FURTHER EVIDENCE THAT THE INHIBITION OF GLYCOGEN-SYNTHASE KINASE-3-BETA BY IGF-1 IS MEDIATED BY PDK1 PKB-INDUCED PHOSPHORYLATION OF SER-9 AND NOT BY DEPHOSPHORYLATION OF TYR-216/

293 cells were transfected with mild-type GSK3 beta (WT-GSK3 beta) or a mutant in which the PKB phosphorylation site (Ser-9) mas altered to Ala (A9-GSK3 beta), Upon stimulation with IGF-1 or insulin, WT-GSK3 be ta was inhibited 75% or 60%, respectively, whereas the activity of the A9-GSK3 beta mutant was unaffected, Incubation of WT-GSK3 beta with P P2A(1) (a Ser/Thr-specific phosphatase) completely reversed the IGF-1- or insulin-induced inhibition. IGF-1 stimulation did not induce any t yrosine dephosphorylation of WT-GSK3 beta or A9-GSK3 beta, Coexpressio n of WT-GSK3 beta in 293 cells with either PKB alpha (also known as AK T) or PDK1 (the 'upstream' activator of PKB) mimicked the IGF-1- or in sulin-induced phosphorylation of Ser-9 and inactivation of GSK3 beta. (C) 1997 Federation of European Biochemical Societies.