A ligand binding domain mutation in the mouse glucocorticoid receptor functionally links chromatin remodeling and transcription initiation

We utilized the mouse mammary tumor virus (MMTV) long terminal repeat (LTR) in vivo to understand how the interaction of the glucocorticoid receptor (GR) with a nucleosome-assembled promoter allows access of factors required for the transition from a repressed promoter to a derepressed, transcriptionally competent promoter. A mutation (C644G) in the ligand binding domain (LBD) of the mouse GR has provided information regarding the steps required in the derepression/activation process and in the functional significance of the two major transcriptional activation domains, AF1 and AF2. The mutant GR activates transcription from a transiently transfected promoter that has a disordered nucleosomal structure, though significantly less well than the wild-type GR. With an integrated, replicated promoter, which is assembled in an ordered nucleosomal array, the mutant GR does not activate transcription, and it fails to induce chromatin remodeling of the MMTV LTR promoter, as indicated by nuclease accessibility assays. Together, these findings support a two-step model for the transition of a nucleosome-assembled, repressed promoter to its transcriptionally active, derepressed form. In addition, we find that the C-terminal GR mutation is dominant over the transcription activation function of the N-terminal GR activation domain. These findings suggest that the primary activation function of the C-terminal activation domain is different from the function of the N-terminal activation domain and that it is required for derepression of the chromatin-repressed MMTV promoter.

Conserved cysteines in the type 1 deiodinase selenoprotein are not essential for catalytic activity

The iodothyronine deiodinases are a family of oxidoreductases that catalyze the removal of iodide from thyroid hormones. Each of the three isoforms contain selenocysteine at its active site and several cysteine residues that may be important for catalytic activity. Of particular interest in the type I deiodinase (D1) is Cys124, which is vicinal to the selenocysteine at position 126, and Cys194, which has been conserved in all deiodinases identified to date. In the present studies, we have characterized the functional properties of C124A, C194A, and C124A/C194A D1 mutants, which were prepared by site-directed mutagenesis and expressed in COS-7 cells. In broken cell preparations, the sensitivity of the mutants to the selective D1 inhibitors propylthiouracil and aurothioglucose were unaltered. Mutagenesis at the Cys124 position was associated with a 7-11-fold increase in the Km of dithiothreitol, whereas Vmax values remained largely unchanged. However, both mutations resulted in marked decreases in Vmax values when glutathione or a reconstituted thioredoxin cofactor system were used in the assay. In contrast to the results of these in vitro studies, no impairment in deiodinating capability was noted in intact cells expressing equivalent levels of the mutant constructs. These studies demonstrate that Cys124 and Cys194 influence the reactivity of the D1 with thiol cofactors in in vitro assay systems but are not determinants of the sensitivity of the enzyme to propylthiouracil and aurothioglucose. Furthermore, the observation that the cysteine mutants are fully active in intact cells demonstrates that the results of commonly used broken cell assays do not accurately predict the activity of the D1 in intact cells and suggests that glutathione and thioredoxin are not the major thiols utilized in vivo to support D1 activity.

Glucocorticoid receptor phosphorylation: overview, function and cell cycle-dependence

All steroid hormone receptors are phosphorylated and undergo hormone-induced hyperphosphorylation. Most phosphorylated residues identified so far are serines in the N-terminal domain. Other residues and domains may also be phosphorylated, e.g. the estrogen receptor is phosphorylated on tyrosine in the hormone-binding domain. Many sites lie in consensus sequences for proline-directed, cell cycle-associated kinases. In some receptors hyperphosphorylation is induced by hormone antagonists as well as agonists, and leads to new phosphorylated sites. With glucocorticoid receptors, hyperphosphorylation is specific for glucocorticoid agonists, follows receptor activation and produces no new sites. Rate studies suggest that hyperphosphorylation is due to accelerated phosphorylation rather than delayed dephosphorylation. Evidence to date indicates that steroid hormone receptor phosphorylation serves not as an on-off switch but modulates function more subtly. Mutations of phosphorylated sites to alanine have been found to decrease activity by 0 to 90%, depending on mutated site, cell type, reporter gene and hormone concentration. With glucocorticoid receptors, some alanine mutants are up to 75% less active in hormone-induced transactivation of certain reporter genes. They are also inactive in hormone-induced repression of transcription of their own gene and down regulation of the receptor protein. Furthermore, they are much less sensitive to degradation. Both basal phosphorylation and hormone-dependent hyperphosphorylation of these receptors are cell cycle-dependent, basal phosphorylation being low in S phase and high in G2/M and hyperphosphorylation the reverse, suggesting a causal relation to the cell cycle-dependence of glucocorticoid activity reported with several cell lines. Hyperphosphorylation appears to be regulated by basal phosphorylation through negative charge in the N-terminal domain, which in S phase is relatively low and permits hyperphosphorylation, but in G2/M is relatively high and blocks hyperphosphorylation.

Mouse glucocorticoid receptor phosphorylation status influences multiple functions of the receptor protein

Although studies have shown that the mouse glucocorticoid receptor (mGR) contains eight phosphorylation sites (Bodwell, J. E., Ortí, E. , Coull, J. M., Pappin, D. J. C., Smith, L. I., and Swift, F. (1991) J. Biol. Chem. 266, 7549-7555), the effect of phosphorylation on receptor function is unclear. We have examined the consequences of single or multiple phosphorylation site mutations on several properties of mGR including receptor expression, ligand-dependent nuclear translocation, hormone-mediated transactivation, ligand-dependent down-regulation of mGR, and receptor protein half-life. Mutations had little effect on receptor expression, subcellular distribution, ligand-dependent nuclear translocation, or on the ability to activate hormone-mediated transcription from a complex (murine mammary tumor virus) promoter. In contrast, the phosphorylation status of the mGR had a profound effect on the ability to transactivate a minimal promoter containing simple glucocorticoid response elements after hormone administration. Similarly, ligand-dependent down-regulation by glucocorticoids of both receptor mRNA and protein was abrogated in mutants containing three or more phosphorylation site alterations. Finally, we show that the phosphorylation status of mGR has a profound effect on the stability of the glucocorticoid receptor protein. Receptors containing seven or eight mutated sites have a markedly extended half-life and do not show the ligand-dependent destabilization seen with wild type receptor. These data show that receptor phosphorylation may play a crucial role in regulating receptor levels and hence control receptor functions.

Control by basal phosphorylation of cell cycle-dependent, hormone-induced glucocorticoid receptor hyperphosphorylation

Mouse glucocorticoid receptors (GRs) are phosphorylated in the N-terminal domain at serine/ threonine residues, most lying in consensus sequences for cell cycle-associated kinases. Glucocorticoid agonists, but not antagonists, induce hyperphosphorylation. Phosphorylation of GRs overexpressed in Chinese hamster ovary (CHO) cells is cell cycle-dependent: basal phosphorylation in S phase is one third that in G2/M; glucocorticoids induce hyperphosphorylation in S but not G2/M, paralleling the reported sensitivity in S and resistance in G2/M of proliferating cells to transcriptional activation by glucocorticoids. This parallel led us to investigate what controls hyperphosphorylation. We tested three hypotheses: hyperphosphorylation is controlled by 1) negative charge due to basal GR phosphorylation, being permitted in S by low charge and blocked in G2/M by high charge; 2) presence in S and absence in G2/M of required kinases; 3) availability in S and lack in G2/M of unoccupied phosphorylatable sites. Our results are inconsistent with 2) and 3), but strongly support 1). GR mutants with alanines (A7GR) or glutamates (E7GR) replacing all but one phosphorylated site were overexpressed in CHO cells. Serine 122 remained intact to report GR phosphorylation. Consistent with hypothesis 1, with A7GRs hormone-induced hyperphosphorylation occurred in both S and G2/M (thus revealing kinase activity for hyperphosphorylation of at least serine 122 in both phases), whereas with E7GRs it occurred in neither phase. We conclude that basal GR phosphorylation controls hormone-induced GR hyperphosphorylation by modulating negative charge in the N-terminal domain and could potentially control other cell cycle-dependent GR properties.

ATP depletion affects the phosphorylation state, ligand binding, and nuclear transport of the 4 S polycyclic aromatic hydrocarbon-binding protein in rat hepatoma cells

In the rat, cytochrome P4501A1 gene expression is thought to be regulated by several trans-acting factors including the 4 S polycyclic aromatic hydrocarbon (PAH)-binding protein. Phosphorylation and dephosphorylation have been suggested to influence the function of many cytosolic receptors and transcription factors. The ATP level within H4IIE rat hepatoma cells could be depleted by treatment with sodium azide or 2,4-dinitrophenol; restoration of the original ATP levels occurred with addition of glucose to the cell culture. ATP depletion reduced the phosphate content of the 4 S protein by approximately 25-30%, which lowered the binding of benzo[a]pyrene (B[a]P) to the 4 S protein by >60%. This effect could not be reversed by the addition of ATP to the binding reaction mixtures. Alkaline phosphatase treatment of the purified 4 S protein in a cell-free system also reduced the B[a]P binding to the protein. Cells treated with a protein phosphatase inhibitor, okadaic acid, and a protein kinase inhibitor, staurosporin, affected the B[a]P binding of the 4 S protein positively and negatively, respectively. These data suggested that phosphorylation is involved in the interaction of the 4 S protein with the PAH. The nuclear translocation of the predominantly cytosolic binding protein has been investigated after ligand binding. Western blots with the immunopurified 4 S PAH-binding protein from cytosolic and nuclear lysates showed significant differences in the distribution of the 4 S receptor between cytosolic and nuclear compartments in control and ATP-depleted cells. Ligand binding stimulated the movement of the receptor into the nucleus, which was completely blocked by reducing the intracellular ATP concentration. These findings provide new information on the role of ATP and phosphorylation on the interaction of B[a]P with 4 S PAH-binding protein and its nuclear translocation.

Immunocytochemical analysis of hormone mediated nuclear translocation of wild type and mutant glucocorticoid receptors

We have analyzed structural and functional features of the human glucocorticoid receptor (hGR) for their effects on receptor subcellular distribution. COS 1 cells transiently transfected with wild type and mutant hGR cDNAs were assessed immunocytochemically using well-characterized antipeptide antibodies to the hGR. The effect of administration of steroid hormones (and the antiglucocorticoid RU486) on receptor localization was evaluated. Unliganded wild type receptors expressed in COS 1 cells were predominately cytoplasmic. Addition of glucocorticoids or the glucocorticoid receptor antagonist, RU486, resulted in complete translocation of these receptors into the nucleus whereas non-glucocorticoid steroids or dibutyryl cAMP were not effective in promoting nuclear translocation. Thus, nuclear translocation was specific for steroids capable of high affinity binding to the hGR. To elucidate the potential role of receptor domains in receptor localization, COS 1 cells transiently transfected with various receptor cDNA mutants were analyzed in a similar manner. Translocation of an hGR deletion mutant lacking the majority of the amino terminus (deletion of amino acids 77-262) was identical to the wild type receptor despite the absence of a transactivation domain. Receptors in which the DNA binding domain was either partially or totally deleted showed an impaired capacity to undergo hormone-inducible nuclear translocation. Deletion of the hinge region of the hGR (which also contains part of the nuclear localization signal, NL1) resulted in receptor localization in the cytoplasm. Mutants in the ligand binding domain exhibited two localization phenotypes, exclusively nuclear or cytoplasmic. Receptor mutants truncated after amino acid 550 were found in the nucleus in the presence and absence of hormone consistent with the existence of nuclear localization inhibitory sequences in the ligand binding domain of the receptor. However, a linker insertion mutant (at amino acid 582) which results in a receptor deficient in ligand binding did not undergo nuclear translocation indicating that nuclear localization inhibitory sequences were intact in this mutant. The role of receptor phosphorylation on hormone induced nuclear translocation was also examined. Mouse glucocorticoid receptors which contained mutations of certain hormone inducible phosphorylation sites exhibited translocation properties similar to wild type mGR indicating that these phosphorylation sites on the receptor do not play a major role in hormone inducible nuclear translocation.

Hormone-induced hyperphosphorylation of specific phosphorylated sites in the mouse glucocorticoid receptor

The glucocorticoid receptor (GR) is phosphorylated in its basal state, and rapidly undergoes hormone-induced hyperphosphorylation after binding glucocorticoids. Previously, we have identified seven phosphorylated sites in the mouse GR. Most of the sites are located in the regions of the N-terminal domain that are necessary for maximum transcriptional activity and reduce nonspecific binding to DNA. Using WCL2 cells, which overexpress mouse GRs, we now quantitate hormone-induced hyperphosphorylation at each of these sites. Addition of triamcinolone acetonide to WCL2 cells results in significant hyperphosphorylation at the majority of the sites. The hyperphosphorylation ratio, i.e. the 32P incorporation into GRs from hormone-treated cells divided by 32P incorporation into GRs from untreated cells, was above 1.0 for all sites but serine 150 and threonine 159. Serine 220 displays marked hormone dependence, with a ratio of 3. For most sites the ratio was about 1.5. Hormone-induced hyperphosphorylation not only increases the charge at selected phosphorylated sites but also provides a substantial increase in the overall negative charge around the region of the N-terminal domain that is involved in transactivation.

Cell cycle-dependent glucocorticoid receptor phosphorylation and activity

Proliferating cells display striking cell cycle dependence in sensitivity to gene activation by glucocorticoids; they are sensitive in late gap 1/synthesis (G1/S) (late G1 and S phases) but resistant in gap 2/mitotic (G2/M). Here we describe large cell cycle-dependent variations in glucocorticoid receptor (GR) phosphorylation that accompany, and may account for, the changes in sensitivity. GRs are basally phosphorylated and undergo hyperphosphorylation after hormone-induced activation. Identified phosphorylated sites are all in the N-terminal domain. Several lie in a region required for full transactivating activity and reduction of nonspecific binding to DNA. Most are in consensus sequences for cell cycle-associated kinases, suggesting that such kinases phosphorylate GRs. We now show with WCL2 cells (Chinese hamster ovary cells with overexpressed GRs) that: 1) glucocorticoid treatment fails to hyperphosphorylate GRs in G2/M but doubles phosphorylation in S, more than seen with unsynchronized cells; and 2) basal GR phosphorylation is almost three times higher in G2/M than S. These results, along with earlier observations, implicate GR phosphorylation with mechanisms of glucocorticoid resistance in G2/M. Such mechanisms might underlie some forms of glucocorticoid resistance in inflammatory and lymphoproliferative diseases. HPLC phosphopeptide maps of GRs from S and G2/M reveal no significant qualitative differences in phosphorylated sites, consistent with a general increase during G2/M in negative charge of the N-terminal domain. We also show that the previously described increase in GR hormone-binding capacity from G1 to S is accompanied by a parallel increase in GR protein.

Glucocorticoid receptors: ATP-dependent cycling and hormone-dependent hyperphosphorylation

The dependence of hormone binding to glucocorticoid receptors (GRs) on cellular ATP levels led us to propose that GRs normally traverse an ATP-dependent cycle, possibly involving receptor phosphorylation, and that without ATP they accumulate in a form that cannot bind hormone. We identified such a form, the null receptor, in ATP-depleted cells. GRs are basally phosphorylated, and become hyperphosphorylated after treatment with hormone (but not RU486). In mouse receptors we have identified 7 phosphorylated sites, all in the N-terminal domain. Most are on serines and lie within a transactivation region. The time-course of hormone-induced hyperphosphorylation indicates that the primary substrates for hyperphosphorylation are the activated receptors; unliganded and hormone-liganded nonactivated receptors become hyperphosphorylated more slowly. After dissociation of substrates for hyperphosphorylation are the activated receptors; unliganded and hormone-liganded nonactivated receptors become hyperphosphorylated more slowly. After dissociation of hormone, most receptors appear to be recycled and reutilized in hyperphosphorylated form. From these and related observations, we have concluded that the postulated ATP-dependent cycle can be accounted for by hormone-induced or spontaneous dissociation of receptor-Hsp90 complexes, followed by reassociation of unliganded receptors with Hsp90 via an ATP-dependent reaction like that demonstrated in cell-free systems. Other steroid hormone receptors might traverse a similar cycle. Four of the 7 phosphorylated sites in the N-terminal domain are in consensus sequences for p34cdc2 kinases important in cell cycle regulation. This observation, along with the known cell cycle-dependence of sensitivity to glucocorticoids and other evidence, point to a role for receptor phosphorylation in controlling responses to glucocorticoids through the cell cycle.

Phosphorylation of steroid hormone receptors

Many observations with intact cells as well as cell-free systems suggest that receptors of the steroid hormone superfamily, along with other transcription factors, are regulated by phosphorylation. All receptors that have been analyzed carefully so far have turned out to be phosphoproteins. They are basally phosphorylated in the absence of ligand, and in many cases become hyperphosphorylated in the presence of hormone or other agonists, and sometimes of antagonists. Several studies indicate that hyperphosphorylation of receptors follows activation, and may require nuclear binding of the receptor. Serines are the predominant phosphorylated residues detected in receptors, with minor amounts of threonine. Tyrosine phosphorylation of the estrogen receptor is a subject of controversy. With various receptors, evidence has been found for phosphorylation in vivo of the N-terminal, hormone-binding, and DNA-binding domains, as well as of the hinge region. All but one of the phosphorylated sites identified in progesterone and glucocorticoid receptors by phosphopeptide mapping and sequencing are in the N-terminal domain; one is in the hinge region. Even after hormone treatment most of those sites are only partly phosphorylated, which means that several subpopulations of receptors, characterized by different states of phosphorylation and potentially different biological activities, must coexist. The majority of identified phosphorylated sites lie in consensus sequences for the PDPK. Many parallels can be discerned between phosphorylation of receptors and of other transcription factors. For example, several transcription factors become hyperphosphorylated on stimulation, and much indirect evidence points to regulation of both receptors and transcription factors by kinases and phosphatases, with cycling between different phosphorylated states. Functions of receptors that are regulated by phosphorylation are only beginning to be investigated. With transcription factors a substantial body of information is already available, and functions that appear to be thus regulated include dimerization, interactions with other proteins, binding to DNA, nuclear-cytoplasmic localization, and transcriptional activity. These and other functions may be found to be regulated by phosphorylation of receptors.

Identification of phosphorylated sites in the mouse glucocorticoid receptor

Glucocorticoid receptors in vivo are phosphorylated in the absence of hormone and become hyperphosphorylated in the presence of glucocorticoid agonist but not antagonists (Ortí, E., Mendel, D.B., Smith, L.I., and Munck, A. (1989) J. Biol. Chem. 264, 9728-9731). As a preliminary step to elucidating the functional significance of receptor phosphorylation, we have identified seven phosphorylated sites on the mouse receptor. Tryptic phosphopeptides from 32P-labeled receptors were purified from glucocorticoid-treated mouse thymoma cells (WEHI-7) and from stably transfected Chinese hamster ovary cells (WCL2) that express large numbers of mouse receptors. Phosphopeptide maps of receptors from these two cell types were almost indistinguishable. Solid phase sequencing revealed phosphorylation at serines 122, 150, 212, 220, 234, and 315 and threonine 159. Serines 122, 150, 212, 220, and 234 and the sequences surrounding them are conserved in the homologous regions of the rat and human receptors, but threonine 159 and serine 315 have no homologues in the human receptor. The seven phosphorylated sites are in the amino-terminal domain of the receptor. All but serine 315 are within transactivation domains identified in the human and/or rat receptors. Serines 212, 220, and 234 are in a highly acidic region that in the mouse receptor is necessary for full transcription initiation activity and reduces nonspecific DNA binding. Serines 212, 220, and 234 and threonine 159 are in consensus sequences for proline-directed kinase and/or p34cdc2 kinase. Serine 122 is in a consensus sequence for casein kinase II whereas serines 150 and 315 do not appear to be in any known kinase consensus sequence. The location of many of these sites suggests a role of phosphorylation in transactivation.

In vivo response of secretory component in the rat uterus to antigen, IFN-gamma, and estradiol

Intrauterine immunization of ovariectomized rats with SRBC is known to elicit pronounced IgA and IgG antibody responses in uterine secretions of immunized uteri. To determine whether secretory component (SC), the receptor for transporting polymeric IgA from tissues to mucosal surfaces, was also influenced by Ag, ovariectomized rats were immunized and boosted by placing SRBC into the lumena of individual uterine horns. In response to Ag, the levels of polymeric IgA, as well as free SC and SC bound to polymeric IgA, increased in uterine secretions. When ovariectomized animals were treated with estradiol, a fivefold increase in SC levels was observed in the immunized horns, indicating that a hormone response is superimposed on the Ag-induced stimulation of uterine SC. To determine whether IFN-gamma influences the presence of SC in uterine secretions, IFN-gamma was placed in the uterine lumena of ovariectomized nonimmunized rats. When uterine secretions were analyzed, significantly higher levels of SC were found in IFN-gamma-exposed uteri than were present in saline treated control animals. In contrast, intrauterine instillation of IFN-gamma had no effect on the levels of IgA in uterine secretions. This response was specific for IFN-gamma in that IFN-alpha/beta had no effect on uterine SC or IgA levels. These results indicate that intrauterine instillation of Ag, in addition to evoking pronounced antibody responses, stimulates the production of SC, which may be responsible for the transport of polymeric IgA from tissue to uterine secretions. Furthermore, they indicate that IFN-gamma placed in the uterine lumen stimulates SC production and suggest that the uterine SC response to Ag may be mediated by the action of IFN-gamma on uterine epithelial cells.

In vivo response of secretory component in the rat uterus to antigen, IFN-gamma, and estradiol

Intrauterine immunization of ovariectomized rats with SRBC is known to elicit pronounced IgA and IgG antibody responses in uterine secretions of immunized uteri. To determine whether secretory component (SC), the receptor for transporting polymeric IgA from tissues to mucosal surfaces, was also influenced by Ag, ovariectomized rats were immunized and boosted by placing SRBC into the lumena of individual uterine horns. In response to Ag, the levels of polymeric IgA, as well as free SC and SC bound to polymeric IgA, increased in uterine secretions. When ovariectomized animals were treated with estradiol, a fivefold increase in SC levels was observed in the immunized horns, indicating that a hormone response is superimposed on the Ag-induced stimulation of uterine SC. To determine whether IFN-gamma influences the presence of SC in uterine secretions, IFN-gamma was placed in the uterine lumena of ovariectomized nonimmunized rats. When uterine secretions were analyzed, significantly higher levels of SC were found in IFN-gamma-exposed uteri than were present in saline treated control animals. In contrast, intrauterine instillation of IFN-gamma had no effect on the levels of IgA in uterine secretions. This response was specific for IFN-gamma in that IFN-alpha/beta had no effect on uterine SC or IgA levels. These results indicate that intrauterine instillation of Ag, in addition to evoking pronounced antibody responses, stimulates the production of SC, which may be responsible for the transport of polymeric IgA from tissue to uterine secretions. Furthermore, they indicate that IFN-gamma placed in the uterine lumen stimulates SC production and suggest that the uterine SC response to Ag may be mediated by the action of IFN-gamma on uterine epithelial cells.

Phosphorylated sites within the functional domains of the approximately 100-kDa steroid-binding subunit of glucocorticoid receptors

The steroid-binding subunit of the glucocorticoid receptor is known to be a approximately 100-kDa phosphoprotein composed of an immunogenic, DNA-binding, and steroid-binding domain. When isolated from WEHI-7 cells, this protein contains between two and three phosphoryl groups per steroid-binding site (Mendel WEHI-7 cells, this protein contains between two and three phosphoryl groups per steroid-binding site (Mendel et al., 1987). To identify the domains that contain these phosphorylated sites, we have analyzed the phosphate content of selected proteolytic fragments of the approximately 100-kDa steroid-binding protein from nonactivated and activated receptors. The approximately 100-kDa steroid-binding protein from WEHI-7 cells grown in the presence of [32P]orthophosphate was covalently labeled with [3H]dexamethasone 21-mesylate, purified with the BuGR2 monoclonal antibody, digested with chymotrypsin or trypsin, and analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). Chymotrypsin digestion of this protein yields a approximately 45-kDa fragment containing both the steroid-binding and DNA-binding domains, which contained both 32P and 3H. Trypsin digestion of the protein yields a approximately 29-kDa fragment encompassing the steroid-binding domain but not the DNA-binding domain of the approximately 100-kDa protein, which also contained both 32P and 3H. The 32P/3H ratio of each fragment provides a measure of phosphate content per steroid-binding site and indicated that each fragment has approximately 30% of the phosphate content of the intact protein. This is sufficient to account for one of the three receptor phosphoryl groups.(ABSTRACT TRUNCATED AT 250 WORDS)

A dynamic model of glucocorticoid receptor phosphorylation and cycling in intact cells

Glucocorticoid receptors have been proposed to undergo an ATP-dependent recycling process in intact cells, and a functional role for receptor phosphorylation has been suggested. To further investigate this possibility we have examined the phosphate content of the steroid-binding protein of all glucocorticoid receptor forms which have been isolated from WEHI-7 mouse thymoma cells. By labeling of intact cells with 32Pi for 18-20 h in the absence of hormone, covalent binding of [3H]dexamethasone 21-mesylate, immunopurification and SDS-PAGE analysis, the steroid binding protein was found to contain, on average, 2-3 phosphates as phosphoserine. One third of the phosphates were associated with proteolytic fragments encompassing the C-terminal steroid-binding domain. The central DNA-binding domain was not phosphorylated, leaving the other two thirds of the phosphates localized in the N-terminal domain. The phosphate content of various receptor forms from cells incubated with 32Pi and [35S]methionine was compared using 35S to normalize for quantity of protein. In ATP-depleted cells a non-steroid-binding form of the receptor (the "null" receptor) is found tightly bound to the nucleus, even without steroid. The phosphate content of null receptors was two thirds that of cytosolic receptors from normal cells, suggesting phosphorylation-dependent cycling in the absence of hormone. Addition of glucocorticoid agonists, but not antagonist, to 32P- and 35S-labeled cells increased the phosphate content of the cytosolic steroid-binding protein up to 170%, indicating an average increase in the phosphates from about 3 to 5. After 30 min of hormone treatment the phosphate content of the steroid-binding protein of cytosolic activated (DNA-binding) and nonactivated receptors, and that of nuclear receptors extractable with high salt concentrations and/or DNase I digestion, was the same. No change in the phosphate content of the 90-kDa heat shock protein associated with unliganded and nonactivated receptors was detected following association of the free protein with the receptor and following hormone binding of the receptor. Analysis of the unextractable nuclear receptors indicated that they contained less phosphate (60% of that of cytosolic receptors), similarly to null receptors, indicating that dephosphorylation is associated with the unextractable nuclear fraction. The rate of hormone-dependent phosphorylation appeared to be much faster than the rate of dephosphorylation in the presence of hormone, the latter determined by a chase of the 32P label with unlabeled phosphate. Our results show that phosphorylation and dephosphorylation are involved in the mechanism of action of glucocorticoid receptors.(ABSTRACT TRUNCATED AT 400 WORDS)

Identification of cysteine-644 as the covalent site of attachment of dexamethasone 21-mesylate to murine glucocorticoid receptors in WEHI-7 cells

Dexamethasone 21-mesylate is a highly specific synthetic glucocorticoid derivative that binds covalently to glucocorticoid receptors via sulfhydryl groups. We have identified the amino acid that reacts with the dexamethasone 21-mesylate by using enzymatic digestion and microsequencing for radiolabel. Nonactivated glucocorticoid receptors obtained from labeling intact WEHI-7 mouse thymoma cells with [3H]dexamethasone 21-mesylate were immunopurified and analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The purified approximately 100-kDa steroid-binding subunit was eluted from gel slices and subjected to enzymatic digestion. Trypsin digestion followed by reversed-phase high-performance liquid chromatography (reversed-phase HPLC) produced a single [3H]dexamethasone 21-mesylate labeled peptide. Automated Edman degradation of this peptide revealed that the [3H]dexamethasone 21-mesylate was located at position 5 from the amino terminus. Dual-isotope labeling studies with [3H]dexamethasone 21-mesylate and [35S]methionine demonstrated that this peptide contained methionine. Staphylococcus aureus V8 protease digestion of [3H]dexamethasone 21-mesylate labeled steroid-binding subunits generated a different radiolabeled peptide containing label at position 7 from the amino terminus. On the basis of the published amino acid sequence of the murine glucocorticoid receptor, our data clearly identify cysteine-644 as the single residue in the steroid-binding domain that covalently binds dexamethasone 21-mesylate.(ABSTRACT TRUNCATED AT 250 WORDS)

Activation of cytosolic glucocorticoid-receptor complexes in intact WEHI-7 cells does not dephosphorylate the steroid-binding protein

In order to determine the ratio of phosphates to hormone-binding sites on nonactivated (non-DNA-binding) glucocorticoid receptors in WEHI-7 mouse thymoma cells, we have extracted these receptors from cells grown to a steady state with 32P, labeled them with a saturating concentration of [3H]dexamethasone 21-mesylate, purified them using a monoclonal antibody, and analyzed them by polyacrylamide gel electrophoresis under denaturing and reducing conditions. The complexes contained approximately 5 mol of phosphate/mol of bound steroid. Only half of the phosphates were associated with the approximately 100-kDa protein which is labeled with [3H]dexamethasone 21-mesylate. The remaining phosphates were associated with the approximately 90-kDa non-steroid-binding component of the nonactivated complex. Dual label studies, using [35S]methionine to measure receptor protein and 32P to measure receptor phosphates, have enabled us to determine the phosphate content, relative to receptor protein, of both nonactivated and activated cytosolic complexes generated in intact WEHI-7 cells exposed to triamcinolone acetonide at 37 degrees C. The total amount of phosphate associated with the activated complex is roughly half of that associated with the nonactivated complex, the decrease being accounted for by dissociation of the approximately 90-kDa phosphoprotein which accompanies activation. However, the ratio of 32P to 35S counts associated with the approximately 100-kDa steroid-binding protein is the same for the activated and nonactivated complexes. These results indicate that there is no net change in the phosphorylation of the approximately 100-kDa steroid-binding component of the cytosolic glucocorticoid-receptor complex upon activation in the intact cell.

Glucocorticoid receptors lacking hormone-binding activity are bound in nuclei of ATP-depleted cells

The glucocorticoid receptor binding capacity of rat thymus cells disappears when the cells are depleted of ATP by anaerobiosis, and rapidly reappears when ATP levels are restored. Loss and recovery of binding capacity occurs even when protein synthesis is suppressed with cycloheximide. In view of this and similar work in other cell systems, we proposed that in cells deprived of ATP the receptor is present in a form--the 'null receptor' form, as we shall call it--that cannot bind hormone. Although many subsequent observations support this idea, no direct evidence has appeared for the existence of the null receptor. We have attempted to detect the null receptor in WEHI-7 mouse thymoma cells with a monoclonal antibody to the glucocorticoid receptor. Here we report that the null receptor is bound in the nuclei of ATP-depleted cells, and is present in amounts comparable to those of receptors in normal cells.

Molybdate-stabilized nonactivated glucocorticoid-receptor complexes contain a 90-kDa non-steroid-binding phosphoprotein that is lost on activation

We have used a monoclonal antibody to purify glucocorticoid-receptor complexes from WEHI-7 mouse thymoma cells. Molybdate-stabilized, nonactivated complexes were found to contain two distinct proteins which could be separated by polyacrylamide gel electrophoresis under denaturing and reducing conditions. One of the proteins, 100 kDa, was labeled when cytosol was incubated with the affinity ligand [3H]dexamethasone 21-mesylate. The second protein, 90 kDa, was not labeled. Several lines of evidence, including Western blot analysis of purified nonactivated complexes, indicate that only the 100-kDa protein is directly recognized by the antibody. The 90-kDa protein appears to be purified as a component of the nonactivated complex due to noncovalent association with the 100-kDa protein. Both the 100-kDa and 90-kDa components of the nonactivated complex become labeled with 35S when cells are grown in medium containing [35S]methionine. Using cells labeled in this manner, we have shown that activated (i.e. DNA-binding) cytosolic complexes, formed by warming either in intact cells or under cell-free conditions, contain only the 100-kDa protein. Complexes extracted from nuclei of warmed cells similarly contain only the 100-kDa protein. These results indicate that the 100-kDa and 90-kDa components of nonactivated complexes separate upon activation. Purification of nonactivated complexes from cells grown in medium containing [32P]orthophosphoric acid indicates that both the 100-kDa and 90-kDa components are phosphoproteins which can be labeled with 32P. Therefore, resolution of the two proteins will be essential in order to determine whether the receptor is dephosphorylated on activation.

Degradation without apparent change in size of molybdate-stabilized nonactivated glucocorticoid-receptor complexes in rat thymus cytosol

We have investigated the stability of the [3H]dexamethasone 21-mesylate-labeled nonactivated glucocorticoid-receptor complex in rat thymus cytosol containing 20 mM sodium molybdate. Cytosol complexes were analyzed under nondenaturing conditions by gel filtration chromatography in the presence of molybdate and under denaturing conditions by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate. When analyzed under nondenaturing conditions, complexes from fresh cytosol and from cytosol left for 2 h at 3 degrees C eluted from gel filtration as a single peak of radioactivity with a Stokes radius of approximately 7.7 nm, suggesting that no proteolysis of the complexes had occurred in either cytosol. When analyzed under denaturing conditions, however, whereas the fresh cytosol gave a receptor band on sodium dodecyl sulfate-polyacrylamide gel electrophoresis at Mr approximately 90,000 (corresponding to the intact complex), the cytosol that had been left for 2 h at 3 degrees C gave only a fragment (Mr approximately 50,000). This fragment, just as the intact complex, could be thermally activated to a DNA-binding form. Proteolysis of the receptor could be blocked by preparing the cytosol in the presence of EGTA, leupeptin, or a heat-stable factor present in the cytosol of rat liver and WEHI-7 mouse thymoma cells. From these results we conclude: (i) 20 mM molybdate does not protect the nonactivated glucocorticoid-receptor complex present in rat thymus cytosol against proteolysis under conditions which are commonly used for cell-free labeling of the receptor, and (ii) the demonstration of a Stokes radius of approximately 8 nm for the nonactivated glucocorticoid-receptor complex is not sufficient to indicate that the receptor complex is present in its intact form.

Stabilization of thymic glucocorticoid-receptor complexes by the calcium-activated protease inhibitor, calpastatin

We previously described a heat-stable factor from WEHI-7 mouse thymoma, rat liver, spleen, and human chronic lymphocytic leukemia cells that prevents degradation of glucocorticoid-receptor complexes (GRC) in cytosols from rat thymus and acute non-lymphocytic leukemia cells. We now show that the factor has many properties in common with calpastatin, a naturally occurring inhibitor of a family of neutral calcium-activated proteases called calpains. Liver GRC-stabilizing activity and calpastatin activity, in addition to surviving boiling, co-chromatography on columns of DEAE-cellulose ion exchange or agarose A-0.5M gel filtration matrices, and have identical isoelectric points of 5.1. This factor should be especially useful for studying GRC function in the presence of calcium.

Evidence for distinct sulfhydryl groups associated with steroid- and DNA-binding domains of rat thymus glucocorticoid receptors

We have found that nonactivated and activated forms of the rat thymus glucocorticoid-receptor complex (GRC) will react with reactive sulfhydryl matrices to form covalently immobilized complexes that can subsequently be eluted with reducing agents. The interaction of GRCs with these matrices depends on the nature of both the immobilized sulfhydryl group and the type of leaving group attached. One matrix, agarose CL-4B-diaminoethyl-succinyl-thioethylamine-2-thiopyridyl+ ++ (DSTT), binds total receptor-bound steroid. A second matrix, agarose CL-4B-diaminoethyl-succinyl-cysteinyl-2-thiobenzoic acid (DSCT), binds activated but not nonactivated complexes. The reaction of activated complexes with the DSCT matrix is apparently through a sulfhydryl group located near the DNA binding domain, as soluble DNA interferes with the reaction. This sulfhydryl group(s) appears to be located in a portion of the GRC that is resistant to degradation, since proteolytic digestion of activated GRC to a point where DNA binding is lost results in only a moderate decrease in binding with the DSCT matrix. Purified receptor, covalently labeled with [3H]dexamethasone to the sulfhydryl associated with the steroid binding domain, was able to bind to DSCT matrix, providing evidence for distinct sulfhydryl groups associated with the steroid and DNA binding domains.

Sulfhydryl-modifying reagents reversibly inhibit binding of glucocorticoid-receptor complexes to DNA-cellulose

Glucocorticoid -receptor complexes from intact rat thymus cells incubated with [3H]dexamethasone at 0 degree C are in the nonactivated form and do not bind to DNA-cellulose. Upon being warmed, they are transformed to activated complexes that bind to DNA-cellulose at 0 degree C. We have found that treatment of dexamethasone-receptor complexes with the sulfhydryl-modifying reagents methyl methanethiosulfonate ( MMTS ) and 5,5'-dithiobis(2-nitrobenzoic acid) (DTNB), either before or after the warming, inhibits subsequent binding to DNA-cellulose. The effects of these reagents can be reversed at 0 degree C by dithioerythritol and other sulfhydryl-containing compounds. These results provide the first clear evidence that sulfhydryl-modifying reagents inhibit the binding of activated dexamethasone-receptor complexes to DNA-cellulose and suggest that sulfhydryl groups may be located in or near the DNA binding domain of the rat thymus glucocorticoid-receptor complex. Furthermore, addition of dithioerythritol at 0 degree C to nonactivated receptor complexes that have been treated with MMTS or DTNB produces a substantial increase in the capacity of these complexes to bind to DNA-cellulose, raising the possibility that sulfhydryl groups may be associated with a region on the receptor that plays a critical role in the activation process.

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 Rs = 8-9 nm, and that activation is accompanied by a reduction in size to Rs = 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 Rs = 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 degrees 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.

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 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.

Purification and characterization of carboxypeptidase A from rat skeletal muscle

Carboxypeptidase A (EC 3.4.17.1) has been purified 44 000-fold in 33% yield from rat skeletal muscle by a four-step procedure. Purification in the presence of dichlorovinyl dimethyl phosphate conveniently inactivates an accompanying chymotrypsin-like enzyme and other serine protease(s) to ensure isolation of pure carboxypeptidase A free of polypeptide contaminants. The enzyme preparation consists of two components with molecular weights of approximately 39 300 and 37 800. The rat muscle carboxypeptidase is very similar to bovine pancreatic carboxypeptidase A in terms of (1) substrate specificity, (2) kinetics and molecular activity, (3) influence of metal ions on catalysis, (4) interaction with inhibitors, (5) effects of ionic strength on activity, and (6) stability and activity as functions of pH. Both muscle and pancreatic carboxypeptidases exhibit enhanced esterolytic activity when assayed in the presence of a variety of indoles and imidazoles or after incubation at relatively high concentrations of MnSO4. The muscle enzyme is substantially less stable than its pancreatic homologue, and in impure preparations is very much less soluble. The latter property is attributable to a binding substance present in such preparations which renders muscle but not pancreatic carboxypeptidase A insoluble until ionic strength is increased to values near 2 M.

MLC-conditioned media stimulate an increase in Fc receptors on human macrophages

Macrophage Fc receptors (FcR) are essential for antibody-dependent cellular cytotoxicity and for optimal phagocytosis of opsonized particulate antigens. Culture in the presence of conditioned medium from mixed leukocyte cultures (MLC-CM) resulted in a dose- and time-dependent increase (up to 10-fold) in FcR-dependent binding of 125I-labeled IgG1 to promyelocytic HL-60 cells, macrophage-like U-937 cells, and normal cultured human monocytes. FcR increase in HL-60 cells was blocked by cycloheximide (100 microM) and was accompanied by a slight decrease in binding affinity. Since cell volume did not change, the increase in FcR probably represents an increase in the surface density of FcR sites. MLC-CM prepared with or without serum were equally effective in augmenting FcR sites, whereas only serum-containing MLC-CM caused morphologic change of U-937 and HL-60 cells.

MLC-conditioned media stimulate an increase in Fc receptors on human macrophages

Macrophage Fc receptors (FcR) are essential for antibody-dependent cellular cytotoxicity and for optimal phagocytosis of opsonized particulate antigens. Culture in the presence of conditioned medium from mixed leukocyte cultures (MLC-CM) resulted in a dose- and time-dependent increase (up to 10-fold) in FcR-dependent binding of 125I-labeled IgG1 to promyelocytic HL-60 cells, macrophage-like U-937 cells, and normal cultured human monocytes. FcR increase in HL-60 cells was blocked by cycloheximide (100 microM) and was accompanied by a slight decrease in binding affinity. Since cell volume did not change, the increase in FcR probably represents an increase in the surface density of FcR sites. MLC-CM prepared with or without serum were equally effective in augmenting FcR sites, whereas only serum-containing MLC-CM caused morphologic change of U-937 and HL-60 cells.