RB1 and glioblastoma: First, we tested whether RB regulated Na+-K+-ATPase enzyme activity in GBM cells and found that treatment with 4 μM RB significantly enhanced Na+-K+-ATPase activity by ∼4-fold in U251 cells and by ∼6-fold in A172 cells (Figure 5B), indicating that RB significantly increased Na+-K+-ATPase activity in GBM cells.