Differences in nuclear retention characteristics of agonist-activated glucocorticoid receptor may determine specific responses

Down-regulation of GRα expression and inhibition of its nuclear translocation by hypoxia

AIMS

Glucocorticoids are the most effective anti-inflammatory agent in treating pulmonary diseases typically accompanied by hypoxia. Our previous study has demonstrated that glucocorticoid receptor α (GRα) expression is reduced in hypoxia but the underlying mechanism remains elusive. In this study we aim to identify the signaling pathway involved in hypoxia-induced down-regulation of GRα, and whether hypoxia affects nuclear translocation of GRα.

MAIN METHODS

Female C57BL/6 mice were sensitized with saline or ovalbumin (OVA) as the in vivo model. Mice were divided into control and OVA groups, and their lung histology and the expression of hypoxia inducible factor (HIF-1) and GRα were examined. A549 cells were exposed to chemical hypoxia as the in vitro model, where mitogen-activated protein kinases (MAPKs) were inhibited specifically by SB203580. Next, under normal or hypoxic conditions, the expression of GRα, MAPKs and HIF-1 signal protein were determined by Western blot analysis, and GRα translocation were observed through live-cell imaging.

KEY FINDINGS

In OVA challenged mice the expression of GRα was down-regulated whereas HIF-1 was up-regulated. Hypoxia caused a time-dependent decrease of GRα expression, and activated multiple signaling pathways including MAPKs and HIF-1. Moreover, GRα expression increased with MAPK inhibition. Interestingly, only MAPK inhibitor SB203580, but not JNK inhibitor SP600125 or ERK inhibitor U0126 improved the expression of GRα under hypoxic condition. GRα nuclear translocation was also significantly inhibited by hypoxia.

SIGNIFICANCE

Hypoxia down-regulated the expression of GRα through p38 signaling pathway, as well as inhibited GRα nuclear translocation significantly.

Glucocorticoid receptor concentration and the ability to dimerize influence nuclear translocation and distribution

Glucocorticoid receptor (GR) concentrations and the ability of the GR to dimerize are factors which influence sensitivity to glucocorticoids. Upon glucocorticoid binding, the GR is actively transported into the nucleus, a crucial step in determining GR function. We examined the effects of GR concentration and the ability to dimerize on GR nuclear import, export and nuclear distribution using both live cell microscopy of GFP-tagged GR and immunofluorescence of untagged GR, with both wild type GR (GRwt) and dimerization deficient GR (GRdim). We found that the observed rate of GR nuclear import increases significantly at higher GR concentrations, at saturating concentrations of dexamethasone (10(-6) M) using GFP-tagged GR, while with untagged GR it is only discernable at sub-saturating ligand concentrations (10(-10)-10(-9) M). Loss of dimerization results in a slower observed rate of nuclear import (2.5- to 3.3-fold decrease for GFP-GRdim) as well as a decreased extent of GR nuclear localization (18-27% decrease for untagged GRdim). These results were linked to an increased rate of GR export at low GR concentrations (1.4- to 1.6-fold increase for untagged GR) and where GR dimerization is abrogated (1.5- to 1.7-fold increase for GFP-GRdim). Furthermore, GR dimerization was shown to be required for the appearance of discrete GC-dependent GR nuclear foci, the loss of which may explain the increased rate of GR export for the GRdim. The reduction in the observed rate of nuclear import and increased rate of nuclear export displayed at low GR concentrations and by the GRdim could explain the lowered glucocorticoid response under these conditions.

Role of molecular chaperones and TPR-domain proteins in the cytoplasmic transport of steroid receptors and their passage through the nuclear pore

In the absence of hormone, corticosteroid receptors such as GR (glucocorticoid receptor) and (mineralocorticoid receptor) are primarily located in the cytoplasm. Upon steroid-binding, they rapidly accumulate in the nucleus. Regardless of their primary location, these receptors and many other nuclear factors undergo a constant and dynamic nucleocytoplasmic shuttling. All members of the steroid receptor family are known to form large oligomeric structures with the heat-shock proteins of 90-kDa (hsp90) and 70-kDa (hsp70), the small acidic protein p23, and a tetratricopeptide repeat (TPR) -domain protein such as FK506-binding proteins (FKBPs), cyclophilins (CyPs) or the serine/threonine protein phosphatase 5 (PP5). It has always been stated that the dissociation of the chaperone heterocomplex (a process normally referred to as receptor "transformation") is the first step that permits the nuclear import of steroid receptors. However the experimental evidence is consistent with a model where the chaperone machinery is required for the retrotransport of the receptor through the cytoplasm and also facilitates the passage through the nuclear pore. Recent evidence indicates that the hsp90-based chaperone system also interacts with structures of the nuclear pore such as importin β and the integral nuclear pore glycoprotein Nup62 facilitating the passage of the untransformed receptor through the nuclear pore.

Differential nuclear localisation and promoter occupancy play a role in glucocorticoid receptor ligand-specific transcriptional responses

The glucocorticoid receptor (GR) is a ligand-activated transcription factor for which a number of endogenous and synthetic ligands exist. A key question in steroid receptor biology is how different ligands elicit different maximal transcriptional responses via the same receptor and on the same promoter. This question was addressed quantitatively for the GR, using a panel of agonists, partial agonists and antagonists, on the endogenous GILZ gene in two different human cell lines. It was found that the extent of GR nuclear localization correlated with the efficacy for GILZ transactivation by the GR in U2OS cells. However, in A549 cells there was no significant correlation, with all ligands resulting in similar levels of GR nuclear localization, despite different levels of transcriptional activation of the GILZ gene. Chromatin immunoprecipitation analysis on the other hand, revealed ligand-specific differences in GILZ promoter occupancy in the A549 cells, which correlated with the transcriptional efficacy of the subset of ligands investigated. This suggests that ligand-specific differences in promoter occupancy by activated GR play a major role in discrimination between agonist, partial agonist and antagonist responses on the endogenous GILZ gene in A549 cells, while differences in nuclear localisation of liganded GR play a role in determining the transcriptional outcome in U2OS cells. These cell line-specific differences were not dependent on the amount of GR present, since transient overexpression of GR in U2OS did not alter the relative ligand-selective nuclear localisation. Our results show that there is a relationship between ligand-specific transactivation efficacy, extent of nuclear translocation and recruitment of GR to the promoter. However, the relative contribution of nuclear translocation and GR promoter recruitment to ligand-specific transactivation efficacy is cell-specific.

Management of cytoskeleton architecture by molecular chaperones and immunophilins

Cytoskeletal structure is continually remodeled to accommodate normal cell growth and to respond to pathophysiological cues. As a consequence, several cytoskeleton-interacting proteins become involved in a variety of cellular processes such as cell growth and division, cell movement, vesicle transportation, cellular organelle location and function, localization and distribution of membrane receptors, and cell-cell communication. Molecular chaperones and immunophilins are counted among the most important proteins that interact closely with the cytoskeleton network, in particular with microtubules and microtubule-associated factors. In several situations, heat-shock proteins and immunophilins work together as a functionally active heterocomplex, although both types of proteins also show independent actions. In circumstances where homeostasis is affected by environmental stresses or due to genetic alterations, chaperone proteins help to stabilize the system. Molecular chaperones facilitate the assembly, disassembly and/or folding/refolding of cytoskeletal proteins, so they prevent aberrant protein aggregation. Nonetheless, the roles of heat-shock proteins and immunophilins are not only limited to solve abnormal situations, but they also have an active participation during the normal differentiation process of the cell and are key factors for many structural and functional rearrangements during this course of action. Cytoskeleton modifications leading to altered localization of nuclear factors may result in loss- or gain-of-function of such factors, which affects the cell cycle and cell development. Therefore, cytoskeletal components are attractive therapeutic targets, particularly microtubules, to prevent pathological situations such as rapidly dividing tumor cells or to favor the process of cell differentiation in other cases. In this review we will address some classical and novel aspects of key regulatory functions of heat-shock proteins and immunophilins as housekeeping factors of the cytoskeletal network.

Visualization by BiFC of different C/EBPβ dimers and their interaction with HP1α reveals a differential subnuclear distribution of complexes in living cells

How the co-ordinated events of gene activation and silencing during cellular differentiation are influenced by spatial organization of the cell nucleus is still poorly understood. Little is known about the molecular mechanisms controlling subnuclear distribution of transcription factors, and their interplay with nuclear proteins that shape chromatin structure. Here we show that C/EBPβ not only associates with pericentromeric heterochromatin but also interacts with the nucleoskeleton upon induction of adipocyte differentiation of 3T3-L1 cells. Different C/EBPβ dimers localize in different nuclear domains. Using BiFC in living cells, we show that LAP (Liver Activating Protein) homodimers localize in euchromatin and heterochromatin. In contrast, LIP (Liver Inhibitory Protein) homodimers localize exclusively in heterochromatin. Importantly, their differential subnuclear distribution mirrors the site for interaction with HP1α. HP1α inhibits LAP transcriptional capacity and occupies the promoter of the C/EBPβ-dependent gene c/ebpα in 3T3-L1 preadipocytes. When adipogenesis is induced, HP1α binding decreases from c/ebpα promoter, allowing transcription. Thus, the equilibrium among different pools of C/EBPβ associated with chromatin or nucleoskeleton, and dynamic changes in their interaction with HP1α, play key roles in the regulation of C/EBP target genes during adipogenesis.

The hsp90-FKBP52 complex links the mineralocorticoid receptor to motor proteins and persists bound to the receptor in early nuclear events

In this study, we demonstrate that the subcellular localization of the mineralocorticoid receptor (MR) is regulated by tetratricopeptide domain (TPR) proteins. The high-molecular-weight immunophilin (IMM) FKBP52 links the MR-hsp90 complex to dynein/dynactin motors favoring the cytoplasmic transport of MR to the nucleus. Replacement of this hsp90-binding IMM by FKBP51 or the TPR peptide favored the cytoplasmic localization of MR. The complete movement machinery, including dynein and tubulin, could be recovered from paclitaxel/GTP-stabilized cytosol and was fully reassembled on stripped MR immune pellets. The whole MR-hsp90-based heterocomplex was transiently recovered in the soluble fraction of the nucleus after 10 min of incubation with aldosterone. Moreover, cross-linked MR-hsp90 heterocomplexes accumulated in the nucleus in a hormone-dependent manner, demonstrating that the heterocomplex can pass undissociated through the nuclear pore. On the other hand, a peptide that comprises the DNA-binding domain of MR impaired the nuclear export of MR, suggesting the involvement of this domain in the process. This study represents the first report describing the entire molecular system that commands MR nucleocytoplasmic trafficking and proposes that the MR-hsp90-TPR protein heterocomplex is dissociated in the nucleus rather than in the cytoplasm.

Nuclear import of the glucocorticoid receptor-hsp90 complex through the nuclear pore complex is mediated by its interaction with Nup62 and importin beta

Glucocorticoid receptor (GR) is cytoplasmic in the absence of ligand and localizes to the nucleus after steroid binding. Previous evidence demonstrated that the hsp90-based heterocomplex bound to GR is required for the efficient retrotransport of the receptor to the nuclear compartment. We examined the putative association of GR and its associated chaperone heterocomplex with structures of the nuclear pore. We found that importin beta and the integral nuclear pore glycoprotein Nup62 interact with hsp90, hsp70, p23, and the TPR domain proteins FKBP52 and PP5. Nup62 and GR were able to interact in a more efficient manner when chaperoned by the hsp90-based heterocomplex. Interestingly, the binding of hsp70 and p23 to Nup62 does not require the presence of hsp90, whereas the association of FKBP52 and PP5 is hsp90 dependent, as indicated by the results of experiments where the hsp90 function was disrupted with radicicol. The ability of both FKBP52 and PP5 to interact with Nup62 was abrogated in cells overexpressing the TPR peptide. Importantly, GR cross-linked to the hsp90 heterocomplex was able to translocate to the nucleus in digitonin-permeabilized cells treated with steroid, suggesting that GR could pass through the pore in its untransformed state.

Mechanisms involved in tissue-specific apopotosis regulated by glucocorticoids

Physiological cell turnover is under the control of a sharp and dynamic balance of different homeostatic mechanisms such as the equilibrium between cell proliferation and cell death. These mechanisms play an important role in maintaining normal tissue function and architecture. It is well known that apoptosis is the prevalent mode of physiological cell loss in most tissues. Steroid hormones like glucocorticoids have been identified as key signals controlling cell turnover by modulating programmed cell death in a tissue- and cell-specific manner. In this sense, several reports have demonstrated that glucocorticoids are able to induce apoptosis in cells of the hematopoietic system such as monocytes, macrophages, and T lymphocytes. In contrast, they protect against apoptotic signals evoked by cytokines, cAMP, tumor suppressors, in glandular cells such as the mammary gland epithelia, endometrium, hepatocytes, ovarian follicular cells, and fibroblasts. Although several studies have provided significant information on hormone-dependent apoptosis in an specific tissue, a clearly defined pathway that mediates cell death in response to glucocorticoids in different cell types is still misunderstood. The scope of this review is held to those mechanisms by which glucocorticoids control apoptosis, emphasizing tissue-specific expression of genes that are involved in the apoptotic pathway.

Glucocorticoids Repress bcl-X Expression in Lymphoid Cells by Recruiting STAT5B to the P4 Promoter

The bcl-X gene plays a critical role in apoptosis. Six different isoforms generated by tissue-specific promoter usage and alternative splicing were described. Some of them exert opposite effects on cell death. In mammary epithelial cells glucocorticoids induce bcl-X expression and increase the ratio bcl-X(L) (antiapoptotic)/bcl-X(S) (apoptotic) by activating P4 promoter, which contains two hormone response elements. Here we show that, on mouse thymocytes and T lymphocyte derivative S49 cells, glucocorticoids inhibited transcription from P4 and decreased the ratio bcl-X(L)/bcl-X(S) favoring apoptosis. Upon hormonal treatment, glucocorticoid receptor (GR), steroid receptor coactivator-1, and RNA polymerase II were transiently recruited to P4 promoter, whereas STAT5B was also recruited but remained bound. Concomitant with the release of GR, silencing mediator for retinoic acid receptor and thyroid hormone receptor and histone deacetylase 3 were recruited, histone H3 was deacetylated, and RNA polymerase II left the promoter. Inhibition of STAT5 activity reverted glucocorticoid repression to activation of transcription and was accompanied by stable recruitment of GR and RNA polymerase II to P4.

Different steroids co-regulate long-term expansion versus terminal differentiation in primary human erythroid progenitors

Outgrowth, long-term self-renewal, and terminal maturation of human erythroid progenitors derived from umbilical cord blood in serum-free medium can be modulated by steroid hormones. Homogeneous erythroid cultures, as characterized by flow cytometry and dependence on a specific mixture of physiologic proliferation factors, were obtained within 8 days from a starting population of mature and immature mononuclear cells. Due to previous results in mouse and chicken erythroblasts, the proliferation-promoting effect of glucocorticoids was not unexpected. Surprisingly, however, androgen had a positive effect on the sustained expansion of human female but not male erythroid progenitors. Under optimal conditions, sustained proliferation of erythroid progenitors resulted in a more than 10(9)-fold expansion within 60 days. Terminal erythroid maturation was significantly improved by adding human serum and thyroid hormone (3,5,3'-triiodothyronine [T3]) to the differentiation medium. This resulted in highly synchronous differentiation of the cells toward enucleated erythrocytes within 6 days, accompanied by massive size decrease and hemoglobin accumulation to levels comparable to those in peripheral blood erythrocytes. Thus, obviously, different ligand-activated nuclear hormone receptors massively influence the decision between self-renewal and terminal maturation in the human erythroid compartment.

Activation of the ligand-mineralocorticoid receptor functional unit by ancient, classical, and novel ligands. Structure-activity relationship

The mineralocorticoid effect on epithelial cells is the resultant of an intricate net of biochemical regulations that ultimately leads to the maintenance of electrolyte homeostasis. Two key protagonists in this plot are the ligand, which broadcasts the information, and the receptor, which functions as a receiver and transducer. Therefore, the responsibility for the final biological effect is not limited to each individual component but to both of them, so they constitute a functional unit. In addition, several prereceptor regulatory mechanisms are also determinant factors for the final biological response. Because steroids are present in both animals and plants and are derived from common precursors, it is intriguing how these simple molecules have acquired specialization to shape biological development and differentiation. This is particularly true for the function of aldosterone in mammals, which is mimicked by glucocorticoids or progesterone in some particular cases. Inasmuch as the most potent mineralocorticoid in nature, aldosterone, shows a poorly angled steroid nucleus at the A?B-ring junction, and because steroids that possess identical functional groups and different steroidal frames elicit different mineralocorticoid effects, we postulate that a planar conformation of the ligand is a key requirement to acquire potent sodium retention properties. The model takes into consideration all the mechanisms involved in the regulation of the final biological effect, although it does not provide a definitive answer to the original question. It is also discussed how the use of novel mineralocorticoid ligands may shed light on the still obscure mechanism of action of the mineralocorticoid receptor.

Molecular mechanism of activation and nuclear translocation of the mineralocorticoid receptor upon binding of pregnanesteroids

The mineralocorticoid receptor (MR) is primarily localized in the cytoplasm of the cell in the absence of ligand. The first step in the genomic-dependent mechanism of action of mineralocorticoids is the binding of steroid to the MR, which in turn triggers MR nuclear translocation. The regulation of hormone-binding to MR is complex and involves a multifactorial mechanism, making it difficult to determine the optimal structure of a steroid for activating the MR and promoting its nuclear translocation. Here we review the structure-activity relationship for several pregnanesteroids that possess various functional groups, and suggest that a flat conformation of the ligand rather than the presence of particular chemical groups is a critical parameter for the final biological effect in vivo. We also discuss how the MR undergoes differential conformational changes according to the nature of the bound ligand, which in turn affects the dynein-dependent retrograde rate of movement for the steroid/receptor complex.

Involvement of Bax protein in the prevention of glucocorticoid-induced thymocytes apoptosis by melatonin

The antiapoptotic effect of melatonin has been described in several systems. In this study, the antagonistic effect of the methoxyindole on dexamethasone-induced apoptosis in mouse thymocytes was examined. Melatonin decreased both DNA fragmentation, and the number of annexin V-positive cells incubated in the presence of dexamethasone. Analysis of the expression of the members of the Bcl-2 family indicated that the synthetic glucocorticoid increased Bax protein levels without affecting the levels of Bcl-2, Bcl-XL, Bcl-XS, or Bak. This effect correlated with an increase in thymocytes bax mRNA levels. Dexamethasone also increased the release of cytochrome C from mitochondria. All of these effects were reduced in the presence of melatonin, which was ineffective per se on these parameters. In addition, the involvement of cAMP on glucocorticoid/melatonin antagonism was examined. Both melatonin and dexamethasone decreased the levels of this nucleotide in mouse thymocytes, indicating that the antagonistic action between both hormones involves a cAMP-independent pathway. In summary, the present results suggest that the antiapoptotic effect of melatonin on glucocorticoid-treated thymocytes would be a consequence of an inhibition of the mitochondrial pathway, presumably through the regulation of Bax protein levels.

Steroid hormones induce bcl-X gene expression through direct activation of distal promoter P4

Bcl-X exists in at least five different isoforms with complex effects on programmed cell death. Glucocorticoids and progestins control bcl-X expression and influence the ratio between bcl-X(L) (antiapoptotic isoform) and bcl-X(S) (proapoptotic isoform) in different tissues. The 5'-UTR region of the mouse bcl-X gene contains at least five different promoters, which exhibit a tissue-specific pattern of promoter usage. Several mRNAs with different 5'-leading exons can be generated upon promoter activation. Here we explore the potential of the various bcl-X gene promoters to be regulated by glucocorticoids or progestins. We found that the region located immediately upstream of promoter 4 (P4) contains two hormone response element (HRE)-like sequences at positions -3040 (HRE I) and -3001 (HRE II) relative to the translation initiation codon. These HRE-like sequences confer hormone responsiveness to a core promoter and bind glucocorticoid or progesterone receptors in vitro. Point mutations of both HREs that prevent steroid receptor binding also eliminate hormonal inducibility. In cells treated with glucocorticoids, the hormone receptor is recruited to the P4 region containing the HREs. Analysis of the products of the endogenous bcl-X in epithelial mammary cells showed that only transcripts originating from P4 increased upon hormone treatment. This observation correlates with the induction of the bcl-X(L) mRNA, suggesting that P4 is one of the bcl-X promoters responsible for the generation of this antiapoptotic isoform.

Subnuclear localization of C/EBP beta is regulated by growth hormone and dependent on MAPK

Localization of transcription regulatory proteins in the nucleus is dynamically regulated, and may alter nucleoplasmic concentrations and/or assembly of multimolecular transcription regulatory complexes, which ultimately regulate gene expression. Since growth hormone (GH) regulates multiple transcription factors including C/EBP beta, the effect of GH on the subcellular localization of C/EBP beta was examined in 3T3-F442A preadipocytes. Indirect immunofluorescence shows that C/EBP beta is diffusely distributed in nuclei of quiescent cells. Within 5 min of GH treatment, the diffuse pattern dramatically becomes punctate. The relocalization of C/EBP beta coincides with DAPI staining of heterochromatin. Further, C/EBP beta and heterochromatin protein (HP)-1 alpha colocalize in the nucleus, consistent with localization of C/EBP beta to pericentromeric heterochromatin. In contrast, C/EBP delta exhibits a diffuse distribution in the nucleus that is not modified by GH treatment. C/EBP beta is rapidly and transiently phosphorylated on a conserved MAPK consensus site in response to GH (Piwien-Pilipuk, G., MacDougald, O. A., and Schwartz, J. (2002) J. Biol. Chem. 277, 44557-44565). Indirect immunofluorescence using antibodies specific for C/EBP beta phosphorylated on the conserved MAPK site shows that GH also rapidly induces a punctate pattern of staining for the phosphorylated C/EBP beta. In addition, phosphorylated C/EBP beta colocalizes to pericentromeric heterochromatin. The satellite DNA present in heterochromatin contains multiple C/EBP binding sites. DNA binding analysis shows that C/EBP beta, C/EBP delta, and C/EBP alpha (p42 and p30 forms) can bind to satellite DNA as homo- or heterocomplexes in vitro. Importantly, GH rapidly and transiently increases binding of endogenous C/EBP beta from 3T3-F442A cells to satellite DNA. Further, the GH-promoted nuclear relocalization of C/EBP beta to pericentromeric heterochromatin was prevented by the MEK inhibitor U0126. This observation suggests that GH-dependent MAPK activation plays a role in the regulation of nuclear relocalization of C/EBP beta. Nuclear redistribution introduces a new level of transcriptional regulation in GH action, since GH-mediated phosphorylation and nuclear redistribution of C/EBP beta may be coordinated to achieve spatial-temporal control of gene expression.

The interplay between the glucocorticoid receptor and nuclear factor-kappaB or activator protein-1: molecular mechanisms for gene repression

The inflammatory response is a highly regulated physiological process that is critically important for homeostasis. A precise physiological control of inflammation allows a timely reaction to invading pathogens or to other insults without causing overreaction liable to damage the host. The cellular signaling pathways identified as important regulators of inflammation are the signal transduction cascades mediated by the nuclear factor-kappaB and the activator protein-1, which can both be modulated by glucocorticoids. Their use in the clinic includes treatment of rheumatoid arthritis, asthma, allograft rejection, and allergic skin diseases. Although glucocorticoids have been widely used since the late 1940s, the molecular mechanisms responsible for their antiinflammatory activity are still under investigation. The various molecular pathways proposed so far are discussed in more detail.