Binding of IRS proteins to calmodulin is enhanced in insulin resistance.

The IRS proteins, major endogenous targets of the insulin receptor, bind to calmodulin in a Ca(2+)-dependent manner. Here, we have examined the interaction between these proteins in animal and cultured cell models of insulin resistance. Both IRS-1 and IRS-2 co-immunoprecipitate with calmodulin from insulin target tissues in rats. The interaction between calmodulin and IRS proteins in rat soleus muscle was enhanced when insulin resistance was induced in rats by treatment with dexamethasone for 5 days. Moreover, injection of angiotensin II into the inferior vena cava enhanced the binding in rat cardiac muscle. Similarly, increased binding between calmodulin and IRS-1 was observed in isolated cells incubated with tumor necrosis factor-alpha. Overexpression of calmodulin in Chinese hamster ovary cells reduced the tyrosine phosphorylation of IRS-1 induced by insulin, with a concomitant decrease in insulin-stimulated association of IRS-1 with the 85-kDa regulatory subunit of phosphatidylinositol 3-kinase. Insulin-stimulated phosphatidylinositol 3-kinase activity associated with IRS-1 was also reduced in cells overexpressing calmodulin, while this activity was increased in cells incubated with the cell-permeable calmodulin antagonist trifluoperazine. These data demonstrate an enhanced interaction between calmodulin and IRS proteins in models of insulin resistance and suggest a possible mechanism by which increased intracellular Ca(2+) concentrations may contribute to impaired insulin sensitivity.