Abstract 5584: The critical role of CLIC1 in mediating metformin's antitumoral effects on glioblastoma

Glioblastoma (GBM) is the most common and lethal brain tumor and there is a general agreement that GBM stem cells (GSCs) are primarily responsible for its high invasiveness and recurrence. It has been demonstrated that several membrane proteins are upregulated in GSCs and support tumor relapse. However, most of these membrane resident proteins are also involved in essential physiological processes, limiting their use as selective anticancer targets. The ideal target will be a membrane protein only present on the tumor surface. CLIC1 is a metamorphic protein that can transition between cytoplasmic and transmembrane forms (tmCLIC1), with the latter associated with chloride conductance. tmCLIC1 is enriched in the plasma membrane of GSCs, while it is largely absent in healthy cells. Previous experiments demonstrated that impairment of tmCLIC1 downregulated tumor growth and invasion. Thus, this protein could represent a promising antitumoral target. It has been demonstrated that tmCLIC1 is sensitive to metformin, a widely used antidiabetic drug. Previous data showed contradictory results about the use of metformin against solid tumor growth. We have recently demonstrated that in human primary GSCs, tmCLIC1 is the principal target of metformin. There are two main problems to adopt metformin as a chemotherapic molecules on glioblastoma patients: the high drug concentration needed to be effective and the time at which tumor cells are exposed to the drug. The final aim of the present study is to improve metformin-CLIC1 interaction, trying to lower metformin effective concentration. The first step is to uncover the molecular mechanism of metformin binding to tmCLIC1. Our results show how mutating a specific single arginine of tmCLIC1 (R29) leads to the loss of metformin’s antitumoral properties. Nonetheless, since arginine and metformin carry the same charge, we tried to investigate more deeply this interaction through a molecular docking simulation. We found that this residue coordinates different negatively charged amino acids, that might create the correct environment in which metformin could bind the protein. The electrophysiological test confirmed that mutating only one of these negatively charged residues partially impairs the effect of metformin on the tmCLIC1 current. Finally, we demonstrated that CLIC1 is essential in metformin antitumoral mechanism against GBM also in zebrafish embryos and in murine model. The project’s long-term goal is to develop a personalized medicine against GBM based on the use of metformin.