Insulin and Diabetes Part 1

Insulin and Diabetes Part 1

1. Chronic Symptoms of Diabetes

   • Retinopathy - most common cause of blindness in people of working age
   • Macrovascular Disease - 2/3 x increased risk of heart disease or stroke
   • Foot Problems - 15% develop foot ulcers, 5-15% of them require amputation
   • Erectile Dysfunction - can affect up to 50% of men
   • Nephropathy - 16% of patients needing renal replacement therapy have diabetes

2. Pancreas

   • Exocrine tissue
     • Alkaline fluid with digestive enzymes
       • Small intestine
   • Endocrine tissue
     • Produce insulin and glucagon
       • Regulates blood glucose
   • 1-2% of total pancreas mass is Islets of Langerhans
     • Surrounded by rings of acinar cells which coat fine branches of the pancreatic duct
     • Include multiple cell types
       • Alpha (A) cells
         • Secrete glucagon
         • Found in islet mantle (periphery)
         • Common in splenic lobe (tail end)
       • Beta (B) cells
         • Secrete insulin (and IAPP/amylin - involved in glycemic regulation)
         • Make up 60-80% of islet endocrine cells
         • Primarily in islet medulla (core)
       • Delta (D) cells
         • Secrete somatostatin
         • Make up 5% of islet cells
       • PP (F) cells
         • Secret pancreatic polypeptide
         • Found in islet mantle (periphery)
         • Common in duodenal lobe (head end)
     • Islet blood flow
       • Islets have a rich blood supply
       • Arterioles ender islet peripheries at breaks and emerge at beta cell-rich regions in the islet core
         • Coalesce into collecting venules outside of islet
       • Blood flow is BAD (beta-alpha-delta)
     • Islet cell interactions
       • Intra-islet interactions
         • Via circulation within the islet
       • Paracrine actions
         • Between neighbouring cells via interstitial space
       • Gap junctions
         • Ions and small molecules (>1000 Da) pass directly between cells
     • Islet innervation
       • Richly innervated by autonomic system
       • Parasympathetic
         • Via vagus nerve
         • Transmitters - acetylcholine and neuropeptides (VIP - vasoactive intestinal peptide)
       • Sympathetic
         • Via coeliac ganglion
         • Transmitters - Noradrenaline and neuropeptides (NPY)
         • Neurohormonal (adrenal)
     • Islet Hormones
       • Somatastatin
         • Found in hypothalamus, gut, stomach and islets
         • Major islet form SS-14
         • Released in response to nutrients
         • Has inhibitory actions on most tissues
       • Islet amyloid polypeptide (IAPP/amylin)
         • Co-secreted with insulin (1:1 molecular ratio)
         • May inhibit gastric emptying - decreases appetite
         • Forms islet amyloid deposits in type 2 diabetes
       • Pancreatic polypeptide
         • Vagus nerve activation stimulates release
         • May inhibit pancreatic exocrine secretions

3. Insulin

   • 6 kDa peptide
     • Made of 2 chains - A chain and B chain
       • Linked by disulphide bonds
   • Formation
     • Proteolysis converts preproinsulin to proinsulin
       • Release of a signal peptide
     • Proinsulin
       • 9 kDa
       • Synthesised in beta cells
       • Proinsulin contains A and B chains and C peptide
     • 2 routes - same but in opposite directions
       • Proinsulin is split between arginine 32 and 33 to give Split 32, 33 proinsulin
         • By PC3 (type-I) endopeptidase
         • Arg 31 and 32 removed to give Des 31,32 proinsulin
           • By carboxypeptidse-H
           • Split and Arg 64 and 65 removed to give insulin and C peptide
             • By PC2 endopeptidase and carboxypeptidase-H
       • Proinsulin is split between arginine 65 and 66 to give split 65, 66 proinsulin
         • By PC2 (type-II) endopeptidase
         • Arg 64 and 65 removed to give Des 64, 65 proinsulin
           • Carboxypeptidase-H
           • Split and Arg 31 and 32 removed to give insulin and C peptide
             • By PC3 endopeptidase and carboxypeptidase-H
   • Regulation
     • Major stimulus is glucose
     • Other agents - neural, hormonal, nutrient
       • Most neural and hormonal stimulators and glucose dependent
     • Neural Stimulation
       • Parasympathetic
         • Neurotransmitters - Ach, VIP
           • Stimulate insulin secretion
       • Sympathetic
         • Neurotransmitters - Adrenaline, Noradrenaline
           • Inhibits insulin secretion
             • Alpha adrenergic
     • Hormonal
       • Incretins (GI hormones) released from the gut following food ingestion
         • GIP (gastric inhibitory polypeptide) - stimulatory
         • GLP-1 (glucagon-like peptide-1) - stimulatory
         • Somatastatin-28 from gut - inhibitory
       • Islet hormones (paracrine?)
         • Glucagon - stimulatory
         • Somatostatin - inhibitory
     • Nutrient Control
       • Glucose - stimulatory
         • Transported into beta-cell by GLUT-2
           • Undergoes glycolysis (glucokinase has low affinity but high specificity for glucose)
             • Change in ATP:ADP inhibits ATP K+ channels
               • Decrease in K+ efflux depolarises cell
                 • Voltage-depended calcium channel opens
                   • Ca2+ release (100-500 nM) into cell
                     • Insulin release by exocytosis
       • Leucine and arginine - stimulatory
       • Free fatty acids - stimulatory
     • Neurotransmitters and hormones act via specific receptors which activate different intracellular pathways
       • Activation is weak in the absence of glucose
         • Activation is very strong in the presence of glucose (and Ca2+) - synergistic
       • Ach binds to receptor - Gq and PLC bind
         • PIP2 becomes IP3 and DAG
           • IP3 stimulates ER which causes Ca2+ release
             • Ca2+ and DAG cause increase of PKCs (and Ca2+ increases CaMKs)
               • Increase of trafficking
                 • Exocytosis of insulin
       • GIP and GLP-1 (7-36) bind to receptor - Gs and AC bind
         • ATP converted to cAMP
           • Increase of PKAs
             • proteins

4. Glucagon

   • Single chain, 39 amino acid peptide - 3845 kDa
   • Has a large precursor - proglucagon
   • Involved in tissue-specific processing
     • Glucagon is found in islet alpha cells (PC2)
     • GLP-1 (7-36) and GLP-2 found in intestinal endocrine cells (PC1/3)
   • Main site of action is liver
     • Increased hepatic output by glycogenolysis and gluconeogenesis
     • Increased blood glucose
   • Regulation
     • Neural
       • Parasympathetic and sympathetic stimulate glucagon release
     • Hormonal
       • GI hormones
         • Cholecystokinin (CCK) and GIP - stimulatory
         • GLP-1 (7-36) and somatostatin - inhibitory
       • Islet hormones
         • Insulin - inhibitory (via islet circulation)
         • Somatostatin - inhibitory (via paracrine action)
     • Nutritional
       • Low glucose - stimulation
       • Arginine - stimulation
       • Fatty acids - inhibition

5. Biological Action of Insulin

   • Anabolic - energy storage
   • Has a critical role in growth and development
   • Major target tissues - muscle, fat, liver
   • Glucose uptake in muscle and adipose tissue
     • Insulin binds to insulin receptor, causing receptor phosphorylation
       • Stimulates GLUT-4 containing vesicle to move to the cell surface
         • Glucose enters cell via GLUT-4 receptors
           • In muscle - glucose is used as energy/stored as glycogen
           • In adipocyte - glucose is stored as fat (glycolysis provides glycerol for triglyceride synthesis)
             • Alpha-glycerophosphate becomes glycerol (and free fatty acids) which forms triglycerides
               • Inhibition of hormone sensitive lipase (which hydrolyses triglycerides into free fatty acids)
               • Stimulation of fatty acid synthesis from glucose
                 • Liver glycogen reaches 5-6%
             • Stimulation of lipoprotein lipase (on capillary wall in fat tissue)
   • Glucose metabolism
     • Insulin
       • Stimulation
         • Glucose-1-phosphate to glycogen
         • Glucose to glucose-6-phosphate
         • Fructose-6-phosphate to fructose-1,6-bisphosphate
         • Phosphoenolpyruvate
       • Inhibition
         • Glycogen to glucose-1-phosphate
         • Fructose-1,6-bisphosphate to fructose-6-phosphate
         • Pyruvate to oxaloacetate
     • Glycogen
       • Stimulation
         • Glycogen to glucose-6-phosphate
         • Pyruvate to oxaloacetate
         • Oxaloacetate to phosphoenolpyruvate
       • Inhibition
         • Phosphoenolpyruvate to pyruvate
   • Carbohydrate metabolism in liver
     • Stimulation of glycogen synthesis (glucose storage)
     • Stimulation of glycolysis (phosphorylation of glucose kinase, trapping glucose in cell and stimulating glucose uptake)
     • Inhibition of glycogenolysis
     • Inhibition of gluconeogenesis
   • Inhibition - gluconeogenesis, glucogenolysis, lipolysis, ketogenesis, proteolysis
   • Stimulation - Glucose uptake (in muscle/adipose), glycolysis, glycogen synthesis, protein synthesis, uptake of ions (K+ and PO4-3)

6. Insulin Receptor

   • Glycoprotein
   • 2 alpha and 2 beta subunits
     • Alpha subunits (MW 135K) are extracellular - joined by disulfide bonds
     • Beta subunits (MW 95K) span cell membrane
     • Each beta subunit bound to alpha subunit by a disulphide bond
   • Tyrosine kinase
   • Insulin binds - phosphorylation of tyrosine on beta subunit
     • Growth signal - Shc phosphorylation
       • Ras-MAPK pathway
         • Cell proliferation
         • Protein synthesis
     • Metabolic signal
       • Phosphorylation of IRS family proteins (IRS-1/-2/-3/-4
         • Cascade of serine and tyrosine phosphorylation/dephosphorylation (involving PI3K)
           • Protein synthesis
           • Glycogen synthesis
           • Lipid metabolism
           • Glucose uptake (via GLUT-4 translocation)