Novel CAR-mediated mechanism for synergistic activation of two distinct elements within the human cytochrome P450 2B6 gene in HepG2 cells

Effects of marine biotoxins on drug-metabolizing cytochrome P450 enzymes and their regulation in mammalian cells

Marine biotoxins are a heterogenous group of natural toxins, which are able to trigger different types of toxicological responses in animals and humans. Health effects arising from exposure to marine biotoxins are ranging, for example, from gastrointestinal symptoms to neurological effects, depending on the individual toxin(s) ingested. Recent research has shown that the marine biotoxin okadaic acid (OA) can strongly diminish the expression of drug-metabolizing cytochrome P450 (CYP) enzymes in human liver cells by a mechanism involving proinflammatory signaling. By doing so, OA may interfere with the metabolic barrier function of liver and intestine, and thus alter the toxico- or pharmacokinetic properties of other compounds. Such effects of marine biotoxins on drug and xenobiotic metabolism have, however, not been much in the focus of research yet. In this review, we present the current knowledge on the effects of marine biotoxins on CYP enzymes in mammalian cells. In addition, the role of CYP-regulating nuclear receptors as well as inflammatory signaling in the regulation of CYPs by marine biotoxins is discussed. Strong evidence is available for effects of OA on CYP enzymes, along with information about possible molecular mechanisms. For other marine biotoxins, knowledge on effects on drug metabolism, however, is scarce.

A high-throughput cell-based gaussia luciferase reporter assay for measurement of CYP1A1, CYP2B6, and CYP3A4 induction

The induction of cytochrome P450s can result in reduced drug efficacy and lead to potential drug-drug interactions. The xenoreceptors-aryl hydrocarbon receptor (AhR), constitutive androstane receptor (CAR), and pregnane X receptor (PXR)-play key roles in CYP induction by xenobiotics. In order to be able to rapidly screen for the induction of three enzymes (CYP1A1, CYP2B6, and CYP3A4), we generated a stable AhR-responsive HepG2 cell line, a stable CAR-responsive HepG2 cell line, and a stable PXR-responsive HepG2 cell line.To validate these stable xenoreceptor-responsive HepG2 cell lines, we evaluated the induction of the different Gaussia reporter activities, as well as the mRNA and protein expression levels of endogenous CYPs in response to different inducers.The induction of luciferase activity in the stable xenoreceptor-responsive HepG2 cell lines by specific inducers occurred in a concentration dependent manner. There was a positive correlation between the induction of luciferase activities and the induction endogenous CYP mRNA expression levels. These xenoreceptor-responsive HepG2 cell lines were further validated with known CYP1A1, CYP2B6, and CYP3A4 inducers.These stable xenoreceptor-responsive HepG2 cell lines may be used in preclinical research for the rapid and sensitive detection of AhR, CAR, and PXR ligands that induce CYP450 isoforms.

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.

DL5050, a Selective Agonist for the Human Constitutive Androstane Receptor

The constitutive androstane receptor (CAR) is a xenobiotic sensor governing the transcription of genes involved in drug disposition, energy homeostasis, and cell proliferation. However, currently available human CAR (hCAR) agonists are nonselective, which commonly activate hCAR along with other nuclear receptors, especially the closely related human pregnane X receptor (hPXR). Using a well-known hCAR agonist CITCO as a template, we report our efforts in the discovery of a potent and highly selective hCAR agonist. Two of the new compounds of the series, 18 and 19 (DL5050), demonstrated excellent potency and selectivity for hCAR over hPXR. DL5050 preferentially induced the expression of CYP2B6 (target of hCAR) over CYP3A4 (target of hPXR) on both the mRNA and protein levels. The selective hCAR agonist DL5050 represents a valuable tool molecule to further define the biological functions of hCAR, and may also be used as a new lead in the discovery of hCAR agonists for various therapeutic applications.

Inhibitory and Inductive Effects of Opuntia ficus indica Extract and Its Flavonoid Constituents on Cytochrome P450s and UDP-Glucuronosyltransferases

Opuntia ficus indica (OFI) is grown abundantly in arid areas and its fruits are regarded as an important food and nutrient source owing to the presence of flavonoids, minerals, and proteins. The previous report that OFI exerts phytoestrogenic activity makes it plausible for OFI-containing supplements to be used as alternative estrogen replacement therapy. In the case of polypharmacy with the consumption of OFI-containing botanicals in post- or peri-menopausal women, it is critical to determine the potential drug-OFI interaction due to the modulation of drug metabolism. In the present study, the modulating effects on the hepatic drug metabolizing enzymes (DMEs) by OFI and its flavonoid constituents (kaempferol, quercetin, isorhamnetin, and their glycosidic forms) were investigated using the liver microsomal fractions prepared from ovariectomized (OVX) rats, human liver microsomes, and human hepatocarcinoma cell line (HepG2). As a result, the oral administration of extracts of OFI (OFIE) in OVX rats induced hepatic CYP2B1, CYP3A1, and UGT2B1. OFIE, hydrolyzed (hdl) OFIE, and several flavonols induced the transcriptional activities of both CYP2B6 and CYP3A4 genes in HepG2 cells. Finally, OFIE did not inhibit activities of cytochrome P450 (CYPs) or uridine diphosphate (UDP)-glucuronosyltransferases (UGTs), whereas hdl OFIE or flavonol treatment inhibited CYP1A2 and CYP3A1/3A4 in rat and human liver microsomes. Our data demonstrate that OFIE may induce or inhibit certain types of DMEs and indicate that drug-OFI interaction may occur when the substrate or inhibitor drugs of specific CYPs or UGTs are taken concomitantly with OFI-containing products.

Antiproliferation of berberine is mediated by epigenetic modification of constitutive androstane receptor (CAR) metabolic pathway in hepatoma cells

Constitutive androstane receptor (CAR) regulates hepatic xenobiotic and energy metabolism, as well as promotes cell growth and hepatocarcinogenesis. Berberine is an ancient multipotent alkaloid drug which derived from Coptis chinensis plants. Here we report that berberine is able to be cellular uptake and accessible to chromatin in human hepatoma HepG2 cells. Berberine induces more apoptosis, cell cycle arrest, but less ROS production in CAR overexpressed mCAR-HepG2 cells. Moreover, berberine inhibits expressions of CAR and its target genes CYP2B6 and CYP3A4. Furthermore, berberine enhances DNA methylation level in whole genome but reduces that in promoter regions CpG sites of CYP2B6 and CYP3A4 genes under the presence of CAR condition. These results indicated that the antiproliferation of berberine might be mediated by the unique epigenetic modifying mechanism of CAR metabolic pathway, suggesting that berberine is a promising candidate in anticancer adjuvant chemotherapy, due to its distinct pharmacological properties in clinic.

Phenobarbital and Insulin Reciprocate Activation of the Nuclear Receptor Constitutive Androstane Receptor through the Insulin Receptor

Phenobarbital (PB) antagonized insulin to inactivate the insulin receptor and attenuated the insulin receptor downstream protein kinase B (AKT)-forkhead box protein O1 and extracellular signal-regulated kinase 1/2 signals in mouse primary hepatocytes and HepG2 cells. Hepatic AKT began dephosphorylation in an early stage of PB treatment, and blood glucose levels transiently increased in both wild-type and constitutive androstane receptor (CAR) knockout (KO) mice. On the other hand, blood glucose levels increased in wild-type mice, but not KO mice, in later stages of PB treatment. As a result, PB, acting as an insulin receptor antagonist, elicited CAR-independent increases and CAR-dependent decreases of blood glucose levels at these different stages of treatment, respectively. Reciprocally, insulin activation of the insulin receptor repressed CAR activation and induction of its target CYP2B6 gene in HepG2 cells. Thus, PB and insulin cross-talk through the insulin receptor to regulate glucose and drug metabolism reciprocally.

Mechanisms of xenobiotic receptor activation: Direct vs. indirect

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

Regulation of gene expression by CAR: an update

The constitutive androstane receptor (CAR), a member of the nuclear receptor superfamily, is a well-known xenosensor that regulates hepatic drug metabolism and detoxification. CAR activation can be elicited by a large variety of xenobiotics, including phenobarbital (PB) which is not a directly binding CAR ligand. The mechanism of CAR activation is complex and involves translocation from the cytoplasm into the nucleus, followed by further activation steps in the nucleus. Recently, epidermal growth factor receptor (EGFR) has been identified as a PB-responsive receptor, and PB activates CAR by inhibiting the EGFR signaling. In addition to regulation of drug metabolism, activation of CAR has multiple biological end points such as modulation of xenobiotic-elicited liver injury, and the role of CAR in endobiotic functions such as glucose metabolism and cholesterol homeostasis is increasingly recognized. Thus, investigations on the molecular mechanism of CAR activation are critical for the real understanding of CAR-mediated processes. Here, we summarize the current understanding of mechanisms by which CAR activators regulate gene expression through cellular signaling pathways and the roles of CAR on xenobiotic-elicited hepatocellular carcinoma, liver injury, glucose metabolism and cholesterol homeostasis.

Coordinated regulation of nuclear receptor CAR by CCRP/DNAJC7, HSP70 and the ubiquitin-proteasome system

The constitutive active/androstane receptor (CAR) plays an important role as a coordinate transcription factor in the regulation of various hepatic metabolic pathways for chemicals such as drugs, glucose, fatty acids, bilirubin, and bile acids. Currently, it is known that in its inactive state, CAR is retained in the cytoplasm in a protein complex with HSP90 and the tetratricopeptide repeat protein cytosoplasmic CAR retention protein (CCRP). Upon activation by phenobarbital (PB) or the PB-like inducer 1,4-bis[2-(3,5-dichloropyridyloxy)]-benzene (TCPOBOP), CAR translocates into the nucleus. We have identified two new components to the cytoplasmic regulation of CAR: ubiquitin-dependent degradation of CCRP and protein-protein interaction with HSP70. Treatment with the proteasome inhibitor MG132 (5 µM) causes CAR to accumulate in the cytoplasm of transfected HepG2 cells. In the presence of MG132, TCPOBOP increases CCRP ubiquitination in HepG2 cells co-expressing CAR, while CAR ubiquitination was not detected. MG132 treatment of HepG2 also attenuated of TCPOBOP-induced CAR transcriptional activation on reporter constructs which contain CAR-binding DNA elements derived from the human CYP2B6 gene. The elevation of cytoplasmic CAR protein with MG132 correlated with an increase of HSP70, and to a lesser extent HSP60. Both CCRP and CAR were found to interact with endogenous HSP70 in HepG2 cells by immunoprecipitation analysis. Induction of HSP70 levels by heat shock also increased cytoplasmic CAR levels, similar to the effect of MG132. Lastly, heat shock attenuated TCPOBOP-induced CAR transcriptional activation, also similar to the effect of MG132. Collectively, these data suggest that ubiquitin-proteasomal regulation of CCRP and HSP70 are important contributors to the regulation of cytoplasmic CAR levels, and hence the ability of CAR to respond to PB or PB-like inducers.

p38 Mitogen-activated protein kinase regulates nuclear receptor CAR that activates the CYP2B6 gene

The constitutive active/androstane receptor (CAR) regulates hepatic drug metabolism by activating genes, such as cytochrome P450, and certain transferases. p38 Mitogen-activated protein kinase (MAPK) is highly activated in human primary hepatocytes but barely in human hepatoma cell lines including HepG2 cells. Liganded-CAR induced CYP2B6 mRNA in human primary hepatocytes far more effectively than in HepG2 cells ectopically expressing CAR. In the present study, we found that activation of p38 MAPK by anisomycin potentiated induction of CYP2B6 mRNA by CAR ligand in HepG2 cells to levels observed in ligand-treated human primary hepatocytes. siRNA knockdown of p38 MAPK abrogated the ability of anisomycin to synergistically induce CYP2B6 mRNA. In addition to CYP2B6, anisomycin cotreatment potentiated an increase in CYP2A7 and CYP2C9 mRNAs but not CYP3A4 or UDP-glucuronosyltransferase 1A1 mRNAs. Thus, activated p38 MAPK is required for liganded-CAR to selectively activate a set of genes that encode drug-metabolizing enzymes. Our present results suggest that CAR-mediated induction of these enzymes cannot be understood by ligand binding alone because the specificity and magnitude of induction are codetermined by a given cell signaling, such as p38 MAPK; both physiologic and pathophysiological states of cell signaling may have a strong impact in hepatic drug-metabolizing capability during treatments.

CREBH determines the severity of sulpyrine-induced fatal shock

Although the pyrazolone derivative sulpyrine is widely used as an antipyretic analgesic drug, side effects, including fatal shock, have been reported. However, the molecular mechanism underlying such a severe side effect is largely unclear. Here, we report that the transcription factor CREBH that is highly expressed in the liver plays an important role in fatal shock induced by sulpyrine in mice. CREBH-deficient mice were resistant to experimental fatal sulpyrine shock. We found that sulpyrine-induced expression of cytochrome P450 2B (CYP2B) family genes, which are involved in sulpyrine metabolism, in the liver was severely impaired in CREBH-deficient mice. Moreover, introduction of CYP2B in CREBH-deficient liver restored susceptibility to sulpyrine. Furthermore, ectopic expression of CREBH up-regulated CYP2B10 promoter activity, and in vivo knockdown of CREBH in wild-type mice conferred a significant resistance to fatal sulpyrine shock. These data demonstrate that CREBH is a positive regulator of CYP2B in response to sulpyrine administration, which possibly results in fatal shock.

Estradiol induces cytochrome P450 2B6 expression at high concentrations: implication in estrogen-mediated gene regulation in pregnancy

Pregnancy alters the rate and extent of drug metabolism, but little is known about the underlying molecular mechanism. We have found that 17β-estradiol (E2) upregulates expression of the major drug-metabolizing enzyme CYP2B6 in primary human hepatocytes. Results from promoter reporter assays in HepG2 cells revealed that E2 activates constitutive androstane receptor (CAR) and enhances promoter activity of CYP2B6, for which high concentrations of E2 reached during pregnancy were required. E2 triggered nuclear translocation of CAR in primary rat hepatocytes that were transiently transfected with human CAR as well as in primary human hepatocytes, further confirming transactivation of CAR by E2. E2-activated estrogen receptor (ER) also enhanced CYP2B6 promoter activity. The DNA-binding domain of ER was not required for the induction of CYP2B6 promoter activity by E2, suggesting involvement of a non-classical mechanism of ER action. Results from deletion and mutation assays as well as electrophorectic mobility shift and supershift assays revealed that two AP-1 binding sites (-1782/-1776 and -1664/-1658 of CYP2B6) are critical for ER-mediated activation of the CYP2B6 promoter by E2. Concurrent activation of both ER and CAR by E2 enhanced CYP2B6 expression in a synergistic manner. Our data demonstrate that at high concentrations reached during pregnancy, E2 activates both CAR and ER that synergistically induce CYP2B6 expression. These results illustrate pharmacological activity of E2 that would likely become prominent during pregnancy.

Polyphenols in alcoholic beverages activating constitutive androstane receptor CAR

The constitutive androstane receptor CAR is a xenosensing nuclear receptor that can be activated by natural polyphenols such as flavonoids and catechins. We examined alcoholic beverage phytochemicals for their ability to activate CAR. HepG2 cells were transfected with CAR expression vector and its reporter gene, and then treated with trans-resveratrol, ellagic acid, β-caryophyllene, myrcene, and xanthohumol. A luciferase assay revealed that ellagic acid and trans-resveratrol activated both human and mouse CAR. Since CAR regulates many genes involved in energy metabolism, the possibility exists that these polyphenols would reduce the risk of certain alcohol-induced metabolic disorders with the help of CAR.

Cigarette smoke extract induces CYP2B6 through constitutive androstane receptor in hepatocytes

Smoking induces a wide range of drug-metabolizing enzymes. Among them, CYP2B6 as well as CYP1A2 is well known to be up-regulated in smokers. Although the induction of CYP1A2 is mediated by the aryl hydrocarbon receptor, the molecular mechanisms of CYP2B6 induction by smoking remain to be fully elucidated. In this study, by preparing cigarette smoke extract (CSE), we addressed the possibility that human constitutive androstane receptor (hCAR) is involved in smoking-mediated induction of CYP2B6. In HepG2 cells, CSE induced CYP1A2 but not CYP2B6, suggesting that CYP2B6 expression is differentially regulated from CYP1A2. Compared with liver in vivo, hCAR expression is dramatically reduced in cultured hepatocytes, such as HepG2. Therefore, to reconstitute hCAR signaling pathways in vitro, we generated adenovirus vector expressing hCAR. Real-time reverse transcription-polymerase chain reaction analyses revealed that the adenoviral transfection of hCAR resulted in the up-regulation of CYP2B6 mRNA, even in the absence of CSE. It is interesting to note that CSE stimulation augmented hCAR-mediated induction of CYP2B6. In contrast, the expression of CYP2B6 was not enhanced by adenovirus vector expressing β-galactosidase, a control vector, either in the presence or absence of CSE. In summary, hCAR mediated the CYP2B6 induction by CSE in Hep2G cells. These data suggest that smoking up-regulates CYP2B6 through hCAR in vivo.

Constitutive active/androstane receptor promotes hepatocarcinogenesis in a mouse model of non-alcoholic steatohepatitis

The nuclear receptor constitutive active/androstane receptor (CAR) acts as a sensor of toxic byproducts derived from the endogenous metabolism and exogenous chemicals. We previously reported that CAR is responsible for exacerbating hepatic injury and fibrosis in a dietary model of non-alcoholic steatohepatitis (NASH) via upregulation of lipid peroxidation. In this study, we investigated the pathological roles of the CAR in the development of hepatocellular carcinoma in NASH model. CAR+/+ and CAR-/- mice were fed methionine- and choline-deficient (MCD) diet after tumor initiation with a single dose of the genotoxic carcinogen diethylnitrosamine (DEN) at 2 weeks of age. Interestingly, the MCD diet dramatically promoted DEN-induced hepatocarcinogenesis in CAR+/+ mice. However, the deletion of CAR leads to a significantly lower tumor incidence and smaller tumor diameter. Hepatocytes of MCD-treated-CAR+/+ mice showed a significantly higher staining frequency of Ki-67, a marker of cell proliferation, and exhibited a higher expression of c-Myc and FoxM1 transcripts compared with MCD-treated CAR-/- mice. Immunohistochemistry revealed the nuclear translocation of CAR thus suggesting that the activation of CAR signaling increased in the hepatocytes of CAR+/+ mice fed MCD diet. In addition, in vitro experiments using the CAR stably expressed cell line with TCPOBOP have suggested that CAR activation directly leads to cell proliferation. Survival was significantly lower in the CAR+/+ mice fed the MCD diet in comparison with the CAR-/- mice. Taken together, these results suggest that CAR may therefore play a critical role in the hepatocarcinogenesis of the murine NASH model via the upregulation of cell proliferation.

Activation of xenobiotic receptors: driving into the nucleus

IMPORTANCE OF THE FIELD

Xenobiotic receptors (XRs) play pivotal roles in regulating the expression of genes that determine the clearance and detoxification of xenobiotics, such as drugs and environmental chemicals. Recently, it has become increasingly evident that most XRs shuttle between the cytoplasm and nucleus, and activation of such receptors is directly associated with xenobiotic-induced nuclear import.

AREAS COVERED IN THIS REVIEW

The scope of this review covers research literature that discusses nuclear translocation and activation of XRs, as well as unpublished data generated from this laboratory. Specific emphasis is given to the constitutive androstane receptor (CAR), the pregnane X receptor and the aryl hydrocarbon receptor.

WHAT THE READERS WILL GAIN

A number of molecular chaperons presumably associated with cellular localization of XRs have been identified. Primary hepatocyte cultures have been established as a unique model retaining inactive CAR in the cytoplasm. Moreover, several splicing variants of human CAR exhibit altered cellular localization and chemical activation.

TAKE HOME MESSAGE

Nuclear accumulation is an essential step in the activation of XRs. Although great strides have been made, much remains to be understood concerning the mechanisms underlying intracellular localization and trafficking of XRs, which involve both direct ligand-binding and indirect pathways.

Synergistically enhanced CYP2B6 inducibility between a polymorphic mutation in CYP2B6 promoter and pregnane X receptor activation

CYP2B6 is a highly inducible and polymorphic enzyme involved in the metabolism of an increasing number of clinically important drugs. Significant interindividual variability in CYP2B6 expression has been attributed to either genetic polymorphisms or chemical-mediated induction through the activation of constitutive androstane receptor and/or pregnane X receptor (PXR). It was reported that the -82T→C substitution within the CYP2B6*22 allele creates a functional CCAAT/enhancer-binding protein (C/EBP) binding site and enhances the basal expression of the CYP2B6 gene. Here, we explored whether this polymorphic mutation could affect drug-mediated induction of CYP2B6. Cell-based promoter reporter assays demonstrated that CYP2B6 luciferase activity was synergistically enhanced in the presence of both -82T→C mutation and rifampicin (RIF)-activated PXR. On the other hand, this synergism was attenuated by disrupting the C/EBP binding site or knocking down C/EBPα expression. Mechanistic studies revealed that C/EBPα plays an important role in such synergism by directly interacting with PXR; enhancing RIF-mediated recruitment of PXR to the -82T→C harboring CYP2B6 promoter; and looping the PXR-bound distal phenobarbital-responsive enhancer module toward the proximal C/EBP binding site. Furthermore, the genotype-phenotype association was evaluated in cultured human primary hepatocytes from 44 donors. Interestingly, RIF-mediated induction of CYP2B6 in four -82T/C carriers was higher compared with that in the reference -82T/T homozygotes. Together, our results demonstrate, for the first time, a synergistic interplay between a CYP2B6 polymorphism and PXR-mediated induction, which may contribute to the large individual variations and inducibility of CYP2B6 in humans.

CCAAT/enhancer-binding protein alpha (C/EBPalpha) and hepatocyte nuclear factor 4alpha (HNF4alpha) synergistically cooperate with constitutive androstane receptor to transactivate the human cytochrome P450 2B6 (CYP2B6) gene: application to the development of a metabolically competent human hepatic cell model

The transcription of tissue-specific and inducible genes is usually subject to the dynamic control of multiple activators. Dedifferentiated hepatic cell lines lose the expression of tissue-specific activators and many characteristic hepatic genes, such as drug-metabolizing cytochrome P450. Here we demonstrate that by combining adenoviral vectors for CCAAT/enhancer-binding protein alpha (C/EBPalpha), hepatocyte nuclear factor 4alpha (HNF4alpha), and constitutive androstane receptor, the CYP2B6 expression and inducibility by CITCO are restored in human hepatoma HepG2 cells at levels similar to those in cultured human hepatocytes. Moreover, several other phase I and II genes are simultaneously activated, which suggests that this is an effective approach to endow dedifferentiated human hepatoma cells with a particular metabolic competence and response to inducers. In order to gain insight into the molecular mechanism, we examined the cooperation of these three transcription factors on the CYP2B6 5'-flanking region. We show new CYP2B6-responsive sequences for C/EBPalpha and HNF4alpha and a novel synergistic regulatory mechanism whereby C/EBPalpha, HNF4alpha, and constitutive androstane receptor bind and cooperate through proximal and distal response elements to confer a maximal level of expression. The results obtained from human liver also suggest that important differences in the expression and binding of C/EBPalpha and HNF4alpha could account for the large interindividual variability of the hepatic CYP2B6 enzyme, which metabolizes commonly used drugs.

Histone deacetylase inhibitors induce cytochrome P450 2B by activating nuclear receptor constitutive androstane receptor

Valproic acid, a histone deacetylase (HDAC) inhibitor, induces the cytochrome P450 2B subfamily. However, the effects of HDAC inhibitors on CYP2B induction are still not fully understood. Nuclear receptor constitutive androstane receptor (CAR) is a key regulator of CYP2B induction. In this study, we investigated the effect of HDAC inhibitors on CAR-mediated CYP2B induction. The expression of CYP2B6 mRNA was induced in HepG2 cells stably expressing mouse CAR (Ym17) by HDAC inhibitors including valproic acid, phenylbutyrate, and trichostatin A. HDAC inhibitors activated the phenobarbital-responsive enhancer module of the CYP2B6 promoter in transient transfection reporter assays with Ym17 cells. Furthermore, HDAC inhibitors synergistically augmented the effect of the CAR ligand, 1,4-bis[2-(3,5-dichloropyridyloxy)]benzene, in the transactivation of CYP2B6 mRNA and the promoter assay in Ym17 cells. Intraperitoneal injection of HDAC inhibitors induced Cyp2b10 mRNA in wild-type mice. However, such induction was not observed in CAR(-/-) mice. Immunoprecipitation demonstrated that CAR formed a complex with HDACs. HDAC inhibitors diminished the binding between CAR and HDAC1 and augmented the binding of steroid receptor coactivator-1 (SRC-1) to CAR. Furthermore, small interfering RNA knockdown of HDAC1 increased CYP2B6 mRNA expression. These results provide novel insight into the mechanism by which HDAC inhibitors affect gene expression of CYP2B6. HDAC inhibitors have the potential to up-regulate CYP2B6 through the dissociation of HDAC1 and recruitment of SRC-1 to receptor CAR.

Dietary flavonoids activate the constitutive androstane receptor (CAR)

The constitutive androstane receptor (CAR) is known as a xeno-sensor that regulates genes involved in xenobiotic excretion and energy metabolism. This study tested a variety of polyphenols for their ability to modulate CAR activity. HepG2 cells were transfected with a CAR expression plasmid and a reporter plasmid containing the human CYP2B6 regulatory region and then treated with flavonoids, catechins, and other bioactive polyphenols. Luciferase assays revealed that baicalein (5,6,7-OH flavone) was a potent activator of both human and mouse CAR. Catechin gallates also activated human and mouse CAR. Wild-type and CAR knockout mice were treated with baicalein and chrysin (5,7-OH flavone), and their liver mRNA was analyzed by real-time polymerase chain reaction (PCR). A significant increase in cyp2b10 mRNA content was observed only in wild-type mice fed chrysin. These results suggest that dietary flavonoids regulate CAR activity and thereby accelerate both detoxification and energy metabolism.

Glucocorticoids and phenobarbital induce murine CYP2B genes by independent mechanisms

BACKGROUND

Genes for CYP of the 2B subfamily (CYP2B genes) have long been known to be inducible in murine liver by phenobarbital and phenobarbital-like inducers. More recently, it has become clear that glucocorticoids can also induce these genes by a mechanism independent of that of phenobarbital-like inducers.

OBJECTIVE

To summarize the evidence for the existence of two distinct molecular mechanisms for induction of murine CYP2B genes and to analyze the wider implications of this situation for inducible xenobiotic metabolism.

METHODS

The mechanism of action of phenobarbital-like inducers of murine CYP2B genes is first briefly summarized. The role of glucocorticoids in the induction of various proteins, particularly rat phosphoenolpyruvate carboxykinase, where transcriptional activation is achieved via a glucocorticoid response unit, is also discussed. Finally, recent results are presented on glucocorticoid induction of murine CYP2B genes, including evidence for the presence of a functional glucocorticoid response unit in the rat CYP2B2 gene and for the role of constitutive androstane receptor as an accessory factor in this response.

RESULTS/CONCLUSION

Murine CYP2B genes are seen to respond to two distinct regulatory mechanisms, but much remains to be learned concerning the interactions between these two regulatory loops, as well as the details of glucocorticoid induction.

Retinoids activate RXR/CAR-mediated pathway and induce CYP3A

Retinoids and carotenoids are frequently used as antioxidants to prevent cancer. In this study, a panel of retinoids and carotenoids was examined to determine their effects on activation of RXR/CAR-mediated pathway and regulation of CYP3A gene expression. Transient transfection assays of HepG2 cells revealed that five out of thirteen studied retinoids significantly induced RXRalpha/CAR-mediated activation of luciferase activity that is driven by the thymidine kinase promoter linked with a PXR binding site in the CYP3A4 gene [tk-(3A4)(3)-Luc reporter]. All-trans retinoic acid (RA) and 9-cis RA were more effective than CAR agonist TCBOPOP in induction of the tk-(3A4)(3)-Luc reporter. Addition of retinoid and TCBOPOP further enhanced the inducibility and the induction was preferentially mediated by RXRalpha/CAR and RXRgamma/CAR heterodimer. Chromatin immunoprecipitation assay showed that retinoids recruit RXRalpha and CAR to the proximal ER6 and distal XREM nuclear receptor response elements of the CYP3A4 gene promoter. The experimental data demonstrate that retinoids can effectively regulate CYP3A gene expression through the RXR/CAR-mediated pathway.

Early growth response 1 loops the CYP2B6 promoter for synergistic activation by the distal and proximal nuclear receptors CAR and HNF4alpha

Nuclear xenobiotic receptor CAR activates transcription of the CYP2B6 gene by directly binding to the distal enhancer PB responsive enhancer module (PBREM). This CAR-mediated activation is synergized by transcription factors early growth response 1 (EGR1) and hepatocyte-enriched nuclear factor 4alpha (HNF4alpha) that bind to the proximal element OA response element KI (OARE(KI)) [Inoue, K., & Negishi, M. (2008). Nuclear receptor CAR requires early growth response 1 to activate the human cytochrome P450 2B6 gene. J. Biol. Chem. 283, 10425-10432]. Two additional EGR1 binding sites have now been found just downstream from PBREM. Internal deletion of EGR1 sites within the context of the -1.8 kb CYP2B6 promoter, which contains both PBREM and OARE(KI), revealed that the distal and proximal EGR1 sites are essential for EGR1 to synergize CAR-mediated transcription. Chromatin conformation capture 3C assays demonstrated that ERG1 may loop the distal PBREM towards the proximal OARE(KI) so that together, CAR and HNF4alpha synergistically activate the CYP2B6 promoter.

Activation of CAR and PXR by Dietary, Environmental and Occupational Chemicals Alters Drug Metabolism, Intermediary Metabolism, and Cell Proliferation

The constitutive androstane receptor (CAR) and the pregnane × receptor (PXR) are activated by a variety of endogenous and exogenous ligands, such as steroid hormones, bile acids, pharmaceuticals, and environmental, dietary, and occupational chemicals. In turn, they induce phase I-III detoxification enzymes and transporters that help eliminate these chemicals. Because many of the chemicals that activate CAR and PXR are environmentally-relevant (dietary and anthropogenic), studies need to address whether these chemicals or mixtures of these chemicals may increase the susceptibility to adverse drug interactions. In addition, CAR and PXR are involved in hepatic proliferation, intermediary metabolism, and protection from cholestasis. Therefore, activation of CAR and PXR may have a wide variety of implications for personalized medicine through physiological effects on metabolism and cell proliferation; some beneficial and others adverse. Identifying the chemicals that activate these promiscuous nuclear receptors and understanding how these chemicals may act in concert will help us predict adverse drug reactions (ADRs), predict cholestasis and steatosis, and regulate intermediary metabolism. This review summarizes the available data on CAR and PXR, including the environmental chemicals that activate these receptors, the genes they control, and the physiological processes that are perturbed or depend on CAR and PXR action. This knowledge contributes to a foundation that will be necessary to discern interindividual differences in the downstream biological pathways regulated by these key nuclear receptors.

CYP2B6: new insights into a historically overlooked cytochrome P450 isozyme

Human CYP2B6 has been thought to account for a minor portion (<1%) of total hepatic cytochrome P450 (CYP) content and to have a minor function in human drug metabolism. Recent studies, however, indicate that the average relative contribution of CYP2B6 to total hepatic CYP content ranges from 2% to 10%. An increased interest in CYP2B6 research has been stimulated by the identification of an ever-increasing substrate list for this enzyme, polymorphic and ethnic variations in expression levels, and evidence for cross-regulation with CYP3A4, UGT1A1 and several hepatic drug transporters by the nuclear receptors pregnane X receptor and constitutive androstane receptor. Moreover, 20- to 250-fold interindividual variation in CYP2B6 expression has been demonstrated, presumably due to transcriptional regulation and polymorphisms. These individual differences may result in variable systemic exposure to drugs metabolized by CYP2B6, including the antineoplastics cyclophosphamide and ifosfamide, the antiretrovirals nevirapine and efavirenz, the anesthetics propofol and ketamine, the synthetic opioid methadone, and the anti-Parkinsonian selegiline. The potential clinical significance of CYP2B6 further enforces the need for a comprehensive review of this xenobiotic metabolizing enzyme. This communication summarizes recent advances in our understanding of this traditionally neglected enzyme and provides an overall picture of CYP2B6 with respect to expression, localization, substrate-specificity, inhibition, regulation, polymorphisms and clinical significance. Emphasis is given to nuclear receptor mediated transcriptional regulation, genetic polymorphisms, and their clinical significance.

Phenobarbital Confers its Diverse Effects by Activating the Orphan Nuclear Receptor Car

In the early 1960s, phenobarbital (PB) was shown to induce hepatic drug metabolism and the induction was implicated in the molecular mechanism of drug tolerance development. Since then, it has become evident that PB not only induces drug metabolism, but also triggers pleiotropic effects on liver function, such as cell growth and communication, proliferation of the endoplasmic reticulum, tumor promotion, glucose metabolism, steroid/thyroid hormone metabolism, and bile acid synthesis. Upon activation by PB and numerous PB-type inducers, the nuclear receptor CAR mediates those pleiotropic actions by regulating various hepatic genes, utilizing multiple regulatory mechanisms.

The antiapoptotic factor growth arrest and DNA-damage-inducible 45 beta regulates the nuclear receptor constitutive active/androstane receptor-mediated transcription

The nuclear receptor constitutive active/androstane receptor (CAR) up-regulated expression of the apoptotic growth arrest and DNA-damage-inducible 45 beta (GADD45B) gene in HepG2 cells. Overexpression of GADD45B augmented CAR-mediated induction of the human CYP2B gene by the CAR activator 1,4-bis[2-(3,5-dichloropyridyloxy)]benzene (TCPOBOP) and coactivated CAR-dependent transcription of the NR1-luciferase reporter gene. Small interfering RNA knockdown of GADD45B resulted in repression of both the induction and the coactivation. Induction of the mouse Cyp2b10 gene by TCPOBOP was profoundly attenuated in the primary hepatocytes prepared from GADD45B-knockout mice compared with those from wild-type mice. Because CAR is a key transcription factor that activates the genes that encode for xenobiotic metabolizing enzymes and transporters, GADD45B, acting as a CAR coactivator and coregulating CAR target genes, may be involved in hepatic drug metabolism and excretion of xenobiotics.

Nuclear receptor CAR requires early growth response 1 to activate the human cytochrome P450 2B6 gene

The nuclear receptor CAR (constitutive active/androstane receptor) is a drug-sensing transcription factor, regulating the hepatic genes that encode various drug-metabolizing enzymes. We have now characterized the novel regulatory mechanism by which the signal molecule EGR1 (early growth response 1) determines CAR-mediated activation of the human CYP2B6 (cytochrome P450 2B6) gene. The CYP2B6 enzyme metabolizes commonly used therapeutics and also activates pro-drugs. The CAR directly binds to the distal enhancer element of the CYP2B6 promoter, which is essential in converging to its drug-sensing function onto promoter activity. However, this binding alone is not sufficient to activate the CYP2B6 promoter; the promoter requires EGR1 to enable CAR to activate the CYP2B6 promoter. Upon stimulation by protein kinase C, EGR1 directly binds to the proximal promoter and coordinates the nearby HNF4alpha (hepatocyte-enriched nuclear factor 4alpha) with CAR at the distal enhancer element to activate the promoter. Thus, synergy of drug activation and the stimulation of cellular signal are necessary for CAR to activate the CYP2B6 gene.

Rifaximin is a gut-specific human pregnane X receptor activator

Rifaximin, a rifamycin analog approved for the treatment of travelers' diarrhea, is also beneficial in the treatment of multiple chronic gastrointestinal disorders. However, the mechanisms contributing to the effects of rifaximin on chronic gastrointestinal disorders are not fully understood. In the current study, rifaximin was investigated for its role in activation of the pregnane X receptor (PXR), a nuclear receptor that regulates genes involved in xenobiotic and limited endobiotic deposition and detoxication. PXR-humanized (hPXR), Pxr-null, and wild-type mice were treated orally with rifaximin, and rifampicin, a well characterized human PXR ligand. Rifaximin was highly concentrated in the intestinal tract compared with rifampicin. Rifaximin treatment resulted in significant induction of PXR target genes in the intestine of hPXR mice, but not in wild-type and Pxr-null mice. However, rifaximin treatment demonstrated no significant effect on hepatic PXR target genes in wild-type, Pxr-null, and hPXR mice. Consistent with the in vivo data, cell-based reporter gene assay revealed rifaximin-mediated activation of human PXR, but not the other xenobiotic nuclear receptors constitutive androstane receptor, peroxisome proliferator-activated receptor (PPAR)alpha, PPARgamma, and farnesoid X receptor. Pretreatment with rifaximin did not affect the pharmacokinetics of the CYP3A substrate midazolam, but it increased the C(max) and decreased T(max) of 1'-hydroxymidazolam. Collectively, the current study identified rifaximin as a gut-specific human PXR ligand, and it provided further evidence for the utility of hPXR mice as a critical tool for the study of human PXR activators. Further human studies are suggested to assess the potential role of rifaximin-mediated gut PXR activation in therapeutics of chronic gastrointestinal disorders.

CAR and PXR: the xenobiotic-sensing receptors

The xenobiotic receptors CAR and PXR constitute two important members of the NR1I nuclear receptor family. They function as sensors of toxic byproducts derived from endogenous metabolism and of exogenous chemicals, in order to enhance their elimination. This unique function of CAR and PXR sets them apart from the steroid hormone receptors. In contrast, the steroid receptors, exemplified by the estrogen receptor (ER) and glucocorticoid receptor (GR), are the sensors that tightly monitor and respond to changes in circulating steroid hormone levels to maintain body homeostasis. This divergence of the chemical- and steroid-sensing functions has evolved to ensure the fidelity of the steroid hormone endocrine regulation while allowing development of metabolic elimination pathways for xenobiotics. The development of the xenobiotic receptors CAR and PXR also reflect the increasing complexity of metabolism in higher organisms, which necessitate novel mechanisms for handling and eliminating metabolic by-products and foreign compounds from the body. The purpose of this review is to discuss similarities and differences between the xenobiotic receptors CAR and PXR with the prototypical steroid hormone receptors ER and GR. Interesting differences in structure explain in part the divergence in function and activation mechanisms of CAR/PXR from ER/GR. In addition, the physiological roles of CAR and PXR will be reviewed, with discussion of interactions of CAR and PXR with endocrine signaling pathways.

Extracellular signal-regulated kinase is an endogenous signal retaining the nuclear constitutive active/androstane receptor (CAR) in the cytoplasm of mouse primary hepatocytes

The nuclear receptor constitutive active/androstane receptor (CAR) is sequestered in the cytoplasm of liver cells before its activation by therapeutic drugs and xenobiotics such as phenobarbital (PB) and 1,4-Bis[2-(3,5-dichloropyridyloxy)]benzene (TCPOBOP) in mouse liver, the regulatory mechanism of which remains poorly understood. Given the finding that epidermal growth factor repressed PB activation of CAR-mediated transcription (Mol Pharmacol 65:172-180, 2004), here we investigated the regulatory role of hepatocyte growth factor (HGF)-mediated signal in sequestering CAR in the cytoplasm of mouse primary hepatocytes. HGF treatment effectively repressed the induction of endogenous CYP2b10 gene by PB and TCPOBOP in mouse primary hepatocytes. On the other hand, inhibition by 1,4-diamino-2,3-dicyano-1,4-bis(methylthio)butadiene (U0126) of an HGF downstream kinase mitogen-activated protein kinase kinase (MEK) induced the Cyp2b10 gene and up-regulated the CAR-regulated promoter activity in the absence of TCPOBOP. HGF treatment increased phosphorylation of extracellular signal-regulated kinase (ERK) 1/2 in the cytosol, thus decreasing the TCPOBOP-induced nuclear accumulation of CAR. In contrast, U0126 dephosphorylated ERK1/2 and increased nuclear CAR accumulation in the absence of TCPOBOP. These results are consistent with the conclusion that the HGF-dependent phosphorylation of ERK1/2 is the endogenous signal that sequesters CAR in the cytoplasm of mouse primary hepatocytes.

Cohesin protein SMC1 represses the nuclear receptor CAR-mediated synergistic activation of a human P450 gene by xenobiotics

CAR (constitutive active/androstane receptor) regulates both the distal enhancer PBREM (phenobarbital-responsive enhancer module) and the proximal element OARE [OA (okadaic acid) response element] to synergistically up-regulate the endogenous CYP2B6 (where CYP is cytochrome P450) gene in HepG2 cells. In this up-regulation, CAR acts as both a transcription factor and a co-regulator, directly binding to and enhancing PBREM upon activation by xenobiotics such as TCPOBOP {1,4-bis-[2-(3,5-dichloropyridyloxy)]benzene} and indirectly associating with the OARE in response to OA [Swales, Kakizaki, Yamamoto, Inoue, Kobayashi and Negishi (2005) J. Biol. Chem. 280, 3458-3466]. We have now identified the cohesin protein SMC1 (structural maintenance of chromosomes 1) as a CAR-binding protein and characterized it as a negative regulator of OARE activity, thus repressing synergy. Treatment with SMC1 small interfering RNA augmented the synergistic up-regulation of CYP2B6 expression 20-fold in HepG2 cells, while transient co-expression of spliced form of SMC1 abrogated the synergistic activation of a 1.8 kb CYP2B6 promoter. SMC1 indirectly binds to a 19 bp sequence (-236/-217) immediately downstream from the OARE in the CYP2B6 promoter. Both DNA affinity and chromatin immunoprecipitation assays showed that OA treatment dissociates SMC1 from the CYP2B6 promoter, reciprocating the indirect binding of CAR to OARE. These results are consistent with the conclusion that SMC1 binding represses OARE activity and its dissociation allows the recruitment of CAR to the OARE, synergizing PBREM activity and the expression of the CYP2B6 gene.

Growth hormone regulation of sex-dependent liver gene expression

The liver is a primary target for the action of GH, a pituitary protein hormone that regulates a broad range of physiological processes, including long bone growth, fatty acid oxidation, glucose uptake, and hepatic steroid and foreign compound metabolism. GH exerts sex-dependent effects on the liver in many species, with many hepatic genes, most notably genes coding for cytochrome P450 (CYP) enzymes, being transcribed in a sex-dependent manner. Sex differences in CYP expression are most striking in rats and mice (up to 500-fold male-female differences), but are also seen, albeit to a much smaller degree, in humans, where they are an important determinant of the sex dependence of hepatic drug and steroid metabolism. This article examines the mechanisms whereby GH, via its sex-dependent temporal patterns of pituitary release, activates intracellular signaling leading to the sexually dimorphic transcription of CYPs and other liver-expressed genes. Recent findings implicating the GH-regulated transcription factor STAT5b (signal transducer and activator of transcription 5b), hepatocyte nuclear factors 3beta, 4alpha and 6, and sex differences in DNA methylation and chromatin structure in the sex-dependent actions of GH are reviewed, and current mechanistic models are evaluated.

Serine 202 regulates the nuclear translocation of constitutive active/androstane receptor

The constitutive active receptor (CAR) in mouse primary hepatocytes undergoes okadaic acid (OA)-sensitive nuclear translocation after activation by xenobiotics such as phenobarbital (PB) and 1,4 bis[2-(3,5-dichloropyridyloxy)]benzene (TCPOBOP). We have now mimicked this TCPOBOP-dependent and OA-sensitive translocation of mouse CAR (mCAR) in HepG2 cells and have demonstrated that protein phosphatase 2A regulates this nuclear translocation. Site-directed mutagenesis analysis of various Ser and Thr residues delineated the translocation activity to Ser-202. Mutation of Ser-202 to Asp (S202D) prevented mCAR translocation into the nucleus of TCPOBOP-treated HepG2 cells. In addition, in the livers of Car-/- mice, the YFP-tagged S202D mutant did not translocate into the nucleus after PB treatment. To examine whether Ser-202 can be phosphorylated, flag-tagged wild-type mCAR or flag-tagged S202A mutant was expressed in HepG2 cells and subjected to Western blot analysis using an antibody specific to a peptide containing phospho-Ser-202. A high molecular weight phosphorylated form of CAR was detected only with the wild-type mCAR. These results are consistent with the conclusion that the dephosphorylation of Ser-202 is a required step that regulates the xenobiotic-dependent nuclear translocation of mCAR.

The nuclear receptors constitutive androstane receptor and pregnane X receptor cross-talk with hepatic nuclear factor 4alpha to synergistically activate the human CYP2C9 promoter

CYP2C9 is an important human drug-metabolizing enzyme that is expressed primarily in liver. Recent studies in our laboratory have shown that the nuclear receptor pregnane X receptor (PXR) is important in the transcriptional activation of the CYP2C9 promoter by drugs such as rifampicin and that the essential element is a constitutive androstane receptor (CAR)/PXR site -1839 bp upstream of the translation start site. Both CAR and PXR transcriptionally up-regulate the CYP2C9 promoter via these elements. In the present study, we ask whether additional sites in the proximal promoter also play a role in this induction. We identify two proximal hepatic nuclear factor (HNF) 4alpha binding sites at -152 and -185 bp of the CYP2C9 promoter, both of which bind HNF4alpha in gel shift assays and transcriptionally up-regulate this promoter in response to HNF4alpha in HepG2 cells. HNF4alpha synergizes with CAR and with PXR in HepG2 cells treated with rifampicin. The synergy only occurs when the CAR/PXR binding site at -1839 bp is present. Mutation of the two HNF4alpha binding sites differentially prevented up-regulation of CYP2C9 promoter by both CAR as well as HNF4alpha, synergy between the two receptors, and essentially abolished induction by rifampicin in HepG2 cells transfected with PXR. These studies strongly support the hypothesis that there is cross talk between distal CAR/PXR sites and HNF4alpha binding sites in the CYP2C9 promoter and that the HNF4alpha sites are required for maximal induction of the CYP2C9 promoter.