Biology of Incretins: GLP-1 and GIP

This review focuses on the mechanisms regulating the synthesis, secretion, biological actions, and therapeutic relevance of the incretin peptides glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1). The published literature was reviewed, with emphasis on recent advances in our understanding of the biology of GIP and GLP-1. GIP and GLP-1 are both secreted within minutes of nutrient ingestion and facilitate the rapid disposal of ingested nutrients. Both peptides share common actions on islet β-cells acting through structurally distinct yet related receptors. Incretin-receptor activation leads to glucose-dependent insulin secretion, induction of β-cell proliferation, and enhanced resistance to apoptosis. GIP also promotes energy storage via direct actions on adipose tissue, and enhances bone formation via stimulation of osteoblast proliferation and inhibition of apoptosis. In contrast, GLP-1 exerts glucoregulatory actions via slowing of gastric emptying and glucose-dependent inhibition of glucagon secretion. GLP-1 also promotes satiety and sustained GLP-1–receptor activation is associated with weight loss in both preclinical and clinical studies. The rapid degradation of both GIP and GLP-1 by the enzyme dipeptidyl peptidase-4 has led to the development of degradation-resistant GLP-1–receptor agonists and dipeptidyl peptidase-4 inhibitors for the treatment of type 2 diabetes. These agents decrease hemoglobin A1c (HbA1c) safely without weight gain in subjects with type 2 diabetes. GLP-1 and GIP integrate nutrient-derived signals to control food intake, energy absorption, and assimilation. Recently approved therapeutic agents based on potentiation of incretin action provide new physiologically based approaches for the treatment of type 2 diabetes. This review focuses on the mechanisms regulating the synthesis, secretion, biological actions, and therapeutic relevance of the incretin peptides glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1). The published literature was reviewed, with emphasis on recent advances in our understanding of the biology of GIP and GLP-1. GIP and GLP-1 are both secreted within minutes of nutrient ingestion and facilitate the rapid disposal of ingested nutrients. Both peptides share common actions on islet β-cells acting through structurally distinct yet related receptors. Incretin-receptor activation leads to glucose-dependent insulin secretion, induction of β-cell proliferation, and enhanced resistance to apoptosis. GIP also promotes energy storage via direct actions on adipose tissue, and enhances bone formation via stimulation of osteoblast proliferation and inhibition of apoptosis. In contrast, GLP-1 exerts glucoregulatory actions via slowing of gastric emptying and glucose-dependent inhibition of glucagon secretion. GLP-1 also promotes satiety and sustained GLP-1–receptor activation is associated with weight loss in both preclinical and clinical studies. The rapid degradation of both GIP and GLP-1 by the enzyme dipeptidyl peptidase-4 has led to the development of degradation-resistant GLP-1–receptor agonists and dipeptidyl peptidase-4 inhibitors for the treatment of type 2 diabetes. These agents decrease hemoglobin A1c (HbA1c) safely without weight gain in subjects with type 2 diabetes. GLP-1 and GIP integrate nutrient-derived signals to control food intake, energy absorption, and assimilation. Recently approved therapeutic agents based on potentiation of incretin action provide new physiologically based approaches for the treatment of type 2 diabetes. The concept that certain factors produced by the intestinal mucosa in response to nutrient ingestion are capable of stimulating the release of substances from the endocrine pancreas and thereby reducing blood glucose levels was first introduced in the early 1900s.1Bayliss W.M. Starling E.H. On the causation of the so-called ‘peripheral reflex secretion’ of the pancreas.Proc R Soc Lond Biol. 1902; 69: 352-353Google Scholar, 2Moore B. Edie E.S. Abram J.H. On the treatment of diabetes mellitus by acid extract of duodenal mucous membrane.Biochem J. 1906; 1: 28-38Crossref PubMed Google Scholar The term incretin subsequently was used to denote these glucose-lowering, intestinal-derived factors.3La Barre J. Sur les possibilites d’un traitement du diabete par l’incretine.Bull Acad R Med Belg. 1932; 12: 620-634Google Scholar With the development of the radioimmunoassay, this communication between the intestine and the endocrine pancreas was confirmed when it was shown that oral glucose administration is associated with a much greater increase in plasma insulin levels when compared with the same amount of glucose given intravenously.4McIntyre N. Holsworth D.C. Turner D.S. New interpretation of oral glucose tolerance.Lancet. 1964; 2: 20-21Abstract PubMed Google Scholar, 5Elrick H. Stimmler L. Hlad Jr, C.J. Arai Y. Plasma insulin response to oral and intravenous glucose administration.J Clin Invest. 1964; 24: 1076-1082Google Scholar This phenomenon has been dubbed the incretin effect, and is estimated to account for approximately 50%–70% of the total insulin secreted after oral glucose administration. Thus, incretins are hormones that are secreted from the gastrointestinal tract into the circulation in response to nutrient ingestion that enhances glucose-stimulated insulin secretion. The first incretin hormone to be identified was isolated from crude extracts of porcine small intestine and initially were named gastric inhibitory polypeptide (GIP), based on its ability to inhibit gastric acid secretion in dogs.6Brown J.C. Dryburgh J.R. Ross S.A. Dupre J. Identification and actions of gastric inhibitory polypeptide.Recent Prog Horm Res. 1975; 31: 487-532PubMed Google Scholar However, subsequent studies using more purified preparations of GIP revealed that GIP could also stimulate insulin secretion in animals and humans. Because the inhibitory effect of GIP on gastric acid secretion was seen only at pharmacologic doses, whereas its incretin action occurred at physiologic levels, GIP was renamed glucose-dependent insulinotropic polypeptide, to reflect its physiologic action yet retain the acronym. In accordance with its role as an incretin hormone, GIP is released from K-cells of the small intestine, primarily in response to glucose or fat ingestion, and potentiates glucose-stimulated insulin secretion. It was recognized, however, that GIP alone could not fully account for the incretin effect in vivo. This was based on the observations that immunoneutralization of endogenous GIP activity attenuates but does not abolish the incretin effect in rodents and in humans surgical resection of the ileum is associated with diminished incretin activity, despite preservation of normal plasma GIP levels.7Lauritsen K.B. Moody A.J. Christensen K.C. Lindkaer Jensen S. Gastric inhibitory polypeptide (GIP) and insulin release after small-bowel resection in man.Scand J Gastroenterol. 1980; 15: 833-840Crossref PubMed Google Scholar The discovery of a second incretin hormone, glucagon-like peptide-1 (GLP-1), followed the cloning and sequencing of mammalian proglucagon genes and complementary DNAs (cDNAs). In addition to glucagon, the proglucagon gene also encoded 2 peptides that were approximately 50% homologous to glucagon and thus aptly were named glucagon-like peptide-1 and glucagon-like peptide-2. Based on their homology to glucagon, both peptides were tested for insulinotropic activity, but only GLP-1 was capable of stimulating insulin secretion. GLP-1 is a tissue-specific posttranslational proteolytic product of the proglucagon gene that is released from intestinal L-cells in response to nutrient ingestion and enhances glucose-stimulated insulin secretion.8Mojsov S. Weir G.C. Habener J.F. Insulinotropin: glucagon-like peptide I (7-37) co-encoded in the glucagon gene is a potent stimulator of insulin release in the perfused rat pancreas.J Clin Invest. 1987; 79: 616-619Crossref PubMed Google Scholar, 9Kreymann B. Williams G. Ghatei M.A. Bloom S.R. Glucagon-like peptide-1 7-36: a physiological incretin in man.Lancet. 1987; 2: 1300-1304Abstract PubMed Google Scholar To date, only GIP and GLP-1 fulfill the definition of an incretin hormone in humans. Furthermore, studies have shown that these 2 peptides potentiate glucose-stimulated insulin secretion in an additive manner, likely contribute equally to the incretin effect, and together can fully account for the incretin effect in humans. The following sections provide an overview of GLP-1 and GIP structure, regulation, biological actions, and therapeutic potential for the treatment of type 2 diabetes (T2DM). The proglucagon gene is located on the long arm of human chromosome 2 and comprises 6 exons and 5 introns, with the entire coding sequence for GLP-1 contained within exon 4 (Figure 1A).10White J.W. Saunders G.F. Structure of the human glucagon gene.Nucl Acids Res. 1986; 14: 4719-4730Crossref PubMed Google Scholar The proglucagon gene is expressed in the α-cells of the endocrine pancreas, the L-cells of the intestine, and neurons located in the caudal brainstem and hypothalamus; mammalian proglucagon gene transcription generates a single messenger RNA (mRNA) transcript that is structurally identical in all 3 cell types (Figure 1B).11Mojsov S. Heinrich G. Wilson I.B. Ravazzola M. Orci L. Habener J.F. Preproglucagon gene expression in pancreas and intestine diversifies at the level of post-translational processing.J Biol Chem. 1986; 261: 11880-11889Abstract Full Text PDF PubMed Google Scholar, 12Drucker D.J. Asa S. Glucagon gene expression in vertebrate brain.J Biol Chem. 1988; 263: 13475-13478Abstract Full Text PDF PubMed Google Scholar In the pancreas, proglucagon gene expression is up-regulated by fasting and hypoglycemia and is inhibited by insulin. Activation of the protein kinase C (PKC) signaling pathway increases islet proglucagon mRNA levels and activators of the cAMP/protein kinase A (PKA) pathway stimulate pancreatic proglucagon gene transcription via a cAMP response element located within the proglucagon gene promoter. Membrane depolarization and calcium influx also stimulate proglucagon gene transcription in islet cells. Gastrin stimulates proglucagon gene expression in glucagon-producing pancreatic cells that stably express the cholecystokinin-2 (CCK-2) receptor in an Egr-1–dependent manner; however, levels of proglucagon mRNA transcripts are normal in gastrin-/- mice. Transgenic mouse studies have indicated that approximately 1.3 kb of rat proglucagon gene 5′-flanking sequences are sufficient to direct pancreatic α-cell– and brain-specific rat proglucagon gene expression.13Efrat S. Teitelman G. Anwar M. Ruggiero D. Hanahan D. Glucagon gene regulatory region directs oncoprotein expression to neurons and pancreatic alpha cells.Neuron. 1988; 1: 605-613Abstract Full Text PDF PubMed Google Scholar Specific sequences located within the proximal promoter region of the rodent proglucagon gene bind the transcription factors Pax-6, Foxa1, Cdx-2/3, Isl-1, Brn4, and c-Maf to mediate pancreatic α-cell–specific proglucagon gene expression. Pax-6 and Cdx-2/3 associate with p300, a co-activator protein, to synergistically regulate islet-specific proglucagon gene transcription. Genetic inactivation of the murine Pax-6 gene results in defective formation of islet cell lineages,14St-Onge L. Sosa-Pineda B. Chowdhury K. Mansouri A. Gruss P. Pax6 is required for differentiation of glucagon-producing a-cells in mouse pancreas.Nature. 1997; 387: 406-409Crossref PubMed Scopus (507) Google Scholar and mice that are homozygous for a dominant-negative version of Pax-6 (small eye [SEYNeu]) have significant reductions in pancreatic proglucagon mRNA transcript levels.15Sander M. Neubuser A. Kalamaras J. Ee H.C. Martin G.R. German M.S. Genetic analysis reveals that PAX6 is required for normal transcription of pancreatic hormone genes and islet development.Genes Dev. 1997; 11: 1662-1673Crossref PubMed Google Scholar Mice with genetic inactivation of the Foxa1 (hepatocyte nuclear factor 3α [HNF-3α]) gene are hypoglycemic and exhibit reduced pancreatic proglucagon mRNA transcript and plasma glucagon levels, implicating a key role for Foxa1 in pancreatic proglucagon gene expression. However, in contrast, the importance of Brn4 for proglucagon gene regulation is unclear because Brn4 potently activates pancreatic α-cell–specific proglucagon gene expression in vitro, but mice with targeted inactivation of the Brn4 gene have normal α-cell development and pancreatic proglucagon mRNA levels.16Heller R.S. Stoffers D.A. Liu A. Schedl A. Crenshaw 3rd, E.B. Madsen O.D. Serup P. The role of Brn4/Pou3f4 and Pax6 in forming the pancreatic glucagon cell identity.Dev Biol. 2004; 268: 123-134Crossref PubMed Scopus (45) Google Scholar Sequences within the 5′-flanking region immediately upstream of the proglucagon gene promoter contain islet cell–specific enhancer-like elements and bind the transcription factor Beta2/NeuroD as well as members of the Foxa, HNF, and Ets families of transcription factors to enhance or repress proglucagon gene expression in a tissue-specific manner. Insulin-mediated inhibition of pancreatic α-cell–specific proglucagon gene expression is regulated via an insulin-responsive element located within the gene promoter, as well as through synergistic interactions between proximal promoter elements and more distal enhancer-like elements. RNA silencing and overexpression studies have shown that insulin inhibits proglucagon gene expression in α-cells via nuclear exclusion of the transcription factor FoxO1.17McKinnon C.M. Ravier M.A. Rutter G.A. FoxO1 is required for the regulation of preproglucagon gene expression by insulin in pancreatic alpha -(TC1-9) cells.J Biol Chem. 2006; 281: 39358-39369Crossref PubMed Scopus (18) Google Scholar Studies using primary intestinal cell cultures or transformed enteroendocrine tumor cell lines have shown that, similar to proglucagon gene expression in the pancreas, the level of intracellular cAMP and activation of cAMP/PKA signaling are major determinants of intestinal proglucagon gene expression.18Drucker D.J. Brubaker P.L. Proglucagon gene expression is regulated by a cyclic AMP-dependent pathway in rat intestine.Proc Natl Acad Sci U S A. 1989; 86: 3953-3957Crossref PubMed Scopus (105) Google Scholar, 19Drucker D.J. Jin T. Asa S.L. Young T.A. Brubaker P.L. Activation of proglucagon gene transcription by protein kinase-A in a novel mouse enteroendocrine cell line.Mol Endocrinol. 1994; 8: 1646-1655Crossref PubMed Google Scholar, 20Brubaker P.L. Schloos J. Drucker D.J. Regulation of glucagon-like peptide-1 synthesis and secretion in the GLUTag enteroendocrine cell line.Endocrinology. 1998; 139: 4108-4114Crossref PubMed Google Scholar The Wnt signaling pathway is a potential mediator of PKA-dependent proglucagon gene transcription in the intestine and Wnt signaling mediates proglucagon gene expression in L-cells via expression of the transcription factor TCF-4.21Yi F. Brubaker P.L. Jin T. TCF-4 mediates cell type-specific regulation of proglucagon gene expression by beta-catenin and glycogen synthase kinase-3beta.J Biol Chem. 2005; 280: 1457-1464Crossref PubMed Scopus (181) Google Scholar Increased levels of cAMP may up-regulate intestinal proglucagon gene transcription by activation of PKA or via the cAMP-regulated guanine nucleotide exchange factor II exchange protein directly activated by cAMP (Epac2)/mitogen-activated protein kinase (MAPK) pathway. A primary regulator of intestinal proglucagon gene expression in vivo is nutrient ingestion.22Rountree D.B. Ulshen M.H. Selub S. Fuller C.R. Bloom S.R. Ghatei M.A. Lund P.K. Nutrient-independent increases in proglucagon and ornithine decarboxylase messenger RNAs after jejunoileal resection.Gastroenterology. 1992; 103: 462-468PubMed Google Scholar Fasting reduces whereas refeeding stimulates proglucagon gene expression in the rat intestine,23Hoyt E.C. Lund P.K. Winesett D.E. Fuller C.R. Ghatei M.A. Bloom S.R. Ulshen M.H. Effects of fasting, refeeding, and intraluminal triglyceride on proglucagon expression in jejunum and ileum.Diabetes. 1996; 45: 434-439Crossref PubMed Google Scholar and diets that are high in fiber24Reimer R.A. McBurney M.I. Dietary fiber modulates intestinal proglucagon messenger ribonucleic acid and postprandial secretion of glucagon-like peptide-1 and insulin in rats.Endocrinology. 1996; 137: 3948-3956Crossref PubMed Scopus (118) Google Scholar or short-chain fatty acids25Tappenden K.A. Thomson A.B. Wild G.E. McBurney M.I. Short-chain fatty acids increase proglucagon and ornithine decarboxylase messenger RNAs after intestinal resection in rats.JPEN J Parenter Enteral Nutr. 1996; 20: 357-362Crossref PubMed Google Scholar increase intestinal proglucagon mRNA levels. Gastrin-releasing peptide (GRP) and GIP increase intestinal proglucagon mRNA levels in mouse enteroendocrine cells and primary fetal rat intestinal cultures, respectively. Surgical removal of portions of the small bowel is associated with increased proglucagon mRNA levels in the remnant intestine. Adenoviral-mediated overexpression of the transcription factor Pax-6 in primary intestinal cultures or rat colonic epithelium is associated with enhanced endogenous proglucagon gene expression,26Trinh D.K. Zhang K. Hossain M. Brubaker P.L. Drucker D.J. Pax-6 activates endogenous proglucagon gene expression in the rodent gastrointestinal epithelium.Diabetes. 2003; 52: 425-433Crossref PubMed Scopus (26) Google Scholar whereas homozygous mice that express the dominant-negative (SEYNeu) form of Pax-6 exhibit significant reductions in proglucagon mRNA levels in the small and large intestines.27Hill M.E. Asa S.L. Drucker D.J. Essential requirement for Pax6 in control of enteroendocrine proglucagon gene transcription.Mol Endocrinol. 1999; 13: 1474-1486Crossref PubMed Google Scholar Hence, Pax-6 is essential for proglucagon gene expression in the intestine and pancreas. In contrast to proglucagon gene expression in the endocrine pancreas and brain, transgenic mouse studies suggest that a much larger region (≈2.3 kb) of rat proglucagon 5′-flanking sequences is required for proglucagon gene expression in the pancreas, brain, and intestine,28Lee Y.C. Asa S.L. Drucker D.J. Glucagon gene 5′-flanking sequences direct expression of SV40 large T antigen to the intestine producing carcinoma of the large bowel in transgenic mice.J Biol Chem. 1992; 267: 10705-10708Abstract Full Text PDF PubMed Google Scholar indicating that DNA sequences located between -2.3 and -1.3 kb in the rat proglucagon promoter are important for specifying intestinal proglucagon gene expression. The sequences situated between -2.3 and -1.3 kb have been designated the proglucagon gene upstream enhancer element, and cell transfection and electrophoretic mobility shift assay (EMSA) studies using enteroendocrine cell lines have indicated that the proglucagon gene upstream enhancer element is composed of multiple positive and negative cis-acting DNA regulatory subdomains and plays an integrative role in regulating tissue-specific proglucagon gene transcription.29Jin T. Drucker D.J. The proglucagon gene upstream enhancer contains positive and negative domains important for tissue-specific proglucagon gene transcription.Mol Endocrinol. 1995; 9: 1306-1320Crossref PubMed Google Scholar Although the majority of studies to date have focused primarily on the regulation of proglucagon gene expression in rodents, a limited number of studies have examined transcriptional regulation of the human proglucagon gene. In transgenic mice, approximately 1.6 kb of human proglucagon gene 5′-flanking sequences can direct proglucagon gene transcription to the brain and intestine, but not pancreatic islets,30Nian M. Drucker D.J. Irwin D. Divergent regulation of human and rat proglucagon gene promoters in vivo.Am J Physiol. 1999; 277: G829-G837PubMed Google Scholar whereas transfection of rodent islet cell lines with human proglucagon promoter-reporter plasmids indicates that sequences within the first 6 kb of the human proglucagon gene 5′-flanking region are required for pancreas-specific gene expression.30Nian M. Drucker D.J. Irwin D. Divergent regulation of human and rat proglucagon gene promoters in vivo.Am J Physiol. 1999; 277: G829-G837PubMed Google Scholar A combination of cell transfection and transgenic reporter studies have identified a conserved region within intron 1, designated ECR3, as critical for expression of the human proglucagon gene in islet α-cells. These studies suggest that the human proglucagon gene likely uses a distinct set of transcription factors and DNA sequences to specify tissue-specific proglucagon gene transcription. The proglucagon mRNA is translated into a single 180 amino acid precursor protein that undergoes tissue-specific posttranslational processing to yield specific peptide profiles in the pancreas, intestine, and brain (Figure 1C and D).11Mojsov S. Heinrich G. Wilson I.B. Ravazzola M. Orci L. Habener J.F. Preproglucagon gene expression in pancreas and intestine diversifies at the level of post-translational processing.J Biol Chem. 1986; 261: 11880-11889Abstract Full Text PDF PubMed Google Scholar Although several prohormone convertase (PC) enzymes have been identified, only PC1/3 and PC2 appear to be important for proglucagon processing.31Rouille Y. Martin S. Steiner D.F. Differential processing of proglucagon by the subtilisin-like prohormone convertases PC2 and PC3 to generate either glucagon or glucagon-like peptide.J Biol Chem. 1995; 270: 26488-26496Crossref PubMed Scopus (99) Google Scholar In pancreatic α-cells, the predominant proglucagon posttranslational processing products are glicentin-related polypeptide, glucagon, intervening peptide-1, and the major proglucagon fragment (Figure 1D). Glucagon, the major counterregulatory hormone to insulin, regulates hepatic glucose production via activation of glycogenolysis and gluconeogenesis and inhibition of glycolysis, and is essential for maintaining glucose homeostasis in the fasting state. The physiologic importance of glucagon for blood glucose regulation is exemplified by the hypoglycemic phenotype of mice that harbor a targeted inactivation of the glucagon receptor gene.32Parker J.C. Andrews K.M. Allen M.R. Stock J.L. McNeish J.D. Glycemic control in mice with targeted disruption of the glucagon receptor gene.Biochem Biophys Res Commun. 2002; 290: 839-843Crossref PubMed Scopus (72) Google Scholar, 33Gelling R.W. Du X.Q. Dichmann D.S. Romer J. Huang H. Cui L. Obici S. Tang B. Holst J.J. Fledelius C. Johansen P.B. Rossetti L. Jelicks L.A. Serup P. Nishimura E. Charron M.J. Lower blood glucose, hyperglucagonemia, and pancreatic alpha cell hyperplasia in glucagon receptor knockout mice.Proc Natl Acad Sci U S A. 2003; 100: 1438-1443Crossref PubMed Scopus (162) Google Scholar To date, no physiologic actions have been identified for glicentin-related polypeptide, intervening peptide-1, or major proglucagon fragment. Islet α-cell–specific posttranslational processing of proglucagon to glucagon is mediated, at least in part, by PC2, and mice that lack active PC2 exhibit mild hypoglycemia and deficient processing of proglucagon to mature glucagon.34Furuta M. Yano H. Zhou A. Rouille Y. Holst J.J. Carroll R. Ravazzola M. Orci L. Furuta H. Steiner D.F. Defective prohormone processing and altered pancreatic islet morphology in mice lacking active SPC2.Proc Natl Acad Sci U S A. 1997; 94: 6646-6651Crossref PubMed Scopus (265) Google Scholar Posttranslational processing of proglucagon in enteroendocrine L-cells and the central nervous system (CNS) liberates glicentin, oxyntomodulin, GLP-1, intervening peptide-2, and GLP-2 (Figure 1D). The physiologic actions of glicentin are not well defined but it exerts trophic effects in the rodent small intestine.35Myojo S. Tsujikawa T. Sasaki M. Fujiyama Y. Bamba T. Trophic effects of glicentin on rat small-intestinal mucosa in vivo and in vitro.J Gastroenterol. 1997; 32: 300-305Crossref PubMed Google Scholar Oxyntomodulin inhibits gastrointestinal secretion and motility and stimulates pancreatic enzyme secretion and intestinal glucose uptake.36Schjoldager B. Mortensen P.E. Myhre J. Christiansen J. Holst J.J. Oxyntomodulin from distal gut Role in regulation of gastric and pancreatic functions.Dig Dis Sci. 1989; 34: 1411-1419Crossref PubMed Google Scholar More recent studies in rodents and humans have identified roles for oxyntomodulin in promoting satiety and regulating intrinsic heart rate.37Dakin C.L. Gunn I. Small C.J. Edwards C.M. Hay D.L. Smith D.M. Ghatei M.A. Bloom S.R. Oxyntomodulin inhibits food intake in the rat.Endocrinology. 2001; 142: 4244-4250Crossref PubMed Scopus (157) Google Scholar, 38Baggio L.L. Huang Q. Brown T.J. Drucker D.J. Oxyntomodulin and glucagon-like peptide-1 differentially regulate murine food intake and energy expenditure.Gastroenterology. 2004; 127: 546-558Abstract Full Text Full Text PDF PubMed Scopus (155) Google Scholar, 39Sowden G.L. Drucker D.J. Weinshenker D. Swoap S.J. Oxyntomodulin increases intrinsic heart rate in mice independent of the glucagon-like peptide-1 receptor.Am J Physiol. 2006; 292: R962-R970Google Scholar GLP-1 exerts a number of actions that are important for regulating glucose homeostasis (described in detail later). To date, no physiologic actions have been identified for intervening peptide-2. GLP-2 stimulates cell proliferation and inhibits apoptosis in the intestinal crypt compartment.40Drucker D.J. Erlich P. Asa S.L. Brubaker P.L. Induction of intestinal epithelial proliferation by glucagon-like peptide 2.Proc Natl Acad Sci U S A. 1996; 93: 7911-7916Crossref PubMed Scopus (415) Google Scholar GLP-2 also up-regulates intestinal glucose transport, improves intestinal barrier function, and inhibits food intake,41Tang-Christensen M. Larsen P.J. Thulesen J. Romer J. Vrang N. 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Extrahypothalamic expression of the glucagon-like peptide-2 receptor is coupled to reduction of glutamate-induced cell death in cultured hippocampal cells.Endocrinology. 2004; 145: 3495-3506Crossref PubMed Scopus (28) Google Scholar The prohormone convertase PC1/3 has been localized to intestinal L-cells and shown to be both necessary and sufficient for posttranslational processing of proglucagon in the intestine.44Rothenberg M.E. Eilertson C.D. Klein K. Zhou Y. Lindberg I. McDonald J.K. Mackin R.B. Noe B.D. Processing of mouse proglucagon by recombinant prohormone convertase 1 and immunopurified prohormone convertase 2 in vitro.J Biol Chem. 1995; 270: 10136-10146Crossref PubMed Scopus (53) Google Scholar, 45Rouille Y. Kantengwa S. Irminger J.C. Halban P.A. Role of the prohormone convertase PC3 in the processing of proglucagon to glucagon-like peptide 1.J Biol Chem. 1997; 272: 32810-32816Crossref PubMed Scopus (54) Google Scholar PC1/3 null mice exhibit increased levels of intestinal proglucagon accompanied by marked decreases in proglucagon processing to glicentin, oxyntomodulin, GLP-1, and GLP-2.46Ugleholdt R. Poulsen M.L. Holst P.J. Irminger J.C. Orskov C. Pedersen J. Rosenkilde M.M. Zhu X. Steiner D.F. Holst J.J. Prohormone convertase 1/3 is essential for processing of the glucose-dependent insulinotropic polypeptide precursor.J Biol Chem. 2006; 281: 11050-11057Crossref PubMed Scopus (27) Google Scholar The prohormone convertase enzymes responsible for the posttranslational processing of proglucagon in the CNS are not well established; however, high levels of PC1/3 and PC2 are present throughout the CNS, including the hypothalamus, where neurons that express proglucagon also can be found. GLP-1 is secreted from intestinal endocrine L-cells, which are located mainly in the distal ileum and colon. In contrast, GIP is released from intestinal K-cells that are localized to more proximal regions (duodenum and jejunum) of the small intestine. However, endocrine cells that produce GLP-1 or GIP, as well as cells that produce both peptides, can be found throughout all regions of the porcine, rat, and human small intestine.47Mortensen K. Christensen L.L. Holst J.J. Orskov C. GLP-1 and GIP are colocalized in a subset of endocrine cells in the small intestine.Regul Pept. 2003; 114: 189-196Crossref PubMed Scopus (141) Google Scholar, 48Theodorakis M.J. Carlson O. Michopoulos S. Doyle M.E. Juhaszova M. Petraki K. Egan J.M. Human duodenal enteroendocrine cells: source of both incretin peptides, GLP-1 and GIP.Am J Physiol. 2006; 290: E550-E559Google Scholar The L-cell is an open-type intestinal epithelial endocrine cell that directly contacts luminal nutrients through its apical surface and neural and vascular tissue through its basolateral surface. Accordingly, GLP-1 secretion from intestinal L-cells is stimulated by a variety of nutrient, neural, and endocrine factors. Meal ingestion, particularly one rich in fats and carbohydrates, is the primary physiologic stimulus for GLP-1 secretion.49Brubaker P.L. The glucagon-like peptides: pleiotropic regulators of nutrient homeostasis.Ann N Y Acad Sci. 2006; 1070: 10-26Crossref PubMed Scopus (32) Google Scholar GLP-1 release can be stimulated by mixed meals or individual nutrien