Nonlinearities in 2-acetylaminofluorene exposure responses for genotoxic and epigenetic effects leading to initiation of carcinogenesis in rat liver.

Abstract

The dose responses for several effects of low-level limited exposures to 2-acetylaminofluorene (AAF) in the livers of male Fischer 344 rats were measured and a subsequent phenobarbital tumor promotion regimen was used to manifest initiation of carcinogenesis. Three doses over a 10-fold range yielding cumulative total exposures of 0.126, 0.42, and 1.26 mmol AAF/kg body weight were achieved by daily intragastric instillation for up to 12 weeks with interim terminations. This was followed by 24 weeks administration of 500 ppm phenobarbital (PB) in the diet to promote liver tumor development. At 12 weeks at the end of AAF administration, all exposures produced adducts in liver DNA, measured by 32P postlabeling, and the level of adducts increased with exposure, except that the high exposure did not produce a dose proportional increase. Measurement of arylsulfotransferase activity, a key enzyme in the metabolic activation of AAF, revealed that in livers from the high exposure animals, the enzyme was inhibited. To assess for toxicity, the centrilobular zone of glutamine synthetase-positive hepatocytes was quantified immunohistochemically at 12 weeks. The area of the zone was reduced in the high exposure group and there was a trend to reduction in relationship to exposure. The two lower exposures to AAF produced no increase in cell proliferation, whereas the high exposure resulted in a marked increase, about 8-fold over controls. Initiation was assessed by induction of hepatocellular altered foci (HAF) that expressed the placental form of glutathione S-transferase. AAF induced HAF in the high exposure group, 9-fold at 8 weeks and 170-fold at 12 weeks compared to controls. In rats maintained on PB for 24 weeks after exposure, the multiplicity of HAF increased in controls and comparably in the low and mid exposure groups, but remained at the about the same high level in the high exposure group. The high exposure produced a substantial incidence of benign neoplasms by 12 weeks, and with promotion by 36 weeks, all rats developed hepatocellular neoplasia. In the mid exposure group, only one adenoma occurred at 36 weeks in 17 rats, while in the low exposure group, no liver tumor occurred in 23 rats. Thus, these findings document nonlinearities for some of the effects of AAF, with supralinear effects at the high exposure for cell proliferation and induction of HAF, and a no-observed-effect level for induction of promotable liver neoplasms at the lowest cumulative exposure of 0.126 mmol/kg, in spite of the formation of DNA adducts. We conclude that the effects of this DNA-reactive hepatocarcinogen leading to initiation exhibit nonlinearities and possible thresholds.