Cyclooxygenase Pathway of the Arachidonate Cascade

Francesca Seta, Boston University School of Medicine, Boston, Massachusetts, USA
Markus Bachschmid, Boston University School of Medicine, Boston, Massachusetts, USA

Published online: April 2012

DOI: 10.1002/9780470015902.a0023401

Prostanoids are potent lipid mediators produced from arachidonic acid by the action of prostaglandin endoperoxide H2 synthases (PGHS‐1 and ‐2), also known as cyclooxygenases (COX‐1 and ‐2). PGHS‐1, by virtue of its constitutive expression, is associated with homeostatic functions, such as platelet aggregation and gastric cytoprotection, whereas PGHS‐2, induced by a variety of inflammatory and mitogenic stimuli, is important in inflammation and cancer. However, this paradigm has been reevaluated over the years thanks to sophisticated genetic and pharmacological tools that have expanded tremendously our knowledge about the COX cascade. Despite identical structure and catalytic activities, subtle biochemical and metabolic differences account for a functional segregation of the two isoforms. Interestingly, in some biological processes, they seem not functionally interchangeable. Nonsteroidal anti‐inflammatory drugs are popular anti‐inflammatory, analgesic and antipyretic medications that block the formation of prostanoids. The search for specific inhibitors of the COX cascade, devoid of unwanted side effects, is still the subject of ongoing research.

Key Concepts:

  • Arachidonic acid (AA, 5,8,11,14‐eicosatetraenoic acid), a 20‐carbon, polyunsaturated fatty acid esterified into membrane phospholipids, can be metabolised via lipoxygenases, to produce leukotrienes; CYP450 monooxygenases to produce epoxyeicosatrienoic and hydroxyeicosatetraenoic acids (EETs and HETEs), and prostaglandin endoperoxide H2 synthase (PGHS) to produce prostanoids.
  • Prostanoids are a group of lipid autacoids including prostaglandins (PGD2, PGE2 and PGF2α), prostacyclin (PGI2) and thromboxanes (TXA2), that act through specific receptors (DP1−2, EP1−4, FPA−B, IP and TP) and, in some cases, nuclear receptors (PPAR), to elicit a variety of physiological and pathophysiological effects.
  • Prostaglandin H2 synthase (PGHS), commonly known as cyclooxygenase (COX), exists in two isoforms: PGHS‐1 and ‐2, or COX‐1 and ‐2 that catalyses the bis‐oxygenation of AA into PGH2, further reduced by cell‐specific downstream PGH2 reductases and isomerases (prostanoid synthases) into prostanoids.
  • PGHS‐1 and PGHS‐2 have identical catalytic activity that is best explained by a branched‐chain reaction between the peroxidase site (POX) and the cyclooxygenase site (COX), both present in the catalytic active site of the enzyme.
  • PGHS‐1 and PGHS‐2 differ in peroxide requirement, substrate utilisation, subcellular localisation and translational and post‐translational regulation, suggesting a segregated activity of the two isoforms when both are present.
  • PGHS‐1, constitutively expressed in many cell types is associated with homeostatic functions, such as platelet aggregation and gastric cytoprotection and PGHS‐2, mainly induced by inflammatory and mitogenic stimuli, is linked to inflammation and cancer. However, it is now clear that both isoforms are important for cardiovascular, renal and reproductive function and both play a role in inflammation and pain sensitisation.
  • PGHS‐2, by virtue of a larger side pocket in the substrate‐binding site, can metabolise polyunsaturated fatty acids other than AA with higher efficiency than PGHS‐1, yielding lipid mediators different from prostanoids, such as glycerol‐prostaglandins and aspirin‐triggered lipoxins.
  • Nonsteroidal anti‐inflammatory drugs (NSAIDs), such as aspirin, diclofenac and coxibs, are anti‐inflammatory, analgesic and antipyretic drugs that block the formation of prostanoids by competitive or irreversible inhibition of AA binding to COX.
  • NSAIDs are a heterogeneous class of organic acids that differ considerably in pharmacokinetics and pharmacodynamics, including the selectivity for PGHS‐1 and PGHS‐2. Individual isoform selectivity attained in vivo is crucial for therapeutic and side effects, and this can vary among individuals and the drug used.

Keywords: eicosanoids; cyclooxygenase; prostaglandin H2 endoperoxidase; prostanoids; peroxides; tyrosyl radical; arachidonic acid; nonsteroideal anti‐inflammatory drugs; coxibs

Figure 1. Structure of Prostaglandin endoperoxide H2 synthase (PGHS). The key enzyme in prostanoid synthesis is the homodimeric PGHS, which consists of two spatially distinct catalytic sites, the peroxidase (left panel) and the cycylooxygenase domain (right panel). Its substrate arachidonic acid (AA) is oxidised at the cyclooxygenase site by the tyrosyl radical (Tyr) into PGG2, which is further converted by haem (Iron Protoporphyrin IX) catalysis into PGH2 at the peroxidase domain. The activity of PGHS is regulated by the ‘peroxide tone’, which describes the requirement of certain levels of peroxides to oxidise the haem, thus forming a tyrosyl radical at the cyclooxygenase via intramolecular electron transfer. Peroxides and peroxynitrite are potent endogenous activators of PGHS. The background depicts the X‐ray structure of the monomeric PGHS, showing (yellow) the iron protoporphyrin on the left panel and the bound AA on the right. PP, protoporphyrin; Fe, iron; R‐OOH, peroxide; R‐OH, alcohol; AA, arachidonic acid; PGG2, prostaglandin endoperoxide G2; PGH2, prostaglandin endoperoxide H2. The epidermal growth factor domain (EGF) is indicated in green and the membrane‐binding domain (MBD) in orange, on the right. Reproduced with permission from Schildknecht et al. (2008). Copyright Federation of American Societies for Experimental Biology.
Figure 2. The cyclooxygenase pathway of the arachidonate cascade. In response to chemical and mechanical stimuli, arachidonic acid, a 20‐carbon fatty acid with four double bonds (20:4) is released from membrane phospholipids by phospholipase A2. Prostaglandin endoperoxide H2 synthase (PGHS) catalyses the bis‐oxygenation of free AA into the unstable endoperoxide PGG2 and the reduction of PGG2 into PGH2, by the coordinated activity of the cyclooxygenase (COX) and the peroxidase domain (POX). PGH2 is further metabolised by cell‐specific terminal isomerases and reductases to yield prostanoids. TXS, thromboxane (Tx) A2 synthase; PGDS, prostaglandin (PG) D2 synthase; PGES, prostaglandin (PG) E2 synthase; PGFS, prostaglandin (PG) F2a synthase; PGIS, prostaglandin (PG) I2 synthase. TXA2 and PGI2 are unstable metabolites and hydrolised within minutes from their synthesis into the inactive metabolites, TxB2 and 6‐keto‐PGF1α.
Figure 3. Nonsteroideal anti‐inflammatory drugs (NSAIDs) classified by PGHS isoform selectivity. Arrow indicates degree of selectivity. Classic or traditional NSAIDs (tNSAIDs) are indicated in black and coxibs (COX‐2 inhibitors) in red. (*) All NSAIDs block prostanoid formation by competitive inhibition with arachidonic acid except aspirin, which irreversibly acetylates COX, and acetaminophen, which inhibits POX, but not COX, activity. To note, meloxicam, diclofenac and etorolac are tNSAIDs with similar selectivity towards PGHS‐2 as coxibs. Valdecoxib and Rofecoxib were withdrawn from the market worldwide, whereas etoricoxib and lumiracoxib never got approval in USA, for evidence of adverse cardiovascular events.
close
 References
    Bresalier RS, Sandler RS, Quan H et al. (2005) Cardiovascular events associated with rofecoxib in a colorectal adenoma chemoprevention trial. The New England Journal of Medicine 352: 1092–1102.
    Chakrabarty A, Tranguch S, Daikoku T et al. (2007) MicroRNA regulation of cyclooxygenase‐2 during embryo implantation. Proceedings of the National Academy of Sciences of the USA 104: 15144–15149.
    Cheng Y, Austin SC, Rocca B et al. (2002) Role of prostacyclin in the cardiovascular response to thromboxane A2. Science 296: 539–541.
    Crofford LJ, Wilder RL, Ristimaki AP et al. (1994) Cyclooxygenase‐1 and ‐2 expression in rheumatoid synovial tissues. Effects of interleukin‐1 beta, phorbol ester, and corticosteroids. The Journal of Clinical Investigation 93: 1095–1101.
    Dinchuk JE, Car BD, Focht RJ et al. (1995) Renal abnormalities and an altered inflammatory response in mice lacking cyclooxygenase II. Nature 378: 406–409.
    Dubois RN, Abramson SB, Crofford L et al. (1998) Cyclooxygenase in biology and disease. The FASEB Journal: Official Publication of the Federation of American Societies for Experimental Biology 12: 1063–1073.
    Eling TE, Glasgow WC, Curtis JF, Hubbard WC and Handler JA (1991) Studies on the reduction of endogenously generated prostaglandin G2 by prostaglandin H synthase. The Journal of Biological Chemistry 266: 12348–12355.
    Forsberg L, Leeb L, Thoren S, Morgenstern R and Jakobsson P (2000) Human glutathione dependent prostaglandin E synthase: gene structure and regulation. FEBS Letters 471: 78–82.
    Fries S, Grosser T, Price TS et al. (2006) Marked interindividual variability in the response to selective inhibitors of cyclooxygenase‐2. Gastroenterology 130: 55–64.
    Ghosh M, Wang H, Ai Y et al. (2007) COX‐2 suppresses tissue factor expression via endocannabinoid‐directed PPARdelta activation. The Journal of Experimental Medicine 204: 2053–2061.
    Hamberg M, Svensson J and Samuelsson B (1975) Thromboxanes: a new group of biologically active compounds derived from prostaglandin endoperoxides. Proceedings of the National Academy of Sciences of the USA 72: 2994–2998.
    Hao CM and Breyer MD (2008) Physiological regulation of prostaglandins in the kidney. Annual Review of Physiology 70: 357–377.
    Hao CM, Redha R, Morrow J and Breyer MD (2002) Peroxisome proliferator‐activated receptor delta activation promotes cell survival following hypertonic stress. The Journal of Biological Chemistry 277: 21341–21345.
    Harris RC, McKanna JA, Akai Y et al. (1994) Cyclooxygenase‐2 is associated with the macula densa of rat kidney and increases with salt restriction. The Journal of Clinical Investigation 94: 2504–2510.
    Hoff T, DeWitt D, Kaever V, Resch K and Goppelt‐Struebe M (1993) Differentiation‐associated expression of prostaglandin G/H synthase in monocytic cells. FEBS Letters 320: 38–42.
    Ishikawa TO and Herschman HR (2006) Conditional knockout mouse for tissue‐specific disruption of the cyclooxygenase‐2 (Cox‐2) gene. Genesis 44: 143–149.
    Ishikawa TO, Jain NK, Taketo MM and Herschman HR (2006) Imaging cyclooxygenase‐2 (Cox‐2) gene expression in living animals with a luciferase knock‐in reporter gene. Molecular Imaging and Biology: MIB: The Official Publication of the Academy of Molecular Imaging 8: 171–187.
    Kamei K, Ishikawa TO and Herschman HR (2006) Transgenic mouse for conditional, tissue‐specific Cox‐2 overexpression. Genesis 44: 177–182.
    Kawka DW, Ouellet M, Hetu PO, Singer II and Riendeau D (2007) Double‐label expression studies of prostacyclin synthase, thromboxane synthase and COX isoforms in normal aortic endothelium. Biochimica et Biophysica Acta 1771: 45–54.
    Khan Z, Khan N, Tiwari RP et al. (2011) Biology of Cox‐2: an application in cancer therapeutics. Current Drug Targets 12: 1082–1093.
    Kliewer SA, Lenhard JM, Willson TM et al. (1995) A prostaglandin J2 metabolite binds peroxisome proliferator‐activated receptor gamma and promotes adipocyte differentiation. Cell 83: 813–819.
    Langenbach R, Morham SG, Tiano HF et al. (1995) Prostaglandin synthase 1 gene disruption in mice reduces arachidonic acid‐induced inflammation and indomethacin‐induced gastric ulceration. Cell 83: 483–492.
    Lim H and Dey SK (2000) PPAR delta functions as a prostacyclin receptor in blastocyst implantation. Trends in Endocrinology and Metabolism: TEM 11: 137–142.
    Lim H, Paria BC, Das SK et al. (1997) Multiple female reproductive failures in cyclooxygenase 2‐deficient mice. Cell 91: 197–208.
    Liou JY, Shyue SK, Tsai MJ et al. (2000) Colocalization of prostacyclin synthase with prostaglandin H synthase‐1 (PGHS‐1) but not phorbol ester‐induced PGHS‐2 in cultured endothelial cells. The Journal of Biological Chemistry 275: 15314–15320.
    Mbonye UR, Yuan C, Harris CE et al. (2008) Two distinct pathways for cyclooxygenase‐2 protein degradation. The Journal of Biological Chemistry 283: 8611–8623.
    McAdam BF, Catella‐Lawson F, Mardini IA et al. (1999) Systemic biosynthesis of prostacyclin by cyclooxygenase (COX)‐2: the human pharmacology of a selective inhibitor of COX‐2. Proceedings of the National Academy of Sciences of the USA 96: 272–277.
    McAdam BF, Mardini IA, Habib A et al. (2000) Effect of regulated expression of human cyclooxygenase isoforms on eicosanoid and isoeicosanoid production in inflammation. The Journal of Clinical Investigation 105: 1473–1482.
    Mizuno K, Yamamoto S and Lands WE (1982) Effects of non‐steroidal anti‐inflammatory drugs on fatty acid cyclooxygenase and prostaglandin hydroperoxidase activities. Prostaglandins 23: 743–757.
    Moncada S, Gryglewski R, Bunting S and Vane JR (1976) An enzyme isolated from arteries transforms prostaglandin endoperoxides to an unstable substance that inhibits platelet aggregation. Nature 263: 663–665.
    Morham SG, Langenbach R, Loftin CD et al. (1995) Prostaglandin synthase 2 gene disruption causes severe renal pathology in the mouse. Cell 83: 473–482.
    Morita I, Schindler M, Regier MK et al. (1995) Different intracellular locations for prostaglandin endoperoxide H synthase‐1 and ‐2. The Journal of Biological Chemistry 270: 10902–10908.
    Naraba H, Murakami M, Matsumoto H et al. (1998) Segregated coupling of phospholipases A2, cyclooxygenases, and terminal prostanoid synthases in different phases of prostanoid biosynthesis in rat peritoneal macrophages. Journal of Immunology 160: 2974–2982.
    Norata GD, Callegari E, Inoue H et al. (2004) HDL3 induces cyclooxygenase‐2 expression and prostacyclin release in human endothelial cells via a p38 MAPK/CRE‐dependent pathway: effects on COX‐2/PGI‐synthase coupling. Arteriosclerosis, Thrombosis, and Vascular Biology 24: 871–877.
    Ricciotti E and FitzGerald GA (2011) Prostaglandins and inflammation. Arteriosclerosis, Thrombosis, and Vascular Biology 31: 986–1000.
    Rocca B, Spain LM, Pure E et al. (1999) Distinct roles of prostaglandin H synthases 1 and 2 in T‐cell development. The Journal of Clinical Investigation 103: 1469–1477.
    Rouzer CA and Marnett LJ (2008) Non‐redundant functions of cyclooxygenases: oxygenation of endocannabinoids. The Journal of Biological Chemistry 283: 8065–8069.
    Sala A, Folco G and Murphy RC (2010) Transcellular biosynthesis of eicosanoids. Pharmacological Reports: PR 62: 503–510.
    Schildknecht S, Bachschmid M, Baumann A and Ullrich V (2004) COX‐2 inhibitors selectively block prostacyclin synthesis in endotoxin‐exposed vascular smooth muscle cells. FASEB Journal 18: 757–759.
    Schildknecht S, Daiber A, Ghisla S, Cohen RA and Bachschmid MM (2008) Acetaminophen inhibits prostanoid synthesis by scavenging the PGHS‐activator peroxynitrite. The FASEB Journal: Official Publication of the Federation of American Societies for Experimental Biology 22: 215–224.
    Schonbeck U, Sukhova GK, Graber P, Coulter S and Libby P (1999) Augmented expression of cyclooxygenase‐2 in human atherosclerotic lesions. The American Journal of Pathology 155: 1281–1291.
    Schuster VL (1998) Molecular mechanisms of prostaglandin transport. Annual Review of Physiology 60: 221–242.
    Serhan CN and Petasis NA (2011) Resolvins and protectins in inflammation resolution. Chemical Reviews 111: 5922–5943.
    Seta F, Chung AD, Turner PV et al. (2009) Renal and cardiovascular characterization of COX‐2 knockdown mice. American Journal of Physiology. Regulatory, Integrative and Comparative Physiology 296: R1751–R1760.
    Smith WL (2008) Nutritionally essential fatty acids and biologically indispensable cyclooxygenases. Trends in Biochemical Sciences 33: 27–37.
    Smith WL, Garavito RM and DeWitt DL (1996) Prostaglandin endoperoxide H synthases (cyclooxygenases)‐1 and ‐2. The Journal of Biological Chemistry 271: 33157–33160.
    Song I, Ball TM and Smith WL (2001) Different suicide inactivation processes for the peroxidase and cyclooxygenase activities of prostaglandin endoperoxide H synthase‐1. Biochemical and Biophysical Research Communications 289: 869–875.
    Swinney DC, Mak AY, Barnett J and Ramesha CS (1997) Differential allosteric regulation of prostaglandin H synthase 1 and 2 by arachidonic acid. The Journal of Biological Chemistry 272: 12393–12398.
    Thoren S and Jakobsson PJ (2000) Coordinate up‐ and down‐regulation of glutathione‐dependent prostaglandin E synthase and cyclooxygenase‐2 in A549 cells. Inhibition by NS‐398 and leukotriene C4. European Journal of Biochemistry/FEBS 267: 6428–6434.
    Thuresson ED, Lakkides KM, Rieke CJ et al. (2001) Prostaglandin endoperoxide H synthase‐1: the functions of cyclooxygenase active site residues in the binding, positioning, and oxygenation of arachidonic acid. The Journal of Biological Chemistry 276: 10347–10357.
    Topper JN, Cai J, Falb D and Gimbrone MA Jr (1996) Identification of vascular endothelial genes differentially responsive to fluid mechanical stimuli: cyclooxygenase‐2, manganese superoxide dismutase, and endothelial cell nitric oxide synthase are selectively up‐regulated by steady laminar shear stress. Proceedings of the National Academy of Sciences of the USA 93: 10417–10422.
    Trebino CE, Stock JL, Gibbons CP et al. (2003) Impaired inflammatory and pain responses in mice lacking an inducible prostaglandin E synthase. Proceedings of the National Academy of Sciences of the USA 100: 9044–9049.
    Tsai AL and Kulmacz RJ (2010) Prostaglandin H synthase: resolved and unresolved mechanistic issues. Archives of Biochemistry and Biophysics 493: 103–124.
    Uddin MJ, Crews BC and Blobaum AL (2010) Selective visualization of cyclooxygenase‐2 in inflammation and cancer by targeted fluorescent imaging agents. Cancer Research 70: 3618–3627.
    Ueno N, Takegoshi Y, Kamei D, Kudo I and Murakami M (2005) Coupling between cyclooxygenases and terminal prostanoid synthases. Biochemical and Biophysical Research Communications 338: 70–76.
    Vane JR (1971) Inhibition of prostaglandin synthesis as a mechanism of action for aspirin‐like drugs. Nature: New Biology 231: 232–235.
    Yu Y, Fan J, Hui Y et al. (2007) Targeted cyclooxygenase gene (ptgs) exchange reveals discriminant isoform functionality. The Journal of Biological Chemistry 282: 1498–1506.
    Yu Y, Stubbe J, Ibrahim S et al. (2010) Cyclooxygenase‐2‐dependent prostacyclin formation and blood pressure homeostasis: targeted exchange of cyclooxygenase isoforms in mice. Circulation Research 106: 337–345.
    Yuan C, Sidhu RS, Kuklev DV et al. (2009) Cyclooxygenase allosterism, fatty acid‐mediated cross‐talk between monomers of cyclooxygenase homodimers. The Journal of Biological Chemistry 284: 10046–10055.
    Zidar N, Odar K, Glavac D et al. (2009) Cyclooxygenase in normal human tissues – is COX‐1 really a constitutive isoform, and COX‐2 an inducible isoform? Journal of Cellular and Molecular Medicine 13: 3753–3763.
 Further Reading
    book Grosser T, Smyth E and Fitzgerald GA (2011) "Goodman and Gilman's the pharmacological basis of therapeutics". In: Anti‐inflammatory, Antipyretic, and Analgesic Agents; Pharmacotherapy of Gout, 12th edn, chap. 34, section IV. New York: McGraw Hill.
    book Smyth E, Grosser T and Fitzgerald GA (2011) "Goodman and Gilman's the pharmacological basis of therapeutics". In: Lipid‐Derived Autacoids and Platelet‐Activating Factor, 12th edn, chap. 33, section IV. New York: McGraw‐Hill.

Related Sub-Topics:

Enzymes: Structure and Action Mechanism
Contact Editor close
Submit a note to the editor about this article by filling in the form below.

* Required Field

Article Title: Cyclooxygenase Pathway of the Arachidonate Cascade
 
How to Cite close
Seta, Francesca, and Bachschmid, Markus(Apr 2012) Cyclooxygenase Pathway of the Arachidonate Cascade. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0023401]