AbstractBACKGROUND 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 125I-labelled GIP(1–42), GIP(1–30)NH2, GIP(2–30)NH2 or GIP(3–30)NH2.KEY RESULTS
GIP(1–30)NH2 displaced 125I-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 (125I-GIP(1–30)NH2), partial agonist (125I-GIP(2–30)NH2) and competitive antagonist (125I-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.