Abstract
Glioblastoma stem cells (GSC) are a significant cell model for explaining brain tumor recurrence. However, mechanisms underlying their radiochemoresistance remain obscure. Here we show that most clonogenic cells in GSC cultures are sensitive to radiation treatment (RT) with or without temozolomide (TMZ). Only a few single cells survive treatment and regain their self-repopulating capacity. Cells re-populated from treatment-resistant GSC clones contain more clonogenic cells compared to those grown from treatment-sensitive GSC clones, and repeated treatment cycles rapidly enriched clonogenic survival. When compared to sensitive clones, resistant clones exhibited slower tumor development in animals. Upregulated genes identified in resistant clones via comparative expression microarray analysis characterized cells under metabolic stress, including blocked glucose uptake, impaired insulin/Akt signaling, enhanced lipid catabolism and oxidative stress, and suppressed growth and inflammation. Moreover, many upregulated genes highlighted maintenance and repair activities, including detoxifying lipid peroxidation products, activating lysosomal autophagy/ubiquitin-proteasome pathways, and enhancing telomere maintenance and DNA repair, closely resembling the anti-aging effects of caloric/glucose restriction (CR/GR), a nutritional intervention that is known to increase lifespan and stress resistance in model organisms. Although treatment–introduced genetic mutations were detected in resistant clones, all resistant and sensitive clones were subclassified to either proneural (PN) or mesenchymal (MES) glioblastoma subtype based on their expression profiles. Functional assays demonstrated the association of treatment resistance with energy stress, including reduced glucose uptake, fatty acid oxidation (FAO)-dependent ATP maintenance, elevated reactive oxygen species (ROS) production and autophagic activity, and increased AMPK activity and NAD+ levels accompanied by upregulated mRNA levels of SIRT1/PGC-1α axis and DNA repair genes. These data support the view that treatment resistance may arise from quiescent GSC exhibiting a GR-like phenotype, and suggest that targeting stress response pathways of resistant GSC may provide a novel strategy in combination with standard treatment for glioblastoma.
Highlights
Glioblastoma (World Health Organization/WHO grade IV) is the most common and aggressive type of primary malignant brain tumor in adults, killing nearly every patient within two years
We found that long-term passaged Glioblastoma stem cells (GSC) lines typically contain 3-30% CD133+ cells even initiated by purified CD133+ cells, and suggested that slow self-renewal and fast proliferative division/differentiation naturally occur during passaging in serum-free media containing stem cell growth factors [7]
We found that the majority of tumorigenic GSC clones derived from patient tumors are sensitive to treatment, and only a minority of clones is able to survive treatment
Summary
Glioblastoma (World Health Organization/WHO grade IV) is the most common and aggressive type of primary malignant brain tumor in adults, killing nearly every patient within two years. Patients whose tumors have a methylated promoter for the gene encoding for O6-methylguanine-DNA methyltransferase (MGMT) are more likely to benefit from the addition of TMZ to RT, they become resistant to the treatment. The development of resistance suggests that there is a remnant of cancer cells possessing tumorigenic capacity with extraordinary defense mechanisms, enabling them to survive treatment. Glioblastoma stem cells (GSC) have become a significant experimental model for explaining tumor recurrence because they possess a tumorigenic capacity [2,3,4,5,6,7], a highly migratory nature [7,8], and a radiochemoresistant phenotype [9,10,11]. Some authors did not find different DNA repair mechanisms in stem and non-stem glioma cells [19,20]
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