Epithelial cell proliferation contributes to airway remodeling in severe asthma

Overcoming steroid unresponsiveness in airways disease

Most of the patients with asthma are found to be successfully treated with conventional therapy. However, there are a small proportion of asthmatic patients who fail to respond to corticosteroids even at high doses or with supplementary therapy. In addition, even high doses of corticosteroids have a minimal effect on the inexorable decline in lung function in COPD (chronic obstructive pulmonary disease) and only a small effect in reducing exacerbations. Corticosteroid-insensitivity therefore presents a profound management problem. Corticosteroids act through a cytosolic receptor [GR (glucocorticoid receptor)], which is activated and translocates to the nucleus. Once in the nucleus, it either binds to DNA and switches on the expression of anti-inflammatory genes or represses the activity of distinct signalling pathways such as NF-kappaB (nuclear factor kappaB), AP-1 (activator protein-1) or MAPKs (mitogen-activated protein kinases). This latter step requires the recruitment of co-repressor molecules. A failure to respond to corticosteroids may therefore result from lack of binding to GR, reduced GR expression, lack of co-repressor activity or enhanced activation of inflammatory pathways. These events can be modulated by oxidative stress or high levels of inflammatory cytokines, which may lead to a reduced clinical outcome. Understanding the molecular mechanisms of GR action, and inaction, may lead to the development of new anti-inflammatory drugs or reverse the relative corticosteroid-insensitivity that is characteristic of these diseases.

Inflammatory biomarkers in airways of patients with severe asthma compared with non-severe asthma

BACKGROUND

About 5-10% of patients with asthma suffer from poorly-controlled disease despite corticosteroid (CS) therapy.

OBJECTIVE

We determined whether there were any differences in inflammatory biomarkers between severe and non-severe asthma patients.

METHODS

Nineteen severe and 20 non-severe asthma patients were recruited and underwent collection of induced sputum, bronchoalveolar lavage (BAL) fluid and bronchial biopsies.

RESULTS

Biopsy results showed no differences in eosinophils (major basic protein positive), neutrophils, macrophages, T cells and mast cells in the bronchial submucosa. However, subbasement membrane (SBM) thickness and smooth muscle area were increased in the biopsies. No significant differences were observed in the induced sputum inflammatory cells. In BAL fluid, there was a significant increase in neutrophils but a significant decrease in macrophages. Eosinophil counts were non-significantly increased threefold in both sputum and BAL in severe asthma. Levels of IL-8 and IL-13 in sputum supernatants were similar in both groups of asthma patients. There was a significant inverse correlation between post-bronchodilator forced expiratory volume in 1 s and provocative concentration of methacholine causing a 20% fall in FEV(1) with SBM thickness.

CONCLUSION

Differences in inflammatory cells were observed mainly in terms of increased neutrophils and reduction in macrophage numbers in BAL fluid with a trend towards increased eosinophils in severe asthma compared with non-severe asthma. However, the most notable features are the increase in features of airway wall remodelling of SBM thickness and smooth muscle area.

Inhaled corticosteroids as combination therapy with beta-adrenergic agonists in airways disease: present and future

Inhaled corticosteroid (ICS) therapy in combination with long-acting beta-adrenergic agonists represents the most important treatment for chronic airways diseases such as asthma and chronic obstructive pulmonary disease (COPD). ICS therapy forms the basis for treatment of asthma of all severities, improving asthma control, lung function and preventing exacerbations of disease. Use of ICS has also been established in the treatment of COPD, particularly symptomatic patients, who experience useful gains in quality of life, likely from an improvement in symptoms such as breathlessness and in reduction in exacerbations, and an attenuation of the yearly rate of deterioration in lung function. The addition of long-acting beta-agonist (LABA) therapy with ICS increases the efficacy of ICS effects in moderate-to-severe asthma. Thus, a 800 mug daily dose of the ICS budesonide reduced severe exacerbation rates by 49% compared to a low dose of 200 mug daily, and addition of the LABA formoterol to budesonide (800 mug) led to a 63% reduction. In COPD, the effects of ICS are less prominent but there are beneficial effects on the decline in FEV(1) and the rate of exacerbations. A reduction in the rate of decline in FEV(1) of 16 ml/year with a 25% reduction in exacerbation rate has been reported with the salmeterol and fluticasone combination. A non-significant 17.5% reduction in all-cause mortality rate with ICS and LABA is reported. Chronic inflammation is a feature of both asthma and COPD, although there are site and characteristic differences. ICS targets this inflammation although this effect of ICS is less effective in patients with severe asthma and with COPD; however, addition of LABA may potentiate the anti-inflammatory effects of ICS. An important consideration is the presence of corticosteroid insensitivity in these patients. Currently available ICS have variably potent binding activities to specific glucocorticoid receptors, leading to inhibition of gene expression by either binding to DNA and inducing anti-inflammatory genes or by repressing the induction of pro-inflammatory mediators. Local side effects of ICS include oral candidiasis, hoarseness and dysphonia, while systemic side effects, such as easy bruising and reduction in growth velocity or bone mineral densitometry, are usually restricted to doses above maximally recommended doses. Use of LABA alone in patients with asthma increases the risk of asthma-related events including deaths, but this is less observed with the combination of ICS and LABA. Therefore, use of LABA alone is not recommended for asthma therapy. Future progress in ICS development will be characterised by the introduction of ICS with greater efficacy with a limited side-effect profile, and by longer-acting ICS that can be used in combination with once-daily LABAs. Other agents that could improve the efficacy of corticosteroids or reverse corticosteroid insensitivity may be added to ICS. ICS in combination with LABAs will continue to remain the main focus of treatment of airways diseases.

Acidic mammalian chitinase regulates epithelial cell apoptosis via a chitinolytic-independent mechanism

Acidic mammalian chitinase (AMCase) is produced during and plays an important role in the pathogenesis of Th2-mediated diseases and antiparasite responses. However, the effector responses of AMCase in these settings have not been adequately defined and the relationship(s) between its chitinolytic and other biologic properties have not been investigated. In these studies, we demonstrate that AMCase protects airway epithelial cells from Fas ligand- and growth factor withdrawal-induced apoptosis. This cytoprotection was associated with Akt phosphorylation and abrogated when the PI3K/Akt pathway was inhibited. Comparable cytoprotection was also seen in experiments comparing wild-type AMCase and mutant AMCase that lacked chitinolytic activity. Importantly, the apoptosis-inhibiting effect of enzymatically active and inactive AMCase was abrogated by treatment with allosamidin. These studies demonstrate that secreted AMCase feeds back in an autocrine and/or paracrine manner to protect pulmonary epithelial cells from growth factor withdrawal- and Fas ligand-induced apoptosis. They also demonstrate that the cytoprotection is mediated via a PI3K/Akt-dependent and allosamidin-sensitive pathway that is independent of the chitinolytic activity of this chitinase.

Sphingosine-1-phosphate increases human alveolar epithelial IL-8 secretion, proliferation and neutrophil chemotaxis

Sphingosine-1-phosphate (S1P) has been presented recently as a pro-inflammatory agent in the airway epithelium since S1P levels are increased in bronchoalveolar lavage fluid of human asthmatics. However, the effects of S1P over the alveolar epithelium and neutrophil interactions are poorly understood. Here, we show that S1P increased interleukin 8 (IL-8) gene expression and protein secretion and proliferation in alveolar epithelial cells A549 at physiological concentrations (1 microM). At the same time, S1P increased intracellular Ca2+ concentration (potency 17.91 microM, measured by epifluorescence microscopy), phospholipase D (PLD) activity (measured by chemiluminiscence method) and extracellular matrix-regulated kinase1/2 (ERK1/2) phosphorylation (measured by western blot) via G(i)-coupled receptor (inhibited by pertussis toxin 100 ng/ml) in A549 cells. Both, IL-8 secretion and A549 proliferation were dependent of PLD activity (inhibited by 1-butanol 0.5%), intracellular Ca2+ (inhibited by acetoxymethyl 1,2-bis(2-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid (BAPTA-AM) 100 microM), ERK1/2 phosphorylation (inhibited by 2-[2-amino-3-methoxyphenyl]-4H-1-benzopyran-4-one (PD98059) 10 microM) and G(i)-coupled receptors (blocked by pertussis toxin 100 ng/ml). Moreover, S1P increased intercellular adhesion molecule I (ICAM-1) expression and failed in vascular cell adhesion molecule I (VCAM-1) modification (measured by flow cytometer) in A549. Indirectly, A549 supernatant fluids arising from A549-S1P 1 microM stimulation decreased L-selectin expression without CD11b/CD18 integrin modification in human neutrophils. In the same way, A549-S1P supernatant fluids increased neutrophil chemotaxis (Boyden chamber), which was inhibited by antibody against IL-8. This study demonstrates for the first time that S1P participates in the alveolar epithelial interactions in vitro.

Airway epithelial cells: current concepts and challenges

The adult human bronchial tree is covered with a continuous layer of epithelial cells that play a critical role in maintaining the conduit for air, and which are central to the defenses of the lung against inhaled environmental concomitants. The epithelial sheet functions as an interdependent unit with the other lung components. Importantly, the structure and/or function of airway epithelium is deranged in major lung disorders, including chronic obstructive pulmonary disease, asthma, and bronchogenic carcinoma. Investigations regarding the airway epithelium have led to many advances over the past few decades, but new developments in genetics and stem cell/progenitor cell biology have opened the door to understanding how the airway epithelium is developed and maintained, and how it responds to environmental stress. This article provides an overview of the current state of knowledge regarding airway epithelial stem/progenitor cells, gene expression, cell-cell interactions, and less frequent cell types, and discusses the challenges for future areas of investigation regarding the airway epithelium in health and disease.

Molecular mechanisms of corticosteroid resistance

Most patients with asthma are successfully treated with conventional therapy. Nevertheless, there is a small proportion of asthmatic patients, including present cigarette smokers and former cigarette smokers, who fail to respond well to therapy with high-dose glucocorticoids (GCs) or with supplementary therapy. In addition, high doses of steroids have a minimal effect on the inexorable decline in lung function in COPD patients and only a small effect on reducing exacerbations. GC insensitivity, therefore, presents a profound management problem in these patients. GCs act by binding to a cytosolic GC receptor (GR), which is subsequently activated and is able to translocate to the nucleus. Once in the nucleus, the GR either binds to DNA and switches on the expression of antiinflammatory genes or acts indirectly to repress the activity of a number of distinct signaling pathways such as nuclear factor (NF)-kappaB and activator protein (AP)-1. This latter step requires the recruitment of corepressor molecules. Importantly, this latter interaction is mutually repressive in that high levels of NF-kappaB and AP-1 attenuate GR function. A failure to respond may therefore result from reduced GC binding to GR, reduced GR expression, enhanced activation of inflammatory pathways, or lack of corepressor activity. These events can be modulated by oxidative stress, T-helper type 2 cytokines, or high levels of inflammatory mediators, all of which may lead to a reduced clinical outcome. Understanding the molecular mechanisms of GR action, and inaction, may lead to the development of new antiinflammatory drugs or may reverse the relative steroid insensitivity that is characteristic of patients with these diseases.

New targets for drug development in asthma

Asthma is a chronic inflammatory disease that affects about 300 million people worldwide, a total that is expected to rise to about 400 million over the next 15-20 years. Most asthmatic individuals respond well to the currently available treatments of inhaled corticosteroids and beta-adrenergic agonists; however, 5-10% have severe disease that responds poorly. Improved knowledge of asthma mechanisms has led to the recognition of different asthma phenotypes that might reflect distinct types of inflammation, explaining the effectiveness of anti-leucotrienes and the anti-IgE monoclonal antibody omalizumab in some patients. However, more knowledge of the inflammatory mechanisms within the airways is required. Improvements in available therapies-such as the development of fast-onset, once-a-day combination drugs with better safety profiles-will occur. Other drugs, such as inhaled p38 MAPK inhibitors and anti-oxidants, that target specific pathways or mediators could prove useful as monotherapies, but could also, in combination with corticosteroids, reduce the corticosteroid insensitivity often seen in severe asthma. Biological agents directed against the interleukin-13 pathway and new immunoregulatory agents that modulate functions of T-regulatory and T-helper-17 cells are likely to be successful. Patient-specific treatments will depend on the development of discriminatory handprints of distinct asthma subtypes and are probably over the horizon. Although a cure is unlikely to be developed in the near future, a greater understanding of disease mechanisms could bring such a situation nearer to reality.

Airway remodeling measured by multidetector CT is increased in severe asthma and correlates with pathology

BACKGROUND

To prospectively apply an automated, quantitative three-dimensional approach to imaging and airway analysis to assess airway remodeling in asthma patients.

METHODS

Using quantitative software (Pulmonary Workstation, version 0.139; VIDA Diagnostics; Iowa City, IA) that enables quantitative airway segment measurements of low-dose, thin-section (0.625 to 1.25 mm), multidetector-row CT (MDCT) scans, we compared airway wall thickness (WT) and wall area (WA) in 123 subjects participating in a prospective multicenter cohort study, the National Institutes of Health Severe Asthma Research Program (patients with severe asthma, n = 63; patients with mild-to-moderate asthma, n = 35); and healthy subjects, n = 25). A subset of these subjects underwent fiberoptic bronchoscopy and endobronchial biopsies (n = 32). WT and WA measurements were corrected for total airway diameter and area: WT and WA, respectively.

RESULTS

Subjects with severe asthma had a significantly greater WT% than patients with mild-to-moderate asthma and healthy subjects (17.2 +/- 1.5 vs 16.5 +/- 1.6 [p = 0.014] and 16.3 +/- 1.2 [p = 0.031], respectively) and a greater WA percentage (WA%) compared to patients with mild-to-moderate asthma and healthy subjects (56.6 +/- 2.9 vs 54.7 +/- 3.3 [p = 0.005] and 54.6 +/- 2.4 [p = 0.003], respectively). Both WT% and WA% were inversely correlated with baseline FEV(1) percent predicted (r = -0.39, p < 0.0001 and r = -0.40, p < 0.0001, respectively) and positively correlated with response to a bronchodilator (r = 0.28, p = 0.002 and r = 0.35, p < 0.0001, respectively). The airway epithelial thickness measure on the biopsy sample correlated with WT% (r = 0.47; p = 0.007) and WA% (r = 0.52; p = 0.003). In the same individual, there is considerable regional heterogeneity in airway WT.

CONCLUSION

Patients with severe asthma have thicker airway walls as measured on MDCT scan than do patients with mild asthma or healthy subjects, which correlates with pathologic measures of remodeling and the degree of airflow obstruction. MDCT scanning may be a useful technique for assessing airway remodeling in asthma patients, but overlap among the groups limits the diagnostic value in individual subjects.

Alterations of the arginine metabolome in asthma

RATIONALE

As the sole nitrogen donor in nitric oxide (NO) synthesis and key intermediate in the urea cycle, arginine and its metabolic pathways are integrally linked to cellular respiration, metabolism, and inflammation.

OBJECTIVES

We hypothesized that arginine (Arg) bioavailability would be associated with airflow abnormalities and inflammation in subjects with asthma, and would be informative for asthma severity.

METHODS

Arg bioavailability was assessed in subjects with severe and nonsevere asthma and healthy control subjects by determination of plasma Arg relative to its metabolic products, ornithine and citrulline, and relative to methylarginine inhibitors of NO synthases, and by serum arginase activity. Inflammatory parameters, including fraction of exhaled NO (Fe(NO)), IgE, skin test positivity to allergens, bronchoalveolar lavage, and blood eosinophils, were also evaluated.

MEASUREMENTS AND MAIN RESULTS

Subjects with asthma had greater Arg bioavailability, but also increased Arg catabolism compared with healthy control subjects, as evidenced by higher levels of Fe(NO) and serum arginase activity. However, Arg bioavailability was positively associated with Fe(NO) only in healthy control subjects; Arg bioavailability was unrelated to Fe(NO) or other inflammatory parameters in severe or nonsevere asthma. Inflammatory parameters were related to airflow obstruction and reactivity in nonsevere asthma, but not in severe asthma. Conversely, Arg bioavailability was related to airflow obstruction in severe asthma, but not in nonsevere asthma. Modeling confirmed that measures of Arg bioavailabilty predict airflow obstruction only in severe asthma.

CONCLUSIONS

Unlike Fe(NO), Arg bioavailability is not a surrogate measure of inflammation; however, Arg bioavailability is strongly associated with airflow abnormalities in severe asthma.

Steroid resistance in asthma: mechanisms and treatment options

Glucocorticoid insensitivity presents a profound management problem in patients with asthma because conventional therapies are not effective. Glucocorticoids, acting through the glucocorticoid receptor (GR), are able to selectively repress inflammatory gene expression by utilizing several distinct mechanisms targeting nuclear factor-varphiB and activator protein-1 activation complexes and by effects on mitogen-activated protein kinases. Different model systems often activate distinct sets of signaling molecules and different glucocorticoid responsiveness may result from differences in concentrations and timing of steroid treatment of cells, GR expression levels, and the precise inflammatory stimulus used. Thus, abnormal activation of many signaling pathways may affect corticosteroid responsiveness in patients with corticosteroid-resistant asthma. Understanding the molecular mechanisms of GR action and inaction may lead to the development of new anti-inflammatory drugs or enable clinicians to reverse the relative steroid-insensitivity that is characteristic of some patients with severe asthma.

Airway modeling and remodeling in the pathogenesis of asthma

PURPOSE OF REVIEW

Asthma remains a severe health problem since current therapies are directed to suppressing, rather than preventing or reversing, the primary disease process. Clearly, a greater understanding of the pathogenesis of asthma is critical to the development of better therapeutic modalities. In this review, we discuss the recent advancements in research targeting the role of airway remodeling in asthma.

RECENT FINDINGS

Epithelial fragility and abnormalities are being recognized as important facets of asthma, as are other features of remodeling such as angiogenesis, goblet cell hyperplasia and thickened lamina reticularis. Significantly, these anomalies occur early in disease pathogenesis. However, their impact on disease severity remains unclear.

SUMMARY

Although an altered immune response is undoubtedly important to the pathogenesis of asthma, there is increasing evidence that the tissue-specific manifestations occur independently of inflammation and significantly impact on disease development and severity.

Role of airway epithelial cells in development of asthma and allergic rhinitis

Asthma and allergic rhinitis frequently coexist in the same patient. There is a similarity and variation as well as potential relationship between asthma and allergic rhinitis. There is an increasing evidence to suggest a major involvement of airway epithelial cells in the pathogenesis of asthma and allergic rhinitis. The present review describes the importance of the airway epithelial cell in the development of allergic airway diseases, its role as the primary airway defense against exposure of the airway and lung to inflammatory stimuli and antigens and as an important player through activation of epithelial Toll-like receptors (TLRs) to provide an important link between innate immunity and allergic disease. Additionally, airway epithelial cells can act as inflammatory promoters capable of directing dendritic cells (DCs) towards a T helper 2 (Th2) response, and as active producers of several inflammatory/anti-inflammatory mediators. It is hypothesized that airway epithelial cells may play as both inflammatory initiator and immuno-pathological feedback regulation between allergic rhinitis and asthma via release of systemic inflammatory mediators. Thus, airway epithelial cells may be valuable therapeutic targets for discovery and development of new drugs and/or new therapeutic strategies to treat asthma and allergic rhinitis.

The airway epithelium is central to the pathogenesis of asthma

Asthma is an inflammatory disorder principally involving the conducting airways and characterised by infiltration of the airway wall with a range of inflammatory cells driven in large part by activation of Th2-type lymphocytes, mast cells and eosinophils. However a key component of asthma is the structural change that involves all of the elements of the airway wall. Here evidence is presented to suggest that the airway epithelium in asthma is fundamentally abnormal with increased susceptibility to environmental injury and impaired repair associated with activation of the epithelial-mesenchymal trophic unit (EMTU). In addition to adopting an activated phenotype, the barrier function of the epithelium is impaired through defective tight junction formation thereby facilitating penetration of potentially toxic or damaging environmental insults. Activated and repairing epithelial cells generate a range of growth factors that are involved in the early life origins of this disease as well as its progression in the form of mucous metaplasia and airway wall remodeling. By placing the epithelium at the forefront of asthma pathogenesis, different approaches to treatment can be devised focused more on protecting vulnerable airways against environmental injury rather than focusing on suppressing airway inflammation or manipulating the immune response.

Glucocorticoid actions on airway epithelial responses in immunity: functional outcomes and molecular targets

Research on the biology of airway epithelium in the last decades has progressively uncovered the many roles of this cell type during the immune response. Far from the early view of the epithelial layer simply as a passive barrier, the airway epithelium is now considered a central player in mucosal immunity, providing innate mechanisms of first-line host defense as well as facilitating adaptive immune responses. Alterations of the epithelial phenotype are primarily involved in the pathogenesis of allergic airways disease, particularly in severe asthma. Appreciation of the epithelium as target of glucocorticoid therapy has also grown, because of studies defining the pathways and mediators affected by glucocorticoids, and studies illustrating the relevance of the control of the response from epithelium in the overall efficacy of topical and systemic therapy with glucocorticoids. Studies of the mechanism of action of glucocorticoids within the biology of the immune response of the epithelium have uncovered mechanisms of gene regulation involving both transcriptional and posttranscriptional events. The view of epithelium as therapeutic target therefore has plenty of room to evolve, as new knowledge on the role of epithelium in immunity is established and novel pathways mediating glucocorticoid regulation are elucidated.

Epithelium dysfunction in asthma

Although asthma is an inflammatory disorder of the conducting airways involving T(H)2-type T cells, there is increasing evidence for an important role played by the epithelium in orchestrating the inflammatory response by interacting with multiple environmental factors to produce a chronic wound scenario involving tissue injury and aberrant repair. Part of this abnormal response is the consequence of impaired barrier function caused by a primary disruption of epithelial tight junctions that allows inhaled substances to pass more easily into the airway wall to interact with immune and inflammatory cells. Aberrant communication between the damaged and stressed epithelium leads to the generation of growth factors that interact with the underlying mesenchyme to promote airway remodeling responses and a more chronic and persistent inflammatory phenotype. Disordered epithelial function with reduced antioxidant defense and impaired capacity to produce primary IFNs may also account for asthmatic susceptibility to air pollution and respiratory virus infection, respectively. Considering asthma as a disease of impaired barrier function opens new opportunities for therapeutic intervention or prevention by agents that could increase the airways resistance to the inhaled environment rather than suppressing the immune or inflammatory response.