HK2 and glioblastoma: GBM cells possess a hyperactivated glycolytic phenotype supported by the overexpression of glucose transporters GLUT1 and GLUT3, as well as glycolytic enzymes such as hexokinase 2 (HK2), lactate dehydrogenase A (LDHA) and pyruvate dehydrogenase kinase 1 (PDK1) [31,32,33].