The effect of BMI-1 inhibition on brain cancer stem cells.

Abstract

e13543 Background: Glioblastoma (GBM) is the most frequent and deadly brain cancer. Despite tolerance doses of radiation, control of tumor growth within the brain remains a formidable failure. Since the identification of brain cancer stem cells (BCSCs), efforts are underway to target the pathways regulating these cells. The role of Bmi-1 (B-cell specific MMLV insertion site-1), a polycomb member of chromatin-remodeling complex, in BCSCs self-renewal was elucidated. Here we utilize shRNA targeting or pharmacological inhibition of Bmi-1 in GBM cell lines and primary cells as a radiosensitizer to examine the effects of combination therapy on cell death and BCSCs differentiation. Methods: Cells were pre-treated with a Bmi-1 inhibitor before being irradiated. Serial neurosphere assay, a measure of self-renewal potential, was employed to study the effects of radiation, Bmi-1 inhibition, or the combination on BCSCs. The efficacy of this combination on cell death was assessed with MTT and clonogenic assays. Next, the abilities of the inhibitor and radiation to induce differentiation in GBM cell lines and primary cells were quantified. Further, by utilizing a novel zebrafish orthotropic xenograft model, small molecules targeting Bmi-1 and other BCSC pathways can be identified, and used to predict response to combination therapies. Results: Targeting of Bmi-1 in combination with radiation, specifically as a radiosensitizer, induced significant cell death in GBM cells, and was five-fold more effective than radiation only. Importantly, the neurosphere forming ability of BCSCs was severely compromised when the cells were treated with the combination, indicating a potent effect on the stem cell constituency. These effects may be due to loss of BCSC self-renewal potential, increased differentiation, and/or apoptosis as cells treated with the combination exhibited decreased expression of neural stem cell markers and abnormal phenotypes compared to single treatment. Conclusions: Targeting of Bmi-1 may eliminate the subpopulation of radioresistant BCSCs. Bmi-1 inhibition when combined with radiotherapy might provide an effective therapy for GBM patients specifically through its effect on BCSCs by affecting their survival, proliferation, and stem cell features.