The environmental estrogen, nonylphenol, activates the constitutive androstane receptor

Endocrine-disrupting chemicals as prostate carcinogens

Endocrine-disrupting chemicals (EDCs) are natural or synthetic compounds that are ubiquitous in the environment and in daily-usage products and interfere with the normal function of the endocrine system leading to adverse health effects in humans. Exposure to these chemicals might elevate the risk of metabolic disorders, developmental and reproductive defects, and endocrine-related cancers. Prostate cancer is the most common hormone-dependent cancer in men, and the fifth leading cause of cancer-related mortality, partly owing to a lack of knowledge about the mechanisms that lead to aggressive castration-resistant forms. In addition to the dependence of early-stage prostate cancer on androgen actions, the prostate is a target of oestrogenic regulation. This hormone dependence, along with the fact that exogenous influences are major risk factors for prostate cancer, make the prostate a likely target of harmful actions from EDCs. Various sources of EDCs and their different modes of action might explain their role in prostate carcinogenesis.

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.

Pharmacological activation of constitutive androstane receptor induces female-specific modulation of hepatic metabolism

Background & Aims

The constitutive androstane receptor (CAR) is a nuclear receptor that binds diverse xenobiotics and whose activation leads to the modulation of the expression of target genes involved in xenobiotic detoxification and energy metabolism. Although CAR hepatic activity is considered to be higher in women than in men, its sex-dependent response to an acute pharmacological activation has seldom been investigated.

Methods

The hepatic transcriptome, plasma markers, and hepatic metabolome, were analysed in Car+/+ and Car-/- male and female mice treated either with the CAR-specific agonist 1,4-bis[2-(3,5-dichloropyridyloxy)]benzene (TCPOBOP) or with vehicle.

Results

Although 90% of TCPOBOP-sensitive genes were modulated in a sex-independent manner, the remaining 10% showed almost exclusive female liver specificity. These female-specific CAR-sensitive genes were mainly involved in xenobiotic metabolism, inflammation, and extracellular matrix organisation. CAR activation also induced higher hepatic oxidative stress and hepatocyte cytolysis in females than in males. Hepatic expression of flavin monooxygenase 3 (Fmo3) was almost abolished and was associated with a decrease in hepatic trimethylamine-N-oxide (TMAO) concentration in TCPOBOP-treated females. In line with a potential role in the control of TMAO homeostasis, CAR activation decreased platelet hyper-responsiveness in female mice supplemented with dietary choline.

Conclusions

More than 10% of CAR-sensitive genes are sex-specific and influence hepatic and systemic responses such as platelet aggregation. CAR activation may be an important mechanism of sexually-dimorphic drug-induced liver injury.

Impact and implications

CAR is activated by many drugs and pollutants. Its pharmacological activation had a stronger impact on hepatic gene expression and metabolism in females than in males, and had a specific impact on liver toxicity and trimethylamine metabolism. Sexual dimorphism should be considered when testing and/or prescribing xenobiotics known to activate CAR.

Exposure to endocrine disruptor alkylphenols and the occurrence of endometrial cancer

Exposure to environmental chemicals with oestrogenic effects has been associated with the development of endometrial cancer (EMCa). EMCa has become the most commonly diagnosed cancer of the female genital tract. To further understand the potential association between exposure to environmental endocrine disruptors and the occurrence of EMCa, we performed a case-control study between 2011 and 2014. We aimed to detect and compare concentrations of a known hormone disruptor, alkylphenol, between women diagnosed with either EMCa or uterine leiomyoma, and those who did not have either of these. Subjects were women diagnosed with either EMCa or uterine leiomyoma (LM) and healthy controls. A structured questionnaire was administered to collect information on lifestyle and health status. Gas chromatography/mass spectrometry was used to measure urinary NP and OP concentrations in participants. Multiple regression analysis was used to examine the association between exposure and outcomes. Overall, 397 women were recruited, including 49 with EMCa, 247 with LM, and 101 controls. Among them, 73.6% showed detectable levels of NP and 61.0% showed detectable levels of OP. The EMCa group had a significantly higher NP concentration than the control group. Higher OP concentrations were also found in participants with EMCa than those with LM and controls. In addition, women in the upper tertile of the NP group had a significantly increased risk of EMCa occurrence (odds ratio [95% confidence interval] = 4.47 [1.69-11.84] for EMCa vs. control). The same was found in the group of women with more than the median level of OP (odds ratio [95% confidence interval] = 4.32 [2.01-9.30] for EMCa vs. LM). Stratification of pre- and post-menopausal groups resulted in a similar association. The results show that NP/OP exposure is associated with EMCa. Further investigations and exposure minimisation are suggested.

Regulation of CAR and PXR Expression in Health and Disease

Pregnane X receptor (PXR, NR1I2) and constitutive androstane receptor (CAR, NR1I3) are members of the nuclear receptor superfamily that mainly act as ligand-activated transcription factors. Their functions have long been associated with the regulation of drug metabolism and disposition, and it is now well established that they are implicated in physiological and pathological conditions. Considerable efforts have been made to understand the regulation of their activity by their cognate ligand; however, additional regulatory mechanisms, among which the regulation of their expression, modulate their pleiotropic effects. This review summarizes the current knowledge on CAR and PXR expression during development and adult life; tissue distribution; spatial, temporal, and metabolic regulations; as well as in pathological situations, including chronic diseases and cancers. The expression of CAR and PXR is modulated by complex regulatory mechanisms that involve the interplay of transcription factors and also post-transcriptional and epigenetic modifications. Moreover, many environmental stimuli affect CAR and PXR expression through mechanisms that have not been elucidated.

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.

Phase 0 of the Xenobiotic Response: Nuclear Receptors and Other Transcription Factors as a First Step in Protection from Xenobiotics

This mini-review examines the crucial importance of transcription factors as a first line of defense in the detoxication of xenobiotics. Key transcription factors that recognize xenobiotics or xenobiotic-induced stress such as reactive oxygen species (ROS), include AhR, PXR, CAR, MTF, Nrf2, NF-κB, and AP-1. These transcription factors constitute a significant portion of the pathways induced by toxicants as they regulate phase I-III detoxication enzymes and transporters as well as other protective proteins such as heat shock proteins, chaperones, and anti-oxidants. Because they are often the first line of defense and induce phase I-III metabolism, could these transcription factors be considered the phase 0 of xenobiotic response?

In Silico Molecular Docking and In Vivo Validation with Caenorhabditis elegans to Discover Molecular Initiating Events in Adverse Outcome Pathway Framework: Case Study on Endocrine-Disrupting Chemicals with Estrogen and Androgen Receptors

Molecular docking is used to analyze structural complexes of a target with its ligand for understanding the chemical and structural basis of target specificity. This method has the potential to be applied for discovering molecular initiating events (MIEs) in the Adverse Outcome Pathway framework. In this study, we aimed to develop in silico–in vivo combined approach as a tool for identifying potential MIEs. We used environmental chemicals from Tox21 database to identify potential endocrine-disrupting chemicals (EDCs) through molecular docking simulation, using estrogen receptor (ER), androgen receptor (AR) and their homology models in the nematode Caenorhabditis elegans (NHR-14 and NHR-69, respectively). In vivo validation was conducted on the selected EDCs with C. elegans reproductive toxicity assay using wildtype N2, nhr-14, and nhr-69 loss-of-function mutant strains. The chemicals showed high binding affinity to tested receptors and showed the high in vivo reproductive toxicity, and this was further confirmed using the mutant strains. The present study demonstrates that the binding affinity from the molecular docking potentially correlates with in vivo toxicity. These results prove that our in silico–in vivo combined approach has the potential to be applied for identifying MIEs. This study also suggests the potential of C. elegans as useful in the in vivo model for validating the in silico approach.

RNA sequencing indicates that atrazine induces multiple detoxification genes in Daphnia magna and this is a potential source of its mixture interactions with other chemicals

Atrazine is an herbicide with several known toxicologically relevant effects, including interactions with other chemicals. Atrazine increases the toxicity of several organophosphates and has been shown to reduce the toxicity of triclosan to D. magna in a concentration dependent manner. Atrazine is a potent activator in vitro of the xenobiotic-sensing nuclear receptor, HR96, related to vertebrate constitutive androstane receptor (CAR) and pregnane X-receptor (PXR). RNA sequencing (RNAseq) was performed to determine if atrazine is inducing phase I-III detoxification enzymes in vivo, and estimate its potential for mixture interactions. RNAseq analysis demonstrates induction of glutathione S-transferases (GSTs), cytochrome P450s (CYPs), glucosyltransferases (UDPGTs), and xenobiotic transporters, of which several are verified by qPCR. Pathway analysis demonstrates changes in drug, glutathione, and sphingolipid metabolism, indicative of HR96 activation. Based on our RNAseq data, we hypothesized as to which environmentally relevant chemicals may show altered toxicity with co-exposure to atrazine. Acute toxicity tests were performed to determine individual LC₅₀ and Hillslope values as were toxicity tests with binary mixtures containing atrazine. The observed mixture toxicity was compared with modeled mixture toxicity using the Computational Approach to the Toxicity Assessment of Mixtures (CATAM) to assess whether atrazine is exerting antagonism, additivity, or synergistic toxicity in accordance with our hypothesis. Atrazine-triclosan mixtures showed decreased toxicity as expected; atrazine-parathion, atrazine-endosulfan, and to a lesser extent atrazine-p-nonylphenol mixtures showed increased toxicity. In summary, exposure to atrazine activates HR96, and induces phase I-III detoxification genes that are likely responsible for mixture interactions.

Agonistic effects of diverse xenobiotics on the constitutive androstane receptor as detected in a recombinant yeast-cell assay

The constitutive androstane receptor (CAR) is a nuclear receptor and transcription factor regulating proteins involved in xenobiotic metabolism. Agonist activation of the CAR can trigger metabolic activation and toxification as well as detoxification and clearance; accordingly, xenobiotic substances acting as CAR ligands may pose a threat to human and animal health. We used yeast cells transduced with the human CAR and the response pathway to measure the CAR-agonistic activities of 549 synthetic or natural compounds: 216 of the tested compounds exhibited CAR-agonistic effects. Eighty-four percent of CAR-activating compounds were aromatic compounds, and >65% of these active compounds were aromatic hydrocarbons, bisphenols, monoalkyl phenols, phthalates, styrene dimers, diphenyl ethers, organochlorines, and organophosphates. The ten most potent compounds were 4-tert-octylphenol (4tOP; reference substance), 4-nonylphenol, diethylstilbestrol, benzyl n-butyl phthalate, 2-(4-hydroxyphenyl)-2,4,4-trimethylchroman, o,p'-DDT, methoxychlor, di-n-propyl phthalate, hexestrol, and octachlorostyrene. The activities of these nine non-reference compounds exceeded 10% of the 4tOP activity. Analysis of para-monoalkyl phenols suggests that branching of the alkyl group and chlorination at the ortho position raises potency. This study provides critical information for identifying the potential of CAR-mediated toxic hazards and for understanding the relevant mechanism.

Authorization and Toxicity of Veterinary Drugs and Plant Protection Products: Residues of the Active Ingredients in Food and Feed and Toxicity Problems Related to Adjuvants

Chemical substances applied in animal husbandry or veterinary medicine and in crop protection represent substantial environmental loads, and their residues occur in food and feed products. Product approval is governed differently in these two sectors in the European Union (EU), and the occurrence of veterinary drug (VD) and pesticide residues indicated by contamination notification cases in the Rapid Alert System for Food and Feed of the EU also show characteristic differences. While the initial high numbers of VD residues reported in 2002 were successfully suppressed to less than 100 cases annually by 2006 and on, the number of notification cases for pesticide residues showed a gradual increase from a low (approximately 50 cases annually) initial level until 2005 to more than 250 cases annually after 2009, with a halt occurring only in 2016. Main notifiers of VD residues include Germany, Belgium, the UK, and Italy (63, 59, 42, and 31 notifications announced, respectively), and main consigning countries of non-compliances are Vietnam, India, China, and Brazil (88, 50, 34, and 23 notifications, respectively). Thus, countries of South and Southeast Asia are considered a vulnerable point with regard to VD residues entering the EU market. Unintended side effects of VDs and plant protection products may be caused not only by the active ingredients but also by various additives in these preparations. Adjuvants (e.g., surfactants) and other co-formulants used in therapeutic agents and feed additives, as well as in pesticide formulations have long been considered as inactive ingredients in the aspects of the required main biological effect of the pharmaceutical or pesticide, and in turn, legal regulations of the approval and marketing of these additives specified significantly less stringent risk assessment requirements, than those specified for the active ingredients. However, numerous studies have shown additive, synergistic, or antagonistic side effects between the active ingredients and their additives in formulated products; moreover, toxicity has been evidenced for various additives. Therefore, toxicological evaluation of surfactants and other additives is essential for proper environmental risk assessment of formulations used in agriculture including animal husbandry and plant protection.

Compensatory changes in CYP expression in three different toxicology mouse models: CAR-null, Cyp3a-null, and Cyp2b9/10/13-null mice

Targeted mutant models are common in mechanistic toxicology experiments investigating the absorption, metabolism, distribution, or elimination (ADME) of chemicals from individuals. Key models include those for xenosensing transcription factors and cytochrome P450s (CYP). Here we investigated changes in transcript levels, protein expression, and steroid hydroxylation of several xenobiotic detoxifying CYPs in constitutive androstane receptor (CAR)-null and two CYP-null mouse models that have subfamily members regulated by CAR; the Cyp3a-null and a newly described Cyp2b9/10/13-null mouse model. Compensatory changes in CYP expression that occur in these models may also occur in polymorphic humans, or may complicate interpretation of ADME studies performed using these models. The loss of CAR causes significant changes in several CYPs probably due to loss of CAR-mediated constitutive regulation of these CYPs. Expression and activity changes include significant repression of Cyp2a and Cyp2b members with corresponding drops in 6α- and 16β-testosterone hydroxylase activity. Further, the ratio of 6α-/15α-hydroxylase activity, a biomarker of sexual dimorphism in the liver, indicates masculinization of female CAR-null mice, suggesting a role for CAR in the regulation of sexually dimorphic liver CYP profiles. The loss of Cyp3a causes fewer changes than CAR. Nevertheless, there are compensatory changes including gender-specific increases in Cyp2a and Cyp2b. Cyp2a and Cyp2b were down-regulated in CAR-null mice, suggesting activation of CAR and potentially PXR following loss of the Cyp3a members. However, the loss of Cyp2b causes few changes in hepatic CYP transcript levels and almost no significant compensatory changes in protein expression or activity with the possible exception of 6α-hydroxylase activity. This lack of a compensatory response in the Cyp2b9/10/13-null mice is probably due to low CYP2B hepatic expression, especially in male mice. Overall, compensatory and regulatory CYP changes followed the order CAR-null > Cyp3a-null > Cyp2b-null mice.

RXRα, PXR and CAR xenobiotic receptors mediate the apoptotic and neurotoxic actions of nonylphenol in mouse hippocampal cells

In the present study, we investigated the role of the retinoid X receptor (RXR), the pregnane X receptor (PXR) and the constitutive androstane receptor (CAR), in the apoptotic and toxic effects of nonylphenol in mouse primary neuronal cell cultures. Our study demonstrated that nonylphenol activated caspase-3 and induced lactate dehydrogenase (LDH) release in hippocampal cells, which was accompanied by an increase in the mRNA expression and protein levels of RXRα, PXR and CAR. Nonylphenol stimulated Rxra, Pxr, and Car mRNA expression. These effects were followed by increase in the protein levels of particular receptors. Immunofluorescence labeling revealed the cellular distribution of RXRα, PXR and CAR in hippocampal neurons in response to nonylphenol, shortening of neurites and cytoplasmic shrinking, as indicated by MAP2 staining. It also showed NP-induced translocation of receptor-specific immunofluorescence from cytoplasm to the nucleus. The use of specific siRNAs demonstrated that Rxra-, Pxr-, and Car-siRNA-transfected cells were less vulnerable to nonylphenol-induced activation of caspase-3 and LDH, thus confirming the key involvement of RXRα/PXR/CAR signaling pathways in the apoptotic and neurotoxic actions of nonylphenol. These new data give prospects for the targeting xenobiotic nuclear receptors to protect the developing nervous system against endocrine disrupting chemicals.

Hormonal and chemical regulation of paraoxonases in mice

In humans and rodents, paraoxonase (PON/Pon) 1 expression and activity in livers and serum are higher in females than in males, and some drugs increase paraoxonase's expression. However, the underlining mechanisms of gender-divergent expression and chemical regulation of Pon1 remain largely unknown. The present study determined the regulatory mechanisms contributing to gender-divergent and chemically altered Pon expression in mouse livers. Pon1 mRNA was much more abundant in the livers of mice than other tissues, with higher levels in female livers than male livers at mRNA and protein levels. Pon2 mRNA was ubiquitously expressed in mouse tissues, but minimally in mouse liver. Pon3 mRNA was most abundant in mouse lung and liver and less abundant in other tissues. Pon1 mRNA was lowest in fetal liver, markedly increased at parturition, and remained relatively constant thereafter. Pon2 and Pon3 mRNA are highly expressed in fetal liver and decreased after birth. Male-pattern growth hormone (GH) administration in hypophysectomized and lit/lit mice decreased Pon1 expression. Sex hormones and female-pattern GH administration had no effect on Pon1 expression, indicating the importance of male-pattern GH in regulating Pon1. Aryl hydrocarbon receptor, pregnane X receptor, and NF-E2-related factor activators had no effect on Pon1 mRNA. A constitutive androstane receptor (CAR) activator decreased Pon1 expression in wild-type but not CAR-null mice. In conclusion, Pon1 mRNA was most abundant in adult mouse livers, whereas Pon2 and Pon3 mRNAs were most abundant in fetal mouse livers. Female-predominant Pon1 expression in mouse livers is caused by the inhibitory effects of male-pattern GH secretion, and CAR activation decreases Pon1 expression.

Daphnia HR96 is a promiscuous xenobiotic and endobiotic nuclear receptor

Daphnia pulex is the first crustacean to have its genome sequenced. The genome project provides new insight and data into how an aquatic crustacean may respond to environmental stressors, including toxicants. We cloned Daphnia pulex HR96 (DappuHR96), a nuclear receptor orthologous to the CAR/PXR/VDR group of nuclear receptors. In Drosophila melanogaster, (hormone receptor 96) HR96 responds to phenobarbital exposure and has been hypothesized as a toxicant receptor. Therefore, we set up a transactivation assay to test whether DappuHR96 is a promiscuous receptor activated by xenobiotics and endobiotics similar to the constitutive androstane receptor (CAR) and the pregnane X-receptor (PXR). Transactivation assays performed with a GAL4-HR96 chimera demonstrate that HR96 is a promiscuous toxicant receptor activated by a diverse set of chemicals such as pesticides, hormones, and fatty acids. Several environmental toxicants activate HR96 including estradiol, pyriproxyfen, chlorpyrifos, atrazine, and methane arsonate. We also observed repression of HR96 activity by chemicals such as triclosan, androstanol, and fluoxetine. Nearly 50% of the chemicals tested activated or inhibited HR96. Interestingly, unsaturated fatty acids were common activators or inhibitors of HR96 activity, indicating a link between diet and toxicant response. The omega-6 and omega-9 unsaturated fatty acids linoleic and oleic acid activated HR96, but the omega-3 unsaturated fatty acids alpha-linolenic acid and docosahexaenoic acid inhibited HR96, suggesting that these two distinct sets of lipids perform opposing roles in Daphnia physiology. This also provides a putative mechanism by which the ratio of dietary unsaturated fats may affect the ability of an organism to respond to a toxic insult. In summary, HR96 is a promiscuous nuclear receptor activated by numerous endo- and xenobiotics.

Lentiviral-mediated RNAi knockdown yields a novel mouse model for studying Cyp2b function

There are few in vivo knockout models available to study the function of Cyp2 members involved in the metabolism of endogenous and exogenous chemicals. These models may help provide insight into the cytochrome P450s (CYPs) responsible for the detoxification and activation of drugs, environmental toxicants, and endobiotics. The aim of this work is to produce a potent Cyp2b-knockdown (KD) mouse for subsequent study of Cyp2b function. We made a quintuple Cyp2b-KD mouse using lentiviral-promoted short hairpin RNA (shRNA) homologous to all five murine Cyp2b subfamily members (Cyp2b9, 2b10, 2b13, 2b19, and 2b23). The Cyp2b-KD mice are viable, fertile, and without obvious gross abnormalities except for an increase in liver weight. Expression of the three hepatic Cyp2b members, 2b9, 2b10, and 2b13, is significantly repressed as demonstrated by quantitative real-time PCR and Western blotting. The constitutive androstane receptor activator, 1,4-Bis[2-(3,5-dichloropyridyloxy)] benzene (TCPOBOP), was used to determine if shRNA-mediated Cyp2b10 repression could be outcompeted by Cyp2b10 induction. TCPOBOP-treated Cyp2b-KD mice show 80-90% less Cyp2b protein expression than TCPOBOP-treated wild-type (WT) mice, demonstrating that Cyp induction does not outcompete the repressive function of the shRNA. Untreated and TCPOBOP-treated Cyp2b-KD mice are poor metabolizers of parathion compared with WT mice. Furthermore, Cyp2b-KD mice are sensitive to parathion, an organophosphate insecticide primarily metabolized by Cyp2b enzymes, when compared with WT mice. In summary, we designed an shRNA construct that repressed the expression and activity of multiple Cyp2b enzymes. We foresee that this novel Cyp2b-KD mouse model will significantly improve our understanding of the role of Cyp2b enzymes in chemical sensitivity and drug metabolism.

Gadd45β is an inducible coactivator of transcription that facilitates rapid liver growth in mice

The growth arrest and DNA damage-inducible 45 (Gadd45) proteins act in many cellular processes. In the liver, Gadd45b (encoding Gadd45β) is the gene most strongly induced early during both compensatory regeneration and drug-induced hyperplasia. The latter response is associated with the dramatic and rapid hepatocyte growth that follows administration of the xenobiotic TCPOBOP (1,4-bis[2-(3,5)-dichoropyridyloxy] benzene), a ligand of the nuclear receptor constitutive androstane receptor (CAR). Here, we have shown that Gadd45b-/- mice have intact proliferative responses following administration of a single dose of TCPOBOP, but marked growth delays. Moreover, early transcriptional stimulation of CAR target genes was weaker in Gadd45b-/- mice than in wild-type animals, and more genes were downregulated. Gadd45β was then found to have a direct role in transcription by physically binding to CAR, and TCPOBOP treatment caused both proteins to localize to a regulatory element for the CAR target gene cytochrome P450 2b10 (Cyp2b10). Further analysis defined separate Gadd45β domains that mediated binding to CAR and transcriptional activation. Although baseline hepatic expression of Gadd45b was broadly comparable to that of other coactivators, its 140-fold stimulation by TCPOBOP was striking and unique. The induction of Gadd45β is therefore a response that facilitates increased transcription, allowing rapid expansion of liver mass for protection against xenobiotic insults.

Ortho-aminoazotoluene activates mouse constitutive androstane receptor (mCAR) and increases expression of mCAR target genes

2'-3-dimethyl-4-aminoazobenzene (ortho-aminoazotoluene, OAT) is an azo dye and a rodent carcinogen that has been evaluated by the International Agency for Research on Cancer (IARC) as a possible (class 2B) human carcinogen. Its mechanism of action remains unclear. We examined the role of the xenobiotic receptor Constitutive Androstane Receptor (CAR, NR1I3) as a mediator of the effects of OAT. We found that OAT increases mouse CAR (mCAR) transactivation in a dose-dependent manner. This effect is specific because another closely related azo dye, 3'-methyl-4-dimethyl-aminoazobenzene (3'MeDAB), did not activate mCAR. Real-time Q-PCR analysis in wild-type C57BL/6 mice revealed that OAT induces the hepatic mRNA expression of the following CAR target genes: Cyp2b10, Cyp2c29, Cyp3a11, Ugt1a1, Mrp4, Mrp2 and c-Myc. CAR-null (Car(-/-)) mice showed no increased expression of these genes following OAT treatment, demonstrating that CAR is required for their OAT dependent induction. The OAT-induced CAR-dependent increase of Cyp2b10 and c-Myc expression was confirmed by Western blotting. Immunohistochemistry analysis of wild-type and Car(-/-) livers showed that OAT did not acutely induce hepatocyte proliferation, but at much later time points showed an unexpected CAR-dependent proliferative response. These studies demonstrate that mCAR is an OAT xenosensor, and indicate that at least some of the biological effects of this compound are mediated by this nuclear receptor.

Endocrine disruptors: from endocrine to metabolic disruption

Synthetic chemicals currently used in a variety of industrial and agricultural applications are leading to widespread contamination of the environment. Even though the intended uses of pesticides, plasticizers, antimicrobials, and flame retardants are beneficial, effects on human health are a global concern. These so-called endocrine-disrupting chemicals (EDCs) can disrupt hormonal balance and result in developmental and reproductive abnormalities. New in vitro, in vivo, and epidemiological studies link human EDC exposure with obesity, metabolic syndrome, and type 2 diabetes. Here we review the main chemical compounds that may contribute to metabolic disruption. We then present their demonstrated or suggested mechanisms of action with respect to nuclear receptor signaling. Finally, we discuss the difficulties of fairly assessing the risks linked to EDC exposure, including developmental exposure, problems of high- and low-dose exposure, and the complexity of current chemical environments.

Nonylphenol-mediated CYP induction is PXR-dependent: The use of humanized mice and human hepatocytes suggests that hPXR is less sensitive than mouse PXR to nonylphenol treatment

Nonylphenol (NP), a by-product of alkylphenol ethoxylates, is a pervasive surfactant that activates the xenosensing nuclear receptor, the pregnane X-receptor (PXR) in transactivation assays in vitro. We are interested in determining if NP activates PXR in vivo, determining if hPXR and mPXR act similarly, and investigating the role of PXR in protecting individuals from NP. Wild-type (WT), PXR-null, and humanized PXR (hPXR) mice were treated with NP at 0, 50 or 75mg/kg/day for one week, and cytochrome P450 (CYP) induction, liver histopathology, and serum NP concentrations were examined. WT mice treated with NP showed induction of Cyp2b, and male-specific induction of Cyp2c and Cyp3a. CYPs were not induced in PXR-null mice, demonstrating that PXR is necessary for NP-mediated CYP induction. CAR-mediated CYP induction was not observed in the PXR-null mice despite previous data demonstrating that NP is also a CAR activator. hPXR mice only showed moderate Cyp induction, suggesting that hPXR is not as sensitive to NP as mPXR in vivo. NP-mediated Cyp3a induction from three human hepatocyte donors was not significant, confirming that hPXR is not very sensitive to NP-mediated CYP induction. Lastly, mice with PXR (mPXR and hPXR) showed lower NP serum concentrations than PXR-null mice treated with NP suggesting that PXR plays a role in decreasing liver toxicity by basally regulating phase I-III detoxification enzymes that promote the metabolism and elimination of NP. In summary, PXR is required for NP-mediated CYP-induction, mPXR mediates greater CYP induction than hPXR in vivo, and the presence of PXR, especially mPXR, is associated with altered histopathology and increased clearance of NP.

Modulation of constitutive androstane receptor (CAR) and pregnane X receptor (PXR) by 6-arylpyrrolo[2,1-d][1,5]benzothiazepine derivatives, ligands of peripheral benzodiazepine receptor (PBR)

Constitutive androstane receptor (CAR) and pregnane X receptor (PXR) regulate xenobiotic sensing and metabolism through interactions with multiple exogenous and endogenous chemicals. Compounds that activate CAR are often ligands of PXR; attention is therefore given to discovery of new, receptor-specific chemical entities that may be exploited for therapeutic and basic research purposes. Recently, ligands of the peripheral benzodiazepine receptor (PBR), PK11195 and FGIN-1-27, were shown to modulate both CAR and PXR. PBR is a mitochondrial transport protein responsible for multiple regulatory functions, including heme biosynthesis, a major component in cytochrome P450 (CYP) enzymes. To investigate possible new roles for PBR involvement in metabolic regulation, expression of the CAR and PXR target genes, CYP2B6 and CYP3A4, was measured in human hepatocytes following treatment with a targeted PBR ligand set. Luciferase reporter assays with transiently expressed wild-type CAR (CAR1), splice variant CAR3, or PXR in HuH-7 cells were used to further study activation of these receptors. Four structurally related PBR ligands (benzothiazepines) differentially modulate CAR1, CAR3 and PXR activity. Benzothiazepine NF49 is an agonist ligand of CAR3, a partial agonist of PXR, exhibits greater inverse agonist activity on CAR1 than does PK11195, and is a new tool for studying these closely related nuclear receptors.

Selective phthalate activation of naturally occurring human constitutive androstane receptor splice variants and the pregnane X receptor

Phthalates and other endocrine-disruptive chemicals are manufactured in large quantities for use as plasticizers and other commercial applications, resulting in ubiquitous human exposure and thus, concern regarding their toxicity. Innate defense against small molecule exposures is controlled in large part by the constitutive androstane receptor (CAR) and the pregnane X receptor (PXR). The human CAR gene undergoes multiple alternative splicing events resulting in the CAR2 and CAR3 variant receptors. Recent studies from our laboratory show that CAR2 is potently and specifically activated by di(2-ethylhexyl) phthalate (DEHP). We hypothesized that alternative splicing is a mechanism for increasing CAR's functional diversity, broadening the human receptors' repertoire of response to environmental xenobiotics. In these studies, we examine the interaction of alternatively spliced CARs and PXR with a range of suspected endocrine disruptors, including phthalates, bisphenol A (BPA), and 4-N-nonylphenol (NP). Transactivation and two-hybrid studies in COS-1 cells revealed differential selectivity of endocrine-disrupting chemicals for the variant CAR and PXR. Ex vivo studies showed DEHP and di-isononyl phthalate potently induced CYP2B6 and CYP3A4 expression in human hepatocytes. Mutation analysis of CAR2, in silico modeling, and ligand docking studies suggested that the SPTV amino acid insertion of CAR2 creates a unique ligand-binding pocket. Alternative gene splicing results in variant CAR receptors that selectively recognize phthalates and BPA. The interaction of phthalates with CAR and PXR suggests a xenobiotic response that is complex and biologically redundant.

Rational quantitative structure-activity relationship (RQSAR) screen for PXR and CAR isoform-specific nuclear receptor ligands

Constitutive androstane receptor (CAR) and pregnane X receptor (PXR) are closely related orphan nuclear receptor proteins that share several ligands and target overlapping sets of genes involved in homeostasis and all phases of drug metabolism. CAR and PXR are involved in the development of certain diseases, including diabetes, metabolic syndrome and obesity. Ligand screens for these receptors so far have typically focused on steroid hormone analogs with pharmacophore-based approaches, only to find relatively few new hits. Multiple CAR isoforms have been detected in human liver, with the most abundant being the constitutively active reference, CAR1, and the ligand-dependent isoform CAR3. It has been assumed that any compound that binds CAR1 should also activate CAR3, and so CAR3 can be used as a ligand-activated surrogate for CAR1 studies. The possibility of CAR3-specific ligands has not, so far, been addressed. To investigate the differences between CAR1, CAR3 and PXR, and to look for more CAR ligands that may be of use in quantitative structure-activity relationship (QSAR) studies, we performed a luciferase transactivation assay screen of 60 mostly non-steroid compounds. Known active compounds with different core chemistries were chosen as starting points and structural variants were rationally selected for screening. Distinct differences in agonist versus inverse agonist/antagonist effects were seen in 49 compounds that had some ligand effect on at least one receptor and 18 that had effects on all three receptors; eight were CAR1 ligands only, three were CAR3 only ligands and four affected PXR only. This work provides evidence for new CAR ligands, some of which have CAR3-specific effects, and provides observational data on CAR and PXR ligands with which to inform in silico strategies. Compounds that demonstrated unique activity on any one receptor are potentially valuable diagnostic tools for the investigation of in vivo molecular targets.

Constitutive androstane receptor -null mice are sensitive to the toxic effects of parathion: association with reduced cytochrome p450-mediated parathion metabolism [corrected]

Constitutive androstane receptor [corrected](CAR) is activated by several chemicals and in turn regulates multiple detoxification genes. Our research demonstrates that parathion is one of the most potent, environmentally relevant CAR activators with an EC(50) of 1.43 microM. Therefore, animal studies were conducted to determine whether CAR was activated by parathion in vivo. Surprisingly, CAR-null mice, but not wild-type (WT) mice, showed significant parathion-induced toxicity. However, parathion did not induce Cyp2b expression, suggesting that parathion is not a CAR activator in vivo, presumably because of its short half-life. CAR expression is also associated with the expression of several drug-metabolizing cytochromes P450 (P450). CAR-null mice demonstrate lower expression of Cyp2b9, Cyp2b10, Cyp2c29, and Cyp3a11 primarily, but not exclusively in males. Therefore, we incubated microsomes from untreated WT and CAR-null mice with parathion in the presence of esterase inhibitors to determine whether CAR-null mice show perturbed P450-mediated parathion metabolism compared with that in WT mice. The metabolism of parathion to paraoxon and p-nitrophenol (PNP) was reduced in CAR-null mice with male CAR-null mice showing reduced production of both paraoxon and PNP, and female CAR-null mice showing reduced production of only PNP. Overall, the data indicate that CAR-null mice metabolize parathion slower than WT mice. These results provide a potential mechanism for increased sensitivity of individuals with lower CAR activity such as newborns to parathion and potentially other chemicals due to decreased metabolic capacity.

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

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

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.

Sexually dimorphic regulation and induction of P450s by the constitutive androstane receptor (CAR)

The constitutive androstane receptor (CAR) is a xenosensing nuclear receptor and regulator of cytochrome P450s (CYPs). However, the role of CAR as a basal regulator of CYP expression nor its role in sexually dimorphic responses have been thoroughly studied. We investigated basal regulation and sexually dimorphic regulation and induction by the potent CAR activator TCPOBOP and the moderate CAR activator Nonylphenol (NP). NP is an environmental estrogen and one of the most commonly found environmental toxicants in Europe and the United States. Previous studies have demonstrated that NP induces several CYPs in a sexually dimorphic manner, however the role of CAR in regulating NP-mediated sexually dimorphic P450 expression and induction has not been elucidated. Therefore, wild-type and CAR-null male and female mice were treated with honey as a carrier, NP, or TCPOBOP and CYP expression monitored by QPCR and Western blotting. CAR basally regulates the expression of Cyp2c29, Cyp2b13, and potentially Cyp2b10 as demonstrated by QPCR. Furthermore, we observed a shift in the testosterone 6alpha/15alpha-hydroxylase ratio in untreated CAR-null female mice to the male pattern, which indicates an alteration in androgen status and suggests a role for androgens as CAR inverse agonists. Xenobiotic-treatments with NP and TCPOBOP induced Cyp2b10, Cyp2c29, and Cyp3a11 in a CAR-mediated fashion; however NP only induced these CYPs in females and TCPOBOP induced these CYPs in both males and females. Interestingly, Cyp2a4, was only induced in wild-type male mice by TCPOBOP suggesting Cyp2a4 induction is not sensitive to CAR-mediated induction in females. Overall, TCPOBOP and NP show similar CYP induction profiles in females, but widely different profiles in males potentially related to lower sensitivity of males to either indirect or moderate CAR activators such as NP. In summary, CAR regulates the basal and chemically inducible expression of several sexually dimorphic xenobiotic metabolizing P450s in a manner that varies depending on the ligand.

A concentration addition model for the activation of the constitutive androstane receptor by xenobiotic mixtures

The effects of contaminants are typically studied in individual exposures; however, environmental exposures are rarely from a single contaminant. Therefore, the study of chemical mixtures is important in determining the effects of xenobiotics. The constitutive androstane receptor (CAR) responds to endobiotics and xenobiotics, and in turn induces detoxification enzymes involved in their elimination. First, we compared several androgens as inverse agonists, including androgens allegedly used by Bay Area Laboratory Co-operative to enhance athletic performance. CAR inverse agonists ranked in order of potency were dihydroandrosterone (DHA) > tetrahydrogestrinone (THG) > androstanol > norbolethone. Therefore, we used DHA as an inverse agonist during transactivation assays. Next, we examined the effects of several pesticides, plasticizers, steroids, and bile acids on CAR activation. Our data demonstrates that several pesticides and plasticizers, including diethylhexylphthalate, nonylphenol, cypermethrin, and chlorpyrifos activate CAR. Both full and partial CAR activators were discovered, and EC(50) values and Hillslopes were determined for use in the concentration addition models. Concentration addition models with and without restraint values to account for partial activators were developed. Measured results from transactivation assays with a mixture of two to five chemicals indicate that the concentration addition model without restraints correctly predicts activity unless all of the chemicals in the mixture are partial activators, and then restraint values be considered. Overall, our data indicates that it is important to consider that we are exposed to a milieu of chemicals, and the efficacy of each individual chemical is not the sole factor in determining CAR's activity in mixture modeling.