Knockdown of Cathepsin L promotes radiosensitivity of glioma stem cells both in vivo and in vitro.

The presence of glioma stem cells (GSCs) in tumor is relevant for glioma treatment resistance. This study assessed whether knockdown of Cathepsin L can influence GSC growth, tumor radiosensitivity, and clinical outcome. Protein levels of Cathepsin L and stem cell markers (CD133 and Nestin) were analyzed in samples from 90 gliomas of different WHO grades and 6 normal brain tissues by immunohistochemistry. Two glioma stem cell lines with overexpressed Cathepsin L were stably transfected with Cathepsin L short hairpin RNA expression vectors. The effects of Cathepsin L inhibition on radiosensitivity, self-renewal, stemness, DNA damage, and apoptosis were evaluated. In addition, an intracranial animal model and subcutaneous tumor xenografts in nude mice were used to assess tumor response to Cathepsin L inhibition in vivo. Our results proved that expressions of Cathepsin L and CD133, but not of Nestin, correlated with malignant grades of glioma tissues. GSCs with high Cathepsin L and CD133 co-expression were extraordinarily radioresistant. Cathepsin L inhibition with radiotherapy significantly reduced GSC growth, promoted apoptosis, and improved radiosensitivity. Knockdown of Cathepsin L resulted in a dramatic reduction of CD133 expression, as well as the decreased phosphorylation of DNA repair checkpoint proteins (ATM and DNA-PKcs). Furthermore, combination of Cathepsin L inhibition and radiotherapy potently blocked tumor growth and decreased blood vessel formation in vivo. Taken together, these findings suggest Cathepsin L as a promising therapeutic target for clinical therapy in GBM patients.

Resveratrol, a potential radiation sensitizer for glioma stem cells both in vitro and in vivo.

Glioblastoma is a malignant human cancer that confers a dismal prognosis. Ionizing radiation (IR) is applied as the standard treatment for malignant gliomas. However, radiotherapy remains merely palliative because of the existence of glioma stem cells (GSCs), which are regarded as highly radioresistant "seed" cells. In this study, the effect and possible mechanisms of radiotherapy in combination with resveratrol (Res) were investigated in a radioresistant GSC line, SU-2. Our results showed that Res inhibited SU-2 proliferation and enhanced radiosensitivity as indicated by clonogenic survival assay. We also observed a decrease in the expression of neural stem cell marker CD133, which indicated that treatment with Res and IR induced SU-2 cell differentiation. In addition, the combination of Res with IR significantly increased autophagy and apoptosis levels in both in vitro cells and nude mouse model. Finally, Res significantly attenuated the repair of radiation-induced DNA damage. Taken together, the present study demonstrated that the significant radiosensitization ability of Res both in vitro and in vivo was attributed to its synergistic antitumor effects, including inhibition of self-renewal and stemness, induction of autophagy, promotion of apoptosis, and prevention of DNA repair. Therefore, Res may function as a radiation sensitizer for malignant glioma treatment.

NVP-BEZ235, a novel dual PI3K/mTOR inhibitor, enhances the radiosensitivity of human glioma stem cells in vitro.

AIM: NVP-BEZ235 is a novel dual PI3K/mTOR inhibitor and shows dramatic effects on gliomas. The aim of this study was to investigate the effects of NVP-BEZ235 on the radiosensitivity and autophagy of glioma stem cells (GSCs) in vitro. METHODS: Human GSCs (SU-2) were tested. The cell viability and survival from ionizing radiation (IR) were evaluated using MTT and clonogenic survival assay, respectively. Immunofluorescence assays were used to identify the formation of autophagosomes. The apoptotic cells were quantified with annexin V-FITC/PI staining and flow cytometry, and observed using Hoechst 33258 staining and fluorescence microscope. Western blot analysis was used to analyze the expression levels of proteins. Cell cycle status was determined by measuring DNA content after staining with PI. DNA repair in the cells was assessed using a comet assay. RESULTS: Treatment of SU-2 cells with NVP-BEZ235 (10-320 nmol/L) alone suppressed the cell growth in a concentration-dependent manner. A low concentration of NVP-BEZ235 (10 nmol/L) significantly increased the radiation sensitivity of SU-2 cells, which could be blocked by co-treatment with 3-MA (50 µmol/L). In NVP-BEZ235-treated SU-2 cells, more punctate patterns of microtubule-associated protein LC3 immunoreactivity was observed, and the level of membrane-bound LC3-II was significantly increased. A combination of IR with NVP-BEZ235 significantly increased the apoptosis of SU-2 cells, as shown in the increased levels of BID, Bax, and active caspase-3, and decreased level of Bcl-2. Furthermore, the combination of IR with NVP-BEZ235 led to G1 cell cycle arrest. Moreover, NVP-BEZ235 significantly attenuated the repair of IR-induced DNA damage as reflected by the tail length of the comet. CONCLUSION: NVP-BEZ235 increases the radiosensitivity of GSCs in vitro by activating autophagy that is associated with synergistic increase of apoptosis and cell-cycle arrest and decrease of DNA repair capacity.

Curcumin promotes differentiation of glioma-initiating cells by inducing autophagy.

Glioblastoma (GBM) is a highly aggressive brain tumor characterized by increased proliferation and resistance to chemotherapy and radiotherapy. Recently, a growing body of evidence suggests that glioma-initiating cells (GICs) are responsible for the initiation and recurrence of GBM. However, the factors determining the differential development of GICs remain poorly defined. In the present study, we show that curcumin, a natural compound with low toxicity in normal cells, significantly induced differentiation of GICs in vivo and in vitro by inducing autophagy. Moreover, curcumin also suppressed tumor formation on intracranial GICs implantation into mice. Our results suggest that autophagy plays an essential role in the regulation of GIC self-renewal, differentiation, and tumorigenic potential, suggesting autophagy could be a promising therapeutic target in a subset of glioblastomas. This is the first evidence that curcumin has differentiating and tumor-suppressing actions on GICs.

Rapamycin induces differentiation of glioma stem/progenitor cells by activating autophagy.

Glioma stem/progenitor cells (GSPCs) are considered to be responsible for the initiation, propagation, and recurrence of gliomas. The factors determining their differentiation remain poorly defined. Accumulating evidences indicate that alterations in autophagy may influence cell fate during mammalian development and differentiation. Here, we investigated the role of autophagy in GSPC differentiation. SU-2 cells were treated with rapamycin, 3-methyladenine (3-MA) plus rapamycin, E64d plus rapamycin, or untreated as control. SU-2 cell xenografts in nude mice were treated with rapamycin or 3-MA plus rapamycin, or untreated as control. Western blotting and immunocytochemistry showed up-regulation of microtubule-associated protein light chain-3 (LC3)-II in rapamycin-treated cells. The neurosphere formation rate and the number of cells in each neurosphere were significantly lower in the rapamycin treatment group than in other groups. Real-time PCR and immunocytochemistry showed down-regulation of stem/progenitor cell markers and up-regulation of differentiation markers in rapamycin-treated cells. Transmission electron microscopy revealed autophagy activation in rapamycin-treated tumor cells in mice. Immunohistochemistry revealed decreased Nestin-positive cells and increased GFAP-positive cells in rapamycin-treated tumor sections. These results indicate that rapamycin induces differentiation of GSPCs by activating autophagy.

Induction of autophagy promotes differentiation of glioma-initiating cells and their radiosensitivity.

Glioblastoma (GBM) is a highly aggressive brain tumor characterized by increased proliferation and resistance to chemotherapy and radiotherapy. Recently, the identification of tumor-initiating cells with stem-like properties in diverse human cancers including GBM represents an important conceptual advance in cancer biology with therapeutic implications. However, the factors determining the differential development and radiosensitization of glioma-initiating cells (GICs) remain poorly defined. Here, we report that rapamycin induced differentiation of GICs and increased their sensitivity to radiation by activating autophagy. Transient in vitro exposure to rapamycin and radiation abolished the capacity of transplanted GICs to establish intracerebral GBMs. Most importantly, in vivo combination of rapamycin and radiation effectively blocked the tumor growth and associated mortality that occurs in mice after intracerebral grafting of human GICs. We demonstrate that rapamycin activated their autophagy and triggers the differentiation cascade in GICs isolated from human GBMs. This was followed by a reduction in proliferation, cell viability, clonogenic ability and increased expression of neural differentiation markers after radiation. Our results suggest that autophagy plays an essential role in the regulation of self-renewal, differentiation, tumorigenic potential and radiosensitization of GICs, suggesting autophagy could be a promising therapeutic target in a subset of GBMs. We propose that autophagy defect in GICs contributes to radioresistance of GICs by desensitizing GICs to normal differentiation cues. Activating autophagy may abrogate the resistance of GICs to radiation and could lead to the development of novel therapeutic approaches for the treatment of GBMs.

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

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.