INSULIN BUT NOT PROGESTERONE PROMOTES THE BIOSYNTHESIS OF GLYCOGEN INXENOPUS-LAEVIS OOCYTES - IMPLICATIONS ON THE CONTROL OF GLYCOGEN-SYNTHASE BY PHOSPHORYLATION, DEPHOSPHORYLATION

Insulin, the well-known hypoglycemic hormone, mimics progesterone in p romoting the resumption of meiosis within the oocyte of Xenopus laevis . Both hormones exert their action through the inhibition of protein k inases arid the activation of protein phosphatases. Because glycogen s ynthase is an enzyme regulated by a kinases/phosphatases cascade, we i nvestigated the effect of insulin and progesterone on the regulation o f glycogen synthesis and glycogen synthase throughout the oogenesis of Xenopus laevis oocytes. In this framework the maximal activity of syn thase ''a'' is concomitant with the vitellogenic period characterized by a drastic increase in the amount df glycogen. Oocyte glycogen synth ase is inhibited by cAMP-dependent phosphorylation and stimulated by 2 0 mM Mg2+. The magnesium effect is inhibited by mu molar concentration s of okadaic acid and suggests that oocyte glycogen synthase is activa ted by dephosphorylation achieved by protein phosphatase-1. The okadai c acid effect is itself thwarted by the specific inhibitor of protein kinase A, confirming the role of this kinase in the regulation of glyc ogen synthase. Finally, working on intact ripe oocytes, we showed that insulin but not progesterone increases glycogen synthesis and glycoge n synthase ''a'' activity and lowers the rates of phosphorylation, esp ecially in the glycogen-bound proteins. (C) 1994 Wiley-Liss, Inc.