Abstract A02: YAP1 opposes differentiation in mesenchymal tumors
Authors: Liu Ying , Ciotti Gabrielle , Eisinger-Mathason T.S. Karin 
Published: September 2020

Abstract
Soft tissue sarcomas are aggressive mesenchymal malignancies diagnosed in ~15,000 Americans annually. Unlike in epithelial cancers, where novel targeted therapies have improved patient survival, the treatment approach for sarcomas has not changed significantly in 25 years. Our work revealed that deregulation of the Hippo pathway enhances sarcomagenesis in an aggressive muscle tumor called undifferentiated pleomorphic sarcoma (UPS). UPS is a commonly diagnosed and metastatic sarcoma subtype found most frequently in adult muscle tissues. We have observed that expression of Angiomotin (AMOT), a crucial mediator of Hippo-associated growth restriction, is commonly silenced in UPS. Among human tissues AMOT is most highly expressed in differentiated human muscle tissue. Ectopic re-expression of the p130 isoform of AMOT in muscle-derived UPS cells significantly inhibits proliferation in vitro. This finding is consistent with the central function of AMOT in cancer cells, which is to sequester the Hippo pathway effector YAP1 and facilitate its degradation, thereby inhibiting growth. YAP1 is a proproliferation transcriptional regulator whose deletion in an autochthonous genetically engineered mouse model (GEMM) of UPS significantly decreased tumorigenesis. Together these data suggest that AMOT loss promotes YAP1-mediated sarcomagenesis in muscle-derived UPS. We investigated the downstream effects of YAP1 signaling in UPS by gene expression analysis of control and Yap1-deficient murine tumors. Mechanistically, we found that Yap1 controls NF-κB transcriptional activity in UPS by inhibiting expression of Usp31, a negative regulator of NF-κB activity. Furthermore, using ChIP-seq of patient samples, we found that both YAP1 and NF-κB activity are substantially upregulated in human UPS. Consistent with these findings, UPS cell proliferation is highly sensitive to YAP1 and NF-κB inhibition. Importantly, loss of NF-κB completely prevents the formation in our murine UPS GEMM. In our effort to identify key transcriptional outputs of YAP1/NF-κB and determine how this signaling pathway controls sarcomagenesis, we found that it represses circadian clock gene expression (PER1, CRY2, ARNTL) and activity. Intriguingly, clock function is activated during muscle differentiation, whereas YAP1 and NF-κB inhibit differentiation is known to enhance proliferation of muscle precursor cells. These observations highlight a role for the YAP1/NF-κB axis in circadian regulation of muscle differentiation. Further studies revealed that YAP1/NF-κB-mediated suppression of the clock leads to downregulation of the unfolded protein response (UPR), a critical component of muscle cell differentiation. UPR transcriptional targets including ATF6 and CHOP are necessary for pruning of myoblasts and apoptosis of differentiation-incompetent muscle precursor cells. Together, our findings suggest a mechanism underlying YAP1-mediated sarcomagenesis via suppression of normal muscle differentiation pathways.
Citation Format: Ying Liu, Gabrielle Ciotti, T.S. Karin Eisinger-Mathason. YAP1 opposes differentiation in mesenchymal tumors [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 A02.