The downregulation of SnoN expression in human renal proximal tubule epithelial cells under high-glucose conditions is mediated by an increase in Smurf2 expression through TGF-β1 signaling

摘要

Transforming growth factor?(TGF)-β1 is a profibrotic cytokine that plays a critical role in the progression of diabetic nephropathy?(DN). Previous studies have demonstrated that the Smad transcriptional co-repressor, Ski-related novel protein?N?(SnoN), an antagonizer of TGF-β1/Smad signaling, is downregulated in the kidneys of diabetic rats; however, the underlying molecular mechanisms remain elusive. In the present study, we demonstrated that the upregulation of Smad ubiquitination regulatory factor-2?(Smurf2), through TGF-β1/Smad signaling, contributes to the downregulation of SnoN under high-glucose conditions in primary human renal proximal tubule epithelial cells?(hRPTECs). The hRPTECs were cultured in high-glucose?(30?mmol/l D-glucose) medium in the presence or absence of either the proteasome inhibitor, MG132, or the TGF-β type?I receptor kinase inhibitor, SB-431542. Small interfering RNA?(siRNA) was used to silence Smurf2. The expression levels of SnoN, Smurf2, Smad2 and phosphorylated?(p-)Smad2 were measured by western blot analysis and RT-qPCR. The protein levels of SnoN were markedly downregulated, while its mRNA levels were increased in the hRPTECs cultured under high-glucose conditions. The protein and mRNA levels of Smurf2 were significantly increased under high-glucose conditions. The knockdown of Smurf2 increased SnoN expression in the hRPTECs cultured in high-glucose medium. Moreover, MG132 partially inhibited SnoN degradation in the hRPTECs under high-glucose conditions and SB-431542 decreased the phosphorylation of Smad2 and the expression of Smurf2 induced under high-glucose conditions. Taken together, the findings of this study demonstrate that the downregulation of SnoN expression in hRPTECs under high-glucose conditions is mediated by the increased expression of Smurf2 through the TGF-β1/Smad signaling pathway.