Phosphorylation of L-cell glucocorticoid receptors in immune complexes: evidence that the receptor is not a protein kinase

Phosphorylation of immunopurified rat liver glucocorticoid receptor by the catalytic subunit of cAMP-dependent protein kinase

We have examined phosphorylation of the rat liver glucocorticoid receptor (GR) and GR-associated protein kinase (PK) activity in the immunopurified receptor preparations. Affinity labeling of hepatic cytosol with [3H]dexamethasone 21-mesylate showed a covalent association of the steroid with a 94 kDa protein. GR was immunopurified with antireceptor monoclonal antibody BuGR2 (Gametchu & Harrison, Endocrinology 114: 274-279, 1984) to near homogeneity. A 23 degrees C incubation of the immunoprecipitated protein A-Sepharose adsorbed GR with [gamma-32P]ATP,Mg2+ and the catalytic subunit of cAMP-dependent PK (cAMP-PK) from bovine heart, led to an incorporation of radioactivity in the 94 kDa protein. Phosphorylation of GR was not evident in the absence of the added kinase. Of the radioinert nucleotides (ATP, GTP, UTP or CTP) tested, only ATP successfully competed with [gamma-32P]ATP demonstrating a nucleotide specific requirement for the phosphorylation of GR. Other divalent cations, such as Mn2+ or Ca2+, could not be substituted for Mg2+ during the phosphorylation reaction. Phosphorylation of GR was sensitive to the presence of the protein kinase inhibitor, H-8, an isoquinoline sulfonamide derivative. In addition, the incorporation of radioactivity into GR was both time- and temperature-dependent. The phosphorylation of GR by cAMP-PK was independent of the presence of hsp-90 and transformation state of the receptor. The results of this study demonstrate that GR is an effective substrate for action of cAMP-PK and that the immunopurified protein A-Sepharose adsorbed GR lacks intrinsic kinase activity but can be conveniently used for the characterization of the phosphorylation reaction in the presence of an exogenous kinase.

Phosphorylation of chicken oviduct progesterone receptor by cAMP-dependent protein kinase

Phosphorylation of immunopurified chicken oviduct progesterone receptor (PR) was studied in intact cells and under cell-free conditions. Cytosol PR was isolated by incubation with anti-PR monoclonal antibody alpha PR22 adsorbed to protein A-Sepharose and suspended in a reaction mixture containing 10 mM Mg2+, 0.1 mM [gamma-32P]ATP, and the catalytic subunit of cAMP-dependent protein kinase (cAMP-PK) from bovine heart. All three major proteins of avian PR (PR-A, 79 kDa; PR-B, 110 kDa; 90 kDa) incorporated 32P-radioactivity on serine residues. The phosphorylation reaction was inhibited by synthetic inhibitors of protein kinases, H-8 and 20-residue peptide IP20. A 40 degrees C preexposure of PR oligomer increased phosphorylation of the 90-kDa protein, known to be a heat-shock protein (hsp-90). The extent of the phosphorylation reaction was temperature-dependent as the 32P-incorporation into PR-A and PR-B increased gradually, showing a maximum at 37 degrees C. Multiple phosphopeptides (4-7) were resolved by two-dimensional electrophoresis chromatography following cleavage of 32P-labeled peptides with trypsin. Both A and B forms of receptor showed similar phosphorylation patterns with B receptor digestion exhibiting two to three additional peptides. Under physiological conditions, preincubation of oviduct mince with forskolin, a regulator of intracellular cAMP levels, caused a greater extent of phosphorylation of PR-A and PR-B proteins. The results of this study demonstrate that chicken oviduct PR is an excellent substrate for the action of cAMP-PK in vitro and that this enzyme may be a physiological regulator of progesterone action in the oviduct.

Hormone-induced progesterone receptor phosphorylation consists of sequential DNA-independent and DNA-dependent stages: analysis with zinc finger mutants and the progesterone antagonist ZK98299

Human progesterone receptors (hPRs) are phosphorylated at multiple serine residues, first in a basal step and then in a hormone-induced step. To determine whether hormone-induced phosphorylation precedes or follows the interaction of hPRs with DNA two strategies were used. (i) DNA binding was prevented or altered with site-specific mutants of the A form of hPR; (ii) DNA binding of wild-type hPR forms A and B was prevented with the progesterone antagonist ZK98299. Two hPRA mutants were constructed: DBDCys, which lacks a critical cysteine residue in the first zinc finger, and DBDsp, which is mutated at three discriminatory amino acids to change its DNA binding specificity from a progesterone response element to an estrogen response element. Receptors were transiently expressed in PR-negative cells and were intranuclear. DBDCys did not bind DNA in vitro and DBDsp bound only the estrogen response element. Transiently expressed hPRA and DBDsp showed the upward shift in electrophoretic mobility characteristic of hormone-induced phosphorylation; it was absent with DBDCys. Hormone-induced [32P] orthophosphate incorporation into transiently expressed DBDCys was reduced 60% compared to hPRA and DBDsp but was not eliminated. ZK98299 binds hPRs but prevents their interaction with DNA. Compared to R5020, the antagonist reduced phosphorylation of hPRB and hPRA in T47D breast cancer cells by 60% and totally prevented the mobility shift. We conclude that the hormone-induced phosphorylation of hPR includes DNA-independent and DNA-dependent stages and that only DNA-dependent sites contribute to the mobility shift.

Studies on the arrangement of glucocorticoid receptors in the plasma membrane of S-49 lymphoma cells

The presence of glucocorticoid receptors is required for glucocorticoid-mediated lymphocytolysis to take place. However, the explicit mechanism of involvement of this receptor continues to be debated. We have recently presented evidence that this response is mediated by a specialized form of the glucocorticoid receptor that resides in the plasma membrane (mGR). Using sequential cell separation techniques ("immunopanning," fluorescent cell sorting, and soft agar cloning), a resultant population of membrane receptor-enriched cells have remained stable and provided material for further analysis. The mGR patching and capping phenomenon originally observed with fluoresceinated monoclonal antibody techniques was verified here with electron micrographic analysis using colloidal gold-conjugated antibody. Using 3H-labeled monoclonal antibody, a radioimmunoassay for membrane receptors was developed. Trypsin treatment removed the membrane receptor antigenic site from the surface of cells. Peptide mapping of receptor purified from plasma membranes reveals several trypsin and alpha-chymotrypsin cleavage sites. Larger fragments resulted from cleavage of the membrane receptor of cells enriched for mGR versus those found in cells depleted of the membrane form, although most of the resulting fragments are shared by the two forms. Confirmation of previous studies correlating membrane receptor with the mechanism of glucocorticoid sensitivity is now extended to include elimination of the lymphocytolysis effect in membrane receptor-stripped (trypsinized) S-49 cells.

Size and steroid-binding characterization of membrane-associated glucocorticoid receptor in S-49 lymphoma cells

The precise mechanism for glucocorticoid-mediated lymphocytolysis is not understood, although it is presumed to be receptor mediated. We have recently presented evidence that this response is mediated by a specialized form of the glucocorticoid receptor (GR) that resides in the plasma membrane (mGR). Confirmation of the previous receptor identification studies in a population of S-49 cells enriched for mGR is now made using another antibody specific for the rodent GR, BUGR-2. The membrane resident receptor could be labeled competitively with the affinity ligand dexamethasone 21-mesylate, and Scatchard analysis of whole cell binding revealed that receptor number, but not the affinity for hormone, varied between the mGR-enriched and -deficient cell populations. Steroid specificity displacement analyses showed an order of affinities as follows: triamcinolone acetonide greater than progesterone greater than dexamethasone greater than testosterone = estrogen. Studies of mGR by one- and two-dimensional gel electrophoresis, immunoblot, autoradiography, and density gradients revealed a species with an equivalent size to cytosolic receptor as well as multiple higher molecular weight species, confirming earlier studies. To offer a possible explanation for the nucleic acid origins of the mGR, RNA from the mGR-enriched cells was probed with rat GR cDNA; mGR-enriched cells contained higher levels of GR mRNA. Possible molecular etiologies of larger receptor species in membrane are discussed.

Steroid hormone receptors

In the three decades since the original discovery of receptors for steroid hormones, much has been learned about the biochemical processes by which these regulatory agents exert their effects in target tissues. The intracellular receptor proteins are potential transcription factors, needed for optimal gene expression in hormone-dependent cells. They are present in an inactive form until association with the hormone converts them to a functional state that can react with target genes. Transformation of the receptor protein to the nuclear binding form appears to involve the removal of both macromolecular and micromolecular factors that act to keep the receptor form reacting with DNA. Much of the native receptor is present in the nucleus, loosely bound and readily extractable, but for some and possibly all steroid hormones, some receptor is in the cytoplasm, perhaps in equilibrium with a nuclear pool. Methods have been developed for the stabilization, purification, and characterization of receptor proteins, and through cloning and sequencing of their cDNAs, primary structures for these receptors are now known. This has led to the recognition of structural similarities among the family of receptors for the different steroid hormones and to the identification of regions in the protein molecule responsible for the various aspects of their function. Monoclonal antibodies recognizing specific molecular domains are available for most receptors. Despite the knowledge that has been acquired, many important questions remain unsolved. How does association with the steroid remove factors keeping the receptor protein in its native state, and how does binding of the transformed receptor to the response element in the promoter region enhance gene transcription? Once it has converted the receptor to the nuclear binding state, is there a further role for the steroid in modulating transcription? Still not entirely clear is the involvement of phosphorylation and/or dephosphorylation in hormone binding, receptor transformation, and transcriptional activation. Less vital to basic understanding but important in the overall picture is whether the native receptors for gonadal hormones are entirely confined to the nucleus or whether there is an intracellular distribution equilibrium. With the effort now being devoted to this field, and with the application of new experimental techniques, especially those of molecular biology, our understanding of receptor function is progressing rapidly. The precise mechanism of steroid hormone action should soon be completely established.

Phosphorylation of calf uterine progesterone receptor by cAMP-dependent protein kinase

We have examined the potential for using calf uterine progesterone receptor (PR) as a substrate for phosphorylation by cAMP-dependent protein kinase (cAMP-PK), PR was found to interact with anti-PR monoclonal antibody alpha PR6 (Sullivan et al., 1986), which was to immunopurify the receptor. Protein staining of the purified preparation revealed the presence of two major bands corresponding to 114 kDa and 90 kDa peptides; only 114 kDa peptide could be photoaffinity-labeled with R5020. The 90 kDa peptide co-migrated with 90 kDa heat shock protein (hsp-90) precipitated by anti-hsp-90 monoclonal antibody AC88 (Riehl et al., 1985). Incubation of the immunopurified protein-A-Sepharose-adsorbed PR with the catalytic subunit of cAMP-PK in the presence of gamma-[32P]ATP and divalent cations resulted in a Mg++-dependent incorporation of 32P-radioactivity into both the 114 kDa and the hsp-90 peptides. Small 32P-incorporation was also seen in the 114 kDa peptide in the presence of Mn++. A 60 degrees C preincubation of immunopurified PR increased the extent of phosphorylation of the hsp-90 peptide. A pretreatment with alkaline phosphatase reduced the ability of PR to act as a substrate while the steroid occupancy of PR appeared to enhance the phosphorylation of the 114 kDa peptide. The differential cation requirement for the phosphorylation of 114 kDa and hsp-90 peptides and a selective hormone-dependent increase in the phosphorylation of the 114 kDa peptide suggest a possible role of phosphorylation in mediating progesterone action in the calf uterus.

The structure and function of steroid receptor proteins

This review has highlighted several topics in the study of steroid hormone action. The unanswered questions regarding the mechanism of ligand-controlled LRF activity, the extent of evolutionary conservation and specificity of DNA binding, and the validity of various models of transcriptional regulation mediated through gene networks point to the future direction of research in this field. Steroid hormones are used extensively in clinical treatments, especially glucocorticoids. Our laboratory is attempting to determine which gene networks are responsible for some of these clinical phenotypes. Figure 5 points out that the study of glucocorticoid action holds a unique position because it spans both the basic sciences and the field of applied molecular biology. Now that we have a fundamental knowledge of the necessary elements required for steroid-dependent regulation of gene expression, we can better investigate the clinical responses to steroid therapy (which include devastating side effects) by isolating and characterizing the important target gene(s). In this author's opinion, future directions in the study of steroid responsiveness will have to include a systematic approach toward deciphering a variety of these LRF-regulated gene networks in experimentally feasible systems. Hopefully, work in this area may be revealing and perhaps beneficial to ongoing clinical studies. In addition, the study of mechanisms of transcriptional induction and repression, using the model system of LRFs, could be applicable to many gene regulatory systems which are controlled by such processes as development and differentiation.

Effect of the 90 kDa heat shock protein, HSP90, on glucocorticoid receptor binding to DNA-cellulose

Glucocorticoid receptors in the IM-9 human lymphoblastoid cell line were affinity labeled with [3H]dexamethasone 21-mesylate and activated to a DNA-binding form by filtration through a Bio-Gel A-1.5m column. The 90 kDa heat shock protein, HSP90, was identified by labeling IM-9 cells with 35S-methionine at both 37 degrees C and 42 degrees C and purified to near homogeneity by sequential chromatography through DE52 and hydroxyapatite. Addition of purified HSP90 to activated, affinity labeled glucocorticoid receptors in a molecular ratio of 16 to 1 inhibited the binding of the receptors to DNA-cellulose. HSP90 did not affect the binding of other proteins to DNA-cellulose, indicating that the inhibitory effect of HSP90 was specific for the glucocorticoid receptor. These results suggest that HSP90 may associate with the glucocorticoid receptor, masking its DNA-binding site and thereby inhibiting receptor interaction with DNA.

Characterization of a casein kinase which interacts with the rabbit progesterone receptor. Differences with the in vivo hormone-dependent phosphorylation

Previous in vivo studies have shown that the rabbit progesterone receptor undergoes two phosphorylation reactions: one basal and a second one which is hormone-dependent. We report here on the presence and characteristics of a kinase activity found in receptor preparations highly purified by immunoaffinity chromatography. 1. This kinase activity is not due to the receptor molecule itself since the two proteins may be separated by several chromatographic and immunological methods. 2. The presence of the kinase in receptor preparations is not an artefact of the purification procedure. The kinase binds to the receptor as shown by coelution in immunoaffinity experiments and during various chromatographies. This interaction probably takes place in vivo and is not artefactually formed during solubilization of the receptor since the kinase also copurifies with receptors isolated from the uterine nuclei of progestin-treated rabbits. 3. This enzyme may be classified as a casein kinase since it readily phosphorylates the latter substrate (Km approximately equal to 0.15 mg/ml) and is not regulated by cyclic nucleotides, Ca2+ and calmodulin or phospholipids. Its classification as a casein kinase I or II is difficult since on the one hand it is inhibited by heparin, activated by polyamines and may use both ATP and GTP, but on the other hand it modifies only serine residues, and is not inhibited by heparin when the receptor itself is employed as a substrate. 4. The kinase which copurifies with the receptor does not mimic in vitro the effects of the hormone-dependent phosphorylation of the receptor observed in vivo: there is no enhancement of kinase activity by the hormone, and the phosphorylated receptor does not exhibit the characteristic "upshift" in its electrophoretic mobility. Thus either this kinase is not the enzyme responsible for the hormone-dependent receptor phosphorylation or, during purification, a factor has been lost which is necessary for retaining the hormone dependency of the reaction.

Carrier-free 8-azidoadenosine 5'-[gamma-32P]triphosphate

We found 8-azidoadenosine 5'-diphosphate to be a phosphoryl acceptor in the enzymatic conversion of 1,3-diphosphoglyceric acid to 3-phosphoglycerate. This has allowed us to synthesize in a single-step procedure carrier-free 8-azidoadenosine 5'-[gamma-32P]triphosphate, requiring no further purification of the end product. The synthesized 8-azidoadenosine 5'-[gamma-32P]triphosphate has been characterized and shown to meet all the criteria for a specific photoreactive ATP analogue.

Characterization of estrogen receptors and associated protein kinase activity by high-performance hydrophobic-interaction chromatography

We have determined that high-performance hydrophobic-interaction chromatography (HPHIC) with weakly hydrophobic columns permit the rapid separation of the labile isoforms of estrogen receptor proteins. Previously we reported the use of the SynChrom propyl 500 column for HPHIC of steroid receptors. However, due to the strongly hydrophobic characteristics of the ligand, [125I]iodoestradiol-17 beta, and the receptor protein, organic solvent was required in the mobile phase for greater recovery of receptor proteins. Here, we report separation of steroid receptors from human breast tumors and rat uteri, using the Beckman CAA-HIC, a non-ionic polyether-bonded column, without the need for organic solvents and with virtually 100% recoveries. Receptors were extracted in 10 mM phosphate buffer (pH 7.4). Maximum resolution and separation were achieved when a descending salt gradient of ammonium sulfate in phosphate buffer (pH 7.4) was used (2-0 M in 30 min). Estrogen receptor (ER) was resolved into two isoforms with tR = 22 +/- 1 min (n = 16, designated as peak I) and 27.5 +/- 0.5 min (n = 14, designated as peak II) and a purification of five- to twenty-fold in a single pass. Free steroid was eluted at tR = 35 +/- 1 min (n = 4). Separation was dependent on adjusting the ionic strength of cytosol to 1.5 M ammonium sulfate. ER, purified by HPHIC, retained ligand binding capacity and exhibited protein kinase activity, which was dominant in the less hydrophobic peak I (tR = 22 min) when immunoprecipitated with the monoclonal antibody D547. This method of rapidly purifying ER with high retention of biological activity may now be applied to the study of the molecular interrelationships of steroid receptor isoforms.

Chick oviduct progesterone receptor phosphorylation: characterization of a copurified kinase and phosphorylation in primary cultures

A protein kinase activity was copurified with the chick oviduct progesterone receptor. The enzyme is magnesium dependent and can use the B subunit of progesterone receptor or histones as substrates. The physiochemical parameters of the kinase were determined [pI approximately 5.3; Stokes radius approximately 7.2 nm; sedimentation coefficient (S 20,w) approximately 5.6] and compared to those of the purified B subunit. The results were consistent with the presence of an unique enzyme distinct from the receptor itself. The physiological significance of receptor phosphorylation was investigated in oviduct cells grown in primary culture. Cells were labeled with [32P]orthophosphate in presence or absence of progesterone and the receptor components were immunoprecipitated with a specific polyclonal antibody. Although progesterone treatment lead to the attachment of most of the receptor (approximately 80%) to nuclear structures, the 32P-labeled B subunit was only recovered in the cytosol fraction. Different procedures to extract the nuclear receptor did not allow detection of any 32P-labeled form in the nuclear-soluble fractions, suggesting that the B subunit was not further phosphorylated upon the exposure of cells to progesterone.

Glucocorticoid receptor phosphorylation in mouse L-cells

This paper summarizes our observations on the phosphorylation state of untransformed and transformed glucocorticoid receptors isolated from 32P-labeled L-cells. The 300-350-kDa 9S untransformed murine glucocorticoid receptor complex is composed of a 100-kDa steroid-binding phosphoprotein and one or possibly two units of the 90-kDa heat shock protein (hsp90), which is also a phosphoprotein. Transformation of this complex to the 4S DNA-binding state is accompanied by dissociation of hsp90. When receptors in cytosol are transformed by heating at 25 degrees C, there is no gross change in the degree of phosphorylation of the steroid-binding protein. Both receptors that are bound to DNA after transformation under cell-free conditions and receptors that are located in the nucleus of cells incubated at 37 degrees C in the presence of glucocorticoid are labeled with 32P. The results of experiments in which the 32P-labeled receptor was submitted to limited proteolysis suggest that the 16-kDa DNA-binding domain is phosphorylated and that the 28-kDa steroid-binding domain is not.

The molybdate-stabilized glucocorticoid binding complex of L-cells contains a 98-100 kdalton steroid binding phosphoprotein and a 90 kdalton nonsteroid-binding phosphoprotein that is part of the murine heat-shock complex

This paper summarizes our work performed with glucocorticoid-binding complexes in molybdate-stabilized cytosol prepared from 32P-labeled L-cells. In our early work, we showed that cytosol prepared from 32P-labeled L-cells contains two phosphoproteins (a 90 and a 98-100 kdalton protein) that elute from an affinity resin of deoxycorticosterone agarose in a manner consistent with the predicted behavior of the glucocorticoid receptor. Both phosphoproteins are immunoadsorbed onto protein-A-Sepharose from molybdate-stabilized cytosol incubated with a monoclonal antibody against the receptor. The 98-100 kdalton phosphoprotein binds steroid and the 90 kdalton phosphoprotein is a structurally different, nonsteroid-binding protein that is bound to the untransformed, molybdate-stabilized glucocorticoid receptor. The 90 kdalton protein reacts on Western blots with a monoclonal antibody raised against a 90 kdalton protein from the water mold Achlya ambisexualis. This antibody recognizes an epitope that is conserved in 90 kdalton phosphoproteins from rodent and human cells, and it reacts with the 90 kdalton phosphoprotein that copurifies with the molybdate-stabilized, untransformed chick oviduct progesterone receptor. The 90 kdalton nonsteroid-binding phosphoprotein is an abundant cytosolic protein that dissociates from the glucocorticoid receptor when it is transformed, and unlike the steroid-binding protein, it does not bind to DNA. The 90 kdalton phosphoprotein determines the acidic behavior of the untransformed glucocorticoid receptor on DEAE-cellulose. This abundant cytosolic 90 kdalton phosphoprotein reacts with rabbit antiserum raised against the gel purified 89 kdalton chicken heat-shock protein (hsp89). This antiserum recognizes 90 kdalton heat-shock proteins in human, rodent, frog and Drosophila cells. Immunoadsorption of molybdate-stabilized cytosol with antibody directed against the 98-100 kdalton steroid receptor results in the immune-specific adsorption of a 90 kdalton phosphoprotein that reacts with anti-hsp89 antibody on Western blots. These observations suggest that, like the transforming proteins from several avian sarcoma viruses, the untransformed glucocorticoid receptor exists in a complex with the 90 kdalton heat-shock protein.