Suppression of weight gain by glucagon in obese Zucker rats

Pharmacological modulation of adaptive thermogenesis: new clues for obesity management?

BACKGROUND

Adaptive thermogenesis represents the main mechanism through which the body generates heat in response to external stimuli, a phenomenon that includes shivering and non-shivering thermogenesis. The non-shivering thermogenesis is mainly exploited by adipose tissue characterized by a brown aspect, which specializes in energy dissipation. A decreased amount of brown adipose tissue has been observed in ageing and chronic illnesses such as obesity, a worldwide health problem characterized by dysfunctional adipose tissue expansion and associated cardiometabolic complications. In the last decades, the discovery of a trans-differentiation mechanism ("browning") within white adipose tissue depots, leading to the generation of brown-like cells, allowed to explore new natural and synthetic compounds able to favour this process and thus enhance thermogenesis with the aim of counteracting obesity. Based on recent findings, brown adipose tissue-activating agents could represent another option in addition to appetite inhibitors and inhibitors of nutrient absorption for obesity treatment.

PURPOSE

This review investigates the main molecules involved in the physiological (e.g. incretin hormones) and pharmacological (e.g. β3-adrenergic receptors agonists, thyroid receptor agonists, farnesoid X receptor agonists, glucagon-like peptide-1, and glucagon receptor agonists) modulation of adaptive thermogenesis and the signalling mechanisms involved.

Hormonal Gut-Brain Signaling for the Treatment of Obesity

The brain, particularly the hypothalamus and brainstem, monitors and integrates circulating metabolic signals, including gut hormones. Gut-brain communication is also mediated by the vagus nerve, which transmits various gut-derived signals. Recent advances in our understanding of molecular gut-brain communication promote the development of next-generation anti-obesity medications that can safely achieve substantial and lasting weight loss comparable to metabolic surgery. Herein, we comprehensively review the current knowledge about the central regulation of energy homeostasis, gut hormones involved in the regulation of food intake, and clinical data on how these hormones have been applied to the development of anti-obesity drugs. Insight into and understanding of the gut-brain axis may provide new therapeutic perspectives for the treatment of obesity and diabetes.

A glucagon analogue decreases body weight in mice via signalling in the liver

Glucagon receptor agonists show promise as components of next generation metabolic syndrome pharmacotherapies. However, the biology of glucagon action is complex, controversial, and likely context dependent. As such, a better understanding of chronic glucagon receptor (GCGR) agonism is essential to identify and mitigate potential clinical side-effects. Herein we present a novel, long-acting glucagon analogue (GCG104) with high receptor-specificity and potent in vivo action. It has allowed us to make two important observations about the biology of sustained GCGR agonism. First, it causes weight loss in mice by direct receptor signalling at the level of the liver. Second, subtle changes in GCG104-sensitivity, possibly due to interindividual variation, may be sufficient to alter its effects on metabolic parameters. Together, these findings confirm the liver as a principal target for glucagon-mediated weight loss and provide new insights into the biology of glucagon analogues.

Glucagon Regulation of Energy Expenditure

Glucagon's ability to increase energy expenditure has been known for more than 60 years, yet the mechanisms underlining glucagon's thermogenic effect still remain largely elusive. Over the last years, significant efforts were directed to unravel the physiological and cellular underpinnings of how glucagon regulates energy expenditure. In this review, we summarize the current knowledge on how glucagon regulates systems metabolism with a special emphasis on its acute and chronic thermogenic effects.

Glucagon Control on Food Intake and Energy Balance

Glucagon exerts pleiotropic actions on energy balance and has emerged as an attractive target for the treatment of diabetes and obesity in the last few years. Glucagon reduces body weight and adiposity by suppression of appetite and by modulation of lipid metabolism. Moreover, this hormone promotes weight loss by activation of energy expenditure and thermogenesis. In this review, we cover these metabolic actions elicited by glucagon beyond its canonical regulation of glucose metabolism. In addition, we discuss recent developments of therapeutic approaches in the treatment of obesity and diabetes by dual- and tri-agonist molecules based on combinations of glucagon with other peptides. New strategies using these unimolecular polyagonists targeting the glucagon receptor (GCGR), have become successful approaches to evaluate the multifaceted nature of glucagon signaling in energy balance and metabolic syndrome.

Glucagon, GLP-1 and Thermogenesis

Brown adipose tissue (BAT) thermogenesis is a conserved mechanism to maintain body temperature in mammals. However, since BAT contribution to energy expenditure can represent a relevant modulator of metabolic homeostasis, many studies have focused on the nervous system and endocrine factors that control the activity of this tissue. There is long-established evidence that the counter-regulatory hormone glucagon negatively influences energy balance, enhances satiety, and increases energy expenditure. Despite compelling evidence showing that glucagon has direct action on BAT thermogenesis, recent findings are questioning this conventional attribute of glucagon action. Glucagon like peptide-1 (GLP-1) is an incretin secreted by the intestinal tract which strongly decreases feeding, and, furthermore, improves metabolic parameters associated with obesity and diabetes. Therefore, GLP-1 receptors (GLP-1-R) have emerged as a promising target in the treatment of metabolic disorders. In this short review, we will summarize the latest evidence in this regard, as well as the current therapeutic glucagon- and GLP-1-based approaches to treating obesity.

Sustained effect of glucagon on body weight and blood glucose: Assessed by continuous glucose monitoring in diabetic rats

Insulin is a vital part of diabetes treatment, whereas glucagon is primarily used to treat insulin-induced hypoglycemia. However, glucagon is suggested to have a central role in the regulation of body weight, which would be beneficial for diabetic patients. Since the glucagon effect on blood glucose is known to be transient, it is relevant to investigate the pharmacodynamics of glucagon after repeated dosing. In the present study, we used telemetry to continuously measure blood glucose in streptozotocin induced diabetic Sprague-Dawley rats. This allowed for a more detailed analysis of glucose regulation compared to intermittent blood sampling. In particular, we evaluated the blood glucose-lowering effect of different insulin doses alone, and in combination with a long acting glucagon analog (LAG). We showed how the effect of the LAG accumulated and persisted over time. Furthermore, we found that addition of the LAG decreased body weight without affecting food intake. In a subsequent study, we focused on the glucagon effect on body weight and food intake during equal glycemic control. In order to obtain comparable maximum blood glucose lowering effect to insulin alone, the insulin dose had to be increased four times in combination with 1 nmol/kg of the LAG. In this set-up the LAG prevented further increase in body weight despite the four times higher insulin-dose. However, the body composition was changed. The insulin group increased both lean and fat mass, whereas the group receiving four times insulin in combination with the LAG only significantly increased the fat mass. No differences were observed in food intake, suggesting a direct effect on energy expenditure by glucagon. Surprisingly, we observed decreased levels of FGF21 in plasma compared to insulin treatment alone. With the combination of insulin and the LAG the blood glucose-lowering effect of insulin was prolonged, which could potentially be beneficial in diabetes treatment.

The New Biology and Pharmacology of Glucagon

In the last two decades we have witnessed sizable progress in defining the role of gastrointestinal signals in the control of glucose and energy homeostasis. Specifically, the molecular basis of the huge metabolic benefits in bariatric surgery is emerging while novel incretin-based medicines based on endogenous hormones such as glucagon-like peptide 1 and pancreas-derived amylin are improving diabetes management. These and related developments have fostered the discovery of novel insights into endocrine control of systemic metabolism, and in particular a deeper understanding of the importance of communication across vital organs, and specifically the gut-brain-pancreas-liver network. Paradoxically, the pancreatic peptide glucagon has reemerged in this period among a plethora of newly identified metabolic macromolecules, and new data complement and challenge its historical position as a gut hormone involved in metabolic control. The synthesis of glucagon analogs that are biophysically stable and soluble in aqueous solutions has promoted biological study that has enriched our understanding of glucagon biology and ironically recruited glucagon agonism as a central element to lower body weight in the treatment of metabolic disease. This review summarizes the extensive historical record and the more recent provocative direction that integrates the prominent role of glucagon in glucose elevation with its under-acknowledged effects on lipids, body weight, and vascular health that have implications for the pathophysiology of metabolic diseases, and the emergence of precision medicines to treat metabolic diseases.

The Characteristics of Hepatic Gsα-cAMP Axis in HSHF Diet-Fed Obese Insulin Resistance Rats and Genetic Diabetic Mice

Stimulatory G protein α-subunit (Gsα) mediated cAMP signal is required for elevated hepatic glucose production (HGP) in diabetic patients. However, it remains obscure of the exact characteristics of hepatic Gsα-cAMP signal axis (including Gsα, glucagon receptor, β₂-adrenergic receptor, cAMP, phosphoenolpyruvate carboxykinase and glucose-6-phosphatase) in insulin resistance (IR) and type 2 diabetes mellitus (T2DM). In current study, we investigated the changing characteristics of hepatic Gsα-cAMP signal axis and blood glucose in high-sugar-high-fat (HSHF)-diet-induced IR Wistar rats and db/db diabetic mice. As expected, the HSHF-diet rats were characterized by hyperinsulinemia, hyperglycemia and impaired glucose tolerance. According to a threshold (1.7) of homeostasis model assessment ratio (HOMA-R), the process of IR in HSHF-diet rats could be divided into slight and high IR stages, with the week-23 as the cut-off point. In early slight IR stage, key molecules expressions of hepatic Gsα-cAMP signal axis in HSHF-diet rats were up-regulated with significantly elevated fasting blood glucose (FBG) from 18 to 23 weeks. Unexpectedly, in high IR stage, hepatic Gsα-cAMP signal axis was recovered comparatively to that of normal chow-diet rats, and no significant differences in FBG levels were found. However, in diabetic db/db mice, up-regulation of hepatic Gsα-cAMP signal axis was responsible for its severely increased fasting hyperglycaemia. Our data revealed a positive correlation between hepatic Gsα-cAMP signal axis and FBG in slight IR stage of HSHF-diet rats and diabetic db/db mice. The current finding thus suggested hepatic Gsα-cAMP signal axis plays a central role in regulating of FBG during the developing and development of T2DM.

Comparison of appetite responses to high- and low-glycemic index postexercise meals under matched insulinemia and fiber in type 1 diabetes

BACKGROUND

Patients with type 1 diabetes face heightened risk of hypoglycemia after exercise. Subsequent overfeeding, as a preventative measure against hypoglycemia, negates the energy deficit after exercise. Patients are also required to reduce the insulin dose administered with postexercise foods to further combat hypoglycemia. However, the insulin dose is dictated solely by the carbohydrate content, even though postprandial glycemia is vastly influenced by glycemic index (GI). With a need to control the postexercise energy balance, appetite responses after meals differing in GI are of particular interest.

OBJECTIVES

We assessed the appetite response to low-glycemic index (LGI) and high-glycemic index (HGI) postexercise meals in type 1 diabetes patients. This assessment also offered us the opportunity to evaluate the influence of GI on appetite responses independently of insulinemia, which confounds findings in individuals without diabetes.

DESIGN

Ten physically active men with type 1 diabetes completed 2 trials in a randomized crossover design. After 45 min of treadmill exercise at 70% of the peak oxygen uptake, participants consumed an LGI (GI ∼37) or HGI (GI ∼92) meal with a matched macronutrient composition, negligible fiber content, and standardized insulin-dose administration. The postprandial appetite response was determined for 180 min postmeal. During this time, circulating glucose, insulin, glucagon, and glucagon-like peptide-1 (GLP-1) concentrations and subjective appetite ratings were determined.

RESULTS

The HGI meal produced an ∼60% greater postprandial glucose area under the curve (AUC) than did the LGI meal (P = 0.008). Insulin, glucagon, and GLP-1 did not significantly differ between trials (P > 0.05). The fullness AUC was ∼25% greater after the HGI meal than after the LGI meal (P < 0.001), whereas hunger sensations were ∼9% lower after the HGI meal than after the LGI meal (P = 0.001).

CONCLUSION

Under conditions of matched insulinemia and fiber, an HGI postexercise meal suppresses feelings of hunger and augments postprandial fullness sensations more so than an otherwise equivalent LGI meal in type 1 diabetes patients.

Agreement between fingertip-capillary and antecubital-venous appetite-related peptides

This study examined the agreement between fingertip-capillary and antecubital-venous measures of appetite-related peptides. Simultaneous fingertip-capillary and antecubital-venous blood samples were collected from 19 participants. The samples were obtained at baseline, 30, 60, 90, and 120 min following breakfast for the determination of plasma GLP17-36, glucagon, insulin and leptin. Between-day reproducibility of fingertip-capillary-derived estimates was assessed in 18 participants. Deming regression, limits of agreement (LOA) and typical error as a coefficient of variation (CV) were used to quantify agreement (CVa) and reproducibility (CVr). Deming regression revealed no systematic bias for any of the analytes studied, but for insulin there was evidence of a proportional difference at higher concentrations. Measures of GLP17-36 (CVa=24.0%, LOA ±2.5 pg m/l per h), leptin (CVa=9.0%, LOA ×/÷1.19) and glucagon (CVa=21.0%, LOA, ±31.5 pg m/l per h) revealed good agreement between methodological approaches. Fingertip-capillary glucagon was highly reproducible between days (CVr=8.2%). GLP17-36 and leptin demonstrated modest reproducibility (CVr=22.7 and 25.0% respectively). For insulin, agreement (CVa=36.0%, LOA ×/÷1.79) and reproducibility were poor (CVr=36.0%). Collectively, the data demonstrate that fingertip-capillary blood sampling provides a comparable and reproducible alternative to antecubital-venous blood sampling for the quantification of glucagon, and to a lesser extent for GLP17-36 and leptin. Caution should be exercised when utilising fingertip-capillary blood sampling for insulin quantification, and consequently should not be employed interchangeably with antecubital-venous blood sampling.

Nutrient regulation of glucagon secretion: involvement in metabolism and diabetes

Glucose homeostasis is precisely regulated by glucagon and insulin, which are released by pancreatic α- and β-cells, respectively. While β-cells have been the focus of intense research, less is known about α-cell function and the actions of glucagon. In recent years, the study of this endocrine cell type has experienced a renewed drive. The present review contains a summary of established concepts as well as new information about the regulation of α-cells by glucose, amino acids, fatty acids and other nutrients, focusing especially on glucagon release, glucagon synthesis and α-cell survival. We have also discussed the role of glucagon in glucose homeostasis and in energy and lipid metabolism as well as its potential as a modulator of food intake and body weight. In addition to the well-established action on the liver, we discuss the effects of glucagon in other organs, where the glucagon receptor is expressed. These tissues include the heart, kidneys, adipose tissue, brain, small intestine and the gustatory epithelium. Alterations in α-cell function and abnormal glucagon concentrations are present in diabetes and are thought to aggravate the hyperglycaemic state of diabetic patients. In this respect, several experimental approaches in diabetic models have shown important beneficial results in improving hyperglycaemia after the modulation of glucagon secretion or action. Moreover, glucagon receptor agonism has also been used as a therapeutic strategy to treat obesity.

Co-agonist of glucagon and GLP-1 reduces cholesterol and improves insulin sensitivity independent of its effect on appetite and body weight in diet-induced obese C57 mice

Dual agonism of glucagon and glucagon-like peptide-1 (GLP-1) receptors reduce body weight without inducing hyperglycemia in rodents. However, the effect of a co-agonist on insulin sensitivity and lipid metabolism has not been thoroughly assessed. Diet-induced obese (DIO) mice received 0.5 mg·kg(-1) of co-agonist or 2.5 mg·kg(-1) of glucagon or 8 μg·kg(-1) of exendin-4 by subcutaneous route, twice daily, for 28 days. A separate group of mice was pair-fed to the co-agonist-treated group for 28 days. Co-agonist treatment reduced food intake and reduced body weight up to 28 days. In addition, it reduced leptin levels and increased fibroblast growth factor 21 (FGF21) levels in plasma, when compared with control and pair-fed groups. Co-agonist treatment decreased triglyceride levels in serum and liver and reduced serum cholesterol, mainly due to reduction in low-density lipoprotein (LDL) cholesterol. These changes were not seen with pair-fed controls. Co-agonist treatment improved glucose tolerance and increased insulin sensitivity, as observed during glucose and insulin-tolerance test, hyperinsulinemic clamp, and reduced gluconeogenesis, as observed in pyruvate-tolerance test. The effects on insulin sensitivity and lipid levels are mostly independent of the food intake or body weight lowering effect of the co-agonist.

The future of incretin-based therapy: novel avenues--novel targets

Incretin-based therapy for type 2 diabetes is based on the antidiabetic effects of glucagon-like peptide-1 (GLP-1) and instituted by GLP-1 receptor agonists and dipeptidyl peptidase-4 inhibitors targeting the key islet defects of the disease. The treatment is clinically efficient and safe, and associated with a low risk of adverse events. It can be used both in early and late stages of the disease and both as monotherapy and add-on to other therapies. Current research on the future of incretin-based therapy focuses on optimizing its place in diabetes treatment and examines its potential in type 1 diabetes, in subjects with obesity without type 2 diabetes and in cardiovascular and neurodegenerative disorders. Other studies aim at prolonging the duration of action of the GLP-1 receptor agonists to allow weekly administration, and to develop orally GLP-1 receptor agonists. Furthermore, other investigators focus on stimulation of GLP-1 secretion by activating GLP-1-producing L-cells or using gene therapy. Finally, also other gastro-entero-pancreatic bioactive peptides are potential targets for drug development as are synthetic peptides engineered as co-agonists stimulating more than one receptor. We can therefore expect a dynamic development within this field in the coming years.

Glucagon regulation of energy metabolism

Glucagon has long been known as a counter-regulatory hormone to insulin of fundamental importance to glucose homeostasis. Its prominent ability to stimulate glycogenolysis and gluconeogenesis, has historically cast this peptide as one hormone where the metabolic consequences of increasing blood glucose levels, especially in obesity, are viewed largely as being deleterious. This perspective may be changing in light of emerging data and reconsideration of historic studies, which suggest that glucagon has beneficial effects on body fat mass, food intake, and energy expenditure. In this review, we discuss the mechanisms of glucagon-mediated body weight regulation as well as possible novel therapeutic approaches in the treatment of obesity and glucose intolerance that may arise from these findings. The paper represents an invited review by a symposium, award winner or keynote speaker at the Society for the Study of Ingestive Behavior [SSIB] Annual Meeting in Portland, July 2009.

The role of gut hormones and the hypothalamus in appetite regulation

The World Health Organisation has estimated that by 2015 approximately 2.3 billion adults will be overweight and more than 700 million obese. Obesity is associated with an increased risk of diabetes, cardiovascular events, stroke and cancer. The hypothalamus is a crucial region for integrating signals from central and peripheral pathways and plays a major role in appetite regulation. In addition, there are reciprocal connections with the brainstem and higher cortical centres. In the arcuate nucleus of the hypothalamus, there are two major neuronal populations which stimulate or inhibit food intake and influence energy homeostasis. Within the brainstem, the dorsal vagal complex plays a role in the interpretation and relaying of peripheral signals. Gut hormones act peripherally to modulate digestion and absorption of nutrients. However, they also act as neurotransmitters within the central nervous system to control food intake. Peptide YY, pancreatic polypeptide, glucagon-like peptide-1 and oxyntomodulin suppress appetite, whilst ghrelin increases appetite through afferent vagal fibres to the caudal brainstem or directly to the hypothalamus. A better understanding of the role of these gut hormones may offer the opportunity to develop successful treatments for obesity. Here we review the current understanding of the role of gut hormones and the hypothalamus on food intake and body weight control.

A new glucagon and GLP-1 co-agonist eliminates obesity in rodents

We report the efficacy of a new peptide with agonism at the glucagon and GLP-1 receptors that has potent, sustained satiation-inducing and lipolytic effects. Selective chemical modification to glucagon resulted in a loss of specificity, with minimal change to inherent activity. The structural basis for the co-agonism appears to be a combination of local positional interactions and a change in secondary structure. Two co-agonist peptides differing from each other only in their level of glucagon receptor agonism were studied in rodent obesity models. Administration of PEGylated peptides once per week normalized adiposity and glucose tolerance in diet-induced obese mice. Reduction of body weight was achieved by a loss of body fat resulting from decreased food intake and increased energy expenditure. These preclinical studies indicate that when full GLP-1 agonism is augmented with an appropriate degree of glucagon receptor activation, body fat reduction can be substantially enhanced without any overt adverse effects.