Isoproterenol Inhibits Insulin‐Stimulated Tyrosine Phosphorylation of the Insulin Receptor Without Increasing its Serine/Threonine Phosphorylation

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Abstract
The effect of a beta-adrenergic agonist (isoproterenol) on the tyrosine kinase activity of the insulin receptor was studied in intact adipocytes. Isoproterenol treatment rapidly (5 min) inhibited the insulin-induced autophosphorylation of the insulin receptor on tyrosine residues in intact adipocytes. The effect of insulin on the phosphorylation of cellular proteins on tyrosine residues was also inhibited by isoproterenol. In order to understand the mechanism responsible for this inhibition, two-dimensional phosphopeptide mapping of the insulin receptor was performed. The pattern of phosphorylation of the insulin receptor in freshly isolated adipocytes showed marked differences from that previously observed in cultured cells overexpressing insulin receptors. These differences include a larger proportion of receptors being phosphorylated on the three tyrosines from the kinase domain and no apparent phosphorylation of the two tyrosines close to the C-terminus after insulin stimulation. Isoproterenol markedly inhibited the effect of insulin on the phosphorylation of the three tyrosines from the kinase domain. However, this inhibition was not associated with an increase in the phosphorylation of serine/threonine peptides. Thus, this direct analysis of insulin receptor phosphorylation sites in intact adipocytes does no support the idea that beta-adrenegic agents inhibit the tyrosine kinase activity of the receptor through a serine/threonine phosphorylation-dependent mechanism.

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