Abstract 1429: Forkhead box M1 (FoxM1) is a direct transcriptional target of EWS/FLI1 and a regulator of Ewing's sarcoma cell morphology, adhesiveness and tumorigenicity

Abstract

Abstract Ewing's sarcomas (EWS) are tumors typically formed by undifferentiated small round cells that predominantly affect children and young adults. The aggressiveness of EWS is illustrated by both the fact that about 25-30% patients present with metastases at diagnosis and the elevated rate of lethal recurrence among them. In spite of therapeutic combinations including chemotherapy, radiation and surgery, the 5-year disease-free survival rate for EWS patients has remained low for many years. Poor prognosis, lethal recurrence and long-term side effects point to the urgent requirement of novel EWS targeted therapies. EWS is cytogenetically characterized by the presence of chromosomal translocations resulting in the fusion of the EWS gene with several ETS genes, most frequently with FLI1, leading to the expression of chimeric proteins that contain the transcriptional activation domain of EWS and the FLI1 DNA binding domain. The EWS/FLI1 aberrant transcription factors are responsible for the malignant phenotype of EWS and regulate a large repertoire of target genes, many of which are critical determinants for EWS development, and thus have the potential of providing novel therapeutic options for EWS. In order to understand mechanisms of cell cycle regulation by EWS/FLI1, we performed comparative array analyses of EWS cells before and after knocking down EWS/FLI1 expression. Results showed that EWS/FLI1 knockdown consistently down-regulated FoxM1, and chromatin immunoprecipitation (ChIP) assays demonstrated that FoxM1 is a novel direct transcriptional target of EWS/FLI1. Western hybridization and immunohistochemical analyses demonstrated that FoxM1 is indeed abundantly expressed in EWS cell lines, patient tumors and EWS cell derived tumors in nude mouse xenografts. Down-regulation of FoxM1 expression in by shRNA-mediated gene knockdown caused morphological alterations, slowed the growth of EWS cells, and increased their adhesion to the substrate in culture-plates. Accordingly, FoxM1 down-regulation in EWS cells diminished their migration and invasiveness as well as their tumorigenicity in nude mice. Alterations in the pattern of integrin expression observed in FoxM1 knocked-down EWS cells may likely contribute to their altered morphology, increased adhesion and the overall downgrading of their malignant phenotype. These data identify FoxM1 as a direct EWS/FLI1 target and as an important regulator of the proliferation, morphology and tumorigenicity of EWS cells, and imply that targeting FoxM1 may allow the development of new molecular therapeutic strategies for EWS patients. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 1429. doi:1538-7445.AM2012-1429