Multiple forms of nuclear estrogen receptor in the hypothalamus and pituitary after in vitro exchange with [3H]estradiol or [3H]hydroxytamoxifen

Mapping on the calf estrogen receptor of the binding domain for an antibody interfering with receptor activation

The localization on the calf estrogen receptor of the binding domain for B36 (an IgM antibody which prevents and reverses the effects of receptor activation) has been studied by means of controlled proteolysis of the receptor-estradiol complex using trypsin, chymotrypsin, and papain. We successively determined for intact and proteolyzed receptor-estradiol complex (i) the abilities of estradiol-binding species to aggregate in low salt medium, to bind to nonspecific DNA absorbed onto cellulose, and to interact with B36 antibody in sucrose gradients; (ii) the hydrodynamic properties of estradiol-binding species, by gel permeation chromatography and sucrose gradient centrifugation in high salt media and (iii) the molecular weights of B36-reactive species, by immunoblot analysis. Three tryptic receptor fragments of Mr 36,000, 34,000, and 33,000 and two chymotryptic fragments of Mr 36,000 and 33,000 included both the hormone- and B36-binding domains but did not interact with DNA, whereas at least two receptor fragments resulting from the action of chymotrypsin and papain bound estradiol with high affinity but interacted neither with DNA nor with B36. Taking into account these results and assuming that structure of the calf estrogen receptor is similar to those of sequenced estrogen receptors (which show a highly conserved organization with considerable homologies in the functional domains), we propose that the B36-binding domain is located either between the DNA- and hormone-binding domains (model I) or at the C-terminal end of the estrogen receptor (model II). The regions that include the main proteolytic cleavage sites of the receptor are also specified, and the abilities of the two models of the calf estrogen receptor to account for the effect of B36 on receptor activation are discussed.

Differential extraction of estradiol- and tamoxifen-receptor complexes from hypothalamic cell nuclei

These experiments compared nuclear estradiol- and tamoxifen-receptor complexes from brain by measuring the ability of a series of ionic agents and DNA intercalators to release estrogen receptors from hypothalamic cell nuclei following in vivo administration of the agonist or antagonist. Over the concentration ranges tested, intercalators released 50-80% of the total estrogen receptors from hypothalamic cell nuclei in a concentration-independent fashion whereas extraction of nuclear receptors by the ionic agents was concentration dependent. In general, brain cell nuclear estrogen receptors were extracted more easily by both ionic agents and intercalators following injections of tamoxifen than of estradiol. It was also found that the polyanion heparin releases a small population of nuclear estrogen receptors that are not extracted by 0.5 M KCl, suggesting the existence of KCl-resistant chromatin acceptor sites in hypothalamic nuclei. These data support the hypothesis that estrogen agonists and antagonists promote different conformational changes when they bind neural estrogen receptors, resulting in the formation of ligand-receptor complexes with different biological activity.

Inhibition of sexual receptivity by anesthesia during estrogen priming

Anesthetics such as pentobarbital have been used to infer a role of neural activity in several neuroendocrine responses. The present experiments tested the hypothesis that neural activity during estrogen priming is necessary for the induction of sexual receptivity. Sexual behavior was induced in ovariectomized female rats by two pulses of unesterified estradiol 6 h apart, followed by progesterone at 19 h and testing at 24 h. Anesthetizing animals for approximately 1 h during each exposure to estradiol inhibited the induction of receptivity. Anesthesia immediately before each exposure to estradiol did not interfere with the induction of sexual receptivity. The induction of neural progestin receptors by estradiol was not prevented by anesthesia. Anesthesia had no effect on the uterine weight response to estradiol. These results suggest that neural activity during estrogen priming is required for the induction of sexual receptivity, and point to differences in the mechanism of action of estrogens in the brain and the uterus.

Effects of sulfhydryl reagents on uterine nuclear estrogen receptors labeled by in vitro exchange with [3H]estradiol or [3H]4-hydroxytamoxifen

We have attempted to convert 4 S uterine nuclear estrogen receptors obtained after in vitro labeling with [3H]antiestrogens to 3 S, the form observed after in vitro exchange with [3H]estradiol, in order to examine the possible relationship between these forms. Treatment of nuclear extracts labeled with the high affinity antiestrogen, [3H]4-hydroxytamoxifen, with a variety of nucleases, phosphatases, or proteases either had no effect on the 4 S antiestrogen-receptor complex or led to loss of ligand binding. The sulfhydryl reducing agents, cysteine or reduced glutathione, on the other hand, brought about conversion of 4 S estrogen receptors to components sedimenting at about 3 S. Conversely, when oxidized glutathione was included in all buffers used for preparation and labeling of nuclear estrogen receptors with [3H]estradiol, more rapidly sedimenting (approximately 4.6 S) forms of estrogen-receptor complex predominated. Cysteine still effected the 4 S to 3 S conversion when nuclear estrogen receptors, partially purified by sucrose gradient centrifugation, were used as substrate, suggesting a direct action of the sulfhydryl reagents on receptor molecules. From these results we propose that nuclear estrogen and antiestrogen-receptor complexes may differ in conformation such that the former may be more sensitive to the action of an endogenous reducing agent which contributes to formation of 3 S [3H]estradiol-receptor complexes.