Transcriptional control of the rat hepatic CYP2E1 gene

Altered cytochrome 2E1 and 3A P450-dependent drug metabolism in advanced ovarian cancer correlates to tumour-associated inflammation

BACKGROUND AND PURPOSE

Previous work has focussed on changes in drug metabolism caused by altered activity of CYP3A in the presence of inflammation and, in particular, inflammation associated with malignancy. However, drug metabolism involves a number of other P450s, and therefore, we assessed the effect of cancer-related inflammation on multiple CYP enzymes using a validated drug cocktail.

EXPERIMENTAL APPROACH

Patients with advanced stage ovarian cancer and healthy volunteers were recruited. Participants received caffeine, chlorzoxazone, dextromethorphan, and omeprazole as in vivo probes for CYP1A2, CYP2E1, CYP2D6, CYP3A, and CYP2C19. Blood was collected for serum C-reactive protein and cytokine analysis.

KEY RESULTS

CYP2E1 activity was markedly up-regulated in cancer (6-hydroxychlorzoxazone/chlorzoxazone ratio of 1.30 vs. 2.75), while CYP3A phenotypic activity was repressed in cancer (omeprazole sulfone/omeprazole ratio of 0.23 vs. 0.49). Increased activity of CYP2E1 was associated with raised serum levels of IL-6, IL-8, and TNF-α. Repression of CYP3A correlated with raised levels of serum C-reactive protein, IL-6, IL-8, and TNF-α.

CONCLUSIONS AND IMPLICATIONS

CYP enzyme activity is differentially affected by the presence of tumour-associated inflammation, affecting particularly CYP2E1- and CYP3A-mediated drug metabolism, and may have profound implications for drug development and prescribing in oncological settings.

Analysis of the promoter of the cytochrome P-450 2B2 gene in the rat

About 3 kb of the promoter region of the gene encoding cytochrome P-450 2B2 (CYP2B2) in the rat were sequenced and searched for potential cis-acting elements. Apart from putative binding sites for (liver-specific) protein factors, a region showing homology with the LINE 1 retrotransposon element was also found. Three proximal promoter fragments, encompassing nucleotides -579 to -372, -372 to -211, and -211 to +1, respectively, were shown to contain binding sites for multiple protein factors by bandshift analyses. The strongest protein-binding element, designated BRE (basic regulatory element), occurs between -103 to -66. Its structure is very similar to a negative control element in the murine cmyc promoter and displays a composite feature having a tandemly repeated sequence homology with the BTE (basic transcription element; Yanagida et al., 1990) separated by a CCAAA-box. The use of a deletion series of this template in in vitro transcription assays, provided evidence that the BRE serves as a major cis-acting element in the (regulated) transcription activation of the CYP2B2 gene.

Coordinate regulation of Cyp2e1 by β-catenin- and hepatocyte nuclear factor 1α-dependent signaling

Depending on their position within the liver lobule, hepatocytes fulfill different metabolic functions. Cytochrome P450 (CYP) 2E1 is a drug-metabolizing enzyme which is exclusively expressed in hepatocytes surrounding branches of the hepatic central vein. Previous publications have shown that signaling through the Wnt/β-catenin pathway, a major determinant of liver zonation, and the hepatocyte-enriched transcription factor HNF (hepatocyte nuclear factor) 1α participate in the regulation of the gene. This study was aimed to decipher the molecular mechanisms by which the two transcription factors, β-catenin and HNF1α, jointly regulate CYP2E1 at the gene promoter level. Chromatin immunoprecipitation identified a conserved Wnt/β-catenin-responsive site (WRE) in the murine Cyp2e1 promoter adjacent to a known HNF1α response element (HNF1-RE). In vitro analyses demonstrated that both, activated β-catenin and HNF1α, are needed for the full response of the promoter. The WRE was dispensable for β-catenin-mediated effects on the Cyp2e1 promoter, while activity of β-catenin was integrated into the promoter response via the HNF1-RE. Physical interaction of β-catenin and HNF1α was demonstrated by co-immunoprecipitation. In conclusion, present data the first time identify and characterize the interplay of HNF1α and β-catenin and elucidate molecular determinants of CYP2E1 expression in the liver.

Bortezomib alleviates drug-induced liver injury by regulating CYP2E1 gene transcription

Acute liver failure, i.e., the fatal deterioration of liver function, is the most common indication that emergency liver transplantation is necessary. Moreover, in the USA, drug-induced liver injury (DILI), including acetaminophen (APAP)-induced hepatotoxicity, is the main cause of acute liver failure. Matching a donor for liver transplantation is extremely difficult, and thus the development of a novel therapy for DILI is urgently needed. Following recent approval by the FDA of the proteasomal inhibitor bortezomib, its therapeutic effects on various human diseases, including solid and hematologic malignancies, have been validated. However, the specific action of proteasomal inhibition in cases of DILI had not been elucidated prior to this study. To examine the effects of proteasomal inhibition in DILI experimentally, male C56Bl/6 mice were injected with 1 mg bortezomib/kg before APAP treatment. Bortezomib not only alleviated APAP-induced hepatotoxicity in a time- and dose-dependent manner, it also alleviated CCl4- and thioacetamide-induced hepatotoxicity. We also noted that bortezomib significantly reduced cytochrome P450 2E1 (CYP2E1) expression and activity in the liver, which was accompanied by the induction of endoplasmic reticulum (ER) stress. In addition, bortezomib decreased hepatocyte nuclear factor‑1α-induced promoter activation of CYP2E1 in Hep3B cells. By contrast, another proteasome inhibitor, MG132, did not cause ER stress and did not markedly affect CYP2E1 enzyme activity. Liver injury induced by APAP was aggravated by MG132, possibly via elevation of connexin 32 expression. This study suggests that proteasome inhibition has different effects in cases of DILI depending on the specific inhibitor being used. Furthermore, results from the mouse model indicated that bortezomib, but not MG132, was effective in alleviating DILI. ER stress induced by proteasome inhibition has previously been shown to exert various effects on DILI patients, and thus each available proteasomal inhibitor should be evaluated individually in order to determine its potential for clinical application.

Recent advances in 2D and 3D in vitro systems using primary hepatocytes, alternative hepatocyte sources and non-parenchymal liver cells and their use in investigating mechanisms of hepatotoxicity, cell signaling and ADME

This review encompasses the most important advances in liver functions and hepatotoxicity and analyzes which mechanisms can be studied in vitro. In a complex architecture of nested, zonated lobules, the liver consists of approximately 80 % hepatocytes and 20 % non-parenchymal cells, the latter being involved in a secondary phase that may dramatically aggravate the initial damage. Hepatotoxicity, as well as hepatic metabolism, is controlled by a set of nuclear receptors (including PXR, CAR, HNF-4α, FXR, LXR, SHP, VDR and PPAR) and signaling pathways. When isolating liver cells, some pathways are activated, e.g., the RAS/MEK/ERK pathway, whereas others are silenced (e.g. HNF-4α), resulting in up- and downregulation of hundreds of genes. An understanding of these changes is crucial for a correct interpretation of in vitro data. The possibilities and limitations of the most useful liver in vitro systems are summarized, including three-dimensional culture techniques, co-cultures with non-parenchymal cells, hepatospheres, precision cut liver slices and the isolated perfused liver. Also discussed is how closely hepatoma, stem cell and iPS cell-derived hepatocyte-like-cells resemble real hepatocytes. Finally, a summary is given of the state of the art of liver in vitro and mathematical modeling systems that are currently used in the pharmaceutical industry with an emphasis on drug metabolism, prediction of clearance, drug interaction, transporter studies and hepatotoxicity. One key message is that despite our enthusiasm for in vitro systems, we must never lose sight of the in vivo situation. Although hepatocytes have been isolated for decades, the hunt for relevant alternative systems has only just begun.

Primary hepatocyte cultures for pharmaco-toxicological studies: at the busy crossroad of various anti-dedifferentiation strategies

Continuously increasing understanding of the molecular triggers responsible for the onset of diseases, paralleled by an equally dynamic evolution of chemical synthesis and screening methods, offers an abundance of pharmacological agents with a potential to become new successful drugs. However, before patients can benefit of newly developed pharmaceuticals, stringent safety filters need to be applied to weed out unfavourable drug candidates. Cost effectiveness and the need to identify compound liabilities, without exposing humans to unnecessary risks, has stimulated the shift of the safety studies to the earliest stages of drug discovery and development. In this regard, in vivo relevant organotypic in vitro models have high potential to revolutionize the preclinical safety testing. They can enable automation of the process, to match the requirements of high-throughput screening approaches, while satisfying ethical considerations. Cultures of primary hepatocytes became already an inherent part of the preclinical pharmaco-toxicological testing battery, yet their routine use, particularly for long-term assays, is limited by the progressive deterioration of liver-specific features. The availability of suitable hepatic and other organ-specific in vitro models is, however, of paramount importance in the light of changing European legal regulations in the field of chemical compounds of different origin, which gradually restrict the use of animal studies for safety assessment, as currently witnessed in cosmetic industry. Fortunately, research groups worldwide spare no effort to establish hepatic in vitro systems. In the present review, both classical and innovative methodologies to stabilize the in vivo-like hepatocyte phenotype in culture of primary hepatocytes are presented and discussed.

Systematic functional characterization of cytochrome P450 2E1 promoter variants in the Chinese Han population

CYP2E1 promoter polymorphisms can lead to significant interindividual differences in expression of CYP2E1. Using a database of CYP2E1 gene polymorphisms established in 2010, our study aimed to functionally characterize the single nucleotide polymorphisms (SNPs) of the promoter region and corresponding haplotypes in the Chinese Han population. Six novel SNPs and seven haplotypes with a frequency equal to or greater than 0.01 were constructed on a luciferase reporter system on the basis of site-directed mutagenesis. Dual luciferase reporter systems were used to analyze regulatory activity. The constructs including single novel SNP mutations exhibited insignificant change in luciferase activity, whereas, the activity produced by Haplo1(GTTGCTATAT), Haplo2 (CTTGCTATAT) and Haplo7 (GAGCTCACAT), containing a −333T>A polymorphism was significantly greater than for the wild type in Hep G2 cells (p<0.05), being 1.5−, 2.0− and 1.4− times greater respectively. These findings suggest the possibility of significant clinical prediction of adverse drug reaction and the facilitation of personalized medicine.

Zonation of heme synthesis enzymes in mouse liver and their regulation by β-catenin and Ha-ras

Cytochrome P450 (CYP) hemoproteins play an important role in hepatic biotransformation. Recently, β-catenin and Ha-ras signaling have been identified as players controlling transcription of various CYP genes in mouse liver. The aim of the present study was to analyze the role of β-catenin and Ha-ras in the regulation of heme synthesis. Heme synthesis-related gene expression was analyzed in normal liver, in transgenic mice expressing activated β-catenin or Ha-ras, and in hepatomas. Regulation of the aminolevulinate dehydratase promoter was studied in vitro. Elevated expression of mRNAs and proteins involved in heme biosynthesis was linked to β-catenin activation in perivenous hepatocytes, in transgenic hepatocytes, and in hepatocellular tumors. Stimulation of the aminolevulinate dehydratase promoter by β-catenin was independent of the β-catenin/T-cell-specific transcription factor dimer. By contrast, activation of Ha-ras repressed heme synthesis-related gene expression. The present data suggest that β-catenin enhances the expression of both CYPs and heme synthesis-related genes, thus coordinating the availability of CYP apoprotein and its prosthetic group heme. The reciprocal regulation of heme synthesis by β-catenin and Ha-ras-dependent signaling supports our previous hypothesis that antagonistic action of these pathways plays a major role in the control of zonal gene expression in healthy mouse liver and aberrant expression patterns in hepatocellular tumors.

IL-4-mediated transcriptional regulation of human CYP2E1 by two independent signaling pathways

Cytochrome P450 2E1 (CYP2E1), the alcohol-inducible member of the cytochrome P450 super family, plays an important role in both physiological and pathophysiological processes. The present study focused on the induction of human CYP2E1 transcription by the anti-inflammatory cytokine interleukin-4 (IL-4) in human hepatoma B16A2 cells and revealed that this regulation is mediated by two independent pathways. RNA interference and overexpression of STAT6, indicated that the JAK-STAT signaling pathway is involved in IL-4-dependent induction and mutagenesis revealed the presence of a STAT6 binding site in CYP2E1 proximal promoter region (-583/-574-bp). However, inhibition of the JAK-STAT6 pathway using JAK1 siRNA constructs could only partially inhibit the induction of CYP2E1 promoter constructs indicating the presence of a second IL-4 responsive element. Indeed by using a series of truncated CYP2E1 promoter constructs a second more distal IL-4 responsive element (-1604/-1428-bp) was identified, which was further shown to involve the activation of IRS1/2. This induction was dependent on the transcription factor NFATc1 as IL-4-induced CYP2E1 expression was altered by silencing or overexpressing NFATc1. A NFATc1 binding site was identified in the second distal IL-4 responsive element (-1551/-1545-bp) by chromatin immunoprecipitation (ChIP) analysis. Finally simultaneous siRNA-mediated down-regulation of both STAT6 and NFATc1 or mutation of both STAT6 and NFATc1 binding sites abolished the IL-4-dependent transcriptional induction of CYP2E1, demonstrating that both pathways are required for maximal activation. In conclusion, the present study indicates that the induction of CYP2E1 transcription by IL-4 is mediated through two independent parallel pathways, involving JAK-STAT6 and IRS1/2 and NFATc1.

Histone modification-mediated CYP2E1 gene expression and apoptosis of HepG2 cells

The incidence of hepatocellular carcinoma is rising due to alcohol drinking, hepatitis C viral infection and metabolic syndrome. Differential expression of CYP2E1 may play a pleiotropic role in the multistep process of liver carcinogenesis. Considerable attention has focused on the antitumor effect of trichostatin A (TSA) as well as CYP2E1 expression-induced apoptosis of cancer cells. However, very few studies have examined the mechanisms by which TSA has an antitumor effect and its association to CYP2E1 expression. The current study examined the action of TSA on CYP2E1 expression and the role of CYP2E1 in inducing apoptosis of HepG2 cells. Our data showed that TSA selectively induced CYP2E1 in four studied human hepatocellular carcinoma (HCC) cell lines (Huh7, PLC/PRF/5, Hep3B and HepG2), but not in normal primary human hepatocytes. TSA-mediated up-regulation of CYP2E1 expression was associated with histone H3 acetylation and the recruitment of HNF-1 and HNF-3beta to the CYP2E1 promoter in HepG2 cells. siRNA-mediated knockdown experiments showed that TSA-induced caspase-3 cleavage was decreased due to reduced expression of CYP2E1 in HepG2 cells. Moreover, down-regulation of CYP2E1 was accompanied by decreased production of mitochondrial reactive oxygen species. These results suggest that histone modification is involved in CYP2E1 gene expression and that CYP2E1-dependent mitochondrial oxidative stress plays a role in TSA-induced apoptosis.

Wnt/beta-catenin signaling activates and determines hepatic zonal expression of glutathione S-transferases in mouse liver

Glutathione S-transferases (GSTs) play an essential role in the elimination of xenobiotic-derived electrophilic metabolites and also catalyze certain steps in the conversion of endogenous molecules. Their expression is controlled by different transcription factors, such as the antioxidant-activated Nrf2 or the constitutive androstane receptor. Here, we show that the Wnt/beta-catenin pathway is also involved in the transcriptional regulation of GSTs: GSTm2, GSTm3, and GSTm6 are overexpressed in mouse hepatomas with activating Ctnnb1 (encoding beta-catenin) mutations and in transgenic hepatocytes expressing activated beta-catenin. Inversely, GSTm expression is reduced in mice with hepatocyte-specific knock out of Ctnnb1. Activation of beta-catenin-dependent signaling stimulates GSTm expression in vitro. Activation of beta-catenin in mouse hepatoma cells activates GSTm3 promoter-driven reporter activity, independently of beta-catenin/T-cell factor sites, via a retinoid X receptor-binding site. By contrast, GSTm expression is inhibited upon Ras activation in mouse liver tumors and transgenic hepatocytes. Recent studies by different groups have shown that beta-catenin-dependent signaling is involved in the transcriptional control of "perivenous" expression of various cytochrome P450s in mouse liver, whereas Ras signaling was hypothesized to antagonize the perivenous hepatocyte phenotype. In synopsis with our present results, it now appears that the Wnt/beta-catenin pathway functions as a master regulator of the expression of both phase I and phase II drug-metabolizing enzymes in perivenous hepatocytes from mouse liver.

[Dosing time based on molecular mechanism of biological clock of hepatic drug metabolic enzyme]

The mammalian circadian pacemaker stays in the paired suprachiasmatic nuclei (SCN). Recent several studies reveal that the circadian rhythms of physiology and behavior are controlled by clock genes. In addition, the effectiveness and toxicity of many drugs vary depending on dosing time associated with 24-h rhythms of biochemical, physiological, and behavioral processes under the control of the circadian clock. Acetaminophen (APAP) is a widely used analgesic drug, and is mainly biotransformed and eliminated as nontoxic conjugates with glucuronic acid and sulfuric acid. Only a small portion of the dose is mainly bioactivated by CYP2E1 to N-acetyl-p-benzoquinone imine (NAPQI), a reactive toxic intermediate. For APAP overdose, glucuronidation and sulfation are saturated and the formation of NAPQI increases. However, the exact mechanisms underlying the chronotoxicity of APAP have not been clarified yet. In the present study, we have clarified that there was a significant dosing time-dependent difference in hepatotoxicity induced by APAP in mice. The mechanism may be related to the rhythmicity of CYP2E1 activity and GSH conjugation. In additon, we investigated whether the liver transcription factor hepatic nuclear factor-1alpha (HNF-1alpha) and clock genes undergoing astriking 24-h rhythm in mouse liver contribute to the 24-h regulation of CYP2E1 activity. A significant 24-h rhythmicity was demonstrated for CYP2E1 activity, protein levels and mRNA levels. HNF-1alpha and clock genes may contribute to produce the 24-h rhythm of CYP2E1 mRNA levels. Metabolism by CYP and GSH conjugation are common metabolic pathways for many drugs such as APAP. These findings support the concept that choosing the most appropriate time of day to administer the drugs associated with metabolic rhythmicity such as CYP and GSH conjugation may reduce hepatotoxicity in experimental and clinical situations. 24-h rhythm of CYP2E1 activity was controlled by HNF-1alpha and clock gene, in a transcriptional level. Identification of rhythmic marker for selecting dosing time will lead improved progress and diffusion of chronopharmacotherapy.

Differential induction of ethanol-metabolizing CYP2E1 and nicotine-metabolizing CYP2B1/2 in rat liver by chronic nicotine treatment and voluntary ethanol intake

Alcohol and nicotine are frequently co-used and co-abused, and use of both drugs alone can affect hepatic drug metabolism. We investigated the influences of chronic nicotine treatment and voluntary ethanol intake on the induction of rat hepatic cytochrome P450 (CYP) enzymes that metabolize ethanol and nicotine. Rats were trained to voluntarily drink ethanol (6% v/v, 1 h) with nicotine pretreatment for 10 days. Another group of rats were treated with the same nicotine doses alone. Hepatic CYP2E1, CYP2B1/2 and CYP2D1 proteins were assessed by immunoblotting. Nicotine pretreatment (0.4, 0.8 and 1.2 mg/kg) increased voluntary ethanol intake on day 10 by 1.8, 2.0, and 1.4 fold respectively compared to saline pretreatment (P<0.01-0.3). CYP2E1 was increased 1.7, 1.8, and 1.4 fold by the three doses of nicotine alone (P<0.02-0.21); CYP2E1 levels were increased by voluntary ethanol intake alone and a further 2.4, 2.2, and 1.8 fold by 0.4, 0.8, and 1.2 mg/kg nicotine respectively versus saline pretreatment (P<0.002-0.06). CYP2B1/2 proteins were not induced by nicotine alone, but were increased by 2.2-2.5 fold by ethanol drinking (P<0.05). CYP2E1 (r=0.67, P<0.001) and CYP2B1/2 levels (r=0.49, P=0.007) correlated with alcohol consumption on day 10. There was no change in CYP2D1. Chronic nicotine increased voluntary ethanol intake thereby enhancing CYP2E1 and CYP2B1/2 levels. Thus CYPs are regulated not only directly by nicotine and ethanol, but also indirectly via an increase in the ethanol consumption in the presence of nicotine pretreatment. Together this may contribute to the co-abuse of these drugs and alter the metabolism of clinical drugs and endogenous substrates.

The molecular mechanism regulating 24-hour rhythm of CYP2E1 expression in the mouse liver

Cytochrome P450 2E1 (CYP2E1) is clinically and toxicologically important and exhibits 24-hour periodicity in its activity. In the present study, we investigated whether hepatic nuclear factor-1alpha (HNF-1alpha) and clock genes with a striking 24-hour rhythm in mouse liver contributed to the 24-hour regulation of CYP2E1 expression. The results demonstrated that the expression of CYP2E1 messenger RNA (mRNA) in the liver was affected by HNF-1alpha and the circadian organization of molecular clocks. The mRNA levels of CYP2E1 in the liver increased from the late light phase to the early dark phase. Luciferase reporter gene analysis revealed that HNF-1alpha activated CYP2E1 promoter activity, which was restricted by CRY1, a member of the circadian organization of molecular clocks. Repressor activity of CRY1 was observed on the HNF-1alpha binding site of the CYP2E1 promoter region with mutated E-box. Serum shock induced approximately 24-hour oscillation in CYP2E1 mRNA in HepG2. Transfection of HNF-1alpha and CRY1 small interfering RNA dampened the oscillation of CYP2E1 mRNA in HepG2. Chromatin immunoprecipitation assay in the CYP2E1 promoter indicated that HNF-1alpha binding to the CYP2E1 promoter increased from the late light phase to the early dark phase. Using the chromatin immunoprecipitation reimmunoprecipitation assay, time-dependent differences were demonstrated for CRY1 protein interaction with HNF-1alpha transcriptional complexes, including coactivator p300 on the HNF-1alpha binding site in the CYP2E1 promoter.

CONCLUSION

Our results suggest that the transcription activator of HNF-1alpha acts periodically and the negative limbs of molecular clocks periodically inhibit CYP2E1 transcription, resulting in the 24-hour rhythm of its mRNA expression.

Hepatic nuclear factor 1alpha inhibitor ursodeoxycholic acid influences pharmacokinetics of the organic anion transporting polypeptide 1B1 substrate rosuvastatin and bilirubin

Expression of the organic anion transporting polypeptide 1B1 (OATP1B1) is regulated by transcription factor hepatic nuclear factor (HNF) 1alpha. The aim of this study was to investigate the effect of ursodeoxycholic acid (UDCA), an inhibitor of transcription factor HNF1alpha, on rosuvastatin and bilirubin kinetics in human healthy volunteers. Both substances are substrates of OATP1B1. Twelve subjects with OATP1B1(*)1b/(*)1b genotype predicting high transport activity were recruited for this randomized, crossover study. Each subject received a single p.o. dose of 20 mg of rosuvastatin after 14 days of p.o. intake of either 500 mg of UDCA or placebo. Plasma concentrations of rosuvastatin were determined on days 15 to 18 of each study period. Subjects were randomly assigned to UDCA or placebo group. Intake of UDCA led to a significant increase in rosuvastatin area under the curve (AUC)(0-72) from 128.5 ng/ml.h to 182.1 ng/ml.h(P = 0.008) compared with the control group. The oral clearance decreased from 155.2 l/h with placebo to 109.8 l/h with UDCA. In addition, the mean values of total bilirubin, conjugated bilirubin, and unconjugated bilirubin significantly increased to 139 +/- 39% (P = 0.003), 127 +/- 29% (P = 0.005), and 151 +/- 52% (P = 0.004), respectively, after UDCA treatment. These results in healthy volunteers confirm the findings from in vitro studies that UDCA inhibits OATP1B1 activity by inhibition of the transcription factor HNF1alpha. They highlight a novel mechanism of OATP1B1-based interaction that is mediated by transcription factor HNF1alpha.

Identification and functional analysis of a novel human CYP2E1 far upstream enhancer

Both transcriptional and post-transcriptional CYP2E1 regulatory mechanisms are known, resulting in 20-fold or greater variation in CYP2E1 expression. To evaluate functional regulatory elements controlling transcription, CYP2E1 promoter constructs were used to make adenovirus vectors containing CYP2E1 promoter-driven luciferase reporters for analyses in both primary human hepatocytes and HepG2 cells. A 1.2-kilobase pair portion of the CYP2E1 promoter was associated with 5- to 10-fold greater luciferase activity. This upstream region contained five direct repeats of 59 base pairs (bp) that increased thymidine kinase-driven luciferase reporter activity in HepG2 cells more than 5-fold, regardless of orientation. Electrophoretic mobility shift assays (EMSAs) identified sequence-specific nuclear protein binding to the 59-bp repeats that was dependent on a 17-bp sequence containing a canonical GATA binding site (WGATAR). Competitive and supershift EMSA identified the participation of GATA4, another GATA family member or GATA-like factor, and a third factor unrelated to the GATA family. Involvement of the tricho-rhino-phalangeal syndrome-1 factor, which also binds a GATA sequence, was eliminated. Rather, competitive EMSA using known binding sequences for the orphan nuclear receptors, steroidogenic factor-1 (or NR5A1), and fetoprotein transcription factor (or NR5A2) implicated an NR5A member in binding a sequence overlapping the canonical GATA. Chromatin immunoprecipitation assay demonstrated in vivo binding of NR5A2 to the enhancer sequence in human hepatocytes. The enhancer sequence is conserved within the human population but seems species-specific. The identification of this novel enhancer and its putative mechanism adds to the complexities of human CYP2E1 regulation.

The 2006 Bernard B. Brodie Award Lecture. Cyp2e1

Bernard B. Brodie's laboratory was the first to examine the mechanisms of drug-induced toxicity at the molecular level. They found that acetaminophen hepatotoxicity was due to the metabolic activation of the drug to a highly reactive toxic metabolite that depleted cellular glutathione and covalently bound to protein. Subsequent studies revealed that activation of acetaminophen to an active metabolite is primarily carried out by CYP2E1, an ethanol-inducible cytochrome P450 that was first suggested by characterization of the microsomal ethanol oxidation system. CYP2E1 is developmentally regulated, under liver-specific control, and undergoes substrate-induced protein stabilization. It is also regulated by starvation and diabetes through insulin-dependent mRNA stabilization. In addition to acetaminophen, CYP2E1 metabolically activates a large number of low M(r) toxicants and carcinogens and thus is of great toxicological importance. The mechanism of regulation CYP2E1 and its role in acetaminophen toxicity will be discussed.

Ageing is associated with increased expression but decreased activity of CYP2E1 in male Wistar rats

The effect of ageing on CYP2E1 activity and its protein and mRNA contents was investigated in both adult (9 months) and senescent (24 months) male Wistar rats. The CYP2E1 activity (as measured by chlorzoxazone hydroxylation) was significantly decreased by 36% in senescent rats as compared to adult rats. However, this decrease of activity was not associated with a loss of protein content because the amount of both CYP2E1 protein and CYP2E1 mRNA did not decrease in senescent rats but rather increased, by 79% and 64% respectively, as compared to adult rats. Lipid peroxidation was increased significantly by 140% with ageing. The decrease in CYP2E1 activity could be explained by post-translational modification of CYP2E1 proteins, due to an increase in oxidative stress in senescent animals, leading to a loss of their functionality. However, no changes in the extent of protein carbonyls were observed in the adult versus senescent rats (16.2 +/- 9.6 vs. 12.7 +/- 7.3 nmol/mg prot) and the major proteasome activity remained unchanged. With regards to the increase of CYP2E1 expression, our results showed that the amount of hepatocyte nuclear factor 1alpha mRNA, a transcription factor that positively regulates CYP2E1, was strongly increased (154%) in senescent rats.

Cloning and mapping of the porcine cytochrome-p450 2E1 gene and its association with skatole levels in the domestic pig

The porcine cytochrome-p450 2E1 (CYP2E1) gene was isolated by screening a pig BAC library and partially sequenced. This sequence information was used to identify six single nucleotide polymorphisms (SNPs) within the CYP2E1 gene and its promoter. In addition, a microsatellite marker tightly linked to the CYP2E1 gene was subcloned from the BAC. One of these markers was used to map the CYP2E1 gene distal of SWC27 on SSC14, well outside reported quantitative trait loci on SSC14 for skatole, indole and taste test measures of boar taint. However, in a population of commercial pigs scored for backfat skatole levels, there was evidence of association between a SNP in the CYP2E1 promoter and skatole deposition, although there was no significant association between this SNP and skatole levels in the experimental cross.

The effects of pregnancy on ethanol clearance

We have studied the effects of pregnancy on ethanol clearance rates and on blood and urine ethanol concentrations (BECs and UECs) in adult Sprague-Dawley rats infused with ethanol intragastrically. Pregnant rats had greater ethanol clearance following an intragastric or intravenous ethanol bolus (3 or 0.75 g/kg, respectively) relative to non-pregnant rats (p<0.05). Pregnant rats infused with ethanol-containing diets for several days had lower (p<0.05) UECs than non-pregnant rats when given the same dose of ethanol. Non-pregnant rats infused ethanol-containing diets at two levels of calories (the higher caloric intake required by pregnant rats [220 kca/kg75/d] or the normal calories required for non-pregnant rats [187 kcal/kg75/d]) had statistically equal UECs, suggesting that increased caloric intake was not responsible for the effect of pregnancy. While the activity of hepatic alcohol dehydrogenase (ADH) did not differ with pregnancy, gastric ADH activity was increased (p<0.001). Furthermore, total hepatic aldehyde dehydrogenase (ALDH) and hepatic mitrochrondrial protein were increased (p<0.05) and hepatic CYP2E1 activity was suppressed (p<0.05). The results suggest that pregnancy increases ethanol elimination in pregnant rats by: 1) induction of gastric ADH; 2) elevated hepatic ALDH activity; and 3) increased mitochondrial respiration. The greater ethanol clearance results in lower tissue ethanol concentrations achieved during pregnancy for a given dose, and this may have clinical significance as a mechanism to protect the growing fetus from ethanol toxicity.

Expression of cytochrome P450 enzymes in hepatic organoid reconstructed by rat small hepatocytes

BACKGROUND AND AIMS

Small hepatocytes (SH), which are hepatic progenitor cells, were isolated from an adult rat liver. SH in a colony sometimes change their shape from small to large and from flat to rising/piled-up. The morphological changes of SH may be correlated with hepatic maturation. Cytochrome P450s (CYP) are drug-metabolizing enzymes and the expression is one of hepatic differentiated functions. However, it is well known that the re-expression and maintenance of CYP activity are very difficult in cultured hepatocytes. We investigated the expression of CYP and the enzymatic activities in long-term cultured SH.

METHODS

SH were isolated from adult rat livers and SH colonies were collected, replated on new dishes, and then cultured. CYP1A1/2, CYP2B1, CYP3A2, CYP4A1, and CYP2E1 were induced by the addition of 3-methylcholanthrene, phenobarbital, pregnenolone-16alpha-carbonitrile, clofibric acid, and ethanol, respectively. Immunocytochemistry, immunoblots, and enzyme activities were examined.

RESULTS

SH could differentiate into mature hepatocytes by the addition of Matrigel and re-express constitutive CYPs. The expression of CYP1A1/2, CYP2B1, CYP3A2, and CYP4A1 dose-dependently increased and the amounts gradually increased with time in culture, especially in the cells treated with Matrigel. Activities of CYP1A, CYP2B, CYP3A and CYP2E in SH treated with Matrigel induced by each of the inducers were approximately 120-fold, 2.8-fold, 6.4-fold and 0.8-fold higher than in the control.

CONCLUSION

The matured SH could re-express the constitutive CYP and recover inducibility, not only of protein expression but also of enzyme activities.

The discovery of the microsomal ethanol oxidizing system and its physiologic and pathologic role

Oxidation of ethanol via alcohol dehydrogenase (ADH) explains various metabolic effects of ethanol but does not account for the tolerance. This fact, as well as the discovery of the proliferation of the smooth endoplasmic reticulum (SER) after chronic alcohol consumption, suggested the existence of an additional pathway which was then described by Lieber and DeCarli, namely the microsomal ethanol oxidizing system (MEOS), involving cytochrome P450. The existence of this system was initially challenged but the effect of ethanol on liver microsomes was confirmed by Remmer and his group. After chronic ethanol consumption, the activity of the MEOS increases, with an associated rise in cytochrome P450, especially CYP2E1, most conclusively shown in alcohol dehydrogenase negative deer mice. There is also cross-induction of the metabolism of other drugs, resulting in drug tolerance. Furthermore, the conversion of hepatotoxic agents to toxic metabolites increases, which explains the enhanced susceptibility of alcoholics to the adverse effects of various xenobiotics, including industrial solvents. CYP2E1 also activates some commonly used drugs (such as acetaminophen) to their toxic metabolites, and promotes carcinogenesis. In addition, catabolism of retinol is accelerated resulting in its depletion. Contrasting with the stimulating effects of chronic consumption, acute ethanol intake inhibits the metabolism of other drugs. Moreover, metabolism by CYP2E1 results in a significant release of free radicals which, in turn, diminishes reduced glutathione (GSH) and other defense systems against oxidative stress which plays a major pathogenic role in alcoholic liver disease. CYP1A2 and CYP3A4, two other perivenular P450s, also sustain the metabolism of ethanol, thereby contributing to MEOS activity and possibly liver injury. CYP2E1 has also a physiologic role which comprises gluconeogenesis from ketones, oxidation of fatty acids, and detoxification of xenobiotics other than ethanol. Excess of these physiological substrates (such as seen in obesity and diabetes) also leads to CYP2E1 induction and nonalcoholic fatty liver disease (NAFLD), which includes nonalcoholic fatty liver and nonalcoholic steatohepatitis (NASH), with pathological lesions similar to those observed in alcoholic steatohepatitis. Increases of CYP2E1 and its mRNA prevail in the perivenular zone, the area of maximal liver damage. CYP2E1 up-regulation was also demonstrated in obese patients as well as in rat models of obesity and NASH. Furthermore, NASH is increasingly recognized as a precursor to more severe liver disease, sometimes evolving into "cryptogenic" cirrhosis. The prevalence of NAFLD averages 20% and that of NASH 2% to 3% in the general population, making these conditions the most common liver diseases in the United States. Considering the pathogenic role that up-regulation of CYP2E1 also plays in alcoholic liver disease (vide supra), it is apparent that a major therapeutic challenge is now to find a way to control this toxic process. CYP2E1 inhibitors oppose alcohol-induced liver damage, but heretofore available compounds are too toxic for clinical use. Recently, however, polyenylphosphatidylcholine (PPC), an innocuous mixture of polyunsaturated phosphatidylcholines extracted from soybeans (and its active component dilinoleoylphosphatidylcholine), were discovered to decrease CYP2E1 activity. PPC also opposes hepatic oxidative stress and fibrosis. It is now being tested clinically.

A novel lipopolysaccharide-modulated Jun binding repressor in intron 2 of CYP2E1

Cytochrome P450 2E1 (CYP2E1) exhibits a pronounced oxidase activity that may mediate apoptotic injury in glial cells as well as hepatocytes. Strict regulation of CYP2E1 and it's activity is therefore thought to be crucial. We have studied CYP2E1 transcriptional regulation in primary cortical glial cells and have identified a novel repressor element at +1452/+1460 in intron 2 of the rat CYP2E1 gene. The element very potently repressed CYP2E1 and SV40 promoters and consisted of the non-palindromic core sequence 5'-TTCCACTCA-3'. Jun proteins were found to interact with the site. The protein complexes were also found to contain an as yet unidentified protein of approximately 60 kDa, probably with DNA binding properties similar to G-box binding factors found in, e.g. Arabidopsis thaliana. Stimulation with lipopolysaccharide, or overexpression of the mitogen-activated protein kinase kinase kinase, MEKK-1, further deepened the repression in primary cortical glial cells. It is suggested that this novel Jun binding repressor helps to control basal expression levels of CYP2E1, and modulates the response to inflammatory factors. Future in vivo experiments will, however, be required for a full appreciation of the role of this repressor in the complex regulation of CYP2E1 during inflammatory conditions.

Characterization of the human lung CYP2F1 gene and identification of a novel lung-specific binding motif

The CYP2F1 gene encodes a cytochrome P450 enzyme capable of bioactivating a number of pulmonary-selective toxicants. The expression of CYP2F1 is highly tissue-selective; the highest expression is observed in the lung with little or no hepatic expression. The objective of these studies was to elucidate the mechanisms that govern the unique tissue-specific regulation of CYP2F1. Cosmid and bacterial artificial chromosome clones were screened and sequenced to identify a gene that spanned 14 kbp containing 10 exons, including an untranslated exon 1. Primer extension analysis and 5'-rapid amplification of cDNA ends were used to identify the transcription start site. Several sequences homologous to known cis-elements were identified in the 5'-upstream region of the CYP2F1 promoter. Transient transfection studies with luciferase reporter constructs demonstrated a significant functional lung cell-specific CYP2F1 promoter region (from position -129 to +115). DNase footprinting analysis of 1.6 kbp of the upstream sequence with nuclear extracts from human lung tissues revealed one strong DNA-protein complex at -152 to -182. This nuclear protein (called lung-specific factor, LSF) was present only in lung but not liver or heart tissues. Competitive electrophoretic mobility shift assays characterized a DNA consensus site, within the LSF-binding domain, that was highly similar to two E box motifs, but no known "E box" trans-factors were identified. These studies identified a novel LSF and its consensus sequence that may control tissue-specific expression of CYP2F1.

Identification of repressor element 1 in cytochrome P450 genes and their negative regulation by RE1 silencing transcription factor/neuron-restrictive silencer factor

RE1 silencing transcription factor/neuron-restrictive silencing factor (REST/NRSF) mediates transcriptional repression in many neuron-specific genes by interaction with the repressor element 1/neuron-restrictive silencing element (RE1/NRSE). This element has been identified at least in 20 neuron specific genes. REST/NRSF is highly expressed in non-neuronal tissues, where it is thought to repress gene transcription. We performed a BLAST search to look for the presence of RE1/NRSE elements in the rat cytochrome P450 genes. We identified the presence of RE1/NRSE element in the cytochrome P450 genes CYP1A1, 2A2, 2E1 and 3A2. Electrophoretic mobility shift assay and supershift assays were carried out to prove functionality of these sites and detect the interaction of REST/NRSF with this sequence. Cotransfection studies in PC12 cells with a plasmid containing the RE1 element of the CYP genes, cloned upstream of the minimal type II sodium channel promoter, in the presence of REST/NRSF, showed a marked expression inhibition of the CAT reporter gene. These data suggest that the RE1 elements that exist in these four CYP genes might be a target for the REST/NRSF transcription factor and such an interaction might play a role in the negative regulation of these genes.

Mechanisms of down-regulation of CYP2E1 expression by inflammatory cytokines in rat hepatoma cells

CYP2E1 is one of the major cytochrome P450 forms whose expression is strongly inhibited by inflammatory cytokines in humans and rodents. In the present study, we have used the Fao rat hepatoma cell line that constitutively expresses CYP2E1 enzyme to investigate mechanisms of cytokine action. The cells were treated with interleukin (IL)-1beta, tumor necrosis factor-alpha (TNFalpha), or IL-6 for 24 or 72 h, and the expression of CYP2E1 was monitored at the transcriptional, mRNA, and protein levels. All three cytokines decreased the CYP2E1 mRNA levels after 24 h, and the effect was even stronger after 72 h. In contrast, significant inhibition of CYP2E1 protein was seen only after 72 h. In transfection assays using a CYP2E1 5' -3685 to +29-luciferase construct, it was found that IL-6 inhibited gene transcription after 24 h, but a similar effect by IL-1beta and TNFalpha was registered only after 72 h. Using 5' deletions of the CYP2E1 5'-reporter construct a responsive region for the IL-6 effect was located to -669 to -507 base pairs in the CYP2E1 5'-flanking region. Interestingly, IL-1beta, but not TNFalpha, was found to reduce hepatocyte nuclear factor (HNF)-1alpha binding to the CYP2E1 promotor. However, the transactivation function of HNF-1alpha was found to be impaired in Fao cells. In mouse primary hepatocytes, IL-1beta decreased HNF-1alpha-mediated transactivation. In conclusion, our data indicate that inflammatory cytokines inhibit CYP2E1 expression by multiple mechanisms, including control of HNF-1alpha function and regulation of other transcriptional factors acting on the CYP2E1 5'-upstream regulatory region. In addition, regulation of factors of importance for the CYP2E1 mRNA stability may be involved.

Ontogeny of hepatic and renal systemic clearance pathways in infants: part I

Dramatic developmental changes in the physiological and biochemical processes that govern drug pharmacokinetics and pharmacodynamics occur during the first year of life. These changes may have significant consequences for the way infants respond to and deal with drugs. The ontogenesis of systemic clearance mechanisms is probably the most critical determinant of a pharmacological response in the developing infant. In recent years, advances in molecular techniques and an increased availability of fetal and infant tissues have afforded enhanced insight into the ontogeny of clearance mechanisms. Information from these studies is reviewed to highlight the dynamic and complex nature of developmental changes in clearance mechanisms in infants during the first year of life. Hepatic and renal elimination mechanisms constitute the two principal clearance pathways of the developing infant. Drug metabolising enzyme activity is primarily responsible for the hepatic clearance of many drugs. In general, when compared with adult activity levels normalised to amount of hepatic microsomal protein, hepatic cytochrome P450-mediated metabolism and the phase II reactions of glucuronidation, glutathione conjugation and acetylation are deficient in the neonate, but sulfate conjugation is an efficient pathway at birth. Parturition triggers the dramatic development of drug metabolising enzymes, and each enzyme demonstrates an independent rate and pattern of maturation. Marked interindividual variability is associated with their developmental expression, making the ontogenesis of hepatic metabolism a highly variable process. By the first year of life, most enzymes have matured to adult activity levels. When compared with adult values, renal clearance mechanisms are compromised at birth. Dramatic increases in renal function occur in the ensuing postpartum period, and by 6 months of age glomerular filtration rate normalised to bodyweight has approached adult values. Maturation of renal tubular functions exhibits a more protracted time course of development, resulting in a glomerulotubular imbalance. This imbalance exists until adult renal tubule function values are approached by 1 year of age. The ontogeny of hepatic biliary and renal tubular transport processes and their impact on the elimination of drugs remain largely unknown. The summary of the current understanding of the ontogeny of individual pathways of hepatic and renal elimination presented in this review should serve as a basis for the continued accruement of age-specific information concerning the ontogeny of clearance mechanisms in infants. Such information can only help to improve the pharmacotherapeutic management of paediatric patients.

The ontogeny of human drug-metabolizing enzymes: phase II conjugation enzymes and regulatory mechanisms

Changes in phase II drug-metabolizing enzyme expression during development, as well as the balance between phase I and phase II enzymes, can significantly alter the pharmacokinetics for a given drug or toxicant. Although our knowledge is incomplete, many of the phase II enzymes are expressed early in development. There is evidence for glutathione S-transferase A1/A2 (GSTA1/A2), GSTM, and GSTP1 in fetal liver, lung and kidney, although tissue-specific patterns and changes with time are observed. N-Acetyltransferase 1 (NAT1) activity also has been reported throughout gestation in fetal liver, adrenal glands, lung, kidney, and intestine. Only postnatal changes in NAT1 expression were apparent. Nothing is known about human NAT2 developmental expression. Some UDP-glucuronosyltransferase and sulfotransferase isoforms also are detectable in fetal liver and other tissues by the first or second trimester, and substantial changes in isoform expression patterns, as well as overall expression levels, are observed with increasing maturity. Finally, expression of both epoxide hydrolases 1 and 2 (EPHX1 and EPHX2) is observed in fetal liver, and for the former, increased expression with time has been documented. Less is known about ontogenic molecular control mechanisms. Limited data suggest that the hepatocyte nuclear factor and CCAAT/enhancer binding protein families are critical for fetal liver drug-metabolizing enzyme expression whereas D element binding protein and related factors may regulate postnatal hepatic expression. There is a paucity of data regarding mechanisms for the onset of extrahepatic fetal expression or specific mechanisms determining temporal switches, such as those observed within the CYP3A and flavin-containing monooxygenase families.

Regulation of flavin-containing monooxygenase 1 expression by ying yang 1 and hepatic nuclear factors 1 and 4

The flavin-containing monooxygenases (FMOs) are important for the oxidation of a variety of environmental toxicants, natural products, and therapeutics. Consisting of six family members (FMO1-5), these enzymes exhibit distinct but broad and overlapping substrate specificity and are expressed in a highly tissue- and species-selective manner. Corresponding to previously identified regulatory domains, a YY1 binding site was identified at the major rabbit FMO1 promoter, position -8 to -2, two overlapping HNF1alpha sites, position -132 to -105, and two HNF4alpha sites, position -467 to -454 and -195 to -182. Cotransfection studies with HNF1alpha and HNF4alpha expression vectors demonstrated a major role for each of these factors in enhancing FMO1 promoter activity. In contrast, YY1 was shown by site-directed mutagenesis to be dispensable for basal promoter activity but suppressed the ability of the upstream domains to enhance transcription. Finally, comparisons between rabbit and human FMO1 demonstrated conservation of each of these regulatory elements. With the exception of the most distal HNF4alpha site, each of the orthologous human sequences also was able to compete with rabbit FMO1 cis-elements for specific protein binding. These data are consistent with these same elements being important for regulating human FMO1 developmental- and tissue-specific expression.

Promoter function and the role of cytokines in the transcriptional regulation of rabbit CYP2E1 and CYP2E2

Rabbit CYP2E1 and CYP2E2 show considerable similarity in the 5' flanking region, but a 32-base-pair element (32-BPE) that is repeated in 2E1 is present only as a single inexact copy in 2E2. In the present investigation, footprinting disclosed two specific binding sites for liver nuclear proteins, and the DNase I sensitivity profiles of the two genes were found to be different. Several positive and negative regulatory elements were identified by transfection with a series of constructs of upstream CYP2E sequences fused to the luciferase gene. Both genes have an HNF-1 consensus motif with one nucleotide mismatch, which affects binding affinity and promoter activity. Investigation of DNA-protein interactions revealed that Sp1 and NFkappaB bind exclusively to the 32-BPE of 2E1 and 2E2, respectively, suggesting a possible regulatory role for the 32-BPE. Interleukin-1alpha (IL-1alpha) gave rise to a 2.5-fold increase in the promoter activity of 2E1 in HepG2 cells, and the IL-1alpha-mediated induction of reporter gene expression was almost completely prevented when the 32-BPE was deleted. Increased DNA binding and Sp1 protein content as a result of IL-1alpha treatment, as well as cotransfection experiments with pPacSp1, suggest that Sp1 is a transcription activator for the induction of 2E1 by IL-1alpha in HepG2 cells.

The alcohol-inducible form of cytochrome P450 (CYP 2E1): role in toxicology and regulation of expression

Cytochrome P450 (CYP) 2E1 catalyzes the metabolism of a wide variety of therapeutic agents, procarcinogens, and low molecular weight solvents. CYP2E1-catalyzed metabolism may cause toxicity or DNA damage through the production of toxic metabolites, oxygen radicals, and lipid peroxidation. CYP2E1 also plays a role in the metabolism of endogenous compounds including fatty acids and ketone bodies. The regulation of CYP2E1 expression is complex, and involves transcriptional, post-transcriptional, translational, and post-translational mechanisms. CYP2E1 is transcriptionally activated in the first few hours after birth. Xenobiotic inducers elevate CYP2E1 protein levels through both increased translational efficiency and stabilization of the protein from degradation, which appears to occur primarily through ubiquitination and proteasomal degradation. CYP2E1 mRNA and protein levels are altered in response to pathophysiologic conditions by hormones including insulin, glucagon, growth hormone, and leptin, and growth factors including epidermal growth factor and hepatocyte growth factor, providing evidence that CYP2E1 expression is under tight homeostatic control.

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.

Characterization of hepatic cytochrome P450 isozyme composition in the transgenic rat expressing low level human growth hormone

1. The present authors have previously developed a transgenic rat carrying a chimeric gene of the mouse whey acidic protein promoter and the structural portion of human growth hormone (GH) gene. Among this (hGH-TG) rat, a line (low GH rat) missing a male-specific pulsatile GH secretary pattern due to suppression of endogenous GH secretion and having a continuous low GH (hGH and rat GH) level in the peripheral circulation was identified. The latter rat was also characterized as having severe obesity with age. This strain (low Gh rat) was used to correlate the sex-specific secretory pattern of GH with the sex-specific expression of cytochrome P450 (CYP) in rat. 2. Comparisons were made between the low GH rat and the non-transgenic rat as to the expression of liver microsomal CYP isozymes. The following enzyme activities were assessed: testosterone (T) hydroxylation and oxidation; ethoxyresorufin O-dealkylation (EROD); bunitrolol (BTL) 4-hydroxylation and T5 alpha-reduction. Protein expression of CYP1A, CYP2C11, CYP2D, CYP2E1, CYP3A2 and CYP4A1 were also assessed by Western blot analysis. 3. Enzyme activities and protein expression of CYP2C11 (T16 alpha and 2alpha-hydroxylase and 17-oxidase activities) and CYP3A2 (T6beta and 2beta-hydroxylase activities) levels, which are known to be higher in the male than in the female rat, were significantly lower in the adult male low GH rat than in the control male rat. In contrast, CYP2A1 (T7 alpha-hydroxylase) and T5-alpha-reductase activities, which are known to be specifically elevated in the female, were significantly higher in the adult male low GH rat than in the control male rat. Thus, the loss of male-specific secretory pattern of GH results in feminization of the pattern of expression of CYP and T5 alpha-reductase activity in the liver. 4. In contrast to other GH-deficient models so far studied, an increase in CYP4A1 and a decrease in CYP2E1 protein expression were observed in the low GH rat. These trends are consistent with the characteristic phenotype of obesity in the transgenic rat because CYP4A1 and CYP2E1 enhance fatty acid excretion and glyconeogenesis from fatty acids respectively.

NADPH-dependent microsomal electron transfer increases degradation of CYP2E1 by the proteasome complex: role of reactive oxygen species

Increased levels of cytochrome P450 2E1 (CYP2E1) produced by low-molecular-weight compounds is mostly due to stabilization of the enzyme against proteolytic degradation. CYP2E1, in the absence of substrate or ligand, normally has a short half-life, but the factors which regulate CYP2E1 turnover or trigger its rapid degradation are not known. Since CYP2E1 is active in producing reactive oxygen species, experiments were carried out to evaluate whether reactive oxygen species modulated the degradation of CYP2E1. CYP2E1 present in human liver microsomes was very stable. Addition of the cytosol fraction produced degradation of CYP2E1, and this was enhanced when NADPH was present in the reaction system. Antioxidants or iron chelators which prevent lipid peroxidation, prevented the degradation of CYP2E1 by the cytosolic fraction. Similarly, diphenyleneiodonium chloride, which inhibits NADPH-dependent electron transfer, prevented the degradation of CYP2E1, as did 4-methylpyrazole, a ligand which increases the level of CYP2E1. If microsomes were first incubated with NADPH for 30 min, followed by the addition of these agents, there was no protection against CYP2E1 degradation. Lactacystin, an inhibitor of the proteasome, decreased the degradation of CYP2E1. In intact HepG2 cells transduced to express CYP2E1, proteasome inhibitors elevated steady-state levels of CYP2E1. Steady-state levels of CYP2E1 were increased by about 50% when the cells were incubated with trolox. Trolox decreased the rate of loss of CYP2E1 protein when the cells were treated with cycloheximide. These results suggest that NADPH-dependent production of reactive oxygen species may result in oxidative modification of CYP2E1, followed by rapid degradation of the labilized CYP2E1 by the proteasome complex. It is interesting to speculate that one consequence of the high rates of production of reactive oxygen species by CYP2E1 is its own labilization and subsequent rapid degradation, and this may be a regulatory mechanism to prevent high levels of the enzyme from accumulating within the cell.

Structural and functional characterization of the 5'-flanking region of the rat and human cytochrome P450 2E1 genes: identification of a polymorphic repeat in the human gene

Cytochrome P450 2E1 (CYP2E1) is a toxicologically very important enzyme with a high extent of interindividual variability in expression. We sequenced and characterized the 5'-flanking region of the human and rat CYP2E1 genes. The identity between the human and rat sequences (-3.8 kb to +1 kb) was generally between 35 and 60%, and the most similar regions were found in the proximal part of the sequence. Two more distant regions at -1.6 to -2.0 kb and -2.5 to -2. 8 kb in the human sequence were also found to have high identity to the rat sequence. A polymorphic repeat sequence in the human gene was found between -2178 to -1945 bp. The common allele (CYP2E1*1C) contained 6 repeats (each 42-60 bp long) and the rare allele (CYP2E1*1D) had 8 repeats with an allele frequency of 1% among Caucasians and 23% among Chinese. The CYP2E1 5'-flanking regions of the human (-3712 bp to +10 bp) and rat (-3685 bp to +28 bp) genes were ligated in front of a luciferase reporter gene and transfected into rat hepatoma Fao and human hepatoma B16A2 cells. Important species specificity was noted in the control of gene expression and regions of negative and positive cis-acting elements were localized. No difference was seen in the constitutive expression between the two polymorphic forms. The importance of this repeat polymorphism for high and low inducible CYP2E1 phenotypes is discussed.

Changes of liver-enriched nuclear transcription factors for albumin gene in starvation in rats

The regulatory mechanism of albumin gene transcription was examined using male Donryu rats (7 wk old) starved for 1 or 3 d. At the designated times, the rats were sacrificed to harvest the liver and to measure the serum albumin level. Neither the serum albumin nor the albumin messenger RNA (mRNA) level showed a significant change for these starvation periods. Among nuclear factors binding to the D site of albumin gene promoter, the CCAAT/enhancer binding protein alpha (C/EBP alpha) mRNA level showed a decrease and the D site binding protein (DBP) mRNA level tended to decrease after 3 d of starvation. In contrast, the C/EBP beta mRNA level showed a significant increase at day 1. As a B site binding nuclear factor, the hepatocyte nuclear factor 1 (HNF-1) mRNA level significantly increased at day 1. Gel mobility-shift analysis combined with Western immunoblotting confirmed the presence of D site binding proteins composed of DBP and C/EBP alpha and beta in both groups subjected to oral feeding and to 3-d starvation, though quantitative analysis could not be done. In conclusion, the nuclear transcription factors binding to the albumin gene promoter undergo regulatory changes during 3 d of starvation, whereas there is no significant decrease in the albumin mRNA level.

Expression of CYP2E1 in human lung and kidney during development and in full-term placenta: a differential methylation of the gene is involved in the regulation process

Methylation of dinucleotide CG residues located in the 5' end of the CYP2E1 gene has been demonstrated to play a role in the control of gene expression in the human developing liver. This study was undertaken to examine the CYP2E1 RNA content of human lung, kidney and full-term placenta and to determine whether the expression of CYP2E1 was controlled by its methylation status in these tissues. CYP2E1 was expressed at a very low level in the lung and kidney at whatever age, and at a variable level in full-term placentas. The restriction profile of genomic DNA was identical in lung and kidney and corresponded to a heavy methylation of HpaII/MspI sites located within the promoter, the first exon and first intron of the CYP2E1 gene. A different pattern of methylation was obtained in full-term placentas, indicating that CpG residues located in the 5' end of the gene were predominantly but not fully demethylated. However, the variable level of CYP2E1 RNA in full-term placentas suggests the involvement of other elements in the regulation process of CYP2E1 in this tissue.

Identification and characterization of a novel tissue-specific transcriptional activating element in the 5'-flanking region of the CYP2A3 gene predominantly expressed in rat olfactory mucosa

CYP2A3 is expressed preferentially in rat olfactory mucosa and is believed to play important roles in maintaining cellular homeostasis in the chemosensory tissue. DNase I footprinting analysis revealed a single protected region in the proximal promoter of the CYP2A3 gene with nuclear extracts from olfactory mucosa, but not from liver, lung, kidney, or brain. The core sequence of the binding site, named the nasal predominant transcriptional activating (NPTA) element, is similar to that of nuclear factor 1, but it interacted with unique proteins detected only in the olfactory mucosa in electrophoretic mobility shift assays or on Southwestern blots. The NPTA element is conserved in rat CYP2A3, mouse Cyp2a5, and human CYP2A6 genes and was found to be essential for transcriptional activity of the CYP2A3 promoter in in vitro transcription assays. NPTA-binding proteins were detectable at day 1 and were much more abundant at day 8 than at day 60 after birth. Furthermore, their levels decreased dramatically during chemically induced degeneration of the olfactory epithelium, paralleling the disappearance of CYP2A3 protein, and rebounded to higher than pretreatment levels during recovery. Thus, we have identified a novel transcriptional activation element potentially responsible for the olfactory mucosa-predominant expression of the CYP2A3 gene in rats and orthologous genes in mice and humans.

Gene expression of albumin and liver-specific nuclear transcription factors in liver of protein-deprived rats

Protein-energy malnutrition causes hypoalbuminemia. Recent work has suggested that this may be partly due to decreased transcription of the albumin gene. This study examined the role of cis-acting and transacting elements of the albumin gene during protein deprivation. Male 7-wk-old Donryu rats were fed a protein-free diet (0% casein diet) for 10 d or given restricted (pair-fed control) or free access (freely fed control) to a 25% casein diet. Serum albumin concentrations were significantly lower in the protein-deprived rats (29 +/- 1 g/L) than in the pair-fed controls (42 +/- 3 g/L) or the freely fed controls (45 +/- 3 g/L). The albumin mRNA level was also significantly lower in livers of protein-deprived rats (36% of pair-fed control). However, gel mobility shift analysis using liver nuclear extracts did not show any significant difference between the protein-deprived rats and the pair-fed controls in the binding activity to the B and D sites of the albumin promoter. Furthermore, gel mobility shift-Western blot analysis showed no significant difference between the two groups in the protein levels of nuclear transcription factors binding to the D sites. The amounts of mRNA of hepatocyte nuclear factor-1 binding to the B site were not significantly different between these two groups. These results suggest that the proximal promoter region may not play a major role in the down-regulation of the albumin gene during protein deprivation.

Regulation of the hepatic CYP 2E1 gene during chronic alcohol exposure: lack of an ethanol response element in the proximal 5'-flanking sequence

Chronic exposure to ethanol is known to cause a dramatic increase in the level of CYP 2E1 apoprotein. More recently it has been demonstrated that under certain conditions the mRNA encoding cytochrome P450 2E1(CYP 2E1) is inducible; however, the mechanisms by which these increases occur are not well understood. In the current study, DNase I footprinting assays performed on the first kilobase of the CYP 2E1 5'-flanking sequences resulted in the identification of 13 sequence-specific protected regions using rat liver nuclear extracts isolated from either control or ethanol-treated animals. No differences were observed in the DNase I footprint patterns produced by the two different nuclear extracts. In addition, analysis by electrophoretic mobility shift assays (EMSA) revealed that with one exception, there were no differences in the level of binding complexes between the two extracts. However, EMSA analysis with an oligonucleotide to one footprint site (designated Site C) revealed that in nuclear extracts isolated from ethanol-treated animals there was a 2.9-fold increase in this binding complex when compared to control nuclear extracts. This site was previously shown to contain an HNF-1alpha binding site, and here we demonstrate that bacterially expressed HNF-1alpha in footprint assays bind Site C sequences and that HNF-1alpha transactivates the CYP 2E1 promoter in co-transfection experiments with HNF-1alpha expression plasmid and plasmids containing CYP 2E1 promoter sequences coupled to the chloramphenicol acetyl transferase gene. Furthermore, in contrast to the increase observed by EMSA in Site C binding, no increase was detected in the CYP 2E1 transcriptional rate supported by nuclear extracts from ethanol-treated animals over controls using in vitro transcription assays, suggesting that the increase by ethanol in CYP 2E1 transcription is not mediated through the HNF-1alpha site.

Characterization of the human cytochrome P4502D6 promoter. A potential role for antagonistic interactions between members of the nuclear receptor family

The functional mapping of the human cytochrome P4502D6 (CYP2D6) promoter in HepG2 cells revealed the presence of both positive and negative regulatory elements. One of these regulatory elements overlapped a sequence that is highly conserved in most members of the CYP2 family. This element, which consists of a degenerate AGGTCA direct repeat spaced by 1 base pair (DR1) and is known to be a target for members of the steroid receptor superfamily, was found to bind in vitro translated hepatocyte nuclear factor 4 (HNF4) in gel retardation analysis. Using HepG2 nuclear extracts, three protein-DNA complexes were formed on the DR1 element, one of which was confirmed to be dependent on the binding of HNF4. The other DR1 complexes were shown to be due to the interaction of the orphan receptor chicken ovalbumin upstream promoter transcription factor I (COUP-TFI). Experiments in COS-7 cells showed that HNF4 could activate the CYP2D6 promoter 30-fold. Surprisingly, mutation of the DR1 element produced a relatively minor 23% decrease in activity in HepG2 cells. Additionally, COUP-TFI was shown to inhibit HNF4 stimulation of the CYP2D6 promoter in COS-7 cells, suggesting that COUP-TFI could attenuate the effect of HNF4 in HepG2 cells. However, when HNF4 levels were increased in HepG2 cells by co-transfection, it resulted in the enhancement of CYP2D6 promoter activity, indicating that HNF4 could overcome the repressive effect of COUP-TFI. Therefore, the contribution of the DR1 element in controlling the transcription of the CYP2D6 gene depends on the balance between positively and negatively acting transcription factors.

Developmental expression of CYP2E1 in the human liver. Hypermethylation control of gene expression during the neonatal period

Cytochromes P-450 are responsible for the biotransformation of drugs and other hydrophobic molecules by the liver. Several isoforms coexist which display an asynchronous onset during the perinatal period suggesting the involvement of multiple mechanisms of regulation. In this paper, we have shown that the CYP2E1 protein and its associated activity could not be detected in the fetal liver and rise during the first few hours following birth independently of the gestational age (between 25-40 weeks). During this period, the CYP2E1 RNA content remains fairly low: the stabilization of the low amount of existing CYP2E1 protein by endogenous ketone bodies could explain the early neonatal rise of the protein level. From 1 month to 1 year, the protein content gradually increases and is accompanied by the accumulation of CYP2E1 RNA, suggesting a transcriptional activation of the gene during the late neonatal period. We examined the methylation status of CpG residues in the 5' flanking region, first exon and first intron of CYP2E1 gene cleaved with HpaII/MspI. Genomic DNA from fetal liver shows several hypermethylated spots in the first-exon-first-intron region, which progressively disappear in neonatal samples. We conclude that during the neonatal period, the accumulation of hepatic CYP2E1 RNA is correlated with the degree of methylation at the 5' end of the CYP2E1 gene.

trans-Acting factors, detoxication enzymes and hepatitis B virus replication in a novel set of human hepatoma cell lines

A panel of four novel human hepatoma cell lines was isolated from a single tumor from a male individual. BC1, B16 and B16A2 lines were well differentiated, while cells of the B9 line were only poorly differentiated, being essentially negative for the functions analyzed. These cell lines have been surveyed for expression of a large set of plasma proteins, accumulation of liver-specific mRNAs and DNA-binding activity of ubiquitous and liver-enriched transcription factors. BC1 cells expressed the highest levels of albumin mRNA, whereas B16 and B16A2 cells accumulated the largest amounts of haptoglobin mRNA. In addition, B16 and B16A2 cells were unique in that they expressed CYP2E1 mRNA, a species absent from the available human liver cells, including HepG2 hepatoma cells, and 3-methylcholanthrene-inducible CYP1A2 mRNA. The activities of genes encoding transcription factors were evidenced in all four cell lines which expressed mRNAs for nuclear factor interleukin 6 and hepatocyte nuclear factor 1 (HNF) together with the DNA-binding activity of NFY and AP1 nuclear proteins. Strikingly, HNF-1 and HNF-4-like DNA-binding activities were restricted to BC1, B16 and B16A2 cells, supporting the idea of the potential role of these (or closely related) factors in the maintenance and/or in the establishment of the differentiated phenotype. B9 cells contained variant HNF1-like DNA-binding activity, similar to dedifferentiated rat hepatoma cells of the H5 line. CCAAT/enhancer-binding protein and HNF-3-like activities were found in all cell lines, although at a lower level and/or activity in B9 cells. Finally, transfection experiments of plasmids containing the whole hepatitis-B virus genome demonstrated that B16 cells, but not B9 cells, were able to support hepatitis-B virus replication and virion production, in agreement with the notion that HNF-1 activity is necessary for viral replication. We believe that the specific complement of transcription factors expressed in the differentiated BC1, B16 and B16A2 cells, and in the poorly differentiated B9 cells, will allow studies on the regulation of hepatic gene expression in these human lines, and will also aid the analysis of xenobiotic metabolism and the biology of hepatitis-B virus replication.

Role of the liver-enriched transcription factor HNF-1 alpha in expression of the CYP2E1 gene

The role of the trans-acting factor HNF-1 alpha in activating CYP2E1 gene expression was confirmed by transient co-transfection of an HNF-1 alpha expression plasmid and the CYP2E1 promoter fused to the chloramphenicol acetyl transferase (CAT) reporter gene. Only HNF-1 alpha, and not HNF-1 beta, HNF-4, C/EBP alpha, C/EBP beta, or DBP, was able to activate the CYP2E1 promoter. The extent of activation was proportional to the number of copies of the HNF-1 binding sequence upstream of the promoter. Removal or mutation of the HNF-1 binding sequence led to inactivation of the promoter in response to HNF-1 alpha. Gel-shift Western blot analysis using a synthetic HNF-1 binding sequence derived from CYP2E1 and rat liver nuclear extract revealed that the protein-DNA complex obtained with adult rat liver nuclear extract consisted of both HNF-1 alpha and HNF-1 beta proteins. The shifted bands produced by nuclear extracts from adult, where the endogenous CYP2E1 gene is active, and fetal rat liver, where the gene is inactive, were found to migrate differently, suggesting that the population of factors, possibly including different ratios of HNF-1 alpha and HNF-1 beta proteins, may change during development. However, the co-transfection study did not show cooperativity between the two factors. Elements upstream of the HNF-1 binding site were found to affect the activity of the promoter negatively in the transfection assay. DNase I hypersensitive site mapping revealed a hypersensitive site in this inhibiting element in the adult rat liver sample but not in liver from newborn animals.

Extracellular matrix proteins modulate cytochrome P450 2D6 expression in human hepatocytes

The role of extracellular matrix proteins on the expression of human liver-specific genes has been investigated. Cytochrome P450 2D6 and albumin are two liver-specific genes which are constitutively expressed in hepatocytes. Primary human hepatocytes were cultured on different extracellular matrix proteins, namely collagen, fibronectin, a combination of collagen/fibronectin and EHS-laminin rich gel, and under different culture conditions. After 48 h in culture, primary human hepatocytes showed a peak in DNA-synthesis associated with a downregulation of liver-specific expression of albumin and P450 2D6, indicating the dedifferentiation of the hepatocytes. Differentiation and expression of liver-specific genes started to increase subsequently. Redifferentiation as determined by albumin and P450 2D6 levels was more pronounced if hepatocytes were seeded on fibronectin, compared to cells seeded on EHS-laminin rich gel, collagen or a combination of collagen and fibronectin.