Mechanisms of anti-carcinogenesis by indole-3-carbinol: Studies of enzyme induction, electrophile-scavenging, and inhibition of aflatoxin B 1 activation

Abstract

The induction of oxidation and conjugation enzymes, the scavenging of carcinogen electrophiles, and the inhibition of aflatoxin B 1 (AFB 1) activation were examined as possible mechanisms of anti-carcinogenesis by indole-3-carbinol (I3C). Liver microsomal 7-ethoxycoumarin O-deethylase and 7-ethoxyresorufin O-deethylase activities were not induced significantly in rainbow trout fed diets containing 500–2000 ppm I3C for 8 days compared to trout fed the control diet. Furthermore, no detectable changes in the specific contents of cytochrome P-450 isozymes LM 2 and LM 4b as measured by Western-blotting and immunoquantitation, were found in liver microsomes following dietary I3C administration. Dietary I3C had no significant effect on liver microsomal uridine diphosphate-glucuronyl-transferase activity, measured using the substrates 1-naphthol and testosterone, or on cytosolic glutathione S-transferase activity, measured using the substrate styrene oxide. The ability of I3C or its acid reaction products (RXM; generated by the reaction of I3C with HC1) to act as scavengers for the direct alkylating agent AFB 1-8,9-Cl 2 was examined. Addition of I3C or RXM to in vitro incubations did not inhibit the covalent binding of AFB 1-8,9-Cl 2 to calf thymus DNA. Kinetic analyses of microsome-mediated binding of AFB 1 to DNA in vitro indicated that RXM inhibited the metabolic activation of AFB 1. RXM increased the apparent K m for the AFB 1-DNA binding reaction without changing the associated V max; the apparent K m values at 0, 3.5, 35, and 350 μM RXM were 35, 38, 66, and 86 μM for trout liver microsomes. RXM also inhibited the activation of AFB 1 by rat liver microsomes, but I3C was not an effective inhibitor against AFB 1-DNA binding mediated by either rat or trout liver microsomes. The results of the present study indicate that inhibition of microsome-activated AFB 1 binding to DNA by I3C products may be of significant importance in I3C inhibition of hepatocarcinogenesis in trout and other species. The inhibition of carcinogen activation by I3C is contrasted with the mechanism of anti-carcinogenesis by β-naphthoflavone, which involves induction of xenobiotic metabolizing enzymes.