Glucagon-like peptide-1 inhibits gastropancreatic function by inhibiting central parasympathetic outflow

Pancreatic Safety of Tirzepatide and Its Effects on Islet Cell Function: A Systematic Review and Meta-Analysis

Background

Endogenous glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) regulate islet cell function. GLP-1 receptor agonists (GLP-1RAs) have been associated with an elevated risk of acute pancreatitis. Data on the pancreatic safety of tirzepatide (a dual GLP-1 and GIP agonist) and its effects on islet cell function in randomized controlled trials (RCTs) are scarce. Moreover, no meta-analysis has comprehensively examined such effects of tirzepatide.

Methods

Electronic databases were searched for RCTs with tirzepatide as the intervention and a placebo or active comparator as the control. The primary outcome was adjudication-confirmed pancreatitis; secondary outcomes were the percent changes from baseline in serum pancreatic amylase, lipase, insulin, C-peptide, glucagon, and homeostasis model assessment of insulin resistance (HOMA2-IR).

Results

Seventeen RCTs with 18 published reports involving 14,645 subjects were analyzed. Over a follow-up duration of 12-72 weeks, tirzepatide had identical risks of pancreatitis to placebo (tirzepatide 5 mg: RR 2.04, 95% CI [0.27-15.69], p = 0.49; 10 mg: RR 0.63, 95% CI [0.08-5.12], p = 0.67; and 15 mg: RR 1.26, 95% CI [0.36-4.98], p = 0.72). Tirzepatide was also associated with comparable risks of pancreatitis to insulin and GLP-1RAs. However, tirzepatide (at all doses) caused greater increases in pancreatic amylase and lipase than placebo and insulin. Individuals on tirzepatide 15 mg and GLP-1RAs had similar risks of having increased lipase levels. The percent reductions in fasting insulin were greater with tirzepatide 10 and 15 mg than with placebo. All doses of tirzepatide caused greater percent reductions in fasting insulin, C-peptide, and glucagon than GLP-1RAs. Compared to placebo and GLP-1RAs, the percent reductions in HOMA2-IR were greater with all doses of tirzepatide.

Conclusion

The meta-analysis provides evidence of the safety of tirzepatide regarding pancreatitis and establishes its positive effect on islet cell functions and insulin resistance.

Glucagon-like peptide-1 receptor agonists in neoplastic diseases

Glucagon-like peptide-1 receptor agonist (GLP-1RA), a novel hypoglycemic agent for the treatment of type 2 diabetes, has well-known effects such as lowering blood sugar, ameliorating inflammation, reducing weight, and lowering blood lipids. It has also been shown that it can influence the proliferation and survival of cells and has a certain effect on the prognosis of some neoplastic diseases. In this study, the potential effects of GLP-1RAs on the occurrence and development of tumors were reviewed to provide new ideas for the prevention and treatment of tumors in patients.

Overlapping approach Proton Pump Inhibitors/Nux vomica-Heel as new intervention for gastro-esophageal reflux management: Delphi consensus study

BACKGROUND

Gastro-esophageal reflux disease (GERD) may affect the upper digestive tract; up to 20% of population in Western nations are affected by GERD. Antacids, histamine H2-receptor antagonists, and Proton Pump Inhibitors (PPIs) are considered the referring medications for GERD. Nevertheless, PPIs must be managed carefully because their use, especially chronic, could be linked with some adverse effects. An effective and safe alternative pharmacological tool for GERD is needed. After the identification of potentially new medications to flank PPIs, it is mandatory to revise and improve good clinical practices even through a consensus process.

AIM

To optimize diagnosis and treatment guidelines for GERD through a consensus based on Delphi method.

METHODS

The availability of clinical studies describing the action of the multicomponent/multitarget medication Nux vomica-Heel, subject of the consensus, is the basic prerequisite for the consensus itself. A modified Delphi process was used to reach a consensus among a panel of Italian GERD specialists on the overlapping approach PPIs/Nux vomica-Heel as a new intervention model for the management of GERD. The Voting Consensus group was composed of 49 Italian Medical Doctors with different specializations: Gastroenterology, otolaryngology, geriatrics, and general medicine. A scientific committee analyzed the literature, determined areas that required investigation (in agreement with the multiple-choice questionnaire results), and identified two topics of interest: (1) GERD disease; and (2) GERD treatment. Statements for each of these topics were then formulated and validated. The Delphi process involved two rounds of questioning submitted to the panel experts using an online platform.

RESULTS

According to their routinary GERD practice and current clinical evidence, the panel members provided feedback to each questionnaire statement. The experts evaluated 15 statements and reached consensus on all 15. The statements regarding the GERD disease showed high levels of agreement, with consensus ranging from 70% to 92%. The statements regarding the GERD treatment also showed very high levels of agreement, with consensus ranging from 90% to 100%. This Delphi process was able to reach consensus among physicians in relevant aspects of GERD management, such as the adoption of a new approach to treat patients with GERD based on the overlapping between PPIs and Nux vomica-Heel. The consensus was unanimous among the physicians with different specializations, underlying the uniqueness of the agreement reached to identify in the overlapping approach between PPIs and Nux vomica-Heel a new intervention model for GERD management. The results support that an effective approach to deprescribe PPIs through a progressive decalage timetable (reducing PPIs administration to as-needed use), should be considered.

CONCLUSION

Nux vomica-Heel appears to be a valid opportunity for GERD treatment to favor the deprescription of PPIs and to maintain low disease activity together with the symptomatology remission.

The GLP-1-mediated gut-kidney cross talk in humans: mechanistic insight

Incretin-based therapy is an antidiabetic and antiobesity approach mimicking glucagon-like peptide-1 (GLP-1) with additional end-organ protection. This review solely focuses on randomized, controlled mechanistic human studies, investigating the renal effects of GLP-1. There is no consensus about the localization of GLP-1 receptors (GLP-1Rs) in human kidneys. Rodent and primate data suggest GLP-1R distribution in smooth muscle cells in the preglomerular vasculature. Native GLP-1 and GLP-1R agonists elicit renal effects. Independently of renal plasma flow and glomerular filtration rate, GLP-1 has a natriuretic effect but only during volume expansion. This is associated with high renal extraction of GLP-1, suppression of angiotensin II, and increased medullary as well as cortical perfusion. These observations may potentially indicate that impaired GLP-1 sensing could establish a connection between salt sensitivity and insulin resistance. It is concluded that a functional GLP-1 kidney axis exists in humans, which may play a role in renoprotection.

The importance of glucose-dependent insulinotropic polypeptide receptor activation for the effects of tirzepatide

Tirzepatide is a unimolecular co-agonist of the glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) receptors recently approved for the treatment of type 2 diabetes by the US Food and Drug Administration and the European Medicine Agency. Tirzepatide treatment results in an unprecedented improvement of glycaemic control and lowering of body weight, but the contribution of the GIP receptor-activating component of tirzepatide to these effects is uncertain. In this review, we present the current knowledge about the physiological roles of the incretin hormones GLP-1 and GIP, their receptors, and previous results of co-targeting the two incretin hormone receptors in humans. We also analyse the molecular pharmacological, preclinical and clinical effects of tirzepatide to discuss the role of GIP receptor activation for the clinical effects of tirzepatide. Based on the available literature on the combination of GLP-1 and GIP receptor activation, tirzepatide does not seem to have a classical co-activating mode of action in humans. Rather, in vitro studies of the human GLP-1 and GIP receptors reveal a biased GLP-1 receptor activation profile and GIP receptor downregulation. Therefore, we propose three hypotheses for the mode of action of tirzepatide, which can be addressed in future, elaborate clinical trials.

The expanding incretin universe: from basic biology to clinical translation

Incretin hormones, principally glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1(GLP-1), potentiate meal-stimulated insulin secretion through direct (GIP + GLP-1) and indirect (GLP-1) actions on islet β-cells. GIP and GLP-1 also regulate glucagon secretion, through direct and indirect pathways. The incretin hormone receptors (GIPR and GLP-1R) are widely distributed beyond the pancreas, principally in the brain, cardiovascular and immune systems, gut and kidney, consistent with a broad array of extrapancreatic incretin actions. Notably, the glucoregulatory and anorectic activities of GIP and GLP-1 have supported development of incretin-based therapies for the treatment of type 2 diabetes and obesity. Here we review evolving concepts of incretin action, focusing predominantly on GLP-1, from discovery, to clinical proof of concept, to therapeutic outcomes. We identify established vs uncertain mechanisms of action, highlighting biology conserved across species, while illuminating areas of active investigation and uncertainty that require additional clarification.

Glucagon-like peptide-1 stimulates acute secretion of pro-atrial natriuretic peptide from the isolated, perfused pig lung exposed to warm ischemia

Glucagon-like peptide-1 (GLP-1) has proven to be protective in animal models of lung disease but the underlying mechanisms are unclear. Atrial natriuretic peptide (ANP) is mainly produced in the heart. As ANP possesses potent vaso- and bronchodilatory effects in pulmonary disease, we hypothesised that the protective functions of GLP-1 could involve potentiation of local ANP secretion from the lung. We examined whether the GLP-1 receptor agonist liraglutide was able to improve oxygenation in lungs exposed to 2 h of warm ischemia and if liraglutide stimulated ANP secretion from the lungs in the porcine ex vivo lung perfusion (EVLP) model. Pigs were given a bolus of 40 µg/kg liraglutide or saline 1 h prior to sacrifice. The lungs were then left in vivo for 2 h, removed en bloc and placed in the EVLP machinery. Lungs from the liraglutide treated group were further exposed to liraglutide in the perfusion buffer (1.125 mg). Main endpoints were oxygenation capacity, and plasma and perfusate concentrations of proANP and inflammatory markers. Lung oxygenation capacity, plasma concentrations of proANP or concentrations of inflammatory markers were not different between groups. ProANP secretion from the isolated perfused lungs were markedly higher in the liraglutide treated group (area under curve for the first 30 min in the liraglutide group: 635 ± 237 vs. 38 ± 38 pmol/L x min in the saline group) (p < 0.05). From these results, we concluded that liraglutide potentiated local ANP secretion from the lungs.

Glucagon-like peptide-1: Are its roles as endogenous hormone and therapeutic wizard congruent?

Glucagon-like peptide-1 (GLP-1) is a peptide derived from differential processing of the precursor for the hormone glucagon. It is secreted predominantly by endocrine cells in the gut epithelium in response to nutrient stimulation. Studies from the last 35 years have given us an idea about its physiological functions. On the basis of some of its many actions, it has also been developed into a pharmaceutical agent for the treatment of obesity and type 2 diabetes (T2DM). It is currently positioned as the most effective anti-obesity agent available and is recommended in both national and international guidelines as an effective second-in line treatment for T2DM, in particular in patients with increased cardiovascular risk. In this review, I first discuss whether the processing of proglucagon may also result in GLP-1 formation in the pancreas and in glucagon in the gut. Next, I discuss the relationship between the physiological actions of GLP-1 and the therapeutic effects of the GLP-1 receptor agonists, which are far from being congruent and generally poorly understood. These relationships illustrate both the difficulties and the benefits of bridging results obtained in the laboratory with those emerging from the clinic.

Pancreas-Brain Crosstalk

The neural regulation of glucose homeostasis in normal and challenged conditions involves the modulation of pancreatic islet-cell function. Compromising the pancreas innervation causes islet autoimmunity in type 1 diabetes and islet cell dysfunction in type 2 diabetes. However, despite the richly innervated nature of the pancreas, islet innervation remains ill-defined. Here, we review the neuroanatomical and humoral basis of the cross-talk between the endocrine pancreas and autonomic and sensory neurons. Identifying the neurocircuitry and neurochemistry of the neuro-insular network would provide clues to neuromodulation-based approaches for the prevention and treatment of diabetes and obesity.

Actions of glucagon-like peptide-1 receptor ligands in the gut

The incretin hormone glucagon-like peptide-1 (GLP-1) is inactivated by the enzyme dipeptidyl peptidase-4 even before it leaves the gut, but it seems to act predominantly via activation of intestinal sensory neurons expressing GLP-1 receptors. Thus, activation of vagal afferents is probably responsible for its effects on appetite and food intake, gastrointestinal secretion and motility, and pancreatic endocrine secretion. However, GLP-1 receptors are widely expressed in the gastrointestinal (GI) tract, including epithelial cells in the stomach, and the Brunner glands, in endocrine cells of the gut epithelium, and on mucosal lymphocytes. In this way, GLP-1 may have important local actions of epithelial protection and endocrine signalling and may interact with the immune system. We review the formation and release of GLP-1 from the endocrine L cells and its fate after release and describe the localization of its receptor throughout the GI tract and discuss its direct or indirect actions in the GI tract.

GLP-1 and Intestinal Diseases

Accumulating evidence implicates glucagon-like peptide-1 (GLP-1) to have, beyond glucose maintenance, a beneficial role in the gastrointestinal tract. Here, we review emerging data investigating GLP-1 as a novel treatment for intestinal diseases, including inflammatory bowel diseases, short-bowel syndrome, intestinal toxicities and coeliac disease. Possible beneficial mechanisms for these diseases include GLP-1′s influence on gastric emptying, its anti-inflammatory properties and its intestinotrophic effect. The current knowledge basis derives from the available GLP-1 agonist treatments in experimental animals and small clinical trials. However, new novel strategies including dual GLP-1/GLP-2 agonists are also in development for the treatment of intestinal diseases.

Polymeric microparticle systems for modified release of glucagon-like-peptide-1 receptor agonists

Type 2 diabetes is a fast-growing worldwide epidemic. Despite the multiple therapies available to treat type 2 diabetes, the disease is not correctly managed in over half of patients, mainly due to non-compliance with prescribed treatment regimes. The development of analogues to the glucagon-like peptide 1 (GLP-1) has resulted in the extension of its half-life and associated benefits. Further benefits in the use of peptide-based GLP-1 receptor agonists have been achieved by the use of controlled-release systems based on polymeric microparticles. In this review, we focus on commercially available formulations and others that remain in development, discussing the preparation methods and the relationship between in vitro and in vivo kinetic release behaviours.

Inferior vagal ganglion galaninergic response to gastric ulcers

Galanin is a neuropeptide widely expressed in central and peripheral nerves and is known to be engaged in neuronal responses to pathological changes. Stomach ulcerations are one of the most common gastrointestinal disorders. Impaired stomach function in peptic ulcer disease suggests changes in autonomic nerve reflexes controlled by the inferior vagal ganglion, resulting in stomach dysfunction. In this paper, changes in the galaninergic response of inferior vagal neurons to gastric ulceration in a pig model of the disease were analyzed based on the authors' previous studies. The study was performed on 24 animals (12 control and 12 experimental). Gastric ulcers were induced by submucosal injections of 40% acetic acid solution into stomach submucosa and bilateral inferior vagal ganglia were collected one week afterwards. The number of galanin-immunoreactive perikarya in each ganglion was counted to determine fold-changes between both groups of animals and Q-PCR was applied to verify the changes in relative expression level of mRNA encoding both galanin and its receptor subtypes: GalR1, GalR2, GalR3. The results revealed a 2.72-fold increase in the number of galanin-immunoreactive perikarya compared with the controls. Q-PCR revealed that all studied genes were expressed in examined ganglia in both groups of animals. Statistical analysis revealed a 4.63-fold increase in galanin and a 1.45-fold increase in GalR3 mRNA as compared with the controls. No differences were observed between the groups for GalR1 or GalR2. The current study confirmed changes in the galaninergic inferior vagal ganglion response to stomach ulcerations and demonstrated, for the first time, the expression of mRNA encoding all galanin receptor subtypes in the porcine inferior vagal ganglia.

No effects of a 6-week intervention with a glucagon-like peptide-1 receptor agonist on pancreatic volume and oedema in obese men without diabetes

AIM

To investigate the effect of a glucagon-like peptide-1 receptor agonist (GLP-1RA), liraglutide, on pancreatic volume, oedema, cellularity and DNA synthesis in humans.

MATERIALS AND METHODS

We performed an open-label study in 14 obese men (age 38 ± 11 years, body mass index 32 ± 4 kg/m² ) without diabetes. Subjects were examined at baseline, during titration (week 4) of liraglutide towards 3.0 mg/day, and 2 weeks after steady-state treatment (week 6) of a final dose of liraglutide. The primary endpoint was pancreatic volume determined by magnetic resonance imaging. Secondary endpoints included pancreatic oedema and cellularity, positron emission tomography-based [¹⁸ F]fluorothymidine (FLT) uptake (DNA synthesis) and plasma pancreatic enzymes.

RESULTS

Plasma amylase (+7 U/L [95% confidence intervals 3-11], P < .01) and lipase (+19 U/L [7-30], P < .01) increased during liraglutide treatment. Pancreatic volume did not change from baseline to steady state of treatment (+0.2 cm³ [-8-8], P = .96) and no change in pancreatic cellular infiltration was found (P = .22). During titration of liraglutide, FLT uptake in pancreatic tissue increased numerically (+0.08 [0.00-0.17], P = .0507).

CONCLUSIONS

Six weeks of treatment with liraglutide did not affect pancreatic volume, oedema or cellularity in obese men without diabetes.

From the Incretin Concept and the Discovery of GLP-1 to Today's Diabetes Therapy

Researchers have been looking for insulin-stimulating factors for more than 100 years, and in the 1960ties it was definitively proven that the gastrointestinal tract releases important insulinotropic factors upon oral glucose intake, so-called incretin hormones. The first significant factor identified was the duodenal glucose-dependent insulinotropic polypeptide, GIP, which however, turned out not to stimulate insulin secretion in patients with type 2 diabetes. But resection experiments clearly indicated the presence of an additional incretin, and in 1986, an unexpected processing fragment of the recently identified glucagon precursor, proglucagon, namely truncated glucagon-like peptide 1 (GLP-1 7-36 amide), was isolated from the gut and found to both stimulate insulin secretion and inhibit glucagon secretion. The peptide also inhibited appetite and food intake. Unlike GIP, this peptide had preserved effects in patients with type 2 diabetes and it was soon documented to have powerful antidiabetic effects in clinical studies. Its utility was limited, however, because of an extremely short half-life in humans, but this problem had two solutions, both of which gave rise to important antidiabetic drugs: (1) orally active inhibitors of the enzyme dipeptidylpeptidase 4 (DPP-4 inhibitors), which was responsible for the rapid degradation; the inhibitors protect endogenous GLP-1 from degradation and thereby unfold its antidiabetic activity, and (2) long-acting injectable analogs of GLP-1 protected against DPP-4 degradation. Particularly, the latter, the GLP-1 receptor agonists, either alone or in various combinations, are so powerful that treatment allows more than 2/3 of type 2 diabetes patients to reach glycemic targets. In addition, these agents cause a weight loss which, with the most successful compounds, may exceed 10% of body weight. Most recently they have also been shown to be renoprotective and reduce cardiovascular risk and mortality.

Acute effects of glucagon-like peptide-1, GLP-19-36 amide, and exenatide on mesenteric blood flow, cardiovascular parameters, and biomarkers in healthy volunteers

Glucagon‐like peptide‐1 (GLP‐1, GLP‐1_7–36amide) and its sister peptide glucagon‐like peptide 2 (GLP‐2) influence numerous intestinal functions and GLP‐2 greatly increases intestinal blood flow. We hypothesized that GLP‐1 also stimulates intestinal blood flow and that this would impact on the overall digestive and cardiovascular effects of the hormone. To investigate the influence of GLP‐1 receptor agonism on mesenteric and renal blood flow and cardiovascular parameters, we carried out a double‐blinded randomized clinical trial. A total of eight healthy volunteers received high physiological subcutaneous injections of GLP‐1, GLP‐1_{9–36 amide} (bioactive metabolite), exenatide (stable GLP‐1 agonist), or saline on four separate days. Blood flow in mesenteric, celiac, and renal arteries was measured by Doppler ultrasound. Blood pressure, heart rate, cardiac output, and stroke volume were measured continuously using an integrated system. Plasma was analyzed for glucose, GLP‐1 (intact and total), exenatide and Pancreatic polypeptide (PP), and serum for insulin and C‐peptide. Neither GLP‐1, GLP‐1_{9–36 amide}, exenatide nor saline elicited any changes in blood flow parameters in the mesenteric or renal arteries. GLP‐1 significantly increased heart rate (two‐way ANOVA, injection [P = 0.0162], time [P = 0.0038], and injection × time [P = 0.082]; Tukey post hoc GLP‐1 vs. saline and GLP‐1_9–36amide [P < 0.011]), and tended to increase cardiac output and decrease stroke volume compared to GLP‐1_{9–36 amide} and saline. Blood pressures were not affected. As expected, glucose levels fell and insulin secretion increased after infusion of both GLP‐1 and exenatide.

Incretin Kinetics Before and After Miglitol in Japanese Patients With Late Dumping Syndrome

Background

In patients with late dumping syndrome following gastrectomy, it has been reported that hypoglycemia occurs due to inhibition of glucagon secretion as a result of excessive insulin production facilitated by an increase in glucagon-like peptide-1 (GLP-1).

Methods

To determine the kinetics of incretins in Japanese patients with late dumping syndrome, an oral glucose tolerance test was carried out before and after miglitol administration, and the kinetics of insulin and incretins were analyzed.

Results

After miglitol administration, there was improvement of hypoglycemia and early phase insulin secretion, with persistent excessive insulin secretion being minimized. These findings revealed that miglitol inhibited rapid excessive influx of carbohydrates into the blood and persistent elevation of GLP-1, resulting in improvement of early phase insulin secretion and minimizing persistent excessive insulin secretion.

Conclusions

Eating frequent small meals is generally effective for late dumping syndrome, but patients often find it difficult to continue such a regimen. Based on the present analysis of incretin kinetics, miglitol may be a useful treatment option for late dumping syndrome.

The role of somatostatin in GLP-1-induced inhibition of glucagon secretion in mice

AIMS/HYPOTHESIS

Glucagon-like peptide-1 (GLP-1) receptor agonists are currently used for the treatment of type 2 diabetes. Their main mechanism of action is enhancement of glucose-induced insulin secretion (from increased beta cell glucose sensitivity) and inhibition of glucagon secretion. The latter has been demonstrated to account for about half of their blood glucose-lowering activity. Whereas the effect of GLP-1 on insulin secretion is clearly dependent on ambient glucose concentrations and has been described in detail, the mechanism responsible for the inhibitory effect of GLP-1 on glucagon secretion is heavily debated. Glucagon inhibition is also said to be glucose-dependent, although it is unclear what is meant by this. We hypothesise here that GLP-1 does not inhibit glucagon secretion during hypoglycaemia because the inhibition depends on somatostatin secretion, which in turn is dependent on glucose levels.

METHODS

We used the perfused mouse pancreas model to investigate this hypothesis.

RESULTS

We found that, in this model, GLP-1 was able to significantly inhibit glucagon secretion from pancreatic alpha cells at all glucose levels tested: 6.0, 1.5 and 0.5 mmol/l (-27.0%, -37.1%, and -23.6%, respectively), and the decrease in glucagon secretion was invariably accompanied by an increase in somatostatin secretion (+286.8%, +158.7%, and +118.8%, respectively). Specific blockade of somatostatin receptor 2 increased glucagon secretion (+118.8% at 1.5 mmol/l glucose and +162.9% at 6.0 mmol/l glucose) and completely eliminated the inhibitory effect of GLP-1.

CONCLUSIONS/INTERPRETATION

We have shown here that the glucagon-lowering effect of GLP-1 is entirely mediated through the paracrine actions of somatostatin in the perfused mouse pancreas. However, in this model, the inhibitory effect of GLP-1 was preserved at hypoglycaemic levels, leaving unanswered the question of how this is avoided in vivo in individuals treated with GLP-1 receptor agonists.

Glucagon-like peptide-1 and glucagon-like peptide-1 receptor agonists in the treatment of type 2 diabetes

The prevalence of type 2 diabetes (T2D) is increasing worldwide. Patients with T2D suffer from various diabetes-related complications. Since there are many patients with T2D that cannot be controlled by previously developed drugs, it has been necessary to develop new drugs, one of which is a glucagon-like peptide-1 (GLP-1) based therapy. GLP-1 has been shown to ameliorate diabetes-related conditions by augmenting pancreatic β-cell insulin secretion and having the low risk of causing hypoglycemia. Because of a very short half-life of GLP-1, many researches have been focused on the development of GLP-1 receptor (GLP-1R) agonists with long half-lives such as exenatide and dulaglutide. Now GLP-1R agonists have a variety of dosing-cycle forms to meet the needs of various patients. In this article, we review the physiological features of GLP-1, the effects of GLP-1 on T2D, the features of several GLP-1R agonists, and the therapeutic effect on T2D.

GLP-1 receptor agonist promotes brown remodelling in mouse white adipose tissue through SIRT1

AIMS/HYPOTHESIS

Accumulating evidence has revealed the significant role of glucagon-like peptide-1 (GLP-1) in weight loss. Sirtuin 1 (SIRT1) plays a vital role in the regulation of lipid metabolism. Here, we investigated the contribution of lipolytic and oxidative changes in white adipose tissue (WAT) to the weight-lowering effect induced by the GLP-1 receptor (GLP-1R) agonist exenatide (exendin-4) in mice. We also looked at the role of SIRT1 in this process.

METHODS

C57BL/6J mice and Sirt1 (+/-) mice were treated with exenatide (24 nmol/kg) or an NaCl solution (154 mmol/l) control i.p. for 8 weeks while receiving a high-fat diet (HFD) after a 12 week HFD challenge. Systemic phenotypic evaluations were carried out during and after the intervention. A lentivirus-mediated short hairpin (sh)RNA vector of the Sirt1 gene was transfected into differentiated 3T3-L1 adipocytes. An in vitro model system used adipocytes induced from Sirt1-null mouse embryonic fibroblasts (MEFs).

RESULTS

Exenatide reduced fat mass and enhanced the lipolytic and oxidative capacity of WAT in diet-induced obese C57BL/6J mice. However, these effects were significantly impaired in Sirt1 (+/-) mice compared with wild-type controls. In vitro, exendin-4 increased lipolysis and fatty acid oxidation by upregulating SIRT1 expression and activity in differentiated 3T3-L1 adipocytes. Conversely, RNA interference (i)-induced knockdown of SIRT1 attenuated the lipolytic and oxidative responses to exendin-4 in differentiated 3T3-L1 adipocytes. Again, these responses were entirely abolished in Sirt1-null MEFs after induction into adipocytes.

CONCLUSIONS/INTERPRETATION

These data highlight that a GLP-1R agonist promotes brown remodelling of WAT in a SIRT1-dependent manner; this might be one of the mechanisms underlying its effect on weight loss.

Glucagon-like peptide-1 receptor is present in pancreatic acinar cells and regulates amylase secretion through cAMP

Glucagon-like peptide-1 (GLP-1) is a glucoincretin hormone that can act through its receptor (GLP-1R) on pancreatic β-cells and increase insulin secretion and production. GLP-1R agonists are used clinically to treat type 2 diabetes. GLP-1 may also regulate the exocrine pancreas at multiple levels, including inhibition through the central nervous system, stimulation indirectly through insulin, and stimulation directly on acinar cells. However, it has been unclear whether GLP-1R is present in pancreatic acini and what physiological functions these receptors regulate. In the current study we utilized GLP-1R knockout (KO) mice to study the role of GLP-1R in acinar cells. RNA expression of GLP-1R was detected in acutely isolated pancreatic acini. Acinar cell morphology and expression of digestive enzymes were not affected by loss of GLP-1R. GLP-1 induced amylase secretion in wild-type (WT) acini. In GLP-1R KO mice, this effect was abolished, whereas vasoactive intestinal peptide-induced amylase release in KO acini showed a pattern similar to that in WT acini. GLP-1 stimulated cAMP production and increased protein kinase A-mediated protein phosphorylation in WT acini, and these effects were absent in KO acini. These data show that GLP-1R is present in pancreatic acinar cells and that GLP-1 can regulate secretion through its receptor and cAMP signaling pathway.

GLP-1 mimetic drugs and the risk of exocrine pancreatic disease: Cell and animal studies

BACKGROUND

Glucagon Like Peptide 1 (GLP-1) mimetic drugs or degradation inhibitors mimic the action of native GLP-1 as a incretin hormone and have become a common second line of therapy for Type 2 diabetes. However, an important clinical issue is whether these drugs increase the incidence of pancreatitis and pancreatic cancer.

OBJECTIVE

This paper reviews the physiology of GLP-1 including its synthesis, secretion and action of the peptide. Reported effects of the mimetic drugs on the exocrine pancreas in animal studies are also reviewed.

RESULTS

GLP-1 is synthesized in a specific class of enteroendocrine cell, the L-cell, by post-translational processing of proglucagon. It is released in response to the presence of nutrients in the small intestine and stimulates vagal afferent nerve endings as well as entering the blood where it is rapidly degraded by dipeptidyl peptidase IV. Its actions are mediated by specific G-protein coupled receptors. The major target tissues are the pancreatic islet beta cells, the brain and the heart but GLP-1 also affects gastrointestinal motility and secretion including the exocrine pancreas where its major systemic action is to inhibit secretion. In some animal, as well as human studies, the GLP-1 mimetic drugs are associated with pancreatitis or precursor lessions to pancreatic cancer but a mechanism is not clear. The most common occurrence of pathology in rodents is when the drugs are combined with a high fat diet.

CONCLUSIONS

There is nothing in the physiology of GLP-1 or animal toxicology studies to support a mechanism of action or a major concern about the action of GLP-1 mimetic drugs on the exocrine pancreas. Further studies are warranted using animal models of disease and high fat diets.

Efficacy and Clinical Characteristics of Liraglutide in Japanese Patients With Type 2 Diabetes

Background

Liraglutide was first released in Japan as a long-acting once-daily glucagon-like peptide-1 receptor agonist. The maximum dose in Japan is 0.9 mg/day, which is half of that used in the United States and the European Union (1.8 mg/day). The efficacy of this maximum allowable dose of liraglutide for Japanese patients and the profiles of those patients for whom this agent should be recommended remain unclear.

Methods

This study aimed to examine the effective use of liraglutide in Japanese type 2 diabetic patients. We administered liraglutide to 60 patients, who had been managed with oral hypoglycemic agents or diet and exercise therapy only, during a period of 6 months.

Results

Though HbA1c levels significantly decreased, by approximately 1.5%, after 6 months of liraglutide administration, no significant changes in body weights were observed. The 0.6 mg dose was effective in approximately 40% of patients. In contrast, the effects of a dose increase from 0.6 mg to 0.9 mg were small. The greatest efficacy, as shown by a 2.5% HbA1c decrease, was achieved in non-obese patients. Thus, efficacy decreased as the degree of obesity increased. In addition, efficacy was higher in patients who had a diabetes duration of less than 10 years and was also higher in the group that had a low sulfonylurea (SU) index, when we define the SU index as mg/glimepiride × years of treatment.

Conclusions

As appetite suppressions and associated decreases in body weights were not observed in obese patients, the efficacy of liraglutide at 0.9 mg did not appear to be high. Rather, it appeared to be highly effective for patients who were non-obese and for whom amelioration of blood glucose elevations could be anticipated via the stimulation of insulin secretion. Therefore, we found that liraglutide at doses of 0.9 mg was highly effective in non-obese patients who were in the early stages of diabetes and was particularly effective in patients who had not yet been administered SU agents.

Renal extraction and acute effects of glucagon-like peptide-1 on central and renal hemodynamics in healthy men

The present experiments were performed to elucidate the acute effects of intravenous infusion of glucagon-like peptide (GLP)-1 on central and renal hemodynamics in healthy men. Seven healthy middle-aged men were examined on two different occasions in random order. During a 3-h infusion of either GLP-1 (1.5 pmol·kg⁻¹·min⁻¹) or saline, cardiac output was estimated noninvasively, and intraarterial blood pressure and heart rate were measured continuously. Renal plasma flow, glomerular filtration rate, and uptake/release of hormones and ions were measured by Fick's Principle after catheterization of a renal vein. Subjects remained supine during the experiments. During GLP-1 infusion, both systolic blood pressure and arterial pulse pressure increased by 5±1 mmHg (P=0.015 and P=0.002, respectively). Heart rate increased by 5±1 beats/min (P=0.005), and cardiac output increased by 18% (P=0.016). Renal plasma flow and glomerular filtration rate as well as the clearance of Na⁺ and Li⁺ were not affected by GLP-1. However, plasma renin activity decreased (P=0.037), whereas plasma levels of atrial natriuretic peptide were unaffected. Renal extraction of intact GLP-1 was 43% (P<0.001), whereas 60% of the primary metabolite GLP-1 9-36amide was extracted (P=0.017). In humans, an acute intravenous administration of GLP-1 leads to increased cardiac output due to a simultaneous increase in stroke volume and heart rate, whereas no effect on renal hemodynamics could be demonstrated despite significant extraction of both the intact hormone and its primary metabolite.

Stabilization of postprandial blood glucose fluctuations by addition of glucagon like polypeptide-analog administration to intensive insulin therapy

Aims/Introduction

The nature of the action of concomitant liraglutide to stabilize postprandial blood glucose level (PBG) in patients on intensive insulin therapy with unstable PBG remains unclear. The aim was to identify the nature of liraglutide's actions to stabilize PBGs.

Materials and Methods

The study participants consisted of 20 diabetes patients showing unstable PBGs after dinner despite undergoing intensive insulin therapy. The dose of bolus insulin was reduced by three units for each meal, and 0.9 mg/day of liraglutide was added and used in combination. We evaluated the participants' data after the first evaluation (immediately before using liraglutide in combination) and the second evaluation (16 weeks after starting concomitant therapy). PBGs after dinner were measured every day for a period of 28 days immediately before carrying out both evaluations. The mean value of the 28 sets of blood glucose data and their standard deviation (SD) values were established as PBGs after dinner, as well as the SD for each participant. The changes in the mean values of the 20 participants, as well as their SD between before and after concomitant therapy, were evaluated.

Results

The mean value of PBGs (12.0 ± 1.0 to 10.1 ± 0.9 mmol/L) and SD values (5.1 ± 0.7–3.5 ± 0.8) after dinner both declined. A multiple regression analysis showed that the combined use of liraglutide was a significant independent variable of the SD values of PBGs after dinner.

Conclusion

The treatment of reducing the dose of insulin and using liraglutide in combination not only suppresses PBGs, but also stabilizes their blood glucose fluctuations.

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.

Effects of GLP-1 on appetite and weight

Glucagon-like peptide 1 (GLP-1) is a cleavage product of the pre-proglucagon gene which is expressed in the α-cells of the pancreas, the L-cells of the intestine, and neurons located in the caudal brainstem and hypothalamus. GLP-1 is of relevance to appetite and weight maintenance because it has actions on the gastrointestinal tract as well as the direct regulation of appetite. It delays gastric emptying and gut motility in humans. In addition, interventricular injections of GLP-1 inhibit food intake, independent of the presence of food in the stomach or gastric emptying. Peripherally administered GLP-1 also affects the central regulation of feeding. It is therefore the synergistic actions of GLP-1 in the gut and brain, acting on both central and peripheral receptors that seem responsible for the effects of the hormone on satiety.

Exogenous glucagon-like peptide 1 reduces contractions in human colon circular muscle

Glucagon-like peptide 1 (GLP1) is a naturally occurring peptide secreted by intestinal L-cells. Though its primary function is to serve as an incretin, GLP1 reduces gastrointestinal motility. However, only a handful of animal studies have specifically evaluated the influence of GLP1 on colonic motility. Consequently, the aims of this study were to investigate the effects induced by exogenous GLP1, to analyze the mechanism of action, and to verify the presence of GLP1 receptors (GLP1Rs) in human colon circular muscular strips. Organ bath technique, RT-PCR, western blotting, and immunofluorescence were used. In human colon, exogenous GLP1 reduced, in a concentration-dependent manner, the amplitude of the spontaneous contractions without affecting the frequency and the resting basal tone. This inhibitory effect was significantly reduced by exendin (9-39), a GLP1R antagonist, which per se significantly increased the spontaneous mechanical activity. Moreover, it was abolished by tetrodotoxin, a neural blocker, or Nω-nitro-l-arginine - a blocker of neuronal nitric oxide synthase (nNOS). The biomolecular analysis revealed a genic and protein expression of the GLP1R in the human colon. The double-labeling experiments with anti-neurofilament or anti-nNOS showed, for the first time, that immunoreactivity for the GLP1R was expressed in nitrergic neurons of the myenteric plexus. In conclusion, the results of this study suggest that GLP1R is expressed in the human colon and, once activated by exogenous GLP1, mediates an inhibitory effect on large intestine motility through NO neural release.

Study of postprandial lipaemia in type 2 diabetes mellitus: exenatide versus liraglutide

Therapeutic approaches based on the actions of the incretin hormone GLP-1 have been widely established in the management of T2DM. Nevertheless, much less research has been aimed at elucidating the role of GLP-1 in lipid metabolism and in particular postprandial dyslipidemia. Exenatide and liraglutide are two GLP-1 receptor agonists which are currently available as subcutaneously administered treatment for T2DM but their chronic effects on postprandial lipaemia have not been well investigated. The aim of this study is to examine the effect of treatment with either liraglutide or exenatide for two weeks on postprandial lipaemia in obese subjects with T2DM. This study was a single-center, two-armed, randomized, controlled 2-week prospective intervention trial in 20 subjects with T2DM. Patients were randomized to receive either liraglutide or exenatide treatment and underwent a standardized meal tolerance test early in the morning after 10 h fast at baseline (visit 1, beginning of treatment) and after a two-week treatment period (visit 2). Exenatide and liraglutide both appear to be equally effective in lowering postprandial lipaemia after the first administration and after a two-week treatment. The mechanisms which lead to this phenomenon, which seem to be independent of gastric emptying, are yet to be studied.

Incretin hormones and the satiation signal

Recent research has indicated that appetite-regulating hormones from the gut may have therapeutic potential. The incretin hormone, glucagon-like peptide-1 (GLP-1), appears to be involved in both peripheral and central pathways mediating satiation. Several studies have also indicated that GLP-1 levels and responses to meals may be altered in obese subjects. Clinical trial results have shown further that two GLP-1 receptor agonists (GLP-1 RAs), exenatide and liraglutide, which are approved for the treatment of hyperglycemia in patients with type 2 diabetes, also produce weight loss in overweight subjects without diabetes. Thus, GLP-1 RAs may provide a new option for pharmacological treatment of obesity.

Intraportal GLP-1 stimulates insulin secretion predominantly through the hepatoportal-pancreatic vagal reflex pathways

We previously reported that glucagon-like peptide-1 (GLP-1) appearance in the portal vein facilitates hepatic vagal afferent activity, and this further augments reflexively the pancreatic vagal efferents in anesthetized rats, suggesting a neuroincretin effect of GLP-1. To determine whether the GLP-1-induced vagal pathways lead to a neuronal-mediated component (NMC) of insulin secretion, we infused GLP-1 at a physiological or pharmacological dose (1 or 3 pmol·kg(-1)·min(-1), respectively) into the portal vein in conscious rats with selective hepatic vagotomy (Vagox) or sham operation (Sham). The experiments consisted of two sequential 10-min intraportal infusions (P1 and P2): glucose at a physiological rate (56 μmol·kg(-1)·min(-1)) in P1 and the glucose plus GLP-1 or vehicle in P2. Under arterial isoglycemia across the groups, the physiological GLP-1 infusion in Sham augmented promptly and markedly arterial insulin levels, approximately twofold the levels in glucose alone infusion (P < 0.005), and insulin levels in Vagox diminished apparently (P < 0.05). Almost 60% of the GLP-1-induced insulin secretion (AUC) in Sham met the NMC, i.e., difference between insulin secretion in Sham and Vagox, (AUC 976 ± 65 vs. 393 ± 94 pmol·min/l, respectively, P < 0.005). Intraportal pharmacological GLP-1 infusion further augmented insulin secretion in both groups, but the NMC remained in 46% (NS; Sham vs. Vagox). In contrast, "isoglycemic" intravenous GLP-1 infusion (3 pmol·kg(-1)·min(-1)) evoked an equal insulin secretion in both groups. Thus, the present results indicate that GLP-1 appearing in the portal vein evokes a powerful neuronal-mediated insulinotropic effect, suggesting the neuroincretin effect.

The effect of exogenous GLP-1 on food intake is lost in male truncally vagotomized subjects with pyloroplasty

Rapid degradation of glucagon-like peptide-1 (GLP-1) by dipeptidyl peptidase-4 suggests that endogenous GLP-1 may act locally before being degraded. Signaling via the vagus nerve was investigated in 20 truncally vagotomized subjects with pyloroplasty and 10 matched healthy controls. Subjects received GLP-1 (7-36 amide) or saline infusions during and after a standardized liquid mixed meal and a subsequent ad libitum meal. Despite no effect on appetite sensations, GLP-1 significantly reduced ad libitum food intake in the control group but had no effect in the vagotomized group. Gastric emptying was accelerated in vagotomized subjects and was decreased by GLP-1 in controls but not in vagotomized subjects. Postprandial glucose levels were reduced by the same percentage by GLP-1 in both groups. Peak postprandial GLP-1 levels were approximately fivefold higher in the vagotomized subjects. Insulin secretion was unaffected by exogenous GLP-1 in vagotomized subjects but was suppressed in controls. GLP-1 significantly reduced glucagon secretion in both groups, but levels were approximately twofold higher and were nonsuppressible in the early phase of the meal in vagotomized subjects. Our results demonstrate that vagotomy with pyloroplasty impairs the effects of exogenous GLP-1 on food intake, gastric emptying, and insulin and glucagon secretion, suggesting that intact vagal innervation may be important for GLP-1's actions.

Glucagon-like peptide 1 and appetite

Glucagon-like peptide 1 (GLP-1) and GLP-1 analogs have received much recent attention due to the success of GLP-1 mimetics in treating type II diabetes mellitus (T2DM), but these compounds may also have the potential to treat obesity. The satiety effect of GLP-1 may involve both within-meal enteroenteric reflexes, and across-meal central signaling mechanisms, that mediate changes in appetite and promote satiety. Here, we review data supporting the role of both peripheral and central GLP-1 signaling in the control of gastrointestinal motility and food intake. Understanding the mechanisms underlying the appetite-suppressive effects of GLP-1 may help in developing targeted treatments for obesity.

Rapid tachyphylaxis of the glucagon-like peptide 1-induced deceleration of gastric emptying in humans

OBJECTIVE

Glucagon-like peptide (GLP)-1 lowers postprandial glycemia primarily through inhibition of gastric emptying. We addressed whether the GLP-1-induced deceleration of gastric emptying is subject to rapid tachyphylaxis and if so, how this would alter postprandial glucose control.

RESEARCH DESIGN AND METHODS

Nine healthy volunteers (25 ± 4 years old, BMI: 24.6 ± 4.7 kg/m(2)) were examined with intravenous infusion of GLP-1 (0.8 pmol · kg(-1) · min(-1)) or placebo over 8.5 h. Two liquid mixed meals were administered at a 4-h interval. Gastric emptying was determined, and blood samples were drawn frequently.

RESULTS

GLP-1 decelerated gastric emptying significantly more after the first meal compared with the second meal (P = 0.01). This was associated with reductions in pancreatic polypeptide levels (marker of vagal activation) after the first but not the second meal (P < 0.05). With GLP-1, glucose concentrations declined after the first meal but increased after the second meal (P < 0.05). The GLP-1-induced reductions in postprandial insulin and C-peptide levels were stronger during the first meal course (P < 0.05). Likewise, glucagon levels were lowered by GLP-1 after the first meal but increased after the second test meal (P < 0.05).

CONCLUSIONS

The GLP-1-induced delay in gastric emptying is subject to rapid tachyphylaxis at the level of vagal nervous activation. As a consequence, postprandial glucose control by GLP-1 is attenuated after its chronic administration.

Glucagon-like peptide-1 relaxes gastric antrum through nitric oxide in mice

Glucagon-like-peptide-1 (GLP-1) is a proglucagon-derived peptide expressed in the intestinal enteroendocrine-L cells and released after meal ingestion. GLP-1 reduces postprandial glycemia not only by its hormonal effects, but also by its inhibitory effects on gastrointestinal motility. Recently, we showed that GLP-1 acts in the enteric nervous system of mouse intestine. Therefore our working hypothesis was that GLP-1 may have also a direct influence on the gastric mechanical activity since the major part of experimental studies about its involvement in the regulation of gastric motility have been conducted in in vivo conditions. The purposes of this study were (i) to examine exogenous GLP-1 effects on mouse gastric mechanical activity using isolated whole stomach; (ii) to clarify the regional activity of GLP-1 using circular muscular strips from gastric fundus or antrum; (iii) to analyze the mechanism of action underlying the observed effects; (iv) to verify regional differences of GLP-1 receptors (GLP-1R) expression by RT-PCR. In the whole stomach GLP-1 caused concentration-dependent relaxation significantly anatagonized by exendin (9-39), an antagonist of GLP-1R and abolished by tetrodotoxin (TTX) or N(ω)-nitro-l-arginine methyl ester (l-NAME), inhibitor of nitric oxide (NO) synthase. GLP-1 was without any effect in fundic strips, but it induced concentration-dependent relaxation in carbachol-precontracted antral strips. The effect was abolished by TTX or l-NAME. RT-PCR analysis revealed a higher expression of GLP-1R mRNA in antrum than in fundus. These results suggest that exogenous GLP-1 is able to reduce mouse gastric motility by acting peripherally in the antral region, through neural NO release.

GLP-1 playing the role of a gut regulatory compound

Gastric emptying is the first step in the metabolic endocrine cascade that takes place after food intake. The incretin hormones originating in the gut, particularly GLP-1, exert multiple antihyperglycaemic actions such as enhancement of glucose-dependent insulin secretion, suppression of glucagon secretion, slowing of gastric emptying with an ensuing decrease in food intake and weight loss. From extensive studies in experimental animals and humans we have found that GLP-1 also exerts a motility-inhibiting and antispasmodic effect in the gut that was verified in healthy volunteers and patients with irritable bowel syndrome (IBS). In order to further investigate the effect of GLP-1 in humans, we used the dipeptidyl peptidase-IV resistant GLP-1 analogue ROSE-010, thereby extending its biological activity. A randomized, double-blinded, prospective clinical trial was carried out in order to investigate the effect of two doses of ROSE-010 in 166 patients suffering from pain attacks of IBS. We found that injections of ROSE-010 were twice as effective as placebo in terms of total pain relief response in those affected by pain attacks due to IBS. Our results show that basal physiological research studies can be translated into clinical use. The current pharmaceutical incentive with incretin mimetics, such as GLP-1 analogues and exenatide, is an interesting development that apart from its obvious use in diabetes type 2, may also be useful in terms of gut motility-regulating effects with effects on appetite, food intake and motility disorders that may provide an opportunity to bring about new improvements in medical care.

Exenatide does not evoke pancreatitis and attenuates chemically induced pancreatitis in normal and diabetic rodents

The risk of developing pancreatitis is elevated in type 2 diabetes and obesity. Cases of pancreatitis have been reported in type 2 diabetes patients treated with GLP-1 (GLP-1R) receptor agonists. To examine whether the GLP-1R agonist exenatide potentially induces or modulates pancreatitis, the effect of exenatide was evaluated in normal or diabetic rodents. Normal and diabetic rats received a single exenatide dose (0.072, 0.24, and 0.72 nmol/kg) or vehicle. Diabetic ob/ob or HF-STZ mice were infused with exenatide (1.2 and 7.2 nmol·kg(-1)·day(-1)) or vehicle for 4 wk. Post-exenatide treatment, pancreatitis was induced with caerulein (CRN) or sodium taurocholate (ST), and changes in plasma amylase and lipase were measured. In ob/ob mice, plasma cytokines (IL-1β, IL-2, IL-6, MCP-1, IFNγ, and TNFα) and pancreatitis-associated genes were assessed. Pancreata were weighed and examined histologically. Exenatide treatment alone did not modify plasma amylase or lipase in any models tested. Exenatide attenuated CRN-induced release of amylase and lipase in normal rats and ob/ob mice but did not modify the response to ST infusion. Plasma cytokines and pancreatic weight were unaffected by exenatide. Exenatide upregulated Reg3b but not Il6, Ccl2, Nfkb1, or Vamp8 expression. Histological analysis revealed that the highest doses of exenatide decreased CRN- or ST-induced acute inflammation, vacuolation, and acinar single cell necrosis in mice and rats, respectively. Ductal cell proliferation rates were low and similar across all groups of ob/ob mice. In conclusion, exenatide did not modify plasma amylase and lipase concentrations in rodents without pancreatitis and improved chemically induced pancreatitis in normal and diabetic rodents.

Bile-induced secretion of glucagon-like peptide-1: pathophysiological implications in type 2 diabetes?

During the last decades it has become clear that bile acids not only act as simple fat solubilizers, but additionally represent complex hormonal metabolic integrators. Bile acids activate both nuclear receptors (controlling transcription of genes involved in for example bile acid, cholesterol, and glucose metabolism) and the cell surface G protein-coupled receptor TGR5 (modulating energy expenditure in brown fat and muscle cells). It has been shown that TGR5 is expressed in enteroendocrine L cells, which secrete the potent glucose-lowering incretin hormone glucagon-like peptide-1 (GLP-1). Recently it was shown that bile acid-induced activation of TGR5 results in intestinal secretion of GLP-1 and that enhanced TGR5 signaling improves postprandial glucose tolerance in diet-induced obese mice. This Perspectives article presents these novel findings in the context of prior studies on nutrient-induced GLP-1 secretion and outlines the potential implications of bile acid-induced GLP-1 secretion in physiological, pathophysiological, and pharmacological perspectives.

Peripheral motor action of glucagon-like peptide-1 through enteric neuronal receptors

BACKGROUND

Glucagon-like peptide-1 (GLP-1) is a proglucagon-derived peptide expressed in the enteroendocrine-L cells of small and large intestine and released in response to meal ingestion. Glucagon-like peptide-1 exerts inhibitory effects on gastrointestinal motility through vagal afferents and central nervous mechanisms; however, no data is available about a direct influence on the gastrointestinal wall. Our aim was to investigate the effects of GLP-1 on the spontaneous and evoked mechanical activity of mouse duodenum and colon and to identify the presence and distribution of GLP-1 receptors (GLP-1R) in the muscle coat.

METHODS

Organ bath recording technique and immunohistochemistry were used.

KEY RESULTS

Glucagon-like peptide-1 (up to the concentration of 1 mumol L(-1)) failed to affect spontaneous mechanical activity. It caused concentration-dependent reduction of the electrically evoked cholinergic contractions in circular smooth muscle of both intestinal segments, without affecting the longitudinal muscle responses. Glucagon-like peptide-1 inhibitory effect was significantly antagonized by exendin (9-39), an antagonist of GLP-1R. In both intestinal preparations, GLP-1 effect was not affected by guanethidine, a blocker of adrenergic neurotransmission, but it was significantly reduced by N(omega)-nitro-l-arginine methyl ester, inhibitor of nitric oxide (NO) synthase. Glucagon-like peptide-1 failed to affect the contractions evoked by exogenous carbachol. Immunohistochemistry demonstrated GLP-1R expression in the enteric neurons. Furthermore, 27% of GLP-1R immunoreactive (IR) neurons in the duodenum and 79% of GLP-1R-IR neurons in the colon, co-expressed nNOS.

CONCLUSIONS & INFERENCES

The present results suggest that GLP-1 is able to act in the enteric nervous system by decreasing the excitatory cholinergic neurotransmission through presynaptic GLP-1Rs, which modulate NO release.

Glucagon-like peptide-1 receptor activation modulates pancreatitis-associated gene expression but does not modify the susceptibility to experimental pancreatitis in mice

OBJECTIVE

Clinical reports link use of the glucagon-like peptide-1 receptor (GLP-1R) agonists exenatide and liraglutide to pancreatitis. However, whether these agents act on the exocrine pancreas is poorly understood.

RESEARCH DESIGN AND METHODS

We assessed whether the antidiabetic agents exendin (Ex)-4, liraglutide, the dipeptidyl peptidase-4 inhibitor sitagliptin, or the biguanide metformin were associated with changes in expression of genes associated with the development of experimental pancreatitis. The effects of Ex-4 when administered before or after the initiation of caerulein-induced experimental pancreatitis were determined. The importance of endogenous GLP-1R signaling for gene expression in the exocrine pancreas and the severity of pancreatitis was assessed in Glp1r(-/-) mice.

RESULTS

Acute administration of Ex-4 increased expression of egr-1 and c-fos in the exocrine pancreas. Administration of Ex-4 or liraglutide for 1 week increased pancreas weight and induced expression of mRNA transcripts encoding the anti-inflammatory proteins pancreatitis-associated protein (PAP) (RegIIIbeta) and RegIIIalpha. Chronic Ex-4 treatment of high-fat-fed mice increased expression of PAP and reduced pancreatic expression of mRNA transcripts encoding for the proinflammatory monocyte chemotactic protein-1, tumor necrosis factor-alpha, and signal transducer and activator of transcription-3. Sitagliptin and metformin did not significantly change pancreatic gene expression profiles. Ex-4 administered before or after caerulein did not modify the severity of experimental pancreatitis, and levels of pancreatic edema and serum amylase were comparable in caerulein-treated Glp1r(-/-) versus Glp1r(+/+) mice.

CONCLUSIONS

These findings demonstrate that GLP-1 receptor activation increases pancreatic mass and selectively modulates the expression of genes associated with pancreatitis. However, activation or genetic elimination of GLP-1R signaling does not modify the severity of experimental pancreatitis in mice.

Vagally mediated effects of glucagon-like peptide 1: in vitro and in vivo gastric actions

Glucagon-like peptide-1 (GLP-1) is a neuropeptide released following meal ingestion that, among other effects, decreases gastric tone and motility. The central targets and mechanism of action of GLP-1 on gastric neurocircuits have not, however, been fully investigated. A high density of GLP-1 containing neurones and receptors are present in brainstem vagal circuits, suggesting that the gastroinhibition may be vagally mediated. We aimed to investigate: (1) the response of identified gastric-projecting neurones of the dorsal motor nucleus of the vagus (DMV) to GLP-1 and its analogues; (2) the effects of brainstem application of GLP-1 on gastric tone; and (3) the vagal pathway utilized by GLP-1 to induce gastroinhibition. We conducted our experiments using whole-cell recordings from identified gastric-projecting DMV neurones and microinjection in the dorsal vagal complex (DVC) of anaesthetized rats while monitoring gastric tone. Perfusion with GLP-1 induced a concentration-dependent excitation of a subpopulation of gastric-projecting DMV neurones. The GLP-1 effects were mimicked by exendin-4 and antagonized by exendin-9-39. In an anaesthetized rat preparation, application of exendin-4 to the DVC decreased gastric tone in a concentration-dependent manner. The gastroinhibitory effects of exendin-4 were unaffected by systemic pretreatment with the pro-motility muscarinic agonist bethanechol, but were abolished by systemic administration of the nitric oxide synthase (NOS) inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME), or by bilateral vagotomy. Our data indicate that GLP-1 activates selective receptors to excite DMV neurones mainly and that the gastroinhibition observed following application of GLP-1 in the DVC is due to the activation of an inhibitory non-adrenergic, non-cholinergic input to the stomach.

GLP-1 regulates gastroduodenal motility involving cholinergic pathways

The gut-born incretin hormone glucagon-like peptide-1 (GLP-1) delays gastric emptying. To elucidate the mechanisms by which GLP-1 affects gastroduodenal motility and glycaemia, we studied the effects of exendin(9-39), a potent GLP-1 receptor antagonist, on gastroduodenal motility and pancreatic hormones. In this randomized, double-blind, placebo-controlled, four-arm, cross-over trial, 10 healthy volunteers were studied during the interdigestive period followed by duodenal perfusion of a mixed liquid meal (250 kcal). On four separate days, exendin(9-39), atropine, exendin(9-39) + atropine or saline were infused intravenously. Antro-pyloro-duodenal and fundic motility were assessed. The compliance of the proximal stomach was determined by isobaric distensions. During fasting, exendin(9-39) did not influence proximal gastric volume, pyloric tone, and duodenal contractility. Exendin(9-39) significantly increased antral waves only in the absence of atropine. During duodenal meal perfusion, exendin(9-39) significantly reduced proximal gastric volume accommodation, abbreviated postprandial antral inhibition, reduced the postprandial increase in pyloric tone, and reduced gastric compliance. Atropine abolished the effects of exendin(9-39) on gastric volume accommodation but did not affect its effects on postprandial antroduodenal motility and on gastric compliance. Exendin(9-39) increased fasting and postprandial glycaemia and plasma glucagon but not insulin concentrations. Atropine did not affect GLP-1 secretion. Cholinergic mechanisms mediate the effects of GLP-1 on postprandial gastric accommodation but not on antro-pyloro-duodenal motility. GLP-1 reduces fasting and postprandial glycaemia, in part by reducing glucagon secretion.

Orlistat accelerates gastric emptying and attenuates GIP release in healthy subjects

Orlistat, an inhibitor of digestive lipases, is widely used for the treatment of obesity. Previous reports on the effect of orally ingested orlistat together with a meal on gastric emptying and secretion of gut peptides that modulate postprandial responses are controversial. We investigated the effect of ingested orlistat on gastric emptying and plasma responses of gut peptides in response to a solid mixed meal with a moderate energy load. In healthy subjects, gastric emptying was determined using scintigraphy and studies were performed without and with 120 mg of orlistat in pellet form in random order. Orlistat shortened t lag and t half and decreased the area under the gastric emptying curve. Orlistat significantly attenuated the secretion of glucose-dependent insulinotropic polypeptide (GIP) but did not alter the plasma responses of cholecystokinin (CCK), glucagon-like peptide-1 (GLP-1), pancreatic polypeptide (PP), and insulin. There was no peptide YY (PYY) response. Area under the curve of gastric emptying was positively correlated with integrated secretion of GIP (r=0.786) in orlistat and was negatively correlated with integrated plasma response of GLP-1 (r=-0.75) in control experiments, implying that inhibition of fat absorption modifies determinants of gastric emptying of a meal. Orlistat administered similar to its use in obesity treatment accelerates gastric emptying of a solid mixed meal with a moderate energy load and profoundly attenuates release of GIP without appreciably altering plasma responses of CCK, GLP-1, and PP. Since GIP is being implemented in the development of obesity, its role in weight control attained by orlistat awaits further investigation.

Genetic variation in Glp1r expression influences the rate of gastric emptying in mice

We demonstrated previously that food intake traits map to a quantitative trait locus (QTL) on proximal chromosome 17, which encompasses Glp1r (glucagon-like peptide 1 receptor), encoding an important modulator of gastric emptying. We then confirmed this QTL in a B6.CAST-17 congenic strain that consumed 27% more carbohydrate and 17% more total calories, yet similar fat calories, per body weight compared with the recipient C57BL/6J. The congenic strain also consumed greater food volume. The current aims were to 1) identify genetic linkage for total food volume in F(2) mice, 2) perform gene expression profiling in stomach of B6.CAST-17 congenic mice using oligonucleotide arrays, 3) test for allelic imbalance in Glp1r expression, 4) evaluate gastric emptying rate in parental and congenic mice, and 5) investigate a possible effect of genetic variation in Glp1r on gastric emptying. A genome scan revealed a single QTL for total food volume (Tfv1) (log of the odds ratio = 7.6), which was confirmed in B6.CAST-17 congenic mice. Glp1r exhibited allelic imbalance in stomach, which correlated with accelerated gastric emptying in parental CAST and congenic B6.CAST-17 mice. Moreover, congenic mice displayed an impaired gastric emptying response to exendin-(9-39). These results suggest that genetic variation in Glp1r contributes to the strain differences in gastric emptying rate.

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.