Metabolic effects of intestinal absorption and enterohepatic cycling of bile acids

The classical functions of bile acids include acting as detergents to facilitate the digestion and absorption of nutrients in the gut. In addition, bile acids also act as signaling molecules to regulate glucose homeostasis, lipid metabolism and energy expenditure. The signaling potential of bile acids in compartments such as the systemic circulation is regulated in part by an efficient enterohepatic circulation that functions to conserve and channel the pool of bile acids within the intestinal and hepatobiliary compartments. Changes in hepatobiliary and intestinal bile acid transport can alter the composition, size, and distribution of the bile acid pool. These alterations in turn can have significant effects on bile acid signaling and their downstream metabolic targets. This review discusses recent advances in our understanding of the inter-relationship between the enterohepatic cycling of bile acids and the metabolic consequences of signaling via bile acid-activated receptors, such as farnesoid X nuclear receptor (FXR) and the G-protein-coupled bile acid receptor (TGR5).KEY WORDS: Bile acids, Liver, Intestine, Transporters, Lipid metabolism, Energy homeostasisAbbreviations: ACCII, acetyl-CoA carboxylase 2; APO, apolipoproteins; ASBT, apical sodium-dependent bile acid transporter; BSEP, bile salt export pump; CYP7A1, cholesterol 7α-hydroxylase; DIO2, deiodinase 2; FAS, fatty acid synthase; FGF, fibroblast growth factor; FOXO1, forkhead box protein O1; FGFR4, fibroblast growth factor receptor 4; FXR, farnesoid X-receptor; G6Pase, glucose-6-phosphatase; GLP-1, glucagon-like polypeptide-1; HNF4α, hepatocyte nuclear factor 4 alpha; IBABP, ileal bile acid binding protein; LDL, low density lipoprotein; NTCP, Na⁺-taurocholate transporting polypeptide; OATP, organic anion transporting polypeptide; OST, organic solute transporter; PEPCK, phosphoenolpyruvate carboxykinase; PGC1α, peroxisome proliferator-activated receptor gamma coactivator 1 alpha; PPAR, peroxisome proliferator-activated receptor; SHP, small heterodimer partner; SREBP1c, sterol regulatory element binding protein-1c; T4, thyroid hormone; TGR5, G-protein-coupled bile acid receptor; VLDL, very low density lipoprotein     

Bile acids and sphingosine-1-phosphate receptor 2 in hepatic lipid metabolism

The liver is the central organ involved in lipid metabolism. Dyslipidemia and its related disorders, including non-alcoholic fatty liver disease (NAFLD), obesity and other metabolic diseases, are of increasing public health concern due to their increasing prevalence in the population. Besides their well-characterized functions in cholesterol homoeostasis and nutrient absorption, bile acids are also important metabolic regulators and function as signaling hormones by activating specific nuclear receptors, G-protein coupled receptors, and multiple signaling pathways. Recent studies identified a new signaling pathway by which conjugated bile acids (CBA) activate the extracellular regulated protein kinases (ERK1/2) and protein kinase B (AKT) signaling pathway via sphingosine-1-phosphate receptor 2 (S1PR2). CBA-induced activation of S1PR2 is a key regulator of sphingosine kinase 2 (SphK2) and hepatic gene expression. This review focuses on recent findings related to the role of bile acids/S1PR2-mediated signaling pathways in regulating hepatic lipid metabolism.Key words: Bile acid, Sphingosine-1 phosphate receptor, Heptic lipid metabolismAbbreviations: ABC, ATP-binding cassette; AKT/PKB, protein kinase B; BSEP/ABCB11, bile salt export protein; CA, cholic acid; CBA, conjugated bile acids; CDCA, chenodeoxycholic acid; CYP27A1, sterol 27-hydroxylase; CYP7A1, cholesterol 7α-hydroxylase; CYP7B1, oxysterol 7α-hydroxylase; CYP8B1, 12α-hydroxylase; DCA, deoxycholic acid; EGFR, epidermal growth factor receptor; ERK, extracellular regulated protein kinases; FGF15/19, fibroblast growth factor 15/19; FGFR, fibroblast growth factor receptor; FXR, farnesoid X receptor; G-6-Pase, glucose-6-phophatase; GPCR, G-protein coupled receptor; HDL, high density lipoprotein; HNF4α, hepatocyte nuclear factor-4α; IBAT, ileal sodium-dependent bile acid transporter; JNK1/2, c-Jun N-terminal kinase; LCA, lithocholic acid; LDL, low-density lipoprotein; LRH-1, liver-related homolog-1; M1–5, muscarinic receptor 1–5; MMP, matrix metalloproteinase; NAFLD, non-alcoholic fatty liver disease; NK, natural killer cells; NTCP, sodium taurocholate cotransporting polypeptide; PEPCK, PEP carboxykinse; PTX, pertussis toxin; S1P, sphingosine-1-phosphate; S1PR2, sphingosine-1-phosphate receptor 2; SHP, small heterodimer partner; SphK, sphingosine kinase; SPL, S1P lyase; Spns2, spinster homologue 2; SPPs, S1P phosphatases; SRC, proto-oncogene tyrosine-protein kinase; TCA, taurocholate; TGR5, G-protein-coupled bile acid receptor; TNFα, tumor necrosis factor α; VLDL, very-low-density lipoprotein     

Bile acid receptors in non-alcoholic fatty liver disease

Accumulating data have shown that bile acids are important cell signaling molecules, which may activate several signaling pathways to regulate biological processes. Bile acids are endogenous ligands for the farnesoid X receptor (FXR) and TGR5, a G-protein coupled receptor. Gain- and loss-of-function studies have demonstrated that both FXR and TGR5 play important roles in regulating lipid and carbohydrate metabolism and inflammatory responses. Importantly, activation of FXR or TGR5 lowers hepatic triglyceride levels and inhibits inflammation. Such properties of FXR or TGR5 have indicated that these two bile acid receptors are ideal targets for treatment of non-alcoholic fatty liver disease, one of the major health concerns worldwide. In this article, we will focus on recent advances on the role of both FXR and TGR5 in regulating hepatic triglyceride metabolism and inflammatory responses under normal and disease conditions.     

The human gut sterolbiome: bile acid-microbiome endocrine aspects and therapeutics

The human body is now viewed as a complex ecosystem that on a cellular and gene level is mainly prokaryotic. The mammalian liver synthesizes and secretes hydrophilic primary bile acids, some of which enter the colon during the enterohepatic circulation, and are converted into numerous hydrophobic metabolites which are capable of entering the portal circulation, returned to the liver, and in humans, accumulating in the biliary pool. Bile acids are hormones that regulate their own synthesis, transport, in addition to glucose and lipid homeostasis, and energy balance. The gut microbial community through their capacity to produce bile acid metabolites distinct from the liver can be thought of as an “endocrine organ” with potential to alter host physiology, perhaps to their own favor. We propose the term “sterolbiome” to describe the genetic potential of the gut microbiome to produce endocrine molecules from endogenous and exogenous steroids in the mammalian gut. The affinity of secondary bile acid metabolites to host nuclear receptors is described, the potential of secondary bile acids to promote tumors, and the potential of bile acids to serve as therapeutic agents are discussed.Abbreviations: APC, adenomatous polyposis coli; BA, bile acids; BSH, bile salt hydrolases; CA, cholic acid; CDCA, chenodeoxycholic acid; COX-2, cyclooxygenase-2; CRC, colorectal cancer; CYP27A1, sterol-27-hydroxylase; CYP7A1, cholesterol 7α-hydroxylase; CYP8B1, sterol 12α-hydroxylase; DCA, deoxycholic acid; EGFR, epidermal growth factor receptor; FAP, familial adenomatous polyposis; FGF15/19, fibroblast growth factor 15/19; FXR, farnesoid X receptor; GABA, γ-aminobutyric acid; GPCR, G-protein coupled receptors; HMP, Human Microbiome Project; HSDH, hydroxysteroid dehydrogenase; LCA, lithocholic acid; LOX, lipooxygenase; MetaHIT, Metagenomics of the Human Intestinal Tract; NSAIDs, non-steroidal anti-inflammatory drugs; PKC, protein kinase C; PSC, primary sclerosing cholangitis; PXR, pregnane X receptor; UDCA, ursodeoxycholic acid; VDR, vitamin D receptorKEY WORDS: Sterolbiome, Gut microbiome, Bile acids, Metabolite, Therapeutic agent     

Effects of chemical modification of ursodeoxycholic acid on TGR5 activation

The aim of this study is to examine the ability of the bile acid analogues obtained by chemical modification of ursodeoxycholic acid (UDCA) for TGR5 activation. Eleven UDCA analogues including 3- or 7-methylated UDCAs and amino acid conjugates were investigated as to their ability to activate TGR5 by means of the luciferase assay. It was noteworthy that 7α-methylated UDCA, namely 3α,7β-dihydroxy-7α-methyl-5β-cholanoic acid, had a significantly high affinity for and ability to activate TGR5 as compared to UDCA. Additionally, FXR activation ability of 7α-methylated UDCA was low relative to that of UDCA. However, other modification of UDCA, such as the introduction of methyl group at its C-3 position and oxidation or epimerization of hydroxyl group in the C-3 position, could not elicit such remarkable effect. The present findings would provide a useful strategy for the development of TGR5-selective agonist.     

Obeticholic acid and budesonide for the treatment of primary biliary cirrhosis

Introduction: Primary biliary cirrhosis (PBC) is a chronic cholestatic liver disease of adults. Treatments are needed when patients have incomplete response to ursodeoxycholic acid (UDCA).

Areas covered: Discoveries of the key role played by bile acids (BAs) and nuclear receptors (NRs) in regulating liver and metabolic homeostasis have led to promising therapeutic approaches in liver diseases. A PubMed search for the recent literature on NRs in liver disease was conducted. In particular, obeticholic acid (OCA) is a farnesoid X receptor (FXR) agonist that has an important role in the enterohepatic circulation of BAs. Preliminary studies of OCA in patients with PBC have demonstrated marked biochemical improvement when administered in combination with UDCA and alone. Pruritus is the most common side effect, limiting treatment at higher doses. Budesonide is a glucocorticoid receptor/pregnane X receptor (PXR) agonist also involved in BA synthesis, metabolism and transport. Studies with budesonide have shown positive effects of short-term combination therapy in selected patients with early stage disease and overlapping features of autoimmune hepatitis.

Expert opinion: Though larger studies are needed, preliminary results of agents targeting FXR and PXR have been encouraging, particularly in subsets of patients with PBC and may mark a new therapeutic era.     

Farnesoid X receptor contributes to oleanolic acid-induced cholestatic liver injury in mice

Farnesoid X receptor (FXR) is a nuclear receptor involved in the metabolism of bile acid. However, the molecular signaling of FXR in bile acid homeostasis in cholestatic drug-induced liver injury remains unclear. Oleanolic acid (OA), a natural triterpenoid, has been reported to produce evident cholestatic liver injury in mice after a long-term use. The present study aimed to investigate the role of FXR in OA-induced cholestatic liver injury in mice using C57BL/6J (WT) mice and FXR knockout (FXR^−/−) mice. The results showed that a significant alleviation in OA-induced cholestatic liver injury was observed in FXR^−/− mice as evidenced by decreases in serum alanine aminotransferase, aspartate aminotransferase, and alkaline phosphatase as well as reduced hepatocyte necrosis. UPLC-MS analysis of bile acids revealed that the contents of bile acids decreased significantly in liver and serum, while increased in the bile in FXR^−/− mice compared with in WT mice. In addition, the mRNA expressions of hepatic transporter Bsep, bile acid synthesis enzymes Bacs and Baat, and bile acids detoxifying enzymes Cyp3a11, Cyp2b10, Ephx1, Ugt1a1, and Ugt2b5 were increased in liver tissues of FXR^−/− mice treated with OA. Furthermore, the expression of membrane protein BSEP was significantly higher in livers of FXR^−/− mice compared with WT mice treated with OA. These results demonstrate that knockout of FXR may alleviate OA-induced cholestatic liver injury in mice by decreasing accumulation of bile acids both in the liver and serum, increasing the export of bile acids via the bile, and by upregulation of bile acids detoxification enzymes.     

Cholic acid and ursodeoxycholic acid therapy in primary biliary cirrhosis

Summary We treated 6 patients with Stage II primary biliary cirrhosis with cholic acid (CA) 10 mg · kg^–1 per day for 3 months and then with the same dose of ursodeoxycholic acid (UDCA). A matching group of 6 patients was observed for 3 months without any therapy. Liver function tests and serum and stool bile acids were investigated before, during and at the end of CA and UDCA therapy.The results of liver function tests deteriorated after 6–8 weeks of CA therapy and the changes were correlated (r=0.92) with an increase in -dihydroxy-bile acids (chenodeoxycholic acid and deoxycholic acid) in the serum. The 24 h excretion of DCA in 24 h faeces was markedly increased.Ursodeoxycholic acid treatment improved liver function tests; after 4 weeks glutamate dehydrogenase (GLDH) had decreased. After 8–12 weeks of therapy ursodeoxycholic acid had increased to 50–60% of the total serum bile acids whereas the more apolar bile acids were significantly decreased. No changes in liver function tests or bile acid metabolism were found in the untreated group.Since CA and UDCA are non-toxic in man, this trial indicates that the apolar bile acids chenodeoxycholic acid and deoxycholic acid may be responsible for the deterioration of liver function in primary biliary cirrhosis. However, the therapeutic effect of UDCA cannot be explained merely by the decrease in -dihydroxy-bile acids in the serum, since the laboratory results had improved prior to the decrease in the serum apolar bile acids.     

The protective effect of obeticholic acid on lipopolysaccharide-induced disorder of maternal bile acid metabolism in pregnant mice

The disorder of bile acid metabolism is a common feature during pregnancy, which leads to adverse birth outcomes and maternal damage effects. However, the cause and therapy about the disorder of bile acid metabolism are still poor. Microbial infection often occurs in pregnant women, which can induce the disorder of bile acid metabolism in adult mice. Here, this study observed the acute effect of lipopolysaccharide (LPS) on maternal bile acid of pregnant mice at gestational day 17 and the protective effect of obeticholic acid (OCA) pretreatment, a potent agonist of bile acid receptor farnesoid X receptor (FXR). The results showed LPS significantly increased the level of maternal serum and disordered bile acids components of maternal serum and liver, which were ameliorated by OCA pretreatment with obviously reducing the contents of CA, TCA, DCA, TCDCA, CDCA, GCA and TDCA in maternal serum and DCA, TCA, TDCA, TUDCA, CDCA and TCDCA in maternal liver. Furthermore, we investigated the effects of OCA on LPS-disrupted bile acid metabolism in maternal liver. LPS disrupted maternal bile acid profile by decreasing transport and metabolism with hepatic tight junctions of bile acid in pregnant mice. OCA obviously increased the protein level of nuclear FXR and regulated its target genes involving in the metabolism of bile acid, which was characterized by the lower expression of bile acid synthase CYP7A1, the higher expression of CYP3A and the higher mRNA level of transporter Mdr1a/b. This study provided the evidences that LPS disrupted bile acid metabolism in the late stage of pregnant mice and OCA pretreatment played the protective role on it by activating FXR.     

Modulation of hepatocyte apoptosis: cross-talk between bile acids and nuclear steroid receptors

The efficient removal of unwanted cells, such as senescent, damaged, mutated or infected cells is crucial for the maintenance of normal liver function. In fact, apoptosis has emerged as a potential contributor to the pathogenesis of a number of hepatic disorders, such as viral hepatitis, autoimmune diseases, ethanol-induced injury, cholestasis, and hepatocellular carcinoma. In contrast to the effect of cytotoxic bile acids in the liver, ursodeoxycholic acid (UDCA) has increasingly been used for the treatment of various liver disorders. The clinical efficacy of this hydrophilic bile acid was first recognized by its use in traditional Asian medicine. However, many studies have subsequently confirmed that UDCA improves liver function by three major mechanisms of action, including protection of cholangiocytes against the cytotoxicity of hydrophobic bile acids, stimulation of hepatobiliary secretion, and inhibition of liver cell apoptosis. UDCA acts as a potent inhibitor of the classical mitochondrial pathway of apoptosis, but also interferes with alternate and upstream molecular targets such as the E2F-1/p53 pathway. Together, there is growing evidence that this hydrophilic bile acid may modulate gene expression to prevent cell death. Curiously, as a cholesterol-derived molecule, UDCA interacts with nuclear steroid receptors, such as the glucocorticoid receptor. Nuclear steroid receptors play crucial roles in mediating steroid hormone signaling involved in many biological processes, including apoptosis. Here, we review the anti-apoptotic mechanisms of UDCA in hepatic cells, and discuss a potential involvement of nuclear steroid receptors in mediating the survival effects of UDCA.     

Upregulation of early growth response factor-1 by bile acids requires mitogen-activated protein kinase signaling

Cholestasis results when excretion of bile acids from the liver is interrupted. Liver injury occurs during cholestasis, and recent studies showed that inflammation is required for injury. Our previous studies demonstrated that early growth response factor-1 (Egr-1) is required for development of inflammation in liver during cholestasis, and that bile acids upregulate Egr-1 in hepatocytes. What remains unclear is the mechanism by which bile acids upregulate Egr-1. Bile acids modulate gene expression in hepatocytes by activating the farnesoid X receptor (FXR) and through activation of mitogen-activated protein kinase (MAPK) signaling. Accordingly, the hypothesis was tested that bile acids upregulate Egr-1 in hepatocytes by FXR and/or MAPK-dependent mechanisms. Deoxycholic acid (DCA) and chenodeoxycholic acid (CDCA) stimulated upregulation of Egr-1 to the same extent in hepatocytes isolated from wild-type mice and FXR knockout mice. Similarly, upregulation of Egr-1 in the livers of bile duct-ligated (BDL) wild-type and FXR knockout mice was not different. Upregulation of Egr-1 in hepatocytes by DCA and CDCA was prevented by the MEK inhibitors U0126 and SL-327. Furthermore, pretreatment of mice with U0126 prevented upregulation of Egr-1 in the liver after BDL. Results from these studies demonstrate that activation of MAPK signaling is required for upregulation of Egr-1 by bile acids in hepatocytes and for upregulation of Egr-1 in the liver during cholestasis. These studies suggest that inhibition of MAPK signaling may be a novel therapy to prevent upregulation of Egr-1 in liver during cholestasis.     

Differential activation of the human farnesoid X receptor depends on the pattern of expressed isoforms and the bile acid pool composition

The farnesoid X receptor (FXR) is a key sensor in bile acid homeostasis. Although four human FXR isoforms have been identified, the physiological role of this diversity is poorly understood. Here we investigated their subcellular localization, agonist sensitivity and response of target genes. Measurement of mRNA revealed that liver predominantly expressed FXRα1(+/−), whereas FXRα2(+/−) were the most abundant isoforms in kidney and intestine. In all cases, the proportion of FXRα(1/2)(+) and FXRα(1/2)(−) isoforms, i.e., with and without a 12 bp insert, respectively, was approximately 50%. When FXR was expressed in liver and intestinal cells the magnitude of the response to GW4064 and bile acids differs among FXR isoforms. In both cell types the strongest response was that of FXRα1(−). Different efficacy of bile acids species to activate FXR was found. The four FXR isoforms shared the order of sensitivity to bile acids species. When in FXR-deficient cells FXR was transfected, unconjugated, but not taurine- and glycine-amidated bile acids, were able to activate FXR. In contrast, human hepatocytes and cell lines showing an endogenous expression of FXR were sensitive to both unconjugated and conjugated bile acids. This suggests that to activate FXR conjugated, but not unconjugated, bile acids require additional component(s) of the intracellular machinery not related with uptake processes, which are missing in some tumor cells. In conclusion, cell-specific pattern of FXR isoforms determine the overall tissue sensitivity to FXR agonists and may be involved in the differential response of FXR target genes to FXR activation.     

Avicholic Acid: A Lead Compound from Birds on the Route to Potent TGR5 Modulators

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