previous study, we observed that racemic equol [(±)-equol] shows the highest antiproliferative effect on human hepatocellular carcinoma SMMC-7721 cells compared with other cells, including human breast cancer MCF-7 and MDA-MB-231 cell lines. In the present study, we use the SMMC-7721 cancer cell line to investigate the mechanisms of (±)-equol-induced, R-(+)-equol-induced, and S-(-)-equol-induced apoptosis. Our purpose was to provide some guidelines to introduce equol into a clinical situation. R-(+)-equol and S-(-)-equol were prepared from (±)-equol by chiral stationary phase high performance liquid chromatography. The antiproliferative effect of equol on SMMC-7721 cells was investigated by crystal violet staining. Equol-induced apoptosis was detected by acridine orange/ethidium bromide staining and by flow cytometry. Western blotting was performed to study the molecular mechanisms of equol-induced apoptosis. The results showed that (±)-equol, R-(+)-equol, and S-(-)-equol inhibited the proliferation of SMMC-7721 cells in a concentration-dependent manner. Exposure of SMMC-7721 cells to equol caused significant cell cycle arrest in the S-phase. In addition, equol was shown to induce endoplasmic reticulum stress-mediated apoptosis by activating caspase-12 and caspase-8, and by upregulating Chop and Bip. Mitochondrion-mediated apoptosis was caused by upregulation of Bax and downregulation of Bcl-2, followed by activation of caspase-9, caspase-3, and cleaved poly (ADP-ribose) polymerase, respectively. This is the first report that shows that R-(+)-equol, S-(-)-equol, and (±)-equol can induce apoptosis of human hepatocellular carcinoma SMMC-7721 cells through the intrinsic pathway and the endoplasmic reticulum stress pathway.