Revisiting the role of glucagon in health, diabetes mellitus and other metabolic diseases

Insulin and glucagon exert opposing effects on glucose metabolism and, consequently, pancreatic islet β-cells and α-cells are considered functional antagonists. The intra-islet hypothesis has previously dominated the understanding of glucagon secretion, stating that insulin acts to inhibit the release of glucagon. By contrast, glucagon is a potent stimulator of insulin secretion and has been used to test β-cell function. Over the past decade, α-cells have received increasing attention due to their ability to stimulate insulin secretion from neighbouring β-cells, and α-cell-β-cell crosstalk has proven central for glucose homeostasis in vivo. Glucagon is not only the counter-regulatory hormone to insulin in glucose metabolism but also glucagon secretion is more susceptible to changes in the plasma concentration of certain amino acids than to changes in plasma concentrations of glucose. Thus, the actions of glucagon also include a central role in amino acid turnover and hepatic fat oxidation. This Review provides insights into glucagon secretion, with a focus on the local paracrine actions on glucagon and the importance of α-cell-β-cell crosstalk. We focus on dysregulated glucagon secretion in obesity, non-alcoholic fatty liver disease and type 2 diabetes mellitus. Lastly, the future potential of targeting hyperglucagonaemia and applying dual and triple receptor agonists with glucagon receptor-activating properties in combination with incretin hormone receptor agonism is discussed.

The glucagon receptor antagonist LY2409021 does not affect gastrointestinal-mediated glucose disposal or the incretin effect in individuals with and without type 2 diabetes

OBJECTIVE

Gastrointestinal-mediated glucose disposal (GIGD) during oral glucose tolerance test (OGTT) reflects the percentage of glucose disposal caused by mechanisms elicited by the oral route of glucose administration. GIGD is reduced in patients with type 2 diabetes (T2D) due to a reduced incretin effect and possibly also due to inappropriate suppression of glucagon after oral glucose. We investigated the effect of glucagon receptor antagonism on GIGD, the incretin effect and glucose excursions in patients with T2D and controls without diabetes.

DESIGN

A double-blind, randomised, placebo-controlled crossover study was conducted.

METHODS

Ten patients with T2D and 10 gender-, age- and BMI-matched controls underwent two 50 g OGTTs and 2 isoglycaemic i.v. glucose infusions, succeeding (~10 h) single-dose administration of 100 mg of the glucagon receptor antagonist LY2409021 or placebo, respectively.

RESULTS

Compared to placebo, LY2409021 reduced fasting plasma glucose in patients with T2D and controls. Plasma glucose excursions after oral glucose assessed by baseline-subtracted area under the curve were increased by LY2409021 compared to placebo in both groups, but no effect of LY2409021 on GIGD or the incretin effect was observed. LY2409021 increased fasting glucagon concentrations three-fold compared to placebo concentrations.

CONCLUSIONS

Glucagon receptor antagonism with LY2409021 had no effect on the impaired GIGD or the impaired incretin effect in patients with T2D and did also not affect these parameters in the controls. Surprisingly, we observed reduced oral glucose tolerance with LY2409021 which may be specific for this glucagon receptor antagonist.

The glucagon receptor antagonist LY2409021 has no effect on postprandial glucose in type 2 diabetes

OBJECTIVE

Type 2 diabetes (T2D) pathophysiology includes fasting and postprandial hyperglucagonemia, which has been linked to hyperglycemia via increased endogenous glucose production (EGP). We used a glucagon receptor antagonist (LY2409021) and stable isotope tracer infusions to investigate the consequences of hyperglucagonemia in T2D.

DESIGN

A double-blinded, randomized, placebo-controlled crossover study was conducted.

METHODS

Ten patients with T2D and ten matched non-diabetic controls underwent two liquid mixed meal tests preceded by single-dose administration of LY2409021 (100 mg) or placebo. Double-tracer technique was used to quantify EGP. Antagonist selectivity toward related incretin receptors was determined in vitro.

RESULTS

Compared to placebo, LY2409021 lowered the fasting plasma glucose (FPG) from 9.1 to 7.1 mmol/L in patients and from 5.6 to 5.0 mmol/L in controls (both P < 0.001) by mechanisms involving reduction of EGP. Postprandial plasma glucose excursions (baseline-subtracted area under the curve) were unaffected by LY2409021 in patients and increased in controls compared to placebo. Glucagon concentrations more than doubled during glucagon receptor antagonism. The antagonist interfered with both glucagon-like peptide 1 and glucose-dependent insulinotropic polypeptide receptors, complicating the interpretation of the postprandial data.

CONCLUSIONS

LY2409021 lowered FPG concentrations but did not improve postprandial glucose tolerance after a meal in patients with T2D and controls. The metabolic consequences of postprandial hyperglucagonemia are difficult to evaluate using LY2409021 because of its antagonizing effects on the incretin receptors.

The Role of Glucagon in the Acute Therapeutic Effects of SGLT2 Inhibition

Sodium-glucose cotransporter 2 inhibitors (SGLT2i) effectively lower plasma glucose (PG) concentration in patients with type 2 diabetes, but studies have suggested that circulating glucagon concentrations and endogenous glucose production (EGP) are increased by SGLT2i, possibly compromising their glucose-lowering ability. To tease out whether and how glucagon may influence the glucose-lowering effect of SGLT2 inhibition, we subjected 12 patients with type 2 diabetes to a randomized, placebo-controlled, double-blinded, crossover, double-dummy study comprising, on 4 separate days, a liquid mixed-meal test preceded by single-dose administration of either 1) placebo, 2) the SGLT2i empagliflozin (25 mg), 3) the glucagon receptor antagonist LY2409021 (300 mg), or 4) the combination empagliflozin + LY2409021. Empagliflozin and LY2409021 individually lowered fasting PG compared with placebo, and the combination further decreased fasting PG. Previous findings of increased glucagon concentrations and EGP during acute administration of SGLT2i were not replicated in this study. Empagliflozin reduced postprandial PG through increased urinary glucose excretion. LY2409021 reduced EGP significantly but gave rise to a paradoxical increase in postprandial PG excursion, which was annulled by empagliflozin during their combination (empagliflozin + LY2409021). In conclusion, our findings do not support that an SGLT2i-induced glucagonotropic effect is of importance for the glucose-lowering property of SGLT2 inhibition.

Efficacy and Safety of Glimepiride With or Without Linagliptin Treatment in Patients With HNF1A Diabetes (Maturity-Onset Diabetes of the Young Type 3): A Randomized, Double-Blinded, Placebo-Controlled, Crossover Trial (GLIMLINA)

OBJECTIVE

Sulfonylureas are first-line treatment of hepatocyte nuclear factor 1-α (HNF1A) diabetes (maturity-onset diabetes of the young type 3), but many patients do not achieve optimal glycemic control without episodes of hypoglycemia. We investigated the combination of the sulfonylurea glimepiride and the dipeptidyl peptidase 4 inhibitor linagliptin versus glimepiride monotherapy with respect to glycemic variability, glycemic control, and risk of hypoglycemia.

RESEARCH DESIGN AND METHODS

In a randomized, double-blinded, crossover trial, patients with HNF1A diabetes (n = 19; mean ± SD age 43 ± 14 years, BMI 24.8 ± 2.8 kg/m², and glycated hemoglobin [HbA_1c] 7.4 ± 0.2% [57.1 ± 7.3 mmol/mol]) were randomly assigned to treatment with glimepiride + linagliptin 5 mg (16 weeks), washout (4 weeks), and glimepiride + placebo (16 weeks) (or vice versa). Glimepiride was titrated targeting a fasting plasma glucose of 4.5-6.0 mmol/L without hypoglycemia. Treatments were evaluated by continuous glucose monitoring (CGM), HbA_1c, and meal test.

RESULTS

Compared with glimepiride + placebo, glimepiride + linagliptin did not significantly improve the primary end point, mean amplitude of glycemic excursions (MAGE) (mean difference -0.7 mmol/L, P = 0.1540), but displayed significant reductions in coefficient of variation on CGM (-3.6%, P = 0.0401), HbA_1c (-0.5%, P = 0.0048), and glimepiride dose (-0.7 mg/day, P = 0.0099). β-cell glucose sensitivity (assessed as C-peptide-to-glucose ratio) during meal test improved with glimepiride + linagliptin. Incidences of hypoglycemia were similar with both treatments.

CONCLUSIONS

Linagliptin as add-on treatment to glimepiride improved glycemic variability and control without increasing risk of hypoglycemia in patients with HNF1A diabetes.

GIP and GLP-1 Potentiate Sulfonylurea-Induced Insulin Secretion in Hepatocyte Nuclear Factor 1α Mutation Carriers

Sulfonylureas (SUs) provide an efficacious first-line treatment in patients with hepatocyte nuclear factor 1α (HNF1A) diabetes, but SUs have limitations due to risk of hypoglycemia. Treatment based on the incretin hormones glucose-dependent insulinotropic peptide (GIP) and glucagon-like peptide 1 (GLP-1) is characterized by their glucose-dependent insulinotropic actions without risk of hypoglycemia. The effect of SUs together with GIP or GLP-1, respectively, on insulin and glucagon secretion in patients with HNF1A diabetes is currently unknown. To investigate this, 10 HNF1A mutation carriers and 10 control subjects without diabetes were recruited for a double-blinded, placebo-controlled, crossover study including 6 experimental days in a randomized order involving 2-h euglycemic-hyperglycemic clamps with coadministration of: 1) SU (glimepiride 1 mg) or placebo, combined with 2) infusions of GIP (1.5 pmol/kg/min), GLP-1 (0.5 pmol/kg/min), or saline (NaCl). In HNF1A mutation carriers, we observed: 1) hypoinsulinemia, 2) insulinotropic effects of both GIP and GLP-1, 3) additive to supra-additive effects on insulin secretion when combining SU+GIP and SU+GLP-1, respectively, and 4) increased fasting and arginine-induced glucagon levels compared with control subjects without diabetes. Our study suggests that a combination of SU and incretin-based treatment may be efficacious in patients with HNF1A diabetes via potentiation of glucose-stimulated insulin secretion.

Glimepiride monotherapy versus combination of glimepiride and linagliptin therapy in patients with HNF1A-diabetes: a protocol for a randomised, double-blinded, placebo-controlled trial

INTRODUCTION

Hepatocyte nuclear factor 1α (HNF1A)-diabetes is the most common monogenetic subtype of diabetes. Strict glycaemic control is crucial for a good prognosis for patients with HNF1A-diabetes. Sulfonylurea (SU) is used as a first-line therapy in HNF1A-diabetes. However, SU therapy may be problematic as it confers a high risk of hypoglycaemia. We hypothesise that low dose of SU in combination with a dipeptidyl peptidase 4 inhibitor provides a safer and more efficacious treatment in patients with HNF1A-diabetes compared with SU as monotherapy.

METHODS AND ANALYSIS

In a randomised, double-blinded, crossover study, patients with HNF1A-diabetes will randomly be assigned to 16 weeks of treatment with glimepiride+linagliptin, 4 weeks of washout and 16 weeks of treatment with glimepiride+placebo (or vice versa). Treatment will be evaluated with continuous glucose monitoring and combined meal and bicycle tests conducted at baseline and at the end of each of the two treatment periods. The primary end point is the absolute difference in the mean amplitude of glycaemic excursions between the two treatments (glimepiride+linagliptin vs glimepiride+placebo) at the end of each treatment period.

ETHICS AND DISSEMINATION

The study protocol is approved by the Danish Medicines Agency, The Scientific-Ethical Committee of the Capital Region of Denmark (H-17014518) and the Danish Data Protection Agency. The trial will be carried out and monitored in compliance with Good Clinical Practice guidelines and in accordance with the latest version of the Declaration of Helsinki. Positive, negative and inconclusive results will be published at scientific conferences and as one or more scientific manuscripts in peer-reviewed journals with authorship in accordance with the International Committee of Medical Journal Editors' recommendations.

TRIAL REGISTRATION NUMBER

2017-000204-15.

[Diagnosis and treatment of maturity onset diabetes of the young type 3]

Maturity onset diabetes of the young type 3 (MODY3) is the most prevalent type of monogenetic diabetes. Treatment guidelines differ from both Type 1 diabetes and Type 2 diabetes. First-line treatment is a long-acting sulphonylurea, which lowers the plasma glucose level effectively, however with the risk of hypoglycaemia. When hypoglycaemia is a problem, short-acting sulphonylureas, glucagon-like peptide-1 receptor agonists and dipeptidyl peptidase-4 inhibitors may be used as alternatives. Metformin, glitazones and sodium glucose transporter 2-inhibitors have only limited applicability in MODY3. Further research needs to evaluate combinational therapy.

The Role of Glucagon in the Pathophysiology and Treatment of Type 2 Diabetes

Type 2 diabetes is a disease involving both inadequate insulin levels and increased glucagon levels. While glucagon and insulin work together to achieve optimal plasma glucose concentrations in healthy individuals, the usual regulatory balance between these 2 critical pancreatic hormones is awry in patients with diabetes. Although clinical discussion often focuses on the role of insulin, glucagon is equally important in understanding type 2 diabetes. Furthermore, an awareness of the role of glucagon is essential to appreciate differences in the mechanisms of action of various classes of glucose-lowering therapies. Newer drug classes such as dipeptidyl peptidase-4 inhibitors and glucagon-like peptide-1 receptor agonists improve glycemic control, in part, by affecting glucagon levels. This review provides an overview of the effect of glucose-lowering therapies on glucagon on the basis of an extensive PubMed literature search to identify clinical studies of glucose-lowering therapies in type 2 diabetes that included assessment of glucagon. Clinical practice currently benefits from available therapies that impact the glucagon regulatory pathway. As clinicians look to the future, improved treatment strategies are likely to emerge that will either use currently available therapies whose mechanisms of action complement each other or take advantage of new therapies based on an improved understanding of glucagon pathophysiology.

Hyperglucagonemia correlates with plasma levels of non-branched-chain amino acids in patients with liver disease independent of type 2 diabetes

Patients with type 2 diabetes (T2D) and patients with nonalcoholic fatty liver disease (NAFLD) frequently exhibit elevated plasma concentrations of glucagon (hyperglucagonemia). Hyperglucagonemia and α-cell hyperplasia may result from elevated levels of plasma amino acids when glucagon's action on hepatic amino acid metabolism is disrupted. We therefore measured plasma levels of glucagon and individual amino acids in patients with and without biopsy-verified NAFLD and with and without type T2D. Fasting levels of amino acids and glucagon in plasma were measured, using validated ELISAs and high-performance liquid chromatography, in obese, middle-aged individuals with I) normal glucose tolerance (NGT) and NAFLD, II) T2D and NAFLD, III) T2D without liver disease, and IV) NGT and no liver disease. Elevated levels of total amino acids were observed in participants with NAFLD and NGT compared with NGT controls (1,310 ± 235 µM vs. 937 ± 281 µM, P = 0.03) and in T2D and NAFLD compared with T2D without liver disease (1,354 ± 329 µM vs. 511 ± 235 µM, P < 0.0001). Particularly amino acids with known glucagonotropic effects (e.g., glutamine) were increased. Plasma levels of total amino acids correlated to plasma levels of glucagon also when adjusting for body mass index (BMI), glycated hemoglobin (Hb_A1c), and cholesterol levels (β = 0.013 ± 0.007, P = 0.024). Elevated plasma levels of total amino acids associate with hyperglucagonemia in NAFLD patients independently of glycemic control, BMI or cholesterol - supporting the potential importance of a "liver-α-cell axis" in which glucagon regulates hepatic amino acid metabolism. Fasting hyperglucagonemia as seen in T2D may therefore represent impaired hepatic glucagon action with increasing amino acids levels. NEW & NOTEWORTHY Hypersecretion of glucagon (hyperglucagonemia) has been suggested to be linked to type 2 diabetes. Here, we show that levels of amino acids correlate with levels of glucagon. Hyperglucagonemia may depend on hepatic steatosis rather than type 2 diabetes.