Phenobarbital Meets Phosphorylation of Nuclear Receptors

Liver matrin-3 protects mice against hepatic steatosis and stress response via constitutive androstane receptor

OBJECTIVE

The prevalence of metabolic dysfunction-associated steatotic liver disease (MASLD) continues to rise with the increasing obesity epidemic. Rezdiffra as an activator of a thyroid hormone receptor-beta is the only Food and Drug Administration approved therapy. As such, there is a critical need to improve our understanding of gene expression regulation and signaling transduction in MASLD to develop new therapies. Matrin-3 is a DNA- and RNA-binding protein involved in the pathogenesis of human diseases. Here we examined its previously uncharacterized role in limiting hepatic steatosis and stress response via the constitutive androstane receptor (CAR).

METHODS

Matrin-3 floxed and liver-specific knockout mice were fed either a chow diet or 60 kcal% high-fat diet (HFD) for up to 16 weeks. The mice were euthanized for different analysis including liver histology, lipid levels, and gene expression. Bulk RNA-seq, bulk ATAC-seq, and single-nucleus Multiome were used to examine changes of transcriptome and chromatin accessibility in the liver. Integrative bioinformatics analysis of our data and publicly available datasets and different biochemical assays were performed to identify underlying the molecular mechanisms mediating matrin-3's effects. Liver-tropic adeno-associated virus was used to restore the expression of CAR for lipid, acute phase genes, and histological analysis.

RESULTS

Matrin-3 expression is induced in the steatotic livers of mice. Liver-specific matrin-3 deletion exacerbated HFD-induced steatosis, acute phase response, and inflammation in the liver of female mice. The transcriptome and chromatin accessibility were re-programmed in the liver of these mice with signatures indicating that CAR signaling is dysregulated. Mechanistically, matrin-3 interacts with CAR mRNA, and matrin-3 deficiency promotes CAR mRNA degradation. Consequently, matrin-3 deletion impaired CAR signaling by reducing CAR expression. Matrin-3 levels positively correlate with CAR expression in human livers. Ces2a and Il1r1 were identified as new target genes of CAR. Interestingly, we found that CAR discords with the expression of its target genes including Cyp2b10 and Ces2a in response to HFD, indicating CAR signaling is dysregulated by HFD despite increased CAR expression. Dysregulated CAR signaling upon matrin-3 deficiency reduced Ces2a and de-repressed Il1r1 expression. CAR restoration partially abrogated the dysregulated gene expression, exacerbated hepatic steatosis, acute phase response, and inflammation in liver-specific matrin-3 knockout mice fed a HFD.

CONCLUSIONS

Our findings demonstrate that matrin-3 is a key upstream regulator maintaining CAR signaling upon metabolic stress, and the matrin-3-CAR axis limits hepatic steatosis and stress response signaling that may give insights for therapeutic intervention.

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.

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.

The Function of Xenobiotic Receptors in Metabolic Diseases

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

[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.

Clinical Relevance of the Constitutive Androstane Receptor

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

Searching for Constitutive Androstane Receptor Modulators

The constitutive androstane receptor (CAR; NR1I3) has been established as one of the main drug- and xenobiotic-responsive transcriptional regulators, collectively called xenosensors. CAR activates the expression of several oxidative, hydrolytic, and conjugative drug-metabolizing enzymes and drug transporters, and therefore, it contributes to drug and xenobiotic elimination, drug interactions, and toxicological processes. This minireview introduces mechanisms that modulate CAR activity and focuses on the recent approaches used to search and characterize CAR agonists, inverse agonists, and indirect activators. This minireview is dedicated to Dr. Masahiko Negishi to celebrate his scientific achievements during his long service at the National Institutes of Health. SIGNIFICANCE STATEMENT: Discovery and characterization of human constitutive androstane receptor (CAR) modulators is important for drug development, toxicity studies, and in generation of chemical tools to dissect biological functions of CAR. This minireview focuses on the main methods used to search for these compounds and discusses their essential features.

Distinct Roles of the Sister Nuclear Receptors PXR and CAR in Liver Cancer Development

Pregnane X receptor (PXR) and constitutively active receptor/constitutive androstane receptor (CAR) are xenobiotic-responsible transcription factors belonging to the same nuclear receptor gene subfamily and highly expressed in the liver. These receptors are activated by a variety of chemicals and play pivotal roles in many liver functions, including xenobiotic metabolism and disposition. Phenobarbital, an enzyme inducer and liver tumor promoter, activates both rodent and human CAR but causes liver tumors only in rodents. Although the precise mechanism for phenobarbital/CAR-mediated liver tumor formation remains to be established, intracellular pathways, including the Hippo pathway/Yes-associated protein-TEA-domain family members system and β-catenin signaling, seem to be involved. In contrast to CAR, previous findings by our group suggest that PXR activation does not promote hepatocyte proliferation but it enhances the proliferation induced by various stimuli. Moreover, and surprisingly, PXR may have antitumor effects in both rodents and humans by targeting inflammatory cytokine signals, angiogenesis and epithelial-mesenchymal transition. In this review, we summarize the current knowledge on the associations of PXR and CAR with hepatocyte proliferation and liver tumorigenesis and their molecular mechanisms and species differences. SIGNIFICANCE STATEMENT: Pregnane X receptor and constitutively active receptor/constitutive androstane receptor have very similar functions in the gene regulation associated with xenobiotic disposition, as suggested by their identification as xenosensors for enzyme induction. In contrast, recent reports clearly suggest that these receptors play distinct roles in the control of hepatocyte proliferation and liver cancer development. Understanding these differences at the molecular level may help us evaluate the human safety of chemical compounds and develop novel drugs targeting liver cancers.

Sex-specific expression mechanism of hepatic estrogen inactivating enzyme and transporters in diabetic women

Circulating estrogens levels significantly decrease in menopause and levels off in postmenopausal women. Accordingly, the liver represses levels of enzymes and membrane transporters, thereby decreasing capability of inactivating and excreting estrogens. Women increasingly develop type 2 diabetes during or after menopause. Estrogens are known to promote liver diseases in these women. Here, we have found that the estrogen inactivating sulfotransferase (SULT1E1) and an ATP-binding cassette subfamily G member 2 (ABCG2), a gene encoding breast cancer resistance protein that exports sulfated estrogens, increased their expression levels in diabetic women but not men. For the sulfotransferase gene, phosphorylated nuclear receptors ERα and RORα, at Ser212 and Ser100, respectively, bind their response elements to activate the SULT1E1 promoter in women. This coordinated increase in estrogen inactivation and excretion, and the phosphorylated nuclear receptor-mediated gene activation could be a defense mechanism against toxicities of estrogens through inactivation and excretion in the livers of women.

Effect of exposure to 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) and polychlorinated biphenyls (PCBs) on mitochondrial DNA (mtDNA) copy number in rats

Mitochondria are intracellular organelles responsible for biological oxidation and energy production. These organelles are susceptible to damage from oxidative stress and compensate for damage by increasing the number of copies of their own genome, mitochondrial DNA (mtDNA). Cancer and environmental exposure to some pollutants have also been associated with altered mtDNA copy number. Since exposures to polychlorinated biphenyls (PCBs) and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) have been shown to increase oxidative stress, we hypothesize that mtDNA copy number will be altered with exposure to these compounds. mtDNA copy number was measured in DNA from archived frozen liver and lung specimens from the National Toxicology Program (NTP) study of female Harlan Sprague Dawley rats exposed to TCDD (3, 10, or 100 ng/kg/day), dioxin-like (DL) PCB 126 (10, 100, or 1000 ng/kg/day), non-DL PCB 153 (10, 100, or 1000 μg/kg/day), and PCB 126 + PCB 153 (10 ng/kg/day + 10 μg/kg/day, 100 ng/kg/day + 100 μg/kg/day, or 1000 ng/kg/day + 1000 μg/kg/day, respectively) for 13 and 52 weeks. An increase in mtDNA copy number was observed in the liver and lung of rats exposed to TCDD and the lung of rats exposed to the mixture of PCB 126 and PCB 153. A statistically significant positive dose-dependent trend was also observed in the lung of rats exposed to PCB 126 and a mixture of PCB 153 and PCB 126, although in neither case was the control copy number significantly exceeded at any dose level. These exposures produced a range of pathological responses in these organs in the two-year NTP studies. Conversely, there was a significant decrease or no change in mtDNA copy number in the liver and lung of rats exposed to non-DL PCB 153. This is consistent with a general lack of PCB 153 mediated liver or lung injury in the NTP study, with the exception of liver hypertrophy. Together, the results suggest that an increase in mtDNA copy number may serve as a sensitive, early biomarker of mitochondrial injury and oxidative stress that contributes to the development of the toxicity of dioxin-like compounds.

Phosphorylation of vaccinia-related kinase 1 at threonine 386 transduces glucose stress signal in human liver cells

Vaccinia-related kinase 1 (VRK1) is a chromatin-associated Ser-Thr kinase that regulates numerous downstream factors including DNA repair as well as stress factors c-Jun and p53. Both c-Jun and p53 are phosphorylated at Ser63 and Thr18, respectively, in response to low glucose (40 mg/dl of medium) but not high glucose (140 mg/dl of medium) in human hepatoma-derived Huh-7 cells. Here, we have determined the molecular mechanism by which VRK1 phosphorylates these residues in response to glucose in Huh-7 cells. Human VRK1 auto-phosphorylates Ser376 and Thr386 in in vitro kinase assays. In Huh-7 cells, this auto-phosphorylation activity is regulated by glucose signaling; Thr386 is auto-phosphorylated only in low glucose medium, while Ser376 is not phosphorylated in either medium. A correlation of this low glucose response phosphorylation of Thr386 with the phosphorylation of c-Jun and p53 suggests that VRK1 phosphorylated at Thr386 catalyzes this phosphorylation. In fact, VRK1 knockdown by siRNA decreases and over-expression of VRK1 T386D increases phosphorylated c-Jun and p53 in Huh-7 cells. Phosphorylation by VRK1 of c-Jun but not p53 is regulated by cadherin Plakophilin-2 (PKP2). The PKP2 is purified from whole extracts of Huh-7 cells cultured in low glucose medium and is characterized to bind a C-terminal peptide of the VRK1 molecules to regulate its substrate specificity toward c-Jun. siRNA knockdowns show that PKP2 transduces low glucose signaling to VRK1 only to phosphorylate c-Jun, establishing the low glucose-PKP2-VRK1-c-Jun pathway as a glucose stress signaling 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.

Reduced hepatic global hydroxymethylation in mice treated with non-genotoxic carcinogens is transiently reversible with a methyl supplemented diet

Non-genotoxic carcinogens (NGCs) are known to cause perturbations in DNA methylation, which can be an early event leading to changes in gene expression and the onset of carcinogenicity. Phenobarbital (PB) has been shown to alter liver DNA methylation and hydroxymethylation patterns in mice in a time dependent manner. The goals of this study were to assess if clofibrate (CFB), a well-studied rodent NGC, would produce epigenetic changes in mice similar to PB, and if a methyl donor supplementation (MDS) would modulate epigenetic and gene expression changes induced by phenobarbital. CByB6F1 mice were treated with 0.5% clofibrate or 0.14% phenobarbital for 7 and 28 days. A subgroup of PB treated and control mice were also fed MDS diet. Liquid Chromatography-Ionization Mass Spectrometry (LC-MS) was used to quantify global liver 5-methylcytosine (5mC) and 5-hydroxymethylcytosine (5hmC) levels. Gene expression analysis was conducted using Affymetrix microarrays. A decrease in liver 5hmC but not 5mC levels was observed upon treatment with both CFB and PB with varying time of onset. We observed moderate increases in 5hmC levels in PB-treated mice when exposed to MDS diet and lower expression levels of several phenobarbital induced genes involved in cell proliferation, growth, and invasion, suggesting an early modulating effect of methyl donor supplementation. Overall, epigenetic profiling can aid in identifying early mechanism-based biomarkers of non-genotoxic carcinogenicity and increases the quality of cancer risk assessment for candidate drugs. Global DNA methylation assessment by LC-MS is an informative first step toward understanding the risk of carcinogenicity.

Antiepileptic Drug-Activated Constitutive Androstane Receptor Inhibits Peroxisome Proliferator-Activated Receptor α and Peroxisome Proliferator-Activated Receptor γ Coactivator 1α-Dependent Gene Expression to Increase Blood Triglyceride Levels

Long-term administration of some antiepileptic drugs often increases blood lipid levels. In this study, we investigated its molecular mechanism by focusing on the nuclear receptors constitutive active/androstane receptor (CAR) and peroxisome proliferator-activated receptor α (PPARα), which are key transcription factors for enzyme induction and lipid metabolism, respectively, in the liver. Treatment of mice with the CAR activator phenobarbital, an antiepileptic drug, increased plasma triglyceride levels and decreased the hepatic expression of PPARα target genes related to lipid metabolism. The increase in PPARα target gene expression induced by fenofibrate, a PPARα ligand, was inhibited by cotreatment with phenobarbital. CAR suppressed PPARα-dependent gene transcription in HepG2 cells but not in COS-1 cells. The mRNA level of peroxisome proliferator-activated receptor γ coactivator 1α (PGC1α), a coactivator for both CAR and PPARα, in COS-1 cells was much lower than in HepG2 cells. In reporter assays with COS-1 cells overexpressing PGC1α, CAR suppressed PPARα-dependent gene transcription, depending on the coactivator-binding motif. In mammalian two-hybrid assays, CAR attenuated the interaction between PGC1α and PPARα Chemical inhibition of PGC1α prevented phenobarbital-dependent increases in plasma triglyceride levels and the inhibition of PPARα target gene expression. These results suggest that CAR inhibits the interaction between PPARα and PGC1α, attenuating PPARα-dependent lipid metabolism. This might explain the antiepileptic drug-induced elevation of blood triglyceride levels. SIGNIFICANCE STATEMENT: Constitutive active/androstane receptor activated by antiepileptic drugs inhibits the peroxisome proliferator-activated receptor α-dependent transcription of genes related to lipid metabolism and upregulates blood triglyceride levels. The molecular mechanism of this inhibition involves competition between these nuclear receptors for coactivator peroxisome proliferator-activated receptor γ coactivator-1α binding.

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.

Telomeres as targets for the toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and polychlorinated biphenyls (PCBs) in rats

Telomere length (TL) can be affected by various factors, including age and oxidative stress. Changes in TL have been associated with chronic disease, including a higher risk for several types of cancer. Environmental exposure of humans to PCBs and dioxins has been associated with longer or shorter leukocyte TL. Relative telomere length (RTL) may serve as a biomarker associated with neoplastic and/or non-neoplastic responses observed with chronic exposures to TCDD and PCBs. RTL was measured in DNA isolated from archived frozen liver and lung tissues from the National Toxicology Program (NTP) studies conducted in female Harlan Sprague Dawley rats exposed for 13, 30, and 52 weeks to TCDD, dioxin-like (DL) PCB 126, non-DL PCB 153, and a mixture of PCB 126 and PCB 153. RTL was assessed by quantitative polymerase chain reaction (qPCR). Consistent with literature, decreased liver and lung RTL was seen with aging. Relative to time-matched vehicle controls, RTL was increased in both the liver and lung tissues of rats exposed to TCDD, PCB 126, PCB 153, and the mixture of PCB 126 and PCB 153, which is consistent with most epidemiological studies that found PCB exposures were associated with increased leukocyte RTL. Increased RTL was observed at doses and/or time points where little to no pathology was observed. In addition to serving as a biomarker of exposure to these compounds in rats and humans, increases in RTL may be an early indicator of neoplastic and non-neoplastic responses that occur following chronic exposure to TCDD and PCBs.

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.

The Power of Resolution: Contextualized Understanding of Biological Responses to Liver Injury Chemicals Using High-throughput Transcriptomics and Benchmark Concentration Modeling

Prediction of human response to chemical exposures is a major challenge in both pharmaceutical and toxicological research. Transcriptomics has been a powerful tool to explore chemical-biological interactions, however, limited throughput, high-costs, and complexity of transcriptomic interpretations have yielded numerous studies lacking sufficient experimental context for predictive application. To address these challenges, we have utilized a novel high-throughput transcriptomics (HTT) platform, TempO-Seq, to apply the interpretive power of concentration-response modeling with exposures to 24 reference compounds in both differentiated and non-differentiated human HepaRG cell cultures. Our goals were to (1) explore transcriptomic characteristics distinguishing liver injury compounds, (2) assess impacts of differentiation state of HepaRG cells on baseline and compound-induced responses (eg, metabolically-activated), and (3) identify and resolve reference biological-response pathways through benchmark concentration (BMC) modeling. Study data revealed the predictive utility of this approach to identify human liver injury compounds by their respective BMCs in relation to human internal exposure plasma concentrations, and effectively distinguished drug analogs with varied associations of human liver injury (eg, withdrawn therapeutics trovafloxacin and troglitazone). Impacts of cellular differentiation state (proliferated vs differentiated) were revealed on baseline drug metabolizing enzyme expression, hepatic receptor signaling, and responsiveness to metabolically-activated toxicants (eg, cyclophosphamide, benzo(a)pyrene, and aflatoxin B1). Finally, concentration-response modeling enabled efficient identification and resolution of plausibly-relevant biological-response pathways through their respective pathway-level BMCs. Taken together, these findings revealed HTT paired with differentiated in vitro liver models as an effective tool to model, explore, and interpret toxicological and pharmacological interactions.

Nuclear receptor CAR-mediated liver cancer and its species differences

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

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.

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.

Widespread epigenetic changes to the enhancer landscape of mouse liver induced by a specific xenobiotic agonist ligand of the nuclear receptor CAR

CAR (Nr1i3), a liver nuclear receptor and xenobiotic sensor, induces drug, steroid and lipid metabolism and dysregulates genes linked to hepatocellular carcinogenesis, but its impact on the liver epigenome is poorly understood. TCPOBOP, a halogenated xenochemical and highly specific CAR agonist ligand, induces localized chromatin opening or closing at several thousand mouse liver genomic regions, discovered as differential DNase-hypersensitive sites (ΔDHS). Active enhancer and promoter histone marks induced by TCPOBOP were enriched at opening DHS and TCPOBOP-inducible genes. Enrichment of CAR binding and CAR motifs was seen at opening DHS and their inducible drug/lipid metabolism gene targets, and at many constitutively open DHS located nearby. TCPOBOP-responsive cell cycle and DNA replication genes co-dependent on MET/EGFR signaling for induction were also enriched for CAR binding. A subset of opening DHS and many closing DHS mapping to TCPOBOP-responsive target genes did not bind CAR, indicating an indirect mechanism for their changes in chromatin accessibility. TCPOBOP-responsive DHS were also enriched for induced binding of RXRA, CEBPA and CEBPB, and for motifs for liver-enriched factors that may contribute to liver-specific transcriptional responses to TCPOBOP exposure. These studies elucidate the enhancer landscape of TCPOBOP-exposed liver and the widespread epigenetic changes that are induced by both direct and indirect mechanisms linked to CAR activation. The global maps of thousands of environmental chemical-induced epigenetic changes described here constitute a rich resource for further research on xenochemical effects on liver chromatin states and the epigenome.

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.

Proteomic Analysis Reveals Novel Mechanisms by Which Polychlorinated Biphenyls Compromise the Liver Promoting Diet-Induced Steatohepatitis

Environmental pollution contributes to fatty liver disease pathogenesis. Polychlorinated biphenyl (PCB) exposures have been associated with liver enzyme elevation and suspected steatohepatitis in cohort studies. Male mice treated with the commercial PCB mixture, Aroclor 1260 (20 mg/kg), and fed high fat diet (HFD) for 12 weeks developed steatohepatitis. Receptor-based modes of action including inhibition of the epidermal growth factor (EGF) receptor were previously proposed, but other mechanisms likely exist. Objectives were to identify and validate the pathways, transcription factors, and mechanisms responsible for the steatohepatitis associated with PCB and HFD coexposures. Comparative proteomics analysis was performed in archived mouse liver samples from the aforementioned chronic exposure study. Pathway and transcription factor analysis (TFA) was performed, and selected results were validated. Liver proteomics detected 1103 unique proteins. Aroclor 1260 upregulated 154 and downregulated 93 of these. Aroclor 1260 + HFD coexposures affected 55 pathways including glutathione metabolism, intermediary metabolism, and cytoskeletal remodeling. TFA of Aroclor 1260 treatment demonstrated alterations in the function of 42 transcription factors including downregulation of NRF2 and key nuclear receptors previously demonstrated to protect against steatohepatitis (e.g., HNF4α, FXR, PPARα/δ/γ, etc.). Validation studies demonstrated that Aroclor 1260 significantly reduced HNF4α protein levels, while Aroclor 1260 + HFD reduced expression of the HNF4α target gene, albumin, in vivo. Aroclor 1260 attenuated EGF-dependent HNF4α phosphorylation and target gene activation in vitro. Aroclor 1260 reduced levels of NRF2, its target genes, and glutathione in vivo. Aroclor 1260 attenuated EGF-dependent NRF2 upregulation, in vitro. Aroclor 1260 indirectly activated hepatic stellate cells in vitro via induction of hepatocyte-derived TGFβ. PCB exposures adversely impacted transcription factors regulating liver protection, function, and fibrosis. PCBs, thus, compromised the liver by reducing its protective responses against nutritional stress to promote diet-induced steatohepatitis. The identified mechanisms by which environmental pollutants influence fatty liver disease pathogenesis require confirmation in humans.

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.

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.

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.