Abstract 3183: Exosomes secreted under hypoxia enhance aggressiveness in Ewing's sarcoma
Authors: Kling Matthew , Coulter Donald , Chaturvedi Nagendra , McGuire Timothy , Sharp John , Joshi Shantaram 
Published: August 2018

Abstract
Ewing's sarcoma (EWS) is a malignant tumor that primarily develops in children and adolescents. The main driver in EWS tumor progression is the result of a chromosomal translocation, t(11;22)(q24;q12), leading to the EWS-FLI1 fusion protein that functions as an oncogenic transcription factor. EWS is curable if diagnosed early, however, patients presenting with metastasis upon diagnosis have the worst prognosis of all the bone tumors. There is emerging evidence that a low oxygen (hypoxic) microenvironment contributes to increased EWS-FLI1 expression in EWS tumors and induces cellular changes observed in many cancers leading to the selection of clones with tumor initiating cell (TIC) properties. TICs are widely considered to be responsible for relapse, metastasis, and therapeutic resistance in many tumors. The effects of hypoxia are mediated primarily by hypoxia inducible factors HIF-1α and HIF-2α and have been demonstrated to upregulate EWS-FLI1 and stem cell-related factors, but the mechanism by which hypoxia promotes aggressiveness of this normally treatable cancer is unclear. In our study, we hypothesize that exosomes secreted under hypoxic conditions enhance the aggressiveness in Ewing's sarcoma by promoting tumor initiating cells in EWS cells and in stromal cells of the tumor microenvironment. This hypothesis is based on evidence in the literature showing that small vesicles (exosomes) released from cancer cells can reprogram metabolic and cellular function of both tumor cells and cells that compose the tumor microenvironment. In our study, RNAseq and pathway analysis of exosomes secreted from hypoxic EWS cells revealed high levels of the EWS-FLI1 fusion transcripts and stem cell related factors CD133, SOX2, OCT4, NANOG and MYC. Moreover, various co-culturing assays demonstrated that hypoxic EWS exosomes can increase cell motility, anchorage independent growth, chemoresistance and TIC formation in EWS cells, and transform microenvironment cells. Our in vivo model revealed that EWS exosomes injected into developing EWS tumors in mice significantly increased tumor formation. EWS tumors injected with hypoxic exosomes had increased expression of the CD133 TIC marker when analyzed by flow cytometry. These findings suggest that hypoxic EWS exosomes can enhance aggressiveness in EWS by promoting TIC formation, therapy resistance, invasiveness and motility in their target cells. Here, we describe a mechanism where EWS-FLI1 containing exosomes enhance TIC formation and introduce a new pathway that must be considered to effectively treat aggressive EWS tumors.
Citation Format: Matthew Kling, Donald Coulter, Nagendra Chaturvedi, Timothy McGuire, John Sharp, Shantaram Joshi. Exosomes secreted under hypoxia enhance aggressiveness in Ewing's sarcoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 3183.