MicroRNA-124 expression counteracts pro-survival stress responses in glioblastoma.

Glioblastomas are aggressive adult brain tumors, characterized by inadequately organized vasculature and consequent nutrient and oxygen (O2)-depleted areas. Adaptation to low nutrients and hypoxia supports glioblastoma cell survival, progression and therapeutic resistance. However, specific mechanisms promoting cellular survival under nutrient and O2 deprivation remain incompletely understood. Here, we show that miR-124 expression is negatively correlated with a hypoxic gene signature in glioblastoma patient samples, suggesting that low miR-124 levels contribute to pro-survival adaptive pathways in this disease. As miR-124 expression is repressed in various cancer types (including glioblastoma), we quantified miR-124 abundance in normoxic and hypoxic regions in glioblastoma patient tissue, and investigated whether ectopic miR-124 expression compromises cell survival during tumor ischemia. Our results indicate that miR-124 levels are further diminished in hypoxic/ischemic regions within individual glioblastoma patient samples, compared with regions replete in O2 and nutrients. Importantly, we also show that increased miR-124 expression affects the ability of tumor cells to survive under O2 and/or nutrient deprivation. Moreover, miR-124 re-expression increases cell death in vivo and enhances the survival of mice bearing intracranial xenograft tumors. miR-124 exerts this phenotype in part by directly regulating TEAD1, MAPK14/p38α and SERP1, factors involved in cell proliferation and survival under stress. Simultaneous suppression of these miR-124 targets results in similar levels of cell death as caused by miR-124 restoration. Importantly, we further demonstrate that SERP1 reintroduction reverses the hypoxic cell death elicited by miR-124, indicating the importance of SERP1 in promoting tumor cell survival. In support of our experimental data, we observed a significant correlation between high SERP1 levels and poor patient outcome in glioblastoma patients. Collectively, among the many pro-tumorigeneic properties of miR-124 repression in glioblastoma, we delineated a novel role in promoting tumor cell survival under stressful microenvironments, thereby supporting tumor progression.