RCSI Glucagon & Incretin Synthesis, Release & Action PDF

RCSI Royal College of Surgeons in Ireland Coláiste Ríoga na Máinleá in Éirinn Glucagon & Incretin Synthesis, Release & Action Class Year 2 Course Biochemistry Lecturer Dr Jeevan Date 20/01/2025  Learning objectives 1. Describe the structure, synthesis & role of g...

RCSI Royal College of Surgeons in Ireland Coláiste Ríoga na Máinleá in Éirinn Glucagon & Incretin Synthesis, Release & Action Class Year 2 Course Biochemistry Lecturer Dr Jeevan Date 20/01/2025  Learning objectives 1. Describe the structure, synthesis & role of glucagon in the human body 2. Outline the factors that stimulate glucagon & incretin release 3. Describe the metabolic effects of glucagon & incretin 4. Outline the pathways involved in the propagation of glucagon signalling 5. Describe the symptoms of hypoglycaemia & body’s response to it 6. Outline the different forms of hypoglycaemia – i) insulin dependent, ii) post-prandial, iii) fasting and iv) alcohol-induced  Alpha (glucagon) Glucagon LO1 29 amino acids Glucagon: Peptide hormone produced by alpha cells of the pancreas. Regulates Blood Glucose: Increases blood sugar levels when they fall. Acts as a hyperglycemic hormone (opposes insulin). Stimulated by: Low blood glucose (e.g., fasting, exercise). High amino acid levels (prevents hypoglycemia after protein-rich meals). Epinephrine (stress response). Inhibited by: High blood glucose and insulin. Normal blood glucose 70-120 mg/dL (3.9-7.1 mmol/L)  Glucagon synthesis LO1 Rough ER (pre-proglucagon) → Golgi (proglucagon) → Secretory vesicles (mature glucagon). 1. Pre-proglucagon: Synthesized in the rough endoplasmic reticulum (ER) of pancreatic alpha cells. 2. Proglucagon Formation: Proteolytic processing in the rough ER generates proglucagon. 3. Mature Glucagon: 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 is triggered by low blood glucose or amino acids.  Glucagon – Structure & Tissue-Specific LO1 Processing of Proglucagon Proglucagon undergoes tissue-specific post-translational processing to yield different peptides depending on the site of synthesis. Same gene, different outcomes 1.Pancreatic Processing (Alpha Cells): Glucagon (primary active hormone). Glicentin-related pancreatic polypeptide (GRPP). Intervening peptide 1 (IP1). Major proglucagon fragment (MPGF). 2. Intestinal Processing (L Cells): Glucagon-like peptide 1 (GLP-1): Enhances insulin secretion (incretin effect). Glucagon-like peptide 2 (GLP-2): Promotes intestinal growth and absorption. Oxyntomodulin: Regulates appetite and energy expenditure. Intervening peptide 2 (IP2). Glicentin: Regulates gastric motility and insulin secretion.  Glucagon Release LO1 Mechanism of Release: Secretory Vesicles: 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 (hypoglycemia). High amino acids (after protein-rich meals). Epinephrine (during stress/exercise). Inhibited by: High blood glucose (hyperglycemia). Insulin (paracrine inhibition from beta cells). Low glucose → Alpha cell activation → Glucagon release High glucose → Inhibition of glucagon release  Mechanism of Glucagon Release LO1 1. Low Glucose (Glucagon Release): Low ATP (due to low glucose) → K⁺ channels remain open. This maintains a membrane potential that keeps voltage-dependent Ca²⁺ channels open. Ca²⁺ influx raises intracellular calcium → triggers exocytosis of glucagon. 2. High Glucose (Inhibition of Release): High ATP (from increased glucose) → K⁺ channels close. Membrane depolarizes, closing Ca²⁺ channels. No Ca²⁺ influx → glucagon release is inhibited. Journal of Endocrinology (2008) 199, 5-19  LO1 Comparison of Insulin & Glucagon Release Mechanisms Beta Cells (Insulin): Depolarization opens Ca²⁺ channels → insulin release. Alpha Cells (Glucagon): Depolarization closes Ca²⁺ channels → glucagon inhibition. Insulin Release (Beta Cells): High glucose → Increased ATP. Mechanism: ATP closes K⁺ channels → Membrane depolarization. Depolarization opens voltage-gated Ca²⁺ channels. Ca²⁺ influx triggers insulin exocytosis. Glucagon Release (Alpha Cells): High glucose: Increased ATP → K⁺ channels close → Membrane depolarization. Depolarization closes Ca²⁺ channels, inhibiting glucagon release. Low glucose → Low ATP. Mechanism: K⁺ channels remain open at low ATP levels. Membrane potential keeps Ca²⁺ channels open. Ca²⁺ influx allows glucagon secretion. Journal of Endocrinology (2008) 199, 5-19  Factors Increasing Glucagon Secretion LO2 Glucagon is continuously secreted at low levels to maintain glucose homeostasis during fasting. Factors that Increase Glucagon Secretion: 1. Low Blood Glucose: Opposite to insulin. Prevents hypoglycemia by stimulating glycogenolysis (Breakdown of Glycogen) and gluconeogenesis (synthesis of Glucose). 2. Low Plasma Fatty Acids: Signals an energy deficit → glucagon promotes fat breakdown to restore energy balance. 3. High Plasma Amino Acids: Particularly after protein-rich meals. Stimulates glucagon and insulin release to balance glucose Low glucose, amino acids, levels. and adrenaline → Glucagon 4. Increased Adrenaline (Epinephrine): secretion → Glycogen “Fight or flight” response → exercise, stress, trauma, and breakdown and glucose fasting. release. Prepares the body for energy demand by promoting glucose availability.  LO2 Factors Decreasing Glucagon Secretion Factors that Decrease Glucagon Secretion: 1. High Blood Glucose: Directly inhibits glucagon release by signalling sufficient energy availability. Opposite to insulin’s stimulation. 2. Insulin (Paracrine Effect): Insulin from beta cells inhibits glucagon release by acting directly on neighbouring alpha cells. This effect is difficult to distinguish from glucose’s inhibitory effect. 3. Somatostatin (Paracrine Effect): Released by delta cells in response to food intake. Inhibits both glucagon and insulin secretion. May prolong nutrient absorption by slowing hormone release, ensuring glucose availability over time.  Major Effects of Glucagon LO3 (Summary): Primary Goal: Increase blood glucose levels to maintain energy supply for the brain and muscles. Key Actions: ↑ Glycogenolysis – Breakdown of liver glycogen. ↑ Gluconeogenesis – Synthesis of glucose from non-carbohydrate sources (amino acids, lactate). ↓ Glycolysis – Inhibits glucose breakdown to preserve glucose. ↓ Glycogenesis – Inhibits glycogen formation. ↑ Lipolysis – Fatty acid breakdown. Glucagon is key in providing ↑ Ketogenesis – Ketone body formation (alternate adequate circulating fuel during fasting). glucose for brain function & for working muscle during exercise  Some additional Effects of glucagon: Liver: ↑ Amino acid uptake to fuel gluconeogenesis. ↑ Triglyceride hydrolysis, beta-oxidation, and ketogenesis. Adipose Tissue: Catecholamines (adrenaline), growth hormone (GH), and corticosteroids exert stronger effects than glucagon.  Signal Transduction Pathway LO4 (Mechanism of Action): 1 Glucagon receptor is a G-protein coupled receptor (GPCR). 2 Signal Cascade: 1. Glucagon binds to GPCR. 3 4 2. Adenylate cyclase is activated by the G-protein. 3. cAMP is produced, stimulating protein kinase A (PKA). 4. PKA phosphorylates downstream enzymes, triggering glycogen breakdown and glucose release.  LO2 Incretin Release & Effects LO3 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 LO2 LO3 Release: Stimulated by: Food intake (especially glucose and fats). Nutrients passing through the small intestine. Inhibited by: Fasting and low glucose states.  Key Effects of Incretins: LO2 LO3 1. Increase Insulin Secretion (Glucose-Dependent): 2. Inhibit Glucagon Secretion (Glucose-Dependent): Suppress glucagon release from pancreatic alpha cells when glucose is high. Prevents unnecessary glucose production during post-meal states. 3. Slow Gastric Emptying: Delays gastric emptying, which prolongs nutrient absorption and reduces postprandial glucose spikes. 4. Promote Satiety (Reduced Food Intake): GLP-1 agonists (eg Semaglutide) used in Acts on the brain to reduce appetite and food treatment of Diabetes Mellitus intake, contributing to weight control.  LO5 Clinical Application – When Glucose Balance Fails  Case Overview: LO5 Patient Profile:68-year-old man with confusion, vomiting, and inability to eat. Relevant History: Type 2 Diabetes Mellitus (T2DM), hypertension, hyperlipidemia. Current Medications: Insulin glargine, insulin aspart (diabetes). Metformin (diabetes). Losartan (hypertension). Simvastatin (lipid-lowering).  Hypoglycemia – Overview LO5 ADA Standard of Care 2022 Normal Range: 70-100 mg/dL (3.9-5.5 mmol/L). Hypoglycemia: