Abstract
Background: Cancer stem cells (CSCs) are associated with tumor development, chemoresistance, recurrence, metastasis, and even prognosis. Interleukin (IL)-6 overexpression has been implicated in the development of various cancers, including osteosarcoma. This study aimed to investigate the role of IL-6 in modulating clinicopathological features, malignant traits, and stemness in osteosarcoma, and to determine the mechanisms underlying IL-6-mediated osteosarcoma progression. Methods: Patients with osteosarcoma (n = 54) and healthy controls (n = 50) were selected. No patients received any pre-operative cancer treatment. Serum levels of IL-6 were determined in patients with osteosarcoma by ELISA and their relationship with pathological features and prognosis analyzed. The 3-(4,5-dimethyl -2-thiazolyl)- 2,5-diphenyl-2H-tetrazolium bromide (MTT) and colony formation assays were used to evaluate cell proliferation, transwell assays were used to assess the invasive potential of cells, and cell migration rates were analyzed using a wound healing assay. Tumor self-renewal was detected using a spheroid formation assay and CD133 and CD44 expression assessed by flow cytometry. Protein levels of NANOG, SOX2, OCT3/4, OPN, and epithelial-to-mesenchymal transition (EMT)-related markers, and the phosphorylation status of STAT3, were determined by western blotting. Finally, cell viability was determined with or without cisplatin (cis-dichlorodiammineplatinum [DDP])/adriamycin (ADR) treatment. Xenograft tumor models were established by subcutaneous injection of osteosarcoma spheroids, with or without IL-6. Results: Serum IL-6 levels were higher in osteosarcoma patients than controls. There was no significant association of serum IL-6 level with age, sex and tumor size; however, it was associated with TNM stage, and lung metastasis (P < 0. 05). IL-6 significantly increased proliferation and colony formation of osteosarcoma cells, and enhanced their invasion and migratory potential, thus promoting an EMT-like phenotype and elevated chemoresistance of to DDP/ADR. Spheroid size/proportion of CD133(+)CD44(+) cells and SOX2, OCT3/4, and NANOG protein levels were elevated by IL-6 treatment in a time-dependent manner; however, IL-6 did not substantially influence any of these features in hFOB 1.19 and T98G cells. Knockdown of IL-6 reduced cell viability, colony formation, and invasion/migration ability, and reversed EMT, whereas it increased chemosensitivity to DDP/ADR. Blocking IL-6 expression with siRNA also caused loss of stemness, including reducing self-renewal ability, and reduced the proportion of CD133/CD44-positive cells, and expression of stemness-related genes. Pretreatment with the STAT3 inhibitor, S3I-201, decreased sphere size, and downregulated NANOG, SOX2, and OCT3/4 protein levels, compared with IL-6 treatment alone. Furthermore, OPN levels were elevated in response to IL-6 and an anti-OPN antibody effectively blocked IL-6-induced spheroid formation and STAT3 phosphorylation. In vivo, tumor size and weight were higher in IL-6 treated mice than controls. Conclusions: IL-6 mediates promotion of osteosarcoma spheroid stemness by activating OPN/STAT3 signaling.