Lack of acetylaminofluorene-DNA adduct formation in enzyme-altered foci of rat liver

Formation of the N-(deoxyguanosin-8-yl)-aminofluorene adductwas studied in enzyme-altered foci induced by four differentliver carcinogenesis models. Foci were detected and scored forenzyme phenotype by a computer-aided image overlay technique.Localization of the enzymes -glutamyl transpeptidase, canalicularATPase and glucose-6-phosphatase was performed by enzyme histochemistry,allowing identification of foci of seven different phenotypes.Patterns of foci obtained by image overlay were compared toin situ 2-acetylaminofluorene-DNA adduct distribution obtainedby immunofluorescence. Foci were induced by the following models:(1) chronic feeding of 0.02% 2-acetylaminofluorene (2-AAF) for8 weeks; (2) intubation of diethylnitrosamine (DEN) (10 mg/kg)24 h after a 70% partial hepatectomy (PH), followed 8 weekslater by a diet containing 0.05% phenobar-bital for 9 months;(3) intubation of DEN (10 mg/kg) 24 h after PH, followed bya diet containing 0.01% ciprofibrate for 5 months, and afteran additional 4 months a diet containing 0.05% phenobarbitalfor 2 months; (4) maintenance for 7.5, 16.5 or 19.5 months aftertransplantation of DEN/2-AAF/PH (‘Solt-Farber’ protocol)donor liver cells into host rats receiving a brief 2-AAF/PHselective regimen then no further treatment until sacrifice.To test the capacity of both foci and morphologically normallivers to form DNA adducts, the animals in models 2–4received a diet containing 0.02% 2-AAF for 5 or 6 days beforesacrifice. In all of the enzyme-altered foci identified in models1–3 there were no DNA adducts visible by immunofluorescence.Scattered groups of positive cells were occasionally seen inthe otherwise dark foci induced by model 4. For technical reasonssome enzyme-altered foci were not identifiable on the fluorescence-stainedslides. In liver serial sections from rats in models 1–4,there were 75, 304, 125 and 68 enzyme-altered foci of sevendifferent phenotypes which were identified as AF-DNA negative.In models 1 and 4 there were some additional adduct-negativefoci not associated with any of the seven identified focus phenotypes.These studies demonstrate that loss of the ability to form DNAadducts in hepatic enzyme-altered foci is a common and veryearly biochemical adaptation to xenobiotic exposure in differenthepatocarcinogenesis models. This adaptation also is retainedby the majority of foci in later stages of hepatocarcinogenesis.