In contrast to the glucocorticoid receptor, the thyroid hormone receptor is translated in the DNA binding state and is not associated with hsp90

Identification of a Thyroid Hormone Binding Site in Hsp90 with Implications for Its Interaction with Thyroid Hormone Receptor Beta

While many proteins are known clients of heat shock protein 90 (Hsp90), it is unclear whether the transcription factor, thyroid hormone receptor beta (TRb), interacts with Hsp90 to control hormonal perception and signaling. Higher Hsp90 expression in mouse fibroblasts was elicited by the addition of triiodothyronine (T3). T3 bound to Hsp90 and enhanced adenosine triphosphate (ATP) binding of Hsp90 due to a specific binding site for T3, as identified by molecular docking experiments. The binding of TRb to Hsp90 was prevented by T3 or by the thyroid mimetic sobetirome. Purified recombinant TRb trapped Hsp90 from cell lysate or purified Hsp90 in pull-down experiments. The affinity of Hsp90 for TRb was 124 nM. Furthermore, T3 induced the release of bound TRb from Hsp90, which was shown by streptavidin-conjugated quantum dot (SAv-QD) masking assay. The data indicate that the T3 interaction with TRb and Hsp90 may be an amplifier of the cellular stress response by blocking Hsp90 activity.

A New Perspective on Thyroid Hormones: Crosstalk with Reproductive Hormones in Females

Accumulating evidence has shown that thyroid hormones (THs) are vital for female reproductive system homeostasis. THs regulate the reproductive functions through thyroid hormone receptors (THRs)-mediated genomic- and integrin-receptor-associated nongenomic mechanisms, depending on TH ligand status and DNA level, as well as transcription and extra-nuclear signaling transduction activities. These processes involve the binding of THs to intracellular THRs and steroid hormone receptors or membrane receptors and the recruitment of hormone-response elements. In addition, THs and other reproductive hormones can activate common signaling pathways due to their structural similarity and shared DNA consensus sequences among thyroid, peptide, and protein hormones and their receptors, thus constituting a complex and reciprocal interaction network. Moreover, THs not only indirectly affect the synthesis, secretion, and action of reproductive hormones, but are also regulated by these hormones at the same time. This crosstalk may be one of the pivotal factors regulating female reproductive behavior and hormone-related diseases, including tumors. Elucidating the interaction mechanism among the aforementioned hormones will contribute to apprehending the etiology of female reproductive diseases, shedding new light on the treatment of gynecological disorders.

A coregulator shift, rather than the canonical switch, underlies thyroid hormone action in the liver

In this study, Shabtai et al. investigated the mechanism of thyroid hormone (TH)-dependent gene repression, generated a mouse line in which endogenous thyroid hormone receptor TRβ1 was epitope-tagged to allow precise chromatin immunoprecipitation at the low physiological levels of thyroid hormone receptors (TR), and defined high-confidence binding sites where TR functioned at enhancers regulated in the same direction as the nearest gene in a TRβ-dependent manner. Their results demonstrate that, in contrast to the canonical “all or none” coregulator switch model, TH regulates gene expression by orchestrating a shift in the relative binding of corepressors and coactivators.

Thyroid hormones (THs) are powerful regulators of metabolism with major effects on body weight, cholesterol, and liver fat that have been exploited pharmacologically for many years. Activation of gene expression by TH action is canonically ascribed to a hormone-dependent “switch” from corepressor to activator binding to thyroid hormone receptors (TRs), while the mechanism of TH-dependent repression is controversial. To address this, we generated a mouse line in which endogenous TRβ1 was epitope-tagged to allow precise chromatin immunoprecipitation at the low physiological levels of TR and defined high-confidence binding sites where TRs functioned at enhancers regulated in the same direction as the nearest gene in a TRβ-dependent manner. Remarkably, although positive and negative regulation by THs have been ascribed to different mechanisms, TR binding was highly enriched at canonical DR4 motifs irrespective of the transcriptional direction of the enhancer. The canonical NCoR1/HDAC3 corepressor complex was reduced but not completely dismissed by TH and, surprisingly, similar effects were seen at enhancers associated with negatively as well as positively regulated genes. Conversely, coactivator CBP was found at all TH-regulated enhancers, with transcriptional activity correlating with the ratio of CBP to NCoR rather than their presence or absence. These results demonstrate that, in contrast to the canonical “all or none” coregulator switch model, THs regulate gene expression by orchestrating a shift in the relative binding of corepressors and coactivators.

Hsp90 chaperones PPARγ and regulates differentiation and survival of 3T3-L1 adipocytes

Adipose tissue dysregulation has a major role in various human diseases. The peroxisome proliferator-activated receptor-γ (PPARγ) is a key regulator of adipocyte differentiation and function, as well as a target of insulin-sensitizing drugs. The Hsp90 chaperone stabilizes a diverse set of signaling 'client' proteins, thereby regulates various biological processes. Here we report a novel role for Hsp90 in controlling PPARγ stability and cellular differentiation. Specifically, we show that the Hsp90 inhibitors geldanamycin and novobiocin efficiently impede the differentiation of murine 3T3-L1 preadipocytes. Geldanamycin at higher concentrations also inhibits the survival of both developing and mature adipocytes, respectively. Further, Hsp90 inhibition disrupts an Hsp90-PPARγ complex, leads to the destabilization and proteasomal degradation of PPARγ, and inhibits the expression of PPARγ target genes, identifying PPARγ as an Hsp90 client. A similar destabilization of PPARγ and a halt of adipogenesis also occur in response to protein denaturing stresses caused by a single transient heat-shock or proteasome inhibition. Recovery from stress restores PPARγ stability and adipocyte differentiation. Thus, our findings reveal Hsp90 as a critical stress-responsive regulator of adipocyte biology and offer a potential therapeutic target in obesity and the metabolic syndrome.

Chaperoning steroidal physiology: lessons from mouse genetic models of Hsp90 and its cochaperones

The molecular chaperone Hsp90 is abundant, ubiquitous, and catholic to biological processes in eukaryotes, controlling phosphorylation cascades, protein stability and turnover, client localization and trafficking, and ligand-receptor interactions. Not surprisingly, Hsp90 does not accomplish these activities alone. Instead, an ever-growing number of cochaperones have been identified, leading to an explosion of reports on their molecular and cellular effects on Hsp90 chaperoning of client substrates. Notable among these clients are many members of the steroid receptor family, such as glucocorticoid, androgen, estrogen and progesterone receptors. Cochaperones typically associated with the mature, hormone-competent states of these receptors include p23, the FK506-binding protein 52 (FKBP52), FKBP51, protein phosphatase 5 (PP5) and cyclophilin 40 (Cyp40). The ultimate relevance of these cochaperones to steroid receptor action depends on their physiological effects. In recent years, the first mouse genetic models of these cochaperones have been developed. This work will review the complex and intriguing phenotypes so far obtained in genetically-altered mice and compare them to the known molecular and cellular impacts of cochaperones on steroid receptors. This article is part of a Special Issue entitled: Heat Shock Protein 90 (HSP90).

Hsp90 and client protein maturation

Heat-shock protein 90 (Hsp90) is a molecular chaperone that assists in the maturation of a limited set of substrate proteins that are collectively referred to as clients. The majority of identified Hsp90 clients are involved in signal transduction, including many steroid hormone receptors and kinases. A handful of Hsp90 clients can be classified as nonsignal transduction proteins, including telomerase, cystic fibrosis transmembrane conductance regulator, and antigenic peptides destined for major histocompatibility complex. Because Hsp90 clients are causative agents in cancer and cystic fibrosis, research on Hsp90 has intensified in recent years. We review the historical path of Hsp90 research within each class of client (kinase, hormone receptor, and nonsignal transduction clients) and highlight current areas of active investigation.

Susceptibility to diet-induced hepatic steatosis and glucocorticoid resistance in FK506-binding protein 52-deficient mice

Although FK506-binding protein 52 (FKBP52) is an established positive regulator of glucocorticoid receptor (GR) activity, an in vivo role for FKBP52 in glucocorticoid control of metabolism has not been reported. To address this question, FKBP52(+/-) mice were placed on a high-fat (HF) diet known to induce obesity, hepatic steatosis, and insulin resistance. Tissue profiling of wild-type mice showed high levels of FKBP52 in the liver but little to no expression in muscle or adipose tissue, predicting a restricted pattern of FKBP52 effects on metabolism. In response to HF, FKBP52(+/-) mice demonstrated a susceptibility to hyperglycemia and hyperinsulinemia that correlated with reduced insulin clearance and reduced expression of hepatic CEACAM1 (carcinoembryonic antigen-related cell adhesion molecule 1), a mediator of clearance. Livers of HF-fed mutant mice had high lipid content and elevated expression of lipogenic genes (peroxisome proliferator-activated receptor gamma, fatty acid synthase, and sterol regulatory element-binding protein 1c) and inflammatory markers (TNFalpha). Interestingly, mutant mice under HF showed elevated serum corticosterone, but their steatotic livers had reduced expression of gluconeogenic genes (phosphoenolpyruvate carboxy kinase, glucose 6 phosphatase, and pyruvate dehydrogenase kinase 4), whereas muscle and adipose expressed normal to elevated levels of glucocorticoid markers. These data suggest a state of glucocorticoid resistance arising from liver-specific loss of GR activity. Consistent with this hypothesis, reduced expression of gluconeogenic genes and CEACAM1 was observed in dexamethasone-treated FKBP52-deficient mouse embryonic fibroblast cells. We propose a model in which FKBP52 loss reduces GR control of gluconeogenesis, predisposing the liver to steatosis under HF-diet conditions attributable to a shunting of metabolism from glucose production to lipogenesis.

Hormone binding and co-regulator binding to the glucocorticoid receptor are allosterically coupled

The glucocorticoid receptor initiates the cellular response to glucocorticoid steroid hormones in vertebrates. Co-regulator proteins dock to the receptor in response to hormone binding and potentiate the transcriptional activity of the receptor by modifying DNA and recruiting essential transcription factors like RNA polymerase II. Hormones and co-regulators bind at distinct sites in the ligand binding domain yet function cooperatively to mediate transcriptional control. This study reveals and quantifies energetic coupling between two binding sites using purified components. Using a library of peptides taken from co-regulator proteins, we determine the pattern of co-regulator binding to the glucocorticoid receptor ligand binding domain. We show that peptides from co-regulators differ in their effects on hormone binding and kinetics. Peptides from DAX1 and SRC1 bind with similar affinity, but DAX1 binding is coupled to hormone binding, and SRC1 is not. Mechanistic details of co-regulator binding and coupling to the hormone binding pocket are uncovered by analysis of properties endowed by mutation of a key residue in the allosteric network connecting the sites.

Targeted ablation reveals a novel role of FKBP52 in gene-specific regulation of glucocorticoid receptor transcriptional activity

FKBP52 is a tetratricopeptide repeat (TPR) protein with peptidyl-prolyl isomerase activity and is found in steroid receptor complexes, including glucocorticoid receptor (GR). It is generally accepted that FKBP52 has a stimulatory effect on GR transcriptional activity. However, the mechanism by which FKBP52 controls GR is not yet clear, with reports showing effects on GR hormone-binding affinity and/or hormone-induced nuclear translocation. To address this issue, we have generated mice with targeted ablation of the FKBP52 gene. To date, no overt defects of GR-regulated physiology have been found in these animals, demonstrating that FKBP52 is not an essential regulator of global GR activity. To better assess the impact of FKBP52 on GR, mouse embryonic fibroblasts (MEFs) were generated from wild-type (WT) and FKBP52-deficient (KO) animals. Analysis of GR activity at reporter genes showed an approximate 70% reduction of activity in 52KO MEF cells, with no effect of FKBP52 loss on thyroid receptor. Interestingly, GR activity at endogenous genes was not globally affected in 52KO cells, with reduced activity at GILZ and FKBP51, but not at SGK and p21. Thus, FKBP52 appears to be a gene-specific modulator of GR. To investigate the mechanism of this action, analyses of GR heterocomplex composition, hormone-binding affinity, and ability to undergo hormone-induced nuclear translocation and DNA-binding were performed. Interestingly, no effect of FKBP52 loss was found for any of these GR properties, suggesting that the main function of FKBP52 is a heretofore-unknown ability to control GR activity at target genes. Lastly, loss of FKBP52 did not affect the ability of GR to undergo hormone-induced autologous down-regulation, showing that FKBP52 does not contribute to all branches of GR signaling. The implications of these results to the potential actions of FKBP52 on GR activity in vivo are discussed.

Heat shock protein 90beta: a novel mediator of vitamin D action

We investigated the role of Heat shock protein 90 (Hsp90) in vitamin D action in Caco-2 cells using geldanamycin (GA) to block Hsp90 function and RNA interference to reduce Hsp90beta expression. When cells were exposed to GA, vitamin D-mediated gene expression and transcriptional activity were inhibited by 69% and 54%, respectively. Gel shift analysis indicated that GA reduced vitamin D-mediated DNA binding activity of the vitamin D receptor (VDR). We tested the specific role of Hsp90beta by knocking down its expression with stably expressed short hairpin RNA. Vitamin D-induced gene expression and transcriptional activity were reduced by 90% and 80%, respectively, in Hsp90beta-deficient cells. Nuclear protein for VDR and RXRalpha, its heterodimer partner, were not reduced in Hsp90beta-deficient cells. These findings indicate that Hsp90beta is needed for optimal vitamin D responsiveness in the enterocyte and demonstrate a specific role for Hsp90beta in VDR signaling.

Thyroid hormone availability and activity in avian species: a review

The intracellular thyroid hormone (TH) availability is influenced by different metabolic pathways. Some of the changes in intracellular TH availability can be linked to changes in local deiodination and sulfation capacities. The secretion of the chicken thyroid consists predominantly of thyroxine (T4). TH receptors (TRs) preferentially bind 3,5,3'-triiodothyronine (T3). Therefore, the metabolism of T4 secreted by the thyroid gland in peripheral tissues, resulting in the production and degradation of receptor-active T3, plays a major role in thyroid function. Food restriction in growing chickens increases hepatic type III deiodinase (D3) levels but decreases growth hormone (GH)-dependent variables such as plasma insulin-like growth factor-I (IGF-I) and T3 concentrations. Refeeding restores hepatic D3 and plasma T3 to control levels within a few hours. It can be concluded that the tissue and time dependent regulation of the balance between TH activating and inactivating enzymes plays an essential role in the control of local T3 availability and hence in TH activity. Two separate genes encode multiple TR isoforms, i.e. TRalpha and TRbeta. These TRs consist of a DNA-binding domain, a ligand-binding domain, a hinge region and an amino-terminal (A/B) domain. TRs mediate their effects on transcription by binding as homodimers or heterodimers to the TH response elements (TREs). Also, unliganded TRs can bind to TREs and may so modulate transcription of target genes.

Interaction of nuclear receptors with hsp90 in living cells

The ubiquitous heat shock protein 90 (hsp90) has been shown to participate directly in the function of a wide variety of cellular signal transduction components, including steroid receptors (SRs). However, there is still no direct evidence for an in vivo association of SRs with hsp90. This study utilizes the mammalian two-hybrid system to study the ability of hsp90 to interact with various (non)liganded nuclear receptors (NRs) in vivo in mammalian cells. As bait, we used ligand-binding domain (LBD) of various NRs fused with the GAL4-DBD. hsp90/Receptor interactions were monitored in COS cells. When NR-LBDs were co-transfected along with hsp90/VP16, none (RxR(2)-LBD) or only minimal (SR-LBDs) transcription inductions were observed (1.9-4.7-fold) in the absence of ligand. Addition of ligand further abolished the observed minimal induction. As a positive control for interaction we used TIF-2, which interacts with SRs in a ligand inducible manner. When co-transfected with NR-LBDs in the absence of ligand TIF-2/VP16 induced minimal activation of transcription (1.6-4.5-fold) that was comparable to the activation induced by the NR-LBDs. In contrast, in the presence of the ligand, the activation ranged between 62- and 134-fold depending on the receptor. The results suggest that the interaction of SRs with the hsp90 is minimal when compared to a bona fide type of interaction with the co-factors.

Heat shock protein-90 (Hsp90) acts as a repressor of peroxisome proliferator-activated receptor-alpha (PPARalpha) and PPARbeta activity

The nuclear receptor (NR) peroxisome proliferator-activated receptor-alpha (PPARalpha) mediates the effects of several hypolipidemic drugs, endogenous fatty acids, and peroxisome proliferators. Despite belonging to a class of NR not known to interact with cytosolic chaperone complexes, we have recently shown that PPARalpha interacts with heat shock protein 90 (Hsp90), although the biological consequence of this association was unknown. In the present study, PPARalpha directly associated with Hsp90 in vitro to a much greater extent than either PPARbeta or PPARgamma. This interaction is similar to other NR-Hsp90 complexes with association occurring between the middle of Hsp90 and the hinge (D) and ligand binding domain (EF) of PPARalpha. Using several different approaches to disrupt Hsp90 complexes within the cell, we demonstrate that Hsp90 is a repressor of both PPARalpha and PPARbeta activity. Treatment with geldanamycin (GA) increased the activity of PPARalpha and in the presence of ligand in transient transfection assays. PPARalpha-response element (PPRE)-reporter assays in a stable cell line treated with GA resulted in enhanced expression of a known target gene, acyl-CoA oxidase. Similarly, overexpression of the tetratricopeptide repeat (TPR) of protein phosphatase 5 (PP5) increased PPARalpha or PPARbeta activity in a PPRE-reporter assay and decreased the interaction between PPARalpha or PPARbeta and Hsp90 in a mammalian two-hybrid assay. Finally, cotransfection with the C-terminal hsp-interacting protein (CHIP) construct, a TPR-containing ubiquitin ligase that interacts with hsp90, increased PPARalpha's and decreased PPARbeta's ability to regulate PPRE-reporter activity upon ligand activation. All three methods to disrupt Hsp90 function (GA, PP5-TPR, CHIP) resulted in an alteration in PPARalpha or PPARbeta activity to a much greater extent than PPARgamma. While FKBP52 had no effect on PPARalpha activity, p23 greatly enhanced constitutive and Wy14 643 induced PPRE-reporter activity. Thus, we describe the chaperone complex as being a regulator of PPARalpha and PPARbeta activity and have identified a novel, subtype-specific, inhibitory role for Hsp90.

Evidence that peroxisome proliferator-activated receptor alpha is complexed with the 90-kDa heat shock protein and the hepatitis virus B X-associated protein 2

The peroxisome proliferator-activated receptor alpha (PPARalpha) is a ligand-inducible transcription factor, which belongs to the nuclear receptor superfamily. PPARalpha mediates the carcinogenic effects of peroxisome proliferators in rodents. In humans, PPARalpha plays a fundamental role in regulating energy homeostasis via control of lipid metabolism. To study the possible role of chaperone proteins in the regulation of PPARalpha activity, a monoclonal antibody (mAb) was made against PPARalpha and designated as 3B6/PPAR. The specificity of mAb 3B6/PPAR in recognizing PPARalpha was tested in immunoprecipitations using in vitro translated PPAR subtypes. The mAb 3B6/PPAR recognized PPARalpha, failed to bind to PPARbeta or PPARgamma, and is efficient in both immunoprecipitating and visualizing the receptor on protein blots. The immunoprecipitation of PPARalpha in mouse liver cytosol using mAb 3B6/PPAR has resulted in the detection of two co-immunoprecipitated proteins, which are heat shock protein 90 (hsp90) and the hepatitis B virus X-associated protein 2 (XAP2). The concomitant depletion of PPARalpha in hsp90-depleted mouse liver cytosol was also detected. Complex formation between XAP2 and PPARalpha/FLAG was also demonstrated in an in vitro translation binding assay. hsp90 interacts with PPARalpha in a mammalian two-hybrid assay and binds to the E/F domain. Transient expression of XAP2 co-expressed with PPARalpha resulted in down-regulation of a peroxisome proliferator response element-driven reporter gene activity. Taken together, these results indicate that PPARalpha is in a complex with hsp90 and XAP2, and XAP2 appears to function as a repressor. This is the first demonstration that PPARalpha is stably associated with other proteins in tissue extracts and the first nuclear receptor shown to functionally interact with XAP2.

A glucocorticoid/retinoic acid receptor chimera that displays cytoplasmic/nuclear translocation in response to retinoic acid. A real time sensing assay for nuclear receptor ligands

Members of the nuclear receptor superfamily play key roles in a host of physiologic and pathologic processes from embryogenesis to cancer. Some members, including the retinoic acid receptor (RAR), are activated by ligand binding but are unaffected in their subcellular distribution, which is predominantly nuclear. In contrast, several members of the steroid receptor family, including the glucocorticoid receptor, are cytoplasmic and only translocate to the nucleus after ligand binding. We have constructed chimeras between RAR and glucocorticoid receptor that selectively respond to RAR agonists but display cytoplasmic localization in the absence of ligand. These chimeric receptors manifest both nuclear translocation and gene activation functions in response to physiological concentrations of RAR ligands. The ability to achieve regulated subcellular trafficking with a heterologous ligand binding domain has implications both for current models of receptor translocation and for structural-functional conservation of ligand binding domains broadly across the receptor superfamily. When coupled to the green fluorescent protein, chimeric receptors offer a powerful new tool to 1) study mechanisms of steroid receptor translocation, 2) detect dynamic and graded distributions of ligands in complex microenvironments such as embryos, and 3) screen for novel ligands of "orphan" receptors in vivo.

Physiological and molecular basis of thyroid hormone action

Thyroid hormones (THs) play critical roles in the differentiation, growth, metabolism, and physiological function of virtually all tissues. TH binds to receptors that are ligand-regulatable transcription factors belonging to the nuclear hormone receptor superfamily. Tremendous progress has been made recently in our understanding of the molecular mechanisms that underlie TH action. In this review, we present the major advances in our knowledge of the molecular mechanisms of TH action and their implications for TH action in specific tissues, resistance to thyroid hormone syndrome, and genetically engineered mouse models.

Nuclear cytoplasmic shuttling by thyroid hormone receptors. multiple protein interactions are required for nuclear retention

In this report, we have studied the intracellular dynamics and distribution of the thyroid hormone receptor-beta (TRbeta) in living cells, utilizing fusions to the green fluorescent protein. Wild-type TRbeta was mostly nuclear in both the absence and presence of triiodothyronine; however, triiodothyronine induced a nuclear reorganization of TRbeta. By mutating defined regions of TRbeta, we found that both nuclear corepressor and retinoid X receptor are involved in maintaining the unliganded receptor within the nucleus. A TRbeta mutant defective in DNA binding had only a slightly altered nuclear/cytoplasmic distribution compared with wild-type TRbeta; thus, site-specific DNA binding is not essential for maintaining TRbeta within the nucleus. Both ATP depletion studies and heterokaryon analysis demonstrated that TRbeta rapidly shuttles between the nuclear and the cytoplasmic compartments. Cotransfection of nuclear corepressor and retinoid X receptor markedly decreased the shuttling by maintaining unliganded TRbeta within the nucleus. In summary, our findings demonstrate that TRbeta rapidly shuttles between the nucleus and the cytoplasm and that protein-protein interactions of TRbeta with various cofactors, rather than specific DNA interactions, play the predominant role in determining the intracellular distribution of the receptor.

Trafficking of nuclear receptors in living cells

Most of the steroid receptor family, with the exception of the estrogen receptor, are classically viewed as 'translocating receptors'. That is, they move from an exclusively, or principally, cytoplasmic distribution in the absence of hormone to a predominately nuclear localization in hormone stimulated cells. The estrogen receptor and the nuclear receptor family are found exclusively in the nucleus, both in hormone stimulated and hormone free cells. This behavior has now been studied with GFP-fusions in living cells, and has in general been confirmed. However, there are important exceptions, and new findings, particularly with regard to sub-nuclear localization. We propose that the intracellular distribution of both receptor classes is dependent not only on subcellular localization signals directly encoded in the receptors, but also on the nature and composition of the large, macromolecular complexes formed by each receptor. Furthermore, we find that most members of the receptor superfamily form focal accumulations within the nucleus in response to ligand, and suggest that these structures may participate in the biological life cycle of the receptors. Finally, we propose that receptor movement in the nucleus is highly dynamic, with the receptors undergoing constant exchange between genomic regulatory elements, multi-protein complexes with other transcription factor partners, and subnuclear structures that are as yet poorly defined.

Molecular chaperones activate the Drosophila ecdysone receptor, an RXR heterodimer

The steroid hormone 20-hydroxyecdysone coordinates the stages of Drosophila development by activating a nuclear receptor heterodimer consisting of the ecdysone receptor, EcR, and the Drosophila RXR receptor, USP. We show that EcR/USP DNA binding activity requires activation by a chaperone heterocomplex like that required for activation of the vertebrate steroid receptors, but not previously shown to be required for activation of RXR heterodimers. Six proteins normally present in the chaperone complex were individually purified and shown to be sufficient for this activation. We also show that two of the six (Hsp90 and Hsc70) are required in vivo for ecdysone receptor activity, and that EcR is the primary target of the chaperone complex.

Discrimination between NL1- and NL2-mediated nuclear localization of the glucocorticoid receptor

Glucocorticoid receptor (GR) cycles between a free liganded form that is localized to the nucleus and a heat shock protein (hsp)-immunophilin-complexed, unliganded form that is usually localized to the cytoplasm but that can also be nuclear. In addition, rapid nucleocytoplasmic exchange or shuttling of the receptor underlies its localization. Nuclear import of liganded GR is mediated through a well-characterized sequence, NL1, adjacent to the receptor DNA binding domain and a second, uncharacterized motif, NL2, that overlaps with the ligand binding domain. In this study we report that rapid nuclear import (half-life [t1/2] of 4 to 6 min) of agonist- and antagonist-treated GR and the localization of unliganded, hsp-associated GRs to the nucleus in G0 are mediated through NL1 and correlate with the binding of GR to pendulin/importin alpha. By contrast, NL2-mediated nuclear transfer of GR occurred more slowly (t1/2 = 45 min to 1 h), was agonist specific, and appeared to be independent of binding to importin alpha. Together, these results suggest that NL2 mediates the nuclear import of GR through an alternative nuclear import pathway. Nuclear export of GR was inhibited by leptomycin B, suggesting that the transfer of GR to the cytoplasm is mediated through the CRM1-dependent pathway. Inhibition of GR nuclear export by leptomycin B enhanced the nuclear localization of both unliganded, wild-type GR and hormone-treated NL1(-) GR. These results highlight that the subcellular localization of both liganded and unliganded GRs is determined, at least in part, by a flexible equilibrium between the rates of nuclear import and export.

Association of the rat liver glucocorticoid receptor with Hsp90 and Hsp70 upon whole body hyperthermic stress

The influence of whole body hyperthermic stress (41 degrees C, 15 min) on association of the glucocorticoid receptor (GR) with heat shock proteins Hsp90 and Hsp70 was followed in rat liver cytosol during a 24 h period after the stress. Total cytosolic concentration of the GR, Hsp90 and Hsp70 and the amounts of Hsp90 and Hsp70 co-immunopurified with the GR were determined by a quantitative Western blotting using appropriate monoclonal antibodies. A significant decrease in the cytosolic GR level in response to the stress was noticed. The ratio of the amount of the GR to Hsp90 recruited by the GR was found to be unaltered by hyperthermia, in spite of the stress-induced increase in the total Hsp90 concentration in the cytosol. Hsp70 was also found in association with the GR and its 2.5-fold induction by the stress was accompanied by about 3-fold increase in its relative amount that co-immunopurified with the GR. The results suggest that heat stress influences the interaction of the GR with Hsp70 through the mechanisms controlling the untransformed rat liver GR heterocomplexes assembly process.

Hormone-induced translocation of thyroid hormone receptors in living cells visualized using a receptor green fluorescent protein chimera

Thyroid hormone nuclear receptors (TRs) are ligand-dependent transcription factors that regulate growth, differentiation, and development. To understand the role of the hormone, 3,3', 5-triiodo-L-thyronine (T3), in the nuclear translocation and targeting of TRs to the regulatory sites in chromatin, we appended green fluorescent protein (GFP) to the human TR subtype beta1 (TRbeta1). The fusion of GFP to the amino terminus of TRbeta1 protein did not alter T3 binding or transcriptional activities of the receptor. The subcellular localization of GFP-TRbeta1 in living cells was visualized by laser-scanning confocal microscopy. In the presence of T3, the expressed GFP-TRbeta1 was predominately localized in the nucleus, exhibiting a nuclear/cytoplasmic ratio of approximately 5.5. No GFP-TRbeta1 was detected in the nucleolus. In the absence of T3, more GFP-TRbeta1 was present in the cytoplasm, exhibiting a nuclear/cytoplasmic ratio of approximately 1.5. In these cells, cytoplasmic GFP-TRbeta1 could be induced to enter the nucleus by T3. The T3-induced translocation was blocked when Lys184-Arg185 in domain D of TRbeta1 was mutated to Ala184-Ala185. Furthermore, the inability of the mutant TR to translocate to the nucleus correlated with the loss of most of its transcriptional activity. These results suggest that TR functions may, in part, be regulated by T3-induced nuclear entry.

Chicken thyroid hormone receptor alpha requires the N-terminal amino acids for exclusive nuclear localization

The subcellular localization of natural and engineered forms of the chicken thyroid hormone receptor (cTR alpha) is dependent on amino acids encoded in the N-terminal region. The full length receptor protein, cTR alpha-p46, was found to localize exclusively to the nucleus, whereas the N-terminally shorter variant, cTR alpha-p40, localizes to both the nucleus and the cytoplasm. The exclusive nuclear localization of cTR alpha-p46 is dependent on the presence of the first 11 N-terminal amino acids, but independent of the phosphorylation of the serine at position 12. Our data identify a novel role for an N-terminal domain of the full length thyroid hormone receptor.

Steroid receptor interactions with heat shock protein and immunophilin chaperones

We have provided a historical perspective on a body of steroid receptor research dealing with the structure and physiological significance of the untransformed 9S receptor that has often confused both novice and expert investigators. The frequent controversies and equivocations of earlier studies were due to the fact that the native, hormone-free state of these receptors is a large multiprotein complex that resisted description for many years because of its unstable and dynamic nature. The untransformed 9S state of the steroid and dioxin receptors has provided a unique system for studying the function of the ubiquitous, abundant, and conserved heat shock protein, hsp90. The hormonal control of receptor association with hsp90 provided a method of manipulating the receptor heterocomplex in a manner that was physiologically meaningful. For several steroid receptors, binding to hsp90 was required for the receptor to be in a native hormone-binding state, and for all of the receptors, hormone binding promoted dissociation of the receptor from hsp90 and conversion of the receptor to the DNA-binding state. Although the complexes between tyrosine kinases and hsp90 were discovered earlier, the hormonal regulation or steroid receptor association with hsp90 permitted much more rapid and facile study of hsp90 function. The observations that hsp90 binds to the receptors through their HBDs and that these domains can be fused to structurally different proteins bringing their function under hormonal control provided a powerful linkage between the hormonal regulation of receptor binding to hsp90 and the initial step in steroid hormone action. Because the 9S receptor hsp90 heterocomplexes could be physically stabilized by molybdate, their protein composition could be readily studied, and it became clear that these complexes are multiprotein structures containing a number of unique proteins, such as FKBP51, FKBP52, CyP-40, and p23, that were discovered because of their presence in these structures. Further analysis showed that hsp90 itself exists in a variety of native multiprotein heterocomplexes independent of steroid receptors and other 'substrate' proteins. Cell-free systems can now be used to study the formation of receptor heterocomplexes. As we outlined in the scheme of Fig. 1, the multicomponent receptor-hsp90 heterocomplex assembly system is being reconstituted, and the importance of individual proteins, such as hsp70, p60, and p23, in the assembly process is becoming recognized. It should be noted that our understanding of the mechanism and purpose of steroid receptor heterocomplex assembly is still at an early stage. We can now speculate on the roles of receptor-associated proteins in receptor action, both as individuals and as a group, but their actual functions are still vague or unknown. We can make realistic models about the chaperoning and trafficking of steroid receptors, but we don't yet know how these processes occur, we don't know where chaperoning occurs in the cell (e.g. Is it limited to the cytoplasm? Is it a diffuse process or does chaperoning occur in association with structural elements?), and, with the exception of the requirement for hormone binding, we don't know the extent to which the hsp90-based chaperone system impacts on steroid hormone action. It is not yet clear how far the discovery of this hsp90 heterocomplex assembly system will be extended to the development of a general understanding of protein processing in the cell. Because this assembly system is apparently present in all eukaryotic cells, it probably performs an essential function for many proteins. The bacterial homolog of hsp90 is not an essential protein, but hsp90 is essential in eukaryotes, and recent studies indicate that the development of the cell nucleus from prokaryotic progenitors was accompanied by the duplication of genes for hsp90 and hsp70 (698). (ABSTRACT TRUNCATED)

Heat shock protein 84 forms a complex with mutant p53 protein predominantly within a cytoplasmic compartment of the cell

Cellular DNA damage results in the increased expression and accumulation of the p53 tumor suppressor protein within the nucleus which leads to cell cycle arrest or apoptosis. In some cases, however, wild-type p53 and some mutant forms of p53 reside in the cytoplasm of cancer cells. To understand the mechanism responsible for its cytoplasmic retention, studies were undertaken to determine if unique proteins form a complex with mutant p53 within the cytoplasm of transformed cells. One protein, with an apparent molecular mass of 92 kDa (p92), was observed to form a complex with a temperature-sensitive mutant p53 (TSp53(Val-135)) in the cytoplasm of transformed rat embryo fibroblasts at the non-permissive temperature. p92 copurified with TSp53(Val-135) on a p53-specific immunoaffinity column and a gel filtration column. The protein was purified to homogeneity and identified as hsp84 by partial amino acid sequence analysis. hsp84 is a member of the hsp90 class of proteins. At the non-permissive temperature, TSp53(Val-135) and hsp84 colocalized in the cytoplasm near the nuclear envelope. At the permissive temperature, TSp53(Val-135) resides in the nucleus and expresses a "wild-type like" conformation. Under these conditions hsp84 continued to reside in the cytoplasm and little or no hsp84 formed a complex with p53. The results suggest that hsp84 binds mutant p53 in a spatial and/or conformation dependent manner.

Animal and plant cell lysates share a conserved chaperone system that assembles the glucocorticoid receptor into a functional heterocomplex with hsp90

The hormone-binding domain of the glucocorticoid receptor must be bound to heat shock protein (hsp) 90 for it to have a high-affinity steroid-binding conformation. Cell-free assembly of a glucocorticoid receptor-hsp90 heterocomplex is brought about in reticulocyte lysate by a preformed protein-folding complex containing hsp90, hsp70, and other proteins [Hutchison, K.A., Dittmar, K. D., & Pratt, W.B. (1994) J. Biol. Chem. 269, 27894-27899]. In this "foldosome" system, hsp70 is required for assembly of the receptor-hsp90 complex and concomitant activation of steroid-binding activity [Hutchison, K.A., Dittmar, K.D., Czar, M.J., & Pratt, W.B. (1994) J. Biol. Chem. 269, 22157-22161]. All previous experiments involving cell-free assembly of both receptor-hsp90 and protein kinase-hsp90 heterocomplexes have been carried out with the protein-folding system in rabbit reticulocyte lysate. In this work, we show that concentrated lysates of receptor-free mouse (L cells) and insect (Sf9) cells and also a plant (wheat germ) lysate fold the immunopurified glucocorticoid receptor into a functional (i.e., steroid binding) heterocomplex with hsp90. Receptor heterocomplex formation in animal lysates and in the plant lysate are not identical in that the dynamics of complex assembly are different, but both systems produce a functional complex that binds steroid. Also, in contrast to animal and insect complexes, receptor-plant hsp90 complexes are not stabilized by molybdate. When added to the other lysate, purified plant and animal hsp90s show partial complementarity, in that a receptor-hsp90 complex is formed but the receptor is not converted to the steroid-binding conformation. When added to rabbit reticulocyte lysate that has been depleted of endogenous hsp70, purified wheat germ and mouse hsp70's are equally active in promoting both assembly of receptor-hsp90 heterocomplexes and conversion of receptor to the steroid-binding conformation. Thus, hsp70 from the plant kingdom has conserved the ability to interact functionally with chaperone proteins of the animal kingdom to cooperate in protein folding as evidenced by formation of a functional receptor-hsp90 heterocomplex.

Nuclear factors specifically favor thyroid hormone binding to c-ErbA alpha 1 protein (thyroid hormone receptor alpha) over-expressed in E. coli

A recombinant rat thyroid hormone receptor alpha (TR alpha or c-ErbA alpha 1) was produced in E. coli as a non-mutated, nonfusioned protein and obtained as an efficient DNA and T3 binding protein that could be easily handled in a buffer-soluble state (rec-TR alpha). It was found that nuclear extracts (NE) added to rec-TR alpha markedly amplified not only DNA binding, which has been well documented, but also T3 binding (increased binding site concentration), which has not yet been reported. This T3 binding amplifying effect on rec-TR alpha occurs at low NE protein concentrations that produce no or minimal endogenous TR with respect to rec-TR, while similar concentrations of other proteins (e.g. ovalbumin or cytosol) only moderately enhanced T3 binding. The T3 binding amplifying nuclear factors, which are partly heat-labile, appeared as necessary auxiliaries in the analyses of partially purified rec-TR alpha. A protective effect of NE against a loss of affinity for T3 under the action of antibodies directed to certain sequences in the TR alpha D domain suggests that nuclear factors help rec-TR alpha to acquire and/or stabilize a conformation that allows the high affinity T3 binding. The nature of this nuclear amplifying factor is still unknown: RXR alpha which, produced in vitro, could amplify binding of the rec-TR alpha to a DNA thyroid response element, was unable to display such a rescue of high affinity binding sites.

Microtubules are not required for glucocorticoid receptor mediated gene induction

Steroid-free glucocorticoid receptors are generally considered to reside in the cytoplasm of cells. After the binding of steroids, the receptors translocate into the nucleus in a manner that has been proposed to involve microtubules. However, some results with inhibitors of microtubule assembly argue to the contrary. In all of these studies, only the whole cell localization of receptors has been examined; the biological activity of these receptors has not been determined. We now report that steroid-induced gene expression is maintained in the absence of intact microtubules. This argues that microtubules are not required for either the nuclear translocation or biological activity of glucocorticoid receptors.

Steroid hormone receptors: activators of gene transcription

Over the past three decades, a great deal of evidence has accumulated in favor of the hypothesis that steroid hormones act via regulation of gene expression. The action is mediated by specific nuclear receptor proteins, which belong to a superfamily of ligand-modulated transcription factors that regulate homeostasis, reproduction, development and differentiation. This family includes receptors for steroid hormones, thyroid hormones, hormonal forms of vitamin A and D, peroxisomal activators, and ecdysone. Molecular cloning and structure/function analyses have revealed that all members of the steroid/thyroid hormone/retinoic acid receptor family have a similar functional domain structure: a variable N-terminal region, which is involved in modulation of gene expression; a short well-conserved DNA-binding domain, which is crucial for recognition of specific DNA sequences and for receptor dimerization; and a partially conserved C-terminal ligand-binding domain, which is important for hormone binding and also for receptor dimerization and transactivation. In contrast to other members of the receptor superfamily steroid hormone receptors form transient complexes with several heat shock proteins. This interaction promotes proper folding and stability of the receptor molecule. Hormone binding induces a conformational change in the receptor molecule and simultaneously a dissociation of all heat shock proteins, which results in DNA-binding of the hormone-receptor complex.(ABSTRACT TRUNCATED AT 250 WORDS)

Oligomeric structures of cytosoluble estrogen-receptor complexes as studied by anti-estrogen receptor antibodies and chemical crosslinking of intact cells

The structure of estrogen-receptor complexes recovered in cytosolic extracts of MCF-7 cells treated with hormone at 2 degrees was probed by chemical crosslinking of intact cells and sample analysis with four monoclonal anti-estrogen receptor antibodies. When MCF-7 cells were treated with either glutaraldehyde or dithiobis(succinimidyl propionate), cytosoluble estrogen-receptor complexes consisted of two major forms sedimenting as 4 S monomers and 8-9 S salt-resistant oligomers. By high salt sucrose density gradient centrifugation, we could observe that the four monoclonal anti-estrogen receptor antibodies bound different forms of receptor complexes from crosslinked cells. While H222 and H226 antibodies could interact with any form we detected, the D75 and D547 monoclonals could only recognize those showing sedimentation coefficients lower than 7 S. When cytosolic extracts from [35S]-methionine-labeled cells were subjected to immunoprecipitation with H222 and D75 anti-estrogen receptor antibodies, electrophoretic analysis of material extracted from immunoprecipitates revealed the presence of 65 kDa estrogen receptors. If extracts were prepared from crosslinked cells, instead, two more components with estimated molecular masses of 220 and 100 kDa were specifically immunoprecipitated by the H222 antibody, whereas only the 100 kDa component and the estrogen receptor were found in immunoprecipitates obtained with the D75 monoclonal. When estrogen-receptor complexes were immunopurified from extracts prepared after cells had been crosslinked with dithiobis(succinimidyl propionate), and the oligomers were dissociated by treatment with beta-mercaptoethanol, electrophoretic analysis of our samples showed that only the 65 kDa estrogen receptor and a 50 kDa protein were selectively immunoprecipitated by anti-estrogen receptor antibodies. We concluded that the structures of cytosoluble estrogen-receptor complexes in MCF-7 cells treated with hormone at 2 degrees C, include oligomeric forms which contain a 50 kDa non-steroid binding protein.

Physiology of the steroid-thyroid hormone nuclear receptor superfamily

Glucocorticoids, other steroid hormones, thyroid hormones and vitamin-derived hormones (including retinoids) all exert their effects by the regulation of hormone-responsive target genes within the cell nucleus. These hormones bind to a series of specific nuclear receptor proteins that function as hormone-inducible transcription factors. The receptors are structurally homologous, are related to the avian erythroblastosis oncogene v-erbA, and exhibit remarkable evolutionary conservation. Together they form the steroid-thyroid hormone nuclear receptor superfamily. This chapter describes the structure and functions of the various family members and highlights the differences and similarities that occur between individual receptor proteins. Type I receptors, which include glucocorticoid receptor and other steroid receptor proteins, interact as homodimers with target sequences of DNA containing two receptor binding sites arranged as a palindrome. Type II receptors, which include receptors for retinoids, thyroid hormone and vitamin D3, bind as heterodimers (or homodimers) to DNA sequences in which two or more receptor-binding sites are arranged as a direct repeat or as other more complex configurations. The complexity of both receptor-DNA and receptor-receptor interactions predicts the potential for considerable cross-talk between various hormone-activated pathways. Thus, the specificity of hormone action and its regulation is discussed in relation to the structural and functional characteristics of the receptors and their molecular mechanisms of action. Finally, potential sites of regulation of hormone action, from circulating hormone levels in the periphery to their delivery to the cell and final site of action in the nucleus, are highlighted to provide a perspective for the following chapters in this volume and to indicate their clinical significance.

Heat-shock proteins as molecular chaperones

Functional proteins within cells are normally present in their native, completely folded form. However, vital processes of protein biogenesis such as protein synthesis and translocation of proteins into intracellular compartments require the protein to exist temporarily in an unfolded or partially folded conformation. As a consequence, regions buried when a polypeptide is in its native conformation become exposed and interact with other proteins causing protein aggregation which is deleterious to the cell. To prevent aggregation as proteins become unfolded, heat-shock proteins protect these interactive surfaces by binding to them and facilitating the folding of unfolded or nascent polypeptides. In other instances the binding of heat-shock proteins to interactive surfaces of completely folded proteins is a crucial part of their regulation. As heat shock and other stress conditions cause cellular proteins to become partially unfolded, the ability of heat-shock proteins to protect cells against the adverse effects of stress becomes a logical extension of their normal function as molecular chaperones.

Regulation of glucocorticoid receptor function through assembly of a receptor-heat shock protein complex

Incubation of immunopurified, hormone-free mouse glucocorticoid receptors with rabbit reticulocyte lysate results in ATP-dependent and monovalent cation-dependent assembly of the GR into a heterocomplex with hsp90, hsp70, and hsp56. Heterocomplex assembly is accompanied by conversion of the receptor from a form that does not bind steroid to a high affinity steroid-binding conformation. Reticulocyte lysate also promotes ATP-dependent dissociation of unliganded receptors from a prebound receptor-DNA complex. Receptor released from DNA has been reconstituted into the heat shock protein heterocomplex and converted to the non-DNA-binding state. The reticulocyte lysate also reconstitutes pp60v-src into a heterocomplex containing hsp90 and p50, both of which are components of the native heterocomplex form of the tyrosine kinase in cytoplasm. Although the c-Raf-1 serine/threonine kinase has never been found in native association with hsp90, it can be assembled into a heat shock protein heterocomplex by the ATP-dependent system in reticulocyte lysate.

Control of steroid receptor function and cytoplasmic-nuclear transport by heat shock proteins

As targeted proteins that move within the cell, the steroid receptors have become very useful probes for understanding the linked phenomena of protein folding and transport. From the study of steroid receptor-associated proteins it has become clear over the past two years that these receptors are bound to a multiprotein complex containing at least two heat shock proteins, hsp90 and hsp56. Attachment of receptors to this complex in a cell-free system appears to require the protein unfolding/folding activity of a third heat shock protein, hsp70. Like the oncogenic tyrosine kinase pp60src, steroid receptors bind to this complex of chaperone proteins at the time of their translation. Binding of the receptor to the hsp90 component of the system occurs through the hormone binding domain and is under strict hormonal control. The hormone binding domain of the receptor acts as a transferable regulatory unit that confers both tight hormonal control and hsp90 binding onto chimaeric proteins. The model of folding and transport being developed for steroid receptors leads to some general suggestions regarding the folding and transport of targeted proteins in the cell.

Steroid receptor folding by heat-shock proteins and composition of the receptor heterocomplex

Over the past 2 years, reports from several laboratories have supported the proposal that the steroid receptors are bound through the hormone-binding domain to a protein complex that contains three heat-shock proteins-hsp90, hsp70, and hsp56. This receptor-heat-shock-protein heterocomplex accounts for the behavior of the classic 9 S, non-DNA-binding form of the adrenocorticoid, sex hormone, and dioxin receptors. The receptor heterocomplex has now been reconstituted by an enzymatic system in reticulocyte lysate. This represents the first in vitro system for reversing receptor transformation, and this ability to reconstitute the receptor heterocomplex promises rapid advances in our understanding of how these receptors are folded, transported, and regulated by hormone in the cell.

Association of the 90-kDa heat shock protein does not affect the ligand-binding ability of androgen receptor

An N-terminal truncated androgen receptor with putative DNA- and ligand-binding domains (AR438) and that with a ligand-binding domain (AR612) were expressed under control of the T7 promoter in E. coli or translated in vitro with rabbit reticulocyte lysate, and their ligand-binding properties and the interaction with HSP90 were investigated. Bacterially expressed AR438 and AR612 bound a synthetic androgen, [3H]R1881, with apparent dissociation constants of 2.6 +/- 0.2 and 3.1 +/- 0.7 nM, respectively, values which are comparable to those of androgen receptor in target tissues. The recombinant androgen receptors sedimented at the 4-5 S region irrespective of the presence of 10 mM tungstate, indicating that the receptor exists free from HtpG, which is the bacterial homolog of eukaryotic HSP90. The apparent dissociation constant of truncated androgen receptors translated in vitro was 0.1 nM for AR438 and 0.2 nM for AR612. Sedimentation coefficients of in vitro translated molecules were converted from 7-8 S in the presence of tungstate to 3 S in the absence of tungstate. Both AR438 and AR612 translated in vitro were retained by anti-rat HSP90 antibody-protein A Sepharose. Exposure to 0.3 M NaCl in the presence of ligand caused dissociation of AR438 and AR612 from HSP90, and concomitantly, the DNA-cellulose binding ability of AR438 was enhanced. Thus, we conclude that the androgen receptor associates with HSP90 through the ligand-binding domain and that this association prevents the interaction of the androgen receptor with DNA. However, HSP90 seems to have little effect on the ligand-binding characteristics of the androgen receptor.

A model of glucocorticoid receptor unfolding and stabilization by a heat shock protein complex

It has recently been reported that incubation of avian progesterone receptors, mouse glucocorticoid receptors, or the viral tyrosine kinase pp60src with rabbit reticulocyte lysate reconstitutes their association with the 90 kDa heat shock protein, hsp90. The reassociation is thought to require unfolding of the steroid receptor or pp60src before hsp90 can bind. The unfoldase activity may be provided by hsp70, which is also present in the reconstituted receptor heterocomplex. In this paper we review evidence that hsp70 and hsp90 are associated in cytosolic heterocomplexes that contain a limited number of other proteins. From an analysis of known receptor-hsp interactions and a predicted direct interaction between hsp90 and hsp70 we have developed an admittedly very speculative model of glucocorticoid receptor unfolding and stabilization. One important feature of the model is that the receptor becomes attached to a heat shock protein heterocomplex rather than undergoing independent unfolding and stabilization events. The model requires that hsp70 and hsp90 bind directly to the receptor at independent sites. Importantly, the model accommodates the stoichiometry of 2 hsp90 per 1 molecule of receptor that has been assayed in the untransformed GR heterocomplex in cytosols prepared from hormone-free cells.