Effects of Th-2 type cytokines on human airway epithelial cells: interleukins-4, -5, and -13

mTOR signaling regulates aberrant epithelial cell proliferative and migratory behaviors characteristic of airway mucous metaplasia in asthma

In asthma, the airway epithelium is hyperplastic, hypertrophied, and lined with numerous large MUC5AC-containing goblet cells (GC). Furthermore, the normal epithelial architecture is disorganized with numerous, what we here describe as, ectopic goblet cells (eGC) deep within the thickened epithelial layer disconnected from the lumenal surface. mTOR is a highly conserved pathway that regulates cell size and proliferation. We hypothesized that the balance between mTOR and autophagy signaling regulates key features of the asthma epithelial layer. Airway histological sections from subjects with asthma had increased frequency of eGC and increased levels of mTOR phosphorylation target-Ribosomal S6. Using human airway epithelial cells (hAECs) with IL-13 stimulation and timed withdrawal to stimulate resolution, we found that multiple key downstream phosphorylation targets downstream from the mTOR complex were increased during early IL-13-mediated mucous metaplasia, and then significantly declined during resolution. The IL-13-mediated changes in mTOR signaling were paralleled by morphologic changes with airway epithelial hypertrophy, hyperplasia, and frequency of eGC. We then examined the relationship between mTOR and autophagy using mice deficient in autophagy protein Atg16L1. Despite having increased cytoplasmic mucins, mouse AECs from Atg16L1 deficient mice had no significant difference in mTOR downstream signaling. mTOR inhibition with rapamycin led to a loss of IL-13-mediated epithelial hypertrophy, hyperplasia, ectopic GC distribution, and reduction in cytoplasmic MUC5AC levels. mTOR inhibition was also associated with a reduction in aberrant IL-13-mediated hAEC proliferation and migration. Our findings demonstrate that mTOR signaling is associated with mucous metaplasia and is crucial to the disorganized airway epithelial structure and function characteristic of muco-obstructive airway diseases such as asthma.

Graphical Abstract Key Concepts

The airway epithelium in asthma is disorganized and characterized by cellular proliferation, aberrant migration, and goblet cell mucous metaplasia.mTOR signaling is a dynamic process during IL-13-mediated mucous metaplasia, increasing with IL-13 stimulation and declining during resolution.mTOR signaling is strongly increased in the asthmatic airway epithelium.mTOR signaling is associated with the development of key features of the metaplastic airway epithelium including cell proliferation and ectopic distribution of goblet cells and aberrant cellular migration.Inhibition of mTOR leads to decreased epithelial hypertrophy, reduced ectopic goblet cells, and cellular migration.

Epigen is induced during the interleukin-13-stimulated cell proliferation in murine primary airway epithelial cells

Airway remodeling in bronchial asthma is characterized by epithelial detachment and proliferation, subepithelial fibrosis, increased smooth muscle mass, and goblet cell hyperplasia. These features are mediated by T-helper type 2 (Th2) cytokines including interleukin (IL)-13. However, the direct effects of IL-13 on the functions of airway epithelial cells are not fully understood. Murine primary airway epithelial (MPAE) cells were cultured in an air-liquid interface (ALI) culture system. AG1478, a specific inhibitor of epidermal growth factor receptor (EGFR) tyrosine kinase, was used to examine whether EGFR was involved in the IL-13-stimulated proliferation of MPAE cells. The expressions of EGFR ligands were investigated by reverse transcriptase-polymerase chain reaction, Western blotting, and immunohistochemical analyses. The cell counting in cross-sections and [(3)H]thymidine incorporation assays revealed a significant increase in the number of MPAE cells cultured with IL-13 compared with a phosphate-buffered saline (PBS) control. AG1478 abolished the IL-13-stimulated proliferation of MPAE cells. The expression of epigen, one of the EGFR ligands, was enhanced in MPAE cells cultured with IL-13. The findings suggest that EGFR is involved in the IL-13-stimulated proliferation of MPAE cells, and that epigen is important for the proliferation process in airway remodeling.

Integrin beta4 was downregulated on the airway epithelia of asthma patients

The shedding of airway epithelial cells and loss of epithelial functional homeostasis are major pathological characteristics of asthma; however, the mechanism underlying these pathologies remains obscure. Our previous work showed that there were three variation sites in 5' flanking region of integrin beta4 in asthma patients, which was correlated with decreased expression of integrin beta4 in peripheral leukocytes. Integrin beta4 is an important structural adhesion molecule on airway epithelia to keep the structural adhesion of epithelial cells. In this work, we further demonstrated that integrin beta4 expression was downregulated in airway epithelia of asthma patients. To probe the relationship between imbalanced expression of integrin beta4 and dysfunction of the airway epithelial cells in asthma, integrin beta4 was silenced in human bronchial epithelium cells (16HBE14O) by integrin beta4 small-interfering RNA lentivirus vector. Upon silencing of integrin beta4, 16HBE14O cells showed reduced proliferation and wound repair. Most cells were shown to be arrested in G1 phase after integrin beta4 silencing, and increased apoptosis was induced in the integrin beta4-silenced cells. In summary, our results provided compelling evidence that integrin beta4 was involved in the structural integrity and functional homeostasis of airway epithelial cells. It is likely that downregulation of integrin beta4 on asthma airway epithelia contributes to the structural disruption and dysfunction of airway epithelial cells.

Alcohol primes the airway for increased interleukin-13 signaling

BACKGROUND

Using an experimental model of airway fibrosis following lung transplantation, we recently showed that chronic alcohol ingestion by donor rats amplifies airway fibrosis in the recipient. Associated with alcohol-mediated amplification of airway fibrosis is increased transforming growth factor beta-1(TGFbeta(1)) and alpha-smooth muscle actin expression. Other studies have shown that interleukin-13 (IL-13) modulates TGFbeta(1) signaling during experimentally-induced airway fibrosis. Therefore, we hypothesized that IL-13 is a component of alcohol-mediated amplification of pro-fibrotic mediators in the alcoholic lung.

METHODS

To test this hypothesis, we analyzed tracheal epithelial cells and type II alveolar cells from control- or alcohol-fed rats, alcohol-treated mouse lung fibroblasts, and human bronchial epithelial cells in vitro for expression of various components of the IL-13 signaling pathway. Signaling via the IL-13 pathway was assessed by measuring levels of phosphorylated signal transducers and activators of transcription-6 (STAT6). In addition, we performed heterotopic tracheal transplantation using control-fed and alcohol-fed donor rats and analyzed tracheal allografts for expression of components of the IL-13 signaling pathway by RT-PCR and immunocytochemical analyses.

RESULTS

Interleukin-13 expression was detected in type II alveolar epithelial cells and human bronchial epithelial cells, but not in lung fibroblasts. IL-13 expression was decreased in whole lung and type II cells in response to alcohol exposure. In all cell types analyzed, expression of IL-13 signaling receptor (IL-13R alpha(1)) mRNA was markedly increased. In contrast, mRNA and protein expression of the IL-13 decoy receptor (IL-13R alpha(2)) were decreased in all cells analyzed. Exposure to alcohol also increased STAT6 phosphorylation in response to IL-13 and lipopolysaccharide.

CONCLUSIONS

Data from multiple cell types in the pulmonary system suggest that IL-13 and its receptors play a role in alcohol-mediated activation of pro-fibrotic pathways. Taken together, these data suggest that alcohol primes the airway for increased IL-13 signaling and subsequent tissue remodeling upon injury such as transplantation.

The role of epithelial injury and repair in the origins of asthma

PURPOSE OF REVIEW

We currently understand little about the mechanisms that lead to asthma. The bronchial epithelium is the first cell layer of contact with the environment and as such is an especially attractive target in which to identify novel mechanisms and new therapeutic strategies in disease development. We discuss the role of epithelial injury and wound repair in the origins of asthma.

RECENT FINDINGS

The presence of inflammation, thickening of the basement membrane and angiogenesis have been described in bronchial biopsies from asthmatic children. We and others have demonstrated the utility of bronchial brushings from children for the isolation, characterization and culture of primary epithelial cells. The results of these experiments suggest that intrinsic differences exist between asthmatic and nonasthmatic epithelial cells.

SUMMARY

It is becoming increasingly clear from studies involving adults and, more recently, children, that the epithelium orchestrates inflammatory and remodeling responses of the airway. Equally clear is that the asthmatic epithelium responds inappropriately to challenge and displays signs of dysregulated repair. Understanding the regulatory mechanisms involved in these processes, including the role of resident/recruited progenitor cells, is crucial if we are to halt the progression of asthma when the disease first manifests in childhood.

IL-17A promotes the growth of airway epithelial cells through ERK-dependent signaling pathway

The effects of IL-17A on mucin production and growth of airway epithelial cells were examined. Histological and immunohistochemical analyses revealed that IL-17A increased the mucin production and number of tracheal epithelial cells in air-liquid interface cultures. The biological property of IL-17A to stimulate the mucin production by tracheal epithelial cells was determined using an ELISA. The mitogenic effect of IL-17A on tracheal epithelial cells was confirmed with Calcein-AM assay. The growth-stimulatory effect of IL-17A was dose-dependent and mediated via the ERK MAP kinase pathway. Inhibitors of MEK abrogated the mitogenic effect of IL-17A, whereas an inhibitor of p38 or JNK displayed no significant inhibitory effect. Moreover, relatively lower doses of IL-13 also significantly increased the growth of tracheal epithelial cells through a distinct signaling pathway from that of IL-17A. These findings provide the first evidence that IL-17A stimulates the growth of airway epithelial cells through the ERK MAP kinase pathway.

Airway smooth muscle growth from the perspective of animal models

Airway smooth muscle maintains airway tone and may assist in adjusting ventilation distribution within the normal lung. Alterations in the properties or the quantity of ASM are likely responsible for some instances of airways hyperresponsiveness to bronchoconstrictive stimuli that is a characteristic of diseases such as asthma. Morphometric studies have shown an increase in the mass of ASM in human asthmatic airways. Animal models have been developed that confirm that ASM can be induced to grow by allergic sensitization and challenge. Growth is in large part by hyperplasia as measured by incorporation of bromodeoxyuridine as a marker of the S-phase of the cell cycle. T cells, in particular CD4+ cells, may participate in the stimulation of growth of ASM by allergen challenge. The growth factors responsible for the increase in ASM are as yet unidentified but two mediators associated with allergic airway responses, cysteinyl leukotrienes and endothelin, have been implicated using specific receptor antagonists. The links between T cells and the biochemical mediators of growth have not been established.