Glucagon Regulation and Synthesis

• Glucagon and incretin are key in regulating blood glucose.
• The presentation outlines their synthesis, release, actions, and how they relate to hypoglycemia.
• The lecture was presented by Dr. Jeevan on January 20th, 2025, for Year 2 Biochemistry students.

Learning Objectives

• Describe the structure, synthesis, and role of glucagon in the human body.
• Outline the factors that stimulate glucagon and incretin release.
• Describe the metabolic effects of glucagon and incretin.
• Outline the pathways involved in glucagon signaling.
• Describe the symptoms of hypoglycemia and the body's response.
• Outline the different forms of hypoglycemia: insulin-dependent, post-prandial, fasting, and alcohol-induced.

Glucagon

• Glucagon is a peptide hormone produced by the alpha cells of the pancreas and consists of 29 amino acids.
• Glucagon regulates blood glucose levels by increasing blood sugar when they fall.
• Acts as a hyperglycemic hormone, opposing insulin.
• Stimulated by low blood glucose (fasting, exercise), high amino acid levels (after protein-rich meals), and epinephrine (stress response).
• Inhibited by high blood glucose and insulin.
• Glucagon regulates blood glucose levels by stimulating the breakdown of glycogen to glucose.
• Normal blood glucose range is 70-120 mg/dL (3.9-7.1 mmol/L).

Glucagon Synthesis

• Pre-proglucagon is synthesized in the rough endoplasmic reticulum (ER) of pancreatic alpha cells.
• Proteolytic processing in the rough ER generates proglucagon.
• Prohormone convertase 2 (PC2) cleaves proglucagon in the Golgi to produce the mature glucagon hormone.
• The mature hormone is packaged into secretory vesicles and stored for release.
• Mature glucagon is stored in secretory vesicles until release, triggered by low blood glucose or amino acids.

Glucagon Structure and Tissue-Specific Processing

• Proglucagon undergoes tissue-specific post-translational processing depending on the site of synthesis.

Pancreatic Processing (Alpha cells)

• Glucagon is the primary active hormone.
• Also produces glicentin-related pancreatic polypeptide (GRPP), intervening peptide 1 (IP1), and major proglucagon fragment (MPGF).

Intestinal Processing (L cells)

• Produces glucagon-like peptide 1 (GLP-1), which enhances insulin secretion (incretin effect).
• Produces glucagon-like peptide 2 (GLP-2), which promotes intestinal growth and absorption.
• Oxyntomodulin regulates appetite and energy expenditure.
• Also produces intervening peptide 2 (IP2) and glicentin, which regulates gastric motility and insulin secretion.

Glucagon Release

• Vesicles containing mature glucagon fuse with the cell membrane.
• Exocytosis releases glucagon into circulation.
• Constant low-level secretion maintains glucose homeostasis during fasting.
• Stimulated by low blood glucose, high amino acids, epinephrine.
• Inhibited by high blood glucose and insulin (paracrine inhibition from beta cells).
• Low glucose leads to alpha cell activation and glucagon release.
• High glucose leads to inhibition of glucagon release.

Mechanism of Glucagon Release: Low Glucose

• Low ATP (due to low glucose) results in K+ channels remaining open.
• Maintains a membrane potential that keeps voltage-dependent Ca2+ channels open.
• Ca2+ influx raises intracellular calcium, triggering exocytosis of glucagon.

Mechanism of Glucagon Release: High Glucose

• High ATP (from increased glucose) causes K+ channels to close.
• The membrane depolarizes, closing Ca2+ channels.
• No Ca2+ influx, so glucagon release is inhibited.

Insulin and Glucagon Release

• Beta cells (insulin): Depolarization opens Ca2+ channels, leading to insulin release.
• Alpha cells (glucagon): Depolarization closes Ca2+ channels, leading to glucagon inhibition.

Insulin Release (Beta Cells)

• High glucose leads to increased ATP.
• ATP closes K+ channels, causing membrane depolarization.
• Depolarization opens voltage-gated Ca2+ channels.
• Ca2+ influx triggers insulin exocytosis.

Glucagon Release (Alpha Cells)

• High glucose increases ATP, closing K+ channels, leading to membrane depolarization.
• Depolarization closes Ca2+ channels, inhibiting glucagon release.

Factors Increasing Glucagon Secretion

• Glucagon is continuously secreted at low levels to maintain glucose homeostasis during fasting.
• Low blood glucose: Opposite to insulin, prevents hypoglycemia by stimulating glycogenolysis and gluconeogenesis.
• Low plasma fatty acids signal an energy deficit, promoting fat breakdown.
• High plasma amino acids, particularly after protein-rich meals, stimulate glucagon and insulin to balance glucose levels.
• Increased adrenaline (epinephrine) during "fight or flight" responses prepares the body for energy demand by promoting glucose availability.

Factors Decreasing Glucagon Secretion

• High blood glucose: Directly inhibits glucagon release, signaling sufficient energy availability (opposite to insulin).
• Insulin (paracrine effect): Insulin from beta cells inhibits glucagon release by acting directly to neighboring alpha cells.
• Somatostatin (paracrine effect): Released by delta cells in response to food intake, inhibits both glucagon and insulin secretion.
• Somatostatin may prolong nutrient absorption by slowing hormone release, ensuring glucose availability over time.

Major Effects of Glucagon

• Primary Goal: Increase blood glucose levels to maintain energy supply for the brain and muscles.
• Key Actions:
  • Increase glycogenolysis which is breakdown of liver glycogen.
  • Increase gluconeogenesis which is synthesis of glucose from non-carbohydrate sources.
  • Decrease glycolysis – inhibits glucose breakdown to preserve glucose.
  • Decrease glycogenesis, inhibiting glycogen formation.
  • Increase lipolysis which is breakdown of fatty acids.
  • Increase ketogenesis which is ketone body formation for alternative fuel during fasting.
• Glucagon is key in circulating adequate glucose for brain function and working muscles.

Additional Effects of Glucagon

• Liver: Increases amino acid uptake to fuel gluconeogenesis and triglyceride hydrolysis, beta-oxidation, and ketogenesis.
• Adipose Tissue: Catecholamines (adrenaline), growth hormone (GH), and corticosteroids exert stronger effects than glucagon.

Signal Transduction Pathway

• Glucagon receptor is a G-protein coupled receptor (GPCR).
• Glucagon binds to GPCR.
• Adenylate cyclase is activated by the G-protein.
• cAMP is produced, stimulating protein kinase A (PKА).
• PKA phosphorylates downstream enzymes, triggering glycogen breakdown and glucose release.

Decreased Activity due to Glucagon

• Glycogen synthase
• Phosphofructokinase
• Pyruvate kinase
• Acetyl CoA carboxykinase
• HMG CoA reductase

Resulting function due to decreased activity

• Glycogenesis
• Glycolysis
• Synthesis of fatty acids and fats

Increased Activity due to Glucagon

• Glycogen phosphorylase
• Glucose-6-phosphatase
• Fructose-1,6-bisphosphatase
• PEP carboxy kinase
• Transaminases

Resulting function due to increased activity

• Glycogenolysis
• Gluconeogenesis

Incretin Release and Effects

• Incretins are gut-derived hormones released in response to food intake.
• The two primary incretins: Glucagon-like peptide-1 (GLP-1) secreted by L-cells in the small intestine.
• Glucose-dependent insulinotropic peptide (GIP) secreted by K-cells in the duodenum.

Factors Stimulating Incretin Release

• Stimulated by food intake (especially glucose and fats) and nutrients passing through the small intestine.
• Inhibited by fasting and low glucose states.

Key Effects of Incretins

• Increase insulin secretion which is glucose-dependent.
• Inhibits glucagon which secretion is glucose-dependent, suppresses release from pancreatic alpha cells when glucose is high.
• Slowes gastric emptying which prolongs nutrient and reduces prostprandial glucose spikes
• Improves satiety and reduces food intake which contributes to weight control
• GLP-1 agonists like Semaglutide are used in the treatment of Diabetes Mellitus

Clinical Application: Hypoglycemia

• Normal blood glucose range: 70-100 mg/dL (3.9-5.5 mmol/L).
• Hypoglycemia: below 70 mg/dL (3.9 mmol/L); Severe: associated with morbidity and mortality.
• Mild Symptoms: Shakiness, irritability, tachycardia, hunger.
• Severe (Neuroglycopenia): Confusion, seizures, loss of consciousness (LOC), and death.

ADA Standard of Care 2022

• Level 1: Glucose <70 mg/dL (3.9 mmol/L) and ≥54 mg/dL (3.0 mmol/L).
• Level 2: Glucose <54 mg/dL (3.0 mmol/L).
• Level 3: Altered mental and/or physical status requiring assistance.

Counter-regulatory Response to Hypoglycemia

• Goal: Prevent or correct hypoglycemia by increasing blood glucose levels when glucose falls below ~70 mg/dL (3.9 mmol/L).
• Liver: Glycogen storage, glucose production (gluconeogenesis), and lipid metabolism.
• Liver responds to insulin and glucagon to maintain blood sugar levels.
• Hypothalamus activates the ANS to promote epinephrine release and glucose mobilization in severe hypoglycemia.
• Hormones: Glucagon (primary), adrenaline (epinephrine), released by ANS, increases glycogen breakdown, inhibits insulin release, mobilizes glucose quickly.
• Cortisol (long-term response): Reduces glucose utilization, promotes gluconeogenesis.
• Growth hormone (GH): Decreases glucose uptake by muscle, promotes lipolysis.

Hypoglycemia-Associated Autonomic Failure (HAAF)

• Recurrent hypoglycemia lowers the threshold for counterregulatory responses.
• Patients experience hypoglycemia unawareness (lack of symptoms like sweating, tachycardia), leading to delayed response.
• Can cause rapid progression to severe hypoglycemia (coma, seizures).

Causes of Hypoglycemia

• Insulin-Induced Hypoglycemia, Postprandial Hypoglycemia, Fasting Hypoglycemia, Ethanol-Induced Hypoglycemia.

Whipple's Triad

• diagnostic criteria for true hypoglycemic disorders (e.g., insulinoma)
• Symptoms of hypoglycemia.
• Low plasma glucose (lab confirmed during symptoms).
• Symptom resolution after glucose administration.
• Helps differentiate clinical from normal glucose fluctuations.

Insulin-Induced

• Hypoglycemia occurs in T1D or T2D patients on insulin when there is too much in bloodstream
• leads to to low blood sugar levels (skpping meals after insulin injection)
• Oral glucose is recommended if the patient is conscious, otherwise give glucagon

Postprandial hypoglycemia

• Hypoglycemia occurs within 4 hours of eating, with response post-meal and is exagerrated
• Causes include: Post-gastric bypass (bariatric surgery), and Insulin autoimmune hypoglycemia

Fasting hypoglycemia

• occurs during prolongued fasting when gluconeogenesis fails
• Rare in healthy indivduals
• Symptoms include neuroglycopenia, confusion, seizures, LOC and C-peptide
• Causes includes pancreatic Tumour, Adrenal Insufficiency and liver damage

Alcohol-induced hypoglycemia

• Related to alcohol abuse disorder and is binge drinking with poor nutrition/fasting
• alcohol metabolism creates excess NADH, diverting Glucose
• glycogenolysis may happen intially to compensate but glycogen depletes

Hypoglycemia Treatment: Rule of 15

• If conscious, administer 15 g of glucose tablets or glucose gel.
• Check blood glucose in 15 minutes.
• If still below 3.9 mmol/L (<70 mg/dL), administer another 15 g of glucose tablets.
• If the patient is not tolerating oral intake, administer glucagon subcutaneously, intravenously, intramuscularly, or intranasally.

This quiz covers glucagon's role in blood glucose regulation, its synthesis pathway involving prohormone convertase 2 (PC2), and its opposition to insulin. It also addresses the metabolic changes and tissue-specific processing of proglucagon and related genetic defects.