Chemoprevention of colorectal neoplasms. Ascorbic acid and beta-carotene

Vitamin status in patients with acute pancreatitis

The serum levels of 6 vitamins were prospectively evaluated in 20 patients with acute biliary pancreatitis and in 20 patients with acute alcoholic pancreatitis. Twenty healthy subjects acted as controls. There were no statistically significant differences in the levels of vitamin B12 and D among the groups. Patients with alcoholic pancreatitis had significantly lower levels of vitamins A, E (p < 0.01) and C (p < 0.001) than those of the control group. Plasma levels of vitamin K were decreased or undetectable in 6 patients with biliary and 3 with alcoholic pancreatitis. The pathophysiological and clinical implications of this abnormal vitamin status in patients with acute pancreatitis are unknown. Nevertheless, multi-vitamin supplementation seems justified, especially in patients affected by alcoholic pancreatitis. Vitamin K should be administered in both types of pancreatitis when blood coagulation tests are disturbed.

How good are rodent models of carcinogenesis in predicting efficacy in humans? A systematic review and meta-analysis of colon chemoprevention in rats, mice and men

Tumours in rodent and human colon share many histological and genetic features. To know if rodent models of colon carcinogenesis are good predictors of chemopreventive efficacy in humans, we conducted a meta-analysis of aspirin, beta-carotene, calcium, and wheat bran studies. Controlled intervention studies of adenoma recurrence in human volunteers were compared with chemoprevention studies of carcinogen-induced tumours in rats, and of polyps in Min (Apc(+/-)) mice: 6714 volunteers, 3911 rats and 458 mice were included in the meta-analyses. Difference between models was small since most global relative risks were between 0.76 and 1.00. A closer look showed that carcinogen-induced rat studies matched human trials for aspirin, calcium, carotene, and were compatible for wheat bran. Min mice results were compatible with human results for aspirin, but discordant for calcium and wheat bran (no carotene study). These few results suggest that rodent models roughly predict effect in humans, but the prediction is not accurate for all agents. Based on three cases only, the carcinogen-induced rat model seems better than the Min mouse model. However, rodent studies are useful to screen potential chemopreventive agents, and to study mechanisms of carcinogenesis and chemoprevention.

Impact of water-dispersible beadlets as a vehicle for the delivery of carotenoids to cultured cells

Water-dispersible beadlets of carotenoids were used as supplements for human umbilical vein endothelial cells (HUVECs), human peripheral blood mononuclear cells (PBMCs) and human monocytes. Stability, cellular association and cytotoxicity of the carotenoid beadlets were compared with carotenoids delivered with tetrahydrofuran (THF). Incubations with lycopene, beta-carotene, lutein and astaxanthin dissolved in THF resulted in a lower stability in the medium, lower cellular association, and a higher standard deviation. Beadlets provided 60, 4, 6, and 2 times greater accumulation of lycopene, beta-carotene, lutein and astaxanthin, respectively, by PBMCs than THF. The cellular association of carotenoids delivered by THF seems to be more carotenoid-specific than when carotenoids are delivered by beadlets. After 48 h of incubation under cell culture conditions all of the four carotenoids (1 microM) delivered by beadlets to the medium showed a reduction less than 30%. In addition, no cytotoxic effect of the carotenoid beadlets or the vehicle alone was detected in a concentration range of 0.5-5 microM. The results show that beadlets are a non-toxic vehicle for supplementing and stabilizing carotenoids in culture media offering a reasonable compromise in term of cell accumulation efficiency.

Ascorbic acid and gastrointestinal cancer

A literature review was made to critically evaluate the ability of ascorbic acid to modulate the incidence of gastrointestinal cancer. A comparison of preclinical, clinical, and epidemiological studies indicated that evidence for ascorbic acid as an inhibitor of carcinogenesis is stronger with regard to gastric cancer and weaker with regard to esophageal and colon/rectal cancer. Insufficient evidence currently exists regarding the oral cavity and the use of ascorbic acid in precancerous conditions such as polyposis and leukoplakia.

beta-Carotene uptake, metabolism, and distribution in BALB/c 3T3 cells

Although a growing number of epidemiological studies indicate that dietary beta-carotene has anticarcinogenic activity, the mechanism(s) of beta-carotene protection remains to be definitively established. In this context, in vitro studies of beta-carotene have been, and continue to be, valuable. We examined the following critical features in designing an in vitro system for studying the protection action of beta-carotene: 1) form of beta-carotene used for cellular uptake, 2) cellular metabolism of beta-carotene, and 3) subcellular distribution of beta-carotene. It was determined that beta-carotene added to medium in a water-dispersible formulation is readily taken up by BALB/c 3T3 cells and is located predominantly in cellular membranes. Cellular uptake of beta-carotene added to medium in an organic solvent is greatly reduced. It was also found that intracellular retinol increased significantly after a three-day exposure of BALB/c 3T3 cells to media containing beta-carotene. This result suggests that the ability to metabolize beta-carotene to retinoids is not limited to cells of intestinal origin. The results and methodology described here will be useful in the rational design of in vitro assays for elucidating the mechanism(s) of beta-carotene protective effects at the cellular level.

Vitamin A and lung cancer

In a dozen case-control and cohort studies, high intake of fruits and vegetables containing carotenoids has been associated with a reduced risk of lung cancer. In contrast, little relation has been found between intake of preformed vitamin A and this disease. Although initial studies suggested that persons with lower levels of serum retinol have higher future rates of lung cancer, this idea was not confirmed in subsequent investigations. Prediagnostic levels of beta-carotene in blood, however, have been inversely related with risk of lung cancer. Available data thus strongly support the hypothesis that dietary carotenoids reduce the risk of lung cancer, but the data are also compatible with the possibility that some other factor in these foods is responsible for the lower risk. Even if ultimately shown to be casual, the relation between diet and lung cancer is modest compared with the deleterious effect of cigarette smoking.

Antioxidants--carcinogenic and chemopreventive properties

Synthetic and naturally occurring antioxidants have a wide variety of biological actions in rodents in addition to their primary antioxidant activity. Some of the included biological effects are of direct interest in relation to studies of carcinogenicity and/or modulation of carcinogenesis. Since the synthetic antioxidant BHA was first found to exert carcinogenic potential in rat and hamster forestomach epithelium, many other synthetic and naturally occurring antioxidants have been examined for their ability to induce proliferative activity in the alimentary canal. These studies have revealed that caffeic acid and sesamol are also tumorigenic for rat forestomach epithelium, whereas catechol and p-methylcatechol induce neoplasia in rat glandular stomach epithelium. Although the proliferative response is very rapid, with inflammation and ulceration, it takes a very long time before carcinomas develop. The proliferative lesions in the forestomach induced by BHA or caffeic acid are largely reversible, in contrast to those induced by genotoxic carcinogens, which generally persist and develop into cancer. Therefore, chronic irritation is considered to be responsible for the induction of stomach cancer by antioxidants. Butylated hydroxyanisole can undergo oxidative metabolism in vitro, and some of the metabolites formed have the potential for binding to proteins. Neither BHA nor its metabolites binds to DNA in vivo, but protein binding in the forestomach was greater than 10 times higher than that in the glandular stomach. It is thus conceivable that BHA is oxidatively metabolized in the forestomach epithelium (possibly entering into redox cycling), and reactive metabolites including semiquinone radicals or active oxygen species are responsible for the carcinogenesis by a mechanism involving binding to macromolecules. Many antioxidants have been shown to modify carcinogenesis, and as a rule, they inhibit the initiation stage by reducing the interaction between carcinogen and DNA. However, both promotion and inhibition have been reported for second-stage carcinogenesis, depending on the organ site, species of animal, or initiating carcinogen. They can also block reaction of amine and nitrite to form nitrosamines or reduce TPA promotion of skin carcinogenesis. Generally high doses of antioxidants are required for carcinoma induction or modification of chemical carcinogenesis. The significance of the reported tumorigenicity and strong promoting activity of antioxidants for forestomach epithelium of animals to the development of human cancer appears limited mainly because humans do not have a forestomach. The carcinogenic and strong promoting activities of catechol and its structurally related compounds on rat glandular stomach epithelium are of greater concern because this tissue is directly analogous to human gastric epithelium.(ABSTRACT TRUNCATED AT 400 WORDS)

[Nutrition and the etiology of colon cancer: from descriptive epidemiology to dietary prevention]

Colon cancer is one of the most frequent forms of cancer in the Federal Republic of Germany and in most Western countries, but is, however, generally rare in Africa, Asia, and Latin America. Based on epidemiological investigations, differing dietary practices are considered to be main reason for these differences. A high fat and protein consumption was identified as a risk factor, while a high fibre content of the diet was found to be protective. Existing hypotheses of the etiology of colon cancer, which are based on the two-stage initiation-promotion model, regard interactions between initiators, promotors, and inhibitors as decisive for the final outcome of colon cancer. Possible initiators are pyrolysis products of protein-rich food (meat or fish), generated by heating, and products of metabolism of intestinal bacteria (e.g. faecal mutagens, N-nitroso compounds, transformation products of bile acids). Fats probably exert their influence only at the promotional stage. The diet-dependent bacterial formation of deoxycholic and lithocholic acids is a possible mechanism which has been experimentally substantiated. The protective effect of a diet rich in fibre seems to be mediated in particular by dilution and adsorption of harmful compounds. Further protective factors in human diet may be calcium, selenium, vitamin A and beta-carotene. In this paper, evidence, both supporting and refuting the existing hypotheses, is discussed, as well as the possibilities of dietary prevention of colon cancer.