Abstract 3039: The MELK/FOXM1 axis is a master regulator of proneural to mesenchymal transition (PMT) in glioma stem cells by controlling EZH2 transcriptional activity

Abstract

Abstract In the Central Nervous System, stem cells are present in the normal brain and primary brain tumor, glioblasotoma multiforme (GBM). Both normal and oncogenic stem cells play central roles in organogenesis and tumorigenesis, respectively. Previously we identified that the mitotic kinase MELK binds and phosphorylates the oncogenic transcription factor FOXM1 to regulate the growth of both brain-derived and GBM-derived stem cells (GSCs). Here we demonstrate that the Enhancer of Zeste Homolog 2 (EZH2), the lysine methyltransferase of Polycomb repressive complex 2, is the major transcriptional target of the MELK-FOXM1 complex in GSCs. PCR-based analysis of FOXM1 immunoprecipitated chromatin determined the binding of FOXM1 to the EZH2 promoter region to drive its transcriptional activity. This signaling is mediated through the protein complex formation of FOXM1 and MELK in a MELK kinase-dependent manner. The MELK-FOXM1 driven EZH2 signals are required to retain the immature stem cell trait and mesenchymal identity in GBM, as well as GSCs' radiation resistant phenotype in a MELK kinase-dependent manner. Clinically, GBM tumors co-express MELK and EZH2 with significant elevation in post-irradiation recurrent tumors. This post-irradiation elevation of MELK and EZH2 was observed specifically in GSCs. Irradiation induces transformation of proneural (PN) GSCs into a Mesenchymal (Mes)-like GSC phenotype (PMT) that is highly resistant to radiation treatment, and inhibition of EZH2 by shRNA reverses the radiation-resistant phenotype of Mes GSCs. Elimination of MELK or FOXM1 by shRNA reduced EZH2 expression resulting in attenuated stemness and radiation resistance in vitro and in vivo. These phenotypes were completely restored by exogenous expression of EZH2. To our knowledge, this study provides the first evidence of the role of MELK-FOXM1-EZH2 signaling axis for maintenance of stem cell trait and PMT regulation in GSCs. Identification of the role of transcriptional regulator EZH2 in PMT by the mitotic kinase MELK and transcription factor FOXM1 complex sheds lights on a novel therapeutic target for aggressive cancers including GBM. Citation Format: Kaushal Joshi, Sunghak Kim, Jeongwu Lee, Ronald Waclaw, Lisa Salcini, Ichiro Nakano. The MELK/FOXM1 axis is a master regulator of proneural to mesenchymal transition (PMT) in glioma stem cells by controlling EZH2 transcriptional activity. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 3039. doi:10.1158/1538-7445.AM2014-3039