glucagon-like peptide 1

Structure

GLP-1 is a 31-aa peptide hormone and is secreted from intestinal endocrine L cells in two major molecular forms, GLP-1(7–37) and GLP-1(7–36)-amide. Amidation is not always important for its biological activity, and these two molecular forms have similar biological activities. The majority of circulating GLP-1 is found to be GLP-1(7–36) amide, with lesser amounts of GLP-1 (7–37). GLP-1 shares a highly conserved alanine at position 9 with GIP and GLP-2, making these peptides ideal substrates for dipeptidyl peptidase 4 (DPP-4).

Gene, mRNA, and precursor

The human proglucagon gene, GCG, location 2q36– q37, spans approximately 9.4 kb and comprises six exons and five introns. It encodes a preproglucagon of 180 aa residues that contains glucagon, GLP-1, and GLP-2. The expression of proglucagon has been reported in pancreatic α cells, intestinal L cells, and the brain.

Synthesis and release

Glucagon, GLP-1, and GLP-2 are processed from proglucagon in pancreatic α cells and intestinal L cells in a tissue-specific manner. Prohormone convertases (PCs) are responsible for the tissue-specific processing. Glucagon is liberated from pancreatic α cells by PC2, wherea GLP-1 and GLP-2 are liberated from the intestinal L cells by PC1/3. As a result, GLP-1 and GLP-2 are coreleased in a 1:1 ratio following nutrient ingestion. The secretion of GLP-1 is stimulated by nutrients such as lipids and carbohydrates. In addition, several hormones such as cholecystokinin (CCK), GIP, and somatostatin as well as numerous neuromediators regulate GLP-1 secretion. Insulin has been reported to inhibit GLP-1 release, indicating a feedback regulation of GLP-1 secretion.

Receptors

The receptor of GLP-1 (GLP1R) is a seventransmembrane GPCR that belongs to a subclass of the family B. The GLP-1 receptor was first identified by ligand binding experiments and measurements of cyclic AMP accumulation using the rat insulinoma cell line. The human GLP-1 receptor gene, GLP1R, is located on chromosome 6 (6p21). The GLP-1 receptor contains a large hydrophilic extracellular domain and seven hydrophobic transmembrane domains. In addition, the GLP-1 receptor has three potential N-linked glycosylation sites, and glycosylation may modulate the receptor function.

Agonists

Exendin-4 (subsequently renamed exenatide, the first FDA approved “incretin mimetic”), Liraglutide (a longacting DPP-4-resistant GLP-1 receptor agonist), CJC-1131 (a GLP-1 analog engineered for covalent coupling to albumin), Albiglutide (originally referred to as albugon, a recombinant human albumin-GLP-1 protein), ZP10 (an exendin-4 derivative), BIM51077 (subsequently renamed taspoglutide), LY315902 (a DPP-4-resistant GLP-1 analog), LY2428757 (a pegylated GLP-1), LY2199265 (Dulaglutide, an Fc immunoglobulin fusion protein), Semaglutide (a synthetic peptide similar to GLP-1), Lixisenatide (a synthetic peptide similar to exandin-4), and Tirzepatide (LY3298176, a dual agonist of GIP and GLP-1 receptors).

Clinical implications

GLP and GIP have implications in the following diseases and conditions:
Diabetes: Type 2 diabetes is characterized by a severely reduced or absent incretin effect. The incretin defect is due to the nearly complete loss of the insulinotropic effect of GIP, whereas the secretion of GIP is normal. In contrast to GIP, GLP-1 was found to retain its insulinotropic effect in patients, and this fact provides part of the background for the clinical use of GLP-1 receptor agonists in diabetes treatment.
Obesity: A role for GLP-1 in the development of obesity was suggested partly because of the apparent physiological effects of GLP-1 on appetite and food intake, and reduced GLP-1 secretion in obesity.
Dumping syndrome: If food reaches the distal intestine very rapidly, for example, after gastrectomy, an exaggerated plasma concentration of GLP-1 may lead to hyperinsulinemia and hypoglycemia.

Description

GLP-1 stimulates pancreatic β cells to secrete insulin as an incretin hormone. GLP-1 also has additional actions, including suppression of glucagon secretion, inhibition of gastric motility, and promotion of satiety. GLP-1 was first identified in the early 1980s, following the cloning of the proglucagon gene. The biological activity of GLP-1 was investigated initially by using the N-terminal extended forms of GLP-1(1–37 and 1–36 amide). However, these forms were devoid of biological activity. In the late 1980s, bioactive GLP-1 peptides were purified from gut extracts and were identified to be GLP-1(7–37) and GLP-1(7–36)-amide. In the current literature, the unqualified designation of GLP-1 corresponds to the truncated peptides.

Biological Functions

Glucagon-like peptide-1 (GLP-1) is an incretin, a natural peptide hormone secreted in response to food intake. Incretins have multiple physiological effects to lower blood sugar, including the stimulation of insulin release and the inhibition of glucagon release following meals.

Clinical Use

GLP-1 enhances glucose-stimulated insulin release. GLP-1 agonists are resistant to degradation by DPP-4; thus these agonists are used clinically for the treatment of diabetes. DPP-4 inhibitors are also acceptable for use in diabetes patients because GLP-1 is rapidly deactivated by DPP-4.