PTEN and glioblastoma: To Further Clarify the Roles of ERK and Akt Signaling in PTEN-Induced Sensitivity of GBM Cells to Palbociclib, We Transfected Constitutive Active ERK (ERK-CA) and Akt (Akt-CA) Plasmids Into U87MG-PTEN/Vector Isogenic Cells.