N-terminally and C-terminally Truncated Forms of Glucose-Dependent Insulinotropic Polypeptide Are High-Affinity Competitive Antagonists of the Human GIP Receptor

Br J Pharmacol. 2016 Mar;173(5):826-38. doi: 10.1111/bph.13384. Epub 2016 Jan 30.

Abstract

Background and purpose: 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 .

Experimental approach: 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 .

Key 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 and implications: 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.

MeSH terms

  • Animals
  • COS Cells
  • Chlorocebus aethiops
  • Cyclic AMP / metabolism
  • Gastric Inhibitory Polypeptide / pharmacology*
  • Humans
  • Peptide Fragments / pharmacology*
  • Receptors, Gastrointestinal Hormone / agonists
  • Receptors, Gastrointestinal Hormone / antagonists & inhibitors*
  • Receptors, Gastrointestinal Hormone / genetics
  • Receptors, Gastrointestinal Hormone / metabolism

Substances

  • Peptide Fragments
  • Receptors, Gastrointestinal Hormone
  • Gastric Inhibitory Polypeptide
  • gastric inhibitory polypeptide receptor
  • Cyclic AMP