Multiple forms of molybdate-stabilized glucocorticoid-receptor complexes from HeLa cell cytosol

Summary The investigation on hydrodynamic parameters of molybdate-stabilized glucocorticoid-receptor complexes from HeLa cell cytosol permitted resolution of four distinct forms. The first one could be detected in concentrated cytosols at low salt concentrations, and had the following properties: sedimentation coefficient = 9 S; R _s = 9.3 nm; M _r = 357,800; f/f _o = 1.83; axial ratio (prolate ellipsoid) = 16. When these cytosol extracts were diluted, a second form could be detected with sedimentation coefficient = 8.3 S; R _s = 9.05 nm; M _r = 320,700;f/f _o = 1.84; axial ratio = 16. Under high salt conditions, glucocorticoid-receptor complexes in concentrated cytosol had the following properties: sedimentation coefficient = 6.4 S; R _s, = 6.7 nm; M _r = 183,100;f/f _o = 1.64; axial ratio = 12. When either these cytosol extracts were diluted, or glucocorticoid-receptor complexes were subjected to repeated analysis, a fourth form was detected with sedimentation coefficient = 3.76 S; R _s = 5.67; M _r = 91,000; f/f _o = 1.75; axial ratio = 14. Besides salt concentration and dilution, the time elapsed between sample dilution and analysis appeared to affect the hydrodynamic properties of glucocorticoid-receptor complexes. On the basis of our findings, it has been concluded that the most likely structure of molybdate-stabilized glucocorticoid-receptor complexes of HeLa cell cytosol can be represented by association of monomers in homodimers, and homotetramers. A homotrimer form could not be deduced from our findings, and the 320,700 glucocorticoid-receptor complex we observed has been suggested to represent an unresolved mixture of trimers and tetramers.     

Characterization of nonactivated and activated glucocorticoid-receptor complexes from intact rat thymus cells

In cells exposed to glucocorticoids at 37 degrees C activated glucocorticoid-receptor complexes (complexes with affinity for nuclei and DNA) are formed after nonactivated complexes. Activation thus appears to be an obligatory physiological process. To investigate this process we have characterized cytoplasmic complexes formed in rat thymocytes at 0 and 37 degrees C. Complexes in cytosols stabilized with molybdate were analyzed by sucrose gradient centrifugation and by chromatography on DNA-cellulose, DEAE-cellulose, and agarose gels. Two major complexes were observed: the nonactivated complex, eluted from DEAE at approximately 200 mM KCl, was formed at 0 and 37 degrees C, gave S20,w = 9.2 S, Stokes radius = 8.3 nm, and calculated Mr = 330,000; the activated complex, eluted from DEAE at approximately 50 mM KCl, appeared only at 37 degrees C, gave S20,w = 4.8 S, Stokes radius = 5.0 nm, and Mr = 100,000. A third, minor complex, probably mero-receptor, which appeared mainly at 37 degrees C, bound to neither DNA nor DEAE, and gave S20,w = 2.9 S, Stokes radius = 2.3 nm, and Mr = 27,000. With three small columns in series (DNA-cellulose, DEAE-cellulose and hydroxylapatite), the three complexes can be separated in 5-10 min. By this method we have examined the stability of complexes under our conditions. We conclude that in intact thymus cells glucocorticoid-receptor complexes occur principally in two forms, nonactivated and activated, and that activation is accompanied by a large reduction in size. The origin of the mero-receptor complex remains uncertain.     

Stabilization of glucocorticoid-receptor complexes in rat thymus cytosol by a factor from WEHI-7 cells

Using a variety of physico-chemical techniques we have recently characterized three distinct forms of glucocorticoid-receptor complexes present in the cytosol from rat thymus cells incubated with glucocorticoid; the relative proportions of these complexes are dependent on the conditions to which the cells or cytosols are exposed. Two of these complexes correspond to the well established nonactivated and activated receptor forms, while the third has properties consistent with mero-receptor. Based on their differential affinities for DNA- and DEAE-cellulose we have developed a rapid mini-column chromatographic procedure for separating these three forms and have used it to examine the stability of complexes in cytosol preparations. We have found that activated glucocorticoid-receptor complexes from rat thymus cells are relatively unstable under cell-free conditions in that they undergo time-dependent losses in DNA binding and are converted to mero-receptor. In contrast, cytosolic glucocorticoid-receptor complexes prepared from WEHI-7 mouse thymoma cells are remarkably stable under similar conditions. Mixing experiments with equal portions of rat thymus and WEHI-7 cytosol revealed that the difference between the two tissues cannot be accounted for merely by differences in amounts of proteolytic enzymes, since addition of rat thymus cytosol to WEHI-7 cytosol containing activated glucocorticoid-receptor complexes does not result in their conversion to mero-receptor. However, the WEHI-7 cytosol affords considerable protection to activated glucocorticoid-receptor complexes in thymus cytosol. The stabilizing factor from WEHI-7 cytosol is heat stable (survives 100 degrees C for 30 min), insensitive to pH over a wide range (4.0-10.0), and appears to be macromolecular. It does not inhibit activation, and thus appears distinct from the previously described endogenous glucocorticoid receptor stabilizing factor responsible for stabilization of thymocyte receptor binding capacity (Leach et al., J. Biol. Chem. 257: 381-388, 1982). We propose that the factor is an endogenous inhibitor of the protease(s) responsible for mero-receptor formation.     

Nonactivated and activated glucocorticoid-receptor complexes in WEHI-7 and rat thymus cells

Our own results and those of others have indicated that nonactivated glucocorticoid-receptor complexes are oligomeric proteins with Stokes radius R_s = 8–9 nm, and that activation is accompanied by a reduction in size to R_s = 5–6 nm. The most convincing evidence for the large size of the nonactivated compared to the activated complex has been obtained with cytosols stabilized with molybdate. It has been suggested, however, that molybdate causes aggregation of complexes. Here we show that nonactivated rat thymus complexes in cytosols with molybdate and 400 M KCl have R_s = 8 nm. Furthermore, cytosols from WEHI-cells, which are exceptionally stable, show clear indications of 8 nM nonactivated complexes even without molybdate.The principal complexes in thymus cells under physiological conditions are the nonactivated, activated and nuclear-bound forms. We have studied the rapid kinetics of formation and interconversion of these complexes in intact cells at 37°C, using our newly-developed mini-column procedure to assay nonactivated and activated complexes. These kinetic results, along with many earlier results, can be accounted for quantitatively with a simple cyclic (irreversible) model in which the dissociation rate constant of the steroid plays a key role. The model predicts correctly the different degrees of activation in the cell with glucocorticoids such as triamcinolone acetonide and dexamethasone on the one hand, and cortisol and corticosterone on the other, without assuming steroid-specific allosteric influences of each of these steroids on the receptor.     

The effect of alkaline phosphatase on the activation of glucocorticoid-receptor complexes in rat liver cytosol

Calf intestinal alkaline phosphatase was found to stimulate the rate of in vitro activation of rat liver glucocorticoid-receptor complexes. This effect was registered both at 0 and 25 degrees C and could be prevented by sodium molybdate. The resulting change in sedimentation behaviour (shift of sedimentation coefficient from 9.6 S to 4.8 S for molybdate-stabilized and alkaline phosphatase-treated complexes, respectively) was similar to that observed after heat activation.     

Effects of ATP and pyrophosphate on properties of glucocorticoid-receptor complexes from rat thymus cells

Cytosols from rat thymus cells incubated with glucocorticoid contain nonactivated and activated receptors and mero-receptor complexes, in relative amounts that depend on the incubation conditions. These forms can be separated by a rapid minicolumn chromatographic technique based on their differential affinities for DNA, DEAE, and hydroxylapatite. We have used this method to examine the effects of ATP, pyrophosphate (PPi), and related compounds on cytosolic complexes. In addition to ATP, already known to promote activation at 0 degrees C, PPi, ADP, and other triphosphates at millimolar concentrations promoted activation of nonactivated complexes. AMP and Pi had little effect. ATP and PPi at millimolar concentrations also reduced binding of activated complexes to DNA. Characterization of the ATP- and PPi-activated complexes by gel filtration and ion exchange chromatography revealed two DNA-binding forms. One was essentially identical (Stokes radius of approximately 5.4 nm, elution from DEAE at approximately 50 mM KCl) to the normal activated complex obtained directly from cells incubated at 37 degrees C. The other had a Stokes radius of approximately 3.1 nm and had no affinity for DEAE. Analysis by minicolumns and gel filtration showed that ATP and PPi prevented formation of mero-receptor complexes, a process which occurs relatively rapidly in untreated thymus cytosols. These compounds did not alter properties of preformed mero-receptor. The accumulation of 3.1-nm complexes in thymus cytosols in which formation of mero-receptor is prevented suggests that this form is an intermediate, normally short-lived, in the conversion of 5.4 nm complexes to mero-receptor.     

Characterization of the molybdate-stabilized glucocorticoid receptor from rat thymus.

The untransformed glucocorticoid receptor of rat thymus cytosol was characterized in the form of its complex with [1,2,4-3H]triamcinolone acetonide by ion-exchange chromatography and by gel filtration and sucrose-density-gradient ultracentrifugation at different ionic strengths. Molybdate (10 mM) was present throughout all experimental procedures and prevented receptor inactivation and degradation as well as transformation. At low ionic strength the molybdate-stabilized steroid-receptor complex was detected as a single highly asymmetric entity with a Stokes radius of 5.85 nm, a sedimentation coefficient of 9.6 S and an apparent molecular weight of 236 000. This form was converted into a smaller, even more asymmetric, form in increasing proportion as the ionic strength was increased. In the presence of 0.4 M-KCl, the smaller form had a Stokes radius of 4.95 nm, a sedimentation coefficient of 4.6 S and an apparent molecular weight of 95 500. It is concluded that the glucocorticoid-receptor complex exists at low ionic strengths as a homodimer or as a heterodimer in which only one subunit possesses a steroid-binding site, and that the process of dissociation into subunits brought about by increasing the ionic strength is a process distinct from, but possibly preceding, the transformation phenomenon responsible for conferring DNA-binding properties on the complex.     

Characteristics of cytosol androgen receptor in rat thymus

Male and female rat thymic cytosol contained specific androgen receptor. The apparent dissociation constants (Kd) were 2.4 nM in males and 2.5 nM in females, and the number of binding sites (NBS) were 23.7 fmol/mg protein in males and 34.2 fmol/mg protein in females. Transformation of receptor to the DNA binding state was achieved by heat or KCl treatment of [3H]R1881-receptor complex, and the characteristics of transformed and nontransformed receptors were investigated. The nontransformed androgen-receptor complex eluted at 0.20-0.25 M KCl from DEAE-Sephacel and sedimented at 9.1 S and its molecular weight was 255,000 on agarose gel chromatography, while the transformed receptor complex eluted at 0.03-0.15 M KCl with a broad peak and sedimented at 4.5 S and its molecular weight was 80,000-85,000. The minicolumn binding assay revealed that approximately 57% of the total receptor complexes bound to DNA-cellulose following heat treatment (20 degrees C, 1 h). Castration exerted no effect on the physicochemical properties of cytosol androgen receptor, but it increased the number of binding site to the female level.     

Identification of a macromolecular inhibitor of glucocorticoid-receptor complex activation in rat liver cytosol

We have identified an endogenous regulator of the glucocorticoid receptor following fractionation of dialyzed rat liver cytosol on DEAE-cellulose. The macromolecular regulator, purified approximately 20-fold as judged by Lowry-reactive material, inhibits activation of glucocorticoid-receptor complexes when assayed by DNA-cellulose binding and by chromatography on DEAE-cellulose minicolumns. In addition the active DEAE-cellulose fraction stabilizes the unoccupied glucocorticoid receptor against heat inactivation. Evidence is presented that the observed inhibition of activation by the active DEAE-cellulose fraction is not due to concentration of cytosolic proteases or RNA. The inhibitory molecule in the active fraction is not stable to heating at 90 degrees C (15 min) and is partially inactivated at 45 degrees C (15-60 min).     

Interaction of sodium thiocyanate with rat hepatic glucocorticoid-receptor complexes

0.1–0.3 M sodium thiocyanate greatly enhanced the rate of inactivation of unbound rat hepatic glucocorticoid receptors in vitro at 4°C. Prior treatment of the unbound glucocorticoid receptor with 10 nM molybdate (at 25°C for 30 min) protected the receptor from 0.3 M KCl, but not from 0.3 M NaSCN inactivation. When the [³H]dexamethasone-receptor complex was examined on sucrose density gradients containing 0.1 M NaSCN, the receptor sedimented as a 4 S complex rather than the 7 S form observed in 0.1 M KCl gradients. NaSCN was found to be more effective in the extraction of both in vivo and in vitro nuclear-bound [³H]dexamethasone-receptor complexes than KCl. At a concentration of 0.3 M, NaSCN extracted most of the specific nuclear-bound receptor. 50 mM NaSCN significantly blocked the thermal activation of preformed [³H]dexamethasone-receptor complexes. The chaotropic salt, NaSCN, appears therefore to have significant effects on glucocorticoid receptors in vitro. In addition, NaSCN appears to be a useful agent in quantitative extraction of steroid from nuclear-bound steroid-receptor complexes.     

Temperature dependence of the dissociation rate constants for the nonactivated (molybdate-stabilized) and activated progesterone receptor-hormone complexes from the chick oviduct

Both the nonactivated and activated forms of the chick oviduct cytosol progesterone receptor-hormone complexes displayed first-order dissociation kinetics at temperatures between 0 and 25 degrees C. The rate constant was always 2-3-times greater for the nonactivated than for the activated complex. The thermodynamic parameters calculated from the Eyring plot for the nonactivated and activated forms, respectively, were: delta H+ = 28.6 +/- 0.2 and 29.9 +/- 1.5 kcal/mol; -T delta S+ = 7.4 +/- 0.6 and 7.7 +/- 1.6 kcal/mol; and delta G+ = 21.3 +/- 0.5 and 22.1 +/- 0.1 kcal/mol. These values suggest that activation results in an increase in enthalpy of the ligand-receptor interaction, thus stabilizing the complex. The dissociation rate constants for the native complex obtained by two different experimental approaches, namely, isotope dilution ('chase') and dissociation against charcoal, indicated the absence of cooperativity in the receptor-ligand binding.     

Chromatographic separation of nontransformed and transformed glucocorticoid-receptor complexes from rat liver cytosol. Use of tungstate in the purification, inactivation, and resolution of receptor forms

Aliquots of rat liver cytosol glucocorticoid-receptor complexes (GRc) were transformed by an incubation with 8-10 mM ATP at 0 degrees C and were compared with those transformed by an exposure to 23 degrees C. The extent of receptor transformation was measured by chromatography of the samples over columns of DEAE-Sephacel. The ATP-transformed complexes, like those which were heat-transformed, exhibited lower affinity for the positively charged ion-exchange resin and were eluted with 0.12 M KCl (peak-I): the nontransformed complexes appeared to possess higher affinity and required 0.21 M KCl (peak II) for their elution. As expected, the receptor in the peak-I exhibited the DNA-cellulose binding capacity and sedimented as 4S in sucrose gradients. Peak II contained an 8-9S glucocorticoid receptor (GR) form that showed reduced affinity for DNA-cellulose. Presence of sodium tungstate (5 mM) prevented both heat and ATP transformation of the GRc resulting in the elution of the complexes in the region of nontransformed receptors. When parallel experiments were performed, binding of the cytosol GRc to rat liver nuclei or DNA-cellulose was seen to increase 10-15 fold upon transformation by heat or ATP: tungstate treatment blocked this process completely. The transformed and nontransformed GRc were also differentially fractionated by (NH4)2SO4: tungstate-treated (nontransformed) receptor required higher salt concentration and was precipitated at 55% saturation. In addition, the GRc could be extracted from DNA-cellulose by an incubation of the affinity resin with sodium tungstate resulting in approximately 500-fold purification of the receptor with a 30% yield. These studies show that the nontransformed, and the heat-, salt-, and ATP-transformed GRc from the rat liver cytosol can be separated chromatographically, and that the use of tungstate facilitates the resolution of these different receptor forms. In addition, extraction of the receptor from DNA-cellulose by tungstate provides another new and efficient method of partial receptor purification.     

Immunofluorescence microscopy of the intracellular translocation of glucocorticoid-receptor complexes in rat hepatoma (HTC) cells

Antibodies to the two dexamethasone-binding proteins from rat liver cytosol have been elicited in rabbits. These antibodies precipitate the dexamethasone binding activities from rat liver cytosol as weil as cytosol from Hepatoma Tissue Culture (HTC) cells. Antibodies to the 45 000 D protein have been used for demonstration of the intracellular dynamics of the glucocorticoid receptor complex by immunofluorescence microscopy, comparing HTC cells treated with dexamethasone at 4 and 37 °C.     

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.     

Transformation of glucocorticoid-receptor complexes is accompanied by dissociation of oligomers in intact cells

The association of glucocorticoid-receptor complexes with other components in vivo has been evaluated by chemical crosslinking of hormone-treated cells. When cells were incubated with hormone at 2 degrees C, before being subjected to crosslinking, most glucocorticoid-receptor complexes were found untransformed, as judged by DEAE-cellulose chromatography, and sedimented as 11-6 S oligomers in sucrose gradients containing 0.3 M NaCl. If crosslinking was performed after cells were treated with hormone at 37 degrees C, about 60% of cytosolic glucocorticoid-receptor complexes were found transformed, and sedimented as 4 S monomers.