Glioblastoma (GBM) represents the most common and aggressive histologic subtype among malignant astrocytoma and is associated with poor outcomes because of heterogeneous tumour cell population including mature non-stem-like cell and immature stem-like cells within the tumour. Thus, it is critical to find new target-specific therapeutic modalities. Protein arginine methyltransferase enzyme 5 (PRMT5) regulates many cellular processes through its methylation activity and its overexpression in GBM is associated with more aggressive disease. Previously, we have shown that silencing of PRMT5 expression in differentiated GBM cell lines results in apoptosis and reduced tumour growth in mice. Here, we report the critical role of PRMT5 in GBM differentiated cells (GBMDC) grown in serum and GBM neurospheres (GBMNS) grown as neurospheres in vitro. Our results uncover a very significant role for PRMT5 in GBMNS self-renewal capacity and proliferation. PRMT5 knockdown in GBMDC led to apoptosis, knockdown in GBMNS led to G1 cell cycle arrest through upregulation of p27 and hypophoshorylation of retinoblastoma protein, leading to senescence. Comparison of impact of PRMT5 on cellular signalling by the Human Phospho-Kinase Array and chromatin immunoprecipitation-PCR revealed that unlike GBMDC, PRMT5 regulates PTEN expression and controls Akt and ERk activity in GBMNS. In vivo transient depletion of PRMT5 decreased intracranial tumour size and growth rate in mice implanted with both primary tumour-derived GBMNS and GBMDC. This is the first study to identify PTEN as a potential downstream target of PRMT5 and PRMT5 is vital to support both mature and immature GBM tumour cell populations.

Background Both the proteasome inhibitor bortezomib and an oncolytic herpes simplex virus-1 (oHSV) expressing GMCSF are currently FDA-approved. While proteasome blockade can increase oHSV replication, immunological consequences and consequent immunotherapy potential are unknown. In this study, we investigated the impact of bortezomib combined with oHSV on tumor cell death and sensitivity to Natural Killier (NK) cell immunotherapy. Experimental Design Western blot, flow cytometry, and caspase 3/7 activity assays were used to evaluate the induction of apoptosis/autophagy and/or necroptotic cell death. Cellular and mitochondrial reactive oxygen species (ROS) production was measured utilizing CellROX™ and MitoSOX. Inhibitors/shRNA targeting ROS, JNK and RIP1 kinase (RIPK1) were utilized to investigate the mechanism of cell killing. The synergistic interaction between oHSV and bortezomib was calculated using a Chou-Talalay analysis. NK cells isolated from normal human blood were co-cultured with tumor cells to evaluate cellular interactions. Q-PCR, ELISA, and FACS analysis were used to evaluate NK cell activation. Intracranial tumor xenografts were utilized to evaluate anti-tumor efficacy. Results Combination treatment with bortezomib and oHSV induced necroptotic cell death and increased the production of mitochondrial ROS and JNK phosphorylation. Inhibitors/shRNA of RIPK1 and JNK rescued synergistic cell killing. Combination treatment also significantly enhanced NK cell activation and adjuvant NK cell therapy of mice treated with bortezomib and oHSV improved anti-tumor efficacy. Conclusions This study provides a significant rationale for triple combination therapy with bortezomib, oHSV, and NK cells to improve efficacy, in glioblastoma patients.

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