C-peptide prevents SMAD3 binding to alpha promoters to inhibit collagen type IV synthesis

Activation of transforming growth factor β1 (TGFB1)/SMAD3 signaling may lead to additional synthesis of collagen type IV (COL4), which is a major contributor to extracellular matrix (ECM) accumulation in diabetic nephropathy (DN). C-peptide can attenuate fibrosis to have unique beneficial effects in DN. However, whether and how C-peptide affects TGFB1/SMAD3-activated COL4 synthesis is unclear. In this study, pathological changes, expression of COL4 a1-a5 chains (Col4a1-a5), COL4 distribution and protein and TGFB1 and SMAD3 protein were first assessed in a rat model of diabetes. Then, rat mesangial cells were treated with high glucose (HG) and/or C-peptide to investigate the underlying mechanism.Col4a1-a5expression, COL4 protein and secretion, TGFB1 protein, SMAD3 nuclear translocation and binding of SMAD3 to its cognate sites in the promoters ofCol4a1a2,Col4a3a4andCol4a5were measured. It was found that C-peptide attenuated glomerular pathological changes and suppressed renalCol4a1-a5mRNA expression, COL4 protein content and TGFB1 protein content. C-peptide had a dose-dependent effect to inhibitCol4a1-a5mRNA expression, COL4 protein content and secretion, in HG-stimulated mesangial cells. In addition, the HG-induced increase in TGFB1 protein content was significantly reduced by C-peptide. Although not apparently affecting SMAD3 nuclear translocation, C-peptide prevented SMAD3 from binding to its sites in theCol4a1a2,Col4a3a4andCol4a5promoters in HG-stimulated mesangial cells. In conclusion, C-peptide could prevent SMAD3 from binding to its sites in theCol4a1a2,Col4a3a4andCol4a5promoters, to inhibit COL4 generation. These results may provide a mechanism for the alleviation of fibrosis in DN by C-peptide.