Assessment of human exposure and response to N-nitroso compounds: a new view on the etiology of digestive tract cancers

Microbiota do megaesôfago e carcinogênese

INTRODUÇÃO: O risco de desenvolvimento de carcinoma esofágico em portadores de megaesôfago é 33 vezes superior ao da população em geral. Possível explicação para este fenômeno poderia estar relacionada à produção de compostos N-nitrosos na luz do órgão, a partir da transformação de nitratos da dieta em nitritos, mediada por bactérias em suspensão no líquido de estase e com o contato crônico destes carcinógenos com a mucosa esofágica. OBJETIVO: Analisar a microbiota esofágica em pacientes portadores de megaesôfago de etiologia chagásica, com especial atenção para a presença de bactérias com capacidade de redução de nitratos. CASUÍSTICA: Foram estudados prospectivamente 15 pacientes portadores de megaesôfago chagásico com idades variando de 28 a 73 anos, sendo 9 do sexo feminino e 6 do sexo masculino, que foram divididos em 3 grupos iguais de 5, de acordo com o grau de dilatação do esôfago, segundo a classificação de Rezende et al. (Grau I, Grau II e Grau III). MÉTODO: A coleta do líquido de estase para estudo microbiológico era realizada através de sonda de Levine nº 14, que era passada pela boca, por dentro de uma cânula de intubação orotraqueal nº 7,5, mantendo-se sua extremidade escondida, a fim de evitar sua contaminação. RESULTADOS: Foram obtidas 93,3% de culturas positivas com grande variedade de microrganismos e predomínio de aeróbios Gram-positivos e anaeróbios. As concentrações de microrganismos foram tanto maiores, quanto maior o grau de dilatação do esôfago. Entre os microrganismos encontrados, o Staphylococcus sp, Corynebacterium sp, Peptostreptococcus sp e a Veillonella sp foram aqueles identificados como tendo a capacidade de redução de nitratos a nitritos. CONCLUSÃO: No megaesôfago chagásico há bactérias na luz do órgão com capacidade de redução de nitratos da dieta, passo importante na produção de compostos N-nitrosos.

From carcinogenesis to clinical interventions for cancer prevention

During the last three decades, the scientific community has made immense progress in acquiring the knowledge needed to prevent cancer. Pioneering research helped to identify potential causes of cancer, particularly environmental factors such as diet, and provided insight regarding their mechanisms-of-action. Concurrently, promising inhibitors of cancer that appeared able to either arrest or reverse cancer development by interfering with one or more steps in the process of carcinogenesis were identified and systematically evaluated for their potential as chemopreventive agents. Numerous agents determined to be safe and effective in preclinical trials have been and continue to be tested in Phase I,II, and III clinical interventions for cancers at various sites, including breast, colon, prostate, esophagus, mouth, lung, cervix, endometrium, ovary, liver, bladder, and skin. The development of valid intermediate biomarkers that can serve as surrogate endpoints for clinical disease is urgently needed to accelerate advances in clinical trials for cancer prevention.

Hepatotoxicity and hepatocarcinogenicity in rats fed squid with or without exogenous nitrite

The popular seafood squid contains high levels of naturally occurring amines such as dimethylamine (DMA) trimethylamine and trimethylamine-N-oxide (TMAO). The hepatotoxicity and hepatocarcinogenicity of squid with or without exogenous nitrite were investigated in rats. Acute necrosis including polymorphogenic neutrophil infiltration, haemorrhage and cholangiofibrosis were observed in the livers of most rats fed squid. Hepatocellular carcinoma (HCC) was induced in two out of 12 rats (16%) by feeding 10% squid in Purina rat chow for 10 months. The incidence of HCC was increased to four out of 10 rats (33%) when 0.3% NaNO2 was added to the above diet. At the end of the experiment a marked elevation of serum gamma-glutamate transferase was observed in treated groups, but no significant changes in the activities of serum glutamic-oxaloacetic transaminase and glutamic-pyruvic transaminase were detected. Vitamin C (0.3%) gave partial protection against hepatic damage. The concentration of DMA in squid is estimated to be 0.19%; this concentration did not induce HCC under the experimental conditions used. Therefore it is suggested that another major naturally occurring amine in squid, TMAO, could be one of the important factors involved in the induction of hepatotoxicity and hepatocarcinogenicity in rats.

Classification of carcinogens: polemics, pedantics, or progress?

Rodent carcinogens may, for physiological or other reasons, induce cancer by a variety of mechanisms which vary in their ability to affect humans. While the current approach of some regulatory agencies to carcinogen risk assessment and regulation may possibly be justified with most genotoxic carcinogens, this is not true with all nongenotoxic carcinogens. Mechanisms attributable to high dose toxicity occasioned by misuse of the maximum tolerated dose concept, imbalancing of homeostasis, unphysiological conditions, and induced cellular proliferation are reviewed. The greatest present need for meaningful regulation of carcinogens is to obtain public acceptance of the fact that some carcinogens are species specific and probably will not exert their effects in humans.

International Commission for Protection Against Environmental Mutagens and Carcinogens. ICPEMC publication No. 19. The classification of carcinogens identified in the rodent bioassay as potential risks to humans: what type of substance should be tested next?

The relevance of rodent cancer bioassay data to humans is discussed in relation to the needs of regulatory agencies. The usefulness of in vivo and in vitro genotoxicity testing in this connection is also discussed. In the case of rodent carcinogens that do not elicit genotoxicity, it is suggested that homeostatic imbalance, cell proliferation, and other processes may play a major role in tumor development and its importance to the possible ability of the test agent to induce human cancer. These possibilities need to be evaluated on a case by case basis. The methods by which chemicals are selected for the rodent cancer bioassay are also discussed and it is pointed out that naturally-occurring constituents of human foods should in future receive greater priority as a consequence of anticipated changes resulting from biotechnology.

Gastric cancer: an epidemiological review

An attempt is made to synthesize the epidemiological literature and identify salient factors from the multitude of potential antecedents of gastric cancer, factors which to a greater or lesser degree create nonrandom variations in the distribution of the disease. Implicit in this approach is the notion that observation of spatial variations in the incidence of gastric cancer may lead to hypotheses relating to the biological, personal and physical environmental factors. An extensive bibliography accompanies the text.

Potentiation of ferrous sulphate and ascorbate on the microbial transformation of endogenous trimethylamine N-oxide to trimethylamine and dimethylamine in squid extracts

The levels of trimethylamine N-oxide (TMAO) in the New Zealand (Nototodarus sloani) species of squid extracts were extremely high (above 9200 ppm). When the extracts were incubated for 2 days at 25 degrees C, approximately 60% TMAO was converted to trimethylamine (TMA) and dimethylamine (DMA). This conversion was very low or negligible at 4 degrees C, but was potentiated by the presence of ferrous sulphate (0.014 M) and ascorbate (0.014 M). Citrobacter freundii and Aeromonas hydrophilia were isolated from the extracts. Cultures of these two micro-organisms and of Escherichia coli were active in catalysing the conversion of TMAO to TMA and DMA either in extract or in aqueous solution. Chloramphenicol (0.416 mg/ml) completely inhibited the growth of these micro-organisms and also effectively blocked the conversion of endogenous TMAO to TMA in the extracts. The present findings suggest that gastro-intestinal flora and dietary ferrous salts and ascorbate may play important roles in the conversion of TMAO to TMA and DMA in man following the ingestion of squid and other TMAO-containing seafoods.

Thermal conversion of trimethylamine-N-oxide to trimethylamine and dimethylamine in squids

The levels of dimethylamine-nitrogen (DMA-N), trimethylamine-nitrogen (TMA-N) and trimethylamine-N-oxide-nitrogen (TMAO-N) were determined in five species of dried squid. Each sample contained extremely high levels of TMAO-N (2558-8064 ppm) and moderate amounts of TMA-N (121-503 ppm) and DMA-N (124-373 ppm). Over 90% of TMAO-N in squid was converted to TMA-N and DMA-N after heating at 200 degrees C for 1 hr; approximately 50% of the volatile TMA-N and DMA-N was lost during the course of the heating. The thermal conversions were accelerated by heat, and possibly involved catalysis by certain tissue constituents. Squids are a popular seafood in most oriental countries, but before appearing on the market they are subjected to a long food-processing procedure. Therefore, a high concentration of TMAO in squids is an important problem, for food technology as well as toxicology.

Primary portal venopathy in the golden hamster treated with low doses of dimethylnitrosamine

Ingestion of dimethylnitrosamine in a low dose and for a limited period by hamsters resulted in primary intrahepatic pylephlebitis. The process began at 24 h and progressed within 2-4 weeks to narrowing and, eventually, to partial obstruction of the portal bed. The lesions were characterized by endothelial necrosis followed by infiltrations of lymphoid cells extending into the portal tracts. Superficial lesions of terminal hepatic veins and parenchymal changes occurred only after a lapse of, respectively, 2 and 4 weeks when restrictions of portal blood had been assumed. The parenchymal changes consisted of regional regenerative hyperplasia accompanied by subendothelial prolapse of hepatocytes into the wall of terminal veins. The lesions of the portal veins persisted for a prolonged period of time after the exposure to dimethylnitrosamine had ceased. These findings, associated with characteristic histological aspects, suggested secondary immune reaction following the initial toxic pylephlebitis. The possible relevance of the experimental results to understanding the nature of endemic portal venopathy in man is discussed with regard to pathogenesis and etiology.

In vivo formation of nitrosocarbamates in the stomach of rats and guinea pigs

The N-nitrosocarbamates are potent mutagens and carcinogens, and have been synthesized under acid conditions that prevail in the human stomach. However, it has never been documented that nitrosocarbamates are actually formed in vivo in the stomach of any mammalian species. Using 14C-labeled carbaryl and carbofuran, attempts were made to isolate the nitroso derivatives from the stomach contents of rats and guinea pigs treated orally with the carbamate and sodium nitrite. Only trace quantities of nitrocarbamate were recovered from the rat stomach, whereas 0.5 to 2.0% of the carbamate doses were isolated as the nitroso derivative from the contents of the guinea pig stomach. The rather low apparent yields resulted in part from the instability of the nitrosocarbamates and from absorption of the carbamate and/or nitrosocarbamate from the stomach. Higher rates of synthesis were indicated by incubating the carbamates with sodium nitrite in the presence of the stomach contents at 37 degrees C for 15 min. About 30% nitrosation occurred with the guinea pig and about 0.5% with the rat. The difference was attributed to the pH of the gastric contents. For the rat, the pH ranged from 3 to 5; gastric contents of the guinea pig had a pH between 1 and 2. Since the pH of the human stomach is also in the pH 1-2 range, it is likely that nitrosation of carbamates in humans would be very similar to that in the guinea pig.

Toxicity of nitrite and dimethylamine in rats

Weanling male Wistar strain rats were administered, through the drinking water, nitrite (0.2%) and dimethylamine (DMA) (0.2%), either singly or in combination for 9 months. Some animals also received 0.5% butylated hydroxytoluene (BHT) in the diet. Nitrite, as well as DMA, caused higher in vitro lipoperoxidation, free lysosomal enzyme activities and cytosolic superoxide dismutase activity in liver. Some of these increases viz., the enzyme activities in liver, were counteracted to a significant extent in the rats receiving a dietary supplement of BHT. The results indicate that nitrite and DMA may induce toxicity through some free radical reactions and that BHT can provide some protection.

High concentrations of dimethylamine and methylamine in squid and octopus and their implications in tumour aetiology

The levels of the common secondary amines in various squid, in octopus and in 17 other seafoods were determined by HPLC. Ammonia and dimethylamine were found in all of the seafoods tested and some of them also contained methylamine and/or ethylamine. Particularly high levels of dimethylamine (946-2043 ppm) and methylamine (38-255 ppm) were detected in various species of squid and in the octopus. Reaction of nitrite in acidic medium with aqueous extract of squid yielded appreciable amounts of N-nitrosodimethylamine. The optimum pH for this reaction was around 2.4. Dimethylamine in dried squid tissues was readily extracted with water or 1% sodium carbonate solution. Heat treatment of dried squid at 200 degrees C was found to increase its amine content dramatically. It appeared that pyrolytic decarboxylation of some amino acids might cause this increase. Squid is a popular seafood in Japan and other oriental countries. The high incidence of stomach cancer in Japan and China is thought by epidemiologists to be associated with traditional Japanese and Chinese diets. Our present finding that squid and other seafoods contain unusually high levels of dimethylamine and other amines adds to the evidence that dietary factors may have an important role in the aetiology of stomach cancer and other gastro-intestinal tumours.

Putative mutagens and carcinogens in foods. IV. Malonaldehyde (malondialdehyde)

Malonaldehyde or malondialdehyde (MA) is an ubiquitous dietary constituent formed by the peroxidation of unsaturated fatty acids and of the food additive, sorbate. The average US citizen consumes between 0.2 and 0.6 microgram of MA each day from fresh and processed meats alone. This figure might have been considerably higher in past decades when meat was packaged and stored under less optimal circumstances. No studies seem to have been performed to determine if free MA is released under physiological conditions, such as in simulated gastric juice. Although pure enzymologically synthesized MA is a weak mutagen for a new Salmonella frameshift tester strain containing a polyadenine sequence, reports of more pronounced mutagenicity and carcinogenicity are very likely erroneous. The impure chemically synthesized MA samples used in the strongly positive tests for genotoxic activity were contaminated with mutagenic methoxy- and ethoxyacroleins that would not be expected to constitute important components of foodstuffs. Thus, the greatest importance of MA and other carbonyls in foodstuffs may be their ability to catalyze nitrosation reactions in the presence of free nitrite and as indicators of the presence of other lipid oxidation products.

Nitrate nitrogen levels in drinking water of urban areas with high- and low-risk populations for stomach cancer: an environmental epidemiology study

A correlation study between mean nitrate nitrogen levels (ppm) in drinking water samples (N = 1389) of Chilean urban areas and age-adjusted death rates per 100 000 population from stomach cancer, by province or region and sex, was made. Drinking water samples from all provinces (N = 25) had a weighed mean of 1.446 ppm (S.E.M. 0.068) with a range of 0.00--30.00 ppm. Nitrate nigrogen levels showed a positive but not significant association with male death rates. The correlation coefficient was +0.0335. Similarly, such levels did exhibit a positive but not significant correlation with female death rates (r = +0.0486). When NO3-N levels and male (r = +0.1367) or female (r = +0.1143) death rates were studied, by region, positive but insignificant correlations were detected. Using Cochran's approximation, mean nitrate nitrogen levels in drinking water samples from six provinces with 50% of the Chilean population (period 1953--55 versus 1973--75), showed a decrease from 1.835 to 1.291 ppm, but there was no significant difference (t = 1.32) between the two values, except in samples from Santiago Province (t = 2.11, P less than 0.05). Provinces (south central area) showing the highest gastric cancer mortality rates in the world for females (up to 40.8/100,000), and ranking second for males (up to 84.1/100,000), exhibited a very low mean level (0.825 ppm).