CDB15:0000719 GIP — GIPR

ABSTRACT: Incretins, endogenous polypeptide hormones released in response to food intake, potentiate insulin secretion from pancreatic beta cells after oral glucose ingestion (the incretin effect). This response is signaled by the two peptide hormones glucose-dependent insulinotropic polypeptide (GIP) (also known as gastric inhibitory polypeptide) and glucagon-like peptide 1 through binding and activation of their cognate class 2 G protein-coupled receptors (GPCRs). Because the incretin effect is lost or significantly reduced in patients with type 2 diabetes mellitus, glucagon-like peptide 1 and GIP have attracted considerable attention for their potential in antidiabetic therapy. A paucity of structural information precludes a detailed understanding of the processes of hormone binding and receptor activation, hampering efforts to develop novel pharmaceuticals. Here we report the crystal structure of the complex of human GIP receptor extracellular domain (ECD) with its agonist, the incretin GIP(1-42). The hormone binds in an alpha-helical conformation in a surface groove of the ECD largely through hydrophobic interactions. The N-terminal ligand residues would remain free to interact with other parts of the receptor. Thermodynamic data suggest that binding is concomitant with structural organization of the hormone, resulting in a complex mode of receptor-ligand recognition. The presentation of a well structured, alpha-helical ligand by the ECD is expected to be conserved among other hormone receptors of this class.

ABSTRACT: Glucose-dependent insulinotropic polypeptide (GIP) plays an important role in the regulation of postprandial insulin secretion and proinsulin gene expression of pancreatic beta-cells. This study demonstrates the molecular cloning of a cDNA for the GIP-receptor from a human insulinoma lambda gt11 cDNA library. The cloned cDNA encoded a seven transmembrane domain protein of 466 amino acids which showed high homology (41%) to the human glucagon-like peptide 1 (GLP-1) receptor. Homology to the GIP receptor from rat or hamster was 79% and 81%, respectively. When transfected stably into fibroblast CHL-cells a high affinity receptor was expressed which coupled to the adenylate cyclase with normal basal cAMP and increasing intracellular cAMP levels under stimulation with human GIP-1-42 (EC50 = 1.29 x 10(-13) M). The receptor accepted only human GIP 1-42 (Kd = 1.93 +/- 0.2 x 10(-8) M) and porcine truncated GIP 1-30 (Kd = 1.13 +/- 0.1 x 10(-8) M) as high affinity ligands. At 1 microM, exendin-4 and (9-39)amide weakly reduced GIP-binding (25%) whereas secretin, glucagon, glucagon-like peptide-1, vasoactive intestinal polypeptide, peptide histidine-isoleucine, and pituitary adenylyl cyclase activating peptide were without effect. In transfected CHL cells, GIP-1-42 did not increase intracellular calcium. Northern analysis revealed one transcript of human GIP receptor mRNA with an apparent size of 5.5 kb. The exact understanding of GIP receptor regulation and signal transduction will aid in the understanding of the incretin hormone's failure to exert its biological action at the pancreatic B-cell in type II diabetes mellitus.

ABSTRACT: The gastrointestinal hormones glucagon-like peptide-1 (GLP-1) and gastric inhibitory polypeptide (GIP) strongly stimulate insulin release. Despite their high N-terminal sequence similarity, GLP-1 does not bind to the GIP receptor and vice versa. To characterize the domains required for interaction of the peptide ligands with their specific receptors, we performed displacement studies with various synthetic GLP-1/GIP hybrid peptides on RINm5F insulinoma cells. Displacement of 125I-GIP and 125I-GLP-1 was measured using GLP-1/GIP chimeras which comprised GIP and GLP-1 sequences at different positions. The binding affinity to the GLP-1 receptor was found to be sensitive to GIP-like exchanges in the N-terminal 22 amino acids as well as in positions 13 and 15 (loss of affinity 280-fold to more than 1000-fold). C-terminal substitution of the GLP-1 sequence by GIP diminished the affinity towards the GLP-1 receptor only 20-fold. All hybrid peptides investigated showed minimal binding affinity for the GIP receptor, indicating that the entire GIP-sequence (1-31) is important for receptor recognition. These findings provide insight into the structural requirements for the specific interaction of two important insulinotropic peptides with their specific receptors.