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

Abstract Purpose: Chemoresistance remains a key hurdle in osteosarcoma (OS) therapy. This study aims to delineate the role and underlying mechanisms of spermidine (SPD) in OS chemoresistance. Experimental Design: Using OS 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 pharmacological 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 OS cells exhibit an upregulation of ornithine decarboxylase 1 (ODC1) and spermidine synthase (SRM), key enzymes involved in SPD synthesis, resulting in heightened levels of this polyamine. SPD diminishes the therapeutic efficacy of CDP and DOX in OS 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 (ABC) transporters, both of which are implicated in drug resistance. Furthermore, pharmacological inhibition of SPD synthesis using DFMO markedly increases the chemosensitivity of OS 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 OS.