The peripheral benzodiazepine receptor ligand 1-(2-chlorophenyl-methylpropyl)-3-isoquinoline-carboxamide is a novel antagonist of human constitutive androstane receptor

The intestinal functions of PXR and CAR

Pregnane X receptor (PXR) and constitutive androstane receptor (CAR) are so-called xenobiotic nuclear receptors that play pivotal roles in xenobiotic metabolism and detoxification. Both receptors, highly expressed in the liver and intestine, also have endobiotic functions by regulating the homeostasis of endogenous chemicals. While their hepatic functions are well-documented, the functional roles of PXR and CAR in the gastrointestinal tract are less understood. This review highlights the intestinal functions of PXR and CAR, focusing on their involvement in colon cancer, host-microbiome interactions, inflammation, and gut barrier integrity. PXR exhibits dual roles in colon cancer, acting either as a tumor suppressor by inducing cell-cycle arrest or as a promoter of cancer aggressiveness through activating the FGF19 signaling. CAR, on the other hand, regulates intestinal barrier integrity and immune responses, particularly in the context of inflammatory bowel disease (IBD). Both PXR and CAR interact with gut microbiota, modulating microbial composition and the production of metabolites, such as indole-3-propionic acid (IPA) that influences the gut barrier function and inflammation. Activation of PXR also mitigates intestinal inflammation by antagonizing the NF-κB signaling, while CAR activation affects bile acid metabolism and T-cell homeostasis. These findings underscore the complex and context-dependent roles of PXR and CAR in the intestinal tracts, offering potential therapeutic targets for gastrointestinal diseases.

Activation of Nuclear Receptor CAR: A Pathway to Delay Aging through Enhanced Capacity for Xenobiotic Resistance

Environmental factors are linked to aging and age-related diseases. Emerging evidence suggests that enhancing body's resistance to xenobiotics might be an anti-aging strategy. The constitutive androstane receptor (CAR) regulates drug-metabolizing enzymes and transporters, coordinating metabolism and immune responses to adapt to stress triggered by exogenous exposure. However, the impact of activating CAR on aging remains unknown. In this study, Caenorhabditis elegans (C. elegans), drug-induced premature aging mice, and senescence accelerated P8 (SAMP8) mice are used as models to explore the effects of CAR activation on lifespan and healthspan, along with the underlying mechanisms. The results showed that hCAR agonist CITCO and mCAR agonist TCPOBOP prolonged the lifespan and healthspan in model organism. The longevity effects of CITCO and TCPOBOP were attenuated in CAR homozygous nhr-8/daf-12 mutant C. elegans as well as CAR-/- mice. In C. elegans, CITCO activated both anti-stress and detoxification genes, and increased the resistance to environmental adversities. Additionally, the lifespan-extending and xenobiotic resistant effects of CITCO might be related to the regulation of age-related pathways. Furthermore, CITCO improved age-related neurodegeneration in C. elegans models. Taken together, the results suggest that the longevity effects of CAR agonists may be related to the enhancement of xenobiotic resistance of animals.

Development of BODIPY FL SNS 032 as a Versatile Probe for Constitutive Androstane Receptor and Multiple Kinases

Human constitutive androstane receptor (hCAR) regulates xenobiotic metabolism. Its large and flexible ligand binding pocket can accommodate structurally diverse compounds. An assay for characterizing the binding of ligands to hCAR is needed but has not been reported. Here, we first discovered the promiscuous kinase inhibitor SNS-032 and its derivative THAL-SNS-032 as binders of hCAR, then developed BODIPY FL SNS 032 (14) as a high-affinity hCAR fluorescent probe (K d: 300 ± 30 nM) in a TR-FRET binding assay and used it to characterize hCAR ligands for their competitive binding activities. BODIPY FL SNS 032 also displayed high binding affinities to multiple kinases, such as hGSK3A (K d: 4.5 ± 0.2 nM), hCDK9/CycT1 (K d: 5.1 ± 0.6 nM), hMAPK15 (K d: 340 ± 20 nM), hCASK (K d: 550 ± 30 nM), and hCAMKK2 (K d: 530 ± 40 nM). BODIPY FL SNS 032 is therefore a versatile probe for hCAR and multiple kinases.

Diindoles produced from commensal microbiota metabolites function as endogenous CAR/Nr1i3 ligands

Numerous studies have demonstrated the correlation between human gut bacteria and host physiology, mediated primarily via nuclear receptors (NRs). Despite this body of work, the systematic identification and characterization of microbe-derived ligands that regulate NRs remain a considerable challenge. In this study, we discover a series of diindole molecules produced from commensal bacteria metabolites that act as specific agonists for the orphan constitutive androstane receptor (CAR). Using various biophysical analyses we show that their nanomolar affinities are comparable to those of synthetic CAR agonists, and that they can activate both rodent and human CAR orthologues, which established synthetic agonists cannot. We also find that the diindoles, diindolylmethane (DIM) and diindolylethane (DIE) selectively up-regulate bona fide CAR target genes in primary human hepatocytes and mouse liver without causing significant side effects. These findings provide new insights into the complex interplay between the gut microbiome and host physiology, as well as new tools for disease treatment.

2-Substituted quinazolines: Partial agonistic and antagonistic ligands of the constitutive androstane receptor (CAR)

Following the discovery of 2-(3-methoxyphenyl)-3,4-dihydroquinazoline-4-one and 2-(3-methoxyphenyl)quinazoline-4-thione as potent, but non-specific activators of the human Constitutive Androstane Receptor (CAR, NR1I3), a series of quinazolinones substituted at the C2 phenyl ring was prepared to examine their ability to selectively modulate human CAR activity. Employing cellular and in vitro TR-FRET assays with wild-type CAR or its variant 3 (CAR3) ligand binding domains (LBD), several novel partial human CAR agonists and antagonists were identified. 2-(3-Methylphenyl) quinazolinone derivatives 7d and 8d acted as partial agonists with the recombinant CAR LBD, the former in nanomolar units (EC50 = 0.055 μM and 10.6 μM, respectively). Moreover, 7d did not activate PXR, and did not show any signs of cytotoxicity. On the other hand, 2-(4-bromophenyl)quinazoline-4-thione 7l possessed significant CAR antagonistic activity, although the compound displayed no agonistic or inverse agonistic activities. A compound possessing purely antagonistic effect was thus identified for the first time. These and related compounds may serve as a remedy in xenobiotic intoxication or, conversely, in suppression of undesirable hepatic CAR activation.

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.

Mitochondrial translocator protein deficiency exacerbates pathology in acute experimental ulcerative colitis

In human patients and animal models of ulcerative colitis (UC), upregulation of the mitochondrial translocator protein (TSPO) in the colon is consistent with inflammation. Although the molecular function for TSPO remains unclear, it has been investigated as a therapeutic target for ameliorating UC pathology. In this study, we examined the susceptibility of Tspo gene-deleted (Tspo ^-/- ) mice to insults as provided by the dextran sodium sulfate (DSS)-induced acute UC model. Our results show that UC clinical signs and pathology were severely exacerbated in Tspo ^-/- mice compared to control Tspo ^fl/fl cohorts. Histopathology showed extensive inflammation and epithelial loss in Tspo ^-/- mice that caused an aggravated disease. Colonic gene expression in UC uncovered an etiology linked to precipitous loss of epithelial integrity and disproportionate mast cell activation assessed by tryptase levels in Tspo ^-/- colons. Evaluation of baseline homeostatic shifts in Tspo ^-/- colons revealed gene expression changes noted in elevated epithelial Cdx2, mast cell Cd36 and Mcp6, with general indicators of lower proliferation capacity and elevated mitochondrial fatty acid oxidation. These findings demonstrate that intact physiological TSPO function serves to limit inflammation in acute UC, and provide a systemic basis for investigating TSPO-targeting mechanistic therapeutics.

Targeting CAR and Nrf2 improves cyclophosphamide bioactivation while reducing doxorubicin-induced cardiotoxicity in triple-negative breast cancer treatment

Cyclophosphamide (CPA) and doxorubicin (DOX) are key components of chemotherapy for triple-negative breast cancer (TNBC), although suboptimal outcomes are commonly associated with drug resistance and/or intolerable side effects. Through an approach combining high-throughput screening and chemical modification, we developed CN06 as a dual activator of the constitutive androstane receptor (CAR) and nuclear factor erythroid 2-related factor 2 (Nrf2). CN06 enhances CAR-induced bioactivation of CPA (a prodrug) by provoking hepatic expression of CYP2B6, while repressing DOX-induced cytotoxicity in cardiomyocytes in vitro via stimulating Nrf2-antioxidant signaling. Utilizing a multicellular coculture model incorporating human primary hepatocytes, TNBC cells, and cardiomyocytes, we show that CN06 increased CPA/DOX-mediated TNBC cell death via CAR-dependent CYP2B6 induction and subsequent conversion of CPA to its active metabolite 4-hydroxy-CPA, while protecting against DOX-induced cardiotoxicity by selectively activating Nrf2-antioxidant signaling in cardiomyocytes but not in TNBC cells. Furthermore, CN06 preserves the viability and function of human iPSC-derived cardiomyocytes by modulating antioxidant defenses, decreasing apoptosis, and enhancing the kinetics of contraction and relaxation. Collectively, our findings identify CAR and Nrf2 as potentially novel combined therapeutic targets whereby CN06 holds the potential to improve the efficacy/toxicity ratio of CPA/DOX-containing chemotherapy.

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.

Phenobarbital Induces SLC13A5 Expression through Activation of PXR but Not CAR in Human Primary Hepatocytes

Phenobarbital (PB), a widely used antiepileptic drug, is known to upregulate the expression of numerous drug-metabolizing enzymes and transporters in the liver primarily via activation of the constitutive androstane receptor (CAR, NR1I3). The solute carrier family 13 member 5 (SLC13A5), a sodium-coupled citrate transporter, plays an important role in intracellular citrate homeostasis that is associated with a number of metabolic syndromes and neurological disorders. Here, we show that PB markedly elevates the expression of SLC13A5 through a pregnane X receptor (PXR)-dependent but CAR-independent signaling pathway. In human primary hepatocytes, the mRNA and protein expression of SLC13A5 was robustly induced by PB treatment, while genetic knockdown or pharmacological inhibition of PXR significantly attenuated this induction. Utilizing genetically modified HepaRG cells, we found that PB induces SLC13A5 expression in both wild type and CAR-knockout HepaRG cells, whereas such induction was fully abolished in the PXR-knockout HepaRG cells. Mechanistically, we identified and functionally characterized three enhancer modules located upstream from the transcription start site or introns of the SLC13A5 gene that are associated with the regulation of PXR-mediated SLC13A5 induction. Moreover, metformin, a deactivator of PXR, dramatically suppressed PB-mediated induction of hepatic SLC13A5 as well as its activation of the SLC13A5 luciferase reporter activity via PXR. Collectively, these data reveal PB as a potent inducer of SLC13A5 through the activation of PXR but not CAR in human primary hepatocytes.

Sphingolipid metabolism as a marker of hepatotoxicity in drug-induced liver injury

Drug-induced liver injury (DILI) has a substantial impact on human health and is a major monetary burden on the drug development process. Presently, there is a lack of robust and analytically validated markers for predicting and early diagnosis of DILI. Sphingolipid metabolism and subsequent disruption of sphingolipid homeostasis has been documented to play a key role contributing to hepatocellular death and subsequent liver injury. A more comprehensive understanding of sphingolipid metabolism in response to liver toxicity has great potential to gain mechanistic insight into hepatotoxicity and define molecular markers that are responsible for hepatocyte dysfunction. Here, we present an analytical platform that provides multidimensional mass spectrometry-based datasets for comprehensive structure characterization of sphingolipids extracted from human primary hepatocytes (HPH) exposed to toxic levels of acetaminophen (APAP). Sphingolipid metabolism as measured by characterization of individual sphingolipid structure was sensitive to APAP toxicity displaying a concentration-dependent response. A number of sphingolipid structures were differentially expressed across varying APAP exposures highlighting the unique role sphingolipid metabolism has in response to hepatotoxicity and its potential use as a molecular marker in DILI.

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.

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.

Translocator protein (TSPO): the new story of the old protein in neuroinflammation

Translocator protein (TSPO), also known as peripheral benzodiazepine receptor, is a transmembrane protein located on the outer mitochondria membrane (OMM) and mainly expressed in glial cells in the brain. Because of the close correlation of its expression level with neuropathology and therapeutic efficacies of several TSPO binding ligands under many neurological conditions, TSPO has been regarded as both biomarker and therapeutic target, and the biological functions of TSPO have been a major research focus. However, recent genetic studies with animal and cellular models revealed unexpected results contrary to the anticipated biological importance of TSPO and cast doubt on the action modes of the TSPO-binding drugs. In this review, we summarize recent controversial findings on the discrepancy between pharmacological and genetic studies of TSPO and suggest some future direction to understand this old and mysterious protein.

Metformin Disrupts Bile Acid Efflux by Repressing Bile Salt Export Pump Expression

PURPOSE

The bile salt export pump (BSEP), a key player in hepatic bile acid clearance, has been the center of research on drug-induced cholestasis. However, such studies focus primarily on the direct inhibition of BSEP, often overlooking the potential impact of transcriptional repression. This work aims to explore the disruption of bile acid efflux caused by drug-induced BSEP repression.

METHODS

BSEP activity was analyzed in human primary hepatocytes (HPH) using a traditional biliary-clearance experiment and a modified efflux assay, which includes a 72-h pretreatment prior to efflux measurement. Relative mRNA and protein expressions were examined by RT-PCR and Western blotting, respectively.

RESULTS

Metformin concentration-dependently repressed BSEP expression in HPH. Although metformin did not directly inhibit BSEP activity, longer metformin exposure reduced BSEP transport function in HPH by down-regulating BSEP expression. BSEP repression by metformin was found to be AMP-activated protein kinase-independent. Additional screening of 10 reported cholestatic non-BSEP inhibitors revealed that the anti-cancer drug tamoxifen also markedly repressed BSEP expression and reduced BSEP activity in HPH.

CONCLUSIONS

Repression of BSEP alone is sufficient to disrupt hepatic bile acid efflux. Metformin and tamoxifen appear to be prototypes of a class of BSEP repressors that may cause drug-induced cholestasis through gene repression instead of direct BSEP inhibition.

Cellular Lysosomes' Activity for Melanin Reduction on Artificial Skin Tissue

All eukaryotes have lysosomes which contain hydrolytic enzymes such as protease to degrade waste materials and cellular fragments. As a cellular organelle, lysosomes function as the digestive system of the cell, serving both to degrade material taken up from outside the cell and to digest obsolete components of the cell itself. Conversely, melanin has photochemical functions to protect tissue from the harmful effects of ultraviolet rays. However, too much of melanin leads to problems such as hyperpigmentation, requiring materials to maintain and control the amount of melanin. In this study, we found evidence of correlation between lysosome and melanin in a new eco-friendly material, MelanoDerm, a reconstituted 3D human skin model containing normal melanocytes and keratinocytes. Melanin content assay and cell viability were measured, using 2% kojic acid as positive control, while MelanoDerm was exposed to various concentrations of lysosome. Our results indicate that lysosome may be a useful cosmetic agent for the treatment of hyperpigmentation.

Mechanistic Insights of Phenobarbital-Mediated Activation of Human but Not Mouse Pregnane X Receptor

Phenobarbital (PB), a broadly used antiseizure drug, was the first to be characterized as an inducer of cytochrome P450 by activation of the constitutive androstane receptor (CAR). Although PB is recognized as a conserved CAR activator among species via a well-documented indirect activation mechanism, conflicting results have been reported regarding PB regulation of the pregnane X receptor (PXR), a sister receptor of CAR, and the underlying mechanisms remain elusive. Here, we show that in a human CAR (hCAR)-knockout (KO) HepaRG cell line, PB significantly induces the expression of CYP2B6 and CYP3A4, two shared target genes of hCAR and human PXR (hPXR). In human primary hepatocytes and hCAR-KO HepaRG cells, PB-induced expression of CYP3A4 was markedly repressed by genetic knockdown or pharmacological inhibition of hPXR. Mechanistically, PB concentration dependently activates hPXR but not its mouse counterpart in cell-based luciferase assays. Mammalian two-hybrid assays demonstrated that PB selectively increases the functional interaction between the steroid receptor coactivator-1 and hPXR but not mouse PXR. Moreover, surface plasmon resonance binding affinity assay showed that PB directly binds to the ligand binding domain of hPXR (KD = 1.42 × 10-05). Structure-activity analysis further revealed that the amino acid tryptophan-299 within the ligand binding pocket of hPXR plays a key role in the agonistic binding of PB and mutation of tryptophan-299 disrupts PB activation of hPXR. Collectively, these data reveal that PB, a selective mouse CAR activator, activates both hCAR and hPXR, and provide novel mechanistic insights for PB-mediated activation of hPXR.

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

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

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

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

Drug discovery technologies to identify and characterize modulators of the pregnane X receptor and the constitutive androstane receptor

The pregnane X receptor (PXR) and the constitutive androstane receptor (CAR) are ligand-activated nuclear receptors (NRs) that are notorious for their role in drug metabolism, causing unintended drug-drug interactions and decreasing drug efficacy. They control the xenobiotic detoxification system by regulating the expression of an array of drug-metabolizing enzymes and transporters that excrete exogenous chemicals and maintain homeostasis of endogenous metabolites. Much effort has been invested in recognizing potential drugs for clinical use that can activate PXR and CAR to enhance the expression of their target genes, and in identifying PXR and CAR inhibitors that can be used as co-therapeutics to prevent adverse effects. Here, we present current technologies and assays used in the quest to characterize PXR and CAR modulators, which range from biochemical to cell-based and animal models.

Identification of Modulators That Activate the Constitutive Androstane Receptor From the Tox21 10K Compound Library

The constitutive androstane receptor (CAR; NR1I3) is a nuclear receptor involved in all phases of drug metabolism and disposition. However, recently it's been implicated in energy metabolism, tumor progression, and cancer therapy as well. It is, therefore, important to identify compounds that induce human CAR (hCAR) activation to predict drug-drug interactions and potential therapeutic usage. In this study, we screen the Tox21 10,000 compound collection to characterize hCAR activators. A potential novel structural cluster of compounds was identified, which included nitazoxanide and tenonitrozole, whereas known structural clusters, such as flavones and prazoles, were also detected. Four compounds, neticonazole, diphenamid, phenothrin, and rimcazole, have been identified as novel hCAR activators, one of which, rimcazole, shows potential selectivity toward hCAR over its sister receptor, the pregnane X receptor (PXR). All 4 compounds translocated hCAR from the cytoplasm into the nucleus demonstrating the first step to CAR activation. Profiling these compounds as hCAR activators would enable an estimation of drug-drug interactions, as well as identify prospective therapeutically beneficial drugs.

Canvass: A Crowd-Sourced, Natural-Product Screening Library for Exploring Biological Space

Natural products and their derivatives continue to be wellsprings of nascent therapeutic potential. However, many laboratories have limited resources for biological evaluation, leaving their previously isolated or synthesized compounds largely or completely untested. To address this issue, the Canvass library of natural products was assembled, in collaboration with academic and industry researchers, for quantitative high-throughput screening (qHTS) across a diverse set of cell-based and biochemical assays. Characterization of the library in terms of physicochemical properties, structural diversity, and similarity to compounds in publicly available libraries indicates that the Canvass library contains many structural elements in common with approved drugs. The assay data generated were analyzed using a variety of quality control metrics, and the resultant assay profiles were explored using statistical methods, such as clustering and compound promiscuity analyses. Individual compounds were then sorted by structural class and activity profiles. Differential behavior based on these classifications, as well as noteworthy activities, are outlined herein. One such highlight is the activity of (-)-2(S)-cathafoline, which was found to stabilize calcium levels in the endoplasmic reticulum. The workflow described here illustrates a pilot effort to broadly survey the biological potential of natural products by utilizing the power of automation and high-throughput screening.

Profiling of bisphenol A and eight its analogues on transcriptional activity via human nuclear receptors

Several bisphenol A (BPA) analogues have been detected in environmental samples, foodstuffs, and/or human biological samples, and there is concern regarding their potential endocrine-disrupting effects. In this study, we characterized the agonistic and/or antagonistic activities of BPA and eight its analogues against human estrogen receptors (ERα/β), androgen receptor (AR), glucocorticoid receptor (GR), pregnane X receptor (PXR), and constitutive androstane receptor (CAR). All the test compounds, except for bisphenol P (BPP), showed both ERα and ERβ agonistic activities, with bisphenol AF (BPAF) being the most potent. On the other hand, BPAF and BPP showed ERα and ERβ antagonistic activities. Interestingly, their ER activities demonstrated a preference toward ERβ. All the test compounds, except for bisphenol S, showed AR antagonistic activities, with bisphenol E being the most potent. Weak GR antagonistic activities were also found in BPA and five its analogues. PXR agonistic activity was observed in the six compounds, with bisphenol Z being the most potent. Results of the CAR assay revealed that BPA and five its analogues acted as CAR inverse agonists. Taken together, these results suggested that BPA analogues demonstrate multiple effects via human nuclear receptors in a similar manner to BPA, and several analogues might have more potent endocrine-disrupting activity than does BPA.

CINPA1 binds directly to constitutive androstane receptor and inhibits its activity

The constitutive androstane receptor (CAR) and pregnane X receptor (PXR) are xenobiotic sensors that regulate the expression of drug-metabolizing enzymes and efflux transporters. CAR activation promotes drug elimination, thereby reducing therapeutic effectiveness, or causes adverse drug effects via toxic metabolites. CAR inhibitors could be used to attenuate these adverse drug effects. CAR and PXR share ligands and target genes, confounding the understanding of the regulation of receptor-specific activity. We previously identified a small-molecule inhibitor, CINPA1, that inhibits CAR (without activating PXR at lower concentrations) by altering CAR-coregulator interactions and reducing CAR recruitment to DNA response elements of regulated genes. However, solid evidence was not presented for the direct binding of CINPA1 to CAR. In this study, we demonstrate direct interaction of CINPA1 with the CAR ligand-binding domain (CAR-LBD) and identify key residues involved in such interactions through a combination of biophysical and computational methods. We found that CINPA1 resides in the ligand-binding pocket to stabilize the CAR-LBD in a more rigid, less fluid state. Molecular dynamics simulations, together with our previously reported docking model, enabled us to predict which CAR residues were critical for interactions with CINPA1. The importance of these residues for CINPA1 binding were then validated by directed mutations and testing the mutant CAR proteins in transcription reporter and coregulatory interaction assays. We demonstrated strong hydrogen bonding of CINPA1 with N165 and H203 and identified other residues involved in hydrophobic contacts with CINPA1. Overall, our data confirm that CINPA1 directly binds to CAR.

Using TR-FRET to Investigate Protein-Protein Interactions: A Case Study of PXR-Coregulator Interaction

Time-resolved fluorescence resonance energy transfer (TR-FRET) protein-protein interaction assays, especially in the format of receptor coregulator (coactivator and corepressor) recruitment/repression assays, have been widely used in nuclear receptor research to characterize the modes of action, efficacies, and binding affinities of ligands (including their properties as agonists, antagonists, and inverse agonists). However, there has been only limited progress in using this assay format for pregnane X receptor (PXR). In this chapter, we discuss TR-FRET protein-protein interaction assays and focus on a novel PXR TR-FRET coactivator interaction assay that we have developed based on a PXR coactivator cocrystal study. This new PXR TR-FRET coactivator interaction assay can characterize the binding affinities of PXR ligands and also differentiate antagonists from agonists. This assay is very robust, with the signal remaining stable over a long incubation time (up to 300min has been tested). It can tolerate high concentrations of DMSO (up to 5%) and has a high signal-to-noise ratio (six under typical assay conditions). This newly developed PXR TR-FRET coactivator interaction assay has potential application in high-throughput screening to identify and characterize novel PXR agonists and antagonists.

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

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

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

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

Current status and future perspectives: TSPO in steroid neuroendocrinology

The mitochondrial translocator protein (TSPO), previously known as the peripheral benzodiazepine receptor (PBR), has received significant attention both as a diagnostic biomarker and as a therapeutic target for different neuronal disease pathologies. Recently, its functional basis believed to be mediating mitochondrial cholesterol import for steroid hormone production has been refuted by studies examining both in vivo and in vitro genetic Tspo-deficient models. As a result, there now exists a fundamental gap in the understanding of TSPO function in the nervous system, and its putative pharmacology in neurosteroid production. In this review, we discuss several recent findings in steroidogenic cells that are in direct contradiction to previous studies, and necessitate a re-examination of the purported role for TSPO in de novo neurosteroid biosynthesis. We critically examine the pharmacological effects of different TSPO-binding drugs with particular focus on studies that measure neurosteroid levels. We highlight the basis of key misconceptions regarding TSPO that continue to pervade the literature, and the need for interpretation with caution to avoid negative impacts. We also summarize the emerging perspectives that point to new directions that need to be investigated for understanding the molecular function of TSPO, only after which the true potential of this therapeutic target in medicine may be realized.

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.

Differences in Gene Regulation by Dual Ligands of Nuclear Receptors Constitutive Androstane Receptor (CAR) and Pregnane X Receptor (PXR) in HepG2 Cells Stably Expressing CAR/PXR

The constitutive androstane receptor (CAR) and pregnane X receptor (PXR) regulate various genes involved in xenobiotics and drug metabolism. In many cases, CAR/PXR share ligands termed dual ligands of CAR/PXR. It is difficult to investigate the effect of CAR/PXR dual ligands in cell lines because CAR and PXR expression is scarcely detected in cultured cell lines. Here, we established a tetracycline-inducible human CAR and stably human PXR-overexpressing HepG2 cell line (HepTR/hCAR/hPXR) to examine CAR/PXR dual ligands. In the present study, we investigated the regulation of CYP2B6, CYP2C9, CYP3A4, and UDP-glucuronosyl transferase, which are target genes of CAR/PXR, by dual ligands of CAR/PXR in two transfectants. Activation of CAR and PXR in cells treated with a high dose of CITCO [6-(4-chlorophenyl)-imidazo(2,1-b)thiazole-5-carbaldehyde] or cotreated with rifampicin and tetracycline resulted in synergistic enhancement of CYP3A4, but not CYP2B6, CYP2C9, or UGT1A1, mRNA expression in HepTR/hCAR/hPXR cells. In contrast, this synergistic effect was not observed in HepTR/hCAR cells. These observations were also demonstrated in human primary hepatocytes. Taken together, our results suggest that dual ligands of CAR/PXR show distinct gene regulation patterns by cross-talk between CAR and PXR. Furthermore, the two newly established cell lines are useful tools to investigate dual ligands of CAR/PXR.

Dimethylesculetin ameliorates maternal glucose intolerance and fetal overgrowth in high-fat diet-fed pregnant mice via constitutive androstane receptor

The constitutive androstane receptor (CAR) has been reported to decrease insulin resistance along with obesity. 6,7-dimethylesculetin (DE) is an active component of Yin Zhi Huang which is a traditional Asian medicine used to treat neonatal jaundice via CAR. In this study, we examined whether DE could affect the expression of gluconeogenic and lipogenic genes via human CAR pathway using human HepG2 cells in vitro. We also studied whether DE treatment during pregnancy could prevent maternal hypertension, glucose intolerance and hyperlipidemia, and fetal overgrowth in high-fat diet (HFD)-induced obese pregnant mice. Dimethylesculetin suppressed the mRNA expression of gluconeogenic genes, phosphoenolpyruvate carboxykinase and glucose-6-phosphatase, and lipogenic genes, sterol regulatory element-binding protein 1 and stearoyl-CoA desaturase 1, and enhanced CAR-mediated transcription. Blocking the CAR-mediated pathway abolished the effect of DE in vitro. DE treatment during pregnancy could prevent maternal hypertension, glucose intolerance and hyperlipidemia, and fetal overgrowth in HFD-induced obese pregnant mice in vivo. Our data indicate that DE might be a potential therapeutic agent for obese pregnant patients with insulin resistance through CAR to prevent the perinatal outcomes such as preeclampsia, gestational diabetes, and macrosomia. Further analysis of possible complications and side effects using animal models is required.

p38 MAP Kinase Links CAR Activation and Inactivation in the Nucleus via Phosphorylation at Threonine 38

Nuclear receptor constitutive androstane receptor (CAR, NR1I3), which regulates hepatic drug and energy metabolisms as well as cell growth and death, is sequestered in the cytoplasm as its inactive form phosphorylated at threonine 38. CAR activators elicit dephosphorylation, and nonphosphorylated CAR translocates into the nucleus to activate its target genes. CAR was previously found to require p38 mitogen-activated protein kinase (MAPK) to transactivate the cytochrome P450 2B (CYP2B) genes. Here we have demonstrated that p38 MAPK forms a complex with CAR, enables it to bind to the response sequence, phenobarbital-responsive enhancer module (PBREM), within the CYP2B promoter, and thus recruits RNA polymerase II to activate transcription. Subsequently, p38 MAPK elicited rephosphorylation of threonine 38 to inactivate CAR and exclude it from the nucleus. Thus, nuclear p38 MAPK exerted dual regulation by sequentially activating and inactivating CAR-mediated transcription through phosphorylation of threonine 38.

Translocator Protein (TSPO) Affects Mitochondrial Fatty Acid Oxidation in Steroidogenic Cells

Translocator protein (TSPO), also known as the peripheral benzodiazepine receptor, is a highly conserved outer mitochondrial membrane protein present in specific subpopulations of cells within different tissues. In recent studies, the presumptive model depicting mammalian TSPO as a critical cholesterol transporter for steroidogenesis has been refuted by studies examining effects of Tspo gene deletion in vivo and in vitro, biochemical testing of TSPO cholesterol transport function, and specificity of TSPO-mediated pharmacological responses. Nevertheless, high TSPO expression in steroid-producing cells seemed to indicate an alternate function for this protein in steroidogenic mitochondria. To seek an explanation, we used CRISPR/Cas9-mediated TSPO knockout steroidogenic MA-10 Leydig cell (MA-10:TspoΔ/Δ) clones to examine changes to core mitochondrial functions resulting from TSPO deficiency. We observed that 1) MA-10:TspoΔ/Δ cells had a shift in substrate utilization for energy production from glucose to fatty acids with significantly higher mitochondrial fatty acid oxidation (FAO), and increased reactive oxygen species production; and 2) oxygen consumption rate, mitochondrial membrane potential, and proton leak were not different between MA-10:TspoΔ/Δ and MA-10:Tspo+/+ control cells. Consistent with this finding, TSPO-deficient adrenal glands from global TSPO knockout (Tspo(-/-)) mice also showed up-regulation of genes involved in FAO compared with the TSPO floxed (Tspo(fl/fl)) controls. These results demonstrate the first experimental evidence that TSPO can affect mitochondrial energy homeostasis through modulation of FAO, a function that appears to be consistent with high levels of TSPO expression observed in cell types active in lipid storage/metabolism.

Mechanisms of xenobiotic receptor activation: Direct vs. indirect

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

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.

Evodia alkaloids suppress gluconeogenesis and lipogenesis by activating the constitutive androstane receptor

The constitutive androstane receptor (CAR) is a key sensor in xenobiotic detoxification and endobiotic metabolism. Increasing evidence suggests that CAR also plays a role in energy metabolism by suppressing the hepatic gluconeogenesis and lipogenesis. In this study, we investigated the effects of two evodia alkaloids, rutaecarpine (Rut) and evodiamine (Evo), on gluconeogenesis and lipogenesis through their activation of the human CAR (hCAR). We found that both Rut and Evo exhibited anti-lipogenic and anti-gluconeogenic effects in the hyperlipidemic HepG2 cells. Both compounds can potently activate hCAR, and treatment of cells with hCAR antagonists reversed the anti-lipogenic and anti-gluconeogenic effects of Rut and Evo. The anti-gluconeogenic effect of Rut and Evo was due to the CAR-mediated inhibition of the recruitment of forkhead box O1 (FoxO1) and hepatocyte nuclear factor 4α (HNF4α) onto the phosphoenolpyruvate carboxykinase (PEPCK) and glucose-6-phosphatase (G6Pase) gene promoters. In vivo, we showed that treatment of mice with Rut improved glucose tolerance in a CAR-dependent manner. Our results suggest that the evodia alkaloids Rut and Evo may have a therapeutic potential for the treatment of hyperglycemia and type 2 diabetes. This article is part of a Special Issue entitled: Xenobiotic nuclear receptors: New Tricks for An Old Dog, edited by Dr. Wen Xie.

The changing landscape in translocator protein (TSPO) function

Translocator protein (TSPO), previously known as the peripheral benzodiazepine receptor (PBR), is an outer mitochondrial membrane protein. TSPO has been shown to cooperate with steroidogenic acute regulatory protein (StAR) and function in the transport of cholesterol into mitochondria. TSPO has also been considered as a structural component of the mitochondrial permeability transition pore (MPTP). However, recent advances have changed these views of TSPO's functions and have prompted a re-evaluation of established concepts. This review summarizes the history of TSPO, key elements of the debate, and functional experiments that have changed our understanding. Moving forward, we examine how this fundamental change impacts our understanding of TSPO and affects the future of TSPO as a therapeutic and diagnostic target.

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.

CINPA1 is an inhibitor of constitutive androstane receptor that does not activate pregnane X receptor

Constitutive androstane receptor (CAR) and pregnane X receptor (PXR) are xenobiotic sensors that enhance the detoxification and elimination of xenobiotics and endobiotics by modulating the expression of genes encoding drug-metabolizing enzymes and transporters. Elevated levels of drug-metabolizing enzymes and efflux transporters, resulting from CAR activation in various cancers, promote the elimination of chemotherapeutic agents, leading to reduced therapeutic effectiveness and acquired drug resistance. CAR inhibitors, in combination with existing chemotherapeutics, could therefore be used to attenuate multidrug resistance in cancers. Interestingly, all previously reported CAR inverse-agonists are also activators of PXR, rendering them mechanistically counterproductive in tissues where both these xenobiotic receptors are present and active. We used a directed high-throughput screening approach, followed by subsequent mechanistic studies, to identify novel, potent, and specific small-molecule CAR inhibitors that do not activate PXR. We describe here one such inhibitor, CINPA1 (CAR inhibitor not PXR activator 1), capable of reducing CAR-mediated transcription with an IC50 of ∼70 nM. CINPA1 1) is a specific xenobiotic receptor inhibitor and has no cytotoxic effects up to 30 µM; 2) inhibits CAR-mediated gene expression in primary human hepatocytes, where CAR is endogenously expressed; 3) does not alter the protein levels or subcellular localization of CAR; 4) increases corepressor and reduces coactivator interaction with the CAR ligand-binding domain in mammalian two-hybrid assays; and 5) disrupts CAR binding to the promoter regions of target genes in chromatin immunoprecipitation assays. CINPA1 could be used as a novel molecular tool for understanding CAR function.

The Use of the LanthaScreen TR-FRET CAR Coactivator Assay in the Characterization of Constitutive Androstane Receptor (CAR) Inverse Agonists

The constitutive androstane receptor (CAR) is a critical nuclear receptor in the gene regulation of xenobiotic and endobiotic metabolism. The LanthaScreen^TM TR-FRET CAR coactivator assay provides a simple and reliable method to analyze the affinity of a ligand to the human CAR ligand-binding domain (LBD) with no need to use cellular models. This in silico assay thus enables the study of direct CAR ligands and the ability to distinguish them from the indirect CAR activators that affect the receptor via the cell signaling-dependent phosphorylation of CAR in cells. For the current paper we characterized the pharmacodynamic interactions of three known CAR inverse agonists/antagonists—PK11195, clotrimazole and androstenol—with the prototype agonist CITCO (6-(4-chlorophenyl)imidazo[2,1-b][1,3]thiazole-5-carbaldehyde-O-(3,4-dichlorobenzyl)oxime) using the TR-FRET LanthaScreen^TM assay. We have confirmed that all three compounds are inverse agonists of human CAR, with IC₅₀ 0.51, 0.005, and 0.35 μM, respectively. All the compounds also antagonize the CITCO-mediated activation of CAR, but only clotrimazole was capable to completely reverse the effect of CITCO in the tested concentrations. Thus this method allows identifying not only agonists, but also antagonists and inverse agonists for human CAR as well as to investigate the nature of the pharmacodynamic interactions of CAR ligands.

Minireview: translocator protein (TSPO) and steroidogenesis: a reappraisal

The 18-kDa translocator protein (TSPO), also known as the peripheral benzodiazepine receptor, is a transmembrane protein in the outer mitochondrial membrane. TSPO has long been described as being indispensable for mitochondrial cholesterol import that is essential for steroid hormone production. In contrast to this initial proposition, recent experiments reexamining TSPO function have demonstrated that it is not involved in steroidogenesis. This fundamental change has forced a reexamination of the functional interpretations made for TSPO that broadly impacts both basic and clinical research across multiple fields. In this minireview, we recapitulate the key studies from 25 years of TSPO research and concurrently examine their limitations that perhaps led towards the incorrect association of TSPO and steroid hormone production. Although this shift in understanding raises new questions regarding the molecular function of TSPO, these recent developments are poised to have a significant positive impact for research progress in steroid endocrinology.

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.

Indirect activation of the SV23 and SV24 splice variants of human constitutive androstane receptor: analysis with 3-hydroxyflavone and its analogues

BACKGROUND AND PURPOSE

Naturally occurring splice variants of human CAR (hCAR), including hCAR-SV23 (insertion of amino acids SPTV) and hCAR-SV24 (APYLT), have been shown to be expressed in liver. However, little is known regarding how hCAR-SV23 and hCAR-SV24 are activated. Therefore, we investigated the mode of activation of these hCAR splice variants.

EXPERIMENTAL APPROACH

Cell-based reporter gene assays, including ligand-binding domain transactivation assays and coactivator recruitment assays, were conducted on cultured HepG2 cells transfected with various constructs and treated with 3-hydroxyflavone or a hydroxylated (galangin, datiscetin, kaempferol, morin, quercetin or myricetin) or methylated (isorhamnetin, tamarixetin, or syringetin) analogue.

KEY RESULTS

Among the flavonols investigated, only 3-hydroxyflavone increased hCAR-SV23 and hCAR-SV24 activities. 3-Hydroxyflavone did not transactivate the ligand-binding domain of these isoforms or recruit steroid receptor coactivators (SRC-1, SRC-2, or SRC-3). By comparison, 3-hydroxyflavone, galangin, datiscetin, kaempferol, quercetin, isorhamnetin and tamarixetin activated hCAR-WT, whereas none of the flavonols activated hCAR-SV25 (both SPTV and APYLT insertions). The flavonols 3-Hydroxyflavone, galangin, quercetin and tamarixetin transactivated the ligand-binding domain of hCAR-WT, but only 3-hydroxyflavone recruited SRC-1, SRC-2 and SRC-3 to the receptor.

CONCLUSION AND IMPLICATIONS

hCAR-SV23 and hCAR-SV24 can be activated by a mechanism that does not involve the ligand-binding domain of the receptor or recruitment of SRC-1, SRC-2, or SRC-3. 3-Hydroxyflavone and its structural analogues activated hCAR in an isoform-selective and chemical-specific manner. Overall, our study provides insight into a novel mode of ligand activation of hCAR-SV23 and hCAR-SV24.

Nigramide C is a natural agonist of human pregnane x receptor

Pregnane X receptor (PXR) is known as a xenosensor, playing a key role in response to xenochemical stimuli. Activation of PXR enhanced expression of various drug-metabolizing enzymes and transporters such as cytochrome P450 3A4 (CYP3A4). During a screening of a natural compounds library for novel ligands of human xenosensing receptors by the mammalian one-hybrid assay, two cyclohexene-type amide alkaloids were isolated, with nigramide C (NigC) showing the most potent activation of human PXR (hPXR). NigC-mediated hPXR activation was enhanced by overexpression of steroid receptor coactivator 1 (SRC1), peroxisome proliferator-activated receptor γ, coactivator 1α, and protein arginine methyltransferase 1. A direct interaction between the ligand-binding domain of hPXR and the receptor interaction domain of SRC1 was observed. NigC induced the expression of endogenous CYP3A4 mRNA and protein in both cultured hepatoma cells and primary hepatocytes. However, in primary hepatocytes, the relative agonist activity of NigC was not as potent as that of rifampicin, probably because of lower metabolic stability of NigC in these cells. In conclusion, NigC was found to be an effective agonist of hPXR. NigC is a useful tool for investigation of hPXR function.

Investigational small-molecule drug selectively suppresses constitutive CYP2B6 activity at the gene transcription level: physiologically based pharmacokinetic model assessment of clinical drug interaction risk

The glycogen synthase kinase-3 inhibitor LY2090314 specifically impaired CYP2B6 activity during in vitro evaluation of cytochrome P450 (P450) enzyme induction in human hepatocytes. CYP2B6 catalytic activity was significantly decreased following 3-day incubation with 0.1-10 μM LY2090314, on average by 64.3% ± 5.0% at 10 μM. These levels of LY2090314 exposure were not cytotoxic to hepatocytes and did not reduce CYP1A2 and CYP3A activities. LY2090314 was not a time-dependent CYP2B6 inhibitor, did not otherwise inhibit enzyme activity at concentrations ≤10 μM, and was not metabolized by CYP2B6. Thus, mechanism-based inactivation or other direct interaction with the enzyme could not explain the observed reduction in CYP2B6 activity. Instead, LY2090314 significantly reduced CYP2B6 mRNA levels (Imax = 61.9% ± 1.4%; IC50 = 0.049 ± 0.043 μM), which were significantly correlated with catalytic activity (r(2) = 0.87, slope = 0.77; Imax = 57.0% ± 10.8%, IC50 = 0.057 ± 0.027 μM). Direct inhibition of constitutive androstane receptor by LY2090314 is conceptually consistent with the observed CYP2B6 transcriptional suppression (Imax = 100.0% ± 10.8% and 57.1% ± 2.4%; IC50 = 2.5 ± 1.2 and 2.1 ± 0.4 μM for isoforms 1 and 3, respectively) and may be sufficiently extensive to overcome the weak but potent activation of pregnane X receptor by ≤10 μM LY2090314 (19.3% ± 2.2% of maximal rifampin response, apparent EC50 = 1.2 ± 1.1 nM). The clinical relevance of these findings was evaluated through physiologically based pharmacokinetic model simulations. CYP2B6 suppression by LY2090314 is not expected clinically, with a projected <1% decrease in hepatic enzyme activity and <1% decrease in hydroxybupropion exposure following bupropion coadministration. However, simulations showed that observed CYP2B6 suppression could be clinically relevant for a drug with different pharmacokinetic properties from LY2090314.