〔Abstract〕 Objective To investigate the effects of cathepsin S(Cat S) on proliferation, migration and invasion of osteosarcoma cells and its potential regulatory mechanism.Methods Normal osteoblasts(hFOB) and osteosarcoma cells(SAOS2 and MG63) were selected as the subjects of this study.Cat S small interfering(si) RNA(si-Cat S) and negative control sequence(si-NC) were transfected into SAOS2 and MG63 cells to modulate the expression of Cat S in osteosarcoma cells.The experimental cells were randomly divided into four groups: hFOB group(hFOB cells),Control group(untransfected SAOS2 or MG63 cells),si-NC group(SAOS2 or MG63 cells transfected with si-NC) and si-Cat S group(SAOS2 or MG63 cells transfected with si-Cat S).The expression of Cat S in SAOS2 and MG63 cells was detected by RT-PCR and Western blot.Effect of Cat S knockdown on the proliferation of SAOS2 and MG63 cells was assessed by CCK-8 and clone formation assays.And effects of Cat S knockdown on the migration and invasion of SAOS2 and MG63 cells were determined by wound-healing and Transwell assays, respectively.Western blot assay was performed to measure the effects of SAOS2 knockdown on the expressions of apoptosis-related proteins(Bcl-2,Bax and Caspase-3) and Wnt/β-catenin pathway related proteins(LRP5,β-catenin, C-myc and Cyclin D1) in SAOS2 cells.Results Compared with hFOB group, the expression of Cat S in SAOS2 group and MG63 group was upregulated(P<0.001).In addition, compared with si-NC group, the proliferation, migration and invasion of cells in si-Cat S group were reduced(P<0. 001). Results of Western blot showed that compared with si-NC group,the expression of Bcl-2 in si-Cat S group was downregulated,while the expression of Bax and Caspase-3 were upregulated(P<0. 001). Meanwhile,compared with si-NC group,the expression of LRP5,β-catenin,C-myc and Cyclin D1 in si-Cat S group was downregulated(P<0. 001).Conclusion Cat S siRNA knockdown can inhibit the proliferation,migration and invasion of osteosarcoma cells and induce apoptosis by regulating Wnt/β-catenin pathway,indicating that Cat S may be one of the potential targets for the treatment of osteosarcoma.