Modulation of Glucocorticoid Receptor Phosphorylation and

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Molecular Endocrinology 21(3):625–634 Copyright © 2007 by The Endocrine Society doi: 10.1210/me.2005-0338

Modulation of Glucocorticoid Receptor Phosphorylation and Transcriptional Activity by a C-Terminal-Associated Protein Phosphatase Zhen Wang, Weiwei Chen, Evelyn Kono, Thoa Dang, and Michael J. Garabedian Departments of Microbiology (Z.W., E.K., T.D., M.J.G.), Urology (M.J.G.), and Pharmacology (W.C.), New York University Cancer Institute, New York University School of Medicine, New York, New York 10016 The glucocorticoid receptor (GR) is phosphorylated at three major sites on its N terminus (S203, S211, and S226), and phosphorylation modulates GR-regulatory functions in vivo. We examined the phosphorylation site interdependence, the contribution of the receptor C-terminal ligand-binding domain, and the participation of protein phosphatases in GR N-terminal phosphorylation and gene expression. We found that GR phosphorylation at S203 was greater when S226 was not phosphorylated and vice versa, indicative of intersite dependency. We also observed that a GR derivative lacking the ligand-binding domain, which no longer binds the heat shock protein 90 (Hsp90) complex, exhibits increased GR phosphorylation at all three sites as compared with the full-length receptor. A GR mutation (F602S) that produces a receptor less dependent on Hsp90 for function as well as treatment with the Hsp90 inhibitor geldanamycin also

increased basal GR phosphorylation at a subset of sites. Pharmacological inhibition of serine/threonine protein phosphatases increased GR basal phosphorylation. Likewise, a reduction in protein phosphatase 5 protein levels enhanced GR phosphorylation at a subset of sites and selectively reduced the induction of endogenous GR target genes. Together, our findings suggest that GR undergoes a phosphorylation/dephosphorylation cycle that maintains steady-state receptor phosphorylation at a low basal level in the absence of ligand. Our findings also suggest that the ligand-dependent increase in GR phosphorylation results, in part, from the dissociation of a ligand-binding domain-linked protein phosphatase(s), and that changes in the intracellular concentration of protein phosphatase 5 differentially affect GR target gene expression. (Molecular Endocrinology 21: 625–634, 2007)

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GR is expressed in virtually all tissues, yet has the capacity to regulate genes in a cell-specific manner. Although steroid hormones, such as cortisol, act as the primary signal in activating the receptor’s transcriptional regulatory functions, GR-mediated transcriptional activation is also modulated both positively and negatively by phosphorylation (5). GR is phosphorylated in the absence of hormone, and additional phosphorylation events occur in conjunction with agonist, but not antagonist, binding (6– 8). The hormone-dependent increase in receptor phosphorylation has fueled speculation that phosphorylation may modulate GR transcriptional regulatory functions. Consistent with this notion is the finding that GR is phosphorylated at three major serine sites within the N-terminal region of the receptor involved in transcriptional regulation (S203, S211, and S226 using the human numbering scheme). Serine to alanine mutations of S203 and S211, individually or in combination, decrease GR transcriptional activation in mammalian cells, suggesting that phosphorylation of these residues is required for full GR activity (9–11). In contrast, an alanine substitution for S226 increases GR transcriptional activity relative to the wild-type receptor, suggesting that phosphorylation of S226 is inhibitory to GR function (12, 13). Thus, phosphorylation

HE GLUCOCORTICOID RECEPTOR (GR) is a hormone-dependent transcription factor that induces differentiation, regulates metabolic processes, and suppresses the inflammatory response (1). In the absence of hormone, the heat shock protein 90 (Hsp90)based chaperone complex represses GR-regulatory activities (2). Hormone binding relieves this repression and results in a conformational change in the receptor, which promotes DNA binding as well as an association with regulatory cofactors to enhance or repress the transcription of target genes (3, 4).

First Published Online December 21, 2006 Abbreviations: AF1, Activation function 1; cdk, cyclin-dependent kinase; Dex, dexamethasone; ER, estrogen receptor; GA, geldanamycin; GILZ, glucocorticoid-inducible leucine zipper; GR, glucocorticoid receptor; HA, hemagglutinin; Hsp90, heat shock protein 90; IGFBP-1, IGF-binding protein 1; IRF8, interferon regulatory factor 8; JNK, c-Jun N-terminal kinase; Lad1, ladinin 1; LBD, ligand-binding domain; OA, okadaic acid; PP5, protein phosphatase 5; SDS, sodium dodecyl sulfate; siRNA, small interfering RNA; TBS, Tris-buffered saline. Molecular Endocrinology is published monthly by The Endocrine Society (http://www.endo-society.org), the foremost professional society serving the endocrine community. 625

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appears to provide both positive and negative regulatory inputs with respect to GR transcriptional activation. We have previously identified two cyclin-dependent kinases (Cdks) and c-Jun N-terminal kinase (JNK) as kinases that target the three major N-terminal GR phosphorylation sites in vitro: cyclin E/Cdk2 and cyclin A/Cdk2 phosphorylate S203 and, S203 and S211, respectively; whereas JNK phosphorylates S226 (12– 14). Two sites, S203 and S211, which can be phosphorylated by Cdks in vitro, display increased phosphorylation upon hormone binding in vivo. In addition, mutations of particular Cdk genes in yeast yield phenotypes similar to those conferred by mutation of the sites themselves (14). Mammalian cells lacking p27KIP1 demonstrate a concomitant rise in cyclin/ Cdk2 activity, and increased GR phosphorylation at the Cdk sites, as well as enhanced receptor transcriptional activity, further strengthening the role for Cdks in GR phosphorylation and activity (15). In addition, Miller et al. (11) recently demonstrated that GR S211 also appears to be substrate for p38 MAPK, and that a mutation of this site to alanine reduced GR-mediated transcriptional activation and apoptosis in a human leukemia cell line. This suggests a role for p38 MAPK signaling in glucocorticoid-induced apoptosis of lymphoid cells. GR phosphorylation at S226 in response to JNK activation decreases GR-dependent transcriptional enhancement in the presence of hormone (12), suggesting that phosphorylation at S226 modulates GR activity by either increasing its affinity for a corepressor or decreasing its interaction with a coactivator involved in transcriptional regulation. JNK phosphorylation of GR has also been reported to increase receptor nuclear export under conditions of hormone withdrawal (13). Thus, JNK phosphorylation inhibits receptor activity by at least two distinct mechanisms: in the presence of hormone GR phosphorylation by JNK affects receptor interaction with factors involved in transcriptional activation, whereas in the absence of hormone it enhances receptor nuclear export. These findings strongly suggest that changes in the activity of the kinases that affect GR phosphorylation render the receptor differentially responsive to glucocorticoid treatment. Perturbations in protein phosphatase activity have also been shown to affect GR function. Treatment of cells with okadaic acid (OA), a protein phosphatase inhibitor that targets protein phosphatase 2A and protein phosphatase 1, results in GR hyperphosphorylation, the accumulation of GR in the cytoplasm, and a corresponding reduction in receptor-mediated transcriptional activation in mammalian cells (16, 17). Protein phosphatase 5 (PP5) is part of the GR aporeceptor complex and is associated with GR through Hsp90 via its tetratricopeptide repeat domain (18). PP5 has also been shown to be associated with ligand-free GR in the nucleus (18). Elimination of endogenous PP5 protein through antisense oligonucleotides increased

Wang et al. • PP5 Affects GR Phosphorylation

dexamethasone (Dex)-mediated induction of a GRresponsive mouse mammary tumor virus reporter gene in A549 cells (19). Expression of a dominant negative PP5 derivative in CV-1 cells had the opposite effect and decreased GR reporter gene transcriptional activity (20). More recently, immunosuppressive ligands such as FK506 have been shown to increase the hormone-binding affinity of GR by exchanging FKBP51 for PP5 (21). Thus, the effect of phosphatases on GR signal transduction and transcriptional regulation appear complex and warrant further examination. Using a series of GR phosphorylation site-specific antibodies that recognize GR only when it is phosphorylated at S203, S211, or S226, we studied the phosphorylation site interdependence, and the presence or absence of the GR ligand-binding domain (LBD), Hsp90, and protein phosphatase activity on the phosphorylation of the GR N terminus. Our findings suggest that, in the absence of ligand, GR is being continually phosphorylated and dephosphorylated through LBD-linked protein phosphatases, including PP5, which in turn can modulate GR target gene expression.

RESULTS Interdependence among the Sites of GR Phosphorylation Given that the major GR N-terminal phosphorylation sites are located relatively close to one another (S203, S211, and S226) (Fig. 1A), we examined whether phosphorylation at one site influenced the ability of a neighboring site to be modified. Using a series of GR phosphorylation site-specific antibodies that recognize GR only when it is phosphorylated at S203, S211, or S226, we compared the phosphorylation status of GR from U2OS cells expressing either wild-type GR, or phospho-deficient serine to alanine (S203A, S211A, S226A) mutants. The level of GR is similar among U2OS cells expressing the alanine substitutions, with the exception of the S226A line, which expresses roughly twice as much receptor. As shown in Fig. 1B, the GR S203A substitution showed a considerable enhancement of phosphorylation of S226, with a slight reduction of phosphorylation at S211. The S211A mutation increased S226 phosphorylation but decreased the phosphorylation of S203. The S226A substitution increased S203 phosphorylation and also augmented phosphorylation at S211, albeit to a lesser extent than S203. Our findings suggest that S211 phosphorylation is only slightly influenced by phosphorylation at S203 and S226. In contrast, phosphorylation of S203 and S226 substantially influences the phosphorylation of the other residue in a reciprocal manner: one site has a greater tendency to be phosphorylated when the other is not phosphorylated (Fig. 1C). This suggests that S203 phosphorylation is a gatekeeper for S226 phosphorylation and vice versa.


Fig. 1. Interdependence among the GR Phosphorylation Sites A, Functional domains and major N-terminal phosphorylation sites of hGR. Shown is a schematic representation of hGR with phosphorylation sites corresponding to those in mouse GR (34). B, Interdependence among the GR phosphorylation sites. Whole-cell extracts prepared from U2OS cells stably expressing an HA-tagged hGR (U2OS-hGR), either wild type (WT) or phosphorylation site mutant derivatives S203A, S211A, or S226A untreated or treated with 100 nM Dex for 1 h were analyzed by immunoblotting with phospho-S203 (GR-P-203), phosphoS211 (GR-P-211), phospho-S226 (GR-P-226), HA antibodies as a measure of total GR or Hsp90 antibody as a control for protein loading. C, Quantitative analysis of immunoblot results in panel B normalized to total GR detected by HA and plotted as fold change in GR phosphorylation relative to WT GR. The data shown are from a single experiment that is representative of at least three independent experiments. D, Immunoprecipitation of the phosphorylated forms of GR by phospho-S203 (S203P), phospho-S211, and phospho-S226 antibodies. U2OS-hGR cells were treated with either ethanol or 100 nM Dex for 1 h and whole-cell extracts containing equal amounts of protein were immunoprecipitated with either total GR (GR), phospho-S203 (S203P), phospho-S211 (S211P), or phospho-S226 (S226P) or total GR antibodies. Immunoprecipitates were analyzed by immunoblotting with total GR, phospho-S203, phospho-S211, and phospho-S226 antibodies. IP, Immunoprecipitation.

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It should be noted that the changes in GR phosphorylation appear not to be a result of incomplete receptor solubilization (22). We do not see differences in the phosphorylation of GR between whole-cell lysates and cells fractionated into nuclear and cytoplasmic extracts (data not shown). Also, immunofluorescence studies with the GR phospho-specific antibodies are consistent with the fractionation studies (6) (data not shown). Therefore, the signal we observe from whole-cell lysates using the GR phosphorylation site-specific antibodies is likely an accurate reflection of receptor phosphorylation. The finding that the S203A mutation enhances phosphorylation at the S226, and vice versa, suggests an inverse relationship between phosphorylation at these two sites. To determine whether receptor molecules are phosphorylated at S203 and S226 simultaneously, GR was immunoprecipitated with phosphoS203, phospho-S211, phospho-S226, or total GR from lysates of U2OS-human (h)GR cells that had been cultured with or without Dex for 1 h. The immunoprecipitates were analyzed by immunoblotting with total GR and the GR phospho-specific antisera. As expected, the GR antibody immunoprecipitated an equivalent amount of GR from Dex-treated or untreated cells (Fig. 1D; top panel, lanes 1 and 2). The phospho-S211 antiserum preferentially immunoprecipitated GR from Dex-treated cells and supports the idea that phosphorylation at S211 is largely hormonedependent. The phospho-S203, and phospho-226 antibody also recognized the GR S211 immunoprecipitates, which suggests that GR can be simultaneously phosphorylated on S203 and S211, as well as S211 and S226 (Fig. 1D, lane 6). The phospho-S203 and phopho-226 antibody immunoprecipitated equivalent amounts GR from untreated and hormone-treated and cells (Fig. 1D, top panel, lanes 3 and 4 and 7 and 8). The GR S203 immunoprecipitate showed less phosphorylation at S226 in the absence of hormone relative to GR immunoprecipitated with phospho-S226 (Fig. 1D; bottom panel, lanes 3 and 7). Likewise, the GR 226 immunoprecipitation demonstrated that phosphorylation at S203 is lower relative to the phospho-S203 immunoprecipitate in the absence of Dex treatment (Fig. 1D; second panel, lanes 3 and 7). This is consistent with the immunoblotting results with the GR serine to alanine substitutions in untreated cells, where one site has a greater tendency to be phosphorylated when the other is not. Thus, it appears that within a population of GR in the absence of hormone, some receptors are phosphorylated at S203 or S226, whereas others are phosphorylated on S203 and S226 simultaneously. However, for the wild-type GR in hormone-treated cells, the S203 and S226 reciprocal site interdependency is mitigated. This likely reflects the fact that the GR serine to alanine substitutions represent a homogenously nonphosphorylated pool of GR that amplify the effect of intersite dependency. This suggests that for the wild-type GR, both kinases and phosphatase are involved in tailoring GR phosphory-

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lation to its steady-state level and that the C-terminal LBD may control this process. The GR C Terminus Restricts Receptor Phosphorylation at Its N Terminus To determine the contribution of the GR C terminus to receptor phosphorylation, U2OS cells expressing fulllength GR or a receptor derivative lacking the GR LBD, but containing the activation function 1 (AF1) and DNA-binding domains (GR-N), were analyzed for GR phosphorylation using site-specific antibodies. This derivative has been shown previously to be a constitutively active transcription factor (23). The full-length GR exhibits a low basal phosphorylation at S203, S211, and S226, and phosphorylation of all three sites, particularly S211, increased upon ligand stimulation. Interestingly, GR-N displayed increased phosphorylation as compared with the full-length receptor under basal conditions (Fig. 2; compare lanes 1 and 3). The level of GR-N phosphorylation is comparable to that observed for the ligand-stimulated full-length GR (Fig. 2; compare lanes 2 and 4). This suggests that the GR LBD is playing an inhibitory role with respect to basal

Fig. 2. The GR C Terminus Restricts Receptor Phosphorylation at the N Terminus in the Absence of Hormone A, Schematic of GR C-terminal deletion construct. GR full length (GR) contains the N terminus, the DNA binding domain, and the ligand-binding domain. The GR-N derivative contains the N terminus and the DNA binding domain, but lacks the ligand binding domain. Whole-cell extracts prepared from U2OS expressing GR FL or GR N, untreated or treated with 100 nM Dex for 1 h, were analyzed by immunoblotting with phosphorylation-specific antibodies, (phosphoS203, phospho-S211, phospho-S226, or total GR, from top to bottom panel) or N499 as a measure of total GR. The basal phosphorylation of GR FL is lower than GR N (compare lane 1 to lane 3), whereas GR-N is similar to ligand-dependent GR phosphorylation of GR FL (compare lane 2 with lane 4). DBD, DNA-binding domain.


receptor phosphorylation and implies a functional interaction between the GR C terminus and N-terminal receptor phosphorylation. Given that a GR derivative lacking the LBD no longer binds Hsp90, we next examined the contribution of Hsp90-based chaperone complex in GR N-terminal phosphorylation. This complex contains Hsp90, Hsc70, p60/HOP, p23, FKBP52 as well as the protein phosphatase PP5 and binds GR through the LBD (18, 20). Treatment with the Hsp90 inhibitor geldanamycin (GA) or introduction of the F602S mutation that reduced the GR dependence on Hsp90 for function (24) results in an increase in basal GR phosphorylation, particularly at S203 and S226 (Fig. 3). Our results suggest that the GR LBD is reducing receptor basal phosphorylation at these sites in an Hsp90-dependent manner, perhaps via an associated protein phosphatase. Serine/Threonine Protein Phosphatases Negatively Regulate GR Basal Phosphorylation We next examined basal GR phosphorylation in the presence of protein phosphatase inhibitors, okadaic acid (OA) and calyculin A. OA inhibits the activity of PP2A and, to a lesser extent, PP1 and PP2B (25), whereas calyculin A targets PP1, PP2A, PP2B, and PP5 and is a more potent inhibitor than OA (26, 27). We found an increase in the basal GR phosphorylation of S211 and S226 in U2OS-hGR cells treated with OA in a dose-dependent manner (Fig. 4A). Likewise, the basal phosphorylation of GR S211 and S226 increased to levels similar to that observed in the presence of ligand with calyculin A treatment (Fig. 4B). The mobility of GR is also affected by calyculin A treatment with a slower migrating species evident upon treatment with this potent phosphatase inhibitor. This likely reflects additional GR sites that are being modified. The kinetics of basal GR phosphorylation in response to calyculin A is rapid and reaches near hormonedependent levels within 30 min of treatment. Under basal conditions, calyculin A has no effect on GR steady-state levels; however, upon hormone treatment, a reduction in total GR levels is observed, suggesting that GR hyperphosphorylation signals liganddependent receptor degradation. Taken together, our results demonstrate that inhibition of protein phosphatase activity increases basal GR phosphorylation. PP5 Negatively Regulates GR Phosphorylation and Differentially Affects GR Target Gene Response PP5 is a tetratricopeptide repeat domain protein that binds to Hsp90 and has been found to be part of the GR-Hsp90 aporeceptor complex (18, 20). Moreover, calyculin A inhibits the activity of PP5 (27). To clarify the role of PP5 in GR phosphorylation, we employed small interfering RNA (siRNA) to reduce the expression of PP5 in U2OS-hGR cells, and examined full-length GR and GR-N phosphorylation. The level of PP5 pro-


Fig. 3. Perturbation of the Hsp90 Activity Affects GR NTerminal Phosphorylation A, GR F602S, a mutation that is less dependent upon Hsp90 for function, affects GR phosphorylation. Whole-cell extracts prepared from U2OS cells expressing an HA-tagged hGR (U2OS-hGR), either wild type (WT) or GR F602S untreated or treated with 100 nM Dex for 1 h, were analyzed by immunoblotting with phospho-S203 (GR-P-203), phosphoS211 (GR-P-211), phospho-S226 (GR-P-226), or HA antibodies as a measure of total GR. B, The Hsp90 inhibitor GA augments basal GR phosphorylation. Whole-cell extracts prepared from U2OS-hGR cells treated with GA for the times indicated (in hours) in the absence of Dex were analyzed by immunoblotting with GR phospho-specific or total GR antibodies as indicated, as well as with an antibody against Hsp90. C, Quantitative analysis of immunoblot results in panel B normalized to total GR using NIH Image. The data shown is the average of three independent experiments, and the error bar represents SD.

tein was reduced in cells treated with the specific PP5 siRNA compared with control cells treated with a nonspecific siRNA duplex (Fig. 5A). We also showed by coimmunoprecipitation that a reduced amount of PP5 is associated with GR when PP5 levels are reduced by siRNA (Fig. 5B). Increased basal phosphorylation of full-length GR on S203 and S226 is observed when PP5 levels are reduced, whereas basal phosphoryla-


Fig. 4. Serine/Threonine Protein Phosphatase Inhibitors Affect GR Basal and Ligand-Dependent Phosphorylation A, GR basal phosphorylation upon OA treatment. Wholecell extracts prepared from U2OS-hGR cells treated with increasing concentrations of OA (lane 1, 0 nM; lane 2, 100 nM; lane 3, 500 nM; lane 4, 1000 nM) for 2 h in the absence of Dex were analyzed by immunoblotting with GR phospho-specific or total GR antibodies as indicated. B, GR phosphorylation upon calyculin A treatment. U2OS-hGR cells treated with 100 nM calyculin A for 30 min in the absence or presence of 100 nM Dex for 1 h and blotted as above. C, Time course of GR phosphorylation of calyculin A-treated cells. Immunoblot images of GR phosphorylated at S203, S211, and S226 in the first hour of calyculin A treatment in the absence (⫺) or presence of Dex. Cal, Calyculin; DMSO, dimethylsulfoxide.

tion of S211 increases only slightly (Fig. 5A, top three panels; compare lanes 1 and 3). A small increase in ligand-dependent phosphorylation at S203, S211, and S226 was also observed with the full-length GR (Fig. 5A; compare lanes 2 and 4). By contrast, GR phosphorylation of the GR-N derivative was largely unaffected by a reduction in PP5 protein. Therefore, PP5 appears to negatively regulate basal phosphorylation of S203 and S226 and, to a lesser extent, hormonedependent GR phosphorylation at S203, S211, and S226 in an LBD-dependent manner. To understand the contribution of PP5 to GR genespecific regulation in vivo, the level of PP5 was again reduced using siRNAs in U2OS-hGR and the mRNA level of four endogenous primary GR target genes, interferon regulatory factor 8 (IRF8), ladinin 1 (Lad1), IGF-binding protein 1 (IGFBP-1), and glucocorticoidinducible leucine zipper (GILZ) were examined by realtime PCR after Dex treatment (29). Interestingly, these target genes responded differently to the reduction of



PP5 levels. Hormone-dependent mRNA expression of Lad1, IRF8, and IGFBP-1 was decreased by 70%, 50%, and 35%, respectively, when the level of PP5 was reduced (Fig. 5C). In contrast, the induction of GILZ was largely unaffected by the reduction in PP5 levels. Therefore, PP5 appears to play a distinct role in GR hormone-dependent gene activation, most likely by modulating receptor and/or cofactor phosphorylation and activity.

DISCUSSION

Fig. 5. Effects on GR Phosphorylation and Gene Expression by Depletion of Endogenous PP5 Protein A, Reduced PP5 protein expression increases GR phosphorylation. U2OS-HA-hGR cells were transfected with either full-length GR (GR FL) or GR-N expression constructs, along with 100 pmol of a specific PP5 siRNA duplex or nonspecific luciferase siRNA (luc) duplex as a control. whole-cell extracts were prepared and analyzed by immunoblotting 48 h after the transfection, with GR phospho-specific, total GR, PP5, or PP2Ac antibodies as indicated. B, Depletion of PP5 reduces its association with GR. U2OS-HA-hGR cells transfected with nonspecific luciferase siRNA (luc) or siRNA directed against PP5 (PP5) and immunoprecipitated with HA under nondenaturing conditions from whole-cell extracts and associated proteins were resolved by SDS-PAGE. PP5 associated with GR was detected by immunoblotting with a mouse monoclonal antibody against PP5 and polyclonal antibody that recognizes total GR. The left panel shows the expression of PP5 and GR before immunoprecipitation (input), and the right panel reveals that PP5 was immunoprecipitated (IP) with GR in the control, but not PP5 siRNA-treated cells. NS and HC denote nonspecific and heavy chain bands, respectively. C, Depletion of endogenous PP5 protein selectively affects endogenous GR target gene expression. U2OS-hGR cells were transfected with 100 pmol of a specific PP5 siRNA duplex or nonspecific luciferase siRNA (luc) duplex as a control. Cells were maintained in stripped serum for 48 h and then treated

In this study, we examined the phosphorylation site interdependence of hGR in the absence and presence of Dex. Our experiments demonstrate an unexpected relationship among the GR phosphorylation sites. Whereas the phosphorylation of S211 appears largely independent of the status of phosphorylation at either S203 or S226, the lack of phosphorylation at S203 augments phosphorylation at the S226 site and vice versa. This might reflect a conformational change induced by phosphorylation at one site that renders the other a poor substrate for a kinase. Alternatively, phosphorylation at one site could result in the recruitment of a phosphatase that prevents hyperphosphorylation at the other site. These findings suggest an inverse relationship between the phosphorylation at S203and S226 to influence receptor activity. Such inverse intersite dependences may reflect interactions among signaling cascades, leading to the opposing phosphorylation of the GR protein. Indeed, S203 is a substrate for CDK phosphorylation, whereas S226 is a target for JNK. In addition, GR S023P does not accumulate in the nucleus, but is localized to the perinuclear region, whereas S226P is nuclear upon ligand binding. It is tempting to speculate that such reciprocal regulation of S203 and S226 phosphorylation could differentially modulate nuclear and nonnuclear GR signaling depending on the stimulus. Our experiments also show that the C terminus of GR restricts receptor N-terminal phosphorylation. A GR derivative lacking the C-terminal LBD exhibited increased GR phosphorylation as compared with the full-length receptor under basal conditions. This suggests that the C terminus of GR is not needed for kinase recruitment, but is required for phosphatase contact and tailoring of the receptor N-terminal phosphorylation.

with the ethanol vehicle or Dex (100 nM) for 2 h. Total RNA was extracted, and mRNA expression was quantified by realtime PCR with primer pairs specific for each gene. The data were transformed using deltCC method with RPL19 as internal control. Gene expression was then normalized to control siRNA-treated sample. Data were averaged from three independent experiments and the error bar represents SD.


Pioneering work from the DeFranco laboratory (16) demonstrated that inhibition of protein phosphatase activity by OA affected GR nuclear retention and transcriptional activation. Suppression of PP5 activity using antisense oligonucleotides has also been shown to increase the association of GR with its cognate DNAbinding sequence and increased basal and hormonedependent GR activity of a synthetic reporter gene (19). These findings are consistent with the effect of OA treatment on GR transcriptional activity (17). Moreover, GR nuclear accumulation in the absence of glucocorticoids was observed when the activity of PP5 was suppressed in A549 lung epithelial cells (28), further suggesting that PP5 might be involved in nuclearcytoplasmic shuttling via dephosphorylation of GR. However, our findings suggest that a reduction in PP5 levels by siRNA decreases GR transcriptional induction at a subset of endogenous GR target genes in U2OS-hGR cells. Consistent with our findings are those from Chen et al. (29), which demonstrated that inhibition of endogenous PP5 binding to receptor using a tetratricopeptide repeat domain from PP5 as a dominant negative, inhibited GR transactivation. These differences may reflect cell type or methodological differences used to reduce phosphatase activity. Our findings from the limited set of endogenous target genes examined indicate that PP5 plays a positive role in GR transcriptional activation, suggesting that enhanced GR phosphorylation as a consequence of reduced PP5 levels at S226, for example, is inhibitory. From the elegant work of Rogatsky et al. (30), we know that the GR target genes Lad1, IRF8, and IGFBP-1 are sensitive to a mutation in GR AF1, the region harboring the major GR N-terminal phosphorylation sites. These same genes are also affected by a reduction of PP5 protein. In contrast, GILZ, a GR target gene that is independent of AF1 and therefore would be predicted to be unaffected by changes in GR N-terminal phosphorylation, is not sensitive to reduced PP5 expression. Thus, changes in PP5 levels selectively influence gene activity most likely through alterations in GR and/or cofactor phosphorylation. Interestingly, it has recently been demonstrated that GR expression in the rat brain is observed in both neurons and glial cells, whereas PP5 expression appears restricted to neurons (31). This suggests that phosphatase action occurs in distinct populations of cells within the central nervous system to potentially affect GR responses. It will be interesting to examine whether GR phosphorylation is elevated in glial cells that lack PP5, as compared with neurons that express this phosphatase. PP5 has also been shown to interact with the estrogen receptor (ER)␣ LBD (32). Although the authors suggested that the interaction between ER and PP5 is direct, it is likely that the interaction is mediated via Hsp90, because the binding assays were performed in cells or in lysates that contain abundant Hsp90. Growth factor- and estrogen-induced phosphorylation of ER at serine 118 in MCF7 cells was reduced by PP5


overexpression and enhanced by a reduction in PP5 protein level. The same relationship holds for ER transcriptional activation: when PP5 is overexpressed, activation of the ER target genes c-myc, cyclin D1, and pS2 is reduced, whereas decreased PP5 increases ER transactivation, suggesting that PP5 is inhibitory to ER transcriptional activation. Although the reduction of PP5 would be expected to increase cellular proliferation through enhanced c-myc, cyclin D1 expression, Urban et al. found, to the contrary, that a reduction of PP5 eliminated estrogen-dependent growth in MCF7 cells (33). Thus, the physiological consequences of PP5 reduction with respect to cellular proliferation appear complex. Nevertheless, changes in the activity of PP5 render both GR and ER differentially responsive to ligand treatment. From our findings we present a model whereby GR is synthesized and constitutively phosphorylated at S203, S211, and S226 (Fig. 6). The receptor is then rapidly dephosphorylated as a function of an LBDassociated protein phosphatase(s), thereby reducing GR N-terminal phosphorylation. PP5 is likely responsible for the dephosphorylation of S203 and S226, because reduced PP5 expression enhances phosphorylation at S203 and S226, but not S211. Given that pharmacological inhibition of phosphatase activity and a GR LBD deletion mutant show increased phosphorylation all three sites, we speculate that an additional LBD-linked protein phosphatase, such as PP1 or PP2A (16), may be responsible for dephosphorylating S211, although this remains to be tested. Upon ligand binding, the phosphatase dissociates and receptor phosphorylation increases, which has selective effects on GR target gene induction. Thus, modulating phosphatase level or activity may represent a mechanism of coordinating GR ligand binding and receptor N-terminal phosphorylation with GR genespecific transcriptional responses.

MATERIALS AND METHODS Cell Culture and Generation of Stable Cell Lines Human osteosarcoma cell line U2OS (HTB96) was obtained from the American Type Culture Collection (Manassas, VA) and cultured in DMEM (Cellgro, Herndon, VA), supplemented with 10% fetal bovine serum (HyClone Laboratories, Inc., Logan, UT), 2 mM L-glutamine, 50 ␮g/ml penicillin, and 50 ␮g/ml streptomycin (Cellgro). Generation of stable cell lines ectopically expressing wild-type hGR (WT) and phosphorylation site mutants, S203A, S211A, and S226A with a hemagglutinin (HA)-epitope at their amino termini were performed as previously described (6). Clones homogeneously expressing HA-tagged hGR were verified by indirect immunofluorescence and immunoblotting with HA- and GR-specific antibodies and were maintained with 500 ␮g/ml G418 (Geneticin; Invitrogen, Carlsbad, CA). Site-Directed Mutagenesis Site mutations of hGR S203A, S211A, S226A, and F602S were performed using the QuikChange Site-Directed Mu-

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synthesized by Dharmacon, Inc. (Lafayette, CO). siRNA (100 pmol) was transfected into U2OS-hGR cells by using LipofectAMINE and Transfection Plus (Invitrogen, CA). The siRNA for luciferase was used as a negative control. Cells were cultured for 2 d before Dex treatment. For the GR and PP5 coimmunoprecipitation, a 10-cm dish of U2OS-hGR cells approximately 50% confluent were transfected twice with the siRNA duplexes against PP5 or luciferase control using Oligofectamine (Invitrogen) according to the manufacturer’s instructions. Cells were placed on ice 20 h after the second transfection, washed twice in PBS, and lysed in 0.5 ml of buffer described in “Preparation of cell extracts.” Extracts containing 1 mg of protein in 500 ␮l were used for each immunoprecipitation. An antibody against HA (10 ␮g; Roche Applied Science, Indianapolis, IN) was added and the samples rocked at 4 C for 16 h. Protein G agarose equilibrated in lysis buffer (50 ␮l slurry) was added, and the samples were rocked for an additional 1 h. The agarose beads were collected by centrifugation for 2 min at 3000 rpm and washed five times in 500 ␮l of lysis buffer. After the last wash the samples were spun for 4 min at 3000 rpm, and the residual lysis buffer was removed. The beads were resuspended in 25 ␮l 2⫻ sodium dodecyl sulfate (SDS)-sample buffer, boiled for 5 min, and pelleted, and a portion of the supernatant was loaded onto a 10% SDS-polyacrylamide gel. Preparation of Cell Extracts

Fig. 6. Regulation of Basal GR Phosphorylation by LBDAssociated Protein Phosphatases A model whereby protein phosphatases associate with the GR LBD and restrict N-terminal phosphorylation. A, In the absence of ligand, newly synthesized GR is phosphorylated at all three sites (S203, S211, S226) and then is rapidly dephosphorylated by protein phosphatase(s) associated with the LBD, including PP5 in complex with Hsp90. B, Ligand binding releases the LBD-associated protein phosphatase(s), resulting in hyperphosphorylation of the GR N terminus. C, The GR N-terminal derivative is also phosphorylated after synthesis but is not subject to dephosphorylation because it lacks the LBD and associated protein phosphatases. DBD, DNA-binding domain. tagenesis kit (Stratagene, La Jolla, CA) according to the manufacturer’s recommendations. All mutations were confirmed by sequencing. Transient Transfection and siRNA Knock-Down Assays U2OS cells were seeded onto 6-cm plates at a density of 2 ⫻ 105 ml in phenol red-free DMEM supplemented with 10% charcoal-stripped fetal bovine serum and 2 mM L-glutamine. Transfection of pCMV-HA-hGR and phosphorylation site mutants, pCMV-rat GR and rat GR N-terminal derivative (GR-N) contain the first 525 amino acids of the rat GR including AF1 and the DNA binding domains, using the LipofectAMINE and Transfection Plus reagent (Invitrogen) were performed according to manufacturer’s recommendation. An empty vector was used as a negative control for the transfection assays. Cells were allowed to recover for 18 h and treated with 100 nM Dex or ethanol (vehicle control) for 1 h. The siRNA for PP5 was directed against the sequence of 5⬘-TATTCGAGCTCAACGGTTT-3⬘. The siRNA duplex was

Extracts for immunoblotting were prepared from a subconfluent 10-cm plate of U2OS-hGR cells treated with 100 nM Dex or equal volume of the ethanol vehicle for 1 h, or treated with 1 ␮M of the Hsp90 inhibitor GA (Sigma Chemical Co., St. Louis, MO), phosphatase inhibitor calyculin A (100 nM for 30 min) (Cell Signaling Technology, Beverly, MA), and okadaic acid (1, 500, 1000 nM for 60 min) (OA) (Sigma). After treatment with these drugs, cells were placed on ice, washed twice with PBS, and lysed in 200 ␮l of lysis buffer containing 50 mM HEPES (pH 7.5), 150 mM NaCl, 1 mM EDTA, 1 mM EGTA, 1 mM NaF, 1% Triton X-100, 10% glycerol, and additional protease and phosphatase inhibitors: 1 mM PMSF, 20 mM ␤-glycerophosphate, 8 mM sodium pyrophosphate, 1 ␮g/ml leupeptin, 1 ␮g/ml pepstatin A, and 1 ␮g/ml aprotinin (Roche). Lysates were centrifuged at 12,000 rpm at 4 C for 15min. The soluble supernatant was normalized for total protein using the BioRad protein assay (Bio-Rad Laboratories, Hercules, CA), and the samples were boiled for 3 min in 2⫻ SDS sample buffer and stored at ⫺20 C. Immunoblotting Cell extracts were separated by 10% SDS-PAGE and transferred to Immobilon paper (Millipore Corp., Bedford, MA) at 110 V for 80 min in Tris-Glycine transfer buffer. The membranes were blocked overnight with 5% BSA in Tris-buffered saline (TBS, pH 7.4) at 4 C and incubated in the blocking buffer with primary antibody at room temperature (RT) for 2–4 h (using 1:1,000 dilutions for phospho-GR antibodies S211 and S226, N499 polyclonal antibody for total hGR and rat GR, and 1:10,000 dilution for phospho-GR antibody S203). The membranes were washed three times for 10 min in TBS/0.1% Triton X-100 and twice in TBS and incubated for 1 h at room temperature with 0.2 ␮g/ml protein A conjugated to horseradish peroxidase (Kirkegaard & Perry Laboratories, Gaithersburg, MD). For blots using monoclonal antibodies (antiHA, from Roche), anti-hsp90 (catalog no. H38220–50; BD Transduction Laboratories. Lexington, KY); anti-PP2Ac (catalog no. 611020; BD Transduction Laboratories), anti-PP5 (catalog no. 610555; BD Transduction Laboratories), horseradish peroxidase-conjugated goat-antimouse IgG was used as the secondary antibody. Blots were then washed three times for 10 min in TBS/0.1% Triton X-100 and twice in TBS


and developed using enhanced chemiluminescence reagents according to manufacturer’s instructions. Quantitative analysis of immunoblots was performed using the NIH Image software package (version 1.63). Real Time PCR Total RNA from U2OS-hGR cells was isolated using Rneasy Mini kits (QIAGEN, Chatsworth, CA). cDNA was synthesized using the Enhanced Avian Reverse Transcriptase (Sigma) and random primer hexamers (Pharmacia Biotech, Piscataway, NJ) following the manufacturer’s instructions. Gene-specific cDNA was amplified in a 20 ␮l reaction containing SYBR Green Taq ReadyMix for Quantitative PCR (Sigma) and 300 nM of each primer. Primer pairs used for IGFBP-1 were 5⬘CCAAGGCACAGGAGACATCAG-3⬘, and 5⬘-AGGGTAGACGCACCAGCAGAGT-3⬘; for IRF8 were 5⬘-CGCACTCCATCTCTGT-3⬘, and 5⬘-GAAGACGAGGTTACGCT-3⬘. Primers used to amplify GILZ, LAD1, and RPL19 have been described elsewhere (32). Real-time PCR was performed using a LightCycler (Roche) under the following conditions: 94 C for 30 sec, 45 cycles of 5 sec at 94 C, 5 sec at 55 C for IRF8, 59 C for GLIZ and IGFBP-1, or 61 C for Lad1 and Rpl19, 10 sec at 72 C, and 2 sec at 76 C for reading. A melting curve of products was performed from 63 C to 94 C with 0.2 C per reading to ensure that a single amplicon was obtained. The expression level of GR target genes was normalized to RPL19 expression.

Acknowledgments We thank Dr. K. Yamamoto for the GR antibody N499. We are also grateful to Drs. I Rogatsky and M.-J. Lee for critically reading the manuscript and J. Fredrick for technical assistance.

Received August 22, 2005. Accepted December 12, 2006. Address all correspondence and requests for reprints to: Michael J. Garabedian, Departments of Microbiology, Urology, New York University Cancer Institute, New York University School of Medicine, 550 First Avenue, New York, New York 10016. E-mail: [email protected]. This work is supported by grants from the National Institutes of Health (DK54836) and American Cancer Society (to M.J.G.). Disclosure Statement: The authors have nothing to disclose.

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