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

Differential expression of NAT1 pharmacogene in hormone receptor positive vs. negative female breast tumors may affect drug treatment

Studies have reported overexpression of NAT1 gene for xenobiotic metabolizing arylamine N-acetyltransferase type 1 in estrogen receptor positive breast tumors, and this association has been linked to patient chemoresistance and response to tamoxifen. We probed the expression of NAT1, using quantitative reverse transcription PCR to screen clinically characterized breast cancer tissue cDNA arrays. Primers detecting all NAT1 alternative transcripts were used, and the protocol and results are reported according to consensus guidelines. The clinical information about 166 tumor samples screened is provided, including tumor stage, estrogen and progesterone receptor status and HER2 expression. NAT1 was found to be significantly (P < 0.001) upregulated in hormone receptor positive vs. negative tumors. No correlation was apparent between NAT1 and tumor stage or HER2 expression. Our findings demonstrate a strong correlation between the expression of NAT1 and steroid hormone receptors in breast tumors, supporting its possible utility as a pharmacogenetic biomarker or drug target. Of the two polymorphic NAT genes, NAT1 is the one primarily expressed in breast tissue, and is subjected to regulation by two differential promoters and more than one polyadenylation signal. Hormonal factors may enhance NAT1 gene expression at the transcriptional or epigenetic level, and tamoxifen has additionally been shown to inhibit NAT1 enzymatic activity. The outcome of tamoxifen treatment is also more favorable in patients with NAT1 overexpressing tumors. The study adds to the growing body of evidence implicating NAT1 in breast cancer and its pharmacological treatment.

Human arylamine N-acetyltransferase 2 genotype-dependent protein expression in cryopreserved human hepatocytes

Human N-acetyltransferases (NAT; EC 2.3.1.5) catalyze the N-acetylation of arylamine and hydrazine drugs and the O-acetylation of N-hydroxylated metabolites of aromatic and heterocyclic amines. Two different isoforms of this protein, N-acetyltransferase 1 (NAT1) and N-acetyltransferase 2 (NAT2), are expressed in human hepatocytes. Both are encoded by a single 870-bp open reading frame that exhibits genetic polymorphisms in human populations. NAT1 and NAT2 share more than 85% gene and protein sequence, making it challenging to produce antibodies with high specificity for NAT1 or NAT2. In the present study, we compared methods for the quantification of immunoreactive NAT1 and NAT2 with seven different antibodies and investigated the relationship of NAT2 genotype to NAT2 mRNA and protein expression in cryopreserved human hepatocytes. Sulfamethazine (NAT2-selective substrate) and NAT2 protein expression differed significantly with NAT2 acetylator genotype (p < 0.0001). NAT2 protein expression and sulfamethazine NAT2 catalytic activity correlated highly across the cryopreserved human hepatocytes of rapid, intermediate, and slow acetylator NAT2 genotypes. In conclusion, our data describe a specific analytical method for the quantification of NAT1 and NAT2 protein expression. We showed that the NAT2 activity in human hepatocytes is directly correlated to expression levels of NAT2 protein but not mRNA.

Functional expression of human arylamine N-acetyltransferase NAT1*10 and NAT1*11 alleles: a mini review

The arylamine N-acetyltransferase (NAT) nomenclature committee assigns functional phenotypes for human arylamine N-acetyltransferase 1 (NAT1) alleles in those instances in which the committee determined a consensus has been achieved in the scientific literature. In the most recent nomenclature update, the committee announced that functional phenotypes for NAT1*10 and NAT1*11 alleles were not provided owing to a lack of consensus. Phenotypic inconsistencies observed among various studies for NAT1*10 and NAT1*11 may be owing to variable allelic expression among different tissues, the limitations of the genotyping assays (which mostly relied on techniques not involving direct DNA sequencing), the differences in recombinant protein expression systems used (bacteria, yeast, and mammalian cell lines) and/or the known inherent instability of human NAT1 protein, which requires very careful handling of native and recombinant cell lysates. Three recent studies provide consistent evidence of the mechanistic basis underlying the functional phenotype of NAT1*10 and NAT1*11 as 'increased-activity' alleles. Some NAT1 variants (e.g. NAT1*14, NAT1*17, and NAT1*22) may be designated as 'decreased-activity' alleles and other NAT1 variants (e.g. NAT1*15 and NAT1*19) may be designated as 'no-activity' alleles compared with the NAT1*4 reference allele. We propose that phenotypic designations as 'rapid' and 'slow' acetylator should be discontinued for NAT1 alleles, although these designations remain very appropriate for NAT2 alleles.

Metabolic Activation of the Cooked Meat Carcinogen 2-Amino-1-Methyl-6-Phenylimidazo[4,5-b]Pyridine in Human Prostate

2-Amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP), an heterocyclic aromatic amine (HAA) formed in cooked meat, is a rodent and possible human prostate carcinogen. Recently, we identified DNA adducts of PhIP in the genome of prostate cancer patients, but adducts of 2-amino-3, 8-dimethylmidazo[4,5-f]quinoxaline (MeIQx) and 2-amino-9 H-pyrido[2,3-b]indole (AαC), other prominent HAAs formed in cooked meats, were not detected. We have investigated the bioactivation of HAAs by Phase I and II enzymes in the human prostate (LNCaP) cell line using cytotoxicity and DNA adducts as endpoints. PhIP, MeIQx, and 2-amino-3-methylimidazo[4,5-f]quinoline, another HAA found in cooked meats, were poorly bioactivated and not toxic. The synthetic genotoxic N-hydroxylated-HAAs were also assayed in LNCaP cells with Phase II enzyme inhibitors. Notably, 2-hydroxy-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (HONH-PhIP), but not other HONH-HAAs, induced cytotoxicity. Moreover, PhIP-DNA adduct formation was 20-fold greater than adducts formed with other HONH-HAAs. Pretreatment of LNCaP cells with mefenamic acid, a specific inhibitor of sulfotransferase (SULT1A1), decreased PhIP-DNA adducts by 25%, whereas (Z)-5-(2'-hydroxybenzylidene)-2-thioxothiazolidin-4-one and pentachlorophenol, inhibitors of SULTs and N-acetyltransferases (NATs), decreased the PhIP-DNA adduct levels by 75%. NATs in cytosolic fractions of LNCaP cells and human prostate catalyzed DNA binding of HONH-PhIP by up to 100-fold greater levels than for SULT and kinase activities. Recombinant NAT2 is catalytically superior to recombinant NAT1 in the bioactivation of HONH-PhIP; however, the extremely low levels of NAT2 activity in prostate suggest that NAT1 may be the major isoform involved in PhIP-DNA damage. Thus, the high susceptibility of LNCaP cells recapitulates the DNA-damaging effect of HONH-PhIP in rodent and human prostate.

Retrospective analysis of estrogen receptor 1 and N‑acetyltransferase gene expression in normal breast tissue, primary breast tumors, and established breast cancer cell lines

The expression levels of estrogen receptor 1 (ESR1), arylamine N‑acetyltransferase 1 (NAT1), and arylamine N‑acetyltransferase 2 (NAT2) are implicated in breast cancer; however, their co-expression profiles in normal breast tissue, primary breast tumors and established breast cancer cell lines are undefined. NAT1 expression is widely reported to be associated with ESR1 expression and is frequently investigated in breast cancer etiology. Furthermore, the NAT2 phenotype has been reported to modify breast cancer risk in molecular epidemiological association studies. Understanding the relationships between the expression levels of these genes is essential to understand their role in breast cancer etiology and treatment. In the present study, NAT1, NAT2 and ESR1 expression data were accessed from repositories of RNA‑Seq data covering 57 breast cancer cell lines, 1,043 primary breast tumors and 99 normal breast tissues. The relationships between gene expression, and between NAT1 activity and RNA expression in breast cancer cell lines were evaluated using non-parametric statistical analyses. Differences in gene expression in each dataset, as well as gene expression differences in normal breast tissue compared to primary breast tumors, and stratification by estrogen receptor status were determined. NAT1 and NAT2 mRNA expression were detected in normal and primary breast tumor tissues; NAT1 expression was much higher than NAT2. NAT1 and ESR1 expression were strongly associated, whereas NAT2 and ESR1 expression were not. Although NAT1 and NAT2 expression were associated, the magnitude was moderate. NAT1, NAT2, and ESR1 expression were increased in primary breast tumor tissue compared with normal breast tissue; however, the magnitude and significance of the differences were lower for NAT2. Analysis of NAT1, NAT2, and ESR1 expression in normal and primary breast tissues and breast cancer cell lines suggested that NAT1 and NAT2 expression are regulated by distinctive mechanisms, whereas NAT1 and ESR1 expression may have overlapping regulation. Defining these relationships is important for future investigations into breast cancer prevention.

2-amino-1-methyl-6-phenylimidazo(4,5-b) pyridine (PhIP) induces gene expression changes in JAK/STAT and MAPK pathways related to inflammation, diabetes and cancer

Background

2-amino-1-methyl-6-phenylimidazo(4,5-b)pyridine (PhIP), a heterocyclic aromatic amine (HCA) formed in meat that is cooked at high temperatures and then ingested, can potentially be retained in human adipose tissues.

Methods

To determine if PhIP is bioactive in the adipocyte, we exposed a human adipocyte cell line,HepG2 and Caco-2 cells to low dose PhIP. Uptake and retention of PhIP was determined and cytotoxicity was assessed by the TUNEL assay. Relative expression of PhIP-activating genes (CYP1A1, CYP1A2, SULT1A1 and UGT1A1) was determined by RT-PCR and global expression changes were also examined.

Results

The percent retention of 0.1 μCi [¹⁴C]-PhIP over a 24 h period was significantly higher in the adipocyte than the HepG2 (p = 0.0001) and Caco-2 (p = 0.0007) cell lines. Cytotoxicity rates were 14.4 and 2.6 % higher compared to controls in Caco-2 and HepG2 cells (p < 0.001 and 0.054, respectively); no significant differences were detected in adipocyte cells (p = 0.18). Caco-2 and HepG2 cells, respectively, had significantly higher basal expression of CYP1A1 (p = 0.001, p = 0.003), SULT1A1 (p = 0.04, p < 0.001) and UGT1A1 (p < 0.001, p = 0.01) compared to the adipocyte. Exposure to 5nM PhIP did not significantly induce expression of these genes in any of the cell lines. Global gene expression analysis of mature adipocytes exposed to 5nM PhIP for 72 h resulted in statistically significant changes in 8 genes (ANGPTL2, CD14, CIDEA, EGR1, FOS, IGFBP5, PALM and PSAT1). Gene-gene interaction and pathway analysis indicates that PhIP modulates genes controlled by the STAT3 transcriptional factor and initiates leptin signaling via the JAK/STAT and MAPK pathway cascades. Early growth response 1 (EGR1) and prostaglandin synthase 2 (COX-2) were down-regulated via c-Fos, while insulin binding protein 5 (IBP5) was up regulated. Expression of transcription factors (ANGPTL2, HP, LEP, SAA1, SAA2), genes related to inflammation (SAA1, LEP), diabetes (IGFBP5) and cancer risk (SAA2) were also elevated upon exposure to 5 nM PhIP..

Conclusions

PhIP mediates gene expression changes within the adipocyte, and the pathways most affected are related to cancer and other chronic diseases. Further studies are needed on the relationship between dietary carcinogens such as PhIP with cancer, obesity and diabetes.

Electronic supplementary material

The online version of this article (doi:10.1186/s12986-016-0111-0) contains supplementary material, which is available to authorized users.

Arylamine N-acetyltransferases: from drug metabolism and pharmacogenetics to drug discovery

Arylamine N-acetyltransferases (NATs) are polymorphic drug-metabolizing enzymes, acetylating arylamine carcinogens and drugs including hydralazine and sulphonamides. The slow NAT phenotype increases susceptibility to hydralazine and isoniazid toxicity and to occupational bladder cancer. The two polymorphic human NAT loci show linkage disequilibrium. All mammalian Nat genes have an intronless open reading frame and non-coding exons. The human gene products NAT1 and NAT2 have distinct substrate specificities: NAT2 acetylates hydralazine and human NAT1 acetylates p-aminosalicylate (p-AS) and the folate catabolite para-aminobenzoylglutamate (p-abaglu). Human NAT2 is mainly in liver and gut. Human NAT1 and its murine homologue are in many adult tissues and in early embryos. Human NAT1 is strongly expressed in oestrogen receptor-positive breast cancer and may contribute to folate and acetyl CoA homeostasis. NAT enzymes act through a catalytic triad of Cys, His and Asp with the architecture of the active site-modulating specificity. Polymorphisms may cause unfolded protein. The C-terminus helps bind acetyl CoA and differs among NATs including prokaryotic homologues. NAT in Salmonella typhimurium supports carcinogen activation and NAT in mycobacteria metabolizes isoniazid with polymorphism a minor factor in isoniazid resistance. Importantly, nat is in a gene cluster essential for Mycobacterium tuberculosis survival inside macrophages. NAT inhibitors are a starting point for novel anti-tuberculosis drugs. Human NAT1-specific inhibitors may act in biomarker detection in breast cancer and in cancer therapy. NAT inhibitors for co-administration with 5-aminosalicylate (5-AS) in inflammatory bowel disease has prompted ongoing investigations of azoreductases in gut bacteria which release 5-AS from prodrugs including balsalazide.

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.

Expression of sulfotransferases and sulfatases in human breast cancer: impact on resveratrol metabolism

Resveratrol is a naturally occurring anticancer compound present in grapes and wine that undergoes pronounced metabolism in human intestine and liver. In order to determine whether resveratrol is also bio-transformed in human breast carcinoma, metabolism experiments were conducted in breast tumor and adjacent non-tumorous specimens from 13 patients. Resveratrol was metabolized in cytosolic tissue fractions to resveratrol-3-O-sulfate: the formation rates were up to 33.5-fold higher in cancer samples than in peritumoral tissue. Further quantitative real-time RT-PCR analysis revealed similar expression of sulfotransferases SULT1A2, 1A3, and 1E1 in the paired control and tumor tissues. Sulfotransferase SULT1A1 expression was below the detection limit in all samples. Interestingly, mRNA expression of steroid sulfatase STS, but not of arylsulfatases ARS-A and ARS-B, was significantly higher (p<0.0017) in non-malignant specimens than in tumor tissue samples, which might explain the higher resveratrol-3-O-sulfate concentrations in breast cancer specimens. Cellular localization of SULT1A3 and STS was also assessed by indirect immunofluorescence on paraffin-embedded sections from control and malignant breast tissue clearly showing a correlation of qRT-PCR data with protein expression of these two enzymes. Our data elucidate the metabolism of resveratrol in malignant and non-malignant breast tissue, which must be considered in humans after oral uptake of dietary resveratrol as a chemopreventive agent.

Expression and functional activities of selected sulfotransferase isoforms in BeWo cells and primary cytotrophoblast cells

Several cytosolic sulfotransferase enzyme isoforms are functional in placenta but there is limited information available on the utility of cultured trophoblast cells for studying sulfation. The trophoblast cell layer constitutes the rate-determining barrier for trans-placental transfer. The objective of this work was to examine the mRNA expression and enzyme activities of four sulfotransferase isoforms reported to be functional in human placenta (SULT1A1, SULT1A3, SULT1E1, and SULT2A1) in primary cytotrophoblast cells and the trophoblast-like BeWo cell line. Reverse transcription polymerase chain reaction (RT-PCR) was performed to determine mRNA expression. Enzyme activities were assessed using the following substrates: 4-nitrophenol for SULT1A1, dopamine for SULT1A3, 17beta-estradiol for SULT1E1, and dehydroepiandrosterone for SULT2A1. For 4-nitrophenol and dopamine sulfation, apparent K(m) values, response to inhibitors (2,6-dichloro-4-nitrophenol and sodium chloride), and thermal stability profiles indicated that 4-nitrophenol and dopamine sulfation in BeWo cells were being mediated by SULT1A1 and SULT1A3, respectively. SULT1A1 and SULT1A3 were also functional in the cytotrophoblast cells. Both at the protein and at the mRNA levels, SULT1A1 was more abundant in BeWo cells in comparison to the primary cytotrophoblast cells. SULT1E1 and SULT2A1 mRNA were not detected in the cytotrophoblasts. SULT1E1 mRNA was weakly expressed in BeWo but there was negligible functional activity. Although SULT2A1 mRNA was abundantly expressed in BeWo, Western blot and enzyme activities revealed that the protein is not expressed in BeWo cells. The results suggest that the BeWo cells and the cytotrophoblast cells can be used to examine the roles of SULT1A1 and SULT1A3 in placental metabolism.

Dietary intake of meat and meat-derived heterocyclic aromatic amines and their correlation with DNA adducts in female breast tissue

It was the aim of this study to examine the association of the consumption of meat in general, meat prepared by different cooking methods and the dietary intake of heterocyclic aromatic amines (HCA) with the level of DNA adducts in the breast tissue of women undergoing reduction mammoplasty. Dietary intake of meat and HCA were assessed via questionnaire in 44 women undergoing reduction mammoplasty. DNA adduct analysis in breast tissue was performed by (32)P-postlabelling analysis. Spearman rank correlation coefficients (r) were calculated to examine the association of meat consumption and dietary HCA intake with tissue DNA adduct levels. A median DNA adduct level of 18.45 (interquartile range 12.81-25.65) per 10(9) nucleotides in breast tissue was observed; median HCA intake was 40.43 ng/day (interquartile range 19.55-102.33 ng/day). Total HCA intake (r = 0.33, P = 0.03), consumption of fried meat (r = 0.39, P = 0.01), beef (r = 0.32, P = 0.03) and processed meat (r = 0.51, P = 0.0004) were statistically significantly correlated with the level of DNA adducts in breast tissue. The detected DNA adducts could not be confirmed to be specific HCA-derived DNA adducts by comparison with external standards, using the (32)P-postlabelling assay. We observed strong correlations of dietary HCA intake and consumption of fried and processed meat with DNA adduct levels in breast tissue of 44 women. Since the detected DNA adducts were not necessarily specific only for HCA, it is possible that HCA intake is a surrogate of other genotoxic substances, such as polycyclic aromatic hydrocarbons, in meat prepared at high temperatures.

ArylamineN-Acetyltransferases: What We Learn from Genes and Genomes

Arylamine N-acetyltransferases (NATs) are phase II xenobiotic metabolizing enzymes, catalyzing acetyl-CoA-dependent N- and O-acetylation reactions. All NATs have a conserved cysteine protease-like Cys-His-Asp catalytic triad inside their active site cleft. Other residues determine substrate specificity, while the C-terminus may control hydrolysis of acetyl-CoA during acetyltransfer. Prokaryotic NAT-like coding sequences are found in >30 bacterial genomes, including representatives of Actinobacteria, Firmicutes and Proteobacteria. Of special interest are the nat genes of TB-causing Mycobacteria, since their protein products inactivate the anti-tubercular drug isoniazid. Targeted inactivation of mycobacterial nat leads to impaired mycolic acid synthesis, cell wall damage and growth retardation. In eukaryotes, genes for NAT are found in the genomes of certain fungi and all examined vertebrates, with the exception of canids. Humans have two NAT isoenzymes, encoded by highly polymorphic genes on chromosome 8p22. Syntenic regions in rodent genomes harbour two Nat loci, which are functionally equivalent to the human NAT genes, as well as an adjacent third locus with no known function. Vertebrate genes for NAT invariably have a complex structure, with one or more non-coding exons located upstream of a single, intronless coding region. Ubiquitously expressed transcripts of human NAT1 and its orthologue, murine Nat2, are initiated from promoters with conserved Sp1 elements. However, in humans, additional tissue-specific NAT transcripts may be expressed from alternative promoters and subjected to differential splicing. Laboratory animals have been widely used as models to study the effects of NAT polymorphism. Recently generated knockout mice have normal phenotypes, suggesting no crucial endogenous role for NAT. However, these strains will be useful for understanding the involvement of NAT in carcinogenesis, an area extensively investigated by epidemiologists, often with ambiguous results.

Mouse N-acetyltransferase type 2, the homologue of human N-acetyltransferase type 1

There is increasing evidence that human arylamine N-acetyltransferase type 1 (NAT1, EC 2.3.1.5), although first identified as a homologue of a drug-metabolising enzyme, appears to be a marker in human oestrogen receptor positive breast cancer. Mouse Nat2 is the mouse equivalent of human NAT1. The development of mouse models of breast cancer is important, and it is essential to explore the biological role of mouse Nat2. We have therefore produced mouse Nat2 as a recombinant protein and have investigated its substrate specificity profile in comparison with human NAT1. In addition, we have tested the effects of inhibitors on mouse Nat2, including compounds which are endogenous and exogenous steroids. We show that tamoxifen, genistein and diethylstilbestrol inhibit mouse Nat2. The steroid analogue, bisphenol A, also inhibits mouse Nat2 enzymic activity and is shown by NMR spectroscopy, through shifts in proton peaks, to bind close to the active site. A three-dimensional structure for human NAT1 has recently been released, and we have used this crystal structure to generate a model of the mouse Nat2 structure. We propose that a conformational change in the structure is required in order for ligands to bind to the active site of the protein.

Arylamine N-Acetyltransferases: Characterization of the Substrate Specificities and Molecular Interactions of Environmental Arylamines with Human NAT1 and NAT2

Arylamine N-acetyltransferases (NATs) are phase II xenobiotic metabolism enzymes that catalyze the detoxification of arylamines by N-acetylation and the bioactivation of N-arylhydroxylamines by O-acetylation. Endogenous and recombinant mammalian NATs with high specific activities are difficult to obtain in substantial quantities and in a state of homogeneity. This paper describes the overexpression of human wild-type NAT2 as a dihydrofolate reductase fusion protein containing a TEV protease-sensitive linker. Treatment of the partially purified fusion protein with TEV protease, followed by chromatographic purification, afforded 2.8 mg of homogeneous NAT2 from 2 L of cell culture. The kinetic specificity constants ( k cat/ K m) for N-acetylation of arylamine environmental contaminants, some of which are associated with bladder cancer risk, were determined with NAT2 and NAT1. The NAT1/NAT2 ratio of the specificity constants varied almost 1000-fold for monosubstituted and disubstituted alkylanilines containing methyl and ethyl ring substituents. 2-Alkyl substituents depressed N-acetylation rates but were more detrimental to catalysis by NAT1 than by NAT2. 3-Alkyl groups caused substrates to be preferentially N-acetylated by NAT2, and both 4-methyl- and 4-ethylaniline were better substrates for NAT1 than NAT2. NMR-based models were used to analyze the NAT binding site interactions of the alkylanilines. The selectivity of NAT1 for acetylation of 4-alkylanilines appears to be due to binding of the substituents to V216, which is replaced by S216 in NAT2. The contribution of 3-alkyl substituents to NAT2 substrate selectivity is attributed to multiple bonding interactions with F93, whereas a single bonding interaction occurs with V93 in NAT1. Unfavorable steric clashes between 2-methyl substituents and F125 of NAT1 may account for the selective NAT2-mediated N-acetylation of 2-alkylanilines; F125 is replaced by S125 in NAT2. These results provide insight into the structural basis for the substrate specificity of two NATs that play major roles in the biotransformation of genotoxic environmental arylamines.

Functional analysis of the human N-acetyltransferase 1 major promoter: quantitation of tissue expression and identification of critical sequence elements

Arylamine N-acetyltransferase 1 (NAT1) plays an important role in the biotransformation of xenobiotics, and genetic variants have been implicated in susceptibility to cancer and birth defects. A specific and quantitative reverse transcription-polymerase chain reaction assay for transcription from the major NAT1 promoter detected high expression with limited variability in human tissues. A 213-base pair (bp) minimal promoter was identified by transfection of luciferase reporter constructs into MCF-7 and HepG2 cell lines. Alignment of the 213-bp region with paralogous and orthologous promoters revealed two conserved region segments, one of which overlaps a 16-bp perfect palindrome. Transfection of luciferase constructs with artificial mutations in the minimal promoter defined two sites important for promoter function. One of these sites included a close match to the Sp1 transcription factor binding consensus sequence. Electrophoretic mobility shift assays (EMSAs), followed by competitive and supershift analyses, confirmed the Sp1 binding. Mutation of the highly conserved portion of the 16-bp palindrome reduced promoter activity more than 3-fold, and an EMSA shift was detected with an oligonucleotide, 200L29, which spans this segment. The 200L29 EMSA shift could not be competed by consensus Sp1 or AP-2 oligonucleotides, and may represent binding of a transcription factor that is common to N-acetyltransferase genes in humans and other species.

Functional characterization of single-nucleotide polymorphisms and haplotypes of human N-acetyltransferase 2

Human N-acetyltransferase 2 (NAT2) is polymorphic in humans and may associate with cancer risk by modifying individual susceptibility to cancers from carcinogen exposure. Since molecular epidemiological studies investigating these associations usually include determining NAT2 single-nucleotide polymorphisms (SNPs), haplotypes or genotypes, their conclusions can be compromised by the uncertainty of genotype-phenotype relationships. We characterized NAT2 SNPs and haplotypes by cloning and expressing recombinant NAT2 allozymes in mammalian cells. The reference and variant recombinant NAT2 allozymes were characterized for arylamine N-acetylation and O-acetylation of N-hydroxy-arylamines. SNPs and haplotypes that conferred reduced enzymatic activity did so by reducing NAT2 protein without changing NAT2 mRNA levels. Among SNPs that reduced catalytic activity, G191A (R64Q), G590A (R197Q) and G857A (G286E) reduced protein half-life but T341C (I114T), G499A (E167K) and A411T (L137F) did not. G857A (G286E) and the major haplotype possessing this SNP (NAT2 7B) altered the affinity to both substrate and cofactor acetyl coenzyme A, resulting in reduced catalytic activity toward some substrates but not others. Our results suggest that coding region SNPs confer slow acetylator phenotype by multiple mechanisms that also may vary with arylamine exposures.

Variable sulfation of dietary polyphenols by recombinant human sulfotransferase (SULT) 1A1 genetic variants and SULT1E1

Human cytosolic sulfotransferases (SULTs) catalyze the sulfate conjugation of several important endo- and xenobiotics. Among the superfamily of SULT enzymes, SULT1A1 catalyzes the sulfation of small planar phenolic compounds, whereas SULT1E1 has a major role in estrogen conjugation. The human SULT1A1 gene has common single nucleotide polymorphisms that define three allozymes, SULT1A1*1, *2, and *3. The enzyme kinetics of SULT1A1 allozymes and SULT1E1 were characterized for the polyphenolic substrates apigenin, chrysin, epicatechin, quercetin, and resveratrol. Purified recombinant SULT proteins were generated in a baculoviral-insect cell system, and incubated in vitro with each substrate to determine catalytic activity. The effect of polyphenol sulfation was examined in mammalian cell lines stably expressing SULT1E1. For all polyphenols investigated, "normal-activity" SULT1A1*1 allozyme had significantly greater Vmax estimates than SULT1E1, and allele-specific differences in SULT1A1-mediated sulfation were observed. The polymorphic SULT1A1*2 allozyme exhibited low activity toward apigenin, epicatechin, and resveratrol. SULT1A1*1 and *3 acted as normal-activity allozymes for these substrates. Altered cellular proliferation was observed in MCF-7 cells stably expressing SULT1E1 upon treatment with chrysin, quercetin, or resveratrol, thus suggesting inactivation of these compounds by SULT1E1. These results suggest an important role for SULT isozymes and their pharmacogenetics in polyphenol disposition.

Reductive detoxification of arylhydroxylamine carcinogens by human NADH cytochrome b5 reductase and cytochrome b5

Heterocyclic and aromatic amine carcinogens are thought to lead to tumor initiation via the formation of DNA adducts, and bioactivation to arylhydroxylamine metabolites is necessary for reactivity with DNA. Carcinogenic arylhydroxylamine metabolites are cleared by a microsomal, NADH-dependent, oxygen-insensitive reduction pathway in humans, which may be a source of interindividual variability in response to aromatic amine carcinogens. The purpose of this study was to characterize the identity of this reduction pathway in human liver. On the basis of our findings with structurally similar arylhydroxylamine metabolites of therapeutic drugs, we hypothesized that the reductive detoxification of arylhydroxylamine carcinogens was catalyzed by NADH cytochrome b5 reductase (b5R) and cytochrome b5 (cyt b5). We found that reduction of the carcinogenic hydroxylamines of the aromatic amine 4-aminobiphenyl (4-ABP; found in cigarette smoke) and the heterocyclic amine 2-amino-1-methyl-6-phenylimidazo [4,5-b] pyridine (PhIP; found in grilled meats) was indeed catalyzed by a purified system containing only human b5R and cyt b5. Specific activities were 56-346-fold higher in the purified system as compared to human liver microsomes (HLM), with similar Michaelis-Menten constants (K(m) values) in both systems. The stoichiometry for b5R and cyt b5 that yielded the highest activity in the purified system was also similar to that found in native HLM ( approximately 1:8 to 1:10). Polyclonal antisera to either b5R or cyt b5 significantly inhibited N-hydroxy-4-aminobiphenyl (NHOH-4-ABP) reduction by 95 and 89%, respectively, and immunoreactive cyt b5 protein content in individual HLM was significantly correlated with individual reduction of both NHOH-4-ABP and N-hydroxy-PhIP (NHOH-PhIP). Finally, titration of HLM into the purified b5R/cyt b5 system did not enhance the efficiency of reduction activity. We conclude that b5R and cyt b5 are together solely capable of the reduction of arylhydroxylamine carcinogens, and we further hypothesize that this pathway may be a source of individual variability with respect to cancer susceptibility following 4-ABP or PhIP exposure.

Arylamine N-acetyltransferase I

Arylamine N-acetyltransferase I (NAT1) is a phase II enzyme that acetylates a wide range of arylamine and hydrazine substrates. The NAT1 gene is located on chromosome 8 and shares homology to NAT genes found in most mammalian species. Gene expression occurs from at least two promoters and a number of tissue-specific transcripts have been identified. The gene is polymorphic with most mutations identified to date producing an unstable protein that is subject to polyubiquitination. The NAT1 protein contains a catalytic triad similar to a number of cysteine proteases and transglutaminases. NAT1 is widely distributed in the body, but the only endogenous substrate identified to date is the folate catabolite p-aminobenzoylglutamate. Recent links between NAT1 genotypes and susceptibility to spina bifida suggests that the enzyme has an important role in folate homeostasis.

The expression of xenobiotic-metabolizing enzymes in human prostate and in prostate epithelial cells (PECs) derived from primary cultures

BACKGROUND

Dietary heterocyclic amines (HCAs) are carcinogenic in rodent prostate requiring activation by enzymes such as cytochrome P450 (CYP) and N-acetyltransferase (NAT).

METHODS

We investigated by Western blotting and immunohistochemistry the expression of CYP1A1, CYP1A2, and NAT1 in human prostate and in prostate epithelial cells (PECs) derived from primary cultures and tested their ability to activate the dietary carcinogen 2-amino-3-methylimidazo[4,5-f]quinoline (IQ) and its N-hydroxy metabolite (N-OH-IQ) to DNA-damaging moieties.

RESULTS

Western blotting identified CYP1A1, CYP1A2, and NAT1. Immunohistochemistry localized NAT1 to the cytoplasm of PECs. Inter-individual variation was observed in the expression levels of CYP1A1, 1A2, and NAT1 (11, 75, and 35-fold, respectively). PECs expressed CYP1A1 and NAT1 but not CYP1A2. When incubated with IQ or N-OH-IQ, PECs formed DNA adducts indicating their ability to metabolically activate these compounds.

CONCLUSIONS

Prostate cells possess the capacity to activate dietary carcinogens. PECs may provide a useful model system to study their role in prostate carcinogenesis.

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.

Cytochrome P450 expression and metabolic activation of cooked food mutagen 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) in MCF10A breast epithelial cells

The cytochrome P450 expression profile was determined in the MCF10A human breast epithelial cell line, as was the ability of this cell line to catalyze the bioactivation of the cooked food mutagen, 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP). Using non-quantitative reverse transcription-polymerase chain reaction (RT-PCR), transcripts for CYP1B1, CYP2J2, CYP2R1, CYP2U1, CYP2W1, CYP4B1, CYP4F, CYP4V2, CYP4X1 and CYP4Z1 were detected in both sub-confluent and confluent MCF10A cells. By contrast, CYP1A2 mRNA was detected only in confluent MCF10A cells, while CYP1A1, CYP2S1 and CYP2F1 were detected predominantly or exclusively in sub-confluent cultures. 2,3,7,8-Tetrachlorodibenzo-p-dioxin treatment of confluent MCF10A cells markedly induced microsomal ethoxyresorufin O-deethylase activity and CYP1A1, CYP1A2 and CYP1B1 mRNA levels, as determined by real-time RT-PCR, while treatment with 10(-4)M PhIP had little effect on these P450 transcript levels. Treatment of confluent MCF10A cells with PhIP (10(-4)M) for 24, 48 or 72 h produced time-dependent increases in the amounts of DNA adducts, as measured by (32)P-post-labeling. These results indicate that multiple P450s, including those known to catalyze PhIP N-oxidation, are expressed in MCF10A cells, and that this non-neoplastic human breast epithelial cell line contains sufficient enzymatic machinery to support PhIP bioactivation and generate DNA damage.

Integration of hepatic drug transporters and phase II metabolizing enzymes: Mechanisms of hepatic excretion of sulfate, glucuronide, and glutathione metabolites

The liver is the primary site of drug metabolism in the body. Typically, metabolic conversion of a drug results in inactivation, detoxification, and enhanced likelihood for excretion in urine or feces. Sulfation, glucuronidation, and glutathione conjugation represent the three most prevalent classes of phase II metabolism, which may occur directly on the parent compounds that contain appropriate structural motifs, or, as is usually the case, on functional groups added or exposed by phase I oxidation. These three conjugation reactions increase the molecular weight and water solubility of the compound, in addition to adding a negative charge to the molecule. As a result of these changes in the physicochemical properties, phase II conjugates tend to have very poor membrane permeability, and necessitate carrier-mediated transport for biliary or hepatic basolateral excretion into sinusoidal blood for eventual excretion into urine. This review summarizes sulfation, glucuronidation, and glutathione conjugation reactions, as well as recent progress in elucidating the hepatic transport mechanisms responsible for the excretion of these conjugates from the liver. The discussion focuses on alterations of metabolism and transport by chemical modulators, and disease states, as well as pharmacodynamic and toxicological implications of hepatic metabolism and/or transport modulation for certain active phase II conjugates. A brief discussion of issues that must be considered in the design and interpretation of phase II metabolite transport studies follows.

Sulfotransferase (SULT) 1A1 Polymorphic Variants *1, *2, and *3 Are Associated with Altered Enzymatic Activity, Cellular Phenotype, and Protein Degradation

The superfamily of sulfotransferase (SULT) enzymes catalyzes the sulfate conjugation of several pharmacologically important endo- and xenobiotics. SULT1A1 catalyzes the sulfation of small planar phenols such as neurotransmitters, steroid hormones, acetaminophen, and p-nitrophenol (PNP). Genetic polymorphisms in the human SULT1A1 gene define three alleles, SULT1A1*1, *2, and *3. The enzyme activities of the SULT1A1 allozymes were studied with a variety of substrates, including PNP, 17beta-estradiol, 2-methoxyestradiol, catecholestrogens, the antiestrogen 4-hydroxytamoxifen (OHT), and dietary flavonoids. Using purified recombinant SULT1A1 protein, marked differences in *1, *2, and *3 activity toward every substrate studied were noted. Substrate inhibition was observed for most substrates. In general, the trend in V(max) estimates was *1 > *3 > *2; however, V(max)/K(m) estimate trends varied with substrate. In MCF-7 cells stably expressing either SULT1A1*1 or *2, the antiestrogenic response to OHT was found to be allele-specific: the cells expressing *2 exhibited a better antiproliferative response. The intracellular stability of the *1 and *2 allozymes was examined in insect as well as mammalian cells. The SULT1A1*2 protein had a shorter half-life than the *1 protein. In addition, the *2 protein was ubiquitinated to a greater extent than *1, suggesting increased degradation via a proteasome pathway. The results of this study suggest marked differences in activity of polymorphic SULT1A1 variants, including SULT1A1*3, toward a variety of substrates. These differences are potentially critical for understanding interindividual variability in drug response and toxicity, as well as cancer risk and incidence.

Structural Analysis of the Genes for Human Arylamine N-Acetyltransferases and Characterisation of Alternative Transcripts

Arylamine N-acetyltransferases are polymorphic drug-metabolising enzymes. The human isoforms, NAT1 and NAT2, are encoded by two genes with intronless coding regions. Human NAT1 protein is found in many tissues, unlike NAT2 which is present predominantly in the intestine and liver. We describe the exon-intron structure of the human NAT genes by analysing data from genomic databases. Comparison of expressed sequence tags, matching NAT gene sequences, with the sequence of human chromosome 8 implied the presence of 8 non-coding exons located 51.5, 51.4, 12.3, 11.9, 10.8, 9.6, 5.2 and 2.6 kb upstream of the single coding exon of the NAT1 gene. A number of expressed sequence tags also indicated transcription initiation from the upstream region adjacent to the NAT1 coding exon, consistent with earlier studies. The NAT2 gene consists of one previously described non-coding and one coding exon, located 8.6 kb apart. These findings were also confirmed by RT-PCR, using cDNA from heart, brain, placenta, lung, liver, skeletal muscle, kidney and pancreas. Alternatively spliced NAT1 transcripts were found in all tissues. Transcription of the NAT2 gene was also detected in these tissues and was demonstrated to start either from the non-coding exon or from immediately upstream of the coding exon. Comparison of the RT-PCR products provided an initial estimate of the relative amounts of the different NAT transcripts expressed in each tissue. Finally, both expressed sequence tag analysis and RT-PCR demonstrated the presence of two differentially utilised polyadenylation signals for NAT1 and NAT2, located about 0.2 and 0.3 kb downstream of the coding region of each gene.

Interspecies metabolism of heterocyclic aromatic amines and the uncertainties in extrapolation of animal toxicity data for human risk assessment

Heterocyclic aromatic amines (HAAs) are potent bacterial mutagens that are formed in cooked meats, tobacco smokes condensate, and diesel exhaust. Many HAAs are carcinogenic in experimental animal models. Because of their wide-spread occurrence in the diet and environment, HAAs may contribute to some common types of human cancers. The extrapolation of animal toxicity data on HAAs to asses human health risk has many uncertainties, which can lead to tenuous risk assessment estimates. Perhaps the most critical and variable parameters in interspecies extrapolation are the effects of dose, species differences in catalytic activities of xenobiotic metabolism enzymes (XMEs), human XME polymorphisms that lead to interindividual differences in carcinogen metabolism and dietary constituents that may either augment or diminish the carcinogenic potency of these genotoxins. The impact of these parameters on the metabolism and toxicological properties of HAAS and uncertainties in extrapolation of animal toxicity data for human risk assessment are presented in this article.

SULT1A1 genotype, active and passive smoking, and breast cancer risk by age 50 years in a German case-control study

INTRODUCTION

Sulfotransferase 1A1 (encoded by SULT1A1) is involved in the metabolism of procarcinogens such as heterocyclic amines and polycyclic aromatic hydrocarbons, both of which are present in tobacco smoke. We recently reported a differential effect of N-acetyltransferase (NAT) 2 genotype on the association between active and passive smoking and breast cancer. Additional investigation of a common SULT1A1 genetic polymorphism associated with reduced enzyme activity and stability might therefore provide deeper insight into the modification of breast cancer susceptibility.

METHODS

We conducted a population-based case-control study in Germany. A total of 419 patients who had developed breast cancer by age 50 years and 884 age-matched control individuals, for whom risk factor information and detailed smoking history were available, were included in the analysis. Genotyping was performed using a fluorescence-based melting curve analysis method. Multivariate logistic regression analysis was used to estimate breast cancer risk associated with the SULT1A1 Arg213His polymorphism alone and in combination with NAT2 genotype in relation to smoking.

RESULTS

The overall risk for breast cancer in women who were carriers of at least one SULT1A1*2 allele was not significantly different from that for women with the SULT1A1*1/*1 genotype (adjusted odds ratio 0.83, 95% confidence interval 0.66-1.06). Risk for breast cancer with respect to several smoking variables did not differ substantially between carriers of the *2 allele and noncarriers. However, among NAT2 fast acetylators, the odds ratio associated with passive smoking only (3.23, 95% confidence interval 1.05-9.92) was elevated in homozygous carriers of the SULT1A1*1 allele but not in carriers of the SULT1A1*2 allele (odds ratio 1.28, 95% confidence interval 0.50-3.31).

CONCLUSION

We found no evidence that the SULT1A1 genotype in itself modifies breast cancer risk associated with smoking in women up to age 50 years. In combination with NAT2 fast acetylator status, however, the SULT1A1*1/*1 genotype might increase breast cancer risk in women exposed to tobacco smoke.

Single-track sequencing for genotyping of multiple SNPs in the N-acetyltransferase 1 (NAT1) gene

Background

Fast, cheap and reliable methods are needed to identify large populations, which may be at risk in relation to environmental exposure. Polymorphisms in NAT1 (N-acetyl transferase) may be suitable markers to identify individuals at risk.

Results

A strategy allowing to address simultaneously 24 various genetic variants in the NAT1 gene using the single sequencing reaction method on the same PCR product is described. A modified automated DNA sequencing using only one of the sequence terminators was used to genotype PCR products in single-track sequencing reactions of NAT1 and was shown to be universal for both DNA sequencing using labeled primers and labeled nucleotides. By this method we detected known SNPs at site T640G, which confers the NAT1*11 allele with frequency of 0.036, further T1088A and C1095A with frequency of 0.172 and 0.188, respectively and a deletion of TAATAATAA in the poly A signal area with a frequency 0.031. All observed frequencies were in Hardy Weinberg equilibrium and comparable to those in Caucasian population. The single-track signatures of the variant genotypes were verified on samples previously genotyped by RLFP.

Conclusions

The method could be of great help to scientists in the field of molecular epidemiology of screening of large populations for known informative biomarkers of susceptibility, such as NAT1.

Genetic Variants of the Sulfotransferase 1A1 and Breast Cancer Risk

Sulfotransferase 1A1 (SULT1A1), also designated as phenol-preferring sulfotransferase, is involved in the bioactivation and detoxification of a variety of potential carcinogens, including iodothyronines, hydroxylated aromatic amines, and phenolic xenobiotics. A common arginine (R) to histidine (H) polymorphism at amino acid position 213 influences SULT1A1 activity and has been suggested as risk factor for a different types of cancers. To investigate the role of this polymorphism for breast cancer risk, SULT1A1 genotype was determined in 500 women with clinically verified breast cancer and 500 female age-matched healthy control subjects. Frequencies of heterozygous (controls: 42.5% patients: 50.2%) or homozygous (controls: 12.6%; patients: 9.4%) carriers of the 213H variant were not significantly different between groups. The SULT1A1 genotype was furthermore not associated with tumor size, histological grading, estrogen or progesterone receptor status and age at diagnosis. The SULT1A1 213H variant was associated with the presence of lymph node metastases (p = 0.002). We conclude that the SULT1A1 R213H polymorphism is not a general risk factor for breast cancer, but may be involved in lymph node metastazing in breast cancer patients.

Relationship between intratumoral expression of genes coding for xenobiotic-metabolizing enzymes and benefit from adjuvant tamoxifen in estrogen receptor alpha-positive postmenopausal breast carcinoma

INTRODUCTION

Little is known of the function and clinical significance of intratumoral dysregulation of xenobiotic-metabolizing enzyme expression in breast cancer. One molecular mechanism proposed to explain tamoxifen resistance is altered tamoxifen metabolism and bioavailability.

METHODS

To test this hypothesis, we used real-time quantitative RT-PCR to quantify the mRNA expression of a large panel of genes coding for the major xenobiotic-metabolizing enzymes (12 phase I enzymes, 12 phase II enzymes and three members of the ABC transporter family) in a small series of normal breast (and liver) tissues, and in estrogen receptor alpha (ERalpha)-negative and ERalpha-positive breast tumors. Relevant genes were further investigated in a well-defined cohort of 97 ERalpha-positive postmenopausal breast cancer patients treated with primary surgery followed by adjuvant tamoxifen alone.

RESULTS

Seven of the 27 genes showed very weak or undetectable expression in both normal and tumoral breast tissues. Among the 20 remaining genes, seven genes (CYP2A6, CYP2B6, FMO5, NAT1, SULT2B1, GSTM3 and ABCC11) showed significantly higher mRNA levels in ERalpha-positive breast tumors than in normal breast tissue, or showed higher mRNA levels in ERalpha-positive breast tumors than in ERalpha-negative breast tumors. In the 97 ERalpha-positive breast tumor series, most alterations of these seven genes corresponded to upregulations as compared with normal breast tissue, with an incidence ranging from 25% (CYP2A6) to 79% (NAT1). Downregulation was rare. CYP2A6, CYP2B6, FMO5 and NAT1 emerged as new putative ERalpha-responsive genes in human breast cancer. Relapse-free survival was longer among patients with FMO5-overexpressing tumors or NAT1-overexpressing tumors (P = 0.0066 and P = 0.000052, respectively), but only NAT1 status retained prognostic significance in Cox multivariate regression analysis (P = 0.0013).

CONCLUSIONS

Taken together, these data point to a role of genes coding for xenobiotic-metabolizing enzymes in breast tumorigenesis, NAT1 being an attractive candidate molecular predictor of antiestrogen responsiveness.

Effect of Dietary Constituents With Chemopreventive Potential on Adduct Formation of a Low Dose of the Heterocyclic Amines PhIP and IQ and Phase II Hepatic Enzymes

We conducted a study to evaluate dietary chemopreventive strategies to reduce genotoxic effects of the carcinogens 2-amino-1-methyl-6-phenyl-imidazo[4,5-b]pyridine (PhIP) and 2-amino-3-methylimidazo[4,5-f]quinoline (IQ). PhIP and IQ are heterocyclic amines (HCAs) that are found in cooked meat and may be risk factors for cancer. Typical chemoprevention studies have used carcinogen doses many thousand-fold higher than usual human daily intake. Therefore, we administered a low dose of [14C]PhIP and [3H]IQ and utilized accelerator mass spectrometry to quantify PhIP adducts in the liver, colon, prostate, and blood plasma and IQ adducts in the liver and blood plasma with high sensitivity. Diets supplemented with phenethylisothiocyanate (PEITC), genistein, chlorophyllin, or lycopene were evaluated for their ability to decrease adduct formation of [14C]PhIP and [3H]IQ in rats. We also examined the effect of treatments on the activity of the phase II detoxification enzymes glutathione S-transferase (GST), UDP-glucuronyltransferase (UGT), phenol sulfotransferase (SULT) and quinone reductase (QR). PEITC and chlorophyllin significantly decreased PhIP-DNA adduct levels in all tissues examined, which was reflected by similar changes in PhIP binding to albumin in the blood. In contrast, genistein and lycopene tended to increase PhIP adduct levels. The treatments did not significantly alter the level of IQ-DNA or -protein adducts in the liver. With the exception of lycopene, the treatments had some effect on the activity of one or more hepatic phase II detoxification enzymes. We conclude that PEITC and chlorophyllin are protective of PhIP-induced genotoxicity after a low exposure dose of carcinogen, possibly through modification of HCA metabolism.

Mammary physiology and milk secretion

The presence of drugs or other potentially toxic materials in milk is an obvious public health risk, especially to infants and neonates. There is also increasing concern that human breast cancer is principally epigenetic in origin and results from environmentally produced lesions. Little is known about the mechanisms by which toxic substances enter milk or mammary tissue but knowledge of these processes is important to toxicologists and researchers involved in drug design and metabolism. Five general pathways have been described for transport of proteins, lipids, ions, nutrients and water into milk. Four of these pathways are transcellular, involving transport across at least two membrane barriers; the fifth is paracellular and allows direct exchange of interstitial and milk components. Solute transport by these pathways is mediated by a diverse, and complex array of transport and secretory processes that are regulated by hormonal, developmental, and physiological factors. Current research is beginning to define the mechanisms underlying some of these processes, however the regulation and coordination of solute transport mechanisms remains poorly understood. In this article we review our current understanding of the normal solute transport and secretory processes involved in milk production, and discuss potential regulatory mechanisms.

Sulfotransferase 1A1 (SULT1A1) polymorphism, PAH-DNA adduct levels in breast tissue and breast cancer risk in a case-control study

Gene-environment interactions are hypothesized to be major contributors to susceptibility to environmental carcinogens and interindividual variability in cancer risk. We present findings on associations between genetic susceptibility due to inherited polymorphisms of the Phase II detoxification enzyme sulfotransferase 1A1 (SULT1A1), breast cancer risk, and polycyclic aromatic hydrocarbon (PAH)-DNA adducts. A hospital based case-control study was conducted at the New York-Presbyterian Medical Center (NYPMC). The study utilized two control groups: one comprised of women with benign breast disease (BBD) and the other comprised of women visiting NYPMC for routine gynecologic checkups (healthy controls). Blood samples were collected from cases and controls; and breast tissue from pathology blocks was collected from cases (tumor and non-tumor tissue) and BBD controls (benign tissue). PAH-DNA adduct levels were measured by immunohistochemistry in breast tissue samples, and the SULT1A1 (Arg/His) polymorphism at codon 213 was determined by PCR RFLP analyses using DNA from white blood cells. Increasing number of His alleles was modestly associated with breast cancer case-control status, when cases were compared to healthy controls (p for trend = 0.08), when cases were compared to BBD controls (p for trend = 0.08) and when cases were compared to both control groups combined (p for trend = 0.07). Contrary to our hypothesis PAH-DNA adduct levels in breast tissue were not associated with SULT1A1 genotype. Our findings are consistent with a prior report that the Arg/His polymorphism in SULT1A1 is associated with breast cancer risk.

Lavage and nipple aspiration of breast ductal fluids: a source of biomarkers for environmental mutagenesis

The acquisition of breast ductal fluid by nipple aspiration and ductal lavage are simple noninvasive procedures to sample breast tissue. Nipple aspiration fluid (NAF) obtained with gentle suction and a simple syringe-adapted apparatus may evaluate the secretory components that bathe the ductal epithelial cells. Evaluations have included the quantification of soluble markers (carcinoembryonic antigen and prostatic-specific antigen), DNA amplification, protein gel electrophoresis, and mutagenesis assays. It has been suggested that environmental mutagens in the breast ductal system may contribute to carcinogenesis. The feasibility of mutagenesis assays on NAF has been limited by the small size of the samples obtained. Three small clinical studies detected mutagens in 6-14% of the samples using the Salmonella Ames assay. Ductal lavage collects more of a cellular aspirate from the ductal system utilizing a microcatheter. Early studies on ductal lavage fluid have included cytology and methylation-specific PCR. Ductal lavage in a high-risk group has identified cellular atypia in 21% of those sampled. Samples obtained through the nipple, by aspiration or lavage, are the proteinaceous secretions from the ductal system and ductal epithelial cells. The fluid represents the cellular events and the dynamic secretory process of the breast and may include potential initiators of the carcinogenesis process in the cellular microenvironment. Fluid obtained by ductal lavage may allow for more detailed studies of the role of mutagens in breast cancer.

The pharmacogenetics of NAT: structural aspects

Arylamine N-acetyltransferases (NATs) catalyze the transfer of an acetyl group from acetyl-CoA to arylhydrazines and to arylamine drugs and carcinogens or to their N-hydroxylated metabolites. NAT plays an important role in detoxification and metabolic activation of xenobiotics and was first identified as the enzyme responsible for inactivation of the antitubercular drug isoniazid, an arylhydrazine. The rate of inactivation was polymorphically distributed in the population: the first example of interindividual pharmacogenetic variation. Polymorphism in NAT activity is primarily due to single nucleotide polymorphisms (SNPs) in the coding region of NAT genes. NAT enzymes are widely distributed in eukaryotes and genome sequences have revealed many homologous members of this enzyme family in prokaryotes. The structures of S almonella typhimurium and Mycobacterium smegmatis NATs have been determined, revealing a unique fold in which a catalytic triad (Cys-His-Asp) forms the active site. Determination of prokaryotic and eukaryotic NAT structures could lead to a better understanding of their role in xenobiotics and endogenous metabolism.

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.