RCSI Insulin Synthesis, Release and Action PDF

RCSI Royal College of Surgeons in Ireland Coláiste Ríoga na Máinleá in Éirinn Insulin Synthesis, Release and Action Class Year 2 Course Biochemistry Lecturer Dr Jeevan Shetty Date 19/01/2025  Lecture Learning Outcomes 1. Describe the structur...

RCSI Royal College of Surgeons in Ireland Coláiste Ríoga na Máinleá in Éirinn Insulin Synthesis, Release and Action Class Year 2 Course Biochemistry Lecturer Dr Jeevan Shetty Date 19/01/2025  Lecture Learning Outcomes 1. Describe the structure of insulin & mechanisms involved in its synthesis & processing 2. Describe the regulation, modulation & mechanism of insulin secretion 3. Outline the different actions of insulin 4. Define & classify Diabetes Mellitus  Insulin & Diabetes LO1 Insulin is a peptide hormone, produced in the pancreas Its principal function is to allow the entry of glucose from the blood into certain tissues, including muscle & adipose tissue, & to enable cells to efficiently utilize glucose This has the effect of reducing blood glucose levels Diabetes Mellitus, which results from inability to synthesise insulin (type 1), or from an inability to respond correctly or sufficiently to insulin (type 2), results in raised blood glucose levels  Pancreas LO1 The pancreas is composed of exocrine & endocrine cells 1.Exocrine cells: acinar cells – secretion of digestive juices into duodenum 2. Endocrine cells: Islets of Langerhans – secrete insulin & glucagon (important for blood glucose control), somatostatin, & pancreatic polypeptide Islet cells are highly vascularized (10-15% of blood flow) & innervated by parasympathetic & sympathetic neurons  Pancreas: Tissues & Cells LO1 There are 5 types of endocrine Acinar somatostatin cells in the Islet of Langerhans, cell digestive each synthesizes & secretes a enzymes specific hormone glucagon 1. alpha cells – glucagon 2. beta cells – insulin insulin 3. delta cells – somatostatin 4. PP cells (F cells) – pancreatic polypeptide 5. Epsilon cells – ghrelin  Paracrine signals in the islet LO1 Alpha (α) Cells: Produce glucagon. Alpha Glucagon can stimulate β-cells to release (glucagon) insulin, but at the same time, insulin inhibits - glucagon release from α-cells. Somatostatin - - Beta Beta (β) Cells: Produce insulin. Insulin Insulin acts on α-cells to inhibit glucagon (insulin) release. Glucagon + + It also acts on δ-cells to modulate somatostatin release. + Delta (somatostatin) Delta (δ) Cells: Produce somatostatin. Somatostatin inhibits both insulin and glucagon release, acting as a local regulatory feedback mechanism.  Beta cells (insulin) Synthesis of insulin LO1 Polypeptide hormone, two chains (A and B) , 51 amino acids. Synthesis Pathway: 1. Pre-proinsulin: Synthesized in the rough ER ribosomes as a single polypeptide chain. 1 2. Proinsulin Formation: Pre-proinsulin is cleaved in the ER to form proinsulin. Proinsulin is transported to the Golgi apparatus. 2 3. Processing in Golgi: 3 Disulfide bonds form between A and B chains. Packaged into secretory granules. 4 4. Mature Insulin: In secretory granules, proinsulin is cleaved into equimolar amounts of insulin and C-peptide. Ready for exocytosis.  Synthesis of insulin: processing LO1 Maturation: Processing Time: 30–120 minutes. Proteases cleave C-peptide from proinsulin to yield mature insulin. Mature insulin forms a crystalline granule with zinc inside vesicles. Secretion: Vesicles fuse with the plasma membrane → Exocytosis of insulin and C-peptide. Half-life: Insulin: 3–5 minutes (removed by insulinases in liver/kidney). C-Peptide: 35 minutes (not removed during portal first pass). Insulinases ensure glucose levels are regulated by degrading excess insulin.  C-Peptide (Clinical Relevance) LO1 C-Peptide as a Marker: Reflects insulin secretion (produced in a 1:1 ratio with insulin). Longer half-life (35 minutes) than insulin (3–8 minutes). C-Peptide is retained during portal circulation, unlike insulin. Useful in assessing endogenous insulin secretion in diabetic patients Mechanism of Insulin Secretion Sulfonylureas- close the channel LO2 1 1.High blood glucose → glucose uptake via facilitated transport through GLUT2 → increase in 2 glycolysis & respiration → increase in ATP → 2.ATP-sensitive K channel closes → reduced efflux of potassium → membrane depolarization → 3 4 3.Voltage-gated Ca++ channel opens → influx of Ca++ → 4.Fusion of vesicle to membrane → exocytosis of insulin & C-Peptide Guyton and Hall 11th ed fig 78-9 Insulin is released in response to elevated blood glucose  Modulation of Insulin Secretion – Key Pathways LO2 1. Glucose Metabolism (Primary Trigger): Glucokinase: Acts as a glucose sensor; stimulated by insulin. ATP Production: Breakdown of glucose, amino acids, and fats increases ATP. High ATP closes K⁺ channels, leading to beta-cell depolarization and insulin release. 2. K⁺ Channel Regulation: Mutations in K⁺ channels → Cause neonatal hyperinsulinemia. Sulfonylurea drugs → Close K⁺ channels, enhancing insulin release by mimicking glucose action. 3. Voltage-Gated Calcium Channels (VGCC): Activation: Gut hormones (GIP, GLP-1): Increase cAMP → enhance calcium influx and insulin secretion. Inhibition: Adrenaline and somatostatin → Inhibit adenylate cyclase, reducing cAMP and calcium influx, suppressing insulin release. Glucokinase → ATP → K⁺ Channels → VGCC → Insulin Secretion  Regulation of Insulin Secretion – Factors Involved LO2 1.Meal Constituents (Directly Stimulate 2.Gastrointestinal Hormones (Enhance Secretion): Response): Glucose – Primary driver. GIP (Gastric Inhibitory Peptide). Amino Acids. GLP-1 (Glucagon-like Peptide-1). Free Fatty Acids. Cholecystokinin (CCK).  Phases of Insulin Release LO2 1. Insulin release occurs in 2 phases: Immediate Phase (3–5 minutes): Triggered by acute glucose rise (2-3x normal fasting levels). Plasma insulin spikes 10-fold. Initial burst drops halfway shortly after. Delayed Phase (Over ~1 hour): Gradual increase follows initial drop. Reflects release of preformed insulin and ongoing new synthesis. 2. Potential Mechanism of Release: Insulin is released in two stages (Guyton and Phase 1 (Immediate): Hall 11th ed fig 78-8) Insulin granules near capillaries are rapidly exocytosed. Phase 2 (Delayed): Granules further from capillaries + newly synthesized insulin contribute to sustained release.  Insulin Receptor Activation and LO3 Effects Activation of Insulin Receptor (IR): Insulin Binding: Insulin binds to the receptor’s alpha subunits → induces autophosphorylation of beta subunits. Activates tyrosine kinase activity. Signal Transduction: Phosphorylation of insulin receptor substrate (IRS) proteins. Initiates downstream signalling cascades  LO3 Effect of insulin Glucose Uptake: Insulin binding to its receptor on liver cells triggers glucose uptake. Glycogenesis: Insulin promotes glycogen production and storage by upregulating enzymes involved in glycogenesis. Glycolysis: Insulin increases glycolysis, breaking down glucose for energy. Lipogenesis: Insulin stimulates the synthesis of fats (lipogenesis) for energy storage. Protein Synthesis: Insulin also promotes protein synthesis.  Glucose Transport and Insulin Action LO3 1. Insulin and Glucose Uptake: Primary Effect: Increases glucose uptake in muscle and adipose tissue. Exception: Brain neurons do not require insulin for glucose uptake. 2. Mechanism of Glucose Transport: GLUT4 (Insulin-Dependent Glucose Transporter): Guyton and Hall 11th ed fig 78-4 Insulin is necessary for efficient Stored in vesicles within muscle and adipose cells. transport of glucose into muscle cells Insulin binding triggers vesicle fusion with the cell membrane. GLUT4 transporters are inserted into the membrane → Glucose uptake begins within seconds. Reversal Process (When Insulin is Removed): GLUT4 vesicles return to intracellular storage. Glucose uptake decreases as transporters are recycled. Insulin Receptor Recycling: Insulin-bound receptors are endocytosed into the cell. Insulin is degraded, and the receptor is recycled back to the membrane for reuse.  Glucose Transporters LO3 Transporter Tissue GLUT1 Ubiquitous, RBC, Facilitated diffusion placenta, colon, kidney GLUT2 Liver, small intestine, Facilitated diffusion pancreas bidirectional GLUT3 Ubiquitous, brain, Facilitated diffusion placenta, kidney GLUT4 Skeletal muscle, heart Facilitated diffusion muscle, adipose tissue (insulin dependent) GLUT5 Jejunum, Intestine, Secondary active kidney tubules transport using Na+ gradient Effects of Insulin on Metabolism LO3 Carbohydrate Metabolism: ↑ Glucose Uptake into muscle and adipose (via GLUT4). ↑ Glycogen Synthesis: Activates glycogen synthase. ↓ Glycogen Breakdown by inhibiting glycogen phosphorylase. ↑ Liver Glucokinase Activity: Promotes glucose storage. ↑ Pentose Phosphate Pathway: Supports NADPH production for biosynthesis. ↓ Gluconeogenesis: Reduces hepatic glucose production. Effects of Insulin on Metabolism LO3 Lipid Metabolism: ↑ Lipogenesis (Fat Storage): Activates capillary lipoprotein lipase → breaks down VLDL & chylomicrons → FA uptake into adipocytes. ↑ Glucose transport into adipose → glycerol-3-phosphate production → triglyceride synthesis. ↓ Lipolysis (Fat Breakdown): Inhibits hormone-sensitive lipase in adipose tissue. Liver: Glucose directed to fatty acid synthesis when glycogen stores are full → VLDL formation.  Insulin deficiency & lipid metabolism LO3 In the absence of insulin, use of stored fat for energy is increased Insulin Deficiency: ↑ Lipolysis: FA release from adipose → plasma free fatty acid levels rise. ↑ Plasma Cholesterol & Lipoproteins. Risk of Ketosis/Acidosis: Uncontrolled fat breakdown. Guyton and Hall 11th ed fig 78-5 Effects of Insulin on Metabolism LO3 Protein Metabolism: ↑ Protein Synthesis: Increases transcription of genes Stimulates amino acid uptake by cells. ↓ Protein Catabolism: Reduces the breakdown of proteins. ↓ Amino Acid Release from muscle and other tissues. ↓ Gluconeogenesis (Liver): Amino acids are key substrates for gluconeogenesis. Insulin inhibits enzymes involved in gluconeogenesis, preserving amino acids for protein synthesis.  SUMMARY SLIDE LO3 Carbohydrate Metabolism: Lipid Metabolism: Protein Metabolism: ↑ Glucose Uptake into muscle and ↑ Lipogenesis (Fat Storage): ↑ Protein Synthesis: adipose (via GLUT4). Activates capillary lipoprotein lipase Increases transcription of genes ↑ Glycogen Synthesis: → breaks down VLDL & chylomicrons → Stimulates amino acid uptake by cells. Activates glycogen synthase. FA uptake into adipocytes. ↓ Protein Catabolism: ↓ Glycogen Breakdown by ↑ Glucose transport into adipose → Reduces the breakdown of proteins. inhibiting glycogen phosphorylase. glycerol-3-phosphate production → ↓ Amino Acid Release from muscle and other ↑ Liver Glucokinase Activity: Promotes triglyceride synthesis. tissues. glucose storage. ↓ Lipolysis (Fat Breakdown): ↓ Gluconeogenesis (Liver): ↑ Pentose Phosphate Pathway: Supports Inhibits hormone-sensitive lipase in Amino acids are key substrates for NADPH production for biosynthesis. adipose tissue. gluconeogenesis. ↓ Gluconeogenesis: Reduces hepatic Liver: Insulin inhibits enzymes involved in glucose production. Glucose directed to fatty acid gluconeogenesis, preserving amino acids for synthesis when glycogen stores are full protein synthesis. → VLDL formation.  Insulin Counterregulatory hormones LO3 1. Catecholamines (Epinephrine & Norepinephrine): ↑ Glycogenolysis – Breaks down glycogen in muscle (myocytes) and liver (hepatocytes). ↑ Lipolysis – Mobilizes fatty acids from adipose tissue. ↑ Plasma Lactate – From glycolysis in muscle. 2. Glucocorticoids (Cortisol): Catabolic Effects: ↑ Protein Breakdown – In muscle and liver. ↑ FA Mobilization – Releases fatty acids from adipose tissue. ↑ Gluconeogenesis – Produces glucose from amino acids and glycerol. 3. Growth Hormone (GH): ↑ Lipolysis – Increases fat breakdown in adipocytes. ↑ Gluconeogenesis – Stimulates hepatic glucose production. ↓ Glucose Uptake – Reduces glucose entry into muscle cells (promotes insulin resistance).  Glucose Homeostasis – Hyperglycemia & LO4 Hypoglycemia Normal Range: Fasting blood glucose 3.9–5.6 mmol/L (70–100 mg/dL). Hypoglycemia (