Cyclooxygenase

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COX-1 and COX-2, also called PGHS-1 and PGHS-2, regulate a key step in prostaglandins and thromboxanes synthesis and are the targets of nonsteroidal antiinflammatory drugs (NSAIDs) ^{[1] [2] [3]}. Prostaglandins are implicated in various pathophysiological processes such as inflammatory reaction, gastrointestinal cytoprotection, hemostasis and thrombosis, as well as renal hemodynamics ^{[1] [3] [4]}. Whereas COX-1 presents a widespread constitutive expression, COX-2 is undetectable in most normal tissues (except for the central nervous system, kidneys, and seminal vesicles), but is induced by various inflammatory and mitogenic stimuli ^{[4] [3] [5]}. More recently, a third isoform named COX-3 was identified as a COX-1 splicing variant. This new isoform may play a role in fever and pain processes ^{[3] [2]}. Additionally, a high level of COX-2 expression is found usually in cancer cells ^[3]. For example, COX-2 overexpression is related to poor-prognosis breast cancer ^{[6] [7]} and endometrial adenocarcinomas ^[8].

Contents 1 Function 2 Structure [9] [4] 3 Physiological Regulation 3.1 Transcriptional regulation [5] 3.2 Post-transcriptional regulation [5] 3.2.1 Via 3’-UTR 3.2.2 miRNAs (microRNAs) 3.2.3 Alternative polyadenylation 4 NSAIDs 5 Reference

Function

COX-2, unlike COX-1, is induced in inflammatory cells when they are activated by various inflammatory and mitogenic stimuli ^[5]. Its function is to produce the prostanoid mediators of the inflammation, although there are some significant exceptions. For example, there is a considerable pool of “constitutive” COX-2 present in the central nervous system (CNS) and some other tissues, although its function is not yet completely clear ^[3].

Moreover, COX-1, that is present in most tissues, has a “housekeeping” role in the body, being involved in tissue homeostasis, and is responsible for the production of prostaglandins involved in gastric cytoprotection, platelet aggregation, renal blood flow autoregulation and the initiation of parturition ^[3].

Structure ^{[9] [4]}

spinqualitylabelsall models Structure of muCOX-2 (PDB entry 5cox) Show:Asymmetric Unit Biological Assembly Drag the structure with the mouse to rotate   Export Animated Image In 1994, Picot et al. published the first three-dimensional structure of a COX enzyme, the ovine COX-1 complexed with the NSAID flurbiprofen. Soon afterward, the crystal structures of human and murine COX-2 followed. First, the three-dimensional (3D) structure of human COX-2 was assessed by means of sequence homology modeling, but in 1996, Luong, C. et al [10] and Kurumbail, R.G et al [11] published two crystal structures of the recombinant human and mouse COX-2 isozymes, respectively, complexed with different selective inhibitors. Drug interactions with COX were one of the first issues to be addressed, and complexes containing a number of different NSAIDs have been studied crystallographically. The structural analysis of COX complexed with substrates or products was more difficult to pursue for a number of technical reasons, particularly the sensitivity of polyunsaturated fatty acids to oxidation. However, within the past years, crystal structures of murine COX-2 complexed with AA and EPA have also been determined. PGHSs are bifunctional . Both COX-1 and COX-2 are membrane-bound enzymes and are present on the lumenal surfaces of the endoplasmic reticulum and of the inner and outer membranes of the nuclear envelope. However, recently, it has been demonstrated in cultured endothelial cells and fibroblasts that a fraction of COX-2 protein is localized to plasma membrane in caveolae-like structures [12]. The primary structure of nascent COX-2 is of 604 amino acids and then it is processed into a mature form by removal of signal peptides giving a protein of 587 amino acids. PGHS-2 is variably glycosylated at two to four sites, leading to the formation of doublets or sometimes triplets on SDS-PAGE. In murine PGHS-2 linked to Asn-68, Asn-144, and Asn-410 in each monomer [13]. The COX monomer consists of : the N-terminal EGF domain, a membrane binding domain (MBD) and a large C-terminal globular catalytic domain containing the heme binding site. The C-terminal segments beyond Pro583 (35 amino acids in COX-2) have not been resolved crystallographically. Collectively, these domains are made up of 25 , seven 310 , four as well as five disulfide bonds which contribute to the interface binding of the two individual monomers to complete the enzyme. Contents 1 Protein domains 1.1 Epidermal Growth Factor Domain 1.2 Membrane Binding Domain 1.3 Catalytic Domain 1.3.1 Peroxidase Active Site Structure 1.3.2 Cyclooxygenase Active Site Structure Protein domains Epidermal Growth Factor Domain The EGF and catalytic domains create the subunit interface in the dimer and place the two MBDs in a homodimer about 25 amstrongs apart. The EGF domains create a substantial portion of the dimer interface. EGF domains are common in several families of membrane proteins and secreted proteins. Typically, the EGF domain occurs at a position in the primary sequence N-terminal to a membrane anchor, such that these domains always occur on the extracytoplasmic face of the membrane. Some authors have suggested that the EGF domains may play a role in the insertion of COX into the lipid bilayer. Membrane Binding Domain PGHS-2 associate with only one face of the membrane bilayer through a monotopic membrane binding domain (MBD) that is comprised of four short, consecutive, amphipathic α-helices (helices A–D) that include residues 59-111 in human PGHS-2 [14]. Three of the four helices lie roughly in the same plane while the last helix angles “upward” into the catalytic domain. Hydrophobic and aromatic residues protrude from these helices to create a hydrophobic surface that would interact with only one face of the lipid bilayer. Catalytic Domain The catalytic domain comprises the bulk of the COX monomer and is almost entirely composed of α-helical secondary structure. As said before COX are bifunctional proteins so we can discern two types of reactions: the heme-dependent bis-oxygenase or COX reaction that converts AA to PGG2 and the subsequent peroxidase (POX) reaction that reduces the 15-hydroperoxide of PGG2 to form PGH2. Peroxidase Active Site Structure The is in a large groove on the side opposite of the MBD. The structures of the peroxidase active sites of PGHSs are similar to those of other heme peroxidases. This site includes a heme group and the iron (III) in the center of this heme is coordinated by His-388 and by His-207. Heme-dependent peroxidase activity is implicated in the formation of a proposed Tyr-385 radical, which is required for cyclooxygenase activity. Gln203 is also important in catalysis, although its function has not been resolved. Mutations of Gln203, His207, or His388 lead to a reduction or elimination of peroxidase activity. The COXs bind 1 mole of ferric-protoporphyrin IX per mole monomer for full activity, as expected for a heme-dependent peroxidase. Cyclooxygenase Active Site Structure PGHS-1 and 2 monomers each contain a 25-°A hydrophobic channel that originates at the membrane binding domain and extends into the core of the globular domain. The MBD forms the mouth and the first half of the channel and allows arachidonate and O2 to enter directly from the apolar compartment of the lipid bilayer. Several amino acids composing the upper half of the channel are uniquely important in cyclooxygenase catalysis. Twenty-four residues line the hydrophobic with only one difference between the isozymes—Ile at position 523 in PGHS-1 and Val at position 523 in PGHS-2. Amino acids lining the hydrophobic cyclooxygenase active site channel include Leu117, Arg120, Phe205, Phe209, Val344, Ile345, Tyr348, Val349, Leu352, Ser353, Tyr355, Leu359, Phe381, Leu384, Tyr385, Trp387, Phe518, Ile/Val523, Gly526, Ala527, Ser530, Leu531, Gly533, Leu534. are polar (Arg120, Ser353, and Ser530). in its radical form is the responsible for abstracting a proton from arachidonic acid during its conversion to PGG2. is the site of acetylation by aspirin and , which is positioned about midway between the mouth and the apex of the active site [15], binds to the carboxylate groups of fatty acids and many NSAIDs.

Physiological Regulation

COX-2 overexpression is very important because it has significant tissue-specific consequences and is associated with inflammatory diseases, cancers and term/preterm labour, thus making COX-2 an important target for pharmacological intervention ^[16].

It is important to know that the expression of COX-2 in many specialized cell types appears to be differentially sensitive to the different stimuli that regulate the unique physiological activities of each tissue ^[4].

It is known that the physiological regulation can be produced at various levels ^[5]:

- Transcriptional regulation

- Post-transcriptional regulation: via 3’UTR, miRNAs (microRNAs) and alternative polyadenylation

Transcriptional regulation ^[5]

Transcriptional activation of COX-2 occurs quickly and transiently in response to different stimuli, for example: pathogens, cytokines, nitric oxide, irradiation, growth factors and various extracellular ligands.

The 5-UTR (untranslated region) of the COX-2 gene has several transcription factor response elements, including two NF-κB (nuclear factor κB) motifs, two AP-1 (activator protein 1) sites and two CREs (cAMP-response elements), among others ^[3]. Transcriptional regulation of COX-2 may also be physically influenced by chromatin remodelling events such as changes in acetylation status of histones and non-histone proteins. For example, the acetylation of NF-κB components by the transcriptional coativator p300 (histone acetyltransferase [HAT]) can activate the COX-2 expression ^[17], while the hypermethylation of the CpG islands results in transcriptional silencing ^[18]. It is also known that the histone deacetylase inhibitors (iHDAC) suppress the activation of the expression in human primary myometrtial cells ^[19] and in cancer cell lines ^[20], by preventing the binding of the transcription factor, c-Jun, to the COX-2 promoter ^[20].

Post-transcriptional regulation ^[5]

Via 3’-UTR

The 3’-UTR of COX-2 is a complex region that contains multiple copies of AREs (AU-rich elements) throughout sequence, which, when bound by specific trans-acting ARE-binding factors, influence COX-2 mRNA stability and also translational efficiency ^[21]. A lot of studies have introduced a new model to the gene regulation of COX-2 by investigating the combined contribution of both transcription and mRNA stability events. For example, one group has reported that the binding of the protein CUGBP2 (CUG triplet repeat, RNA-binding protein 2) in specific AREs within the first 60 nucleotides of the COX-2 3’-UTR can stabilize the COX-2 mRNA inhibiting its translation ^[22]. Also, there is evidence that mitogenic inhibitors (e.g. taxanes) can control COX-2 transcription via PKC (Protein Kinase C)-p38 MAPK (Mitogen-Activated Protein Kinase) signaling cascade and it is known that the stability of COX-2 mRNA can be controlled by the binding of HuR (a mRNA-stabilizing factor) to AREs in 3’-UTR of COX-2.

miRNAs (microRNAs)

A recent study has demonstrated that the microRNAs miR-101a and miRNA-199a can interact with the COX-2 3’-UTR in vitro repressing its translation ^[23].

Alternative polyadenylation

The human COX-2 3’-UTR has several polyadenylation sites. COX-2 uses two alternative polyadenylation sites, in a tissue-specific manner, which derives in the formation of 2 COX-2 mRNAs: one with 2.8 kb and another one with 4.6 kb ^[24]. It is known that selection of the proximal polyadenylation signal is enhanced by presence of additional USEs (Upstream Sequence Elements) where four RNA-binding proteins (U1A, PTB, p54nrb and PSF) can bind, enhancing the recruitment and stabilization of core adenylation factors on the COX-2 mRNA ^[25].

NSAIDs

Non-steroid anti-inflammatory drugs are a chemically heterogeneous group of compounds whose major function is the inhibition of cyclooxygenases (Table 1). Apart from their anti-inflammatory effect, they also present analgesic and antipyretic properties ^[5].

Table 1: Chemical variety of NSAIDs

Pharmacologic group	Drug
Salicylates	Acetylsalicylic acid
Propionic	Naproxen
	Ibuprofen
Para-aminophenols	Paracetamol
Indolacetic	Indometacin
Pirrolacetic	Ketorolac
Phenilacetic	Diclofenac
Piranoidacetic	Etodolac
Anthranilic	Mefenamic acid
Nicotinic	Clonixin
Sulfonanilides	Nimesulide

Classical NSAIDs, as salicylate or phenoprofen, are mostly inhibitors of both isoenzymes, although each isoform is inhibited in a different level (Table 2). Chronic users of NSAIDs develop gastric ulcers or gastrointestinal complications, explained by the inhibition of COX-1. For this reason, selective inhibitors of COX-2, as , valdecoxib and etoricoxib, have been developed ^{[3] [5]}. They don’t cause gastric pathology, but it has been proved to be responsible of nephrotoxicity in some patients.

Table 2: Selectivity of some NSAIDs (adapted from ^[5])

Drug	Coefficient of selectivity (IC50Cox-1/IC50Cox-2)
Ketorolac	100-1000
Naproxen	1-10
Ibuprofen	1-10
Indometacin	1-10
Acetylsalicylic acid	1
Diclofenac	1-0.1
Valdecoxib	0.01-0.001
Etoricoxib	0.01-0.001

spinqualitylabelsall models Insert caption here Show:Asymmetric Unit Biological Assembly Drag the structure with the mouse to rotate   Export Animated Image

The majority of NSAIDs inhibit competitively the initial dioxygenation ^{[3] [5]}. In general, these drugs block COX-1 in a quicker manner, whereas COX-2 inhibition is a more time-dependant event, and usually irreversible ^{[3] [5]}. The new COX-2 inhibitors exhibit PGHS-2 selectivity because they inhibit this isoform by a time-dependent ^{[26] [4]}, pseudoirreversible mechanism, whereas they inhibit PGHS-1 by a rapid, competitive, and reversible mechanism ^[4].

The inhibition mechanism consists of the entrance of the drug by the hydrophobic channel and the formation of hydrogen bonds with Arg120. This interaction prevents the fatty acids from entering the catalytic site. Selectivity of COX-2 inhibitors is mainly mediated by the substitution of Ile523 in COX-1 with Val523 in COX-2, which results in the presence of a small side pocket adjacent to the active site channel, appreciably increasing the volume of the COX-2 active site ^{[3] [5] [4]}. The effect of this change is compounded by the substitution of Val434 in COX-2 for Ile434 in COX-1 within the second group of amino acids conforming the active site ^[15]. The combination of these two substitutions in COX-2 allows a neighboring amino acid, Phe518, to swing out of the way, which further increases access to the side pocket ^[15].

In addition, other NSAIDs present alternative inhibition mechanisms. Acetylsalicylic acid, for example, makes its function by irreversible acetylation of COX-2 in Ser516 ^[5].

Finally, paracetamol is considered an atypical NSAIDs, not only because of its lack of anti-inflammatory properties but also because it does not interact neither with COX-1 nor with COX-2 ^[5]. It has beeb proposed that paracetamol may act as an analgesic and antipyretic drug by inhibition of COX-3 ^[5].

Reference

1. ↑ ^{1.0 1.1} Smith WL, Langenbach R. Why there are two cyclooxygenase isozymes. J Clin Invest. 2001 Jun;107(12):1491-5. PMID:11413152 doi:10.1172/JCI13271
2. ↑ ^{2.0 2.1} Chandrasekharan NV, Dai H, Roos KL, Evanson NK, Tomsik J, Elton TS, Simmons DL. COX-3, a cyclooxygenase-1 variant inhibited by acetaminophen and other analgesic/antipyretic drugs: cloning, structure, and expression. Proc Natl Acad Sci U S A. 2002 Oct 15;99(21):13926-31. Epub 2002 Sep 19. PMID:12242329 doi:10.1073/pnas.162468699
3. ↑ ^{3.00 3.01 3.02 3.03 3.04 3.05 3.06 3.07 3.08 3.09 3.10 3.11} Ghosh N, Chaki R, Mandal V, Mandal SC. COX-2 as a target for cancer chemotherapy. Pharmacol Rep. 2010 Mar-Apr;62(2):233-44. PMID:20508278
4. ↑ ^{4.0 4.1 4.2 4.3 4.4 4.5 4.6} Smith WL, DeWitt DL, Garavito RM. Cyclooxygenases: structural, cellular, and molecular biology. Annu Rev Biochem. 2000;69:145-82. PMID:10966456 doi:10.1146/annurev.biochem.69.1.145
5. ↑ ^{5.00 5.01 5.02 5.03 5.04 5.05 5.06 5.07 5.08 5.09 5.10 5.11 5.12 5.13} Rang HP, Dale MM, Ritter JM, Flower RJ. 2008. Pharmacology. Elsevier. 6th edition. UK. 844 p.
6. ↑ Barnes NL, Warnberg F, Farnie G, White D, Jiang W, Anderson E, Bundred NJ. Cyclooxygenase-2 inhibition: effects on tumour growth, cell cycling and lymphangiogenesis in a xenograft model of breast cancer. Br J Cancer. 2007 Feb 26;96(4):575-82. Epub 2007 Feb 6. PMID:17285134 doi:10.1038/sj.bjc.6603593
7. ↑ Boland GP, Butt IS, Prasad R, Knox WF, Bundred NJ. COX-2 expression is associated with an aggressive phenotype in ductal carcinoma in situ. Br J Cancer. 2004 Jan 26;90(2):423-9. PMID:14735188 doi:10.1038/sj.bjc.6601534
8. ↑ Sales KJ, Grant V, Jabbour HN. Prostaglandin E2 and F2alpha activate the FP receptor and up-regulate cyclooxygenase-2 expression via the cyclic AMP response element. Mol Cell Endocrinol. 2008 Mar 26;285(1-2):51-61. Epub 2008 Feb 3. PMID:18316157 doi:10.1016/j.mce.2008.01.016
9. ↑ Perrone G, Zagami M, Altomare V, Battista C, Morini S, Rabitti C. COX-2 localization within plasma membrane caveolae-like structures in human lobular intraepithelial neoplasia of the breast. Virchows Arch. 2007 Dec;451(6):1039-45. Epub 2007 Sep 13. PMID:17851687 doi:10.1007/s00428-007-0506-4
10. ↑ Luong C, Miller A, Barnett J, Chow J, Ramesha C, Browner MF. Flexibility of the NSAID binding site in the structure of human cyclooxygenase-2. Nat Struct Biol. 1996 Nov;3(11):927-33. PMID:8901870
11. ↑ Kurumbail RG, Stevens AM, Gierse JK, McDonald JJ, Stegeman RA, Pak JY, Gildehaus D, Miyashiro JM, Penning TD, Seibert K, Isakson PC, Stallings WC. Structural basis for selective inhibition of cyclooxygenase-2 by anti-inflammatory agents. Nature. 1996 Dec 19-26;384(6610):644-8. PMID:8967954 doi:http://dx.doi.org/10.1038/384644a0
12. ↑ Spencer AG, Thuresson E, Otto JC, Song I, Smith T, DeWitt DL, Garavito RM, Smith WL. The membrane binding domains of prostaglandin endoperoxide H synthases 1 and 2. Peptide mapping and mutational analysis. J Biol Chem. 1999 Nov 12;274(46):32936-42. PMID:10551860
13. ↑ Vecchio AJ, Simmons DM, Malkowski MG. Structural basis of fatty acid substrate binding to cyclooxygenase-2. J Biol Chem. 2010 Jul 16;285(29):22152-63. Epub 2010 May 12. PMID:20463020 doi:10.1074/jbc.M110.119867
14. ↑ Luong C, Miller A, Barnett J, Chow J, Ramesha C, Browner MF. Flexibility of the NSAID binding site in the structure of human cyclooxygenase-2. Nat Struct Biol. 1996 Nov;3(11):927-33. PMID:8901870
15. ↑ ^{15.0 15.1 15.2} Garavito RM, DeWitt DL. The cyclooxygenase isoforms: structural insights into the conversion of arachidonic acid to prostaglandins. Biochim Biophys Acta. 1999 Nov 23;1441(2-3):278-87. PMID:10570255
16. ↑ Harper KA, Tyson-Capper AJ. Complexity of COX-2 gene regulation. Biochem Soc Trans. 2008 Jun;36(Pt 3):543-5. PMID:18482003 doi:10.1042/BST0360543
17. ↑ Deng WG, Zhu Y, Wu KK. Up-regulation of p300 binding and p50 acetylation in tumor necrosis factor-alpha-induced cyclooxygenase-2 promoter activation. J Biol Chem. 2003 Feb 14;278(7):4770-7. Epub 2002 Dec 5. PMID:12471036 doi:10.1074/jbc.M209286200
18. ↑ Song SH, Jong HS, Choi HH, Inoue H, Tanabe T, Kim NK, Bang YJ. Transcriptional silencing of Cyclooxygenase-2 by hyper-methylation of the 5' CpG island in human gastric carcinoma cells. Cancer Res. 2001 Jun 1;61(11):4628-35. PMID:11389100
19. ↑ Tyson-Capper AJ, Cork DM, Wesley E, Shiells EA, Loughney AD. Characterization of cellular retinoid-binding proteins in human myometrium during pregnancy. Mol Hum Reprod. 2006 Nov;12(11):695-701. Epub 2006 Sep 7. PMID:16959971 doi:10.1093/molehr/gal070
20. ↑ ^{20.0 20.1} Yamaguchi K, Lantowski A, Dannenberg AJ, Subbaramaiah K. Histone deacetylase inhibitors suppress the induction of c-Jun and its target genes including COX-2. J Biol Chem. 2005 Sep 23;280(38):32569-77. Epub 2005 Jul 1. PMID:15994313 doi:10.1074/jbc.M503201200
21. ↑ Dixon DA, Kaplan CD, McIntyre TM, Zimmerman GA, Prescott SM. Post-transcriptional control of cyclooxygenase-2 gene expression. The role of the 3'-untranslated region. J Biol Chem. 2000 Apr 21;275(16):11750-7. PMID:10766797
22. ↑ Mukhopadhyay D, Houchen CW, Kennedy S, Dieckgraefe BK, Anant S. Coupled mRNA stabilization and translational silencing of cyclooxygenase-2 by a novel RNA binding protein, CUGBP2. Mol Cell. 2003 Jan;11(1):113-26. PMID:12535526
23. ↑ Subbaramaiah K, Marmo TP, Dixon DA, Dannenberg AJ. Regulation of cyclooxgenase-2 mRNA stability by taxanes: evidence for involvement of p38, MAPKAPK-2, and HuR. J Biol Chem. 2003 Sep 26;278(39):37637-47. Epub 2003 Jun 25. PMID:12826679 doi:10.1074/jbc.M301481200
24. ↑ Hall-Pogar T, Zhang H, Tian B, Lutz CS. Alternative polyadenylation of cyclooxygenase-2. Nucleic Acids Res. 2005 May 4;33(8):2565-79. Print 2005. PMID:15872218 doi:10.1093/nar/gki544
25. ↑ Hall-Pogar T, Liang S, Hague LK, Lutz CS. Specific trans-acting proteins interact with auxiliary RNA polyadenylation elements in the COX-2 3'-UTR. RNA. 2007 Jul;13(7):1103-15. Epub 2007 May 16. PMID:17507659 doi:10.1261/rna.577707
26. ↑ FitzGerald GA, Loll P. COX in a crystal ball: current status and future promise of prostaglandin research. J Clin Invest. 2001 Jun;107(11):1335-7. PMID:11390412 doi:10.1172/JCI13037