You are in: eMedicine Specialties > Endocrinology > Metabolic Disorders Cyclooxygenase DeficiencyArticle Last Updated: Nov 2, 2005AUTHOR AND EDITOR INFORMATIONSection 1 of 9
  Author: John R Witherow, MD, Staff Physician, Department of Diagnostic Radiology, Yale University School of Medicine John R Witherow is a member of the following medical societies: Radiological Society of North America Editors: Dimitris A Papanicolaou, MD, Department of Medicine/Endocrinology, Assistant Professor, Emory University; Francisco Talavera, PharmD, PhD, Senior Pharmacy Editor, eMedicine; Arthur B Chausmer, MD, PhD, FACP, FACE, FACN, CNS, Affiliate Research Professor, School of Computational Sciences; Principal, Bioinformatics and Computational Biology Program, C/A Informatics, LLC; Mark Cooper, MD, Head, Vascular Division, Baker Medical Research Institute; Professor of Medicine, Monash University; George T Griffing, MD, Professor of Medicine, Director of General Internal Medicine, St Louis University Author and Editor Disclosure Synonyms and related keywords: cyclooxygenase deficiency, COX deficiency, prostaglandin-endoperoxide synthase deficiency, PTGS deficiency, fatty acid cyclooxygenase deficiency, prostaglandin H synthase deficiency, PGH synthase deficiency, EC 1.14.99.1 deficiency, eicosanoid, seminal fluid, COX inhibition, COX-1, PTGS-1,COX-2, PTGS-2 CYCLOOXYGENASE DEFICIENCYSection 2 of 9
  Cyclooxygenase (COX), also known as prostaglandin-endoperoxide synthase (PTGS), fatty acid COX, PGH synthase, and EC 1.14.99.1, is a key regulatory enzyme in the synthetic pathway of eicosanoid production. Eicosanoids are responsible for multiple inflammatory, mitogenic, and angiogenic activities in various tissue and organ systems. Therefore, COX is implicated in the production of fever, inflammation, and pain. Regardless of the etiology, a deficiency of COX results in both beneficial and detrimental physiological conditions relative to imbalances of the eicosanoids. Thus, tracing research of the COX pathway is essential to an understanding of COX deficiency and examining the variable effects of COX inhibition are advantageous. In 1930, American gynecologists Kurzok and Lieb first described the stimulatory effects of seminal fluid on human uterine muscle tissue. A few years later, von Euler of Sweden independently discovered similar effects of human seminal fluid on smooth muscle tissue. He localized the biologic activity to a fraction of lipid soluble acids that he termed "prostaglandin," hypothesizing that these substances originate in the prostate gland. Two decades later, the prostaglandins were deduced to be a family of related compounds that contained 20-carbon polyunsaturated fatty acids with a cyclopentane ring (see Image 1). By 1964, after recognition of this basic structure, Bergstrom and colleagues successfully synthesized series 2 prostaglandins from arachidonic acid using sheep seminal fluid. However, the physiologic significance of prostaglandin production did not unfold until 1971 when Vane, Smith, and Willis discovered that aspirin and indomethacin inhibited prostaglandin biosynthesis. Further investigations by Smith concluded that aspirin and indomethacin inhibited synthesis by specifically blocking oxygenation of arachidonic acid. Collectively, these landmark discoveries have provided initial insight into the COX pathway of arachidonate metabolism. Recent advances in genomic analysis have led to a clearer understanding of the COX pathway. Initial investigations by Miyamoto in 1976 and Simmons in 1989 have demonstrated that 2 isoforms exist: COX-1 (PTGS-1) and COX-2 (PTGS-2) respectively. The transcription of COX-1 yields a 2.7-kilobase (kb) mRNA that encodes a 576-residue, 65-kd protein. Conversely, the transcription of COX-2 yields a 4.5-kb mRNA that encodes a 70-kd protein with roughly 70-75% homology to the COX-1 protein. By 1991, Funk and colleagues localized COX-1 to 9q32-q33.3 via somatic hybrid DNA analysis. Three years later, Tay and colleagues localized COX-2 to 1q25.2-q25.3 via fluorescein in situ hybridization. Furthermore, using sequence analysis of human genomic DNA, researchers have concluded that the amino acids important for catalysis by COX-1 are conserved and equally important for catalysis by COX-2. Evidence suggests that COX-1 and COX-2 are similar in structure and function but exist as 2 distinct enzymatic entities. They have been defined as monotropic integral membrane proteins located primarily in the endoplasmic reticulum (COX-1) and perinuclear envelope (COX-2). Their distinct biosynthetic activity includes an endoperoxidase synthase reaction that oxygenates and cyclizes polyunsaturated fatty acid precursors (eg, arachidonic acid) to form prostaglandin G2 (PGG2) and a peroxidase reaction that converts PGG2 to prostaglandin H2 ([PGH2] see Image 2). In turn, PGH2 is converted to biologically active products (ie, prostaglandin E2 [PGE2]) by individual synthase and reductase reactions. Recent studies have demonstrated that the eicosanoids produced by cytosolic COX-1 participate in autocrine and paracrine activities, while those produced by perinuclear COX-2 result in intracrine activity. This subcellular localization of the COX enzyme using new microscopy techniques has helped to explain why 2 isoforms exist. Furthermore, researchers have proposed that COX-1 and COX-2 acquire arachidonic acid from different phospholipases, suggesting participation through separate pathways. COX-1 is expressed constitutively and isolated throughout most cell lines in almost all mammalian tissues. It is described as a housekeeping enzyme, responsible for cell-to-cell signaling, tissue homeostasis, and cytoprotection. In view of this, researchers hypothesize that COX-1 is a rapid responder to various physiologic conditions. Conversely, COX-2 is described as an inducible isoform influenced by a plethora of proinflammatory mediators. Isolation of the COX-2 protein is negligible in most tissues without appropriate stimulation. However, newer literature reveals that COX-2 is expressed constitutively in some cell lines of the brain, kidney, and trachea. Although vaguely described, COX-2 is considered to be a principle mediator of inflammation, mitogenesis, and angiogenesis. COX deficiency typically denotes an acquired cause-and-effect relationship between the protein's enzymatic activity and the nonsteroidal anti-inflammatory drugs (NSAIDs). However, Nyma, while studying bleeder families on the Aland Islands, reported an autosomal dominant pattern of inheritance for COX deficiency. Horellou and colleagues (1983) also described 3 family members of 2 successive generations with bleeding tendencies and concluded that their platelets were compatible with a COX deficiency. This study also described an autosomal dominant pattern of inheritance but deduced that COX was not imperative to aggregation and adenosine triphosphate (ATP) release in the face of high collagen concentrations. Sporadically documented familial and constitutional bleeding disorders have been associated with platelet COX-1 deficiency. However, the physiologic consequences of COX deficiency are best described as acquired, rather than inherited, disorders. The effect of COX-1 deficiency varies from tissue to tissue and largely is determined by the fate of the end products. Several studies have concluded that direct inhibition of COX-1 coincides with a loss of cytoprotection. In turn, COX-1 deficiency has been deemed as a significant participant in gastric and renal pathology. However, Langenbach (1999) reported that mice deficient in COX-1 live uneventful lives despite a 99% reduction in overall prostaglandin production. He also emphasized that a lack of gastric and renal pathology existed in spite of observed prostaglandin deficiency. Confirming earlier literature, he reported that female mice deficient in COX-1 had difficulties with parturition, while male mice deficient in COX-1 were unimpaired. Langenbach also has demonstrated that, contrary to popular belief, COX-1 deficiency correlates with reduced edema in the face of inflammatory mediators that increase arachidonate, illuminating its significance in the inflammatory cascade. Without significant controversy, the consensus has been that inhibition of COX-2 is responsible, in part, for the antipyretic, analgesic, and anti-inflammatory properties of NSAIDs. However, novel investigations using genetically altered COX knockout mice have uncovered new ramifications associated with COX-2. First, Lim and colleagues (1997) observed that COX-2 deficient female mice incurred reproductive failures via abnormalities of ovulation, fertilization, implantation, and decidualization, although follicular development was normal. Independently, Dinchuck (1995) described reduced neonatal viability in mice deficient in COX-2 as a consequence of renal dysplasia, cardiac fibrosis, infertility, and endotoxin-induced hepatocellular cytotoxicity. He concluded that COX-2 deficiency does not affect viability in utero and that heterozygous and wild type mice do not have disease. More recently, Cheng (1999) has cited the importance of COX-2 as a "mediator of increased renin production in response to inhibition of angiotensin II production." As a result, he proposed that a component of the renin-angiotensin feedback system may be modulated by COX-2 expression. Through the evolving study of COX, new theories have been integrated into previously well-described pathways. Nevertheless, the influence that COX has on each cell, tissue, organ, and system results from precursor, product, and receptor variability. Inevitably, additional ramifications of COX deficiency will unfold as knockout mice are investigated further. PRECURSORSSection 3 of 9
The eicosanoids, which include prostaglandins, leukotrienes, thromboxanes, and lipoxins, are derived from the oxygenation of 20-carbon polyunsaturated essential fatty acids via the COX and lipoxygenase pathways. However, only a fraction of these 20-carbon polyenoic acid precursors are the substrates that actually yield eicosanoids. The most significant of the precursors include the following: 8,11,14-eicosatetraienoic acid (dihomo-g-linolenic acid); 5,8,11,14-eicosatetraenoic acid (arachidonic acid); and 5,8,1,14,17-eicosapentaenoic acid. Omega-6 fatty acids, the essential substrates of dihomo-g-linolenic acid and arachidonic acid, are derivatives of linoleic acid. Omega-3 fatty acids, the essential substrates of eicosapentaenoic acid, are derivatives of a-linolenic acid. As a result, linoleic and linolenic fatty acid pathways of desaturation and elongation provide the essential precursors for COX and lipoxygenase that ultimately result in the production of eicosanoids. PRODUCTSSection 4 of 9
As previously noted, COX metabolizes arachidonic acid (5,8,11,14-eicosatetraenoic acid) into PGH2 intermediates. Subsequently, PGH2 is converted into biologically active products via cell-specific enzymatic reactions. The products include not only classic prostaglandins (ie, prostaglandin D2 [PGD2], PGE2, prostaglandin F2-alpha [PGF2-alpha]) but also prostacyclin (PGI2) and thromboxane A2 (TXA2). Besides these bienoic products, COX metabolizes analogous fatty acids into monoenoic and trienoic prostaglandins and thromboxanes as a consequence of the number of precursor double bonds. Collectively, their diverse properties are implicated in several physiologic processes as receptor-dependent mediators and as intracellular secondary messengers. However, the potencies of different monoenoic, bienoic, and trienoic prostaglandins and thromboxanes vary with respect to each family. Studies emphasize that the products are particularly influential in local biologic environments because of their rapid conversion to inactive metabolites. Hence, precursor availability and enzyme kinetics play a key role in the regulation of individual responses. TXA2, the predominate product of COX in platelets and macrophages, is converted from PGH2 by thromboxane synthase. The structural characteristics include a 6-membered ring containing an ether (see Image 3). The functional characteristics include platelet aggregation, vascular smooth muscle constriction, and bronchial smooth muscle constriction with a corresponding 30-second tissue half-life. Furthermore, TXA2 has a different affinity for each eicosanoid-specific receptor because of distinct receptor ligands. As a result, the physiologic responses via TXA2 are tailored to the situation and are less haphazard when stimulated. PGI2, the predominate product of COX in microvascular endothelium, is converted from PGH2 by PGI2 synthase. The structural characteristics include not only a 5-member ring similar to all prostaglandins but also an enzyme-specific arrangement of respective hydroxyl and carbonyl groups (see Image 4). The functional characteristics include inhibition of platelet aggregation, inhibition of platelet and neutrophil adhesion, dilation of bronchial and vascular smooth muscle, and modulation of cholesterol efflux from arterial walls with a corresponding 3-minute tissue half-life. Furthermore, the biosynthesis of PGI2 is enhanced in the face of thrombogenesis and vasoconstriction to balance the physiologic milieu, not unlike several well-known stimulation/inhibition processes. PGD2, the predominate product of COX in mast cells, is converted from PGH2 by endoperoxide-D isomerase. Again, the structural characteristics are similar to all prostaglandins. The functional characteristics include symptoms associated with histamine release (eg, hypotension) and poorly defined roles associated with immunologic processes. However, the formation of 9-alpha,11-beta-prostaglandin F (PGF) metabolites by preferential conversion of PGD2 results in the inhibition of platelet aggregation and the contraction of vascular smooth muscle tissue. This might explain the occasional hypertensive patient observed with systemic mastocytosis. PGE2 is a significant product of COX in the gastric mucosa, renal medulla, microvascular endothelium, and in some tumors. This biologically active product is converted from PGH2 by endoperoxide-E isomerase. Although the products are structurally related (see Image 5), each functional temperament is curiously diverse. In the gastric mucosa, PGE2 preserves integrity by influencing mucus and bicarbonate secretion. It also maintains mucosal blood flow and participates in cellular repair. In the renal medulla, PGE2 enhances vasodilatation and inhibits tubular sodium absorption. Hence, a deficiency of PGE2, as observed in essential hypertension, results in unopposed vasoconstriction. Further attributes include stimulation of local osteoclasts, relaxation of bronchial smooth muscle tissue, contraction of uterine smooth muscle tissue, and modulation of presynaptic adrenergic neuron receptors. Despite vague interpretation, PGE2 also is hypothesized to be a key participant in local inflammatory responses. CYCLOOXYGENASE EXPRESSION AND PLATELET FUNCTIONSection 5 of 9
Platelets have 3 distinct roles in coagulation: (1) initial adhesion, (2) phospholipid externalization, and (3) platelet aggregation. Research suggests that TXA2, the final product of platelet arachidonic acid metabolism, is necessary for normal platelet function. Aggregation occurs as TXA2 diffuses from the platelet and binds to glycoprotein IIb-IIIa (GPIIb-IIIa) platelet membrane receptors to enhance alpha granule and dense granule secretion. The alpha granules store platelet activation markers, whereas the dense granules store calcium, serotonin, and ATP. Thus, inhibition of COX diminishes platelet secretion and inevitably disrupts normal platelet function. Platelets also are unique in that they do not regenerate COX. This becomes apparent in the face of irreversible inhibitors, notoriously aspirin. Therefore, COX inhibition by aspirin results in diminished TXA2 production and inevitably in the loss of platelet aggregatory properties for the life of the platelet (ie, 7-10 d). However, recent studies demonstrate that high concentrations of strong promoters (eg, thrombin, collagen) are not dependent upon TXA2. In fact, they report that total inhibition of platelet COX does not alter a complete platelet response, suggesting that multiple pathways of platelet activation exist. Perhaps this observation accounts for the normal hemostasis observed in humans taking aspirin. CYCLOOXYGENASE EXPRESSION AND COLON CARCINOMASection 6 of 9
COX expression has been implicated in colon carcinoma since epidemiologic studies first described reduced mortality rates in subjects taking aspirin. Initially, Tsujii and DuBois (1995) studied the effects of COX-2 overexpression by transfecting rat intestinal epithelium. They demonstrated that overexpression increased adhesion and inhibited apoptosis, perhaps enhancing the tumorigenic potential. However, in 1996, Oshima and colleagues bred mice with APC mutations similar to humans and COX was uncovered as a significant factor in colon carcinoma. They reported that mice carrying an APC mutation and were wild type for COX-2 expression developed an average of 652 polyps at 10 weeks, whereas heterozygous mice developed 224 polyps, and null mice 93 polyps. They also found that drugs that inhibited COX-2, but not COX-1, greatly reduced polyp formation. Further research by Langenbach (1999) that used multiple intestinal neoplasia mice (which led to 100% incidence of carcinoma) showed that COX-1 and COX-2 deficiency decreased polyps by 70-80%. Although a correlation exists between COX expression and colon carcinoma, the mechanism of action remains unknown. These studies may represent only one of many antitumorigenic pathways affected by NSAIDs irrespective of COX dependence. INFLUENCE OF NONSTEROIDAL ANTI-INFLAMMATORY DRUGSSection 7 of 9
Since Reverend Stone of England professed "the success of the bark of the willow in the cure of agues" (fever) in the mid-eighteenth century, the anti-inflammatory properties of related compounds have been exploited for medicinal purposes. Once Vane and colleagues deduced that aspirin and indomethacin inhibited the COX pathway of arachidonate metabolism, numerous investigations were performed to better understand the mechanism by which NSAIDs interacted with COX. Smith and Dewitt (1995) explained fundamental differences among the NSAIDs based upon their interaction with COX active sites. Consequently, they described 3 distinct classes of inhibitors that currently are recognized. Class I NSAIDs (ie, ibuprofen, piroxicam, naproxen, mefenamic acid, sulindac sulfide, flufenamic acid) are defined as simple competitive COX inhibitors. They interact rapidly and reversibly with COX active sites to form an enzyme-inhibitor (EI) complex. Recently, Smith and Dewitt (1996) determined the IC50 of several class I NSAIDs and observed that an overall higher affinity for the COX-1 isoform exists. However, they emphasize that the relative potency of individual class I NSAIDs may vary from in vitro to in vivo models. Class II NSAIDs (ie, indomethacin, flurbiprofen, meclofenamate, diclofenac) are defined as competitive, time-dependent COX inhibitors. Their interaction with COX active sites is laggard relative to class I inhibitors. After initial formation of the EI complex, the inhibitor causes a slow but reversible conformational change in the protein to form an EI* complex. This non–covalently bound semistable complex is less susceptible to proteases and dissociates from COX active sites at a slower rate than EI complexes. Several studies have identified that different class II inhibitors possess different k2 and k-2 values for COX-1 and COX-2 active sites based upon in vivo selectivity. Although the biochemical mechanism of EI* complex formation is unknown, researchers attribute the kinetic properties of class II inhibitors to structural differences. Class III NSAIDs (ie, aspirin, salicylic acid, valeryl salicylate) are defined as irreversible, competitive, and time-dependent COX inhibitors. Although these NSAIDs are categorized together, only aspirin covalently modifies COX-1 and COX-2. This modification results after aspirin binds to the COX active site and transfers its acetyl group to a definitive serine residue. The acetylation of COX-1 specifically interferes with the endoperoxidase reaction that normally oxygenates and cyclizes arachidonic acid to form PGG2. Conversely, acetylation of COX-2 results in isoform modification that yields 15R-hydroxyeicosatetraenoic acid (15R-HETE) instead of PGG2. MULTIMEDIASection 8 of 9
REFERENCESSection 9 of 9
Cyclooxygenase Deficiency excerpt Article Last Updated: Nov 2, 2005 | ||||||||||||||||||||||||||||||||||||||
