On the aetiology of gastric cancer: mutagenicity of food extracts after incubation with nitrite

Mutagenic activity of nitrosated foods in an area with a high risk for stomach cancer

In a search for possible sources of mutagens in human foods, a study was made of six common foods in the diet of a population at high risk for gastric cancer (in Nariño, Colombia). No mutagenic activity was demonstrated in nonnitrosated foods. After nitrosation, only one of the foods, fava beans, demonstrated a mutagenic effect. This effect was direct and concentrated in the germ and pulp of the beans.

Formation of N-nitroso-N-methylurea in various samples of smoked/dried fish, fish sauce, seafoods, and ethnic fermented/pickled vegetables following incubation with nitrite under acidic conditions

In continuation of our previous studies on N-nitroso-N-methylurea (NMU) formation in cured meats following incubation with nitrite at gastric pH, we extended the investigation to other foods mentioned in the title of this paper. The main objective was to determine whether these foods have the potential to form NMU at pH's that can be found in the human stomach. This was done by nitrosating an aliquot (5 g for fish sauce, 10 g for the others) of each with 7.25 microM to 1.59 mM levels of sodium nitrite for 2 h at room temperature at pH 0.8--1.5 and measuring the amounts of NMU formed. Of the samples tested, fish sauce formed 2--712 ng of NMU, followed in decreasing order by herring (<0.3--688 ng); dried anchovy, shrimp, and other fishes (<0.3--134 ng); crab and lobster paté (<0.3--342 ng); sardines (6--59 ng); oysters and mussels (11--31 ng); dried squid (3--47 ng); kimchi (7--107 ng); and Japanese pickled radish (<0.3--72 ng). Incorporation of 200-2000 ppm of ascorbic acid in the fish sauce and other foods, prior to nitrosation, appreciably inhibited such NMU formation. Although previous researchers in China reported NMU formation in nitrosated samples of fish sauce, this is the first reported formation of NMU upon nitrosation of the other foods mentioned above, and the first reported inhibition of such formation by added ascorbic acid.

Genotoxicity of instant coffee and of some phenolic compounds present in coffee upon nitrosation

Instant coffee exhibits genotoxic activity upon nitrosation at acidic pH values in the Ames tester strain TA100. Using adsorption chromatography (Amberlit XAD-2) it was observed that the major fraction of molecules responsible for the genotoxic activity upon nitrosation was not retained on this resin, suggesting that the polar molecules present in instant coffee could be responsible for the genotoxicity observed upon nitrosation. Some phenolic molecules present in instant coffee (catechol, caffeic acid, and chlorogenic acid) were also genotoxic upon nitrosation under the same experimental conditions. The concentrations of nitrosatable phenolic compounds in the studied coffee were determined by HPLC and their contributions to the total genotoxicity observed were studied. The results obtained suggest that besides phenolic compounds other molecules were also involved in the genotoxicity of this beverage upon nitrosation. Teratogenesis Carcinog. Mutagen. 20:241-249, 2000.

Gastric carcinogenesis: 2-chloro-4-methylthiobutanoic acid, a novel mutagen in salted, pickled Sanma hiraki fish, or similarly treated methionine

The customary salting and pickling of fish in high risk gastric cancer regions were modeled to explore the relevant causative chemicals. The fish Sanma hiraki was treated with sodium chloride and sodium nitrite at pH 3. Previously, it had been found that an extract of the treated fish was mutagenic in Salmonella typhimurium TA 1535 without S9 and also that it induced glandular stomach cancer upon gavage to rats. We now demonstrate that the mutagenicity was enhanced by preincubation of the raw meat for several days before salt-nitrite treatment. HPLC techniques showed that three mutagens were present in the fish extract. One of the mutagens was found to be stable over the pH range of 1.0-9.0. This mutagen was purified by silica gel solid phase extraction, followed by a series of reverse phase HPLC steps, and was characterized by low and high resolution MS, NMR, and FT-IR. While N-nitroso compounds were generally believed to be associated with gastric carcinogenesis, it was unexpectedly found that the mutagen has the novel structure 2-chloro-4-methylthiobutanoic acid (CMBA). Based on the structure, it seemed likely that methionine might be the precursor, and this was, indeed, proven. Both salt and nitrite are essential factors for forming this mutagen. The yield of CMBA was linear for chloride concentrations from 0 to 800 mM NaCl. Of 20 amino acids reacted with nitrite and chloride at pH 3, only methionine generated a mutagen for S. typhimurium TA 1535. Tryptophan gave a product mutagenic in S. typhimurium TA 100 and TA 98, but not TA 1535, and in the case of tyrosine, the mutagen was active only for TA 100. These results suggest an important role for salt in gastric carcinogenesis and provide new approaches for exploring the formation of mutagens/carcinogens for specific target organs.

Nutrition and stomach cancer

Epidemiologic evidence on the relation between nutrition and stomach cancer is reviewed. Stomach cancer shows a distinct international variation and dramatic worldwide decline. These descriptive features suggest that dietary factors are important in determining the risk of stomach cancer. The authors assessed relevant data regarding specific dietary hypotheses in the etiology of stomach cancer. A negative association with fresh vegetables and fruits is highly consistent in numerous case-control studies in different populations. Both epidemiologic and experimental data suggest that vitamins C and carotenoids lower risk of stomach cancer. Evidence is sparse and inconsistent as to protective effects of vitamin E and selenium. Epidemiologic studies have not lent, and will not provide, supportive evidence for an etiologic role of nitrate intake. High salt intake has been associated with an increased risk in many case-control studies and limited cohort studies. Taken together with animal data, it is considered that high salt intake is a risk factor for stomach cancer. Both epidemiologic and experimental data are inconclusive as to whether high-starch diets confer an increased risk. Cohort studies using quantitative dietary assessment and biologic measurement of micronutrients are needed for further understanding of etiologic roles of dietary factors in the causation of stomach cancer.

Teas and tea components as inhibitors of carcinogen formation in model systems and man

BACKGROUND

Epidemiological evidence points to a cancer protective role of green-yellow-orange vegetables and fruits. The involvement of teas as a protective factor in carcinogenesis has not received the attention it seems to merit. To gain relevant information, attempts were made to stimulate in vitro those conditions to which human groups are actually exposed.

METHODS

The inhibitory effects of infusions of Chinese, Japanese, and Ceylonese teas were examined by adding them to a nitrosation mixture consisting of 0.8 mg sodium nitrite and 340 mg equivalent of a widely consumed salt-preserved fish (Pak Wik) and estimating the frequency of mutants in TA 1535 strain of Salmonella typhimurium.

RESULTS

The tea samples exhibited a strong inhibitory effect at concentrations that are actually ingested by man. A comparable inhibition was obtained by several tea phenolics. A second series of experiments dealt with the formation of nitrosoproline (NPRO) which can be strongly inhibited in vitro by the tea infusions and tea phenolics. The effects of the tea infusions and caffeic acid on the endogeneous formation of NPRO in man were examined by having volunteers ingest 300 mg sodium nitrate and 30 min later 300 mg proline, collecting urine samples over a 24-hr period, and estimating the excreted NPRO. The tested teas, at doses regularly consumed, again exerted a strong inhibitory effect on endogeneous NPRO formation in humans. Comparable inhibitory effects were obtained by ingesting caffeic acid, chlorogenic acid, or ferulic acid with the nitrosation mixture.

CONCLUSIONS

These results indicate that the simultaneous intake of teas with food products that are being nitrosated within the stomach of human subjects should exert a protective, beneficial effect.

N-nitrosamine and mutagenicity formation in Chinese salted fish after digestion

Salted and dried fish (Nemipterus virgatus), acquired from Hong Kong, was treated with 0.43-110 mM nitrite during in vitro digestion using gastric enzymes and the volatile N-nitrosamine content and mutagenicity on Salmonella typhimurium TA100 assayed without concentration. N-Nitrosodimethylamine (NDMA; the only nitrosamine detected) formation was second order in nitrite concentration. When 10 g of fish was treated with 6.96 mM nitrite, 394 nM NDMA was formed. Thiocyanate was catalytic for NDMA formation at nitrite concentration greater than 0.87 mM and when the ratio of thiocyanate to nitrite was greater than 1. Approximately a 50% inhibition in NDMA formation by ascorbic acid was seen when the ratio of ascorbate to nitrite was approximately 2 or greater and the nitrite concentration was 1.74 mM. Mutagenicity increased with increasing nitrite concentration but the addition of thiocyanate did not increase mutagenicity over nitrite alone. Ascorbate increased mutagenicity even though NDMA formation was inhibited. Even at nitrite concentrations greater than 100-fold higher than expected in vivo, there was insufficient NDMA formed to account for the observed mutagenicity. These data do not exclude the possibility that the observed mutagenicity was due to non-volatile N-nitroso compounds, however, this possibility seems unlikely given the effects of ascorbate and thiocyanate which would be expected to inhibit and enhance non-volatile N-nitroso compound formation.

In-vitro testing and the carcinogenic potential of several nitrosated indole compounds

4-chloro-methoxyindole is a naturally occurring compound in Vicia faba which can easily react with nitrite to form a N-nitroso compound. In this in vitro study, the potential genotoxic effects of nitrosated 4-chloro-6-methoxyindole and its structural analogue 4-chloroindole were evaluated for the first time by using both Salmonella and Chinese hamster V79 cells. Additionally, the inhibition of gap junctional intercellular communication in V79 cells by these compounds was determined; this is a validated parameter for tumor-promoting activity. Most assays were also performed with nitrosated indole-3-acetonitrile, a naturally occurring compound in brassicas. Both nitrosated chloroindoles were highly mutagenic to Salmonella typhimurium TA100 without the need of exogenous metabolic activation and were potent inducers of Sister Chromatid Exchanges. Nitrosated indole-3-acetonitrile generated the same effects, although at much higher concentrations. Equivocal results were obtained for the nitrosated chloroindoles in a forward mutation assay using the hypoxanthine guaninephosphoribosyltransferase locus. All nitrosated indole compounds significantly inhibited gap junctional intercellular communication. These results indicate that nitrosated chloroindoles and nitrosated indole-3-acetonitrile should be considered as mutagens and agents with potential tumor-promoting capacity.

Mutagenicity of soy sauce treated with a physiologically feasible concentration of nitrite

Soy sauce treated with nitrite was found to be more mutagenic to Escherichia coli WP2 uvrA/pKM101 than Salmonella typhimurium TA100 without S9 mix. The mutagenicity of soy sauce treated with nitrite was affected by the concentration of soy sauce in the nitrosation mixture, and a concentration of 5% resulted in the highest specific activity (revertants/ml soy sauce equivalent). By incubating soy sauce at a concentration of 5% in a solution of 1 mM nitrite at pH 3 for 1 h at 37 degrees C, the equivalent of 1 ml of soy sauce induced 2790 revertants of E. coli WP2 uvrA/pKM101 without S9 mix.

Mutagenicity screening of foods. II. Results with fruits and vegetables

A survey of the mutagenic potential of a wide variety of food products has been initiated with results for 28 different beverages reported previously [Stoltz et al, 1982b]. Here, results for samples of 46 widely consumed fruits and vegetables from six general categories are given. Each sample was concentrated and fractionated by polarity and solubility to give five fractions, each of which was assayed for mutagenic potential with Salmonella typhimurium TA98 and TA100. Although statistical analysis of the data resulted in positive findings for 22 fruit and vegetable samples, only six products (grapes, onions, peaches, raisins, raspberries, strawberries) demonstrated potent mutagenic activity.

Inhibitory effects of phenolics, teas and saliva on the formation of mutagenic nitrosation products of salted fish

The objectives of this study were to simulate in vitro some of the conditions that may prevail in man during the ingestion of a meal and to quantitate the inhibitory effect of phenolics and phenolic-containing beverages on the formation of mutagenic nitrosation products. The test system consisted of nitrosating (pH 2, 1 h, 37 degrees C) an aqueous fraction of a salt-preserved Chinese fish (Pak Wik) with or without the inhibitors to be tested and estimating the frequency of his+ revertants per survivor of Salmonella typhimurium (strain TA1535). The phenolics and teas were added to the nitrosation mixture. Catechin, chlorogenic acid, gallic acid, pyrogallol and tannic acid suppressed the formation of mutagenic nitrosation products. The inhibitory efficiency was comparable to that of ascorbic acid. A Japanese, a Chinese and a Ceylonese tea also prevented the formation of mutagenic nitrosated fish products at doses which are usually consumed by man. Moreover, saliva exerted an inhibitory effect. The inhibitory effect was not additive when the phenolics or saliva were added concurrently to the nitrosation mixture. The possibility that phenolics are involved in the apparent chemopreventive effect of fruits and vegetables is discussed.

Mutagenicity screening of foods I. Results with beverages

Following a number of recent reports on the presence of mutagens in certain foods, a general survey of the mutagenic potential of a wide variety of food products has been initiated. Here, results for samples of 28 widely consumed beverages from 13 general categories are reported. Each sample was concentrated and fractionated by polarity and solubility to give up to seven fractions, each of which was assayed for mutagenic potential with Salmonella typhimurium TA98 and TA100 +/- fortified liver homogenate. Fractions showing evidence of either mutagenicity or toxicity were retested at the same and lower concentrations. The utility of the fractionation procedure and the sensitivity of the screening strategy were established by assaying six beverages spiked with known mutagens prior to fractionation. Statistical analysis of the data resulted in positive findings for seven beverages, although confirmation of these results through analysis of a second sample was obtained only for red wine, grape juice, and instant coffee. The remaining 21 beverages showed no strong evidence of mutagenic activity. For those foods for which the variation among replicate plates was largest, the false-positive rate for the two-stage screening procedure employed was estimated to be less than 1% while the false-negative rate for a beverage inducing a threefold increase in the background mutation rate was conservatively estimated to be limited to 14%.

Vitamins and cancer prevention: issues and dilemmas

Vitamins are a class of organic compounds that are components of an adequate diet. They or their derivatives function as coenzymes, cellular antioxidants, and/or regulators of gene expression. Fourteen vitamins are recognized in human nutrition (Vitamins A, D, E, K, B1, B2, B6, B12, C, niacin, folacin, pantothenic acid, biotin, choline), with deficiencies or excesses in intake leading to changes in protein, nucleic acid, carbohydrates, fat and/or mineral metabolism. Thus, the integrity of physiological systems, including those associated with detoxification, cellular repair, immune processes, and neural and endocrine function, depends upon the nutritional and vitamin status of the host. For these reasons, it may be anticipated that the adequacy of the vitamin supply to cells and tissues would affect the development, progress, and outcome of cancers. In this review, the definition and functions of and requirements and recommended allowance for vitamins are discussed briefly before exploring the evidence, largely from studies in experimental animals, that indicates the nature of the link between vitamins and cancer. Although evidence based on studies in animal systems reveals that vitamin intake and status can modulate the outcome of experimental carcinogenesis, the findings are often conflicting and difficult to interpret. Furthermore, it is not yet possible to develop a suitable prediction of the role of the individual vitamins in tumor development. The significance of these observations for human nutrition and cancer prevention, particularly in reference to ascorbic acid (vitamin C), vitamin E, and B-complex vitamins is considered. Vitamin A and retinoid compounds are discussed elsewhere in the symposium. The many popular misconceptions and unsound advice concerning vitamins and health, including "fake" vitamins-pangamic acid ("vitamin B15") and laetrile ("vitamin B17")-are also discussed. On the basis of current evidence, it would be inappropriate to recommend either substantial changes in habitual vitamin intakes, as provided by an adequate, well-balanced diet, or promotion of megavitamin intakes, as a means of reducing risk from cancers in the human population. However, a prudent approach toward diet and food habits, as a means of better optimizing the health consequences of our complex lifestyle is to be recommended.

Mutagenic factors in cooked foods

The charred surface of fish and beef showed strong mutagenic activity in Salmonella typhimurium test strains when activated by S-9 mix of rat liver. The pyrolysis products of proteins and amino acids were also highly mutagenic. Among the pyrolysis products of amino acids, those of tryptophan, serine, and glutamic acid were most active. The new gamma-carboline derivatives, 3-amino-1,4-dimethyl-5H-pyrido[4,3-b]indole and 3-amino-1-methyl-5H-pyrido[4,3-b]indole, were purified from the pyrolysis products of tryptophan. These new compounds were stronger mutagens than aflatoxin B1 towards S. typhimurium TA98, a frameshift type mutant, and they also transformed cryopreserved Syrian hamster embryo cells in vitro. Tryptophan pyrolysate also contained the beta-carboline derivatives, norharman and harman, which are not mutagenic alone, but act as comutagens. A mixture of norharman or harman and nonmutagenic aniline or o-toluidine was strongly mutagenic. The mutagenicities of charred products of other foods, such as seaweed and garlic, are reviewed in this article. Flavonoids, such as kaempferol and quercetin, and glycosides of these flavonoles were mutagenic. The mutagenicity of cooked vegetables depends partly on these flavonoid derivatives. The already-known existence of benzol[a]pyrene and nitroso compounds in cooked food is also reviewed.

Mutagens in the feces of 3 South-African populations at different levels of risk for colon cancer

The incidence of mutagens in the feces of 3 South-African populations at different risk levels for colon cancer has been determined. Lyophilized fecal samples were extracted with ether and the mutagenicity of the extracts determined using the Salmonella/mammalian microsome mutagenicity test. 19% of the samples from urban white South-Africans, a population at a high risk for colon cancer, were mutagenic using Salmonella typhimurium strain TA100. This incidence was significantly greater (p less than 0.001) than the incidence of mutagen excretion in the low-risk populations of urban blacks (2%) and rural blacks (0%). This pattern was also obtained using Salmonella typhimurium strain TA98. The incidence of mutagen excretion for urban whites was 10%, as compared to 5% and 2% for urban and rural blacks, respectively.

Mechanism of action of diet as a carcinogen

Stomach cancer in the United States has decreased over the last 50 years. It is still a major type of neoplasm in Japan, Eastern and Northern Europe, and parts of Latin America. Current concepts suggest that the reduction of gastric cancer in the U. S. stems from an increased consumption of foods with vitamin C on a year round basis, which is shown to antagonize the formation of putative gastric carcinogens. Risk factors for large bowel, breast, and prostate cancer are totally different from those for gastric cancer and thus are amenable to independent controls, with the goal of ultimately reducing the risk and preventing these major cancers in man. Current research aims to identify the nature of the mutagenic materials obtained during the frying of protein-containing foods. This process may be involved in the generation of carcinogens for cancer of the colon, breast, and prostate. Cancer of the colon is subject to somewhat different controlling elements than cancer of the breast because of the nature of the cell kinetics governing these tissues. Thus, the mechanism of action of diet involves lifestyle. The type, quality, and mode of cooking of food, particularly, play important roles in the etiology of the main human cancers in the gastrointestinal tract and the endocrine-sensitive organs.

Dietary and nutritional implications in the multifactorial etiology of certain prevalent human cancers

It has been estimated that 80--90% of the cancer rate in the U. S. can be attributed to environmental factors. For the last 20 years the role of smoking has been recognized by the scientific community. However it is only recently that the role of diet, i.e., food and beverages in carcinogenesis has begun to be recognized. It is likely that diet is more important than smoking in cancer causation. Both human and animal studies support this assumption. The study of special populations in the U. S. such as the Mormons and the Seventh Day Adventists also point to the potential of reducing U. S. cancer rates and individual risk factors through the modification of dietary habits. The major hypotheses of the role of dietary and nutritional factors in cancer etiology are examined in light of current scientific knowledge. General guideline for the reduction of risk from the major chronic diseases are also discussed.

Gastric carcinoma promoted by alkaline reflux gastritis -- with special reference to bile and other surfactants as promoters of postoperative gastric cancer

For reasons that are not known, the incidence of stomach cancer is declining in most countries. In contrast, cancer of the gastric remnant after partial gastrectomy for peptic ulcer disease seems to be increasing. "Stump cancer" has been claimed to be a separate disease entity and has recently been mentioned as an iatrogenic cancer. Alkaline bile reflux, dietary surfactants and maybe the abuse of tobacco are postulated as promoting or initiating agents, in causing malignant transformation of the operated as well as non-operated stomach. Variations in gastric pH, microflora and emptying rate may be of importance in the carcinogenic process. The decline in cancer incidence in the non-operated stomach has mainly been restricted to the intestinal type of cancer. Thus, it is important to note that stump cancer often is of the diffuse type. Different target cells and/or modes of carcinogen exposure may be of etiological importance in these histologically and clinically separate neoplastic diseases.

The relation of repair phenomena to mutation induction in bacteria

The relation of various processes to mutation induction by radiation and chemicals is discussed for for various species of bacteria. A variety of repair processes have been identified at the molecular level that can eliminate many kinds of potentially mutagenic lesions before they can be converted to final mutation. Fixation often but not always occurs at replication. A number of mutagens, including UV light, ionizing radiation, and a number of chemicals, induce an error-prone process, perhaps a modification of the proof-reading system, that allows bacteria to survive after potentially lethal damage at the expense of making errors. Some mutagens, notably monofunctional alkylating agents and base analogues, produce mutations by other processes. Even in these cases, repair processes play an important role. There is some evidence that error-free as well as error-prone repair processes can be induced. A brief discussion is given of the relation of these findings to the practical problems of hazards estimations.