Rhinovirus-induced exacerbations of asthma: How is the {beta}2-adrenoceptor implicated?

Rhinovirus induces airway remodeling: what are the physiological consequences?

BACKGROUND

Rhinovirus infections commonly evoke asthma exacerbations in children and adults. Recurrent asthma exacerbations are associated with injury-repair responses in the airways that collectively contribute to airway remodeling. The physiological consequences of airway remodeling can manifest as irreversible airway obstruction and diminished responsiveness to bronchodilators. Structural cells of the airway, including epithelial cells, smooth muscle, fibroblasts, myofibroblasts, and adjacent lung vascular endothelial cells represent an understudied and emerging source of cellular and extracellular soluble mediators and matrix components that contribute to airway remodeling in a rhinovirus-evoked inflammatory environment.

MAIN BODY

While mechanistic pathways associated with rhinovirus-induced airway remodeling are still not fully characterized, infected airway epithelial cells robustly produce type 2 cytokines and chemokines, as well as pro-angiogenic and fibroblast activating factors that act in a paracrine manner on neighboring airway cells to stimulate remodeling responses. Morphological transformation of structural cells in response to rhinovirus promotes remodeling phenotypes including induction of mucus hypersecretion, epithelial-to-mesenchymal transition, and fibroblast-to-myofibroblast transdifferentiation. Rhinovirus exposure elicits airway hyperresponsiveness contributing to irreversible airway obstruction. This obstruction can occur as a consequence of sub-epithelial thickening mediated by smooth muscle migration and myofibroblast activity, or through independent mechanisms mediated by modulation of the β2 agonist receptor activation and its responsiveness to bronchodilators. Differential cellular responses emerge in response to rhinovirus infection that predispose asthmatic individuals to persistent signatures of airway remodeling, including exaggerated type 2 inflammation, enhanced extracellular matrix deposition, and robust production of pro-angiogenic mediators.

CONCLUSIONS

Few therapies address symptoms of rhinovirus-induced airway remodeling, though understanding the contribution of structural cells to these processes may elucidate future translational targets to alleviate symptoms of rhinovirus-induced exacerbations.

Asthmatic bronchial smooth muscle increases rhinovirus replication within the bronchial epithelium

Rhinovirus (RV) infection of the bronchial epithelium is implicated in the vast majority of severe asthma exacerbations. Interestingly, the susceptibility of bronchial epithelium to RV infection is increased in persons with asthma. Bronchial smooth muscle (BSM) remodeling is an important feature of severe asthma pathophysiology, and its reduction using bronchial thermoplasty has been associated with a significant decrease in the exacerbation rate. We hypothesized that asthmatic BSM can play a role in RV infection of the bronchial epithelium. Using an original co-culture model between bronchial epithelium and BSM cells, we show that asthmatic BSM cells increase RV replication in bronchial epithelium following RV infection. These findings are related to the increased production of CCL20 by asthmatic BSM cells. Moreover, we demonstrate an original downregulation of the activity of the epithelial protein kinase RNA-activated (PKR) antiviral pathway. Finally, we identify a direct bottom-up effect of asthmatic BSM cells on bronchial epithelium susceptibility to RV infection.

Airway smooth muscle pathophysiology in asthma

The airway smooth muscle (ASM) cell plays a central role in the pathogenesis of asthma and constitutes an important target for treatment. These cells control muscle tone and thus regulate the opening of the airway lumen and air passage. Evidence indicates that ASM cells participate in the airway hyperresponsiveness as well as the inflammatory and remodeling processes observed in asthmatic subjects. Therapeutic approaches require a comprehensive understanding of the structure and function of the ASM in both the normal and disease states. This review updates current knowledge about ASM and its effects on airway narrowing, remodeling, and inflammation in asthma.

Time to Say Goodbye to Bronchiolitis, Viral Wheeze, Reactive Airways Disease, Wheeze Bronchitis and All That

The diagnosis and management of infants and children with a significant viral lower respiratory tract illness remains the subject of much debate and little progress. Over the decades various terms for such illnesses have been in and fallen out of fashion or have evolved to mean different things to different clinicians. Terms such as "bronchiolitis," "reactive airways disease," "viral wheeze," and many more are used to describe the same condition and the same term is frequently used to describe illnesses caused by completely different dominant pathologies. This lack of clarity is due, in large part, to a failure to understand the basic underlying inflammatory and associated processes and, in part, due to the lack of a simple test to identify a condition such as asthma. Moreover, there is a lack of insight into the fact that the same pathology can produce different clinical signs at different ages. The consequence is that terminology and fashions in treatment have tended to go around in circles. As was noted almost 60 years ago, amongst pre-school children with a viral LRTI and airways obstruction there are those with a "viral bronchitis" and those with asthma. In the former group, a neutrophil dominated inflammation response is responsible for the airways' obstruction whilst amongst asthmatics much of the obstruction is attributable to bronchoconstriction. The airways obstruction in the former group is predominantly caused by airways secretions and to some extent mucosal oedema (a "snotty lung"). These patients benefit from good supportive care including supplemental oxygen if required (though those with a pre-existing bacterial bronchitis will also benefit from antibiotics). For those with a viral exacerbation of asthma, characterized by bronchoconstriction combined with impaired b-agonist responsiveness, standard management of an exacerbation of asthma (including the use of steroids to re-establish bronchodilator responsiveness) represents optimal treatment. The difficulty is identifying which group a particular patient falls into. A proposed simplified approach to the nomenclature used to categorize virus associated LRTIs is presented based on an understanding of the underlying pathological processes and how these contribute to the physical signs.

Asthma: A Loss of Post-natal Homeostatic Control of Airways Smooth Muscle With Regression Toward a Pre-natal State

The defining feature of asthma is loss of normal post-natal homeostatic control of airways smooth muscle (ASM). This is the key feature that distinguishes asthma from all other forms of respiratory disease. Failure to focus on impaired ASM homeostasis largely explains our failure to find a cure and contributes to the widespread excessive morbidity associated with the condition despite the presence of effective therapies. The mechanisms responsible for destabilizing the normal tight control of ASM and hence airways caliber in post-natal life are unknown but it is clear that atopic inflammation is neither necessary nor sufficient. Loss of homeostasis results in excessive ASM contraction which, in those with poor control, is manifest by variations in airflow resistance over short periods of time. During viral exacerbations, the ability to respond to bronchodilators is partially or almost completely lost, resulting in ASM being "locked down" in a contracted state. Corticosteroids appear to restore normal or near normal homeostasis in those with poor control and restore bronchodilator responsiveness during exacerbations. The mechanism of action of corticosteroids is unknown and the assumption that their action is solely due to "anti-inflammatory" effects needs to be challenged. ASM, in evolutionary terms, dates to the earliest land dwelling creatures that required muscle to empty primitive lungs. ASM appears very early in embryonic development and active peristalsis is essential for the formation of the lungs. However, in post-natal life its only role appears to be to maintain airways in a configuration that minimizes resistance to airflow and dead space. In health, significant constriction is actively prevented, presumably through classic negative feedback loops. Disruption of this robust homeostatic control can develop at any age and results in asthma. In order to develop a cure, we need to move from our current focus on immunology and inflammatory pathways to work that will lead to an understanding of the mechanisms that contribute to ASM stability in health and how this is disrupted to cause asthma. This requires a radical change in the focus of most of "asthma research."

Asthmatic Bronchial Smooth Muscle Increases CCL5-Dependent Monocyte Migration in Response to Rhinovirus-Infected Epithelium

Asthma exacerbations, a major concern in therapeutic strategies, are most commonly triggered by viral respiratory infections, particularly with human rhinovirus (HRV). Infection of bronchial epithelial (BE) cells by HRV triggers inflammation, notably monocyte recruitment. The increase of bronchial smooth muscle (BSM) mass in asthma, a hallmark of bronchial remodeling, is associated with the annual rate of exacerbations. The aim of the present study was to assess whether or not BSM could increase monocyte migration induced by HRV-infected BE. We used an advanced in vitro model of co-culture of human BE cells in air-liquid interface with human BSM cells from control and asthmatic patients. Inflammation triggered by HRV infection (HRV-16, MOI 0.1, 1 h) was assessed at 24 h with transcriptomic analysis and multiplex ELISA. In vitro CD14⁺ monocyte migration was evaluated with modified Boyden chamber. Results showed that HRV-induced monocyte migration was substantially increased in the co-culture model with asthmatic BSM, compared with control BSM. Furthermore, the well-known monocyte migration chemokine, CCL2, was not involved in this increased migration. However, we demonstrated that CCL5 was further increased in the asthmatic BSM co-culture and that anti-CCL5 blocking antibody significantly decreased monocyte migration induced by HRV-infected BE. Taken together, our findings highlight a new role of BSM cells in HRV-induced inflammation and provide new insights in mucosal immunology which may open new opportunities for prevention and/or treatment of asthma exacerbation.

Dietary omega-6, but not omega-3, polyunsaturated or saturated fatty acids increase inflammation in primary lung mesenchymal cells

Obesity is an important risk factor for developing severe asthma. Dietary fatty acids, which are increased in sera of obese individuals and after high-fat meals, activate the innate immune system and induce inflammation. This study investigated whether dietary fatty acids directly cause inflammation and/or synergize with obesity-induced cytokines in primary human pulmonary fibroblasts in vitro. Fibroblasts were challenged with BSA-conjugated fatty acids [ω-6 polyunsaturated fatty acids (PUFAs) and ω-3 PUFAs or saturated fatty acids (SFAs)], with or without TNF-α, and release of the proinflammatory cytokines, IL-6 and CXCL8, was measured. We found that the ω-6 PUFA arachidonic acid (AA), but not ω-3 PUFAs or SFAs, upregulates IL-6 and CXCL8 release. Combined AA and TNF-α challenge resulted in substantially greater cytokine release than either alone, demonstrating synergy. Synergistic upregulation of IL-6, but not CXCL8, was mainly mediated via cyclooxygenase (COX). Inhibition of p38 MAPK reduced CXCL8 release, induced by AA and TNF-α alone, but not in combination. Synergistic CXCL8 release, following AA and TNF-α challenge, was not medicated via a single signaling pathway (MEK1, JNK, phosphoinositide 3-kinase, and NF-κB) nor by hyperactivation of NF-κB or p38. To investigate if these findings occur in other airway cells, effects of AA in primary human airway smooth muscle (ASM) cells and human bronchial epithelial cells were also investigated. We found proinflammatory effects in ASM cells but not epithelial cells. This study suggests that diets rich in ω-6 PUFAs might promote airway inflammation via multiple pathways, including COX-dependent and -independent pathways, and in an obese person, may lead to more severe airway inflammation.

β2-Adrenoceptor Function in Asthma

β2-adrenoceptor agonists, often used in combination with corticosteroids, have been extensively used for the treatment of asthma. However, concerns have been raised regarding their adverse effects and safety including poor asthma control, life-threatening exacerbations, exacerbations that often require hospitalization, and asthma-related deaths. The question as to whether these adverse effects relate to the loss of their bronchoprotective action remains an interesting possibility. In the chapter, we will review the experimental evidence that describes the different potential factors and associated mechanisms that can blunt the therapeutic action of β2-adrenoceptor agonists in asthma. We show here evidence that various key inflammatory cytokines, growth factors, some respiratory viruses, certain allergens, unknown factors present in serum from atopic asthmatics have the capacity to impair β2-adrenoceptor function in airway smooth muscle, the main target of these drugs. More importantly, we present our latest research describing the role played by mast cells in impairing β2-adrenoceptor function. Although no definitive conclusion could be made regarding the implication of one single mechanism, receptor uncoupling, or receptor desensitization due to phosphorylation represents the main inhibitory pathways associated with a loss of β2-adrenoceptor function in airway smooth muscle. Targeting the pathways leading to β2-adrenoceptor dysfunction will likely provide novel therapies to improve the efficacy of β2-agonists in asthma.

Cyclooxygenase 2: its regulation, role and impact in airway inflammation

Cyclooxygenase 2 (COX-2: official gene symbol - PTGS2) has long been regarded as playing a pivotal role in the pathogenesis of airway inflammation in respiratory diseases including asthma. COX-2 can be rapidly and robustly expressed in response to a diverse range of pro-inflammatory cytokines and mediators. Thus, increased levels of COX-2 protein and prostanoid metabolites serve as key contributors to pathobiology in respiratory diseases typified by dysregulated inflammation. But COX-2 products may not be all bad: prostanoids can exert anti-inflammatory/bronchoprotective functions in airways in addition to their pro-inflammatory actions. Herein, we outline COX-2 regulation and review the diverse stimuli known to induce COX-2 in the context of airway inflammation. We discuss some of the positive and negative effects that COX-2/prostanoids can exert in in vitro and in vivo models of airway inflammation, and suggest that inhibiting COX-2 expression to repress airway inflammation may be too blunt an approach; because although it might reduce the unwanted effects of COX-2 activation, it may also negate the positive effects. Evidence suggests that prostanoids produced via COX-2 upregulation show diverse actions (and herein we focus on prostaglandin E2 as a key example); these can be either beneficial or deleterious and their impact on respiratory disease can be dictated by local concentration and specific interaction with individual receptors. We propose that understanding the regulation of COX-2 expression and associated receptor-mediated functional outcomes may reveal number of critical steps amenable to pharmacological intervention. These may prove invaluable in our quest towards future development of novel anti-inflammatory pharmacotherapeutic strategies for the treatment of airway diseases.

Effect of Sphingosine 1-Phosphate on Cyclo-Oxygenase-2 Expression, Prostaglandin E2 Secretion, and β2-Adrenergic Receptor Desensitization

Tachyphylaxis of the β2-adrenergic receptor limits the efficacy of bronchodilatory β2-agonists in respiratory disease. Cellular studies in airway smooth muscle (ASM) have shown that inflammatory mediators and infectious stimuli reduce β2-adrenergic responsiveness in a cyclo-oxygenase (COX)-2-mediated, prostaglandin E2 (PGE2)-dependant manner. Herein, we show that sphingosine 1-phosphate (S1P), a bioactive sphingolipid that plays an important role in pathophysiology of asthma, also induces β2-adrenergic receptor desensitization in bronchial ASM cells and exerts hyporesponsiveness to β2-agonists. We treated ASM cells with S1P (1 μM) for up to 24 hours and then examined the temporal kinetics of COX-2 mRNA expression, protein up-regulation, and PGE2 secretion. S1P significantly enhanced COX-2 expression and PGE2 secretion, and this was repressed by the selective COX-2 inhibitor celecoxib, the corticosteroid dexamethasone, or small interfering RNA (siRNA) knockdown of COX-2 expression. In combination with another proinflammatory mediator found elevated in asthmatic airways, the cytokine TNF-α, we observed that S1P-induced COX-2 mRNA expression and protein up-regulation and PGE2 secretion from ASM cells were significantly enhanced. Notably, S1P induced heterologous β2-adrenergic desensitization, as measured by inhibition of cyclic adenosine monophosphate production in response to the short-acting β2-agonist, salbutamol, and the long-acting β2-agonist, formoterol. Taken together, these data indicate that S1P represses β2-adrenergic activity in ASM cells by increasing COX-2-mediated PGE2 production, and suggest that this bioactive sphingolipid found elevated in asthma may contribute to β2-adrenergic desensitization.

IL-17A increases TNF-α-induced COX-2 protein stability and augments PGE2 secretion from airway smooth muscle cells: impact on β2 -adrenergic receptor desensitization

BACKGROUND

IL-17A plays an important role in respiratory disease and is a known regulator of pulmonary inflammation and immunity. Recent studies have linked IL-17A with exacerbation in asthma and COPD. We have shown that the enzyme cyclooxygenase-2 (COX-2) and its prostanoid products, prostaglandin E2 (PGE2 ) in particular, are key contributors in in vitro models of infectious exacerbation; however, the impact of IL-17A was not known.

METHODS AND RESULTS

We address this herein and show that IL-17A induces a robust and sustained upregulation of COX-2 protein and PGE2 secretion from airway smooth muscle (ASM) cells. COX-2 can be regulated at transcriptional, post-transcriptional and/or post-translational levels. We have elucidated the underlying molecular mechanisms responsible for the sustained upregulation of TNF-α-induced COX-2 by IL-17A in ASM cells and show that is not via increased COX-2 gene expression. Instead, TNF-α-induced COX-2 upregulation is subject to regulation by the proteasome, and IL-17A acts to increase TNF-α-induced COX-2 protein stability as confirmed by cycloheximide chase experiments. In this way, IL-17A acts to amplify the COX-2-mediated effects of TNF-α and greatly enhances PGE2 secretion from ASM cells.

CONCLUSION

As PGE2 is a multifunctional prostanoid with diverse roles in respiratory disease, our studies demonstrate a novel function for IL-17A in airway inflammation by showing for the first time that IL-17A impacts on the COX-2/PGE2 pathway, molecules known to contribute to disease exacerbation.

TLR3 activation increases chemokine expression in human fetal airway smooth muscle cells

Viral infections, such as respiratory syncytial virus and rhinovirus, adversely affect neonatal and pediatric populations, resulting in significant lung morbidity, including acute asthma exacerbation. Studies in adults have demonstrated that human airway smooth muscle (ASM) cells modulate inflammation through their ability to secrete inflammatory cytokines and chemokines. The role of ASM in the developing airway during infection remains undefined. In our study, we used human fetal ASM cells as an in vitro model to examine the effect of Toll-like receptor (TLR) agonists on chemokine secretion. We found that fetal ASM express multiple TLRs, including TLR3 and TLR4, which are implicated in the pathogenesis of respiratory syncytial virus and rhinovirus infection. Cells were treated with TLR agonists, polyinosinic-polycytidylic acid [poly(I:C)] (TLR3 agonist), lipopolysaccharide (TLR4 agonist), or R848 (TLR7/8 agonist), and IL-8 and chemokine (C-C motif) ligand 5 (CCL5) secretion were evaluated. Interestingly, poly(I:C), but neither lipopolysaccharide nor R848, increased IL-8 and chemokine (C-C motif) ligand 5 secretion. Examination of signaling pathways suggested that the poly(I:C) effects in fetal ASM involve TLR and ERK signaling, in addition to another major inflammatory pathway, NF-κB. Moreover, there are variations between fetal and adult ASM with respect to poly(I:C) effects on signaling pathways. Pharmacological inhibition suggested that ERK pathways mediate poly(I:C) effects. Overall, our data show that poly(I:C) initiates activation of proinflammatory pathways in developing ASM, which may contribute to immune responses to infection and exacerbation of asthma.

Progression of Irreversible Airflow Limitation in Asthma: Correlation with Severe Exacerbations

BACKGROUND

Severe exacerbations of asthma are periods of excess functional and pathological changes in the airways that have been proposed to induce airway remodeling.

OBJECTIVE

The objective of this study was to explore whether severe exacerbations are correlated with the decline in post-bronchodilator forced expiratory volume in 1 second (FEV1) and loss of bronchodilator reversibility (BDR).

METHODS

We examined the changes in FEV1 and BDR in 140 nonsmoking patients with well-controlled asthma at baseline and correlated these changes with the frequency of severe asthma exacerbations.

RESULTS

A 3-year follow-up assessment was completed in 128 patients. A total of 28 (21.9%) patients experienced at least 1 severe exacerbation with a mean rate of 0.16 year(-1). The exacerbation rate was significantly correlated with an annual rate of decline in FEV1 (ρ = 0.49, P < .0001). Both patients with 1 exacerbation and those with 2 or more exacerbations had greater declines in FEV1 than patients with no exacerbations (no exacerbation, 13.6 mL/year; 1 exacerbation, 41.3 mL/year; 2 or more exacerbations, 58.3 mL/year; P < .01 and P < .0001, respectively). The changes in BDR from baseline to the end of the study in patients who did or did not experience an exacerbation were -1.2% and 0.1%, respectively (P < .0005). The changes in BDR were significantly correlated with the annual rates of change in FEV1 (r = 0.40, P < .0001).

CONCLUSION

The occurrence of severe exacerbations of asthma is correlated with the progression of irreversible airflow limitation over time. This suggests that asthma exacerbations could have the long-term adverse consequences of structural and functional changes in the airways.

Viral infections and asthma: an inflammatory interface?

Asthma is a chronic inflammatory disease of the airways in which the majority of patients respond to treatment with corticosteroids and β₂-adrenoceptor agonists. Acute exacerbations of asthma substantially contribute to disease morbidity, mortality and healthcare costs, and are not restricted to patients who are not compliant with their treatment regimens. Given that respiratory viral infections are the principal cause of asthma exacerbations, this review article will explore the relationship between viral infections and asthma, and will put forward hypotheses as to why virus-induced exacerbations occur. Potential mechanisms that may explain why current therapeutics do not fully inhibit virus-induced exacerbations, for example, β₂-adrenergic desensitisation and corticosteroid insensitivity, are explored, as well as which aspects of virus-induced inflammation are likely to be attenuated by current therapy.

TLR2 activation causes tachyphylaxis to β2 -agonists in vitro and ex vivo: modelling bacterial exacerbation

BACKGROUND

Asthma is a widespread chronic health problem exacerbated by common viral and bacterial infections. Further research is required to understand how infection worsens asthma control in order to advance therapeutic options in the future. Recent research has revealed that β2 -adrenergic receptor (β2 -AR) agonists lose bronchodilatory efficacy because the receptor-mediated molecular pathways responsible for their beneficial actions are desensitized by infection. To date, most studies have focussed on viral infection, leaving the impact of bacterial infection on β2 -AR desensitization relatively under-investigated. We address this in this study.

METHODS AND RESULTS

Utilizing an in vitro model of bacterial exacerbation in airway smooth muscle (ASM) cells, we show that activation of toll-like receptor 2 (TLR2; mimicking bacterial infection) in the presence of an inflammatory stimulus leads to β2 -AR desensitization. This occurs via TLR2-dependent upregulation of cyclooxygenase 2 (COX-2) mRNA expression and increased secretion of PGE2 . Importantly, PGE2 causes heterologous β2 -AR desensitization and reduces cAMP production in response to short-acting (salbutamol) and long-acting (formoterol) β2 -agonists. Thus, bacterial infectious stimuli act in a PGE2 -dependent manner to severely curtail the beneficial actions of β2 -agonists. The impact of β2 -AR desensitization is demonstrated by reduced gene expression of the critical anti-inflammatory molecule MKP-1 in response to β2 -agonists, as well as impaired bronchodilation in a mouse lung slices.

CONCLUSIONS

Taken together, our results show that, like viruses, bacteria induce prostanoid-dependent β2 -AR desensitization on ASM cells. Notably, COX-2 inhibition with the specific inhibitor celecoxib represses PGE2 secretion, presenting a feasible pharmacological option for treatment of infectious exacerbation in asthma in the future.

Characterising the mechanism of airway smooth muscle β2 adrenoceptor desensitization by rhinovirus infected bronchial epithelial cells

Rhinovirus (RV) infections account for approximately two thirds of all virus-induced asthma exacerbations and often result in an impaired response to β2 agonist therapy. Using an in vitro model of RV infection, we investigated the mechanisms underlying RV-induced β2 adrenoceptor desensitization in primary human airway smooth muscle cells (ASMC). RV infection of primary human bronchial epithelial cells (HBEC) for 24 hours produced conditioned medium that caused β2 adrenoceptor desensitization on ASMCs without an effect on ASMCs viability. Less than 3 kDa size fractionation together with trypsin digestion of RV-induced conditioned medium did not prevent β2 adrenoceptor desensitization, suggesting it could potentially be mediated by a small peptide or lipid. RV infection of BECs, ASMCs and fibroblasts produced prostaglandins, of which PGE2, PGF2α and PGI2 had the ability to cause β2 adrenoceptor desensitization on ASMCs. RV-induced conditioned medium from HBECs depleted of PGE2 did not prevent ASMC β2 adrenoceptor desensitization; however this medium induced PGE2 from ASMCs, suggesting that autocrine prostaglandin production may be responsible. Using inhibitors of cyclooxygenase and prostaglandin receptor antagonists, we found that β2 adrenoceptor desensitization was mediated through ASMC derived COX-2 induced prostaglandins. Since ASMC prostaglandin production is unlikely to be caused by RV-induced epithelial derived proteins or lipids we next investigated activation of toll-like receptors (TLR) by viral RNA. The combination of TLR agonists poly I:C and imiquimod induced PGE2 and β2 adrenoceptor desensitization on ASMC as did the RNA extracted from RV-induced conditioned medium. Viral RNA but not epithelial RNA caused β2 adrenoceptor desensitization confirming that viral RNA and not endogenous human RNA was responsible. It was deduced that the mechanism by which β2 adrenoceptor desensitization occurs was by pattern recognition receptor activation of COX-2 induced prostaglandins.

Emerging mediators of airway smooth muscle dysfunction in asthma

Phenotypic changes in airway smooth muscle are integral to the pathophysiological changes that constitute asthma - namely inflammation, airway wall remodelling and bronchial hyperresponsiveness. In vitro and in vivo studies have shown that the proliferative, secretory and contractile functions of airway smooth muscle are dysfunctional in asthma. These functions can be modulated by various mediators whose levels are altered in asthma, derived from inflammatory cells or produced by airway smooth muscle itself. In this review, we describe the emerging roles of the CXC chemokines (GROs, IP-10), Th17-derived cytokines (IL-17, IL-22) and semaphorins, as well as the influence of viral infection on airway smooth muscle function, with a view to identifying new opportunities for therapeutic intervention in asthma.

Targeting airway smooth muscle in airways diseases: an old concept with new twists

Airway smooth muscle (ASM) manifests a hyper-responsive phenotype in airway disorders such as asthma. ASM also modulates immune responses by secreting mediators and expressing cell-surface molecules that promote recruitment of inflammatory cells to the lungs. The aim of the current article is to highlight therapeutics that may modulate ASM responses in airway disorders and exacerbations.

β2-Agonists upregulate PDE4 mRNA but not protein or activity in human airway smooth muscle cells from asthmatic and nonasthmatic volunteers

β(2)-Adrenergic receptor (β2AR) agonists induce airway relaxation via cAMP. Phosphodiesterase (PDE)s degrade and regulate cAMP, and in airway smooth muscle (ASM) cells PDE4D degrades cAMP. Long-acting β(2)-agonists are now contraindicated as monotherapy for asthma, and increased PDE4D has been speculated to contribute to this phenomenon. In this study we investigated the expression of PDE4D in asthmatic and nonasthmatic ASM cells and its regulation by formoterol and budesonide. Primary ASM cells from people with or without asthma were stimulated with transforming growth factor (TGF)-β(1), formoterol, and/or budesonide. PDE4D mRNA was assessed by real-time PCR, or PCR to assess splice variant production. PDE4D protein was assessed by Western blotting, and we investigated the effect of formoterol on cAMP production and PDE activity. Interleukin (IL)-6 was assessed using ELISA. PDE4D mRNA was dose dependently upregulated by formoterol, with a single splice variant, PDE4D5, present. Formoterol did not induce PDE4D protein at time points between 3 to 72 h, whereas it did induce and increase IL-6 secretion. We pretreated cells with actinomycin D and a proteasome inhibitor, MG132, and found no evidence of alterations in mRNA, protein expression, or degradation of PDE4D. Finally PDE activity was not altered by formoterol. This study shows, for the first time, that PDE4D5 is predominantly expressed in human ASM cells from people with and without asthma and that formoterol does not upregulate PDE4D protein production. This leads us to speculate that continual therapy with β2AR agonists is unlikely to cause PDE4-mediated tachyphylaxis.

Airway smooth muscle and immunomodulation in acute exacerbations of airway disease

Airway smooth muscle (ASM) manifests a hyperresponsive phenotype in airway disorders such as asthma, chronic obstructive pulmonary disease (COPD), and cystic fibrosis. Current evidence also suggests that ASM modulates immune responses by secreting mediators and expressing cell surface molecules. Such processes amplify or dampen inflammation by inflammatory cells in the airways or by altering cellular responses to viruses, bacteria, or pathogens known to exacerbate airways diseases.

β2-Agonist induced cAMP is decreased in asthmatic airway smooth muscle due to increased PDE4D

Background and Objective

Asthma is associated with airway narrowing in response to bronchoconstricting stimuli and increased airway smooth muscle (ASM) mass. In addition, some studies have suggested impaired β-agonist induced ASM relaxation in asthmatics, but the mechanism is not known.

Objective

To characterize the potential defect in β-agonist induced cAMP in ASM derived from asthmatic in comparison to non-asthmatic subjects and to investigate its mechanism.

Methods

We examined β₂-adrenergic (β₂AR) receptor expression and basal β-agonist and forskolin (direct activator of adenylyl cyclase) stimulated cAMP production in asthmatic cultured ASM (n = 15) and non-asthmatic ASM (n = 22). Based on these results, PDE activity, PDE4D expression and cell proliferation were determined.

Results

In the presence of IBMX, a pan PDE inhibitor, asthmatic ASM had ∼50% lower cAMP production in response to isoproterenol, albuterol, formoterol, and forskolin compared to non-asthmatic ASM. However when PDE4 was specifically inhibited, cAMP production by the agonists and forskolin was normalized in asthmatic ASM. We then measured the amount and activity of PDE4, and found ∼2-fold greater expression and activity in asthmatic ASM compared to non-asthmatic ASM. Furthermore, inhibition of PDE4 reduced asthmatic ASM proliferation but not that of non-asthmatic ASM.

Conclusion

Decreased β-agonist induced cAMP in ASM from asthmatics results from enhanced degradation due to increased PDE4D expression. Clinical manifestations of this dysregulation would be suboptimal β-agonist-mediated bronchodilation and possibly reduced control over increasing ASM mass. These phenotypes appear to be “hard-wired” into ASM from asthmatics, as they do not require an inflammatory environment in culture to be observed.

Beta-Adrenergic Agonists

Inhaled β₂-adrenoceptor (β₂-AR) agonists are considered essential bronchodilator drugs in the treatment of bronchial asthma, both as symptoms-relievers and, in combination with inhaled corticosteroids, as disease-controllers. In this article, we first review the basic mechanisms by which the β₂-adrenergic system contributes to the control of airway smooth muscle tone. Then, we go on describing the structural characteristics of β₂-AR and the molecular basis of G-protein-coupled receptor signaling and mechanisms of its desensitization/ dysfunction. In particular, phosphorylation mediated by protein kinase A and β-adrenergic receptor kinase are examined in detail. Finally, we discuss the pivotal role of inhaled β₂-AR agonists in the treatment of asthma and the concerns about their safety that have been recently raised.