Protein 14-3-3sigma interacts with and favors cytoplasmic subcellular localization of the glucocorticoid receptor, acting as a negative regulator of the glucocorticoid signaling pathway

Crosstalk between p38 MAPK and GR Signaling

The p38 MAPK is a signaling pathway important for cells to respond to environmental and intracellular stress. Upon activation, the p38 kinase phosphorylates downstream effectors, which control the inflammatory response and coordinate fundamental cellular processes such as proliferation, apoptosis, and differentiation. Dysregulation of this signaling pathway has been linked to inflammatory diseases and cancer. Secretion of glucocorticoids (GCs) is a classical endocrine response to stress. The glucocorticoid receptor (GR) is the primary effector of GCs and plays an important role in the regulation of cell metabolism and immune response by influencing gene expression in response to hormone-dependent activation. Its ligands, the GCs or steroids, in natural or synthetic variation, are used as standard therapy for anti-inflammatory treatment, severe asthma, autoimmune diseases, and several types of cancer. Several years ago, the GR was identified as one of the downstream targets of p38, and, at the same time, it was shown that glucocorticoids could influence p38 signaling. In this review, we discuss the role of the crosstalk between the p38 and GR in the regulation of gene expression in response to steroids and comprehend the importance and potential of this interplay in future clinical applications.

Glucocorticoid receptor Thr524 phosphorylation by MINK1 induces interactions with 14-3-3 protein regulators

The glucocorticoid receptor (GR) is a ligand-dependent transcription factor that plays a central role in inflammation. The GR activity is also modulated via protein–protein interactions, including binding of 14-3-3 proteins induced by GR phosphorylation. However, the specific phosphorylation sites on the GR that trigger these interactions and their functional consequences are less clear. Hence, we sought to examine this system in more detail. We used phosphorylated GR peptides, biophysical studies, and X-ray crystallography to identify key residues within the ligand-binding domain of the GR, T524 and S617, whose phosphorylation results in binding of the representative 14-3-3 protein 14-3-3ζ. A kinase screen identified misshapen-like kinase 1 (MINK1) as responsible for phosphorylating T524 and Rho-associated protein kinase 1 for phosphorylating S617; cell-based approaches confirmed the importance of both GR phosphosites and MINK1 but not Rho-associated protein kinase 1 alone in inducing GR–14-3-3 binding. Together our results provide molecular-level insight into 14-3-3-mediated regulation of the GR and highlight both MINK1 and the GR–14-3-3 axis as potential targets for future therapeutic intervention.

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.

14-3-3 modulation of the inflammatory response

Regulation of inflammation is a central part of the maintenance of homeostasis by the immune system. One important class of regulatory protein that has been shown to have effects on the inflammatory process are the 14-3-3 proteins. Herein we describe the roles that have been identified for 14-3-3 in regulation of the inflammatory response. These roles encompass regulation of the response that affect inflammation at the genetic, molecular and cellular levels. At a genetic level 14-3-3 is involved in the regulation of multiple transcription factors and affects the transcription of key effectors of the immune response. At a molecular level many of the constituent parts of the inflammatory process, such as pattern recognition receptors, protease activated receptors and cytokines are regulated through phosphorylation and recognition by 14-3-3 whilst disruption of the recognition processes has been observed to result in clinical syndromes. 14-3-3 is also involved in the regulation of cell proliferation and differentiation, this has been shown to affect the immune system, particularly T- and B-cells. Finally, we discuss how abnormal levels of 14-3-3 contribute to undesirable immune responses and chronic inflammatory conditions.

Molecular pathways involved in the transport of nuclear receptors from the nucleus to cytoplasm

Nuclear receptors (NRs) are transcription regulators that direct the expression of many genes linked to cellular processes, such as proliferation, differentiation, and apoptosis. Additionally, some cellular events are also modulated by signaling pathways induced by NRs outside of the nucleus. Hence, the subcellular transport of NRs is dynamic and is modulated by several signals, protein-protein interactions, and posttranslational modifications. Particularly, the exit of NRs from the nucleus to cytoplasm and/or other compartments is transcendental, as it is this export event, which determines their abundance in the cells' compartments, the activation or attenuation of nuclear or extranuclear pathways, and the magnitude and duration of their effects inside or outside of the nucleus. Consequently, an adequate control of the distribution of NRs is critical for homeostasis, because a deregulation in the nucleo-cytoplasmic transport of NRs could be involved in diseases including cancer as well as metabolic and vascular alterations. In this review, we investigated the pathways and molecular and biological aspects that have been described for the nuclear export of NRs so far and their functional relevance in some diseases. This information suggests that the transport of NRs out of the nucleus is a key mechanism for the identification of new therapeutic targets for alterations associated with the deregulation of the function of NRs.

SIRT1 is a transcriptional enhancer of the glucocorticoid receptor acting independently to its deacetylase activity

Glucocorticoids have strong effects on diverse human activities through the glucocorticoid receptor (GR). Sirtuin 1 (SIRT1) is a NAD+-dependent histone deacetylase and promotes longevity by influencing intermediary metabolism and other regulatory activities including mitochondrial function. In this study, we examined the effects of SIRT1 on GR-mediated transcriptional activity. We found that SIRT1 enhanced GR-induced transcriptional activity on endogenous and exogenous glucocorticoid-responsive genes, whereas knockdown of SIRT1 attenuated it. This effect of SIRT1 was independent to its deacetylase activity, as the SIRT1 mutant defective in this activity (H363Y) enhanced GR transcriptional activity, and the compounds inhibiting or activating the SIRT1 deacetylase activity did not influence it. RNA-seq analysis revealed that SIRT1 knockdown influenced ∼30% of the glucocorticoid-responsive transcriptome for most of which it acted as an enhancer for positive/negative effects of this hormone. SIRT1 physically interacted with GR, and was attracted to GR-bound glucocorticoid response elements in a glucocorticoid-dependent fashion. SIRT1 cooperatively activated GR transcriptional activity with the PPARγ coactivator-1α also in its deacetylase activity-independent fashion. Thus, SIRT1 is a novel transcriptional enhancer of GR-induced transcriptional activity possibly by functioning as a scaffold for the transcriptional complex formed on GR.

Proteome Analysis of Drosophila Mutants Identifies a Regulatory Role for 14-3-3ε in Metabolic Pathways

The evolutionary conserved family of 14-3-3 proteins appears to have a role in integrating numerous intracellular pathways, including signal transduction, intracellular trafficking, and metabolism. However, little is known about how this interactive network might be affected by the direct abrogation of 14-3-3 function. The loss of Drosophila 14-3-3ε resulted in reduced survival of mutants during larval-to-adult transition, which is known to depend on an energy supply coming from the histolysis of fat body tissue. Here we report a differential proteomic analysis of larval fat body tissue at the onset of larval-to-adult transition, with the loss of 14-3-3ε resulting in the altered abundance of 16 proteins. These included proteins linked to protein biosynthesis, glycolysis, tricarboxylic acid cycle, and lipid metabolic pathways. The ecdysone receptor (EcR), which is responsible for initiating the larval-to-adult transition, colocalized with 14-3-3ε in wild-type fat body tissues. The altered protein abundance in 14-3-3ε mutant fat body tissue was associated with transcriptional deregulation of alcohol dehydrogenase, fat body protein 1, and lamin genes, which are known targets of the EcR. This study indicates that 14-3-3ε has a critical role in cellular metabolism involving either molecular crosstalk with the EcR or direct interaction with metabolic proteins.

AKT1 has dual actions on the glucocorticoid receptor by cooperating with 14-3-3

Glucocorticoids are important therapeutic compounds for acute lymphoblastic leukemia (ALL). AKT1 or the protein kinase B is frequently activated in ALL, and contributes to the development of glucocorticoid resistance. We examined impact of AKT1 on glucocorticoid receptor (GR)-induced transcriptional activity in cooperation with phospho-serine/threonine-binding protein 14-3-3. AKT1 has two distinct actions on GR transcriptional activity, one through segregation of GR in the cytoplasm by phosphorylating GR at Ser-134 and subsequent association of 14-3-3, and the other through direct modulation of GR transcriptional activity in the nucleus. For the latter, AKT1 and 14-3-3 are attracted to DNA-bound GR, accompanied by AKT1-dependent p300 phosphorylation, H3S10 phosphorylation and H3K14 acetylation at the DNA site. These two actions of AKT1 regulate distinct sets of glucocorticoid-responsive genes. Our results suggest that specific inhibition of the AKT1/14-3-3 activity on the cytoplasmic retention of GR may be a promising target for treating glucocorticoid resistance observed in ALL.

Epigenetic and Glucocorticoid Receptor-Mediated Regulation of Glutathione Peroxidase 3 in Lung Cancer Cells

Glutathione peroxidase 3 (GPx3), an antioxidant enzyme, acts as a modulator of redox signaling, has immunomodulatory function, and catalyzes the detoxification of reactive oxygen species (ROS). GPx3 has been identified as a tumor suppressor in many cancers. Although hyper-methylation of the GPx3 promoter has been shown to down-regulate its expression, other mechanisms by which GPx3 expression is regulated have not been reported. The aim of this study was to further elucidate the mechanisms of GPx3 regulation. GPx3 gene analysis predicted the presence of ten glucocorticoid response elements (GREs) on the GPx3 gene. This result prompted us to investigate whether GPx3 expression is regulated by the glucocorticoid receptor (GR), which is implicated in tumor response to chemotherapy. The corticosteroid dexamethasone (Dex) was used to examine the possible relationship between GR and GPx3 expression. Dex significantly induced GPx3 expression in H1299, H1650, and H1975 cell lines, which exhibit low levels of GPx3 expression under normal conditions. The results of EMSA and ChIP-PCR suggest that GR binds directly to GRE 6 and 7, both of which are located near the GPx3 promoter. Assessment of GPx3 transcription efficiency using a luciferase reporter system showed that blocking formation of the GR-GRE complexes reduced luciferase activity by 7-8-fold. Suppression of GR expression by siRNA transfection also induced down-regulation of GPx3. These data indicate that GPx3 expression can be regulated independently via epigenetic or GR-mediated mechanisms in lung cancer cells, and suggest that GPx3 could potentiate glucocorticoid (GC)-mediated anti-inflammatory signaling in lung cancer cells.

The Interactome of the Glucocorticoid Receptor and Its Influence on the Actions of Glucocorticoids in Combatting Inflammatory and Infectious Diseases

Glucocorticoids (GCs) have been widely used for decades as a first-line treatment for inflammatory and autoimmune diseases. However, their use is often hampered by the onset of adverse effects or resistance. GCs mediate their effects via binding to glucocorticoid receptor (GR), a transcription factor belonging to the family of nuclear receptors. An important aspect of GR's actions, including its anti-inflammatory capacity, involves its interactions with various proteins, such as transcription factors, cofactors, and modifying enzymes, which codetermine receptor functionality. In this review, we provide a state-of-the-art overview of the protein-protein interactions (PPIs) of GR that positively or negatively affect its anti-inflammatory properties, along with mechanistic insights, if known. Emphasis is placed on the interactions that affect its anti-inflammatory effects in the presence of inflammatory and microbial diseases.

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.

Circulating cortisol-associated signature of glucocorticoid-related gene expression in subcutaneous fat of obese subjects

OBJECTIVE

Serum cortisol concentrations fluctuate in a circadian fashion, and glucocorticoids exert strong effects on adipose tissue and induce obesity through the glucocorticoid receptor.

DESIGN AND METHODS

To examine the impact of physiologic levels of circulating cortisol on subcutaneous adipose tissue, 25 overweight and obese subjects were employed, and their serum levels of morning (AM) and evening (PM) cortisol, AM/PM cortisol ratios, and 24-h urinary-free cortisol (UFC) were compared with their clinical parameters, serum cytokine levels, and mRNA expression of 93 receptor action-regulating and 93 glucocorticoid-responsive genes in abdominal subcutaneous fat.

RESULTS AND CONCLUSIONS

AM cortisol levels did not correlate with mRNA expression of the all genes examined, whereas PM cortisol levels, AM/PM cortisol ratios, and 24-h UFC were associated with distinct sets of these genes. Body mass index did not significantly correlate with the four cortisol parameters employed. These results suggest that physiologic levels of AM serum cortisol do not solely represent biological effects of circulating cortisol on the expression of glucocorticoid-related genes in subcutaneous adipose tissue, whereas PM levels, amplitude, and net amounts of the diurnally fluctuating serum cortisol have distinct effects. Through the genes identified in this study, glucocorticoids appear to influence intermediary metabolism, energy balance, inflammation, and local circadian rythmicity in subcutaneous fat. Our results may also explain in part the development of metabolic abnormality and obesity in subjects under stress or patients with melancholic/atypical depression who demonstrate elevated levels of PM serum cortisol.

Proteomic profiling of rapid non-genomic and concomitant genomic effects of acute restraint stress on rat thymocytes

In order to investigate rapid non-genomic effects of acute stress, rats were restrained for 15 min which was sufficient to activate the hypothalamus-pituitary-adrenal (HPA) axis but too short to induce massive genomic effects of cortisol. Subcellular fractions of thymocytes (cytosol, nucleus, membrane) were investigated using quantitative 2D DIGE with MALDI-TOF/TOF mass spectrometry. In total, 108 proteins with differential subcellular localizations were identified. The specificity of the changes induced by psychological stress was reflected by the prominent modulation of proteins involved in the HPA and sympathoadrenal medullar (SAM) axis such as HMGB1 and NHERF1. Intracellular trafficking was characterized by a dominant protein exodus from the cytosol. Real translocation was observed for 9 proteins with 6 that shuttled from the cytosol to the nucleus (HYOU1, HNRPF, HNRPC, STRAP, PSA1, PPA1) and 3 from the nucleus to the cytosol (HMGB1, NHERF1, PSMA1). Proteins showing subcellular reshuffling were largely involved in transcription and translation processes (39 of 108) with a significant enrichment of RNA splicing factors. Bioinformatics analysis revealed significant enrichment for protein kinase A and 14-3-3 signaling, probably reflecting real non-genomic effects. This is the first study investigating rapid effects of stress-induced HPA activation in vivo at the proteome level.

Ligand-independent phosphorylation of the glucocorticoid receptor integrates cellular stress pathways with nuclear receptor signaling

Glucocorticoids are stress hormones that maintain homeostasis through gene regulation mediated by nuclear receptors. We have discovered that other cellular stressors are integrated with glucocorticoid signaling through a new hormone-independent phosphorylation site, Ser134, on the human glucocorticoid receptor (GR). Ser134 phosphorylation is induced by a variety of stress-activating stimuli in a p38 mitogen-activated protein kinase (MAPK)-dependent manner. Cells expressing a mutant glucocorticoid receptor incapable of phosphorylation at Ser134 (S134A-GR) had significantly altered hormone-dependent genome-wide transcriptional responses and associated hormone-mediated cellular functions. The phosphorylation of Ser134 significantly increased the association of the GR with the zeta isoform of the 14-3-3 class of signaling proteins (14-3-3zeta) on chromatin promoter regions, resulting in a blunted hormone-dependent transcriptional response of select genes. These data argue that the phosphorylation of Ser134 acts as a molecular sensor on the GR, monitoring the level of cellular stress to redirect glucocorticoid-regulated signaling through altered 14-3-3zeta cofactor binding and promoter recruitment. This posttranslational modification allows prior cellular stress signals to dictate the transcriptional response to glucocorticoids.

Glucocorticoid receptor mutants: man-made tools for functional research

The glucocorticoid receptor (GR) is a ligand-dependent transcription factor that can bind to glucocorticoids (GCs). Upon ligand binding, GR sheds its cytoplasmic chaperoning complex and translocates to the nucleus, where it can act as a ligand-dependent transcription factor, transactivating or transrepressing specific gene promoters. Often, GR interacts with specific cofactors to implement a variety of gene promoter effects. GR activity and function is further modulated by post-translational modifications. To assess the diverse aspects of GR mechanisms of activation and gene regulation, researchers continue to use a range of artificial GR mutants. In this review we analyze the characteristics of GR mutants with the aim of assisting the design and interpretation of GR mutant-based experiments.

Molecular chaperones, essential partners of steroid hormone receptors for activity and mobility

Steroid hormone receptors (SHRs) are notorious intracellular travellers, transiting among different cellular compartments as they mature, are subjected to regulation and exert their biological functions. Understanding the processes governing the intracellular traffic of SHRs is important, since their unbalanced or erroneous localization could lead to the development of diseases. In this review, we not only explore the functions of the heat-shock protein 90 (Hsp90) molecular chaperone machine for the intracellular transport of SHRs, but also for the regulation of their nuclear mobility, for their recycling and for the regulation of their transcriptional output.

Glucocorticoid signaling in the cell. Expanding clinical implications to complex human behavioral and somatic disorders

Glucocorticoids contribute to the maintenance of basal and stress-related homeostasis in all higher organisms, and influence a large proportion of the expressed human genome, and their effects spare almost no organs or tissues. Glucocorticoids regulate many functions of the central nervous system, such as arousal, cognition, mood, sleep, the activity and direction of intermediary metabolism, the maintenance of a proper cardiovascular tone, the activity and quality of the immune and inflammatory reaction, including the manifestations of the sickness syndrome, and growth and reproduction. The numerous actions of glucocorticoids are mediated by a set of at least 16 glucocorticoid receptor (GR) isoforms forming homo- or hetero-dimers. The GRs consist of multifunctional domain proteins operating as ligand-dependent transcription factors that interact with many other cell signaling systems, including large and small G proteins. The presence of multiple GR monomers and homo- or hetero-dimers expressed in a cell-specific fashion at different quantities with quantitatively and qualitatively different transcriptional activities suggest that the glucocorticoid signaling system is highly stochastic. Glucocorticoids are heavily involved in human pathophysiology and influence life expectancy. Common behavioral and/or somatic complex disorders, such as anxiety, depression, insomnia, chronic pain and fatigue syndromes, obesity, the metabolic syndrome, essential hypertension, diabetes type 2, atherosclerosis with its cardiovascular sequelae, and osteoporosis, as well as autoimmune inflammatory and allergic disorders, all appear to have a glucocorticoid-regulated component.

Mechanisms regulating the susceptibility of hematopoietic malignancies to glucocorticoid-induced apoptosis

Glucocorticoids (GCs) are commonly used in the treatment of hematopoietic malignancies owing to their ability to induce apoptosis of these cancerous cells. Whereas some types of lymphoma and leukemia respond well to this drug, others are resistant. Also, GC-resistance gradually develops upon repeated treatments ultimately leading to refractory relapsed disease. Understanding the mechanisms regulating GC-induced apoptosis is therefore uttermost important for designing novel treatment strategies that overcome GC-resistance. This review discusses updated data describing the complex regulation of the cell's susceptibility to apoptosis triggered by GCs. We address both the genomic and nongenomic effects involved in promoting the apoptotic signals as well as the resistance mechanisms opposing these signals. Eventually we address potential strategies of clinical relevance that sensitize GC-resistant lymphoma and leukemia cells to this drug. The major target is the nongenomic signal transduction machinery where the interplay between protein kinases determines the cell fate. Shifting the balance of the kinome towards a state where Glycogen synthase kinase 3alpha (GSK3alpha) is kept active, favors an apoptotic response. Accumulating data show that it is possible to therapeutically modulate GC-resistance in patients, thereby improving the response to GC therapy.

Proteins interact with the cytosolic mineralocorticoid receptor depending on the ligand

Steroid receptors belonging to the superfamily of nuclear receptors do not exist as single monomeric proteins but mediate their effects by the interaction with numerous other proteins, e.g., cofactors for transcription, but also other proteins involved in cellular signaling. This interaction may be ligand dependent, which explains the differential effects of receptor ligands. Whereas some receptors, e.g., the estrogen receptor, have been studied in great detail, much less is known about proteins interacting with the mineralocorticoid receptor (MR). In this study, we aimed to identify interacting proteins using a proteomics approach involving tagged receptor constructs. After affinity isolation of MR complexes, blue native electrophoresis revealed the presence of several populations of MR complexes differing in size and composition. During the identification of interacting proteins, various heat shock proteins but also several previously undescribed potential interactors were found, including 14-3-3-ε. We also demonstrate here that the cytosolic MR in the presence of detergent interacts in a ligand-selective manner with glucose-regulated protein 78 and propionyl-CoA carboxylase-β precursor, which are found in the unliganded or aldosterone-containing complex but not with spironolactone.

Nucleocytoplasmic transport of proteins

In eukaryotic cells, the movement of macromolecules between the nucleus and cytoplasm occurs through the nuclear pore complex (NPC)--a large protein complex spanning the nuclear envelope. The nuclear transport of proteins is usually mediated by a family of transport receptors known as karyopherins. Karyopherins bind to their cargoes via recognition of nuclear localization signal (NLS) for nuclear import or nuclear export signal (NES) for export to form a transport complex. Its transport through NPC is facilitated by transient interactions between the karyopherins and NPC components. The interactions of karyopherins with their cargoes are regulated by GTPase Ran. In the current review, we describe the NPC structure, NLS, and NES, as well as the model of classic Ran-dependent transport, with special emphasis on existing alternative mechanisms; we also propose a classification of the basic mechanisms of protein transport regulation.

Gene expression and biological processes influenced by deletion of Stat3 in pulmonary type II epithelial cells

Background

The signal transducer and activator of transcription 3 (STAT3) mediates gene expression in response to numerous growth factors and cytokines, playing an important role in many cellular processes. To better understand the molecular mechanisms by which Stat3 influences gene expression in the lung, the effect of pulmonary epithelial cell specific deletion of Stat3 on genome wide mRNA expression profiling was assessed. Differentially expressed genes were identified from Affymetrix Murine GeneChips analysis and subjected to gene ontology classification, promoter analysis, pathway mapping and literature mining.

Results

Total of 791 mRNAs were significantly increased and 314 mRNAs were decreased in response to the deletion of Stat3^Δ/Δ in the lung. STAT is the most enriched cis-elements in the promoter regions of those differentially expressed genes. Deletion of Stat3 induced genes influencing protein metabolism, transport, chemotaxis and apoptosis and decreased the expression of genes mediating lipid synthesis and metabolism. Expression of Srebf1 and 2, genes encoding key regulators of fatty acid and steroid biosynthesis, was decreased in type II cells from the Stat3^Δ/Δ mice, consistent with the observation that lung surfactant phospholipids content was decreased. Stat3 influenced both pro- and anti-apoptotic pathways that determine cell death or survival. Akt, a potential transcriptional target of Stat3, was identified as an important participant in Stat3 mediated pathways including Jak-Stat signaling, apoptosis, Mapk signaling, cholesterol and fatty acid biosynthesis.

Conclusion

Deletion of Stat3 from type II epithelial cells altered the expression of genes regulating diverse cellular processes, including cell growth, apoptosis and lipid metabolism. Pathway analysis indicates that STAT3 regulates cellular homeostasis through a complex regulatory network that likely enhances alveolar epithelial cell survival and surfactant/lipid synthesis, necessary for the protection of the lung during injury.

Glucocorticoid receptor action in beneficial and side effects of steroid therapy: lessons from conditional knockout mice

Glucocorticoids (GCs) are potent immune suppressive drugs with unfortunately severe side effects. Different molecular modes of actions of the GC receptor (GR) have been identified. Transcriptional transactivation by binding of a dimerized GR protein complex to the promoter of GC regulated genes or interference with activity of pro-inflammatory transcription factors by GR monomers are considered as the two major mechanisms. It has been hypothesized that selective GR agonists (SEGRAs) addressing dimer-independent function would reveal potent steroid therapeutic activity with reduced side effects. Recent studies of a mouse knock-in strain with a dimerization-deficient GR demonstrate that some inflammatory processes can be suppressed by GCs, while others cannot. Also side effects of GCs occur in these mice. Thus, depending on the process that is treated, SEGRA could be therapeutically more or less effective and not all side effects of steroid therapy may be reduced.

An essential role for 14-3-3 proteins in brassinosteroid signal transduction in Arabidopsis

Brassinosteroids (BRs) are essential hormones for plant growth and development. BRs regulate gene expression by inducing dephosphorylation of two key transcription factors, BZR1 and BZR2/BES1, through a signal transduction pathway that involves cell-surface receptors (BRI1 and BAK1) and a GSK3 kinase (BIN2). How BR-regulated phosphorylation controls the activities of BZR1/BZR2 is not fully understood. Here, we show that BIN2-catalyzed phosphorylation of BZR1/BZR2 not only inhibits DNA binding, but also promotes binding to the 14-3-3 proteins. Mutations of a BIN2-phosphorylation site in BZR1 abolish 14-3-3 binding and lead to increased nuclear localization of BZR1 protein and enhanced BR responses in transgenic plants. Further, BR deficiency increases cytoplasmic localization, and BR treatment induces rapid nuclear localization of BZR1/BZR2. Thus, 14-3-3 binding is required for efficient inhibition of phosphorylated BR transcription factors, largely through cytoplasmic retention. This study demonstrates that multiple mechanisms are required for BR regulation of gene expression and plant growth.

Glucocorticoid action networks and complex psychiatric and/or somatic disorders

Glucocorticoids contribute fundamentally to the maintenance of basal and stress-related homeostasis in all higher organisms. These hormones influence a large percentage of the expressed human genome and their effects spare almost no organs or tissues. Glucocorticoids influence many functions of the central nervous system, such as arousal, cognition, mood and sleep, the activity and direction of intermediary metabolism, the maintenance of a normal cardiovascular tone, the activity and quality of the immune and inflammatory reaction, including the manifestations of the sickness syndrome, as well as growth and reproduction. The numerous actions of glucocorticoids are mediated by a set of at least 16 glucocorticoid receptor (GR) isoforms forming homo- or hetero-dimers. The GRs consist of multifunctional domain proteins operating as ligand-dependent transcription factors that interact with many other cell signaling systems. The presence of multiple GR monomers and dimers expressed in a cell-specific fashion at different quantities with quantitatively and qualitatively different transcriptional activities suggests that the glucocorticoid signaling system is highly stochastic. Based on ample evidence, we present our conception that glucocorticoids are heavily involved in human pathophysiology and influence life expectancy. Common psychiatric and/or somatic complex disorders, such as anxiety, depression, insomnia, chronic pain and fatigue syndromes, obesity, the metabolic syndrome, essential hypertension, diabetes type 2, atherosclerosis with its cardiovascular sequelae, and osteoporosis, as well as autoimmune inflammatory and allergic disorders, all appear to have a glucocorticoid component.

The glucocorticoid receptor is increased in Atm-/- thymocytes and in Atm-/- thymic lymphoma cells, and its nuclear translocation counteracts c-myc expression

We have previously demonstrated that spontaneous DNA synthesis in immature thymocytes of Atm-/- mice is elevated, and that treatment with the glucocorticoid dexamethasone (Dex) attenuates this increased DNA synthesis and prevents the development of thymic lymphomas. Deregulation of c-myc may drive the uncontrolled proliferation of Atm-/- thymocytes, since upregulation of c-myc parallels the elevated DNA synthesis in the cells. In this study, we show that the glucocorticoid receptor (GR) is expressed at high levels in Atm-/- thymocytes and in Atm-/- thymic lymphoma cells, although serum glucocorticoid (GC) levels in Atm-/- mice are similar to those in Atm+/+ mice. In cultured Atm-/- thymic lymphoma cells treated with Dex, GR nuclear translocation occurs, resulting in suppression of DNA synthesis and c-myc expression at both the mRNA and protein levels. Interestingly, the GR antagonist RU486 also causes GR nuclear translocation, but does not affect DNA synthesis and c-myc expression in Atm-/- thymic lymphoma cells. As expected, RU486 reverses the suppressive effects of Dex on DNA synthesis and c-myc expression. Administration of Dex to Atm-/- mice decreases the elevated c-Myc protein levels in their thymocytes. These findings suggest that GC/GR signaling plays an important role in regulating c-myc expression in Atm-/- thymocytes and thymic lymphoma cells.

14-3-3 sigma increases the transcriptional activity of the androgen receptor in the absence of androgens

The androgen receptor (AR) is a ligand-activated transcription factor that regulates numerous target genes, including prostate-specific antigen (PSA). We examined the ability of each member of the 14-3-3 family to modulate transcription of PSA through the AR. Despite significant homology within the 14-3-3 family we observed differences in the ability of each isoform to alter the transcriptional activity of the AR. Significantly, 14-3-3 sigma activated PSA-luciferase reporters not only at castrate levels of androgens, but also in the complete absence of androgens. 14-3-3 sigma also increased expression of the endogenous PSA gene in the absence of androgens. Knockdown of the AR by siRNA oligonucleotides abolished activation of these reporters by 14-3-3 sigma. These findings may have greatest significance in hormone refractory prostate cancer where the AR may be activated in a ligand-independent manner.

CAR and PXR: the xenobiotic-sensing receptors

The xenobiotic receptors CAR and PXR constitute two important members of the NR1I nuclear receptor family. They function as sensors of toxic byproducts derived from endogenous metabolism and of exogenous chemicals, in order to enhance their elimination. This unique function of CAR and PXR sets them apart from the steroid hormone receptors. In contrast, the steroid receptors, exemplified by the estrogen receptor (ER) and glucocorticoid receptor (GR), are the sensors that tightly monitor and respond to changes in circulating steroid hormone levels to maintain body homeostasis. This divergence of the chemical- and steroid-sensing functions has evolved to ensure the fidelity of the steroid hormone endocrine regulation while allowing development of metabolic elimination pathways for xenobiotics. The development of the xenobiotic receptors CAR and PXR also reflect the increasing complexity of metabolism in higher organisms, which necessitate novel mechanisms for handling and eliminating metabolic by-products and foreign compounds from the body. The purpose of this review is to discuss similarities and differences between the xenobiotic receptors CAR and PXR with the prototypical steroid hormone receptors ER and GR. Interesting differences in structure explain in part the divergence in function and activation mechanisms of CAR/PXR from ER/GR. In addition, the physiological roles of CAR and PXR will be reviewed, with discussion of interactions of CAR and PXR with endocrine signaling pathways.

An active nuclear retention signal in the glucocorticoid receptor functions as a strong inducer of transcriptional activation

The glucocorticoid receptor (GR) cycles between a naive chaperone-complexed form in the cytoplasm and a transcriptionally active steroid-bound nuclear form. Nuclear import of GR occurs rapidly and is mediated through the importin alpha/beta karyopherin import pathway. By contrast, nuclear export of GR occurs only slowly under most conditions, despite a dependence on active signaling. In this study we have defined a nuclear retention signal (NRS) in the hinge region of GR that actively opposes the nuclear export of GR as well as the nuclear export mediated through an ectopic CRM1-dependent nuclear export signal (NES). The GR NRS overlaps closely with the basic NL1 nuclear localization signal (NLS) but can be distinguished from NL1 by targeted mutagenesis. Substitution of the classical NLS from SV40 T antigen for the GR NL1 results in a receptor in which nuclear export is accelerated. Remarkably, although the SV40-modified GR remains predominantly nuclear in the presence of steroid and is recruited to transcriptional regulatory regions indistinguishably from wild-type GR, the substitution dramatically weakens the ability of GR to activate transcription of a mouse mammary tumor virus reporter gene. These results suggest that active nuclear retention of GR plays an integral role in glucocorticoid signaling.

Proteomic analysis in human breast cancer: Identification of a characteristic protein expression profile of malignant breast epithelium

Gene expression analysis has become a promising tool in predicting the clinical course of malignant disease and the response to antineoplastic therapy. Surprisingly, only little is known about the protein expression pattern of human tumors. Recent advances in proteomic analysis allow proteins of interest to be identified by their expression and/or modification pattern in 2-DE rather than using the traditional approach of translating gene expression data. To identify a proteomic pattern that is characteristic for malignant breast epithelium, we performed differential 2-DE analysis in sets of microdissected malignant breast epithelia and corresponding adjacent normal breast epithelia from five patients with invasive breast carcinoma. Thirty-two protein spots were found to be selectively regulated in malignant epithelium, and were subjected to MALDI-TOF and/or immunoblotting for protein identification. Thirteen of the identified proteins had previously not been associated with breast cancer. The validity of these findings was confirmed by literature review and immunohistochemistry for identified proteins in an independent cohort of 50 breast cancer specimens. We here describe, for the first time, a proteomic analysis of matched normal and malignant epithelia from invasive breast carcinomas. This strategy leads to a better understanding of oncogenesis at an operational level and helps to characterize the malignant phenotype of individual tumors, and thereby to identify novel targets for antineoplastic therapy.

Analysis of the influence of subcellular localization of the HIV Rev protein on Rev-dependent gene expression by multi-fluorescence live-cell imaging

The human immunodeficiency virus Rev protein is a post-transcriptional activator of HIV gene expression. Rev is a nucleocytoplasmic shuttle protein that displays characteristic nuclear/nucleolar subcellular localization in various cell lines. Cytoplasmic localization of Rev occurs under various conditions disrupting Rev function. The goal of this study was to investigate the relationship between localization of Rev and its functional activity in living cells. A triple-fluorescent imaging assay, called AQ-FIND, was established for automatic quantitative evaluation of nucleocytoplasmic distribution of fluorescently tagged proteins. This assay was used to screen 500 rev genes generated by error-prone PCR for Rev mutants with different localization phenotypes. Activities of the Rev mutants were determined with a second quantitative, dual-fluorescent reporter assay. In HeLa cells, the majority of nuclear Rev mutants had activities similar to wild-type Rev. The activities of Rev mutants with abnormal cytoplasmic localization ranged from moderately impaired to nonfunctional. There was no linear correlation between subcellular distribution and levels of Rev activity. In astrocytes, nuclear Rev mutants showed similar impaired activities as the cytoplasmic wild-type Rev. Our data suggest that steady-state subcellular localization is not a primary regulator of Rev activity but may change as a secondary consequence of altered Rev function. The methodologies described here have potential for studying the significance of subcellular localization for functions of other regulatory factors.

Rho family Guanine nucleotide exchange factor Brx couples extracellular signals to the glucocorticoid signaling system

Glucocorticoids regulate many crucial biologic functions through their cytoplasmic/nuclear glucocorticoid receptors (GR). Excess, deficiency, or alteration in tissue sensitivity to glucocorticoids has been associated with major causes of human morbidity and mortality. Brx, a cytoplasmic Rho family guanine nucleotide exchange factor, binds to and influences the activity of several nuclear hormone receptors. We examined the functional and molecular interactions between GR and Brx. The glucocorticoid sensitivity of lymphocytes obtained from mice haplo-insufficient for Brx was significantly decreased. Conversely, GR-mediated transcriptional activity of a glucocorticoid response element (GRE)-mediated glucocorticoid-responsive promoter was enhanced by Brx in a guanine nucleotide exchange factor domain-dependent fashion. Brx interacted with GR, forming a ternary complex with RhoA. In a chromatin immunoprecipitation assay, Brx and RhoA were co-precipitated with GREs only in the presence of ligand-activated GR. Extracellularly administered lysophosphatidic acid, which activates its signaling cascade through a specific membrane GTP-binding protein (G-protein)-coupled receptor in a G-protein alpha(13)-, Brx-, and RhoA-dependent fashion, enhanced GR transcriptional activity, whereas depletion of endogenous Brx attenuated this effect. These findings suggest that glucocorticoid signaling and, hence, the tissue sensitivity to glucocorticoids, may be coupled to extracellular signals via Brx and small G-proteins. Nuclear Brx might act as a local GRE-GR-transcriptosome activator by mediating the effect of small G-proteins on glucocorticoid-regulated genes.

Role of mitochondrial glucocorticoid receptor in glucocorticoid-induced apoptosis

The mechanisms by which glucocorticoid receptor (GR) mediates glucocorticoid (GC)-induced apoptosis are unknown. We studied the role of mitochondrial GR in this process. Dexamethasone induces GR translocation to the mitochondria in GC-sensitive, but not in GC-resistant, T cell lines. In contrast, nuclear GR translocation occurs in all cell types. Thymic epithelial cells, which cause apoptosis of the PD1.6 T cell line in a GR-dependent manner, induce GR translocation to the mitochondria, but not to the nucleus, suggesting a role for mitochondrial GR in eliciting apoptosis. This hypothesis is corroborated by the finding that a GR variant exclusively expressed in the mitochondria elicits apoptosis of several cancer cell lines. A putative mitochondrial localization signal was defined to amino acids 558-580 of human GR, which lies within the NH2-terminal part of the ligand-binding domain. Altogether, our data show that mitochondrial and nuclear translocations of GR are differentially regulated, and that mitochondrial GR translocation correlates with susceptibility to GC-induced apoptosis.

Negative regulatory role of Annexin-A1 in 14-3-3η-mediated glucocorticoid receptor transcriptional activation

Annexin-A1 (ANX-1) is involved in glucocorticoid receptor (GR)-mediated signal transduction. However, the molecular mechanism by which ANX-1 plays a role in GR signaling is not fully understood. Recently, we reported that 14-3-3eta inhibits degradation of GR, resulting in an increase in GR transcriptional activity. In this study, we have addressed the role of ANX-1 in 14-3-3eta-induced enhancement of GR transactivation. ANX-1 abolished the increase in GR transcriptional activity due to 14-3-3eta. ANX-1 had no effect on the protein level of GR. However, ANX-1 abrogated the inhibitory effect of 14-3-3eta on GR degradation, which causes a decrease in GR stability in the nucleus. Our results indicate that ANX-1 functions as a negative regulator of GR transcriptional activation by inhibiting 14-3-3eta-induced up-regulation of GR.

Intracellular glucocorticoid signaling: a formerly simple system turns stochastic

Glucocorticoids contribute fundamentally to the maintenance of basal and stress-related homeostasis in all higher organisms. The major roles of these steroids in physiology are amply matched by their remarkable contributions to pathology. Glucocorticoids influence about 20% of the expressed human genome, and their effects spare almost no organs or tissues. For many years we thought that the numerous actions of glucocorticoids were mediated by a single receptor molecule: the classic glucocorticoid receptor (GR) isoform alpha, a complex, multifunctional domain protein, operating as a ligand-dependent transcription factor. The GR gene, however, encodes two 3' splicing variants, GRalpha and GRbeta, from alternative use of two distinct terminal exons (9alpha and 9beta), and each variant mRNA is translated from at least eight initiation sites into multiple GRalpha and possibly GRbeta isoforms, amounting to a minimum of 16 GR monomers and 256 different homo- or heterodimers. The translational GRalpha isoforms may be produced variably in target tissues, have varying intrinsic transcriptional activities, and influence different complements of glucocorticoid-responsive genes. It is likely that expression and functional differences might also be present between the putative GRbeta translational isoforms. The presence of multiple GR monomers and dimers in different quantities with quantitatively and qualitatively different transcriptional activities suggests that the glucocorticoid signaling system is highly stochastic.

Statin-induced blockade of prenylation alters nucleocytoplasmic shuttling of GTP-binding proteins gamma2 and beta2 and enhances their suppressive effect on glucocorticoid receptor transcriptional activity

BACKGROUND

We previously reported that the guanine tri-phosphate-binding proteins (G) beta and gamma are both localized in the nucleus, in addition to their expected cytoplasmic/plasma membrane localization. These proteins, as a heterodimeric complex, suppress glucocorticoid response element-mediated transcriptional activity of the glucocorticoid receptor through direct physical interactions between Gbeta and the glucocorticoid receptor.

MATERIALS AND METHODS

As Ggamma is prenylated at a cysteine residue in its C-terminal portion, and as this post-translational modification is required for many of the known Gbeta/Ggamma activities, we examined the effect of its absence or diminution on Gbeta/Ggamma-induced suppression of glucocorticoid receptor-induced transcriptional activity.

RESULTS

In a functional reporter assay, Ggamma2C68S, which is defective at the prenylation site, was more potent than the wild-type Ggamma2 at increasing Gbeta2-induced suppression of glucocorticoid receptor transactivation. Interestingly, the enhanced green fluorescent protein fusion of this mutant Ggamma2 was localized preferentially in the nucleus, while it was absent from the plasma membrane. Lovastatin, a 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitor that abrogates the prenylation of Ggamma, shifted the subcellular localization of enhanced green fluorescence protein-fused Ggamma2 and Gbeta2 from the cytoplasm/plasma membrane to the nucleus and further suppressed glucocorticoid receptor-induced transcriptional activity.

CONCLUSIONS

These findings indicate that not only is the natural covalent addition of the prenyl residue to Ggamma unnecessary for the transcriptional suppression induced by Gbeta/Ggamma on the glucocorticoid receptor, but rather helps retain the Gbeta/Ggamma complex away from the nucleus decreasing its antiglucocorticoid actions.

Myoepithelial cells: pathology, cell separation and markers of myoepithelial differentiation

Until recently the myoepithelial cell has been studied relatively little in terms of its role in breast cancer. A number of malignancies showing myoepithelial differentiation have been reported in the literature, although they are still thought to be relatively rare and only limited studies are published. As a result of recent expression profiling experiments, one type of tumor with myoepithelial features, the so-called 'basal' breast cancer, has received a renewed interest, although it has been known to pathologists for more than two decades. These tumors, which express markers of both luminal and myoepithelial cells, are now being studied using antibodies against some new molecules that have emerged from studies of sorted normal luminal and myoepithelial cells. These immunohistochemical data, combined with genomic studies, may lead to better identification and management of patients with 'basal' tumors.

G protein beta interacts with the glucocorticoid receptor and suppresses its transcriptional activity in the nucleus

Extracellular stimuli that activate cell surface receptors modulate glucocorticoid actions via as yet unclear mechanisms. Here, we report that the guanine nucleotide-binding protein (G protein)-coupled receptor-activated WD-repeat Gbeta interacts with the glucocorticoid receptor (GR), comigrates with it into the nucleus and suppresses GR-induced transactivation of the glucocorticoid-responsive genes. Association of Ggamma with Gbeta is necessary for this action of Gbeta. Both endogenous and enhanced green fluorescent protein (EGFP)-fused Gbeta2 and Ggamma2 proteins were detected in the nucleus at baseline, whereas a fraction of EGFP-Gbeta2 and DsRed2-GR comigrated to the nucleus or the plasma membrane, depending on the exposure of cells to dexamethasone or somatostatin, respectively. Gbeta2 was associated with GR/glucocorticoid response elements (GREs) in vivo and suppressed activation function-2-directed transcriptional activity of the GR. We conclude that the Gbetagamma complex interacts with the GR and suppresses its transcriptional activity by associating with the transcriptional complex formed on GR-responsive promoters.

Mechanisms of receptor-mediated nuclear import and nuclear export

Nuclear transport of proteins and RNA occurs through the nuclear pore complex and is mediated by a superfamily of transport receptors known collectively as karyopherins. Karyopherins bind to their cargoes by recognition of specific nuclear localization signals or nuclear export signals. Transport through the nuclear pore complex is facilitated by transient interactions between the karyopherins and the nuclear pore complex. The interactions of karyopherins with their cargoes are regulated by the Ras-related GTPase Ran. Ran is assisted in this process by proteins that regulate its GTPase cycle and subcellular localization. In this review, we describe several of the major transport pathways that are conserved in higher and lower eukaryotes, with particular emphasis on the role of Ran. We highlight the latest advances in the structure and function of transport receptors and discuss recent examples of steroid hormone receptor import and regulation by signal transduction pathways. Understanding the molecular basis of nuclear transport may provide insight into human diseases by revealing how nucleocytoplasmic trafficking regulates protein activity.

Enhancement of steroid receptor-mediated transcription for the development of highly responsive bioassays

We have previously generated several transformed human mammary cell lines for the detection of steroid receptor-mediated activities and used these cell lines to detect and characterize steroid hormone (ant)agonistic compounds. In this report, we describe the specific optimization procedures used to enhance receptor-mediated transcription through the human glucocorticoid, progesterone and androgen receptors, respectively. Sodium arsenite-induced chemical stress leads to a substantial and specific increase in the glucocorticoid receptor-mediated transcription, resulting in maximal stimulations of more than 2000-fold by the agonist dexamethasone. Similarly, a combined treatment with forskolin (an activator of adenylate cyclase) and trichostatin A (an inhibitor of histone deacetylases) leads to a synergistic enhancement of progesterone or androgen stimulation, resulting in a maximal induction of more than 200-fold or about 100-fold, respectively. The enhanced responses to specific steroids are mediated by the corresponding nuclear receptor. We show that by using these enhanced transcriptional stimulation protocols, it is possible to detect lower amounts of steroid hormones without substantially affecting the relative biological activities of various agonists. Finally, the application of these enhanced reporter cell assays to real biological samples from meat-producing animals is evaluated, and some validation parameters are presented.

Molecular mechanisms of glucocorticoids in the control of inflammation and lymphocyte apoptosis

The immune system must be tightly controlled not only to guarantee efficient protection from invading pathogens and oncogenic cells but also to avoid exaggerated immune responses and autoimmunity. This is achieved through interactions amongst leukocytes themselves, by signals from stromal cells and also by various hormones, including glucocorticoids. The glucocorticoids are a class of steroid hormones that exert a wide range of anti-inflammatory and immunosuppressive activities after binding to the glucocorticoid receptor. The power of these hormones was acknowledged many decades ago, and today synthetic derivatives are widely used in the treatment of inflammatory disorders, autoimmunity and cancer. In this review, we summarize our present knowledge of the molecular mechanisms of glucocorticoid action, their influence on specific leukocytes and the induction of thymocyte apoptosis, with an emphasis on how molecular genetics has contributed to our growing, although still incomplete, understanding of these processes.

Targeted proteomic analysis of 14-3-3 sigma, a p53 effector commonly silenced in cancer

To comprehensively identify proteins interacting with 14-3-3 sigma in vivo, tandem affinity purification and the multidimensional protein identification technology were combined to characterize 117 proteins associated with 14-3-3 sigma in human cells. The majority of identified proteins contained one or several phosphorylatable 14-3-3-binding sites indicating a potential direct interaction with 14-3-3 sigma. 25 proteins were not previously assigned to any function and were named SIP2-26 (for 14-3-3 sigma-interacting protein). Among the 92 interactors with known function were a number of proteins previously implicated in oncogenic signaling (APC, A-RAF, B-RAF, and c-RAF) and cell cycle regulation (AJUBA, c-TAK, PTOV-1, and WEE1). The largest functional classes comprised proteins involved in the regulation of cytoskeletal dynamics, polarity, adhesion, mitogenic signaling, and motility. Accordingly ectopic 14-3-3 sigma expression prevented cellular migration in a wounding assay and enhanced mitogen-activated protein kinase signaling. The functional diversity of the identified proteins indicates that induction of 14-3-3 sigma could allow p53 to affect numerous processes in addition to the previously characterized inhibitory effect on G2/M progression. The data suggest that the cancer-specific loss of 14-3-3 sigma expression by epigenetic silencing or p53 mutations contributes to cancer formation by multiple routes.

Mechanisms of glucocorticoid-induced apoptosis in hematologic malignancies: updates

PURPOSE OF REVIEW

Glucocorticoids remain a central component of the therapeutic armamentarium for a broad spectrum of hematologic malignancies. There is an extensive body of evidence suggesting that the efficacy of glucocorticoids stems from their ability to mediate apoptosis in leukemia, lymphoma, and myeloma cells.

RECENT FINDINGS

Traditionally, glucocorticoid-induced apoptosis is divided into three stages: an initiation stage, which involves glucocorticoid receptor activation and glucocorticoid receptor-mediated gene regulation; a decision stage, which engages the prosurvival and proapoptotic factors at the mitochondrial level; and an execution stage, which implicates caspases and endonuclease activation. Recent discoveries have clarified many aspects of the apoptotic pathway, including activation of the caspases cascade and multicatalytic proteasome, suppression of prosurvival transcription factors such as AP-1, c-myc, nuclear factor-kappaB, as well as cross-talk between the T-cell receptor and cytokine signaling pathways.

SUMMARY

This review focuses primarily on insights gained during recent years into the mechanism of the signaling pathways responsible for mediating glucocorticoid-induced apoptosis in hematologic malignancies. This information provides a scientific basis to explore synergistic approaches that may enhance glucocorticoid-induced apoptosis and may bypass mechanism of resistance.

HIV-1 accessory protein Vpr inhibits the effect of insulin on the Foxo subfamily of forkhead transcription factors by interfering with their binding to 14-3-3 proteins: potential clinical implications regarding the insulin resistance of HIV-1-infected patients

HIV-1 accessory protein Vpr arrests host cells at the G2/M phase of the cell cycle by interacting with members of the protein family 14-3-3, which regulate the activities of "partner" molecules by binding to their phosphorylated serine or threonine residues and changing their intracellular localization and/or stability. Vpr does this by facilitating the association of 14-3-3 to its partner protein Cdc25C, independent of the latter's phosphorylation status. Here we report that the same viral protein interfered with and altered the activity of another 14-3-3 partner molecule, Foxo3a, a subtype of the forkhead transcription factors, by inhibiting its association with 14-3-3. Foxo3a's transcriptional activity is normally suppressed by insulin-induced translocation of this protein from the nucleus into the cytoplasm. Vpr inhibited the ability of insulin or its downstream protein kinase Akt to change the intracellular localization of Foxo3a preferentially to the cytoplasm. This HIV-1 protein also interfered with insulin-induced coprecipitation of 14-3-3 and Foxo3a in vivo and antagonized the negative effect of insulin on Foxo3a-induced transactivation of a FOXO-responsive promoter. Moreover, Vpr antagonized insulin-induced suppression of the mRNA expression of the glucose 6-phosphatase, manganese superoxide dismutase, and sterol carrier protein 2 genes, which are known targets of insulin and FOXO, in HepG2 cells. These findings indicate that Vpr interferes with the suppressive effects of insulin on FOXO-mediated transcription of target genes via 14-3-3. Vpr thus may contribute to the tissue-selective insulin resistance often observed in HIV-1-infected individuals.

Dynamic interactions between 14-3-3 proteins and phosphoproteins regulate diverse cellular processes

14-3-3 proteins exert an extraordinarily widespread influence on cellular processes in all eukaryotes. They operate by binding to specific phosphorylated sites on diverse target proteins, thereby forcing conformational changes or influencing interactions between their targets and other molecules. In these ways, 14-3-3s 'finish the job' when phosphorylation alone lacks the power to drive changes in the activities of intracellular proteins. By interacting dynamically with phosphorylated proteins, 14-3-3s often trigger events that promote cell survival--in situations from preventing metabolic imbalances caused by sudden darkness in leaves to mammalian cell-survival responses to growth factors. Recent work linking specific 14-3-3 isoforms to genetic disorders and cancers, and the cellular effects of 14-3-3 agonists and antagonists, indicate that the cellular complement of 14-3-3 proteins may integrate the specificity and strength of signalling through to different cellular responses.

Dexamethasone resistance in B-cell precursor childhood acute lymphoblastic leukemia occurs downstream of ligand-induced nuclear translocation of the glucocorticoid receptor

Glucocorticoids are among the most effective agents used in the treatment of childhood acute lymphoblastic leukemia (ALL), and patient response to treatment is an important determinant of long-term outcome. Despite its clinical significance, the molecular basis of glucocorticoid resistance in lymphoid malignancies is still poorly understood. We have recently developed a highly clinically relevant experimental model of childhood ALL, in which primary childhood ALL biopsies were established as xenografts in nonobese diabetic/severe combined immunodeficient (NOD/SCID) mice. The in vivo and in vitro responses of a panel of these xenografts to the glucocorticoid, dexamethasone, reflected the outcome of the patients from whom they were derived. In this report we show that glucocorticoid resistance in B-cell precursor (BCP) ALL xenografts was not due to down-regulation of the glucocorticoid receptor (GR) nor to defective ligand binding of the GR. Moreover, dexamethasone-induced GR translocation from the cytoplasm to the nucleus was comparable in all xenografts. However, glucocorticoid resistance was associated with profoundly attenuated induction of the BH3-only proapoptotic protein, Bim, when xenograft cells were exposed to dexamethasone. These results show that dexamethasone resistance in BCP ALL xenografts occurs downstream of ligand-induced nuclear translocation of the GR, but upstream of Bim induction.

Proteomic identification of 14-3-3 sigma as a common component of the androgen receptor and the epidermal growth factor receptor signaling pathways of the human prostate epithelial cell line M12

The epidermal growth factor receptor and androgen receptor (AR) both play major roles in the control of prostate growth. Our hypothesis is that shared downstream components of these two signaling pathways are significant participants in androgen-independent growth. Our first objective was to identify proteins whose activation and/or expression in AR-positive prostate epithelial cells are induced by both epidermal growth factor (EGF) and dihydrotestosterone (DHT). AR expression was induced in a tumorigenic, metastatic subline of the SV40 large T-antigen immortalized human prostate epithelial subline M12 by stable transfection with human wild-type AR cDNA. These M12AR (+) cells with functional AR were treated in parallel with EGF (10 ng/ml) or DHT (10(-8) M) for 24 h before 2D gel electrophoresis and Western immunoblotting with antiphosphotyrosine monoclonal antibody. Coomassie blue-stained spots on a 2D gel run in parallel were aligned with the phosphoproteins on the Western immunoblot, and identified by matrix-assisted laser desorption ionization/time-of-flight mass spectroscopy. The most interesting of the seven proteins that appeared to be phosphorylated by these criteria was 14-3-3 protein sigma. Protein extracted after either EGF or DHT treatment, immunoprecipitated with antiphosphotyrosine monoclonal antibody, and immunoblotted by anti-14-3-3 sigma confirmed phosphorylation of 14-3-3 sigma. Addition of either DHT or EGF to the M12AR(+) cells induced subcellular migration of 14-3-3 sigma and activated a 14-3-3 sigma reporter construct. Immunohistochemical analysis revealed nuclear localization of 14-3-3 sigma in higher Gleason grade prostate cancers relative to benign glands. These findings implicate 14-3-3 sigma in the development of human prostate cancer cells and could provide a new target for intervention in prostate cancer.

Importin 7 and importin alpha/importin beta are nuclear import receptors for the glucocorticoid receptor

The vertebrate glucocorticoid receptor (GR) is cytoplasmic without hormone and localizes to the nucleus after hormone binding. GR has two nuclear localization signals (NLS): NL1 is similar in sequence to the SV40 NLS; NL2 is poorly defined, residing in the ligand-binding domain. We found that GR displayed similar hormone-regulated compartmentalization in Saccharomyces cerevisiae and required the Sxm1 nuclear import receptor for NL2-mediated import. Two metazoan homologues of Sxm1, importin 7 and importin 8, bound both NL1 and NL2, whereas importin alpha selectively bound NL1. In an in vitro nuclear import assay, both importin 7 and the importin alpha-importin beta heterodimer could import a GR NL1 fragment. Under these conditions, full-length GR localized to nuclei in the presence but not absence of an unidentified component in cell extracts. Interestingly, importin 7, importin 8, and importin alpha bound GR even in the absence of hormone; thus, hormonal control of localization is exerted at a step downstream of import receptor binding.