Methoxyamine sensitizes the resistant glioblastoma T98G cell line to the alkylating agent temozolomide

Chemoresistance represents a major obstacle to successful treatment for malignant glioma with temozolomide. N7-methyl-G and N3-methyl-A adducts comprise more than 80 % of DNA lesions induced by temozolomide and are processed by the base excision repair, suggesting that the cellular resistance could be caused, in part, by this efficient repair pathway, although few studies have focused on this subject. The aim of this study was to evaluate the cellular responses to temozolomide treatment associated with methoxyamine (blocker of base excision repair) in glioblastoma cell lines, in order to test the hypothesis that the blockage of base excision repair pathway might sensitize glioblastoma cells to temozolomide. For all the tested cell lines, only T98G showed significant differences between temozolomide and temozolomide plus methoxyamine treatment, observed by reduced survival rates, enhanced the levels of DNA damage, and induced an arrest at G2-phase. In addition, ~10 % of apoptotic cells (sub-G1 fraction) were observed at 48 h. Western blot analysis demonstrated that APE1 and FEN1 presented a slightly reduced expression levels under the combined treatment, probably due to AP sites blockade by methoxyamine, thus causing a minor requirement of base excision repair pathway downstream to the AP removal by APE1. On the other hand, PCNA expression in temozolomide plus methoxyamine-treated cells does not rule out the possibility that such alteration might be related to the blockage of cell cycle (G2-phase), as observed at 24 h of recovery time. The results obtained in the present study demonstrated the efficiency of methoxyamine to overcome glioblastoma resistance to temozolomide treatment.