Hyperglycemia and inhibition of glycogen synthase in streptozotocin-treated mice: role of O-linked N-acetylglucosamine.

Glycogen synthase is post-translationally modified by both phosphate and O-linked N-acetylglucosamine (O-GlcNAc). In 3T3-L1 adipocytes exposed to high concentrations of glucose, O-GlcNAc contributes to insulin resistance of glycogen synthase. We sought to determine whether O-GlcNAc also regulates glycogen synthase in vivo. Glycogen synthase activity in fat pad extracts was inhibited in streptozotocin (STZ)-treated diabetic mice. The half-maximal activation concentration for glucose 6-phosphate (A(0.5)) was increased to 830 +/- 120 microm compared with 240 +/- 20 microm in control mice (C, p < 0.01), while the basal glycogen synthase activity (%I-form) was decreased to 2.4 +/- 1.4% compared with 10.1 +/- 1.8% in controls (p < 0.01). Glycogen synthase activity remained inhibited after compensatory insulin treatment. After insulin treatment kinetic parameters of glycogen synthase were more closely correlated with blood glucose (A(0.5), r(2) = 0.70; %I-form, r(2) = 0.59) than insulin levels (A(0.5), r(2) = 0.04; %I-form, r(2) = 0.09). Hyperglycemia also resulted in an increase in the level of O-GlcNAc on glycogen synthase (16.1 +/- 1.8 compared with 7.0 +/- 0.9 arbitrary intensity units for controls, p < 0.01), even though the level of phosphorylation was identical in diabetic and control mice either with (STZ: 2.9 +/- 1.0 and C: 3.2 +/- 0.8) or without (STZ: 12.2 +/- 2.8 and C: 13.8 +/- 3.0 arbitrary intensity units) insulin treatment. In all mice the percent activation of glycogen synthase that could be achieved in vitro by recombinant protein phosphatase 1 (230 +/- 30%) was significantly greater in the presence of beta-d-N-acetylglucosaminidase (410 +/- 60%, p < 0.01). This synergistic stimulation of glycogen synthase due to codigestion by protein phosphatase 1 and beta-d-N-acetylglucosaminidase was more pronounced in STZ-diabetic mice (310 +/- 70%) compared with control mice (100 +/- 10%, p < 0.05). The findings demonstrate that O-GlcNAc has a role in the regulation of glycogen synthase both in normoglycemia and diabetes.