The Hypoxic Microenvironment and Evasive Resistance to Antiangiogenesis in Brain Tumors

Background/Purpose: Diffuse invasion of the surrounding brain parenchyma is a major obstacle in the treatment of gliomas. Recent evidence suggests that increased invasiveness also allows gliomas to evade antiangiogenic therapies. Hypoxia is a key regulator of hallmarks of cancer including tumor invasion, however, the molecular machinery that controls the invasive process in brain tumors is incompletely understood. Here, we identify the epi-/genetic and microenvironmental downregulation of ephrinB2 as a crucial step that promotes tumor invasion by abrogation of repulsive signals.

Methods: Using gain and loss of function experiments, we analyzed the role of ephrinB2 and ZEB2 for tumor growth in cell culture and animal models. Through biochemical analyses, luciferase reporter assays and immunostaining we further characterized the function of EphrinB2 and ZEB2 in tumor invasion.

Results: We demonstrate that ephrinB2 is downregulated in human gliomas as a consequence of promoter hypermethylation and gene deletion. Consistently, genetic deletion of ephrinB2 in a murine high-grade glioma model increases invasion. Importantly, ephrinB2 gene silencing is complemented by a hypoxia induced transcriptional repression. Mechanistically, hypoxia-inducible factor (HIF)-1α induces the EMT repressor ZEB2, which directly downregulates ephrinB2 through promoter binding to enhance tumor invasiveness. This mechanism is activated following anti-angiogenic treatment of gliomas and is efficiently blocked by disrupting ZEB2 activity.

Conclusion: Taken together, our results identify ZEB2 as an attractive therapeutic target to inhibit tumor invasion and counteract tumor resistance mechanisms induced by antiangiogenic treatment strategies.