Cyclooxygenases: structural and functional insights

Piroxicam reverses endotoxin-induced hypotension in rats: contribution of vasoactive eicosanoids and nitric oxide

Nitric oxide (NO) produced by inducible NO synthase (iNOS) is responsible for endotoxin-induced vascular hyporeactivity and hypotension resulting in multiple organ failure. Endotoxic shock is also characterized by decreased expression of constitutive cyclooxygenase (COX-1), cytochrome P450 (CYP) 4A and endothelial NOS (eNOS). Our previous studies demonstrated that dual inhibition of iNOS and COX with a selective COX-2 inhibitor, NS-398, or a non-selective COX inhibitor, indomethacin, restores blood pressure presumably because of increased production of 20-hydroxyeicosatetraenoic acid (20-HETE) derived from arachidonic acid (AA) by CYP4A in endotoxaemic rats. The aim of this study was to investigate the effects of piroxicam, a preferential COX-1 inhibitor, on the endotoxin-induced changes in blood pressure, expression of COX-1, inducible COX (COX-2), CYP4A1, eNOS, iNOS and heat shock protein 90 (hsp90), and production of PGI(2), PGE(2), 20-HETE and NO. Injection of endotoxin (10 mg/kg, i.p.) to male Wistar rats caused a fall in blood pressure and an increase in heart rate associated with elevated renal 6-keto-PGF(1α) and PGE(2) levels as well as an increase in COX-2 protein expression. Endotoxin also caused an elevation in systemic and renal nitrite levels associated with increased renal iNOS protein expression. In contrast, systemic and renal 20-HETE levels and renal expression of eNOS, COX-1 and CYP4A1 were decreased in endotoxaemic rats. The effects of endotoxin, except for renal COX-1 and eNOS protein expression, were prevented by piroxicam (10 mg/kg, i.p.), given 1 hr after injection of endotoxin. Endotoxin did not change renal hsp90 protein expression. These data suggest that a decrease in the expression and activity of COX-2 and iNOS associated with an increase in CYP4A1 expression and 20-HETE synthesis contributes to the effect of piroxicam to prevent the hypotension during rat endotoxaemia.

The structural basis of endocannabinoid oxygenation by cyclooxygenase-2

The cyclooxygenases (COX-1 and COX-2) oxygenate arachidonic acid (AA) in the committed step of prostaglandin biogenesis. Substitutions of I434V, H513R, and I523V constitute the only differences in residues lining the cyclooxygenase channel between COX-1 and COX-2. These changes create a hydrophobic pocket in COX-2, with Arg-513 located at the base of the pocket, which has been exploited in the design of COX-2-selective inhibitors. Previous studies have shown that COX-2, but not COX-1, can oxygenate endocannabinoid substrates, including 2-arachidonoyl glycerol (2-AG). To investigate the isoform-specific structural basis of endocannabinoid binding to COX-2, we determined the crystal structure of the 2-AG isomer 1-arachidonoyl glycerol (1-AG) in complex with wild type and R513H murine (mu) COX-2 to 2.2 and 2.35 Å, respectively, and R513H muCOX-2 in complex with AA to 2.45 Å resolution. The 2,3-dihydroxypropyl moiety of 1-AG binds near the opening of the cyclooxygenase channel in the space vacated by the movement of the Leu-531 side chain, validating our previous hypothesis implicating the flexibility of the Leu-531 side chain as a determinant for the ability of COX-2 to oxygenate endocannabinoid substrates. Functional analyses carried out to compliment our structural findings indicated that Y355F and R513H muCOX-2 constructs had no effect on the oxygenation of 1-AG and 2-AG, whereas substitutions that resulted in a shortened side chain for Leu-531 had only modest effects. Both AA and 1-AG bind to R513H muCOX-2 in conformations similar to those observed in the co-crystal structures of these substrates with wild type enzyme.

Biosynthesis of hemiketal eicosanoids by cross-over of the 5-lipoxygenase and cyclooxygenase-2 pathways

The prostaglandin and leukotriene families of lipid mediators are formed via two distinct biosynthetic pathways that are initiated by the oxygenation of arachidonic acid by either cyclooxygenase-2 (COX-2) or 5-lipoxygenase (5-LOX), respectively. The 5-LOX product 5S-hydroxyeicosatetraenoic acid, however, can also serve as an efficient substrate for COX-2, forming a bicyclic diendoperoxide with structural similarities to the arachidonic acid-derived prostaglandin endoperoxide PGH(2) [Schneider C, et al. (2006) J Am Chem Soc 128:720-721]. Here we identify two cyclic hemiketal (HK) eicosanoids, HKD(2) and HKE(2), as the major nonenzymatic rearrangement products of the diendoperoxide using liquid chromatography-mass spectrometry analyses as well as UV and NMR spectroscopy. HKD(2) and HKE(2) are furoketals formed by spontaneous cyclization of their respective 8,9-dioxo-5S,11R,12S,15S-tetrahydroxy- or 11,12-dioxo-5S,8S,9S,15S-tetrahydroxy-eicosadi-6E,13E-enoic acid precursors, resulting from opening of the 9S,11R- and 8S,12S-peroxide rings of the diendoperoxide. Furthermore, the diendoperoxide is an efficient substrate for the hematopoietic type of prostaglandin D synthase resulting in formation of HKD(2), equivalent to the enzymatic transformation of PGH(2) to PGD(2). HKD(2) and HKE(2) were formed in human blood leukocytes activated with bacterial lipopolysaccharide and calcium ionophore A23187, and biosynthesis was blocked by inhibitors of 5-LOX or COX-2. HKD(2) and HKE(2) stimulated migration and tubulogenesis of microvascular endothelial cells, implicating a proangiogenic role of the hemiketals in inflammatory sites that involve expression of 5-LOX and COX-2. Identification of the highly oxygenated hemiketal eicosanoids provides evidence for a previously unrecognized biosynthetic cross-over of the 5-LOX and COX-2 pathways.

Synthesis and structure-activity relationship studies of urea-containing pyrazoles as dual inhibitors of cyclooxygenase-2 and soluble epoxide hydrolase

A series of dual inhibitors containing a 1,5-diarylpyrazole and a urea were designed, synthesized, and evaluated as novel COX-2/sEH dual inhibitors in vitro using recombinant enzyme assays and in vivo using a lipopolysaccharide (LPS) induced model of pain in rats. The best inhibition potencies and selectivity for sEH and COX-2 over COX-1 were obtained with compounds (21b, 21i, and 21j) in which both the 1,5-diaryl-pyrazole group and the urea group are linked with a three-methylene group. Compound 21i showed the best pharmacokinetic profiles in both mice and rats (higher AUC and longer half-life). Following subcutaneous administration at 10 mg/kg, compound 21i exhibited antiallodynic activity that is more effective than the same dose of either a COX-2 inhibitor (celecoxib) or a sEH inhibitor (t-AUCB) alone, as well as coadministration of both inhibitors. Thus, these novel dual inhibitors exhibited enhanced in vivo antiallodynic activity in a nociceptive behavioral assay.

Human cyclooxygenase-2 is a sequence homodimer that functions as a conformational heterodimer

Prostaglandin endoperoxide H synthases 1 and 2, also known as cyclooxygenases (COXs) 1 and 2, convert arachidonic acid (AA) to prostaglandin endoperoxide H(2). Prostaglandin endoperoxide H synthases are targets of nonspecific nonsteroidal anti-inflammatory drugs and COX-2-specific inhibitors called coxibs. PGHS-2 is a sequence homodimer. Each monomer has a peroxidase and a COX active site. We find that human PGHS-2 functions as a conformational heterodimer having a catalytic monomer (E(cat)) and an allosteric monomer (E(allo)). Heme binds tightly only to the peroxidase site of E(cat), whereas substrates, as well as certain inhibitors (e.g. celecoxib), bind the COX site of E(cat). E(cat) is regulated by E(allo) in a manner dependent on what ligand is bound to E(allo). Substrate and nonsubstrate fatty acids (FAs) and some COX inhibitors (e.g. naproxen) preferentially bind to the COX site of E(allo). AA can bind to E(cat) and E(allo), but the affinity of AA for E(allo) is 25 times that for E(cat). Palmitic acid, an efficacious stimulator of human PGHS-2, binds only E(allo) in palmitic acid/murine PGHS-2 co-crystals. Nonsubstrate FAs can potentiate or attenuate actions of COX inhibitors depending on the FA and whether the inhibitor binds E(cat) or E(allo). Our studies suggest that the concentration and composition of the free FA pool in the environment in which PGHS-2 functions in cells, the FA tone, is a key factor regulating PGHS-2 activity and its responses to COX inhibitors. We suggest that differences in FA tone occurring with different diets will likely affect both base-line prostanoid synthesis and responses to COX inhibitors.

Synthesis and evaluation of carbaborane derivatives of indomethacin as cyclooxygenase inhibitors

Nonsteroidal anti-inflammatory drugs (NSAIDs) exert their pharmacological activities by inhibiting cyclooxygenase (COX)-1 and COX-2. Previous studies have shown that esters and amides of non-selective inhibitors such as indomethacin are selective against COX-2, which is the therapeutically relevant isoform. Structure-activity analysis indicates that substituted phenyl rings are tolerated as ester components. In the present study, the introduction of inorganic ortho- and meta-carbaborane moieties was explored with the aim to create COX-2 inhibitors and more importantly to investigate the validity of using these boron clusters as drug entities. Interestingly, only the ortho-carbaborane ester was active whereas the meta isomer was not. A similar lack of inhibitory potency was observed when an adamantyl substituent or alkylene spacers at the carbaborane were introduced in the ester functionality.

Platelets participate in synovitis via Cox-1-dependent synthesis of prostacyclin independently of microparticle generation

In addition to the well-described role of platelets in thrombosis, a growing body of evidence implicates platelets in diverse inflammatory responses. We recently showed platelets can contribute to the pathophysiology of inflammatory arthritis via IL-1- containing microparticles. In this study, we demonstrate that platelets, and not platelet microparticles, actively contribute to synovitis via production of proinflammatory prostacyclin in an autoimmune arthritis model. Using both genetic and pharmacologic approaches, we establish that paracrine production of prostacyclin proceeds in the absence of cyclooxygenase-2. Furthermore, we also demonstrate that prostacyclin generation can arise via transcellular collaboration between platelets and fibroblast-like synoviocytes. In addition to shedding light on an unappreciated pathway of lipid synthesis in arthritis, we further delineate a novel effector activity by which platelets can contribute to inflammatory disease.

Reduced choroidal neovascular membrane formation in cyclooxygenase-2 null mice

PURPOSE

To assess the degree of laser-induced choroidal neovascular membrane formation in wild-type (WT) and COX-2 null mice and to measure vascular endothelial growth factor (VEGF), interleukin (IL)-1β, and tumor necrosis factor (TNF)-α levels in the retina and choroid.

METHODS

Four laser burns were placed in each eye of WT and COX-2 null mice to induce choroidal neovascularization. Fluorescein angiography (FA) was performed at 14 days, and retinal pigment epithelium-choroid-sclera (choroidal) flat mounts were prepared. The retina and choroid were isolated from WT and COX-2 null mice at 24, 72, and 168 hours after laser photocoagulation and from unlasered eyes and were tested for VEGF, IL-1β, and TNF-α.

RESULTS

COX-2 null mice demonstrated 58% (P = 0.001) and 48% (P = 0.001) reductions in CNV formation on FA and choroidal flat mounts, respectively, compared with WT mice. For unlasered mice, mean VEGF concentrations in the retina and choroid were 1.2 ± 0.42 pg/mg protein for WT but only 0.42 ± 0.2 pg/mg protein for COX-2 null mice (P < 0.05). After laser photocoagulation, WT mice showed significantly greater VEGF and IL-β expression in the retina and choroid by 168 hours (P < 0.05) and 72 hours (P < 0.05), respectively, compared with COX-2 null mice.

CONCLUSIONS

COX-2 null mice exhibited significantly less choroidal neovascular membrane formation associated with reduced expression of VEGF. The results of this study suggest that COX-2 modulates VEGF expression in CNV and implicates a potential therapeutic role for nonsteroidal anti-inflammatory drugs.

Attenuation of LPS-induced apoptosis in NGF-differentiated PC12 cells via NF-κB pathway and regulation of cellular redox status by an oxazine derivative

Neuronal cell death due to apoptosis is a common characteristic of neurodegenerative diseases. In this study, we report protective effect of 2-ethoxy-4,5-diphenyl-1,3-oxazine-6-one (EDPOO) against lipopolysaccharide (LPS)-induced cell death in rat pheochromocytoma (PC12) cells, as assessed by MTT test, acridine orange/ethidium bromide staining, determination of Bax, Bcl-2 and caspase-3 levels. We further show that this compound could increase heat shock proteins Hsp-70 and Hsp-32 levels. EDPOO also modulates nuclear levels of Nrf2 and NF-κB, transcription factors that are activated by intracellular reactive oxygen species and/or mediators generated due to chemical exposure of cells. Pretreatment of the cells with this oxazine derivative also increases γ-GCS level, as well as antioxidant enzyme activities, in a dose-dependent manner. Protective effect of this compound could represent a promising approach for treatment of neurodegenerative diseases.

Analysis of omega-3 and omega-6 fatty acid-derived lipid metabolite formation in human and mouse blood samples

Mass spectrometry techniques have enabled the identification of different lipid metabolites and mediators derived from omega-6 and omega-3 polyunsaturated fatty acids (n-6 and n-3 PUFA) that are implicated in various biological processes. However, the broad-spectrum assessment of physiologically formed lipid metabolites and mediators in blood samples has not been presented so far. Here lipid mediators and metabolites of the n-6 PUFA arachidonic acid as well as the long-chain n-3 PUFA eicosapentaenoic acids (EPA) and docosahexaenoic acid (DHA) were measured in human blood samples as well as in mouse blood. There were detectable but mostly very low amounts of the assayed compounds in human native plasma samples, whereas in vitro activation of whole blood with the calcium ionophore A23187 led to highly significant increases of metabolite formation, with a predominance of the 12-lipoxygenase (12-LOX) products 12-hydroxyeicosatetraenoic acid (12-HETE), 12-hydroxyeicosapentaenoic acid (12-HEPE) and 14-hydroxydocosahexaenoic acid (14-HDHA). A23187 activation also led to significant increases in the formation of 5-LOX products including leukotriene B(4) (LTB(4)), leukotriene B(5) (LTB(5)) as well as of 15-LOX products and prostaglandin E(2) (PGE(2)) and thromboxane B(2) (TXB(2)). Levels were similar or even higher in A23187-activated mouse blood. The approach presented here thus provides a protocol for the comprehensive and concomitant assessment of the generation capacity of n-3 and n-6 PUFA-derived lipid metabolites as well as thromboxanes and prostaglandins in human and murine blood samples. Further studies will now have to evaluate lipid metabolite generation capacity in different physiological and pathophysiological contexts.

Protective effects of Korean mistletoe lectin on radical-induced oxidative stress

The radical scavenging effects and protective activities against oxidative stress of Korean mistletoe (Viscum album coloratum) lectin were investigated in vitro and with a cellular system using LLC-PK(1) renal epithelial cells. The Korean mistletoe lectin (KML) showed 1,1-diphenyl-2-picrylhydrazyl radical scavenging activity with an IC(50) value of 42.6 microg/ml. It also exerted nitric oxide (NO), superoxide anion (O(2)(-)), and hydroxyl radical scavenging activities in concentration-dependent manners. These results suggest that KML is a promising antioxidant by scavenging free radicals. Furthermore, under the LLC-PK(1) cellular model, the cells showed declines in viability and increases in lipid peroxidation through oxidative stress induced by sodium nitroprusside (SNP) and pyrogallol, generators of NO and O(2)(-), respectively. However, KML significantly and dose-dependently inhibited cell cytotoxicity and lipid peroxidation. In addition, 3-morpholinosydnonimnie (SIN-1), a generator of peroxynitrite (ONOO(-)) formed by simultaneously releases of NO and O(2)(-), caused cytotoxicity, lipid peroxidation, and NO overproduction in the LLC-PK(1) cells while KML ameliorated ONOO(-)-induced oxidative damage. Furthermore, overexpressions of cyclooxygenase-2 and inducible NO synthase induced by SIN-1 were observed, but KML down-regulated the expression levels of both genes. KML also reduced SIN-1-induced nuclear factor kappa B expression and the phosphorylation of inhibitor kappa B alpha in LLC-PK(1) cells. These results indicate that KML has protective activities against oxidative damage induced by free radicals.

Protective actions of des-aspartate-angiotensin I in mice model of CEES-induced lung intoxication

The present study investigated the protective actions of des-aspartate-angiotensin I (DAA-I) in mice that were intranasally administered 2-chloroethyl ethyl sulfide (CEES), a half sulfur mustard. The protection was dose-dependent, and an oral dose of 75 mg kg⁻¹ per day administered 18 h post exposure and for the following 13 days, offered maximum protection that increased survival by a third. DAA-I attenuated the early processes of inflammation seen in the CEES-inoculated mice. DAA-I attenuated (i) elevated pulmonary ROS, and gp91-phox protein of NADPH oxidase, a non phagocytic enzyme that generates superoxide and subsequent ROS; (ii) intercellular adhesion molecule-1 (ICAM⁻¹) that is involved in the extravasation of circulating leucocytes; and (iii) myeloperoxidase activity, which is a surrogate enzymatic measurement of neutrophil infiltration. These actions led to improved histological lung structures, and survival of type-1 pneumocytes. The action of DAA-I on animal survival was blocked by losartan, a selective angiotensin AT1 receptor blocker, indicting that the AT1 receptor mediates the protection. The presence of elevated PGE2 and PGI2 in lung supernatants of DAA-I treated CEES-inoculated mice indicates that the two prostaglandins are involved in signaling the protective actions of DAA-I. This finding complements earlier studies showing that DAA-I acts on an indomethacin-sensitive angiotensin AT1 receptor. The findings of the present study are the first demonstration of an angiotensin peptide as an effective antidote for CEES intoxication. DAA-I is also an effective therapeutic intervention against CEES that was instituted at 18 h post exposure, and challenges conventional assumptions of limited efficacy with delayed action against alkylating agents.

Comparison of cyclooxygenase-1 crystal structures: cross-talk between monomers comprising cyclooxygenase-1 homodimers

Prostaglandin endoperoxide H synthases (PGHSs)-1 and -2 (also called cyclooxygenases (COXs)-1 and -2) catalyze the committed step in prostaglandin biosynthesis. Both isoforms are targets of nonsteroidal antiinflammatory drugs (NSAIDs). PGHSs are homodimers that exhibit half-of-sites COX activity; moreover, some NSAIDs cause enzyme inhibition by binding only one monomer. To learn more about the cross-talk that must be occurring between the monomers comprising each PGHS-1 dimer, we analyzed structures of PGHS-1 crystallized under five different conditions including in the absence of any tightly binding ligand and in the presence of nonspecific NSAIDs and of a COX-2 inhibitor. When crystallized with substoichiometric amounts of an NSAID, both monomers are often fully occupied with inhibitor; thus, the enzyme prefers to crystallize in a fully occupied form. In comparing the five structures, we only observe changes in the positions of residues 123-129 and residues 510-515. In cases where one monomer is fully occupied with an NSAID and the partner monomer is incompletely occupied, an alternate conformation of the loop involving residues 123-129 is seen in the partially occupied monomer. We propose, on the basis of this observation and previous cross-linking studies, that cross-talk between monomers involves this mobile 123-129 loop, which is located at the dimer interface. In ovine PGHS-1 crystallized in the absence of an NSAID, there is an alternative route for substrate entry into the COX site different than the well-known route through the membrane binding domain.

Structural basis for certain naturally occurring bioflavonoids to function as reducing co-substrates of cyclooxygenase I and II

Background

Recent studies showed that some of the dietary bioflavonoids can strongly stimulate the catalytic activity of cyclooxygenase (COX) I and II in vitro and in vivo, presumably by facilitating enzyme re-activation. In this study, we sought to understand the structural basis of COX activation by these dietary compounds.

Methodology/Principal Findings

A combination of molecular modeling studies, biochemical analysis and site-directed mutagenesis assay was used as research tools. Three-dimensional quantitative structure-activity relationship analysis (QSAR/CoMFA) predicted that the ability of bioflavonoids to activate COX I and II depends heavily on their B-ring structure, a moiety known to be associated with strong antioxidant ability. Using the homology modeling and docking approaches, we identified the peroxidase active site of COX I and II as the binding site for bioflavonoids. Upon binding to this site, bioflavonoid can directly interact with hematin of the COX enzyme and facilitate the electron transfer from bioflavonoid to hematin. The docking results were verified by biochemical analysis, which reveals that when the cyclooxygenase activity of COXs is inhibited by covalent modification, myricetin can still stimulate the conversion of PGG₂ to PGE₂, a reaction selectively catalyzed by the peroxidase activity. Using the site-directed mutagenesis analysis, we confirmed that Q189 at the peroxidase site of COX II is essential for bioflavonoids to bind and re-activate its catalytic activity.

Conclusions/Significance

These findings provide the structural basis for bioflavonoids to function as high-affinity reducing co-substrates of COXs through binding to the peroxidase active site, facilitating electron transfer and enzyme re-activation.

Targeting COX-2 expression by natural compounds: a promising alternative strategy to synthetic COX-2 inhibitors for cancer chemoprevention and therapy

Cyclooxygenase (COX)-2 is a pro-inflammatory immediate early response protein, chronically up-regulated in many pathological conditions. In autoimmune diseases, it is responsible for degenerative effects whereas in cancer, it correlates with poor prognosis. A constitutive expression of COX-2 is triggered since the earliest steps of carcinogenesis. Consequently, strategies aimed at inhibiting COX-2 enzymatic activity have been clinically applied for the treatment of autoimmune disorders; in addition, the same approaches are currently investigated for anti-cancer purposes. However, COX-2 protein inhibitors (i.e., NSAIDs and COXIBs) are not amenable to prolonged administration since they may cause severe side effects, and efforts are underway to identify alternative approaches for chemoprevention/therapy. COX-2 expression is a multi-step process, highly regulated at transcriptional and post-transcriptional levels. Defects in the modulation of one or both of these steps may be found in pathological conditions. Targeting COX-2 expression may therefore represent a promising strategy, by which the same preventive and therapeutic benefits may be gained while avoiding the severe side effects of COX-2 enzymatic inhibition. Naturally occurring compounds derived from plants/organisms represent a huge source of biologically active molecules, that remains largely unexplored. Derived from plants/organisms used in traditional forms of medicine or as dietary supplements, these compounds have been experimentally investigated for their anti-inflammatory and anti-cancer potential. In this review, we will analyze how natural compounds may modulate the multistep regulation of COX-2 gene expression and discuss their potential as a new generation of COX-2 targeting agents alternative to the synthetic COX-2 inhibitors.

In Silico Screening of Nonsteroidal Anti-Inflammatory Drugs and Their Combined Action on Prostaglandin H Synthase-1

The detailed kinetic model of Prostaglandin H Synthase-1 (PGHS-1) was applied to in silico screening of dose-dependencies for the different types of nonsteroidal anti-inflammatory drugs (NSAIDs), such as: reversible/irreversible, nonselective/selective to PGHS-1/PGHS-2 and time dependent/independent inhibitors (aspirin, ibuprofen, celecoxib, etc.) The computational screening has shown a significant variability in the IC50s of the same drug, depending on different in vitro and in vivo experimental conditions. To study this high heterogeneity in the inhibitory effects of NSAIDs, we have developed an in silico approach to evaluate NSAID action on targets under different PGHS-1 microenvironmental conditions, such as arachidonic acid, reducing cofactor, and peroxide concentrations. The designed technique permits translating the drug IC₅₀, obtained in one experimental setting to another, and predicts in vivo inhibitory effects based on the relevant in vitro data. For the aspirin case, we elucidated the mechanism underlying the enhancement and reduction (aspirin resistance) of its efficacy, depending on PGHS-1 microenvironment in in vitro/in vivo experimental settings. We also present the results of the in silico screening of the combined action of sets of two NSAIDs (aspirin with ibuprofen, aspirin with celecoxib), and study the mechanism of the experimentally observed effect of the suppression of aspirin-mediated PGHS-1 inhibition by selective and nonselective NSAIDs. Furthermore, we discuss the applications of the obtained results to the problems of standardization of NSAID test assay, dependence of the NSAID efficacy on cellular environment of PGHS-1, drug resistance, and NSAID combination therapy.

The nociceptive and anti-nociceptive effects of bee venom injection and therapy: a double-edged sword

Bee venom injection as a therapy, like many other complementary and alternative medicine approaches, has been used for thousands of years to attempt to alleviate a range of diseases including arthritis. More recently, additional theraupeutic goals have been added to the list of diseases making this a critical time to evaluate the evidence for the beneficial and adverse effects of bee venom injection. Although reports of pain reduction (analgesic and antinociceptive) and anti-inflammatory effects of bee venom injection are accumulating in the literature, it is common knowledge that bee venom stings are painful and produce inflammation. In addition, a significant number of studies have been performed in the past decade highlighting that injection of bee venom and components of bee venom produce significant signs of pain or nociception, inflammation and many effects at multiple levels of immediate, acute and prolonged pain processes. This report reviews the extensive new data regarding the deleterious effects of bee venom injection in people and animals, our current understanding of the responsible underlying mechanisms and critical venom components, and provides a critical evaluation of reports of the beneficial effects of bee venom injection in people and animals and the proposed underlying mechanisms. Although further studies are required to make firm conclusions, therapeutic bee venom injection may be beneficial for some patients, but may also be harmful. This report highlights key patterns of results, critical shortcomings, and essential areas requiring further study.

Structural basis of fatty acid substrate binding to cyclooxygenase-2

The cyclooxygenases (COX-1 and COX-2) are membrane-associated heme-containing homodimers that generate prostaglandin H(2) from arachidonic acid (AA). Although AA is the preferred substrate, other fatty acids are oxygenated by these enzymes with varying efficiencies. We determined the crystal structures of AA, eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA) bound to Co(3+)-protoporphyrin IX-reconstituted murine COX-2 to 2.1, 2.4, and 2.65 A, respectively. AA, EPA, and docosahexaenoic acid bind in different conformations in each monomer constituting the homodimer in their respective structures such that one monomer exhibits nonproductive binding and the other productive binding of the substrate in the cyclooxygenase channel. The interactions identified between protein and substrate when bound to COX-1 are conserved in our COX-2 structures, with the only notable difference being the lack of interaction of the carboxylate of AA and EPA with the side chain of Arg-120. Leu-531 exhibits a different side chain conformation when the nonproductive and productive binding modes of AA are compared. Unlike COX-1, mutating this residue to Ala, Phe, Pro, or Thr did not result in a significant loss of activity or substrate binding affinity. Determination of the L531F:AA crystal structure resulted in AA binding in the same global conformation in each monomer. We speculate that the mobility of the Leu-531 side chain increases the volume available at the opening of the cyclooxygenase channel and contributes to the observed ability of COX-2 to oxygenate a broad spectrum of fatty acid and fatty ester substrates.

Prostaglandins in bone: bad cop, good cop?

Prostaglandins (PGs) are multifunctional regulators of bone metabolism that stimulate both bone resorption and formation. PGs have been implicated in bone resorption associated with inflammation and metastatic bone disease, and also in bone formation associated with fracture healing and heterotopic ossification. Recent studies have identified roles for inducible cyclooxygenase (COX)-2 and PGE(2) receptors in these processes. Although the effects of PGs have been most often associated with cAMP production and protein kinase A activation, PGs can engage an extensive G-protein signaling network. Further analysis of COX-2 and PG receptors and their downstream G-protein signaling in bone could provide important clues to the regulation of skeletal cell growth in both health and disease.

Cyclooxygenase-2 and cancer treatment: understanding the risk should be worth the reward

Targeting the prostaglandin (PG) pathway is potentially a critical intervention for the prevention and treatment of cancer. Central to PG biosynthesis are two isoforms of cyclooxygenase (COX 1 and 2), which produce prostaglandin H(2) (PGH(2)) from plasma membrane stores of fatty acids. COX-1 is constitutively expressed, whereas COX-2 is an inducible isoform upregulated in many cancers. Differences between COX-1 and COX-2 catalytic sites enabled development of selective inhibitors. Downstream of the COX enzymes, prostaglandin E(2) synthase converts available PGH(2) to prostaglandin E(2) (PGE(2)), which can stimulate cancer progression. Significant research efforts are helping identify more selective targets and fully elucidate the downstream targets of prostaglandin E(2)-mediated oncogenesis. Nonetheless, as a key rate-limiting control point of PG biosynthesis, COX-2 continues to be an important anticancer target. As we embark upon a new era of individualized medicine, a better understanding of the individual risk and/or benefit involved in COX-2 selective targeting is rapidly evolving. This review endeavors to summarize developments in our understanding of COX-2 and its downstream targets as vital areas of anticancer research and to provide the current status of an exciting aspect of molecular medicine.

Posttranscriptional and posttranslational determinants of cyclooxygenase expression

Cyclooxygenases (COX-1 and COX-2) are ER-resident proteins that catalyze the committed step in prostanoid synthesis. COX-1 is constitutively expressed in many mammalian cells, whereas COX-2 is usually expressed inducibly and transiently. Abnormal expression of COX-2 has been implicated in the pathogenesis of chronic inflammation and various cancers; therefore, it is subject to tight and complex regulation. Differences in regulation of the COX enzymes at the posttranscriptional and posttranslational levels also contribute significantly to their distinct patterns of expression. Rapid degradation of COX-2 mRNA has been attributed to AU-rich elements (AREs) at its 3' UTR. Recently, microRNAs that can selectively repress COX-2 protein synthesis have been identified. The mature forms of these COX proteins are very similar in structure except that COX-2 has a unique 19-amino acid (19-aa) segment located near the C-terminus. This C-terminal 19-aa cassette plays an important role in mediation of the entry of COX-2 into the ER-associated degradation (ERAD) system, which transports ER proteins to the cytoplasm for degradation by the 26S proteasome. A second pathway for COX-2 protein degradation is initiated after the enzyme undergoes suicide inactivation following cyclooxygenase catalysis. Here, we discuss these molecular determinants of COX-2 expression in detail. [BMB reports 2009; 42(9): 552-560].

Two pathways for cyclooxygenase-2 protein degradation in vivo

COX-2, formally known as prostaglandin endoperoxide H synthase-2 (PGHS-2), catalyzes the committed step in prostaglandin biosynthesis. COX-2 is induced during inflammation and is overexpressed in colon cancer. In vitro, an 18-amino acid segment, residues 595-612, immediately upstream of the C-terminal endoplasmic reticulum targeting sequence is required for N-glycosylation of Asn(594), which permits COX-2 protein to enter the endoplasmic reticulum-associated protein degradation system. To determine the importance of this COX-2 degradation pathway in vivo, we engineered a del595-612 PGHS-2 (Delta 18 COX-2) knock-in mouse lacking this 18-amino acid segment. Delta 18 COX-2 knock-in mice do not exhibit the renal or reproductive abnormalities of COX-2 null mice. Delta 18 COX-2 mice do have elevated urinary prostaglandin E(2) metabolite levels and display a more pronounced and prolonged bacterial endotoxin-induced febrile response than wild type (WT) mice. Normal brain tissue, cultured resident peritoneal macrophages, and cultured skin fibroblasts from Delta 18 COX-2 mice overexpress Delta 18 COX-2 relative to WT COX-2 expression in control mice. These results indicate that COX-2 can be degraded via the endoplasmic reticulum-associated protein degradation pathway in vivo. Treatment of cultured cells from WT or Delta 18 COX-2 mice with flurbiprofen, which blocks substrate-dependent degradation, attenuates COX-2 degradation, and treatment of normal mice with ibuprofen increases the levels of COX-2 in brain tissue. Thus, substrate turnover-dependent COX-2 degradation appears to contribute to COX-2 degradation in vivo. Curiously, WT and Delta 18 COX-2 protein levels are similar in kidneys and spleens from WT and Delta 18 COX-2 mice. There must be compensatory mechanisms to maintain constant COX-2 levels in these tissues.

Thematic Review Series: Proteomics. An integrated omics analysis of eicosanoid biology

Eicosanoids have been implicated in a vast number of devastating inflammatory conditions, including arthritis, atherosclerosis, pain, and cancer. Currently, over a hundred different eicosanoids have been identified, with many having potent bioactive signaling capacity. These lipid metabolites are synthesized de novo by at least 50 unique enzymes, many of which have been cloned and characterized. Due to the extensive characterization of eicosanoid biosynthetic pathways, this field provides a unique framework for integrating genomics, proteomics, and metabolomics toward the investigation of disease pathology. To facilitate a concerted systems biology approach, this review outlines the proteins implicated in eicosanoid biosynthesis and signaling in human, mouse, and rat. Applications of the extensive genomic and lipidomic research to date illustrate the questions in eicosanoid signaling that could be uniquely addressed by a thorough analysis of the entire eicosanoid proteome.

Role of the cyclooxygenase pathway in chemotherapy-induced oral mucositis: a pilot study

GOALS

Oral mucositis can be a significant and dose-limiting complication of high-dose cancer therapy. Mucositis is a particularly severe problem in patients receiving myeloablative chemotherapy prior to bone marrow or hematopoetic stem cell transplant (HSCT). The cyclooxygenase (COX) pathway mediates tissue injury and pain through upregulation of pro-inflammatory prostaglandins, including prostaglandin E2 (PGE2) and prostacyclin (PGI2). The objective of this small (n = 3) pilot study was to examine the role of the COX pathway in causing mucosal injury and pain in chemotherapy-induced oral mucositis.

MATERIALS AND METHODS

We collected blood, saliva, and oral mucosal biopsy specimens from three autologous HSCT patients at the following time-points before and after administration of conditioning chemotherapy: Day -10, +10, +28, and +100, where day 0 is day of transplant. RNA extracted from full-thickness tissue samples was measured by RT-PCR for the following: COX-1, COX-2, microsomal prostaglandin E synthase (mPGES), IL-1beta, and TNF-alpha. Blood and saliva samples were measured by ELISA for PGE2 and PGI2, which are markers of COX activity. Severity of oral mucositis was determined using the Oral Mucositis Index. Severity of pain due to oral mucositis was measured using a Visual Analog Scale. Relationships between the different variables were examined using Spearman rank correlation coefficients.

MAIN RESULTS

Mean mucositis and pain scores increased significantly after administration of chemotherapy and then gradually declined. The correlation between changes in mucositis and pain scores was strong and statistically significant. The following additional correlations were statistically significant: between tissue COX-1 and pain; between tissue mPGES and pain; between salivary PGE1 and pain; between salivary PGI2 and pain. Other relationships were not statistically significant.

CONCLUSIONS

Our finding of significant associations of pain scores with tissue COX-1 and mPGES, as well as salivary prostaglandins, is suggestive of a role for the cyclooxygenase pathway in mucositis, possibly via upregulation of pro-inflammatory prostaglandins. However, our small sample size may have contributed to the lack of significant associations between COX-2 and other inflammatory mediators with mucosal injury and pain. Thus, additional studies with larger numbers of subjects are warranted to confirm the involvement of the cyclooxygenase pathway in chemotherapy-induced mucositis.