Molecular Characterization of Glioblastoma Cancer Stem Cells

Malignant gliomas are locally aggressive, highly vascular tumors that have an overall survival time less than 14 months, and current therapies provide little improvement in the disease course and outcome. While glioblastoma multiforme (GBM) patients present uniform histological phenotypes, the molecular determinants of the disease vary considerably between individual cases resulting in complicated prognosis. The heterogeneity, aggressiveness and rapid tumor relapse of GBM is believed to be sustained by cancer stem-like cell populations that are able to initiate and maintain tumors. Although CSCs represent only a small fraction of cells within a tumor, their high tumor-initiating capacity and therapeutic resistance is believed to drive tumorigenesis. Therefore, it is imperative to improve our molecular understanding of the tumor initiating cells and identify pathways associated with CSCs in order to devise innovative strategies to selectively target them. In this study, Glioma CSCs were isolated and maintained in vitro using an adherent culture system and shown to have constitutive activation of the STAT3/NF-κB signaling pathways and upregulation of STAT3 and NF-κB-dependent genes. Gene expression profiling also found components of the Notch pathway as being deregulated in glioma CSC, which were sensitive to treatment with STAT3 inhibitors. We also identified two CSC populations that produce Classical or Mesenchymal GBM tumors but display identical histological features. Adherent CSC-derived Mesenchymal GBM xenografts were found to exhibit high STAT3 and ANGPTL4 expression levels compared to Classical tumors. This subpopulation of glioma CSCs formed tumors with histopathological features of GBM and were enriched for stem cell markers, transcriptional networks and pro-angiogenic markers characteristic of the Mesenchymal subtype. Molecular characterization of aggressive Mesenchymal GBM xenografts identified high STAT3 and ANGPTL4 expression levels within the CD133+ CSC subpopulation, and these proteins were shown to colocalize within GBM stem cells. Elevated STAT3 and ANGPTL4 expression was found to correlate to short-term survival of human GBM patients, and a link in expression levels was observed between the genes in individual patient samples. The deregulated expression of these genes in glioma CSCs was sensitive to the kinase inhibitors, WP1066 and Sorafenib, and targeted inhibition of STAT3 and ANGPTL4 was found to decrease stem cell marker expression within tumors and lead to tumor regression. Taken together, these studies reveal that multiple CSC populations exist within GBM that drive molecular heterogeneity and tumorigenesis. Establishing an adherent CSC culture that maintains a Mesenchymal GBM signature provides a valuable and accurate model of the human disease, which will give insight into the role in tumor progression of novel genes as well as their utility as new therapeutic targets. The constitutive activation of STAT3 and NF-κB signaling pathways found in glioma CSCs that regulates Notch signaling, as well as, the important relationship established between STAT3 and ANGPTL4 in Mesenchymal GBM stem cells provides potential therapeutic value as biomarkers in targeting the CSC subpopulation of GBM