STEM-16. DUAL INHIBITION OF PROTEIN ARGININE METHYLTRANSFERASE 5 AND PROTEIN PHOSPHATASE 2A ENHANCES THE ANTI-TUMOR EFFICACY IN PRIMARY GLIOBLASTOMA NEUROSPHERES

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Abstract INTRODUCTION Glioblastoma (GBM) is the most common malignant primary brain tumor and has a heterogeneous tumor cell population. The median survival of GBM patients is less than two years with current multi-model therapy of maximal surgical resection followed by chemotherapy, radiation and tumor treating fields. Protein Arginine Methyltransferase 5 (PRMT5) regulates cellular functions through symmetric di-methylation of arginine residues of histone and non-histone proteins. Our recent findings show that PRMT5 is overexpressed in GBM; its inhibition causes apoptosis and senescence of mature and immature GBM tumor cells respectively. Protein Phosphatase 2A (PP2A), a serine-threonine phosphatase, is associated with senescent tumor cells that contribute to therapy resistance; LB100 is a first-in-class small molecule inhibitor of PP2A that can sensitize tumor cells that are chemo- and radiation-therapy resistant. In this study, using patient-derived primary GBM neurospheres (GBMNS), we tested the anti-GBM effect of concurrent PRMT5 and PP2A inhibition. METHODS Patient-derived primary GBM neurospheres transfected with PRMT5 target-specific siRNA were treated with LB100 and subjected to in vitro assays such as proliferation assay, cell cycle analysis, cytotoxicity assay, PP2A activity and western blot. RESULTS LB100 treatment significantly reduced the viability of PRMT5-depleted GBM cells as compared to PRMT5 intact cells. LB100 caused G1 cell cycle arrest through inactivation of Rb protein; this effect is further enhanced in combination with PRMT5-depletion. Combination therapy also increased the expression of phospho-MLKL; Necrostatin-1 rescued PRMT5-depleted cells from the cytotoxic effects of LB100 indicating that necroptosis caused the enhanced cytotoxicity in combination therapy. We also found that PRMT5-depletion increased the PP2A activity in GBM neurospheres. CONCLUSION Combining the PRMT5 inhibition and PP2A inhibition produced greater antitumor effects than with PRMT5 inhibition alone.