Abstract A26: Classification of Glioblastoma Tumorsphere Depending on the Regulatory Mechanisms of the Hippo Pathway

Abstract Background: Although glioblastoma (GBM) is the most common primary brain tumor, the best available treatment options are still associated with poor prognosis. Recently, the Hippo/YAP signaling pathway has emerged as an important driver of GBM. Nevertheless, extensive studies have not yet been focused on the importance of phosphorylation event in regulating endogenous YAP activity in GBM. Here, we sought to elucidate that the modulation and stabilization of YAP/TAZ in Hippo pathway promote GBM progression. Methods: A core gene expression signature reflecting Silence of Hippo pathway (SOH) was developed in GBM patient samples to determine whether SOH enhances the tumor progression. In GBM tumorsphere (TS), mRNA levels of YAP1 were determined by microarray, and nuclear YAP1 expression level and its correlation with tumor aggressiveness were assessed through nucleus cytosolic fractionation, quantitative confocal microscopy, Western blot, and TEAD4 reporter assay. Cell proliferation, stemness, and invasive properties were also measured after treatment of YAP siRNA or its inhibitor verteporfin (VP). Results: GBM patients with SOH signature associated with poor prognosis and shorter survival. GBM TSs exhibited various phosphorylation states of YAP and could be distinguished by its phosphorylation status. GBM TSs with reduced YAP phosphorylation, including TS15-88, and other GBM TSs with higher YAP phosphorylation, which includes TS13-64, showed aggressive or less-aggressive cancer phenotypes, respectively. The siRNA-mediated knockdown of YAP1 significantly suppressed proliferation of TSs, and the results were confirmed using VP. Knockdown of YAP1 not only attenuated the invasiveness and stemness of TSs. Conclusion: We showed that reduced phosphorylation of endogenous YAP contributes to GBM progression. Moreover, we found that GBM TSs can be classified into two groups based on their YAP phosphorylation. These associations emphasize the YAP signaling network as new therapeutic opportunities in diagnosing and treating GBM. Citation Format: Yoojung Oh, Eui Hyun Kim, Dongkyu Lee, Junseong Park, Ju Hyung Moon, Hyun Woo Park, Jong Hee Chang, Seok-Gu Kang. Classification of glioblastoma tumorsphere depending on the regulatory mechanisms of the Hippo pathway [abstract]. In: Proceedings of the AACR Special Conference on the Hippo Pathway: Signaling, Cancer, and Beyond; 2019 May 8-11; San Diego, CA. Philadelphia (PA): AACR; Mol Cancer Res 2020;18(8_Suppl):Abstract nr A26.