Stabilization of glucocorticoid receptor association with RNA by a low molecular weight factor from rat liver cytosol

Stabilization of glucocorticoid-receptor interactions in vitro by removal of RNA bound to receptor complexes in vivo

The dissociation of the steroid from glucocorticoid-receptor-RNA complexes at 5 degrees C was evaluated in cytosolic and nuclear extracts prepared from Hela cells crosslinked in vivo with glutaraldehyde. Sample treatment with catalytically active RNase A prevented the dissociation of the steroid which was induced by sample dilution with buffer. Dilution of the extracts with boiled cytosol, instead, stabilized steroid-receptor interactions. We conclude that some heat-stable factor should be also associated with glucocorticoid-receptor-RNA complexes from crosslinked cells, stabilizing steroid-receptor interactions, and we propose that it could counteract the labilizing effect of RNA.

Mapping contacts between unpurified human progesterone receptor and the hormone response element of mouse mammary tumor virus

Binding of steroid hormone receptors to specific recognition sites of hormone-inducible genes is one of the events required for hormonal regulation of gene transcription. We have employed an immunoprecipitation assay to map the interaction between unpurified human progesterone receptors from crude nuclear extracts of T47D cells and the hormone response element of the mouse mammary tumor virus (MMTV). DNase I footprints and methylation interference patterns are similar to those reported with highly purified rabbit progesterone receptors, suggesting that both human and rabbit receptors recognize similar features in the hormone response element. More importantly, these patterns suggest that if other factors are associated with unpurified nuclear receptor, they do not alter the contacts made by receptor nor do they make contacts themselves with MMTV DNA in a manner detected by DNase I or methylation interference assays. The sites of interaction of receptors bound with the clinically important progestin antagonist, RU 486, are comparable to those observed with an agonist-receptor complex. These results suggest that the antagonist prevents receptor action at a step after its recognition and binding to specific sites on a hormone-responsive enhancer element.

Evidence that 5 S intermediate state in glucocorticoid receptor transformation contains hsp90 in addition to the steroid-binding protein

Previous studies have shown that the exposure of molybdate-stabilized nontransformed glucocorticoid receptor (GR) of the chick embryonic neural retina to 0.4 M KCl dissociated the 9.5 S complex to a 5 S GR complex, which is an intermediate state in GR transformation. The present study was designed to characterize the 5 S GR complex. It shows that molybdate-stabilized nontransformed 9.5 S GR complex and 5 S GR interact with monoclonal antibodies (MAb) directed against 90 kDa heat shock protein (hsp90), as evidenced by the increase in the sedimentation velocity of these GR-complexes. Electrofocusing of the partially purified molybdate-stabilized nontransformed GR, prepared from [32P]-labeled neural retinas, and of the 5 S GR (derived from molybdate-stabilized preparation) showed that nontransformed GR complex, which has an apparent pI (pI') value of 5.0 +/- 0.2, and 5 S GR, which was resolved in a major peak with a pI' value of 5.8, are phosphorylated. Partially purified 5 S GR, cleared of molybdate and exposed to 25 degrees C, was resolved by electrofocusing into two phosphorylated fractions, one with a pI' value of 6.5, representing the monomeric GR form and the other with a pI' value of 5.1, apparently representing the acidic hsp90. The dissociation of hsp90 from the molybdate-cleared 5 S heterodimer seems to account for the decrease in the negative net charge of 5 S GR from pI' 6.5. Monomeric GR, derived from a molybdate-cleared, partially purified GR preparation, by the exposure to 25 degrees C, did not retain glucocorticoid-binding activity. Molybdate-stabilized 5 S GR was apparently re-assembled into the oligomeric nontransformed state when the salt concentration was reduced. This phenomenon was evident under the low-salt conditions of electrofocusing, by the shift in pI' value of GR from 5.8 to 5.0; and in glycerol density gradients containing 0.15 M KCl, by the shift in the sedimentation of the GR complex from 5 S to 9.5 S.

Androgen receptors: structures, mutations, antibodies and cellular dynamics

An overview of recent studies from this and other laboratories on the structures and intracellular dynamics of androgen receptors is presented. Human and rat androgen receptors are unique in that, aside from their DNA and androgen binding domains, they have amino terminal regions rich in oligo- and poly(amino acids) motifs as in some regulatory and homeotic genes. Point mutations that cause sequence changes or deletion of regions of androgen receptors appear to be responsible for some cases of androgen-insensitivity. Monoclonal antibodies produced against specific regions of the androgen receptor bind to androgen receptors but not other major steroid receptors. Androgen receptors in the human and rat prostate, and monkey seminal vesicle were localized to the nucleus of target cells in these tissues with these antibodies; androgen receptors also were found in the cytoplasm of some target cells. Actinomycin D and 3'-deoxyadenosine, inhibitors of transcription, RNA processing and nucleo-cytoplasmic transport of RNA, interfere with the intracellular dynamics of androgen receptors, suggesting as we have proposed previously that androgen receptors may function not only at the site of transcription but also are involved in posttranscriptional regulation of mRNA stability and utilization.

Identification of a protein from rat liver cytosol that enhances activation of the glucocorticoid receptor

We have identified a factor from rat liver cytosol that enhances the DNA-cellulose-binding ability of the glucocorticoid receptor and lowers the sedimentation value from 9-10 S to 4-5 S. Cytosol is prepared in the presence of molybdate, and unactivated receptor is isolated by chromatography on DEAE-cellulose in the presence of molybdate. This receptor sediments at 9-10 S and has little affinity for DNA. If the molybdate is removed and the receptor is incubated at 25 degrees C with the low-salt wash of the DEAE-cellulose column, DNA binding is enhanced by 50-600% relative to controls incubated with buffer only. In addition, the factor present in the low-salt wash converts the 9-10 S receptor into a mixture of 5 S and 4 S forms. The factor must be present during the incubation in order to exert its maximal effect. Factor added after the incubation has only marginal effects on the DNA-binding ability of the receptor, indicating that the factor does not increase the DNA-binding ability of activated receptor. Moreover, the factor is significantly less effective on receptor that has been activated before incubation with the factor. These results suggest that the factor acts as an activation enhancer. Preliminary characterization indicates that the activation enhancer is a trypsin-sensitive protein of approx. 70,000 Da, whose activation-enhancing properties are inhibited by ATP. RNAase A, which has effects similar to those described above on the 7-8 S receptor, does not mimic the effects of the activation enhancer on the 9-10 S receptor.

Evidence that the modulator of the glucocorticoid-receptor complex is the endogenous molybdate factor

We have recently purified the modulator of the glucocorticoid-receptor complex from rat liver. Purified modulator inhibits glucocorticoid-receptor complex activation and stabilizes the steroid-binding ability of the unoccupied glucocorticoid receptor. Since these activities are shared by exogenous sodium molybdate, modulator appears to be the endogenous factor that sodium molybdate mimics. In this report, we present additional evidence for the mechanism of action of purified modulator. (i) Molybdate and modulator inhibit receptor activation as measured by DNA-cellulose binding, DEAE-cellulose chromatography, and Sepharose 4B gel filtration. (ii) The ability of molybdate and modulator to inhibit receptor activation and stabilize the unoccupied receptor appears to be additive. (iii) Scatchard analysis of heat-destabilized unoccupied receptors indicates that the number of steroid-binding sites is reduced during destabilization, whereas the steroid dissociation constant remains unchanged. Molybdate and modulator stabilize the receptor by maintaining the number of steroid-binding sites. (iv) Molybdate and modulator do not inhibit alkaline phosphatase-induced destabilization of the unoccupied receptor. However, alkaline phosphatase-induced destabilization is reversed by the addition of dithiothreitol in the presence, but not in the absence, of molybdate or modulator. These results suggest that the mechanism of action for modulator is identical to that of sodium molybdate, and we propose that modulator is the endogenous molybdate factor for the glucocorticoid receptor.

Transformation of the glucocorticoid receptor in the cell-free cytosol of the neural retina of the chick embryo: changes in the size and charge of the receptor complex during transformation suggest a multistage process

The physicochemical properties of the glucocorticoid receptors (GR), and the molecular changes induced during their transformation in the cell-free cytosol of the neural retina of the chick embryo, were investigated. The surface charge of the various size forms of the GR complex was determined on gel filtration and/or glycerol density gradient-isolated GR, by electrofocusing under nondenaturing conditions. The nontransformed molybdate-stabilized GR in hypotonic buffer (containing PMSF) appears as a 350 kilodalton (kDa) complex (Rs = 8.6 nm, S = 9.5), with an apparent pI value (pI') of 4.4 +/- 0.1. The GRs in heat or salt-activated cytosols appear as a 90 kDa hormone-receptor complex (Rs = 5.6 +/- 0.2, S = 3.9 +/- 0.1), which is resolved as a major peak with a pI' value of 6.2 +/- 0.1 and a minor peak with a pI' value of 5.4. The transformation of the 350 kDa oligomer to the 90 kDa monomer occurs in three stages. Two distinct dissociation steps were induced by 0.4 M KCl: (a) the dissociation of the 350 kDa complex to a 170 kDa complex (Rs = 7.8 +/- 0.2, S = 5.1 +/- 0.2), exhibiting a pI' value of 5.6 +/- 0.2, induced by salt and not inhibited by molybdate; and (b) the dissociation of the 170 kDa complex to the 102 kDa complex (Rs = 5.6 +/- 0.2, S = 4.4), also exhibiting a pI' value of 5.6 +/- 0.2, which is blocked by molybdate. The third step, the transition of the 102 kDa complex to the activated (nuclear-like), 90 kDa form, is dependent on cytosolic factors. It is induced in the isotonic milieu by physiological temperatures, and in the cold by exposing the crude cytosol to 0.4 M KCl. The nature of this cytosolic processing step is unknown. It occurs in the presence of PMSF, which presumably inhibits proteolytic GR degradation in the cytosol of the neural retina. Activated GR complexes tend to aggregate. Molybdate inhibits activation-induced GR-aggregation.

Sodium molybdate converts the RNA-associated transformed, oligomeric form of the glucocorticoid receptor into the transformed, monomeric form

The glucocorticoid receptor from rat liver cytosol prepared in 2 ml buffer/g tissue sedimented at approximately 10 S in low salt density gradient centrifugation without molybdate. When the receptor was heated at 25 degrees C, both approximately 10 S and approximately 7 S forms were seen in low salt gradient. The approximately 10 S form was not capable of binding to DNA-cellulose and was stabilized by sodium molybdate, namely it corresponded to untransformed receptor. The approximately 7 S form was capable of binding to DNA-cellulose and regarded as transformed receptor. On the other hand, partially-purified transformed receptor labeled with [3H]dexamethasone-21-mesylate sedimented at approximately 5 S, which migrated as a approximately 94 kDa species in SDS-polyacrylamide gel electrophoresis. The reconstitution analysis of this partially-purified approximately 5 S receptor and liver cytosol, showed the shift to approximately 7 S form. RNase A or T1 converted approximately 7 S transformed form into approximately 5 S but it did not affect approximately 10 S untransformed form. 5-20 mM sodium molybdate also shifted approximately 7 S to approximately 5 S. These results indicate that the approximately 7 S transformed form of the glucocorticoid receptor observed in low salt conditions might be an oligomer, probably including both approximately 5 S steroid-binding component and RNA/ribonucleoprotein, and that molybdate dissociates these interactions in a specific manner.

Enhanced inhibition of estrogen receptor nuclear binding in the uterus of aged mice

We have studied why uteri from aging mice show a decrease nuclear concentration of estrogen receptors (UER). While 50-60% of available cytosolic UER from ovariectomized (OVX) mice aged 4-8 months, upon physiochemical activation, are able to bind either to DNA-cellulose or nuclear suspensions from young animals, only 20-30% of comparable concentrations of cytosolic UER from mice aged 15-18 months did so under identical experimental conditions. Nuclear dilutions with uterine cytosolic fractions from estrogen treated OVX mice prior to determination of [3H] UER binding sites in nuclear suspensions decreased the number of nuclear ER sites in both age groups. However, we observed that cytosols from aged animals showed a greater ability to prevent [3H]E2 binding to nuclear sites when compared to young ones (inhibition index: 0.286 +/- 0.013 (SE) vs. 0.137 +/- 0.025, P less than 0.05). These changes occur independently of protein concentration and result from dilution of a specific endogenous inhibitor of [3H]E2 binding to nuclear sites. The significance of these observed differences is discussed.

A comparison of the glucocorticoid receptor system in the spinal cord and hippocampus

Studied under in vivo conditions, uptake of [3H]corticosterone (CORT) by purified cell nuclei of the hippocampus was much higher than in the spinal cord, although the latter may contain in cytosol up to 50% of glucocorticoid receptors found in cytosol of hippocampus. Experiments were undertaken to explain these differences. First, the in vivo affinity of receptors for exogenous CORT was comparable in both tissues. Second, an inhibitor of translocation, although present, was not preferentially concentrated in the spinal cord as compared to the hippocampus. However, the sensitivity towards RNAase A, an enzyme that increased binding to DNA-cellulose (taken as a measure of increased affinity for nuclear components), was preserved in the hippocampus but absent in the cord. We discuss the possibility that refractoriness to RNAase A may play a role in the reduced nuclear uptake of [3H]CORT shown by the spinal cord in vivo, but also consider possible that heterogeneity of receptor types binding CORT in the spinal cord and hippocampus may account for the differences observed in both tissues.

The glucocorticoid receptor protein binds to transfer RNA

The glucocorticoid receptor from mouse AtT-20 cells exists in three forms: the untransformed receptor (9.1S; Mr of 319,000), a large oligomeric molecule that does not bind to DNA; the transformed receptor (4S; Mr of 96,000), which is formed by dissociation of untransformed receptor after steroid binding and which binds to DNA to modulate gene expression; and an intermediate size receptor (6S; Mr of 132,000), which also binds to DNA and contains a bound small RNA molecule. This RNA species has now been purified and identified as transfer RNA (tRNA). The three tRNA's for the basic amino acids accounted for about 78% of the total amino acid-accepting activity [arginine (52%), lysine (17%), and histidine (9%)], while the remaining 22% was represented by six other tRNA species. This tRNA-binding activity of the glucocorticoid receptor may reflect post-transcriptional mechanisms of regulating gene expression, such as alterations in the translational efficiency of or the modulation of the stability of hormone-induced proteins.

Properties of an intermediate-sized androgen receptor: association with RNA

This study identifies an intermediate-sized androgen receptor and characterizes its relationship with the 9.1S and 4.4S receptor forms. Under low ionic conditions, at 2-4 degrees C, there exists a 9.1S (+/- 0.17) (n = 30) oligomeric form which does not bind to DNA. Under high ionic conditions, this form dissociates to a 4.4S (+/- 0.08) (n = 18) monomeric form. When the salt concentration is lowered, the 4.4S monomer converts to a species with an intermediate sedimentation coefficient of 7.7S (+/- 0.15) (n = 17) which binds to DNA. Unlike the 9.1S oligomer the 7.7S form is not maintained by sodium molybdate under high ionic conditions but rather dissociates to the 4.4S monomer. To determine whether these forms were associated with RNA, the 7.7S form was incubated with RNase A and analyzed by density gradient centrifugation. The 7.7S form was digested fully by RNase to the 4.4S monomer. The 7.7S form demonstrated a buoyant density of 1.2459 +/- 0.014 g/cm3 (n = 6) in metrizamide gradients, suggesting a ribonucleoprotein component. The sedimentation coefficient of the 9.1S form was unaffected by RNase. These data suggest that the intermediate 7.7S receptor form is composed of 4.4S monomer associated with a ribonucleoprotein molecule(s).

Studies with chymotrypsin and RNAase showing a heterooligomeric structure of the glucocorticoid receptor complex from rat liver which is stabilized by a low molecular weight factor

The glucocorticoid receptor from rat liver displays a differential sensitivity toward digestion by chymotrypsin and RNAase A that is dependent on its activation state. Unactivated (9-10 S) receptor is not digested by these enzymes, while activated 7-8 S receptor is. Chymotrypsin treatment yields an approx. 3 S form, while RNAase treatment yields a 4.9 S form that is distinct from the high-salt 4 S form. To firmly establish that the results are due to specific hydrolytic activities of the particular enzymes, we show that the chymotrypsin effect is inhibited by diisopropylfluorophosphate and not RNAasin, while the reverse is true for RNAase A. We further show that the differential sensitivity toward chymotrypsin is due to the association of a proteinase-resistant, heat-stable low molecular weight factor with the unactivated glucocorticoid receptor. When this factor is removed by warming, dialysis or molecular sieving of the receptor complex, the complex becomes sensitive to chymotrypsin. We also show that moderate chymotrypsin treatment yields a 6-7 S form of the receptor which is composed of, at least, RNA and the 4 S receptor. On the basis of these results, we propose that the 9-10 S receptor is composed of a low molecular weight stabilizing factor whose presence apparently alters the conformation of the complex such that the RNA and the RNA-binding site of the receptor are protected, a chymotrypsin-sensitive factor, RNA and the 4 S receptor itself.