The goal of this study was to investigate a novel mechanism by which neural extracellular matrix (ECM) proteins regulate the invasive behavior of malignant gliomas, which are the most common primary brain tumors in adults. Invasion of glioma cells is facilitated by secreted metalloproteases that cleave ECM proteoglycans that inhibit cell migration. Surprisingly, glioma cells also produce these proteoglycans at higher levels than normal neural cells, which appears paradoxical. One of these proteoglycans, brevican (BCAN), is uniquely expressed in the CNS, is the most abundant chondroitin-sulfate proteoglycan in the adult brain ECM, and is upregulated in all low-grade gliomas and glioblastomas independently of their molecular subtype. The functions of brevican, both in normal neural cells or glioma cells, remain unknown and are assumed to be purely structural. We analyzed the signaling mechanisms of brevican using gain- and loss-of-function approaches in differentiated glioblastoma cell lines as well as tumor stem cells, combined with experiments of cell adhesion and invasion in vitro and in vivo. We discovered that a fragment of brevican, but not the full-length protein, interacts with cell-surface sulfatides and activates Src kinase, resulting in trans-activation of EGFR/MAPK signaling even in absence of the native EGFR ligands (TGF-alpha or EGF). Brevican-enhanced EGFR/MAPK activation resulted in increased cell adhesion -via fibronectin production- and motility, which were reversed using Src inhibitors or by treating the cells with aryl-sulfatase that removes sulfatides from the cell surface. Importantly, we observed that brevican secreted by glioblastoma cells was cleaved not only by these cells but also by normal astrocytes that were co-opted by the tumor cells. Absence of this cooperative effect was observed when tumors were implanted in an EGFR-deficient mouse model in which astrocytes did not process glioma-derived brevican, resulting in significantly reduced tumor dispersion. These results resolve the paradoxical production of "anti-migratory" proteoglycans by tumor cells; establish for the first time the entire signaling axis for the proteoglycan brevican in glioma cells; and reveal how this proteoglycan mediates a cooperative interaction between tumor cells and astrocytes that is needed for glioma invasion. These fundamental studies may be leveraged to advance novel anti-invasive strategies that could potentiate the efficacy of current glioma therapies.
Citation Format: Somanath Kundu, Hosung Sim, Bin Hu, Mohan S. Nandhu, Russell T. Matthews, Mariano S. Viapiano. A novel mechanism of glioblastoma cell invasion regulated by a neural proteoglycan that activates Src/EGFR signaling and mediates tumor-astroglia cooperation [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 3148.