Abstract
Adult ovariectomized Sprague-Dawley rats were administered a single initiating dose of 200 mg diethylnitrosamine (DEN)/kg, i.p. 17α-ethynylestradiol (EE2) was then chronically administered to the rats by means of s.c. Silastic implants at an estimated dose of 90 µg EE2/kg/day. Hepatic γ-glutamyltranspeptidase-positive foci were evaluated after 20–60 (+20,+30, ...,+60) weeks of chronic EE2 treatment and after 20 or 30 weeks of EE2 followed by 20 or 30 weeks with no EE2 [(+20,-20), (+30,-20), (+30,-30)] to determine the effects of withdrawal of the promoting agent on the persistence or reversibility of these focal lesions. Our results show that γ-glutamyltranspeptidase-positive foci are no longer dependent on exogenous EE2 administration for their continued growth in initiated animals given EE2 chronically for 20 weeks. In DEN-initiated, EE2-promoted animals the number of foci per cc (Nr) seen at 20 weeks increased over the next 20 weeks in the absence of further EE2 treatment, but was not statistically different than Nr for continued EE2 treatment. The proportion of total liver volume occupied by foci (Vv) was 0.0054 (+30), 0.0191 (+30,-20), and 0.0135 (+50). The (+30,-20) Vv was significantly different than that for (+30) (P < 0.01). Hepatocellular adenomas and carcinomas were detected in DEN-initiated and EE2-promoted animals as early as 30 weeks. Hepatic tumor incidence continued to increase after withdrawal of EE2 in initiated animals which had received only 30 weeks of promotion. Within the framework of our studies, the promoting effects of EE2 do not appear to be reversible by withdrawal of EE2 after 20 weeks of treatment. It may be that events or factors which were estrogen dependent in the early stages of promotion are now constitutive.
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