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, and lungs. Our understanding of GLP-1 actions in these tissues is incomplete, but likely to grow over the next few years.
GLP-1 and the immune system
GLP-1 has been used in short term studies to treat subjects with type 1 diabetes, and several preclinical studies have examined the efficacy of GLP-1R agonists in models of T1DM such as the NOD mouse. Intriguingly, the results of some of these studies suggest that GLP-1 therapy may have immunomodulatory actions. Initiation of exendin-4 treatmentt alone after the development of diabetes had little therapeutic benefit in NOD mice. In contrast, exendin-4 together with lysophiline for 28 days, markedly improved glucose control in NOD mice, even 6-14 weeks after cessation of therapy. Furthermore, the combination therapy preserved the number of intact islets, and appeared to reduce the extent of inflammatory cell infiltration in the remaining islets. See Combined treatment with lisofylline and exendin-4 reverses autoimmune diabetes. Biochem Biophys Res Commun. 2006 Jun 9;344(3):1017-22. Similarly, continuous administration of nativeGLP-1to 8 week old female NOD mice for 4 or 8 weeks lowered blood glucose, increased formation of new b-cells, suppressed b-cell apoptosis and delayed the onset of diabetes as described in Continuous stimulation of human glucagon-like peptide-1 (7-36) amide in a mouse model (NOD) delays onset of autoimmune type 1 diabetes. Diabetologia. 2007 Jul 14; [Epub ahead of print]. In contrast, twice daily administration of exendin-4 at two different doses, 100 ng, and 2 ug twice daily, prior to the development of clinical diabetes in NOD mice, produced only modest effects on the numbers of disease-free mice, with detectable but small increased in beta cell mass and reductions in insulitis. Intriguingly,GLP-1receptor expression was also detected in different immune compartments as outlined in Exendin-4 modulates diabetes onset in non obese diabetic mice Endocrinology. 2008 Mar;149(3):1338-49. Nevertheless, the biology of GLP-1 action in immune cells is poorly understood.
Hadjiyanni and colleagues detected widespread expression of the GLP-1R in immune cells from bone marow, spleen, thymus and peripherla lymph nodes. Immune GLP-1R expression appeared to be relatively higher in cells from NOD mice compared to cells ontained from C57BL/6 mice. GLP-1R activation increased cyclic AMP formation in immune populations but failed to affect lymphocyte survival, proliferation, or migration. Although no major changes in cell numbers or immune cell function were observed in Glp1r-/- lymphocytes, Glp1r −/− thymocytes exhibited a hypoproliferative response, whilst peripheral Glp1r −/− lymphocytes were hyperproliferative in response to mitogenic stimulation. As these studies were carried out in "normal mice", further analysis of the role of the GLP-1R in the context of immune dysfunction, such as T1DM, appears warranted. Glucagon-like peptide-1 receptor (GLP-1R) signaling selectively regulates murine lymphocyte proliferation and maintenance of peripheral regulatory T-cells Diabetologia 2010 10.1007/s00125-009-1643-x
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. GGLP-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 and mice, 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 esential for intstinal 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 vildagliptinVildagliptin therapy reduces postprandial intestinal triglyceride-rich lipoprotein particles in patients with type 2 diabetes Diabetologia. 2006 Sep;49(9):2049-57
GLP-1 and bone
Several gut peptides, including GIP, PYY, and GLP-2, regulate bone resorption and/or bone formation. 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.
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 Feb 15. [Epub ahead of print]
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
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 GLP-1R-/- 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 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.
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
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.
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. 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
Nevertheless, 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 action in the exocrine pancreas, liver 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 preclinical studies of GLP-1R 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]