Functional analysis of the purified glucocorticoid receptor

Modulation of steroid hormone receptor activity

Classical steroid hormones (SHs) - estrogens, androgens, progestins, glucocorticoids and mineralocorticoids - play critical roles in the regulation of reproduction, metabolism and cancer. SHs act via their cognate steroid hormone receptors (SHRs) in multiple target tissues throughout the body, exerting their physiological effects through nuclear receptor (NR)-mediated gene transcription. Since SHRs are the mediators of steroid hormone signalling in cells, regulation of their expression and function is critical for appropriate physiological responses to SHs. Cells regulate SHRs by determining the cellular concentration of SHR proteins in the cell and by tightly regulating their activity through post-translational modifications and interactions with coactivator protein complexes. In this chapter we will examine each of these regulatory mechanisms and assess their functional impact on the activity of SHRs.

An active nuclear retention signal in the glucocorticoid receptor functions as a strong inducer of transcriptional activation

The glucocorticoid receptor (GR) cycles between a naive chaperone-complexed form in the cytoplasm and a transcriptionally active steroid-bound nuclear form. Nuclear import of GR occurs rapidly and is mediated through the importin alpha/beta karyopherin import pathway. By contrast, nuclear export of GR occurs only slowly under most conditions, despite a dependence on active signaling. In this study we have defined a nuclear retention signal (NRS) in the hinge region of GR that actively opposes the nuclear export of GR as well as the nuclear export mediated through an ectopic CRM1-dependent nuclear export signal (NES). The GR NRS overlaps closely with the basic NL1 nuclear localization signal (NLS) but can be distinguished from NL1 by targeted mutagenesis. Substitution of the classical NLS from SV40 T antigen for the GR NL1 results in a receptor in which nuclear export is accelerated. Remarkably, although the SV40-modified GR remains predominantly nuclear in the presence of steroid and is recruited to transcriptional regulatory regions indistinguishably from wild-type GR, the substitution dramatically weakens the ability of GR to activate transcription of a mouse mammary tumor virus reporter gene. These results suggest that active nuclear retention of GR plays an integral role in glucocorticoid signaling.

Majority of RNA which co-purifies with unactivated rat hepatic glucocorticoid-receptor complexes is nonspecific and not receptor-associated

Molybdate-stabilized, unactivated rat hepatic glucocorticoid-receptor complexes were purified by a three-step procedure which includes affinity chromatography, gel filtration and anion exchange chromatography. Following elution of unactivated steroid-receptor complexes from the final DEAE-cellulose column, RNA which remained bound to the anion exchange resin was eluted with 1 M KCl. This RNA was small and heterogeneous in size. Equivalent amounts of RNA were detected after a mock purification which was devoid of receptors, suggesting that the presence of this RNA is not dependent on that of receptors. Both a [32P]DNA complementary to the RNA eluted from DEAE-cellulose and a [32P]DNA probe synthesized from total rat liver RNA gave similar results when hybridized to total rat liver RNA. These data indicated that the RNA which co-purified with unactivated receptors through the first two steps was very similar to total RNA in overall composition. Virtually identical hybridization patterns were also detected when end-labeled probes generated from the DEAE-cellulose eluted RNA or total liver RNA were hybridized to total genomic rat DNA, suggesting that the RNA eluted from the anion exchange resin is not specific or unique. Although these results do not exclude the possibility that there could be specific RNA species associated with the unactivated glucocorticoid receptor, they do indicate that the majority of the RNA eluted from DEAE-cellulose following elution of receptor complexes appears indistinguishable from total rat liver RNA and can be detected in parallel mock purifications.

Purification, monoclonal antibody production and structural analyses of human progesterone receptors

Progesterone receptors (PR) from human breast cancer cells have been purified, and used as antigens to generate anti-PR monoclonal antibodies. Immunologic and in situ photoaffinity labeling methods were then used to study receptor structure, and we conclude that native human PR consist of two independent 8S receptors: one 8S subtype contains B-proteins (120,000 kDa) and the other contains A-proteins (94,000 kDa).

Enhanced phosphorylation of progesterone receptor by protein kinase C in human breast cancer cells

Affinity-isolated progesterone receptor (PR) from human breast cancer cells incubated with [32P]orthophosphate was shown to exist as a phosphoprotein. Exposure of the cells to 10 nM phorbol-12-myristate-13-acetate (PMA) for 10 min increased by 30-40% the amount of label incorporated into the 116-kDa receptor protein. A two-fold increase in the total number of steroid binding sites was also observed in cells receiving PMA treatment. This apparent unmasking of PR binding sites by phosphorylation probably involved conformational changes to existing receptor complexes and affected the eventual state of receptor dissociation or transformation. An increase primarily in the 8 S sedimenting molecular species was observed but PMA treatment also led to the appearance of a smaller, 2-3 S form of receptor (10% of total) that was not present in control samples. When cytosols were partially transformed in vitro by ATP and salt, all molecular species of receptor (8, 4, and 2-3 S) from the PMA-treated samples consistently migrated faster in sucrose gradients. The larger amount of 2-3 S receptor in PMA-treated samples disappeared when ATP, but not salt, was the transforming agent. These results suggest a major role for phosphorylating reactions in the receptor-mediated action of steroids by regulating hormone-binding and influencing receptor transformation. Tumor promoters such as the phorbol esters may act by artificially increasing the level of processing of steroid receptor.

Dynamics of cortisol receptor activity in the gills of the brook trout, Salvelinus fontinalis, during seawater adaptation

Injection of cortisol into freshwater-adapted brook trout caused a rapid increase in plasma cortisol and a significant drop in plasma osmolality, thyroxine (T4), and chloride concentrations, but a nonsignificant drop in plasma triiodothyronine (T3). Concomitantly, cortisol cytosolic receptor activity decreased and cortisol nuclear extract receptor activity increased significantly. Transfer of brook trout into 30% seawater evoked a significant rise in plasma cortisol, osmotic, and chloride concentrations and a significant drop in plasma T4 and T3. Cortisol cytosol receptor activity rapidly dropped following entry into seawater, and cortisol nuclear extract receptor activity rose significantly in a manner similar to the effects of the injection of cortisol. Although there was a lack of correlation between the cytosol cortisol concentration and the cytosol receptor concentration, a significant and positive correlation occurred between the nuclear extract cortisol concentration and the nuclear extract cortisol receptor activity. This positive correlation may reflect an important role for the putative nuclear receptor. These results support the hypothesis that cortisol has an important role in the adaptation of brook trout to the marine environment.

The presence of corticosteroid receptor activity in the gills of the brook trout, Salvelinus fontinalis

Gill tissue from brook trout was examined for the presence of cortisol receptors. Both cytosolic and nuclear preparations from the gills manifested high equilibrium association constants (Ka) and low maximum binding capacities (Nmax) indicative of high-affinity and low-capacity receptor activity (cytosol: Ka = 0.31 +/- 0.02 X 10(9)/M, Nmax = 223.9 +/- 22.8 fmol/mg protein; nuclear extract: Ka = 0.02 +/- 0.003 X 10(9)/M, Nmax = 424.6 +/- 96.3 fmol/mg protein). Gel permeation (Sephacryl S-300) column chromatography gave two incompletely separated peaks at 326,000 and 189,000 Da and Stokes radii of 5.96 and 4.81 nm using [3H]triamcinolone acetonide and only one peak at 219,000 Da and 5.4 nm using [3H]cortisol. The binding of the synthetic compounds, triamcinolone acetonide and dexamethasone, appears to be different from that of the natural steroid, cortisol. The receptor activity appears to be highly specific for cortisol since cortisone and 11 beta,17 alpha,21-trihydroxy-4-pregnen-3,20-dione-21-phosphate bind with much lower affinity. The gill tissue cytosol fractions had the highest cortisol-binding activity, followed by liver, intestine, and muscle. The association constants for the liver, intestine, and muscle were the same order of magnitude as that for the gill. These results are consistent with the concept of nonmembrane steroid receptors of target organs.