Abstract 5930: Targeting Skp2 in synovial sarcoma reduces tumor-initiating properties and promotes apoptosis

Background. Despite the knowledge that most synovial sarcoma (SS) is driven by the fusion oncoprotein SS18-SSX, efforts in generating therapeutic agents that target the fusion oncoproteins have not been successful. The oncogenic role of S phase kinase-associated protein (Skp2) has been demonstrated in several cancers. However, the functional involvement of this F-box protein in SS is not well understood. Our goals are to examine the oncogenic role of Skp2 in SS and assess the therapeutic potential of inhibiting Skp2. Methods. The expression of Skp2 in established cell lines, patient specimens, SS18-SSX transgenic mouse tumors was assessed using RT-PCR, western blot, and immunochemistry. Genetic depletion of Skp2 was achieved by lentiviral shRNA. Further proliferation assay, sphere formation assay, invasion assay, and ALDH assay were performed along with western blot. The efficacy of flavokawain A (FKA), an skp2 inhibitor was also analyzed correspondingly. A mouse xenograft model was used to validate in vitro results. Results. Skp2 is frequently overexpressed in established cell lines, clinical specimens, and SS18-SSX transgenic mouse tumors. High levels of Skp2 correlated with more advanced clinical stages in SS. Next, we showed that genetic depletion of Skp2 reduced mesenchymal and stemness markers and inhibited the invasive and proliferative capacities of SS cell lines, as well as increased apoptotic markers and p27. Furthermore, Skp2 depletion markedly suppressed the growth of SS xenografts tumors, which indicates that Skp2 is essential in maintaining SS tumorigenesis. Treatment of SS cell lines with FKA reduced Skp2 expression in a dose-dependent manner and resulted in G2/M phase cell cycle arrest and apoptosis. In addition, FKA also suppressed the invasion and tumor-initiating properties in SS, similar to the effects of Skp2 knockdown.Conclusions. Taken together, our results suggest that Skp2 plays an essential role in the biology of SS by promoting the mesenchymal state and cancer stemness. Given that chemoresistance is often associated with cancer stemness, strategies of combining Skp2 inhibitors with conventional chemotherapy in SS may be desirable. Citation Format: Jichuan Wang, Kenji Sato, Ed O9Donnell, Amit Singla, Simon Yaguare, Osama Aldahamsheh, Brian Batko, Hasibagan Borjihan, Janet Tingling, Daniel A Weiser, David M Loeb, Richard Gorlick, Edward L Schwartz, Rui Yang, Xiaolin Zi, Hongling Zhao, David S Geller, Bang H Hoang. Targeting Skp2 in synovial sarcoma reduces tumor-initiating properties and promotes apoptosis [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 5930.