Identification of a bioactive domain in the amino-terminus of glucose-dependent insulinotropic polypeptide (GIP)

The incretins are a class of hormones released from the small bowel that ac t on the endocrine pancreas to potentiate insulin secretion in a glucose-de pendent manner. Due to the requirement for an elevated glucose concentratio n for activity, the incretins, glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-ii have potential in the treatment of non- insulin-dependent diabetes mellitus. A series of synthetic peptide GIP frag ments was generated for the purpose of elucidating the bioactive domain of the molecule. Peptides were screened for stimulation of cyclic AMP (cAMP) a ccumulation in Chinese hamster ovary cells transfected with the rat islet G IP receptor. Of the GIP fragments tested, GIP(1-14) and GIP(19-30) demonstr ated the greatest cAMP-stimulating ability over the range of concentrations tested (up to 20 muM). In contrast, GIP fragments corresponding to amino a cids 15-42, 15-30, 16-30 and 17-30 all demonstrated weak antagonism of GIP( 1-42) activity. Competitive-binding displacement studies indicated that the se peptides were low-affinity ligands for the GIP receptor. To examine biol ogical activity in vivo, a bioassay was developed in the anesthetized rat. Intravenous infusion of GIP(1-42) (1 pmol/min/100 g) with a concurrent intr aperitoneal glucose load (1 g/kg) significantly reduced circulating blood g lucose excursions through stimulation of insulin release. Higher doses of G IP(1-14) and GIP(19-30) (100 pmol/min/100 g) also reduced blood glucose exc ursions. (C) 2001 Elsevier Science B.V. All rights reserved.