Glucagon-Like Peptide-1
Glucagon-like peptide-1 (GLP-1) is a 30-amino acid incretin hormone that enhances glucose-dependent insulin secretion and serves as the basis for widely prescribed antidiabetic and anti-obesity therapies.
Glucagon-Like Peptide-1
Overview
Glucagon-like peptide-1 (GLP-1) is an incretin hormone derived from the proglucagon gene, expressed in intestinal L-cells and hypothalamic neurons. The two major bioactive forms are GLP-1(1-37) and the truncated, more potent GLP-1(7-37), with the amidated form GLP-1(7-36)NH2 being the predominant circulating species in humans. GLP-1 is released from intestinal L-cells in response to nutrient ingestion, particularly glucose and fatty acids.
Structure and Processing
The proglucagon gene encodes a 160-amino acid precursor that undergoes tissue-specific post-translational processing. In intestinal L-cells, prohormone convertase 1/3 (PC1/3) cleaves proglucagon to produce GLP-1, GLP-2, oxyntomodulin, and glicentin-related pancreatic polypeptide (GRPP). In pancreatic alpha cells, prohormone convertase 2 (PC2) produces glucagon instead, demonstrating how differential processing yields distinct hormonal products from the same precursor.
The mature GLP-1(7-36)NH2 peptide is a 30-amino acid amidated peptide that adopts an alpha-helical conformation upon receptor binding.
Incretin Effect and Receptor Signaling
GLP-1 exerts its metabolic effects through the GLP-1 receptor (GLP-1R), a class B G-protein-coupled receptor coupled to adenylyl cyclase. The incretin effect refers to the observation that oral glucose elicits a greater insulin response than intravenous glucose of equivalent glycemic magnitude, with GLP-1 accounting for approximately 50-70% of this effect in healthy individuals.
GLP-1 enhances glucose-dependent insulin secretion from pancreatic beta cells by elevating intracellular cAMP, which amplifies glucose-stimulated insulin exocytosis. Critically, this insulinotropic action is glucose-dependent, minimizing the risk of hypoglycemia.
Degradation and Pharmacology
Endogenous GLP-1 is rapidly degraded by dipeptidyl peptidase-4 (DPP-4), which cleaves the N-terminal dipeptide, rendering the peptide biologically inactive. This enzymatic degradation results in a circulating half-life of approximately 2 minutes for intact GLP-1.
DPP-4 inhibitors (gliptins) extend endogenous GLP-1 half-life and were the first incretin-based therapies approved for type 2 diabetes. GLP-1 receptor agonists (semaglutide, liraglutide, dulaglutide, exenatide) are structurally modified to resist DPP-4 degradation, achieving half-lives of hours to days. These agents promote weight loss through central appetite suppression and delay gastric emptying.
Clinical Impact
GLP-1 receptor agonists have demonstrated cardiovascular benefit in large outcome trials, reducing major adverse cardiovascular events in patients with type 2 diabetes and established cardiovascular disease. High-dose semaglutide has shown substantial efficacy in obesity management, achieving weight reductions exceeding 15% in clinical trials.
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