Transcriptional regulation of rat liver epoxide hydratase, NADPH-Cytochrome P-450 oxidoreductase, and cytochrome P-450b genes by phenobarbital

Xenobiotica-metabolizing enzyme induction potential of chemicals in animal studies: NanoString nCounter gene expression and peptide group-specific immunoaffinity as accelerated and economical substitutions for enzyme activity determinations?

Xenobiotica-metabolizing enzyme (XME) induction is a relevant biological/biochemical process vital to understanding the toxicological profile of xenobiotics. Early recognition of XME induction potential of compounds under development is therefore important, yet its determination by traditional XME activity measurements is time consuming and cost intensive. A proof-of-principle study was therefore designed due to the advent of faster and less cost-intensive methods for determination of enzyme protein and transcript levels to determine whether two such methods may substitute for traditional measurement of XME activity determinations. The results of the study show that determination of enzyme protein levels by peptide group-specific immunoaffinity enrichment/MS and/or determination of gene expression by NanoString nCounter may serve as substitutes for traditional evaluation methodology and/or as an early predictor of potential changes in liver enzymes. In this study, changes of XME activity by the known standard XME inducers phenobarbital, beta-naphthoflavone and Aroclor 1254 were demonstrated by these two methods. To investigate the applicability of these methods to demonstrate XME-inducing activity of an unknown, TS was also examined and found to be an XME inducer. More specifically, TS was found to be a phenobarbital-type inducer (likely mediated by CAR rather than PXR as nuclear receptor), but not due to Ah receptor-mediated or antioxidant response element-mediated beta-naphthoflavone-type induction. The results for TS were confirmed via enzymatic activity measurements. The results of the present study demonstrate the potential applicability of NanoString nCounter mRNA quantitation and peptide group-specific immunoaffinity enrichment/MS protein quantitation for predicting compounds under development to be inducers of liver XME activity.

Nuclear receptor co-repressor RIP140 regulates diurnal expression of cytochrome P450 2b10 in mouse liver

Elucidating the mechanisms for circadian expression of drug-metabolizing enzymes is essential for a better understanding of dosing time-dependent drug metabolism and pharmacokinetics. CYP2B6 (Cyp2b10 in mice) is an important enzyme responsible for metabolism and detoxification of approximately 10% of drugs. Here, we aimed to investigate a potential role of nuclear receptor co-repressor RIP140 in circadian regulation of Cyp2b10 in mice.We first uncovered diurnal rhythmicity in hepatic RIP140 mRNA and protein with peak values at ZT10 (ZT, zeitgeber time). RIP140 ablation up-regulated Cyp2b10 expression and blunted its rhythm in mice and in AML-12 cells. Consistent with a negative regulatory effect, overexpression of RIP140 inhibited Cyp2b10 promoter activity and reduced cellular Cyp2b10 expression.Furthermore, RIP140 suppressed Car- and Pxr-mediated transactivation of Cyp2b10, and the suppressive effects were attenuated when the RIP140 gene was silenced. Chromatin immunoprecipitation assays revealed that recruitment of RIP140 protein to the Cyp2b10 promoter was circadian time-dependent in wild-type mice. More extensive recruitment was observed at ZT10 than at ZT2 consistent with the rhythmic pattern of RIP140 protein. However, the time-dependency of RIP140 recruitment was lost in RIP140^-/- mice.Additionally, we identified a D-box and a RORE cis-element in RIP140 promoter. D-box- and RORE-acting clock components such as Dbp, E4bp4, Rev-erbα/β and Rorα transcriptionally regulated RIP140, potentially accounting for its rhythmic expression.In conclusion, RIP140 regulates diurnal expression of Cyp2b10 in mouse liver through periodical repression of Car- and Pxr-mediated transactivation. This co-regulator-driven mechanism represents a novel source of diurnal rhythmicity in drug-metabolizing enzymes.

Insights into CYP2B6-mediated drug-drug interactions

Mounting evidence demonstrates that CYP2B6 plays a much larger role in human drug metabolism than was previously believed. The discovery of multiple important substrates of CYP2B6 as well as polymorphic differences has sparked increasing interest in the genetic and xenobiotic factors contributing to the expression and function of the enzyme. The expression of CYP2B6 is regulated primarily by the xenobiotic receptors constitutive androstane receptor (CAR) and pregnane X receptor (PXR) in the liver. In addition to CYP2B6, these receptors also mediate the inductive expression of CYP3A4, and a number of important phase II enzymes and drug transporters. CYP2B6 has been demonstrated to play a role in the metabolism of 2%–10% of clinically used drugs including widely used antineoplastic agents cyclophosphamide and ifosfamide, anesthetics propofol and ketamine, synthetic opioids pethidine and methadone, and the antiretrovirals nevirapine and efavirenz, among others. Significant inter-individual variability in the expression and function of the human CYP2B6 gene exists and can result in altered clinical outcomes in patients receiving treatment with CYP2B6-substrate drugs. These variances arise from a number of sources including genetic polymorphism, and xenobiotic intervention. In this review, we will provide an overview of the key players in CYP2B6 expression and function and highlight recent advances made in assessing clinical ramifications of important CYP2B6-mediated drug–drug interactions.

Microsomal epoxide hydrolase 1 (EPHX1): Gene, structure, function, and role in human disease

Microsomal epoxide hydrolase (EPHX1) is an evolutionarily highly conserved biotransformation enzyme for converting epoxides to diols. Notably, the enzyme is able to either detoxify or bioactivate a wide range of substrates. Mutations and polymorphic variants in the EPHX1 gene have been associated with susceptibility to several human diseases including cancer. This review summarizes the key knowledge concerning EPHX1 gene and protein structure, expression pattern and regulation, and substrate specificity. The relevance of EPHX1 for human pathology is especially discussed.

Application of a substrate cocktail approach in the assessment of cytochrome P450 induction using cultured human hepatocytes

Induction of the cytochrome P450 (CYP) family of enzymes by coadministered compounds can result in drug-drug interactions, as in the case of the coadministration of rifampicin with many CYP3A substrates, including midazolam. Identification of potential drug-drug interactions due to CYP induction during drug discovery is critical. We present a substrate cocktail method that was applied to assess the induction of CYP1A, CYP2B6, CYP2C9, and CYP3A using a 96-well high-throughput format. Viable cell counts were determined using a high-content screening system to normalize activities. Substrate cocktail incubations demonstrated a similar fold induction for known inducers as compared with discrete probe incubations. The system was further validated by determining the induction potency of rifampicin. The E(max) and EC(50) values in two separate lots of hepatocytes for CYP3A induction by rifampicin in a 96-well format were similar when discrete probe was compared with the probe cocktail. This system has been demonstrated to be suitable for high-throughput assessments of CYP induction.

Werner's syndrome helicase participates in transcription of phenobarbital-inducible CYP2B genes in rat and mouse liver

Werner's syndrome (WS) is a rare human autosomal recessive segmental progeroid syndrome clinically characterized by atherosclerosis, cancer, osteoporosis, type 2 diabetes mellitus and ocular cataracts. The WRN gene codes for a RecQ helicase which is present in many tissues. Although the exact functions of the WRN protein remain unclear, accumulating evidence suggests that it participates in DNA repair, replication, recombination and telomere maintenance. It has also been proposed that WRN participates in RNA polymerase II-dependent transcription. However no promoter directly targeted by WRN has yet been identified. In this work, we report mammalian genes that are WRN targets. The rat CYP2B2 gene and its closely related mouse homolog, Cyp2b10, are both strongly induced in liver by phenobarbital. We found that there is phenobarbital-dependent recruitment of WRN to the promoter of the CYP2B2 gene as demonstrated by chromatin immunoprecipitation analysis. Mice homozygous for a Wrn mutation deleting part of the helicase domain showed a decrease in basal and phenobarbital-induced CYP2B10 mRNA levels compared to wild type animals. The phenobarbital-induced level of CYP2B10 protein was also reduced in the mutant mice. Electrophoretic mobility shift assays showed that WRN can participate in the formation of a complex with a specific sequence within the CYP2B2 basal promoter. Hence, there is a WRN binding site in a region of DNA sequence to which WRN is recruited in vivo. Taken together, these results suggest that WRN participates in transcription of CYP2B genes in liver and identifies the first physical interaction between a specific promoter sequence and WRN.

Focus on the intermediate state: immature mRNA of cytochromes P450--methods and insights

The scattered and limited data on hnRNAs (pre-mRNAs) of cytochromes P450 (CYP) are compiled and discussed for the first time. The methods for determination and quantification of hnRNAs are compared. In most cases, CYP hnRNA levels were determined as a parameter of transcriptional activity. It is known, however, that some CYPs, in particular CYP2E1, are in addition specifically and extensively regulated by post-transcriptional processes. Obviously, these processes also influence the processing of CYP hnRNAs so that their levels cannot be considered a mere result of transcription. The underlying mechanisms of post-transcriptional CYP hnRNA and mRNA regulation are not well understood. It is our aim therefore to bring together available data on CYP hnRNA and to discuss them in the light of recent advances in knowledge concerning pre-mRNA processing and interactions between RNA and low molecular weight interfering RNAs. By doing this, we hope to drive research in a direction which appears promising in providing some long-awaited answers with respect to mechanisms of post-transcriptional CYP regulation.

Cell shape, cell-cell contact, cell-extracellular matrix contact and cell polarity are all required for the maximum induction of CYP2B1 and CYP2B2 gene expression by phenobarbital in adult rat cultured hepatocytes

The effect of cell shape, cell density, contact with extracellular matrix and cell polarity on the phenobarbital (PB)-induced gene expression of CYP2B1 and CYP2B2 (CYP2B1/2B2) in adult rat hepatocytes was investigated. High cell density enhanced the induction of CYP2B1/2B2 gene expression by PB. Hepatocytes cultured on EHS gel showed a spherical cell shape and highly enhanced the induction of CYP2B1/2B2 gene expression by PB. Although monolayer hepatocytes cultured on type I collagen (TIC) and type IV collagen exhibited poor induction of CYP2B1/2B2 gene expression by PB, monolayer cells on laminin showed substantial induction. The addition of soluble laminin to media did not show any effect on induction in monolayer hepatocytes cultured on TIC. Dishes coated with different concentrations of immovable laminin demonstrated complicated effects. Coating with higher concentrations of laminin resulted in greater induction of CYP2B1/2B2 gene expression by PB. On the other hand, when hepatocytes were cultured on dishes coated with lower concentrations of laminin, they became round and greater induction of CYP2B1/2B2 gene expression by PB was observed. Spherical hepatocytes cultured on low concentrations of TIC also showed highly enhanced induction of CYP2B1/2B2 gene expression by PB. EHS gel overlay to hepatocytes cultured on TIC and collagen sandwich configurations that are known to induce cell polarity enhanced the induction by PB. The induction of CYP2B1/2B2 gene expression needed cytoskeleton organization, such as actin filament, microtubule filament and intermediate filament. These results demonstrate that cell shape, cell density, contact with extracellular matrix and cell polarity all play critical roles in the induction of CYP2B1/2B2 gene expression by PB.

The relationship among microsomal enzyme induction, liver weight, and histological change in cynomolgus monkey toxicology studies

The purpose of this investigation was to examine the relationship among hepatic microsomal enzyme induction, liver weight, histological evidence of hepatic injury, and serum clinical chemistry markers of hepatic origin in the cynomolgus monkey. We report here the results from independent toxicology studies for 10 investigative drug candidates representing four therapeutic classes. Study conditions were selected to elicit target organ toxicity. We found that six of the 10 compounds altered cytochrome P450-associated activities in both male and female monkeys, two in females only, and one altered similar activities in males only. Frequently, significant treatment-related elevations in NADPH cytochrome c reductase and ethylmorphine N-demethylase were noted. When the results from all 10 studies were pooled, 14 cytochrome P450-associated activities were significantly increased and five were decreased in males compared to 15 significantly increased and three decreased in the females. Treatment-associated liver weight increases were noted in four studies. Except for hepatocellular hypertrophy in one study, no significant treatment-related microscopic changes in liver and no elevations of serum biomarkers commonly associated with liver toxicity were observed in any of the studies that demonstrated significant hepatic enzyme induction. Compared to parallel rat studies, one compound was an inducer only in monkeys and one was an inducer only in rats. Significant elevations of microsomal drug-metabolizing enzymes in the cynomolgus monkey liver are not accompanied by substantial hepatic changes except for hepatomegaly. These alterations in the hepatic drug-metabolizing enzyme system were benign based the absence of histopathological lesions and serum biomarkers of hepatobiliary toxicity.

Role for mitogen-activated protein kinases in phenobarbital-induced expression of cytochrome P450 2B in primary cultures of rat hepatocytes

Phenobarbital (PB) alters expression of numerous hepatic genes, including genes of cytochrome P450 2B1 and 2B2 (CYP2B). However, the intracellular mechanisms remain to be fully elucidated. The present study investigated the involvement of mitogen-activated protein kinases (MAPKs) in rat hepatocytes in primary culture. We showed that PB induced an early, dose-dependent activation of ERK (extracellular signal-regulated kinase), JNK (c-Jun N-terminal kinase) and p38 MAPKs. Regarding the PB (1mM) induction of CYP2B mRNA expression, while chemically inhibiting JNK had no effect, specific inhibitors of the ERK (U0-126) and p38 (SB-203580) pathways up- and down-regulated this expression, respectively. However, although such a regulation was confirmed when testing the effect of a dominant negative mutant of the ERK pathway on the CYP2B2 enhancer-promoter activity, no such transcriptional role was found with the p38 pathway. Moreover, upon arrest of transcription, the stability of CYP2B mRNA remained unaffected by SB-203580. In conclusion, we show that the ERK pathway negatively regulates CYP2B2 enhancer-promoter activity and that, despite p38 activation upon PB exposure, the sensitivity of CYP2B mRNA expression to SB-203580 appears to be unrelated to this kinase.

Epoxide hydrolases: their roles and interactions with lipid metabolism

The epoxide hydrolases (EHs) are enzymes present in all living organisms, which transform epoxide containing lipids by the addition of water. In plants and animals, many of these lipid substrates have potent biologically activities, such as host defenses, control of development, regulation of inflammation and blood pressure. Thus the EHs have important and diverse biological roles with profound effects on the physiological state of the host organisms. Currently, seven distinct epoxide hydrolase sub-types are recognized in higher organisms. These include the plant soluble EHs, the mammalian soluble epoxide hydrolase, the hepoxilin hydrolase, leukotriene A4 hydrolase, the microsomal epoxide hydrolase, and the insect juvenile hormone epoxide hydrolase. While our understanding of these enzymes has progressed at different rates, here we discuss the current state of knowledge for each of these enzymes, along with a distillation of our current understanding of their endogenous roles. By reviewing the entire enzyme class together, both commonalities and discrepancies in our understanding are highlighted and important directions for future research pertaining to these enzymes are indicated.

Alternative promoters determine tissue-specific expression profiles of the human microsomal epoxide hydrolase gene (EPHX1)

Microsomal epoxide hydrolase (EPHX1) catalyzes hydration reactions that determine the cellular disposition of reactive epoxide derivatives. Whereas the previously defined EPHX1 exon 1 sequence (E1) is derived from a promoter proximal to exon 2 of the EPHX1 coding region, in this investigation, we identified an alternative EPHX1 exon 1 sequence, E1-b, originating from a gene promoter localized approximately 18.5 kb upstream of exon 2. Northern hybridizations demonstrated that the E1-b variant is widely expressed and that the E1-b promoter functions as the primary driver of EPHX1 expression in human tissues. In contrast, the E1 promoter directs expression only in the liver. To examine the basis for liver-specific usage of the E1 promoter, we identified several potential cis-regulatory elements that included GATA (-110/-105) and hepatocyte nuclear factor 3 (HNF3) (-96/-88) motifs. GATA-4 was the principal GATA family member interacting with its respective motif, whereas both HNF3alpha and HNF3beta were capable of interacting with the HNF3 element. GATA-4 and HNF3alpha/HNF3beta DNA binding complexes were enriched in hepatic cells. Site-directed mutagenesis and transactivation analyses of the E1 promoter revealed that GATA-4 is probably a principal factor that regulates liver-specific expression of the E1 variant, with HNF3alpha and HNF3beta acting to negatively regulate GATA-4 function in hepatic cells.

Spurious observation of splenic cyp2b1 expression

Phenobarbital (PB) induction of the CYP2B subfamily was studied in the livers and spleens of male and female rats. Animals were treated with either PB (10 mg/kg) or vehicle for 4 consecutive days. A reverse transcriptase-polymerase chain reaction (RT-PCR), quantitative Northern blotting, Western blotting, and a radioenzymatic assay were used to observe differential levels of CYP2B1 and CYP2B2 mRNAs, proteins, and catalytic activities. CYP2B2 expression was limited to the livers of PB-treated male and female rats and was not detected in spleen. Low constitutive levels of CYP2B1 mRNA were markedly induced approximately 7- to 17-fold in the livers of PB-treated male and female rats, respectively. However, using the same standard oligonucleotide probe for CYP2B1 mRNA, we observed considerably greater constitutive concentrations of the transcript in spleen than in liver. Putative splenic CYP2B1 mRNA was significantly elevated by the PB treatment, although not as profoundly as the hepatic response. In contrast, only the livers of the barbiturate-treated rats expressed CYP2B1 proteins or specific catalytic activity (androstenedione 16beta-hydroxylase). Protein and catalytic activities of the isoforms were undetectable in spleen of either male or female vehicle- and PB-treated rats. In agreement, RT-PCR was unable to demonstrate the expression of splenic CYP2B1 mRNAs. Investigating the possibility that the Northern probe for CYP2B1 was identifying a similar sequence isoform, we performed RT-PCR using primers for CYP2B12 and CYP2B15. Since neither of these isoforms was expressed in spleen, we conclude that the spurious results using the Northern probe for CYP2B1 mRNA were due to the presence of a cross-reacting, PB-responsive transcript not currently identifiable in existing databases.

Regulation of P450 genes by liver-enriched transcription factors and nuclear receptors

Cytochrome P450s (P450s) constitute a superfamily of heme-proteins that play an important role in the activation of chemical carcinogens, detoxification of numerous xenobiotics as well as in the oxidative metabolism of endogenous compounds such as steroids, fatty acids, prostaglandins, and leukotrienes. In addition, some P450s have important roles in physiological processes, such as steroidogenesis and the maintenance of bile acid and cholesterol homeostasis. Given their importance, the molecular mechanisms of P450 gene regulation have been intensely studied. Direct interactions between transcription factors, including nuclear receptors, with the promoters of P450 genes represent one of the primary means by which the expression of these genes is controlled. In this review, several liver-enriched transcription factors that play a role in the tissue-specific, developmental, and temporal regulation of P450s are discussed. In addition, the nuclear receptors that play a role in the fine control of cholesterol and bile acid homeostasis, in part, through their modulation of specific P450s, are discussed.

Evaluation of thyroid hormone effects on liver P450 reductase translation

The expression of NADPH cytochrome P450 oxidoreductase (P450R) in rat liver is positively regulated by thyroid hormone (T3), at both the transcriptional and post-transcriptional levels. Here we investigate the effects of T3-induced hyperthyroidism on the regulation of P450R protein synthesis. T3 treatment of adult male rats led to a strong induction (up to approximately 10-fold) of liver P450R mRNA but little or no change in P450R protein and activity. Investigation of this discrepancy revealed that the association of hepatic P450R mRNA with polysomes was not altered by T3 treatment, suggesting that the discoordinate changes in P450R mRNA and protein levels do not reflect decreased recruitment of T3-induced P450R mRNA into polysomes. Moreover, polysome size distribution analysis of P450R mRNA did not show any T3-dependent changes. When assayed in an in vitro translation system, T3-induced and uninduced P450R mRNAs were translated with similar efficiencies. Moreover, liver cell extract from T3-treated rats did not selectively inhibit in vitro translation of T3-induced P450R mRNA. Thus, neither structural changes in P450R mRNA nor trans-acting binding proteins in liver cytosol were found to control translation of P450R mRNA in response to T3 treatment. Taken together, these data suggest that P450R may in part be regulated at the level of protein stability in hyperthyroid rat liver.

Induction of CYP2B1/2 and nicotine metabolism by ethanol in rat liver but not rat brain

A higher proportion of alcoholics than non-alcoholics smoke (>80 vs 30%). In animals, chronic administration of alcohol induces tolerance to some effects of nicotine. To investigate if chronic ethanol (EtOH) induces alterations in CYP2B1/2 and nicotine C-oxidation activity, male rats (N = 4-6/group) were treated once daily with saline or EtOH (0.3, 1.0, and 3.0 g/kg, p.o./by gavage) for 7 days. A quantitative immunoblotting assay was developed to detect CYP2B1/2 in the brain, where constitutive expression is low, and in the liver. Using this method, it was determined that EtOH did not alter CYP2B1/2 protein expression significantly in six brain regions (olfactory bulbs, olfactory tubercles, frontal cortex, hippocampus, cerebellum, and brainstem). However, a dose-dependent induction of CYP2B1/2 protein expression was detected in the liver. Significant induction of 2-, 3-, and 2.7-fold were observed for the 0.3, 1.0, and 3.0 g/kg doses, respectively. Increases were also observed in CYP2B1 mRNA, which was induced by 14, 38, and 43% at the same doses. Liver microsomal nicotine C-oxidation also was increased (1.3 to 4.5-fold). CYP2B selective inactivators demonstrated that approximately 70% of nicotine C-oxidation was mediated by CYP2B1/2 in both EtOH-induced and uninduced hepatic microsomes. In summary, chronic, behaviorally relevant doses of EtOH induce CYP2B1/2 protein, mRNA, and nicotine C-oxidation activity in rat liver but not in rat brain, and these increases could contribute to cross-tolerance and co-abuse of ethanol and nicotine.

Phenobarbital response elements of cytochrome P450 genes and nuclear receptors

Phenobarbital (PB) response elements are composed of various nuclear receptor (NR)-binding sites. A 51-bp distal element PB-responsive enhancer module (PBREM) conserved in the PB-inducible CYP2B genes contains two NR-binding direct repeat (DR)-4 motifs. Responding to PB exposure in liver, the NR constitutive active receptor (CAR) translocates to the nucleus, forms a dimer with the retinoid X receptor (RXR), and activates PBREM via binding to DR-4 motifs. For CYP3A genes, a common NR site [DR-3 or everted repeat (ER)-6] is present in proximal promoter regions. In addition, the distal element called the xenobiotic responsive module (XREM) is found in human CYP3A4 genes, which contain both DR-3 and ER-6 motifs. Pregnane X receptor (PXR) could bind to all of these sites and, upon PB induction, a PXR:RXR heterodimer could transactivate XREM. These response elements and NRs are functionally versatile, and capable of responding to distinct but overlapping groups of xenochemicals.

Induction of drug metabolism by nuclear receptor CAR: molecular mechanisms and implications for drug research

Recent findings indicate that induction of drug metabolism is regulated by activation of specific members of the nuclear receptor gene family. This minireview deals with the mechanisms by which phenobarbital and phenobarbital-type chemicals induce cytochrome P450 and other genes, and summarises the knowledge on the role of the constitutively active receptor CAR in the induction process. The potential implications of CAR-mediated induction for drug research and possible uses of CAR are also discussed.

Molecular basis for cell-specific regulation of the NADPH-cytochrome P450 oxidoreductase gene

NADPH-cytochrome P450 oxidoreductase (CYPOR), a flavoprotein localized in the nuclear envelope and endoplasmic reticulum of most cell types, is responsible for transferring electrons from NADPH to the cytochromes P450 as well as heme oxygenase, squalene epoxidase, and cytochrome b(5). CYPOR is encoded by a single gene and, similar to many housekeeping genes, has a TATA-less, GC-rich promoter with multiple Sp1 consensus sites. The current work has delineated the importance of multiple cis-acting elements contained within the proximal promoter for basal expression of the CYPOR gene. Transcription factor binding sites within this region included two upstream Sp1 motifs, a SEC element containing overlapping Sp1/Egr-1/CACCC box motifs, and a novel site designated the OxidoReductase Upstream element (ORU). Mutational modification of the ORU element, leading to a loss of protein binding, resulted in an approximately 90% decrease in transcriptional activity in H4IIE cells. Similarly, inactivation of the Egr-1/CACCC segment of the SEC element dramatically reduced promoter activity to less than 10% of wild-type, while mutagenesis of the contiguous Sp1 site did not affect basal transcription. Although both Sp1 sites contained within the minimal promoter were required for optimal expression in H4IIE cells, loss of these sites was compensated for by those Sp1 motifs located upstream of position 206, suggesting that Sp1 was acting as a position-independent enhancer. Hence, the CYPOR promoter was distinguished from the majority of TATA-less promoters in that Sp1 was not a primary transcriptional regulator and by the fact that the Sp1 binding site closest to the transcription start site was nonfunctional. Furthermore, both the SEC and ORU elements were essential for basal expression; however, the ORU element exhibited cell-specific differences in regulatory activity. Thus, several mechanisms appear to be in place to selectively alter the expression of the CYPOR gene.

Lack of modulation by phenobarbital of cyclic AMP levels or protein kinase A activity in rat primary hepatocytes

Results of previous studies have substantiated a negative modulatory role for cyclic AMP (cAMP) and protein kinase A (PKA) dependent processes on the phenobarbital (PB) induction response in hepatocytes. The current study was conducted to further examine the potential role of second messenger pathways in the initial phases of induction, specifically addressing the effects of PB on the expression of intracellular cAMP levels and associated PKA activity. Using a highly differentiated primary rat hepatocyte system, cells were exposed to PB for various intervals (30 sec to 48 hr), and levels of intracellular cAMP and subsequent PKA activity were determined. Although PB markedly induced CYP2B expression, exposure to this agent produced no detectable increases in cAMP levels and PKA activity at any of the times examined. These results demonstrated that the initial events stimulated by PB in rat hepatocytes do not include alterations of cAMP levels or associated PKA activities.

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.

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.

Mechanism of the synergistic induction of CYP2H by isopentanol plus ethanol: comparison to glutethimide and relation to induction of 5-aminolevulinate synthase

We had previously found that combined treatment with isopentanol and ethanol synergistically induced CYP2H protein and activity in cultured chick nepatoytes. Here we investigated the mechanism of induction of CYP2H by the alcohols and whether they caused a coordinate induction of 5-aminolevulinate synthase (ALAS) mRNA. Treatment with isopentanol alone or in combination with ethanol resulted in coordinate increases in CYP2H1 and ALAS mRNAs. With isopentanol alone, the amounts of CYP2H1 and ALAS mRNAs at 4 to 6 h were similar to those observed after treatment with the alcohol combination, but declined by 11 h. Readdition of isopentanol at 11 h again increased the expression of both mRNAs, indicating that the decreases at 11 h were due to limiting amounts of inducer. Similar results were observed in cells exposed to low concentrations of glutethimide. In the combined alcohol treatment, increases in CYP2H1 and ALAS mRNAs were sustained from 4 h to 11 h after addition of the alcohols, but decreased to control levels by 24 h. Using pulse labeling to measure de novo synthesis of CYP2H1/2 protein, we found that the increases in CYP2H1/2 protein reflected the increases in CYP2H1 mRNA. The half-life of CYP2H1/2 protein, measured from pulse-chase experiments, was approximately twofold greater than the half-life of CYP2H1 mRNA. Our results indicate that the alcohols and glutethimide coordinately increase ALAS and CYP2H1 mRNA, and that increases in CYP2H1/2 protein arise from increases in its mRNA.

Evidence for involvement of cAMP-dependent pathway in the phenobarbital-induced expression of a novel hamster cytochrome P450, CYP3A31

Recently, we isolated a novel Syrian hamster cDNA clone that encodes a protein which has been named CYP3A31. In primary hepatocyte cultures, CYP3A31 is dramatically induced by phenobarbital. To elucidate the mechanism of this induction, we first studied the effects of cAMP on phenobarbital-induced CYP3A31 expression using forskolin and N6,O2'-dibutyryl cAMP in hepatocyte cultures. At 100 microM, forskolin significantly inhibited both the phenobarbital-induced CYP3A31 mRNAs expression and the testosterone 6beta-hydroxylation activity related to the CYP3A subfamily in rats, whereas 0.1 microM forskolin potentiated the phenobarbital induction of CYP3A31 mRNA and the testosterone 6beta-hydroxylation activity. Treatment with N6,O2'-dibutyryl cAMP resulted in an inhibition of phenobarbital-induced CYP3A31 gene expression and testosterone 6beta-hydroxylation activity. Increasing amounts of transfected cAMP-response element binding proteins (CREB) or CREB-binding proteins in hamster hepatocytes reduced the phenobarbital-induction of CYP3A31 mRNAs expression. These results suggest that in vitro induction of CYP3A31 by phenobarbital in Syrian hamster hepatocytes is regulated by a cAMP-dependent pathway.

Effect of phenobarbital and beta-naphthoflavone on activities of different rat esterases after paraoxon exposure

1. The effects of two model inducers of the cytochrome P450 system, phenobarbital (PB) and beta-naphthoflavone (NF), on the toxicity of paraoxon were studied in rats. 2. Paraoxon toxicity was measured by inhibition of brain acetylcholinesterase (AChE) activity. 3. PB treatment did not affect the toxicity of paraoxon, whereas NF increased the inhibition of brain AChE. PB administration slightly increased the activities of some peripheral cholinesterases and carboxylesterases, as well as liver microsomal paraoxonase (Pxase). 4. NF administration, in contrast, decreased the activities of peripheral esterases. Serum Pxase activity was reduced by both inducers. 5. Hepatic CYP2B and CYP1A were markedly induced by PB and NF, respectively. 6. Cytochrome P450 isoenzymes induced by PB or NF seemed not to be critical in the detoxification of paraoxon in vivo. NF caused a general reduction of peripheral esterases, which led to an increase in paraoxon toxicity. 7. The results indicated the great importance of peripheral cholinesterases and carboxylesterases as a detoxifying mechanism of paraoxon. The role of serum paraoxonase was not critical.

Induction time course of cytochromes P450 by phenobarbital and 3-methylcholanthrene pretreatment in liver microsomes of Alligator mississippiensis

Alligator mississippiensis has at least two classes of inducible hepatic microsomal cytochromes P450 (CYP): (1) those induced by 3-methylcholanthrene (3MC), and (2) those induced by phenobarbital (PB). The rates of induction by these xenobiotic compounds are significantly slower than those reported for mammals. Carbon monoxide binding, western blots, and enzymatic activity measurements indicated that at least 48-72 hr are required to reach full induction. A methoxy-, ethoxy-, pentoxy, and benzyloxyphenoxazone (resorufin) O-dealkylation (MROD, EROD, PROD, and BROD) profile was indicative of substrate selectivity typical of 3MC- and PB-induced P450s. MROD and BROD showed the greatest ability to discriminate between alligator hepatic microsomes induced by 3MC and PB, respectively. This is in contrast to mammals, in which EROD is a biomarker of polycyclic aromatic hydrocarbon exposure because of its ability to discriminate the induction of CYP 1A. In a similar manner, PROD is a highly preferred activity of CYP 2B in mammals; thus, it is used to indicate CYP 2B induction. The induction of P450 by PB is a general phenomenon in mammals and birds. To the best of our knowledge, this is the first report demonstrating PB induction of P450 activities typical of the mammalian CYP 2 family isoforms in alligator or any reptilian liver. The importance of this finding to the evolution of CYP 2 family regulation by PB is heightened by the fact that induction by this xenobiotic is not common to fish and other lower vertebrates (Ertl RP and Winston GW, Comp Biochem Physiol, in press). Although indicating the presence of CYP 1A- and CYP 2B-like isoforms in alligator, it remains to be established how closely related these alligator P450s are to mammalian isoforms.

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

Although the induction of drug-metabolizing systems by phenobarbital has been recognized for about 40 years, the mechanism by which cytochrome P450 gene expression is increased is still not well understood. A 163-bp fragment at about -2.2 Kb in CYP2B2 has been shown to mediate phenobarbital induction in primary rat hepatocytes (Trottier, et al. [1995] Gene 158:263-268) and by an in situ transient transfection assay in rat liver (Park, Y., et al. [1996]. J. Biol. Chem. 271:23725-23728). Deletion mutations of this fragment indicated that the 88-bp stretch from -2258 to -2170 was the minimal sequence that could mediate phenobarbital induction in the in situ system if single copies of the deleted fragments fused to the CYP2C1 proximal promoter were assayed. If three copies of the fragments were present, 5' and 3' deletions defined a minimal 37-bp core fragment, which, although necessary for phenobarbital responsiveness, was not sufficient unless additional sequence was present at either end, suggesting that redundant elements were present in the two flanking regions. Site-specific mutagenesis of an NF-1 site within the 88-bp fragment and linker scanning mutagenesis across the fragment indicated that the NF-1 site and a region to the 5' side of the site contributed to the magnitude of the response, but neither the NF-1 mutations nor any of the linker scanning mutations eliminated the response to phenobarbital. Mutation in a region 3' of the NF-1 site resulted in elevated basal expression without substantial effects on phenobarbital-induced expression. Binding of NF-1 to the 37-bp core fragment was established by gel-shift competition studies and by supershifts of the protein-DNA complexes by antisera to NF-1. Additional protein-DNA complexes were detected in the regions flanking the NF-1 site. These studies indicate that the CYP2B2 phenobarbital-responsive enhancer contains multiple constitutive and phenobarbital-responsive elements. Binding of nuclear proteins from control or phenobarbital-treated animals in vitro to this region was very similar. The only difference detected was a complex that was substantially reduced by phenobarbital treatment and mapped to the 3' side of the NF-1 site.

The CYP2B2 phenobarbital response unit contains an accessory factor element and a putative glucocorticoid response element essential for conferring maximal phenobarbital responsiveness

Hepatic cytochrome P450s play a critical role in the metabolism of hydrophobic xenobiotics. One of the major unsolved problems in xenobiotic metabolism is the molecular mechanism whereby phenobarbital induces hepatic enzymes, particularly CYP2B1 and CYP2B2 in rat liver. By using primary rat hepatocytes for transfection analyses, we previously identified in the CYP2B2 5'-flank a 163-base pair Sau3AI fragment that confers phenobarbital inducibility on a cat reporter gene and that has the properties of a transcriptional enhancer. Transfection experiments with sub-regions of the Sau3AI fragment now indicate that a central core together with an upstream or downstream accessory element within the fragment can confer phenobarbital responsiveness. One such accessory element, AF1, was identified and localized. DNase I footprinting analysis revealed the presence of a footprint overlapping this AF1 element. It also identified three other major protected regions, two of which are putative recognition sites for known transcription factors. Site-directed mutagenesis indicated that a putative glucocorticoid response element as well as a nuclear factor 1 site and an associated nuclear receptor hexamer half-site are essential for conferring maximal phenobarbital inducibility. Taken together, the results indicate that phenobarbital induction of CYP2B2 requires interactions among multiple regulatory proteins and cis-acting elements constituting a phenobarbital response unit.

Protein synthesis inhibitors exhibit a nonspecific effect on phenobarbital-inducible cytochome P450 gene expression in primary rat hepatocytes

Previous investigations have indicated that de novo protein synthesis is a critical requirement for phenobarbital (PB) induction. We reexamined this issue in PB-responsive primary rat hepatocyte cultures using a broader array of protein synthesis inhibitors and experimental end points. Anisomycin, cycloheximide, emetine, puromycin, and puromycin aminonucleoside, a negative analog, were evaluated for their respective effects on protein synthesis and the PB-induction process. All of the inhibitors effectively repressed de novo protein synthesis in the cells in a concentration-dependent manner. However, anisomycin only minimally effected PB induction, ascertained though the measure of CYP2B1, CYP2B2, and CYP3A1 mRNA levels. The inactive agent, puromycin aminonucleoside, produced marked repression of the PB-induction response. Results from further experiments demonstrated that these protein synthesis inhibitors stimulated rapid and differential phosphorylation of the stress-activated protein kinase/c-Jun kinase (SAPK/JNK) pathway, indicating nonselective actions on cellular processes. Puromycin aminonucleoside was without effect on these pathways, despite its efficacy as an inhibitor of PB induction. These results demonstrate that de novo protein synthesis is not a requirement for PB induction, nor is activation of the SAPK/JNK kinase cascade responsible for down-regulating PB responsiveness in primary hepatocytes.

Liver-enriched transcription factors, HNF-1, HNF-3, and C/EBP, are major contributors to the strong activity of the chicken CYP2H1 promoter in chick embryo hepatocytes

Chicken CYP2H1 promoter constructs express strongly in chick embryo hepatocytes at a level comparable with that of Rous sarcoma viral promoter. We have identified the transcription factors responsible for the active CYP2H1 promoter. Binding sites for transcription factors were located within the first 160 bp of promoter sequence using promoter deletion experiments and DNase I footprint analysis. Sequence analysis revealed characteristic sites for the liver-enriched transcription factors of the HNF-1, HNF-3, and C/EBP families and for the ubiquitous factor, USF. Protein binding to these sites was established by gel mobility shift assays. Mutagenesis and transient transfection experiments demonstrated that these sites, in combination, were responsible for the strong promoter activity with a substantial contribution from HNF-1 and HNF-3. The promoter was also active in mammalian HepG2 and COS-1 cell lines where expression was dependent on the identified transcription factor binding sites but promoter activity in the HeLa cells was low. Transactivation experiments revealed that promoter expression could be activated through the appropriate binding sites by exogenously expressed rat HNF-1alpha or HNF-1beta, rat HNF-3alpha or HNF-3beta and chicken C/EBP alpha. Transcriptional synergism between HNF-1 and C/EBP was observed in these transactivation experiments. A Barbie box-like sequence overlapped the USF element but was not functional. The results demonstrate that liver-enriched transcription factors and USF direct strong expression of the CYP2H1 promoter in transiently transfected cells. By comparison, in vivo expression of this gene in uninduced chick embryo hepatocytes is low but markedly increased by phenobarbital. Drug induction may therefore substantially reflect derepression of this inherently active promoter.

Contributions of hepatic and intestinal metabolism and P-glycoprotein to cyclosporine and tacrolimus oral drug delivery

The objective of this section is to evaluate the contributions of hepatic metabolism, intestinal metabolism and intestinal p-glycoprotein to the pharmacokinetics of orally administered cyclosporine and tacrolimus. Cyclosporine and tacrolimus are metabolized primarily by cytochrome P450 3A4 (CYP3A4) in the liver and small intestine. There is also evidence that cyclosporine is metabolized to a lesser extent by cytochrome P450 3A5 (CYP3A5). Cyclosporine and tacrolimus are also substrates for p-glycoprotein, which acts as a counter-transport pump, actively transporting cyclosporine and tacrolimus back into the intestinal lumen. Traditional teaching of clinical drug metabolism has been that hepatic metabolism is of primary importance, and other sites of metabolism play a relatively minor role. It appears as though intestinal metabolism plays a much greater role in the pharmacokinetics of orally administered drugs than previously thought. Intestinal metabolism may account for as much as 50% of oral cyclosporine metabolism. There are at least two components of intestinal metabolism for cyclosporine and tacrolimus, intestinal CYP3A4/CYP3A5 and intestinal p-glycoprotein activities. The quantity of intestinal enzymes, although highly variable, do not appear to be the key to explaining the variability of oral cyclosporine pharmacokinetics in kidney transplant patients. However, the quantity of intestinal p-glycoprotein accounts for approximately 17% of the variability in oral cyclosporine pharmacokinetics. It may be that p-glycoprotein maximizes drug exposure to intestinal enzymes, thus decreasing the importance of enzyme quantity. Since cyclosporine's FDA approval in 1983, there have been many reports of clinically significant drug interactions of other agents when given concomitantly with cyclosporine. With the FDA approval of tacrolimus in 1994, a similar pattern of clinically significant drug interactions appears to be emerging. It seems that compounds that alter (either induce or inhibit) CYP3A4 and/or p-glycoprotein will alter the oral pharmacokinetics of cyclosporine and tacrolimus. It should be expected that, until further data are available, the drugs which interact with cyclosporine will also interact with tacrolimus.

Thyroid regulation of NADPH:cytochrome P450 oxidoreductase: identification of a thyroid-responsive element in the 5'-flank of the oxidoreductase gene

The current study demonstrates that T3-activated transcription of the NADPH:cytochrome P450 oxidoreductase (P450R) gene is dependent on the thyroid hormonal status of the animal, with both transcriptional and post-transcriptional pathways being important in regulating the cellular P450R mRNA level. The region required for transcriptional activation of the P450R gene by T3 has been identified. Nuclear run-on experiments demonstrated that the effects of T3 on P450R transcription are dependent on thyroid status, with a transcriptional enhancement obtained in T3-treated hypothyroid rat liver (1.8-fold increase) but not in T3-treated euthyroid animals. Transient cotransfection of P450R promoter/chloramphenicol acetyl transferase (CAT) constructs and the thyroid hormone receptor beta1 (TR beta1) expression plasmid into rat hepatoma H4IIE cells resulted in a 2.4-fold induction of promoter activity that was both T3 and TR beta1 dependent. Analysis of promoter deletion constructs identified a P450R-thyroid response region (P450R-TRE; bases, -564 to -536) containing three imperfect direct repeats of the thyroid response motif, AGGTCA. Mutational analysis further established that T3 induction was dependent only on the upstream direct repeat, having the sequence AGGTGAgctgAGGCCA. Footprint analysis showed that all three motifs were protected by proteins present in rat liver nuclear extracts, and a direct interaction between P450R-TRE and T3 receptors TR alpha1 and TR beta1 was demonstrated by gel-shift analysis. In vitro binding studies with P450R-TRE revealed the formation of heterodimeric complexes when TR alpha1 was coincubated with either the retinoic X receptor alpha or nuclear extract from rat liver, COS, or H4IIE cells. In addition, placement of the P450R-TRE upstream of the T3-nonresponsive heterologous thymidine kinase promoter resulted in a 2.7-fold transcriptional enhancement that was both T3 and TR beta1 dependent. Previous studies have demonstrated that T3 augments P450R mRNA levels approximately 20-30-fold and approximately 12-fold, respectively, in hypothyroid and euthyroid rats. Hence, for the hypothyroid state, transcriptional and post-transcriptional events contribute to the T3-induced mRNA increases; however, the marked increase in message level in T3-treated euthyroid animals depends primarily on post-transcriptional pathways.

Ethylbenzene modulates the expression of different cytochrome P-450 isozymes by discrete multistep processes

Ethylbenzene (EB) treatment to male Holtzman rats was shown to alter the expression of cytochrome P-450s 1A1, 2B, 2C11, 2E1, and 3A, with several isozymes exhibiting complex multiphasic induction patterns when treated for 1 and 3 days with the alkylbenzene. Male rats were treated with daily i.p. injections of EB for either one or three days, and the effects on P-450 dependent activities, P-450 immunoreactive protein levels and their corresponding mRNA levels were measured. Although levels of P-450 2B, 2C11, 2E1, and 3A were all modulated by EB treatment, each exhibited different temporal characteristics. P-450 2B1/2B2 were induced after a single EB exposure and continued to be elevated after EB treatment for 3 days. However, P-450 2B1 and 2B2 mRNA levels were elevated about 50-fold after a single injection, and returned to control values after continued EB administration. P-450 2C11 expression was decreased to about 45% of controls after either single or repeated EB exposure with corresponding changes being observed in the levels of 2C11 mRNA. P-450 2E1 was induced by EB according to a complex multistep induction pattern. Both P-450 2E1 protein and RNA levels were increased 2-4-fold after a single EB treatment but returned to control values after continued administration. P-450 3A-dependent testosterone 2beta-hydroxylation and P-450 3A immunoreactive protein levels were both increased about 3-fold after a single EB treatment, whereas levels were only elevated 2-fold after EB treatment for 3 days. In contrast, P-450 3A2 mRNA was unaffected by a single EB injection but was increased 3.5-fold with repeated administration. Changes in P-450 3A1/2 were similar to those observed with P-450 3A2, whereas changes in P-450 3A1/23 and 3A23 mRNAs were not detectable. These data indicate that while EB can influence the expression of several P-450 isozymes, the hydrocarbon appears to alter P-450 expression by acting at different regulatory steps.

Characterization of the microsomal epoxide hydrolase of hepatic microsomes of the common dab,Limanda limanda

Epoxide hydrolase of microsomal membranes of the common dab (Limanda limanda) has been characterized using p-nitrostyrene oxide as substrate. Under the conditions of assay used, the turnover number with this substrate was higher than found for the more frequently used styrene oxide and steady state kinetics were observed. The enzyme had a KM of 0.12 mM and optima for pH and temperature between pH 8-10.2 and 50-60°C respectively. Enzyme activity was unaffected by low concentrations of ionic and non-ionic detergents but was inhibited by higher concentrations of Lubrol and Brij. The enzyme protein did not react with monospecific antibodies to rat or human microsomal epoxide hydrolase during Western blotting. Large inter-individual variation in enzyme activity was found but the enzyme does not appear to be expressed in a gender-specific way. Fish were administered a wide range of hydrocarbons which are known to alter the expression of cytochrome P450 1A but these had no effect other than benzothiophene which caused a small increase in enzyme activity.

The CYP2B1 proximal promoter contains a functional C/EBP regulatory element

Cytochromes P450 2B1 and 2B2 (CYP2B1 and CYP2B2) are well-known phenobarbital-inducible genes in rat liver. Potential transcriptional regulatory elements in the proximal promoter regions of rat CYP2B genes were analyzed by transfection in HepG2 hepatoma cells and by binding of nuclear proteins. Deletion of sequences from -1,400 to -110 had modest effects on promoter activity, but further deletion to -57 decreased the transcriptional activity by more than 90%, suggesting the presence of strong cis-acting elements in this region. Sequences similar to a basal transcription element (BTE) in CYP1A1 and a proposed phenobarbital responsive element (Barbie box) are present from -89 to -67. However, no protection was detected in these regions by DNase I footprinting assay. Instead, a region (FP1) from -64 to -45 was protected by liver nuclear extracts. Mutation of either the BTE or FP1 sequences of CYP2B1, or both, reduced transcriptional activity by 70-80% in HepG2 cells. FP1 was identified as a functional C/EBP site by co-transfection of C/EBP expression vectors and supershift assays with C/EBP antisera. Binding of liver nuclear proteins to sequences within the -110 to +1 region was not detectably altered by pretreatment of rats with phenobarbital.

Transcriptional regulation of the CYP2B1 and CYP2B2 genes by C/EBP-related proteins

Cytochrome P450 (CYP) 2B1 and 2B2 are encoded by two closely related genes, CYP2B1 and CYP2B2, that are expressed at low levels in adult rat liver but are induced markedly by the administration of the drug phenobarbital (PB) or other structurally unrelated hydrophobic compounds to animals. Very little is understood about the molecular mechanisms that control both basal and induced transcription of these genes. We have identified two liver specific DNase I hypersensitive sites associated with the CYP2B1 and CYP2B2 (CYP2B) genes. One site, which maps to a region in the 5'-flanking region between -2.2 and -2.3 kb, became more resistant to DNase I cleavage in nuclei from PB-treated rats; the converse was true of the other hypersensitive site, which maps to the proximal promoter region between -0.05 and -0.15 kb. DNase I footprint analysis revealed three prominent and one weak footprinted regions in the promoter region in the vicinity of the proximal hypersensitive site. Using competitor oligonucleotides, we determined that one footprinted region (FT2), between -42 and -66 bp, is likely to represent a binding site for CCAATT enhancer binding protein (C/EBP) family members. Indeed, bacterial expressed recombinant C/EBP alpha bound at this site and formed a footprint pattern identical to the pattern observed with liver nuclear extract. In vitro transcription assays demonstrated that the FT2 site contributed strongly to promoter activity, since its mutation reduced transcription by 80%. Two other sites identified by footprint analysis (FT1 and FT3) are also required to maintain high basal transcription of CYP2B2 promoter constructs in an in vitro transcription assay. Transient transfection experiments confirmed the expectation that C/EBP alpha could activate the 1.4 kb CYP2B promoter constructs, with mutation of the FT2 site impairing both basal transcription and transactivation by exogenous C/EBP alpha.

The chemopreventive agent diallyl sulfide. A structurally atypical phenobarbital-type inducer

Diallyl sulfide (DAS), a known chemopreventive agent, was administered i.g. (200 or 500 mg/kg body wt/day) to male F344/NCr rats for 4 days. Livers were removed, and hepatic levels of a variety of drug-metabolizing enzymes were determined with either catalytic assays or by quantifying levels of total cellular RNA coding for the individual genes of interest. The high dose of DAS induced the cytochrome P450 (CYP) 2B subfamily to near maximal levels [i.e. similar to those induced by phenobarbital (PB)] and induced the CYP3A subfamily, while having minimal effects on the levels of the CYP1A subfamily. In addition, DAS induced the glutathione S-transferase alpha subfamily, the glutathione S-transferase mu subfamily, and epoxide hydrolase. Unlike PB, however, DAS was also able to induce quinone oxidoreductase. In fact, the pleiotropic hepatic response to DAS appeared to be similar to that elicited by PB, with the exception that only DAS induced quinone oxidoreductase. Finally, we determined that DAS induced the levels of a specific nuclear binding protein that appears to be associated with the induction of various genes that are part of the pleiotropic response caused by PB-type inducers.

Gene structure and promoter analysis of the rat constitutive CYP2C23 gene

The CYP2C23 gene is expressed constitutively in the rat liver and kidney. It exhibits different profiles of expression in the two tissues, suggesting that several regulation processes could exist. In this paper, we report the structure of the 5'-flanking region of the CYP2C23 gene; 4.5 kbp were sequenced and analyzed. The CYP2C23 gene is present as a single copy into the rat genome and an unique transcription start site is used in both liver and kidney. Four DNase I hypersensitive sites have been mapped to the distal part of the hepatic promoter and three are detected in the kidney: only one site is present in the two tissues (L3/K1). In the proximal region, one site is specific for the kidney and one is detected in all the tissues tested. Footprint experiments allowed precise identification of the sequence of protected regions: HNF4 and CREB binding motifs are present in the distal liver-specific sites, motifs for AP-1, NF-1, and XRE-Bf are in the distal kidney site, and a Tf-LF1 binding site is localized in the L3/K1 protected site. In the proximal region, a sequence protected in all tissues contains a SP1/NF kappa B motif, whereas a sequence containing a HNF-4 binding motif is exclusively protected by kidney nuclear extracts. Altogether, the data clearly demonstrate that trans-acting factors involved in CYP2C23 gene expression differ in liver and kidney.

Pentobarbital-induced changes in Drosophila glutathione S-transferase D21 mRNA stability

The Drosophila glutathione S-transferase (gstD) genes are a family of divergently transcribed, intronless genes and pseudogenes. Under control conditions, the steady-state level of gstD1 mRNA is 20-fold higher than that of the gstD21 mRNA despite a lower transcription rate of the gstD1 gene. The GST D1 protein level is four times as abundant as the GST D21 protein. The gstD1 and gstD21 genes responded rapidly to pentobarbital (PB) as changes in mRNA levels were detectable within 30 min of treatment. Maximal induction of gstD1 and gstD21 resulted in 3-fold and 20-fold elevation of their respective mRNA levels. The major mechanism for the increase in gstD1 mRNAs appears to be transcriptional activation. The 2-fold increase in the rate of gstD21 transcription, however, cannot fully account for the 20-fold increase in the steady-state level of gstD21 mRNA. Therefore, post-transcriptional mechanism(s) should also be responsible for the increase of gstD21 mRNA by PB. Because the gstD21 mRNA is relatively unstable under control conditions, induction of the intronless gstD21 mRNA by PB occurs mainly at the level of enhanced mRNA stability. The GST D1 protein level in adult Drosophila was increased approximately 2-fold after PB treatment, whereas the GST D21 level remained relatively the same. Thus, an increase in gstD21 mRNA stability by PB treatment is probably coupled to a regulatory effect at the translational level.

Localization of a phenobarbital-responsive element (PBRE) in the 5'-flanking region of the rat CYP2B2 gene

Cytochrome P450 2B1, encoded by CYP2B1, and cytochrome P450 2B2, encoded by CYP2B2, are inducible in rat liver by phenobarbital (PB). We have used cultured adult rat hepatocytes to study molecular mechanisms regulating CYP2B1/CYP2B2 transcription. Northern blot analysis demonstrated that the endogenous CYP2B1/CYP2B2 genes were inducible by PB in such cultures. A PB-responsive element (PBRE) conferring PB inducibility on a reporter gene was identified in the CYP2B2 5'-flanking region. The PBRE was localized to a 163-bp Sau3AI fragment situated between 2155 and 2318 bp upstream from the CYP2B2 transcription start point (tsp). The PBRE also conferred PB responsiveness on an enhancerless heterologous promoter and was active in both orientations both upstream and downstream from the heterologous promoter; hence, it has the properties of a transcriptional enhancer. Gel-retardation assays showed that nuclear extracts of liver cells of untreated and PB-treated rats contained sequence-specific DNA-binding factors that interact with a PBRE-containing DNA fragment. These results may open the way to identifying one or more transcription factors mediating induction of CYP2B2 and CYP2B1 in rat liver.

cAMP-associated inhibition of phenobarbital-inducible cytochrome P450 gene expression in primary rat hepatocyte cultures

The effects of elevated intracellular cyclic adenosine monophosphate (cAMP) in regulating phenobarbital (PB)-inducible gene expression in primary rat hepatocyte cultures were investigated. Cells were exposed to various concentrations (0.1-100 microM) of cAMP analogs and/or activators of intracellular cAMP-dependent pathways. Effects of these treatments were assessed either using a 1-h pulse prior to PB (100 microM) exposure or in conjunction with PB during a 24-h exposure period. PB-inducible responses were measured in hepatocytes by hybridization to cytochrome P450 (CYP) CYP2B1, CYP2B2, and CYP3A1 mRNAs. The cAMP analogs, 8-bromo-cAMP, 8-(4-chlorophenylthio)-cAMP, dibutyryl cAMP, and (Sp)-5,6-DCl-cBiMPS ((Sp)-5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole-3', 5'-monophosphorothioate), and the activators of adenylate cyclase, forskolin and glucagon, dramatically inhibited PB-mediated induction of CYP2B1 and CYP2B2 in a concentration-dependent manner. A similar inhibition of PB-induced CYP3A1 mRNA levels was effected by the cAMP analogs and glucagon. The phosphodiesterase inhibitors isobutylmethylxanthine and RO 201724 potentiated the cAMP responses. Increasing the concentration of PB (0.05-1.00 mM) did not alleviate the cAMP-mediated repression. A requirement for protein kinase A (PKA) was demonstrated by the use of (Sp)-cAMPS, a highly specific activator of PKA, whereas the inactive diastereoisomer, (Rp)-cAMPS, was ineffective in modulating PB induction. The response to cAMP was specific since elevated intracellular cAMP levels did not perturb beta-naphtholflavone-mediated induction of CYP1A1, CYP1A2, microsomal epoxide hydrolase, or dexamethasone-mediated induction of CYP3A1 gene expression. Nor did elevated intracellular cAMP modulate the liver-selective albumin gene expression levels. The results of the present study demonstrated striking inhibition of PB-mediated CYP gene induction by cAMP and PKA activators, indicating a negative regulatory role for the cAMP signal transduction pathway on PB gene induction.

Neonatal phenobarbital-induced defects in age- and sex-specific growth hormone profiles regulating monooxygenases

Growth hormone was secreted in sexually dimorphic patterns in both 65- and 150-day-old rats (i.e., "on-off" pulsatile for males and "continuous" pulsatile for females), but as a result of a 200-400% increase in pulse levels the mean concentration of hormone in the circulation was about two times as great in the younger animals. Neonatal exposure to phenobarbital at anticonvulsant therapeutic doses for the rat reduced the pulse amplitudes of circulating growth hormone in both the 65- and 150-day-old males but only in the 65-day-old females. As expected, neonatal administration of the barbiturate produced an almost immediate increase in the activities of the hepatic monooxygenases, as measured by hexobarbital metabolism, which declined to noninduction levels after treatment ceased. Contrary to the well-known transient effects of phenobarbital, at around the time of sexual maturity when gender-dependent differences in hepatic monooxygenases appear (males > females), we observed a second "round" of enzyme induction that persisted in both sexes for the remainder of the study (180 days). Because growth hormone is the primary regulator of sex-dependent hepatic monooxygenases, we have proposed that the abnormal plasma growth hormone profiles produced by neonatal phenobarbital are responsible for the permanent induction of hepatic monooxygenases.

Relationship between hydrocarbon structure and induction of P450: effect on RNA levels

1. Exposure to simple aromatic hydrocarbons has been shown to induce P450-dependent activities and the expression of particular P450 isozymes in a manner related to the molecular structure of the inducing hydrocarbon. In an attempt to identify the structural relationship controlling P450 induction, the effect of hydrocarbon treatment on the RNA levels for specific P450 isozymes was examined. 2. Rats were treated with daily injections of hydrocarbons (benzene, toluene, ethylbenzene, n-propylbenzene, m- and p-xylene) for 3 days, and the effects on specific RNA levels were examined by Northern blot hybridization. 3. Although P4502B1 mRNA was not elevated after hydrocarbon treatment, a significant elevation in 2B2 mRNA was observed after exposure to the larger aromatic hydrocarbons, ethylbenzene and m-xylene. It is interesting to note that despite the substantial elevation of P4502B protein levels, only a small elevation of P4502B1 and 2B2 RNA was observed. 4. P4502C11 mRNA was only suppressed by ethylbenzene administration, despite the depression of 2C11 protein levels by several hydrocarbons. 5. P4501A1 mRNA was not detectable and 2E1 mRNA was not changed by any aromatic hydrocarbon treatment investigated in this study. 6. The data indicate that the levels of mRNA species for a number of P450 isozymes are differentially regulated by exposure to hydrocarbons, and that small changes in hydrocarbon size or isomeric structure can influence the levels of these mRNA species.

Phenobarbital induction of alpha 1-acid glycoprotein in primary rat hepatocyte cultures

The serum level of rat alpha 1-acid glycoprotein is significantly increased by treatment with phenobarbital, and in vivo studies have shown that phenobarbital seems to act mainly at the transcriptional level. To show the direct mediating effect of phenobarbital on alpha 1-acid glycoprotein gene expression, we investigated the ability of primary cultured rat hepatocytes to respond to in vitro phenobarbital administration. Phenobarbital increased both alpha 1 acid glycoprotein secretion and corresponding mRNA levels in primary rat hepatocytes cultured on matrigel. Used in combination with interleukin-1, interleukin-6 and dexamethasone, phenobarbital had an additive or synergistic effect on alpha 1-acid glycoprotein synthesis. These results show that (a) phenobarbital acts directly on hepatocytes by increasing alpha 1-acid glycoprotein gene expression and (b) this effect is mediated by a specific mechanism independent of pathways involved in alpha 1-acid glycoprotein induction by interleukin-1, interleukin-6 and glucocorticoids.

Induction of rat alpha-1-acid glycoprotein by phenobarbital is independent of a general acute-phase response

Phenobarbital (PB) induces transcription of the alpha 1-acid glycoprotein (AGP) gene, one of the major positive acute-phase proteins, the expression of which is controlled by a specific combination of glucocorticoids and cytokines. This raises questions as to the involvement of glucocorticoids and cytokine pathways in the PB-mediated effect on AGP gene expression. We found that the pattern of whole-serum proteins in PB-treated rats differed markedly from that observed during a typical acute inflammatory response (in turpentine-treated rats): levels of some positive acute-phase proteins (APP) increased slightly (alpha 1-acid glycoprotein, haptoglobin, hemopexin and T-kininogen), while levels of alpha 2 macroglobulin, the most sensitive marker of the acute-phase reaction, decreased. Among the negative APP, neither albumin nor prealbumin decreased while CBG increased. The cytokines involved in AGP gene regulation (mainly IL1, IL6 and TNF alpha) do not therefore seem to mediate the effect of PB on acute-phase protein expression. Glucocorticoid involvement is also ruled out by the observed enhancement of the effect of PB on AGP expression in adrenalectomized animals. Our results suggest that phenobarbital acts on AGP expression by a mechanism independent of the inflammatory pathway.