Pancreas Bi1100en Hormones – Cellular and Molecular Mechanisms ▪ behind stomach and under liver, the head of pancreas is next to duodenum ▪ approx. 15 cm; 60-90 g ▪ endocrine (1.5 - 4.5 % of volume) and exocrine function (production of pancreatic juice containing HCO3 - and precursors of digestive enzymes) Pancreas ▪ fibrous sheath on the surface and septa reinforcing the inner tissue ▪ dense network of capillaries along septa ▪ exocrine alveolar gland divided into lobes (acini) – acinar cells + centroacinous cells ▪ Islets of Langerhans (approx. 1-3 mil.): α-cells > glucagon β-cells > insulin PP (γ- / F) cells > pancreatic polypeptide δ-cells > somatostatin ε-cells > ghrelin Microanatomy of pancreas Pancreas - endocrine function ▪ hormones travel through the portal blood to the liver: 1. nutrient storage (glycogen, storage lipids) 2. mobilization of energy reserves during starvation, physical activity and stress (glucagon, adrenaline) 3. regulation of glycemia 4. growth stimulation ▪ humoral and paracrine regulation: adrenaline activates α-cells (glucagon) and inhibits β-cells (insulin) glucose inhibits α-cells (glucagon) and activates β-cells (insulin) glycogen activates α-cells (glucagon) somatostatin inhibits α- (glucagon) and β-cells (insulin) insulin inhibits α-cells (glucagon) located at the edge of islets 1869 Paul Langerhans - described islets of Langerhans in the pancreas 1889 Oscar Minkowski - connection between pancreas and diabetes (dog surgery) 1920 Frederick Banting and Charles Best - pure isletin extracted 1922 the world's first insulin-treated diabetic patient 1923 Nobel Prize in Physiology or Medicine (Banting and Macleod) 1958 Nobel Prize in Chemistry (Frederick Sanger for describing the structure of insulin) Insulin ▪ a peptide composed of 51 AMK (6 kDa) ▪ chain A and B connected by two disulfide bonds ▪ preproinsulin > proinsulin (84 AMK) > cleavage of chain C > insulin ▪ half-life 5-8 min ▪ degraded in liver and kidneys (endocytosis of the insulin-receptor complex) ▪ insulin released in pulses, the main stimulus is increase in blood glucose ▪ mechanism of insulin release: ↑ glucose in plasma > ↑ glucose in β-cells (GLUT2) > ↑ glc oxidation (Krebs cycle) > ↑ ATP > closing the ATP-controlled K+ channels > depolarization > opening the potential-driven Ca2+ channels> ↑ Ca2+ in the cell > insulin exocytosis and opening of K+ channels ▪ stimulation through vagal nerve, gastrin, secretin, GIP (gastric inhibitory polypeptide/enterogastron), GLP-1 (glucagon-like peptide/enteroglucagon) ▪ some AAs, free fatty acids, some pituitary and steroid hormones increase the secretion of insulin Insulin Insulin: synthesis translation (β-cells) ↓ preproinsulin ↓ translocation to the ER lumen (cleavage of 24 AA signal peptide) ↓ proinsulin ↓ transport to GA ↓ convertases cleave out C-peptide and stimulates synthesis of insulin ↓ stored in secretory granules before stimulating exocytosis Insulin: synthesis ▪ chain A of 21 AAs stabilized by a disulfide bridge ▪ chain B of 30 AAs ▪ chains inter-connected by two disulfide bridges C-peptide ▪ pulse release based on glycemia ▪ pancreas of a healthy adult contains about 6-10 mg of insulin, of which about 2 mg is used daily Insulin Insulin PKB insulin binding > autophosphorylation of receptor β subunit > insulin receptor substrate 1 (IRS-1) and its phosphorylation > phosphorylation of intracellular proteins with SH2-domains (protein kinase B = Akt) > increase of glycogen synthase activity and incorporation of GLUT4 glucose transporters into the cell membrane ▪ concentration of insulin in between meals is about 57-79 pmol/l Functions: ▪ lowers blood glucose ▪ supports growth (Ras-MAPK) and anabolism (fat formation, supports storing of glc in liver and AAs in the form of proteins in skeletal muscles) ▪ synthesis of glycogen in the liver ▪ incorporation of GLUT4 into the membrane of skeletal muscle ▪ stimulates Na+/K+-ATPase and thus supports re-uptake of dietary K+ Insulin ▪ insulin deficiency ▪ damaged β-cells, eg. after exposure to toxic substances or due to an autoimmune disease (often caused by a viral infection) ▪ most patients have detectable antibodies to islets of Langerhans or insulin ▪ genetic predispositions (more common in certain types of HLA) ▪ patients are given insulin Diabetes mellitus type I: insulin-dependent (IDDM), juvenile diabetes ▪ reduced sensitivity of target organs to insulin > relative insulin deficiency ▪ associated with normal or even higher insulin production ▪ genetic predisposition, obesity, autoantibodies to insulin or its receptors ▪ type II diabetes can develop even at a young age (MODY = maturity onset diabetes of the young) ▪ lifestyle modification, insulin administration only in more severe cases Diabetes mellitus type II: non-insulin-dependent (NIDDM), senile diabetes ▪ combined activity with hormones supporting growth and response to stress conditions: • somatotropin (growth hormone) • thyroid hormones • glucocorticoids (Cushing disease - steroid diabetes) • adrenaline • progestogens and human placental lactogen (gestational diabetes) • glucagon ▪ under normal circumstances, these hormones act synergistically and, by acting against insulin, keep glycemia normal Other types of diabetes mellitus: LAG - hyperthyroidism, pregnancy ▪ the main consequence is hyperglycemia ▪ high blood sugar > hyperosmolarity in extracellular environment > excretion of glucose in urine is associated with loss of water, Na+ and K+ > dehydration > feeling thirsty ▪ breakdown of muscle proteins to release AA > weight loss and muscle weakness ▪ lipolysis in adipose tissue > higher levels of fatty acids in the blood > conversion to other acidic metabolites in the liver > acidosis > deep breathing (Kussmaul) and breakdown into ketonic substances (acetone in breath) ▪ disorders of metabolism, electrolytes and osmolarity can cause hyperosmolar or ketoacidic coma ▪ type I diabetes: hyperglycemia, hyperosmolarity ▪ type II diabetes: hyperglycemia, hyperosmolarity, increased proteolysis and lipolysis (ketoacidosis) Insulin: acute deficiency Insulin: acute deficiency ▪ hyperglycemia leads to irreversible damage to the body after several years to decades ▪ glucose in cells reduced to sorbitol > accumulation in cells > osmotic edema (clouding of the eye lens - cataract; nerve transmission disorders) ▪ cells not absorbing glucose (eg. leucocytes) in hyperosmolar environment > weakened immunity > higher risk of infection ▪ glycated erythrocytes and walls of blood vessels > microangiopathy > blindness, kidney damage ▪ macroangiopathy > heart attack, stroke, kidney damage ▪ diabetic foot syndrome (microangiopathy + ischemia + infection) Insulin: chronic deficiency ▪ pure insulin is administered in solution with zinc (six insulin molecules forming hexamer binds to two zinc atoms), peptide stabilizers and preservatives ▪ formerly porcine or bovine insulin extracted from the pancreas (minimal differences in AA chain); today predominantly human (HM insulin) is used produced by genetic engineering in Escherichia coli or Saccharomyces cerevisiae ▪ fast and depot insulins ▪ administration subcutaneously using insulin syringes, pens or pumps ▪ application 4 μg of insulin per 1 kg of human weight intravenously reduces blood glucose by about half (assuming a normal effect of the insulin receptor) ▪ it is not possible to administer orally due to degradation! Therapy - synthetic insulin Therapy - synthetic insulin Insulin pen ▪ cheaper ▪ repeated injections ▪ more manipulation ▪ different types of insulin Insulin pump ▪ more expensive ▪ bolus doses when eating ▪ controlled by software ▪ fast insulin ▪ most often caused by high level of insulin (eg. decreased insulin requirement during exercise) ▪ oral administration of antidiabetics ▪ genetic disorders, less frequently tumors or autoimmune disorders (antibodies that bind and gradually release insulin) ▪ hypoglycemia occurs naturally during intense work ▪ hypoglycemia can also be caused by a lack of insulin antagonists (glucagon, growth hormone, glucocorticoids and others) ▪ glucose disorders ▪ alcoholism ▪ hypoglycaemia > hunger > sympathetic activation > increased heart rate, sweating, tremor > convulsions, loss of consciousness > irreversible brain damage Insulin: excess and hypoglycemia ▪ binding to endothelial cells, nerve cells, fibroblasts and renal duct cells ▪ action via G proteins ▪ ↑eNOS, ↑ Na+/K+-ATPases and others (eg vasodilation, nerve transmission) ▪ positive effects in patients with diabetes I (nervous activity, blood flow…) C-peptide ▪ α-cells of the islets of Langerhans ▪ 29 AA (3.5 kDa) ▪ secretin protein family (sekretin, somatoliberin, gastric inhibitory polypeptide GIP, vasoactive intestinal peptide and others) ▪ precursor called proglucagon > alternative products > some of them inhibit glucagon production and increase insulin production ▪ stored in secretory granules and released by exocytosis ▪ insulin antagonist Regulation: ▪ hypoglycemia (released during starvation and prolonged exercise) ▪ stimulation by some AA from food (alanine, arginine) ▪ sympathetic stimulation via β2-adrenergic receptors, cholecystokinin ▪ suppressed by glucose, high plasma levels of free AA, insulin and somatostatin Glucagon ▪ acts through G proteins, cAMP, CREB ▪ regulation of glycemia (securing the energy source in the time between food intake and under increased activity) ▪ increases glycogenolysis in the liver (glucagon activates the enzyme glycogen phosphorylase a; not in the muscles!) ▪ gluconeogenesis from lactate, AA and glycerol (lipolysis) Glucagon glycogen phosphorylase b = Insulin, glucagon and glycemia Insulin, glucagon and glycogen metabolism Akt = Synergistic action of hormones in the regulation of glycemia ▪ δ-cells of the islets of Langerhans, duodenum and intestine + neurosecretion in the hypothalamus (inhibits growth hormone secretion in the adenohypophysis) ▪ encoded by a single gene in humans (other vertebrates mostly 6) ▪ homolog of cortistatin ▪ released with increased concentration of glucose and some AA (arginine) in the blood after a meal, induced by a low pH in the stomach ▪ endocrine and paracrine function ▪ acts through G proteins ▪ inhibits secretion of hormones in adenohypophysis (see earlier) ▪ inhibits release of insulin, glucagon, histamine, cholecystokinin, gastrin, secretin, motilin and other gastrointestinal hormones ▪ inhibition of gastric acid production (histamine antagonist), gastric emptying, smooth muscle contractions, intestinal blood flow and exocrine function of the pancreas Somatostatin (growth hormone–inhibiting hormone) ▪ PP (γ- /F) islets of Langerhans (especially the head of the pancreas) ▪ 36 AA (4.2 kDa) ▪ increased secretion during starvation, exercise, acute hypoglycemia and after protein intake x decreased secretion due to somatostatin and intravenous glucose ▪ stimulated by vagal nerve, cholecystokinin and gastrin ▪ regulation of endocrine and exocrine pancreatic secretion (antagonist of cholecystokinin), gastrointestinal secretion and hepatic glycogen levels ▪ inhibits digestion, including intestinal motility and gastric emptying ▪ the exact physiological function is not yet clear ▪ PP levels increased in patients with anorexia; administration of PP to rodents reduces food intake Pancreatic polypeptide (PP) Gastrointestinal tract Hormones regulating the digestion: ▪ ghrelin/leptin ▪ cholecystokinin (CCK) ▪ gastrin ▪ secretin ▪ motilin ▪ vasoactive intestinal peptide (VIP) ▪ gastric inhibitory polypeptide (GIP) ▪ glucagon-like peptide (GLP-1, enteroglucagon) and other hormones ▪ the digestive tract is the largest endocrine organ ▪ endocrine cells are diffuse in the GI tract ▪ CNS-affecting peptide ▪ belongs to the family of motilin peptides ▪ produced in stomach and duodenum, pancreas, small intestine, lungs, gonads, adrenal cortex, kidneys, placenta and brain ▪ produced by cleavage of preproghrelin (homolog of promotilin) > proghrelin > ghrelin (28 AA) and C-ghrelin (it is thought to produce hormone obestatin) ▪ secretion on an empty stomach x stops when the stomach stretches (secretion stops faster after intaking proteins or sugars than after lipid intake) ▪ ghrelin is able to cross the blood-brain barrier Ghrelin ("Hunger hormone") DOI: 10.1056/NEJMoa012908 ▪ G protein-coupled receptors (the same target cells have also leptin and insulin receptors) ▪ acts on the hypothalamic cells and increases hunger ▪ activation of cholinergic-dopaminergic intermediate circuit mediating reward reactions and appetite ▪ signals to orexigenic neuropeptide Y (NPY) and agouti-related protein (AgRP) neurons > food intake ▪ motivation to search for food sources (confirmed by the effect of ghrelin injection), body weight regulator ▪ increases gastric acid production and intestinal motility (preparing body for food intake) ▪ energy management (reduced ATP production, fat and glycogen storage, heat production) ▪ antagonist of leptin and insulin Ghrelin Ghrelin ▪ protein of 167 AA (16 kDa) ▪ produced by white adipose tissue, but also in brown adipose tissue, placenta, ovaries, skeletal muscles, stomach, bone marrow and other tissues ▪ leptin production grows exponentially with the amount of white fat ▪ the highest concentration in the blood between midnight and morning ▪ insulin and emotional stress increase level of leptin; reduced in sleep deprivation and starvation Leptin ("Satiety hormone") ▪ acts against ghrelin ▪ receptors in the arcuate nucleus of the hypothalamus > regulation of appetite and energy balance ▪ 6 receptors encoded by one gene ▪ intracellular action eg. via JAK-STAT and MAPK ▪ reduced sensitivity to leptin observed in obese people ▪ stimulates satiety by irritating the nerves in the hypothalamus (inhibition of neuropeptide Y and agouti-related peptide) and inhibits hunger ▪ outside the hypothalamus modulates energy expenditure, activates immune cells, pancreatic β-cells and acts as a growth factor Leptin Ghrelin,leptin and metabolic control Ghrelin Leptin ↑ food intake ↓ food intake ↓ energy expenditure ↑ energy expenditure ↓ lipid catabolism ↑ lipid catabolism ↑ glucose in plasma ↓ glucose in plasma ▪ peptides ▪ produced in endocrine cells of the mucosa, diffusely spread ▪ very similar in structure (peptide families) > similar effects at higher concentrations ▪ regulation neuronal, humoral, paracrine and endocrine ▪ affect motility, secretion, blood supply and growth of GI tract Hormones produced in GI tract ▪ stomach antrum, duodenum (G cells) ▪ released when the stomach dilates, when levels of peptides and AA increase due to protein breakdown, nerve stimuli (parasympathetic, vagal nerve > gastrin-releasing peptide) ▪ release inhibited by low pH in the stomach and duodenum, by somatostatin ▪ promotes the production of gastric juice (HCl; directly by translocation of K+/ H+ ATPase pumps into the cell membrane, indirectly via histamine), pepsinogen secretion and gastric mucosal growth Gastrin ▪ mucosa of the whole small intestine (I cells) ▪ gastrin/cholecystokinin family ▪ preprocholecystokinin 33 AA (posttranslational modification to produce many forms) ▪ stimulation via long chain fatty acids, AA, peptides in the lumen of the small intestine, nerve stimuli ▪ causes contractions of the gallbladder, secretion of pancreas (digestive enzymes) and suppresses gastric emptying Cholecystokinin (CCK, pancreozymin) ▪ production mainly in the duodenum (S cells) ▪ 27 AA, stored as inactive prosecretin (low pH activation) ▪ stimulated by acidic chymus ▪ suppresses secretion of HCl and growth of the gastric mucosa, stimulates HCO3 - secretion Secretin Endocrine regulation of digestion - summary ▪ gastrin (HCl, pepsinogen) ▪ CCK (secretion of bile, pancreas): processing fat- and protein-rich diet ▪ secretin (secretion of the pancreas): neutralization of acid digestion ▪ enterogastron (gastric / digestive inhibitory polypeptide): negative regulation of digestion