Changes in the phosphorylation state of the inhibitory guanine-nucleotide-binding protein Gi-2 in hepatocytes from lean (Fa/Fa) and obese (fa/fa) Zucker rats

Eur J Biochem. 1990 Sep 11;192(2):537-42. doi: 10.1111/j.1432-1033.1990.tb19258.x.


Treatment of intact, 32Pi-labelled hepatocytes from lean Zucker rats with a range of agents including 12-O-tetradecanoyl-phorbol 13-acetate (TPA), vasopressin, and angiotensin II elicited substantial increases in the phosphorylation of the alpha-subunit of the inhibitory G protein of adenylate cyclase (alpha Gi-2). These agonist-induced phosphorylations of alpha Gi-2 were associated with loss of Gi function as assessed by the ability of low concentrations of guanylyl 5'-[beta,gamma imido]triphosphate (p[NH]ppG) to inhibit forskolin-stimulated adenylate cyclase activity. Hepatocytes from obese Zucker rats displayed a resistance to both agonist-induced phosphorylation of alpha Gi-2 and to p[NH]ppG-mediated inhibition of adenylate cyclase. The basal level of alpha Gi-2 phosphorylation in hepatocytes from obese Zucker rats was considerably greater at 1.06 +/- 0.09 mol phosphate/mol alpha Gi-2 than in hepatocytes from lean animals which gave 0.54 +/- 0.09 mol phosphate/mol alpha Gi-2. Incubation with TPA (10 ng/ml, 15 min) approximately doubled the level of phosphorylation of alpha Gi-2 in the hepatocytes from lean animals but had little effect on the phosphorylation of alpha Gi-2 in hepatocytes from obese animals. Incubation of hepatocytes from lean animals with ligands which lead to the phosphorylation of alpha Gi-2 abolished the ability of low concentrations of p[NH]ppG to inhibit adenylate cyclase expressed in isolated membranes. Treatment of hepatocyte plasma membranes from lean but not obese Zucker rats with pure protein kinase C led to the phosphorylation of alpha Gi-2. The resistance to protein-kinase-C-mediated phosphorylation in hepatocyte membranes from obese animals could be overcome by treatment of the membranes with alkaline phosphatase. These results indicate that the defect in guanine-nucleotide-mediated 'Gi function' seen in obese Zucker rats may be due to an inactivating phosphorylation of alpha Gi-2.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Adenylyl Cyclases / metabolism
  • Angiotensin II / pharmacology
  • Animals
  • Cell Membrane / metabolism
  • Cells, Cultured
  • GTP-Binding Proteins / metabolism*
  • Kinetics
  • Liver / metabolism*
  • Male
  • Phosphates / metabolism
  • Phosphorylation
  • Rats
  • Rats, Zucker / metabolism*
  • Reference Values
  • Tetradecanoylphorbol Acetate / pharmacology
  • Vasopressins / pharmacology


  • Phosphates
  • Vasopressins
  • Angiotensin II
  • GTP-Binding Proteins
  • Adenylyl Cyclases
  • Tetradecanoylphorbol Acetate