Glycogen synthesis in mouse hepatocytes is negatively regulated by GRK2

Abstract

The primary biological effect of insulin is to maintain glucose homeostasis. The biological effects of insulin are mediated by binding to insulin receptor that belongs to the tyrosine kinase receptor family. There is increasing evidence that G-protein coupled receptor kinase 2 (GRK2), known to desensitize agonist occupied G-protein coupled receptors (GPCRs), regulates insulin receptor signaling. We have earlier shown that GRK2 is translocated to the plasma membrane in response to insulin treatment suggesting that GRK2 is activated by insulin. The present study was designed to test the hypothesis that the activated GRK2 negatively regulates the insulin receptor signaling and glycogen synthesis. To test this hypothesis, we downregulated GRK2 in hepatocytes using siRNA technique. The insulin induced glycogen synthesis, which is the end point of insulin mediated metabolic pathway in hepatocytes was determined using radio labeled D-[U-14C] glucose incorporation into glycogen, in control and GRK2 deficient hepatocytes. We observed an increase in the glycogen synthesis in response to insulin treatment in GRK2 deficient hepatocytes as compared to the control. We also determined the phosphorylation of the GSK3 alpha and GSK3 beta, a serine/threonine kinase known to inhibit glycogen synthase. We observed a decrease in the activity of both GSK3 alpha and GSK3 beta in GRK2 deficient hepatocytes suggesting that the absence of GRK enhances insulin receptor signaling. Thus our results suggest that GRK2 has an inhibitory effect on insulin receptor signaling and glycogen synthesis in mouse hepatocytes.