SMAD2 and renal fibrosis: Regarding the mechanism of RRR against renal fibrosis, some studies have shown that its nephroprotective effect was to reduce the expressions of TGF-β1, TGF-β receptor I (TGF-β RI), TGF-β RII, Smad2, p-Smad2, Smad3, p-Smad3, and Smad4, meanwhile increasing Smad7 (Zhang ZH.