Estrogenic stimulation of the antiestrogen specific binding site in rat uterus and liver

Tamoxifen and other antiestrogens bind to the estrogen receptor and to a specific antiestrogen binding site (AEBS). We have confirmed the existence of an AEBS by saturation and competitive binding analyses with [3H]tamoxifen as the radiolabeled ligand. The quantities of AEBS (pmol/g tissue) present in a low speed cytosol (25,000 X g X 30 min) of the liver and uterus vary with species and physiological state: liver, mature female rat, 82.7 +/- 6.7; ovariectomized mature rat, 46.1 +/- 2.9; immature female rat, 40.0 +/- 1.8; mature male rat, 57.3 +/- 2.7; and mature female mouse, 63.0 +/- 5.0; uterus, mature rat, 17.9 +/- 1.7; ovariectomized mature rat, 13.3 +/- 0.5; immature rat, 8.2 +/- 0.8; and mature mouse, 49.0 +/- 2.1. The dissociation constants did not differ significantly in any of the cases and were 1.5 and 2.5 nM for the liver and uterus, respectively. Developmental studies demonstrated that the liver AEBS gradually increases in both sexes from 21.0 pmol/g at birth to their respective adult levels with the only divergence occurring approximately at the time of puberty. The AEBS concentration also varied with the stage of the estrous cycle [highest at estrus (20.6 +/- 1.8) and metestrus (91.7 +/- 5.9) for the uterus and liver, respectively, and lowest at diestrus (uterus, 15.9 +/- 0.2; liver, 60.2 +/- 4.1)]. Furthermore, estradiol treatment of ovariectomized rats significantly increased both liver and uterine AEBS levels, whereas tamoxifen increased uterine AEBS concentration although having no significant effect on the level of these sites in liver. These results demonstrate the presence of a cytosolic AEBS in the liver and uterus which is distinct from the estrogen receptor and appears to be under estrogenic regulation.