Displacement of 17-beta-estradiol from uterine receptor sites by an estrogen antagonist

Modulation of uterine morphology and growth by estradiol-17beta and an estrogen antagonist

The estrogen antagonist C1628 maintains sustained hypertrophy of the uterine epithelium and the synthesis of many proteins including peroxidase. C1628 is a progestogen, inducing secretion of the protein by surface epithelial and glandular cells. C1628 is a connective tissue mitogen, inducing DNA synthesis in fibroblasts and the endothelium. C1628 and estrogen share these properties mentioned above. Estrogen, however, induced moderate growth of the mucosa within a 24-h period and massive hyperplasia of the mucosa within a 24-h period thereafter. C1628 alone, or in combination with estradiol, does not have mitogenic effect on the mucosa, and in fact blocks the mitotic response normally induced by estrogen alone.

Estrogen-receptor interaction

The interaction of estradiol with uterine cells involves the association of the hormone with an extranuclear receptor protein, followed by temperature dependent translocation of the resulting complex to the nucleus. During this process, the steroid binding unit of the protein undergoes an alteration, called "receptor transformation," that can be recognized by an increase in its sedimentation rate from 3.8S to 5.2S, and by its acquisition of the ability to bind to isolated uterine nuclei and to alleviate a tissue specific deficiency in the RNA synthesizing capacity of such nuclei. Receptor transformation can be effected in the absence of nuclei by warming uterine cytosol with estradiol. This preparation of transformed complex resembles that extracted from nuclei both in its sedimentation rate (5.3S) and in its ability to bind to uterine nuclei and augment RNA synthesis, properties that are not shown by the native complex. It is proposed that receptor transformation is an important step in estrogen action and that a principal role of the hormone is to induce conversion of the receptor protein to a biochemically functional form.

Estrogen "receptors" in brain: an unsolved problem

Various investigators have postulated that estrogen-sensitive structures contain "receptors" which accumulate and bind estradiol. Further, it has been implied that brain receptors function in a similar way to uterine receptors, even though the evidence for this conclusion is not strong. The present study was designed to compare the accumulation of radioactivity by brain and uterus following (3)H-estradiol treatment in estrous, diestrous, and ovariectomized rats. Uterus exhibited high levels of radioactivity in diestrus, but not in estrus or the ovariectomized condition. The accumulation of hormone by brain did not fluctuate with the various levels of endogenous hormone stimulation, but rather showed a pattern of uptake similar to muscle, a nontarget tissue. It was concluded that if brain hormone receptors exist, they do not respond to estradiol as do uterine receptors.