Bile Acid Synthesis Gene Expression Research Articles

Global chemical effects of the microbiome include new bile-acid conjugations.

A mosaic of cross-phylum chemical interactions occurs between all metazoans and their microbiomes. A number of molecular families that are known to be produced by the microbiome have a marked effect on the balance between health and disease1-9. Considering the diversity of the human microbiome (which numbers over 40,000operational taxonomic units10), the effect of the microbiome on the chemistry of an entire animal remains underexplored. Here we use mass spectrometry informatics and data visualization approaches11-13 to provide an assessment of the effects of the microbiome on the chemistry of an entire mammal by comparing metabolomics data from germ-free and specific-pathogen-free mice. We found that the microbiota affects the chemistry of all organs. This included the amino acid conjugations of host bile acids that were used to producephenylalanocholic acid, tyrosocholic acid and leucocholic acid, which have not previously been characterized despite extensive research on bile-acid chemistry14. These bile-acid conjugates were also found in humans, and were enriched in patients with inflammatory bowel disease or cystic fibrosis. These compounds agonized the farnesoidX receptor in vitro, and mice gavaged with the compounds showed reduced expression of bile-acid synthesis genes in vivo. Further studies are required to confirm whether these compounds have a physiological role in the host, and whether they contribute to gut diseases that are associated with microbiome dysbiosis.

Deficiency of cholesterol 7α-hydroxylase in bile acid synthesis exacerbates alcohol-induced liver injury in mice.

Alcoholic fatty liver disease (AFLD) is a major risk factor for cirrhosis‐associated liver diseases. Studies demonstrate that alcohol increases serum bile acids in humans and rodents. AFLD has been linked to cholestasis, although the physiologic relevance of increased bile acids in AFLD and the underlying mechanism of increasing the bile acid pool by alcohol feeding are still unclear. In this study, we used mouse models either deficient of or overexpressing cholesterol 7α‐hydroxylase (Cyp7a1), the rate‐limiting and key regulatory enzyme in bile acid synthesis, to study the effect of alcohol drinking in liver metabolism and inflammation. Mice were challenged with chronic ethanol feeding (10 days) plus a binge dose of alcohol by oral gavage (5 g/kg body weight). Alcohol feeding reduced bile acid synthesis gene expression but increased the bile acid pool size, hepatic triglycerides and cholesterol, and inflammation and injury in wild‐type mice and aggravated liver inflammation and injury in Cyp7a1‐deficient mice. Interestingly, alcohol‐induced hepatic inflammation and injury were ameliorated in Cyp7a1 transgenic mice. Conclusion: Alcohol feeding alters hepatic bile acid and cholesterol metabolism to cause liver inflammation and injury, while maintenance of bile acid and cholesterol homeostasis protect against alcohol‐induced hepatic inflammation and injury. Our findings indicate that CYP7A1 plays a key role in protection against alcohol‐induced steatohepatitis. (Hepatology Communications 2018;2:99–112)

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

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.

Requirement for MLL3 in p53 Regulation of Hepatic Expression of Small Heterodimer Partner and Bile Acid Homeostasis

The histone H3-lysine-4 methyltransferase mixed-lineage leukemia 3 (MLL3) belongs to a large complex that functions as a coactivator of multiple transcription factors, including the bile acid (BA)-activated nuclear receptor, farnesoid X receptor (FXR), a critical player in BA homeostasis. BA-activated FXR induces hepatic expression of small heterodimer partner (SHP), which in turn suppresses expression of BA synthesis genes, Cyp7a1 and Cyp8b1. Thus, MLL3(Δ/Δ) mice that express a catalytically inactive mutant form of MLL3 display increased BA levels. Recently, we have discovered a distinct regulatory pathway for BA homeostasis, in which p53 independently up-regulates SHP expression in the liver. Here, we show that the MLL3 complex is also essential for p53 transactivation of SHP. Although activated p53 signaling in MLL3(+/+) mice results in decreased BA levels through hepatic up-regulation of SHP, these changes are abolished in MLL3(Δ/Δ) mice. For both HepG2 cells and mouse liver, we also demonstrate that p53 directs the recruitment of different components of the MLL3 complex to the p53-response elements of SHP and that p53-dependent H3-lysine-4-trimethylation of SHP requires MLL3. From these results, we conclude that both FXR- and p53-dependent regulatory pathways for SHP expression in BA homeostasis require the MLL3 complex; thus, the MLL3 complex is likely a master regulator of BA homeostasis. Using a common coregulator complex for multiple transcription factors, which independently control expression of the same gene, might be a prevalent theme in gene regulation and may also play critical roles in assigning a specific biological function to a coregulator complex.