Chrysin, baicalein and galangin are indirect activators of the human constitutive androstane receptor (CAR)

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.

Natural compounds targeting nuclear receptors for effective cancer therapy

Human nuclear receptors (NRs) are a family of forty-eight transcription factors that modulate gene expression both spatially and temporally. Numerous biochemical, physiological, and pathological processes including cell survival, proliferation, differentiation, metabolism, immune modulation, development, reproduction, and aging are extensively orchestrated by different NRs. The involvement of dysregulated NRs and NR-mediated signaling pathways in driving cancer cell hallmarks has been thoroughly investigated. Targeting NRs has been one of the major focuses of drug development strategies for cancer interventions. Interestingly, rapid progress in molecular biology and drug screening reveals that the naturally occurring compounds are promising modern oncology drugs which are free of potentially inevitable repercussions that are associated with synthetic compounds. Therefore, the purpose of this review is to draw our attention to the potential therapeutic effects of various classes of natural compounds that target NRs such as phytochemicals, dietary components, venom constituents, royal jelly-derived compounds, and microbial derivatives in the establishment of novel and safe medications for cancer treatment. This review also emphasizes molecular mechanisms and signaling pathways that are leveraged to promote the anti-cancer effects of these natural compounds. We have also critically reviewed and assessed the advantages and limitations of current preclinical and clinical studies on this subject for cancer prophylaxis. This might subsequently pave the way for new paradigms in the discovery of drugs that target specific cancer types.

Comparative analysis of YAP/TEAD inhibitors in 2D and 3D cultures of primary human hepatocytes reveals a novel non-canonical mechanism of CYP induction

Induction of cytochrome P450 (CYP) genes constitutes an important cause of drug-drug interactions and preclinical evaluation of induction liability is mandatory for novel drug candidates. YAP/TEAD signaling has emerged as an attractive target for various oncological indications and multiple chemically distinct YAP/TEAD inhibitors are rapidly progressing towards clinical stages. Here, we tested the liability for CYP induction of a diverse set of YAP/TEAD inhibitors with different modes of action and TEAD isoform selectivity profiles in monolayers and 3D spheroids of primary human hepatocytes (PHH). We found that YAP/TEAD inhibition resulted in broad induction of CYPs in 2D monolayers, whereas, if at all, only marginal induction was seen in spheroid culture. Comprehensive RNA-Seq indicated that YAP/TEAD signaling was increased in 2D culture compared to spheroids, which was paralleled by elevated activities of the interacting transcription factors LXR and ESRRA, likely at least in part due to altered mechanosensing. Inhibition of this YAP/TEAD hyperactivation resulted in an overall reduction of hepatocyte dedifferentiation marked by increased hepatic functionality, including CYPs. These results thus demonstrate that the observed induction is due to on-target effects of the compounds rather than direct activation of xenobiotic sensing nuclear receptors. Combined, the presented data link hepatocyte dedifferentiation to YAP/TEAD dysregulation, reveal a novel non-canonical pathway of CYP induction and highlight the advantage of organotypic 3D cultures to predict clinically relevant pharmacokinetic properties, particularly for atypical induction mechanisms.

Why Do Dietary Flavonoids Have a Promising Effect as Enhancers of Anthracyclines? Hydroxyl Substituents, Bioavailability and Biological Activity

Anthracyclines currently play a key role in the treatment of many cancers, but the limiting factor of their use is the widespread phenomenon of drug resistance and untargeted toxicity. Flavonoids have pleiotropic, beneficial effects on human health that, apart from antioxidant activity, are currently considered small molecules-starting structures for drug development and enhancers of conventional therapeutics. This paper is a review of the current and most important data on the participation of a selected series of flavonoids: chrysin, apigenin, kaempferol, quercetin and myricetin, which differ in the presence of an additional hydroxyl group, in the formation of a synergistic effect with anthracycline antibiotics. The review includes a characterization of the mechanism of action of flavonoids, as well as insight into the physicochemical parameters determining their bioavailability in vitro. The crosstalk between flavonoids and the molecular activity of anthracyclines discussed in the article covers the most important common areas of action, such as (1) disruption of DNA integrity (genotoxic effect), (2) modulation of antioxidant response pathways, and (3) inhibition of the activity of membrane proteins responsible for the active transport of drugs and xenobiotics. The increase in knowledge about the relationship between the molecular structure of flavonoids and their biological effect makes it possible to more effectively search for derivatives with a synergistic effect with anthracyclines and to develop better therapeutic strategies in the treatment of cancer.

Integrating Network Pharmacology and Experimental Validation Deciphers the Mechanism of Guizhi Fuling Wan against Adenomyosis

Aim

This study aimed to predict the key targets and endocrine mechanisms of Guizhi Fuling Wan (GZFLW) in treating adenomyosis (AM) through network pharmacology, molecular docking, and animal experiment verification.

Methods

The related ingredients and targets of GZFLW in treating AM were screened out using TCMSP, BATMAN-TCM, SwissTargetPrediction, and PubChem Database. Then, the protein-protein interaction (PPI) analysis and the network of compound-hub targets were constructed. At the same time, the key targets were uploaded to the Metascape Database for KEGG pathway enrichment analysis. After that, the molecular docking technology of the main active components and hub targets was performed. Furthermore, animal experiments were used to verify the results of network pharmacology analysis.

Results

A total of 55 active ingredients of GZFLW and 44 overlapping targets of GZFLW in treating AM were obtained. After screening, 25 hub targets were collected, including ESR1, EGF, and EGFR. Then, the KEGG pathway enrichment analysis results indicated that the endocrine therapeutic mechanism of GZFLW against AM is mainly associated with the estrogen signaling pathway, endocrine resistance, and an EGFR tyrosine kinase signaling pathway. Then, molecular docking showed that the significant compounds of GZFLW had a strong binding ability with ERα and EGFR. More importantly, the animal experiments confirmed that the GZFLW could downregulate the abnormal infiltration of the endometrial epithelium into the myometrium and had no interference with the normal sexual cycle. This effect may be directly related to intervening the local estrogen signaling pathway of the endometrial myometrial interface (EMI). It may also be associated with the myometrium cells' estrogen resistance via GPER/EGFR signaling pathway.

Conclusion

The endocrine mechanism of GZFLW in treating AM was explored based on network pharmacology, molecular docking, and animal experiments, which provided a theoretical basis for the clinical application of GZFLW.

Metabolism-Disrupting Chemicals and the Constitutive Androstane Receptor CAR

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

Nuclear receptor phosphorylation in xenobiotic signal transduction

Nuclear pregnane X receptor (PXR, NR1I2) and constitutive active/androstane receptor (CAR, NR1I3) are nuclear receptors characterized in 1998 by their capability to respond to xenobiotics and activate cytochrome P450 (CYP) genes. An anti-epileptic drug, phenobarbital (PB), activates CAR and its target CYP2B genes, whereas PXR is activated by drugs such as rifampicin and statins for the CYP3A genes. Inevitably, both nuclear receptors have been investigated as ligand-activated nuclear receptors by identifying and characterizing xenobiotics and therapeutics that directly bind CAR and/or PXR to activate them. However, PB, which does not bind CAR directly, presented an alternative research avenue for an indirect ligand-mediated nuclear receptor activation mechanism: phosphorylation-mediated signal regulation. This review summarizes phosphorylation-based mechanisms utilized by xenobiotics to elicit cell signaling. First, the review presents how PB activates CAR (and other nuclear receptors) through a conserved phosphorylation motif located between two zinc fingers within its DNA-binding domain. PB-regulated phosphorylation at this motif enables nuclear receptors to form communication networks, integrating their functions. Next, the review discusses xenobiotic-induced PXR activation in the absence of the conserved DNA-binding domain phosphorylation motif. In this case, phosphorylation occurs at a motif located within the ligand-binding domain to transduce cell signaling that regulates hepatic energy metabolism. Finally, the review delves into the implications of xenobiotic-induced signaling through phosphorylation in disease development and progression.

Effects of Naringin on the Activity and mRNA Expression of CYP Isozymes in Rats

Naringin (NRG) is a common dietary flavonoid in citrus fruits and has been documented to possess multiple pharmacological activities, including anti-oxidant, anti-inflammatory, and neuroprotective effects. Naringin is frequently consumed in combination with common clinical drugs. To date, the effects of NRG on cytochrome P450 enzymes have not been fully investigated yet. In this study, the activities of hepatic CYP1A2, CYP2D2, CYP2C9, CYP2C19, and CYP2E1 in rats after the continuous oral administration of NRG (50 and 500 mg/kg) were evaluated using cocktail probe-drug method. The concentrations of 5 probe drugs (phenacetin, dextromethorphan, diclofenac sodium, omeprazole, and chlorzoxazone) in rat plasma were simultaneously determined with a validated HPLC-MS/MS (high performance liquid chromatography-tandem mass spectrometry) method and then used to calculate corresponding pharmacokinetic parameters. Compared with the control group, the AUC(0- t), AUC(0-∞), t 1/2, and C max of each probe drug in treatment groups showed no significant differences. Meanwhile, fluorescence quantitative polymerase chain reaction (FQ-PCR) analysis revealed that NRG did not significantly affect the mRNA expressions of genes CYP1a2, CYP2d2, CYP2c6, CYP2c11, and CYP2e1 in rat liver. Based on these results, it could be concluded that NRG showed no significant effects on the activities and mRNA expressions of tested CYP450 in rats.

Stilbene compound trans-3,4,5,4´-tetramethoxystilbene, a potential anticancer drug, regulates constitutive androstane receptor (Car) target genes, but does not possess proliferative activity in mouse liver

The constitutive androstane receptor(CAR) activation is connected with mitogenic effects leading to liver hyperplasia and tumorigenesis in rodents. CAR activators, including phenobarbital, are considered rodent non-genotoxic carcinogens. Recently, trans-3,4,5,4´-tetramethoxystilbene(TMS), a potential anticancer drug (DMU-212), have been shown to alleviate N-nitrosodiethylamine/phenobarbital-induced liver carcinogenesis. We studied whether TMS inhibits mouse Car to protect from the PB-induced tumorigenesis. Unexpectedly, we identified TMS as a murine CAR agonist in reporter gene experiments, in mouse hepatocytes, and in C57BL/6 mice in vivo. TMS up-regulated Car target genes Cyp2b10, Cyp2c29 and Cyp2c55 mRNAs, but down-regulated expression of genes involved in gluconeogenesis and lipogenesis. TMS did not change or down-regulate genes involved in liver proliferation or apoptosis such as Mki67, Foxm1, Myc, Mcl1, Pcna, Bcl2, or Mdm2, which were up-regulated by another Car ligand TCPOBOP. TMS did not increase liver weight and had no significant effect on Ki67 and Pcna labeling indices in mouse liver in vivo. In murine hepatic AML12 cells, we confirmed a Car-independent proapoptotic effect of TMS. We conclude that TMS is a Car ligand with limited effects on hepatocyte proliferation, likely due to promoting apoptosis in mouse hepatic cells, while controlling Car target genes involved in xenobiotic and endobiotic metabolism.

Marine Ligands of the Pregnane X Receptor (PXR): An Overview

Pregnane X Receptor (PXR) is a ligand-activated transcription factor which binds many structurally different molecules. The receptor is able to regulate the expression of a wide array of genes and is involved in cancer and different key physiological processes such as the metabolism of drugs/xenobiotics and endogenous compounds including lipids and carbohydrates, and inflammation. Algae, sponges, sea squirts, and other marine organisms are some of the species from which structurally new molecules have been isolated that have been subsequently identified in recent decades as ligands for PXR. The therapeutic potential of these natural compounds is promising in different areas and has recently resulted in the registration of trabectedin by the FDA as a novel antineoplastic drug. Apart from being potentially novel drugs, these compounds can also serve as models for the development of new molecules with improved activity. The aim of this review is to succinctly summarize the currently known natural molecules isolated from marine organisms with a proven ability to interact with PXR.

Reviewing the mechanisms of natural product-drug interactions involving efflux transporters and metabolic enzymes

The World Health Organization (WHO) and other worldwide health agencies have recently taken initiatives to encourage the use of traditional medicine and/or complementary/alternative medicine in order to promote well-being and public health. In this way, one of the WHO's concerns is the safe use of these therapies. Phytotherapy is a strategy consisting of the use of medicinal plants (MP) and/or herbal medicinal products (HMP) for medicinal purposes. The use of phytotherapy concomitantly with drugs may cause interactions compromising the expected pharmacological action or generating toxic effects. These interactions are complex processes that may occur with multiple medications targeting different metabolic pathways, and involving different compounds present in MP and HMP. Thus, the aim of this review was to summarize the main MP- and HMP-drug interactions that involve specific transporters (P-glycoprotein and BCRP) and CYP450 enzymes (CYP3A4 and CYP2D6), which play relevant roles in the mechanisms of interactions. Firstly, multiple databases were used to search studies describing in vitro or in vivo MP and HMP-drug interactions and, after that, a systematic note-taking and appraisal of the literature was conducted. It was observed that several MP and HMP, metabolic pathways and transcription factors are involved in the transporters and enzymes expression or in the modulation of their activity having the potential to provide such interactions. Thus, the knowledge of MP- and HMP-drug interaction mechanisms could contribute to prevent harmful interactions and can ensure the safe use of these products to help the establishment of the therapeutic planning in order to certify the best treatment strategy to be used.

Galangin Activates Nrf2 Signaling and Attenuates Oxidative Damage, Inflammation, and Apoptosis in a Rat Model of Cyclophosphamide-Induced Hepatotoxicity

Cyclophosphamide (CP) is a widely used chemotherapeutic agent; however, its clinical application is limited because of its multi-organ toxicity. Galangin (Gal) is a bioactive flavonoid with promising biological activities. This study investigated the hepatoprotective effect of Gal in CP-induced rats. Rats received Gal (15, 30 and 60 mg/kg/day) for 15 days followed by a single dose of CP at day 16. Cyclophosphamide triggered liver injury characterized by elevated serum transaminases, alkaline phosphatase (ALP) and lactate dehydrogenase (LDH), and histopathological manifestations. Increased hepatic reactive oxygen species, malondialdehyde, nitric oxide, and oxidative DNA damage along with declined glutathione and antioxidant enzymes were demonstrated in CP-administered rats. CP provoked hepatic nuclear factor-kappaB (NF-κB) phosphorylation and increased mRNA abundance of inducible nitric oxide synthase (iNOS) and cyclooxygenase-2 (COX-2), and tumor necrosis factor-alpha (TNF-α) and interleukin-1 beta (IL-1β) both expression and serum levels. Gal prevented CP-induced liver injury, boosted antioxidants and suppressed oxidative stress, DNA damage, NF-κB phosphorylation and pro-inflammatory mediators. Gal diminished Bax and caspase-3, and increased B-cell lymphoma-2 (Bcl-2) in liver of CP-administered rats. In addition, Gal increased peroxisome proliferator-activated receptor gamma (PPARγ) expression and activated hepatic nuclear factor erythroid 2-related factor 2 (Nrf2) signaling showed by the increase in Nrf2, NAD(P)H: quinone acceptor oxidoreductase-1 (NQO-1) and heme oxygenase 1 (HO-1) in CP-administered rats. These findings suggest that Gal prevents CP hepatotoxicity through activation of Nrf2/HO-1 signaling and attenuation of oxidative damage, inflammation and cell death. Therefore, Gal might represent a promising adjuvant therapy to prevent hepatotoxicity in patients on CP treatment.

Two Approaches for Evaluating the Effects of Galangin on the Activities and mRNA Expression of Seven CYP450

Galangin is a marker compound of honey and Alpinia officinarum Hance that exhibits great potential for anti-microbial, anti-diabetic, anti-obesity, anti-tumour and anti-inflammatory applications. Galangin is frequently consumed in combination with common clinical drugs. Here, we evaluated the effects of galangin on cytochrome P450 (CYP)-mediated metabolism, using two different approaches, to predict drug⁻drug interactions. Male Sprague Dawley rats were administered galangin daily for 8 weeks. A "cocktail-probes" approach was employed to evaluate the activities of different CYP450 enzymes. Blood samples of seven probe drugs were analysed using liquid chromatography-tandem mass spectrometry in positive and negative electrospray-ionisation modes. Pharmacokinetic parameters were calculated to identify statistical differences. CYP mRNA-expression levels were investigated in real-time quantitative polymerase chain reaction experiments. The galangin-treated group showed significantly decreased AUC_0⁻∞ and C_max values for CYP1A2, and CYP2B3. The galangin-treated group showed significantly increased AUC_0⁻∞ and C_max values for CYP2C13 and CYP3A1. No significant influences were observed in the pharmacokinetic profiles of CYP2C11, CYP2D4 and CYP2E1. The mRNA-expression results were consistent with the pharmacokinetic results. Thus, CYP450 enzyme activities may be altered by long-term galangin administration, suggesting galangin to be a promising candidate molecule for enhancing oral drug bioavailability and chemoprevention and reversing multidrug resistance.

Systematic Investigation of Scutellariae Barbatae Herba for Treating Hepatocellular Carcinoma Based on Network Pharmacology

As the fifth most common type of malignant cancers globally, hepatocellular carcinoma (HCC) is the second leading cause of cancer-related mortality worldwide. As a long-time medicinal herb in Traditional Chinese Medicine (TCM), Scutellariae Barbatae Herba (SBH) has also been used for treating various cancers including HCC, but its underlying mechanisms have not been completely clarified. Presently, an innovative network-pharmacology platform was introduced to systematically elucidate the pharmacological mechanisms of SBH against HCC, adopting active ingredients prescreening, target fishing, and network analysis. The results revealed that SBH appeared to work on HCC probably through regulating 4 molecular functions, 20 biological processes, and hitting on 21 candidate targets involved in 40 pathways. By in-depth analysis of the first-ranked signaling pathway and hit genes, only TTR was highly and specially expressed in the liver tissue. TTR might play a crucial role in neutrophil degranulation pathway during SBH against HCC. Hence, TTR might become a therapeutic target of HCC. The study investigated the anti-hepatoma mechanisms of SBH from a holistic perspective, which provided a theoretical foundation for further experimental research and rational clinical application of SBH.

Nuclear Receptor Metabolism of Bile Acids and Xenobiotics: A Coordinated Detoxification System with Impact on Health and Diseases

Structural and functional studies have provided numerous insights over the past years on how members of the nuclear hormone receptor superfamily tightly regulate the expression of drug-metabolizing enzymes and transporters. Besides the role of the farnesoid X receptor (FXR) in the transcriptional control of bile acid transport and metabolism, this review provides an overview on how this metabolic sensor prevents the accumulation of toxic byproducts derived from endogenous metabolites, as well as of exogenous chemicals, in coordination with the pregnane X receptor (PXR) and the constitutive androstane receptor (CAR). Decrypting this network should provide cues to better understand how these metabolic nuclear receptors participate in physiologic and pathologic processes with potential validation as therapeutic targets in human disabilities and cancers.

Teriflunomide Is an Indirect Human Constitutive Androstane Receptor (CAR) Activator Interacting With Epidermal Growth Factor (EGF) Signaling

The constitutive androstane receptor (CAR) is a nuclear receptor involved mainly in xenobiotic and endobiotic metabolism regulation. CAR is activated directly by its ligands via the ligand binding domain (LBD) or indirectly by inhibition of the epidermal growth factor (EGF) signaling. We found that leflunomide (LEF) and its main metabolite teriflunomide (TER), both used for autoimmune diseases treatment, induce the prototype CAR target gene CYP2B6 in primary human hepatocytes. As TER was discovered to be an EGF receptor antagonist, we sought to determine if TER is an indirect activator of CAR. In primary human hepatocytes and in differentiated HepaRG cells, we found that LEF and TER up-regulate CAR target genes CYP2B6 and CYP3A4 mRNAs and enzymatic activities. TER stimulated CAR+A mutant translocation into the nucleus but neither LEF nor TER activated the CAR LBD, CAR3 variant or pregnane X receptor (PXR) in gene reporter assays. Interestingly, TER significantly up-regulated CAR mRNA expression, a result which could be a consequence of both EGF receptor and ELK-1 transcription factor inhibition by TER or by TER-mediated activation of glucocorticoid receptor (GR), an upstream hormonal regulator of CAR. We can conclude that TER is a novel indirect CAR activator which through EGF inhibition and GR activation controls both detoxification and some intermediary metabolism genes.

A review on the ethnomedicinal uses, phytochemistry and pharmacology of Alpinia officinarum Hance

ETHNOPHARMACOLOGICAL RELEVANCE

Alpinia officinarum Hance is a perennial plant that has been traditionally used for many decades to treat several ailments including inflammation, pain, stomach-ache, cold, amongst others. Pharmacological studies over the years have demonstrated remarkable bioactivities that could be further explored for development of new therapeutic agents against various ailments.

AIM OF THE STUDY

The paper critically reviewed the ethno-medicinal uses, pharmacology, and phytochemistry of A. officinarum.

METHODS

Keywords including A. officinarum and its synonyms were searched using electronic databases including ISI web of knowledge, Science direct, Scopus, PubMed, Google scholar and relevant database for Masters and Doctoral theses.

RESULTS

A. officinarum is prepared in Asia, Turkey, Morocco and Iran as a decoction, infusion or juice as a single preparation or in combination with other herbs, food or drinks for the treatment of general health problems including cold, inflammation, digestive disorders, etc. Pharmacological studies revealed the potent in vitro and in vivo bioactivities of various parts of A. officinarum that include anti-inflammatory, cytotoxicity, homeostasis, lipid regulation, antioxidant, antiviral, antimicrobial, antiosteoporosis, etc. Over 90 phytochemical constituents have been identified and isolated from A. officinarum comprising vastly of phenolic compounds especially diarylheptanoids isolated from the rhizome and considered the most active bioactive components.

CONCLUSION

In vitro and in vivo studies have confirmed the potency of A. officinarum. However, further studies are required to establish the mechanisms mediating its bioactivities in relation to the medicinal uses as well as investigating any potential toxicity for future clinical studies.

Effect of Endocannabinoid Oleamide on Rat and Human Liver Cytochrome P450 Enzymes in In Vitro and In Vivo Models

The endocannabinoid system is important for many physiologic and pathologic processes, but its role in the regulation of liver cytochromes P450 (P450s) remains unknown. We studied the influence of the endocannabinoid oleamide on rat and human liver P450s. Oleamide was administered intraperitoneally to rats at doses of 0.1, 1, and 10 mg/kg per day for 7 days. The content and activity of key P450s were evaluated in rat liver microsomes. Moreover, interactions with nuclear receptors regulating P450 genes and serum levels of their ligands (prolactin, corticosterone, and free triiodothyronine) were tested in in vitro P450 inhibition assays. Decreased protein levels and metabolic activities of CYP1A2, CYP2B, and CYP2C11, along with a drop in metabolic activity of CYP2D2, were observed in animals treated with oleamide (10 mg/kg per day). The activities of CYP2C6, CYP2A, and CYP3A and the levels of hormones were not altered. In vitro, oleamide exhibited a weak inhibition of rat CYP1A2, CYP2D2, and CYP2C6. The activities of rat CYP2A, CYP2B, CYP2C11, and CYP3A and human CYP1A2, CYP2B6, CYP2C9, and CYP3A4 were not altered. Oleamide did not interact with human pregnane X, constitutive androstane, or aryl hydrocarbon receptors in reporter gene experiments and did not regulate their target P450 genes in primary human hepatocytes. Our results indicate that oleamide caused the downregulation of some rat liver P450s, and hormones are not mediators of this effect. In vitro oleamide inhibits mainly rat CYP2C6 and is neither an agonist nor antagonist of major human nuclear receptors involved in the regulation of xenobiotic metabolism.

Natural product modulators of human sensations and mood: molecular mechanisms and therapeutic potential

Humans perceive physical information about the surrounding environment through their senses. This physical information is registered by a collection of highly evolved and finely tuned molecular sensory receptors. A multitude of bioactive, structurally diverse ligands have evolved in nature that bind these molecular receptors. The complex, dynamic interactions between the ligands and the receptors lead to changes in our sensory perception or mood. Here, we review our current knowledge of natural products and their derived analogues that interact specifically with human G protein-coupled receptors, ion channels, and nuclear hormone receptors to modulate the sensations of taste, smell, temperature, pain, and itch, as well as mood and its associated behaviour. We discuss the molecular and structural mechanisms underlying such interactions and highlight cases where subtle differences in natural product chemistry produce drastic changes in functional outcome. We also discuss cases where a single compound triggers complex sensory or behavioural changes in humans through multiple mechanistic targets. Finally, we comment on the therapeutic potential of the reviewed area of research and draw attention to recent technological developments in genomics, metabolomics, and metabolic engineering that allow us to tap the medicinal properties of natural product chemistry without taxing nature.

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.

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.

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.

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.

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

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

Development of CINPA1 analogs as novel and potent inverse agonists of constitutive androstane receptor

Constitutive androstane receptor (CAR, NR1I3) and pregnane X receptor (PXR, NR1I2) are master regulators of endobiotic and xenobiotic metabolism and disposition. Because CAR is constitutively active in certain cellular contexts, inhibiting CAR might reduce drug-induced hepatotoxicity and resensitize drug-resistant cancer cells to chemotherapeutic drugs. We recently reported a novel CAR inhibitor/inverse agonist CINPA1 (11). Here, we have obtained or designed 54 analogs of CINPA1 and used a time-resolved fluorescence resonance energy transfer (TR-FRET) assay to evaluate their CAR inhibition potency. Many of the 54 analogs showed CAR inverse agonistic activities higher than those of CINPA1, which has an IC50 value of 687 nM. Among them, 72 has an IC50 value of 11.7 nM, which is about 59-fold more potent than CINPA1 and over 10-fold more potent than clotrimazole (an IC50 value of 126.9 nM), the most potent CAR inverse agonist in a biochemical assay previously reported by others. Docking studies provide a molecular explanation of the structure-activity relationship (SAR) observed experimentally. To our knowledge, this effort is the first chemistry endeavor in designing and identifying potent CAR inverse agonists based on a novel chemical scaffold, leading to 72 as the most potent CAR inverse agonist so far. The 54 chemicals presented are novel and unique tools for characterizing CAR's function, and the SAR information gained from these 54 analogs could guide future efforts to develop improved CAR inverse agonists.

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.

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.