Lipid-derived Autacoids

5

LIPID-DERIVED AUTACOIDSEicosanoids and Platelet-Activating Factor

Prof. Dr. Zeliha Yazc

EICOSANOIDS

formed from certain polyunsaturated fatty acids extremely prevalentreleased locally and act as local hormonesproduce a broad spectrum of biological effectvery potent and have wide range of propertiesrabidly synthesized and have a high turnovernot stored

Biosynthesis

Released from tissues by a variety of stimuli

Hormones and chemical stimuli

Receptors-G proteins

Activation of PLs cytosolic Ca++ Physical stimuli

influx of Ca++

Activation of PLs

PLB PLA1

CH2 - O - CO - R1 R2 - CO - O - C O CH2 - O - PO - O - R3+ PLA2 PLC PLD R1, R2 = Long-chain fatty acid esters R3 = -CH2 - CH2 - N(CH3)3+ Phosphocholine R3 = -CH2 - CH2 - NH3+ Phosphoethanolamine

Phospholipases which hydrolyse fatty acids

Cardiac PLA2 (cPLA2)Cytocolic PLA2 Ca++ -dependentSecretory PLA2 Ca++ -dependent

Essential Membrane PLA2fatty acids phospholipids ARACHIDONIC + Lyso-PAF PAF Triglyceride ACID

Lypoxygenase Cyclooxygenases Cytochrome P450 Free radicals

HpETEs Prostaglandins (PGs) Epoxides Isoprostanes Leukotrienes Thromboxanes OH-PGs Lipoxins O-PGs HETEs EETs

The arachidonic acid cascade HpETEs/HETEs: Hydroperoxy/Hydroxyeicosatetraenoic acids EETs: Epoxyeicosatetraenoic acids

The type of eicosanoidsThe nature of PUFAThe type of cellThe species

n-6 series fatty acids n-3 series fatty acids

Meats Fish oil Animal fats Green leaves Soya bean oil Soya bean oil Corn oil Linseed oil Sunflower seed oil Safflower oil

Linoleic acid -linolenic acid 18:2n-6 18:3n-3

delta-6-desaturation

-linolenic acid Octadecatetraenoik acid 18:3n-6 18:4n-3

elongation

Dihomo--linolenic acid Eicosatetraenoic acid 20:3n-6 20:4n-3

1.series PGs delta-5-desaturation3.series LTs

Arachidonic acid Eicosapentaenoic acid 20:4n-6 20:5n-3

2.series PGs elongation 3.series PGs4.series LTs 5.series LTsOH fatty acids OH fatty acids

Adrenic acid Docosapentaenoic acid 22:4n-6 22:5n-3

delta-4-desaturation

Docosapentaenoic asid Docosahexaenoic acid 22:5n-6 22:6n-3

The main pathways of desaturation and elongation of the essential fatty acids.

Products of Cyclooxygenases

ARACHIDONIC ACID

COX-1, COX-3 COX-2

PGG2 hydroperoxidase prostacyclin PGH2 thromboxsane synthase synthase PGI2 isomerases TXA2 hydrolysis 6-keto-PGF1 TXB2 oxidation oxidation PGE2 PGF2 PGD2 2,3-dinor-6-keto 2,3-dinor-TXB2 PGF1 PGDH 11-dehidro-TXB2 13 reduction liver oxidation oxidation PGE-M PGF-M

Products of cyclooxygenases

Cyclooxygenase-1 (PGHS-1, COX-1)located in the luminal side of the endoplasmic reticulum (ER)PGs are produced in the ER, exit the cell to interact with cell surface receptors and involved in the rapid modulation of cellular responses to hormonal stimulation.Cyclooxygenase-2 (PGHS-2, COX-2)

It is not normally present.Inflammatory compounds (e.g. cytokines) induce COX-2 expressionlocated in the luminal side of the ER and on the luminal side of nuclear envelope.PGs are produced in the ER and nucleus and involved in pathological and specific physiological events (inflammation, cancer, apoptosis, mitogenesis and ovulation)exit the cell and help coordinate cellular responses via extracellular receptors.may regulate nuclear events (transcription, apoptosis)

Cyclooxygenase-3 (COX-3)Expressed in the heart and in the cerebral cortexInhibited by acetaminophen, phenacetin and dipyrone

Products of Lipoxygenases

Lipoxygenases:Cytosolic enzymescatalyze the oxygenation of polyenic fatty acidsdiffer in their specificity for placing the hydroperoxy groupTissues differ in the lipoxygenase that they contain (platelets 12-lipoxygenase, leukocytes5 &12- lipoxygenases)

Activated inflammatory cells Ca++ 5-lipoxygenase binds to (macrophages, mast cells, FLAP (5-lipoxygenase activating protein)PMNLs, basophils, eosinophils).

11-HpETE 11-lipoxygenase 12-lipoxygenase ARACHIDONIC 12-HpETE 12-HETE ACID 15-lipoxygenase 15-HpETE 15-HETE FLAP 5-lipoxygenase Lipoksin A Lipoksin B 5,15-DHETE 5-HpETE 5-HETE

LTA synthase

glutathione LTA4 LTC4 S-transferase -glutamyl LTA transpeptidase hydrolase LTD4 LTB4 Dipeptidase lung, kidney, liver LTE4 -glutamyl transpeptidase LTF4

Products of lipoxygenases

LTC4, LTD4 and LTE4 peptidoleukotrienens or cysteinyl LTsLTC4 and LTD4 slow-reacting substance of anaphylaxis (SRS-A) secreted in asthma and anaphylaxis.

Products of Cytochrome P450active on smooth muscle cells and in cell signalling pathwaysthought to have vascular, endocrine, renal and ocular effects

Products of Free Radicals (Isoprostans)Stereoisomers of PGsThe free radicals catalyze peroxidation of arachidonate independent of the COX (Nonenzymatic pathway of arachidonate conversion)Their production is not blocked by steroidal and nonsteroidal anti-inflammatory agentsstored as part of the membrane phospholipidsten-fold greater in blood and urine than the COX-derived PGsthought to play an important role in the hepato-renal syndrome

Inhibitors and Activators of Eicosanoid Biosynthesis

PLA2 activated by Ca++ and calmodulin, serine protease, bradykinin, neomycin sulphate inhibited by Ca++ channel blockers, glucocorticoids (annexins and lipocortin), mepacrine, chloroquine

PLC inhibited by glucocorticoids, neomycin sulphatePLD inhibited by propranolol, demethoxyviridin, neomycin sulphate

COXs inhibited by NSAIDs

COX-1/COX-2 IC50(M/L) (The concentration of drug that inhibits the enzyme activity 50%)high in selective inhibitors of COX-2 Nonselective NSAIDs 0.5-3.0 (e.g. indomethacin, ketoprofen, flurbiprofen) Relatively selective COX-2 inhibitors 10-20 (e.g. meloxicam, nimesulide)Highly selective COX-2 inhibitors >100 (e.g. celecoxib, refecoxib)

Aspirinonly NSAID known covalently modify COXscompletely inhibits cyclooxygenase activity in COX-1dose not affect peroxidase activity in COX-1dose not inhibit cyclooxygenase activity in COX-2 (modifies COX-2 15R-HETE)Valerylsalicylate is a relatively selective inhibitors of COX-1

Dazoxiben and Pirmagrel inhibit thromboxane synthaseRidogrel inhibit thromboxane synthase and thromboxane receptors TXA2 (platelet aggregator and vasoconstrictor) PGI2 (antithrombogenic and vasodilator)

ETYA competitive inhibitorZileuton inhibitor of 5-lipoxygenase ulcerative colitis, asthma, allergic rhinitis

Pharmacological Properties of Eicosanoids

Cardiovascular SystemPGEs Potent vasodilators (arterioles, precapillary sphincters, postcapillary venules)Large veins are not affectedSystemic blood pressure Blood flow to organs Cardiac output Weak, direct inotropic effectsForce of contraction Heart rate

PGF2A potent vasoconstrictor (pulmonary arteries and veins)dose not alter blood pressure8-epi- PGF2 a potent vasoconstrictor (increased in cirrhosis)

PGD2Vasodilator in the mesenteric, coronary and renal vascular badVasoconstrictor in the pulmonary arteries

PGI2 Vasodilatorcauses hypotension (iv)causes a reflex increase in heart ratenot inactivated in the lungsA physiological modulator of vascular tone

TXA2 A only smooth muscle cell mitogen eicosanoidA potent vasoconstrictor

LTC4 LTD4Intravascular volume, cardiac contractility, coronary blood flow cause hypotensionVasoconstrictor (in coronary arteries, distal segment of the pulmonary arteries, mesenteric vasculature)constrict arterioles and reduce exudation of plasmaact on the endothelial lining of postcapillary venules andcause exudation of plasma

BloodPGI2 synthesized by the vascular endotheliuminhibits the aggregation of platelets

TXA2 A major product of arachidonate in plateletsA powerful inducer of platelet aggregation X aspirin

LTB4 A potent chemotactic agent for PMNLs, eosinophils and monocytes

PGE2inhibits lymphocyte function and proliferation suppress the cellular immune response

Kidney and Urine FormationPGs synthesized in the renal medulla and renal cortexincrease renin releaseinhibit Na+, Cl- and water reabsorptionincrease renal blood flowincrease water and Na+ excretion X NSAIDs

TXA2 decreases renal blood flow, the rate of glomerular filtration and water excretion

Renal blood flow Na excretion Blood pressure

Na restriction

Renin Vasopressin

Angiotensin II PG release X NSAIDs Aldosterone Na retention

Prostaglandins and the renin-angiotensin release

Gastrointestinal System

Longitudinal muscle is contracted by PGE2 and PGF2Circular muscle is contracted by PGI2 and PGF2, relaxed by PGE2PGs reduce transit times in the small intestine and colon (diarrhea, cramps, reflux of bile)

PGEs and PGI2cAMP and gastrin volume of secretion, acidity and pepsin mucus secretionHCO3 secretion cytoprotectant propertieslocal blood flow

Respiratory System

PGF2, PGD2, LTC4, LTD4, PG endoperoxides and TXA2 bronchoconstrictorsPGE2 and PGI2 cause bronchodilatation

Aspirin COX PGE LTs microvascular permeability plasma exudation Bronchospasm mucus secretion

Reproduction and ParturitionPGF2, PGE and TXA2elevated in blood and amniotic fluid during labourhave potent oxytocic actions X aspirinNSAIDs increase the length of gestation, prolong the duration of spontaneous labour and interrupt premature labour

PGE and PGI2 (high dose)produce relaxation in the uterusPGF2 and PGE increase during menstruationcontract uterine and gastrointestinal smooth musclesensitize afferent pain fiberscontribute to primary dysmenorrhoeaPGF2 a luteolytic hormone in some subprimate speciesuses in synchronizing ovulation in animals

Central and Peripheral Nervous SystemsPGEsPyrogens release IL-1 PGE2 increases body temperature X NSAIDsinhibit the release of NA from sympathetic nerve ending X NSAIDscause pain (id)LTB4produces hyperalgesia

Endocrine SystemPGE2 (systemic)increases circulating concentration of ACTH, GH, prolactin, gonadotropins, Ca++ mobilizationstimulates steroid production, insulin release, progesterone secretion, thyrotropin-like effects, LH-like effectsinhibits the basal rate of lipolysis

Mechanism of Action of Eicosanoids

The diversity of the effects of PGs is explained by the existence of a number of distinct receptors that mediate their actions.

Second messenger systems

Receptors - G proteins

Adenyl cyclase Stimulation of PLC

Accumulation of cAMP DAG, IP3

Cytocolic Ca++

Receptors of prostanoids and leukotrienes

Receptor subtype PlateletaggregationSmooth muscleNaturalagonistSecondmessengerDP-

PGD2 cAMP EP1

+PGE, F2 IP3/DAG/Ca+2EP2 -PGE cAMP EP3+/ +PGE cAMP /EP4

cAMP FP

+PGF2 IP3/DAG/Ca+2IP--PGI2(PGE) cAMP TP

+TXA2, PGH2 (PGD2,PGF2)IP3/DAG/Ca+2

TP+

TXA2,PGH2 IP3/DAG/Ca+2 cys-LT1

+LTC4,LTD4 IP3/DAG/Ca+2cys-LT2

+LTC4,D4,E4 IP3/DAG/Ca+2cys-LT3

+LTC4,LTD4 IP3/DAG/Ca+2BLT ( PMNL )

LTB4 IP3/DAG/Ca+2

cAMP: adenosin 3,5-monophosphate, IP3: inositol 1,4,5-triphosphate, DAG: diacylglycerol,PMNL: polimorphonuclear leucocytes, +: stimulation, : inhibition.

Montelukast and Zafirlukastinhibit cys-LT1 Receptors used in asthma

Therapeutic Uses

In the clinical application of eicosanoidsStable long-acting analogs of PGs have been developed.Specific enzyme inhibitors and receptor antagonists have been develop.Dietary manipulation is used to change the FA composition of the cell membrane phospholipids.

Transfusion and Platelet StoragePGE1 and PGI2improve the harvest and storage of platelets in therapeutic transfusion.

PGI2prevents the platelet aggregation in extracorporeal circulation systems.may be used in place of heparin during dialysis to prevent the contraindication of heparin.

Enhancement of Pulmonary Blood FlowPGE1 and PGI2increase pulmonary blood flow and oxygenation of blood in infant with congenital heart defects that restrict pulmonary or systemic blood flow (dilation of ductus arteriosus)Indomethacincloses the ductus arteriosus in delayed closure of it.

IschemiaPGI2 and its analogs(carbacyclin, iloprost) decrease arterial pressureinhibit platelet aggregationreduce pulmonary resistance

Gastric CytoprotectionPGE1 (rioprostil, misoprostil)suppress gastric acid secretion and heal gastric ulcer as effectively as H2 antagonists.

PGE2 (enprostil, arbaprostil, trimoprostil)used for the prevention of ulcers that occur during NSAIDs treatment

Therapeutic AbortionDinoprost (PGF2),Carboprost

Dinoprostone(PGE2), Sulprostan and Meteneprost

Alprostadil (PGE1), Gemeprostuse for the induction of midtrimestr abortionDuring early pregnancy, the methotrexate-PG combination induces abortion in 96% of cases, mifepristone-PG combination causes abortion of the fetus 99% of the time.

ImpotancePGE1Intracavernous injection of Alprostadil (PGE1) causes complete or partial erection.

Cell-mediated Organ Transplant Rejection PGE2 and PGI2attenuate T cell proliferation and rejection.

LTB4stimulates B cell proliferation.

Preparations

PGE1 and analogsAlprostadil (PGE1), CaverjectEnisoprostGemeprostLimaprostMisoprostol, CytotecRioprostil

PGE2 and analogsDinoprostone(PGE2)Albaprostil methyl, EnprostilTrimoprost, SulprostanMeteneprost, Viprostal

PGF2 and analogsLatanoprost, XalatanTravoprost, TravatanBimatoprost, LumiganDinoprost (PGF2)CarboprostAlphaprostol, Chloroprostenolsodium, Lubrostiol, Fenprostalen

PGI2 and analogsEpoprostenol (PGI2)Iloprost, IlomedinCyprosten, Beraprost,Carbacycline

PLATELET-ACTIVATING FACTOR (PAF)a modified phospholipid.formed by a smaller number of cell types (leukocytes, platelets and endothelial cells).can effect on every organ and tissue.

CH2 - O - CO - R1 R2 - CO - O - CH O CH2 - O - PO - coline 1-O-alkyl-2-acyl-glycerophosphocoline PLA2 Acyltransferase

CH2 - O - CO - R1 HO - CH O CH2 O - PO - coline Lyso-PAF

PAF acetyltransferase PAF acetylhydrolase

CH2 - O - CO - R1 H3C- CO - O - CH O CH2 - O - PO - coline

PAF

Synthesis and degradation of platelet-activating factor (PAF)

Pharmacological Properties

a potent vasodilator lowers vascular resistance and systemic blood pressureincreases coronary blood flow at small dosesdecreases coronary blood flow at higher doses

a potent stimulator of in vitro platelet aggregation iv injection of PAF causes platelet aggregation and thrombocytopenia

promotes aggregations of PMNLs and monocytes, and degranulation of eosinophilsa chemotactic factor for eosinophils, neutrophils and monocytes

contracts gastrointestinal, uterine and pulmonary smooth muscle

the most potent known ulcerogen

decreases renal blood flow, glomerular filtration rate, urine volume and excretion of sodium.

Physiological and Pathological Functions of PAF

PlateletsA potent stimulator of invitro platelet aggregation iv injection of PAF causes platelet aggregation and thrombocytopenia.

Reproduction and ParturitionThe administration of PAF restores ovulationThe embryo begins to produce PAF following ovulation and fertilization promotes platelet aggregation and the release of platelet factors stimulate activation and implantation of the blastocyst.Contracts the myometrium directly or by promoting the release of PGE2

Inflammatory and Allergic ResponsesId injection of PAF duplicates the signs and symptoms of inflammation.Inhalation of PAF causes tracheal and bronchial edema, mucus secretion, bronchocontraction, and an increase in the accumulation of eosinophils, which are important in asthma.The plasma concentration of PAF is increased in anaphylactic shock.

Mechanism of Action of PAFstimulates G protein-linked cell-surface receptors activation of PLs formation of IP3, DAG, AA and eicosanoids PGs, TXA2 or LTs may function as extracellular mediators of the effects of PAF It also may exert actions without exiting the cell

Receptor AntagonistsThey inhibit the binding of PAF to its receptor and block its action selectively.They migth be usefull in the therapy of disorders postulated by PAF such as asthma and sepsis.Ginkgolide B (Ginkgo biloba)(Bilokan, Craton, Gingobil, Seremaks fort, Tanakan, Tebokan): Potent and selectiveTriazolobenzodiazepines (apafant, alprazolam(Xanax), triazolam)

Case 1

A 26-year-old woman was brought to the emergency department with gross edema of both legs and some syanosis of the feet.

She had started using oral contraceptives at the age 16, but after 9 months, she developed bilateral deep vein thrombosis and small pulmonary embolus. She responded well to anticoagulant therapy and discontination of the contraceptive pills.

At age 22, she had two spontaneus abortions.

A third pregnancy was terminated at 30 weeks because of intrauterine death, and two weeks later while she was still on heparin, a large thrombus was removed surgically, from the lower aorta and both iliac and femoral arteries. She developed renal failure as a result of the thrombus into the renal arteries.

I.v. prostacyclin (PGI2) was therefore combined with the heparin, and renal function improved progressively. After 6 weeks, both drugs were discontinued. For the past 2 years, she had been maintained on warfarin, aspirin, and dipyrimadole and has remained asymptomatic.

Case 2

A 55-year-old women was brought to hospital with an 8-year history of Raynauds phenomenon assosiated with intractable pain and ischemic skin ulcer of the extremities. In addition, she had borderline hypertention (185/85 mmHg), and mild angina on effort.

She had been treated with systemic and topical antibiotic to care for the ulcer, and nifedipine as a vasodilator, but results had not been satisfactory.

On this admission, all other medications were discontinued for 24 hours, and she was given alprostadil (PGE1) by constant infusion pump (6 ng/kg/min for 12 hrs than 10 ng/kg/min for 60 hrs). After 4 weeks, there was a remarkable improvement in the pain and increased warmth on the hands. Vasospasm attacts were much less severe, and ulcers were heald. She was followed up for a period of 8 months, during which time the Raynauds syndroms remained much improved, and the cutaneus ulcers did not recur.