The critical role of carboxy-terminal amino acids in ligand-dependent and -independent transactivation of the constitutive androstane receptor

Induction of Constitutive Androstane Receptor during the Development of Oxidative Stress

We studied the effect of 3-, 24-, and 72-h exposure to H₂O₂ in concentrations of 0.1-100.0 μM on the level of constitutive androstane receptor in Caco-2 cells. It was shown that 3- and 24-h incubation with Н₂О₂ in all concentrations had no effect on the level of constitutive androstane receptors. Increasing the incubation time to 72 h led to an increase in the level of constitutive androstane receptor at H₂O₂ concentrations of 5, 10, and 50 μM and to a decrease at a concentration of 100 μM. Antioxidant glutathione (1 mM) in parallel to the prooxidant neutralized these changes.

Regulatory Mechanics of Constitutive Androstane Receptors: Basal and Ligand-Directed Actions

Constitutive androstane receptor (CAR) is a nuclear hormone receptor that primarily functions in sensing and metabolizing xenobiotics. The basal activity of this receptor is relatively high, and CAR is deemed active in the absence of ligand. The (over)activation can promote drug toxicity and tumor growth. Thus, therapeutic treatments seek inverse agonists to inhibit or modulate CAR activities. To advance our understanding of the regulatory mechanisms of CAR, we used computational and experimental approaches to elucidate three aspects of CAR activation and inactivation: (1) ligand-dependent actions, (2) ligand-orthologue specificity, and (3) constitutive activity. For ligand-dependent actions, we examined the ligand-bound simulations and identified two sets of ligand-induced contacts promoting CAR activation via coactivator binding (H11-H12 contact) or inactivation via corepressor binding (H4-H11 contact). For orthologue specificity, we addressed a puzzling fact that murine CAR (mCAR) and human CAR (hCAR) respond differently to the same ligand (CITCO), despite their high sequence homology. We found that the helix H7 of hCAR is responsible for a stronger binding of the ligand CITCO compared to mCAR, hence a stronger CITCO-induced activation. For basal activity, we reported computer-generated unliganded CAR structures and critical mutagenesis (mCAR's V209A and N333D) results of a cell-based transcription assay. Our results reveal that the basal conformation of CAR shares prominent features with the agonist-bound form, and helix HX has an important contribution to the constitutive activity. These findings altogether can be useful for the understanding of constitutively active receptors and the design of drug molecules targeting them.

High-content analysis of constitutive androstane receptor (CAR) translocation identifies mosapride citrate as a CAR agonist that represses gluconeogenesis

The constitutive androstane receptor (CAR) plays an important role in hepatic drug metabolism and detoxification but has recently been projected as a potential drug target for metabolic disorders due to its repression of lipogenesis and gluconeogenesis. Thus, identification of physiologically-relevant CAR modulators has garnered significant interest. Here, we adapted the previously characterized human CAR (hCAR) nuclear translocation assay in human primary hepatocytes (HPH) to a high-content format and screened an FDA-approved drug library containing 978 compounds. Comparison of hCAR nuclear translocation results with the Tox21 hCAR luciferase reporter assay database in 643 shared compounds revealed significant overlap between these two assays, with approximately half of hCAR agonists also mediating nuclear translocation. Further validation of these compounds in HPH and/or using published data from literature demonstrated that hCAR translocation exhibits a higher correlation with the induction of hCAR target genes, such as CYP2B6, than the luciferase assay. In addition, some CAR antagonists which repress CYP2B6 mRNA expression in HPH, such as sorafenib, rimonabant, and CINPA1, were found to translocate hCAR to the nucleus of HPH. Notably, both the translocation assay and the luciferase assay identified mosapride citrate (MOS), a gastroprokinetic agent that is known to reduce fasting blood glucose levels in humans, as a novel hCAR activator. Further studies with MOS in HPH uncovered that MOS can repress the expression of gluconeogenic genes and decrease glucose output from hepatocytes, providing a previously unidentified liver-specific mechanism by which MOS modulates blood glucose levels.

Molecular Basis of Metabolism-Mediated Conversion of PK11195 from an Antagonist to an Agonist of the Constitutive Androstane Receptor

The constitutive androstane receptor (CAR) plays an important role in xenobiotic metabolism, energy homeostasis, and cell proliferation. Antagonism of the CAR represents a key strategy for studying its function and may have potential clinical applications. However, specific human CAR (hCAR) antagonists are limited and conflicting data on the activity of these compounds have been reported. 1-(2-chlorophenyl)-N-methyl-N-(1-methylpropyl)-3-isoquinolinecarboxamide (PK11195), a typical peripheral benzodiazepine receptor ligand, has been established as a potent hCAR deactivator in immortalized cells; whether it inhibits hCAR activity under physiologically relevant conditions remains unclear. Here, we investigated the effects of PK11195 on hCAR in metabolically competent human primary hepatocytes (HPH) and HepaRG cells. We show that although PK11195 antagonizes hCAR in HepG2 cells, it induces the expression of CYP2B6 and CYP3A4, targets of hCAR and the pregnane X receptor (PXR), in HPH, HepaRG, and PXR-knockout HepaRG cells. Utilizing a HPH-HepG2 coculture model, we demonstrate that inclusion of HPH converts PK11195 from an antagonist to an agonist of hCAR, and such conversion was attenuated by potent CYP3A4 inhibitor ketoconazole. Metabolically, we show that the N-desmethyl metabolite is responsible for PK11195-mediated hCAR activation by facilitating hCAR interaction with coactivators and enhancing hCAR nuclear translocation in HPHs. Structure-activity analysis revealed that N-demethylation alters the interaction of PK11195 with the binding pocket of hCAR to favor activation. Together, these results indicate that removal of a methyl group switches PK11195 from a potent antagonist of hCAR to an agonist in HPH and highlights the importance of physiologically relevant metabolism when attempting to define the biologic action of small molecules.

Development of pharmacophore-based classification model for activators of constitutive androstane receptor

Constitutive androstane receptor (CAR) is predominantly expressed in the liver and is important for regulating drug metabolism and transport. Despite its biological importance, there have been few attempts to develop in silico models to predict the activity of CAR modulated by chemical compounds. The number of in silico studies of CAR may be limited because of CAR's constitutive activity under normal conditions, which makes it difficult to elucidate the key structural features of the interaction between CAR and its ligands. In this study, to address these limitations, we introduced 3D pharmacophore-based descriptors with an integrated ligand and structure-based pharmacophore features, which represent the receptor-ligand interaction. Machine learning methods (support vector machine and artificial neural network) were applied to develop an in silico model with the descriptors containing significant information regarding the ligand binding positions. The best classification model built with a solvent accessibility volume-based filter and the support vector machine showed good predictabilities of 87%, and 85.4% for the training set and validation set, respectively. This demonstrates that our model can be used to accurately predict CAR activators and offers structural information regarding ligand/protein interactions.

Mechanisms of xenobiotic receptor activation: Direct vs. indirect

The so-called xenobiotic receptors (XRs) have functionally evolved into cellular sensors for both endogenous and exogenous stimuli by regulating the transcription of genes encoding drug-metabolizing enzymes and transporters, as well as those involving energy homeostasis, cell proliferation, and/or immune responses. Unlike prototypical steroid hormone receptors, XRs are activated through both direct ligand-binding and ligand-independent (indirect) mechanisms by a plethora of structurally unrelated chemicals. This review covers research literature that discusses direct vs. indirect activation of XRs. A particular focus is centered on the signaling control of the constitutive androstane receptor (CAR), the pregnane X receptor (PXR), and the aryl hydrocarbon receptor (AhR). We expect that this review will shed light on both the common and distinct mechanisms associated with activation of these three XRs. This article is part of a Special Issue entitled: Xenobiotic nuclear receptors: New Tricks for An Old Dog, edited by Dr. Wen Xie.

Constitutive activity and ligand-dependent activation of the nuclear receptor CAR-insights from molecular dynamics simulations

The constitutive androstane receptor (CAR) possesses, unlike most other nuclear receptors, a pronounced basal activity in vitro whose structural basis is still not fully understood. Using comparative molecular dynamics simulations of CAR X-ray crystal structures, we evaluated the molecular basis for constitutive activity and ligand-dependent receptor activation. Our results suggest that a combination of van der Waals interactions and hydrogen bonds is required to maintain the activation helix in the active conformation also in absence of a ligand. Furthermore, we identified conformational rearrangements within the ligand-binding pocket upon agonist binding and an influence of CAR inducers pregnanedione and CITCO on the helical conformation of the activation helix. Based on the results a model for ligand-dependent CAR activation is suggested.

The vitamin D sterol-vitamin D receptor ensemble model offers unique insights into both genomic and rapid-response signaling

Steroid hormones serve as chemical messengers in a wide number of species and target tissues by transmitting signals that result in both genomic and nongenomic responses. Genomic responses are mediated by the formation of a ligand-receptor complex with its cognate steroid hormone nuclear receptor (NR). Nongenomic responses can be mediated at the plasma membrane by a membrane-localized NR. The focus of this Review is on the structural attributes and molecular mechanisms underlying vitamin D sterol (VDS)-vitamin D receptor (VDR) selective and stereospecific regulation of nongenomic and genomic signaling. The VDS-VDR conformational ensemble model describes how VDSs can selectively initiate or block either nongenomic or genomic biological responses by interacting with two VDR ligand-binding pockets, one kinetically favored by 1alpha,25(OH)(2)D(3) (1,25D) and the other thermodynamically favored. We describe the variables that affect the three major elements of the model: the conformational flexibility of the unliganded (apo) protein, the flexibility of the VDS, and the physicochemical selectivity of the VDR genomic pocket (VDR-GP) and alternative pocket (VDR-AP). We also discuss how these three factors collectively provide a rational explanation for the complexities of VDS regulation of cell biology and highlight the current limitations of the model.

Ligand specificity of constitutive androstane receptor as probed by induced-fit docking and mutagenesis

Constitutive androstane receptor (CAR, NR1I3) belongs to the nuclear receptor family of transcription factors and acts as a chemical sensor of drugs and endogenous compounds. The ligand-binding preferences of CAR are diverse, and more importantly, there are significant species differences in ligand specificity. Here, we show that while certain residues are critical for the basal activity of mouse CAR (mCAR) and/or affect the binding of all tested ligands, mutation of some ligand-binding pocket (LBP) residues (e.g., F171 and Y336) paradoxically decreased the activity of a specific ligand while increasing that of others. Comparisons to previously reported human CAR (hCAR) residues indicated that the function of key CAR residues (e.g., N175, L253) is dramatically different between species. The docking results provide some mechanistic rationale for the ability of 17alpha-ethinyl-3,17beta-estradiol (EE2) to both activate mCAR and repress hCAR.

Pharmacogenetics of the constitutive androstane receptor

The constitutive androstane receptor (CAR), a member of the NR1I group of nuclear hormone receptors, has been implicated in regulating the expression of genes that are critical in xenobiotic and endobiotic metabolism, uptake and elimination as well as genes involved in various other physiological processes. Hence, functional variation in CAR associated with its expression and/or activity can influence the transcriptional activation of its target genes and could contribute to the observed variation in drug response and toxicity. Moreover, coadministration of agents that are also CAR activators contributes to clinically relevant drug-drug interactions in patients receiving certain combination therapies. This review will discuss the functional significance of known genetic variants in CAR and the most common alternatively spliced isoforms of CAR. We will also discuss the influence of gender and ethnicity on CAR and its target genes. Although genetic polymorphisms in CAR may have an indirect effect on drug disposition, understanding the association of genetic polymorphisms in CAR with the expression of its target genes might help us to better understand the molecular mechanisms underlying the interindividual variation in drug disposition in addition to drug-drug interactions.

Ab Initio Fragment Molecular Orbital Study of Molecular Interactions between Liganded Retinoid X Receptor and Its Coactivator: Roles of Helix 12 in the Coactivator Binding Mechanism

On the basis of the fragment molecular orbital method we addressed molecular interactions of liganded retinoid X receptor (RXR) with steroid receptor co-activating factor-1 (SRC1) coactivator to examine the contribution of helix 12 (H12), which contains the core of the transcriptional activation function 2 activating domain, to the coactivator binding of RXR. The interaction between H12 and SRC1 was proved to be the main cause for the stabilization of the coactivator binding. In particular, highly conserved charged (Glu453) and hydrophobic (Phe450) residues in H12 were found to have stronger electrostatic and dispersion interactions with SRC1 than the other charged and hydrophobic residues in H12, respectively. In addition, the charge transfer (CT) from RXR to SRC1 was found to occur mainly by the changes in charges of H12 residues. Large positive and negative charge changes were observed especially for Glu453 and for Lys631 and Ile632 in SRC1, respectively, indicating that Glu453 is an electron donor for Lys631 and Ile632 in this CT. Taken together, our findings quantitatively demonstrated that H12 and its highly conserved residues significantly contribute to the coactivator binding not only by the Coulomb and dispersion interactions but also by the CT described with the quantum-mechanical framework.

New insights into Vitamin D sterol-VDR proteolysis, allostery, structure-function from the perspective of a conformational ensemble model

Recently, we have developed a Vitamin D sterol (VDS)-VDR conformational ensemble model. This model can be broken down into three individual, yet interlinked parts: (a) the conformationally flexible VDS, (b) the apo/holo-VDR helix-12 (H12) conformational ensemble, and (c) the presence of two VDR ligand binding pockets (LBPs); one thermodynamically favored (the genomic pocket, G-pocket) and the other kinetically favored by VDSs (the alternative pocket, A-pocket). One focus of this study is to use directed VDR mutagenesis to (1) demonstrate H12 is stabilized in the transcriptionally active closed conformation (hVDR-c1) by three salt-bridges that span the length of H12 (cationic residues R154, K264 and R402), (2) to elucidate the VDR trypsin sites [R173 (hVDR-c1), K413 (hVDR-c2) and R402 (hVDR-c3)] and (3) demonstrate the apo-VDR H12 equilibrium can be shifted. The other focus of this study is to apply the model to generate a mechanistic understanding to discrepancies observed in structure-function data obtained with a variety of 1alpha,25(OH)(2)-Vitamin D(3) (1,25D) A-ring and side-chain analogs, and side-chain metabolites. We will demonstrate that these structure-function conundrums can be rationalized, for the most part by focusing on alterations in the VDS conformational flexibility and the elementary interaction between the VDS and the VDR A- and G-pockets, relative to the control, 1,25D.

Combined simulation and mutagenesis analyses reveal the involvement of key residues for peroxisome proliferator-activated receptor alpha helix 12 dynamic behavior

The dynamic properties of helix 12 in the ligand binding domain of nuclear receptors are a major determinant of AF-2 domain activity. We investigated the molecular and structural basis of helix 12 mobility, as well as the involvement of individual residues with regard to peroxisome proliferator-activated receptor alpha (PPARalpha) constitutive and ligand-dependent transcriptional activity. Functional assays of the activity of PPARalpha helix 12 mutants were combined with free energy molecular dynamics simulations. The agreement between the results from these approaches allows us to make robust claims concerning the mechanisms that govern helix 12 functions. Our data support a model in which PPARalpha helix 12 transiently adopts a relatively stable active conformation even in the absence of a ligand. This conformation provides the interface for the recruitment of a coactivator and results in constitutive activity. The receptor agonists stabilize this conformation and increase PPARalpha transcription activation potential. Finally, we disclose important functions of residues in PPARalpha AF-2, which determine the positioning of helix 12 in the active conformation in the absence of a ligand. Substitution of these residues suppresses PPARalpha constitutive activity, without changing PPARalpha ligand-dependent activation potential.

PXR and CAR: nuclear receptors which play a pivotal role in drug disposition and chemical toxicity

Xenobiotic metabolism and detoxification is regulated by receptors (e.g., PXR, CAR) whose characterization has contributed significantly to our understanding of drug responses in humans. Technologies facilitating the screening of compounds for receptor interactions provide valuable tools applicable in drug development. Most use in vitro systems or mice humanized for receptors in vivo. In vitro assays are limited by the reporter systems and cell lines chosen and are uninformative about effects in vivo. Humanized mouse models provide novel, exciting ways of understanding the functions of these genes. This article evaluates these technologies and current knowledge on PXR/CAR-mediated regulation of gene expression.

Comparison of homology models and X-ray structures of the nuclear receptor CAR: assessing the structural basis of constitutive activity

The constitutive androstane receptor (CAR) possesses an intrinsic basal activity whose structural basis has been analysed during the last decade. Recently, we published a homology model of the CAR ligand binding domain (LBD) based on the X-ray structures of the closely related pregnane X (PXR) and vitamin D (VDR) receptor. A detailed analysis of the homology model and molecular dynamics (MD) simulations afforded us to propose a potential mechanism underlying the constitutive activity of CAR. Almost simultaneously, X-ray structures of human and mouse CAR LBD were released. In the present study, a detailed analysis and comparison of homology model and X-ray structures is carried out in order to evaluate the quality and reliability of our homology modelling procedure. The hypothesis of the constitutive activity which we proposed on the basis of our modelling results was tested for consistency with the crystal structures. In addition, the features stated to be essential for the basal activity based on the X-ray data were investigated by means of molecular dynamics simulations. Our results show that the homology modelling procedure was able to predict the CAR LBD structure with high accuracy. Structural features that have been revealed as critical for constitutive activity in the model are also observed in the X-ray structures. Furthermore, the MD simulations of the CAR X-ray structures and a detailed analysis of other NRs clarify the role of distinct structural features that have been assigned an important role for the constitutive activity.

Structural Determinants of the Agonist-independent Association of Human Peroxisome Proliferator-activated Receptors with Coactivators

Lipid homeostasis is controlled by various nuclear receptors (NRs), including the peroxisome proliferator-activated receptors (PPARalpha, delta, and gamma), which sense lipid levels and regulate their metabolism. Here we demonstrate that human PPARs have a high basal activity and show ligand-independent coactivator (CoA) association comparable with the NR constitutive androstane receptor. Using PPARgamma as an example, we found that four different amino acid groups contribute to the ligand-independent stabilization of helix 12 of the PPAR ligand-binding domain. These are: (i) Lys329 and Glu499, mediating a charge clamp-type stabilization of helix 12 via a CoA bridge; (ii) Glu352, Arg425, and Tyr505, directly stabilizing the helix via salt bridges and hydrogen bonds; (iii) Lys347 and Asp503, interacting with each other as well as contacting the CoA; and (iv) His351, Tyr(355), His477, and Tyr501, forming a hydrogen bond network. These amino acids are highly conserved within the PPAR subfamily, suggesting that the same mechanism may apply for all three PPARs. Phylogenetic trees of helix 12 amino acid and nucleotide sequences of all crystallized NRs and all human NRs, respectively, indicated a close relationship of PPARs with constitutive androstane receptor and other constitutive active members of the NR superfamily. Taking together, the ligand-independent tight control of the position of the PPAR helix 12 provides an effective alternative for establishing an interaction with CoA proteins. This leads to high basal activity of PPARs and provides an additional view on PPAR signaling.

Antagonist- and inverse agonist-driven interactions of the vitamin D receptor and the constitutive androstane receptor with corepressor protein

Ligand-dependent signal transduction by nuclear receptors (NRs) includes dynamic exchanges of coactivator (CoA) and corepressor (CoR) proteins. Here we focused on the structural determinants of the antagonist- and inverse agonist-enhanced interaction of the endocrine NR vitamin D receptor (VDR) and the adopted orphan NR constitutive androstane receptor (CAR) from two species with the CoR NR corepressor. We found that the pure VDR antagonist ZK168281 and the human CAR inverse agonist clotrimazole are both effective inhibitors of the CoA interaction of their respective receptors, whereas ZK168281 resembled more the mouse CAR inverse agonist androstanol in its ability to recruit CoR proteins. Molecular dynamics simulations resulted in comparable models for the CoR receptor interaction domain peptide bound to VDR/antagonist or CAR/inverse agonist complexes. A salt bridge between the CoR and a conserved lysine in helix 4 of the NR is central to this interaction, but also helix 12 was stabilized by direct contacts with residues of the CoR. Fixation of helix 12 in the antagonistic/inverse agonistic conformation prevents an energetically unfavorable free floatation of the C terminus. The comparable molecular mechanisms that explain the similar functional profile of antagonist and inverse agonists are likely to be extended from VDR and CAR to other members of the NR superfamily and may lead to the design of even more effective ligands.

A structural basis for the species-specific antagonism of 26,23-lactones on vitamin D signaling

The 26,23-lactone derivative of 1alpha,25-dihydroxyvitamin D3, TEI-9647, is a partial antagonist of the of human vitamin D receptor (VDR). However, we found that TEI-9647 in rat cells behaves as a weak VDR agonist. This behavior could be mimicked in human cells by the double mutagenesis of human VDR (specifically C403S and C410N). The increased agonistic action of TEI-9647 correlates to a gain in the interaction of the VDR with coactivator protein and a decreased stabilization of the antagonistic conformation of the receptor. Molecular dynamics simulations indicated that TEI-9647 acts as antagonist of human VDR by reducing the stability of helix 12 of the ligand binding domain. In contrast, N410 of the rat VDR stabilized, via backbone contacts, the interaction between helices 11 and 12. This results in TEI-9647 becoming a weak agonist in this organism.

Molecular dynamics simulations of the human CAR ligand-binding domain: deciphering the molecular basis for constitutive activity

The constitutive androstane receptor (CAR) belongs to the superfamily of nuclear-hormone receptors that function as ligand-activated transcription factors. CAR plays an essential role in the metabolism of xenobiotics and shows--in contrast to related receptors--constitutive activity. However, the molecular basis for the constitutive activity remains unclear. In the present study, homology models of the ligand binding domain (LBD) were generated based on the crystal structures of the related pregnane X (PXR) and the vitamin D receptor (VDR). The models were used to investigate the basal activity of CAR and the effect of coactivator binding. Molecular dynamics (MD) simulations of complexed and uncomplexed receptor revealed a hypothesis for the activation mechanism. The suggested mechanism is supported by experimental results from site-directed mutagenesis. The basal activity of CAR can be explained by specific van-der-Waals interactions between amino acids on the LBD and its C-terminal activation domain (AF-2). Docking studies with the GOLD program yielded the interaction modes of structurally diverse agonists, giving insight into mechanisms by which ligands enhance CAR activity.

Structural determinants of constitutive androstane receptor required for its glucocorticoid receptor interacting protein-1-mediated nuclear accumulation

Nuclear translocation of constitutive androstane receptor (CAR) is a primary mechanism for the induction of cytochrome P450 genes by phenobarbital (PB). We have shown that exogenous expression of the p160 coactivator glucocorticoid receptor interacting protein-1 (GRIP1) in hepatocytes in vivo can mediate PB-independent nuclear accumulation of murine CAR (mCAR). To understand the mechanism of this PB-independent nuclear accumulation, we have examined the mCAR structural determinants of its GRIP1-mediated nuclear localization. Mutations of the xenobiotic response sequence (XRS), which had been shown to block PB-dependent nuclear translocation of human CAR in mouse hepatocytes in vivo, also blocked GRIP1-mediated nuclear accumulation of mCAR in mouse hepatocytes in vivo and further blocked nuclear localization in cultured HepG2 cells. A leucine 326 XRS mutant retained partial transcriptional activity, but mutations of three leucines in the XRS eliminated transcriptional activity in HepG2 cells, suggesting that the translocation function of the XRS overlaps with transcriptional functions. Mutation of the activation function 2 motif, by deletion of the C-terminal 8 amino acids, also reduced nuclear localization by both PB treatment and GRIP1 expression in hepatocytes in vivo, suggesting that either interaction with GRIP1 through this motif or active CAR was required for the nuclear localization. The localization of a DNA-binding domain mutant was essentially unchanged by coexpression of GRIP1, although without GRIP1 coexpression, this mutant expressed exhibited a more nuclear localization compared with wild type. The results are most consistent with a model in which GRIP1 interaction and activation of mCAR in the nucleus result in retention and accumulation of mCAR in the nucleus in untreated animals. The model requires that mCAR is constantly shuttling between the nucleus and cytoplasm even in untreated animals in which mCAR is predominantly cytoplasmic.

Amino acids important for ligand specificity of the human constitutive androstane receptor

The human constitutive androstane receptor (CAR, NR1I3) is an important ligand-activated regulator of oxidative and conjugative enzymes and transport proteins. Because of the lack of a crystal structure of the ligand-binding domain (LBD), wide species differences in ligand specificity and the scarcity of well characterized ligands, the factors that determine CAR ligand specificity are not clear. To address this issue, we developed highly defined homology models of human CAR LBD to identify residues lining the ligand-binding pocket and to perform molecular dynamics simulations with known human CAR modulators. The roles of 22 LBD residues for basal activity, ligand selectivity, and interactions with co-regulators were studied using site-directed mutagenesis, mammalian co-transfection, and yeast two-hybrid assays. These studies identified several amino acids within helices 3 (Asn(165)), 5 (Val(199)), 11 (Tyr(326), Ile(330), and Gln(331)), and 12 (Leu(343) and Ile(346)) that contribute to the high basal activity of human CAR. Unique residues within helices 3 (Ile(164) and Asn(165)), 5 (Cys(202) and His(203)), and 7 (Phe(234) and Phe(238)) were found control the selectivity for CAR activators and inhibitors. A single residue in helix 7 (Phe(243)) appears to explain the human/mouse species difference in response of CAR to 17alpha-ethynyl-3,17beta-estradiol.

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.

Expression of Constitutive Androstane Receptor Splice Variants in Human Tissues and Their Functional Consequences

The constitutive androstane receptor (CAR) NR1I3 is a transcription factor that upon activation by xenobiotics induces transcription of drug-metabolizing and drug transporter genes. Our goal was to identify whether alternative splicing of CAR makes an important contribution to the generation of novel CAR proteins. The wild-type CAR mRNA (CAR.1) and splice variants (SVs) were amplified from human liver cDNAs and in a panel of cDNAs from 36 human tissues, using exon 1- and 3'-untranslated region primers, cloned and sequenced. Twenty-two unique hCAR splice variants (CAR-SVs) containing different combinations of splicing (deletion of exons 2, 4, 5, 7, partial deletion of exon 9, or insertion of 12 or 15 base pairs from introns 6 or 7) were identified. CAR mRNAs were expressed in small intestine, kidney, testis, adrenal, and brain caudate nucleus. Intestine expressed only CAR.1 mRNA, whereas spleen, heart, and prostate expressed only CAR-SVs. In vitro transcription and translation of CAR-SVs lacking exon 2 (missing ATG start site) generated CAR proteins that differed in M(r) from CAR.1. These CAR-SVs used a translation initiation site in exon 1, generating CAR with a unique amino-terminal sequence. Transcripts lacking part of exon 9 altered the CAR reading frame generating CAR proteins with a unique carboxy-terminal end. CAR-SVs demonstrated compromised binding to CYP2B6 NR elements and transcriptional activation of a CYP2B6 luciferase reporter. The expression of CAR in additional human cell types increases the range of tissue specific transcriptional responses regulated by this receptor, suggesting additional biological roles for CAR and CAR-SV proteins in these tissues.

Agonist-dependent and agonist-independent transactivations of the human constitutive androstane receptor are modulated by specific amino acid pairs

The constitutive androstane receptor (CAR) is an interesting member of the nuclear receptor superfamily because of its exceptionally high constitutive activity due to ligand-independent interaction of the ligand-binding domain with co-activator proteins. This study compares the agonist-dependent and agonist-independent activities of human CAR with those of mouse CAR and the vitamin D receptor and demonstrates that the constitutive activity of CAR is mediated by at least three contacts between the amino acids of helix 12, partner amino acids in helices 4 and 11, and a charge clamp between helices 12 and 3. The stabilization of helix 12 by a contact between its C terminus and the lysine of helix 4 has the same impact in human and mouse CARs. In addition, the charge clamp between the glutamate in helix 12 and the lysine in helix 3 is also important for the constitutive activity of both receptor orthologs but less critical for the agonist-dependent stabilization of their respective helices 12. Interestingly, Cys-357 in mouse CAR has significantly more impact on the stabilization of helix 12 than does the orthologous position Cys-347 in human CAR. This deficit appears to be compensated by a more dominant role of Ile-330 in human CAR over Leu-340 in mouse CAR because it is more efficient than Cys-347 in controlling the flexibility of helix 12 in the presence of an agonist. The constitutive activity of other members of the nuclear receptor superfamily could be explained by a homologous hydrophobic interaction between large, non-polar amino acids of helices 11 and 12.

Ligand-mediated conformational changes of the VDR are required for gene transactivation

The central element of the molecular switch of nuclear 1alpha,25-dihydroxyvitamin D(3) (1alpha,25(OH)(2)D(3)) signaling is the ligand-binding domain (LBD) of the Vitamin D receptor (VDR), which can be stabilized by 1alpha,25(OH)(2)D(3) or its analogues in to agonistic, antagonistic or inverse agonistic conformations. The positioning of helix 12 of the LBD is of most critical importance for these conformations, because it determines the distance between the charge clamp amino acids K246 and E420 that are essential for VDR-coactivator (CoA) interaction. Most VDR ligands have been identified as agonists and only a few (e.g., ZK168281 and TEI-9647) as pure or partial antagonists. Antagonists induce corepressor (CoR) dissociation from the VDR but prevent completely or partially CoA interaction and thus transactivation. Gemini is a 1alpha,25(OH)(2)D(3) analogue with two identical side chains that despite its significantly increased volume binds to the VDR and acts under most conditions as an agonist. Interestingly, supramolar CoR concentrations shift Gemini from an agonist to an inverse agonist, which actively recruits CoR to the VDR and thus mediates repression of 1alpha,25(OH)(2)D(3) target genes. Gemini is the first described (conditional) inverse agonist to an endocrine nuclear receptor (NR) and may function as a sensor for cell-specific CoA/CoR ratios.

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.