The nuclear receptor PXR is a lithocholic acid sensor that protects against liver toxicity

The vitamin D-antimicrobial peptide pathway and its role in protection against infection

Vitamin D deficiency has been correlated with increased rates of infection. Since the early 19th century, both environmental (i.e., sunlight) and dietary sources (cod liver) of vitamin D have been identified as treatments for TB. The recent discovery that vitamin D induces antimicrobial peptide gene expression explains, in part, the 'antibiotic' effect of vitamin D and has greatly renewed interest in the ability of vitamin D to improve immune function. Subsequent work indicates that this regulation is biologically important for the response of the innate immune system to wounds and infection and that deficiency may lead to suboptimal responses toward bacterial and viral infections. The regulation of the cathelicidin antimicrobial peptide gene is a human/primate-specific adaptation and is not conserved in other mammals. The capacity of the vitamin D receptor to act as a high-affinity receptor for vitamin D and a low-affinity receptor for secondary bile acids and potentially other novel nutritional compounds suggests that the evolutionary selection to place the cathelicidin gene under control of the vitamin D receptor allows for its regulation under both endocrine and xenobiotic response systems. Future studies in both humans and humanized mouse models will elucidate the importance of this regulation and lead to the development of potential therapeutic applications.

Sterol-protein interactions in cholesterol and bile acid synthesis

Cholesterol and other cholesterol related metabolites, oxysterols, and bile acids, establish specific interactions with enzymes and other proteins involved in cholesterol and bile acid homeostasis, triggering a variety of biological responses. The substrate-enzyme binding represents the best-characterized type of complementary interaction between proteins and small molecules. Key enzymes in the pathway that converts cholesterol to bile acids belong to the cytochrome P450 superfamily. In contrast to the majority of P450 enzymes, those acting on cholesterol and related metabolites exhibit higher stringency with respect to substrate molecules. This stringency, coupled with the specificity of the reactions, dictates the chemical features of intermediate metabolites (oxysterols) and end products (bile acids). Both oxysterols and bile acids have emerged in recent years as new signalling molecules due to their ability to interact and activate nuclear receptors, and consequently to regulate the transcription of genes involved in cholesterol and bile acid homeostasis and metabolism, but also in glucose and fatty acid metabolism. Interestingly, other proteins function as bile acid or sterol receptors. New findings indicate that bile acids also interact with a membrane G protein-coupled receptor, triggering a signalling cascade that ultimately promote energy expenditure. On the other end, cholesterol and side chain oxysterols establish specific interactions with different proteins residing in the endoplasmic reticulum that result in controlled protein degradation and/or trafficking to the Golgi and the nucleus. These regulatory pathways converge and contribute to adapt cholesterol uptake and synthesis to the cellular needs.

Intestinal detoxification limits the activation of hepatic pregnane X receptor by lithocholic acid

The intestinal-derived secondary bile acid (BA) lithocholic acid (LCA) is hepatotoxic and is implicated in the pathogenesis of cholestatic diseases. LCA is an endogenous ligand of the xenobiotic nuclear receptor pregnane X receptor (PXR), but there is currently no consensus on the respective roles of hepatic and intestinal PXR in mediating protection against LCA in vivo. Under the conditions reported here, we show that mice lacking Pxr are resistant to LCA-mediated hepatotoxicity. This unexpected phenotype is found in association with enhanced urinary BA excretion and elevated basal expression of drug metabolism enzymes and the hepatic sulfate donor synthesis enzyme Papss2 in Pxr(-/-) mice. By subsequently comparing molecular responses to dietary and intraperitoneal administration of LCA, we made two other significant observations: 1) LCA feeding induces intestinal, but not hepatic, drug-metabolizing enzymes in a largely Pxr-independent manner; and 2) in contrast to LCA feeding, bypassing first-pass gut transit by intraperitoneal administration of LCA did induce hepatic detoxification machinery and in a Pxr-dependent manner. These data reconcile important discrepancies in the reported molecular responses to this BA and suggest that Pxr plays only a limited role in mediating responses to gut-derived LCA. Furthermore, the route of administration must be considered in the future planning and interpretation of experiments designed to assess hepatic responses to BAs, orally administered pharmaceuticals, and dietary toxins.

Farnesoid X receptor-Acting through bile acids to treat metabolic disorders

Farnesoid X receptor (FXR) is a member of the nuclear receptor superfamily and plays an important role in maintaining bile acid, lipid and glucose homeostasis. Bile acids are endogenous ligands for FXR. However, bile acids may also activate pathways independent of FXR. The development of specific FXR agonists has provided important insights into the role of FXR in metabolism. Recent data have demonstrated that FXR is a therapeutic target for treatment of certain metabolic disorders. This review will focus on recent advances in the role of FXR in metabolic disease.

Transcriptional activation of PPARalpha by phenobarbital in the absence of CAR and PXR

The nuclear receptors CAR (constitutive androstane receptor) and PXR (pregnane X receptor) mediate the effects of phenobarbital on gene transcription. To investigate the relative contribution of these nuclear receptors to the expression of specific genes we studied the effect of phenobarbital in livers of wild type, CAR(-/-), PXR(-/-) and CAR/PXR(-/-) knockout mice. Spotted Steroltalk v1 cDNA arrays were applied containing probes for genes involved in drug metabolism, sterol biosynthesis, steroid synthesis/transport and heme synthesis. In the absence of CAR and PXR, phenobarbital unexpectedly induced mRNAs of several nuclear receptors, including PPARalpha and its target genes Cyp4a10 and Cyp4a14. Interestingly, in primary cultures of hepatocytes isolated from CAR/PXR(-/-) knockout mice, phenobarbital increased HNF-4alpha levels. In further experiments in these hepatocyte cultures we provide evidence that phenobarbital directly induces transcription of the PPARalpha gene via its HNF-4alpha response element, and indirectly by lack of inhibitory crosstalk of AMPK, CAR and PXR with HNF-4alpha. Our results provide further insight into CAR and PXR-independent effects of phenobarbital and the crosstalk between different nuclear receptor signaling pathways.

Bile acid-activated receptors in the treatment of dyslipidemia and related disorders

Dyslipidemia is a metabolic disorder that constitutes a major risk factor for cardiovascular diseases and stroke and is often associated with diabetes mellitus and atherosclerosis. In recent years a number of ligand-activated receptors have been found to exert a role in integrating essential steps of lipid and glucose metabolism. Bile acid-activated receptors are a defined subset of nuclear and G-protein coupled receptors mainly expressed in entero-hepatic tissues for which bile acids function as signaling molecules. Primary bile acids (chenodeoxycholic acid and cholic acid) are physiological ligands/activators of farnesoid-X-receptor (FXR), pregnane-X-receptor (PXR) and constitutive androstane receptor (CAR), while litocholic acid is a ligand for the Vitamin D receptor (VDR) and the G-protein coupled receptor TGR5. Despite FXR demonstrates a high selectivity for bile acids, PXR and CAR are relatively promiscuous receptors integrating lipid homeostasis with xenobiotic metabolism. FXR, PXR, CAR and TGR exert synergistic activities in regulating lipid and glucose homeostasis and energy expenditure and liver and peripheral insulin sensitivity. Ligands for these receptors hold promise in the treatment of dyslipidemic conditions as revealed by results of a number of preclinical models but carry a defined risk for potential side effects.

Drug-induced liver injury: insights from genetic studies

Drug-induced liver injury (DILI) is an increasing health problem and a challenge for physicians, regulatory bodies and the pharmaceutical industry, not only because of its potential severity and elusive pathogenesis but also because it is often inaccurately diagnosed, commonly missed entirely and more often not reported. The general view is that idiosyncratic DILI, which is not predictable whether based on the pharmacology of the drug or on the dose administered, is determined by the presence in the recipient of variants in, or expression of, genes coding for key metabolic pathways and/or the immune response, and the interaction of these genetic variants with environmental variables. Furthermore, idiosyncratic DILI is an example of a complex-trait disease with two or more susceptibility loci, as reflected by the frequency of genetic variants in the population often being higher than the occurrence of significant liver injury. Polymorphisms of bioactivation/toxification pathways via the CYP450 enzymes (Phase I), detoxification reactions (Phase II) and excretion/transport (Phase III), together with immunological factors that might determine DILI are reviewed. Challenges such as gene-trait association studies and whole-genome studies, and future approaches to the study of DILI are explored. Better knowledge of the candidate genes involved could provide further insight for the prospective identification of susceptible patients at risk of developing drug-induced hepatotoxicity, development of new diagnostic tools and new treatment strategies with safer drugs.

Long-term virus-induced alterations of CYP3A-mediated drug metabolism: a look at the virology, immunology and molecular biology of a multi-faceted problem

Virus infections are on the rise. Although the first description of CYP expression during virus infection was recorded 50 years ago, mechanistic studies of this phenomenon only began to appear in the last decade due to breakthroughs in molecular biology, genomic and transgenic technology. This review describes the relationship(s) among CYP-mediated drug metabolism, virus infection and the immune response and evaluates in vitro and in vivo models for mechanistic studies. The first studies that assessed CYP expression during infection focused on inflammatory mediators and the innate immune response at early time points. Recent studies assessing virus infection and its effect on hepatic CYP expression noted more long-term effects. An obvious approach toward understanding how viruses affect hepatic CYP3A expression and function would be to assess key regulators of CYP during infection. Improvements in techniques to identify post-translational modifications of CYP and systems that focus on virus-receptor interactions which allow subtraction and addition of immunological and regulatory elements that drive CYP will demonstrate that long-term changes in drug metabolism start from the time the virus enters the circulation, are reinforced by virus binding to cellular targets and further solidified by changes in cellular processes long after the virus is cleared.

Urinary metabolomics in Fxr-null mice reveals activated adaptive metabolic pathways upon bile acid challenge

Farnesoid X receptor (FXR) is a nuclear receptor that regulates genes involved in synthesis, metabolism, and transport of bile acids and thus plays a major role in maintaining bile acid homeostasis. In this study, metabolomic responses were investigated in urine of wild-type and Fxr-null mice fed cholic acid, an FXR ligand, using ultra-performance liquid chromatography (UPLC) coupled with electrospray time-of-flight mass spectrometry (TOFMS). Multivariate data analysis between wild-type and Fxr-null mice on a cholic acid diet revealed that the most increased ions were metabolites of p-cresol (4-methylphenol), corticosterone, and cholic acid in Fxr-null mice. The structural identities of the above metabolites were confirmed by chemical synthesis and by comparing retention time (RT) and/or tandem mass fragmentation patterns of the urinary metabolites with the authentic standards. Tauro-3alpha,6,7alpha,12alpha-tetrol (3alpha,6,7alpha,12alpha-tetrahydroxy-5beta-cholestan-26-oyltaurine), one of the most increased metabolites in Fxr-null mice on a CA diet, is a marker for efficient hydroxylation of toxic bile acids possibly through induction of Cyp3a11. A cholestatic model induced by lithocholic acid revealed that enhanced expression of Cyp3a11 is the major defense mechanism to detoxify cholestatic bile acids in Fxr-null mice. These results will be useful for identification of biomarkers for cholestasis and for determination of adaptive molecular mechanisms in cholestasis.

The potential influence of genetic variants in genes along bile acid and bile metabolic pathway on blood cholesterol levels in the population

The liver is currently known to be the major organ to eliminate excess cholesterol from our body. It accomplishes this function in two ways: conversion of cholesterol molecules into bile acids (BAs) and secretion of unesterified cholesterol molecules into bile. BAs are synthesized in the hepatocytes, secreted into bile and delivered to the lumen of the small intestine where they act as detergents to facilitate absorption of fats and fat-soluble vitamins. About 95% of BAs are recovered in the ileum during each cycle of the enterohepatic circulation. Five percent are lost and replaced by newly synthesized BAs, which amounts to approximately 500 mg/day in adult humans. In contrast to the efficiency of the BAs' enterohepatic circulation, 50% of the 1000 mg of cholesterol secreted daily into bile is lost in feces. It is known that rare human mutations in certain genes in bile acid and bile metabolic pathway influence blood cholesterol levels. With the recent success of genome-wide association studies, we are convinced that common genetic variants also play a role in the genetic architecture of plasma lipid traits. In this review, we summarized the current state of knowledge about genetic variations in bile acid and bile metabolic pathway, and assessed their impact on blood cholesterol levels and cholesterol metabolic kinetics in the population.

Gene expression profiling in rat liver treated with compounds inducing elevation of bilirubin

We have constructed a large-scale transcriptome database of rat liver treated with various drugs. In an effort to identify a biomarker for the diagnosis of elevated total bilirubin (TBIL) and direct bilirubin (DBIL), we extracted 59 probe sets of rat hepatic genes from the data for seven typical drugs, gemfibrozil, phalloidin, colchicine, bendazac, rifampicin, cyclosporine A, and chlorpromazine, which induced this phenotype from 3 to 28 days of repeated administration in the present study. Principal component analysis (PCA) using these probes clearly separated dose- and time-dependent clusters in the treated groups from their controls. Eighteen more drugs in the database, reported to elevate TBIL and DBIL, were estimated by PCA using these probe sets. Of these, 12 drugs, that is methapyrilene, thioacetamide, ticlopidine, ethinyl estradiol, alpha-naphthylisothiocyanate, indomethacin, methyltestosterone, penicillamine, allyl alcohol, aspirin, iproniazid, and isoniazid were also separated from the control clusters, as were the seven typical drugs causing elevation of TBIL and DBIL. The principal component 1 (PC1) value showed high correlation with TBIL and DBIL. In the cases of colchicine, bendazac, chlorpromazine, gemfibrozil, and phalloidin, the possible elevation of TBIL and DBIL could be predicted by expression of these genes 24 h after single administration. We conclude that these identified 59 probe sets could be useful to diagnose the cause of elevation of TBIL and DBIL, and that toxicogenomics would be a promising approach for prediction of this type of toxicity.

Antibiotics suppress Cyp3a in the mouse liver by reducing lithocholic acid-producing intestinal flora

We previously demonstrated that ciprofloxacin (CPX), a new quinolone antibiotic, suppresses Cyp3a in the mouse liver by reducing the hepatic level of lithocholic acid (LCA) produced by intestinal flora. The present study investigated the possibility that other antibiotics with antibacterial activity against LCA-producing bacteria also cause a decrease in the LCA level in the liver, leading to reduced expression of Cyp3a11. While the mRNA expression of Cyp3a11 in the liver was significantly reduced when SPF mice were administered antibiotics such as ampicillin, CPX, levofloxacin, or a combination of vancomycin and imipenem, no significant changes were observed after antibiotic treatment of GF mice lacking intestinal flora. LCA-producing bacteria in the feces as well as the hepatic level of the taurine conjugate of LCA were significantly reduced in the antibiotic-treated SPF mice, suggesting that the decrease in Cyp3a11 expression can be attributed to the reduction in LCA-producing intestinal flora following antibiotic administration. These results suggest that the administration of antibiotics with activity against LCA-producing bacteria can also cause a decrease in the LCA level in humans, which may lower CYP3A4 expression. The intestinal flora are reported to be altered not only by drugs, such as antibiotics, but also by stress, disease, and age. The findings of the present study suggest that these changes in intestinal flora could modify CYP expression and contribute to the individual differences in pharmacokinetics.

The aldo-keto reductase Akr1b7 gene is a common transcriptional target of xenobiotic receptors pregnane X receptor and constitutive androstane receptor

Aldo-keto reductase (AKR) family 1, member 7 (AKR1B7), a member of the AKR superfamily, has been suggested to play an important role in the detoxification of lipid peroxidation by-products. The nuclear receptors pregnane X receptor (PXR) and constitutive androstane receptor (CAR) are xenosensors postulated to alleviate xeno- and endobiotic chemical insults. In this study, we show that the mouse Akr1b7 is a shared transcriptional target of PXR and CAR in the liver and intestine. Treatment of wild-type mice with the PXR agonist pregnenolone-16alpha-carbonitrile (PCN) activated Akr1b7 gene expression, whereas the effect was abrogated in PXR(-/-) mice. Similarly, the activation of Akr1b7 gene expression by the CAR agonist 1,4-bis[2-(3,5-dichlorpyridyloxyl)]-benzene, seen in wild-type mice, was abolished in CAR(-/-) mice. The promoter of Akr1b7 gene was activated by PXR and CAR, and this activation was achieved through the binding of PXR-retinoid X receptor (RXR) or CAR-RXR heterodimers to direct repeat-4 type nuclear receptor-binding sites found in the Akr1b7 gene promoter. At the functional level, treatment with PCN in wild-type mice, but not PXR(-/-) mice, led to a decreased intestinal accumulation of malondialdehyde, a biomarker of lipid peroxidation. The regulation of Akr1b7 by PXR was independent of the liver X receptor (LXR), another nuclear receptor known to regulate this AKR isoform. Because a major function of Akr1b7 is to detoxify lipid peroxidation, the PXR-, CAR-, and LXR-controlled regulatory network of Akr1b7 may have contributed to alleviate toxicity associated with lipid peroxidation.

Role of hepatic transporters in prevention of bile acid toxicity after partial hepatectomy in mice

The enterohepatic recirculation of bile acids (BAs) is important in several physiological processes. Although there has been considerable research on liver regeneration after two-thirds partial hepatectomy (PHx), little is known about how the liver protects itself against BA toxicity during regeneration. In this study, various BAs in plasma and liver, the composition of micelle-forming bile constituents, as well as gene expression of the main hepatobiliary transporters were quantified in sham-operated and PHx mice 24 and 48 h after surgery. PHx did not influence the hepatic concentrations of taurine-conjugated BAs (T-BA) but increased the concentration of glycine-conjugated (G-BA) and unconjugated BAs. Total BA excretion (microg x min(-1) x g liver wt(-1)) increased 2.4-fold and was accompanied by a 55% increase in bile flow after PHx. The plasma concentrations of T-BAs (402-fold), G-BAs (17-fold), and unconjugated BAs (500-fold) increased. The mRNA and protein levels of the BA uptake transporter Ntcp were unchanged after PHx, whereas the canalicular Bsep protein increased twofold at 48 h. The basolateral efflux transporter Mrp3 was induced at the mRNA (2.6-fold) and protein (3.1-fold) levels after PHx, which may contribute to elevated plasma BA and bilirubin levels. Biliary phospholipid excretion was nearly doubled in PHx mice, most likely owing to increased mRNA expression of the phospholipid transporter, Mdr2. In conclusion, the remnant liver after PHx excretes 2.5-fold more BAs and three times more phospholipids per gram liver than the sham-operated mouse liver. Upregulation of phospholipid transport may be important in protecting the biliary tract from BA toxicity during PHx.

Circadian dysregulation disrupts bile acid homeostasis

BACKGROUND

Bile acids are potentially toxic compounds and their levels of hepatic production, uptake and export are tightly regulated by many inputs, including circadian rhythm. We tested the impact of disrupting the peripheral circadian clock on integral steps of bile acid homeostasis.

METHODOLOGY/PRINCIPAL FINDINGS

Both restricted feeding, which phase shifts peripheral clocks, and genetic ablation in Per1(-/-)/Per2(-/-) (PERDKO) mice disrupted normal bile acid control and resulted in hepatic cholestasis. Restricted feeding caused a dramatic, transient elevation in hepatic bile acid levels that was associated with activation of the xenobiotic receptors CAR and PXR and elevated serum aspartate aminotransferase (AST), indicative of liver damage. In the PERDKO mice, serum bile acid levels were elevated and the circadian expression of key bile acid synthesis and transport genes, including Cyp7A1 and NTCP, was lost. This was associated with blunted expression of a primary clock output, the transcription factor DBP, which transactivates the promoters of both genes.

CONCLUSIONS/SIGNIFICANCE

We conclude that disruption of the circadian clock results in dysregulation of bile acid homeostasis that mimics cholestatic disease.

Intestinal cell-specific vitamin D receptor (VDR)-mediated transcriptional regulation of CYP3A4 gene

CYP3A4 is the most important drug-metabolizing enzyme that is involved in biotransformation of more than 50% of drugs. Pregnane X receptor (PXR) dominantly controls CYP3A4 inducibility in the liver, whereas vitamin D receptor (VDR) transactivates CYP3A4 in the intestine by secondary bile acids. Four major functional PXR-binding response elements of CYP3A4 have been discovered and their cooperation was found to be crucial for maximal up-regulation of the gene in hepatocytes. VDR and PXR recognize similar response element motifs and share DR3(XREM) and proximal ER6 (prER6) response elements of the CYP3A4 gene. In this work, we tested whether the recently discovered PXR response elements DR4(eNR3A4) in the XREM module and the distal ER6 element in the CLEM4 module (CLEM4-ER6) bind VDR/RXRalpha heterodimer, whether the elements are involved in the intestinal transactivation, and whether their cooperation with other elements is essential for maximal intestinal expression of CYP3A4. Employing a series of gene reporter plasmids with various combinations of response element mutations transiently transfected into four intestinal cell lines, electrophoretic mobility shift assay (EMSA) and chromatin immunoprecipitation assay (ChIP), we found that the CLEM4-ER6 motif interacts with VDR/RXRalpha heterodimer and partially cooperates with DR3(XREM) and prER6 in both basal and VDR-mediated inducible CYP3A4 regulation in intestinal cells. In contrast, eNR3A4 is involved only in the basal transactivation in intestinal cells and in the PXR-mediated rifampicin-induced transactivation of CYP3A4 in LS174T intestinal cells. We thus describe a specific ligand-induced VDR-mediated transactivation of the CYP3A4 gene in intestinal cells that differs from PXR-mediated CYP3A4 regulation in hepatocytes.

[Cholestasis and cholestatic liver diseases]

The main determinant of bile formation is an osmotic filtration process resulting from active transport of bile acids and other osmotic solutes (glutathion). Most of the membrane transporters ensuring bile formation have now been identified. The expression of these membrane transporters is regulated through transcriptional and post-traductional mechanisms. Transcriptional regulation is under the control of nuclear receptors activated by ligands such as bile acids, which act as endogenous steroids synthesized from cholesterol in hepatocytes. Cholestatic liver diseases comprise genetic diseases resulting from the complex interaction between genetic and environmental factors. Monogenic cholestatic diseases recently identified illustrate the key role of membrane transporters in biliary function. Bile acids and inflammatory mediators are potent modulators of transporters and nuclear receptor genes and thus trigger an adaptative response to cholestasis. The extent of this adaptative response could explain the compelling phenotypic variability of cholestatic diseases in childhood and adults. The first-line medical treatment is currently ursodeoxycholic acid and in case of failure of this medical treatment, liver transplantation is required. Recent progress in the molecular pathogenesis of bile formation and cholestatic liver diseases is expected to provide the design of drugs targeted to the molecular abnormalities typical of cholestatic diseases.

Bile acids as regulatory molecules

In the past, bile acids were considered to be just detergent molecules derived from cholesterol in the liver. They were known to be important for the solubilization of cholesterol in the gallbladder and for stimulating the absorption of cholesterol, fat-soluble vitamins, and lipids from the intestines. However, during the last two decades, it has been discovered that bile acids are regulatory molecules. Bile acids have been discovered to activate specific nuclear receptors (farnesoid X receptor, preganane X receptor, and vitamin D receptor), G protein coupled receptor TGR5 (TGR5), and cell signaling pathways (c-jun N-terminal kinase 1/2, AKT, and ERK 1/2) in cells in the liver and gastrointestinal tract. Activation of nuclear receptors and cell signaling pathways alter the expression of numerous genes encoding enzyme/proteins involved in the regulation of bile acid, glucose, fatty acid, lipoprotein synthesis, metabolism, transport, and energy metabolism. They also play a role in the regulation of serum triglyceride levels in humans and rodents. Bile acids appear to function as nutrient signaling molecules primarily during the feed/fast cycle as there is a flux of these molecules returning from the intestines to the liver following a meal. In this review, we will summarize the current knowledge of how bile acids regulate hepatic lipid and glucose metabolism through the activation of specific nuclear receptors and cell signaling pathways.

Technical pentabromodiphenyl ether and hexabromocyclododecane as activators of the pregnane-X-receptor (PXR)

Technical pentabrominated diphenyl ether (pentaBDE mix) is a mixture of polybrominated diphenyl ethers (PBDEs) which has been widely used as a flame retardant. Since its ban in several countries it has been replaced by other brominated flame retardants such as hexabromocyclododecane (HBCD). Both certain PBDE congeners and HBCD are present in environmental and human samples reflecting their persistent and bioaccumulative properties. PentaBDE mix and HBCD have recently been found to induce cytochrome P450 (CYP) 3 enzymes in rat liver. In this study we tested both technical pentaBDE mix and HBCD for their potency to induce CYP3A enzymes in rat hepatocytes in primary culture, and in rat H4IIE and human HepG2 hepatoma cells. In rat hepatocytes, HBCD was a more effective CYP3A1 inducer than pentaBDE mix, being less effective, however, than the prototype inducer dexamethasone. In human HepG2 cells, both compounds and the prototype inducer rifampicin were about equally effective. In contrast, in HepG2 cells, HBCD failed to induce luciferin-PFBE dealkylase, a common catalytic activity of a number of CYP3A enzymes, possibly reflecting enzyme inhibition. A significant induction of catalytic activity was observed in rat hepatocytes with both compounds. Analysis of a XREM-driven reporter gene activity in transfected cells confirmed that both compounds act as agonists of the human and rat pregnane-X-receptor, which was detectable in all cell types used.

Bile acids as endogenous etiologic agents in gastrointestinal cancer

Bile acids are implicated as etiologic agents in cancer of the gastrointestinal (GI) tract, including cancer of the esophagus, stomach, small intestine, liver, biliary tract, pancreas and colon/rectum. Deleterious effects of bile acid exposure, likely related to carcinogenesis, include: induction of reactive oxygen and reactive nitrogen species; induction of DNA damage; stimulation of mutation; induction of apoptosis in the short term, and selection for apoptosis resistance in the long term. These deleterious effects have, so far, been reported most consistently in relation to esophageal and colorectal cancer, but also to some extent in relation to cancer of other organs. In addition, evidence is reviewed for an association of increased bile acid exposure with cancer risk in human populations, in specific human genetic conditions, and in animal experiments. A model for the role of bile acids in GI carcinogenesis is presented from a Darwinian perspective that offers an explanation for how the observed effects of bile acids on cells contribute to cancer development.

Hepatic bile acid metabolism in the neonatal hamster: expansion of the bile acid pool parallels increased Cyp7a1 expression levels

Intraluminal concentrations of bile acids are low in newborn infants and increase rapidly after birth, at least partly owing to increased bile acid synthesis rates. The expansion of the bile acid pool is critical since bile acids are required to stimulate bile flow and absorb lipids, a major component of newborn diets. The purpose of the present studies was to determine the mechanism responsible for the increase in bile acid synthesis rates and the subsequent enlargement of bile acid pool sizes (BAPS) during the neonatal period, and how changes in circulating hormone levels might affect BAPS. In the hamster, pool size was low just after birth and increased modestly until 10.5 days postpartum (dpp). BAPS increased more significantly ( approximately 3-fold) between 10.5 and 15.5 dpp. An increase in mRNA and protein levels of cholesterol 7alpha-hydroxylase (Cyp7a1), the rate-limiting step in classical bile acid synthesis, immediately preceded an increase in BAPS. In contrast, levels of oxysterol 7alpha-hydroxylase (Cyp7b1), a key enzyme in bile acid synthesis by the alternative pathway, were relatively elevated by 1.5 dpp. farnesyl X receptor (FXR) and short heterodimeric partner (SHP) mRNA levels remained relatively constant at a time when Cyp7a1 levels increased. Finally, although simultaneous increases in circulating cortisol and Cyp7a1 levels occurred, precocious expression of Cyp7a1 could not be induced in neonatal hamsters with dexamethasone. Thus the significant increase in Cyp7a1 levels in neonatal hamsters is due to mechanisms independent of the FXR and SHP pathway and cortisol.

Regulation of tissue-specific carboxylesterase expression by pregnane x receptor and constitutive androstane receptor

The liver- and intestine-enriched carboxylesterase 2 (CES2) enzyme catalyzes the hydrolysis of several clinically important anticancer agents administered as prodrugs. For example, irinotecan, a carbamate prodrug used in the treatment of colorectal cancer, is biotransformed in vivo by CES2 in intestine and liver, thereby producing a potent topoisomerase I inhibitor. Pregnane X receptor (PXR) and constitutive androstane receptor (CAR), two members of the nuclear receptor superfamily of ligand-activated transcription factors, mediate gene activation in response to xenobiotic stress. Together, these receptors comprise a protective response in mammals that coordinately regulate hepatic transport, metabolism, and elimination of numerous xenobiotic compounds. In the present study, microarray analysis was used to identify PXR target genes in duodenum in mice. Here, we show that a gene encoding a member of the CES2 subtype of liver- and intestine-enriched CES enzymes, called Ces6, is induced after treatment with pregnenolone 16alpha-carbonitrile in a PXR-dependent manner in duodenum and liver in mice. Treatment of mice with the CAR activator 1,4-bis[2-(3,5-dichloropyridyloxy)] benzene also induced expression of Ces6 in duodenum and liver in a CAR-dependent manner, whereas treatment with phenobarbital produced induction of Ces6 exclusively in liver. These data identify a key role for PXR and CAR in regulating the drug-inducible expression and activity of an important CES enzyme in vivo. Future studies should focus on determining whether these signaling pathways governing drug-inducible CES expression in intestine and liver are conserved in humans.

Ligand diversity of human and chimpanzee CYP3A4: activation of human CYP3A4 by lithocholic acid results from positive selection

For currently unknown reasons, the evolution of CYP3A4 underwent acceleration in the human lineage after the split from chimpanzee. We investigated the significance of this event by comparing Escherichia coli-expressed CYP3A4 from humans, chimpanzee, and their most recent common ancestor. The expression level of chimpanzee CYP3A4 was approximately 50% of the human CYP3A4, whereas ancestral CYP3A4 did not express in E. coli. Steady-state kinetic analysis with 7-benzyloxyquinoline, 7-benzyloxy-4-(trifluoromethyl)coumarin (7-BFC), and testosterone showed no significant differences between human and chimpanzee CYP3A4. Upon addition of alpha-naphthoflavone (25 microM), human CYP3A4 showed a slightly decreased substrate concentration at which 50% of the maximal rate V(max) is reached for 7-BFC, whereas chimpanzee CYP3A4 showed a >2-fold increase. No significant differences in inhibition/activation were found for a panel of 43 drugs and endogenous compounds, suggesting that the wide substrate spectrum of human CYP3A4 precedes the human-chimpanzee split. A striking exception was the hepatotoxic secondary bile acid lithocholic acid, which at saturation caused a 5-fold increase in 7-BFC debenzylation by human CYP3A4 but not by chimpanzee CYP3A4. Mutagenesis of human CYP3A4 revealed that at least four of the six amino acids positively selected in the human lineage contribute to the activating effect of lithocholic acid. In summary, the wide functional conservation between chimpanzee and human CYP3A4 raises the prospect that phylogenetically more distant primate species such as rhesus and squirrel monkey represent suitable models of the human counterpart. Positive selection on the human CYP3A4 may have been triggered by an increased load of dietary steroids, which led to a novel defense mechanism against cholestasis.

Intestinal flora induces the expression of Cyp3a in the mouse liver

In order to determine the effects of intestinal flora on the expression of cytochrome P450 (CYP), the mRNA expression of CYP was compared between specific pathogen-free (SPF) and germ-free (GF) mice. Most of the major CYP isozymes showed higher expression in the livers of SPF mice compared with GF mice. Nuclear factors such as pregnane X receptor (PXR) and constitutive androstane receptor (CAR), as well as transporters and conjugation enzymes involved in the detoxification of lithocholic acid (LCA), also showed higher expression in SPF mice. The findings suggest that in the livers of SPF mice, LCA produced by intestinal flora increases the expression of CYPs via activation of PXR and CAR. Drugs such as antibiotics, some diseases and ageing, etc. are known to alter intestinal flora. The present findings suggest that such changes also affect CYP and are one of the factors responsible for individual differences in pharmacokinetics.

Activation of CAR and PXR by Dietary, Environmental and Occupational Chemicals Alters Drug Metabolism, Intermediary Metabolism, and Cell Proliferation

The constitutive androstane receptor (CAR) and the pregnane × receptor (PXR) are activated by a variety of endogenous and exogenous ligands, such as steroid hormones, bile acids, pharmaceuticals, and environmental, dietary, and occupational chemicals. In turn, they induce phase I-III detoxification enzymes and transporters that help eliminate these chemicals. Because many of the chemicals that activate CAR and PXR are environmentally-relevant (dietary and anthropogenic), studies need to address whether these chemicals or mixtures of these chemicals may increase the susceptibility to adverse drug interactions. In addition, CAR and PXR are involved in hepatic proliferation, intermediary metabolism, and protection from cholestasis. Therefore, activation of CAR and PXR may have a wide variety of implications for personalized medicine through physiological effects on metabolism and cell proliferation; some beneficial and others adverse. Identifying the chemicals that activate these promiscuous nuclear receptors and understanding how these chemicals may act in concert will help us predict adverse drug reactions (ADRs), predict cholestasis and steatosis, and regulate intermediary metabolism. This review summarizes the available data on CAR and PXR, including the environmental chemicals that activate these receptors, the genes they control, and the physiological processes that are perturbed or depend on CAR and PXR action. This knowledge contributes to a foundation that will be necessary to discern interindividual differences in the downstream biological pathways regulated by these key nuclear receptors.

Nuclear receptors as therapeutic targets in cholestatic liver diseases

Cholestasis results in intrahepatic accumulation of cytotoxic bile acids, which cause liver damage ultimately leading to biliary fibrosis and cirrhosis. Cholestatic liver injury is counteracted by a variety of adaptive hepatoprotective mechanisms including alterations in bile acid transport, synthesis and detoxification. The underlying molecular mechanisms are mediated mainly at a transcriptional level via a complex network involving nuclear receptors including the farnesoid X receptor, pregnane X receptor, vitamin D receptor and constitutive androstane receptor, which target overlapping, although not identical, sets of genes. Because the intrinsic adaptive response to bile acids cannot fully prevent liver injury in cholestasis, therapeutic targeting of these receptors via specific and potent agonists may further enhance the hepatic defence against toxic bile acids. Activation of these receptors results in repression of bile acid synthesis, induction of phases I and II bile acid hydroxylation and conjugation and stimulation of alternative bile acid export while limiting hepatocellular bile acid import. Furthermore, the use of nuclear receptor ligands may not only influence bile acid transport and metabolism but may also directly target hepatic fibrogenesis and inflammation. Many drugs already used to treat cholestasis and its complications such as pruritus (e.g. ursodeoxycholic acid, rifampicin, fibrates) may act via activation of nuclear receptors. More specific and potent nuclear receptor ligands are currently being developed. This article will review the current knowledge on nuclear receptors and their potential role in the treatment of cholestatic liver diseases.

Activation of PXR induces hypercholesterolemia in wild-type and accelerates atherosclerosis in apoE deficient mice

The nuclear hormone receptor pregnane X receptor (PXR; also called SXR) functions as a xenobiotic sensor to coordinately regulate xenobiotic metabolism via transcriptional regulation of xenobiotic-detoxifying enzymes and transporters. Although many clinically relevant PXR ligands have been shown to affect cholesterol levels, the role of PXR in cholesterol homeostasis and atherosclerosis has not been thoroughly investigated. Here, we report that activation of PXR by feeding the PXR agonist pregnenolone 16alpha-carbonitrile (0.02%) for 2 weeks to wild-type (WT) mice significantly increased total cholesterol levels and atherogenic lipoproteins VLDL and LDL levels, but had no effect in PXR knockout (PXR(-/-)) mice. Chronic PXR activation in atherosclerosis prone apolipoprotein E deficient (ApoE(-/-)) mice was found to decrease HDL levels and increase atherosclerotic cross-sectional lesion area at both the aortic root and in the brachiocephalic artery by 54% (P < 0.001) and 116% (P < 0.01), respectively. PXR activation significantly regulated genes in the liver involved in lipoprotein transportation and cholesterol metabolism, including CD36, ApoA-IV, and CYP39A1, in both WT and ApoE(-/-) mice. Furthermore, PXR activation can increase CD36 expression and lipid accumulation in peritoneal macrophages of ApoE(-/-) mice. In summary, PXR activation in WT mice increases levels of the atherogenic lipoproteins VLDL and LDL, whereas in ApoE(-/-) mice, PXR increases atherosclerosis, perhaps by diminishing levels of the antiatherogenic ApoA-IV and increasing lipid accumulation in macrophages.

Nuclear receptors CAR and PXR: Molecular, functional, and biomedical aspects

Nuclear receptors (NRs) are ligand-activated transcription factors sharing a common evolutionary history and having similar sequence features at the protein level. Selective ligand(s) for some NRs is not known, therefore these NRs have been named "orphan receptors". Whenever ligands have been recognized for any of the orphan receptor, it has been categorized and grouped as "adopted" orphan receptor. This group includes the constitutive androstane receptor (CAR) and the pregnane X receptor (PXR). They function as sensors of toxic byproducts derived from endogenous metabolites and of exogenous chemicals, in order to enhance their elimination. This unique function of CAR and PXR sets them apart from the steroid hormone receptors. The broad response profile has established that CAR and PXR are xenobiotic sensors that coordinately regulate xenobiotic clearance in the liver and intestine via induction of genes involved in drug and xenobiotic metabolism. In the past few years, research has revealed new and mostly unsuspected roles for CAR and PXR in modulating hormone, lipid, and energy homeostasis as well as cancer and liver steatosis. The purpose of this review is to highlight the structural and molecular bases of CAR and PXR impact on human health, providing information on mechanisms through which diet, chemical exposure, and environment ultimately impact health and disease.

Human CYP3A4 and murine Cyp3A11 are regulated by equol and genistein via the pregnane X receptor in a species-specific manner

Pregnane X receptor (PXR) is an important component of the body's adaptive defense system responsible for the elimination of various toxic xenobiotics. PXR activation by endogenous and exogenous chemicals, including steroids, antibiotics, bile acids, and herbal compounds, results in induction of drug metabolism. We investigated the ability of the isoflavones genistein, daidzein, and the daidzein metabolite equol to activate human and mouse PXR in vitro using cell-based transient transfection studies and primary hepatocytes and in vivo in a mouse model. In transient transfection assays, the isoflavones genistein and daidzein activate full-length, wild-type mouse PXR, but not a mutant form, with genistein being the most potent. In contrast, equol was a more potent activator of human PXR than genistein or daidzein. In a mammalian 2-hybrid assay, isoflavones induced recruitment of the coactivator steroid receptor coactivator 1 to PXR. When tested against the native human Cytochrome P450 3A4 (CYP3A4) promoter, equol was the more potent activator and treatment of human hepatocytes with equol increased CYP3A4 mRNA and immunoreactive protein expression. Treatment of wild-type, but not PXR(-/-), mouse hepatocytes showed that genistein and daidzein induced the expression of Cytochrome P450 3A11 (Cyp3A11) mRNA, whereas equol had no effect. Cyp3A11 mRNA was also induced in vivo in mice fed a soy protein-containing diet. The results presented herein demonstrate that there is a species-specific difference in the activation of PXR by isoflavones and equol.

Constitutive androstane receptor-mediated changes in bile acid composition contributes to hepatoprotection from lithocholic acid-induced liver injury in mice

Pharmacological activation of the constitutive androstane receptor (CAR) protects the liver during cholestasis. The current study evaluates how activation of CAR influences genes involved in bile acid biosynthesis as a mechanism of hepatoprotection during bile acid-induced liver injury. CAR activators phenobarbital (PB) and 1,4-bis[2-(3,5-dichloropyridyloxy)]benzene (TCPOBOP) or corn oil (CO) were administered to C57BL/6 wild-type (WT) and CAR knockout (CAR-null) mice before and during induction of intrahepatic cholestasis using the secondary bile acid, lithocholic acid (LCA). In LCA-treated WT and all the CAR-null groups (excluding controls), histology revealed severe multifocal necrosis. This pathology was absent in WT mice pretreated with PB and TCPOBOP, indicating CAR-dependent hepatoprotection. Decreases in total hepatic bile acids and hepatic monohydroxy, dihydroxy, and trihydroxy bile acids in PB- and TCPOBOP-pretreated WT mice correlated with hepatoprotection. In comparison, concentrations of monohydroxylated and dihydroxylated bile acids were increased in all the treated CAR-null mice compared with CO controls. Along with several other enzymes (Cyp7b1, Cyp27a1, Cyp39a1), Cyp8b1 expression was increased in hepatoprotected mice, which could be suggestive of a shift in the bile acid biosynthesis pathway toward the formation of less toxic bile acids. In CAR-null mice, these changes in gene expression were not different among treatment groups. These results suggest CAR mediates a shift in bile acid biosynthesis toward the formation of less toxic bile acids, as well as a decrease in hepatic bile acid concentrations. We propose that these combined CAR-mediated effects may contribute to the hepatoprotection observed during LCA-induced liver injury.

Compensatory induction of liver efflux transporters in response to ANIT-induced liver injury is impaired in FXR-null mice

Alpha-naphthyl isothiocyanate (ANIT) is a hepatotoxicant that produces acute intrahepatic cholestasis in rodents. Farnesoid X receptor (FXR) and pregnane X receptor (PXR) are two major bile acid sensors in liver. The purpose of this study was to characterize the regulation of hepatic transporters by FXR and PXR during ANIT-induced liver injury. Wild-type, FXR-null, and PXR-null mice were administered ANIT (75 mg/kg, po) and evaluated 48 h later for hepatotoxicity and messenger RNA (mRNA) expression of basolateral uptake (sodium taurocholate-cotransporting polypeptide, organic anion transporting polypeptide [Oatp] 1a1, Oatp1a4, Oatp1b2) and efflux transporters (organic solute transporter [Ost] alpha, Ostbeta, multidrug resistance-associated protein [Mrp] 3, Mrp4), as well as canalicular transporters (bile salt export pump [Bsep], Mrp2, multidrug resistance protein 2 [Mdr2], ATPase, class I, type 8B, member 1 [Atp8b1]). Livers from wild-type and PXR-null mice had comparable multifocal necrosis 48 h after ANIT. However, ANIT-treated FXR-null mice have fewer and smaller necrotic foci than wild-type mice but had scattered single-cell hepatocyte necrosis throughout the liver. Serum alanine transaminase, alkaline phosphatase (ALP), and direct bilirubin were increased in all genotypes, with higher ALP levels in FXR-null mice. Serum and liver unconjugated bile acids were higher in ANIT-treated FXR-null mice than the other two genotypes. ANIT induced mRNA expression of Mdr2, Bsep, and Atp8b1 in wild-type and PXR-null mice but failed to upregulate these genes in FXR-null mice. mRNA expression of uptake transporters declined in livers of all genotypes following ANIT treatment. ANIT increased Ostbeta and Mrp3 mRNA in livers of wild-type and PXR-null mice but did not alter Ostbeta mRNA in FXR-null mice. In conclusion, FXR deficiency enhances susceptibility of mice to ANIT-induced liver injury, likely a result of impaired induction of hepatobiliary efflux transporters and subsequent hepatic accumulation of unconjugated bile acids.

Function of retinoic acid receptors during embryonic development

Retinoids, the active metabolites of vitamin A, regulate complex gene networks involved in vertebrate morphogenesis, growth, cellular differentiation and homeostasis. Studies performed in vitro, using either acellular systems or transfected cells, have shown that retinoid actions are mediated through heterodimers between the RAR and RXR nuclear receptors. However, in vitro studies indicate what is possible, but not necessarily what is actually occurring in vivo, because they are performed under non-physiological conditions. Therefore, genetic approaches in the animal have been be used to determine the physiological functions of retinoid receptors. Homologous recombination in embryonic stem cells has been used to generate germline null mutations of the RAR- and RXR-coding genes in the mouse. As reviewed here, the generation of such germline mutations, combined with pharmacological approaches to block the RA signalling pathway, has provided genetic evidence that RAR/RXR heterodimers are indeed the functional units transducing the RA signal during prenatal development. However, due to (i) the complexity in “hormonal” signalling through transduction by the multiple RARs and RXRs, (ii) the functional redundancies (possibly artefactually generated by the mutations) within receptor isotypes belonging to a given family, and (iii) in utero or early postnatal lethality of certain germline null mutations, these genetic studies have failed to reveal all the physiological functions of RARs and RXRs, notably in adults. Spatio-temporally-controlled somatic mutations generated in given cell types/tissues and at chosen times during postnatal life, will be required to reveal all the functions of RAR and RXR throughout the lifetime of the mouse.

Bile acids: regulation of synthesis

Bile acids are physiological detergents that generate bile flow and facilitate intestinal absorption and transport of lipids, nutrients, and vitamins. Bile acids also are signaling molecules and inflammatory agents that rapidly activate nuclear receptors and cell signaling pathways that regulate lipid, glucose, and energy metabolism. The enterohepatic circulation of bile acids exerts important physiological functions not only in feedback inhibition of bile acid synthesis but also in control of whole-body lipid homeostasis. In the liver, bile acids activate a nuclear receptor, farnesoid X receptor (FXR), that induces an atypical nuclear receptor small heterodimer partner, which subsequently inhibits nuclear receptors, liver-related homolog-1, and hepatocyte nuclear factor 4alpha and results in inhibiting transcription of the critical regulatory gene in bile acid synthesis, cholesterol 7alpha-hydroxylase (CYP7A1). In the intestine, FXR induces an intestinal hormone, fibroblast growth factor 15 (FGF15; or FGF19 in human), which activates hepatic FGF receptor 4 (FGFR4) signaling to inhibit bile acid synthesis. However, the mechanism by which FXR/FGF19/FGFR4 signaling inhibits CYP7A1 remains unknown. Bile acids are able to induce FGF19 in human hepatocytes, and the FGF19 autocrine pathway may exist in the human livers. Bile acids and bile acid receptors are therapeutic targets for development of drugs for treatment of cholestatic liver diseases, fatty liver diseases, diabetes, obesity, and metabolic syndrome.

Regulation of the proprotein convertase subtilisin/kexin type 9 in intestinal epithelial cells

Proprotein convertase subtilisin/kexin type 9 (PCSK9) posttranslationally promotes the degradation of the low-density lipoprotein receptor (LDLr) in hepatocytes and increases plasma LDL cholesterol. It is not clear, however, whether PCSK9 plays a role in the small intestine. Here, we characterized the patterns of variations of PCSK9 and LDLr in fully differentiated Caco-2/15 cells as a function of various potential effectors. Cholesterol (100 microM) solubilized in albumin or micelles significantly downregulated PCSK9 gene (30%, P<0.05) and protein expression (50%, P<0.05), surprisingly in concert with a decrease in LDLr protein levels (45%, P<0.05). Cells treated with 25-hydroxycholesterol (50 microM) also displayed significant reduction in PCSK9 gene (37%, P<0.01) and protein (75% P<0.001) expression, whereas LDLr showed a decrease at the gene (30%, P<0.05) and protein (57%, P<0.01) levels, respectively. The amounts of PCSK9 mRNA and protein in Caco-2/15 cells were associated to the regulation of 3-hydroxy-3-methylglutaryl-CoA reductase and sterol regulatory element binding protein-2 (SREBP-2) that can transcriptionally activate PCSK9 via sterol-regulatory elements located in its proximal promoter region. On the other hand, depletion of cholesterol content by hydroxypropyl-beta-cyclodextrin upregulated PCSK9 transcripts (20%, P<0.05) and protein mass (540%, P<0.001), in parallel with SREBP-2 protein levels. The addition of bile acids (BA) taurocholate and deoxycholate to the apical culture medium lowered PCSK9 gene expression (25%, P<0.01) and raised PCSK9 protein expression (30%, P<0.01), respectively, probably via the modulation of farnesoid X receptor. Furthermore, unconjugated and conjugated BA exhibited different effects on PCSK9 and LDLr. Altogether, these data indicate that intestinal PCSK9 is highly modulated by sterols and emphasize the distinct effects of BA species.

A phosphomimetic mutation at threonine-57 abolishes transactivation activity and alters nuclear localization pattern of human pregnane x receptor

The pregnane X receptor (PXR) plays crucial roles in multiple physiological processes. However, the signaling mechanisms responsible are not well defined; it is most likely that multiple functions of PXR are modulated by its phosphorylation. Therefore, we sought to determine whether mutation at a highly conserved Thr(57) affects human PXR (hPXR) function. Site-directed mutagenesis was performed to generate phosphorylation-deficient (hPXR(T57A)) and phosphomimetic (hPXR(T57D)) mutants. Gene reporter, Western blotting, immunocytochemistry, mammalian two-hybrid, and electrophoretic mobility shift assays were used to study cytochrome P450 3A4 (CYP3A4) promoter activation, protein levels, localization, cofactor interaction, and CYP3A4 promoter binding of the hPXR mutants, respectively. hPXR(T57D), but not hPXR(T57A), lost its transcriptional activity. Neither mutation altered hPXR's protein levels and interaction with steroid receptor coactivator-1. hPXR and hPXR(T57A) exhibited a homogenous nuclear distribution, whereas hPXR(T57D) exhibited a distinctive punctate nuclear localization pattern similar to that of hPXR mutants with impaired function that colocalize with silencing mediator of retinoid and thyroid receptors (SMRT), although silencing of SMRT did not rescue the altered function of hPXR(T57D). However, hPXR(T57D), but not hPXR(T57A), impaired hPXR's ability to bind to the CYP3A4 promoter, consistent with the mutant's transactivation function. Furthermore, the 70-kDa form of ribosomal protein S6 kinase (p70 S6K) phosphorylated hPXR in vitro and inhibited its transcriptional activity, whereas hPXR(T57A) partially resisted the inhibitory effect of p70 S6K. Our studies identify a functionally significant phosphomimetic mutant (hPXR(T57D)) and show p70 S6K phosphorylation and regulation of hPXR transactivation to support the notion that phosphorylation plays important roles in regulating hPXR function.

Human pregnane X receptor activation and CYP3A4/CYP2B6 induction by 2,3-oxidosqualene:lanosterol cyclase inhibition

The effects of [4'-(6-allyl-methyl-amino-hexyloxy)-2'-fluoro-phenyl]-(4-bromophenyl)-methanone fumarate (Ro 48-8071), an inhibitor of 2,3-oxidosqualene:lanosterol cyclase (cyclase), were evaluated on CYP3A4 and CYP2B6 mRNA content in primary cultured human hepatocytes. In seven hepatocyte culture preparations, 24-h treatment with 3, 10, or 30 microM Ro 48-8071 produced median increases in CYP3A4 mRNA content that were 2.2-, 7.1-, and 8.5-fold greater than untreated control, respectively, and produced increases in CYP2B6 mRNA content that were 3.0-, 4.6-, and 3.4-fold greater than control, respectively. Increases in CYP3A4 immunoreactive protein content were also measured in Ro 48-8071-treated hepatocytes. To evaluate the effects of cyclase inhibitor treatments further, a pregnane X receptor (PXR)-responsive transactivation assay in HepG2 cells was used. Ro 48-8071, trans-N-(4-chlorobenzoyl)-N-methyl-(4-dimethylaminomethylphenyl)-cyclohexylamine (BIBX 79), and 3beta-(2-diethylaminoethoxy)androst-5-en-17-one HCl (U18666A) induced luciferase expression from a PXR-responsive reporter with EC(50)s of 0.113, 0.916, and 0.294 microM, respectively. Treatment of the HepG2 system with (E)N-ethyl-N-(6,6-dimethyl-2-hepten-4-ynyl)-3-[(3,3'-bithiophen-5-yl)methoxy]benzenemethanamine (NB-598), an inhibitor of squalene monooxygenase, at concentrations sufficient to achieve cholesterol biosynthesis inhibition significantly inhibited cyclase inhibitor-mediated, but not rifampicin-mediated, reporter induction. Direct treatment of the HepG2 system with 1 to 10 microM squalene 2,3:22,23-dioxide, but not squalene 2,3-oxide, significantly activated PXR-responsive reporter expression. Also, squalene 2,3:22,23-dioxide bound to human PXR in vitro with an IC(50) of 3.35 microM. These data indicate that cyclase inhibitors are capable of producing CYP3A4 and CYP2B6 induction in primary cultured human hepatocytes, and that an endogenous squalene metabolite is a conserved intracrine activator of PXR.

Hybrid scoring and classification approaches to predict human pregnane X receptor activators

PURPOSE

The human pregnane X receptor (PXR) is a transcriptional regulator of many genes involved in xenobiotic metabolism and excretion. Reliable prediction of high affinity binders with this receptor would be valuable for pharmaceutical drug discovery to predict potential toxicological responses

MATERIALS AND METHODS

Computational models were developed and validated for a dataset consisting of human PXR (PXR) activators and non-activators. We used support vector machine (SVM) algorithms with molecular descriptors derived from two sources, Shape Signatures and the Molecular Operating Environment (MOE) application software. We also employed the molecular docking program GOLD in which the GoldScore method was supplemented with other scoring functions to improve docking results.

RESULTS

The overall test set prediction accuracy for PXR activators with SVM was 72% to 81%. This indicates that molecular shape descriptors are useful in classification of compounds binding to this receptor. The best docking prediction accuracy (61%) was obtained using 1D Shape Signature descriptors as a weighting factor to the GoldScore. By pooling the available human PXR data sets we revealed those molecular features that are associated with human PXR activators.

CONCLUSIONS

These combined computational approaches using molecular shape information may assist scientists to more confidently identify PXR activators.

Medical treatment of primary sclerosing cholangitis: a role for novel bile acids and other (post-)transcriptional modulators?

Primary sclerosing cholangitis (PSC) is a rare chronic cholestatic disease of the liver and bile ducts that is associated with inflammatory bowel disease, generally leads to end-stage liver disease, and is complicated by malignancies of the biliary tree and the large intestine. The pathogenesis of PSC remains enigmatic, making the development of targeted therapeutic strategies difficult. Immunosuppressive and antifibrotic therapeutic agents were ineffective or accompanied by major side effects. Ursodeoxycholic acid (UDCA) has consistently been shown to improve serum liver tests and might lower the risk of colon carcinoma and cholangiocarcinoma by yet unknown mechanisms. Whether "high dose" UDCA improves the long-term prognosis in PSC as suggested by small pilot trials remains to be demonstrated. The present overview discusses potential therapeutic options aside of targeted immunological therapies and UDCA. The C23 bile acid norUDCA has been shown to markedly improve biochemical and histological features in a mouse model of sclerosing cholangitis without any toxic effects. Studies in humans are eagerly being awaited. Nuclear receptors like the farnesoid-X receptor (FXR), pregnane-X receptor (PXR), vitamin D receptor (VDR), and peroxisome-proliferator-activator receptors (PPARs) have been shown to induce expression of diverse carriers and biotransformation enzymes of the intestinal and hepatic detoxification machinery and/or to modulate fibrogenesis. Pros and cons of respective receptor agonists for the future treatment of PSC are discussed in detail. In our view, the novel bile acid norUDCA and agonists of PPARs, VDR, and PXR appear particularly attractive for further studies in PSC.

Mechanism of vitamin D receptor inhibition of cholesterol 7alpha-hydroxylase gene transcription in human hepatocytes

Lithocholic acid (LCA) is a potent endogenous vitamin D receptor (VDR) ligand. In cholestasis, LCA levels increase in the liver and intestine. The objective of this study is to test the hypothesis that VDR plays a role in inhibiting cholesterol 7alpha-hydroxylase (CYP7A1) gene expression and bile acid synthesis in human hepatocytes. Immunoblot analysis has detected VDR proteins in the nucleus of the human hepatoma cell line HepG2 and human primary hepatocytes. 1alpha, 25-Dihydroxy-vitamin D(3) or LCA acetate-activated VDR inhibited CYP7A1 mRNA expression and bile acid synthesis, whereas small interfering RNA to VDR completely abrogated VDR inhibition of CYP7A1 mRNA expression in HepG2 cells. Electrophoretic mobility shift assay and mutagenesis analyses have identified the negative VDR response elements that bind VDR/retinoid X receptor alpha in the human CYP7A1 promoter. Mammalian two-hybrid, coimmunoprecipitation, glutathione S-transferase pull-down, and chromatin immunoprecipitation assays show that ligand-activated VDR specifically interacts with hepatocyte nuclear factor 4alpha (HNF4alpha) to block HNF4alpha interaction with coactivators or to compete with HNF4alpha for coactivators or to compete for binding to CYP7A1 chromatin, which results in the inhibition of CYP7A1 gene transcription. This study shows that VDR is expressed in human hepatocytes and may play a critical role in the inhibition of bile acid synthesis, thus protecting liver cells during cholestasis.

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.

TGR5: an emerging bile acid G-protein-coupled receptor target for the potential treatment of metabolic disorders

Over the past decade, new roles for bile acids in paracrine and endocrine regulation of cholesterol homeostasis, lipid and carbohydrate metabolism and immunomodulatory functions have been discovered. Most of the early discoveries focused on the genomic actions of bile acids through the activation of families of nuclear receptors, such as the farnesoid X receptor and vitamin D receptors, until a new chapter in the bile acid receptor discovery unfolded in the form of TGR5; a novel G-protein-coupled receptor mediating several non-genomic functional responses induced by binding of bile acids. The key involvement of TGR5 in mediating energy homeostasis and glucose homeostasis made it an attractive target for the potential treatment of metabolic disorders.

Gene expression profiling in the liver and lung of perfluorooctane sulfonate-exposed mouse fetuses: comparison to changes induced by exposure to perfluorooctanoic acid

Perfluorooctane sulfonate (PFOS) and perfluorooctanoic acid (PFOA) are environmental contaminants found in the tissues of humans and wildlife. They are activators of peroxisome proliferator-activated receptor-alpha (PPAR alpha) and exhibit hepatocarcinogenic potential in rats. PFOS and PFOA are also developmental toxicants in rodents and PFOS has been shown to induce pulmonary deficits in rat offspring. Pregnant CD-1 mice were dosed with 0, 5, or 10mg/kg PFOS from gestation days 1-17. Transcript profiling was conducted on the fetal liver and lung. Results were contrasted to data derived from a previous PFOA study. PFOS-dependent changes were primarily related to activation of PPAR alpha. No remarkable differences were found between PFOS and PFOA. Given that PPAR alpha signaling is required for neonatal mortality in PFOA-treated mice but not those exposed to PFOS, the neonatal mortality observed for PFOS may reflect functional deficits related to the physical properties of the chemical rather than to transcript alterations.

Significance and mechanism of CYP7a1 gene regulation during the acute phase of liver regeneration

Cholesterol 7alpha-hydroxylase (CYP7a1) is the rate-limiting enzyme in the classic pathway of bile acid synthesis. Expression of CYP7a1 is regulated by a negative feedback pathway of bile acid signaling. Previous studies have suggested that bile acid signaling is also required for normal liver regeneration, and CYP7a1 expression is strongly repressed after 70% partial hepatectomy (PH). Both the effect of CYP7a1 suppression on liver regrowth and the mechanism by which 70% PH suppresses CYP7a1 expression are unknown. Here we show that liver-specific overexpression of an exogenous CYP7a1 gene impaired liver regeneration after 70% PH, which was accompanied by increased hepatocyte apoptosis and liver injury. CYP7a1 expression was initially suppressed after 70% PH in an farnesoid X receptor/ small heterodimer partner-independent manner; however, both farnesoid X receptor and small heterodimer partner were required to regulate CYP7a1 expression at the later stage of liver regeneration. c-Jun N-terminus kinase and hepatocyte growth factor signaling pathways are activated during the acute phase of liver regeneration. We determined that hepatocyte growth factor and c-Jun N-terminus kinase pathways were involved in the suppressing of the CYP7a1 expression in the acute phase of live regeneration. Taken together, our results provide the significance that CYP7a1 suppression is required for liver protection after 70% PH and there are two distinct phases of CYP7a1 gene regulation during liver regeneration.

Unsaturated fatty acid regulation of cytochrome P450 expression via a CAR-dependent pathway

The liver is responsible for key metabolic functions, including control of normal homoeostasis in response to diet and xenobiotic metabolism/detoxification. We have shown previously that inactivation of the hepatic cytochrome P450 system through conditional deletion of POR (P450 oxidoreductase) induces hepatic steatosis, liver growth and P450 expression. We have exploited a new conditional model of POR deletion to investigate the mechanism underlying these changes. We demonstrate that P450 induction, liver growth and hepatic triacylglycerol (triglyceride) homoeostasis are intimately linked and provide evidence that the observed phenotypes result from hepatic accumulation of unsaturated fatty acids, which mediate these phenotypes by activation of the nuclear receptor CAR (constitutive androstane receptor) and, to a lesser degree, PXR (pregnane X receptor). To our knowledge this is the first direct evidence that P450s play a major role in controlling unsaturated fatty acid homoeostasis via CAR. The regulation of P450s involved in xenobiotic metabolism by this mechanism has potentially significant implications for individual responses to drugs and environmental chemicals.

The steroid and xenobiotic receptor (SXR), beyond xenobiotic metabolism

The steroid and xenobiotic receptor (SXR) (also known as pregnane X receptor or PXR) is a nuclear hormone receptor activated by a diverse array of endogenous hormones, dietary steroids, pharmaceutical agents, and xenobiotic compounds. SXR has an enlarged, flexible, hydrophobic ligand binding domain (LBD) which is remarkably divergent across mammalian species and SXR exhibits considerable differences in its pharmacology among mammals. The broad response profile of SXR has led to the development of "the steroid and xenobiotic sensor hypothesis". SXR has been established as a xenobiotic sensor that coordinately regulates xenobiotic clearance in the liver and intestine via induction of genes involved in drug and xenobiotic metabolism. In the past few years, research has revealed new and mostly unsuspected roles for SXR in modulating inflammation, bone homeostasis, vitamin D metabolism, lipid homeostasis, energy homeostasis and cancer. The identification of SXR as a xenobiotic sensor has provided an important tool for studying new mechanisms through which diet, chemical exposure, and environment ultimately impact health and disease. The discovery and pharmacological development of new PXR modulators might represent an interesting and innovative therapeutic approach to combat various diseases.

Metabolomics reveals a novel vitamin E metabolite and attenuated vitamin E metabolism upon PXR activation

Pregnane X receptor (PXR) is an important nuclear receptor xenosensor that regulates the expression of metabolic enzymes and transporters involved in the metabolism of xenobiotics and endobiotics. In this study, ultra-performance liquid chromatography (UPLC) coupled with electrospray time-of-flight mass spectrometry (TOFMS), revealed altered urinary metabolomes in both Pxr-null and wild-type mice treated with the mouse PXR activator pregnenolone 16alpha-carbonitrile (PCN). Multivariate data analysis revealed that PCN significantly attenuated the urinary vitamin E metabolite alpha-carboxyethyl hydroxychroman (CEHC) glucuronide together with a novel metabolite in wild-type but not Pxr-null mice. Deconjugation experiments with beta-glucuronidase and beta-glucosidase suggested that the novel urinary metabolite was gamma-CEHC beta-D-glucoside (Glc). The identity of gamma-CEHC Glc was confirmed by chemical synthesis and by comparing tandem mass fragmentation of the urinary metabolite with the authentic standard. The lower urinary CEHC was likely due to PXR-mediated repression of hepatic sterol carrier protein 2 involved in peroxisomal beta-oxidation of branched-chain fatty acids (BCFA). Using a combination of metabolomic analysis and a genetically modified mouse model, this study revealed that activation of PXR results in attenuated levels of the two vitamin E conjugates, and identification of a novel vitamin E metabolite, gamma-CEHC Glc. Activation of PXR results in attenuated levels of the two vitamin E conjugates that may be useful as biomarkers of PXR activation.

Bile Acid sulfation: a pathway of bile acid elimination and detoxification

Sulfotransferase-2A1 catalyzes the formation of bile acid-sulfates (BA-sulfates). Sulfation of BAs increases their solubility, decreases their intestinal absorption, and enhances their fecal and urinary excretion. BA-sulfates are also less toxic than their unsulfated counterparts. Therefore, sulfation is an important detoxification pathway of BAs. Major species differences in BA sulfation exist. In humans, only a small proportion of BAs in bile and serum are sulfated, whereas more than 70% of BAs in urine are sulfated, indicating their efficient elimination in urine. The formation of BA-sulfates increases during cholestatic diseases. Therefore, sulfation may play an important role in maintaining BA homeostasis under pathologic conditions. Farnesoid X receptor, pregnane X receptor, constitutive androstane receptor, and vitamin D receptor are potential nuclear receptors that may be involved in the regulation of BA sulfation. This review highlights current knowledge about the enzymes and transporters involved in the formation and elimination of BA-sulfates, the effect of sulfation on the pharmacologic and toxicologic properties of BAs, the role of BA sulfation in cholestatic diseases, and the regulation of BA sulfation.