Mutagenicities of N-nitrosamines on Salmonella

The mutagenic activities of 11 N-nitrosamines were tested using Salmonella typhimurium TA100 and TA98. All the carcinogenic N-nitrosamines were mutagenic on TA100 with a drug-activating system from the rat liver, whereas N,N-diphenylnitrosamine, a non-carcinogen, was not mutagenic. None of the N-nitrosamines was mutagenic on TA98, except N,N-diethylnitrosamine which was weakly mutagenic. To detect the mutagenicity of N,N-dimethylnitrosamine, the pre-incubation of bacteria and N,N-dimethylnitrosamine with S-9 Mix before if was poured onto plates was obligatorily required. Dimethyl sulfoxide inhibited the mutagenic effect of N,N-dimethylnitrosamine.

Phenobarbital-induced synthesis of the microsomal drug-metabolizing enzyme system and its relationship to the proliferation of endoplasmic membranes. A morphological and biochemical study

Liver microsomes, isolated from rats which had been treated with phenobarbital in vivo, were found to exhibit increased activities of oxidative demethylation and TPNH-cytochrome c reductase and an increased amount of CO-binding pigment. Simultaneous administration of actinomycin D or puromycin abolished the phenobarbital-induced enzyme synthesis. Increased rate of P(i) (32) incorporation into microsomal phospholipid was the first sign of phenobarbital stimulation and appeared 3 hours after a single injection of this drug. Microsomes were divided into smooth-surfaced and rough-surfaced vesicle fractions. The fraction consisting of smooth-surfaced vesicles exhibited the greatest increase in protein content and oxidative demethylation activity after phenobarbital administration in vivo. Ultrastructural studies revealed that drug treatment also gave rise to proliferation of the endoplasmic reticulum in the hepatic parenchymal cells, first noticed after two phenobarbital injections. The phenobarbital-induced synthesis of the metabolizing enzymes is discussed with special reference to the relationship to the stimulated synthesis of the endoplasmic membranes.

Evaluation of genetic risks of alkylating agents. II. Haemoglobin as a dose monitor

The degree of alkylation of haemoglobin was determined at different times after treatment of mice with one directly active alkylating agent, ethylene oxide, and one agent that requires metabolic activation, dimethylnitrosamine. Because of the random alkylation of red blood cells of various ages and the stability of alkylated haemoglobin, the amount of alkylated amino acids in haemoglobin decreases linearly with time, reaching the value zero after about 40 days, the life-span of erythrocytes in the mouse. This provides a basis for the use of haemoglobin as a monitor for integral doses of genotoxic environmental chemicals.

MRP8, ATP-binding cassette C11 (ABCC11), is a cyclic nucleotide efflux pump and a resistance factor for fluoropyrimidines 2',3'-dideoxycytidine and 9'-(2'-phosphonylmethoxyethyl)adenine

MRP8 (ABCC11) is a recently identified cDNA that has been assigned to the multidrug resistance-associated protein (MRP) family of ATP-binding cassette transporters, but its functional characteristics have not been determined. Here we examine the functional properties of the protein using transfected LLC-PK1 cells. It is shown that ectopic expression of MRP8 reduces basal intracellular levels of cAMP and cGMP and enhances cellular extrusion of cyclic nucleotides in the presence or absence of stimulation with forskolin or SIN-1A. Analysis of the sensitivity of MRP8-overexpressing cells revealed that they are resistant to a range of clinically relevant nucleotide analogs, including the anticancer fluoropyrimidines 5'-fluorouracil (approximately 3-fold), 5'-fluoro-2'-deoxyuridine (approximately 5-fold), and 5'-fluoro-5'-deoxyuridine (approximately 3-fold), the anti-human immunodeficiency virus agent 2',3'-dideoxycytidine (approximately 6-fold) and the anti-hepatitis B agent 9'-(2'-phosphonylmethoxynyl)adenine (PMEA) (approximately 5-fold). By contrast, increased resistance was not observed for several natural product chemotherapeutic agents. In accord with the notion that MRP8 functions as a drug efflux pump for nucleotide analogs, MRP8-transfected cells exhibited reduced accumulation and increased efflux of radiolabeled PMEA. In addition, it is shown by the use of in vitro transport assays that MRP8 is able to confer resistance to fluoropyrimidines by mediating the MgATP-dependent transport of 5'-fluoro-2'-deoxyuridine monophosphate, the cytotoxic intracellular metabolite of this class of agents, but not of 5'-fluorouracil or 5'-fluoro-2'-deoxyuridine. We conclude that MRP8 is an amphipathic anion transporter that is able to efflux cAMP and cGMP and to function as a resistance factor for commonly employed purine and pyrimidine nucleotide analogs.

The 399Gln polymorphism in the DNA repair gene XRCC1 modulates the genotoxic response induced in human lymphocytes by the tobacco-specific nitrosamine NNK

DNA repair plays a critical role in protecting the genome of the cell from the insults of cancer-causing agents such as those found in tobacco smoke. Reduced DNA repair capacity would, therefore, constitute a significant risk factor for smoking-related cancers. Recently, a number of polymorphisms in several DNA repair genes have been discovered, and it is possible that these polymorphisms may affect DNA repair capacity and thus modulate cancer susceptibility in exposed populations. In the current study, we explored the relationship between two polymorphisms in the DNA repair gene XRCC1 (polymorphisms in codons 194 and 399) and the genotoxic response induced by the tobacco-specific nitrosamine, 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK). The sister chromatid exchange (SCE) assay was used as a marker of genetic damage. Our results, using whole blood cultures from 47 volunteers, indicated that treatment of cells with 0.24, 0.72 and 1.44 mM of NNK induced a concentration-dependent increase in the mean number of SCE (P<0.001). There was a significant difference (P<0.05) in response to NNK treatment between cells from individuals with the 399Gln allele (either homozygous or heterozygous) and cells from individuals with the homozygous 399 Arg/Arg genotype. Treatment of cells that have the 399Gln allele with 0.24, 0.72 and 1.44 mM NNK resulted in 22.8, 35.8 and 52.8% increases in NNK-induced SCE, respectively. Treatment of cells with the 399 Arg/Arg genotype using the same NNK concentrations resulted in 16.0, 15.5 and 32.6% increases in NNK-induced SCE, respectively. In contrast, no significant difference in NNK-induced SCE was observed between cells with the codon 194 Arg/Arg genotype and cells with the codon 194 Arg/Trp genotype at all concentrations of NNK tested. These data suggest that the Arg399Gln amino acid change may alter the phenotype of the XRCC1 protein, resulting in deficient DNA repair. Our study underscores the important role of polymorphisms in DNA repair genes in influencing the genotoxic responses to environmental mutagens, and justifies additional studies to investigate their potential role in susceptibility to cancer.

Cytogenetic effects of mutagens/carcinogens after activation in a microsomal system in vitro I. Induction of chromosome aberrations and sister chromatid exchanges by diethylnitrosamine (DEN) and dimethylnitrosamine (DMN) in CHO cells in the presence of rat-liver microsomes

A rat-liver microsomal system in vitro has been used to activate two indirectly acting carcinogens, DMN and DEN. On activation, both compounds were extremely potent in inducing chromosomal aberrations as well as sister chromatid exchanges in Chinese hamster cells. The implications of these findings and the potential utility of this technique to detect mutagens/carcinogens are discussed.

Transport of the beta -O-glucuronide conjugate of the tobacco-specific carcinogen 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL) by the multidrug resistance protein 1 (MRP1). Requirement for glutathione or a non-sulfur-containing analog

Nitrosamine 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) and its metabolite 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL) play a crucial role in the induction of lung cancer, and NNAL-O-glucuronide formation and elimination are important steps in detoxification of these compounds. In the present study, we investigated the ATP-binding cassette (ABC) protein, MRP1 (ABCC1), as a candidate transporter responsible for NNAL-O-glucuronide export. MRP1 mediates the active transport of numerous GSH-, sulfate-, and glucuronide-conjugated organic anions and can transport certain xenobiotics by a mechanism that may involve co-transport with GSH. Using membrane vesicles prepared from transfected cells, we found that MRP1 transports [3H]NNAL-O-glucuronide but is dependent on the presence of GSH (Km 39 microm, Vmax 48 pmol x mg(-1) x min(-1)). We also found that the sulfur atom in GSH was dispensable because transport was supported by the GSH analog, gamma-glutamyl-alpha-aminobutyryl-glycine. Despite stimulation of NNAL-O-glucuronide transport by GSH, there was no detectable reciprocal stimulation of [3H]GSH transport. Moreover, whereas the MRP1 substrates leukotriene C4 (LTC4) and 17beta-estradiol 17beta-(d-glucuronide) (E(2)17betaG) inhibited GSH-dependent uptake of [3H]NNAL-O-glucuronide, only [3H]LTC4 transport was inhibited by NNAL-O-glucuronide (+GSH) and the kinetics of inhibition were complex. A mutant form of MRP1, which transports LTC4 but not E(2)17betaG, also did not transport NNAL-O-glucuronide suggesting a commonality in the binding elements for these two glucuronidated substrates, despite their lack of reciprocal transport inhibition. Finally, the related MRP2 transported NNAL-O-glucuronide with higher efficiency than MRP1 and unexpectedly, GSH inhibited rather than stimulated uptake. These studies provide further insight into the complex interactions of the MRP-related proteins with GSH and their conjugated organic anion substrates, and extend the range of xenotoxins transported by MRP1 and MRP2 to include metabolites of known carcinogens involved in the etiology of lung and other cancers.

Calcium-independent modulation of cyclic GMP and activation of guanylate cyclase by nitrosamines

Nitrosamines markedly increase concentrations of guanosine 3', 5' - monophosphate (cyclic GMP) in several tissues from the rat and in human colonic mucosa. These agents are effective in the absence of extracellular calcium and enhance guanylate cyclase activity in tissue homogenates. Stimulation of cyclic GMP was greatest in liver, where the carcinogenic activity of nitrosamines is also most pronounced.

Tobacco-specific carcinogenic nitrosamines. Ligands for nicotinic acetylcholine receptors in human lung cancer cells

Lung cancer demonstrates a strong etiologic association with smoking. Of the two most common histologic lung cancer types, small cell carcinoma (SCLC) is found almost exclusively in smokers, whereas peripheral adenocarcinoma (PAC) also develops in a significant number of nonsmokers. N'-Nitrosonornicotine (NNN) and 4(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), nicotine-derived nitrosamines, are potent lung carcinogens contained in tobacco products. Because of the structural similarity of NNN and NNK with nicotine, we hypothesized that these carcinogens are ligands for nicotinic acetylcholine receptors (nAChRs). Using cell lines derived from human small cell lung carcinoma and pulmonary adenocarcinoma with the site-selective ligands alpha-bungarotoxin (alpha-BTX) and epibatidine (EB) in receptor binding and cell proliferation assays, we found that SCLC expressed neuronal nicotinic receptors with high affinity to alpha-BTX, whereas PAC cells expressed nicotinic receptors with high affinity to EB. NNK bound with high affinity to alpha-BTX-sensitive nAChRs in SCLC cells, while NNN bound with high affinity to EB sensitive nAChRs in PAC cells. The affinity of each nitrosamine to these receptors was several orders of magnitude greater than that of nicotine. NNK stimulated the proliferation of SCLC cells via this mechanism. Our findings suggest that NNK may contribute to the genesis of SCLC in smokers via chronic stimulation of the alpha BTX-sensitive nAChR-subtype expressed in these cells. Both nitrosamines may also contribute to a host of nicotine-related diseases that are currently thought to be caused by the chronic interaction of nicotine with nAChRs expressed in a large spectrum of mammalian cells.

Endogenous versus exogenous exposure to N-nitroso compounds and gastric cancer risk in the European Prospective Investigation into Cancer and Nutrition (EPIC-EURGAST) study

The risk of gastric cancer (GC) associated with dietary intake of nitrosodimethylamine (NDMA) and endogenous formation of nitroso compounds (NOCs) was investigated in the European Prospective Investigation into Cancer and Nutrition (EPIC). The study included 521,457 individuals and 314 incident cases of GC that had occurred after 6.6 average years of follow-up. An index of endogenous NOC (ENOC) formation was estimated using data of the iron content from meat intake and faecal apparent total NOC formation according to previous published studies. Antibodies to Helicobacter pylori and vitamin C levels were measured in a sub-sample of cases and matched controls included in a nested case-control within the cohort. Exposure to NDMA was < 1 microg on average compared with 93 mug on average from ENOC. There was no association between NDMA intake and GC risk (HR, 1.00; 95% CI, 0.7-1.43). ENOC was significantly associated with non-cardia cancer risk (HR, 1.42; 95% CI, 1.14-1.78 for an increase of 40 microg/day) but not with cardia cancer (HR, 0.96; 95% CI, 0.69-1.33). Although the number of not infected cases is low, our data suggest a possible interaction between ENOC and H.pylori infection (P for interaction = 0.09). Moreover, we observed an interaction between plasma vitamin C and ENOC (P < 0.02). ENOC formation may account for our previously reported association between red and processed meat consumption and gastric cancer risk.

Approaches to cancer prevention based on an understanding of N-nitrosamine carcinogenesis

Understanding carcinogenesis is critical for development of rational approaches to cancer prevention. This paper uses N-nitrosamine carcinogenesis as an example. N-Nitrosamines are a large group of potent carcinogens. Approximately 300 different N-nitrosamines are carcinogenic. At least 30 animal species are responsive to their effects. There is little doubt that humans exposed to sufficient amounts of N-nitrosamines would also be susceptible to their carcinogenic effects. Human exposure to preformed N-nitrosamines occurs through the diet, in certain occupational settings, and through the use of tobacco products, cosmetics, pharmaceutical products, and agricultural chemicals. Diminishing human exposure to these carcinogens is one approach to prevention of cancer, and this has been accomplished in many instances, although exposure to N-nitrosamines in tobacco products is still unacceptably high. Human exposure to N-nitrosamines also occurs by nitrosation of amines in the body, via their acid or bacterial catalyzed reaction with nitrite, or by reaction with products of nitric oxide generated during inflammation or infection. A second approach toward prevention of N-nitrosamine carcinogenesis is inhibition of this endogenous N-nitrosamine formation. Substantial reductions have been achieved with ascorbic acid and other nitrite scavengers. N-Nitrosamines undergo a simple cytochrome P450-mediated metabolic activation step, which is critical for their carcinogenicity. The third approach involves the use of chemopreventive agents that block this step, or other steps in the carcinogenic process. A large number of potent chemopreventive agents against nitrosamine carcinogenesis have been identified. Chemoprevention of lung cancer induced by the tobacco-specific nitrosamine 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) is discussed as an example of this approach.

Comparative studies of the hepatocarcinogen N,N-dimethylnitrosamine in vivo: reaction sites in rat liver DNA and the significance of their relative stabilities

The reaction of the hepatocarcinogen N,N-dimethylnitrosamine has been compared with that of methyl methanesulphonate, a methylating agent which is not a liver carcinogen. Consistent differences have been observed in the reaction of rat liver DNA in vivo with these agents; O(6)-alkylation and the production of unidentified acid-labile products were distinctive features of the reaction with the carcinogenic nitroso compound but were undetectable or in low yield, respectively, after reaction with the alkyl sulphonate. Evidence has been obtained for the excision of these reaction products in animals treated with the hepatocarcinogen and the significance of their relative stabilities is discussed.

Preferential effects of nicotine and 4-(N-methyl-N-nitrosamine)-1-(3-pyridyl)-1-butanone on mitochondrial glutathione S-transferase A4-4 induction and increased oxidative stress in the rat brain

We have investigated the in vivo effects of the tobacco-specific toxins nicotine and 4-(N-methyl-N-nitrosamino)-1-(3-pyridyl)-1-butanone (NNK) on antioxidant defense systems in the mitochondrial, microsomal, and cytosolic compartments of rat brain, lung, and liver. Nicotine induced maximum oxidative stress in brain mitochondria, as seen from a 1.9-fold (P < 0.001) increase in thiobarbituric acid-reactive substance (TBARS) and a 2-fold (P < 0.001) increase in glutathione S-transferase (GST) A4-4 (also referred to as rGST 8-8) activities. These changes were accompanied by a 25-40% increase in reactive oxygen species and a 20-30% decrease in alcohol dehydrogenase activities. The 4-(N-methyl-N-nitrosamino)-1-(3-pyridyl)-1-butanone-induced oxidative damage was apparent in the microsomal fraction of brain, lung, and liver, and it also increased 4-hydroxynonenal specific GST A4-4 activity in the brain and lung mitochondrial matrix fraction. The levels of microsomal thiobarbituric acid reactive substance, cytochrome P4502E1 activity, and reactive oxygen species were also increased significantly (P < 0.001) in all tissues. Both of these toxins induced the level of GST A4-4 mRNA in the brain, while they caused a marked reduction in the liver GST A4-4 mRNA pool. Additionally, the brain mitochondrial matrix showed a markedly higher level of 4-hydroxynonenal specific GST activity and mGST A4-4 antibody-reactive protein than did the cytosolic fraction. In conclusion, the present study provides evidence for the occurrence of GST A4-4 enzyme activity in mammalian mitochondria, in addition to demonstrating that both mitochondria and microsomes are intracellular targets for nicotine- and NNK-induced organ toxicity.

Nitric oxide synthase-independent long-term potentiation in area CA1 of hippocampus

In order to elucidate the role of NO in LTP, we have investigated a biochemical effector of NO action at hippocampal synapses, guanylyl cyclase. We have observed that LTP-inducing stimuli elicit an increase in the activity of guanylyl cyclase, an effect blocked by inhibitors of NO synthase. Extracellular application of hemoglobin, which binds NO and thereby blocks its actions, also attenuated the increase in guanylyl cyclase activity. Taken together, these results provide direct biochemical evidence for an elevation of NO levels with LTP-inducing stimulation, and support the hypothesis that NO can function as a transcellular messenger in the hippocampus. These findings also implicate guanylyl cyclase as a target of NO and demonstrate that while NO is elevated with LTP-inducing stimuli, the activity of NO synthase is not necessary for induction of LTP by all stimulus paradigms.

Carcinogenic nitrosamines: free radical aspects of their action

NDMA and other nitrosamines may be activated into DNA binding intermediates by a cytochrome P450-dependent formation of alpha-nitrosamino radicals or photochemically. Within the catalytic site of cytochrome P450, these radical intermediates either combine with HO. to form alpha-hydroxynitrosamines or decompose into nitric oxide and N-methylformaldimine. In the presence of phosphate, nutagenic alpha-phosphonooxy derivatives are formed from radicals generated chemically/photochemically. Studies on lipid peroxidation, in vivo and in vitro, have further suggested that radicals are formed as intermediates from N-nitrosodialkylamines. The level of nitrosamine-induced lipid peroxidation parallels hepatocarcinogenicity in rats. These data, although preliminary, provide further evidence that free radical damage and DNA alkylation are involved in carcinogenesis induced by nitrosamines.

Nicotinic receptors mediate tumorigenic action of tobacco-derived nitrosamines on immortalized oral epithelial cells

Frequent users of smokeless tobacco (ST) have an increased risk for developing oral cancer. Nicotine and its derivatives may contribute to tumorigenesis through stimulation of nicotinic acetylcholine receptors (nAChRs) in target cells. Emerging evidence indicates that nAChRs can be stimulated by the nicotine-derived nitrosamines 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) and N'-nitrosonornicotine (NNN) that can induce oral cavity tumors in laboratory animals. This study was designed to elucidate the receptor-mediated mechanisms of the initiation and progression of NNK-, and NNN-induced oral cancers. We used Het-1A cells that were found to express alpha3, alpha5, alpha7, alpha9, beta2 and beta4 nAChR subunits. Both NNK and NNN competed with nicotinic radioligands for binding to Het-1A cells. NNK showed a higher than NNN affinity to the [3H]nicotine-labeled binding sites, and NNN-to the [3H]epibatidine-sensitive nAChRs. NNK and NNN increased proliferative potential of Het-1A cells and produced an anti-apoptotic effect, which was alleviated by antagonists. alpha-Bungarotoxin was most effective against NNK and mecamylamine against NNN. Treatment of Het-1A cells with either NNK or NNN led to acquisition of capability of anchorage independent growth and ability to produce tumors in nude mice, both of which can be by inhibited by antagonists. To elucidate the signaling mechanisms, we studied transcription of the genes encoding the cell cycle, apoptosis and signal transduction regulators at both the mRNA and protein levels. The Het-1A cells stimulated with nitrosamines showed multifold increases of the mRNA transcripts encoding PCNA and Bcl-2, and upregulated expression of the transcription factors GATA3, nuclear factor-kappaB, and STAT-1. The STAT-1 protein-binding activity induced by NNK and NNN correlated with elevated gene expression. The obtained results establish the role of specific nAChR subtypes in tobacco-related carcinogenesis and open a novel avenue for oral cancer chemoprevention.

Differentiation of regenerating pancreatic cells into hepatocyte-like cells

Differentiation is the process by which multicellular organisms achieve the specialized functions necessary for adaptation and survival. An in vivo model in the Syrian golden hamster is described in which regenerating pancreatic cells are converted into hepatocyte-like cells, as evidenced by the presence of albumin, peroxisomes, and a variety of morphological markers. These cells are stable after the conversion is triggered by a single dose of the carcinogen N-nitrosobis(2-oxopropyl)amine administered during the S phase in regenerating pancreatic cells. This suggests that, given the proper stimulus, regenerating cells in adult pancreas can be redirected into a totally different pathway of differentiation.