Smad Anchor for Receptor Activation Regulates High Glucose-Induced EMT via Modulation of Smad2 and Smad3 Activities in Renal Tubular Epithelial Cells

Abstract

Background: SARA is an essential adaptor protein in TGF-β1 signaling and it is also involved in the process of Epithelial-mesenchymal transition (EMT) and fibrosis. Our aim was to investigate the effect of SARA on high glucose (HG)-induced EMT and extracellular matrix (ECM) synthesis in renal tubular epithelial cells. Methods: The cells were transfected with the following plasmids: wild-type SARA (SARA-WT), SARA-dSBD (SARA with Smad binding domain deletion) and then subjected to HG ambience (30 m<smlcap>M</smlcap>). The expression levels were assessed by Western-blot, real-time PCR and immunofluorescence. Results: HG-induced EMT phenotype with increased expression of ECM protein in HK-2 cells. This was associated with the decreased expression of SARA and Smad2. In comparison with the HG group, overexpression of SARA in HK-2 cells, a relatively high upregulation of ZO-1 expression was seen, while that of Vimentin, fibronectin and collagen I was decreased. The Smad2 protein expression was increased in HK-2 cells after transfection with SARA (WT) plasmid. Interestingly, the overexpression of SARA prolonged the activity period of Smad2 and shortened that of Smad3, which seemed consistent with the change of EMT phenotype and ECM changes in HK-2 cell induced by HG. Conclusions: SARA regulates HG-induced EMT and ECM excretion via modulation of the activation of Smad2 and Smad3 in renal tubular epithelial cells. In view of these findings, it is conceivable that SARA may serve as a potential novel target in pre-EMT states for the amelioration renal fibrosis seen in chronic kidney diseases.