Cross-talk between tyrosine kinase and G-protein-linked receptors: Phosphorylation of β₂-adrenergic receptors in response to insulin

Protein kinases play a pivotal role in the propagation and modulation of transmembrane signaling pathways. Two major classes of receptors, G-protein-linked and tyrosine kinase receptors not only propagate signals but also are substrates for phosphorylation in response to stimulation by agonist ligands. Insulin (operating via tyrosine kinase receptors) and catecholamines (operating by G-protein-linked receptors) are counterregulatory with respect to lipid and carbohydrate metabolism. How, on a cellular level, these two distinct classes of receptors may cross-regulate each other remains controversial. In the present work we identify a. novel cross-talk between members of two distinct classes of receptors, tyrosine kinase (insulin) and G-protein-linked (β-adrenergic) receptors. Treatment of DDT₁ MF-2 hamster vas deferens smooth muscle cells with insulin promoted a marked attenuation (desensi-tization) of β-adrenergic receptor-mediated activation of adenylylcyclase. Measured by immune precipitation of β₂-adrenergic receptors from cells metabolically labeled with [³²P]orthophosphate, the basal state of receptor phosphorylation was increased 2-fold by insulin. Phosphoamino acid analysis revealed that for insulin-stimulated cells, the β₂-adrenergic receptors showed increased phosphorylation on tyrosyl and decreased phosphorylation on threonyl residues. Phosphorylation of the β-adrenergic receptor was rapid and peaked at 30 min following stimulation of cells by insulin, β- Adrenergic receptor phosphorylation and attenuation of catecholamine-sensitive adenylylcyclase provide a biochemical basis for the counterregulatory effects of insulin upon catecholamine action.