Glucagon acts through its own receptors in the presence of functional glucagon-like peptide-1 receptors on hamster insulinoma (original) (raw)
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Glucagon-Like Peptide-1: Regulation of Insulin Secretion and Therapeutic Potential
Basic <html_ent glyph="@amp;" ascii="&"/> Clinical Pharmacology <html_ent glyph="@amp;" ascii="&"/> Toxicology, 2004
Glucagon-like peptide-1 (GLP-1) is an intestinally derived insulinotropic hormone currently under investigation for use as a novel therapeutic agent in the treatment of type 2 diabetes. One of several important effects of GLP-1 is on nutrient-induced pancreatic hormone release and is mediated by binding to a specific G-protein coupled receptor resulting in the activation of adenylate cyclase and an increase in cAMP generation. In the b-cell, cAMP binds and modulates activities of both protein kinase A and cAMP-regulated guanine nucleotide exchange factor II, thereby enhancing glucosedependent insulin secretion. The stimulatory action of GLP-1 on insulin secretion involves interaction with a plethora of signal transduction processes including ion channel activity, intracellular Ca 2π handling and exocytosis of the insulincontaining granules. In this review we focus principally on recent advances in our understanding on the cellular mechanisms proposed to underlie GLP-1's insulinotropic effect and attempt to incorporate this knowledge into a working model for the control of insulin secretion. Lastly, this review discusses the applicability of GLP-1 as a therapeutic agent for the treatment of type 2 diabetes.
FEBS Letters, 1988
We tested the truncated 7-37 glucagon-like peptide 1 (TGLP-1), a naturally occurring porcine intestinal peptide, and other members of the glucagon family, including pancreatic glucagon (G-29), GLP-1 and GLP-2 for their ability to activate the cAMP generating system in rat gastric glands and HGT-1 human gastric cancer cells. In rat fundic glands, TGLP-1 was about 100 times more potent (EC50 = 2.8 X 10(-9) M) than GLP-1 of G-29, and 10 times more potent than G-29 in the HGT-1 cell line. Our results support the notion that TGLP-1 plays a direct role in the regulation of acid secretion in rat and human gastric mucosa.
Signal transduction of the GLP-1-receptor cloned from a human insulinoma
FEBS Letters, 1994
GLP-1 (glucagon-like peptide 1 (7-36) amide) plays an important role in the regulation of insulin secretion and proinsulin gene expression of pancreatic @ells. Patients with insulinoma tumors show uncontrolled insulin hypersecretion. This study demonstrates the molecular cloning of a cDNA for the GLP-1 receptor from a human insulinoma employing a I-gtl 1 cDNA library. The cloned cDNA encoded a seven transmembrane domain protein of 463 amino acids which showed high homology to the GLP-I receptor in normal human pancreas. Four amino acid exchanges were found in comparison to a receptor sequence obtained from regular pancreatic islets. When transfected transiently into COS-7 or stably into fibroblast CHL cells a high affinity receptor was expressed which coupled to the adenylate cyclase with normal basal CAMP and increasing intracellular CAMP levels under GLP-1 stimulation. The receptor accepted GLP-1 and the non-mammalian agonist exendin-4 as high affinity ligands. In transfected COS-7 cells, GLP-1 did not influence intracellular calcium, whereas in the stably transfected fibroblasts GLP-1 transiently increased intracellular calcium to a small extent. The understanding of GLP-1 receptor regulation and signal transduction will aid in the discovery of compounds that act as agonists of the GLP-1 receptor for potential use in the treatment of diabetes and will facilitate the understanding of its expression under normal and pathophysiological conditions.
Glucagon-like peptide-1 (GLP-1) and glucose metabolism in human myocytes
Journal of Endocrinology, 2002
Glucagon-like peptide-1 (GLP-1) has been shown to have insulin-like effects upon the metabolism of glucose in rat liver, muscle and fat, and on that of lipids in rat and human adipocytes. These actions seem to be exerted through specific receptors which, unlike that of the pancreas, are not -at least in liver and muscle -cAMP-associated. Here we have investigated the effect, its characteristics, and possible second messengers of GLP-1 on the glucose metabolism of human skeletal muscle, in tissue strips and primary cultured myocytes. In muscle strips, GLP-1, like insulin, stimulated glycogen synthesis, glycogen synthase a activity, and glucose oxidation and utilization, and inhibited glycogen phosphorylase a activity, all of this at physiological concentrations of the peptide. In cultured myotubes, GLP-1 exerted, from 10 13 mol/l, a doserelated increase of the -[U-14 C]glucose incorporation into glycogen, with the same potency as insulin, together with an activation of glycogen synthase a; the effect of 10 11 mol/l GLP-1 on both parameters was additive to that induced by the equimolar amount of insulin. Synthase a was still activated in cells after 2 days of exposure to GLP-1, as compared with myotubes maintained in the absence of peptide. In human muscle cells, exendin-4 and its truncated form 9-39 amide (Ex-9) are both agonists of the GLP-1 effect on glycogen synthesis and synthase a activity; but while neither GLP-1 nor exendin-4 affected the cellular cAMP content after 5-min incubation in the absence of 3-isobutyl-1-methylxantine (IBMX), an increase was detected with Ex-9. GLP-1, exendin-4, Ex-9 and insulin all induced the prompt hydrolysis of glycosylphosphatidylinositols (GPIs). This work shows a potent stimulatory effect of GLP-1 on the glucose metabolism of human skeletal muscle, and supports the long-term therapeutic value of the peptide. Further evidence for a GLP-1 receptor in this tissue, different from that of the pancreas, is also illustrated, suggesting a role for an inositolphosphoglycan (IPG) as at least one of the possible second messengers of the GLP-1 action in human muscle. 173, 465-473 3 HOH release, and lactate production, were measured M A LUQUE and others · GLP-1 and human skeletal muscle 466
A Human Cellular Model for Studying the Regulation of Glucagon-Like Peptide-1 Secretion
Endocrinology, 2001
GLP-1 (glucagon-like peptide-1) is a potent insulin secretagogue released from L cells in the intestine. The regulation of GLP-1 secretion has been described both in vivo and in vitro in several animal species, but data from human cellular models are lacking. For this purpose, factors and cell-signaling pathways regulating GLP-1 secretion were investigated in the NCI-H716 human intestinal cell line. After differentiation, these cells homogeneously produced 16.8 pmol GLP-1/mg protein with a basal release of 4.2% during a 2-h incubation period. Nutrients, such as palmitic acid, oleic acid, and meat hydrolysate, stimulated GLP-1 secretion in a dose-dependent manner, as did the cholinergic agonist carbachol and the neu-romediator gastrin-releasing peptide. Along with stimulating GLP-1 release, gastrin-releasing peptide, like ionomycin, increased intracellular calcium levels. Activators of PKA and PKC were able to increase GLP-1 secretion in NCI-H716 cells. However, neither PKA activators nor meat hydrolysate increased proglucagon mRNA levels. These findings indicate that the NCI-H716 cell line constitutes a unique model to study the cellular mechanism of GLP-1 secretion in humans and suggest potential interspecies divergence in the regulation of proglucagon gene expression in enteroendocrine cells.
Diabetes, Obesity and Metabolism
Glucagon has a noble history in the annals of metabolic disease, 1 even though, to a layperson, insulin is its more famous counterregulatory partner. For decades medical students have been taught that glucagon raises blood glucose by increasing hepatic glucose output and that alleviation of hypoglycaemia is its primary function. 2 Thus, inhibition of glucagon secretion 3 or action 4 are logical approaches to the development of therapeutics that improve glycaemic control in both type 2 and type 1 diabetes mellitus; indeed, this strategy has been pursued for nearly 4 decades. The situation, however, is complex. The preproglucagon gene product can be cleaved at various points by specific prohormone convertases to yield several bioactive peptides, including glucagon, in a tissue-selective manner. 1 Prohormone convertase 2 is expressed in pancreatic islets to generate mainly glucagon in α cells. By contrast, prohormone convertase 1/3 processes proglucagon in the gut and brain to glucagon-like peptide-1 (GLP-1), glucagon-like peptide-2 (GLP-2), glicentin and oxyntomodulin. These boundaries are not absolute, however, and the relative abundance of these peptides in a tissue, and therefore its secretory potential, can vary depending on the differential expression of the processing enzymes. 5 There is evidence that GLP-1 is expressed in pancreatic islets, 6,7 while glucagon may be secreted by enteroendocrine cells in the gut. 8 Enteroendocrine cells also express other peptides such as peptide-tyrosinetyrosine (PYY), 9,10 and it now appears that secretion can be modulated by feedback inhibition by homotypic (GLP-1 reducing GLP-1 secretion) as well as heterotypic mechanisms (GLP-1 reducing PYY secretion). 11,12 Glucagon receptor pharmacology is also multifaceted. The classic
Endocrinology, 1998
Glucagon-like peptide-1(7-36)amide (GLP-1) is a potent insulinotropic peptide released from the small intestine. To investigate the regulation of GLP-1 secretion, we established a GLP-1 release assay based on primary canine intestinal L-cells. The ileal mucosa was digested with collagenase/EDTA to a single cell suspension and enriched for L-cells by counterstream centrifugal elutriation. We performed release assays on the cultured cells after 36 h, and GLP-1 in the supernatant was determined by enzyme-linked immunoabsorbent assay (ELISA). Glucose-dependent insulinotropic peptide (GIP) dose dependently stimulated the release of GLP-1 and resulted in a 2-fold increase at 100 nM GIP. This effect was fully inhibited by 10 nM somatostatin. However, neither basal or GIP stimulated GLP-1 secretion were affected by ambient glucose concentrations from 5-25 mM. The receptorindependent secretagogues  phorbol myristate acetate and forskolin dose dependently increased the secretion of GLP-1; effects inhibited by staurosporine and H8 respectively. Costimulation with GIP and phorbol ester, but not forskolin, resulted in an additive response. Furthermore, the effect of GIP could be inhibited by H8 but not by staurosporine. These results indicate that glucose does not directly stimulate canine L-cells. It is more probable that glucose releases GIP from the upper intestine that in turn stimulates GLP-1 secretion. The ability of GIP to stimulate GLP-1 secretion is probably mediated through activation of protein kinase A.