SPHK2 and glioblastoma: The targeting of the sphingolipid system as a therapeutic direction for glioblastoma has been promisingly investigated, since representative oncopromoter as sphingosine-1-phosphate (S1P) and its kinases (SPHK1 and SPHK2) have been clearly recognized as key actors of tumor progression and aggressiveness [22,23,24,25,26].