Dual regulation of glycogen synthase kinase-3β by the α1A-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-3 beta was investigated in Rat-1 fibroblasts stably expressing the alpha (1A)-adrenergic receptor. Treatment of these cells with either insulin or phenylephrine (PE), an al-adrenergic receptor agonist, induced Ser-9 phosphorylation of GSK-3 beta and inhibited GSK-3 beta activity. Insulin-induced GSK-3 beta 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-3 beta phosphorylation. Experiments using protein kinase C (PKC) inhibitors suggest that phorbol ester-sensitive novel PKC and Go 6983-sensitive atypical PKC isoforms are involved in the PE-induced phosphorylation of GSK-3 beta. Indeed, PE treatment of Rat-1 cells increased the activity of atypical PKC zeta, and expression of PKC zeta in COS-7 cells stimulated GSK-3 beta Ser-9 phosphorylation. In addition, PE-induced GSK-3 beta phosphorylation was reduced in Rat-1 cells treated with a cell-permeable PKC zeta pseudosubstrate peptide inhibitor. These results suggest that the alpha (1A)-adrenergic receptor regulates GSK-3 beta through two signaling pathways. One pathway inhibits insulin-induced GSK-3 beta phosphorylation by blocking insulin activation of Akt. The second pathway stimulates Ser-9 phosphorylation of GSK3 beta, probably via PKC.