Supplementary Material for: Indoxyl sulfate aggravates podocyte damage through the TGF-β1/Smad/ROS signaling pathway

Introduction: Hyperglycaemia induces the production of a large quantity of reactive oxygen species (ROS) and activates the transforming growth factor β1(TGF-β1)/Smad signalling pathway, which is the main initiating factor in the formation of diabetic nephropathy. Indoxyl sulfate (IS) is a protein-binding gut-derived uraemic toxin that localizes to podocytes, induces oxidative stress and inflames podocytes. The involvement of podocyte damage in diabetic nephropathy through the TGF-β1/) signalling pathway is still unclear. Methods: In this study, we cultured differentiated rat podocytes in vitro and measured the expression levels of nephrin, synaptopodin, CD2AP, SRGAP2a and α-SMA by quantitative real-time PCR (qRT‒PCR) and western blotting after siRNA-mediated TGF-β1 silencing, TGF-β1 overexpression and the presence of the ROS inhibitor acetylcysteine. We detected the expression levels of nephrin, synaptopodin, CD2AP, SRGAP2a, SRGAP2a in the Smad2/3, phosphorylated-Smad2/3 (p-Smad2/3), Smad7, NADPH oxidase 4 (NOX4), and ROS levels under high glucose (HG) and IS conditions. Results: The results indicated that nephrin, synaptopodin, CD2AP and SRGAP2a expressions were significantly upregulated and α-SMA expression was significantly downregulated in the presence of HG under siRNA-mediated TGF-β1 silencing or after the addition of acetylcysteine. However, in the presence of HG, the expressions of nephrin, synaptopodin, CD2AP and SRGAP2a were significantly downregulated, and the expression of α-SMA was significantly upregulated with the overexpression of TGF-β1. IS supplementation under HG conditions further significantly reduced the expressions of nephrin, synaptopodin, CD2AP and SRGAP2a; altered the expressions of Smad2/3, p-Smad2/3, Smad7 and NOX4; and increased ROS production in podocytes. Conclusions: This study suggests that IS may modulate the expression of nephrin, synaptopodin, CD2AP and SRGAP2a by regulating the ROS and TGF-β1/Smad signalling pathways, providing new theoretical support for the treatment of diabetic nephropathy.