Progress and challenges of selective Farnesoid X Receptor modulation

(E)-7-Ethylidene-lithocholic Acid (7-ELCA) Is a Potent Dual Farnesoid X Receptor (FXR) Antagonist and GPBAR1 Agonist Inhibiting FXR-Induced Gene Expression in Hepatocytes and Stimulating Glucagon-like Peptide-1 Secretion From Enteroendocrine Cells

Bile acids (BAs) are key signaling steroidal molecules that regulate glucose, lipid, and energy homeostasis via interactions with the farnesoid X receptor (FXR) and G-protein bile acid receptor 1 (GPBAR1). Extensive medicinal chemistry modifications of the BA scaffold led to the discovery of potent selective or dual FXR and GPBAR1 agonists. Herein, we discovered 7-ethylidene-lithocholic acid (7-ELCA) as a novel combined FXR antagonist/GPBAR1 agonist (IC₅₀ = 15 μM/EC₅₀ = 26 nM) with no off-target activation in a library of 7-alkyl substituted derivatives of BAs. 7-ELCA significantly suppressed the effect of the FXR agonist obeticholic acid in BSEP and SHP regulation in human hepatocytes. Importantly, 7-ELCA significantly stimulated the production of glucagon-like peptide-1 (GLP-1), an incretin with insulinotropic effect in postprandial glucose utilization, in intestinal enteroendocrine cells. We can suggest that 7-ELCA may be a prospective approach to the treatment of type II diabetes as the dual modulation of GPBAR1 and FXR has been supposed to be effective in the synergistic regulation of glucose homeostasis in the intestine.

Secondary (iso)BAs cooperate with endogenous ligands to activate FXR under physiological and pathological conditions

IsoBAs, stereoisomers of primary and secondary BAs, are found in feces and plasma of human individuals. BA signaling via the nuclear receptor FXR is crucial for regulation of hepatic and intestinal physiology/pathophysiology.

AIM

Investigate the ability of BA-stereoisomers to bind and modulate FXR under physiological/pathological conditions.

METHODS

Expression-profiling, luciferase-assays, fluorescence-based coactivator-association assays, administration of (iso)-BAs to WT and cholestatic mice.

RESULTS

Compared to CDCA/isoCDCA, administration of DCA/isoDCA, UDCA/isoUDCA only slightly increased mRNA expression of FXR target genes; the induction was more evident looking at pre-mRNAs. Notably, almost 50% of isoBAs were metabolized to 3-oxo-BAs within 4 h in cell-based assays, making it difficult to study their actions. FRET-based real-time monitoring of FXR activity revealed that isoCDCA>CDCA stimulated FXR, and isoDCA and isoUDCA allowed fully activated FXR to be re-stimulated by a second dose of GW4064. In vivo co-administration of a single dose of isoBAs followed by GW4064 cooperatively activated FXR, as did feeding of UDCA in a background of endogenous FXR ligands. However, in animals with biliary obstruction and concomitant loss of intestinal BAs, UDCA was unable to increase intestinal Fgf15. In contrast, mice with an impaired enterohepatic circulation of BAs (Asbt-/-, Ostα-/-), administration of UDCA was still able to induce ileal Fgf15 and repress hepatic BA-synthesis, arguing that UDCA is only effective in the presence of endogenous FXR ligands.

CONCLUSION

Secondary (iso)BAs cooperatively activate FXR in the presence of endogenous BAs, which is important to consider in diseases linked to disturbances in BA enterohepatic cycling.

The Farnesoid X Receptor Agonist Tropifexor Prevents Liver Damage in Parenteral Nutrition-fed Neonatal Piglets

OBJECTIVES

Intestinal failure-associated liver disease (IFALD) is a life-threatening complication for patients with intestinal failure who receive long-term parenteral nutrition (PN). We evaluated the effects of the farnesoid X receptor agonist tropifexor on a neonatal piglet model of IFALD fed with PN.

METHODS

The piglets received PN and tropifexor for 14 days, then levels of liver enzymes, bile acid metabolism, inflammation, and intestinal barrier markers were assessed using quantitative real-time PCR. Fibroblast growth factor (FGF) 19 serum levels were determined using enzyme-linked immunosorbent assays. Bile acids were determined in liver, serum, and intestinal contents, and the microbiome was sequenced in different intestinal segments.

RESULTS

The PN model was established in newborn piglets. The levels of serum liver enzymes, pro-inflammatory factors, and oxidative stress increased in the livers of piglets fed with PN, but not in those fed with PN and tropifexor. Tropifexor stimulated FGF19 expression in ileal epithelial cells, increased portal FGF19 levels, then inhibited cholesterol 7α-hydroxylase expression in the liver. Tropifexor increased the relative abundance of bacteria associated with bile salt hydrolase and 7α-dehydrogenation in the contents of ileum and altered the composition of bile acids in serum, liver, and intestinal contents. Tropifexor also inhibited intestinal inflammation, alleviated intestinal mucosal atrophy, and improved the intestinal barrier.

CONCLUSIONS

Tropifexor might prevent liver damage in neonatal piglets receiving PN by altering the composition of intestinal microbiota and bile acids. Tropifexor also alleviates intestinal inflammation and preserves the intestinal barrier.

Critical roles of bile acids in regulating intestinal mucosal immune responses

Bile acids are a class of cholesterol derivatives that have been known for a long time for their critical roles in facilitating the digestion and absorption of lipid from the daily diet. The transformation of primary bile acids produced by the liver to secondary bile acids appears under the action of microbiota in the intestine, greatly expanding the molecular diversity of the intestinal environment. With the discovery of several new receptors of bile acids and signaling pathways, bile acids are considered as a family of important metabolites that play pleiotropic roles in regulating many aspects of human overall health, especially in the maintenance of the microbiota homeostasis and the balance of the mucosal immune system in the intestine. Accordingly, disruption of the process involved in the metabolism or circulation of bile acids is implicated in many disorders that mainly affect the intestine, such as inflammatory bowel disease and colon cancer. In this review, we discuss the different metabolism profiles in diseases associated with the intestinal mucosa and the diverse roles of bile acids in regulating the intestinal immune system. Furthermore, we also summarize recent advances in the field of new drugs that target bile acid signaling and highlight the importance of bile acids as a new target for disease intervention.

Network pharmacology-based mechanism prediction and pharmacological validation of Xiaoyan Lidan formula on attenuating alpha-naphthylisothiocyanate induced cholestatic hepatic injury in rats

ETHNOPHARMACOLOGICAL RELEVANCE

The well-known Chinese prescription, Xiaoyan Lidan Formula (XYLDF), possesses efficiency of heat-clearing, dampness-eliminating and jaundice-removing. It has long been used clinically for the treatment of hepatobiliary diseases due to intrahepatic cholestasis (IHC). However, the mechanism of XYLDF for its therapeutic effects remains elusive.

AIM OF THE STUDY

The study aimed to explore the potential targets for liver protective mechanism of XYLDF based on network pharmacology and experimental assays in ANIT-induced cholestatic hepatic injury (CHI) in rats.

MATERIALS AND METHODS

On the basis of the 29 serum migrant compounds of XYLDF elucidated by UPLC-TOF-MS/MS, a network pharmacology approach was applied for the mechanism prediction. Systematic networks were constructed to identify potential molecular targets, biological processes, and signaling pathways. And the interactions between significantly potential targets and active compounds were simulated by molecular docking. For the mechanism validation, an ANIT-induced rat model was used to evaluate the effects of XYLDF on CHI according to serum biochemistry, bile flow rates, histopathological examination, and the gene and protein expression including enzymes related to synthesis, export, and import of bile acid in liver and ileum, and those of inflammatory cytokines, analyzed by RT-qPCR and WB.

RESULTS

The results of network pharmacology research indicated TNF (TNF-α), RELA (NF-κB), NR1H4 (FXR), and ICAM1 (ICAM-1) to be the important potential targets of XYLDF for cholestatic liver injury, which are related to bile metabolism and NF-κB-mediated inflammatory signaling. And the molecular docking had pre-validated the prediction of network pharmacology, as the core active compounds of XYLDF had shown strong simulation binding affinity with FXR, followed by NF-κB, TNF-α, and ICAM-1. Meanwhile, the effects of XYLDF after oral administration on ANIT-induced CHI in rats exhibited the decreased levels of transaminases (ALT and AST), TBA, and TBIL in serum, raised bile flow rates, and markedly improved hepatic histopathology. Furthermore, consistent to the above targets prediction and molecular docking, XYLDF significantly up-regulated the expression of FXR, SHP, BSEP, and MRP2, and down-regulated CYP7A1 and NTCP in liver, and promoted expression of IBABP and OSTα/β in ileum, suggesting the activation of FXR-mediated pathway referring to bile acid synthesis, transportation, and reabsorption. Moreover, the lower levels of TNF-α in plasma and liver, as well as the reduced hepatic gene and protein expression of NF-κB, TNF-α, and ICAM-1 after XYLDF treatment revealed the suppression of NF-κB-mediated inflammatory signaling pathway, as evidenced by the inhibition of nuclear translocation of NF-κB.

CONCLUSIONS

XYLDF exhibited an ameliorative liver protective effect on ANIT-induced cholestatic hepatic injury. The present study has confirmed its mechanism as activating the FXR-regulated bile acid pathway and inhibiting inflammation via the NF-κB signaling pathway.

FXR in liver physiology: Multiple faces to regulate liver metabolism

The liver is the central metabolic hub which coordinates nutritional inputs and metabolic outputs. Food intake releases bile acids which can be sensed by the bile acid receptor FXR in the liver and the intestine. Hepatic and intestinal FXR coordinately regulate postprandial nutrient disposal in a network of interacting metabolic nuclear receptors. In this review we summarize and update the "classical roles" of FXR as a central integrator of the feeding state response, which orchestrates the metabolic processing of carbohydrates, lipids, proteins and bile acids. We also discuss more recent and less well studied FXR effects on amino acid, protein metabolism, autophagic turnover and inflammation. In addition, we summarize the recent understanding of how FXR signaling is affected by posttranslational modifications and by different FXR isoforms. These modifications and variations in FXR signaling might be considered when FXR is targeted pharmaceutically in clinical applications.

Multi-Target Approaches in Metabolic Syndrome

Metabolic syndrome (MetS) is a highly prevalent disease cluster worldwide. It requires polypharmacological treatment of the single conditions including type II diabetes, hypertension, and dyslipidemia, as well as the associated comorbidities. The complex treatment regimens with various drugs lead to drug-drug interactions and inadequate patient adherence, resulting in poor management of the disease. Multi-target approaches aim at reducing the polypharmacology and improving the efficacy. This review summarizes the medicinal chemistry efforts to develop multi-target ligands for MetS. Different combinations of pharmacological targets in context of in vivo efficacy and future perspective for multi-target drugs in MetS are discussed.

The Hypoglycemic Effect of Berberine and Berberrubine Involves Modulation of Intestinal Farnesoid X Receptor Signaling Pathway and Inhibition of Hepatic Gluconeogenesis

Our previous study suggests that berberine (BBR) lowers lipids by modulating bile acids and activating intestinal farnesoid X receptor (FXR). However, to what extent this pathway contributes to the hypoglycemic effect of BBR has not been determined. In this study, the glucose-lowering effects of BBR and its primary metabolites, berberrubine (M1) and demethyleneberberine, in a high-fat diet-induced obese mouse model were studied, and their modulation of the global metabolic profile of mouse livers and systemic bile acids was determined. The results revealed that BBR (150 mg/kg) and M1 (50 mg/kg) decreased mouse serum glucose levels by 23.15% and 48.14%, respectively. Both BBR and M1 markedly modulated the hepatic expression of genes involved in gluconeogenesis and metabolism of amino acids, fatty acids, and purine. BBR showed a stronger modulatory effect on systemic bile acids than its metabolites. Moreover, molecular docking and gene expression analysis in vivo and in vitro suggest that BBR and M1 are FXR agonists. The mRNA levels of gluconeogenesis genes in the liver, glucose-6-phosphatase and phosphoenolpyruvate carboxykinase, were significantly decreased by BBR and M1. In summary, BBR and M1 modulate systemic bile acids and activate the intestinal FXR signaling pathway, which reduces hepatic gluconeogenesis by inhibiting the gene expression of gluconeogenesis genes, achieving a hypoglycemic effect. BBR and M1 may function as new, natural, and intestinal-specific FXR agonists with a potential clinical application to treat hyperglycemia and obesity. SIGNIFICANCE STATEMENT: This investigation revealed that BBR and its metabolite, berberrubine, significantly lowered blood glucose, mainly through activating intestinal farnesoid X receptor signaling pathway, either directly by themselves or indirectly by modulating the composition of systemic bile acids, thus inhibiting the expression of gluconeogenic genes in the liver and, finally, reducing hepatic gluconeogenesis and lowering blood glucose. The results will help elucidate the mechanism of BBR and provide a reference for mechanism interpretation of other natural products with low bioavailability.

Linderae Radix Ethanol Extract Alleviates Diet-Induced Hyperlipidemia by Regulating Bile Acid Metabolism Through gut Microbiota

Hyperlipidemia is a common metabolic disorder and regarded as one of the main risk factors for cardiovascular disease. The gut microbiota has been identified as a potential contributor to hyperlipidemia as it can greatly regulate bile acid metabolism. Linderae radix is a natural medicine widely used in the treatment of a variety of diseases and is also a common drug for hyperlipidemia. Recently, the lipid-lowering effect of Linderae radix are receiving increasing attention but the underlying mechanism remains unknown. The study aimed to investigate the effects of Linderae radix ethanol extract (LREE) on gut microbiota in rats with hyperlipidemia syndrome. We established a hyperlipidemia rat model using a high-fat diet and used LREE as the intervention. Blood lipid levels and pathological examination were measured to assess the effects of LREE on hyperlipidemia. The gut microbiota was determined by 16s rDNA sequencing and the bile acid metabolism-related proteins were detected by western blot to discover the underlying correlations. The results show that LREE lowered TC, TG, and LDL levels effectively, and it also alleviated liver injury by reducing ALT and AST activity. Meanwhile, LREE improved gut microbiota disturbance caused by HFD via increasing intestinal microbiota diversity and changing the abundance of the Firmicutes, Bacteroidetes, and Actinobacteria. In addition, LREE can increase bile acid reabsorption and promote fecal excretion through farnesoid X receptor (FXR), apical sodium-dependent bile acid transporter (ASBT), organic solute transporter alpha (OST-α), and cytochrome P450 family 7 Subfamily A Member 1 (CYP7A1) thus restoring abnormal bile acid metabolism caused by hyperlipidemia.

The Liver under the Spotlight: Bile Acids and Oxysterols as Pivotal Actors Controlling Metabolism

Among the myriad of molecules produced by the liver, both bile acids and their precursors, the oxysterols are becoming pivotal bioactive lipids which have been underestimated for a long time. Their actions are ranging from regulation of energy homeostasis (i.e., glucose and lipid metabolism) to inflammation and immunity, thereby opening the avenue to new treatments to tackle metabolic disorders associated with obesity (e.g., type 2 diabetes and hepatic steatosis) and inflammatory diseases. Here, we review the biosynthesis of these endocrine factors including their interconnection with the gut microbiota and their impact on host homeostasis as well as their attractive potential for the development of therapeutic strategies for metabolic disorders.

Baicalin and the liver-gut system: Pharmacological bases explaining its therapeutic effects

With the development of high-throughput screening and bioinformatics technology, natural products with a range of pharmacological targets in multiple diseases have become important sources of new drug discovery. These compounds are derived from various plants, including the dried root of Scutellaria baicalensis Georgi, which is often used as a traditional Chinese herb named Huangqin, a popular medication used for thousands of years in China. Many studies have shown that baicalin, an extract from Scutellaria baicalensis Georgi, exerts various protective effects on liver and gut diseases. Baicalin plays a therapeutic role mainly by mediating downstream apoptosis and immune response pathways induced by upstream oxidative stress and inflammation. During oxidative stress regulation, PI3K/Akt/NRF2, Keap-1, NF-κB and HO-1 are key factors associated with the healing effects of baicalin on NAFLD/NASH, ulcerative colitis and cholestasis. In the inflammatory response, IL-6, IL-1β, TNF-α, MIP-2 and MIP-1α are involved in the alleviation of NAFLD/NASH, cholestasis and liver fibrosis by baicalin, as are TGF-β1/Smads, STAT3 and NF-κB. Regarding the apoptosis pathway, Bax, Bcl-2, Caspase-3 and Caspase-9 are key factors related to the suppression of hepatocellular carcinoma and attenuation of liver injury and colorectal cancer. In addition to immune regulation, PD-1/PDL-1 and TLR4-NF-κB are correlated with the alleviation of hepatocellular carcinoma, ulcerative colitis and colorectal cancer by baicalin. Moreover, baicalin regulates intestinal flora by promoting the production of SCFAs. Furthermore, BA is involved in the interactions of the liver-gut axis by regulating TGR5, FXR, bile acids and the microbiota. In general, a comprehensive analysis of this natural compound was conducted to determine the mechanism by which it regulates bile acid metabolism, the intestinal flora and related signaling pathways, providing new insights into the pharmacological effects of baicalin. The mechanism linking the liver and gut systems needs to be elucidated to draw attention to its great clinical importance.

Pharmacophore modeling and virtual screening studies for discovery of novel farnesoid X receptor (FXR) agonists

Farnesoid X receptor (FXR) agonists would be considered as an important therapeutic strategy for several chronic liver and metabolic diseases. Here we have employed an integrated virtual screening by combining ligand-based pharmacophore mapping and molecular docking to identify novel nonsteroidal FXR agonists. Eighteen compounds were selected for in vitro FXR agonistic activity assay, and results showed five compounds exhibiting promising FXR agonistic activity. Among these compounds, compounds F4 and F17 were the most remarkable in vitro activity by using homogeneous time resolved fluorescence (HTRF) assay and the full-length FXR reporter gene assay in HepG2 cells. Real-time PCR assay was performed to measure the expression of FXR target genes. Compounds F4 and F17 increased small heterodimer partner (SHP), in turn, suppress mRNA levels of cholesterol 7-alpha-hydroxylase (CYP7A1). The obtained compounds F4 and F17 from this study may be potential leads for developing novel FXR agonists in the treatment of metabolic diseases.

Targeting the alternative bile acid synthetic pathway for metabolic diseases

The gut microbiota is profoundly involved in glucose and lipid metabolism, in part by regulating bile acid (BA) metabolism and affecting multiple BA-receptor signaling pathways. BAs are synthesized in the liver by multi-step reactions catalyzed via two distinct routes, the classical pathway (producing the 12α-hydroxylated primary BA, cholic acid), and the alternative pathway (producing the non-12α-hydroxylated primary BA, chenodeoxycholic acid). BA synthesis and excretion is a major pathway of cholesterol and lipid catabolism, and thus, is implicated in a variety of metabolic diseases including obesity, insulin resistance, and nonalcoholic fatty liver disease. Additionally, both oxysterols and BAs function as signaling molecules that activate multiple nuclear and membrane receptor-mediated signaling pathways in various tissues, regulating glucose, lipid homeostasis, inflammation, and energy expenditure. Modulating BA synthesis and composition to regulate BA signaling is an interesting and novel direction for developing therapies for metabolic disease. In this review, we summarize the most recent findings on the role of BA synthetic pathways, with a focus on the role of the alternative pathway, which has been under-investigated, in treating hyperglycemia and fatty liver disease. We also discuss future perspectives to develop promising pharmacological strategies targeting the alternative BA synthetic pathway for the treatment of metabolic diseases.

Structure-guided modification of isoxazole-type FXR agonists: Identification of a potent and orally bioavailable FXR modulator

Farnesoid X receptor (FXR) agonists are emerging as potential therapeutics for the treatment of various metabolic diseases, as they display multiple effects on bile acid, lipid, and glucose homeostasis. Although the steroidal obeticholic acid, a full FXR agonist, was recently approved, several side effects probably due to insufficient pharmacological selectivity impede its further clinical application. Activating FXR in a partial manner is therefore crucial in the development of novel FXR modulators. Our efforts focusing on isoxazole-type FXR agonists, common nonsteroidal agonists for FXR, led to the discovery a series of novel FXR agonists bearing aryl urea moieties through structural simplification of LJN452 (phase 2). Encouragingly, compound 11k was discovered as a potent FXR agonist which exhibited similar FXR agonism potency but lower maximum efficacy compared to full agonists GW4064 and LJN452 in cell-based FXR transactivation assay. Extensive in vitro evaluation further confirmed partial efficacy of 11k in cellular FXR-dependent gene modulation, and revealed its lipid-reducing activity. More importantly, orally administration of 11k in mice exhibited desirable pharmacokinetic characters resulting in promising in vivo FXR agonistic activity.

Characterization of a Structural Leoligin Analog as Farnesoid X Receptor Agonist and Modulator of Cholesterol Transport

The ligand-activated farnesoid X receptor is an emerging therapeutic target for the development of drugs against metabolic syndrome-related diseases. In this context, selective bile acid receptor modulators represent a novel concept for drug development. Selective bile acid receptor modulators act in a target gene- or tissue-specific way and are therefore considered less likely to elicit unwanted side effects. Based on leoligin, a lignan-type secondary plant metabolite from the alpine plant Leontopodium nivale ssp. alpinum, 168 synthesized structural analogs were screened in a farnesoid X receptor in silico pharmacophore-model. Fifty-six virtual hits were generated. These hits were tested in a cell-based farnesoid X receptor transactivation assay and yielded 7 farnesoid X receptor-activating compounds. The most active one being LT-141A, with an EC₅₀ of 6 µM and an E_max of 4.1-fold. This analog did not activate the G protein-coupled bile acid receptor, TGR5, and the metabolic nuclear receptors retinoid X receptor α, liver X receptors α/β, and peroxisome proliferator-activated receptors β/γ. Investigation of different farnesoid X receptor target genes characterized LT-141A as selective bile acid receptor modulators. Functional studies revealed that LT-141A increased cholesterol efflux from THP-1-derived macrophages via enhanced ATP-binding cassette transporter 1 expression. Moreover, cholesterol uptake in differentiated Caco-2 cells was significantly decreased upon LT-141A treatment. In conclusion, the leoligin analog LT-141A selectively activates the nuclear receptor farnesoid X receptor and has an influence on cholesterol transport in 2 model systems.

3β-Isoobeticholic acid efficiently activates the farnesoid X receptor (FXR) due to its epimerization to 3α-epimer by hepatic metabolism

Bile acids (BAs) are important signaling molecules acting via the farnesoid X nuclear receptor (FXR) and the membrane G protein-coupled bile acid receptor 1 (GPBAR1). Besides deconjugation of BAs, the oxidoreductive enzymes of colonic bacteria and hepatocytes enable the conversion of BAs into their epimers or dehydrogenated forms. Obeticholic acid (OCA) is the first-in-class BA-derived FXR agonist approved for the treatment of primary biliary cholangitis. Herein, a library of OCA derivatives, including 7-keto, 6-ethylidene derivatives and 3β-epimers, was synthetized and investigated in terms of interactions with FXR and GPBAR1 in transaction assays and evaluated for FXR target genes expression in human hepatocytes and C57BL/6 mice. The derivatives were further subjected to cell-free analysis employing in silico molecular docking and a TR-FRET assay. The conversion of the 3βhydroxy epimer and its pharmacokinetics in mice were studied using LC-MS. We found that only the 3β-hydroxy epimer of OCA (3β-isoOCA) possesses significant activity to FXR in hepatic cells and mice. However, in a cell-free assay, 3β-isoOCA had about 9-times lower affinity to FXR than did OCA. We observed that 3β-isoOCA readily epimerizes to OCA in hepatocytes and murine liver. This conversion was significantly inhibited by the hydroxy-Δ⁵-steroid dehydrogenase inhibitor trilostane. In addition, we found that 3,7-dehydroobeticholic acid is a potent GPBAR1 agonist. We conclude that 3β-isoOCA significantly activates FXR due to its epimerization to the more active OCA by hepatic metabolism. Other modifications as well as epimerization on the C3/C7 positions and the introduction of 6-ethylidene in the CDCA scaffold abrogate FXR agonism and alleviate GPBAR1 activation.

Farnesoid X Receptor Agonists as Therapeutic Target for Cardiometabolic Diseases

Cardiometabolic diseases are characterized as a combination of multiple risk factors for cardiovascular disease (CVD) and metabolic diseases including diabetes mellitus and dyslipidemia. Cardiometabolic diseases are closely associated with cell glucose and lipid metabolism, inflammatory response and mitochondrial function. Farnesoid X Receptor (FXR), a metabolic nuclear receptor, are found to be activated by primary BAs such as chenodeoxycholic acid (CDCA), cholic acid (CA) and synthetic agonists such as obeticholic acid (OCA). FXR plays crucial roles in regulating cholesterol homeostasis, lipid metabolism, glucose metabolism, and intestinal microorganism. Recently, emerging evidence suggests that FXR agonists are functional for metabolic syndrome and cardiovascular diseases and are considered as a potential therapeutic agent. This review will discuss the pathological mechanism of cardiometabolic disease and reviews the potential mechanisms of FXR agonists in the treatment of cardiometabolic disease.

Nuclear receptor FXR impairs SK-Hep-1 cell migration and invasion by inhibiting the Wnt/β-catenin signaling pathway

Recently, the nuclear receptor farnesoid X receptor (FXR) has been considered to be a liver tumor suppressor. However, the role of FXR in liver cancer invasion and metastasis remains unclear. The results of the current study demonstrated that FXR suppressed the migratory and invasive capacities of SK-Hep-1 cells in vitro and that FXR overexpression inhibited local invasion and lung metastasis of SK-Hep-1 ×enografts in vivo. Bioinformatics analysis of the gene expression profile of SK-Hep-1 cells with different FXR levels indicated that FXR may regulate the Wnt/β-catenin pathway. Compared with controls, FXR-overexpressing SK-Hep-1 cells exhibited decreased expression of β-catenin target genes and reduced nuclear translocation of β-catenin proteins in vitro and in vivo. In conclusion, these results indicated that FXR may suppress SK-Hep-1 cell invasion and metastasis by suppressing the Wnt/β-catenin signaling pathway. The current study provided novel insight into the diagnosis and treatment of liver cancer.

Bile acids mediated potential functional interaction between FXR and FATP5 in the regulation of Lipid Metabolism

Perturbation in lipid homeostasis is one of the major bottlenecks in metabolic diseases, especially Non-alcoholic Fatty Liver Disease (NAFLD), which has emerged as a leading global cause of chronic liver disease. The bile acids (BAs) and their derivatives exert a variety of metabolic effects through complex and intertwined pathways, thus becoming the attractive target for metabolic syndrome treatment. To modulate the lipid homeostasis, the role of BAs, turn out to be paramount as it is essential for the absorption, transport of dietary lipids, regulation of metabolic enzymes and transporters that are essential for lipid modulation, flux, and excretion. The synthesis and transport of BAs (conjugated and unconjugated) is chiefly controlled by nuclear receptors and the uptake of long-chain fatty acids (LCFA) and BA conjugation via transporters. Among them, from in-vivo studies, farnesoid X receptor (FXR) and liver-specific fatty acid transport protein 5 (FATP5) have shown convincing evidence for their key roles in lipid homeostasis and reversal of fatty liver disease substantially. BAs have a wider range of biological effects as they are identified as modulators for FXR and FATP5 both and therefore hold a significant promise for altering the lipid content in the treatment of a metabolic disorder. BAs also have received noteworthy interest in drug delivery research due to its peculiar physicochemical properties and biocompatibility. Here, we are highlighting the connecting possibility of BAs as an agonist for FXR and antagonist for FATP5, paving an avenue to target them for designing synthetic small molecules for lipid homeostasis.

Discovery, Structural Refinement and Therapeutic Potential of Farnesoid X Receptor Activators

Farnesoid X receptor acts as bile acid sensing transcription factor and has been identified as valuable molecular drug target to treat severe liver disorders, such as non-alcoholic steatohepatitis (NASH). Preclinical and clinical data indicate anti-fibrotic effects obtained with FXR activation that also appear promising for other fibrotic diseases beyond NASH. Strong efforts in FXR ligand discovery have yielded potent steroidal and non-steroidal FXR activators, some of which have been studied in clinical trials. While the structure–activity relationship of some FXR agonist frameworks have been studied extensively, the structural diversity of potent FXR activator chemotypes is still limited to a handful of well-studied compound classes. Together with safety concerns related to full therapeutic activation of FXR, this indicates the need for novel innovative FXR ligands with selective modulatory properties. This chapter evaluates FXR's value as drug target with emphasis on fibrotic diseases, analyses FXR ligand recognition and requirements and focuses on the discovery and structural refinement of leading FXR activator chemotypes.

Pose Filter-Based Ensemble Learning Enables Discovery of Orally Active, Nonsteroidal Farnesoid X Receptor Agonists

Farnesoid X receptor (FXR) agonists can reverse dysregulated bile acid metabolism, and thus, they are potential therapeutics to prevent and treat nonalcoholic fatty liver disease. The low success rate of FXR agonists' R&D and the side effects of clinical candidates such as obeticholic acid make it urgent to discover new chemotypes. Unfortunately, structure-based virtual screening (SBVS) that can speed up drug discovery has rarely been reported with success for FXR, which was likely hindered by the failure in addressing protein flexibility. To address this issue, we devised human FXR (hFXR)-specific ensemble learning models based on pose filters from 24 agonist-bound hFXR crystal structures and coupled them to traditional SBVS approaches of the FRED docking plus Chemgauss4 scoring function. It turned out that the hFXR-specific pose filter ensemble (PFE) was able to improve ligand enrichment significantly, which rendered 3RUT-based SBVS with its PFE the ideal approach for FXR agonist discovery. By screening of the Specs chemical library and in vitro FXR transactivation bioassay, we identified a new class of FXR agonists with compound XJ034 as the representative, which would have been missed if the PFE was not coupled. Following that, we performed in-depth biological studies which demonstrated that XJ034 resulted in a downtrend of intracellular triglyceride in vitro, significantly decreased the serum/liver TG in high fat diet-induced C57BL/6J obese mice, and more importantly, showed metabolic stabilities in both plasma and liver microsomes. To provide insight into further structure-based lead optimization, we solved the crystal structure of hFXR complexed with compound XJ034, uncovering a unique hydrogen bond between compound XJ034 and residue Y375. The current work highlights the power of our pose filter-based ensemble learning approach in terms of scaffold hopping and provides a promising lead compound for further development.

Clinical translation of liver regeneration therapies: A conceptual road map

The increasing incidence of severe liver diseases worldwide has resulted in a high demand for curative liver transplantation. Unfortunately, the need for transplants by far eclipses the availability of suitable grafts leaving many waitlisted patients to face liver failure and often death. Routine use of smaller grafts (for example left lobes, split livers) from living or deceased donors could increase the number of life-saving transplants but is often limited by the graft versus recipient weight ratio defining the safety margins that minimize the risk of small for size syndrome (SFSS). SFSS is a severe complication characterized by failure of a small liver graft to regenerate and occurs when a donor graft is insufficient to meet the metabolic demand of the recipient, leading to liver failure as a result of insufficient liver mass. SFSS is not limited to transplantation but can also occur in the setting of hepatic surgical resections, where life-saving large resections of tumors may be limited by concerns of post-surgical liver failure. There are, as yet no available pro-regenerative therapies to enable liver regrowth and thus prevent SFSS. However, there is optimism around targeting factors and pathways that have been identified as regulators of liver regeneration to induce regrowth in vivo and ex vivo for clinical use. In this commentary, we propose a roadmap for developing such pro-regenerative therapy and for bringing it into the clinic. We summarize the clinical indications, preclinical models, pro-regenerative pathways and safety considerations necessary for developing such a drug.

Synthesis of Novel Farnesoid X Receptor Agonists and Validation of Their Efficacy in Activating Differentiation of Mouse Bone Marrow-Derived Mesenchymal Stem Cells into Osteoblasts

The modulators of farnesoid X receptor (FXR), a bile acid receptor, regulate various biological processes including bile acid metabolism, and are associated with the control of fatty liver and osteoporosis. Thus, the control of FXR activity and development of FXR modulators are critical not only for research, but also for clinical application. In this study, we synthesized novel FXR agonists 1–4 possessing isoxazole and N-substituted benzimidazole moieties, and compared their effects on osteoblast differentiation with the known FXR agonists, chenodeoxycholic acid and a synthetic compound, GW4064. Two (3 and 4) of the four novel FXR agonists 1–4 showed high specificities for FXR. Computer-assisted modeling suggested that the binding of the FXR agonist 3 with ligand binding domain of FXR was similar to GW4064. FXR was expressed in mouse bone marrow-derived mesenchymal stem cell (MSC)-like ST2 cells (ST-2 MSCs). The FXR agonists activated the BMP-2-induced differentiation of ST-2 MSCs into osteoblasts and enhanced the expression of RUNX2. Moreover, the potency of the FXR agonist 3 was comparable to GW4064 in promoting osteoblast differentiation of ST-2 MSCs. These results indicate that FXR activation enhanced the BMP-2-induced differentiation of MSCs into osteoblasts through activating RUNX2 expression. FXR could be a potential therapeutic target for the treatment of bone diseases such as osteoporosis.

Recent advances from metabolomics and lipidomics application in alzheimer's disease inspiring drug discovery

Introduction: Although age is a major risk factor for Alzheimer's disease (AD), it is not an inevitable consequence of aging nor is it exclusively an old-age disease. Several other major risk factors for AD are strongly associated with metabolism and include lack of exercise, obesity, diabetes, high blood pressure and cholesterol, over-consumption of alcohol and depression in addition to low educational level, social isolation, and cognitive inactivity. Approaches for Alzheimer prevention and treatment through manipulation of metabolism and utilization of active metabolites have great potential either as a primary or secondary treatment avenue or as a preventative strategy in high-risk individuals.Areas covered: This review outlines the current knowledge concerning the relationship between AD and metabolism and the novel treatments attempting to correct changes in AD patients determined through metabolomics or lipidomic analyses.Expert opinion: Metabolites are one of the main driving factors and indicators of AD and can offer many possible avenues for prevention and treatment. However, with the highly interconnected effects of metabolites and metabolism, as well as the many different routes for metabolism dysfunction, successful treatment would have to include the correction of metabolic errors as well as errors in transport and metabolite processing in order to affect and revert AD progression.

Targeting FXR in Cholestasis

The farnesoid X receptor (FXR, NR1H4) is a bile acid (BA)-activated transcription factor, which is essential for BA homeostasis. FXR and its hepatic and intestinal target genes, small heterodimer partner (SHP, NR0B2) and fibroblast growth factor 15/19 (Fgf15 in mice, FGF19 in humans), transcriptionally regulate BA synthesis, detoxification, secretion, and absorption in the enterohepatic circulation. Furthermore, FXR modulates a large variety of physiological processes, such as lipid and glucose homeostasis as well as the inflammatory response. Targeted deletion of FXR renders mice highly susceptible to cholic acid feeding resulting in cholestatic liver injury, weight loss, and increased mortality. Combined deletion of FXR and SHP spontaneously triggers early-onset intrahepatic cholestasis in mice resembling human progressive familial intrahepatic cholestasis (PFIC). Reduced expression levels and activity of FXR have been reported in human cholestatic conditions, such as PFIC type 1 and intrahepatic cholestasis of pregnancy. Recently, two pairs of siblings with homozygous FXR truncation or deletion variants were identified. All four children suffered from severe, early-onset PFIC and liver failure leading to death or need for liver transplantation before the age of 2. These findings underscore the central role of FXR as regulator of systemic and hepatic BA levels. Therefore, targeting FXR has been exploited in different animal models of both intrahepatic and obstructive cholestasis, and the first FXR agonist obeticholic acid (OCA) has been approved for the treatment of primary biliary cholangitis (PBC). Further FXR agonists as well as a FGF19 analogue are currently tested in clinical trials for different cholestatic liver diseases. This chapter will summarize the current knowledge on the role of FXR in cholestasis both in rodent models and in human diseases.

Chronic activation of FXR-induced liver growth with tissue-specific targeting Cyclin D1

The nuclear receptor (FXR) plays essential roles in maintaining bile acid and lipid homeostasis by regulating diverse target genes. And its agonists were promising agents for treating various liver diseases. Nevertheless, the potential side effect of chronic FXR activation by specific agonists is not fully understood. In this study, we investigated the mechanism of FXR agonist WAY-362450 induced liver enlargement during treating liver diseases. We demonstrated that chronic ingestion of WAY-362450 induced liver hypertrophy instead of hyperplasia in mouse. Global transcriptional pattern was also examined in mouse livers after treatment with WAY-362450 by RNA-seq assay. Through GO and KEGG enrichment analyses, we demonstrated that the expression of Cyclin D1 (Ccnd1) among the cell cycle-regulating genes was notably increased in WAY-362450-treated mouse liver. Activation of FXR-induced Ccnd1 expression in hepatocyte in a time-dependent manner in vivo and in vitro. Through bioinformatics analysis and ChIP assay, we identified FXR as a direct transcriptional activator of Ccnd1 through binding to a potential enhancer, which was specifically active in livers. We also found active histone acetylation was essential for Ccnd1 induction by FXR. Thus, our study indicated that activation of FXR-induced harmless liver hypertrophy with spatiotemporal modulation of Ccnd1. With a better understanding of the mechanism of tissue-specific gene regulation by FXR, it is beneficial for development and appropriate application of its specific agonist in preventing hepatic diseases.

Dissecting the allosteric FXR modulation: a chemical biology approach using guggulsterone as a chemical tool

Guggulsterone is a promiscuous ligand for endocrine and metabolic lipid receptors traditionally used to treat a number of diseases including diabesity, hyperlipidemia, atherosclerosis, and osteoarthritis. Although relatively weak, its activity at the farnesoid X receptor (FXR) is particularly intriguing as guggulsterone acts as an antagonist with a peculiar ability of gene selective modulation. We report here a chemical biology study with the aim to further characterize the biological action of guggulsterone at the FXR and to obtain further insights into the functional role played by noncanonical FXR binding pockets S2 and S3. Our results suggest that the FXR accessory pockets might act as potential targets for small molecules able to modulate the metabolic activation of the receptor without affecting the anti-inflammatory activity thus revealing a new approach for disclosing selective FXR modulators that might bypass potential side-effects from chronic treatments.

A molecular biomarker for prediction of clinical outcome in children with ASD, constipation, and intestinal inflammation

In children with autism spectrum disorder (ASD) who present to the gastroenterologist with chronic constipation on a background of colonic inflammation, we have identified two distinct clinical subtypes: (1) patients who experience a sustained state of GI symptomatic remission while on maintenance anti-inflammatory therapy (fast responders) and, (2) those with recurrent right-sided fecal loading requiring regular colon cleanouts during treatment for enterocolitis (slow responders). We hypothesized that a detailed molecular analysis of tissue from the affected region of the colon would provide mechanistic insights regarding the fast versus slow response to anti-inflammatory therapy. To test this, ascending colon biopsy tissues from 35 children with ASD (20 slow responders and 15 fast responders) were analyzed by RNAseq. Hierarchical cluster analysis was performed to assign samples to clusters and gene expression analysis was performed to identify differentially expressed transcripts (DETs) between samples within the clusters. Significant differences were found between the two clusters with fast responder-predominant cluster showing an upregulation of transcripts involved in the activation of immune and inflammatory response and the slow responder-predominant cluster showing significant over-representation of pathways impacting colonic motility (e.g. genes involved in tryptophan and serotonin degradation and mitochondrial dysfunction). Regression analysis identified a single long non-coding RNA that could predict cluster assignment with a high specificity (0.88), sensitivity (0.89) and accuracy (0.89). Comparison of gene expression profiles in the ascending colon from a subset of patients with ASD, chronic right-sided fecal loading constipation and a slow versus fast response to therapy has identified molecular mechanisms that likely contribute to this differential response following the primary therapeutic intervention (i.e. treatment for colonic inflammation with brief induction immunosuppression followed by maintenance non-steroidal anti-inflammatory therapy). Importantly, we have identified a transcript that, if validated, may provide a biomarker that can predict from the outset which patients will be slow responders who would benefit from an alternate therapeutic strategy in treating their constipation.

Identification of FDA-approved drugs targeting the Farnesoid X Receptor

The farnesoid X receptor (FXR) belongs to the nuclear receptor family and is activated by bile acids. Multiple, chemically rather diverse, FXR agonists have been developed and several of these compounds are currently tested in clinical trials for NAFLD and cholestasis. Here, we investigated possible FXR-agonism or antagonism of existing FDA/EMA-approved drugs. By using our recently developed FRET-sensor, containing the ligand binding domain of FXR (FXR-LBD), 1280 FDA-approved drugs were screened for their ability to activate FXR in living cells using flow cytometry. Fifteen compounds induced the sensor for more than twenty percent above background. Real-time confocal microscopy confirmed that avermectin B1a, gliquidone, nicardipine, bepridil and triclosan activated the FRET sensor within two minutes. These compounds, including fluticasone, increased mRNA expression of FXR target genes OSTα and OSTβ in Huh7 cells, and in most cases also of MRP2, SHP and FGF19. Finally, avermectin B1a, gliquidone, nicardipine and bepridil significantly increased IBABP promoter activity in a luciferase reporter assay in a dose-dependent manner. In conclusion, six FDA/EMA-approved drugs currently used in the clinical practice exhibit moderate agonistic FXR activity. This may on the one hand explain (undesired) side-effects, but on the other hand may form an opportunity for polypharmacology.

Calculus Bovis Sativus Improves Bile Acid Homeostasis via Farnesoid X Receptor-Mediated Signaling in Rats With Estrogen-Induced Cholestasis

Cholestatic diseases are characterized by toxic bile acid (BA) accumulation, and abnormal BA composition, which subsequently lead to liver injury. Biochemical synthetic Calculus Bovis Sativus (CBS) is derived from natural Calculus Bovis, a traditional Chinese medicine, which has been used to treat hepatic diseases for thousands of years. Although it has been shown that CBS administration to 17α-ethinylestradiol (EE)-induced cholestatic rats improves bile flow and liver injury, the involved underlying mechanism is largely unknown. In this study, we showed that CBS administration to EE-induced cholestatic rats significantly decreased serum and hepatic BA levels and reversed hepatic BA composition. DNA microarray analysis suggested that the critical pathways enriched by CBS treatment were bile secretion and primary BA synthesis. These findings led us to focus on the effects of CBS on regulating BA homeostasis, including BA transport, synthesis and metabolism. CBS enhanced hepatic BA secretion by inducing efflux transporter expression and inhibiting uptake transporter expression. Moreover, CBS reduced BA synthesis by repressing the expression of BA synthetic enzymes, CYP7A1 and CYP8B1, and increased BA metabolism by inducing the expression of metabolic enzymes, CYP3A2, CYP2B10, and SULT2A1. Mechanistic studies indicated that CBS increased protein expression and nuclear translocation of hepatic and intestinal farnesoid X receptor (FXR) to regulate the expression of these transporters and enzymes. We further demonstrated that beneficial effects of CBS administration on EE-induced cholestatic rats were significantly blocked by guggulsterone, a FXR antagonist. Therefore, CBS improved BA homeostasis through FXR-mediated signaling in estrogen-induced cholestatic rats. Together, these findings suggested that CBS might be a novel and potentially effective drug for the treatment of cholestasis.

Hepatoprotection of auraptene from the peels of citrus fruits against 17α-ethinylestradiol-induced cholestasis in mice by activating farnesoid X receptor

Auraptene protects against estrogen-induced cholestasis in mice.

Structural Insight into the Binding Mode of FXR and GPBAR1 Modulators

In this chapter we provide an exhaustive overview of the binding modes of bile acid (BA) and non-BA ligands to the nuclear farnesoid X receptor (FXR) and the G-protein bile acid receptor 1 (GPBAR1). These two receptors play a key role in many diseases related to lipid and glucose disorders, thus representing promising pharmacological targets. We pay particular attention to the chemical and structural features of the ligand-receptor interaction, providing guidelines to achieve ligands endowed with selective or dual activity towards the receptor and paving the way to future drug design studies.

Potential of Intestine-Selective FXR Modulation for Treatment of Metabolic Disease

Farnesoid X receptor controls bile acid metabolism, both in the liver and intestine. This potent nuclear receptor not only maintains homeostasis of its own ligands, i.e., bile acids, but also regulates glucose and lipid metabolism as well as the immune system. These findings have led to substantial interest for FXR as a therapeutic target and to the recent approval of an FXR agonist for treating primary biliary cholangitis as well as ongoing clinical trials for other liver diseases. Given that FXR biology is complex, including moderate expression in tissues outside of the enterohepatic circulation, temporal expression of isoforms, posttranscriptional modifications, and the existence of several other bile acid-responsive receptors such as TGR5, clinical application of FXR modulators warrants thorough understanding of its actions. Recent findings have demonstrated remarkable physiological effects of targeting FXR specifically in the intestine (iFXR), thereby avoiding systemic release of modulators. These include local effects such as improvement of intestinal barrier function and intestinal cholesterol turnover, as well as systemic effects such as improvements in glucose homeostasis, insulin sensitivity, and nonalcoholic fatty liver disease (NAFLD). Intriguingly, metabolic improvements have been observed with both an iFXR agonist that leads to production of enteric Fgf15 and increased energy expenditure in adipose tissues and antagonists by reducing systemic ceramide levels and hepatic glucose production. Here we review the recent findings on the role of intestinal FXR and its targeting in metabolic disease.

Signaling from Intestine to the Host: How Bile Acids Regulate Intestinal and Liver Immunity

Primary bile acids (BAs) are generated in the liver as the end products of cholesterol catabolism; they are then conjugated and accumulated in the gallbladder. After a meal ingestion, BAs are reversed into the duodenum to facilitate the lipid absorption. At the intestinal level, the 95% of BAs are reabsorbed and redirected into enterohepatic circulation; indeed only a small amount of them are then subjected to chemical modifications by the intestinal microbiota, which plays a very important role in the generation of secondary bile acids and in regulating host's metabolism and activity of the immune system. Behind their role in nutrients absorption, bile acids act as signaling molecules, activating several receptors, known as bile acid-activated receptors (BARs), including the farnesoid-X-receptor (FXR) and the G protein-coupled bile acid receptor 1 (GPBAR1 or Takeda G-protein receptor 5). Both receptors appear to contribute to maintain the tolerogenic state of the liver and intestine immunity. In particular, FXR and GPBAR1 are highly expressed in cells of innate immunity including intestinal and liver macrophages, dendritic cells, and natural killer T cells. In this chapter, we provide an overview on mechanisms through which FXR and GPBAR1 modulate the signaling between microbiota and intestinal and liver innate immune system. This overview could help to explain beneficial effects exerted by GPBAR1 and FXR agonists in the treatment of metabolic and immuno-mediated diseases.

Developments in bile salt based therapies: A critical overview

Bile acids, amphipathic molecules known for their facilitating role in fat absorption, are also recognized as signalling molecules acting via nuclear and membrane receptors. Of the bile acid-activated receptors, the Farnesoid X Receptor (FXR) and the G protein-coupled bile acid receptor-1 (Gpbar1 or TGR5) have been studied most extensively. Bile acid signaling is critical in the regulation of bile acid metabolism itself, but it also plays a significant role in glucose, lipid and energy metabolism. Activation of FXR and TGR5 leads to reduced hepatic bile salt load, improved insulin sensitivity and glucose regulation, increased energy expenditure, and anti-inflammatory effects. These beneficial effects render bile acid signaling an interesting therapeutic target for the treatment of diseases such as cholestasis, non-alcoholic fatty liver disease, and diabetes. Here, we summarize recent findings on bile acid signaling and discuss potential and current limitations of bile acid receptor agonist and modulators of bile acid transport as future therapeutics for a wide-spectrum of diseases.

Nuclear Receptor Metabolism of Bile Acids and Xenobiotics: A Coordinated Detoxification System with Impact on Health and Diseases

Structural and functional studies have provided numerous insights over the past years on how members of the nuclear hormone receptor superfamily tightly regulate the expression of drug-metabolizing enzymes and transporters. Besides the role of the farnesoid X receptor (FXR) in the transcriptional control of bile acid transport and metabolism, this review provides an overview on how this metabolic sensor prevents the accumulation of toxic byproducts derived from endogenous metabolites, as well as of exogenous chemicals, in coordination with the pregnane X receptor (PXR) and the constitutive androstane receptor (CAR). Decrypting this network should provide cues to better understand how these metabolic nuclear receptors participate in physiologic and pathologic processes with potential validation as therapeutic targets in human disabilities and cancers.

FXR modulators for enterohepatic and metabolic diseases

INTRODUCTION

Farnesoid X receptor (FXR), a nuclear receptor mainly expressed in enterohepatic tissues, is a master for bile acid, lipid and glucose homeostasis. Additionally, it acts as a cell protector with unclear mechanism but may be implicated in combating against inflammation, fibrosis and cancers. FXR is thus accepted as a promising target particularly for the enterohepatic diseases, and numerous FXR modulators have been patented and developed.

AREAS COVERED

This review provides an update on the development of FXR modulators for enterohepatic diseases and offers an in-depth perspective on new strategies for the development of novel FXR modulators.

EXPERT OPINION

Despite the development of numerous FXR modulators, which culminated in the successful launch of obeticholic acid (OCA), it remains a matter of debate on how the function of FXR should be exploited for therapeutic purposes. The improvement for obesity achieved by either FXR agonists or antagonists is still in confusion. Whether the side effect of pruritus induced by OCA could be exempted for non-steroidal FXR agonists needs further validation. Apart from the development of conventional FXR ligands, emerging evidence support that restoration of FXR protein level may represent a new strategy in targeting FXR for enterohepatic and metabolic diseases.