Steroid receptor phosphorylation: a key modulator of multiple receptor functions

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.

Lysine methylation of progesterone receptor at activation function 1 regulates both ligand-independent activity and ligand sensitivity of the receptor

Progesterone receptor (PR) exists in two isoforms, PRA and PRB, and both contain activation functions AF-1 and AF-2. It is believed that AF-1 is primarily responsible for the ligand-independent activity, whereas AF-2 mediates ligand-dependent PR activation. Although more than a dozen post-translational modifications of PR have been reported, no post-translational modification on AF-1 or AF-2 has been reported. Using LC-MS/MS-based proteomic analysis, this study revealed AF-1 monomethylation at Lys-464. Mutational analysis revealed the remarkable importance of Lys-464 in regulating PR activity. Single point mutation K464Q or K464A led to ligand-independent PR gel upshift similar to the ligand-induced gel upshift. This upshift was associated with increases in both ligand-dependent and ligand-independent PR phosphorylation and PR activity due to the hyperactivation of AF-1. In contrast, mutation of Lys-464 to the bulkier phenylalanine to mimic the effect of methylation caused a drastic decrease in PR activity. Importantly, PR-K464Q also showed heightened ligand sensitivity, and this was associated with increases in its functional interaction with transcription co-regulators NCoR1 and SRC-1. These results suggest that monomethylation of PR at Lys-464 probably has a repressive effect on AF-1 activity and ligand sensitivity.

Prolactin activates ERα in the absence of ligand in female mammary development and carcinogenesis in vivo

Resistance of estrogen receptor positive (ERα+) breast cancers to antiestrogens is a major factor in the mortality of this disease. Although activation of ERα in the absence of ligand is hypothesized to contribute to this resistance, the potency of this mechanism in vivo is not clear. Epidemiologic studies have strongly linked prolactin (PRL) to both development of ERα+ breast cancer and resistance to endocrine therapies. Here we employed genetically modified mouse models to examine the ability of PRL and cross talk with TGFα to activate ERα, using a mutated ERα, ERα(G525L), which is refractory to endogenous estrogens. We demonstrate that PRL promotes pubertal ERα-dependent mammary ductal elongation and gene expression in the absence of estrogen, which are abrogated by the antiestrogen, ICI 182,780 (ICI). PRL and TGFα together reduce sensitivity to estrogen, and 30% of their combined stimulation of ductal proliferation is inhibited by ICI, implicating ligand-independent activation of ERα as a component of their interaction. However, PRL/TGFα-induced heterogeneous ERα+ tumors developed more rapidly in the presence of ICI and contained altered transcripts for surface markers associated with epithelial subpopulations and increased signal transducer and activator of transcription 5b expression. Together, these data support strong interactions between PRL and estrogen on multiple levels. Ligand-independent activation of ERα suggests that PRL may contribute to resistance to antiestrogen therapies. However, these studies also underscore ERα-mediated moderation of tumor phenotype. In light of the high expression of PRL receptors in ERα+ cancers, understanding the actions of PRL and cross talk with other oncogenic factors and ERα itself has important implications for therapeutic strategies.

Glucocorticoid sensitivity in health and disease

Glucocorticoids regulate many physiological processes and have an essential role in the systemic response to stress. For example, gene transcription is modulated by the glucocorticoid-glucocorticoid receptor complex via several mechanisms. The ultimate biologic responses to glucocorticoids are determined by not only the concentration of glucocorticoids but also the differences between individuals in glucocorticoid sensitivity, which is influenced by multiple factors. Differences in sensitivity to glucocorticoids in healthy individuals are partly genetically determined by functional polymorphisms of the gene that encodes the glucocorticoid receptor. Hereditary syndromes have also been identified that are associated with increased and decreased sensitivity to glucocorticoids. As a result of their anti-inflammatory properties, glucocorticoids are widely used in the treatment of allergic, inflammatory and haematological disorders. The variety in clinical responses to treatment with glucocorticoids reflects the considerable variation in glucocorticoid sensitivity between individuals. In immune-mediated disorders, proinflammatory cytokines can induce localized resistance to glucocorticoids via several mechanisms. Individual differences in how tissues respond to glucocorticoids might also be involved in the predisposition for and pathogenesis of the metabolic syndrome and mood disorders. In this Review, we summarize the mechanisms that influence glucocorticoid sensitivity in health and disease and discuss possible strategies to modulate glucocorticoid responsiveness.

Short tandem repeats in the inhibitory domain of the mineralocorticoid receptor: prediction of a β-solenoid structure

Background

The human mineralocorticoid receptor (MR) is one of the main components of the renin-angiotensin-aldosterone system (RAAS), the system that regulates the body exchange of water and sodium. The evolutionary origins of this protein predate those of renin and the RAAS; accordingly it has other roles, which are being characterized. The MR has two trans-activating ligand independent domains and one inhibitory domain (ID), which modulates the activity of the former. The structure of the ID is currently unknown.

Results

Here we report that the ID contains at least 15 tandem repeats of around 10 amino acids, which we computationally characterize in the human MR and in selected orthologs. This ensemble of repeats seems to have emerged around 450 million years ago, after the divergence of the MR from its close homolog, the glucocorticoid receptor, which does not possess the repeats. The region would have quickly expanded by successive duplication of the repeats stabilizing at its length in human MR shortly after divergence of tetrapoda from bony fishes 400 million years ago. Structural predictions, in combination with molecular dynamics simulations suggest that the repeat ensemble forms a β-solenoid, namely a β-helical fold with a polar core, stabilized by hydrogen-bonded ladders of polar residues. Our 3D-model, in conjunction with previous experimental data, implies a role of the β-helical fold as a scaffold for multiple intra-and inter-molecular interactions and that these interactions are modulated via phosphorylation-dependent conformational changes.

Conclusions

We, thus, propose that the structure of the repeat ensemble plays an important role in the coordination and sequential interactions of various MR partners and therefore in the functionality and specificity of MR.

Inhaled long-acting β2 agonists enhance glucocorticoid receptor nuclear translocation and efficacy in sputum macrophages in COPD

BACKGROUND

Combination inhaled therapy with long-acting β2 agonists (LABAs) and corticosteroids is beneficial in treating asthma and chronic obstructive pulmonary disease (COPD).

OBJECTIVE

In asthma, LABAs enhance glucocorticoid receptor (GR) nuclear translocation in the presence of corticosteroids. Whether this biological mechanism occurs in COPD, a relatively corticosteroid-resistant disease, is uncertain.

METHODS

Eight patients with mild/moderate COPD participated in a double-blind, placebo-controlled, crossover study and inhaled single doses of fluticasone propionate (FP) 100 μg, FP 500 μg, salmeterol xinafoate (SLM) 50 μg, and combination FP 100 μg + SLM 50 μg. One hour postinhalation, sputum was induced, nuclear proteins isolated from purified macrophages, and levels of activated nuclear GR quantified by using a GR-glucocorticoid response element ELISA-based assay.

RESULTS

Nuclear GR significantly increased after the inhalation of FP 500 μg (P < .01), but not after the inhalation of FP 100 μg or SLM 50 μg, compared with placebo. Interestingly, SLM in combination with FP 100 μg increased nuclear GR levels equivalent to those of FP 500 μg alone. This was significantly greater than either FP 100 μg (P < .05) or SLM 50 μg (P < .01) alone. In vitro in a human macrophage cell line, SLM (10(-8) mol/L) enhanced FP (10(-9) mol/L)-induced mitogen-activated protein kinase phosphatase-1 mRNA (5.8 ± 0.6 vs 8.4 ± 1.1 × 10(-6) copies, P < .05) and 2 × glucocorticoid response element-luciferase reporter gene activity (250.1 ± 15.6 vs 103.1 ± 23.6-fold induction, P < .001). Addition of SLM (10(-9) mol/L) to FP (10(-11) mol/L) significantly enhanced FP-mediated suppression of IL-1β-induced CXCL8 (P < .05).

CONCLUSIONS

Addition of SLM 50 μg to FP 100 μg enhanced GR nuclear translocation equivalent to that seen with a 5-fold higher dose of FP in sputum macrophages from patients with COPD. This may account for the superior clinical effects of combination LABA/corticosteroid treatment compared with either as monotherapy observed in COPD.

Protein Kinase C α Modulates Estrogen-Receptor-Dependent Transcription and Proliferation in Endometrial Cancer Cells

Endometrial cancer is the most common invasive gynecologic malignancy in developed countries. The most prevalent endometrioid tumors are linked to excessive estrogen exposure and hyperplasia. However, molecular mechanisms and signaling pathways underlying their etiology and pathophysiology remain poorly understood. We have shown that protein kinase C α (PKC α ) is aberrantly expressed in endometrioid tumors and is an important mediator of endometrial cancer cell survival, proliferation, and invasion. In this study, we demonstrate that expression of active, myristoylated PKC α conferred ligand-independent activation of estrogen-receptor- (ER-) dependent promoters and enhanced responses to estrogen. Conversely, knockdown of PKC α reduced ER-dependent gene expression and inhibited estrogen-induced proliferation of endometrial cancer cells. The ability of PKC α to potentiate estrogen activation of ER-dependent transcription was attenuated by inhibitors of phosphoinositide 3-kinase (PI3K) and Akt. Evidence suggests that PKC α and estrogen signal transduction pathways functionally interact, to modulate ER-dependent growth and transcription. Thus, PKC α signaling, via PI3K/Akt, may be a critical element of the hyperestrogenic environment and activation of ER that is thought to underlie the development of estrogen-dependent endometrial hyperplasia and malignancy. PKC α -dependent pathways may provide much needed prognostic markers of aggressive disease and novel therapeutic targets in ER positive tumors.

Eph receptor B4 is a regulator of estrogen receptor alpha in breast cancer cells

BACKGROUND

Estrogen receptor alpha (ER-α) plays an important role in breast cancer initiation and progression and represents a major target in cancer therapy. The expression and activity of ER-α is regulated by multiple mechanisms at the transcriptional and post-translational level. Interaction of tyrosine kinase receptor-activated signaling pathways with ER-α function has been reported. We previously performed a kinome-wide small interfering RNA high-throughput screen to identify novel protein kinases involved in the regulation of ER-α transcriptional activity in human breast cancer cells. Our screening analysis identified the Eph receptor tyrosine kinases (Eph) as potential positive regulators of ER-α.

RESULTS

In this study, we demonstrate Eph receptor B4 (EphB4), a member of Eph kinase family, a positive regulator of ER-α in human breast cancer cell lines (MCF-7, T-47D and BT-474). Down-regulation of EphB4 by RNA interference technology impairs estrogen-dependent ER-α transcriptional activity in breast cancer cells. Decreased activity of ER-α after EphB4 knockdown is the consequence of diminished ER-α messenger RNA and protein expression. Furthermore, phosphorylation of Akt, a downstream mediator of EphB4, is reduced following EphB4 silencing.

CONCLUSIONS

Our data suggests EphB4 as an upstream regulator of ER-α in human breast cancer cells by modulating ER-α transcription. The results also suggest Akt as a relevant downstream signaling molecule in this novel EphB4-ER-α pathway.

Interaction of JAK with steroid receptor function

The function of steroid receptors is not only regulated by steroid hormones, but also by multiple cellular signaling cascades activated by membrane-bound receptors which are stimulated by growth factors or cytokines. Cross-talk between JAK and steroid receptors plays a central role in the regulation of a multitude of physiological processes and aberrant signaling is involved in the development of numerous diseases including cancer. In this review we provide a brief summary of the knowledge of interactions between JAK and the function of steroid receptors in normal cells and tissues and in diseases.

Glucocorticoids and their receptors: insights into specific roles in mitochondria

Glucocorticoids (GCs) affect most physiological systems and are the most frequently used drugs for multiple disorders and organ transplantation. GC functions depend on a balance between circulating GC and cytoplasmic glucocorticoid receptor II (GR). Mitochondria individually enclose circular, double-stranded DNA that is expressed and replicated in response to nuclear-encoded factors imported from the cytoplasm. Fine-tuning and response to cellular demands should be coordinately regulated by the nucleus and mitochondria; thus mitochondrial-nuclear interaction is vital to optimal mitochondrial function. Elucidation of the direct and indirect effects of steroids, including GCs, on mitochondria is an important and emerging field of research. Mitochondria may also be under GC control because GRs are present in mitochondria, and glucocorticoid response elements (GREs) reside in the mitochondrial genome. Therefore, mitochondrial gene expression can be regulated by GCs via at least two different mechanisms: direct action on mitochondrial DNA and oxidative phosphorylation (OXPHOS) genes, or by an indirect effect through interaction with nuclear genes. In this review, we outline possible mechanisms of regulation of mitochondrial genes in response to GCs in view of translocation of the GR into mitochondria and the possible regulation of OXPHOS genes by GREs in the mitochondrial genome.

The role of intracrine androgen metabolism, androgen receptor and apoptosis in the survival and recurrence of prostate cancer during androgen deprivation therapy

Prostate cancer (CaP) is the most frequently diagnosed cancer and leading cause of cancer death in American men. Almost all men present with advanced CaP and some men who fail potentially curative therapy are treated with androgen deprivation therapy (ADT). ADT is not curative and CaP recurs as the lethal phenotype. The goal of this review is to apply our current understanding of CaP and castration-recurrent CaP (CR-CaP) to earlier studies that characterized ADT and the molecular mechanisms that facilitate the transition from androgen-stimulated CaP to CR-CaP. Reexamination of earlier studies also may provide a better understanding of how more newly recognized mechanisms, such as intracrine metabolism, may be involved with the early events that allow CaP survival after initiation of ADT and subsequent development of CR-CaP.

Multidomain integration in the structure of the HNF-4α nuclear receptor complex

The hepatocyte nuclear factor 4 alpha (HNF4α, NR2A1) is a member of the nuclear receptor (NR) family of transcription factors that use conserved DNA binding domains (DBDs) and ligand binding domains (LBDs)^1,2. HNF4α is the most abundant DNA-binding protein in the liver, where some 40% of the actively transcribed genes have a HNF4α response element ^1,3,4. These regulated genes are largely involved in the hepatic gluconeogenic program and lipid metabolism^3,5,6. In the pancreas too, HNF4α is a master regulator controlling an estimated 11% of islet genes⁷. HNF4α protein mutations are linked to Maturity Onset of Diabetes in Young 1 (MODY1) and hyperinsulinemic hypoglycemia (HH)^8–11. Prior structural analyses of NRs, while productive with individual domains, have lagged in revealing the connectivity patterns of NR domains. Here, we describe the 2.9 Å crystal structure of the multi-domain HNF4α homodimer bound to its DNA response element and coactivator-derived peptides. A convergence zone connects multiple receptor domains in an asymmetric fashion joining distinct elements from each monomer. An arginine target of PRMT1 methylation protrudes directly into this convergence zone and sustains its integrity. A serine target of protein kinase C is also responsible for maintaining domain-domain interactions. These post-translational modifications manifest into changes in DNA binding by communicating through the tightly connected surfaces of the quaternary fold. We find that some MODY1 mutations, positioned on the LBD and hinge regions of the receptor, compromise DNA binding at a distance by communicating through the inter-junctional surfaces of the complex. The overall domain representation of the HNF4α homodimer is different from that of the PPARγ-RXRα heterodimer, even when both NR complexes are assembled on the same DNA element. Our findings suggest that unique quaternary folds and inter-domain connections in NRs could be exploited by small-molecule allosteric modulators that impact distal functions in these polypeptides.

Corticosteroid resistance in patients with asthma and chronic obstructive pulmonary disease

Reduced responsiveness to the anti-inflammatory effects of corticosteroids is a major barrier to effective management of asthma in smokers and patients with severe asthma and in the majority of patients with chronic obstructive pulmonary disease (COPD). The molecular mechanisms leading to steroid resistance are now better understood, and this has identified new targets for therapy. In patients with severe asthma, several molecular mechanisms have been identified that might account for reduced steroid responsiveness, including reduced nuclear translocation of glucocorticoid receptor (GR) α after binding corticosteroids. This might be due to modification of the GR by means of phosphorylation as a result of activation of several kinases (p38 mitogen-activated protein kinase α, p38 mitogen-activated protein kinase γ, and c-Jun N-terminal kinase 1), which in turn might be due to reduced activity and expression of phosphatases, such as mitogen-activated protein kinase phosphatase 1 and protein phosphatase A2. Other mechanisms proposed include increased expression of GRβ, which competes with and thus inhibits activated GRα; increased secretion of macrophage migration inhibitory factor; competition with the transcription factor activator protein 1; and reduced expression of histone deacetylase (HDAC) 2. HDAC2 appears to mediate the action of steroids to switch off activated inflammatory genes, but in patients with COPD, patients with severe asthma, and smokers with asthma, HDAC2 activity and expression are reduced by oxidative stress through activation of phosphoinositide 3-kinase δ. Strategies for managing steroid resistance include alternative anti-inflammatory drugs, but a novel approach is to reverse steroid resistance by increasing HDAC2 expression, which can be achieved with theophylline and phosphoinositide 3-kinase δ inhibitors. Long-acting β2-agonists can also increase steroid responsiveness by reversing GRα phosphorylation. Identifying the molecular mechanisms of steroid resistance in asthmatic patients and patients with COPD can thus lead to more effective anti-inflammatory treatments.

Progesterone and related compounds in hepatocellular carcinoma: basic and clinical aspects

Primary liver cancer is the fifth most common cancer worldwide and the third most common cause of cancer mortality. Hepatocellular carcinoma (HCC) accounts for 85% to 90% of primary liver cancers. Major risk factors for HCC include infection with HBV or HCV, alcoholic liver disease, and most probably nonalcoholic fatty liver disease. In general, men are two to four times more often associated with HCC than women. It can be suggested that sex hormones including progesterone may play some roles in HCC. Rather, very limited information discusses its potential involvement in HCC. This paper thus collects some recent studies of the potential involvement of progesterone and related compounds in HCC from basic and clinical aspects. In addition, two synthetic progestins, megestrol acetate (MA) and medroxyprogesterone acetate (MPA), will be discussed thoroughly. It is noted that progesterone can also serve as the precursor for androgens and estrogens produced by the gonadal and adrenal cortical tissues, while men have a higher incidence of HCC than women might be due to the stimulatory effects of androgen and the protective effects of estrogen. Eventually, this paper suggests a new insight on the associations of progesterone and related compounds with HCC development and treatment.

Corticosteroid resistance and novel anti-inflammatory therapies in chronic obstructive pulmonary disease: current evidence and future direction

Corticosteroids are widely used in the treatment of chronic obstructive pulmonary disease (COPD). However, in contrast to their use in mild-to-moderate asthma, they are much less effective in enhancing lung function and have little or no effect on controlling the underlying chronic inflammation. In most clinical trials in COPD patients, corticosteroids have shown little benefit as monotherapy, but have shown a greater clinical effect in combination with long-acting bronchodilators. Several mechanisms of corticosteroid resistance have been postulated, including a reduction in histone deacetylase (HDAC)-2 activity and expression, impaired corticosteroid activation of the glucocorticoid receptor (GR) and increased pro-inflammatory signalling pathways. Reversal of corticosteroid resistance in COPD patients by restoring HDAC2 levels has proved effective in a small study, and long-term studies are needed to determine whether novel HDAC2 activators or theophylline improve disease progression, exacerbations or mortality. Advances in the understanding of the cellular and molecular mechanisms of corticosteroid resistance in COPD pathophysiology have supported the development of new emerging classes of anti-inflammatory drugs in COPD treatment. These include treatments such as inhibitors of phosphoinositide-3-kinase-delta (PI3Kδ), phosphodiesterase-4 (PDE4), p38 mitogen-activated protein kinase (MAPK) and nuclear factor kappa B (NF-κB), and therapeutic agents such as chemokine receptor antagonists. Of these, PI3Kδ, PDE4, p38 MAPK inhibitors and chemokine receptor antagonists are in clinical patient trials. Of importance, patient adverse effects associated with oral administration of these novel agents needs to be addressed in order to optimize therapy and patient compliance. Combinations of these drugs with corticosteroids may have additional benefits.

PKC-mediated USP phosphorylation at Ser35 modulates 20-hydroxyecdysone signaling in Drosophila

The nuclear receptor complex of the steroid hormone, 20-hydroxyecdysone (20E), is a heterodimer composed of EcR and USP. Our previous studies in Drosophila suggest that PKC modulates 20E signaling by phosphorylating EcR-USP. However, the exact phosphorylation sites in EcR and USP have not been identified. Using LC-MS/MS analysis, we first identified Ser35 of USP as a PKC phosphorylation site. Mutation of USP Ser35 to Ala35 in S2 cells not only eliminated USP phosphorylation, but also attenuated the 20E-induced luciferase activity, mimicking the treatment with a PKC-specific inhibitor chelerythrine chloride in Kc cells. In the larval salivary glands (SG), inhibition of PKC activity with the binary GAL4/UAS system reduced USP phosphorylation and down-regulated the 20E primary-response genes, E75B and Br-C, and RNAi knockdown of Rack1 had stronger inhibitory effects than overexpression of PKCi. Moreover, RNAi knockdown of four PKC isozyme genes expressed in the SG exhibited a variety of inhibitory effects on USP phosphorylation and expression of E75B and Br-C, with the strongest inhibitory effects occurring when aPKC was knocked down by RNAi. Taken together, we conclude that PKC-mediated USP phosphorylation at Ser35 modulates 20E signaling in Drosophila.

Non-genomic effect of glucocorticoids on cardiovascular system

Glucocorticoids (GCs) are essential steroid hormones for homeostasis, development, metabolism, and cognition and possess anti-inflammatory and immunosuppressive actions. Since glucocorticoid receptor II (GR) is nearly ubiquitous, chronic activation or depletion of GCs leads to dysfunction of diverse organs, including the heart and blood vessels, resulting predominantly from changes in gene expression. Most studies, therefore, have focused on the genomic effects of GC to understand its related pathophysiological manifestations. The nongenomic effects of GCs clearly differ from well-known genomic effects, with the former responding within several minutes without the need for protein synthesis. There is increasing evidence that the nongenomic actions of GCs influence various physiological functions. To develop a GC-mediated therapeutic target for the treatment of cardiovascular disease, understanding the genomic and nongenomic effects of GC on the cardiovascular system is needed. This article reviews our current understanding of the underlying mechanisms of GCs on cardiovascular diseases and stress, as well as how nongenomic GC signaling contributes to these conditions. We suggest that manipulation of GC action based on both GC and GR metabolism, mitochondrial impact, and the action of serum- and glucocorticoid-dependent kinase 1 may provide new information with which to treat cardiovascular diseases.

Effects of sorafenib on C-terminally truncated androgen receptor variants in human prostate cancer cells

Recent evidence suggests that the development of castration resistant prostate cancer (CRPCa) is commonly associated with an aberrant, ligand-independent activation of the androgen receptor (AR). A putative mechanism allowing prostate cancer (PCa) cells to grow under low levels of androgens, is the expression of constitutively active, C-terminally truncated AR lacking the AR-ligand binding domain (LBD). Due to the absence of a LBD, these receptors, termed ARΔLBD, are unable to respond to any form of anti-hormonal therapies. In this study we demonstrate that the multikinase inhibitor sorafenib inhibits AR as well as ARΔLBD-signalling in CRPCa cells. This inhibition was paralleled by proteasomal degradation of the AR- and ARΔLBD-molecules. In line with these observations, maximal antiproliferative effects of sorafenib were achieved in AR and ARΔLBD-positive PCa cells. The present findings warrant further investigations on sorafenib as an option for the treatment of advanced AR-positive PCa.

Stable inhibition of specific estrogen receptor α (ERα) phosphorylation confers increased growth, migration/invasion, and disruption of estradiol signaling in MCF-7 breast cancer cells

Elevated phosphorylation of estrogen receptor α (ERα) at serines 118 (S118) and 167 (S167) is associated with favorable outcome for tamoxifen adjuvant therapy and may serve as surrogate markers for a functional ERα signaling pathway in breast cancer. It is possible that loss of phosphorylation at S118 and/or S167 could disrupt ERα signaling, resulting in aggressive ERα-independent breast cancer cells. To this end, MCF-7 breast cancer cells were stably transfected with an ERα-specific short hairpin RNA that reduced endogenous ERα. The resulting cell line was stably transfected with wild-type ERα (ER-AB cells), or ERα containing serine to alanine mutation at S118 or S167 (S118A cells and S167A cells, respectively). These stable cell lines expressed approximately equivalent ERα compared with parental MCF-7 cells and were evaluated for growth, morphology, migration/invasion, and ERα-regulated gene expression. S118A cells and S167A cells exhibited increased growth and migration/invasion in vitro. Forward- and side-scatter flow cytometry revealed that S167A cells were smaller in size, and both S118A and S167A cells exhibited less cellular complexity. S118A and S167A cells expressed pancytokeratin and membrane localization of β-catenin and did not express vimentin, indicating retention of epithelial lineage markers. Expression of ERα-target genes and other genes regulated by ERα signaling or involved in breast cancer were markedly altered in both S118A and S167A cells. In summary, attenuated phosphorylation of ERα at S118 and S167 significantly affected cellular physiology and behavior in MCF-7 breast cancer cells, resulting in increased growth, migration/invasion, compromised expression of ERα target genes, and markedly altered gene expression patterns.

Differential ubiquitylation of the mineralocorticoid receptor is regulated by phosphorylation

Aldosterone stimulation of the mineralocorticoid receptor (MR) is involved in numerous physiological responses, including Na+ homeostasis, blood pressure control, and heart failure. Aldosterone binding to MR promotes different post-translational modifications that regulate MR nuclear translocation, gene expression, and finally receptor degradation. Here, we show that aldosterone stimulates rapid phosphorylation of MR via ERK1/2 in a dose-dependent manner (from 0.1 to 10 nM) in renal epithelial cells. This phosphorylation induces an increase of MR apparent molecular weight, with a maximal upward shift of 30 kDa. Strikingly, these modifications are critical for the regulation of the MR ubiquitylation state. Indeed, we find that MR is monoubiquitylated in its basal state, and this status is sustained by the tumor suppressor gene 101 (Tsg101). Phosphorylation leads to disruption of MR/Tsg101 association and monoubiquitin removal. These events prompt polyubiquitin-dependent destabilization of MR and degradation. Preventing MR phosphorylation by ERK1/2 inhibition or mutation of target serines affects the sequential mechanisms of MR ubiquitylation and inhibits the aldosterone-mediated degradation. Our data provide a novel model of negative feedback of aldosterone signaling, involving sequential phosphorylation, monoubiquitin removal and subsequent polyubiquitylation/degradation of MR.

Ligand binding promotes CDK-dependent phosphorylation of ER-alpha on hinge serine 294 but inhibits ligand-independent phosphorylation of serine 305

Phosphorylation of estrogen receptor-α (ERα) is critical for its transcription factor activity and may determine its predictive and therapeutic value as a biomarker for ERα-positive breast cancers. Recent attention has turned to the poorly understood ERα hinge domain, as phosphorylation at serine 305 (Ser305) associates with poor clinical outcome and endocrine resistance. We show that phosphorylation of a neighboring hinge domain site, Ser294, analyzed by multiple reaction monitoring mass spectrometry of ERα immunoprecipitates from human breast cancer cells is robustly phosphorylated exclusively by ligand (estradiol and tamoxifen) activation of ERα and not by growth factor stimulation (EGF, insulin, heregulin-β). In a reciprocal fashion, Ser305 phosphorylation is induced by growth factors but not ligand activation of ERα. Phosphorylation at Ser294 and Ser305 is suppressed upon co-stimulation by EGF and ligand, respectively, unlike the N-terminal (AF-1) domain Ser118 and Ser167 sites of ERα where phosphorylation is enhanced by ligand and growth factor co-stimulation. Inhibition of cyclin-dependent kinases (CDK) by roscovitine or SNS-032 suppresses ligand-activated Ser294 phosphorylation without affecting Ser118 or Ser104/Ser106 phosphorylation. Likewise, cell-free studies using recombinant ERα and specific cyclin-CDK complexes suggest that Ser294 phosphorylation is primarily induced by the transcription-regulating and cell-cycle-independent kinase CDK7. Thus, CDK-dependent phosphorylation at Ser294 differentiates ligand-dependent from ligand-independent activation of Ser305 phosphorylation, showing that hinge domain phosphorylation patterns uniquely inform on the various ERα activation mechanisms thought to underlie the biologic and clinical diversity of hormone-dependent breast cancers.

Allosteric modulators of steroid hormone receptors: structural dynamics and gene regulation

Steroid hormones are synthesized from cholesterol primarily in the adrenal gland and the gonads and play vital roles in normal physiology, the control of development, differentiation, metabolic homeostasis, and reproduction. The actions of these small lipophilic molecules are mediated by intracellular receptor proteins. It is just over 25 yr since the first cDNA for steroid receptors were cloned, a development that led to the birth of a superfamily of ligand-activated transcription factors: the nuclear receptors. The receptor proteins share structurally and functionally related ligand binding and DNA-binding domains but possess distinct N-terminal domains and hinge regions that are intrinsically disordered. Since the original cloning experiments, considerable progress has been made in our understanding of the structure, mechanisms of action, and biology of this important class of ligand-activated transcription factors. In recent years, there has been interest in the structural plasticity and function of the N-terminal domain of steroid hormone receptors and in the allosteric regulation of protein folding and function in response to hormone, DNA response element architecture, and coregulatory protein binding partners. The N-terminal domain can exist as an ensemble of conformers, having more or less structure, which prime this region of the receptor to rapidly respond to changes in the intracellular environment through hormone binding and posttranslation modifications. In this review, we address the question of receptor structure and function dynamics with particular emphasis on the structurally flexible N-terminal domain, intra- and interdomain communications, and the allosteric regulation of receptor action.

Coordinate regulation of estrogen receptor β degradation by Mdm2 and CREB-binding protein in response to growth signals

The biological actions of estrogen are mediated via estrogen receptors ERα and ERβ. Yet, other cellular signaling events that also impact ER functions have an important role in breast carcinogenesis. Here, we show that activation of ErbB2/ErbB3 tyrosine kinase receptors with growth factor heregulin-β prompts ERβ degradation by the 26S proteasome, a mechanism that requires the coactivator cAMP response element-binding (CREB)-binding protein (CBP). We found that CBP promotes ERβ ubiquitination and degradation through enhancement of the PI3-K/Akt pathway by heregulin-β, an effect potentiated by a negatively charged hinge region of ERβ. Activated Akt triggered the recruitment of E3 ubiquitin ligase Mdm2 to ERβ, which was further stabilized by CBP, resulting in ERβ poly-ubiquitination. Mutation of CBP Thr-1872 or Mdm2 Ser-186/188 Akt sites resulted in a dissociation of the ERβ-CBP-Mdm2 complex and reduced ERβ turnover. We found that the decrease in ERβ induced by heregulin-β was associated with reduced target gene promoter occupancy and enhanced proliferation of breast cancer cells. However, knockdown of Mdm2 restored endogenous ERβ levels resulting in reduction of breast cancer cell growth. These studies identify a tripartite Akt-regulated phosphorylation mechanism that functions to hamper normal ERβ activity and turnover through the concerted actions of CBP and Mdm2 in response to growth factor signaling pathways in breast cancer cells.

Human gene control by vital oncogenes: revisiting a theoretical model and its implications for targeted cancer therapy

An important assumption of our current understanding of the mechanisms of carcinogenesis has been the belief that clarification of the cancer process would inevitably reveal some of the crucial mechanisms of normal human gene regulation. Since the momentous work of Bishop and Varmus, both the molecular and the biochemical processes underlying the events in the development of cancer have become increasingly clear. The identification of cellular signaling pathways and the role of protein kinases in the events leading to gene activation have been critical to our understanding not only of normal cellular gene control mechanisms, but also have clarified some of the important molecular and biochemical events occurring within a cancer cell. We now know that oncogenes are dysfunctional proto-oncogenes and that dysfunctional tumor suppressor genes contribute to the cancer process. Furthermore, Weinstein and others have hypothesized the phenomenon of oncogene addiction as a distinct characteristic of the malignant cell. It can be assumed that cancer cells, indeed, become dependent on such vital oncogenes. The products of these vital oncogenes, such as c-myc, may well be the Achilles heel by which targeted molecular therapy may lead to truly personalized cancer therapy. The remaining problem is the need to introduce relevant molecular diagnostic tests such as genome microarray analysis and proteomic methods, especially protein kinase identification arrays, for each individual patient. Genome wide association studies on cancers with gene analysis of single nucleotide and other mutations in functional proto-oncogenes will, hopefully, identify dysfunctional proto-oncogenes and allow the development of more specific targeted drugs directed against the protein products of these vital oncogenes. In 1984 Willis proposed a molecular and biochemical model for eukaryotic gene regulation suggesting how proto-oncogenes might function within the normal cell. That model predicted the existence of vital oncogenes and can now be used to hypothesize the biochemical and molecular mechanisms that drive the processes leading to disruption of the gene regulatory machinery, resulting in the transformation of normal cells into cancer.

Regulation of estrogen receptor α N-terminus conformation and function by peptidyl prolyl isomerase Pin1

Estrogen receptor alpha (ERα), a key driver of growth in the majority of breast cancers, contains an unstructured transactivation domain (AF1) in its N terminus that is a convergence point for growth factor and hormonal activation. This domain is controlled by phosphorylation, but how phosphorylation impacts AF1 structure and function is unclear. We found that serine 118 (S118) phosphorylation of the ERα AF1 region in response to estrogen (agonist), tamoxifen (antagonist), and growth factors results in recruitment of the peptidyl prolyl cis/trans isomerase Pin1. Phosphorylation of S118 is critical for Pin1 binding, and mutation of S118 to alanine prevents this association. Importantly, Pin1 isomerizes the serine118-proline119 bond from a cis to trans isomer, with a concomitant increase in AF1 transcriptional activity. Pin1 overexpression promotes ligand-independent and tamoxifen-inducible activity of ERα and growth of tamoxifen-resistant breast cancer cells. Pin1 expression correlates with proliferation in ERα-positive rat mammary tumors. These results establish phosphorylation-coupled proline isomerization as a mechanism modulating AF1 functional activity and provide insight into the role of a conformational switch in the functional regulation of the intrinsically disordered transactivation domain of ERα.

Janus kinase 2--a novel negative regulator of estrogen receptor α function

Estrogen receptor α (ERα) functions as a transcription factor to regulate a wide range of cellular activities in response to 17β-estradiol (E2). The regulation of ERα transcriptional activity is highly complex and not yet fully understood. In this respect, recent studies have highlighted the importance of certain cellular protein kinases. To identify novel protein kinases regulating ERα activity, we performed a high-throughput siRNA screening in combination with a luciferase reporter assay in an ERα positive breast cancer cell line. Among the vast majority of potential positive regulators, we found Janus kinase 2 (JAK2), a member of the Janus kinase family of non-receptor tyrosine kinases, to have a negative regulatory effect on E2 induced luciferase activity. In addition, silencing of JAK2 resulted in increased expression of endogenous ERα target genes, pS2 and GREB1. In an attempt to understand the mechanism underlying JAK2 mediated regulation of ERα transcriptional activity, we found that JAK2 negatively regulates ERα protein level. Gene expression analysis revealed no significant influence of JAK2 on ERα mRNA level. Subsequently, a role of JAK2 in regulating ERα protein degradation was analyzed. Inhibition of the lysosome did not alter JAK2 mediated downregulation of ERα. In contrast, using proteasome inhibitors MG132 and lactacystin, we demonstrated that JAK2 governs ERα protein stability via the ubiquitin-proteasome pathway. In contrast to JAK2, the two other members of the JAK family expressed in the breast (JAK1 and TYK2) had no influence on ERα function. In addition, we found that prolonged E2 treatment upregulates JAK2 mRNA and protein levels. These results suggest a novel negative regulation of ERα activity and protein by JAK2 in breast cancer cells and indicate a potential new cross-talk.

Glucocorticosteroids: current and future directions

Glucocorticoids are the most effective anti-inflammatory therapy for asthma yet are relatively ineffective in chronic obstructive pulmonary disease. Glucocorticoids suppress inflammation via several molecular mechanisms. Glucocorticoids suppress the multiple inflammatory genes that are activated in chronic inflammatory diseases, such as asthma, by reversing histone acetylation of activated inflammatory genes through binding of ligand-bound glucocorticoid receptors (GR) to co-activator molecules and recruitment of histone deacetylase-2 to the activated inflammatory gene transcription complex (trans-repression). At higher concentrations of glucocorticoids GR homodimers interact with DNA recognition sites to activate transcription through increased histone acetylation of anti-inflammatory genes and transcription of several genes linked to glucocorticoid side effects (trans-activation). Glucocorticoids also have post-transcriptional effects and decrease stability of some pro-inflammatory mRNA species. Decreased glucocorticoid responsiveness is found in patients with severe asthma and asthmatics who smoke, as well as in all patients with chronic obstructive pulmonary disease. Several molecular mechanisms of glucocorticoid resistance have now been identified which involve post-translational modifications of GR. Histone deacetylase-2 is markedly reduced in activity and expression as a result of oxidative/nitrative stress so that inflammation becomes resistant to the anti-inflammatory actions of glucocorticoids. Dissociated glucocorticoids and selective GR modulators which show improved trans-repression over trans-activation effects have been developed to reduce side effects, but so far it has been difficult to dissociate anti-inflammatory effects from adverse effects. In patients with glucocorticoid resistance alternative anti-inflammatory treatments are being investigated as well as drugs that may reverse the molecular mechanisms of glucocorticoid resistance.

Testostérone et contrôle central de l’érection

La testostérone orchestre l’organisation périnatale et l’activation adulte des structures nerveuses cérébrales et spinales impliquées dans l’expression du comportement sexuel mâle. Cette revue décrit brièvement les différents effets de la testostérone dans la régulation de la motivation sexuelle et de l’érection, et les modèles génétiques générés, jusqu’à présent, dans le but d’élucider ses mécanismes d’action centraux.

Phosphorylation of Estrogen Receptor α at serine 118 directs recruitment of promoter complexes and gene-specific transcription

Phosphorylation of estrogen receptor α (ERα) is important for receptor function, although the role of specific ERα phosphorylation sites in ERα-mediated transcription remains to be fully evaluated. Transcriptional activation by ERα involves dynamic, coordinate interactions with coregulators at promoter enhancer elements to effect gene expression. To determine whether ERα phosphorylation affects recruitment of unique protein complexes at gene-specific promoters, changes in ERα Ser118 phosphorylation were assessed for effects on receptor and coregulator recruitment and transcription of ERα-regulated genes. Chromatin immunoprecipitation assays to measure promoter association found a 17β-estradiol (E2)-dependent recruitment of ERα at 150 min to ERα-regulated promoters, whereas ERα phosphorylated at Ser118 was dissociated from promoters after E2 treatment. Mutation of Ser118 to alanine (S118A) altered unliganded and ligand-induced association of ERα and p160 coregulators with ERα target promoters when compared with wild-type (WT)-ERα transfection. S118A and WT-ERα exhibited a similar level of recruitment to the estrogen response element-driven pS2 promoter and induced pS2 mRNA after E2 treatment. Although WT-ERα was recruited to c-myc and cyclin D1 promoters after E2 treatment and induced mRNA expression, S118A exhibited reduced interaction with c-myc and cyclin D1 promoters, and E2 did not induce c-myc and cyclin D1 mRNA. In addition, S118A resulted in increased recruitment of steroid receptor coactivator-1, glucocorticoid receptor interacting protein-1, and activated in breast cancer-1 to pS2, c-myc, and cyclin D1 irrespective of the presence of E2. Together, these data indicate that site specific phosphorylation of ERα directs gene-specific recruitment of ERα and transcriptional coregulators to ERα target gene promoters.

Regulation of progesterone receptor activity by cyclin dependent kinases 1 and 2 occurs in part by phosphorylation of the SRC-1 carboxyl-terminus

We described previously a novel role for cyclin A2/Cdk2 as a progesterone receptor (PR) coactivator. In reporter gene assays, cyclin A2 overexpression enhanced PR activity while inhibition of Cdk2 activity using the chemical inhibitor roscovitine or Cdk2 siRNA strongly inhibited PR activity. We demonstrate here that both Cdk1 and Cdk2 contribute to maximal induction of endogenous progestin responsive genes in T47D breast cancer cells. Our earlier studies suggested that the mechanism by which cyclin A2/Cdk2 enhances PR activity is via phosphorylation of steroid receptor coactivator-1 (SRC-1), which increases PR-SRC-1 interactions. To assess the importance of SRC-1 phosphorylation in the regulation of PR activity, SRC-1 was phosphorylated by cyclin A2/Cdk2 in vitro and seventeen phosphorylation sites were identified using biochemical techniques. We show that one of these sites, T1426 (adjacent to the C-terminal LXXLL nuclear receptor interaction motif), is an in vivo target of Cdks in mammalian cells and an in vitro target of Cdk1 and Cdk2. Phosphorylation of T1426 also contributes to SRC-1 coactivation potential, as mutation of the threonine target site to alanine results in reduced stimulation of PR activity by SRC-1. Together, these results suggest a role for Cdk1 and Cdk2 in the regulation of endogenous PR activity in part through phosphorylation of SRC-1.

KX-01, a novel Src kinase inhibitor directed toward the peptide substrate site, synergizes with tamoxifen in estrogen receptor α positive breast cancer

KX-01 is the first clinical Src inhibitor of the novel peptidomimetic class that targets the peptide substrate site of Src providing more specificity toward Src kinase. The present study was designed to evaluate the effects of KX-01 as a single agent and in combination with tamoxifen (TAM) on cell growth and apoptosis of ERα positive breast cancer in vitro and in vivo. Flow cytometry demonstrated that KX-01 induced cell cycle arrest in G2/M phase. Immunofluorescent staining for mitotic phase markers and TUNEL staining indicated that cells had arrested in the mitotic phase and mitotic arrested cells were undergoing apoptosis. KX-01 induced nuclear accumulation of cyclin B1, and activation of CDK1, MPM2, and Cdc25C that is required for progression past the G2/M checkpoint. Apoptosis resulted from activation of caspases 6, 7, 8, and 9. Combinational index analysis revealed that combinations of KX-01 with TAM resulted in synergistic growth inhibition of breast cancer cell lines. KX-01 combined with TAM resulted in decreased ERα phosphorylation at Src-regulated phosphorylation sites serines 118 and 167 that were associated with reduced ERα transcriptional activity. Orally administered KX-01 resulted in a dose dependent growth inhibition of MCF-7 tumor xenografts, and in combination with TAM exhibited synergistic growth inhibition. Immunohistochemical analysis revealed that combinational treatment reduced angiogenesis, and ERα signaling in tumors compared to either drug alone that may underlie the synergistic tumor growth inhibition. Combinations of KX-01 with endocrine therapy present a promising new strategy for clinical management of ERα positive breast cancer.

The phosphorylation of the androgen receptor by TFIIH directs the ubiquitin/proteasome process

In response to hormonal stimuli, a cascade of hierarchical post-translational modifications of nuclear receptors are required for the correct expression of target genes. Here, we show that the transcription factor TFIIH, via its cdk7 kinase, phosphorylates the androgen receptor (AR) at position AR/S515. Strikingly, this phosphorylation is a key step for an accurate transactivation that includes the cyclic recruitment of the transcription machinery, the MDM2 E3 ligase, the subsequent ubiquitination of AR at the promoter of target genes and its degradation by the proteasome machinery. Impaired phosphorylation disrupts the transactivation, as observed in cells either overexpressing the non-phosphorylated AR/S515A, isolated from xeroderma pigmentosum patient (bearing a mutation in XPD subunit of TFIIH), or in which cdk7 kinase was silenced. Indeed, besides affecting the cyclic recruitment of the transcription machinery, the AR phosphorylation defect favourizes to the recruitment of the E3 ligase CHIP instead of MDM2, at the PSA promoter, that will further attract the proteasome machinery. These observations illustrate how the TFIIH phosphorylation might participate to the transactivation by regulating the nuclear receptors turnover.

Binding of the ERalpha nuclear receptor to DNA is coupled to proton uptake

Nuclear receptors act as ligand-modulated transcription factors and orchestrate a plethora of cellular functions central to health and disease. Although studied for more than half a century, many mysteries surrounding the mechanism of action of nuclear receptors remain unresolved. Herein, using isothermal titration calorimetry (ITC) in conjunction with macromolecular modeling (MM), we provide evidence that the binding of the ERalpha nuclear receptor to its DNA response element is coupled to proton uptake by two ionizable residues, H196 and E203, located at the protein-DNA interface. Alanine substitution of these ionizable residues decouples protonation and hampers the binding of ERalpha to DNA by nearly 1 order of magnitude. Remarkably, H196 and E203 are predominantly conserved across approximately 50 members of the nuclear receptor family, implying that proton-coupled equilibrium may serve as a key regulatory switch for modulating protein-DNA interactions central to nuclear receptor function and regulation. Taken together, our findings unearth an unexpected but critical step in the molecular action of nuclear receptors and suggest that they may act as sensors of intracellular pH.

Bile acids regulate hepatic gluconeogenic genes and farnesoid X receptor via G(alpha)i-protein-coupled receptors and the AKT pathway

Bile acids are important regulatory molecules that can activate specific nuclear receptors and cell signaling pathways in the liver and gastrointestinal tract. In the current study, the chronic bile fistula (CBF) rat model and primary rat hepatocytes (PRH) were used to study the regulation of gluconeogenic genes phosphoenolpyruvate carboxykinase (PEPCK) and glucose-6-phosphatase (G-6-Pase) and the gene encoding short heterodimeric partner (SHP) by taurocholate (TCA). The intestinal infusion of TCA into the CBF rat rapidly (1h) activated the AKT (approximately 9-fold) and ERK1/2 (3- to 5-fold) signaling pathways, downregulated (approximately 50%, 30 min) the mRNA levels of PEPCK and G-6-Pase, and induced (14-fold in 3 h) SHP mRNA. TCA rapidly ( approximately 50%, 1-2 h) downregulated PEPCK and G-6-Pase mRNA levels in PRH. The downregulation of these genes by TCA was blocked by pretreatment of PRH with pertussis toxin (PTX). In PRH, TCA plus insulin showed a significantly stronger inhibition of glucose secretion/synthesis from lactate and pyruvate than either alone. The induction of SHP mRNA in PRH was strongly blocked by inhibition of PI3 kinase or PKCzeta by specific chemical inhibitors or knockdown of PKCzeta by siRNA encoded by a recombinant lentivirus. Activation of the insulin signaling pathway appears to be linked to the upregulation of farnesoid X receptor functional activity and SHP induction.

CCAAT/Enhancer-binding protein beta DNA binding is auto-inhibited by multiple elements that also mediate association with p300/CREB-binding protein (CBP)

Signaling through Ras GTPases controls the activity of many transcription factors including CCAAT/enhancer-binding protein (C/EBPbeta), which regulates oncogenic H-Ras(V12)-induced senescence and growth arrest. Here we report that C/EBPbeta (LAP) DNA binding is inhibited by N-terminal sequences and derepressed by oncogenic Ras signaling. Sequence and mutational analyses showed that auto-repression involves two LXXLF (phiXXphiphi)-like motifs (LX1 and LX2) and a third element, auto-inhibitory domain (AID), located within conserved region CR5. LX1 is a critical component of the transactivation domain and has been shown to mediate C/EBPbeta binding to the TAZ2 region of p300/CREB-binding protein coactivators. C/EBPbeta auto-repression also involves a C-terminal regulatory domain (CRD) adjacent to the leucine zipper. CRD contains a third phiXXphiphi motif (LX3) and a short sequence, KQL, which has similarity to a region in the protein-binding site of TAZ2. The C/EBPbeta N- and C-terminal domains physically associate in a manner that requires the basic region and CRD. We propose a model in which the regulatory sequences form a hydrophobic core that reciprocally inhibits DNA binding and transactivation. We also suggest a mechanism for C/EBPbeta derepression involving several recently identified modifications within AID and CRD. Finally, we show that association of activated C/EBPbeta with p300/CREB-binding protein requires the LX2 and AID auto-inhibitory elements. Thus, the N-terminal regulatory elements have dual roles in auto-inhibition and coactivator binding.

Mechanisms and resistance in glucocorticoid control of inflammation

Glucocorticoids are the most effective anti-inflammatory therapy for many chronic inflammatory and immune diseases, such as asthma, but are relatively ineffective in other diseases such as chronic obstructive pulmonary disease (COPD). Glucocorticoids suppress inflammation by several mechanisms. Glucocorticoids suppress the multiple inflammatory genes that are activated in chronic inflammatory diseases, such as asthma, by reversing histone acetylation of activated inflammatory genes through binding of liganded glucocorticoid receptors (GR) to coactivator molecules and recruitment of histone deacetylase-2 (HDAC2) to the activated transcription complex. At higher concentrations of glucocorticoids GR homodimers interact with DNA recognition sites to activate transcription through increased histone acetylation of anti-inflammatory genes and transcription of several genes linked to glucocorticoid side effects. Decreased glucocorticoid responsiveness is found in patients with severe asthma and asthmatics who smoke, as well as in all patients with COPD and cystic fibrosis. Several molecular mechanisms of glucocorticoid resistance have now been identified. HDAC2 is markedly reduced in activity and expression as a result of oxidative/nitrative stress so that inflammation becomes resistant to the anti-inflammatory actions of glucocorticoids. Dissociated glucocorticoids have been developed to reduce side effects but so far it has been difficult to dissociate anti-inflammatory effects from adverse effects. In patients with glucocorticoid resistance alternative anti-inflammatory treatments are being investigated as well as drugs that may reverse the molecular mechanism of glucocorticoid resistance.

The role of glucocorticoid receptor phosphorylation in Mcl-1 and NOXA gene expression

Background

The cyclin-dependent kinase (CDK) and mitogen-activated protein kinase (MAPK) mediated phosphorylation of glucocorticoid receptor (GR) exerts opposite effects on GR transcriptional activity and affects other posttranslational modifications within this protein. The major phosphorylation site of human GR targeted by MAPK family is the serine 226 and multiple kinase complexes phosphorylate receptor at the serine 211 residue. We hypothesize that GR posttranslational modifications are involved in the determination of the cellular fate in human lymphoblastic leukemia cells. We investigated whether UV signalling through alternative GR phosphorylation determined the cell type specificity of glucocorticoids (GCs) mediated apoptosis.

Results

We have identified putative Glucocorticoid Response Elements (GREs) within the promoter regulatory regions of the Bcl-2 family members NOXA and Mcl-1 indicating that they are direct GR transcriptional targets. These genes were differentially regulated in CEM-C7-14, CEM-C1-15 and A549 cells by glucocorticoids and JNK pathway. In addition, our results revealed that the S211 phosphorylation was dominant in CEM-C7-14, whereas the opposite was the case in CEM-C1-15 where prevalence of S226 GR phosphorylation was observed. Furthermore, multiple GR isoforms with cell line specific patterns were identified in CEM-C7-14 cells compared to CEM-C1-15 and A549 cell lines with the same antibodies.

Conclusions

GR phosphorylation status kinetics, and site specificity as well as isoform variability differ in CEM-C7-14, CEM-C1-15, and A549 cells. The positive or negative response to GCs induced apoptosis in these cell lines is a consequence of the variable equilibrium of NOXA and Mcl-1 gene expression potentially mediated by alternatively phosphorylated GR, as well as the balance of MAPK/CDK pathways controlling GR phosphorylation pattern. Our results provide molecular base and valuable knowledge for improving the GC based therapies of leukaemia.

Regulation of Dauer formation by O-GlcNAcylation in Caenorhabditis elegans

Modification of proteins at serine or threonine residues with N-acetylglucosamine, termed O-GlcNAcylation, plays an important role in most eukaryotic cells. To understand the molecular mechanism by which O-GlcNAcylation regulates the entry of Caenorhabditis elegans into the non-aging dauer state, we performed proteomic studies using two mutant strains: the O-GlcNAc transferase-deficient ogt-1(ok430) strain and the O-GlcNAcase-defective oga-1(ok1207) strain. In the presence of the dauer pheromone daumone, ogt-1 showed suppression of dauer formation, whereas oga-1 exhibited enhancement of dauer formation. Consistent with these findings, treatment of wild-type N2 worms with low concentrations of daumone and the O-GlcNAcase inhibitor O-(2-acetamido-2-deoxy-d-glucopyranosylidene)amino-N-phenylcarbamate (PUGNAc) enhanced dauer formation, which was dependent on intact O-GlcNAcylation metabolism. We also found that the treatment of daumone enhanced O-GlcNAcylation in vivo. Seven proteins, identified by coupled two-dimensional electrophoresis/liquid chromatography-mass spectroscopy (LC-MS) analysis, were differentially expressed in oga-1(ok1207) worms compared with wild-type N2 worms. The identities of these proteins suggest that O- GlcNAcylation influences stress resistance, protein folding, and mitochondrial function. Using O-GlcNAc labeling with fluorescent dye combined with two-dimensional electrophoresis/LC-MS analysis, we also identified five proteins that were differentially O-GlcNAcylated during dauer formation. Analysis of these candidate O-GlcNAcylated proteins suggests that O-GlcNAcylation may regulate cytoskeleton modifications and protein turnover during dauer formation.

Diversity of mechanisms involved in aromatase regulation and estrogen action in the brain

BACKGROUND

The mechanisms through which estrogens modulate neuronal physiology, brain morphology, and behavior in recent years have proven to be far more complex than previously thought. For example, a second nuclear estrogen receptor has been identified, a new family of coregulatory proteins regulating steroid-dependent gene transcriptions was discovered and, finally, it has become clear that estrogens have surprisingly rapid effects based on their actions on cell membranes, which in turn result in the modulation of intracellular signaling cascades.

SCOPE OF REVIEW

This paper presents a selective review of new findings in this area related to work in our laboratories, focusing on the role of estrogens in the activation of male sexual behavior. Two separate topics are considered. We first discuss functions of the steroid receptor coactivator-1 (SRC-1) that has emerged as a key limiting factor for behavioral effects of estradiol. Knocking-down its expression by antisense oligonucleotides drastically inhibits male-typical sexual behaviors. Secondly, we describe rapid regulations of brain estradiol production by calcium-dependent phosphorylations of the aromatase enzyme, themselves under the control of neurotransmitter activity.

MAJOR CONCLUSIONS

These rapid changes in estrogen bioavailability have clear behavioral consequences. Increases or decreases in estradiol concentrations respectively obtained by an acute injection of estradiol itself or of an aromatase inhibitor lead within 15-30 min to parallel changes in sexual behavior frequencies.

GENERAL SIGNIFICANCE

These new controls of estrogen action offer a vast array of possibilities for discrete local controls of estrogen action. They also represent a formidable challenge for neuroendocrinologists trying to obtain an integrated view of brain function in relation to behavior.

Absence of ligand-independent transcriptional activation of the estrogen receptor via the estrogen response element in pituitary lactotrophs in primary culture

The estrogen receptor (ER) is a ligand-activated transcription factor that enhances gene expression by binding to specific regulatory DNA sequences called estrogen response elements (EREs). In some cell lines, the ER is also activated in a ligand-independent manner by multiple signaling pathways. In this study, we developed a novel adenovirus-mediated assay for promoter activation, termed LASETA, which we then used to examine whether ligand-independent activation of the ER occurred in normal pituitary lactotrophs in primary culture. In the LASETA adenovirus vector, the loxP-flanked stop sequence was deleted by prolactin (PRL) promoter-regulated expression of Cre recombinase. This led to lactotroph-specific expression of a reporter gene driven by an ERE-containing promoter. Estrogen-induced expression of the reporter protein luciferase in LASETA was specific for lactotrophs and was ER-dependent. LASETA was shown to be reliable even with varying Cre recombinase expression levels, which were caused by changes in PRL promoter activity. Using LASETA, we observed no change in ERE-mediated ER activity in the absence of estrogen after treatment of normal lactotrophs with agents such as insulin-like growth factor-1, epidermal growth factor, the adenylate cyclase activator forskolin, the extracellular signal-regulated kinase kinase inhibitor U0126, and the protein kinase A inhibitor H89. The ERE-mediated ligand-independent ER activity was induced by the growth factors and forskolin in the somatolactotroph tumor cell line GH4C1 cells. These results suggest that ERE-mediated ligand-independent activation of ER does not occur in normal lactotrophs in primary culture, and is a phenomenon likely restricted to transformed cells.

Phosphorylation control of nuclear receptors

Most transcription factors including nuclear receptors (NRs) act as sensors of the extracellular and intracellular compartments. As such, NRs serve as integrating platforms for a variety of stimuli and are targets for Post-translational modifications such as phosphorylations. During the last decade, knowledge of NRs phosphorylation advanced considerably because of the emergence of new technologies. Indeed, the development of a wide range of phosphorylation site databases, high accuracy mass spectrometry, and phospho-specific antibodies allowed the identification of multiple novel phosphorylation sites in NRs. New and improved methods also emerge to connect these data with the downstream consequences of phosphorylation on NRs structure (computational prediction, NMR), intracellular localization (FRAP), interaction with coregulators (proteomics, FRET, FLIM), and affinity for DNA (ChIP, ChIP-seq, FRAP). In the future, such integrated strategies should provide data with a treasure-trove of information about the integration of numerous signaling events by NRs.

Identification of four novel phosphorylation sites in estrogen receptor alpha: impact on receptor-dependent gene expression and phosphorylation by protein kinase CK2

Background

Estrogen receptor α (ERα) phosphorylation is important for estrogen-dependent transcription of ER-dependent genes, ligand-independent receptor activation and endocrine therapy response in breast cancer. However ERα phosphorylation at the previously identified sites does not fully account for these receptor functions. To determine if additional ERα phosphorylation sites exist, COS-1 cells expressing human ERα were labeled with [³²P]H₃PO₄ in vivo and ERα tryptic phosphopeptides were isolated to identify phosphorylation sites.

Results

Previously uncharacterized phosphorylation sites at serines 46/47, 282, 294, and 559 were identified by manual Edman degradation and phosphoamino acid analysis and confirmed by mutagenesis and phospho-specific antibodies. Antibodies detected phosphorylation of endogenous ERα in MCF-7, MCF-7-LCC2, and Ishikawa cancer cell lines by immunoblot. Mutation of Ser-282 and Ser-559 to alanine (S282A, S559A) resulted in ligand independent activation of ERα as determined by both ERE-driven reporter gene assays and endogenous pS2 gene expression in transiently transfected HeLa cells. Mutation of Ser-46/47 or Ser-294 to alanine markedly reduced estradiol dependent reporter activation. Additionally protein kinase CK2 was identified as a kinase that phosphorylated ERα at S282 and S559 using motif analysis, in vitro kinase assays, and incubation of cells with CK2 kinase inhibitor.

Conclusion

These novel ERα phosphorylation sites represent new means for modulation of ERα activity. S559 represents the first phosphorylation site identified in the extreme C-terminus (F domain) of a steroid receptor.

Partial agonist activity of the progesterone receptor antagonist RU486 mediated by an amino-terminal domain coactivator and phosphorylation of serine400

Jun dimerization protein-2 (JDP-2) is a progesterone receptor (PR) coregulatory protein that acts by inducing structure and transcriptional activity in the disordered amino-terminal domain (NTD) of PR. JDP-2 can also potentiate the partial agonist activity of the PR antagonist RU486 by mechanisms that have not been defined. Functional mutagenesis experiments revealed that a subregion of the NTD (amino acids 323-427) was required for the partial agonist activity of RU486 induced by PR interaction with JDP-2. However, this subregion was not required for JDP-2 enhancement of the activity of progestin agonists. Mutation of phosphorylation sites within this region of the NTD showed that phosphorylation of serine 400 was required for the partial agonist activity of RU486 stimulated by JDP-2, but was not required for activity of hormone agonist, either in the presence or absence of JDP-2. Cyclin-dependent kinase 2 (Cdk2)/cyclin A is a novel PR coregulator that binds the NTD and acts by phosphorylating steroid receptor coactivator-1 and modulating steroid receptor coactivator-1 interaction with PR. Cdk2/cyclin A also potentiated the partial agonist activity of RU486; however, phosphorylation of serine 400 was not required, indicating that JDP-2 and Cdk2/cyclin A act by distinct mechanisms. We conclude that PR bound to RU486 and associated with JDP-2 adopts an active conformation in a subregion of the NTD requiring phosphorylation of serine 400 that is distinct from that promoted by progestin agonists. These data underscore the structural flexibility of the NTD of PR, and the ability of steroid ligands together with interacting proteins to affect the conformation and activity of the NTD.

Importance of steroid receptor coactivators in the modulation of steroid action on brain and behavior

Steroid receptors such as estrogen and androgen receptors are nuclear receptors involved in the transcriptional regulation of a large number of target genes. Steroid-dependent protein expression in the brain controls a large array of biological processes including spatial cognition, copulatory behavior and neuroprotection. The discovery of a competition, or squelching, between two different nuclear receptors introduced the notion that common cofactors may be involved in the modulation of transcriptional activity of nuclear receptors. These cofactors or coregulatory proteins are functionally divided into coactivators and corepressors and are involved in chromatin remodeling and stabilization of the general transcription machinery. Although a large amount of information has been collected about the in vitro function of these coregulatory proteins, relatively little is known regarding their physiological role in vivo, particularly in the brain. Our laboratory and others have demonstrated the importance of SRC-1 in the differentiation and activation of steroid-dependent sexual behaviors and the related neural genes. For example, we report that the inhibition of SRC-1 expression blocks the activating effects of exogenous testosterone on male sexual behaviors and increases the volume of the median preoptic area. Other coactivators are likely to be involved in the modulation in vivo of steroid receptor activity and it seems that the presence of a precise subset of coactivators could help define the phenotype of the cell by modulating a specific downstream pathway after steroid receptor activation. The very large number of coactivators and their association into preformed complexes potentially allows the determination of hundreds of different phenotypes. The study of the expression of the coactivator and their function in vivo is required to fully understand steroid action and specificity in the brain.

Specific involvement of glycogen synthase kinase-3 in the function and activity of sex steroid hormone receptors reveals the complexity of their regulation

Protein kinases represent key nodes for the integration of multiple intracellular signalling pathways, resulting in modulation of both ligand-dependent and ligand-independent mechanisms of sex steroid receptor (sSR) signalling cascades. The proline-directed Ser/Thr kinases including mitogen-activated protein kinases and cyclin dependent kinases were especially reported to contribute to the function and activity of sSRs. The relevant effects of these kinases are well-documented but the impact of glycogen synthase kinase-3 (GSK-3), another member of this kinase family, has been underestimated. Indeed, the specific role of GSK-3 regarding the different sSRs will help to understand further the complexity of sSR signalling. So far, AR and ERalpha were identified as GSK-3 substrates. Additionally, the docking properties of GSK-3 were demonstrated to play a crucial role in sSR signal transduction. Reciprocally, GSK-3 was described as a potential target of non-genomic effects of sSRs. Therefore, GSK-3 regulates and is regulated by sSRs. This review focuses on the emerging and promising involvements of GSK-3 regarding the signalling cascade of the respective sSRs. This review represents a necessary complement of information to highlight the importance of GSK-3 regarding sSR function and activity.

Phosphorylation and protein-protein interactions in PXR-mediated CYP3A repression

BACKGROUND

The expression of drug-metabolizing enzymes CYPs is controlled by pregnane X receptor (PXR), and, therefore, understanding how PXR modulates CYP expression is important to minimize adverse drug interactions, one type of preventable adverse drug reaction.

OBJECTIVE

We review the mechanisms of PXR-mediated repression of CYP expression.

METHODS

We discuss the clinical implications of CYP repression and the role of signal cross-talks, including protein-protein interactions and phosphorylation of PXR and coregulators, in inhibiting PXR and repressing CYP expression.

RESULTS/CONCLUSION

Kinases such as cyclin-dependent kinase 2, protein kinase A, PKC and 70 kDa form of ribosomal protein S6 kinase repress CYP expression by phosphorylating and inhibiting PXR. Growth factor signaling represses CYP expression by phosphorylating and inhibiting forkhead in rhabdomyosarcoma, a co-activator of PXR. During inflammation, NF-kappaB represses both PXR and CYP expression through protein-protein interactions with the PXR pathway.