Mutual influence of ROS, pH and CLIC1 membrane protein in the regulation of G1/S phase progression in human glioblastoma stem cells

Glioblastoma (GB) is the most lethal, aggressive and diffuse brain tumor. The main challenge for successful treatment is targeting the cancer stem cell (CSC) sub-population responsible for tumor origin, progression and recurrence. Chloride Intracellular Channel 1 (CLIC1), highly expressed in CSCs, is constitutively present in the plasma membrane where it is associated with chloride ion permeability. In vitro, CLIC1 inhibition leads to a significant arrest of GB CSCs in G1 phase of the cell cycle. Furthermore, CLIC1 knockdown impairs tumor growth in vivo. Here we demonstrate that CLIC1 membrane localization and function is specific for GB CSCs. Mesenchymal stem cells (MSCs) do not show CLIC1-associated chloride permeability and inhibition of CLIC1 protein function has no influence on MSC cell cycle progression. Investigation of the basic functions of GB CSCs reveals a constitutive state of oxidative stress and cytoplasmic alkalinization compared with MSCs. Both intracellular oxidation and cytoplasmic pH changes have been reported to affect CLIC1 membrane functional expression. We now report that in CSCs these three elements are temporally linked during CSC G1/S transition. Impeding CLIC1-mediated chloride current prevents both intracellular ROS accumulation and pH changes. CLIC1 membrane functional impairment results in GB CSCs resetting from an allostatic tumorigenic condition to a homeostatic steady state. In contrast, inhibiting NADPH oxidase and NHE1 proton pump results in cell death of both GB CSCs and MSCs. Our results show that CLIC1 membrane protein is crucial and specific for GB CSC proliferation, and is a promising pharmacological target for successful brain tumor therapies.