Inhibition of Notch Signaling Induces Neural Differentiation in Ewing Sarcoma

Frank Baliko^*, Tamara Bright^*, Raymond Poon^*, Brenda Cohen^*, Sean E. Egan^* and Benjamin A. Alman^*

From the Program in Developmental and Stem Cell Biology,^* The Hospital for Sick Children, and the Department of Surgery, University of Toronto, Toronto, Ontario, Canada

Cells from Ewing sarcoma exhibit cellular features and expressmarkers, suggesting that the tumor is of neuroectodermal origin.Because Notch signaling regulates the differentiation of neuroectodermalcells during development, we examined the role of Notch signalingin Ewing sarcomas. We found that Ewing sarcomas express Notchreceptors, ligands, and the Notch target gene HES1. To determinethe functional implications of Notch signaling, we expressedtetracycline-regulated constitutively active, dominant-negative(DN), or wild-type Notch-1 receptors in two Ewing sarcoma celllines, or we treated the cell lines with a ${gamma}$ -secretase inhibitor.Expression of the constitutively active Notch-1 reduced proliferationand expression of the DN Notch-1 reduced apoptosis in vitro.However, there was only a small difference in the volume oftumors that formed when the cell lines expressing these constructswere implanted in nude mice. Xenograft tumors derived from thecell lines expressing DN Notch-1 exhibited a neural phenotype.Treatment with a ${gamma}$ -secretase inhibitor caused similar changesas expression of the DN construct. Notch signaling plays a rolein cell differentiation, proliferation, and apoptosis in Ewingsarcoma, but its inhibition is only associated with a smallchange in tumor growth potential.