title: Endoplasmic Reticulum Stress and Store-Operated Calcium Entry Contribute to Usnic Acid-Induced Toxicity in Hepatic Cells.
abstract: The use of usnic acid as a weight loss agent is a safety concern due to reports of acute liver failure in humans. Previously we demonstrated that usnic acid induces apoptosis and cytotoxicity in hepatic HepG2 cells. We also demonstrated that usnic acid induces autophagy as a survival mechanism against its cytotoxicity. In this study, we investigated and characterized further molecular mechanisms underlying the toxicity of usnic acid in HepG2 cells. We found that usnic acid causes endoplasmic reticulum (ER) stress demonstrated by the increased expression of typical ER stress markers, including CHOP, ATF-4, p-eIF2α, and spliced XBP1. Usnic acid inhibited the secretion of Gaussia luciferase measured by an ER stress reporter assay. An ER stress inhibitor 4-phenylbutyrate attenuated usnic acid-induced apoptosis. Moreover, usnic acid significantly increased the cytosolic free Ca(2+) concentration. Usnic acid increased the expression of calcium release-activated calcium channel protein 1 (CRAM1 or ORAI1) and stromal interaction molecule 1, two key components of store-operated calcium entry (SOCE), which is the major Ca(2+) influx pathway in non-excitable cells, this finding was also confirmed in primary rat hepatocytes. Furthermore, knockdown of ORAI1 prevented ER stress and ATP depletion in response to usnic acid. In contrast, overexpression of ORAI1 increased ER stress and ATP depletion caused by usnic acid. Taken together, our results suggest that usnic acid disturbs calcium homeostasis, induces ER stress, and that usnic acid-induced cellular damage occurs at least partially via activation of the Ca(2+) channel of SOCE.
authors: *Division of Biochemical Toxicology, Division of Genetic and Molecular Toxicology, National Center for Toxicological Research/U.S. FDA, Jefferson, AR 72079, Tianjin Medical University General Hospital, Tianjin 300052, China and Division of Systems Biology, National Center for Toxicological Research/U.S. FDA, Jefferson, AR 72079.
authors: Si
authors: S
authors: Chen
authors: *Division of Biochemical Toxicology, Division of Genetic and Molecular Toxicology, National Center for Toxicological Research/U.S. FDA, Jefferson, AR 72079, Tianjin Medical University General Hospital, Tianjin 300052, China and Division of Systems Biology, National Center for Toxicological Research/U.S. FDA, Jefferson, AR 72079 *Division of Biochemical Toxicology, Division of Genetic and Molecular Toxicology, National Center for Toxicological Research/U.S. FDA, Jefferson, AR 72079, Tianjin Medical University General Hospital, Tianjin 300052, China and Division of Systems Biology, National Center for Toxicological Research/U.S. FDA, Jefferson, AR 72079.
authors: Zhuhong
authors: Z
authors: Zhang
authors: *Division of Biochemical Toxicology, Division of Genetic and Molecular Toxicology, National Center for Toxicological Research/U.S. FDA, Jefferson, AR 72079, Tianjin Medical University General Hospital, Tianjin 300052, China and Division of Systems Biology, National Center for Toxicological Research/U.S. FDA, Jefferson, AR 72079.
authors: Yuanfeng
authors: Y
authors: Wu
authors: *Division of Biochemical Toxicology, Division of Genetic and Molecular Toxicology, National Center for Toxicological Research/U.S. FDA, Jefferson, AR 72079, Tianjin Medical University General Hospital, Tianjin 300052, China and Division of Systems Biology, National Center for Toxicological Research/U.S. FDA, Jefferson, AR 72079.
authors: Qiang
authors: Q
authors: Shi
authors: *Division of Biochemical Toxicology, Division of Genetic and Molecular Toxicology, National Center for Toxicological Research/U.S. FDA, Jefferson, AR 72079, Tianjin Medical University General Hospital, Tianjin 300052, China and Division of Systems Biology, National Center for Toxicological Research/U.S. FDA, Jefferson, AR 72079.
authors: Hua
authors: H
authors: Yan
authors: *Division of Biochemical Toxicology, Division of Genetic and Molecular Toxicology, National Center for Toxicological Research/U.S. FDA, Jefferson, AR 72079, Tianjin Medical University General Hospital, Tianjin 300052, China and Division of Systems Biology, National Center for Toxicological Research/U.S. FDA, Jefferson, AR 72079.
authors: Nan
authors: N
authors: Mei
authors: *Division of Biochemical Toxicology, Division of Genetic and Molecular Toxicology, National Center for Toxicological Research/U.S. FDA, Jefferson, AR 72079, Tianjin Medical University General Hospital, Tianjin 300052, China and Division of Systems Biology, National Center for Toxicological Research/U.S. FDA, Jefferson, AR 72079.
authors: William H
authors: WH
authors: Tolleson
authors: *Division of Biochemical Toxicology, Division of Genetic and Molecular Toxicology, National Center for Toxicological Research/U.S. FDA, Jefferson, AR 72079, Tianjin Medical University General Hospital, Tianjin 300052, China and Division of Systems Biology, National Center for Toxicological Research/U.S. FDA, Jefferson, AR 72079 lei.guo@fda.hhs.gov.
authors: Lei
authors: L
authors: Guo
mesh_terms: D001572:Benzofurans / Q000633:toxicity*; D002118:Calcium / Q000378:metabolism*; D004721:Endoplasmic Reticulum / Q000378:metabolism*; D056945:Hep G2 Cells; D006801:Humans; D013312:Stress, Physiological
chemical_list: D001572:Benzofurans; C073339:usnic acid; D002118:Calcium
keywords: ER stress; apoptosis; liver toxicity; store operated calcium entry; usnic acid