title: N-terminally and C-terminally truncated forms of glucose-dependent insulinotropic polypeptide are high-affinity competitive antagonists of the human GIP receptor.
abstract: OBJECTIVE
  Glucose-dependent insulinotropic polypeptide (GIP) affects lipid, bone and glucose homeostasis. High-affinity ligands for the GIP receptor are needed to elucidate the physiological functions and pharmacological potential of GIP in vivo. GIP(1-30)NH2 is a naturally occurring truncation of GIP(1-42). Here, we have characterized eight N-terminal truncations of human GIP(1-30)NH2 .
  
  
  METHODS
  COS-7 cells were transiently transfected with human GIP receptors and assessed for cAMP accumulation upon ligand stimulation or competition binding with (125) I-labelled GIP(1-42), GIP(1-30)NH2 , GIP(2-30)NH2 or GIP(3-30)NH2 .
  
  
  RESULTS
  GIP(1-30)NH2 displaced (125) I-GIP(1-42) as effectively as GIP(1-42) (Ki 0.75 nM), whereas the eight truncations displayed lower affinities (Ki 2.3-347 nM) with highest affinities for GIP(3-30)NH2 and GIP(5-30)NH2 (5-30)NH2 . Only GIP(1-30)NH2 (Emax 100% of GIP(1-42)) and GIP(2-30)NH2 (Emax 20%) were agonists. GIP(2- to 9-30)NH2 displayed antagonism (IC50 12-450 nM) and Schild plot analyses identified GIP(3-30)NH2 and GIP(5-30)NH2 as competitive antagonists (Ki 15 nM). GIP(3-30) NH2 was a 26-fold more potent antagonist than GIP(3-42). Binding studies with agonist ((125) I-GIP(1-30)NH2 ), partial agonist ((125) I-GIP(2-30)NH2 ) and competitive antagonist ((125) I-GIP(3-30)NH2 ) revealed distinct receptor conformations for these three ligand classes.
  
  
  CONCLUSIONS
  The N-terminus is crucial for GIP agonist activity. Removal of the C-terminus of the endogenous GIP(3-42) creates another naturally occurring, more potent, antagonist GIP(3-30)NH2 , which like GIP(5-30)NH2 , was a high-affinity competitive antagonist. These peptides may be suitable tools for basic GIP research and future pharmacological interventions.
authors.affiliation: Department of Neuroscience and Pharmacology, Faculty of Health and Medical Sciences, University of Copenhagen, Copenhagen, Denmark.|Department of Biomedical Sciences, Faculty of Health and Medical Sciences, University of Copenhagen, Copenhagen, Denmark.|Center for Diabetes Research, Gentofte Hospital, University of Copenhagen, Copenhagen, Denmark.
authors.forename: L S
authors.initials: LS
authors.lastname: Hansen
authors.affiliation: Department of Neuroscience and Pharmacology, Faculty of Health and Medical Sciences, University of Copenhagen, Copenhagen, Denmark.|Department of Biomedical Sciences, Faculty of Health and Medical Sciences, University of Copenhagen, Copenhagen, Denmark.
authors.forename: A H
authors.initials: AH
authors.lastname: Sparre-Ulrich
authors.affiliation: Center for Diabetes Research, Gentofte Hospital, University of Copenhagen, Copenhagen, Denmark.
authors.forename: M
authors.initials: M
authors.lastname: Christensen
authors.affiliation: Department of Biomedical Sciences, Faculty of Health and Medical Sciences, University of Copenhagen, Copenhagen, Denmark.|Center for Diabetes Research, Gentofte Hospital, University of Copenhagen, Copenhagen, Denmark.
authors.forename: F K
authors.initials: FK
authors.lastname: Knop
authors.affiliation: NNF Center for Basic Metabolic Research, University of Copenhagen, Copenhagen, Denmark.|Department of Biomedical Sciences, Faculty of Health and Medical Sciences, University of Copenhagen, Copenhagen, Denmark.
authors.forename: B
authors.initials: B
authors.lastname: Hartmann
authors.affiliation: NNF Center for Basic Metabolic Research, University of Copenhagen, Copenhagen, Denmark.|Department of Biomedical Sciences, Faculty of Health and Medical Sciences, University of Copenhagen, Copenhagen, Denmark.
authors.forename: J J
authors.initials: JJ
authors.lastname: Holst
authors.affiliation: Department of Neuroscience and Pharmacology, Faculty of Health and Medical Sciences, University of Copenhagen, Copenhagen, Denmark.
authors.forename: M M
authors.initials: MM
authors.lastname: Rosenkilde
mesh_terms: D000818:Animals; D019556:COS Cells; D002522:Chlorocebus aethiops; D000242:Cyclic AMP / Q000378:metabolism; D005749:Gastric Inhibitory Polypeptide / Q000494:pharmacology*; D006801:Humans; D010446:Peptide Fragments / Q000494:pharmacology*; D011964:Receptors, Gastrointestinal Hormone / Q000819:agonists / Q000037:antagonists & inhibitors* / Q000235:genetics / Q000378:metabolism
chemical_list: D010446:Peptide Fragments; D011964:Receptors, Gastrointestinal Hormone; D005749:Gastric Inhibitory Polypeptide; C041086:gastric inhibitory polypeptide receptor; D000242:Cyclic AMP