This study measured the effect of precise doses of 2-acetylaminofluorene (AAF) in inducing DNA damage, functional changes and neoplastic conversion in rat liver. Groups of male F344 rats at 9 weeks of age were exposed to cumulative doses of 0.5 or 2.0 mmol AAF per kg body weight given by gavage daily 5 days per week over an 8-week period and maintained with no further exposure for up to 8 weeks. Administration of AAF resulted in the formation of N-deoxyguanosin-(8-yl)-2-aminofluorene in liver DNA in relationship to dose. In centrilobular hepatocytes the zone of glutamine synthetase-expressing cells was reduced by exposure. By 8 weeks, but not at 4 weeks, the higher of the two doses of AAF provoked an increase in cell proliferation measured by immunohistochemical incorporation of bromodeoxyuridine. Altered hepatocellular foci expressing the placental form of glutathione transferase were induced by the high dose of AAF at 4 weeks, but not at the low dose. At 8 weeks the incidence of foci at the high dose was 79 times that induced by the low dose. These foci were highly proliferative. In animals exposed to AAF for 8 weeks and maintained for 4 weeks with no exposure, DNA adducts decreased by 80% and cell proliferation subsided by 80%, although the glutamine synthetase zone remained diminished. After discontinuation of AAF, the number of foci diminished by 50% and their proliferation subsided by 80% at 4 weeks, indicating a phenotypic reversion of many foci. With this protocol of administration of precise doses of AAF, we have established non-linearity of effects and a lack of correlation between DNA adduct formation and induction of cellular lesions. We suggest that doses in the range of those reported can be used to study the contribution of epigenetic and genotoxic effects in carcinogenesis and to study threshold events.
