The Impact of Hypoxia and Mesenchymal Transition on Glioblastoma Pathogenesis and Cancer Stem Cells Regulation

Glioblastoma (GBM) is an aggressive primary brain tumor with potential for wide dissemination and resistance to standard treatments. Although GBM represents a single histopathologic diagnosis under current World Health Organization criteria, data from multiplatform molecular profiling efforts, including The Cancer Genome Atlas, indicate that multiple subgroups with distinct markers and biology exist. It remains unclear whether treatment resistance differs based on subgroup. Recent evidence suggests that hypoxia, or absence of normal tissue oxygenation, is important in generating tumor resistance through a signaling cascade driven by hypoxia-inducible factors and vascular endothelial growth factor. Hypoxia can result in isolation of tumor cells from therapeutic agents and activation of downstream tumor protective mechanisms. In addition, there are links between hypoxia and the phenomenon of mesenchymal transition in gliomas. Mesenchymal transformation in gliomas resembles at many levels the epithelial-mesenchymal transition that has been described in other solid tumors in which epithelial cells lose their epithelial characteristics and take on a more mesenchymal phenotype, but the mesenchymal transition in brain tumors is also distinct, perhaps related to the unique cell types and cellular organization in the brain and brain tumors. Cancer stem cells, which are specific cell populations involved in self-renewal, differentiation, and GBM pathophysiology, are also importantly regulated by hypoxia signaling pathways. In this review, we discuss the interplay of hypoxia and mesenchymal signaling in GBM including the key pathway regulators and downstream genes, the effect of these processes in regulation of the tumor microenvironment and cancer stem cells, and their role in treatment resistance.