Metabolism of carcinogenic amino derivatives in various species and DNA alkylation by their metabolites

New Aminobiphenylcysteine Derivatives in Globin and Urine of Rats Dosed with 4-Aminobiphenyl, a Tobacco Smoke Carcinogen

The 4-biphenylnitrenium ion (BPN), a reactive metabolic intermediate of the tobacco smoke carcinogen 4-aminobiphenyl (4-ABP), can react with nucleophilic sulfanyl groups in glutathione (GSH) as well as in proteins. The main site of attack of these S-nucleophiles was predicted using simple orientational rules of aromatic nucleophilic substitution. Thereafter, a series of presumptive 4-ABP metabolites and adducts with cysteine were synthesized, namely, S-(4-amino-3-biphenyl)cysteine (ABPC), N-acetyl-S-(4-amino-3-biphenyl)cysteine (4-amino-3-biphenylmercapturic acid, ABPMA), S-(4-acetamido-3-biphenyl)cysteine (AcABPC), and N-acetyl-S-(4-acetamido-3-biphenyl)cysteine (4-acetamido-3-biphenylmercapturic acid, AcABPMA). Then, globin and urine of rats dosed with a single ip dose of 4-ABP (27 mg/kg b.w.) was analyzed by HPLC-ESI-MS². ABPC was identified in acid-hydrolyzed globin at levels of 3.52 ± 0.50, 2.74 ± 0.51, and 1.25 ± 0.12 nmol/g globin (mean ± S.D.; n = 6) on days 1, 3, and 8 after dosing, respectively. In the urine collected on day 1 (0-24 h) after dosing, excretion of ABPMA, AcABPMA, and AcABPC amounted to 1.97 ± 0.88, 3.09 ± 0.75, and 3.69 ± 1.49 nmol/kg b.w. (mean ± S.D.; n = 6), respectively. On day 2, excretion of the metabolites decreased by one order of magnitude followed by a slower decrease on day 8. Regarding the possible formation of AcABPC from ABPC, N-acetylation of the amino group at the biphenyl moiety prior to that at cysteine appears to be very unlikely. Thus, the structure of AcABPC indicates the involvement of N-acetyl-4-biphenylnitrenium ion (AcBPN) and/or its reactive ester precursors in in vivo reactions with GSH and protein-bound cysteine. ABPC in globin might become an alternative biomarker of the dose of toxicologically relevant metabolic intermediates of 4-ABP.

Cloudy apple juice protects against chemical-induced oxidative stress in rat

BACKGROUND

Apples abundant in phenolic compounds show a variety of biological activities that may contribute to beneficial effects against some chronic diseases.

PURPOSE

The aim of our study was to assess the protective effect of cloudy apple juice against chemical-induced oxidative stress in rats.

METHODS

Male Wistar rats were treated with apple juice per os, 10 mL/kg/day for 28 days and with a single dose of N-nitrosodiethylamine (NDEA), 150 mg/kg or carbon tetrachloride (CCl(4)), 2 mL/kg, 24 h before killing. Two groups of rats not pretreated with juice were administered each of the xenobiotics alone.

RESULTS

Microsomal lipid peroxidation in the liver was decreased in rats pretreated with juice by 52-87% when compared to animals given NDEA or CCl(4) alone. Pretreatment with juice protected antioxidant enzymes: catalase, glutathione peroxidase and glutathione reductase but not superoxide dismutase. Their activity was recovered by 49-173% when compared to that in rats given either toxicant alone. The plasma activity of paraoxonase 1 was reduced by both toxicants and was increased by 23% in the apple/CCl(4) group. A rise in plasma protein carbonyls caused by the xenobiotics was reduced by 20% only in apple/NDEA-treated rats. Also, in this group of animals, a 9% decrease in DNA damage in blood leukocytes was observed.

CONCLUSION

Phytochemicals in commonly consumed apple juice may protect some macromolecules against oxidative insult induced by xenobiotics.

Protective effect of red beetroot against carbon tetrachloride- and N-nitrosodiethylamine-induced oxidative stress in rats

The aim of the study was to investigate the potential protective effect of beetroot juice in a model of oxidative stress induced by N-nitrosodiethylamine (NDEA) and carbon tetrachloride (CCl(4)). Male Wistar rats were treated with beetroot juice per os, 8 mL/kg/day for 28 days, and a single i.p. dose of the xenobiotics: 150 mg/kg NDEA or 2 mL/kg CCl(4). Simultaneously, two groups of rats not pretreated with juice were given only each of the xenobiotics. The level of microsomal lipid peroxidation in the liver, expressed as TBARS concentration, was increased several fold in rats administered only NDEA or CCl(4). TBARS were decreased by 38% only in rats pretreated with beetroot juice before the administration of CCl(4). In animals pretreated with juice and receiving NDEA, a further increase in TBARS occurred. All of the investigated antioxidant enzymes were inhibited by the administration of either toxicant alone by 26%-77% as compared to controls. Pretreatment with juice caused a partial recovery in the activity of glutathione peroxidase and glutathione reductase, by 35% and 66%, respectively. Superoxide dismutase activity was increased about 3-fold in animals pretreated with juice. Both xenobiotics caused a rise in plasma protein carbonyls, which were reduced by 30% in rats pretreated with juice and then injected with NDEA. Similarly, DNA damage in blood leukocytes caused by either toxicant was slightly diminished, by 20%, in the rats treated with juice before NDEA administration. It could be concluded that pretreatment with beetroot juice can counteract, to some extent, xenobiotic-induced oxidative stress in rats.

LF, Lopes C. Chemical carcinogenesis

The use of chemical compounds benefits society in a number of ways. Pesticides, for instance, enable foodstuffs to be produced in sufficient quantities to satisfy the needs of millions of people, a condition that has led to an increase in levels of life expectancy. Yet, at times, these benefits are offset by certain disadvantages, notably the toxic side effects of the chemical compounds used. Exposure to these compounds can have varying effects, ranging from instant death to a gradual process of chemical carcinogenesis. There are three stages involved in chemical carcinogenesis. These are defined as initiation, promotion and progression. Each of these stages is characterised by morphological and biochemical modifications and result from genetic and/or epigenetic alterations. These genetic modifications include: mutations in genes that control cell proliferation, cell death and DNA repair - i.e. mutations in proto-oncogenes and tumour suppressing genes. The epigenetic factors, also considered as being non-genetic in character, can also contribute to carcinogenesis via epigenetic mechanisms which silence gene expression. The control of responses to carcinogenesis through the application of several chemical, biochemical and biological techniques facilitates the identification of those basic mechanisms involved in neoplasic development. Experimental assays with laboratory animals, epidemiological studies and quick tests enable the identification of carcinogenic compounds, the dissection of many aspects of carcinogenesis, and the establishment of effective strategies to prevent the cancer which results from exposure to chemicals.

A metabolic activation mechanism of 7H-dibenzo[c,g]carbazole via o-quinone. Part 2: covalent adducts of 7H-dibenzo[c,g]carbazole-3,4-dione with nucleic acid bases and nucleosides

7H-dibenzo[c,g]carbazole (DBC) is a potent multispecies, multisite carcinogen present in the environment. The metabolic activation pathways of DBC are not completely known. It is hypothesized that DBC may be metabolically activated by oxidation to the reactive Michael acceptor o-quinones, which can form stable and depurinating DNA adducts. The synthesis of DBC-3,4-dione has been previously reported by this research group. In the present article, we describe the synthesis and chemical structural elucidation of nine DBC-nucleic acid adducts produced from reactions of DBC-3,4-dione with Ade, Cyt, 2'-deoxyguanosine (dGuo), 2'-deoxycytidine (dCyd), and Guo. Adducts were isolated from reaction mixtures by HPLC and analyzed using MS including elemental compositions and collision-activated dissociation (CAD), (1)H NMR, and two-dimensional chemical shift correlation spectroscopy (COSY) NMR. The adducts, 7-[3,4-dione-DBC-1-yl]-Ade, N4-[3,4-dione-DBC-1-yl]-Cyt, 5-[3,4-dione-DBC-1-yl]-Cyt, two conformational isomers of N2-[3,4-dihydroxy-DBC-1-yl]-dGuo, and two conformational isomers of N2-[3,4-dihydroxy-DBC-1-yl]-Guo, were characterized. Two adducts from reactions of DBC-3,4-dione with dCyd were identified by MS but not fully characterized by NMR due to instability of the adducts. Under similar conditions, the reactions of DBC-3,4-dione with Gua and 2'-deoxyadenosine (dAdo) did not result in an identifiable adduct. Liver DNA adducts from mice treated topically with DBC-3,4-dione (100 microg) in dimethyl sulfoxide/acetone (15/85, 100 microL) were identified with 32P-postlabeling. The major adduct chromatographically matched one of the adducts formed from livers of DBC-treated mouse (adduct 3) using identical conditions.

Application of the in vitro rat hepatocyte micronucleus assay in genetic toxicology testing

The investigation of micronuclei in mitogenic stimulated hepatocytes in vitro is a quite new area of research. Nevertheless, a relatively large database comprising more than 40 tested compounds of various classes has been generated up to now. This paper reviews the available data for the in vitro rat hepatocyte micronucleus assay, showing a sensitivity of this assay in identifying mutagens and genotoxic liver carcinogens of about 85%. Additionally, all of the tested non-carcinogens gave negative results. The use of primary hepatocytes instead of permanently dividing mammalian cell lines for the investigation of micronucleus induction has several advantages. (1) The broad spectrum of metabolizing enzymes expressed in primary hepatocytes ensures an adequate activation of most xenobiotics. (2) No transfer of activated metabolites via the culture medium is necessary in this system, since the metabolizing cells are the target cells themselves. (3) Whilst in experiments with permanently dividing cells the use of S9-mix restricts the treatment period with the test compounds to 2-6 h in the hepatocyte micronucleus assay continuous treatment of up to 48 h is possible. Investigations with the pyrrolizidine alkaloids retrorsine, monocrotaline and isatidine, strong mutagens and liver carcinogens, clearly showed that at least for isatidine a prolonged exposure period is essential to detect its mutagenic potential. This compound gave positive results in rat hepatocytes but not in V79-cells/S9-mix cultures. (4) The results obtained with the hepatocyte micronucleus assay are in good agreement with the genotoxic profiles of most of the compounds tested. Only three polycyclic aromatic hydrocarbons led to 'false-negative' results, since they strongly inhibited hepatocyte proliferation and thereby prevented micronucleus formation. (5) Hepatocytes are target cells of special interest when compounds are investigated which act specifically in the liver. Especially for hepatocarcinogens classified as non-genotoxins in standard genotoxicity tests or for chemicals showing DNA-repair induction in hepatocytes but no mutagenicity in standard tests, the hepatocyte micronucleus assay can contribute to clarify the situation. (6) The rat hepatocyte micronucleus assay can be performed easily and without great efforts in parallel to the in vitro hepatocyte DNA repair test (UDS-test), using the same hepatocyte batches. (7) Similar to the two versions of the UDS-test, the hepatocyte micronucleus assay can be performed following an in vivo-in vitro protocol. In order to further validate the hepatocyte micronucleus assay, as a next step controlled interlaboratory studies should be initiated.

Formation and persistence of DNA adducts of 2-amino-3-methylimidazo[4,5-f]quinoline (IQ) in CDF1 mice fed a high omega-3 fatty acid diet

The potent bacterial mutagen 2-amino-3-methylimidazo[4,5-f]quinoline (IQ) is carcinogenic in the CDF1 mouse, affecting the liver, lungs and forestomach. IQ forms DNA adducts in both target- and non-target organs of the CDF1 mouse. The chemopreventive effects of menhaden oil (MO), a fish oil high in omega-3 fatty acids, are well known. Because DNA adduct formation is considered to be a critical event in the initiation of carcinogenesis, we have assessed the effects of dietary MO on IQ-DNA adduct formation. For the duration of the study, young adult, male CDF1 mice were maintained on either powdered chow diet, AIN-76A diet, or AIN-76A diet modified to contain 19% MO (19% MO diet). After 2 weeks on these diets, all animals received 0.01% (w/w) IQ in the diet for the next 3 weeks. Groups of 4 animals were killed 1, 2, 4, 6, 8 or 12 days thereafter for analysis of IQ-DNA adducts by 32P-postlabeling. IQ-DNA adduct patterns were qualitatively similar in the liver, lungs, stomach, small intestine, cecum, colon, kidneys, heart and spleen. Adduct levels in the liver, lungs, stomach and colon decreased significantly during the 12-day study period, but only to a relatively small extent and only with certain of the diets. On day 1, the 19% MO diet significantly decreased (35.8-90.0%) adduct levels in the stomach, cecum, colon and kidneys, when compared to chow diet or AIN-76A diet. On day 12, adduct levels in the liver, stomach, heart and spleen were decreased (36.5-64.7%) as a result of MO feeding. With the exception of the liver, heart and spleen on day 12, there were no significant differences in organ adduct levels between the chow diet and the AIN-76A diet. It is concluded that feeding 0.01% (w/w) IQ in the diet for 3 weeks results in a relatively slow rate of adduct removal and that this rate is largely independent of the type of diet. Dietary MO inhibits IQ-DNA adduct formation only in certain target- and non-target organs of the CDF1 mouse, a finding similar to our previous results in the F344 rat. MO may affect the initiation phase of IQ tumorigenesis by inhibiting IQ-DNA adduct formation in certain target organs.

DNA adducts of heterocyclic amines: formation, removal and inhibition by dietary components

The dietary mutagens 2-amino-3-methylimidazo[4,5-f]quinoline (IQ) and 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) are carcinogenic in rodents. In F344 rats PhIP induces mammary tumors in females and colon tumors in males, while IQ induces tumors principally in the liver, Zymbal gland and intestines. In CDF1 mice, IQ induces liver, lung and forestomach tumors. We have evaluated the dynamics of formation, removal and inhibition of PhIP- and IQ-DNA adducts in these rodents. After bolus doses (50 mg/kg, by gavage) of IQ or PhIP, both IQ- and PhIP-DNA adducts were removed rapidly from both target and nontarget organs, while after 3-4 weeks of feeding IQ or PhIP (0.01-0.04%) adduct removal was much slower. Gavaging of male F344 rats with PhIP (0.1-1000 micrograms/kg/day) for 23 days resulted in accumulation of PhIP-DNA adducts in various organs, but adducts were detectable only at 100 or 1000 micrograms/kg/day. Urinary excretion of unchanged PhIP was a constant proportion (1.6-2.1%) of the daily dose over the entire dose range and was independent of duration of exposure. When weanling female F344 rats were exposed to dietary PhIP (0.01-0.04%) for 1-4 weeks, the presence of either conjugated linoleic acid (CLA; 0.1-1.0%) or indole-3-carbinol (13C; 0.1%) in the diet inhibited PhIP-DNA adduct formation (58-99%) in various organs, including the mammary gland and the colon. Similarly, the inclusion of 0.075% 4-ipomeanol (IPO) in the diet of male CDF1 mice exposed for 3 weeks to dietary IQ (0.01%) resulted in inhibition of IQ-DNA adduct formation (30-59%) in the target organs (liver, lungs, stomach) but not in a number of other organs. It is concluded that (1) the rate of PhIP- and IQ-DNA adduct removal depends on the dose and frequency of administration, (2) urinary PhIP may be a good biomarker of recent PhIP exposure and (3) CLA, I3C and IPO are potential chemopreventive agents against PhIP- or IQ-induced tumors in rodents.

Comparative carcinogenicity, metabolism, mutagenicity, and DNA binding of 7H-dibenzo[c,g]carbazole and dibenz[a,j]acridine

Complex mixtures that are produced from the combustion of organic materials have been associated with increased cancer mortality. These mixtures contain homocyclic and heterocyclic polycyclic aromatic hydrocarbons (PAHs), many of which are known carcinogens. In particular, N-heterocyclic aromatic compounds (NHA) are present in these mixtures. Studies to determine the metabolic activation of these compounds have been undertaken. The purpose of this review is to compare and contrast the metabolic activation and biological effects of two NHA, 7H-dibenzo[c,g]carbazole (DBC) and dibenz[a,j]acridine (DBA), in order to better assess the contribution of NHA to the carcinogenic potency of complex mixtures and to develop biomarkers of the carcinogenic process. DBC has both local and systemic effects in the mouse; it is a potent skin and liver carcinogen following topical application and a lung carcinogen following i.p. application. On the other hand, DBA is a moderate mouse skin carcinogen following topical application and a lung carcinogen following subcutaneous injection. The biological differences for DBC and DBA are reflected in target organ-specific proximate and mutagenic metabolites and DNA adduct patterns.

Dimethylnitrosamine-induced DNA damage and toxic cell death in cultured mouse hepatocytes

Chronic exposure to dimethylnitrosamine produces hepatic tumors through recurrent DNA alkylation, whereas acute exposure can cause liver necrosis through mechanisms that remain largely unknown. Our laboratory recently demonstrated that DNA fragmentation occurs early on and may be a causal event in dimethylnitrosamine-induced necrosis in liver. A challenge to interpreting these results is that up to 30% of liver cells are non-parenchymal and could account for the observed DNA fragmentation. In the present study, we have examined whether dimethylnitrosamine induces early genomic DNA fragmentation in cultured mouse hepatocytes. Hepatic parenchymal cells isolated from male ICR mice were cultured in Williams E medium. DNA damage was assessed quantitatively as a fragmented fraction that was not sedimented at 27,000 x g, and qualitatively from agarose gel electrophoresis. Cellular response to DNA damage was assessed by measuring induction of the DNA repair enzyme DNA ligase. Toxic cell death was estimated from release of lactate dehydrogenase (LDH) or adenine nucleotides from cells prelabeled with [3H]adenine. Dimethylnitrosamine produced a twofold increase in [3H]adenine release by 6 h and LDH release at 36 h. DNA fragmentation and DNA ligase activity increased by as early as 1 h. The Ca(2+)-endonuclease inhibitor aurintricarboxylic acid and the Ca2+ chelator ethylenediamine tetraacetic acid (EDTA) prevented DNA fragmentation through 6 h and virtually abolished cytotoxicity through 30 h. DNA ligase induction was strongly associated with DNA fragmentation. Early increases in DNA fragmentation and DNA ligase were highly correlated with later toxic cell death. Such results strongly suggest that dimethylnitrosamine-induced fragmentation of DNA in target parenchymal cells is a causal factor in the toxic death of these liver cells.

Characterization of the basal and carcinogen regulatory elements of the rat mdr1b promoter

In this report we characterized the transcriptional regulation of the rat mdr1b gene by xenobiotics. The expression of this gene was increased in primary rat hepatocytes and in the H4-II-E hepatoma cell line by exposure to carcinogens such as aflatoxin B1, N-acetoxy-2-acetylaminofluorene, and methyl methanesulfonate. Nuclear run-on experiments indicated that the higher steady-state levels of mdr1b mRNA were due to an increase in transcription. The 5'-flanking region of the mdr1b gene was isolated, sequenced, and functionally characterized in transient and stable transfection assays. A single transcription start site was identified for this gene; no alternate start sites were used after induction with aflatoxin B1. Deletion analysis of this promoter demonstrated that the sequence between nt -214 and -178 was critical for basal promoter activity. This region did not contain any consensus-binding sites for previously identified transcription factors. A negative regulatory region was also identified between nt -940 and -250. No specific carcinogen-responsive element was identified; the xenobiotic response required a large part of the promoter. These data suggest that the carcinogen induction of mdr1b expression is mediated through sequences that overlap or that are identical to the basal promoter element.

Genetic toxicity of 2-acetylaminofluorene, 2-aminofluorene and some of their metabolites and model metabolites

2-Acetylaminofluorene and 2-aminofluorene are among the most intensively studied of all chemical mutagens and carcinogens. Fundamental research findings concerning the metabolism of 2-acetylaminofluorene to electrophilic derivatives, the interaction of these derivatives with DNA, and the carcinogenic and mutagenic responses that are associated with the resulting DNA damage have formed the foundation upon which much of genetic toxicity testing is based. The parent compounds and their proximate and ultimate mutagenic and carcinogenic derivatives have been evaluated in a variety of prokaryotic and eukaryotic assays for mutagenesis and DNA damage. The reactive derivatives are active in virtually all systems, while 2-acetylaminofluorene and 2-aminofluorene are active in most systems that provide adequate metabolic activation. Knowledge of the structures of the DNA adducts formed by 2-acetylaminofluorene and 2-aminofluorene, the effects of the adducts on DNA conformation and synthesis, adduct distribution in tissues, cells and DNA, and adduct repair have been used to develop hypotheses to understand the genotoxic and carcinogenic effects of these compounds. Molecular analysis of mutations produced in cell-free, bacterial, in vitro mammalian, and intact animal systems have recently been used to extend these hypotheses.

Formation of N-hydroxy-N-arylacetamides from nitroso aromatic compounds by the mammalian pyruvate dehydrogenase complex

Bovine, human and porcine heart mitochondria and isolated porcine heart pyruvate dehydrogenase complex (PDHC) pyruvate-dependently form N-hydroxy-N-arylacetamides from nitroso aromatic compounds, including carcinogenic 4-biphenyl and 2-fluorenyl derivatives. The PDHC-catalysed formation of N-hydroxyacetanilide (N-OH-AA) from nitrosobenzene (NOB), through a Ping Pong mechanism, is optimum at pH 6.8 and is accelerated by thiamin pyrophosphate, but is inhibited by thiamin thiazolone pyrophosphate and ATP. Km pyruvate in the reaction is independent of pH over the range tested, whereas KmNOB increases at lower pH, owing to ionization of an active-site functional group of pKa 6.3. The enzymic ionization decreases log (Vmax/KmNOB). Isolated pyruvate dehydrogenase (E1), a constitutive enzyme of PDHC, forms N-OH-AA by itself and has comparable kinetic parameters to those of the PDHC-catalysed N-OH-AA formation. The catalytic efficiency of PDHC in the formation of N-hydroxy-N-arylacylamides, due to the steric limitation of the active site of E1, is lowered both by bulky alkyl groups of alpha-oxo acids and by p-substituents (but not an o-substituent) on nitrosobenzenes. These nitroso compounds serve as electrophiles in the reaction in which the reductive acetylation step is rate-limiting. The reaction mechanism and other factors affecting N-hydroxy-N-arylacylamide formation are discussed.

Inhibition of 2-amino-3-methylimidazo[4,5-f]quinoline-DNA adduct formation in CDF1 mice by heat-altered derivatives of linoleic acid

Grilled ground beef contains a number of carcinogens, including aminoimidazoazaarenes, such as 2-amino-3-methylimidazo[4,5-f]quinoline (IQ), as well as anticarcinogenic substances, such as heat-generated derivatives of linoleic acid (CLA). In the present study, CLA was administered by gavage every other day to young adult CDF1 mice for a period of 45 days (50 microliters/48 hr for days 1-24 and 100 microliters/48 hr for days 25-45), using trioctanoin as a control. On day 46 all animals received a single oral dose (50 mg/kg) of IQ and tissues were collected 24 hr later. Tissue DNA was purified and analysed for IQ-DNA adducts by 32P-postlabelling assays. Compared with controls, CLA treatment caused a 43.1 and 31.8% inhibition of adduct formation in the livers of male and female mice, respectively. In the lung and large intestine CLA had a 74.2 and 39.4% inhibitory effect, respectively, in the female only, whereas there was no effect in the stomach or small intestine of either sex. In the kidneys of females, CLA treatment inhibited IQ-DNA adduct formation almost completely (95.2%), whereas in the kidneys of males CLA had no effect. It is concluded that CLA inhibits IQ-DNA adduct formation in certain IQ target organs (liver and lung) and non-target organs (large intestine, kidney), but is inactive in other target organs (stomach) and non-target organs (small intestine) of the CDF1 mouse.

Modulation of 4-(methylnitrosamino)-1-(3-pyridyl)-1 butanone demethylation and denitrosation by rat liver microsomes

The nicotine-derived N-nitrosamine 4-(methylnitrosamino)-1-(3-pyridyl)-1 butanone (NNK) is abundant in tobacco smoke and is a potent carcinogen in laboratory animals. We have characterized and compared methyl hydroxylation leading to formaldehyde formation (activation pathway) and denitrosation (putative deactivation pathway) of NNK using rat liver microsomes. Methyl hydroxylation was linear between 10 and 50 min and denitrosation was linear between 20 and 65 min. The ratio of nitrite/formaldehyde formation was smaller with 4 mM NNK (0.016) than with N-nitrosodimethylamine (NDMA) (0.037). Liver microsomes exhibited at least two apparent Km values (0.005 and 0.534 mM) for NNK methyl hydroxylation. In vitro methyl hydroxylation of NDMA and NNK were induced by pretreatment of the rat with isopropanol. Butylated hydroxytoluene pretreatment doubled the rate of microsomal denitrosation but did not affect the rate of methyl hydroxylation (+)-Catechin is a flavonoid present in tea, wine and apple skin. We observed that 0.10 mM (+)-catechin inhibited methyl hydroxylation and denitrosation of 4 mM NNK by 40% and 62%, respectively. These results indicated that NNK could be deactivated by denitrosation. Hydroxylation of the NNK methyl group by rat liver microsomes was 34 times more extensive than denitrosation.

Cytochrome P4501A2 constitutively expressed from transduced DNA mediates metabolic activation and DNA-adduct formation of aromatic amine carcinogens in NIH 3T3 cells

We transduced mouse cytochrome P4501A2 DNA into NIH 3T3 cells by retrovirus-mediated gene transfer. The capacity of the transduced cytochrome P4501A2 for metabolic activation and DNA-carcinogen adduct formation of aromatic amine carcinogens was investigated. Clones of NIH 3T3 cells that constitutively express cytochrome P4501A2 and controls were exposed to a prototype food-derived carcinogenic heterocyclic amine, 2-amino-3-methylimidazo[4,5-f]quinoline (IQ), and an aromatic amine, 2-acetylaminofluorene (AAF), and their genomic DNAs were analyzed for adducts by 32P-postlabeling assays. Kinetic analysis of DNA-carcinogen adducts indicated that adduct formation was dependent on the level of the enzyme, the dose of carcinogen, and the duration of exposure. Addition of 7,8-benzoflavone, an inhibitor of P4501A2, blocked both the enzyme activity and DNA-adduct formation, indicating the specific role of P4501A2 in metabolic activation and adduct formation. Three specific IQ-DNA adducts were detected in cells expressing P4501A2. Fingerprints of the in situ DNA adducts were similar to those of the in vivo adducts in rodent hepatic DNA after the administration of IQ. A single AAF-DNA adduct was observed in cells exposed to AAF, but other minor adducts were also detected in vivo. These results show that cells expressing constitutive levels of single cytochrome P450s provide an excellent in situ model system for analyzing the catalytic specificity, metabolic activation, and genotoxicity of putative toxic, mutagenic, and carcinogenic substances.

Binding of oxamniquine to the DNA of schistosomes

Hycanthone-sensitive and hycanthone-resistant schistosomes (which are also sensitive and resistant to oxamniquine) were exposed in vitro to tritium-labelled oxamniquine. The initial uptake of the drug into the schistosomes was essentially the same for the 2 strains. The homogenate of worms incubated with tritiated oxamniquine was fractionated and a purified DNA fraction was obtained by ethanol precipitation, RNAase and protease digestion, repeated phenolchloroform extractions, CsC1 gradient centrifugation and extensive dialysis. The DNA fraction from sensitive worms contained radioactive oxamniquine at a level corresponding to about 1 drug molecule per 50,000 base pairs, while the DNA from resistant worms contained essentially no drug. The results support the hypothesis that oxamniquine, like hycanthone, exerts its activity by alkylating macromolecules of sensitive schistosomes. The possibility is discussed that oxamniquine may lack the mutagenic properties of hycanthone because it is not an intercalating agent.

Binding of tritiated hycanthone and hycanthone N-methylcarbamate to macromolecules of drug-sensitive and drug-resistant schistosomes

Adult Schistosoma mansoni of the hycanthone-sensitive and of the hycanthone-resistant strain were exposed in vitro to tritium-labeled hycanthone. The drug was taken up in similar amounts by the two strains, a result which is not compatible with hypothetical mechanisms of resistance based on reduced drug entry into the schistosomes. Labeled hycanthone was found to bind irreversibly to macromolecules of sensitive schistosomes, whereas the binding was minimal in resistant worms. In particular, the DNA of sensitive schistosomes showed high levels of tightly bound hycanthone, while the corresponding fraction of resistant schistosomes failed to do so. Female schistosomes and immature worms, which are less sensitive to hycanthone, showed a diminished drug-DNA binding with respect to adult males. Tritiated hycanthone N-methylcarbamate, which is effective against sensitive and resistant schistosomes, bound in similar amounts to the DNA of both strains. These results strongly support a previously proposed mechanism of action of hycanthone, which is based essentially on the alkylation of worm macromolecules by a drug derivative produced in sensitive schistosomes.

Rat and hamster hepatocyte-mediated induction of SCEs and mutation in V79 cells and mutation of salmonella by aminofluorene and dimethylnitrosamine

Aminofluorene (AF) and dimethylnitrosamine (DMN) were examined for their ability to induce multiple genetic endpoints after rat and hamster hepatocyte metabolic activation. The endpoints measured included mutations at the Na+/K+-ATPase (ouabain resistance) and hypoxanthine-guanine-phosphoribosyltransferase (6-thioguanine resistance) loci, and sister-chromatid exchanges (SCEs) in Chinese hamster V79 cells, and mutation of Salmonella typhimurium strains TA98 and TA100. AF, with rat and hamster hepatocyte activation, induced only low levels of mutations at either loci in V79 cells but did induce SCEs. Mutation of Salmonella by AF after hepatocyte activation also occurred and was a sensitive endpoint for detecting this aromatic amine. DMN induced high levels of mutations at both loci in V79 cells in addition to SCEs in the presence of hepatocytes from both species. DMN was also mutagenic to Salmonella, but only with hamster hepatocytes. Salmonella did not respond as strongly to DMN as the V70 cells. Hamster hepatocytes were more active than rat hepatocytes in activating both carcinogens. The results indicate the variable sensitivity of the genetic endpoints and species differences in activation for two potent chemical carcinogens.

Metabolism and DNA binding of 2,6-dinitrotoluene in Fischer-344 rats and A/J mice

2,6-Dinitrotoluene (2,6-DNT) is a potent hepatocarcinogen in Fischer-344 rats, while its 2,4-isomer is believed to be noncarcinogenic. Neither 2,6-DNT nor 2,4-DNT is carcinogenic in the strain A mouse lung tumor bioassay. To explore the possible reasons for these differences in tumor responses, we have studied the in vitro metabolism and DNA binding of 2,6-DNT in cultured hepatocytes of the Fischer-344 rat and the A/J mouse, and have also investigated the in vivo DNA binding of 2,6-DNT and 2,4-DNT in these two species. In vitro metabolism of 2,6-DNT by rat and mouse hepatocytes was similar and resulted mainly in the formation of 2,6-dinitrobenzyl alcohol, either unconjugated or as a glucuronide (57.5 to 85.5% of the total per fraction), with smaller amounts of polar, acidic metabolites (8.4 to 38.7%) and minor amounts (1.2 to 5.3%) of 2-amino-6-nitrotoluene. Anaerobic metabolism of 2,6-DNT by an extract of rat or mouse cecal contents resulted mainly in the formation of 2-amino-6-nitrotoluene and 2-(N-acetylamino)-6-nitrotoluene, and minor amounts of 2,6-diaminotoluene. Ip administration of 2,6-DNT or 2,4-DNT (150 mg/kg each) to Fischer-344 rats resulted, after 24 hr, in covalent binding to DNA of the liver (131.1 to 259.9 pmol 2,6-DNT/mg DNA; 215.4 to 226.8 pmol 2,4-DNT/mg DNA), and lower binding to DNA of the lungs and the intestine (14.9 to 22.7 pmol 2,6-DNT/mg DNA; 45.0 to 75.0 pmol 2,4-DNT/mg DNA). Similar treatment of A/J mice resulted in lower binding in the liver (25.9 to 31.9 pmol 2,6-DNT/mg DNA; 42.6 to 58.9 pmol 2,4-DNT/mg DNA), no detectable binding of 2,6-DNT in extrahepatic tissues and low amounts of binding of 2,4-DNT to lung and intestinal DNA (9.7 to 39.0 pmol/mg DNA). In vitro binding of 2,6-DNT to DNA of cultured hepatocytes from both A/J mice and Fischer-344 rats required prior metabolism of 2,6-DNT by the respective extracts from cecal contents. DNA binding was non-detectable in hepatocytes incubated with 2,6-DNT only. It is concluded that binding of 2,6-DNT to liver DNA requires its prior reductive metabolism, probably by intestinal microorganisms, and that the higher binding of 2,6-DNT in the Fischer-344 rat than in the A/J mouse may, in part, be responsible for the high susceptibility of the Fischer-344 rat to 2,6-DNT carcinogenesis.(ABSTRACT TRUNCATED AT 400 WORDS)

Selective activation of carcinogenic aromatic amines to bacterial mutagens in the marine mussel Mytilus galloprovincialis

The postmitochondrial fraction of the marine mussel Mytilus galloprovincialis digestive gland activates selectively precarcinogenic aromatic amines, but not precarcinogenic benzo[a]pyrene, to Salmonella typhimurium TA 98 mutagens. This activation potential is NADPH-dependent, is not inducible by exposure to Diesel 2 oil and a polluted environment, and is inhibited by methimazole. The characteristics of this activation potential are consistent with the recent finding of the presence of FAD-containing-, and lack of cytochrome P-450 dependent-, monooxygenase activity in Mytilus edulis. The presence of such selective potential in marine invertebrate(s) may bring new insight into our understanding of the fate and the effects of carcinogens in the marine environment.

DNA modification by chemical carcinogens

The chemistry and molecular biology of DNA adducts is only one part of the carcinogenic process. Many other factors will determine whether a particular chemical will exert a carcinogenic effect. For example, the size of particles upon which a carcinogenic may be adsorbed will influence whether or not, and if so where, deposition within the lung will occur. The simultaneous exposure to several different agents may enhance or inhibit the metabolism of a chemical to its ultimate carcinogenic form (Rice et al., 1984; Smolarek and Baird, 1984). The ultimate carcinogenic metabolites may be influenced in their ability to react with DNA by a number of factors such as internal levels of detoxifying enzymes, the presence of other metabolic intermediates such as glutathione with which they could react either enzymatically or non-enzymatically, and the state of DNA which is probably most heavily influenced by whether or not the cell is undergoing replication or particular sequences being expressed. Replicating forks have been shown to be more extensively modified than other areas of DNA. Another critical factor which can influence the final outcome of the DNA damage is whether or not the modifications can be repaired. If this occurs with high fidelity and the cell has not previously undergone replication then the effect of the damage by the carcinogen is likely to be minimal. The major area in which progress is needed is an understanding of what this damage really does to the cell such that after an additional period of time, which may be as long as twenty or more years, these prior events are expressed and cell proliferation occurs. Clearly additional stimulatory factors, for example tumor promoting agents such as the phorbol esters or phenobarbital, are often needed. After such prolonged periods it seems likely that the DNA adducts would no longer be present. However, the way in which their earlier presence is remembered is not clear. Simple mutations do not explain all the characteristics of tumor progression and, when it occurs, regression. Even if a specific site mutation does occur then its expression must be under other types of control. Any explanation of the action of DNA modification at the molecular level also requires that account be taken of the diverse nature of the DNA adducts from simple modifications such as methylation to bulkier adducts such as benzo[a]pyrene, aflatoxin or aromatic amines.(ABSTRACT TRUNCATED AT 400 WORDS)