Effects of ethinyl estradiol and tamoxifen on liver DNA turnover and new synthesis and appearance of gamma glutamyl transpeptidase-positive foci in female rats

We investigated possible mechanisms associated with promotion of hepatocarcinogenesis by the synthetic estrogens mestranol (M) and ethinyl estradiol (EE). Our first objective was to determine whether chronic EE treatment was associated with hepatotoxicity accompanied by regenerative hyperplasia. Female Sprague-Dawley (SD) rats were partially bepatectomized and their liver DNA prelabeled with [³H]thymidine. Two weeks later the rats were treated with EE at three doses or M (using timed-release tablets implanted s.c.) or with 0.05% phenobarbital (PB) in the diet. The rats were killed 6 weeks later and the amount of [³H]thymidine remaining in liver DNA determined. The results showed that none of the promoters caused hepatotoxkty as detected by the loss of prelabeled DNA. EE but not M or PB caused a dramatic dose-dependent enlargement of the pituitary. Subsequent experiments were carried out to define the dose-time response of liver DNA synthesis to treatment with EE, M and PB. Female SD rats were treated with these agents and [³H]-thymidine incorporation into DNA was determined at various times thereafter. The results showed that EE at 2.5 μig/day and PB (0.05%) caused a rapid increase in liver DNA synthesis which peaked between 24 and 72 h and remained elevated for at least the next 7 days. Dose-response experiments with EE- and M-treated rats demonstrated that 24 h after beginning treament, significant increased in liver DNA synthesis could be detected at an EE dose as low as 0.1 μig/day; DNA synthesis was also significantly increased by M. The anti-estrogen tamoxifen (T), did not cause increased liver DNA synthesis. However, T at 15 μig/day did inhibit the induction of DNA synthesis caused by EE and M at 2.5 μig/day but not by PB (0.05%). The effect of T on promotion by EE of the appearance of gamma glutamyl trans-peptidase (GGT)-positive foci was determined. Female SD rats were initiated with a single i.p. dose of diethylnitrosamine at 20 mg/kg given 24 h after partial hepatectomy. One week later the rats were treated with EE (5 μig/day), T (15 or 50 μg/day) or EE plus T at both doses using timed-release tablets. The rats were killed after 4 months and the incidence, number and size of the GGT foci determined. The results revealed that, as expected, EE enhanced the appearance of GGT foci. Unexpectedly, T alone at both doses also enhanced the appearance of the foci. However, in rats treated with EE plus T at either dose, the number of foci was similar to that in the non-promoted placebo-treated controls. T also prevented the pituitary hyperplasia and hypertrophy associated with EE treatment.