The human glioma cell lines U373, A172, M059K were obtained from the American Type Culture Collection (ATCC, Manassas, VA, USA); H4, U87, U251 were purchased from the Cell Culture Center (Chinese Academy of Medical Sciences, Beijing, China). All these cell lines were maintained in Dulbecco's modified Eagle's medium (DMEM; Invitrogen, Carlsbad, CA, USA) supplemented with 10% fetal bovine serum (FBS; HyClone, Logan, UT, USA) and at 37°C in 5% CO2.