Abstract: Objective The F-box protein S-phase kinase-associated protein 2(Skp2)is one of the positive regulators of the cell cycle that promotes ubiquitin mediated proteolysis of some cyclin-dependent kinase inhibitors.In this study,we investigated the mechanism of apoptosis induced by down-regulation Skp2 expression using siRNA in MCF-7 human breast cancer cell lines.Methods We downregulated Skp2 in MCF-7 cells using siRNA(small interfering RNA).After 48h,cells were treated with epirubicin.TUNEL assay and Hoechst33258 stain were used to detected apoptosis.We also investigated the mechanism of apoptosis by detecting the expression of some factors associated with cell cycle using Western blot analysis.Results Knockdown of Skp2 by RNAi increased cellular apoptosis in vitro.Down regulation of Skp2 increased the level of p27,p21 and CyclinE protein.Epirubicin also decreased Skp2 protein levels in MCF-7 cells.Treatment with Skp2 siRNA followed by treatment with epirubicin not only increased apoptosis synergistically,but also activated p53 in MCF-7 cells.Conclusion In vitro,Skp2 siRNA induced apoptosis in breast cancer cells.p27,p21 and CyclinE regulated apoptosis by downregulation of Skp2 in breast cancer cells.Skp2 RNAi sensitized MCF-7 cells to apoptosis induced by epirubicin potentially through p53-dependent way.Skp2 could be a therapeutic target in breast cancer.