Epithelial-mesenchymal transition (EMT), a process whereby fully differentiated epithelial cells undergo transition to a mesenchymal phenotype giving rise to fibroblasts and myofibroblasts, is increasingly recognized as playing an important role in repair and scar formation following epithelial injury. The extent to which this process contributes to fibrosis following injury in the lung is a subject of active investigation. Recently, it was demonstrated that transforming growth factor (TGF)-β induces EMT in alveolar epithelial cells (AEC) in vitro and in vivo, and epithelial and mesenchymal markers have been colocalized to hyperplastic type II (AT2) cells in lung tissue from patients with idiopathic pulmonary fibrosis (IPF), suggesting that AEC may exhibit extreme plasticity and serve as a source of fibroblasts and/or myofibroblasts in lung fibrosis. In this review, we describe the characteristic features of EMT and its mechanistic underpinnings. We further describe the contribution of EMT to fibrosis in adult tissues following injury, focusing especially on the critical role of TGF-β and its downstream mediators in this process. Finally, we highlight recent descriptions of EMT in the lung and the potential implications of this process for the treatment of fibrotic lung disease. Treatment for fibrosis of the lung in diseases such as IPF has heretofore focused largely on amelioration of potential inciting processes such as inflammation. It is hoped that this review will stimulate further consideration of the cellular mechanisms of fibrogenesis in the lung and especially the role of the epithelium in this process, potentially leading to innovative avenues of investigation and treatment.

Idiopathic pulmonary fibrosis (IPF)/usual interstitial pneumonia is a ravaging condition of progressive lung scarring and destruction. Anti-inflammatory therapies including corticosteroids have limited efficacy in this ultimately fatal disorder. An important unmet need is to identify new agents that interact with key molecular pathways involved in the pathogenesis of pulmonary fibrosis to prevent progression or reverse fibrosis in these patients. Because aberrant activation of the Wnt/β-catenin signaling cascade occurs in lungs of patients with IPF, we have targeted this pathway for intervention in pulmonary fibrosis using ICG-001, a small molecule that specifically inhibits T-cell factor/β-catenin transcription in a cyclic AMP response-element binding protein binding protein (CBP)-dependent fashion. ICG-001 selectively blocks the β-catenin/CBP interaction without interfering with the β-catenin/p300 interaction. We report here that ICG-001 (5 mg/kg per day) significantly inhibits β-catenin signaling and attenuates bleomycin-induced lung fibrosis in mice, while concurrently preserving the epithelium. Administration of ICG-001 concurrent with bleomycin prevents fibrosis, and late administration is able to reverse established fibrosis and significantly improve survival. Because no effective treatment for IPF exists, selective inhibition of Wnt/β-catenin-dependent transcription suggests a potential unique therapeutic approach for pulmonary fibrosis.

An understanding of the mechanisms underlying pulmonary fibrosis remains elusive. Once believed to result primarily from chronic inflammation, it is now clear that inflammation and chronic fibrosis, especially in diseases such as idiopathic pulmonary fibrosis/usual interstitial pneumonia, are often dissociated, and that inflammation is neither necessary nor sufficient to induce fibrosis. The origin of the primary effector cell of fibrosis in the lung, the myofibroblast, is not clearly established. Three potential sources have been hypothesized. Although conversion of resident fibroblasts and differentiation of circulating bone marrow–derived progenitors likely play a role, the possible contribution of alveolar epithelial cells (AECs), through a process termed “epithelial–mesenchymal transition” (EMT), has only recently received consideration. A process by which epithelial cells lose cell–cell attachment, polarity and epithelial-specific markers, undergo cytoskeletal remodeling, and gain a mesenchymal phenotype, EMT plays a prominent role in fibrogenesis in adult tissues such as the kidney. This review summarizes the evidence supporting a central role for EMT in the pathogenesis of lung fibrosis, the potential for EMT in AECs in vitro and in vivo and role of transforming growth factor-β1 in this process, and the implications of epithelium-driven fibrosis on future research and treatment. Potential pathways involved in EMT are also discussed. It is hoped that a major shift in current paradigms regarding the genesis of pulmonary fibrosis and dissection of the relevant pathways may allow development of targeted interventions that could potentially reverse the process and ameliorate the debilitating effects of abnormal repair and progressive fibrosis.

Cystic fibrosis (CF), a disorder characterized by mutations of the CF transmembrane regulator gene, is characterized in the lung by chronic inflammation, leading to progressive damage to the airway epithelium, bronchiectasis, and chronic obstructive lung disease. One process contributing to the airway derangement is the chronic burden of oxidants released by inflammatory cells on the respiratory epithelial surface. With this background, we hypothesized that glutathione in respiratory epithelial lining fluid (ELF) in CF patients might be oxidized and/or diminished in amount compared with that in normal subjects. Recovery of ELF by bronchoalveolar lavage from young adults with CF (n = 21) and normal subjects (n = 25) demonstrated marked neutrophil-dominated inflammation in ELF in CF patients. As predicted, ELF in CF patients was characterized by a deficiency of glutathione (P < 0.001), but this was secondary to a reduction in reduced glutathione (P < 0.001), inasmuch as there were no differences in ELF levels of oxidized glutathione (P > 0.2). Unexpectedly, there was also a marked deficiency of reduced glutathione in plasma (P < 0.02); i.e., the glutathione "deficiency" observed in ELF in CF patients is not limited to the site of the inflammation but is systemic. Although the etiology of this generalized deficiency of extracellular glutathione is unknown, it is important in considering options for treating the concomitant and devastating lung pathology in this disorder.

Interactions between transforming growth factor-β (TGF-β) and Wnt are crucial to many biological processes, although specific targets, rationale for divergent outcomes (differentiation versus block of epithelial proliferation versus epithelial-mesenchymal transition (EMT)) and precise mechanisms in many cases remain unknown. We investigated β-catenin-dependent and transforming growth factor-β1 (TGF-β1) interactions in pulmonary alveolar epithelial cells (AEC) in the context of EMT and pulmonary fibrosis. We previously demonstrated that ICG-001, a small molecule specific inhibitor of the β-catenin/CBP (but not β-catenin/p300) interaction, ameliorates and reverses pulmonary fibrosis and inhibits TGF-β1-mediated α-smooth muscle actin (α-SMA) and collagen induction in AEC. We now demonstrate that TGF-β1 induces LEF/TCF TOPFLASH reporter activation and nuclear β-catenin accumulation, while LiCl augments TGF-β-induced α-SMA expression, further confirming co-operation between β-catenin- and TGF-β-dependent signaling pathways. Inhibition and knockdown of Smad3, knockdown of β-catenin and overexpression of ICAT abrogated effects of TGF-β1 on α-SMA transcription/expression, indicating a requirement for β-catenin in these Smad3-dependent effects. Following TGF-β treatment, co-immunoprecipitation demonstrated direct interaction between endogenous Smad3 and β-catenin, while chromatin immunoprecipitation (ChIP)-re-ChIP identified spatial and temporal regulation of α-SMA via complex formation among Smad3, β-catenin, and CBP. ICG-001 inhibited α-SMA expression/transcription in response to TGF-β as well as α-SMA promoter occupancy by β-catenin and CBP, demonstrating a previously unknown requisite TGF-β1/β-catenin/CBP-mediated pro-EMT signaling pathway. Clinical relevance was shown by β-catenin/Smad3 co-localization and CBP expression in AEC of IPF patients. These findings suggest a new therapeutic approach to pulmonary fibrosis by specifically uncoupling CBP/catenin-dependent signaling downstream of TGF-β.