Interplay between insulin and nutrients in the regulation of translation factors

Protein synthesis in mammalian cells is regulated through alterations in the states of phosphorylation of eukaryotic initiation factors and elongation factors (eIFs and eEFs respectively) and of other regulatory proteins. This modulates their activities or their abilities to interact with one another. Insulin activates several of these proteins including the following: the guanine-nucleotide exchange factor eIF2B; the eIF4F complex, which (through eIF4E) interacts with the cap of the mRNA; p70 S6 kinase; and elongation factor eEF2, which mediates the translocation step of elongation. Control of the last three of these is linked to mTOR (mammalian target of rapamycin). In Chinese hamster ovary cells, regulation of all these proteins by insulin is modulated by the presence of amino acids and/or glucose in the medium. For example, p70 S6 kinase activity declines in the absence of amino acids and cannot be stimulated by insulin under this condition. The readdition of amino acids, especially leucine, restores activity and sensitivity to insulin. With eIF2B and eEF2, both amino acids and glucose must be provided for insulin to regulate their activities. In contrast, insulin-stimulation of the formation of eIF4F complexes requires glucose but not amino acids. Glucose metabolism is required for this permissive effect. Our recent studies have also identified the mechanism by which mTOR signalling regulates the phosphorylation of eEF2. eEF2 kinase is phosphorylated by p70 S6 kinase at Ser-366; this results in the inactivation of eEF2 kinase, especially at low (micromolar) Ca concentrations.