AMINO-ACID AVAILABILITY REGULATES P70 S6 KINASE AND MULTIPLE TRANSLATION FACTORS

Incubation of Chinese hamster ovary cells without amino acids for up t o 60 min caused a rapid marked decrease in p70 S6 kinase activity and increased binding of initiation factor eIF4E to its inhibitory regulat or protein 4E-BP1. This was associated with dephosphorylation of 4E-BP 1 and eIF4E and dissociation of eIF4E from eIF4G. All these effects we re rapidly reversed by resupplying a mixture of amino acids and this w as blocked by rapamycin and by inhibitors of phosphatidylinositol 3-ki nase, implying a role for phosphatidylinositol 3-kinase in the signall ing pathway linking amino acids with the control of p70 S6 kinase acti vity and the phosphorylation of these translation factors. Amino acid withdrawal also led to changes in the phosphorylation of other transla tion factors; phosphorylation of eIF4E decreased whereas elongation fa ctor eEF2 became more heavily phosphorylated, each of these changes be ing associated with decreased activity of the factor in question. Earl ier studies have suggested that protein kinase B (PKB) may act upstrea m of p70 S6 kinase. However, amino acids did not affect the activity o f PKB, indicating that amino acids activate p70 S6 kinase through a pa thway independent of this enzyme. Studies with individual amino acids suggested that the effects on p70 S6 kinase activity and translation-f actor phosphorylation were independent of cell swelling. The data show that amino acid supply regulates multiple translation factors in mamm alian cells.