Cytochrome P450 CYP1B1 determines susceptibility to 7, 12-dimethylbenz[a]anthracene-induced lymphomas

The use of gene knockout mice to unravel the mechanisms of toxicity and chemical carcinogenesis

Metabolism of toxins and carcinogens is carried out by large groups of xenobiotic-metabolizing enzymes. These enzymes are generally considered to be required for elimination of xenobiotics such as drugs, dietary chemicals and environmental pollutants, and to be required for chemical toxicity and carcinogenicity. An important role for these enzymes in metabolism of endogenous chemicals has not been established. Mouse lines in which the genes encoding several xenobiotic-metabolizing enzymes were knocked out were produced and are being used to determine the role of metabolism in carcinogenesis, and acute and chronic toxicities in vivo. Mouse lines lacking the P450s CYP1A1, CYP1A2, CYP1B1 and CYP2E1, microsomal epoxide hydrolase (mEH), NADPH:quinone oxidoreductase and the glutathione S-transferase P1 have no deleterious phenotypes, indicating that these enzymes are not required for mammalian development and physiological homeostasis. However, when challenged with toxins and carcinogens, they respond differently from their wild-type (WT) counterparts. For example, mice lacking CYP1A2 and CYP2E1 are totally resistant to acetaminophen-induced hepatotoxicity. Mice lacking CYP1B1 or mEH are less responsive to tumorigenesis by 7,12-dimethybenz[a]anthracene. However, CYP1A2-null mice do not significantly differ from WT mice in their response to the hepatocarcinogen 4-aminobiphenyl. These and other studies indicate that the xenobiotic-metabolism null mice are of great value in the study of the mechanisms of chemical injury.

The use of toxicodynamics in risk assessment

Risk assessment of xenobiotics is a qualitative and quantitative assessment of toxic properties conventionally based on data resulting from tests in animals exposed to the substance. The assessment of dose-effect relationship includes evaluation of exposure at the site of action. More recently, emphasis is put on understanding the relationship between exposure at the site of action and the resulting effect, i.e. toxicodynamic. In this respect, results from genotoxicity studies may be a measure for exposure and at the same time of an effect. Results of toxicodynamic endpoints such as binding to receptors or release of hormones have been used when replacing default values for interspecies extrapolation. It may also be envisaged to use toxicodynamic endpoints in order to get an estimate of intraspecies variability. It was demonstrated that this approach may be helpful only if the relationship between the toxicodynamic endpoint and the definite endpoint is known by using the example of bisphenol A. Whereas there are clear effects of bisphenol A in in vitro and ex vivo studies, the classical two generation study has not been able to detect an effect on reproduction and/or fertility. Looking in the future development of toxicodynamic endpoints, gene profiling and the analysis of proteins ('proteomics') may be helpful tools employed in screening and being related to the mode of action are explored for their suitability in terms of toxicodynamic endpoints.

Genetic variability in susceptibility and response to toxicants

Xenobiotic metabolism is carried out by phase I and phase II enzymes which are to a large extent polymorphic. The majority of cytochrome P450 (CYP) enzymes involved in xenobiotic metabolism are polymorphic and inducible, resulting in abolished, quantitatively or qualitatively altered or enhanced drug metabolising activity. Stable duplication, multiduplication or amplification of active genes have been described. In mouse models it is apparent that inactivation of specific enzymes active in xenobiotic metabolism can affect the risk for cancer development in relation to specific xenobiotic exposure, whereas the situation in humans is far more complex. The polymorphism of CYP enzymes is expected to influence individual sensitivity and toxicity for different environmental agents, although there is as yet no real consensus in the literature about specific firm relationships in this regard. The incidence of serious and fatal adverse drug reactions (ADRs) has been found to be very high among hospitalised patients, the cost of ADRs to society is large and they are responsible for 5-10% of all hospital admissions. It is likely that predictive genotyping could avoid 10-20% of ADRs. In the present contribution an overview is presented regarding our present knowledge about the polymorphism of phase I enzymes, with emphasis on xenobiotic metabolising CYPs and the importance for metabolic activation of xenobiotics.

Metabolic activation in drug allergies

Drug allergies are a major problem in the clinic and during drug development. At the present time, it is not possible to predict the potential of a new chemical entity to produce an allergic reaction (hypersensitivity) in patients in preclinical development. Such adverse reactions, because of their idiosyncratic nature, only become apparent once the drug has been licensed. Our present chemical understanding of drug hypersensitivity is based on the hapten hypothesis, in which covalent binding of the drug (metabolite) plays a central role in drug immunogenicity and antigenicity. If this theory is correct, then it should be possible to develop in vitro systems to assess the potential of drugs to bind to critical proteins, either directly or indirectly after metabolic activation to protein-reactive metabolites (bioactivation) and initiate hypersensitivity. The purpose of this review is to assess critically the evidence to support the hapten mechanism, and also to consider alternative mechanisms by which drugs cause idiosyncratic toxicity.

Human CYP1B1 Leu432Val gene polymorphism: ethnic distribution in African-Americans, Caucasians and Chinese; oestradiol hydroxylase activity; and distribution in prostate cancer cases and controls

Cytochrome P4501B1 (CYP1B1) is involved in the activation of many carcinogens and in the metabolism of steroid hormones, including 17beta-oestradiol (E2) and testosterone. We report a significant difference in the allele frequencies of two point mutations in the coding region of the CYP1B1 gene among Caucasian (n = 189), African-American (n = 52) and Chinese (Linxian) (n = 109) populations. A (C to G) transversion at position 1666 in exon 3, which results in an amino acid substitution of Leu432 to Val, was present in African-Americans with an allele frequency for Va1432 of 0.75, in Caucasians of 0.43, and in Chinese of 0.17. A (C to T) transition at position 1719 in exon 3, with no amino acid change (Asp449), appeared to be closely linked with the Val432 variant. Results using human lung microsomal preparations from individuals with the CYP1B1Val/Val and CYP1B1Leu/Leu genotypes indicate that Val432 variant may be a high activity allele and thus may contribute to the interindividual differences in CYP1B1 activity. Because CYP1B1 is involved in hormone and carcinogen metabolism, and given the disparate rates of prostate cancer among ethnic groups, we also evaluated the association of the CYP1B1 Leu432Val polymorphism with prostate cancer risk in a pilot case-control study. Among Caucasians, 34% of men with cancer (n = 50) were homozygous for the Val432 polymorphism, while only 12% of matched control subjects (n = 50) had this genotype. These preliminary data indicate that genetic polymorphisms in CYP1B1 might play an important role in human prostate carcinogenesis.

Trans-resveratrol modulates the catalytic activity and mRNA expression of the procarcinogen-activating human cytochrome P450 1B1

The present study was performed to determine if trans-resveratrol (3,5,4'-trihydroxy-trans-stilbene) modulates the catalytic activity and gene expression of cytochrome P450 1B1 (CYP1B1). In vitro, trans-resveratrol decreased human recombinant CYP1B1-catalyzed 7-ethoxyresorufin O-dealkylation activity, with an IC50 value of 1.4 ± 0.2 µM (mean ± SEM). Enzyme kinetic analysis indicated that trans-resveratrol inhibited CYP1B1 enzyme activity by a mixed-type inhibition and the apparent Ki was 0.75 ± 0.06 µM. To determine if trans-resveratrol modulates constitutive CYP1B1 gene expression, cultured MCF-7 human breast carcinoma cells were treated with trans-resveratrol. As indicated by RT-PCR analysis, treatment of MCF-7 cells with 10 µM trans-resveratrol decreased relative CYP1B1 mRNA levels after 5 h, but not after 1.5 or 3 h, of exposure. trans-Resveratrol treatment at 5, 7.5, 10, or 20 µM for 5 h produced a concentration-dependent decrease in CYP1B1 mRNA levels. The extent of suppression was ~50% at 20 µM concentration. The suppressive effect was not a consequence of a toxic response to the compound as assessed by a cell proliferation assay. Overall, our novel finding that trans-resveratrol inhibits the catalytic activity and suppresses the constitutive gene expression of CYP1B1 leads to the possibility that this nutraceutical confers protection against toxicity and carcinogenicity induced by compounds that undergo CYP1B1-catalyzed bioactivation.Key words: cytochrome P450, CYP1B1, 7-ethoxyresorufin, nutraceutical, trans-resveratrol.

Using cytochrome P-450 gene knock-out mice to study chemical metabolism, toxicity, and carcinogenicity

Cytochrome P-450 (CYP) enzymes are heme-containing proteins that carry out oxidative metabolism of a wide range of structurally diverse exogenous chemicals and therapeutic agents as well as endogenous compounds. For some of these xenobiotics, oxidative metabolism results in the formation of toxic, mutagenic, or carcinogenic metabolites. In the past, the role of CYP enzymes in metabolism and chemical-induced toxicity was studied indirectly through use of specific antibodies or inducers and inhibitors of these enzymes. Progress in molecular biology and the ability to bioengineer animal models that do not express CYP1A2, CYP1A1, CYP1B1, CYP2E1, or both CYP1A2 and CYP2E1 isozymes has allowed for direct investigations of the in vivo role of these enzymes in the metabolism, toxicity, and carcinogenicity of xenobiotics. This article reviews research conducted to date that utilizes these genetically bioengineered mice in metabolism, toxicity, or carcinogenicity studies of chemicals. Some studies showed a positive correlation between in vivo results and in vitro predictions for the role of a specific CYP in chemical-induced effects, whereas other studies did not support in vitro predictions. Work reviewed herein demonstrates the importance of using animal models for investigating the role of specific CYP enzymes in metabolism and chemical-induced toxicity or carcinogenicity rather than relying solely on in vitro techniques. Eventually, studies of this nature will facilitate a more accurate assessment of human risks with regard to chemicals by helping us to understand the relationships between chemical metabolism, carcinogenicity, and polymorphisms in CYP enzymes.

A cytochrome P4501B gene from a fish, Pleuronectes platessa

Tetrapod cytochrome P4501 family (CYP1A1, CYP1A2 and CYP1B1) enzymes are most active in hydroxylating a variety of environmental contaminants including polyaromatic hydrocarbons (PAH), planar polychlorinated biphenyls and arylamines and thus play a pivotal role in the toxicology of these compounds. Mammalian CYP1A1 and CYP1A2 genes appear to have diverged after the evolutionary emergence of mammals, whereas fish species apparently possess only one CYP1A family gene, and fish CYP1A enzymes exhibit properties of both of the mammalian isoforms. We have isolated a further CYP1 family gene from a marine flatfish (plaice; Pleuronectes platessa), which, on the basis of exon organisation and sequence similarity, can be assigned as a piscine CYP1B. Its deduced amino acid sequence shows the closest (54%) identity to mammalian CYP1B1 proteins and, on the basis of molecular modeling studies, shows a high degree of positional and structural conservation of the substrate contacting amino acid residues in its putative active site when compared to other CYP1 enzymes. Phylogenetic analysis of fish and mammalian CYP1 family sequences indicates that the plaice CYP1B and mammalian CYP1B1 genes share a common ancestry. Plaice CYP1B has a more restricted tissue expression profile than the previously isolated plaice CYP1A, only being detectable, by Northern blotting, in gill tissue. In contrast to CYP1A, which shows extensive PAH-dependent induction in a variety of tissues, plaice CYP1B appears unresponsive to treatment with a prototypical PAH-type inducer, beta-naphthoflavone (BNF).

Vanadium chemoprevention of 7,12-dimethylbenz(a)anthracene-induced rat mammary carcinogenesis: probable involvement of representative hepatic phase I and II xenobiotic metabolizing enzymes

Vanadium, a non-platinum group metal and dietary micronutrient, is now proving to act as a promising antitumor agent. The present study was conducted to ascertain its antineoplastic potential against an experimental mammary carcinogenesis. Female Sprague-Dawley rats, at 50 days of age, were treated with 7,12-dimethylbenz(a)anthracene (DMBA) (0.5 mg/100 g body weight) by a single tail vein injection in an oil emulsion. Vanadium (ammonium monovanadate) at the concentration of 0.5 ppm was supplemented in drinking water and given ad libitum to the experimental group immediately after the carcinogen treatment and it continued until the termination of the study (24 weeks for histological and biochemical observations and 35 weeks for morphological findings). It was found that vanadium treatment brought about a substantial protection against DMBA-induced mammary carcinogenesis. This was evident from histological findings that showed no sign of hyperplasia or abnormality after vanadium treatment. There was a significant reduction in incidence (P < 0.05), total number, multiplicity (P < 0.01) and size of palpable mammary tumors and delay in mean latency period of tumor appearance (P < 0.001) following vanadium supplementation compared to DMBA control. From the cumulative results of various hepatic biochemical indices namely, lipid peroxidation, reduced glutathione level, superoxide dismutase activity, cytochrome P450 content and glutathione S-transferase activity, the anticarcinogenic potential of vanadium was well reflected through stabilization of these parameters. Results of the study indicate that the anticarcinogenic activity of vanadium during DMBA-initiated mammary carcinogenesis is mediated through alteration of hepatic antioxidant status as well as modulation of phase I and II drug metabolizing enzymes. On the basis of the observed results, vanadium can be considered as a readily available, promising and novel cancer chemopreventive agent.

Applications of genetically manipulated mice in pharmacogenetics and pharmacogenomics

Gene knockout mice have proven to be very useful in understanding the role of xenobiotic-metabolizing enzymes in chemical toxicity and carcinogenesis. The combination of gene knockout technology with transgenic mouse technology should provide more versatile and suitable animal models to study the risks of chemical exposures in humans in terms of toxicity and carcinogenesis, as well as development and design of new therapeutic drugs. Recent studies using genetically manipulated mice are summarized.

Molecular genetics of cancer susceptibility

Studies on the role of genetically polymorphic enzymes like cytochrome P450 1A1, arylamine N-acetyltransferase 2 or glutathione S-transferase M1 as cancer susceptibility factors date back more than 20 years, and some associations have been confirmed in several studies and meta-analyses. Overall, the extent of risk modulation due to these polymorphisms is only moderate but remains epidemiologically relevant. The role of some of these polymorphisms in human health may even be ambiguous: rapid acetylation, for example, protects from urinary bladder cancer but appears to increase the risk of laryngeal, lung and colon cancer. The first genetic polymorphisms in xenobiotics transporters such as P-gp (MDR1) and MRP2 have recently been identified. These polymorphisms may have great impact as cancer susceptibility factors as well as factors modulating the outcome of cancer treatment. Enzymes involved in generation or detoxification of reactive oxygen species also have to be considered; one of these enzymes, myeloperoxidase, constitutes a relatively strong lung cancer risk factor, as confirmed in 4 independent studies. Other genes, including those coding for DNA repair enzymes, signal transduction and cell growth regulation, may ultimately prove more important than the metabolic enzymes as cancer susceptibility factors. Study designs in molecular genetic epidemiology are evolving; large ongoing prospective trials increasingly allow confirmatory nested case control studies to be performed. However, carefully controlled, large case-control studies will remain the mainstay in molecular genetic epidemiology. Molecular genetic epidemiological evaluation of response to chemoprevention as well as response to the adverse events of cancer chemotherapy are likely to provide results that may be useful for individualized prevention and treatment in the near future. Since routine genotyping of all persons is now feasible, something like a genotype passport may soon become reality, and molecular and clinical epidemiological studies will have to provide the basis for understanding how to use genotype data for the benefit of the population.

Identification of cytochrome P450 1B-like sequences in two teleost fish species (scup, Stenotomus chrysops and plaice, Pleuronectes platessa) and in a cetacean (striped dolphin, Stenella coeruleoalba)

The cytochromes P450 (CYP) constitute a multigene family of enzymes playing a critical role in the oxidation of many endogenous and xenobiotic substrates. The CYP1 family is of particular interest in environmental toxicology because its members are dominant in the metabolism of polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs) and aryl amines. Three members of the CYP1 family, CYP1A1, CYP1A2, and CYP1B1, have been identified in mammals. We report here on the identification and cloning of cytochrome P4501B-like sequences from two teleost fish species and a marine mammal. Sequences clustering with CYP1B1 in phylogenetic analysis were obtained from liver cDNA of scup (Stenotomus chrysops), genomic DNA of plaice (Pleuronectes platessa), and liver cDNA of striped dolphin (Stenella coeruleoalba).

Characterization and functional analysis of two common human cytochrome P450 1B1 variants

Cytochrome P450 1B1 (CYP1B1) is a human extrahepatic P450 that activates procarinogens, metabolizes 17beta-estradiol, and may well have a role in the pathogenesis of some forms of cancer. Besides rare deleterious mutations reported for the CYP1B1 gene, six single-nucleotide polymorphisms have been reported, of which four cause amino acid exchanges. We have expressed two of the common CYP1B1 alleles in yeast cells and mammalian COS-1 cells in order to functionally characterize the alleles with respect to kinetic properties and protein stability. The CYP1B1.2 variant contains the two linked amino acid substitutions R48G and A119S compared to CYP1B1.1. The kinetic parameters of two structurally unrelated CYP1B1 substrates for the two variants were examined. No kinetic differences were seen of 17beta-estradiol hydroxylation activities between the two CYP1B1 variants and an only minor increase in the apparent Km for ethoxyresorufin was observed for CYP1B1.2. It therefore appears that they have very similar catalytic properties and the substitutions do not appear to alter CYP1B1 catalytic function. The two CYP1B1 variants were similarly stable when expressed in mammalian COS-1 cells, indicating that the substitutions have no effect on protein folding or stability. The combined results indicate that these two CYP1B1 variants show very similar properties with respect to catalytic activities and protein stability and do not alter CYP1B1 function.

Molecular genetics of primary congenital glaucoma

Molecular genetic studies conducted during the last several years have thrown some light on the basic molecular defects in primary congenital glaucoma (PCG) and the rationale behind the clinical and genetic presentation of this paediatric eye condition. The existence of a hereditary form of PCG segregating as an autosomal recessive trait with high penetrance is now confirmed. The primary molecular defect underlying the majority of PCG cases has been identified as mutations in the cytochrome P4501B1 (CYP1B1) gene. This gene is expressed in tissues of the anterior chamber angle of the eye. Molecular modelling experiments suggest that mutations observed in PCG patients interfere with the integrity of the CYP1B1 molecule as well as its ability to adopt a normal conformation and bind haem. On the basis of these observations, we hypothesised that CYP1B1 participates in the normal development and function of the eye by metabolising essential molecules that are perhaps used in a signalling pathway. Revealing the identity of this molecule is our major objective since it can lead to as yet unknown biochemical cascades controlling the terminal stages of anterior chamber angle development.

Polymorphisms in P450 CYP1B1 affect the conversion of estradiol to the potentially carcinogenic metabolite 4-hydroxyestradiol

Most drug metabolizing cytochrome P450s (P450) are predominantly expressed in the liver. In contrast, human CYP1B1 is an extrahepatic P450 which is overexpressed in many tumours and has been strongly implicated in the activation of carcinogens. Rare allelic variants of the CYP1B1 gene which encode an inactive protein have been identified. However, four polymorphisms which most likely do not abolish functionality have been described. In this report, we have characterized the functional consequences of these. A CYP1B1 cDNA, identical to a cDNA published previously, served as a template to introduce allelic changes either separately or in combination. The resulting effects on CYP1B1 activity were determined in membranes isolated from Escherichia coli which coexpressed CYP1B1 together with P450 reductase. None of the allelic changes affected the CYP1B1 expression level. The allelic changes Arg48 to Gly, Ala19 to Ser and Asn453 to Ser had little influence on the Vmax and the Km of the CYP1B1 mediated 2- and 4-hydroxylation of estradiol. In contrast, the Km of these metabolic pathways was increased at least three-fold by the allelic change Va432 to Leu or by simultaneously changing Val432 to Leu and Asn453 to Ser. However, these alterations had little effect on the kinetic parameters of other CYP1B1 mediated reactions such as the epoxidation of (-)-trans-(7R,8R)-benzo[a]pyrene 7,8-dihydrodiol as determined by (r-7,t-8,t-9,c-10)-benzo[a]pyrene tetraol formation, or such as the O-dealkylation of ethoxyresorufin and the 1'-hydroxylation of bufuralol. Molecular modelling suggests that amino acid residue 432 of CYP1B1 may be involved in the interaction between CYP1B1 and P450 reductase. Since 4-hydroxyestradiol has been implicated in hormonal carcinogenesis and CYP1B1 is expressed in target tissues, the data presented demonstrate that polymorphisms in CYP1B1 have the potential to affect disease susceptibility.

Transcriptional regulation of the human CYP1B1 gene. Evidence for involvement of an aryl hydrocarbon receptor response element in constitutive expression

The cytochrome P450 1B1 gene (CYP1B1) is expressed constitutively and is inducible by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in the human breast adenocarcinoma cell line MCF-7 but not in the human hepatoma cell line HepG2. Genomic DNA isolated from both cell lines was digested with the methylation-sensitive restriction enzyme isoschizomers MspI and HpaII, and subjected to Southern analysis with a probe for the CYP1B1 promoter/enhancer region. Although differences were observed in methylation patterns for the CYP1B1 gene from MCF-7 and HepG2 cells, treatment with the demethylating agent 5-azacytidine (10 microM for 6 days) did not activate CYP1B1 mRNA expression in HepG2 cells. Furthermore, treatment with the histone deacetylase inhibitor trichostatin A (100 nM for 24 h) did not activate CYP1B1 mRNA expression in HepG2 cells. Comparative analysis of the constitutive expression of luciferase/1B1 reporter constructs containing a series of deletions in the 5' enhancer region indicated that in MCF-7 cells the region from -987 to -732 (relative to the transcription start site) was necessary for maximal levels of activity. Mutation of the aryl hydrocarbon receptor response elements (dioxin response elements) in this region showed that the dioxin response elements located at -833 is essential for constitutive gene expression in MCF-7 cells. In HepG2 cells, reporter gene activity was at least equal or greater than the activity observed in MCF-7 cells, which is in marked contrast to the expression of the native CYP1B1 gene. Taken together these findings indicate that the observed cell-specific differences in CYP1B1 constitutive expression are not mediated by DNA promoter/enhancer methylation, but are likely due to either 1) inaccessibility of the 5'-enhancer region in HepG2 cells to transcriptional activators due to a higher order chromatin structure that does not involve histone acetylation, or 2) the action of a repressor protein at cis-elements located outside of the -2296 to +25 region examined with the CYP1B1 reporter constructs. Furthermore, at least one of the dioxin response elements in the enhancer region is required for constitutive expression of CYP1B1.

Serum concentrations of organochlorine compounds and K-ras mutations in exocrine pancreatic cancer. PANKRAS II Study Group

BACKGROUND

Organochlorine compounds such as 1,1,1-trichloro-2,2-bis(p-chlorophenyl)-ethane (p,p'-DDT), 1,1-dichloro-2,2-bis(p-chlorophenyl) ethylene (p,p'-DDE), and some polychlorinated biphenyls (PCBs) are carcinogenic to animals and possibly also to human beings. Occupational exposure to DDT may increase the risk of pancreas cancer. The high frequency of K-ras mutations in pancreatic cancer remains unexplained. We analysed the relation between serum concentrations of selected organochlorine compounds and mutations in codon 12 of the K-ras gene in patients with exocrine pancreatic cancer.

METHODS

Cases were prospectively identified in five hospitals. Mutations in K-ras were analysed by PCR and artificial restriction fragment length polymorphism. Cases of pancreatic cancer with wild-type K-ras (n=17) were frequency matched for age and sex to cases of pancreatic cancer with a K-ras mutation (n=34, case-case study). These 51 cases were further compared with 26 hospital controls (case-control comparison). Serum organochlorine concentrations were measured by high-resolution gas chromatography with electron-capture detection and negative ion chemical ionisation mass spectrometry.

FINDINGS

Serum concentrations of p,p'-DDT were significantly higher in pancreatic cancer cases with a K-ras mutation than in cases without a mutation (odds ratio for upper tertile 8.7 [95% CI 1.6-48.5], p for trend=0.005). For p,p'-DDE the corresponding figures were 5.3 (1.1-25.2, p for trend=0.031). These estimates held after adjusting for total lipids, other covariates, and total PCBs. A specific association was observed between a glycine to valine substitution at codon 12 and both p,p'-DDT and p,p'-DDE concentrations (odds ratio 15.9, p=0.044 and odds ratio 24.1, p=0.028; respectively). A similar pattern was shown for the major di-ortho-chlorinated PCBs (congeners 138, 153, and 180), even after adjustment for p,p'-DDE, but without a specific association with spectrum. Concentrations of p,p'-DDT and p,p'-DDE were similar among wild-type cases and controls, but significantly higher for K-ras mutated cases than for controls (p<0.01).

INTERPRETATION

Organochlorine compounds such as p,p'-DDT, p,p'-DDE, and some PCBs could play a part in the pathogenesis of exocrine pancreatic cancer through modulation of K-ras activation. The results require replication, but they suggest new roles for organochlorines in the development of several cancers in human beings.

Metabolism-based polycyclic aromatic acetylene inhibition of CYP1B1 in 10T1/2 cells potentiates aryl hydrocarbon receptor activity

We have used polycyclic aromatic hydrocarbon (PAH) alkyne metabolism-based inhibitors to test whether CYP1B1 metabolism is linked to aryl hydrocarbon receptor (AhR) activation in mouse embryo fibroblasts (MEF). 1-ethynylpyrene (1EP) selectively inactivated CYP1B1 dimethylbenzanthracene (DMBA) metabolism in C3H10T1/2 MEFs; whereas 1-(1-propynyl)pyrene (1PP) preferentially inhibited CYP1A1 activity in Hepa-1c1c7 mouse hepatoma cells (Hepa). In each cell type >90% inhibition of DMBA metabolism after 1 h treatment with each inhibitor (0.1 microM) was progressively reversed and then increased to levels seen with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) induction (fourfold stimulation). It was found that 0.1 microM 1EP and 1PP maximally induce CYP1B1 and CYP1A1 mRNA levels in10T1/2 and Hepa cells, respectively, after 6 h. 1-Ethylpyrene (EtP), which lacks the activatable acetylene moiety, was far less effective as an inhibitor and as an inducer. AhR activation is essential for 1EP induction as evidenced by the use of AhR antagonists and AhR-deficient MEFs and absence of induction following inhibition of DMBA metabolism with carbon monoxide (CO). Inhibition of CYP1B1 was linked to enhanced AhR activation even at early stages prior to significant ligand depletion. 1EP and EtP were similarly effective in stimulating AhR nuclear translocation, though 5-10 times slower compared with TCDD, and produced no significant down-regulation of the AhR. TCDD activated AhR/Arnt complex formation with an oligonucleotide xenobiotic response element far more extensively than 1EP or EtP, even at concentrations of 1EP that increased CYP1B1 mRNA to similar levels. CO did not influence these responses to EtP, event hough CO treatment potentiated EtP induction of CYP1B1 mRNA. These differences suggest a fundamental difference between PAH/AhR and TCDD/AhR complexes where CYP1B1 metabolic activity regulates the potency, rather than the formation of the AhR/Arnt complex.

Targeted disruption of the microsomal epoxide hydrolase gene. Microsomal epoxide hydrolase is required for the carcinogenic activity of 7,12-dimethylbenz[a]anthracene

Microsomal epoxide hydrolase (mEH) is a conserved enzyme that is known to hydrolyze many drugs and carcinogens, and a few endogenous steroids and bile acids. mEH-null mice were produced and found to be fertile and have no phenotypic abnormalities thus indicating that mEH is not critical for reproduction and physiological homeostasis. mEH has also been implicated in participating in the metabolic activation of polycyclic aromatic hydrocarbon carcinogens. Embryonic fibroblast derived from the mEH-null mice were unable to produce the proximate carcinogenic metabolite of 7,12-dimethylbenz[a]anthracene (DMBA), a widely studied experimental prototype for the polycylic aromatic hydrocarbon class of chemical carcinogens. They were also resistant to DMBA-mediated toxicity. Using the two-stage initiation-promotion skin cancer bioassay, the mEH-null mice were found to be highly resistant to DMBA-induced carcinogenesis. In a complete carcinogenesis bioassay, the mEH mice were totally resistant to tumorigenesis. These data establish in an intact animal model that mEH is a key genetic determinant in DMBA carcinogenesis through its role in production of the ultimate carcinogenic metabolite of DMBA, the 3,4-diol-1,2-epoxide.