INFLUENCE OF VITAMIN-A, VITAMIN-C, AND VITAMIN-E AND BETA-CAROTENE ON AFLATOXIN B-1 BINDING TO DNA IN WOODCHUCK HEPATOCYTES

Background. There is extensive epidemiologic evidence suggesting a protective role for micronutrients in cancer incidence. This evidence comes from studies of fruit and vegetable intake and serum levels of specific micronutrients. There also is limited in vitro evidence demonstrating that micronutrients can influence the first step in carcinogenesis, binding of chemical carcinogens to DNA. These in vitro studies allow the determination of specific effects of individual micronutrients. The influence of micronutrients on DNA binding of aflatoxin B1 (AFB(1)), a potent hepatocarcinogen, in mammalian cells is unknown. Woodchuck hepatocytes were used as a model to investigate the effects of vitamin A (all-trans retinol), C (ascorbic acid), ascorbyl palmitate (a synthetic lipophilic derivative of ascorbic acid), vitamin E (alpha-tocopherol), and beta-carotene on AFB(1)-DNA binding. Methods. Woodchuck hepatocytes were treated with 4 doses (0.080, 0.40, 2.0, and 10 mu M) of [H-3]AFB(1) or with different combinations of AFB(1) and the vitamins for 6 hours, and adduct levels determined. Western blot analysis of protein extracts of treated cells was used to determine the effects of vitamin A and beta-carotene on glutathione-S- transferase M1 levels. Results. Vitamin A inhibited formation of AFB(1)-DNA adducts in a dose-dependent manner throughout a concentration range of 34-122 mu M by 40-80%. Vitamin C (0.080-10 mM) was much less effective than vitamin A as an inhibitor of AFB(1)-DNA binding. Treatment with 6.0-48.3 mu M ascorbyl palmitate reduced adduct levels at lower AFB(1) concentrations but had no significant effect at higher AFB(1) concentrations. beta-Carotene and vitamin E enhanced covalent binding of AFB(1) to DNA. Enhancement with beta-carotene was observed when both tetrahydrofuran or liposomes were used as the administration vehicle. Western blot analysis indicated that neither the vitamin A nor beta-carotene treatment affected glutathione-S-transferase M1 protein levels. Conclusions. These results demonstrate that micronutrients play a complex role in the process of chemical carcinogenesis. Although protective effects were seen with several antioxidant vitamins, increased DNA adduct formation was observed with beta-carotene and vitamin E. This antioxidant activity may be unrelated to the inhibition of DNA adduct formation. Additional studies are needed to understand the mechanism of enhanced adduct formation.