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4E-BP1, a repressor of mRNA translation, is phosphorylated and inactivated by the Akt(PKB) signaling pathway
A C Gingras1, S G Kennedy, M A O'Leary
1Department of Biochemistry, McGill University Montreal, Quebec, Canada H3G 1Y6.
Abstract:
Growth factors and hormones activate protein translation by phosphorylation and inactivation of the translational repressors, the eIF4E-binding proteins (4E-BPs), through a wortmannin- and rapamycin-sensitive signaling pathway. The mechanism by which signals emanating from extracellular signals lead to phosphorylation of 4E-BPs is not well understood. Here we demonstrate that the activity of the serine/threonine kinase Akt/PKB is required in a signaling cascade that leads to phosphorylation and inactivation of 4E-BP1. PI 3-kinase elicits the phosphorylation of 4E-BP1 in a wortmannin- and rapamycin-sensitive manner, whereas activated Akt-mediated phosphorylation of 4E-BP1 is wortmannin resistant but rapamycin sensitive. A dominant negative mutant of Akt blocks insulin-mediated phosphorylation of 4E-BP1, indicating that Akt is required for the in vivo phosphorylation of 4E-BP1. Importantly, an activated Akt induces phosphorylation of 4E-BP1 on the same sites that are phosphorylated upon serum stimulation. Similar to what has been observed with serum and growth factors, phosphorylation of 4E-BP1 by Akt inhibits the interaction between 4E-BP1 and eIF-4E. Furthermore, phosphorylation of 4E-BP1 by Akt requires the activity of FRAP/mTOR. FRAP/mTOR may lie downstream of Akt in this signaling cascade. These results demonstrate that the PI 3-kinase-Akt signaling pathway, in concert with FRAP/mTOR, induces the phosphorylation of 4E-BP1.
Insights
Growth factors activate protein translation by inactivating 4E-BPs. The study shows the PI 3-kinase-Akt pathway, with FRAP/mTOR, mediates this phosphorylation, linking extracellular signals to protein synthesis regulation.
Area of Science:
- Molecular Biology
- Cell Signaling
- Biochemistry
Background:
- Growth factors and hormones regulate protein translation via phosphorylation of eIF4E-binding proteins (4E-BPs).
- The precise signaling mechanisms linking extracellular signals to 4E-BP phosphorylation remain unclear.
Purpose of the Study:
- To elucidate the signaling pathway responsible for 4E-BP phosphorylation and inactivation.
- To determine the role of the Akt/PKB kinase in this process.
Main Methods:
- Utilized wortmannin and rapamycin sensitivity assays.
- Employed dominant-negative Akt mutants.
- Investigated the effect of activated Akt on 4E-BP1 phosphorylation sites.
- Assessed the interaction between 4E-BP1 and eIF-4E.
- Examined the requirement for FRAP/mTOR activity.
Main Results:
- The PI 3-kinase-Akt pathway is essential for 4E-BP1 phosphorylation.
- Activated Akt induces 4E-BP1 phosphorylation at sites identical to serum stimulation.
- Akt-mediated phosphorylation of 4E-BP1 inhibits its interaction with eIF-4E.
- FRAP/mTOR activity is required for Akt-mediated 4E-BP1 phosphorylation, suggesting it acts downstream of Akt.
Conclusions:
- The PI 3-kinase-Akt signaling pathway, in conjunction with FRAP/mTOR, mediates the phosphorylation and inactivation of 4E-BP1.
- This pathway connects extracellular signals to the regulation of protein translation initiation.