Focal adhesion kinase promotes biphasic differentiation and induces infiltration and metastasis of synovial sarcoma

Synovial sarcoma is a malignant tumor of mesenchymal origin, which is characterized by the simultaneous expression of epithelia and mesenchyme. Tumor microenvironment plays an important role in the development of synovial sarcoma. Exon microarray showed that there were significant differences in cell adhesion-related genes of tumor microenvironment between epithelial cells and mesenchymal cells of synovial sarcoma. In this study, we measured the expression of focal adhesion kinase (FAK) and epithelial-mesenchymal transition (EMT) related proteins in biphasic synovial sarcoma and preliminarily explored the tumorigenic mechanism of FAK in synovial sarcoma. FAK expression in sarcoma and normal mesenchymal tissues was analyzed using online databases. Laser capture microdissection and tissue microarray were employed to isolate epithelial and spindle cell components from synovial sarcoma specimens. Immunohistochemistry and quantitative real-time PCR (qRT-PCR) were performed to detect the expression of FAK and EMT-related molecules in the epithelial and mesenchymal components of synovial sarcoma tissues. In cellular functional experiments, FAK-overexpressing HS-SY-II cells were established using lentiviral transduction, and cell proliferation, migration, and invasion were assessed by CCK-8, colony formation, and Transwell assays. Immunofluorescence staining was performed to evaluate EMT marker expression. For in vivo validation, an experimental lung metastasis model was established via tail vein injection in nude mice. Finally, RNA-seq was used to detect transcriptomic alterations following FAK inhibition by TAE226, and expression of EMT-related factors was validated by Western blot and qRT-PCR. Multiple online databases revealed high FAK expression in sarcoma. In synovial sarcoma tissues, both protein and mRNA expression levels of FAK were higher in the spindle cell area than in the epithelial area. FAK expression was negatively correlated with E-cadherin and positively correlated with N-cadherin and Snail. High FAK expression was significantly associated with poor prognostic factors including recurrence, metastasis, FNCLCC grade III, and TNM stage IV. Functional assays demonstrated that FAK overexpression significantly enhanced proliferation, migration, and invasion of HS-SY-II cells. Immunofluorescence staining showed that FAK upregulation induced EMT, as evidenced by decreased E-cadherin and increased N-cadherin and Vimentin expression. In vivo, FAK overexpression increased lung metastasis incidence (3/5 vs. 0/5 in controls). Conversely, inhibition of FAK by TAE226 in SW982 cells significantly decreased cell motility, migration, and proliferation. RNA-seq analysis revealed significant alterations in EMT-related pathways following FAK inhibition, which was confirmed by Western blot and qRT-PCR showing decreased mesenchymal markers and increased epithelial markers. Our data demonstrate that FAK induces EMT and is associated with poor prognosis in synovial sarcoma. FAK regulates the expression of EMT-related factors and the EMT process, leading to changes in tumor cell invasion, migration, proliferation, and metastasis. These findings suggest that FAK could be a promising therapeutic target for synovial sarcoma.