SMAD2 and renal fibrosis: Among various mediators, transforming growth factor-β (TGF-β) plays a critical role in tissue fibrosis by diverse mechanisms, such as activating downstream Smad2/3/7 proteins to regulate renal fibrosis through the classical pathway or through the non-classical pathway, regulating the activity of PI3K/RhoA/TAK1/Ras and other proteins to mediate renal fibrosis.