Miniglucagon (glucagon 19-29): a novel regulator of the pancreatic islet physiology

The role of glucagon after bariatric/metabolic surgery: much more than an "anti-insulin" hormone

The biological activity of glucagon has recently been proposed to both stimulate hepatic glucose production and also include a paradoxical insulinotropic effect, which could suggest a new role of glucagon in the pathophysiology type 2 diabetes mellitus (T2DM). An insulinotropic role of glucagon has been observed after bariatric/metabolic surgery that is mediated through the GLP-1 receptor on pancreatic beta cells. This effect appears to be modulated by other members of the proglucagon family, playing a key role in the beneficial effects and complications of bariatric/metabolic surgery. Glucagon serves a dual role after sleeve gastrectomy (SG) and Roux-en-Y gastric bypass (RYGB). In addition to maintaining blood glucose levels, glucagon exhibits an insulinotropic effect, suggesting that glucagon has a more complex function than simply an "anti-insulin hormone".

The effects of curcumin, mangiferin, resveratrol and other natural plant products on aminopeptidase B activity

Aminopeptidase B (Ap-B) is a Zn²⁺-aminopeptidase of the M1 family which is implicated, in conjunction with the nardilysin endoprotease, in the generation of miniglucagon, a peptide involved in the maintenance of glucose homeostasis. Other in vivo physiological roles have been established for this vertebrate enzyme, such as the processing of Arg-extended forms of human insulin and cholecystokinin 9 and the degradation of viral epitopes in the cytoplasm. Among M1 family members, Ap-B is phylogenetically close to leukotriene A₄ hydrolase (LTA₄H), a bi-functional aminopeptidase also able to transform LTA₄ in LTB₄ (a lipid mediator of inflammation). As the activities of LTA₄H are reported to be inhibited by resveratrol, a polyphenolic molecule from red wine, the effect of this molecule was investigated on the Ap-B activity. Several other active phenolic compounds produced in plants were also tested. Among them, curcumin and mangiferin are the most effective inhibitors. Dixon analysis indicates that curcumin is a non-competitive inhibitor with a Ki value of 46 μmol.L^-1. Dixon and Lineweaver-Burk representations with mangiferin show a mixed non-competitive inhibition with Ki' and Ki values of 194 μmol.L^-1 and 105 μmol.L^-1, respectively. At 200 μmol.L^-1, no significant effect was observed with caffeic, chlorogenic, ferulic, salicylic and sinapic acids as well as with resveratrol. Analyses on the 3D-structure of LTA₄H with resveratrol (pdb: 3FTS) and the Ap-B 3D-model allow hypothesis to explain theses results.

Dissecting the Physiology and Pathophysiology of Glucagon-Like Peptide-1

An aging world population exposed to a sedentary life style is currently plagued by chronic metabolic diseases, such as type-2 diabetes, that are spreading worldwide at an unprecedented rate. One of the most promising pharmacological approaches for the management of type 2 diabetes takes advantage of the peptide hormone glucagon-like peptide-1 (GLP-1) under the form of protease resistant mimetics, and DPP-IV inhibitors. Despite the improved quality of life, long-term treatments with these new classes of drugs are riddled with serious and life-threatening side-effects, with no overall cure of the disease. New evidence is shedding more light over the complex physiology of GLP-1 in health and metabolic diseases. Herein, we discuss the most recent advancements in the biology of gut receptors known to induce the secretion of GLP-1, to bridge the multiple gaps into our understanding of its physiology and pathology.

Exendin-4 Upregulates Adiponectin Level in Adipocytes via Sirt1/Foxo-1 Signaling Pathway

Glucagon-like peptide-1 (GLP-1) receptor plays an essential role in regulating glucose metabolism. GLP-1 receptor agonists have been widely used for treating diabetes and other insulin resistance-related diseases. However, mechanisms underlying the anti-diabetic effects of GLP-1 receptor agonists remain largely unknown. In this study, we investigated the effects of GLP-1 agonist exendin-4 on the expression of adiponectin, an insulin sensitizing hormone. We found that exendin-4 increased the expression and secretion of adiponectin both in vitro and in vivo. Our data showed that exendin-4 upregulated adiponectin expression at both mRNA and protein levels in adipocytes and adipose tissues. The effects of exendin-4 on adiponectin expression were dependent on the GLP-1 receptor. We further demonstrated important roles of Sirt1 and transcriptional factor Foxo-1 in mediating the function of exendin-4 in regulating adiponectin expression. Suppression of Sirt1 or Foxo-1 expression significantly impaired exendin-4-induced adiponectin expression. Consistently, exendin-4 up-regulated Sirt1 and Foxo-1 expression in vivo. Our work is the first study demonstrating the role of Sirt1/Foxo-1 in regulating the regulatory function of a GLP-1 receptor agonist in adiponectin expression both in vitro and in vivo. The results provide important information for the mechanism underlying the function of GLP-1R on improving insulin resistance and related diseases.

Vascular Biology of Glucagon Receptor Superfamily Peptides: Mechanistic and Clinical Relevance

Regulatory peptides produced in islet and gut endocrine cells, including glucagon, glucagon-like peptide-1 (GLP-1), GLP-2, and glucose-dependent insulinotropic polypeptide, exert actions with considerable metabolic importance and translational relevance. Although the clinical development of GLP-1 receptor agonists and dipeptidyl peptidase-4 inhibitors has fostered research into how these hormones act on the normal and diseased heart, less is known about the actions of these peptides on blood vessels. Here we review the effects of these peptide hormones on normal blood vessels and highlight their vascular actions in the setting of experimental and clinical vascular injury. The cellular localization and signal transduction properties of the receptors for glucagon, GLP-1, GLP-2, and glucose-dependent insulinotropic polypeptide are discussed, with emphasis on endothelial cells and vascular smooth muscle cells. The actions of these peptides on the control of blood flow, blood pressure, angiogenesis, atherosclerosis, and vascular inflammation are reviewed with a focus on elucidating direct and indirect mechanisms of action. How these peptides traverse the blood-brain barrier is highlighted, with relevance to the use of GLP-1 receptor agonists to treat obesity and neurodegenerative disorders. Wherever possible, we compare actions identified in cell lines and primary cell culture with data from preclinical studies and, when available, results of human investigation, including studies in subjects with diabetes, obesity, and cardiovascular disease. Throughout the review, we discuss pitfalls, limitations, and challenges of the existing literature and highlight areas of controversy and uncertainty. The increasing use of peptide-based therapies for the treatment of diabetes and obesity underscores the importance of understanding the vascular biology of peptide hormone action.

Allostery and Biased Agonism at Class B G Protein-Coupled Receptors

Class B G protein-coupled receptors (GPCRs) respond to paracrine or endocrine peptide hormones involved in control of bone homeostasis, glucose regulation, satiety, and gastro-intestinal function, as well as pain transmission. These receptors are targets for existing drugs that treat osteoporosis, hypercalcaemia, Paget's disease, type II diabetes, and obesity and are being actively pursued as targets for numerous other diseases. Exploitation of class B receptors has been limited by difficulties with small molecule drug discovery and development and an under appreciation of factors governing optimal therapeutic efficacy. Recently, there has been increasing awareness of novel attributes of GPCR function that offer new opportunity for drug development. These include the presence of allosteric binding sites on the receptor that can be exploited as drug binding pockets and the ability of individual drugs to enrich subpopulations of receptor conformations to selectively control signaling, a phenomenon termed biased agonism. In this review, current knowledge of biased signaling and small molecule allostery within class B GPCRs is discussed, highlighting areas that have progressed significantly over the past decade, in addition to those that remain largely unexplored with respect to these phenomena.

Physiology of proglucagon peptides: role of glucagon and GLP-1 in health and disease

The preproglucagon gene (Gcg) is expressed by specific enteroendocrine cells (L-cells) of the intestinal mucosa, pancreatic islet α-cells, and a discrete set of neurons within the nucleus of the solitary tract. Gcg encodes multiple peptides including glucagon, glucagon-like peptide-1, glucagon-like peptide-2, oxyntomodulin, and glicentin. Of these, glucagon and GLP-1 have received the most attention because of important roles in glucose metabolism, involvement in diabetes and other disorders, and application to therapeutics. The generally accepted model is that GLP-1 improves glucose homeostasis indirectly via stimulation of nutrient-induced insulin release and by reducing glucagon secretion. Yet the body of literature surrounding GLP-1 physiology reveals an incompletely understood and complex system that includes peripheral and central GLP-1 actions to regulate energy and glucose homeostasis. On the other hand, glucagon is established principally as a counterregulatory hormone, increasing in response to physiological challenges that threaten adequate blood glucose levels and driving glucose production to restore euglycemia. However, there also exists a potential role for glucagon in regulating energy expenditure that has recently been suggested in pharmacological studies. It is also becoming apparent that there is cross-talk between the proglucagon derived-peptides, e.g., GLP-1 inhibits glucagon secretion, and some additive or synergistic pharmacological interaction between GLP-1 and glucagon, e.g., dual glucagon/GLP-1 agonists cause more weight loss than single agonists. In this review, we discuss the physiological functions of both glucagon and GLP-1 by comparing and contrasting how these peptides function, variably in concert and opposition, to regulate glucose and energy homeostasis.

The forgotten members of the glucagon family

From proglucagon, at least six final biologically active peptides are produced by tissue-specific post-translational processing. While glucagon and GLP-1 are the subject of permanent studies, the four others are usually left in the shadow, in spite of their large biological interest. The present review is devoted to oxyntomodulin and miniglucagon, not forgetting glicentin, although much less is known about it. Oxyntomodulin (OXM) and glicentin are regulators of gastric acid and hydromineral intestinal secretions. OXM is also deeply involved in the control of food intake and energy expenditure, properties that make this peptide a credible treatment of obesity if the question of administration is solved, as for any peptide. Miniglucagon, the C-terminal undecapeptide of glucagon which results from a secondary processing of original nature, displays properties antagonistic to that of the mother-hormone glucagon: (a) it inhibits glucose-, glucagon- and GLP-1-stimulated insulin release at sub-picomolar concentrations, (b) it reduces the in vivo insulin response to glucose with no change in glycemia, (c) it displays insulin-like properties at the cellular level using only a part of the pathway used by insulin, making it a good basis for developing a pharmacological workaround of insulin resistance.

The avian proglucagon system

Understanding how the proglucagon system functions in maintaining glycemic control and energy balance in birds, as well as defining its specific roles in regulating metabolism, gastrointestinal tract function and food intake requires detailed knowledge of the components that comprise this system. These include proglucagon, a precursor protein from which glucagon and two glucagon-like peptide hormones (GLP-1 and GLP-2) are derived, and the membrane bound G-protein-coupled receptors that specifically bind glucagon, GLP-1 and GLP-2 to mediate their individual physiological actions. Another key feature of the proglucagon system that is important for regulating its activity in different tissues involves post-translational processing of the proglucagon precursor protein and the individual peptide hormones derived from it. Currently, there is limited information about the proglucagon system in birds with the majority of that coming from studies involving chickens. By summarizing what is currently known about the proglucagon system in birds, this review aims to provide useful background information for future investigations that will explore the nature and actions of this important hormonal system in different avian species.

Immunohistochemical localization of glucagon and pancreatic polypeptide on rat endocrine pancreas: coexistence in rat islet cells

We used immunofluorescence double staining method to investigate the cellular localization of glucagon and pancreatic polypeptide (PP) in rat pancreatic islets. The results showed that both A-cells (glucagon-secreting cells) and PP-cells (PP-secreting cells) were located in the periphery of the islets. However, A-cells and PP-cells had a different regional distribution. Most of A-cells were located in the splenic lobe but a few of them were in the duodenal lobe of the pancreas. In contrast, the majority of PP-cells were found in the duodenal lobe and a few of them were in the splenic lobe of the pancreas. Furthermore, we found that 67.74% A-cells had PP immunoreactivity, 70.92% PP-cells contained glucagon immunoreactivity with immunofluorescence double staining. Our data support the concept of a common precursor stem cell for pancreatic hormone-producing cells.

Expression of proglucagon and proglucagon-derived peptide hormone receptor genes in the chicken

To better understand how the proglucagon system functions in birds, we utilized a molecular cloning strategy to sequence and characterize the chicken proglucagon gene that encodes glucagon, glucagon-like peptide (GLP)-1 and GLP-2. This gene has seven exons and six introns with evidence for an additional (alternate) first exon and two promoter regions. We identified two distinct classes of proglucagon mRNA transcripts (PGA and PGB) produced by alternative splicing at their 3'-ends. These were co-expressed in all tissues examined with pancreas and proventriculus showing the highest levels of each. Although both mRNA classes contained coding sequence for glucagon and GLP-1, class A mRNA lacked that portion of the coding region (CDS) containing GLP-2; whereas, class B mRNA had a larger CDS that included GLP-2. Both classes of mRNA transcripts exhibited two variants, each with a different 5'-end arising from alternate promoter and alternate first exon usage. Fasting and refeeding had no effect on proglucagon mRNA expression despite significant changes in plasma glucagon levels. To investigate potential differences in proglucagon precursor processing among tissues, mRNA expression for two prohormone convertase (PC) genes was analyzed. PC2 mRNA was predominantly expressed in pancreas and proventriculus, whereas PC1/3 mRNA was more highly expressed in duodenum and brain. We also determined mRNA expression of the specific receptor genes for glucagon, GLP-1 and GLP-2 to help define major sites of hormone action. Glucagon receptor mRNA was most highly expressed in liver and abdominal fat, whereas GLP-1 and GLP-2 receptor genes were highly expressed in the gastrointestinal tract, brain, pancreas and abdominal fat. These results offer new insights into structure and function of the chicken proglucagon gene, processing of the precursor proteins produced from it and potential activity sites for proglucagon-derived peptide hormones mediated by their cognate receptors.

Pro-protein convertases in intermediary metabolism: islet hormones, brain/gut hormones and integrated physiology

Many peptide hormones implicated in the regulation of intermediary metabolism arise from larger precursors called prohormones. These precursors are cut into pieces by proprotein convertases, more precisely those called prohormone convertases (PCs) that cleave at the C terminus of basic doublets. The remaining basic amino acids are eliminated by a specialized carboxypeptidase, leading to the active hormone. This processing may provide, from a single precursor, several peptides with different biological activities depending on the site(s) of cleavage on the precursor. When the processing is tissue-specific, this mechanism allows to produce, from a single protein, different sets of hormones depending on the tissue considered, leading to novel regulatory processes. The archetype of such a pluripotent prohormone in the field of intermediary metabolism is pro-glucagon that, when cut by PC1 in intestinal L cells, produces four different peptides with different specificities [glicentin, oxyntomodulin (OXM), glucagon-like peptide-1, and glucagon-like peptide-2], whereas, when cut by PC2 in the alpha cells of the endocrine pancreas, glucagon is produced and, through the supplementary action of NRD convertase, a fragment of glucagon (miniglucagon) with original properties.

Overexpression of a dominant negative GIP receptor in transgenic mice results in disturbed postnatal pancreatic islet and beta-cell development

The expression of a dominant negative glucose-dependent insulinotropic polypeptide receptor (GIPRdn) under the control of the rat pro-insulin gene promoter induces severe diabetes mellitus in transgenic mice. This study aims to gain further insight into the effect of the expression of a dominant negative GIPR on glucose homeostasis and postnatal development of the endocrine pancreas. The diabetic phenotype of GIPRdn transgenic animals was first observed between 14 and 21 days of age (urine glucose>1000 mg/dl). After onset of diabetes, serum glucose was significantly higher and insulin values were significantly lower in GIPRdn transgenic mice vs. non-transgenic littermate controls. Morphometric studies of pancreatic islets and their endocrine cell types were carried out at 10, 30 and 90 days of age. The total islet and total beta-cell volume of transgenic mice was severely reduced as compared to control mice, irrespective of the age at sampling (p<0.05). The total volume of isolated insulin positive cells that were not contained within established islets was significantly reduced in transgenic mice, indicating disturbed islet neogenesis. These findings demonstrate in vivo evidence that intact signaling of G-protein coupled receptors is involved in postnatal islet and beta-cell development and neogenesis of the pancreatic islets.

Significance of prohormone convertase 2, PC2, mediated initial cleavage at the proglucagon interdomain site, Lys70-Arg71, to generate glucagon

To define the biological significance of the initial cleavage at the proglucagon (PG) interdomain site, K70-R71 downward arrow, we created two interdomain mutants, K70Q-R71Q and R71A. Cotransfection studies in GH4C1 cells show significant amounts of glucagon production by PC2 along with some glicentin, glicentin-related polypeptide-glucagon (GRPP-glucagon) and oxyntomodulin from wild-type PG. In contrast, a larger peptide, PG 33-158, and low amounts of GRPP-glucagon are predominantly generated from interdomain mutants. HPLC analysis shows a 5-fold increase in glucagon production by PC2 from wild-type PG and a corresponding 4-fold lower accumulation and secretion of unprocessed precursor relative to interdomain mutants. PC2 generates significant levels of glucagon from a glicentin (PG 1-69) expression plasmid, whereas PC1/3 produces only modest amounts of oxyntomodulin. Employing a major PG fragment (PG 72-158) expression plasmid, we show that PC1/3 predominantly generates glucagon-like peptide (GLP)-1, whereas PC2 produces only N-terminally extended GLP-1. Surprisingly, production of GLP-1 and GLP-2 by PC1/3 from interdomain mutants, compared with wild-type PG, is not significantly impaired. In addition to PC2 and PC1/3, PC5/6A and furin are also able to cleave the sites, K70-R71 downward arrow and R107-X-R-R110 downward arrow in PG. We show a much greater ability of furin to cleave the monobasic site, R77 downward arrow, than at the dibasic site, R124-R125 downward arrow, which is also weakly processed by PC5/6A, indicating overlapping specificities of these two convertases mainly with PC1/3. We propose here a trimer-like model of the spatial organization of the hormonal sequences within the PG molecule in which the accessibility to prohormone convertase action of most cleavage sites is restricted with the exception of the interdomain site, K70-R71, which is maximally accessible.

Glucagon promotes cAMP-response element-binding protein phosphorylation via activation of ERK1/2 in MIN6 cell line and isolated islets of Langerhans

By using the MIN6 cell line and pancreatic islets, we show that in the presence of a low glucose concentration, corresponding to physiological glucagon release from alpha cells, glucagon treatment of the beta cell caused a rapid, time-dependent phosphorylation and activation of p44/p42 mitogen-activated protein kinase (ERK1/2) independently from extracellular calcium influx. Inhibition of either cAMP-dependent protein kinase (PKA) or MEK completely blocked ERK1/2 activation by glucagon. However, no significant activation of several upstream activators of MEK, including Shc-p21(Ras) and phosphatidylinositol 3-kinase, was observed in response to glucagon treatment. Chelation of intracellular calcium (intracellular [Ca(2+)]) reduced glucagon-mediated ERK1/2 activation. In addition, internalization of glucagon receptors through clathrin-coated pits formation is required for ERK1/2 activation. Remarkably, glucagon promotes the nuclear translocation of ERK1/2 and induces the phosphorylation of cAMP-response element-binding protein (CREB). Miniglucagon, produced from glucagon and released together with the mother hormone from the alpha cells in low glucose situations, blocks the insulinotropic effect of glucagon, whereas it does not inhibit the glucagon-induced PKA/ERK1/2/CREB pathway. We conclude that glucagon-induced ERK1/2 activation is mediated by PKA and that an increase in [Ca(2+)](i) is required for maximal ERK activation. Our results uncover a novel mechanism by which the PKA/ERK1/2 signaling network engaged by glucagon, in situation of low glucose concentration, regulates phosphorylation of CREB, a transcription factor crucial for normal beta cell function and survival.

International Union of Pharmacology. XXXV. The glucagon receptor family

Peptide hormones within the secretin-glucagon family are expressed in endocrine cells of the pancreas and gastrointestinal epithelium and in specialized neurons in the brain, and subserve multiple biological functions, including regulation of growth, nutrient intake, and transit within the gut, and digestion, energy absorption, and energy assimilation. Glucagon, glucagon-like peptide-1, glucagon-like peptide-2, glucose-dependent insulinotropic peptide, growth hormone-releasing hormone and secretin are structurally related peptides that exert their actions through unique members of a structurally related G protein-coupled receptor class 2 family. This review discusses advances in our understanding of how these peptides exert their biological activities, with a focus on the biological actions and structural features of the cognate receptors. The receptors have been named after their parent and only physiologically relevant ligand, in line with the recommendations of the International Union of Pharmacology Committee on Receptor Nomenclature and Drug Classification (NC-IUPHAR).