Here, we also show that astrocytes exposed to hypoxia upregulate the gene and protein expression levels of VEGF-A, which, once secreted to the microenvironment, can increase the invasion of glioma cells by activating C-X-C chemokine receptor type 4 (CXCR4) signaling [61,62]. Here, CXCR4 is linked to central nervous system cancer.