Therapeutic approaches to eradicate glioblastoma stem cells

Glioblastoma (GBM) is the most common primary malignant brain tumour in adults. Median life expectancy for GBM patients is less than 2 years. The limited outcome of neurosurgery [1] is mainly due to the high ability of GBM cells to rapidly and extensively invade the brain parenchyma; through the activity of self-renewing cancer stem cells (CSCs) [2]. Treatment efficacy is also prevented by the high degree of inter- and intra-tumoral heterogeneity, sustained by CSCs and non-CSC progeny, and intrinsic radio- and chemo-resistance of GBM CSCs. In different tumours, including GBM, growth rate, migratory ability, drug resistance and tumorigenicity of CSCs have been associated to the expression of the cellular prion protein (PrPC), a membrane-associated protein whose physiological function is largely unknown [3]. PrPC expression affects different CNS cell functions: stress- and apoptosis-protection, cell morphology, differentiation, proliferation, motility, and adhesion [4]. Moreover, PrPC down-regulation inhibits GSC proliferation, self-renewal, and tumorigenicity. In addition, PrPC plays a role in cell adhesion and cytoskeleton integrity involved in the invasiveness of cancer cell [5], [6]. We evaluated the intracellular pathways activated by PrPC expression to control GSC proliferation and differentiation. We report that downregulation of PrPC expression reduces proliferation and migration rate in several GSC cultures, impairing stemness-related gene expression (Sox2, OCT4, NANOG, Nestin). These effects are dependent on the impairment of Wnt/β‐catenin signalling, which resulted to be down-regulated in the expression of its components and of target genes. WNT/β-catenin pathway (or canonical WNT pathway) is highly active in the early phase of brain development, regulating neuronal migration and polarization, axon guidance and dendrite development. Moreover, it plays a central role in the control of proliferation, self-renewal, stemness maintenance of stem cells and is a major regulator of epithelial-mesenchymal transition (EMT). To get insights into the mechanisms controlling the interactions between PrPC and intracellular signalling machinery in GSCs and normal astrocytes, we analysed these cells for the presence of a transmembrane immature form (Pro-PrP) identified in some solid tumours and virtually absent in CNS cells. A prevalent expression of the transmembrane Pro-PrP form in GSCs could determine a direct and aberrant activation of Wnt/β-catenin pathway, which is not present in normal cells. Another protein overexpressed in GSCs is the chloride channel CLIC1, that is involved in the regulation of their proliferation and invasive capacity. CLIC1 is one of the Wnt-induced proteins up-regulated [7]. CLIC1 is a prognostic and predictive marker in GBM, and its down regulation implies the proliferative inhibition of GSCs [8], [9]. We searched for novel repositioned drugs with anticancer and CLIC1 inhibitory effects. Among this compounds triamterene (TMT) is a potassium-sparing diuretic used in hypertensive and heart failure patients.