N‐terminally and C‐terminally truncated forms of glucose‐dependent insulinotropic polypeptide are high‐affinity competitive antagonists of the human GIP receptor

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)NH(2) is a naturally occurring truncation of GIP(1–42). Here, we have characterized eight N‐terminal truncations of human GIP(1–30)NH(2). 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)NH(2), GIP(2–30)NH(2) or GIP(3–30)NH(2). KEY RESULTS: GIP(1–30)NH(2) displaced (125)I‐GIP(1–42) as effectively as GIP(1–42) (K(i) 0.75 nM), whereas the eight truncations displayed lower affinities (K(i) 2.3–347 nM) with highest affinities for GIP(3–30)NH(2) and GIP(5–30)NH(2) (5–30)NH(2). Only GIP(1–30)NH(2) (E(max) 100% of GIP(1–42)) and GIP(2–30)NH(2) (E(max) 20%) were agonists. GIP(2‐ to 9–30)NH(2) displayed antagonism (IC(50) 12–450 nM) and Schild plot analyses identified GIP(3–30)NH(2) and GIP(5–30)NH(2) as competitive antagonists (K(i) 15 nM). GIP(3–30) NH(2) was a 26‐fold more potent antagonist than GIP(3–42). Binding studies with agonist ((125)I‐GIP(1–30)NH(2)), partial agonist ((125)I‐GIP(2–30)NH(2)) and competitive antagonist ((125)I‐GIP(3–30)NH(2)) 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)NH(2), which like GIP(5–30)NH(2), was a high‐affinity competitive antagonist. These peptides may be suitable tools for basic GIP research and future pharmacological interventions.