Objective To investigate the mechanism of inducing production of vascular endothelial growth factors (VEGF) by recombinant human S100 calcium binding protein A4 (rhS100A4) in rheumatoid arthritis fibroblast-like synoviocytes (RAFLSs).Methods Synovial tissue was sampled from the patients with rheumatoid arthritis (RA) undergoing knee arthroplasty for in vitro culture of RAFLSs.CCK-8 assay was conducted to detect the effect of rhS100A4 and the effect of its interaction with Rapamycin (Rap),an inhibitor of mammalian rapamycin target 1 (mTORC1) signaling pathway,on the proliferation of RAFLSs.The effects of rhS100A4 and its interaction with Rap on the expression of VEGF in RAFLSs were detected by immunofluorescence.After rhS100A4 and its cooperation with Rap stimulated the conditioned medium (CM)produced by RAFLSs,the effect of CM on formation of lumen in human unbilical vein endothelial cells (HUVECs) in vitro was observed to detect the angiogenic ability of rhS100A4.Western blot was used to detect the effect of rhS100A4 on the phosphorylation of downstream ribosomal protein S6 (S6) in the mTORC1 signaling pathway in RAFLSs and to analyze the effects of rhS100A4 and Rap on phosphorylation of S6 protein and expression of VEGF protein in RAFLSs.Results rhS100A4 promoted cell proliferation and expression of VEGF protein in RAFLSs,and the CM formed by rhS100A4 promoted HUVECs to form blood vessels in vitro.Rap inhibited the above biological effects of rhS100A4,rhS100A4 activated the downstream protein S6 in the mTORC1 signaling pathway in RAFLSs cells to increase their phosphorylation levels.The effects of rhS100A4 on the phosphorylation of S6 protein and on the expression of VEGF protein in RAFLSs were inhibited by Rap.Conclusion rhS10OA4 promotes production of VEGF in RAFLSs by activating the mTORC 1 signaling pathway.