Steroid receptors, nuclear localization

Understanding of the intracellular localization of steroid receptors has gone through a number of phases, beginning with the view that receptors translocated from cytoplasm to nucleus in the presence of hormone. Indeed, with the exception of thyroid hormone receptors, which are exclusively nuclear in location, cell fractionation studies have revealed that in the absence of hormone, steroid receptors are extracted in the soluble or cytosolic fraction. However, when steroid is present in the cell, many occupied receptors are retained by purified cell nuclei. Histological procedures, such as immunocytochemistry, have confirmed the largely nuclear localization of occupied receptors, but... 

Steroid hormones achieve their effects on target tissues through intracellular receptor proteins. According to recent views, oestrogen and progestin receptors are localized in the nuclear compartment of the cells, whereas glucocorticoid receptors may reside in both the cytoplasm and the nucleus. Determination of the intracellular localization of androgen receptors awaits the development of (monoclonal) antibodies which will enable immunohistochemical studies. The molecular aspects of the mechanism of action of steroid hormones will be covered in other chapters [1-3] in this volume. The present chapter deals with the characterization, assay and purification of steroid receptors. 

Whether unoccupied GR are totally nuclear is less clear. Immunocytochemical studies with anti-GR antibodies show both cytoplasmic and nuclear localization at the light level, and an increase in nuclear staining simultaneous with decreased cytoplasmic staining in response to addition of glucocorticoids [106-109]. It may well be that GR have a different intracellular distribution from the sex-steroid receptors. 

In contrast to most receptors, most ligand-free steroid receptors are not membrane bound. They can be cytoplasmic or nuclear, depending on the receptor and the time scale. At a given instant, ERs are mostly nuclear, like the nuclear receptors. Eor the other receptors, the amount of nuclear localization is PR > MR > AR and GR. However, the dynamic picture is that receptors are shuttling back and forth across the nuclear membrane (53)... 

Figure 3.35 Nuclear localization of a steroid receptor. (A) The receptor, made visible by attachment of the green fluorescent protein, is located predominantly in the cytoplasm of the cultured cell. (B) Subsequent to the addition of corticosterone (a glucocorticoid steroid), the receptor moves into the nucleus. [Courtesy of Dr. William. B. Pratt.]...

The exact role of PPIases is not yet fully understood. Their catalytic activity does not seem to be implicated in the assembly and composition of heterocomplexes, since these are insensitive to treatment with PPIase inhibitors. It has been proposed that PPIases might control the nuclear targeting of the receptor, a role consistent with the nuclear localization of hFKBP52 and hCyp40. It is noteworthy that the relative abundance of multidomain immunophilins may change in response to varied hormonal stimuli that upregulate the incorporation of TPR immunophilins into steroid receptor complexes [3,34],... 

Essentially the functional counterpart of coactivators, corepressor proteins bind to many NRs in the absence of ligand and serve to repress basal transcriptive activity [62], Corepressors play a particularly important role for NRs that are found almost exclusively in the nucleus, unlike the apo steroid receptors that are cytoplasmically localized. Studies involving the nuclear-localized receptors TR and RAR led to the identification of silencing mediator of retinoid and thyroid (SMRT) receptors and nuclear receptor corepressor (NCoR) [63, 64]. Both SMRT and NCoR recruit histone deacetylases (HDACs), namely, HDAC3, which function to reverse the chromatin unwinding result of the coactivator-recruited histone acetylases [65],... 

Recently, evidence for action of CAR at an additional site has been presented" . It should be emphasized that the action of CAR is somewhat different than other receptors from the steroid receptor superfamily, in that it may be enhanced in the absence of a bound ligand and some of the control is at the level of nuclear translocation . Other factors involved are HNF-4 (ref [405]) and C/EBPa (ref [338]), accounting at least in part for hepatic localization. 

Fig. 16.13. Steroid hormone receptors. A. Domains of the steroid hormone receptor. The transactivation domain (TAD) binds coactivators DNA-binding domain (DBD) binds to hormone response element in DNA ligand-binding domain (LBD) binds hormone NLS is the nuclear localization signal the dimerization sites are the portions of the protein involved in forming a dimer. The inhibitor binding site binds heat shock proteins and masks the nuclear localization signal. B. Transcriptional regulation by steroid hormone receptors. Additional abbreviations HSP, heat shock proteins GRE, glucocorticoid response element GIZ, glucocorticoid receptor.

All steroid hormones act through nuclear receptors to change the level of expression of specific genes (p. 465). Recent evidence indicates that they also have more rapid effects, mediated by receptors localized in the plasma membrane. 

Milgrom and co-workers [105] recently developed an immunogold method for detection of PR in the rabbit uterus and have examined the effect of hormone addition on receptor localization at the ultrastructural level. PR were found to be predominantly nuclear in the presence and absence of hormone, but a small amount was detectable in the cytoplasm which was not apparent at the light microscopical level. These cytoplasmic PR were localized over endoplasmic reticulum and clusters of free ribosomes and may likely represent newly synthesized protein. No PR were located in the plasma membrane. Within the nucleus, unoccupied PR were associated with condensed chromatin which became more dispersed after hormone addition. These ultrastructural studies indicate that steroid-free PR translocate from their site of synthesis in the cytoplasm to the nucleus in a hormone independent manner, and that addition of hormone changes their intranuclear localization. 

Based on the receptor activation mechanism, the nuclear receptors may be divided into two basic groups. In the first group (those including most of the steroid hormone receptors), the receptors can be localized in the nucleus or in the cytoplasm. The receptors of the other group (discussed in Section 4.7) are always localized in the nucleus. Representative ligands of these receptors are the derivatives of retinoic acid, the T3 hormone and VitD3. 

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