2-AMINO-1-METHYL-6-PHENYLIMIDAZO[4,5-B]PYRIDINE IS A POTENT MUTAGEN IN THE MOUSE SMALL-INTESTINE

Mutations in long lived stem cells are critical events in carcinogenes is. The Dlb-1 assay detects intestinal stem cell mutation at the Dlb-1 locus in Dlb-1a/b heterozygous mice by visualizing mutated clones of epithelial cells in situ which do not bind the lectin Dolichos bifloru s agglutinin. We have used this assay to show that the food-derived he terocyclic amine 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP ) is a potent intestinal mutagen when administered either i.p. or p.o. This contrasts with the inactivity of the structurally related mutage n 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline in the assay which we have described previously. Immunocytochemical localization of the P450 enzyme CYP1A2, which is responsible for the primary activation of the se mutagens, shows that in untreated mice it is present in liver hepat ocytes and in occasional villus epithelial cells but is absent from th e target intestinal stem cell population. In addition, liver microsome s, unlike intestinal microsomes, were able to convert PhIP to the prox imate mutagen N-hydroxy-PhIP. CYP1A2 immunoreactivity in beta-napthofl avone-induced animals was elevated in liver hepatocytes and increased to a lesser extent in duodenal villus epithelial cells. Treatment with beta-napthoflavone produced an unexpected 46% decrease in the number of Dlb-1 mutations in response to PhIP. Following treatment with PhIP, there was no difference in the number of Dlb-1 locus mutations betwee n the proximal and distal ends of the small intestine in uninduced ani mals, indicating that the bile duct is unlikely to be responsible for transport of mutation inducing metabolites of PhIP to the small intest ine. Our results demonstrate that metabolic activation of an indirect acting genotoxic agent can occur at a site other than the target tissu e, and absence of the enzymes required for activation of a mutagen doe s not necessarily protect that tissue from its genotoxic effects.