Use of comprehensive screening methods to detect selective human CAR activators

Filling the Blank Space: Branched 4-Nonylphenol Isomers Are Responsible for Robust Constitutive Androstane Receptor (CAR) Activation by Nonylphenol

4-Nonylphenol (4-NP), a para-substituted phenolic compound with a straight or branched carbon chain, is a ubiquitous environmental pollutant and food contaminant. 4-NP, particularly the branched form, has been identified as an endocrine disruptor (ED) with potent activities on estrogen receptors. Constitutive Androstane Receptor (CAR) is another crucial nuclear receptor that regulates hepatic lipid, glucose, and steroid metabolism and is involved in the ED mechanism of action. An NP mixture has been described as an extremely potent activator of both human and rodent CAR. However, detailed mechanistic aspects of CAR activation by 4-NP are enigmatic, and it is not known if 4-NP can directly interact with the CAR ligand binding domain (LBD). Here, we examined interactions of individual branched (22NP, 33NP, and 353NP) and linear 4-NPs with CAR variants using molecular dynamics (MD) simulations, cellular experiments with various CAR expression constructs, recombinant CAR LBD in a TR-FRET assay, or a differentiated HepaRG hepatocyte cellular model. Our results demonstrate that branched 4-NPs display more stable poses to activate both wild-type CAR1 and CAR3 variant LBDs in MD simulations. Consistently, branched 4-NPs activated CAR3 and CAR1 LBD more efficiently than linear 4-NP. Furthermore, in HepaRG cells, we observed that all 4-NPs upregulated CYP2B6 mRNA, a relevant hallmark for CAR activation. This is the first study to provide detailed insights into the direct interaction between individual 4-NPs and human CAR-LBD, as well as its dominant variant CAR3. The work could contribute to the safer use of individual 4-NPs in many areas of industry.

Environmental chemical TCPOBOP exposure alters milk liposomes and offspring growth trajectories in mice

Exposure to environmental endocrine disruptors (EEDs) has become a global health concern, and EEDs are known to be potent inducers of constitutive androstane receptor (CAR). 1,4-bis [2-(3,5-dichloropyridyloxy)] benzene (TCPOBOP, hereafter abbreviated as TC), a specific ligand for CAR, has been considered as a potential EED. Here, we analyzed the effect of TC exposure to female mice on the histological morphology of their alveoli in the basic unit of lactation. We quantified differences in the milk metabolome of the control and TC-exposed group while assessing the correlations between metabolites and neonatal growth. Mammary histological results showed that TC exposure inhibited alveolar development. Based on the milk metabolomic data, we identified a total of 1505 differential metabolites in both the positive and negative ion mode, which indicated that TC exposure affected milk composition. As expected, the differential metabolites were significantly enriched in the drug metabolism pathway. Further analyses revealed that differential metabolites were significantly enriched in multiple lipid metabolic pathways, such as fatty acid biosynthesis, suggesting that most differential metabolites were concentrated in lipids. Simultaneously, a quantitative analysis showed that TC exposure led to a decrease in the relative abundance of total milk lipids, affecting the proportion of some lipid subclasses. Notably, a portion of lipid metabolites were associated with neonatal growth. Taken together, these findings suggest that TC exposure may affect milk lipidomes, resulting in the inability of mothers to provide adequate nutrients, ultimately affecting the growth and health of their offspring.

Cross-species applicability of an adverse outcome pathway network for thyroid hormone system disruption

Thyroid hormone system disrupting compounds are considered potential threats for human and environmental health. Multiple adverse outcome pathways (AOPs) for thyroid hormone system disruption (THSD) are being developed in different taxa. Combining these AOPs results in a cross-species AOP network for THSD which may provide an evidence-based foundation for extrapolating THSD data across vertebrate species and bridging the gap between human and environmental health. This review aimed to advance the description of the taxonomic domain of applicability (tDOA) in the network to improve its utility for cross-species extrapolation. We focused on the molecular initiating events (MIEs) and adverse outcomes (AOs) and evaluated both their plausible domain of applicability (taxa they are likely applicable to) and empirical domain of applicability (where evidence for applicability to various taxa exists) in a THSD context. The evaluation showed that all MIEs in the AOP network are applicable to mammals. With some exceptions, there was evidence of structural conservation across vertebrate taxa and especially for fish and amphibians, and to a lesser extent for birds, empirical evidence was found. Current evidence supports the applicability of impaired neurodevelopment, neurosensory development (eg, vision) and reproduction across vertebrate taxa. The results of this tDOA evaluation are summarized in a conceptual AOP network that helps prioritize (parts of) AOPs for a more detailed evaluation. In conclusion, this review advances the tDOA description of an existing THSD AOP network and serves as a catalog summarizing plausible and empirical evidence on which future cross-species AOP development and tDOA assessment could build.

Discovery of Novel Human Constitutive Androstane Receptor Agonists with the Imidazo[1,2-a]pyridine Structure

The nuclear constitutive androstane receptor (CAR, NR1I3) plays significant roles in many hepatic functions, such as fatty acid oxidation, biotransformation, liver regeneration, as well as clearance of steroid hormones, cholesterol, and bilirubin. CAR has been proposed as a hypothetical target receptor for metabolic or liver disease therapy. Currently known prototype high-affinity human CAR agonists such as CITCO (6-(4-chlorophenyl)imidazo[2,1-b][1,3]thiazole-5-carbaldehyde-O-(3,4-dichlorobenzyl)oxime) have limited selectivity, activating the pregnane X receptor (PXR) receptor, a related receptor of the NR1I subfamily. We have discovered several derivatives of 3-(1H-1,2,3-triazol-4-yl)imidazo[1,2-a]pyridine that directly activate human CAR in nanomolar concentrations. While compound 39 regulates CAR target genes in humanized CAR mice as well as human hepatocytes, it does not activate other nuclear receptors and is nontoxic in cellular and genotoxic assays as well as in rodent toxicity studies. Our findings concerning potent human CAR agonists with in vivo activity reinforce the role of CAR as a possible therapeutic target.

Metabolism-Disrupting Chemicals and the Constitutive Androstane Receptor CAR

During the last two decades, the constitutive androstane receptor (CAR; NR1I3) has emerged as a master activator of drug- and xenobiotic-metabolizing enzymes and transporters that govern the clearance of both exogenous and endogenous small molecules. Recent studies indicate that CAR participates, together with other nuclear receptors (NRs) and transcription factors, in regulation of hepatic glucose and lipid metabolism, hepatocyte communication, proliferation and toxicity, and liver tumor development in rodents. Endocrine-disrupting chemicals (EDCs) constitute a wide range of persistent organic compounds that have been associated with aberrations of hormone-dependent physiological processes. Their adverse health effects include metabolic alterations such as diabetes, obesity, and fatty liver disease in animal models and humans exposed to EDCs. As numerous xenobiotics can activate CAR, its role in EDC-elicited adverse metabolic effects has gained much interest. Here, we review the key features and mechanisms of CAR as a xenobiotic-sensing receptor, species differences and selectivity of CAR ligands, contribution of CAR to regulation hepatic metabolism, and evidence for CAR-dependent EDC action therein.

Identification of novel agonists by high-throughput screening and molecular modelling of human constitutive androstane receptor isoform 3

Prediction of drug interactions, based on the induction of drug disposition, calls for the identification of chemicals, which activate xenosensing nuclear receptors. Constitutive androstane receptor (CAR) is one of the major human xenosensors; however, the constitutive activity of its reference variant CAR1 in immortalized cell lines complicates the identification of agonists. The exclusively ligand-dependent isoform CAR3 represents an obvious alternative for screening of CAR agonists. As CAR3 is even more abundant in human liver than CAR1, identification of its agonists is also of pharmacological value in its own right. We here established a cellular high-throughput screening assay for CAR3 to identify ligands of this isoform and to analyse its suitability for identifying CAR ligands in general. Proof-of-concept screening of 2054 drug-like compounds at 10 µM resulted in the identification of novel CAR3 agonists. The CAR3 assay proved to detect the previously described CAR1 ligands in the screened libraries. However, we failed to detect CAR3-selective compounds, as the four novel agonists, which were selected for further investigations, all proved to activate CAR1 in different cellular and in vitro assays. In primary human hepatocytes, the compounds preferentially induced the expression of the prototypical CAR target gene CYP2B6. Failure to identify CAR3-selective compounds was investigated by molecular modelling, which showed that the isoform-specific insertion of five amino acids did not impact on the ligand binding pocket but only on heterodimerization with retinoid X receptor. In conclusion, we demonstrate here the usability of CAR3 for screening compound libraries for the presence of CAR agonists.

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.

Human constitutive androstane receptor agonist DL5016: A novel sensitizer for cyclophosphamide-based chemotherapies

The DNA alkylating prodrug cyclophosphamide (CPA), alone or in combination with other agents, is one of the most commonly used anti-cancer agents. As a prodrug, CPA is activated by cytochrome P450 2B6 (CYP2B6), which is transcriptionally regulated by the human constitutive androstane receptor (hCAR). Therefore, hCAR agonists represent novel sensitizers for CPA-based therapies. Among known hCAR agonists, compound 6-(4-chlorophenyl)imidazo-[2,1-b]thiazole-5-carbaldehyde-O-(3,4-dichlorobenzyl)oxime (CITCO) is the most potent and broadly utilized in biological studies. Through structural modification of CITCO, we have developed a novel compound DL5016 (32), which has an EC₅₀ value of 0.66 μM and E_MAX value of 4.9 when activating hCAR. DL5016 robustly induced the expression of hCAR target gene CYP2B6, at both the mRNA and protein levels, and caused translocation of hCAR from the cytoplasm to the nucleus in human primary hepatocytes. The effects of DL5016 were highlighted by dramatically enhancing the efficacy of CPA-based cytotoxicity to non-Hodgkin lymphoma cells.

DNA elements for constitutive androstane receptor- and pregnane X receptor-mediated regulation of bovine CYP3A28 gene

The regulation of cytochrome P450 3A (CYP3A) enzymes is established in humans, but molecular mechanisms of its basal and xenobiotic-mediated regulation in cattle are still unknown. Here, ~10 kbp of the bovine CYP3A28 gene promoter were cloned and sequenced, and putative transcription factor binding sites were predicted. The CYP3A28 proximal promoter (PP; -284/+71 bp) contained DNA elements conserved among species. Co-transfection of bovine nuclear receptors (NRs) pregnane X and constitutive androstane receptor (bPXR and bCAR) with various CYP3A28 promoter constructs into hepatoma cell lines identified two main regions, the PP and the distal fragment F3 (-6899/-4937 bp), that were responsive to bPXR (both) and bCAR (F3 fragment only). Site-directed mutagenesis and deletion of NR motif ER6, hepatocyte nuclear factor 1 (HNF-1) and HNF-4 binding sites in the PP suggested either the involvement of ER6 element in bPXR-mediated activation or the cooperation between bPXR and liver-enriched transcription factors (LETFs) in PP transactivation. A putative DR5 element within the F3 fragment was involved in bCAR-mediated PP+F3 transactivation. Although DNA enrichment by anti-human NR antibodies was quite low, ChIP investigations in control and RU486-treated BFH12 cells, suggested that retinoid X receptor α (RXRα) bound to ER6 and DR5 motifs and its recruitment was enhanced by RU486 treatment. The DR5 element seemed to be recognized mainly by bCAR, while no clear-cut results were obtained for bPXR. Present results point to species-differences in CYP3A regulation and the complexity of bovine CYP3A28 regulatory elements, but further confirmatory studies are needed.

A Reverse Transfection Method for Screening of Nuclear Receptor Activators

Reporter assays are useful to study nuclear receptor activation and for example to evaluate the propensity of novel drug candidates to cause induction of drug-metabolizing cytochrome P450 enzymes. Here, we describe a protocol for a reverse transfection system to study the activation of human nuclear receptors constitutive androstane receptor and pregnane X receptor. The system provides long-term stability and uniformity of DNA-carrier complexes, thus avoiding the inherent variation in conventional transfection methods. Further, the system is easily adaptable for different studies. It offers reproducible and reliable results for early drug development and mechanistic studies related to nuclear receptor activation and resulting changes in gene expression.

Health-Relevant Phenotypes in the Offspring of Mice Given CAR Activators Prior to Pregnancy

Hepatic induction in response to drugs and environmental chemicals affects drug therapies and energy metabolism. We investigated whether the induction is transmitted to the offspring. We injected 3-day- and 6-week-old F0 female mice with TCPOBOP, an activator of the nuclear receptor constitutive androstane receptor (CAR, NR1I3), and mated them 1-6 weeks afterward. We detected in the offspring long-lasting alterations of CAR-mediated drug disposition, energy metabolism, and lipid profile. The transmission to the first filial generation (F1) was mediated by TCPOBOP transfer from the F0 adipose tissue via milk, as revealed by embryo transfer, crossfostering experiments, and liquid chromatography-mass spectrometry analyses. The important environmental pollutant PCB153 activated CAR in the F1 generation in a manner similar to TCPOBOP. Our findings indicate that chemicals accumulating and persisting in adipose tissue may exert liver-mediated, health-relevant effects on F1 offspring simply via physical transmission in milk. Such effects may occur even if treatment has been terminated far ahead of conception. This should be considered in assessing developmental toxicity and in the long-term follow-up of offspring of mothers exposed to both approved and investigational drugs, and to chemicals with known or suspected accumulation in adipose tissue.

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.

2-(3-Methoxyphenyl)quinazoline Derivatives: A New Class of Direct Constitutive Androstane Receptor (CAR) Agonists

Constitutive androstane receptor (CAR) is a key regulator of xenobiotic and endobiotic metabolism. Together with pregnane X (PXR) and aryl hydrocarbon (AHR) receptors, it is referred to as "xenobiotic receptor". The unique properties of human CAR, such as its high constitutive activity, both direct (ligand-binding domain-dependent) and indirect activation have hindered the discovery of direct selective human CAR ligands. Herein, we report a novel class of direct human CAR agonists in a group of 2-(3-methoxyphenyl)quinazoline derivatives. The compounds are even more potent activators of human CAR than is prototype 6-(4-chlorophenyl)imidazo[2,1-b][1,3]thiazole-5-carbaldehyde O-(3,4-dichlorobenzyl)oxime (CITCO). The three most potent ligands are at the same time extremely potent activators of the other xenobiotic or hormonal receptors, namely PXR, AHR, and vitamin D receptor, which regulate major xenobiotic-metabolizing enzymes and efflux transporters. Thus, the novel CAR ligands can be also considered as constituting the first class of potent pan-xenobiotic receptor ligands that can serve as potential antidotes boosting overall metabolic elimination of xenobiotic or toxic compounds.

Improved assays for xenosensor activation based on reverse transfection

Discovery of receptor-dependent mechanisms for regulation of drug metabolism has provided a new way to evaluate the propensity of drug candidates to cause induction of cytochrome P450 enzymes. Therefore, receptor-based reporter assays have become common in early stages of drug development projects and in mechanistic studies. Here, we report a reverse transfection system to conduct activation assays for human xenosensors AhR, CAR and PXR. The assay format is based on long-term stability and uniformity of DNA/carrier complexes on culture plates, avoiding multiple stages and variation inherent in conventional transfection methods. Consequently, these improved assays are streamlined, reproducible and formally validated with Z' factors exceeding 0.5. This novel reverse transfection system is expected to find use in diverse areas of early drug development such prediction of CYP induction, evaluation of species differences and in mechanistic studies.

Regulation of gene expression by CAR: an update

The constitutive androstane receptor (CAR), a member of the nuclear receptor superfamily, is a well-known xenosensor that regulates hepatic drug metabolism and detoxification. CAR activation can be elicited by a large variety of xenobiotics, including phenobarbital (PB) which is not a directly binding CAR ligand. The mechanism of CAR activation is complex and involves translocation from the cytoplasm into the nucleus, followed by further activation steps in the nucleus. Recently, epidermal growth factor receptor (EGFR) has been identified as a PB-responsive receptor, and PB activates CAR by inhibiting the EGFR signaling. In addition to regulation of drug metabolism, activation of CAR has multiple biological end points such as modulation of xenobiotic-elicited liver injury, and the role of CAR in endobiotic functions such as glucose metabolism and cholesterol homeostasis is increasingly recognized. Thus, investigations on the molecular mechanism of CAR activation are critical for the real understanding of CAR-mediated processes. Here, we summarize the current understanding of mechanisms by which CAR activators regulate gene expression through cellular signaling pathways and the roles of CAR on xenobiotic-elicited hepatocellular carcinoma, liver injury, glucose metabolism and cholesterol homeostasis.

Small-molecule modulators of the constitutive androstane receptor

INTRODUCTION

The constitutive androstane receptor (CAR) induces drug-metabolizing enzymes for xenobiotic metabolism.

AREAS COVERED

This review covers recent advances in elucidating the biological functions of CAR and its modulation by a growing number of agonists and inhibitors.

EXPERT OPINION

Extrapolation of animal CAR function to that of humans should be carefully scrutinized, particularly when rodents are used in evaluating the metabolic profile and carcinogenic properties of clinical drugs and environmental chemicals. Continuous efforts are needed to discover novel CAR inhibitors, with extensive understanding of their inhibitory mechanism, species selectivity, and discriminating power against other xenobiotic sensors.

Interactions of endosulfan and methoxychlor involving CYP3A4 and CYP2B6 in human HepaRG cells

Humans are usually exposed to several pesticides simultaneously; consequently, combined actions between pesticides themselves or between pesticides and other chemicals need to be addressed in the risk assessment. Many pesticides are efficient activators of pregnane X receptor (PXR) and/or constitutive androstane receptor (CAR), two major nuclear receptors that are also activated by other substrates. In the present work, we searched for interactions between endosulfan and methoxychlor, two organochlorine pesticides whose major routes of metabolism involve CAR- and PXR-regulated CYP3A4 and CYP2B6, and whose mechanisms of action in humans remain poorly understood. For this purpose, HepaRG cells were treated with both pesticides separately or in mixture for 24 hours or 2 weeks at concentrations relevant to human exposure levels. In combination they exerted synergistic cytotoxic effects. Whatever the duration of treatment, both compounds increased CYP3A4 and CYP2B6 mRNA levels while differently affecting their corresponding activities. Endosulfan exerted a direct reversible inhibition of CYP3A4 activity that was confirmed in human liver microsomes. By contrast, methoxychlor induced this activity. The effects of the mixture on CYP3A4 activity were equal to the sum of those of each individual compound, suggesting an additive effect of each pesticide. Despite CYP2B6 activity being unchanged and increased with endosulfan and methoxychlor, respectively, no change was observed with their mixture, supporting an antagonistic effect. Altogether, our data suggest that CAR and PXR activators endosulfan and methoxychlor can interact together and with other exogenous substrates in human hepatocytes. Their effects on CYP3A4 and CYP2B6 activities could have important consequences if extrapolated to the in vivo situation.

Dichlorodiphenyltrichloroethane technical mixture regulates cell cycle and apoptosis genes through the activation of CAR and ERα in mouse livers

Dichlorodiphenyltrichloroethane (DDT) is a widely used organochlorine pesticide and a xenoestrogen that promotes rodent hepatomegaly and tumours. A recent study has shown significant correlation between DDT serum concentration and liver cancer incidence in humans, but the underlying mechanisms remain elusive. We hypothesised that a mixture of DDT isomers could exert effects on the liver through pathways instead of classical ERs. The acute effects of a DDT mixture containing the two major isomers p,p'-DDT (85%) and o,p'-DDT (15%) on CAR and ERα receptors and their cell cycle and apoptosis target genes were studied in mouse livers. ChIP results demonstrated increased CAR and ERα recruitment to their specific target gene binding sites in response to the DDT mixture. The results of real-time RT-PCR were consistent with the ChIP data and demonstrated that the DDT was able to activate both CAR and ERα in mouse livers, leading to target gene transcriptional increases including Cyp2b10, Gadd45β, cMyc, Mdm2, Ccnd1, cFos and E2f1. Western blot analysis demonstrated increases in cell cycle progression proteins cMyc, Cyclin D1, CDK4 and E2f1 and anti-apoptosis proteins Mdm2 and Gadd45β. In addition, DDT exposure led to Rb phosphorylation. Increases in cell cycle progression and anti-apoptosis proteins were accompanied by a decrease in p53 content and its transcriptional activity. However, the DDT was unable to stimulate the β-catenin signalling pathway, which can play an important role in hepatocyte proliferation. Thus, our results indicate that DDT treatment may result in cell cycle progression and apoptosis inhibition through CAR- and ERα-mediated gene activation in mouse livers. These findings suggest that the proliferative and anti-apoptotic conditions induced by CAR and ERα activation may be important contributors to the early stages of hepatocarcinogenesis as produced by DDT in rodent livers.

Identification of novel activators of constitutive androstane receptor from FDA-approved drugs by integrated computational and biological approaches

PURPOSE

The constitutive androstane receptor (CAR, NR1I3) is a xenobiotic sensor governing the transcription of numerous hepatic genes associated with drug metabolism and clearance. Recent evidence suggests that CAR also modulates energy homeostasis and cancer development. Thus, identification of novel human (h) CAR activators is of both clinical importance and scientific interest.

METHODS

Docking and ligand-based structure-activity models were used for virtual screening of a database containing over 2000 FDA-approved drugs. Identified lead compounds were evaluated in cell-based reporter assays to determine hCAR activation. Potential activators were further tested in human primary hepatocytes (HPHs) for the expression of the prototypical hCAR target gene CYP2B6.

RESULTS

Nineteen lead compounds with optimal modeling parameters were selected for biological evaluation. Seven of the 19 leads exhibited moderate to potent activation of hCAR. Five out of the seven compounds translocated hCAR from the cytoplasm to the nucleus of HPHs in a concentration-dependent manner. These compounds also induce the expression of CYP2B6 in HPHs with rank-order of efficacies closely resembling that of hCAR activation.

CONCLUSION

These results indicate that our strategically integrated approaches are effective in the identification of novel hCAR modulators, which may function as valuable research tools or potential therapeutic molecules.

Evaluation of Ginkgo biloba extract as an activator of human glucocorticoid receptor

ETHNOPHARMACOLOGICAL RELEVANCE

Ginkgo biloba, which is one of the most frequently used herbal medicines, is commonly used in the management of several conditions, including memory impairment. Previously, it was reported to decrease the expression of peripheral benzodiazepine receptor and the biosynthesis of glucocorticoids, thereby regulating glucocorticoid levels. However, it is not known whether Ginkgo biloba extract regulates the function of the glucocorticoid receptor.

AIM OF THE STUDY

We determined whether Ginkgo biloba extract and several of its chemical constituents affect the activity of human glucocorticoid receptor (hGR).

MATERIALS AND METHODS

A hGR-dependent reporter gene assay was conducted in HepG2 human hepatocellular carcinoma cells and hGR target gene expression assays were performed in primary cultures of human hepatocytes.

RESULTS

Multiple lots and concentrations of the extract and several of its chemical constituents (ginkgolide A, ginkgolide B, ginkgolide C, ginkgolide J, and bilobalide) did not increase hGR activity, as assessed by a cell-based luciferase reporter gene assay. The extract did not influence the expression of hGR target genes, including tyrosine aminotransferase (hTAT), constitutive androstane receptor (hCAR), or pregnane X receptor (hPXR), in primary cultures of human hepatocytes. Moreover, hGR antagonism by mifepristone (also known as RU486) did not attenuate the extent of induction of hCAR- and hPXR-regulated target genes CYP2B6 and CYP3A4 by Ginkgo biloba extract.

CONCLUSION

Ginkgo biloba extract, ginkgolide A, ginkgolide B, ginkgolide C, ginkgolide J, and bilobalide are not activators of hGR. Furthermore, the extract does not influence the hGR-hCAR or the hGR-hPXR signaling pathway in primary cultures of human hepatocytes.

Xenobiotic receptor humanized mice and their utility

The nuclear receptors pregnane X receptor, constitutive androstane receptor, and peroxisome proliferator-activated receptor alpha have important endogenous functions and are also involved in the induction of drug-metabolizing enzymes and transporters in response to exogenous xenobiotics. Though not belonging to the same protein family, the Per-Sim-ARNT domain receptor aryl hydrocarbon receptor functionally overlaps with the three nuclear receptors in many aspects and is therefore included in this review. Significant species differences in ligand affinity and biological responses as a result of activation of these receptors have been described. Several xenobiotic receptor humanized mice have been created to overcome these species differences and to provide in vivo models that are more predictive for human responses. This review provides an overview of the different xenobiotic receptor humanized mouse models described to date and will summarize how these models can be applied in basic research and improve drug discovery and development. Some of the key applications in the evaluation of drug induction, drug-drug interactions, nongenotoxic carcinogenicity, other toxicity, or efficacy studies are described. We also discuss relevant considerations in the interpretation of such data and potential future directions for the use of xenobiotic receptor humanized mice.

New in vitro tools to study human constitutive androstane receptor (CAR) biology: discovery and comparison of human CAR inverse agonists

The human constitutive androstane receptor (CAR, NR1I3) is one of the key regulators of xenobiotic and endobiotic metabolism. The unique properties of human CAR, such as the high constitutive activity and the complexity of signaling, as well as the lack of functional and predictive cell-based assays to study the properties of the receptor, have hindered the discovery of selective human CAR ligands. Here we report a novel human CAR inverse agonist, 1-[(2-methylbenzofuran-3-yl)methyl]-3-(thiophen-2-ylmethyl) urea (S07662), which suppresses human CAR activity, recruits the corepressor NCoR in cell-based assays, and attenuates the phenytoin- and 6-(4-chlorophenyl)imidazo[2,1-b][1,3]thiazole-5-carbaldehyde O-(3,4-dichlorobenzyl)oxime (CITCO)-induced expression of CYP2B6 mRNA in human primary hepatocytes. The properties of S07662 are also compared with those of known human CAR inverse agonists by using an array of different in vitro and in silico assays. The identified compound S07662 can be used as a chemical tool to study the biological functions of human CAR and also as a starting point for the development of new drugs for various conditions involving the receptor.