Regulation of bile acid synthesis: pathways, nuclear receptors, and mechanisms

LXRS and FXR: the yin and yang of cholesterol and fat metabolism

Liver X receptors (LXRs) and farnesoid X receptor (FXR) are nuclear receptors that function as intracellular sensors for sterols and bile acids, respectively. In response to their ligands, these receptors induce transcriptional responses that maintain a balanced, finely tuned regulation of cholesterol and bile acid metabolism. LXRs also permit the efficient storage of carbohydrate- and fat-derived energy, whereas FXR activation results in an overall decrease in triglyceride levels and modulation of glucose metabolism. The elegant, dual interplay between these two receptor systems suggests that they coevolved to constitute a highly sensitive and efficient system for the maintenance of total body fat and cholesterol homeostasis. Emerging evidence suggests that the tissue-specific action of these receptors is also crucial for the proper function of the cardiovascular, immune, reproductive, endocrine pancreas, renal, and central nervous systems. Together, LXRs and FXR represent potential therapeutic targets for the treatment and prevention of numerous metabolic and lipid-related diseases.

Fxr(-/-) mice adapt to biliary obstruction by enhanced phase I detoxification and renal elimination of bile acids

Farnesoid X receptor knockout (Fxr(-/-)) mice cannot upregulate the bile salt export pump in bile acid loading or cholestatic conditions. To investigate whether Fxr(-/-) mice differ in bile acid detoxification compared with wild-type mice, we performed a comprehensive analysis of bile acids extracted from liver, bile, serum, and urine of naive and common bile duct-ligated wild-type and Fxr(-/-) mice using electrospray and gas chromatography mass spectrometry. In addition, hepatic and renal gene expression levels of Cyp2b10 and Cyp3a11, and protein expression levels of putative renal bile acid-transporting proteins, were investigated. We found significantly enhanced hepatic bile acid hydroxylation in Fxr(-/-) mice, in particular hydroxylations of cholic acid in the 1beta, 2beta, 4beta, 6alpha, 6beta, 22, or 23 position and a significantly enhanced excretion of these metabolites in urine. The gene expression level of Cyp3a11 was increased in the liver of Fxr(-/-) mice, whereas the protein expression levels of multidrug resistance-related protein 4 (Mrp4) were increased in kidneys of both genotypes during common bile duct ligation. In conclusion, Fxr(-/-) mice detoxify accumulating bile acids in the liver by enhanced hydroxylation reactions probably catalyzed by Cyp3a11. The metabolites formed were excreted into urine, most likely with the participation of Mrp4.

Mitochondrial cholesterol transport: A possible target in the management of hyperlipidemia

Sterol 27-hydroxylase (CYP27A1) may defend cells against accumulation of excess cholesterol, making this enzyme a possible target in the management of hyperlipidemia. The study objective was to analyze cholesterol homeostatic responses to increases in CYP27A1 activity in HepG2 cells and primary human hepatocytes. Increasing CYP27A1 activity by increasing enzyme expression led to significant increases in bile acid synthesis with compensatory increases in HMG-CoA reductase (HMGR) activity/protein, LDL receptor (LDLR) mRNA, and LDLR-mediated cholesterol uptake. Under these conditions, only a small increase in cellular 27-hydroxycholesterol (27OH-Chol) concentration was observed. No changes were detected in mature sterol regulatory element-binding proteins (SREBP) 1 or 2. Increasing CYP27A1 activity by increasing mitochondrial cholesterol transport (i.e., substrate availability) led to greater increases in bile acid synthesis with significant increases in cellular 27OH-Chol concentration. Mature SREBP 2 protein decreased significantly with compensatory decreases in HMGR protein. No change was detected in mature SREBP 1 protein. Despite increasing 27OH-Chol and lowering SREBP 2 protein concentrations, LDLR mRNA increased significantly, suggesting alternative mechanisms of LDLR transcriptional regulation. These findings suggest that regulation of liver mitochondrial cholesterol transport represents a potential therapeutic strategy in the treatment of hyperlipidemia and atherosclerosis.

Cholestyramine feeding lowers number of colonic apoptotic cells in rat

Secondary bile acids that are formed in the colon by bacterial action have the potential property of eliciting pathological conditions. Apoptosis of mucosal epithelial cells is recognized as an adaptation that may counteract such pathologies. Cholestyramine, an anion exchange resin that sequesters bile salts in the gut, could decrease levels of secondary bile salt stress and thus conserve the potency of the protective action. Two groups of rats were studied: those fed 4% cholestyramine and those fed regular rat food. Rats were fed cholestyramine for 7, 14, 21, or 28 d. All animals were evaluated for cell death (apoptosis) using in situ TUNEL staining, and confirmed with single-stranded DNA (ssDNA). The effect of cholestyramine on the proliferating cell nuclear antigen (PCNA) in colonic crypt cells was also examined. Our data shows that animals fed cholestyramine for 28 d show evidence of a significant decrease in the levels of apoptotic cells in their large intestines, particularly goblet cells, when compared with the control animals and no change in cell proliferation. Thus, cholestyramine may serve as an alternative in attenuating apoptosis associated with inflammatory disorders that can result in significant enterocyte and goblet-cell death.

Regulation of bile acid biosynthesis by hepatocyte nuclear factor 4alpha

Hepatocyte nuclear factor 4alpha (HNF4alpha) regulates many genes that are preferentially expressed in liver. Mice lacking hepatic expression of HNF4alpha (HNF4alphaDeltaL) exhibited markedly increased levels of serum bile acids (BAs) compared with HNF4alpha-floxed (HNF4alphaF/F) mice. The expression of genes involved in the hydroxylation and side chain beta-oxidation of cholesterol, including oxysterol 7alpha-hydroxylase, sterol 12alpha-hydroxylase (CYP8B1), and sterol carrier protein x, was markedly decreased in HNF4alphaDeltaL mice. Cholesterol 7alpha-hydroxylase mRNA and protein were diminished only during the dark cycle in HNF4alphaDeltaL mice, whereas expression in the light cycle was not different between HNF4alphaDeltaL and HNF4alphaF/F mice. Because CYP8B1 expression was reduced in HNF4alphaDeltaL mice, it was studied in more detail. In agreement with the mRNA levels, CYP8B1 enzyme activity was absent in HNF4alphaDeltaL mice. An HNF4alpha binding site was found in the mouse Cyp8b1 promoter that was able to direct HNF4alpha-dependent transcription. Surprisingly, cholic acid-derived BAs, produced as a result of CYP8B1 activity, were still observed in the serum and gallbladder of these mice. These studies reveal that HNF4alpha plays a central role in BA homeostasis by regulation of genes involved in BA biosynthesis, including hydroxylation and side chain beta-oxidation of cholesterol in vivo.

Weaving betaKlotho into bile acid metabolism

Bile acids are natural detergents that assist in the absorption and digestion of fats in the intestine. In liver, the synthesis of bile acids from cholesterol is regulated by multiple signaling cascades that repress transcription of the gene encoding cholesterol 7alpha-hydroxylase (CYP7A1), the rate-limiting enzyme in the classic bile acid synthesis pathway. In this issue of the JCI, Ito and coworkers demonstrate that mice lacking betaKlotho, a membrane protein with 2 putative glycosidase domains, have increased Cyp7a1 mRNA levels and bile acid concentrations. betaKlotho-KO mice also have small gallbladders and are resistant to cholesterol gallstone formation. These findings highlight the central role of betaKlotho in bile acid homeostasis and raise the possibility that this protein could be a pharmacologic target for the treatment of gallstones.

Telling the liver (not) to make bile acids: a new voice from the gut?

The maintenance of adequate amounts of bile acids in the liver, biliary tract, and intestine requires a finely tuned control of their synthesis. A paper in this issue of Cell Metabolism by Inagaki et al. (2005) indicates that sensing of the levels of bile acids in the intestine may trigger the secretion of a hormone which regulates bile acid production in the liver.

Fibroblast growth factor 15 functions as an enterohepatic signal to regulate bile acid homeostasis

The liver and intestine play crucial roles in maintaining bile acid homeostasis. Here, we demonstrate that fibroblast growth factor 15 (FGF15) signals from intestine to liver to repress the gene encoding cholesterol 7alpha-hydroxylase (CYP7A1), which catalyzes the first and rate-limiting step in the classical bile acid synthetic pathway. FGF15 expression is stimulated in the small intestine by the nuclear bile acid receptor FXR and represses Cyp7a1 in liver through a mechanism that involves FGF receptor 4 (FGFR4) and the orphan nuclear receptor SHP. Mice lacking FGF15 have increased hepatic CYP7A1 mRNA and protein levels and corresponding increases in CYP7A1 enzyme activity and fecal bile acid excretion. These studies define FGF15 and FGFR4 as components of a gut-liver signaling pathway that synergizes with SHP to regulate bile acid synthesis.

Altered hepatic cholesterol metabolism compensates for disruption of phosphatidylcholine transfer protein in mice

Phosphatidylcholine transfer protein (PC-TP) is a member of the steroidogenic acute regulatory transfer protein-related domain superfamily and is enriched in liver. To explore a role for PC-TP in hepatic cholesterol metabolism, Pctp-/- and wild-type C57BL/6J mice were fed a standard chow diet or a high-fat, high-cholesterol lithogenic diet. In chow-fed Pctp-/- mice, acyl CoA:cholesterol acyltransferase (Acat) activity was markedly increased, 3-hydroxy-3-methylglutaryl-CoA reductase activity was unchanged, and cholesterol 7alpha-hydroxylase activity was reduced. Consistent with increased Acat activity, esterified cholesterol concentrations in livers of Pctp-/- mice were increased, whereas unesterified cholesterol concentrations were reduced. Hepatic phospholipid concentrations were also decreased in the absence of PC-TP and consequently, unesterified cholesterol-to-phospholipid ratios in liver remained unchanged. The lithogenic diet downregulated 3-hydroxy-3-methylglutaryl-CoA reductase in wild-type and Pctp-/- mice, whereas Acat was increased only in wild-type mice. In response to the lithogenic diet, a greater reduction in cholesterol 7alpha-hydroxylase activity in Pctp-/- mice could be attributed to increased size and hydrophobicity of the bile salt pool. Despite higher hepatic phospholipid concentrations, the unesterified cholesterol-to-phospholipid ratio increased. The lack of Acat upregulation suggests that, in the setting of the dietary challenge, the capacity for esterification to defend against hepatic accumulation of unesterified cholesterol was exceeded in the absence of PC-TP expression. We speculate that regulation of cholesterol homeostasis is a physiological function of PC-TP in liver, which can be overcome with a cholesterol-rich lithogenic diet.

Changes in the expression of genes related to bile acid synthesis and transport by the rat liver during hepatocarcinogenesis

The relationship between BA (bile acid) secretion (measured by GC–MS) and the expression of genes (measured by reverse transcription real-time PCR) involved in liver BA transport and metabolism was investigated at 20 and 32 weeks during rat hepatocarcinogenesis. A progressive loss of mRNA for transporters (more marked for Ntcp, Bsep and Mrp2 than for Oatp1/Oatp1a1, Oatp2/Oatp1a4 and Oatp4/Oatp1b2) was found. The mRNA levels of Cyp7a1 and the nuclear receptors FXR (farnesoid X receptor), SHP (small heterodimer partner) and FTF (α-fetoprotein transcription factor) were not modified, whereas those of Cyp8b1 were enhanced and those of Cyp27 were reduced. Biliary secretion of CA (cholic acid) remained unchanged, whereas that of CDCA (chenodeoxycholic acid) and other non-C12-hydroxylated BAs was diminished. The re-appearance of ‘flat-BAs’ (mainly allo-BAs at 20 weeks and Δ4-unsaturated-BAs at 32 weeks) probably reflects the progressive decrease observed in the expression of 3-oxo-Δ4-steroid 5β-reductase, together with the maintenance of steroid 5α-reductase type I. A significant correlation between the 5α-reductase/5β-reductase ratio and bile output of ‘flat-BAs’ was found. In conclusion, during rat hepatocarcinogenesis, the expression of transporters/enzymes responsible for BA homoeostasis is changed due to mechanisms other than those controlled by FXR/SHP/FTF. These modifications result in the re-appearance of ‘flat-BAs’, together with an increased CA/CDCA ratio in bile.

Regulation of hepatic metabolic pathways by the orphan nuclear receptor SHP

SHP (small heterodimer partner) is an important component of the feedback regulatory cascade, which controls the conversion of cholesterol to bile acids. In order to identify the bona fide molecular targets of SHP, we performed global gene expression profiling combined with chromatin immunoprecipitation assays in transgenic mice constitutively expressing SHP in the liver. We demonstrate that SHP affects genes involved in diverse biological pathways, and in particular, several key genes involved in consecutive steps of cholesterol degradation, bile acid conjugation, transport and lipogenic pathways. Sustained expression of SHP leads to the depletion of hepatic bile acid pool and a concomitant accumulation of triglycerides in the liver. The mechanism responsible for this phenotype includes SHP-mediated direct repression of downstream target genes and the bile acid sensor FXRalpha, and an indirect activation of PPARgamma and SREBP-1c genes. We present evidence for the role of altered chromatin configurations in defining distinct gene-specific mechanisms by which SHP mediates differential transcriptional repression. The multiplicity of genes under its control suggests that SHP is a pleiotropic regulator of diverse metabolic pathways.

Feedback regulation of bile acid synthesis in human liver: importance of HNF-4alpha for regulation of CYP7A1

A great number of nuclear factors are involved in the negative feedback mechanism regulating bile acid synthesis. There are two major ways for the negative feedback to effect the synthesis; the SHP-dependent, involving FXR, and the SHP-independent way, affecting HNF-4alpha. We studied 23 patients with gallstone disease. Eight patients were treated with chenodeoxycholic acid, 7 with cholestyramine prior to operation, and 8 served as controls. Liver biopsies were analyzed with Real-time-PCR. In the cholestyramine-treated group mRNA levels of CYP7A1 were increased about 10-fold. Treatment with CDCA decreased the mRNA levels of CYP7A1 by about 70%. The mRNA levels of CYP8B1, CYP27A1, and CYP7B1 were not significantly altered in the treated groups. The analysis of mRNA levels for HNF-4alpha showed 64% higher levels in the cholestyramine-treated group compared to the controls. These levels showed positive and highly significant correlation to the levels of mRNA of CYP7A1 when studied in all three groups together. FXR, SHP, and LRH-1/FTF were not significantly affected by the different treatments. Our results indicate that when bile acid synthesis is upregulated by cholestyramine treatment the SHP-independent pathway for controlling CYP7A1 transcription dominates over the SHP-dependent pathway.

Hologram quantitative structure–activity relationships for a series of farnesoid X receptor activators

The farnesoid X receptor (FXR) is an attractive drug target for the development of novel therapeutic agents for the treatment of dyslipidemia and cholestasis. Hologram quantitative structure-activity relationship (HQSAR) studies were conducted on a series of potent FXR activators originated from natural product-like libraries. A training set containing 82 compounds served to establish the models. The best HQSAR model was generated using atoms, bonds, connections, chirality, and donor and acceptor as fragment distinction and fragment size default (4-7) with six components. The model was used to predict the potency of 20 test set compounds that were not included in the training set, and the predicted values were in good agreement with the experimental results. The final HQSAR model and the information obtained from HQSAR 2D contribution maps should be useful for the design of novel FXR ligands having improved potency.

Herbal or natural medicines as modulators of peroxisome proliferator-activated receptors and related nuclear receptors for therapy of metabolic syndrome

The use of herbal or natural medicines for the treatment of various disorders has a long and extensive history. Many of these herbal medicines are finding their way onto the world market as alternatives to prescribed drugs currently available to treat various disorders/ailments. In particular, hyperlipidaemia is a major risk factor for atherosclerotic coronary vascular disease, which can culminate in mortality in diabetes mellitus. There is overwhelming evidence that patients with type 2 diabetes mellitus often have metabolic syndrome and require a multifactorial intervention including aggressive treatment of arterial hypertension and dyslipidaemia to prevent cardiovascular complications. One of the most active areas of metabolic research into potential treatments is in the role of nuclear receptors as therapeutic targets for both glucose and lipid metabolism. The purpose of this review is to highlight the recent advances made by pharmaceutical and research organizations in identifying biologically active compounds from natural plant products capable of modulating nuclear receptors such as peroxisome proliferator-activated receptors and, to a lesser extent, liver X receptor and farnesoid X receptor. The specific features presented by these receptors provide an in-depth insight into the pathogenesis of metabolic disease and thus, a means of establishing potential mechanisms of action with traditional medicine. In hindsight, the review offers valuable information for rational drug design using known active compounds of plant origin. Further research may ultimately lead to a reduction in both the chronic microvascular complications of type 2 diabetes mellitus and the risk of cardiovascular disease and metabolic syndrome with the use of traditional medicine.

Cholesterol 7alpha-hydroxylase is phosphorylated at multiple amino acids

The activity of cholesterol 7alpha-hydroxylase (gpCYP7A1), the rate limiting enzyme in bile acid synthesis, has been postulated to be regulated by phosphorylation/dephosphorylation. This study has found that several kinase activators rapidly reduce the amount of bile acid produced by the human hepatoma cell line, HepG2, and that gpCYP7A1 from HepG2 cell extracts eluted in the phosphoprotein fraction of FeIII columns. After incubating the HepG2 cells with radioactive orthophosphate, the band identified as gpCYP7Al on immunoblots was strongly labeled. Recombinant gpCYP7A was expressed as 6xHIS fusion polypeptides and subjected to kinase assays. The locations of phosphorylation were mapped further by screening synthetic peptides against AMP-activated protein kinase (AMPK), c-Jun N-terminal kinase, protein kinase A, and a panel of nine protein kinase C isoforms. AMPK, also known as 3-hydroxy-3-methylglutaryl coenzyme A reductase kinase, phosphorylated cholesterol 7alpha-hydroxylase, suggesting a potential mechanism of coordination of cholesterol synthesis and degradation.

Transient changes in the expression pattern of key enzymes for bile acid synthesis during rat liver regeneration

Changes in the expression patterns of genes involved in bile acid (BA) synthesis were investigated during rat liver regeneration that follows two-thirds partial hepatectomy. BAs in bile were measured by GC-MS and the absolute and relative abundance of specific mRNAs in the liver by RT-real-time quantitative PCR. Cyclin E mRNA, used as an indicator of liver cell proliferation, peaked at day 1. The levels of mRNA of alpha-fetoprotein transcription factor (FTF) and small heterodimer partner (SHP) were first reduced (day 1) and then (days 2-3) increased, when those of farnesoid X receptor (FXR) were also transiently enhanced. The early (day 1) up-regulation of Cyp7a1, and Cyp8b1, together with the down-regulation of Cyp27, was consistent with an increased proportion of cholic acid versus chenodeoxycholic acid and a progressive recovery in total BAs secretion. The transient appearance of flat BAs (allo-BAs plus Delta4-unsaturated-BAs) during rat liver regeneration was probably due to the changes in the expression ratio of steroid 5alpha- versus 5beta-reductase. Both were first (day 1) down-regulated and then up-regulated (5alpha-reductase more than 5beta-reductase). In conclusion, changes in the expression patterns of nuclear receptors and enzymes involved in BA synthesis are consistent with the transient modifications that occur in BA pool during rat liver regeneration.

Illuminating drug discovery with biological pathways

Systems biology promises to impact significantly on the drug discovery process. One of its ultimate goals is to provide an understanding of the complete set of molecular mechanisms describing an organism. Although this goal is a long way off, many useful insights can already come from currently available information and technology. One of the biggest challenges in drug discovery today is the high attrition rate: many promising candidates prove ineffective or toxic owing to a poor understanding of the molecular mechanisms of biological systems they target. A "systems" approach can help identify pathways related to a disease and can suggest secondary effects of drugs that might cause these problems and thus ultimately improve the drug discovery pipeline.

Species-specific mechanisms for cholesterol 7alpha-hydroxylase (CYP7A1) regulation by drugs and bile acids

The gene encoding cholesterol 7alpha-hydroxylase (CYP7A1) is tightly regulated in order to control intrahepatic cholesterol and bile acid levels. Ligands of the xenobiotic-sensing pregnane X receptor inhibit CYP7A1 expression. To retrace the evolution of the molecular mechanisms underlying CYP7A1 inhibition, we used a chicken hepatoma cell system that retains the ability to be induced by phenobarbital and other drugs. Whereas bile acids regulate CYP7A1 via small heterodimer partner and liver receptor homolog-1, mRNA expression of these nuclear receptors is unchanged by xenobiotics. Instead, drugs repress chicken hepatic nuclear factor 4alpha (HNF4alpha) transcript levels concomitant with a reduction in CYP7A1 expression. Importantly, no reduction of HNF4alpha levels is found in mouse liver in vivo and in human primary hepatocyte cultures, respectively. Thus, besides the importance of HNF4alpha in CYP7A1 regulation in all species, birds and mammals use different signaling pathways to adjust CYP7A1 levels after exposure to xenobiotics.

Grape seed procyanidins improve atherosclerotic risk index and induce liver CYP7A1 and SHP expression in healthy rats

Moderate consumption of red wine reduces risk of death from cardiovascular disease. The polyphenols in red wine are ultimately responsible for this effect, exerting antiatherogenic actions through their antioxidant capacities and modulating intracellular signaling pathways and transcriptional activities. Lipoprotein metabolism is crucial in atherogenesis, and liver is the principal organ controlling lipoprotein homeostasis. This study was intended to identify the primary effects of procyanidins, the most abundant polyphenols in red wine, on both plasma lipoprotein profile and the expression of genes controlling lipoprotein homeostasis in the liver. We show that procyanidins lowered plasma triglyceride, free fatty acids, apolipoprotein B (apoB), LDL-cholesterol and nonHDL:nonLDL-cholesterol levels and slightly increased HDL-cholesterol. Liver mRNA levels of small heterodimer partner (SHP), cholesterol 7alpha-hydroxylase (CYP7A1), and cholesterol biosynthetic enzymes increased, whereas those of apoAII, apoCI, and apoCIII decreased. Lipoprotein lipase (LPL) mRNA levels increased in muscle and decreased in adipose tissue. In conclusion, procyanidins improve the atherosclerotic risk index in the postprandial state, inducing in the liver the overexpression of CYP7A1 (suggesting an increase of cholesterol elimination via bile acids) and SHP, a nuclear receptor emerging as a key regulator of lipid homeostasis at the transcriptional level. These results could explain, at least in part, the beneficial long-term effects associated with moderate red wine consumption.

Mechanism of rifampicin and pregnane X receptor inhibition of human cholesterol 7 alpha-hydroxylase gene transcription

Bile acids, steroids, and drugs activate steroid and xenobiotic receptor pregnane X receptor (PXR; NR1I2), which induces human cytochrome P4503A4 (CYP3A4) in drug metabolism and cholesterol 7 alpha-hydroxylase (CYP7A1) in bile acid synthesis in the liver. Rifampicin, a human PXR agonist, inhibits bile acid synthesis and has been used to treat cholestatic diseases. The objective of this study is to elucidate the mechanism by which PXR inhibits CYP7A1 gene transcription. The mRNA expression levels of CYP7A1 and several nuclear receptors known to regulate the CYP7A1 gene were assayed in human primary hepatocytes by quantitative real-time PCR (Q-PCR). Rifampicin reduced CYP7A1 and small heterodimer partner (SHP; NR02B) mRNA expression suggesting that SHP was not involved in PXR inhibition of CYP7A1. Rifampicin inhibited CYP7A1 reporter activity and a PXR binding site was localized to the bile acid response element-I. Mammalian two-hybrid assays revealed that PXR interacted with hepatic nuclear factor 4 alpha (HNF4 alpha, NR2A1) and rifampicin was required. Coimmunoprecipitation assay confirmed PXR interaction with HNF4 alpha. PXR also interacted with peroxisome proliferator-activated receptor gamma coactivator (PGC-1 alpha), which interacted with HNF4 alpha and induced CYP7A1 gene transcription. Rifampicin enhanced PXR interaction with HNF4 alpha and reduced PGC-1 alpha interaction with HNF4 alpha. Chromatin immunoprecipitation assay showed that PXR, HNF4 alpha, and PGC-1 alpha bound to CYP7A1 chromatin, and rifampicin dissociated PGC-1 alpha from chromatin. These results suggest that activation of PXR by rifampicin promotes PXR interaction with HNF4 alpha and blocks PGC-1 alpha activation with HNF4 alpha and results in inhibition of CYP7A1 gene transcription. Rifampicin inhibition of bile acid synthesis may be a protective mechanism against drug and bile acid-induced cholestasis.

T0901317 is a dual LXR/FXR agonist

We characterize the ability of the liver X receptor (LXRalpha [NR1H3] and LXRbeta [NR1H2]) agonist, T0901317, to activate the farnesoid X receptor (FXR [NR4H4]). Although T0901317 is a much more potent activator of LXR than FXR, this ligand actually activates FXR more potently than a natural bile acid FXR ligand, chenodeoxycholic acid. Thus, the FXR activity of T0901317 must be considered when utilizing this agonist as a pharmacological tool to investigate LXR function.