Recruitment of the p160 coactivators by the glucocorticoid receptor: dependence on the promoter context and cell type but not hypoxic conditions

Stress molecular signaling in interaction with cognition

Exposure to stressful life events is associated with a high risk of developing psychiatric disorders with a wide variety of symptoms. Cognitive symptoms in stress-related psychiatric disorders can be particularly challenging to understand, both for those experiencing them and for healthcare providers. To gain insights, it is important to capture stress-induced structural, epigenomic, transcriptomic, and proteomic changes in relevant brain regions such as the amygdala, hippocampus, locus coeruleus and prefrontal cortex, resulting in long-lasting alterations in brain function. In this review, we will emphasize a subset of stress molecular mechanisms altering neuroplasticity, neurogenesis, and balance between excitatory and inhibitory neurons. We then discuss how to identify genetic risk factors that may accelerate stress-driven or stress-induced cognitive impairment. Despite the development of new technologies such as single-cell resolution sequencing, our understanding of the molecular effects of stress in the brain remains to be deepened. A better understanding of the diversity of stress effects in different brain regions and cell types is a pre-requisite to open new avenues for mechanism-informed prevention and treatment of stress-related cognitive symptoms.

Bidirectional Crosstalk Between Hypoxia Inducible Factors and Glucocorticoid Signalling in Health and Disease

Glucocorticoid-induced (GC) and hypoxia-induced transcriptional responses play an important role in tissue homeostasis and in the regulation of cellular responses to stress and inflammation. Evidence exists that there is an important crosstalk between both GC and hypoxia effects. Hypoxia is a pathophysiological condition to which cells respond quickly in order to prevent metabolic shutdown and death. The hypoxia inducible factors (HIFs) are the master regulators of oxygen homeostasis and are responsible for the ability of cells to cope with low oxygen levels. Maladaptive responses of HIFs contribute to a variety of pathological conditions including acute mountain sickness (AMS), inflammation and neonatal hypoxia-induced brain injury. Synthetic GCs which are analogous to the naturally occurring steroid hormones (cortisol in humans, corticosterone in rodents), have been used for decades as anti-inflammatory drugs for treating pathological conditions which are linked to hypoxia (i.e. asthma, ischemic injury). In this review, we investigate the crosstalk between the glucocorticoid receptor (GR), and HIFs. We discuss possible mechanisms by which GR and HIF influence one another, in vitro and in vivo, and the therapeutic effects of GCs on HIF-mediated diseases.

Epigenetic Mechanisms of the Glucocorticoid Receptor

The glucocorticoid receptor (GR) has been shown to be important for mediating cellular responses to stress and circulating glucocorticoids. Ligand-dependent transcriptional changes induced by GR are observed across numerous tissues. However, the mechanisms by which GR achieves cell and tissue-specific effects are less clear. Epigenetic mechanisms have been proposed to explain some of these differences as well as some of the lasting, even transgenerational, effects of stress and glucocorticoid action. GR functions in tandem with epigenetic cellular machinery to coordinate transcription and shape chromatin structure. Here, we describe GR interactions with these effectors and how GR acts to reshape the epigenetic landscape in response to the environment.

Glucocorticoid receptor control of transcription: precision and plasticity via allostery

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

A gradient of glucocorticoid sensitivity among helper T cell cytokines

Helper T (Th) cells secret specific cytokines that promote immune responses whereas glucocorticoids limit the extent of immune responses by inhibiting cytokine secretion and other functions of Th cells. However, glucocorticoid resistance develops in subgroups of patients with Th cell-driven diseases such as asthma and Crohn's disease. Recent evidence supports that Th1, Th2, and Th17 cells have distinct glucocorticoid sensitivity. Th1 cells are sensitive to glucocorticoid-induced apoptosis and cytokine suppression while Th2 cells are sensitive to the latter but not the former and Th17 cells are resistant to both. This gradient of glucocorticoid sensitivity of Th cells corresponds to the glucocorticoid sensitivity of the diseases they underlie. We identify the mechanisms contributing to distinct glucocorticoid sensitivity of Th cells and their cytokines in the literature, as this information is useful to improve treatment strategies for glucocorticoid resistant immunological disorders.

Understanding stress-effects in the brain via transcriptional signal transduction pathways

Glucocorticoid hormones exert crucial effects on the brain in relation to physiology, endocrine regulation, mood and cognition. Their two receptor types, glucocorticoid and mineralocorticoid receptors (GR and MR), are members of the nuclear receptor superfamily and act in large measure as transcription factors. The outcome of MR/GR action on the genome depends on interaction with members from different protein families, which are of crucial importance for cross-talk with other neuronal and hormonal signals that impinge on the glucocorticoid sensitive circuitry. Relevant interacting proteins include other transcription factors that may either tether the receptor to the DNA, or that bind in the vicinity of GR and MR to tune the transcriptional response. In addition, transcriptional coregulator proteins constitute the actual signal transduction pathway to the transcription machinery. We review the current evidence for involvement of individual coregulators in GR-dependent effects on stress responses, and learning and memory. We discuss the use of in vitro and in silico tools to predict those coregulators that are of importance for particular brain processes. Finally, we discuss the potential of selective receptor modulators that may only allow a subset of all interactions, thus allowing more selective targeting of glucocorticoid-dependent processes in the brain.

Transcriptome remodeling in hypoxic inflammation

Hypoxia is an integral component of the inflamed tissue microenvironment. Today, the influence of hypoxia on the natural evolution of inflammatory responses is widely accepted; however, many molecular and cellular mechanisms mediating this relationship remain to be clarified. Hypoxic stress affects several independent transcriptional regulators related to inflammation in which HIF-1 and NF-kappaB play central roles. Transcription factors interact with both HATs and HDACs, which are components of large multiprotein co-regulatory complexes. This review summarizes the current knowledge on hypoxia-responsive transcriptional pathways in inflammation and their importance in the etiology of chronic inflammatory diseases, with the primary focus on transcriptional co-regulators and histone modifications in defining gene-specific transcriptional responses in hypoxia, and on the recent progress in the understanding of hypoxia-mediated epigenetic reprogramming. Furthermore, this review discusses the molecular cross-talk between glucocorticoid anti-inflammatory pathways and hypoxia.

Hypoxia down-regulates glucocorticoid receptor alpha and attenuates the anti-inflammatory actions of dexamethasone in human alveolar epithelial A549 cells

AIMS

Glucocorticoids (GCs) are frequently used to treat various pulmonary diseases, which are typically accompanied by hypoxia. Whether hypoxia influences the effects of GCs on human airway cells remains unclear. The aim of the present study was to characterize changes in the expression levels of two isoforms of the glucocorticoid receptor (GR) and to evaluate the anti-inflammatory actions of GCs under hypoxic conditions in A549 cells.

MAIN METHODS

A549 cells were exposed to normoxic or hypoxic conditions for 24, 48 and 72 h. Morphological alterations of cells were captured using a differential interference contrast microscope (DIC), and cell cycle distribution was estimated by flow cytometry. Real-time quantitative polymerase chain reaction and western blot were used to determine the mRNA and protein expression levels of GRalpha and GRbeta. Radioimmunoassay (RIA) for interleukin (IL)-8 was used to assess the anti-inflammatory actions of GCs after cells were stimulated with lipopolysaccharide (LPS).

KEY FINDINGS

After cells were exposed to hypoxic conditions for 48 h, visible morphological alterations in the cells were observed. Cell cycle analysis showed that the number of cells in G1 phase increased significantly under hypoxia compared to the normoxic conditions. Hypoxia caused a time-dependent decrease in both mRNA and protein expression levels for GRalpha, but not GRbeta. Furthermore, when exposed to hypoxia for 48 h, the inhibitory effects of dexamethasone on LPS-stimulated IL-8 release were attenuated.

SIGNIFICANCE

These results indicate that hypoxia impairs the anti-inflammatory actions of GCs in A549 cells, which could be attributed to down-regulation of GRalpha expression under hypoxic conditions.

Modulation of transcription parameters in glucocorticoid receptor-mediated repression

Glucocorticoid receptors (GRs) affect both gene induction and gene repression. The disparities of receptor binding to DNA and increased vs. decreased gene expression have suggested significant mechanistic differences between GR-mediated induction and repression. Numerous transcription factors are known to modulate three parameters of gene induction: the total activity (Vmax) and position of the dose-response curve with glucocorticoids (EC50) and the percent partial agonist activity with antiglucocorticoids. We have examined the effects on GR-mediated repression of five modulators (coactivators TIF2 [GRIP1, SRC-2] and SRC-1, corepressor SMRT, and comodulators STAMP and Ubc9), a glucocorticoid steroid (deacylcortivazol [DAC]) of very different structure, and an inhibitor of histone deacetylation (trichostatin A [TSA]). These factors interact with different domains of GR and thus are sensitive topological probes of GR action. These agents altered the Vmax, EC50, and percent partial agonist activity of endogenous and exogenous repressed genes similarly to that previously observed for GR-regulated gene induction. Collectively, these results suggest that GR-mediated induction and repression share many of the same molecular interactions and that the causes for different levels of gene transcription arise from more distal downstream steps.

Steroid receptor coactivator-1 from brain physically interacts differentially with steroid receptor subtypes

In vitro studies reveal that nuclear receptor coactivators enhance the transcriptional activity of steroid receptors, including estrogen (ER) and progestin receptors (PR), through ligand-dependent interactions. Whereas work from our laboratory and others shows that steroid receptor coactivator-1 (SRC-1) is essential for efficient ER and PR action in brain, very little is known about receptor-coactivator interactions in brain. In the present studies, pull-down assays were used to test the hypotheses that SRC-1 from hypothalamic and hippocampal tissue physically associate with recombinant PR or ER in a ligand-dependent manner. SRC-1, from hypothalamus or hippocampus, interacted with PR-A and PR-B in the presence of an agonist, but not in the absence of ligand or in the presence of a selective PR modulator, RU486. Interestingly, SRC-1 from brain associated more with PR-B, the stronger transcriptional activator, than with PR-A. In addition, SRC-1 from brain, which was confirmed by mass spectrometry, interacted with ERalpha and ERbeta in the presence of agonist but not when unliganded or in the presence of the selective ER modulator, tamoxifen. Furthermore, SRC-1 from hypothalamus, but not hippocampus, interacted more with ERalpha than ERbeta, suggesting distinct expression patterns of other cofactors in these brain regions. These findings suggest that interactions of SRC-1 from brain with PR and ER are dependent on ligand, receptor subtype, and brain region to manifest the pleiotropic functional consequences that underlie steroid-regulated behaviors. The present findings reveal distinct contrasts with previous cell culture studies and emphasize the importance of studying receptor-coactivator interactions using biologically relevant tissue.

Differential modulation of glucocorticoid and progesterone receptor transactivation

The determinants of the different biological activities of progesterone receptors (PRs) vs. glucocorticoid receptors (GRs), which bind to the same DNA sequences, remain poorly understood. The mechanisms by which differential expression of a common target gene can be achieved by PR and GR include unequal agonist steroid concentrations for half maximal induction (EC50) and dissimilar amounts of residual partial agonist activity for antisteroids in addition to the more common changes in total gene induction, or Vmax. Several factors are known to alter some or all of these three parameters for GR-regulated gene induction and some (i.e., the corepressors NCoR and SMRT) modulate the EC50 and partial agonist activity for GR and PR induction of the same gene in opposite directions. The current study demonstrates that other factors known to modulate GR properties (GME, GMEB-2, Ubc9, and STAMP) can also differentially interact with PRs or alter several of the above induction parameters under otherwise identical conditions. These results support the hypothesis that the modulation of EC50, partial agonist activity, and Vmax by a given factor is not limited to one receptor in a specific cell line. Furthermore, the number of factors that unequally modulate PR and GR induction parameters is now greatly expanded, thereby increasing the possible mechanisms for differential gene regulation by PRs vs. GRs.