The role of glucose metabolites in the activation and translocation of glycogen synthase by insulin in 3T3-L1 adipocytes

The role of increased glucose transport in the hormonal regulation of glyco gen synthase by insulin was investigated in 3T3-L1 adipocytes. Insulin trea tment stimulated glycogen synthase activity 4-5-fold in these cells. Cytoso lic glycogen synthase levels decreased by 75% in response to insulin, where as, conversely, the glycogenolytic agent isoproterenol increased cytosolic enzyme levels by 200%. Removal of extracellular glucose reduced glycogen sy nthase activation by 40% and completely blocked enzymatic translocation, Ad dition of 5 mM 2-deoxyglucose did not restore glycogen synthase translocati on but did augment dephosphorylation of the protein by insulin. The translo cation event could be reconstituted in vitro only by the addition of UDP-gl ucose to basal cell lysates. Amylase pretreatment of the extracts suppresse d glycogen synthase translocation, indicating that the enzyme was binding t o glycogen. Incubation of 3T3-L1 adipocytes with 10 mM glucosamine induced a state of insulin resistance, blocked the translocation of glycogen syntha se, and inhibited insulin-stimulated glycogen synthesis by 50%. Surprisingl y, glycogen synthase activation by insulin was enhanced 4-fold, in part due to allosteric activation by a glucosamine metabolite. In vitro, glucosamin e 6-phosphate and glucose 6-phosphate stimulated glycogen synthase activity with similar concentration curves. These results indicate that glucose met abolites have an impact on the regulation of glycogen synthase activation a nd localization by insulin.