Intercellular metabolic and signaling mechanisms of brain cancer-glia communication driving tumor progression - PROJECT SUMMARY/ABSTRACT Glioblastoma multiforme (GBM) is the most common primary malignant brain tumor in adults and has an extremely poor prognosis, with a median survival of only ~12–18 months after diagnosis and no available curative therapies. A hallmark of GBM is its highly immunosuppressive tumor microenvironment (TME), characterized by the accumulation of tumor-associated myeloid cells (TAMCs) polarized into a tumor-supportive and immunosuppressive state. Until the blood-brain barrier is disrupted by advanced disease and/or therapeutic intervention, microglia, the brain’s resident immune cells, are the predominant TAMC type in GBM. We and others have found that GBM-associated microglia promote GBM progression by fostering immunosuppression within the TME and moreover by directly supporting GBM cell proliferation. However, the underlying mechanisms governing microglial reprogramming in GBM remain poorly understood, and strategies to reverse this process are lacking. We have recently discovered an actionable intercellular axis, which simultaneously drives immunosuppressive microglial polarization and provides GBM cells with metabolic support. Specifically, we found that branched-chain á-ketoacids (BCKAs), which are generated from branched-chain amino acids (BCAAs) by BCAT1 and avidly secreted by GBM cells, function as signaling metabolites to drive microglial reprogramming via specific molecular sensors. Moreover, microglia consume GBM cell-derived BCKAs and process them through two pathways, recycling them into BCAAs that are secreted to establish a putative nitrogen shuttle, and generating 3-hydroxyisobutyrate (3-HIB), a paracrine regulator of fatty acid uptake. As such, the key role of BCAT1 in GBM is non-cell autonomous. Correspondingly, selective blockade of BCAT1 in GBM cells does not affect their proliferation in monoculture, but relieves microglial immunosuppression in the TME in vivo and potently inhibits tumor growth. This project aims to elucidate the mechanisms by which BCKAs drive immunosuppressive microglial reprogramming in GBM and to define the broader consequences for the TME (Aim 1), to determine the contribution of intercellular BCAA-BCKA cycling to GBM anabolism (Aim 2), and finally to test the hypothesis that BCAT1 blockade, by relieving microglial reprogramming in GBM, will synergize with immune and/or radiation therapy (Aim 3). To achieve these goals, we will use a multidisciplinary approach that combines advanced genetic and molecular methods, cutting edge metabolomics, and newly developed genetically engineered mouse models (GEMMs). Together, these studies will provide pioneering insights into the mechanisms by which microglia promote GBM progression and will establish the potential of targeting the GBM cell-microglia BCAA/BCKA axis for GBM therapy. Animal use is scientifically justified since there is currently no alternative model that recapitulates the complex interactions among GBM cells, microglia, and the immune system that occur in vivo. All studies will adhere to the principles of replacement, reduction, and refinement.
