Spermidine Secreted by Apoptotic Cells Enhances Chemotherapy Resistance by Modulating β-Catenin Activity in Osteosarcoma

PURPOSE: Chemoresistance remains a key hurdle in osteosarcoma therapy. This study aims to delineate the role and underlying mechanisms of spermidine (SPD) in osteosarcoma chemoresistance. EXPERIMENTAL DESIGN: Using osteosarcoma cell lines and xenografts, we combined flow cytometry, Western blotting, proteomic mass spectrometry, and RNA sequencing to characterize SPD-driven changes in cellular pathways and resistance signatures. We tested whether pharmacologic inhibition of SPD biosynthesis, alone or in combination with standard chemotherapy, improves therapeutic response in vivo. RESULTS: Following chemotherapy, either cisplatin (CDP) or doxorubicin (DOX), apoptotic osteosarcoma cells exhibit an upregulation of ornithine decarboxylase 1 and SPD synthase, key enzymes involved in SPD synthesis, resulting in heightened levels of this polyamine. SPD diminishes the therapeutic efficacy of CDP and DOX in osteosarcoma cells, both in vitro and in vivo. Mechanistically, SPD enhances β-catenin activity, which subsequently upregulates genes associated with cancer stemness and ATP-binding cassette transporters, both of which are implicated in drug resistance. Furthermore, pharmacologic inhibition of SPD synthesis using α-difluoromethylornithine markedly increases the chemosensitivity of osteosarcoma cells to CDP and DOX. CONCLUSIONS: These findings illuminate the critical role of apoptotic cell metabolites in mediating treatment resistance and suggest that targeting SPD may offer a promising therapeutic strategy to augment the effectiveness of chemotherapy in osteosarcoma.