Glucagon-like Peptide-1 Receptor Activation Research Articles

GLP-1 Receptor Activation Abrogates β-Cell Dysfunction by PKA Cα-Mediated Degradation of Thioredoxin Interacting Protein.

Glucagon-like peptide 1 receptor (GLP-1R) agonist (Exendin-4) is a well-known agent used to improve β-cell dysfunctions via protein kinase A (PKA), but the detailed downstream molecular mechanisms are still elusive. We have now found that PKA Cα mediated- TXNIP phosphorylation and degradation played a vital role in the β-cell protective role of exendin-4. After PKA activator (Exendin-4 or FSK) treatment, PKA Cα could directly interact with TXNIP by bimolecular fluorescence complementation and Co-IP assays in INS-1 cells. And PKA Cα overexpression decreased TXNIP level, whereas TXNIP level was largely increased in PKA Cα-KO β-cells by CRISPR-Cas9. Interestingly, TXNIP overexpression or PKA Cα-KO has impaired β-cell functions, including loss of insulin secretion and activation of inflammation. PKA Cα directly phosphorylated TXNIP at Ser307 and Ser308 positions, leading to its degradation via activation of cellular proteasome pathway. Consistent with this observation, TXNIP (S307/308A) mutant resisted the degradation effects of PKA Cα. However, exendin-4 neither affected TXNIP level in TXNIP (S307/308A) mutant overexpressed β-cells nor in PKA Cα-KO β-cells. Moreover, exendin-4 treatment reduced the inflammation gene expression in TXNIP overexpressed β-cells, but exendin-4 treatment has no effect on the inflammation gene expression in TXNIP (S307/308A) overexpressed β-cells. In conclusion, our study reveals the integral role of PKA Cα/TXNIP signaling in pancreatic β-cells and suggests that PKA Cα-mediated TXNIP degradation is vital in β-cell protective effects of exendin-4.

Activation of GLP-1 receptors attenuates oxycodone taking and seeking without compromising the antinociceptive effects of oxycodone in rats.

Despite the effectiveness of current medications to treat opioid use disorder, there is still a high rate of relapse following detoxification. Thus, there is critical need for innovative studies aimed at identifying novel neurobiological mechanisms that could be targeted to treat opioid use disorder. A growing body of preclinical evidence indicates that glucagon-like peptide-1 (GLP-1) receptor agonists reduce drug reinforcement. However, the efficacy of GLP-1 receptor agonists in attenuating opioid-mediated behaviors has not been thoroughly investigated. Using recently established models of opioid-taking and -seeking behaviors, we showed that systemic administration of the GLP-1 receptor agonist exendin-4 reduced oxycodone self-administration and the reinstatement of oxycodone-seeking behavior in rats. We also identified behaviorally selective doses of exendin-4 that reduced opioid-taking and -seeking behaviors and did not produce adverse feeding effects in oxycodone-experienced rats. To identify a central site of action, we showed that systemic exendin-4 penetrated the brain and bound putative GLP-1 receptors on dopamine D1 receptor- and dopamine D2 receptor-expressing medium spiny neurons in the nucleus accumbens shell. Consistent with our systemic studies, infusions of exendin-4 directly into the accumbens shell attenuated oxycodone self-administration and the reinstatement of oxycodone-seeking behavior without affecting ad libitum food intake. Finally, exendin-4 did not alter the analgesic effects of oxycodone, suggesting that activation of GLP-1 receptors attenuated opioid reinforcement without reducing the thermal antinociceptive effects of oxycodone. Taken together, these findings suggest that GLP-1 receptors could serve as potential molecular targets for pharmacotherapies aimed at reducing opioid use disorder.

P5997GLP-1(28-36) prevents progression of ischemic heart failure in mice

Abstract Background Glucagon-like peptide-1 (GLP-1), its metabolites and related drugs have demonstrated cardioprotective benefits in several animal models of cardiovascular disease (CVD) and select clinical trials. Indeed, large cardiovascular outcome trials (CVOT) of GLP-1 analogs showed significant reductions in major adverse cardiovascular events (MACE). However, smaller studies in patients with heart failure (HF) (e.g. FIGHT), and secondary analyses of some CVOT (e.g. LEADER), have suggested that the cardiovascular benefits of GLP-1 analogs may be muted in select patients. Speculating on how this may be due to undesirable increases in heart rate caused by activation of sinoatrial GLP-1 receptors (Glp1r), we have explored the Glp1r-independent cardioprotective actions of GLP-1(28–36), a neutral endopeptidase (NEP)-derived metabolite of GLP-1. We have shown that the cardioprotective effects of GLP-1(28–36) are mediated by mitochondrial trifunctional protein-α (MTPα)-dependent metabolic shift from fatty acid- to glucose oxidation in coronary vascular cells. As metabolic perturbations are believed to contribute to the pathophysiology of HF, we hypothesized that treatment with GLP-1(28–36) may have beneficial effects on this condition. Purpose To evaluate if treatment with GLP-1(28–36) can prevent onset of HF and/or reverse established HF in a post-MI mouse model. Methods and results Permanent LAD ligation was performed in 10–12wk old male C57BL/6J wild-type mice (wt). Immediately post-MI, mice were assigned to receive either GLP-1(28–36) or scrambled peptide [Scram(28–36)] at 18.5nmol/kg/d (N=30/group) subcutaneously (s.c.) via osmotic mini-pumps for 4wk. Although, treatment with GLP-1(28–36) did not improve post-MI survival, triphenyltetrazolium chloride (TTC)-stained hearts 28d post-MI reveal smaller infarct size in GLP-1(28–36)- vs. Scram(28–36)-treated mice (35.3±1.9% vs. 41.2±1.7%, N=12–15/group, P<0.05). Echocardiography at 28d post-MI showed improved LVEF in mice treated with GLP-1(2–36) (30.1±2.3% vs. 24.2±3.7%, N=12–15/group, P<0.05). Similarly, treatment with GLP-1(28–36) reduced heart/body weight ratio (7.9±0.3 vs. 8.8±0.4 mg/g, N=12–15/group, P<0.05). Next, we tested if GLP-1(28–36) might reverse ischemic HF in this model. After permanent LAD ligation of 10–12wk old male wt mice, only those with echocardiography-defined LVEF between 20–35% at 28d post-MI were randomized to treatment with GLP-1(28–36) or Scram(28–36) [18.5nmol/kg/d (N=15/group)] via s.c. mini-pumps for 4wk. Echocardiography at 56d post-MI (i.e. 28d post-treatment start) revealed that GLP-1(28–36) preserved LV function with no deterioration in LVEF vs. Scram(28–36)-treated controls [−3.1±4.9% vs. −22.8±4%, relative change, N=15/group, P<0.001]. Conclusion In a post-MI mouse model of HF, treatment with GLP-1(28–36) prevents progression to HF, and preserves LV function after the development of established HF. Acknowledgement/Funding This study was funded by a Fellowship from Ted Rogers Centre for Heart Research and a Project Grant from Heart and Stroke Foundation, Canada

Glucagon-like peptide-1 receptor pathway inhibits extracellular matrix production by mesangial cells through store-operated Ca2+ channel.

Glomerular mesangial cell is the major source of mesangial matrix. Our previous study demonstrated that store-operated Ca2+channel signaling suppressed extracellular matrix protein production by mesangial cells. Recent studies demonstrated that glucagon-like peptide-1 receptor (GLP-1R) pathway had renoprotective effects. However, the underlying mechanism(s) remains unclear. The present study was aimed to determine if activation of GLP-1R decreased extracellular matrix protein production by mesangial cells through upregulation of store-operated Ca2+function. Experiments were conducted in cultured human mesangial cells. Liraglutide and exendin 9–39 were used to activate and inhibit GLP-1R, respectively. Store-operated Ca2+function was estimated by evaluating the SOC-mediated Ca2+entry (SOCE). We found that liraglutide treatment reduced high glucose-stimulated production of fibronectin and collagen IV. The inhibitory effects of liraglutide were not observed in the presence of exendin 9–39. Exendin-4, another GLP-1R agonist also blunted high glucose-stimulated fibronectin and collagen IV production. Treatment of human mesangial cells with liraglutide for 24 h significantly attenuated the high glucose-induced reduction of Orai1 protein. Consistently, Ca2+imaging experiments showed that the inhibition of high glucose on SOCE was significantly attenuated by liraglutide. However, in the presence of exendin 9–39, liraglutide failed to reverse the high glucose effect. Furthermore, liraglutide effects on fibronectin and collagen IV protein abundance were significantly attenuated by GSK-7975A, a selective blocker of store-operated Ca2+. Taken together, our findings suggest that GLP-1R signaling inhibited high glucose-induced extracellular matrix protein production in mesangial cells by restoring store-operated Ca2+function.Impact statementDiabetic kidney disease continues to be a major challenge to health care system in the world. There are no known therapies currently available that can cure the disease. The present study provided compelling evidence that activation of GLP-1R inhibited extracellular matrix protein production by glomerular mesangial cells. We further showed that the beneficial effect of GLP-1R was attributed to upregulation of store-operated Ca2+channel function. Therefore, we identified a novel mechanism contributing to the renal protective effects of GLP-1R pathway. Activation of GLP-1R pathway and/or store-operated Ca2+channel signaling in MCs could be an option for patients with diabetic kidney disease.

DPP-4 inhibition by linagliptin prevents cardiac dysfunction and inflammation by targeting the Nlrp3/ASC inflammasome.

We compared the effects of linagliptin (Lina, a DPP4 inhibitor) and GLP-1 receptor activation by exenatide followed by exendin-4 in an infusion pump (EX) on infarct size (IS), post-infarction activation of the inflammasome and remodeling in wild-type (WT) and db/db diabetic mice. Mice underwent 30min ischemia followed by 24h reperfusion. IS was assessed by TTC. Additional mice underwent permanent coronary artery occlusion. Echocardiography was performed 2w after infarction. Activation of the inflammasome in the border zone of the infarction was assessed by rt-PCR and ELISA 2w after reperfusion. Further in vitro experiments were done using primary human cardiofibroblasts and cardiomyocytes exposed to simulated ischemia-reoxygenation. Lina and EX limited IS in both the WT and the db/db mice. Lina and EX equally improved ejection fraction in both the WT and the db/db mice. mRNA levels of ASC, NALP3, IL-1β, IL-6, Collagen-1, and Collagen-3 were higher in the db/db mice than in the WT mice. Infarction increased these levels in the WT and db/db mice. Lina more than EX attenuated the increase in ASC, NALP3, IL-1β, IL-6, Collagen-1 and Collagen-3, TNFα and IL-1β, and decreased apoptosis, especially in the db/db mice. In vitro experiments showed that Lina, but not EX, attenuated the increase in TLR4 expression, an effect that was dependent on p38 activation with downstream upregulation of Let-7i and miR-146b levels. Lina and EX had similar effects on IS and post-infarction function, but Lina attenuated the activation of the inflammasome and the upregulation of collagen-1 and collagen-3 more than direct GLP-1 receptor activation. This effect depends on p38 activation with downstream upregulation of miR-146b levels that suppresses TLR4 expression.

Peptides come to the rescue of pancreatic β cells

Insulin and glucagon are well-known peptide hormones that keep glucose levels within a healthy range in the body. But they are only part of a complex network that controls concentrations of this ubiquitous sugar in blood and tissues. Other molecules regulate glucose by controlling insulin secretion from the pancreas or protecting pancreatic β cells against stresses that lead to cellular dysfunction or cell death (1). One of these protective regulators is glucagon-like peptide 1 (GLP-1), a 30-amino-acid-long peptide produced in specialized epithelial cells of the intestine, called L cells, and also in the brain and other organs and tissues (2). GLP-1 belongs to a group of peptides that mediate the “incretin effect,” an endocrine response to glucose arising from food digestion in the intestines (2, 3). This response helps regulate food intake and the fate of dietary glucose. Specifically, GLP-1 is released from the intestinal cells when food is ingested and then binds to and activates the GLP-1 receptor (GLP-1R), a G protein–coupled receptor on many cell types, including β cells in which GLP-1R signaling stimulates insulin synthesis and secretion (3). Notably, the incretin effect stimulates insulin secretion from pancreatic β cells more strongly than exposure to glucose alone. An article published in the Journal of Biological Chemistry (4), recognized as a Classic here, added to our understanding of the incretin effect by showing that GLP-1R signaling protects β cells from cell death (Fig. 1). This finding was significant for preventing or managing type 2 diabetes, in which β-cell apoptosis occurs (5) and may contribute to insufficient pancreatic insulin production (6). Open in a separate window Figure 1. Li et al. (4) have shown that binding of the receptor agonist exendin-4 to GLP-1R on pancreatic β cells protects the cells from cellular injury and cytokine-induced apoptosis and thereby preserves glucose homeostasis in mice. Binding of GLP-1 to its cognate receptor on pancreatic β cells up-regulates intracellular cAMP levels, in turn reducing streptozotocin-induced β-cell death. Images of exendin-4 and GLP-1R (with GLP-1 bound) are from Ref. 7; image of cAMP is from Wikimedia, used under Creative Commons.

High-throughput identification of peptide agonists against GPCRs by co-culture of mammalian reporter cells and peptide-secreting yeast cells using droplet microfluidics

Since G-protein coupled receptors (GPCRs) are linked to various diseases, screening of functional ligands against GPCRs is vital for drug discovery. In the present study, we developed a high-throughput functional cell-based assay by combining human culture cells producing a GPCR, yeast cells secreting randomized peptide ligands, and a droplet microfluidic device. We constructed a reporter human cell line that emits fluorescence in response to the activation of human glucagon-like peptide-1 receptor (hGLP1R). We then constructed a yeast library secreting an agonist of hGLP1R or randomized peptide ligands. We demonstrated that high-throughput identification of functional ligands against hGLP1R could be performed by co-culturing the reporter cells and the yeast cells in droplets. We identified functional ligands, one of which had higher activity than that of an original sequence. The result suggests that our system could facilitate the discovery of functional peptide ligands of GPCRs.

2-Iodo-4'-Methoxychalcone Attenuates Methylglyoxal-Induced Neurotoxicity by Activation of GLP-1 Receptor and Enhancement of Neurotrophic Signal, Antioxidant Defense and Glyoxalase Pathway.

Methylglyoxal (MG) acts as a reactive precursor of advanced glycation end products (AGEs). This compound is often connected with pathologies such as diabetes, neurodegenerative processes and diseases of aging. 2-iodo-4′-methoxychalcone (CHA79), a synthetic halogen-containing chalcone derivative, has been reported its anti-diabetic activity. This study aims to investigate the potential protective capability of CHA79 against MG-mediated neurotoxicity in SH-SY5Y cells. Results indicated CHA79 increased viability of cells and attenuated the rate of apoptosis in MG-exposed SH-SY5Y. CHA79 up-regulated expression of anti-apoptotic protein (Bcl-2) and down-regulated apoptotic proteins (Bax, cytochrome c, caspase-3, caspase-9). Moreover, CHA79 significantly up-regulated expression of neurotrophic factors, including glucagon-like peptide-1 receptor (GLP-1R), brain derived neurotrophic factor (BDNF), p75NTR, p-TrkB, p-Akt, p-GK-3β and p-CREB. CHA79 attenuated MG-induced ROS production and enhanced the antioxidant defense including nuclear factor erythroid 2-related factor 2 (Nrf2), HO-1, SOD and GSH. Furthermore, CHA79 attenuated MG-induced reduction of glyoxalase-1 (GLO-1), a vital enzyme on removing AGE precursors. In conclusion, CHA79 is the first novel synthetic chalcone possessing the GLP-1R and GLO-1 activating properties. CHA 79 also exhibits neuroprotective effects against MG toxicity by enhancing neurotrophic signal, antioxidant defense and anti-apoptosis pathway.

1027-P: A Phase 1, Open-Label, Single-Dose Study to Evaluate Pharmacokinetics of MEDI0382 in Patients with Renal Impairment

MEDI0382, an oxyntomodulin-like peptide with targeted GLP-1 and glucagon receptor activity, is in development for T2DM. This open-label, parallel-group study (NCT03235375) evaluated PK of SC MEDI0382 (100 µg; single dose) in subjects (N = 37) grouped by renal function: Group 1, ESRD or severe renal impairment (RI; CrCl < 20 mL/min); Group 2, severe RI (CrCl ≥ 20 mL/min to < 30 mL/min); Group 3, healthy controls (CrCl ≥ 90 mL/min); Group 4a, moderate RI (CrCl ≥ 30 mL/min to < 45 mL/min); and Group 4b, moderate RI (CrCl ≥ 45 mL/min to < 60 mL/min). Coprimary endpoints were geometric mean ratio AUC48h and Cmax of MEDI0382 (Table 1). MEDI0382 had similar PK profiles across all groups (Table 2). This suggests that PK of MEDI0382 is unaffected by renal function, allowing dosing in subjects with RI in future trials. Disclosure G.K. Klein: None. M. Petrone: Employee; Self; MedImmune. Y. Yang: Employee; Self; MedImmune. T. Hoang: Employee; Self; MedImmune. S. DiCostanza: Employee; Self; AstraZeneca. L. Hansen: Employee; Self; AstraZeneca, MedImmune. Stock/Shareholder; Self; Bristol-Myers Squibb Company, Novo Nordisk A/S. A. Flor: Employee; Self; AstraZeneca.

1030-P: Effect of MEDI0382 on the Pharmacodynamics and Pharmacokinetics of Warfarin or Esmolol in Healthy Volunteers

MEDI0382 is an oxyntomodulin-like peptide with targeted GLP-1 and glucagon receptor activity under development for T2DM treatment. Patients with T2DM have a high risk for cardiovascular disease. In an open-label, crossover, phase 1 study (NCT03347968), we evaluated the effect of MEDI0382 (SC; titrated up to 300 µg for 25 days) on the pharmacodynamics (PD) and PK of warfarin (PO; 25 mg on day [d] 2 [warfarin alone] and d 27 [warfarin + MEDI0382]) and PD of esmolol (IV; 200 µg/kg/min for 90 min on d 1 [esmolol alone] and d 26 [esmolol + MEDI0382]) in healthy volunteers (N = 22). Coprimary endpoints were prothrombin time (PT) maximum international normalized ratio (INRmax) and INRAUC of warfarin with MEDI0382, and change in heart rate from rest to after a 9-minute treadmill test (Bruce Protocol) with esmolol and MEDI0382. The coprimary endpoints were analyzed using paired t-tests. INRmax and INRAUC values were log transformed for analysis. MEDI0382 had no effect on PT INRmax, INRAUC (Table), or the PK profile of warfarin. Similar changes in heart rate with the treadmill test were observed with esmolol alone or with esmolol and MEDI0382 (Table). MEDI0382 resulted in an increased incidence of nausea and vomiting. There were no serious or severe adverse events. These results support co-administration of MEDI0382 and warfarin or esmolol and do not suggest safety concerns. Disclosure H.A. Coleman: None. M. Petrone: Employee; Self; MedImmune. T.M. Petrohoy: None. Y. Yang: Employee; Self; MedImmune. L.J. Sanford: None. L. Hansen: Employee; Self; AstraZeneca, MedImmune. Stock/Shareholder; Self; Bristol-Myers Squibb Company, Novo Nordisk A/S. A. Flor: Employee; Self; AstraZeneca.

1839-P: GLP-1 Receptor Antagonism Using Exendin-9,39 in the Fasting State Modulates a-Cell Function in Humans with Type 2 Diabetes

Glucagon-Like Peptide-1 (GLP-1) potentiates insulin secretion in response to an oral challenge. Exendin-(9,39) a competitive antagonist of GLP-1, has been used to quantify the contribution of endogenous GLP-1 to glucose tolerance. In nondiabetic subjects, Exendin-(9,39) infused for three hours altered the subsequent β-cell response to an i.v. glucose challenge. Similar results were observed using perifused human islets from nondiabetic donors. We hypothesized that the effects of fasting GLP-1 secretion on β-cell response would be similar in people with type 2 diabetes (T2D). We studied 9 subjects with T2D in random order. Glucose containing [3-3H]-glucose was infused to mimic the systemic appearance of glucose after a meal. On one occasion, saline, was infused for 3 hours prior (-180 to 0 minutes) to the I.V. challenge, while on the other Exendin-(9,39) was infused at 750pmol/kg/minutes. Exendin-(9,39) infusion prior to the i.v. challenge increased fasting (8.7 ± 0.6 vs. 7.8 ± 0.5 mmol/l, p < 0.01) and peak (13.3 ± 0.8 vs. 12.5 ± 0.6 mmol/l, p = 0.05) glucose concentrations. These differences were not explained by altered insulin secretion: insulin and C-peptide concentrations did not differ between study days. On the other hand, fasting glucagon concentrations were increased (70 ± 8 vs. 53 ± 1 ng/l, p = 0.01) by Exendin-(9,39) and this was accompanied by increased endogenous glucose production (23 ± 1 vs. 20 ± 1 μmol/kg/minutes, p = 0.02). In separate experiments using perifused isolated human islets from diabetic donors, pretreatment with Exendin-(9,39) did not alter integrated insulin secretion in response to hyperglycemia (0.6 ± 0.2 vs. 0.9 ± 0.4 nmol/Islet/30 minutes, p = 0.49). Taken together these results suggest that in T2D fasting GLP-1 receptor activation may augment glucagon suppression in response to subsequent challenges. This may represent an adaptive mechanism not seen in nondiabetic subjects that restrains α-cell function to decrease postprandial hyperglycemia. Disclosure M.D. Hurtado: None. A.M. Egan: None. M.C. Laurenti: Employee; Self; GlySens Incorporated. J. Adams: None. R.A. Rizza: Advisory Panel; Self; Lilly Diabetes, Metavention, Novo Nordisk A/S, vTv Therapeutics. C. Cobelli: None. A. Matveyenko: None. C. Dalla Man: None. A. Vella: Advisory Panel; Self; vTv Therapeutics. Research Support; Self; Novo Nordisk Inc. Funding Endocrine Fellows Foundation; National Institutes of Health

Effects of Exendin-4 on the differentiation of neural stem cells from subventricular zone of adult mice in vitro

To study the effect of exendin-4(Ex-4) on the differentiation of neural stem cells(NSCs) in adult mouse subventricular zone(SVZ)and its mechanism . NSCs in the SVZ were derived from 5-week C57BL/6J mice and the expression of nestin was detected by immunofluorescence. The cell morphology was observed after the cells treatmed with 100 nmol/L Ex-4 for 14 days.The expressions of nestin and glucagon-like peptide-1 receptor (GLP-1R) were detected by immunofluorescence. GLP-1R was knocked down by using shRNA and the study was divided into four groups: control group, Ex-4 group, GLP-1R knockdown group, GLP-1R knockdown + Ex-4 group. After treatment with 100 nmol/L Ex-4 for 14 d, β-tublin III and glial fibrillary acidic protein (GFAP) were labeled by immunofluorescence and then the proportion of β-tublin III positive cells were counted. Western blot was used to detect the activation of cAMP-response element binding protein (CREB) in NSCs. In order to further study the effects of Ex-4 on mitogen-activated protein kinase(MAPK) and phosphatidylinositol 3-hydroxy kinase (PI3K) pathways, the cells were pretreated with MAPK inhibitor U0126 at a concentration of 0.07 μmol/L for 30 min or PI3K inhibitor LY294002 at 50 μmol for 2 h, respectively. The study was divided into six groups: control group, Ex-4 group, U0126 group, U0126 + Ex-4 group, LY294002 group, LY294002 + Ex-4 group. The activation of CREB in each group was detected by Western blot. The experiment was repeated three times independently. NSCs were successfully extracted from SVZ of C57BL/6J mice. Immunofluorescence showed that nestin and GLP-1R were positive in NSCs. Compared with the control group, the proportion of neurons differentiated from Ex-4 group was higher. The percentage of neurons in GLP-1R knockdown + Ex-4 group was basically the same as that in control group (P＜0.01). The positive cells of beta-tublin III showed positive activation of GLP-1R and CREB. Western blot showed that CREB was significantly activated in the Ex-4 group, and knockdown of GLP-1R abolished its activation (P＜0.01). U0126 did not affect Ex-4-mediated CERB activation, and LY294002 significantly reduced Ex-4-mediated CREB activation (P＜0.01). Ex-4 promotes the differentiation of NSCs into neurons in SVZ of adult mice through GLP-1R receptor, which may be achieved through PI3K/CREB pathway.

Topical ocular administration of the GLP-1 receptor agonist liraglutide arrests hyperphosphorylated tau-triggered diabetic retinal neurodegeneration via activation of GLP-1R/Akt/GSK3β signaling

Diabetic retinal neurodegeneration, in particular synaptic neurodegeneration of retinal ganglion cells (RGCs) occurring before RGCs apoptosis, may represent the earliest event in the pathogenesis of diabetic retinopathy (DR). Our previous study identified hyperphosphorylated-tau as a critical toxic mediator in diabetic RGCs synaptic neurodegeneration. Thus, therapeutic agents targeting to tau may appear as a promising strategy to arrest the progression of DR. The glucagon-like-peptide 1 receptor (GLP-1R) agonists, including liraglutide, can ameliorate neurodegenerative features in models of Alzheimer's disease and diabetes by decreasing tau hyperphosphorylation in the brain. Liraglutide has also been found to prevent retinal neural apoptosis/loss in diabetic mice. However, whether liraglutide can prevent diabetic synapse degeneration of RGCs, and its neuroprotective role, if any, is due to alleviating retinal tau hyperphosphorylation remain unknown. Here, using a well characterized high-fat diet (HFD)-induced diabetes mouse model, we showed that topical ocular administration of liraglutide reversed hyperphosphorylated tau-triggered RGCs synaptic degeneration in HFD-induced diabetes. The neuroprotective effect of liraglutide on diabetic retinae was abolished when GLP-1R or Akt was inhibited by topically co-administration with a GLP-1R antagonist, exendin-(9–39), or an Akt inhibitor MK2206, respectively. However, knock-down of GSK3β by intravitreal injection of si-GSK3β restored the neuroprotective effects of liraglutide abrogated by Akt inactivation. Thus, our present study demonstrated that liraglutide can arrest hyperphosphorylated tau-triggered retinal neurodegeneration via activation of GLP-1R/Akt/GSK3β signaling. Our results also propose that topical ocular application of liraglutide can be envisaged as a potentially useful strategy for the treatment of retinal tauopathy at the early onset of DR.

Emerging incretin hormones actions: focus on bone metabolism.

Incretin hormones, namely glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) are gastro-intestinal hormones released from different enteroendocrine cells after nutrient intake. Incretins exert their actions though binding to and activation of specific GIP and GLP-1 receptors which are present in several target tissues. Incretin receptor activation in the pancreas leads to the incretin effect and other significant non-insulinotropic effects. Extra-pancreatic effects of incretin hormones in several other target tissues, such as their role in the pathophysiology of obesity and their potential relation with cardiovascular function, cognitive function, triglyceride storage in adipose tissue and bone metabolism, have attracted scientific interest. In the current review we intend to summarize existing knowledge on specific effects of GIP and GLP-1 in bone cells and bone metabolism. Starting from the identification of GIP receptor and GLP-1 receptor in animal and human bone cells, continuing with the skeletal effects of incretin deficiency or overexpression in animals, ending to the latest data on incretin and incretin agonists administration in cells, animals and humans, incretins play a significant yet complex role in the pathophysiology of bone metabolism affecting both formation and resorption. Although existing evidence seem strong and concrete, there is still a long way to go until their possible therapeutic or adjuvant use as bone modulating drugs can be considered.

Endogenous GLP-1 in lateral septum promotes satiety and suppresses motivation for food in mice

Glucagon-like peptide 1 receptors (GLP-1R) are expressed in the lateral septum (LS) of rats and mice, and we have published that endogenous LS GLP-1 affects feeding and motivation for food in rats. Here we asked if these effects are also observed in mice. In separate dose-response studies using male C57Bl6J mice, intra-LS GLP-1 or the GLP-1R antagonist Exendin 9 (Ex9) was delivered shortly before dark onset, at doses subthreshold for effect when injected intracerebroventricularly (icv). Intra-LS GLP-1 significantly suppressed chow intake early in the dark phase and tended to reduce overnight intake. However, blockade of LS GLP-1R with Ex9 had no effect on ad libitum dark onset chow intake. We then asked if LS GLP-1R blockade blunts nutrient preload-induced intake suppression. Mice were trained to consume Ensure immediately before dark onset, which suppressed subsequent chow intake, and intra-LS Ex9 attenuated that preload-induced intake suppression. We also found that restraint stress robustly activates hindbrain GLP-1-producing neurons, and that LS GLP-1R blockade attenuates 30-min restraint stress-induced hypophagia in mice. Furthermore, we have reported that in the rat, GLP-1R in the dorsal subregion of the LS (dLS) affect motivation for food. We examined this in food-restricted mice responding for sucrose pellets on a progressive ratio (PR) schedule. Intra-dLS GLP-1R stimulation significantly suppressed, and Ex9 significantly increased, operant responding, and the Ex9 effect remained after mice returned to ad libitum conditions. Similarly, we found that stimulation of dLS GLP-1 suppressed licking for sucrose and conversely, Ex9 increased licking under ad libitum feeding conditions. Together, our data suggest that endogenous activation of LS GLP-1R plays a role in feeding in mice under some but not all conditions, and that these receptors strongly influence motivation for food.

Activation of GLP-1 and Glucagon Receptors Regulates Bile Homeostasis Independent of Thyroid Hormone.

Balanced coagonists of glucagon-like peptide-1 (GLP-1) and glucagon receptors are emerging therapies for the treatment of obesity and diabetes. Such coagonists also regulate lipid metabolism, independent of their body weight lowering effects. Many actions of the coagonists are partly mediated by fibroblast growth factor 21 (FGF21) signaling, with the major exception of bile homeostasis. Since thyroid hormone is an important regulator of bile homeostasis, we studied the involvement of thyroid hormone in coagonist-induced changes in lipid and bile metabolism. We evaluated the effect of a single dose of coagonist Aib2 C24 chimera2 at 150 to 10000 µg/kg on tetraiodothyronine (T4) and triiodothyronine (T3) in high-fat diet-induced obese (DIO) mice and chow-fed mice. Repeated dose treatment of coagonist (150 µg/kg, subcutaneously) was assessed in four mice models namely, on lipid and bile homeostasis in DIO mice, propylthiouracil (PTU)-treated DIO mice, methimazole (MTM)-treated DIO mice and choline-deficient, L-amino acid-defined, highfat diet (CDAHFD)-induced nonalcoholic steatohepatitis (NASH). Single dose treatment of coagonist did not alter serum T3 and T4 in chow-fed mice and DIO mice. Coagonist treatment improved lipid metabolism and biliary cholesterol excretion. Chronic treatment of GLP-1 and glucagon coagonist did not alter serum T3 in hypothyroid DIO mice and CDAHFDinduced NASH. Coagonist increased serum T4 in DIO mice after 4 and 40 weeks of treatment, though no change in T4 levels was observed in hypothyroid mice or mice with NASH. Our data demonstrate that coagonist of GLP-1 and glucagon receptors does not modulate bile homeostasis via thyroid signaling.

Glucagon-like peptide-1 receptor activation alleviates lipopolysaccharide-induced acute lung injury in mice via maintenance of endothelial barrier function

Glucagon-like peptide-1 (GLP-1), which is well known for regulating glucose homeostasis, exhibits multiple actions in cardiovascular disorders and renal injury. However, little is known about the effect of GLP-1 receptor (GLP-1R) activation on acute lung injury (ALI). In this study, we investigated the effect of GLP-1R on ALI and the potential underlying mechanisms with the selective agonist liraglutide. Our results show that GLP-1 levels decreased in serum, though they increased in bronchoalveolar lavage fluid (BALF) and lung tissue in a mouse model of lipopolysaccharide (LPS)-induced ALI. Liraglutide prevented LPS-induced polymorphonuclear neutrophil (PMN) extravasation, lung injury, and alveolar-capillary barrier dysfunction. In cultured human pulmonary microvascular endothelial cells (HPMECs), liraglutide protected against LPS-induced endothelial barrier injury by restoring intercellular tight junctions and adherens junctions. Moreover, liraglutide prevented PMN–endothelial adhesion by inhibiting the expression of intercellular adhesion molecule-1 (ICAM-1) and vascular cell adhesion molecule-1 (VCAM-1), and thereafter suppressed PMN transendothelial migration. Furthermore, liraglutide suppressed LPS-induced activation of Rho/NF-κB signaling in HPMECs. In conclusion, our results show that GLP-1R activation protects mice from LPS-induced ALI by maintaining functional endothelial barrier and inhibiting PMN extravasation. These results also suggest that GLP-1R may be a potential therapeutic target for the treatment of ALI.

DPP-4 inhibition enhanced renal tubular and myocardial GLP-1 receptor expression decreased in CKD with myocardial infarction

BackgroundChronic kidney disease (CKD) is strongly associated with cardiovascular disease and is a significant risk factor for increased morbidity and mortality. In contrast, GLP-1 receptor (GLP-1R) activation has been shown to confer both renal and cardiovascular protection, though its relationship with CKD and CKD with myocardial ischemia/reperfusion (MI/R) remains poorly understood. Here, we investigated changes in renal and myocardial GLP-1R expression in the CKD rat model with MI/R.MethodsMale Sprague Dawley rats with 5/6 nephrectomy were used as a rat model of CKD and CKD with MI/R. For myocardial ischemia, the left coronary artery was ligated and released for 30 min 1 week after 5/6 nephrectomy. Dipeptidyl-peptidase 4 (DPP-4) inhibitors were administered orally with linagliptin once daily for 8 weeks. Renal cortical and myocardial GLP-1R expression were measured via immunohistochemistry and western blot analysis.ResultsDPP-4 activity was increased in CKD. Western blot density of GLP-1R in renal cortex extracts revealed increased abundance 2 weeks after 5/6 nephrectomy, followed by a decrease at 8 weeks. In contrast, CKD and CKD with MI/R rats showed decreases in renal and cardiac expression of GLP-1R; these effects were attenuated in rats treated with linagliptin.ConclusionsIn CKD with MI/R, linagliptin attenuated renal injury and increased renal and myocardial GLP-1R expression. These data suggest that activation of renal and myocardial GLP-1R expression may provide both cardio- and renoprotective effects.

Liraglutide and its Neuroprotective Properties-Focus on Possible Biochemical Mechanisms in Alzheimer's Disease and Cerebral Ischemic Events.

Liraglutide is a GLP-1 analog (glucagon like peptide-1) used primarily in the treatment of diabetes mellitus type 2 (DM2) and obesity. The literature starts to suggest that liraglutide may reduce the effects of ischemic stroke by activating anti-apoptotic pathways, as well as limiting the harmful effects of free radicals. The GLP-1R expression has been reported in the cerebral cortex, especially occipital and frontal lobes, the hypothalamus, and the thalamus. Liraglutide reduced the area of ischemia caused by MCAO (middle cerebral artery occlusion), limited neurological deficits, decreased hyperglycemia caused by stress, and presented anti-apoptotic effects by increasing the expression of Bcl-2 and Bcl-xl proteins and reduction of Bax and Bad protein expression. The pharmaceutical managed to decrease concentrations of proapoptotic factors, such as NF-κB (Nuclear Factor-kappa β), ICAM-1 (Intercellular Adhesion Molecule 1), caspase-3, and reduced the level of TUNEL-positive cells. Liraglutide was able to reduce the level of free radicals by decreasing the level of malondialdehyde (MDA), and increasing the superoxide dismutase level (SOD), glutathione (GSH), and catalase. Liraglutide may affect the neurovascular unit causing its remodeling, which seems to be crucial for recovery after stroke. Liraglutide may stabilize atherosclerotic plaque, as well as counteract its early formation and further development. Liraglutide, through its binding to GLP-1R (glucagon like peptide-1 receptor) and consequent activation of PI3K/MAPK (Phosphoinositide 3-kinase/mitogen associated protein kinase) dependent pathways, may have a positive impact on Aβ (amyloid beta) trafficking and clearance by increasing the presence of Aβ transporters in cerebrospinal fluid. Liraglutide seems to affect tau pathology. It is possible that liraglutide may have some stem cell stimulating properties. The effects may be connected with PKA (phosphorylase kinase A) activation. This paper presents potential mechanisms of liraglutide activity in conditions connected with neuronal damage, with special emphasis on Alzheimer’s disease and cerebral ischemia.

Glucagon Switches between GLP-1 and Glucagon Receptors in Normal and Hyperglycemic Mice

Despite the well-known role of the GLP-1 receptor (GLP-1R) in potentiating glucose-stimulated insulin secretion (GSIS) and as a therapeutic drug target in diabetes treatment, how GLP-1R is activated under physiological and pathological conditions remains elusive. Against preconceptions in the field, we prove that glucagon released from pancreatic α-cells serves as the principal ligand for GLP-1R on β-cells and is essential for GSIS in normal mice. Activation of the cognate glucagon receptor (GCGR) by glucagon amplifies GSIS evoked by intermediate concentrations of glucose only, as high glucose bypasses GCGR in increasing cytosolic cAMP from the same downstream pool. As the endogenous dual-receptor agonist, glucagon switches from GLP-1R to GCGR in β-cells of mice fed a high-fat diet, which significantly enhances GSIS and reduces body weight. Therefore, developing dual-receptor agonists that target balanced activation of GCGR and GLP-1R in vivo may better preserve glycemic control than the activation of GLP-1R alone.