Key Event Title
|Level of Biological Organization|
Key Event Components
|prostaglandin-endoperoxide synthase activity||prostaglandin G/H synthase 1||decreased|
|prostaglandin-endoperoxide synthase activity||prostaglandin G/H synthase 2||decreased|
Key Event Overview
AOPs Including This Key Event
|AOP Name||Role of event in AOP|
|Cyclooxygenase inhibition leading reproductive failure||MolecularInitiatingEvent|
|Cyclooxygenase inhibition 2||MolecularInitiatingEvent|
|Cyclooxygenase inhibition 3||MolecularInitiatingEvent|
|Cyclooxygenase inhibition 5||MolecularInitiatingEvent|
|Cyclooxygenase inhibition 1||MolecularInitiatingEvent|
|Cyclooxygenase inhibition 4||MolecularInitiatingEvent|
Key Event Description
Prostaglandin-endoperoxide synthase (PTGS; KEGG ID E.C. 18.104.22.168; ) is an enzyme that has two catalytic sites. Cyclooxygenase site (COX) catalyzes conversion of arachidonic acid into endoperoxide prostaglandin G2 (Simmons et al., 2004). Peroxidase active site converts PGG2 to PGH2 (KEGG reactions 1599, 1590, ). PGH2 is a precursor for synthesis of other prostaglandins (e.g., PGEs, PGFs; ), prostacyclin and thromboxanes (Simmons et al., 2004; Botting and Botting 2011). Two of the COX isoforms (COX-1 and COX-2) encoded by two different genes (ptgs1 and ptgs2) are well characterized. Ptgs1 is typically expressed constitutively and is involved in maintenance of homeostatic functions. Ptgs2 is largely inducible (e.g., by inflammation, during discrete stages of gamete maturation etc.), but can also be constitutively expressed (e.g., kidney; Green et al, 2012). In mammals, COX-3 (a splice of COX-1) has also been identified (Chandrasekharan et al., 2002), but its function is not well characterized and it is not likely to have prostaglandin producing capacity (Bacchi et al., 2012).
Most COX inhibitors interfere with COX site via competitive inhibition (compete for active site with arachidonic acid), but some are capable of covalent modification of COX (Simmons et al., 2004; Willoughby et al., 2011). The inhibition of COX can lead to reduced efficiency of converting arachidonic acid to PGG2. Therefore inhibition of COX can decrease the rate of prostaglandin production (reviewed Simmons et al, 2004; Bacchi et al., 2012).
How It Is Measured or Detected
Multiple methods have been developed to investigate inhibition of COX activity - the cyclooxygenase (COX) reaction can be monitored by measurement of oxygen consumption, peroxidase co-substrate oxidation or prostaglandin (PG) detection (e.g., Jang and Pezzuto, 1997; Cuendet et al., 2006). Commercial kits from many suppliers deploying a variety of methods are available for purchase (e.g., Cayman Chemicals, Ann Arbor, MI). Repeatability and reproducibility of these commercial assays is well documented – the data generated by assays is reproducible and interassay variation is typically below 5%. The preparation of fish ovarian tissue for COX activity assay is described by Lister and Van der Kraak (2008).
- COX1 activity - US EPA ToxCast assay id: NVS_ENZ_oCOX1
- COX2 activity - US EPA ToxCast assay id: NVS_ENZ_oCOX2
Domain of Applicability
There is a high level of conservation of this molecular target (i.e., COX), as well as its function, especially across vertebrates (Havird et al., 2008, 2015), indicating that many vertebrate taxa may be susceptible to COX inhibition. Typically, teleost fish genomes contain more than one COX-1 and/or COX -2 gene, likely a result of genome duplication after divergence of teleosts from tetrapods (e.g., Ishikawa et al., 2007; Havird et al., 2015). In invertebrates, COX is found in most crustaceans, the majority of molluscs, but only in specific taxa/lineages within Cnidaria and Annelida. COX genes are not found in Hemichordata, Echinodermata, or Platyhelminthes. Insecta COX genes lack in homology, but may function as COX enzymes based on structural analyses (Havird et al., 2015).
Evidence for Perturbation by Stressor
Overview for Molecular Initiating Event
Non-steroidal anti-inflammatory drugs have been specifically designed to inhibit cyclooxygenase active site of PTGS; these mechanisms of inhibition are well characterized (Simmons et al, 2004). NSAIDs interfere with COX site via multiple mechanisms including competitive inhibition (most NSAIDs compete for active site with arachidonic acid) and covalent modification (irreversible acetylation) of COX (e.g., aspirin) (Simmons et al., 2004; Willoughby et al., 2011). NSAIDs display different levels of selectivity for the COX-1 vs. COX-2 isoforms (Simmons et al., 2004). Majority of NSAIDs inhibit both isoforms (with variable levels of selectivity for COX-1 vs. COX-2), but several have been designed to preferentially inhibit COX-2 (Bacchi et al., 2012). Recently, COX-1 specific inhibitors have been developed and their therapeutic potential is being explored (Liedtke et al., 2012). Most extensive evidence regarding chemical initiation of this event comes from the mammalian literature and relates to NSAIDs.
In addition to NSAIDs, common environmental contaminants of diverse chemical structures and uses (e.g., parabens, phthalates, benzophenones) have been postulated to inhibit prostaglandin synthesis via COX inhibition (Kristensen et al., 2011). U.S. EPA’s high throughput screening program (ACToR, epa.gov) indicated COX as a frequent contaminant target - 61% of 143 tested chemicals inhibited COX-1 and 59% of 106 inhibited COX-2 activity. Several chemicals were either similar in potency (e.g., monobutylphthalate) or more potent than NSAIDs (e.g., insecticide emamectin benzoate and industrial intermediary 1-Chloro-4-nitrobenzene were more potent inhibitors of COX2 than NSAID celecoxib, which was specifically designed to inhibit COX-2). Mechanisms of inhibition for these chemicals are not well elucidated.
Bacchi, S., Palumbo, P., Sponta, A., & Coppolino, M. F. (2012). Clinical pharmacology of non-steroidal anti-inflammatory drugs: a review. Anti-Inflammatory & Anti-Allergy Agents in Medicinal Chemistry (Formerly Current Medicinal Chemistry-Anti-Inflammatory and Anti-Allergy Agents), 11(1), 52-64.
Botting, R. M., & Botting, J. H. (2011). C14 Non-steroidal anti-inflammatory drugs. In Principles of Immunopharmacology (pp. 573-584). Birkhäuser Basel.
Cao, H., Yu, R., Tao, Y., Nikolic, D., & van Breemen, R. B. (2011). Measurement of cyclooxygenase inhibition using liquid chromatography–tandem mass spectrometry. Journal of pharmaceutical and biomedical analysis, 54(1), 230-235.
Chandrasekharan, N. V., Dai, H., Roos, K. L. T., Evanson, N. K., Tomsik, J., Elton, T. S., & Simmons, D. L. (2002). COX-3, a cyclooxygenase-1 variant inhibited by acetaminophen and other analgesic/antipyretic drugs: cloning, structure, and expression. Proceedings of the National Academy of Sciences,99(21), 13926-13931.
Cuendet, M., Mesecar, A. D., DeWitt, D. L., & Pezzuto, J. M. (2006). An ELISA method to measure inhibition of the COX enzymes. Nature protocols,1(4), 1915-1921. Green, T., Gonzalez, A. A., Mitchell, K. D., & Navar, L. G. (2012). The Complex Interplay between COX-2 and Angiotensin II in Regulating Kidney Function. Current opinion in nephrology and hypertension, 21(1), 7.
Havird, J. C., Kocot, K. M., Brannock, P. M., Cannon, J. T., Waits, D. S., Weese, D. A., ... & Halanych, K. M. (2015). Reconstruction of Cyclooxygenase Evolution in Animals Suggests Variable, Lineage-Specific Duplications, and Homologs with Low Sequence Identity. Journal of molecular evolution, 1-16.
Havird, J. C., Miyamoto, M. M., Choe, K. P., & Evans, D. H. (2008). Gene duplications and losses within the cyclooxygenase family of teleosts and other chordates. Molecular biology and evolution, 25(11), 2349-2359.
Ishikawa, T. O., Griffin, K. J., Banerjee, U., & Herschman, H. R. (2007). The zebrafish genome contains two inducible, functional cyclooxygenase-2 genes.Biochemical and biophysical research communications, 352(1), 181-187.
Jang, M. S., & Pezzuto, J. M. (1997). Assessment of cyclooxygenase inhibitors using in vitro assay systems. Methods in cell science, 19(1), 25-31.
Kristensen, D. M., Skalkam, M. L., Audouze, K., Lesné, L., Desdoits-Lethimonier, C., Frederiksen, H., ... & Leffers, H. (2011). Many putative endocrine disruptors inhibit prostaglandin synthesis. Environmental health perspectives, 119(4), 534-41.
Liedtke, A. J., Crews, B. C., Daniel, C. M., Blobaum, A. L., Kingsley, P. J., Ghebreselasie, K., & Marnett, L. J. (2012). Cyclooxygenase-1-selective inhibitors based on the (E)-2′-des-methyl-sulindac sulfide scaffold. Journal of medicinal chemistry, 55(5), 2287-2300.
Lister, A. L., & Van Der Kraak, G. (2008). An investigation into the role of prostaglandins in zebrafish oocyte maturation and ovulation. General and comparative endocrinology, 159(1), 46-57.
Simmons, D. L., Botting, R. M., & Hla, T. (2004). Cyclooxygenase isozymes: the biology of prostaglandin synthesis and inhibition. Pharmacological reviews,56(3), 387-437.
Willoughby, D. A., Moore, A. R., & Colville-Nash, P. R. (2000). COX-1, COX-2, and COX-3 and the future treatment of chronic inflammatory disease. The Lancet, 355(9204), 646-648.