An essential role for nuclear receptors SXR/PXR in detoxification of cholestatic bile acids

Lithocholic acid feeding induces segmental bile duct obstruction and destructive cholangitis in mice

We determined the mechanisms of hepatobiliary injury in the lithocholic acid (LCA)-fed mouse, an increasingly used model of cholestatic liver injury. Swiss albino mice received control diet or 1% (w/w) LCA diet (for 1, 2, and 4 days), followed by assessment of liver morphology and ultrastructure, tight junctions, markers of fibrosis and key proteins of hepatobiliary function, and bile flow and composition. As expected LCA feeding led to bile infarcts, which were followed by a destructive cholangitis with activation and proliferation of periductal myofibroblasts. At the ultrastructural level, small bile ducts were frequently obstructed by crystals. Biliary-excreted fluorescence-labeled ursodeoxycholic acid accumulated in bile infarcts, whereas most infarcts did not stain with India ink injected into the common bile duct; both findings are indicative of partial biliary obstruction. Expression of the main basolateral bile acid uptake proteins (sodium-taurocholate cotransporter and organic anion-transporting polypeptide 1) was reduced, the canalicular transporters bile salt export pump and multidrug-related protein 2 were preserved, and the basolateral transporter multidrug-related protein 3 and the detoxifying enzyme sulfotransferase 2a1 were induced. Thus, we demonstrate that LCA feeding in mice leads to segmental bile duct obstruction, destructive cholangitis, periductal fibrosis, and an adaptive transporter and metabolic enzyme response.

Traditional Chinese medicines Wu Wei Zi (Schisandra chinensis Baill) and Gan Cao (Glycyrrhiza uralensis Fisch) activate pregnane X receptor and increase warfarin clearance in rats

The traditional Chinese medicines (TCMs) are essential components of alternative medicines. Many TCMs are known to alter the expression of hepatic drug-metabolizing enzymes and transporters. The molecular mechanism by which TCMs and/or their constituents regulate enzyme and transporter expression, however, has remained largely unknown. In this report, we show that two TCMs, Wu Wei Zi (Schisandra chinensis Baill) and Gan Cao (Glycyrrhiza uralensis Fisch), and their selected constituents activate the xenobiotic orphan nuclear receptor pregnane X receptor (PXR). Treatment with TCM extracts and the Schisandrol and Schisandrin constituents of Wu Wei Zi induced the expression of drug-metabolizing enzymes and transporters in reporter gene assays and in primary hepatocyte cultures. The affected enzymes and transporters include CYP3A and 2C isozymes and the multidrug resistance-associated protein 2. In transient transfection and reporter gene assays, the Schisandrin constituents of Wu Wei Zi had an estimated EC50 of 2 and 1.25 microM on hPXR and mPXR, respectively. Interestingly, mutations that were intended to alter the pore of the ligand-binding cavity of PXR had species-specific effects on the activities of the individual Schisandrols and Schisandrins. In rats, the administration of Wu Wei Zi and Gan Cao increased the metabolism of the coadministered warfarin, reinforcing concerns involving the safe use of herbal medicines and other nutraceuticals to avoid PXR-mediated drug-drug interactions. Meanwhile, the activation of PXR and induction of detoxifying enzymes provide a molecular mechanism for the hepatoprotective effects of certain TCMs.

The role of pregnane X receptor in 2-acetylaminofluorene-mediated induction of drug transport and -metabolizing enzymes in mice

Activation of the pregnane X receptor (PXR) mediates the induction of several drug transporters and -metabolizing enzymes. In vitro studies have reported that several of these genes are induced after exposure to the hepatocarcinogen, 2-acetylaminofluorene (2-AAF). Thus, we hypothesized that PXR may play a role in the in vivo induction of gene expression by 2-AAF. We examined the expression of the drug-metabolizing enzymes CYP1A2 and CYP3A11 and the drug transporters breast cancer resistance protein (BCRP), MRP2, and OATP2. Wild-type (PXR+/+) and PXR-null (PXR-/-) C57BL/6 mice were injected daily for 7 days with 150 or 300 mg/kg 2-AAF suspended in corn oil (i.p.), whereas the control group received corn oil vehicle. Levels of mRNA isolated from liver were measured by reverse transcription-polymerase chain reaction and normalized to beta-actin. Treatment of PXR+/+ mice resulted in a dose-dependent 2- to 4-fold induction (p<0.001) of MRP2, OATP2, BCRP, CYP3A11, and CYP1A2, but no induction was observed in PXR-/- mice. Induction of PXR mRNA was observed in the 2-AAF-treated PXR+/+ mice. Furthermore, a dose-dependent increase in CYP3A4 promoter construct activity was observed in HepG2 cells cotransfected with human or rat PXR, indicating that 2-AAF does indeed activate PXR. These results suggest that PXR is responsible for 2-AAF-mediated induction of drug efflux transporters and biotransformation enzymes in the liver. Moreover, novel findings demonstrate that PXR plays a role in regulation of the drug efflux transporter, BCRP, in mice.

Functional inhibitory cross-talk between constitutive androstane receptor and hepatic nuclear factor-4 in hepatic lipid/glucose metabolism is mediated by competition for binding to the DR1 motif and to the common coactivators, GRIP-1 and PGC-1alpha

The role of the constitutive androstane receptor (CAR) in xenobiotic metabolism by inducing expression of cytochromes P450 is well known, but CAR has also been implicated in the down-regulation of key genes involved in bile acid synthesis, gluconeogenesis, and fatty acid beta-oxidation by largely unknown mechanisms. Because a key hepatic factor, hepatic nuclear factor-4 (HNF-4), is crucial for the expression of many of these genes, we examined whether CAR could suppress HNF-4 transactivation. Expression of CAR inhibited HNF-4 transactivation of CYP7A1, a key gene in bile acid synthesis, in HepG2 cells, and mutation of the DNA binding domain of CAR impaired this inhibition. Gel shift assays revealed that CAR competes with HNF-4 for binding to the DR1 motif in the CYP7A1 promoter. TCPOBOP, a CAR agonist that increases the interaction of CAR with coactivators, potentiated CAR inhibition of HNF-4 transactivation. Furthermore, inhibition by CAR was reversed by expression of increasing amounts of GRIP-1 or PGC-1alpha, indicating that CAR competes with HNF-4 for these coactivators. Treatment of mice with phenobarbital or TCPOBOP resulted in decreased hepatic mRNA levels of the reported genes down-regulated by CAR, including Cyp7a1 and Pepck. In vivo recruitment of endogenous CAR to the promoters of Cyp7a1 and Pepck was detected in mouse liver after phenobarbital treatment, whereas association of HNF-4 and coactivators, GRIP-1, p300, and PGC-1alpha, with these promoters was significantly decreased. Our data suggest that CAR inhibits HNF-4 activity by competing with HNF-4 for binding to the DR1 motif and to the common coactivators, GRIP-1 and PGC-1alpha, which may be a general mechanism by which CAR down-regulates key genes in hepatic lipid and glucose metabolism.

Soy protein isolate induces CYP3A1 and CYP3A2 in prepubertal rats

Feeding soy diets has been shown to induce cytochrome P450s in gene family CYP3A in Sprague-Dawley rat liver. We compared expression of CYP3A enzymes on postnatal Day 33 (PND33) rats fed casein or soy protein isolate (SPI+)-based AIN-93G diets continuously from gestational Day 4 through PND33 or the diets were switched on PND15 (n = 3-6 litters) to examine the potential imprinting effects of soy on drug metabolism. In addition rats were fed casein, SPI+, SPI+ stripped of phytochemicals (SPI-), or casein diets supplemented with the soy-associated isoflavones genistein or daidzein from weaning through PND33 to examine the hypothesis that the isoflavones are responsible for CYP3A induction by soy feeding. Feeding SPI either continuously or from weaning induced hepatic CYP3A1 and CYP3A2 mRNA, apoprotein, and CYP3A-dependent testosterone 6beta-hydroxylase activity in liver microsomes 2- to 5-fold (P < 0.05). CYP3A mRNA expression was also elevated 2- to 3-fold in the jejunum of SPI-fed rats (P < 0.05). CYP3A was not induced in livers of rats switched to casein from soy at weaning. Induction of CYP3A1 also did not occur in rats fed SPI-, but CYP3A2 mRNA and apoprotein were induced (P < 0.05) in females fed SPI-. Offspring weaned onto genistein-supplemented diets had no elevation of CYP3A mRNAs or apoproteins. Weaning onto daidzein diets increased CYP3A2 mRNA and apoprotein expression in male rats (P < 0.05). These data suggest that early soy consumption may increase the metabolism of a wide variety of CYP3A substrates, but that soy does not imprint the expression of CYP3A enzymes. Effects on CYP3A1 expression appear to be primarily due to phytochemical components of SPI other than isoflavones. In contrast, consumption of soy protein and daidzein appear to be associated with the induction of CYP3A2.

Pregnane X receptor-mediated transcription

The pregnane X receptor (PXR, receptor NR1I2) is a ligand-activated transcription factor that is activated by structurally diverse endogenous steroids and foreign chemicals and serves as an important steroid and xenobiotic sensor. This member of the nuclear receptor superfamily is highly expressed in liver and in the gastrointestinal tract, where it regulates transcription of a large set of genes that contribute to foreign compound metabolism and to the metabolism and transcellular transport of steroid hormones, bile acids, and other endogenous substances. This chapter summarizes studies of PXR and its biological functions and describes a cell culture-based luciferase reporter gene assay for determination of PXR transcriptional activity. This assay can be used to identify novel drugs and environmental chemicals that serve as PXR ligands and thereby modulate PXR activity and may aid in the prediction of drug-drug interactions and foreign chemical-induced toxicities associated with the activation of PXR transcriptional responses.

Identification of regulatory sites in the human PXR (NR1I2) promoter region

The human pregnane X receptor (hPXR, NR1I2) is a member of the nuclear receptor superfamily and a key regulator of genes encoding several major cytochrome P450 enzymes and transporters. However, the transcriptional regulation of hPXR itself remains unclear. We recently reported significant diversity in the 5' region of human hepatic PXR transcripts and identified the major transcription initiation site. Here, we investigate the transcriptional regulatory sites in the hPXR 5'-flanking region. Luciferase reporter constructs containing various lengths of 5'-flanking region, up to 10.5 kb upstream of the major transcription initiation site, were assessed for promoter activity in HepG2 cells. We mapped the minimal essential region for promoter activity to a 160 bp region upstream of the transcription initiation site, an area that also showed nuclear protein binding. Constructs with mutations introduced into these protein-binding sites demonstrated reduced promoter activity concomitant with reduced DNA-protein binding activity. hPXR promoter activity was observed in HepG2 cells but not in HeLa cells. Likewise, nuclear protein binding to promoter elements was also observed in HepG2 but not HeLa cells. The present study provides basic information on the transcriptional regulation of hPXR and may help elucidate the regulatory mechanisms of hPXR target genes.

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.

Downregulation of endothelin-1 by farnesoid X receptor in vascular endothelial cells

The farnesoid X receptor (FXR) is a member of the nuclear receptor superfamily that is highly expressed in liver, kidney, adrenals, and intestine. FXR may play an important role in the pathogenesis of cardiovascular diseases via regulating the metabolism and transport of cholesterol. In this study, we report that FXR is also expressed in rat pulmonary artery endothelial cells (EC), a "nonclassical" bile acid target tissue. FXR is functional in EC, as demonstrated by induction of its target genes such as small heterodimer partner (SHP) after treatment with chenodeoxycholic acid, a FXR agonist. Interestingly, activation of FXR in EC led to downregulation of endothelin (ET)-1 expression. Reporter assays showed that activation of FXR inhibited transcriptional activation of the human ET-1 gene promoter and also repressed the activity of a heterologous promoter driven by activator protein (AP)-1 response elements. Electrophoretic mobility-shift and chromatin immunoprecipitation assays indicated that FXR reduced the binding activity of AP-1 transcriptional factors, suggesting that FXR may suppress ET-1 expression via negatively interfering with AP-1 signaling. These studies suggest that FXR may play a role in endothelial homeostasis and may serve as a novel molecular target for manipulating ET-1 expression in vascular EC.

Orphan nuclear receptors, PXR and LXR: new ligands and therapeutic potential

Found in almost all animal species, orphan nuclear receptors (NRs) represent a unique and pivotal resource to uncover new regulatory systems that impact on both health and human diseases. Some of the current marketed drugs are known to target orphan NRs. Examples include the anticancer and retinoic X receptor (RXR)-targeting bexarotene (Targretin, Ligand Pharmaceuticals, Inc.) and the antidiabetic and peroxisome proliferator-activated receptor (PPAR)-gamma-targeting thiaozolidinediones. Several studies presented at a recent conference (Orphan and Nuclear Receptors - New Therapeutic Developments) have provided new insights into several orphan NRs, including the pregnane X receptor (PXR), the liver X receptor (LXR), the constitutive androstane receptor (CAR), PPAR and the RXR. This paper will focus on PXR and LXR, whose recent target gene analysis and ligand identification have raised both promises and practical concerns as to whether or not these receptors can be used as therapeutic targets.

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

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

Pregnane and Xenobiotic Receptor (PXR/SXR) resides predominantly in the nuclear compartment of the interphase cell and associates with the condensed chromosomes during mitosis

Pregnane and Xenobiotic Receptor (PXR) is a transcription factor that is activated by a diverse range of xenobiotics and endogenous metabolites including steroids, bile acids and about 50% of the prescription drugs. In specific cell types (e.g. liver and intestine) it serves as a 'xenosensor' by regulating expression of a network of genes involved in xenobiotic clearance from the body. PXR expression in several cancerous tissues and its regulated expression of multi-drug resistance proteins highlight its significance in prognosis of malignancies. The view that subcellular localization and ligand induced movements of transcription factors is one of the major phenomena in regulating transcriptional activity, we used a green fluorescent protein tagged PXR chimera to study its dynamic behaviour in living cells. Under all experimental conditions, PXR was observed to be a predominantly nuclear protein maintaining a dynamic equilibrium between the nuclear and cytoplasmic compartments of the interphase cells. Interestingly, for the first time, a member of the nuclear receptor superfamily, PXR, has been observed to be associated with condensed chromosomes during all the mitotic stages of cell division. The significance of PXR association with mitotic chromosomes is discussed.

Regulation of bile acid biosynthesis by hepatocyte nuclear factor 4alpha

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

Molecular and functional comparison of 1,25-dihydroxyvitamin D(3) and the novel vitamin D receptor ligand, lithocholic acid, in activating transcription of cytochrome P450 3A4

The vitamin D receptor (VDR) binds to and mediates the effects of the 1,25-dihydroxyvitamin D(3) (1,25(OH)(2)D(3)) hormone to alter gene transcription. A newly recognized VDR ligand is the carcinogenic bile acid, lithocholic acid (LCA). We demonstrate that, in HT-29 colon cancer cells, both LCA and 1,25(OH)(2)D(3) induce expression of cytochrome P450 3A4 (CYP3A4), an enzyme involved in cellular detoxification. We also show that LCA-VDR stimulates transcription of gene reporter constructs containing DR3 and ER6 vitamin D responsive elements (VDREs) from the human CYP3A4 gene. Utilizing gel mobility shift, pulldown, and mammalian two-hybrid assays, we observe that: (i) 1,25(OH)(2)D(3) enhances retinoid X receptor (RXR) heterodimerization with VDR more effectively than LCA, (ii) the 1,25(OH)(2)D(3)-liganded VDR-RXR heterodimer recruits full-length SRC-1 coactivator, whereas this interaction is minimal with LCA unless LXXLL-containing fragments of SRC-1 are employed, and (iii) both 1,25(OH)(2)D(3) and LCA enhance the binding of VDR to DRIP205/mediator, but unlike 1,25(OH)(2)D(3)-VDR, LCA-VDR does not interact detectably with NCoA-62 or TRIP1/SUG1, suggesting a different pattern of LCA-VDR comodulator association. Finally, residues in the human VDR (hVDR) ligand binding domain (LBD) were altered to create mutants unresponsive to 1,25(OH)(2)D(3)- and/or LCA-stimulated transactivation, identifying S237 and S225/S278 as critical for 1,25(OH)(2)D(3) and LCA action, respectively. Therefore, these two VDR ligands contact distinct residues in the binding pocket, perhaps generating unique receptor conformations that determine the degree of RXR and comodulator binding. We propose that VDR is a bifunctional regulator, with the 1,25(OH)(2)D(3)-liganded conformation facilitating high affinity endocrine actions, and the LCA-liganded configuration mediating local, lower affinity cellular detoxification by upregulation of CYP3A4 in the colon.

Expression of the pregnane X receptor in mice antagonizes the cholic acid-mediated changes in plasma lipoprotein profile

OBJECTIVE

Modification of lipoprotein metabolism by bile acids has been mainly explained by activation of the farnesyl X receptor (FXR). The aim of the present study was to determine the relative contribution of the pregnane X receptor (PXR), another bile acid-activated nuclear receptor to changes in plasma lipoprotein profile.

METHODS AND RESULTS

Wild-type mice, Pxr-deficient mice, and Pxr-null mice expressing human PXR (Pxr-null SXR-Tg mice) were fed a cholic acid-containing diet, and consequences on plasma lipoprotein profiles and target gene expression were assessed. Cholic acid produced significant decreases in high-density lipoprotein (HDL) cholesterol, plasma apolipoprotein (apo)A-I and hepatic apoA-I mRNA in wild-type mice. Interestingly, the effect of cholic acid was significantly more pronounced in Pxr-deficient mice, indicating that PXR contributes to the weakening of the effect of bile acids on lipoprotein metabolism. Reciprocally, changes in HDL/apoA-I profiles were abolished in Pxr-null SXR-Tg mice in which PXR-responsive genes, particularly those involved in bile acid detoxification were readily activated after cholic acid treatment.

CONCLUSIONS

PXR expression in mice antagonizes the cholic acid-mediated downregulation of plasma HDL cholesterol and apoA-I, and magnification of PXR/SXR-mediated changes may constitute a new mean to counteract the effects of bile acids on plasma lipoproteins.

Induction of phase I, II and III drug metabolism/transport by xenobiotics

Drug metabolizing enzymes (DMEs) play central roles in the metabolism, elimination and detoxification of xenobiotics and drugs introduced into the human body. Most of the tissues and organs in our body are well equipped with diverse and various DMEs including phase I, phase II metabolizing enzymes and phase III transporters, which are present in abundance either at the basal unstimulated level, and/or are inducible at elevated level after exposure to xenobiotics. Recently, many important advances have been made in the mechanisms that regulate the expression of these drug metabolism genes. Various nuclear receptors including the aryl hydrocarbon receptor (AhR), orphan nuclear receptors, and nuclear factor-erythoroid 2 p45-related factor 2 (Nrf2) have been shown to be the key mediators of drug-induced changes in phase I, phase II metabolizing enzymes as well as phase III transporters involved in efflux mechanisms. For instance, the expression of CYP1 genes can be induced by AhR, which dimerizes with the AhR nuclear translocator (Arnt), in response to many polycyclic aromatic hydrocarbon (PAHs). Similarly, the steroid family of orphan nuclear receptors, the constitutive androstane receptor (CAR) and pregnane X receptor (PXR), both heterodimerize with the retinoid X receptor (RXR), are shown to transcriptionally activate the promoters of CYP2B and CYP3A gene expression by xenobiotics such as phenobarbital-like compounds (CAR) and dexamethasone and rifampin-type of agents (PXR). The peroxisome proliferator activated receptor (PPAR), which is one of the first characterized members of the nuclear hormone receptor, also dimerizes with RXR and has been shown to be activated by lipid lowering agent fibrate-type of compounds leading to transcriptional activation of the promoters on CYP4A gene. CYP7A was recognized as the first target gene of the liver X receptor (LXR), in which the elimination of cholesterol depends on CYP7A. Farnesoid X receptor (FXR) was identified as a bile acid receptor, and its activation results in the inhibition of hepatic acid biosynthesis and increased transport of bile acids from intestinal lumen to the liver, and CYP7A is one of its target genes. The transcriptional activation by these receptors upon binding to the promoters located at the 5-flanking region of these CYP genes generally leads to the induction of their mRNA gene expression. The physiological and the pharmacological implications of common partner of RXR for CAR, PXR, PPAR, LXR and FXR receptors largely remain unknown and are under intense investigations. For the phase II DMEs, phase II gene inducers such as the phenolic compounds butylated hydroxyanisol (BHA), tert-butylhydroquinone (tBHQ), green tea polyphenol (GTP), (-)-epigallocatechin-3-gallate (EGCG) and the isothiocyanates (PEITC, sulforaphane) generally appear to be electrophiles. They generally possess electrophilic-mediated stress response, resulting in the activation of bZIP transcription factors Nrf2 which dimerizes with Mafs and binds to the antioxidant/electrophile response element (ARE/EpRE) promoter, which is located in many phase II DMEs as well as many cellular defensive enzymes such as heme oxygenase-1 (HO-1), with the subsequent induction of the expression of these genes. Phase III transporters, for example, P-glycoprotein (P-gp), multidrug resistance-associated proteins (MRPs), and organic anion transporting polypeptide 2 (OATP2) are expressed in many tissues such as the liver, intestine, kidney, and brain, and play crucial roles in drug absorption, distribution, and excretion. The orphan nuclear receptors PXR and CAR have been shown to be involved in the regulation of these transporters. Along with phase I and phase II enzyme induction, pretreatment with several kinds of inducers has been shown to alter the expression of phase III transporters, and alter the excretion of xenobiotics, which implies that phase III transporters may also be similarly regulated in a coordinated fashion, and provides an important mean to protect the body from xenobiotics insults. It appears that in general, exposure to phase I, phase II and phase III gene inducers may trigger cellular "stress" response leading to the increase in their gene expression, which ultimately enhance the elimination and clearance of these xenobiotics and/or other "cellular stresses" including harmful reactive intermediates such as reactive oxygen species (ROS), so that the body will remove the "stress" expeditiously. Consequently, this homeostatic response of the body plays a central role in the protection of the body against "environmental" insults such as those elicited by exposure to xenobiotics.

A Novel Polymorphic Cytochrome P450 Formed by Splicing of CYP3A7 and the Pseudogene CYP3AP1

The cytochrome P450 3A7 (CYP3A7) is the most abundant CYP in human liver during fetal development and first months of postnatal age, playing an important role in the metabolism of endogenous hormones, drugs, differentiation factors, and potentially toxic and teratogenic substrates. Here we describe and characterize a novel enzyme, CYP3A7.1L, encompassing the CYP3A7.1 protein with the last four carboxyl-terminal amino acids replaced by a unique sequence of 36 amino acids, generated by splicing of CYP3A7 with CYP3AP1 RNA. The corresponding CYP3A7-3AP1 mRNA had a significant expression in liver, kidney, and gastrointestinal tract, and its presence was found to be tissue-specific and dependent on the developmental stage. Heterologous expression in yeast revealed that CYP3A7.1L was a functional enzyme with a specific activity similar to that of CYP3A7.1 and, in some conditions, a different hydroxylation specificity than CYP3A7.1 using dehydroepiandrosterone as a substrate. CYP3A7.1L was found to be polymorphic due to a mutation at position -6 of the first splicing site of CYP3AP1 (CYP3A7_39256T-->A), which abrogates the pseudogene splicing. This polymorphism had pronounced interethnic differences and was in linkage disequilibrium with other functional polymorphisms described in the CYP3A locus: CYP3A7*2 and CYP3A5*1. Therefore, the resulting CYP3A haplotypes express different sets of enzymes within the population. In conclusion, a novel mechanism, consisting of the splicing of the pseudogene CYP3AP1 to CYP3A7, causes the formation of the novel CYP3A7.1L having a different tissue distribution and functional properties than the parent CYP3A7 enzyme, with possible developmental, physiological, and toxicological consequences.

A pregnane X receptor agonist with unique species-dependent stereoselectivity and its implications in drug development

Pregnane X receptor (PXR) is an orphan nuclear receptor that regulates the expression of genes encoding drug-metabolizing enzymes and transporters. In addition to affecting drug metabolism, potent and selective PXR agonists may also have therapeutic potential by removing endogenous and exogenous toxins. In this article, we report the synthesis and identification of novel PXR agonists from a library of peptide isosteres. Compound S20, a C-cyclopropylalkylamide, was found to be a PXR agonist with both enantiomer- and species-specific selectivity. S20 has three chiral carbons and was resolved into its two enantiomers. The individual S20 enantiomers exhibited striking mouse/human-specific PXR activation, whereby enantiomer (+)-S20 preferentially activated hPXR, and enantiomer (-)-S20 was a better activator for mPXR. As a human PXR (hPXR) agonist, (+)-S20 was more potent and efficacious than rifampicin. Mutagenesis studies revealed that the ligand binding domain residue Phe305 is critical for the preference for the (-)-S20 enantiomer by the rodent PXR. Treatment of S20 induced the expression of drug-metabolizing enzymes and transporters in reporter gene assays, in primary human hepatocytes, and in "humanized" hPXR transgenic mice. To our knowledge, S20 represents the first compound whose enantiomers have opposite species preference in activating a xenobiotic receptor. The stereoselectivity may be used to guide the development of safer drugs to avoid drug-drug interactions or to achieve human-specific therapeutic effects when a xenobiotic receptor is being used as a drug target.

Cyclic AMP-independent activation of CYP3A4 gene expression by forskolin

Forskolin and cAMP have been shown to have paradoxical effects in the regulation of expression levels of mRNA of cytochrome P450 3A (CYP3A) family members. We demonstrate in this study that forskolin upregulated the promoter for CYP3A4 independent of its ability to increase cAMP levels. This activity was explained showing forskolin directly activated the pregnane-X-receptor, a known regulator of CYP3A genes.

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.

Hormonal regulation of hepatic organic anion transporting polypeptides

Organic anion transporting polypeptides (Oatp) mediate the transport of a wide variety of amphipathic organic substrates. Rat Oatp1b2 and human OATP1B3 are members of a liver-specific subfamily of Oatps/OATPs. We investigated whether prolactin (PRL) and growth hormone (GH) regulated Oatp1b2 and OATP1B3 gene expression via signal transducers and activators of transcription 5 (Stat5). Binding sites for Stat5 transcription factors were located in the promoters of Oatp1b2 and OATP1B3 at -209 to -201 (5'-TTCTGGGAA-3') and -170 to -162 (5'-TTCTGAGAA-3'), respectively. In primary hepatocytes from female and male rats treated with PRL or GH, Oatp1b2 mRNA measured by real-time polymerase chain reaction was significantly induced 2-fold. HepG2 cells were transiently transfected with expression vectors containing Oatp1b2 or OATP1B3 promoter fragments, cDNAs for Stat5a, and the receptors for PRL (PRLR(L)) or GH (GHR), and treated with PRL or GH. PRL and GH induction of Oatp1b2 and OATP1B3 promoter activity required cotransfection of Stat5a and PRLR(L) or GHR. Mutation of the Stat5 binding site in both promoters eliminated hormonal induction. In DNA binding assays, HepG2 cells transfected with cDNAs for Stat5a and PRLR(L) were treated with PRL, and nuclear extracts were probed with a (32)P-labeled oligomer corresponding to -177 to -157 of the OATP1B3 promoter. PRL enhanced the binding of Stat5a to the OATP1B3 promoter and DNA-protein binding was inhibited in competition assays by excess OATP1B3 and Stat5 consensus oligomers but not by mutant Stat5 oligomers. These findings indicate that PRL and GH can regulate Oatp1b2 and OATP1B3 gene expression via the Stat5 signal-transduction pathway.

Lithocholic acid decreases expression of UGT2B7 in Caco-2 cells: a potential role for a negative farnesoid X receptor response element

Human UDP-glucuronosyltransferase (UGT) 2B7 is the major isoform catalyzing the glucuronidation of a variety of endogenous compounds including bile acids. To determine the role of bile acids in the regulation of UGT2B7 expression, Caco-2 cells were incubated with the natural human farnesoid X receptor (hFXR) ligand, chenodeoxycholic acid, as well as the secondary bile acid, lithocholic acid, derived from chenodeoxycholic acid. Incubation of Caco-2 cells with lithocholic acid in the absence of exogenous hFXR resulted in a dose-dependent down-regulation of UGT2B7 mRNA levels, with an IC(50) of 13 microM. Similar down-regulation was also observed with chenodeoxycholic acid; however, much higher concentrations were required. Transient transfection of Caco-2 cells with hFXR suppressed UGT2B7 mRNA expression both in the absence and presence of ligand. UGT2B7 promoter transfection experiments and deletion/mutation analysis showed that lithocholic acid-activated hFXR decreased UGT2B7 promoter activity via a negative hFXR response element (NFRE) located between nucleotides -148 and -134. Cotransfection with hFXR and/or human retinoid X receptor further enhanced the repression. Electrophoretic mobility shift assays additionally confirmed the role of NFRE in UGT2B7 down-regulation by lithocholic acid. These findings suggest that lithocholic acid, an activator of nuclear hFXR, acts as a negative regulator of UGT2B7 expression, indicating that hFXR may play an essential role in lithocholic acid homeostasis through negative regulation of this UGT that is involved in lithocholic acid biotransformation. Therefore, it is postulated that lithocholic acid toxicity may be due to down-regulation of genes involved in its detoxification, including UGT2B7, leading to limited excretion of lithocholic acid from the body.

Pregnane X receptor-agonists down-regulate hepatic ATP-binding cassette transporter A1 and scavenger receptor class B type I

Pregnane X receptor (PXR) is the molecular target for a wide variety of endogenous and xenobiotic compounds. It regulates the expression of genes central to the detoxification (cytochrome P-450 enzymes) and excretion (xenobiotic transporters) of potentially harmful compounds. The aim of the present investigation was to determine the role of PXR in regulation of high-density lipoprotein (HDL) cholesterol metabolism by studying its impact on ATP-binding cassette transporter A1 (ABCA1) and scavenger receptor class B type I (SR-BI) expression in hepatocytes. ABCA1 and SR-BI are major factors in the exchange of cholesterol between cells and HDL. Expression analyses were performed using Western blotting and quantitative real time RT-PCR. Luciferase reporter gene assays were used to measure promoter activities. Total cholesterol was measured enzymatically after lipid extraction (Folch's method). The expression of ABCA1 and SR-BI was inhibited by the PXR activators rifampicin and lithocholic acid (LCA) in HepG2 cells and pregnenolone 16alpha-carbonitrile (PCN) in primary rat hepatocytes. Thus, PXR appears to be a regulator of hepatic cholesterol transport by inhibiting genes central to cholesterol uptake (SR-BI) and efflux (ABCA1).

Vitamin D Receptor-dependent Regulation of Colon Multidrug Resistance-associated Protein 3 Gene Expression by Bile Acids

The multidrug resistance-associated protein 3 (MRP3) is a multispecific anion transporter that is capable of transporting a number of conjugated and unconjugated bile acids. Expression of the MRP3 gene is increased during pathological states associated with elevated bile acid concentrations indicating a role for this transporter in adaptive and homeostatic bile acid metabolism. Analysis of Mrp3 mRNA levels in various mouse tissues with known relevance and/or exposure to bile acids revealed the highest levels of basal expression in the colon followed in order by the liver, duodenum, jejunum, ileum, and kidney. Functional analysis of a murine Mrp3 promoter reporter construct revealed vitamin D receptor (VDR)-dependent activation by 1,25-dihydroxyvitamin D(3) (VD3), 9-cis-retinoic acid (RA), and the cholestatic secondary bile acid, lithocholic acid (LCA). Using a series of deletion constructs combined with sequence analysis, a candidate VDR response element (VDRE) was identified between -1028 and -1014 bp of the Mrp3 promoter. Activation of the Mrp3 promoter in response to VD3, RA, or LCA, as well as binding of VDR/RXR heterodimers, was attenuated substantially by mutation of this VDRE. Treatment of mice with VD3 or LCA demonstrated in vivo modulation of the Mrp3 gene in colon but not in the liver. Reduction of endogenous VDR expression in colon adenocarcinoma MCA-38 cells by siRNA transfection was associated with reduced constitutive and inducible expression of the Mrp3 gene. These data support a regulatory role for the VDR in the protection of colon cells from bile acid toxicity through regulation of the Mrp3 expression.

Clofibrate and perfluorodecanoate both upregulate the expression of the pregnane X receptor but oppositely affect its ligand-dependent induction on cytochrome P450 3A23

The pregnane X receptor (PXR) interacts with a vast array of structurally dissimilar chemicals and confers induction of several major types of drug metabolizing enzymes such as cytochrome P450s (CYP). We previously reported that the expression of PXR was markedly increased in rats treated with clofibrate and perfluorodecanoic acid (PFDA). The present study was undertaken to test the hypothesis that induced expression of PXR increases PXR ligand-dependent induction on CYP3A23. Rat hepatocytes were treated with clofibrate or PFDA individually, or along with PXR ligand pregnenolone 16alpha-carbonitrile (PCN), and the levels of PXR and CYP3A23 were determined by Western blots. Both clofibrate and PFDA markedly increased the expression of PXR with PFDA being more potent, and the induction was abolished by actinomycin D, an inhibitor for mRNA synthesis. As expected, PCN alone markedly induced the expression of CYP3A23. Interestingly, co-treatment with clofibrate enhanced the induction, whereas co-treatment with PFDA suppressed it. Clofibrate and PFDA represent multi-classes of chemicals called peroxisome proliferators including many therapeutic agents and industrial pollutants. The opposing effects of clofibrate and PFDA on the PCN-induced expression of CYP3A23 suggest that peroxisome proliferators likely increase the expression of PXR but differentially alter its ligand-dependent induction. The interaction between PXR inducer and ligand provides a novel mechanism on how functionally and structurally distinct chemicals cooperatively regulate the expression of xenobiotic-metabolizing enzymes and transporters.

Novel pathways of bile acid metabolism involving CYP3A4

The hepatic predominating cytochrome P450, CYP3A4, plays an essential role in the detoxification of bile acids and is important in pathological conditions such as cholestasis where CYP3A4 is adaptively up-regulated. However, the mechanism that triggers the up-regulation of CYP3A4 is still not clear. In this study, using recombinant CYP3A4 and human liver microsomes, we demonstrate that CYP3A4 can metabolise lithocholic acid into 3-dehydrolithocholic acid, a potent activator of the nuclear receptors, pregnane X receptor and 1,25-dihydroxy vitamin D3 receptor, which are known to regulate the expression of CYP3A4. This process thus provides a feed-forward metabolism of toxic bile acid that may be of importance in maintaining bile acid homeostasis. We also provide evidence for a novel CYP3A4-mediated metabolic pathway of the secondary bile acid deoxycholic acid. Patients treated with the antiepileptic drug carbamazepine, a CYP3A4 inducer, had markedly elevated urinary excretion of 1beta-hydroxydeoxycholic acid compared to healthy controls. The importance of CYP3A4 in this process was verified by incubations with recombinant CYP3A4 and human liver microsomes, both of which efficiently converted deoxycholic acid into 1beta-hydroxydeoxycholic acid. Interestingly, CYP3A4 was also found to be active against the secondary bile acid ursodeoxycholic acid.

Molecular regulation of hepatobiliary transport systems: clinical implications for understanding and treating cholestasis

Hepatobiliary transport systems are responsible for hepatic uptake and excretion of bile salts and other biliary constituents (eg, bilirubin) into bile. Hereditary transport defects can result in progressive familial and benign recurrent intrahepatic cholestasis. Exposure to acquired cholestatic injury (eg, drugs, hormones, proinflammatory cytokines, biliary obstruction or destruction) also results in altered expression and function of hepatic uptake and excretory systems, changes that may maintain and contribute to cholestasis and jaundice. Recruitment of alternative efflux pumps and induction of phase I and II detoxifying enzymes may limit hepatic accumulation of potentially toxic biliary constituents in cholestasis by providing alternative metabolic and escape routes. These molecular changes are mediated by bile salts, proinflammatory cytokines, drugs, and hormones at a transcriptional and posttranscriptional level. Alterations of hepatobiliary transporters and enzymes are not only relevant for a better understanding of the pathophysiology of cholestatic liver diseases, but may also represent important targets for pharmacotherapy. Drugs (eg, ursodeoxycholic acid, rifampicin) used to treat cholestatic liver diseases and pruritus may counteract cholestasis via stimulation of defective transporter expression and function. In addition, therapeutic strategies may be aimed at supporting and stimulating alternative detoxification pathways and elimination routes for bile salts in cholestasis.

Human and animal hepatocytes in vitro with extrapolation in vivo

Human and animal hepatocytes are now being used as an in vitro technique to aid drug discovery by predicting the in vivo metabolic pathways of drugs or new chemical entities (NCEs), identifying drug-metabolizing enzymes and predicting their in vivo induction. Because of the difficulty of establishing whether the cytotoxic susceptibility of human hepatocytes to xenobiotics/drugs in vitro could be used to predict in vivo human hepatotoxicity, a comparison of the susceptibility of the hepatocytes of human and animal models to six chemical classes of drugs/xenobiotics in vitro have been related to their in vivo hepatotoxicity and the corresponding activity of their metabolizing enzymes. This study showed that the cytotoxic effectiveness of 16 halobenzenes towards rat hepatocytes in vitro using higher doses and short incubation times correlated well with rat hepatotoxic effectiveness in vivo with lower doses/longer times. The hepatic/hepatocyte xenobiotic metabolizing enzyme activities of various animal species and human have been reviewed for use by veterinarians and research scientists. Where possible, recommendations have been made regarding which animal hepatocyte model is most applicable for modeling the susceptibility to xenobiotic induced hepatotoxicity of those humans with slow versus rapid metabolizing enzyme polymorphisms. These recommendations are based on the best human fit for animal drug/xenobiotic metabolizing enzymes in terms of activity, kinetics and substrate/inhibitor specificity. The use of human hepatocytes from slow versus rapid metabolizing individuals for drug metabolism/cytotoxicity studies; and the research use of freshly isolated rat hepatocytes and "Accelerated Cytotoxicity Mechanism Screening" (ACMS) techniques for identifying drug/xenobiotic reactive metabolites are also described. Using these techniques the molecular hepatocytotoxic mechanisms found in vitro for seven classes of xenobiotics/drugs were found to be similar to the rat hepatotoxic mechanisms reported in vivo.

Regulation of Bcl-2 and Bcl-xL anti-apoptotic protein expression by nuclear receptor PXR in primary cultures of human and rat hepatocytes

The pregnane X receptor (PXR) plays a major role in the protection of the body by regulating the genes involved in the metabolism and elimination of potentially toxic xeno- and endobiotics. We previously described that PXR activator dexamethasone protects hepatocytes from spontaneous apoptosis. We hypothesise a PXR-dependent co-regulation process between detoxication and programmed cell death. Using primary cultured human and rat hepatocytes, we investigated to determine if PXR is implicated in the regulation of Bcl-2 and Bcl-xL, two crucial apoptosis inhibitors. In the present study we demonstrated that the treatment of primary cultured hepatocytes with PXR agonists increased hepatocyte viability and protects them from staurosporine-induced apoptosis. The anti-apoptotic capacity of PXR activation was correlated with Bcl-2 and Bcl-xL induction at both the transcriptional and protein levels in man and rats, respectively. The inhibition of PXR expression by antisense oligonucleotide abolished PXR activators Bcl-xL induction. Accordingly, PXR overexpression in HepG2 cells led to bcl-2 induction upon clotrimazole treatment and protects cells against Fas-induced apoptosis. Our results demonstrate that PXR expression is required for Bcl-2 and Bcl-xL up-regulation upon PXR activators treatment in human and rat hepatocytes. They also suggest that PXR may protect the liver against chemicals by simultaneously regulating detoxication and the apoptotic pathway.

Role of cytochromes P450 in chemical toxicity and oxidative stress: studies with CYP2E1

Cytochromes P450 are responsible for metabolism of most xenobiotics and are required for the efficient elimination of foreign chemicals from the body. Paradoxically, these enzymes also metabolically activate biologically inert compounds to electrophilic derivatives that can cause toxicity, cell death and sometimes cellular transformation resulting in cancer. To establish the role of these enzymes in toxicity and carcinogenicity in vivo, gene knockout mice have been developed. To illustrate the role of P450s in toxicity, CYP2E1-null mice were employed with the commonly used analgesic drug acetaminophen. CYP2E1 is the rate-limiting enzyme that initiates the cascade of events leading to acetaminophen hepatotoxicity; in the absence of this P450, toxicity will only be apparent at high concentrations. Other enzymes and nuclear receptors are also involved in activation or inactivating chemicals. CYP2E1 is induced by alcohol and the primary P450 that carries out ethanol oxidation that can lead to the production of activated oxygen species and oxidative stress that elevate ERK1/2 phosphorylation through EGRF/c-Raf signaling. Paradoxically, activation of this pathway inhibits apoptotic cell death stimulated by reactive oxygen generating chemicals but accelerates necrotic cell death produced by polyunsaturated fatty acids. CYP2E1 is thought to contribute to liver pathologies that result from alcoholic liver disease and non-alcoholic steatohepatitis.

Dual role of orphan nuclear receptor pregnane X receptor in bilirubin detoxification in mice

The pregnane X receptor (PXR) and the constitutive androstane receptor (CAR) are implicated in xenobiotic and endobiotic detoxification, including the clearance of toxic bilirubin. Previous studies have suggested both overlapping and preferential regulation of target genes by these receptors, but the mechanism of cross-talk remains elusive. Here we reveal a dual role of PXR in bilirubin detoxification in that both the loss and activation of PXR led to protection from hyperbilirubinemia induced by bilirubin infusion or hemolysis. The increased bilirubin clearance in PXR-null mice was associated with selective upregulation of detoxifying enzymes and transporters, and the pattern of regulation is remarkably similar to that of transgenic mice expressing the activated CAR. Interestingly, the increased bilirubin clearance and associated gene regulation were absent in the CAR-null or double-knockout mice. In cell cultures, ligand-free PXR specifically suppressed the ability of CAR to induce the multidrug resistance associated protein 2 (MRP2), a bilirubin-detoxifying transporter. This suppression was, at least in part, the result of the disruption of ligand-independent recruitment of coactivator by CAR. In conclusion, PXR plays both positive and negative roles in regulating bilirubin homeostasis, and this provides a novel mechanism that may govern receptor cross-talk and the hierarchy of xenobiotic and endobiotic regulation. PXR is a potential therapeutic target for clinical treatment of jaundice. (HEPATOLOGY 2005;41:497-505.).

How to make a Barrett esophagus: pathophysiology of columnar metaplasia of the esophagus

Barrett esophagus is defined as a specialized intestinal replacing the squamous epithelium of the esophageal mucosa in response to gastroesophageal reflux. Barrett metaplasia is a healing process that develops to protect the esophagus from further damage. Although mechanisms by which Barrett metaplasia evolves toward dysplasia and adenocarcinoma have been extensively studied, the process by which squamous epithelium is replaced by specialized intestinal metaplasia is poorly understood. Barrett esophagus develops when defense mechanisms in the esophageal mucosa (luminal secretion of mucus, bicarbonate, growth factors, etc.) are overwhelmed by an ongoing cycle of mucosal injury and repair. Hydrogen ion, pepsin, trypsin, and bile acids are considered harmful agents that synergistically invade the esophageal mucosa. Areas of destroyed squamous epithelium are then progressively reepithelized by a columnar epithelium that may originate from multipotent stem cells located within the basal layer of the normal esophageal mucosa or in the ducts of submucosal glands.

Evolution of the pregnane x receptor: adaptation to cross-species differences in biliary bile salts

The pregnane X receptor (PXR) regulates the metabolism and elimination of bile salts, steroids, and xenobiotics. The sequence of the PXR ligand-binding domain diverges extensively between different animals, suggesting interspecies differences in ligands. Of the endogenous ligands known to activate PXR, biliary bile salts vary the most across vertebrate species, ranging from 27-carbon (C27) bile alcohol sulfates (early fish, amphibians) to C24 bile acids (birds, mammals). Using a luciferase-based reporter assay, human PXR was activated by a wide variety of bile salts. In contrast, zebrafish PXR was activated efficiently only by cyprinol sulfate, the major zebrafish bile salt, but not by recent bile acids. Chicken, mouse, rat, and rabbit PXRs were all activated by species-specific bile acids and by early fish bile alcohol sulfates. In addition, phylogenetic analysis using maximum likelihood demonstrated evidence for nonneutral evolution of the PXR ligand-binding domain. PXR activation by bile salts has expanded from narrow specificity for C27 bile alcohol sulfates (early fish) to a broader specificity for recent bile acids (birds, mammals). PXR specificity for bile salts has thus paralleled the increasing complexity of the bile salt synthetic pathway during vertebrate evolution, an unusual example of ligand-receptor coevolution in the nuclear hormone receptor superfamily.

Nuclear receptors as targets for drug development: regulation of cholesterol and bile acid metabolism by nuclear receptors

Nuclear receptors are ligand-dependent transcription factors that recently have been shown to play important roles in the metabolism of cholesterol and bile acids. Cholesterol homeostasis is maintained by de novo synthesis, absorption from diet, catabolism to bile acids and other steroids, and excretion into bile. Dysregulation of this mechanism leads to atherosclerosis and its life-threatening coronary and cerebrovascular sequelae. Conversion of cholesterol to bile acids in the liver is positively regulated by liver X receptor (LXR) alpha, a nuclear receptor for oxysterols. LXRalpha and LXRbeta, a second oxysterol receptor, regulate intestinal absorption and biliary excretion of cholesterol by inducing target gene expression. LXRs stimulate reverse cholesterol transport from peripheral tissues and exhibit antiatherogenic activity. Farnesoid X receptor (FXR), a bile acid receptor, represses bile acid synthesis and import in hepatocytes, stimulates bile acid export from cells, and protects hepatocytes from bile acid toxicity. Pregnane X receptor (PXR) and vitamin D receptor (VDR) respond to secondary bile acids and induce their catabolism. Thus, nuclear receptors play important roles in regulation of cholesterol and bile acid metabolism.

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

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

The involvement of the pregnane X receptor in hepatic gene regulation during inflammation in mice

Inflammation and proinflammatory cytokines suppress the expression of several hepatic transporters and metabolic enzymes, often resulting in cholestatic liver disease. However, mechanism(s) of this down-regulation have not been fully elucidated. As the pregnane X receptor (PXR) is involved in inducing many of these hepatic proteins, it is possible that PXR is also involved in their down-regulation during inflammation. Thus, we compared the effect of inflammation on hepatic gene regulation in wild-type (PXR(+/+)) versus PXR-null (PXR(-/-)) mice. Treatment of PXR(+/+) but not PXR(-/-) mice with the PXR activators 5-pregnen-3beta-ol-20-one-16alpha-carbonitrile (PCN) or 17beta-hydroxy-11beta-[4-dimethylamino phenyl]-17alpha-[1-propynyl] estra-4,9-dien-3-one (RU486) resulted in increased mRNA levels of bsep, mdr1a, mrp2, mrp3, oatp2, and cyp3a11, indicating involvement of PXR in their regulation. Significantly lower mRNA levels of bsep, mdr2, mrp2, mrp3, ntcp, oatp2, and cyp3a11 were found in endotoxin-treated PXR(+/+) mice. In endotoxin-treated PXR(-/-) mice, the extent of mrp2 suppression was significantly diminished. Changes in MRP2 expression were supported by Western blot analysis. Although interleukin (IL)-6 imposed significant decreases in the expression of bsep, mrp2, and cyp3a11 in PXR(+/+) mice, this was not observed in PXR(-/-) mice. Of note, significantly lower levels of PXR mRNA and protein were detected in endotoxin- and IL-6-treated PXR(+/+) mice. In addition, endotoxin and IL-6 were also able to suppress PCN-mediated induction of bsep, mrp2, cyp3a11, and PXR. Taken together, our results suggest that PXR plays a role in the down-regulation of several hepatic proteins during inflammation.

Simultaneous substitution of phenylalanine-305 and aspartate-318 of rat pregnane X receptor with the corresponding human residues abolishes the ability to transactivate the CYP3A23 promoter

The pregnane X receptor (PXR) is a key regulator on the expression of genes involved in the elimination of chemicals. As one of the most divergent members in the nuclear receptor family, PXR is activated in a highly species-dependent manner by certain chemicals. Pregnenolone 16alpha-carbonitrile (PCN), a glucocorticoid antagonist, efficaciously activates rodent but not human PXR. This study was undertaken to investigate the structural basis for PCN-mediated activation of rat PXR. A series of rat-human chimeric PXRs were prepared to gradually replace the ligand-binding domain of human PXR with the corresponding rat sequence at an increasing length of 20 residues. Cotransfection experiments established that region(306-326) acted as a transitional conjunction from none to full PCN responsive status. Site-directed mutagenesis study identified two residues (Phe-305 and Asp-318) that were critical in supporting PCN-mediated activation, and simultaneous substitution of both residues abolished the ability of rat PXR to transactivate the CYP3A23 promoter. In addition, substitutions on Phe-305, Asp-318, or both markedly reduced the basal transcriptional activity, and the reduction occurred with the CYP3A4 but not CYP3A23 promoter. Further study with CYP3A4 and CYP3A23 hybrid reporters demonstrated that the region harboring the distal PXR element in the CYP3A4 promoter mediated the repressive activity. PXR has been shown to interact with corepressors in the absence of ligand. The decreased responsiveness toward PCN and reduced basal transcriptional activity suggest that Phe-305 and Asp-318 are involved in both ligand-binding and corepressor interactions.

Role of farnesoid X receptor in the enhancement of canalicular bile acid output and excretion of unconjugated bile acids: a mechanism for protection against cholic acid-induced liver toxicity

Mice lacking the farnesoid X receptor (FXR) involved in the maintenance of hepatic bile acid levels are highly sensitive to cholic acid-induced liver toxicity. Serum aspartate aminotransferase (AST) activity was elevated 15.7-fold after feeding a 0.25% cholic acid diet, whereas only slight increases in serum AST (1.7- and 2.5-fold) were observed in wild-type mice fed 0.25 and 1% cholic acid diet, respectively. Bile salt export pump mRNA and protein levels were increased in wild-type mice fed 1% cholic acid diet (2.1- and 3.0-fold) but were decreased in FXR-null mice fed 0.25% cholic acid diet. The bile acid output rate was 2.0- and 3.7-fold higher after feeding of 0.25 and 1.0% cholic acid diet in wild-type mice, respectively. On the other hand, no significant increase in bile acid output rate was observed in FXR-null mice fed 0.25% cholic acid diet in contrast to a significant decrease observed in mice fed a 1.0% cholic acid diet in spite of the markedly higher levels of hepatic tauro-conjugated bile acids. Unconjugated cholic acid was not detected in the bile of wild-type mice fed a control diet, but it was readily detected in wild-type mice fed 1% cholic acid diet. The ratio of biliary unconjugated cholic acid to total cholic acid (unconjugated cholic acid and tauro-conjugated cholic acid) reached 30% under conditions of hepatic taurine depletion. These results suggest that the cholic acid-induced enhancement of canalicular bile acid output rates and excretion of unconjugated bile acids are involved in adaptive responses for prevention of cholic acid-induced toxicity.

Nuclear receptors constitutive androstane receptor and pregnane X receptor ameliorate cholestatic liver injury

Cholestasis is associated with accumulation of bile acids and lipids, and liver injury. The constitutive androstane receptor (CAR) and pregnane X receptor (PXR) are xenobiotic nuclear receptors that coordinate protective hepatic responses to potentially toxic stimuli, including bile acids. We investigated the role of these receptors in the regulation of bile acid and lipid metabolism in a bile duct ligation (BDL) model of cholestasis applied to receptor knockout mice. Hepatic damage from bile acid accumulation was increased in both CAR knockout (CARKO) and PXR knockout mice, but bile acid concentrations were lower in CARKO mice. High-density lipoprotein (HDL) cholesterol was elevated in CARKO mice, and serum total cholesterol increased less in CARKO or PXR knockout mice than WT mice after BDL. Gene expression analysis of the BDL knockout animals demonstrated that, in response to cholestasis, PXR and CAR both repressed and induced the specific hepatic membrane transporters Oatp-c (organic anion transporting polypeptide C) and Oatp2 (Na+-dependent organic anion transporter 2), respectively. Induction of the xenobiotic transporter multidrug resistance protein 1 in cholestasis was independent of either PXR or CAR, in contrast to the known pattern of induction of multidrug resistance protein 1 by xenobiotics. These results demonstrate that CAR and PXR influence cholesterol metabolism and bile acid synthesis, as well as multiple detoxification pathways, and suggest their potential role as therapeutic targets for the treatment of cholestasis and lipid disorders.

Coordinate transcriptional regulation of bile acid homeostasis and drug metabolism

Drugs and bile acids are taken up into hepatocytes by specialized transport proteins localized at the basolateral membrane, e.g., organic anion transporting polypeptides . Following intracellular metabolism by cytochrome P450 (CYP) enzymes, drug metabolites are excreted into bile or urine via ATP-dependent multidrug resistance proteins (MDR1 and MRPs). Bile acids are excreted mainly via the bile salt export pump (BSEP, ABCB11). The genes coding for drug and bile acid transporters and CYP enzymes are regulated by a complex network of transcriptional cascades, notably by the ligand-activated nuclear receptors FXR, PXR, and CAR and by the ligand-independent nuclear receptor HNF-4alpha. The bile acid synthesizing enzymes CYP7A1, CYP8B1, and CYP27A1 are subject to negative feedback regulation by bile acids, which is partly mediated through the transcriptional repressor SHP. The role of transcriptional cofactors, such as SRC-1 and PGC-1, in mediating the gene-specific effects of individual nuclear receptors is becoming increasingly evident.

Pregnane X receptor prevents hepatorenal toxicity from cholesterol metabolites

Efficient detoxification and clearance of cholesterol metabolites such as oxysterols, bile alcohols, and bile acids are critical for survival because they can promote liver and cardiovascular disease. We report here that loss of the nuclear xenobiotic receptor PXR (pregnane X receptor), a regulator of enterohepatic drug metabolism and clearance, results in an unexpected acute lethality associated with signs of severe hepatorenal failure when mice are fed with a diet that elicits accumulation of cholesterol and its metabolites. Induction of a distinct drug clearance program by a high-affinity ligand for the related nuclear receptor, the constitutive androstane receptor, does not overcome the lethality, indicating the unique requirement of PXR for detoxification. We propose that the PXR signaling pathway protects the body from toxic dietary cholesterol metabolites, and, by extension, PXR ligands may ameliorate human diseases such as cholestatic liver diseases and the associating acute renal failure.

The Identification of Ligand Features Essential for PXR Activation by Pharmacophore Modeling

Drug metabolizing enzymes and transporters are often involved in clinically relevant drug-drug interactions. These functional proteins can be induced by a wide range of xenobiotics. The induction is mediated by a group of receptors known as orphan nuclear receptors. The pregnane X receptor (PXR) is a member of this receptor family and regulates the expression of multiple Cytochrome P450 enzyme families (e.g. CYP 3A and 2B), phase II enzymes (e.g. UDP glucuronosyl transferases), and transporters (e.g. multidrug resistance protein 1). The software package Catalyst was employed to derive pharmacophore models for PXR activation. A structure based pharmacophore hypothesis and several ligand based ones were compared in order to identify ligand receptor interactions essential for receptor activation. The results suggest that hydrogen bonding to Gln285 is indispensable for PXR activation. Most ligands were found to form a second hydrogen bond to His407. Hydrophobic interactions are not essential for receptor activation but contribute to ligand affinity. Highly active compounds share up to five hydrophobic features that allow the ligand to occupy large areas of the predominantly hydrophobic binding pocket.

Combined loss of orphan receptors PXR and CAR heightens sensitivity to toxic bile acids in mice

Efficient detoxification of bile acids is necessary to avoid pathological conditions such as cholestatic liver damage and colon cancer. The orphan nuclear receptors PXR and CAR have been proposed to play an important role in the detoxification of xeno- and endo-biotics by regulating the expression of detoxifying enzymes and transporters. In this report, we showed that the combined loss of PXR and CAR resulted in a significantly heightened sensitivity to bile acid toxicity in a sex-sensitive manner. A regimen of lithocholic acid treatment, which was tolerated by wild-type and PXR null mice, caused a marked accumulation of serum bile acids and histological liver damage as well as an increased hepatic lipid deposition in double knockout males. The increased sensitivity in males was associated with genotype-specific suppression of bile acid transporters and loss of bile acid-mediated downregulation of small heterodimer partner, whereas the transporter suppression was modest or absent in females. The double knockout mice also exhibited gene- and tissue-specific dysregulation of PXR and CAR target genes in response to PXR and CAR agonists. In conclusion, althoughthe cross-regulation of target genes by PXR and CAR has b een proposed, the current study represents in vivo evidence of the combined loss of both receptors causing a unique pattern of gene regulation that can be translated into physiological events such as sensitivity to toxic bile acids.

Animal models of xenobiotic receptors in drug metabolism and diseases

Drug-metabolizing enzymes, including phase II conjugating enzymes, play an important role in both drug metabolism and human diseases. The genes that encode these enzymes and transporters are inducible by numerous xenobiotics and endobiotics and the inducibility shows clear species specificity. In the past several years, orphan nuclear receptors, such as PXR and CAR, have been established as species-specific "xenobiotic receptors" that regulate the expression of phase I and phase II enzymes and drug transporters. The creation of xenobiotic receptor transgenic and knockout mice has not only provided an opportunity to dissect the transcriptional control of drug metabolizing enzymes, but also offered a unique opportunity to study the xenobiotic receptor-mediated enzyme regulation in both drug metabolism and diseases. "Humanized" hPXR transgenic mice represent a major step forward in the creation and utilization of humanized rodent models for toxicological assessment that may aid in the development of safer drugs.