Cyclooxygenase Pathway of the Arachidonate Cascade

Abstract

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.. 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.

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Further Reading

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.

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.

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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]