Farnesoid X Receptor Activation by Chenodeoxycholic Acid Induces Detoxifying Enzymes through AMP-Activated Protein Kinase and Extracellular Signal-Regulated Kinase 1/2-Mediated Phosphorylation of CCAAT/Enhancer Binding Protein beta

Abstract

Farnesoid X receptor (FXR) regulates redox homeostasis and elicits a cytoprotective effect. CCAAT/enhancer binding protein-beta (C/EBP beta) plays a role in regulating the expression of hepatocyte-specific genes and contributes to hepatocyte protection and liver regeneration. In view of the role of FXR in xenobiotic metabolism and hepatocyte survival, this study investigated the potential of FXR to activate C/EBP beta for the induction of detoxifying enzymes and the responsible regulatory pathway. Chenodeoxycholic acid (CDCA), a major component in bile acids, activates FXR. In HepG2 cells, CDCA treatment activated C/EBP beta, as shown by increases in its phosphorylation, nuclear accumulation, and expression. 3-(2,6-Dichlorophenyl)-4-(3'-carboxy-2-chlorostilben-4-yl-)oxymethyl-5-isopropyl-isoxazole (GW4064), a synthetic FXR ligand, had similar effects. In addition, CDCA enhanced luciferase gene transcription from the construct containing -1.65-kb GSTA2 promoter, which contained C/EBP response element (pGL-1651). Moreover, CDCA treatment activated AMP-activated protein kinase (AMPK), which led to extracellular signal-regulated kinase 1/2 (ERK1/2) activation, as evidenced by the results of experiments using a dominant-negative mutant of AMPK alpha and chemical inhibitor. The activation of ERK1/2 was responsible for the activating phosphorylation of C/EBP beta. FXR knockdown attenuated the ability of CDCA to activate AMPK and ERK1/2 and phosphorylate C/EBP beta. Consistently, enforced expression of FXR promoted the phosphorylation of AMPK alpha, ERK1/2, and C/EBP beta, verifying that C/EBP beta phosphorylation elicited by CDCA results from the activation of AMPK and ERK1/2 by FXR. In mice, CDCA treatment activated C/EBP beta with the induction of detoxifying enzymes in the liver. Our results demonstrate that CDCA induces antioxidant and xenobiotic-metabolizing enzymes by activating C/EBP beta through AMPK-dependent ERK1/2 pathway downstream of FXR.