Ewing sarcoma cells, of which over 85% retain chimeric fusion gene EWS/Fli-1, are by and large more resistant to chemotherapeutics compared to nonneoplastic cells. The purpose of this study is to determine the role of EWS/Fli-1 fusion and its downstream targets regarding the cells’ resistance against actinomycin D (ActD), which is one of the most commonly used antitumor agents in combination chemotherapy of Ewing sarcomas.

Cytotoxicity was measured by WST-8 assay. Caspase-dependent and -independent cell death was examined by fluorescence microscope. Protein expression was analyzed by western blotting. Caspase activity was determined by Caspase-Glo assay.

ActD-induced caspase-dependent apoptotic cell death to Ewing sarcoma TC-135 cells in a dose- and time- dependent manner. Knockdown of EWS/Fli-1 fusion by siRNA resulted in enhancement of ActD-induced apoptosis. ActD treatment activated both mitogen-activated protein kinase kinase (MEK)/extracellular signal-regulated kinase (ERK) and phosphatidylinositol 3-kinase (PI3K)/Akt pathways although in a distinctive manner. Combined administration of U0126 (MEK inhibitor) and LY294002 (PI3K inhibitor) significantly enhanced ActD-induced apoptosis in vitro and suppressed xenograft tumor growth in vivo.

The present study demonstrated for the first time that combination of U0126 and LY294002 can augment the cytotoxicity of ActD against Ewing sarcoma cells in vitro and in vivo. Our results indicate that further study on combination of conventional chemotherapies with MEK and PI3K inhibitors may be considered for innovative treatments of Ewing sarcoma patients.