Glucagon-like peptide-1 receptor signaling modulates beta cell apoptosis

Physiology and emerging biochemistry of the glucagon-like peptide-1 receptor

The glucagon-like peptide-1 (GLP-1) receptor is one of the best validated therapeutic targets for the treatment of type 2 diabetes mellitus (T2DM). Over several years, the accumulation of basic, translational, and clinical research helped define the physiologic roles of GLP-1 and its receptor in regulating glucose homeostasis and energy metabolism. These efforts provided much of the foundation for pharmaceutical development of the GLP-1 receptor peptide agonists, exenatide and liraglutide, as novel medicines for patients suffering from T2DM. Now, much attention is focused on better understanding the molecular mechanisms involved in ligand induced signaling of the GLP-1 receptor. For example, advancements in biophysical and structural biology techniques are being applied in attempts to more precisely determine ligand binding and receptor occupancy characteristics at the atomic level. These efforts should better inform three-dimensional modeling of the GLP-1 receptor that will help inspire more rational approaches to identify and optimize small molecule agonists or allosteric modulators targeting the GLP-1 receptor. This article reviews GLP-1 receptor physiology with an emphasis on GLP-1 induced signaling mechanisms in order to highlight new molecular strategies that help determine desired pharmacologic characteristics for guiding development of future nonpeptide GLP-1 receptor activators.

Glucagon-like peptides 1 and 2 and vasoactive intestinal peptide are neuroprotective on cultured and mast cell co-cultured rat myenteric neurons

BACKGROUND

Neuropathy is believed to be a common feature of functional and inflammatory intestinal diseases. Vasoactive intestinal peptide (VIP) is an acknowledged neuroprotective agent in peripheral, including enteric, and central neurons. The proglucagon-like hormones glucagon-like peptide 1 and 2 (GLP1 and GLP2) belong to the secretin/glucagon/VIP superfamily of peptides and GLP1 and GLP2 receptors are expressed in enteric neurons. Possible neuroprotective effects of these peptides were investigated in the present study.

METHODS

GLP1, GLP2 and VIP were added to cultured myenteric neurons from rat small intestine or to co-cultures of myenteric neurons and rat peritoneal mast cells. Receptor selectivity was tested by the simultaneous presence of a GLP1 receptor antagonist (exendin (9-39) amide) or a VIP receptor antagonist (hybrid of neurotensin 6-11 and VIP 7-28). Neuronal survival was examined using immunocytochemistry and cell counting.

RESULTS

GLP1, GLP2 and VIP significantly and concentration-dependently enhanced neuronal survival. In addition the peptides efficiently counteracted mast cell-induced neuronal cell death in a concentration-dependent manner. Exendin(9-39)amide reversed GLP1-induced neuroprotection while GLP2- and VIP-induced enhanced neuronal survival were unaffected. The VIP receptor antagonist reversed GLP1- and VIP-induced neuroprotection while the GLP2-induced effect on neuronal survival was unaffected.

CONCLUSIONS

By activating separate receptors VIP, GLP1 and GLP2 elicit neuroprotective effects on rat myenteric neurons cultured with or without mast cells. This implies a powerful therapeutic potential of these peptides in enteric neuropathies with a broad spectrum of applications from autoimmunity to functional disorders.

Deletion of GαZ protein protects against diet-induced glucose intolerance via expansion of β-cell mass

Insufficient plasma insulin levels caused by deficits in both pancreatic β-cell function and mass contribute to the pathogenesis of type 2 diabetes. This loss of insulin-producing capacity is termed β-cell decompensation. Our work is focused on defining the role(s) of guanine nucleotide-binding protein (G protein) signaling pathways in regulating β-cell decompensation. We have previously demonstrated that the α-subunit of the heterotrimeric G(z) protein, Gα(z), impairs insulin secretion by suppressing production of cAMP. Pancreatic islets from Gα(z)-null mice also exhibit constitutively increased cAMP production and augmented glucose-stimulated insulin secretion, suggesting that Gα(z) is a tonic inhibitor of adenylate cyclase, the enzyme responsible for the conversion of ATP to cAMP. In the present study, we show that mice genetically deficient for Gα(z) are protected from developing glucose intolerance when fed a high fat (45 kcal%) diet. In these mice, a robust increase in β-cell proliferation is correlated with significantly increased β-cell mass. Further, an endogenous Gα(z) signaling pathway, through circulating prostaglandin E activating the EP3 isoform of the E prostanoid receptor, appears to be up-regulated in insulin-resistant, glucose-intolerant mice. These results, along with those of our previous work, link signaling through Gα(z) to both major aspects of β-cell decompensation: insufficient β-cell function and mass.

Self-inducible secretion of glucagon-like peptide-1 (GLP-1) that allows MIN6 cells to maintain insulin secretion and insure cell survival

Based on the hypothesis that MIN6 cells could produce glucagon-like peptide-1 (GLP-1) to maintain cell survival, we analyzed the effects of GLP-1 receptor agonist, exendin-4 (Ex4), and antagonist, exendin-(9-39) (Ex9) on cell function and cell differentiation. MIN6 cells expressed proglucagon mRNAs and produced GLP-1, which was accelerated by Ex4 and suppressed by Ex9. Moreover, Ex4 further enhanced glucose-stimulated GLP-1 secretion, suggesting autocrine loop-contributed amplification of the GLP-1 signal. Ex4 up-regulated cell differentiation- and cell function-related CREBBP, Pdx-1, Pax6, proglucagon, and PC1/3 gene expressions. The confocal laser scanning images revealed that GLP-1 positive cells were dominant in the early stage of cells, but positive for insulin were more prominent in the mature stage of cells. Ex4 accelerated cell viability, while Ex9 and anti-GLP-1 receptor antibody enhanced cell apoptosis. MIN6 cells possess a mechanism of GLP-1 signal amplification in an autocrine fashion, by which the cells maintained insulin production and cell survival.

Making progress: preserving beta cells in type 1 diabetes

The clinical care of patients with type 1 diabetes (T1D) has greatly improved over the past few decades; however, it remains impossible to completely normalize blood sugar utilizing currently available tools. Research is underway with a goal to improve the care and, ultimately, to cure T1D by preserving beta cells. This review will outline the progress that has been made in trials aimed at preserving insulin secretion in T1D by modifying the immune assault on the pancreatic beta cell. Although not yet ready for clinical use, successful trials have been conducted in new-onset T1D that demonstrated utility of three experimental agents with disparate modes of action (anti-T cell, anti-B cell, and costimulation blockade) to preserve insulin secretion. In contrast, prevention studies have so far failed to produce positive results but have shown that such studies are feasible and have identified new promising agents for study.

Neuroprotective effect of the glucagon-like peptide-1 receptor agonist, synthetic exendin-4, in streptozotocin-induced diabetic rats

BACKGROUND AND PURPOSE

Glucagon-like peptide-1 (GLP-1) receptors are widely expressed in neural tissues and diminish neuronal degeneration or induce neuronal differentiation. The aim of this study was to investigate the effect of the GLP-1 pathway on peripheral nerves in streptozotocin-induced diabetic rats.

EXPERIMENTAL APPROACH

Diabetic and nondiabetic rats were treated with the GLP-1 receptor agonist, synthetic exendin-4 (i.p., 1 nmol·kg(-1)·day(-1)) or placebo for 24 weeks, and current perception threshold values, cAMP levels and nerve fibre size in the sciatic nerve were measured. We also investigated GLP-1 receptor expression, quantitative changes in PGP9.5-positive intraepidermal nerve fibres and cleaved caspase 3-stained Schwann cells by immunohistochemistry.

KEY RESULTS

GLP-1 receptor expression was detected in the sciatic nerve and skin. After exendin-4 treatment, the increase seen in current perception threshold values at 2000 and 250 Hz in diabetic rats was reduced. Also, the decrease in myelinated fibre size or axon/fibre area ratio in the sciatic nerve and the loss of intraepidermal nerve fibre in the skin of diabetic rats were ameliorated. These responses were closely associated with the attenuation of Schwann cell apoptosis and improvement in the cAMP level in exendin-4-treated diabetic rats, compared with placebo-treated animals.

CONCLUSION AND IMPLICATIONS

Synthetic exendin-4 may prevent peripheral nerve degeneration induced by diabetes in an animal model, supporting the hypothesis that GLP-1 may be useful in peripheral neuropathy. The neuroprotection is probably attributable to GLP-1 receptor activation, antiapoptotic effects and restoration of cAMP content.

Towards PET imaging of intact pancreatic beta cell mass: a transgenic strategy

PURPOSE

We have generated transgenic mouse lines expressing the positron emission tomography (PET) reporter gene, sr39tk, under the control of the mouse insulin I promoter (MIP-sr39tk) to image endogenous islets using PET.

PROCEDURES

The MIP-sr39tk transgene was constructed using the 8.3 kb fragment of the mouse insulin I promoter and the sr39tk coding sequence from the mrfp-hrl-ttk trifusion construct. Expression of sr39TK in beta cells was confirmed by fluorescence immunohistochemistry of pancreatic sections. Histological sections were used to determine beta cell mass, islet area and islet number. Beta cell function was determined using intraperitoneal glucose tolerance tests. For ex vivo biodistrubution, mice were injected i.v. with 9.25 MBq [(18)F]fluorohydroxymethyl-butyl-guanine (FHBG), euthanized 1 h later and pancreata and other organs were collected and counted. For PET scans, mice were injected i.v. with 9.25 MBq [(18)F]FHBG, and dynamic scans were conducted for 1 h, followed by a 30 min static acquisition. To induce type 1 diabetes-like symptoms, MIP-sr39tk mice were injected i.p. with 40 mg/kg streptozotocin (STZ) once per day for five consecutive days, and biodistribution and PET scans were conducted thereafter.

RESULTS

Ex vivo quantification of [(18)F]FHBG uptake in the pancreas showed a 4.5-fold difference in transgenic vs. non-transgenics, confirming that expression of sr39TK results in the retention of the PET tracer. In STZ-treated MIP-sr39tk mice, impairments in glucose tolerance and decreases in beta cell mass correlated significantly with a diminishment in [(18)F]FHBG uptake before fasting hyperglycemia became apparent.

CONCLUSIONS

The MIP-sr39tk mouse demonstrates that PET imaging can detect changes in beta cell mass that precede the onset of diabetes.

Reduction of both beta cell death and alpha cell proliferation by dipeptidyl peptidase-4 inhibition in a streptozotocin-induced model of diabetes in mice

AIMS/HYPOTHESIS

Incretins stimulate insulin secretion in a glucose-dependent manner but also promote pancreatic beta cell protection. Dipeptidyl peptidase-4 (DPP-4) inhibitors are a new glucose-lowering treatment that blocks incretin degradation by DPP-4. We assessed whether DPP-4 inhibition suppresses the progression to hyperglycaemia in a low-dose streptozotocin (STZ)-induced diabetic mouse model, and then investigated how DPP-4 inhibition affects islet function and morphology.

METHODS

The DPP-4 inhibitor, des-fluoro-sitagliptin (SITA), was administered to mice during and after STZ injections, and in some mice also before STZ.

RESULTS

In control mice, STZ resulted in hyperglycaemia associated with impaired insulin secretion and excess glucagon secretion. In SITA-treated STZ mice, these metabolic abnormalities were improved, particularly when SITA administration was initiated before STZ injections. We observed beta cell loss and dramatic alpha cell expansion associated with decreased insulin content and increased glucagon content after STZ administration. In SITA-treated mice, islet architecture and insulin content were preserved, and no significant increase in glucagon content was observed. After STZ exposure, beta cell apoptosis increased before hyperglycaemia, and SITA treatment reduced the number of apoptotic beta cells. Interestingly, alpha cell proliferation was observed in non-treated mice after STZ injection, but the proliferation was not observed in SITA-treated mice.

CONCLUSIONS/INTERPRETATION

Our results suggest that the ability of DPP-4 inhibition to suppress the progression to STZ-induced hyperglycaemia involves both alleviation of beta cell death and alpha cell proliferation.

COUP-TFII controls mouse pancreatic β-cell mass through GLP-1-β-catenin signaling pathways

Background

The control of the functional pancreatic β-cell mass serves the key homeostatic function of releasing the right amount of insulin to keep blood sugar in the normal range. It is not fully understood though how β-cell mass is determined.

Methodology/Principal Findings

Conditional chicken ovalbumin upstream promoter transcription factor II (COUP-TFII)-deficient mice were generated and crossed with mice expressing Cre under the control of pancreatic duodenal homeobox 1 (pdx1) gene promoter. Ablation of COUP-TFII in pancreas resulted in glucose intolerance. Beta-cell number was reduced at 1 day and 3 weeks postnatal. Together with a reduced number of insulin-containing cells in the ductal epithelium and normal β-cell proliferation and apoptosis, this suggests decreased β-cell differentiation in the neonatal period. By testing islets isolated from these mice and cultured β-cells with loss and gain of COUP-TFII function, we found that COUP-TFII induces the expression of the β-catenin gene and its target genes such as cyclin D1 and axin 2. Moreover, induction of these genes by glucagon-like peptide 1 (GLP-1) via β-catenin was impaired in absence of COUP-TFII. The expression of two other target genes of GLP-1 signaling, GLP-1R and PDX-1 was significantly lower in mutant islets compared to control islets, possibly contributing to reduced β-cell mass. Finally, we demonstrated that COUP-TFII expression was activated by the Wnt signaling-associated transcription factor TCF7L2 (T-cell factor 7-like 2) in human islets and rat β-cells providing a feedback loop.

Conclusions/Significance

Our findings show that COUP-TFII is a novel component of the GLP-1 signaling cascade that increases β-cell number during the neonatal period. COUP-TFII is required for GLP-1 activation of the β-catenin-dependent pathway and its expression is under the control of TCF7L2.

A simple matter of life and death-the trials of postnatal Beta-cell mass regulation

Pancreatic beta-cells, which secrete the hormone insulin, are the key arbiters of glucose homeostasis. Defective beta-cell numbers and/or function underlie essentially all major forms of diabetes and must be restored if diabetes is to be cured. Thus, the identification of the molecular regulators of beta-cell mass and a better understanding of the processes of beta-cell differentiation and proliferation may provide further insight for the development of new therapeutic targets for diabetes. This review will focus on the principal hormones and nutrients, as well as downstream signalling pathways regulating beta-cell mass in the adult. Furthermore, we will also address more recently appreciated regulators of beta-cell mass, such as microRNAs.

Phosphoproteins in stress-induced disease

The integrated stress response (ISR) is an evolutionarily conserved homeostatic program activated by specific pathological states. These include amino acid deprivation, viral infection, iron deficiency, and the misfolding of proteins within the endoplasmic reticulum (ER), the so-called ER stress. Although apparently disparate, each of these stresses induces phosphorylation of a translation initiation factor, eIF2α, to attenuate new protein translation while simultaneously triggering a transcriptional program. This is achieved by four homologous stress-sensing kinases: GCN2, PKR, HRI, and PERK. In addition to these kinases, mammals possess two specific eIF2α phosphatases, GADD34 and CReP, which play crucial roles in the recovery of protein synthesis following the initial insult. They are not only important in embryonic development but also appear to play important roles in disease, particularly cancer. In this chapter, we discuss each of the eIF2α kinases, in turn, with particular emphasis on their regulation and the new insights provided by recent structural studies. We also discuss the potential for developing novel drug therapies that target the ISR.

GLP-1, the gut-brain, and brain-periphery axes

Glucagon-like peptide 1 (GLP-1) is a gut hormone which directly binds to the GLP-1 receptor located at the surface of the pancreatic β-cells to enhance glucose-induced insulin secretion. In addition to its pancreatic effects, GLP-1 can induce metabolic actions by interacting with its receptors expressed on nerve cells in the gut and the brain. GLP-1 can also be considered as a neuropeptide synthesized by neuronal cells in the brain stem that release the peptide directly into the hypothalamus. In this environment, GLP-1 is assumed to control numerous metabolic and cardiovascular functions such as insulin secretion, glucose production and utilization, and arterial blood flow. However, the exact roles of these two locations in the regulation of glucose homeostasis are not well understood. In this review, we highlight the latest experimental data supporting the role of the gut-brain and brain-periphery axes in the control of glucose homeostasis. We also focus our attention on the relevance of β-cell and brain cell targeting by gut GLP-1 for the regulation of glucose homeostasis. In addition to its action on β-cells, we find that understanding the physiological role of GLP-1 will help to develop GLP-1-based therapies to control glycemia in type 2 diabetes by triggering the gut-brain axis or the brain directly. This pleiotropic action of GLP-1 is an important concept that may help to explain the observation that, during their treatment, type 2 diabetic patients can be identified as 'responders' and 'non-responders'.

Dipeptidyl peptidase IV inhibitor attenuates kidney injury in streptozotocin-induced diabetic rats

Dipeptidyl peptidase (DPP) IV inhibitors are probably beneficial for preventing diabetic complication and modulating glucagon-like peptide-1 receptor (GLP-1R) expression. The aim of this study was to determine whether the DPP IV inhibitor LAF237 (vildagliptin) has renoprotective qualities in streptozotocin-induced diabetic rats. Diabetic and nondiabetic rats were treated with an oral dose of 4 or 8 mg/kg/day LAF237 or placebo for 24 weeks, and renal injury was observed by light and electron microscopy. We also assessed DPP IV activity, active GLP-1 level, cAMP and 8-hydroxy-deoxyguanosine excretion, and GLP-1R, cleaved caspase 3, and transforming growth factor-β1 (TGF-β1) expression. LAF237 significantly decreased proteinuria, albuminuria, and urinary albumin/creatinine ratio, improved creatinine clearance, and dose-dependently inhibited interstitial expansion, glomerulosclerosis, and the thickening of the glomerular basement membrane in diabetic rats. It is noteworthy that LAF237 markedly down-regulated DPP IV activity and increased active GLP-1 levels, which probably prevented oxidative DNA damage and renal cell apoptosis by activating the GLP-1R and modulating cAMP. Renoprotection was also associated with a reduction in TGF-β1 overexpression. Our study suggests that DPP IV inhibitors may ameliorate diabetic nephropathy as well as reduce the overproduction of TGF-β1. The observed renoprotection is probably attributable to inhibition of DPP IV activity, mimicking of incretin action, and activation of the GLP-1R.

Repeated administration of exendin-4 reduces focal cerebral ischemia-induced infarction in rats

Exendin-4 is a GLP-1 agonist that is clinically used for the treatment of diabetes mellitus and may also have neuroprotective effect. We explored the effect of repeated administration of exendin-4 (0.5 μg/kg, intraperitoneal twice a day for 7 days) on infarct volume, neurological deficit (neurological score, grip test, foot fault and rota rod tests), oxidative stress parameters (malondialdehyde, reduced glutathione, and superoxide dismutase) and expression of endothelin (ET) ET(A) and ET(B) receptors following cerebral ischemia produced in rats by permanent middle cerebral artery occlusion (MCAO). Since ET(A) receptors in the central nervous system (CNS) are involved in cerebral ischemia, we determined the effect of a specific ET(A) receptor antagonist, BQ123 (1mg/kg, intravenously administered thrice: 30 min, 2h and 4h after MCAO for a total dose of 3 mg/kg) on cerebral ischemia in control and exendin-4 treated rats. Results indicate that exendin-4 treated rats had significant protection following MCAO induced cerebral ischemia. The infarct volume was 27% less compared to vehicle treated rats. The neurological deficit following MCAO was lower and oxidative stress parameters were improved in exendin-4 treated rats compared to control. BQ123 significantly improved infarct volume, oxidative stress parameters and neurological deficit in ischemic rats treated with vehicle or exendin-4. BQ123 induced protection from cerebral ischemia was similar in vehicle or exendin-4 treated rats. Expression of ET(A) receptors was significantly increased following cerebral ischemia which was not affected by exendin-4 treatment or by BQ123 administration. No change in expression of ET(B) receptors was observed following cerebral ischemia or any treatment. It is concluded that exendin-4 protects the CNS from damage due to cerebral ischemia by reducing oxidative stress and is independent of ET receptor involvement.

Glucagon-like peptide analogues for type 2 diabetes mellitus

BACKGROUND

Glucagon-like peptide analogues are a new class of drugs used in the treatment of type 2 diabetes that mimic the endogenous hormone glucagon-like peptide 1 (GLP-1). GLP-1 is an incretin, a gastrointestinal hormone that is released into the circulation in response to ingested nutrients. GLP-1 regulates glucose levels by stimulating glucose-dependent insulin secretion and biosynthesis, and by suppressing glucagon secretion, delayed gastric emptying and promoting satiety.

OBJECTIVES

To assess the effects of glucagon-like peptide analogues in patients with type 2 diabetes mellitus.

SEARCH STRATEGY

Studies were obtained from electronic searches of The Cochrane Library (last search issue 1, 2011), MEDLINE (last search March 2011), EMBASE (last search March 2011), Web of Science (last search March 2011) and databases of ongoing trials.

SELECTION CRITERIA

Studies were included if they were randomised controlled trials of a minimum duration of eight weeks comparing a GLP-1 analogue with placebo, insulin, an oral anti-diabetic agent, or another GLP-1 analogue in people with type 2 diabetes.

DATA COLLECTION AND ANALYSIS

Data extraction and quality assessment of studies were done by one reviewer and checked by a second. Data were analysed by type of GLP-1 agonist and comparison treatment. Where appropriate, data were summarised in a meta-analysis (mean differences and risk ratios summarised using a random-effects model).

MAIN RESULTS

Seventeen randomised controlled trials including relevant analyses for 6899 participants were included in the analysis. Studies were mostly of short duration, usually 26 weeks.In comparison with placebo, all GLP-1 agonists reduced glycosylated haemoglobin A1c (HbA1c) levels by about 1%. Exenatide 2 mg once weekly and liraglutide 1.8 mg reduced it by 0.20% and 0.24% respectively more than insulin glargine. Exenatide 2 mg once weekly reduced HbA1c more than exenatide 10 μg twice daily, sitagliptin and pioglitazone. Liraglutide 1.8 mg reduced HbA1c by 0.33% more than exenatide 10 μg twice daily. Liraglutide led to similar improvements in HbA1c compared to sulphonylureas but reduced it more than sitagliptin and rosiglitazone.Both exenatide and liraglutide led to greater weight loss than most active comparators, including in participants not experiencing nausea. Hypoglycaemia occurred more frequently in participants taking concomitant sulphonylurea. GLP-1 agonists caused gastrointestinal adverse effects, mainly nausea. These adverse events were strongest at the beginning and then subsided. Beta-cell function was improved with GLP-1 agonists but the effect did not persist after cessation of treatment.None of the studies was long enough to assess long-term positive or negative effects.

AUTHORS' CONCLUSIONS

GLP-1 agonists are effective in improving glycaemic control.

Reprogramming gut and pancreas endocrine cells to treat diabetes

Multiple approaches have been investigated with the ultimate goal of providing insulin independence to patients with either type 1 or type 2 diabetes. Approaches to produce insulin-secreting cells in culture, convert non-β-cells into functional β-cells or engineer autologous cells to express and secrete insulin in a meal-responsive manner have all been described. This research has been facilitated by significant improvements in both viral and non-viral gene delivery approaches that have enabled new experimental strategies. Many studies have examined possible avenues to confer islet cytoprotection against immune rejection, inflammation and apoptosis by genetic manipulation of islet cells prior to islet transplantation. Here we review several reports based on the reprogramming of pancreas and gut endocrine cells to treat diabetes.

Understanding the cardiovascular effects of incretin

Cardiovascular disease (CVD), a leading cause of death in patients with diabetes mellitus, has several pathogenic mechanisms that are well established. However, the traditional hypoglycemic agents do not have proven positive effects on macrovascular disease. Novel therapeutic agents target the incretin pathway including the glucagon-like peptide 1 (GLP-1) receptor (GLP-1R) agonists and the dipeptidyl peptidase-4 inhibitors. The glucose-regulatory actions of these agents function by increasing insulin secretion and suppressing glucagon. They also act to increase weight loss not only by inhibiting gastric emptying, but also by reducing appetite. Although GLP-1 and GLP-1R agonists have demonstrated beneficial effects on myocardium and vascular endothelium including coronary and peripheral mouse vessels, they also have anti-inflammatory and anti-atherogenic actions. These agents also have positive effects on the lipid profile and blood pressure. Although these cardioprotective actions seem to be beyond the effects of glucose control and weight loss, they are mediated through GLP-1R- or GLP-1R-independent actions of cleaved GLP-1 (9-36). Larger randomized controlled trials are necessary to elucidate the clinical promise of these beneficial CVD effects.

Beneficial effects of exendin-4 on experimental polyneuropathy in diabetic mice

OBJECTIVE

The therapeutic potential of exendin-4, an agonist of the glucagon-like peptide-1 receptor (GLP-1R), on diabetic polyneuropathy (DPN) in streptozotocin (STZ)-induced diabetic mice was investigated.

RESEARCH DESIGN AND METHODS

The presence of the GLP-1R in lumbar dorsal root ganglion (DRG) was evaluated by immunohistochemical analyses. DRG neurons were dissected from C57BL6/J mice and cultured with or without Schwann cell-conditioned media in the presence or absence of GLP-1 (7-37) or exendin-4. Then neurite outgrowth was determined. In animal-model experiments, mice were made diabetic by STZ administration, and after 12 weeks of diabetes, exendin-4 (10 nmol/kg) was intraperitoneally administered once daily for 4 weeks. Peripheral nerve function was determined by the current perception threshold and motor and sensory nerve conduction velocity (MNCV and SNCV, respectively). Sciatic nerve blood flow (SNBF) and intraepidermal nerve fiber densities (IENFDs) also were evaluated.

RESULTS

The expression of the GLP-1R in DRG neurons was confirmed. GLP-1 (7-37) and exendin-4 significantly promoted neurite outgrowth of DRG neurons. Both GLP-1R agonists accelerated the impaired neurite outgrowth of DRG neurons cultured with Schwann cell-conditioned media that mimicked the diabetic condition. At the doses used, exendin-4 had no effect on blood glucose or HbA(1c) levels. Hypoalgesia and delayed MNCV and SNCV in diabetic mice were improved by exendin-4 without affecting the reduced SNBF. The decreased IENFDs in sole skins of diabetic mice were ameliorated by exendin-4.

CONCLUSIONS

Our findings indicate that exendin-4 ameliorates the severity of DPN, which may be achieved by its direct actions on DRG neurons and their axons.

Role of endogenous ROS production in impaired metabolism-secretion coupling of diabetic pancreatic β cells

One of the characteristics of type 2 diabetes is that the insulin secretory response of β cells is selectively impaired to glucose. In the Goto-Kakizaki (GK) rat, a genetic model of type 2 diabetes mellitus, glucose-induced insulin secretion is selectively impaired due to deficient ATP production derived from impaired glucose metabolism. In addition, islets in GK rat and human type 2 diabetes are oxidatively stressed. In this issue, role of endogenous reactive oxygen species (ROS) production in impaired metabolism-secretion coupling of diabetic pancreatic β cells is reviewed. In β cells, ROS is endogenously produced by activation of Src, a non-receptor tyrosine kinase. Src inhibitors restore the impaired insulin release and impaired ATP elevation by reduction in ROS production in diabetic islets. Src is endogenously activated in diabetic islets, since the level of Src pY416 in GK islets is higher than that in control islets. In addition, exendin-4, a glucagon-like peptide-1 (GLP-1) receptor agonist, decreases Src pY416 and glucose-induced ROS production and ameliorates impaired ATP production dependently on Epac in GK islets. These results indicate that GLP-1 signaling regulates endogenous ROS production due to Src activation and that incretin has unique therapeutic effects on impaired glucose metabolism in diabetic β cells.

Glucagon-like peptide-1 analog liraglutide in combination with sulfonylurea safely improves blood glucose measures vs sulfonylurea monotherapy in Japanese patients with type 2 diabetes: Results of a 52-week, randomized, multicenter trial

Aims/Introduction:  Sulfonylurea (SU) agents are the most effective drugs at lowering blood glucose when used alone. However, their effectiveness declines after a certain period. The addition of liraglutide to existing SU therapy might reverse some of the known drawbacks of SU.

MATERIALS AND METHODS

  This multicenter, randomized, 52-week study assessed the long-term efficacy and safety of adding liraglutide at 0.6 or 0.9 mg/day to existing SU therapy in Japanese patients with inadequately controlled type 2 diabetes.

RESULTS

  In total, 264 patients were enrolled and received treatment. At week 52, HbA1c in the liraglutide 0.6 mg, liraglutide 0.9 mg and placebo groups was reduced from 9.00 to 7.91%, from 8.61 to 7.33%, and from 8.85 to 8.79%, respectively. The mean difference of HbA1c (95% CI) in the liraglutide 0.6 and 0.9 mg groups vs the placebo group was 0.96 (-1.25 to -0.67) and -1.33 (-1.62 to -1.04), respectively. For the liraglutide 0.6 mg, 0.9 mg and placebo groups, the Japanese Diabetes Society target HbA1c of <6.9% was achieved by 15.1, 39.1 and 4.5% of patients, respectively. Mean fasting plasma glucose at week 52 was lower in the liraglutide groups compared with the placebo group, and mean bodyweight remained unchanged in the liraglutide groups. Most subjects in all three treatment groups reported mild adverse events. No major hypoglycemic episode was reported.

CONCLUSIONS

  Once-daily administration of liraglutide in combination with SU for 52 weeks provided favorable metabolic control, a safety profile and did not alter bodyweight. This trial was registered with ClinicalTrial.gov (no. NCT00395746). (J Diabetes Invest,doi: 10.1111/j.2040-1124.2011.00103.x, 2011).

Glucagon-like peptide-1 and candesartan additively improve glucolipotoxicity in pancreatic β-cells

Glucagon-like peptide-1 (GLP-1) and angiotensin II type 1 receptor blocker reduce β-cell apoptosis in diabetes, but the underlying mechanisms are not fully understood. We examined the combination effects of GLP-1 and candesartan, an angiotensin II type 1 receptor blocker, on glucolipotoxicity-induced β-cell apoptosis; and we explored the possible mechanisms of the antiapoptotic effects. The effects of GLP-1 and/or candesartan on glucolipotoxicity-induced apoptosis and the phosphorylation of insulin receptor substrate-2 (IRS-2), protein kinase B (PKB), and forkhead box O1 (FoxO1) were evaluated by using MIN6 cells and isolated mouse pancreatic islets. Although palmitate significantly enhanced the high-glucose-induced apoptosis in both islets and MIN6 cells, GLP-1 and candesartan significantly inhibited apoptosis; and combination treatment additively prevented apoptosis. Whereas palmitate significantly decreased the phosphorylation of IRS-2, PKB, and FoxO1 in MIN6 cells, these changes were significantly inhibited by treatment with GLP-1 and/or candesartan. In addition, wortmannin, an inhibitor of phosphoinositide 3-kinase, markedly inhibited GLP-1- and/or candesartan-mediated PKB and FoxO1 phosphorylation. The present results suggest that GLP-1 and candesartan additively prevent glucolipotoxicity-induced apoptosis in pancreatic β-cells through the IRS-2/phosphoinositide 3-kinase/PKB/FoxO1 signaling pathway.

Glucolipotoxicity and beta cells in type 2 diabetes mellitus: target for durable therapy?

Type 2 diabetes mellitus (T2DM) is characterised by beta-cell failure in the setting of obesity-related insulin resistance. Progressive beta-cell dysfunction determines the course of the disease, regardless of the treatment used. There is mounting evidence that chronically elevated circulating levels of glucose and fatty acids contribute to relentless beta-cell function decline, by endorsing processes commonly referred to as glucolipotoxicity. Mechanisms related to glucolipotoxicity include endoplasmic reticulum (ER) stress, oxidative stress, mitochondrial dysfunction and islet inflammation. The most commonly prescribed blood-glucose lowering agents, metformin and sul-fonylurea, may temporarily improve glycaemic control, however, these drugs do not alter the continuous decline in beta-cell function in T2DM patients. Evidence exists that novel classes of drugs, the thiazolidinediones (TZDs) and incretin-based therapies, may be able to preserve beta-cell function and functional beta-cell mass, amongst others by reducing glucolipotoxicity in the beta cell. The durability of the effects of TZDs and incretin-based therapies on beta-cell function, whether given as monotherapy or combined with other treatment, should be addressed in future, long-term clinical studies.

Incretin effect: GLP-1, GIP, DPP4

The term incretin effect was used to describe the fact that oral glucose load produces a greater insulin response than that of an isoglycemic intravenous glucose infusion. This difference has been attributed to gastrointestinal peptides GLP-1 and GIP. Since incretin effect is reduced in subjects with type 2 diabetes, despite GLP-1 activity preservation, two forms of incretin-based treatment have emerged: GLP-1R agonists, administered subcutaneously and DPP-4 inhibitors, administered orally. There is a great interest whether incretin-based treatment will be associated with sustained long-term control and improvement in β-cell function. The observation that GLP-1R agonists improve myocardial function and survival of cardiomyocytes highlights the need for further studies. Incretin-based therapies offer a new option and show great promise for the treatment of type 2 diabetes.

Stromal cell-derived factor-1 (SDF-1)/chemokine (C-X-C motif) receptor 4 (CXCR4) axis activation induces intra-islet glucagon-like peptide-1 (GLP-1) production and enhances beta cell survival

AIMS/HYPOTHESIS

The endogenous production of stromal cell-derived factor-1 (SDF-1) in beta cells in transgenic mice attenuates the development of diabetes in response to streptozotocin. Here we propose that beta cell injury induces SDF-1 production, and the SDF-1/chemokine (C-X-C motif) receptor 4 (CXCR4) interaction auto-activates Sdf1 expression, resulting in the autocrine production of SDF-1 by beta cells and the paracrine activation of glucagon-like peptide-1 (GLP-1) production by alpha cells.

METHODS

SDF-1 production in adult mouse and human islets and rat INS-1 cells was measured in models of beta cell injury. The paracrine actions of SDF-1 on GLP-1 production in alpha cells were explored. The potential synergism between the growth-promoting actions of GLP-1 and the pro-survival actions of SDF-1 on the preservation of cell mass was evaluated by cell viability assays.

RESULTS

In adult islets and INS-1 cells, Sdf1 expression was re-induced in response to injury. The interaction of SDF-1 with its receptor on alphaTC1 cells activated protein kinase Akt, stimulated cell proliferation and induced the expression of prohormone convertase 1/3 and the consequent production of GLP-1 in alpha cells. The combination of GLP-1 and SDF-1 additively enhanced both the growth and longevity of INS-1 beta cells.

CONCLUSIONS/INTERPRETATION

The results of these studies suggest that in response to beta cell injury and the ensuing induction of SDF-1, the biological function of alpha cells switches from the production of glucagon to the provision of the local growth factor GLP-1 which, in combination with SDF-1, promotes the growth, survival and viability of the beta cells.

Receptors and effects of gut hormones in three osteoblastic cell lines

Background

In recent years the interest on the relationship of gut hormones to bone processes has increased and represents one of the most interesting aspects in skeletal research. The proportion of bone mass to soft tissue is a relationship that seems to be controlled by delicate and subtle regulations that imply "cross-talks" between the nutrient intake and tissues like fat. Thus, recognition of the mechanisms that integrate a gastrointestinal-fat-bone axis and its application to several aspects of human health is vital for improving treatments related to bone diseases. This work analysed the effects of gut hormones in cell cultures of three osteoblastic cell lines which represent different stages in osteoblastic development. Also, this is the first time that there is a report on the direct effects of glucagon-like peptide 2, and obestatin on osteoblast-like cells.

Methods

mRNA expression levels of five gut hormone receptors (glucose-dependent insulinotropic peptide [GIP], glucagon-like peptide 1 [GLP-1], glucagon-like peptide 2 [GLP-2], ghrelin [GHR] and obestatin [OB]) were analysed in three osteoblastic cell lines (Saos-2, TE-85 and MG-63) showing different stages of osteoblast development using reverse transcription and real time polymerase chain reaction. The responses to the gut peptides were studied using assays for cell viability, and biochemical bone markers: alkaline phosphatase (ALP), procollagen type 1 amino-terminal propeptides (P1NP), and osteocalcin production.

Results

The gut hormone receptor mRNA displayed the highest levels for GIP in Saos-2 and the lowest levels in MG-63, whereas GHR and GPR39 (the putative obestatin receptor) expression was higher in TE-85 and MG-63 and lower in Saos-2. GLP-1 and GLP-2 were expressed only in MG-63 and TE-85. Treatment of gut hormones to cell lines showed differential responses: higher levels in cell viability in Saos-2 after GIP, in TE-85 and MG-63 after GLP-1, GLP-2, ghrelin and obestatin. ALP showed higher levels in Saos-2 after GIP, GHR and OB and in TE-85 after GHR. P1NP showed higher levels after GIP and OB in Saos-2. Decreased levels of P1NP were observed in TE-85 and MG-63 after GLP-1, GLP-2 and OB. MG-63 showed opposite responses in osteocalcin levels after GLP-2.

Conclusions

These results suggest that osteoblast activity modulation varies according to different development stage under different nutrition related-peptides.

Exendin-4 increases islet amyloid deposition but offsets the resultant beta cell toxicity in human islet amyloid polypeptide transgenic mouse islets

AIMS/HYPOTHESIS

In type 2 diabetes, aggregation of islet amyloid polypeptide (IAPP) into amyloid is associated with beta cell loss. As IAPP is co-secreted with insulin, we hypothesised that IAPP secretion is necessary for amyloid formation and that treatments that increase insulin (and IAPP) secretion would thereby increase amyloid formation and toxicity. We also hypothesised that the unique properties of the glucagon-like peptide-1 (GLP-1) receptor agonist exendin-4 to maintain or increase beta cell mass would offset the amyloid-induced toxicity.

METHODS

Islets from amyloid-forming human IAPP transgenic and control non-transgenic mice were cultured for 48 h in 16.7 mmol/l glucose alone (control) or with exendin-4, potassium chloride (KCl), diazoxide or somatostatin. Human IAPP and insulin release, amyloid deposition, beta cell area/islet area, apoptosis and AKT phosphorylation levels were determined.

RESULTS

In control human IAPP transgenic islets, amyloid formation was associated with increased beta cell apoptosis and beta cell loss. Increasing human IAPP release with exendin-4 or KCl increased amyloid deposition. However, while KCl further increased beta cell apoptosis and beta cell loss, exendin-4 did not. Conversely, decreasing human IAPP release with diazoxide or somatostatin limited amyloid formation and its toxic effects. Treatment with exendin-4 was associated with an increase in AKT phosphorylation compared with control and KCl-treated islets.

CONCLUSIONS/INTERPRETATION

IAPP release is necessary for islet amyloid formation and its toxic effects. Thus, use of insulin secretagogues to treat type 2 diabetes may result in increased islet amyloidogenesis and beta cell death. However, the AKT-associated anti-apoptotic effects of GLP-1 receptor agonists such as exendin-4 may limit the toxic effects of increased islet amyloid.

Improved glycemic control and reduced bodyweight with exenatide: A double-blind, randomized, phase 3 study in Japanese patients with suboptimally controlled type 2 diabetes over 24 weeks

Aims/Introduction:  To evaluate the efficacy and safety of the glucagon-like peptide-1 receptor agonist, exenatide, in Japanese patients with type 2 diabetes mellitus suboptimally controlled despite therapeutic doses of a sulfonylurea alone or combined with a biguanide or thiazolidinedione.

MATERIALS AND METHODS

  Patients were randomized to a placebo or exenatide, either 5 or 10 μg, given subcutaneously b.i.d. in addition to oral therapy. Patients randomized to 10 μg exenatide received 5 μg b.i.d. for the first 4 weeks, followed by 10 μg b.i.d. for the last 20 weeks.

RESULTS

  A total of 179 patients received the study drug and composed the full analysis set (n = 35, placebo; n = 72, exenatide 5 μg; n = 72, exenatide 10 μg; 68% male; 58 ± 10 years; body mass index 25.5 ± 4.1 kg/m(2); HbA1c 8.2 ± 0.9%; means ± standard deviations). Baseline to end-point (least-squares means ± standard errors) HbA1c changes (%) were -0.28 ± 0.15 (placebo), -1.34 ± 0.11 (exenatide 5 μg) and -1.62 ± 0.11 (exenatide 10 μg) (both P < 0.001, exenatide vs placebo). Baseline to end-point bodyweight changes (kg) were -0.47 ± 0.39 (placebo), -0.39 ± 0.28 (exenatide 5 μg) and -1.54 ± 0.27 (exenatide 10 μg; P = 0.026, exenatide 10 μg vs placebo). Nausea, generally mild to moderate, was reported in 8.6% (placebo), 25.0% (exenatide 5 μg) and 36.1% (exenatide 10 μg) of patients. Mild to moderate hypoglycemia was reported in 22.9% (placebo), 51.4% (exenatide 5 μg) and 58.3% (exenatide 10 μg) of patients.

CONCLUSIONS

  Over 24 weeks, exenatide vs the placebo improved glycemic control, reduced bodyweight (10 μg) and was well tolerated in Japanese patients with type 2 diabetes mellitus suboptimally controlled, despite oral therapy including a sulfonylurea. This trial was registered with ClinicalTrials.gov (no. NCT00577824). (J Diabetes Invest, doi: 10.1111/j.2040-1124.2010.00084.x, 2011).

Activation of the GLP-1 receptor signalling pathway: a relevant strategy to repair a deficient beta-cell mass

Recent preclinical studies in rodent models of diabetes suggest that exogenous GLP-1R agonists and DPP-4 inhibitors have the ability to increase islet mass and preserve beta-cell function, by immediate reactivation of beta-cell glucose competence, as well as enhanced beta-cell proliferation and neogenesis and promotion of beta-cell survival. These effects have tremendous implication in the treatment of T2D because they directly address one of the basic defects in T2D, that is, beta-cell failure. In human diabetes, however, evidence that the GLP-1-based drugs alter the course of beta-cell function remains to be found. Several questions surrounding the risks and benefits of GLP-1-based therapy for the diabetic beta-cell mass are discussed in this review and require further investigation.

The human glucagon-like peptide-1 analogue liraglutide preserves pancreatic beta cells via regulation of cell kinetics and suppression of oxidative and endoplasmic reticulum stress in a mouse model of diabetes

AIMS/HYPOTHESIS

We investigated the molecular mechanism by which the human glucagon-like peptide-1 analogue liraglutide preserves pancreatic beta cells in diabetic db/db mice.

METHODS

Male db/db and m/m mice aged 10 weeks received liraglutide or vehicle for 2 days or 2 weeks. In addition to morphological and biochemical analysis of pancreatic islets, gene expression profiles in the islet core area were investigated by laser capture microdissection and real-time RT-PCR.

RESULTS

Liraglutide treatment for 2 weeks improved metabolic variables and insulin sensitivity in db/db mice. Liraglutide also increased glucose-stimulated insulin secretion (GSIS) and islet insulin content in both mouse strains and reduced triacylglycerol content in db/db mice. Expression of genes involved in cell differentiation and proliferation in both mouse strains was regulated by liraglutide, which, in db/db mice, downregulated genes involved in pro-apoptosis, endoplasmic reticulum (ER) stress and lipid synthesis, and upregulated genes related to anti-apoptosis and anti-oxidative stress. In the 2 day experiment, liraglutide slightly improved metabolic variables in db/db mice, but GSIS, insulin and triacylglycerol content were not affected. In db/db mice, liraglutide increased gene expression associated with cell differentiation, proliferation and anti-apoptosis, and suppressed gene expression involved in pro-apoptosis; it had no effect on genes related to oxidative stress or ER stress. Morphometric results for cell proliferation, cell apoptosis and oxidative stress in db/db mice islets were consistent with the results of the gene expression analysis.

CONCLUSIONS/INTERPRETATION

Liraglutide increases beta cell mass not only by directly regulating cell kinetics, but also by suppressing oxidative and ER stress, secondary to amelioration of glucolipotoxicity.

Therapy in the early stage: incretins

The complex pathological mechanisms responsible for development of type 2 diabetes are not fully addressed by conventional drugs, which are also associated with inconvenient side effects such as weight gain or hypoglycemia. Two types of incretin-based therapies are now in use: incretin mimetics (glucagon-like peptide-1 [GLP-1] receptor agonists that bind specific receptors and mimic the action of natural GLP-1) and incretin enhancers (inhibitors of the enzyme that degrade the incretin hormones and thus prolong their activity). Both offer important advantages over previous agents. In addition to the proven glucose-lowering efficacy, they promote weight loss (or are weight neutral) by slowing gastric emptying and inducing satiety, inhibit glucagon secretion with maintenance of counterregulatory mechanisms, and exhibit cardiovascular benefits, while having a low risk profile. Importantly, short-term studies have shown that incretins/incretin-based therapies protect β-cells (by enhancing cell proliferation and differentiation and inhibiting apoptosis) and stimulate their function (by recruiting β-cells to the secretory process and increasing insulin biosynthesis/secretion). These therapies have the opportunity to interfere with the disease progression if used as an early intervention, when enough β-cell mass/function can still be preserved or restored.

Taspoglutide, a novel human once-weekly GLP-1 analogue, protects pancreatic β-cells in vitro and preserves islet structure and function in the Zucker diabetic fatty rat in vivo

AIM

Glucagon-like peptide-1 (GLP-1) has protective effects on pancreatic β-cells. We evaluated the effects of a novel, long-acting human GLP-1 analogue, taspoglutide, on β-cells in vitro and in vivo.

METHODS

Proliferation of murine pancreatic β (MIN6B1) cells and rat islets in culture was assessed by imaging of 5-ethynyl-2'-deoxyuridine-positive cells after culture with taspoglutide. Apoptosis was evaluated with the transferase-mediated 2'-deoxyuridine 5'-triphosphate nick-end labelling assay in rat insulinoma (INS-1E) cells and isolated human islets exposed to cytokines (recombinant interleukin-1β, interferon-γ, tumour necrosis factor-α) or lipotoxicity (palmitate) in the presence or absence of taspoglutide. Islet morphology and survival and glucose-stimulated insulin secretion in perfused pancreata were assessed 3-4 weeks after a single application of taspoglutide to prediabetic 6-week-old male Zucker diabetic fatty (ZDF) rats.

RESULTS

Proliferation was increased in a concentration-dependent manner up to fourfold by taspoglutide in MIN6B1 cells and was significantly stimulated in isolated rat islets. Taspoglutide almost completely prevented cytokine- or lipotoxicity-induced apoptosis in INS-1E cells (control 0.5%, cytokines alone 2.2%, taspoglutide + cytokines 0.6%, p < 0.001; palmitate alone 8.1%, taspoglutide + palmitate 0.5%, p < 0.001) and reduced apoptosis in isolated human islets. Treatment of ZDF rats with taspoglutide significantly prevented β-cell apoptosis and preserved healthy islet architecture and insulin staining intensity as shown in pancreatic islet cross sections. Basal and glucose-stimulated insulin secretion of in situ perfused ZDF rat pancreata was normalized after taspoglutide treatment.

CONCLUSIONS

Taspoglutide promoted β-cell proliferation, prevented apoptosis in vitro and exerted multiple β-cell protective effects on islet architecture and function in vivo in ZDF rats.

Bimodal effect on pancreatic β-cells of secretory products from normal or insulin-resistant human skeletal muscle

OBJECTIVE

Type 2 diabetes is characterized by insulin resistance with a relative deficiency in insulin secretion. This study explored the potential communication between insulin-resistant human skeletal muscle and primary (human and rat) β-cells.

RESEARCH DESIGN AND METHODS

Human skeletal muscle cells were cultured for up to 24 h with tumor necrosis factor (TNF)-α to induce insulin resistance, and mRNA expression for cytokines was analyzed and compared with controls (without TNF-α). Conditioned media were collected and candidate cytokines were measured by antibody array. Human and rat primary β-cells were used to explore the impact of exposure to conditioned media for 24 h on apoptosis, proliferation, short-term insulin secretion, and key signaling protein phosphorylation and expression.

RESULTS

Human myotubes express and release a different panel of myokines depending on their insulin sensitivity, with each panel exerting differential effects on β-cells. Conditioned medium from control myotubes increased proliferation and glucose-stimulated insulin secretion (GSIS) from primary β-cells, whereas conditioned medium from TNF-α-treated insulin-resistant myotubes (TMs) exerted detrimental effects that were either independent (increased apoptosis and decreased proliferation) or dependent on the presence of TNF-α in TM (blunted GSIS). Knockdown of β-cell mitogen-activated protein 4 kinase 4 prevented these effects. Glucagon-like peptide 1 protected β-cells against decreased proliferation and apoptosis evoked by TMs, while interleukin-1 receptor antagonist only prevented the latter.

CONCLUSIONS

Taken together, these data suggest a possible new route of communication between skeletal muscle and β-cells that is modulated by insulin resistance and could contribute to normal β-cell functional mass in healthy subjects, as well as the decrease seen in type 2 diabetes.

Neuroprotective properties of GLP-1: theoretical and practical applications

In the past few years, the development of pharmaceutical agents that enhance the physiological effects of glucagon-like peptide-1 (GLP-1), either through GLP-1 receptor agonism (GLP-1 agonists) or by inhibiting GLP-1 degradation (dipeptidylpeptidase-4 inhibitors) has broadened the range of treatment options for individuals with type 2 diabetes. It has been recognized for some time that GLP-1 also has extra-pancreatic effects, notably targeting the brain, where it regulates appetite and satiety, as well as peripheral functions highly controlled by the autonomic nervous system, such as gastric emptying. Furthermore, data are beginning to emerge that indicate a potential role for GLP-1 in neuroprotection. The increased risk of Alzheimer's disease, Parkinson's disease and stroke in people with type 2 diabetes suggests that shared mechanisms/pathways of cell death, possibly related to insulin dysregulation, may underlie all of these disorders. Although the disease anatomy varies with each disorder, a wide range of genetic and environmental triggers result in activation of similar biochemical pathways in all of them, suggesting a complex network of biochemical events that feed in to a final common path towards cellular dysfunction and death. This article summarizes the evidence for neuronal activity of GLP-1 and examines the limited data that currently exist on the therapeutic potential of GLP-1 in specific neurological and neurodegenerative conditions, namely Alzheimer's disease, Parkinson's disease, Huntingdon's disease, stroke and peripheral sensory neuropathy.

Antiapoptotic effects of GLP-1 in murine HL-1 cardiomyocytes

Activation of apoptosis contributes to cardiomyocyte dysfunction and death in diabetic cardiomyopathy. The peptide glucagon-like peptide-1 (GLP-1), a hormone that is the basis of emerging therapy for type 2 diabetic patients, has cytoprotective actions in different cellular models. We investigated whether GLP-1 inhibits apoptosis in HL-1 cardiomyocytes stimulated with staurosporine, palmitate, and ceramide. Studies were performed in HL-1 cardiomyocytes. Apoptosis was induced by incubating HL-1 cells with staurosporine (175 nM), palmitate (135 μM), or ceramide (15 μM) for 24 h. In staurosporine-stimulated HL-1 cardiomyocytes, phosphatidylserine exposure, Bax-to-Bcl-2 ratio, Bad phosphorylation (Ser(136)), BNIP3 expression, mitochondrial membrane depolarization, cytochrome c release, caspase-3 activation, DNA fragmentation, and mammalian target of rapamycin (mTOR)/p70S6K phosphorylation (Ser(2448) and Thr(389), respectively) were assessed. Apoptotic hallmarks were also measured in the absence or presence of low (5 mM) and high (10 mM) concentrations of glucose. In addition, phosphatidylserine exposure and DNA fragmentation were analyzed in palmitate- and ceramide-stimulated cells. Staurosporine increased apoptosis in HL-1 cardiomyocytes. GLP-1 (100 nM) partially inhibited staurosporine-induced mitochondrial membrane depolarization and completely blocked the rest of the staurosporine-induced apoptotic changes. This cytoprotective effect was mainly mediated by phosphatidylinositol 3-kinase (PI3K) and partially dependent on ERK1/2. Increasing concentrations of glucose did not influence GLP-1-induced protection against staurosporine. Furthermore, GLP-1 inhibited palmitate- and ceramide-induced phosphatidylserine exposure and DNA fragmentation. Incretin GLP-1 protects HL-1 cardiomyocytes against activation of apoptosis. This cytoprotective ability is mediated mainly by the PI3K pathway and partially by the ERK1/2 pathway and seems to be glucose independent. It is proposed that therapies based on GLP-1 may contribute to prevent cardiomyocyte apoptosis.

Ranolazine increases β-cell survival and improves glucose homeostasis in low-dose streptozotocin-induced diabetes in mice

In addition to its anti-ischemic and antianginal effects, ranolazine has been shown to lower hemoglobin A(1c) (HbA(1c)) in patients with coronary artery disease and diabetes. The present study was undertaken to test the hypothesis that ranolazine lowers HbA(1c) because of improved glucose homeostasis in an animal model. Diabetes in mice was induced by giving multiple low doses of streptozotocin. Ranolazine was given twice daily via an oral gavage (20 mg/kg) for 8 weeks. Fasting plasma glucose levels were significantly lower in the ranolazine-treated group (187 ± 19 mg/dl) compared with the vehicle group (273 ± 23 mg/dl) at 8 weeks. HbA(1c) was 5.8 ± 0.4% in the vehicle group and 4.5 ± 0.2% in the ranolazine-treated group (p < 0.05). Glucose disposal during the oral glucose tolerance test (OGTT) and insulin tolerance test were not different between the two groups; however, during OGTT, peak insulin levels were significantly (p < 0.05) higher in ranolazine-treated mice. Mice treated with ranolazine had healthier islet morphology and significantly (p < 0.01) higher β-cell mass (69 ± 2% per islet) than the vehicle group (50 ± 5% per islet) as determined from hematoxylin and eosin staining. The number of apoptotic cells was significantly (p < 0.05) less in the pancreas of the ranolazine-treated group (14 ± 2% per islet) compared with the vehicle group (24 ± 4% per islet). In addition, ranolazine increased glucose-stimulated insulin secretion in rat and human islets in a glucose-dependent manner. These data suggest that ranolazine may be a novel antidiabetic agent that causes β-cell preservation and enhances insulin secretion in a glucose-dependent manner in diabetic mice.

Sitagliptin prevents the development of metabolic and hormonal disturbances, increased β-cell apoptosis and liver steatosis induced by a fructose-rich diet in normal rats

The aim of the present study was to test the effect of sitagliptin and exendin-4 upon metabolic alterations, β-cell mass decrease and hepatic steatosis induced by F (fructose) in rats. Normal adult male Wistar rats received a standard commercial diet without (C) or with 10% (w/v) F in the drinking water (F) for 3 weeks; animals from each group were randomly divided into three subgroups: untreated (C and F) and simultaneously receiving either sitagliptin (CS and FS; 115.2 mg/day per rat) or exendin-4 (CE and FE; 0.35 nmol/kg of body weight, intraperitoneally). Water and food intake, oral glucose tolerance, plasma glucose, triacylglycerol (triglyceride), insulin and fructosamine concentration, HOMA-IR [HOMA (homoeostasis model assessment) for insulin resistance], HOMA-β (HOMA for β-cell function) and liver triacylglycerol content were measured. Pancreas immunomorphometric analyses were also performed. IGT (impaired glucose tolerance), plasma triacylglycerol, fructosamine and insulin levels, HOMA-IR and HOMA-β indexes, and liver triacylglycerol content were significantly higher in F rats. Islet β-cell mass was significantly lower in these rats, due to an increase in the percentage of apoptosis. The administration of exendin-4 and sitagliptin to F animals prevented the development of all the metabolic disturbances and the changes in β-cell mass and fatty liver. Thus these compounds, useful in treating Type 2 diabetes, would also prevent/delay the progression of early metabolic and tissue markers of this disease.

A novel dipeptidyl peptidase IV inhibitor DA-1229 ameliorates streptozotocin-induced diabetes by increasing β-cell replication and neogenesis

We studied the effect of a novel dipeptidyl peptidase IV (DPP IV) inhibitor, DA-1229, on blood glucose profile and pancreatic β-cell mass in established diabetes after streptozotocin (STZ) treatment. Mice that developed diabetes after administration of STZ 100mg/kg were treated with DA-1229 for 13 weeks. DA-1229 significantly reduced plasma DPP IV activity, and enhanced glucagon-like peptide 1 (GLP-1) levels. In STZ-treated mice fed DA-1229 (STZ-DA), blood glucose levels were significantly lower than those in diabetic mice fed normal chow (STZ-NC). Basal and glucose-stimulated insulin secretion and glucose tolerance assessed by intraperitoneal glucose tolerance test were significantly improved by DA-1229 administration. Volume density of β-cell was significantly increased in STZ-DA mice compared to STZ-NC mice, suggesting that DA-1229-mediated amelioration of established diabetes was due to beneficial effect of DA-1229 on β-cell mass. The number of replicating β-cells and that of scattered small β-cell unit representing β-cell neogenesis were significantly increased in STZ-DA mice compared to STZ-NC mice, explaining increased β-cell mass by DA-1229. The expression of PDX-1, a downstream mediator of GLP-1 action, was increased in islets of STZ-DA mice compared to STZ-NC mice. These results suggest a therapeutic potential of DA-1229 in diabetes, particularly that associated with decreased β-cell mass.

Exendin-4 suppresses SRC activation and reactive oxygen species production in diabetic Goto-Kakizaki rat islets in an Epac-dependent manner

OBJECTIVE

Reactive oxygen species (ROS) is one of most important factors in impaired metabolism secretion coupling in pancreatic β-cells. We recently reported that elevated ROS production and impaired ATP production at high glucose in diabetic Goto-Kakizaki (GK) rat islets are effectively ameliorated by Src inhibition, suggesting that Src activity is upregulated. In the present study, we investigated whether the glucagon-like peptide-1 signal regulates Src activity and ameliorates endogenous ROS production and ATP production in GK islets using exendin-4.

RESEARCH DESIGN AND METHODS

Isolated islets from GK and control Wistar rats were used for immunoblotting analyses and measurements of ROS production and ATP content. Src activity was examined by immunoprecipitation of islet lysates followed by immunoblotting. ROS production was measured with a fluorescent probe using dispersed islet cells.

RESULTS

Exendin-4 significantly decreased phosphorylation of Src Tyr416, which indicates Src activation, in GK islets under 16.7 mmol/l glucose exposure. Glucose-induced ROS production (16.7 mmol/l) in GK islet cells was significantly decreased by coexposure of exendin-4 as well as PP2, a Src inhibitor. The Src kinase-negative mutant expression in GK islets significantly decreased ROS production induced by high glucose. Exendin-4, as well as PP2, significantly increased impaired ATP elevation by high glucose in GK islets. The decrease in ROS production by exendin-4 was not affected by H-89, a PKA inhibitor, and an Epac-specific cAMP analog (8CPT-2Me-cAMP) significantly decreased Src Tyr416 phosphorylation and ROS production.

CONCLUSIONS

Exendin-4 decreases endogenous ROS production and increases ATP production in diabetic GK rat islets through suppression of Src activation, dependently on Epac.

Glucagon-like peptide analogues for type 2 diabetes mellitus: systematic review and meta-analysis

BACKGROUND

Glucagon-like peptide (GLP-1) analogues are a new class of drugs used in the treatment of type 2 diabetes. They are given by injection, and regulate glucose levels by stimulating glucose-dependent insulin secretion and biosynthesis, suppressing glucagon secretion, and delaying gastric emptying and promoting satiety. This systematic review aims to provide evidence on the clinical effectiveness of the GLP-1 agonists in patients not achieving satisfactory glycaemic control with one or more oral glucose lowering drugs.

METHODS

MEDLINE, EMBASE, the Cochrane Library and Web of Science were searched to find the relevant papers. We identified 28 randomised controlled trials comparing GLP-1 analogues with placebo, other glucose-lowering agents, or another GLP-1 analogue, in patients with type 2 diabetes with inadequate control on a single oral agent, or on dual therapy. Primary outcomes included HbA1c, weight change and adverse events.

RESULTS

Studies were mostly of short duration, usually 26 weeks. All GLP-1 agonists reduced HbA1c by about 1% compared to placebo. Exenatide twice daily and insulin gave similar reductions in HbA1c, but exenatide 2 mg once weekly and liraglutide 1.8 mg daily reduced it by 0.20% and 0.30% respectively more than glargine. Liraglutide 1.2 mg daily reduced HbA1c by 0.34% more than sitagliptin 100 mg daily. Exenatide and liraglutide gave similar improvements in HbA1c to sulphonylureas. Exenatide 2 mg weekly and liraglutide 1.8 mg daily reduced HbA1c by more than exenatide 10 μg twice daily and sitagliptin 100 mg daily. Exenatide 2 mg weekly reduced HbA1c by 0.3% more than pioglitazone 45 mg daily.Exenatide and liraglutide resulted in greater weight loss (from 2.3 to 5.5 kg) than active comparators. This was not due simply to nausea. Hypoglycaemia was uncommon, except when combined with a sulphonylurea. The commonest adverse events with all GLP-1 agonists were initial nausea and vomiting. The GLP-1 agonists have some effect on beta-cell function, but this is not sustained after the drug is stopped.

CONCLUSIONS

GLP-1 agonists are effective in improving glycaemic control and promoting weight loss.

Role of the glucose-dependent insulinotropic polypeptide and its receptor in the central nervous system: therapeutic potential in neurological diseases

Glucose-dependent insulinotropic polypeptide (GIP) is a 42-amino acid hormone, secreted from the enteroendocrine K cells, which has insulin-releasing and extra-pancreatic actions. GIP and its receptor present a widespread distribution in the mammalian brain where they have been implicated with synaptic plasticity, neurogenesis, neuroprotection and behavioral alterations. This review attempts to provide a comprehensive picture of the role of GIP in the central nervous system and to highlight recent findings from our group showing its potential involvement in neurological illnesses including epilepsies, Parkinson's disease and Alzheimer's disease.

Diabetes as a disease of endoplasmic reticulum stress

Endoplasmic reticulum (ER) stress is an integral part of life for all professional secretory cells, but it has been studied to greatest depth in the pancreatic β-cell. This reflects both the crucial role played by ER stress in the pathogenesis of diabetes and also the exquisite vulnerability of these cells to ER dysfunction. The adaptive cellular response to ER stress, the unfolded protein response, comprises mechanisms to both regulate new protein translation and a transcriptional program to allow adaptation to the stress. The core of this response is a triad of stress-sensing proteins: protein kinase R-like endoplasmic reticulum kinase (PERK), inositol-requiring enzyme 1 (IRE1) and activating transcription factor 6. All three regulate portions of the transcriptional unfolded protein response, while PERK also attenuates protein synthesis during ER stress and IRE1 interacts directly with the c-Jun amino-terminal kinase stress kinase pathway. In this review we shall discuss these processes in detail, with emphasis given to their impact on diabetes and how recent findings indicate that ER stress may be responsible for the loss of β-cell mass in the disease.

Gastrointestinal hormones and the regulation of β-cell mass

Type 2 diabetes occurs due to a relative deficit in β-cell mass or function. Glucagon-like peptide 1 (GLP-1), glucose-dependent insulinotropic polypeptide (GIP), cholecystokinin (CCK), and gastrin are gastrointestinal hormones that are secreted in response to nutrient intake, regulating digestion, insulin secretion, satiety, and β-cell mass. In this review, we focus upon β-cell mass regulation. β-cell mass expands through β-cell proliferation and islet neogenesis; β-cell mass is lost via apoptosis. GLP-1 and GIP are well-studied gastrointestinal hormones and influence β-cell proliferation, apoptosis, and islet neogenesis. CCK regulates β-cell apoptosis and mitogenesis, and gastrin stimulates islet neogenesis. GLP-1 and GIP bind to G protein-coupled receptors and regulate β-cell mass via multiple signaling pathways. The protein kinase A pathway is central to this process because it directly regulates proliferative and anti-apoptotic genes and transactivates several signaling cascades, including Akt and mitogen-activated protein kinases. However, the signaling pathways downstream of G protein-coupled CCK receptors that influence β-cell mass remain unidentified. Gastrointestinal hormones integrate nutrient signals from the gut to the β-cell, regulating insulin secretion and β-cell mass adaptation.

Treatment of diabetes with glucagon-like peptide-1 gene therapy

IMPORTANCE OF THE FIELD

Glucagon-like peptide (GLP)-1 receptor agonists are in widespread clinical use for the treatment of diabetes. While effective, these peptides require frequent injections to maintain efficacy. Therefore, alternative delivery methods including gene therapy are currently being evaluated.

AREAS COVERED IN THIS REVIEW

Here, we review the biology of GLP-1, evidence supporting the clinical use of the native peptide as well as synthetic GLP-1 receptor agonists, and the rationale for their delivery by gene therapy. We then review progress made in the field of GLP-1 gene therapy for both type 1 and type 2 diabetes.

WHAT THE READER WILL GAIN

Efforts to improve the biological half-life of GLP-1 receptor agonists are discussed. We focus on the development of both viral and non-viral gene delivery methods, highlighting vector designs and the strengths and weaknesses of these approaches. We also discuss the utility of targeting regulated GLP-1 production to tissues including the liver, muscle, islet and gut.

TAKE HOME MESSAGE

GLP-1 is a natural peptide possessing several actions that effectively combat diabetes. Current delivery methods for GLP-1-based drugs are cumbersome and do not recapitulate the normal secretion pattern of the native hormone. Gene therapy offers a useful method for directing long-term production and secretion of the native peptide. Targeted production of GLP-1 using tissue-specific promoters and delivery methods may improve therapeutic efficacy, while also eliminating the burden of frequent injections.

Chronic treatment of exendin-4 affects cell proliferation and neuroblast differentiation in the adult mouse hippocampal dentate gyrus

Exendin-4 isolated from Heloderma suspectum venom acts via glucagon-like peptide 1 (GLP-1) receptor and has clinically been used in the type 2 diabetes. In this study, we investigated the effects of exendin-4 on cell proliferation and neuroblast differentiation in the subgranular zone (SGZ) of the dentate gyrus in mice. Exendin-4 was treated intraperitoneally to male ICR mice twice a day for 21 days. The exendin-4-treated group showed a significantly higher number of Ki67- (1.51-fold), doublecortin (DCX)- (2.5-fold) and 5-bromo-2'-deoxyuridine (BrdU)+DCX- (2.46-fold) immunoreactive cells in the SGZ of the dentate gyrus compared to the control group. The results of this study showed that treatment with exendin-4 increased cell proliferation neuroblast differentiation in the SGZ of the dentate gyrus, suggesting that exendin-4 promotes structural plasticity in the dentate gyrus.