Astin B, a Cyclic Pentapeptide from Aster Tataricus, Induces Apoptosis and Autophagy in Human Hepatic L-02 Cells

Astins (including astin B) are a class of halogenated cyclic pentapeptides isolated from the medicinal herb of Aster tataricus. However, our previous works showed that the herbal medicine was hepatotoxic in vivo, and a toxicity-guided isolation method led to the identification of a cyclopeptide astin B. Astin B is structurally similar to cyclochlorotine, a well-known hepatotoxic mycotoxin. Thus, the aim of this study was to determine the potential cytotoxic effects and the underlying mechanism of astin B on human normal liver L-02 cells. We found that astin B has hepatotoxic effects in vitro and in vivo and that hepatic injury was primarily mediated by apoptosis in a mitochondria/caspase-dependent manner. Astin B provoked oxidative stress-associated inflammation in hepatocytes as evidenced by increased levels of reactive oxygen species (ROS), reduced contents of intracellular glutathione (GSH), and enhanced phosphorylation of c-Jun N-terminal kinase (JNK). Furthermore, the mitochondria-dependent apoptosis was evidenced by the depolarization of the mitochondrial membrane potential, the release of cytochrome c into cytosol, the increased ratio of Bax/Bcl-2, and the increased activities of caspases-9 and -3. Interestingly, astin B treatment also induces autophagy in L-02 cells, characterized by acidic-vesicle fluorescence, increased LC3-II and decreased p62 expression. Autophagy is a protective mechanism that is used to protect cells from apoptosis. The presence of autophagy is further supported by the increased cytotoxicity and the enhanced cleaved caspase-3 after co-treatment of cells with an autophagy inhibitor, also by increased LC3-II and decreased p62 after co-treatment with a caspase inhibitor. Taken together, astin B, most likely together with other members of astins, is the substance that is primarily responsible for the hepatotoxicity of A.tataricus.