GLP-2 has a number of actions in the intestine including:
Stimulation of mucosal growth in the small and large intestine
Inhibition of enterocyte and crypt cell apoptosis
Stimulation of enterocyte glucose transport and GLUT-2 expression
Increased nutrient absorption
Inhibition of gastric emptying and gastric acid secretion
Reduction of intestinal permeability
Stimulation of intestinal blood flow
Relaxation of intestinal smooth muscle
Gastrointestinal actions of GLP-2
What are the molecular targets for GLP-2 action? The IGF-1 system is important for GLP-2 action in mice, as GLP-2 induces the expression of IGF-1 in vitro and IGF-1-/- mice exhibit markedly impaired responses to GLP-2 in vivo. Furthermore, IGF-2-/- mice also exhibited an impairment of selective GLP-2 actions as described in The essential role of insulin-like growth factor-1 in the intestinal tropic effects of glucagon-like Peptide-2 in mice. Gastroenterology. 2006 Aug;131(2):589-605. Rowland and colleagues provided further information about the importance of the intestinal IGF-1 system for GLP-2 action through generation and analysis of mice with selective intestinal deletion of the IGF-1 receptor using the vilin-Cre system. The majority of actions of GLP-2 that are traditionally associated with an induction of mucosal growth were preserved in mice with intestinal-specific Igf-1R deletion, however the ability of GLP-2 to stimulate crypt cell proliferation was reduced in these mice Loss of glucagon-like peptide-2-induced proliferation following intestinal epithelial insulin-like growth factor-1-receptor deletion. Gastroenterology. 2011 Dec;141(6):2166-2175.e7.
Murali and colleagues studied IGF-1 and GLP-2 action in mice with combined disruption of IGFBP-3 and IGFBP-5, the key IGF binding proteins that potentiate the action of IGF-1 in vivo. The double KO mice exhibited lower plasma levels of IGF-1, and increased mass of the small and large bowel. No effect of exogenous IGF-1 or GLP-2 on colonic mass was detected, however GLP-2, but not IGF-1 significantly increased small bowel mass and crypt cell proliferation in IGFBP3/5-/- mice. GLP-2 had no effect on the muscularis layer in WT or KO mice. GLP-2 reduced the intestinal expression of IGF-1, IGF1R, proglucagon and the Glp2r and increased expression of epiregulin in KO mice Exogenous GLP-2 and IGF-I induce a differential intestinal response in IGF binding protein-3 and -5 double knockout mice Am J Physiol Gastrointest Liver Physiol. 2012 Jan 26.
GLP-2 also appears to exert effects on the gut independent of IGF-1 or KGF action. GLP-2 administration in mice produces a spectrum of action comparable to that described for EGF, including stimulation of crypt cell proliferation, and both EGF and GLP-2 activate a number of common downstream targets in the small and large bowel. Moreover, GLP-2 , but not IGF-1 or KGF activates a subset of ErbB family members in the murine gut, and many of the actions of GLP-2 in the GI tract are significantly diminished or eliminated following elimination of ErbB signaling. See ErbB signaling is required for the proliferative actions of GLP-2 in the murine gut doi:10.1053/j.gastro.2009.05.057
Infusion of GLP-2 in TPN-fed rats resulted in upregulation of sucrase-isomaltase gene expression in the small bowel, but no change in the levels of cdx-2, a key transcription factor that regulates SI gene expression. Hence, the mechanism for these findings remains uncertain, but is likely related to the GLP-2-induction of a coordinated molecular program of small bowel epithelial growth. To review the data, see Am J Physiol Gastrointest Liver Physiol 2000 Mar;278(3):G425-G428 Glucagon-like peptide-2 increases sucrase-isomaltase but not caudal-related homeobox protein-2 gene expression.
GLP-2 and nutrient absorption
Of the actions in the GI tract delineated above, the most rapid effects detected following GLP-2 infusion are localized to the enterocyte, with enhanced hexose transport mediated in part by increased GLUT-2 localization to the basolateral membrane in the rat. See Upregulation of SGLT-1 transport activity in rat jejunum induced by GLP-2 infusion in vivo. Am J Physiol. 1997 Dec;273(6 Pt 2):R1965-71 and Basolateral D-glucose transport activity along the crypt-villus axis in rat jejunum and upregulation induced by gastric inhibitory peptide and glucagon-like peptide-2. Exp Physiol. 1998 Sep;83(5):605-16 and Rapid insertion of GLUT2 into rat Jejunal brush-border membrane promoted by glucagon-like peptide 2. Biochem J. 2002 Jul 3
Similarly, GLP-2 rapidly promotes absorption of enteral lipids, and increases secretion of triglyceride (TG)-rich lipoprotein (TRL)-apoB48. GLP-2 also stimulated secretion of TRL-rich particles from jejunal fragments. The actions of GLP-2 on intestinal lipid synthesis/secretion were diminished in CD36-/- mice. See Glucagon-like Peptide-2 Increases Intestinal Lipid Absorption and Chylomicron Production via CD36 Gastroenterology. 2009 May 28. [Epub ahead of print]
Anti-inflammatory actions of GLP-2
A large body of evidence demonstrates activation of GLP-2R signaling attenuates experimental inflammation, particularly in preclinical models of intestinal injury. These actions are most likely indirect, as GLP-2 has not been demonstrated to act directly on cells in the immune system. Several studies have implicated a role for VIP as a downstream anti-inflammatory mediator of GLP-2 action in the setting of experimental inflammation. Sigalet and colleagues demonstrated that GLP-2 activated a subset of VIP+ neurons in rats with TNBS- or DS-inducec colitis, and the anti-inflammatory actions of GLP-2 were significantly attenuated by co-administration of a VIP peptide hybrid antagonist Enteric neural pathways mediate the anti-inflammatory actions of glucagon-like peptide 2 Am J Physiol Gastrointest Liver Physiol. 2007 Jul;293(1):G211-21. Similarly, GLP-2 administration preserved the number of enteric neurons and Vip+ neurons in rats with TNBS colitis and increased the number of VIP+ neurons in control rats without inflammation The effects of glucagon-like peptide 2 on enteric neurons in intestinal inflammation Neurogastroenterol Motil. 2010 Dec;22(12):1318-e350. doi: 10.1111/j.1365-2982.2010.01585.x
Yusta and colleagues examined the importance of the Vip gene for the intestinal actions of GLP-2 in Vip-/- mice. Surprisingly, Vip-/- mice exhibit enhanced cypt cell proliferation, crypt compartment hyperplasia, reduced lysozyme expression, and abnormal intervillous bridging. These phenotypes were not reproduced by treatment of WT mice with a VIP antagonist, nor were they reversed by treatment of Vip-/- mice with VIP. Furthermore, Vip-/- mice exhibited evidence for compensation with increased intestinal growth factor and PACAP gene expression. Exogenous GLP-2 administration increased intestinal gene expression, ErbB ligands and stimulated crypt cell proliferation and gut growth in both Vip+/+ and Vip-/- mice. Surprisingly, GLP-2 did not attenuate intestinal inflammation or prevent weight loss in mice with DS colitis, independent of Vip genotype. Intestinotrophic Glucagon-Like Peptide-2 (GLP-2) Activates Intestinal Gene Expression and Growth Factor-Dependent Pathways Independent of the Vasoactive Intestinal Peptide Gene in Mice Endocrinology April 24, 2012 en.2012-1069
Does GLP-2 have a role in the maturation of the fetal or neonatal gut? The GLP-2 receptor is expressed in the fetal gut and treatment of neonatal rats, from the first day of birth, with GLP-2, promotes enhanced gut growth. Similarly, treatment of premature pigs, infused with TPN, with GLP-2, also enhanced DNA synthesis, increased villus height, reduced apoptosis and reduced proteolysis in the developing pig gut. See GLP-2 stimulates intestinal growth in premature TPN-fed pigs by suppressing proteolysis and apoptosis AJP Vol. 279, Issue 6, G1249-G1256, December 2000.
In contrast, although the GLP-2 receptor is expressed in the fetal pig and rat intestine, biologically active GLP-2 (1-33) is not detected in the circulation of 98-day gestation fetuses, but is detectable by day 115. Furthermore, exogenous GLP-2 did not stimulate growth of the fetal pig gut, but was trophic to the neonatal pig intestine. These findings imply that GLP-2 is unlikely to play a major if any role in gut development. To review these elegant experiments, see GLP-2 has differential effects on small intestine growth and function in fetal and neonatal pigs. Am J Physiol Regul Integr Comp Physiol. 2001 Dec;281(6):R1986-93
Administration of GLP-2 twice daily alone or in combination with dexamethasone to rats beginning at day 11 after birth for 10 days produced selective increases in lipid absorption assessed ex vivo. Animals were analyzed at postnatal day 21 and at 7 weeks of age. Although no change in body weight was observed with GLP-2 alone, GLP-2 prevented the reduction in body weight observed with dexamethasone treatment. Similarly, GLP-2 alone had no effect on intestinal or mucosal weight. GLP-2 alone did not change lipid uptake in the jejunum or ileum of suckling or weanling rats. See Treatment of suckling rats with GLP-2 plus dexamethasone increases the ileal uptake of fatty acids in later life. Am J Physiol Gastrointest Liver Physiol. 2005 Jan;288 (1): G54-9
GLP-2 also stimulates the induction of PC2/TIS7, an immediate early gene, in both IEC cells cultured in vitro, and in the murine small bowel following exogenous [Gly2]-GLP-2 treatment in vivo. The molecular mechanisms coupling GLP-2 activated signaling pathways to induction of PC2/TIS7 expression remain to be determined. See Growth factor regulation of PC4/TIS7, an immediate early gene expressed during gut adaptation after resection. JPEN J Parenter Enteral Nutr. 2003 Mar-Apr;27(2):123-31.
Studies in rodents have demonstrated that GLP-2 improves survival and decreases bacterial infection following intestinal injury. Experiments from the lab of Mary Perdue in Hamilton now show that GLP-2 reduces intestinal permeability via effects on the transcellular and paracellular pathways in mice. GLP-2 increases the number of microvilli and increases cell length. Some of the functional effects on permeability were rapid and detected as early as 4 hours after GLP-2 administration. Subsequent studies employing a murine model of hypersensitivity and antigen sensitization demonstrated GLP-2 reduced permeability, decreased the number of inflammatory cells, and attenuated intestinal macromolecular uptake. See Gut 2000;47:112-119 ( July ) Glucagon-like peptide-2 enhances intestinal epithelial barrier function of both transcellular and paracellular pathways in the mouse. Gut. 2000 Jul;47(1):112-9 and Glucagon-like peptide-2-enhanced barrier function reduces pathophysiology in a model of food allergy. Am J Physiol Gastrointest Liver Physiol. 2003 Jun;284(6):G905-G912. Although GLP-2 reduces gut permeability in NOD mice, it does not prevent the development of autoimmune diabetes Glucagon like peptide-2 reduces intestinal permeability but does not modify the onset of type 1 diabetes in the non obese diabetic mouse. Endocrinology. 2009 Feb;150(2):592-9. Epub 2008 Oct 9
GLP-2 may also play a role in the development of experimental inflammation via effects on gut permeability as outlined in studies of ob:ob mice. Administration of the GLP-2 antagonist GLP-2(3-33) abolished the beneficial effects of prebiotics on inflammatory mediators in ob:ob mice, whereas treatment of ob:ob mice with GLP-2 reduced plasma levels of LPS, decreased circulating levels of inflammatory cytokines, and upregulated expression, as assessed by immunohistochemistry of tight junction proteins occludin and zonula occludens-1, and plasma levels of FTIC dextran. These findings invoke a role for endogenous and exogenous GLP-2 action in modulation of gut permeability, indirectly influencing the level of systemic inflammation in obese mice. See Changes in gut microbiota control inflammation in obese mice through a mechanism involving GLP-2-driven improvement of gut permeability Gut. 2009 Aug;58(8):1091-103
Does GLP-2 exert trophic effects in humans? An indirect answer to this question derives from the historical observation that a single patient with a glucagon-producing tumor presented clinically with massive small bowel enlargement, as described in Endocrine tumour in kidney affecting small bowel structure, motility, and absorptive function. Gut. 1971 Oct;12(10):773-82. To review the data on human patients with glucagonomas and intestinal growth, see GLP-2 and Human Tumors. Subsequent studies with native GLP-2 or GLP-2 analogues such as teduglutide are consistent with the notion that activation of GLP-2R signaling also promotes an intestinotrophich response in the human small bowel. Teduglutide (ALX-0600), a dipeptidyl peptidase IV resistant glucagon-like peptide 2 analogue, improves intestinal function in short bowel syndrome patient Gut. 2005 Sep;54(9) :1224-31
Extra-intestinal actions of GLP-2
Islets and glucose homeostasis
Although plasma glucose does not change following GLP-2 administration in rodents or humans, pharmacological levels of GLP-2 (~10-fold higher than normal) are associated with increased circulating levels of glucagon in the fasted and postprandial state in normal human subjects, as shown in Glucagon-like Peptide 2 stimulates glucagon secretion, enhances lipid absorption, and inhibits gastric Acid secretion in humans. Gastroenterology. 2006 Jan;130(1):44-54. Moreover, deHeer and colleagues have demonstrated GLP-2 receptor expression in rat islets by RT-PCR and have demonstrated localization of GLP-2R immunoreactive protein to α-cells in human and rat islets by immunocytochemistry. Perfused rat pancreas experiments demonstrated that 10 nM GLP-2 stimulates glucagon secretion See The alpha cell expresses glucagon-like peptide-2 receptors and glucagon-like peptide-2 stimulates glucagon secretion from the rat pancreas. Diabetologia. 2007 Oct;50(10):2135-42
Subsequent studies examined the importance of GLP-2 action for control of islet function and beta cell mass in lean, high fat fed, diabetic, and obese mice. Although no significant phenotype was observed in characterization of glucose homeotasis in lean, HFD, or STZ Glp2r-/- mice, ob/ob:Glp2r-/- mice exhibited a modest deterioration in glucose tolerance, a-cell hyperplasia, increased glucagon secretion, and reduced b-cell mass relative to ob/ob:Glp2r+/+ mice. A trend towards a reductin in gut barrier function was observed in ob/ob:Glp2r-/- mice, suggesting that increased islet inflammation may have contributed to these findings. See The glucagon-like peptide-2 receptor modulates islet adaptation to metabolic stress in the ob/ob mouse Gastroenterology 2010 Sep;139(3):857-68
GLP-2 and bone
A single injection of GLP-2 inhibited the liberation of C-terminal telopeptide region of type I collagen, a marker of bone resorption, in postmenopausal women. Similarly, the levels of urine DPD/creatinine, a marker of bone resorption, were also significantly reduced in patients receiving a single 800-microg dose of GLP-2. Hence GLP-2 pharmacologically modulates bone resorption, as outlined in Role of gastrointestinal hormones in postprandial reduction of bone resorption. J Bone Miner Res. 2003 Dec;18(12):2180-9
The effect of GLP-2 on bone turnover at different time points during the 24h cycle has been examined in part in short term studies of healthy female postmenopausal subjects. GLP-2 administration produced a dose-related reduction of s-CTX, a marker of bone resorption, and osteocalcin levels were increased. Hence, acute GLP-2 administration may have effects on bone turnover that spans both inhibition of resorption, and possibly, stimulation of bone formation. See Reduction of nocturnal rise in bone resorption by subcutaneous GLP-2. Bone. 2004 Jan;34(1):140-7. The predominant effect of GLP-2 on inhibition of bone resorption was confirmed in a 14 day study of postmenopausal women who received either 1.6 or 3.2 mg of GLP-2 as a subcutaneous injection. GLP-2 significantly inhibited bone resorption as measured by nocturnal serum and urine concentrations of fragments derived from the degradation of the C-terminal telopeptide region of collagen type I (s-CTX and u-CTX) and u-DPD. In contrast, no change in levels of bone formation, serum osteocalcin and procollagen type I N-terminal propeptide (PINP), were observed following GLP-2 treatment. See Disassociation of bone resorption and formation by GLP-2 A 14-day study in healthy postmenopausal women. Bone. 2006 Oct 31; [Epub ahead of print]
A longer and larger study examined the effects of GLP-2 administration at doses of 0.4 mg, 1.6 mg and 3.2 mg GLP-2, administered nightly for 4 months in postmenopausal women with osteoporosis. GLP-2 treatment reduced the nocturnal rise in the bone resorption marker s-CTX, but had no effect on osteocalcin, a marker of bone formation. Treatment with GLP-2 modestly but significantly increased total hip BMD in a dose-dependent manner Four-Month Treatment with GLP-2 Significantly Increases Hip BMD A randomized, placebo-controlled, dose-ranging study in postmenopausal women with low BMD Bone. 2009 Nov;45(5):833-42
GLP-2 and control of blood flow
Infusion of GLP-2 in 20 day old TPN-fed piglets to achieve pharmacological levels of GLP-2 (~ 400 pM) produces a rapid increase in portal-visceral blood flow (detectable within 10 minutes) and intestinal blood volume. GLP-2 also increased intestinal cNOS protein and activity and co-infusion of GLP-2 and L-NAME abrogated the stimulation of blood flow or intestinal glucose uptake. See GLP-2-mediated up-regulation of intestinal blood flow and glucose uptake is nitric oxide-dependent in TPN-fed piglets 1. Gastroenterology. 2003 Jul;125(1):136-147. The actions of GLP-2 to stimulate blood flow are more evident in the duodenum and jejunum but not in the distal small bowel or colon. High dose GLP-2 (2000 pmol/kg/hr) also increased pancreatic blood flow in pigs, consistent with a predominant effect localized the the superior mesenteric artery Glucagon-like peptide-2 acutely increases proximal small intestinal blood flow in TPN-fed neonatal piglets Am J Physiol Regul Integr Comp Physiol. 2006 Feb; 290(2):R283-9. The co-localization of eNOS and the GLP-2R in subsets of porcine enteric neurons, together with the detection of increased eNOS protein in porcine jejunum provide further evidence for the importance of eNOS as a downstream target for GLP-2 action GLP-2 receptor localizes to enteric neurons and endocrine cells expressing vasoactive peptides and mediates increased blood flow Gastroenterology. 2006 Jan;130(1):150-64.
GLP-2 infusion also increased SMA blood flow in anesthetized rats. Modest increases in blood flow were also seen in the carotid artery. L-NAME only partially abolished the effect of GLP-2 on blood flow and indomethacin, CCK antagonists or the GLP-1 antagonist exendin(9-39) did not attenuate the effect of GLP-2 on SMA blood flow. Mediators of glucagon-like peptide 2-induced blood flow: responses in different vascular sites Regul Pept. 2007 Jul 5;142(1-2):7-15.
The effects of GLP-2 on blood flow have also been demonstrated in healthy human subjects in the fasting state, where intravenous or s.c GLP-2 administration rapidly increased SMA blood flow, to the same extent as meal ingestion Glucagon-like peptide-2 increases mesenteric blood flow in humans Scand J Gastroenterol. 2009;44(3):314-9.
Whether GLP-2 exerts actions on the heart independent of changes in blood flow remains unclear. Bremholm and colleagues injected native GLP-2 into 10 healthy subjects and assessed cardiovascular function. Although modest changes in cardiac output and HR were observed, these changes were not significantly different from placebo-treated subjects and thought to be secondary to increases in SMA blood flow. The effect of glucagon-like peptide-2 on arterial blood flow and cardiac parameters Regul Pept. 2010 Jan 8;159(1-3):67-71.
The ability of GLP-2 to augment blood flow has also been demonstrated in 8 human subjects with short bowel syndrome, with less than 200 cm of remnant small bowel. The effect of GLP-2 to acutely augment blood flow was proportional to the length of remnant intestine, and occurred predominantly in the SMA, with little change in the celiac artery and no significant changes in cardiac hemodynamics, although small trends were observed. The effect of Glucagon-Like Peptide-2 on mesenteric blood flow and cardiac parameters in end-jejunostomy short bowel patients Regul Pept. 2011 Mar 21. [Epub ahead of print]
Cardiovascular actions of GLP-2
Angelone and colleagues reported the expression of the GLP-2 receptor in rat heart using a combination of Real Time PCR and Western blotting, using an antibody from MBL International Corporation. Remarkably, a very strong single band was obtained in Western blotting using extracts frm rat heart and intestine. Infusion of GLP-2 in an isolated Langendorff preparation increased LV contractlity at the lowest dose tested, then progressively reduced LV contractility at higher doses, associated with a decreased in cardiac levels of cAMP, reduced PLB(phospholamban) phosphorylation and increased Erk1/2 phosphorylation. Receptor identification and physiological characterisation of glucagon-like peptide-2 in the rat heart Nutr Metab Cardiovasc Dis. 2012 Jun;22(6):486-94.
A related study from the same research group examined the effects of GLP-2 administration on recovery of LV function post ischemia in the isolated rat heart. In contrast to the study reported above, infusion of GLP-2 immediately after induction of ischemia in the reperfusion phase was associated with a significnt reduction of infarct size, and associated with a robust increase in LV function. These salutary effects were blocked by multiple kinase inhibitors
Postconditioning with glucagon like peptide-2 reduces ischemia/reperfusion injury in isolated rat hearts: role of survival kinases and mitochondrial KATP channels. Basic Res Cardiol. 2012 Jul;107(4):272
GLP-2 action in the lung
Arda-Pirincci and colleagues assessed the effects of teduglutide (200 ug/kg every 12 hrs) administered for 10 days) in Balb/C mice with lung injury pursuant to administration of TNF-a and actinomycin-D. Teduglutide pre-treatment reduced the pulmonary damage relative to findings in mice treated with TNF-a and actinomycin-D alone. The authors used immunocytochemistry and an anti-GLP-2 receptor antibody from Chemicon to identify immunopositive cells for the GLP-2R in type 2 pneumocytes and mesenchymal cells in connective tissue. No effect of teduglutide was detected in healthy lungs from control mice. Teduglutide, a glucagon-like peptide 2 analogue: A novel protective agent with anti-apoptotic and anti-oxidant properties in mice with lung injury Peptides. 2012 Oct 8. pii: S0196-9781(12)00419-6
Does GLP-2 regulate calcium metabolism or bone turnover?
In the pilot study of GLP-2 administration to human subjects with short bowel syndrome, 400 ug GLP-2 twice daily for 5 weeks reduced stomal calcium excretion, increased bone mass and a trend to increased calcium absorption was noted in 6 patients. A F/U paper reported parameters of calcium absorption and bone turnover in more detail in these patients. GLP-2 treatment increased spinal bone mineral density (BMD), and increased calcium absorption in patients not receiving parenteral nutrition. Markers of bone turnover decreased in most of the GLP-2-treated patients. See Short-term administration of glucagon-like peptide-2. Effects on bone mineral density and markers of bone turnover in short-bowel patients with no colon. Scand J Gastroenterol. 2002 Apr;37(4):392-8
GLP-2 actions on intestinal smooth muscle
GLP-2 produces concentration-dependent relaxation of murine gastric muscle, actions that were reduced by pharmacological blockade of neuronal N-type voltage operated calcium channels, and by a VIP receptor antagonist. GLP-2 also produced direct relaxation of carbachol-precontracted muscle strips from the mouse fundus. See GLUCAGON-LIKE PEPTIDE-2 RELAXES MOUSE STOMACH THROUGH VASOACTIVE INTESTINAL PEPTIDE RELEASE. Am J Physiol Gastrointest Liver Physiol. 2009;296 G678-G684