Although the principal focus of GLP-1 research has been on pleiotropic actions that ultimately converge on regulation of nutrient intake and disposal through effects on CNS satiety centers, gastrointestinal motility, islet function and β cell growth, the GLP-1 receptor is widely expressed in several tissues not considered classic metabolic regulators of energy homeostasis, such as the heart, kidney, blood vessels. intestine, immune cells and the lungs. Our understanding of GLP-1 actions in these tissues is incomplete, but continues to expand.
GLP-1 and the gastrointestinal tract
Koehler and colleagues demonstrated that GLP-1R agonists, including liraglutide and exenatide, were potent inducers of small and large bowel growth in mice, findings consistent with previous reports from Kissow and Deacon. Furthermore, elevated levels of endogenous GLP-1, as occur in Gcgr-/- mice, substantially contributes to small and large bowel growth in this model, as double knockout Gcgr-/-:Glp1r-/- mice exhibit significantly reduced bowel growth, most evident in the colon, compared to Gcgr-/- mice alone. The mechanism through which GLP-1R signaling promotes expansion of the intestinal mucosa is not via crypt cell proliferation, but rather through promotion of crypt fission. Notably, the actions of GLP-1R agonists to promote gut growth were additive in combination with GLP-2, albeit through different mechanisms. The mechanism through which GLP-1R signaling promotes gut growth proceeds via Fgf7. Microarray analyses revealed that Fgf7 and exendin-4 co-regulated the expression of several dozen genes, including genes regulating intestinal growth, in the murine gut. GLP-1R agonists induce Fgf7 expression in the gut and in colonic polyps and failed to induce small or large bowel growth in Ffg7-/- mice. In contrast, both EGF and GLP-2 robustly stimulaed gut growth in Fgf7-/- mice. Although the GLP-1R is robustly expressed in small and large bowel intraepithelial lymphocytes (IELs), bone marrow transplantation revealed that the IEL GLP-1R does not mediate the growth responses of GLP-1R agonists. In contrast, reconstitution of GLP-1R+ IELs in Glp1r-/- mice almost completely normalized the previously abnormal intestinal proinflammatory gene expression signature. Although the human GLP1R was not overexpressed in RNA samples from several dozen human colon cancers, GLP-1R agonists significantly increased the number of polyps in ApcMin+/- mice, whereas genetic loss of the Glp1r reduced the number of polyps in Glp1r-/-:ApcMin+/- mice. See GLP-1R agonists promote normal and neoplastic intestinal growth through mechanisms requiring Fgf7 Cell Metabolism 2015. 21;379-391
GLP-1 also regulates/inhibits gut motility in multiple species. Kunkel and colleagues administered exenatide, 5 ug twice daily to five patients with short bowel syndrome and less than 90 cm of residual small bowel. In this retrospecive analysis, exenatide reduced diarrhea, normalized small bowel motility as assessed by manometry, and 3 patients were able to discontinue their requirements for TPN. Remarkably, the patients discontinued TPN on the same day they started exenatide. The authors attributed the therapeutic effects of 'GLP-1' therapy to the anti-motility effects of exenatide. Little information was provided about quantitative energy balance prior to or during exenatide use in this report See Efficacy of the glucagon-like peptide-1 agonist exenatide in the treatment of short bowel syndrome Neurogastroenterol Motil. 2011 May 10. doi: 10.1111/j.1365-2982.2011.01723.x.
Kissow and colleagues examined the gastrointestinal tract in normal CD1 mice, or after administration of the carcinogen 1,2 dimethylhyrazine (DMH-once weekly for 12 weeks), after treatment with GLP-1R agonists, DPP-4 inhibitors (sitagliptin), or a GLP-2 analogue. Liraglutide was given as 15 mg/kg twice daily, exenatide as 625 ug/kg twice daily, and h[Gly]2-GLP-2 as 1.25 mg/kg twice daily. Sitagliptin was administered as 8 mg per mouse per os twice daily. Liraglutide, exenatide and h[Gly]2-GLP-2 all increased small bowel weight, with the effect of liraglutide and h[Gly]2-GLP-2 similar in magnitude; after 30 days, both h[Gly]2-GLP-2 and liraglutide increased colon weight. For all 3 peptides, the trophic effect was most pronounced in the small bowel. After administration of the carcinogen, DMH, h[Gly2]-GLP-2 unexpectedly decreased the numbers of ACF, but exhibited rats increased numbers of non-malignant adenomas and mucin-depleted foci. Liraglutide did not increase the numbers of ACF or adenomas. Glucagon-like peptide-1 (GLP-1) receptor agonists or DPP-4 inhibition does not accelerate neoplasia in carcinogen treated mice Regul Pept. 2012 Sep 15. pii: S0167-0115(12)00232-7.
Kissow also compared the actions of liraglutide, (600 and 300 ug twice daily), vs. a GLP-2R agonist NNC-103-0066. NNC-103-0066 is a GLP-2 analogue with two substitutions, Leu17 has been replaced by Lys, and Lys30 has been replaced by Arg, as compared to the native GLP-2. Furthermore, the analogue is acylated with a β-alanine (spacer) and a C16 fatty acid at the ε-amino group of Lys17. NNC-103-0066 was administered 25 ug twice daily, in mice with mucositis induced by intraperitoneal 5-FU. A subset of mice also received exendin(9-39), which blunted recovery of mucosal weight in mice given 5-FU, but paradoxiclly increased small bowel weight in mice without chemotherapy administration. GLP-2(3-33) had no effect on parameters of gut growth or injury in 5-FU-treated mice. NNC-103-0066 prevented chemotherapy associated weight loss, whereas all liraglutide-treated mice lost significant (more than 10%) amounts of weight. Both liraglutide and NNC-103-0066 increased small bowel weight, villus height, and cross-sectional area and decreased mucositis scores after 5-FU. The mechanisms for the protective effects of liraglutide or NNC-103-0066 were not identified Glucagon-like peptide-1 as a treatment for chemotherapy-induced mucositis Gut. 2012 Oct 20.
GLP-1 and human mesenchymal stem cells
The GLP-1 receptor was detected by RT-PCR in mesenchymal stem cells isolated from "normal" human bone marrow. Short term 24h administration of GLP-1 did not affect gene expression of stem cell markers. GLP-1 added to hMSCs during the differentiation process reduced the expression of C/EBPb, LPL, and PPARg; GLP-1 also exerted detectable effects on cell proliferation of undifferentiated cells and reduced apoptosis in serum-starved cells in this culture system SIGNALLING AND BIOLOGICAL EFFECTS OF GLUCAGON-LIKE PEPTIDE 1 (GLP-1) ON THE DIFFERENTIATION OF MESENCHYMAL STEM CELLS FROM HUMAN BONE MARROW. Am J Physiol Endocrinol Metab. 2009 Dec 29. [Epub ahead of print]
GLP-1, plasma lipids, and lipid absorption
Very little is currently known about the effects of GLP-1 on levels of circulating lipids, lipid biosynthesis, or lipid clearance. Qin and colleagues examined the effects of intravenous GLP-1 on lipid metabolism in rats following intraduodenal lipid infusion. GLP-1 reduced lymph flow, inhibited intestinal triolein absorption, and reduced lymphatic apolipoprotein output. See GLP-1 reduces intestinal lymph flow, triglyceride absorption, and apolipoprotein production in rats. Am J Physiol Gastrointest Liver Physiol. 2005 May;288(5):G943-9.
GLP-1R agonists and DPP-4 inhibitors also reduce postprandial lipoprotein levels in hamsters mice and humans, independent of changes in body weight, predominantly through inhibition of intestinal chylomicron synthesis. These actions of GLP-1 are mediated through the known GLP-1 receptor, and are direct actions on the gut. Furthermore, transient blockade of GLP-1R signaling with exendin(9-39), or genetic elimination of GLP-1R signaling in Glp1r-/- mice, results in enhanced appearance of postprandial triglyceride rich lipoproteins. Hence, basal GLP-1R signaling is essential for intestinal control of chylomicron synthesis in vivo. See The glucagon-like peptide 1 receptor is essential for postprandial lipoprotein synthesis and secretion in hamsters and mice. Diabetologia. 2009 Dec 3. [Epub ahead of print] These findings are also consistent with analysis of postprandial lipoproteins in human diabetic subjects treated with vildagliptin Vildagliptin therapy reduces postprandial intestinal triglyceride-rich lipoprotein particles in patients with type 2 diabetes Diabetologia. 2006 Sep;49(9):2049-57. Similar findings were obtained by Tremblay and colleagues who studied 36 subjects with T2DM randomized to treatment with sitagliptin or placebo for 6 weeks, following which an oral lipid tolerance test was performed. Sitagliptin treatment reduced HbA1c from 6.9-6.6%, decreased postprandial levels of ApoB, apoB-48, TGs, VLDL cholesterol, FFAs and glucose, in association with increased levels of active GLP-1 and GIP and reduced levels of glucagon but did not change levels of insulin or C-peptide Effect of sitagliptin therapy on postprandial lipoprotein levels in patients with type 2 diabetes Diabetes Obes Metab. 2011 Jan 12. doi: 10.1111/j.1463-1326.2011.01362.
Parlevliet employed 4 weeks of therapy using either CNTO3649, a long-acting GLP-1R agonist, or exendin-4 to demonstrate that GLP-1R activation reduces hepatic VLDL synthesis in high fat fed APOE*3-Leiden transgenic mice. GLP-1R activation reduced fasting glucose and insulin, decreased hepatic triglyceride content, with little effect on body weight. The contribution of intestinal chylomicron/lipid delivery to the liver was not examined, and whether the mechanisms coupling GLP-1R activation to reduced hepatic VLDL secretion were direct or indirect was not examined.
Eliasson and colleagues examined the effects of placebo (24), alogliptin 25 mg alone (25), or alogliptin plus pioglitazone (22)in patients with type 2 diabetes treated with the different agents for 16 weeks. Reductions in A1c were greater with combination of pioglitazone/alogliptin (0.95% reduction) then with alogliptin alone (0.39%). Alogliptin alone reduced fasting triglyceride levels by 0.56 mM, whereas the Alo/pio combination reduced triglycerides by 0.81 mM. Fasting total and LDL cholesterol did not change significantly in any of the groups. Both alogliptin alone and alogliptin/pioglitaone produced significant reductions in postprandial triglycerides, chylomicrons, and VLDL triglycerides, detectable within 4 weeks of therapy. Postprandial ApoB48 levels and VLDL1 ApoB48 and apoB-100 were reduced to a greater extent by alogliptin alone. Lowering of postprandial lipids in individuals with type 2 diabetes treated with alogliptin and/or pioglitazone: a randomised double-blind placebo-controlled study Diabetologia. 2012 Jan 12. [Epub ahead of print
Acute administration of a single 10 ug dose of exenatide to subjects with IGT or recently diagnosed T2DM produced highly significant reductions in postprandial triglycerides, apoB48 and CIII, and RLP-cholesterol and triglyceride. The mechanisms mediating this effect in humans require further study Improvement of postprandial endothelial function after a single dose of exenatide in individuals with impaired glucose tolerance and recent-onset type 2 diabetes. Atherosclerosis. 2010 May 25. [Epub ahead of print] and are discussed in an accompanying Editorial Newly appreciated therapeutic effect of GLP-1 receptor agonists: Reduction in postprandial lipemia Atherosclerosis. 2010 Jun 25.
The consequences of acute GLP-1R activation on triglyceride synthesis and clearance in normal healthy non-diabetic human subjects was reported by Xiao and colleagues. A duodenal tube was placed in the proximal small bowel for continuous nutrient delivery distal to the stomach, eliminating the acute actions of exenatide on gastric emptying. Somatostatin was infused to clamp levels of insulin and glucagon, allowing for a direct assessment of the actions of exenatide on TRLs independent of changes in islet hormones. Continuous exenatide infusion had no effect on the appearance or disappearance of ApoB100-enriched TRLs, but significantly reduced the appearance, but not the clearance of ApoB48 TRLs. These findings are consistent with preclinical studies demonstrating inhibition of the secretion and appearance of ApoB48-containing TRLs with GLP-1R agonists or DPP-4 inhibitors. See Exenatide, a Glucagon-like Peptide Receptor Agonist, Acutely Inhibits Intestinal Lipoprotein Production in Healthy Humans Arterioscler Thromb Vasc Biol. 2012 Apr 5. [Epub ahead of print]
The suppression of the meal-stimulated increase in plasma chylomicrons and triglycerides has also been reported after therapy with liraglutide. Administration of increasing doses of liraglutide (0.6, 1.2 and 1.8 mg daily) for 3 weeks to human subjects with T2D suppressed levels of postprandial trigylcerides and apoB48 after a fat rich meal independent of changes in gastric emptying Liraglutide suppresses postprandial triglyceride and apolipoprotein B48 elevations after a fat-rich meal in patients with type 2 diabetes: A randomized, double-blind, placebo-controlled, crossover trial Diabetes Obes Metab. 2013 May 17. doi: 10.1111/dom.12133
Farr and colleagues examined the importance of acute central nevous system GLP-1 action for control of intestinal lipoprotein release in regular chow fed fat loaded Syrian hamsters. Acute icv administration of either exendin-4 or MK-0626 (DPP4 inhibitor) reduced levels of TRL and ApoB48 particles; the actions of exendin-4, but curiously, not MK-0626, were blocked by icv co-administration of the GLP-1R antagonist exendin(9-39). The inhibitory actions of exendin-4 were also blocked by peripheral adrenergic receptor blockade, or central inhibition of MCR4 signaling. In contrast, central exendin(9-39) had no effect on the ability of peripheral exendin-4 to acutely inhibit intestinal lipoprotein release, however gastric emptying was not examined in these specific experiments Central Nervous System Regulation of Intestinal Lipoprotein Metabolism by Glucagon-Like Peptide-1 via a Brain-Gut Axis Arterioscler Thromb Vasc Biol. 2015 Feb 12. pii: ATVBAHA.114.304873
GLP-1 and bone
Several gut peptides, including GIP, PYY, and GLP-2, regulate bone resorption and/or bone formation, primarily in rodents. Although GLP-1 is not known to directly regulate skeletal homeostasis, Chizumi Yamada and colleagues have shown that the Glp1r-/- mouse exhibits a number of defects in bone density, including cortical osteopenia and enhanced bone fragility, likely attributable to increased bone resorption. Moreover, the actions of GLP-1R agonists appear to be indirect, possibly associated with reduced GLP-1R-dependent calcitonin production by thyroid C cells. Whether treatment with GLP-1R agonists will modify bone density or strength has not been determined. See The Murine Glp1r is essential for control of bone resorption Endocrinology 2008 Feb;149(2):574-9.
Intriguingly, many of these hormones, specifically GLP-1, GIP, PYY, and GLP-2, are all DPP-4 substrates, hence changes, specifically reduction of DPP-4 activity, may result in potential perturbation in bone biology perhaps through indirect modulation of active peptide hormone activity. As patients with type 2 diabetes exhibit increased fracture rates due to a reduction in bone quality, Kyle and colleagues examined multiple parameters of bone strength and bone quality in Dpp4+/+ vs Dpp4-/- mice, with or without estrogen deficiency induced by ovariectomy (OVX). Kyle also studied the same parameters in wildtype mice placed on a high fat diet, with or without OVX, and treated with the DPP-4 inhibitor sitagliptin, or the thiazolidinedione, pioglitazone. The degree of glucose control achieved with the two different anti-diabetic agents was comparable for sitagliptin vs. pioglitazone, as assessed by glucose tolerance and HbA1c, however pioglitazone-treated mice exhibited enhanced insulin sensitivity, significant weight gain and fat accumulation. Consistent with previous reports, the TZD pioglitazone reduced bone mineral density, and bone mineral volume, and decreased bone quality as demonstrated by significant adverse changes in vertebral compression parameters. Pioglitazone also reduced the mineral apposition rate and increased bone marrow adiposity. In contrast, sitagliptin treatment of WT mice was largely neutral on multiple parameters of bone mineral density and bone quality, however modest increases in bone mineral density, trabecular bone mineralization profiles and trabecular architecture were observed with sitagliptin. Similarly, Dpp4-/- mice exhibited very little perturbation in bone mineral density or quality, however both sitagliptin-treated or Dpp4-/- mice did experience further reductions in BMD and bone quality following OVX. See Kyle, K.A., Willett, T.L., Baggio, L.L., Drucker, D. J., Grynpas, M. Differential Effects of PPAR-g Activation vs. Chemical or Genetic reduction of DPP-4 Activity on Bone Quality in Mice Endocrinology 2011 Feb;152(2):457-67
Cusick and colleagues compared the effects of sitagliptin, pioglitazone, rosiglitazone, combinations of sitagliptin with pioglitazone, or vehicle alone in non-diabetic female rats following OVX. Rats were 18 weeks age at the start of the study, and drugs were administered for a period of 12 weeks. Bone loss after alendronate was prevented by alendronate, whereas sitagliptin had no effect or modestly preserved bone density (significantly increased in the lumbar spine), whereas bone density was reduced in rats treated with pioglitazone or rosiglitazone. Sitagliptin did not improve bone density in rats treated with pioglitazone. Bone Loss in the Estrogen-Depleted Rat is Not Exacerbated by Sitagliptin, Either Alone or in Combination with a Thiazolidinedione Diabetes Obes Metab. 2013 Apr 3. doi: 10.1111/dom.12109
The effects of short term administration of native GLP-1 administered by continuous subcutaenous infusion were examined in male Wistar rats after administration of STZ (type 1 DM model) or fructose feeding to induce insulin resistance (T2DM). GLP-1 produced modest increases in levels of osteocalcin and osteoprotegerin RNAs in tibial RNA from normal and diabetic rats, small increases in bone mineral density, and modest but significant changes in some parameters of trabecular bone structure. The mechanism that mediates these effects was not explored. See Effect of GLP-1 Treatment on Bone Turnover in Normal, Type 2 Diabetic, and Insulin-Resistant States Calcif Tissue Int. 2009 Jun;84(6):453-61
Kim and colleagues described detection of an immunoreactive GLP-1R protein in osteocyte-like MLO-Y4 cells and in osteocytes of rat femurs, using PCR and 2 different GLP-1R antisera. However it remains unclear whether these cells express a full length functional GLP-1R. Treatment of OLETF rats with exendin-4 reduced body weight, improved glycemic control and increased BMD, increased osteocalcin and decreased sclerostin levels, but no controls were employed for changes in glycemia or body weight. Exendin-4 Increases Bone Mineral Density in Type 2 Diabetic OLETF Rats Potentially Through the Down-Regulation of SOST/Sclerostin in Osteocytes Life Sci. 2013 Jan 25. doi:pii: S0024-3205(13)00002-7
GLP-1 action in the exocrine pancreas, and biliary tree
The putative mechanism through which GLP-1R agonists may act on the exocrine pancreas remains uncertain, and pancreatitis has not been described in the majority of large preclinical studies of http://www.ncbi.nlm.nih.gov/pubmed/22363635R agonists. Infusion of GLP-1 inhibits pancreatic exocrine secretions in short term studies of normal human subjects Truncated GLP-1 (proglucagon 78-107-amide) inhibits gastric and pancreatic functions in man. Dig Dis Sci. 1993 Apr;38(4):665-73. These effects are likely indirect, due to the effecst of GLP-1 on reduction of gastric emptying, and hence reduced transit of food into the small bowel.
Analysis of the biological activity of exendin-4 in guinea pig pancreatic exocrine slices revealed that exendin-4 stimulates cyclic AMP release through incompletely characterized mechanisms as outlined in Isolation and characterization of exendin-4, an exendin-3 analogue, from Heloderma suspectum venom. Further evidence for an exendin receptor on dispersed acini from guinea pig pancreas. J Biol Chem. 1992 Apr 15;267(11):7402-5. Susequent experiments revelaed that although exendin-4 alone did not stimulate amylase release, exendin-4 potentiated amylase release induced by CCK, carbamylcholine, bombesin or a calcium ionophore, A23187 Exendin-4, a new peptide from Heloderma suspectum venom, potentiates cholecystokinin-induced amylase release from rat pancreatic acini. Regul Pept. 1992 Sep 22;41(2):149-56. These interactions of exendin-4 with pancreatic acinar cells are also seen with native GLP-1, which also increased cyclic AMP release in comparable studies, actions which were blocked by the GLP-1R antagonist exendin(9-39); similarly, binding sites for both exendin-4 and GLP-1 were detected in experiments using radiolabelled peptides and guinea pig pancreatic acinar cells Truncated glucagon-like peptide-1 interacts with exendin receptors on dispersed acini from guinea pig pancreas. Identification of a mammalian analogue of the reptilian peptide exendin-4. J Biol Chem. 1992 Oct 25;267(30):21432-7. and Use of 125I-[Y39]exendin-4 to characterize exendin receptors on dispersed pancreatic acini and gastric chief cells from guinea pig. Regul Pept. 1994 Aug 31;53(1):47-59
There is very little information available about the possible effects of GLP-1/exendin-4 on the gall bladder or biliary tract. Preclinical studies in rats demonstrate that GLP-1/exendin-4 stimulates cholangiocyte growth, and that proliferating cholangiocytes may be capable of actually synthesizing GLP-1 Glucagon-like peptide-1 and its receptor agonist exendin-4 modulate cholangiocyte adaptive response to cholestasis. Gastroenterology. 2007 Jul;133(1):244-55. Subsequent studies demonstrated that exendin-4 is also capable of exerting anti-apoptotic effects on rat cholangiocytes cultured in vitro, and in a bile duct ligation/toxin (Ccl4)-induced model of cholangiocyte apoptosis in vivo Exendin-4, a Glucagon-Like Peptide 1 receptor agonist, protects cholangiocytes from apoptosis. Gut. 2008 Oct 1. [Epub ahead of print] For an overview of reports linking Glp1r activation to pancreatitis, see GLP-1 SAFETY
GLP-1 action in the liver
Studies of GLP-1 action in the liver continue to generate uncertainty in regard to whether the classical GLP-1 receptor is actually expressed in hepatocytes.
Tomas and colleagues provide evidence that a carboxyterminal nonapeptide derived from GLP-1 via NEP24.11 cleavage, GLP-1(28-36) amide, is biologically active in hepatocytes where it suppresses hepatic glucose production. Fluorescent-label (FAM was added to the N-terminus) GLP-1(28-36)amide entered mouse hepatocytes where it appeared to localize to mitochondria, whereas 3 control peptides did not exhibit the same pattern of localization. The stimulation of gluconeogenesis with cAMP, dexamethasone and lactate was inhibited in a dose-dependent manner by GLP-1(28-36)amide at concentrations o 0.1-10 mM through mechanisms not blocked by exendin(9-39) although Ex(9-39) also weakly inhibited gluconeogenesis. Induction of oxidative stress in mouse hepatocytes or H4IIE cells with tBHP or hydrogen peroxide produced a fall in ATP that was prevented by GLP-1(28-36). Similarly, tBHP increased levels of ROS that were decreased by GLP-1(28-36). See GLP-1-Derived Nonapeptide GLP-1(28-36)amide Targets To Mitochondria and Suppresses Glucose Production and Oxidative Stress in Isolated Mouse Hepatocytes Regul Pept. 2011 Apr 11;167(2-3):177-84
Evidence for a structurally and functionally distinct GLP-1 receptor also derives from studies of GLP-1 action in the liver. Several studies have shown that GLP-1 binds to liver membranes, and is displaced by GLP-1(1-36)amide Glucagon-like peptide-1 binding to rat hepatic membranes. J Endocrinol. 1995 Jul;146(1):183-9. Experiments using human hepatoma cells have shown that GLP-1 modulates the cell content of radiolabelled glycosylphosphatidylinositols (GPIs), Inositolphosphoglycans are possible mediators of the glucagon-like peptide 1 (7-36)amide action in the liver. J Endocrinol Invest. 1996 Feb;19(2):114-8. Incubation of rat hepatocytes with GLP-1 resulted in inhibition of glucagon-stimulated glycogenolysis, an effect additive with insulin. The authors postulated that this effect is through a distinct receptor, as GLP-1 did not displace glucagon binding from liver membranes. See Glucagon-like peptide-1 inhibits glucagon-induced glycogenolysis in perivenous hepatocytes specifically. Regul Pept. 2003 Mar 28;111(1-3):207-10.
Similarly, GLP-1activated PI3K/PKB, p70s6k, p44 and p42 MAP-kinase in rat liver cells and the activation of PI3K/PKB, PKC and PP-1, but not PP-2A, appeared to mediate the GLP-1 stimulatory action on glycogen synthase a in rat hepatocytes, while MAPKs and p70s6k were postulated to participate in other GLP-1effects. Whether these cells expressed the known GLP-1 receptor remains unclear. See Cell signalling of the GLP-1 action in rat liver. Mol Cell Endocrinol. 2003 Jun 30;204(1-2):43-50.
Exendin-4 has also been shown to promote transdifferentiation of liver cells in the context of ectopic pdx-1 expression. Although expression of the known GLP-1R was not identified in liver cells, either prior to or following the transdifferentiation process, Ex-4 increased levels of cAMP in human liver cells, and increased levels of phosphoCREB, Akt, PKC, and ERK1/2. Following Pdx-1 transduction, Ex-4 increased insulin promoter activity and insulin gene expression in transduced liver cells. Similarly, Ex-4 induced expression of a broad number of transcription factors and proteins associated with specialized beta cell function. Ex-4 was also found to increase liver cell proliferation and likely increases the proportion of cells susceptible to transduction by Pdx-1 Exendin-4 promotes liver cell proliferation and enhances PDX-1-induced liver to pancreas transdifferentiation J Biol Chem. 2009 Sep 15. [Epub ahead of print]
Raab and colleagues assessed the actions of exendin-4 after treatment of neonatal rats with intrauterine growth retardation. Exendin-4 reduced basal hepatic glucose output and increased liver insulin sensitivity. Remarkably, the direct effects of exendin-4 on neonatal rat hepatocytes, namely reduce PEPCK expression and improved insulin signaling, were observed in the absence of detectable expression of the known GLP-1R-See Neonatal Exendin-4 Treatment Reduces Oxidative Stress and Prevents Hepatic Insulin Resistance in Intrauterine Growth Retarded Rats Am J Physiol Regul Integr Comp Physiol. 2009 Oct 21. [Epub ahead of print]
In contrast, Svegliati-Baroni and co-authors report detection of PCR products (142 bp) corresponding to human Glp1r in RNA from normal liver, and Glp1r expression levels were reduced in livers from subjects with Nash. A GLP-1R-immunoreactive protein was easily detected using whole extracts from human pancreas, human HepG2 cells, and human liver using an antibody from Alpha Diagnostics. A 453 b.p Glp1r PCR product was also detected in RNA from rat liver and the abundance of the transcripts and immunoreactive protein was reduced after several months of high fat feeding. The immunoreactive GLP-1R protein was also detected in isolated rat hepatocytes frm rats with NASH and a 24h incubation with exenatide induced the expression of Pparg, Ppara, Cpt1a and Acox1. Exenatide also increased the activity of pAKT, PKA and AMPK in rat hepatocytes. The induction of pAKT was sensitive to inhibitors of cAMP and PI3K. Glucagon-like peptide-1 receptor activation stimulates hepatic lipid oxidation and restores hepatic signalling alteration induced by a high-fat diet in nonalcoholic steatohepatitis Liver Int. 2011 Feb 15. doi: 10.1111/j.1478-3231.2011.02462.x.
The effects of pioglitazone and exenatide in combination, with or without metformin in human subjects with T2DM and NAFLD were examined in an open label study of pioglitazone alone (30 mg initially, increased to 45 mg for 50 weeks(n=10); or pioglitazone plus exenatide for 50 weeks (n=11). Exenatide therapy abrogated the weight gain experienced by the patients receiving pioglitazone alone (3.7 kg), and the reduction in hepatic fat (61%) was significantly greater in subjects treated with exenatide plus pioglitazone. Fasting plasma FGF-21 levels were also reduced (2.3 to 1.1 ng/ml) in patients treated with both pio plus exenatide. Continuous delivery of exenatide alone via osmotic minipump to high fat fed mice for 4 weeks resulted in very high plasma levels of exenatide (1,734 pM), improved glucose tolerance and a significant reduction in liver weight and hepatic fat (triacylglycerol) content and decreased levels of hepatic and plasma FGF-21. Exenatide decreases hepatic fibroblast growth factor 21 resistance in non-alcoholic fatty liver disease in a mouse model of obesity and in a randomised controlled trial Diabetologia. 2011 Sep 29. [Epub ahead of print
Sharma and colleagues assessed the direct effect of exenatide in human hepatocytes exposed to different fatty acids and in high fat fed mice treated with liraglutide. Human hepatocytes loaded with fatty acids exhibited a reduction in fatty acid storage as quantified by Oil Red O staining. Hepatocyte viability, quantified by cleavage of tetrazolium salt XTT, DNA condensation, and levels of cleaved caspase-3 and CHOP, were also improved with 24 hrs of exendin-4 treatment. Exendin-4 treated human hepatocytes and liver from liraglutide-treated high fat fed mice also exhibited changes in markers of autophagy, lysosomal turnover, and increased numbers of autophagic vacuoles. The signal transduction pathways and receptor(s) mediating these observations were not identified in these studies. GLP-1 Analogs Reduce Hepatocyte Steatosis and Improve Survival by Enhancing the Unfolded Protein Response and Promoting Macroautophagy PLoS One. 2011;6(9):e25269. Epub 2011 Sep 21
A follow-up study by the same group examined the consequences of liraglutide therapy, 200 ug/kg daily,for 4 weeks in mice fed a "Western" diet (45% calories from fat; 40% of the fat representing hydrogenated vegetable oil previously shown to produce hepatic injury in C57BL/6 mice. Liraglutide improved hepatic insulin sensitivity, reduced blood pressure, adiposity, and decreased liver weight, with only modest reductions in body weight after 4 weeks. Liraglutide reduced plasma cholesterol and triglycerides and reduced hepatic fat accumulation. Liraglutide therapy was asssociated with a 400-fold increase in mRNA transcripts for LFABP, reduced levels of CPT-1alpha, and preservation of levels of MTTP, and ApoB hepatic mRNA transcripts. GLP-1 analogue, liraglutide ameliorates hepatic steatosis and cardiac hypertrophy in C57BL/6J mice fed a western diet Am J Physiol Gastrointest Liver Physiol. 2011 Oct 28
Lee and colleagues examined the effects of exendin-4 on the development of fatty liver in high fat fed mice. Treatment with Ex-4 for 10 weeks (1 nmol/kg/day) reduced levels of triglycerides and FFAs in HF fed mice. Immunoreactive GLP-1R was detected in the liver, with reduced levels in HF fed mice, and levels restored towards normal in mice treated with Ex-4. Similarly, Ex-4 increased the hepatic mRNA expression of Nampt, Sirt1, Lkb1, AMPKa1, and AMPKa2, AdipoQ, Acox, and MCAD, and decreased expression of SREBP-1c, SCD-1. Fasn, and Acaca. Glp1r mRNA and protein was also detected in the liver HepG2 and Huh7 cell lines, and treatment of these cell lines with Ex-4 decreased lipid accumulation, in association with increased levels of Glp1r, Sirt1 and AMPK expression/phosphorylation. Exendin-4 Improves Steatohepatitis by Increasing Sirt1 Expression in High-Fat Diet-Induced Obese C57BL/6J Mice PLoS One. 2012;7(2):e31394.
Gupta et al assessed the actions of exendin-4 in the setting of ischemic vascular injury of the liver in lean and high fat fed mice. Administration of Ex-4 prior to and after induction of hepatic necrosis reduced the extent of liver injury in fat fed mice, evident both histologically, and by analysis of liver enzymes 24 h later. Exendin-4 also reduced hepatic apoptosis in lean mice after ischemic injury. Similarly, exendin-4 directly reduced the extent of apoptosis in HuH7 human hepatocytes incubated with fatty acids under hypoxic conditions. Exendin-4-treated livers also exhibited reduced triglyceride levels, smaller fat droplets, with electron microscopy appearances consistent with lipid droplet fission. Exendin-4 also increased levels of phosphorylated HSL and perilipin, assessed by immunohistochemistry, in adipocytes and HuH7 cells in vitro. The Glucagon-Like Peptide-1 Receptor Agonist, Exendin 4, Has a Protective Role in Ischemic Injury of Lean and Steatotic Liver by Inhibiting Cell Death and Stimulating Lipolysis. Am J Pathol. 2012 Sep 5.
Edgerton examined whether direct portal exendin-4 infusion, at two different doses, modulated hepatic glucose uptake and production in dogs while arterial glucose was maintained at ~ 160 mg/dl. Although peripheral glucose utilization was enhanced after exendin-4 infusion, insulin levels were increased and glucagon levels were suppressed; after correction for changes in insulin, the authors concluded that there was no evidence for an independent direct effect of exendin-4 on heptic glucose metabolism. Effects of intraportal exenatide on hepatic glucose metabolism in the conscious dog Am J Physiol Endocrinol Metab. 2013 May 14
Taher and colleagues examined mechanisms through which GLP-1R agonists reduced fasting and postprandial lipemia in hamsters and mice. Both exendatide and liraglutide reduced hepatic lipid content and decreased plasma VLDL following both peripheral and intraventrocular peptide administratio. No Glp1r mRNA transcripts were detected in liver, and the lipid lowering effects were disrupted in hamsters following vagotomy. GLP-1 Receptor Agonism Ameliorates Hepatic VLDL Overproduction and de novo Lipogenesis in Insulin Resistance doi:10.1016/j.molmet.2014.09.005
Armstrong and colleagues carried out clinical studies of 14 patients with bopsy-proven NASH randomized to 1.8 mg liraglutide or placebo for 12 weeks. Liraglutide reduced body weight, glucose, plasma LDL, liver enzymes, adiponectin, leptin and CCL-2, and increased total body and hepatic insulin sensitivity in clamp studies. Liraglutide-treated patienst also exhibited a small reduction (1.26%) in de novo hepatic lipogenesis, assessed by measuring incorporation of 14-C-acetate into the lipid component of human hepatocytes ex vivo. The authors suggest that GLP-1 has direct effects on hepatocyte and adipocyte lipid metabolism Glucagon-Like Peptide 1 Decreases Lipotoxicity in Non-Alcoholic Steatophepatitis J Hepatol. 2015 Sep 19. pii: S0168-8278(15)00624-8
In a longer study, Armstrong and colleagues studied the efficacy and safety of liraglutide 1.8mg daily in a randomized double blind placebo-controlled (26 study subjects in each group) over 48 weeks. Study endpoints included changes in inflammation and fibrosis via analysis of liver histology. The primary histological outcome was pre-defined as resolution of steatohepatitis (disappearance of hepatocyte ballooning) without worsening of fibrosis, assessed using the Kleiner fibrosis classification. Liraglutide-treated subjects experienced a mean reduction n body weight (5.5% from baseline), reduced levels of A1c and circulating levels of liver enzymes. 9/23 liraglutide-treated vs. 2/22 placebo-treated subjects had histological resolution of NASH. Improvement was detected in subjects with and without T2DM. Fewer patients in the liraglutide group (compared to placebo exhibited histological progressio of fibrosis (9% vs. 36%) No differences were seen in quantitation of lobular inflammation or NAFLD scores. A very modest in levels of the serum enhanced liver fibrosis test (an ECM marker set consisting of tissue inhibitor of metalloproteinases 1 (TIMP-1), amino-terminal propeptide of type III procollagen (PIIINP) and hyaluronic acid (HA)) was detected following liraglutide administration. Liraglutide safety and efficacy in patients with non-alcoholic steatohepatitis (LEAN): a multicentre, double-blind, randomised, placebo-controlled phase 2 study Lancet. 2015 Nov 19. pii: S0140-6736(15)00803-X
GLP-1 and hypothalamic-pituitary function
Although much recent attention has focused on the role of hypothalamic GLP-1 in the control of food intake, GLP-1 may also regulate the hypothalamic pituitary axis (HPA) via effects on LH, TSH, CRH, oxytocin and vasopressin secretion. To review the data generated using cell lines and rodents, see Glucagon-like peptide-1 (GLP-1) releases thyrotropin (TSH): characterization of binding sites for GLP-1 on alpha-TSH cells. Endocrinology. 1996 Oct;137(10):4130-8.and Glucagon-like peptide-1 stimulates luteinizing hormone-releasing hormone secretion in a rodent hypothalamic neuronal cell line. J Clin Invest. 1998 Mar 15;101(6):1334-41 and Central administration of glucagon-like peptide-1 activates hypothalamic neuroendocrine neurons in the rat. Endocrinology. 1997 Oct;138(10):4445-55
These GLP-1 actions do not appear to be essential for HPA function, as Glp1r-/- mice cycle normally, are fertile, and exhibit normal basal levels of plasma osmolarity, corticosterone, thyroid hormones, estradiol, and testosterone Neuroendocrine function and response to stress in mice with complete disruption of glucagon-like peptide-1 receptor signaling. Endocrinology. 2000 Feb;141(2):752-62. Conversely, transgenic mice with sustained elevations in circulating exendin-4 are fertile and do not exhibit significant disturbances in eating or drinking behavior Sustained expression of exendin-4 does not perturb glucose homeostasis, beta-cell mass, or food intake in metallothionein-preproexendin transgenic mice. J Biol Chem. 2000 Nov 3;275(44):34471-7
GLP-1 and the kidney
The GLP-1 receptor is expressed in the kidney (the precise cellular sites of GLP-1R expression remain incompletely understood) and GLP-1R activation rapidly promotes natriuresis through incompletely understood mechanisms Glucagon-like peptide 1 induces natriuresis in healthy subjects and in insulin-resistant obese men J Clin Endocrinol Metab. 2004 Jun;89(6):3055-61. The importance of neurotransmission for the effects of GLP-1 on kidney function was demonstrated in studies using denervated kidney preparations, where the natriuretic and diuretic response to GLP-1 was attenuated in the denervated kidney. The natriuretic effects of GLP-1 appear to be associated with inhibition of Na+ reabsorption in the proximal tubule Renal effects of glucagon-like peptide in rats Eur J Pharmacol. 2002 Jan 11;434(3):163-7. Consistent with these findings, GLP-1R mRNA transcripts were detected im primary cultures of porcine kidney epithelial cells and GLP-1 directly inhibited Na+ but not glucose reabsorption Glucagon-like peptide 1 receptor expression in primary porcine proximal tubular cells Regul Pept. 2007 Jun 7;141(1-3):120-8. Infusion of native GLP-1 into salt-senstive Dahl rats for 14 days reduced the development of hypertension and proteinuria, improved Na+ excretion and was associated with decreased histological evidence for cardiac and renal damage Antihypertensive effect of glucagon-like peptide 1 in Dahl salt-sensitive rats J Hypertens. 2003 Jun;21(6):1125-35. Similar findings were then obtained in studies using GLP-1 or an exendin-4 analogue infused, with or without captopril, for 4 weeks in Dahl rats, with histological evidence for attenuation of renal damage The exenatide analogue AC3174 attenuates hypertension, insulin resistance, and renal dysfunction in Dahl salt-sensitive rats Cardiovasc Diabetol. 2010 Aug 3;9:3. These data are also consistent with findings reported in in salt-sensitive obese db/db mice and angiotensin II (angII)-infused C57BLK6/J mice, where exendin-4 treatment attenuated the development of hypertension and enhanced sodium excretion Exendin-4 has an anti-hypertensive effect in salt-sensitive mice model Biochem Biophys Res Commun. 2009 Feb 27;380(1):44-9
In preclinical studies, Kim and colleagues demonstrated that GLP-1R agonists such as exendin-4 and liraglutide increase atrial natriuretic factor (ANF) secretion from the cardiac atria, which in turn promotes blood pressure reduction and enhanced urine sodium excretion. The BP reduction and natriuresis were eliminate in Nppa-/- mice. In contrast, Lovshin and colleagues examined whether the same mechanisms are preserved in hypertensive human subjects treated with liraglutide vs. placebo for 21 days in a randomized crossover design in the Blood pressure Outcomes with Liraglutide study (BOLT). Liraglutide had only modest effects on acute ANF secretion, however a significant natriuresis was detected after 21 days, suggesting that mechanisms regulating GLP-1R-dependent sodium excretion and BP reduction diverge in mice vs. humans. See
Multiple preclinical studies support the concept that GLP-1R activation may be renoprotective. Treatment of diabetic db/db mice with exendin-4 reduced blood glucose, decreased urine albumin excretion and reduced histological parameters of glomerular injury, TGF-beta1 expression, and type IV collagen accumulation with fewer infiltrating inflammatory cells and apoptotic cells in the glomeruli Long-term treatment of glucagon-like peptide-1 analog exendin-4 ameliorates diabetic nephropathy through improving metabolic anomalies in db/db mice J Am Soc Nephrol. 2007 Apr;18(4):1227-38. Similarly, treatment of rats with STZ-induced diabetes with exendin-4, 10mg/kg once daily for 8 weeks had no significant effect on body weight or HbA1c, but reduced albumin excretion, decreased glomerular hypertrophy, and reduced markers of inflammation, fibrosis, and oxidative stress in the kidney. GLP-1 receptor mRNA was detected in rat glomeruli, glomerular endothelial cells, human monocytes, and THP-1 macrophages and exendin-4 directly attenuated the expression of high glucose-stimulated TNFa and IL-1b in THP-1 cells and in human glomerular microvascular endothelial cells Glucagon-like peptide-1 receptor agonist ameliorates renal injury through its anti-inflammatory action without lowering blood glucose level in a rat model of type 1 diabetes Diabetologia. 2011 Jan 21
A direct role for exendin-4 in modulation of Na+ exchange was also suggested in experiments using renal proximal tubular LLC-PK(1) cells. Treatment with exendin-4 modulated the activity of the activity of Na(+)/H(+) exchanger NHE3 in part via cAMP-dependent mechanisms associated with NHE3 phosphorylation. The porcine GLP-1 receptor was detected by RT-PCR in LLC-PK(1) cells and in cells isolated from the porcine kidney cortex, but not the medulla and exendin-4 increased cAMP accumulation in LLC-PK(1) cells, effects that were blocked by the antagonist exendin(9-39) Regulation of Na+/H+ exchanger NHE3 by glucagon-like peptide 1 receptor agonist exendin-4 in renal proximal tubule cells Am J Physiol Renal Physiol. 2009 Dec;297(6):F1647-55.
Mima and colleagues demonstrated, using a GLP-1R antisera, that renal GLP-1R expression was predominantly localized to glomerular endothelial cells (GEC)and downregulated in diabetic mice in mice. PKC activation increased GLP-1R degradation in GECs. Exendin-4 treatment of GECs opposed the Ang2-induction of c-Raf/Erk1/2/PAI-1 via a PKA-dependent pathway in GECs in vitro. Similar interactions on pc-Raf and pERK1/2 were noted in kidneys of mice infused with Ang 2 and treated with exendin-4. Protective Effects of GLP-1 on Glomerular Endothelium and Its Inhibition by PKCβ Activation in Diabetes Diabetes. 2012 Nov;61(11):2967-79
Katagiri and colleagues demonstrated that acute treatment with either exendin-4 or alogliptin reduced the extent of cis-platinum-induced acute renal injury in mice. The authors attributed the renoprotective actions to GLP-1R signaling, and systemic administration of GLP-1R siRNA significantly reduced renal GLP-1R protein expression (assessed by Western blotting and immunhistochemistry in proximal tubular cells using an antibody from Acris) and reversed the benefit seen with alogliptin Protection of Glucagon-Like Peptide-1 in Cisplatin-Induced Renal Injury Elucidates Gut-Kidney Connection J Am Soc Nephrol. 2013 Oct 3.
GLP-1 and the lung
GLP-1 receptor mRNA transcripts have been localized to the lung in rodents and humans, and several studies have confirmed the presence of GLP-1 binding sites using rat lung membrane preparations. Nevertheless, the precise pulmonary cell types that express the GLP-1R remain incompletely identified. Several reports have suggested that GLP-1 may exert actions both on airways (tracheal rings) and on pulmonary vasculature. Addition of GLP-1 to lung preparations increased macromolecule secretion and relaxed preconstricted pulmonary arteries. See GLP-1 stimulates secretion of macromolecules from airways and relaxes pulmonary artery. Am J Physiol. 1993 Oct;265(4 Pt 1):L374-81. Subsequent studies demonstrated that GLP-1 increases pulmonary surfactant production from isolated rat pneumocytes Glucagon-like peptide-1-(7-36)amide increases pulmonary surfactant secretion through a cyclic adenosine 3',5'- monophosphate- dependent protein kinase mechanism in rat type II pneumocytes. Endocrinology. 1998 May;139(5):2363-8 and similar studies have also been carried out using human lung cells Glucagon-like Peptide-1(7-36) Amide Stimulates Surfactant Secretion in Human Type II Pneumocytes. Am J Respir Crit Care Med. 2001 Mar 15;163(4):840-846.
Romani-Perez demonstrated a progressive upregulation of Glp1r expression (partial Glp1r mRNA transcript by RT-PCR)in the developing rat lung, with lower levels in the fetal lung increasing to adult levels by neonatal day 1. Administration of exendin-4 (5 ug/kg q 12h) to timed pregnant rats starting at day 14 of gestation increased levels of Surfactant Protein- RNA and protein in rat lung at E20 and P1. These effects were not reproduced by the GLP-1R agonist liraglutide. Ex-4 also augmented the SP levels in amniotic fluid at day 18 but not day 20. Ex-4 had similarly modest inductive effects on the expression of SP-B expression. Using a model of nitrofen (NTF)-induced lung injury (delayed lung maturation), liraglutide but not Ex-4 increased lung weights, enhanced SP-A and SP-B expression, increased Glp1r mRNA transcripts in the lung, and reduced histological evidence of lung hypoplasia. Remarkably, liraglutide, but not dexamethasone improved survival of NTF-treated rat neonatal pups. Whether exendin-4 or liraglutide crossed the placenta and circulated as intact peptides in the rat fetus was not examined. Pulmonary GLP-1 Receptor Increases at Birth and Exogenous GLP-1 Receptor Agonists Augmented Surfactant-Protein Levels in Litters from Normal and Nitrofen-Treated Pregnant Rats Endocrinology. 2013 Jan 25
Viby and colleagues examined the consequences of manipulating GLP-1R signaling in an experimental model of murine lung injury. Exenatide or liraglutide-treated mice with a modified ovalbumin/LPS lung injury exhibited improved survival, improved paramters of lung function using plethysmography, and reduced histological evidence of lung inflammation, through incompletely understood mechanisms Glucagon-like peptide-1 (GLP-1) reduces mortality and improves lung function in a model of experimental obstructive lung disease in female mice. Endocrinology. 2013 Oct 3
GLP-1 action in fat and muscle cells
GLP-1 has been shown to exert modest effects on fat and muscle cells in vitro, however convincing evidence for a major physiologically relevant action of GLP-1 on lipolysis has not been forthcoming. GLP-1 binding sites have been identified in human adipose tissue however the molecular identity of the adipose tissue GLP-1R remains unclear Presence of glucagon and glucagon-like peptide-1-(7-36)amide receptors in solubilized membranes of human adipose tissue. J Clin Endocrinol Metab. 1993 Dec;77(6):1654-7.. Studies using rat explants studying the incorporation of [14C]acetate into saponifiable fat demonstrated that both GIP and GLP-1(7-36) stimulated fatty acid synthesis within the physiological range of the circulating hormones. At lower concentrations of the hormones, GLP-1(7-36) amide was a more potent stimulator of fatty acid synthesis than GIP in omental adipose tissue culture. See Effect of the entero-pancreatic hormones, gastric inhibitory polypeptide and glucagon-like polypeptide-1(7-36) amide, on fatty acid synthesis in explants of rat adipose tissue. J Endocrinol. 1991 Aug; 130 (2):267-72
Lockie and colleagues examined the importance of GLP-1R signaling for brown adipose tissue (BAT) thermogenesis. While BAT function in respnse to cold seems intact in Glp1r-/- mice, central administration of GLP-1R agonists such as GLP-1 or oxyntomodulin reduced food intake yet increased energy expenditure, iBAT temperature and thermogenesis, actions abolished in Glp1r-/- mice. Intraperitoneal administration of the peptides had no effect on thermogenesis. Sympathetic nerve activity (SNA) innervating BAT was increased following icv administration of glucagon, oxyntomodulin, or GLP-1. Central GLP-1R activation also increased the expression of multiple BAT genes important for thermogenesis. Hence, the brain GLP-1R system is capable of communicating with BAT to regulate thermogenesis in mice. Direct Control of Brown Adipose Tissue Thermogenesis by Central Nervous System Glucagon-Like Peptide-1 Receptor Signaling Diabetes. 2012 Aug 28
Bertin and colleagues have studied the effects of GLP-1 infusion on lipolysis and local blood flow in nine healthy human volunteers infused with either epinephrine, glucagon, GLP-1, or saline. These investigators analyzed dialysate glycerol content, ethanol ratio, and blood flow in sc abdominal adipose tissue and the gastrocnemius skeletal muscle. Neither glucagon or GLP-1 affected the rate of lipolysis in fat or muscle. See Action of Glucagon and Glucagon-Like Peptide-1-(7-36) Amide on Lipolysis in Human Subcutaneous Adipose Tissue and Skeletal Muscle in Vivo. J Clin Endocrinol Metab. 2001 Mar 1;86(3):1229-1234
Similarly, Chai and colleagues demonstrated that GLP-1 enhances muscle glucose uptake in rats by augmented muscle microvascular blood flow independent of insulin. A GLP-1 infusion increased plasma NO levels, tissue oxygem saturation, and muscle glucose uptake, in a NO-sensitive manner as these actions were abolished by L-NAME. In bovine endothelial cells, native GLP-1 enhanced Akt and NOS phosphorylation in a GLP-1R-dependent manner as these actions were abolished by exendin(9-39). Whether the actions of native GLP-1 to augment muscle blood flow in vivo are attributable to native GLP-1, or GLP-1(9-36), was not explored Glucagon-Like Peptide 1 Recruits Microvasculature and Increases Glucose Use in Muscle via a Nitric Oxide–Dependent Mechanism Diabetes published ahead of print February 22, 2012, doi:10.2337/db11-1073
The actions of exendin-4, GLP-1, and exendin(9-39) have been studied in human adipocytes in vitro. GLP-1, Ex-4 and insulin, but not Ex-9, increased glucose uptake in adipocytes isolated from "normal subjects", actions that were blocked by inhibitors of PI3K and MAPKs blocked the stimulatory action of GLP-1, Ex-4 and insulin. Surprisingly, the GLP-1R antagonist exendin(9-39) enhanced PI3K and stimulated p42 MAPK. The actions of GLP-1 and exendin-4 were impaired in adipocytes from obese subjects. The receptor delineating these actions was not delineated. See The action of GLP-1 and exendins upon glucose transport in normal human adipocytes, and on kinase activity as compared to morbidly obese patients. Int J Mol Med. 2007 Jun;19(6):961-6
The relationship between GLP-1 and visfatin, a polyfunctional adipokine with enzymatic activity have been described in studies in normal human subjects. Both normal weight and obese subjects exhibited a decrease in serum visfatin levels following oral glucose administration. Insulin and GLP-1 significantly reduced visfatin secretion in 3T3L1 cells in vitro. GLP-1 also produced a modest decrease on levels of phospho ERK1/2 in 3T3 L1 cells. Whether GLP-1 significantly contributes to control of visfatin secretion in vivo remains unclear. In Vivo Suppression of Visfatin by Oral Glucose Uptake: Evidence for a Novel Incretin-Like Effect by Glucagon-Like Peptide-1 (GLP-1). J Clin Endocrinol Metab. 2011 Jun 15.
Secreted frizzled-related protein-5 (Sfrp-5) is an adipocyte-enriched protein that exerts anti-inflammatory activity in mice. Plasma levels of Sfrp5 were lower in human subjects with IGT and T2D, and appear to be glucose-regulated. Administration of liraglutide for 16 weeks was associated with reduction of HbA1c and body weight and a significant increase in circulating levels of adiponectin and of Sfrp5 from 52.8 to 84.8 ug/L. Whether regulation of Sfrp5 reflects changes in glucose, body weight or GLP-1R signaling requires further study. Circulating Sfrp5 Is a Signature of Obesity-Related Metabolic Disorders and Is Regulated by Glucose and Liraglutide in Humans JCEM jc.2012-2466; doi:10.1210/jc.2012-246
There continue to be reports describing actions of GLP-1 on muscle, including studies with human muscle cells and strips, suggesting actions of glucagon-like peptide agonists and antagonists. In muscle strips, GLP-1 stimulated glycogen synthesis, glycogen synthase a activity, and glucose oxidation and utilization, and inhibited glycogen phosphorylase a activity. In cultured myotubes, GLP-1 at very low doses of 0.1-1 pM stimulated glucose incorporation into glycogen. Curiously, exendin-4 and its truncated form 9-39 amide (Ex-9) both exert the same types of effects on glycogen synthesis and synthase a activity without stimulating an increase in cAMP accumulation. The identity of the receptor that transduces these intriguing effects remains unknown. See Glucagon-like peptide-1 (GLP-1) and glucose metabolism in human myocytes. J Endocrinol. 2002 Jun; 173( 3): 465-73
GLP-1 and taste receptors
Taste receptors have been shown to be expressed on gut GLP-1-producing L cells and these receptors modulate GLP-1 secretion in mice. Moreover, GLP-1, may also be produced in subsets of murine taste cells, together with its receptor expressed on adjacent nerve endings, where it appears to contribute to gustatory discrimination. Moreover, Glp1r-/- mice exhibit reduced taste responses to sweeteners, implicating a functional role for the GLP-1 system in murine taste discrimination. See Modulation of taste sensitivity by GLP-1 signaling J Neurochem. 2008 Apr 5; [Epub ahead of print] and Expression of glucagon-like peptide-1 in the taste buds of rat circumvallate papillae Acta Histochem. 2008;110(2):151-4. Epub 2007 Dec 3