Phosphorylation of glucocorticoid receptor-associated and free forms of the ∼90-kDa heat shock protein before and after receptor activation

Comprehensive overview of the structure and regulation of the glucocorticoid receptor

Glucocorticoids are among the most prescribed drugs worldwide for the treatment of numerous immune and inflammatory disorders. They exert their actions by binding to the glucocorticoid receptor (GR), a member of the nuclear receptor superfamily. There are several GR isoforms resulting from alternative RNA splicing and translation initiation of the GR transcript. Additionally, these isoforms are all subject to several transcriptional, post-transcriptional, and post-translational modifications, all of which affect the protein's stability and/or function. In this review, we summarize recent knowledge on the distinct GR isoforms and the processes that generate them. We also review the importance of all known transcriptional, post-transcriptional, and post-translational modifications, including the regulation of GR by microRNAs. Moreover, we discuss the crucial role of the putative GR-bound DNA sequence as an allosteric ligand influencing GR structure and activity. Finally, we describe how the differential composition and distinct regulation at multiple levels of different GR species could account for the wide and diverse effects of glucocorticoids.

Post-translational modification of heat-shock protein 90: impact on chaperone function

Heat-shock protein 90 (Hsp90) is a molecular chaperone required for the stability and function of many signaling proteins that are often activated, mutated or overexpressed in cancer cells and that underly cancer cell proliferation and survival. Hsp90 is a conformationally flexible protein that associates with a distinct set of cochaperones depending on ATP or ADP occupancy of an N-terminal binding pocket. Nucleotide exchange and ATP hydrolysis by Hsp90 itself, with the assistance of cochaperones, drive the Hsp90 chaperone machine to bind, chaperone and release client proteins. Cycling of the Hsp90 chaperone machine is critical to its function. Although ATP binding and hydrolysis have been convincingly implicated in regulating the Hsp90 cycle, growing evidence suggests that various post-translational modifications of Hsp90, including phosphorylation, acetylation and other modifications, provide an additional overlapping or parallel level of regulation. A more complete understanding of how these various protein modifications are regulated and interact with each other at the cellular level to modulate Hsp90 chaperone activity is critical to the design of novel approaches to inhibit this medically important molecular target.

Synergistic effect of estramustine and [3'-keto-Bmtl]-[Val2]-cyclosporine (PSC 833) on the inhibition of androgen receptor phosphorylation in LNCaP cells

Estramustine phosphate has been used frequently alone or in combination with other drugs for the treatment of hormone-refractory prostate cancer. Estramustine is one of the major active metabolites of estramustine phosphate in vivo. We recently demonstrated that estramustine acts as an androgen antagonist, and the combination of estramustine with [3'-keto-Bmtl]-[Val2]-cyclosporine (PSC 833) results in synergistic cytotoxicity. Unlike other regulators of microtubules, such as paclitaxel, the present study demonstrated that estramustine alone or in combination with PSC 833 did not induce bcl-2 phosphorylation in LNCaP cells. No synergism between estramustine and PSC 833 in the induction of bcl-2 phosphorylation was obtained in MCF-7 cells exposed for 16 hr to estramustine (5-15 microM) and PSC 833 (5 microM). A significant synergistic antiandrogenic effect as measured by the inhibition of dihydrotestosterone-induced reporter gene luciferase expression in both wild-type and mutated androgen receptor (AR) cDNA-transfected HeLa cells was observed when the cells were exposed to estramustine and PSC 833. Treatment of LNCaP cells with estramustine alone (5-15 microM) resulted in a decrease of AR expression and phosphorylation. This effect was enhanced markedly by PSC 833. A strong correlation between AR phosphorylation and expression of the AR target gene PSA was obtained in dihydrotestosterone-stimulated LNCaP cells. The up-regulated PSA expression is a function of the level of the phosphorylated AR (r = 0.9814), but not the dephosphorylated form of the receptor protein (r = 0.4808). Thus, our studies suggest that the synergism between estramustine and PSC 833 in LNCaP cells is a consequence of inhibition of AR expression and phosphorylation, thus leading to interruption of AR-mediated gene expression.

Dexamethasone and triamcinolone acetonide accumulation in mouse fibroblasts is differently modulated by the immunosuppressants cyclosporin A, FK506, rapamycin and their analogues, as well as by other P-glycoprotein ligands

In mouse fibroblasts (LMCAT cells) stably transfected with the reporter gene chloramphenicol acetyl transferase under the control of the mouse mammary tumor virus promoter (MMTV-CAT), cyclosporin A (CsA), FK506, and rapamycin (Rap) at micromolar concentrations potentiate dexamethasone- (Dex) induced CAT gene activity in a dose-dependent way (Renoir J.-M., Mercier-Bodard C., Hoffmann K., Le Bihan S., Ning Y. M., Sanchez E. R., Handschumacher R. E. and Baulieu E. E., Proc. Natl. Acad. Sci. U.S.A., 92, 1995, 4977-4981). In this work, we used LMCAT and 1471.1 cells, another mouse fibroblast cell line stably transfected with the MMTV-CAT construct, and found that exposure to immunosuppressants affected steroid-induced transcription differently. Indeed, all immunosuppressants, including inactive analogues, potentiated not only Dex- but also TA-induced CAT gene expression in LMCAT cells. The extent of this potentiation was 3 times lower for TA than for Dex. These immunosuppressants have no effect in 1471.1 cells. In addition, no difference of glucocorticosteroid affinity for the GR was observed in 1471.1 cells, in contrast to LMCAT cells. In both cell lines, the drugs tested increased [3H] Dex and [3H] TA (although to a lesser extent) accumulation. Since it is known that immunosuppressants can reverse the membrane Phospho-glycoprotein (P-gp) activity responsible for an active efflux of small hydrophobic molecules from numerous cell types, we therefore measured the relative efficiency of other P-gp ligands (including vinca alkaloids and the inactive CsA analogue, PSC833), on [3H] Dex and [3H] TA accumulation. In both cell lines, and depending on the drugs, reversal of Dex export was more pronounced than that of TA export (approximately 11 times in LMCAT and approximately 2 times in 1471.1 cells). However, the antiprogestin/antiglucocorticosteroid RU 38 486 and its 17beta derivatives RU 49 953 which does not bind to GR, both identified as strong reversal molecules of P-gp activity, had respectively, no and a strong inhibiting effect on steroid accumulation in both cell lines. These results suggest that a mechanism resembling but different from P-gp can modulate steroid entry into these mouse fibroblasts. This is confirmed by the failure to demonstrate the presence of P-gp by immunoprecipitation and Western blot experiments in membrane preparations from both cell lines. From these data, we conclude: (i) that the two synthetic GR ligands do not accumulate similarly in mouse fibroblasts, (ii) that RU 49 953 increases steroid efflux, in contrast to other agents known to reverse P-gp activity (iii) that cellular entry and export of Dex and TA can be modulated by membrane efflux mechanism(s), different from P-gp, and (iiii) that immunosuppressant potentiation of Dex- and TA-induced CAT activity involves such a mechanism in LMCAT cells. In 1471.1 cells, the lack of any enhancing effect upon steroid-induced transcription of all the drugs tested, although they all increase steroid accumulation, suggests involvement of immunosuppressant-influenced factor(s) acting downstream from steroid entry, in the hormone receptor-mediated transcription pathway(s).

Phosphorylation and inhibition of rat glucocorticoid receptor transcriptional activation by glycogen synthase kinase-3 (GSK-3). Species-specific differences between human and rat glucocorticoid receptor signaling as revealed through GSK-3 phosphorylation

Transcriptional activation by the glucocorticoid receptor (GR) is regulated by both glucocorticoid binding and phosphorylation. The rat GR N-terminal transcriptional regulatory domain contains four major phosphorylation sites: threonine 171 (Thr171), serine 224 (Ser224), serine 232 (Ser232), and serine 246 (Ser246). We have previously demonstrated that Ser224 and Ser232 are phosphorylated by cyclin-dependent kinases, while Ser246 is phosphorylated by the c-Jun N-terminal kinase. We report here that the remaining GR phosphorylation site, Thr171, is a target for glycogen synthase kinase-3 (GSK-3) in vitro and in cultured mammalian cells. Increasing GSK-3 activity through its overexpression in cultured cells inhibits GR transcriptional enhancement, an effect dependent upon Thr171. Correspondingly, overexpression of a constitutively active form of the GSK-3 inhibitor, protein kinase B/Akt, increases GR transcriptional enhancement. Overexpression of GSK-3 had no effect on GR-mediated transcriptional repression of AP1-dependent gene expression. Importantly, transcriptional activation by the human GR (hGR), which contains an alanine (Ala150) at the position equivalent to Thr171 in rat GR, is not affected by GSK-3 overexpression. Introduction of a threonine residue at this position (A150T) establishes GSK-3-mediated inhibition of hGR transcriptional activation. These findings demonstrate species-specific differences in GR signaling, as revealed through GSK-3 phosphorylation, which suggests that GR function in rodents may not fully recapitulate receptor action in humans and that hGR is capable of adopting the GSK-3 signaling pathway through a somatic mutation.

Cloning and characterization of a porcine protein kinase gene and relationship to a class of heat shock proteins

We have determined the genomic sequence of a porcine protein kinase (PPK) gene, including 1,844 bp upstream of the transcription initiation site. The gene spans over 19 kb and consists of 18 exons and 17 introns. The 5' regulatory region contains a characteristic heat shock element in the first intron, a weak heat shock element 1,464 bp upstream of the transcription initiation site, an atypical TATA box, and further consensus sequences typical for eukaryotic promoters such as an SP-1 binding site. Southern blot analysis indicates that PPK exists as a single-copy gene in the porcine haploid genome. The PPK gene is transcribed in all investigated tissues as shown by Northern blotting and reverse transcriptase polymerase chain reaction. Comparison of the protein and cDNA sequences of PPK to other sequences in DNA and protein databases indicates significant homology to a class of heat shock proteins, the glucose-regulated proteins (GRP94). In addition, nucleotide sequences at the 5' terminus of the PPK gene show strong homology to the GRP94 family. Domains highly conserved with human tumor rejection antigen (GP96) or glucose-regulated protein (GRP94) genes are identified within the 5' terminus and the first intron of the PPK gene. These findings suggest that these proteins are either identical or represent a family of closely related proteins.

A protein kinase isolated from porcine brain microvessels is similar to a class of heat-shock proteins

To further characterize a protein kinase present in porcine brain microvessels, a cDNA library using porcine microvessel poly(A) RNA was screened with polyclonal antibodies raised against the native protein kinase. Since no full-length cDNA clone could be obtained, the missing sequence information was completed using two subsequent polymerase chain reactions. The amplified transcripts were cloned and the sequence determined. Additionally, a genomic DNA library from porcine kidney was screened to substantiate the results obtained from the polymerase chain reaction. Earlier hints of a relation to a subclass of the family of heat-shock proteins (HSPs) based upon a close sequence similarity at its amino-terminus could be confirmed by comparison of the full-length cDNA sequences. Common protein kinase consensus sequences, a targeting sequence for proteins of the endoplasmic reticulum at the carboxy-terminus as well as a hydrophobic leader sequence in the amino-terminal region of the protein could also be identified. Furthermore, a set of membrane-associated substrate proteins of this enzyme could be detected in brain capillaries. The results indicate that at least some members of the HSP 90 subfamily undergo autophosphorylation and show protein kinase activity by phosphorylating substrate proteins in vitro.

Glucocorticoid receptors and resistance to glucocorticoids in hematologic malignancies

Glucocorticoids are highly effective in inducing the cytolysis of cells of lymphocytic origin. This property has resulted in their incorporation into chemotherapy regimens used in the treatment of hematologic malignancies. Studies at the molecular and cellular levels have demonstrated that the hormone-induced cytolytic response is mediated through a highly specific cytoplasmic glucocorticoid receptor (GR). The GR has been cloned and sequenced and found to be organized into a discrete series of domains which mediate the receptor functions of hormone binding, nuclear translocation, DNA binding and transcriptional modulation. Thus, the binding of glucocorticoids by the GR induces a series of cellular events which result in the activation or repression of a network of glucocorticoid responsive genes and produces a specific cellular response. Prolonged exposure to glucocorticoids ultimately causes resistance to develop; thereby limiting the usefulness of this class of drugs. Studies addressing the mechanism of resistance have shown that the GR is the primary target of genetic alterations that lead to resistance to cytolysis. Using mouse and human cell lines as model systems, it has been shown that the vast majority of glucocorticoid resistant mutants express low levels or altered forms of the GR. Similarly, in vivo studies on patients have suggested that low GR levels are associated with a poor response to glucocorticoid based therapies. Recently, aberrant GR isolated from a patient with multiple myeloma resistant to glucocorticoids were found to harbor deletions in their hormone binding domains. Sequencing of the receptors suggested that each arose as a result of alternate splicing events. In both cases, the latter event produces a receptor unable to bind hormone leading to the speculation that alternate splicing may serve as a mechanism by which a cell evades the effects of glucocorticoids. The therapeutic implications for patients expressing aberrant receptors is discussed.

Interaction of antiandrogen-androgen receptor complexes with DNA and transcription activation

Human androgen receptor (hAR) is a ligand-dependent transcription factor that mediates androgen-induced actions on target tissues. Transfection studies in receptor deficient monkey kidney cells CV-1 in culture examine the ability of dihydrotestosterone (DHT) and of the antiandrogens hydroxylflutamide (HO-FLU), cyproterone acetate (Cypro.A) and RU 23908-10 to stimulate or to inhibit the transcription activation of mouse mammary tumor virus-bacterial chloramphenicol acetyltransferase (MMTV-CAT). CV-1 cells cotransfected with wild type hAR (hAR1-910) and MMTV-CAT, were treated with varying concentrations of DHT. DHT stimulated transcription activation of MMTV-CAT gene in a dose-dependent fashion. Cypro.A though only partially, also stimulated the transcription activation of MMTV-CAT. In the absence of steroids, HO-FLU induced the MMTV-CAT transcription in transfectants only 4% above the basal level. RU 23908-10 revealed the least agonistic activity at concentrations between 10 nM and 1 microM. Despite this, 100- to 1000-fold molar excess of all antiandrogens inhibited the agonistic activity of 10 nM DHT in this system. Receptor binding assays confirmed that HO-FLU, Cypro.A and RU 23908-10 competed with [3H]DHT for AR binding with hAR expressed in CV-1 cells. Western blot analysis using AR antipeptide antibodies raised in rabbits revealed the presence of two AR protein bands in extracts prepared from hAR1-910 transfected CV-1 cells. Incubation of labeled synthetic palindromic androgen responsive element (ARE) with the hAR containing CV-1 cell extracts followed by u.v. cross-linking demonstrated the specificity of AR-DNA interaction. Analysis by gel mobility shift assays showed that the interaction of AR-antiandrogen complexes with labeled ARE was specific.

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.

Estradiol increases phosphorylation of the 90 kDa heat shock protein not associated with estradiol receptor in MCF-7 cells in culture

MCF-7 cells in monolayer culture were incubated with [32P]orthophosphate for 18 h followed by covalent whole cell labelling of the estradiol receptor with tritiated tamoxifen aziridine [( 3H]TA). The heat shock protein (hsp-90) bound to receptor was precipitated with monoclonal antibodies H222 or JS 34/32, coupled to protein A-Sepharose and purified by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) under reducing conditions. Hsp-90 not associated with receptor was similarly purified after isolation with the monoclonal antibody AC88. It was found that estradiol treatment of the cells markedly increased phosphate incorporation in the free hsp-90, without affecting heat shock protein bound to receptor. A 6-fold increase in phosphate content was observed after 10 min incubation of the cells with estradiol. A similar effect was seen after treatment of the cells with the phorbol ester 12-O-tetradecanoylphorbol 13-acetate (TPA). The calcium ionophore A23187 had no influence on hsp-90 phosphorylation, and treatment of the cells with forskolin to increase the cellular content of cAMP had a reverse effect. A 50% reduction of the phosphate content in the free hsp-90 was observed after 15 min treatment. The observation that estradiol, TPA and forskolin had effect only on hsp-90 not bound to receptor is an indication that the receptor-hsp-90 complex exists in vivo. Time course studies show that the effect of estradiol is non-genomic. Two possible explanations of the results seem to exist. Either estradiol induces an increase in the degree of phosphorylation of hsp-90, or hsp-90 is translocated to the cytosol from a different cellular compartment.

The state transitions of normal and mutant androgen-receptor complexes in human genital skin fibroblasts

We have incubated cells from controls and subjects with receptor-defective androgen resistance with 3H-labelled testosterone (T), methyltrienolone (MT), dihydrotestosterone (DHT) or mibolerone (MB) and studied the temperature dependence of the dissociation rate constants of these various androgen-receptor (A-R) complexes both within cells and after they were extracted from them. In control cells, Arrhenius plots for T-, MT-, DHT- and MB-R complexes were linear and formed a hierarchy of dissociation states with energies of state IV greater than III greater than II, greater than I, respectively. Relative to this hierarchy, the dissociation states of the MB-, DHT- and MT-R complexes in mutant cells were displaced to higher, androgen-inappropriate energies in a mutant-distinctive pattern. When extracted from cells control or mutant T- or MT-R complexes, and mutant (but not control) DHT- or MB-R complexes lowered their respective dissociation rates by undergoing state transitions in conformity with the hierarchy. Hence we propose that different A-R complexes reach different dissociative states by undergoing sequential transitions along a common pathway, and that these transitions are co-regulated both by the chemical characteristics of the bound androgen and by other cellular non-receptor factors.

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)

The non-activated glucocorticoid receptor: structure and activation

Glucocorticoid hormone receptors are present in the soluble fraction of target cell homogenates as large entities (Mr approximately 300,000) that are unable to interact with DNA. These large complexes contain an Mr approximately 94,000 steroid- and DNA-binding polypeptide, in association with an Mr approximately 90,000 non-ligand-binding entity, which has been identified as a heat shock protein, hsp90. This protein has been purified to near homogeneity as a component of the non-activated receptor complex. Characterization of the purified protein revealed its presence as a dimer in the large receptor form. Dissociation of the receptor-hsp90 complex can be induced by heat treatment only when ligand is bound to the receptor, as demonstrated by specific DNA-binding assay and sucrose gradient ultracentrifugation, hsp90 represents ca 1% of total proteins in rat liver cytosol, and milligram amounts were purified using a combination of high performance ion exchange and gel permeation chromatography. Monospecific antibodies were raised in rabbits. They were found to precipitate the intact non-activated glucocorticoid receptor, as well as the Mr approximately 27,000 steroid-binding fragment of the receptor generated by trypsin treatment, indicating that hsp90 interacts with the steroid-binding domain of the glucocorticoid receptor. Finally, translation of glucocorticoid receptor mRNA in reticulocyte lysate yields a protein which also interacts with hsp90 and binds to DNA only after ligand-binding and heat treatment. Thus, the glucocorticoid receptor is synthesized in a non-activated form also in vitro.

Androgen receptors: structures, mutations, antibodies and cellular dynamics

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