Dephosphorylation of threonine 38 is required for nuclear translocation and activation of human xenobiotic receptor CAR (NR1I3)

Effects of di-(2-ethylhexyl) phthalate and its metabolites on transcriptional activity via human nuclear receptors and gene expression in HepaRG cells

Di-(2-ethylhexyl) phthalate (DEHP) is widely used as a plasticizer in polyvinyl chloride products. DEHP exposure in humans is of great concern due to its endocrine-disrupting properties. In this study, we characterized the agonistic activities of DEHP and its five metabolites, mono-(2-ethylhexyl) phthalate (MEHP), 5OH-MEHP, 5oxo-MEHP, 5cx-MEPP and 2cx-MMHP against human nuclear receptors, peroxisome proliferator-activated receptor α (PPARα), pregnane X receptor (PXR), and constitutive androstane receptor (CAR) using transactivation assays. In the PPARα assay, the order of the agonistic activity was MEHP >> 5cx-MEPP >5OH-MEHP, 5oxo-MEHP >2cx-MMHP > DEHP, with DEHP significantly inhibiting MEHP-induced PPARα agonistic activity. This finding was compared to the results from in silico docking simulation. In the PXR assay, DEHP showed PXR agonistic activity more potent than that of MEHP, whereas the other metabolites showed little activity. In the CAR assay, none of the tested compounds showed agonistic activity. Moreover, the expression levels of PPARα-, PXR-, and CAR-target genes in HepaRG cells exposed to DEHP or MEHP were investigated using qRT-PCR analysis. As a result, exposure to these compounds significantly upregulated PXR/CAR target genes (CYP3A4 and CYP2B6), but not PPARα target genes (CYP4A11, etc.) in HepaRG cells. Taken together, these results suggest that direct PXR and/or indirect CAR activation by several DEHP metabolites may be involved in the endocrine disruption by altering hormone metabolism.

Mechanism of Panax notoginseng saponins modulation of miR-214-3p/NR1I3 affecting the pharmacodynamics and pharmacokinetics of warfarin

Background

With the prevalence of dietary supplements, the use of combinations of herbs and drugs is gradually increasing, together with the risk of drug interactions. In our clinical work, we unexpectedly found that the combination of Panax notoginseng and warfarin, which are herbs that activate blood circulation and remove blood stasis, showed antagonistic effects instead. The purpose of this study was to evaluate the drug interaction between Panax notoginseng saponins (PNS) and warfarin, the main active ingredient of Panax notoginseng, and to explore the interaction mechanism.

Methods

The effects and mechanisms of PNS on the pharmacodynamics and pharmacokinetics of warfarin were explored mainly in Sprague-Dawley rats and HepG2 cells. Elisa was used to detect the concentrations of coagulation factors, HPLC-MS to detect the blood concentrations of warfarin in rats, immunoblotting was employed to examine protein levels, qRT-PCR to detect mRNA levels, cellular immunofluorescence to detect the localization of NR1I3, and dual luciferase to verify the binding of miR-214-3p and NR1I3.

Results

PNS significantly accelerated warfarin metabolism and reduced its efficacy, accompanied by increased expression of NR1I3 and CYP2C9. Interference with NR1I3 rescued the accelerated metabolism of warfarin induce by PNS co-administration. In addition, we demonstrated that PNS significantly reduced miR-214-3p expression, whereas miR-214-3p overexpression reduced NR1I3 and CYP2C9 expression, resulting in a weakened antagonistic effect of PNS on warfarin. Additionally, we found that miR-214-3p bound directly to NR1I3 3'-UTR and significantly downregulated NR1I3 expression.

Conclusion

Our study demonstrated that PNS accelerates warfarin metabolism and reduces its pharmacodynamics by downregulating miR-214-3p, leading to increased expression of its target gene NR1I3, these findings provide new insights for clinical drug applications to avoid adverse effects.

Timosaponin AⅢ induces drug-metabolizing enzymes by activating constitutive androstane receptor (CAR) via dephosphorylation of the EGFR signaling pathway

The current study aimed to assess the effect of timosaponin AⅢ (T-AⅢ) on drug-metabolizing enzymes during anticancer therapy. The in vivo experiments were conducted on nude and ICR mice. Following a 24-day administration of T-AⅢ, the nude mice exhibited an induction of CYP2B10, MDR1, and CYP3A11 expression in the liver tissues. In the ICR mice, the expression levels of CYP2B10 and MDR1 increased after a three-day T-AⅢ administration. The in vitro assessments with HepG2 cells revealed that T-AⅢ induced the expression of CYP2B6, MDR1, and CYP3A4, along with constitutive androstane receptor (CAR) activation. Treatment with CAR siRNA reversed the T-AⅢ-induced increases in CYP2B6 and CYP3A4 expression. Furthermore, other CAR target genes also showed a significant increase in the expression. The up-regulation of murine CAR was observed in the liver tissues of both nude and ICR mice. Subsequent findings demonstrated that T-AⅢ activated CAR by inhibiting ERK1/2 phosphorylation, with this effect being partially reversed by the ERK activator t-BHQ. Inhibition of the ERK1/2 signaling pathway was also observed in vivo. Additionally, T-AⅢinhibited the phosphorylation of EGFR at Tyr1173 and Tyr845, and suppressed EGF-induced phosphorylation of EGFR, ERK, and CAR. In the nude mice, T-AⅢ also inhibited EGFR phosphorylation. These results collectively indicate that T-AⅢ is a novel CAR activator through inhibition of the EGFR pathway.

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.

Phosphorylation of nuclear receptors: Novelty and therapeutic implications

Nuclear receptors (NR) collectively regulate several biological functions in various organs. While NRs can be characterized by activation of the transcription of their signature genes, they also have other diverse roles. Although most NRs are directly activated by ligand binding, which induces cascades of events leading to gene transcription, some NRs are also phosphorylated. Despite extensive investigations, primarily focusing on unique phosphorylation of amino acid residues in different NRs, the role of phosphorylation in the biological activity of NRs in vivo has not been firmly established. Recent studies on the phosphorylation of conserved phosphorylation motifs within the DNA- and ligand-binding domains confirmed has indicated the physiologically relevance of NR phosphorylation. This review focuses on estrogen and androgen receptors, and highlights the concept of phosphorylation as a drug target.

Drug-drug interactions that alter the exposure of glucuronidated drugs: Scope, UDP-glucuronosyltransferase (UGT) enzyme selectivity, mechanisms (inhibition and induction), and clinical significance

Drug-drug interactions (DDIs) arising from the perturbation of drug metabolising enzyme activities represent both a clinical problem and a potential economic loss for the pharmaceutical industry. DDIs involving glucuronidated drugs have historically attracted little attention and there is a perception that interactions are of minor clinical relevance. This review critically examines the scope and aetiology of DDIs that result in altered exposure of glucuronidated drugs. Interaction mechanisms, namely inhibition and induction of UDP-glucuronosyltransferase (UGT) enzymes and the potential interplay with drug transporters, are reviewed in detail, as is the clinical significance of known DDIs. Altered victim drug exposure arising from modulation of UGT enzyme activities is relatively common and, notably, the incidence and importance of UGT induction as a DDI mechanism is greater than generally believed. Numerous DDIs are clinically relevant, resulting in either loss of efficacy or an increased risk of adverse effects, necessitating dose individualisation. Several generalisations relating to the likelihood of DDIs can be drawn from the known substrate and inhibitor selectivities of UGT enzymes, highlighting the importance of comprehensive reaction phenotyping studies at an early stage of drug development. Further, rigorous assessment of the DDI liability of new chemical entities that undergo glucuronidation to a significant extent has been recommended recently by regulatory guidance. Although evidence-based approaches exist for the in vitro characterisation of UGT enzyme inhibition and induction, the availability of drugs considered appropriate for use as 'probe' substrates in clinical DDI studies is limited and this should be research priority.

Phenobarbital in Nuclear Receptor Activation: An Update

Phenobarbital (PB) is a commonly prescribed anti-epileptic drug that can also benefit newborns from hyperbilirubinemia. Being the first drug demonstrating hepatic induction of cytochrome P450 (CYP), PB has since been broadly used as a model compound to study xenobiotic-induced drug metabolism and clearance. Mechanistically, PB-mediated CYP induction is linked to a number of nuclear receptors, such as the constitutive androstane receptor (CAR), pregnane X receptor (PXR), and estrogen receptor α, with CAR being the predominant regulator. Unlike prototypical agonistic ligands, PB-mediated activation of CAR does not involve direct binding with the receptor. Instead, dephosphorylation of threonine 38 in the DNA-binding domain of CAR was delineated as a key signaling event underlying PB-mediated indirect activation of CAR. Further studies revealed that such phosphorylation sites appear to be highly conserved among most human nuclear receptors. Interestingly, while PB is a pan-CAR activator in both animals and humans, PB activates human but not mouse PXR. The species-specific role of PB in gene regulation is a key determinant of its implication in xenobiotic metabolism, drug-drug interactions, energy homeostasis, and cell proliferation. In this review, we summarize the recent progress in our understanding of PB-provoked transactivation of nuclear receptors with a focus on CAR and PXR. SIGNIFICANCE STATEMENT: Extensive studies using PB as a research tool have significantly advanced our understanding of the molecular basis underlying nuclear receptor-mediated drug metabolism, drug-drug interactions, energy homeostasis, and cell proliferation. In particular, CAR has been established as a cell signaling-regulated nuclear receptor in addition to ligand-dependent functionality. This mini-review highlights the mechanisms by which PB transactivates CAR and PXR.

[Understanding the Underlying Mechanism of Xenobiotic-Sensing Nuclear Receptor Activation]

The nuclear receptor superfamily comprises 48 members in humans. In various organs, nuclear receptors regulate a variety of physiological functions through transcription of target genes. They are associated with the development and progression of endocrine and metabolic disorders, as well as with cancer development. Therefore, agonists and antagonists targeting nuclear receptors are currently being developed as therapeutic drugs for these diseases. Nuclear receptors can be activated through ligand binding or phosphorylation, which is mediated by various cellular signaling pathways. Activation of a nuclear receptor necessitates significant structural modifications in each of its domains. My research has been focused on unraveling the intricate mechanisms underlying the activation of nuclear receptors using constitutive androstane receptor (CAR) and pregnane X receptor (PXR) as model nuclear receptor proteins. CAR and PXR are highly expressed in the liver and are activated by a wide range of xenobiotics. Given their crucial roles in the metabolism and disposition of xenobiotics, as well as their potential in mediating drug-drug interactions, it is imperative to extensively study the mechanisms of xenobiotic-induced activation of these receptors. Such studies are essential for advancements in drug development, as well as for ensuring food and chemical safety. In this review, I elucidate the molecular basis underlying the activation of xenobiotic-responsive nuclear receptors.

Activation of Alternative Bilirubin Clearance Pathways Partially Reduces Hyperbilirubinemia in a Mouse Model Lacking Functional Ugt1a1 Activity

Bilirubin is a heme catabolite and Ugt1a1 is the only enzyme involved in the biological elimination of bilirubin. Partially functional or non-functional Ugt1a1 may result in neuronal damage and death due to the accumulation of unconjugated bilirubin in the brain. The understanding of the role of alternative bilirubin detoxification mechanisms that can reduce bilirubin toxicity risk is crucial for developing novel therapeutic strategies. To provide a proof-of-principle showing whether activation of alternative detoxification pathways could lead to life-compatible bilirubin levels in the absence of Ugt1a1 activity, we used Ugt1^−/− hyperbilirubinemic mice devoid of bilirubin glucuronidation activity. We treated adult Ugt1^−/− mice with TCPOBOP, a strong agonist of the constitutive androstane receptor (CAR). TCPOBOP treatment decreased plasma and liver tissue bilirubin levels by about 38%, and resulted in the transcriptional activation of a vast array of genes involved in bilirubin transport and metabolism. However, brain bilirubin level was unaltered. We observed ~40% degradation of bilirubin in the liver microsomes from TCPOBOP treated Ugt1^−/− mice. Our findings suggest that, in the absence of Ugt1a1, the activation of alternative bilirubin clearance pathways can partially improve hyperbilirubinemic conditions. This therapeutic approach may only be considered in a combinatorial manner along with other treatments.

Molecular basis of crosstalk in nuclear receptors: heterodimerization between PXR and CAR and the implication in gene regulation

The 48 human nuclear receptors (NRs) form a superfamily of transcription factors that regulate major physiological and pathological processes. Emerging evidence suggests that NR crosstalk can fundamentally change our understanding of NR biology, but detailed molecular mechanisms of crosstalk are lacking. Here, we report the molecular basis of crosstalk between the pregnane X receptor (PXR) and constitutive androstane receptor (CAR), where they form a novel heterodimer, resulting in their mutual inhibition. PXR and CAR regulate drug metabolism and energy metabolism. Although they have been broadly perceived as functionally redundant, a growing number of reports suggests a mutual inhibitory relation, but their precise mode of coordinated action remains unknown. Using methods including RNA sequencing, small-angle X-ray scattering and crosslinking mass spectrometry we demonstrate that the mutual inhibition altered gene expression globally and is attributed to the novel PXR–CAR heterodimerization via the same interface used by each receptor to heterodimerize with its functional partner, retinoid X receptor (RXR). These findings establish an unexpected functional relation between PXR, CAR and RXR, change the perceived functional relation between PXR and CAR, open new perspectives on elucidating their role and designing approaches to regulate them, and highlight the importance to comprehensively investigate nuclear receptor crosstalk.

Detection and Functional Analysis of Estrogen Receptor α Phosphorylated at Serine 216 in Mouse Neutrophils

Serine 216 constitutes a protein kinase C phosphorylation motif located within the DNA binding domain of estrogen receptor α (ERα). In this chapter, we present experimental procedures confirming that mouse ERα is phosphorylated at serine 216 in peripheral blood neutrophils and in neutrophils that infiltrate the uterus, as well as the role of phosphoserine 216 in neutrophil migration. A phospho-peptide antibody (αP-S216) was utilized in Western blot, immunohistochemistry, and double immunofluorescence staining to detect this phosphorylation of an endogenous ERα. Both immunohistochemistry (with αP-S216 or neutrophil marker Ly6G antibody) and double immunofluorescence staining of mouse uterine sections prepared from C3H/HeNCrIBR females revealed that phosphorylated ERα was expressed in all infiltrating neutrophils during hormonal cycles but not in any other of the other uterine cells. Neutrophils infiltrate the uterus from the bloodstream. White blood cells (WBC) were prepared from peripheral blood of C3H/HeNCrIBR females or males and double immunostained. Blood neutrophils also expressed phosphorylated ERα but in only about 20% of cells in both sexes. Only the neutrophils expressing phosphorylated ERα spontaneously migrated in in vitro Transwell migration assays and infiltrated the uterus in mice.

Protein Kinase N Family Negatively Regulates Constitutive Androstane Receptor-Mediated Transcriptional Induction of Cytochrome P450 2b10 in the Livers of Mice

In receptor-type transcription factors-mediated cytochrome P450 (P450) induction, few studies have attempted to clarify the roles of protein kinase N (PKN) in the transcriptional regulation of P450s. This study aimed to examine the involvement of PKN in the transcriptional regulation of P450s by receptor-type transcription factors, including the aryl hydrocarbon receptor, constitutive androstane receptor (CAR), and pregnane X receptor. The mRNA and protein levels and metabolic activity of P450s in the livers of wild-type (WT) and double-mutant (D) mice harboring both PKN1 kinase-negative knock-in and PKN3 knockout mutations [PKN1 ^T778A/T778A; PKN3 ^-/-] were determined after treatment with activators for receptor-type transcription factors. mRNA and protein levels and metabolic activity of CYP2B10 were significantly higher in D mice treated with the CAR activator phenobarbital (PB) but not with 1,4-bis((3,5-dichloropyridin-2-yl)oxy)benzene compared with WT mice. We examined the CAR-dependent pathway regulated by PKN after PB treatment because the extent of CYP2B10 induction in WT and D mice was notably different in response to treatment with different CAR activators. The mRNA levels of Cyp2b10 in primary hepatocytes from WT and D mice treated with PB alone or in combination with Src kinase inhibitor 1 (SKI-1) or U0126 (a mitogen-activated protein kinase inhibitor) were evaluated. Treatment of hepatocytes from D mice with the combination of PB with U0126 but not SKI-1 significantly increased the mRNA levels of Cyp2b10 compared with those from the corresponding WT mice. These findings suggest that PKN may have inhibitory effects on the Src-receptor for activated C kinase 1 (RACK1) pathway in the CAR-mediated induction of Cyp2b10 in mice livers. SIGNIFICANCE STATEMENT: This is the first report of involvement of PKN in the transcriptional regulation of P450s. The elucidation of mechanisms responsible for induction of P450s could help optimize the pharmacotherapy and improve drug development. We examined whether the mRNA and protein levels and activities of P450s were altered in double-mutant mice harboring both PKN1 kinase-negative knock-in and PKN3 knockout mutations. PKN1/3 negatively regulates CAR-mediated induction of Cyp2b10 through phosphorylation of a signaling molecule in the Src-RACK1 pathway.

RORα phosphorylation by casein kinase 1α as glucose signal to regulate estrogen sulfation in human liver cells

Estrogen sulfotransferase (SULT1E1) metabolically inactivates estrogen and SULT1E1 expression is tightly regulated by multiple nuclear receptors. Human fetal, but not adult, livers express appreciable amounts of SULT1E1 protein, which is mimicked in human hepatoma-derived HepG2 cells cultured in high glucose (450 mg/dl) medium. Here, we have investigated this glucose signal that leads to phosphorylation of nuclear receptor RORα (NR1F1) at Ser100 and the transcription mechanism by which phosphorylated RORα transduces this signal to nuclear receptor HNF4α, activating the SULT1E1 promoter. The promoter is repressed by non-phosphorylated RORα which binds a distal enhancer (−943/−922 bp) and interacts with and represses HNF4α-mediated transcription. In response to high glucose, RORα becomes phosphorylated at Ser100 and reverses its repression of HNF4α promoter activation. Moreover, the casein kinase CK1α, which is identified in an enhancer-bound nuclear protein complex, phosphorylates Ser100 in in vitro kinase assays. During these dynamic processes, both RORα and HNF4α remain on the enhancer. Thus, RORα utilizes phosphorylation to integrate HNF4α and transduces the glucose signal to regulate the SULT1E1 gene in HepG2 cells and this phosphorylation-mediated mechanism may also regulate SULT1E1 expressions in the human liver.

The xenobiotic receptors PXR and CAR in liver physiology, an update

Pregnane X receptor (PXR) and constitutive androstane receptor (CAR) are two nuclear receptors that are well-known for their roles in xenobiotic detoxification by regulating the expression of drug-metabolizing enzymes and transporters. In addition to metabolizing drugs and other xenobiotics, the same enzymes and transporters are also responsible for the production and elimination of numerous endogenous chemicals, or endobiotics. Moreover, both PXR and CAR are highly expressed in the liver. As such, it is conceivable that PXR and CAR have major potentials to affect the pathophysiology of the liver by regulating the homeostasis of endobiotics. In recent years, the physiological functions of PXR and CAR in the liver have been extensively studied. Emerging evidence has suggested the roles of PXR and CAR in energy metabolism, bile acid homeostasis, cell proliferation, to name a few. This review summarizes the recent progress in our understanding of the roles of PXR and CAR in liver physiology.

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.

Noncanonical Constitutive Androstane Receptor Signaling in Gene Regulation

The constitutive androstane receptor (CAR, NR1I3) is extremely important for the regulation of many physiological processes, especially xenobiotic (drug) metabolism and transporters. CAR differs from steroid hormone receptors in that it can be activated using structurally unrelated chemicals, both through direct ligand-binding and ligand-independent (indirect) mechanisms. By binding to specific responsive elements on DNA, CAR increases the expression of its target genes encoding drug-metabolizing enzymes and transporters. Therefore, CAR is mainly characterized as a ligand-dependent or ligand-independent transcription factor, and the induction of gene expression is considered the canonical mode of CAR action. Consistent with its central role in xenobiotic metabolism, CAR signaling includes a collection of mechanisms that are employed alongside the core transcriptional machinery of the receptor. These so-called noncanonical CAR pathways allow the receptor to coordinate the regulation of many aspects of cell biology. In this mini-review, we review noncanonical CAR signaling, paying special attention to the role of CAR in energy homeostasis and cell proliferation.

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.

Estrogen receptor α phosphorylated at Ser216 confers inflammatory function to mouse microglia

BACKGROUND

Estrogen receptor α (ERα) has been suggested to regulate anti-inflammatory signaling in brain microglia, the only resident immune cells in the brain. ERα conserves the phosphorylation motif at Ser216 within the DNA binding domain. Previously, Ser216 was found to be phosphorylated in neutrophils infiltrating into the mouse uterus and to enable ERα to regulate migration. Given the implication of this phosphorylation in immune regulation, ERα was examined in mouse microglia to determine if Ser216 is phosphorylated and regulates microglia's inflammation. It was found that Ser216 was constitutively phosphorylated in microglia and demonstrated that in the absence of phosphorylated ERα in ERα KI brains microglia inflamed, confirming that phosphorylation confers ERα with anti-inflammatory capability. ERα KI mice were obese and weakened motor ability.

METHODS

Mixed glia cells were prepared from brains of 2-days-old neonates and cultured to mature and isolate microglia. An antibody against an anti-phospho-S216 peptide of ERα (αP-S216) was used to detect phosphorylated ERα in double immunofluorescence staining with ERα antibodies and a microglia maker Iba-1 antibody. A knock-in (KI) mouse line bearing the phosphorylation-blocked ERα S216A mutation (ERα KI) was generated to examine inflammation-regulating functions of phosphorylated ERα in microglia. RT-PCR, antibody array, ELISA and FACS assays were employed to measure expressions of pro- or anti-inflammatory cytokines at their mRNA and protein levels. Rotarod tests were performed to examine motor connection ability.

RESULTS

Double immune staining of mixed glia cells showed that ERα is phosphorylated at Ser216 in microglia, but not astrocytes. Immunohistochemistry with an anti-Iba-1 antibody showed that microglia cells were swollen and shortened branches in the substantial nigra (SN) of ERα KI brains, indicating the spontaneous activation of microglia as observed with those of lipopolysaccharide (LPS)-treated ERα WT brains. Pro-inflammatory cytokines were up-regulated in the brain of ERα KI brains as well as cultured microglia, whereas anti-inflammatory cytokines were down-regulated. FACS analysis showed that the number of IL-6 producing and apoptotic microglia increased in those prepared from ERα KI brains. Times of ERα KI mice on rod were shortened in Rotarod tests.

CONCLUSIONS

Blocking of Ser216 phosphorylation aggravated microglia activation and inflammation of mouse brain, thus confirming that phosphorylated ERα exerts anti-inflammatory functions. ERα KI mice enable us to further investigate the mechanism by which phosphorylated ERα regulates brain immunity and inflammation and brain diseases. Video abstract.

Nuclear receptor CAR-ERα signaling regulates the estrogen sulfotransferase gene in the liver

Estrogen sulfotransferase (SULT1E1) inactivates estrogen and regulates its metabolic homeostats. Whereas SULT1E1 is expressed low in the liver of adult mice, it is induced by phenobarbital (PB) treatment or spontaneously in diabetic livers via nuclear receptors. Utilizing constitutive active/androstane receptor (CAR) KO, estrogen receptor α (ERα KO, phosphorylation-blocked ERα S216A KI mice, it is now demonstrated that, after being activated by PB, CAR binds and recruits ERα onto the Sulte1 promoter for subsequent phosphorylation at Ser216. This phosphorylation tightens CAR interacting with ERα and to activates the promoter. Hepatic SULT1E1 mRNA levels are constitutively up-regulated in type 1 diabetic Akita mice; CAR spontaneously accumulates in the nucleus and activates the Sult1e1 promoter by recruiting phosphorylated ERα in the liver as observed with PB-induced livers. Thus, this CAR-phosphorylated ERα signaling enables these two nuclear receptors to communicate, activating the Sult1e1 gene in response to either PB or diabetes in mice. ERα phosphorylation may integrate CAR into estrogen actions, providing insights into understanding drug-hormone interactions in clinical therapy.

Ser100-Phosphorylated RORα Orchestrates CAR and HNF4α to Form Active Chromatin Complex in Response to Phenobarbital to Regulate Induction of CYP2B6

We have previously shown that the retinoid-related orphan receptor alpha (RORα) phosphorylation plays a pivotal role in sulfotransferase 1E1 gene regulation within mouse liver. Here, we found serine 100-phosphorylated RORα orchestrates constitutive androstane receptor (CAR) and hepatocyte nuclear factor 4 alpha (HNF4α) to induce CYP2B6 by phenobarbital (PB) in human primary hepatocytes (HPHs). RORα knockdown using small interfering RNAs suppressed CYP2B6 mRNAs in HPH, whereas transient expression of RORα in COS-1 cells activated CYP2B6 promoter activity in reporter assays. Through chromatin immunoprecipitation (IP) and gel shift assays, we found that RORα in the form of phosphorylated (p-) S100 directly bound to a newly identified RORα response element (RORα response element on CYP2B6 promoter, -660/-649) within the CYP2B6 promoter in untreated or treated HPH. In PB-treated HPH, p-Ser100 RORα was both enriched in the distal phenobarbital response element module (PBREM) and the proximal okadaic acid response element (OARE), a known HNF4α binding site. Chromatin conformation capture assay revealed direct contact between the PBREM and OARE only in PB-treated HPH. Moreover, CAR preferably interacted with phosphomimetically mutated RORα at Ser100 residue in co-IP assay. A gel shift assay with a radiolabeled OARE module and nuclear extracts prepared from PB-treated mouse liver confirmed that HNF4α formed a complex with Ser 100-phosphorylated RORα, as shown by supershifted complexes with anti-p-Ser100 RORα and anti-HNF4α antibodies. Altogether, the results established that p-Ser100 RORα bridging the PBREM and OARE orchestrates CAR and HNF4α to form active chromatin complex during PB-induced CYP2B6 expression in human primary hepatocytes. SIGNIFICANCE STATEMENT: CYP2B6 is a vital enzyme for the metabolic elimination of xenobiotics, and it is prone to induction by xenobiotics, including phenobarbital via constitutive androstane receptor (CAR) and hepatocyte nuclear factor 4 alpha (HNF4α). Here, we show that retinoid-related orphan receptor alpha (RORα), through phosphorylated S100 residue, orchestrated CAR-HNF4α interaction on the CYP2B6 promoter in human primary hepatocyte cultures. These results signify not only the role of RORα in the molecular process of CYP2B6 induction, but it also reveals the importance of conserved phosphorylation sites within the DNA-binding domain of the receptor.

Computational prediction of cytochrome P450 inhibition and induction

Cytochrome P450 (CYP) enzymes play an important role in the phase I metabolism of many xenobiotics. Most drug-drug interactions (DDIs) associated with CYP are caused by either CYP inhibition or induction. The early detection of potential DDIs is highly desirable in the pharmaceutical industry because DDIs can cause serious adverse events, which can lead to poor patient health and drug development failures. Recently, many computational studies predicting CYP inhibition and induction have been reported. The current computational modeling approaches for CYP metabolism are classified as ligand- and structure-based; various techniques, such as quantitative structure-activity relationships, machine learning, docking, and molecular dynamic simulation, are involved in both the approaches. Recently, combining these two approaches have resulted in improvements in the prediction accuracy of DDIs. In this review, we present important, recent developments in the computational prediction of the inhibition of four clinically crucial CYP isoforms (CYP1A2, 2C9, 2D6, and 3A4) and three nuclear receptors (aryl hydrocarbon receptor, constitutive androstane receptor, and pregnane X receptor) involved in the induction of CYP1A2, 2B6, and 3A4, respectively.

A phosphorylation-deficient mutant of retinoid X receptor α at Thr 167 alters fasting response and energy metabolism in mice

Retinoid X receptor α (RXRα) has a conserved phosphorylation motif at threonine 162 (humans) and threonine 167 (mice) within the DNA-binding domain. Here we have generated RXRα knock-in mice (Rxrα^T167A) bearing a single mutation of Thr 167 to alanine and examined the roles of Thr 167 in the regulation of energy metabolism within adipose, muscle, and liver tissues. Rxrα^T167A mice exhibited down-regulation of metabolic pathways converting glucose to fatty acids, such as acetyl-CoA carboxylase in the white adipose tissue (WAT) and ATP citrate lyase in the muscle. They also reduced gene expression for genes related to fatty acid catabolism and triglyceride synthesis in WAT and controlled heat factors such as adrenergic receptor β1 in muscles. In contrast, hepatic gluconeogenic pathways and synthetic pathways related to fatty acids remained unaffected by this mutation. Expression of multiple genes that were affected by the Thr 167 mutation in adipose tissue exhibited clear response to LG100268, a synthetic RXR agonist. Thus, the altered gene expression in mutant mice adipose appeared to be a direct effect of RXRα Thr 167 mutation and by some secondary effect of the mutation. Blood glucose levels remained normal in Rxrα^T167A during feeding, as observed with RXRα wild-type mice. However, Rxrα^T167A mice exhibited an attenuated decrease of blood glucose levels that occurred after fasting. This attenuation correlated with a concomitant down-regulation of lipid metabolism in WAT and was associated with RXRα phosphorylation at Thr 167. Thus, Thr 167 enabled RXRα to coordinate these three organs for regulation of energy metabolism and maintenance of glucose homeostasis.

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.

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.

Influence of signaling kinases on functional dynamics of nuclear receptor CAR

Constitutive androstane receptor (CAR) is a xenobiotic nuclear receptor known to regulate genes involved in key physiological processes like drug metabolism, maintenance of energy homeostasis, and cell proliferation. Owing to the diverse regulatory roles played by the receptor, it is critical to understand the precise cellular signals that dictate functional dynamics of CAR. With the objective of exploring the hitherto unknown regulatory pathways modulating CAR, we subjected the CAR protein sequence to a kinase prediction tool and identified several kinases recognizing CAR as a substrate. Using fluorescence live cell imaging and specific inhibitors it was observed that CAR functions under the regulation of mitogen-activated protein kinase (MAPK) and glycogen synthase kinase 3 (GSK3) signaling cascade. Additionally, insulin-like growth factor 1 (IGF1)-mediated inhibition of GSK3 also induced nuclear translocation of CAR linking CAR to the Akt signaling pathway. Identification of T38 residue of CAR as the GSK3 target site further substantiated our observations. Taking cues from these findings, we propose a hypothetical model elucidating the GSK3-mediated regulation of CAR dynamics through the involvement of Akt pathway. Further research into this area is expected to provide novel therapeutic targets in disease conditions like type 2 diabetes and hepatocellular carcinoma.

Indirect activation of constitutive androstane receptor in three-dimensionally cultured HepG2 cells

Constitutive androstane receptor (CAR), a member of the nuclear receptor superfamily, is retained as an inactive form phosphorylated at threonine in the cytoplasm of hepatocytes. Upon activation, CAR is dephosphorylated to move into the nucleus and induces the transcription of genes. Thus, nuclear translocation is a key step for CAR activation in hepatocytes. However, this nuclear translocation has not been demonstrated in conventional two-dimensionally-cultured immortalized cell lines such as HepG2, in which CAR spontaneously accumulates in the nucleus. In this study, we showed that treatment with the indirect CAR activator phenobarbital activated transcription of the CYP3A4 gene in three-dimensionally (3D)-cultured HepG2 cells. CAR was retained as its phosphorylated form in the cytoplasm and was translocated to the nucleus in 3D-cultured HepG2 cells in response to treatment with phenobarbital. Moreover, okadaic acid and epidermal growth factor, were found to repress phenobarbital-induced CAR nuclear translocation and subsequent activation of the CYP3A4 gene promoter. These results suggested that 3D-cultured HepG2 cells properly regulated CAR activation as has been observed in hepatocytes.

Constitutive androstane receptor 1 is constitutively bound to chromatin and 'primed' for transactivation in hepatocytes

The constitutive androstane receptor (CAR) is a xenobiotic sensor expressed in hepatocytes that activates genes involved in drug metabolism, lipid homeostasis, and cell proliferation. Much progress has been made in understanding the mechanism of activation of human CAR by drugs and xenobiotics. However, many aspects of the activation pathway remain to be elucidated. In this report, we have used viral constructs to express human CAR, its splice variants, and mutant CAR forms in hepatocytes from Car-/- mice in vitro and in vivo. We demonstrate CAR expression rescued the ability of Car-/- hepatocytes to respond to a wide range of CAR activators including phenobarbital. Additionally, two major splice isoforms of human CAR, CAR2 and CAR3, were inactive with almost all the agents tested. In contrast to the current model of CAR activation, ectopic CAR1 is constitutively localized in the nucleus and is loaded onto Cyp2b10 gene in the absence of an inducing agent. In studies to elucidate the role of threonine T38 in CAR regulation, we found that the T38D mutant was inactive even in the presence of CAR activators. However, the T38A mutant was activated by CAR inducers, showing that T38 is not essential for CAR activation. Also, using the inhibitor erlotinib, we could not confirm a role for the epidermal growth factor receptor in CAR regulation. Our data suggest that CAR is constitutively bound to gene regulatory regions and is regulated by exogenous agents through a mechanism which involves protein phosphorylation in the nucleus.

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.

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

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

Phenobarbital-induced phosphorylation converts nuclear receptor RORα from a repressor to an activator of the estrogen sulfotransferase gene Sult1e1 in mouse livers

The estrogen sulfotransferase SULT1E1 sulfates and inactivates estrogen, which is reactivated via desulfation by steroid sulfatase, thus regulating estrogen homeostasis. Phenobarbital (PB), a clinical sedative, activates Sult1e1 gene transcription in mouse livers. Here, the molecular mechanism by which the nuclear receptors CAR, which is targeted by PB, and RORα communicate through phosphorylation to regulate Sult1e1 activation has been studied. RORα, a basal activity repressor of the Sult1e1 promoter, becomes phosphorylated at serine 100 and converts to an activator of the Sult1e1 promoter in response to PB. CAR regulates both the RORα phosphorylation and conversion. Our findings suggest that PB signals CAR to communicate with RORα via serine 100 phosphorylation, converting RORα from transcription repressor to activator of the Sult1e1 gene and inducing SULT1E1 expression in mouse livers.

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

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

The Roles of Xenobiotic Receptors: Beyond Chemical Disposition

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

Nuclear Receptor CAR Suppresses GADD45B-p38 MAPK Signaling to Promote Phenobarbital-induced Proliferation in Mouse Liver

Phenobarbital, a nongenotoxic hepatocarcinogen, induces hepatic proliferation and promotes development of hepatocellular carcinoma (HCC) in rodents. Nuclear receptor constitutive active/androstane receptor (NR1I3/CAR) regulates the induction and promotion activities of phenobarbital. Here, it is demonstrated that phenobarbital treatment results in dephosphorylation of a tumor suppressor p38 MAPK in the liver of C57BL/6 and C3H/HeNCrlBR mice. The molecular mechanism entails CAR binding and inhibition of the growth arrest and DNA-damage-inducible 45 beta (GADD45B)-MAPK kinase 6 (MKK6) scaffold to repress phosphorylation of p38 MAPK. Phenobarbital-induced hepatocyte proliferation, as determined by BrdUrd incorporation, was significantly reduced in both male and female livers of GADD45B knockout (KO) mice compared with the wild-type mice. The phenobarbital-induced proliferation continued until 48 hours after phenobarbital injection in only the C57BL/6 males, but neither in males of GADD45B KO mice nor in females of C57BL/6 and GADD45B KO mice. Thus, these data reveal nuclear receptor CAR interacts with GADD45B to repress p38 MAPK signaling and elicit hepatocyte proliferation in male mice.Implications: This GADD45B-regulated male-predominant proliferation can be expanded as a phenobarbital promotion signal of HCC development in future studies.Visual Overview: http://mcr.aacrjournals.org/content/molcanres/16/8/1309/F1.large.jpg Mol Cancer Res; 16(8); 1309-18. ©2018 AACR.

Mechanisms of Action of Dehydroepiandrosterone

Dehydroepiandrosterone (3β-hydroxy-5-androsten-17-one, DHEA) and its sulfated metabolite DHEA-S are the most abundant steroids in circulation and decline with age. Rodent studies have shown that DHEA has a wide variety of effects on liver, kidney, adipose, reproductive tissues, and central nervous system/neuronal function. The mechanisms by which DHEA and DHEA-S impart their physiological effects may be direct actions on plasma membrane receptors, including a DHEA-specific, G-protein-coupled receptor in endothelial cells; various neuroreceptors, e.g., aminobutyric-acid-type A, N-methyl-d-aspartate (NMDA), and sigma-1 (S1R) receptors; by binding steroid receptors: androgen and estrogen receptors (ARs, ERα, or ERβ); or by their metabolism to more potent sex steroid hormones, e.g., testosterone, dihydrotestosterone, and estradiol, which bind with higher affinity to ARs and ERs. DHEA inhibits voltage-gated T-type calcium channels. DHEA activates peroxisome proliferator-activated receptor (PPARα) and CAR by a mechanism apparently involving PP2A, a protein phosphatase dephosphorylating PPARα and CAR to activate their transcriptional activity. We review our recent study showing DHEA activated GPER1 (G-protein-coupled estrogen receptor 1) in HepG2 cells to stimulate miR-21 transcription. This chapter reviews some of the physiological, biochemical, and molecular mechanisms of DHEA and DHEA-S activity.

Allosteric pathways in nuclear receptors - Potential targets for drug design

The nuclear receptor family of transcription factor proteins mediates endocrine function and plays critical roles in the development, physiology and pharmacology. Malfunctioning nuclear receptors are associated with several disease states. The functional activity of nuclear receptors is regulated by small molecular hormonal and synthetic molecules. Multiple sources of evidence have identified and distinguished between the different allosteric pathways initiated by ligands, DNA and cofactors such as co-activators and co-repressors. Also, these biophysical studies are attempting to determine how these pathways that regulate co-activator and DNA recognition can control gene transcription. Thus, there is a growing interest in determining the genome-scale impact of allostery in nuclear receptors. Today, it is accepted that a detailed understanding of the allosteric regulatory pathways within the nuclear receptor molecular complex will enable the development of efficient drug therapies in the long term.

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

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

Interaction of the phosphorylated DNA-binding domain in nuclear receptor CAR with its ligand-binding domain regulates CAR activation

The nuclear protein constitutive active/androstane receptor (CAR or NR1I3) regulates several liver functions such as drug and energy metabolism and cell growth or death, which are often involved in the development of diseases such as diabetes and hepatocellular carcinoma. CAR undergoes a conversion from inactive homodimers to active heterodimers with retinoid X receptor α (RXRα), and phosphorylation of the DNA-binding domain (DBD) at Thr-38 in CAR regulates this conversion. Here, we uncovered the molecular mechanism by which this phosphorylation regulates the intramolecular interaction between CAR's DBD and ligand-binding domain (LBD), enabling the homodimer-heterodimer conversion. Phosphomimetic substitution of Thr-38 with Asp increased co-immunoprecipitation of the CAR DBD with CAR LBD in Huh-7 cells. Isothermal titration calorimetry assays also revealed that recombinant CAR DBD-T38D, but not nonphosphorylated CAR DBD, bound the CAR LBD peptide. This DBD-LBD interaction masked CAR's dimer interface, preventing CAR homodimer formation. Of note, EGF signaling weakened the interaction of CAR DBD T38D with CAR LBD, converting CAR to the homodimer form. The DBD-T38D-LBD interaction also prevented CAR from forming a heterodimer with RXRα. However, this interaction opened up a CAR surface, allowing interaction with protein phosphatase 2A. Thr-38 dephosphorylation then dissociated the DBD-LBD interaction, allowing CAR heterodimer formation with RXRα. We conclude that the intramolecular interaction of phosphorylated DBD with the LBD enables CAR to adapt a transient monomer configuration that can be converted to either the inactive homodimer or the active heterodimer.

Cellular adaptation to xenobiotics: Interplay between xenosensors, reactive oxygen species and FOXO transcription factors

Cells adapt to an exposure to xenobiotics by upregulating the biosynthesis of proteins involved in xenobiotic metabolism. This is achieved largely via activation of cellular xenosensors that modulate gene expression. Biotransformation of xenobiotics frequently comes with the generation of reactive oxygen species (ROS). ROS, in turn, are known modulators of signal transduction processes. FOXO (forkhead box, class O) transcription factors are among the proteins deeply involved in the cellular response to stress, including oxidative stress elicited by the formation of ROS. On the one hand, FOXO activity is modulated by ROS, while on the other, FOXO target genes include many that encode antioxidant proteins – thereby establishing a regulatory circuit. Here, the role of ROS and of FOXOs in the regulation of xenosensor transcriptional activities will be discussed. Constitutive androstane receptor (CAR), pregnane X receptor (PXR), peroxisome proliferator-activated receptors (PPARs), arylhydrocarbon receptor (AhR) and nuclear factor erythroid 2-related factor 2 (Nrf2) all interact with FOXOs and/or ROS. The two latter not only fine-tune the activities of xenosensors but also mediate interactions between them. As a consequence, the emerging picture of an interplay between xenosensors, ROS and FOXO transcription factors suggests a modulatory role of ROS and FOXOs in the cellular adaptive response to xenobiotics.

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Exposure of cells to xenobiotics may trigger formation of reactive oxygen species.

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Xenosensors respond to xenobiotics by upregulation of xenobiotic metabolism.

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FOXO transcription factors modulate the activities of several xenosensors.

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ROS affect FOXO activity, and FOXO target genes include antioxidant proteins.

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FOXOs bridge xenobiotic-induced ROS generation and xenosensor regulation.

Targeting nuclear receptors for the treatment of fatty liver disease

Ligand-activated nuclear receptors, including peroxisome proliferator-activated receptor alpha (PPARα), pregnane X receptor, and constitutive androstane receptor, were first identified as key regulators of the responses against chemical toxicants. However, numerous studies using mouse disease models and human samples have revealed critical roles for these receptors and others, such as PPARβ/δ, PPARγ, farnesoid X receptor (FXR), and liver X receptor (LXR), in maintaining nutrient/energy homeostasis in part through modulation of the gut-liver-adipose axis. Recently, disorders associated with disrupted nutrient/energy homeostasis, e.g., obesity, metabolic syndrome, and non-alcoholic fatty liver disease (NAFLD), are increasing worldwide. Notably, in NAFLD, a progressive subtype exists, designated as non-alcoholic steatohepatitis (NASH) that is characterized by typical histological features resembling alcoholic steatohepatitis (ASH), and NASH/ASH are recognized as major causes of hepatitis virus-unrelated liver cirrhosis and hepatocellular carcinoma. Since hepatic steatosis is basically caused by an imbalance between fat/energy influx and utilization, abnormal signaling of these nuclear receptors contribute to the pathogenesis of fatty liver disease. Standard therapeutic interventions have not been fully established for fatty liver disease, but some new agents that activate or inhibit nuclear receptor signaling have shown promise as possible therapeutic targets. In this review, we summarize recent findings on the roles of nuclear receptors in fatty liver disease and discuss future perspectives to develop promising pharmacological strategies targeting nuclear receptors for NAFLD/NASH.

Phosphorylated Nuclear Receptor CAR Forms a Homodimer To Repress Its Constitutive Activity for Ligand Activation

The nuclear receptor CAR (NR1I3) regulates hepatic drug and energy metabolism as well as cell fate. Its activation can be a critical factor in drug-induced toxicity and the development of diseases, including diabetes and tumors. CAR inactivates its constitutive activity by phosphorylation at threonine 38. Utilizing receptor for protein kinase 1 (RACK1) as the regulatory subunit, protein phosphatase 2A (PP2A) dephosphorylates threonine 38 to activate CAR. Here we demonstrate that CAR undergoes homodimer-monomer conversion to regulate this dephosphorylation. By coexpression of two differently tagged CAR proteins in Huh-7 cells, mouse primary hepatocytes, and mouse livers, coimmunoprecipitation and two-dimensional gel electrophoresis revealed that CAR can form a homodimer in a configuration in which the PP2A/RACK1 binding site is buried within its dimer interface. Epidermal growth factor (EGF) was found to stimulate CAR homodimerization, thus constraining CAR in its inactive form. The agonistic ligand CITCO binds directly to the CAR homodimer and dissociates phosphorylated CAR into its monomers, exposing the PP2A/RACK1 binding site for dephosphorylation. Phenobarbital, which is not a CAR ligand, binds the EGF receptor, reversing the EGF signal to monomerize CAR for its indirect activation. Thus, the homodimer-monomer conversion is the underlying molecular mechanism that regulates CAR activation, by placing phosphorylated threonine 38 as the common target for both direct and indirect activation of CAR.

Phenobarbital Meets Phosphorylation of Nuclear Receptors

Phenobarbital was the first therapeutic drug to be characterized for its induction of hepatic drug metabolism. Essentially at the same time, cytochrome P450, an enzyme that metabolizes drugs, was discovered. After nearly 50 years of investigation, the molecular target of phenobarbital induction has now been delineated to phosphorylation at threonine 38 of the constitutive androstane receptor (NR1I3), a member of the nuclear receptor superfamily. Determining this mechanism has provided us with the molecular basis to understand drug induction of drug metabolism and disposition. Threonine 38 is conserved as a phosphorylation motif in the majority of both mouse and human nuclear receptors, providing us with an opportunity to integrate diverse functions of nuclear receptors. Here, I review the works and accomplishments of my laboratory at the National Institutes of Health National Institute of Environmental Health Sciences and the future research directions of where our study of the constitutive androstane receptor might take us.

Role of Nuclear Constitutive Androstane Receptor in Regulation of Hepatocyte Proliferation and Hepatocarcinogenesis

Activation of the constitutive androstane receptor (CAR) in hepatocytes occurs as a body adaptation in response to a number of external influences, and its functional activity is primarily related to induction of enzymes detoxifying xenobiotics. However, special attention was recently given to CAR due to the fact that its key role becomes unveiled in various physiological and pathophysiological processes occurring in the liver: gluconeogenesis, metabolism of fatty acids and bilirubin, hormonal regulation, proliferation of hepatocytes, and hepatocarcinogenesis. Here we review the main pathways and mechanisms that elevate hepatocyte proliferative activity related to CAR and whose disturbance may be a pivotal factor in hepatocarcinogenesis.

Transactivation Assays to Assess Canine and Rodent Pregnane X Receptor (PXR) and Constitutive Androstane Receptor (CAR) Activation

The pregnane X receptor (PXR/SXR, NR1I2) and constitutive androstane receptor (CAR, NR1I3) are nuclear receptors (NRs) involved in the regulation of many genes including cytochrome P450 enzymes (CYPs) and transporters important in metabolism and uptake of both endogenous substrates and xenobiotics. Activation of these receptors can lead to adverse drug effects as well as drug-drug interactions. Depending on which nuclear receptor is activated will determine which adverse effect could occur, making identification important. Screening for NR activation by New Molecular Entities (NMEs) using cell-based transactivation assays is the singular high throughput method currently available for identifying the activation of a particular NR. Moreover, screening for species-specific NR activation can minimize the use of animals in drug development and toxicology studies. With this in mind, we have developed in vitro transactivation assays to identify compounds that activate canine and rat PXR and CAR3. We found differences in specificity for canine and rat PXR, with the best activator for canine PXR being 10 μM SR12813 (60.1 ± 3.1-fold) and for rat PXR, 10 μM dexamethasone (60.9 ± 8.4 fold). Of the 19 test agents examined, 10 and 9 significantly activated rat and canine PXR at varying degrees, respectively. In contrast, 5 compounds exhibited statistically significant activation of rat CAR3 and 4 activated the canine receptor. For canine CAR3, 50 μM artemisinin proved to be the best activator (7.3 ± 1.8 and 10.5 ± 2.2 fold) while clotrimazole (10 μM) was the primary activator of the rat variant (13.7 ± 0.8 and 26.9 ± 1.3 fold). Results from these studies demonstrated that cell-based transactivation assays can detect species-specific activators and revealed that PXR was activated by at least twice as many compounds as was CAR3, suggesting that there are many more agonists for PXR than CAR.

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.

Phosphorylation of Farnesoid X Receptor at Serine 154 Links Ligand Activation With Degradation

Comparison of 11 human nuclear receptor amino acid sequences revealed a conserved phosphorylation motif within their DNA-binding domains as an intramolecular signal that regulates proteolytic degradation. Nuclear receptors use this signal to either degrade or proscribe degradation through either the proteasome or nonproteasome pathways. A phosphomimetic farnesoid X receptor (FXR) S154D mutant neither bound to nor trans-activated an FXR-response element-driven reporter gene and was rapidly degraded in COS-1 cells. Ectopically expressed FXR had increased Ser154 phosphorylation in COS-1 cells after ligand treatment, and knock-down of the nuclear vaccinia-related kinase 1 (VRK1) greatly reduced this phosphorylation. FXR was phosphorylated at Ser154 in the nucleus of centrilobular hepatocytes only in ligand-treated mice. Thus, FXR Ser154 phosphorylation is a rheostat for activation and subsequent degradation that controls receptor levels and activity.