Insulin receptor substrate-1/2 mediates IL-4-induced migration of human airway epithelial cells

Type 2 inflammation drives an airway basal stem cell program through insulin receptor substrate signaling

BACKGROUND

Chronic rhinosinusitis with nasal polyposis (CRSwNP) is a type 2 (T2) inflammatory disease associated with an increased number of airway basal cells (BCs). Recent studies have identified transcriptionally distinct BCs, but the molecular pathways that support or inhibit human BC proliferation and differentiation are largely unknown.

OBJECTIVE

We sought to determine the role of T2 cytokines in regulating airway BCs.

METHODS

Single-cell and bulk RNA sequencing of sinus and lung airway epithelial cells was analyzed. Human sinus BCs were stimulated with IL-4 and IL-13 in the presence and absence of inhibitors of IL-4R signaling. Confocal analysis of human sinus tissue and murine airway was performed. Murine BC subsets were sorted for RNA sequencing and functional assays. Fate labeling was performed in a murine model of tracheal injury and regeneration.

RESULTS

Two subsets of BCs were found in human and murine respiratory mucosa distinguished by the expression of basal cell adhesion molecule (BCAM). BCAM expression identifies airway stem cells among P63+KRT5+NGFR+ BCs. In the sinonasal mucosa, BCAMhi BCs expressing TSLP, IL33, CCL26, and the canonical BC transcription factor TP63 are increased in patients with CRSwNP. In cultured BCs, IL-4/IL-13 increases the expression of BCAM and TP63 through an insulin receptor substrate-dependent signaling pathway that is increased in CRSwNP.

CONCLUSIONS

These findings establish BCAM as a marker of airway stem cells among the BC pool and demonstrate that airway epithelial remodeling in T2 inflammation extends beyond goblet cell metaplasia to the support of a BC stem state poised to perpetuate inflammation.

The Role of Insulin Receptor Substrate Proteins in Bronchopulmonary Dysplasia and Asthma: New Potential Perspectives

Insulin receptor substrates (IRSs) are proteins that are involved in signaling through the insulin receptor (IR) and insulin-like growth factor (IGFR). They can also interact with other receptors including growth factor receptors. Thus, they represent a critical node for the transduction and regulation of multiple signaling pathways in response to extracellular stimuli. In addition, IRSs play a central role in processes such as inflammation, growth, metabolism, and proliferation. Previous studies have highlighted the role of IRS proteins in lung diseases, in particular asthma. Further, the members of the IRS family are the common proteins of the insulin growth factor signaling cascade involved in lung development and disrupted in bronchopulmonary dysplasia (BPD). However, there is no study focusing on the relationship between IRS proteins and BPD yet. Unfortunately, there is still a significant gap in knowledge in this field. Thus, in this review, we aimed to summarize the current knowledge with the major goal of exploring the possible roles of IRS in BPD and asthma to foster new perspectives for further investigations.

Genome-wide DNA methylation analysis of pulmonary function in middle and old-aged Chinese monozygotic twins

BACKGROUND

Previous studies have determined the epigenetic association between DNA methylation and pulmonary function among various ethnics, whereas this association is largely unknown in Chinese adults. Thus, we aimed to explore epigenetic relationships between genome-wide DNA methylation levels and pulmonary function among middle-aged Chinese monozygotic twins.

METHODS

The monozygotic twin sample was drawn from the Qingdao Twin Registry. Pulmonary function was measured by three parameters including forced expiratory volume the first second (FEV1), forced vital capacity (FVC), and FEV1/FVC ratio. Linear mixed effect model was used to regress the methylation level of CpG sites on pulmonary function. After that, we applied Genomic Regions Enrichment of Annotations Tool (GREAT) to predict the genomic regions enrichment, and used comb-p python library to detect differentially methylated regions (DMRs). Gene expression analysis was conducted to validate the results of differentially methylated analyses.

RESULTS

We identified 112 CpG sites with the level of P < 1 × 10^-4 which were annotated to 40 genes. We identified 12 common enriched pathways of three pulmonary function parameters. We detected 39 DMRs located at 23 genes, of which PRDM1 was related to decreased pulmonary function, and MPL, LTB4R2, and EPHB3 were related to increased pulmonary function. The gene expression analyses validated DIP2C, ASB2, SLC6A5, and GAS6 related to decreased pulmonary function.

CONCLUSION

Our DNA methylation sequencing analysis on identical twins provides new references for the epigenetic regulation on pulmonary function. Several CpG sites, genes, biological pathways and DMRs are considered as possible crucial to pulmonary function.

Insulin-Like Growth Factor-1 Signaling in Lung Development and Inflammatory Lung Diseases

Insulin-like growth factor-1 (IGF-1) was firstly identified as a hormone that mediates the biological effects of growth hormone. Accumulating data have indicated the role of IGF-1 signaling pathway in lung development and diseases such as congenital disorders, cancers, inflammation, and fibrosis. IGF-1 signaling modulates the development and differentiation of many types of lung cells, including airway basal cells, club cells, alveolar epithelial cells, and fibroblasts. IGF-1 signaling deficiency results in alveolar hyperplasia in humans and disrupted lung architecture in animal models. The components of IGF-1 signaling pathways are potentiated as biomarkers as they are dysregulated locally or systemically in lung diseases, whereas data may be inconsistent or even paradoxical among different studies. The usage of IGF-1-based therapeutic agents urges for more researches in developmental disorders and inflammatory lung diseases, as the majority of current data are collected from limited number of animal experiments and are generally less exuberant than those in lung cancer. Elucidation of these questions by further bench-to-bedside researches may provide us with rational clinical diagnostic approaches and agents concerning IGF-1 signaling in lung diseases.

Chemokine expression in the early response to injury in human airway epithelial cells

Basal airway epithelial cells (AEC) constitute stem/progenitor cells within the central airways and respond to mucosal injury in an ordered sequence of spreading, migration, proliferation, and differentiation to needed cell types. However, dynamic gene transcription in the early events after mucosal injury has not been studied in AEC. We examined gene expression using microarrays following mechanical injury (MI) in primary human AEC grown in submersion culture to generate basal cells and in the air-liquid interface to generate differentiated AEC (dAEC) that include goblet and ciliated cells. A select group of ~150 genes was in differential expression (DE) within 2-24 hr after MI, and enrichment analysis of these genes showed over-representation of functional categories related to inflammatory cytokines and chemokines. Network-based gene prioritization and network reconstruction using the PINTA heat kernel diffusion algorithm demonstrated highly connected networks that were richer in differentiated AEC compared to basal cells. Similar experiments done in basal AEC collected from asthmatic donor lungs demonstrated substantial changes in DE genes and functional categories related to inflammation compared to basal AEC from normal donors. In dAEC, similar but more modest differences were observed. We demonstrate that the AEC transcription signature after MI identifies genes and pathways that are important to the initiation and perpetuation of airway mucosal inflammation. Gene expression occurs quickly after injury and is more profound in differentiated AEC, and is altered in AEC from asthmatic airways. Our data suggest that the early response to injury is substantially different in asthmatic airways, particularly in basal airway epithelial cells.

New insights into the mechanisms of pulmonary edema in acute lung injury

Appearance of alveolar protein-rich edema is an early event in the development of acute respiratory distress syndrome (ARDS). Alveolar edema in ARDS results from a significant increase in the permeability of the alveolar epithelial barrier, and represents one of the main factors that contribute to the hypoxemia in these patients. Damage of the alveolar epithelium is considered a major mechanism responsible for the increased pulmonary permeability, which results in edema fluid containing high concentrations of extravasated macromolecules in the alveoli. The breakdown of the alveolar-epithelial barrier is a consequence of multiple factors that include dysregulated inflammation, intense leukocyte infiltration, activation of pro-coagulant processes, cell death and mechanical stretch. The disruption of tight junction (TJ) complexes at the lateral contact of epithelial cells, the loss of contact between epithelial cells and extracellular matrix (ECM), and relevant changes in the communication between epithelial and immune cells, are deleterious alterations that mediate the disruption of the alveolar epithelial barrier and thereby the formation of lung edema in ARDS.

Amnion Epithelial Cells Promote Lung Repair via Lipoxin A4

Human amnion epithelial cells (hAECs) have been shown to possess potent immunomodulatory properties across a number of disease models. Recently, we reported that hAECs influence macrophage polarization and activity, and that this step was dependent on regulatory T cells. In this study, we aimed to assess the effects of hAEC-derived proresolution lipoxin-A₄ (LXA4) on T-cell, macrophage, and neutrophil phenotype and function during the acute phase of bleomycin-induced lung injury. Using C57Bl6 mice, we administered 4 million hAECs intraperitoneally 24 hours after bleomycin challenge. Outcomes were measured at days 3, 5, and 7. hAEC administration resulted in significant changes to T-cell, macrophage, dendritic cell, and monocyte/macrophage infiltration and phenotypes. Endogenous levels of lipoxygenases, LXA4, and the lipoxin receptor FPR2 were elevated in hAEC-treated animals. Furthermore, we showed that the effects of hAECs on macrophage phagocytic activity and T-cell suppression are LXA4 dependent, whereas the inhibition of neutrophil-derived myleoperoxidase by hAECs is independent of LXA4. This study provides the first evidence that lipid-based mediators contribute to the immunomodulatory effects of hAECs and further supports the growing body of evidence that LXA4 is proresolutionary in lung injury. This discovery of LXA4-dependent communication between hAECs, macrophages, T cells, and neutrophils is important to the understanding of hAEC biodynamics and would be expected to inform future clinical applications. Stem Cells Translational Medicine 2017;6:1085-1095.

Peripheral blood IRF1 expression as a marker for glucocorticoid sensitivity

OBJECTIVE

Despite of the common usage of glucocorticoids (GCs), a significant portion of asthma patients exhibit GC insensitivity. This could be mediated by diverse mechanisms, including genomics. Recent work has suggested that measuring changes in gene expression may provide more predictive information about GC insensitivity than baseline gene expression alone, and that expression changes in peripheral blood may be reflective of those in the airway.

METHODS

We performed in silico discovery using gene expression omnibus (GEO) data that evaluated GC effect on gene expression in multiple tissue types. Subsequently, candidate genes whose expression levels are affected by GC were examined in cell lines and in primary cells derived from human airway and blood.

RESULTS

Through gene expression omnibus analysis, we identified interferon regulator factor 1 (IRF1), whose expression is affected by GC treatment in airway smooth muscle cells, normal human bronchial epithelial (NHBE) cells, and lymphoblastoid cell lines (LCLs). Significant IRF1 downregulation post GC exposure was confirmed in two cultured airway epithelial cell lines and primary NHBE cells (P<0.05). We observed large interindividual variation in GC-induced IRF1 expression changes among primary NHBE cells tested. Significant downregulation of IRF1 was also observed in six randomly selected LCLs (P<0.05), with variable degrees of downregulation among different samples. In peripheral blood mononuclear cells obtained from healthy volunteers, variable downregulation of IRF1 by GC was also shown. NFKB1, a gene whose expression is known to be downregulated by GC and the degree of downregulation of which is reflective of GC response, was used as a control in our study. IRF1 shows more consistent downregulation across tissue types when compared with NFKB1.

CONCLUSION

Our results suggest that GC-induced IRF1 gene expression changes in peripheral blood could be used as a marker to reflect GC response in the airway.

Regulation of macrophage motility by the water channel aquaporin-1: crucial role of M0/M2 phenotype switch

The water channel aquaporin-1 (AQP1) promotes migration of many cell types. Although AQP1 is expressed in macrophages, its potential role in macrophage motility, particularly in relation with phenotype polarization, remains unknown. We here addressed these issues in peritoneal macrophages isolated from AQP1-deficient mice, either undifferentiated (M0) or stimulated with LPS to orientate towards pro-inflammatory phenotype (classical macrophage activation; M1). In non-stimulated macrophages, ablation of AQP1 (like inhibition by HgCl2) increased by 2-3 fold spontaneous migration in a Src/PI3K/Rac-dependent manner. This correlated with cell elongation and formation of lamellipodia/ruffles, resulting in membrane lipid and F4/80 recruitment to the leading edge. This indicated that AQP1 normally suppresses migration of resting macrophages, as opposed to other cell types. Resting Aqp1-/- macrophages exhibited CD206 redistribution into ruffles and increased arginase activity like IL4/IL13 (alternative macrophage activation; M2), indicating a M0-M2 shift. In contrast, upon M1 orientation by LPS in vitro or peritoneal inflammation in vivo, migration of Aqp1-/- macrophages was reduced. Taken together, these data indicate that AQP1 oppositely regulates macrophage migration, depending on stimulation or not by LPS, and that macrophage phenotypic and migratory changes may be regulated independently of external cues.

Delay of airway epithelial wound repair in COPD is associated with airflow obstruction severity

BACKGROUND

Airway epithelium integrity is essential to maintain its role of mechanical and functional barrier. Recurrent epithelial injuries require a complex mechanism of repair to restore its integrity. In chronic obstructive pulmonary disease (COPD), an abnormal airway epithelial repair may participate in airway remodeling. The objective was to determine if airway epithelial wound repair of airway epithelium is abnormal in COPD.

METHODS

Patients scheduled for lung resection were prospectively recruited. Demographic, clinical data and pulmonary function tests results were recorded. Emphysema was visually scored and histological remodeling features were noted. Primary bronchial epithelial cells (BEC) were extracted and cultured for wound closure assay. We determined the mean speed of wound closure (MSWC) and cell proliferation index, matrix metalloprotease (MMP)-2, MMP-9 and cytokines levels in supernatants of BEC 18 hours after cell wounding. In a subset of patients, bronchiolar epithelial cells were also cultured for wound closure assay for MSWC analyze.

RESULTS

13 COPD and 7 non COPD patients were included. The severity of airflow obstruction and the severity of emphysema were associated with a lower MSWC in BEC (p = 0.01, 95% CI [0.15-0.80]; p = 0.04, 95% CI [-0.77;-0.03] respectively). Cell proliferation index was decreased in COPD patients (19 ± 6% in COPD vs 27 ± 3% in non COPD, p = 0.04). The severity of COPD was associated with a lower level of MMP-2 (7.8 ± 2 10(5) AU in COPD GOLD D vs 12.8 ± 0.13 10(5) AU in COPD GOLD A, p = 0.04) and a lower level of IL-4 (p = 0.03, 95% CI [0.09;0.87]). Moreover, higher levels of IL-4 and IL-2 were associated with a higher MSWC (p = 0.01, 95% CI [0.17;0.89] and p = 0.02, 95% CI [0.09;0.87] respectively). Clinical characteristics and smoking history were not associated with MSWC, cell proliferation index or MMP and cytokines levels. Finally, we showed an association of the MSWC of bronchial and corresponding bronchiolar epithelial cells obtained from the same patients (p = 0.02, 95% CI [0.12;0.89]).

CONCLUSION

Our results showed an abnormal bronchial epithelial wound closure process in severe COPD. Further studies are needed to elucidate the contribution and the regulation of this mechanism in the complex pathophysiology of COPD.

Interleukin-1 receptor-associated kinase M (IRAK-M) promotes human rhinovirus infection in lung epithelial cells via the autophagic pathway

Human rhinovirus (HRV) is the most common viral etiology in acute exacerbations of asthma. However, the exact mechanisms underlying HRV infection in allergic airways are poorly understood. IL-13 increases interleukin-1 receptor associated kinase M (IRAK-M) and subsequently inhibits airway innate immunity against bacteria. However, the role of IRAK-M in lung HRV infection remains unclear. Here, we provide the first evidence that IRAK-M over-expression promotes lung epithelial HRV-16 replication and autophagy, but inhibits HRV-16-induced IFN-β and IFN-λ1 expression. Inhibiting autophagy reduces HRV-16 replication. Exogenous IFN-β and IFN-λ1 inhibit autophagy and HRV-16 replication. Our data indicate the enhancing effect of IRAK-M on epithelial HRV-16 infection, which is partly through the autophagic pathway. Impaired anti-viral interferon production may serve as a direct or an indirect (e.g., autophagy) mechanism of enhanced HRV-16 infection by IRAK-M over-expression. Targeting autophagic pathway or administrating anti-viral interferons may prevent or attenuate viral (e.g., HRV-16) infections in allergic airways.

Experimental analysis of risk factors for ulcerative dermatitis in mice

Ulcerative dermatitis (UD) is a severe inflammatory skin disorder with an unknown aetiology. Recently, insulin receptor substrate 1 KO mice were shown to be fully resistant to UD. In this study, we showed that high-fat diet (HFD) feeding significantly increased incidence of UD in wild type (WT) C57BL/6 mice, as did lithium-mediated inhibition of GSK3-β, which is a key negative regulator of IRS1. In contrast to WT mice, resistance to UD was fully preserved in HFD-fed Irs1-KO mice. Our results identify IRS1 as a key determinant of UD pathogenesis and establish a direct link between diet composition, obesity-induced inflammation and chronic ulceration.

Human leukocyte antigen-G expression in differentiated human airway epithelial cells: lack of modulation by Th2-associated cytokines

Background

Human leukocyte antigen (HLA)-G is a nonclassical class I antigen with immunomodulatory roles including up-regulation of suppressor T regulatory lymphocytes. HLA-G was recently identified as an asthma susceptibility gene, and expression of a soluble isoform, HLA-G5, has been demonstrated in human airway epithelium. Increased presence of HLA-G5 has been demonstrated in bronchoalveolar lavage fluid recovered from patients with mild asthma; this suggests a role for this isoform in modulating airway inflammation though the mechanisms by which this occurs is unclear. Airway inflammation associated with Th2 cytokines such as IL-4 and IL-13 is a principal feature of asthma, but whether these cytokines elicit expression of HLA-G is not known.

Methods

We examined gene and protein expression of both soluble (G5) and membrane-bound (G1) HLA-G isoforms in primary differentiated human airway epithelial cells collected from normal lungs and grown in air-liquid interface culture. Cells were treated with up to 10 ng/ml of either IL-4, IL-5, or IL-13, or 100 ng/ml of the immunomodulatory cytokine IL-10, or 10,000 U/ml of the Th1-associated cytokine interferon-beta, for 24 hr, after which RNA was isolated for evaluation by quantitative PCR and protein was collected for Western blot analysis.

Results

HLA-G5 but not G1 was present in dAEC as demonstrated by quantitative PCR, western blot and confocal microscopy. Neither G5 nor G1 expression was increased by the Th2-associated cytokines IL-4, IL-5 or IL-13 over 24 hr, nor after treatment with IL-10, but was increased 4.5 ± 1.4 fold after treatment with 10,000 U/ml interferon-beta.

Conclusions

These data demonstrate the constitutive expression of a T lymphocyte regulatory molecule in differentiated human airway epithelial cells that is not modulated by Th2-associated cytokines.

Knowledge translation: airway epithelial cell migration and respiratory diseases

Airway epithelial cell migration is essential for lung development and growth, as well as the maintenance of respiratory tissue integrity. This vital cellular process is also important for the repair and regeneration of damaged airway epithelium. More importantly, several lung diseases characterized by aberrant tissue remodeling result from the improper repair of damaged respiratory tissue. Epithelial cell migration relies upon extracellular matrix molecules and is further regulated by numerous local, neuronal, and hormonal factors. Under inflammatory conditions, cell migration can also be stimulated by certain cytokines and chemokines. Many well-known environmental factors involved in the pathogenesis of chronic lung diseases (e.g., cigarette smoking, air pollution, alcohol intake, inflammation, viral and bacterial infections) can inhibit airway epithelial cell migration. Further investigation of cellular and molecular mechanisms of cell migration with advanced techniques may provide knowledge that is relevant to physiological and pathological conditions. These studies may eventually lead to the development of therapeutic interventions to improve lung repair and regeneration and to prevent aberrant remodeling in the lung.

Acute lung injury: how macrophages orchestrate resolution of inflammation and tissue repair

Lung macrophages are long living cells with broad differentiation potential, which reside in the lung interstitium and alveoli or are organ-recruited upon inflammatory stimuli. A role of resident and recruited macrophages in initiating and maintaining pulmonary inflammation in lung infection or injury has been convincingly demonstrated. More recent reports suggest that lung macrophages are main orchestrators of termination and resolution of inflammation. They are also initiators of parenchymal repair processes that are essential for return to homeostasis with normal gas exchange. In this review we will discuss cellular cross-talk mechanisms and molecular pathways of macrophage plasticity which define their role in inflammation resolution and in initiation of lung barrier repair following lung injury.

Interleukin(IL)-4 promotion of CXCL-8 gene transcription is mediated by ERK1/2 pathway in human pulmonary artery endothelial cells

Interleukin-4 is central to allergic pulmonary inflammatory responses, but its contribution to airway neutrophilia remains controversial. The endothelium plays a critical role in regulating leukocyte recruitment and migration during inflammation. However, its response to IL-4 is reported to either increase or decrease the production of neutrophil chemotactic factors. We hypothesized that these conflicting findings may be due to the tissue and the size of the vessels from which endothelial cells have been derived. The expression of CXCL-8 by human primary culture umbilical veins endothelial cells (HUVECs), human pulmonary artery endothelial cells (HPAECs), and human pulmonary microvascular endothelial cells (HPMECs) when stimulated with recombinant human IL-4 (rhIL-4) was studied. The chemoattractant property of the cells' supernatants for neutrophils was evaluated using Boyden chambers. The role of the nuclear factor-κB (NF-κB), and mitogen-activated protein kinases (MAPK) in IL-4-induced HPAECs was studied using Western blotting and electrophoretic mobility shift assay (EMSA). We demonstrated that IL-4 increased the mRNA expression and the protein production of CXCL-8 in HPAECs, but not in HUVECs and HPMECs. The supernatants of HAPECs stimulated by IL-4 significantly promoted neutrophils migration in a dose-dependent manner, and was significantly attenuated by an inhibitor of CXCL-8. We also found that extracellular-regulated protein kinase1/2 (ERK1/2) is activated by IL-4 in HPAECs, but not JUN-N-terminal protein kinase (JNK) or p38 MAPK pathway. Furthermore, NF-κB-DNA binding activity, phosphorylation of IκBα and p65 levels were not affected by rhIL-4 in HAPECs. These findings indicate marked functional differences in the response of micro and macro-ECs to IL-4. ERK1/2, rather than NF-κB, JNK and p38 MAPK signaling, plays a role in IL-4 induced chemokine activation. Our results suggest that inhibition of ERK1/2 may be a possible target for airway neutrophilia in allergic lung diseases.

Expression of IL-4/IL-13 receptors in differentiating human airway epithelial cells

IL-4 and IL-13 elicit several important responses in airway epithelium including chemokine secretion and mucous secretion that may contribute to airway inflammation, cell migration, and differentiation. These cytokines have overlapping but not identical effector profiles likely due to shared subunits in their receptor complexes. These receptors are variably described in epithelial cells, and the relative expression, localization, and function of these receptors in differentiated and repairing epithelial cells are not clear. We examined IL-4/IL-13 receptor expression and localization in primary airway epithelial cells collected from normal human lungs and grown under conditions yielding both undifferentiated and differentiated cells inclusive of basal, goblet, and ciliated cell phenotypes. Gene expression of the IL-4Rα, IL-2Rγc, IL-13Rα1, and IL-13Rα2 receptor subunits increased with differentiation, but different patterns of localization and protein abundance were seen for each subunit based on both differentiation and the cell subtypes present. Increased expression of receptor subunits observed in more differentiated cells was associated with more substantial functional responses to IL-4 stimulation including increased eotaxin-3 expression and accelerated migration after injury. We demonstrate substantial differences in IL-4/IL-13 receptor subunit expression and responsiveness to IL-4 based on the extent of airway epithelial cell differentiation and suggest that these differences may have functional consequences in airway inflammation.

Epithelial repair mechanisms in the lung

The recovery of an intact epithelium following lung injury is critical for restoration of lung homeostasis. The initial processes following injury include an acute inflammatory response, recruitment of immune cells, and epithelial cell spreading and migration upon an autologously secreted provisional matrix. Injury causes the release of factors that contribute to repair mechanisms including members of the epidermal growth factor and fibroblast growth factor families (TGF-alpha, KGF, HGF), chemokines (MCP-1), interleukins (IL-1beta, IL-2, IL-4, IL-13), and prostaglandins (PGE(2)), for example. These factors coordinate processes involving integrins, matrix materials (fibronectin, collagen, laminin), matrix metalloproteinases (MMP-1, MMP-7, MMP-9), focal adhesions, and cytoskeletal structures to promote cell spreading and migration. Several key signaling pathways are important in regulating these processes, including sonic hedgehog, Rho GTPases, MAP kinase pathways, STAT3, and Wnt. Changes in mechanical forces may also affect these pathways. Both localized and distal progenitor stem cells are recruited into the injured area, and proliferation and phenotypic differentiation of these cells leads to recovery of epithelial function. Persistent injury may contribute to the pathology of diseases such as asthma, chronic obstructive pulmonary disease, and pulmonary fibrosis. For example, dysregulated repair processes involving TGF-beta and epithelial-mesenchymal transition may lead to fibrosis. This review focuses on the processes of epithelial restitution, the localization and role of epithelial progenitor stem cells, the initiating factors involved in repair, and the signaling pathways involved in these processes.