Identification of Thymidine Phosphorylase as a Potential Therapeutic Target for Bone Loss Associated Periprosthetic Osteolysis

Abstract

Background: Macrophage-derived factors are generally thought to play a key role in the pathogenesis of aseptic loosening though initiating periprosthetic inflammation and pathological bone resorption. Therefore, the aim of this study was to identify macrophage-derived factors that promote osteoclast differentiation and periprosthetic bone destruction. Methods: We examined the effects of 12 macrophage-derived factors that were identified by RNA sequencing analysis of stimulated macrophages on osteoclast differentiation in vitro. The identified molecule was characterized as osteoclastogenic factor in vitro and in vivo. RNA-seq for the induced-osteoclasts was performed in an effort to understand its action mode. Therapeutic effects of Src kinase inhibitor were evaluated in implant particles-induced osteolysis murine model. Findings: Thymidine phosphorylase (TYMP) was found to induce the production of significant numbers of osteoclasts that exhibited resorbing activities on dentine slices. Functionally, TYMP silencing by siRNA reduced the number of osteoclasts in macrophages that had been stimulated with implant particles, suggesting a potential role in periprosthetic osteolysis. High levels of TYMP were detected in the serum and synovial tissues of patients that had been diagnosed with aseptic loosening. Moreover, the administration of TYMP onto calvariae of mice induced pathological bone resorption that was accompanied by an excessive infiltration of inflammatory cells and osteoclasts. Interestingly, TYMP stimulation appeared to activate the tyrosine kinase Fyn signaling associated with cell fusion and osteoclast formation. The oral administration of saracatinib, an FYN kinase inhibitor, significantly and consistently suppressed the formation of bone osteolytic lesions in implant wear particles-induced osteolysis model. Interpretation: Our findings highlight a novel molecular target for therapeutic intervention in implant loosening. Funding Information: Japan Society for the Promotion of Science (Grant-in-Aid for Scientific Research C;17K10993), Akiyama Life Science, Foundation and Kobayashi Foundation, and Japan Agencyfor Medical Research and Development (JP20gm6210004). Declaration of Interests: The authors declare no competing interests. Ethics Approval Statement: The institutional review boards of Hokkaido University and hospital approved the study protocols for the use of human samples (approval ID: 016-0002). All participants signed an informed consent form for research use. Procedures for animal experiments were performed following protocols approved by the Institute of Animal Care and Use Committee of the Hokkaido University Graduate School of Medicine (no. 17-0085).