Structure-Function of the Glucagon Receptor Family of G Protein-Coupled Receptors: The Glucagon, GIP, GLP-1, and GLP-2 Receptors

The Ubiquitination Status of the Glucagon Receptor determines Signal Bias

The pancreatic hormone glucagon activates the glucagon receptor (GCGR), a class B seven-transmembrane G protein-coupled receptor (GPCR) that couples to the stimulatory heterotrimeric Gs protein and provokes protein kinase A-dependent signaling cascades vital to hepatic glucose metabolism and islet insulin secretion. Glucagon-stimulation also initiates recruitment of the endocytic adaptors, β-arrestin1 and β-arrestin2, which regulate desensitization and internalization of the GCGR. Unlike many other GPCRs, the GCGR expressed at the plasma membrane is constitutively ubiquitinated and upon agonist-activation, internalized GCGRs are deubiquitinated at early endosomes and recycled via Rab4-containing vesicles. Herein we report a novel link between the ubiquitination status and signal transduction mechanism of the GCGR. In the deubiquitinated state, coupling of the GCGR to Gs is diminished, while binding to β-arrestin is enhanced with signaling biased to a β-arrestin1-dependent p38 mitogen activated protein kinase (MAPK) pathway. This ubiquitin-dependent signaling bias arises through the modification of lysine333 (K333) on the cytoplasmic face of transmembrane helix V. Compared with the GCGR-WT, the mutant GCGR-K333R has impaired ubiquitination, diminished G protein coupling and protein kinase A signaling, but unimpaired potentiation of glucose-stimulated-insulin secretion in response to agonist-stimulation, which involves p38 MAPK signaling. Both WT and GCGR-K333R promote the formation of glucagon-induced β-arrestin1-dependent p38 signaling scaffold that requires canonical upstream MAPK-Kinase3, but is independent of Gs, Gi and β-arrestin2. Thus ubiquitination/deubiquitination at K333 in the GCGR defines the activation of distinct transducers with the potential to influence various facets of glucagon signaling in health and disease.

Cross-Talk between Obesity and Diabetes: Introducing Polyphenols as an Effective Phytomedicine to Combat the Dual Sword Diabesity

: Obesity-associated diabetes mellitus, a chronic metabolic affliction accounting for 90% of all diabetic patients, has been affecting humanity extremely badly and escalating the risk of developing other serious disorders. It is observed that 0.4 billion people globally have diabetes, whose major cause is obesity. Currently, innumerable synthetic drugs like alogliptin and rosiglitazone are being used to get through diabetes, but they have certain complications, restrictions with severe side effects, and toxicity issues. Recently, the frequency of plant-derived phytochemicals as advantageous substitutes against diabesity is increasing progressively due to their unparalleled benefit of producing less side effects and toxicity. Of these phytochemicals, dietary polyphenols have been accepted as potent agents against the dual sword "diabesity". These polyphenols target certain genes and molecular pathways through dual mechanisms such as adiponectin upregulation, cannabinoid receptor antagonism, free fatty acid oxidation, ghrelin antagonism, glucocorticoid inhibition, sodium-glucose cotransporter inhibition, oxidative stress and inflammation inhibition etc. which sequentially help to combat both diabetes and obesity. In this review, we have summarized the most beneficial natural polyphenols along with their complex molecular pathways during diabesity.

Potential Effect of DPP-4 Inhibitors Towards Hepatic Diseases and Associated Glucose Intolerance

Dipeptidyl-peptidase-4 (DPP-4) is an enzyme having various properties and physiological roles in lipid accumulation, resistance to anticancer agents, and immune stimulation. DPP-4 includes membrane-bound peptidases and is a kind of enzyme that cleaves alanine or proline-containing peptides such as incretins, chemokines, and appetite-suppressing hormones (neuropeptide) at their N-terminal dipeptides. DPP-4 plays a role in the final breakdown of peptides produced by other endo and exo-peptidases from nutritious proteins and their absorption in these tissues. DPP-4 enzyme activity has different modes of action on glucose metabolism, hunger regulation, gastrointestinal motility, immune system function, inflammation, and pain regulation. According to the literature survey, as DPP-4 levels increase in individuals with liver conditions, up-regulation of hepatic DPP-4 expression is likely to be the cause of glucose intolerance or insulin resistance. This review majorly focuses on the cleavage of alanine or proline-containing peptides such as incretins by the DPP-4 and its resulting conditions like glucose intolerance and cause of DPP-4 level elevation due to some liver conditions. Thus, we have discussed the various effects of DPP-4 on the liver diseases like hepatitis C, non-alcoholic fatty liver, hepatic regeneration and stem cell, hepatocellular carcinoma, and the impact of elevated DPP-4 levels in association with liver diseases as a cause of glucose intolerance and their treatment drug of choices. In addition, the effect of DPP-4 inhibitors on obesity and their negative aspects are also discussed in brief.

The Human Islet: Mini-Organ With Mega-Impact

This review focuses on the human pancreatic islet-including its structure, cell composition, development, function, and dysfunction. After providing a historical timeline of key discoveries about human islets over the past century, we describe new research approaches and technologies that are being used to study human islets and how these are providing insight into human islet physiology and pathophysiology. We also describe changes or adaptations in human islets in response to physiologic challenges such as pregnancy, aging, and insulin resistance and discuss islet changes in human diabetes of many forms. We outline current and future interventions being developed to protect, restore, or replace human islets. The review also highlights unresolved questions about human islets and proposes areas where additional research on human islets is needed.

Glucagon-like peptide-1 attenuated carboxymethyl lysine induced neuronal apoptosis via peroxisome proliferation activated receptor-γ

Backgrounds and aims: The role of peroxisome proliferator activated receptor-γ (PPAR-γ) in neuronal apoptosis remains unclear. We aim to investigate the role of PPAR-γ in glucagon-like peptide-1 (GLP-1) alleviated neuronal apoptosis induced by carboxymethyl-lysine (CML).

Materials and Methods: In vitro, PC12 cells were treated by CML/GLP-1. Moreover. the function of PPAR-γ was blocked by GW9662. In vivo, streptozotocin (STZ) was used to induce diabetic rats with neuronal apoptosis. The cognitive function of rats was observed by Morris water maze. Apoptosis was detected by TUNEL assay. Bcl2, Bax, PPAR-γ and receptor of GLP-1 (GLP-1R) were measured by western blotting or immunofluorescence.

Results: In vitro experiment, CML triggered apoptosis, down-regulated GLP-1R and PPAR-γ. Moreover, GLP-1 not only alleviated the apoptosis, but also increased levels of PPAR-γ. GW9662 abolished the neuroprotective effect of GLP-1 on PC12 cells from apoptosis. Furthermore, GLP-1R promoter sequences were detected in the PPAR-γ antibody pulled mixture. GPL-1 levels decreased, while CML levels increased in diabetic rats, compared with control rats. Additionally, we observed elevated bax, decreased bcl2, GLP-1R and PPAR-γ in diabetic rats.

Conclusions: GLP-1 could attenuate neuronal apoptosis induced by CML. Additionally, PPAR-γ involves in this process.

Chronic treatment with anti-GIPR mAb alone and combined with DPP-4 inhibitor correct obesity, dyslipidemia and nephropathy in rodent animals

OBJECTIVE

Glucose-dependent insulinotropic polypeptide receptor (GIPR) has been identified as a contributor to obesity, and GIPR knockout mice are protected against diet-induced obesity (DIO). Therefore, we developed the anti-GIPR antagonistic monoclonal antibody (mAb) alone and in combination with DPP-4 inhibitor as potential therapeutic strategy for treating obesity and dyslipidemia based on this genetic evidence.

METHODS

Fully neutralized GIPR activity of GIPR-monoclonal antibody (mAb) was assessed by regulating the in vitro production of cAMP in the mouse GIPR stably expressing cells. Chronic efficacies of GIPR-mAb alone and in combination with DPP-4 inhibitor Sitagliptin in diabetic or DIO mice were both investigated. Multiple metabolic parameters including body weight, glucose level, fat mass, lipid metabolism-related indicators as well as H&E staining and immunohistochemical analysis were performed. Role of GIPR in pancreatic cells on regulating fat metabolism was explored in GIPR β-cell knockout mouse model.

RESULTS

Chronic treatment of GIPR-mAb improved body weight control, glucose metabolism, and was associated with reduced fat mass, enhanced pancreatic function and exchange ratio of the resting respiratory in diabetic mice. In addition, further study of anti-GIPR mAb combined with Sitagliptin in DIO mice demonstrated significantly improved weight loss compare to the both monomer treatment. Furthermore, we demonstrated important role of GIPR in β-cell in regulating the fat mass and response to antagonistic GIPR-mAb in a conditional GIPR-knockout mouse.

CONCLUSION

Chronic treatment with anti-GIPR mAb alone and combined with DPP-4 inhibitor provide preclinical therapeutic approaches to treat obesity.

Emerging Role of Caveolin-1 in GLP-1 Action

Glucagon-like peptide-1 (GLP-1) is a gut hormone mainly produced in the intestinal epithelial endocrine L cells, involved in maintaining glucose homeostasis. The use of GLP-1 analogous and dipeptidyl peptidase-IV (DPP-IV) inhibitors is well-established in Type 2 Diabetes. The efficacy of these therapies is related to the activation of GLP-1 receptor (GLP-1R), which is widely expressed in several tissues. Therefore, GLP-1 is of great clinical interest not only for its actions at the level of the beta cells, but also for the extra-pancreatic effects. Activation of GLP-1R results in intracellular signaling that is regulated by availability of downstream molecules and receptor internalization. It has been shown that GLP-1R co-localizes with caveolin-1, the main component of caveolae, small invagination of the plasma membrane, which are involved in controlling receptor activity by assembling signaling complexes and regulating receptor trafficking. The aim of this review is to outline the important role of caveolin-1 in mediating biological effects of GLP-1 and its analogous.

Repositioning the Alpha Cell in Postprandial Metabolism

Glucose homeostasis is maintained in large part due to the actions of the pancreatic islet hormones insulin and glucagon, secreted from β- and α-cells, respectively. The historical narrative positions these hormones in opposition, with insulin primarily responsible for glucose-lowering and glucagon-driving elevations in glucose. Recent progress in this area has revealed a more complex relationship between insulin and glucagon, highlighted by data demonstrating that α-cell input is essential for β-cell function and glucose homeostasis. Moreover, the common perception that glucagon levels decrease following a nutrient challenge is largely shaped by the inhibitory effects of glucose administration alone on the α-cell. Largely overlooked is that a mixed nutrient challenge, which is more representative of typical human feeding, actually stimulates glucagon secretion. Thus, postprandial metabolism is associated with elevations, not decreases, in α-cell activity. This review discusses the recent advances in our understanding of how α-cells regulate metabolism, with a particular focus on the postprandial state. We highlight α- to β-cell communication, a term that describes how α-cell input into β-cells is a critical axis that regulates insulin secretion and glucose homeostasis. Finally, we discuss the open questions that have the potential to advance this field and continue to evolve our understanding of the role that α-cells play in postprandial metabolism.

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

AIM

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

RESULTS AND DISCUSSION

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

CONCLUSION

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

GLP-1 Receptor Agonists in Type 2 Diabetes Mellitus

The rising epidemic of type 2 diabetes mellitus & associated complications is a serious cause of concern for humanity. Glucagon-like peptide-1 receptor agonists commonly abbreviated as GLP-1 RAs, emerged as a promising therapeutic class based on incretin therapy that regulates glucose metabolism through multiple mechanisms. In the present study, various investigational & clinically used GLP-1 RAs have been reviewed with emphasis on their efficacy, structural modifications, adverse effects and toxicities. Various clinical trials justifying their efficacy have also been included, which highlighted the potential of GLP-1 RAs over conventional anti hyperglycaemic agents through a study of pooled effect on glycemic efficacy and weight-loss. The significant potency and appreciable safety of GLP-1 RAs manifested their potential as a logical approach for the management of type 2 diabetes.

β Cell tone is defined by proglucagon peptides through cAMP signaling

Paracrine interactions between pancreatic islet cells have been proposed as a mechanism to regulate hormone secretion and glucose homeostasis. Here, we demonstrate the importance of proglucagon-derived peptides (PGDPs) for α to β cell communication and control of insulin secretion. Signaling through this system occurs through both the glucagon-like peptide receptor (Glp1r) and glucagon receptor (Gcgr). Loss of PGDPs, or blockade of their receptors, decreases insulin secretion in response to both metabolic and nonmetabolic stimulation of mouse and human islets. This effect is due to reduced β cell cAMP and affects the quantity but not dynamics of insulin release, indicating that PGDPs dictate the magnitude of insulin output in an isolated islet. In healthy mice, additional factors that stimulate cAMP can compensate for loss of PGDP signaling; however, input from α cells is essential to maintain glucose tolerance during the metabolic stress induced by high-fat feeding. These findings demonstrate an essential role for α cell regulation of β cells, raising the possibility that abnormal paracrine signaling contributes to impaired insulin secretion in diabetes. Moreover, these findings support reconsideration of the role for α cells in postprandial glucose control.

Hemodynamic Effects of Glucagon: A Literature Review

CONTEXT

Glucagon's effects on hemodynamic parameters, most notably heart rate and cardiac contractility, are often overlooked. The glucagon receptor is a central target in novel and anticipated type 2 diabetes therapies, and hemodynamic consequences of glucagon signaling have therefore become increasingly important. In this review, we summarize and evaluate published studies on glucagon pharmacology with a focus on clinical hemodynamic effects in humans.

EVIDENCE ACQUISITION

PubMed, Embase, and the Cochrane Library were searched for clinical studies concerning hemodynamic effects of glucagon (no year restriction). Papers reporting effects of a defined glucagon dose on any hemodynamic parameter were included. Reference searches were conducted in retrieved articles.

EVIDENCE SYNTHESIS

Hemodynamic effects of glucagon have been investigated mainly in cohort studies of patients suffering from heart failure receiving large glucagon bolus injections. The identified studies had shortcomings related to restricted patient groups, lack of a control group, randomization, or blinding. We identified no properly conducted randomized clinical trials. The majority of human studies report stimulating effects of pharmacological glucagon doses on heart rate, cardiac contractility, and blood pressure. The effects were characterized by short duration, interindividual variation, and rapid desensitization. Some studies reported no measurable effects of glucagon.

CONCLUSIONS

The level of evidence regarding hemodynamic effects of glucagon is low, and observations in published studies are inconsistent. Actual effects, interindividual variation, dose-response relationships, and possible long-term effects of supraphysiological glucagon levels warrant further investigation.

COMPARISON OF THE LEVELS OF C-REACTIVE PROTEIN, GLP-1 AND GLP-2 AMONG INDIVIDUALS WITH DIABETES, MORBID OBESITY AND HEALTHY CONTROLS: AN EXPLORATORY STUDY

BACKGROUND

The glucagon-like peptides 1 and 2 (GLP-1/GLP-2) are gut hormones that may directly affect the glucose homeostasis and their activity seems to be significantly affected by chronic inflammation.

OBJECTIVE

To evaluate the postprandial levels of glucagon-like peptides 1 and 2 (GLP-1/GLP-2), C-reactive protein (CRP), and the postprandial glucose and insulin levels among individuals with obesity, type 2 diabetes, and healthy controls.

METHODS

An exploratory cross-sectional study, which involved individuals awaiting for bariatric/metabolic surgery and healthy controls. Postprandial levels of GLP-1, GLP-2, glucose, and insulin were obtained after a standard meal tolerance test. Inflammation was assessed by means of CRP.

RESULTS

There were 30 individuals enrolled in the study, divided into three groups: non-diabetic with morbid obesity (NDO; n=11 individuals), diabetic with mild obesity (T2D; n=12 individuals), and healthy controls (C; n=7 individuals). The mean CRP levels were significantly higher in the NDO group (6.6±4.7 mg/dL) than in the T2D (3.3±2.2 mg/dL) and C groups (2.5±3.2 mg/dL) (P=0.038). The GLP-1 levels following standard meal tolerance test and the area under the curve of GLP-1 did not differ among the three groups. The GLP-2 levels were significantly lower in the NDO and T2D than in the C group following standard meal tolerance test at all the times evaluated. The area under the curve of the GLP-2 was significantly lower in the NDO and T2D groups than in the C group (P=0.05 and P=0.01, respectively).

CONCLUSION

GLP-2 levels were impaired in the individuals with obesity and diabetes. This mechanism seems to be enrolled in preventing the worsening of the glucose homeostasis in these individuals.

Characterization of the Glucagonlike Peptide-1 Receptor in Male Mouse Brain Using a Novel Antibody and In Situ Hybridization

Glucagonlike peptide 1 (GLP-1) is a physiological regulator of appetite, and long-acting GLP-1 receptor (GLP-1R) agonists lower food intake and body weight in both human and animal studies. The effects are mediated through brain GLP-1Rs, and several brain nuclei expressing the GLP-1R may be involved. To date, the mapping of the complete location of GLP-1R protein in the brain has been challenged by lack of good antibodies and the discrepancy between mRNA and protein, especially relevant in neuronal axonal processes. Here, we present a specific monoclonal GLP-1R antibody for immunohistochemistry with murine tissue and show detailed distribution of GLP-1R expression, as well as mapping of GLP-1R mRNA by nonradioactive in situ hybridization. Semiautomated image analysis was performed to map the GLP-1R distribution to atlas plates from the Allen Institute for Brain Science. The GLP-1R was abundantly expressed in numerous regions, including the septal nucleus, hypothalamus, and brain stem. GLP-1R protein expression was also observed on neuronal projections in brain regions devoid of any mRNA that has not been observed in earlier reports. Taken together, these findings provide knowledge on GLP-1R expression in neuronal cell bodies and neuronal projections.

Pharmacokinetic drug evaluation of exenatide for the treatment of type 2 diabetes

INTRODUCTION

Glucagon-like peptide-1 (GLP-1) receptor analogs are a group of therapeutic agents which mimic endogenous GLP-1, exerting their effect by the stimulation of the GLP-1 receptor with a wide distribution. Its activation increases insulin releasing dependent on blood glucose levels, suppression of glucagon secretion and a reduction of hepatic glucose output. It delays gastric emptying and increases satiety. Exenatide is the synthetic version of exendin-4, a natural peptide with similar properties to human GLP-1. There are two pharmaceutical forms, for subcutaneous injection: twice daily and once weekly. Clinical practice guidelines recommend them because of a high efficacy reducing hyperglycemia, low risk of hypoglycemia and a significative weight loss effect. Gastrointestinal adverse events are the most common beside injection site-related. Their cost is the main limitation to use. Areas covered: We review the recent literature investigating the pharmacokinetics and pharmacodynamics and efficacy-safety studies of exenatide twice daily and once weekly in type 2 diabetes Expert opinion: GLP-1 receptor analogs are now positioned as an effective and safe drug for the treatment of type 2 diabetes. Exenatide significally reduces HbA1c and fasting plasma glucose. Additionally, it produces moderate weight loss and decreases blood pressure. One weekly formulation may improve compliance while cost is still a limitation. EXSCEL trial has shown that, despite cardiovascular safety, exenatide do not exhibits cardiovascular benefits.

Empagliflozin and linagliptin combination therapy for treatment of patients with type 2 diabetes mellitus

INTRODUCTION

Many patients with type 2 diabetes mellitus (T2DM) fail to achieve the desired A1c goal because the antidiabetic medications used do not correct the underlying pathophysiologic abnormalities and monotherapy is not sufficiently potent to reduce the A1c to the 6.5 - 7.0% range. Insulin resistance and islet (beta and alpha) cell dysfunction are major pathophysiologic abnormalities in T2DM. We examine combination therapy with linagliptin plus empagliflozin as a therapeutic approach for the treatment of inadequately controlled T2DM patients.

AREAS COVERED

A literature search of all human diabetes, metabolism and general medicine journals from year 2000 to the present was conducted. Glucagon like peptide-1 (GLP-1) deficiency/resistance contributes to islet cell dysfunction by impairing insulin secretion and increasing glucagon secretion. DPP-4 inhibitors (DPP4i) improve pancreatic islet function by augmenting glucose-dependent insulin secretion and decreasing elevated plasma glucagon levels. Linagliptin, a DPP-4 inhibitor, reduces HbA1c, is weight neutral, has an excellent safety profile and a low risk of hypoglycemia. The expression of sodium-glucose cotransporter-2 (SGLT2) in the proximal renal tubule is upregulated in T2DM, causing excess reabsorption of filtered glucose. The SGLT2 inhibitor (SGLT2i), empagliflozin, improves HbA1c by causing glucosuria and ameliorating glucotoxicity. It also decreases weight and blood pressure, and has a low risk of hypoglycemia.

EXPERT OPINION

The once daily oral combination of linagliptin plus empagliflozin does not increase the risk of hypoglycemia and tolerability and discontinuation rates are similar to those with each as monotherapy. At HbA1c values below 8.5% linagliptin/empagliflozin treatment produces an additive effect, whereas above 8.5%, there is a less than additive reduction with combination therapy compared with the effect of each agent alone. Linagliptin/empagliflozin addition is a logical combination in patients with T2DM, especially those with an HbA1c < 8.5%.

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

INTRODUCTION

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

AREAS COVERED

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

EXPERT OPINION

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