Cytoplasmic accumulation of the nuclear receptor CAR by a tetratricopeptide repeat protein in HepG2 cells

PPP1R16A, the membrane subunit of protein phosphatase 1beta, signals nuclear translocation of the nuclear receptor constitutive active/androstane receptor

Constitutive active/androstane receptor (CAR), a member of the nuclear steroid/thyroid hormone receptor family, activates transcription of numerous hepatic genes upon exposure to therapeutic drugs and environmental pollutants. Sequestered in the cytoplasm, this receptor signals xenobiotic exposure, such as phenobarbital (PB), by translocating into the nucleus. Unlike other hormone receptors, translocation can be triggered indirectly without binding to xenobiotics. We have now identified a membrane-associated subunit of protein phosphatase 1 (PPP1R16A, or abbreviated as R16A) as a novel CAR-binding protein. When CAR and R16A are coexpressed in mouse liver, CAR translocates into the nucleus. Close association of R16A and CAR molecule on liver membrane was shown by fluorescence resonance energy transfer (FRET) analysis using expressed yellow fluorescent protein (YFP)-CAR and CFP-R16A fusion proteins. R16A can form dimer through its middle region, where protein kinase A phosphorylation sites are recently identified. Translocation of CAR by R16A correlates with the ability of R16A to form an intermolecular interaction via the middle region. Moreover, this interaction is enhanced by PB treatment in mouse liver. R16A specifically interacted with PP1beta in HepG2 cells despite the highly conserved structure of PP1 family molecules. PP1beta activity was inhibited by R16A in vitro and coexpression of PP1beta in liver can prevent YFP-CAR translocation into mouse liver. Taken together, R16A at the membrane may mediate the PB signal to initiate CAR nuclear translocation, through a mechanism including its dimerization and inhibition of PP1beta activity, providing a novel model for the translocation of nuclear receptors in which direct interaction of ligands and the receptors may not be crucial.

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.

Comparison of the induction profile for drug disposition proteins by typical nuclear receptor activators in human hepatic and intestinal cells

BACKGROUND AND PURPOSE

Certain nuclear receptors (NRs) such as the constitutive androstane receptor (CAR), pregnane X receptor (PXR) and farnesoid X receptor (FXR) mediate induction of some cytochrome P450 enzymes and ABC transporters but conflicting reports exist. The purpose of this study was to assess the reasons for these discrepancies and use a standardized approach to compare activators of NRs.

EXPERIMENTAL APPROACH

Dexamethasone, pregnenolone 16alpha-carbonitrile, rifampicin, phenobarbital and chenodeoxycholic acid were incubated with HepG2, Caco-2 and cryopreserved human hepatocytes prior to analysis of mRNA and protein for CYP2B6, CYP3A4, CYP3A5, ABCB1, ABCC1, ABCC2, PXR, CAR and FXR.

KEY RESULTS

Dexamethasone significantly up-regulated PXR, CYP3A4 and ABCB1 expression in HepG2 and Caco-2 cells. As a result, including dexamethasone as a media supplement masked the induction of these genes by pregnenolone 16alpha-carbonitrile, which may explain discrepancies between previous reports. In the absence of dexamethasone, significant activator-dependent induction was observed in all cell types. Significant correlations were observed between the fold increase in mRNA and in protein, which were, for most instances, logarithmic. Changes in mRNA expression were greater in cell lines than primary cells but for most transcripts correlations were observed between fold increases in HepG2 and hepatocytes.

CONCLUSIONS AND IMPLICATIONS

Clearly, no in vitro system can imitate the physiology of a hepatocyte or intestinal cell within an intact organ in vivo, but these data explain some of the discrepancies reported between laboratories and have important implications for study design.

Human nuclear pregnane X receptor cross-talk with CREB to repress cAMP activation of the glucose-6-phosphatase gene

The nuclear PXR (pregnane X receptor) was originally characterized as a key transcription factor that activated hepatic genes encoding drug-metabolizing enzymes. We have now demonstrated that PXR also represses glucagon-activated transcription of the G6Pase (glucose-6-phosphatase) gene by directly binding to CREB [CRE (cAMP-response element)-binding protein]. Adenoviral-mediated expression of human PXR (hPXR) and its activation by rifampicin strongly repressed cAMP-dependent induction of the endogenous G6Pase gene in Huh7 cells. Using the -259 bp G6Pase promoter construct in cell-based transcription assays, repression by hPXR of PKA (cAMP-dependent protein kinase)-mediated promoter activation was delineated to CRE sites. GST (glutathione transferase) pull-down and immunoprecipitation assays were employed to show that PXR binds directly to CREB, while gel-shift assays were used to demonstrate that this binding prevents CREB interaction with the CRE. These results are consistent with the hypothesis that PXR represses the transcription of the G6Pase gene by inhibiting the DNA-binding ability of CREB. In support of this hypothesis, treatment with the mouse PXR activator PCN (pregnenolone 16alpha-carbonitrile) repressed cAMP-dependent induction of the G6Pase gene in primary hepatocytes prepared from wild-type, but not from PXR-knockout, mice, and also in the liver of fasting wild-type, but not PXR-knockout, mice. Moreover, ChIP (chromatin immunoprecipitation) assays were performed to show a decreased CREB binding to the G6Pase promoter in fasting wild-type mice after PCN treatment. Thus drug activation of PXR can repress the transcriptional activity of CREB, down-regulating gluconeogenesis.

Induction of nuclear translocation of constitutive androstane receptor by peroxisome proliferator-activated receptor alpha synthetic ligands in mouse liver

Peroxisome proliferators activate nuclear receptor peroxisome proliferator-activated receptor alpha (PPARalpha) and enhance the transcription of several genes in liver. We report here that synthetic PPARalpha ligands Wy-14,643, ciprofibrate, clofibrate, and others induce the nuclear translocation of constitutive androstane receptor (CAR) in mouse liver cells in vivo. Adenoviral-enhanced green fluorescent protein-CAR expression demonstrated that PPARalpha synthetic ligands drive CAR into the hepatocyte nucleus in a PPARalpha- and PPARbeta-independent manner. This translocation is dependent on the transcription coactivator PPAR-binding protein but independent of coactivators PRIP and SRC-1. PPARalpha ligand-induced nuclear translocation of CAR is not associated with induction of Cyp2b10 mRNA in mouse liver. PPARalpha ligands interfered with coactivator recruitment to the CAR ligand binding domain and reduced the constitutive transactivation of CAR. Both Wy-14,643 and ciprofibrate occupied the ligand binding pocket of CAR and adapted a binding mode similar to that of the CAR inverse agonist androstenol. These observations, therefore, provide information for the first time to indicate that PPARalpha ligands not only serve as PPARalpha agonists but possibly act as CAR antagonists.

Redundant enhancement of mouse constitutive androstane receptor transactivation by p160 coactivator family members

Constitutive androstane receptor (CAR) transactivation is enhanced by p160 coactivators, which include three members, SRC-1, SRC-2, and SRC-3. Each of the p160 coactivators enhanced mouse CAR (mCAR) transactivation of the CYP2B1 phenobarbital (PB)-responsive enhancer in transfected cultured cells and mouse hepatocytes in vivo. The cellular localization of the p160 coactivators in hepatocytes in vivo was not altered by PB treatment, nor did any of the p160 coactivators selectively colocalize with mCAR in the nucleus. Exogenous expression of each p160 coactivator mediated the PB-independent nuclear accumulation of mCAR in hepatocytes in vivo. Induction of Cyp2b10 gene expression by PB was equivalent or greater in mice null for each of the p160 coactivators than in wild type mice. These results indicate that the p160 coactivators are redundant with regard to enhancing CAR-mediated induction of cytochrome P450 genes. SRC-3 alone of the p160 coactivators enhanced CAR transactivation in hepatic cells without PB treatment.

Subcellular trafficking signals of constitutive androstane receptor: evidence for a nuclear export signal in the DNA-binding domain

Translocation of constitutive androstane receptor (CAR) from the cytoplasm to the nucleus is induced by phenobarbital-like drugs. Nuclear localization signals (NLSs) and a sequence [xenochemical response signal (XRS)] required for xenobiotic-induced nuclear translocation have been defined in rat and human CAR, but a nuclear export signal (NES) has not been identified. To identify cellular localization signals of CAR, the localization of fragments and mutants of mouse CAR expressed in mouse hepatocytes in vivo was examined. Consistent with other studies, an NLS in the hinge region, a diffuse NLS in the ligand-binding domain, and a cytoplasmic retention sequence were identified, and mutation of the XRS blocked nuclear accumulation both in phenobarbital-treated mice in vivo and in untreated HepG2 cells. Fusing the simian virus 40 NLS to the mutant proteins reversed the localization defect resulting from mutation of the hinge NLS but not that from mutation of the XRS, indicating that the XRS is not simply a novel phenobarbital-responsive NLS. In the DNA-binding domain, a sequence in CAR is conserved with an NES identified in other nuclear receptors. Mutation of two conserved phenylalanines in this sequence resulted in increased nuclear localization of both full-length CAR and a CAR fragment containing the DNA-binding domain. The DNA-binding domain sequence, therefore, may contain an NES, which is consistent with nucleocytoplasmic shuttling of CAR. The results demonstrate that regulation of the cellular localization of CAR is complex, with multiple sequences mediating nuclear import and export and retention in the cytoplasm.

Aryl-hydrocarbon receptor activation regulates constitutive androstane receptor levels in murine and human liver

The aryl-hydrocarbon receptor (AhR) is a basic helix-loop-helix/Per-Arnt-Sim transcription factor that can be activated by exogenous as well as endogenous ligands. AhR is traditionally associated with xenobiotic metabolism. In an attempt to identify novel target genes, C57BL/6J mice were treated with beta-naphthoflavone (BNF), a known AhR ligand, and genome-wide expression analysis studies were performed using high-density microarrays. Constitutive androstane receptor (CAR) was found to be one of the differentially regulated genes. Real-time quantitative polymerase chain reaction (qPCR) verified the increase in CAR messenger RNA (mRNA) level. BNF treatment did not increase CAR mRNA in AhR-null mice. Time-course studies in mice revealed that the regulation of CAR mRNA mimicked that of Cyp1A1, a known AhR target gene. To demonstrate that the increase in CAR mRNA translates to an increase in functional CAR protein, mice were sequentially treated with BNF (6 hours) followed by the selective CAR agonist, TCPOBOP (3 hours). qPCR revealed an increase in the mRNA level of Cyp2b10, previously known to be regulated by CAR. This also suggests that CAR protein is present in limiting amounts with respect to its transactivation ability. Finally, CAR was also up-regulated in primary human hepatocytes in response to AhR activation by 2,3,7,8-tetrachlorodibenzo-p-dioxin and benzo[a]pyrene.

CONCLUSION

This study identifies a mode of up-regulating CAR and potentially expands the role of AhR in drug metabolism. This study also demonstrates in vivo up-regulation of CAR through chemical exposure.

Decreased PXR and CAR inhibit transporter and CYP mRNA Levels in the liver and intestine of mice with collagen-induced arthritis

Nuclear receptors, such as pregnane X receptor (PXR) and constitutive androstane receptor (CAR), regulate the transcription of transporters and cytochrome P450s (CYPs). We investigated whether quantitative and functional changes in PXR and CAR affected the transporters and CYPs in a mouse model of chronic arthritis. The mRNA levels of PXR were significantly decreased in the intestine of mice with collagen-induced arthritis (CIA) compared with control mice. The mRNA levels of CAR were significantly decreased in both the liver and intestine of CIA mice. The mRNA levels of Mdr1a/1b, Mrp3, BCRP and Cyp2b10 were decreased in the liver of CIA mice, while little change in the mRNA levels was observed for Cyp3a11 in the liver and the transporters in the intestine. Taken together, the present results reveal that the effects of CAR mRNA suppression on the regulation of transporters and CYPs differ between the liver and intestine in chronic arthritis.

Difference in nucleocytoplasmic shuttling sequences of rat and human constitutive active/androstane receptor

Fluorescence recovery after photobleaching (FRAP) in spontaneous multinuclear cells shows that both rat and human constitutive active/androstane receptors (CARs) are shuttling proteins with both nuclear localization signals (NLSs) and nuclear export signals (NESs). We previously identified two NLSs in rat CAR: NLS1 in the hinge region (residues 100-108) and NLS2 in the ligand-binding domain (residues 111-320). In the present study, we compared the intracellular localization signals between rat and human CARs. There was a marked difference in their intracellular localization in COS-7 cells because, unlike rat CAR, human CAR does not contain NLS1 due to an amino acid change at position 106. A CRM1-dependent leucine-rich NES, which is sensitive to an inhibitory effect of leptomycin B, was found in the cytoplasmic retention region previously identified within the ligand-binding domain of rat CAR (residues 220-258). We found that human CAR instead has a NES in the ligand-binding domain between residues 170 and 220. Also, we detected CRM1-independent C-terminal NESs between residues 317-358 of rat and human CARs. Removal of NLS1 by N-terminal truncation and mutation of xenochemical response signal caused rat CAR to localize in the cytoplasm of COS-7 cells, which we suspect is due to the masking of NLS2.

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.

Transactivation of a DR-1 PPRE by a human constitutive androstane receptor variant expressed from internal protein translation start sites

Downstream in-frame start codons produce amino-terminal-truncated human constitutive androstane receptor protein isoforms (ΔNCARs). The ΔNCARs are expressed in liver and in vitro cell systems following translation from in-frame methionine AUG start codons at positions 76, 80, 125, 128, 168 and 265 within the full-length CAR mRNA. The resulting CAR proteins lack the N-terminal DNA-binding domain (DBD) of the receptor, yielding ΔNCAR variants with unique biological function. Although the ΔNCARs maintain full retinoid X receptor alpha (RXRα) heterodimerization capacity, the ΔNCARs are inactive on classical CAR-inducible direct repeat (DR)-4 elements, yet efficiently transactivate a DR-1 element derived from the endogenous PPAR-inducible acyl-CoA oxidase gene promoter. RXRα heterodimerization with CAR1, CAR76 and CAR80 isoforms is necessary for the DR-1 PPRE activation, a function that exhibits absolute dependence on both the respective RXRα DBD and CAR activation (AF)-2 domains, but not the AF-1 or AF-2 domain of RXRα, nor CAR's DBD. A new model of CAR DBD-independent transactivation is proposed, such that in the context of a DR-1 peroxisome proliferator-activated response element, only the RXRα portion of the CAR-RXRα heterodimer binds directly to DNA, with the AF-2 domain of tethered CAR mediating transcriptional activation of the receptor complex.

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.

CAR2 displays unique ligand binding and RXRalpha heterodimerization characteristics

The constitutive androstane receptor (CAR; NR1I3) regulates the expression of genes involved in xenobiotic metabolism. Alternative splicing of the human CAR gene yields an array of mRNAs that encode structurally diverse proteins. One form of CAR, termed CAR2, contains an additional four amino acids (SPTV) that are predicted to reshape the ligand-binding pocket. The current studies show a marked, ligand-independent, CAR2-mediated transactivation of reporters containing optimal DR-3, DR-4, and DR-5 response elements, and reporters derived from the natural CYP2B6 and CYP3A4 gene promoters. Overexpression of the RXRalpha ligand binding domain was critical for achieving these effects. CAR2 interaction with SRC-1 was similarly dependent on the coexpression of RXRalpha. Mutagenesis of Ser233 (SPTV) to an alanine residue yielded a receptor possessing higher constitutive activity. Alternatively, mutating Ser233 to an aspartate residue drastically reduced the transactivation capacity of CAR2. The respective abilities of these mutagenized forms of CAR2 to transactivate a DR-4 x 3 reporter element correlated with their ability to interact with RxRalpha and to recruit SRC-1 in a ligand-regulated manner. Together, these results demonstrate a robust RXRalpha-dependent recruitment of coactivators and transactivation by CAR2. In addition, CAR2 displays novel dose responses to clotrimazole and androstanol compared with the reference form of the receptor while at the same time retaining the ability to bind CITCO. This result supports a hypothesis whereby the four-amino-acid insertion in CAR2 structurally modifies its ligand binding pocket, suggesting that CAR2 is regulated by a set of ligands distinct from those governing the activity of reference CAR.

Effects of alterations in CAR on bilirubin detoxification in mouse collagen-induced arthritis

Nuclear receptors such as constitutive androstane receptor (CAR) and pregnane X receptor (PXR) regulate the transcription of cytochromes P450 and transporters. We investigated whether quantitative and functional changes in CAR and PXR could affect bilirubin detoxification in chronic arthritis. The CAR mRNA level was significantly decreased in the liver of mice with collagen-induced arthritis (CIA) compared with control mice. In normal mice treated with CAR agonists, relatively rapid elimination of bilirubin was observed after its intravenous injection. Next, we investigated the effects of CAR on bilirubin-detoxifying enzymes and transporters in arthritis. The mRNA levels of organic anion transporter peptide (OATP) 2, glutathione S-transferase (GST) A1, and GSTA2 were decreased in CIA mice, whereas the mRNA levels of OATP4, UDP-glucuronosyl-transferase 1A1, and multidrug resistance-associated protein 2 remained unchanged. The protein levels and transport activities of OATP2 were also decreased in CIA mice. Furthermore, the CIA mice actually exhibited retarded elimination of bilirubin after its intravenous injection. These results indicate that alterations to CAR during arthritis affect the elimination of bilirubin because of changes in multiple bilirubin-detoxifying enzymes and transporters.

Nuclear pregnane X receptor cross-talk with FoxA2 to mediate drug-induced regulation of lipid metabolism in fasting mouse liver

Upon drug activation, the nuclear pregnane X receptor (PXR) regulates not only hepatic drug but also energy metabolism. Using Pxr(-/-) mice, we have now investigated the PXR-mediated repression of lipid metabolism in the fasting livers. Treatment with PXR activator pregnenolone 16alpha-carbonitrile (PCN) down-regulated the mRNA levels of carnitine palmitoyltransferase 1A (in beta-oxidation) and mitochondrial 3-hydroxy-3-methylglutarate-CoA synthase 2 (in ketogenesis) in wild-type (Pxr(+/+)) mice only. In contrast, the stearoyl-CoA desaturase 1 (in lipogenesis) mRNA was up-regulated in the PCN-treated Pxr(+/+) mice. Reflecting these up- and down-regulations and consistent with decreased energy metabolism, the levels of hepatic triglycerides and of serum 3-hydroxybutylate were increased and decreased, respectively, in the PCN-treated Pxr(+/+) mice. Using gel shift, glutathione S-transferase pull-down and cell-based reporter assays, we then examined whether PXR could cross-talk with the insulin response forkhead factor FoxA2 to repress the transcription of the Cpt1a and Hmgcs2 genes, because FoxA2 activates these genes in fasting liver. PXR directly bound to FoxA2 and repressed its activation of the Cpt1a and Hmgcs2 promoters. Moreover, ChIP assays showed that PCN treatment attenuated the binding of FoxA2 to these promoters in fasting Pxr(+/+) but not Pxr(-/-) mice. These results are consistent with the conclusion that PCN-activated PXR represses FoxA2-mediated transcription of Ctp1a and Hmgcs2 genes in fasting liver.

Drug-induced changes in P450 enzyme expression at the gene expression level: a new dimension to the analysis of drug-drug interactions

Drug-drug interactions (DDIs) caused by direct chemical inhibition of key drug-metabolizing cytochrome P450 enzymes by a co-administered drug have been well documented and well understood. However, many other well-documented DDIs cannot be so readily explained. Recent investigations into drug and other xenobiotic-mediated expression changes of P450 genes have broadened our understanding of drug metabolism and DDI. In order to gain additional information on DDI, we have integrated existing information on drugs that are substrates, inhibitors, or inducers of important drug-metabolizing P450s with new data on drug-mediated expression changes of the same set of cytochrome P450s from a large-scale microarray gene expression database of drug-treated rat tissues. Existing information on substrates and inhibitors has been updated and reorganized into drug-cytochrome P450 matrices in order to facilitate comparative analysis of new information on inducers and suppressors. When examined at the gene expression level, a total of 119 currently marketed drugs from 265 examined were found to be cytochrome P450 inducers, and 83 were found to be suppressors. The value of this new information is illustrated with a more detailed examination of the DDI between PPARalpha agonists and HMG-CoA reductase inhibitors. This paper proposes that the well-documented, but poorly understood, increase in incidence of rhabdomyolysis when a PPARalpha agonist is co-administered with a HMG-CoA reductase inhibitor is at least in part the result of PPARalpha-induced general suppression of drug metabolism enzymes in liver. The authors believe this type of information will provide insights to other poorly understood DDI questions and stimulate further laboratory and clinical investigations on xenobiotic-mediated induction and suppression of drug metabolism.

Transcription coactivator PRIP, the peroxisome proliferator-activated receptor (PPAR)-interacting protein, is redundant for the function of nuclear receptors PParalpha and CAR, the constitutive androstane receptor, in mouse liver

Disruption of the genes encoding for the transcription coactivators, peroxisome proliferator-activated receptor (PPAR)-interacting protein (PRIP/ASC-2/RAP250/TRBP/NRC) and PPAR-binding protein (PBP/TRAP220/DRIP205/MED1), results in embryonic lethality by affecting placental and multiorgan development. Targeted deletion of coactivator PBP gene in liver parenchymal cells (PBP(LIV-/-)) results in the near abrogation of the induction of PPARalpha and CAR (constitutive androstane receptor)-regulated genes in liver. Here, we show that targeted deletion of coactivator PRIP gene in liver (PRIP(LIV-/-)) does not affect the induction of PPARalpha-regulated pleiotropic responses, including hepatomegaly, hepatic peroxisome proliferation, and induction of mRNAs of genes involved in fatty acid oxidation system, indicating that PRIP is not essential for PPARalpha-mediated transcriptional activity. We also provide additional data to show that liver-specific deletion of PRIP gene does not interfere with the induction of genes regulated by nuclear receptor CAR. Furthermore, disruption of PRIP gene in liver did not alter zoxazolamine-induced paralysis, and acetaminophen-induced hepatotoxicity. Studies with adenovirally driven EGFP-CAR expression in liver demonstrated that, unlike PBP, the absence of PRIP does not prevent phenobarbital-mediated nuclear translocation/retention of the receptor CAR in liver in vivo and cultured hepatocytes in vitro. These results show that PRIP deficiency in liver does not interfere with the function of nuclear receptors PPARalpha and CAR. The dependence of PPARalpha- and CAR-regulated gene transcription on coactivator PBP but not on PRIP attests to the existence of coactivator selectivity in nuclear receptor function.

Stimulation of AMP-activated protein kinase is essential for the induction of drug metabolizing enzymes by phenobarbital in human and mouse liver

Our previous studies have suggested a role for AMP-activated protein kinase (AMPK) in the induction of CYP2B6 by phenobarbital (PB) in hepatoma-derived cells (Rencurel et al., 2005). In this study, we showed in primary human hepatocytes that: 1) 5'-phosphoribosyl-5-aminoimidazol-4-carboxamide 1-beta-d-ribofuranoside and the biguanide metformin, known activators of AMPK, dose-dependently increase the expression of CYP2B6 and CYP3A4 to an extent similar to that of PB. 2) PB, but not the human nuclear receptor constitutive active/androstane receptor (CAR) ligand 6-(4-chlorophenyl)imidazol[2,1-6][1,3]thiazole-5-carbaldehyde, dose-dependently increase AMPK activity. 3) Pharmacological inhibition of AMPK activity with compound C or dominant-negative forms of AMPK blunt the inductive response to phenobarbital. Furthermore, in transgenic mice with a liver-specific deletion of both the alpha1 and alpha2 AMPK catalytic subunits, basal levels of Cyp2b10 and Cyp3a11 mRNA were increased but not in primary culture of mouse hepatocytes. However, phenobarbital or 1,4 bis[2-(3,5-dichloropyridyloxy)]benzene, a mouse CAR ligand, failed to induce the expression of these genes in the liver or cultured hepatocytes from mice lacking hepatic expression of the alpha1 and alpha2 subunits of AMPK. The distribution of CAR between the nucleus and cytosol was not altered in hepatocytes from mice lacking both AMPK catalytic subunits. These data highlight the essential role of AMPK in the CAR-mediated signal transduction pathway.

PXR and CAR: nuclear receptors which play a pivotal role in drug disposition and chemical toxicity

Xenobiotic metabolism and detoxification is regulated by receptors (e.g., PXR, CAR) whose characterization has contributed significantly to our understanding of drug responses in humans. Technologies facilitating the screening of compounds for receptor interactions provide valuable tools applicable in drug development. Most use in vitro systems or mice humanized for receptors in vivo. In vitro assays are limited by the reporter systems and cell lines chosen and are uninformative about effects in vivo. Humanized mouse models provide novel, exciting ways of understanding the functions of these genes. This article evaluates these technologies and current knowledge on PXR/CAR-mediated regulation of gene expression.

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.

The DHR96 nuclear receptor regulates xenobiotic responses in Drosophila

Exposure to xenobiotics such as plant toxins, pollutants, or prescription drugs triggers a defense response, inducing genes that encode key detoxification enzymes. Although xenobiotic responses have been studied in vertebrates, little effort has been made to exploit a simple genetic system for characterizing the molecular basis of this coordinated transcriptional response. We show here that approximately 1000 transcripts are significantly affected by phenobarbital treatment in Drosophila. We also demonstrate that the Drosophila ortholog of the human SXR and CAR xenobiotic receptors, DHR96, plays a role in this response. A DHR96 null mutant displays increased sensitivity to the sedative effects of phenobarbital and the pesticide DDT as well as defects in the expression of many phenobarbital-regulated genes. Metabolic and stress-response genes are also controlled by DHR96, implicating its role in coordinating multiple response pathways. This work establishes a new model system for defining the genetic control of xenobiotic stress responses.

Peroxisome proliferator-activated receptor (PPAR)-binding protein (PBP) but not PPAR-interacting protein (PRIP) is required for nuclear translocation of constitutive androstane receptor in mouse liver

The constitutive androstane receptor (CAR) regulates transcription of phenobarbital-inducible genes that encode xenobiotic-metabolizing enzymes in liver. CAR is localized to the hepatocyte cytoplasm but to be functional, it translocates into the nucleus in the presence of phenobarbital-like CAR ligands. We now demonstrate that adenovirally driven EGFP-CAR, as expected, translocates into the nucleus of normal wild-type hepatocytes following phenobarbital treatment under both in vivo and in vitro conditions. Using this approach we investigated the role of transcription coactivators PBP and PRIP in the translocation of EGFP-CAR into the nucleus of PBP and PRIP liver conditional null mouse hepatocytes. We show that coactivator PBP is essential for nuclear translocation of CAR but not PRIP. Adenoviral expression of both PBP and EGFP-CAR restored phenobarbital-mediated nuclear translocation of exogenously expressed CAR in PBP null livers in vivo and in PBP null primary hepatocytes in vitro. CAR translocation into the nucleus of PRIP null livers resulted in the induction of CAR target genes such as CYP2B10, necessary for the conversion of acetaminophen to its hepatotoxic intermediate metabolite, N-acetyl-p-benzoquinone imine. As a consequence, PRIP-deficiency in liver did not protect from acetaminophen-induced hepatic necrosis, unlike that exerted by PBP deficiency. These results establish that transcription coactivator PBP plays a pivotal role in nuclear localization of CAR, that it is likely that PBP either enhances nuclear import or nuclear retention of CAR in hepatocytes, and that PRIP is redundant for CAR function.

Roles of Nuclear Receptors in the Gene Expression of Drug-metabolizing Enzymes under Various Physiological Conditions

The nuclear receptor constitutive androstane receptor (CAR), a key transcription factor for the expression of cytochrome P450 (CYP) 2B genes, resides in the cytoplasm under untreated conditions and translocates into the nucleus upon xenobiotic exposure. CAR forms a multiprotein complex including heat shock protein 90 in the cytoplasm as the glucocorticoid receptor, and it is likely that protein phosphatase 2A plays a critical role in the first step of CAR nuclear translocation. In addition to the xenobiotic induction of CYP2Bs, our recent studies have indicated that CAR is important for sex and strain differences and obesity/diabetes-associated changes in the expression of CYP2B genes. These results have raised the hypothesis that the expression of nuclear receptors varies depending on the physiologic condition, leading to the dysregulation of CYP expression. In obese mice fed a high-fat diet, however, hepatic CYP3A levels are drastically decreased without any significant changes in the expression of nuclear receptors including the pregnane X receptor and hepatocyte nuclear factor-4, which are known to be key transcription factors in the expression of CYP3A genes. These results indicate that it is important to investigate the mechanism of the transcriptional regulation of nuclear receptor genes as well as the activation of nuclear receptors to understand the CYP expression system fully.

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.

Pregnane and Xenobiotic Receptor (PXR/SXR) resides predominantly in the nuclear compartment of the interphase cell and associates with the condensed chromosomes during mitosis

Pregnane and Xenobiotic Receptor (PXR) is a transcription factor that is activated by a diverse range of xenobiotics and endogenous metabolites including steroids, bile acids and about 50% of the prescription drugs. In specific cell types (e.g. liver and intestine) it serves as a 'xenosensor' by regulating expression of a network of genes involved in xenobiotic clearance from the body. PXR expression in several cancerous tissues and its regulated expression of multi-drug resistance proteins highlight its significance in prognosis of malignancies. The view that subcellular localization and ligand induced movements of transcription factors is one of the major phenomena in regulating transcriptional activity, we used a green fluorescent protein tagged PXR chimera to study its dynamic behaviour in living cells. Under all experimental conditions, PXR was observed to be a predominantly nuclear protein maintaining a dynamic equilibrium between the nuclear and cytoplasmic compartments of the interphase cells. Interestingly, for the first time, a member of the nuclear receptor superfamily, PXR, has been observed to be associated with condensed chromosomes during all the mitotic stages of cell division. The significance of PXR association with mitotic chromosomes is discussed.

Localization of the nuclear receptor CAR at the cell membrane of mouse liver

The nuclear receptor constitutive active/androstane receptor CAR is a drug-sensing transcription factor. Upon activation by various drugs such as phenobarbital (PB), CAR translocates from the cytoplasm into the nucleus to regulate the genes that encode enzymes and proteins involved in hepatic metabolism. Here, we have shown the presence of CAR at the cell membrane of mouse livers, using Car+/+ and Car-/- mice. Levels of the cell membrane CAR increased after PB treatment. The CAR exists as a large approximately 160 kDa complex. Thus, CAR undergoes PB-induced translocation to the cell membrane, indicating that CAR may exert a non-genomic action.

Characterization of nuclear localization signals and cytoplasmic retention region in the nuclear receptor CAR

The constitutive androstane receptor (CAR) is a ligand/activator-dependent transactivation factor that resides in the cytoplasm and forms part of an as yet unidentified protein complex. Upon stimulation, CAR translocates into the nucleus where it modulates the transactivation of target genes. However, CAR exogenously expressed in rat liver RL-34 cells is located in the nucleus even in the absence of activators. By transiently transfecting RL-34 cells with various mutated rat CAR segments, we identified two nuclear localization signals: a basic amino acid-rich sequence (RRARQARRR) between amino acids 100 and 108; and an assembly of noncontiguous residues widely spread over amino acid residues 111 to 320 within the ligand binding domain. A C-terminal leucine-rich segment corresponding to a previously reported murine xenochemical response signal was not found to exhibit nuclear import activity in cultured cells. Using rat primary hepatocytes transfected with various CAR segments, we identified the region required for the cytoplasmic retention of CAR. Based on these results, the intracellular localization of CAR would be determined by the combined effects of nuclear localization signals, the xenochemical response signal, and the cytoplasmic retention region.

Nuclear receptors and drug disposition gene regulation

In this minireview, the role of various nuclear receptors and transcription factors in the expression of drug disposition genes is summarized. Specifically, the molecular aspects and functional impact of the aryl hydrocarbon receptor (AhR), nuclear factor-E2 p45-related factor 2 (N(r)f2), hepatocyte nuclear factor 1alpha (HNF1alpha), constitutive androstane receptor (LAR), pregnane X receptor (PXR), farnesoid X receptor (FXR), peroxisome proliferator-activated receptor alpha (PPAR(alpha)), hepatocyte nuclear factor 4alpha (HNF4alpha), vitamin D receptor (VDR), liver receptor homolog 1 (LRH1), liver X receptor (LXR(alpha)), small heterodimer partner-1 (SHP-1), and glucocorticoid receptor (GR) on gene expression are detailed. Finally, we discuss some current topics and themes in nuclear receptor-mediated regulation of drug metabolizing enzymes and drug transporters.

Transcription coactivator peroxisome proliferator-activated receptor-binding protein/mediator 1 deficiency abrogates acetaminophen hepatotoxicity

Peroxisome proliferator-activated receptor-binding protein (PBP), also known as thyroid hormone receptor-associated protein 220/vitamin D receptor-interacting protein 205/mediator 1, an anchor for multisubunit mediator transcription complex, functions as a transcription coactivator for nuclear receptors. Disruption of the PBP gene results in embryonic lethality around embryonic day 11.5 by affecting placental and multiorgan development. Here, we report that targeted deletion of PBP in liver parenchymal cells (PBP(Liv-/-)) results in the abrogation of hypertrophic and hyperplastic influences in liver mediated by constitutive androstane receptor (CAR) ligands phenobarbital (PB) and 1,4-bis-[2-(3,5-dichloropyridyloxy)]benzene, and of acetaminophen-induced hepatotoxicity. CAR interacts with the two nuclear receptor-interacting LXXLL (L, leucine; X, any amino acid) motifs in PBP in a ligand-dependent manner. We also show that PBP interacts with the C-terminal portion of CAR, suggesting that PBP is involved in the regulation of CAR function. Although the full-length PBP only minimally increased CAR transcriptional activity, a truncated form of PBP (amino acids 487-735) functioned as a dominant negative repressor, establishing that PBP functions as a coactivator for CAR. A reduction in CAR mRNA and protein level observed in PBP(Liv-/-) mouse liver suggests that PBP may regulate hepatic CAR expression. PBP-deficient hepatocytes in liver failed to reveal PB-dependent translocation of CAR to the nucleus. Adenoviral reconstitution of PBP in PBP(Liv-/-) mouse livers restored PB-mediated nuclear translocation of CAR as well as inducibility of CYP1A2, CYP2B10, CYP3A11, and CYP7A1 expression. We conclude that transcription coactivator PBP/TRAP220/MED1 is involved in the regulation of hepatic CAR function and that PBP deficiency in liver abrogates acetaminophen hepatotoxicity.

Impaired nuclear translocation of CAR in hepatic preneoplastic lesions: Association with an attenuated CYP2B induction by phenobarbital

Phenobarbital (PB) induction of CYP2B, a representative target gene of constitutive androstane receptor (CAR), has been observed to be attenuated in preneoplastic lesions of rat liver; however, molecular basis for this attenuation is poorly understood. In this report, we provide evidence indicating that the CAR expressed in the hepatic preneoplastic lesions of rats and mice was resistant to nuclear translocation and transactivation of the PB-responsive enhancer module upon PB treatment. These observations suggest that the attenuation of the induction of CYP2B by PB in hepatic preneoplastic lesions is evidently a consequence of impaired nuclear translocation of CAR.

Structure of the murine constitutive androstane receptor complexed to androstenol: a molecular basis for inverse agonism

The nuclear receptor CAR is a xenobiotic responsive transcription factor that plays a central role in the clearance of drugs and bilirubin while promoting cocaine and acetaminophen toxicity. In addition, CAR has established a "reverse" paradigm of nuclear receptor action where the receptor is active in the absence of ligand and inactive when bound to inverse agonists. We now report the crystal structure of murine CAR bound to the inverse agonist androstenol. Androstenol binds within the ligand binding pocket, but unlike many nuclear receptor ligands, it makes no contacts with helix H12/AF2. The transition from constitutive to basal activity (androstenol bound) appears to be associated with a ligand-induced kink between helices H10 and H11. This disrupts the previously predicted salt bridge that locks H12 in the transcriptionally active conformation. This mechanism of inverse agonism is distinct from traditional nuclear receptor antagonists thereby offering a new approach to receptor modulation.

A structural basis for constitutive activity in the human CAR/RXRalpha heterodimer

The X-ray crystal structure of the human constitutive androstane receptor (CAR, NR1I3)/retinoid X receptor alpha (RXRalpha, NR2B1) heterodimer sheds light on the mechanism of ligand-independent activation of transcription by nuclear receptors. CAR contains a single-turn Helix X that restricts the conformational freedom of the C-terminal AF2 helix, favoring the active state of the receptor. Helix X and AF2 sit atop four amino acids that shield the CAR ligand binding pocket. A fatty acid ligand was identified in the RXRalpha binding pocket. The endogenous RXRalpha ligand, combined with stabilizing interactions from the heterodimer interface, served to hold RXRalpha in an active conformation. The structure suggests that upon translocation, CAR/RXRalpha heterodimers are preorganized in an active conformation in cells such that they can regulate transcription of target genes. Insights into the molecular basis of CAR constitutive activity can be exploited in the design of inverse agonists as drugs for treatment of obesity.

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

The constitutive active receptor (CAR) regulates the induction of the cytochrome P450 2B6 (CYP2B6) gene by phenobarbital-type inducers, such as 1,4 bis[2-(3,5-dichloropyridyloxy)]benzene (TCPOBOP) via the distal phenobarbital-responsive enhancer module (PBREM, at -1732/-1685 bp). Activation of the PBREM by TCPOBOP generated a 10-fold induction of CYP2B6 mRNA in HepG2 cells stably expressing mouse CAR (Ym17). Co-treatment with the protein phosphatase inhibitor okadaic acid (OA) synergistically increased this induction over 100-fold without directly activating CAR or the PBREM. Although OA synergy required the presence of PBREM, deletion assays delineated the OA-responsive activity to a proximal 24-bp (-256/-233) sequence (OARE) in the CYP2B6 promoter. CAR did not directly bind to the OARE in electrophoretic mobility shift assays. However, both DNA affinity and chromatin immunoprecipitation assays showed a significant increase in CAR association with the OARE after co-treatment with TCPOBOP and OA, indicating the indirect binding of CAR to the OARE. The two cis-acting elements, the distal PBREM and the proximal OARE, within the chromatin structure are both regulated by CAR in response to TCPOBOP and OA, respectively, to maximally induce the CYP2B6 promoter. This functional interaction between the two sites expands the current understanding of the mechanism of CAR-mediated inducible transcription.

Coordinate transcriptional regulation of bile acid homeostasis and drug metabolism

Drugs and bile acids are taken up into hepatocytes by specialized transport proteins localized at the basolateral membrane, e.g., organic anion transporting polypeptides . Following intracellular metabolism by cytochrome P450 (CYP) enzymes, drug metabolites are excreted into bile or urine via ATP-dependent multidrug resistance proteins (MDR1 and MRPs). Bile acids are excreted mainly via the bile salt export pump (BSEP, ABCB11). The genes coding for drug and bile acid transporters and CYP enzymes are regulated by a complex network of transcriptional cascades, notably by the ligand-activated nuclear receptors FXR, PXR, and CAR and by the ligand-independent nuclear receptor HNF-4alpha. The bile acid synthesizing enzymes CYP7A1, CYP8B1, and CYP27A1 are subject to negative feedback regulation by bile acids, which is partly mediated through the transcriptional repressor SHP. The role of transcriptional cofactors, such as SRC-1 and PGC-1, in mediating the gene-specific effects of individual nuclear receptors is becoming increasingly evident.

Structural determinants of constitutive androstane receptor required for its glucocorticoid receptor interacting protein-1-mediated nuclear accumulation

Nuclear translocation of constitutive androstane receptor (CAR) is a primary mechanism for the induction of cytochrome P450 genes by phenobarbital (PB). We have shown that exogenous expression of the p160 coactivator glucocorticoid receptor interacting protein-1 (GRIP1) in hepatocytes in vivo can mediate PB-independent nuclear accumulation of murine CAR (mCAR). To understand the mechanism of this PB-independent nuclear accumulation, we have examined the mCAR structural determinants of its GRIP1-mediated nuclear localization. Mutations of the xenobiotic response sequence (XRS), which had been shown to block PB-dependent nuclear translocation of human CAR in mouse hepatocytes in vivo, also blocked GRIP1-mediated nuclear accumulation of mCAR in mouse hepatocytes in vivo and further blocked nuclear localization in cultured HepG2 cells. A leucine 326 XRS mutant retained partial transcriptional activity, but mutations of three leucines in the XRS eliminated transcriptional activity in HepG2 cells, suggesting that the translocation function of the XRS overlaps with transcriptional functions. Mutation of the activation function 2 motif, by deletion of the C-terminal 8 amino acids, also reduced nuclear localization by both PB treatment and GRIP1 expression in hepatocytes in vivo, suggesting that either interaction with GRIP1 through this motif or active CAR was required for the nuclear localization. The localization of a DNA-binding domain mutant was essentially unchanged by coexpression of GRIP1, although without GRIP1 coexpression, this mutant expressed exhibited a more nuclear localization compared with wild type. The results are most consistent with a model in which GRIP1 interaction and activation of mCAR in the nucleus result in retention and accumulation of mCAR in the nucleus in untreated animals. The model requires that mCAR is constantly shuttling between the nucleus and cytoplasm even in untreated animals in which mCAR is predominantly cytoplasmic.

Cytoplasmic Localization of Pregnane X Receptor and Ligand-dependent Nuclear Translocation in Mouse Liver

The pregnane X receptor (PXR) plays an important role in the response to xenobiotics and endogenous toxins. We have used a specific anti-PXR antibody in the Western blotting of mouse liver nuclear extracts to show that PXR is accumulated in the nucleus after treatment with 5-pregnen-3beta-ol-20-one-16alpha-carbonitrile (PCN), followed by an increase in Cyp3a11 mRNA. Expression of wild type PXR and various mutants as green fluorescent fusion proteins in mouse livers showed that PXR was retained in the cytoplasm from where PCN treatment translocated PXR into the nucleus. Furthermore, the xenochemical response signal, the nuclear translocation signal, and the activation function 2 domain were all required for the nuclear translocation to occur. Immunoprecipitation experiments using the hsp90 antibody demonstrated the presence of PXR in a complex with the endogenous cytoplasmic constitutive active/androstane receptor retention protein (CCRP) in HepG2 cells. Fluorescence resonance energy transfer analysis of mouse liver sections after co-expression of cyan fluorescent protein-CCRP and yellow fluorescent protein-PXR also indicated that CCRP and PXR were closely associated in vivo. Overexpression of exogenous CCRP increased the cytoplasmic level of the PXR.CCRP.hsp90 complex, whereas a decrease in endogenous CCRP by treatment with small interfering RNA for CCRP repressed the PXR-mediated reporter activity in HepG2 cells. We conclude that the CCRP mediates the retention of PXR in the cytosol and modulates the activation of PXR in response to PCN treatment.

The evolution of drug-activated nuclear receptors: one ancestral gene diverged into two xenosensor genes in mammals

BACKGROUND: Drugs and other xenobiotics alter gene expression of cytochromes P450 (CYP) by activating the pregnane X receptor (PXR) and constitutive androstane receptor (CAR) in mammals. In non-mammalian species, only one xenosensor gene has been found. Using chicken as a model organism, the aim of our study was to elucidate whether non-mammalian species only have one or two xenosensors like mammals. RESULTS: To explore the evolutionary aspect of this divergence, we tried to identify additional xenobiotic sensing nuclear receptors in chicken using various experimental approaches. However, none of those revealed novel candidates. Ablation of chicken xenobiotic receptor (CXR) function by RNAi or dominant-negative alleles drastically reduced drug-induction in a chicken hepatoma cell line. Subsequently, we functionally and structurally characterized CXR and compared our results to PXR and CAR. Despite the high similarity in their amino acid sequence, PXR and CAR have very distinct modes of activation. Some aspects of CXR function, e.g. direct ligand activation and high promiscuity are very reminiscent of PXR. On the other hand, cellular localization studies revealed common characteristics of CXR and CAR in terms of cytoplasmic-nuclear distribution. Finally, CXR has unique properties regarding its regulation in comparison to PXR and CAR. CONCLUSION: Our finding thus strongly suggest that CXR constitutes an ancestral gene which has evolved into PXR and CAR in mammals. Future studies should elucidate the reason for this divergence in mammalian versus non-mammalian species.

Nuclear receptors CAR and PXR cross talk with FOXO1 to regulate genes that encode drug-metabolizing and gluconeogenic enzymes

The nuclear receptors CAR and PXR activate hepatic genes in response to therapeutic drugs and xenobiotics, leading to the induction of drug-metabolizing enzymes, such as cytochrome P450. Insulin inhibits the ability of FOXO1 to express genes encoding gluconeogenic enzymes. Induction by drugs is known to be decreased by insulin, whereas gluconeogenic activity is often repressed by treatment with certain drugs, such as phenobarbital (PB). Performing cell-based transfection assays with drug-responsive and insulin-responsive enhancers, glutathione S-transferase pull down, RNA interference (RNAi), and mouse primary hepatocytes, we examined the molecular mechanism by which nuclear receptors and FOXO1 could coordinately regulate both enzyme pathways. FOXO1 was found to be a coactivator to CAR- and PXR-mediated transcription. In contrast, CAR and PXR, acting as corepressors, downregulated FOXO1-mediated transcription in the presence of their activators, such as 1,4-bis[2-(3,5-dichloropyridyloxy)]benzene (TCPOBOP) and pregnenolone 16alpha-carbonitrile, respectively. A constitutively active mutant of the insulin-responsive protein kinase Akt, but not the kinase-negative mutant, effectively blocked FOXO1 activity in cell-based assays. Thus, insulin could repress the receptors by activating the Akt-FOXO1 signal, whereas drugs could interfere with FOXO1-mediated transcription by activating CAR and/or PXR. Treatment with TCPOBOP or PB decreased the levels of phosphoenolpyruvate carboxykinase 1 mRNA in mice but not in Car(-/-) mice. We conclude that FOXO1 and the nuclear receptors reciprocally coregulate their target genes, modulating both drug metabolism and gluconeogenesis.

CAR: detailing new models

Functional analysis has broadened our understanding of the physiological roles of the two related nuclear receptors pregnane X receptor (PXR; NR1I2) and constitutive androstane receptor (CAR; NR1I3). Initial research focused on the role of these two receptors in xenobiotic detoxification and, more recently, additional functional roles for CAR have been identified. Specifically, CAR activity has been shown to ameliorate the effects of hyperbilirubinemia, caloric restriction and toxic bile acids. Thus, the physiological role of CAR has broadened to include responses to metabolic and nutritional stress. These data highlight potential new opportunities in targeting CAR for drug discovery.

CAR, Driving into the Future

The nuclear orphan receptor CAR is active in the absence of ligand with the unique capability to be further regulated by activators. A number of these activators, including phenobarbital, do not directly bind to the receptor. Considered a xenobiotic sensing receptor, CAR transcriptionally modifies the expression of genes involved in the metabolism and elimination of xenobiotics and steroids in response to these compounds and other cellular metabolites. Its hepatic expression pattern endows the liver with the ability to protect against not only exogenous but also endogenous insults. The mechanism of CAR activation is complex, involving translocation from the cytoplasm to the nucleus in the presence of activators, followed by further activation steps in the nucleus. Although this mechanism remains under investigation, we have summarized here the cellular signaling pathways elucidated so far and speculate on the mechanism by which CAR activators regulate gene expression through this network.

The constitutive active/androstane receptor regulates phenytoin induction of Cyp2c29

Many cytochrome P450 isoforms are known to be drug-inducible. The anticonvulsant phenytoin has been reported to be an inducer of human CYP2B6, CYP3A4, and murine CYP2C29. However, the molecular mechanism mediating phenytoin induction remains unclear. Herein, we used in vivo and in vitro gene reporter assays of the Cyp2c29 promoter to delineate the phenytoin-response activity to a phenytoin-responsive module located at -1371 kb upstream of the Cyp2c29 translation start site. The phenytoin-responsive module, consisting of two motifs of two imperfect direct repeat hexamers spaced by four nucleotides and a putative CCAAT/enhancer-binding protein-binding site, mediated luciferase reporter induction by phenytoin in mouse livers in vivo and was activated by CAR in HepG2 cells. Hepatic CYP2C29 mRNA was induced by phenytoin in wild-type but not in CAR-null mice, indicating that constitutive active or androstane receptor (CAR) regulates phenytoin-induced transcription of the Cyp2c29 gene. Furthermore, the constitutive levels of CYP2C29 mRNA were reduced approximately 77-fold in CAR-null mice compared with those in the wild-type mice, suggesting that CAR may also regulate the constitutive expression of the Cyp2c29 gene either directly or indirectly.

Human constitutive androstane receptor mediates induction of CYP2B6 gene expression by phenytoin

Compared with its rodent orthologs, little is known about the chemical specificity of human constitutive androstane receptor (hCAR) and its regulation of hepatic enzyme expression. Phenytoin (PHY), a widely used antiepileptic drug, is a potent inducer of CYP2B6 in primary human hepatocytes, but does not activate human pregnane X receptor (PXR) significantly in cell-based transfection assays at the same concentrations associated with potent induction of CYP2B6. Based on this observation, we hypothesized that PHY may be a selective activator of hCAR. In primary human hepatocytes, expression of CYP2B6 reporter genes containing phenobarbital-responsive enhancer module (PBREM) or PBREM/xenobiotic-responsive enhancer module (XREM) response elements were activated up to 14- and 28-fold, respectively, by 50 microm PHY. By contrast, parallel experiments in HepG2 cell lines co-transfected with an hPXR expression vector did not show increased reporter activity. These results indicated that a PXR-independent pathway, which is retained in primary hepatocytes, is responsible for PHY induction of CYP2B6. Further experiments revealed that PHY effectively translocates hCAR from the cytoplasm into the nucleus in both primary human hepatocytes and CAR(-/-) mice. Compared with vehicle controls, PHY administration significantly increased CYP2B6 reporter gene expression, when this reporter construct was delivered together with hCAR expression vector into CAR(-/-) mice. However, PHY did not increase reporter gene expression in CAR(-/-) mice in the absence of hCAR vector, implying that CAR is essential for mediating PHY induction of CYP2B6 gene expression. Taken together, these observations demonstrate that, in contrast to most of the known CYP2B6 inducers, PHY is a selective activator of CAR in humans.

Alternative splicing affects the function and tissue-specific expression of the human constitutive androstane receptor

Background

The constitutive androstane receptor (CAR) plays a key role in the control of drug metabolism and transport by mediating the phenobarbital-type induction of many phase I and II drug metabolizing enzymes and drug transporters.

Results

We identified transcripts generated by four different alternative splicing events in the human CAR gene. Two of the corresponding ligand binding domain isoforms demonstrated novel functional properties: First, CAR(SV3), which is encoded by a transcript containing an lengthened exon 7, differentially transactivated target gene promoters. Second, CAR(SV2), which results from the use of an alternative 3' splice site lengthening exon 8, showed ligand-dependent instead of constitutive interaction with coactivators. Furthermore, alternatively spliced transcripts demonstrated a tissue-specific expression pattern. In most tissues, only transcripts generated by alternative splicing within exon 9 were expressed. The encoded variant demonstrated a loss-of-function phenotype. Correct splicing of exon 8 to exon 9 is restricted to only a few tissues, among them liver and small intestine for which CAR function has been demonstrated, and is associated with the induction of CAR expression during differentiation of intestinal cells.

Conclusion

Due to their specific activities, CAR variant proteins SV2 and SV3 may modulate the activity of reference CAR(SV1). Furthermore, we propose that transcriptional activation and regulation of splicing of exon 9 may be coupled to ensure appropriate tissue- and differentiation state-specific expression of transcripts encoding functional CAR protein. Altogether, alternative splicing seems to be of utmost importance for the regulation of CAR expression and function.