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Epithelial-mesenchymal transition (EMT) plays key roles during lung development and many lung diseases such as chronic obstructive pulmonary disease (COPD), lung cancer, and pulmonary fibrosis. Here, integrating morphological observations with underlying molecular mechanisms, we highlight the functional role of EMT in lung development and injury repair, and discuss how it can contribute to pathogenesis of chronic lung disease. We discuss the evidence of manifestation of EMT and its potential driving role in COPD, idiopathic pulmonary fibrosis (IPF), bronchiolitis obliterans syndrome (BOS), and lung cancer, while noting that all cells need not display a full EMT in any of these contexts, i.e., often cells co-express epithelial and mesenchymal markers but do not fully convert to extracellular matrix (ECM) -producing fibroblasts. Finally, we discuss recent therapeutic attempts to restrict EMT in chronic lung disease. Developmental Dynamics 247:346-358, 2018. © 2017 Wiley Periodicals, Inc., (© 2017 Wiley Periodicals, Inc.)

Unfortunately, the research effort directed into chronic obstructive pulmonary disease (COPD) has been disproportionately weak compared to its social importance, and indeed it is the least researched of all common chronic conditions. Tobacco smoking is the major etiological factor. Only 25% of smokers will develop "classic" COPD; in these vulnerable individuals the progression of airways disease to symptomatic COPD occurs over two or more decades. We know surprisingly little about the pathobiology of COPD airway disease, though small airway fibrosis and obliteration are likely to be the main contributors to physiological airway dysfunction and these features occur earlier than any subsequent development of emphysema. One potential mechanism contributing to small airway fibrosis/obliteration and change in extracellular matrix (ECM) is epithelial mesenchymal transition (EMT), so called Type-II EMT. When associated with angiogenesis (Type-III EMT) it may well also be a link with the development of lung (airway) cancer which is closely associated with COPD. Active EMT in COPD may help to explain why lung cancer is so common in smokers and also the core pathophysiology of small airway fibrosis. Better understanding may lead to new markers for incipient neoplasia, and better preventive management of patients. There is serious need to understand key components of airway EMT in smokers and COPD, and to demarcate novel drug targets for the prevention of lung cancer and airway fibrosis, as well as better secondary management of COPD. Since over 90% of human cancer arises in epithelia and the involvement of EMT in all of these may be a central paradigm, insights gained in COPD may have important generalizable value.