Investigation of antiestrogenic properties of unleaded gasoline in female mice.

Chronic exposure of female B6C3F1 mice to a high concentration of unleaded gasoline (UG) vapor induced liver tumors and caused uterine changes suggestive of estrogen antagonism. These effects of UG may be related, since estrogens inhibit hepatocarcinogenesis in mice. The purpose of this study was to determine if antiestrogenic properties of UG could be demonstrated in sensitive short-term assays. Competitive binding to estrogen receptors was assayed in vitro in uterine cytosols prepared from ovariectomized (OVEX) mice. UG did not inhibit specific binding of 17 beta-[3H]estradiol (E2) to uterine cytosols. To determine if UG induced estrogen metabolism, hepatocyte suspensions were prepared from female mice treated by intragastric intubation (ig) for 3 days with corn oil (control) or UG (1800 mg/kg/day). In a quantitative in vitro assay, hepatocytes isolated from UG-treated mice converted E2 and 17 alpha-ethinyl estradiol to water soluble metabolites at a three-fold faster rate than control hepatocytes. Dose-response studies confirmed the induction of E2 metabolism by UG doses as low as 600 mg/kg/day. In a 3-day uterotrophic assay, immature female mice cotreated with UG (600 or 1800 mg/kg/day, ig) and E2 (1 microgram/day, sc) had similar relative uterus weights and uterine peroxidase activity as mice cotreated with corn oil and E2. In a modified uterotrophic assay, OVEX mice treated with corn oil or UG (2400 mg/kg/day, ig) on Days 1-4 and cotreated with E2 (4 micrograms/kg/day, sc) on Days 3-4 had similar uterus weights on Day 5. Thus, while ig treatment of mice with UG induced estrogen metabolism in isolated hepatocytes, this induction did not have functional antiestrogenic consequences as measured by uterotrophic assays. These data suggest that the uterine effects caused by chronic exposure of mice to UG vapor may not be due to direct antiestrogenic effects of UG.