Objective To investigate the molecular mechanisms of protective effects of thioredoxin (Trx) on human vascular endothelial cells in atherosclerosis.Methods The cell models of Trx-overexpressing cells (Ad Trx) and the control cells (Ad-con) were established by adenovirus vector gene transfer technology in human umbilical vein endothelial cells (HUVECs).The oxidized low density lipoprotein,a risk factor of atherosclerosis,was used as a stimulator.Western blot and indirect immunofluorescence were used to detect the protein expression levels and the cellular localization of Trx,adhesion molecules (ICAM-1,VCAM-1) and the upstream signal pathways.Trx activity was detected by insulin disulfide reduction assay,and cellular reactive oxygen species (ROS)production was detected by fluorescent probe DCFH-DA.Results As compared with control group,Trx protein expression level was enhanced in Ad-trx group and the Trx activity in Ad-Trx group was upregulated by (26.2 ±3.3)％.The result of ROS detection showed that overexpression of Trx significantly inhibited the cellular ROS generation.As compared with control group,overexpression of Trx obviously inhibited the adhesion molecules expression but markedly promoted the phosphorylation of Smad3 in endothelial cells with or without oxLDL stimulation (P＜0.05).Pretreatment of cells with SIS3,a specific inhibitor of Smad3 phosphorylation,reversed Trx-induced inhibition of adhesion molecules expression.Further studies showed that pretreatment of cells with SIS3 enhanced oxLDL-induced AP-1 subunit c-fos nuclear expression.Conclusions The enhancement of Smad3 phosphorylation and c-Fos nuclear expression are mainly responsible for the Trx-induced downregulation of adhesion molecules.