Xanthatin induces apoptosis by activating endoplasmic reticulum stress in hepatoma cells

Hepatocellular carcinoma (HCC) has high incidence and mortality in patients with chronic liver diseases worldwide. However, there are limited chemotherapeutic agents for HCC in clinic. Xanthatin, a natural sesquiterpene lactone, has significant antitumor activity against a variety of cancers, but little is known about its effects on HCC and the underlying mechanism. Here, we evaluated the antitumor effects of xanthatin on human hepatoma cells. We found that xanthatin caused morphological changes and reduced cell viability in three HCC cell lines in concentration- and time-dependent manners. Xanthatin at 10 μM significantly arrested cell cycle at the G2/M checkpoint, and at 40 μM significantly arrested cell cycle at the S phase in hepatoma cells. Additionally, xanthatin induced apoptosis associated with activation of caspase-3 in hepatoma cells, but did not apparently induce apoptosis in human normal LO2 hepatocytes. We also demonstrated that the three primary signaling pathways of unfolded protein response (UPR) were activated by xanthatin to different extents. Notably, the PERK/eIF-2α/ATF4 axis was most significantly activated by xanthatin. More importantly, both xanthatin and tunicamycin, an endoplasmic reticulum stress (ERS) inducing compound, increased the levels of CHOP and cleaved-caspase-3 in HepG2 cells, but their effects were significantly abolished by siRNA-mediated knockdown of CHOP. Further experiments validated that xanthatin more potently activated ATF4 by promoting its nuclear translocation in hepatoma cells. Taken together, we discovered that xanthatin induced apoptosis in human hepatoma cells by activating ERS. Our current data revealed a novel mechanism for xanthatin as a promising anti-tumor candidate for HCC therapy.