Determination of the enzymes responsible for activation of the heterocyclic amine 2-amino-3-methylimidazo[4,5-f]quinoline in the human breast

Human Breast Milk Contamination with Aflatoxins, Impact on Children's Health, and Possible Control Means: A Review

Aflatoxins are natural toxicants produced mainly by species of the Aspergillus genus, which contaminate virtually all feeds and foods. Apart from their deleterious health effects on humans and animals, they can be secreted unmodified or carried over into the milk of lactating females, thereby posing health risks to suckling babies. Aflatoxin M1 (AFM1) is the major and most toxic aflatoxin type after aflatoxin B1 (AFB1). It contaminates human breast milk upon direct ingestion from dairy products or by carry-over from the parent molecule (AFB1), which is hydroxylated in the liver and possibly in the mammary glands by cytochrome oxidase enzymes and then excreted into breast milk as AFM1 during lactation via the mammary alveolar epithelial cells. This puts suckling infants and children fed on this milk at a high risk, especially that their detoxifying activities are still weak at this age essentially due to immature liver as the main organ responsible for the detoxification of xenobiotics. The occurrence of AFM1 at toxic levels in human breast milk and associated health conditions in nursing children is well documented, with developing countries being the most affected. Different studies have demonstrated that contamination of human breast milk with AFM1 represents a real public health issue, which should be promptly and properly addressed to reduce its incidence. To this end, different actions have been suggested, including a wider and proper implementation of regulatory measures, not only for breast milk but also for foods and feeds as the upstream sources for breast milk contamination with AFM1. The promotion of awareness of lactating mothers through the organization of training sessions and mass media disclosures before and after parturition is of a paramount importance for the success of any action. This is especially relevant that there are no possible control measures to ensure compliance of lactating mothers to specific regulatory measures, which can yet be appropriate for the expansion of breast milk banks in industrialized countries and emergence of breast milk sellers. This review attempted to revisit the public health issues raised by mother milk contamination with AFM1, which remains undermined despite the numerous relevant publications highlighting the needs to tackle its incidence as a protective measure for the children physical and mental health.

Human Family 1-4 cytochrome P450 enzymes involved in the metabolic activation of xenobiotic and physiological chemicals: an update

This is an overview of the metabolic activation of drugs, natural products, physiological compounds, and general chemicals by the catalytic activity of cytochrome P450 enzymes belonging to Families 1-4. The data were collected from > 5152 references. The total number of data entries of reactions catalyzed by P450s Families 1-4 was 7696 of which 1121 (~ 15%) were defined as bioactivation reactions of different degrees. The data were divided into groups of General Chemicals, Drugs, Natural Products, and Physiological Compounds, presented in tabular form. The metabolism and bioactivation of selected examples of each group are discussed. In most of the cases, the metabolites are directly toxic chemicals reacting with cell macromolecules, but in some cases the metabolites formed are not direct toxicants but participate as substrates in succeeding metabolic reactions (e.g., conjugation reactions), the products of which are final toxicants. We identified a high level of activation for three groups of compounds (General Chemicals, Drugs, and Natural Products) yielding activated metabolites and the generally low participation of Physiological Compounds in bioactivation reactions. In the group of General Chemicals, P450 enzymes 1A1, 1A2, and 1B1 dominate in the formation of activated metabolites. Drugs are mostly activated by the enzyme P450 3A4, and Natural Products by P450s 1A2, 2E1, and 3A4. Physiological Compounds showed no clearly dominant enzyme, but the highest numbers of activations are attributed to P450 1A, 1B1, and 3A enzymes. The results thus show, perhaps not surprisingly, that Physiological Compounds are infrequent substrates in bioactivation reactions catalyzed by P450 enzyme Families 1-4, with the exception of estrogens and arachidonic acid. The results thus provide information on the enzymes that activate specific groups of chemicals to toxic metabolites.

Cancer chemoprevention revisited: Cytochrome P450 family 1B1 as a target in the tumor and the microenvironment

Cancer chemoprevention is the use of synthetic, natural or biological agents to prevent or delay the development or progression of malignancies. Intriguingly, many phytochemicals with anti-inflammatory and anti-angiogenic effects, recently proposed as chemoprevention strategies, are inhibitors of Cytochrome P450 family 1B1 (CYP1B1), an enzyme overexpressed in a wide variety of tumors and associated with angiogenesis. In turn, pro-inflammatory cytokines were reported to boost CYP1B1 expression, suggesting a key role of CYP1B1 in a positive loop of inflammatory angiogenesis. Other well-known pro-tumorigenic activities of CYP1B1 rely on metabolic bioactivation of xenobiotics and steroid hormones into their carcinogenic derivatives. In contrast to initial in vitro observations, in vivo studies demonstrated a protecting role against cancer for the other CYP1 family members (CYP1A1 and CYP1A2), suggesting that the specificity of CYP1 family inhibitors should be carefully taken into account for developing potential chemoprevention strategies. Recent studies also proposed a role of CYP1B1 in multiple cell types found within the tumor microenvironment, including fibroblasts, endothelial and immune cells. Overall, our review of the current literature suggests a positive loop between inflammatory cytokines and CYP1B1, which in turn may play a key role in cancer angiogenesis, acting on both cancer cells and the tumor microenvironment. Strategies aiming at specific CYP1B1 inhibition in multiple cell types may translate into clinical chemoprevention and angioprevention approaches.

Contributions of human enzymes in carcinogen metabolism

Considerable support exists for the roles of metabolism in modulating the carcinogenic properties of chemicals. In particular, many of these compounds are pro-carcinogens that require activation to electrophilic forms to exert genotoxic effects. We systematically analyzed the existing literature on the metabolism of carcinogens by human enzymes, which has been developed largely in the past 25 years. The metabolism and especially bioactivation of carcinogens are dominated by cytochrome P450 enzymes (66% of bioactivations). Within this group, six P450s--1A1, 1A2, 1B1, 2A6, 2E1, and 3A4--accounted for 77% of the P450 activation reactions. The roles of these P450s can be compared with those estimated for drug metabolism and should be considered in issues involving enzyme induction, chemoprevention, molecular epidemiology, interindividual variations, and risk assessment.

Identification of mammary epithelial cells subject to chronic oxidative stress in mammary epithelium of young women and teenagers living in USA: implication for breast carcinogenesis

Current knowledge of changes in the mammary epithelium relevant to breast carcinogenesis is limited to when histological changes are already present because of a lack of biomarkers needed to identify where such molecular changes might be ongoing at earlier during the of decades-long latent stages of breast carcinogenesis. Breast reduction tissues from young women and teenagers, representative of USA's high breast cancer incidence population, were studies using immunocytochemistry and targeted PCR arrays in order to learn whether a marker of chronic oxidative-stress [protein adducts of 4-hydroxy-2-nonenal (4HNE)] can identify where molecular changes relevant to carcinogenesis might be taking place prior to any histological changes. 4HNE-immunopositive (4HNE+) mammary epithelial cell-clusters were identified in breast tissue sections from most women and from many teenagers (ages 14-30 y) and, in tissues from women ages 17-27 y with many vs. few 4HNE+ cells, the expression of 30 of 84 oxidative-stress associated genes was decreased and only one was increased > 2-fold. This is in contrast to increased expression of many of these genes known to be elicited by acute oxidative-stress. The findings validate using 4HNE-adducts to identify where molecular changes of potential relevance to carcinogenesis are taking place in histologically normal mammary epithelium and highlight differences between responses to acute vs. chronic oxidative-stress. We posit that the altered gene expression in 4HNE+ tissues reflect adaptive responses to chronic oxidative-stress that enable some cells to evade mechanisms that have evolved to prevent propagation of cells with oxidatively-damaged DNA and to accrue heritable changes needed to establish a cancer.

Metabolism and biomarkers of heterocyclic aromatic amines in molecular epidemiology studies: lessons learned from aromatic amines

Aromatic amines and heterocyclic aromatic amines (HAAs) are structurally related classes of carcinogens that are formed during the combustion of tobacco or during the high-temperature cooking of meats. Both classes of procarcinogens undergo metabolic activation by N-hydroxylation of the exocyclic amine group to produce a common proposed intermediate, the arylnitrenium ion, which is the critical metabolite implicated in toxicity and DNA damage. However, the biochemistry and chemical properties of these compounds are distinct, and different biomarkers of aromatic amines and HAAs have been developed for human biomonitoring studies. Hemoglobin adducts have been extensively used as biomarkers to monitor occupational and environmental exposures to a number of aromatic amines; however, HAAs do not form hemoglobin adducts at appreciable levels, and other biomarkers have been sought. A number of epidemiologic studies that have investigated dietary consumption of well-done meat in relation to various tumor sites reported a positive association between cancer risk and well-done meat consumption, although some studies have shown no associations between well-done meat and cancer risk. A major limiting factor in most epidemiological studies is the uncertainty in quantitative estimates of chronic exposure to HAAs, and thus, the association of HAAs formed in cooked meat and cancer risk has been difficult to establish. There is a critical need to establish long-term biomarkers of HAAs that can be implemented in molecular epidemioIogy studies. In this review, we highlight and contrast the biochemistry of several prototypical carcinogenic aromatic amines and HAAs to which humans are chronically exposed. The biochemical properties and the impact of polymorphisms of the major xenobiotic-metabolizing enzymes on the biological effects of these chemicals are examined. Lastly, the analytical approaches that have been successfully employed to biomonitor aromatic amines and HAAs, and emerging biomarkers of HAAs that may be implemented in molecular epidemiology studies are discussed.

Ochratoxin A induces CYP3A4, 2B6, 3A5, 2C9, 1A1, and CYP1A2 gene expression in primary cultured human hepatocytes: a possible activation of nuclear receptors

Ochratoxin A (OTA) is a mycotoxin produced by fungi of two genera: Penicillium and Aspergillus. OTA has been shown to be nephrotoxic, hepatotoxic, teratogenic, and immunotoxic to several species of animals and to cause kidney and liver tumors in mice and rats. Biotransformation of OTA has not been entirely elucidated. Several metabolites have been characterized in vitro and/or in vivo, whereas other metabolites remain to be characterized. At present, data available regarding OTA metabolism and cytochrome inductions concern only rodents or in vitro systems. The aim of the present study was to explore the effect of OTA on mRNA expression of some cytochromes known to be regulated by pregnane X receptor (PXR), constitutive androstane receptor (CAR), and aryl hydrocarbon receptor (AhR), using primary cultures of human hepatocytes. Our results showed that OTA reduced hepatocyte viability in a dose-dependent manner. Using quantitative real-time reverse-transcription polymerase chain reaction, our study showed that treatment of primary cultured human hepatocytes with noncytotoxic increasing concentrations of OTA for 24 hours caused a significant upregulation of CYP3A4, CYP2B6, and, to a lesser extent, CYP3A5 and CYP2C9. PXR mRNA expression increased in only 1 treated liver, whereas CAR mRNA expression was not affected. OTA was found also to induce an overexpression of CYP1A1 and CYP1A2 genes accompanied by an increase in AhR mRNA expression. These findings suggest that OTA could activate PXR and AhR; however, further investigations are needed to confirm nuclear-receptor activation by OTA.

Individual variability in the detoxification of carcinogenic arylhydroxylamines in human breast

Cytochrome b(5) (b5) and NADH cytochrome b(5) reductase (b5R) detoxify reactive hydroxylamine (NHOH) metabolites of known arylamine and heterocyclic amine mammary carcinogens. The aim of this study was to determine whether NHOH reduction for the prototypic arylamine 4-aminobiphenyl (4-ABP) was present in human breast and to determine whether variability in activity was associated with single nucleotide polymorphisms (SNPs) in the coding, promoter, and 3'untranslated region (UTR) regions of the genes encoding b5 (CYB5A) and b5R (CYB5R3). 4-ABP-NHOH reduction was readily detected in pooled human breast microsomes, with a K(m) (280μM) similar to that found with recombinant b5 and b5R, and a V(max) of 1.12 ± 0.19 nmol/min/mg protein 4-ABP-NHOH reduction varied 75-fold across 70 individual breast samples and correlated significantly with both b5 (80-fold variability) and b5R (14-fold) immunoreactive protein. In addition, wide variability in b5 protein expression was significantly associated with variability in CYB5A transcript levels, with a trend toward the same association between b5R and CYB5R3. Although a sample with a novel coding SNP in CYB5A, His22Arg, was found with low reduction and b5 expression, no other SNPs in either gene were associated with outlier activity or protein expression. We conclude that b5 and b5R catalyze the reduction of 4-ABP-NHOH in breast tissue, with very low activity, protein, and messenger RNA expression in some samples, which cannot be attributed to promoter, coding, or 3'UTR SNPs. Further studies are underway to characterize the transcriptional regulation of CYB5A and CYB5R3 and begin to understand the mechanisms of individual variability in this detoxification pathway.

NQO1, MPO, and the risk of lung cancer: a HuGE review

The aim of this study is to summarize the available molecular epidemiologic studies of lung cancer and metabolic genes, such as NAD(P)H quinone reductase 1 (NQO1) and myeloperoxidase (MPO). NQO1 plays a dual role in the detoxification and activation of procarcinogens whereas MPO has Phase I activity by converting lipophilic carcinogens into hydrophilic forms. Variant genotypes of both NQO1 Pro187 Ser and MPO G-463A polymorphisms may be related to low enzyme activity. The Pro/Ser and Ser/Ser genotypes combined of NQO1 was significantly associated with decreased risk of lung cancer in Japanese [random effects odds ratio (OR) = 0.70, 95% confidence interval (CI) = 0.56-0.88] among whom the variant allele is common. The variant genotype of MPO was associated with decreased risk of lung cancer among Caucasians (random effects OR = 0.70, 95% CI = 0.47-1.04). Gene-environment interactions in both polymorphisms may be hampered by inaccurate categorization of tobacco exposure. Evidence on gene-gene interactions is extremely limited. As lung cancer is a multifactorial disease, an improved understanding of such interactions may help identify individuals at risk for developing lung cancer. Such a study should include larger sample size and other polymorphisms in the metabolism of tobacco-derived carcinogens and address interactions with smoking status. The effects of polymorphisms are best represented by their haplotypes. In future studies on lung cancer, the development of haplotype-based approaches will facilitate the evaluation of haplotypic effects, either for selected polymorphisms physically close to each other or for multiple genes within the same drug-metabolism pathway.

N-Acetyltransferase and sulfotransferase activity in human prostate: potential for carcinogen activation

BACKGROUND

N-Acetyltransferases (NATs) and sulfotransferases (SULTs) are key phase II metabolizing enzymes that can be involved both in the detoxification and in the activation of many human promutagens and procarcinogens.

METHODS AND RESULTS

We investigated the expression of NATs and SULTs in human prostate and tested their role in the activation the N-hydroxy (N-OH) metabolite of 2-amino-3-methylimidazo[4,5-f]quinoline (IQ), a dietary carcinogen, to form DNA adducts. Western blotting showed detectable levels of NAT1, SULT1A1 and SULT1A3 with marked inter-individual variation. NAT2 and other SULT enzymes were not detectable. NAT1 was localized by immunohistochemistry to the cytoplasm of epithelial cells. The presence of acetyl Co-enzyme A (acetyl CoA) and 3'-phosphoadenosine-5'-phosphosulfate (PAPS), NAT and SULT cofactors, respectively, significantly increased the level of DNA adducts, detected by P-postlabelling analysis, in calf thymus DNA incubated with N-OH-IQ and prostate cytosolic fractions. The enhancement in the level of DNA adducts in the presence of PAPS correlated with the level of SULT1A1 protein. A single prostate cytosol with the SULT1A1*2/*2 genotype produced less DNA adducts than cytosols with the *1/*2 and *1/*1 genotypes. No significant correlation was observed between NAT1 protein level and the formation of DNA adducts, even in the presence of acetyl CoA.

CONCLUSIONS

In conclusion, we demonstrated that NAT1, SULT1A1 and SULT1A3 are present in human prostate and that both enzyme classes significantly contribute to the activation of N-hydroxylated heterocyclic amines to DNA-damaging species in this tissue. Variation in expression levels, in combination with dietary and/or environmental exposure to carcinogens, could be influential in determining individual susceptibility to prostate cancer.

Bioactivation of 3-aminobenzanthrone, a human metabolite of the environmental pollutant 3-nitrobenzanthrone: evidence for DNA adduct formation mediated by cytochrome P450 enzymes and peroxidases

3-Nitrobenzanthrone (3-NBA) is a suspected human carcinogen found in diesel exhaust and ambient air pollution. The main metabolite of 3-NBA, 3-aminobenzanthrone (3-ABA), was detected in the urine of salt mining workers occupationally exposed to diesel emissions. We evaluated the role of hepatic cytochrome P450 (CYP) enzymes in the activation of 3-ABA in vivo by treating hepatic cytochrome P450 oxidoreductase (POR)-null mice and wild-type littermates intraperitoneally with 0.2 and 2mg/kg body weight of 3-ABA. Hepatic POR-null mice lack POR-mediated CYP enzyme activity in the liver. Using the (32)P-postlabelling method, multiple 3-ABA-derived DNA adducts were observed in liver DNA from wild-type mice, qualitatively similar to those formed in incubations using human hepatic microsomes. The adduct pattern was also similar to those formed by the nitroaromatic counterpart 3-NBA and which derive from reductive metabolites of 3-NBA bound to purine bases in DNA. DNA binding by 3-ABA in the livers of the null mice was undetectable at the lower dose and substantially reduced (by up to 80%), relative to wild-type mice, at the higher dose. These data indicate that POR-mediated CYP enzyme activities are important for the oxidative activation of 3-ABA in livers, confirming recent results indicating that CYP1A1 and -1A2 are mainly responsible for the metabolic activation of 3-ABA in human hepatic microsomes. No difference in DNA binding was found in kidney and bladder between null and wild-type mice, suggesting that cells in these extrahepatic organs have the metabolic capacity to oxidize 3-ABA to species forming the same 3-ABA-derived DNA adducts, independently from the CYP-mediated oxidation in the liver. We determined that different model peroxidases are able to catalyse DNA adduct formation by 3-ABA in vitro. Horseradish peroxidase (HRP), lactoperoxidase (LPO), myeloperoxidase (MPO), and prostaglandin H synthase (PHS) were all effective in activating 3-ABA in vitro, forming DNA adducts qualitatively similar to those formed in vivo in mice treated with 3-ABA and to those found in DNA reacted with N-hydroxy-3-aminobenzanthrone (N-OH-ABA). Collectively, these results suggest that both CYPs and peroxidases may play an important role in metabolizing 3-ABA to reactive DNA adduct forming species.

Naturally occurring coumarins inhibit 7,12-dimethylbenz[a]anthracene DNA adduct formation in mouse mammary gland

Naturally occurring coumarins (NOCs) are anti-carcinogenic in the mouse skin model. To characterize the chemopreventive potential of NOCs against breast cancer, we first examined their effects on 7,12-dimethylbenz[a]anthracene (DMBA)-DNA adduct formation in mouse mammary gland. We hypothesized that those NOCs that both inhibited cytochrome P450 1A1/1B1 and induced hepatic glutathione S-transferases (GSTs) would be the most effective in blocking DMBA-DNA adduct formation in mouse mammary gland. To address this hypothesis, simple coumarins (e.g. coumarin and limettin, which induced mouse hepatic GSTs but had little effect on P4501A1/1B1) and linear furanocoumarins (e.g. imperatorin and isopimpinellin, which induced hepatic GSTs and were potent inhibitors of P4501A1/1B1) were compared. Mice were pretreated with NOCs (150 mg/kg body wt, by gavage) prior to either a single dose of DMBA (50 microg) or multiple doses of DMBA (20 microg daily for 3 and 6 weeks). Mammary DMBA-DNA adduct formation was quantitated by the nuclease P1-enhanced 32P-postlabeling assay. With the single dose of DMBA, coumarin, limettin, imperatorin and isopimpinellin inhibited DMBA-DNA adduct formation by 50, 41, 79 and 88%, respectively. Coumarin, limettin and imperatorin blocked DMBA-DNA adduct formation by 36, 60, and 66% at 3 weeks, and by 0, 49 and 55% at 6 weeks of DMBA dosing, respectively. In a 6 week dose-response study of select NOCs and 7,8-benzoflavone (a potent P4501 inhibitor that had little effect on GSTs), DMBA-DNA adduct formation was inhibited by 0, 43 and 24% in the limettin groups; by 26, 26 and 69% in the isopimpinellin groups; and by 80, 96 and 97% in the 7,8- benzoflavone groups at 35, 70 and 150 mg/kg, respectively. Taken together, these results suggest that linear furanocoumarins had a greater inhibitory effect on DMBA-DNA adduct formation in mouse mammary glands compared with simple coumarins, and that the predominant effect may be P4501 inhibition.

2-Nitrosoamino-3-methylimidazo[4,5-f]quinoline activated by the inflammatory response forms nucleotide adducts

Heterocyclic amines and inflammation have been implicated in the etiology of colon cancer. We have recently demonstrated that during autoxidation of the inflammatory mediator nitric oxide 2-amino-3-methylimidazo[4,5-f]quinoline (IQ) undergoes nitrosation to form 2-nitrosoamino-3-methylimidazo[4,5-f]quinoline (N-NO-IQ). This study evaluates the genotoxicity of N-NO-IQ and compares the adducts it forms to those of 2-hydroxyamino-3-methylimidazo[4,5-f]quinoline (N-OH-IQ). N-NO-IQ was incubated with 2'-deoxyguanosine 3'-monophosphate (dGp) under a variety of inflammatory conditions. 32P-Postlabeling demonstrated the presence of multiple adducts. Incubation of N-OH-IQ with dGp at pH 7.4, 5.5, or 2.0 resulted in the formation of a single major adduct, N-(deoxyguanosin-8-yl)-IQ (dG-C8-IQ). Using a combination of 32P-postlabeling, HPLC, and nuclease P1 treatment, N-NO-IQ was shown to produce dG-C8-IQ under several different conditions. HOCl oxidation of N-NO-IQ increased dG-C8-IQ formation, and this was further increased as pH decreased from 7.4 to 5.5. Oxidation of N-NO-IQ formed a new adduct, adduct 2, while in the absence of oxidants adduct m was the major adduct. Adducts 2 and m were not formed by N-OH-IQ and not further identified. The results demonstrate that N-NO-IQ forms N-(deoxyguanosin-8-yl)-IQ, is genotoxic, is activated by conditions that mediate inflammatory responses, and is a possible cancer risk factor for individuals with colitis, inflammation of the colon.

Genetic susceptibility to tobacco-related cancer

Although cigarette smoking is the dominant risk factor for several epithelial cancers, only a small fraction of individuals with tobacco exposure develop cancer. The underlying hypothesis is that genetic factors may render certain smokers more susceptible to cancer than others. Genetic alterations in critical regulatory pathways may predispose cells to carcinogenesis. These pathways include regulation of xenobiotic metabolism; control of genomic stability, including DNA repair mechanisms, cell-cycle checkpoints, apoptosis and telomere length; and control of microenvironmental factors, such as matrix metalloproteinases, inflammation and growth factors. In addition, epigenetic events, such as promoter hypermethylation and loss of imprinting, are also involved in carcinogenesis. In this review, we will summarize recent advances in genetic susceptibility to tobacco-related cancer. Emphasizing on risk assessment, we will describe how genetic variations in the above-mentioned genetic pathways modify the tobacco-related cancer risk. In addition, we will discuss how genetic variations may assist in predicting clinical outcome, such as the natural history of cancer and treatment response. The measurements of genetic susceptibility by both genotypic and phenotypic assays are covered in the text. Finally, we present a number of current concerns that need to be addressed as the exciting field of molecular cancer epidemiology advances rapidly.

Cigarette smoking, cytochrome P4501A1 polymorphisms, and breast cancer among African-American and white women

INTRODUCTION

Previous epidemiologic studies suggest that women with variant cytochrome P4501A1 (CYP1A1) genotypes who smoke cigarettes are at increased risk for breast cancer.

METHODS

We evaluated the association of breast cancer with CYP1A1 polymorphisms and cigarette smoking in a population-based, case-control study of invasive breast cancer in North Carolina. The study population consisted of 688 cases (271 African Americans and 417 whites) and 702 controls (285 African Americans and 417 whites). Four polymorphisms in CYP1A1 were genotyped using PCR/restriction fragment length polymorphism analysis: M1 (also known as CYP1A1*2A), M2 (CYP1A1*2C), M3 (CYP1A1*3), and M4 (CYP1A1*4)

RESULTS

No associations were observed for CYP1A1 variant alleles and breast cancer, ignoring smoking. Among women who smoked for longer than 20 years, a modest positive association was found among women with one or more M1 alleles (odds ratio [OR] = 2.1, 95% confidence interval [CI] = 1.2-3.5) but not among women with non-M1 alleles (OR = 1.2, 95% CI = 0.9-1.6). Odds ratios for smoking longer than 20 years were higher among African-American women with one or more M3 alleles (OR = 2.5, 95% CI = 0.9-7.1) compared with women with non-M3 alleles (OR = 1.3, 95% CI = 0.8-2.2). ORs for smoking in white women did not differ appreciably based upon M2 or M4 genotypes.

CONCLUSIONS

Cigarette smoking increases breast cancer risk in women with CYP1A1 M1 variant genotypes and in African-American women with CYP1A1 M3 variant genotypes, but the modifying effects of the CYP1A1 genotype are quite weak.

Formation and analysis of heterocyclic aromatic amine–DNA adducts in vitro and in vivo

The detection and quantification of heterocyclic aromatic amine (HAA)-DNA adducts, critical biomarkers in interspecies extrapolation of toxicity data for human risk assessment, remains a challenging analytical problem. The two main analytical methods currently in use to screen for HAA-DNA adducts are the 32P-postlabeling assay and mass spectrometry, using either accelerated mass spectrometry (AMS) or liquid chromatography and electrospray ionization mass spectrometry (LC-ESI-MS). In this review, the principal methods to synthesize and characterize DNA adducts, and the methods applied to measure HAA-DNA adduct in vitro and vivo are discussed.

Combined effect of MPO, GSTM1 and GSTT1 polymorphisms on chromosome aberrations and lung cancer risk

The role of myeloperoxidase (MPO), and glutathione S-transferase mu and theta (GSTM1 and GSTT1) genetic polymorphisms on lung cancer risk was investigated in 110 Caucasian patients and 119 matched controls. Single genotype variants were not significantly associated with lung cancer risk. However, inheritance of the combined GSTM1 and GSTT1 null genotypes showed a significant increase in risk (crude OR = 2.32, 95% CI = 1.01-6.04). Based on adjustment by age, gender and smoking history, the MPO GA interacted with the presence of GSTM1 and GSTT1 genotypes to significantly reduce the risk (OR = 0.17, 95% CI = 0.03-0.98). From the chromosome aberration (CA) study in a subgroup of 79 patients and 69 matched controls, patients had significantly more CA than the controls. Among the patients, GSTM1 null was associated with a significant increase of CA and MPO AA was associated with a significant decrease of CA compared to their respective wild-type genotypes. After stratifying by smoking history (< or = and > 40 pack-years) and genotype, patients still had significantly more CA than the respective controls in most genotype categories. This indicates that the patients had additional contributing factors such as other susceptibility genes and/or different styles of smoking compared with the controls. In conclusion, our study indicates that CA is a useful biomarker to show the functional characteristics of genotypes and the interactive effects from combined genotypes. Therefore, our study strengthens the combined use of genotype and biomarkers for genetic susceptibility to environmental cancer.

Use of human liver S9 in the Ames test: assay of three procarcinogens using human S9 derived from multiple donors

The purpose of the present study was to examine the inter-individual variation in the mutagenicity of chemicals using a variety of human S9 fractions. For this purpose, three procarcinogens, 2-amino-3-methylimidazo[4,5-f]quinoline (IQ), benzo[a]pyrene (BP), and dimethylnitrosamine (DMN), were selected for the Ames test and their mutagenicity was examined using human liver S9 fractions prepared from 18 different donors and one pooled liver S9 fraction prepared from 15 different donors. In addition, rat S9 fraction prepared from male rats pretreated with phenobarbital and 5,6-benzoflavone (PB/BF) was used as reference in order to examine the mutagenic differences between human and rat (PB/BF) S9 fractions. The data demonstrate a large inter-individual diversity in the mutagenic response to procarcinogens. The mutagenicity of IQ and BP in the presence of a human liver S9 fraction (lot HLS-014) was equal to that observed in the presence of rat (PB/BF) S9 fraction. The mutagenicity of IQ and BP in the presence of a pooled human liver S9 fraction was lower (90 and 95%, respectively) than that observed in the presence of rat (PB/BF) S9. On the contrary, the mutagenicity of DMN in the presence of either a selected human liver S9 fraction (lot HLS-014) or pooled fraction was 8-fold higher than that found in the presence of rat (PB/BF) S9 fraction. Human liver S9 fraction (lot HLS-014) had one of the highest cytochrome P450 enzyme activities among the 18 different donors and higher than the pooled human liver S9 fraction. These results suggest that the use of both selected human liver S9 fractions with high metabolic activity (e.g., lot HLS-014 as used in this study) and a pooled S9 fraction with moderate metabolic activity could be used as a means to evaluate the inter-individual variability in mutagenic response to chemicals and to confirm positive responses from studies completed with rodent S9.

Genetic polymorphisms and lung cancer susceptibility: a review

Lung cancer is a major cause of cancer-related death in the developed countries and the overall survival rate has still an extremely poor. Cigarette smoking is an established risk factor for lung cancer although a possible role for genetic susceptibility in the development of lung cancer has been inferred from familial clustering of the disease and segregation analyzes. Everyone may have a unique combination of polymorphic traits that modify genetic susceptibility and response to drugs, chemicals and carcinogens. Developments in molecular biology have led to growing interest in investigation of biological markers, which may increase predisposition to lung carcinogenesis. Therefore, the high-risk genotype of an individual could be determined easily. As there are the great number of carcinogen-activating and -detoxifying enzymes, the variation in their expression and the complexity of exposures to tobacco carcinogens, the existence of multiple alleles at loci of those enzymes may result in differential susceptibilities of individuals. This review summarize data addressing the relationships of lung cancer to markers of genetic susceptibility genes, including metabolic polymorphisms other than well-investigated cytochrome P450s or glutathione S-transferases, DNA repair genes and the p53 tumor suppressor gene. Among genetic polymorphisms reviewed here, myeloperoxidase gene (a G to A mutation) and microsomal epoxide hydrolase exon 4 polymorphism (substitution of Arg for His) were significantly associated with lung cancer risk. As lung cancer is a multifactorial disease, an improved understanding of the interplay of environmental and genetic polymorphisms at multiple loci may help identify individuals who are at increased risk for lung cancer. Hopefully, in the future we will be able to screen for lung cancer susceptibility by using specific biomarkers.

Metabolism of tamoxifen by recombinant human cytochrome P450 enzymes: formation of the 4-hydroxy, 4'-hydroxy and N-desmethyl metabolites and isomerization of trans-4-hydroxytamoxifen

The cytochrome P450 (P450)-mediated biotransformation of tamoxifen is important in determining both the clearance of the drug and its conversion to the active metabolite, trans-4-hydroxytamoxifen. Biotransformation by P450 forms expressed extrahepatically, such as in the breast and endometrium, may be particularly important in determining tissue-specific effects of tamoxifen. Moreover, tamoxifen may serve as a useful probe drug to examine the regioselectivity of different forms. Tamoxifen metabolism was investigated in vitro using recombinant human P450s. Forms CYP1A1, 1A2, 1B1, 2A6, 2B6, 2C9, 2C19, 2D6, 2E1, 3A4, 3A5, and 3A7 were coexpressed in Escherichia coli with recombinant human NADPH-cytochrome P450 reductase. Bacterial membranes were harvested and incubated with tamoxifen or trans-4-hydroxytamoxifen under conditions supporting P450-mediated catalysis. CYP2D6 was the major catalyst of 4-hydroxylation at low tamoxifen concentrations (170 +/- 20 pmol/40 min/0.2 nmol P450 using 18 microM tamoxifen), but CYP2B6 showed significant activity at high substrate concentrations (28.1 +/- 0.8 and 3.1 +/- 0.5 nmol/120 min/0.2 nmol P450 for CYP2D6 and CYP2B6, respectively, using 250 microM tamoxifen). These two forms also catalyzed 4'-hydroxylation (13.0 +/- 1.9 and 1.4 +/- 0.1 nmol/120 min/0.2 nmol P450, respectively, for CYP2B6 and CYP2D6 at 250 microM tamoxifen; 0.51 +/- 0.08 pmol/40 min/0.2 nmol P450 for CYP2B6 at 18 microM tamoxifen). Tamoxifen N-demethylation was mediated by CYP2D6, 1A1, 1A2, and 3A4, at low substrate concentrations, with contributions by CYP1B1, 2C9, 2C19 and 3A5 at high concentrations. CYP1B1 was the principal catalyst of 4-hydroxytamoxifen trans-cis isomerization but CYP2B6 and CYP2C19 also contributed.

Summary of information on human CYP enzymes: human P450 metabolism data

This chapter is an update of the data on substrates, reactions, inducers, and inhibitors of human CYP enzymes published previously by Rendic and DiCarlo (1), now covering selection of the literature through 2001 in the reference section. The data are presented in a tabular form (Table 1) to provide a framework for predicting and interpreting the new P450 metabolic data. The data are formatted in an Excel format as most suitable for off-line searching and management of the Web-database. The data are presented as stated by the author(s) and in the case when several references are cited the data are presented according to the latest published information. The searchable database is available either as an Excel file (for information contact the author), or as a Web-searchable database (Human P450 Metabolism Database, www.gentest.com) enabling the readers easy and quick approach to the latest updates on human CYP metabolic reactions.

Mutagens in human breast lipid and milk: the search for environmental agents that initiate breast cancer

Epidemiological studies indicate the involvement of environmental factors in the etiology of breast cancer, but have not provided clear indications of the nature of the agents responsible. Several environmental carcinogens are known to induce mammary tumors in rodents, and the abundance of adipose tissue in the human breast suggests that the epithelial cells, from which breast tumors commonly arise, could be exposed to lipid-soluble carcinogens sequestered by the adipose tissue. In this report we review our studies in which we have examined human mammary lipid, obtained from elective reduction mammoplasties from healthy donors, and human milk from healthy mothers, for the presence of components with genotoxic activity in several in vitro assays. A significant proportion of lipid extracts induced mutations in bacteria and micronuclei in mammalian cells. They also caused DNA damage, detected as single-strand breaks in the alkaline single-cell gel electrophoresis (comet) assay, in both the MCL-5 cell line and in primary cultures of human mammary epithelial cells. Genotoxic activity was also found in a significant proportion of extracts of human breast milk. Viable cells recovered from milk samples showed evidence of DNA damage and were susceptible to comet formation by genotoxic agents in vitro. Genotoxic activity was found to be less prevalent in milk samples from countries of lower breast cancer incidence (the Far East) compared with that in samples from the UK. The agents responsible for the activity in milk appear to be moderately polar lipophilic compounds and of low molecular weight. Identification of these agents and their sources may hold clues to the origins of breast cancer.

N-Acetyltransferases, sulfotransferases and heterocyclic amine activation in the breast

Heterocyclic amines are mammary carcinogens in rats and their N-hydroxy metabolites are substrates for subsequent metabolic activation by N-acetyltransferases (NAT) and sulfotransferases (SULT) in man. We investigated the expression of these enzymes in human breast tissue and the relationship between NAT genotype and NAT mRNA expression or enzyme activity. Immunohistochemical staining of sections of breast tissue identified expression of NAT1 and NAT2 protein in human mammary epithelial cells, but not in the stroma. We also measured the formation of DNA adducts of the heterocyclic amines 2-amino-3-methylimidazo[4,5-f]quinoline (IQ) and 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine in calf thymus DNA after incubation of their promutagenic N-hydroxy metabolites with mammary cytosols prepared from reduction mammoplasty tissue. Experimental observations gained from use of enzyme cofactors and NAT and/or SULT inhibitors on cytosolic enzyme activity, recombinant NAT1 activity and heterocyclic amine-DNA adduct formation suggest that both NAT1 and SULT1A enzymes contribute significantly to the activation of N-hydroxylated heterocyclic amines in mammary tissue. NAT1 mRNA transcript levels were found to be two- to three-fold higher than mRNA transcripts of the NAT2 gene in reduction mammoplasty tissue and mammary epithelial cells. NAT1-specific p-aminobenzoic acid acetylation activity, but not NAT2-specific sulfamethazine acetylation activity, was detectable in mammary cytosols. There was no association apparent between NAT genotype and the levels of NAT mRNA or NAT enzyme activity, or between NAT1 genotype and IQ-DNA adduct formation mediated by mammary cytosols. Western blot analysis of mammary cytosolic protein showed detectable levels of SULT1A1 and SULT1A3.

Single nucleotide polymorphisms, metabolic activation and environmental carcinogenesis: why molecular epidemiologists should think about enzyme expression

This commentary was written to stimulate thoughts on, and consideration of, enzyme expression data in target organs when investigating possible associations between polymorphisms in carcinogen activation enzymes, lifestyle/dietary factors and cancer risk. The lung and breast are taken as examples. There is overwhelming evidence for a genotoxic mechanism in lung cancer development, and compelling evidence for the contribution of genotoxins to breast cancer aetiology. A consistent association has been shown where lung cancer risk is decreased by a G-->A polymorphism in the myeloperoxidase (MPO) gene, which is expressed in neutrophils recruited to the lung after chemical or immunological insults. In the breast, a consistent lack of association has been observed for women who are fast N:-acetyltransferase type 2 (NAT2) acetylators consuming cooked meat. This could be explained by the lack of detectable NAT2-associated sulfamethazine acetylation activity in cytosols prepared from mammary tissue, suggesting a minor contribution to carcinogen activation. The recent identification in mammary cytosols of detectable sulfotransferase isoforms (SULT1A1 and SULT1A3), which have high catalytic efficiency for activating N:-hydroxylated heterocyclic amines (HCAs, mutagens in cooked meat), offers a more important role for these enzymes in the metabolic activation of genotoxins in the breast. The possible contribution of MPO and lactoperoxidase enzymes to carcinogen activation in mammary tissue is also considered. Sulfotransferases and peroxidases have wide substrate specificity in terms of carcinogen activation (HCAs, aromatic amines and polycyclic aromatic hydrocarbons-all present in cooked meat and tobacco smoke) compared with NATs (HCAs and aromatic amines only). For gene-environment interactions, investigations into functional polymorphisms in SULT and peroxidase genes may, therefore, offer new evidence for the involvement of genotoxins in the initiation of carcinogenesis. Identification of the isoforms (if any) of carcinogen activation enzymes that are expressed in the organs of interest will help to determine which genes to investigate in these studies.

Activation of genotoxins to DNA-damaging species in exfoliated breast milk cells

Exfoliated cells, isolated from breast milk samples donated by UK-resident women (n=15), were incubated, either immediately or after culture for 7 days, with one of a series of genotoxins, either in the presence or absence of the DNA-repair inhibitors, hydroxyurea (HU), and cytosine arabinoside (ara-C). The numbers of DNA single-strand breaks induced were then assessed as comet tail length (CTL) (microm) using the alkaline single cell-gel electrophoresis ('Comet') assay; cell viability was measured by trypan blue exclusion. The heterocyclic aromatic amines (HAAs) (2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) (0.4 mM), 3-amino-1,4-dimethyl-5H-pyrido[4,3-b]indole (Trp-P-1) (1.67 mM), 3-amino-1-methyl-5H-pyrido[4,3-b]indole (Trp-P-2) (1.77 mM)), a polycyclic aromatic hydrocarbon (benzo[a]pyrene (B[a]P) (0.36 mM)), a nitro-polycyclic aromatic hydrocarbon (1-nitropyrene (1-NP) (1.84 mM)) and aromatic amines (o-toluidine (0.85 mM), p-chloroaniline (0. 71 mM)) each induced statistically significant (P<0.0001, Mann-Whitney test) increases in median CTLs in breast milk cells from all the donors examined when incubated (30 min, 37 degrees C) in the presence of HU/ara-C. In some cases, these compounds were also active in the absence of the repair inhibitors. There were marked variations in comet formation between donors and between genotoxins. Cell culture appeared to increase the epithelial cell proportion and cultured cells retained their ability to activate genotoxins. The results suggest that breast milk is a valuable source of human mammary cells for the study of the metabolic activation of possible carcinogens.

Metabolic activation of carcinogens and expression of various cytochromes P450 in human prostate tissue

Epidemiological evidence suggests a link between meat consumption and prostate cancer. In this study, benign prostatic hyperplasia tissues, obtained by transurethral resection or radical retropubic prostatectomy from UK-resident individuals (n = 18), were examined for CYP1 expression and for their ability, in short-term organ culture, to metabolically activate carcinogens found in cooked meat. Semi-quantitative RT-PCR analysis of CYP1 expression detected CYP1A2 mRNA transcripts in the prostates of four individuals, as well as mRNA transcripts from CYP1A1 and CYP1B1. The compounds tested for metabolic activation were 2-amino-1-methyl-6-phenylimidazo[4, 5-b]pyridine (PhIP; 500 microM, n = 9) and its metabolite N:-hydroxy PhIP (20 microM, n = 8), 2-amino-3-methylimidazo[4,5-f]quinoline (IQ; 500 microM, n = 6) and benzo[a]pyrene (B[a]P; 50 microM, n = 5). After incubation (PFMR medium, 22 h, 37 degrees C), DNA was isolated from tissue fragments and DNA adducts were detected and quantified by (32)P-postlabelling analysis. DNA adduct formation was detected in all samples incubated with PhIP (mean, adducts per 10(8) nucleotides), N:-hydroxy-PhIP (2736/10(8)) or B[a]P (1/10(8)). IQ-DNA adducts were detected in 5/6 tissues (mean, 1/10(8)). The CYP1 inhibitor alpha-naphthoflavone (10 microM) reduced B[a]P-DNA adduct formation in tissues from two individuals by 96 and 64%, respectively. This pilot study shows that human prostate tissue can metabolically activate 'cooked meat' carcinogens, a process that could contribute to prostate cancer development.

DNA damage in human breast milk cells and its induction by 'early' and 'late' milk extracts

Environmental and dietary factors are thought to be significant in breast cancer aetiology. The alkaline single-cell gel electrophoresis ('Comet') assay was used to examine breast milk cells for DNA damage and to measure the activity of extracts of the milk in causing such damage. UK-resident women were recruited as donors (n = 16) and provided 'early' ( approximately 4 weeks post-partum) and/or 'late' ( approximately 4 months post-partum) milk samples. Cells (79-94% viable, trypan blue exclusion) were either examined immediately for DNA damage or were cultured for 1 week prior to treatment with a breast milk extract. DNA damage in the form of single-strand breaks was quantified as comet tail length (CTL). Cell preparations examined immediately exhibited interindividual variation in median CTL (range 2.0-40.0 microm) with or without the DNA repair inhibitors hydroxyurea (HU) and cytosine arabinoside (ara-C). DNA damage decreased following culture, suggesting either DNA repair or death of DNA-damaged cells. Some donors' breast milk extracts induced DNA damage in their cultured cells and increases in median CTL were significantly greater with HU/ara-C (range 4.0-72.5 microm) than without (range 2.5-27.5 microm). Genotoxicity occurred without cytotoxicity (81-97% viability after treatment). Comparisons between cells and extracts from 'early' and 'late' milk samples did not support the idea of a progressive clearance of genotoxins from mammary lipid during lactation. Donors whose untreated cells contained the most DNA damage tended to yield genotoxic breast milk extracts. Cells isolated from milk activated the rodent mammary carcinogens o-toluidine and 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP). The relevance of genotoxic exposures to breast cancer initiation requires further investigation.