The Effect of Endothelin-1 on Myofibroblast Transformation of Pleural Mesothelial Cells and Its Implications for Pleural and Pulmonary Fibrosis

Idiopathic pulmonary fibrosis (IPF) and pleural fibrosis is a group of progressive and irreversible thoracic diseases characterized by cellular and structural changes in the pleura and lung parenchyma and the formation of myofibroblasts which produce abundant extracellular matrix (ECM) deposited particularly in the pleura and subpleural foci. Recent studies demonstrated that epithelial to mesenchymal transition (EMT) into myofibroblasts and circulating fibrocytes are critical cellular mechanisms in the regulation of fibrosis. Myofibroblasts are the key effector cells in pulmonary fibrosis. Persistence of transforming growth factor (TGF)-β expression and ECM deposition promote a pro-survival/anti-apoptotic phenotype so that myofibroblasts persist to propagate fibrosis. It was postulated that myofibroblasts may be derived from the circulating fibrocytes, the resident lung fibroblasts and epithelial cells undergoing EMT. In response to injury, alveolar epithelial cells may transform into myofibroblasts and contribute to fibrosis. However, other epithelial origins of myofibroblasts in pulmonary and pleural fibrosis have not fully investigated. The pleural mesothelium composes of a metabolically active monolayer of cells, pleural mesothelial cells (PMC), covering the chest wall and lungs. Overexpression of TGF-β1 in pleural mesothelium causes PMC transformation into myofibroblasts, and induces fibrosis of the pleura and subpleural pulmonary fibrosis. Furthermore, transformed PMCs could migrate via haptotaxis to the deeper layer of lung parenchyma, raising the possibility that the PMC may be an important player involved in the development of the subpleural fibrosis known to be a hallmark of IPF. Previous studies demonstrated that endothelin (ET)-1 acts with other profibrotic mediators to recruit fibroblasts and allow for their differentiation to myofibroblasts. Elevated levels of circulating ET-1 and increased ET-1 production in bronchoalveolar lavage fluid exist in patients with pulmonary fibrosis and correlated with the severity of fibrosis. ET-1 binds with two G protein-coupled receptors, ETA and ETB, and produces significant mitogenic activity for smooth muscle cells and fibroblasts. ET-1 is the principal effector of the many profibrotic roles of TGF-. Furthermore, ET-1 may induce EMT in alveolar epithelial cells, acting through ETA-mediated TGF-β1 production. We hypothesized that PMC may be another epithelial origin of myofibroblasts in the pleura and lung parenchyma and may respond to various profibrotic cytokines induced by tissue injury, including ET-1, and play an important role in the pathogenesis of IPF and pleural fibrosis. The aims of this project are to verify the ET-1-induced EMT and ECM production in PMC, to investigate the activated receptors and signalings, to study the associated development in pulmonary and pleural fibrosis, using in vitro and in vivo methods; moreover, the ET-1 levels and EMT response of PMCs in pleural fluids in patients with or without pleuropulmonary fibrosis will be measured. Furthermore, the therapeutic potential of inhibitors of EMT of PMC for the treatment of pleuropulmonary fibrosis will hopefully be explored in this study.