Overexpression of ZNF217 in glioblastoma contributes to the maintenance of glioma stem cells regulated by hypoxia-inducible factors

Glioblastoma multiforme (GBM) is the most aggressive and common kind of primary brain tumor in adults, and is thought to be driven by a subpopulation of glioma stem cells (GSCs). GSCs reside in a specialized hypoxic niche, which can regulate the tumorigenic capacity of GSCs primarily through the hypoxia-inducible factors (HIFs), HIF1α and HIF2α. ZNF217 is an oncogene frequently amplified in many kinds of tumors. It is associated with aggressive tumor behavior and poor clinical prognosis, but its role in gliomas is poorly known. Gene expression and copy number analysis from TCGA data reveal that ZNF217 is amplified in 32% and overexpressed in 71.2% of GBMs. Quantitative RT-PCR and western blotting of a cohort of glioma samples showed that ZNF217 was highly expressed in gliomas and increased with tumor grade. Analysis of a molecular database demonstrated that ZNF217 expression correlated with poor survival of glioma patients. Investigation of ZNF217 expression in GSCs, non-GSCs and normal neural stem cells (NSCs) indicated that ZNF217 was more highly expressed in GSCs than in non-GSCs and NSCs. Knockdown of ZNF217 in GSCs by small-interfering RNA (siRNA) inhibited their growth and promoted their differentiation. Interestingly, ZNF217 was upregulated in GSCs and the GBM cell line U87 when exposed to the hypoxic environment of 1% oxygen. Knockdown of either HIF1α or HIF2α, which has a central role in the hypoxia-induced responses of these cells, inhibited ZNF217 expression. In addition, ZNF217 upregulation was compromised under hypoxia in U87 and GSCs when either HIF1α or HIF2α was targeted by siRNA. HIF2α knockdown inhibited ZNF217 expression more efficiently in both normoxia and hypoxia than HIF1α knockdown. Therefore, ZNF217 is overexpressed in GBMs and contributes to the maintenance of GSCs, which is regulated by HIFs released by the hypoxic environment of the tumor.