Modulation of glucocorticoid receptor phosphorylation and transcriptional activity by a C-terminal-associated protein phosphatase

Flow Cytometry FRET Reveals Posttranslational Modifications Drive Protein Phosphatase-5 Conformational Changes in Mammalian Cells

The serine/threonine protein phosphatase 5 (PP5) plays an essential role in regulating hormone and stress-induced signaling networks as well as extrinsic apoptotic pathways in cells. Unlike other protein phosphatases, PP5 possesses both regulatory and catalytic domains, and its function is further modulated through post-translational modifications (PTMs). PP5 contains a tetratricopeptide repeat (TPR) domain, which usually inhibits its phosphatase activity by blocking the active site (closed conformation). Certain activators bind to the PP5-TPR domain, alleviating this inhibition and allowing the catalytic domain to adopt an active (open) conformation. While this mechanism has been proposed based on structural and biophysical studies, PP5 conformational changes and activity has yet to be observed in cells. Here, we designed and developed a flow cytometry-based fluorescence resonance energy transfer (FC-FRET) method, enabling real-time observation of PP5 autoinhibition and activation within live mammalian cells. By quantifying FRET efficiency using sensitized emission, we established a standardized and adaptable data acquisition workflow. Our findings revealed that, in a cellular context, PP5 exists in multiple conformational states, none of which alone fully predict its activity. Additionally, we have demonstrated that PTMs such as phosphorylation and SUMOylation impact PP5 conformational changes, representing a significant advancement in our understanding of its regulatory mechanisms.

SUMOylation of protein phosphatase 5 regulates phosphatase activity and substrate release

The serine/threonine protein phosphatase 5 (PP5) regulates hormone and stress-induced signaling networks. Unlike other phosphoprotein phosphatases, PP5 contains both regulatory and catalytic domains and is further regulated through post-translational modifications (PTMs). Here we identify that SUMOylation of K430 in the catalytic domain of PP5 regulates phosphatase activity. Additionally, phosphorylation of PP5-T362 is pre-requisite for SUMOylation, suggesting the ordered addition of PTMs regulates PP5 function in cells. Using the glucocorticoid receptor, a well known substrate for PP5, we demonstrate that SUMOylation results in substrate release from PP5. We harness this information to create a non-SUMOylatable K430R mutant as a 'substrate trap' and globally identified novel PP5 substrate candidates. Lastly, we generated a consensus dephosphorylation motif using known substrates, and verified its presence in the new candidate substrates. This study unravels the impact of cross talk of SUMOylation and phosphorylation on PP5 phosphatase activity and substrate release in cells.

Effects of Antipsychotics on the Hypothalamus-Pituitary-Adrenal Axis in a Phencyclidine Animal Model of Schizophrenia

Schizophrenia (SCH) is a mental disorder that requires long-term antipsychotic treatment. SCH patients are thought to have an increased sensitivity to stress. The dysregulation of the hypothalamic-pituitary-adrenal (HPA) axis, observed in SCH, could include altered levels of glucocorticoids, glucocorticoid receptors (GRs), and associated proteins. The perinatal administration of phencyclidine (PCP) to rodents represents an animal model of SCH. This study investigated the effects of perinatal PCP exposure and subsequent haloperidol/clozapine treatment on corticosterone levels measured by ELISA and the expression of GR-related proteins (GR, pGR, HSP70, HSP90, FKBP51, and 11β-Hydroxysteroid dehydrogenase-11β-HSD) determined by Western blot, in different brain regions of adult rats. Six groups of male rats were treated on the 2nd, 6th, 9th, and 12th postnatal days (PN), with either PCP or saline. Subsequently, one saline and one PCP group received haloperidol/clozapine from PN day 35 to PN day 100. The results showed altered GR sensitivity in the rat brain after PCP exposure, which decreased after haloperidol/clozapine treatment. These findings highlight disturbances in the HPA axis in a PCP-induced model of SCH and the potential protective effects of antipsychotics. To the best of our knowledge, this is the first study to investigate the effects of antipsychotic drugs on the HPA axis in a PCP animal model of SCH.

Glucocorticoids, their uses, sexual dimorphisms, and diseases: new concepts, mechanisms, and discoveries

The normal stress response in humans is governed by the hypothalamic-pituitary-adrenal (HPA) axis through heightened mechanisms during stress, raising blood levels of the glucocorticoid hormone cortisol. Glucocorticoids are quintessential compounds that balance the proper functioning of numerous systems in the mammalian body. They are also generated synthetically and are the preeminent therapy for inflammatory diseases. They act by binding to the nuclear receptor transcription factor glucocorticoid receptor (GR), which has two main isoforms (GRα and GRβ). Our classical understanding of glucocorticoid signaling is from the GRα isoform, which binds the hormone, whereas GRβ has no known ligands. With glucocorticoids being involved in many physiological and cellular processes, even small disruptions in their release via the HPA axis, or changes in GR isoform expression, can have dire ramifications on health. Long-term chronic glucocorticoid therapy can lead to a glucocorticoid-resistant state, and we deliberate how this impacts disease treatment. Chronic glucocorticoid treatment can lead to noticeable side effects such as weight gain, adiposity, diabetes, and others that we discuss in detail. There are sexually dimorphic responses to glucocorticoids, and women tend to have a more hyperresponsive HPA axis than men. This review summarizes our understanding of glucocorticoids and critically analyzes the GR isoforms and their beneficial and deleterious mechanisms and the sexual differences that cause a dichotomy in responses. We also discuss the future of glucocorticoid therapy and propose a new concept of dual GR isoform agonist and postulate why activating both isoforms may prevent glucocorticoid resistance.

Treating the Side Effects of Exogenous Glucocorticoids; Can We Separate the Good From the Bad?

It is estimated that 2% to 3% of the population are currently prescribed systemic or topical glucocorticoid treatment. The potent anti-inflammatory action of glucocorticoids to deliver therapeutic benefit is not in doubt. However, the side effects associated with their use, including central weight gain, hypertension, insulin resistance, type 2 diabetes (T2D), and osteoporosis, often collectively termed iatrogenic Cushing's syndrome, are associated with a significant health and economic burden. The precise cellular mechanisms underpinning the differential action of glucocorticoids to drive the desirable and undesirable effects are still not completely understood. Faced with the unmet clinical need to limit glucocorticoid-induced adverse effects alongside ensuring the preservation of anti-inflammatory actions, several strategies have been pursued. The coprescription of existing licensed drugs to treat incident adverse effects can be effective, but data examining the prevention of adverse effects are limited. Novel selective glucocorticoid receptor agonists and selective glucocorticoid receptor modulators have been designed that aim to specifically and selectively activate anti-inflammatory responses based upon their interaction with the glucocorticoid receptor. Several of these compounds are currently in clinical trials to evaluate their efficacy. More recently, strategies exploiting tissue-specific glucocorticoid metabolism through the isoforms of 11β-hydroxysteroid dehydrogenase has shown early potential, although data from clinical trials are limited. The aim of any treatment is to maximize benefit while minimizing risk, and within this review we define the adverse effect profile associated with glucocorticoid use and evaluate current and developing strategies that aim to limit side effects but preserve desirable therapeutic efficacy.

Phosphatases maintain low catalytic activity of SGK1: DNA damage resets the balance in favor of phosphorylation

The serum- and glucocorticoid-induced kinase 1 (SGK1) promotes cell survival under stress conditions and facilitates the emergence of drug resistance in cancer. The underlying mechanisms of these observations are not fully understood. In this study, we found that SGK1 activity is suppressed by the action of the S/T phosphatases PP5 and PP2A, which constantly dephosphorylate SGK1. Using newly developed anti-phospho SGK1 antibodies and inhibitors of phosphatases, we determined that the high degree of dephosphorylation is caused by two factors: the tendency of SGK1 to unfold, which makes it dependent on Hsp90 chaperone complexes composed of four proteins, Hsp90/CDC37/PP5/SGK1, and where the phosphatase PP5 persistently dephosphorylates SGK1 within the complex. SGK1 binding to PP2A regulatory subunits B55γ and B55δ brings PP2A catalytic subunit close to exposed SGK1 phosphoresidues. A further association of phosphorylated pS37-FAM122A-an endogenous inhibitor of PP2A-to the holoenzyme diminishes dephosphorylation of SGK1 mediated by PP2A. Our study also reveals that genotoxic stress can reverse the dominant impact of phosphatases over kinases by activating the DNA-dependent protein kinase, which enhances mTORC2 activity directed to SGK1. Thus, our results provide insight into a molecular pathway that enables SGK1 to gain phosphorylation and catalytic activity and promote cell survival, potentially diminishing the efficacy of cancer treatments. As the DNA damage response operates in many cancer cells and is further induced by chemotherapies, the findings of this study could have significant implications for the development of novel cancer therapies targeting SGK1.

Emerging Epigenetic and Posttranslational Mechanisms Controlling Resistance to Glucocorticoids in Acute Lymphoblastic Leukemia

Glucocorticoids are extensively used for the treatment of acute lymphoblastic leukemia as they pressure cancer cells to undergo apoptosis. Nevertheless, glucocorticoid partners, modifications, and mechanisms of action are hitherto poorly characterized. This hampers our understanding of therapy resistance, frequently occurring in leukemia despite the current therapeutic combinations using glucocorticoids in acute lymphoblastic leukemia. In this review, we initially cover the traditional view of glucocorticoid resistance and ways of targeting this resistance. We discuss recent progress in our understanding of chromatin and posttranslational properties of the glucocorticoid receptor that might be proven beneficial in our efforts to understand and target therapy resistance. We discuss emerging roles of pathways and proteins such as the lymphocyte-specific kinase that antagonizes glucocorticoid receptor activation and nuclear translocation. In addition, we provide an overview of ongoing therapeutic approaches that sensitize cells to glucocorticoids including small molecule inhibitors and proteolysis-targeting chimeras.

Dual function of protein phosphatase 5 (PPP5C): An emerging therapeutic target for drug discovery

Phosphorylation of proteins is reversibly controlled by the kinases and phosphatases in many posttranslational regulation patterns. Protein phosphatase 5 (PPP5C) is a serine/threonine protein phosphatase showing dual function by simultaneously exerting dephosphorylation and co-chaperone functions. Due to this special role, PPP5C was found to participate in many signal transductions related to various diseases. Abnormal expression of PPP5C results in cancers, obesity, and Alzheimer's disease, making it a potential drug target. However, the design of small molecules targeting PPP5C is struggling due to its special monomeric enzyme form and low basal activity by a self-inhibition mechanism. Through realizing the PPP5C's dual function as phosphatase and co-chaperone, more and more small molecules were found to regulate PPP5C with a different mechanism. This review aims to provide insights into PPP5C's dual function from structure to function, which could provide efficient design strategies for small molecules targeting PPP5C as therapeutic candidates.

HSP70-HSP90 Chaperone Networking in Protein-Misfolding Disease

Molecular chaperones and their associated co-chaperones are essential in health and disease as they are key facilitators of protein-folding, quality control and function. In particular, the heat-shock protein (HSP) 70 and HSP90 molecular chaperone networks have been associated with neurodegenerative diseases caused by aberrant protein-folding. The pathogenesis of these disorders usually includes the formation of deposits of misfolded, aggregated protein. HSP70 and HSP90, plus their co-chaperones, have been recognised as potent modulators of misfolded protein toxicity, inclusion formation and cell survival in cellular and animal models of neurodegenerative disease. Moreover, these chaperone machines function not only in folding but also in proteasome-mediated degradation of neurodegenerative disease proteins. This chapter gives an overview of the HSP70 and HSP90 chaperones, and their respective regulatory co-chaperones, and explores how the HSP70 and HSP90 chaperone systems form a larger functional network and its relevance to counteracting neurodegenerative disease associated with misfolded proteins and disruption of proteostasis.

A specialized Hsp90 co-chaperone network regulates steroid hormone receptor response to ligand

Heat shock protein-90 (Hsp90) chaperone machinery is involved in the stability and activity of its client proteins. The chaperone function of Hsp90 is regulated by co-chaperones and post-translational modifications. Although structural evidence exists for Hsp90 interaction with clients, our understanding of the impact of Hsp90 chaperone function toward client activity in cells remains elusive. Here, we dissect the impact of recently identified higher eukaryotic co-chaperones, FNIP1/2 (FNIPs) and Tsc1, toward Hsp90 client activity. Our data show that Tsc1 and FNIP2 form mutually exclusive complexes with FNIP1, and that unlike Tsc1, FNIP1/2 interact with the catalytic residue of Hsp90. Functionally, these co-chaperone complexes increase the affinity of the steroid hormone receptors glucocorticoid receptor and estrogen receptor to their ligands in vivo. We provide a model for the responsiveness of the steroid hormone receptor activation upon ligand binding as a consequence of their association with specific Hsp90:co-chaperone subpopulations.

Glucocorticoid Receptor Signaling in Diabetes

Stress and depression increase the risk of Type 2 Diabetes (T2D) development. Evidence demonstrates that the Glucocorticoid (GC) negative feedback is impaired (GC resistance) in T2D patients resulting in Hypothalamic-Pituitary-Adrenal (HPA) axis hyperactivity and hypercortisolism. High GCs, in turn, activate multiple aspects of glucose homeostasis in peripheral tissues leading to hyperglycemia. Elucidation of the underlying molecular mechanisms revealed that Glucocorticoid Receptor (GR) mediates the GC-induced dysregulation of glucose production, uptake and insulin signaling in GC-sensitive peripheral tissues, such as liver, skeletal muscle, adipose tissue, and pancreas. In contrast to increased GR peripheral sensitivity, an impaired GR signaling in Peripheral Blood Mononuclear Cells (PBMCs) of T2D patients, associated with hyperglycemia, hyperlipidemia, and increased inflammation, has been shown. Given that GR changes in immune cells parallel those in brain, the above data implicate that a reduced brain GR function may be the biological link among stress, HPA hyperactivity, hypercortisolism and hyperglycemia. GR polymorphisms have also been associated with metabolic disturbances in T2D while dysregulation of micro-RNAs—known to target GR mRNA—has been described. Collectively, GR has a crucial role in T2D, acting in a cell-type and context-specific manner, leading to either GC sensitivity or GC resistance. Selective modulation of GR signaling in T2D therapy warrants further investigation.

Oxidative Stress Promotes Corticosteroid Insensitivity in Asthma and COPD

Corticosteroid insensitivity is a key characteristic of patients with severe asthma and COPD. These individuals experience greater pulmonary oxidative stress and inflammation, which contribute to diminished lung function and frequent exacerbations despite the often and prolonged use of systemic, high dose corticosteroids. Reactive oxygen and nitrogen species (RONS) promote corticosteroid insensitivity by disrupting glucocorticoid receptor (GR) signaling, leading to the sustained activation of pro-inflammatory pathways in immune and airway structural cells. Studies in asthma and COPD models suggest that corticosteroids need a balanced redox environment to be effective and to reduce airway inflammation. In this review, we discuss how oxidative stress contributes to corticosteroid insensitivity and the importance of optimizing endogenous antioxidant responses to enhance corticosteroid sensitivity. Future studies should aim to identify how antioxidant-based therapies can complement corticosteroids to reduce the need for prolonged high dose regimens in patients with severe asthma and COPD.

Lycii radicis cortex inhibits glucocorticoid‑induced bone loss by downregulating Runx2 and BMP‑2 expression

Lycii radicis cortex (LRC) has been used to regulate high blood pressure, body temperature, pain and bone disorders in East Asia. Glucocorticoids (GCs), also known as steroids, are potent immunity regulators widely used in the treatment of inflammatory diseases. However, despite their effectiveness, GC usage is strictly controlled due to severe side‑effects, such as osteoporosis. However, further research is required as to date, at least to the best of our knowledge, there is no appropriate model to overcome secondary osteoporosis as a side‑effect of GC use. Thus, the aim of the present study was to establish an experimental model of osteoporosis induced by GC. Furthermore, the present study aimed to establish the research methodology for medical evaluations of the effectiveness and side‑effects of GCs. A secondary osteoporosis animal model was established, and the animals were divided into two groups as follows: The allergic contact dermatitis (ACD)‑induced group and the non‑ACD‑induced group. In the ACD‑induced group, a GC topical application group was compared with a GC subcutaneous injection group. The results revealed that the presence of ACD affected the induction of GC‑mediated osteoporosis. Therefore, the group exhibiting induced ACD that was treated with a topical application of GC was selected for examining the side‑effects of GCs. The effects of LRC on secondary osteoporosis were confirmed in vivo and in vitro. The results indicated that LRC regulated dexamethasone‑induced osteoblast apoptotic markers, including caspase‑6, caspase‑9, X‑linked inhibitor of apoptosis, apoptosis inhibitor 1 and apoptosis inhibitor 2, and increased the expression of osteoblast differentiation‑related genes, such as Runt‑related transcription factor 2 and bone morphogenetic protein 2 in the MC3T3E‑1 cell line. LRC also significantly reduced GC‑induced osteoporosis and exerted anti‑inflammatory effects in vivo. In addition, LRC inhibited the reduction of calbindin‑D28k in the kidney. Overall, the results of the present study suggest that the use of LRC alleviates GC‑induced secondary osteoporosis.

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.

The role of PP5 and PP2C in cardiac health and disease

Protein phosphatases are important, for example, as functional antagonists of β-adrenergic stimulation of the mammalian heart. While β-adrenergic stimulations increase the phosphorylation state of regulatory proteins and therefore force of contraction in the heart, these phosphorylations are reversed and thus force is reduced by the activity of protein phosphatases. In this context the role of PP5 and PP2C is starting to unravel. They do not belong to the same family of phosphatases with regard to sequence homology, many similarities with regard to location, activation by lipids and putative substrates have been worked out over the years. We also suggest which pathways for regulation of PP5 and/or PP2C described in other tissues and not yet in the heart might be useful to look for in cardiac tissue. Both phosphatases might play a role in signal transduction of sarcolemmal receptors in the heart. Expression of PP5 and PP2C can be increased by extracellular stimuli in the heart. Because PP5 is overexpressed in failing animal and human hearts, and because overexpression of PP5 or PP2C leads to cardiac hypertrophy and KO of PP5 leads to cardiac hypotrophy, one might argue for a role of PP5 and PP2C in heart failure. Because PP5 and PP2C can reduce, at least in vitro, the phosphorylation state of proteins thought to be relevant for cardiac arrhythmias, a role of these phosphatases for cardiac arrhythmias is also probable. Thus, PP5 and PP2C might be druggable targets to treat important cardiac diseases like heart failure, cardiac hypertrophy and cardiac arrhythmias.

Differential regulation of the glucocorticoid receptor nucleocytoplasmic shuttling by TPR-domain proteins

A dimer of the heat-shock protein of 90-kDa (Hsp90) represents the critical core of the chaperone complex associated to the glucocorticoid receptor (GR) oligomer. The C-terminal end of the Hsp90 dimer shapes a functional acceptor site for co-chaperones carrying tetratricopeptide repeat (TPR) domains, where they bind in a mutually exclusive and competitive manner. They impact on the biological properties of the GR•Hsp90 complex and are major players of the GR transport machinery. Recently, we showed that the overexpression of a chimeric TPR peptide influences the subcellular distribution of GR. In this study, the functional role of endogenous proteins carrying TPR or TPR-like sequences on GR subcellular distribution was characterized. It is demonstrated that, contrarily to the positive influence of FKBP52 on GR nuclear accumulation, FKBP51 and 14-3-3 impaired this property. While SGT1α showed no significant effect, the overexpression of the Ser/Thr phosphatase PP5 resulted in a nearly equal nuclear-cytoplasmic redistribution of GR rather than its typical cytoplasmic localization in the absence of steroid. This observation led to analyse the influence of the phosphorylation status of GR, which resulted not linked to its nucleo-cytoplasmic shuttling mechanism. Nonetheless, it was evidenced that both PP5 and FKBP52 are related to the anchorage of the GR to nucleoskeleton structures. The influence of these TPR domain proteins on the steroid-dependent transcriptional activity of GR was also characterized. It is postulated that the pleiotropic actions of the GR in different cell types may be the consequence of the relative abundance of different TPR-domain interacting co-chaperones.

Structure and function of the co-chaperone protein phosphatase 5 in cancer

Protein phosphatase 5 (PP5) is a serine/threonine protein phosphatase that regulates many cellular functions including steroid hormone signaling, stress response, proliferation, apoptosis, and DNA repair. PP5 is also a co-chaperone of the heat shock protein 90 molecular chaperone machinery that assists in regulation of cellular signaling pathways essential for cell survival and growth. PP5 plays a significant role in survival and propagation of multiple cancers, which makes it a promising target for cancer therapy. Though there are several naturally occurring PP5 inhibitors, none is specific for PP5. Here, we review the roles of PP5 in cancer progression and survival and discuss the unique features of the PP5 structure that differentiate it from other phosphoprotein phosphatase (PPP) family members and make it an attractive therapeutic target.

Role of Phosphorylation in the Modulation of the Glucocorticoid Receptor's Intrinsically Disordered Domain

Protein phosphorylation often switches cellular activity from one state to another, and this post-translational modification plays an important role in gene regulation by the nuclear hormone receptor superfamily, including the glucocorticoid receptor (GR). Cell signaling pathways that regulate phosphorylation of the GR are important determinants of GR actions, including lymphoid cell apoptosis, DNA binding, and interaction with coregulatory proteins. All major functionally important phosphorylation sites in the human GR are located in its N-terminal domain (NTD), which possesses a powerful transactivation domain, AF1. The GR NTD exists as an intrinsically disordered protein (IDP) and undergoes disorder-order transition for AF1’s efficient interaction with several coregulatory proteins and subsequent AF1-mediated GR activity. It has been reported that GR’s NTD/AF1 undergoes such disorder-order transition following site-specific phosphorylation. This review provides currently available information regarding the role of GR phosphorylation in its action and highlights the possible underlying mechanisms of action.

Formoterol counteracts the inhibitory effect of cigarette smoke on glucocorticoid-induced leucine zipper (GILZ) transactivation in human bronchial smooth muscle cells

Cigarette smokers with asthma and chronic obstructive pulmonary disease (COPD) are less responsive to glucocorticoids (GCs). The anti-inflammatory action of GCs depends also on their ability to transactivate genes such as GC-induced leucine zipper (GILZ). We investigated the effects of aqueous cigarette smoke extract (CSE) on GILZ transactivation evoked by 17-beclomethasone monopropionate (BMP) or fluticasone propionate (FP) in the presence or absence of the long acting β2-adrenoceptor agonist (LABA) bronchodilator formoterol or salmeterol in human primary cultures of human bronchial smooth muscle cells (HBSMC). We monitored GC receptor Ser211 phosphorylation by western blot analysis and GC receptor nuclear translocation by immunostaining followed high-content imaging analysis. BMP, as well as FP, induced GILZ expression in a concentration-dependent manner (EC₅₀ of 0.87 and 0.16 nM respectively). Pre-incubation with CSE inhibited GC-evoked GILZ transactivation (>50%), GC receptor Ser211 phosphorylation and nuclear translocation. Both formoterol and salmeterol counteracted the effect of CSE on GC-induced GILZ expression but not on nuclear translocation or phosphorylation. The effect of formoterol was mimicked by the cAMP-elevating agent forskolin and blocked by ICI 118,551, a selective β2-adrenoceptor antagonist. Pre-incubation with TNF-α also reduced GC-evoked GILZ transactivation but was not counteracted by formoterol undercovering a different responsiveness to LABAs of TNF-α in comparison to CSE. In sum, CSE inhibits GC-evoked transactivation of GILZ and such effect is counteracted by LABAs, through β2-adrenoceptors and a cAMP-dependent mechanism. This study sheds light on a mechanism underlying complementary interactions between LABAs and inhaled GCs that could be relevant in smokers with asthma and COPD.

Hsp90 Heterocomplexes Regulate Steroid Hormone Receptors: From Stress Response to Psychiatric Disease

The hypothalamus-pituitary-adrenal (HPA) axis directly controls the stress response. Dysregulation of this neuroendocrine system is a common feature among psychiatric disorders. Steroid hormone receptors, like glucocorticoid receptor (GR), function as transcription factors of a diverse set of genes upon activation. This activity is regulated by molecular chaperone heterocomplexes. Much is known about the structure and function of these GR/heterocomplexes. There is strong evidence suggesting altered regulation of steroid receptor hormones by chaperones, particularly the 51 kDa FK506-binding protein (FKBP51), may work with environmental factors to increase susceptibility to various psychiatric illnesses including post-traumatic stress disorder (PTSD), major depressive disorder (MDD), and anxiety. This review highlights the regulation of steroid receptor dynamics by the 90kDa heat shock protein (Hsp90)/cochaperone heterocomplexes with an in depth look at how the structural regulation and imbalances in cochaperones can cause functional effects on GR activity. Links between the stress response and circadian systems and the development of novel chaperone-targeting therapeutics are also discussed.

Corticosteroid Receptors in the Brain: Transcriptional Mechanisms for Specificity and Context-Dependent Effects

Corticosteroid hormones act in the brain to support adaptation to stress via binding to mineralocorticoid and glucocorticoid receptors (MR and GR). These receptors act in large measure as transcription factors. Corticosteroid effects can be highly divergent, depending on the receptor type, but also on brain region, cell type, and physiological context. These differences ultimately depend on differential interactions of MR and GR with other proteins, which determine ligand binding, nuclear translocation, and transcriptional activities. In this review, we discuss established and potential mechanisms that confer receptor and cell type-specific effects of the MR and GR-mediated transcriptional effects in the brain.

Possible roles for glucocorticoid signalling in breast cancer

Our understanding of breast cancer biology, and our ability to manipulate breast cancers have grown exponentially in the last 20 years. Much of that expansion has focused on the roles of steroids in driving these neoplasms. Initially this research focused on estrogens and progesterone receptors, and more recently on androgen actions in breast cancers. This review aims to make the case for glucocorticoids as the next essential steroid subclass that contributes significantly to our understanding of steroidogenic regulation of these neoplasms. Glucocorticoids have the potential to play multiple roles in the regulation of breast cancers including their control of cellular differentiation, apoptosis and proliferation. Beyond this they also act as a master integrator of organ homeostats in relation to such as circadian rhythms and stress responses. Therefore a better understanding of glucocorticoids and breast cancer could help to explain some of the epidemiological links between circadian disruption and/or stress and breast cancer development. Finally glucocorticoids are currently used during chemotherapeutic treatment in breast cancer therapy and yet results of various studies suggest that this may have an adverse impact on treatment success. This review aims to summarise the current evidence for glucocorticoids as actors in breast cancer and then suggest future essential approaches in order to determine the roles of glucocorticoids in this disease.

Glucocorticoid receptor activation is associated with increased resistance to heat-induced hyperthermia and injury

AIM

Anti-inflammatory mediators likely play a key role in maintaining thermal homeostasis and providing protection against heat stress. The aim of this study was to investigate the association between activation of the glucocorticoid receptor (GR) and resistance to heat-induced hyperthermia and injury.

METHODS

Effects of heat exposure on core body temperature, muscle GR phosphorylation status and subcellular expression were examined in control mice and thermal acclimation (TA)-exposed mice. In addition, effects of TA and corticosterone on C2C12 mouse myoblast viability and subcellular GR were assessed during heat exposure.

RESULTS

Phosphorylated, nuclear and mitochondrial GR levels were significantly higher in the gastrocnemius muscles of mice with mild hyperthermia (tolerant), compared to mice with severe hyperthermia (intolerant) during a heat exposure test. Similar changes were found in mice after TA, compared to non-TA-exposed controls. Additional groups of TA and non-TA-exposed mice underwent a heat exposure test. TA mice presented a significantly lower hyperthermic response during heat exposure than non-TA-exposed control. C2C12 cells exposed to TA incubation had higher viability against heat shock and showed higher GR levels in their mitochondria and nuclei detected by Western blot analysis and fluorescence microscopy, compared to cells exposed to normal incubation. Furthermore, pre-incubation with 0.1 μM corticosterone increased C2C12 cell viability during heat exposure and mitochondrial and nuclear GR expression.

CONCLUSION

The results of these in vivo and in vitro studies suggest that GR activation is associated with increased resistance against heat-induced hyperthermia and injury.

Site-specific phosphorylation regulates the structure and function of an intrinsically disordered domain of the glucocorticoid receptor

Intrinsically disordered (ID) regions of the transcription factor proteins have much larger frequency of phosphorylation sites than ordered regions, suggesting an important role in their regulatory capacity. Consistent with this phenomenon, most of the functionally known phosphorylation sites in the steroid receptor family of transcription factors are located in the ID N-terminal domain that contains a powerful activation function (AF1) region. In this study, we determined the structural and functional consequences of functionally known phosphorylation residues (Ser203, 211, and 226) located in the human glucocorticoid receptor’s (GR’s) ID AF1 domain. We report the relative importance of each phosphorylation site in inducing a functionally active ordered conformation in GR’s ID AF1 domain. Our data demonstrate a mechanism through which ID domain of the steroid receptors and other similar transcription factors may adopt a functionally active conformation under physiological conditions.

Impaired Dual-Specificity Protein Phosphatase DUSP4 Reduces Corticosteroid Sensitivity

We have reported that phosphorylation of the glucocorticoid receptor (GR) at Ser²²⁶ reduces GR nuclear translocation, resulting in corticosteroid insensitivity in patients with severe asthmas. A serine/threonine protein phosphatase 2A, which regulates c-Jun N-terminal kinase (JNK) 1 and GR-Ser²²⁶ signaling, is involved in this mechanism. Here, we further explored protein kinase dual-specificity phosphatases (DUSPs) with the ability to dephosphorylate JNK, and identified DUSP4 as a phosphatase involved in the regulation of corticosteroid sensitivity. The effects of knocking down DUSPs (DUSP1, 4, 8, 16, and 22) by small interfering RNA (siRNA) were evaluated in a monocytic cell line (U937). Corticosteroid sensitivity was determined by dexamethasone enhancement of FK506-binding protein 51 or inhibition of tumor necrosis factor α (TNFα)-induced interferon γ and interleukin 8 expression and GR translocation from cell cytoplasm to nucleus. The nuclear/cytoplasmic GR, phosphorylation levels of GR-Ser²²⁶ and JNK1, coimmunoprecipitated GR-JNK1-DUSP4, and DUSP4 expression were analyzed by western blotting and/or imaging flow cytometry. Phosphatase activity of immunoprecipitated (IP)-DUSP4 was measured by fluorescence-based assay. Knockdown of DUSP4 enhanced phosphorylation of GR-Ser²²⁶ and JNK1 and reduced GR nuclear translocation and corticosteroid sensitivity. Coimmunoprecipitation experiments showed that DUSP4 is associated with GR and JNK1. In peripheral blood mononuclear cells from severe asthmatics, DUSP4 expression was reduced versus healthy subjects and negatively correlated with phosphorylation levels of GR-Ser²²⁶ and JNK1. Formoterol enhanced DUSP4 activity and restored corticosteroid sensitivity reduced by DUSP4 siRNA. In conclusion, DUSP4 regulates corticosteroid sensitivity via dephosphorylation of JNK1 and GR-Ser²²⁶ DUSP4 activation by formoterol restores impaired corticosteroid sensitivity, indicating that DUSP4 is crucial in regulating corticosteroid sensitivity, and therefore might be a novel therapeutic target in severe asthma.

Glucocorticoid receptor control of transcription: precision and plasticity via allostery

The glucocorticoid receptor (GR) is a constitutively expressed transcriptional regulatory factor (TRF) that controls many distinct gene networks, each uniquely determined by particular cellular and physiological contexts. The precision of GR-mediated responses seems to depend on combinatorial, context-specific assembly of GR-nucleated transcription regulatory complexes at genomic response elements. In turn, evidence suggests that context-driven plasticity is conferred by the integration of multiple signals, each serving as an allosteric effector of GR conformation, a key determinant of regulatory complex composition and activity. This structural and mechanistic perspective on GR regulatory specificity is likely to extend to other eukaryotic TRFs.

Cav1.2 channels mediate persistent chronic stress-induced behavioral deficits that are associated with prefrontal cortex activation of the p25/Cdk5-glucocorticoid receptor pathway

Chronic stress is known to precipitate and exacerbate neuropsychiatric symptoms, and exposure to stress is particularly pathological in individuals with certain genetic predispositions. Recent genome wide association studies have identified single nucleotide polymorphisms (SNPs) in the gene CACNA1C, which codes for the Ca_v1.2 subunit of the L-type calcium channel (LTCC), as a common risk variant for multiple neuropsychiatric conditions. Ca_v1.2 channels mediate experience-dependent changes in gene expression and long-term synaptic plasticity through activation of downstream calcium signaling pathways. Previous studies have found an association between stress and altered Ca_v1.2 expression in the brain, however the contribution of Ca_v1.2 channels to chronic stress-induced behaviors, and the precise Ca_v1.2 signaling mechanisms activated are currently unknown. Here we report that chronic stress leads to a delayed increase in Ca_v1.2 expression selectively within the prefrontal cortex (PFC), but not in other stress-sensitive brain regions such as the hippocampus or amygdala. Further, we demonstrate that while Ca_v1.2 heterozygous (Ca_v1.2^+/−) mice show chronic stress-induced depressive-like behavior, anxiety-like behavior, and deficits in working memory 1–2 days following stress, they are resilient to the effects of chronic stress when tested 5–7 days later. Lastly, molecular studies find a delayed upregulation of the p25/Cdk5-glucocorticoid receptor (GR) pathway in the PFC when examined 8 days post-stress that is absent in Ca_v1.2^+/− mice. Our findings reveal a novel Ca_v1.2-mediated molecular mechanism associated with the persistent behavioral effects of chronic stress and provide new insight into potential Ca_v1.2 channel mechanisms that may contribute to CACNA1C-linked neuropsychiatric phenotypes.

Cytokine-Induced Glucocorticoid Resistance from Eosinophil Activation: Protein Phosphatase 5 Modulation of Glucocorticoid Receptor Phosphorylation and Signaling

The mechanisms contributing to persistent eosinophil activation and poor eosinopenic response to glucocorticoids in severe asthma are poorly defined. We examined the effect of cytokines typically overexpressed in the asthmatic airways on glucocorticoid signaling in in vitro activated eosinophils. An annexin V assay used to measure eosinophil apoptosis showed that cytokine combinations of IL-2 plus IL-4 as well as TNF-α plus IFN-γ, or IL-3, GM-CSF, and IL-5 alone significantly diminished the proapoptotic response to dexamethasone. We found that IL-2 plus IL-4 resulted in impaired phosphorylation and function of the nuclear glucocorticoid receptor (GCR). Proteomic analysis of steroid sensitive and resistant eosinophils identified several differentially expressed proteins, namely protein phosphatase 5 (PP5), formyl peptide receptor 2, and annexin 1. Furthermore, increased phosphatase activity of PP5 correlated with impaired phosphorylation of the GCR. Importantly, suppression of PP5 expression with small interfering RNA restored proper phosphorylation and the proapoptotic function of the GCR. We also examined the effect of lipoxin A4 on PP5 activation by IL-2 plus IL-4. Similar to PP5 small interfering RNA inhibition, pretreatment of eosinophils with lipoxin A4 restored GCR phosphorylation and the proaptoptotic function of GCs. Taken together, our results showed 1) a critical role for PP5 in cytokine-induced resistance to GC-mediated eosinophil death, 2) supported the dependence of GCR phosphorylation on PP5 activity, and 3) revealed that PP5 is a target of the lipoxin A4-induced pathway countering cytokine-induced resistance to GCs in eosinophils.

MiR-21a-5p suppresses bisphenol A-induced pre-adipocyte differentiation by targeting map2k3 through MKK3/p38/MAPK

Childhood obesity is a metabolic disease characterized by accumulation of excessive fat. Bisphenol A (BPA), a potential obesogen compound, possesses an estrogen mimetic activity and endocrine disruption effect. MicroRNA-21a-5p (miR-21a-5p) is reported to regulate the adipogenic differentiation. Our study showed that miR-21a-5p overexpression significantly decreased the red lipid droplets and triglyceride level in BPA-induced 3T3-L1 cells. BPA induced the mRNA and protein expression levels of PPARγ, C/EBPα and adiponectin, and the induction was inhibited by miR-21a-5p mimics transfection. MiR-21a-5p mimics inhibited the GR activity, GR phosphorylation (S220, S21a-5p2, and S234), and the activation of p38/MAPK pathway, which are elevated by BPA treatment in 3T3-L1 cells. MiR-21a-5p overexpression inhibited the protein level of MKK3, but not in the mRNA level. Luciferase activity assay showed that miR-21a-5p directly targeted map2k3 3'-UTR. MKK3 overexpression attenuated the effect of miR-21a-5p mimics transfection on 3T3-L1 differentiation. We also assessed the body weight, fat mass and the content of serum lipid in rats subcutaneous injected with BPA and miR-21a-5p mimics. MiR-21a-5p overexpression attenuated BPA-induced obesity in vivo. These findings suggested that miR-21a-5p inhibited BPA induced adipocyte differentiation by targeting map2k3 through MKK3/p38/MAPK in 3T3-L1 cells, providing a potential therapeutic strategy for BPA induced obesity.

Effects of histamine H1 receptor signaling on glucocorticoid receptor activity. Role of canonical and non-canonical pathways

Histamine H1 receptor (H1R) antagonists and glucocorticoid receptor (GR) agonists are used to treat inflammatory conditions such as allergic rhinitis, atopic dermatitis and asthma. Consistent with the high morbidity levels of such inflammatory conditions, these receptors are the targets of a vast number of approved drugs, and in many situations their ligands are co-administered. However, this drug association has no clear rationale and has arisen from clinical practice. We hypothesized that H1R signaling could affect GR-mediated activity, impacting on its transcriptional outcome. Indeed, our results show a dual regulation of GR activity by the H1R: a potentiation mediated by G-protein βγ subunits and a parallel inhibitory effect mediated by Gαq-PLC pathway. Activation of the H1R by its full agonists resulted in a composite potentiating effect. Intriguingly, inactivation of the Gαq-PLC pathway by H1R inverse agonists resulted also in a potentiation of GR activity. Moreover, histamine and clinically relevant antihistamines synergized with the GR agonist dexamethasone to induce gene transactivation and transrepression in a gene-specific manner. Our work provides a delineation of molecular mechanisms underlying the widespread clinical association of antihistamines and GR agonists, which may contribute to future dosage optimization and reduction of well-described side effects associated with glucocorticoid administration.

The activity of protein phosphatase 5 towards native clients is modulated by the middle- and C-terminal domains of Hsp90

Protein phosphatase 5 is involved in the regulation of kinases and transcription factors. The dephosphorylation activity is modulated by the molecular chaperone Hsp90, which binds to the TPR-domain of protein phosphatase 5. This interaction is dependent on the C-terminal MEEVD motif of Hsp90. We show that C-terminal Hsp90 fragments differ in their regulation of the phosphatase activity hinting to a more complex interaction. Also hydrodynamic parameters from analytical ultracentrifugation and small-angle X-ray scattering data suggest a compact structure for the Hsp90-protein phosphatase 5 complexes. Using crosslinking experiments coupled with mass spectrometric analysis and structural modelling we identify sites, which link the middle/C-terminal domain interface of C. elegans Hsp90 to the phosphatase domain of the corresponding kinase. Studying the relevance of the domains of Hsp90 for turnover of native substrates we find that ternary complexes with the glucocorticoid receptor (GR) are cooperatively formed by full-length Hsp90 and PPH-5. Our data suggest that the direct stimulation of the phosphatase activity by C-terminal Hsp90 fragments leads to increased dephosphorylation rates. These are further modulated by the binding of clients to the N-terminal and middle domain of Hsp90 and their presentation to the phosphatase within the phosphatase-Hsp90 complex.

Neurotrophic-priming of glucocorticoid receptor signaling is essential for neuronal plasticity to stress and antidepressant treatment

Neurotrophins and glucocorticoids are robust synaptic modifiers, and deregulation of their activities is a risk factor for developing stress-related disorders. Low levels of brain-derived neurotrophic factor (BDNF) increase the desensitization of glucocorticoid receptors (GR) and vulnerability to stress, whereas higher levels of BDNF facilitate GR-mediated signaling and the response to antidepressants. However, the molecular mechanism underlying neurotrophic-priming of GR function is poorly understood. Here we provide evidence that activation of a TrkB-MAPK pathway, when paired with the deactivation of a GR-protein phosphatase 5 pathway, resulted in sustained GR phosphorylation at BDNF-sensitive sites that is essential for the transcription of neuronal plasticity genes. Genetic strategies that disrupted GR phosphorylation or TrkB signaling in vivo impaired the neuroplasticity to chronic stress and the effects of the antidepressant fluoxetine. Our findings reveal that the coordinated actions of BDNF and glucocorticoids promote neuronal plasticity and that disruption in either pathway could set the stage for the development of stress-induced psychiatric diseases.

The mechanism and significance of synergistic induction of the expression of plasminogen activator inhibitor-1 by glucocorticoid and transforming growth factor beta in human ovarian cancer cells

Plasminogen activator inhibitor-1 (PAI-1) plays a key role in tissue remodeling and tumor development by suppression of plasminogen activator function. Glucocorticoids (GCs) and transforming growth factor beta (TGF-β) signal pathways cross-talk to regulate gene expression, but the mechanism is poorly understood. Here we investigated the mechanism and significance of co-regulation of PAI-1 by TGF-β and dexamethasone (DEX), a synthetic glucocorticoid in ovarian cancer cells. We found that TGF-β and DEX showed rapidly synergistic induction of PAI-1 expression, which contributed to the early pro-adhesion effects. The synergistic induction effect was accomplished by several signal pathways, including GC receptor (GR) pathway and TGF-β-activated p38MAPK, ERK and Smad3 pathways. TGF-β-activated p38MAPK and ERK pathways cross-talked with GR pathway to augment the expression of PAI-1 through enhancing DEX-induced GR phosphorylation at Ser211 in ovarian cancer cells. These findings reveal possible novel mechanisms by which TGF-β pathways cooperatively cross-talk with GR pathway to regulate gene expression.

Selective activators of protein phosphatase 5 target the auto-inhibitory mechanism

This paper describes the identification of compounds, which stimulate the activity of the protein phosphatase PPH-5 and addresses the influence of the identified compounds on the enzymatic properties and the potential mechanism of their action.

Protein phosphatase 5 (PP5) is an evolutionary conserved serine/threonine phosphatase. Its dephosphorylation activity modulates a diverse set of cellular factors including protein kinases and the microtubule-associated tau protein involved in neurodegenerative disorders. It is auto-regulated by its heat-shock protein (Hsp90)-interacting tetratricopeptide repeat (TPR) domain and its C-terminal α-helix. In the present study, we report the identification of five specific PP5 activators [PP5 small-molecule activators (P5SAs)] that enhance the phosphatase activity up to 8-fold. The compounds are allosteric modulators accelerating efficiently the turnover rate of PP5, but do barely affect substrate binding or the interaction between PP5 and the chaperone Hsp90. Enzymatic studies imply that the compounds bind to the phosphatase domain of PP5. For the most promising compound crystallographic comparisons of the apo PP5 and the PP5–P5SA-2 complex indicate a relaxation of the auto-inhibited state of PP5. Residual electron density and mutation analyses in PP5 suggest activator binding to a pocket in the phosphatase/TPR domain interface, which may exert regulatory functions. These compounds thus may expose regulatory mechanisms in the PP5 enzyme and serve to develop optimized activators based on these scaffolds.

Decreased adipogenesis and adipose tissue in mice with inactivated protein phosphatase 5

White adipose tissue levels are decreased and preadipoctye differentiation to adipocytes is retarded in mice with inactivated protein phosphatase 5. Increased phosphate in the glucocorticoid receptor mediates this phenotype by altering expression of several proteins in the pathway of adipogenesis.

Effect of BRAF mutational status on expression profiles in conventional papillary thyroid carcinomas

BACKGROUND

Whereas 40 % to 70 % of papillary thyroid carcinomas (PTCs) are characterized by a BRAF mutation (BRAFmut), unified biomarkers for the genetically heterogeneous group of BRAF wild type (BRAFwt) PTCs are not established yet. Using state-of-the-art technology we compared RNA expression profiles between conventional BRAFwt and BRAFmut PTCs.

METHODS

Microarrays covering 36,079 reference sequences were used to generate whole transcript expression profiles in 11 BRAFwt PTCs including five micro PTCs, 14 BRAFmut PTCs, and 7 normal thyroid specimens. A p-value with a false discovery rate (FDR) < 0.05 and a fold change > 2 were used as a threshold of significance for differential expression. Network and pathway utilities were employed to interpret significance of expression data. BRAF mutational status was established by direct sequencing the hotspot region of exon 15.

RESULTS

We identified 237 annotated genes that were significantly differentially expressed between BRAFwt and BRAFmut PTCs. Of these, 110 genes were down- and 127 were upregulated in BRAFwt compared to BRAFmut PTCs. A number of molecules involved in thyroid hormone metabolism including thyroid peroxidase (TPO) were differentially expressed between both groups. Among cancer-associated molecules were ERBB3 that was downregulated and ERBB4 that was upregulated in BRAFwt PTCs. Two microRNAs were significantly differentially expressed of which miR492 bears predicted functions relevant to thyroid-specific molecules. The protein kinase A (PKA) and the G protein-coupled receptor pathways were identified as significantly related signaling cascades to the gene set of 237 genes. Furthermore, a network of interacting molecules was predicted on basis of the differentially expressed gene set.

CONCLUSIONS

The expression study focusing on affected genes that are differentially expressed between BRAFwt and BRAFmut conventional PTCs identified a number of molecules which are connected in a network and affect important canonical pathways. The identified gene set adds to our understanding of the tumor biology of BRAFwt and BRAFmut PTCs and contains genes/biomarkers of interest.

Analysis of FK506, timcodar (VX-853) and FKBP51 and FKBP52 chaperones in control of glucocorticoid receptor activity and phosphorylation

The immunosuppressive ligand FK506 and the FK506-binding protein FKBP52 are stimulatory to glucocorticoid receptor (GR) activity. Here, we explore the underlying mechanism by comparing GR activity and phosphorylation status in response to FK506 and the novel nonimmunosuppressive ligand timcodar (VX-853) and in the presence and absence of FKBP52 and the closely related protein FKBP51. Using mouse embryonic fibroblast cells (MEFs) deficient knockout (KO) in FKBP51 or FKBP52, we show decreased GR activity at endogenous genes in 52KO cells, but increased activity in 51KO cells. In 52KO cells, elevated phosphorylation occurred at inhibitory serine 212 and decreased phosphorylation at the stimulatory S220 residue. In contrast, 51KO cells showed increased GR phosphorylation at the stimulatory residues S220 and S234. In wild-type (WT) MEF cells, timcodar, like FK506, potentiated dexamethasone-induced GR transcriptional activity at two endogenous genes. Using 52KO and 51KO MEF cells, FK506 potentiated GR activity in 51KO cells but could not do so in 52KO cells, suggesting FKBP52 as the major target of FK506 action. Like FK506, timcodar potentiated GR in 51KO cells, but it also increased GR activity in 52KO cells. Knock-down of FKBP51 in the 52KO cells showed that the latter effect of timcodar required FKBP51. Thus, timcodar appears to have a dual specificity for FKBP51 and FKBP52. This work demonstrates phosphorylation as an important mechanism in FKBP control of GR and identifies the first nonimmunosuppressive macrolide capable of targeting GR action.

Comprehensive overview of the structure and regulation of the glucocorticoid receptor

Glucocorticoids are among the most prescribed drugs worldwide for the treatment of numerous immune and inflammatory disorders. They exert their actions by binding to the glucocorticoid receptor (GR), a member of the nuclear receptor superfamily. There are several GR isoforms resulting from alternative RNA splicing and translation initiation of the GR transcript. Additionally, these isoforms are all subject to several transcriptional, post-transcriptional, and post-translational modifications, all of which affect the protein's stability and/or function. In this review, we summarize recent knowledge on the distinct GR isoforms and the processes that generate them. We also review the importance of all known transcriptional, post-transcriptional, and post-translational modifications, including the regulation of GR by microRNAs. Moreover, we discuss the crucial role of the putative GR-bound DNA sequence as an allosteric ligand influencing GR structure and activity. Finally, we describe how the differential composition and distinct regulation at multiple levels of different GR species could account for the wide and diverse effects of glucocorticoids.

FKBP51 reciprocally regulates GRα and PPARγ activation via the Akt-p38 pathway

FK506-binding protein 51 (FKBP51) is a negative regulator of glucocorticoid receptor-α (GRα), although the mechanism is unknown. We show here that FKBP51 is also a chaperone to peroxisome proliferator-activated receptor-γ (PPARγ), which is essential for activity, and uncover the mechanism underlying this differential regulation. In COS-7 cells, FKBP51 overexpression reduced GRα activity at a glucocorticoid response element-luciferase reporter, while increasing PPARγ activity at a peroxisome proliferator response element reporter. Conversely, FKBP51-deficient (knockout) (51KO) mouse embryonic fibroblasts (MEFs) showed elevated GRα but reduced PPARγ activities compared with those in wild-type MEFs. Phosphorylation is known to exert a similar pattern of reciprocal modulation of GRα and PPARγ. Knockdown of FKBP51 in 3T3-L1 preadipocytes increased phosphorylation of PPARγ at serine 112, a phospho-residue that inhibits activity. In 51KO cells, elevated phosphorylation of GRα at serines 220 and 234, phospho-residues that promote activity, was observed. Because FKBP51 is an essential chaperone to the Akt-specific phosphatase PH domain leucine-rich repeat protein phosphatase, Akt signaling was investigated. Elevated Akt activation and increased activation of p38 kinase, a downstream target of Akt that phosphorylates GRα and PPARγ, were seen in 51KO MEFs, causing activation and inhibition, respectively. Inactivation of p38 with PD169316 reversed the effects of FKBP51 deficiency on GRα and PPARγ activities and reduced PPARγ phosphorylation. Last, loss of FKBP51 caused a shift of PPARγ from cytoplasm to nucleus, as previously shown for GRα. A model is proposed in which FKBP51 loss reciprocally regulates GRα and PPARγ via 2 complementary mechanisms: activation of Akt-p38-mediated phosphorylation and redistribution of the receptors to the nucleus for direct targeting by p38.

Functional proteomics for the characterization of impaired cellular responses to glucocorticoids in asthma

In chronic airway inflammatory disorders, such as asthma, glucocorticoid (GC) insensitivity is a challenging clinical problem associated with life-threatening disease progression and the potential development of serious side effects. The mechanism of steroid resistance in asthma remains unclear and may be multifactorial. Excluding noncompliance with GC treatment, abnormal steroid pharmacokinetics, and rare genetic defects in the glucocorticoid receptor (GR), the majority of GC insensitivity in asthma can be attributed to secondary defects related to GR function. Airway inflammatory cells obtained from patients with GC-resistant asthma show a number of abnormalities in cell immune responses to GC, which suggests that there is a causative defect in GR signaling in GC-resistant cells that could be further elucidated by a functional and molecular proteomics approach. Since T cells, eosinophils, and monocytes play a major role in the pathogenesis of airway inflammation, most of the work published to date has focused on these cell types as the primary therapeutic targets in GC-insensitive asthma. We herein review several distinct techniques for the assessment of (1) the cellular response to GCs including the effect of GCs on cell viability, adhesion, and mediator release; (2) the functionality of GC receptors, including phosphorylation of the GR, nuclear translocation, and binding activities; and (3) the characterization of proteins differentially expressed in steroid-resistant cells by comparative 2DE-gel electrophoresis-based techniques and mass spectrometry. These comprehensive approaches are expected to reveal novel candidates for biomarkers of steroid insensitivity, which may lead to the development of effective therapeutic interventions for patients with chronic steroid-resistant asthma.

Basal p38 mitogen-activated protein kinase regulates unliganded glucocorticoid receptor function in airway smooth muscle cells

Like many steroid receptors, the glucocorticoid (GC) receptor (GR) is a phosphoprotein. Although there are multiple phosphorylation sites critical for GR transcriptional activity (i.e., serine [S]203, S211, and S226), their respective role in driving GR functions is highly cell specific. We have recently identified protein phosphatase 5 as an essential Ser/Thr phosphatase responsible for impairing GR function via S211 dephosphorylation in airway smooth muscle (ASM) cells. Because p38 mitogen-activated protein kinase (MAPK) directly phosphorylates GR in different cell types in a stimulus- and cell-dependent manner, we investigated the role of p38 MAPK on GR phosphorylation and function in ASM cells. Cells were transfected with 100 nM p38 MAPK small interfering RNA or 2 μg MAPK kinase 3 expression vector (a specific kinase that directly activates p38 MAPK) in the presence or absence of fluticasone (100 nM) and/or p38 MAPK pharmacological inhibitor SB203580. We found that p38 MAPK blockade positively regulates GR nuclear translocation and GR-dependent induction of the steroid-target gene GC-induced leucine zipper in a hormone-independent manner. We also found that p38 MAPK-dependent regulation of GR functions was associated with a differential action on GR phosphorylation at S203 and S211 residues. This study demonstrated that the inactive state of GR in resting conditions is not only ensured by the absence of the GC ligand but also by p38 MAPK-dependent phosphorylation of unliganded GR at specific residues, which appears to be important in determining the overall GC responsiveness of ASM cells.

Prenatal alcohol exposure modifies glucocorticoid receptor subcellular distribution in the medial prefrontal cortex and impairs frontal cortex-dependent learning

Prenatal alcohol exposure (PAE) has been shown to impair learning, memory and executive functioning in children. Perseveration, or the failure to respond adaptively to changing contingencies, is a hallmark on neurobehavioral assessment tasks for human fetal alcohol spectrum disorder (FASD). Adaptive responding is predominantly a product of the medial prefrontal cortex (mPFC) and is regulated by corticosteroids. In our mouse model of PAE we recently reported deficits in hippocampal formation-dependent learning and memory and a dysregulation of hippocampal formation glucocorticoid receptor (GR) subcellular distribution. Here, we examined the effect of PAE on frontal cortical-dependent behavior, as well as mPFC GR subcellular distribution and the levels of regulators of intracellular GR transport. PAE mice displayed significantly reduced response flexibility in a Y-maze reversal learning task. While the levels of total nuclear GR were reduced in PAE mPFC, levels of GR phosphorylated at serines 203, 211 and 226 were not significantly changed. Cytosolic, but not nuclear, MR levels were elevated in the PAE mPFC. The levels of critical GR trafficking proteins, FKBP51, Hsp90, cyclophilin 40, dynamitin and dynein intermediate chain, were altered in PAE mice, in favor of the exclusion of GR from the nucleus, indicating dysregulation of GR trafficking. Our findings suggest that there may be a link between a deficit in GR nuclear localization and frontal cortical learning deficits in prenatal alcohol-exposed mice.

Mitogen-activated protein kinases and protein phosphatase 5 mediate glucocorticoid-induced cytotoxicity in pancreatic islets and β-cells

Glucocorticoid excess is associated with glucose intolerance and diabetes. In addition to inducing insulin resistance, glucocorticoids impair β-cell function and cause β-cell apoptosis. In this study we show that dexamethasone activates mitogen-activated protein kinases (MAPKs) signaling in MIN6 β-cells, as evident by enhanced phosphorylation of p38 MAPK and c-Jun N-terminal kinase (JNK). In contrast, the integrated stress response pathway was inhibited by dexamethasone. A p38 MAPK inhibitor attenuated dexamethasone-induced apoptosis in β-cells and isolated islets and decreased glucocorticoid receptor phosphorylation at S220. In contrast, a JNK inhibitor augmented DNA fragmentation and dexamethasone-induced formation of cleaved caspase 3. We also show that inhibition of protein phosphatase 5 (PP5) augments apoptosis in dexamethasone-exposed islets and β-cells, with a concomitant activation of p38 MAPK. In conclusion, our data provide evidence that in islets and β-cells, p38 MAPK and JNK phosphorylation work in concert with PP5 to regulate the cytotoxic effects exerted by glucocorticoids.

Lipid raft- and protein kinase C-mediated synergism between glucocorticoid- and gonadotropin-releasing hormone signaling results in decreased cell proliferation

Cross-talk between the glucocorticoid receptor (GR) and other receptors is emerging as a mechanism for fine-tuning cellular responses. We have previously shown that gonadotropin-releasing hormone (GnRH) ligand-independently activates the GR and synergistically modulates glucocorticoid-induced transcription of an endogenous gene in LβT2 pituitary gonadotrope precursor cells. Here, we investigated GR and GnRH receptor (GnRHR) cross-talk that involves co-localization with lipid rafts in LβT2 cells. We report that the GnRHR and a small population of the GR co-localize with the lipid raft protein flotillin-1 (Flot-1) at the plasma membrane and that the GR is present in a complex with Flot-1, independent of the presence of ligands. We found that the SGK-1 gene is up-regulated by Dex and GnRH alone, whereas a combination of both ligands resulted in a synergistic increase in SGK-1 mRNA levels. Using siRNA-mediated knockdown and antagonist strategies, we show that the gene-specific synergistic transcriptional response requires the GR, GnRHR, and Flot-1 as well as the protein kinase C pathway. Interestingly, although several GR cofactors are differentially recruited to the SGK-1 promoter in the presence of Dex and GnRH, GR levels remain unchanged compared with Dex treatment alone, suggesting that lipid raft association of the GR has a role in enhancing its transcriptional output in the nucleus. Finally, we show that Dex plus GnRH synergistically inhibit cell proliferation in a manner dependent on SGK-1 and Flot-1. Collectively the results support a mechanism whereby GR and GnRHR cross-talk within Flot-1-containing lipid rafts modulates cell proliferation via PKC activation and SGK-1 up-regulation.

A preliminary evaluation of leukocyte phospho-glucocorticoid receptor as a potential biomarker of depressogenic vulnerability in healthy adults

The mechanism of maladaptive chronic stress response involves altered phosphorylation of the glucocorticoid receptor (GR). In this study, we investigated if important depressogenic vulnerability factors, such as neuroticism and self-reports of negative affective states, may be associated with alterations in levels of the GR and GR phosphoisoforms in peripheral blood mononuclear cells (PBMC) of healthy adults. In 21 women and 16 men we evaluated PMBC levels of total GR (tGR), GR phosphorylated at serine 211 (pGR-S211) and serine 226 (pGR-S226) and correlated these data with personality traits and current reports of stress, anxiety and depression. Also, we assessed plasma cortisol levels in all tested subjects. Our results showed that in women nuclear pGR-S226 was positively correlated with neuroticism and current reports of depression, anxiety and stress, while the ratio of nuclear pGR-S211/pGR-S226 was negatively correlated with reports of depression. None of the aforementioned correlations were significant in men. No significant relations between cortisol levels and any of GR parameters were observed. These preliminary findings highlight the value of GR phosphorylation-related research in identifying molecular biomarkers of depressogenic vulnerability, at least in women.