α1A Adrenergic Receptor Induces Eukaryotic Initiation Factor 4E-binding Protein 1 Phosphorylation via a Ca2+-dependent Pathway Independent of Phosphatidylinositol 3-kinase/Akt*

Phosphorylation of the translation repressor eukaryotic initiation factor 4E-binding protein 1 (4E-BP1) is thought to be partly responsible for increased protein synthesis induced by growth factors. This study investigated the effect of a Gq-coupled receptor on protein synthesis and the phosphorylation state and function of 4E-BP1 in Rat-1 fibroblasts expressing the human α1A adrenergic receptor. Treatment of cells with phenylephrine (PE), a specific α1adrenergic receptor agonist, increased protein synthesis and induced the phosphorylation of 4E-BP1 and its release from translation initiation factor 4E. Although the PE-induced phosphorylation of 4E-BP1 was blocked by the phosphatidylinositol 3-kinase inhibitor LY294002, neither phosphatidylinositol 3-kinase nor Akt, its downstream effector, is activated in cells treated with PE (Ballou, L. M., Cross, M. E., Huang, S., McReynolds, E. M., Zhang, B. X., and Lin, R. Z., J. Biol. Chem. 275, 4803–4809). The effect of PE on 4E-BP1 phosphorylation was also abolished in cells depleted of intracellular Ca2+ and in cells pretreated with calmodulin antagonists. By contrast, phosphorylation of 4E-BP1 still occurred in cells in which the Ca2+- and diacylglycerol-dependent isoforms of protein kinase C were down-regulated by prolonged exposure to a phorbol ester. We conclude that activation of the α1A adrenergic receptor in Rat-1 fibroblasts leads to phosphorylation of 4E-BP1 via a pathway that is Ca2+- and calmodulin-dependent. Phosphatidylinositol 3-kinase, Akt, and phorbol ester-sensitive protein kinase C isoforms do not appear to be required in this signaling pathway.