Estrogen receptors in rat bone: Their interaction with estrogen receptor modulators

Estrogen receptors (ER) were studied in rat bone cytosol using immunoprecip itation, and Western blot technique. Ligand specificity of bone ER was stud ied using various known modulators of ER. Competitive experiments were perf ormed under exchange conditions in bone tissue obtained from one day old ra ts. ER alpha and beta subtypes were identified using immunoblotting experim ents compared with that of ovarian and uterine tissues. In competitive bind ing assay, maximum inhibition in specific H-3-E-2 binding was shown by E-2 followed by tamoxifen and diethylstilbestrol. 7-Hydroxycentchroman and 85/2 87 also inhibited specific H-3-E-2 binding but were less potent as compared to tamoxifen and diethylstilbestrol. However, 85/287 was less effective (8 1%) as compared to 7-hydroxycentchroman. Polyacrylamide gel electrophoresis of cytosol and Western blot analysis revealed the presence of 55 kD and 66 kD ER immunoreactive bands corresponding to alpha and beta subtypes, respe ctively, in bone as well as in uterus. Interestingly, the concentration of 55 kD ER was 3-fold higher than that of 66 kD ER. Ovarian cytosol revealed the presence of a 55 kD band only in Western blot analysis. These studies s uggest the action of estrogens/ER modulators on osteoblasts which contain a limited number of classical alpha as well as beta sub types of ER that are known to be structurally different in their hormone-binding domains.