Abstract LB_B15: The antiproliferative activity of metformin in glioblastoma stem cells is mediated by the direct binding and functional inhibition of the membrane configuration of CLIC1 protein
Authors: Mazzanti Michele M , Cianci Francesca M 
Published: December 2023

Abstract
Glioblastoma (GBM) is the deadliest brain tumor and represents a major clinical challenge. Even with the current standard of care and cutting-edge strategies, patients’ life expectancy remains invariably poor. Tumor heterogeneity and its extensive invasiveness make GBM difficult to be radically resected by neurosurgery and recurrence is common. There is scientific consensus that tumor relapse originates from progenitor/stem-like cells known as glioblastoma stem cells (GSCs). GSCs are endowed with distinct tumorigenic potential, with properties of self-renewal and multi-lineage differentiation that contribute to tumor mass heterogeneity. Thus, GSCs represent an appealing target for new therapies. In recent years, metformin's anti-cancer properties have been highlighted. Metformin administration to patients with type 2 diabetes (T2D) has been positively linked to a decrease in the risk of developing several cancers and in cancer-related mortality, including glioblastoma. Additionally, numerous studies on glioblastoma demonstrate that metformin’s anti-cancer properties are directed against GSCs. Despite this evidence, metformin’s mechanism of action on cancer cells has not been clarified, although it has been proposed to target the oxidative phosphorylation (OXPHOS) pathway. GSCs are able to shuttle between glycolysis and OXPHOS energetic pathways, and this metabolic plasticity could be the major reason for metformin’s low effectiveness in clinical trials so far. Chloride Intracellular Channel 1 (CLIC1) is a metamorphic protein that is predominantly cytoplasmic under physiological conditions. In response to persistent stress, it translocates to the plasma membrane, inducing chloride conductance. When GSCs are treated with metformin and agents that impair the activity of the transmembrane form of CLIC1 (tmCLIC1), they show a parallel and non-additive inhibition of proliferation. Our previous studies comprehensively demonstrated that tmCLIC1 contributes to the progression of GBM both in vitro and in vivo. In addition, its specific localization and enrichment at the GSC plasma membrane position tmCLIC1 as a prime pharmacological target for GBM treatment. Our investigation found that in vitro, metformin’s antiproliferative effect is exerted at a millimolar concentration range. However, in vivo, with metformin administered through drinking water, mice exhibit an increase in survival with brain levels of metformin quantified in the nanomolar range. It is possible that the constant fresh drug circulation, continuously reaching the brain during prolonged administration in vivo (up to 100 days) creates a drug steady state with persistent saturation and inhibition of all the active tmCLIC1 expressed by GBM cells. The need for lower concentrations in vivo than in vitro is also important to translate these studies into the clinic; the already-available extended-release formulation may provide the same sustained activity on GBM cells as the drug in mouse drinking water.
Citation Format: Michele M Mazzanti, Francesca M Cianci. The antiproliferative activity of metformin in glioblastoma stem cells is mediated by the direct binding and functional inhibition of the membrane configuration of CLIC1 protein [abstract]. In: Proceedings of the AACR-NCI-EORTC Virtual International Conference on Molecular Targets and Cancer Therapeutics; 2023 Oct 11-15; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2023;22(12 Suppl):Abstract nr LB_B15.