Wnt/β-catenin activated Ewing sarcoma cells promote the angiogenic switch.

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 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 non-active 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 secrete factors that promote angiogenesis. In particular, activation of canonical Wnt signaling leads Ewing sarcoma cells to upregulate expression and secretion of pro-angiogenic 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 TGFBR2, 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.