Nutrients differentially regulate multiple translation factors and their control by insulin

Abstract

Eukaryotic initiation factor eIF2B and eukaryotic elongation factor eEF2 each mediate regulatory steps important for the overall regulation of mRNA translation in mammalian cells and are activated by insulin. Here, we demonstrate that their activation by insulin requires the presence, in the medium in which the cells are maintained, of both amino acids and glucose: insulin only induced activation of eIF2B and the dephosphorylation of eEF2 when cells were exposed to both types of nutrient. Other translational regulators, e.g. the 70 kDa ribosomal protein S6 kinase (p70 S6 kinase) and the eIF4E binding protein 1, 4E-BP1, are also regulated by insulin but their control does not require glucose, only amino acids. The effects of nutrients on the activation of eIF2B do not reflect changes in the phosphorylation of eIF2 (and, by inference, operation of a kinase analogous to yeast Gcn2p), or a requirement for nutrients for inactivation of glycogen synthase kinase-3 or dephosphorylation of eIF2B. Nutrients did not affect the ability of insulin to activate protein kinase B. These data show that activation by insulin of p70 S6 kinase, which modulates the translation of specific mRNAs, depends on the availability of amino acids whereas regulation of factors involved in overall activation of translation (eIF2B, eEF2) requires both amino acids and glucose. These results add substantially to the emerging evidence that nutrients themselves modulate functions of mammalian cells and indicate that (i) nutrients modulate the activation of eIF2B and eEF2 through as-yet unidentified mechanisms and (ii) regulation of p70 S6 kinase and 4E-BP1 by insulin requires other inputs in addition to protein kinase B.