Activation of the constitutive androstane receptor inhibits gluconeogenesis without affecting lipogenesis or fatty acid synthesis in human hepatocytes

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.

CAR Protects Females from Diet-Induced Steatosis and Associated Metabolic Disorders

Non-Alcoholic Fatty Liver Disease (NAFLD) is the most common cause of chronic liver disease worldwide, affecting 70-90% of obese individuals. In humans, a lower NAFLD incidence is reported in pre-menopausal women, although the mechanisms affording this protection remain under-investigated. Here, we tested the hypothesis that the constitutive androstane nuclear receptor (CAR) plays a role in the pathogenesis of experimental NAFLD. Male and female wild-type (WT) and CAR knock-out (CAR-/-) mice were subjected to a high-fat diet (HFD) for 16 weeks. We examined the metabolic phenotype of mice through body weight follow-up, glucose tolerance tests, analysis of plasmatic metabolic markers, hepatic lipid accumulation, and hepatic transcriptome. Finally, we examined the potential impact of HFD and CAR deletion on specific brain regions, focusing on glial cells. HFD-induced weight gain and hepatic steatosis are more pronounced in WT males than females. CAR-/- females present a NASH-like hepatic transcriptomic signature suggesting a potential NAFLD to NASH transition. Transcriptomic correlation analysis highlighted a possible cross-talk between CAR and ERα receptors. The peripheral effects of CAR deletion in female mice were associated with astrogliosis in the hypothalamus. These findings prove that nuclear receptor CAR may be a potential mechanism entry-point and a therapeutic target for treating NAFLD/NASH.

The Function of Xenobiotic Receptors in Metabolic Diseases

Metabolic diseases are a series of metabolic disorders that include obesity, diabetes, insulin resistance, hypertension, and hyperlipidemia. The increased prevalence of metabolic diseases has resulted in higher mortality and mobility rates over the past decades, and this has led to extensive research focusing on the underlying mechanisms. Xenobiotic receptors (XRs) are a series of xenobiotic-sensing nuclear receptors that regulate their downstream target genes expression, thus defending the body from xenobiotic and endotoxin attacks. XR activation is associated with the development of a number of metabolic diseases such as obesity, nonalcoholic fatty liver disease, type 2 diabetes, and cardiovascular diseases, thus suggesting an important role for XRs in modulating metabolic diseases. However, the regulatory mechanism of XRs in the context of metabolic disorders under different nutrient conditions is complex and remains controversial. This review summarizes the effects of XRs on different metabolic components (cholesterol, lipids, glucose, and bile acids) in different tissues during metabolic diseases. As chronic inflammation plays a critical role in the initiation and progression of metabolic diseases, we also discuss the impact of XRs on inflammation to comprehensively recognize the role of XRs in metabolic diseases. This will provide new ideas for treating metabolic diseases by targeting XRs. SIGNIFICANCE STATEMENT: This review outlines the current understanding of xenobiotic receptors on nutrient metabolism and inflammation during metabolic diseases. This work also highlights the gaps in this field, which can be used to direct the future investigations on metabolic diseases treatment by targeting xenobiotic receptors.

Mapping transcriptional heterogeneity and metabolic networks in fatty livers at single-cell resolution

Non-alcoholic fatty liver disease is a heterogeneous disease with unclear underlying molecular mechanisms. Here, we perform single-cell RNA sequencing of hepatocytes and hepatic non-parenchymal cells to map the lipid signatures in mice with non-alcoholic fatty liver disease (NAFLD). We uncover previously unidentified clusters of hepatocytes characterized by either high or low srebp1 expression. Surprisingly, the canonical lipid synthesis driver Srebp1 is not predictive of hepatic lipid accumulation, suggestive of other drivers of lipid metabolism. By combining transcriptional data at single-cell resolution with computational network analyses, we find that NAFLD is associated with high constitutive androstane receptor (CAR) expression. Mechanistically, CAR interacts with four functional modules: cholesterol homeostasis, bile acid metabolism, fatty acid metabolism, and estrogen response. Nuclear expression of CAR positively correlates with steatohepatitis in human livers. These findings demonstrate significant cellular differences in lipid signatures and identify functional networks linked to hepatic steatosis in mice and humans.

Human CYP2B6 produces oxylipins from polyunsaturated fatty acids and reduces diet-induced obesity

Multiple factors in addition to over consumption lead to obesity and non-alcoholic fatty liver disease (NAFLD) in the United States and worldwide. CYP2B6 is the only human detoxification CYP whose loss is associated with obesity, and Cyp2b-null mice show greater diet-induced obesity with increased steatosis than wildtype mice. However, a putative mechanism has not been determined. LC-MS/MS revealed that CYP2B6 metabolizes PUFAs, with a preference for metabolism of ALA to 9-HOTrE and to a lesser extent 13-HOTrE with a preference for metabolism of PUFAs at the 9- and 13-positions. To further study the role of CYP2B6 in vivo, humanized-CYP2B6-transgenic (hCYP2B6-Tg) and Cyp2b-null mice were fed a 60% high-fat diet for 16 weeks. Compared to Cyp2b-null mice, hCYP2B6-Tg mice showed reduced weight gain and metabolic disease as measured by glucose tolerance tests, however hCYP2B6-Tg male mice showed increased liver triglycerides. Serum and liver oxylipin metabolite concentrations increased in male hCYP2B6-Tg mice, while only serum oxylipins increased in female hCYP2B6-Tg mice with the greatest increases in LA oxylipins metabolized at the 9 and 13-positions. Several of these oxylipins, specifically 9-HODE, 9-HOTrE, and 13-oxoODE, are PPAR agonists. RNA-seq data also demonstrated sexually dimorphic changes in gene expression related to nuclear receptor signaling, especially CAR > PPAR with qPCR suggesting PPARγ signaling is more likely than PPARα signaling in male mice. Overall, our data indicates that CYP2B6 is an anti-obesity enzyme, but probably to a lesser extent than murine Cyp2b's. Therefore, the inhibition of CYP2B6 by xenobiotics or dietary fats can exacerbate obesity and metabolic disease potentially through disrupted PUFA metabolism and the production of key lipid metabolites.

Clinical Relevance of the Constitutive Androstane Receptor

Constitutive androstane receptor (CAR) (NR1I3), a xenobiotic receptor, has long been considered a master mediator of drug disposition and detoxification. Accumulating evidence indicates that CAR also participates in various physiologic and pathophysiological pathways regulating the homeostasis of glucose, lipid, and bile acids, and contributing to cell proliferation, tissue regeneration and repair, as well as cancer development. The expression and activity of CAR can be regulated by various factors, including small molecular modulators, CAR interaction with other transcription factors, and naturally occurring genetic variants. Given that the influence of CAR has extended beyond the realm of drug metabolism and disposition and has expanded into a potential modulator of human diseases, growing efforts have centered on understanding its clinical relevance and impact on human pathophysiology. This review highlights the current information available regarding the contribution of CAR to various metabolic disorders and cancers and ponders the possible challenges that might arise from pursuing CAR as a potential therapeutic target for these diseases. SIGNIFICANCE STATEMENT: The growing importance of the constitutive androstane receptor (CAR) in glucose and lipid metabolism as well as its potential implication in cell proliferation emphasizes a need to keenly understand the biological function and clinical impact of CAR. This minireview captures the clinical relevance of CAR by highlighting its role in metabolic disorders and cancer development.

In vitro profiling of toxic effects of environmental polycyclic aromatic hydrocarbons on nuclear receptor signaling, disruption of endogenous metabolism and induction of cellular stress

Polycyclic aromatic hydrocarbons (PAHs) may interact with multiple intracellular receptors and related signaling pathways. We comprehensively evaluated the toxicity profiles of six environmentally relevant PAHs differing in structure, genotoxicity and their ability to activate the aryl hydrocarbon receptor (AhR). We focused particularly on their impact on intracellular hormone-, xenobiotic- and lipid-sensing receptors, as well as on cellular stress markers, combining a battery of human reporter gene assays and qRT-PCR evaluation of endogenous gene expression in human hepatocyte-like HepaRG cells, with LC/MS-MS analysis of cellular sphingolipids. The effects of PAHs included: activation of estrogen receptor α (in case of fluoranthene (Fla), pyrene (Pyr), benz[a]anthracene (BaA), benzo[a]pyrene (BaP)), suppression of androgen receptor activity (Fla, BaA, BaP and benzo[k]fluoranthene (BkF)), enhancement of dexamethasone-induced glucocorticoid receptor activity (chrysene (Chry), BaA, and BaP), and potentiation of triiodothyronine-induced thyroid receptor α activity (all tested PAHs). PAHs also induced transcription of endogenous gene targets of constitutive androstane receptor (Fla, Pyr), or repression of target genes of pregnane X receptor and peroxisome proliferator-activated receptor α (in case of the AhR-activating PAHs - Chry, BaA, BaP, and BkF) in HepaRG cells. In the same cell model, the AhR agonists reduced the expression of glucose metabolism genes (PCK1, G6PC and PDK4), and they up-regulated levels of glucosylceramides, together with a concomitant induction of expression of UGCG, glucosylceramide synthesis enzyme. Finally, both BaP and BkF were found to induce expression of early stress and genotoxicity markers: ATF3, EGR1, GDF15, CDKN1A/p21, and GADD45A mRNAs, while BaP alone increased levels of IL-6 mRNA. Overall, whereas low-molecular-weight PAHs exerted significant effects on nuclear receptors (with CYP2B6 induction observed already at nanomolar concentrations), the AhR activation by 4-ring and 5-ring PAHs appeared to be a key mechanism underlying their impact on nuclear receptor signaling, endogenous metabolism and induction of early stress and genotoxicity markers.

Identifying CAR Modulators Utilizing a Reporter Gene Assay

The constitutive androstane receptor (CAR, NR1I3) controls the transcription of numerous hepatic drug metabolizing enzymes and transporters. There are two possible methods of activation for CAR, direct ligand binding and a ligand-independent method, which makes this a unique nuclear receptor. Both mechanisms require the translocation of CAR from the cytoplasm into the nucleus. Interestingly, CAR is constitutively active and spontaneously localized in the nucleus of most immortalized cell lines. This creates an important challenge in most in vitro assay models because immortalized cells cannot be used without inhibiting the high basal activity. In this book chapter, we go into detail of how to perform quantitative high-throughput screens to identify human CAR modulators through the employment of a double stable cell line. Using this line, we can identify activators, as well as deactivators, of the challenging nuclear receptor, CAR.

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.

Negative Regulation of Human Hepatic Constitutive Androstane Receptor by Cholesterol Synthesis Inhibition: Role of Sterol Regulatory Element Binding Proteins

The squalene synthase inhibitor squalestatin 1 (Squal1) is a potent and efficacious inducer of CYP2B expression in primary cultured rat hepatocytes and rat liver. To determine whether Squal1 is also an inducer of human CYP2B, the effects of Squal1 treatment were evaluated in primary cultured human hepatocytes, differentiated HepaRG cells, and humanized mouse livers. Squal1 treatment did not increase CYP2B6 mRNA levels in human hepatocytes or HepaRG cells and only slightly and inconsistently increased CYP2B6 mRNA content in humanized mouse liver. However, treatment with farnesol, which mediates Squal1's effect on rat CYP2B expression, increased CYP2B6 mRNA levels in HepaRG cells expressing the constitutive androstane receptor (CAR), but not in cells with knocked-down CAR. To determine the impact of cholesterol biosynthesis inhibition on CAR activation, the effects of pravastatin (Prava) were determined on CITCO-mediated gene expression in primary cultured human hepatocytes. Prava treatment abolished CITCO-inducible CYP2B6 expression, but had less effect on rifampicin-mediated CYP3A4 induction, and CITCO treatment did not affect Prava-inducible HMG-CoA reductase (HMGCR) expression. Treatment with inhibitors of different steps of cholesterol biosynthesis attenuated CITCO-mediated CYP2B6 induction in HepaRG cells, and Prava treatment increased HMGCR expression and inhibited CYP2B6 induction with comparable potency. Transfection of HepG2 cells with transcriptionally active sterol regulatory element binding proteins (SREBPs) reduced CAR-mediated transactivation, and inducible expression of transcriptionally active SREBP2 attenuated CITCO-inducible CYP2B6 expression in HepaRG cells. These findings suggest that Squal1 does not induce CYP2B6 in human hepatocytes because Squal1's inhibitory effect on cholesterol biosynthesis interferes with CAR activation. SIGNIFICANCE STATEMENT: The cholesterol biosynthesis inhibitor squalestatin 1 induces rat hepatic CYP2B expression indirectly by causing accumulation of an endogenous isoprenoid that activates the constitutive androstane receptor (CAR). This study demonstrates that squalestatin 1 does not similarly induce CYP2B6 expression in human hepatocytes. Rather, inhibition of cholesterol biosynthesis interferes with CAR activity, likely by activating sterol regulatory element binding proteins. These findings increase our understanding of the endogenous processes that modulate human drug-metabolizing gene expression.

Gadd45b is required in part for the anti-obesity effect of constitutive androstane receptor (CAR)

Crosstalk between xenobiotic metabolism and energy metabolism in the liver has provided a potential opportunity to target xenobiotic receptors to treat metabolic diseases. Activation of constitutive androstane receptor (CAR), a xenobiotic-sensing nuclear receptor, has been shown to inhibit obesity, suppress hepatic gluconeogenesis, and ameliorate hyperglycemia in rodent models of obesity and type 2 diabetes. However, the underlying molecular mechanism remains to be defined. The growth arrest and DNA damage-inducible gene 45b (Gadd45b), a well-known anti-apoptotic factor, has been shown to be an inducible coactivator of CAR in promoting rapid liver growth. It is unknown whether the effect of CAR on energy metabolism depends on GADD45B. In the present study and by using a high fat diet (HFD)-induced obesity model, we show that reduced body weight gain and improved insulin sensitivity by the CAR agonist 1,4-bis[2-(3,5-dichloropyridyloxy)] benzene (TCPOBOP) were markedly blunted in Gadd45b knockout mice. Mechanistically, the TCPOBOP-responsive inhibition of hepatic lipogenesis, gluconeogenesis, and adipose inflammation observed in wild type mice were largely abolished in Gadd45b knockout mice. We conclude that Gadd45b is required in part for the metabolic benefits of CAR activation.

Constitutive Androstane Receptor: A Peripheral and a Neurovascular Stress or Environmental Sensor

Xenobiotic nuclear receptors (NR) are intracellular players involved in an increasing number of physiological processes. Examined and characterized in peripheral organs where they govern metabolic, transport and detoxification mechanisms, accumulating data suggest a functional expression of specific NR at the neurovascular unit (NVU). Here, we focus on the Constitutive Androstane Receptor (CAR), expressed in detoxifying organs such as the liver, intestines and kidneys. By direct and indirect activation, CAR is implicated in hepatic detoxification of xenobiotics, environmental contaminants, and endogenous molecules (bilirubin, bile acids). Importantly, CAR participates in physiological stress adaptation responses, hormonal and energy homeostasis due to glucose and lipid sensing. We next analyze the emerging evidence supporting a role of CAR in NVU cells including the blood-brain barrier (BBB), a key vascular interface regulating communications between the brain and the periphery. We address the emerging concept of how CAR may regulate specific P450 cytochromes at the NVU and the associated relevance to brain diseases. A clear understanding of how CAR engages during pathological conditions could enable new mechanistic, and perhaps pharmacological, entry-points within a peripheral-brain axis.

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.

The Role of Xenobiotic Receptors on Hepatic Glycolipid Metabolism

Background:

PXR (Pregnane X Receptor) and CAR (Constitutive Androstane Receptor) are termed as xenobiotic receptors, which are known as core factors in regulation of the transcription of metabolic enzymes and drug transporters. However, accumulating evidence has shown that PXR and CAR exert their effects on energy metabolism through the regulation of gluconeogenesis, lipogenesis and β-oxidation. Therefore, in this review, we are trying to summary recent advances to show how xenobiotic receptors regulate energy metabolism.

Methods:

A structured search of databases has been performed by using focused review topics. According to conceptual framework, the main idea of research literature was summarized and presented.

Results:

For introduction of each receptor, the general introduction and the critical functions in hepatic glucose and lipid metabolism have been included. Recent important studies have shown that CAR acts as a negative regulator of lipogenesis, gluconeogenesis and β -oxidation. PXR activation induces lipogenesis, inhibits gluconeogenesis and inhabits β-oxidation.

Conclusion:

In this review, the importance of xenobiotic receptors in hepatic glucose and lipid metabolism has been confirmed. Therefore, PXR and CAR may become new therapeutic targets for metabolic syndrome, including obesity and diabetes. However, further research is required to promote the clinical application of this new energy metabolism function of xenobiotic receptors.

The Roles of Xenobiotic Receptors: Beyond Chemical Disposition

Over the past 20 years, the ability of the xenobiotic receptors to coordinate an array of drug-metabolizing enzymes and transporters in response to endogenous and exogenous stimuli has been extensively characterized and well documented. The constitutive androstane receptor (CAR) and the pregnane X receptor (PXR) are the xenobiotic receptors that have received the most attention since they regulate the expression of numerous proteins important to drug metabolism and clearance and formulate a central defensive mechanism to protect the body against xenobiotic challenges. However, accumulating evidence has shown that these xenobiotic sensors also control many cellular processes outside of their traditional realms of xenobiotic metabolism and disposition, including physiologic and/or pathophysiologic responses in energy homeostasis, cell proliferation, inflammation, tissue injury and repair, immune response, and cancer development. This review will highlight recent advances in studying the noncanonical functions of xenobiotic receptors with a particular focus placed on the roles of CAR and PXR in energy homeostasis and cancer development.

The Role of PPAR and Its Cross-Talk with CAR and LXR in Obesity and Atherosclerosis

The prevalence of obesity and atherosclerosis has substantially increased worldwide over the past several decades. Peroxisome proliferator-activated receptors (PPARs), as fatty acids sensors, have been therapeutic targets in several human lipid metabolic diseases, such as obesity, atherosclerosis, diabetes, hyperlipidaemia, and non-alcoholic fatty liver disease. Constitutive androstane receptor (CAR) and liver X receptors (LXRs) were also reported as potential therapeutic targets for the treatment of obesity and atherosclerosis, respectively. Further clarification of the internal relationships between these three lipid metabolic nuclear receptors is necessary to enable drug discovery. In this review, we mainly summarized the cross-talk of PPARs-CAR in obesity and PPARs-LXRs in atherosclerosis.

Epidermal Growth Factor Represses Constitutive Androstane Receptor Expression in Primary Human Hepatocytes and Favors Regulation by Pregnane X Receptor

Growth factors have key roles in liver physiology and pathology, particularly by promoting cell proliferation and growth. Recently, it has been shown that in mouse hepatocytes, epidermal growth factor receptor (EGFR) plays a crucial role in the activation of the xenosensor constitutive androstane receptor (CAR) by the antiepileptic drug phenobarbital. Due to the species selectivity of CAR signaling, here we investigated epidermal growth factor (EGF) role in CAR signaling in primary human hepatocytes. Primary human hepatocytes were incubated with CITCO, a human CAR agonist, or with phenobarbital, an indirect CAR activator, in the presence or absence of EGF. CAR-dependent gene expression modulation and PXR involvement in these responses were assessed upon siRNA-based silencing of the genes that encode CAR and PXR. EGF significantly reduced CAR expression and prevented gene induction by CITCO and, to a lower extent, by phenobarbital. In the absence of EGF, phenobarbital and CITCO modulated the expression of 144 and 111 genes, respectively, in primary human hepatocytes. Among these genes, only 15 were regulated by CITCO and one by phenobarbital in a CAR-dependent manner. Conversely, in the presence of EGF, CITCO and phenobarbital modulated gene expression only in a CAR-independent and PXR-dependent manner. Overall, our findings suggest that in primary human hepatocytes, EGF suppresses specifically CAR signaling mainly through transcriptional regulation and drives the xenobiotic response toward a pregnane X receptor (PXR)-mediated mechanism.

Small-molecule modulators of PXR and CAR

Two nuclear receptors, the pregnane X receptor (PXR) and the constitutive androstane receptor (CAR), participate in the xenobiotic detoxification system by regulating the expression of drug-metabolizing enzymes and transporters in order to degrade and excrete foreign chemicals or endogenous metabolites. This review aims to expand the perceived relevance of PXR and CAR beyond their established role as master xenosensors to disease-oriented areas, emphasizing their modulation by small molecules. Structural studies of these receptors have provided much-needed insight into the nature of their binding promiscuity and the important elements that lead to ligand binding. Reports of species- and isoform-selective activation highlight the need for further scrutiny when extrapolating from animal data to humans, as animal models are at the forefront of early drug discovery. This article is part of a Special Issue entitled: Xenobiotic nuclear receptors: New Tricks for An Old Dog, edited by Dr. Wen Xie.

RNA-Seq reveals common and unique PXR- and CAR-target gene signatures in the mouse liver transcriptome

The pregnane X receptor (PXR) and constitutive androstane receptor (CAR) are well-known xenobiotic-sensing nuclear receptors with overlapping functions. However, there lacks a quantitative characterization to distinguish between the PXR and CAR target genes and signaling pathways in the liver. The present study performed a transcriptomic comparison of the PXR- and CAR-targets using RNA-Seq in livers of adult wild-type mice that were treated with the prototypical PXR ligand PCN (200mg/kg, i.p. once daily for 4days in corn oil) or the prototypical CAR ligand TCPOBOP (3mg/kg, i.p., once daily for 4days in corn oil). At the given doses, TCPOBOP differentially regulated many more genes (2125) than PCN (212), and 147 of the same genes were differentially regulated by both chemicals. As expected, the top pathways differentially regulated by both PCN and TCPOBOP were involved in xenobiotic metabolism, and they also up-regulated genes involved in retinoid metabolism, but down-regulated genes involved in inflammation and iron homeostasis. Regarding unique pathways, PXR activation appeared to overlap with the aryl hydrocarbon receptor signaling, whereas CAR activation appeared to overlap with the farnesoid X receptor signaling, acute-phase response, and mitochondrial dysfunction. The mRNAs of differentially regulated drug-processing genes (DPGs) partitioned into three patterns, namely TCPOBOP-induced, PCN-induced, as well as TCPOBOP-suppressed gene clusters. The cumulative mRNAs of the differentially regulated DPGs, phase-I and -II enzymes, as well as efflux transporters were all up-regulated by both PCN and TCPOBOPOP, whereas the cumulative mRNAs of the uptake transporters were down-regulated only by TCPOBOP. The absolute mRNA abundance in control and receptor-activated conditions was examined in each DPG category to predict the contribution of specific DPG genes in the PXR/CAR-mediated pharmacokinetic responses. The preferable differential regulation by TCPOBOP in the entire hepatic transcriptome correlated with a marked change in the expression of many DNA and histone epigenetic modifiers. In conclusion, the present study has revealed known and novel, as well as common and unique targets of PXR and CAR in mouse liver following pharmacological activation using their prototypical ligands. Results from this study will further support the role of these receptors in regulating the homeostasis of xenobiotic and intermediary metabolism in the liver, and aid in distinguishing between PXR and CAR signaling at various physiological and pathophysiological conditions. This article is part of a Special Issue entitled: Xenobiotic nuclear receptors: New Tricks for An Old Dog, edited by Dr. Wen Xie.

Nonsterol Isoprenoids Activate Human Constitutive Androstane Receptor in an Isoform-Selective Manner in Primary Cultured Mouse Hepatocytes

Our laboratory previously reported that accumulation of nonsterol isoprenoids following treatment with the squalene synthase inhibitor, squalestatin 1 (SQ1) markedly induced cytochrome P450 (CYP)2B1 mRNA and reporter activity in primary cultured rat hepatocytes, which was dependent on activation of the constitutive androstane receptor (CAR). The objective of the current study was to evaluate whether isoprenoids likewise activate murine CAR (mCAR) or one or more isoforms of human CAR (hCAR) produced by alternative splicing (SPTV, hCAR2; APYLT, hCAR3). We found that SQ1 significantly induced Cyp2b10 mRNA (∼3.5-fold) in primary hepatocytes isolated from both CAR-wild-type and humanized CAR transgenic mice, whereas the 3-hydroxy-3-methylglutaryl-CoA reductase inhibitor pravastatin had no effect. In the absence of CAR, basal Cyp2b10 mRNA levels were reduced by 28-fold and the effect of SQ1 on Cyp2b10 induction was attenuated. Cotransfection with an expression plasmid for hCAR1, but not hCAR2 or hCAR3, mediated SQ1-induced CYP2B1 and CYP2B6 reporter activation in hepatocytes isolated from CAR-knockout mice. This effect was also observed following treatment with the isoprenoid trans,trans-farnesol. The direct agonist CITCO increased interaction of hCAR1, hCAR2, and hCAR3 with steroid receptor coactivator-1. However, no significant effect on coactivator recruitment was observed with SQ1, suggesting an indirect activation mechanism. Further results from an in vitro ligand binding assay demonstrated that neither farnesol nor other isoprenoids are direct ligands for hCAR1. Collectively, our findings demonstrate that SQ1 activates CYP2B transcriptional responses through farnesol metabolism in an hCAR1-dependent manner. Further, this effect probably occurs through an indirect mechanism.

Quantitative High-Throughput Luciferase Screening in Identifying CAR Modulators

The constitutive androstane receptor (CAR, NR1I3) is responsible for the transcription of multiple drug metabolizing enzymes and transporters. There are two possible methods of activation for CAR, direct ligand binding and a ligand-independent method, which makes this a unique nuclear receptor. Both of these mechanisms require translocation of CAR from the cytoplasm into the nucleus. Interestingly, CAR is constitutively active in immortalized cell lines due to the basal nuclear location of this receptor. This creates an important challenge in most in vitro assay models because immortalized cells cannot be used without inhibiting the high basal activity. In this book chapter, we go into detail of how to perform quantitative high-throughput screens to identify hCAR1 modulators through the employment of a double stable cell line. Using this line, we are able to identify activators, as well as deactivators, of the challenging nuclear receptor, CAR.

Genome-wide analysis of human constitutive androstane receptor (CAR) transcriptome in wild-type and CAR-knockout HepaRG cells

The constitutive androstane receptor (CAR) modulates the transcription of numerous genes involving drug metabolism, energy homeostasis, and cell proliferation. Most functions of CAR however were defined from animal studies. Given the known species difference of CAR and the significant cross-talk between CAR and the pregnane X receptor (PXR), it is extremely difficult to decipher the exact role of human CAR (hCAR) in gene regulation, relying predominantly on pharmacological manipulations. Here, utilizing a newly generated hCAR-knockout (KO) HepaRG cell line, we carried out RNA-seq analysis of the global transcriptomes in wild-type (WT) and hCAR-KO HepaRG cells treated with CITCO, a selective hCAR agonist, phenobarbital (PB), a dual activator of hCAR and hPXR, or vehicle control. Real-time PCR assays in separate experiments were used to validate RNA-seq findings. Our results indicate that genes encoding drug-metabolizing enzymes are among the main clusters altered by both CITCO and PB. Specifically, CITCO significantly changed the expression of 135 genes in an hCAR-dependent manner, while PB altered the expression of 227 genes in WT cells of which 94 were simultaneously modulated in both cell lines reflecting dual effects of PB on hCAR/PXR. Notably, we found that many genes promoting cell proliferation and tumorigenesis were up-regulated in hCAR-KO cells, suggesting that hCAR may play an important role in cell growth that differs from mouse CAR. Together, our results reveal both novel and known targets of hCAR and support the role of hCAR in maintaining the homeostasis of metabolism and cell proliferation in the liver.

Quantitative high-throughput identification of drugs as modulators of human constitutive androstane receptor

The constitutive androstane receptor (CAR, NR1I3) plays a key role in governing the transcription of numerous hepatic genes that involve xenobiotic metabolism/clearance, energy homeostasis, and cell proliferation. Thus, identification of novel human CAR (hCAR) modulators may not only enhance early prediction of drug-drug interactions but also offer potentially novel therapeutics for diseases such as metabolic disorders and cancer. In this study, we have generated a double stable cell line expressing both hCAR and a CYP2B6-driven luciferase reporter for quantitative high-throughput screening (qHTS) of hCAR modulators. Approximately 2800 compounds from the NIH Chemical Genomics Center Pharmaceutical Collection were screened employing both the activation and deactivation modes of the qHTS. Activators (115) and deactivators (152) of hCAR were identified from the primary qHTS, among which 10 agonists and 10 antagonists were further validated in the physiologically relevant human primary hepatocytes for compound-mediated hCAR nuclear translocation and target gene expression. Collectively, our results reveal that hCAR modulators can be efficiently identified through this newly established qHTS assay. Profiling drug collections for hCAR activity would facilitate the prediction of metabolism-based drug-drug interactions, and may lead to the identification of potential novel therapeutics.

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.

Targeting xenobiotic receptors PXR and CAR in human diseases

Nuclear receptors such as the pregnane X receptor (PXR) and constitutive androstane receptor (CAR) are xenobiotic receptors regulating not only drug metabolism and disposition but also various human diseases such as cancer, diabetes, inflammatory disease, metabolic disease and liver diseases, suggesting that PXR and CAR are promising targets for drug discovery. Consequently, there is an urgent need to discover and develop small molecules that target these PXR- and/or CAR-mediated human-disease-related pathways for relevant therapeutic applications. This review proposes approaches to target PXR and CAR, either individually or simultaneously, in the context of various human diseases, taking into consideration the structural differences between PXR and CAR.