Selectivity of cell cycle regulation of glucocorticoid receptor function

Radiation-induced DNA double-strand breaks in cortisol exposed fibroblasts as quantified with the novel foci-integrated damage complexity score (FIDCS)

Without the protective shielding of Earth's atmosphere, astronauts face higher doses of ionizing radiation in space, causing serious health concerns. Highly charged and high energy (HZE) particles are particularly effective in causing complex and difficult-to-repair DNA double-strand breaks compared to low linear energy transfer. Additionally, chronic cortisol exposure during spaceflight raises further concerns, although its specific impact on DNA damage and repair remains unknown. This study explorers the effect of different radiation qualities (photons, protons, carbon, and iron ions) on the DNA damage and repair of cortisol-conditioned primary human dermal fibroblasts. Besides, we introduce a new measure, the Foci-Integrated Damage Complexity Score (FIDCS), to assess DNA damage complexity by analyzing focus area and fluorescent intensity. Our results show that the FIDCS captured the DNA damage induced by different radiation qualities better than counting the number of foci, as traditionally done. Besides, using this measure, we were able to identify differences in DNA damage between cortisol-exposed cells and controls. This suggests that, besides measuring the total number of foci, considering the complexity of the DNA damage by means of the FIDCS can provide additional and, in our case, improved information when comparing different radiation qualities.

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

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

In vitro, in vivo, and in silico evaluation of the glucocorticoid receptor antagonist activity of 3,6-dibromocarbazole

3,6-Dibromocarbazole is a novel environmental contaminant which is currently detected in several environmental media worldwide. This work aims to investigate the anti-glucocorticoid potency and endocrine disrupting effects of 3,6-dibromocarbazole. In vitro experiments indicated that 3,6-dibromocarbazole possessed glucocorticoid receptor (GR) antagonistic activity and inhibited dexamethasone-induced GR nuclear translocation. 3,6-Dibromocarbazole reduced the expression levels of glucocorticoid responsive genes including glucose-6-phosphatase (G6Pase), phosphoenolpyruvate carboxykinase (PEPCK), fatty acid synthase (FAS), and tyrosine aminotransferase (TAT), and further disrupted the protein expression of two key enzymes PEPCK and FAS in gluconeogenesis. In vivo experiments showed that 3,6-dibromocarbazole induced abnormal development of zebrafish embryos and disrupted the major neurohormones involved in activation of hypothalamic-pituitary-adrenocortical (HPA) axis in zebrafish larvae. The results of molecular docking and molecular dynamics simulation contributed to explain the antagonistic effect of 3,6-dibromocarbazole. Taken together, this work identified 3,6-dibromocarbazole as a GR antagonist, which might exert endocrine disrupting effects by interfering the pathway of gluconeogenesis.

The Effects of Combined Exposure to Simulated Microgravity, Ionizing Radiation, and Cortisol on the In Vitro Wound Healing Process

Human spaceflight is associated with several health-related issues as a result of long-term exposure to microgravity, ionizing radiation, and higher levels of psychological stress. Frequent reported skin problems in space include rashes, itches, and a delayed wound healing. Access to space is restricted by financial and logistical issues; as a consequence, experimental sample sizes are often small, which limits the generalization of the results. Earth-based simulation models can be used to investigate cellular responses as a result of exposure to certain spaceflight stressors. Here, we describe the development of an in vitro model of the simulated spaceflight environment, which we used to investigate the combined effect of simulated microgravity using the random positioning machine (RPM), ionizing radiation, and stress hormones on the wound-healing capacity of human dermal fibroblasts. Fibroblasts were exposed to cortisol, after which they were irradiated with different radiation qualities (including X-rays, protons, carbon ions, and iron ions) followed by exposure to simulated microgravity using a random positioning machine (RPM). Data related to the inflammatory, proliferation, and remodeling phase of wound healing has been collected. Results show that spaceflight stressors can interfere with the wound healing process at any phase. Moreover, several interactions between the different spaceflight stressors were found. This highlights the complexity that needs to be taken into account when studying the effect of spaceflight stressors on certain biological processes and for the aim of countermeasures development.

Examining of some physical and physiological parameters of 10-18 years old male skiers to seasonal cycles

Abstract: Skiing can be identified as a branch with the most important representative power of winter sports. Further, it has been characterized by high popularity and population, branching off in itself. During the year, different physical performance and physiological indicators are sometimes encountered in athletes depending on seasonal cycles, training levels, and living conditions. The aim of this study is to examine some physical performance parameters and the determined hormone levels to seasonal cycles. Methods: 15 male skiers with a mean age of 14.53±2.61 (years), a mean height of 158.53±9.66 (cm), and a mean body mass of 54.20±10.85 (kg) participated in the study. The information about participants' age, height, and body mass was determined by standard methods. Various measurement tools were used including a digital hand dynamometer (TKK 5401) for hand grip strength, a digital dynamometer (TKK 5402) for back and leg strength, a jump meter (Takei TKK 5406) for vertical jump height, and an electronic hand spirometer (firstMED) for respiratory functions. The Wingate anaerobic power test (Monark 894 E bicycle ergometer) was performed to determine the anaerobic power level. Additionally, to determine somatotypes (ectomorph, mesomorph, endomorph) and body fat percentage; skinfold caliper (Holtain), tape measure, and digital caliper (Holtain) were respectively utilized for skinfold thickness measurement, circumference measurements, and diameter measurements. Blood samples (hemogram test, vitamin D, cortisol, and testosterone to be checked) were taken from the antecubital vein in the sitting position. From the blood samples, serum plasma was separated and preserved by centrifugation (+4o) and all samples were analyzed at once. All tests were performed once in September, December, March, and June at an altitude of 2,000 and in pre-season and mid-season. The data were analyzed through IBM SPSS 24.0 package program. Shapiro-Wilk was used to determine the distribution of the data, descriptive and frequency analysis was used to determine the mean of the variables, and one-way analysis of variance (ANOVA) was used to determine the differences between measurements. The results were presented as arithmetic mean and standard deviation (𝑋̅±Ss). Results: In 10-18 years old male skiers, it was determined that vitamin D reached its highest level in autumn, testosterone in summer, and cortisol in winter. In the inter-test comparison results, significant differences were determined in the vertical jump, right and left-hand grip strength, leg strength, testosterone, vitamin D, HCT, FVC, FEV1, and anaerobic power parameters. Conclusions: The results of the study have mostly supported the literature. Keywords: vitamin D; testosterone; cortisol; anaerobic power; respiratory functions

Myeloma-Modified Adipocytes Exhibit Metabolic Dysfunction and a Senescence-Associated Secretory Phenotype

Bone marrow adipocytes (BMAd) have recently been implicated in accelerating bone metastatic cancers, such as acute myelogenous leukemia and breast cancer. Importantly, bone marrow adipose tissue (BMAT) expands with aging and obesity, two key risk factors in multiple myeloma disease prevalence, suggesting that BMAds may influence and be influenced by myeloma cells in the marrow. Here, we provide evidence that reciprocal interactions and cross-regulation of myeloma cells and BMAds play a role in multiple myeloma pathogenesis and treatment response. Bone marrow biopsies from patients with multiple myeloma revealed significant loss of BMAT with myeloma cell infiltration of the marrow, whereas BMAT was restored after treatment for multiple myeloma. Myeloma cells reduced BMAT in different preclinical murine models of multiple myeloma and in vitro using myeloma cell-adipocyte cocultures. In addition, multiple myeloma cells altered adipocyte gene expression and cytokine secretory profiles, which were also associated with bioenergetic changes and induction of a senescent-like phenotype. In vivo, senescence markers were also increased in the bone marrow of tumor-burdened mice. BMAds, in turn, provided resistance to dexamethasone-induced cell-cycle arrest and apoptosis, illuminating a new possible driver of myeloma cell evolution in a drug-resistant clone. Our findings reveal that bidirectional interactions between BMAds and myeloma cells have significant implications for the pathogenesis and treatment of multiple myeloma. Targeting senescence in the BMAd or other bone marrow cells may represent a novel therapeutic approach for treatment of multiple myeloma. SIGNIFICANCE: This study changes the foundational understanding of how cancer cells hijack the bone marrow microenvironment and demonstrates that tumor cells induce senescence and metabolic changes in adipocytes, potentially driving new therapeutic directions.

Growth Hormone(s), Testosterone, Insulin-Like Growth Factors, and Cortisol: Roles and Integration for Cellular Development and Growth With Exercise

Hormones are largely responsible for the integrated communication of several physiological systems responsible for modulating cellular growth and development. Although the specific hormonal influence must be considered within the context of the entire endocrine system and its relationship with other physiological systems, three key hormones are considered the "anabolic giants" in cellular growth and repair: testosterone, the growth hormone superfamily, and the insulin-like growth factor (IGF) superfamily. In addition to these anabolic hormones, glucocorticoids, mainly cortisol must also be considered because of their profound opposing influence on human skeletal muscle anabolism in many instances. This review presents emerging research on: (1) Testosterone signaling pathways, responses, and adaptations to resistance training; (2) Growth hormone: presents new complexity with exercise stress; (3) Current perspectives on IGF-I and physiological adaptations and complexity these hormones as related to training; and (4) Glucocorticoid roles in integrated communication for anabolic/catabolic signaling. Specifically, the review describes (1) Testosterone as the primary anabolic hormone, with an anabolic influence largely dictated primarily by genomic and possible non-genomic signaling, satellite cell activation, interaction with other anabolic signaling pathways, upregulation or downregulation of the androgen receptor, and potential roles in co-activators and transcriptional activity; (2) Differential influences of growth hormones depending on the "type" of the hormone being assayed and the magnitude of the physiological stress; (3) The exquisite regulation of IGF-1 by a family of binding proteins (IGFBPs 1-6), which can either stimulate or inhibit biological action depending on binding; and (4) Circadian patterning and newly discovered variants of glucocorticoid isoforms largely dictating glucocorticoid sensitivity and catabolic, muscle sparing, or pathological influence. The downstream integrated anabolic and catabolic mechanisms of these hormones not only affect the ability of skeletal muscle to generate force; they also have implications for pharmaceutical treatments, aging, and prevalent chronic conditions such as metabolic syndrome, insulin resistance, and hypertension. Thus, advances in our understanding of hormones that impact anabolic: catabolic processes have relevance for athletes and the general population, alike.

Relapse-associated AURKB blunts the glucocorticoid sensitivity of B cell acute lymphoblastic leukemia

Glucocorticoids (GCs) are used in combination chemotherapies as front-line treatment for B cell acute lymphoblastic leukemia (B-ALL). Although effective, many patients relapse and become resistant to chemotherapy and GCs in particular. Why these patients relapse is not clear. We took a comprehensive, functional genomics approach to identify sources of GC resistance. A genome-wide shRNA screen identified the transcriptional coactivators EHMT2, EHMT1, and CBX3 as important contributors to GC-induced cell death. This complex selectively supports GC-induced expression of genes contributing to cell death. A metaanalysis of gene expression data from B-ALL patient specimens revealed that Aurora kinase B (AURKB), which restrains GC signaling by phosphorylating EHMT1-2, is overexpressed in relapsed B-ALL, suggesting it as a potential contributor to relapse. Inhibition of AURKB enhanced GC-induced expression of cell death genes, resulting in potentiation of GC cytotoxicity in cell lines and relapsed B-ALL patient samples. This function for AURKB is distinct from its canonical role in the cell cycle. These results show the utility of functional genomics in understanding mechanisms of resistance and rapidly identifying combination chemotherapeutics.

GR-regulating Serine/Threonine Kinases: New Physiologic and Pathologic Implications

Glucocorticoid hormones, end products of the hypothalamic-pituitary-adrenal axis, virtually influence all human functions both in a basal homeostatic condition and under stress. The glucocorticoid receptor (GR), a nuclear hormone receptor superfamily protein, mediates these actions of glucocorticoids by acting as a ligand-dependent transcription factor. Because glucocorticoid actions are diverse and strong, many biological pathways adjust them in local tissues by targeting the GR signaling pathway as part of the regulatory loop coordinating complex human functions. Phosphorylation of GR protein by serine/threonine kinases is one of the major regulatory mechanisms for this communication. In this review, recent progress in research investigating GR phosphorylation by these kinases is discussed, along with the possible physiologic and pathophysiologic implications.

Cross-talk between the glucocorticoid receptor and MyoD family inhibitor domain-containing protein provides a new mechanism for generating tissue-specific responses to glucocorticoids

Glucocorticoids are primary stress hormones that regulate many physiological processes, and synthetic derivatives of these molecules are widely used in the clinic. The molecular factors that govern tissue specificity of glucocorticoids, however, are poorly understood. The actions of glucocorticoids are mediated by the glucocorticoid receptor (GR). To discover new proteins that interact with GR and modulate its function, we performed a yeast two-hybrid assay. The MyoD family inhibitor domain-containing protein (MDFIC) was identified as a binding partner for GR. MDFIC associated with GR in the cytoplasm of cells, and treatment with glucocorticoids resulted in the dissociation of the GR-MDFIC complex. To investigate the function of the GR-MDFIC interaction, we performed a genome-wide microarray in intact and MDFIC-deficient A549 cells that were treated with glucocorticoids. A large cohort of genes was differentially regulated by GR depending on the presence or absence of MDFIC. These gene changes were strongly associated with inflammation, and glucocorticoid regulation of the inflammatory response was altered in MDFIC-deficient cells. At a molecular level, the interaction of MDFIC with GR altered the phosphorylation status of the receptor. We demonstrate in COS-1 cells that changes in receptor phosphorylation underlie the ability of MDFIC to regulate the transcriptional activity of GR. Finally, we show that GR directly represses the MDFIC gene, revealing a negative feedback loop by which glucocorticoids limit MDFIC activity. These findings identify a new binding partner for cytoplasmic GR that modulates the receptor transcriptome and contributes to the tissue-specific actions of glucocorticoids.

Cell-cycle-dependent regulation of androgen receptor function

The androgen receptor (AR) is a critical oncogene in prostate cancer (PCa) development and progression. In this study, we demonstrate cell-cycle-dependent regulation of AR activity, localization, and phosphorylation. We show that for three AR-target genes, androgen-stimulated AR transactivation is highest during the G1 phase, decreased during S-phase, and abrogated during G2/M. This change in AR transactivation parallels changes in AR localization and phosphorylation. A combination of imaging techniques and quantitative analysis reveals nuclear AR localization during interphase and the exclusion of the majority, but not all, AR from chromatin during mitosis. Flow cytometry analyses using a phospho-S308 AR-specific antibody in asynchronous and chemically enriched G2/M PCa cells revealed ligand-independent induction of S308 phosphorylation in mitosis when CDK1 is activated. Consistent with our flow cytometry data, IP-western blotting revealed an increase in S308 phosphorylation in G2/M, and the results of an in vitro kinase assay indicated that CDK1 was able to phosphorylate the AR on S308. Pharmacological inhibition of CDK1 activity resulted in decreased S308 phosphorylation in PCa cells. Importantly, using a combination of anti-total AR and phospho-S308-specific antibodies in immunofluorescence experiments, we showed that the AR is excluded from condensed chromatin in mitotic cells when it was phosphorylated on S308. In summary, we show that the phosphorylation of the AR on S308 by CDK1 during mitosis regulates AR localization and correlates with changes in AR transcriptional activity. These findings have important implications for understanding the function of AR as an oncogene.

Effects of heat stress on the gene expression of nutrient transporters in the jejunum of broiler chickens (Gallus gallus domesticus)

In broiler chickens, heat stress disrupts nutrient digestion and absorption. However, the underlying molecular mechanism is not clearly understood. Hence, to investigate the effects of high ambient temperatures on the expression levels of nutrient transporters in the jejunum of broiler chickens, seventy-two 35-day-old male broiler chickens with similar body weights were randomly allocated into two groups: control (24 ± 1 °C) and heat-stressed (32 ± 1 °C). The chickens in the heat-stressed group were exposed to 10 h of heat daily from 08:00 to 18:00 and then raised at 24 ± 1 °C. The rectal temperature and feed intake of the chickens were recorded daily. After 7 days, nine chickens per group were sacrificed by exsanguination, and the jejunum was collected. The results show that heat exposure significantly decreased the feed intake and increased the rectal temperature of the broiler chickens. The plasma concentrations of uric acid and triglyceride significantly increased and decreased, respectively, in the heat-stressed group. No significant differences in the levels of plasma glucose, total amino acids, and very low-density lipoprotein were observed between the heat-stressed and control groups. However, the plasma concentration of glucose tended to be higher (P = 0.09) in the heat-stressed group than in the control group. Heat exposure did not significantly affect the mRNA levels of Na(+)-dependent glucose transporter 1 and amino acid transporters y + LAT1, CAT1, r-BAT, and PePT-1. However, the expression levels of GLUT-2, FABP1, and CD36 were significantly decreased by heat exposure. The results of this study provide new insights into the mechanisms by which heat stress affects nutrient absorption in broiler chickens. Our findings suggest that periodic heat exposure might alter the jejunal glucose and lipid transport rather than amino acid transport. However, intestinal epithelial damage and cell loss should be considered when interpreting the effects of heat stress on the expression of intestinal transporters.

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

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

The biology of the glucocorticoid receptor: new signaling mechanisms in health and disease

Glucocorticoids are primary stress hormones necessary for life that regulate numerous physiologic processes in an effort to maintain homeostasis. Synthetic derivatives of these hormones have been mainstays in the clinic for treating inflammatory diseases, autoimmune disorders, and hematologic cancers. The physiologic and pharmacologic actions of glucocorticoids are mediated by the glucocorticoid receptor (GR), a member of the nuclear receptor superfamily of ligand-dependent transcription factors. Ligand-occupied GR induces or represses the transcription of thousands of genes through direct binding to DNA response elements, physically associating with other transcription factors, or both. The traditional view that glucocorticoids act through a single GR protein has changed dramatically with the discovery of a large cohort of receptor isoforms with unique expression, gene-regulatory, and functional profiles. These GR subtypes are derived from a single gene by means of alternative splicing and alternative translation initiation mechanisms. Posttranslational modification of these GR isoforms further expands the diversity of glucocorticoid responses. Here we discuss the origin and molecular properties of the GR isoforms and their contribution to the specificity and sensitivity of glucocorticoid signaling in healthy and diseased tissues.

Cell cycle phase regulates glucocorticoid receptor function

The glucocorticoid receptor (GR) is a member of the nuclear hormone receptor superfamily of ligand-activated transcription factors. In contrast to many other nuclear receptors, GR is thought to be exclusively cytoplasmic in quiescent cells, and only translocate to the nucleus on ligand binding. We now demonstrate significant nuclear GR in the absence of ligand, which requires nuclear localisation signal 1 (NLS1). Live cell imaging reveals dramatic GR import into the nucleus through interphase and rapid exclusion of the GR from the nucleus at the onset of mitosis, which persists into early G(1). This suggests that the heterogeneity in GR distribution is reflective of cell cycle phase. The impact of cell cycle-driven GR trafficking on a panel of glucocorticoid actions was profiled. In G2/M-enriched cells there was marked prolongation of glucocorticoid-induced ERK activation. This was accompanied by DNA template-specific, ligand-independent GR transactivation. Using chimeric and domain-deleted receptors we demonstrate that this transactivation effect is mediated by the AF1 transactivation domain. AF-1 harbours multiple phosphorylation sites, which are consensus sequences for kinases including CDKs, whose activity changes during the cell cycle. In G2/M there was clear ligand independent induction of GR phosphorylation on residues 203 and 211, both of which are phosphorylated after ligand activation. Ligand-independent transactivation required induction of phospho-S211GR but not S203GR, thereby directly linking cell cycle driven GR modification with altered GR function. Cell cycle phase therefore regulates GR localisation and post-translational modification which selectively impacts GR activity. This suggests that cell cycle phase is an important determinant in the cellular response to Gc, and that mitotic index contributes to tissue Gc sensitivity.

Cellular processing of the glucocorticoid receptor gene and protein: new mechanisms for generating tissue-specific actions of glucocorticoids

Glucocorticoids regulate numerous physiological processes and are mainstays in the treatment of inflammation, autoimmune disease, and cancer. The traditional view that glucocorticoids act through a single glucocorticoid receptor (GR) protein has changed in recent years with the discovery of a large cohort of receptor subtypes arising from alternative processing of the GR gene. These isoforms differ in their expression, gene regulatory, and functional profiles. Post-translational modification of these proteins further expands GR diversity. Here, we discuss the origin and molecular properties of the GR isoforms and their contribution to the sensitivity and specificity of the glucocorticoid response.

Comparative effects of microtubules disruption on glucocorticoid receptor functions in proliferating and quiescent cells

We have recently demonstrated that the alkaloid colchicine (COL) inhibits glucocorticoid receptor (GR) transcriptional activity. In addition, we described proteasome-mediated degradation of GR in COL-treated HeLa cells. While these effects were previously attributed to cell cycle arrest in G2/M phase, this explanation is not applicable for nonproliferating cells such as human hepatocytes (HH). In the current study, we compared COL-mediated microtubule disruption and cell cycle arrest with selected GR functions in HeLa cells and HH as models of proliferating and quiescent cells, respectively. Microtubule disruption led to irreversible decrease in GR binding capacity and protein level in HeLa cells. None of the parameters was restored 24 hours after COL withdrawal. In contrast, dexamethasone (DEX) binding was increased in HH at the beginning of the treatment, with following transient activation of extracellular signal-regulated kinase (ERK). The findings of these investigations emphasize the GR-signaling differences between primary and transformed cells.

The glucocorticoid receptor signalling in breast cancer

Glucocorticoids (GCs) are provided as co-medication with chemotherapy in breast cancer, albeit several lines of evidence indicate that their use may have diverse effects and in fact may inhibit chemosensitivity. The molecular basis of GC-induced resistance to chemotherapy in breast cancer remains poorly defined. Recent researchers, in an attempt to clarify some aspects of the underlying pathways, provide convincing evidence that GCs induce effects that are dependent upon the glucocorticoid-receptor (GR)-mediated transcriptional regulation of specific genes known to play key roles in cellular/tissue functions, including growth, apoptosis, differentiation, metastasis and cell survival. In this review, we focus on how GC-induced chemoresistance in breast cancer is mediated by the GR, unravelling the molecular interplay of GR signalling with other signalling cascades prevalent in breast cancer. We also include a detailed description of GR structure and function, summarizing data gained during recent years into the mechanism(s) of the cross-talk between the GR and other signalling cascades and secondary messengers, via which GCs exert their pleiotropic effects.

Glucocorticoid receptor transcriptional isoforms and resistance in multiple myeloma cells

Although glucocorticoids play an important role in the treatment of multiple myeloma, some patients do not respond or develop resistance. The glucocorticoid receptor (GR), a single gene, mediates the effects of glucocorticoids. Using a model system of a multiple myeloma cell line sensitive to glucocorticoids and its early and late resistant variants, we have analyzed mutations in the GR gene, detected the presence of different transcriptional isoforms, quantified their levels of expression, and identified the promoters that regulate their expression. Levels of GR transcripts were comparable with the expression of total GR protein. Development of resistance correlates with an overall reduction in GR mRNA levels. This decrease in GR levels is neither due to mutation of the gene nor due to methylation. GRalpha is the predominant isoform in the sensitive cell line decreasing in expression in the early resistant cells and virtually undetectable in late resistant cells. GR-P is expressed at equivalent levels in both sensitive and early resistant cells, whereas in the late resistant cells, GR-P is the predominant isoform. GR-A is only expressed in the early resistant cell line. GRbeta is the least expressed isoform in all cell lines. Interestingly, the level of expression of exon 1-exon 2 RNA fragments remains similar in sensitive and resistant cell lines. Resistant cells became sensitive to glucocorticoids after GRalpha transfection. In conclusion, we show different patterns of expression of the GR isoforms and provide evidence that a decline in the expression of GRalpha may be associated with development of resistance.

Expression, protein stability and transcriptional activity of retinoic acid receptors are affected by microtubules interfering agents and all-trans-retinoic acid in primary rat hepatocytes

Cellular signaling by glucocorticoid receptor and aryl hydrocarbon receptor is restricted by microtubules interfering agents (MIAs). This leads to down-regulation of drug metabolizing enzymes and drug interactions. Here we investigated the effects of all-trans-retinoic acid (ATRA) and MIAs, i.e. colchicine, nocodazole and taxol on the regulation of retinoic acid receptor (RAR) genes in primary cultures of rat hepatocytes. ATRA (1microM) down-regulated RARalpha and RARgamma mRNAs (decrease 23% and 41%, respectively) whereas it up-regulated RARbeta mRNA (4.3-fold induction). All MIAs diminished the expression of RARs in dose-dependent manner; the potency of MIAs increased in order NOC Collapse

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MESH Headings

• Animals
• Catalysis/drug effects
• Cell Survival/drug effects
• Cells, Cultured
• Colchicine/pharmacology
• Gene Expression Regulation/drug effects
• HeLa Cells
• Hepatocytes/cytology
• Hepatocytes/drug effects
• Hepatocytes/metabolism
• Humans
• Leupeptins/pharmacology
• Microtubules/drug effects
• Proteasome Inhibitors
• RNA, Messenger/genetics
• RNA, Messenger/metabolism
• Rats
• Receptors, Retinoic Acid/genetics
• Receptors, Retinoic Acid/metabolism
• Retinoic Acid Receptor alpha
• Thermodynamics
• Transcription, Genetic/drug effects
• Transglutaminases/metabolism
• Tretinoin/pharmacology
• Tubulin Modulators/pharmacology
• Retinoic Acid Receptor gamma

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Affiliation(s)

Zdenek Dvorák Institute of Medical Chemistry and Biochemistry, Faculty of Medicine, Palacký University Olomouc, Hnevotínská 3, 77515 Olomouc, Czech Republic.
Radim Vrzal
Jitka Ulrichová
Dana Macejová
Slavomíra Ondková
Július Brtko

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Cyclins, cyclin dependent kinases, and regulation of steroid receptor action

Steroid receptors (SR), which are ligand activated transcription factors, and their coactivators are phosphoproteins whose activities are regulated by cell signaling pathways. Many of the identified phosphorylation sites in these proteins contain Ser/Thr-Pro motifs suggesting that they are substrates for cyclin dependent kinases and/or for mitogen activated protein kinases. An analysis of the roles of cyclins and their kinases in regulating receptor action has revealed that there are both stimulatory and inhibitory actions of cyclins, that some of the actions are independent of the partner kinases and that these activities are receptor specific. Consistent with this finding, the limited analyses of receptor activity as a function of cell cycle reveal distinct patterns of activation. SR often regulate cell proliferation. Thus, the cross-talk between cyclins and their kinases and the SR provides a means for integrating the actions of the SR with the cell cycle status of cells.

Phagocytic efficacy of macrophage-like cells as a function of cell cycle and Fcgamma receptors (FcgammaR) and complement receptor (CR)3 expression

Previous studies have shown that the efficiency of phagocytosis is a function of cell cycle and that phagocytosis promotes cell cycle progression. Because phagocytosis is dependent on cellular receptors we hypothesized that Fcgamma receptors (FcgammaR) and complement receptors (CR) expression varied with cell cycle. Consequently, we used centrifugal elutriation of macrophage-like cells, fluorescence activated cell sorting analysis and receptor staining to investigate expression of FcgammaR and CR as a function of cell cycle. We confirmed that FcgammaR expression on macrophage-like cells increased as the cells progressed from G1 to G2 phases. Moreover, CR3 expression varied as a function of cell cycle in a manner similar to FcgammaR. Correlation of receptor expression with cell size showed that FcgammaR and CR3 expression on macrophages was determined largely by cell size enlargement during the cell cycle. The efficacy of both Fc- and complement-mediated phagocytosis of live Cryptococcus neoformans (Cn) showed a biphasic pattern with the efficacy of phagocytosis decreasing when the cells approached the G1-S interface, which paralleled the changes in receptor surface expression when cells exited G1 phase. Live Cn cells were significantly more resistant to phagocytosis than dead cells at all stages of macrophage-like cell cycle. In contrast to live cells, the efficacy of phagocytosis of dead Cn decreased as surface receptor expression increased. Hence, the efficacy of phagocytosis in this system as function of cell cycle is not related to phagocytic receptor expression.

Opposite effects of histone deacetylase inhibitors on glucocorticoid and estrogen signaling in human endometrial Ishikawa cells

Histone deacetylase inhibitors (HDACi), which have emerged as a new class of anticancer agents, act by modulating expression of genes controlling apoptosis or cell proliferation. Here, we compared the effect of HDACi on transcriptional activation by estrogen or glucocorticoid receptors (ER and GR, respectively), two members of the steroid receptor family with cell growth regulatory properties. Like other transcription factors, steroid receptors modulate histone acetylation on target promoters. Using episomal reporter vectors containing minimal promoters to avoid promoter-specific effects, we observed that long-term (24-h) incubation with HDACi strongly stimulated GR-dependent but markedly repressed ER-dependent signaling in ER+/GR+ human endometrial carcinoma Ishikawa cells. These effects were reproduced on endogenous target genes and required incubation periods with HDACi substantially longer than necessary to increase global histone acetylation. Repression of estrogen signaling was due to direct inhibition of transcription from multiple ERalpha promoters and correlated with decreased histone acetylation of these promoters. In contrast, the strong HDACi stimulation of GR-dependent gene regulation was not accounted for by increased GR expression, but it was mimicked by overexpression of the histone acetyltransferase complex component transcriptional intermediary factor 2. Together, our results demonstrate striking and opposite effects of HDACi on ER and GR signaling that involve regulatory events independent of histone hyperacetylation on receptor target promoters.

Cyclin-dependent kinase activity is required for progesterone receptor function: novel role for cyclin A/Cdk2 as a progesterone receptor coactivator

Our studies examining the role of the cell cycle-regulated kinase cyclin A/Cdk2 in progesterone receptor (PR) action have demonstrated that cyclin-dependent kinase activity is required for PR function and that cyclin A/Cdk2 functions as a PR coactivator. Although Cdk2 can phosphorylate PR, elimination of these phosphorylation sites has little effect on the ability of cyclin A/Cdk2 to stimulate PR activity. PR interacts with cyclin A and recruits cyclin A/Cdk2 to progestin-responsive promoters, stimulating transcription. Inhibition of Cdk2 activity abolishes progesterone-dependent activation of PR target genes in part through inhibition of PR-dependent recruitment of steroid receptor coactivator 1 (SRC-1) and subsequent histone H4 acetylation at the target promoter. In vitro studies revealed that the interaction between SRC-1 and PR is dependent upon phosphorylation of SRC-1. This heretofore-unknown mechanism provides a potential means for integrating the regulation of PR activity with cell cycle progression. Moreover, the ability of PR to recruit cyclin A/Cdk2 to target promoters provides locally elevated levels of kinase, which can preferentially facilitate phosphorylation-dependent interactions and enzymatic activities of coactivators at the target promoter.

G protein-coupled receptor 30 down-regulates cofactor expression and interferes with the transcriptional activity of glucocorticoid

G protein-coupled receptor 30 (GPR30) has previously been described to be important in steroid-mediated growth and to inhibit cell proliferation. Here we investigated whether the effect of GPR30 on cell growth is dependent on steroid hormone receptors. We stably introduced GPR30 in immortalized normal mammary epithelial (HME) cells using retroviruses for gene delivery. GPR30 inhibited the growth and proliferation of the cells. They expressed glucocorticoid receptor, but not estrogen or progesterone receptor. GPR30 down-regulated the expression of cofactor transcription intermediary factor 2 (TIF2) analyzed using quantitative RT-PCR analysis, and also diminished the expression of TIF2 at protein level analyzed by Western blotting using nuclear extracts from mammary epithelial cells. When HME cells were transiently transfected with the glucocorticoid response element MMTV-luc reporter plasmid, stable expression of GPR30 resulted in the abolition of ligand-induced transactivation of the promoter. In COS cells, transient transfection of GPR30 with glucocorticoid receptor alpha resulted in an abrogation of the MMTV-luc and GRE-luc reporter activities induced by dexamethasone. The results suggest a novel mechanism by which membrane-initiated signaling interferes with steroid signaling.

Familial/sporadic glucocorticoid resistance: clinical phenotype and molecular mechanisms

Glucocorticoids regulate a variety of biologic processes and exert profound influences on many physiologic functions. Their actions are mediated by the glucocorticoid receptor (GR), which belongs to the nuclear receptor family of ligand-dependent transcription factors. Alterations in tissue sensitivity to glucocorticoids may manifest as states of resistance or hypersensitivity. Glucocorticoid resistance is a rare, familial or sporadic, condition characterized by generalized, partial target-tissue resistance to glucocorticoids. Compensatory elevations in circulating adrenocorticotropic hormone (ACTH) concentrations lead to increased production of adrenal steroids with mineralocorticoid and/or androgenic activity and their corresponding clinical manifestations, as well as increased urinary free-cortisol excretion in the absence of symptomatology suggestive of hypercortisolism. The molecular basis of the condition has been ascribed to mutations in the GR gene, which impair normal glucocorticoid signal transduction, altering tissue sensitivity to glucocorticoids. The present review focuses on the mechanisms of GR action and the clinical manifestations and molecular mechanisms of familial/sporadic glucocorticoid resistance.

Modulation of glucocorticoid receptor function via phosphorylation

The glucocorticoid receptor (GR) is phosphorylated at multiple serine residues in a hormone-dependent manner. It has been suggested that GR phosphorylation affects turnover, subcellular trafficking, or the transcriptional regulatory functions of the receptor, yet the contribution of individual GR phosphorylation sites to the modulation of GR activity remains enigmatic. This review critically evaluates the literature on GR phosphorylation and presents more recent work on the mechanism of GR phosphorylation from studies using antibodies that recognize GR only when it is phosphorylated. In addition, we present support for the notion that GR phosphorylation modifies protein-protein interactions, which can stabilize the hypophosphorylated form of the receptor in the absence of ligand, as well as facilitate transcriptional activation by the hyperphosphorylation of GR via cofactor recruitment upon ligand binding. Finally, we propose that GR phosphorylation also participates in the nongenomic activation of cytoplasmic signaling pathways evoked by GR. Thus, GR phosphorylation is a versatile mechanism for modulating and integrating multiple receptor functions.

Cell cycle-dependent expression of thyroid hormone receptor-beta is a mechanism for variable hormone sensitivity

Thyroid hormone receptors (TRs) are ligand-regulatable transcription factors. Currently, little is known about the expression of TRs or other nuclear hormone receptors during the cell cycle. We thus developed a stable expression system to express green fluorescent protein-TRbeta in HeLa cells under tetracycline regulation, and studied TR expression during the cell cycle by laser scanning cytometry. Only approximately 9-15% of the nonsynchronized cell population expressed TR because the majority of cells were in G(1) phase and did not express detectable amounts of TR. However, when cells were synchronized in early S phase with hydroxyurea and then released, TR expression levels increased in a cell cycle-dependent manner and peaked to 30-40% cells expressing TR at late G(2)/M phase before declining to nonsynchronized levels. Moreover, we observed a direct correlation between transcriptional activity and TR expression during the cell cycle. Similar cell cycle-dependent findings also were observed for endogenous TR in rat pituitary GH(3) cells. Last, cycloheximide studies demonstrated that the increase in TR expression was primarily due to increased translation. These novel observations of cell cycle-dependent expression of TR suggest that differential hormone sensitivity can occur during the cell cycle and may contribute to cell cycle progression during normal development and oncogenesis.

Colchicine down-regulates cytochrome P450 2B6, 2C8, 2C9, and 3A4 in human hepatocytes by affecting their glucocorticoid receptor-mediated regulation

The xenobiotic-mediated induction of three major human liver cytochrome P450 genes, CYP2B6, CYP2C9, and CYP3A4, is known to be regulated by the constitutive androstane receptor (CAR) and the pregnane X receptor (PXR). CAR and PXR are regulated, at least in part, by the glucocorticoid receptor (GR) and the hypothesis of a signal transduction cascade GR-[CAR/PXR]-P450 has been proposed. This study was aimed at testing this hypothesis in primary human hepatocytes by using the tubulin network disrupting agent colchicine. Colchicine (COL) decreased both basal and rifampicin- and phenobarbital-inducible expression of CYP2B6, CYP2C8/9, and CYP3A4. A parallel down-regulation of mRNA expression of CAR, PXR, and tyrosine aminotransferase, a prototypic gene directly regulated by GR, was observed. COL affected neither the level of GR mRNA nor ligand binding to GR. To evaluate the effect of colchicine on GR-mediated gene transactivation, HeLa cells stably or transiently transfected with a GR-responsive element-dependent luciferase reporter gene were used. COL decreased the dexamethasone-induced luciferase expression in stably transfected cell line by 50%, whereas GR transactivation in transiently transfected cells was not affected by COL. In contrast, ligand-dependent GR translocation in the human embryonic kidney 293 cell line transiently transfected with GFP-GR was inhibited by COL. We conclude that alteration of the signal transduction mediated through the GR-[CAR/PXR]-P450 cascade by colchicine is responsible for the down-regulation of CYP2C9 and CYP3A4, implicating cytoskeleton as necessary for correct functioning of this cascade under physiological conditions.

Glucocorticoid regulation of human eosinophil gene expression

Molecular analysis of steroid-regulated gene expression in freshly isolated human eosinophils is difficult due to the inherent high rate of spontaneous apoptosis and elevated levels of endogenous ribonucleases. To circumvent these limitations, we determined if the human eosinophilic cell line EoL-1 could serve as an in vitro model of glucocorticoid signaling. We found by optimizing growth conditions in low serum-containing media that dexamethasone (Dex) treatment of EoL-1 cells induced an apoptotic pathway that was inhibited by interleukin-5 (IL-5). Moreover, gene expression profiling using RNA from untreated EoL-1 cells and from freshly isolated human eosinophils identified 380 commonly expressed genes, including the eosinophil markers granule major basic protein, prostaglandin-endoperoxide synthase 1 and arachidonate 15-lipoxygenase. Expression profiling was performed using EoL-1 cells that had been treated with dexamethasone for 0, 4, 12, 24 and 48h identifying 162 genes as differentially expressed. Two of the most highly upregulated genes based on expression profiling were the transcription factor Ets-2 and the MHC Class II genes (Q, R, and P). Expression of these genes in EoL-1 cells was shown to be dexamethasone-induced at the RNA and protein levels which is consistent with the known function of Ets-2 in controlling cell cycle progression and the role of MHC Class II antigens in mediating eosinophil functions.

Molecular interactions between glucocorticoids and long-acting beta2-agonists

beta(2)-Adrenergic receptor agonists and glucocorticoids are the two most effective treatments for asthma, and used in combination they are more effective than either alone. Glucocorticoids mediate their anti-inflammatory effects through the action of activated glucocorticoid receptors (GRs), with the level of activity being related to the number of nuclear receptors. Glucocorticoids can upregulate the synthesis of several genes in human lung cells through interaction with specific DNA binding regions (glucocorticoid response elements) within the promoter region of glucocorticoid-responsive genes. Many of the down-regulating effects of GRs on the synthesis of cytokines and other inflammatory mediators are due to repression of other transcription factors, such as activator protein-1 and nuclear factor kappaB. GR functions such as nuclear localization and gene activation can be regulated by phosphorylation status. Long-acting beta(2)-agonists may affect GR nuclear localization through modulation of GR phosphorylation and furthermore through priming of GR functions within the nucleus by modifying GR or GR-associated protein phosphorylation. Glucocorticoids in turn may regulate beta(2)-adrenergic receptor function by increasing its expression, acting through glucocorticoid response elements, and, importantly, by restoring G-protein-beta(2)-receptor coupling and inhibiting beta(2)-receptor downregulation, thereby preventing desensitization.

Glucocorticoid receptors in hippocampal neurons that do not engage proteasomes escape from hormone-dependent down-regulation but maintain transactivation activity

The glucocorticoid receptor (GR) protein is subjected to hormone-dependent down-regulation in most cells and tissues. This reduction in receptor levels that accompanies chronic hormone exposure serves to limit hormone responsiveness and operates at transcriptional, posttranscriptional, and posttranslational levels. The ability of glucocorticoid hormones to trigger GR down-regulation may be not universal, particularly in mature and developing neurons in which conflicting results regarding hormone control of GR protein have been reported. We find that endogenous GR is not down-regulated in the HT22 mouse hippocampal cell line and in primary hippocampal neurons derived from embryonic rats. Because GR has the capacity to be ubiquitylated in HT22 cells, receptor down-regulation must be limited by defects in either targeting of polyubiquitylated receptor to the proteasome or processing of the targeted receptor by the proteasome. Despite the lack of GR down-regulation in the HT22 cells, glucocorticoid-induced transcription from transiently transfected templates is attenuated upon prolonged hormone treatment. This termination of GR transactivation is not due to inefficient nuclear import or nuclear retention of the receptor. Furthermore, GR efficiently exports from HT22 cell nuclei in hormone-withdrawn cells, indicating that the receptor has access to both nuclear and cytoplasmic degradation pathways. Our results suggest that appropriate maturation of proteasomal degradative or targeting activities may be required, particularly in hippocampal neurons, for hormone-dependent down-regulation of GR.

Androgen receptor phosphorylation. Regulation and identification of the phosphorylation sites

Activation of signal transduction kinase cascades has been shown to alter androgen receptor (AR) activity. Although it has been suggested that changes in AR phosphorylation might be directly responsible, the basal and regulated phosphorylations of the AR have not been fully determined. We have identified the major sites of AR phosphorylation on ARs expressed in COS-1 cells using a combination of peptide mapping, Edman degradation, and mass spectrometry. We describe the identification of seven AR phosphorylation sites, show that the phosphopeptides seen with exogenously expressed ARs are highly similar to those seen with endogenous ARs in LNCaP cells and show that specific agonists differentially regulate the phosphorylation state of endogenous ARs in LNCaP prostate cancer cells. Treatment of LNCaP cells with the synthetic androgen, R1881, elevates phosphorylation of serines 16, 81, 256, 308, 424, and 650. Ser-94 appears constitutively phosphorylated. Forskolin, epidermal growth factor, and phorbol 12-myristate 13-acetate increase the phosphorylation of Ser-650. The kinetics of phosphorylation of most sites in response to hormone or forskolin is temporally delayed, reaching a maximum at 2 h post-stimulation. The exception is Ser-81, which continues to display increasing phosphorylation at 6 h. These data provide a basis for analyzing mechanisms of cross-talk between growth factor signaling and androgen in prostate development, physiology, and cancer.

The glucocorticoid receptor: coding a diversity of proteins and responses through a single gene

The ability of natural and synthetic glucocorticoids to elicit numerous and diverse physiological responses is remarkable. How the product of a single gene can participate in such a myriad of cell- and tissue-specific pathways has remained largely unknown. The last several years have seen increased description of glucocorticoid receptor (GR) protein isoforms. Here we review the current state of knowledge regarding naturally occurring GR isoforms and discuss how this array of receptor species generates the diversity associated with the glucocorticoid response. We propose that the multiplicity of receptor forms have unique tissue- specific actions on the downstream biology providing a mechanism to create GR signaling networks.

Activity of the GR in G2 and mitosis

To elucidate the mechanisms mediating the reported transient physiological glucocorticoid resistance in G2/M cell cycle phase, we sought to establish a model system of glucocorticoid-resistant cells in G2. We synchronized various cell lines in G2 to measure dexamethasone (DEX)-induced transactivation of either two endogenous promoters (rat tyrosine aminotransferase and mouse metallothionein I) or the mouse mammary tumor virus (MMTV) promoter stably or transiently transfected. To circumvent the need for synchronization drugs, we stably transfected an MMTV-driven green fluorescent protein to directly correlate DEX-induced transactivation with the cell cycle position for each cell of an asynchronous population using flow cytometry. Surprisingly, all promoters tested were DEX-inducible in G2. Even in mitotic cells, only the stably transfected MMTV promoter was repressed, whereas the same promoter transiently transfected was inducible. The use of Hoechst 33342 for synchronization in previous studies probably caused a misinterpretation, because we detected interference of this drug with GR-dependent transcription independent of the cell cycle. Finally, GR activated a simple promoter in G2, excluding a functional effect of cell cycle-dependent phosphorylation of GR, as implied previously. We conclude that GR itself is fully functional throughout the entire cell cycle, but GR responsiveness is repressed in mitosis due to chromatin condensation rather than to specific modification of GR.

The expression of MHC class II genes in macrophages is cell cycle dependent

Using different drugs, we stopped the cell cycle of bone marrow-derived macrophages at different points. After IFN-gamma stimulation, macrophages arrested at the G(1) phase of the cell cycle did not increase cell surface expression of the MHC class II IA. This inhibition is specific, because, under the same conditions, IFN-gamma induces the expression of Fcgamma receptors and the inducible NO synthase mRNA. Treatments that inhibit macrophage proliferation by blocking the cell cycle at the G(1) phase, such as adenosine, forskolin, or LPS, blocked the IFN-gamma induction of IA. Under IFN-gamma treatment, the steady-state levels of IAalpha and IAss mRNA did not increase in cells arrested at the G(1) phase and the half-life of the MHC mRNA was not modified. These data suggest that the cell cycle modulation of IFN-gamma-induced MHC II gene expression occurs at the transcriptional level. The expression of the class II transactivator mRNA induced by IFN-gamma was also blocked when macrophages were arrested at the G(1) phase of the cell cycle, suggesting that the lack of IFN-gamma response occurs at the early steps of MHC class II expression. Finally, macrophages arrested at the G(1) phase showed increased basal levels of cell surface IA due to an increase of the translational efficiency. These data show that the expression of MHC class II genes is regulated by the cell cycle.

Hormone-dependent translocation of vitamin D receptors is linked to transactivation

Vitamin D receptor (VDR) acts as a transcription factor mediating genomic actions of calcitriol. Our earlier studies suggested that calcitriol induces translocation of cytoplasmic VDR, but the physiologic relevance of this finding remained uncertain. Previous studies demonstrated that the activation function 2 domain (AF-2) plays an essential role in VDR transactivation. To elucidate hormone-dependent VDR translocation and its role, we constructed green fluorescent protein (GFP) chimeras with full-length VDR (VDR-GFP), AF-2-truncated VDR (AF-2del-VDR-GFP), and ligand-binding domain (LBD)-truncated VDR (LBDdel-VDR-GFP). COS-7 cells were transiently transfected with these constructs. Western blot analysis, fluorescent microscopy, and transactivation assays showed that the generated chimeras are expressed and fluoresce and that VDR-GFP is transcriptionally active. After hormone treatment, cytoplasmic VDR-GFP translocated to the nucleus in a concentration-, time-, temperature-, and analog-specific manner. Hormone dose-response relationships for translocation and for transactivation were similar. Truncation of LBD and truncation of AF-2 each abolished hormone-dependent translocation and transactivation. Our data confirm a hormone-dependent VDR translocation, demonstrate that an intact AF-2 domain is required for this translocation, and indicate that translocation is part of the receptor activation process.

Phosphorylation of human estrogen receptor alpha by protein kinase A regulates dimerization

Phosphorylation provides an important mechanism by which transcription factor activity is regulated. Estrogen receptor alpha (ERalpha) is phosphorylated on multiple sites, and stimulation of a number of growth factor receptors and/or protein kinases leads to ligand-independent and/or synergistic increase in transcriptional activation by ERalpha in the presence of estrogen. Here we show that ERalpha is phosphorylated by protein kinase A (PKA) on serine-236 within the DNA binding domain. Mutation of serine-236 to glutamic acid prevents DNA binding by inhibiting dimerization by ERalpha, whereas mutation to alanine has little effect on DNA binding or dimerization. Furthermore, PKA overexpression or activation of endogenous PKA inhibits dimerization in the absence of ligand. This inhibition is overcome by the addition of 17beta-estradiol or the partial agonist 4-hydroxy tamoxifen. Interestingly, treatment with the complete antagonist ICI 182,780 does not overcome the inhibitory effect of PKA activation. Our results indicate that in the absence of ligand ERalpha forms dimers through interaction between DNA binding domains and that dimerization mediated by the ligand binding domain only occurs upon ligand binding but that the complete antagonist ICI 182,780 prevents dimerization through the ligand-binding domain. Heterodimer formation between ERalpha and ERbeta is similarly affected by PKA phosphorylation of serine 236 of ERalpha. However, 4-hydroxytamoxifen is unable to overcome inhibition of dimerization by PKA. Thus, phosphorylation of ERalpha in the DNA binding domain provides a mechanism by which dimerization and thereby DNA binding by the estrogen receptor is regulated.

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

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

Nucleocytoplasmic trafficking of steroid-free glucocorticoid receptor

Glucocorticoid receptor (GR) recycles between an inactive form complexed with heat shock proteins (hsps) and localized to the cytoplasm and a free liganded form that regulates specific gene transcription in the nucleus. We report here that, contrary to previous assumptions, association of GR into hsp-containing complexes is not sufficient to prevent the shuttling or trafficking of the GR across the nuclear membrane. Following the withdrawal of treatment with cortisol or the hormone antagonist RU486, GRs recycled rapidly into hsp-associated, hormone-responsive complexes. However, cortisol-withdrawn receptors redistributed to the cytoplasm very slowly (t(1)/(2) = 8-9 h) and RU486-withdrawn receptors not at all. Persistent localization of these GRs to the nucleus was not due to a gross defect in export, since in both instances the complexed nuclear GRs transferred efficiently between heterokaryon nuclei. Moreover, the addition of a nuclear retention signal to the N terminus of GR induced the transfer of naive receptor to the nucleus in the absence of steroid. These results suggest that the localization of GR to the cytoplasm is determined by fine control of the rates of transfer of GR across the nuclear membrane and/or by active retention that occurs independently from the association of GR with hsps.

Airway smooth muscle as a target of glucocorticoid action in the treatment of asthma

Glucocorticoids are highly effective in the control of asthma and suppression of airway inflammation. The cellular and molecular mechanisms involved in the anti-inflammatory actions of glucocorticoids are becoming clearer. Although it is apparent that glucocorticoids have effects on many aspects of inflammation, it is not certain which actions on which cell types are the most critical in controlling asthma. Airway smooth muscle cells represent a significant proportion of all cells present in the airways and might therefore be expected to be a prominent cellular target for inhaled steroids. Despite this, little is known of the action of glucocorticoids on airway smooth muscle. It is becoming clear that in addition to its contractile properties, airway smooth muscle can potentially contribute to the pathogenesis of asthma by increased proliferation and by expression and secretion of pro-inflammatory cytokines and mediators, which in turn may lead to the activation and recruitment of key inflammatory cells in the airways. This review examines the action of glucocorticoids on some of the diverse functions of airway smooth muscle that are implicated in remodeling of the airways in asthma. Glucocorticoids either directly or indirectly modulate contraction of airway smooth muscle by suppressing agonist-induced increases in intracellular calcium levels or by downregulating or uncoupling receptors linked to contraction (e.g., muscarinic M2 or M3, histamine H1 receptors). In addition, glucocorticoids may augment relaxation of airway smooth muscle by increasing activation of either cyclic AMP-dependent (e.g., increased expression of beta2-adrenoceptors, reduced homologous desensitization of beta2-adrenoceptors) or AMP-independent mechanisms (e.g., increased Na+/K+ electrogenic pump activity). In addition to their effects on contraction, glucocorticoids are also effective antiproliferative agents in airway smooth muscle, but under some circumstances may also contribute to proliferation by inhibiting the antiproliferative effect of high concentrations of tumor necrosis factor alpha in these cells. Glucocorticoids also suppress induction of cyclooxygenase-2 in human airway smooth muscle cells and the subsequent synthesis and release of arachidonic acid metabolites, particularly prostaglandin E2. The potential of airway smooth muscle to recruit and activate pro-inflammatory cells such as the eosinophil may also be reduced by glucocorticoids, as they are effective in preventing the release of several cytokines (e.g., RANTES, interleukin-8, and granulocyte macrophage colony-stimulating factor). The possibility exists that as we begin to understand and speculate more about the likely role of airway smooth muscle in the pathogenesis of asthma, it may be necessary to reconsider airway smooth muscle as an important cellular target for the action of glucocorticoids in the treatment of asthma.

Glucocorticoid receptor phosphorylation: overview, function and cell cycle-dependence

All steroid hormone receptors are phosphorylated and undergo hormone-induced hyperphosphorylation. Most phosphorylated residues identified so far are serines in the N-terminal domain. Other residues and domains may also be phosphorylated, e.g. the estrogen receptor is phosphorylated on tyrosine in the hormone-binding domain. Many sites lie in consensus sequences for proline-directed, cell cycle-associated kinases. In some receptors hyperphosphorylation is induced by hormone antagonists as well as agonists, and leads to new phosphorylated sites. With glucocorticoid receptors, hyperphosphorylation is specific for glucocorticoid agonists, follows receptor activation and produces no new sites. Rate studies suggest that hyperphosphorylation is due to accelerated phosphorylation rather than delayed dephosphorylation. Evidence to date indicates that steroid hormone receptor phosphorylation serves not as an on-off switch but modulates function more subtly. Mutations of phosphorylated sites to alanine have been found to decrease activity by 0 to 90%, depending on mutated site, cell type, reporter gene and hormone concentration. With glucocorticoid receptors, some alanine mutants are up to 75% less active in hormone-induced transactivation of certain reporter genes. They are also inactive in hormone-induced repression of transcription of their own gene and down regulation of the receptor protein. Furthermore, they are much less sensitive to degradation. Both basal phosphorylation and hormone-dependent hyperphosphorylation of these receptors are cell cycle-dependent, basal phosphorylation being low in S phase and high in G2/M and hyperphosphorylation the reverse, suggesting a causal relation to the cell cycle-dependence of glucocorticoid activity reported with several cell lines. Hyperphosphorylation appears to be regulated by basal phosphorylation through negative charge in the N-terminal domain, which in S phase is relatively low and permits hyperphosphorylation, but in G2/M is relatively high and blocks hyperphosphorylation.

Immunohistochemical localization of ecdysteroid receptor and ultraspiracle in the epithelial cell line from Chironomus tentans (Insecta, Diptera)

Ecdysteroid receptor (EcR) and its heterodimerization partner, ultraspiracle (USP), were demonstrated in the epithelial cell line from Chironomus tentans by immunohistochemistry. In untreated cells both proteins are present in nuclei as well as in granular compartments of the cytosol. At 1 day after addition of 1-microM 20-OH-ecdysone (20E) total immunofluorescence had increased in the nuclei, whereas the cytoplasmic staining had disappeared. At the 2nd and 3rd days all cells within a vesicle appear identical according to morphological criteria, but the EcR and USP immunoreactivity becomes restricted into patches of neighbouring cells. The hormonally induced changes in the pattern of localization of functional ecdysteroid receptor, the heterodimer of EcR and USP, are discussed in relation to similar effects of 20E on acetylcholinesterase and muscarinic acetylcholine receptor distribution in this cell line.

Regulation of the glucocorticoid receptor gene by the AP-1 transcription factor

The glucocorticoid receptor (GR) is a ligand-activated nuclear transcription factor, and AP- 1 (Fos/Jun or Jun/Jun) is a transcription factor whose components are nuclear proteins encoded by c-fos and c-jun protooncogenes. Serum stimulation of serum-starved NIH 3T3 cells resulted in an approx 188-fold induction of c-fos mRNA at 30 min and an approximately ninefold induction of c-jun mRNA at 1 h, followed by an increase in GR mRNA levels at 3-12 hour (twofold). Sequential induction of cFos, cJun, and GR protein levels also occurred. Overexpression of the cFos protein in NIH 3T3 cells (NIH 3T3 [cFos 3] and NIH 3T3 [cFos 10]) caused an increase in the endogenous GR protein. Previous and present studies showed that a putative AP-1 site within the GR promoter binds AP-1 proteins (both Jun and Fos family members). To address the molecular mechanism involved in transcriptional activation of the GR gene, we investigated the relevance of AP-1 binding complexes in this activation and in overall regulation of GR gene transcription. Transient transfection with a full length GR promoter linked to a luciferase gene into both NIH 3T3 (cFos 3) and NIH 3T3 (cFos 10) cells gave rise to an induction of luciferase activity. This induction was abolished following mutation or deletion of the GR AP-1 site from the promoter. These findings suggest that cFos is responsible for the induction of GR expression in serum-stimulated NIH 3T3 cells, and serum growth factors may stimulate GR transcription by a cFos-dependent mechanism at the putative AP-1 site. These studies support a role for the AP-1 transcription factor in regulating GR gene expression.

Expression and subcellular distribution of the beta-isoform of the human glucocorticoid receptor

Alternative splicing of the human glucocorticoid receptor (hGR) primary transcript produces two highly homologous protein isoforms, termed hGR alpha and hGRbeta, that differ at their carboxy-termini. In contrast to the well characterized hGR alpha isoform, which modulates gene expression in a hormone-dependent fashion, the biological significance of hGRbeta has only recently begun to emerge. We and others have shown that the hGRbeta messenger RNA transcript is widely expressed in human tissues and that the hGRbeta protein functions as a dominant negative inhibitor of hGR alpha in transfected cells. Unfortunately, these initial studies did not determine whether the hGRbeta protein was made in vivo. Such analyses are hindered because available anti-hGR antibodies cannot discriminate between the similarly sized hGR alpha and hGRbeta proteins. Therefore, to investigate the expression of the hGRbeta protein, we have produced an antipeptide, hGRbeta-specific antibody termed BShGR. This antibody was made against the unique 15-amino acid peptide at the carboxy-terminus of hGRbeta and recognizes both the native and denatured conformations of hGRbeta, but does not cross-react with hGR alpha. Using BShGR on Western blots and in immunoprecipitation experiments, we detected the hGRbeta protein in a variety of human cell lines and tissues. Immunocytochemistry was then performed with BShGR on HeLa S3 and CEM-C7 cells and on tissue sections prepared from lung, thymus, and liver to assess the cellular and subcellular distribution of hGRbeta. In all immunopositive cells, hGRbeta was found in the nucleus independent of glucocorticoid treatment. Within tissues, the hGRbeta protein was expressed most abundantly in the epithelial cells lining the terminal bronchiole of the lung, forming the outer layer of Hassall's corpuscle in the thymus, and lining the bile duct in the liver. As a potential in vivo inhibitor of hGR alpha activity, expression of hGRbeta may be an important factor regulating target cell responsiveness to glucocorticoids.

Differential effects of agonist and antagonists on autoregulation of glucocorticoid receptors in a rat colonic adenocarcinoma cell line

The relative abilities of a potent glucocorticoid receptor (GR) agonist (RU 28362), a weak GR agonist (aldosterone) and a potent GR antagonist (RU 38486) to promote in vivo activation/transformation and subsequent down-regulation of GR mRNA and protein levels were compared using the PROb rat colonic adenocarcinoma cell line. In vivo activation, which is followed immediately by nuclear translocation, by these ligands (1 microM) was evaluated in terms of their abilities to deplete cytoplasmic GR protein levels after a 30 min incubation period. Western blotting experiments with the anti-GR monoclonal antibody BuGR2 demonstrated that a brief incubation with RU 28362 resulted in nearly complete depletion of cytoplasmic GR, whereas incubation with aldosterone resulted in a 50% depletion of the cytoplasmic GR protein. Incubation with RU 38486 was even more effective than aldosterone in promoting this key step in the GR pathway. Prolonged treatment (18 h) with RU 28362 resulted in significant down-regulation of GR mRNA and total cellular GR protein levels. Similar incubation with aldosterone resulted in a transient decrease in the GR mRNA level and also down-regulated the total GR protein level. Although prolonged incubation with RU 38486 did not result in detectable down-regulation of the GR mRNA level, this antagonist very effectively down-regulated total cellular GR protein levels. Taken collectively, these data demonstrate that agonists capable of promoting in vivo activation (and subsequent nuclear translocation) of GR are also effective at down-regulating GR at both the mRNA and protein levels. Although the antagonist RU 38486 is also capable of down-regulating GR protein levels by shortening the half-life of the receptor, it appears to be incapable of altering the rate of transcription of the GR gene. Glucocorticoid target tissue sensitivity may thus be decreased via two independent mechanisms: agonist-induced repression of GR gene transcription; and/or ligand-induced degradation of total cellular GR protein levels.

Sequence-specific DNA binding and transcription factor phosphorylation by Ku Autoantigen/DNA-dependent protein kinase. Phosphorylation of Ser-527 of the rat glucocorticoid receptor

NRE1 is a DNA sequence element through which Ku antigen/DNA-dependent protein kinase (DNA-PK) catalytic subunit represses the induction of mouse mammary tumor virus transcription by glucocorticoids. Although Ku is an avid binder of DNA ends and has the ability to translocate along DNA, we report that direct sequence-specific Ku binding occurs with higher affinity (Kd = 0.84 +/- 0.24 nM) than DNA end binding. Comparison of Ku binding to several sequences over which Ku can accumulate revealed two classes of sequence. Sequences with similarity to NRE1 competed efficiently for NRE1 binding. Conversely, sequences lacking similarity to NRE1 competed poorly for Ku and were not recognized in the absence of DNA ends. Phosphorylation of glucocorticoid receptor (GR) fusion proteins by DNA-PK reflected Ku DNA-binding preferences and demonstrated that co-localization of GR with DNA-PK on DNA in cis was critical for efficient phosphorylation. Phosphorylation of the GR fusion protein by DNA-PK mapped to a single site, Ser-527. This site occurs adjacent the GR nuclear localization sequence between the DNA and ligand binding domains of GR, and thus its phosphorylation, if confirmed, has the potential to affect receptor function in vivo.

Steroid hormone receptors and their regulation by phosphorylation

The steroid/thyroid hormone receptor superfamily of ligand-activated transcription factors encompasses not only the receptors for steroids, thyroid hormone, retinoids and vitamin D, but also a large number of proteins whose functions and/or ligands are unknown and which are thus termed orphan receptors. Recent studies have highlighted the importance of phosphorylation in receptor function. Although most of the phosphorylation sites are serine and threonine residues, a few of the family members are also phosphorylated on tyrosine. Those steroid receptor family members that are bound to heat-shock proteins in the absence of ligand typically are basally phosphorylated and exhibit increases in phosphorylation upon ligand binding. Most of these sites contain Ser-Pro motifs, and there is evidence that cyclin-dependent kinases and MAP kinases (mitogen-activated protein kinases) phosphorylate subsets of these sites. In contrast, phosphorylation sites identified thus far in members of the family that bind to DNA in the absence of hormone typically do not contain Ser-Pro motifs and are frequently casein kinase II or protein kinase A sites. Phosphorylation has been implicated in DNA binding, transcriptional activation and stability of the receptors. The finding that some of the steroid receptor family members can be activated in the absence of ligand by growth factors or neurotransmitters that modulate kinase and/or phosphatase pathways underscores the role of phosphorylation in receptor function. Hence this family of transcription factors integrates signals from ligands as well as from signal transduction pathways, resulting in alterations in mRNA and protein expression that are unique to the complex signals received.