Two novel glucagon receptor antagonists prove effective therapeutic agents in high-fat-fed and obese diabetic mice

The glucagon receptor antagonist desHis1Pro4Glu9-glucagon(Lys12PAL) alters alpha-cell turnover and lineage in mice, but does not cause alpha-cell hyperplasia

OBJECTIVE

Glucagon receptor (GCGR) antagonism elicits antihyperglycemic effects in rodents and humans. The present study investigates whether the well characterised peptide-based GCGR antagonist, desHis¹Pro⁴Glu⁹-glucagon (Lys¹²PAL), alters alpha-cell turnover or identity in mice.

METHODS

Multiple low-dose streptozotocin (STZ) treated (50 mg/kg bw, 5 days) transgenic Glu^CreERT2;ROSA26-eYFP mice were employed. STZ mice received twice daily administration of saline vehicle or desHis¹Pro⁴Glu⁹-glucagon (Lys¹²PAL), at low- or high-dose (25 and 100 nmol/kg, respectively) for 11 days.

RESULTS

No GCGR antagonist induced changes in food or fluid intake, body weight or glucose homeostasis were observed. As expected, STZ dramatically reduced (P < 0.001) islet numbers and increased (P < 0.01) alpha-to beta-cell ratio, which was linked to elevated (P < 0.05) levels of beta-cell apoptosis. Whilst treatment with desHis¹Pro⁴Glu⁹-glucagon (Lys¹²PAL) decreased (P < 0.05-P < 0.001) alpha- and beta-cell areas, it also helped restore the classic rodent islet alpha-cell mantle in STZ mice. Interestingly, low-dose desHis¹Pro⁴Glu⁹-glucagon (Lys¹²PAL) increased (P < 0.05) alpha-cell apoptosis rates whilst high dose decreased (p < 0.05) this parameter. This difference reflects substantially increased (P < 0.001) alpha-to beta-cell transdifferentiation following high dose desHis¹Pro⁴Glu⁹-glucagon (Lys¹²PAL) treatment, which was not fully manifest with low-dose therapy.

CONCLUSIONS

Taken together, the present study indicates that peptidic GCGR antagonists can positively influence alpha-cell turnover and lineage in identity in multiple low-dose STZ mice, but that such effects are dose-related.

Upregulated TGF-β1 contributes to hyperglycaemia in type 2 diabetes by potentiating glucagon signalling

AIMS/HYPOTHESIS

Glucagon-stimulated hepatic gluconeogenesis contributes to endogenous glucose production during fasting. Recent studies suggest that TGF-β is able to promote hepatic gluconeogenesis in mice. However, the physiological relevance of serum TGF-β levels to human glucose metabolism and the mechanism by which TGF-β enhances gluconeogenesis remain largely unknown. As enhanced gluconeogenesis is a signature feature of type 2 diabetes, elucidating the molecular mechanisms underlying TGF-β-promoted hepatic gluconeogenesis would allow us to better understand the process of normal glucose production and the pathophysiology of this process in type 2 diabetes. This study aimed to investigate the contribution of upregulated TGF-β1 in human type 2 diabetes and the molecular mechanism underlying the action of TGF-β1 in glucose metabolism.

METHODS

Serum levels of TGF-β1 were measured by ELISA in 74 control participants with normal glucose tolerance and 75 participants with type 2 diabetes. Human liver tissue was collected from participants without obesity and with or without type 2 diabetes for the measurement of TGF-β1 and glucagon signalling. To investigate the role of Smad3, a key signalling molecule downstream of the TGF-β1 receptor, in mediating the effect of TGF-β1 on glucagon signalling, we generated Smad3 knockout mice. Glucose levels in Smad3 knockout mice were measured during prolonged fasting and a glucagon tolerance test. Mouse primary hepatocytes were isolated from Smad3 knockout and wild-type (WT) mice to investigate the underlying molecular mechanisms. Smad3 phosphorylation was detected by western blotting, levels of cAMP were detected by ELISA and levels of protein kinase A (PKA)/cAMP response element-binding protein (CREB) phosphorylation were detected by western blotting. The dissociation of PKA subunits was measured by immunoprecipitation.

RESULTS

We observed higher levels of serum TGF-β1 in participants without obesity and with type 2 diabetes than in healthy control participants, which was positively correlated with HbA_1c and fasting blood glucose levels. In addition, hyperactivation of the CREB and Smad3 signalling pathways was observed in the liver of participants with type 2 diabetes. Treating WT mouse primary hepatocytes with TGF-β1 greatly potentiated glucagon-stimulated PKA/CREB phosphorylation and hepatic gluconeogenesis. Mechanistically, TGF-β1 treatment induced the binding of Smad3 to the regulatory subunit of PKA (PKA-R), which prevented the association of PKA-R with the catalytic subunit of PKA (PKA-C) and led to the potentiation of glucagon-stimulated PKA signalling and gluconeogenesis.

CONCLUSIONS/INTERPRETATION

The hepatic TGF-β1/Smad3 pathway sensitises the effect of glucagon/PKA signalling on gluconeogenesis and synergistically promotes hepatic glucose production. Reducing serum levels of TGF-β1 and/or preventing hyperactivation of TGF-β1 signalling could be a novel approach for alleviating hyperglycaemia in type 2 diabetes.

Alpha-cells and therapy of diabetes: Inhibition, antagonism or death?

Absolute or relative hyperglucagonaemia is a characteristic of both Type 1 and Type 2 diabetes, resulting in fasting hyperglycaemia due in part to increased hepatic glucose production and lack of postprandial suppression of circulating glucagon concentrations. Consequently, therapeutics that target glucagon secretion or biological action may be effective antidiabetic agents. In this regard, specific glucagon receptor (GCGR) antagonists have been developed that exhibit impressive glucose-lowering actions, but unfortunately may cause off-target adverse effects in humans. Further to this, several currently approved antidiabetic agents, including GLP-1 mimetics, DPP-4 inhibitors, metformin, sulphonylureas and pramlintide likely exert part of their glucose homeostatic actions through direct or indirect inhibition of GCGR signalling. In addition to agents that inhibit the release of glucagon, compounds that enhance the transdifferentiation of glucagon secreting alpha-cells towards an insulin positive beta-cell phenotype could also help curb excess glucagon secretion in diabetes. Use of alpha-cell toxins represents another possible strategy to address hyperglucagonaemia in diabetes. In that respect, research from the 1920 s with diguanides such as synthalin A demonstrated effective glucose-lowering with alpha-cell ablation in both animal models and humans with diabetes. However, further clinical use of synthalin A was curtailed due its adverse effects and the increased availability of insulin. Overall, these observations with therapeutics that directly target alpha-cells, or GCGR signaling, highlight a largely untapped potential for diabetes therapy that merits further detailed consideration.

Sustained glucagon receptor antagonism in insulin-deficient high-fat-fed mice

Discerning modification to the amino acid sequence of native glucagon can generate specific glucagon receptor (GCGR) antagonists, that include desHis1Pro4Glu9-glucagon and the acylated form desHis1Pro4Glu9(Lys12PAL)-glucagon. In the current study, we have evaluated the metabolic benefits of once-daily injection of these peptide-based GCGR antagonists for 18 days in insulin-resistant high-fat-fed (HFF) mice with streptozotocin (STZ)-induced insulin deficiency, namely HFF-STZ mice. Administration of desHis1Pro4Glu9-glucagon moderately (P < 0.05) decreased STZ-induced elevations of food intake. Body weight was not different between groups of HFF-STZ mice and both treatment interventions delayed (P < 0.05) the onset of hyperglycaemia. The treatments reduced (P < 0.05-P < 0.001) circulating and pancreatic glucagon, whilst desHis1Pro4Glu9(Lys12PAL)-glucagon also substantially increased (P < 0.001) pancreatic insulin stores. Oral glucose tolerance was appreciably improved (P < 0.05) by both antagonists, despite the lack of augmentation of glucose-stimulated insulin release. Interestingly, positive effects on i.p. glucose tolerance were less obvious suggesting important beneficial effects on gut function. Metabolic benefits were accompanied by decreased (P < 0.05-P < 0.01) locomotor activity and increases (P < 0.001) in energy expenditure and respiratory exchange ratio in both treatment groups. In addition, desHis1Pro4Glu9-glucagon increased (P < 0.01-P < 0.001) O2 consumption and CO2 production. Together, these data provide further evidence that peptidic GCGR antagonists are effective treatment options for obesity-driven forms of diabetes, even when accompanied by insulin deficiency.

Insights into the Role of Glucagon Receptor Signaling in Metabolic Regulation from Pharmacological Inhibition and Tissue-Specific Knockout Models

While glucagon has long been recognized as the primary counter hormone to insulin’s actions, it has recently gained recognition as a metabolic regulator with its effects extending beyond control of glycemia. Recently developed models of tissue-specific glucagon receptor knockouts have advanced our understanding of this hormone, providing novel insight into the role it plays within organs as well as its systemic effects. Studies where the pharmacological blockade of the glucagon receptor has been employed have proved similarly valuable in the study of organ-specific and systemic roles of glucagon signaling. Studies carried out employing these tools demonstrate that glucagon indeed plays a role in regulating glycemia, but also in amino acid and lipid metabolism, systemic endocrine, and paracrine function, and in the response to cardiovascular injury. Here, we briefly review recent progress in our understanding of glucagon’s role made through inhibition of glucagon receptor signaling utilizing glucagon receptor antagonists and tissue specific genetic knockout models.

Role of Glucagon and Its Receptor in the Pathogenesis of Diabetes

Various theories for the hormonal basis of diabetes have been proposed and debated over the past few decades. Insulin insufficiency was previously regarded as the only hormone deficiency directly leading to metabolic disorders associated with diabetes. Although glucagon and its receptor are ignored in this framework, an increasing number of studies have shown that they play essential roles in the development and progression of diabetes. However, the molecular mechanisms underlying the effects of glucagon are still not clear. In this review, recent research on the mechanisms by which glucagon and its receptor contribute to the pathogenesis of diabetes as well as correlations between GCGR mutation rates in populations and the occurrence of diabetes are summarized. Furthermore, we summarize how recent research clearly establishes glucagon as a potential therapeutic target for diabetes.

Weight-reducing, lipid-lowering and antidiabetic activities of a novel arginine vasopressin analogue acting at the V1a and V1b receptors in high-fat-fed mice

AIM

To assess the beneficial metabolic effects of the nonapeptide hormone, arginine vasopressin (AVP), on metabolism.

MATERIALS AND METHODS

We exchanged amino acids at position 3 and 8 of AVP, namely phenylalanine and arginine, with those of oxytocin, to generate novel analogues with altered receptor selectivity. Secondary modification by N-terminal acetylation was used to impart stability to circulating endopeptidases. Analogues were screened for degradation, bioactivity in rodent/human clonal beta cells and primary murine islets, together with evaluation of receptor activation profile.

RESULTS

Analogue Ac3IV, which lacked effects at the V2 receptors responsible for modulation of fluid balance, was selected as the lead compound for assessment of antidiabetic efficacy in high-fat-fed mice. Twice-daily administration of Ac3IV, or the gold standard control exendin-4, for 22 days, reduced energy intake as well as body weight and fat content. Both interventions decreased circulating glucose levels, enhanced insulin sensitivity, and substantially improved glucose tolerance and related insulin secretion in response to an intraperitoneal or oral glucose challenge. The peptides decreased total- and increased HDL-cholesterol, but only Ac3IV decreased LDL-cholesterol, triglyceride and non-fasting glucagon concentrations. Elevations of islet and beta-cell areas were partially reversed, accompanied by suppressed islet cell proliferation, decreased beta-cell apoptosis and, in the case of exendin-4, also decreased alpha-cell apoptosis.

CONCLUSION

AVP-based therapies that exclusively target V1a and V1b receptors may have significant therapeutic potential for the treatment of obesity and related diabetes, and merit further clinical exploration.

Enzymatically stable analogue of the gut-derived peptide xenin on beta-cell transdifferentiation in high fat fed and insulin-deficient Ins1Cre/+ ;Rosa26-eYFP mice

BACKGROUND

The antidiabetic effects of the gut hormone xenin include augmenting insulin secretion and positively affecting pancreatic islet architecture.

METHODS

The current study has further probed pancreatic effects through sub-chronic administration of the long-acting xenin analogue, xenin-25[Lys¹³ PAL], in both high fat fed (HFF) and streptozotocin (STZ)-induced insulin-deficient Ins1^Cre/+ ;Rosa26-eYFP transgenic mice. Parallel effects on metabolic control and pancreatic islet morphology, including islet beta-cell lineage tracing were also assessed.

RESULTS

Xenin-25[Lys¹³ PAL] treatment reversed body weight loss induced by STZ, increased plasma insulin and decreased blood glucose levels. There were less obvious effects on these parameters in HFF mice, but all xenin-25[Lys¹³ PAL] treated mice exhibited decreased pancreatic alpha-cell areas and circulating glucagon. Xenin-25[Lys¹³ PAL] treatment fully, or partially, returned overall islet and beta-cell areas in STZ- and HFF mice to those of lean control animals, respectively, and was consistently associated with decreased beta-cell apoptosis. Interestingly, xenin-25[Lys¹³ PAL] also increased beta-cell proliferation and decreased alpha-cell apoptosis in STZ mice, with reduced alpha-cell growth noted in HFF mice. Lineage tracing studies revealed that xenin-25[Lys¹³ PAL] reduced the number of insulin positive pancreatic islet cells that lost their beta-cell identity, in keeping with a decreased transition of insulin positive to glucagon positive cells. These beneficial effects on islet cell differentiation were linked to maintained expression of Pdx1 within beta-cells. Xenin-25[Lys¹³ PAL] treatment was also associated with increased numbers of smaller sized islets in both models.

CONCLUSIONS

Benefits of xenin-25[Lys¹³ PAL] on diabetes includes positive modulation of islet cell differentiation, in addition to promoting beta-cell growth and survival.

Genetic activation of α-cell glucokinase in mice causes enhanced glucose-suppression of glucagon secretion during normal and diabetic states

Objective

While the molecular events controlling insulin secretion from β-cells have been documented in detail, the exact mechanisms governing glucagon release by α-cells are understood only partially. This is a critical knowledge gap, as the normal suppression of glucagon secretion by elevated glucose levels fails in type 2 diabetes (T2D) patients, contributing to hyperglycemia through stimulation of hepatic glucose production. A critical role of glycolytic flux in regulating glucagon secretion was supported by recent studies in which manipulation of the activity and expression of the glycolytic enzyme glucokinase altered the setpoint for glucose-suppression of glucagon secretion (GSGS). Given this precedent, we hypothesized that genetic activation of glucokinase specifically in α-cells would enhance GSGS and mitigate T2D hyperglucagonemia.

Methods

We derived an inducible, α-cell-specific glucokinase activating mutant mouse model (Gck^{LoxPGck∗/LoxPGck∗}; Gcg-CreER^T2; henceforth referred to as “α-mutGCK”) in which the wild-type glucokinase gene (GCK) is conditionally replaced with a glucokinase mutant allele containing the ins454A activating mutation (Gck∗), a mutation that increases the affinity of glucokinase for glucose by almost 7-fold. The effects of α-cell GCK activation on glucose homeostasis, hormone secretion, islet morphology, and islet numbers were assessed using both in vivo and ex vivo assays. Additionally, the effect of α-cell GCK activation on GSGS was investigated under diabetogenic conditions of high-fat diet (HFD) feeding that dysregulate glucagon secretion.

Results

Our study shows that α-mutGCK mice have enhanced GSGS in vivo and ex vivo, independent of alterations in insulin levels and secretion, islet hormone content, islet morphology, or islet number. α-mutGCK mice maintained on HFD displayed improvements in glucagonemia compared to controls, which developed the expected obesity, glucose intolerance, elevated fasting blood glucose, hyperinsulinemia, and hyperglucagonemia.

Conclusions

Using our novel α-cell specific activation of GCK mouse model, we have provided additional support to demonstrate that the glycolytic enzyme glucokinase is a key determinant in glucose sensing within α-cells to regulate glucagon secretion. Our results contribute to our fundamental understanding of α-cell biology by providing greater insight into the regulation of glucagon secretion through α-cell intrinsic mechanisms via glucokinase. Furthermore, our HFD results underscore the potential of glucokinase as a druggable target which, given the ongoing development of allosteric glucokinase activators (GKAs) for T2D treatment, could help mitigate hyperglucagonemia and potentially improve blood glucose homeostasis.

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Inducible and cell type-specific point mutation in glucokinase enables analysis of glucose suppression of glucagon secretion.

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Glycolytic flux through glucokinase determines the set-point for glucagon secretion in pancreatic α-cells.

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Pancreatic α-cells are a physiologically relevant target of glucokinase activator drugs.

Genistein diet improves body weight, serum glucose and triglyceride levels in both male and female ob/ob mice

PURPOSE

Diabetic obesity in the leptin-deficient ob/ob mouse is associated with weight gain, and hyperglycemia, along with hyperinsulinemia. We have previously examined the effects of genistein (a naturally occurring isoflavone found in soy) on metabolic disturbances in the ob/ob mouse and demonstrated beneficial effects of genistein (600 mg genistein/kg diet, for 4-weeks) on T₃ production and corticosterone status. The goal of this study was to examine whether dietary genistein could prevent, or at least lessen, the typical phenotype in this murine model of diabetic-obesity, and to assess potential sex-differences.

PATIENTS AND METHODS

The ob/ob mice (male and female) aged 4-5 weeks were randomly assigned to one of two diets for a period of 4-weeks: standard rodent diet, or genistein-containing diet (600 mg genistein/kg diet). Comparisons were made to a lean control group.

RESULTS

Genistein diet significantly reduced body weight by 12% in females and 9% in males. Genistein significantly lowered serum glucose levels by 18% in females and 43% in males, yet had no effect on serum insulin. Genistein diet significantly lowered serum triglyceride levels in both ob/ob male and female mice returning them to lean levels. In females only, genistein significantly reduced serum pancreatic polypeptide levels by 56% and increased serum GIP levels 2.3-fold. Genistein had sex-dependent effects on hepatic steatosis: in females, genistein further increased the % fat area and the fat droplet diameter 2.6-fold, along with additionally increasing hepatic TBARS.

CONCLUSION

The results from this study indicate interesting beneficial effects of genistein diet for both male and female ob/ob mice.

Long-term treatment with acylated analogues of apelin-13 amide ameliorates diabetes and improves lipid profile of high-fat fed mice

Previous studies have shown that modified apelin analogues exhibited enzyme resistance in plasma and improved circulating half-life compared to apelin-13. This study investigated the antidiabetic effects of chronic administration of stable long acting fatty acid modified apelin analogues, namely, (Lys8GluPAL)apelin-13 amide and pGlu(Lys8GluPAL)apelin-13 amide, in high-fat fed obese-diabetic mice. Male NIH Swiss mice (groups n = 8) were maintained either on a high-fat diet (45% fat) from 8 to 28 weeks old, or control mice were fed a normal diet (10% fat). When diet induced obesity-diabetes was established after high-fat feeding, mice were injected i.p. once daily with apelin analogues, liraglutide (25 nmol/kg) or saline (controls). Administration of (Lys8GluPAL)apelin-13 amide and pGlu(Lys8GluPAL)apelin-13 amide for 28 days significantly reduced food intake and decreased body weight. Non-fasting glucose was reduced (p<0.01 to p<0.001) and plasma insulin concentrations increased (p<0.01 to p<0.001). This was accompanied by enhanced insulin responses (p<0.01 to p<0.001) and significant reductions in glucose excursion after oral (p<0.01) or i.p. (p<0.01) glucose challenges and feeding. Apelin analogues also significantly improved HbA1c (p<0.01), enhanced insulin sensitivity (p<0.01), reduced triglycerides (p<0.001), increased HDL-cholesterol (p<0.01) and decreased LDL-cholesterol (p<0.01), compared to high-fat fed saline treated control mice. Cholesterol levels were decreased (p<0.01) by pGlu(Lys8GluPAL)apelin-13 amide and both apelin treated groups showed improved bone mineral content, reduced fat deposits and increased plasma GLP-1. Daily treatment with liraglutide mirrored many of these changes (not on bone or adipose tissue), but unlike apelin analogues increased plasma amylase. Consumption of O2, production of CO2, respiratory exchange ratio and energy expenditure were improved by apelin analogues. These results indicate that long-term treatment with acylated analogues (Lys8GluPAL)apelin-13 amide and particularly pGlu(Lys8GluPAL)apelin-13 amide resulted in similar or enhanced therapeutic responses to liraglutide in high-fat fed mice. Fatty acid derived apelin analogues represent a new and exciting development in the treatment of obesity-diabetes.

MafB Is Critical for Glucagon Production and Secretion in Mouse Pancreatic α Cells In Vivo

The MafB transcription factor is expressed in pancreatic α and β cells during development but becomes exclusive to α cells in adult rodents. Mafb-null (Mafb^-/- ) mice were reported to have reduced α- and β-cell numbers throughout embryonic development. To further analyze the postnatal function of MafB in the pancreas, we generated endocrine cell-specific (Mafb^ΔEndo ) and tamoxifen-dependent (Mafb^ΔTAM ) Mafb knockout mice. Mafb^ΔEndo mice exhibited reduced populations of insulin-positive (insulin⁺) and glucagon⁺ cells at postnatal day 0, but the insulin⁺ cell population recovered by 8 weeks of age. In contrast, the Arx⁺ glucagon⁺ cell fraction and glucagon expression remained decreased even in adulthood. Mafb^ΔTAM mice, with Mafb deleted after pancreas maturation, also demonstrated diminished glucagon⁺ cells and glucagon content without affecting β cells. A decreased Arx⁺ glucagon⁺ cell population in Mafb^ΔEndo mice was compensated for by an increased Arx⁺ pancreatic polypeptide⁺ cell population. Furthermore, gene expression analyses from both Mafb^ΔEndo and Mafb^ΔTAM islets revealed that MafB is a key regulator of glucagon expression in α cells. Finally, both mutants failed to respond to arginine, likely due to impaired arginine transporter gene expression and glucagon production ability. Taken together, our findings reveal that MafB is critical for the functional maintenance of mouse α cells in vivo, including glucagon production and secretion, as well as in development.

Alpha cell dysfunction in type 1 diabetes

Type 1 diabetes is characterized by selective loss of beta cells and insulin secretion, which significantly impact glucose homeostasis. However, this progressive disease is also associated with dysfunction of the alpha cell component of the islet, which can exacerbate hyperglycemia due to paradoxical hyperglucagonemia or lead to severe hypoglycemia as a result of failed counterregulation. In this review, the physiology of alpha cell secretion and the potential mechanisms underlying alpha cell dysfunction in type 1 diabetes will be explored. Because type 1 diabetes is a progressive disease, a synthesized timeline of aberrant alpha cell function will be presented as an attempt to delineate the natural history of type 1 diabetes with respect to the alpha cell.

Investigational glucagon receptor antagonists in Phase I and II clinical trials for diabetes

INTRODUCTION

Despite type 2 diabetes (T2D) being recognized as a bihormonal pancreatic disease, current therapies are mainly focusing on insulin, while targeting glucagon has been long dismissed. However, glucagon receptor (GCGr) antagonists are currently investigated in clinical trials. Area covered: Following a brief description of the rationale for antagonizing GCGr in T2D, lessons from GCGr knock-out mice and pharmacological means to antagonize GCGr, a detailed description of the main results obtained with GCGr antagonists in Phase I-II clinical trials is provided. The development of several small molecules has been discontinued, while new ones are currently considered as well as innovative approaches such as monoclonal antibodies or antisense oligonucleotides inhibiting GCGr gene expression. Their potential benefits but also limitations are discussed. Expert opinion: The proof-of-concept that antagonizing GCGr improves glucose control in T2D has been confirmed in humans. Nevertheless, some adverse events led to stopping the development of some of these GCGr antagonists. New approaches seem to have a better benefit/risk balance, although none has progressed to Phase III clinical trials so far. Pharmacotherapy of T2D is becoming a highly competitive field so that GCGr antagonists should provide clear advantages over numerous existing glucose-lowering medications before eventually reaching clinical practice.

Hypoglycemic Effect of Combined Ghrelin and Glucagon Receptor Blockade

Glucagon receptor (GcgR) blockade has been proposed as an alternative to insulin monotherapy for treating type 1 diabetes since deletion or inhibition of GcgRs corrects hyperglycemia in models of diabetes. The factors regulating glycemia in a setting devoid of insulin and glucagon function remain unclear but may include the hormone ghrelin. Not only is ghrelin release controlled by glucose but also ghrelin has many actions that can raise or reduce falls in blood glucose level. Here, we tested the hypothesis that ghrelin rises to prevent hypoglycemia in the absence of glucagon function. Both GcgR knockout (Gcgr^-/-) mice and db/db mice that were administered GcgR monoclonal antibody displayed lower blood glucose levels accompanied by elevated plasma ghrelin levels. Although treatment with the pancreatic β-cell toxin streptozotocin induced hyperglycemia and raised plasma ghrelin levels in wild-type mice, hyperglycemia was averted in similarly treated Gcgr^-/- mice and the plasma ghrelin level was further increased. Notably, administration of a ghrelin receptor antagonist further reduced blood glucose levels into the markedly hypoglycemic range in overnight-fasted, streptozotocin-treated Gcgr^-/- mice. A lowered blood glucose level also was observed in overnight-fasted, streptozotocin-treated ghrelin receptor-null mice that were administered GcgR monoclonal antibody. These data suggest that when glucagon activity is blocked in the setting of type 1 diabetes, the plasma ghrelin level rises, preventing hypoglycemia.

Targeting hepatic glucose metabolism in the treatment of type 2 diabetes

Type 2 diabetes mellitus is characterized by the dysregulation of glucose homeostasis, resulting in hyperglycaemia. Although current diabetes treatments have exhibited some success in lowering blood glucose levels, their effect is not always sustained and their use may be associated with undesirable side effects, such as hypoglycaemia. Novel antidiabetic drugs, which may be used in combination with existing therapies, are therefore needed. The potential of specifically targeting the liver to normalize blood glucose levels has not been fully exploited. Here, we review the molecular mechanisms controlling hepatic gluconeogenesis and glycogen storage, and assess the prospect of therapeutically targeting associated pathways to treat type 2 diabetes.

Glucagon actions on the kidney revisited: possible role in potassium homeostasis

It is now recognized that the metabolic disorders observed in diabetes are not, or not only due to the lack of insulin or insulin resistance, but also to elevated glucagon secretion. Accordingly, selective glucagon receptor antagonists are now proposed as a novel strategy for the treatment of diabetes. However, besides its metabolic actions, glucagon also influences kidney function. The glucagon receptor is expressed in the thick ascending limb, distal tubule, and collecting duct, and glucagon regulates the transepithelial transport of several solutes in these nephron segments. Moreover, it also influences solute transport in the proximal tubule, possibly by an indirect mechanism. This review summarizes the knowledge accumulated over the last 30 years about the influence of glucagon on the renal handling of electrolytes and urea. It also describes a possible novel role of glucagon in the short-term regulation of potassium homeostasis. Several original findings suggest that pancreatic α-cells may express a “potassium sensor” sensitive to changes in plasma K concentration and could respond by adapting glucagon secretion that, in turn, would regulate urinary K excretion. By their combined actions, glucagon and insulin, working in a combinatory mode, could ensure an independent regulation of both plasma glucose and plasma K concentrations. The results and hypotheses reviewed here suggest that the use of glucagon receptor antagonists for the treatment of diabetes should take into account their potential consequences on electrolyte handling by the kidney.

Glucagon receptor antagonist and GIP agonist combination for diet-induced obese mice

Ablation of glucagon receptor signaling represents a potential treatment option for type 2 diabetes (T2DM). Additionally, activation of glucose-dependent insulinotropic polypeptide (GIP) receptor signaling also holds therapeutic promise for T2DM. Therefore, this study examined both independent and combined metabolic actions of desHis(1)Pro(4)Glu(9)(Lys(12)PAL)-glucagon (glucagon receptor antagonist) and d-Ala(2)GIP (GIP receptor agonist) in diet-induced obese mice. Glucagon receptor binding has been linked to alpha-helical structure and desHis(1)Pro(4)Glu(9)(Lys(12)PAL)-glucagon displayed enhanced alpha-helical content compared with native glucagon. In clonal pancreatic BRIN-BD11 beta-cells, desHis(1)Pro(4)Glu(9)(Lys(12)PAL)-glucagon was devoid of any insulinotropic or cAMP-generating actions, and did not impede d-Ala(2)GIP-mediated (P<0.01 to P<0.001) effects on insulin and cAMP production. Twice-daily injection of desHis(1)Pro(4)Glu(9)(Lys(12)PAL)-glucagon or d-Ala(2)GIP alone, and in combination, in high-fat-fed mice failed to affect body weight or energy intake. Circulating blood glucose levels were significantly (P<0.05 to P<0.01) decreased by all treatments regimens, with plasma and pancreatic insulin elevated (P<0.05 to P<0.001) in all mice receiving d-Ala(2)GIP. Interestingly, plasma glucagon concentrations were decreased (P<0.05) by sustained glucagon inhibition (day 28), but increased (P<0.05) by d-Ala(2)GIP therapy, with a combined treatment resulting in glucagon concentration similar to saline controls. All treatments improved (P<0.01) intraperitoneal and oral glucose tolerance, and peripheral insulin sensitivity. d-Ala(2)GIP-treated mice showed increased glucose-induced insulin secretion in response to intraperitoneal and oral glucose. Metabolic rate and ambulatory locomotor activity were increased (P<0.05 to P<0.001) in all desHis(1)Pro(4)Glu(9)(Lys(12)PAL)-glucagon-treated mice. These studies highlight the potential of glucagon receptor inhibition alone, and in combination with GIP receptor activation, for T2DM treatment.

THE ROLE OF GLUCAGON IN THE PATHOPHYSIOLOGY AND MANAGEMENT OF DIABETES

OBJECTIVE

There is general recognition that insulin and glucagon are the main hormones involved in the pathophysiology of diabetes, but the role of glucagon in diabetes is complex and in some circumstances controversial. The increasing appreciation of the role of glucagon in currently used hypoglycemic agents and the ongoing development of glucagon-targeted therapies underscores glucagon's important contribution in optimizing diabetes management. The current review provides a background on glucagon physiology and pathophysiology and an update for investigators, endocrinologists, and other healthcare providers on glucagon-modulating therapies.

METHODS

A literature review was conducted utilizing published literature in PubMed and AccessMedicine including the years 1922-2015 using the following key words: glucagon, bihormonal, diabetes mellitus, glucagon antagonists, glucagon-targeted therapies.

RESULTS

Glucagon is a counterregulatory hormone that promotes hepatic glucose production, thus preventing hypoglycemia in normal physiology. In patients with diabetes mellitus, glucagon secretion may be unregulated, which contributes to problems with glucose homeostasis. Several of the most effective therapies for diabetes have been found to suppress glucagon secretion or action, which may contribute to their success. Additionally, glucagon-specific targeted therapies, such as glucagon receptor antagonists, are being studied at a basic and clinical level.

CONCLUSION

Glucagon plays an important role in contributing to hyperglycemia in patients with diabetes. Utilizing hypoglycemic agents that decrease glucagon secretion or inhibit glucagon action can help improve glycemic control, making these agents a valuable resource in diabetes therapy.

Inhibiting or antagonizing glucagon: making progress in diabetes care

Absolute or relative hyperglucagonaemia has been recognized for years in all experimental or clinical forms of diabetes. It has been suggested that excess secretion of glucagon by the islet α cells is a direct consequence of intra-islet insulin secretory defects. Recent studies have shown that knockout of the glucagon receptor or administration of a monoclonal specific glucagon receptor antibody make insulin-deficient type 1 diabetic rodents thrive without insulin. These observations suggest that glucagon plays an essential role in the pathophysiology of diabetes and that targeting the α cell and glucagon are innovative approaches in the management of diabetes. Despite active research and identification of promising compounds, no one selective glucagon antagonist is presently used in the treatment of diabetes. Interestingly, besides insulin, several drugs used today in the management of diabetes appear to exert their effects, in part, by inhibiting glucagon secretion (glucagon-like peptide-1 receptor agonists, dipeptidyl peptidase-4 inhibitors, α-glucosidase inhibitors and, possibly, sulphonylureas) or glucagon action (metformin). The potential risks associated with total glucagon suppression include α-cell hyperplasia, increased mass of the pancreas, increased susceptibility to hepatosteatosis and hepatocellular injury and increased risk of hypoglycaemia, and these should be considered in the search and development of new compounds reducing glucagon receptor signalling. More than 40 years after its initial description, hyperglucagonaemia in diabetes can no longer be ignored or minimized, and its correction represents an attractive way to improve diabetes management.

Pancreatic α-Cell Dysfunction in Type 2 Diabetes: Old Kids on the Block

Type 2 diabetes (T2D) has been known as 'bi-hormonal disorder' since decades ago, the role of glucagon from α-cell has languished whereas β-cell taking center stage. Recently, numerous findings indicate that the defects of glucagon secretion get involve with development and exacerbation of hyperglycemia in T2D. Aberrant α-cell responses exhibit both fasting and postprandial states: hyperglucagonemia contributes to fasting hyperglycemia caused by inappropriate hepatic glucose production, and to postprandial hyperglycemia owing to blunted α-cell suppression. During hypoglycemia, insufficient counter-regulation response is also observed in advanced T2D. Though many debates still remained for exact mechanisms behind the dysregulation of α-cell in T2D, it is clear that the blockade of glucagon receptor or suppression of glucagon secretion from α-cell would be novel therapeutic targets for control of hyperglycemia. Whereas there have not been remarkable advances in developing new class of drugs, currently available glucagon-like peptide-1 and dipeptidyl peptidase-IV inhibitors could be options for treatment of hyperglucagonemia. In this review, we focus on α-cell dysfunction and therapeutic potentials of targeting α-cell in T2D.