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Lipid Mediators: Synthesis,Lipid Mediators: Synthesis,Metabolism, and FunctionMetabolism, and Function
Biochemistry 201Oct 23, 2007Electa Park
OverviewOverview
• Biological function of lipid mediators• Arachidonic acid (AA) synthesis• AA metabolites and related enzymes
– Prostaglandins, thromboxanes, leukotrienes• EETS, Lipoxins, Isoprostanes
• Inhibiton of AA metabolizing enzymes• Receptors and Metabolism
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• A protective response of the body
• Elicited by cell injury (microbes, toxins)– Mechanical, thermal, chemical, and bacterial insult
• Goal: eradication of harmful agents and initiationof the healing process
• Arachidonic acid (AA) metabolites participate inthe pathogenesis of inflammation and fever
InflammationInflammation• Hallmarks:
– Pain- PGE2, LTB4, LTC4, LTD4, PAF (hyperalgesia)– Fever- PGE2– Vasodilation- PGE2, PGI2– Increased vascular permeability- PGE2, PGD2, PGI2, LTC4,
LTD4, PAF– Chemotaxis and Leukocyte activation- LTB4, PAF– Cell Migration- LTB4– Tissue damage
• Anaphylaxis:– LTC4, LTD4
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Other Effects of AAOther Effects of AAMetabolitesMetabolites
• Bronchioconstrictor- LTs, PAF– Asthma, acute insult
• Platelet aggregation-– Promoted by TXA2
– Inhibited by PGI2
• Gastric cytoprotection-PGI2, PGE2, PGF2, PAF– Smooth muscle contraction, inhibit acid secretion in stomach,
enhance mucus secretion and mucosal blood flow
• Renal function-PGI2, PGE2– Modulate renal blood flow, urine formation
• Reproduction-PGE2, PGF2, PAF– Conception, labor, menstruation, male reproductive
mechanism
• Musculoskeletal-PGs– Bone remodelling (osteoclast and osteoblasts)– Rheumatoid Arthritis
Other Effects of AAOther Effects of AAMetabolitesMetabolites
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PLA2
Arachidonic Acid
Prostaglandins Leukotrienes
Prostacyclin Thromboxanes
COX 1/2 lipoxygenase
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Cellular Membranes:Cellular Membranes:Phospholipid BilayersPhospholipid Bilayers
Phosphatidylcholine majorsource of AA in body;enriched in membranes ofcells of myeloid origin andendothelium
• Secretory - first isolated– ~10 groups, only IIA, V, and X involved in PG
synthesis– Low molecular weight (~14 kDa)– Restricted expression
• IIA - small intestine• V - eye, heart, pancreas• X - testis, stomach
– Have specific cell surface receptors• Are involved in signaling directly; can modulate PG
production via receptor binding and internalization
Phospholipase Phospholipase AA22
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Phospholipase Phospholipase AA22
• Cytosolic– 85 kDa major isoform (cPLA2-α), also β (110 kDa)
and γ (61 kDa)• α, β Ca2+-dependent• γ Ca2+-independent
– Primary source of AA for PGs, etc.– Found in most tissues– Activated by Ca2+ binding and membrane association
• Activation also regulated by phosphorylation,phosphoinositides, scaffold proteins
– Site of action:• Nuclear membrane• Also reported on ER, mitochondrial membranes
Structure of cPLAStructure of cPLA22
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Structure of cPLAStructure of cPLA22
Structure of cPLAStructure of cPLA22
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Structure of cPLAStructure of cPLA22
(Ca(Ca2+2+))22
cPLAcPLA22 Active Site Serine Active Site Serine
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PLA2
Arachidonic Acid
Prostaglandins Leukotrienes
Prostacyclin Thromboxanes
COX 1/2 lipoxygenase
Cyclooxygenase Cyclooxygenase 1 & 21 & 2(Cox 1/2)(Cox 1/2)
• Also known as Prostaglandin Endoperoxide HSynthase 1 & 2
• Bind AA with Km ≈ 5 µM, O2 ≈ 5 µM• Cox 1
– constitutive expression in most tissues• Cox 2
– Expressed in nervous, immune, renal tissue– Expression inducible by inflammatory and proliferative
signals– May play a role in modulating endogenous
cannabinoid signaling• Km for 2-arachidonylglycerol ≈ 5 µM; for
arachidonylethanolamide ≈ 24 µM
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COX 1/2: One Enzyme, TwoCOX 1/2: One Enzyme, TwoActivitiesActivities
• Cyclooxygenase activity-– Dependent on active site tyrosyl radical– Results in PGG2
• Can activate other COX enzymes
• Peroxidase activity-– Heme dependent– Leads to production of PGH2
• Short t1/2• Rapidly metabolized to more stable PGs
Epidermal Growth Factor DomainEpidermal Growth Factor DomainMembrane Binding DomainMembrane Binding DomainCatalytic DomainCatalytic Domain
COX1 COX1 HomodimerHomodimer
FlurbiprofenFlurbiprofenHemeHeme
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COX1 MonomerCOX1 Monomer
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•• Cyclooxygenase Cyclooxygenase activity dependent on activity dependent on tyrosyl tyrosyl radicalradical
•• Peroxidase Peroxidase activity dependent on activity dependent on heme heme groupgroup
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11
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1315
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PGISynthase
PGI2 (prostacyclin)
6-keto-PGF1α
PGD2 PGF2(9α, 11α-PGF2α)
PGDSynthase
PGFSynthase
PGESynthase
PGE2
TXB2
TXA2
TXASynthase
PGH2
PGH2 Metabolizing Enzymes:PGH2 Metabolizing Enzymes:LocalizationLocalization
• PGD2 synthase (isomerase)– Immune cells
• PGE2 synthase (isomerase)– Most cells
• PGF2 synthase (reductase)– Uterus, seminal vesicles
• PGI2 (Prostacyclin) synthase– Endothelial cells
• Thromboxane synthase– Platelets
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Thromboxane SynthaseThromboxane Synthase
Thromboxane SynthaseThromboxane Synthase
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VSMC
Vasoconstriction
Vasodilation
Aggregation
PlateletsDeclumping
Uterine smoothmuscle
Contraction,parturition
Chemotaxis
Th2 lymphocyte
Allergic asthma
Lung epithelial cell
Bone resorptionOsteoclast
Fever Generation
Neurons of OVLT of PDA
Maturation for ovulationand fertilization
OvarianCO cells
Pain response
Spinal neurons
Cell activation: Generic cellcytokines, growth factors, mechanical trauma
X = NSAIDS celecoxib rofecoxib
PLA2
Arachidonic Acid
Prostaglandins Leukotrienes
Prostacyclin Thromboxanes
COX 1/2 lipoxygenase
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LipoxygenaseLipoxygenase• 5-Lipoxygenase 5-HPETE
• 8-Lipoxygenase
• 12-lipoxygenase 12-HPETE
• 15-lipoxygenase 15-HPETE
Leukotrienes
5,12-DHETE
8-HPETE
11-HPETE
8-HETE
11-HETE
12-HETE5,12-DHETE5S,12S-THETE
15-HETE16,15-DHETE8,15-DHETE
DHETE, HETE - Leukocytes, eosinophils,platelets = muscle contraction, cell degranulation,superoxide production, chemotaxis
AA Metabolism byAA Metabolism by5-5-LipoxygenaseLipoxygenase
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Leukotriene Leukotriene (LT) Synthesis(LT) Synthesis
• LTA4 - 5-Lipoxygenase– Neutrophils, basophils, eosinophils, mast cells, monocytes,
macrophages• LTB4 - LTA4 hydrolase
– Most cells• LTC4 - LTC4 synthase (glutathione transferase)
– Basophils, eosinophils, mast cells, monocytes/macrophages,platelets
• LTD4 - γ-glutamyl transpeptidase, γ-glutamylleukotrienase– Extracelluar metabolite of LTC4– LTE4 - aminopeptidase activity towards LTD4
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5
6
10
7
7
LTA4 Synthesis Step 1
5
5
5
6
10
7
7
10
6
10
H
6
LTA410
H
6
H
O-OHHO-O
H2O
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10
H
6
H+
LTA4 Synthesis Step 2
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Leukotriene Leukotriene synthesissynthesis
cPLA2α
cPLA2α
5-LO
5-LO
5-LO
5-LO
FLAP
FLAP LTC4-S
LTA4-H
LTA4-H
LT SynthesisLT Synthesis
• LTA4 - 5-Lipoxygenase– Neutrophils, basophils, eosinophils, mast cells, monocytes,
macrophages• LTB4 - LTA4 hydrolase
– Most cells• LTC4 - LTC4 synthase (glutathione transferase)
– Basophils, eosinophils, mast cells, monocytes/macrophages,platelets
– mGST 2 or mGST 3 (testis)• LTD4 - γ-glutamyl transpeptidase, γ-glutamyl
leukotrienase– Extracelluar metabolite of LTC4
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LTALTA44 HydrolaseHydrolase
LTALTA44-H active site-H active site
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LT SynthesisLT Synthesis
• LTA4 - 5-Lipoxygenase– Neutrophils, basophils, eosinophils, mast cells, monocytes,
macrophages• LTB4 - LTA4 hydrolase
– Most cells• LTC4 - LTC4 synthase (glutathione transferase)
– Basophils, eosinophils, mast cells, monocytes/macrophages,platelets
– mGST 2 or mGST 3 (testis)• LTD4 - γ-glutamyl transpeptidase, γ-glutamyl
leukotrienase– Extracelluar metabolite of LTC4
LTA4 MetabolitesLTA4 MetabolitesNeutrophil
Chemotaxis
Most Tissues
Heart, adrenal,brain, spleen
Bronchioconstriction,edema
Airway SMC,Postcapillary venul endotheliumLTA4
LTB4
Cell activation
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LipoxinsLipoxins
Lipoxin Lipoxin FunctionFunction
• Cell-cell interactions• Endogenous anti-
inflammatory substance• Inhibits neutrophil activation,
migration, and chemotacticsignals
• Inhibits TNF, NFκB,cytoprotective for enterocytes,stimulates macorphagephagocytosis of neutrophils
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Other enzymes thatOther enzymes thatmetabolize AAmetabolize AA
• Cytochrome P450 enzymes– ER resident enzymes– Main actions in renal and vascular systems– Action via receptor binding and modulation of
intracellular signaling pathways
Epoxyeicosatrienoic Epoxyeicosatrienoic acidacid• Mechanism of Action: Activate smooth
muscle large conductance Ca 2+ activatedK + channels (BkCa) which hyperpolarizesthe smooth muscle causingvasorelaxation.
• Endothelium-derived hyperpolarizingfactor.
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IsoprostanesIsoprostanes
• Non-cyclooxygenase isomers ofprostaglandins: auto-oxidation products ofpolyunsaturated fatty acids.
• Stimulate the TP receptors• Markers of lipid peroxidation, stimulate
platelet aggregation, cell proliferation andvasoconstriction.
PLA2
Arachidonic Acid
Prostaglandins Leukotrienes
Prostacyclin Thromboxanes
COX 1/2 lipoxygenase
NSAIDS
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Inhibitors of PG SynthesisInhibitors of PG Synthesis• COX inhibitors-
– Non-Steroidal Anti-Inflammatory Drugs(NSAIDS)
• Salicylates• Arylpropionic acid derivatives• Acetaminophen????
– Selective COX 2inhibitors
HistoryHistory• The bark of willow was prescribed as far back as
400B.C by Hippocrates as a pain reliever andfever reducer
• First purified in 1829 byLeroux
• Introduced as “aspirin” byBayer and Co. of Germanyas an antiinflammatory in 1900
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Mechanisms of ActionMechanisms of Action
• NSAIDS were used for years without anyknowledge of how it worked
• In 1971 Vane et al demonstrate that lowconcentrations of aspirin inhibit theenzymatic production of prostaglandins
SalicylatesSalicylates
• Aspirin• Salycylic acids• Diflunisal (arthritis)
CH2CO
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SalicylatesSalicylates• Mechanism of action
– Acetylates proteins including COX-1 & 2COX-1 & 2– Irreversible inhibition
CH
O-
NH2 O
CH2CO
salicylate CHNH2 OCH2C
OO
Aspirin-Triggered Aspirin-Triggered Lipoxin Lipoxin (ATL)(ATL)
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Inhibitors of PG SynthesisInhibitors of PG Synthesis• COX inhibitors-
– Non-Steroidal Anti-Inflammatory Drugs (NSAIDS)• Salicylates• Arylpropionic acid derivatives
– Ibuprofen (Motrin, Nuprin, Advil)– Naproxin (Aleve, Naprosyn, Anaprox)– Flurbiprofen (Ansaid)– Ketoprophen (Orudis)– Oxaprozin (Daypro) - long half-life
• Acetaminophen????
– Selective COX 2 inhibitors
Arylproprionic Arylproprionic Acid DerivativesAcid Derivatives• Mechanism of Action
– Competitive inhibitors– Block cyclooxygenase activity– Reversibly bind to COX 1 & 2COX 1 & 2 inhibiting
interaction with arachidonic acid
COX 1 andFlurbiprofen
COX 2 andSC-588
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Proprionic Proprionic Acid Derivatives:Acid Derivatives:Competitive COX InhibitorsCompetitive COX Inhibitors
Ibuprofen in AAbinding site ofCOX
NSAID & NSAID & Propionic Propionic acidacidderivative Activitiesderivative Activities
– Antiinflammatory• Treatment of muskuloskeletal disorders
(rheumatoid arthritis, osteoarthritis, andankylosing spondylitis)
• Symptomatic relief only– Antipyretic
• Reduce body temperature in febrile states– Analgesic
• Low to moderate pain caused by inflammation
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AcetominophenAcetominophen
• Analgesic• Antiinflammatory• Not antipyrogenic• Doesn’t inhibit COX1/2; may inhibit COX3• No longer considered an NSAID• Hepatotoxicity when combined with alcohol
– Glutathione depletion
Inhibitors of PG SynthesisInhibitors of PG Synthesis• COX inhibitors-
– Non-Steroidal Anti-Inflammatory Drugs(NSAIDS)
• Salicylates• Arylpropionic acid derivatives• Acetaminophen????
– Selective COX 2inhibitors
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COX-2 InhibitorsCOX-2 Inhibitors
• Celecoxib (Celebrex)• Rofecoxib (Vioxx)• Valdecoxib (Bextra)
• Mechanism of action:– Selectively inhibits COX-2COX-2 which is induced locally at
the site of inflammation– Does not interfere with constitutive form that protects
the gastrointestinal tract– Drug companies thought they had a safer drug to
treat forms of osteoarthritis• Vioxx recall-
– COX-2 metabolites improtant for platelet deaggregationand vascular healing
– Major cardiovascular complications» Heart attack» Stroke
COX-2 InhibitorsCOX-2 Inhibitors
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Merck Announces Voluntary WorldwideWithdrawal of VIOXX®WHITEHOUSE STATION, N.J., Sept. 30, 2004—Merck & Co., Inc. today announced a voluntaryworldwide withdrawal of VIOXX® (rofecoxib), its arthritis and acute pain medication.The company’s decision, which is effective immediately, is based on new, three-year data from a prospective, randomized, placebo-controlled clinical trial, theAPPROVe (Adenomatous Polyp Prevention on VIOXX) trial.
PLA2
Arachidonic Acid
Prostaglandins Leukotrienes
Prostacyclin Thromboxanes
COX 1/2 lipoxygenase
PAF
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Platelet Activating FactorPlatelet Activating Factor• Lipid mediator derived from
phosphatidylcholine• Platelet and leukocyte activation
– Cell polarization– Integrin activation– Priming (for degranulation)– Redistribution of surface ligands
• Displayed on cell surface of endothelialcells
PAF SynthesisPAF Synthesis
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Lipid Mediators elicit theirLipid Mediators elicit theireffects through effects through GPCRsGPCRs
• G Protein-Coupled Receptors– Also called 7-transmembrane (TM) receptors
• Couple to trimeric G-proteins that areactivated by ligand binding
• Have complex biology; not simply on-offswitches
PAF ReceptorPAF Receptor
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TXATXA22 Receptor Receptor
PGE Receptor & PGE Receptor & IsoformsIsoforms
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Prostaglandin Signaling:Prostaglandin Signaling:Receptors and 2nd MessengersReceptors and 2nd Messengers
• PGD2 DP Gs ↑cAMP• PGE EP1 IP3/DAG/Ca2+
• PGE EP2 Gs ↑cAMP• PGE EP3,4 ↑↓cAMP• PGF2 FP IP3/DAG/Ca2+
• PGI IP Go K+,• TXA2 TP IP3/DAG/Ca2+
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EET Receptor systemEET Receptor system
Metabolism of Lipid MediatorsMetabolism of Lipid Mediators
• Cytochrome P450 4A, 4F– ω-oxidation– β-oxidation
• Peptidase activity• Make compounds less hydrophobic →
excretion in urine
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ReferencesReferences• Phospholipases
– Diaz BL, JP Arm Phospholipase A2, Prostaglands Leukot and Essent FattyAcids 69 (2003) 87-97
– Dessen A Structure and Mechanism of human cytosolic phospholipase A2,Biochim Biophys Acta 1488 (2000) 40-47
• COX1/2 & PGs– Rouzer CA, LJ Marnett Structural and functional differences between
cyclooxygenases: Fatty acid oxygenases with a critical role in cell signalingBiochem Biophys Res Comm 338 (2005) 34-44
– Smith WL, I Song The enzymology of prostaglandin endoperoxide H synthases1- and 2, Prostaglands other lipid mediat 68-69 (2002) 115-128
– Garavito RM, MG Malkowski, DL DeWitt the structures of prostaglandinendoperoxide H synthases-1 and -2, Prostaglanding other lipid mediat 68-69(2002) 129-152
– Gupta K, BS Selinsky, CJ Kaub, AK Katz, PJ Loll The 2.0 A Resolution CrystalStructure of Prostaglandin H2 Synthase-1: Structural Insights into an UnusualPeroxidase, J Mol Biol 335 (2004) 503-518
• Thromboxane– Wang L-H, RJ Kulmacz Thromboxane synthase: structure and function of
protein and gene Prostaglandins other lipid mediat 68-69 (2002) 409-422
• Lipoxygenases and LTs– Murphy RC, MA Gijon Biosynthesis and metabolism of leukotrienes Biochem J
405 (2007) 379-395• PAF
– Prescott SM, GA Zimmerman, DM Stafforini, TM McIntyre Platelet-ActivatingFactor and Related Lipid Mediators Annu Rev Biochem 69 (2000) 419-445
• Inhibitors– Bertolini A, A Ottani, M Sandrini Selective COX-2 Inhibitors and Dual Acting
Anti-inflammatory Drugs: Critical Remarks Curr Med Chem 9 (2002) 1033-1043– Rajakariar R, MM Yaqoob, DW Gilroy COX-2 in Inflammation and Resolution
Mol Interv 6 (2006) 199-207• Receptors
– Narumiya S Prostanoid Receptors: Structure, Function, and Distribution Ann NY Acad Sci 744 (1994) 126-138
• CYP 450– Kalsotra A, HW Strobel Cytochrome P45 4F subfamily: At the crossroads of
eicosanoid and drug metabolism Pharmacol Ther 112 (2006) 589-611• PG/LT/TX function
– Miller SB Prostaglandins in Health and Disease: An overview J Semarthrit 3(2006) 37-49
– William KI, GA Higgs Eicosanoids and Inflammation J Pathol 156 (1988) 101-110
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