Eicosanoids Lecture Notes PDF

# Eicosanoids ## Objectives At the end of the lecture, students should be able to discuss: - Eicosanoids - Cyclooxygenase - Lipoxgenase - Biological effect on different tissues - Therapeutic effects - Inhibition of eicosanoid ## Eicosanoids - Derived from 20-carbon polyunsaturated fatty acids -...

# Eicosanoids ## Objectives At the end of the lecture, students should be able to discuss: - Eicosanoids - Cyclooxygenase - Lipoxgenase - Biological effect on different tissues - Therapeutic effects - Inhibition of eicosanoid ## Eicosanoids - Derived from 20-carbon polyunsaturated fatty acids - Paracrine or autocrine messenger molecules - Short half-lives (10 seconds - 5 minutes) so that functions are usually limited to actions on nearby cells. - Bind to specific cell surface G-protein coupled receptors, and generally increase cAMP levels. May also bind to nuclear receptors and alter gene transcription. ## Eicosanoids Classification Eicosanoids are classified into two main groups: - Prostanoids - Leukotrienes and Lipoxins Prostanoids are further sub-classified into three groups: - Prostaglandins (PGs) - Prostacyclins (PGIs) - Thromboxanes (TXs) ## Inflammatory Biomolecules - **Histamine** - Made in almost all tissues - Very short half-life - Act locally on neighbors - Not usually stored up - 20-carbon backbones - Made from arachidonic acid - **Eicosanoids** - Prostaglandins - Thromboxanes - Leukotrienes - **Bradykinins** - **Cytokines** - Interferons - Interleukins - Chemotaxins - Other minor molecules... ## Eicosanoids Synthesis - **Arachidonic acid** - **TXA2** (E,I,F,D) - **PGs** (E,I,F,D) - **LTB4** (C,D,E) - **LTS** (C,D,E) ## Role of Eicosanoids - Prostaglandins, Prostacyclins, Thromboxanes, Leukotrienes, Epoxyeicosatrienoic acids - They have roles in: - Inflammation - Fever - Regulation of blood pressure - Blood clotting - Immune system modulation - Control of reproductive processes & tissue growth - Regulation of sleep/wake cycle ## Effects of Eicosanoids - Induction of inflammation - Mediation of pain signals - Induction of fever - Smooth muscle contraction (including uterus) - Smooth muscle relaxation - Protection of stomach lining - Stimulation of platelet aggregation - Inhibition of platelet aggregation - Sodium and water retention ## Precursors of Eicosanoids - Arachidonic acid (ω6) - Eicosatrienoic acid (γ-linolenic acid, ω6) - Eicosapentaenoic Acid (ω3) ## Pathways of Arachidonic Acid (AA) Release and Metabolism - **Free radicals** - **Phospholipase A2** - **Isoprostanes** - **Arachidonic acid (20:4 cis D5,8,11,14)** - **Lipoxygenases (LOX)** - HETEs - Leukotrienes - Lipoxins - **Cyclooxygenases (COX)** - Prostaglandins - Prostacyclin - Thromboxane - Prostanoids ## Classification - **Cyclooxygenase Pathway** - Prostaglandins - Thromboxanes - Prostacyclins - **Lipoxygenase Pathway** - Leukotrienes - HETES ## Cyclooxygenase (COX) Pathway The cyclooxygenase pathway is a major pathway for the metabolism of arachidonic acid. It is responsible for the production of prostaglandins, prostacyclins, and thromboxanes. **COX-1** - Constitutive - The prostaglandins associated with are: - GI mucosal integrity - Platelet function - Renal function **COX-2** - Inducible - The prostaglandins associated with are: - Gain - Fever - Inflammation ## Prostaglandins (PGs) Prostaglandins are a group of eicosanoids that are produced by the cyclooxygenase pathway. They have a wide range of physiological effects, including: - Inflammation - Pain - Fever - Blood clotting - Vasodilation - Vasoconstriction **Prostaglandins (PGs)** - PG-G and H - PG-F - PG-A - PG-B - PG-E - PG-C and D ## Synthesis of Eicosanoids - **Phospholipid membrane** - **NSAIDS** - **Phospholipase A2** - **Arachidonic acid** - **Cyclooxygenase** - **Prostaglandin H2** - **Thromboxanes** - **Prostaglandins** - **Lipoxygenase** - **Leukotriene A** - **Leukotriene B1** - **Leukotriene C4, D4, E4** ## Cyclooxygenase (COX) Pathway - Prostaglandin H synthase, present as two isoenzymes (PGHS-1/COX-1, PGHS-2/COX-2). - Each possessing two activities: - **Cyclooxygenase** - catalyzes addition of two molecules of $O_2$ to the arachidonic acid molecule, forming $PGG_2$. - **Hydroperoxidase** - converts the hydroperoxy function of $PGG_2$ to an OH group (of $PGH_2$). - The enzyme is also capable of self-catalyzed destruction. ## Effects of Prostaglandins - **Vascular** - Vasoconstrictors - $TXA_2$ - $PGF_{2α}$ - Vasodilators - \(PGI_2\) - \(PGE_2\) - **Gastrointestinal Tract** - Longitudinal muscle: \(PGE_2\), \(PGF_{2α}\) - Circular muscle: \(PGF_{2α}\), \(PGI_2\), relaxed by \(PGE_2\) - **Airways** - Relaxed: \(PGD_2\), \(PGE_1\), \(PGE_2\), \(PGI_2\) - Contracted: \(PGF_{2α}\), \(TXA_2\) ## Role of Prostaglandins - **Inflammation** - **Fever** - PGs induce fever by stimulating the thermoregulatory center in the brain. - **Pain** - PGs sensitize pain receptors to stimulation, as a result increase pain perception - **Swelling** - There is vasodilatation and increased capillary permeability induced by PGs which is responsible for swelling of the inflamed tissue. - **Erythema, wheal and Flare** is also induced by PGs like PGE and PGD2. - **PGD2** is considered an important mediator of anaphylaxis ## Effect on Smooth Muscle - **Intestinal** - \(PGE\) and \(PGF\) produce contraction of the longitudinal smooth muscles producing diarrhea, cramps and reflux of bile. - **Clinical Significance:** Diarrhea and abdominal cramps are the commonly observed as side effects of PGs. - **Bronchial** - \(PGFs\) contract and \(PGEs\) relax bronchial smooth muscles. - **Clinical Significance:** \(PGE_1\) and \(PGE_2\) are therapeutically used as bronchodilators. - **Vascular** - \(PGEs\) cause vasodilatation. - \(PGF_{2α}\), and \(PG A_2\) cause vasoconstriction. - **Clinical Significance:** Systemic blood pressure falls in response to PGEs and PGAS - **Uterine** - \(PGE_1\), \(PGE_2\) and \(PGF_{2α}\) cause uterine contractions. - **Clinical Significance:** - \(PGE_2\) is used for the induction of labor at or near term. - In higher dosage PGES are used as abortifacients in first and second trimester of pregnancy. - They are also responsible for causing dysmenorrhea ## Effect on Platelets - \(PGE_1\) cause inhibition of platelet aggregation. - **Clinical significance:** \(PGE_1\) has been used for harvesting and storage of blood platelets for therapeutic transfusion. - **Inhibition:** \(PGE_1\), \(PGI_2\) - **Aggregator:** \(TXA_2\) ## Blood - \(TXA_2\) - Is a potent inducer of platelet aggregation - \(PGI_2\) and \(PGE_2\) - Inhibit platelet aggregation - \(PGES\) - Induce erythropoiesis by stimulating the renal release of erythropoietin. - **5-HPETE** - Stimulates release of histamine - \(PGI_2\) and \(PGD\) - Inhibit histamine release ## Effect on Kidney - \(PGEs\) cause: - Increased renal plasma flow - Increased GFR - Increased diuresis - Natriuresis - Kaliuresis is also induced by the action of \(PGE_2\) - **Major one:** \(PGE_2\), \(PGI_2\) - \(GFR\) ↑ (vasodilator effect) - \(TXA_2\): renal vasoconstrictor may play a role in renal function? (inflammatory renal disease) ## Effect on Gastrointestinal Secretions - \(PGE_1\) and \(E_2\) inhibit gastric secretions and are required for maintaining the integrity of gastric mucosa. - The effect is opposite on the pancreatic and intestinal secretions. There is increase in the volume, enzyme and electrolyte content of the pancreatic and intestinal secretions in response to \(PGE_1\) - Watery diarrhea results in response to administration of \(PGE_1\). ## Effect on Endocrine Glands - \(PGEs\) have insulin-like effects. They inhibit lipolysis and the effects on carbohydrate metabolism are the same as insulin - \(PTH\) (Parathormone) like effects are also seen on bone metabolism by PGs. They mobilize calcium from bones producing hypercalcemia. - Thyrotropin-like effects are seen on the thyroid gland. - Steroidogenic effects are seen on the adrenal tissue. ## Female Reproductive System - **Abortion** - \(PGF_{2α}\), \(PGE_2\): oxytoxic and ripening and priming of the cervix - Dinoprostone (\(PGE_2\)) - Plasma \(t_{1/2}\): 2-2.5 minutes - Carboprost tromethamine (\(PGF_{2α}\)) - Drawback: pulmonary hypertension - **Facilitation of Labor** - Potency:\(PGE_2\) > \(PGF_{2α}\) - Side effect:\(PGF_{2α}\) > \(PGE_2\) ## Male Reproductive Organ - **Fertility?** \(PGE\), \(PGF\) - **Erectile Dysfunction:** - Alprostadil (\(PGE_1\)) - Intracavernosal injection - Urethral suppository - **Bone Metabolism:** - Increase bone turn over - **Eye** - \(PGE\), \(PGF\) - Increasing outflow of aqueous humor ## Immunological Response - \(PGES\) secreted by macrophages: - Modulate or decrease the functions of - \(T\) and \(B\) lymphocytes ## Prostacyclins and Thromboxanes - **Prostacyclins (PGI)** contain another ring between 6th and 9th carbon atoms. - **Thromboxanes (TX)** have a six-membered oxane ring. - There are three series for Thromboxanes as well as for Prostacyclins. ## Functions of Prostacyclins - Synthesized in the heart and vascular endothelial cells. - Inhibit platelet and leukocyte aggregation - Decrease T-cell proliferation, lymphocyte migration and secretion of IL-1α and IL-2 - Induce vasodilatation and production of cAMP - Prevent clot formation. ## Functions of Thromboxanes - Synthesized by platelets. - Induce platelet aggregation. - Promote vasoconstriction. - Lymphocyte proliferation - Bronchoconstriction and - Promote clot formation. ## Lipoxygenase (LOX) Pathway - **Leukotrienes** - Identified as LTs. - A family of conjugated trienes formed in leukocytes, platelets, and macrophages by the lipoxygenase pathway in response to both immunologic and nonimmunologic stimuli. ## Leukotrienes and Allergies - Leukotrienes are a hundred times more potent than histamine. - Histamine provided a rapid response to an allergen. - In the later stages, leukotrienes are principally responsible for inflammation, smooth muscle constriction, constriction of the airways, and mucous secretion form mucosal epithelium. ## Leukotriene - **Non-peptido Leukotriene (LTB4):** - Role Chemostatic agent (attract certain type of WBCs to fight infection). - Increased vascular permeability, T-cell proliferation, leukocyte aggregation, IL-1, IL-2, INF-γ - **Peptide Leukotriene (LTC4, LTD4, LTE4):** - Increased Bronchoconstriction, Vascular permeability, IFN-γ - Component of Slow Reaction Substances of Anaphylaxis (SRS-A) released from sensitized lung after immunologic reaction. - Mediator of asthma: constrict bronchi & increase mucus secretion. - Involve in immediate hypersensitivity (allergic reaction) and inflammation and heart attack. ## Slow-reacting Substance of Anaphylaxis (SRS-A) - Slow-reacting substance of anaphylaxis (SRS-A) is a mixture of leukotrienes C4, D4, and E4. - This mixture is a potent constrictor of the bronchial airway musculature. - These leukotrienes together with leukotriene B4 also cause vascular permeability. - Chemotaxis and activation of leukocytes and - Are important regulators in many diseases involving inflammatory or immediate hypersensitivity reactions, such as asthma. ## Blood Cell and Inflammation - **Chemoattractant:** - **Neutrophils:** LTB4, lipoxin A - **Eosinophils:** LTC4, LTD4 - **Heart and Smooth Muscle** - **Cardiovascular** - **Smooth Muscle Chemoattractant:** 12(S)-HETE - **Gastrointestinal:** LTB4 - **Airways:** LTC4, LTD4 - **Increased vascular permeability, mucus secretion** ## Lipoxins Lipoxins are produced mainly by leukocytes and platelets stimulated by cytokines (IL-4, TGF-β): - 5-lipoxygenase (5-LO) of neutrophils produces leukotriene LTA4 which enters platelets where it is converted by 15-LO to LXA4 or LXB4 - 15-LO of epithelial cells and monocytes forms 15-HPETE which becomes a substrate of 5-LO and epoxid hydrolase of leukocytes...transcellular biosynthesis ## Biological Roles of Lipoxins Unlike pro-inflammatory eicosanoids, lipoxins attenuate the inflammation and appear to facilitate the resolution of the acute inflammatory response. - Hypothesis: In the first phase of the inflammatory response, leukotrienes are produced (e.g. LTB4) → then, the level of PGs rises and PGs „switch" the syntheses from leukotriene production to the pathway which, in the 2nd phase, produces lipoxins promoting the resolution of inflammation. - Therefore, potential therapeutic use of LXs in the treatment of inflammatory diseases (glomerulonephritis, asthma) is being extensively studied. ## Mediators of Different Phases of Inflammation - **Injury** - Microbial - Physical - Immunological - Hypoxia/Reperfusion - **Acute Inflammation** - Infiltration of inflammatory cells - Vasodilation - Proliferation of resident cells - **Braking Signals, Resolution & Repair** - Clearance of inflammatory cells & debris - Restoration of parenchymal cells - Limit matrix accumulation ## Main Sites of Eicosanoid Biosynthesis - Endothelial cells - Leukocytes - Platelets - Kidney Unlike histamine, eicosanoids are NOT synthesized in advance and stored in granules – when needed, they can be produced very quickly from arachidonate released from membranes. ## Major Sites of Synthesis of Eicosanoids - **Thromboxane A2:** Platelets (thrombocytes) + Lungs - **PGI2:** Vascular Endothelial Cells (blood vessels) + Heart - **PGH2:** heart, Kidney, spleen etc - **Leukotrienes:** WBCs (leukocytes), mast cell, lungs, spleen, heart and brain ## Inhibition of Eicosanoid Synthesis - **Corticosteroids** - By producing annexins, lipocortins - Inhibiting phospholipase A2 - **NSAIDs** - Inhibition of COX activity - May increase the lipoxygenase activity? ## Anti-Inflammatory Drugs Inhibit Eicosanoid Synthesis - **NSAIDs** - **Membrane lipids** - **Steroids** - **Phospholipase A2** - **Arachidonic Acid** - **Cyclooxygenase:** Prostaglandins, thromboxanes - **Lipoxygenase:** Leukotrienes ## Prostanoids Therapeutic Uses - **Uterine Stimulation** - **Dinoprostone (PGE2):** - Prostin E2 vaginal suppositories used to induce abortion between 12th-20th gestational weeks - Prostin E2 oral tablets for elective induction of labor/obliged induction because of HTN, toxemia, intrauterine death - Treatment of duration ≤ 18 hours - **Prostin E2 vaginal gel** used for induction of labor at term or near term (1-2 mg intravaginal, repeated Q 6hrs according to response) ## Inducing Abortion - **Second Trimester:** - Infusion of carboprost tromethamine or - Administration of vaginal suppositories containing dinoprostone - **First Trimester:** - These prostaglandins are combined with mifepristone (RU486) ## Prostanoids Therapeutic Uses GIT - **Misoprostol (CYTOTEC)** is a synthetic methyl ester analogue of \(PGE_1\). - Used to prevent drug-induced gastric ulceration during NSAIDs, corticosteroid or anticoagulant therapy. - It can be used alone or in combination with antacids for duodenal ulcer treatment. - Not used for pregnant women or whom are planning pregnancy. ## Prostanoids Therapeutic Uses Platelet Aggregation - **Epoprostenol (PGI2):** - It is used as a heparin replacement in some hemodialysis patients. - Used to prevent platelet aggregation in extracorporal circulation systems. ## Maintenance of Ductus Arteriosus - Is produced by \(PGE_1\) [Prostin VR] infusion - \(PGE_1\) will maintain patency of the ductus arteriosus, which may be desirable before surgery. ## Impotence & Erectile Dysfunction - Alprostadil (\(PGE_1\)) can be injected directly into the corpus cavernosum or administered as a transurethral suppository to cause vasodilation and enhance tumescence. - Replaced by PDE-V inhibitors ## Leukotrienes Therapeutic Importance - LTs have no therapeutic uses, but LTs antagonists have: - Anti-asthma medications: - 5-Lipoxygenase Inhibitors, e.g., zileutin - Leukotriene-receptor antagonists; montelukast, & zafirlukast ## Adverse Effects of Eicosanoids - Local pain and irritation - Bronchospasm - Gastrointestinal disturbances: nausea, vomiting, cramping, and diarrhea.