Abstract: Objective To study the roles and mechanism of Ku80 in silica-induced cell cycle changes in human embryo lung fibroblasts(HELF).Methods The expression of Ku80 protein was inhibited by siRNA plasmids.Flow cytometry was used to detect cell cycle and Western blot was used to determine expression levels of cyclin D1,cyclin dependent kinase-4(CDK4),estradiol(E2) promoter binding factor-1(E2F1) and phosphorylation levels of retinobastoma protein(Rb) after the cells were treated with silica at different time(0,3,6,12,and 24 hours).Results The proportion of negative control cells in G₁ phase decreased from 89.28±2.19% to 68.93±3.79% after exposed to silica and the proportion of cells in G₁ phase for cells with Ku80 inhibition(H-Ku80) decreased from 85.16±3.73% to 59.92±3.31%.Silica exposure increased the protein levels of cyclin D₁,CDK4,E2F1,and phosphorylation in negative control cells.The expression levels of cyclin D1 and CDK4 were obviously suppressed.However,the protein level of E2F1 and phosphorylation level of Rb-ser780 increased in H-Ku80 cells compared with those in trhe control cells.Conclusion Ku80 may play a role in silica-induced alternations of cell cycle.Ku80 positively regulates silica-induced overexpression of cyclin D1 and CDK4,but negatively regulates silica-induced overexpression of E2F1 and phosphorylation increases the level of Rb in human embryo lung fibroblasts.