Novel signal transduction and peptide specificity of glucagon-like peptide receptor in 3T3-L1 adipocytes

In vitro chronic glycation induces AGEs accumulation reducing insulin-stimulated glucose uptake and increasing GLP1R in adipocytes

Intracellular AGEs accumulation increases RAGE and GLP1R and reduces glucose uptake in adipocytes.

Case Report: Liraglutide for Weight Management in Beckwith-Wiedemann Syndromic Obesity

Genetic obesity, including syndromic and non-syndromic forms, represents a minority of cases compared to essential obesity but gene dysregulations lead to complex clinical conditions that make their management particularly difficult. Among them, Beckwith-Wiedemann syndrome (BWS) is a multisystem human genomic imprinting disorder characterized by overgrowth. We describe the first case of liraglutide treatment in an 18-year-old boy patient affected by BWS complicated by macroglossia, cryptorchidism, nephroblastoma, organomegaly, microscopic lymphocytic colitis, pharmacologically treated arterial hypertension, obesity, and obstructive sleep apnea syndrome. He presented a normal cognitive development. Body mass index at the time of first transition visit in the adult endocrinology department at the age of 18-years-old was 40.6 kg/m² without glucose metabolism impairment. Lifestyle interventions failed because of poor compliance. During 20 months of 3.0 mg liraglutide treatment, a weight loss of 19 kg (-13.3%) and BMI reduction of 6.8 points were registered without side effects. To date, liraglutide treatment was effective on obesity in 7 subjects with Prader Willy Syndrome and 14 with melanocortin-4 receptor mutations. The efficacy of liraglutide in BWS could be related to a crosstalk among glucagon-like peptide (GLP)-1 system, mechanisms related to the cyclin-dependent kinase inhibitor 1C (CDKN1C), and dopamine mesolimbic circuit. Clinical trials aiming at a tailored medicine in genetic obesity are needed.

Identification and characterization of adipose surface epitopes

Adipose tissue is a central regulator of metabolism and an important pharmacological target to treat the metabolic consequences of obesity, such as insulin resistance and dyslipidemia. Among the various cellular compartments, the adipocyte cell surface is especially appealing as a drug target as it contains various proteins that when activated or inhibited promote adipocyte health, change its endocrine function and eventually maintain or restore whole-body insulin sensitivity. In addition, cell surface proteins are readily accessible by various drug classes. However, targeting individual cell surface proteins in adipocytes has been difficult due to important functions of these proteins outside adipose tissue, raising various safety concerns. Thus, one of the biggest challenges is the lack of adipose selective surface proteins and/or targeting reagents. Here, we discuss several receptor families with an important function in adipogenesis and mature adipocytes to highlight the complexity at the cell surface and illustrate the problems with identifying adipose selective proteins. We then discuss that, while no unique adipocyte surface protein might exist, how splicing, posttranslational modifications as well as protein/protein interactions can create enormous diversity at the cell surface that vastly expands the space of potentially unique epitopes and how these selective epitopes can be identified and targeted.

Human Sperm Express the Receptor for Glucagon-like Peptide-1 (GLP-1), Which Affects Sperm Function and Metabolism

AIM

Glucagon-like peptide-1 (GLP-1) produces pleiotropic effects binding to the GLP-1 receptor (GLP1-R), potentiating insulin secretion in the pancreas. GLP1-R is expressed in peripheral tissues and evidence for its role in reproduction has come from knockout mice, although the relationship between GLP-1 and male fertility needs to be clarified. Given that human sperm is an insulin-sensitive and insulin-secreting cell, we hypothesized that the GLP-1/GLP1-R axis may be expressed and functional in these cells.

RESULTS AND DISCUSSION

We revealed the presence of GLP1-R by Western blotting and immunofluorescence analyses. Because Exendin-4 (Ex-4) displays similar functional properties to native GLP-1, we used this agonist to perform a dose-response study on progressive motility and cholesterol efflux, showing that 300 pM Ex-4 was the most effective treatment. These actions are mediated by GLP1-R and independent from sperm-secreted insulin. The exposure to Ex-4 fueled phosphatidylinositol-3-kinase (PI3K)/AKT signaling and was reversed by H89, indicating a protein kinase A (PKA)-dependence of GLP-1/GLP1-R signaling. It emerged that in sperm, insulin secretion regulated by Ex-4 did not occur in a strictly glucose-dependent manner. A stimulatory action of Ex-4/GLP1-R on lactate dehydrogenase and glucose-6-phosphate dehydrogenase (G6PDH) activities was observed. Ex-4/GLP1-R decreased triglycerides content concomitantly to enhanced lipase and acyl-coenzyme A (acyl-CoA) dehydrogenase activities, addressing a lipolytic effect.

CONCLUSION

Collectively, we discovered that human sperm is a new GLP1 incretin target, broadening our knowledge about the effects of the GLP1-R agonist in the male reproductive field. Further findings in humans should be conducted in the future to confirm it and to improve the translational aspect of this study.

Effects of Exenatide and Humalog Mix25 on Fat Distribution, Insulin Sensitivity, and β-Cell Function in Normal BMI Patients with Type 2 Diabetes and Visceral Adiposity

In China, most normal BMI (body mass index of ≥18.5 to <25 kg/m²) adults with type 2 diabetes (T2DM) exhibit visceral adiposity. This study compared the effects of exenatide and humalog Mix25 on normal BMI patients with T2DM and visceral adiposity. A total of 95 patients were randomized to receive either exenatide or humalog Mix25 treatment for 24 weeks. Subcutaneous adipose tissue (SAT) and visceral adipose tissue (VAT) were quantified by magnetic resonance imaging (MRI) and liver fat content (LFC) by liver proton magnetic resonance spectroscopy (¹H MRS). Each patient's weight, waist circumference, BMI, blood glucose, insulin sensitivity, pancreatic β-cell function, and fibroblast growth factor 21 (FGF-21) levels were measured. Data from 81 patients who completed the study (40 and 41 in the exenatide and humalog Mix25 groups, respectively) were analysed. The change in 2 h plasma blood glucose was greater in the exenatide group (P = 0.039). HOMA-IR and MBCI improved significantly after exenatide therapy (P < 0.01, P = 0.045). VAT and LFC decreased in both groups (P < 0.01 for all) but to a greater extent in the exenatide group, while SAT only decreased with exenatide therapy (P < 0.01). FGF-21 levels declined more in the exenatide group (P < 0.01), but were positively correlated with VAT in the entire cohort before (r = 0.244, P = 0.043) and after (r = 0.290, P = 0.016) the intervention. The effects of exenatide on glycaemic metabolism, insulin resistance, pancreatic β-cell function, and fat deposition support its administration to normal BMI patients with T2DM and visceral adiposity.

Liraglutide improves vascular dysfunction by regulating a cAMP-independent PKA-AMPK pathway in perivascular adipose tissue in obese mice

BACKGROUND

Perivascular adipose tissue (PVAT) attenuates its anti-contractile effect through an endothelial-dependent mechanism that aggravates endothelial dysfunction in obesity. The present study was conducted to explore whether liraglutide could improve vascular dysfunction, including the anti-contractile effect of PVAT and endothelial function, by modulating PVAT-related signaling pathways in obesity.

METHODS

C57BL/6 mice were fed a normal-chow diet or a high-fat diet (HFD) with or without liraglutide treatment. Vascular function of the thoracic aorta with or without PVAT were measured. Protein levels of components of the PKA-AMPK-PGC1α and antioxidant signaling pathway in PVAT were determined by western blotting. Brown adipose tissue-related gene in PVAT was measured by qRT-PCR.

RESULTS

Metabolic profiles of HFD-fed mice were improved after treatment with liraglutide. Liraglutide improved PVAT-induced anti-contractile capability and PVAT-induced endothelial dysfunction in HFD-fed mice both in vivo and ex vivo. However, blocking PKA, or AMPK, but not cAMP, attenuated these beneficial effects of liraglutide. Treating HFD-fed mice with liraglutide activated the AMPK/eNOS pathway and induced browning-related gene expression. Moreover, liraglutide increased antioxidant capability. The protective effects were related to activation of a cAMP-independent PKA-AMPK pathway, as demonstrated by western blot and PCR.

CONCLUSIONS

Liraglutide improved vascular dysfunction by modulating a cAMP-independent PKA-AMPK pathway in PVAT in HFD-induced obese mice. The findings provide a novel mechanism for the cardiovascular protection of liraglutide by modulating PVAT function in obesity.

Multiple target tissue effects of GLP-1 analogues on non-alcoholic fatty liver disease (NAFLD) and non-alcoholic steatohepatitis (NASH)

Accumulating experimental and clinical evidences over the last decade indicate that GLP-1 analogues have a series of central nervous system and peripheral target tissues actions which are able to significantly influence the liver metabolism. GLP-1 analogues pleiotropic effects proved to be efficacious in T2DM subjects not only reducing liver steatosis and ameliorating NAFLD and NASH, but also in lowering plasma glucose and liver inflammation, improving cardiac function and protecting from kidney dysfunction. While the experimental and clinical data are robust, the precise mechanisms of action potentially involved in these protective multi-target effects need further investigation. Here we present a systematic review of the most recent literature data on the multi-target effects of GLP-1 analogues on the liver, on adipose and muscular tissue and on the nervous system, all capable of influencing significant aspects of the fatty liver disease physiopathology. From this analysis, we can conclude that the multi-target beneficial action of the GLP-1 analogues could explain the positive effects observed in animal and human models on progression of NAFLD to NASH.

Effects of Liraglutide on Weight Loss, Fat Distribution, and β-Cell Function in Obese Subjects With Prediabetes or Early Type 2 Diabetes

OBJECTIVE

Obesity is associated with an increased risk of type 2 diabetes and cardiovascular complications. The risk depends significantly on adipose tissue distribution. Liraglutide, a glucagon-like peptide 1 analog, is associated with weight loss, improved glycemic control, and reduced cardiovascular risk. We determined whether an equal degree of weight loss by liraglutide or lifestyle changes has a different impact on subcutaneous adipose tissue (SAT) and visceral adipose tissue (VAT) in obese subjects with prediabetes or early type 2 diabetes.

RESEARCH DESIGN AND METHODS

Sixty-two metformin-treated obese subjects with prediabetes or newly diagnosed type 2 diabetes, were randomized to liraglutide (1.8 mg/day) or lifestyle counseling. Changes in SAT and VAT levels (determined by abdominal MRI), insulin sensitivity (according to the Matsuda index), and β-cell function (β-index) were assessed during a multiple-sampling oral glucose tolerance test; and circulating levels of IGF-I and IGF-II were assessed before and after a comparable weight loss (7% of initial body weight).

RESULTS

After comparable weight loss, achieved by 20 patients per arm, and superimposable glycemic control, as reflected by HbA_1c level (P = 0.60), reduction in VAT was significantly higher in the liraglutide arm than in the lifestyle arm (P = 0.028), in parallel with a greater improvement in β-index (P = 0.021). No differences were observed in SAT reduction (P = 0.64). IGF-II serum levels were significantly increased (P = 0.024) only with liraglutide administration, and the increase in IGF-II levels correlated with both a decrease in VAT (ρ = -0.435, P = 0.056) and an increase in the β-index (ρ = 0.55, P = 0.012).

CONCLUSIONS

Liraglutide effects on visceral obesity and β-cell function might provide a rationale for using this molecule in obese subjects in an early phase of glucose metabolism dysregulation natural history.

Is cleaved glucagon-like peptide 1 really inactive? Effects of GLP-1(9-36) on human adipose stem cells

Glucagon-like peptide 1(9-36) [GLP-1(9-36)] is generated by dipeptidyl peptidase-4 (DPP4) cleavage of the gut incretin hormone, GLP-1(7-36). Since GLP-1(9-36) has a very low affinity for the GLP-1 receptor (GLP-1R), it has so far been considered an inactive form of GLP-1. Here we show GLP-1(9-36) activity in human adipose stem cells (ASC) in vitro. GLP-1(9-36) inhibits human ASC proliferation, glucose uptake and adipogenesis, as well as induces cell apoptosis, to a similar extent as GLP-1(7-36) and liraglutide. Since GLP-1(9-36) effects are not reverted by the receptor antagonist exendin(9-39), which conversely reverts the effects of GLP-1(7-36), we hypothesized that the former may be mediated by a GLP-1 receptor different from the classical pancreatic one. This is the first report of GLP-1(9-36) activity in human adipose cells. Nevertheless, these findings deserve further preclinical studies to better elucidate novel and unforeseen GLP-1(9-36) activities, which could allow a better understanding of the clinical profile of DPP4 inhibitors and GLP-1R agonists.

Effects of glucagon-like peptide-1 on the differentiation and metabolism of human adipocytes

BACKGROUND AND PURPOSE

Glucagon-like peptide-1 (GLP-1) analogues improve glycaemic control in type 2 diabetic (T2D) patients and cause weight loss in obese subjects by as yet unknown mechanisms. We recently demonstrated that the GLP-1 receptor, which is present in adipocytes and the stromal vascular fraction of human adipose tissue (AT), is up-regulated in AT of insulin-resistant morbidly obese subjects compared with healthy lean subjects. The aim of this study was to explore the effects of in vitro and in vivo administration of GLP-1 and its analogues on AT and adipocyte functions from T2D morbidly obese subjects.

EXPERIMENTAL APPROACH

We analysed the effects of GLP-1 on human AT and isolated adipocytes in vitro and the effects of GLP-1 mimetics on AT of morbidly obese T2D subjects in vivo.

KEY RESULTS

GLP-1 down-regulated the expression of lipogenic genes when administered during in vitro differentiation of human adipocytes from morbidly obese patients. GLP-1 also decreased the expression of adipogenic/lipogenic genes in AT explants and mature adipocytes, while increasing that of lipolytic markers and adiponectin. In 3T3-L1 adipocytes, GLP-1 decreased free cytosolic Ca2+ concentration ([Ca2+]i). GLP-1-induced responses were only partially blocked by GLP-1 receptor antagonist exendin (9–39). Moreover, administration of exenatide or liraglutide reduced adipogenic and inflammatory marker mRNA in AT of T2D obese subjects.

CONCLUSIONS AND IMPLICATIONS

Our data suggest that the beneficial effects of GLP-1 are associated with changes in the adipogenic potential and ability of AT to expand, via activation of the canonical GLP-1 receptor and an additional, as yet unknown, receptor.

Perspectives in GLP-1 Research: New Targets, New Receptors

The incretin hormone glucagon-like peptide-1 (GLP-1) binds to and activates its G-protein-coupled-receptor GLP-1R to reduce glycaemia through the stimulation of insulin and suppression of pancreatic glucagon secretion. Recently, GLP-1 effects unrelated to glucose homeostasis have been discovered in myocardium, bone, adipose tissue, and other target organs, which appear to be mainly mediated by GLP-1R-independent pathways. Here, we summarize knowledge on GLP-1R agonists (GLP-1RAs) as they relate to the improvement of glucose control, and focus on the most recently described effects, discussing the preclinical evidence of the involvement of alternative receptors and signalling mechanisms. It is now evident that the universe of GLP-1RAs is expanding further from the initial incretin effect, opening new unforeseen avenues for research and clinical applications.

Specific biology of neuroendocrine tumors: Peptide receptors as molecular targets

Neuroendocrine tumors (NET) are characterized by a high over-expression of many different peptide hormone receptors. These receptors represent important molecular targets for imaging and therapy, using either radiolabeled or cold peptide analogs. The clinically best established example is somatostatin receptor targeting. A relatively new application is glucagon-like peptide 1 (GLP-1) receptor-targeted imaging of insulinomas, which is highly sensitive. A potential future candidate for peptide receptor targeting is the gastric inhibitory peptide (GIP) receptor. It was recently found to exhibit a very wide expression in NET and may be a particularly suitable target in somatostatin and GLP-1 receptor negative tumors. With increasing use of peptide receptor targeting, reliable morphologic in vitro tools to assess peptide receptors in tissues are mandatory, such as in vitro receptor autoradiography or thoroughly established immunohistochemical procedures.

Reduction of serum FABP4 level by sitagliptin, a DPP-4 inhibitor, in patients with type 2 diabetes mellitus

Fatty acid binding protein 4 (FABP4), also known as adipocyte FABP or aP2, is secreted from adipocytes in association with lipolysis as a novel adipokine, and elevated serum FABP4 level is associated with obesity, insulin resistance, and atherosclerosis. However, little is known about the modulation of serum FABP4 level by therapeutic drugs. Sitagliptin (50 mg/day), a dipeptidyl peptidase 4 (DPP-4) inhibitor that increases glucagon-like peptide 1 (GLP-1), was administered to patients with type 2 diabetes (n = 24) for 12 weeks. Treatment with sitagliptin decreased serum FABP4 concentration by 19.7% (17.8 ± 1.8 vs. 14.3 ± 1.5 ng/ml, P < 0.001) and hemoglobin A1c without significant changes in adiposity or lipid variables. In 3T3-L1 adipocytes, sitagliptin or exendin-4, a GLP-1 receptor agonist, had no effect on short-term (2 h) secretion of FABP4. However, gene expression and long-term (24 h) secretion of FABP4 were significantly reduced by sitagliptin, which was not mimicked by exendin-4. Treatment with recombinant DPP-4 increased gene expression and long-term secretion of FABP4, and the effects were cancelled by sitagliptin. Furthermore, knockdown of DPP-4 in 3T3-L1 adipocytes decreased gene expression and long-term secretion of FABP4. In conclusion, sitagliptin decreases serum FABP4 level, at least in part, via reduction in the expression and consecutive secretion of FABP4 in adipocytes by direct inhibition of DPP-4.

The Place of Dipeptidyl Peptidase-4 Inhibitors in Type 2 Diabetes Therapeutics: A "Me Too" or "the Special One" Antidiabetic Class?

Incretin-based therapies, the most recent therapeutic options for type 2 diabetes mellitus (T2DM) management, can modify various elements of the disease, including hypersecretion of glucagon, abnormal gastric emptying, postprandial hyperglycaemia, and, possibly, pancreatic β cell dysfunction. Dipeptidyl peptidase-4 (DPP-4) inhibitors (gliptins) increase glucagon-like peptide-1 (GLP-1) availability and correct the "incretin defect" seen in T2DM patients. Clinical studies have shown good glycaemic control with minimal risk of hypoglycaemia or any other adverse effects, despite the reports of pancreatitis, whose association remains to be proved. Recent studies have been focusing on the putative ability of DPP-4 inhibitors to preserve pancreas function, in particular due to the inhibition of apoptotic pathways and stimulation of β cell proliferation. In addition, other cytoprotective effects on other organs/tissues that are involved in serious T2DM complications, including the heart, kidney, and retina, have been increasingly reported. This review outlines the therapeutic potential of DPP-4 inhibitors for the treatment of T2DM, focusing on their main features, clinical applications, and risks, and discusses the major challenges for the future, in particular the possibility of becoming the preferred therapy for T2DM due to their ability to modify the natural history of the disease and ameliorate nephropathy, retinopathy, and cardiovascular complications.

Effect of liraglutide on proliferation and differentiation of human adipose stem cells

Glucagon-Like Peptide-1 (GLP-1) receptor agonists, used as glucose-lowering drugs, also induce weight loss by inhibiting food intake. The present study was aimed at the assessment of the in vitro effects of the GLP-1 receptor agonist liraglutide on proliferation and differentiation of human adipose stem cells (ASC) obtained from subcutaneous adipose tissue of morbidly obese subjects undergoing bariatric surgery. Liraglutide (10-100 nM) significantly inhibited ASC proliferation and viability, with a maximum effect at 6 days of culture (45% and 50%, for liraglutide 10 and 100 nM, respectively); the effect was reverted by exendin 9-39. Glucose uptake was significantly reduced by liraglutide in a dose dependent manner. Treatment with liraglutide reduced intracellular lipid accumulation in differentiating ASC, together with FABP-4 mRNA expression (-18%, -23%, -46%, for 1 nM, 10 nM and 100 nM, respectively), whereas it stimulated adiponectin (APN) expression (1.86-, 2.64-, 2.28-fold increase, for 1 nM, 10 nM and 100 nM, respectively). Liraglutide exerts effects on human adipose cell precursors, inhibiting proliferation and differentiation, while stimulating the expression of the insulin-sensitizing adipokine APN. These effects could contribute to the actions of GLP-1 receptor agonists on body weight and insulin sensitivity.

Molecular mechanisms underlying physiological and receptor pleiotropic effects mediated by GLP-1R activation

The incidence of type 2 diabetes in developed countries is increasing yearly with a significant negative impact on patient quality of life and an enormous burden on the healthcare system. Current biguanide and thiazolidinedione treatments for type 2 diabetes have a number of clinical limitations, the most serious long-term limitation being the eventual need for insulin replacement therapy (Table 1). Since 2007, drugs targeting the glucagon-like peptide-1 (GLP-1) receptor have been marketed for the treatment of type 2 diabetes. These drugs have enjoyed a great deal of success even though our underlying understanding of the mechanisms for their pleiotropic effects remain poorly characterized even while major pharmaceutical companies actively pursue small molecule alternatives. Coupling of the GLP-1 receptor to more than one signalling pathway (pleiotropic signalling) can result in ligand-dependent signalling bias and for a peptide receptor such as the GLP-1 receptor this can be exaggerated with the use of small molecule agonists. Better consideration of receptor signalling pleiotropy will be necessary for future drug development. This is particularly important given the recent failure of taspoglutide, the report of increased risk of pancreatitis associated with GLP-1 mimetics and the observed clinical differences between liraglutide, exenatide and the newly developed long-acting exenatide long acting release, albiglutide and dulaglutide.

The anorexic effect of Ex4/Fc through GLP-1 receptor activation in high-fat diet fed mice

Exendin-4 (Ex4), a peptide initially found in the saliva of the Gila monster, can activate the signaling pathway of the incretin hormone glucagon-like peptide-1 (GLP-1) through the GLP-1 receptor (GLP-1R). We previously reported that a chimera protein consisting of Ex4 and mouse IgG heavy chain constant regions (Ex4/Fc) can exert biological effects of GLP-1, such as improving glycemic control and ameliorating manifestations in diabetic mice. The aim of this study was to determine whether Ex4/Fc is effective in modulating energy homeostasis in mice. Our results showed that in vivo expression of Ex4/Fc by intramuscular injection of the plasmid encoding Ex4/Fc followed by local electroporation effectively decreased food intake in the mice on high-fat diet (HFD) feeding. In addition, the reduced energy intake was associated with the decreased excrements from the Ex4/Fc-treated HFD mice but not the Fc control mice. Remarkably, the Ex4/Fctreated HFD mice displayed significantly lower triglyceride (TG) levels when compared with the control mice. Interestingly, while the leptin levels were not changed, the circulating ghrelin levels were higher in Ex4/Fc mice than those in the Fc control mice. These results suggested that Ex4/Fc can improve energy metabolism and lipid metabolism through GLP-1R in mice under excessive nutrition conditions.

GIP and GLP-1, the two incretin hormones: Similarities and differences

Gastric inhibitory polypeptide (GIP) and glucagon‐like peptide‐1 (GLP‐1) are the two primary incretin hormones secreted from the intestine on ingestion of glucose or nutrients to stimulate insulin secretion from pancreatic β cells. GIP and GLP‐1 exert their effects by binding to their specific receptors, the GIP receptor (GIPR) and the GLP‐1 receptor (GLP‐1R), which belong to the G‐protein coupled receptor family. Receptor binding activates and increases the level of intracellular cyclic adenosine monophosphate in pancreatic β cells, thereby stimulating insulin secretion glucose‐dependently. In addition to their insulinotropic effects, GIP and GLP‐1 play critical roles in various biological processes in different tissues and organs that express GIPR and GLP‐1R, including the pancreas, fat, bone and the brain. Within the pancreas, GIP and GLP‐1 together promote β cell proliferation and inhibit apoptosis, thereby expanding pancreatic β cell mass, while GIP enhances postprandial glucagon response and GLP‐1 suppresses it. In adipose tissues, GIP but not GLP‐1 facilitates fat deposition. In bone, GIP promotes bone formation while GLP‐1 inhibits bone absorption. In the brain, both GIP and GLP‐1 are thought to be involved in memory formation as well as the control of appetite. In addition to these differences, secretion of GIP and GLP‐1 and their insulinotropic effects on β cells have been shown to differ in patients with type 2 diabetes compared to healthy subjects. We summarize here the similarities and differences of these two incretin hormones in secretion and metabolism, their insulinotropic action on pancreatic β cells, and their non‐insulinotropic effects, and discuss their potential in treatment of type 2 diabetes. (J Diabetes Invest, doi: 10.1111/j.2040‐1124.2010.00022.x, 2010)

Glucose-dependent insulinotropic polypeptide and glucagon-like peptide-1: Incretin actions beyond the pancreas

Glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) are the two primary incretin hormones secreted from the intestine on ingestion of various nutrients to stimulate insulin secretion from pancreatic β-cells glucose-dependently. GIP and GLP-1 undergo degradation by dipeptidyl peptidase-4 (DPP-4), and rapidly lose their biological activities. The actions of GIP and GLP-1 are mediated by their specific receptors, the GIP receptor (GIPR) and the GLP-1 receptor (GLP-1R), which are expressed in pancreatic β-cells, as well as in various tissues and organs. A series of investigations using mice lacking GIPR and/or GLP-1R, as well as mice lacking DPP-4, showed involvement of GIP and GLP-1 in divergent biological activities, some of which could have implications for preventing diabetes-related microvascular complications (e.g., retinopathy, nephropathy and neuropathy) and macrovascular complications (e.g., coronary artery disease, peripheral artery disease and cerebrovascular disease), as well as diabetes-related comorbidity (e.g., obesity, non-alcoholic fatty liver disease, bone fracture and cognitive dysfunction). Furthermore, recent studies using incretin-based drugs, such as GLP-1 receptor agonists, which stably activate GLP-1R signaling, and DPP-4 inhibitors, which enhance both GLP-1R and GIPR signaling, showed that GLP-1 and GIP exert effects possibly linked to prevention or treatment of diabetes-related complications and comorbidities independently of hyperglycemia. We review recent findings on the extrapancreatic effects of GIP and GLP-1 on the heart, brain, kidney, eye and nerves, as well as in the liver, fat and several organs from the perspective of diabetes-related complications and comorbidities.

Characterization of [¹²⁵I]GLP-1(9-36), a novel radiolabeled analog of the major metabolite of glucagon-like peptide 1 to a receptor distinct from GLP1-R and function of the peptide in murine aorta

AIMS

Glucagon-like peptide 1 (GLP-1) is an insulin secretagogue, released in response to meal ingestion and efficiently lowers blood glucose in Type 2 diabetic patients. GLP-1(7-36) is rapidly metabolized by dipeptidyl peptidase IV to the major metabolite GLP-1(9-36)-amide, often thought to be inactive. Inhibitors of this enzyme are widely used to treat diabetes. Our aim was to characterize the binding of GLP-1(9-36) to native mouse tissues and to cells expressing GLP1-R as well as to measure functional responses in the mouse aorta compared with GLP-1(7-36).

MAIN METHODS

The affinity of [(125)I]GLP-1(7-36) and [(125)I]GLP-1(9-36) was measured in mouse tissues by saturation binding and autoradiography used to determine receptor distribution. The affinity of both peptides was compared in binding to recombinant GLP-1 receptors using cAMP and scintillation proximity assays. Vasoactivity was determined in mouse aortae in vitro.

KEY FINDINGS

In cells expressing GLP-1 receptors, GLP-1(7-36) bound with the expected high affinities (0.1 nM) and an EC50 of 0.07 nM in cAMP assays but GLP-1(9-36) bound with 70,000 and 100,000 fold lower affinities respectively. In contrast, in mouse brain, both labeled peptides bound with a single high affinity, with Hill slopes close to unity, although receptor density was an order of magnitude lower for [(125)I]GLP-1(9-36). In functional experiments both peptides had similar potencies, GLP-1(7-36), pD2=7.40 ± 0.24 and GLP-1(9-36), pD2=7.57 ± 0.64.

SIGNIFICANCE

These results suggest that GLP-1(9-36) binds and has functional activity in the vasculature but these actions may be via a pathway that is distinct from the classical GLP-1 receptor and insulin secretagogue actions.

Discovery and development of exenatide: the first antidiabetic agent to leverage the multiple benefits of the incretin hormone, GLP-1

INTRODUCTION

The GLP-1 receptor agonist exenatide is synthetic exendin-4, a peptide originally isolated from the salivary secretions of the Gila monster. Exenatide was developed as a first-in-class diabetes therapy, with immediate- and extended-release formulations. In preclinical diabetes models, exenatide enhanced glucose-dependent insulin secretion, suppressed inappropriately elevated glucagon secretion, slowed gastric emptying, reduced body weight, enhanced satiety, and preserved pancreatic β-cell function. In clinical trials, both exenatide formulations reduced hyperglycemia in patients with type 2 diabetes mellitus (T2DM) and were associated with weight loss.

AREAS COVERED

This article reviews the development of exenatide from its discovery and preclinical investigations, to the elucidation of its pharmacological mechanisms of action in mammalian systems. The article also presents the pharmacokinetic profiling and toxicology studies of exenatide, as well as its validation in clinical trials.

EXPERT OPINION

GLP-1 receptor agonists represent a new paradigm for the treatment of patients with T2DM. By leveraging incretin physiology, a natural regulatory system that coordinates oral nutrient intake with mechanisms of metabolic control, these agents address multiple core defects in the pathophysiology of T2DM. Studies have identified unique benefits including improvements in glycemic control and weight, and the potential for beneficial effects on the cardiometabolic system without the increased risk of hypoglycemia associated with insulin therapy. Peptide hormone therapeutics can offer significant advantages over small molecule drug targets when it comes to specificity, potency, and more predictable side effects. As exemplified by exenatide, injectable peptides can be important drugs for the treatment of chronic diseases, such as T2DM.

Regulation of adipocyte formation by GLP-1/GLP-1R signaling

Increased nutrient intake leads to excessive adipose tissue accumulation, obesity, and the development of associated metabolic disorders. How the intestine signals to adipose tissue to adapt to increased nutrient intake, however, is still not completely understood. We show here, that the gut peptide GLP-1 or its long-lasting analog liraglutide, function as intestinally derived signals to induce adipocyte formation, both in vitro and in vivo. GLP-1 and liraglutide activate the GLP-1R, thereby promoting pre-adipocyte proliferation and inhibition of apoptosis. This is achieved at least partly through activation of ERK, PKC, and AKT signaling pathways. In contrast, loss of GLP-1R expression causes reduction in adipogenesis, through induction of apoptosis in pre-adipocytes, by inhibition of the above mentioned pathways. Because GLP-1 and liraglutide are used for the treatment of type 2 diabetes, these findings implicate GLP-1 as a regulator of adipogenesis, which could be an alternate pathway leading to improved lipid homeostasis and controlled downstream insulin signaling.

Study of the potential association of adipose tissue GLP-1 receptor with obesity and insulin resistance

The increase in glucagon-like peptide-1 (GLP-1) activity has emerged as a useful therapeutic tool for the treatment of type 2 diabetes mellitus. The actions of GLP-1 on β-cells and the nervous and digestive systems are well known. The action of this peptide in adipose tissue (AT), however, is still poorly defined. Furthermore, no relationship has been established between GLP-1 receptor (GLP-1R) in AT and obesity and insulin resistance (IR). We provide evidence for the presence of this receptor in AT and show that its mRNA and protein expressions are increased in visceral adipose depots from morbidly obese patients with a high degree of IR. Experiments with the 3T3-L1 cell line showed the lipolytic and lipogenic dose-dependent effect of GLP-1. Moreover, GLP-1 stimulated lipolysis in 3T3-L1 adipocytes in a receptor-dependent manner involving downstream adenylate cyclase/cAMP signaling. Our data also demonstrate that the expression of the GLP-1R in AT correlated positively with the homeostasis model assessment index in obese IR subjects. Furthermore, prospective studies carried out with patients that underwent biliopancreatic diversion surgery showed that subjects with high levels of GLP-1R expression in AT, which indicates a deficit of GLP-1 in this tissue, were those whose insulin sensitivity improved after surgery, suggesting the potential relationship between AT GLP-1R and insulin sensitivity amelioration in obese subjects. Altogether these results indicate that the GLP-1/GLP-1R system in AT represents another potential candidate for improving insulin sensitivity in obese patients.

The peptide-hormone glucagon-like peptide-1 activates cAMP and inhibits growth of breast cancer cells

The incretin hormone glucagon-like peptide (GLP)-1 is secreted from intestinal L cells in response to food intake, and promotes insulin secretion and pancreatic β-cell proliferation. Reduced GLP-1 levels are observed in obesity and type 2 diabetes mellitus (T2DM) and are associated with reduced insulin secretion and increased insulin resistance. GLP-1 mediates its activities through activation of a G-protein coupled receptor, which is expressed in the pancreas, as well as other tissues. Long-acting GLP-1 receptor (GLP-1R) agonists, such as exendin-4, are currently approved for the treatment of T2DM. As obesity and T2DM are associated with increased risk of breast cancer, we aimed to explore the effects of GLP-1 and exendin-4, on breast cancer cells. Treatment with GLP-1 or exendin-4 reduced viability and enhanced apoptosis of breast cancer cells but did not affect viability of nontumorigenic cells. Moreover, exendin-4 attenuated tumor formation by breast cancer cells in athymic mice. Treatment with either GLP-1 or exendin-4 elevated cAMP levels, activated the down-stream target CREB, and enhanced CRE promoter transcription, in breast cancer cells. Moreover, inhibition of exendin-4-induced adenylate cyclase activation restored cell viability, thus suggesting cAMP as a principle mediator of exendin-4 anti-tumorigenic activity. While the pancreatic form of the GLP-1R could not be detected in breast cancer cells, several lines of evidence indicated the existence of an alternative GLP-1R in mammary cells. Thus, internalization of GLP-1 into MCF-7 cells was evidenced, infection of MCF-7 cells with the pancreatic receptor enhanced proliferation, and treatment with exendin-(9-39), a GLP-1R antagonist, further increased cAMP levels. Our studies indicate the incretin hormone GLP-1 as a potent inducer of cAMP and an inhibitor of breast cancer cell proliferation. Reduced GLP-1 levels may, therefore, serve as a novel link between obesity, diabetes mellitus, and breast cancer.

Exendin-4 improves blood glucose control in both young and aging normal non-diabetic mice, possible contribution of beta cell independent effects

Aims

Type 2 diabetes is highly prevalent in the elderly population. Glucagon like Peptide-1 mimetic such as exendin-4 augments post-prandial insulin secretion. However, the potential influence of aging on the therapeutic effects of this peptide has not been well studied. In this study, we examined the glucose regulatory effects of exendin-4 in mice with different ages.

Methods

We treated 3-month and 20 to 22-month old C57/DBA mice with 10 nM/kg exendin-4 for 10 days with measurements of blood glucose and body weight. We performed OGTT and ITT to evaluate the glucose response and insulin sensitivity. Islet morphology and beta cell mass were measured by immuno-staining and beta cell proliferation was evaluated by BrdU incorporation and PCNA staining. Real-time PCR and western blot were used to measure protein changes in the liver tissue after exendin-4 treatment.

Results

Exendin-4 treatment improved glycemic control in both 3-month and 20 to 22-month old mice. In both groups of mice, the blood glucose lowering effect was independent of beta cell function as indicated by unchanged beta cell proliferation, insulin secretion or beta cell mass. Moreover, we found that exendin-4 treatment increased hepatic AKT and FOXO1 phosphorylation and inhibited glucose-6-phosphotase (G6P) and Phosphoenolpyruvate carboxykinase (PEPCK) expression in young mice, but this effect was attenuated in aging mice while the insulin sensitivity showed no change in the young group but significantly improved in aging mice.

Conclusion

Based on these data, we conclude that the glucose lowering effect of exendin-4 in normal non-diabetic mice was not blunted by aging. We further showed that although there was slight difference in the glucose modulating mechanism of exendin-4 therapy in young and aged mice, the improved glucose control seemed uncorrelated with increased beta cell mass or insulin secretion.

Characteristic of GLP-1 effects on glucose metabolism in human skeletal muscle from obese patients

Direct effects of GLP-1, kinase-mediated, on glucose and lipid metabolism in rat and human extrapancreatic tissues, are amply documented and also changes in type-2 diabetic (T2D) patients. Here, we explored the characteristics of the GLP-1 action and those of its analogs Ex-4 and Ex-9, on muscle glucose transport (GT) and metabolism in human morbid obesity (OB), as compared with normal and T2D subjects. In primary cultured myocytes from OB, GT and glycogen synthase a (GSa) activity values were lower than normal, and comparable to those reported in T2D patients; GT was increased by either GLP-1 or Ex-9 in a more efficient manner than in normal or T2D, up to normal levels; the Ex-4 increasing effect on GSa activity was two times that in normal cells, while Ex-9 failed to modify the enzyme activity. In OB, the control value of all kinases analyzed - PI3K, PKB, MAPKs, and p70s6K - although lower than that in normal or T2D subjects, the cells maintained their response capability to GLP-1, Ex-4, Ex-9 and insulin, with some exceptions. GLP-1 and exendins showed a direct normalizing action in the altered glucose uptake and metabolism in the muscle of obese subjects, which in the case of GLP-1 could account, at least in part, for the reported restoration of the metabolic conditions of these patients after restrictive surgery.

Presence of a functional receptor for GLP-1 in osteoblastic cells, independent of the cAMP-linked GLP-1 receptor

Glucagon-like peptide 1 (GLP-1) controls glucose metabolism in extrapancreatic tissues through receptors other than the pancreatic cAMP-linked GLP-1 receptor; also, GLP-1 induces an insulin- and PTH-independent bone anabolic action in insulin-resistant and type-2 diabetic rats. Here we searched for the presence and characteristics of GLP-1 receptors in osteoblastic MC3T3-E1 cells. [(125)I]-GLP-1 specific binding to MC3T3-E1 cells was time- and temperature-dependent, reaching maximal value at 30 min at 25 degrees C; in these conditions, [(125)I]-GLP-1 binding was dissociable, and displaced by GLP-1, partially by GLP-2, but not by exendin-4 (Ex-4), exendin-9 (Ex-9), glucagon or insulin; Scatchard analysis of the unlabeled GLP-1 data showed high and low affinity binding sites; cross-linking of GLP-1 binding revealed an estimated 70 kDa band, almost undetectable in the presence of 10(-6) M GLP-1. GLP-1, Ex-9, insulin or glucagon failed to modify cellular cAMP content, while GLP-2 and Ex-4 increased it. However, GLP-1 induced an immediate hydrolysis of glycosylphosphatidylinositols (GPIs) generating short-lived inositolphosphoglycans (IPGs), and an increase in phosphatidylinositol-3 kinase (PI3K) and mitogen activated protein kinase (MAPK) activities; Ex-4 also affected GPIs, but its action was delayed with respect to that of GLP-1. This incretin was found to decrease Runx2 but increased osteocalcin gene expression, without affecting that of osteoprotegerin or the canonical Wnt pathway activity in MC3T3-E1 cells which do not express the pancreatic GLP-1 receptor. Our data demonstrate for the first time that GLP-1 can directly and functionally interact with osteoblastic cells, possibly through a GPI/IPG-coupled receptor.

GLP-1(7-36)-amide and Exendin-4 stimulate the HPA axis in rodents and humans

Glucagon-like peptide-1 (GLP-1) is a potent insulinotropic peptide expressed in the gut and brain, which is secreted in response to food intake. The levels of GLP-1 within the brain have been related to the activity of the hypothalamic-pituitary-adrenal (HPA) axis, and hence, this peptide might mediate some responses to stress. Nevertheless, there is little information regarding the effects of circulating GLP-1 on the neuroendocrine control of HPA activity. Here, we have studied the response of corticoadrenal steroids to the peripheral administration of GLP-1 (7-36)-amide and related peptides [exendin (Ex)-3, Ex-4, and Ex-4(3-39)] in rats, mice, and humans. GLP-1 increases circulating corticosterone levels in a time-dependent manner, both in conscious and anaesthetized rats, and it has also increased aldosterone levels. Moreover, GLP-1 augmented cortisol levels in healthy subjects and diabetes mellitus (DM)-1 patients. The effects of GLP-1/Ex-4 on the HPA axis are very consistent after distinct means of administration (intracerebroventricular, iv, and ip), irrespective of the metabolic state of the animals (fasting or fed ad libitum), and they were reproduced by different peptides in this family, independent of glycaemic changes and their insulinotropic properties. Indeed, these effects were also observed in diabetic subjects (DM-1 patients) and in the DM-1 streptozotocin-rat or DM-2 muscle IGF-I receptor-lysine-arginine transgenic mouse animal models. The mechanisms whereby circulating GLP-1 activates the HPA axis remain to be elucidated, although an increase in ACTH after Ex-4 and GLP-1 administration implicates the central nervous system or a direct effect on the pituitary. Together, these findings suggest that GLP-1 may play an important role in regulating the HPA axis.

Pleiotropic actions of the incretin hormones

The insulin secretory response to a meal results largely from glucose stimulation of the pancreatic islets and both direct and indirect (autonomic) glucose-dependent stimulation by incretin hormones released from the gastrointestinal tract. Two incretins, Glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1), have so far been identified. Localization of the cognate G protein-coupled receptors for GIP and GLP-1 revealed that they are present in numerous tissues in addition to the endocrine pancreas, including the gastrointestinal, cardiovascular, central nervous and autonomic nervous systems (ANSs), adipose tissue, and bone. At these sites, the incretin hormones exert a range of pleiotropic effects, many of which contribute to the integration of processes involved in the regulation of food intake, and nutrient and mineral processing and storage. From detailed studies at the cellular and molecular level, it is also evident that both incretin hormones act via multiple signal transduction pathways that regulate both acute and long-term cell function. Here, we provide an overview of current knowledge relating to the physiological roles of GIP and GLP-1, with specific emphasis on their modes of action on islet hormone secretion, β-cell proliferation and survival, central and autonomic neuronal function, gastrointestinal motility, and glucose and lipid metabolism. However, it is emphasized that despite intensive research on the various body systems, in many cases there is uncertainty as to the pathways by which the incretins mediate their pleiotropic effects and only a rudimentary understanding of the underlying cellular mechanisms involved, and these are challenges for the future.

Differences in the central anorectic effects of glucagon-like peptide-1 and exendin-4 in rats

OBJECTIVE

Glucagon-like peptide (GLP)-1 is a regulatory peptide synthesized in the gut and the brain that plays an important role in the regulation of food intake. Both GLP-1 and exendin (Ex)-4, a long-acting GLP-1 receptor (GLP-1r) agonist, reduce food intake when administered intracerebroventricularly, whereas Ex4 is much more potent at suppressing food intake when given peripherally. It has generally been hypothesized that this difference is due to the relative pharmacokinetic profiles of GLP-1 and Ex4, but it is possible that the two peptides control feeding via distinct mechanisms.

RESEARCH DESIGN AND METHODS

In this study, the anorectic effects of intracerebroventricular GLP-1 and Ex4, and the sensitivity of these effects to GLP-1r antagonism, were compared in rats. In addition, the GLP-1r dependence of the anorectic effect of intracerebroventricular Ex4 was assessed in GLP-1r(-/-) mice.

RESULTS

Intracerebroventricular Ex4 was 100-fold more potent than GLP-1 at reducing food intake, and this effect was insensitive to GLP-1r antagonism. However, GLP-1r antagonists completely blocked the anorectic effect of intraperitoneal Ex4. Despite the insensitivity of intracerebroventricular Ex4 to GLP-1r antagonism, intracerebroventricular Ex4 failed to reduce food intake in GLP-1r(-/-) mice.

CONCLUSIONS

These data suggest that although GLP-1rs are required for the actions of Ex4, there appear to be key differences in how GLP-1 and Ex4 interact with central nervous system GLP-1r and in how Ex4 interacts with GLP-1r in the brain versus the periphery. A better understanding of these unique differences may lead to expansion and/or improvement of GLP-1-based therapies for type 2 diabetes and obesity.

Exendin-4, a GLP-1 receptor agonist, directly induces adiponectin expression through protein kinase A pathway and prevents inflammatory adipokine expression

Exendin-4 (Ex-4) is a glucagon-like peptide-1 receptor (GLP-1R) agonist that has been used as a drug injected subcutaneously for treatment of type 2 diabetes. Many studies have revealed molecular targets of Ex-4, but its influence on adipokines has not been determined. Our study showed that Ex-4 induced secretion of adiponectin into the culture medium of 3T3-L1 adipocytes. This effect of Ex-4 is due to increased adiponectin mRNA level through the GLP-1R. Both forskolin and 3-isobutyl-1-methylxanthine (IBMX), which may finally elevate cyclic adenosine monophosphate (cAMP) concentration, prevented the induction of adiponectin expression by Ex-4. Moreover, H89, a protein kinase A inhibitor, blocked the effect of Ex-4 on adiponectin. On the other hand, Ex-4 decreased the mRNA levels of inflammatory adipokines. The results indicate that Ex-4 directly promotes adiponectin secretion via the protein kinase A pathway in 3T3-L1 adipocytes and may ameliorate insulin resistance.

Ginsenosides Rb1 and Rg1 suppress triglyceride accumulation in 3T3-L1 adipocytes and enhance beta-cell insulin secretion and viability in Min6 cells via PKA-dependent pathways

Ginseng root is known to induce anti-diabetic activity, but the key components involved are unknown. We investigated which major ginsenosides in ginseng enhanced glucose homeostasis by in vitro studies. Rb1 and Rg1 reduced the triglyceride accumulation in 3T3-L1 adipocytes by activating PKA with increased intracellular cAMP. However, the insulin-stimulated glucose uptake was enhanced by Rb1 and Rg1 via activation of phosphatidylinositol-3 kinase. Rb1 and Rg1 promoted glucose-stimulated insulin secretion and cell viability in Min6 cells through PKA which augmented IRS2 expression to enhance insulin/IGF-1 signaling. These results suggest that Rb1 and Rg1 improved glucose homeostasis through the activation of a PKA like glucagon-like peptide-1 receptor agonist.

Peptide hormone exendin-4 stimulates subventricular zone neurogenesis in the adult rodent brain and induces recovery in an animal model of parkinson's disease

We investigated the effects of exendin-4 on neural stem/progenitor cells in the subventricular zone of the adult rodent brain and its functional effects in an animal model of Parkinson's disease. Our results showed expression of GLP-1 receptor mRNA or protein in the subventricular zone and cultured neural stem/progenitor cells isolated from this region. In vitro, exendin-4 increased the number of neural stem/progenitor cells, and the number of cells expressing the neuronal markers microtubule-associated protein 2, beta-III-tubulin, and neuron-specific enolase. When exendin-4 was given intraperitoneally to naïve rodents together with bromodeoxyuridine, a marker for DNA synthesis, both the number of bromodeoxyuridine-positive cells and the number of neuronal precursor cells expressing doublecortin were increased. Exendin-4 was tested in the 6-hydroxydopamine model of Parkinson's disease to investigate its possible functional effects in an animal model with neuronal loss. After unilateral lesion and a 5-week stabilization period, the rats were treated for 3 weeks with exendin-4. We found a reduction of amphetamine-induced rotations in animals receiving exendin-4 that persisted for several weeks after drug administration had been terminated. Histological analysis showed that exendin-4 significantly increased the number of both tyrosine hydroxylase- and vesicular monoamine transporter 2-positive neurons in the substantia nigra. In conclusion, our results show that exendin-4 is able to promote adult neurogenesis in vitro and in vivo, normalize dopamine imbalance, and increase the number of cells positive for markers of dopaminergic neurons in the substantia nigra in a model of Parkinson's disease.

The physiology of glucagon-like peptide 1

Glucagon-like peptide 1 (GLP-1) is a 30-amino acid peptide hormone produced in the intestinal epithelial endocrine L-cells by differential processing of proglucagon, the gene which is expressed in these cells. The current knowledge regarding regulation of proglucagon gene expression in the gut and in the brain and mechanisms responsible for the posttranslational processing are reviewed. GLP-1 is released in response to meal intake, and the stimuli and molecular mechanisms involved are discussed. GLP-1 is extremely rapidly metabolized and inactivated by the enzyme dipeptidyl peptidase IV even before the hormone has left the gut, raising the possibility that the actions of GLP-1 are transmitted via sensory neurons in the intestine and the liver expressing the GLP-1 receptor. Because of this, it is important to distinguish between measurements of the intact hormone (responsible for endocrine actions) or the sum of the intact hormone and its metabolites, reflecting the total L-cell secretion and therefore also the possible neural actions. The main actions of GLP-1 are to stimulate insulin secretion (i.e., to act as an incretin hormone) and to inhibit glucagon secretion, thereby contributing to limit postprandial glucose excursions. It also inhibits gastrointestinal motility and secretion and thus acts as an enterogastrone and part of the "ileal brake" mechanism. GLP-1 also appears to be a physiological regulator of appetite and food intake. Because of these actions, GLP-1 or GLP-1 receptor agonists are currently being evaluated for the therapy of type 2 diabetes. Decreased secretion of GLP-1 may contribute to the development of obesity, and exaggerated secretion may be responsible for postprandial reactive hypoglycemia.

Resistin Is a Key Mediator of Glucose-dependent Insulinotropic Polypeptide (GIP) Stimulation of Lipoprotein Lipase (LPL) Activity in Adipocytes

Studies on the physiological roles of the incretin hormone, glucose-dependent insulinotropic polypeptide (GIP) have largely focused on its insulinotropic action and ability to regulate beta-cell mass. In previous studies on the stimulatory effect of GIP on adipocyte lipoprotein lipase (LPL), a pathway was identified involving increased phosphorylation of protein kinase B (PKB) and reduced phosphorylation of LKB1 and AMP-activated protein kinase (AMPK). The slow time of onset of the responses suggested that GIP may have induced release of an intermediary molecule, and the current studies focused on the possible contribution of the adipokine resistin. In differentiated 3T3-L1 adipocytes, GIP, in the presence of insulin, increased resistin secretion through a pathway involving p38 mitogen-activated protein kinase (p38 MAPK) and the stress-activated protein kinase/Jun amino-terminal kinase (SAPK/JNK). The other major incretin hormone, glucagon-like peptide-1 (GLP-1), exhibited no significant effects. Chronic elevation of circulating GIP levels in the Vancouver Diabetic Fatty (VDF) Zucker rat resulted in increases in circulating resistin levels and activation of p38 MAPK or SAPK/JNK in epididymal fat tissue, suggesting the existence of identical pathways in vivo as well as in vitro. Administration of resistin to 3T3-L1 adipocytes mimicked the effects of GIP on the PKB/LKB1/AMPK/LPL pathway: increasing phosphorylation of PKB, reducing levels of phosphorylated LKB1 and AMPK, and increasing LPL activity. Knockdown of resistin using RNA interference attenuated the effect of GIP on the PKB/LKB1/AMPK/LPL pathway in 3T3-L1 adipocytes, supporting a role for resistin as a mediator.

Pro-protein convertases in intermediary metabolism: islet hormones, brain/gut hormones and integrated physiology

Many peptide hormones implicated in the regulation of intermediary metabolism arise from larger precursors called prohormones. These precursors are cut into pieces by proprotein convertases, more precisely those called prohormone convertases (PCs) that cleave at the C terminus of basic doublets. The remaining basic amino acids are eliminated by a specialized carboxypeptidase, leading to the active hormone. This processing may provide, from a single precursor, several peptides with different biological activities depending on the site(s) of cleavage on the precursor. When the processing is tissue-specific, this mechanism allows to produce, from a single protein, different sets of hormones depending on the tissue considered, leading to novel regulatory processes. The archetype of such a pluripotent prohormone in the field of intermediary metabolism is pro-glucagon that, when cut by PC1 in intestinal L cells, produces four different peptides with different specificities [glicentin, oxyntomodulin (OXM), glucagon-like peptide-1, and glucagon-like peptide-2], whereas, when cut by PC2 in the alpha cells of the endocrine pancreas, glucagon is produced and, through the supplementary action of NRD convertase, a fragment of glucagon (miniglucagon) with original properties.

Activation of Lipoprotein Lipase by Glucose-dependent Insulinotropic Polypeptide in Adipocytes

Glucose-dependent insulinotropic polypeptide (GIP) has been mainly studied because of its glucose-dependent insulinotropic action and its ability to regulate beta-cell proliferation and survival. Considerably less is known about the effects of GIP on fat metabolism, and the present study was directed at identifying the mechanisms underlying its stimulatory action on lipoprotein lipase (LPL). In differentiated 3T3-L1 adipocytes, GIP, in the presence of insulin, increased LPL activity and triglyceride accumulation through a pathway involving increased phosphorylation of protein kinase B (PKB) and reductions in phosphorylated LKB1 and AMP-activated protein kinase (AMPK). Knockdown of AMPK using RNA interference and application of the AMPK inhibitor, Compound C, supported this conclusion. In contrast, the other major incretin hormone, glucagon-like peptide-1, exhibited no significant effects on LPL activity or PKB, LKB1, or AMPK phosphorylation. Cultured subcutaneous human adipocytes showed similar responses to GIP but with greater sensitivity. Chronic elevation of circulating GIP levels in the Vancouver diabetic fatty Zucker rat in vivo resulted in increased LPL activity and elevated triglyceride accumulation in epididymal fat tissue, combined with a modulation of PKB, LKB1, and AMPK phosphorylation similar to that observed in vitro. This appears to be the first demonstration of a GIP-stimulated signal transduction pathway involved in increasing fat storage in adipocytes.

Exendin-4 potently decreases ghrelin levels in fasting rats

Ghrelin is a potent orexigenic and adipogenic hormone that strongly influences fat deposition and the generation of hunger in obesity. Indeed, hyperghrelinemia appears to promote an increase in food intake as seen in Prader-Willi Syndrome (PWS). Exendin (Ex)-4 is an agonist of the glucagon-like peptide (GLP)-1 receptor (GLP-1r) that has anorexigenic and fat-reducing properties. Here, we report that Ex-4 reduces the levels of ghrelin by up to 74% in fasted rats. These effects are dose dependent and long lasting (up to 8 h), and they can be detected after both central and peripheral administration of Ex-4. Suppression of ghrelin was neither mimicked by GLP-1(7-36)-NH(2) nor blocked by the GLP-1r antagonist Ex-(9-39). Moreover, it was independent of the levels of leptin and insulin. The decrease in ghrelin levels induced by Ex-4 may explain the reduced food intake in fasted rats, justifying the more potent anorexigenic effects of Ex-4 when compared with GLP-1. As well as the potential benefits of Ex-4 in type 2 diabetes, the potent effects of Ex-4 on ghrelin make it tempting to speculate that Ex-4 could offer a therapeutic option for PWS and other syndromes characterized by substantial amounts of circulating ghrelin.

Gastrointestinal hormones regulating appetite

The role of gastrointestinal hormones in the regulation of appetite is reviewed. The gastrointestinal tract is the largest endocrine organ in the body. Gut hormones function to optimize the process of digestion and absorption of nutrients by the gut. In this capacity, their local effects on gastrointestinal motility and secretion have been well characterized. By altering the rate at which nutrients are delivered to compartments of the alimentary canal, the control of food intake arguably constitutes another point at which intervention may promote efficient digestion and nutrient uptake. In recent decades, gut hormones have come to occupy a central place in the complex neuroendocrine interactions that underlie the regulation of energy balance. Many gut peptides have been shown to influence energy intake. The most well studied in this regard are cholecystokinin (CCK), pancreatic polypeptide, peptide YY, glucagon-like peptide-1 (GLP-1), oxyntomodulin and ghrelin. With the exception of ghrelin, these hormones act to increase satiety and decrease food intake. The mechanisms by which gut hormones modify feeding are the subject of ongoing investigation. Local effects such as the inhibition of gastric emptying might contribute to the decrease in energy intake. Activation of mechanoreceptors as a result of gastric distension may inhibit further food intake via neural reflex arcs. Circulating gut hormones have also been shown to act directly on neurons in hypothalamic and brainstem centres of appetite control. The median eminence and area postrema are characterized by a deficiency of the blood-brain barrier. Some investigators argue that this renders neighbouring structures, such as the arcuate nucleus of the hypothalamus and the nucleus of the tractus solitarius in the brainstem, susceptible to influence by circulating factors. Extensive reciprocal connections exist between these areas and the hypothalamic paraventricular nucleus and other energy-regulating centres of the central nervous system. In this way, hormonal signals from the gut may be translated into the subjective sensation of satiety. Moreover, the importance of the brain-gut axis in the control of food intake is reflected in the dual role exhibited by many gut peptides as both hormones and neurotransmitters. Peptides such as CCK and GLP-1 are expressed in neurons projecting both into and out of areas of the central nervous system critical to energy balance. The global increase in the incidence of obesity and the associated burden of morbidity has imparted greater urgency to understanding the processes of appetite control. Appetite regulation offers an integrated model of a brain-gut axis comprising both endocrine and neurological systems. As physiological mediators of satiety, gut hormones offer an attractive therapeutic target in the treatment of obesity.

Role of glucagon-like peptide-1 in the pathogenesis and treatment of diabetes mellitus

Glucagon-like peptide-1 (GLP-1) is an incretin hormone secreted from enteroendocrine L cells in response to ingested nutrients. The first recognized and most important action of GLP-1 is the potentiation of glucose-stimulated insulin secretion in beta-cells, mediated by activation of its seven transmembrane domain G-protein-coupled receptor. In addition to its insulinotropic actions, GLP-1 exerts islet-trophic effects by stimulating replication and differentiation and by decreasing apoptosis of beta-cells. The GLP-1 receptor is expressed in a variety of other tissues important for carbohydrate metabolism, including pancreatic alpha-cells, hypothalamus and brainstem, and proximal intestinal tract. GLP-1 also appears to exert important actions in liver, muscle and fat. Thus, GLP-1 suppresses glucagon secretion, promotes satiety, delays gastric emptying and stimulates peripheral glucose uptake. The impaired GLP-1 secretion observed in type 2 diabetes suggests that GLP-1 plays a role in the pathogenesis of this disorder. Thus, because of its multiple actions, GLP-1 is an attractive therapeutic target for the treatment of type 2 diabetes, and major interest has resulted in the development of a variety of GLP-1 receptor agonists for this purpose. Ongoing clinical trials have shown promising results and the first analogs of GLP-1 are expected to be available in the near future.

Effect of GLP-1 on glucose transport and its cell signalling in human myocytes

Glucagon-like peptide-1 (GLP-1) controls glucose metabolism in extrapancreatic tissues participating in glucose homeostasis, through receptors not associated to cAMP. In rat hepatocytes, activation of PI3K/PKB, PKC and PP-1 mediates the GLP-1-induced stimulation of glycogen synthase. We have investigated the effect of GLP-1 in normal human myocytes, and that of its structurally related peptides exendin-4 (Ex-4) and its truncated form 9-39 (Ex-9) upon glucose uptake, and the participation of cellular enzymes proposed to mediate insulin actions. GLP-1 and both exendins activated, like insulin, PI3K/PKB and p42/44 MAPK enzymes, but p70s6k was activated only by GLP-1 and insulin. GLP-1, Ex-4 and Ex-9, like insulin, stimulated glucose uptake; wortmannin blocked the action of GLP-1, insulin and Ex-9, and reduced that of Ex-4; PD98059 abolished the effect of all peptides/hormones, while rapamycin blocked that of insulin and partially prevented that of GLP-1. H-7 abolished the action of GLP-1, insulin and Ex-4, while Ro 31-8220 prevented only the Ex-4 and Ex-9 effect. In conclusion, GLP-1, like insulin, stimulates glucose uptake, and this involves activation of PI3K/PKB, p44/42 MAPKs, partially p70s6k, and possibly PKC; Ex-4 and Ex-9 both have GLP-1-like effect upon glucose transport, in which both share with GLP-1 an activation of PI3K/PKB--partially in the case of Ex-4--and p44/42 MAPKs but not p70s6k.

Clinical endocrinology and metabolism. Glucagon-like peptide-1 and glucagon-like peptide-2

The glucagon-like peptides (glucagon-like peptide-1 (GLP-1) and glucagon-like peptide-2 (GLP-2)) are released from enteroendocrine cells in response to nutrient ingestion. GLP-1 enhances glucose-stimulated insulin secretion and inhibits glucagon secretion, gastric emptying and feeding. GLP-1 also has proliferative, neogenic and antiapoptotic effects on pancreatic beta-cells. More recent studies illustrate a potential protective role for GLP-1 in the cardiovascular and central nervous systems. GLP-2 is an intestinal trophic peptide that stimulates cell proliferation and inhibits apoptosis in the intestinal crypt compartment. GLP-2 also regulates intestinal glucose transport, food intake and gastric acid secretion and emptying, and improves intestinal barrier function. Thus, GLP-1 and GLP-2 exhibit a diverse array of metabolic, proliferative and cytoprotective actions with important clinical implications for the treatment of diabetes and gastrointestinal disease, respectively. This review will highlight our current understanding of the biology of GLP-1 and GLP-2, with an emphasis on both well-characterized and more novel therapeutic applications of these peptides.

Glucagon-like peptide-2: divergent signaling pathways

BACKGROUND AND AIMS

Glucagon-like peptide 2 (GLP-2) is an endogenous hormone with potent and specific intestinotrophic activity in vivo and in vitro. The aim of this study was to define the initial signal transduction mechanisms mediating the proliferative actions of GLP-2 on intestinal epithelial cells.

METHODS

The proliferative actions of GLP-2 on the human Caco-2 cell line were assessed. Specific G-protein inhibitors, pertussis and cholera toxin, were used to characterize the roles of early signal transduction mechanisms in mediating the proliferative actions of GLP-2 in these cells.

RESULTS

GLP-2 directly stimulated proliferation in the Caco-2 cells. GLP-2 stimulated proliferation was (1) inhibited in a dose-dependent fashion by both pertussis and cholera toxin and (2) augmented by 2',5'-dideoxyadenosine. Proliferation rates were inversely proportional to changes in intracellular cAMP concentration.

CONCLUSIONS

Our findings suggest that a G-protein-linked signaling pathway is involved with GLP-2 bioactivity in the intestinal epithelial cell line Caco-2.

Dual effects of pituitary adenylate cyclase-activating polypeptide and isoproterenol on lipid metabolism and signaling in primary rat adipocytes

Pituitary adenylate cyclase-activating peptide (PACAP) is a neuropeptide that exerts its effects throughout the body by elevating the intracellular amounts of cAMP. In adipocytes, an increased amount of cAMP is associated with increased lipolysis. In this work we evaluated the effects of PACAP38 on triglyceride metabolism in primary rat adipocytes. Stimulation of adipocytes with PACAP (0.1-100 nm) resulted in stimulation of lipolysis to the same extent as isoproterenol. Lipolysis was blocked by 25 microm of the protein kinase A inhibitor H-89 and potentiated in the presence of 10 microm OPC3911, a phosphodiesterase 3 inhibitor. In addition, PACAP38 induced activation of protein kinase A. Insulin efficiently inhibited PACAP38-induced lipolysis in a phosphatidyl inositol 3-kinase and phosphodiesterase 3-dependent manner. Interestingly, we also found that PACAP38, as well as isoproterenol, induced potentiation of lipogenesis in the presence of insulin. These results show that PACAP38 and isoproterenol mediate catabolic as well as anabolic effects in adipocytes, depending on the concentration of insulin present. We speculate that in the early postprandial state and during fasting, when insulin levels are low, PACAP and beta-adrenergic catecholamines induce lipolysis, whereas when higher levels of insulin are present, these agents potentiate the anabolic effect of insulin, i.e. storage of triglycerides.

International Union of Pharmacology. XXXV. The glucagon receptor family

Peptide hormones within the secretin-glucagon family are expressed in endocrine cells of the pancreas and gastrointestinal epithelium and in specialized neurons in the brain, and subserve multiple biological functions, including regulation of growth, nutrient intake, and transit within the gut, and digestion, energy absorption, and energy assimilation. Glucagon, glucagon-like peptide-1, glucagon-like peptide-2, glucose-dependent insulinotropic peptide, growth hormone-releasing hormone and secretin are structurally related peptides that exert their actions through unique members of a structurally related G protein-coupled receptor class 2 family. This review discusses advances in our understanding of how these peptides exert their biological activities, with a focus on the biological actions and structural features of the cognate receptors. The receptors have been named after their parent and only physiologically relevant ligand, in line with the recommendations of the International Union of Pharmacology Committee on Receptor Nomenclature and Drug Classification (NC-IUPHAR).

Role of endogenous glucagon-like peptide-1 in islet regeneration after partial pancreatectomy

A reduction in beta-cell mass is an important causative factor in type 1 and type 2 diabetes. Glucagon-like peptide-1 (GLP-1) and the long-acting agonist exendin 4 (Ex-4) expand beta-cell mass by stimulating neogenesis and proliferation. In the partial pancreatectomy (Ppx) model, exogenous Ex-4 promotes islet regeneration, leading to sustained improvement in glucose tolerance. In this study, we investigate the potential role of endogenous GLP-1 in islet growth. We examined beta-cell mass regeneration after 70% Ppx in mice receiving the GLP-1 antagonist Ex9-39 and in GLP-1R(-/-) mice. In Ex9-39-treated sham-operated mice, persistent fasting hyperglycemia was observed, but beta-cell mass was not diminished. In pancreatectomized mice, persistent glucose intolerance was noted, but this was not further exacerbated by Ex9-39. Accordingly, beta-cell mass recovery of Ppx mice was not impaired by Ex9-39. In contrast, GLP-1R(-/-) CD1 mice showed worse glucose intolerance after Ppx compared with wild-type CD1 Ppx mice, and this correlated with a significant defect in beta-cell mass regeneration. The recovery of beta-cell mass differed markedly in the BALB/c and CD1 control mice, indicating a significant role of genetic background in the regulation of beta-cell mass. These studies point to a role for endogenous GLP-1 in beta-cell regeneration after Ppx in mice.

The diverse roles of specific GLP-1 receptors in the control of food intake and the response to visceral illness

Intracerebroventricular administration of glucagon-like peptide-1 (7-36) amide (GLP-1) reduces food intake and produces symptoms of visceral illness, such as a conditioned taste aversion (CTA). The central hypothesis of the present work is that separate populations of GLP-1 receptors mediate the anorexia and taste aversion associated with GLP-1 administration. To test this hypothesis, we first compared the ability of various doses of GLP-1 to induce anorexia or CTA when administered into either the lateral or fourth ventricle. Lateral and fourth ventricular GLP-1 resulted in reduction of food intake at similar doses, whereas only lateral ventricular GLP-1 resulted in a CTA. Such data indicate that both hypothalamic and caudal brainstem GLP-1 receptors are likely to participate in the ability of GLP-1 to reduce food intake. We also hypothesized that the site that must mediate the ability of GLP-1 to induce visceral illness is in the central nucleus of the amygdala (CeA). Administration of 0.2 or 1.0 microg of GLP-1 (7-36) but not the inactive GLP-1 (9-36) resulted in a strong CTA with no accompanying anorexia. In addition, bilateral CeA administration of 2.5 microg of a GLP-1 receptor antagonist before intraperitoneal administration of the toxin lithium chloride resulted in a diminished CTA. Together, these data indicate that separate GLP-1 receptor populations mediate the multiple responses to GLP-1. These results indicate that GLP-1 is a flexible system that can be activated under various circumstances to alter the ingestion of nutrients and/or produce other visceral illness responses, depending on the ascending pathways of the GLP-1 system that are recruited.

Local, exendin-(9-39)-insensitive, site of action of GLP-1 in canine ileum

Glucagon-like peptide-1 (GLP-1) modulates glucose levels following a meal, including by inhibition of gastric emptying and intestinal transport. Intra-arterial injection of GLP-1 into the gastric corpus, antrum, or pylorus of anesthetized dogs had no effect on the contractile activity of the resting or neurally activated stomach. GLP-1 injected intra-arterially inhibited intestinal segments when activated by enteric nerve stimulation but not by acetylcholine. Isolated ileum segments were perfused intra-arterially, instrumented with strain gauges to record circular muscle activity and with subserosal electrodes to stimulate enteric nerves. GLP-1 caused concentration-dependent inhibition of nerve-stimulated phasic but not tonic activity. This was absent during TTX-induced activity and partly prevented by N(G)-nitro-L-arginine. Exendin-(9-39), the GLP-1 antagonist, had no intrinsic activity and did not affect the actions of GLP-1. Capsaicin mimicked the effects of GLP-1 and may have reduced the effect of subsequent GLP-1. GLP-1 may mediate paracrine action on afferent nerves in the canine ileal mucosa using an unusual receptor.

Exendin-4 increases insulin sensitivity via a PI-3-kinase-dependent mechanism: contrasting effects of GLP-1

The insulinotropic agent, exendin-4, is a long-acting analogue of glucagon-like peptide-1 (GLP-1) which improves glucose tolerance in humans and animals with diabetes, but the underlying mechanisms and the effects of exendin-4 on peripheral (muscle/fat) insulin action are unclear. Previous in vivo and clinical studies have been difficult to interpret because of complex, simultaneous changes in insulin and glucagon levels and possible effects on hepatic metabolism. Thus, the comparative effects of exendin-4 and GLP-1 on insulin-stimulated 2-[3H]deoxyglucose (2-DOG) uptake were measured in fully differentiated L6 myotubes and 3T3-adipocytes, including co-incubation with inhibitors of the PI-3-kinase (wortmannin) and mitogen-activated protein (MAP) kinase (PD098059) pathways. In L6 myotubes, there was a concentration-dependent and PI-3-kinase-dependent increase in insulin-stimulated 2-DOG uptake with exendin-4 and GLP-1, e.g. for exendin-4 the C(I-200) value (concentration of insulin required to increase 2-DOG uptake 2-fold) decreased from 1.3 +/- 1.4 x 10(-7)M (insulin alone, n=16) to 5.9 +/- 1.3 x 10(-8)M (insulin+exendin-4 0.1nM, n=18, P<0.03). A similar insulin-sensitizing effect was observed with exendin-4 in 3T3-adipocytes, but GLP-1 had no effect on adipocyte insulin sensitivity. In conclusion, this is the first direct evidence showing that exendin-4 increases insulin-stimulated glucose uptake in muscle and fat derived cells via a pathway that involves PI-3-kinase activation. Furthermore, the contrasting responses of exendin and GLP-1 in 3T3-adipocytes suggest that the peripheral insulin-sensitizing effect of exendin-4 (in contrast to the insulinotropic effect) does not involve the GLP-1 receptor pathway.