Induction of the STING-dependent DNA damage pathway by cucurbitacin B enhances immunotherapy efficacy in osteosarcoma.

OBJECTIVES: Osteosarcoma (OS) is a highly aggressive bone tumor with limited therapeutic options. Cucurbitacin B (CuB), a natural compound derived from Cucurbitaceae plants, has demonstrated antitumor activity in various malignancies; however, its mechanisms in OS remain unclear. This study aims to elucidate the antitumor effects of CuB in OS and explore its molecular mechanisms. MATERIALS AND METHODS: MG63 and K7M2 OS cells were treated with CuB, and cell viability was assessed using the cell counting kit-8 (CCK8) assay. Colony formation assays were employed to evaluate proliferation, while flow cytometry was used to analyze apoptosis and cell cycle distribution. DNA damage was determined by immunofluorescence staining and comet assay. Western blotting was used to detect proteins involved in activating the stimulator of the interferon genes (STING) pathway. In vivo OS xenograft models were established to monitor tumor growth and immune responses, and the therapeutic efficacy of combination treatment with anti-programmed death-ligand 1 (PD-L1) was evaluated. RESULTS: CuB inhibited OS cell proliferation, induced apoptosis, and caused G2/M cell cycle arrest. It activated the STING pathway and induced DNA damage. In vivo, CuB reduced tumor growth and metastasis, enhanced CD8+ and CD4+ T cell infiltration, and reduced regulatory T cells (Tregs) and myeloid-derived suppressor cells (MDSCs). Combination with anti-PD-L1 further suppressed tumor growth. CONCLUSION: CuB exerts anti-OS effects by inducing DNA damage, activating the STING pathway, enhancing immune responses, and synergizing with anti-PD-L1, highlighting its therapeutic potential.