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Journal ArticleDOI

Impaired intestinal proglucagon processing in mice lacking prohormone convertase 1.

01 Mar 2004-Endocrinology (Endocrine Society)-Vol. 145, Iss: 3, pp 1349-1355

TL;DR: It is strongly suggested that PC1 is essential for intestinal proglucagon processing in vivo and, thereby, plays an important role in production of the incretin hormone GLP-1 and the intestinotrophic hormoneGLP-2.

AbstractThe neuroendocrine prohormone convertases 1 and 2 (PC1 and PC2) are expressed in endocrine intestinal L cells and pancreatic A cells, respectively, and colocalize with proglucagon in secretory granules. Mice lacking PC2 have multiple endocrinopathies and cannot process proglucagon to mature glucagon in the pancreas. Disruption of PC1 results in dwarfism and also multiple neuroendocrine peptide processing defects. This study compares the pancreatic and intestinal processing of proglucagon in mice lacking PC1 expression with that in age-matched wild-type controls. Because proglucagon was found to precipitate in acidic extracts, the intestinal processing profile was analyzed in both acidic and neutral extracts by gel filtration, HPLC, and RIA. Supporting a central role for PC2 in glucagon biosynthesis, we found normal processing of proglucagon to glucagon in the pancreas, whereas the intestinal proglucagon processing showed marked defects. Tissue proglucagon levels in null mice were elevated, and proglucagon processing to glicentin, oxyntomodulin, and glucagon-like peptide-1 and -2 (GLP-1 and GLP-2) was markedly decreased, indicating that PC1 is essential for the processing of all the intestinal proglucagon cleavage sites. This includes the monobasic site R(77) and, thereby, production of mature, biologically active GLP-1. We also found elevated glucagon levels, suggesting that factors other than PC1 that are capable of processing to mature glucagon are present in the secretory granules of the L cell. These findings strongly suggest that PC1 is essential for intestinal proglucagon processing in vivo and, thereby, plays an important role in production of the incretin hormone GLP-1 and the intestinotrophic hormone GLP-2.

Topics: Proglucagon (78%), Glucagon-like peptide-1 (73%), Glucagon-like peptide-2 (70%), Proprotein convertase 1 (62%), Glucagon (59%)

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Citations
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Journal ArticleDOI
TL;DR: The main actions of GLP-1 are to stimulate insulin secretion and to inhibit glucagon secretion, thereby contributing to limit postprandial glucose excursions and acts as an enterogastrone and part of the "ileal brake" mechanism.
Abstract: Glucagon-like peptide 1 (GLP-1) is a 30-amino acid peptide hormone produced in the intestinal epithelial endocrine L-cells by differential processing of proglucagon, the gene which is expressed in ...

2,389 citations


Journal ArticleDOI
TL;DR: Two new classes of drugs based on incretin action have been approved for lowering blood glucose levels in type 2 diabetes (T2DM) and an incretIn enhancer (sitagliptin, which is a DPP4 inhibitor).
Abstract: Incretins are gut hormones that are secreted from enteroendocrine cells into the blood within minutes after eating. One of their many physiological roles is to regulate the amount of insulin that is secreted after eating. In this manner, as well as others to be described in this review, their final common raison d'etre is to aid in disposal of the products of digestion. There are two incretins, known as glucose-dependent insulinotropic peptide (GIP) and glucagon-like peptide-1 (GLP-1), that share many common actions in the pancreas but have distinct actions outside of the pancreas. Both incretins are rapidly deactivated by an enzyme called dipeptidyl peptidase 4 (DPP4). A lack of secretion of incretins or an increase in their clearance are not pathogenic factors in diabetes. However, in type 2 diabetes (T2DM), GIP no longer modulates glucose-dependent insulin secretion, even at supraphysiological (pharmacological) plasma levels, and therefore GIP incompetence is detrimental to beta-cell function, especially after eating. GLP-1, on the other hand, is still insulinotropic in T2DM, and this has led to the development of compounds that activate the GLP-1 receptor with a view to improving insulin secretion. Since 2005, two new classes of drugs based on incretin action have been approved for lowering blood glucose levels in T2DM: an incretin mimetic (exenatide, which is a potent long-acting agonist of the GLP-1 receptor) and an incretin enhancer (sitagliptin, which is a DPP4 inhibitor). Exenatide is injected subcutaneously twice daily and its use leads to lower blood glucose and higher insulin levels, especially in the fed state. There is glucose-dependency to its insulin secretory capacity, making it unlikely to cause low blood sugars (hypoglycemia). DPP4 inhibitors are orally active and they increase endogenous blood levels of active incretins, thus leading to prolonged incretin action. The elevated levels of GLP-1 are thought to be the mechanism underlying their blood glucose-lowering effects.

640 citations


Cites background from "Impaired intestinal proglucagon pro..."

  • ...In taste cells of the tongue, however, both PC1/3 and PC2 (as well as 7B2) are present; consequently, GLP-1, GLP-2, and glucagon are all present (Shin et al., 2008)....

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  • ...It is noteworthy that Wideman et al. (2006) recently demonstrated that up-regulation of PC1/3 expression using adenovirus system in pancreatic cells leads to increase of islet GLP-1 secretion, resulting in improved glucose-stimulated insulin secretion and enhanced survival of islets in response to cytokine treatment....

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  • ...In addition to being present in K cells, where, as stated above, it is required for the cleavage of proGIP, PC1/3 is also present in intestinal L cells and is responsible for the processing of proglucagon to GLP-1 (and GLP-2) (Rothenberg et al., 1995; Rouillé et al., 1995) (Fig....

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  • ...In addition to encoding glucagon, the proglucagon gene encodes two glucagon-like peptides that have approximately 50% amino acid homology to glucagon; these are designated GLP-1 and glucagon-like peptide-2 (GLP-2, which is not insulinotropic, has no glucose-lowering properties, and is therefore not an incretin)....

    [...]

  • ...Similar effects have been observed also in PC1/3-expressing cells derived from mice lacking active PC2 (Wideman et al., 2007)....

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Journal ArticleDOI
TL;DR: The islet cell composition and organization are described in different species and serve as a basis for understanding how the numerous paracrine, hormonal, and nervous signals fine-tune glucagon secretion under different physiological conditions.
Abstract: Glucagon, a hormone secreted from the alpha-cells of the endocrine pancreas, is critical for blood glucose homeostasis. It is the major counterpart to insulin and is released during hypoglycemia to induce hepatic glucose output. The control of glucagon secretion is multifactorial and involves direct effects of nutrients on alpha-cell stimulus-secretion coupling as well as paracrine regulation by insulin and zinc and other factors secreted from neighboring beta- and delta-cells within the islet of Langerhans. Glucagon secretion is also regulated by circulating hormones and the autonomic nervous system. In this review, we describe the components of the alpha-cell stimulus secretion coupling and how nutrient metabolism in the alpha-cell leads to changes in glucagon secretion. The islet cell composition and organization are described in different species and serve as a basis for understanding how the numerous paracrine, hormonal, and nervous signals fine-tune glucagon secretion under different physiological conditions. We also highlight the pathophysiology of the alpha-cell and how hyperglucagonemia represents an important component of the metabolic abnormalities associated with diabetes mellitus. Therapeutic inhibition of glucagon action in patients with type 2 diabetes remains an exciting prospect.

506 citations


Journal ArticleDOI
TL;DR: It is demonstrated that in mouse stomach, prohormone convertase 1/3 (PC1/3) is the endoprotease responsible for the conversion of proghrelin to ghrelin, and the acylation of this peptide is processing-independent.
Abstract: The orexigenic hormone ghrelin is a 28-amino-acid peptide derived from a 99-amino-acid precursor and acylated at Ser-3, which was initially isolated from rat stomach. In addition to stimulating appetite and growth, it also plays various important roles in energy homeostasis and in the cardiovascular and immune systems. Although analysis of its physiological effects has yielded many significant results, much less information is available on its biosynthesis and the mechanism of its acylation. In this report, we have studied the endoproteolytic processing of this molecule from its precursor (proghrelin) into mature ghrelin in various prohormone convertase null mouse strains generated in our laboratory and have identified the convertase responsible for this event. Using Western blotting, mass spectrometry, and immunocytochemical methods, we have demonstrated that (a) in mouse stomach, prohormone convertase 1/3 (PC1/3) is the endoprotease responsible for the conversion of proghrelin to ghrelin, (b) the acylation of this peptide is processing-independent, and (c) the expression of proghrelin mRNA is increased in the processing-deficient (PC1/3 null) mouse.

199 citations


Journal ArticleDOI
TL;DR: Sustained augmentation of GLP-2R signaling reduces the requirement for parenteral nutrition in human subjects with short-bowel syndrome and integrates nutrient-derived signals to optimize mucosal integrity and energy absorption.
Abstract: Glucagon-like peptide-2 (GLP-2) is a 33-amino-acid proglucagon-derived peptide secreted from enteroendocrine L cells. GLP-2 circulates at low basal levels in the fasting period, and plasma levels rise rapidly after food ingestion. Renal clearance and enzymatic inactivation control the elimination of bioactive GLP-2. GLP-2 increases mesenteric blood flow and activates proabsorptive pathways in the gut, facilitating nutrient absorption. GLP-2 also enhances gut barrier function and induces proliferative and cytoprotective pathways in the small bowel. The actions of GLP-2 are transduced via a single G protein–coupled receptor (GLP-2R), expressed predominantly within the gastrointestinal tract. Disruption of GLP-2R signaling increases susceptibility to gut injury and impairs the adaptive mucosal response to refeeding. Sustained augmentation of GLP-2R signaling reduces the requirement for parenteral nutrition in human subjects with short-bowel syndrome. Hence GLP-2 integrates nutrient-derived signals to optimiz...

196 citations


Cites background from "Impaired intestinal proglucagon pro..."

  • ...Prohormone convertase 1 (PC1) liberates GLP-2 from proglucagon in the intestine; mice and humans with inactivating mutations in PC1 exhibit reduced levels of circulating mature GLP-2(1–33) and enteropathies (1, 2)....

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  • ...The anti-inflammatory actions of GLP-2 (reduced intestinal cytokine expression and decreased myeloperoxidase activity) in a rat model of trinitrobenzene sulfonic acid (TNBS)-induced colitis were attenuated by coadministration of the VIP hybrid antagonist Lys(1)-Pro(2,5)-Arg(3,4)-Tyr(6)VIP(7–28) (83); conversely, GLP-2 increased the number of VIP+ neurons in the ileal submucosal plexus....

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References
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Journal ArticleDOI
01 Apr 1994-Diabetes
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855 citations


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TL;DR: The major endoproteolytic processing enzymes of the secretory pathway are the subtilisin-like proprotein convertases (SPCs) and Furin, and recent X-ray studies of the peptidyl alpha-hydroxylating monooxygenase component of PAM have shed new light on the role of copper in the mechanism of this reaction.
Abstract: The major endoproteolytic processing enzymes of the secretory pathway are the subtilisin-like proprotein convertases (SPCs). Furin (SPC1) has emerged as one of the major processing enzymes of the constitutive secretory pathway and its localization in the trans-Golgi network and mechanism of autoactivation have been studied in considerable detail. Recent gene disruption experiments and the study of naturally-occurring mutations underscore the importance of PC2 (prohormones convertase 2, or SPC2) and PC1/PC3 (prohormone convertase 1/3, or SPC3) in the processing of a wide variety of hormone and neuropeptide precursors. The role of Carboxypeptidase E (CPE) in the removal of carboxy-terminal basic residues exposed by the endoproteases was shown to be necessary for efficient endoproteolytic processing of proinsulin and several other protein precursors. Many biologically active peptides are also amidated after their proteolytic processing by peptidylglycine alpha-amidating monooxygenase (PAM) and recent X-ray studies of the peptidyl alpha-hydroxylating monooxygenase component of PAM have shed new light on the role of copper in the mechanism of this reaction.

661 citations


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641 citations



Journal ArticleDOI
TL;DR: The data indicate that glucagon is essential for maintenance of normal glycemia and postnatal regulation of islet and α and δ cell numbers and the lean phenotype of Gcgr−/− mice suggests glucagon action may be involved in the regulation of whole body composition.
Abstract: Glucagon, the counter-regulatory hormone to insulin, is secreted from pancreatic α cells in response to low blood glucose. To examine the role of glucagon in glucose homeostasis, mice were generated with a null mutation of the glucagon receptor (Gcgr−/−). These mice display lower blood glucose levels throughout the day and improved glucose tolerance but similar insulin levels compared with control animals. Gcgr−/− mice displayed supraphysiological glucagon levels associated with postnatal enlargement of the pancreas and hyperplasia of islets due predominantly to α cell, and to a lesser extent, δ cell proliferation. In addition, increased proglucagon expression and processing resulted in increased pancreatic glucogen-like peptide 1 (GLP-1) (1–37) and GLP-1 amide (1–36 amide) content and a 3- to 10-fold increase in circulating GLP-1 amide. Gcgr−/− mice also displayed reduced adiposity and leptin levels but normal body weight, food intake, and energy expenditure. These data indicate that glucagon is essential for maintenance of normal glycemia and postnatal regulation of islet and α and δ cell numbers. Furthermore, the lean phenotype of Gcgr−/− mice suggests glucagon action may be involved in the regulation of whole body composition.

489 citations