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Wnt/β-catenin signaling is active in small subpopulations of Ewing sarcoma cells, and these cells display a more metastatic phenotype, in part due to antagonism of EWS-FLI1-dependent transcriptional activity. Importantly, these β-catenin-activated Ewing sarcoma cells also alter secretion of extracellular matrix (ECM) proteins. We thus hypothesized that, in addition to cell-autonomous mechanisms, Wnt/β-catenin-active tumor cells might contribute to disease progression by altering the tumor microenvironment (TME). Analysis of transcriptomic data from primary patient biopsies and from β-catenin-active versus -nonactive tumor cells identified angiogenic switch genes as being highly and reproducibly upregulated in the context of β-catenin activation. In addition, in silico and in vitro analyses, along with chorioallantoic membrane assays, demonstrated that β-catenin-activated Ewing cells secreted factors that promote angiogenesis. In particular, activation of canonical Wnt signaling leads Ewing sarcoma cells to upregulate expression and secretion of proangiogenic ECM proteins, collectively termed the angiomatrix. Significantly, our data show that induction of the angiomatrix by Wnt-responsive tumor cells is indirect and is mediated by TGF-β. Mechanistically, Wnt/β-catenin signaling antagonizes EWS-FLI1-dependent repression of TGF-β receptor type 2, thereby sensitizing tumor cells to TGF-β ligands. Together, these findings suggest that Wnt/β-catenin-active tumor cells can contribute to Ewing sarcoma progression by promoting angiogenesis in the local TME.

Tumor: tumor microenvironment (TME) interactions are critical for tumor progression and the composition and structure of the local extracellular matrix (ECM) are key determinants of tumor metastasis. We recently reported that activation of Wnt/beta-catenin signaling in Ewing sarcoma cells induces widespread transcriptional changes that are associated with acquisition of a metastatic tumor phenotype. Significantly, ECM protein-encoding genes were found to be enriched among Wnt/beta-catenin induced transcripts, leading us to hypothesize that activation of canonical Wnt signaling might induce changes in the Ewing sarcoma secretome. To address this hypothesis, conditioned media from Ewing sarcoma cell lines cultured in the presence or absence of Wnt3a was collected for proteomic analysis. Label-free mass spectrometry was used to identify and quantify differentially secreted proteins. We then used in silico databases to identify only proteins annotated as secreted. Comparison of the secretomes of two Ewing sarcoma cell lines revealed numerous shared proteins, as well as a degree of heterogeneity, in both basal and Wnt-stimulated conditions. Gene set enrichment analysis of secreted proteins revealed that Wnt stimulation reproducibly resulted in increased secretion of proteins involved in ECM organization, ECM receptor interactions, and collagen formation. In particular, Wnt-stimulated Ewing sarcoma cells up-regulated secretion of structural collagens, as well as matricellular proteins, such as the metastasis-associated protein, tenascin C (TNC). Interrogation of published databases confirmed reproducible correlations between Wnt/beta-catenin activation and TNC and COL1A1 expression in patient tumors. In summary, this first study of the Ewing sarcoma secretome reveals that Wnt/beta-catenin activated tumor cells upregulate secretion of ECM proteins. Such Wnt/beta-catenin mediated changes are likely to impact on tumor: TME interactions that contribute to metastatic progression.

Local and metastatic progression of solid tumors depends on crosstalk between tumor cells and the tumor microenvironment (TME), including both stromal cells and the extracellular matrix (ECM). We recently showed that high Wnt/beta-catenin activity in Ewing sarcoma correlates with diminished patient survival and that canonical Wnt signaling alters the tumor secretome, influencing ECM protein composition. In light of this, we investigated the hypothesis that Wnt/beta-catenin supports tumor progression by modulating tumor: TME crosstalk. Our results reveal that, in discrete tumor cell sub-populations, beta-catenin activation sensitizes cells to TGF-beta ligands through derepression of the TGF-beta receptor, TGFBR2, resulting in canonical Wnt-induced, TGF-beta-dependent upregulation of TGF-beta targets. Significantly, these Wnt/TGF-beta responsive targets include multiple AngioMatrix genes that are known to alter the TME to promote angiogenesis, including tenascin-C and collagens. Studies of Ewing sarcoma models, in vitro and in vivo, as well as in two independent patient cohorts, confirm that a direct relationship exists between beta-catenin and TGF-beta activation in tumor cells and angiogenesis in the local TME. Mechanistically, this is due, in part, to tenascin C-mediated promotion of endothelial cell proliferation. Thus, functional cooperation between canonical Wnt and TGF-beta signaling in Ewing cells induces secretion of pro-angiogenic factors. This study reveals a novel link between Wnt and TGF-beta signaling in Ewing sarcoma and illustrates the critical contribution of tumor cell heterogeneity and tumor: TME crosstalk to sarcoma progression. Citation Format: Allegra G. Hawkins, Elisabeth A. Pedersen, Wei Jiang, Sydney Treichel, Colin Sperring, Jay Read, Brian Magnuson, Rajiv M. Patel, Dafydd Thomas, Rashmi Chugh, Elizabeth R. Lawlor. Cooperation between canonical Wnt and TGF-beta pathways promotes sarcoma angiogenesis [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 3645.

Neuroblastoma (NB) is a malignant tumor that accounts for ~15% of all pediatric cancer mortality. Nearly half of patients present with high-risk disease that has poor outcome, and the proto-oncogene MYCN is amplified in 45% of high-risk cases. Further, relapsed neuroblastoma demonstrates frequent activation of mitogen activated protein kinase (MAPK) signaling, including mutations in tumor suppressor NF1 and epigenetic silencing of the gene. Human induced pluripotent stem cells (iPSCs) represent a valuable tool by which to further dissect the genetic requirements of high-risk NB. Human iPSCs are superior to genetically engineered mouse models (GEMMs) at capturing the telomere biology and chromosomal landscape of primary human neuroblastoma. Further, they can be genetically manipulated to determine the genetic drivers of the disease. As proof of principle, we first differentiated normal human iPSCs toward trunk neural crest cells (NCC), the putative cells of origin for NB. We subsequently introduced established genetic drivers of NB (MYCN, ALK) and orthotopically implanted the resulting trunk NCCs into the renal capsules of immunocompromised mice. Three months post injection, tumors developed in 60% of MYCN/ALK mice, 10% in MYCN alone, and no tumors developed in ALK alone or empty vector. Tumors were transplantable and expressed markers typically found in NB, while lacking markers of other tumors such as rhabdomyosarcoma, Ewing sarcoma, and lymphoma. To show we can use our model to evaluate candidate drivers of NB, we knocked out NF1 in MYCN trunk NCCs using CRISPR/Cas9 and implanted these cells orthotopically in mice. 80% of mice with MYCN/NF1 modified cells developed tumors while NF1 knockout alone did not. Thus, we have generated the first human iPSC model of NB driven by MYCN and have shown the utility of our model in validating cooperating mutations. MH and TZ contributed equally to this work.

Ewing sarcoma is the second most common bone cancer in children and adolescents. Patients with metastatic disease have less than a 25% chance of survival making it imperative to understand the mechanisms that drive metastasis. We previously showed that activation of Wnt/β-catenin signaling in Ewing sarcoma cells alters the tumor cell transcriptome, promoting transition to a more metastatic cellular phenotype. In addition, primary tumors with high-level Wnt activation were associated with worse clinical outcomes, supporting a role for canonical Wnt signaling in metastatic progression. Significantly, extracellular matrix (ECM) protein genes were among the most affected by Wnt/β-catenin, suggesting that activation of the pathway might lead to alterations in the local tumor microenvironment (TME). Crosstalk between the TME and tumor cells regulates fundamental aspects of the metastatic cascade including invasion, migration, and angiogenesis. It is our goal to define the impact of Wnt/β-catenin signaling on the TME and elucidate how these changes contribute to metastasis. Our specific hypothesis is that activation of Wnt/β-catenin in Ewing sarcoma cells leads to changes in the tumor secretome that result in altered TME composition and tumor:TME crosstalk. To address this hypothesis, conditioned media from Ewing sarcoma cell lines cultured in the presence and absence of Wnt3a was collected for proteomic analysis. We used label-free mass spectrometry with spectral counting to quantify proteins that passed a ≤1% FDR threshold. Significance was defined as greater than 4.5 or less than 0.2 fold difference in counts between Wnt3a and control conditions. Western blot of conditioned media was used to validate identified proteins. Preliminary analysis of the TC32 cell line identified 49 proteins up regulated in Wnt3a treated media in comparison to control. Significantly, Tenascin C (TNC), was among the most significantly up regulated proteins, validating our prior data that TNC is a transcriptional target of Wnt/β-catenin and a promoter of the metastatic phenotype in vivo. Gene ontology analysis of the significantly differentially expressed proteins revealed enrichment of proteins involved in ECM-receptor signaling, ECM organization, cell adhesion, and collagen catabolism. These biologic processes all play critical roles in tumor:TME interactions and together contribute to the early steps of adhesion, migration and invasion that are responsible for initiation of metastasis. These preliminary studies confirm that Wnt/β-catenin-dependent changes in the Ewing sarcoma transcriptome are associated with changes in tumor cell secretome. We are now validating these findings in additional cell lines and updated results will be presented. These studies will elucidate how changes in tumor cells can lead to changes in the local TME and in tumor:TME crosstalk that promote metastatic progression of Ewing sarcoma. Citation Format: Allegra Hawkins, Colin Sperring, Venkatesha Basrur, Alexey Nesvizhskii, Elizabeth Lawlor. Activation of Wnt/beta-catenin alters the Ewing sarcoma secretome - Implications for tumor:tumor microenvironment crosstalk [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 5836. doi:10.1158/1538-7445.AM2017-5836