MafG, A Novel Target of FXR that Regulates Bile Acid Homeostasis

Abstract

Bile acids, the major products of hepatic cholesterol catabolism, are indispensable for lipid absorption and cholesterol metabolism. Systemic bile acid metabolism is tightly regulated by various nuclear receptors, including farnesoid X receptor (FXR; Nat Rev Gastroenterol Hepatol 2014;11:55–67). In humans, approximately 500 mg of cholesterol are converted into bile acids every day via the classical pathway initiated by CYP7A1 or an alternative pathway through CYP7B1 (Nat Rev Drug Discov 2008;7:678–693). CYP8B1 is responsible for cholic acid synthesis (J Clin Invest 2002;110:1191–1200). FXR signaling regulates bile acid synthetic pathways by repressing both Cyp7a1 and Cyp8b1 expression. FXR-mediated small heterodimer partner (Shp) induction is a mechanism of the repression of Cyp7a1 expression. However, bile acid synthetic pathways are also regulated by SHP-independent mechanisms. In a recent article published in Cell Metabolism, de Aguiar Vallim et al discovered a novel FXR-regulated transcriptional repressor, MafG (V-Maf avian musculoaponeurotic fibrosarcoma oncogene homolog G) as an important regulator of negative feedback mechanism of bile acids. MafG belongs to small MAF proteins, which is a bidirectional transcriptional regulator. On 1 hand, MafG interacts with transcription factors Nrf2 and HIF1α to increase their transcriptional activity (Biochim Biophys Acta 2008; 1783:1847–1856; FEBS Lett 2008;582:2357–2364). On the other hand, MafG binds to DNA as a homodimer (or sometimes as a heterodimer), which leads to transcriptional repression owing to a lack of transactivation domain (Mol Cell Biol 2006;26:4652–4663). To find the repression mechanism of bile acid synthesis genes by FXR, de Aguiar Vallim et al focused on transcriptional repressors, such as MafG. The ChIP-seq analysis of hepatic FXR from 2 different datasets determined four putative transcriptional repressor genes expressed in the liver MafG, cysteine-rich intestinal protein 2 (Crips2), zinc finger protein 385a (Zfp385a), and Shp. In vivo overexpression of MafG, introduced by injection of adenoviral vector, repressed the expression of Cyp8b1, but not Cyp7a1, in the liver. Crip2 or Zfp385a overexpression did not change either Cyp7a1 or Cyp8b1 expression. Interestingly, MafG overexpression changed the composition of bile acid pool, but not the pool size. The analysis of the hepatic gene profile showed the significant reductions in the expression of bile acid synthesis genes including Cyp7b1 and Cyp27a1 by MafG overexpression. In contrast, the loss-of-gene studies using antisense oligonucleotides or in MafG heterozygous mice showed increased expression of bile acid synthesis genes including Cyp8b1, Cyp7b1, and Cyp27a1, but not Cyp7a1. This confirms that MafG is a repressor of cholic acid synthesis as well as a key regulator of bile acid metabolism. Then, the study investigated to identify MafG binding sites (MAREs) in bile acid synthetic genes through MafG ChIP-seq analysis. The ChIP-seq analysis identified MAREs at loci for multiple bile acid synthetic genes, including Cyp8b1, Cyp27a1, Cyp7b1, Acox2, Akr1d1, Akr1c14, Ntcp, Hsd17b4, and Scp2, but not Cyp7a1. Taken together, the present study demonstrates that FXR signaling regulates bile acid synthesis gene expression through induction of a transcriptional repressor MafG, a novel FXR target gene. MafG binds to MAREs on the promoter and/or intronic regions of multiple bile acid synthetic genes and suppresses their transcription, thereby regulating the bile acid synthesis.