Glucagon

What is the physiological role of glucagon in glucose homeostasis?

Glucagon is a peptide hormone that works in opposition to insulin to maintain blood glucose levels. Primarily, glucagon stimulates the liver to release glucose through increased glycogenolysis and gluconeogenesis when blood glucose levels fall too low. It also decreases glycogen synthesis, increases ketogenesis, and promotes lipolysis and proteolysis . This counter-regulatory role to insulin is essential for preventing hypoglycemia during fasting states. Research has demonstrated that glucagon secretion is inhibited by elevated glucose levels and stimulated by decreased glucose concentrations, creating a feedback loop that maintains glucose homeostasis . The balance between insulin and glucagon actions is crucial for proper metabolic function, with disruptions in this balance contributing to metabolic disorders.

How does the "glucagonocentric hypothesis" differ from traditional views of diabetes pathophysiology?

The glucagonocentric hypothesis, proposed by Unger et al., challenges the traditional insulin-centric view of diabetes. While conventional understanding focused primarily on insulin deficiency, this hypothesis posits that excess glucagon plays a more essential role in the development of diabetes . The evidence supporting this includes: (1) hyperglucagonemia is present in all forms of diabetes, including type 2 diabetes and models of pancreatic destruction; (2) marked hyperglucagonemia occurs when anti-insulin serum is perfused to normal pancreas; and (3) most importantly, the metabolic manifestations of diabetes can be suppressed by glucagon suppressors like somatostatin even during total insulin deficiency . Further evidence comes from global glucagon receptor knockout (Gcgr-/-) mice, which demonstrate that β-cell destruction alone does not cause diabetes in the absence of glucagon signaling . This hypothesis has significant implications for therapeutic approaches, suggesting that targeting glucagon action could be as important as addressing insulin deficiency.

What experimental models are commonly used to study glucagon function?

Researchers employ several experimental models to investigate glucagon function:

• Glucagon receptor knockout (Gcgr-/-) mice: These mice do not respond to glucagon at any concentration, exhibit lower fasting blood glucose levels than wild-type mice, enhanced glucose tolerance, and elevated insulin sensitivity during insulin tolerance testing . This model has been instrumental in establishing the role of glucagon receptor signaling in glucose homeostasis.

• Acute glucagon administration studies: Randomized, crossover, single- or double-blind studies measuring ad libitum meal energy intake, energy expenditure, subjective appetite, glucose, and insulin following acute administration of glucagon compared to a control. These studies have revealed that acute glucagon administration produces substantial increases in energy expenditure (SMD: 0.72; 95% CI, 0.37–1.08; P < 0.001), circulating glucose (SMD: 1.11; 95% CI, 0.60–1.62; P < 0.001), and insulin concentrations (SMD: 1.33; 95% CI, 0.88–1.77; P < 0.001) .

• Organ-specific receptor deletion models: For example, studies removing glucagon receptors specifically from mouse kidneys have revealed the hormone's role in maintaining kidney health .

• Co-infusion models: Studies co-administering glucagon with other hormones like GLP-1 to investigate their combined effects on metabolic parameters .

These models provide complementary approaches to understanding glucagon's multifaceted roles in different physiological contexts.

What is the relationship between glucagon and energy expenditure?

Glucagon has been demonstrated to significantly increase energy expenditure in both animal and human studies. The initial observation of glucagon's effect on energy expenditure was made in 1960 by Salter, who showed through a pair-feeding paradigm that glucagon causes an increase in energy expenditure in rodents . This finding was subsequently confirmed through indirect calorimetry measurements .

In human studies, acute glucagon administration has been shown to produce moderate to large increases in energy expenditure, with a standardized mean difference (SMD) of 0.72 (95% CI, 0.37–1.08; P < 0.001) compared to controls . This effect appears to be consistent across studies and independent of glucagon's effects on food intake. The mechanism involves activation of brown adipose tissue and increased thermogenesis, which contributes to its potential application in obesity treatments. Importantly, when co-administered with GLP-1, glucagon maintains its energy expenditure-enhancing properties while its hyperglycemic effects are mitigated, offering a promising therapeutic avenue .

How do glucagon and GLP-1 receptor signaling pathways interact, and what are the implications for therapeutic development?

Glucagon and GLP-1 receptor signaling pathways exhibit complex interactions with significant therapeutic implications. Both receptors belong to the G protein-coupled receptor family but trigger distinct downstream effects. Glucagon primarily acts through GCGR to promote glucose production, ketogenesis, and energy expenditure, while GLP-1 acts through GLP-1R to enhance insulin secretion and suppress glucagon release .

Interestingly, glucagon can bind to both GCGR and GLP-1R, though with different affinities. In β-cells, glucagon promotes insulin secretion through both receptors, but the insulin-stimulating effect is primarily mediated by GLP-1R . This cross-reactivity has led to the development of dual GCGR and GLP-1R co-agonists as potential therapeutics for diabetes and obesity.

The ratio of GCGR to GLP-1R expression appears particularly crucial to human islet function due to the greater number of mixed α-β cell interfaces in human islets compared to rodent models . This anatomical difference may affect the translational relevance of findings from animal studies.

What is the evidence for extrahepatic functions of glucagon, particularly in kidney physiology?

Emerging research has revealed important extrahepatic functions of glucagon, particularly in kidney physiology. While glucagon's role in hepatic glucose production has been well-established, its function in the kidneys has only recently gained attention. Research from UT Southwestern Medical Center demonstrates that glucagon plays a key role in maintaining kidney health .

When researchers removed glucagon receptors specifically from mouse kidneys, the animals developed symptoms similar to chronic kidney disease (CKD) . This finding suggests that glucagon signaling in the kidneys is essential for normal renal function. According to study leader Philipp Scherer, Ph.D., "Our study defines important protective effects of glucagon for kidney health and normal systemic metabolic well-being of the entire organism" .

The mechanisms behind this protective effect likely involve glucagon's influence on renal blood flow, glomerular filtration rate, and tubular reabsorption processes. Further research is needed to fully elucidate these pathways, but this evidence suggests that therapeutic approaches targeting glucagon must consider its beneficial effects on kidney function to avoid unintended consequences.

This research has significant implications for understanding the pathophysiology of CKD, which affects hundreds of millions of people globally, and may lead to novel therapeutic strategies targeting the glucagon signaling pathway for kidney diseases.

How do the metabolic effects of glucagon differ between acute and chronic administration?

The metabolic effects of glucagon differ significantly between acute and chronic administration, a distinction critical for therapeutic applications. Acute glucagon administration produces well-documented effects including increased energy expenditure, elevated glucose production, stimulation of insulin secretion, and potential reduction in food intake, though the latter effect shows smaller and more variable responses (SMD: –0.19; 95% CI, –0.59 to 0.21; P = 0.345) .

In contrast, chronic elevation of glucagon, as seen in diabetes, contributes to pathological hyperglycemia through sustained hepatic glucose output and resistance to insulin's suppressive effects on gluconeogenesis . This chronic hyperglucagonemia is observed in all forms of diabetes and has been implicated in its pathogenesis .

The divergent effects may be explained by:

• Receptor desensitization: Prolonged exposure to glucagon leads to downregulation and desensitization of glucagon receptors, altering cellular responses over time.

• Compensatory mechanisms: Chronic glucagon elevation triggers compensatory responses in multiple organs that aren't observed with acute administration.

• Altered glucagon-to-insulin ratio: The relative balance between these hormones changes with chronic exposure, affecting metabolic outcomes.

Understanding these differences is crucial for therapeutic development, as treatments targeting acute glucagon effects (such as energy expenditure for obesity) must avoid the detrimental consequences of chronic activation.

What are the current challenges in developing dual glucagon/GLP-1 receptor agonists for obesity treatment?

Development of dual glucagon/GLP-1 receptor agonists for obesity treatment faces several significant challenges despite their promising therapeutic potential:

Recent progress includes SAR425899, which can reduce blood glucose and HbA1c levels while decreasing body weight in patients with type 2 diabetes, although gastrointestinal side effects remain problematic . It improves postprandial glucose control by enhancing β-cell function and slowing glucose absorption rate .

What are the optimal experimental conditions for studying glucagon-induced energy expenditure?

Designing experiments to accurately measure glucagon-induced energy expenditure requires careful consideration of multiple factors based on established methodological approaches:

• Study design: Randomized, crossover, single- or double-blind design is considered optimal, allowing each subject to serve as their own control, thereby reducing inter-individual variability . The washout period between crossover interventions should be sufficient to prevent carryover effects, typically 3-7 days.

• Control selection: Appropriate controls include vehicle solutions with identical composition excluding glucagon, or solutions containing a non-active peptide with similar physical properties.

• Measurement techniques: Indirect calorimetry remains the gold standard for measuring energy expenditure, with continuous measurement for at least 30-60 minutes after glucagon administration . Systems should measure both oxygen consumption and carbon dioxide production to calculate respiratory quotient, providing insights into substrate utilization.

• Glucagon dosing: Doses typically range from 50-150 pmol/kg/min for infusion studies, with lower doses (≥1 μg/kg) for bolus administration. Dose-response curves are valuable for determining threshold effects .

• Timing considerations: Peak effects on energy expenditure typically occur 30-45 minutes following glucagon administration, with measurement continuing until return to baseline .

• Confounding variables: Control for factors affecting energy expenditure including time of day (circadian effects), ambient temperature, prior food intake, physical activity, and body composition. Subjects should be studied after an overnight fast in thermoneutral conditions with limited movement.

• Concurrent measurements: Simultaneously measure plasma glucagon, glucose, insulin, and free fatty acids to correlate energy expenditure changes with hormonal and metabolic parameters.

This methodological approach has been validated in studies demonstrating that acute glucagon administration produces substantial increases in energy expenditure with a standardized mean difference of 0.72 (95% CI, 0.37–1.08; P < 0.001) .

How can researchers effectively differentiate direct glucagon effects from secondary metabolic responses?

Differentiating direct glucagon effects from secondary metabolic responses requires sophisticated experimental approaches:

• Tissue-specific receptor knockout models: Generating organ-specific glucagon receptor knockout models allows researchers to determine whether observed effects are directly mediated through glucagon receptors in the tissue of interest. For example, kidney-specific glucagon receptor knockout mice have revealed direct effects of glucagon signaling on renal function .

• Ex vivo tissue preparations: Isolating tissues and exposing them to controlled glucagon concentrations in the absence of other circulating factors can identify direct tissue responses. This approach has been particularly valuable for studying glucagon's effects on isolated hepatocytes, adipocytes, and pancreatic islets.

• Rapid sampling protocols: Implementing frequent sampling immediately after glucagon administration can capture the temporal sequence of events, distinguishing primary from secondary effects. Primary effects typically occur within minutes, while secondary effects may take 30+ minutes to develop.

• Receptor antagonist studies: Selective glucagon receptor antagonists can block direct effects while allowing secondary effects mediated through other pathways to persist, helping to distinguish the two.

• Combination with GLP-1: Co-administration with GLP-1 can help differentiate effects, as some studies have shown that GLP-1 blunts the rise in glucose seen with glucagon alone while preserving glucagon's effects on energy expenditure . This suggests different pathways for these effects.

• Clamp techniques: Hyperinsulinemic-euglycemic clamps can maintain stable blood glucose and insulin levels while administering glucagon, allowing researchers to observe glucagon effects independent of changes in these parameters.

• Molecular signaling analysis: Examining the activation of specific intracellular signaling pathways (e.g., cAMP, PKA, CREB) can identify the direct molecular consequences of glucagon receptor activation before secondary metabolic adaptations occur.

These approaches have been instrumental in delineating glucagon's direct effects on energy expenditure, appetite regulation, and kidney function from its secondary effects resulting from altered glucose and insulin levels.

What are the key considerations in designing clinical trials for glucagon-based therapeutics?

Designing clinical trials for glucagon-based therapeutics requires careful consideration of multiple factors to ensure safety, efficacy, and meaningful outcomes:

• Target population selection:

  • For diabetes applications: Consider disease type, duration, existing treatments, and baseline glycemic control

  • For obesity applications: Consider BMI range, comorbidities, and previous weight loss attempts

  • For combination therapies: Define appropriate exclusion criteria regarding contraindications to either component

• Dosing strategy optimization:

  • For dual agonists (glucagon/GLP-1): Determine optimal ratio to balance the beneficial weight loss effects of both hormones while minimizing hyperglycemic effects and gastrointestinal side effects

  • Consider dose-escalation protocols to improve tolerability

  • Evaluate both fixed-dose and dose-titration approaches

• Endpoint selection:

  • Primary endpoints:

    • For diabetes: HbA1c reduction, time-in-range for glucose

    • For obesity: Percent weight loss from baseline

    • For kidney applications: Markers of renal function

  • Secondary endpoints should include:

    • Energy expenditure measurements

    • Body composition changes (not just total weight)

    • Cardiovascular parameters

    • Quality of life measures

• Safety monitoring:

  • Hypoglycemia risk assessment

  • Hyperglycemia monitoring with glucagon-based therapies

  • Gastrointestinal tolerability

  • Cardiovascular safety (heart rate, blood pressure)

  • Pancreatitis risk (especially for incretin-based components)

• Trial design considerations:

  • Randomized controlled design with appropriate comparators (placebo and active comparators)

  • Consider adaptive designs to adjust dosing based on interim analyses

  • Sufficient duration to assess both short-term efficacy and long-term safety

    • Minimum 26 weeks for weight loss outcomes

    • 52+ weeks for cardiovascular and renal outcomes

• Pharmacokinetic/pharmacodynamic modeling:

  • Characterize the relationship between plasma concentrations and biological effects

  • Account for potential receptor desensitization with chronic administration

• Biomarker development:

  • Identify predictive biomarkers of response

  • Develop surrogate endpoints for long-term outcomes

Current clinical trials with dual agonists like SAR425899 have shown promising effects on both glucose control and weight reduction, though gastrointestinal side effects remain challenging . These emerging data provide valuable guidance for future trial design.

What techniques are most effective for measuring glucagon's effects on food intake in research settings?

Measuring glucagon's effects on food intake requires precise methodology to obtain reliable, reproducible results. Based on current research approaches, the following techniques are most effective:

• Ad libitum test meal design:

  • Standardized meals with excess food to prevent ceiling effects

  • Foods should be weighed before and after consumption with accuracy to 0.1g

  • Meals should be palatable but not highly hedonistic to avoid non-homeostatic eating

  • Multiple food options with varying macronutrient compositions help detect specific macronutrient preferences

• Timing considerations:

  • Administer glucagon 30-45 minutes before meal presentation for peak effect

  • Standardize time of day to control for circadian influences

  • Ensure consistent pre-study fasting periods (typically 8-12 hours)

• Measurement parameters:

  • Total energy intake (primary outcome)

  • Rate of consumption

  • Macronutrient selection

  • Meal duration

  • Satiety quotient (intake divided by change in hunger)

• Subjective appetite assessment:

  • Visual analog scales (VAS) for hunger, fullness, prospective consumption

  • Administer at baseline, pre-meal, and at regular intervals post-meal

  • Use electronic data capture to improve accuracy and prevent back-filling

• Study design optimization:

  • Randomized, crossover, single or double-blind design

  • Appropriate washout periods between conditions

  • Control for menstrual cycle phase in female participants

  • Consider habituation to laboratory environment before test days

• Contextual controls:

  • Standardize environmental conditions (temperature, noise, distractions)

  • Control for social factors by consistent presence/absence of researchers

  • Screen for eating disorders and highly restrained eaters

Meta-analysis of five studies (77 participants) using these methodologies has shown that the mean intervention effect of acute glucagon administration on energy intake was small (standardized mean difference: –0.19; 95% CI, –0.59 to 0.21; P = 0.345) . This suggests that while glucagon may reduce food intake, the effect is variable and more subtle than its impact on energy expenditure.

How do current approaches to targeting the glucagon pathway for diabetes treatment differ?

Current approaches to targeting the glucagon pathway for diabetes treatment operate from two seemingly contradictory but complementary therapeutic paradigms:

• Glucagon antagonism for glycemic control:

  • Based on the bi-hormonal abnormality hypothesis that hyperglucagonemia contributes to hyperglycemia in diabetes

  • Glucagon receptor antagonists are being developed to block glucagon's hyperglycemic effects

  • Clinical trials are underway using drugs which block glucagon activity to treat type 2 diabetes

  • Potential concerns include risk of hypoglycemia and effects on liver lipid metabolism

• Glucagon agonism for obesity-related diabetes:

  • Utilizes glucagon's effects on energy expenditure and appetite to target obesity, a major risk factor for type 2 diabetes

  • Most approaches combine glucagon receptor activation with incretin activity to mitigate hyperglycemic effects

  • Dual and triple agonists targeting glucagon receptor alongside GLP-1R and/or GIP receptors are in development

  • Example: SAR425899, a dual GCGR/GLP-1R agonist that reduces blood glucose, HbA1c levels, and body weight in patients with type 2 diabetes

• Novel insulin-glucagon combinations:

  • Development of protein molecules that combine insulin and glucagon to help reduce the burden of blood glucose highs and lows

  • Professor Michael Weiss's team at Indiana University has designed and tested a molecule combining both hormones

  • Initial tests in rats with type 1 diabetes found the combination can lower risk of hypoglycemia both at mealtimes and throughout the day

  • Current work focuses on optimizing the design to improve time-in-range and developing manufacturing methods

These diverse approaches reflect the complex role of glucagon in metabolism and the recognition that different aspects of glucagon biology can be targeted depending on the specific diabetes phenotype and comorbidities. The field is increasingly moving toward personalized approaches based on individual metabolic profiles.

What is the evidence for using glucagon in combination with GLP-1 for obesity treatment?

The scientific evidence for combining glucagon with GLP-1 for obesity treatment is substantial and growing:

This evidence supports the continued development of combination therapies targeting both glucagon and GLP-1 receptors as a promising approach for obesity treatment with potential applications in obesity-related metabolic disorders.

What is the potential role of glucagon receptor signaling in novel kidney disease therapeutics?

Recent research has revealed an unexpected and potentially transformative role for glucagon receptor signaling in kidney health, opening new avenues for kidney disease therapeutics:

• Protective effects on kidney function:

  • Studies from UT Southwestern Medical Center have demonstrated that glucagon plays a key role in maintaining kidney health

  • When researchers specifically removed glucagon receptors from mouse kidneys, the animals developed symptoms similar to chronic kidney disease (CKD)

  • According to Philipp Scherer, Ph.D., "Our study defines important protective effects of glucagon for kidney health and normal systemic metabolic well-being of the entire organism"

• Therapeutic implications:

  • These findings suggest that enhancing glucagon receptor signaling in the kidneys could potentially protect against or treat kidney disease

  • This represents a paradigm shift, as previous therapeutic approaches have focused primarily on blocking glucagon action for diabetes

  • Selective kidney-targeted glucagon receptor agonists could potentially provide renal protection without unwanted systemic metabolic effects

• Considerations for existing therapeutics:

  • Current glucagon receptor antagonists being developed for diabetes may need careful evaluation for potential adverse effects on kidney function

  • The dual role of glucagon highlights the importance of tissue-specific drug delivery systems

  • Patients with existing kidney disease may require special monitoring when receiving therapies that modulate glucagon signaling

• Research directions:

  • Further investigation is needed to elucidate the specific mechanisms by which glucagon protects kidney function

  • Determining whether the protective effects extend to human kidneys and various forms of kidney disease

  • Development of kidney-specific glucagon receptor modulators that could selectively enhance renal protection

  • Exploration of the interaction between glucagon signaling and established pathways involved in kidney disease progression

These findings have significant implications for the hundreds of millions of people affected by chronic kidney disease worldwide . As our understanding of glucagon's role in kidney physiology deepens, novel therapeutic approaches targeting this pathway may emerge as an important strategy in the nephrology armamentarium.

How might our understanding of glucagon's effects on energy expenditure inform future obesity treatments?

Glucagon's demonstrated effects on energy expenditure provide a foundation for innovative obesity treatment approaches that go beyond traditional appetite-suppressing strategies:

• Addressing the metabolic adaptation problem:

  • One of the most challenging aspects of weight loss maintenance is the reduction in energy expenditure that accompanies weight loss

  • GLP-1 analogues like liraglutide show initial dramatic weight reduction that often plateaus due to this metabolic adaptation

  • Glucagon can potentially counteract this adaptation by maintaining or increasing energy expenditure even during caloric restriction and weight loss

• Optimizing combination therapies:

  • The most promising approach incorporates glucagon's energy expenditure effects with the appetite-suppressing effects of GLP-1

  • Research shows that co-infusion of glucagon with GLP-1 at doses which, alone, were sub-anorectic, combined to cause significant reduction in food intake and increased energy expenditure

  • This dual approach targets both sides of the energy balance equation simultaneously

  • Future therapies may expand to triple agonists incorporating GIP (glucose-dependent insulinotropic polypeptide) receptor activation for enhanced metabolic benefits

• Targeted delivery systems:

  • Development of tissue-specific delivery systems could maximize glucagon's thermogenic effects in brown adipose tissue and muscle while minimizing unwanted effects in other tissues

  • Exploring pulsatile delivery systems that mimic physiological patterns may optimize efficacy while reducing receptor desensitization

• Personalizing treatment approaches:

  • Research indicates variable individual responses to glucagon's effects on energy expenditure and food intake

  • Genetic and phenotypic factors likely influence responsiveness to glucagon-based therapies

  • Developing biomarkers to predict individual responses could enable targeted use in patients most likely to benefit

• Overcoming current limitations:

  • Managing glucagon's hyperglycemic effects remains a key challenge

  • Recent research demonstrates that when co-administered with GLP-1, glucagon maintains its energy expenditure-enhancing properties while its hyperglycemic effects are mitigated

  • Continued refinement of dual agonist molecules like SAR425899 is needed to reduce gastrointestinal side effects while preserving beneficial metabolic effects

The standardized mean difference in energy expenditure with acute glucagon administration is substantial (SMD: 0.72; 95% CI, 0.37–1.08; P < 0.001) , highlighting the potential impact of harnessing this effect for obesity treatment. As obesity affects approximately 13% of the world's adult population , developing effective glucagon-based therapies represents a significant opportunity to address this global health challenge.

What are the most pressing unanswered questions in glucagon research?

Despite significant advances in our understanding of glucagon physiology and pathophysiology, several critical questions remain that will shape future research priorities:

• Tissue-specific glucagon actions: While glucagon's effects on liver and, more recently, kidney function are increasingly understood, its direct actions on other tissues including brain, muscle, adipose tissue, and the cardiovascular system remain inadequately characterized. Elucidating these tissue-specific effects will be essential for developing targeted therapies with optimal benefit-risk profiles.

• Molecular mechanisms of glucagon resistance: Hyperglucagonemia is present in various metabolic disorders, yet the molecular basis for altered glucagon sensitivity in different tissues remains poorly understood. Identifying the pathways leading to glucagon resistance could reveal novel therapeutic targets.

• Long-term efficacy and safety of dual/triple agonists: While acute studies show promising results for combined GCGR/GLP-1R activation, the long-term efficacy, safety, and potential for receptor desensitization with chronic administration require further investigation.

• Protective mechanisms in kidney function: The newly discovered role of glucagon in maintaining kidney health raises questions about the specific molecular pathways involved and how these might be therapeutically targeted without unwanted metabolic effects.

• Individual variability in response: The factors determining individual responsiveness to glucagon's effects on energy expenditure, appetite, and glucose metabolism remain largely unknown. Identifying genetic, epigenetic, and environmental determinants of this variability will be crucial for personalizing treatment approaches.