Activation of the nuclear receptor FXR induces fibrinogen expression: a new role for bile acid signaling

FXR agonists in NASH treatment

The farnesoid X receptor (FXR), a bile acid (BA)-activated nuclear receptor highly expressed in the liver and intestine, regulates the expression of genes involved in cholesterol and bile acid homeostasis, hepatic gluconeogenesis, lipogenesis, inflammation and fibrosis, in addition to controlling intestinal barrier integrity, preventing bacterial translocation and maintaining gut microbiota eubiosis. Non-alcoholic steatohepatitis (NASH), an advanced stage of non-alcoholic fatty liver disease, is characterized by hepatic steatosis, hepatocyte damage (ballooning) and inflammation, leading to fibrosis, cirrhosis and hepatocellular carcinoma. NASH represents a major unmet medical need, but no pharmacological treatments have yet been approved. The pleiotropic mechanisms involved in NASH development offer a range of therapeutic opportunities and among them FXR activation has emerged as an established pharmacological target. Various FXR agonists with different physicochemical properties, which can be broadly classified as BA derivatives, non-BA-derived steroidal FXR agonists, non-steroidal FXR agonists, and partial FXR agonists, are in advanced clinical development. In this review we will summarize key preclinical and clinical features of the most advanced FXR agonists and critically evaluate their potential in NASH treatment.

EDP-297: A novel, farnesoid X receptor agonist-Results of a phase I study in healthy subjects

EDP-297 is a farnesoid X receptor agonist under development for treating nonalcoholic steatohepatitis. The pharmacokinetic (PK), pharmacodynamic (PD), food effect, and safety were evaluated in a single ascending dose (SAD) and multiple ascending dose (MAD) phase I study. Healthy subjects received single EDP-297 doses of 20-600 μg or once daily doses of 5-90 μg for 14 days. Safety, PKs, and PDs were assessed, including fibroblast growth factor 19 (FGF-19) and 7-α-hydroxy-4-cholesten-3-one (C4). Among 82 subjects, EDP-297 was generally well-tolerated. Pruritus was observed in four subjects in the SAD phase and seven subjects in the MAD phase; four severe cases occurred at 90 μg in the MAD phase, including one that led to drug discontinuation. A grade 2 elevation in alanine aminotransferase occurred with 90 μg. Mean lipid values remained within normal range. Plasma exposures of EDP-297 increased with SADs and MADs, with mean half-life following multiple doses of 9-12.5 h. No food effect was observed. Mean FGF-19 increased and C4 decreased up to 95% and 92%, respectively. EDP-297 was generally well-tolerated up to 60 μg MAD, with linear PKs suitable for once daily oral dosing, target engagement, and no food effect.

EDP-305 in patients with NASH: A phase II double-blind placebo-controlled dose-ranging study

BACKGROUND & AIMS

EDP-305 is an oral farnesoid X receptor (FXR) agonist under development for the treatment of non-alcoholic steatohepatitis (NASH). Herein, we aimed to assess the efficacy, safety and tolerability of EDP-305 in patients with fibrotic NASH.

METHODS

In this double-blind phase II study, patients with fibrotic NASH (without cirrhosis), diagnosed by historical biopsy or phenotypically, were randomized to EDP-305 1 mg, EDP-305 2.5 mg, or placebo, for 12 weeks. The primary endpoint was mean change in alanine aminotransferase (ALT) from baseline to Week 12, and the key secondary endpoint was mean change in liver fat content from baseline to Week 12.

RESULTS

Between January 2018 and July 2019, 134 patients were randomized and 132 were evaluated. At Week 12, the least squares mean reductions from baseline in ALT for patients receiving 2.5 mg EDP-305 and 1 mg EDP-305 were -27.9 U/L (95% CI 0.03 to 24.9; p = 0.049) and -21.7 U/L (-5.8 to 18.3: p = 0.304), respectively, compared to -15.4 U/L for those receiving placebo. Absolute liver fat reduction was -7.1% (2.0-7.5; p = 0.0009) with 2.5 mg EDP-305, -3.3% with EDP-305 1 mg, and -2.4% with placebo. The most common (≥5%) adverse events were pruritus, nausea, vomiting, diarrhea, headache, and dizziness. Pruritus occurred in 50.9%, 9.1%, and 4.2% of patients in the 2.5 mg, 1 mg, and placebo groups, respectively, and led to study drug discontinuation in 20.8% of patients in the 2.5 mg group and 1.8% in the 1 mg group.

CONCLUSIONS

EDP-305 reduced ALT levels and liver fat content, providing support for a longer-term trial assessing histological endpoints in patients with NASH. CLINICALTRIALS.

GOV NUMBER

NCT03421431 LAY SUMMARY: Non-alcoholic fatty liver disease is a chronic hepatic disease that can progress to non-alcoholic steatohepatitis (NASH), which is associated with an increased risk of cirrhosis and liver cancer. Results from this phase II study support continued development of EDP-305, an oral farnesoid X receptor agonist, for the treatment of patients with NASH.

Coagulopathy in Malnourished Mice Is Sexually Dimorphic and Regulated by Nutrient-Sensing Nuclear Receptors

Liver dysfunction, including coagulopathy, is a prominent feature of protein-energy malnutrition. To identify mechanisms underlying malnutrition-associated coagulopathy, we administered a low-protein low-fat diet to lactating dams and examined hepatic transcription and plasma coagulation parameters in young adult weanlings. Malnutrition impacted body composition to a greater extent in male versus female mice. Transcriptional profiles suggested opposing effects of nutrient-sensing nuclear receptors, namely induction of peroxisome proliferator-activated receptor α (PPARα) targets and repression of farnesoid-X-receptor (FXR) targets. Coagulopathy with decreased synthesis of fibrinogen-α (FGA) and factor 11 (F11) was observed in malnourished male animals but not female animals. In primary mouse hepatocytes, FXR agonist increased and PPARα agonist decreased Fga and F11 messenger RNA expression. Nuclear receptor DNA response elements were identified in the Fga and F11 gene regulatory regions, and opposing effects of FXR and PPARα were confirmed with luciferase assays. Unexpectedly, hepatic PPARα protein was markedly depleted in malnourished male liver and was not enriched on Fga or F11 response elements. Rather, there was loss of FXR binding at these response elements. Reduced PPARα protein was associated with loss of hepatocyte peroxisomes, which are necessary for bile acid biosynthesis, and with decreased concentrations of bile acids that function as FXR ligands, most notably the FXR agonist chenodeoxycholic acid. Conclusion: Malnutrition impairs growth and liver synthetic function more severely in male mice than in female mice. Malnourished male mice are coagulopathic and exhibit decreased hepatocyte peroxisomes, FXR agonist bile acids, FXR binding on Fga and F11 gene regulatory elements, and coagulation factor synthesis. These effects are absent in female mice, which have low baseline levels of PPARα, suggesting that nutrient-sensing nuclear receptors regulate coagulation factor synthesis in response to host nutritional status in a sex-specific manner.

Orphan nuclear receptor ERRγ is a key regulator of human fibrinogen gene expression

Fibrinogen, 1 of 13 coagulation factors responsible for normal blood clotting, is synthesized by hepatocytes. Detailed roles of the orphan nuclear receptors regulating fibrinogen gene expression have not yet been fully elucidated. Here, we identified estrogen-related receptor gamma (ERRγ) as a novel transcriptional regulator of human fibrinogen gene expression. Overexpression of ERRγ specially increased fibrinogen expression in human hepatoma cell line. Cannabinoid receptor types 1(CB1R) agonist arachidonyl-2'-chloroethylamide (ACEA) up-regulated transcription of fibrinogen via induction of ERRγ, whereas knockdown of ERRγ attenuated fibrinogen expression. Deletion analyses of the fibrinogen γ (FGG) gene promoter and ChIP assays revealed binding sites of ERRγ on human fibrinogen γ gene promoter. Moreover, overexpression of ERRγ was sufficient to increase fibrinogen gene expression, whereas treatment with GSK5182, a selective inverse agonist of ERRγ led to its attenuation in cell culture. Finally, fibrinogen and ERRγ gene expression were elevated in liver tissue of obese patients suggesting a conservation of this mechanism. Overall, this study elucidates a molecular mechanism linking CB1R signaling, ERRγ expression and fibrinogen gene transcription. GSK5182 may have therapeutic potential to treat hyperfibrinogenemia.

Mutations in the nuclear bile acid receptor FXR cause progressive familial intrahepatic cholestasis

Neonatal cholestasis is a potentially life-threatening condition requiring prompt diagnosis. Mutations in several different genes can cause progressive familial intrahepatic cholestasis, but known genes cannot account for all familial cases. Here we report four individuals from two unrelated families with neonatal cholestasis and mutations in NR1H4, which encodes the farnesoid X receptor (FXR), a bile acid-activated nuclear hormone receptor that regulates bile acid metabolism. Clinical features of severe, persistent NR1H4-related cholestasis include neonatal onset with rapid progression to end-stage liver disease, vitamin K-independent coagulopathy, low-to-normal serum gamma-glutamyl transferase activity, elevated serum alpha-fetoprotein and undetectable liver bile salt export pump (ABCB11) expression. Our findings demonstrate a pivotal function for FXR in bile acid homeostasis and liver protection.

Neonatal cholestasis is a result of elevated bile acid levels, and is associated with mutations in genes regulating bile acid homeostasis. Here the authors identify mutations in the bile acid sensing farnesoid X receptor in four individuals with neonatal cholestasis from two unrelated families.

Nuclear bile acid signaling through the farnesoid X receptor

Bile acids (BAs) are amphipathic molecules produced from cholesterol by the liver. Expelled from the gallbladder upon meal ingestion, BAs serve as fat solubilizers in the intestine. BAs are reabsorbed in the ileum and return via the portal vein to the liver where, together with nutrients, they provide signals to coordinate metabolic responses. BAs act on energy and metabolic homeostasis through the activation of membrane and nuclear receptors, among which the nuclear receptor farnesoid X receptor (FXR) is an important regulator of several metabolic pathways. Highly expressed in the liver and the small intestine, FXR contributes to BA effects on metabolism, inflammation and cell cycle control. The pharmacological modulation of its activity has emerged as a potential therapeutic strategy for liver and metabolic diseases. This review highlights recent advances regarding the mechanisms by which the BA sensor FXR contributes to global signaling effects of BAs, and how FXR activity may be regulated by nutrient-sensitive signaling pathways.

Mechanistic insights into isoform-dependent and species-specific regulation of bile salt export pump by farnesoid X receptor

Expression of bile salt export pump (BSEP) is regulated by the bile acid/farnesoid X receptor (FXR) signaling pathway. Two FXR isoforms, FXRα1 and FXRα2, are predominantly expressed in human liver. We previously showed that human BSEP was isoform-dependently regulated by FXR and diminished with altered expression of FXRα1 and FXRα2 in patients with hepatocellular carcinoma. In this study, we demonstrate that FXRα1 and FXRα2 regulate human BSEP through two distinct FXR responsive elements (FXRE): IR1a and IR1b. As the predominant regulator, FXRα2 potently transactivated human BSEP through IR1a, while FXRα1 weakly transactivated human BSEP through a newly identified IR1b. Relative expression of FXRα1 and FXRα2 affected human BSEP expression in vitro and in vivo. Electrophoretic mobility shift and chromatin immunoprecipitation assays confirmed the binding and recruitment of FXRα1 and FXRα2 to IR1b and IR1a. Sequence analysis concluded that IR1b was completely conserved among species, whereas IR1a exhibited apparent differences across species. Sequence variations in IR1a were responsible for the observed species difference in BSEP transactivation by FXRα1 and FXRα2. In conclusion, FXR regulates BSEP in an isoform-dependent and species-specific manner through two distinct FXREs, and alteration of relative FXR isoform expression may be a potential mechanism for FXR to precisely regulate human BSEP in response to various physiological and pathological conditions.

Regulation of human class I alcohol dehydrogenases by bile acids

Class I alcohol dehydrogenases (ADH1s) are the rate-limiting enzymes for ethanol and vitamin A (retinol) metabolism in the liver. Because previous studies have shown that human ADH1 enzymes may participate in bile acid metabolism, we investigated whether the bile acid-activated nuclear receptor farnesoid X receptor (FXR) regulates ADH1 genes. In human hepatocytes, both the endogenous FXR ligand chenodeoxycholic acid and synthetic FXR-specific agonist GW4064 increased ADH1 mRNA, protein, and activity. Moreover, overexpression of a constitutively active form of FXR induced ADH1A and ADH1B expression, whereas silencing of FXR abolished the effects of FXR agonists on ADH1 expression and activity. Transient transfection studies and electrophoretic mobility shift assays revealed functional FXR response elements in the ADH1A and ADH1B proximal promoters, thus indicating that both genes are direct targets of FXR. These findings provide the first evidence for direct connection of bile acid signaling and alcohol metabolism.

Bile salt export pump is dysregulated with altered farnesoid X receptor isoform expression in patients with hepatocellular carcinoma

As a canalicular bile acid effluxer, the bile salt export pump (BSEP) plays a vital role in maintaining bile acid homeostasis. BSEP deficiency leads to severe cholestasis and hepatocellular carcinoma (HCC) in young children. Regardless of the etiology, chronic inflammation is the common pathological process for HCC development. Clinical studies have shown that bile acid homeostasis is disrupted in HCC patients with elevated serum bile acid level as a proposed marker for HCC. However, the underlying mechanisms remain largely unknown. In this study, we found that BSEP expression was severely diminished in HCC tissues and markedly reduced in adjacent nontumor tissues. In contrast to mice, human BSEP was regulated by farnesoid X receptor (FXR) in an isoform-dependent manner. FXR-α2 exhibited a much more potent activity than FXR-α1 in transactivating human BSEP in vitro and in vivo. The decreased BSEP expression in HCC was associated with altered relative expression of FXR-α1 and FXR-α2. FXR-α1/FXR-α2 ratios were significantly increased, with undetectable FXR-α2 expression in one third of the HCC tumor samples. A similar correlation between BSEP and FXR isoform expression was confirmed in hepatoma Huh7 and HepG2 cells. Further studies showed that intrahepatic proinflammatory cytokines, such as interleukin-6 (IL-6) and tumor necrosis factor alpha (TNF-α), were significantly elevated in HCC tissues. Treatment of Huh7 cells with IL-6 and TNF-α resulted in a marked increase in FXR-α1/FXR-α2 ratio, concurrent with a significant decrease in BSEP expression.

CONCLUSION

BSEP expression is severely diminished in HCC patients associated with alteration of FXR isoform expression induced by inflammation. Restoration of BSEP expression through suppressing inflammation in the liver may reestablish bile acid homeostasis.

Bile acids and coagulation factors: paradoxical association in children with chronic liver disease

OBJECTIVE

To examine whether there is an association between bile acids and coagulation factors in children with chronic liver disease.

METHODS

Forty-five patients (age 2.8 months-18.8 years) were included in this cross-sectional observational study carried out at a single tertiary referral center. Coagulation factors, prothrombin time, albumin, and total fasting serum bile acids were analyzed. The international normalized ratio (INR) and the pediatric end-stage liver disease score were calculated.

RESULTS

The 12 patients with bile acids more than 200 μmol/l showed a significant positive correlation between bile acids and factor V (FV), FVII, and prothrombin time (r(s) = 0.80, 0.72, 0.60, P<0.05) and a significant negative correlation between bile acids and INR (r(s) = -0.58, P<0.05). Conversely, in the group with bile acids less than 200 μmol/l, there was a significant negative correlation between bile acids and FVII and FIX (r(s) = -0.41 and -0.41, P<0.05) and a positive, albeit nonsignificant, correlation between bile acids and INR. No in-vitro analytical interference between bile acids and coagulation factors was found. Patients with bile acids more than 200 μmol/l had a significantly worse outcome than patients with lower levels of bile acids.

CONCLUSION

A positive correlation was found between bile acids and coagulation factors in patients with bile acids more than 200 μmol/l. Coagulation factors may be questionable as prognostic markers in patients with markedly elevated bile acids.

Molecular basis of esophageal cancer development and progression

This article discusses the molecular basis of esophageal cancer development and subsequent progression of disease. Differing epidemiologic factors are associated with esophageal adenocarcinoma and squamous cell carcinoma. These 2 different histologic types have differing putative underlying mechanisms of transdifferentiation from normal esophageal mucosa to malignant histologies via gene dysregulation, biochemical modifications, and altered cell signaling pathways. Our developing understanding of the molecular events underlying esophageal cancer is leading to the establishment of identifiable biomarkers and the clinical use of molecularly targeted treatment agents. The identification of driving genetic mutations and altered signaling pathways has also had favorable outcomes.

Deciphering the nuclear bile acid receptor FXR paradigm

Originally called retinoid X receptor interacting protein 14 (RIP14), the farnesoid X receptor (FXR) was renamed after the ability of its rat form to bind supra-physiological concentrations of farnesol. In 1999 FXR was de-orphanized since primary bile acids were identified as natural ligands. Strongly expressed in the liver and intestine, FXR has been shown to be the master transcriptional regulator of several entero-hepatic metabolic pathways with relevance to the pathophysiology of conditions such as cholestasis, fatty liver disease, cholesterol gallstone disease, intestinal inflammation and tumors. Furthermore, given the importance of FXR in the gut-liver axis feedbacks regulating lipid and glucose homeostasis, FXR modulation appears to have great input in diseases such as metabolic syndrome and diabetes. Exciting results from several cellular and animal models have provided the impetus to develop synthetic FXR ligands as novel pharmacological agents. Fourteen years from its discovery, FXR has gone from bench to bedside; a novel nuclear receptor ligand is going into clinical use.

Role of fibrinogen and protease-activated receptors in acute xenobiotic-induced cholestatic liver injury

Alpha-naphthylisothiocyanate (ANIT)-induced cholestatic liver injury causes tissue factor (TF)-dependent coagulation in mice, and TF deficiency reduces ANIT-induced liver injury. However, the mechanism whereby TF contributes to hepatotoxicity in this model is not known. Utilizing pharmacological and genetic strategies, we evaluated the contribution of fibrinogen and two distinct receptors for thrombin, protease-activated receptor-1 (PAR-1) and PAR-4, in a model of acute ANIT hepatotoxicity. ANIT administration (60 mg/kg, po) caused a marked induction of the genes encoding the three fibrinogen chains (α, β, and γ) in liver, an increase in plasma fibrinogen, and concurrent deposition of thrombin-cleaved fibrin in liver. Partial depletion of circulating fibrinogen with ancrod did not impact ANIT hepatotoxicity. However, complete fibrin(ogen) deficiency significantly reduced serum alanine aminotransferase activity and hepatocellular necrosis in ANIT-treated mice. ANIT-induced hepatocellular necrosis was similar in PAR-1(-/-) mice compared with PAR-1(+/+) mice. Interestingly, the progression of ANIT-induced hepatocellular necrosis was significantly reduced in PAR-4(-/-) mice and by administration of an inhibitory PAR-4 pepducin (P4Pal-10, 0.5 mg/kg, sc) to wild-type mice 8 h after ANIT treatment. Interestingly, a distinct lesion, parenchymal-type peliosis, was also observed in PAR-4(-/-) mice treated with ANIT and in mice that were given P4Pal-10 prior to ANIT administration. The results suggest that fibrin(ogen), but not PAR-1, contributes to the progression of ANIT hepatotoxicity in mice. Moreover, the data suggest a dual role for PAR-4 in ANIT hepatotoxicity, both mediating an early protection against peliosis and contributing to the progression of hepatocellular necrosis.

The molecular pathogenesis of Barrett's esophagus: common signaling pathways in embryogenesis metaplasia and neoplasia

Although Barrett's esophagus has been recognized for over 50 years, the cellular and molecular mechanisms leading to the replacement of squamous esophageal epithelium with a columnar type are largely unknown. Barrett's is known to be an acquired process secondary to chronic gastroesophageal reflux disease and occurs in the presence of severe disruption of the gastroesophageal barrier and reflux of a mixture of gastric and duodenal content. Current hypothesis suggest that epithelial change occurs due to stimulation of esophageal stem cells present in the basal layers of the epithelium or submucosal glands, toward a columnar epithelial differentiation pathway. The transcription factor CDX2 seems to play a key role in promoting the cellular biology necessary for columnar differentiation, and can be induced by bile salt and acid stimulation. Several cellular signaling pathways responsible for modulation of intestinal differentiation have also been identified and include WNT, Notch, BMP, Sonic HH and TGFB. These also have been shown to respond to stimulation by bile acids, acid or both and may influence CDX2 expression. Their relative activity within the stem cell population is almost certainly responsible for the development of the esophageal columnar epithelial phenotype we know as Barrett's esophagus.

Coordinated regulation of dimethylarginine dimethylaminohydrolase-1 and cationic amino acid transporter-1 by farnesoid X receptor in mouse liver and kidney and its implication in the control of blood levels of asymmetric dimethylarginine

Asymmetric dimethylarginine (ADMA) is a potent endogenous inhibitor of endothelial nitric-oxide synthase (eNOS), and increased plasma concentrations of ADMA have been regarded as a risk factor for a number of cardiovascular diseases. Circulating ADMA is largely taken up by liver and kidney via system y(+) carriers of the cationic amino acid (CAT) family and subsequently metabolized by dimethylarginine dimethylaminohydrolases (DDAHs). As such, agents targeted at enhancing ADMA metabolism may prove to be useful in the prevention and/or treatment of various types of cardiovascular disease. Farnesoid X receptor (FXR) is a member of the nuclear receptor superfamily and plays an important role in the maintenance of cholesterol and bile acid homeostasis. We report here that treatment of mice with an FXR agonist 3-(2,6-dichlorophenyl)-4-(3'-carboxy-2-chlorostilben-4-yl)oxymethyl-5-isopropylisoxazole; GW4064) led to increased expression of DDAH-1 and CAT-1 in both liver and kidney. In cultured human hepatocytes and kidney proximal tubular epithelial cells, GW4064 increased CAT-1 expression, and this was associated with a significant increase in the cellular uptake of ADMA. Promoter analyses suggest that CAT-1 is a likely target of FXR, and a functional FXR response element was found in the promoter region of CAT-1 gene. These data suggest that FXR may play an important role in regulating blood levels of ADMA via coordinated regulation of DDAH-1 and CAT-1 in liver and kidney.

Role of bile acids and bile acid receptors in metabolic regulation

The incidence of the metabolic syndrome has taken epidemic proportions in the past decades, contributing to an increased risk of cardiovascular disease and diabetes. The metabolic syndrome can be defined as a cluster of cardiovascular disease risk factors including visceral obesity, insulin resistance, dyslipidemia, increased blood pressure, and hypercoagulability. The farnesoid X receptor (FXR) belongs to the superfamily of ligand-activated nuclear receptor transcription factors. FXR is activated by bile acids, and FXR-deficient (FXR(-/-)) mice display elevated serum levels of triglycerides and high-density lipoprotein cholesterol, demonstrating a critical role of FXR in lipid metabolism. In an opposite manner, activation of FXR by bile acids (BAs) or nonsteroidal synthetic FXR agonists lowers plasma triglycerides by a mechanism that may involve the repression of hepatic SREBP-1c expression and/or the modulation of glucose-induced lipogenic genes. A cross-talk between BA and glucose metabolism was recently identified, implicating both FXR-dependent and FXR-independent pathways. The first indication for a potential role of FXR in diabetes came from the observation that hepatic FXR expression is reduced in animal models of diabetes. While FXR(-/-) mice display both impaired glucose tolerance and decreased insulin sensitivity, activation of FXR improves hyperglycemia and dyslipidemia in vivo in diabetic mice. Finally, a recent report also indicates that BA may regulate energy expenditure in a FXR-independent manner in mice, via activation of the G protein-coupled receptor TGR5. Taken together, these findings suggest that modulation of FXR activity and BA metabolism may open new attractive pharmacological approaches for the treatment of the metabolic syndrome and type 2 diabetes.

FXR: a metabolic regulator and cell protector

Farnesoid X receptor (FXR) is a member of the nuclear receptor superfamily of ligand-activated transcription factors. As a metabolic regulator, FXR plays key roles in bile acid, cholesterol, lipid, and glucose metabolism. Therefore, FXR is a potential drug target for a number of metabolic disorders, especially those related to the metabolic syndrome. More recently, our group and others have extended the functions of FXR to more than metabolic regulation, which include anti-bacterial growth in intestine, liver regeneration, and hepatocarcinogenesis. These new findings suggest that FXR has much broader roles than previously thought, and also highlight FXR as a drug target for multiple diseases. This review summarizes the basic information of FXR but focuses on its new functions.

Bile acids and signal transduction: role in glucose homeostasis

Bile acids are mainly recognized for their role in dietary lipid absorption and cholesterol homeostasis. However, recent progress in bile acid research suggests that bile acids are important signaling molecules that play a role in glucose homeostasis. Among the various supporting evidence, several reports have demonstrated an improvement of the glycemic index of type 2 diabetic patients treated with diverse bile acid binding resins. Herein, we review the diverse interactions of bile acids with various signaling/response pathways, including calcium mobilization and protein kinase activation, membrane receptor-mediated responses, and nuclear receptor responses. Some of the effects of the bile acids are direct through the activation of specific receptors, i.e., TGR5, CAR, VDR, and FXR, while others imply modulation of the hormonal, growth factor and/or neuromediator responses, i.e., glucagon, EGF, and acetylcholine. We also discuss recent evidence implicating the interaction of bile acids with glucose homeostasis mechanisms, with the integration of our understanding of how the signaling mechanisms modulated by bile acid could regulate glucose metabolism.

FXR: a promising target for the metabolic syndrome?

The metabolic syndrome is an insulin-resistant state that is characterized by a cluster of cardiovascular risk factors, including abdominal obesity, hyperglycemia, elevated blood pressure and combined dyslipidemia. In this review, we discuss the role of the bile-acid-activated farnesoid X receptor (FXR) in the modulation of the metabolic syndrome. Owing to its regulatory actions in lipid and glucose homeostasis, FXR is a potential pharmacological target. Moreover, the observation that FXR also influences endothelial function and atherosclerosis indicates a regulatory role in the cardiovascular complications that are associated with the metabolic syndrome. The pharmacological activation of FXR leads to a complex response that integrates beneficial actions and potentially undesirable side-effects. Thus, the identification of selective FXR modulators (selective bile acid receptor modulators) is required for the development of compounds that can be used to treat the metabolic syndrome.

International Union of Pharmacology. LXII. The NR1H and NR1I receptors: constitutive androstane receptor, pregnene X receptor, farnesoid X receptor alpha, farnesoid X receptor beta, liver X receptor alpha, liver X receptor beta, and vitamin D receptor

The nuclear receptors of the NR1H and NR1I subgroups include the constitutive androstane receptor, pregnane X receptor, farnesoid X receptors, liver X receptors, and vitamin D receptor. The newly emerging functions of these related receptors are under the control of metabolic pathways, including metabolism of xenobiotics, bile acids, cholesterol, and calcium. This review summarizes results of structural, pharmacologic, and genetic studies of these receptors.

FXR regulates organic solute transporters alpha and beta in the adrenal gland, kidney, and intestine

Expression of the farnesoid X receptor (FXR; NR1H4) is limited to the liver, intestine, kidney, and adrenal gland. However, the role of FXR in the latter two organs is unknown. In the current study, we performed microarray analysis using RNA from H295R cells infected with constitutively active FXR. Several putative FXR target genes were identified, including the organic solute transporters alpha and beta (OSTalpha and OSTbeta). Electromobility shift assays and promoter-reporter studies identified functional farnesoid X receptor response elements (FXREs) in the promoters of both human genes. These FXREs are conserved in both mouse genes. Treatment of wild-type mice with 3-(2,6-dichlorophenyl)-4-(3'-carboxy-2-chloro-stilben-4-yl)-oxymethyl-5-isopropyl-isoxazole (GW4064), a synthetic FXR agonist, induced OSTalpha and OSTbeta mRNAs in the intestine and kidney. Both mRNAs were also induced when wild-type, but not FXR-deficient (FXR-/-), adrenals were cultured in the presence of GW4064. OSTalpha and OSTbeta mRNA levels were also induced in the adrenals and kidneys of wild-type, but not FXR-/-, mice after the increase of plasma bile acids in response to the hepatotoxin alpha-naphthylisothiocyanate. Finally, overexpression of human OSTalpha and OSTbeta facilitated the uptake of conjugated chenodeoxycholate and the activation of FXR target genes. These results demonstrate that OSTalpha and OSTbeta are novel FXR target genes that are expressed in the adrenal gland, kidney, and intestine.

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.

Alpha-crystallin is a target gene of the farnesoid X-activated receptor in human livers

Alpha-crystallins comprise 35% of soluble proteins in the ocular lens and possess chaperone-like functions. Furthermore, the alphaA subunit (alphaA-crystallin) of alpha crystallin is thought to be "lens-specific" as only very low levels of expression were detected in a few non-lenticular tissues. Here we report that human alphaA-crystallin is expressed in human livers and is regulated by farnesoid X-activated receptor (FXR) in response to FXR agonists. AlphaA-crystallin was identified in a microarray screen as one of the most highly induced genes after treatment of HepG2 cells with the synthetic FXR ligand GW4064. Northern blot and quantitative real-time PCR analyses confirmed that alphaA-crystallin expression was induced in HepG2-derived cell lines and human primary hepatocytes and hepatic stellate cells in response to either natural or synthetic FXR ligands. Transient transfection studies and electrophoretic mobility shift assays revealed a functional FXR response element located in intron 1 of the human alphaA-crystallin gene. Importantly, immunohistochemical staining of human liver sections showed increased alphaA-crystallin expression in cholangiocytes and hepatocytes. As a member of the small heat shock protein family possessing chaperone-like activity, alphaA-crystallin may be involved in protection of hepatocytes from the toxic effects of high concentrations of bile acids, as would occur in disease states such as cholestasis.