Activation of the Trancription Factor Nrf2 prevents Lithocholic acid-induced Hepatotoxicity
Edugie J. Ekuase, Jie Liu, and Curtis D. Klaassen
University of Kansas, School of Medicine, Kansas City, KS, 66160
Oxidative stress triggers a host of responses by activating the nuclear factor erythroid-derived 2- like 2 (Nrf2), which leads to the up-regulation of cytoprotective genes. Lithocholic acid (LCA), a secondary bile acid that is formed by intestinal bacteria from chenodeoxycholic acid, has been linked to liver injury. The importance of Nrf2 in LCA hepatotoxicity is not known. To investigate the role of Nrf2, we developed a Nrf2-gene dose-response model, namely Nrf2-null mice, wild-type mice, Kelch-like ECH associating protein (Keap1)-knockdown (Keap1-KD) mice with enhanced Nrf2 activation, and Keap1-hepatocyte knockout (Keap1-HKO) mice with maximum hepatic Nrf2 activation. These four groups of mice were fed 0.4% LCA and on the seventh day, serum and livers were collected. Serum alanine aminotransferase (ALT), a biomarker of liver injury, was increased in Nrf2-null and wild-type mice, but much less of an increase was observed in Keap1-KD and Keap1-HKO mice. Liver histopathological analysis showed increased cell proliferation, inflammation, and extensive necrosis in Nrf2-null mice and wild-type mice but much less in Keap1-KD and Keap1-HKO mice. Hepatic mRNAs of the bile acid biosynthetic gene (Cyp8b1) was decreased markedly by LCA in the Nrf2-null and wild-type mice, but was near control values in Keap1-KD mice. In addition, LCA increased the mRNAs of the pro-inflammation genes (Egr-1, Mip2, IL-1β, mKC), cell adhesion genes (Icam-1, Vcam-1), and apoptotic genes (Noxa, Bax, Bcl-xl, Mcl) in Nrf2-null mice, but less or not at all in mice with over expression of Nrf2. In conclusion, Nrf2 activation protects against LCA-induced liver toxicity probably through induction of antioxidant defense genes and decreased inflammation.