Insulin regulation of protein translation repressor 4E-BP1, an eIF4E-binding protein, in renal epithelial cells

Background. Augmented protein translation by insulin involves activation of eukaryotic initiation factor 4E (eIF4E) that follows release of eIF4E from a heterodimeric complex by phosphorylation of its inhibitory binding prote in, 4E-BP1. We examined insulin regulation of 4E-BP1 phosphorylation in mur ine proximal tubular epithelial cells. Methods and Results. Insulin (1 nmol/L) increased de novo protein synthesis by 58 +/- 11% (P < 0.001). Insulin also augmented 4E-BP1 phosphorylation a nd phosphatidylinositol S-kinase (PI3-kinase) activity in antiphosphotyrosi ne immunoprecipitates. This could be prevented by PI 3-kinase inhibitors, W ortmannin, and LY294002. Insulin also activated Akt that lies downstream of PI 3-kinase. Rapamycin abrogated 4E-BP1 phosphorylation in response to ins ulin, suggesting involvement of mammalian target of rapamycin (mTOR), a kin ase downstream of Akt. Insulin-stimulated phosphorylation of 4E-BP1 was als o inhibited by PD098059, implying involvement of Erk-1/-2 mitogen-activated protein (MAP) kinase. An increase in Erk-1/-2 type MAP kinase activity by insulin was directly confirmed in an immunokinase assay and was found to be PIS-kinase dependent. Conclusions. In proximal tubular epithelial cells, insulin augments 4E-BP1 phosphorylation, which is PI 3-kinase and mTOR dependent. The requirement f or Erk-1/-2 MAP kinase activation for 4E-BP1 phosphorylation by insulin sug gests a cross-talk between PI 3-kinase and Erk-1/-2-type MAP kinase pathway s.