RAPGEF4 and idiopathic pulmonary fibrosis: Epac can regulate the proliferation, migration, and relaxation of airway smooth muscle cells (ASMC) in IPF, thereby correcting dysfunction and retarding the progression of IPF, which may be due to the reduction of RhoA activity by cAMP/Epac/Rap1 signaling (Roscioni et al., 2011; Zieba et al., 2011).