Residue threonine 350 confers steroid hormone responsiveness to the mouse nuclear orphan receptor CAR

Flame retardant BDE-47 effectively activates nuclear receptor CAR in human primary hepatocytes

Polybrominated diphenyl ether BDE-47 (2,2',4,4'-tetrabromodiphenyl ether) is a thyroid hormone disruptor in mice; hepatic induction of various metabolic enzymes and transporters has been suggested as the mechanism for this disruption. Utilizing Car (-/-) and Pxr (-/-) mice as well as human primary hepatocytes, here we have demonstrated that BDE-47 activated both mouse and human nuclear receptor constitutive activated/androstane receptor (CAR). In mouse livers, CAR, not PXR, was responsible for Cyp2b10 mRNA induction by BDE-47. In human primary hepatocytes, BDE-47 was able to induce translocation of YFP-tagged human CAR from the cytoplasm to the nucleus andCYP2B6 and CYP3A4 mRNAs expressions. BDE-47 activated human CAR in a manner akin to the human CAR ligand CITCO (6-(4-Chlorophenyl)imidazo[2,1-b][1,3]thiazole-5-carbaldehyde-O-(3,4-dichlorobenzyl)oxime) in luciferase-reporter assays using Huh-7 cells. In contrast, mouse CAR was not potently activated by BDE-47 in the same reporter assays. Furthermore, human pregnane X receptor (PXR) was effectively activated by BDE-47 while mouse PXR was weakly activated in luciferase-reporter assays. Our results indicate that BDE-47 induces CYP genes through activation of human CAR in addition to the previously identified pathway through human PXR.

3D structures and ligand specificities of nuclear xenobiotic receptors CAR, PXR and VDR

The nuclear receptors constitutive androstane receptor (CAR), pregnane X receptor (PXR) and vitamin D receptor (VDR) control a large array of genes that code for important proteins in humans including metabolic enzymes and transporters. 3D structures for the ligand-binding domain (LBD) of these receptors are abundantly available, providing valuable insights into the ligand-binding specificity as well as the activation mechanisms. The ligand-binding site of PXR is large and flexible, whereas those of CAR and VDR are compact and rigid, respectively. In general, the ligand profiles of the receptors are in agreement with the LBD structures. The crystal structures have greatly helped us to understand the promiscuity and/or specificity of CAR, PXR and VDR.

Reduced expression of UGT1A1 in intestines of humanized UGT1 mice via inactivation of NF-κB leads to hyperbilirubinemia

BACKGROUND & AIMS

Bilirubin is a natural and potent antioxidant that accumulates in the blood of newborn children and leads to physiological jaundice. Breastfed infants have higher serum levels of bilirubin than formula-fed infants and are at risk for bilirubin-induced neurological dysfunction (BIND). Clearance of bilirubin requires the expression of uridine diphosphate glucuronosyltransferase (UGT) 1A1; we investigated its role in the association between breast feeding with jaundice in mice.

METHODS

We studied mice in which the original Ugt1 locus was disrupted and replaced with the human UGT1 locus (hUGT1 mice); these mice spontaneously develop neonatal hyperbilirubinemia and BIND. We fed human breast milk or formula to neonatal hUGT1 mice and examined activation of the intestinal xenobiotic receptors pregnane X receptor and constitutive androstane receptor. We also examined inflammatory signaling pathways in mice with disruptions in IκB-kinase-α and IκB kinase-β in the intestinal epithelium.

RESULTS

hUGT1 mice that were fed breast milk developed severe hyperbilirubinemia because of suppression of UGT1A1 in the gastrointestinal tract. Formula-fed hUGT1 mice had lower serum levels of bilirubin, which resulted from induction of UGT1A1 in the gastrointestinal tract. hUGT1/Pxr-null mice did not develop severe hyperbilirubinemia, whereas hUGT1/Car-null mice were susceptible to BIND when they were fed breast milk. Breast milk appeared to suppress intestinal IκB kinase α and β, resulting in inactivation of nuclear factor-κB and loss of expression of UGT1A1, leading to hyperbilirubinemia.

CONCLUSIONS

Breast milk reduces expression of intestinal UGT1A1, which leads to hyperbilirubinemia and BIND; suppression of this gene appears to involve inactivation of nuclear factor-κB. Hyperbilirubinemia can be reduced by activation of pregnane X receptor, constitutive androstane receptor, or nuclear factor-κB.

Active ERK1/2 protein interacts with the phosphorylated nuclear constitutive active/androstane receptor (CAR; NR1I3), repressing dephosphorylation and sequestering CAR in the cytoplasm

The nuclear constitutive active/androstane receptor (CAR) is inactivated and sequestered in the cytoplasm when Thr-38 is phosphorylated. Here, we have demonstrated that activated ERK1/2 interacts with phosphorylated CAR to repress dephosphorylation of Thr-38. The phosphorylation-dependent interaction between CAR and ERK1/2 was examined by co-immunoprecipitation experiments of ectopically expressed FLAG-tagged CAR T38A and CAR T38D mutants with endogenous phospho-ERK1/2 in Huh-7 cells. Phospho-ERK1/2 coprecipitated only the phosphorylation-mimicking CAR T38D mutant; this coprecipitation was mediated by the interaction with the xenochemical response signal peptide near the C terminus of CAR. This interaction increased after EGF treatment and decreased after treatment with the MEK inhibitor U0126 as well as after knockdown of MEK1/2 by shRNA in Huh-7 cells. The phosphorylation levels of Thr-38 of CAR decreased in U0126-treated Huh-7 cells. Thus, activated ERK1/2 interacts with CAR and represses dephosphorylation of Thr-38, providing a cell signal-regulated mechanism for CAR activation.

Di-(2-ethylhexyl)-phthalate (DEHP) activates the constitutive androstane receptor (CAR): a novel signalling pathway sensitive to phthalates

Di-(2-ethylhexyl)-phthalate (DEHP), a widely used plasticizer, is detected in consumer's body fluids. Contamination occurs through environmental and food chain sources. In mouse liver, DEHP activates the peroxisome proliferator-activated receptor alpha (PPARalpha) and regulates the expression of its target genes. Several in vitro investigations support the simultaneous recruitment of additional nuclear receptor pathways. We investigated, in vivo, the hepatic impact of low doses of DEHP on PPARalpha activation, and the putative activation of additional signalling pathways. Wild-type and PPARalpha-deficient mice were exposed to different doses of DEHP. Gene expression profiling delineated the role of PPARalpha and revealed a PPARalpha-independent regulation of several prototypic constitutive androstane receptor (CAR) target genes. Thus, we developed an original hepatic cell line expressing CAR to investigate its activation by DEHP. By means of a pharmacological inhibitor or CAR-targeting shRNAs, we established that CAR is required for the effect of DEHP on Cyp2b10, a recognized CAR target gene. Moreover, DEHP dose-dependently induced CYP2B6 in human primary hepatocyte cultures. This finding demonstrates that CAR also represents a transcriptional regulator sensitive to phthalates. CAR-mediated effects of DEHP provide a new rationale for most endpoints of phthalates toxicity described previously, including endocrine disruption, hepatocarcinogenesis and the metabolic syndrome.

The peripheral benzodiazepine receptor ligand 1-(2-chlorophenyl-methylpropyl)-3-isoquinoline-carboxamide is a novel antagonist of human constitutive androstane receptor

As a promiscuous xenobiotic sensor, the constitutive androstane receptor (CAR; NR1I3) regulates the expression of multiple drug-metabolizing enzymes and transporters in liver. The constitutively activated nature of CAR in the cell-based transfection assays has hindered its use as a predictor of metabolism-based drug-drug interactions. Here, we have identified 1-(2-chlorophenylmethylpropyl)-3-isoquinoline-carboxamide (PK11195), a typical peripheral benzodiazepine receptor (PBR) ligand, as a selective and potent inhibitor of human (h) CAR. In cell-based transfection assays, PK11195 inhibited the constitutive activity of hCAR more than 80% at the concentration of 10 microM, and the PK11195-inhibited activity was efficiently reactivated by the direct CAR activator, 6-(4-chlorophenyl)imidazo[2,1-b][1,3]thiazole-5-carbaldehyde-O-(3,4-dichlorobenzyl) oxime, but not by the indirect hCAR activator, phenobarbital. Mammalian two-hybrid and GST pull-down assays showed that PK11195 repressed the interactions of hCAR with the coactivators steroid receptor coactivator-1 and glucocorticoid receptor-interacting protein 1 to inhibit hCAR activity. The inhibition by PK11195 specifically occurred to the hCAR: PK1195 strongly activated human pregnane X receptor (PXR), whereas it did not alter the activity of the mouse CAR and mouse PXR. In addition, PBR played no role in the PK11195 inhibition of hCAR because the inhibition fully occurred in the HeLa cells in which the PBR was knocked down by small interfering RNA. In the Car(-/-) mouse liver, PK11195 translocated enhanced yellow fluorescent protein-hCAR into the nucleus. These results are consistent with the conclusion that PK11195 is a novel hCAR-specific antagonist that represses the CAR-coactivator interactions to inhibit the receptor activity inside the nucleus. Thus, PK11195 can be used as a chemical tool for studying the molecular basis of CAR function.

The antiapoptotic factor growth arrest and DNA-damage-inducible 45 beta regulates the nuclear receptor constitutive active/androstane receptor-mediated transcription

The nuclear receptor constitutive active/androstane receptor (CAR) up-regulated expression of the apoptotic growth arrest and DNA-damage-inducible 45 beta (GADD45B) gene in HepG2 cells. Overexpression of GADD45B augmented CAR-mediated induction of the human CYP2B gene by the CAR activator 1,4-bis[2-(3,5-dichloropyridyloxy)]benzene (TCPOBOP) and coactivated CAR-dependent transcription of the NR1-luciferase reporter gene. Small interfering RNA knockdown of GADD45B resulted in repression of both the induction and the coactivation. Induction of the mouse Cyp2b10 gene by TCPOBOP was profoundly attenuated in the primary hepatocytes prepared from GADD45B-knockout mice compared with those from wild-type mice. Because CAR is a key transcription factor that activates the genes that encode for xenobiotic metabolizing enzymes and transporters, GADD45B, acting as a CAR coactivator and coregulating CAR target genes, may be involved in hepatic drug metabolism and excretion of xenobiotics.

CAR and PXR: the xenobiotic-sensing receptors

The xenobiotic receptors CAR and PXR constitute two important members of the NR1I nuclear receptor family. They function as sensors of toxic byproducts derived from endogenous metabolism 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. In contrast, the steroid receptors, exemplified by the estrogen receptor (ER) and glucocorticoid receptor (GR), are the sensors that tightly monitor and respond to changes in circulating steroid hormone levels to maintain body homeostasis. This divergence of the chemical- and steroid-sensing functions has evolved to ensure the fidelity of the steroid hormone endocrine regulation while allowing development of metabolic elimination pathways for xenobiotics. The development of the xenobiotic receptors CAR and PXR also reflect the increasing complexity of metabolism in higher organisms, which necessitate novel mechanisms for handling and eliminating metabolic by-products and foreign compounds from the body. The purpose of this review is to discuss similarities and differences between the xenobiotic receptors CAR and PXR with the prototypical steroid hormone receptors ER and GR. Interesting differences in structure explain in part the divergence in function and activation mechanisms of CAR/PXR from ER/GR. In addition, the physiological roles of CAR and PXR will be reviewed, with discussion of interactions of CAR and PXR with endocrine signaling pathways.

Orphan Nuclear Receptor Constitutive Active/Androstane Receptor–Mediated Alterations in DNA Methylation during Phenobarbital Promotion of Liver Tumorigenesis

Altered DNA methylation is an epigenetic mechanism that plays a key role in the carcinogenesis process, and the nongenotoxic rodent hepatocarcinogen phenobarbital (PB) alters the methylation status of DNA in mouse liver. The constitutive active/androstane nuclear receptor (CAR) mediates half of the PB-induced hepatic gene expression changes and it is essential for liver tumor promotion in PB-treated mice. Here, a technique involving methylation-sensitive restriction digestion, arbitrarily primed PCR, and capillary electrophoresis was utilized to detect PB-induced regions of altered DNA methylation (RAMs) in CAR wildtype (WT) mice that are sensitive to promotion by PB and resistant CAR knockout (KO) mice. The CAR WT mice developed preneoplastic lesions after 23 weeks of PB treatment (precancerous) and liver tumors after 32 weeks, while the CAR KO mice did not develop tumors (Y. Yamamoto, et al., 2004, Cancer Res. 64, 7197-7200). Our goal was to discern those RAMs which are playing important roles in tumor formation by comparing the RAMs that form in sensitive and resistant groups of mice. Using this novel approach, 42 unique RAMs were identified in the precancerous as compared to the CAR KO, 23-week PB-treated tissue. Of these 42 RAMs, 14 carried forward to the tumor tissue, and additionally, 104 total unique RAMs were observed in the tumor tissue. These results indicate that there are unique RAMs occurring in the sensitive CAR WT mice and that a portion of these are seen in both the precancerous and tumor tissue. We hypothesize that these unique RAMs may be facilitating the tumorigenesis process, and these data support the view that DNA methylation plays a causative role in PB-induced tumorigenesis.

Identification of longevity-associated genes in long-lived Snell and Ames dwarf mice

Recent landmark molecular genetic studies have identified an evolutionarily conserved insulin/IGF-1 signal transduction pathway that regulates lifespan. In C. elegans, Drosophila, and rodents, attenuated insulin/IGF-1 signaling appears to regulate lifespan and enhance resistance to environmental stress. The Ames (Prop1 (df/df)) and Snell (Pit1 (dw/dw)) hypopituitary dwarf mice with growth hormone (GH), thyroid-stimulating hormone (TSH), and prolactin deficiencies live 40-60% longer than control mice. Both mutants are resistant to multiple forms of environmental stress in vitro. Taken collectively, these genetic models indicate that diminished insulin/IGF-l signaling may play a central role in the determination of mammalian lifespan by conferring resistance to exogenous and endogenous stressors. These pleiotropic endocrine pathways control diverse programs of gene expression that appear to orchestrate the development of a biological phenotype that promotes longevity. With the ability to investigate thousands of genes simultaneously, several microarray surveys have identified potential longevity assurance genes and provided information on the mechanism(s) by which the dwarf genotypes (dw/dw) and (df/df), and caloric restriction may lead to longevity. We propose that a comparison of specific changes in gene expression shared between Snell and Ames dwarf mice may provide a deeper understanding of the transcriptional mechanisms of longevity determination. Furthermore, we propose that a comparison of the physiological consequences of the Pit1dw and Prop1df mutations may reveal transcriptional profiles similar to those reported for the C. elegans and Drosophila mutants. In this study we have identified classes of genes whose expression is similarly affected in both Snell and Ames dwarf mice. Our comparative microarray data suggest that specific detoxification enzymes of the P(450) (CYP) family as well as oxidative and steroid metabolism may play a key role in longevity assurance of the Snell and Ames dwarf mouse mutants. We propose that the altered expression of these genes defines a biochemical phenotype which may promote longevity in Snell and Ames dwarf mice.

Orphan nuclear receptors adopted by crystallography

Of the large nuclear hormone receptor superfamily of proteins, orphan nuclear receptors have remained a mystery owing to their lack of identified ligands and their constitutive nature. Now, structures of several ligand-binding domains of orphan receptors have provided some surprising insights that were not anticipated from molecular studies. Therefore, most orphan nuclear receptors have now been 'adopted' and their regulation has been shown to range from true ligand-independence to highly promiscuous ligand-dependence. Former orphan receptors have been found to contain ligand-binding pockets that range in volume from vast (>1600A3) to non-existent and have been shown to generate surface AF2 motifs that range from being multifunctionally active to distinctly inactive. Insights from these new structures illustrate how powerful a structural biology approach can be when integrated with molecular and cellular physiology.

Thr176 regulates the activity of the mouse nuclear receptor CAR and is conserved in the NR1I subfamily members PXR and VDR

The mouse nuclear receptor CAR (constitutively active receptor) is a transcription factor that is activated by phenobarbital-type inducers such as TCPOBOP {1,4 bis[2-(3,5-dichloropyridyloxy)]benzene} in liver in vivo. However, CAR is constitutively active in cell-based transfection assays, the molecular mechanism for which has not been elucidated yet. In the model structure of CAR, Thr176 constitutes a part of the ligand-binding surface, but its side chain is not directed toward the surface, instead it forms a hydrogen bond with Thr350 in the AF2 (activation function 2) domain of CAR. Thr350 is known to regulate CAR activity [Ueda, Kakizaki, Negishi, and Sueyoshi (2002) Mol. Pharmacol. 61, 1284-1288]. Thr176 was mutated to various amino acids to examine whether this interaction played a role in conferring the constitutive activity. Hydrophobic and positively charged amino acids at position 176 abrogated the constitutive activity, whereas polar and negatively charged amino acids retained it. When one of the small hydrophobic amino acids, such as alanine or valine, was substituted for threonine, the mutants were fully activated by TCPOBOP. The co-activator SRC-1 (steroid receptor co-activator-1) regulated the activity changes associated with the mutations. Thr248 and Ser230 are the Thr176-corresponding residues in human pregnane X receptor and mouse vitamin D3 receptor respectively, interacting directly with the conserved threonine in the AF2 domains. Thr248 and Ser230 also regulated the ligand-dependent activity of these receptors by augmenting binding of the receptors to SRC-1. Thr176, Thr248 and Ser230 are conserved residues in the NR1I (nuclear receptor 1I) subfamily members and determine their activity.

Metabolism and Disposition Kinetics of Nicotine

Nicotine is of importance as the addictive chemical in tobacco, pharmacotherapy for smoking cessation, a potential medication for several diseases, and a useful probe drug for phenotyping cytochrome P450 2A6 (CYP2A6). We review current knowledge about the metabolism and disposition kinetics of nicotine, some other naturally occurring tobacco alkaloids, and nicotine analogs that are under development as potential therapeutic agents. The focus is on studies in humans, but animal data are mentioned when relevant to the interpretation of human data. The pathways of nicotine metabolism are described in detail. Absorption, distribution, metabolism, and excretion of nicotine and related compounds are reviewed. Enzymes involved in nicotine metabolism including cytochrome P450 enzymes, aldehyde oxidase, flavin-containing monooxygenase 3, amine N-methyltransferase, and UDP-glucuronosyltransferases are represented, as well as factors affecting metabolism, such as genetic variations in metabolic enzymes, effects of diet, age, gender, pregnancy, liver and kidney diseases, and racial and ethnic differences. Also effects of smoking and various inhibitors and inducers, including oral contraceptives, on nicotine metabolism are discussed. Due to the significance of the CYP2A6 enzyme in nicotine clearance, special emphasis is given to the effects and population distributions of CYP2A6 alleles and the regulation of CYP2A6 enzyme.

The nuclear xenobiotic receptor CAR: structural determinants of constitutive activation and heterodimerization

Constitutive androstane receptor (CAR) induces xenobiotic, bilirubin, and thyroid hormone metabolism as a heterodimer with the retinoid X receptor (RXR). Unlike ligand-dependent nuclear receptors, CAR is constitutively active. Here, we report the heterodimeric structure of the CAR and RXR ligand binding domains (LBDs), which reveals an unusually large dimerization interface and a small CAR ligand binding pocket. Constitutive CAR activity appears to be mediated by the compact nature of the CAR LBD that displays several unique features including a shortened AF2 helix and helix H10, which are linked by a two-turn helix that normally adopts an extended loop in other receptors, and an extended helix H2 that stabilizes the canonical LBD fold by packing tightly against helix H3. These structural observations provide a molecular framework for understanding the atypical transcriptional activation properties of CAR.

CAR, Driving into the Future

The nuclear orphan receptor CAR is active in the absence of ligand with the unique capability to be further regulated by activators. A number of these activators, including phenobarbital, do not directly bind to the receptor. Considered a xenobiotic sensing receptor, CAR transcriptionally modifies the expression of genes involved in the metabolism and elimination of xenobiotics and steroids in response to these compounds and other cellular metabolites. Its hepatic expression pattern endows the liver with the ability to protect against not only exogenous but also endogenous insults. The mechanism of CAR activation is complex, involving translocation from the cytoplasm to the nucleus in the presence of activators, followed by further activation steps in the nucleus. Although this mechanism remains under investigation, we have summarized here the cellular signaling pathways elucidated so far and speculate on the mechanism by which CAR activators regulate gene expression through this network.

Human constitutive androstane receptor mediates induction of CYP2B6 gene expression by phenytoin

Compared with its rodent orthologs, little is known about the chemical specificity of human constitutive androstane receptor (hCAR) and its regulation of hepatic enzyme expression. Phenytoin (PHY), a widely used antiepileptic drug, is a potent inducer of CYP2B6 in primary human hepatocytes, but does not activate human pregnane X receptor (PXR) significantly in cell-based transfection assays at the same concentrations associated with potent induction of CYP2B6. Based on this observation, we hypothesized that PHY may be a selective activator of hCAR. In primary human hepatocytes, expression of CYP2B6 reporter genes containing phenobarbital-responsive enhancer module (PBREM) or PBREM/xenobiotic-responsive enhancer module (XREM) response elements were activated up to 14- and 28-fold, respectively, by 50 microm PHY. By contrast, parallel experiments in HepG2 cell lines co-transfected with an hPXR expression vector did not show increased reporter activity. These results indicated that a PXR-independent pathway, which is retained in primary hepatocytes, is responsible for PHY induction of CYP2B6. Further experiments revealed that PHY effectively translocates hCAR from the cytoplasm into the nucleus in both primary human hepatocytes and CAR(-/-) mice. Compared with vehicle controls, PHY administration significantly increased CYP2B6 reporter gene expression, when this reporter construct was delivered together with hCAR expression vector into CAR(-/-) mice. However, PHY did not increase reporter gene expression in CAR(-/-) mice in the absence of hCAR vector, implying that CAR is essential for mediating PHY induction of CYP2B6 gene expression. Taken together, these observations demonstrate that, in contrast to most of the known CYP2B6 inducers, PHY is a selective activator of CAR in humans.

Induction of Drug Metabolism: The Role of Nuclear Receptors

Induction of drug metabolism was described more than 40 years ago. Progress in understanding the molecular mechanism of induction of drug-metabolizing enzymes was made recently when the important roles of the pregnane X receptor (PXR) and the constitutive androstane receptor (CAR), two members of the nuclear receptor superfamily of transcription factors, were discovered to act as sensors for lipophilic xenobiotics, including drugs. CAR and PXR bind as heterodimeric complexes with the retinoid X receptor to response elements in the regulatory regions of the induced genes. PXR is directly activated by xenobiotic ligands, whereas CAR is involved in a more complex and less well understood mechanism of signal transduction triggered by drugs. Most recently, analysis of these xenobiotic-sensing nuclear receptors and their nonmammalian precursors such as the chicken xenobiotic receptor suggests an important role of PXR and CAR also in endogenous pathways, such as cholesterol and bile acid biosynthesis and metabolism. In this review, recent findings regarding xenosensors and their target genes are summarized and are put into an evolutionary perspective in regard to how a living organism has derived a system that is able to deal with potentially toxic compounds it has not encountered before.

The role of the nuclear receptor CAR as a coordinate regulator of hepatic gene expression in defense against chemical toxicity

The nuclear receptor CAR (constitutive active receptor) mediates the induction of transcription of cytochrome P450 (CYP) genes by phenobarbital (PB) and PB-type inducers. A recent study using CAR-null mice has shown that CAR regulates not only the CYP genes but also other genes encoding various drug/steroid-metabolizing enzymes. In addition to coordinating these enzymes, CAR plays other roles in hepatic gene expression: CAR represses various genes including carnitine palmitoyltransferase 1a and phosphoenolpyruvate carboxykinase 1 in response to PB, and the receptor regulates the constitutive expression of genes such as squalene epoxidase. On the other hand, induction of certain genes such as amino levulinate synthase 1 by PB is not regulated by CAR. Here we describe diverse roles of CAR in hepatic gene expression with a particular focus on endogenous substances such as cholesterol, bilirubin, and steroid hormones.