GLP-1 and the incretin pathway: peptide drugs for metabolic disease

Detailed explanation

The incretin pathway is the neuroendocrine system by which the gut signals nutrient ingestion to pancreatic β-cells, amplifying insulin secretion in a glucose-dependent manner. The phenomenon — first described in the 1960s — is that an oral glucose load triggers substantially more insulin secretion than an isoglycaemic intravenous glucose load, with the difference attributable to two intestinal hormones: glucagon-like peptide-1 (GLP-1), secreted by enteroendocrine L-cells of the distal small bowel and colon, and glucose-dependent insulinotropic peptide (GIP), secreted by K-cells of the duodenum and proximal jejunum. In healthy adults, the incretin effect accounts for 50–70% of the postprandial insulin response. In type-2 diabetes the effect is materially attenuated — predominantly through reduced β-cell GIP responsiveness rather than reduced GLP-1 sensitivity — which led to the central therapeutic hypothesis that GLP-1 receptor agonism could restore the incretin effect in diabetic patients. Native GLP-1 is a 30-amino-acid peptide derived from post-translational processing of the proglucagon gene. Active forms are GLP-1(7-37) and GLP-1(7-36)amide, the latter representing the predominant circulating species. Active GLP-1 binds the GLP-1 receptor (GLP-1R, a 463-residue class B G-protein-coupled receptor) expressed on pancreatic β-cells, intestinal mucosa, central and peripheral neurons, vascular smooth muscle, and other tissues. β-cell GLP-1R activation triggers Gαs/adenylate cyclase signalling, raising intracellular cAMP and triggering glucose-dependent insulin secretion through PKA-mediated phosphorylation of multiple targets including KATP channel closure and Epac2-mediated insulin granule mobilisation. The strictly glucose-dependent nature of incretin-driven insulin secretion is a critical pharmacological feature — at low glucose concentrations, the cAMP-driven insulinotropic signal does not produce hypoglycaemia, distinguishing the class from sulphonylureas. Pancreatic α-cell GLP-1R activation suppresses glucagon secretion in a glucose-dependent manner, providing a complementary glucose-lowering mechanism. Central GLP-1R activation in the hypothalamic arcuate nucleus and area postrema drives appetite suppression and satiety — the principal mechanism behind the dramatic weight-loss efficacy of high-dose GLP-1 agonists. Peripheral GLP-1R effects include slowed gastric emptying (which flattens the postprandial glucose excursion), modest cardiovascular effects on blood pressure, and emerging data on neuroprotection through both central and peripheral pathways. Native GLP-1 has a plasma half-life of approximately two minutes due to rapid degradation by dipeptidyl peptidase-4 (DPP-4), which cleaves the N-terminal His-Ala bond to generate the inactive GLP-1(9-37/36) fragment. This degradation pathway is the principal pharmacological constraint addressed by all clinically successful GLP-1R agonists, which are engineered for DPP-4 resistance through amino-acid substitution, fatty-acid acylation, or other modifications. Glucose-dependent insulinotropic peptide (GIP) is a 42-amino-acid incretin acting through the GIP receptor (GIPR), a class B GPCR expressed on pancreatic β-cells, adipose tissue, central neurons, and gastric mucosa. β-cell GIPR activation potentiates glucose-stimulated insulin secretion alongside GLP-1, but GIPR has distinct effects: in healthy subjects GIP does not suppress glucagon and may stimulate it in hypoglycaemia (providing glucose-stabilising rather than glucose-lowering action), and GIPR is expressed on adipose tissue where it regulates both lipogenesis and lipolysis. The clinical relevance of GIPR agonism was reignited by tirzepatide (Mounjaro/Zepbound), a dual GIPR/GLP-1R co-agonist whose obesity and type-2-diabetes efficacy exceeds GLP-1R monoagonists, suggesting additive metabolic effects from combined incretin pathway activation. Whether GIPR agonism, antagonism, or biased agonism contributes most to this benefit remains an open research question. The principal clinically deployed compounds engaging this pathway include semaglutide (long-acting fatty-acid-acylated GLP-1R agonist, marketed as Ozempic for type-2 diabetes and Wegovy for obesity; oral semaglutide is marketed as Rybelsus), tirzepatide (dual GIPR/GLP-1R agonist marketed as Mounjaro for type-2 diabetes and Zepbound for obesity), retatrutide (triple GLP-1R/GIPR/glucagon-R agonist in Phase III development by Eli Lilly), liraglutide (the daily GLP-1R agonist that preceded semaglutide), and exenatide (the original exendin-4-derived GLP-1R agonist). Additional metabolic peptides engaging adjacent mechanisms include MOTS-c (a mitochondrial-derived peptide modulating AMPK signalling and adipocyte function) and 5-Amino-1MQ (a small-molecule NNMT inhibitor — not strictly an incretin compound but commonly grouped with metabolic research peptides for its adipocyte effects). Standard clinical and pre-clinical endpoints for incretin-pathway research include fasting and postprandial glucose, HbA1c, body weight and body composition, oral glucose tolerance test (OGTT), mixed-meal tolerance test (MMTT) for incretin-effect characterisation, insulin and C-peptide concentrations for β-cell function, glucagon and ghrelin for counter-regulatory hormone profiles, and gastric emptying scintigraphy for mechanistic confirmation of the gastric component. Cardiovascular outcome trials have established the broader vascular benefits of GLP-1 agonist class — MACE reduction in the SUSTAIN-6 (semaglutide) and SURPASS-CVOT (tirzepatide) programmes — beyond glycaemic control.