Article date: March 2016
By: L S Hansen, A H Sparre‐Ulrich, M Christensen, F K Knop, B Hartmann, J J Holst, M M Rosenkilde in Volume 173, Issue 5, pages 826-838
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 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.
DOI: 10.1111/bph.13384
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