Abstract
One of the features of malignant gliomas is their deviant resistance to cellular apoptosis induced by cytotoxic reagents. Bmi-1, an oncoprotein, has been linked to oncogenesis and cancer progression in various types of human cancers including gliomas. However, the mechanisms underlying Bmi-1 antiapoptotic function remain largely unknown. In this study, we report that Bmi-1 renders apoptotic resistance to glioma cells through nuclear factor-kappaB (NF-kappaB). In glioma cells, ectopic expression of Bmi-1 significantly inhibits doxorubicin-, BCNU-, or UV irradiation- induced apoptosis through reduction of activated caspase-3 and PARP, and induction of Bcl-X(L). Cellular depletion of Bmi-1 enhances the sensitivity of glioma cells to apoptosis induced by doxorubicin, BCNU, or UV irradiation. Bmi-1 activates NF-kappaB through stimulation of IkappaB phosphorylation, nuclear translocation, and transcriptional activity of NF-kappaB and expression of downstream genes of NF-kappaB including caspase-3, PARP, Bcl-X(L), and c-Myc. Inhibition of the IKK-NF-kappaB pathway abrogates the antiapoptotic effect of Bmi-1 on glioma cells. In high-grade gliomas, Bmi-1 and NF-kappaB are co-expressed in the cell nucleus. Up-regulation of Bmi-1 also correlates with tumor progression and poor survival of patients with gliomas. Together, our data demonstrate that Bmi-1 bestows apoptotic resistance to glioma cells through the IKK-NF-kappaB pathway and suggest Bmi-1 as a useful indicator for glioma prognosis.