Peptide-based GIP receptor inhibition produces limited metabolic improvements in mice whether alone or combined with GLP-1 agonism.

Abstract

ObjectiveCombinatorial gut hormone therapy is one of the more promising strategies for identifying improved treatments for metabolic disease. Many approaches combine the established benefits of glucagon-like peptide-1 (GLP-1) agonism with one or more additional molecules with the aim of improving metabolic outcomes. Recent attention has been drawn to the glucose-dependent insulinotropic polypeptide (GIP) system due to compelling pre-clinical evidence describing the metabolic benefits of antagonising the GIP receptor (GIPR). We rationalised that benefit might be accrued from combining GIPR antagonism with GLP-1 agonism. To this end we investigated the metabolic effects of co-administration of previously reported peptide-based GIPR antagonists with the GLP-1 agonist liraglutide. MethodsTwo GIPR peptide antagonists, GIPA-1 (mouse GIP(3-30)NH2) and GIPA-2 (NAc-K10[{gamma}E{gamma}E-C16]-Arg18-hGIP(5-42)), were pharmacologically characterised in vitro in an assay measuring cAMP production in CHO-K1 cells overexpressing the mouse GIPR. These peptides were then characterised in vivo in lean mice for their effect on oral glucose tolerance, as well as their ability to antagonize exogenous GIP action. Finally, a mouse model of diet-induced obesity (DIO) was used to investigate the potential metabolic benefits of chronic dosing of peptide-based GIPR antagonists, alone or in combination with liraglutide. ResultsIn vitro, both GIPR peptides exhibited potent antagonistic properties, with GIPA-2 being the more potent of the two. Acute in vivo administration of GIPA-1 during an oral glucose tolerance test (OGTT) had negligible effects on glucose tolerance and circulated insulin in lean mice. In contrast, GIPA-2 impaired glucose tolerance and attenuated circulating insulin levels, with offsetting effects on glycemia noted with co-administration with exogenous mouse GIP, suggesting true antagonism via GIPA-2 at the GIP receptor. Chronic administration studies in a DIO mouse model showed expected effects of GLP-1 agonism (via liraglutide), lowering food intake, body weight, fasting blood glucose and plasma insulin concentrations while improving glucose sensitivity, whereas delivery of either GIPR antagonist alone had negligible effects on these parameters. Interestingly, chronic dual therapy with the GIPR antagonists and GLP-1 showed separation from single intervention arms though augmented insulin sensitizing effects (modestly lowering insulin and HOMA-IR) and lowering plasmas triglycerides and free-fatty acids, with more notable effects observed with GIPA-1 compared to GIPA-2. ConclusionWe conclude that, in contrast to the well-documented effects of GLP-1R agonism, systemic administration of peptide-based GIPR antagonists demonstrate minimal benefit on metabolic parameters in DIO mice, exhibiting no major effects on body weight, food intake and glycaemic parameters. However, the co-administration of both a GIPR antagonist together with a GLP1 agonist uncovers interesting synergistic and beneficial effects on measures of insulin sensitivity, circulating lipids and certain adipose stores that seem influenced by the degree or nature of GIP receptor antagonism.