Nuclear factor-1 motif and redundant regulatory elements comprise phenobarbital-responsive enhancer in CYP2B1/2

Estrogen modulates transactivations of SXR-mediated liver X receptor response element and CAR-mediated phenobarbital response element in HepG2 cells

The nuclear receptors, steroid and xenobiotic receptor (SXR) and constitutive androstane receptor (CAR) play important functions in mediating lipid and drug metabolism in the liver. The present study demonstrates modulatory actions of estrogen in transactivations of SXR-mediated liver X receptor response element (LXRE) and CAR-mediated phenobarbital response element (PBRU). When human estrogen receptor (hERα) and SXR were exogenously expressed, treatment with either rifampicin or corticosterone promoted significantly the SXR-mediated transactivation of LXRE reporter gene in HepG2. However, combined treatment with estrogen plus either rifampicin or corticosterone resulted in less than 50% of the mean values of the transactivation by rifampicin or corticosterone alone. Thus, it is suggested that estrogen may repress the SXR-mediated transactivation of LXRE via functional cross-talk between ER and SXR. The CAR-mediated transactivation of PBRU was stimulated by hERa in the absence of estrogen. However, the potentiation by CAR agonist, TCPOBOP, was significantly repressed by moxestrol in the presence of ER. Thus, ER may play both stimulatory and inhibitory roles in modulating CAR-mediated transactivation of PBRU depending on the presence of their ligands. In summary, this study demonstrates that estrogen modulates transcriptional activity of SXR and CAR in mediating transactivation of LXRE and PBRU, respectively, of the nuclear receptor target genes through functional cross-talk between ER and the corresponding nuclear receptors.

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.

Effects of TK promotor and hepatocyte nuclear factor-4 in CAR-mediated transcriptional activity of phenobarbital responsive unit of CYP2B gene in monkey kidney epithelial-derived cell line COS-7

Previous studies reported that constitutive androstane receptor (CAR) does not transactivate phenobarbital responsive unit (PBRU)2C1luciferase reporter gene in COS cells in which endogenous CYP2B1 gene is not induced with PB. In order to understand molecular mechanism(s) whereby PBRU is transactivated, this article determined if the use of strong thymidine kinase (TK) promotor rather than the minimal CYP2C1 promotor, and hepatocyte nuclear factor-4 (HNF-4) can affect CAR-mediated transactivation of PBRU in the monkey kidney epithelial-derived COS-7 cells. To examine CAR-mediated transactivation, cultured COS-7 cells were transfected with CAR expression plasmid, pEGFP-mCAR1, and confirmed for high level of the protein expression. In COS-7 cells, TK promotor induced CAR-mediated PBRU transactivation in a dose-dependent manner. Whereas expression of HNF-4 slightly promoted PBRU transactivation with low amount of CAR transfected, it repressed PBRU transactivation in a dose-dependent manner with high amount of CAR. Consistent with the previous reports in Hep G2 cells, CAR transactivated PBRU2C1luciferase in a dose-dependent manner and this CAR-mediated transactivation required functional NR-1 and NF-1 sites. However, HNF-4 did not affect CAR-mediated PBRU transactivation in Hep G2 cells. These results suggest that proximal promotor and a trans-acting factor, HNF-4, can affect CAR-mediated transactivation of PBRU in COS-7 cells.

Analysis of multiple nuclear receptor binding sites for CAR/RXR in the phenobarbital responsive unit of CYP2B2

The phenobarbital (PB) responsive enhancers in CYP2B genes contain a core of two direct repeat-4 nuclear receptor binding sites, NR-1 and NR-2, which flank an NF-1 site and appear to be most important for PB responsiveness. Additional sequences outside the core are required for maximal PB responsiveness, including a third direct repeat-4 site, NR-3. The PB response is mediated by constitutive androstane receptor (CAR) which binds as a CAR/RXR heterodimer to the NR sites. To determine the relative importance of the third NR site, each of the NR sites was mutated individually and in all combinations in the rat PB responsive unit (PBRU). Mutation of NR-3 resulted in similar effects on transactivation of the PBRU by CAR in HepG2 cells as did mutations of NR-1 and NR-2. The recruitment of GRIP1/SRC-2 by CAR/RXR to the PBRU assessed by gel shift assays was cooperatively enhanced if more than one NR site in the PBRU was occupied by CAR/RXR. NR-3 in combination with NR-1 or NR-2 was equal to NR-1 and NR-2 in mediating this cooperative recruitment. Recruitment of SRC-1 and GRIP1/SRC-2 was similar for all NR sites, while some selectivity of NR-1 for SRC-3 was observed. SRC-3 also exhibited CAR-independent activation of the PBRU in HepG2 cells. Micrococcal nuclease mapping of nucleosomes revealed that the NR-1/NR-2 core of the PBRU is present in a nucleosome while NR-3 is present in the linker adjacent to the nucleosome. In the linear sequence NR-3 is further from NR-1 than NR-2 is, but in a nucleosomal structure, NR-3 is well positioned for cooperative recruitment of GRIP1/SRC-2 by CAR/RXR that is bound to NR-3 and either NR-1 or NR-2, while NR-1 and NR-2 are on opposite sides of the nucleosome separated by the histone core. These results demonstrate that NR-3 is functionally similar to NR-1 and NR-2 in CAR transactivation of the PBRU in vitro and suggest that NR-3 may have a greater role in a chromatin context in vivo than is apparent from transient transfection studies.

Repression of CAR-mediated transactivation of CYP2B genes by the orphan nuclear receptor, short heterodimer partner (SHP)

The induction of CYP2B gene expression by phenobarbital (PB) is mediated by the translocation of the constitutive androstane receptor (CAR) from the cytoplasm to the nucleus. The CAR/RXR heterodimer binds to two DR-4 sites in a complex phenobarbital responsive unit (PBRU) in the CYP2B gene. The short heterodimer partner (SHP), an orphan nuclear receptor that lacks a conventional DNA binding domain, was initially identified by its interaction with CAR. We have examined the role of SHP in CAR-mediated transactivation of the CYP2B gene. Coexpression of SHP inhibited the transactivation of the CYP2B gene by CAR in cultured hepatoma cells and the p160 coactivator GRIP1 reversed the inhibition. The interaction of CAR with SHP was confirmed by GST pulldown experiments. SHP did not block the binding of either CAR/RXR to the PBRU or binding of GRIP1 to the CAR/RXR complex in gel mobility shift assays, but slightly increased CAR/RXR binding and slightly altered the mobility of the CAR/RXR/GRIP1 complex, suggesting an interaction of SHP with these complexes. The presence of SHP in the complexes, however, could not be detected in an antibody supershift assay. Recombinant corepressors mSin3A, SMRT, and HDAC1, but not NCoR1, interacted with GST-SHP but each of these corepressors in liver nuclear extracts bound to GST-SHP. SMRT and NCoR1 inhibited CAR-mediated activation independent of SHP, but mSin3A and HDAC1 had little effect alone, and were additive with SHP. These studies demonstrate that SHP does not inhibit CAR-mediated trans-activation by interfering with DNA binding or by competition with GRIP1. Instead, SHP may either inhibit recruitment of other coactivators by GRIP1 or actively recruit corepressors directly to the CAR/RXR/PBRU complex.

Induction of Drug Metabolism: The Role of Nuclear Receptors

Induction of drug metabolism was described more than 40 years ago. Progress in understanding the molecular mechanism of induction of drug-metabolizing enzymes was made recently when the important roles of the pregnane X receptor (PXR) and the constitutive androstane receptor (CAR), two members of the nuclear receptor superfamily of transcription factors, were discovered to act as sensors for lipophilic xenobiotics, including drugs. CAR and PXR bind as heterodimeric complexes with the retinoid X receptor to response elements in the regulatory regions of the induced genes. PXR is directly activated by xenobiotic ligands, whereas CAR is involved in a more complex and less well understood mechanism of signal transduction triggered by drugs. Most recently, analysis of these xenobiotic-sensing nuclear receptors and their nonmammalian precursors such as the chicken xenobiotic receptor suggests an important role of PXR and CAR also in endogenous pathways, such as cholesterol and bile acid biosynthesis and metabolism. In this review, recent findings regarding xenosensors and their target genes are summarized and are put into an evolutionary perspective in regard to how a living organism has derived a system that is able to deal with potentially toxic compounds it has not encountered before.

Upregulation of cytochromes P450 2B in rat liver by orphenadrine

1 The alkylamine drug orphenadrine (ORPH) is an inducer and inhibitor of the microsomal cytochrome P450 (CYP) system in mammals. This study evaluated the selectivity of CYP induction by ORPH in rat liver. 2 Immunoblot analysis indicated that ORPH was a selective inducer of the phenobarbitone (PB)-inducible CYP2B in rat liver. CYP2B protein was increased to approximately 14-fold of levels in untreated rat liver. By comparison PB increased CYP2B expression 40-fold. Corresponding increases in the activity of CYP2B-dependent androstenedione 16beta-hydroxylation were measured in microsomes from ORPH and PB-induced rats. 3 Northern analysis indicated that CYP2B1/2 mRNA was increased in ORPH-induced rat liver. Consistent with this finding, ORPH was found to activate a PB-responsive enhancer module in constitutive androstane receptor (CAR)-transfected Hep G2 cells. 4 Other alkylamines like troleandomycin impair CYP turnover. We tested whether ORPH induction of CYP2B may include a post-translational component. In PB-pretreated animals ORPH administration delayed the loss of CYP2B after PB withdrawal, but no evidence for altered turnover was found. 5 These studies establish ORPH as a selective inducer of CYP2B in rat liver. Induction appears to be mediated pretranslationally by CAR activation of CYP2B gene transcription. Post-translational stabilisation by an ORPH metabolite does not elicit induction. Induction of CYP2B may influence pharmacokinetic interactions involving ORPH.

Transcriptional analysis in vivo of the hepatic genes, Cyp2b9 and Cyp2b10, by intravenous administration of plasmid DNA in mice

Phenobarbital (PB) responsiveness of CYP2B genes has been shown to be mediated by a PB responsive unit (PBRU). The core of the PBRU contains two nuclear receptor sites, NR-1 and NR-2, and a nuclear factor-1 (NF 1) binding site, which are required for PB responsiveness, but the importance of sequences flanking the core is not clear. We have used intravenous administration of plasmid DNA in the tail veins of mice to transfect hepatocytes in vivo and analyze sequence requirements for PB induction. In this assay PB treatment increased transactivation by the Cyp2b10 PBRU about 100-fold, which is similar to the increase in the expression of the endogenous gene while the Cyp2b9 PBRU was unresponsive. Analysis of chimeras of the two PBRUs and deletion mutants of the Cyp2b10 PBRU indicated that the core region containing the NR-1, NR-2 and NF-1 core sites is not sufficient for PB responsiveness. Additional sequence at the 3' side of the core sequence, which included a previously defined accessory factor-1 (AF-1) site, partially restored responsiveness. This region contained a binding site for NF-1 only in Cyp2b10 and not in Cyp2b9, but the intact site was not required for PB responsiveness. Purified constitutive androstane receptor (CAR)/retinoid X receptor (RXR) bound to the core NR-1 and NR-2 sites and to a third NR-3 site to the 5' side of the core in Cyp2b10. No binding of CAR/RXR to the Cyp2b9 PBRU was observed. These results indicate that changes in the NR sites which eliminate CAR/RXR binding are sufficient for the non-responsiveness to PB of Cyp2b9, but changes in sequences flanking the core independently eliminate PB responsiveness. The results demonstrate the advantages of transfection of mouse hepatocytes in vivo by tail vein injection of DNA as a method for transcriptional analysis of genes in vivo and show that sequences flanking the core region of the PBRU are required for PB induction in vivo.

Glucocorticoid receptor-interacting protein 1 mediates ligand-independent nuclear translocation and activation of constitutive androstane receptor in vivo

Phenobarbital (PB) induction of CYP2B genes is mediated by translocation of the constitutively active androstane receptor (CAR) to the nucleus. Interaction of CAR with p160 coactivators and enhancement of CAR transactivation by the coactivators have been shown in cultured cells. In the present studies, the interaction of CAR with the p160 coactivator glucocorticoid receptor-interacting protein 1 (GRIP1) was examined in vitro and in vivo. Binding of GRIP1 to CAR was shown by glutathione S-transferase (GST) pull-down and affinity DNA binding. N- or C-terminal fragments of GRIP1 that contained the central receptor-interacting domain bound to GST-CAR, but the presence of ligand increased the binding to GST-CAR of only the fragments containing the C-terminal region. In gel shift analysis, binding to CAR was observed only with GRIP1 fragments containing the C-terminal region, and the binding was increased by a CAR agonist and decreased by a CAR antagonist. Expression of GRIP1 enhanced CAR-mediated transactivation in cultured hepatic-derived cells 2-3-fold. In hepatocytes transfected in vivo, expression of exogenous GRIP1 alone induced transactivation of the CYP2B1 PB-dependent enhancer 15-fold, whereas CAR expression alone resulted in only a 3-fold enhancement in untreated mice. Remarkably, CAR and GRIP1 together synergistically transactivated the enhancer about 150-fold, which is approximately equal to activation by PB treatment. In PB-treated mice, expression of exogenous CAR alone had little effect, expression of GRIP1 increased transactivation about 2-fold, and with CAR and GRIP, a 4-fold activation was observed. In untreated mice, expression of GRIP resulted in nuclear translocation of green fluorescent protein-CAR. These results strongly suggest that a p160 coactivator functions in CAR-mediated transactivation in vivo in response to PB treatment and that the synergistic activation of CAR by GRIP in untreated animals results from both nuclear translocation and activation of CAR.

Functional analysis of the phenobarbital-responsive unit in rat CYP2B211Abbreviations: P450, cytochrome P450; PB, phenobarbital; CYP, P450 gene; NR, nuclear receptor; NF-1, nuclear factor-1; GRE, glucocorticoid response element; CAR, constitutive androgen receptor; RXR, retinoid X receptor; PBRU, PB response element

An 163-bp fragment of the rat cytochrome P450 gene, CYP2B2 has been shown to contain sequences that mediate phenobarbital (PB) responsiveness of this gene. In studies on this rat gene and the orthologous mouse gene, Cyp2b10, the minimal fragment required for near full PB responsiveness has varied from about 50 to 80 bp depending on the gene used and the number of copies of the PB responsive sequences assessed. Since there is a single copy of the CYP genes in the genome, we have evaluated deletion and block mutations across an 84-bp region of the PB responsive unit (PBRU), by in situ transfection in rat liver using single copies of the PBRU sequences. From the 5' end, deletions to -2243 retained more than 50% responsiveness to PB compared to the 163-bp fragment. The fragment -2237 to -2155 retained less than 20% responsiveness even though it contained the nuclear receptor (NR)-1, NR-2, and NF-1 motifs which are present in the core of the PBRU. From the 3' end, deletions from -2170 to -2194 eliminated PB responsiveness indicating that the 74-bp sequence from -2243 to -2170 is able to mediate full PB responsiveness. Block mutations within the NR-1 and NF-1 regions reduced responsiveness most dramatically, but did not abolish it, and mutations 3' of the NF-1 site modestly reduced responsiveness. Protein binding was not affected by mutations in the NR-1 region as assessed by DNase I footprinting in vitro but mutations within the NR-2 region reduced binding to the NF-1 site. Mutations of the 5' half or the 3' half of the bipartite NF-1 site, resulted in loss of protection of the NF-1 site and new footprints to the 3' or 5' side, respectively, of the NF-1 site. These results indicate that sequences in addition to the NR-1 and -2 and the NF-1 sites are required for full responsiveness to PB and suggest that proteins which bind to these sites may interact.

Orphan receptor promiscuity in the induction of cytochromes p450 by xenobiotics

The mechanisms by which different classes of chemicals induce the same cytochrome P450 (CYP) or the same chemical differentially induces more than one CYP are not well understood. We show that in primary hepatocytes and in vivo in liver (transfected by particle-mediated delivery) two orphan nuclear receptors, constitutive androstane receptor and pregnane X receptor (PXR1), transactivate a CYP gene via the same response element in a xenobiotic-specific manner. The constitutive androstane receptor mediates the barbiturate activation of expression of CYP2B1 and CYP3A1. PXR1 activates both genes in response to synthetic steroids. To exert their effect the receptors bind to the same direct repeat site (DR4) within the phenobarbital response element of the CYP2B1 promoter and to the same DR3 site in the pregnane X response element of CYP3A1. The receptors are therefore promiscuous with respect to DNA binding but not ligand binding. Differences in enhancer half-site spacing may influence the efficiency of interactions between the receptor and the transcription machinery and hence form the basis for the differential induction of CYP2B1 and CYP3A1 in response to barbiturates and synthetic steroids.

Xenobiotic induction of cytochrome P450 2B1 (CYP2B1) is mediated by the orphan nuclear receptor constitutive androstane receptor (CAR) and requires steroid co-activator 1 (SRC-1) and the transcription factor Sp1

The constitutive androstane receptor (CAR) activates the expression of a reporter gene attached to the phenobarbital-response element (PBRE) of the cytochrome P450 2B1 (CYP2B1) gene in response to the barbiturate phenobarbital and the plant product picrotoxin. The xenobiotic-mediated increase in transactivation occurs in transfected primary hepatocytes and in liver transfected by biolistic-particle-mediated DNA transfer, but not in the transformed cell lines HepG2, CV-1 and HeLa, which support only constitutive activation of gene expression by CAR. Steroid co-activator 1 (SRC-1) enhances both constitutive and xenobiotic-induced CAR-mediated transactivation via the CYP2B1 PBRE in transfected primary hepatocytes. The nuclear receptor 1 (NR1) site of the PBRE is sufficient for CAR-mediated transactivation, but additional sequences within the PBRE, and hence the proteins that bind to them, are required for the interaction of CAR with SRC-1. The NR2 site of the PBRE binds proteins other than CAR, including an unidentified nuclear receptor heterodimerized with retinoid X receptor alpha. By binding to the proximal promoter of CYP2B1, the transcription factor Sp1 increases both basal transcription and xenobiotic-induced expression via the PBRE. Thus induction of CYP2B1 expression by xenobiotics is mediated by the nuclear receptor CAR and, for optimal expression, requires SRC-1 and Sp1.

Chromatin assembly enhances binding to the CYP2B1 phenobarbital-responsive unit (PBRU) of nuclear factor-1, which binds simultaneously with constitutive androstane receptor (CAR)/retinoid X receptor (RXR) and enhances CAR/RXR-mediated activation of the PBRU

Phenobarbital induction of CYP2B genes is mediated by a complex phenobarbital-responsive enhancer (PBRU), which contains a binding site for nuclear factor-1 (NF-1) flanked by two DR-4 nuclear receptor (NR) binding sites for a heterodimer of constitutive androstane receptor (CAR) and retinoid X receptor (RXR). To examine potential interactions between NF-1 and CAR/RXR, binding of purified recombinant proteins to DNA, or to chromatin assembled using Drosophila embryo extract, was examined. NF-1 and CAR/RXR bound simultaneously and independently to the overlapping NF-1 and NR-1 sites; binding of CAR/RXR to the NR-2 site was modestly increased by NF-1 binding; and CAR/RXR bound to a new site in the PBRU region, designated NR-3. Assembly of plasmid DNA into chromatin using Drosophila extract resulted in linearly phased nucleosomes in the PBRU region. The apparent binding affinity of NF-1 was increased by about 10-fold in assembled chromatin compared with DNA, whereas CAR/RXR binding was decreased. As observed for DNA, however, simultaneous, largely independent, binding to the NF-1 and NR sites was observed. CAR-mediated transactivation of the PBRU in cultured cells of hepatic origin was inhibited by mutations in the NF-1 site, and overexpression of NF-1 increased CAR transactivation in HepG2 cells. These studies demonstrate that NF-1 and CAR/RXR can both bind to the PBRU at the same time and that chromatin assembly increases NF-1 binding, which is consistent with previous in vivo footprinting studies in which the NF-1 site was occupied in untreated animals and the NF-1 and flanking NR sites were occupied after phenobarbital treatment. CAR-mediated trans-activation of the PBRU was increased by NF-1, analogous to NF-1 effects on phenobarbital induction in previous transient transfection studies and consistent with mediation of phenobarbital induction by CAR.

Mutational analysis of the CYP2B2 phenobarbital response unit and inhibitory effect of the constitutive androstane receptor on phenobarbital responsiveness

A 163-base pair enhancer in the CYP2B2 5' flank confers phenobarbital (PB) inducibility and constitutes a PB response unit (PBRU). By transfection of primary hepatocytes, we analyzed the function of elements comprising the PBRU and evaluated the role of the constitutive androstane receptor (CAR) in PB responsiveness. A 51-base pair PB-responsive enhancer module (PBREM) within the PBRU confers near-maximal PB response when fused to a tk promoter. However, replacing the PBRU with the PBREM in the CYP2B2 5' flank in the natural sequence context reduced PB responsiveness by approximately 4-fold. Mutational analysis also demonstrated that PBRU sequence elements outside the PBREM are essential for maximal PB responsiveness. The PBRU contains two putative nuclear receptor binding sites, NR1 and NR2. CAR binds to retinoic acid beta2 response elements (betaRARE) and to the NR1 and NR2 sites of the PBRU and activates transcription of reporter genes in cell lines. However, conversion of NR1 into betaRARE was the equivalent of an inactivating mutation, indicating that CAR does not activate PB-dependent transcription via NR1 in the natural sequence context. A betaRAREx2-tk reporter construct was inducible by all-trans-retinoic acid (at-RA) as expected and also responded to PB. The latter can be attributed to nuclear accumulation of CAR after PB exposure. Exogenous CAR increased both the basal and PB-induced response of betaRAREx2-tk but reduced PBRU-dependent PB response. Furthermore, exogenous CAR also reduced the at-RA response of the betaRAREx2-tk construct. Thus, CAR acts negatively on PB responsiveness mediated by the CYP2B2 PBRU just as it prevents maximal at-RA responsiveness mediated by betaRARE.

Dexamethasone enhances constitutive androstane receptor expression in human hepatocytes: consequences on cytochrome P450 gene regulation

The barbiturate phenobarbital induces the transcription of cytochromes P450 (CYPs) 2B through the constitutive androstane receptor (CAR; NR1I3). CAR is a member of the nuclear receptor family (NR1) mostly expressed in the liver, which heterodimerizes with retinoid X receptor (RXR) and was shown to transactivate both the phenobarbital responsive element module of the human CYP2B6 gene and the CYP3A4 xenobiotic response element. Because previous studies in rodent hepatocyte cultures have shown that the phenobarbital-mediated induction of CYP2B genes is potentiated by glucocorticoids, we examined the role of activated glucocorticoid receptor in this process. We show that submicromolar concentrations of dexamethasone enhance phenobarbital-mediated induction of CYP3A4, CYP2B6, and CYP2C8 mRNA in cultured human hepatocytes. In parallel, we observed that glucocorticoid agonists, such as dexamethasone, prednisolone, or hydrocortisone, specifically increase human car (hCAR) mRNA expression. Accumulation of hCAR mRNA parallels that of tyrosine aminotransferase: both mRNAs reach a maximum at a concentration of 100 nM dexamethasone and are down-regulated by concomitant treatment with the glucocorticoid antagonist RU486. Moreover, the effect of dexamethasone on hCAR mRNA accumulation appears to be of transcriptional origin because the addition of protein synthesis inhibitor cycloheximide has no effect, and dexamethasone does not affect the degradation of hCAR mRNA. Furthermore, dexamethasone increases both basal and phenobarbital-mediated nuclear translocation of CAR immunoreactive protein in human hepatocytes. The up-regulation of CAR mRNA and protein in response to dexamethasone explains the synergistic effect of this glucocorticoid on phenobarbital-mediated induction of CYP2B genes and the controversial role of the glucocorticoid receptor on phenobarbital-mediated CYP gene inductions.

Effect of phenobarbital on intralobular expression of CYP2B1/2 in livers of rats: difference in the expression between single and repetitive administrations

Phenobarbital (PB) was shown to induce the major PB-inducible cytochrome P450 (CYP) isoforms, CYP2B1/2, in perivenular hepatocytes by a single injection, and in midzonal and periportal hepatocytes in addition to perivenular hepatocytes by injections of the same dosage once a day for 3 days in rat livers. The present study was undertaken to determine whether the spread of enzyme induction to midzonal and periportal hepatocytes is caused by the increase in total dose of the drug by repetitive injections or by the repetitive injections of the drug themselves. Male adult rats were administered PB by a single injection (80 mg/kg) or repetitive injections (20 mg/kg once a day for 4 days; a total dose of 80 mg/kg), and the molar content of CYP2B1/2 was measured by quantitative immunohistochemistry in the cytoplasm of perivenular, midzonal, and periportal hepatocytes. In addition, the molar content of total CYP in the cytoplasm was measured by microphotometry, and the expression of CYP2B2 mRNA was examined by in situ hybridization. When animals received the single injection, the isoforms and CYP2B2 mRNA increased markedly in perivenular hepatocytes, increased somewhat in midzonal hepatocytes, and remained unchanged in periportal hepatocytes. If animals received the repetitive injections, however, although the isoforms and the mRNA increased markedly in perivenular hepatocytes, they also increased markedly in midzonal hepatocytes and somewhat in periportal hepatocytes. These findings demonstrated that the enlargement of the sublobular area in which induction of the isoforms occurred was caused by the repetitive injections of PB themselves.

Cell biology of cytochrome P-450 in the liver

Cytochromes P-450 (P-450) are members of a multigene superfamily of hemoproteins consisting the microsomal monooxygenase system with NADPH P-450 reductase (reductase) and/or reducing equivalents. Expression of many P-450 isoforms in hepatocytes is shown to be regulated at the level of transcription through interaction between cis-acting elements in the genes and DNA-binding (transacting) factors. Some isoforms of the CYP1A, 2B, 2E, and 3A subfamilies are regulated at the posttranscriptional level. For the topology of P-450 and reductase molecules in ER membrane of hepatocytes, models from stopped flow analysis and electron spin resonance are proposed. The densities of total P-450 and reductase molecules are revealed to be high enough to support the cluster model, suggesting that about ten P-450 molecules form an aggregate and surround one reductase molecule, and therefore the two enzymes form large micelles. ER proliferation after PB administration, which had been correlated with increase in P-450 level, is shown to be probably independent of the increase in P-450 level. There are considerable discrepancies among results reported on sublobular expression of various P-450 isoforms. Causes of the discrepancies are likely to be differences in experimental conditions of histochemical detection carried out and/or in species, strain, and/or sex.

Identification and functional characterization of a conserved, nuclear factor 1-like element in the proximal promoter region of CYP1A2 gene specifically expressed in the liver and olfactory mucosa

CYP1A2 is a major cytochrome P-450 isoform in the liver and the olfactory mucosa but is essentially not expressed in other tissues. A nuclear factor 1 (NF-1) -like element was identified in the proximal promoter region of rat, mouse, rabbit, and human CYP1A2 genes through data base analysis. In vitro DNase I footprinting with a -211 to +81 probe from the rat CYP1A2 gene and nuclear extracts from rat liver and olfactory mucosa revealed a single protected region corresponding to the NF-1-like element at -129 to -111. Protein binding to this NF-1-like element was tissue-selective and was confirmed by in vivo footprinting in native chromatin from rat liver. Multiple DNA-binding complexes were detected in gel-shift assays using the CYP1A2 NF-1-like element and nuclear extracts from liver and olfactory mucosa, all of which were supershifted in the presence of an anti-NF1 antibody. The NF-1-like element was essential for transcriptional activity of the CYP1A2 gene in an in vitro transcription assay using nuclear extracts from the two tissues. Thus, members of the NF-1 family of transcription factors may play an important role in the tissue-selective expression of the CYP1A2 gene in the liver and olfactory mucosa.

Tissue-specific chromatin structure of the phenobarbital-responsive unit and proximal promoter of CYP2B1/2 and modulation by phenobarbital

Phenobarbital induction of transcription of CYP2B genes is mediated by an enhancer, termed a phenobarbital responsive unit (PBRU), approximately 2000 bp 5' of the transcription start site. To further delineate the mechanism of phenobarbital induction, protein binding in native chromatin and the nucleosomal structure of the PBRU and proximal promoter were examined in liver and kidney, in which the CYP2B1/2 genes are expressed and not expressed, respectively. Protein binding to the PBRU in kidney chromatin was not detected even though in vitro DNase I footprints were not detectably different with nuclear extracts from liver and kidney. Likewise, protein binding to regulatory motifs was not detected in the proximal promoter region in kidney chromatin. In liver chromatin, however, DNase I hypersensitivity and partial protection of the regulatory motifs from DNase I digestion or reaction with dimethyl sulfate was observed and phenobarbital treatment increased the hypersensitivity but only modestly affected protection. Low resolution Southern analysis of micrococcal nuclease-digested chromatin from untreated rats revealed micrococcal nuclease hypersensitive regions in the proximal promoter and PBRU regions in liver, but not in kidney. Phenobarbital treatment increased hyper-sensitivity in liver in both regions. Micrococcal nuclease hypersensitivity in the PBRU was largely restricted to a linker region between phased nucleosomes while in the proximal promoter hypersensitivity extended over approximately 200 bp suggesting disruption of a nucleosome in this region. These data indicate that in liver phenobarbital treatment substantially alters protein binding to regulatory motifs in the PBRU, while not greatly affecting such binding in the proximal promoter, and substantially alters chromatin structure in both regions, presumably as a result of chromatin modifying factors recruited to the PBRU. In the kidney, chromatin is probably in a closed conformation that prevents binding of regulatory factors.

P450 gene induction by structurally diverse xenochemicals: central role of nuclear receptors CAR, PXR, and PPAR

The biochemistry of foreign compound metabolism and the roles played by individual cytochrome P450 (CYP) enzymes in drug metabolism and in the toxification and detoxification of xenochemicals prevalent in the environment are important areas of molecular pharmacology and toxicology that have been widely studied over the past decade. Important advances in our understanding of the mechanisms through which foreign chemicals impact on these P450-dependent metabolic processes have been made during the past 2 years with several key discoveries relating to the mechanisms through which xenochemicals induce the expression of hepatic P450 enzymes. Roles for three "orphan" nuclear receptor superfamily members, designated CAR, PXR, and PPAR, in respectively mediating the induction of hepatic P450s belonging to families CYP2, CYP3, and CYP4 in response to the prototypical inducers phenobarbital (CAR), pregnenolone 16alpha-carbonitrile and rifampicin (PXR), and clofibric acid (PPAR) have now been established. Two other nuclear receptors, designated LXR and FXR, which are respectively activated by oxysterols and bile acids, also play a role in liver P450 expression, in this case regulation of P450 cholesterol 7alpha-hydroxylase, a key enzyme of bile acid biosynthesis. All five P450-regulatory nuclear receptors belong to the same nuclear receptor gene family (family NR1), share a common heterodimerization partner, retinoid X-receptor (RXR), and are subject to cross-talk interactions with other nuclear receptors and with a broad range of other intracellular signaling pathways, including those activated by certain cytokines and growth factors. Endogenous ligands of each of those nuclear receptors have been identified and physiological receptor functions are emerging, leading to the proposal that these receptors may primarily serve to modulate hepatic P450 activity in response to endogenous dietary or hormonal stimuli. Accordingly, P450 induction by xenobiotics may in some cases lead to a perturbation of endogenous regulatory circuits with associated pathophysiological consequences.

Positive regulation of the rat CYP2B2 phenobarbital response unit by the nuclear receptor hexamer half-site.nuclear factor 1 complex

A distal 163-bp fragment mediates phenobarbital responsiveness of the rat CYP2B2 gene. Multiple cis-acting elements in this fragment cooperate to form a phenobarbital response unit (PBRU). A nuclear factor 1 binding site and an associated nuclear receptor hexamer half-site are present in both the rat CYP2B2 PBRU and the homologous mouse Cyp2b10 sequence. Based on mutational analyses, the hexamer half-site has been reported to act positively in CYP2B2 and negatively in Cyp2b10. However, the specific mutations introduced into the rat and mouse hexamer half-sites were different, raising the possibility that the different roles attributed to the element may be a consequence of the different mutations used. We introduced into the rat CYP2B2 hexamer half-site the specific mutational change previously introduced into the Cyp2b10 sequence, where its effect was to increase the basal level of expression and to abolish phenobarbital responsiveness. In the rat context, this mutation reduced but did not abolish phenobarbital responsiveness and decreased, rather than increased, the basal level of expression. The residual phenobarbital responsiveness of the hexamer half-site mutant, as well as that of nuclear factor 1 mutants, indicates that these elements behave as positive accessory sites, suggesting that factors binding to them function as activators of phenobarbital-dependent transcription.

Phenobarbital responsiveness conferred by the 5'-flanking region of the rat CYP2B2 gene in transgenic mice

Phenobarbital (PB) is a prototype for a class of agents that produce marked transcriptional activation of a number of genes, including certain cytochrome P-450s. We used transgenic mouse approaches and multiple gene reporters to assess the functional consequences of specific deletions and site-specific mutations within the 2.5kb 5'-flanking region of the rat CYP2B2 gene. Protein-DNA interactions at the PBRU domain also were characterized. Using the transgenic models, we demonstrate that sequences between -2500 and -1700bp of the CYP2B2 gene are critical for PB induction; mice with 1700 or 800bp of 5'-flanking CYP2B2 sequence are not PB responsive. DNA affinity enrichment techniques and immunoblotting and electromobility shift assays were used to determine that nuclear factor 1 (NF-1) interacts strongly with a site centered at -2200bp in the PB responsive unit (PBRU) of CYP2B2. To test the functional contribution of NF-1 in PB activation, we introduced specific mutations within the PBRU NF-1 element and demonstrated that these mutations completely ablate the binding interaction. However, transgenic mice incorporating the mutant NF-1 sequence within an otherwise wild-type -2500/CYP2B2 transgene maintained full PB responsiveness. These results indicate that, despite the avidity of the respective DNA-protein interaction within the PBRU in vitro, NF-1 interaction is not an essential factor directing PB transcriptional activation in vivo.

NF-kappaB is involved in the induction of the rat hepatic alpha1-acid glycoprotein gene by phenobarbital

Phenobarbital, a classical inducer of the drug-metabolizing cytochrome P450 genes, induces alpha1-acid glycoprotein gene expression through a PB-responsive element (PBRE) located at position -142 to -126 from the transcriptional start site. The aim of this study was to investigate nuclear protein binding to the PBRE sequence after PB treatment. Cycloheximide treatment showed that de novo protein synthesis was not required for PB to induce AGP gene expression, pointing to post-translational modifications. Studies of the DNA-protein complex with the PBRE showed that phosphorylation status is a key regulator of the binding capacity of transactivating proteins involved in PB transcriptional activation. This DNA-protein complex, analyzed by southwestern blotting and UV cross-linking, involves three nuclear factors with molecular weights of 43, 52, and 65 kDa. Supershift and competition experiments showed that the 43-kDa factor can be related to C/EBPalpha and the 52- and 65-kDa factors to the two subunits of NF-kappaB.