Dual Regulation of Glycogen Synthase Kinase-3β by the α _1A-Adrenergic Receptor

Catecholamines, acting through adrenergic receptors, play an important role in modulating the effects of insulin on glucose metabolism. Insulin activation of glycogen synthesis is mediated in part by the inhibitory phosphorylation of glycogen synthase kinase-3 (GSK-3). In this study, catecholamine regulation of GSK-3β was investigated in Rat-1 fibroblasts stably expressing the α_1A^- adrenergic receptor. Treatment of these cells with either insulin or phenylephrine (PE), an α₁-adrenergic receptor agonist, induced Ser-9 phosphorylation of GSK-3β and inhibited GSK-3β activity. Insulin-induced GSK-3β phosphorylation is mediated by the phosphatidylinositol 3-kinase/Akt signaling pathway. PE treatment does not activate phosphatidylinositol 3-kinase or Akt (Ballou, L. M., Cross, M. E., Huang, S., McReynolds, E. M., Zhang, B. X., and Lin, R. Z. (2000) J. Biol. Chem. 275, 4803-4809), but instead inhibits insulin-induced Akt activation and GSK-3β phosphorylation. Experiments using protein kinase C (PKC) inhibitors suggest that phorbol ester-sensitive novel PKC and Gö 6983-sensitive atypical PKC isoforms are involved in the PE-induced phosphorylation of GSK-3β. Indeed, PE treatment of Rat-1 cells increased the activity of atypical PKCζ, and expression of PKCζ in COS-7 cells stimulated GSK-3β Ser-9 phosphorylation. In addition, PE-induced GSK-3β phosphorylation was reduced in Rat-1 cells treated with a cell-permeable PKCζ pseudosubstrate peptide inhibitor. These results suggest that the α_1A-adrenergic receptor regulates GSK-3β through two signaling pathways. One pathway inhibits insulin-induced GSK-3β phosphorylation by blocking insulin activation of Akt. The second pathway stimulates Ser-9 phosphorylation of GSK-3β probably via PKC.