Using yeast to study glucocorticoid receptor phosphorylation

Intense inhibitory avoidance training increases nuclear-phosphorylated glucocorticoid receptors in neurons of CA1 of hippocampus and ventral caudate putamen

Corticosterone (CORT), the principal glucocorticoid in rodents, is released after stressful experiences such as training with high foot-shock intensities in the inhibitory avoidance task (IA). CORT reaches the glucocorticoid receptor (GR) located in almost all brain cells; the GR is subsequently phosphorylated at serine 232 (pGRser232). This has been reported as an indicator of ligand-dependent activation of the GR, as well as a requirement for its translocation into the nucleus for its transcription factor activity. The GR is present in the hippocampus with a high concentration in CA1 and dentate gyrus (DG), and a smaller proportion in CA3, and sparsely present in the caudate putamen (CPu); both structures are involved in memory consolidation of IA. To study the participation of CORT in IA, we quantified the ratio of pGR-positive neurons in both dorsal hippocampus (CA1, CA3 and DG) and dorsal and ventral regions of CPu of rats trained in IA, using different foot-shock intensities. Brains were dissected 60 min after training for immunodetection of pGRser232 positive cells. The results show that the groups trained with 1.0 and 2.0 mA had higher retention latencies than the 0.0 mA or 0.5 mA groups. An increase in the ratio of pGR-positive neurons was found in CA1 and ventral region of CPu only for the 2.0 mA trained group. These findings suggest that activation of GRs in CA1 and ventral CPu is involved in the consolidation of a stronger memory of IA, possibly through the modulation of gene expression.

Site-specific and dose-dependent effects of glucocorticoid receptor phosphorylation in yeast Saccharomyces cerevisiae

The glucocorticoid receptor (GR) signal transduction and transcriptional regulation are efficiently recapitulated when GR is expressed in Saccharomyces cerevisiae. In this report we demonstrate that the in vivo GR phosphorylation pattern, hormone dependency and interdependency of phosphorylation events were similar in yeast and mammalian cells. GR phosphorylation at S246 exhibited inhibitory effect on S224 and S232 phosphorylation, suggesting the conservation of molecular mechanisms that control this interdependence between yeast and mammalian cells. To assess the effects of GR phosphorylation the mutated GR derivatives T171A, S224A, S232A, S246A were overexpressed and their transcriptional activity was analysed. These receptor derivatives displayed significant hormone inducible transcription when overexpressed in S. cerevisiae. We have established an inducible methionine expression system, which allows the close regulation of the receptor protein levels to analyse the dependence of GR function on its phosphorylation and protein abundance. Using this system we observed that GR S246A mutation increased its activity across all of the GR concentrations tested. The activity of the S224A and S246A mutants was mostly independent of GR protein levels, whereas the WT, T171A and S232A mediated transcription diminished with declining GR protein levels. Our results suggest that GR phosphorylation at specific residues affects its transcriptional functions in a site selective manner and these effects were directly linked to GR dosage.

Western blot analysis of glucocorticoid receptor phosphoisoforms by one- and two-dimensional electrophoretic assays

The glucocorticoid receptor (GR) protein is a cytosolic ligand-dependent transcription factor with numerous functions regulated by post-translational modifications, including phosphorylation/dephosphorylation. Among the functions most extensively affected by GR phosphorylation are the modulation of its transcriptional activity, alterations in its interaction pattern with cofactors, nuclear translocation and selective gene transactivation. Intensive analysis of the intracellular distribution of GR phosphoisoforms and their interaction with proteins of other cellular signalling networks required the use of [?-32P]ATP as a phosphate donor, and special laboratory protection measures to avoid external irradiation and contamination. In the present study, simple and easy-to-use non-radioactive protein mobility shift assays (NMS assays) were developed using one- and/or two-dimensional gel electrophoresis based on differences in the pI and molecular mass of GR phosphoisoforms. The GR isoforms were immunodetected with specific monoclonal or polyclonal anti-GR antibodies by Western blot in three diverse systems, namely yeast BJ2168 cells expressing wild-type rat GR, rat hepatoma GRH2 cells grown in culture and brain tissue from Wistar rat experimental animals. The results obtained using the NMS assay were similar to previous results obtained with the [?-32P] ATP standard assay.

Specific activation of the glucocorticoid receptor and modulation of signal transduction pathways in human lens epithelial cells

PURPOSE

Prolonged use of glucocorticoids (GCs) can lead to cataract formation. Lens GC responses have been difficult to elucidate. A previous study showed the presence of the glucocorticoid receptor (GR) in immortalized and primary human lens epithelial cells (hLECs) and GC-induced changes in gene expression. This study demonstrates specific GR activation and identifies the biological effect of GC-induced changes in gene expression in hLECs.

METHODS

HLE B-3 (B-3) and primary cultures of hLECs were transfected with pGRE.Luc and treated with or without dexamethasone (Dex), RU-486, spironolactone, or vehicle. mRNA and protein expression were examined by real-time PCR and Western blot analysis, respectively. Cell proliferation and apoptosis were examined by WST-1 and flow cytometry, respectively.

RESULTS

Dex treatment of B-3 and primary cultures demonstrated specific GR, but not mineralocorticoid receptor (MR), activation and phosphorylation. Pathway analysis revealed GC-induced changes in expression of MAPK regulators. Increased expression of GILZ mRNA and MKP-1 mRNA and protein was observed in immortalized and donor hLECs. This corresponded with a decrease in the phosphorylated forms of RAF, ERK, p38, and AKT, but not in JNK. No net change in LEC proliferation or apoptosis was observed with Dex treatment.

CONCLUSIONS

GC treatment of hLECs activates the GR to modulate the expression of MAPK and PI3K/AKT regulators. This is the first demonstration of GC signaling in hLECs. GCs, MAPK, and PI3K/AKT are involved in cell processes implicated in steroid-induced cataractogenesis. The absence of a net change in cell activity with acute steroid treatment is consistent with the possibility that chronic treatment leads to prolonged modulation of these pathways and steroid-induced cataract.

Stabilization of the unliganded glucocorticoid receptor by TSG101

The glucocorticoid receptor (GR) has been shown to undergo hormone-dependent down-regulation via transcriptional, post-transcriptional, and posttranslational mechanisms. However, the mechanisms involved in modulating GR levels in the absence of hormone remain enigmatic. Here we demonstrate that TSG101, a previously identified GR-interacting protein, stabilizes the hypophosphorylated form of GR in the absence of ligand. We found that a non-phosphorylated version of GR (S203A/S211A) showed enhanced interaction with TSG101 as compared with the wild type GR, suggesting that TSG101 interacts more favorably with GR when it is not phosphorylated. A significant accumulation of GR S203A/S211A protein is detected in the absence of ligand when TSG101 is overexpressed, whereas no increase in the wild type phosphorylated GR or phosphomimetic GR S203E/S211E was observed in mammalian cells. In contrast, down-regulation of TSG101 expression by siRNA renders the hypophosphorylated form of GR unstable. We further show that TSG101 stabilizes GR by impeding its degradation by the proteasome and extending receptor half-life. Thus, in absence of a ligand, TSG101 binds GR and protects the non-phosphorylated receptor from degradation.

Modulation of glucocorticoid receptor function via phosphorylation

The glucocorticoid receptor (GR) is phosphorylated at multiple serine residues in a hormone-dependent manner. It has been suggested that GR phosphorylation affects turnover, subcellular trafficking, or the transcriptional regulatory functions of the receptor, yet the contribution of individual GR phosphorylation sites to the modulation of GR activity remains enigmatic. This review critically evaluates the literature on GR phosphorylation and presents more recent work on the mechanism of GR phosphorylation from studies using antibodies that recognize GR only when it is phosphorylated. In addition, we present support for the notion that GR phosphorylation modifies protein-protein interactions, which can stabilize the hypophosphorylated form of the receptor in the absence of ligand, as well as facilitate transcriptional activation by the hyperphosphorylation of GR via cofactor recruitment upon ligand binding. Finally, we propose that GR phosphorylation also participates in the nongenomic activation of cytoplasmic signaling pathways evoked by GR. Thus, GR phosphorylation is a versatile mechanism for modulating and integrating multiple receptor functions.

Mutation of serines 104, 106, and 118 inhibits dimerization of the human estrogen receptor in yeast

Ligand-dependent dimerization and phosphorylation participate in regulating transcriptional activation of the estrogen receptor-alpha (ER). We investigated the role of serines 104, 106, and 118 located in the activation function-1 (AF-1) domain of ER in ligand-induced receptor dimerization. These serines, previously documented as important sites for transactivation, were mutated to alanine, and yeast genetic systems were used to determine their effect on receptor dimerization and transcriptional activity. The serine to alanine mutants resulted in 50-80% decreased dimerization in response to 17beta-estradiol, while having modest effects on ER-mediated transactivation. We further demonstrated that ER expressed in yeast became hyperphosphorylated in the presence of estradiol, most likely at a site(s) different than the serines under investigation. Ligand-induced phosphorylation was inhibited by U0126 indicating that the ER was phosphorylated via the MAPK pathway. Taken together, these data indicate that serines 104, 106, and 118 are important for ligand-dependent ER dimerization, and that MAP kinase mediated phosphorylation may be important for ER function, in yeast model systems.

Cell-specific regulation of apoptosis by glucocorticoids: implication to their anti-inflammatory action

Glucocorticoids play a major role in attenuation of the inflammatory response. These steroid hormones are able to induce apoptosis in cells of the hematopoietic system such as monocytes, macrophages, and T lymphocytes that are involved in the inflammation reaction. In contrast, it was discovered recently that in glandular cells such as the mammary gland epithelia, hepatocytes, ovarian follicular cells, and in fibroblasts glucocorticoids protect against apoptotic signals evoked by cytokines, cAMP, tumor suppressors, and death genes. The anti-apoptotic effect of glucocorticoids is exerted by modulation of several survival genes such as Bcl-2, Bcl-x(L), and NFkB, in a cell-specific manner. Moreover, upregulation or downregulation of the same gene product can occur in a cell-dependent manner following stimulation by glucocorticoids. This phenomenon is probably due to composite regulatory cross-talk among multiple nuclear coactivators or corepressors, which mediate the transcription regulation of the genes, by their interaction with the glucocorticoid receptor. These observations suggest that the anti-inflammatory action of glucocorticoids is exerted by two complementary mechanisms: on one hand, they induce death of the cells that provoke the inflammation, and on the other hand they protect the resident cells of the inflamed tissue by arresting apoptotic signals. Moreover, the complementary action of glucocorticoids provides a new insight to the therapeutic potential of these hormones.

Deciphering the phosphorylation "code" of the glucocorticoid receptor in vivo

The glucocorticoid receptor (GR) is phosphorylated at multiple serine residues in a hormone-dependent manner, yet progress on elucidating the function of GR phosphorylation has been hindered by the lack of a simple assay to detect receptor phosphorylation in vivo. We have produced antibodies that specifically recognize phosphorylation sites within human GR at Ser(203) and Ser(211). In the absence of hormone, the level of GR phosphorylation at Ser(211) was low compared with phosphorylation at Ser(203). Phosphorylation of both residues increased upon treatment with the GR agonist dexamethasone. Using a battery of agonists and antagonists, we found that the transcriptional activity of GR correlated with the amount of phosphorylation at Ser(211), suggesting that Ser(211) phosphorylation is a biomarker for activated GR in vivo. Mechanistically, the kinetics of Ser(203) and Ser(211) phosphorylation in response to hormone differed, with Ser(211) displaying a more robust and sustained phosphorylation relative to Ser(203). Analysis of GR immunoprecipitates with phospho-GR-specific antibodies indicated that the receptor was phosphorylated heterogeneously at Ser(203) in the absence of hormone, whereas in the presence of hormone, a subpopulation of receptors was phosphorylated at both Ser(203) and Ser(211). Interestingly, biochemical fractionation studies following hormone treatment indicated that the Ser(203)-phosphorylated form of the receptor was predominantly cytoplasmic, whereas Ser(211)-phosphorylated GR was found in the nucleus. Likewise, by immunofluorescence, Ser(203)-phosphorylated GR was located in the cytoplasm and perinuclear regions of the cell, but not in the nucleoplasm, whereas strong phospho-Ser(211) staining was evident in the nucleoplasm of hormone-treated cells. Our results suggest that differentially phosphorylated receptor species are located in unique subcellular compartments, likely modulating distinct aspects of receptor function.

Mammalian transcription factors in yeast: strangers in a familiar land

Many transcription factors in human cells have functional orthologues in yeast, and a common experimental theme has been to define the function of the yeast protein and then test whether the mammalian version behaves similarly. Although, at first glance, this approach does not seem feasible for factors that do not have yeast counterparts, mammalian transcriptional activators or repressors can be expressed directly in yeast. Often, the mammalian factor retains function in yeast, and this allows investigators to exploit the experimental tractability of yeast to ask a diverse set of questions.

Reduced activity of the NPR-A kinase triggers dephosphorylation and homologous desensitization of the receptor

NPR-A, the receptor for the atrial natriuretic peptide (ANP), is a 130-kDa protein presenting an extracellular ANP-binding domain, a single transmembrane domain, an intracellular regulatory kinase homology domain (KHD), and a guanylyl cyclase catalytic domain. Upon stimulation, NPR-A receptors are activated to produce cyclic guanosine monophosphate (cGMP) and are subsequently desensitized through dephosphorylation of residues at their KHD. We used wild-type rat (r) NPR-A (WT) and a disulfide-bridged mutant (C423S) expressed in human embryonic kidney (HEK) 293 cells to study receptor phosphorylation. We have previously characterized the C423S receptor as constitutively active and desensitized. At basal state, 32P incorporation in the rNPR-A(C423S) covalent dimer is about 24 times less efficient than incorporation in the WT rNPR-A. When membranes from WT and rNPR-A(C423S) are incubated with [35S]ATPgammaS, the mutant dimer receptor displays 3.5% of the thiophosphate incorporation found for WT rNPR-A. Since the rNPR-A(C423S) dimer is already extensively dephosphorylated, we then used the WT rNPR-A to study dephosphorylation. As previously documented, adding ANP globally induces time-dependent dephosphorylation of the receptor. However, in pulse-chase experiments with the WT rNPR-A, adding ANP during the chase does not lead to a significant effect on receptor dephosphorylation. On the other hand, thiophosphorylation of the WT rNPR-A previously desensitized with ANP is reduced to 8.3% of the incorporation for untreated receptor, similar to results found with the rNPR-A(C423S) at basal state. These results demonstrate that ANP-induced rNPR-A desensitization is modulated by a significant reduction in the activity or affinity of the rNPR-A kinase that contributes to the low phosphorylation level after induction. Moreover, we further document a close relationship between tight dimerization, dephosphorylation, and desensitization.

Localization of the Glucocorticoid Receptor in Rat Brain Mitochondria

The distribution of glucocorticoid receptor in subcellular fractions of brain cortex and hippocampus, two regions rich in glucocorticoid receptor, has revealed its presence in nuclei, cytosol, mitochondria, synaptosomes, and synaptosomal mitochondria. The identification of glucocorticoid receptor has been accomplished both by Western blotting using antibodies recognizing the carboxy and the amino terminus of the glucocorticoid receptor and by immunogold electron microscopy using the same anti-glucocorticoid receptor antibodies. Antibody-glucocorticoid receptor interaction is abolished by preincubation of each antibody with its competing peptide. In addition to the intact 95-kDa glucocorticoid receptor in all fractions, lower molecular weight glucocorticoid receptor fragments have been also detected by Western blotting. The presence of glucocorticoid receptor in brain mitochondria supports the concept of a direct action of glucocorticoids on mitochondrial gene transcription, parallel to the established primary actions of the hormones on nuclear gene transcription, as a mechanism of coordinate regulation of respiratory enzyme biosynthesis by steroid hormones.