Regulation of Akt mRNA and protein levels by glycogen synthase kinase-3β in adrenal chromaffin cells:: Effects of LiCl and SB216763

In cultured bovine adrenal chromaffin cells, where Akt1 is the predominant isoform over Akt2 and Akt3, chronic (: 12 h) treatment with 1-20 mM LiCl, an inhibitor of glycogen synthase kinase-3, decreased Akt1 level by similar to 52% (EC50=3.7 mM; t(1/2)=12 h); it was associated with LiCl-induced increased levels of Ser(9)-phosphorylated glycogen synthase kinase-3 beta (similar to 37%) and beta-catenin (similar to 59%), two hallmarks of glycogen synthase kinase-3 beta inhibition. The same LiCl treatment did not change phosphoinositide 3-kinase, phosphoinositide-dependent kinase 1, and extracellular signal-regulated kinase-1/2 levels. Treatment with SB216763 [3-(2,4-dichlorophenyl)-4-(1-methyl-1H-indol-3-yl)-1H-pyrrole-2,5-dione], a selective inhibitor of glycogen synthase kinase-3, lowered Akt1 level by similar to 67% (EC50=2 mu M; t(1/2)=12 h), when SB216763 caused concentration- and time-dependent increase of beta-catenin level by similar to 76%. LiCl- or SB216763-induced Akt1 decrease, as well as increases of Ser(9)-phosphorylated glycogen synthase kinase-3 beta and beta-catenin were restored to the control levels of nontreated cells after the washout of LiCl (20 mM for 24 h)- or SB216763 (30 mu M for 24 h)-treated cells. LiCl-induced Akt1 reduction was not prevented by beta-lactone, lactacystin (two inhibitors of proteasome), calpastatin (an inhibitor of calpain), or leupeptin (an inhibitor of lysosome). LiCl decreased Akt1 mRNA level by 20% at 6 h, with no effect on Akt1 mRNA stability. These results suggest that glycogen synthase kinase-3 beta inhibition caused down-regulation of Akt1 mRNA and Akt1 protein levels; conversely, constitutive activity of glycogen synthase kinase-3 beta maintains steady-state level of Akt1 in quiescent adrenal chromaffin cells. (C) 2008 Elsevier B.V. All rights reserved.