Human Constitutive Androstane Receptor (CAR) and Pregnane X Receptor (PXR) Support the Hypertrophic but not the Hyperplastic Response to the Murine Nongenotoxic Hepatocarcinogens Phenobarbital and Chlordane In Vivo

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.

Interaction with YAP underlies the species differences between humans and rodents in CAR-dependent hepatocyte proliferation

Constitutive androstane receptor (CAR), a nuclear receptor predominantly expressed in the liver, is activated by diverse chemicals and induces hepatocyte proliferation and hepatocarcinogenesis in rodents. However, the underlying mechanism responsible for CAR-dependent hepatocyte proliferation remains unclear. Importantly, this phenomenon has not been observed in the human liver. This study aimed to investigate the molecular mechanism underlying CAR-induced hepatocyte proliferation and to explore the species differences in hepatocyte proliferation between humans and rodents. Treatment of mice with the CAR activator TCPOBOP induced hepatocyte proliferation and nuclear accumulation of yes-associated protein (YAP), a known liver cancer inducer. This induction was abolished in CAR-knockout mice. Exogenously expressed YAP in cultured cells was accumulated in the nucleus by the coexpression with mouse CAR but not human CAR. Pull-down analysis of recombinant proteins revealed that mouse CAR interacted with YAP, whereas human CAR did not. Further investigations using YAP deletion mutants identified the WW domain of YAP as essential for interacting with CAR and showed that the PY motif (PPAY) in mouse CAR was crucial for binding to the WW domain, whereas human CAR with its mutated motif (PPAH) failed to interact with YAP. A mouse model harboring the Y150H mutation (PPAY to PPAH) in CAR displayed drastically attenuated TCPOBOP-induced hepatocyte proliferation and nuclear accumulation of YAP. CAR induces the nuclear accumulation of YAP through the PY motif-WW domain interaction to promote hepatocyte proliferation. The absence of this interaction in human CAR contributes to the lack of CAR-dependent hepatocyte proliferation in human livers.

A Review of CYP-Mediated Drug Interactions: Mechanisms and In Vitro Drug-Drug Interaction Assessment

Drug metabolism is a major determinant of drug concentrations in the body. Drug-drug interactions (DDIs) caused by the co-administration of multiple drugs can lead to alteration in the exposure of the victim drug, raising safety or effectiveness concerns. Assessment of the DDI potential starts with in vitro experiments to determine kinetic parameters and identify risks associated with the use of comedication that can inform future clinical studies. The diverse range of experimental models and techniques has significantly contributed to the examination of potential DDIs. Cytochrome P450 (CYP) enzymes are responsible for the biotransformation of many drugs on the market, making them frequently implicated in drug metabolism and DDIs. Consequently, there has been a growing focus on the assessment of DDI risk for CYPs. This review article provides mechanistic insights underlying CYP inhibition/induction and an overview of the in vitro assessment of CYP-mediated DDIs.

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.

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.

Development and Experimental Validation of Regularized Machine Learning Models Detecting New, Structurally Distinct Activators of PXR

The pregnane X receptor (PXR) regulates the metabolism of many xenobiotic and endobiotic substances. In consequence, PXR decreases the efficacy of many small-molecule drugs and induces drug-drug interactions. The prediction of PXR activators with theoretical approaches such as machine learning (ML) proves challenging due to the ligand promiscuity of PXR, which is related to its large and flexible binding pocket. In this work we demonstrate, by the example of random forest models and support vector machines, that classifiers generated following classical training procedures often fail to predict PXR activity for compounds that are dissimilar from those in the training set. We present a novel regularization technique that penalizes the gap between a model’s training and validation performance. On a challenging test set, this technique led to improvements in Matthew correlation coefficients (MCCs) by up to 0.21. Using these regularized ML models, we selected 31 compounds that are structurally distinct from known PXR ligands for experimental validation. Twelve of them were confirmed as active in the cellular PXR ligand-binding domain assembly assay and more hits were identified during follow-up studies. Comprehensive analysis of key features of PXR biology conducted for three representative hits confirmed their ability to activate the PXR.

The role of mouse and human peroxisome proliferator-activated receptor-α in modulating the hepatic effects of perfluorooctane sulfonate in mice

Perfluorooctane sulfonate (PFOS) is a stable environmental contaminant that can activate peroxisome proliferator-activated receptor alpha (PPARα). In the present work, the specific role of mouse and human PPARα in mediating the hepatic effects of PFOS was examined in short-term studies using wild type, Ppara-null and PPARA-humanized mice. Mice fed 0.006 % PFOS for seven days (∼10 mg/kg/day), or 0.003 % PFOS for twenty-eight days (∼5 mg/kg/day), exhibited higher liver and serum PFOS concentrations compared to controls. Relative liver weights were also higher following exposure to dietary PFOS in all three genotypes as compared vehicle fed control groups. Histopathological examination of liver sections from mice treated for twenty-eight days with 0.003 % PFOS revealed a phenotype consistent with peroxisome proliferation, in wild-type and PPARA-humanized mice that was not observed in Ppara-null mice. With both exposures, expression of the PPARα target genes, Acox1, Cyp4a10, was significantly increased in wild type mice but not in Ppara-null or PPARA-humanized mice. By contrast, expression of the constitutive androstane receptor (CAR) target gene, Cyp2b10, and the pregnane X receptor (PXR) target gene, Cyp3a11, were higher in response to PFOS administration in all three genotypes compared to controls for both exposure periods. These results indicate that mouse PPARα can be activated in the liver by PFOS causing increased expression of Acox1, Cyp4a10 and histopathological changes in the liver. While histopathological analyses indicated the presence of mouse PPARα-dependent hepatic peroxisome proliferation in wild-type (a response associated with activation of PPARα) and a similar phenotype in PPARA-humanized mice, the lack of increased Acox1 and Cyp4a10 mRNA by PFOS in PPARA-humanized mice indicates that the human PPARα was not as responsive to PFOS as mouse PPARα with this dose regimen. Moreover, results indicate that hepatomegaly caused by PFOS does not require mouse or human PPARα and could be due to effects induced by activation of CAR and/or PXR.

Off-target lipid metabolism disruption by the mouse constitutive androstane receptor ligand TCPOBOP in humanized mice

The constitutive androstane receptor (CAR) controls xenobiotic clearance, regulates liver glucose, lipid metabolism, and energy homeostasis. These functions have been mainly discovered using the prototypical mouse-specific CAR ligand TCPOBOP in wild-type or CAR null mice. However, TCPOBOP is reported to result in some off-target metabolic effects in CAR null mice. In this study, we compared the metabolic effects of TCPOBOP using lipidomic, transcriptomic, and proteomic analyses in wild-type and humanized CAR-PXR-CYP3A4/3A7 mice. In the model, human CAR retains its constitutive activity in metabolism regulation; however, it is not activated by TCPOBOB. Notably, we observed that TCPOBOP affected lipid homeostasis by elevating serum and liver triglyceride levels and promoted hepatocyte hypertrophy in humanized CAR mice. Hepatic lipidomic analysis revealed a significant accumulation of triglycerides and decrease of its metabolites in humanized CAR mice. RNA-seq analysis has shown divergent gene expression levels in wild-type and humanized CAR mice. Gene expression regulation in humanized mice is mainly involved in lipid metabolic processes and in the PPAR, leptin, thyroid, and circadian clock pathways. In contrast, CAR activation by TCPOBOP in wild-type mice reduced liver and plasma triglyceride levels and induced a typical transcriptomic proliferative response in the liver. In summary, we identified TCPOBOP as a disruptor of lipid metabolism in humanized CAR mice. The divergent effects of TCPOBOP in humanized mice in comparison with the prototypical CAR-mediated response in WT mice warrant the use of appropriate model ligands and humanized animal models during the testing of endocrine disruption and the characterization of adverse outcome pathways.

Suppression of apoptotic signaling in rat hepatocytes by non-dioxin-like polychlorinated biphenyls depends on the receptors CAR and PXR

Non-dioxin-like polychlorinated biphenyls (NDL-PCBs) represent a sub-group of persistent organic pollutants found in food, environmental samples and human and animal tissues. Promotion of pre-neoplastic lesions in rodent liver has been suggested as an indicator for a possible increased risk of liver cancer in humans exposed to NDL-PCBs. In rodent hepatocytes, suppression of DNA damage-triggered apoptosis is a typical mode of action of liver tumor promoters. Here, we report that NDL-PCBs suppress apoptosis in rat hepatocytes treated in culture with an apoptogenic dose of UV light. Suppression became less pronounced when the constitutive androstane receptor (CAR) and/or the pregnane-X-receptor (PXR) where knocked-out using siRNAs, while knocking-out both receptors led to a full reconstitution of apoptosis. In contrast, suppression of apoptosis by the CAR or PXR activators phenobarbital or dexamethasone were CAR- or PXR-specific. Induction and suppression of apoptosis were paralleled by changes in caspase 3/7, 8 and 9 activities. Our findings indicate that NDL-PCBs can suppress UV-induced apoptosis in rat hepatocytes by activating CAR and PXR. It needs further investigation if these mechanisms of action are also of relevance for human liver.

Application of humanized mice to toxicology studies: Evaluation of the human relevance of the mode of action for rodent liver tumor formation by activators of the constitutive androstane receptor (CAR)

The constitutive androstane receptor (CAR)-mediated mode of action (MOA) for phenobarbital (PB)-induced rodent liver tumor formation has been established, with increased hepatocyte proliferation, which is a key event in tumor formation. Previous studies have demonstrated that PB and other CAR-activators stimulate proliferation in cultured rodent hepatocytes, but not in cultured human hepatocytes. However, in the genetically humanized CAR and pregnane X receptor (PXR) mouse (hCAR/hPXR mouse, downstream genes are still mouse), PB increased hepatocyte proliferation and tumor production in vivo. In contrast to the hCAR/hPXR mouse, studies with chimeric mice with human hepatocytes (PXB-mouse, both receptor and downstream genes are human) demonstrated that PB did not increase human hepatocyte proliferation in vivo. PB increased hepatocyte proliferation in a chimeric mouse model with rat hepatocytes, indicating that the lack of human hepatocyte proliferation is not due to any functional defect in the chimeric mouse liver environment. Gene expression analysis demonstrated that the downstream genes of CAR/PXR activation were similar in hCAR/hPXR and CD-1 mice, but differed from those observed in chimeric mice with human hepatocytes. These findings strongly support the conclusion that the MOA for CAR-mediated rodent liver tumor formation is qualitatively implausible for humans. Indeed, epidemiological studies have found no causal link between PB and human liver tumors. There are many similarities with respect to hepatic effects and species differences between rodent CAR and peroxisome proliferator-activated receptor α activators. Based on our research, the chimeric mouse with human hepatocytes (PXB-mouse) is reliable for human cancer risk assessment of test chemicals.

Comparison of the Hepatic Effects of Phenobarbital in Chimeric Mice Containing Either Rat or Human Hepatocytes With Humanized Constitutive Androstane Receptor and Pregnane X Receptor Mice

Using a chimeric mouse humanized liver model, we provided evidence that human hepatocytes are refractory to the mitogenic effects of rodent constitutive androstane receptor (CAR) activators. To evaluate the functional reliability of this model, the present study examined mitogenic responses to phenobarbital (PB) in chimeric mice transplanted with rat hepatocytes, because rats are responsive to CAR activators. Treatment with 1000 ppm PB for 7 days significantly increased replicative DNA synthesis (RDS) in rat hepatocytes of the chimeric mice, demonstrating that the transplanted hepatocyte model is functionally reliable for cell proliferation analysis. Treatment of humanized CAR and pregnane X receptor (PXR) mice (hCAR/hPXR mice) with 1000 ppm PB for 7 days significantly increased hepatocyte RDS together with increases in several mitogenic genes. Global gene expression analysis was performed with liver samples from this and from previous studies focusing on PB-induced Wnt/β-catenin signaling and showed that altered genes in hCAR/hPXR mice clustered most closely with liver tumor samples from a diethylnitrosamine/PB initiation/promotion study than with wild-type mice. However, different gene clusters were observed for chimeric mice with human hepatocytes for Wnt/β-catenin signaling when compared with those of hCAR/hPXR mice, wild-type mice, and liver tumor samples. The results of this study demonstrate clear differences in the effects of PB on hepatocyte RDS and global gene expression between human hepatocytes of chimeric mice and hCAR/hPXR mice, suggesting that the chimeric mouse model is relevant to humans for studies on the hepatic effects of rodent CAR activators whereas the hCAR/hPXR mouse is not.

Splice variants of metabolic nuclear receptors: Relevance for metabolic disease and therapeutic targeting

Metabolic nuclear receptors are ligand-activated transcription factors which control a wide range of metabolic processes and signaling pathways in response to nutrients and xenobiotics. Targeting these NRs is at the forefront of our endeavours to generate novel treatment options for diabetes, metabolic syndrome and fatty liver disease. Numerous splice variants have been described for these metabolic receptors. Structural changes, as a result of alternative splicing, lead to functional differences among NR isoforms, resulting in the regulation of different metabolic pathways by these NR splice variants. In this review, we describe known splice variants of FXR, LXRs, PXR, RXR, LRH-1, CAR and PPARs. We discuss their structure and functions, and elaborate on the regulation of splice variant abundance by nutritional signals. We conclude that NR splice variants pose an intriguing new layer of complexity in metabolic signaling, which needs to be taken into account in the development of treatment strategies for metabolic diseases.

Critical evaluation of the human relevance of the mode of action for rodent liver tumor formation by activators of the constitutive androstane receptor (CAR)

Many nongenotoxic chemicals have been shown to produce liver tumors in mice and/or rats by a mode of action (MOA) involving activation of the constitutive androstane receptor (CAR). Studies with phenobarbital (PB) and other compounds have identified the key events for this MOA: CAR activation; increased hepatocellular proliferation; altered foci formation; and ultimately the development of adenomas/carcinomas. In terms of human relevance, the pivotal species difference is that CAR activators are mitogenic agents in mouse and rat hepatocytes, but they do not stimulate increased hepatocellular proliferation in humans. This conclusion is supported by substantial in vitro studies with cultured rodent and human hepatocytes and also by in vivo studies with chimeric mice with human hepatocytes. Examination of the literature reveals many similarities in the hepatic effects and species differences between activators of rodent CAR and the peroxisome proliferator-activated receptor alpha (PPARα), with PPARα activators also not being mitogenic agents in human hepatocytes. Overall, a critical analysis of the available data demonstrates that the established MOA for rodent liver tumor formation by PB and other CAR activators is qualitatively not plausible for humans. This conclusion is supported by data from several human epidemiology studies.

Role of the constitutive androstane receptor (CAR) in human liver cancer

BACKGROUND

The constitutive androstane receptor (CAR) is a member of the nuclear receptor superfamily (subfamily 1, group I, member 3, also known as NR1I3) that is almost exclusively expressed in the liver. CAR interacts with key signalling pathways such as those involved in drug, energy and bilirubin metabolism. In mouse models, activation of CAR leads to tumorigenesis by inducing pro-proliferative and anti-apoptotic signalling. However, many previous reports have shown species differences between CAR activity in animal models and humans. Recent studies have demonstrated that the mode of action of CAR in rodent liver tumorigenesis is not applicable to humans. Despite this, many studies still continue to study the role of CAR in animal models, hence, there is a need to further explore the role of CAR in human diseases particularly cancers. While there is limited evidence for a role of CAR in human cancers, some studies have proposed a tumour-suppressive role of CAR in liver cancer. In addition, recent studies exploring CAR in human livers demonstrated a hepato-protective role for CAR in and more specifically, its ability to drive differentiation and liver regeneration. This review will discuss the role of CAR in liver cancer, with a focus on species differences and its emerging, tumour-suppressive role in liver cancer and its role in the regulation of liver cancer stem cells.

Diazepam Promotes Translocation of Human Constitutive Androstane Receptor (CAR) via Direct Interaction with the Ligand-Binding Domain

The constitutive androstane receptor (CAR) is the essential regulator of genes involved both in xenobiotic and endobiotic metabolism. Diazepam has been shown as a potent stimulator of CAR nuclear translocation and is assumed as an indirect CAR activator not interacting with the CAR cavity. In this study, we sought to determine if diazepam is a ligand directly interacting with the CAR ligand binding domain (LBD) and if it regulates its target genes in a therapeutically relevant concentration. We used different CAR constructs in translocation and luciferase reporter assays, recombinant CAR-LBD in a TR-FRET assay, and target genes induction studied in primary human hepatocytes (PHHs), HepaRG cells, and in CAR humanized mice. We also used in silico docking and CAR-LBD mutants to characterize the interaction of diazepam and its metabolites with the CAR cavity. Diazepam and its metabolites such as nordazepam, temazepam, and oxazepam are activators of CAR+Ala in translocation and two-hybrid assays and fit the CAR cavity in docking experiments. In gene reporter assays with CAR3 and in the TR-FRET assay, only diazepam significantly interacts with CAR-LBD. Diazepam also promotes up-regulation of CYP2B6 in PHHs and in HepaRG cells. However, in humanized CAR mice, diazepam significantly induces neither CYP2B6 nor Cyp2b10 genes nor does it regulate critical genes involved in glucose and lipids metabolism and liver proliferation. Thus, we demonstrate that diazepam interacts with human CAR-LBD as a weak ligand, but it does not significantly affect expression of tested CAR target genes in CAR humanized mice.

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.

The Connection of Azole Fungicides with Xeno-Sensing Nuclear Receptors, Drug Metabolism and Hepatotoxicity

Azole fungicides, especially triazole compounds, are widely used in agriculture and as pharmaceuticals. For a considerable number of agricultural azole fungicides, the liver has been identified as the main target organ of toxicity. A number of previous studies points towards an important role of nuclear receptors such as the constitutive androstane receptor (CAR), the pregnane-X-receptor (PXR), or the aryl hydrocarbon receptor (AHR), within the molecular pathways leading to hepatotoxicity of these compounds. Nuclear receptor-mediated hepatic effects may comprise rather adaptive changes such as the induction of drug-metabolizing enzymes, to hepatocellular hypertrophy, histopathologically detectable fatty acid changes, proliferation of hepatocytes, and the promotion of liver tumors. Here, we present a comprehensive review of the current knowledge of the interaction of major agricultural azole-class fungicides with the three nuclear receptors CAR, PXR, and AHR in vivo and in vitro. Nuclear receptor activation profiles of the azoles are presented and related to histopathological findings from classic toxicity studies. Important issues such as species differences and multi-receptor agonism and the consequences for data interpretation and risk assessment are discussed.

Nuclear receptor CAR-mediated liver cancer and its species differences

Introduction: The nuclear receptor CAR plays an important role in the regulation of hepatic responses to xenobiotic exposure, including the induction of hepatocyte proliferation and chemical carcinogenesis. Phenobarbital, a well-known liver cancer promoter, has been found to promote hepatocyte proliferation via CAR activation. However, the molecular mechanisms by which CAR induces liver carcinogenesis remain unknown. In addition, it is believed that CAR-mediated liver carcinogenesis shows a species difference; phenobarbital treatment induces hepatocyte proliferation and liver cancer in rodents but not in humans. However, the mechanisms are also unknown.Areas covered: Several reports indicate that the key oncogenic signaling pathways Wnt/β-catenin and Hippo/YAP are involved in CAR-mediated liver carcinogenesis. We introduce current data about the possible molecular mechanisms involved in CAR-mediated liver carcinogenesis and species differences by focusing on these two signaling pathways.Expert opinion: CAR may activate both the Wnt/β-catenin and Hippo/YAP signaling pathways. The synergistic activation of both signaling pathways seems to be important for CAR-mediated liver cancer development. Low homology between the ligand binding domains of human CAR and rodent CAR might cause species differences in the interactions with proteins that control the Wnt/β-catenin and Hippo/YAP pathways as well as liver cancer induction.

Perfluorooctane sulfonate alters gut microbiota-host metabolic homeostasis in mice

Perfluorooctane sulfonate (PFOS) is a persistent environmental chemical whose biological effects are mediated by multiple mechanisms. Recent evidence suggests that the gut microbiome may be directly impacted by and/or alter the fate and effects of environmental chemicals in the host. Thus, the aim of this study was to determine whether PFOS influences the gut microbiome and its metabolism, and the host metabolome. Four groups of male C57BL/6 J mice were fed a diet with or without 0.003 %, 0.006 %, or 0.012 % PFOS, respectively. 16S rRNA gene sequencing, metabolomic, and molecular analyses were used to examine the gut microbiota of mice after dietary PFOS exposure. Dietary PFOS exposure caused a marked change in the gut microbiome compared to controls. Dietary PFOS also caused dose-dependent changes in hepatic metabolic pathways including those involved in lipid metabolism, oxidative stress, inflammation, TCA cycle, glucose, and amino acid metabolism. Changes in the metabolome correlated with changes in genes that regulate these pathways. Integrative analyses also demonstrated a strong correlation between the alterations in microbiota composition and host metabolic profiles induced by PFOS. Further, using isolated mouse cecal contents, PFOS exposure directly affected the gut microbiota metabolism. Results from these studies demonstrate that the molecular and biochemical changes induced by PFOS are mediated in part by the gut microbiome, which alters gene expression and the host metabolome in mice.

Comparative studies on the effects of sodium phenobarbital and two other constitutive androstane receptor (CAR) activators on induction of cytochrome P450 enzymes and replicative DNA synthesis in cultured hepatocytes from wild type and CAR knockout rats

Nongenotoxic chemicals can produce liver tumours in rats and mice by a mitogenic mode of action involving activation of the constitutive androstane receptor (CAR). The aim of this study was to evaluate the usefulness of cultured hepatocytes from normal (wild type; WT) and CAR knockout (KO) rats to screen compounds as potential activators of rat CAR and to validate this test system. Cultured hepatocytes from male Sprague-Dawley WT and CAR KO rats were treated with either 100 and 1000 μM sodium phenobarbital (NaPB), 3-100 μM fluquinconazole (FQZ), or 3-300 μM 3-(difluoromethyl)-1-methyl-N-(3´,4´,6-trifluoro[1,1´-biphenyl]-2-yl)-1H-pyrazole-4-carboxamide (TI1) for 96 h. Induction of cytochrome P450 (CYP) enzymes was monitored by measurement of 7-pentoxyresorufin O-depentylase (PROD), 7-benzyloxyresorufin O-debenzylase (BROD) and 7-benzyloxyquinoline O-debenzylase (BQ) activities. Hepatocytes undergoing replicative DNA synthesis (RDS) were labelled by adding 10 μM 5-bromo-2´-deoxyuridine to the culture medium for determination of the hepatocyte labelling index. The treatment of WT, but not of CAR KO, rat hepatocytes with NaPB, FQZ and TI1 increased hepatocyte RDS and induced CYP2B-dependent PROD activity. In contrast, all three compounds increased CYP2B/3A-dependent BROD and CYP3A-dependent BQ activities in both WT and CAR KO rat hepatocytes. Hepatocyte RDS was increased in both WT and CAR KO rat hepatocytes by treatment with 25 ng/ml epidermal growth factor as a positive control. Overall, these results demonstrate that the effects of three CAR activators on RDS and CYP2B enzyme induction are abolished in cultured CAR KO rat hepatocytes. As demonstrated by this validation study, the CAR KO hepatocyte model is a useful in vitro mechanistic tool for the rapid screening of chemicals as potential activators of rat CAR.

Role of Nuclear Receptors PXR and CAR in Xenobiotic-Induced Hepatocyte Proliferation and Chemical Carcinogenesis

Nuclear receptors pregnane X receptor (PXR) and constitutive active/androstane receptor (CAR) are xenobiotic-responsible transcriptional factors that belong to the same subfamily and are expressed abundantly in the liver. They play crucial roles in various liver functions including xenobiotic disposition and energy metabolism. CAR is also involved in xenobiotic-induced hepatocyte proliferation and hepatocarcinogenesis in rodents. However, there are some open questions on the association between chemical carcinogenesis and these nuclear receptors. These include the molecular mechanism for CAR-mediated hepatocyte proliferation and hepatocarcinogenesis. Another important question is whether PXR is associated with hepatocyte proliferation. We have recently reported a novel and unique function of PXR associated with murine hepatocyte proliferation: PXR activation alone does not induce hepatocyte proliferation but accelerates hepatocyte proliferation induced by various types of stimuli including CAR- or peroxisome proliferator-activated receptor alpha activating compounds, liver injury, and growth factors. We have also reported a role of yes-associated protein (YAP), a transcriptional cofactor controlling organ size and cell growth under the Hippo pathway, in CAR-mediated hepatocyte proliferation in mice. In this review, I will introduce our recent results as well as related studies on the roles of PXR and CAR in xenobiotic-induced hepatocyte proliferation and their molecular mechanisms.

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.

An Extensively Humanized Mouse Model to Predict Pathways of Drug Disposition and Drug/Drug Interactions, and to Facilitate Design of Clinical Trials

Species differences in drug metabolism and disposition can confound the extrapolation of in vivo PK data to man and also profoundly compromise drug efficacy studies owing to differences in pharmacokinetics, in metabolites produced (which are often pharmacologically active), and in differential activation of the transcription factors constitutive androstane receptor (CAR) and pregnane X receptor (PXR), which regulate the expression of such enzymes as P450s and drug transporters. These differences have gained additional importance as a consequence of the use of genetically modified mouse models for drug-efficacy testing and also patient-derived xenografts to predict individual patient responses to anticancer drugs. A number of humanized mouse models for cytochrome P450s, CAR, and PXR have been reported. However, the utility of these models has been compromised by the redundancy in P450 reactions across gene families, whereby the remaining murine P450s can metabolize the compounds being tested. To remove this confounding factor and create a mouse model that more closely reflects human pathways of drug disposition, we substituted 33 murine P450s from the major gene families involved in drug disposition, together with Car and Pxr, for human CAR, PXR, CYP1A1, CYP1A2, CYP2C9, CYP2D6, CYP3A4, and CYP3A7. We also created a mouse line in which 34 P450s were deleted from the mouse genome. Using model compounds and anticancer drugs, we demonstrated how these mouse lines can be applied to predict drug-drug interactions in patients and discuss here their potential application in the more informed design of clinical trials and the personalized treatment of cancer.

Farnesol induces fatty acid oxidation and decreases triglyceride accumulation in steatotic HepaRG cells

Non-alcoholic fatty liver disease is manifested by hepatic accumulation of triglycerides (TG) and is commonly associated with metabolic syndrome. The isoprenoid farnesol (FOH) modulates lipid metabolism and reduces hepatic TG content in rodents. This effect involves activation of at least two nuclear receptors, peroxisome proliferator-activated receptor α (PPARα) and farnesoid X receptor. We evaluated the effects of FOH (100 μM) in a cellular model of human hepatic steatosis by loading hepatocyte-like HepaRG cells with oleic acid (OA, 0.66 mM). FOH treatment decreased OA-induced TG accumulation by ~25%. Using PCR arrays, we found that FOH treatment modulated the mRNA levels of several lipid-metabolizing enzymes, both alone and when cells were loaded with OA. While FOH activated PPARα and the constitutive androstane receptor (CAR), most of the FOH-mediated effects on lipid-metabolizing genes could be attributed to activation of PPARα. In OA-loaded HepaRG cells, FOH increased fatty acid oxidation, which was accompanied by up-regulation of PPARα target genes involved in mitochondrial fatty acid oxidation, including hydroxyacyl-CoA dehydrogenase/3-ketoacyl-CoA thiolase/enoyl-CoA hydratase and acetyl-coenzyme A acyltransferase 2. These effects on gene expression were lost when the cells were co-treated with the PPARα antagonist, GW6471. OA treatment alone decreased the mRNA levels of the drug-metabolizing enzymes, cytochrome P450 (CYP)1A2, 2B6, and 3A4, and increased CYP2E1 expression, all of which were attenuated by FOH co-treatment. These findings show that FOH treatment increases fatty acid oxidation and decreases TG accumulation in steatotic HepaRG cells, which is likely attributable to PPARα-mediated induction of mitochondrial fatty acid oxidation.

In vivo genome-wide binding interactions of mouse and human constitutive androstane receptors reveal novel gene targets

The constitutive androstane receptor (CAR; NR1I3) is a nuclear receptor orchestrating complex roles in cell and systems biology. Species differences in CAR's effector pathways remain poorly understood, including its role in regulating liver tumor promotion. We developed transgenic mouse models to assess genome-wide binding of mouse and human CAR, following receptor activation in liver with direct ligands and with phenobarbital, an indirect CAR activator. Genomic interaction profiles were integrated with transcriptional and biological pathway analyses. Newly identified CAR target genes included Gdf15 and Foxo3, important regulators of the carcinogenic process. Approximately 1000 genes exhibited differential binding interactions between mouse and human CAR, including the proto-oncogenes, Myc and Ikbke, which demonstrated preferential binding by mouse CAR as well as mouse CAR-selective transcriptional enhancement. The ChIP-exo analyses also identified distinct binding motifs for the respective mouse and human receptors. Together, the results provide new insights into the important roles that CAR contributes as a key modulator of numerous signaling pathways in mammalian organisms, presenting a genomic context that specifies species variation in biological processes under CAR's control, including liver cell proliferation and tumor promotion.

Case examples of an evaluation of the human relevance of the pyrethroids/pyrethrins-induced liver tumours in rodents based on the mode of action

Rodent carcinogenicity studies are useful for screening for human carcinogens but they are not perfect. Some modes of action (MOAs) lead to cancers in both experimental rodents and humans, but others that lead to cancers in rodents do not do so in humans. Therefore, analysing the MOAs by which chemicals produce tumours in rodents and determining the relevance of such tumour data for human risk are critical. Recently, experimental data were obtained as case examples of an evaluation of the human relevance of pyrethroid (metofluthrin and momfluorothrin)- and pyrethrins-induced liver tumours in rats based on MOA. The MOA analysis, based on the International Programme on Chemical Safety (IPCS) framework, concluded that experimental data strongly support that the postulated MOA for metofluthrin-, momfluorothrin- and pyrethrins-produced rat hepatocellular tumours is mediated by constitutive androstane receptor (CAR) activation. Since metofluthrin and momfluorothrin are close structural analogues, reproducible outcomes for both chemicals provide confidence in the MOA findings. Furthermore, cultured human hepatocyte studies and humanized chimeric mouse liver studies demonstrated species difference between human hepatocytes (refractory to the mitogenic effects of these compounds) and rat hepatocytes (sensitive to their mitogenic effects). These data strongly support the hypothesis that the CAR-mediated MOA for liver tumorigenesis is of low carcinogenic risk for humans. In this research, in addition to cultured human hepatocyte studies, the usefulness of the humanized chimeric liver mouse models was clearly demonstrated. These data substantially influenced decisions in regulatory toxicology. In this review I comprehensively discuss the human relevance of the CAR-mediated MOA for rodent liver tumorigenesis based on published information, including our recent molecular research on CAR-mediated MOA.

Role of xenobiotics in the induction and progression of fatty liver disease

Non-alcoholic fatty liver disease is a major cause of chronic liver pathology in humans. Fatty liver disease involves the accumulation of hepatocellular fat in hepatocytes that can progress to hepatitis. Steatohepatitis is categorized into alcoholic (ASH) or non-alcoholic (NASH) steatohepatitis based on the etiology of the insult. Both pathologies involve an initial steatosis followed by a progressive inflammation of the liver and eventual hepatic fibrosis (steatohepatitis) and cirrhosis. The involvement of pharmaceuticals and other chemicals in the initiation and progression of fatty liver disease has received increased study. This review will examine not only how xenobiotics initiate hepatic steatosis and steatohepatitis but also how the presence of fatty liver may modify the metabolism and pathologic effects of xenobiotics. The feeding of a high fat diet results in changes in the expression of nuclear receptors that are involved in adaptive and adverse liver effects following xenobiotic exposure. High fat diets also modulate cellular and molecular pathways involved in inflammation, metabolism, oxidative phosphorylation and cell growth. Understanding the role of hepatic steatosis and steatohepatitis on the sequelae of toxic and pathologic changes seen following xenobiotic exposure has importance in defining proper and meaningful human risk characterization of the drugs and other chemical agents.

Human relevance of rodent liver tumour formation by constitutive androstane receptor (CAR) activators

A large number of nongenotoxic chemicals have been shown to increase the incidence of liver tumours in rats and/or mice by a mode of action (MOA) involving activation of the constitutive androstane receptor (CAR). Studies with the model CAR activator phenobarbital (PB) and its sodium salt (sodium phenobarbital; NaPB) have demonstrated that the key and associative events for rat and mouse liver tumour formation include CAR activation, increased hepatocyte replicative DNA synthesis (RDS), induction of cytochrome P450 CYP2B subfamily enzymes, liver hypertrophy, increased altered hepatic foci and hepatocellular adenomas/carcinomas. The key species difference between the rat and mouse compared to humans, is that human hepatocytes are refractory to the mitogenic effects of PB/NaPB and other CAR activators. While PB/NaPB and other CAR activators stimulate RDS in rat and mouse hepatocytes in both in vitro and in vivo studies, such compounds do not stimulate RDS in cultured human hepatocytes and in in vivo studies performed in chimeric mice with humanised livers. In terms of species differences in RDS, unlike the rat and mouse, humans are similar to other species such as the Syrian hamster and guinea pig in being nonresponsive to the mitogenic effects of CAR activators. Overall, the MOA for rat and mouse liver tumour formation by PB/NaPB and other CAR activators is considered qualitatively not plausible for humans. This conclusion is supported by data from a number of epidemiological studies, which demonstrate that chronic treatment with PB does not increase the incidence of liver cancer in humans.

Xenobiotic CAR Activators Induce Dlk1-Dio3 Locus Noncoding RNA Expression in Mouse Liver

Derisking xenobiotic-induced nongenotoxic carcinogenesis (NGC) represents a significant challenge during the safety assessment of chemicals and therapeutic drugs. The identification of robust mechanism-based NGC biomarkers has the potential to enhance cancer hazard identification. We previously demonstrated Constitutive Androstane Receptor (CAR) and WNT signaling-dependent up-regulation of the pluripotency associated Dlk1-Dio3 imprinted gene cluster noncoding RNAs (ncRNAs) in the liver of mice treated with tumor-promoting doses of phenobarbital (PB). Here, we have compared phenotypic, transcriptional ,and proteomic data from wild-type, CAR/PXR double knock-out and CAR/PXR double humanized mice treated with either PB or chlordane, and show that hepatic Dlk1-Dio3 locus long ncRNAs are upregulated in a CAR/PXR-dependent manner by two structurally distinct CAR activators. We further explored the specificity of Dlk1-Dio3 locus ncRNAs as hepatic NGC biomarkers in mice treated with additional compounds working through distinct NGC modes of action. We propose that up-regulation of Dlk1-Dio3 cluster ncRNAs can serve as an early biomarker for CAR activator-induced nongenotoxic hepatocarcinogenesis and thus may contribute to mechanism-based assessments of carcinogenicity risk for chemicals and novel therapeutics.

Comparison of the effects of sodium phenobarbital in wild type and humanized constitutive androstane receptor (CAR)/pregnane X receptor (PXR) mice and in cultured mouse, rat and human hepatocytes

Phenobarbital (PB), a constitutive androstane receptor (CAR) activator, produces liver tumours in rodents by a mitogenic mode of action involving CAR activation. In this study, the hepatic effects of sodium phenobarbital (NaPB) were compared in male C57BL/6J wild type (WT) mice and in humanized mice, where both the mouse CAR and pregnane X receptor (PXR) have been replaced by their human counterparts (hCAR/hPXR mice). Investigations were also performed in cultured male C57BL/6J and CD-1 mouse, male Sprague-Dawley rat and male and female human hepatocytes. The treatment of WT and hCAR/hPXR mice with 186-984 ppm NaPB in the diet for 7 days resulted in increased relative liver weight, hypertrophy and induction of cytochrome P450 (CYP) enzyme activities. Treatment with NaPB also produced dose-dependent increases in hepatocyte replicative DNA synthesis (RDS), with the effect being more marked in WT than in hCAR/hPXR mice. While the treatment of cultured C57BL/6J and CD-1 mouse, Sprague-Dawley rat and human hepatocytes with 100 and/or 1000 μM NaPB for 4 days induced CYP enzyme activities, increased RDS was only observed in mouse and rat hepatocytes. However, as a positive control, epidermal growth factor increased RDS in hepatocytes from all three species. In summary, although human hepatocytes are refractory to the mitogenic effects of NaPB, treatment with NaPB induced RDS in vivo in hCAR/hPXR mice, which is presumably due to the human CAR and PXR receptors operating in a mouse hepatocyte regulatory environment. As the response of the hCAR/hPXR mouse to the CAR activator NaPB differs markedly from that of human hepatocytes, the hCAR/hPXR mouse is thus not a suitable animal model for studies on the hepatic effects of nongenotoxic rodent CAR activators.

Constitutive androstane receptor regulates the intestinal mucosal response to injury

BACKGROUND AND PURPOSE

The pathogenesis of the inflammatory bowel diseases (IBD), comprising Crohn's disease (CD) and ulcerative colitis (UC), involves aberrant interactions between a genetically susceptible individual, their microbiota and environmental factors. Alterations in xenobiotic receptor expression and function are associated with increased risk for IBD. Here, we have assessed the role of the constitutive androstane receptor (CAR), a xenobiotic receptor closely related to the pregnane X receptor, in the regulation of intestinal mucosal homeostasis.

EXPERIMENTAL APPROACH

CAR expression was assessed in intestinal mucosal biopsies obtained from CD and UC patients, and in C57/Bl6 mice exposed to dextran sulphate sodium (DSS; 3.5% w/v in drinking water) to evoke intestinal inflammation and tissue damage. CAR-deficient mice were exposed to DSS and mucosal healing assessed. Modulation of wound healing by CAR was assessed in vitro. The therapeutic potential of CAR activation was evaluated, using 3,3',5,5'-tetrachloro-1,4-bis(pyridyloxy)benzene (TCPOBOP), a selective rodent CAR agonist.

KEY RESULTS

CAR expression was reduced in CD and UC samples, compared with expression in healthy controls. This was reproduced in our DSS studies, where CAR expression was reduced in colitic mice. CAR-deficient mice exhibited reduced healing following DSS exposure. In vitro, CAR activation accelerated intestinal epithelial wound healing by enhancing cell migration. Lastly, treating mice with TCPOBOP, following induction of colitis, enhanced mucosal healing.

CONCLUSION AND IMPLICATIONS

Our results support the notion that xenobiotic sensing is altered during intestinal inflammation, and suggest that CAR activation may prove effective in enhancing mucosal healing in patients with IBD.

RNA Sequencing Reveals Age and Species Differences of Constitutive Androstane Receptor-Targeted Drug-Processing Genes in the Liver

The constitutive androstane receptor (CAR/Nr1i3) is an important xenobiotic-sensing nuclear receptor that is highly expressed in the liver and is well known to have species differences. During development, age-specific activation of CAR may lead to modified pharmacokinetics and toxicokinetics of drugs and environmental chemicals, leading to higher risks for adverse drug reactions in newborns and children. The goal of this study was to systematically investigate the age- and species-specific regulation of various drug-processing genes (DPGs) after neonatal or adult CAR activation in the livers of wild-type, CAR-null, and humanized CAR transgenic mice. At either 5 or 60 days of age, the three genotypes of mice were administered a species-appropriate CAR ligand or vehicle once daily for 4 days (i.p.). The majority of DPGs were differentially regulated by age and/or CAR activation. Thirty-six DPGs were commonly upregulated by CAR activation regardless of age or species of CAR. Although the cumulative mRNAs of uptake transporters were not readily altered by CAR, the cumulative phase I and phase II enzymes as well as efflux transporters were all increased after CAR activation in both species. In general, mouse CAR activation produced comparable or even greater fold increases of many DPGs in newborns than in adults; conversely, humanized CAR activation produced weaker induction in newborns than in adults. Western blotting and enzyme activity assays confirmed the age and species specificities of selected CAR-targeted DPGs. In conclusion, this study systematically compared the effect of age and species of CAR proteins on the regulation of DPGs in the liver and demonstrated that the regulation of xenobiotic biotransformation by CAR is profoundly modified by age and species.

Exposure to PFOA and PFOS and fetal growth: a critical merging of toxicological and epidemiological data

Toxicological and epidemiological evidence on the association between perfluorooctanoic acid (PFOA) or perfluorooctane sulfonic acid (PFOS) and birth/fetal weight was assessed. An extensive search for toxicological information in rats and mice, and a systematic search for epidemiological evidence were conducted. The linear regression coefficient (LRC) of birth weight (BrthW) on PFOA/PFOS was considered, and separate random effects meta-analyses for untransformed (i.e. not mathematically transformed) and log-transformed values were performed. Toxicological evidence: PFOA: 12 studies (21 datasets) in mice showed statistically significant lower birth/fetal weights from 5 mg/kg body weight per day. PFOS: most of the 13 studies (19 datasets) showed lower birth/fetal weights following in utero exposure. Epidemiological evidence: Sixteen articles were considered. The pooled LRC for a 1 ng/mL increase in untransformed PFOA (12 studies) in maternal plasma/serum was -12.8 g (95% CI -23.2; 2.4), and -27.1 g (95% CI -50.6; -3.6) for an increase of 1 log_e ng/mL PFOA (nine studies). The pooled LRC for untransformed PFOS (eight studies) was -0.92 g (95%CI -3.4; 1.6), and for an increase of 1 log_e ng/mL was -46.1(95% CI -80.3; -11.9). No consistent pattern emerged for study location or timing of blood sampling.

CONCLUSIONS

Epidemiological and toxicological evidence suggests that PFOA and PFOS elicit a decrease in BrthW both in humans and rodents. However, the effective animal extrapolated serum concentrations are 10²-10³ times higher than those in humans. Thus, there is no quantitative toxicological evidence to support the epidemiological association, thus reducing the biological plausibility of a causal relationship.

Use of early phenotypic in vivo markers to assess human relevance of an unusual rodent non-genotoxic carcinogen in vitro

Foci of altered hepatocytes (FAH) are considered putative, pre-neoplastic lesions that can occur spontaneously in aging rodents, but can also be induced by chemicals or drugs. Progression of FAH to hepatocellular neoplasms has been reported repeatedly but increases in foci in rodents do not necessarily lead to tumors in carcinogenicity studies and the relevance for humans often remains unclear. Here we present the case of RG3487, a molecule which induced FAH and, later on, tumors in rats. Because the molecule was negative in genotoxicity assays it was classified as a non-genotoxic carcinogen. In order to assess the potential for liver tumor formation in humans, we analyzed treatment-induced changes in vivo to establish a possible mode of action (MoA). In vivo and in vitro gene expression analysis revealed that nuclear receptor signaling was unlikely to be the relevant MoA and no other known mechanism could be established. We therefore took an approach comparing phenotypic markers, including mRNA changes, proliferation and glycogen accumulation, in vitro using cells of different species to assess the human relevance of this finding. Since the alterations observed in rats were not seen in the liver of mice or dogs in vivo, we could validate the relevance of the cell models chosen by use of hepatocytes from these species in vitro. This ultimately allowed for a cross-species comparison, which suggested that the formation of FAH and liver tumors was rat specific and unlikely to translate to human. Our work showed that phenotypic species comparison in vitro is a useful approach for assessment of the human relevance of pre-clinical findings where no known mechanism can be established.

Methyl isobutyl ketone-induced hepatocellular carcinogenesis in B6C3F1 mice: A constitutive androstane receptor (CAR)-mediated mode of action

In a National Toxicology Program (NTP) chronic inhalation study with methyl isobutyl ketone (MIBK), increases in hepatocellular adenomas and hepatocellular adenomas and carcinomas (combined) were observed in male and female B6C3F₁ mice at 1800 ppm. A DNA reactive Mode-of-Action (MOA) for this liver tumor response is not supported by the evidence as MIBK and its major metabolites lack genotoxicity in both in vitro and in vivo studies. Constitutive androstane receptor (CAR) nuclear receptor-mediated activation has been hypothesized as the MOA for MIBK-induced mouse liver tumorigenesis. To further investigate the MOA for MIBK-induced murine liver tumors, male and female B6C3F1, C57BL/6, and CAR/PXR Knockout (KO) mice were exposed to either 0 or 1800 ppm MIBK for 6 h/day, 5 days/week for a total of 10 days. On day 1, mice were implanted with osmotic mini-pumps containing 5-Bromo-2-deoxyuridine (BrdU) 1 h following exposure and humanely euthanized 1-3 h following the final exposure. B6C3F₁ and C57BL/6 mice had statistically significant increases in liver weights compared to controls that corresponded with hepatocellular hypertrophy and increased mitotic figures. Hepatocellular proliferation data indicated induction of S-phase DNA synthesis in B6C3F₁ and C57BL/6 mice exposed to 1800 ppm MIBK compared to control, and no increase was observed in MIBK exposed CAR/PXR KO mice. Liver gene expression changes indicated a maximally-induced Cyp2b10 (CAR-associated) transcript and a slight increase in Cyp3a11(PXR-associated) transcript in B6C3F₁ and C57BL/6 mice exposed to 1800 ppm MIBK compared to controls, but not in Cyp1a1 (AhR-associated) or Cyp4a10 (PPAR-α-associated) transcripts. CAR/PXR KO mice exposed to 1800 ppm MIBK showed no evidence of activation of AhR, CAR, PXR or PPAR-α nuclear receptors via their associated transcripts. MIBK induced hepatic effects are consistent with a phenobarbital-like MOA where the initiating events are activation of the CAR and PXR nuclear receptors and resultant hepatocellular proliferation leading to rodent liver tumors.

Characterizing drug-metabolizing enzymes and transporters that are bona fide CAR-target genes in mouse intestine

Intestine is responsible for the biotransformation of many orally-exposed chemicals. The constitutive androstane receptor (CAR/Nr1i3) is known to up-regulate many genes encoding drug-metabolizing enzymes and transporters (drug-processing genes/DPGs) in liver, but less is known regarding its effect in intestine. Sixty-day-old wild-type and Car^−/− mice were administered the CAR-ligand TCPOBOP or vehicle once daily for 4 days. In wild-type mice, Car mRNA was down-regulated by TCPOBOP in liver and duodenum. Car^−/− mice had altered basal intestinal expression of many DPGs in a section-specific manner. Consistent with the liver data (Aleksunes and Klaassen, 2012), TCPOBOP up-regulated many DPGs (Cyp2b10, Cyp3a11, Aldh1a1, Aldh1a7, Gsta1, Gsta4, Gstm1-m4, Gstt1, Ugt1a1, Ugt2b34, Ugt2b36, and Mrp2–4) in specific sections of small intestine in a CAR-dependent manner. However, the mRNAs of Nqo1 and Papss2 were previously known to be up-regulated by TCPOBOP in liver but were not altered in intestine. Interestingly, many known CAR-target genes were highest expressed in colon where CAR is minimally expressed, suggesting that additional regulators are involved in regulating their expression. In conclusion, CAR regulates the basal expression of many DPGs in intestine, and although many hepatic CAR-targeted DPGs were bona fide CAR-targets in intestine, pharmacological activation of CAR in liver and intestine are not identical.

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.

The regulation of human hepatic drug transporter expression by activation of xenobiotic-sensing nuclear receptors

INTRODUCTION

If a drug is found to be an inducer of hepatic drug metabolizing enzymes via activation of nuclear receptors such as pregnane X receptor (PXR) or constitutive androstane receptor (CAR), it is likely that drug transporters regulated through these same receptors will be induced as well. This review highlights what is currently known about the molecular mechanisms that regulate transporter expression and where the research is directed. Areas covered: This review is focused on publications that describe the role of activated hepatic nuclear receptors in the subsequent regulation of drug uptake and/or efflux transporters following exposure to xenobiotics. Expert opinion: Many of the published studies on the role of nuclear receptors in the regulation of drug transporters involve non-human test animals. But due to species response differences, these associations are not always applicable to humans. For this reason, some relevant human in vitro models have been developed, such as primary or cryopreserved human hepatocytes, human liver slices, or HepG2 or HuH7 cell lines transiently or stably transfected with PXR expression and reporter constructs as well as in vivo models such as PXR-humanized mice. These human-relevant test systems will continue to be developed and applied for the testing of investigational drugs.

Development of an in vitro high content imaging assay for quantitative assessment of CAR-dependent mouse, rat, and human primary hepatocyte proliferation

Rodent liver tumors promoted by constitutive androstane receptor (CAR) activation are known to be mediated by key events that include CAR-dependent gene expression and hepatocellular proliferation. Here, an in vitro high content imaging based assay was developed for quantitative assessment of nascent DNA synthesis in primary hepatocyte cultures from mouse, rat, and human species. Detection of DNA synthesis was performed using direct DNA labeling with the nucleoside analog 5-ethynyl-2'-deoxyuridine (EdU). The assay was multiplexed to enable direct quantitation of DNA synthesis, cytotoxicity, and cell count endpoints. An optimized defined medium cocktail was developed to sensitize hepatocytes to cell cycle progression. The baseline EdU response to defined medium was greatest for mouse, followed by rat, and then human. Hepatocytes from all three species demonstrated CAR activation in response to the CAR agonists TCPOBOP, CITCO, and phenobarbital based on increased gene expression for Cyp2b isoforms. When evaluated for a proliferation phenotype, TCPOBOP and CITCO exhibited significant dose-dependent increases in frequency of EdU labeling in mouse and rat hepatocytes that was not observed in hepatocytes from three human donors. The observed species differences are consistent with CAR activators inducing a proliferative response in rodents, a key event in the liver tumor mode of action that is not observed in humans.

Pregnane X Receptor and Cancer: Context-Specificity is Key

Pregnane X receptor (PXR) is an adopted orphan nuclear receptor that is activated by a wide-range of endobiotics and xenobiotics, including chemotherapy drugs. PXR plays a major role in the metabolism and clearance of xenobiotics and endobiotics in liver and intestine via induction of drug-metabolizing enzymes and drug-transporting proteins. However, PXR is expressed in several cancer tissues and the accumulating evidence strongly points to the differential role of PXR in cancer growth and progression as well as in chemotherapy outcome. In cancer cells, besides regulating the gene expression of enzymes and proteins involved in drug metabolism and transport, PXR also regulates other genes involved in proliferation, metastasis, apoptosis, anti-apoptosis, inflammation, and oxidative stress. In this review, we focus on the differential role of PXR in a variety of cancers, including prostate, breast, ovarian, endometrial, and colon. We also discuss the future directions to further understand the differential role of PXR in cancer, and conclude with the need to identify novel selective PXR modulators to target PXR in PXR-expressing cancers.

Lack of CAR impacts neuronal function and cerebrovascular integrity in vivo

Nuclear receptors (NRs) are a group of transcription factors emerging as players in normal and pathological CNS development. Clinically, an association between the constitutive androstane NR (CAR) and cognitive impairment was proposed, however never experimentally investigated. We wished to test the hypothesis that the impact of CAR on neurophysiology and behavior is underlined by cerebrovascular-neuronal modifications. We have used CAR(-/-) C57BL/6 and wild type mice and performed a battery of behavioral tests (recognition, memory, motor coordination, learning and anxiety) as well as longitudinal video-electroencephalographic recordings (EEG). Brain cell morphology was assessed using 2-photon or electron microscopy and fluorescent immunohistochemistry. We observed recognition memory impairment and increased anxiety-like behavior in CAR(-/-) mice, while locomotor activity was not affected. Concomitantly to memory deficits, EEG monitoring revealed a decrease in 3.5-7Hz waves during the awake/exploration and sleep periods. Behavioral and EEG abnormalities in CAR(-/-) mice mirrored structural changes, including tortuous fronto-parietal penetrating vessels. At the cellular level we found reduced ZO-1, but not CLDN5, tight junction protein expression in cortical and hippocampal isolated microvessel preparations. Interestingly, the neurotoxin kainic acid, when injected peripherally, provoked a rapid onset of generalized convulsions in CAR(-/-) as compared to WT mice, supporting the hypothesis of vascular permeability. The morphological phenotype of CAR(-/-) mice also included some modifications of GFAP/IBA1 glial cells in the parenchymal or adjacent to collagen-IV(+) or FITC(+) microvessels. Neuronal defects were also observed including increased cortical NEUN(+) cell density, hippocampal granule cell dispersion and increased NPY immunoreactivity in the CA1 region in CAR(-/-) mice. The latter may contribute to the in vivo phenotype. Our results indicate that behavioral and electroencephalographic changes in adult CAR(-/-) mice are concomitant to discrete developmental or structural brain defects. The latter could increase the vulnerability to neurotoxins. The possibility that interfering with nuclear receptors during development could contribute to adulthood brain changes is proposed.

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.