Two food-borne heterocyclic amines: Metabolism and DNA adduct formation of amino-α-carbolines

Screening DNA Damage in the Rat Kidney and Liver by Untargeted DNA Adductomics

Air pollution, tobacco smoke, and red meat are associated with renal cell cancer (RCC) risk in the United States and Western Europe; however, the chemicals that form DNA adducts and initiate RCC are mainly unknown. Aristolochia herbaceous plants are used for medicinal purposes in Asia and worldwide. They are a significant risk factor for upper tract urothelial carcinoma (UTUC) and RCC to a lesser extent. The aristolochic acid (AA) 8-methoxy-6-nitrophenanthro-[3,4-d]-1,3-dioxolo-5-carboxylic acid (AA-I), a component of Aristolochia herbs, contributes to UTUC in Asian cohorts and in Croatia, where AA-I exposure occurs from ingesting contaminated wheat flour. The DNA adduct of AA-I, 7-(2'-deoxyadenosin-N6-yl)-aristolactam I, is often detected in patients with UTUC, and its characteristic A:T-to-T:A mutational signature occurs in oncogenes and tumor suppressor genes in AA-associated UTUC. Identifying DNA adducts in the renal parenchyma and pelvis caused by other chemicals is crucial to gaining insights into unknown RCC and UTUC etiologies. We employed untargeted screening with wide-selected ion monitoring tandem mass spectrometry (wide-SIM/MS2) with nanoflow liquid chromatography/Orbitrap mass spectrometry to detect DNA adducts formed in rat kidneys and liver from a mixture of 13 environmental, tobacco, and dietary carcinogens that may contribute to RCC. Twenty DNA adducts were detected. DNA adducts of 3-nitrobenzanthrone (3-NBA), an atmospheric pollutant, and AA-I were the most abundant. The nitrophenanthrene moieties of 3-NBA and AA-I undergo reduction to their N-hydroxy intermediates to form 2'-deoxyguanosine (dG) and 2'-deoxyadenosine (dA) adducts. We also discovered a 2'-deoxycytidine AA-I adduct and dA and dG adducts of 10-methoxy-6-nitro-phenanthro-[3,4-d]-1,3-dioxolo-5-carboxylic acid (AA-III), an AA-I isomer and minor component of the herbal extract assayed, signifying AA-III is a potent kidney DNA-damaging agent. The roles of AA-III, other nitrophenanthrenes, and nitroarenes in renal DNA damage and human RCC warrant further study. Wide-SIM/MS2 is a powerful scanning technology in DNA adduct discovery and cancer etiology characterization.

Metabolism of the Tobacco Carcinogen 2-Amino-9H-pyrido[2,3-b]indole (AαC) in Primary Human Hepatocytes

2-Amino-9H-pyrido[2,3-b]indole (AαC) is the most abundant carcinogenic heterocyclic aromatic amine (HAA) formed in mainstream tobacco smoke. AαC is a liver carcinogen in rodents, but its carcinogenic potential in humans is not known. To obtain a better understanding of the genotoxicity of AαC in humans, we have investigated its metabolism and its ability to form DNA adducts in human hepatocytes. Primary human hepatocytes were treated with AαC at doses ranging from 0.1-50 μM, and the metabolites were characterized by ultra-performance LC/ion trap multistage mass spectrometry (UPLC/MSn). Six major metabolites were identified: a ring-oxidized doubly conjugated metabolite, N2-acetyl-2-amino-9H-pyrido[2,3-b]indole-6-yl-oxo-(β-d-glucuronic acid) (N2-acetyl-AαC-6-O-Gluc); two ring-oxidized glucuronide (Gluc) conjugates: 2-amino-9H-pyrido[2,3-b]indol-3-yl-oxo-(β-d-glucuronic acid) (AαC-3-O-Gluc) and 2-amino-9H-pyrido[2,3-b]indol-6-yl-oxo-(β-d-glucuronic acid) (AαC-6-O-Gluc); two sulfate conjugates, 2-amino-9H-pyrido[2,3-b]indol-3-yl sulfate (AαC-3-O-SO3H) and 2-amino-9H-pyrido[2,3-b]indol-6-yl sulfate (AαC-6-O-SO3H); and the Gluc conjugate, N2-(β-d-glucosidurony1)-2-amino-9H-pyrido[2,3-b]indole (AαC-N2-Gluc). In addition, four minor metabolites were identified: N2-acetyl-9H-pyrido[2,3-b]indol-3-yl sulfate (N2-acetyl-AαC-3-O-SO3H), N2-acetyl-9H-pyrido[2,3-b]indol-6-yl sulfate (N2-acetyl-AαC-6-O-SO3H), N2-acetyl-2-amino-9H-pyrido[2,3-b]indol-3-yl-oxo-(β-d-glucuronic acid) (N2-acetyl-AαC-3-O-Gluc), and O-(β-d-glucosidurony1)-2-hydroxyamino-9H-pyrido[2,3-b]indole (AαC-HN2-O-Gluc). The latter metabolite, AαC-HN2-O-Gluc is a reactive intermediate that binds to DNA to form the covalent adduct N-(2'-deoxyguanosin-8-yl)-2-amino-9H-pyrido[2,3-b]indole (dG-C8-AαC). Preincubation of hepatocytes with furafylline, a selective mechanism-based inhibitor of P450 1A2, resulted in a strong decrease in the formation of AαC-HN2-O-Gluc and a concomitant decrease in DNA adduct formation. Our findings describe the major pathways of metabolism of AαC in primary human hepatocytes and reveal the importance of N-acetylation and glucuronidation in metabolism of AαC. P450 1A2 is a major isoform involved in the bioactivation of AαC to form the reactive AαC-HN2-O-Gluc conjugate and AαC-DNA adducts.

Bioactivation of Heterocyclic Aromatic Amines by UDP Glucuronosyltransferases

2-Amino-9H-pyrido[2,3-b]indole (AαC) and 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) are carcinogenic heterocyclic aromatic amines (HAA) that arise during the burning of tobacco and cooking of meats. UDP-glucuronosyltransferases (UGT) detoxicate many procarcinogens and their metabolites. The genotoxic N-hydroxylated metabolite of AαC, 2-hydroxyamino-9H-pyrido[2,3-b]indole (HONH-AαC), undergoes glucuronidation to form the isomeric glucuronide (Gluc) conjugates N(2)-(β-d-glucosidurony1)-2-hydroxyamino-9H-pyrido[2,3-b]indole (AαC-HON(2)-Gluc) and O-(β-d-glucosidurony1)-2-hydroxyamino-9H-pyrido[2,3-b]indole (AαC-HN(2)-O-Gluc). AαC-HON(2)-Gluc is a stable metabolite but AαC-HN(2)-O-Gluc is a biologically reactive intermediate, which covalently adducts to DNA at levels that are 20-fold higher than HONH-AαC. We measured the rates of formation of AαC-HON(2)-Gluc and AαC-HN(2)-O-Gluc in human organs: highest activity occurred with liver and kidney microsomes, and lesser activity was found with colon and rectum microsomes. AαC-HN(2)-O-Gluc formation was largely diminished in liver and kidney microsomes, by niflumic acid, a selective inhibitor UGT1A9. In contrast, AαC-HON(2)-Gluc formation was less affected and other UGT contribute to N(2)-glucuronidation of HONH-AαC. UGT were reported to catalyze the formation of isomeric Gluc conjugates at the N(2) and N3 atoms of 2-hydroxyamino-1-methyl-6-phenylimidazo[4,5-b]pyridine (HONH-PhIP), the genotoxic metabolite of PhIP. However, we found that the N3-Gluc of HONH-PhIP also covalently bound to DNA at higher levels than HONH-PhIP. The product ion spectra of this Gluc conjugate acquired by ion trap mass spectrometry revealed that the Gluc moiety was linked to the oxygen atom of HONH-PhIP and not the N3 imidazole atom of the oxime tautomer of HONH-PhIP as was originally proposed. UGT1A9, an abundant UGT isoform expressed in human liver and kidney, preferentially forms the O-linked Gluc conjugates of HONH-AαC and HONH-PhIP as opposed to their detoxicated N(2)-Gluc isomers. The regioselective O-glucuronidation of HONH-AαC and HONH-PhIP, by UGT1A9, is a mechanism of bioactivation of these ubiquitous HAAs.

Caffeine and other methylxanthines as interceptors of food-borne aromatic mutagens: inhibition of Trp-P-1 and Trp-P-2 mutagenic activity

Caffeine is one of the most important biologically active food components. In this article, we demonstrate that caffeine and other methylxanthines significantly reduce the mutagenic activity of two food-derived heterocyclic aromatic amines, Trp-P-1 and Trp-P-2 in the Salmonella typhimurium TA98 strain. Moreover, protection against Trp-P-1-induced mutagenicity was independent of liver S9 enzymatic fraction, suggesting that mechanisms other than modulation of mutagen bioactivation can contribute to the observed protective effects. UV-vis spectroscopy and computational studies revealed that methylxanthines intercept Trp-P-1 and Trp-P-2 in noncovalent molecular complexes, with association constants (KAC) in the 10(2) M(-1) range. Enthalpy values (ΔH about -30 kJ·mol(-1)) of mutagen-methylxanthine heterocomplexation obtained microcalorimetrically correspond to stacking (π-π) interactions. Finally, we demonstrated that the biological activity of Trp-P-1 and Trp-P-2 is strictly dependent on the presence of the mutagen in a free (unbound with methylxanthine) form, suggesting that mutagen sequestration in stacking heterocomplexes with methylxanthines can decrease its bioavailability and diminish its biological effects.

QSAR and metabolic assessment tools in the assessment of genotoxicity

In this chapter, a range of computational tools for applying QSAR and grouping/read-across methods are described, and their integrated use in the computational assessment of genotoxicity is illustrated through the application of selected tools to two case-study compounds-2-amino-9H-pyrido[2,3-b]indole (AαC) and 2-aminoacetophenone (2-AAP). The first case study compound (AαC) is an environment pollutant and a food contaminant that can be formed during the cooking of protein-rich food. The second case study compound (2-AAP) is a naturally occurring compound in certain foods and also proposed for use as a flavoring agent. The overall aim is to describe and illustrate a possible way of combining different information sources and software tools for genotoxicity and metabolism prediction by means of a simple stepwise approach. The chapter is aimed at researchers and assessors who have a basic knowledge of computational toxicology and some familiarity with the practical use of computational tools. The emphasis is on how to evaluate the data generated by multiple tools, rather than the practical use of any specific tool.

UDP-glucuronosyltransferase-mediated metabolic activation of the tobacco carcinogen 2-amino-9H-pyrido[2,3-b]indole

2-Amino-9H-pyrido[2,3-b]indole (AαC) is a carcinogenic heterocyclic aromatic amine (HAA) that arises in tobacco smoke. UDP-glucuronosyltransferases (UGTs) are important enzymes that detoxicate many procarcinogens, including HAAs. UGTs compete with P450 enzymes, which bioactivate HAAs by N-hydroxylation of the exocyclic amine group; the resultant N-hydroxy-HAA metabolites form covalent adducts with DNA. We have characterized the UGT-catalyzed metabolic products of AαC and the genotoxic metabolite 2-hydroxyamino-9H-pyrido[2,3-b]indole (HONH-AαC) formed with human liver microsomes, recombinant human UGT isoforms, and human hepatocytes. The structures of the metabolites were elucidated by (1)H NMR and mass spectrometry. AαC and HONH-AαC underwent glucuronidation by UGTs to form, respectively, N(2)-(β-D-glucosidurony1)-2-amino-9H-pyrido[2,3-b]indole (AαC-N(2)-Gl) and N(2)-(β-D-glucosidurony1)-2-hydroxyamino-9H-pyrido[2,3-b]indole (AαC-HON(2)-Gl). HONH-AαC also underwent glucuronidation to form a novel O-linked glucuronide conjugate, O-(β-D-glucosidurony1)-2-hydroxyamino-9H-pyrido[2,3-b]indole (AαC-HN(2)-O-Gl). AαC-HN(2)-O-Gl is a biologically reactive metabolite and binds to calf thymus DNA (pH 5.0 or 7.0) to form the N-(deoxyguanosin-8-yl)-AαC adduct at 20-50-fold higher levels than the adduct levels formed with HONH-AαC. Major UGT isoforms were examined for their capacity to metabolize AαC and HONH-AαC. UGT1A4 was the most catalytically efficient enzyme (V(max)/K(m)) at forming AαC-N(2)-Gl (0.67 μl·min(-1)·mg of protein(-1)), and UGT1A9 was most catalytically efficient at forming AαC-HN-O-Gl (77.1 μl·min(-1)·mg of protein(-1)), whereas UGT1A1 was most efficient at forming AαC-HON(2)-Gl (5.0 μl·min(-1)·mg of protein(-1)). Human hepatocytes produced AαC-N(2)-Gl and AαC-HN(2)-O-Gl in abundant quantities, but AαC-HON(2)-Gl was a minor product. Thus, UGTs, usually important enzymes in the detoxication of many procarcinogens, serve as a mechanism of bioactivation of HONH-AαC.

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.

Heterocyclic aromatic amine formation in barbecued sardines (Sardina pilchardus) and Atlantic salmon (Salmo salar)

The formation of heterocyclic aromatic amines (HAs) during barbecuing of sardines ( Sardina pilchardus ) and Atlantic salmon ( Salmo salar ) to various degrees of doneness and grilling conditions was evaluated by HPLC-diode array (DAD)/fluorescence (FLD) detection. Additionally, the influences of charcoal and electric heat sources on formation of HAs in grilled salmon were compared. With regard to sardine samples barbecued at 280-300 degrees C, "rare" samples produced nondetectable amounts of HAs, "medium" sardines presented IQ, MeIQx, PhIP, and AalphaC at levels of 1.9, 4.4, 3.3, and 2.0 ng/g, respectively, and "well done" sardines presented IQ, MeIQx, Trp-P-1, Trp-P-2, PhIP, AalphaC, and MeAalphaC at levels of 0.9, 2.2, 1.8, 8.2, 6.5, 17.7, and 10.6 ng/g, respectively. Different qualitative and quantitative profiles of HAs were observed in sardine and salmon samples cooked under similar conditions of temperature and doneness. Levels of 13.3, 3.5, 1.13, and 3.18 ng/g were obtained, respectively, for PhIP, AalphaC, MeAalphaC, and Glu-P-1 in salmon samples barbecued at 280-300 degrees C. The contents of HAs were significantly higher in these samples than in salmon samples barbecued at 180-200 degrees C or in the electric device. However, MeIQx content (0.5 ng/g) was lower in salmon samples barbecued at 280-300 degrees C than in the other samples.