title: Desensitization and phosphorylation of the glucagon-like peptide-1 (GLP-1) receptor by GLP-1 and 4-phorbol 12-myristate 13-acetate.
abstract: Glucagon-like peptide-1 (GLP-1) stimulates glucose-induced insulin secretion by binding to a specific G protein-coupled receptor linked to activation of the adenylyl cyclase pathway. Here, using insulinoma cell lines, we studied homologous and heterologous desensitization of GLP-1-induced cAMP production. Preexposure of the cells to GLP-1 induced a decrease in GLP-1-mediated cAMP production, as assessed by a 3- to 5-fold rightward shift of the dose-response curve and an approximately 20 percent decrease in the maximal production of cAMP. Activation of protein kinase C by the phorbol ester phorbol 12-myristate 13-acetate (PMA) also induced desensitization of the GLP-1-mediated response, leading to a 6- to 9-fold shift in the EC50 and a 30% decrease in the maximal production of cAMP. Both forms of desensitization were additive, and the protein kinase C inhibitor RO-318220 inhibited PMA-induced desensitization, but not agonist-induced desensitization. GLP-1- and PMA-dependent desensitization correlated with receptor phosphorylation, and the levels of phosphorylation induced by the two agents were additive. Furthermore, PMA-induced, but not GLP-1-induced, phosphorylation was totally inhibited by RO-318220. Internalization of the GLP-1 receptor did not participate in the desensitization induced by PMA, as a mutant GLP-1 receptor lacking the last 20 amino acids of the cytoplasmic tail was found to be totally resistant to the internalization process, but was still desensitized after PMA preexposure. PMA and GLP-1 were not able to induce the phosphorylation of a receptor deletion mutant lacking the last 33 amino acids of the cytoplasmic tail, indicating that the phosphorylation sites were located within the deleted region. The cAMP production mediated by this deletion mutant was not desensitized by PMA and was only poorly desensitized by GLP-1. Together, our results indicate that the production of cAMP and, hence, the stimulation of insulin secretion induced by GLP-1 can be negatively modulated by homologous and heterologous desensitization, mechanisms that involve receptor phosphorylation.
authors.affiliation: Department of Pharmacology and Toxicology, University of Lausanne, Switzerland.
authors.forename: C
authors.initials: C
authors.lastname: Widmann
authors.forename: W
authors.initials: W
authors.lastname: Dolci
authors.forename: B
authors.initials: B
authors.lastname: Thorens
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D015153:Blotting, Western; D000242:Cyclic AMP / Q000378:metabolism; D004789:Enzyme Activation / Q000187:drug effects; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005934:Glucagon / Q000494:pharmacology*; D052216:Glucagon-Like Peptide 1; D000067757:Glucagon-Like Peptide-1 Receptor; D007211:Indoles / Q000494:pharmacology; D007340:Insulinoma; D008969:Molecular Sequence Data; D010190:Pancreatic Neoplasms; D010446:Peptide Fragments / Q000494:pharmacology*; D010766:Phosphorylation; D011493:Protein Kinase C / Q000037:antagonists & inhibitors / Q000378:metabolism*; D011498:Protein Precursors / Q000494:pharmacology*; D051381:Rats; D018027:Receptors, Glucagon / Q000737:chemistry / Q000187:drug effects / Q000378:metabolism*; D013755:Tetradecanoylphorbol Acetate / Q000494:pharmacology*; D014407:Tumor Cells, Cultured
chemical_list: D004791:Enzyme Inhibitors; C000593835:Glp1r protein, rat; D000067757:Glucagon-Like Peptide-1 Receptor; D007211:Indoles; D010446:Peptide Fragments; D011498:Protein Precursors; D018027:Receptors, Glucagon; D052216:Glucagon-Like Peptide 1; D005934:Glucagon; D000242:Cyclic AMP; D011493:Protein Kinase C; D013755:Tetradecanoylphorbol Acetate; C064758:Ro 31-8220