Studies in gastric carcinogenesis. V. The effects of ascorbic acid on N-nitroso compound formation in human gastric juice in vivo and in vitro

Pathways of Gastric Carcinogenesis, Helicobacter pylori Virulence and Interactions with Antioxidant Systems, Vitamin C and Phytochemicals

Helicobacter pylori is a class one carcinogen which causes chronic atrophic gastritis, gastric intestinal metaplasia, dysplasia and adenocarcinoma. The mechanisms by which H. pylori interacts with other risk and protective factors, particularly vitamin C in gastric carcinogenesis are complex. Gastric carcinogenesis includes metabolic, environmental, epigenetic, genomic, infective, inflammatory and oncogenic pathways. The molecular classification of gastric cancer subtypes has revolutionized the understanding of gastric carcinogenesis. This includes the tumour microenvironment, germline mutations, and the role of Helicobacter pylori bacteria, Epstein Barr virus and epigenetics in somatic mutations. There is evidence that ascorbic acid, phytochemicals and endogenous antioxidant systems can modify the risk of gastric cancer. Gastric juice ascorbate levels depend on dietary intake of ascorbic acid but can also be decreased by H. pylori infection, H. pylori CagA secretion, tobacco smoking, achlorhydria and chronic atrophic gastritis. Ascorbic acid may be protective against gastric cancer by its antioxidant effect in gastric cytoprotection, regenerating active vitamin E and glutathione, inhibiting endogenous N-nitrosation, reducing toxic effects of ingested nitrosodimethylamines and heterocyclic amines, and preventing H. pylori infection. The effectiveness of such cytoprotection is related to H. pylori strain virulence, particularly CagA expression. The role of vitamin C in epigenetic reprogramming in gastric cancer is still evolving. Other factors in conjunction with vitamin C also play a role in gastric carcinogenesis. Eradication of H. pylori may lead to recovery of vitamin C secretion by gastric epithelium and enable regression of premalignant gastric lesions, thereby interrupting the Correa cascade of gastric carcinogenesis.

Concentration Does Matter: The Beneficial and Potentially Harmful Effects of Ascorbate in Humans and Plants

SIGNIFICANCE

Ascorbate (Asc) is an essential compound both in animals and plants, mostly due to its reducing properties, thereby playing a role in scavenging reactive oxygen species (ROS) and acting as a cofactor in various enzymatic reactions. Recent Advances: Growing number of evidence shows that excessive Asc accumulation may have negative effects on cellular functions both in humans and plants; inter alia it may negatively affect signaling mechanisms, cellular redox status, and contribute to the production of ROS via the Fenton reaction.

CRITICAL ISSUES

Both plants and humans tightly control cellular Asc levels, possibly via biosynthesis, transport, and degradation, to maintain them in an optimum concentration range, which, among other factors, is essential to minimize the potentially harmful effects of Asc. On the contrary, the Fenton reaction induced by a high-dose Asc treatment in humans enables a potential cancer-selective cell death pathway.

FUTURE DIRECTIONS

The elucidation of Asc induced cancer selective cell death mechanisms may give us a tool to apply Asc in cancer therapy. On the contrary, the regulatory mechanisms controlling cellular Asc levels are also to be considered, for example, when aiming at generating crops with elevated Asc levels.

The Role of the Natural Antioxidants in the Oxihaemoglobin Oxidation and the Diminution of Nitrite Concentration

The paper includes the study of the inhibition of the process of methemoglobinization at oxidation with nitrites in the presence of sodium dihydroxyfumarate (DFH3Na) and resveratrol (3,4’,5-trihydroxystilben). The experimental study was carried out by treatment of the erythrocyte mass by hemolysis and exposure to nitrite. The kinetic investigations were carried out in following conditions: [Resv] = (5.10-5 – 1.10-3) mol/l, [DFH3Na] = 1.10-6 – 5.10-6 mol/l; [HbO2]=1.10-3 mol/l; pH 7,1; t = 370C. The rate of transformation of HbO2 in the presence of resveratrol and DFH3Na was calculated from kinetic curves of consumption of the substrate and formation of MetHb obtained pectrophotometrically (λmax= 540 nm for HbO2 and λmax=630 for MetHb). It has been found out that the introduction of resveratrol and DFH3Na in the system HbO2 – NO2- causes the decrease of the autooxidation factor φ DFH3Na approximately by 1.1 – 2.5 times and φresveratrol by 1.1 – 1.7 times. The time of achievement of the maximum rate of oxidation of HbO2 dζ/dτ (where ζ is the rate of transformation of HbO2 in MetHb) increases while the phase of fast oxidation of HbO2 decreases with increase of content of inhibitors. The process of interaction of nitrites with reducers (such as DFH4, DFH3Na, resveratrol and (+)-catechine) was carried out as well. It has been established that degree of diminishing of the concentration of nitrites in the system RedH2-NO2- decreases as follows: DFH<4DFH3Na<Resv<(+)Catechol.

Genotoxic and carcinogenic risk to humans of drug-nitrite interaction products

The large majority of N-nitroso compounds (NOC) have been found to produce genotoxic effects and to cause tumor development in laboratory animals; four NOC have been classified by the International Agency for Research on Cancer (IARC) as probably and another 15 as possibly carcinogenic to humans. A considerable fraction of drugs are theoretically nitrosatable due to the presence of amine, amide or other groups which by reacting with nitrite in the gastric environment, or even in other sites, can give rise to the formation of NOC, and in some cases other reactive species. This review provides a synthesis of information on the chemistry of NOC formation, the carcinogenic activity of NOC in animals and humans and the inhibitors of nitrosation reactions. It contains information on the drugs which have been tested for the formation of NOC by reaction with nitrite and the genotoxic-carcinogenic effects of their nitrosation products. In an extensive search we have found that 182 drugs, representing a wide variety of chemical structures and therapeutic activities, were examined in various experimental conditions for their ability to react with nitrite, and 173 (95%) of them were found to form NOC or other reactive species. Moreover, 136 drugs were examined in short-term genotoxicity tests and/or in long-term carcinogenesis assays, either in combination with nitrite or using their nitrosation product, in order to establish whether they produce genotoxic and carcinogenic effects; 112 (82.4%) of them have been found to give at least one positive response. The problem of endogenous drug nitrosation is largely unrecognized. Only a small fraction of theoretically nitrosatable drugs have been examined for the possible formation of genotoxic-carcinogenic NOC, guidelines for genotoxicity testing of pharmaceuticals do not indicate the need of performing the appropriate tests, and patients are not informed that the drug-nitrite interaction and the consequent risk can be reduced to a large extent by consuming the nitrosatable drug with ascorbic acid.

Rationale and impact of vitamin C in clinical nutrition

PURPOSE OF REVIEW

The impact of vitamin C on oxidative stress-related diseases is moderate because of its limited oral bioavailability and rapid clearance. Parenteral administration can increase the benefit of vitamin C supplementation as is evident in critically ill patients. The aim here is to assess recent evidence of the clinical benefit and underlying effects of parenteral vitamin C in conditions of oxidative stress.

RECENT FINDINGS

In critically ill patients and after severe burns, the rapid restoration of depleted ascorbate levels with high-dose parenteral vitamin C may reduce circulatory shock, fluid requirements and oedema.

SUMMARY

Oxidative stress is associated with reduced ascorbate levels. Ascorbate is particularly effective in protecting the vascular endothelium, which is especially vulnerable to oxidative stress. The restoration of ascorbate levels may have therapeutic effects in diseases involving oxidative stress. The rapid replenishment of ascorbate is of special clinical significance in critically ill patients who experience drastic reductions in ascorbate levels, which may be a causal factor in the development of circulatory shock. Supraphysiological levels of ascorbate, which can only be achieved by the parenteral and not by the oral administration of vitamin C, may facilitate the restoration of vascular function in the critically ill patient.

Studies of nitric oxide generation from salivary nitrite in human gastric juice

BACKGROUND

Saliva contains substantial concentrations of nitrite derived from the enterosalivary recirculation of dietary nitrate.

METHODS

We have investigated factors in gastric juice influencing the fate of nitrite in swallowed saliva. When nitrite (100 microM) is added to human gastric juice pH 1.5 or pH 2.5 at 37 degrees C containing physiological concentrations of thiocyanate (1 mM) and ascorbic acid (200 microM), it is converted to nitric oxide within a few seconds.

RESULTS

The reduction of nitrite to nitric oxide is slower at pH 3.5 and very little is generated at pH 4.5. The rate of nitric oxide generation at acid pH increases with increasing thiocyanate concentration. The concentration of nitric oxide generated in the above way is maintained until the ascorbic acid is depleted by the recycling of nitric oxide to nitrite. In gastric juice depleted of ascorbic acid, very little nitrite is reduced to nitric oxide at any pH.

CONCLUSION

These studies indicate that in the healthy acid-secreting stomach most salivary nitrite will be reduced to nitric oxide at the gastro-oesophageal junction and gastric cardia where it first encounters gastric juice.

The physiological role of dehydroascorbic acid

Dehydroascorbic acid (DHA) is abundant in the human diet and also is generated from vitamin C (ascorbic acid, AA) in the lumen of the gastrointestinal tract. DHA is absorbed from the lumen of the small intestine and reduced to AA, which subsequently circulates in the blood. Utilization of AA as an antioxidant and enzyme cofactor causes its oxidation to DHA in extracellular fluid and cells. DHA has an important role in many cell types because it can be used to regenerate AA. Both physiological (e.g. insulin, insulin-like growth factor I, cyclic AMP) and pathological (e.g. oxidative stress, diabetes, sepsis) factors alter the transport and metabolic mechanisms responsible for this DHA recycling.

Glucose modulates vitamin C transport in adult human small intestinal brush border membrane vesicles

The uptake of L-ascorbate (vitamin C) and its oxidized form, dehydro-L-ascorbic acid (DHAA), was evaluated in brush border membrane vesicles isolated from adult human duodenum, jejunum and ileum. Ascorbate was taken up along the entire length of the small intestine with a threefold higher initial uptake rate in distal than proximal segments. Ascorbate uptake was Na(+)-dependent, potential-sensitive and saturable (K(m), 200 micromol/L), whereas DHAA transport involved facilitated diffusion (K(m), 800 micromol/L). Pharmacologic experiments were conducted to characterize further these transport mechanisms. DHAA uptake was not mediated by the fructose carrier GLUT5, the uridine transporter or the 4, 4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS)-sensitive anion exchanger of the apical membrane. DIDS and sulfinpyrazone, an inhibitor of the urate/lactate exchanger, both significantly reduced the initial rate of ascorbate uptake. Acidic pH inhibited ascorbate uptake, and this effect was not due to a transmembrane proton gradient. Increasing concentrations of glucose in the transport media also significantly inhibited ascorbate uptake, but no effect of glucose was seen when glucose internalization was blocked by phlorizin. Preloading the vesicles with glucose inhibited ascorbate uptake similarly, indicating that glucose interferes with the ascorbate transporter from the internal side of the membrane. The results of this study suggest that DHAA crosses the apical membrane by facilitated diffusion, whereas ascorbate transport is a Na(+)-dependent, electrogenic process modulated by glucose.

Micronutrients, vitamins, and cancer risk

There is now considerable evidence that a high intake of fruit and vegetables can decrease the risk of developing cancer. While it is by no means clear how this particular diet alters cancer risk, there is substantial metabolic and experimental evidence to implicate antioxidant micronutrients, The dietary components include some vitamins, such as C and E, the carotenoids, and the flavinoids. In chemical systems, cell culture, and experimental animals, these components have the ability to quench the carcinogenic potential of reactive oxygen species and other carcinogens, such as N-nitrosocompounds. Some of these micronutrients can act synergistically, and high concentrations are often found in tissues, such as the leucocytes and mucosal cells, that are particularly prone to reactive species attack. Experimental systems containing these micronutrients also appear to be able to reduce DNA damage and mutagenesis. However, assessment of individual vitamin intake, as opposed to fruit and vegetable consumption, does not increase the protective association of these components, and the results of intervention studies in man, especially with carotenoids, have been disappointing. We await the results of other clinical trials, but as yet, there is insufficient evidence to recommend supplements of these particular micronutrients for the prevention of cancer. However, it would be prudent to suggest changes in diet that would increase consumption of fruit and vegetables, such as a diet is clearly associated with protection.

Nutrient intake patterns and gastric cancer risk: a case-control study in Belgium

A case-control study on gastric cancer risk in relation to nutrient composition of diet was conducted in the 2 Belgian provinces of Oost-Vlaanderen and Liège as part of a large epidemiological study on cancers of the digestive tract, also including colorectal cancer. The statistical analysis was carried out on a total of 301 men and women aged 35-74 years with histologically confirmed stomach tumors and 2,851 population controls. Dietary intake assessments were obtained by interview, using a dietary history questionnaire. Gastric cancer risk was increased for diets rich in mono- and disaccharides, according to statistical models based on energy-adjusted residuals [odds ratio (OR) = 1.88 for upper vs. lower quartiles of energy-adjusted intakes] or energy decomposition models, but showed no increase for high intake of polysaccharides. Intake of polyunsaturated fats, particularly linoleic acid, was inversely associated with gastric cancer risk. High intake of vitamin C, beta carotene and vitamins B1 (thiamine), B3 (nicotinic acid) or B6 (pyridoxine) was also associated with decreased risk, whereas increased risk was observed for high intake of vitamin A (retinol) or vitamin B2 (riboflavin). Many of these associations between gastric cancer risk and nutrient composition were similar to those found in the analysis of a parallel study on cancers of the colon and rectum.

Volatile N-nitrosamines in gastric juice of patients with various conditions of the gastrointestinal tract determined by gas chromatography-mass spectrometry and related to intragastric pH and nitrate and nitrite levels

Gastric juice samples of 71 patients undergoing upper gastrointestinal endoscopy were collected as well as saliva samples from 40 of these patients. Age, sex, endoscopic diagnosis and medication were recorded. The gastric juice samples were analyzed for the presence and quantity of individual volatile N-nitrosamines, which were detected by gas chromatography-mass spectrometry, without prior derivatization. The samples were screened for eight nitrosamines, i.e. N-nitrosodimethylamine, N-nitrosoethylmethylamine, N-nitrosodiethylamine, N-nitrosodi-n-propylamine, N-nitrosodi-n-butylamine, N-nitrosopyrrolidine, N-nitrosopiperidine and N-nitrosomorpholine. The pH of the fresh gastric juice as well as nitrate and nitrite levels of gastric juice and saliva were determined. The mean total level of volatile N-nitrosamines in gastric juice was found to be 4.84 nmol/l (range 0-17.7 nmol/l). The main N-nitrosamines found were N-nitrosodiethylamine (mean concentration 3.1 nmol/l), N-nitrosodimethylamine (mean concentration 0.90 nmol/l) and N-nitrosopyrrolidine (mean concentration 0.38 nmol/l). Significant correlations between mean intragastric pH values and mean N-nitrosodi-n-butylamine level (P = 0.005) and total volatile N-nitrosamine contents (P = 0.009) were observed.

The role of ascorbate in antioxidant protection of biomembranes: interaction with vitamin E and coenzyme Q

One of the vital roles of ascorbic acid (vitamin C) is to act as an antioxidant to protect cellular components from free radical damage. Ascorbic acid has been shown to scavenge free radicals directly in the aqueous phases of cells and the circulatory system. Ascorbic acid has also been proven to protect membrane and other hydrophobic compartments from such damage by regenerating the antioxidant form of vitamin E. In addition, reduced coenzyme Q, also a resident of hydrophobic compartments, interacts with vitamin E to regenerate its antioxidant form. The mechanism of vitamin C antioxidant function, the myriad of pathologies resulting from its clinical deficiency, and the many health benefits it provides, are reviewed.