Glucagon-like peptide-1

Glucagon-like peptide-1 (GLP-1) is a 30 or 31 amino acid long peptide hormone deriving from the tissue-specific posttranslational processing of the proglucagon peptide. It is produced and secreted by intestinal enteroendocrine L-cells and certain neurons within the nucleus of the solitary tract in the brainstem upon food consumption. The initial product GLP-1 (1–37) is susceptible to amidation and proteolytic cleavage which gives rise to the two truncated and equipotent biologically active forms, GLP-1 (7–36) amide and GLP-1 (7–37). Active GLP-1 composes two α-helices from amino acid position 13–20 and 24–35 separated by a linker region. Glucagon-like peptide-1 (GLP-1) is a 30 or 31 amino acid long peptide hormone deriving from the tissue-specific posttranslational processing of the proglucagon peptide. It is produced and secreted by intestinal enteroendocrine L-cells and certain neurons within the nucleus of the solitary tract in the brainstem upon food consumption. The initial product GLP-1 (1–37) is susceptible to amidation and proteolytic cleavage which gives rise to the two truncated and equipotent biologically active forms, GLP-1 (7–36) amide and GLP-1 (7–37). Active GLP-1 composes two α-helices from amino acid position 13–20 and 24–35 separated by a linker region. Alongside glucose-dependent insulinotropic peptide (GIP), GLP-1 is an incretin; thus, it has the ability to decrease blood sugar levels in a glucose-dependent manner by enhancing the secretion of insulin. Beside the insulinotropic effects, GLP-1 has been associated with numerous regulatory and protective effects. Unlike GIP, the action of GLP-1 is preserved in patients with type 2 diabetes and substantial pharmaceutical research has therefore been directed towards the development of GLP-1-based treatment. However, endogenous GLP-1 is rapidly degraded primarily by dipeptidyl peptidase-4 (DPP-4), but also neutral endopeptidase 24.11 (NEP 24.11) and renal clearance, resulting in a half-life of approximately 2 minutes. Consequently, only 10–15 % of GLP-1 reaches circulation intact, leading to fasting plasma levels of only 0–15 pmol/L. To overcome this, GLP-1 receptor agonists and DPP-4 inhibitors have been developed to increase GLP-1 activity. As opposed to common treatment agents such as insulin and sulphonylurea, GLP-1-based treatment has been associated with weight loss and a lower risk of hypoglycemia, two important considerations for patients with type 2 diabetes. The proglucagon gene is expressed in several organs including the pancreas (α-cells of the islets of Langerhans), gut (intestinal enteroendocrine L-cells) and brain (caudal brainstem and hypothalamus). Pancreatic proglucagon gene expression is promoted upon fasting and hypoglycaemia induction and inhibited by insulin. Conversely, intestinal proglucagon gene expression is reduced during fasting and stimulated upon food consumption. In mammals, the transcription gives rise to identical mRNA in all three cell types, which is further translated to the 180 amino acid precursor called proglucagon. However, as a result of tissue-specific posttranslational processing mechanisms, different peptides are produced in the different cells. In the pancreas (α-cells of the islets of Langerhans), proglucagon is cleaved by prohormone convertase (PC) 2 producing glicentin-related pancreatic peptide (GRPP), glucagon, intervening peptide-1 (IP-1) and major proglucagon fragment (MPGF). In the gut and brain, proglucagon is catalysed by PC 1/3 giving rise to glicentin, which may be further processed to GRPP and oxyntomodulin, GLP-1, intervening peptide-2 (IP-2) and glucagon-like peptide-2 (GLP-2). Initially, GLP-1 was thought to correspond to proglucagon (72–108) suitable with the N-terminal of the MGPF, but sequencing experiments of endogenous GLP-1 revealed a structure corresponding to proglucagon (78–107) from which two discoveries were found. Firstly, the full-length GLP-1 (1–37) was found to be catalysed by endopeptidase to the biologically active GLP-1 (7–37). Secondly, the glycine corresponding to proglucagon(108) was found to serve as a substrate for amidation of the C-terminal arginine resulting in the equally potent GLP-1 (7–36) amide. In humans, almost all (>80%) secreted GLP-1 is amidated, whereas a considerable part remains GLP-1 (7–37) in other species. GLP-1 is packaged in secretory granules and secreted into the hepatic portal system by the intestinal L-cells located primarily in the distal ileum and colon but also found in the jejunum and duodenum. The L-cells are open-type triangular epithelial cells directly in contact with the lumen and neuro-vascular tissue and are accordingly stimulated by various nutrient, neural and endocrine factors. GLP-1 is released in a biphasic pattern with an early phase after 10–15 minutes followed by a longer second phase after 30–60 minutes upon meal ingestion. As the majority of L-cells are located in the distal ileum and colon, the early phase is likely explained by neural signalling, gut peptides or neurotransmitters. Other evidence suggest that the amount of L-cells located in the proximal jejunum is sufficient to account for the early phase secretion through direct contact with luminal nutrients. Less controversially, the second phase is likely caused by direct stimulation of L-cells by digested nutrients. The rate of gastric emptying is therefore an important aspect to consider, as it regulates the entry of nutrients into the small intestines where the direct stimulation occurs. One of the actions of GLP-1 is to inhibit gastric emptying, thus slowing down its own secretion upon postprandial activation. Fasting plasma concentration of biologically active GLP-1 range between 0 and 15 pmol/L in humans and is increased 2- to 3-fold upon food consumption depending on meal size and nutrient composition. Individual nutrients, such as fatty acids, essential amino acids and dietary fibre have also shown to stimulate GLP-1 secretion.