Homeostatic glucocorticoid signaling in airway smooth muscle: A roadmap to asthma pathogenesis.

Homeostasis is the self-regulating process by which the body maintains internal stability within a narrow physiological range (i.e., "normality") as it dynamically adjusts to disruptive influences.

Constitutively active signaling by the G protein βγ-subunit mediates intrinsically increased phosphodiesterase-4 activity in human asthmatic airway smooth muscle cells.

Signaling by the Gβγ subunit of Gi protein, leading to downstream c-Src-induced activation of the Ras/c-Raf1/MEK-ERK1/2 signaling pathway and its upregulation of phosphodiesterase-4 (PDE4) activity, was recently shown to mediate the heightened contractility in proasthmatic sensitized isolated airway smooth muscle (ASM), as well as allergen-induced airway hyperresponsiveness and inflammation in an in vivo animal model of allergic asthma. This study investigated whether cultured human ASM (HASM) cells derived from asthmatic donor lungs exhibit constitutively increased PDE activity that is attributed to intrinsically upregulated Gβγ signaling coupled to c-Src activation of the Ras/MEK/ERK1/2 cascade. We show that, relative to normal cells, asthmatic HASM cells constitutively exhibit markedly increased intrinsic PDE4 activity coupled to heightened Gβγ-regulated phosphorylation of c-Src and ERK1/2, and direct co-localization of the latter with the PDE4D isoform.

Pro-asthmatic cytokines regulate unliganded and ligand-dependent glucocorticoid receptor signaling in airway smooth muscle.

To elucidate the regulation of glucocorticoid receptor (GR) signaling under pro-asthmatic conditions, cultured human airway smooth muscle (HASM) cells were treated with proinflammatory cytokines or GR ligands alone and in combination, and then examined for induced changes in ligand-dependent and -independent GR activation and downstream signaling events. Ligand stimulation with either cortisone or dexamethsone (DEX) acutely elicited GR translocation to the nucleus and, comparably, ligand-independent stimulation either with the Th2 cytokine, IL-13, or the pleiotropic cytokine combination, IL-1β/TNFα, also acutely evoked GR translocation. The latter response was potentiated by combined exposure of cells to GR ligand and cytokine.

IL-13-induced changes in endogenous glucocorticoid metabolism in the lung regulate the proasthmatic response.

Endogenous glucocorticoid (GC) activation is regulated by the intracellular GC-activating and -inactivating enzymes 11β-hydroxysteroid dehydrogenase (11β-HSD)1 and 11β-HSD2, respectively, that catalyze interconversion of inert cortisone and its bioactive metabolite cortisol. Because endogenous GCs are critically implicated in suppressing the asthmatic state, this study examined the roles of the 11β-HSD enzymes in regulating GC activation and bronchoprotection during proasthmatic stimulation. Airway hyperresponsiveness to methacholine and inflammation were assessed in rabbits following inhalation of the proasthmatic/proinflammatory cytokine IL-13 with and without pretreatment with the 11β-HSD inhibitor carbenoxolone (CBX).

G Protein βγ-subunit signaling mediates airway hyperresponsiveness and inflammation in allergic asthma.

Since the Gβγ subunit of Gi protein has been importantly implicated in regulating immune and inflammatory responses, this study investigated the potential role and mechanism of action of Gβγ signaling in regulating the induction of airway hyperresponsiveness (AHR) in a rabbit model of allergic asthma. Relative to non-sensitized animals, OVA-sensitized rabbits challenged with inhaled OVA exhibited AHR, lung inflammation, elevated BAL levels of IL-13, and increased airway phosphodiesterase-4 (PDE4) activity. These proasthmatic responses were suppressed by pretreatment with an inhaled membrane-permeable anti-Gβγ blocking peptide, similar to the suppressive effect of glucocorticoid pretreatment.

Mechanism of glucocorticoid protection of airway smooth muscle from proasthmatic effects of long-acting beta2-adrenoceptor agonist exposure.

Background: Chronic use of long-acting beta2-adrenergic receptor agonists (LABAs), resulting in beta2-adrenergic receptor desensitization, has been associated with increased asthma morbidity. When LABAs are used in combination with inhaled glucocorticoids, however, asthma control is improved, raising the following question: Do glucocorticoids inhibit the proasthmatic mechanism that mediates altered contractility in LABA-exposed airway smooth muscle (ASM)?

Objective: This study aimed to identify the potential protective role and mechanism of action of glucocorticoids in mitigating the effects of prolonged LABA exposure on ASM constrictor and relaxation responsiveness.

Methods: Cultured human ASM cells and isolated rabbit ASM tissues were examined for induced changes in agonist-mediated cyclic adenosine monophosphate accumulation, constrictor and relaxation responsiveness, and expression of specific glucocorticoid-regulated molecules after 24-hour exposure to the LABA salmeterol in the absence and presence of dexamethasone.

Current concepts on the use of glucocorticosteroids and beta-2-adrenoreceptor agonists to treat childhood asthma.

Purpose Of Review: This article reviews current concepts regarding the clinical and scientific rationale for the combined use of glucocorticosteroids and beta-2-adrenoreceptor (beta2AR) agonists in the treatment of childhood asthma.

Recent Findings: Several studies have demonstrated that inhaled corticosteroids (ICS) and beta2AR agonists are the most effective medications for the management of asthma in children. Given substantial evidence of an increased clinical benefit when these agents are used together, new studies are being pursued to establish the efficacy and safety of this combinational therapy in infants and children.

Variants of DENND1B associated with asthma in children.

Background: Asthma is a complex disease that has genetic and environmental causes. The genetic factors associated with susceptibility to asthma remain largely unknown.

Methods: We carried out a genomewide association study involving children with asthma.

Mechanism regulating proasthmatic effects of prolonged homologous beta2-adrenergic receptor desensitization in airway smooth muscle.

Use of long-acting beta(2)-adrenergic receptor (beta2AR) agonists to treat asthma incurs an increased risk of asthma morbidity with impaired bronchodilation and heightened bronchoconstriction, reflecting the adverse effects of prolonged homologous beta2AR desensitization on airway smooth muscle (ASM) function. Since phosphodiesterase 4 (PDE4) regulates ASM relaxation and contractility, we examined whether the changes in ASM function induced by prolonged homologous beta2AR desensitization are attributed to altered expression and action of PDE4. Cultured human ASM cells and isolated rabbit ASM tissues exposed for 24 h to the long-acting beta2AR agonist salmeterol exhibited impaired acute beta2AR-mediated cAMP accumulation and relaxation, respectively, together with ASM constrictor hyperresponsiveness.

Th2 cytokine-induced upregulation of 11beta-hydroxysteroid dehydrogenase-1 facilitates glucocorticoid suppression of proasthmatic airway smooth muscle function.

The anti-inflammatory actions of endogenous glucocorticoids (GCs) are regulated by the activities of the GC-activating and -inactivating enzymes, 11beta-hydroxysteroid dehydrogenase (11beta-HSD)-1 and 11beta-HSD2, respectively, that catalyze the interconversion of the inert GC, cortisone, and its bioactive derivative, cortisol. Proinflammatory cytokines regulate 11beta-HSD1 expression in various cell types and thereby modulate the bioavailability of cortisol to the glucocorticoid receptor (GR). Since endogenous GCs reportedly attenuate the airway asthmatic response to allergen exposure, we investigated whether airway smooth muscle (ASM) exhibits cytokine-induced changes in 11beta-HSD1 expression that enable the ASM to regulate its own bioavailability of GC and, accordingly, the protective effect of GR signaling on airway function under proasthmatic conditions.

Prolonged heterologous beta2-adrenoceptor desensitization promotes proasthmatic airway smooth muscle function via PKA/ERK1/2-mediated phosphodiesterase-4 induction.

Beta2-adrenergic receptor (beta2AR) agonists acutely relieve bronchoconstriction via cAMP-mediated relaxation of airway smooth muscle (ASM). Airway constrictor responsiveness may be significantly heightened, however, following protracted exposure to these agents, presumably reflecting the effects of beta2AR desensitization in ASM accompanying prolonged cAMP signaling. Because cAMP phosphodiesterase (PDE) activity can significantly modulate ASM contractility, we investigated the mechanism regulating PDE expression and its potential role in mediating changes in agonist-induced constrictor and relaxation responsiveness in ASM following its heterologous beta2AR desensitization by prolonged exposure to cAMP-elevating agents.

Superantigen presentation by airway smooth muscle to CD4+ T lymphocytes elicits reciprocal proasthmatic changes in airway function.

Microbial products serving as superantigens (SAgs) have been implicated in triggering various T cell-mediated chronic inflammatory disorders, including severe asthma. Given earlier evidence demonstrating that airway smooth muscle (ASM) cells express MHC class II molecules, we investigated whether ASM can present SAg to resting CD4(+) T cells, and further examined whether this action reciprocally elicits proasthmatic changes in ASM responsiveness. Coincubation of CD4(+) T cells with human ASM cells pulsed with the SAg, staphylococcal enterotoxin A (SEA), elicited adherence and clustering of class II and CD3 molecules at the ASM/T cell interface, indicative of immunological synapse formation, in association with T cell activation.

Regulation of Toll-like receptor 4-induced proasthmatic changes in airway smooth muscle function by opposing actions of ERK1/2 and p38 MAPK signaling.

Activation of Toll-like receptors (TLRs) on immune surveillance cells in the lung has been implicated in the pathobiology of allergic asthma, a condition associated with altered airway smooth muscle (ASM) contractility. Because ASM is known to directly respond to various proasthmatic stimuli, the potential role of TLR signaling in ASM in regulating airway expression of the proasthmatic phenotype was investigated. Cultured human ASM cells were found to express TLR4 and TLR9 mRNA transcripts and, whereas TLR9 stimulation had little effect, TLR4 activation with LPS elicited significant increases in IL-6 release and evoked proasthmatic-like changes in the constrictor and relaxation responsiveness of isolated rabbit ASM tissues.

Proasthmatic effects and mechanisms of action of the dust mite allergen, Der p 1, in airway smooth muscle.

Background: House dust mite allergen exposure is a key risk factor for the development of allergic asthma. Beyond provoking immune cell-mediated allergic responses, house dust mite allergens were recently shown to exert direct effects on airway structural cells secondary to their intrinsic protease activities.

Objective: This study tested the hypothesis that house dust mite allergen exposure can produce changes in airway responsiveness through a direct effect on airway smooth muscle (ASM).

Autocrine regulation of airway smooth muscle responsiveness.

Bronchial asthma is characterized by airway inflammation, exaggerated airway narrowing to bronchoconstrictor agonists, and attenuated beta-adrenoceptor-mediated airway relaxation. Various cytokines/chemokines have been implicated in the pathogenesis of the airway inflammatory response, and certain cytokines, most notably including specific Th2-type cytokines and IL-1beta, have been shown to directly regulate airway smooth muscle (ASM) responsiveness. Recent evidence supports the concept that the ASM itself has the capacity to endogenously express a number of these cytokines under specific conditions of ASM sensitization.

T lymphocyte-mediated changes in airway smooth muscle responsiveness are attributed to induced autocrine release and actions of IL-5 and IL-1beta.

Background: Bidirectional stimulatory cross-talk was recently found to exist between activated T cells and airway smooth muscle (ASM) cells, a process that involves coligation of specific cellular adhesion-costimulatory molecules that results in the induction of proasthmatic-like changes in ASM responsiveness.

Objective: The present study examined whether the cooperative intercellular signaling between activated T cells and ASM cells is coupled to the induced expression and actions of IL-5 and IL-1beta.

Methods: Agonist-induced constrictor and relaxant responses were examined in rabbit ASM segments exposed to resting and anti-CD3-activated T cells in the absence and presence of either an anti-IL-5 receptor mAb or the recombinant human IL-1 receptor antagonist.