Objective:To investigate the effects of tamoxifen on high glucose-induced epithelial-to-mesenchymal transition of rat peritoneal mesothelial cells and the underlying mechanism.Methods:The peritoneal mesothelial cells of normal male SD rats were selected between January 2015 and June 2016 and then cultured and divided into blank control, high-glucose stimulation and drug intervention groups. High-glucose stimulation group: primary cultured rat peritoneal mesothelial cells (RPMCs) were treated with 60 mmol/L high-concentration glucose to induce epithelial-to-mesenchymal transition. Drug intervention group: (1) RPMCs were treated with 60 mmol/L high-concentration glucose and different concentrations (0.5 μmol/L, 2 μmol/L) of tamoxifen. After 72 hours of stimulation, protein was extracted. (2) RPMCs were treated with 60 mmol/L high-concentration glucose and 2 μmol/L tamoxifen with or without 2 μmol/L ER-α antagonist for 1 hour to extract protein and for 6 hours to extract RNA. (3) RPMCs were treated with high-concentration glucose and 2 μmol/L tamoxifen with or without 1 μmol/L 1 μM proteasome inhibitor for 1 hour to extract protein. Western blot analysis was performed to analyze change in E-cadherin, α-SMA, Smad2, p-Smad2, Smad3, p-Smad3 and Smad4 protein. Real-time fluorescence quantitative PCR was performed to detect the change in mRNA expression of Smad2, Smad3, connective tissue growth factor and plasminogen activator inhibitor 1.Results:Tamoxifen attenuated epithelial-to-mesenchymal transition on RPMCs induced by high-level glucose, showing increased expression of epithelial cell marker E-cadherin and decreased expression of α-SMA in a concentration-dependent manner ( tE-cadherin = 2.31, tα-SMA =-2.53, both P < 0.05).TGF-β1/R-Smad signal pathway was activated by high-concentration glucose. Phosphorylation of Smad2/3 and mRNA expression of CTGF and PAI-1 were increased. Tamoxifen remarkably reduced protein and mRNA level of above mentioned protein and related target genes ( tp-Smad2 = -3.38, tCTGF = -3.81, P < 0.05), which could be blocked by ER-α antagonist. Finally, proteasome inhibitor could weaken the inhibitory effects of tamoxifen on p-Smad2/3 and increase p-Smad2/3 protein level ( tp-Smad2 = 3.94, P < 0.05). Conclusion:Tamoxifen activates ER-α on RPMCs, weakens the activation of TGF-β1/R-Smad signal pathway through decreasing p-Smad2 protein level, and effectively inhibits the progression of high-concentration glucose-induced epithelial-to-mesenchymal transition possibly through degrading p-Smad2 protein through proteasome. The role of tamoxifen in epithelial-to-mesenchymal transition may provide a possible guide for research, prevention and treatment of peritoneal fibrosis.