HIGH POTENCY ANTAGONISTS OF THE PANCREATIC GLUCAGON-LIKE PEPTIDE-1 RECEPTOR

GLP-1-(7-36)-amide and exendin-4-(1-39) are glucagon-like peptide-1 (G LP-1) receptor agonists, whereas exendin-(9-39) is the only known anta gonist, To analyze the transition from agonist to antagonist and to id entify the amino acid residues involved in ligand activation of the GL P-1 receptor, we used exendin analogs with successive N-terminal trunc ations, Chinese hamster ovary cells stably transfected with the rat GL P-1 receptor were assayed for changes in intracellular cAMP caused by the test peptides in the absence or presence of half-maximal stimulato ry doses of GLP-1. N-terminal truncation of a single amino acid reduce d the agonist activity of the exendin peptide, whereas N-terminal trun cation of 3-7 amino acids produced antagonists that were 4-10-fold mor e potent than exendin-(9-39), N-terminal truncation of GLP-1 by 2 amin o acids resulted in weak agonist activity, but an 8-amino acid N-termi nal truncation inactivated the peptide, Binding studies performed usin g I-125-labeled GLP-1 confirmed that all bioactive peptides specifical ly displaced tracer with high potency, In a set of exendin/GLP-1 chime ric peptides, substitution of GLP-1 sequences into exendin-(3-39) prod uced loss of antagonist activity with conversion to a weak agonist, Th e results show that receptor binding and activation occur in separate domains of exendin, but they are more closely coupled in GLP-1.