Tissue-specific expression and methylation of the human CYP2E1 gene

Racing against time: leveraging preclinical models to understand pulmonary susceptibility to perinatal acetaminophen exposures

Over the past decade, clinicians have increasingly prescribed acetaminophen (APAP) for patients in the neonatal intensive care unit (NICU). Acetaminophen has been shown to reduce postoperative opiate burden, and may provide similar efficacy for closure of the patent ductus arteriosus (PDA) as nonsteroidal anti-inflammatory drugs (NSAIDs). Despite these potential benefits, APAP exposures have spread to increasingly less mature infants, a highly vulnerable population for whom robust pharmacokinetic and pharmacodynamic data for APAP are lacking. Concerningly, preclinical studies suggest that perinatal APAP exposures may result in unanticipated adverse effects that are unique to the developing lung. In this review, we discuss the clinical observations linking APAP exposures to adverse respiratory outcomes and the preclinical data demonstrating a developmental susceptibility to APAP-induced lung injury. We show how clinical observations linking perinatal APAP exposures to pulmonary injury have been taken to the bench to produce important insights into the potential mechanisms underlying these findings. We argue that the available data support a more cautious approach to APAP use in the NICU until large randomized controlled trials provide appropriate safety and efficacy data.

Pulmonary implications of acetaminophen exposures independent of hepatic toxicity

Both preclinical and clinical studies have demonstrated that exposures to acetaminophen (APAP) at levels that cause hepatic injury cause pulmonary injury as well. However, whether exposures that do not result in hepatic injury have acute pulmonary implications is unknown. Thus, we sought to determine how APAP exposures at levels that do not result in significant hepatic injury impact the mature lung. Adult male ICR mice (8-12 wk) were exposed to a dose of APAP known to cause hepatotoxicity in adult mice [280 mg/kg, intraperitoneal (ip)], as well as a lower dose previously reported to not cause hepatic injury (140 mg/kg, ip). We confirm that the lower dose exposures did not result in significant hepatic injury. However, like high dose, lower exposure resulted in increased cellular content of the bronchoalveolar lavage fluid and induced a proinflammatory pulmonary transcriptome. Both the lower and higher dose exposures resulted in measurable changes in lung morphometrics, with the lower dose exposure causing alveolar wall thinning. Using RNAScope, we were able to detect dose-dependent, APAP-induced pulmonary Cyp2e1 expression. Finally, using FLIM we determined that both APAP exposures resulted in acute pulmonary metabolic changes consistent with mitochondrial overload in lower doses and a shift to glycolysis at a high dose. Our findings demonstrate that APAP exposures that do not cause significant hepatic injury result in acute inflammatory, morphometric, and metabolic changes in the mature lung. These previously unreported findings may help explain the potential relationship between APAP exposures and pulmonary-related morbidity.

Toxic Acetaminophen Exposure Induces Distal Lung ER Stress, Proinflammatory Signaling, and Emphysematous Changes in the Adult Murine Lung

Clinical studies have demonstrated a strong association between both acute toxic exposure and the repetitive, chronic exposure to acetaminophen (APAP) with pulmonary dysfunction. However, the mechanisms underlying this association are unknown. Preclinical reports have demonstrated that significant bronchiolar injury occurs with toxic APAP exposure, but very little information exists on how the distal lung is affected. However, cells in the alveolar space, including the pulmonary epithelium and resident macrophages, express the APAP-metabolizing enzyme CYP2E1 and are a potential source of toxic metabolites and subsequent distal lung injury. Thus, we hypothesized that distal lung injury would occur in a murine model of toxic APAP exposure. Following exposure of APAP (280 mg/kg, IP), adult male mice were found to have significant proximal lung histopathology as well as distal lung inflammation and emphysematous changes. Toxic APAP exposure was associated with increased CYP2E1 expression in the distal lung and accumulation of APAP-protein adducts. This injury was associated with distal lung activation of oxidant stress, endoplasmic reticulum stress, and inflammatory stress response pathways. Our findings confirm that following toxic APAP exposure, distal lung CYP2E1 expression is associated with APAP metabolism, tissue injury, and oxidant, inflammatory, and endoplasmic reticulum signaling. This previously unrecognized injury may help improve our understanding of the relationship between APAP and pulmonary-related morbidity.

Alcoholic Liver Disease: Alcohol Metabolism, Cascade of Molecular Mechanisms, Cellular Targets, and Clinical Aspects

Alcoholic liver disease is the result of cascade events, which clinically first lead to alcoholic fatty liver, and then mostly via alcoholic steatohepatitis or alcoholic hepatitis potentially to cirrhosis and hepatocellular carcinoma. Pathogenetic events are linked to the metabolism of ethanol and acetaldehyde as its first oxidation product generated via hepatic alcohol dehydrogenase (ADH) and the microsomal ethanol-oxidizing system (MEOS), which depends on cytochrome P450 2E1 (CYP 2E1), and is inducible by chronic alcohol use. MEOS induction accelerates the metabolism of ethanol to acetaldehyde that facilitates organ injury including the liver, and it produces via CYP 2E1 many reactive oxygen species (ROS) such as ethoxy radical, hydroxyethyl radical, acetyl radical, singlet radical, superoxide radical, hydrogen peroxide, hydroxyl radical, alkoxyl radical, and peroxyl radical. These attack hepatocytes, Kupffer cells, stellate cells, and liver sinusoidal endothelial cells, and their signaling mediators such as interleukins, interferons, and growth factors, help to initiate liver injury including fibrosis and cirrhosis in susceptible individuals with specific risk factors. Through CYP 2E1-dependent ROS, more evidence is emerging that alcohol generates lipid peroxides and modifies the intestinal microbiome, thereby stimulating actions of endotoxins produced by intestinal bacteria; lipid peroxides and endotoxins are potential causes that are involved in alcoholic liver injury. Alcohol modifies SIRT1 (Sirtuin-1; derived from Silent mating type Information Regulation) and SIRT2, and most importantly, the innate and adapted immune systems, which may explain the individual differences of injury susceptibility. Metabolic pathways are also influenced by circadian rhythms, specific conditions known from living organisms including plants. Open for discussion is a 5-hit working hypothesis, attempting to define key elements involved in injury progression. In essence, although abundant biochemical mechanisms are proposed for the initiation and perpetuation of liver injury, patients with an alcohol problem benefit from permanent alcohol abstinence alone.

An improved in vitro method for screening toxin and medicine targeting CYP2E1

The cytochrome P450 enzyme 2E1 (CYP2E1) presents in both microsome and mitochondrion, which influences the metabolism of many xenobiotics. The mice active liver homogenate was prepared for the medicinal incubation and mitochondrion was extracted for chemical screening targeting CYP2E1 enzyme. Representative CYP2E1 inducers (ethanol and pyrazole) and inhibitors (diallyldisulfide and kaempferol) were applied to evaluate the effectiveness of homogenate-mitochondrial system. In parallel, the in-vitro microsomal method targeting CYP2E1 was also operated for comparison. The results showed that in homogenate-mitochondrial method, the protein level and activity of CYP2E1 were increased by ethanol and pyrazole; reduced by diallyldisulfide and kaempferol, and this homogenate-mitochondrial method is convenient with good repeatability and reproducibility in screening chemicals targeting CYP2E1, especially for the inducers. Thus, the homogenate-mitochondrial method might be effective in screening both CYP2E1 inhibitor and inducer.

Prenatal antidepressant exposure associated with CYP2E1 DNA methylation change in neonates

Some but not all neonates are affected by prenatal exposure to serotonin reuptake inhibitor antidepressants (SRI) and maternal mood disturbances. Distinguishing the impact of these 2 exposures is challenging and raises critical questions about whether pharmacological, genetic, or epigenetic factors can explain the spectrum of reported outcomes. Using unbiased DNA methylation array measurements followed by a detailed candidate gene approach, we examined whether prenatal SRI exposure was associated with neonatal DNA methylation changes and whether such changes were associated with differences in birth outcomes. Prenatal SRI exposure was first associated with increased DNA methylation status primarily at CYP2E1(β(Non-exposed) = 0.06, β(SRI-exposed) = 0.30, FDR = 0); however, this finding could not be distinguished from the potential impact of prenatal maternal depressed mood. Then, using pyrosequencing of CYP2E1 regulatory regions in an expanded cohort, higher DNA methylation status--both the mean across 16 CpG sites (P < 0.01) and at each specific CpG site (P < 0.05)--was associated with exposure to lower 3rd trimester maternal depressed mood symptoms only in the SRI-exposed neonates, indicating a maternal mood x SRI exposure interaction. In addition, higher DNA methylation levels at CpG2 (P = 0.04), CpG9 (P = 0.04) and CpG10 (P = 0.02), in the interrogated CYP2E1 region, were associated with increased birth weight independently of prenatal maternal mood, SRI drug exposure, or gestational age at birth. Prenatal SRI antidepressant exposure and maternal depressed mood were associated with altered neonatal CYP2E1 DNA methylation status, which, in turn, appeared to be associated with birth weight.

Using in vitro and in vivo models to evaluate the oral bioavailability of nutraceuticals

Nutraceuticals are the bioactive compounds found in many dietary sources. Numerous publications have reported their ability to prevent the development of degenerative diseases through modulation of physiological and physiochemical processes in living systems. Having sufficient concentration at the target site of action is the most critical factor for nutraceuticals to deliver the health benefits. For consumers, it is commonly accepted to ingest these bioactive components through oral delivery route because it is convenient and cost-efficient and allows flexible dosing schedule. Thus, it is important to understand the oral bioavailability of nutraceuticals to evaluate their qualifications as disease preventive agents and to calculate the required ingestion dosages. To predict the oral bioavailability of nutraceuticals, many in vitro and in vivo models have been developed to reduce the need for frequent human pharmacokinetic studies, which are costly and time-consuming and involve ethical complications. These models evaluate one or more of the influential factors that contribute to the oral bioavailability and are efficient screening techniques with the potential of predicting the pharmacokinetic process in humans. To accurately predict human oral bioavailability, further research is required to develop not only a better correlation between the in vitro and in vivo models but also an accurate scaling factor that takes into account interspecies variations.

Role and importance of polymorphisms with respect to DNA methylation for the expression of CYP2E1 enzyme

Different individuals possess slightly different genetic information and show genetically-determined differences in several enzyme activities due to genetic variability. Following an integrated approach, we studied the polymorphisms and methylation of sites contained in the 5' flanking region of the metabolizing enzyme CYP2E1 in correlation to its expression in both tumor and non-neoplastic liver cell lines, since to date little is known about the influence of these (epi)genetic elements in basal conditions and under induction by the specific inductor and a demethylating agent. In treated cells, reduced DNA methylation, assessed both at genomic and gene level, was not consistently associated with the increase of enzyme expression. Interestingly, the Rsa/Pst haplotype differentially influenced CYP2E1 enzyme expression. In addition, regarding the Variable Number of Tandem Repeats polymorphism, cells with A4/A4 genotype showed a greater expression inhibition (ranging from 20% to 30%) compared with others carrying the A2/A2 one, while those cells bringing A2/A3 genotype showed an increase of expression (of 25%, about). Finally, we demonstrated for the first time that the A2 and A3 CYP2E1 alleles play a more important role in the expression of the enzyme, compared with other (epi)genetic factors, since they are binding sites for trans-acting proteins.

Role of xenobiotic metabolism in cancer: involvement of transcriptional and miRNA regulation of P450s

Cytochrome P450 enzymes (P450s) are important targets in cancer, due to their role in xenobiotic metabolism. Since P450s are the "bridges" between the environment and our body, their function can be linked in many ways to carcinogenesis: they activate dietary and environmental components to ultimate carcinogens (i), the cancer tissue maintains its drug resistance with altered expression of P450s (ii), P450s metabolize (sometimes activate) drugs used for cancer treatment (iii) and they are potential targets for anticancer therapy (iiii). These highly polymorphic enzymes are regulated at multiple molecular levels. Regulation is as important as genetic difference in the existing individual variability in P450 activity. In this review, examples of the transcriptional (DNA methylation, histone modification, modulation by xenosensors) and post-transcriptional (miRNA) regulation will be presented and thereby introduce potential molecular targets at which the metabolism of anticancer drugs, the elimination of cancerogenes or the progress of carcinogenesis could be affected.

The impact of CYP2E1 genetic variability on risk assessment of VOC mixtures

Humans are simultaneously exposed to multiple chemicals in the environment. Many of the chemicals use the same enzymes in their metabolic pathways. Competitive inhibition may occur as one of the possible interactions between the xenobiotics in human body. For example, many volatile organic compounds (VOCs) are metabolized using P450 enzymes, specifically CYP2E1. Inheritable gene alterations may result in changes of function of the enzymes in different human subpopulations. Variations in quantity and/or quality of particular isoenzymes may cause differences in the metabolism of VOCs. These variations may cause higher sensitivity in certain populations. Using examples of three different mixtures, this review paper outlines the variances in CYP2E1 isoenzymes, effect of exposure to such mixtures on sensitive populations, and approaches to mixtures risk assessment.

Structure, function, regulation and polymorphism and the clinical significance of human cytochrome P450 1A2

Human CYP1A2 is one of the major CYPs in human liver and metabolizes a number of clinical drugs (e.g., clozapine, tacrine, tizanidine, and theophylline; n > 110), a number of procarcinogens (e.g., benzo[a]pyrene and aromatic amines), and several important endogenous compounds (e.g., steroids). CYP1A2 is subject to reversible and/or irreversible inhibition by a number of drugs, natural substances, and other compounds. The CYP1A gene cluster has been mapped on to chromosome 15q24.1, with close link between CYP1A1 and 1A2 sharing a common 5'-flanking region. The human CYP1A2 gene spans almost 7.8 kb comprising seven exons and six introns and codes a 515-residue protein with a molecular mass of 58,294 Da. The recently resolved CYP1A2 structure has a relatively compact, planar active site cavity that is highly adapted for the size and shape of its substrates. The architecture of the active site of 1A2 is characterized by multiple residues on helices F and I that constitutes two parallel substrate binding platforms on either side of the cavity. A large interindividual variability in the expression and activity of CYP1A2 has been observed, which is largely caused by genetic, epigenetic and environmental factors (e.g., smoking). CYP1A2 is primarily regulated by the aromatic hydrocarbon receptor (AhR) and CYP1A2 is induced through AhR-mediated transactivation following ligand binding and nuclear translocation. Induction or inhibition of CYP1A2 may provide partial explanation for some clinical drug interactions. To date, more than 15 variant alleles and a series of subvariants of the CYP1A2 gene have been identified and some of them have been associated with altered drug clearance and response and disease susceptibility. Further studies are warranted to explore the clinical and toxicological significance of altered CYP1A2 expression and activity caused by genetic, epigenetic, and environmental factors.

Influence of cytochrome P450 polymorphisms on drug therapies: pharmacogenetic, pharmacoepigenetic and clinical aspects

The polymorphic nature of the cytochrome P450 (CYP) genes affects individual drug response and adverse reactions to a great extent. This variation includes copy number variants (CNV), missense mutations, insertions and deletions, and mutations affecting gene expression and activity of mainly CYP2A6, CYP2B6, CYP2C9, CYP2C19 and CYP2D6, which have been extensively studied and well characterized. CYP1A2 and CYP3A4 expression varies significantly, and the cause has been suggested to be mainly of genetic origin but the exact molecular basis remains unknown. We present a review of the major polymorphic CYP alleles and conclude that this variability is of greatest importance for treatment with several antidepressants, antipsychotics, antiulcer drugs, anti-HIV drugs, anticoagulants, antidiabetics and the anticancer drug tamoxifen. We also present tables illustrating the relative importance of specific common CYP alleles for the extent of enzyme functionality. The field of pharmacoepigenetics has just opened, and we present recent examples wherein gene methylation influences the expression of CYP. In addition microRNA (miRNA) regulation of P450 has been described. Furthermore, this review updates the field with respect to regulatory initiatives and experience of predictive pharmacogenetic investigations in the clinics. It is concluded that the pharmacogenetic knowledge regarding CYP polymorphism now developed to a stage where it can be implemented in drug development and in clinical routine for specific drug treatments, thereby improving the drug response and reducing costs for drug treatment.

Metabolic and genetic factors contributing to alcohol induced effects and fetal alcohol syndrome

Alcohol-related damages on newborns and infants include a wide variety of complications from facial anomalies to neurodevelopmental delay, known as fetal alcohol syndrome (FAS). However, only less than 10% of women drinking alcohol during pregnancy have children with FAS. Understanding the risk factors increasing the probability for newborn exposed in utero to alcohol to develop FAS is therefore a key issue. The involvement of genetics as a one risk factor in FAS has been suggested by animal models and by molecular epidemiological studies on different populations, bearing allelic variants for those enzymes, such as ADH e CYP2E1, involved in ethanol metabolism. Indeed, one of the major factors determining the peak blood alcohol exposure to the fetus is the metabolic activity of the mother, in addition to placental and fetal metabolism, explaining, at least partially, the risk of FAS. The different rates of ethanol metabolism may be the result of genetic polymorphisms, the most relevant of which have been described in the paper.

Characterisation of the xenobiotic-metabolizing Cytochrome P450 expression pattern in human lung tissue by immunochemical and activity determination

The lung represents an important target for the toxic effects of chemicals. Many of the chemicals require enzymatic activation to exert their adverse effects, which is mostly catalysed by Cytochrome P450 (CYP) enzymes. Although there is considerable evidence that individual members of the xenobiotic-metabolizing P450 family are expressed in human lung tissue at the mRNA level, there is conflicting evidence concerning the following issues: (I) the qualitative expression pattern of CYP isoenzymes; (II) CYP expression at the protein and/or activity level; and (III) interindividual variability of CYP enzymes in human lung. The latter can be the basis for individual susceptibility towards the adverse effects of lung toxicants. In preparing for studying factors to explain interindividual variability of CYP expression in lung tissue, we investigated the qualitative pulmonary expression pattern of xenobiotic-metabolizing CYP enzymes and elaborated the optimal conditions for quantification at the protein and activity level. By using either individual human lung samples or pooled microsomes from different individuals, immunoreactive bands specific for the following CYP enzymes could be determined by Western blotting: CYP1A1, CYP1A2, CYP2E1 and CYP3A5. Western blotting experiments were also supportive of the presence of CYP2A, CYP2B6, CYP2D6 and CYP3A4 in human lung. By using antibodies specific for CYP2C enzymes and CYP1B1, respectively, immunoreactive bands, which differed slightly in mobility from corresponding standards, were detectable. In addition, we measured methoxy- and ethoxyresorufin dealkylase activities and chlorzoxazone (CLX)-hydroxylase activity in human lung and confirmed the specifities of the latter two activities by inhibition experiments. In summary, we have established methodologies to quantify a panel of CYP enzymes in human lung samples among which there are CYP enzymes whose expression at the protein and activity level has not been evidenced so far.

Inhibition by allyl sulfides and phenethyl isothiocyanate of methyl-n-pentylnitrosamine depentylation by rat esophageal microsomes, human and rat CYP2E1, and Rat CYP2A3

Garlic and Cruciferae are associated with reduced risks of several human cancers, and some of their constituents are anticarcinogenic in animals. Here we studied inhibition of in vitro metabolism of the rat esophageal carcinogen methyl-n-pentylnitrosamine (MPN) by garlic-derived allyl sulfides and by Cruciferae-derived phenethyl isothiocyanate (PEITC) and sulforaphane. The test inhibitors were incubated with [3H]-MPN, NADPH-generating system and rat esophageal microsomes (REM) or a cytochrome P450 (CYP). [3H]-MPN activation by depentylation was assayed by HPLC with radiometric determination of [3H]-pentaldehyde 2,4-dinitrophenylhydrazone. IC50 for depentylation of 40 microM MPN by rat CYP2E1 was 5-12 microM for diallyl sulfide (DAS), diallyl disulfide (DADS), and PEITC and 10-20 microM for diallyl sulfone, allyl mercaptan, and diallyl trisulfide. Maximum inhibition required preincubation of rat CYP2E1 with DAS for 15 min and with DADS for 30 min. Using these preincubation times, Ki for MPN depentylation by REM, rat and human CYP2E1, and rat CYP2A3 was 0.6-1.6 microM for inhibition by DAS and 1.7-70 microM for inhibition by DADS. With PEITC, Ki for MPN depentylation by REM, rat CYP2E1, and rat CYP2A3 was 0.4-4.6 microM. These low Ki and IC50 values may help explain how garlic and Cruciferae inhibit carcinogenesis.

CpG methylation of the mouse CYP1A2 promoter

Cytochrome P450 1A2 (CYP1A2) is a xenobiotic metabolizing enzyme that is tissue-specifically regulated in the mammalian liver by arylhydrocarbon receptor (AhR)-dependent and -independent pathways. In this study, CpG methylation of the CYP1A2 promoter was analyzed in mouse tissues and liver-derived cells. Compared to lung and kidney, the CYP1A2 promoter is undermethylated in the liver in a promoter domain-specific manner. The CYP1A2 promoter showed a similar methylation pattern in wild-type and AhR-null liver. At birth, the promoter was hypermethylated and CYP1A2 was negligibly expressed in the liver. However, CYP1A2 expression increased following birth, coincident with the demethylation of the promoter. In hepatoma Hepa1c1c7 cells not expressing CYP1A2, the promoter was hypermethylated at specific CpG sites. In isolated hepatocytes, CYP1A2 expression declined over time and the degree of CYP1A2 methylation increased, albeit only after a delay. Exposure to 5-aza-2'-deoxycytidine did not induce CYP1A2 in Hepa1c1c7 cells and hepatocytes. Taken together, our findings suggest that CpG methylation is involved in the tissue-specific and developmental regulation of CYP1A2, but the de novo methylation of the CYP1A2 promoter is induced by the silent state of the gene rather than causing it.

Epidermal CYP2 family cytochromes P450

Skin is the largest and most accessible drug-metabolizing organ. In mammals, it is the competent barrier that protects against exposure to harmful stimuli in the environment and in the systemic circulation. Skin expresses many cytochromes P450 that have critical roles in exogenous and endogenous substrate metabolism. Here, we review evidence for epidermal expression of genes from the large CYP2 gene family, many of which are expressed preferentially in extrahepatic tissues or specifically in epithelia at the environmental interface. At least 13 CYP2 genes (CYP2A6, 2A7, 2B6, 2C9, 2C18, 2C19, 2D6, 2E1, 2J2, 2R1, 2S1, 2U1, and 2W1) are expressed in skin from at least some human individuals, and the majority of these genes are expressed in epidermis or cultured keratinocytes. Where epidermal expression has been localized in situ by hybridization or immunocytochemistry, CYP2 transcripts and proteins are most often expressed in differentiated keratinocytes comprising the outer (suprabasal) cell layers of the epidermis and skin appendages. The tissue-specific transcriptional regulation of CYP2 genes in the epidermis, and in other epithelia that interface with the environment, suggests important roles for at least some CYP2 gene products in the production and disposition of molecules affecting competency of the epidermal barrier.

Quantitation of cytochrome P450 mRNA levels in human skin

There is considerable interindividual variation in man's ability to metabolize drugs and foreign compounds. These differences can partly be attributed to genetic polymorphisms that result in the generation of multiple phenotypes with different drug-metabolizing capabilities. Genetically derived differences can easily be assessed by genotyping assays in cases where the polymorphism has been identified. However, many of the polymorphisms that result in these are not known, secondly not all the differences can be attributed to genetic polymorphisms, hence genotyping methods cannot be employed. We have therefore, developed real-time (Taqman) PCR assays to quantitate levels of P450 mRNAs in human tissues. These assays are highly sensitive, reproducible, and specific and will allow quantitation of P450 mRNA levels in various human tissues. We have applied these assays to quantitate cytochrome P450 mRNA levels in human skin samples from 27 healthy volunteers. The expression of 13 P450s was assessed. The major enzymes detected were CYP1B1, CYP2B6, CYP2D6, and CYP3A4 with mean values of 2.5, 2.6, 2.7, and 1.1 fg/18S rRNA in 50ng total RNA, respectively. Lower levels of CYP2C18, CYP2C19, and CYP3A5 were also detected while CYP1A2, 2A6, and 2C8 were below limits of detection. There was interindividual variation in the levels of mRNA among the 27 subjects studied although Poisson analysis showed data to be normally distributed, except for CYP2B6, as some individuals completely lacked CYP2B6 mRNA.

The cytochrome P-450 isoenzyme CYP2E1 in the biological processing of industrial chemicals: consequences for occupational and environmental medicine

The importance of the isoform CYP2E1 of the human cytochrome P-450 superfamily of enzymes for occupational and environmental medicine is derived from its unique substrate spectrum that includes a number of highly important high-production chemicals, such as aliphatic and aromatic hydrocarbons, solvents and industrial monomers (i.a. alkanes, alkenes, aromatic and halogenated hydrocarbons). Many polymorphic genes, such as CYP2E1, show considerable differences in allelic distribution between different human populations. The polymorphic nature of the human CYP2E1 gene is significant for inter-individual differences in toxicity of its substrates. Since the substrate spectrum of CYP2E1 includes many compounds of basic relevance to industrial toxicology, a rationale for metabolic interactions of different CYP2E1 substrates is provided. In-depth research into the inter-individual phenotypic differences of human CYP2E1 enzyme activities was enabled by the recognition that the 6-hydroxylation of the drug chlorzoxazone is mediated by CYP2E1. Studies on CYP2E1 phenotyping have pointed to inter-individual variations in enzyme activities. There are consistent ethnic differences in CYP2E1 enzyme expression, mostly demonstrated between European and Japanese populations, which point to a major impact of genetic factors. The most frequently studied genetic polymorphisms are the restriction fragment length polymorphisms PstI/ RsaI (mutant allele: CYP2E1*5B) located in the 5'-flanking region of the gene, as well as the DraI polymorphism (mutant allele: CYP2E1*6) located in intron 6. These polymorphisms are partly related, as they form the common allele designated CYP2E1*5A. Striking inter-ethnic differences between Europeans and Asians appear with respect to the frequencies of the CYP2E1*5A allele (only approximately 5% of Europeans are heterozygous, but 37% of Asians are, whilst 6% of Asians are homozygous). Available studies indicate a wide variation in human CYP2E1 expression, which are very likely based on complex gene-environment interactions. Major inter-ethnic differences are apparent on the genotyping and the phenotyping levels. Selected cases are presented where inter-ethnic variations of CYP2E1 may provide likely explanations for unexplained findings concerning industrial chemicals that are CYP2E1 substrates. Possible consequences of differential inter-individual and inter-ethnic susceptibilities are related to individual expressions of clinical symptoms of chemical toxicity, to results of biological monitoring of exposed workers, and to the interpretation of results of epidemiological or molecular-epidemiological studies.

Expression and regulation of xenobiotic-metabolizing cytochrome P450 (CYP) enzymes in human lung

Pathogenesis of lung diseases, such as lung cancer and chronic obstructive pulmonary disease, is tightly linked to exposure to environmental chemicals, most notably tobacco smoke. Many of the compounds associated with these diseases require an enzymatic activation to exert their deleterious effects on pulmonary cells. These activation reactions are mostly catalyzed by cytochrome P450 (CYP) enzymes. Interindividual differences in the in situ activation and inactivation of chemical toxicants may contribute to the risk of developing lung diseases associated with these compounds. This review summarizes in detail the expression of individual CYP forms in human pulmonary tissue and gives a view on the significance of the pulmonary expression of CYP enzymes. The localization of individual CYP enzymes in various cell types of human lung and the emerging field of regulation of human pulmonary CYP enzymes are discussed. At least CYP1A1 (in smokers), CYP1B1, CYP2B6, CYP2E1, CYP2J2, and CYP3A5 proteins are expressed in human lung, and also other CYP forms are likely to be expressed. Xenobiotic-metabolizing CYP enzymes are mostly expressed in bronchial and bronchiolar epithelium, Clara cells, type II pneumocytes, and alveolar macrophages in human lung, although individual CYP forms have different patterns of localization in pulmonary tissues. Problems in animal to human lung toxicity extrapolation and several specific aspects requiring more detailed assessment are identified.

Cytochrome P450 4A11 expression in human keratinocytes: effects of ultraviolet irradiation

BACKGROUND

The skin is the major interface between the body and its environment. Directly and continuously exposed to a large variety of foreign agents and stimuli such as ultraviolet radiation (UVR), cutaneous cells are active sites of intense metabolism. The cytochromes P450 (P450) are a group of enzymes that play an important part in the protective role of the skin; they are a family of microsomal membrane-bound mono-oxygenases. These haem-containing proteins catalyse the insertion of an atom of molecular oxygen into the substrate. Although generally present at low levels, a certain number of these enzymes have now been characterized in mammalian skin as constitutive or inducible isoforms.

OBJECTIVES

To test the effects of UVR, a source of oxidative stress, on the expression of mRNA coding for several P450 isoforms (CYP), with particular reference to the CYP2E1 and CYP4A11 isoforms, which might play a role in lipid metabolism in human keratinocytes.

METHODS

Human keratinocytes were cultured, irradiated and mRNA expression was analysed by gel electrophoresis after reverse transcriptase polymerase chain reactions. CYP proteins were determined from keratinocyte microsomal fractions by sodium dodecyl sulphate-polyacrylamide gel electrophoresis and immunoperoxidase staining. Thin layer chromatography was used to detect (omega-1)- and (omega)-hydroxylation of lauric acid in the microsomal fractions.

RESULTS

mRNAs for CYP2E1, CYP1A1 and CYP3A5 were expressed in all the keratinocyte preparations tested; however, neither CYP3A4 nor CYP3A7 were detected, either in the presence or absence of UVR treatment. CYP19Aro, CYP2C19 and CYP26 were not expressed constitutively, although some induction of CYP19Aro was seen after combined UVB and UVA irradiation. CYP4A11 mRNA was not detected in any keratinocyte preparations either under control conditions or after UVB treatment. Nevertheless, in non-irradiated keratinocyte microsomes, two protein bands were immunoreactive with anti-CYP4A11 enzyme antibodies, one of which corresponds to CYP4A11 protein. UVA treatment of cultured keratinocytes induced CYP4A11 mRNA expression after 24 h, as well as an increase in immunoreactivity of the two protein bands. Although (omega-1)- and (omega)-hydroxylation of fatty acids is attributed to CYP2E1 and CYP4A11, respectively, in the liver or kidney, no omega-hydroxylation of lauric acid was observed in microsomal preparations from cultured keratinocytes.

CONCLUSIONS

However, CYP4A11 may participate in the defence mechanism against UVA-induced oxidative damage.

Induction and regulation of xenobiotic-metabolizing cytochrome P450s in the human A549 lung adenocarcinoma cell line

Several cytochrome P450 (CYP) enzymes are expressed in the human lung, where they participate in metabolic inactivation and activation of numerous exogenous and endogenous compounds. In this study, the expression pattern of all known xenobiotic-metabolizing CYP genes was characterized in the human alveolar type II cell-derived A549 adenocarcinoma cell line using qualitative reverse transcriptase/polymerase chain reaction (RT-PCR). In addition, the mechanisms of induction by chemicals of members in the CYP1 and CYP3A subfamilies were assessed by quantitative RT-PCR. The expression of messenger RNAs (mRNAs) of CYPs 1A1, 1B1, 2B6, 2C, 2E1, 3A5, and 3A7 was detected in the A549 cells. The amounts of mRNAs of CYPs 1A2, 2A6, 2A7, 2A13, 2F1, 3A4, and 4B1 were below the limit of detection. 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) induced CYP1A1 and CYP1B1 mRNAs 56-fold and 2.5-fold, respectively. CYP3A5 was induced 8-fold by dexamethasone and 11-fold by phenobarbital. CYP3A4 was not induced by any of the typical CYP3A4 inducers used. The tyrosine kinase inhibitor genistein and the protein kinase C inhibitor staurosporine blocked TCDD-elicited induction of CYP1A1, but they did not affect CYP1B1 induction. Protein phosphatase inhibitors okadaic acid and calyculin A enhanced TCDD-induction of CYP1B1 slightly, but had negligible effects on CYP1A1 induction. These results suggest that CYP1A1 and CYP1B1 are differentially regulated in human pulmonary epithelial cells and give the first indication of the induction of CYP3A5 by glucocorticoids in human lung cells. These results establish that having retained several characteristics of human lung epithelial cell CYP expression, the A549 lung cell line is a valuable model for mechanistic studies on induction of the pulmonary CYP system.

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.

Xenobiotic-metabolizing cytochrome P450 enzymes in the human feto-placental unit: role in intrauterine toxicity

Practically all lipid-soluble xenobiotics enter the conceptus through placental transfer. Many xenobiotics, including a number of clinically used drugs, are known to cause unwanted effects in the embryo or fetus, including in utero death, initiation of birth defects, and production of functional abnormalities. It is well established that numerous xenobiotics are not necessarily toxic as such, but are enzymatically transformed in the body to reactive and toxic intermediates. The cytochrome P450 (CYP) enzymes are known to catalyze oxidative metabolism of a vast number of compounds, including many proteratogens, procarcinogens, and promutagens. About 20 xenobiotic-metabolizing CYP forms are known to exist in humans. Most of these forms are most abundant in the liver, but examples of exclusively extrahepatic CYP forms also exist. Unlike rodents, the liver of the human fetus and even embryo possesses relatively well-developed metabolism of xenobiotics. There is experimental evidence for the presence of CYP1A1, CYP1B1, CYP2C8, CYP2D6, CYP2E1, CYP3A4, CYP3A5, and CYP3A7 in the fetal liver after the embryonic phase (after 8 to 9 weeks of gestation). Significant xenobiotic metabolism occurs also during organogenesis (before 8 weeks of gestation). Also, some fetal extrahepatic tissues, most notably the adrenal, contain substantial levels of CYP enzymes. The full-term human placenta is devoid of many CYP activities present in liver. Placental CYP1A1 is highly inducible by maternal cigarette smoking. Other forms present in full-term placenta include CYP4B1 and CYP19 (steroid aromatase), which also contribute to the oxidation of some xenobiotics. At earlier stages of pregnancy, the placenta may express a wider array of CYP genes, including CYP2C, CYP2D6, and CYP3A7. Due to the small size of the fetus and low abundance of CYPs in placenta, the contribution of feto-placental metabolism to overall gestational pharmacokinetics of drugs is probably minor. In contrast, several toxic outcomes have been ascribed to altered metabolic patterns in the feto-placental unit, including a putative association between reduced placental oxidative capacity and birth defects. Examples of human teratogens that are substrates for CYP enzymes include thalidomide, phenytoin, ethanol, and several hormonal agents. Recent studies have improved our understanding of the expression and regulation of individual CYP genes in the fetus and placenta, and the stage is set for applying this knowledge with more precision to the role of xenobiotic metabolism in abnormal intrauterine development in humans.

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.