title: The GIP receptor displays higher basal activity than the GLP-1 receptor but does not recruit GRK2 or arrestin3 effectively.
abstract: OBJECTIVE
  Glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) are important regulators of insulin secretion, and their functional loss is an early characteristic of type 2 diabetes mellitus (T2DM). Pharmacological levels of GLP-1, but not GIP, can overcome this loss. GLP-1 and GIP exert their insulinotropic effects through their respective receptors expressed on pancreatic β-cells. Both the GLP-1 receptor (GLP-1R) and the GIP receptor (GIPR) are members of the secretin family of G protein-coupled receptors (GPCRs) and couple positively to adenylate cyclase. We compared the signalling properties of these two receptors to gain further insight into why GLP-1, but not GIP, remains insulinotropic in T2DM patients.
  
  
  METHODS
  GLP-1R and GIPR were transiently expressed in HEK-293 cells, and basal and ligand-induced cAMP production were investigated using a cAMP-responsive luciferase reporter gene assay. Arrestin3 (Arr3) recruitment to the two receptors was investigated using enzyme fragment complementation, confocal microscopy and fluorescence resonance energy transfer (FRET).
  
  
  RESULTS
  GIPR displayed significantly higher (P<0.05) ligand-independent activity than GLP-1R. Arr3 displayed a robust translocation to agonist-stimulated GLP-1R but not to GIPR. These observations were confirmed in FRET experiments, in which GLP-1 stimulated the recruitment of both GPCR kinase 2 (GRK2) and Arr3 to GLP-1R. These interactions were not reversed upon agonist washout. In contrast, GIP did not stimulate recruitment of either GRK2 or Arr3 to its receptor. Interestingly, arrestin remained at the plasma membrane even after prolonged (30 min) stimulation with GLP-1. Although the GLP-1R/arrestin interaction could not be reversed by agonist washout, GLP-1R and arrestin did not co-internalise, suggesting that GLP-1R is a class A receptor with regard to arrestin binding.
  
  
  CONCLUSIONS
  GIPR displays higher basal activity than GLP-1R but does not effectively recruit GRK2 or Arr3.
authors.affiliation: Department of Pharmacology and Toxicology, Faculty of Medicine, Kuwait University, Safat, Kuwait.
authors.forename: Suleiman
authors.initials: S
authors.lastname: Al-Sabah
authors.affiliation: Department of Pharmacology and Toxicology, Faculty of Medicine, Kuwait University, Safat, Kuwait.
authors.forename: Munya
authors.initials: M
authors.lastname: Al-Fulaij
authors.affiliation: Department of Pharmacology and Toxicology, Faculty of Medicine, Kuwait University, Safat, Kuwait.
authors.forename: Ghina
authors.initials: G
authors.lastname: Shaaban
authors.affiliation: Department of Pharmacology and Toxicology, Faculty of Medicine, Kuwait University, Safat, Kuwait.
authors.forename: Hanadi A
authors.initials: HA
authors.lastname: Ahmed
authors.affiliation: School of Biomedical Sciences, University of Leeds, Leeds, United Kingdom.
authors.forename: Rosalind J
authors.initials: RJ
authors.lastname: Mann
authors.affiliation: School of Biomedical Sciences, University of Leeds, Leeds, United Kingdom.
authors.forename: Dan
authors.initials: D
authors.lastname: Donnelly
authors.affiliation: School of Pharmacy, Institute for Pharmacology and Toxicology, The Philipps University of Marburg, Marburg, Germany.
authors.forename: Moritz
authors.initials: M
authors.lastname: Bünemann
authors.affiliation: School of Pharmacy, Institute for Pharmacology and Toxicology, The Philipps University of Marburg, Marburg, Germany.
authors.forename: Cornelius
authors.initials: C
authors.lastname: Krasel
mesh_terms: D000818:Animals; D019390:Arrestins / Q000378:metabolism*; D016466:CHO Cells; D006224:Cricetinae; D003412:Cricetulus; D003924:Diabetes Mellitus, Type 2 / Q000235:genetics / Q000378:metabolism; D054769:G-Protein-Coupled Receptor Kinase 2 / Q000378:metabolism*; D005749:Gastric Inhibitory Polypeptide / Q000378:metabolism / Q000494:pharmacology; D052216:Glucagon-Like Peptide 1 / Q000378:metabolism / Q000494:pharmacology; D000067757:Glucagon-Like Peptide-1 Receptor / Q000235:genetics / Q000378:metabolism*; D057809:HEK293 Cells; D006801:Humans; D011485:Protein Binding; D011964:Receptors, Gastrointestinal Hormone / Q000819:agonists / Q000235:genetics / Q000378:metabolism*; D000097789:Glucagon-Like Peptide-1 Receptor Agonists
chemical_list: D019390:Arrestins; D000067757:Glucagon-Like Peptide-1 Receptor; D011964:Receptors, Gastrointestinal Hormone; C082333:arrestin3; D005749:Gastric Inhibitory Polypeptide; D052216:Glucagon-Like Peptide 1; C041086:gastric inhibitory polypeptide receptor; C496567:GRK2 protein, human; D054769:G-Protein-Coupled Receptor Kinase 2; D000097789:Glucagon-Like Peptide-1 Receptor Agonists