Augmentation of imatinib-induced cytotoxicity in human malignant glioma cells by suppression of autophagy

AACR Annual Meeting-- Apr 12-16, 2008; San Diego, CA 2269 Background: Malignant gliomas are the most common primary tumors of central nervous system. The prognosis of patients with malignant gliomas is poor in spite of current intensive therapy, including surgery, radiation, and chemotherapy. Therefore, we need to explore novel therapeutic modalities. Imatinib mesylate, a tyrosine kinase inhibitor of bcr-abl, c-abl, c-kit, and PDGFR α and β, has recently been proved effective in the treatment of patients with several types of tumors such as Philadelphia chromosome positive leukemias. However, results of clinical trials for patients with recurrent malignant gliomas indicate that imatinib alone has insufficient activity. Moreover, the molecular mechanisms by which imatinib exhibits cytotoxicity against malignant glioma cells are not yet fully understood. Therefore, in this study, we determined the type of cell death caused by imatinib and explored the feasibility of sensitizing malignant glioma cells to imatinib. Materials and methods: Human malignant glioma cell lines U87-MG and U373-MG were treated with imatinib and assessed by cell viability assay, electron microscopy, and immunoblotting to determine the type of cell death. Because imatinib induced non-apoptotic cell death with autophagic features in tumor cells, autophagy was inhibited at different stages to assess the role of autophagy in cancer therapy: we used a class III phosphatidylinositol 3 kinase inhibitor 3-methyladenine (3-MA) or siRNA against Atg5 that inhibit autophagy at an early stage, or bafilomycin A1 or RTA203 that are vacuolar type H+-ATPase inhibitors and suppress autophagy at a late stage. Results: Imatinib inhibited the viability of malignant glioma cells in a dose- dependent manner. Imatinib caused non-apoptotic autophagic cell death. When imatinib-induced autophagy was suppressed by 3-MA or siRNA against Atg5, the cytotoxicity of imatinib was attenuated. In contrast, bafilomycin A1 or RTA203 inhibited autophagy and enhanced the antitumor effect of imatinib through induction of apoptotic cell death in both tumor cells. Conclusions: Imatinib causes non-apoptotic autophagy in malignant glioma cells. When autophagy was inhibited at a late stage, the cytotoxicity of imatinib was augmented by stimulation of apoptotic signal pathway. Our findings provide a potential strategy to sensitize malignant glioma cells to imatinib by inhibition of autophagy at a late stage.