Roles for Insulin-Like Growth Factor I and Transforming Growth Factor-β in Fibrotic Lung Disease

Idiopathic pulmonary fibrosis (IPF) is a lung disease that is characterized by epithelial cell damage and areas of denuded basement membrane resulting in inflammation, fibroblast proliferation, excessive extracellular matrix (ECM) deposition, and remodeling of alveolar gas exchange units. The progressive loss of lung gas exchange units in patients with IPF leads to respiratory failure and eventually to death. While the etiology of this disease is unknown, for many years studies suggested that chronic inflammation was the underlying factor that caused fibroproliferation and structural alterations of the lung. Recent data show that fibroproliferation and fibrosis can occur independently of inflammation, suggesting that IPF is a disease caused by a mesenchymal, rather than an immune disorder. Mesenchymal growth factors, including transforming growth factor (TGF)-β, insulin-like growth factor (IGF)-I, platelet-derived growth factor, connective tissue growth factor, fibroblast growth factors, and keratinocyte growth factors, as well as proinflammatory cytokines such as tumor necrosis factor-α and interleukin-1β, have been shown to be exaggerated in several fibrotic lung disorders including IPF, ARDS, sarcoidosis, and bronchopulmonary dysplasia, as well as pulmonary manifestations of systemic diseases such as rheumatoid arthritis or progressive systemic sclerosis (scleroderma). We argue that inflammation is required to initiate growth factor production and repair of the damaged alveolar epithelial lining in fibrotic lung diseases and that exaggerated TGF-β production may be responsible for the fibrotic response seen in diseases such as IPF. We recognize the potential role of several growth factors in the fibroproliferative process in the lung, and in this brief report we focus on the possible roles of the growth factors IGF-I and TGF-β in cell migration, proliferation, and ECM synthesis in patients with IPF.