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

Formation of the N-(deoxyguanosin-8-yl)-aminofluorene adduct was studied in enzyme-altered foci induced by four different liver carcinogenesis models. Foci were detected and scored for enzyme phenotype by a computer-aided image overlay technique. Localization of the enzymes gamma-glutamyl transpeptidase, canalicular ATPase 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 to in situ 2-acetylaminofluorene--DNA adduct distribution obtained by immunofluorescence. Foci were induced by the following models: (1) chronic feeding of 0.02% 2-acetylaminofluorene (2-AAF) for 8 weeks; (2) intubation of diethylnitrosamine (DEN) (10 mg/kg) 24 h after a 70% partial hepatectomy (PH), followed 8 weeks later by a diet containing 0.05% phenobarbital for 9 months; (3) intubation of DEN (10 mg/kg) 24 h after PH, followed by a diet containing 0.01% ciprofibrate for 5 months, and after an additional 4 months a diet containing 0.05% phenobarbital for 2 months; (4) maintenance for 7.5, 16.5 or 19.5 months after transplantation of DEN/2-AAF/PH ('Solt-Farber' protocol) donor liver cells into host rats receiving a brief 2-AAF/PH selective regimen then no further treatment until sacrifice. To test the capacity of both foci and morphologically normal livers to form DNA adducts, the animals in models 2-4 received a diet containing 0.02% 2-AAF for 5 or 6 days before sacrifice. In all of the enzyme-altered foci identified in models 1-3 there were no DNA adducts visible by immunofluorescence. Scattered groups of positive cells were occasionally seen in the otherwise dark foci induced by model 4. For technical reasons some enzyme-altered foci were not identifiable on the fluorescence-stained slides. In liver serial sections from rats in models 1-4, there were 75, 304, 125 and 68 enzyme-altered foci of seven different phenotypes which were identified as AF-DNA negative. In models 1 and 4 there were some additional adduct-negative foci not associated with any of the seven identified focus phenotypes. These studies demonstrate that loss of the ability to form DNA adducts in hepatic enzyme-altered foci is a common and very early biochemical adaptation to xenobiotic exposure in different hepatocarcinogenesis models. This adaptation also is retained by the majority of foci in later stages of hepatocarcinogenesis.