CBX3 promotes epithelial-mesenchymal transition in synovial sarcoma via the SHH signaling pathway

Synovial sarcoma (SS) is a malignant mesenchymal tumor of uncertain histogenesis, representing approximately 5% to 10% of all soft tissue sarcomas. It predominantly affects adolescents and young adults. The role of CBX3 (Chromobox homolog 3) in Synovial sarcoma progression, particularly in relation to epithelial mesenchymal transition (EMT), remains unclear. This study aimed to investigate the functional role of CBX3 in synovial sarcoma and to elucidate the underlying molecular mechanisms by which it regulates EMT. Human synovial sarcoma cell lines (SYO-1, HS-SY-II, YaFuSS, and Fuji) and the immortalized human keratinocyte line (HaCaT) were used for in vitro studies. For in vivo modeling, mice were inoculated with Fuji cells. CBX3 expression was significantly upregulated in human synovial sarcoma tumor specimens compared to control tissues. Clinically, patients with high CBX3 expression exhibited significantly shorter overall survival than those with low expression. In vitro, CBX3 promoted cell proliferation, induced EMT, and suppressed mitochondrial oxidative metabolism. Conversely, siRNA-mediated knockdown of CBX3 (si-CBX3) enhanced mitochondrial oxidative activity. Moreover, CBX3 overexpression inhibited ferroptosis in SS cells, whereas its knockdown (sh-CBX3) promoted both ferroptosis and mitochondrial oxidation - effects consistently observed in both in vitro assays and the mouse xenograft model. Mechanistically, CBX3 activated the Sonic Hedgehog (SHH) signaling pathway. Pharmacological inhibition of SHH signaling abrogated CBX3-mediated suppression of ferroptosis and restoration of mitochondrial oxidation. Furthermore, co-immunoprecipitation assays demonstrated that CBX3 physically interacts with SHH protein and stabilizes it by reducing its polyubiquitination. CBX3 drives EMT and tumor progression in SS by activating the SHH/Gli1 signaling axis. Mechanistically, CBX3 binds directly to and SHH and prevents its ubiquitin-mediated degradation, thereby stabilizing the protein. CBX3-SHH subsequently suppresses mitochondrial oxidative metabolism, which in turn inhibits ferroptosis and facilitates EMT - ultimately promoting SS aggressiveness.