Glucocorticoid ligands specify different interactions with NF-kappaB by allosteric effects on the glucocorticoid receptor DNA binding domain

Glucocorticoids inhibit inflammation by acting through the glucocorticoid receptor (GR) and powerfully repressing NF-kappaB function. Ligand binding to the C-terminal of GR promotes the nuclear translocation of the receptor and binding to NF-kappaB through the GR DNA binding domain. We sought how ligand recognition influences the interaction between NF-kappaB and GR. Both dexamethasone (agonist) and RU486 (antagonist) promote efficient nuclear translocation, and we show occupancy of the same intranuclear compartment as NF-kappaB with both ligands. However, unlike dexamethasone, RU486 had negligible activity to inhibit NF-kappaB transactivation. This failure may stem from altered co-factor recruitment or altered interaction with NF-kappaB. Using both glutathione S-transferase pull-down and bioluminescence resonance energy transfer approaches, we identified a major glucocorticoid ligand effect on interaction between the GR and the p65 component of NF-kappaB, with RU486 inhibiting recruitment compared with dexamethasone. Using the bioluminescence resonance energy transfer assay, we found that RU486 efficiently recruited NCoR to the GR, unlike dexamethasone, which recruited SRC1. Therefore, RU486 promotes differential protein recruitment to both the C-terminal and DNA binding domain of the receptor. Importantly, using chromatin immunoprecipitation, we show that impaired interaction between GR and p65 with RU486 leads to reduced recruitment of the GR to the NF-kappaB-responsive region of the interleukin-8 promoter, again in contrast to dexamethasone that significantly increased GR binding. We demonstrate that ligand-induced conformation of the GR C-terminal has profound effects on the functional surface generated by the DNA binding domain of the GR. This has implications for understanding ligand-dependent interdomain communication.     

Differential control of glucocorticoid receptor hormone-binding function by tetratricopeptide repeat (TPR) proteins and the immunosuppressive ligand FK506

Many laboratories have documented the existence of tetratricopeptide repeat (TPR) proteins (also known as immunophilins) in hormone-free steroid receptor complexes. Yet, the distinct roles of these proteins in steroid receptor action are poorly understood. In this work, we have investigated the effects of four TPR proteins (FKBP52, FKBP51, Cyp40, and PP5) on hormone-binding function of glucocorticoid receptor (GR) endogenously expressed in mammalian L929 cells. As a first step, we treated L929 cells with select immunophilin ligands [FK506, rapamycin, cyclosporin A (CsA), and cyclosporin H (CsH)], which are commonly thought to increase the GR response to hormone by inhibiting membrane-based steroid exporters. As expected, all four immunophilin ligands increased both the intracellular concentration of dexamethasone and GR activity at the MMTV-CAT reporter. To determine whether these ligands could target GR function independent of steroid export mechanisms, we performed GR reporter gene assays under conditions of immunophilin ligand and dexamethasone treatment that yielded equal intracellular hormone concentrations. FK506 was found to stimulate GR transactivity beyond the effect of this ligand on hormone retention. In contrast, CsA only affected the GR through upregulation of hormone retention. By Scatchard analysis, FK506 was found to increase GR hormone-binding affinity while decreasing total binding sites for hormone. This result correlated with loss of GR-associated FKBP51 and replacement with PP5. Interestingly, no GR-associated Cyp40 was found in these cells, consistent with the ability of CsA ligand to only affect GR through the hormone export mechanism. To test the role of FKBP52 independent of FK506, FKBP52 was placed under the control of a tetracycline-inducible promoter. Upregulation of FKBP52 caused an increase in both GR hormone-binding affinity and transactivity, even in the absence of FK506. These results show that immunosuppressive ligands can alter GR hormone-binding function by changing the TPR protein composition of receptor complexes and that TPR proteins exert a hierarchical effect on this GR function in the following order: FKBP52 > PP5 > FKBP51.     

Decreased glucocorticoid receptor activity following glucocorticoid receptor antisense RNA gene fragment transfection.

Depression is often characterized by increased cortisol secretion caused by hyperactivity of the hypothalamic-pituitary-adrenal axis and by nonsuppression of cortisol secretion following dexamethasone administration. This hyperactivity of the hypothalamic-pituitary-adrenal axis could result from a reduced glucocorticoid receptor (GR) activity in neurons involved in its control. To investigate the effect of reduced neuronal GR levels, we have blocked cellular GR mRNA processing and/or translation by introduction of a complementary GR antisense RNA strand. Two cell lines were transfected with a reporter plasmid carrying the chloramphenicol acetyltransferase (CAT) gene under control of the mouse mammary tumor virus long terminal repeat (a glucocorticoid-inducible promoter). This gene construction permitted assay of the sensitivity of the cells to glucocorticoid hormones. Cells were also cotransfected with a plasmid containing 1,815 bp of GR cDNA inserted in the reverse orientation downstream from either a neurofilament gene promoter element or the Rous sarcoma virus promoter element. Northern (RNA) blot analysis demonstrated formation of GR antisense RNA strands. Measurement of the sensitivity of CAT activity to exogeneous dexamethasone showed that although dexamethasone increased CAT activity by as much as 13-fold in control incubations, expression of GR antisense RNA caused a 2- to 4-fold decrease in the CAT response to dexamethasone. Stable transfectants bearing the GR antisense gene fragment construction demonstrated a 50 to 70% decrease of functional GR levels compared with normal cells, as evidenced by a ligand-binding assay with the type II glucocorticoid receptor-specific ligand [3H]RU 28362. These results validate the use of antisense RNA to GR to decrease cellular response to glucocorticoids.     

Mechanisms of glucocorticoid function in human leukemic cells: analysis of receptor gene mutants of the activation-labile type using the covalent affinity ligand dexamethasone mesylate

In the cultured acute lymphoblastic leukemic (ALL) cell line, clones of sensitive cells are killed by receptor-occupying concentrations of glucocorticoids. In addition, several types of resistance have been identified. The types of resistance are r- (glucocorticoid binding site loss), ract/l (activation labile receptors) and r+ly- (defective lysis mechanism). The two types of receptor mutants have been examined for the presence and expression of the glucocorticoid receptor (GR) gene. Southern blot analysis, using a full-length cDNA probe for human GR, shows that the gene in both is grossly intact. Examination of the expression of the gene by Northern blots reveals the presence of normal, 7-kb message in both types of receptor mutants, though in amounts somewhat reduced from wild-type. This report focuses on the activation labile mutants. Since characterization of these mutants suggests that they can bind ligand but not retain it during activation, we hypothesized that they would respond normally to a ligand that could not be lost during activation. This seems to be the case. When the covalent affinity ligand dexamethasone mesylate, itself a partial glucocorticoid agonist/antagonist, is used, the ract/l cells are killed to an extent corresponding to that evoked by a sub-optimal concentration of the full agonist dexamethasone. We conclude: (1) that the ract/l receptors can function to kill cells if provided a ligand that they do not lose during activation; (2) that the partial agonist activity of dexamethasone mesylate for cell killing is not due to release of a small amount of free dexamethasone; (3) that the poor agonist activity of dexamethasone mesylate receptor complexes suggests that the role of steroid is strictly to participate in conversion of the receptor to its DNA binding form, after which presence of the steroid actually interferes with proper receptor action.     

The glucocorticoid agonist activities of mifepristone (RU486) and progesterone are dependent on glucocorticoid receptor levels but not on EC50 values

Mifepristone is an antagonist of the glucocorticoid receptor (GR) that also has significant agonist activity in some cell types. We examined the partial agonist activity of mifepristone in COS-7 cells transfected with increasing amounts of a glucocorticoid receptor expression vector pmGR. As pmGR levels increased, the response of the reporter, pMTVCAT to dexamethasone increased, consistent with increasing levels of receptor expression; the response to mifepristone also increased but at a higher rate, resulting in increasing mifepristone agonist and decreasing antagonist activity. In contrast, increasing pMTVCAT levels increased CAT activity induced by both dexamethasone and mifepristone, but did not change the relative agonist activity of mifepristone. We also examined the relationship between agonist activity and receptor level in a series of clones of the E8.2.A3 cell line expressing various levels of GR. Again, the relative agonist activity of mifepristone increased as GR increased. This increase was not due to changes in the dose response curves to these two ligands since their EC50 values were independent of receptor levels. These results indicate that the degree of glucocorticoid agonist activity exhibited by mifepristone is dependent on the concentration of GR in the cell. Similar results were obtained with another partial agonist of the GR, progesterone, whereas the complete antagonist ZK98.299 had no agonist activity under any condition. Taken together, these results suggest that the phenomenon of receptor concentration-dependence is a property of partial GR agonists in general.     

Differential regulation by dexamethasone of glucocorticoid receptor messenger RNA concentrations in neuronal cultures derived from fetal rat hypothalamus and cerebral cortex

1. Differential regulation, by dexamethasone, of glucocorticoid receptor gene expression was studied in three different neuronal cultures derived from hypothalamus amygdala, and cerebral cortex. 2. Cellular glucocorticoid receptor (GR) mRNA concentration was measured by hybridization using a 32P-labeled RNA probe complementary to a 2.2-kb fragment of the glucocorticoid receptor mRNA. Changes in the amount of GR mRNA were evaluated in relation to the content of beta-actin mRNA. 3. In cells derived from either hypothalamus or cerebral cortex, we observed a complex pattern of GR mRNA concentrations which were characterized by cyclic variations of GR mRNA content during continuous treatment with dexamethasone for up to 72 hr. 4. In contrast to cells derived from the hypothalamus where a persistent 30-40% reduction in GR mRNA levels was seen for up to a least 72 hr, we observed, in cells derived from the cerebral cortex, a sustained increased (1.4-fold) of the GR mRNA at this same time interval.     

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

We studied the glucocorticoid response to the synthetic steroid pregna-1,4-diene-11beta-ol-3,20-dione (DeltaHOP) in several cell types and correlated its biological effect with the ability of the glucocorticoid receptor (GR) to be retained in the nuclear compartment. We observed that the DeltaHOP-transformed GR was diffusely distributed in the nucleus compared to the discrete structures observed for the dexamethasone (DEX)-transformed GR. Despite the fact that the receptor was entirely nuclear upon binding of each steroid and exhibited identical nuclear export rates, a greater amount of DeltaHOP-transformed GR was recovered in the cytoplasmic fraction after hypotonic cell lysis. Furthermore, accelerated nuclear export of GR was evidenced in digitonin-permeabilized cells treated with ATP and molybdate. Inasmuch as limited trypsinization of DEX-GR and DeltaHOP-GR complexes yielded different proteolytic products, we conclude that GR undergoes a differential conformational change upon binding of each ligand. We propose that these conformational differences may consequently lead to changes of stability in the interaction of the GR with chromatin. Therefore, the dynamic exchange of liganded GR with chromatin is likely to have significant consequences for the observed pleiotropic physiological responses triggered by glucocorticoid ligands, not only in different tissues but also in the same cell type.     

Molecular determinants of glucocorticoid receptor mobility in living cells: the importance of ligand affinity

The actions of glucocorticoids are mediated by the glucocorticoid receptor (GR), which is activated upon ligand binding, and can alter the expression of target genes either by transrepression or transactivation. We have applied FRAP (fluorescence recovery after photobleaching) to quantitatively assess the mobility of the yellow fluorescent protein (YFP)-tagged human GR alpha-isoform (hGRalpha) in the nucleus of transiently transfected COS-1 cells and to elucidate determinants of its mobility. Addition of the high-affinity agonist dexamethasone markedly decreases the mobility of the receptor in a concentration-dependent manner, whereas low-affinity ligands like corticosterone decrease the mobility to a much lesser extent. Analysis of other hGRalpha ligands differing in affinity suggests that it is the affinity of the ligand that is a major determinant of the decrease in mobility. Similar results were observed for two hGRalpha antagonists, the low-affinity antagonist ZK98299 and the high-affinity antagonist RU486. The effect of ligand affinity on mobility was confirmed with the hGRalpha mutant Q642V, which has an altered affinity for triamcinolone acetonide, dexamethasone, and corticosterone. Analysis of hGRalpha deletion mutants indicates that both the DNA-binding domain and the ligand-binding domain of the receptor are required for a maximal ligand-induced decrease in receptor mobility. Interestingly, the mobility of transfected hGRalpha differs among cell types. Finally, the proteasome inhibitor MG132 immobilizes a subpopulation of unliganded receptors, via a mechanism requiring the DNA-binding domain and the N-terminal part of the ligand-binding domain. Ligand binding makes the GR resistant to the immobilizing effect of MG132, and this effect depends on the affinity of the ligand. Our data suggest that ligand binding induces a conformational change of the receptor which is dependent on the affinity of the ligand. This altered conformation decreases the mobility of the receptor, probably by targeting the receptor to relatively immobile nuclear domains with which it transiently associates. In addition, this conformational change blocks immobilization of the receptor by MG132.     

Dependence of dexamethasone-induced Akt/FOXO1 signaling,upregulation of MAFbx,and protein catabolism upon the glucocorticoid receptor

The muscle ubiquitin ligases MAFbx and MuRF1 are upregulated in and promote muscle atrophy. Upregulation of MAFbx and MuRF1 by glucocorticoids has been linked to activation of FOXO1 and FOXO3A resulting from reduced Akt activity. We determined the requirements for the glucocorticoid receptor (GR) in these biological responses in C2C12 cells in which GR expression was knocked down by stable expression of an shRNA. Loss of GR prevented dexamethasone-induced increases in protein catabolism. Loss of GR, or inhibition of ligand binding to GR with RU486, prevented upregulation of MAFbx and MuRF1 by dexamethasone. Loss of GR also prevented dexamethasone-induced decreases in Akt phosphorylation, and increases in the fraction of FOXO1 that was unphosphorylated. The findings establish a requirement for the GR in activating molecular signals that promote muscle protein catabolism.