CCN2 and idiopathic pulmonary fibrosis: In addition, despite that several involved pro-fibrotic factors have been identified, such as the platelet-derived growth factor (PDGF), the tumor necrosis factor (TNF), endothelin-1, the connective tissue growth factor (CTGF), and osteopontin, transforming growth factor beta-1 (TGF-β1) remains one of the most important molecular drivers for IPF [18].