Alterations in the tyrosine kinase activity of the insulin receptor produced by in vitro hyperinsulinemia.

The tyrosine kinase activity of the insulin receptor was examined in fat cells made insulin resistant by hyperinsulinemia. Adipocytes previously treated for 16 h in vitro with 0.1 microM insulin were markedly insensitive to insulin as shown by the requirement of 2.3-fold higher concentration of insulin to stimulate half-maximal activation of glucose metabolism (glucose oxidation and glucose conversion to lipids). A similar rightward shift in the insulin dose-response curve was also evident for an insulin action not dependent on glucose metabolism, i.e. the inhibition of hormone-stimulated lipolysis. The almost 60% loss of insulin sensitivity occurred in conjunction with a 37% loss in insulin-binding activity, an alteration caused by a reduction in the number of insulin receptors. Studies of tyrosine kinase activity demonstrated a preferential alteration in this insulin receptor property, in addition to the receptor loss. The insulin-stimulated Vmax for the phosphorylation of the artificial substrate, Glu80-Tyr20, was significantly reduced by almost 40%, when the activity was expressed per insulin binding and measured in a receptor preparation purified by wheat germ affinity chromatography and immunoprecipitation. An elevation in basal tyrosine kinase activity was also present, which correlated with a lower apparent Km for ATP (0.025 mM) in comparison to the Km of 0.070 mM for the receptors from control cells. These findings indicate the presence of two types of alterations that involve the insulin receptor and hyperinsulinemia: one constituted by a modest reduction in receptor number and a second by modifications in the tyrosine kinase activity of the remaining receptors. Both alterations are required to explain the magnitude of the insulin resistance that develops after 16 h of insulin treatment.