Glucagon revisited: Coordinated actions on the liver and kidney

Vasopressin, protein metabolism, and water conservation

PURPOSE OF REVIEW

Highlight the mechanisms through which vasopressin and hypertonic stress regulate protein metabolism.

RECENT FINDINGS

Mammals have an 'aestivation-like' response in which hypertonic stress increases muscle catabolism and urea productionVasopressin can directly regulate ureagenesis in the liver and the kidneyIn humans chronic hypertonic stress is associated with premature aging, diabetes, cardiovascular disease, and premature mortality.

SUMMARY

There is an evolutionarily conserved 'aestivation-like' response in humans in which hypertonic stress results in activation of the vasopressin system, muscle catabolism, and ureagenesis in order to promote water conservation.

Increased hepatic glucagon sensitivity in totally pancreatectomised patients

OBJECTIVE

The metabolic phenotype of totally pancreatectomised patients includes hyperaminoacidaemia and predisposition to hypoglycaemia and hepatic lipid accumulation. We aimed to investigate whether the loss of pancreatic glucagon may be responsible for these changes.

METHODS

Nine middle-aged, normal-weight totally pancreatectomised patients, nine patients with type 1 diabetes (C-peptide negative), and nine matched controls underwent two separate experimental days, each involving a 150-min intravenous infusion of glucagon (4 ng/kg/min) or placebo (saline) under fasting conditions while any basal insulin treatment was continued.

RESULTS

Glucagon infusion increased plasma glucagon to similar high physiological levels in all groups. The infusion increased hepatic glucose production and decreased plasma concentration of most amino acids in all groups, with more pronounced effects in the totally pancreatectomised patients compared with the other groups. Glucagon infusion diminished fatty acid re-esterification and tended to decrease plasma concentrations of fatty acids in the totally pancreatectomised patients but not in the type 1 diabetes patients.

CONCLUSION

Totally pancreatectomised patients were characterised by increased sensitivity to exogenous glucagon at the level of hepatic glucose, amino acid, and lipid metabolism, suggesting that the metabolic disturbances characterising these patients may be rooted in perturbed hepatic processes normally controlled by pancreatic glucagon.

The impact of a single dose of whey protein on glucose flux and metabolite profiles in normoglycemic males: insights into glucagon and insulin biology

Protein ingestion concurrently stimulates euglycemic glucagon and insulin secretion, a response that is particularly robust with rapidly absorbing proteins. Previously, we have shown that ingestion of repeated doses of rapidly absorbing whey protein equally stimulated endogenous glucose production (EGP) and glucose disposal (Rd), thus explaining the preservation of euglycemia. Here, we aimed to determine if a smaller single dose of whey could elicit a large enough glucagon and insulin response to stimulate glucose flux. Therefore, in normoglycemic young adult males (n = 10; age ∼26; BMI ∼25), using [6,6-2H2] glucose tracing and quantitative targeted metabolite profiling, we determined the metabolic response to a single 25 g "standard" dose of whey protein. Whey protein ingestion did not alter glycemia, but increased circulating glucagon (peak 4-fold basal), insulin (peak 6-fold basal), amino acids, and urea while also reducing free fatty acid (FFA) and glycerol concentrations. Interestingly, the postprandial insulin response was driven by both a stimulation of insulin secretion and marked reduction in hepatic insulin clearance. Whey protein ingestion resulted in a modest stimulation of EGP and Rd, both peaking at ∼20% above baseline 1 h after protein ingestion. These findings demonstrate that the ingestion of a single standard serving of whey protein can induce a euglycemic glucagon and insulin response that stimulates glucose flux. We speculate on a theory that could potentially explain how glucagon and insulin synergistically provide hardwired control of nitrogen and glucose homeostasis.NEW & NOTEWORTHY Protein ingestion concurrently stimulates glucagon and insulin secretion. Here we show that in normoglycemic males, ingestion of a single "standard" 25 g serving of rapidly absorbing whey protein drives a sufficiently large glucagon and insulin response, such that it simultaneously increases endogenous glucose production and glucose disposal. We speculate on a novel theory that could potentially explain how the antagonistic/synergistic actions of glucagon and insulin simultaneously provide tight control of glucose and nitrogen homeostasis.

The Regulation and Secretion of Glucagon in Response to Nutrient Composition: Unraveling Their Intricate Mechanisms

Glucagon was initially regarded as a hyperglycemic substance; however, recent research has revealed its broader role in metabolism, encompassing effects on glucose, amino acids (AAs), and lipid metabolism. Notably, the interplay of glucagon with nutrient intake, particularly of AAs, and non-nutrient components is central to its secretion. Fasting and postprandial hyperglucagonemia have long been linked to the development and progression of type 2 diabetes (T2DM). However, recent studies have brought to light the positive impact of glucagon agonists on lipid metabolism and energy homeostasis. This review explores the multifaceted actions of glucagon, focusing on its regulation, signaling pathways, and effects on glucose, AAs, and lipid metabolism. The interplay between glucagon and other hormones, including insulin and incretins, is examined to provide a mechanistic understanding of its functions. Notably, the liver-α-cell axis, which involves glucagon and amino acids, emerges as a critical aspect of metabolic regulation. The dysregulation of glucagon secretion and its impact on conditions such as T2DM are discussed. The review highlights the potential therapeutic applications of targeting the glucagon pathway in the treatment of metabolic disorders.

Rethinking Vasopressin: New Insights into Vasopressin Signaling and Its Implications

Vasopressin is a highly conserved peptide hormone that has been traditionally associated with water homeostasis. There is accumulating evidence in both humans and animal models that vasopressin is implicated in the regulation of metabolism. This review focuses on the effects that vasopressin exerts on the regulation of glucose and fatty acids with a particular emphasis on the potential repercussions of metabolic dysregulation in kidney disease.

Glucagon in type 2 diabetes: Friend or foe?

Hyperglucagonemia is one of the 'ominous' eight factors underlying the pathogenesis of type 2 diabetes (T2D). Glucagon is a peptide hormone involved in maintaining glucose homoeostasis by increasing hepatic glucose output to counterbalance insulin action. Long neglected, the introduction of dual and triple agonists exploiting glucagon signalling pathways has rekindled the interest in this hormone beyond its classic effect on glycaemia. Glucagon can promote weight loss by regulating food intake, energy expenditure, and brown and white adipose tissue functions through mechanisms still to be fully elucidated, thus its role in T2D pathogenesis should be further investigated. Moreover, the role of glucagon in the development of T2D micro- and macro-vascular complications is elusive. Mounting evidence suggests its beneficial effect in non-alcoholic fatty liver disease, while few studies postulated its favourable role in peripheral neuropathy and retinopathy. Contrarily, glucagon receptor agonism might induce renal changes resembling diabetic nephropathy, and data concerning glucagon actions on the cardiovascular system are conflicting. This review aims to summarise the available findings on the role of glucagon in the pathogenesis of T2D and its complications. Further experimental and clinical data are warranted to better understand the implications of glucagon signalling modulation with new antidiabetic drugs.

The relationship between glucose and the liver-alpha cell axis - A systematic review

Until recently, glucagon was considered a mere antagonist to insulin, protecting the body from hypoglycemia. This notion changed with the discovery of the liver-alpha cell axis (LACA) as a feedback loop. The LACA describes how glucagon secretion and pancreatic alpha cell proliferation are stimulated by circulating amino acids. Glucagon in turn leads to an upregulation of amino acid metabolism and ureagenesis in the liver. Several increasingly common diseases (e.g., non-alcoholic fatty liver disease, type 2 diabetes, obesity) disrupt this feedback loop. It is important for clinicians and researchers alike to understand the liver-alpha cell axis and the metabolic sequelae of these diseases. While most of previous studies have focused on fasting concentrations of glucagon and amino acids, there is limited knowledge of their dynamics after glucose administration. The authors of this systematic review applied PRISMA guidelines and conducted PubMed searches to provide results of 8078 articles (screened and if relevant, studied in full). This systematic review aims to provide better insight into the LACA and its mediators (amino acids and glucagon), focusing on the relationship between glucose and the LACA in adult and pediatric subjects.

Association of dietary proteins with serum creatinine and estimated glomerular filtration rate in a general population sample: the CHRIS study

BACKGROUND

Diet is known to affect kidney function. However, population-based studies provide contrasting evidence, resulting in a poor understanding of the effect of proteins from specific foods on kidney health.

METHODS

We analyzed the effect of total daily protein intake (TDPI) and source-specific daily protein intake (DPI) on fasting serum creatinine (SCr) and estimated glomerular filtration rate (eGFR) in the Cooperative Health Research In South Tyrol (CHRIS) cross-sectional study (n = 5889), using the GA²LEN food frequency questionnaire for TDPI and DPI estimation. We fitted multivariable adjusted mixed models of SCr and eGFR on TDPI and DPI quartiles (Q1-Q4) in the overall sample, and after removing individuals with known hypertension, diabetes or chronic kidney disease (CKD).

RESULTS

Higher TDPI as well as DPI from overall animal sources, fish, and poultry, were associated with higher SCr (trend test p, p_trend < 0.01), with larger effect after excluding individuals with known hypertension, diabetes or CKD. The eGFR was lower at higher TDPI (Q4 vs Q1: - 1.6 ml/min/1.73 m²; 95% CI - 2.5, - 0.7; p_trend = 3e-4) and DPI from fish (Q4 vs Q1: - 2.1 ml/min/1.73 m²; 95% CI - 2.9, - 1.20; p_trend = 4.3e-6), overall animal source (Q4 vs Q1: - 1.6 ml/min/1.73 m²; 95% CI -2.5, - 0.8), processed meat (Q4 vs Q1: - 1.4 ml/min/1.73 m²; p_trend = 0.027), red meat, offal and processed meat (Q4 vs Q1: - 1.4 ml/min/1.73 m²; p_trend = 0.015) and poultry (Q4 vs Q1: - 0.9 ml/min/1.73 m²; p_trend = 0.015).

CONCLUSIONS

TDPI and DPI from specific animal sources were positively associated with SCr and negatively associated with eGFR. Lacking an alternative marker of kidney function, confounding involving muscle mass metabolism cannot be fully excluded.

Ion transport peptide regulates energy intake, expenditure, and metabolic homeostasis in Drosophila

In mammals, energy homeostasis is regulated by the antagonistic action of hormones insulin and glucagon. However, in contrast to the highly conserved insulin, glucagon is absent in most invertebrates. Although there are several endocrine regulators of energy expenditure and catabolism (such as the adipokinetic hormone), no single invertebrate hormone with all of the functions of glucagon has been described so far. Here, we used genetic gain- and loss-of-function experiments to show that the Drosophila gene Ion transport peptide (ITP) codes for a novel catabolic regulator that increases energy expenditure, lowers fat and glycogen reserves, and increases glucose and trehalose. Intriguingly, Ion transport peptide has additional functions reminiscent of glucagon, such as inhibition of feeding and transit of the meal throughout the digestive tract. Furthermore, Ion transport peptide interacts with the well-known signaling via the Adipokinetic hormone; Ion transport peptide promotes the pathway by stimulating Adipokinetic hormone secretion and transcription of the receptor AkhR. The genetic manipulations of Ion transport peptide on standard and Adipokinetic hormone-deficient backgrounds showed that the Adipokinetic hormone peptide mediates the hyperglycemic and hypertrehalosemic effects of Ion transport peptide, while the other metabolic functions of Ion transport peptide seem to be Adipokinetic hormone independent. In addition, Ion transport peptide is necessary for critical processes such as development, starvation-induced foraging, reproduction, and average lifespan. Altogether, our work describes a novel master regulator of fly physiology with functions closely resembling mammalian glucagon.

High Protein Diets Improve Liver Fat and Insulin Sensitivity by Prandial but Not Fasting Glucagon Secretion in Type 2 Diabetes

Glucagon (GCGN) plays a key role in glucose and amino acid (AA) metabolism by increasing hepatic glucose output. AA strongly stimulate GCGN secretion which regulates hepatic AA degradation by ureagenesis. Although increased fasting GCGN levels cause hyperglycemia GCGN has beneficial actions by stimulating hepatic lipolysis and improving insulin sensitivity through alanine induced activation of AMPK. Indeed, stimulating prandial GCGN secretion by isocaloric high protein diets (HPDs) strongly reduces intrahepatic lipids (IHLs) and improves glucose metabolism in type 2 diabetes mellitus (T2DM). Therefore, the role of GCGN and circulating AAs in metabolic improvements in 31 patients with T2DM consuming HPD was investigated. Six weeks HPD strongly coordinated GCGN and AA levels with IHL and insulin sensitivity as shown by significant correlations compared to baseline. Reduction of IHL during the intervention by 42% significantly improved insulin sensitivity [homeostatic model assessment for insulin resistance (HOMA-IR) or hyperinsulinemic euglycemic clamps] but not fasting GCGN or AA levels. By contrast, GCGN secretion in mixed meal tolerance tests (MMTTs) decreased depending on IHL reduction together with a selective reduction of GCGN-regulated alanine levels indicating greater GCGN sensitivity. HPD aligned glucose metabolism with GCGN actions. Meal stimulated, but not fasting GCGN, was related to reduced liver fat and improved insulin sensitivity. This supports the concept of GCGN-induced hepatic lipolysis and alanine- and ureagenesis-induced activation of AMPK by HPD.

Enzymatically-Synthesized Glycogen Induces Cecal Glucagon-Like Peptide-1 Production and Suppresses Food Intake in Mice

It is well known that dietary fiber stimulates the release of satiety hormones such as glucagon-like peptide-1 (GLP-1), which in turn suppresses appetite. In order to evaluate appetite regulating role of enzymatically synthesized glycogen (ESG, one of the resistant starch), we examined the effects of dietary supplementation of ESG on food intake and cecal proglucagon gene expression in normal and high fat diet-fed mice. Twenty four male ICR mice were weighed and assigned to four groups: normal diet group; normal diet containing 25% ESG group; high-fat diet (HFD) group; HFD containing 25% ESG group. Each group was fed the relevant diets for 3 wk. All data were analyzed by a two-way ANOVA with the main effects of HFD and ESG. ESG significantly decreased food intake and increased the weight of the cecum and cecal content. Plasma total short chain fatty acids concentration was significantly elevated by ESG. The mRNA levels of proglucagon in the cecum and plasma total GLP-1 concentration were significantly increased by ESG. The mRNA levels of appetite regulating neuropeptides such as neuropeptide Y, agouti-related protein, proopiomelanocortin, and cocain- and amphetamine-regulating transcript in the hypothalamus were not influenced by ESG. There is no significant interaction between diet and ESG in any parameters. These results suggest that ESG-induced upregulation of GLP-1 production in the cecum suppresses food intake in mice and that fecal fermentation may be involved in the anorexigenic effect.

Deleterious mutation V369M in the mouse GCGR gene causes abnormal plasma amino acid levels indicative of a possible liver-α-cell axis

Glucagon plays an important role in glucose homeostasis and amino acid metabolism. It regulates plasma amino acid levels which in turn modulate glucagon secretion from the pancreatic α-cell, thereby establishing a liver-α-cell axis described recently. We reported previously that the knock-in mice bearing homozygous V369M substitution (equivalent to a naturally occurring mutation V368M in the human glucagon receptor, GCGR) led to hypoglycemia with improved glucose tolerance. They also exhibited hyperglucagonemia, pancreas enlargement and α-cell hyperplasia. Here, we investigated the effect of V369M/V368M mutation on glucagon-mediated amino acid metabolism. It was found that GcgrV369M+/+ mice displayed increased plasma amino acid levels in general, but significant accumulation of the ketogenic/glucogenic amino acids was observed in animals fed with a high-fat diet (HFD), resulting in deleterious metabolic consequence characteristic of α-cell proliferation and hyperglucagonemia.

Fasting Hormones Synergistically Induce Amino Acid Catabolism Genes to Promote Gluconeogenesis

BACKGROUND & AIMS

Gluconeogenesis from amino acids (AAs) maintains glucose homeostasis during fasting. Although glucagon is known to regulate AA catabolism, the contribution of other hormones to it and the scope of transcriptional regulation dictating AA catabolism are unknown. We explored the role of the fasting hormones glucagon and glucocorticoids in transcriptional regulation of AA catabolism genes and AA-dependent gluconeogenesis.

METHODS

We tested the RNA expression of AA catabolism genes and glucose production in primary mouse hepatocytes treated with fasting hormones (glucagon, corticosterone) and feeding hormones (insulin, fibroblast growth factor 19). We analyzed genomic data of chromatin accessibility and chromatin immunoprecipitation in mice and primary mouse hepatocytes. We performed chromatin immunoprecipitation in livers of fasted mice to show binding of cAMP responsive element binding protein (CREB) and the glucocorticoid receptor (GR).

RESULTS

Fasting induced the expression of 31 genes with various roles in AA catabolism. Of them, 15 were synergistically induced by co-treatment of glucagon and corticosterone. Synergistic gene expression relied on the activity of both CREB and GR and was abolished by treatment with either insulin or fibroblast growth factor 19. Enhancers adjacent to synergistically induced genes became more accessible and were bound by CREB and GR on fasting. Akin to the gene expression pattern, gluconeogenesis from AAs was synergistically induced by glucagon and corticosterone in a CREB- and GR-dependent manner.

CONCLUSIONS

Transcriptional regulation of AA catabolism genes during fasting is widespread and is driven by glucagon (via CREB) and corticosterone (via GR). Glucose production in hepatocytes is also synergistically augmented, showing that glucagon alone is insufficient in fully activating gluconeogenesis.

SGLT2 inhibitors and kidneys: mechanisms and main effects in diabetes mellitus patients

Type 2 diabetes mellitus (T2DM) is the cause of the development of diabetic nephropathy — a complication that determines the high degree of disability and mortality of such patients. Until recently, approaches to normalizing glucose levels did not have a significant possibility of influencing the outcome of kidney damage in diabetes. Type 2 sodium glucose cotransporter inhibitors (SGLT2) are a new class of glucose-lowering drugs that improve glycemic control due to an insulin-independent mechanism of action associated with increased urinary glucose excretion. The review provides an analysis of the results of studies on the assessment of nephroprotective actions — one of the pleiotropic actions of this drugs group. These materials show the properties of SGLT2 inhibitors to reduce the risk of developing and the progression of albuminuria, to save glomerular filtration rate, to reduce the frequency of end-stage renal disease and the need for renal replacement therapy in patients with T2DM. The article gives and analyzes the currently existing hypotheses of the mechanism of action of these glucose-lowering drugs. The risk of the most common renal complications with the use of SGLT2 inhibitors is considered. The practical aspects of the use of SGLT2 inhibitors in modern algorithms for the care of patients with T2DM are indicated, as well as the prospects for new randomized clinical trials. 

The Impact of Amino Acids on Postprandial Glucose and Insulin Kinetics in Humans: A Quantitative Overview

Different amino acids (AAs) may exert distinct effects on postprandial glucose and insulin concentrations. A quantitative comparison of the effects of AAs on glucose and insulin kinetics in humans is currently lacking. PubMed was queried to identify intervention studies reporting glucose and insulin concentrations after acute ingestion and/or intravenous infusion of AAs in healthy adults and those living with obesity and/or type 2 diabetes (T2DM). The systematic literature search identified 55 studies that examined the effects of l-leucine, l-isoleucine, l-alanine, l-glutamine, l-arginine, l-lysine, glycine, l-proline, l-phenylalanine, l-glutamate, branched-chain AAs (i.e., l-leucine, l-isoleucine, and l-valine), and multiple individual l-AAs on glucose and insulin concentrations. Oral ingestion of most individual AAs induced an insulin response, but did not alter glucose concentrations in healthy participants. Specific AAs (i.e., leucine and isoleucine) co-ingested with glucose exerted a synergistic effect on the postprandial insulin response and attenuated the glucose response compared to glucose intake alone in healthy participants. Oral AA ingestion as well as intravenous AA infusion was able to stimulate an insulin response and decrease glucose concentrations in T2DM and obese individuals. The extracted information is publicly available and can serve multiple purposes such as computational modeling.

Glucagon-like peptide-1 receptors in the kidney: impact on renal autoregulation

Glucagon-like peptide-1 (GLP-1) and strategies based on this blood sugar-reducing and appetite-suppressing hormone are used to treat obesity and type 2 diabetes. However, the GLP-1 receptor (GLP-1R) is also present in the kidney, where it influences renal function. The effect of GLP-1 on the kidney varies between humans and rodents. The effect of GLP-1 on kidney function also seems to vary depending on its concentration and the physiological or pathological state of the kidney. In studies with rodents or humans, acute infusion of pharmacological doses of GLP-1 stimulates natriuresis and diuresis. However, the effect on the renal vasculature is less clear. In rodents, GLP-1 infusion increases renal plasma flow and glomerular filtration rate, suggesting renal vasodilation. In humans, only a subset of the study participants exhibits increased renal plasma flow and glomerular filtration rate. Differential status of kidney function and changes in renal vascular resistance of the preglomerular arterioles may account for the different responses of the human study participants. Because renal function in patients with type 2 diabetes is already at risk or compromised, understanding the effects of GLP-1R activation on kidney function in these patients is particularly important. This review examines the distribution of GLP-1R in the kidney and the effects elicited by GLP-1 or GLP-1R agonists. By integrating results from acute and chronic studies in healthy individuals and patients with type 2 diabetes along with those from rodent studies, we provide insight into how GLP-1R activation affects renal function and autoregulation.

Differential Impact of Dietary Branched Chain and Aromatic Amino Acids on Chronic Kidney Disease Progression in Rats

The metabolism of dietary proteins generates waste products that are excreted by the kidney, in particular nitrogen-containing urea, uric acid, ammonia, creatinine, and other metabolites such as phosphates, sulfates, and protons. Kidney adaptation includes an increase in renal plasma flow (RPF) and glomerular filtration rate (GFR) and represents a burden for diseased kidneys increasing the progression rate of CKD. The present study aimed at identifying potential differences between amino acid (AA) groups constituting dietary proteins regarding their impact on RPF, GFR, and CKD progression. We utilized the well-established 5/6 nephrectomy (5/6 Nx) CKD model in rats and submitted the animals for 5 weeks to either the control diet (18% casein protein) or to diets containing 8% casein supplemented with 10% of a mix of free amino acids, representing all or only a subset of the amino acids contained in casein. Whereas the RPF and GFR measured in free moving animals remained stable during the course of the diet in rats receiving the control mix, these parameters decreased in animals receiving the branched chain amino acid (BCAA) supplementation and increased in the ones receiving the aromatic amino acids (AAAs). In animals receiving essential amino acids (EAAs) containing both BCAAs and AAAs, there was only a small increase in RPF. The kidneys of the 5/6 Nx rats receiving the BCAA diet showed the strongest increase in smooth muscle actin and collagen mRNA expression as a result of higher level of inflammation and fibrosis. These animals receiving BCAAs also showed an increase in plasma free fatty acids pointing to a problem at the level of energy metabolism. In contrast, the animals under AAA diet showed an activation of AMPK and STAT3. Taken together, our results demonstrate that subsets of EAAs contained in dietary proteins, specifically BCAAs and AAAs, exert contrasting effects on kidney functional parameters and CKD progression.