(A) RNAi-mediated reduction of YAP and TAZ protein levels in FUJI cells. To exclude unspecific off-target effects, three siRNAs targeting YAP or TAZ; #1-3 respectively were tested. Knockdown of YAP and TAZ was consistently associated with reduced expression levels of FOXM1 and PLK1 compared to non-targeting negative control siRNA (β-Actin used as loading reference). (B) In HS-SY-II, FUJI and 1273/99 SySa cells expression levels of YAP, TAZ and PLK1 were reduced by treatment with increasing concentrations of verteporfin (β-Actin used as loading reference). (C) In HS-SY-II cells TEAD reporter luciferase activity was significantly decreased upon treatment with verteporfin (***P<0.001). (D) In flow cytometric analyses, increased rates of apoptosis (cleaved PARP) were detected in HS-SY-II cells incubated with verteporfin (**P<0.01).
ARTICLE ABSTRACT
Synovial sarcoma is a soft tissue malignancy characterized by a reciprocal t(X;18) translocation. The chimeric SS18-SSX fusion protein acts as a transcriptional dysregulator representing the major driver of the disease; however, the signaling pathways activated by SS18-SSX remain to be elucidated to define innovative therapeutic strategies.
Immunohistochemical evaluation of the Hippo signaling pathway effectors YAP/TAZ was performed in a large cohort of synovial sarcoma tissue specimens. SS18-SSX dependency and biological function of the YAP/TAZ Hippo signaling cascade were analyzed in five synovial sarcoma cell lines and a mesenchymal stem cell model in vitro. YAP/TAZ-TEAD–mediated transcriptional activity was modulated by RNAi-mediated knockdown and the small-molecule inhibitor verteporfin. The effects of verteporfin were finally tested in vivo in synovial sarcoma cell line-based avian chorioallantoic membrane and murine xenograft models as well as a patient-derived xenograft.
A significant subset of synovial sarcoma showed nuclear positivity for YAP/TAZ and their transcriptional targets FOXM1 and PLK1. In synovial sarcoma cells, RNAi-mediated knockdown of SS18-SSX led to significant reduction of YAP/TAZ-TEAD transcriptional activity. Conversely, SS18-SSX overexpression in SCP-1 cells induced aberrant YAP/TAZ-dependent signals, mechanistically mediated by an IGF-II/IGF-IR signaling loop leading to dysregulation of the Hippo effectors LATS1 and MOB1. Modulation of YAP/TAZ-TEAD–mediated transcriptional activity by RNAi or verteporfin treatment resulted in significant growth inhibitory effects in vitro and in vivo.
Our preclinical study identifies an elementary role of SS18-SSX–driven YAP/TAZ signals, highlights the complex network of oncogenic signaling pathways in synovial sarcoma pathogenesis, and provides evidence for innovative therapeutic approaches.
