Selective regulation of bone cell apoptosis by translational isoforms of the glucocorticoid receptor

Emerging roles of glucocorticoid receptor phosphorylation in modulating glucocorticoid hormone action in health and disease

Glucocorticoids (GCs) are hormones naturally released when the body perceives stress and function to return homeostatic balance within various tissues. Synthetic GCs are widely prescribed therapeutics for the treatment of numerous inflammatory disorders and cancers. The effects of GCs are mediated by their binding and activation of the GC receptor (GR), a transcription factor that is subject to hormone-dependent and -independent phosphorylation on several serine and threonine residues. The GR is phosphorylated by kinases such as MAPKs, CDKs, and GSK-3beta, and these modifications modulate the transcriptional activity of the GR within cells. Here, we discuss the phosphorylation status of the GR as a mechanism to dictate how cells will ultimately respond to GCs. In doing so, we will review current knowledge about each phosphorylated residue within the GR and their contributions to modulating GC signaling in normal homeostatic physiology and during the progression of disease.

Mechanisms generating diversity in glucocorticoid receptor signaling

Glucocorticoids regulate diverse biological processes throughout the body via the glucocorticoid receptor (GR). Ligand-bound GR translocates into the nucleus and can elicit changes in gene expression by direct contact with the DNA or by protein-protein interactions with other transcription factors. The GR can also mediate rapid nongenomic signaling events initiated in the cytoplasm. In this chapter, we review the biological and physiological implications of glucocorticoids, the GR, and many of the signal transduction mechanisms that mediate their action.

Crosstalk in inflammation: the interplay of glucocorticoid receptor-based mechanisms and kinases and phosphatases

Glucocorticoids (GCs) are steroidal ligands for the GC receptor (GR), which can function as a ligand-activated transcription factor. These steroidal ligands and derivatives thereof are the first line of treatment in a vast array of inflammatory diseases. However, due to the general surge of side effects associated with long-term use of GCs and the potential problem of GC resistance in some patients, the scientific world continues to search for a better understanding of the GC-mediated antiinflammatory mechanisms. The reversible phosphomodification of various mediators in the inflammatory process plays a key role in modulating and fine-tuning the sensitivity, longevity, and intensity of the inflammatory response. As such, the antiinflammatory GCs can modulate the activity and/or expression of various kinases and phosphatases, thus affecting the signaling efficacy toward the propagation of proinflammatory gene expression and proinflammatory gene mRNA stability. Conversely, phosphorylation of GR can affect GR ligand- and DNA-binding affinity, mobility, and cofactor recruitment, culminating in altered transactivation and transrepression capabilities of GR, and consequently leading to a modified antiinflammatory potential. Recently, new roles for kinases and phosphatases have been described in GR-based antiinflammatory mechanisms. Moreover, kinase inhibitors have become increasingly important as antiinflammatory tools, not only for research but also for therapeutic purposes. In light of these developments, we aim to illuminate the integrated interplay between GR signaling and its correlating kinases and phosphatases in the context of the clinically important combat of inflammation, giving attention to implications on GC-mediated side effects and therapy resistance.

Glucocorticoids and their actions in cells

The biologic responses to the administration of exogenous glucocorticoids (GCs) appear to be specific to cell type. As a result, progress in understanding how to improve the benefit-to-risk ratio of GC therapies for the eye will depend on better characterization of both the receptors and the proteins induced when these receptors are stimulated. The complexity and diversity of the GC receptors in human tissue is underscored by evidence that up to 6,000 genes are expressed or suppressed within hours of GC exposure. The enormous potential to use exogenous GC agents to downregulate processes involved in age-related macular degeneration must be balanced against a similar potential for counterproductive effects. Recent progress predicts an increasing precision with which GC steroids can be used to influence biologic functions.

MicroRNA 18 and 124a down-regulate the glucocorticoid receptor: implications for glucocorticoid responsiveness in the brain

Glucocorticoids (GCs) exert profound effects on a variety of physiological processes, including adaptation to stress, metabolism, immunity, and neuronal development. Cellular responsiveness to GCs depends on numerous factors, including the amount of the glucocorticoid receptor (GR) protein. We tested the hypothesis that micro-RNAs (miRs), a recently discovered group of noncoding RNAs involved in mRNA translation, might control GR activity by reducing GR protein levels in neuronal tissues. We tested a panel of five miRs consisting of 124aa, 328, 524, 22, and 18. We found that miRs 18 and 124a reduced GR-mediated events in addition to decreasing GR protein levels. miR reporter assays revealed binding of miR-124a to the 3' untranslated region of GR. In correspondence, the activation of the GR-responsive gene glucocorticoid-induced leucine zipper was strongly impaired by miR-124a and -18 overexpression. Although miR-18 is expressed widely throughout the body, expression of miR-124a is restricted to the brain. Endogenous miR-124a up-regulation during neuronal differentiation of P19 cells was associated with a decreasing amount of GR protein levels and reduced activity of luciferase reporter constructs bearing GR 3' untranslated regions. Furthermore, we show that miR-124a expression varies over time during the stress hyporesponsive period, a neonatal period when GC signaling is modulated. Our findings demonstrate a potential role for miRs in the regulation of cell type-specific responsiveness to GCs, as may occur during critical periods of neuronal development. Ultimately, our results may provide a better understanding of the etiology of stress-related diseases as well as the efficacy of GC therapy.

Molecular mechanisms regulating glucocorticoid sensitivity and resistance

Glucocorticoid receptor agonists are mainstays in the treatment of various malignancies of hematological origin. Glucocorticoids are included in therapeutic regimens for their ability to stimulate intracellular signal transduction cascades that culminate in alterations in the rate of transcription of genes involved in cell cycle progression and programmed cell death. Unfortunately, subpopulations of patients undergoing systemic glucocorticoid therapy for these diseases are or become insensitive to glucocorticoid-induced cell death, a phenomenon recognized as glucocorticoid resistance. Multiple factors contributing to glucocorticoid resistance have been identified. Here we summarize several of these mechanisms and describe the processes involved in generating a host of glucocorticoid receptor isoforms from one gene. The potential role of glucocorticoid receptor isoforms in determining cellular responsiveness to glucocorticoids is emphasized.

Critical illness-related corticosteroid insufficiency

The diagnosis of adrenal failure and the indications for corticosteroid therapy in critically ill patients are controversial. This controversy is fueled by the complexity of the issues and the paucity of data from high quality clinical trials. Nevertheless, while the use of high-dose corticosteroids in patients with severe sepsis and ARDS failed to improve outcome and was associated with increased complications, an extended course of stress-dose corticosteroids has been reported to increase the occurrence of ventilator-free days and survival in select groups of ICU patients. These patients typically have an exaggerated proinflammatory response. Until recently the exaggerated proinflammatory response that characterizes critically ill patients with systemic inflammation has focused on suppression of the hypothalamic-pituitary-adrenal axis and adrenal failure. However, experimental and clinical data suggest that glucocorticoid tissue resistance may also play an important role. This complex syndrome is referred to as critical illness-related corticosteroid insufficiency (CIRCI) and is defined as inadequate corticosteroid activity for the severity of the illness of a patient. The paper reviews cortisol physiology, CIRCI, and the role of corticosteroid therapy in critically ill patients.

Tissue-specific glucocorticoid action: a family affair

Glucocorticoids exert a wide variety of physiological and pathological responses, most of which are mediated by the ubiquitously expressed glucocorticoid receptor (GR). The glucocorticoid response varies among individuals, as well as within tissues from the same individual, and this phenomenon can be partially explained through understanding the process of generating bioavailable ligand and the molecular heterogeneity of GR. This review focuses on the recent advances in our understanding of prereceptor ligand metabolism, GR subtypes and GR polymorphisms. Furthermore, we evaluate the impact of tissue- and individual-specific diversity in the glucocorticoid pathway on human health and disease.

Glycogen synthase kinase 3beta-mediated serine phosphorylation of the human glucocorticoid receptor redirects gene expression profiles

Aberrant glycogen synthase kinase 3beta (GSK-3beta) activity is associated with the progression of several pathological conditions such as diabetes, Alzheimer's, and cancer. GSK-3beta regulates cellular processes by directly phosphorylating metabolic enzymes and transcription factors. Here, we discovered a new target for GSK-3beta phosphorylation: the human glucocorticoid receptor (GR). Glucocorticoid signaling is essential for life and regulates diverse biological functions from cell growth to metabolism to apoptosis. Specifically, we found hormone-dependent GR phosphorylation on serine 404 by GSK-3beta. Cells expressing a GR that is incapable of GSK-3beta phosphorylation had a redirection of the global transcriptional response to hormone, including the activation of additional signaling pathways, in part due to the altered ability of unphosphorylatable GR to recruit transcriptional cofactors CBP/p300 and the p65 (RelA) subunit of NF-kappaB. Furthermore, GSK-3beta-mediated GR phosphorylation inhibited glucocorticoid-dependent NF-kappaB transrepression and attenuated the glucocorticoid-dependent cell death of osteoblasts. Collectively, our results describe a novel convergence point of the GSK-3beta and the GR pathways, resulting in altered hormone-regulated signaling. Our results also provide a mechanism by which GSK-3beta activity can dictate how cells will ultimately respond to glucocorticoids.

Molecular genetic studies of gene identification for osteoporosis

This review comprehensively summarizes the most important and representative molecular genetics studies of gene identification for osteoporosis published up to the end of September 2007. It is intended to constitute a sequential update of our previously published reviews covering the available data up to the end of 2004. Evidence from candidate gene-association studies, genome-wide linkage and association studies, as well as functional genomic studies (including gene-expression microarray and proteomics) on osteogenesis and osteoporosis, are reviewed separately. Studies of transgenic and knockout mice models relevant to osteoporosis are summarized. The major results of all studies are tabulated for comparison and ease of reference. Comments are made on the most notable findings and representative studies for their potential influence and implications on our present understanding of genetics of osteoporosis. The format adopted by this review should be ideal for accommodating future new advances and studies.