Abstract 427: Sox2 modulation of cancer stem cell behavior in GBM

Abstract

Abstract Background and Objective: Glioblastoma (GBM) is the most aggressive primary brain tumor. GBM cells are phenotypically plastic and can present cancer stem cell (CSC) properties, such as the ability to self-renew and differentiate into the cells comprising the bulk of the tumor, phenocopying the parental tumor in mouse xenografts. GBM CSCs express neural stem cell (NSC) markers, including Sox2, a HMG-box transcription factor that regulates the expression of genes associated with self-renewal and differentiation. Culturing dissociated GBM tumors in serum-containing medium typically results in loss of the CSC phenotype and downregulation of Sox2, while serum-free neurosphere medium (NM) is used for selection and propagation of CSCs. We have previously identified a GBM specimen (GS1) which, contrary to the norm, retained Sox2 expression in low passage serum culture, along with the ability to dedifferentiate into neurospheres in vitro, and to form tumors in rodents (deCarvalho et al., Stem Cells 2010, 28:181-190). Here we investigate the endogenous expression of Sox2 in GBM biopsies, xenograft tumors, and cultured cells, and test the hypothesis that Sox2 may be a driver of the cancer stem cell phenotype in GBMs. Experimental Approaches: Endogenous expression of Sox2, downstream targets, and cell lineage markers, was determined by immunohistochemistry. miRNAs targeting Sox2 were stably expressed in GBM cells. The effect of Sox2 knockdown on clonogenicity, global gene expression, proliferation in vitro, tumor formation and morphology was determined. Results: Sox2 expression was heterogeneous within and among the 26 GBM surgical specimens tested, ranging from 1% to over 80% positive nuclei. Robust Sox2 expression was observed in over 80% of 56 GBM orthotopic xenografts analyzed. Sox2 positive cells co-expressed mesenchymal (vimentin), neural (nestin), and astrocytic (GFAP) markers. Knocking down Sox2 expression significantly impaired the ability of GS1 cells to form neurospheres, without affecting cell proliferation, significantly altered the expression of known target genes, such as Nestin, and genes in various signaling pathways, and affected xenograft tumor formation. Sox2 protein levels in GBMs were further regulated by post-transcriptional mechanisms. Conclusions: Sox2 is part of the signature gene set for the proneural GBM subclass. Our results show that Sox2 protein expression is ubiquitous in GBM tumors and xenografts, independent of patient survival or treatment status. Sox2 is a master regulator of embryonic and neural stem cell phenotype, and our data contributes to the view that it may have an important role in the plasticity of GBM cells in acquiring CSC phenotype, with possible implications for the pathology of GBMs. 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 427. doi:1538-7445.AM2012-427