Molecular mechanisms of pulmonary fibrosis

Pulmonary fibrosis is the end-point of a numerous and heterogeneous group of disorders known as interstitial lung diseases (ILD). Lung fibrotic remodeling is characterized by fibroblast/myofibroblast activation, and excessive extracellular matrix accumulation leading to progressive organ dysfunction and usually terminal outcome. Treatment is largely ineffective primarily because few of the molecular mechanisms have been well defined to design appropriate targets for therapy. While the pathogenesis is incompletely understood, a growing body of evidence suggests two different pathogenic routes for developing pulmonary fibrosis. The inflammatory pathway, where a shift to the so-called T-helper 2 type cytokine networks is critical, and the epithelial pathway represented by idiopathic pulmonary fibrosis, by far the most aggressive ILD. In this pathway the inflammatory process is irrelevant, and the physiopathology seems to be dominated by epithelial cell injury and activation. Both routes may trigger a number of cytokines/growth factors inducing fibroblast migration/proliferation and phenotype change to myofibroblasts, with a consequent accumulation of extracellular matrix. An imbalance in matrix metalloproteinase/tissue inhibitors of metalloproteinases may contribute to alteration in extracellular matrix turnover and remodeling. This review will focus in some of the mechanisms involved in both prefibrotic pathways, as well as those involved in fibroblast activation and abnormal matrix deposition.