Abstract B50: YAP induces development of mesenchymal subtype of high-grade serous ovarian cancer from granulosa cells
Authors: Lv Xiangmin , He Chunbo , Huang Cong , Hua Guohua , Ma Bowen , Chen Xingcheng , Zhou Jin , Wang Zhengfeng , Dong Jixin , Rueda Bo R. , Davis John S. , Wang Cheng 
Published: August 2018

Abstract
The Hippo pathway plays critical roles in both organ development and tumorigenesis. Our previous studies demonstrated that Yes-Associated Protein 1 (YAP), a transcription coactivator and the major effector of the Hippo pathway, plays a critical role in ovarian cancer development. The present study investigates the role of the Hippo/YAP signaling pathway in the tumorigenesis of ovarian granulosa cells. Fluorescent and chromogenic immunohistochemistry were used to examine YAP expression in human and mouse ovaries. Western blot and qRT-PCR were used to determine protein and mRNA expression. Retrovirus-based YAP expressing constructs and YAP siRNAs were utilized to manipulate YAP levels in primary or immortalized human granulosa cells. A new three-dimensional hanging-drop culture system was used to mimic the growth of granulosa cells in vivo. The soft agar assay and xenograft mouse models were employed to investigate the role of YAP in transformation and tumorigenesis of granulosa cells. Immunohistochemical analyses showed that the active form of YAP (nuclear YAP) was expressed in proliferative granulosa cells in human and mouse ovaries, whereas the inactive form of YAP (cytoplasmic YAP) was detected predominantly in terminally differentiated luteal cells, indicating that YAP activity may be involved in the regulation of granulosa cell proliferation and differentiation. Overexpression of wild-type or constitutively active YAP compromised the expression of genes required for steroidogenesis and induced de-differentiation of differentiated granulosa cells. Ectopic expression of the constitutively active form of YAP in less-differentiated human granulosa cells derived from growing follicles induced tumors in a xenograft mouse model. Intriguingly, we found that these tumors resembled human high-grade serous ovarian cancer (HGSOC) morphologically, histologically, and genetically. The tumors were accompanied by accumulation of large amount of ascites in the peritoneal cavity. Highly proliferative and aggressive tumor cells invaded into omentum, visceral fat tissue, diaphragm, pancreas, liver, and gastrointestinal system. Deletion of the BRCA1 gene in these tumor cells promoted tumor progression and significantly reduced survival rate. Interestingly, these tumors have high expression of N-cadherin but very low expression of CA125 and E-cadherin, which are molecular characteristics of a recently identified mesenchymal subtype of HGSOC. Studies from The Cancer Genome Atlas and the Australian Ovarian Cancer Study Group indicate that the mesenchymal subtype of HGSOC has the poorest prognosis among all subtypes of ovarian cancers. In conclusion, results from this study indicate that YAP plays a critical role in regulating granulosa cell proliferation and differentiation. Overactivation of YAP can result in de-differentiation and reprogramming of ovarian granulosa cells, leading to development of mesenchymal subtype of HGSOC. Our results identify early-stage granulosa cells as a cell of origin of newly identified mesenchymal type of HGSOC.
Citation Format: Xiangmin Lv, Chunbo He, Cong Huang, Guohua Hua, Bowen Ma, Xingcheng Chen, Jin Zhou, Zhengfeng Wang, Jixin Dong, Bo R. Rueda, John S. Davis, Cheng Wang. YAP induces development of mesenchymal subtype of high-grade serous ovarian cancer from granulosa cells. [abstract]. In: Proceedings of the AACR Conference: Addressing Critical Questions in Ovarian Cancer Research and Treatment; Oct 1-4, 2017; Pittsburgh, PA. Philadelphia (PA): AACR; Clin Cancer Res 2018;24(15_Suppl):Abstract nr B50.