Recent studies suggest that a small subpopulation of malignant cells with stem-like properties is resistant to chemotherapy and may be responsible for the existence of residual cancer after treatment. We have isolated highly tumorigenic cancer cells with 100-fold increase in tumor initiating capacity from the tumor xenografts of human glioblastoma U87 cells in mice. These cells exhibit stem-like properties and show unique energy metabolic characteristics including low mitochondrial respiration, increased glycolysis for ATP generation, and preference for hypoxia to maintain their stemness and tumor forming capacity. Mechanistically, mitochondrial depression in the highly tumorigenic cells occurs mainly at complex II of the electron transport chain with a down-regulation of the succinate dehydrogenase subunit B, leading to deregulation of hypoxia-inducible factors. Under hypoxia, the stem-like cancer cells are resistant to conventional anticancer agents but are sensitive to glycolytic inhibition. Furthermore, combination of glycolytic inhibition with standard therapeutic agents is effective in killing the tumor-initiating cells in vitro and inhibits tumor formation in vivo. Our study suggests that stem-like cancer cells prefer a low oxygen microenvironment and actively utilize the glycolytic pathway for ATP generation. Inhibition of glycolysis may be an effective strategy to eradicate residual cancer stem cells that are otherwise resistant to chemotherapeutic agents in their hypoxic niches.

Glioblastoma remains the most common and devastating primary brain tumor despite maximal therapy with surgery, chemotherapy, and radiation. The glioma stem cell (GSC) subpopulation has been identified in glioblastoma and likely plays a key role in resistance of these tumors to conventional therapies as well as recurrent disease. GSCs are capable of self-renewal and differentiation; glioblastoma-derived GSCs are capable of de novo tumor formation when implanted in xenograft models. Further, GSCs possess unique surface markers, modulate characteristic signaling pathways to promote tumorigenesis, and play key roles in glioma vascular formation. These features, in addition to microenvironmental factors, present possible targets for specifically directing therapy against the GSC population within glioblastoma. In this review, the authors summarize the current knowledge of GSC biology and function and the role of GSCs in new vascular formation within glioblastoma and discuss potential therapeutic approaches to target GSCs.

Malignant gliomas are common primary tumors of the central nervous system. The prognosis of patients with malignant glioma is poor in spite of current intensive therapy and thus novel therapeutic modalities are necessary. Imatinib mesylate, a tyrosine kinase inhibitor, is effective in the therapy of tumors including leukemias but not as a monotherapy for malignant glioma. Recently, it is thought that the adequate modulation of autophagy can enhance efficacy of anticancer therapy. The outcome of autophagy manipulation, however, seems to depend on the autophagy initiator, the combined stimuli, the extent of cellular damage and the type of cells, and it is not yet fully understood how we should modulate autophagy to augment efficacy of each anticancer therapy. In this study, we examined the effect of imatinib with or without different types of autophagy inhibitors on human malignant glioma cells. Imatinib inhibited the viability of U87-MG and U373-MG cells in a dose dependent manner and caused nonapoptotic autophagic cell death. Suppression of imatinib-induced autophagy by 3-methyladenine or small interfering RNA against Atg5, which inhibit autophagy at an early stage, attenuated the imatinibinduced cytotoxicity. In contrast, inhibition of autophagy at a late stage by bafilomycin A1 or RTA 203 enhanced imatinib-induced cytotoxicity through the induction of apoptosis following mitochondrial disruption. Our findings suggest that therapeutic efficiency of imatinib for malignant glioma may be augmented by inhibition of autophagy at a late stage, and that appropriate modulation of autophagy may sensitize tumor cells to anticancer therapy.

Stem cells including cancer stem cells (CSCs) require niches to maintain stemness, yet it is unclear how CSCs maintain stemness in the suboptimal environment outside their niches during invasion. Postnatal codeletion of Pten and Trp53 in mouse neural stem cells (NSCs) leads to their expansion in the subventricular zone (SVZ) niches but fails to maintain the stemness outside the SVZs. We discovered that QKI is a major regulator of NSC stemness. Qki deletion against Pten−/−Trp53−/− backdrop helps NSCs maintain their stemness outside the SVZs in Nestin-CreERT2 QkiL/LPtenL/LTrp53L/L mice, which develop glioblastoma with a penetrance of 92% and a median survival of 105 days. Mechanistically, Qki deletion decreases endolysosome-mediated degradation and enriches receptors essential for maintaining self-renewal on the cytoplasmic membrane to cope with low ligand levels outside the niches. Thus, downregulating endolysosome level by QKI loss helps glioma stem cells (GSCs) maintain their stemness in suboptimal environments outside the niches.

Autophagy, an evolutionarily conserved response to stress, has recently been implicated in cancer initiation and progression, but the detailed mechanisms and functions have not yet been fully elucidated. One major obstacle to our understanding is lack of an efficient and robust method to specifically monitor autophagic cells in cancer specimens. To identify molecular events associated with autophagy, we performed cDNA microarray analysis of autophagic glioblastoma cell lines. Based on the analysis, we raised a polyclonal antibody against isoform B of human microtubule-associated protein 1 light chain 3 (LC3B). Application of the anti-LC3B antibody revealed the presence of autophagic cells in both in vitro and in vivo settings. Of the 65 glioblastoma tissues, 31 had highly positive cytoplasmic staining of LC3B. The statistical interaction between cytoplasmic staining of LC3B and Karnofsky Performance Scale score was significant. High expression of LC3B was associated with an improved outcome for patients with poorer performance, whereas, for patients with normal performance, survival was better for patients with low staining than with high staining of LC3B. Anti-LC3B antibody provides a useful tool for monitoring the induction of autophagy in cancer cells and tissues.