Mechanisms underlying BAI1/ADGRB1 negative regulation of glioblastoma mesenchymal transition and invasion.

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Project Summary New therapies are urgently needed for patients with malignant gliomas, which are highly invasive and lethal brain tumors. Patients with glioblastoma (GBM) die within 1-2 years of diagnosis despite current conventional therapies, including surgery, radiation and chemotherapy. A major driver of tumor recurrence is the infiltrative nature of the glioma cells into adjacent normal brain, which is driven by activation of a mesenchymal transcription program. This mesenchymal transition is activated through extracellular stimulation of cell surface receptors by growth factors such as TGFβ1. The overall purpose of the present project is to investigate the role of adhesion G protein-coupled receptor B1 (ADGRB1/BAI1) in the mesenchymal switch and invasion, and explore new therapies for GBM based on the related mechanisms. ADGRB1 is an orphan adhesion GPCR specifically expressed in the brain. We previously showed that ADGRB1 expression is significantly reduced in patients with GBM through epigenetic silencing, suggesting that ADGRB1 loss may facilitate tumor formation. Our new preliminary data show that low ADGRB1 expression correlates with invasion and poor outcome in glioma patients. Restoration of ADGRB1 expression in GBM cells suppresses the mesenchymal phenotype in culture and mice xenografts. Our pilot studies further suggest ADGRB1 can inhibit TGFβ1-driven mesenchymal transition through a WxLWxLW motif in its first thrombospondin type 1 repeat (TSR1). This motif mediates ADGRB1 binding to the latent TGFβ1 complex and prevents TGFβ1 maturation. Based on these results, we hypothesize that ADGRB1 acts as a brain tumor suppressor by blocking the TGFβ1-mediated mesenchymal switch and that restoration of its expression with epigenetic therapy will represent a novel therapeutic intervention for GBM. To test our hypothesis, we propose the following aims: (i) define how ADGRB1 negatively regulates the mesenchymal transition and glioma invasion, (ii), determine how BAI1 antagonizes TGFβ1 pro-mesenchymal signaling and (iii) evaluate whether epigenetic restoration of BAI1 expression can inhibit glioma cell invasion in vitro and in vivo, and augment survival post-operation. These studies are important as we identified a specific region in the extracellular domain of BAI1 that antagonizes TGFβ1 maturation and the glioma mesenchymal switch, providing a new mechanism for antagonizing this oncogenic pathway that can be exploited therapeutically. These findings support targeting this new pathway in patients whose cancers are driven by mesenchymal transition.