Insulin-like growth factor-I provokes functional antagonism and internalization of β1-adrenergic receptors

Hormones that activate receptor tyrosine kinases have been shown to regulate G protein-coupled receptors, and herein we investigate the ability of IGF-I to regulate the beta(1)-adrenergic receptor. Treating Chinese hamster ovary cells in culture with IGF-I is shown to functionally antagonize the ability of expressed beta(1)-adrenergic receptors to accumulate intracellular cAMP in response to stimulation by the beta-adrenergic agonist Iso. The attenuation of beta(1)-adrenergic action was accompanied by internalization of beta(1)-adrenergic receptors in response to IGF-I. Inhibiting either phosphatidylinositol 3-kinase or the serine/threonine protein kinase Akt blocks the ability of IGF-I to antagonize and to internalize beta(1)-adrenergic receptors. Mutation of one potential Akt substrate site Ser412Ala, but not another Ser312Ala, of the beta(1)-adrenergic receptor abolishes the ability of IGF-I to functionally antagonize and to sequester the beta(1)-adrenergic receptor. We also tested the ability of IGF-I to regulate beta(1)-adrenergic receptors and their signaling in adult canine cardiac myocytes. IGF-I attenuates the ability of beta(1)-adrenergic receptors to accumulate intracellular cAMP in response to Iso and promotes internalization of beta(1)-adrenergic receptors in these cardiac myocytes.