Exendin-4 agonist and exendin(9-39)amide antagonist of the GLP-1(7-36)amide effects in liver and muscle

GLP-1 Receptor Agonists in Neurodegeneration: Neurovascular Unit in the Spotlight

Defects in brain energy metabolism and proteopathic stress are implicated in age-related degenerative neuronopathies, exemplified by Alzheimer’s disease (AD) and Parkinson’s disease (PD). As the currently available drug regimens largely aim to mitigate cognitive decline and/or motor symptoms, there is a dire need for mechanism-based therapies that can be used to improve neuronal function and potentially slow down the underlying disease processes. In this context, a new class of pharmacological agents that achieve improved glycaemic control via the glucagon-like peptide 1 (GLP-1) receptor has attracted significant attention as putative neuroprotective agents. The experimental evidence supporting their potential therapeutic value, mainly derived from cellular and animal models of AD and PD, has been discussed in several research reports and review opinions recently. In this review article, we discuss the pathological relevance of derangements in the neurovascular unit and the significance of neuron–glia metabolic coupling in AD and PD. With this context, we also discuss some unresolved questions with regard to the potential benefits of GLP-1 agonists on the neurovascular unit (NVU), and provide examples of novel experimental paradigms that could be useful in improving our understanding regarding the neuroprotective mode of action associated with these agents.

Gut Peptide Agonism in the Treatment of Obesity and Diabetes

Obesity is a global healthcare challenge that gives rise to devastating diseases such as the metabolic syndrome, type-2 diabetes (T2D), and a variety of cardiovascular diseases. The escalating prevalence of obesity has led to an increased interest in pharmacological options to counteract excess weight gain. Gastrointestinal hormones such as glucagon, amylin, and glucagon-like peptide-1 (GLP-1) are well recognized for influencing food intake and satiety, but the therapeutic potential of these native peptides is overall limited by a short half-life and an often dose-dependent appearance of unwanted effects. Recent clinical success of chemically optimized GLP-1 mimetics with improved pharmacokinetics and sustained action has propelled pharmacological interest in using bioengineered gut hormones to treat obesity and diabetes. In this article, we summarize the basic biology and signaling mechanisms of selected gut peptides and discuss how they regulate systemic energy and glucose metabolism. Subsequently, we focus on the design and evaluation of unimolecular drugs that combine the beneficial effects of selected gut hormones into a single entity to optimize the beneficial impact on systems metabolism. © 2020 American Physiological Society. Compr Physiol 10:99-124, 2020.

Glucagon, GLP-1 and Thermogenesis

Brown adipose tissue (BAT) thermogenesis is a conserved mechanism to maintain body temperature in mammals. However, since BAT contribution to energy expenditure can represent a relevant modulator of metabolic homeostasis, many studies have focused on the nervous system and endocrine factors that control the activity of this tissue. There is long-established evidence that the counter-regulatory hormone glucagon negatively influences energy balance, enhances satiety, and increases energy expenditure. Despite compelling evidence showing that glucagon has direct action on BAT thermogenesis, recent findings are questioning this conventional attribute of glucagon action. Glucagon like peptide-1 (GLP-1) is an incretin secreted by the intestinal tract which strongly decreases feeding, and, furthermore, improves metabolic parameters associated with obesity and diabetes. Therefore, GLP-1 receptors (GLP-1-R) have emerged as a promising target in the treatment of metabolic disorders. In this short review, we will summarize the latest evidence in this regard, as well as the current therapeutic glucagon- and GLP-1-based approaches to treating obesity.

Diabetes pharmacotherapy and effects on the musculoskeletal system

Persons with type 1 or type 2 diabetes have a significantly higher fracture risk than age-matched persons without diabetes, attributed to disease-specific deficits in the microarchitecture and material properties of bone tissue. Therefore, independent effects of diabetes drugs on skeletal integrity are vitally important. Studies of incretin-based therapies have shown divergent effects of different agents on fracture risk, including detrimental, beneficial, and neutral effects. The sulfonylurea class of drugs, owing to its hypoglycemic potential, is thought to amplify the risk of fall-related fractures, particularly in the elderly. Other agents such as the biguanides may, in fact, be osteo-anabolic. In contrast, despite similarly expected anabolic properties of insulin, data suggests that insulin pharmacotherapy itself, particularly in type 2 diabetes, may be a risk factor for fracture, negatively associated with determinants of bone quality and bone strength. Finally, sodium-dependent glucose co-transporter 2 inhibitors have been associated with an increased risk of atypical fractures in select populations, and possibly with an increase in lower extremity amputation with specific SGLT2I drugs. The role of skeletal muscle, as a potential mediator and determinant of bone quality, is also a relevant area of exploration. Currently, data regarding the impact of glucose lowering medications on diabetes-related muscle atrophy is more limited, although preclinical studies suggest that various hypoglycemic agents may have either aggravating (sulfonylureas, glinides) or repairing (thiazolidinediones, biguanides, incretins) effects on skeletal muscle atrophy, thereby influencing bone quality. Hence, the therapeutic efficacy of each hypoglycemic agent must also be evaluated in light of its impact, alone or in combination, on musculoskeletal health, when determining an individualized treatment approach. Moreover, the effect of newer medications (potentially seeking expanded clinical indication into the pediatric age range) on the growing skeleton is largely unknown. Herein, we review the available literature regarding effects of diabetes pharmacotherapy, by drug class and/or by clinical indication, on the musculoskeletal health of persons with diabetes.

Peptide-based multi-agonists: a new paradigm in metabolic pharmacology

Obesity and its comorbidities, such as type 2 diabetes, are pressing worldwide health concerns. Available anti-obesity treatments include weight loss pharmacotherapies and bariatric surgery. Whilst surgical interventions typically result in significant and sustained weight loss, available pharmacotherapies are far less effective, typically decreasing body weight by no more than 5-10%. An emerging class of multi-agonist drugs may eventually bridge this gap. This new class of specially tailored drugs hybridizes the amino acid sequences of key metabolic hormones into one single entity with enhanced potency and sustained action. Successful examples of this strategy include multi-agonist drugs targeting the receptors for glucagon-like peptide-1 (GLP-1), glucagon and the glucose-dependent insulinotropic polypeptide (GIP). Due to the simultaneous activity at several metabolically relevant receptors, these multi-agonists offer improved body weight loss and glucose tolerance relative to their constituent monotherapies. Further advancing this concept, chimeras were generated that covalently link nuclear acting hormones such as oestrogen, thyroid hormone (T₃ ) or dexamethasone to peptide hormones such as GLP-1 or glucagon. The benefit of this strategy is to restrict the nuclear hormone action exclusively to cells expressing the peptide hormone receptor, thereby maximizing combinatorial metabolic efficacy of both drug constituents in the target cells whilst preventing the nuclear hormone cargo from entering and acting on cells devoid of the peptide hormone receptor, in which the nuclear hormone might have unwanted effects. Many of these multi-agonists are in preclinical and clinical development and may represent new and effective tools in the fight against obesity and its comorbidities.

Pleiotropic Effects of GLP-1 and Analogs on Cell Signaling, Metabolism, and Function

The incretin hormone Glucagon-Like Peptide-1 (GLP-1) is best known for its "incretin effect" in restoring glucose homeostasis in diabetics, however, it is now apparent that it has a broader range of physiological effects in the body. Both in vitro and in vivo studies have demonstrated that GLP-1 mimetics alleviate endoplasmic reticulum stress, regulate autophagy, promote metabolic reprogramming, stimulate anti-inflammatory signaling, alter gene expression, and influence neuroprotective pathways. A substantial body of evidence has accumulated with respect to how GLP-1 and its analogs act to restore and maintain normal cellular functions. These findings have prompted several clinical trials which have reported GLP-1 analogs improve cardiac function, restore lung function and reduce mortality in patients with obstructive lung disease, influence blood pressure and lipid storage, and even prevent synaptic loss and neurodegeneration. Mechanistically, GLP-1 elicits its effects via acute elevation in cAMP levels, and subsequent protein kinase(s) activation, pathways well-defined in pancreatic β-cells which stimulate insulin secretion in conjunction with elevated Ca2+ and ATP. More recently, new studies have shed light on additional downstream pathways stimulated by chronic GLP-1 exposure, findings which have direct relevance to our understanding of the potential therapeutic effects of longer lasting analogs recently developed for clinical use. In this review, we provide a comprehensive description of the diverse roles for GLP-1 across multiple tissues, describe downstream pathways stimulated by acute and chronic exposure, and discuss novel pleiotropic applications of GLP-1 mimetics in the treatment of human disease.

Anti-Obesity Therapy: from Rainbow Pills to Polyagonists

With their ever-growing prevalence, obesity and diabetes represent major health threats of our society. Based on estimations by the World Health Organization, approximately 300 million people will be obese in 2035. In 2015 alone there were more than 1.6 million fatalities attributable to hyperglycemia and diabetes. In addition, treatment of these diseases places an enormous burden on our health care system. As a result, the development of pharmacotherapies to tackle this life-threatening pandemic is of utmost importance. Since the beginning of the 19th century, a variety of drugs have been evaluated for their ability to decrease body weight and/or to improve deranged glycemic control. The list of evaluated drugs includes, among many others, sheep-derived thyroid extracts, mitochondrial uncouplers, amphetamines, serotonergics, lipase inhibitors, and a variety of hormones produced and secreted by the gastrointestinal tract or adipose tissue. Unfortunately, when used as a single hormone therapy, most of these drugs are underwhelming in their efficacy or safety, and placebo-subtracted weight loss attributed to such therapy is typically not more than 10%. In 2009, the generation of a single molecule with agonism at the receptors for glucagon and the glucagon-like peptide 1 broke new ground in obesity pharmacology. This molecule combined the beneficial anorectic and glycemic effects of glucagon-like peptide 1 with the thermogenic effect of glucagon into a single molecule with enhanced potency and sustained action. Several other unimolecular dual agonists have subsequently been developed, and, based on their preclinical success, these molecules illuminate the path to a new and more fruitful era in obesity pharmacology. In this review, we focus on the historical pharmacological approaches to treat obesity and glucose intolerance and describe how the knowledge obtained by these studies led to the discovery of unimolecular polypharmacology.

Oxidative Stress, Intrauterine Growth Restriction, and Developmental Programming of Type 2 Diabetes

Intrauterine growth restriction (IUGR) leads to reduced birth weight and the development of metabolic diseases such as Type 2 diabetes in adulthood. Mitochondria dysfunction and oxidative stress are commonly found in key tissues (pancreatic islets, liver, and skeletal muscle) of IUGR individuals. In this review, we explore the role of oxidative stress in IUGR-associated diabetes etiology.

Liraglutide improves hepatic insulin resistance via the canonical Wnt signaling pathway

Liraglutide, a modified form of glucagon‑like peptide‑1 (GLP‑1), is used in the treatment of diabetes mellitus. However, the underlying mechanism by which liraglutide improves liver insulin resistance remains to be elucidated. The proto‑oncogene Wnt (Wnt) signaling pathway has been reported to be associated with glucose and lipid metabolism. Using in vivo and in vitro models of diabetes and insulin resistance, it was investigated whether the beneficial effects of liraglutide on liver glucose metabolism are mediated by the Wnt signaling pathway. The results of the present study demonstrate that body weight, fasting blood glucose, insulin levels and the homeostasis model assessment for insulin resistance were markedly decreased in db/db mice treated with liraglutide compared with control mice. Liraglutide also improved liver morphology and reduced the accumulation of lipid droplets. Furthermore, the expression of glucose-6-phosphatase and phosphoenolpyruvate carboxykinase was downregulated, whereas the expression of phosphorylated forkhead box O1, Wnt signaling pathway‑associated molecules, β‑catenin, transcription factor 7‑like 2 and phosphorylated glycogen synthase kinase-3β was upregulated in the liver of mice treated with liraglutide. In the in vitro study, increased gluconeogenesis and decreased glucose uptake rates were observed in insulin resistant hepatocytes; treatment with liraglutide significantly reversed this effect. Furthermore, transfection of insulin resistant hepatocytes with β‑catenin small interfering RNA attenuated the effects of liraglutide, suggesting that liraglutide improves insulin resistance via activating the β‑catenin/Wnt signaling pathway. The results of the present study suggest a novel mechanism underlying liraglutide‑mediated improvements in insulin resistance in the liver. The Wnt signaling pathway may be a potential therapeutic target for the treatment of altered hepatic physiology in insulin resistance.

Glucagon-like peptide-1 analogues exenatide and liraglutide exert inhibitory effect on the early phase of liver regeneration after partial hepatectomy in rats

Glucagon-like peptide-1 (GLP-1) is an incretin known for proliferative and antiapoptotic effects on various tissues. Exenatide and Liraglutide are GLP-1 analogues used in clinical practice as antidiabetic drugs. Since GLP-1 and its analogues exert significant effect on liver metabolism and since changes in intermediary metabolism play an important role in the process of liver regeneration, we decided to determine the effect of Exenatide and Liraglutide on the early phase of liver regeneration and selected metabolic parameters in a model of 2/3 partial hepatectomy (PHx) in rats. Animals were submitted either to PHx or laparotomy and received 3 doses of either GLP-1 analogues (Exenatide - 42 microg/kg b.w., Liraglutide - 0.75 mg/kg b.w.) or saline intraperitoneally. We analyzed body and liver weight, liver bromodeoxyuridine incorporation, liver content of DNA, triacylglycerols and cholesterol and biochemical serum parameters. Bromodeoxyuridine labeling was significantly lower in hepatectomized rats receiving either type of GLP-1 analogues when compared to hepatectomized controls. This effect was more pronounced in the Liraglutide group compared to Exenatide (p<0.001). In addition, liver DNA content was lower in hepatectomized rats receiving Liraglutide than in hepatectomized control rats (p<0.001). In conclusion, GLP-1 analogues Exenatide and Liraglutide significantly inhibited an early phase of liver regeneration after PHx in rats. This inhibitory effect was more pronounced in rats receiving Liraglutide.

The GLP-1 receptor agonists exenatide and liraglutide activate Glucose transport by an AMPK-dependent mechanism

Aims/hypothesis

Potentiation of glucose-induced insulin secretion is the main mechanism of exenatide (EXE) antidiabetic action, however, increased glucose utilization by peripheral tissues has been also reported. We here studied the effect of EXE on glucose uptake by skeletal muscle cells.

Methods

2-deoxy-glucose (2DG) uptake and intracellular signal pathways were measured in rat L6 skeletal muscle myotubes exposed to 100 nmol/l EXE for up to 48 h. Mechanisms of EXE action were explored by inhibiting AMPK activity with compound C (CC, 40 μmol/l) or siRNAs (2 μmol/l).

Results

Time course experiments show that EXE increases glucose uptake up to 48 h achieving its maximal effect, similar to that induced by insulin, after 20 min (2- vs 2.5-fold-increase, respectively). Differently from insulin, EXE does not stimulate: (i) IR β-subunit- and IRS1 tyrosine phosphorylation and binding to p85 regulatory subunit of PI-3kinase; (ii) AKT activation; and (iii) ERK1/2 and JNK1/2 phosphorylation. Conversely, EXE increases phosphorylation of α-subunit of AMPK at Thr172 by 2.5-fold (p < 0.01). Co-incubation of EXE and insulin does not induce additive effects on 2DG-uptake. Inhibition of AMPK with CC, and reduction of AMPK protein expression by siRNA, completely abolish EXE-induced 2DG-uptake. Liraglutide, another GLP-1 receptor agonist, also stimulates AMPK phosphorylation and 2DG-uptake. Moreover, EXE stimulates 2DG-uptake also by L6 myotubes rendered insulin-resistant with methylglyoxal. Finally, EXE also induces glucose transporter Glut-4 translocation to the plasma membrane.

Conclusions/interpretation

In L6 myotubes, EXE and liraglutide increase glucose uptake in an insulin-independent manner by activating AMPK.

Electronic supplementary material

The online version of this article (doi:10.1186/s12967-016-0985-7) contains supplementary material, which is available to authorized users.

Gastrointestinal actions of glucagon-like peptide-1-based therapies: glycaemic control beyond the pancreas

The gastrointestinal hormone glucagon-like peptide-1 (GLP-1) lowers postprandial glucose concentrations by regulating pancreatic islet-cell function, with stimulation of glucose-dependent insulin and suppression of glucagon secretion. In addition to endocrine pancreatic effects, mounting evidence suggests that several gastrointestinal actions of GLP-1 are at least as important for glucose-lowering. GLP-1 reduces gastric emptying rate and small bowel motility, thereby delaying glucose absorption and decreasing postprandial glucose excursions. Furthermore, it has been suggested that GLP-1 directly stimulates hepatic glucose uptake, and suppresses hepatic glucose production, thereby adding to reduction of fasting and postprandial glucose levels. GLP-1 receptor agonists, which mimic the effects of GLP-1, have been developed for the treatment of type 2 diabetes. Based on their pharmacokinetic profile, GLP-1 receptor agonists can be broadly categorized as short- or long-acting, with each having unique islet-cell and gastrointestinal effects that lower glucose levels. Short-acting agonists predominantly lower postprandial glucose excursions, by inhibiting gastric emptying and intestinal glucose uptake, with little effect on insulin secretion. By contrast, long-acting agonists mainly reduce fasting glucose levels, predominantly by increased insulin and reduced glucagon secretion, with potential additional direct inhibitory effects on hepatic glucose production. Understanding these pharmacokinetic and pharmacodynamic differences may allow personalized antihyperglycaemic therapy in type 2 diabetes. In addition, it may provide the rationale to explore treatment in patients with no or little residual β-cell function.

Acute but not chronic activation of brain glucagon-like peptide-1 receptors enhances glucose-stimulated insulin secretion in mice

AIM

To investigate the role of brain glucagon-like peptide-1 (GLP-1) in pancreatic β-cell function.

METHODS

To determine the role of brain GLP-1 receptor (GLP-1R) on β-cell function, we administered intracerebroventricular (i.c.v.) infusions of GLP-1 or the specific GLP-1 antagonist exendin-9 (Ex-9), in both an acute and a chronic setting.

RESULTS

We observed that acute i.c.v. GLP-1 infusion potentiates glucose-stimulated insulin secretion (GSIS) and improves glucose tolerance, whereas central GLP-1R blockade with Ex-9 impaired glucose excursion after a glucose load. Sustained activation of central nervous system GLP-1R, however, did not produce any effect on either GSIS or glucose tolerance. Similarly, ex vivo GSIS performed in islets from mice chronically infused with i.c.v. GLP-1 resulted in no differences compared with controls. In addition, in mice fed a high-fat diet we observed that acute i.c.v. GLP-1 infusion improved glucose tolerance without changes in GSIS, while chronic GLP-1R activation had no effect on glucose homeostasis.

CONCLUSIONS

Our results indicate that, under non-clamped conditions, brain GLP-1 plays a functional neuroendocrine role in the acute regulation of glucose homeostasis in both lean and obese rodents.

Direct effect of incretin hormones on glucose and glycerol metabolism and hemodynamics

The objective of this study was to assess the insulin-independent effects of incretin hormones on glucose and glycerol metabolism and hemodynamics under euglycemic and hyperglycemic conditions. Young, healthy men (n=10) underwent three trials in a randomized, controlled, crossover study. Each trial consisted of a two-stage (euglycemia and hyperglycemia) pancreatic clamp (using somatostatin to prevent endogenous insulin secretion). Glucose and lipid metabolism was measured via infusion of stable glucose and glycerol isotopic tracers. Hemodynamic variables (femoral, brachial, and common carotid artery blood flow and flow-mediated dilation of the brachial artery) were also measured. The three trials differed as follows: 1) saline [control (CON)], 2) glucagon-like peptide (GLP-1, 0.5 pmol·kg(-1)·min(-1)), and 3) glucose-dependent insulinotropic polypeptide (GIP, 1.5 pmol·kg(-1)·min(-1)). No between-trial differences in glucose infusion rates (GIR) or glucose or glycerol kinetics were seen during euglycemia, whereas hyperglycemia resulted in increased GIR and glucose rate of disappearance during GLP-1 compared with CON and GIP (P<0.01 for all). However, when normalized to insulin levels, no differences between trials were seen for GIR or glucose rate of disappearance. Besides a higher femoral blood flow during hyperglycemia with GIP (vs. CON and GLP-1, P<0.001), no between-trial differences were seen for the hemodynamic variables. In conclusion, GLP-1 and GIP have no direct effect on whole body glucose metabolism or hemodynamics during euglycemia. On the contrary, during hyperglycemia, GIP increases femoral artery blood flow with no effect on glucose metabolism, whereas GLP-1 increases glucose disposal, potentially due to increased insulin levels.

Liraglutide enhances glucose transporter 4 translocation via regulation of AMP-activated protein kinase signaling pathways in mouse skeletal muscle cells

OBJECTIVE

Liraglutide is an anti-diabetic drug and human glucagon-like peptide-1 (GLP-1) analog that primarily functions in the pancreas. However, its extra-pancreatic functions are not clear. Skeletal muscle tissue is an important determinant of blood glucose and cells take in approximately 80% of dietary glucose via glucose transporter 4 (GLUT4) on the plasma membrane. Insulin and muscle contraction are two physiological stimuli of GLUT4 translocation to the cell membrane from intracellular storage compartments, but the signaling mechanisms that mediate these processes are different. AMP-activated protein kinase (AMPK) and Akt are the key signal molecules mediating the effects of muscle contraction and insulin, respectively, on GLUT4 translocation. Here, we investigate the effect of liraglutide on GLUT4 translocation and the roles of AMPK and Akt in this mechanism in skeletal muscle cells by stably expressing GLUT4myc with an exofacial myc-epitope C(2)C(12)-GLUT4myc.

MATERIALS/METHODS

The cell surface GLUT4myc levels were determined by an antibody-coupled colorimetric assay. The phosphorylation levels of AMPK, Akt, AS160, TBC1D1, and GLUT4 were determined by western blotting. The cAMP levels were measured by an ELISA kit. siRNA was transfected with Lipofectamine 2000. Analysis of variance (ANOVA) was used for data analysis.

RESULTS

Liraglutide stimulated GLUT4 translocation in C(2)C(12)-GLUT4myc myotubes. Liraglutide increased the intracellular cAMP levels and the phosphorylation of AMPK, AS160, and TBC1D1. Akt phosphorylation and GLUT4 expression were not affected. Inhibition of AMPK by siRNA or Compound C reduced liraglutide-induced GLUT4 translocation.

CONCLUSION

Our results suggest that liraglutide may induce GLUT4 translocation by activation of AMPK in muscle cells.

Effects of intraportal exenatide on hepatic glucose metabolism in the conscious dog

Incretins improve glucose metabolism through multiple mechanisms. It remains unclear whether direct hepatic effects are an important part of exenatide's (Ex-4) acute action. Therefore, the objective of this study was to determine the effect of intraportal delivery of Ex-4 on hepatic glucose production and uptake. Fasted conscious dogs were studied during a hyperglycemic clamp in which glucose was infused into the hepatic portal vein. At the same time, portal saline (control; n = 8) or exenatide was infused at low (0.3 pmol·kg⁻¹·min⁻¹, Ex-4-low; n = 5) or high (0.9 pmol·kg⁻¹·min⁻¹, Ex-4-high; n = 8) rates. Arterial plasma glucose levels were maintained at 160 mg/dl during the experimental period. This required a greater rate of glucose infusion in the Ex-4-high group (1.5 ± 0.4, 2.0 ± 0.7, and 3.7 ± 0.7 mg·kg⁻¹·min⁻¹ between 30 and 240 min in the control, Ex-4-low, and Ex-4-high groups, respectively). Plasma insulin levels were elevated by Ex-4 (arterial: 4,745 ± 428, 5,710 ± 355, and 7,262 ± 1,053 μU/ml; hepatic sinusoidal: 14,679 ± 1,700, 15,341 ± 2,208, and 20,445 ± 4,020 μU/ml, 240 min, area under the curve), whereas the suppression of glucagon was nearly maximal in all groups. Although glucose utilization was greater during Ex-4 infusion (5.92 ± 0.53, 6.41 ± 0.57, and 8.12 ± 0.54 mg·kg⁻¹·min⁻¹), when indices of hepatic, muscle, and whole body glucose uptake were expressed relative to circulating insulin concentrations, there was no indication of insulin-independent effects of Ex-4. Thus, this study does not support the notion that Ex-4 generates acute changes in hepatic glucose metabolism through direct effects on the liver.

Exendin-4 reduces glycemia by increasing liver glucokinase activity: an insulin independent effect

Exendin-4 is a stable peptide agonist of GLP-1 receptor that exhibits insulinotropic actions. Some in vivo studies indicated insulin-independent glucoregulatory actions of exendin-4. That finding prompted us to evaluate effects of exendin-4 on liver glucose metabolism. Acute and chronic treatment of exendin-4 resulted in increased hepatic glucokinase activity in db/db mice but not in lean C57 mice. The stimulatory effect of exendin-4 on glucokinase activity was abrogated by exendin 9-39, a GLP-1 antagonist. Exposure of hepatocytes isolated from db/db mice to exendin-4 elicited a rapid increase in cAMP, which was synergized by IBMX, an inhibitor of cAMP degradation. The GLP-1 antagonist, exendin 9-39, has abolished the cAMP generating effects of exendin-4 as well. Furthermore, chronic treatment of exendin-4 in streptozotocin-treated C57 mice resulted in restoration of hepatic glycogen, an indicator of improved glucose metabolism, without apparent changes in serum insulin levels. In conclusion, exendin-4 increased glucokinase enzyme protein and activity in liver via a mechanism parallel to and independent of insulin. Exendin-4-induced increase in hepatic glucokinase activity is more pronounced in the presence of hepatic insulin resistance. This beneficial effect of exendin-4 on liver glucokinase activity may be mediated by GLP-1 receptor.

Glucagon like peptide-1-induced glucose metabolism in differentiated human muscle satellite cells is attenuated by hyperglycemia

BACKGROUND

Glucagon like peptide-1 (GLP-1) stimulates insulin secretion from the pancreas but also has extra-pancreatic effects. GLP-1 may stimulate glucose uptake in cultured muscle cells but the mechanism is not clearly defined. Furthermore, while the pancreatic effects of GLP-1 are glucose-dependent, the glucose-dependency of its extra-pancreatic effects has not been examined.

METHODS

Skeletal muscle satellite cells isolated from young (22.5 ± 0.97 yr), lean (BMI 22.5 ± 0.6 kg/m(2)), healthy males were differentiated in media containing either 22.5 mM (high) or 5 mM (normal) glucose for 7 days in the absence or presence of insulin and/or various GLP-1 concentrations. Myocellular effects of GLP-1, insulin and glucose were assessed by western-blot, glucose uptake and glycogen synthesis.

RESULTS

We firstly show that the GLP-1 receptor protein is expressed in differentiated human muscle satellite cells (myocytes). Secondly, we show that in 5 mM glucose media, exposure of myocytes to GLP-1 results in a dose dependent increase in glucose uptake, GLUT4 amount and subsequently glycogen synthesis in a PI3K dependent manner, independent of the insulin signaling cascade. Importantly, we provide evidence that differentiation of human satellite cells in hyperglycemic (22.5 mM glucose) conditions increases GLUT1 expression, and renders the cells insulin resistant and interestingly GLP-1 resistant in terms of glucose uptake and glycogen synthesis. Hyperglycemic conditions did not affect the ability of insulin to phosphorylate downstream targets, PKB or GSK3. Interestingly we show that at 5 mM glucose, GLP-1 increases GLUT4 protein levels and that this effect is abolished by hyperglycemia.

CONCLUSIONS

GLP-1 increases glucose uptake and glycogen synthesis into fully-differentiated human satellite cells in a PI3-K dependent mechanism potentially through increased GLUT4 protein levels. The latter occurs independently of the insulin signaling pathway. Attenuation of both GLP-1 and insulin-induced glucose metabolism by hyperglycemia is likely to occur downstream of PI3K.

Chronic DPP-IV inhibition with PKF-275-055 attenuates inflammation and improves gene expressions responsible for insulin secretion in streptozotocin induced diabetic rats

Inhibitors of dipeptidyl peptidase-4 (DPP-IV), a key regulator of the actions of incretin hormones, exert antihyperglycemic effects in type 2 diabetic patients. A major question concerns the potential ability of long term DPP-IV inhibition to have beneficial disease-modifying effects, specifically to attenuate loss of pancreatic β-cell mass due to oxidative stress induced inflammation. Here, we investigated the effects of a potent and selective DPP-4 inhibitor, an analog of vildagliptin (PKF-275-055), on glycemic control, pancreatic β-cell mass, genes and proteins expressions, tumor necrosis factor-alpha, and nitric oxide in an n2-STZ diabetic model of rat with defects in insulin sensitivity and secretion. To induce NIDDM, streptozotocin (STZ) 90 mg/kg was administered i.p. to a group of 2 days old pups. Diabetic rats were administered orally with vildagliptin analog PKF-275-055. Saline treated animals served as diabetic control. Significant and dose-dependent correction of postprandial hyperglycemia was observed in diabetic rats following 8 weeks of chronic therapy. Treatment with PKF-275-055 showed increased the number of insulin-positive β-cells in islets and improved the expressions of genes and proteins are responsible for insulin secretions. In addition, treatment of rats with PKF-275-055 significantly increased insulin content, glycogen content and total proteins content; and decreased the inflammatory markers i.e. nitric oxide and TNF-alpha. The present studies indicate that PKF-275-055 is a novel selective DPP-IV inhibitor having potential to reduce inflammation that might be a potential agent for type 2 diabetes.

Vildagliptin selectively ameliorates GLP-1, GLUT4, SREBP-1c mRNA levels and stimulates β-cell proliferation resulting in improved glucose homeostasis in rats with streptozotocin-induced diabetes

OBJECTIVE

Inhibitors of dipeptidyl peptidase-4 (DPP-4), a key regulator of the actions of incretin hormones, exert antihyperglycemic effects in type 2 diabetic patients. A major unanswered question concerns the potential ability of DPP-4 inhibition to have beneficial disease-modifying effects, specifically to attenuate loss of pancreatic β-cell mass and function due to oxidative stress induced inflammation. Here, we investigated the effects of a potent and selective DPP-4 inhibitor vildagliptin on glycemic control, pancreatic β-cell mass and function, genes and proteins expressions, tumor necrosis factor-alpha, and nitric oxide in an n2-STZ diabetic model of rat with defects in insulin sensitivity and secretion.

METHOD

To induce NIDDM, STZ (sigma chemicals, USA) (90 mg/kg) was administered i.p. to a group of 2 days old pups. Another group of pups received only saline. The pups were weaned for 21 days, and 6 weeks after the injection of STZ, the animals were checked for fasting glucose level (FPG) ≥160 mg/dl were considered as diabetic.

RESULTS

Significant and dose-dependent correction of postprandial and fasting hyperglycemia was observed in diabetic rats following 8 weeks of chronic therapy. Treatment with vildagliptin showed increase in the number of insulin-positive β-cells in islets and improved the expressions of genes and proteins are responsible for insulin secretions. In addition, treatment of rats with vildagliptin significantly increased insulin content; and decreased the nitric oxide and TNF-alpha concentration.

CONCLUSION

These findings suggest that DPP-4 inhibitors may offer long-lasting efficacy in the treatment of diabetes mellitus by modifying the courses of the disease.

Incretin-based therapies--review of the physiology, pharmacology and emerging clinical experience

Diabetes therapies based on manipulation of the incretin system are now widely available, with millions of people receiving treatment. The incretin hormones, glucose-dependent insulinotropic peptide and glucagon-like peptide-1 are released from endocrine cells in the small intestinal mucosa primarily in response to oral nutrient ingestion. They have various effects, but those most relevant to metabolic dysfunction include stimulation of insulin and suppression of glucagon secretion, with resultant reduction in fasting and postprandial glucose. Incretin secretion and/or action is impaired in type 2 diabetes, leading to development of strategies aimed at redressing this abnormality. These strategies include pharmacological inhibition of dipeptidyl peptidase-4, the enzyme responsible for the short half-life of endogenous incretins, and administration of long-acting dipeptidyl peptidase-4-resistant peptides that bind to and activate the glucagon-like peptide-1 receptor. In this review, we address aspects of incretin biology and pharmacotherapy with a view to highlighting potentially clinically relevant issues and areas of basic research that may impinge on these.

Normalizing action of exendin-4 and GLP-1 in the glucose metabolism of extrapancreatic tissues in insulin-resistant and type 2 diabetic states

Exendin-4 (Ex-4) mimics glucagon-like peptide-1 (GLP-1 or GCG as listed in the HUGO database), being anti-diabetic and anorectic, in stimulating glucose and lipid metabolism in extrapancreatic tissues. We studied the characteristics of Ex-4 and GLP-1 action, during prolonged treatment, on GLUTs expression (mRNA and protein), glycogen content (GC), glucose transport (GT), glycogen synthase a (GSa), and kinase (PI3K and MAPKs) activity, in liver, muscle, and fat of insulin-resistant (IR, by fructose) and type 2 diabetic (T2D, streptozotocin at birth) rats compared with normal rats. In both IR and T2D, the three tissues studied presented alterations in all measured parameters. In liver, GLP-1 and also Ex-4 normalized the lower than normal Glut2 (Slc2a2) expression and showed a trend to normalize the reduced GC in IR, and GLP-1, like Ex-4, also in T2D, effects mediated by PI3K and MAPKs. In skeletal muscle, neither GLP-1 nor Ex-4 modified Glut4 (Slc2a4) expression in either experimental model but showed normalization of reduced GT and GSa, in parallel with the normalization of reduced PI3K activity in T2D and MAPKs in both models. In adipose tissue, the altered GLUT4 expression in IR and T2D, along with reduced GT in IR and increased GT in T2D, and with hyperactivated PI3K in both, became normal after GLP-1 and Ex-4 treatment; yet, MAPKs, that were also higher, became normal only after Ex-4 treatment. The data shows that Ex-4, as well as GLP-1, exerts a normalizing effect on IR and T2D states through a distinct post-receptor mechanism, the liver being the main target for Ex-4 and GLP-1 to control glucose homeostasis.

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.

Exendin-4 improves hepatocyte injury by decreasing proliferation through blocking NGF/TrkA in diabetic mice

The hepatocytes express nerve growth factor (NGF) and its high affinity receptor tyrosine kinase A (TrkA). However, the link between NGF/TrkA system and hepatocyte proliferation in diabetic animals and the effects of exendin-4, a glucagon like peptide-1 (GLP-1) receptor agonist, on this system are not known. BALB/c male mice were divided into four groups. The first group was given citrate buffer only, the second group was administered exendin-4 alone, the third group received streptozotocin (STZ), and the fourth group was given both STZ and exendin-4. Exendin-4 (3μg/kg) was administered by subcutaneous injection daily for 30 days after the animals were rendered diabetic by administration of STZ (200mg/kg). With treatment of exendin-4 to the diabetic mice the following results were noted (i) NGF, TrkA and proliferating cell nuclear antigen positive hepatocytes were decreased; (ii) p75 neurotrophin receptor and caspase-3 positive hepatocyte could not be detected; (iii) liver alanine transaminase and aspartate transaminase activities, lipid peroxidation, protein carbonyl and myeloperoxidase levels were decreased; (iv) liver catalase, superoxide dismutase, glutathione peroxidase activities and glutathione levels were increased. These data suggest that exendin-4 might exerts its anti-proliferative action through blocking NGF/TrkA system and stimulating oxidative defense system in liver of diabetic mice.

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.

Exendin-4 exerts osteogenic actions in insulin-resistant and type 2 diabetic states

Poor control of glucose homeostasis accounts for diabetes-related bone loss. Incretins - GLP-1 and GIP - have been proposed to affect bone turnover. GLP-1, apart from its anti-diabetic and other actions, has shown to exert a bone anabolic effect in streptozotocin-induced type 2 diabetic (T2D) and fructose-induced insulin-resistant (IR) rats. Exendin-4 (Ex-4), a peptide of non-mammalian nature, is sharing with GLP-1 part of its structural sequence, and also several glucoregulatory effects in mammals in an even more efficient manner. We have explored the effect of continuous administration (3 days by osmotic pump) of Ex-4 or saline (control) on bone turnover factors and bone structure in T2D and IR rats, compared to N, and the possible interaction of Ex-4 with the Wnt signalling pathway. Blood was taken before and after treatment for plasma measurements; tibiae and femurs were collected for gene expression of bone markers (RT-PCR) and structure (microCT) analysis; we also measured the mRNA levels of LRP5 - an activator of the Wnt pathway - and those of DKK1 and sclerostin (SOST) - both blockers of LRP5 activity. Compared to N-control, plasma glucose and insulin were respectively higher and lower in T2D; osteocalcin (OC) and tartrate-resistant alkaline phosphatase 5b (TRAP5b) were lower; after Ex-4, these turnover markers were further reduced in T2D and IR, while TRAP5b increased in N. Bone OC, osteoprogeterin (OPG) and receptor activator of NF-kB ligand (RANKL) mRNA were lower in T2D and IR; Ex-4 increased OC in all groups and OPG in N and IR, reduced RANKL in N and T2D but increased it in IR; the LRP5/DKK1 and LRP5/SOST mRNA ratios were similarly decreased in T2D, but in IR, the latter ratio was reduced while the former was increased; after Ex-4, both ratios augmented in N, and that of LRP5/DKK1 tended to normalize in T2D and IR. In conclusion, Ex-4 exerts osteogenic effects in T2D and IR models, and interacts with the Wnt pathway to promote bone formation.

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.

Clinical review: The extrapancreatic effects of glucagon-like peptide-1 and related peptides

CONTEXT

Glucagon-like peptide-1 (GLP-1) 7-36 amide, an insulinotropic hormone released from the intestinal L cells in response to nutrient ingestion, has been extensively reviewed with respect to beta-cell function. However GLP-1 receptors are abundant in many other tissues. Thus, the function of GLP-1 is not limited to the islet cells, and it has regulatory actions on many other organs.

EVIDENCE ACQUISITION

A review of published, peer-reviewed medical literature (1987 to September 2008) on the extrapancreatic actions of GLP-1 was performed.

EVIDENCE SYNTHESIS

The extrapancreatic actions of GLP-1 include inhibition of gastric emptying and gastric acid secretion, thereby fulfilling the definition of GLP-1 as an enterogastrone. Other important extrapancreatic actions of GLP-1 include a regulatory role in hepatic glucose production, the inhibition of pancreatic exocrine secretion, cardioprotective and cardiotropic effects, the regulation of appetite and satiety, and stimulation of afferent sensory nerves. The primary metabolite of GLP-1, GLP-1 (9-36) amide, or GLP-1m, is the truncated product of degradation by dipeptidyl peptidase-4. GLP-1m has insulinomimetic effects on hepatic glucose production and cardiac function. Exendin-4 present in the salivary gland of the reptile, Gila monster (Heloderma suspectum), is a high-affinity agonist for the mammalian GLP-1 receptor. It is resistant to degradation by dipeptidyl peptidase-4, and therefore has a prolonged half-life.

CONCLUSION

GLP-1 and its metabolite have important extrapancreatic effects particularly with regard to the cardiovascular system and insulinomimetic effects with respect to glucose homeostasis. These effects may be particularly important in the obese state. GLP-1, GLP-1m, and exendin-4 therefore have potential therapeutic roles because of their diffuse extrapancreatic actions.

Exenatide sensitizes insulin-mediated whole-body glucose disposal and promotes uptake of exogenous glucose by the liver

OBJECTIVE

Recent progress suggests that exenatide, a mimetic of glucagon-like peptide-1 (GLP-1), might lower glycemia independent of increased beta-cell response or reduced gastrointestinal motility. We aimed to investigate whether exenatide stimulates glucose turnover directly in insulin-responsive tissues dependent or independent of insulinemia.

RESEARCH DESIGN AND METHODS

An intraportal glucose infusion clamp was used in dogs to measure glucose turnover to encompass potent activation of the putative glucose/GLP-1 sensor in the porto-hepatic circulation with exenatide. The modified glucose clamp was performed in the presence of postprandial hyperinsulinemia and hyperglycemia with exenatide (20 microg) or saline injected at 0 min. Furthermore, the role of hyperglycemia versus hyperinsulinemia in exenatide-mediated glucose disposal was studied.

RESULTS

With hyperinsulinemia and hyperglycemia, exenatide produced a significant increase in total glucose turnover by approximately 30%, as indicated by portal glucose infusion rate (saline 15.9 +/- 1.6 vs. exenatide 20.4 +/- 2.1 mg x kg(-1) x min(-1), P < 0.001), resulting from increased whole-body glucose disposal (R(d), approximately 20%) and increased net hepatic uptake of exogenous glucose ( approximately 80%). Reducing systemic hyperglycemia to euglycemia, exenatide still increased total glucose turnover by approximately 20% (saline 13.2 +/- 1.9 vs. exenatide 15.6 +/- 2.1 mg x kg(-1) x min(-1), P < 0.05) in the presence of hyperinsulinemia, accompanied by smaller increments in R(d) (12%) and net hepatic uptake of exogenous glucose (45%). In contrast, reducing hyperinsulinemia to basal levels, exenatide-increased total glucose turnover was completely abolished despite hyperglycemia (saline 2.9 +/- 0.6 vs. exenatide 2.3 +/- 0.3 mg x kg(-1) x min(-1), P = 0.29).

CONCLUSIONS

Exenatide directly stimulates glucose turnover by enhancing insulin-mediated whole-body glucose disposal and increasing hepatic uptake of exogenous glucose, contributing to its overall action to lower postprandial glucose excursions.

The role of incretins in glucose homeostasis and diabetes treatment

Incretins are gut hormones that are secreted from enteroendocrine cells into the blood within minutes after eating. One of their many physiological roles is to regulate the amount of insulin that is secreted after eating. In this manner, as well as others to be described in this review, their final common raison d'être is to aid in disposal of the products of digestion. There are two incretins, known as glucose-dependent insulinotropic peptide (GIP) and glucagon-like peptide-1 (GLP-1), that share many common actions in the pancreas but have distinct actions outside of the pancreas. Both incretins are rapidly deactivated by an enzyme called dipeptidyl peptidase 4 (DPP4). A lack of secretion of incretins or an increase in their clearance are not pathogenic factors in diabetes. However, in type 2 diabetes (T2DM), GIP no longer modulates glucose-dependent insulin secretion, even at supraphysiological (pharmacological) plasma levels, and therefore GIP incompetence is detrimental to beta-cell function, especially after eating. GLP-1, on the other hand, is still insulinotropic in T2DM, and this has led to the development of compounds that activate the GLP-1 receptor with a view to improving insulin secretion. Since 2005, two new classes of drugs based on incretin action have been approved for lowering blood glucose levels in T2DM: an incretin mimetic (exenatide, which is a potent long-acting agonist of the GLP-1 receptor) and an incretin enhancer (sitagliptin, which is a DPP4 inhibitor). Exenatide is injected subcutaneously twice daily and its use leads to lower blood glucose and higher insulin levels, especially in the fed state. There is glucose-dependency to its insulin secretory capacity, making it unlikely to cause low blood sugars (hypoglycemia). DPP4 inhibitors are orally active and they increase endogenous blood levels of active incretins, thus leading to prolonged incretin action. The elevated levels of GLP-1 are thought to be the mechanism underlying their blood glucose-lowering effects.

Effect of exendin-4 treatment upon glucose uptake parameters in rat liver and muscle, in normal and type 2 diabetic state

Exendin-4, like GLP-1, is insulinotropic, antidiabetic and glucoregulatory among other properties, which are thought to be exerted through the pancreatic GLP-1 receptor; exendin-4 is also an agonist of the GLP-1 stimulatory action upon liver and muscle glucose metabolism, where GLP-1 receptor is distinct from that in the pancreas. We investigated the action of prolonged treatment with exendin-4 upon glucose transport parameters in skeletal muscle and liver of normal rats and streptozotocin-induced type 2 diabetic rats (T2D). Muscle of T2D showed lower than normal glucose transport; exendin-4 did not modify the value in normal but normalized that in the T2D; unlike previously detected with GLP-1, no apparent modification was observed in GLUT-4 expression in either group after exendin-4, except for an increased GLUT-4 protein in normal rats. Yet, exendin-4 significantly stimulated liver GLUT-2-mRNA and -protein in T2D and normal rats, the effect upon GLUT-2-protein in T2D being higher than that in normal animals; this was accompanied by a normalizing action of exendin-4 upon the lower than normal liver glycogen in T2D rats. These data suggest that the liver may represent at least one of the major target organs for exendin-4 to exert its plasma lowering effect in diabetic state.

Arcuate glucagon-like peptide 1 receptors regulate glucose homeostasis but not food intake

OBJECTIVE

Glucagon-like peptide-1 (GLP-1) promotes glucose homeostasis through regulation of islet hormone secretion, as well as hepatic and gastric function. Because GLP-1 is also synthesized in the brain, where it regulates food intake, we hypothesized that the central GLP-1 system regulates glucose tolerance as well.

RESEARCH DESIGN AND METHODS

We used glucose tolerance tests and hyperinsulinemic-euglycemic clamps to assess the role of the central GLP-1 system on glucose tolerance, insulin secretion, and hepatic and peripheral insulin sensitivity. Finally, in situ hybridization was used to examine colocalization of GLP-1 receptors with neuropeptide tyrosine and pro-opiomelanocortin neurons.

RESULTS

We found that central, but not peripheral, administration of low doses of a GLP-1 receptor antagonist caused relative hyperglycemia during a glucose tolerance test, suggesting that activation of central GLP-1 receptors regulates key processes involved in the maintenance of glucose homeostasis. Central administration of GLP-1 augmented glucose-stimulated insulin secretion, and direct administration of GLP-1 into the arcuate, but not the paraventricular, nucleus of the hypothalamus reduced hepatic glucose production. Consistent with a role for GLP-1 receptors in the arcuate, GLP-1 receptor mRNA was found to be expressed in 68.1% of arcuate neurons that expressed pro-opiomelanocortin mRNA but was not significantly coexpressed with neuropeptide tyrosine.

CONCLUSIONS

These data suggest that the arcuate GLP-1 receptors are a key component of the GLP-1 system for improving glucose homeostasis by regulating both insulin secretion and glucose production.

CNS GLP-1 regulation of peripheral glucose homeostasis

Current models hold that peripheral and CNS GLP-1 signaling operate as distinct systems whereby CNS GLP-1 regulates food intake and circulating GLP-1 regulates glucose homeostasis. There is accumulating evidence that the arcuate nucleus, an area of the CNS that regulates energy homeostasis, responds to hormones and nutrients to regulate glucose homeostasis as well. Recent data suggest that GLP-1 may be another signal acting on the arcuate to regulate glucose homeostasis challenging the conventional model of GLP-1 physiology. This review discusses the peripheral and central GLP-1 systems and presents a model whereby these systems are integrated in regulation of glucose homeostasis.

Exenatide Injection

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Insulin secretion-independent effects of GLP-1 on canine liver glucose metabolism do not involve portal vein GLP-1 receptors

Whether glucagon-like peptide (GLP)-1 requires the hepatic portal vein to elicit its insulin secretion-independent effects on glucose disposal in vivo was assessed in conscious dogs using tracer and arteriovenous difference techniques. In study 1, six conscious overnight-fasted dogs underwent oral glucose tolerance testing (OGTT) to determine target GLP-1 concentrations during clamp studies. Peak arterial and portal values during OGTT ranged from 23 to 65 pM and from 46 to 113 pM, respectively. In study 2, we conducted hyperinsulinemic-hyperglycemic clamp experiments consisting of three periods (P1, P2, and P3) during which somatostatin, glucagon, insulin and glucose were infused. The control group received saline, the PePe group received GLP-1 (1 pmol.kg(-1).min(-1)) peripherally, the PePo group received GLP-1 (1 pmol.kg(-1).min(-1)) peripherally (P2) and then intraportally (P3), and the PeHa group received GLP-1 (1 pmol.kg(-1).min(-1)) peripherally (P2) and then through the hepatic artery (P3) to increase the hepatic GLP-1 load to the same extent as in P3 in the PePo group (n = 8 dogs/group). Arterial GLP-1 levels increased similarly in all groups during P2 ( approximately 50 pM), whereas portal GLP-1 levels were significantly increased (2-fold) in the PePo vs. PePe and PeHa groups during P3. During P2, net hepatic glucose uptake (NHGU) increased slightly but not significantly (vs. P1) in all groups. During P3, GLP-1 increased NHGU in the PePo and PeHa groups more than in the control and PePe groups (change of 10.8 +/- 1.3 and 10.6 +/- 1.0 vs. 5.7 +/- 1.0 and 5.4 +/- 0.8 micromol.kg(-1).min(-1), respectively, P < 0.05). In conclusion, physiological GLP-1 levels increase glucose disposal in the liver, and this effect does not involve GLP-1 receptors located in the portal vein.

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.

Metformin causes reduction of food intake and body weight gain and improvement of glucose intolerance in combination with dipeptidyl peptidase IV inhibitor in Zucker fa/fa rats

An incretin hormone, glucagon-like peptide-1 (GLP-1), has been shown to lower plasma glucose via glucose-dependent insulin secretion and to reduce appetite. We previously found that the biguanide metformin, an antidiabetic agent, causes a significant increase of plasma active GLP-1 level in the presence of dipeptidyl peptidase IV (DPPIV) inhibitor in normal rats. This finding suggested that the combination treatment might produce a greater antidiabetic and anorectic effect, based on enhanced GLP-1 action. In this study, we assessed the effects of subchronic treatment with metformin and a DPPIV inhibitor, valine-pyrrolidide (val-pyr), on glycemic control, food intake, and weight gain using Zucker fa/fa rats, a model of obesity and impaired glucose tolerance. The combination treatment caused a significant increase of GLP-1 level in Zucker fa/fa rats. In a subchronic study, val-pyr, metformin, or both compounds were administered orally b.i.d. for 14 days. The combination treatment significantly decreased food intake and body weight gain, although neither metformin nor val-pyr treatment alone had any effect. In an oral glucose tolerance test on day 1, the coadministration caused a greater improvement of glucose tolerance and a prominent increase of plasma active GLP-1 without marked insulin secretion. The 14-day combination treatment produced a potent reduction of fasting blood glucose and plasma insulin levels. These results demonstrate that the combination therapy of metformin with DPPIV inhibitor leads to reduced food intake and body weight gain, most likely through the significant increase of plasma GLP-1 level. The combination therapy seems to be a good candidate for treatment of type 2 diabetes with obesity.

Insulin-independent effects of GLP-1 on canine liver glucose metabolism: duration of infusion and involvement of hepatoportal region

Whether glucagon-like peptide-1 (GLP-1) has insulin-independent effects on glucose disposal in vivo was assessed in conscious dogs by use of tracer and arteriovenous difference techniques. After a basal period, each experiment consisted of three periods (P1, P2, P3) during which somatostatin, glucagon, insulin, and glucose were infused. The control group (C) received saline in P1, P2, and P3, the PePe group received saline in P1 and GLP-1 (7.5 pmol.kg(-1).min(-1)) peripherally (Pe; iv) in P2 and P3, and the PePo group received saline in P1 and GLP-1 peripherally (iv) (P2) and then into the portal vein (Po; P3). Glucose and insulin concentrations increased to two- and fourfold basal, respectively, and glucagon remained basal. GLP-1 levels increased similarly in the PePe and PePo groups during P2 ( approximately 200 pM), whereas portal GLP-1 levels were significantly increased (3-fold) in PePo vs. PePe during P3. In all groups, net hepatic glucose uptake (NHGU) occurred during P1. During P2, NHGU increased slightly but not significantly in all groups. During P3, NHGU increased in PePe and PePo groups to a greater extent than in C, but no significant effect of the route of infusion of GLP-1 was demonstrated (16.61 +/- 2.91 and 14.67 +/- 2.09 vs. 4.22 +/- 1.57 micromol.kg(-1).min(-1), respectively).

IN CONCLUSION

GLP-1 increased glucose disposal in the liver independently of insulin secretion; its full action required long-term infusion. The route of infusion did not modify the hepatic response.

Long-term treatment with dipeptidyl peptidase IV inhibitor improves hepatic and peripheral insulin sensitivity in the VDF Zucker rat: a euglycemic-hyperinsulinemic clamp study

Upon release into circulation, the potent insulin secretagogues glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) are rapidly cleaved and inactivated by the enzyme dipeptidyl peptidase IV (DP IV). Long-term administration of specific DP IV inhibitors, so as to enhance circulating active GIP and GLP-1 levels, has been shown to improve glucose tolerance and beta-cell glucose responsiveness and to reduce hyperinsulinemia in the Vancouver diabetic fatty (VDF) rat model of type 2 diabetes. Using the VDF model, the current study was undertaken to examine the effects of long-term DP IV inhibitor treatment on insulin sensitivity. Euglycemic-hyperinsulinemic clamps were performed on two sets of conscious VDF rats treated with or without the DP IV inhibitor P32/98 (20 mg. kg(-1). day(-1) for 12 weeks). The protocol consisted of three sequential 90-min periods with insulin infusion rates of 0, 5, and 15 mU. kg(-1). min(-1) and included a constant infusion of [ (3)H]glucose for measure of hepatic and peripheral insulin sensitivity. Relative to untreated littermates, the treated animals showed a left shift in the sensitivity of hepatic glucose output to insulin (average reduction approximately 6 micro mol. kg(-1). min(-1)) and a marked gain in peripheral responsiveness to insulin, with glucose disposal rates increasing 105 and 216% in response to the two insulin steps (versus 2 and 46% in controls). These results provide the first demonstration of improved hepatic and peripheral insulin sensitivity after DP IV inhibitor therapy, and coupled with apparent improvements in beta-cell function, they offer strong support for the utility of these compounds in the treatment of diabetes.

Long-term treatment with the dipeptidyl peptidase IV inhibitor P32/98 causes sustained improvements in glucose tolerance, insulin sensitivity, hyperinsulinemia, and beta-cell glucose responsiveness in VDF (fa/fa) Zucker rats

The incretins, glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide 1 (GLP-1) are responsible for >50% of nutrient-stimulated insulin secretion. After being released into the circulation, GIP and GLP-1 are rapidly inactivated by the circulating enzyme dipeptidyl peptidase IV (DP IV). The use of DP IV inhibitors to enhance these insulinotropic hormonal axes has proven effective on an acute scale in both animals and humans; however, the long-term effects of these compounds have yet to be determined. Therefore, we carried out the following study: two groups of fa/fa Zucker rats (n = 6 each) were treated twice daily for 3 months with the DP IV inhibitor P32/98 (20 mg.kg(-1).day(-1), p.o.). Monthly oral glucose tolerance tests (OGTTs), performed after drug washout, revealed a progressive and sustained improvement in glucose tolerance in the treated animals. After 12 weeks of treatment, peak OGTT blood glucose values in the treated animals averaged 8.5 mmol/l less than in the controls (12.0 +/- 0.7 vs. 20.5 +/- 1.3 mmol/l, respectively). Concomitant insulin determinations showed an increased early-phase insulin response in the treated group (43% increase). Furthermore, in response to an 8.8 mmol/l glucose perfusion, pancreata from controls showed no increase in insulin secretion, whereas pancreata from treated animals exhibited a 3.2-fold rise in insulin secretion, indicating enhanced beta-cell glucose responsiveness. Also, both basal and insulin-stimulated glucose uptake were increased in soleus muscle strips from the treated group (by 20 and 50%, respectively), providing direct evidence for an improvement in peripheral insulin sensitivity. In summary, long-term DP IV inhibitor treatment was shown to cause sustained improvements in glucose tolerance, insulinemia, beta-cell glucose responsiveness, and peripheral insulin sensitivity, novel effects that provide further support for the use of DP IV inhibitors in the treatment of diabetes.

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.

Glucagon-like peptide-1 activates the adenylyl cyclase system in rockfish enterocytes and brain membranes

Glucagon-like peptide (GLP) exerts important physiological functions in fish liver, but extrahepatic sites of action and physiological roles have been largely ignored. We show here that GLP activates adenylyl cyclase in isolated brain and enterocyte membranes and increases cellular cyclic adenosine monophosphate (cAMP) levels in isolated enterocytes of rockfish (Sebastes caurinus). Following exposure to synthetic zebrafish GLP (zf-GLP) (1 nM-1 microM), a concentration-dependent increase in enterocyte cAMP is noted. The maximum increase in cAMP levels is observed at 1 microM zf-GLP, and represents a 30% increase above control values. Exendin-4, a GLP receptor agonist in mammals, elicits a similar concentration-dependent increase in enterocyte cAMP. In contrast, norepinephrine or prostaglandin E2 (at 1 microM) increased cAMP levels by 2 and 4-fold, respectively. Brain membrane adenylyl cyclase is activated 20-40% by zf-GLP, and to a smaller extent by zf-glucagon, while exendin-4 is as effective as zf-GLP at a dose of 100 nM. These results suggest potential physiological roles of GLP in brain and intestine in piscine systems analogous to GLP-1 functions in these tissues described for mammals.

Glucagon-like peptide-1 activates the adenylyl cyclase system in rockfish enterocytes and brain membranes

Glucagon-like peptide (GLP) exerts important physiological functions in fish liver, but extrahepatic sites of action and physiological roles have been largely ignored. We show here that GLP activates adenylyl cyclase in isolated brain and enterocyte membranes and increases cellular cyclic adenosine monophosphate (cAMP) levels in isolated enterocytes of rockfish (Sebastes caurinus). Following exposure to synthetic zebrafish GLP (zf-GLP) (1 nM-1 microM), a concentration-dependent increase in enterocyte cAMP is noted. The maximum increase in cAMP levels is observed at 1 microM zf-GLP, and represents a 30% increase above control values. Exendin-4, a GLP receptor agonist in mammals, elicits a similar concentration-dependent increase in enterocyte cAMP. In contrast, norepinephrine or prostaglandin E2 (at 1 microM) increased cAMP levels by 2 and 4-fold, respectively. Brain membrane adenylyl cyclase is activated 20-40% by zf-GLP, and to a smaller extent by zf-glucagon, while exendin-4 is as effective as zf-GLP at a dose of 100 nM. These results suggest potential physiological roles of GLP in brain and intestine in piscine systems analogous to GLP-1 functions in these tissues described for mammals.

Effects of glucagon-like peptide 1 on the kinetics of glycogen synthase a in hepatocytes from normal and diabetic rats

Glucagon-like peptide 1(7-36)amide (GLP-1) is currently under investigation as a possible tool in the treatment of non-insulin-dependent diabetes mellitus. In addition to enhancing nutrient-stimulated insulin release, the peptide also favors glycogen synthesis and glucose use in liver, muscle, and adipose tissue. GLP-1 also activates glycogen synthase a in hepatocytes from both normal and diabetic rats. In the present study, the kinetic aspects of such an activation were investigated in hepatocytes from normal rats and from animals rendered diabetic induced by injection of streptozotocin, either in the adult age (insulin-dependent diabetes mellitus model) or in days 1 or 5 after birth (non-insulin-dependent diabetes mellitus models). GLP-1 increased, in a dose-dependent manner, glycogen synthase a activity in the hepatocytes from all groups studied. The activation of the enzyme reached a steady state within 1 min exposure to GLP-1, which, at 10(-12) M, caused a half-maximal activation. When comparing fed vs. overnight-starved normal rats, a somewhat lower basal activity of glycogen synthase a in fasted animals (P < 0.05) coincided with a greater relative increment in reaction velocity in response to GLP-1. The basal activity of glycogen synthase a and the relative extent of its inhibition by glucagon or activation by insulin and GLP-1 were modulated by the extracellular concentration of D-glucose. The activation of glycogen synthase a by either insulin or GLP-1 resulted not solely in an increase in maximal velocity but also in a decrease in affinity of the enzyme for uridine diphosphate-glucose; in diabetic animals, the capacity of insulin or GLP-1 to increase the maximal velocity and Michaelis-Menten constant were less marked than in normal rats. In conclusion, this study indicates that the GLP-1-induced activation of glycogen synthase a displays attributes of rapidity, sensitivity, and nutritional dependency that are well suited for both participation in the physiological regulation of enzyme activity and therapeutic purpose.

GLP-1 receptors in golden Syrian hamster islets: identification and functional characterization

This study aims at the identification and functional characterization of glucagon-like peptide 1 (7-36) amide (GLP-1) receptor in islets from Golden Syrian hamsters. Using a polyclonal antibody against rat GLP-1 receptors, Western blotting of the islet proteins revealed two major bands of 44 and 70 kDa, similar to those found in rat islets, RINm5F cells, and HIT-T15 cells. In Northern blots, transcripts of 2.7, 3.6 and 3.7 kb were observed in rat islets and RINm5F cells after hybridization with rat GLP-1 receptor cDNA probes of either 21 9 bp or 1.5 kb. Such was not the case in either hamster islets or HIT-T15 cells. However, a single 3.6-kb transcript was observed in the latter two cases when a human GLP-1 receptor cDNA probe of 1.6 kb was used for hybridization. In the isolated perfused pancreas of Golden Syrian hamsters, a rise in D-glucose concentration from 3.3 to 8.3 mM caused a biphasic stimulation of insulin release, which was further increased by either GLP-1 or glucagon (10(-9)M each). The enhancing action of GLP-1 on glucose-stimulated insulin secretion was much more marked than that of glucagon. The rise in D-glucose concentration decreased by 46+/-4% the release of glucagon, but GLP-1 failed to exert any obvious effect on glucagon secretion in the presence of 8.3 mM D-glucose. These results indicate that GLP-1 receptors are expressed in islets of Golden Syrian hamsters with an extracellular part possessing the same immunoreactivity as the rat islet GLP-1 receptors. The expression of the mRNA for the GLP-1 receptor differs, however, from that found in rat or human islets.