Suresh Mohla

Tissue-specific stimulation of RNA synthesis by transformed estradiol-receptor complex.

Summary Exposure to the estradiol-receptor complex of uterine cytosol increases the RNA synthetic capacity of purified uterine nuclei but not of liver or kidney nuclei. This stimulation is effected only by complex in which the binding unit of the receptor has undergone estrogen-induced conversion from the native (4S) to the transformed (5S) form. It is suggested that an important function of estrogenic hormone is to promote transformation of the receptor protein to an active form which can enter the nucleus and stimulate RNA synthesis.

Estrophile to nucleophile in two easy steps

Abstract A variety of self-consistent experimental evidence supports the concept that estrogcnic hormones interact with their target tissues by a multi-stage mechanism in which the hormone first associates with the 4S binding unit of an 8S extranuclear receptor protein, activating it to undergo conversion to a 5S form. This temperature-dependent transformation takes place when the receptor is bound to estradiol, but not when it is uncomplexed or bound to estrone. The transformed hormone-receptor complex migrates to the nucleus where it associates with an acceptor site in the chromatin. Analytical amounts of the 5S nuclear complex and a calciumstabilized 4S cytosol complex have been isolated from calf uterus in apparently pure state, opening the way for the large scale purification of these different forms of the receptor protein. The estrogen-receptor interaction sequence appears to be involved in the acceleration of biosynthetic reactions in hormone-dependent tissues. Exposure of isolated uterine nuclei either to transformed complex or to native complex under conditions where transformation can take place causes a significant increase in their ability to synthesize RNA. Similar treatment does not enhance the already high RNA synthetic capacity of liver or kidney nuclei. It is suggested that an important function of the hormone is to promote the conversion of the receptor protein to an active form which can enter the nucleus and alleviate a defficiency in RNA synthesis, characteristic of estrogen-responsive tissues.

The Role of Estrophilin in Estrogen Action

The role of estrophilin in estrogen action was reviewed. The interaction of uterine cells with estradiol involves the association of estradiol with estrophilin followed by temperature-dependent translocation of the resulting complex to the nucleus. The steroid binding unit of estrophilin undergoes an alteration (receptor transformation) that is recognized by an increase in its sedimentation rate and by its ability to bind to isolated uterine nuclei or chromatin and to alleviate a tissue specific deficiency in the ribonuclei acid (RNA) synthesizing capacity. The estrophilin complex influence on RNA synthesis in isolated nuclei shows the same quantitative, qualitative and tissue specificity characteristics as evoked by the administration of estradiol in vivo. It was concluded that receptor transformation is an important step in estrogen action and that a major role of the hormone is to induce conversion of the native estrophilin to a biochemically functional form.

Estrogen-independent activation of the receptor protein of calf uterine cytosol

Abstract Precipitation of the estrogen receptor protein of calf uterine cytosol by ammonium sulfate at 2°C, either in the presence or absence of estradiol, is accompanied by its conversion from native (3. 8 S) to a transformed (5. 5 S) state. This product, complexed with estradiol, resembles that previously obtained, either by warming uterine cytosol with estradiol or by extracting nuclei from hormone-treated uteri, in its ability to bind to uterine nuclei and to increase their RNA polymerase activity. The transformed protein, uncomplexed with steroid, also stimulates RNA synthesis, supporting the hypothesis that receptor transformation, effected physiologically by the hormone, plays a key role in estrogen action.

Receptor transformation, the key to estrogen action

Abstract The transforrnation of the native (4S) uterine cytosol-receptor complex to an active (5S) form is an estrogen-requiring, temperature-dependent process, the rate of which is affected by the pH and ionic strength of the cytosol. Unlike its 4S precursor, the transformed complex is retained by uterine nuclei and chromatin but by non-target tissue chromatin as well. The specificity of the in vitro nuclear uptake of transformed receptor complex appears to reside in the biochemical response; transformed receptor effects in vitro the same target tissue specific, quantitative stimulation and qualitative change in nuclear RNA synthesis as that observed in the target tissue after administration of the hormone in vivo .

Transformed estrogen receptor in the regulation of RNA synthesis in uterine nuclei

Abstract The interaction of estradiol with uterine tissue 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 receptor undergoes transformation that can be recognized by an increase in its sedimentation rate from 3.8S to 5.2S, as well as acquisition of the ability to bind to isolated uterine nuclei and to alleviate a tissue-specific deficiency in their RNA synthesizing capacity. 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 in its sedimentation rate (5.3S) and its ability to bind to uterine nuclei and augment RNA synthesis. It is proposed that receptor transformation is an important step in estrogen action, and that a principal role of the hormone may be to induce conversion of the receptor protein to a biochemically functional form.

Estrogen Receptor Transformation and Nuclear RNA Synthesis

Studies from many laboratories (l) have demonstrated that the characteristic affinity of estrogen-responsive tissues for the hormone, both in vivo and in vitro, results from their content of specific estrogen-binding proteins called estrogen receptors or estrophiles. The interaction of estradiol with target tissues, such as uterus, takes place by a sequential mechanism in which the hormone first combines with an extranuclear receptor protein to form a complex which, by a temperature-dependent process, is translocated to the nucleus where most of the incorporated steroid becomes localized (Fig. 1).

RECEPTOR TRANSFORMATION, THE KEY TO ESTROGEN ACTION

The transformation of the native (4S) uterine cytosol-receptor complex to an active (5S) form is an estrogen-requiring, temperature-dependent process, the rate of which is affected by the pH and ionic strength of the cytosol. Unlike its 4S precursor, the transformed complex is retained by uterine nuclei and chromatin but by non-target tissue chromatin as well. The specificity of the in vitro nuclear uptake of transformed receptor complex appears to reside in the biochemical response; transformed receptor effects in vitro the same target tissue specific, quantitative stimulation and qualitative change in nuclear RNA synthesis as that observed in the target tissue after administration of the hormone in vivo.

HORMONE-INDUCED TRANSFORMATION OF RECEPTOR PROTEINS*

Publisher Summary This chapter describes hormone-induced transformation of receptor proteins. During the course of steroid-induced translocation to the nuclei of their respective target cells, extra nuclear receptor proteins for estrogenic and androgenic hormones undergo temperature-dependent alteration that can be recognized both by a change in sedimentation properties and the acquisition of an ability to bind to isolated nuclei or chromatin. Warming the uterine cytosol receptor with estradiol increases the sedimentation coefficient of the complex from 3–8 S to 5–3 S, whereas similar incubation of the prostatic cytosol receptor with dihydrotestosterone decreases the sedimentation coefficient from 3–8 S to 3–0 S. Treatment of isolated uterine nuclei with transformed estradiol–receptor complex not only increases their RNA polymerase activity but also changes the nearest neighbor frequency spectrum of the RNA produced, in the same manner as does estradiol administered in vivo.

Receptor Transformation — A Key Step in Estrogen Action

Experiments from many laboratories (1,2) have established that, for all classes of steroid hormones, the hormone-receptor complex found in the nuclei of target cells is derived from an initially formed extranuclear complex, as first proposed for estrogens in uterine tissue (3,4). Evidence is accumulating that the steroid-induced translocation of the hormone-receptor complex to the nucleus is accompanied by a hormone-mediated alteration of the receptor protein, a process that has been called receptor transformation (5–7) or activation (8, 9). This paper summarizes some observations concerning transformation of estrophilin, the receptor protein for estrogens, and describes recent achievements in the purification of the transformed estradiol-receptor complex of calf uterus.