Toxicity of chronic exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin in diethylnitrosamine-initiated ovariectomized rats implanted with subcutaneous 17 beta-estradiol pellets.

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2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a potent hepatocarcinogen in female but not in male rats. Several lines of evidence suggest a key role of ovarian hormones, presumably estrogen, in the mechanism of TCDD-induced hepatocarcinogenesis. The aim of this current study was to determine the toxicity of co-treatment with TCDD and 17 beta-estradiol and assess the efficacy of 90-day subcutaneous constant release 17 beta-estradiol pellets. Ovariectomized (OVX) female Sprague-Dawley rats were initiated with diethylnitrosamine (DEN) and treated with TCDD for 20 or 30 weeks in the presence and absence of 17 beta-estradiol. TCDD concentrations were equivalent in livers of TCDD-treated sham operated and OVX rats following 20 weeks of treatment. Following 30 weeks of TCDD treatment, liver TCDD concentrations were higher in OVX rats than in intact rats. TCDD concentrations in livers of TCDD-treated OVX rats receiving supplemental 17 beta-estradiol were similar to intact rats following either 20 or 30 weeks of treatment. Mean hepatic background TCDD concentrations in untreated rats were 2-fold higher in intact rats compared to OVX rats, regardless of 17-estradiol exposure following 20, but not 30 weeks of treatment. Serum indicators of hepatocellular and hepatobiliary toxicity indicated transient hepatotoxicity in TCDD-treated OVX rats receiving 17 beta-estradiol. Histopathological alterations indicated hepatotoxicity induced by exposure to TCDD following either 20 or 30 weeks of exposure. No excess hepatotoxicity was associated with 17 beta-estradiol-supplementation in TCDD-exposed OVX female Sprague-Dawley rats. Serum 17 beta-estradiol concentrations were not constant and resulted in supra-physiological levels that decreased over time, resulting in target physiological serum 17 beta-estradiol concentrations following several weeks of release. Treatment with 17 beta-estradiol resulted in uterine weights and total body weights comparable to sham-operated female rats. These data confirm the efficacy of supplemental subcutaneous 17 beta-estradiol pellets on the induction of estrogenic responses in TCDD-treated rats and indicate no increased hepatotoxicity associated with 17 beta-estradiol exposure in TCDD-treated rats.