Diet and diabetic state modify glycogen synthase activity and expression in rat hepatocytes

Glycogen synthase (GS), a key regulatory enzyme in glycogen synthesis, is c ontrolled by multisite phosphorylation and allosteric regulation and is act ivated by insulin. This study investigated changes in GS activity and expre ssion in hepatocytes isolated from rats under altered nutritional and diabe tic conditions. Experiments were carried out in healthy rats fed a chow die t, rats on high simple sugar (60% of energy from fructose and sucrose) or h igh fat (46% of energy from fat) diet, and in rats with streptozotocin indu ced diabetes, In the presence of insulin, activated GS activity (GS(1) form ) was increased by 89% in hepatocytes isolated from healthy rats. The stimu latory effect of insulin on GS activity and expression was blunted by cyclo heximide and actinomycin treatment. In rats fed a high simple sugar or high fat diet, insulin stimulation of GS(1) in isolated hepatocytes was impaire d and GS expression was significantly lower in rats fed the high fat diet i n comparison to controls. GLUT-2 protein expression was significantly lower ed by both the high fat and high simple sugar diets. In hepatocytes isolate d from diabetic rats, total GS activity (GS(T)) was lower than in hepatocyt es from healthy animals. Insulin added to the incubation medium did not sti mulate GS activity, demonstrating impaired sensitivity to insulin in diabet ic rats. However, insulin administration significantly increased GS express ion indicating that a defect in synthase phosphorylation may be responsible for impaired GS activity in the diabetic state. The results presented in t his study further confirm that GS activity is affected by both dietary and hormonal factors which can be measured in a rat hepatocyte model. (C) 2001 Elsevier Science Inc. All rights reserved.