Influence of polymorphisms of the human glutathione transferases and cytochrome P450 2E1 enzyme on the metabolism and toxicity of ethylene oxide and acrylonitrile

Study of urinary mercapturic acids as biomarkers of human acrylonitrile exposure

Exposure to the vinyl monomer acrylonitrile (AN) is primarily occupational. AN is also found in cigarette smoke. AN can be detoxified to form N-acetyl-S-(2-cyanoethyl)-cysteine (CEMA) or activated to 2-cyanoethylene oxide (CEO) and detoxified to form N-acetyl-S-(1-cyano-2-hydroxyethyl)-cysteine (CHEMA) and N-acetyl-S-(2-hydroxyethyl)-cysteine (HEMA). These urinary mercapturic acids (MAs) are considered to be potential biomarkers of AN exposure. This study assessed personal AN exposure, urinary MAs (CEMA, CHEMA, and HEMA), and cotinine (a biomarker of cigarette smoke) in 80 AN-exposed and 23 non-exposed factory workers from urine samples provided before and after work shifts. Unambiguous linear correlations were observed between levels of urinary CEMA and CHEMA with personal AN exposures, indicating their potential as chemically-specific biomarkers for AN exposures. AN exposure was the dominant factor in MA formation for AN-exposed workers, whereas urinary cotinine used as a biomarker showed that cigarette smoke exposure was the primary factor for non-exposed workers. The CHEMA/CEMA and (CHEMA+HEMA)/CEMA ratios in this human study differ from those in similar studies of AN-treated rats and mice in literature, suggesting a possible dose- and species-dependent effect in AN metabolic activation and detoxification.

Acrylonitrile induction of rodent neoplasia: Potential mechanism of action and relevance to humans

Acrylonitrile, an industrial chemical, is a multisite carcinogen in rats and mice, producing tumors in four tissues with barrier function, that is, brain, forestomach, Zymbal’s gland, and Harderian gland. To assess mechanism(s) of action (MoA) for induction of neoplasia and to evaluate whether the findings in rodents are indicative of human hazard, data on the potential key effects produced by acrylonitrile in the four rodent target tissues of carcinogenicity were evaluated. A notable finding was depletion of glutathione in various organs, including two target tissues, the brain, and forestomach, suggesting that this effect could be a critical initiating event. An additional combination of oxidative DNA damage and cytotoxic effects of acrylonitrile and its metabolites, cyanide, and 2-cyanoethylene oxide, could initiate pro-inflammatory signaling and sustained cell and tissue injury, leading to compensatory cell proliferation and neoplastic development. The in vivo DNA-binding and genotoxicity of acrylonitrile has been studied in several target tissues with no compelling positive results. Thus, while some mutagenic effects were reported in acrylonitrile-exposed rodents, data to determine whether this mutagenicity stems from direct DNA reactivity of acrylonitrile are insufficient. Accordingly, the induction of tumors in rodents is consistent primarily with a non-genotoxic MoA, although a contribution from weak mutagenicity cannot be ruled out. Mechanistic data to support conclusions regarding human hazard from acrylonitrile exposure is weak. Comparison of metabolism of acrylonitrile between rodents and humans provide little support for human hazard. Three of the tissues affected in bioassays (forestomach, Zymbal’s gland, and Harderian gland) are present only in rodents, while the brain is anatomically different between rodents and humans, diminishing relevance of tumor induction in these tissues to human hazard. Extensive epidemiological data has not revealed causation of human cancer by acrylonitrile.

Possible production of arsenic hemoglobin adducts via exposure to arsine

OBJECTIVES

Arsine is an arsenic compound generated as a by-product in metal refineries. Accidental poisoning occurs sporadically; however, the administrative level for workers has not been established. Thus, it is essential to identify a highly specific biomarker for risk management in the workplace. The aim of this study was to identify an arsenic adduct, a potential biomarker, in the plasma.

METHODS

Preserved mouse blood was exposed to arsine in vitro, and the plasma was separated. The residual clot of the control sample was hemolyzed using ultrapure water, and the supernatant was collected. Plasma from mice exposed to arsine in vivo was also separated from blood. Immunoprecipitation assays were conducted using all samples after ultrafiltration, and three fractions were collected. The total arsenic concentration in each fraction was quantified using inductively coupled plasma mass spectrometry (ICP-MS). The three in vitro samples and the eluate fraction from immunoprecipitation were analyzed by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS).

RESULTS

In the exposed samples, the arsenic concentration in the fraction containing immunocomplexes was higher when immunoprecipitation was conducted with an anti-globin antibody. Three peaks were specifically observed in arsine-exposed samples after MALDI-TOF-MS analysis. Two of them were around m/z 15,000, and the other was m/z 15,700. The latter peak was confirmed even after immunoprecipitation.

CONCLUSIONS

Globin forms an adduct with arsenic after both in vitro and in vivo exposure to arsine. This adduct together with hemoglobinuria could be a candidate biomarker of acute arsine poisoning in plasma.

Susceptibility of lung epithelial cells to alkylating genotoxic insult

Alkylation is one of the most common types of DNA damage that can lead to mutations and cancer. Lung is the primary target organ of airborne alkylators such as ethylene oxide (EO). However, the ability of EO to cause lung cancer has not been clearly demonstrated yet. The aim of this study was to investigate the susceptibility of lung cells to alkylating DNA insult by detecting EO-mediated DNA damage with the alkaline comet assay in human lung epithelial cells, peripheral blood lymphocytes, and keratinocytes. The susceptibility of these cell types toward the alkylating insult induced by EO was compared against the oxidative DNA insult induced by hydrogen peroxide (H2 O2 ). Due to the volatility of EO, its active concentrations were monitored by gas chromatography during exposure and were found to decrease significantly in a time-dependent manner. EO induced a statistically significant genotoxic effect at the lowest concentration used (16.4 µM) in lung epithelial cells and in lymphocytes, while in keratinocytes, a genotoxic effect was not detected until 55.5 µM EO. However, lung epithelial cells demonstrated increased resistance to oxidative insult. In fact, oxidative DNA damage detectable by endonuclease treatment was minimal in lung cells compared with the other cell types. These results suggest an increased sensitivity of lung epithelial cells toward the alkylating effects of EO, which was not observed for oxidative DNA damage. Our findings point out the importance of DNA alkylation and the possible role of EO on the induction of lung cancer.

Urinary biomarkers of smokers' exposure to tobacco smoke constituents in tobacco products assessment: a fit for purpose approach

There are established guidelines for bioanalytical assay validation and qualification of biomarkers. In this review, they were applied to a panel of urinary biomarkers of tobacco smoke exposure as part of a "fit for purpose" approach to the assessment of smoke constituents exposure in groups of tobacco product smokers. Clinical studies have allowed the identification of a group of tobacco exposure biomarkers demonstrating a good doseresponse relationship whilst others such as dihydroxybutyl mercapturic acid and 2-carboxy-1-methylethylmercapturic acid - did not reproducibly discriminate smokers and non-smokers. Furthermore, there are currently no agreed common reference standards to measure absolute concentrations and few inter-laboratory trials have been performed to establish consensus values for interim standards. Thus, we also discuss in this review additional requirements for the generation of robust data on urinary biomarkers, including toxicant metabolism and disposition, method validation and qualification for use in tobacco products comparison studies.

Differential response to acrylonitrile toxicity in rat primary astrocytes and microglia

Acrylonitrile (ACN) is a widely used chemical in the production of plastics, resins, nitriles, acrylic fibers, synthetic rubber and acrylamide. While acute high level exposures to ACN are known to be lethal, chronic low dose exposures causes glial cell tumors in rats. Recently, these glial tumors have been characterized as microglial in origin. While effects of ACN on astrocytes, the more numerous glial cell, have been investigated, the effects on microglia are unknown. This study was conducted to compare the responses of astrocytes and microglia to ACN treatment in vitro to address differential sensitivities and adaptive responses to this toxic chemical. Cell viability, ACN uptake, lipid peroxidation byproducts (F2-isoprostanes), glutathione (GSH) levels and expression of NF-E2-related factor 2 (Nrf2) were evaluated in primary rat microglia and astrocytes following ACN treatment. Results indicate that microglia are more sensitive to ACN than astrocytes, accumulating less ACN while demonstrating higher F2-isoprostane levels. GSH levels were up-regulated in both cell types, as a protective mechanism against ACN-induced oxidative stress, while Nrf2 levels were only induced in microglia. Our data suggest that microglia and astrocytes exhibit different sensitivities and responses to ACN, which are linked to the intracellular thiol status inherent to each of these cell types.

Genetic Polymorphism in Glutathione Transferases (GST): Population distribution of GSTM1, T1, and P1 conjugating activity

Glutathione transferases (GST) catalyze the conjugation of glutathione (GSH) with electrophiles, many of which may otherwise interact with protein or DNA. In select cases such as halogenated solvents, GST-mediated conjugation may lead to a more toxic or mutagenic metabolite. Polymorphisms that exert substantial effects on GST function were noted in human populations for several isozymes. This analysis focuses on three well-characterized isozymes, GSTM1, T1, and P1, in which polymorphisms were extensively studied with respect to DNA adducts and cancer in molecular epidemiologic studies. The current review and analysis focused upon how polymorphisms in these GST contributed to population variability in GST function. The first step in developing this review was to characterize the influence of genotype on phenotype (enzyme function) and the frequency of the polymorphisms across major population groups for all three GST. This information was then incorporated into Monte Carlo simulations to develop population distributions of enzyme function. These simulations were run separately for GSTM1, T1, and P1, and also for the combination of these isozymes, to assess the possibility of overlapping substrate specificity. Monte Carlo simulations indicated large interindividual variability for GSTM1 and T1 due to the presence of the null (zero activity) genotype, which is common in all populations studied. Even for GSTM1 or T1 non-null individuals, there was considerable interindividual variability with a bimodal distribution of enzyme activity evident. GSTP1 polymorphisms are associated with somewhat less variability due to the absence of null genotypes. However, in all cases simulated, the estimated variability is sufficiently large to warrant consideration of GST function distributions in assessments involving GST-mediated activation or detoxification of xenobiotics. Ideally, such assessments would involve physiologically based toxicokinetic (PBTK) modeling to assess population variability in internal dose.

The influence of genetic polymorphisms on population variability in six xenobiotic-metabolizing enzymes

This review provides variability statistics for polymorphic enzymes that are involved in the metabolism of xenobiotics. Six enzymes were evaluated: cytochrome P-450 (CYP) 2D6, CYP2E1, aldehyde dehydrogenase-2 (ALDH2), paraoxonase (PON1), glutathione transferases (GSTM1, GSTT1, and GSTP1), and N-acetyltransferases (NAT1 and NAT2). The polymorphisms were characterized with respect to (1) number and type of variants, (2) effects of polymorphisms on enzyme function, and (3) frequency of genotypes within specified human populations. This information was incorporated into Monte Carlo simulations to predict the population distribution and describe interindividual variability in enzyme activity. The results were assessed in terms of (1) role of these enzymes in toxicant activation and clearance, (2) molecular epidemiology evidence of health risk, and (3) comparing enzyme variability to that commonly assumed for pharmacokinetics. Overall, the Monte Carlo simulations indicated a large degree of interindividual variability in enzyme function, in some cases characterized by multimodal distributions. This study illustrates that polymorphic metabolizing systems are potentially important sources of pharmacokinetic variability, but there are a number of other factors including blood flow to liver and compensating pathways for clearance that affect how a specific polymorphism will alter internal dose and toxicity. This is best evaluated with the aid of physiologically based pharmacokinetic (PBPK) modeling. The population distribution of enzyme activity presented in this series of articles serves as inputs to such PBPK modeling analyses.

Hemoglobin adducts as a marker of exposure to chemical substances, especially PRTR class I designated chemical substances

Many new biomarkers are being studied, in addition to classical biomarkers, such as chemical substances and their metabolites in blood and urine and modified enzymes. Among these new biomarkers, hemoglobin adducts are thought to be especially useful for the estimation of chemical exposures. We review here the use of biomarkers for monitoring exposures to nine substances, mainly focusing on PRTR class I designated chemical substances, styrene, phenyloxirane (styrene oxide), 4,4'-methylendiphenyl diisocyanate (MDI), 4,4'-methylendianiline (MDA), 1,3-butadiene, ethylene oxide, propylene oxide, acrylamide and acrylonitrile. Hemoglobin adduct levels were elevated after exposures to styrene, MDI, MDA, 1, 3-butadiene, ethylene oxide, acrylamide and acrylonitrile. Moreover, hemoglobin adducts of butadiene, ethylene oxide, acrylamide and acrylonitrile have several useful advantages. For example, the hemoglobin adduct of 1,3-butadiene is an even more useful biomarker of exposure than urinary metabolites, and in the case of ethylene oxide, even though the concentration of ethylene oxide-Hb in the blood of workers did not exceed the value of the German exposure equivalent, a significant difference in it was found between workers and a control group. Also hemoglobin adducts of acrylamide and acrylonitrile can reflect their exposures because there are no urinary metabolites of acrylamide and acrylonitrile that are useful for exposure assessment. In addition to these advantages, hemoglobin adducts are superior to DNA adducts with respect to the availability of large amounts, availability of methods for chemical identification, and well-defined life spans due to the absence of repair. Hemoglobin adducts can be effective biomarkers for assessing exposure to and the effects of chemicals.

Role of CYP2D6, CYP1A1, CYP2E1, GSTT1, and GSTM1 genes in the susceptibility to acute leukemias

Acute leukemias (ALs) are heterogeneous diseases. Functional polymorphisms in the genes encoding detoxification enzymes cause inter-individual differences, which contribute to leukemia susceptibility. The CYP2D6, CYP1A1, CYP2E1, GSTT1, and GSTM1 polymorphisms in ALL (n = 156) and AML (n = 94) patients and 140 healthy controls were genotyped by PCR and/or PCR-RFLP using blood or bone marrow samples. No association was observed between the GSTT1 gene deletion and patients (OR = 0.8, 95% CI = 0.4-1.7 for AMLs and OR = 0.9, 95% CI = 0.5-1.6 for ALLs). Patients with ALL and AML had a higher prevalence of the GSTM1 deletions compared to controls but only the difference among adult AML patients (OR = 2.1, 95% CI = 1.0-4.2) was statistically significant. The CYP2D6*3 variant allele frequency was lower in the overall acute leukemia patients (0.6%) compared to controls (P = 0.03). CYP2D6*1/*3 genotype frequency also showed a protective association in AML patients (OR = 0.09, 95% CI = 0.01-1.7; P = 0.04). We also found a risk association for CYP2E1*5 in ALL and AML (OR = 3.6, 95% CI = 1.4-9.4 and OR = 3.9, 95% CI = 1.4-10.5, respectively). No association was found for the studied CYP2D6*4, CYP1A1*2A, and GSTT1"null" variants and the risk of acute leuke-mia (ALL or AML). This case-control study suggests a contribution of CYP2E1, CYP2D6, and GSTM1 "null" variants to the development of acute leukemias.

Cancer dose–response assessment for acrylonitrile based upon rodent brain tumor incidence: Use of epidemiologic, mechanistic, and pharmacokinetic support for nonlinearity

A cancer dose-response assessment was conducted for acrylonitrile (AN) using updated information on mechanism of action, epidemiology, toxicity, and pharmacokinetics. Although more than 10 chronic bioassays indicate that AN produces multiple tumors in rats and mice, a number of large, well-conducted epidemiology studies provide no evidence of a causal association between AN exposure and cancer mortality of any type. The epidemiological data include early industry exposures that are far higher than occur today and that approach or exceed levels found to be tumorigenic in animals. Despite the absence of positive findings in the epidemiology data, a dose-response assessment was conducted for AN based on brain tumors in rats. Mechanistic studies implicate the involvement of oxidative stress in rat brain due to a metabolite (2-cyanoethylene oxide or CEO, cyanide), but do not conclusively rule out a potential role for the direct genotoxicity of CEO. A PBPK model was used to predict internal doses (peak CEO in brain) for 12 data sets, which were pooled together to provide a consistent characterization of the dose-response relationship for brain tumor incidence in the rat. The internal dose corresponding to a 5% increase in extra risk (ED 05=0.017 mg/L brain) and its lower confidence limit (LED 05=0.014 mg/L brain) was used as the point of departure. The weight-of-evidence supports the use of a nonlinear extrapolation for the cancer dose-response assessment. A quantitative comparison of the epidemiology exposure-response data (lung and brain cancer mortality) to the rat brain tumor data in terms of internal dose adds to the confidence in the nonlinear extrapolation. Uncertainty factors of 200 and 220 (for the oral and inhalation routes, respectively) were applied to the LED 05 to account for interspecies variation, intraspecies variation, and the severity of the response measure. Accordingly, oral doses below 0.009 mg/kg-day and air concentrations below 0.1mg/m(3) are not expected to pose an appreciable risk to human populations exposed to AN.

Markers of genetic susceptibility in human environmental hygiene and toxicology: the role of selected CYP, NAT and GST genes

Inherited genetic traits co-determine the susceptibility of an individual to a toxic chemical. Special emphasis has been put on individual responses to environmental and industrial carcinogens, but other chronic diseases are of increasing interest. Polymorphisms of relevant xenobiotic metabolising enzymes may be used as toxicological susceptibility markers. A growing number of genes encoding enzymes involved in biotransformation of toxicants and in cellular defence against toxicant-induced damage to the cells has been identified and cloned, leading to increased knowledge of allelic variants of genes and genetic defects that may result in a differential susceptibility toward environmental toxicants. "Low penetrating" polymorphisms in metabolism genes tend to be much more common in the population than allelic variants of "high penetrating" cancer genes, and are therefore of considerable importance from a public health point of view. Positive associations between cancer and CYP1A1 alleles, in particular the *2C I462V allele, were found for tissues following the aerodigestive tract. Again, in most cases, the effect of the variant CYP1A1 allele becomes apparent or clearer in connection with the GSTM1 null allele. The CYP1B1 codon 432 polymorphism (CYP1B1*3) has been identified as a susceptibility factor in smoking-related head-and-neck squameous cell cancer. The impact of this polymorphic variant of CYP1B1 on cancer risk was also reflected by an association with the frequency of somatic mutations of the p53 gene. Combined genotype analysis of CYP1B1 and the glutathione transferases GSTM1 or GSTT1 has also pointed to interactive effects. Of particular interest for the industrial and environmental field is the isozyme CYP2E1. Several genotypes of this isozyme have been characterised which seem to be associated with different levels of expression of enzyme activity. The acetylator status for NAT2 can be determined by genotyping or by phenotyping. In the pathogenesis of human bladder cancer due to occupational exposure to "classical" aromatic amines (benzidine, 4-aminodiphenyl, 1-naphthylamine) acetylation by NAT2 is regarded as a detoxication step. Interestingly, the underlying European findings of a higher susceptibility of slow acetylators towards aromatic amines are in contrast to findings in Chinese workers occupationally exposed to aromatic amines which points to different mechanisms of susceptibility between European and Chinese populations. Regarding human bladder cancer, the hypothesis has been put forward that genetic polymorphism of GSTM1 might be linked with the occurrence of this tumour type. This supports the hypothesis that exposure to PAH might causally be involved in urothelial cancers. The human polymorphic GST catalysing conjugation of halomethanes, dihalomethanes, ethylene oxide and a number of other industrial compounds could be characterised as a class theta enzyme (GSTT1) by means of molecular biology. "Conjugator" and "non-conjugator" phenotypes are coincident with the presence and absence of the GSTT1 gene. There are wide variations in the frequencies of GSTT1 deletion (GSTT1*0/0) among different ethnicities. Human phenotyping is facilitated by the GST activity towards methyl bromide or ethylene oxide in erythrocytes which is representative of the metabolic GSTT1 competence of the entire organism. Inter-individual variations in xenobiotic metabolism capacities may be due to polymorphisms of the genes coding for the enzymes themselves or of the genes coding for the receptors or transcription factors which regulate the expression of the enzymes. Also, polymorphisms in several regions of genes may cause altered ligand affinity, transactivation activity or expression levels of the receptor subsequently influencing the expression of the downstream target genes. Studies of individual susceptibility to toxicants and gene-environment interaction are now emerging as an important component of molecular epidemiology.

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.

DNA adduct burden and tobacco carcinogenesis

DNA adducts associated with tobacco smoking could provide a marker of biologically effective dose of tobacco carcinogens and improve individual cancer risk prediction. A significant number of clinical and epidemiologic studies have reported associations of increased DNA adduct levels with the occurrence of the prevalent tobacco related cancers including cancer of the lung, head and neck, and bladder. The inducibility of DNA adducts following in vitro treatments using blood lymphocytes also appears to be a risk factor in the development of lung and head and neck cancer. Corroborative evidence pointing to the importance of DNA adducts in tobacco carcinogenesis include numerous studies showing associations of tobacco smoke exposure with the induction of DNA adducts in humans in vivo. Further effort is necessary, however, to more fully characterize the dose-response relationship between smoking and DNA adducts in exposed target and surrogate tissues. The relationship between gene polymorphisms thought to modify tobacco-related cancer risk and DNA adduct levels is complex. Results of some DNA adduct studies (both in vitro and in vivo) appear inconsistent with the epidemiologic findings. This is evident for polymorphisms involving both carcinogen metabolism (e.g. GSTP1) and DNA repair (e.g. XRCC1). Molecular studies of human tumors suggest associations of p53 mutation with DNA adducts and have revealed correlations of DNA adduct levels with somatic alterations (e.g. 3p21 LOH) that are thought to occur at the very earliest stages of tobacco carcinogenesis. More research is needed to assess the relationship between endogenous sources of DNA adducts and tobacco smoke exposure and the relative oncogenic effects of chemically stable versus unstable DNA adducts. Many potentially fruitful new avenues of cancer research are emerging that integrate DNA adduct analyses with assessments of smoking, genetics, diet and ambient air quality. These investigations aim to understand the multifactorial nature of interindividual variability in response to tobacco carcinogens. As these trends continue a variety of innovative study designs and approaches will become important in human populations.

Immunohistochemical study of CYP2E1 in hepatocellular carcinoma carcinogenesis: examination with newly prepared anti-human CYP2E1 antibody

Cytochrome P450 2E1 (CYP2E1) is known as a heme-containing enzyme that produces abundant free radicals, and its involvement in carcinogenesis has been suggested in several organs in vivo. In this study, to clarify the involvement of CYP2E1 in liver cancer and its carcinogenesis process, we investigated the expression of CYP2E1 in 42 surgically resected or biopsied specimens of hepatocellular carcinomas (HCC) and 26 cases with other liver lesions immunohistochemically using a newly prepared anti-human CYP2E1 antibody. When intracellular CYP2E1 expression was investigated in three different regions of HCC specimens, the expression in hepatocytes of the peri-tumor region was the highest (p<0.001) compared with those in the tumor and non-peri-tumor regions. Histologically, the expression of CYP2E1 in tumor cells tended to decrease as the cells were less differentiated (p<0.0001) and was the lowest in poorly differentiated HCC (p<0.01). CYP2E1 expression was highest in the pseudo-glandular type and low in the thick trabecular and solid types of HCC (p<0.0001). In mature regenerative nodules of liver cirrhosis, adenomatous hyperplasia (AH) and atypical adenomatous hyperplasia (AAH) to early-HCC, CYP2E1 expression was notably high as compared with other legions. CYP2E1 has a strong free radical-producing ability, and the cell injury and DNA damages by the free radicals are considered to be involved in carcinogenesis. Therefore, our results suggest that the different expression of CYP2E1 in hepatocytes may play important roles in the multistep carcinogenic process and the histogenesis of hepatocellular carcinoma.

Acrylonitrile exposure: the effect on p53 and p21(WAF1) protein levels in the blood plasma of occupationally exposed workers and in vitro in human diploid lung fibroblasts

Acrylonitrile (ACN) is a compound widely used in the synthesis of a variety of organic products. It has been found that ACN is carcinogenic in rats, and some epidemiological studies also suggest a possible carcinogenic effect of ACN in humans. The aim of the present study was to assess the effect of ACN exposure on the expression of p53 and p21(WAF1) proteins in vitro as well as in vivo. In vitro ACN exposure of human lung fibroblasts resulted in the induction of both p53 and p21(WAF1) proteins. To evaluate the effect of ACN on the levels of p53 and p21(WAF1) proteins in the blood plasma of ACN-exposed workers, samples from 49 subjects (average age 44 years, 88% males, 12% females) exposed to ACN in the petrochemical industry (ACN concentration ranged from 0.05 to 0.3mg/m(3)) were analyzed. Subjects living in the same area (N=24, average age 43 years, 92% males, 8% females), but not working in the petrochemical industry were used as controls. No significant differences in either p53, or p21(WAF1) levels between the exposed and control groups were found. The expression of p53 was significantly higher in exposed non-smokers as compared with smokers (P=0.02). No effect of GSTM1 and GSTT1 genotypes on the expression of either protein was observed. Subjects with an EPHX high activity genotype had significantly higher p21(WAF1) expression as compared with genotypes with low or medium EPHX activity. We conclude that plasma levels of both proteins are not relevant biomarkers for occupational ACN exposure.

Possible impact of human CYP2E1 polymorphisms on the metabolism of acrylonitrile

Case reports of human accidental poisonings point to significant individual differences in human acrylonitrile metabolism and toxicity. A cohort of 59 persons with industrial handling of low levels of acrylonitrile has repetitively been studied from 1994 through 1999 as part of a medical surveillance programme. The analyses included adduct determinations of N-terminal N-(cyanoethyl)valine in haemoglobin and genotypings of the following cytochrome P-450 2E1 (CYP2E1) polymorphisms: G-1259C and C-1019T (two subjects heterozygous), A-316G (three subjects heterozygous), T-297A (15 subjects heterozygous), G-35T (eight subjects heterozygous), G4804A (two subjects heterozygous), T7668A (six subjects heterozygous). N-(Cyanoethyl)valine adduct levels were, if any, only slightly influenced by smoking and mainly determined by the external acrylonitrile exposures. The individual means and medians of N-(cyanoethyl)valine levels over the entire observation period were compared with the CYP2E1 variants (Wilcoxon rank sum test). No influences of the investigated CYP2E1 polymorphisms on the N-(cyanoethyl)valine levels appeared at the 5% level. However, there was a trend, at a level of P approximately 0.1, pointing to higher acrylonitrile-specific adduct levels in persons with the A-316G mutation. Higher adduct levels would be compatible with a slower CYP2E1-mediated metabolism of acrylonitrile and with lower extents of toxification to cyanide.

Concluding remarks: symposium on Genetic Susceptibility to Environmental Toxicants

The symposium on "Genetic Susceptibility to Environmental Toxicants" provided a state-of-the-art forum on the role of genetic susceptibility involving exposures to occupational, dietary, and other environmental toxicants. While much of the work focused on single gene-environmental interactions, there was a clear understanding that multiple gene polymorphisms in a metabolic pathway would prove to be essential knowledge in better assessing health risks. A clear need to couple these advances with better measures of exposure was also appreciated, along with improved methods to conduct individual risk assessment procedures. The ethics of these new paradigms was also discussed.