alpha(1A) 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 4 E-binding protein 1 (4E-BP1) is thought to be partly responsible for increa sed protein synthesis induced by growth factors. This study investigated th e effect of a G(q)-coupled receptor on protein synthesis and the phosphoryl ation state and function of 4E-BP1 in Rat-1 fibroblasts expressing the huma n alpha(1A) adrenergic receptor. Treatment of cells with phenylephrine (PE) , a specific alpha(1) adrenergic receptor agonist, increased protein synthe sis and induced the phosphorylation of 4E-BP1 and its release from translat ion initiation factor 4E. Although the PE-induced phosphorylation of 4E-BP1 was blocked by the phosphatidylinositol 3-kinase inhibitor LY294002, neith er phosphatidylinositol a-kinase nor Akt, its downstream effector, is activ ated in cells treated with PE (Ballou, L. Rf,, Cross, M. E,, Huang, S., McR eynolds, E, M., Zhang, B, L, and Lin, R. Z,, J. Biol. Chem. 275, 4803-4809) . The effect of PE on 4E-BP1 phosphorylation was also abolished in cells de pleted of intracellular Ca2+ and in cells pretreated with calmodulin antago nists. By contrast, phosphorylation of 4E-BP1 still occurred in cells in wh ich 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 alpha(1A) adrenergic receptor in Rat-1 fibroblasts leads to phosphorylation of 4E-BP1 via a pathway that is Ca2+- and calmodulin-dep endent. Phosphatidylinositol 3-kinase, Akt, and phorbol ester-sensitive pro tein kinase C isoforms do not appear to be required in this signaling pathw ay.