2678-P: Understanding the Implications of Constitutive Activity of Proglucagon Peptide Receptors

Introduction and Objective: G protein-coupled receptors (GPCRs) exist in a dynamic equilibrium between active and inactive conformations. However, in the absence of their activating ligand, GPCRs can remain in or spontaneously adopt the active conformation, giving rise to constitutive activity. While it is plausible that all GPCRs have some degree of constitutive activity, whether this is sufficient to impact cellular function is an important distinction. The GLP-1R has been reported to have constitutive activity in immortalized β-cells based on reduced basal cAMP levels and insulin secretion upon GLP-1R antagonist exendin[9-39] (Ex9) treatment. Consequently, Ex9 has been defined as an inverse agonist, an antagonist that also reduces constitutive activity of the GLP-1R. In mice and humans, Ex9 lowers insulin secretion and raises fasting glycemia, when circulating GLP-1 levels are low. This observation could reflect the inverse agonist properties of Ex9. Alternatively, Ex9 could be blocking the paracrine actions of proglucagon derived peptides produced by α-cells. We aimed to establish the implications of constitutive activity at the GLP-1R and the potential for Ex9 to serve as an inverse agonist. Methods: We tested for constitutive activity of the GLP-1 and the glucagon (Gcg) receptors in islets and hepatocytes using Gcg-/- mice, which lack ligands for these receptors by measuring insulin secretion, glucose tolerance, and amino acids. Results: Ex9 blunted insulin secretion and worsened glucose tolerance in control but had no effect in Gcg-/- mice. GcgR antagonism also failed to reduce insulin secretion Gcg-/- islets. However, GcgR antagonism further increased fasting amino acid levels in Gcg-/- mice to levels comparable to control mice. Conclusion: Constitutive activity of the GLP-1R or the GCGR is insufficient to influence β-cell function in mice. Rather, Ex9 reduces insulin secretion as a competitive antagonist by interrupting α- to β-cell communication. There is sufficient constitutive activity of the GCGR in mouse hepatocytes to regulate amino acid levels. Disclosure N. Morrow: None. A. Chen: None. M. Capozzi: None. D.M. D'Alessio: Other - Clinical Trials; Current; Eli Lilly and Company. Consultant; Current; Sun Pharmaceutical Industries Ltd., Vertex Pharmaceuticals Incorporated. J. Campbell: Research Support; Current; Eli Lilly and Company. Research Support; Ended; Novo Nordisk. Research Support; Current; Merck & Co., Inc., Structure Therapeutics, Protagonist Therapeutics, Prostasis. Consultant; Current; Roche Pharmaceuticals, Neurocrine Biosciences, Inc., Arrowhead Pharmaceuticals, Boehringer Ingelheim International GmbH. Funding American Diabetes Association (1-26-PDF-0556), National Institutes of Health (R01DK123075, R01DK125353, R01DK046492, P30K124723)