IMMU-37. TARGETING NON-CATALYTIC ACTIVATORS OF THE PROTEASOME DECREASES TUMOR GROWTH AND ENHANCES ANTIGEN PRESENTATION IN GLIOBLASTOMA

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Abstract Glioblastoma (GBM) is the most common adult primary brain tumor plagued by inevitable recurrence and poor survival. We recently performed a genome wide CRISPR/Cas9 essentiality screen in GBM stem cells, revealing a plethora of potential targets for further exploration. The proteasome is a multimeric protein complex that degrades cellular proteins contributing to homeostatic proteostasis, stress response, and antigen presentation. Most proteasomal subunits are essential for GBM stem cell growth in vitro, however, they are also essential for non-malignant neural cells, suggesting that inhibition of those subunits may lead to toxicity. Indeed, adverse neurological symptoms were prevalent in phase III clinical trials for the brain penetrant proteasome inhibitor, Marizomib, which may be linked to the vital role of proteasome subunits in non-malignant neural counterparts. Proteasome inhibitors target the catalytic subunits of the proteasome, however the role of individual proteasome activators, most of which are non-essential for growth in vitro, have not been fully elucidated in GBM. Here, we examined the functionality of non-essential proteasome activator subunits in GBM stem cells in vitro and in vivo. Surprisingly, despite lack of growth changes in vitro, we observed abrogated stem-cell self-renewal in vitro and improved survival in vivo in orthotopic xenograft models following targeting of specific activator subunits. Molecular profiling of targeted cells revealed an upregulation of interferon-γ signaling and upregulation of antigen presentation machinery. Thus, targeting specific activator subunits may inhibit malignant growth in vivo while sparing normal neural counterparts from proteotoxic stress. We are further investigating enhanced antigen presentation by targeting these proteasome activator subunits and examining changes in the tumor microenvironment and survival in syngeneic immunocompetent models of GBM. Further understanding of this mechanism may provide novel targets for GBM treatment or improve immunotherapies in GBM.

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