Metformin antiproliferative activity is exclusively mediated by the membrane functional expression of the Chloride Intracellular Channel 1 in glioblastoma stem cells

Metformin is the first-line drug for type-2 diabetes. Retrospective analyses, based on diabetic patients’ clinical data, demonstrate that daily assumption of metformin reduces the incidence of several kinds of solid tumors. Even though it is widely agreed that metformin must be internalized to accomplish its pharmacological activity, direct evidence about metformin membrane permeability and/or the presence of a specific membrane receptor in cancer cells is still missing. Here, we show that the transmembrane form of Chloride Intracellular Channel 1 (tmCLIC1) works as a privileged metformin receptor in glioblastoma stem-like cells. We found that metformin impairs tmCLIC1 activity by a specific binding coordinated by arginine 29. Its mutation, preventing metformin to bind and block tmCLIC1, abolishes the biguanide inhibition of glioblastoma cell proliferation in 2D and 3D models and metformin dependent effect on mitochondrial respiration. In addition, we demonstrate the direct binding between the drug and its target, and by in vivo experiments on zebrafish embryos and mice orthotopically engrafted with glioblastoma cells and treated with metformin, we prove that metformin binding to tmCLIC1 is crucial for metformin antineoplastic effect. Considering tmCLIC1’s contribution to glioblastoma progression, the present work provides the fundaments for future development of strategies aimed at improving metformin-tmCLIC1 interaction to further increase metformin therapeutic potential. ### Competing Interest Statement The authors have declared no competing interest.