Antagonism to estradiol in the mouse: reduced entry of receptors complexed with 4-hydroxytamoxifen into a Mg2+-soluble chromatin fraction.

Antagonism to estradiol has been examined in murine uteri. When tamoxifen was administered simultaneously with estradiol (0.05 microgram/mouse), it was able to act as an antagonist over the dosage range 0.05-50 micrograms/mouse. The metabolite 4-hydroxytamoxifen (4OH-tamoxifen) had high affinity for estrogen receptors and was a slightly better antagonist over the dosage range 0.005-1 microgram/mouse. After uteri were exposed to either [3H] estradiol or [3H]4OH-tamoxifen, receptors complexed with [3H] estradiol penetrated a chromatin region, which was released as the Mg2+-soluble chromatin fraction after DNAase I treatment more readily than receptors complexed with [3H]4OH-tamoxifen. [3H]4OH-tamoxifen-receptor complexes could not be driven into the Mg2+-soluble chromatin fraction by increasing the ligand concentration during translocation. Relative to [3H]estradiol, significantly more [3H]4OH-tamoxifen was observed to associate with uterine cells and to penetrate the nucleus so that neither restricted entry nor extranuclear partitioning could explain the failure of [3H]4OH-tamoxifen-receptor complexes to enter the Mg2+-soluble chromatin. Bleomycin, an agent that interrupts DNA continuity, did not interfere with the appearance of estrogen receptor activity in the Mg2+-soluble chromatin fraction. Preincubation of intact uteri in the presence of molybdate (20 mM) did inhibit the appearance of receptor activity in this chromatin fraction; however, this effect did not occur through inhibition of receptor activation, but, rather, through the lowering of receptor activity in all chromatin fractions. In the studies reported here, the chromatin positioning of estrogen receptors complexed with estradiol appeared to be distinct from the positioning of receptors complexed with 4OH-tamoxifen. These observations suggest an additional basis from which the mechanisms separating the actions of estrogen agonists and antagonists can be approached.