The molecular chaperone Hsp90 can negatively regulate the activity of a glucocorticosteroid-dependent promoter

New advances in endocrine hypertension: from genes to biomarkers

Hypertension (HT) is a major cardiovascular risk factor that affects 10% to 40% of the general population in an age-dependent manner. Detection of secondary forms of HT is particularly important because it allows the targeted management of the underlying disease. Among hypertensive patients, the prevalence of endocrine HT reaches up to 10%. Adrenal diseases are the most frequent cause of endocrine HT and are associated with excess production of mineralocorticoids (mainly primary aldosteronism), glucocorticoids (Cushing syndrome), and catecholamines (pheochromocytoma). In addition, a few rare diseases directly affecting the action of mineralocorticoids and glucocorticoids in the kidney also lead to endocrine HT. Over the past years, genomic and genetic studies have allowed improving our knowledge on the molecular mechanisms of endocrine HT. Those discoveries have opened new opportunities to transfer knowledge to clinical practice for better diagnosis and specific treatment of affected subjects. In this review, we describe the physiology of adrenal hormone biosynthesis and action, the clinical and biochemical characteristics of different forms of endocrine HT, and their underlying genetic defects. We discuss the impact of these discoveries on diagnosis and management of patients, as well as new perspectives related to the use of new biomarkers for improved patient care.

p23 and Aha1: Distinct Functions Promote Client Maturation

Hsp90 is a conserved molecular chaperone regulating the folding and activation of a diverse array of several hundreds of client proteins. The function of Hsp90 in client processing is fine-tuned by a cohort of co-chaperones that modulate client activation in a client-specific manner. They affect the Hsp90 ATPase activity and the recruitment of client proteins and can in addition affect chaperoning in an Hsp90-independent way. p23 and Aha1 are central Hsp90 co-chaperones that regulate Hsp90 in opposing ways. While p23 inhibits the Hsp90 ATPase and stabilizes a client-bound Hsp90 state, Aha1 accelerates ATP hydrolysis and competes with client binding to Hsp90. Even though both proteins have been intensively studied for decades, research of the last few years has revealed intriguing new aspects of these co-chaperones that expanded our perception of how they regulate client activation. Here, we review the progress in understanding p23 and Aha1 as promoters of client processing. We highlight the structures of Aha1 and p23, their interaction with Hsp90, and how their association with Hsp90 affects the conformational cycle of Hsp90 in the context of client maturation.

Innovative tumour targeting therapeutics in Cushing's disease

Cushing's disease (CD) is the most frequent form of endogenous hypercortisolism. Management of this devastating condition relies on pituitary surgery, while effective pharmacological treatment mainly focus on periphery targeting pharmaceuticals. Approved tumour-targeting drugs are limited to dopamine agonists and somatostatin analogues with frequently low efficacy and substantial side effects. Discoveries on the genetics and pathophysiology of corticotroph tumorigenesis brought forward new potential pharmacological targets. Compounds such as retinoic acid although promising in preclinical studies, are not as efficient in the clinic. Others, such as, silibinin, gefitinib and roscovitine are effective in preclinical models, but their efficacy and safety still needs to be determined in patients with CD.

Molecular Derangements and the Diagnosis of ACTH-Dependent Cushing's Syndrome

Endogenous Cushing's syndrome (CS) is associated with morbidities (diabetes, hypertension, clotting disorders) and shortens life because of infections, pulmonary thromboembolism, and cardiovascular disease. Its clinical presentation is immensely variable, and diagnosis and treatment are often delayed. Thus, there are many opportunities for basic and clinical research leading to better tests, faster diagnosis, and optimized medical treatments. This review focuses on CS caused by excessive adrenocorticotropin (ACTH) production. It describes current concepts of the regulation of ACTH synthesis and secretion by normal corticotropes and mechanisms by which dysregulation occurs in corticotrope (termed "Cushing's disease") and noncorticotrope (so-called ectopic) ACTH-producing tumors. ACTH causes adrenal gland synthesis and pulsatile release of cortisol; the excess ACTH in these forms of CS leads to the hypercortisolism of endogenous CS. Again, the differences between healthy individuals and those with CS are highlighted. The clinical presentations and their use in the interpretation of CS screening tests are described. The tests used for screening and differential diagnosis of CS are presented, along with their relationship to cortisol dynamics, pathophysiology, and negative glucocorticoid feedback regulation in the two forms of ACTH-dependent CS. Finally, several gaps in current understanding are highlighted in the hope of stimulating additional research into this challenging disorder.

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.

Class 3 phosphoinositide 3-kinase promotes hepatic glucocorticoid receptor stability and transcriptional activity

Aim

Lipid kinase class 3 phosphoinositide 3‐kinase (PI3K) and nuclear receptor transcription factor glucocorticoid receptor (GR) play essential physiological roles in metabolic adaptation to fasting by activating lysosomal degradation by autophagy and metabolic gene expression, yet their functional interaction is unknown. The requirement of class 3 PI3K for GR function was investigated in liver tissue.

Methods

Inactivation of class 3 PI3K was achieved through deletion of its essential regulatory subunit Vps15, by expressing Cre‐recombinase in the livers of Vps15^f/f mice. The response to both 24‐h fasting and synthetic GR ligand, dexamethasone (DEX) was evaluated in control and mutant mice. Liver tissue was analysed by immunoblot, RT‐qPCR, and LC‐MS.

Results

Vps15 mutant mice show decreased transcript levels of GR targets, coupled with lower nuclear levels of total and phosphorylated on Ser211, GR protein. Acute DEX treatment and 24‐h fasting both failed to re‐activate expression of GR targets in the livers of Vps15 mutant mice to the levels observed in controls. Decreased levels of endogenous GR ligand corticosterone and lower expression of 11β‐hydroxysteroid dehydrogenase 1 (11β‐HSD1), a metabolic enzyme that controls corticosterone availability, were found in the livers of Vps15 mutants. Hepatic Vps15 depletion resulted in the activation of nuclear Akt1 signalling, which was paralleled by increased polyubiquitination of GR.

Conclusion

In the liver, class 3 PI3K is required for corticosterone metabolism and GR transcriptional activity.

Effect of EPA on Hsp90 and GRα protein expression in multiple myeloma drug-resistant cells

BACKGROUND

Approximately 20% of MM patients harbor glucocorticoid (GC) resistance and are not responsive to therapeutic effect. Chaperoneheat-shock proteins Hsp90 is needed for ligand docking, The imbalance of Hsp90/GRα (glucocorticoid receptor α) may be an important cause of GC resistance. Recent studies have indicated that EPA could repress cancer cell growth by regulating critical influential factors in progression of cancer, consisting of resistance to drugs, chemosensitivity. The aim of the present study was to test the cytotoxic effects of EPA alone or EPA + Dexamethasone in dexamethasone-resistant MM cell (MM.1R) and investigate whether DHA can induce apoptosis and reverse acquired glucocorticoid resistance in dexamethasone-resistant MM cell (MM.1R).

METHODS

Cell Counting Kit-8 (CCK-8) was used to detect the proliferation of MM.1R cells after treating with EPA alone and EPA combined with DEX. Mitochondrial membrane potential was measured by flow cytometry and GRα and Hsp90 protein expression were assessed by western blot analysis.

RESULTS

EPA alone was able to inhibit cell proliferation as evidenced by CCK-8 assay and the tumor growth was remarkably suppressed by EPA + Dexamethasone, Cell apoptosis after EPA treatment was obviously observed by Flow cytometry analysis of the mitochondrial membrane potential. Analysis of Hsp90 and GRα proteins in MM.1R cells incubated with EPA revealed down-regulation of Hsp90 and up-regulation of GRα. Accordingly, the Hsp90/GRα ratio was significantly decreased with the increase of EPA concentration.

CONCLUSIONS

EPA might be used as a new effective treatment for reversal of glucocorticoid-resistance in multiple myeloma.

Generalized and tissue specific glucocorticoid resistance

Glucocorticoids (GCs) are steroid hormones that influence several physiologic functions and are among the most frequently prescribed drugs worldwide. Resistance to GCs has been observed in the context of the familial generalized GC resistance (Chrousos' syndrome) or tissue specific GC resistance in chronic inflammatory states. In this review, we have summarized the major factors that influence individual glucocorticoid sensitivity/resistance. The fine-tuning of GC action is determined in a tissue-specific fashion that includes the combination of different GC receptor promoters, translation initiation sites, splice isoforms, interacting proteins, post-translational modifications, and alternative mechanisms of signal transduction.

Posttranslational Modifications of the Mineralocorticoid Receptor and Cardiovascular Aging

During aging, the cardiovascular system is especially prone to a decline in function and to life-expectancy limiting diseases. Cardiovascular aging is associated with increased arterial stiffness and vasoconstriction as well as left ventricular hypertrophy and reduced diastolic function. Pathological changes include endothelial dysfunction, atherosclerosis, fibrosis, hypertrophy, inflammation, and changes in micromilieu with increased production of reactive oxygen and nitrogen species. The renin-angiotensin-aldosterone-system is an important mediator of electrolyte and blood pressure homeostasis and a key contributor to pathological remodeling processes of the cardiovascular system. Its effects are partially conveyed by the mineralocorticoid receptor (MR), a ligand-dependent transcription factor, whose activity increases during aging and cardiovascular diseases without correlating changes of its ligand aldosterone. There is growing evidence that the MR can be enzymatically and non-enzymatically modified and that these modifications contribute to ligand-independent modulation of MR activity. Modifications reported so far include phosphorylation, acetylation, ubiquitination, sumoylation and changes induced by nitrosative and oxidative stress. This review focuses on the different posttranslational modifications of the MR, their impact on MR function and degradation and the possible implications for cardiovascular aging and diseases.

Terazosin reduces steroidogenic factor 1 and upregulates heat shock protein 90 expression in LH-induced bovine ovarian theca cells

Hyperthecosis syndrome is a common endocrine system metabolic disorder in women of childbearing age. The main symptoms are elevated androgen levels, abnormal ovulation, and excessive oxidative stress. Currently, there is no effective treatment for hyperthecosis syndrome. α(1)-adrenergic receptor (ADRA1) is involved in the metabolic pathway of ovarian steroid hormone. This study studied the mechanism of the ADRA1 inhibitor terazosin in the LH-induced bovine theca cells in vitro. We found that terazosin regulates the expression of steroidogenic factor 1 (SF1) and downstream genes through the ERK1/2 pathway, reducing androgen content. Terazosin promotes the expression of HSP90 and reduces the activity of iNOS. In addition, Terazosin up-regulates the expression of nuclear factor erythroid 2-related factor 2 (Nrf2) and its downstream gene γ-GCS, which improves the ability of theca cells to resist oxidative stress. This study provides a reference for the treatment of human hyperthecosis syndrome.

The Nuclear and DNA-Associated Molecular Chaperone Network

Maintenance of a healthy and functional proteome in all cellular compartments is critical to cell and organismal homeostasis. Yet, our understanding of the proteostasis process within the nucleus is limited. Here, we discuss the identified roles of the major molecular chaperones Hsp90, Hsp70, and Hsp60 with client proteins working in diverse DNA-associated pathways. The unique challenges facing proteins in the nucleus are considered as well as the conserved features of the molecular chaperone system in facilitating DNA-linked processes. As nuclear protein inclusions are a common feature of protein-aggregation diseases (e.g., neurodegeneration), a better understanding of nuclear proteostasis is warranted.

Impact of USP8 Gene Mutations on Protein Deregulation in Cushing Disease

CONTEXT

Cushing disease (CD) is a rare disorder with severe sequels and incompletely understood pathogenesis. The underlying corticotroph adenomas harbor frequently somatic mutations in the ubiquitin-specific peptidase 8 (USP8) gene. These mutations render USP8 hyperactive and prevent client proteins from degradation.

OBJECTIVE

To investigate the impact of USP8 mutations on proteins deregulated in CD.

DESIGN

One hundred eight pituitary adenomas (75 corticotroph [58 USP8 wild type (WT) and 17 USP8 mutated], 14 somatotroph, and 19 nonfunctioning) were investigated by immunohistochemistry. All evaluated proteins [USP8, arginine vasopressin receptor 1b and 2, corticotropin-releasing hormone receptor, cAMP response element-binding protein (CREB), p27/kip1, cyclin E, heat shock protein 90 (HSP90), orphan nuclear receptor 4, epidermal growth factor receptor, histone deacetylase 2, glucocorticoid receptor, cyclin-dependent kinase 5 and Abelson murine leukemia viral oncogene homolog 1 enzyme substrate 1] were known to be deregulated in CD. Furthermore, AtT20 cells were transfected with USP8 to investigate the expression of possible downstream proteins by immunoblot.

RESULTS

Whereas most of the investigated proteins were not differentially expressed, the cell-cycle inhibitor p27 was significantly reduced in USP8 mutated corticotroph adenoma (H-score 2.0 ± 1.0 vs 1.1 ± 1.1 in WT adenomas; P = 0.004). In contrast, the chaperone HSP90 was expressed higher (0.5 ± 0.4 vs 0.2 ± 0.4; P = 0.29), and the phosphorylation of the transcription factor CREB was increased in USP8 mutated adenomas (1.30.5 ± 0.40.9 vs 0.70.5 ± 0.40.7; P = 0.014). Accordingly, AtT20 cells transfected with the USP8 P720R mutant had higher phosphorylated CREB (pCREB) levels than WT transfected cells (1.3 ± 0.14 vs 1 ± 0.23; P = 0.13).

CONCLUSIONS

We could demonstrate that USP8 mutations are associated with deregulation of p27/kip1, HSP90, and pCREB. These findings suggest that these proteins are direct or indirect clients of USP8 and could therefore be potential targets for therapeutic approaches in patients with CD.

Tumor-Directed Therapeutic Targets in Cushing Disease

CONTEXT

The most frequent cause of endogenous hypercortisolism is Cushing disease (CD), a devastating condition associated with severe comorbidities and high mortality. Effective tumor-targeting therapeutics are limited.

DESIGN

Search in PubMed with key words "corticotroph" and "Cushing's disease" plus the name of the mentioned therapeutic agent and in associated references of the obtained papers. Additionally, potential therapeutics were obtained from ClinicalTrials.gov with a search for "Cushing disease."

RESULTS

At present, the tumor-targeted pharmacological therapy of CD is concentrated on dopamine agonists (cabergoline) and somatostatin analogs (pasireotide) with varying efficacy, escape from response, and considerable side effects. Preclinical studies on corticotroph pathophysiology have brought forward potential drugs such as retinoic acid, silibinin, and roscovitine, whose efficacy and safety remain to be determined.

CONCLUSIONS

For many patients with CD, effective tumor-targeted pharmacological therapy is still lacking. Coordinated efforts are pivotal in establishing efficacy and safety of novel therapeutics in this rare but devastating disease.

Recent advances in understanding corticotroph pituitary tumor initiation and progression

Cushing’s disease is the most frequent form of hypercortisolism and is caused by hypophyseal corticotroph adenomas secreting excessive amounts of adrenocorticotropic hormone. Most of the tumors develop sporadically and only a limited number of corticotroph adenomas have been found to be associated with different neuroendocrine syndromes or with familial isolated pituitary adenomas. The pathogenic mechanisms of corticotroph adenomas are largely unknown, but the discovered aberrant chaperoning activity of heat shock protein 90 on the one hand and the presence of ubiquitin-specific protease 8 mutations on the other hand partially explained the causes of their development. Corticotroph tumors arise initially as benign microadenomas but with time form invasively growing aggressive macroadenomas which can switch to corticotroph carcinomas in extremely rare cases. The mechanisms through which corticotroph tumors escape from glucocorticoid negative feedback are still poorly understood, as are the processes that trigger the progression of benign corticotroph adenomas toward aggressive and malignant phenotypes. This review summarizes recent findings regarding initiation and progression of corticotroph pituitary tumors.

Effects of thermal stress on the expression of glucocorticoid receptor complex linked genes in Senegalese sole (Solea senegalensis): Acute and adaptive stress responses

The present study examined the short and mid-term effects of a rise in temperature from 18°C to 24°C on the expression of genes related to the stress response regulation in juveniles of Senegalese sole, Solea senegalensis. The animals were exposed to a temperature increase of 6°C, after 1month of acclimation at 18°C. After this process, samples of different tissues were collected from a total of 96 fish at four sampling points: 1h, 24h, 3days and 1week. The transcript levels of a set of genes involved in the stress response such as glucocorticoid receptors 1 and 2, corticotrophin-releasing factor, corticotrophin-releasing factor binding proteins, proopiomelanocortin A and B, and cellular stress defense (heat shock protein 70, 90AA and 90AB) were quantified at these sampling points. Additionally, blood samples were also taken to measure the circulating plasma cortisol concentration. Thermal stress induced by increasing temperature prompted an elevation of plasma cortisol levels in juvenile Senegalese sole after 1h as a short-term response, and a consecutive increase after one week, as a mid-term response. Senegalese sole seemed to respond positively in terms of adaptive mechanisms, with a rapid over-expression of grs and hsps in liver and brain, significantly higher after one hour post stress, denoting the fast and acute response of those tissues to a rapid change on temperature. The ratio hsp90/gr also increased 24h after thermal shock, ratio proposed to be an adaptive mechanism to prevent proteosomal degradation of GR. As a mid-term response, the elevation of brain crfbp gene expression one week after thermal shock could be an adaptive mechanism of negative feedback on HPI axis. Taken together, these data suggested an initial up-regulation of the glucocorticoid receptor complex linked genes in response to a temperature increase in Senegalese sole, with heat shock protein 90 potentially being a regulatory factor for the glucocorticoid receptor in the presence of cortisol.

Genetic variation in the glucocorticoid pathway involved in interindividual differences in the glucocorticoid treatment

Glucocorticoids (GCs) are widely used for treating asthma, rheumatoid arthritis, nephrotic syndrome, acute lymphoblastic leukemia and other autoimmune diseases. However, in a subgroup of patients, failure to respond to GCs is known as GC resistance or GC insensitivity. This represents an important barrier to effective treatment and a clinical problem requiring an urgent solution. Genetic variation in the GC pathway is a significant factor in interindividual differences in GC treatment. This article reviews the pharmacogenetics of GCs in diverse diseases based on the GC pathway.

Melatonin modulates glucocorticoid receptor mediated inhibition of antioxidant response and apoptosis in peripheral blood mononuclear cells

Pineal melatonin is known for its immunomodulatory and anti-stress properties. It modulates stress condition by regulating antioxidant responses and apoptosis in the immune cells. Stress causes increased glucocorticoid level that acts through glucocorticoid receptor (GR) and is translocated into nucleus under regulation of HSP90 based chaperone machinery. Melatonin influences glucocorticoid and GR mediated stress condition in rodents, but till date there are no reports which could suggest the effect of melatonin treatment on GR mediated apoptosis and inhibition of Nrf-2/hemeoxygenase-1 (HO-1) induced antioxidant status in immunocompetent cells (peripheral blood mononuclear cells; PBMCs). Therefore, in the present study, we considered GR mediated inhibition of Nrf2 and HO-1 along with anti-apoptotic Bcl-2 expression in PBMCs. The PBMCs were treated with synthetic glucocorticoid; dexamethasone (Dex) and melatonin (Mel), to explore the effect of melatonin treatment in regulation of GR mediated apoptosis and inhibition of antioxidant status in immune cells. It was noted that melatonin treatment retained GR into cytoplasm by inhibiting the dissociation of HSP90 from GR-HSP90 complex and enhanced expression of Nrf2/HO-1 and Bcl-2 expression. This led to increased HO-1 expression and elevated Bcl-2 led to increased Bcl-2/Bax ratio that might ultimately enhanced the cellular antioxidant response and survival under glucocorticoid mediated stress condition. Our observations suggest that the declined GR nuclear translocation upon melatonin treatment might be responsible for the up-regulation of Nrf2 mediated HO-1 activity and increased Bcl-2/Bax ratio in PBMCs to maintain the immune homeostasis under stress condition.

The New Molecular Landscape of Cushing's Disease

Cushing's disease (CD) is caused by corticotropin-secreting pituitary adenomas and results in substantial morbidity and mortality. Its molecular basis has remained poorly understood until the past few years, when several proteins and genes [such as testicular orphan nuclear receptor 4 (TR4) and heat shock protein 90 (HSP90)] were found to play key roles in the disease. Most recently, mutations in the gene of ubiquitin-specific peptidase 8 (USP8) increasing its deubiquination activity were discovered in a high percentage of corticotroph adenomas. Here, we will discuss emerging insights in the molecular alterations that finally result in CD. The therapeutic potential of these findings needs to be carefully evaluated in the near future, hopefully resulting in new treatment options for this devastating disorder.

A C-terminal HSP90 inhibitor restores glucocorticoid sensitivity and relieves a mouse allograft model of Cushing disease

One function of the glucocorticoid receptor (GR) in corticotroph cells is to suppress the transcription of the gene encoding proopiomelanocortin (POMC), the precursor of the stress hormone adrenocorticotropin (ACTH). Cushing disease is a neuroendocrine condition caused by partially glucocorticoid-resistant corticotroph adenomas that excessively secrete ACTH, which leads to hypercortisolism. Mutations that impair GR function explain glucocorticoid resistance only in sporadic cases. However, the proper folding of GR depends on direct interactions with the chaperone heat shock protein 90 (HSP90, refs. 7,8). We show here that corticotroph adenomas overexpress HSP90 compared to the normal pituitary. N- and C-terminal HSP90 inhibitors act at different steps of the HSP90 catalytic cycle to regulate corticotroph cell proliferation and GR transcriptional activity. C-terminal inhibitors cause the release of mature GR from HSP90, which promotes its exit from the chaperone cycle and potentiates its transcriptional activity in a corticotroph cell line and in primary cultures of human corticotroph adenomas. In an allograft mouse model, the C-terminal HSP90 inhibitor silibinin showed anti-tumorigenic effects, partially reverted hormonal alterations, and alleviated symptoms of Cushing disease. These results suggest that the pathogenesis of Cushing disease caused by overexpression of heat shock proteins and consequently misregulated GR sensitivity may be overcome pharmacologically with an appropriate HSP90 inhibitor.

Nuclear shuttling precedes dimerization in mineralocorticoid receptor signaling

The mineralocorticoid receptor (MR), a member of the steroid receptor superfamily, regulates water-electrolyte balance and mediates pathophysiological effects in the renocardiovascular system. Previously, it was assumed that after binding aldosterone, the MR dissociates from HSP90, forms homodimers, and then translocates into the nucleus where it acts as a transcription factor (Guiochon-Mantel et al., 1989; Robertson et al., 1993; Savory et al., 2001). We found that, during aldosterone-induced nuclear translocation, MR is bound to HSP90 both in the cytosol and the nucleus. Homodimerization measured by eBRET and FRET takes place when the MR is already predominantly nuclear. In vitro binding of MR to DNA was independent of ligand but could be partially inhibited by geldanamycin. Overall, here we provide insights into classical MR signaling necessary for elucidating the mechanisms of pathophysiological MR effects and MR specificity.

Estrogen signaling and the aging brain: context-dependent considerations for postmenopausal hormone therapy

Recent clinical studies have spurred rigorous debate about the benefits of hormone therapy (HT) for postmenopausal women. Controversy first emerged based on a sharp increase in the risk of cardiovascular disease in participants of the Women's Health Initiative (WHI) studies, suggesting that decades of empirical research in animal models was not necessarily applicable to humans. However, a reexamination of the data from the WHI studies suggests that the timing of HT might be a critical factor and that advanced age and/or length of estrogen deprivation might alter the body's ability to respond to estrogens. Dichotomous estrogenic effects are mediated primarily by the actions of two high-affinity estrogen receptors alpha and beta (ER α & ER β ). The expression of the ERs can be overlapping or distinct, dependent upon brain region, sex, age, and exposure to hormone, and, during the time of menopause, there may be changes in receptor expression profiles, post-translational modifications, and protein:protein interactions that could lead to a completely different environment for E2 to exert its effects. In this review, factors affecting estrogen-signaling processes will be discussed with particular attention paid to the expression and transcriptional actions of ER β in brain regions that regulate cognition and affect.

A ligand-specific kinetic switch regulates glucocorticoid receptor trafficking and function

The ubiquitously expressed glucocorticoid receptor (GR) is a major drug target for inflammatory disease, but issues of specificity and target tissue sensitivity remain. We now identify high potency, non-steroidal GR ligands, GSK47867A and GSK47869A, which induce a novel conformation of the GR ligand-binding domain (LBD) and augment the efficacy of cellular action. Despite their high potency, GSK47867A and GSK47869A both induce surprisingly slow GR nuclear translocation, followed by prolonged nuclear GR retention, and transcriptional activity following washout. We reveal that GSK47867A and GSK47869A specifically alter the GR LBD structure at the HSP90-binding site. The alteration in the HSP90-binding site was accompanied by resistance to HSP90 antagonism, with persisting transactivation seen after geldanamycin treatment. Taken together, our studies reveal a new mechanism governing GR intracellular trafficking regulated by ligand binding that relies on a specific surface charge patch within the LBD. This conformational change permits extended GR action, probably because of altered GR-HSP90 interaction. This chemical series may offer anti-inflammatory drugs with prolonged duration of action due to altered pharmacodynamics rather than altered pharmacokinetics.

Total substitution of fish oil by vegetable oils in Senegalese sole (Solea senegalensis) diets: effects on fish performance, biochemical composition, and expression of some glucocorticoid receptor-related genes

To study the substitution of fish oil by vegetable oils in fish diets, juveniles Senegalese sole (Solea senegalensis) were fed diets (56 % crude protein, 12 % crude lipid) containing either linseed (100LO) or soybean (100SO) oils in comparison with a 100 % fish oil-based diet (100FO) for 90 days. Samples of muscle, liver, and intestine were collected for biochemical analysis and for glucocorticoid receptor-related genes, including GR1 and GR2, and the associated heat shock proteins HSP70, HSP90AA, and HSP90AB. Besides, basal levels of plasma cortisol were also determined. After the feeding period, a stress test, consisting on 5 min of net chasing, was applied to a selected population of each dietary group. Total replacement of fish oil by vegetable oils did not induced changes in fish growth and performance, but affected fatty acid profile of muscle, liver, and intestine, reflecting those tissues the characteristic fatty acids of each type of dietary oil. A tendency to conserve the ARA/EPA ratio could be observed in the different tissues, despite of the level of these fatty acids in diet. Chasing stress induced an increase of muscle GR1 and a reduction in intestinal GR2 relative expressions at any of the experimental diets assayed. In liver, chasing stress induced an increase in both GR1 and GR2 gene expression in fish fed fish oil diets. Similarly, chasing stress induced an increase of muscle HSP70 and decrease of HSP90AB in liver at any of the experimental diet assayed. Besides, vegetable oils decreased the expression of HSP70 in intestine, being the relative expression of liver HSP90AA increased by the inclusion of linseed oil in the diet, at any of the experimental conditions assayed.

Hsp90 inhibitors are efficacious against Kaposi Sarcoma by enhancing the degradation of the essential viral gene LANA, of the viral co-receptor EphA2 as well as other client proteins

Heat-shock protein 90 (Hsp90) inhibitors exhibit activity against human cancers. We evaluated a series of new, oral bioavailable, chemically diverse Hsp90 inhibitors (PU-H71, AUY922, BIIB021, NVP-BEP800) against Kaposi sarcoma (KS). All Hsp90 inhibitors exhibited nanomolar EC(50) in culture and AUY922 reduced tumor burden in a xenograft model of KS. KS is associated with KS-associated herpesvirus (KSHV). We identified the viral latency associated nuclear antigen (LANA) as a novel client protein of Hsp90 and demonstrate that the Hsp90 inhibitors diminish the level of LANA through proteasomal degradation. These Hsp90 inhibitors also downregulated EphA2 and ephrin-B2 protein levels. LANA is essential for viral maintenance and EphA2 has recently been shown to facilitate KSHV infection; which in turn feeds latent persistence. Further, both molecules are required for KS tumor formation and both were downregulated in response to Hsp90 inhibitors. This provides a rationale for clinical testing of Hsp90 inhibitors in KSHV-associated cancers and in the eradication of latent KSHV reservoirs.

Nuclear HSP90 regulates the glucocorticoid responsiveness of PBMCs in patients with idiopathic nephrotic syndrome

Resistance to glucocorticoid (GC) is a challenge for the treatment of patients with idiopathic nephrotic syndrome (INS). Most of the effects of GC are mediated by the GC receptor (GR). Heat shock protein 90 (HSP90) is an important molecular chaperone for the GR and is supposed to be the key factor in regulating GC effects. In a previous study, we found that both the expression and nuclear distribution of HSP90 were increased in GC resistant INS patients. The aim of this study is to explore how these phenomena contribute to GC resistance in INS patients. Healthy subjects and INS patients with different GC responses were recruited. The total HSP90 expression was determined by reverse transcription-PCR and flow cytometric analysis. Western blot analysis was used to evaluate the expression of nuclear HSP90. Co-immunoprecipitation and electrophoretic mobility gel shift assays were performed to explore the interaction between HSP90 and the GR in the nucleus as well as the DNA-binding activity of GR. We induced the upregulation of the expression of total HSP90 in PBMCs by treatment with interleukin-6 in vitro and found that the nuclear HSP90 level, the DNA-binding activity of the GR and the cell apoptotic responsiveness to GC remained unchanged. Furthermore, an increased nuclear HSP90 was demonstrated mainly by binding to GR in the nucleus, while the DNA-binding activity of the GR dramatically decreased in GC resistant INS patients. The present results suggest that the accumulation of HSP90 in the nucleus potentially hinders DNA-binding activity and transactivation, which may contribute to GC resistance in patients with INS.

Chronic stress decreases availability of heat shock proteins to glucocorticoid receptor in response to novel acute stress in Wistar rat hypothalamus

Chronic psychosocial isolation (CPSI) is known to cause several maladaptive changes in the limbic brain structures, which regulate the hypothalamic-pituitary-adrenal (HPA) axis activity. In this study, we focused our investigation on CPSI effects in the hypothalamus (HT) since it is a major driver of HPA axis activity. We also investigated whether the exposure to CPSI could alter the response to subsequent acute stress (30-min immobilization). In the HT, we followed cytosolic and nuclear levels of the glucocorticoid receptor (GR), as a mediator of HPA axis feedback inhibition, and its chaperones, the heat shock proteins (HSPs), hsp70 and hsp90. The CPSI did not cause any changes in either GR or HSPs levels. However, we observed increase of the GR and hsp70 in both HT cellular compartments as a response of naïve rats to acute stress, whereas the response of CPSI rats to acute stress was associated with elevation of the GR in the cytosol and decrease of HSPs in the nucleus. Thus, our data indicated reduced availability of HSPs to GR in both cytosol and nucleus of the HT under acute stress of CPSI animals, and therefore, pointed out to potentially negative effects of CPSI on GR function in the HT.

The myosin-binding UCS domain but not the Hsp90-binding TPR domain of the UNC-45 chaperone is essential for function in Caenorhabditis elegans

The UNC-45 family of molecular chaperones is expressed in metazoan organisms from Caenorhabditis elegans to humans. The UNC-45 protein is essential in C. elegans for early body-wall muscle cell development and A-band assembly. We show that the myosin-binding UCS domain of UNC-45 alone is sufficient to rescue lethal unc-45 null mutants arrested in embryonic muscle development and temperature-sensitive loss-of-function unc-45 mutants defective in worm A-band assembly. Removal of the Hsp90-binding TPR domain of UNC-45 does not affect rescue. Similar results were obtained with overexpression of the same fragments in wild-type nematodes when assayed for diminution of myosin accumulation and assembly. Titration experiments show that, on a per molecule basis, UCS has greater activity in C. elegans muscle in vivo than full-length UNC-45 protein, suggesting that UNC-45 is inhibited by either the TPR domain or its interaction with the general chaperone Hsp90. In vitro experiments with purified recombinant C. elegans Hsp90 and UNC-45 proteins show that they compete for binding to C. elegans myosin. Our in vivo genetic and in vitro biochemical experiments are consistent with a novel inhibitory role for Hsp90 with respect to UNC-45 action.

17-allyamino-17-demethoxygeldanamycin treatment results in a magnetic resonance spectroscopy-detectable elevation in choline-containing metabolites associated with increased expression of choline transporter SLC44A1 and phospholipase A2

Introduction

17-allyamino-17-demethoxygeldanamycin (17-AAG), a small molecule inhibitor of Hsp90, is currently in clinical trials in breast cancer. However, 17-AAG treatment often results in inhibition of tumor growth rather than shrinkage, making detection of response a challenge. Magnetic resonance spectroscopy (MRS) and spectroscopic imaging (MRSI) are noninvasive imaging methods than can be used to monitor metabolic biomarkers of drug-target modulation. This study set out to examine the MRS-detectable metabolic consequences of Hsp90 inhibition in a breast cancer model.

Methods

MCF-7 breast cancer cells were investigated, and MRS studies were performed both on live cells and on cell extracts. ³¹P and ¹H MRS were used to determine total cellular metabolite concentrations and ¹³C MRS was used to probe the metabolism of [1,2-¹³C]-choline. To explain the MRS metabolic findings, microarray and RT-PCR were used to analyze gene expression, and in vitro activity assays were performed to determine changes in enzymatic activity following 17-AAG treatment.

Results

Treatment of MCF-7 cells with 17-AAG for 48 hours caused a significant increase in intracellular levels of choline (to 266 ± 18% of control, P = 0.05) and phosphocholine (PC; to 181 ± 10% of control, P = 0.001) associated with an increase in expression of choline transporter SLC44A1 and an elevation in the de novo synthesis of PC. We also detected an increase in intracellular levels of glycerophosphocholine (GPC; to 176 ± 38% of control, P = 0.03) associated with an increase in PLA2 expression and activity.

Conclusions

This study determined that in the MCF-7 breast cancer model inhibition of Hsp90 by 17-AAG results in a significant MRS-detectable increase in choline, PC and GPC, which is likely due to an increase in choline transport into the cell and phospholipase activation. ¹H MRSI can be used in the clinical setting to detect levels of total choline-containing metabolite (t-Cho, composed of intracellular choline, PC and GPC). As Hsp90 inhibitors enter routine clinical use, t-Cho could thus provide an easily detectable, noninvasive metabolic biomarker of Hsp90 inhibition in breast cancer patients.

Role of glucocorticoids and the glucocorticoid receptor in metabolism: insights from genetic manipulations

Since the discovery of the beneficial effects of adrenocortical extracts for treating adrenal insufficiency more than 80 years ago, glucocorticoids and their cognate, intracellular receptor, the glucocorticoid receptor have been characterized as critical checkpoints in the delicate hormonal control of energy homeostasis in mammals. Whereas physiological levels of glucocorticoids are required for proper metabolic control, aberrant glucocorticoid action has been linked to a variety of pandemic metabolic diseases, such as type II diabetes and obesity. Based on its importance for human health, studies of the molecular mechanisms of within the glucocorticoid signaling axis have become a major focus in biomedical research. In particular, the understanding of tissue-specific functions of the glucocorticoid receptor pathway has been proven to be of substantial value for the development of novel therapies in the treatment of chronic metabolic disorders. Therefore, this review focuses on the consequences of endogenous and experimental modulation of glucocorticoid receptor expression for metabolic homeostasis and dysregulation, particularly emphasizing tissue-specific contributions of the glucocorticoid pathway to the control of energy metabolism.

Glucocorticoid receptor dynamics and gene regulation

The glucocorticoid receptor regulates the expression of a large number of genes in mammalian cells. The interaction of this receptor with regulatory elements has been discovered to be highly dynamic, with occupancy states measured in seconds, rather than minutes or hours. This finding has led to a paradigm shift in our understanding of receptor function throughout the genome. The mechanisms involved in these rapid exchange events, as well as the implications for receptor function, are discussed.

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.

Indirect participation of Hsp90 in the regulation of the cyclin E turnover

Cyclin E is the Cdk2-regulatory subunit required for the initiation of DNA replication at the G1/S transition. It accumulates in late G1 phase and gets rapidly degraded by the ubiquitin/proteasome pathway during S phase. The degradation of cyclin E is a consequence of its phosphorylation and subsequent isomerization by the peptidyl-prolyl isomerase Pin1. We show that in the colon cancer cells HT-29 the inhibition of the chaperone function of Hsp90 by geldanamycin (GA) enhances the ubiquitinylation of cyclin E and triggers active degradation via the proteasome pathway. As Hsp90 forms multiprotein complexes with and regulates the function and cell contents of numerous signaling proteins, this observation suggests a direct interaction between Hsp90 and cyclin E. However, experiments using cell lysate fractionation did not reveal the presence of complexes containing both Hsp90 and cyclin E. Coupled transcription/translation experiments also failed to detect the formation of complexes between newly synthesized cyclin E and Hsp90. We conclude that Hsp90 can regulate the degradation of cellular proteins without binding to them, by an indirect mechanism. This conclusion postulates a new category of proteins that are affected by the inactivation of Hsp90. Our observations do not support the possible involvement of a PPIase in this indirect mechanism. Besides, we did not observe active geldanamycin-dependent degradation of cyclin E in the prostate cancer-derived cell line DU-145, indicating that the Hsp90-dependent stabilization of cyclin E requires specific regulatory mechanism which may be lost in certain types of cancer cells.

Patients with multiple sclerosis resisted to glucocorticoid therapy: abnormal expression of heat-shock protein 90 in glucocorticoid receptor complex

OBJECTIVE

The majority of patients with multiple sclerosis (MS) respond favorably to glucocorticoids (GS) for their relapse treatment (steroid-sensitive multiple sclerosis). Unfortunately, a small subset of patients with multiple-sclerosis fails to adequately respond even to high dose of GS (steroid-resistant multiple sclerosis). Mechanism of GS therapeutic unresponsiveness is not resolved.

METHODS

Transcripts for glucocorticoid receptor (GR) was assessed in peripheral blood mononuclear cells by real-time polymerase chain reaction in patients with steroid-sensitive and steroid-resistant multiple sclerosis. GR expression was assessed by Western blotting. The amount of heat-shock protein 90 (hsp90) in GR cytoplasmic complex was assessed by immunoprecipitation. Hsp90 was shown to stabilize the GR complex, to prevent its translocation to nucleus, and to inhibit GR transcription.

RESULTS

Peripheral blood mononuclear cells of steroid-resistant multiple sclerosis transcripts for all three isoforms of GR, alpha, beta, and gamma, were reduced by about two-folds compared with patients with steroid-sensitive multiple sclerosis. We have not found an increase in the beta and gamma transcripts of GR, which might serve as a dominant negative mutants, over GR alpha in steroid-resistant multiple sclerosis. The amount of hsp90 in the GR complex in cytoplasm was significantly higher in steroid-resistant multiple sclerosis compared with steroid-sensitive multiple sclerosis.

CONCLUSIONS

Molecular mechanism of GS unresponsiveness in some patients with multiple sclerosis might be related to increased presence of hsp90 in the GR cytoplasmic complex, leading to the inhibition of GR translocation to nucleus and reduction in its transcription.

Time-course changes in nuclear translocation of hepatic glucocorticoid receptor in rats after burn trauma and its pathophysiological significance

Nuclear translocation is a determining step for the glucocorticoid receptor (GR) to exert its functions in response to traumatic conditions. This study was designed to observe the nuclear translocation changes of hepatic GR in severely burned rats during early postburn stage, and to explore the effects of high-dose dexamethasone on GR nuclear translocation. Rats with 35% total body surface area full-thickness burn injury, parallelized with a sham-burn group, were killed at consecutive time points to examine the changes in plasma corticosterone and expression of hepatic GR at both whole-cell and nuclear levels. The effects of high-dose dexamethasone on GR nuclear translocation and suppression of proinflammatory cytokine overproduction were subsequently analyzed. In burned rats, plasma corticosterone increased remarkably soon after burn injury. On the contrary, the hepatic GR levels showed an initial phase of decrease as measured in both whole-cell and nucleus by Western blot, followed by a rapid elevation in the nucleus but a slow recovery at whole-cell level. By comparing the changes of GR in both whole-cell and nuclear levels, we found that GR nuclear translocation was relatively enhanced in the early postburn period. High-dose dexamethasone administered at 1 or 48 h postburn did not further elevate GR nuclear translocation, neither did it restrain the increased release of proinflammatory cytokines such as TNF-alpha and IL-1 beta. These studies suggest that although the whole-cell level of hepatic GR is decreased, GR nuclear translocation is relatively enhanced at early postburn stage. High-dose exogenous glucocorticoids may not promote more nuclear translocation of GR to reinforce its functions.

The glucocorticoid responses are shaped by molecular chaperones

The glucocorticoid receptor is a known regulator of a variety of physiological processes. Its mode of action is well defined: upon hormone binding, it undergoes a conformational change, translocates to the nucleus and modulates the transcription of target genes. Molecular chaperones have a widely recognized role in the folding of newly made proteins, but their participation in further maturation of folded proteins to their active states and beyond tends to be underestimated. This review presents the current knowledge on how the Hsp70 and Hsp90 chaperone machines help to shape the responses to glucocorticoids. We discuss the contributions of these molecular chaperones to folding, activation, intracellular transport, transcriptional regulation, and decay of the glucocorticoid receptor.

Glucocorticoid resistance syndrome: A diagnostic and therapeutic approach

In the past decades, several cases of the syndrome of generalized glucocorticoid (GC) resistance have been reported. This familial disease is characterized by reduced cortisol effects, due to a GC receptor (GR) defect, which is compensated by hyperactivity of the hypothalamic-pituitary-adrenal (HPA) axis. As a consequence, patients present with signs of adrenal overproduction of mineralocorticoids (hypertension and hypokalaemic alkalosis) and, in females, of androgens (hirsutism, male pattern of baldness, menstrual irregularities). In a few kindreds the underlying molecular basis has been revealed--e.g. mutations in the gene coding for the GR--but in a substantial number of patients the cause of GC resistance has not yet been elucidated. In this chapter we also discuss some other determinants which can lead to GC resistance. Diagnosis of generalized GC resistance can be difficult. This review highlights the diagnostic process and therapeutic options for treating patients with this disease.

Unliganded and hormone-bound glucocorticoid receptors interact with distinct hydrophobic sites in the Hsp90 C-terminal domain

Unlike most chaperones, heat-shock protein 90 (Hsp90) interacts with a select group of "client proteins" that regulate essential biological processes. Little is known about how Hsp90 recognizes and binds these proteins. The glucocorticoid receptor (GR) is a well characterized Hsp90 client protein, whose hormone binding, nuclear-cytoplasmic trafficking, and transcriptional activity are regulated by Hsp90. Here, we provide evidence that unliganded and hormone-bound GR interact with two distinct, solvent-exposed hydrophobic sites in the Hsp90 C-terminal domain that contain the sequences "MxxIM" (HM10) and "L/MxxIL" (HM9). Our results indicate that binding of Hsp90 HM10 to unliganded GR stabilizes the unliganded ligand-binding pocket of GR indirectly by promoting an intramolecular interaction between the C-terminal alpha-helix (H12) and a solvent-exposed hydrophobic groove in the GR ligand binding domain. In the presence of hormone, Hsp90 appears to bind the hydrophobic groove of GR directly by mimicking the interactions of GR with transcriptional coactivators. The identified interactions provide insights into the mechanisms that enable Hsp90 to regulate the activity of both unliganded and hormone-bound GR and to sharpen the cellular response to hormone.

Abnormal expression and distribution of heat shock protein 90: potential etiologic immunoendocrine mechanism of glucocorticoid resistance in idiopathic nephrotic syndrome

Resistance to glucocorticoid (GC) treatment in some patients with idiopathic nephrotic syndrome (INS) is a significant clinical problem. Heat shock protein 90 (HSP90) is the chaperon protein of the GC receptor, which is supposed to be the key factor of GC response. Therefore, we conducted this study to define the mechanisms of GC resistance related to HSP90. INS patients and cell lines with differing GC responses were included in the present study. We found that the level of HSP90 mRNA expression in INS patients was significantly higher than that in healthy controls and that HSP90 expression in GC-resistant INS patients was higher than that in GC-sensitive INS patients. A confocal immunofluorescence test was performed to investigate the subcellular localization of HSP90, and we found that the distribution of HSP90 in the GC-resistant INS group was greater in the nuclei than that of the GC-sensitive INS group. When the function of HSP90 was blocked by the HSP90-specific inhibitor, the GC sensitivity of GC-sensitive cells decreased remarkably. These results indicate that HSP90 plays a vital role in GC response. In addition, the abnormality in the mRNA level and subcellular distribution of HSP90 in GC-resistant INS patients may be etiologically significant in terms of endogenous/synthetic GC resistance. On one hand, it may disturb immunoendocrine regulation via endogenous GC and immune homeostasis and thus be involved in the occurrence of the immune-mediated disease; on the other hand, it may influence the patient's response to synthetic GC treatment and result in treatment failure.

mRNA expression levels of (co)chaperone molecules of the glucocorticoid receptor are not involved in glucocorticoid resistance in pediatric ALL

Resistance to glucocorticoids (GC) is an important adverse risk factor in the treatment of acute lymphoblastic leukemia (ALL). To induce apoptosis, GC bind to the GC receptor (GR), which is regulated by various (co)chaperone proteins such as heat-shock protein 70 (HSP-70), HSP-40, HIP (HSP-70-interacting protein), BAG-1 (BCL-2-associated gene product-1), HOP (HSP-70/HSP-90-Organizing protein), HSP-90, P-23, FKBP-51, FKBP-52 and CYP-40. In this study, we tested the hypothesis that mRNA expression levels of these molecules are determinants of GC resistance in childhood ALL. In all, 20 children with ALL cells in vitro sensitive to prednisolone (LC(50) < 0.1 microg/ml) were compared each with a resistant patient (LC(50) >150 mug/ml), matched for immunophenotype, age and white blood cell count. mRNA expression levels of the (co)chaperone molecules were measured by quantitative real-time RT-PCR and normalized to GAPDH and RNaseP levels. In vitro resistance to prednisolone was measured by MTT assay. HSP-90 mRNA expression levels were 2000-fold higher as compared to HSP-70. Using matched pair analysis, mRNA expression levels of the various (co)chaperone molecules were not significantly different between in vitro-sensitive and -resistant patients. GC resistance in childhood ALL cannot be attributed to different mRNA expression levels of the investigated (co)chaperone molecules involved in GC binding and transport to the nucleus.

A novel mode of chaperone action: heme activation of Hap1 by enhanced association of Hsp90 with the repressed Hsp70-Hap1 complex

Molecular chaperones Hsp90 and Hsp70 control many signal transducers, including cyclin-dependent kinases and steroid receptors. The yeast heme-responsive transcriptional activator Hap1 is a native substrate of both Hsp90 and Hsp70. Hsp90 and Hsp70 are critical for the precise regulation of Hap1 activity by heme. Here, to decipher the molecular events underlying the actions of Hsp90 and Hsp70 in heme regulation, we purified various multichaperone-Hap1 complexes and characterized the complexes linked to Hap1 repression and activation by two-dimensional electrophoresis analysis. Notably, we found that in vitro Hap1 is associated continuously with Ssa and its co-chaperones, and this association is not weakened by heme. Heme enhances the interaction between Hap1 and Hsp90. In vivo, defective Ssa, Ydj1, or Sro9 function causes Hap1 derepression in the absence of heme, whereas defective Hsp90 function causes reduced Hap1 activity at high heme concentrations. These results show that continuous association of Hap1 with Ssa, Ydj1, and Sro9 confers Hap1 repression, whereas enhanced association of Hsp90 with the repressed Hap1-Ssa-Ydj1-Sro9 complex by heme causes Hap1 activation. This novel mechanism of chaperone action may operate to control the activity of other important signal transducers.

Functional Role for Heat Shock Factors in the Transcriptional Regulation of Human RANK Ligand Gene Expression in Stromal/Osteoblast Cells

RANK Ligand (RANKL) is a critical osteoclastogenic factor that is expressed on stromal cells and osteoblasts. Most resorption stimuli induce osteoclast formation by modulating RANKL gene expression in marrow stromal/osteoblast cells. However, it is unclear how these stimuli modulate RANKL gene expression in the bone microenvironment. To characterize the transcriptional control of human RANKL gene expression in stromal/osteoblast cells, we PCR-amplified and cloned a 2-kb 5'-flanking sequence of the RANKL gene, using normal human osteoblast derived genomic DNA as a template. Sequence analysis identified the presence of several potential Heat Shock Factor (HSF) responsive elements (HSE) in the human RANKL gene promoter region. Co-expression of HSF-1 or HSF-2 with the RANKL gene promoter-luciferase reporter plasmid in human osteoblastic cells (NOBC) demonstrated a 2-fold and 4.5-fold increase in promoter activity, respectively. RT-PCR analysis for HSF-1 and 2 mRNA expression in human bone marrow-derived stromal cells (SAKA-T) and osteoblast cells detected only HSF-2 expression. As evident from EMSA analysis, in contrast to 1,25(OH)(2)D(3) SAKA-T cells treated with b-FGF demonstrated increased levels of HSF-2 binding to the HSE present in the RANKL gene promoter region. Immunocytochemical staining further confirmed nuclear localization of HSF-2 in both SAKA-T transformed stromal cells and human bone marrow derived primary stromal/preosteoblastic cells in response to b-FGF treatment. Furthermore, b-FGF treatment of SAKA-T cells transfected with the luciferase reporter plasmid containing the hRANKL HSE region (-2 kb to -1275 bp) upstream to a heterologous promoter showed increased levels of transactivation. Western blot analysis further demonstrated enhanced levels of RANKL expression and HSP-27 phosphorylation in SAKA-T cells treated with b-FGF. In addition, overexpression of HSF-2 in SAKA-T cells resulted in a 5-fold increase in the levels of RANKL expression in these cells. These data further suggest that HSF-2 is a downstream target of b-FGF to induce RANKL expression in stromal/osteoblast cells, and that HSF may play an important role in modulating RANKL gene expression in the bone microenvironment.

Regulation of nuclear retention of glucocorticoid receptor by nuclear Hsp90

Heat shock protein 90 (Hsp90) has been demonstrated in both cytoplasm and nucleus, and regulates cytoplasmic retention of glucocorticoid receptor (GR). However, the role of nuclear Hsp90 in GR trafficking is less characterized. The present study examined the role of Hsp90 in nuclear retention of GR after ligand withdrawal. Hsp90 inhibitors; geldanamycin (GA) and radicicol (Rad), significantly accelerated nuclear export of GR after withdrawal of ligands including dexamethasone, corticosterone and RU486. GA accelerated relocalization of GR in the cytoplasm even when reimport of GR into the nucleus was inhibited by okadaic acid or when novel GR synthesis was inhibited by cycloheximide. Overexpression of wild type or nuclear-targeted Hsp90 attenuated Hsp90 inhibitor-induced acceleration of GR nuclear export, although nuclear Hsp90 showed higher activity than the wild type. Only nuclear-targeted Hsp90 prolonged basal nuclear retention of GR after withdrawal of dexamethasone and corticosterone. These results suggest that nuclear Hsp90 regulates the nuclear retention of GR.

Heat and heavy metal stress synergize to mediate transcriptional hyperactivation by metal-responsive transcription factor MTF-1

Mammalian cells react to heavy metal stress by transcribing a number of genes that contain metal-response elements (MREs) in their promoter/enhancer region; this activation is mediated by metal-responsive transcription factor-1 (MTF-1). Well-known target genes of MTF-1 are those encoding metallothioneins, small, cysteine-rich proteins with a high affinity for heavy metals. The response to heat shock, another cell stress, is mediated by heat shock transcription factor 1 (HSF1), which activates a battery of heat shock genes. Little is known about the cross-talk between the different anti-stress systems of the cell. Here we report a synergistic activation of metal-responsive promoters by heavy metal load (zinc or cadmium) and heat shock. An obvious explanation, cooperativity between MTF-1 and HSF1, seems unlikely: transfected HSF1 boosts the activity of an Hsp70 promoter but hardly affects an MRE-containing promoter upon exposure to metal and heat shock. A clue to the mechanism is given by our finding that heat shock leads to intracellular accumulation of heavy metals. We propose that the known anti-apoptotic effect of heat shock proteins allows for cell survival despite heavy metal accumulation and, consequently, results in a hyperactivation of the metal response pathway.

Corticosteroid receptors, 11 beta-hydroxysteroid dehydrogenase, and the heart

Mineralocorticoid and glucocorticoid hormones are known as corticosteroid hormones and are synthesized mainly in the adrenal cortex; however, more recently the enzymes involved in their synthesis have been found in a variety of cells and tissues, including the heart. The effects of these hormones are mediated via both cytoplasmic mineralocorticoid receptors (MRs) and glucocorticoid receptors (GRs), which act as ligand-inducible transcription factors. In addition, rapid, nongenomically mediated effects of these steroids can occur that may be via novel corticosteroid receptors. The lipophilic nature of these hormones allows them to pass freely through the cell membrane, although the intracellular concentration of mineralocorticoids and glucocorticoids is dependent on several cellular factors. The main regulators of intracellular glucocorticoid levels are 11 beta-hydroxysteroid dehydrogenase (11 beta HSD) isoforms. 11 beta HSD1 acts predominantly as a reductase in vivo, facilitating glucocorticoid action by converting circulating receptor-inactive 11-ketoglucocorticoids to active glucocorticoids. In contrast, 11 beta HSD 2 acts exclusively as an 11 beta-dehydrogenase and decreases intracellular glucocorticoids by converting them to their receptor-inactive 11-ketometabolites. Furthermore, P-glycoproteins, by actively pumping steroids out of cells, can selectively decrease steroids and local steroid synthesis can increase steroid concentrations. Receptor concentration, receptor modification, and receptor-protein interactions can also significantly impact on the corticosteroid response. This review details the receptors and possible mechanisms involved in both mediating and modulating corticosteroid responses. In addition, direct effects of corticosteroids on the heart are described including a discussion of the corticosteroid receptors and the mechanisms involved in mediating their effects.

Autoregulation of glucocorticoid receptor by cortisol in rainbow trout hepatocytes

We used primary cultures of trout hepatocytes and a physiological dose of cortisol (100 ng/ml), mimicking stressed levels in salmonid fish, to address the impact of glucocorticoid stimulation on glucocorticoid receptor (GR) mRNA abundance and protein content. Cortisol significantly elevated GR mRNA content over a 24-h period; this increase was abolished by actinomycin D, suggesting transcriptional control of GR. However, cortisol significantly decreased GR protein content, leading us to hypothesize that lower GR protein content may be regulating GR mRNA abundance. Indeed, treatment of hepatocytes with MG-132, a proteasomal inhibitor shown to prevent GR degradation by cortisol, abolished cortisol-mediated GR mRNA upregulation. Also, geldanamycin, a heat shock protein 90-specific inhibitor, abolished the GR mRNA increase evident with cortisol but did not modify cortisol-induced increases in abundance of mRNA for phosphoenolpyruvate carboxykinase, a glucocorticoid-responsive gene, or hepatocyte glucose release. Together, our results suggest a negative feedback loop for GR gene regulation by cortisol in trout hepatocytes. The autoregulation of GR may be a crucial step in the physiological stress response process, especially in modulating energy-dependent processes that are glucocorticoid dependent, including gluconeogenesis.

Ligand-dependent genetic recombination in fibroblasts : a potentially powerful technique for investigating gene function in fibrosis

Strategies for conditional induction of transgene expression in mice are likely to be valuable for testing the role of candidate genes in disease pathogenesis. We have developed a system for lineage-specific, ligand-dependent, induction of sustained transgene expression in fibroblastic cells in mice using a chimeric gene encoding the Cre-ER(T) fusion protein, under the control of a fibroblast-specific regulatory sequence from the pro alpha 2(I)collagen gene. Cre-ER(T) operates as a tamoxifen-dependent DNA recombinase to excise fragments flanked by specific LoxP consensus sequences. To test efficiency and ligand dependency of this strategy, Cre-ER(T)-expressing mice were backcrossed with heterozygous ROSA26-LacZ reporter mice, in which a floxed-STOP cassette has been introduced upstream of a bacterial beta-galactosidase (LacZ) reporter gene at a ubiquitously expressed locus. Constitutive or tamoxifen-induced LacZ expression was examined in embryonic, neonatal, and adult compound-transgenic mice. When pregnant ROSA26-LacZ females received a single dose of tamoxifen, high-level expression of LacZ in the skin was demonstrable from 24 hours after injection in double-transgenic embryos harboring both the Cre-ER(T) transgene and the target ROSA26-LacZ allele. High-level expression of LacZ was also induced postnatally by tamoxifen specifically in dermal and visceral fibroblasts. By allowing efficient embryonic or postnatal modification of alleles that have been targeted to incorporate LoxP sites, or to switch on transgenes cloned downstream of the floxed-STOP cassette, this system will allow fibroblast-specific genetic perturbations to be induced at predetermined embryonic or postnatal time points. This should greatly assist in in vivo functional studies of candidate genes in fibrotic diseases such as systemic sclerosis.

The molecular chaperone Hsp90 mediates heme activation of the yeast transcriptional activator Hap1

Hsp90 plays critical roles in the proper functioning of a wide array of eukaryotic signal transducers such as steroid receptors and tyrosine kinases. Hap1 is a naturally occurring substrate of Hsp90 in Saccharomyces cerevisiae. Hap1 transcriptional activity is precisely and stringently controlled by heme. Previous biochemical studies suggest that in the absence of heme, Hap1 is bound to Hsp90 and other proteins, forming a higher order complex termed HMC (high molecular weight complex), and is repressed. Heme promotes the disruption of the HMC and activates Hap1, permitting Hap1 to bind to DNA with high affinity and to stimulate transcription. By lowering the expression levels of wild-type Hsp90, using a highly specific Hsp90 inhibitor, and by examining the effects of various Hsp90 mutants on Hap1, we show that Hsp90 is critical for Hap1 activation by heme. Furthermore, we show that many Hsp90 mutants exert differential effects on Hap1 and steroid receptors. Notably, mutant G313N weakens Hsp90 steroid receptor interaction but strongly enhances Hsp90-Hap1 interaction and increases Hap1 resistance to protease digestion. Additionally, we found that a heme-independent Hap1 mutant still depends on Hsp90 for high activity. These experiments together suggest that Hsp90 promotes Hap1 activation by inducing or maintaining Hap1 in a transcriptionally active conformation.

The Hsp70-Ydj1 molecular chaperone represses the activity of the heme activator protein Hap1 in the absence of heme

In Saccharomyces cerevisiae, heme directly mediates the effects of oxygen on transcription through the heme activator protein Hap1. In the absence of heme, Hap1 is bound by at least four cellular proteins, including Hsp90 and Ydj1, forming a higher-order complex, termed HMC, and its activity is repressed. Here we purified the HMC and showed by mass spectrometry that two previously unidentified major components of the HMC are the Ssa-type Hsp70 molecular chaperone and Sro9 proteins. In vivo functional analysis, combined with biochemical analysis, strongly suggests that Ssa proteins are critical for Hap1 repression in the absence of heme. Ssa may repress the activities of both Hap1 DNA-binding and activation domains. The Ssa cochaperones Ydj1 and Sro9 appear to assist Ssa in Hap1 repression, and only Ydj1 residues 1 to 172 containing the J domain are required for Hap1 repression. Our results suggest that Ssa-Ydj1 and Sro9 act together to mediate Hap1 repression in the absence of heme and that molecular chaperones promote heme regulation of Hap1 by a mechanism distinct from the mechanism of steroid signaling.