2-Hydroxyestrone (2-OHE(1)) and 16alpha-hydroxyestrone (16alpha-OHE(1)) have been reported to be risk factors for negative bone balance and breast cancer, respectively. The roles of these two metabolites of estrone as estrogen agonists or antagonists with respect to estrogen target tissues, or both, are poorly defined. The purpose of this study was to characterize metabolite and tissue-specific differences between the actions of hydroxylated estrones on selected reproductive and non-reproductive estrogen target tissues in growing rats. First, the effects of ovariectomy were determined. Ovariectomy had the expected effects, including increases in all dynamic bone measurements at the proximal tibial epiphysis, without induction of bone loss. Second, ovariectomized growing rats were continuously treated for 3 weeks with 2-OHE(1), 16alpha-OHE(1), 17beta-estradiol (E(2)), a combination of E(2) and 2-OHE(1) (E(2)+2-OHE(1)), or a combination of E(2) and 16alpha-OHE(1) (E(2)+16alpha-OHE(1)), using controlled release subcutaneous implanted pellets containing 5 mg 2-OHE(1), 5 mg 16alpha-OHE(1), 0.05 mg E(2) or placebo. E(2) reduced body weight gain and radial and longitudinal bone growth as well as indices of cancellous bone turnover, and increased serum cholesterol, uterine wet weight and epithelial cell height, and proliferative cell nuclear antigen labeling in mammary gland. The hydroxylated estrones did not alter uterine wet weight and 16alpha-OHE(1) antagonized the E(2)-stimulated increase in epithelial cell height. 2-OHE(1) had no effect on cortical bone, whereas 16alpha-OHE(1) was an estrogen agonist with respect to all cortical bone measurements. 16alpha-OHE(1) also behaved as an estrogen agonist with respect to serum cholesterol and cancellous bone measurements. 2-OHE(1) had no effect on most E(2)-regulated indices of cancellous bone growth and turnover, but was a weak estrogen agonist with respect to mineral apposition rate and bone formation rate. Neither estrogen metabolite influenced body weight gain. Third, weanling rats were treated for 1 week with vehicle, E(2) (200 microg/kg per day) or 16alpha-OHE(1) (30, 100, 300, 1000 and 3000 microg/kg per day) to confirm uterotropic effects of daily subcutaneous (s.c.) administration of 16alpha-OHE(1). 16alpha-OHE(1) increased uterine weight in a dose-response manner to values that did not differ from rats treated with E(2). We conclude that the estrogen metabolites 2-OHE(1) and 16alpha-OHE(1) have target tissue-specific biological activities which differ from one another as well as from E(2). These findings add further support to the concept that there are several classes of estrogens with distinct biological activities. Furthermore, differences in the route of administration could influence the tissue specificity of estrogen metabolites.

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