Transcriptional control of O6-methylguanine DNA methyltransferase expression and temozolomide resistance in glioblastoma

O-6-methylguanine DNA methyltransferase (MGMT) promoter methylation is a predictive biomarker for benefit from alkylating chemotherapy, specifically temozolomide (TMZ), in glioblastoma, the most common malignant intrinsic brain tumor. Glioma-initiating cells (GIC) with stem-like properties have been associated with resistance to therapy and progression. We assessed the levels of MGMTmRNA and MGMT protein by real-time PCR and immunoblot and evaluated the impact of MGMT on TMZ sensitivity in clonogenicity assays in GIC sphere cultures (S) or differentiated adherent monolayer cultures (M). Nuclear factor kappa B (NF-kappa B) signaling was assessed by reporter assay and immunoblot. Compared to M cells, S cells expressed higher levels of MGMT. Differentiation of GIC induced by S-to-M transition resulted in a gradual loss of MGMT expression and increased TMZ sensitivity. This transcriptional regulation of MGMT was restricted to cell lines without MGMT promoter methylation and was not coupled to any specific neurobasal (NB) stem cell medium supplement or loss of cell adhesion. Expression levels of p50/p65 subunits of NF-kappa B, a transcriptional regulator of MGMT, were increased in S cells. Inhibition of NF-kappa B by the small molecule inhibitor, BAY 11-7082, or siRNA-mediated gene silencing, reduced MGMT levels. In summary, alkylator resistance of S cells is mainly promoted by over-expression of MGMT which results from increased activity of the NF-kappa B pathway in this cell culture model of glioma stem-like cells.