Exp Clin Endocrinol Diabetes 2005; 113 - V9_74
DOI: 10.1055/s-2005-862858

Impaired activation of proteinkinase B by insulin in primary cultured hepatocytes from NZO-mice, a model system for the metabolic syndrome

V Schippmann 1, R Kluge 2, WA Scherbaum 3, SR Bornstein 4, HG Joost 2, A Barthel 5
  • 1RWTH Aachen, Institut für Pharmakologie und Toxikologie, Aachen, Germany
  • 2Deutsches Institut für Ernährungsforschung, Pharmakologie, Potsdam-Rehbrücke, Germany
  • 3Heinrich-Heine-Universität, Klinik für Endokrinologie, Diabetiologie und Rheumatologie, und Deutsches Diabetes Forschungs-Institut, Düsseldorf, Germany
  • 4Universitätsklinikum Dresden, Medizinische Klinik III, Dresden, Germany
  • 5Heinrich-Heine-Universität, Klinik für Endokrinologie, Diabetologie und Rheumatologie, Düsseldorf, Germany

In type-2-diabetes, increased glucose production by the liver due to insulin resistance is a central pathophysiological event and fasting hyperglycemia is the typical clinical correlate of this phenomenon. Insulin is known to inhibit hepatic glucose production by decreasing the expression of the two key regulatory gluconeogenic enzymes, phosphoenolpyruvate carboxykinase (PEPCK) and the glucose-6-phosphatase catalytic subunit (G6Pase). This effect of insulin is mainly mediated via activation of phosphatidylinositol (PI) 3-kinase and proteinkinase B (PKB). The pharmacological intervention in signaling events which regulate hepatic glucose production is generally regarded as a potential strategy for the treatment of the metabolic aberrations associated with this disease. However, the identification of the underlying signaling defects is a major prerequisite for a specific approach.

In order to analyze insulin resistance of the liver and the underlying defect, we characterized the signal transduction of insulin in isolated hepatocytes from NZO/SJL-mice. The NZO-strain represents a polygenic mouse model with increased incidence for the development of obesity and type-2-diabetes with metabolic abberations closely resembling the metabolic syndrome in humans.

In isolated hepatocytes from NZO-mice, activation of PKB by insulin was clearly decreased (EC50 5×10–8M vs. 5×10–9M insulin, max. induction 2,5-fold vs. 5-fold over basal) as compared to metabolically normal SJL control mice. Similarly, G6Pase enzymatic activity was increased in hepatocytes from NZO mice. In contrast, the levels of insulin dependent tyrosine-phosphorylation of the insulin receptor ß-subunit were not significantly different between hepatocytes from both groups.

In summary, our data suggest that in this model (1) an endogenous defect resulting in insulin resistance of the liver is localized in the hepatocyte itself. Furthermore, these data suggest that (2) the defect in insulin signaling is localized on the level of PKB or upstream in the signaling cascade.