Knockdown of the DNA-dependent protein kinase catalytic subunit radiosensitizes glioma-initiating cells by inducing autophagy

Abstract

Glioblastoma (GBM) is a highly aggressive brain tumor characterized by increased proliferation and resistance to chemotherapy and radiotherapy. A growing body of evidence suggests that only a small subpopulation of malignant glioma cells, called glioma stem cells or glioma-initiating cells (GICs), have true tumorigenic potential and confer glioma radioresistance. DNA-dependent protein kinase catalytic subunit (DNA-PKcs) plays a major role in the repair of DNA double-strand breaks induced by ionizing radiation (IR). Suppression of one of these components of the DNA-PK complex can inhibit the DNA double-strand break repair and radiosensitize the cells. In general, the cell death induced by IR is considered to be apoptotic. Recently, autophagy, an alternative form of programmed cell death, has been shown to contribute significantly to anti-neoplastic effects of radiation therapy. Autophagy is independent of phagocytes and differs from apoptosis by the presence of autophagosomes, autolysosomes, and an intact nucleus in the cell. Little is known, however, regarding the relationship between DNA-PKcs and IR-induced autophagy in GICs. In the present study, we constructed plasmids encoding short hairpin RNA (shRNA) targeting DNA-PKcs, which were then transfected into GICs. Then, we used GICs and DNA-PKcs-RNAi transfected cells to investigate the role of DNA-PKcs in IR-induced apoptotic and autophagic cell death. IR induced massive autophagic cell death in DNA-PKcs-RNAi transfected cells, but only occasional apoptotic cells were detected among GICs. Specific inhibition of DNA-PKcs in GICs induced autophagy and radiosensitized the cells. Our results suggest that such radiation-induced autophagy may enhance the effect of glioma therapies.