Opposing effects of chronic glucagon receptor agonism and antagonism on amino acids, hepatic gene expression, and alpha cells

Hepatic PKA Mediates Liver and Pancreatic α-Cell Cross Talk

Glucagon stimulates hepatic glucose production, in part by promoting the uptake and catabolism of amino acids. Inhibition of the liver glucagon receptor (GCGR) results in elevated plasma amino acids, which triggers the proliferation of pancreatic α-cells, forming a liver-α-cell loop. This study aims to delineate hepatic signaling molecules downstream of GCGR that mediate the liver-α-cell loop. We knocked down liver GCGR, its G-coupled protein GNAS, and two GNAS downstream effectors, PKA and EPAC2 (RAPGEF4). Mice with GCGR, GNAS, and PKA knockdown had similar suppression of hepatic amino acid catabolism genes, hyperaminoacidemia, and α-cell hyperplasia, but those with EPAC2 knockdown did not. We then demonstrated that activating liver PKA was sufficient to reverse hyperaminoacidemia and α-cell hyperplasia caused by GCGR blockade. These results suggest that liver GCGR signals through PKA to control amino acid metabolism and that hepatic PKA plays a critical role in the liver-α-cell loop.

ARTICLE HIGHLIGHTS

A liver-α-cell loop exists, where inhibition of the liver glucagon receptor (GCGR) causes hyperaminoacidemia and pancreatic α-cell hyperplasia, but the GCGR downstream factors responsible for these effects are not clear. We silenced GCGR, its G-coupled protein GNAS, and two GNAS downstream effectors, PKA and EPAC2, to assess their effects on the liver-α-cell loop. Inhibition of the GCGR-GNAS-PKA pathway suppresses amino acid catabolism and causes α-cell hyperplasia, whereas PKA activation promotes amino acid catabolism and reduces alpha cell mass even when GCGR is blocked. Our study establishes hepatic PKA as the critical regulator of the liver-α-cell loop.

Allosteric communication mechanism in the glucagon receptor

Recent drug development suggests agonists may be more promising against glucagon receptor dysregulation in metabolic disorders. Allosteric modulation may pave an alternative way to initiate responses that are required to target these metabolic disorders. Here, we investigated the allosteric communication mechanisms within the glucagon receptor using molecular dynamics simulations on five receptor states. Results highlighted that the extracellular domain is dynamic in the absence of an orthosteric agonist. In the presence of a partial agonist, we observed increased flexibility in the N terminus of the receptor compared with the full agonist-bound receptor. Class B1 G protein-coupled receptor (GPCR) microswitches showed repacking going from the inactive state to the active state, allowing for G protein coupling. In the full agonist- and G protein-bound state, Gαs showed both translational and rotational movement in the N terminus, core, and α5-helix, thereby forming key interactions between the core of the G protein and the receptor. Finally, the allosteric communication from the extracellular region to the G protein coupling region of the receptor was the strongest in the intracellular negative allosteric modulator-bound state, the full agonist and G protein-bound state, and the full agonist-bound G protein-free state. The residue positions predicted to play a significant role in the allosteric communication mechanism showed overlap with disease-associated mutations. Overall, our study provides insights into the allosteric communication mechanism in a class B1 GPCR, which sets the foundation for future design of allosteric modulators targeting the glucagon receptor.

The neglected PCK1/glucagon (inter)action in nutrient homeostasis beyond gluconeogenesis: Disease pathogenesis and treatment

BACKGROUND

Glucagon plays a central role in hepatic adaptation during fasting, with the upregulation of hepatic phosphoenolpyruvate carboxykinase 1 (PCK1) traditionally associated with increased gluconeogenesis. However, recent experimental models and clinical studies have challenged this view, suggesting a more complex interplay between PCK1 and glucagon, which extends beyond gluconeogenesis and has broader implications for metabolic regulation in health and disease.

SCOPE OF REVIEW

This review provides a comprehensive overview of the current evidence on the multifaceted roles of PCK1 in glucagon-dependent hepatic adaptation during fasting, which is crucial for maintaining systemic homeostasis not only of glucose, but also of lipids and amino acids. We explore the relationship between PCK1 deficiency and glucagon resistance in metabolic disorders, including inherited PCK1 deficiency and metabolic dysfunction-associated steatotic liver disease (MASLD), and compare findings from experimental animal models with whole-body or tissue-specific ablation of PCK1 or the glucagon receptor. We propose new research platforms to advance the therapeutic potential of targeting PCK1 in metabolic diseases.

MAJOR CONCLUSIONS

We propose that hepatic PCK1 deficiency might be an acquired metabolic disorder linking alterations in lipid metabolism with impaired glucagon signaling. Our findings highlight interesting links between glycerol, PCK1 deficiency, elevated plasma alanine levels and glucagon resistance. We conclude that the roles of PCK1 and glucagon in metabolic regulation are more complex than previously assumed. In this (un)expected scenario, hepatic PCK1 deficiency and glucagon resistance appear to exert limited control over glycemia, but have broader metabolic effects related to lipid and amino acid dysregulation. Given the shift in glucagon research from receptor inhibition to activation, we propose that a similar paradigm shift is needed in the study of hepatic PCK1. Understanding PCK1 expression and activity in the glucagon-dependent hepatic adaptation to fasting might provide new perspectives and therapeutic opportunities for metabolic diseases.

Glucagon, Metabolic Dysfunction-Associated Steatotic Liver Disease and Amino Acids in Humans and Animals without Diabetes Mellitus-An Evidence Map

INTRODUCTION

Health systems are confronted with not only the growing worldwide childhood obesity epidemic but also associated comorbidities. These subsequently cause variations in distinct metabolic pathways, leading to metabolic dysfunction-associated steatotic liver disease (MASLD). The aim of this evidence map is to systematically evaluate the evidence and to identify research gaps on glucagon-induced amino acid (AA) turnover and its metabolic interaction with MASLD.

METHODOLOGY

A systematic literature search was conducted up to April 2023 in three electronic databases. Studies were required to include at least two of the main research areas, glucagon, AA metabolism and MASLD. Two independent reviewers screened titles and abstracts according to prespecified eligibility criteria, as well as full-text articles. Results are summarized in tables stratified by human and animal studies and study population age.

RESULTS

Thirty-four references were ultimately included. The publication years dated back to 1965 showed a great increase from 2012 to 2023. In total, there were 19 animal studies and 15 human studies. Among the human studies, except for two studies in adolescents, all the studies were conducted in adults. In human studies, the methods used to evaluate metabolic changes differed among hyperinsulinemic-euglycemic clamp and oral glucose tolerance tests. Thirteen studies focused on the metabolic effects of MASLD, while only two studies explored the interaction between MASLD, glucagon and AA metabolism in humans. The other 19 studies focused on metabolomics, beta cell function or just one topic of a research area and not on interactions between one another.

CONCLUSION

Research on the interaction between MASLD, glucagon and AA metabolism in humans is sparse and complete lacking in pediatrics. Furthermore, longitudinal studies with a focus on hyperglucagonemia independent of diabetes but related to MASLD present an unambiguous research gap.

Glucagon Resistance in Individuals With Obesity and Hepatic Steatosis Can Be Measured Using the GLUSENTIC Test and Index

Increased plasma levels of glucagon (hyperglucagonemia) promote diabetes development but are also observed in patients with metabolic dysfunction-associated steatotic liver disease (MASLD). This may reflect hepatic glucagon resistance toward amino acid catabolism. A clinical test for measuring glucagon resistance has not been validated. We evaluated our glucagon sensitivity (GLUSENTIC) test, which consists of 2 study days: a glucagon injection and measurements of plasma amino acids and an infusion of mixed amino acids and subsequent calculation of the GLUSENTIC index (primary outcome measure) from measurements of glucagon and amino acids. To distinguish glucagon-dependent from insulin-dependent actions on amino acid metabolism, we also studied patients with type 1 diabetes (T1D). The δ-decline in total amino acids was 49% lower in MASLD following exogenous glucagon (P = 0.01), and the calculated GLUSENTIC index was 34% lower in MASLD (P < 0.0001) but not T1D (P > 0.99). In contrast, glucagon-induced glucose increments were similar in control participants and participants with MASLD (P = 0.41). The GLUSENTIC test and index may be used to measure glucagon resistance in individuals with obesity and MASLD.

ARTICLE HIGHLIGHTS

Hepatic steatosis and not type 2 diabetes, body mass index, or hepatic fibrosis associates with hyperglucagonemia in individuals with steatotic liver disease

Increased plasma concentrations of glucagon (hyperglucagonemia) are reported in patients with type 2 diabetes (T2D) and are considered a diabetogenic risk factor. Emerging evidence suggests that hepatic steatosis in obesity is causing a condition of resistance toward glucagon's effects on amino acid metabolism, resulting in an amino acid-induced hyperglucagonemia. We investigated the presence of hyperglucagonemia in individuals with biopsy-verified metabolic dysfunction-associated steatotic liver disease (MASLD), and whether body mass index (BMI), T2D, hepatic steatosis, and/or fibrosis contribute to this relationship. To dissect potential mechanisms, we also determined hepatic gene expression related to amino acid transport and catabolism. Individuals with MASLD had hyperglucagonemia {controls (n = 74) vs. MASLD (n = 106); median [Q1, Q3]; 4 [3, 7] vs. 8 [6, 13] pM), P < 0.0001} and were glucagon resistant (assessed by the glucagon-alanine index) {1.3 [0.9, 2.1] vs. 3.3 [2.1, 5.3] pM·mM, P < 0.0001}. These changes were associated with hepatic steatosis (P < 0.001, R2 > 0.25) independently of BMI, sex, age, and T2D. Plasma levels of glucagon were similar in individuals with MASLD when stratified on T2D status {MASLD-T2D (n = 52) vs. MASLD + T2D (n = 54); 8 [6, 11] vs. 8 [6, 13] pM, P = 0.34} and hepatic fibrosis {MASLD + F0 (n = 25) vs. MASLD + F1-F3 (n = 67); 8.4 [7.0, 13.3] vs. 7.9 [5.2, 11.6] pM, P = 0.43}. Obesity (BMI = 30 kg/m2) did not alter glucagon levels (P = 0.65) within groups (control/MASLD). The mRNA expression of proteins involved in amino acid transport and catabolism was downregulated in MASLD. Thus, relative hyperglucagonemia is present in individuals with biopsy-verified MASLD, and hepatic steatosis partially drives hyperglucagonemia and glucagon resistance, irrespective of T2D, BMI, and hepatic fibrosis.NEW & NOTEWORTHY Individuals with metabolic dysfunction-associated steatotic liver disease (MASLD) present with increased plasma levels of glucagon (hyperglucagonemia), irrespective of body mass index (BMI) and type 2 diabetes. Therefore, MASLD and the resultant hyperglucagonemia may act as a diabetogenic risk factor. Notably, hepatic steatosis was a significant contributor to the hyperglucagonemia in MASLD, potentially unveiling a pathway for the hyperglucagonemia in some patients with type 2 diabetes.

Phosphoproteomics-directed manipulation reveals SEC22B as a hepatocellular signaling node governing metabolic actions of glucagon

The peptide hormone glucagon is a fundamental metabolic regulator that is also being considered as a pharmacotherapeutic option for obesity and type 2 diabetes. Despite this, we know very little regarding how glucagon exerts its pleiotropic metabolic actions. Given that the liver is a chief site of action, we performed in situ time-resolved liver phosphoproteomics to reveal glucagon signaling nodes. Through pathway analysis of the thousands of phosphopeptides identified, we reveal "membrane trafficking" as a dominant signature with the vesicle trafficking protein SEC22 Homolog B (SEC22B) S137 phosphorylation being a top hit. Hepatocyte-specific loss- and gain-of-function experiments reveal that SEC22B was a key regulator of glycogen, lipid and amino acid metabolism, with SEC22B-S137 phosphorylation playing a major role in glucagon action. Mechanistically, we identify several protein binding partners of SEC22B affected by glucagon, some of which were differentially enriched with SEC22B-S137 phosphorylation. In summary, we demonstrate that phosphorylation of SEC22B is a hepatocellular signaling node mediating the metabolic actions of glucagon and provide a rich resource for future investigations on the biology of glucagon action.

Glucagon infusion alters the circulating metabolome and urine amino acid excretion in dogs

Glucagon plays a central role in amino acid (AA) homeostasis. The dog is an established model of glucagon biology, and recently, metabolomic changes in people associated with glucagon infusions have been reported. Glucagon also has effects on the kidney; however, changes in urinary AA concentrations associated with glucagon remain under investigation. Therefore, we aimed to fill these gaps in the canine model by determining the effects of glucagon on the canine plasma metabolome and measuring urine AA concentrations. Employing two constant rate glucagon infusions (CRI) - low-dose (CRI-LO: 3 ng/kg/min) and high-dose (CRI-HI: 50 ng/kg/min) on five research beagles, we monitored interstitial glucose and conducted untargeted liquid chromatography-tandem mass spectrometry (LC-MS/MS) on plasma samples and urine AA concentrations collected pre- and post-infusion. The CRI-HI induced a transient glucose peak (90-120 min), returning near baseline by infusion end, while only the CRI-LO resulted in 372 significantly altered plasma metabolites, primarily reductions (333). Similarly, CRI-HI affected 414 metabolites, with 369 reductions, evidenced by distinct clustering post-infusion via data reduction (PCA and sPLS-DA). CRI-HI notably decreased circulating AA levels, impacting various AA-related and energy-generating metabolic pathways. Urine analysis revealed increased 3-methyl-l-histidine and glutamine, and decreased alanine concentrations post-infusion. These findings demonstrate glucagon's glucose-independent modulation of the canine plasma metabolome and highlight the dog's relevance as a translational model for glucagon biology. Understanding these effects contributes to managing dysregulated glucagon conditions and informs treatments impacting glucagon homeostasis.

Chronic treatment with glucagon-like peptide-1 and glucagon receptor co-agonist causes weight loss-independent improvements in hepatic steatosis in mice with diet-induced obesity

OBJECTIVES

Co-agonists at the glucagon-like peptide-1 and glucagon receptors (GLP1R/GCGR) show promise as treatments for metabolic dysfunction-associated steatotic liver disease (MASLD). Although most co-agonists to date have been heavily GLP1R-biased, glucagon directly acts on the liver to reduce fat content. The aims of this study were to investigate a GCGR-biased co-agonist as treatment for hepatic steatosis in mice.

METHODS

Mice with diet-induced obesity (DIO) were treated with Dicretin, a GLP1/GCGR co-agonist with high potency at the GCGR, Semaglutide (GLP1R monoagonist) or food restriction over 24 days, such that their weight loss was matched. Hepatic steatosis, glucose tolerance, hepatic transcriptomics, metabolomics and lipidomics at the end of the study were compared with Vehicle-treated mice.

RESULTS

Dicretin lead to superior reduction of hepatic lipid content when compared to Semaglutide or equivalent weight loss by calorie restriction. Markers of glucose tolerance and insulin resistance improved in all treatment groups. Hepatic transcriptomic and metabolomic profiling demonstrated many changes that were unique to Dicretin-treated mice. These include some known targets of glucagon signaling and others with as yet unclear physiological significance.

CONCLUSIONS

Our study supports the development of GCGR-biased GLP1/GCGR co-agonists for treatment of MASLD and related conditions.

Glucagon resistance and metabolic-associated steatotic liver disease: a review of the evidence

Metabolic-associated steatotic liver disease (MASLD) is closely associated with obesity. MASLD affects over 1 billion adults globally but there are few treatment options available. Glucagon is a key metabolic regulator, and its actions include the reduction of liver fat through direct and indirect means. Chronic glucagon signalling deficiency is associated with hyperaminoacidaemia, hyperglucagonaemia and increased circulating levels of glucagon-like peptide 1 (GLP-1) and fibroblast growth factor 21 (FGF-21). Reduction in glucagon activity decreases hepatic amino acid and triglyceride catabolism; metabolic effects include improved glucose tolerance, increased plasma cholesterol and increased liver fat. Conversely, glucagon infusion in healthy volunteers leads to increased hepatic glucose output, decreased levels of plasma amino acids and increased urea production, decreased plasma cholesterol and increased energy expenditure. Patients with MASLD share many hormonal and metabolic characteristics with models of glucagon signalling deficiency, suggesting that they could be resistant to glucagon. Although there are few studies of the effects of glucagon infusion in patients with obesity and/or MASLD, there is some evidence that the expected effect of glucagon on amino acid catabolism may be attenuated. Taken together, this evidence supports the notion that glucagon resistance exists in patients with MASLD and may contribute to the pathogenesis of MASLD. Further studies are warranted to investigate the direct effects of glucagon on metabolism in patients with MASLD.

Adaptive infusion of a glucagon-like peptide-1/glucagon receptor co-agonist G3215, in adults with overweight or obesity: Results from a phase 1 randomized clinical trial

AIMS

To determine whether a continuous infusion of a glucagon-like peptide receptor (GLP-1R)/glucagon receptor (GCGR) co-agonist, G3215 is safe and well tolerated in adults with overweight or obesity.

METHODS

A phase 1 randomized, double blind, placebo-controlled trial of G3215 in overweight or obese participants, with or without type 2 diabetes.

RESULTS

Twenty-six participants were recruited and randomized with 23 completing a 14-day subcutaneous infusion of G3215 or placebo. The most common adverse events were nausea or vomiting, which were mild in most cases and mitigated by real-time adjustment of drug infusion. There were no cardiovascular concerns with G3215 infusion. The pharmacokinetic characteristics were in keeping with a continuous infusion over 14 days. A least-squares mean body weight loss of 2.39 kg was achieved with a 14-day infusion of G3215, compared with 0.84 kg with placebo infusion (p < .05). A reduction in food consumption was also observed in participants receiving G3215 and there was no deterioration in glycaemia. An improved lipid profile was seen in G3215-treated participants and consistent with GCGR activation, a broad reduction in circulating amino acids was seen during the infusion period.

CONCLUSION

An adaptive continuous infusion of the GLP-1/GCGR co-agonist, G3215, is safe and well tolerated offering a unique strategy to control drug exposure. By allowing rapid, response-directed titration, this strategy may allow for mitigation of adverse effects and afford significant weight loss within shorter time horizons than is presently possible with weekly GLP-1R and multi-agonists. These results support ongoing development of G3215 for the treatment of obesity and metabolic disease.

Integration of metabolic flux with hepatic glucagon signaling and gene expression profiles in the conscious dog

Glucagon rapidly and profoundly stimulates hepatic glucose production (HGP), but for reasons that are unclear, this effect normally wanes after a few hours, despite sustained plasma glucagon levels. This study characterized the time course of glucagon-mediated molecular events and their relevance to metabolic flux in the livers of conscious dogs. Glucagon was either infused into the hepato-portal vein at a sixfold basal rate in the presence of somatostatin and basal insulin, or it was maintained at a basal level in control studies. In one control group, glucose remained at basal, whereas in the other, glucose was infused to match the hyperglycemia that occurred in the hyperglucagonemic group. Elevated glucagon caused a rapid (30 min) and largely sustained increase in hepatic cAMP over 4 h, a continued elevation in glucose-6-phosphate (G6P), and activation and deactivation of glycogen phosphorylase and synthase activities, respectively. Net hepatic glycogenolysis increased rapidly, peaking at 15 min due to activation of the cAMP/PKA pathway, then slowly returned to baseline over the next 3 h in line with allosteric inhibition by glucose and G6P. Glucagon's stimulatory effect on HGP was sustained relative to the hyperglycemic control group due to continued PKA activation. Hepatic gluconeogenic flux did not increase due to the lack of glucagon's effect on substrate supply to the liver. Global gene expression profiling highlighted glucagon-regulated activation of genes involved in cellular respiration, metabolic processes, and signaling, as well as downregulation of genes involved in extracellular matrix assembly and development.NEW & NOTEWORTHY Glucagon rapidly stimulates hepatic glucose production, but these effects are transient. This study links the molecular and metabolic flux changes that occur in the liver over time in response to a rise in glucagon, demonstrating the strength of the dog as a translational model to couple findings in small animals and humans. In addition, this study clarifies why the rapid effects of glucagon on liver glycogen metabolism are not sustained.

Markers of Glucagon Resistance Improve With Reductions in Hepatic Steatosis and Body Weight in Type 2 Diabetes

Context

Hyperglucagonemia may develop in type 2 diabetes due to obesity-prone hepatic steatosis (glucagon resistance). Markers of glucagon resistance (including the glucagon-alanine index) improve following diet-induced weight loss, but the partial contribution of lowering hepatic steatosis vs body weight is unknown.

Objective

This work aimed to investigate the dependency of body weight loss following a reduction in hepatic steatosis on markers of glucagon resistance in type 2 diabetes.

Methods

A post hoc analysis was conducted from 2 previously published randomized controlled trials. We investigated the effect of weight maintenance (study 1: isocaloric feeding) or weight loss (study 2: hypocaloric feeding), both of which induced reductions in hepatic steatosis, on markers of glucagon sensitivity, including the glucagon-alanine index measured using a validated enzyme-linked immunosorbent assay and metabolomics in 94 individuals (n = 28 in study 1; n = 66 in study 2). Individuals with overweight or obesity with type 2 diabetes were randomly assigned to a 6-week conventional diabetes (CD) or carbohydrate-reduced high-protein (CRHP) diet within both isocaloric and hypocaloric feeding-interventions.

Results

By design, weight loss was greater after hypocaloric compared to isocaloric feeding, but both diets caused similar reductions in hepatic steatosis, allowing us to investigate the effect of reducing hepatic steatosis with or without a clinically relevant weight loss on markers of glucagon resistance. The glucagon-alanine index improved following hypocaloric, but not isocaloric, feeding, independently of macronutrient composition.

Conclusion

Improvements in glucagon resistance may depend on body weight loss in patients with type 2 diabetes.

Integration of metabolic flux with hepatic glucagon signaling and gene expression profiles in the conscious dog

Glucagon rapidly and profoundly simulates hepatic glucose production (HGP), but for reasons which are unclear, this effect normally wanes after a few hours, despite sustained plasma glucagon levels. This study characterized the time course and relevance (to metabolic flux) of glucagon mediated molecular events in the livers of conscious dogs. Glucagon was either infused into the hepato-portal vein at a 6-fold basal rate in the presence of somatostatin and basal insulin, or it was maintained at a basal level in control studies. In one control group glucose remained at basal while in the other glucose was infused to match the hyperglycemia that occurred in the hyperglucagonemic group. Elevated glucagon caused a rapid (30 min) but only partially sustained increase in hepatic cAMP over 4h, a continued elevation in G6P, and activation and deactivation of glycogen phosphorylase and synthase activities, respectively. Net hepatic glycogenolysis and HGP increased rapidly, peaking at 30 min, then returned to baseline over the next 3h (although glucagons stimulatory effect on HGP was sustained relative to the hyperglycemic control group). Hepatic gluconeogenic flux did not increase due to lack of glucagon effect on substrate supply to the liver. Global gene expression profiling highlighted glucagon-regulated activation of genes involved in cellular respiration, metabolic processes, and signaling, and downregulation of genes involved in extracellular matrix assembly and development.

100 years of glucagon and 100 more

The peptide hormone glucagon, discovered in late 1922, is secreted from pancreatic alpha cells and is an essential regulator of metabolic homeostasis. This review summarises experiences since the discovery of glucagon regarding basic and clinical aspects of this hormone and speculations on the future directions for glucagon biology and glucagon-based therapies. The review was based on the international glucagon conference, entitled 'A hundred years with glucagon and a hundred more', held in Copenhagen, Denmark, in November 2022. The scientific and therapeutic focus of glucagon biology has mainly been related to its role in diabetes. In type 1 diabetes, the glucose-raising properties of glucagon have been leveraged to therapeutically restore hypoglycaemia. The hyperglucagonaemia evident in type 2 diabetes has been proposed to contribute to hyperglycaemia, raising questions regarding underlying mechanism and the importance of this in the pathogenesis of diabetes. Mimicry experiments of glucagon signalling have fuelled the development of several pharmacological compounds including glucagon receptor (GCGR) antagonists, GCGR agonists and, more recently, dual and triple receptor agonists combining glucagon and incretin hormone receptor agonism. From these studies and from earlier observations in extreme cases of either glucagon deficiency or excess secretion, the physiological role of glucagon has expanded to also involve hepatic protein and lipid metabolism. The interplay between the pancreas and the liver, known as the liver-alpha cell axis, reflects the importance of glucagon for glucose, amino acid and lipid metabolism. In individuals with diabetes and fatty liver diseases, glucagon's hepatic actions may be partly impaired resulting in elevated levels of glucagonotropic amino acids, dyslipidaemia and hyperglucagonaemia, reflecting a new, so far largely unexplored pathophysiological phenomenon termed 'glucagon resistance'. Importantly, the hyperglucagonaemia as part of glucagon resistance may result in increased hepatic glucose production and hyperglycaemia. Emerging glucagon-based therapies show a beneficial impact on weight loss and fatty liver diseases and this has sparked a renewed interest in glucagon biology to enable further pharmacological pursuits.

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.

Development of a glucagon sensitivity test in humans: Pilot data and the GLUSENTIC study protocol

A physiological feedback system exists between hepatocytes and the alpha cells, termed the liver-alpha cell axis and refers to the relationship between amino acid-stimulated glucagon secretion and glucagon-stimulated amino acid catabolism. Several reports indicate that non-alcoholic fatty liver disease (NAFLD) disrupts the liver-alpha cell axis, because of impaired glucagon receptor signaling (glucagon resistance). However, no experimental test exists to assess glucagon resistance in humans. The objective was to develop an experimental test to determine glucagon sensitivity with respect to amino acid and glucose metabolism in humans. The proposed glucagon sensitivity test (comprising two elements: 1) i.v. injection of 0.2 mg glucagon and 2) infusion of mixed amino acids 331 mg/hour/kg) is based on nine pilot studies which are presented. Calculation of a proposed glucagon sensitivity index with respect to amino acid catabolism is also described. Secondly, we describe a complete study protocol (GLUSENTIC) according to which the glucagon sensitivity test will be applied in a cross-sectional study currently taking place. 65 participants including 20 individuals with a BMI 18.6-25 kg/m², 30 individuals with a BMI ≥ 25-40 kg/m², and 15 individuals with type 1 diabetes with a BMI between 18.6 and 40 kg/m² will be included. Participants will be grouped according to their degree of hepatic steatosis measured by whole-liver magnetic resonance imaging (MRI). The primary outcome measure will be differences in the glucagon sensitivity index between individuals with and without hepatic steatosis. Developing a glucagon sensitivity test and index may provide insight into the physiological and pathophysiological mechanism of glucagon action and glucagon-based therapies.

The feedback cycles between glucose, amino acids and lipids and alpha cell secretion and their role in metabolic fatty liver disease

PURPOSE OF REVIEW

Glucagon increases hepatic glucose production and in patients with metabolic diseases, glucagon secretion is increased contributing to diabetic hyperglycemia. This review explores the role of amino acids and lipids in the regulation of glucagon secretion and how it may be disturbed in metabolic diseases such as obesity and metabolic associated fatty liver disease (MAFLD).

RECENT FINDINGS

Human and animal studies have shown that MAFLD is associated with glucagon resistance towards amino acid catabolism, resulting in elevated plasma levels of amino acids. A recent clinical study showed that MAFLD is also associated with glucagon resistance towards lipid metabolism. In contrast, MAFLD may not decrease hepatic sensitivity to the stimulatory effects of glucagon on glucose production.

SUMMARY

Elevated plasma levels of amino acids and lipids associated with MAFLD may cause diabetogenic hyperglucagonemia. MAFLD and glucagon resistance may therefore be causally linked to hyperglycemia and the development of type 2 diabetes.

Glucagon receptor antagonism impairs and glucagon receptor agonism enhances triglycerides metabolism in mice

OBJECTIVE

Treatment with glucagon receptor antagonists (GRAs) reduces blood glucose but causes dyslipidemia and accumulation of fat in the liver. We investigated the acute and chronic effects of glucagon on lipid metabolism in mice.

METHODS

Chronic effects of glucagon receptor signaling on lipid metabolism were studied using oral lipid tolerance tests (OLTTs) in overnight fasted glucagon receptor knockout (Gcgr^-/-) mice, and in C57Bl/6JRj mice treated with a glucagon receptor antibody (GCGR Ab) or a long-acting glucagon analogue (GCGA) for eight weeks. Following treatment, liver tissue was harvested for RNA-sequencing and triglyceride measurements. Acute effects were studied in C57Bl/6JRj mice treated with a GRA or GCGA 1 h or immediately before OLTTs, respectively. Direct effects of glucagon on hepatic lipolysis were studied using isolated perfused mouse liver preparations. To investigate potential effects of GCGA and GRA on gastric emptying, paracetamol was, in separate experiments, administered immediately before OLTTs.

RESULTS

Plasma triglyceride concentrations increased 2-fold in Gcgr^-/- mice compared to their wild-type littermates during the OLTT (P = 0.001). Chronic treatment with GCGR Ab increased, whereas GCGA treatment decreased, plasma triglyceride concentrations during OLTTs (P < 0.05). Genes involved in lipid metabolism were upregulated upon GCGR Ab treatment while GCGA treatment had opposite effects. Acute GRA and GCGA treatment, respectively, increased (P = 0.02) and decreased (P = 0.003) plasma triglyceride concentrations during OLTTs. Glucagon stimulated hepatic lipolysis, evident by an increase in free fatty acid concentrations in the effluent from perfused mouse livers. In line with this, GCGR Ab treatment increased, while GCGA treatment decreased, liver triglyceride concentrations. The effects of glucagon appeared independent of changes in gastric emptying of paracetamol.

CONCLUSIONS

Glucagon receptor signaling regulates triglyceride metabolism, both chronically and acutely, in mice. These data expand glucagon´s biological role and implicate that intact glucagon signaling is important for lipid metabolism. Glucagon agonism may have beneficial effects on hepatic and peripheral triglyceride metabolism.