Signaling downstream of mTOR is required for IGF-1-induced MDM2 up-regulation.

<p>(A) H1299 cells were transiently transfected with wild-type 4EBP1, 4EBP1-5A, or vector plasmid. Twenty-four hours after transfection, cells were split and maintained for an additional 24 hours prior to serum starvation for 24 hours. Cells were then treated with or without IGF-1 (50 ng/mL) for six hours. Cells were lysed and subjected to western blotting. Quantitative analyses of MDM2 protein levels were performed by densitometry scanning for each individual MDM2 protein band and the corresponding actin protein band. The ratio of MDM2 over actin in the control sample (lane 1) was arbitrarily set as 1.0. (B) U2-OS cells were transiently transfected with siRNA specific against eIF4E (si4E) or a scrambled control (siScr). Cell lysates were subjected to western blotting, as shown. (C) U2-OS cells transfected with si4E or siScr were serum-starved for 24 hours prior to treatment with or without 5 ng/mL IGF-1 for six hours. Cell lysates were subjected to western blotting, as shown. (D) U2-OS cells were transiently transfected with siRNA specific against S6K (siS6K) or a scrambled siRNA (siScr). Cell lysates were subjected to western blotting, as shown. (E) U2-OS cells transfected with siS6K or siScr were serum-starved for 24 hours prior to treatment with or without 5 ng/mL IGF-1 for six hours. Cell lysates were subjected to western blotting, as shown.