Effects of Chronic Dietary Ethanol on in Vivo and in Vitro Metabolism of Methylazoxymethanol and on Methylazoxymethanol-induced DNA Methylation in Rat Colon and Liver1

We examined the metabolism of "C-labeled methylazoxymethanol (MAM) in male F344 rats pair-fed for 21.0 days either a liquid control diet, an isocaloric liquid diet containing 6.6% ethanol by volume (contin uous ethanol diet), or the ethanol diet for 20.5 days followed by the control diet for 0.5 day (interrupted ethanol diet). Compared to rats fed the control liquid diet, metabolism of |1,2-"C]MAM acetate to exhaled "CO? was inhibited by 25 to 42% in rats fed the continuous ethanol diet, but was initially stimulated by 90% in rats given the interrupted ethanol diet. MAM-induced DNA methylation, as reflected in 7-methylguanine and 06-methylguanine content 24 h after carcinogen administration, was inhibited in the colon mucosae of rats fed the interrupted ethanol diet by 52 to 54%, and an even greater inhibition (71 to 86%) of DNA methyla tion occurred in the colon mucosae of rats fed the continuous ethanol diet. Liver DNA methylation was significantly inhibited (by 32 to 42%) only in those rats fed the continuous ethanol diet. Liver microsomes isolated from rats fed the 3 diets metabolized MAM to formic acid and methanol in vitro, but liver microsomes from rats fed the continuous ethanol diet were 12 to 15 times more active than liver microsomes from rats fed the control diet. Liver microsomes isolated from rats fed the interrupted ethanol diet were only 3 to 5 times more active in MAM metabolism than liver microsomes from rats fed the control diet, indicat ing very rapid turnover of the ethanol-induced enzyme(s) catalyzing the oxidation of MAM. Although chronic ethanol feeding enhanced the activity of liver microsomes for MAM metabolism, ethanol was found to inhibit the reaction competitively. Hepatocytes isolated from rats fed the continuous ethanol diet were considerably more sensitive to MAMinduced unscheduled DNA synthesis than hepatocytes isolated from rats given the control liquid diet, indicating that the stimulation of MAM metabolism by dietary ethanol results in increased DNA damage, observ able in an in vitro system. Thus, the increased metabolic activation of MAM, due to enzyme induction by ethanol which is observed in vitro, is not reflected in increased liver DNA methylation in vivo. In addition, the great degree of similarity between the present observations involving MAM and the results of our previous studies involving azoxymethane and A'-nitrosodimethylamine (O. S. Sohn et al., Cancer Res., 47:31233129, 1987) raises the question of whether all three carcinogens might not be metabolically activated by the same form of ethanol-induced rat liver microsomal enzyme(s).