Glycogen synthase is a key regulatory enzyme in hepatic glycogen metabolism, perhaps the most important pathway in regulation of glucose homeostasis. Therefore, it is extremely important that the hepatic regulatory process be understood. While it is known that different isozyme forms of glycogen synthase and phosphorylase exist in muscle and liver, our knowledge of the mechanisms through which the liver isozymes are regulated is limited. The long term goal of this research is the elucidation of the mechanisms of hormones and other effectors in the regulation of hepatic glycogen metabolism and of the defects in glycogen metabolism associated with the diabetic state or severe insulin deficiency. The hypotheses to be tested are that insulin, cortisol and triiodothyronine facilitate hepatic glycogen synthesis chronically by maintenance of glycogenic enzymes in a responsive state, and that hepatic glycogen synthase is acutely regulated by insulin and glucose through changes in the phosphorylation state of specific serine residues. The premise is that a complete understanding of hepatic defects in glycogen synthesis observed in the diabetic state will lead to a better understanding of the mechanism of insulin action.
Specific aims are to 1) Study diabetic defects and the acute and chronic regulation of glycogen synthetic enzymes using the primary adult rat hepatocyte culture system. 2) Use the antibodies which we have developed to study the acute effects of insulin and glucose on the phosphorylation state and chronic control of synthase. 3) Determine specific amino acid sequences around phosphorylation sites in rat liver synthase and site specific phosphorylation/dephosphorylation as effected by glucose and insulin.4) Determine which liver isozyme of phosphoprotein phosphatase is the physiologically meaningful form of glycogen synthase phosphatase. 5) Determine if glycogen synthase phosphatase is physically missing in the diabetic state or is in some way altered.and 6) Determine if synthase phosphatase is regulated acutely, and if so, how. Results of these studies should clarify the roles of hormonal interactions in diabetes- related abnormalities of hepatic glycogen metabolism, and should clarify the mechanistic roles of insulin and glucose in acute and chronic regulation of hepatic glycogen synthesis. These studies should lead to an eventual understanding of the mechanism of hormone action and improved clinical prognosis for diabetics.
