Figure 5.
Reciprocal interactions between ERK and PI3K regulate the mTOR pathway at the level of PDK1. Immunoblots were performed on CA1 regions from slices frozen 5 min after HFS. A, The HFS-induced phosphorylation of PDK1 requires ERK activity. HFS induced the phosphorylation of PDK1 at S241, and this effect was prevented by either 50 μm PD98059 (PD) or 20 μm U0126. B, Phosphorylation of Akt at T308 was increased after HFS, and this effect was blocked by preincubation with 50 μm PD98059 (PD). C, The ERK substrate RSK became hyperphosphorylated after HFS, and this effect was sensitive to the MEK inhibitors U0126 (20 μm) and PD98059 (PD; 50 μm). All asterisks indicate p < 0.05 versus controls (Con); group values of n ≥ 6. D, PI3K activity mediates a component of MEK/ERK phosphorylation. Incubation with the PI3K inhibitor LY294002 (LY; 20 μm) reduced both basal and HFS-induced phosphorylation of MEK1/2 at S218/S222 of MEK1 (black bars) and ERK2 at T202/Y204 (gray bars). Data are from excised CA1 regions, and stimulated slices were frozen 5 min after HFS. Sample immunoblots are shown below. Single and double asterisks indicate p < 0.05 and p < 0.01, respectively; all values of n ≥ 4. E, Schematic of the interactions between the ERK and PI3K–mTOR pathways in regulating TOP mRNA translation, based on the experiments summarized in Figures 1 –5. Components in black are coregulated by the PI3K and ERK pathways (shown in blue and green, respectively). The arrows do not necessarily represent direct interactions [e.g., the connection between mTOR and p70S6K might involve the inhibition of a protein phosphatase (Peterson et al., 1999)], and some known interactions have been excluded for the sake of clarity (e.g., mTOR facilitates cap-dependent translation by phosphorylating 4EBP).