Mechanisms of anti-carcinogenesis by indole-3-carbinol: effect on the distribution and metabolism of aflatoxin B1 in rainbow trout

Abstract

Indole-3-carbinol (I3C), a component of cruciferous vegetables, was previously shown to inhibit aflatoxin B ₁ (AFB ₁ ) carcinogenesis in trout. The purpose of this study was to examine the effect of I3C on AFB ₁ metabolism and hepatic DNA adduct formation in vivo and in vitro . When fed at 0.2%, I3C produced a 70% reduction in average in vivo hepatic DNA binding of injected AFB ₁ over a 21-day period when compared to controls. A 24-h distribution study of injected tritiated AFB ₁ in I3C fish showed less total radioactivity in the blood and liver at all times examined, compared to controls. These reductions were due primarily to reduced levels of AFB ₁ bound to red blood cell DNA, reduced plasma levels of the primary metabolite aflatoxicol (AFL), and decreased levels of AFB ₁ and polar metabolites present in the liver of I3C fish. In contrast to blood, total radioactivity was significantly elevated in the bile of I3C fish resulting from a 7-fold increase in aflatoxicol-M ₁ glucuronide levels over controls. No difference was observed in concentration of AFL glucuronide, the primary conjugate present in control fish. There was no difference in total radioactivity remaining in the carcass of I3C or control fish. AFB ₁ metabolism in freshly isolated hepatocytes from I3C fish showed 20% less DNA binding in a 1-h assay, with a 2-fold increase in aflatoxin M ₁ production. Addition of I3C to control hepatocytes at levels of 1, 10 or 100 μM had no effect on AFB ₁ DNA binding. These findings indicate that I3C inhibition of AFB ₁ hepatocar-cinogenesis in trout involves substantial changes in the phar-macokinetics of carcinogen distribution, metabolism and elmination, leading to significantly reduced initial hepatic-nuclear DNA damage in vivo .