Johanna Zilliacus

Structure and function of the glucocorticoid receptor

Glucocorticoids cause changes in the expression of target genes via interaction with an intracellular receptor protein, the glucocorticoid receptor. This signal transduction process can be divided into a number of steps, each of which represents a functional facet of the receptor protein. These steps include (i) receptor transformation to an active form resulting from specific interaction with glucocorticoid steroid hormones, (ii) homo-dimerization, (iii) DNA-binding to specific hormone response elements in the genome and (iv) modulation of the expression levels of linked genes. These aspects of glucocorticoid receptor function have been studied using a combination of tertiary structure determination, biochemical assays and a genetic approach using a yeast system to screen for mutant receptors that are altered in function. The results show that contacts involving both the DNA and steroid binding domains are involved in dimerization and high affinity DNA binding. Genetic experiments have illuminated the role of amino acids within the recognition helix of the DNA-binding domain in discriminating between cognate DNA response elements for the glucocorticoid receptor and closely related binding sites for other nuclear receptors. Squelching experiments suggest that the N-terminal transactivation domain of the receptor contacts components of the general transcriptional machinery that appear to be distinct from the TATA binding protein, TFIID, during transactivation of gene expression by the DNA-bound receptor.

Interaction of the Ligand-activated Glucocorticoid Receptor with the 14-3-3η Protein

The glucocorticoid receptor (GR) is a ligand-activated transcription factor. In this study, we used the yeast two-hybrid system to isolate cDNAs encoding proteins that interact with the human GR ligand-binding domain (LBD) in a ligand-dependent manner. One isolated cDNA from a HeLa cell library encoded the COOH-terminal portion of the η-isoform of the 14-3-3 protein (residues 187–246). Glucocorticoid agonists, triamcinolone acetonide and dexamethasone, induced the GR LBD/14-3-3η protein fragment interaction, but an antagonist, RU486, did not. Glutathione S-transferase pull-down experiments in vitro showed that full-length 14-3-3η protein also interacted with the activated GR. Transient transfection studies using COS-7 cells revealed a stimulatory effect of 14-3-3η protein on transcriptional activation by the GR. The 14-3-3 family members have recently been found to associate with a number of important signaling proteins, such as protein kinase C and Raf-1, as functional modulators. Our findings suggest a novel regulatory role of 14-3-3η protein in GR-mediated signaling pathways and also point to a mechanism whereby GR may cross-talk with other signal transduction systems.

Glucocorticoid Receptor Interaction with 14-3-3 and Raf-1, a Proposed Mechanism for Cross-talk of Two Signal Transduction Pathways

The glucocorticoid receptor (GR) functions as a ligand-dependent transcription factor. In the present study we describe a specific immunoaffinity chromatography purification of GR from liver cytosol from adrenalectomized rats that may be used to identify hitherto unknown cytosolic GR interacting proteins. We have identified the ubiquitously expressed 14-3-3 as well as Raf-1, a downstream effector of Ras, as GR co-purifying proteins. In our semi-quantitative analysis liganded/activated GR showed the strongest interaction with 14-3-3 and Raf-1, but 14-3-3 was also found to co-purify with GR in a nonliganded/nonactivated state. By extensive salt washes we were also able to demonstrate that the glucocorticoid induced interaction between GR, 14-3-3, and Raf-1, respectively, is remarkably stable and withstood 2.4 m salt. The interaction between GR and 14-3-3 was also verified by 14-3-3 co-immunoprecipitation studies. Our observations that GR and Raf-1 are found within the same protein complex (“receptosome”) in the cytoplasm of rat liver cells could provide a mechanistic explanation for glucocorticoid effects on the Raf-1-Ras signaling pathway.

Regulation of Subnuclear Localization Is Associated with a Mechanism for Nuclear Receptor Corepression by RIP140

ABSTRACT Regulation of gene transcription by nuclear receptors involves association with numerous coregulators. Receptor-interacting protein 140 (RIP140) is a corepressor that negatively regulates the ligand-induced activity of several nuclear receptors, including the glucocorticoid receptor (GR). In the present study, we have characterized the role of the intranuclear localization of RIP140 in its corepressor activity. In the absence of ligand-activated GR, RIP140 is localized in small nuclear foci targeted by a 40-amino-acid-long sequence. Although the focus-targeting domain overlaps with a binding sequence for the corepressor CtBP (C-terminal binding protein), interaction with CtBP is not involved in the localization. RIP140 foci do not correspond to PML bodies but partly colocalize with domains harboring the corepressor SMRT. Upon ligand binding, GR and RIP140 are redistributed to large nuclear domains distinct from the RIP140 foci. The redistribution requires regions of RIP140 with corepressor activity, as well as the DNA-binding domain of GR. Furthermore, we show that full RIP140 corepressor activity is contributed both by C-terminal receptor-binding LXXLL motifs and interaction with the CtBP corepressor. In conclusion, our results suggest that the corepressor function of RIP140 is multifaceted and involves binding to nuclear receptors, as well as additional functions mediated by the formation and intranuclear relocalization of a repressive protein complex.

The nuclear-receptor interacting protein (RIP) 140 binds to the human glucocorticoid receptor and modulates hormone-dependent transactivation.

The glucocorticoid receptor (GR) regulates target gene expression in response to corticosteroid hormones. We have investigated the mechanism of transcriptional activation by the GR by studying the role of the receptor interacting protein RIP140. Both in vivo and in vitro protein-protein interaction assays revealed a ligand-dependent interaction between the GR and RIP140. The ligand binding domain of the GR was sufficient for this interaction, while both the N- and C-terminal regions of RIP140 bound to the receptor. In a yeast transactivation assay RIP140 and SRC-1, a member of the steroid receptor coactivator family of proteins, both enhanced the transactivation activity of a GR protein (GRA-1) in which the potent N-terminal tau1 transactivation domain has been deleted. In contrast, in COS-7 cells increasing amounts of RIP140 significantly inhibited GRdeltatau1 function. In cotransfection studies in COS-7 cells, RIP140 also inhibited receptor activity in presence of both SRC-1 and the coactivator protein CBP together. Thus, in yeast cells a stimulation of receptor activity was observed, while in mammalian cells RIP140 repressed GR function. Taken together, these data suggest that, (1) RIP140 is a target protein for the GR and (2) RIP140 can modulate the transactivation activity of the receptor.

Interaction of the parathyroid hormone receptor with the 14-3-3 protein.

The receptor for parathyroid hormone (PTH) and PTH-related protein (PTHrP) regulates calcium homeostasis, bone remodeling and skeletal development. 14-3-3 proteins bind to signaling proteins and act as molecular scaffolds and regulators of subcellular localization. We show that the parathyroid hormone receptor (PTHR) interacts with 14-3-3 and the proteins colocalize within the cell. 14-3-3 interacts with the C-terminal tail of the receptor containing a consensus 14-3-3 binding motif, but additional binding sites are also used. Protein kinase-A treatment of the receptor and especially the C-terminal tail reduces 14-3-3 binding. The expressed C-terminal tail is primarily localized in the nucleus, supporting the function of a putative nuclear localization signal that could be involved in the previously described nuclear localization of PTHR. The observed interaction between PTHR and the 14-3-3 protein implies that 14-3-3 could contribute to regulation of PTHR signaling.

Molecular basis for repression of liver X receptor-mediated gene transcription by receptor-interacting protein 140.

Similarities in physiological roles of LXR (liver X receptors) and co-repressor RIP140 (receptor-interacting protein 140) in regulating energy homoeostasis and lipid and glucose metabolism suggest that the effects of LXR could at least partly be mediated by recruitment of the co-repressor RIP140. In the present study, we have elucidated the molecular basis for regulation of LXR transcriptional activity by RIP140. LXR is evenly localized in the nucleus and neither the N-terminal domain nor the LBD (ligand-binding domain) is necessary for nuclear localization. Both LXR subtypes, LXRalpha and LXRbeta, interact with RIP140 and co-localize in diffuse large nuclear domains. Interaction and co-localization are dependent on the LBD of the receptor. The C-terminal domain of RIP140 is sufficient for full repressive effect. None of the C-terminal NR (nuclear receptor)-boxes is required for the co-repressor activity, whereas the NR-box-like motif as well as additional elements in the C-terminal region are required for full repressive function. The C-terminal NR-box-like motif is necessary for interaction with LXRbeta, whereas additional elements are needed for strong interaction with LXRalpha. In conclusion, our results suggest that co-repression of LXR activity by RIP140 involves an atypical binding mode of RIP140 and a repression element in the RIP140 C-terminus.

Zinc coordination scheme for the C-terminal zinc binding site of nuclear hormone receptors

The DNA-binding domain of the glucocorticoid receptor contains two zinc ions which are important for the structure and function of the protein. The zinc ions are tetrahedrally coordinated by cysteine residues within the DNA-binding domain. The DNA-binding domain of the glucocorticoid receptor, as well as of the other nuclear hormone receptors, contains nine highly conserved cysteine residues. It has not been clearly established which of these nine cysteine residues are involved in the coordination of zinc. Two models have been proposed for the zinc coordination scheme. We present evidence in favour of the model which excludes the most C-terminal cysteine residue (Cys-481 of the human glucocorticoid receptor) from the zinc coordination scheme. Mutation of this residue in the context of the glucocorticoid receptor DNA-binding domain expressed in E. coli does not significantly reduce the structural integrity of the protein or its DNA-binding properties. These in vitro results are also confirmed by in vivo transactivation assays in yeast.

FMS-like tyrosine kinase 3 interacts with the glucocorticoid receptor complex and affects glucocorticoid dependent signaling

The glucocorticoid receptor (GR) forms part of a multiprotein complex consisting of chaperones and proteins active in glucocorticoid signaling and other pathways. By immunoaffinity purification of GR, followed by Edman sequencing and Western blotting, we identified the FMS-like tyrosine kinase 3 (Flt3) as a GR-interacting protein in rat liver and hepatoma cells. Flt3 interacts with both non-liganded and liganded GR. The DNA-binding domain of GR is sufficient for Flt3 interaction as shown by GST-pull down experiments. Studies of the effects of Flt3 and its ligand FL in glucocorticoid-driven reporter-gene assays in Cos7 cells, show that co-transfection with Flt3 and FL potentiates glucocorticoid effects. Treatment with FL had no effect on GR location and Dex induced translocation of GR was unaffected by FL. In summary, GR and Flt3 interact, affecting GR signaling. This novel cross-talk between GR and a hematopoietic growth factor might also imply glucocorticoid effects on Flt3-mediated signaling.

Functional interaction between the pro-apoptotic DAP3 and the glucocorticoid receptor.

Apoptosis is an essential process for functions such as organ development and the immune response, and glucocorticoids are one of the important regulators of the cellular functions underlying these events. We have previously shown that the pro-apoptotic death-associated protein 3 (DAP3) directly interacts with the glucocorticoid receptor (GR), leading to the enhancement of the activity of the ligand-induced receptor. Here, we show that coexpression of DAP3 and GR results in an increased amount of cellular GR, as well as in partial translocation of DAP3 to the nucleus. Although the N-terminal domain of DAP3 is sufficient for interaction with GR, the full-length DAP3 is needed to efficiently increase GR levels and enhance the transcriptional activity of GR. Since full-length DAP3 is also necessary for the pro-apoptotic effect, the interplay between the N- and C-termini of DAP3 is probably essential for its cellular function.