Increased glucocorticoid receptor beta alters steroid response in glucocorticoid-insensitive asthma

Effects of glucocorticoids on inflammation and arthritis

PURPOSE OF REVIEW

Glucocorticoids are used in the treatment of most rheumatic diseases, and are used chronically in up to 50% of cases of rheumatoid arthritis. The frequency of their use has in the past been incompletely supported by trial evidence of their benefit and by incomplete understanding of their mechanisms of action. The present review will examine significant recent publications relating to glucocorticoids of relevance to rheumatology

RECENT FINDINGS

The basic science of glucocorticoid action has advanced considerably in the review period. Major advances include demonstration of the function of glucocorticoid-induced anti-inflammatory proteins such as mitogen-activated protein kinase phosphatase-1, expanded understanding of alternately spliced isoforms of the glucocorticoid receptor, and major steps in understanding factors regulating glucocorticoid sensitivity in disease. Clinical studies have also had major developments, most strikingly with the publication of two randomized prospective studies on the efficacy of glucocorticoids in preventing erosions in rheumatoid arthritis.

SUMMARY

Despite over 50 years of glucocorticoid use in rheumatology, basic science and clinical studies continue to yield intriguing new data. The quest for therapeutic agents with the beneficial effects of glucocorticoids but lacking their harmful effects is aided by this progress.

Glucocorticoid resistance syndrome: A diagnostic and therapeutic approach

In the past decades, several cases of the syndrome of generalized glucocorticoid (GC) resistance have been reported. This familial disease is characterized by reduced cortisol effects, due to a GC receptor (GR) defect, which is compensated by hyperactivity of the hypothalamic-pituitary-adrenal (HPA) axis. As a consequence, patients present with signs of adrenal overproduction of mineralocorticoids (hypertension and hypokalaemic alkalosis) and, in females, of androgens (hirsutism, male pattern of baldness, menstrual irregularities). In a few kindreds the underlying molecular basis has been revealed--e.g. mutations in the gene coding for the GR--but in a substantial number of patients the cause of GC resistance has not yet been elucidated. In this chapter we also discuss some other determinants which can lead to GC resistance. Diagnosis of generalized GC resistance can be difficult. This review highlights the diagnostic process and therapeutic options for treating patients with this disease.

Dexamethasone inhibition of trabecular meshwork cell phagocytosis and its modulation by glucocorticoid receptor beta

Glucocorticoid treatment can lead to the development of glaucomatous ocular hypertension and a secondary open-angle glaucoma due to increased aqueous humor outflow resistance that is associated with morphological and biochemical changes in the trabecular meshwork (TM). The cellular responses of glucocorticoids are achieved by binding to the glucocorticoid receptor alpha (GRalpha), a ligand-activated transcription factor. An alternatively spliced variant, glucocorticoid receptor beta (GRbeta), has dominant negative activity on GRalpha and has been implicated in a variety of steroid-resistant diseases. We previously showed that GRbeta can block dexamethasone (DEX) responsiveness in TM cells. TM cells are actively phagocytic and function in the removal of debris, pigment and other materials from the aqueous outflow drainage pathway. A decrease in phagocytic activity has been proposed in the pathogenesis of glaucoma and glucocorticoid-induced glaucoma. In this study, we investigated the effect of DEX and GRbeta on phagocytosis in normal and glaucomatous TM cells. Human transformed normal NTM-5 and primary normal NTM174-00 cells, which express relatively high amounts of GRbeta, and transformed glaucomatous GTM-3 and primary glaucomatous GTM520-05 cells, which have lower GRbeta expression, were treated with 100 nM DEX or vehicle control for 24h. NTM cells also were transfected with a control or GRbeta expression plasmid to examine the effect of GRbeta on phagocytic activity. The cells were incubated with Alexa 488 conjugated Staphylococcus aureus bioparticles opsonized with rabbit IgG for 1h, followed by fixation and incubation with Alexa 633 conjugated goat anti-rabbit IgG to distinguish ingested from extracellular bioparticles. DAPI nuclear staining was used to quantify cell numbers. Cells and bioparticles were visualized by confocal microscopy. We found that NTM-5 cells ingested more bioparticles than GTM-3 cells. DEX treatment significantly decreased the phagocytosis of bioparticles in NTM-5 and GTM-3 cells, while GTM-3 cells were more responsive to DEX, compared to NTM-5 cells. In primary cell culture, NTM174-00 also engulfed more bioparticles than GTM520-05 cells. DEX treatment significantly decreased the phagocytic activity in GTM520-05, but not in NTM174-00 cells. Transient transfection of pCMX-hGRbeta plasmid increased the expression of GRbeta and consequently maintained the phagocytotic activity of NTM-5 cells in the presence of DEX. Our data demonstrated that the expression level of GRbeta in TM cells can regulate DEX-induced suppression of phagocytotic activity. The lower expression of GRbeta in glaucomatous TM cells may contribute to the altered phagocytic function of TM cells, and may lead to the increased aqueous humor outflow resistance mediated by glucocorticoids.

Methotrexate regulates the expression of glucocorticoid receptor alpha and beta isoforms in normal human peripheral mononuclear cells and human lymphocyte cell lines in vitro

MTX is an effective therapy for autoimmune-inflammatory diseases. The mechanisms that mediate these actions are not completely clear. It is accepted that many of these effects are mediated through the release of adenosine with the activation of the adenosine receptor A2. MTX is used as a steroid sparing agent. An improved in vitro GC cell sensitivity in GC insensitive asthma patients has been demonstrated after MTX treatment. Most GC actions are mediated by the GCR. The effect of MTX on GCRs expression has not been previously evaluated. Therefore, we evaluate if MTX regulates the expression of glucocorticoid receptors, increasing the expression of the active receptor (GCR alpha) and/or decreasing the expression of the dominant negative receptor (GCR beta). We show that MTX increases the mRNA and protein levels of GCR alpha and decreases or leaves unchanged the protein expression of the GCR beta in CEM cells in culture. This effect was also observed in other lymphocytes (Jurkat and Raji) and in PBMNC from healthy volunteers. We also show that upon MTX treatment PBMC from normal volunteers exhibit a higher sensitivity to DEX inhibition on LPS-induced TNF alpha release. To explore if these actions are mediated by adenosine through the adenosine receptor A2 we evaluate the effect of adenosine on the GCRs expression and the effect of an A2 receptor blocker (DMPX) on MTX effects on GCRs expression. Our results show that adenosine does not mimic and DMPX can enhance MTX effects on these receptors. We conclude that MTX increases the GCR alpha/GCR beta ratio of expression in lymphocytes which could mediate its previously reported effects in improving cell glucocorticoid sensitivity. These actions are not mediated by the adenosine receptor A2.

Modulatory role for retinoid-related orphan receptor alpha in allergen-induced lung inflammation

RATIONALE

Nuclear receptors play a critical role in the regulation of inflammation, thus representing attractive targets for the treatment of asthma.

OBJECTIVE

In this study, we assess the potential regulatory function of retinoid-related orphan receptor alpha (RORalpha) in the adaptive immune response using ovalbumin (OVA)-induced airway inflammation as a model.

METHODS

Allergen-induced inflammation was compared between wild-type (WT) and staggerer (RORalpha(sg/sg)) mice, a natural mutant strain that is deficient in RORalpha expression.

MEASUREMENTS AND MAIN RESULTS

Despite robust increases in OVA-specific IgE, RORalpha(sg/sg) mice developed significantly less pulmonary inflammation, mucous cell hyperplasia, and eosinophilia compared with similarly treated WT animals. Induction of Th2 cytokines, including interleukin (IL)-4, IL-5, and IL-13, was also significantly less in RORalpha(sg/sg) mice. Microarray analysis using lung RNA showed increased expression of many genes, previously implicated in inflammation, in OVA-treated WT mice. These include mucin Muc5b, the chloride channel calcium-activated 3 (Clca3), macrophage inflammatory protein (MIP) 1alpha and 1beta, eotaxin-2, serum amyloid A3 (Saa3), and insulin-like growth factor 1 (Igf1). These genes were induced to a greater extent in OVA-treated WT mice relative to RORalpha(sg/sg) mice.

CONCLUSIONS

Our study demonstrates that mice deficient in RORalpha exhibit an attenuated allergic inflammatory response, indicating that RORalpha plays a critical role in the development of Th2-driven allergic lung inflammation in mice, and suggests that this nuclear receptor should be further evaluated as a potential asthma target.

Structure-function relationship of estrogen receptor alpha and beta: impact on human health

17Beta-estradiol (E2) controls many aspects of human physiology, including development, reproduction and homeostasis, through regulation of the transcriptional activity of its cognate receptors (ERs). The crystal structures of ERs with agonists and antagonists and the use of transgenic animals have revealed much about how hormone binding influences ER conformation(s) and how this conformation(s), in turn, influences the interaction of ERs with co-activators or co-repressors and hence determines ER binding to DNA and cellular outcomes. This information has helped to shed light on the connection between E2 and the development or progression of numerous diseases. Current therapeutic strategy in the treatment of E2-related pathologies relies on the modulation of ER trancriptional activity by anti-estrogens; however, data accumulated during the last five years reveal that ER activities are not only restricted to the nucleus. ERs are very mobile proteins continuously shuttling between protein targets located within various cellular compartments (e.g., membrane, nucleus). This allows E2 to generate different and synergic signal transduction pathways (i.e., non-genomic and genomic) which provide plasticity for cell response to E2. Understanding the structural basis and the molecular mechanisms by which ER transduce E2 signals in target cells will allow to create new pharmacologic therapies aimed at the treatment of a variety of human diseases affecting the cardiovascular system, the reproductive system, the skeletal system, the nervous system, the mammary gland, and many others.

Importin 13 regulates nuclear import of the glucocorticoid receptor in airway epithelial cells

Antiinflammatory effects of glucocorticoids are critical to treatment of airway inflammation in such common disorders as asthma. There is considerable variation in responsiveness to glucocorticoid, and prolonged exposure can result in glucocorticoid resistance. We cloned LGL2, a glucocorticoid-inducible gene in fetal rat lung. We described the characterization of lgl2 as a nuclear transport protein, classified as importin 13 (IPO13), and demonstrated developmental regulation of IPO13 nucleocytoplasmic shuttling. We now report on the identification of the glucocorticoid receptor (GR) as a cargo substrate for IPO13. Binding of GR and IPO13 was demonstrated by GR-GST pulldown and coimmunoprecipitation. To investigate the role of IPO13 in modulating GR signaling in the lung, we studied IPO13-regulated GR transport in airway epithelial cells. Small interfering RNAs that inhibited IPO13 synthesis prevented nuclear translocation of GR. Silencing of IPO13 also abrogated the ability of cortisol to inhibit synthesis of the inflammatory cytokine IL-8 after stimulation with TNF-alpha. Our findings support a role for IPO13 in promoting nuclear occupancy of GR in a way that strongly potentiates the antiinflammatory effects of glucocorticoids. We speculate that variation in cellular levels of IPO13 and intracellular IPO13 shuttling rates may contribute to glucocorticoid resistance.

Update on glucocorticoid action and resistance

Extensive development of inhaled and oral glucocorticoids has resulted in highly potent molecules that have been optimized to target activity to the lung and minimize systemic exposure. These have proved highly effective for most asthmatic subjects, but despite these developments, there are a number of subjects with asthma who fail to respond to even high doses of inhaled or even oral glucocorticoids. Advances in delineating the fundamental mechanisms of glucocorticoid pharmacology, especially the concepts of transactivation and transrepression and cofactor recruitment, have resulted in better understanding of the molecular mechanisms whereby glucocorticoids suppress inflammation. The existence of multiple mechanisms underlying glucocorticoid insensitivity raises the possibility that this might indeed reflect different diseases with a common phenotype, and studies examining the efficacy of potential new agents should be targeted toward subgroups of patients with severe corticosteroid-resistant asthma who clearly require effective new drugs and other approaches to improved asthma control.

Relative corticosteroid insensitivity of peripheral blood mononuclear cells in severe asthma

RATIONALE AND OBJECTIVES

Patients with severe asthma have a poor therapeutic response to corticosteroid therapy, and corticosteroid responsiveness cannot be easily measured in these patients. We hypothesized that this poor response is associated with a reduced effect of corticosteroids to inhibit cytokine release from activated peripheral blood mononuclear cells (PBMCs).

METHODS

Patients with severe asthma were defined by American Thoracic Society criteria. We compared the suppression of LPS-induced cytokine release (monocyte chemotactic protein-1 [MCP-1], macrophage inflammatory protein [MIP] 1alpha, RANTES, tumor necrosis factor alpha, interleukin 1beta (IL-1beta), IL-8, IFN-gamma, IL-6, IL-10, and granulocyte-macrophage colony-stimulating factor [GM-CSF]) by dexamethasone from PBMCs of patients with severe asthma (n = 16), patients with nonsevere asthma (n = 19), and normal volunteers (n = 10).

RESULTS

There was no difference in baseline spontaneous or stimulated release of these cytokines among groups. LPS-induced release of 10 cytokines was less suppressed by dexamethasone (10(-6) M) in patients with severe asthma compared with patients with nonsevere asthma, with statistical significance achieved for IL-1beta (p < 0.03), IL-8 (p < 0.03), and MIP-1alpha (p < 0.003), and borderline significance for IL-6 (p = 0.054). There was less difference between the two groups for dexamethasone at 10(-8) M. Nuclear histone deacetylase (HDAC) and histone acetyltransferase activities were reduced in patients with severe asthma compared with patients with nonsevere asthma (p < 0.01). HDAC activity reduction correlated directly to the degree of steroid insensitivity of GM-CSF (r = 0.57, p < 0.01) and IFN-gamma (r = 0.56, p < 0.05) release. Reduction in histone acetyltransferase activity related to corticosteroid use rather than asthma severity.

CONCLUSIONS

Patients with severe asthma have diminished corticosteroid sensitivity of PBMCs when compared with patients with nonsevere asthma, associated with a reduction in HDAC activity that parallels the impaired corticosteroid sensitivity.