Cyclooxygenases, microsomal prostaglandin E synthase-1, and cardiovascular function

mPGES-1 as a target for cancer suppression: A comprehensive invited review "Phospholipase A2 and lipid mediators"

Prostaglandin E(2) (PGE(2)) is a bioactive lipid that can elicit a wide range of biological effects associated with inflammation and cancer. The physiological roles of PGE(2) are diverse, mediated in part through activation of key downstream signaling cascades via transmembrane EP receptors located on the cell surface. Elevated levels of COX-2 and concomitant overproduction of PGE(2) are often found in human cancers. These observations have led to the use of non-steroidal anti-inflammatory drugs (NSAIDs) as chemopreventive agents, particularly for colorectal cancer (CRC). Their long-term use, however, may be associated with gastrointestinal toxicity and increased risk of adverse cardiovascular events, prompting the development of other enzymatic targets in this pathway. This review will focus on recent efforts to target the terminal synthase, mPGES-1, for cancer chemoprevention. The role of mPGES-1 in the pathogenesis of various cancers is discussed. In addition, an overview of recent efforts to develop small molecule inhibitors that target the protein with high selectivity is also be reviewed.

Expression of cyclooxygenase-2 and microsomal prostagalandin E synthase-1 in head and neck squamous cell carcinoma

OBJECTIVE

Our aim was to examine the expression of COX-2 and microsomal prostaglandin E synthase-1 (mPGES-1), and compare the results with clinical data.

PATIENTS AND METHODS

Expression of COX-1, COX-2, and mPGS-1 was evaluated by immunohistochemistry in head and neck squamous cell carcinoma (HNSCC) from 25 patients and mRNA levels were determined by the quantitative reverse transcription polymerase chain reaction.

RESULTS

COX-2 overexpression was noted in tumor cells from 19 of the 25 carcinoma patients and mPGES was expressed by tumor cells from 21 patients. Localization of COX-2 and mPGES-1 was very similar, and well-differentiated tumors showed stronger expression than poorly/moderately differentiated tumors. The COX-2/GAPDH and mPGES/GAPDH mRNA ratios were significantly higher in well-differentiated carcinomas and N0 patients.

CONCLUSIONS

There was overexpression of COX-2 and its downstream enzyme mPGES-1, and their localization in tumor cells was similar, suggesting that these enzymes play both an important role in the development and proliferation of HNSCC. Furthermore, COX-2 and mPGES-1 were strongly expressed by well-differentiated carcinomas, suggesting their involvement in the differentiation of cancer.

Emotion recollected in tranquility: lessons learned from the COX-2 saga

Nonsteroidal antinflammatory drugs (NSAIDs) inhibit prostaglandin formation by cyclooxygenases (COX) 1 and 2. NSAIDs selective for inhibition of COX-2 are less likely than traditional drugs to cause serious gastrointestinal adverse effects, but predispose to adverse cardiovascular events, such as heart failure, myocardial infarction, and stroke. Evidence from human pharmacology and genetics, genetically manipulated rodents, and other animal models and randomized trials indicates that this is consequent to suppression of COX-2-dependent cardioprotective prostagladins, particularly prostacyclin. Lessons drawn from how this saga unfolded are relevant to how we approach drug surveillance and regulation, integrate diversifed forms of information and might pursue a more personalized approach to drug efficacy and risk.

The impact of microsomal prostaglandin e synthase 1 on blood pressure is determined by genetic background

Prostaglandin (PG)E(2) has multiple actions that may affect blood pressure. It is synthesized from arachidonic acid by the sequential actions of phospholipases, cyclooxygenases, and PGE synthases. Although microsomal PGE synthase (mPGES)1 is the only genetically verified PGE synthase, results of previous studies examining the consequences of mPGES1 deficiency on blood pressure (BP) are conflicting. To determine whether genetic background modifies the impact of mPGES1 on BP, we generated mPGES1(-/-) mice on 2 distinct inbred backgrounds, DBA/1lacJ and 129/SvEv. On the DBA/1 background, baseline BP was similar between wild-type (WT) and mPGES1(-/-) mice. By contrast, on the 129 background, baseline BPs were significantly higher in mPGES1(-/-) animals than WT controls. During angiotensin II infusion, the DBA/1 mPGES1(-/-) and WT mice developed mild hypertension of similar magnitude, whereas 129-mPGES1(-/-) mice developed more severe hypertension than WT controls. DBA/1 animals developed only minimal albuminuria in response to angiotensin II infusion. By contrast, WT 129 mice had significantly higher levels of albumin excretion than WT DBA/1 and the extent of albuminuria was further augmented in 129 mPGES1(-/-) animals. In WT mice of both strains, the increase in urinary excretion of PGE(2) with angiotensin II was attenuated in mPGES1(-/-) animals. Urinary excretion of thromboxane was unaffected by angiotensin II in the DBA/1 lines but increased more than 4-fold in 129 mPGES1(-/-) mice. These data indicate that genetic background significantly modifies the BP response to mPGES1 deficiency. Exaggerated production of thromboxane may contribute to the robust hypertension and albuminuria in 129 mPGES1-deficient mice.

mPGES-1 protects against DOCA-salt hypertension via inhibition of oxidative stress or stimulation of NO/cGMP

Microsomal prostaglandin E synthase-1 (mPGES-1) is a recently characterized cytokine-inducible enzyme critically involved in pain and inflammatory response. However, its role in blood pressure regulation is still debatable. The present study was undertaken to examine the effect of mPGES-1 deletion on DOCA-salt hypertension. After 2 weeks of DOCA plus 1% NaCl as drinking fluid, hypertension and sodium retention were more severe in mPGES-1 knockout (KO) mice than in wild-type (WT) controls. The indices of oxidative stress including urinary 8-isprostane and renal thiobarbituric acid-reactive substances were only modestly increased or unchanged in the WT mice but more significantly increased in the KO mice after DOCA-salt. Conversely, in response to DOCA-salt, the indices of antioxidant systems including renal expression of superoxide dismutase-3 and urinary nitrate/nitrite excretion were all significantly elevated in the WT mice but remarkably suppressed in the KO mice. Tempol treatment (50 mg/kg per day) in DOCA-salt KO mice produced a marked attenuation of hypertension, sodium retention, and kidney injury. Immunoblotting demonstrated increased renal expression of mPGES-1 in DOCA-salt WT mice. DOCA-salt induced a nearly 5-fold increase in urinary PGE(2) excretion in the WT mice, and this increase was completely abolished in the KO mice. Together, these results suggest that mPGES-1-derived PGE(2) confers protection against DOCA-salt hypertension likely via inhibition of oxidative stress or stimulation of superoxide dismutase-3 and urinary nitrate/nitrite system.

Vascular biology of eicosanoids and atherogenesis

Atherosclerosis is a chronic and progressive inflammatory vascular disease that is characterized by a complex interplay between some components of the bloodstream and the arterial wall. The lipid derivatives eicosanoids have been identified as important mediators that contribute to mechanisms of atherogenesis. Prostaglandins and thromboxane A2 are members of the eicosanoid family synthesized from arachidonic acid by the combined action of cyclooxygenases and prostaglandins and thromboxane A2 synthase. Thromboxane A2, a potent platelet activator and vasoconstrictor and prostacyclin, a platelet inhibitor and vasodilator, are the most important in the development of cardiovascular diseases. Several pro-atherogenic biological effects have also been attributed to isoprostanes, a class of eicosanoid isomers formed via a free radical-mediated oxidation of fatty acids esterified in membrane phospholipids. Both groups of lipids manifest their biological activities by binding to specific receptors in target cells. In this article, we will describe the biological roles of prostacyclin, thromboxane A2 and isoprostanes in atherogenesis and discuss the latest pharmacological studies assessing the therapeutic effects of drugs that specifically target their biosynthesis and/or biological activities on vascular inflammation and atherosclerotic lesion development.

Cyclooxygenase-2-dependent prostacyclin formation and blood pressure homeostasis: targeted exchange of cyclooxygenase isoforms in mice

RATIONALE

Cyclooxygenase (COX)-derived prostanoids (PGs) are involved in blood pressure homeostasis. Both traditional nonsteroidal antiinflammatory drugs (NSAIDs) that inhibit COX-1 and COX-2 and NSAIDs designed to be selective for inhibition of COX-2 cause sodium retention and elevate blood pressure.

OBJECTIVE

To elucidate the role of COX-2 in blood pressure homeostasis using COX-1>COX-2 mice, in which the COX-1 expression is controlled by COX-2 regulatory elements.

METHODS AND RESULTS

COX-1>COX-2 mice developed systolic hypertension relative to wild types (WTs) on a high-salt diet (HSD); this was attenuated by a PGI(2) receptor agonist. HSD increased expression of COX-2 in WT mice and of COX-1 in COX-1>COX-2 mice in the inner renal medulla. The HSD augmented in all strains urinary prostanoid metabolite excretion, with the exception of the major PGI(2) metabolite that was suppressed on regular chow and unaltered by the HSD in both mutants. Furthermore, inner renal medullary expression of the receptor for PGI(2), but not for other prostanoids, was depressed by HSD in WT and even more so in both mutant strains. Increasing osmolarity augmented expression of COX-2 in WT renal medullary interstitial cells and again the increase in formation of PGI(2) observed in WTs was suppressed in cells derived from both mutants. Intramedullary infusion of the PGI(2) receptor agonist increased urine volume and sodium excretion in mice.

CONCLUSIONS

These studies suggest that dysregulated expression of the COX-2 dependent, PGI(2) biosynthesis/response pathway in the renal inner renal medulla undermines the homeostatic response to a HSD. Inhibition of this pathway may contribute directly to the hypertensive response to NSAIDs.

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.

The search for new COX-2 inhibitors: a review of 2002 - 2008 patents

BACKGROUND

Two COX isoenzymes are known, COX-1 and COX-2, for which the main inhibitors are the NSAIDs. The common anti-inflammatory drugs (such as aspirin, ibuprofen and naproxen) all act by blocking the action of both the COX-1 and COX-2 enzymes. The COX-2 inhibitors represent a new class of drugs that do not affect COX-1 but selectively block COX-2. This selective action provides the benefits of reducing inflammation without irritating the stomach and cardiovascular effects.

OBJECTIVE

This review focuses on patents published in the field during 2002 - 2008, paying particular attention to promising COX-2 inhibitors.

CONCLUSION

Structural analogues of the COX-2 inhibitors celecoxib and valdecoxib, and novel potential pyridazine, triazole, indole and thione derivatives emerge as promising leads for the treatment of inflammation, pain and other diseases.

Heritability of thromboxane A2 and prostaglandin E2 biosynthetic machinery in a Spanish population

OBJECTIVE

Prostanoids play a critical role in clinical areas such as inflammation, thrombosis, immune response, and cancer. Although some studies suggest that there are genes that determine variability of some prostanoid-related phenotypes, the genetic influence on these traits has not been evaluated.

METHODS AND RESULTS

The relative contributions of genetic and environmental influences to the prostanoid biosynthetic pathway-related phenotypes, cyclooxygenase isoenzymes, microsomal-PGE-synthase-1 and TxA-synthase expression, and thromboxane-A(2) and prostaglandin-E(2) production by stimulated whole blood, were assessed in a sample of 308 individuals in 15 extended families. The effects of measured covariates (such as sex, age, and smoking), genes, and environmental variables shared by members of a household were quantified. Heritabilities ranging from 0.406 to 0.634 for enzyme expression and from 0.283 to 0. 751 for prostanoid production were found.

CONCLUSIONS

These results demonstrate clearly the importance of genetic factors in determining variation in phenotypes that are components of the prostanoid biosynthetic pathways. The presence of such strong genetic effects suggest that it will be possible to localize previously unknown genes that influence quantitative variation in these phenotypes, some of which affect multiple aspects of cell biology, with important clinical implications.

Increased type IIA secretory phospholipase A(2) expression contributes to oxidative stress in end-stage renal disease

End-stage renal disease (ESRD) patients exhibit increased in vivo oxidative stress conceivably contributing to cardiovascular mortality. The type IIA secretory phospholipase A(2) (sPLA(2)) has proatherogenic activity. We explored the hypothesis that sPLA(2) contributes to oxidative stress generation and endothelial dysfunction in ESRD patients and transgenic (tg) mice. Patients with ESRD had increased in vivo oxidative stress as assessed by plasma isoprostane levels (p < 0.001). Active sPLA(2) in plasma was substantially increased compared with healthy controls (1,156 +/- 65 versus 184 +/- 5 ng/dL, p < 0.001) and correlated with plasma isoprostanes (r = 0.61, p < 0.001). Correspondingly, human sPLA(2) tg mice display increased generation of reactive oxygen species within aortic vascular smooth muscle cells, leading to severe endothelial dysfunction (maximal vasodilation in response to 10 micromol/L acetylcholine, sPLA(2) 36 +/- 8%, controls 80 +/- 2% of phenylephrine-induced vasoconstriction). Increased vascular oxidative stress in sPLA(2) tg mice is dependent on the induction of vascular cyclooxygenase (COX)2 expression. Conversely, ESRD patients show increased formation of COX2-derived prostaglandins (p < 0.05) correlated with plasma sPLA(2) (r = 0.71, p < 0.05). Our data indicate that increased expression of sPLA(2) might represent a novel causative risk factor contributing to the increased cardiovascular disease morbidity and mortality in ESRD.

Prostaglandin E(2) synthase inhibition as a therapeutic target

BACKGROUND

Most NSAIDs function by inhibiting biosynthesis of PGE(2) by inhibition of COX-1 and/or COX-2. Since COX-1 has a protective function in the gastro-intestinal tract (GIT), non-selective inhibition of both cycloxy genases leads to moderate to severe gastro-intestinal intolerance. Attempts to identify selective inhibitors of COX-2, led to the identification of celecoxib and rofecoxib. However, long-term use of these drugs has serious adverse effects of sudden myocardial infarction and thrombosis. Drug-mediated imbalance in the levels of prostaglandin I(2) (PGI(2)) and thromboxane A(2) (TXA(2)) with a bias towards TXA(2) may be the primary reason for these events. This resulted in the drugs being withdrawn from the market, leaving a need for an effective and safe anti-inflammatory drug.

METHODS

Recently, the focus of research has shifted to enzymes downstream of COX in the prosta glandin biosynthetic pathway such as prostaglandin E(2) synthases. Microsomal prostaglandin E(2) synthase-1 (mPGES-1) specifically isomerizes PGH(2) to PGE(2), under inflammatory conditions. In this review, we examine the biology of mPGES-1 and its role in disease. Progress in designing molecules that can selectively inhibit mPGES-1 is reviewed.

CONCLUSION

mPGES-1 has the potential to be a target for anti-inflammatory therapy, devoid of adverse GIT and cardiac effects and warrants further investigation.

The effects of microsomal prostaglandin E synthase-1 deletion in acute cardiac ischemia in mice

The goal of the present study was to assess how genetic loss of microsomal prostaglandin E(2) synthase-1 (mPGES-1) affects acute cardiac ischemic damage after coronary occlusion in mice. Wild type (WT), heterozygous (mPGES-1(+/-)), and homozygous (mPGES-1(-/-)) knockout mice were subjected to left coronary artery occlusion. At 24h, myocardial infarct (MI) volume was measured histologically. Post-MI survival, plasma levels of creatine phosphokinase (CPK) and cardiac troponin-I, together with MI size, were similar in WT, mPGES-1(+/-) and mPGES-1(-/-) mice. In contrast, post-MI survival was reduced in mPGES-1(-/-) mice pretreated with I prostanoid receptor (IP) antagonist (12/16) compared with vehicle-treated controls (13/13 mPGES-1(-/-)) together with increased CPK and cardiac troponin-I release. The deletion of mPGES-1 in mice results in increased prostacyclin I(2) (PGI(2)) formation and marginal effects on the circulatory prostaglandin E(2) (PGE(2)) level. We conclude that loss of mPGES-1 results in increased PGI(2) formation, and in contrast to inhibition of PGI(2), without worsening acute cardiac ischemic injury.

Comparison of microsomal prostaglandin E synthase-1 deletion and COX-2 inhibition in acute cardiac ischemia in mice

The aim of the present study was to compare the effects of genetic mPGES-1 loss and COX-2 inhibition on myocardial damage after coronary occlusion. mPGES-1(-/-) mice and their wild-type littermates were injected with vehicle or COX-2 inhibitor (celecoxib), and 30min later the left coronary artery was surgically occluded. At 24h, myocardial infarct (MI) volume was measured histologically. Post-MI survival was reduced in WT mice receiving celecoxib (12/20) compared with vehicle-treated controls (12/12) or the loss of mPGES-1 (13/13) together with increased phosphokinase (CPK) and cardiac troponin-I release. Endogenous mPGES-1 expression was unchanged by ischemia in WT mice and absent in mPGES-1(-/-) hearts. COX-2 expression was markedly increased at 24h after MI in WT hearts; this upregulation was largely attenuated in mPGES-1(-/-) mice. We conclude that loss of mPGES-1 prevents the upregulation of COX-2 after myocardial infarct, and in contrast to inhibition of COX-2, does not increase ischemic myocardial damage.

Renal and cardiovascular characterization of COX-2 knockdown mice

Selective cyclooxygenase-2 (COX-2) inhibitors (coxibs) increase the incidence of cardiovascular and cerebrovascular events. Complete disruption of the murine gene encoding COX-2 ( Ptgs2) leads to renal developmental problems, as well as female reproductive anomalies and patent ductus arteriosus of variable penetrance in newborns, thus rendering this genetic approach difficult to compare with coxib administration. Here, we created hypomorphic Ptgs2 (COX-2Neo/Neo) mice in which COX-2 expression is suppressed to an extent similar to that achieved with coxibs, but not eliminated, in an attempt to circumvent these difficulties. In LPS-challenged macrophages and cytokine-stimulated endothelial cells obtained from COX-2Neo/Neo mice, COX-2 expression was reduced 70–90%, and these mice developed a mild renal phenotype compared with COX-2 mice possessing an active site mutation (COX-2Y385F/Y385F), with minimal signs of renal dysfunction as measured by FITC-inulin clearance and blood urea nitrogen. These COX-2 knockdown mice displayed an increased propensity for thrombogenesis compared with their wild-type (COX-2+/+) littermates observed by intravital microscopy in cremaster muscle arterioles upon ferric chloride challenge. Measurement of urinary prostanoid metabolites indicated that COX-2Neo/Neo mice produced 50% less prostacyclin but similar levels of PGE2 and thromboxane compared with COX-2+/+ mice in the absence of any blood pressure and ex vivo platelet aggregation abnormalities. COX-2Neo/Neo mice, therefore, provide a genetic surrogate of coxib therapy with disrupted prostacyclin biosynthesis that predisposes to induced arterial thrombosis.

Microsomal prostaglandin E synthase protein levels correlate with prognosis in colorectal cancer patients

The aim of this study is to investigate the expression of three prostaglandin E synthase (PGES) isomers in colorectal cancer (CRC) tissue and to evaluate their relationship to clinicopathological factors and patient prognosis. Microsomal PGES (mPGES)-1, mPGES-2, cytosolic PGES (cPGES) and cyclooxygenase (COX)-2 protein expression were analyzed by real-time polymerase chain reaction and Western blot. The localization of each PGES and COX-2 protein was examined by immunohistochemistry in 155 surgical resections and correlated to clinicopathological factors and patient prognosis. mPGES-1 mRNA and protein levels were significantly higher in CRC than in paired normal tissues. mPGES-1 immunoreactivity localized in cancer cells in 43% of cases. mPGES-2 immunoreactivity was significantly more pronounced in cancer cells than in adjacent normal epithelium in 36% of cases. cPGES immunoreactivity was homogeneous in cancer cells and thus determined constitutive. mPGES-1 and mPGES-2 correlated with significantly worse prognosis in stage I-III patients. These results indicate that mPGES-1 and mPGES-2 may each play a role in CRC progression.

Cardiomyocyte cyclooxygenase-2 influences cardiac rhythm and function

Nonsteroidal anti-inflammatory drugs selective for inhibition of COX-2 increase heart failure and elevate blood pressure. The COX-2 gene was floxed and crossed into merCremer mice under the alpha-myosin heavy-chain promoter. Tamoxifen induced selective deletion of COX-2 in cardiomyocytes depressed cardiac output, and resulted in weight loss, diminished exercise tolerance, and enhanced susceptibility to induced arrhythmogenesis. The cardiac dysfunction subsequent to pressure overload recovered progressively in the knockouts coincident with increasing cardiomyocyte hypertrophy and interstitial and perivascular fibrosis. Inhibition of COX-2 in cardiomyocytes may contribute to heart failure in patients receiving nonsteroidal anti-inflammatory drugs specific for inhibition of COX-2.

Anchoring fusion thrombomodulin to the endothelial lumen protects against injury-induced lung thrombosis and inflammation

RATIONALE

Endothelial thrombomodulin (TM) regulates thrombosis and inflammation. Diverse forms of pulmonary and vascular injury are accompanied by down-regulation of TM, which aggravates tissue injury. We postulated that anchoring TM to the endothelial surface would restore its protective functions.

OBJECTIVES

To design an effective and safe strategy to treat pulmonary thrombotic and inflammatory injury.

METHODS

We synthesized a fusion protein, designated scFv/TM, by linking the extracellular domain of mouse TM to a single-chain variable fragment of an antibody to platelet endothelial cell adhesion molecule-1 (PECAM-1). The targeting and protective functions of scFv/TM were tested in mouse models of lung ischemia-reperfusion and acute lung injury (ALI) caused by intratracheal endotoxin and hyperoxia, both of which caused approximately 50% reduction in the endogenous expression of TM.

MEASUREMENTS AND MAIN RESULTS

Biochemical assays showed that scFv/TM accelerated protein C activation by thrombin and bound mouse PECAM-1 and cytokine high mobility group-B1. After intravenous injection, scFv/TM preferentially accumulated in the mouse pulmonary vasculature. In a lung model of ischemia-reperfusion injury, scFv/TM attenuated elevation of early growth response-1, inhibited pulmonary deposition of fibrin and leukocyte infiltration, and preserved blood oxygenation more effectively than soluble TM. In an ALI model, scFv/TM, but not soluble TM, suppressed activation of nuclear factor-kappaB, inflammation and edema in the lung and reduced mortality without causing hemorrhage.

CONCLUSIONS

Targeting TM to the endothelium using an scFv anchor enhances its antithrombotic and antiinflammatory effectiveness in models of ALI.

Cyclooxygenase-2 inhibitors, nonsteroidal anti-inflammatory drugs, and cardiovascular risk

Cyclooxygenase-2 (cox-2) inhibitors, also known as coxibs, were introduced with the promise that they would provide pain relief similar to that of traditional nonsteroidal anti-inflammatory drugs (NSAIDs) but would be better tolerated with lower risk of gastrointestinal (GI) side effects. Although coxibs were associated with lower GI risk, experimental and observational data raised the specter of increased cardiovascular risk associated with this class of drugs. This article describes the pharmacologic and biologic basis of cardiovascular risk associated with coxibs, summarizes the evidence for cardiovascular risk associated with cox-2 inhibitors, and weighs the risks and potential benefits of pain management with these agents.

Prostanoids in health and disease

The prostanoids are a family of lipid mediators generated by the action of cyclooxygenase on a 20-carbon unsaturated fatty acid, arachidonic acid. Prostanoids are generated widely in response to diverse stimuli and, acting in a paracrine or autocrine manner, play important roles in normal physiology and disease. This review summarizes the current knowledge on prostanoid generation and the roles of individual mediators, their biosynthetic pathways, and their receptors in health and disease.

Prostaglandins, NSAIDs, and gastric mucosal protection: why doesn't the stomach digest itself?

Except in rare cases, the stomach can withstand exposure to highly concentrated hydrochloric acid, refluxed bile salts, alcohol, and foodstuffs with a wide range of temperatures and osmolarity. This is attributed to a number of physiological responses by the mucosal lining to potentially harmful luminal agents, and to an ability to rapidly repair damage when it does occur. Since the discovery in 1971 that prostaglandin synthesis could be blocked by aspirin and other nonsteroidal anti-inflammatory drugs (NSAIDs), there has been great interest in the contribution of prostaglandins to gastric mucosal defense. Prostaglandins modulate virtually every aspect of mucosal defense, and the importance of this contribution is evident by the increased susceptibility of the stomach to injury following ingestion of an NSAID. With chronic ingestion of these drugs, the development of ulcers in the stomach is a significant clinical concern. Research over the past two decades has helped to identify some of the key events triggered by NSAIDs that contribute to ulcer formation and/or impair ulcer healing. Recent research has also highlighted the fact that the protective functions of prostaglandins in the stomach can be carried out by other mediators, in particular the gaseous mediators nitric oxide and hydrogen sulfide. Better understanding of the mechanisms through which the stomach is able to resist injury in the presence of luminal irritants is helping to drive the development of safer anti-inflammatory drugs, and therapies to accelerate and improve the quality of ulcer healing.

Role of microsomal prostaglandin E synthase-1 (mPGES1)-derived PGE2 in patency of the ductus arteriosus in the mouse

Prostaglandin E2 (PGE2) plays a key role in the ductus arteriosus, prenatally by maintaining patency and postnatally by promoting tissue remodeling for closure. Here, by using near-term mouse fetuses with (wild-type, WT) and without microsomal PGE synthase-1 (mPGES1-/-), we have examined the importance of this enzyme for PGE2 formation and function. mPGES1-/- ductus, unlike WT ductus, contracted little, or not all, to indomethacin in vitro. Coincidentally, as evident from responses to NG-nitro-L-arginine methyl ester and zinc photoporphyrin, the mutant showed no significant enhancement of nitric oxide (NO)- and carbon monoxide (CO)-based relaxation. mPGES1 suppression differs, therefore, from cyclooxygenase (COX) suppression, whether genetically or pharmacologically induced, where NO is markedly up-regulated. In vivo, the ductus was patent, albeit occasionally with a narrowed lumen, in all mPGES1-/- fetuses. Conversely, postnatal closure progressed regularly in mPGES1-/- animals thanks to residual PGE2 originating via mPGES2. We conclude that mPGES1 is critical for PGE2 formation in the ductus but its loss does not entail compensatory up-regulation of other relaxing mechanisms. Accordingly, an mPGES1 inhibitor stands out as a prospective better tool, compared with the currently used COX inhibitors, for the management of premature infants with persistent ductus.

Consequences of altered eicosanoid patterns for nociceptive processing in mPGES-1-deficient mice

Cyclooxygenase-2 (COX-2)-dependent prostaglandin (PG) E₂ synthesis in the spinal cord plays a major role in the development of inflammatory hyperalgesia and allodynia. Microsomal PGE₂ synthase-1 (mPGES-1) isomerizes COX-2-derived PGH₂ to PGE₂. Here, we evaluated the effect of mPGES-1-deficiency on the noci-ceptive behavior in various models of nociception that depend on PGE₂ synthesis. Surprisingly, in the COX-2-dependent zymosan-evoked hyperalgesia model, the nociceptive behavior was not reduced in mPGES-1-deficient mice despite a marked decrease of the spinal PGE₂ synthesis. Similarly, the nociceptive behavior was unaltered in mPGES-1-deficient mice in the formalin test. Importantly, spinal cords and primary spinal cord cells derived from mPGES-1-deficient mice showed a redirection of the PGE₂ synthesis to PGD₂, PGF_2α and 6-keto-PGF_1α (stable metabolite of PGI₂). Since the latter prostaglandins serve also as mediators of noci-ception they may compensate the loss of PGE₂ synthesis in mPGES-1-deficient mice.

Prostanoid function and cardiovascular disease

Prostanoids, including prostaglandins (PGs) and thromboxanes (TXs) are synthesized from arachidonic acid by the combined action of cyclooxygenases (COXs) and PG and TX synthases. Finally after their synthesis, prostanoids are quickly released to the extracellular medium exerting their effects upon interaction with prostanoid receptors present in the neighbouring cells. These agents exert important actions in the cardiovascular system, modulating vascular homeostasis and participating in the pathogenesis of vascular diseases as thrombosis and atherosclerosis. Among prostanoids, Tromboxane (TX)A(2), a potent platelet activator and vasoconstrictor and prostacyclin (PGI2), a platelet inhibitor and vasodilator, are the most important in controlling vascular homeostasis. Although multiple studies using pharmacological inhibitors and genetically deficient mice have demonstrated the importance of prostanoid-mediated actions on cardiovascular physiology, further analysis on the prostanoid mediated actions in the vascular system are required to better understand the benefits and risks for the use of COX inhibitors in cardiovascular diseases.

Genetic deletion of mPGES-1 suppresses intestinal tumorigenesis

Elevated levels of prostaglandin E(2) (PGE(2)) are often found in colorectal cancers. Thus, nonsteroidal anti-inflammatory drugs, including selective cyclooxygenase-2 (COX-2) inhibitors, are among the most promising chemopreventive agents for colorectal cancer. However, their long-term use is restricted by the occurrence of adverse events believed to be associated with a global reduction in prostaglandin production. In the present study, we evaluated the chemopreventive efficacy of targeting the terminal synthase microsomal PGE(2) synthase 1 (mPGES-1), which is responsible for generating PGE(2), in two murine models of intestinal cancer. We report for the first time that genetic deletion of mPGES-1 in Apc-mutant mice results in marked and persistent suppression of intestinal cancer growth by 66%, whereas suppression of large adenomas (>3 mm) was almost 95%. This effect occurred despite loss of Apc heterozygosity and beta-catenin activation. However, we found that mPGES-1 deficiency was associated with a disorganized vascular pattern within primary adenomas as determined by CD31 immunostaining. We also examined the effect of mPGES-1 deletion on carcinogen-induced colon cancer. The absence of mPGES-1 reduced the size and number of preneoplastic aberrant crypt foci (ACF). Importantly, mPGES-1 deletion also blocked the nuclear accumulation of beta-catenin in ACF, confirming that beta-catenin is a critical target of PGE(2) procarcinogenic signaling in the colon. Our data show the feasibility of targeting mPGES-1 for cancer chemoprevention with the potential for improved tolerability over traditional nonsteroidal anti-inflammatory drugs and selective COX-2 inhibitors.

Microsomal prostaglandin E synthase-1 inhibition in cardiovascular inflammatory disease

Prostaglandins (PGs), particularly PGE2 and prostacyclin (PGI2), are potent mediators of pain and inflammation. Both atherosclerosis and aortic aneurysm exhibit the hallmarks of inflammation. However, randomized trials of inhibitors of PG synthesis--nonsteroidal anti-inflammatory drugs--reveal that they predispose to cardiovascular risk. This appears to be consequent to inhibition of PGI2 and PGE2 formed by cyclooxygenase-2 (COX-2). Inhibitors of microsomal PGE synthase-1 (mPGES-1) are being developed for relief of pain and interest has focused on their potential impact on the cardiovascular system. Deletion of mPGES-1 retards atherogenesis and limits aortic aneurysm formation in hyperlipidaemic mice. However, it does not predispose to thrombogenesis and has a limited impact on blood pressure compared to inhibition of COX-2. This occurs despite the potential of the suppressed PGE2 in affording cardioprotection via its EP2 and EP4 receptors. However, deletion of mPGES-1 permits rediversion of the PGH2 substrate to other PG synthases and augmented formation of PGI2 and PGD2 mitigates this effect. However, increased PGI2 may also attenuate relief of pain. Pain relief seems likely to be a nuanced indication for mPGES-1 inhibitors, but they have therapeutic potential in syndromes of cardiovascular inflammation, cancer and perhaps in neurodegenerative disease. However, as the products of substrate rediversion vary according to cell type, these drugs may have contrasting impact amongst individuals at varied stages of disease evolution.

Microsomal Prostaglandin E 2 Synthase-1 Deletion Leads to Adverse Left Ventricular Remodeling After Myocardial Infarction

Background— Pharmacological inhibition of cyclooxygenase-2 increases the risk of myocardial infarction (MI) and stroke. Microsomal prostaglandin (PG) E 2 synthase-1 (mPGES-1), encoded by the Ptges gene, functions downstream from cyclooxygenase-2 in the inducible PGE 2 biosynthetic pathway. We caused acute MI in Ptges +/+ and Ptges −/− mice to define the role of mPGES-1 in cardiac ischemic injury.

Methods and Results— Twenty-eight days after MI, Ptges −/− mice develop more left ventricular (LV) dilation, have worse LV systolic and diastolic function, and have higher LV end-diastolic pressure than Ptges +/+ mice but have similar pulmonary wet-to-dry weight ratios, cardiac mass, infarct size, and mortality. The length-to-width ratio of individual cardiomyocytes is significantly greater in Ptges −/− than Ptges +/+ mice after MI, a finding consistent with eccentric cardiomyocyte hypertrophy in Ptges −/− mice. Expression of atrial natriuretic peptide, brain natriuretic peptide, and α- and β-myosin heavy chain, markers of ventricular hypertrophy, is higher in the LV of Ptges −/− than Ptges +/+ mice after MI. Ptges +/+ mice express cyclooxygenase-2 and mPGES-1 protein in inflammatory cells adjacent to the infarct after MI but do not express these proteins in cardiomyocytes. Ptges −/− mice express cyclooxygenase-2 in inflammatory cells adjacent to the infarct and do not express mPGES-1 in any cells in the heart. Levels of PGE 2 but not PGD 2 , thromboxane A 2 , PGI 2 , or PGF 2α are higher in the infarct and LV remote from the infarct after MI in Ptges +/+ than Ptges −/− mice.

Conclusions— In Ptges +/+ mice, mPGES-1 in inflammatory cells catalyzes PGE 2 biosynthesis in the LV after MI. Deletion of mPGES-1 leads to eccentric cardiac myocyte hypertrophy, LV dilation, and impaired LV contractile function after acute MI.

Prostaglandin E2 and F2alpha activate the FP receptor and up-regulate cyclooxygenase-2 expression via the cyclic AMP response element

In endometrial adenocarcinomas COX-2 and F-series prostanoid (FP) receptor expression and prostanoid biosynthesis (PGE(2) and PGF(2alpha)) are elevated. In the present study, we investigated the effect of PGE(2) and PGF(2alpha) on the expression of COX-2 via the FP receptor in endometrial adenocarcinoma cells stably expressing the FP receptor (FPS cells). Using chemical inhibitors of intracellular signaling pathways, reporter gene assays and quantitative RT-PCR analysis, we show that PGE(2) and PGF(2alpha) can mobilize inositol 1,4,5-trisphosphate, induce ERK1/2 phosphorylation via the phospholipase Cbeta-protein kinase A-epidermal growth factor receptor pathway and induce cyclooxygenase-2 (COX-2) expression via the FP receptor. In addition we show that the PGE(2) or PGF(2alpha)-regulation of COX-2 via the FP receptor is mediated via the cAMP response element (CRE) binding site on the COX-2 promoter. These data indicate that PGE(2) and PGF(2alpha) biosynthesized locally within endometrial adenocarcinomas can regulate tumor cell function in an autocrine/paracrine manner via the FP receptor.

Association of microsomal prostaglandin E synthase 1 deficiency with impaired fracture healing, but not with bone loss or osteoarthritis, in mouse models of skeletal disorders

OBJECTIVE

Prostaglandin E synthase (PGES) functions as the terminal enzyme in the biosynthesis of prostaglandin E(2) (PGE(2)) and is a potent regulator of bone and cartilage metabolism. Among the 3 isozymes of PGES, microsomal PGES-1 (mPGES-1) is known to play the most critical role in the production of PGE(2) in pathophysiologic events. This study investigated the roles of mPGES-1 under normal physiologic and pathophysiologic conditions in the skeletons of mPGES-1-deficient (mPGES-1(-/-)) mice.

METHODS

Skeletons of mPGES-1(-/-) mice and their wild-type littermates were compared by radiologic and histologic analyses. Four models of skeletal disorders were created: bone loss induced by ovariectomy, bone loss induced by hind limb unloading, osteoarthritis (OA) induced by instability in the knee joint, and bone fracture by osteotomy at the tibial midshaft. Expression of the PGES enzymes was examined by immunohistochemistry and real-time reverse transcription-polymerase chain reaction. The cellular mechanism of fracture healing was examined in ex vivo cultures of costal cartilage chondrocytes.

RESULTS

Microsomal PGES-1(-/-) mice had unaffected skeletal phenotypes under normal physiologic conditions. In the bone fracture model, fracture healing was impaired by the mPGES-1 deficiency, with half of the mice remaining in a non-bone union state even after 21 days; normal fracture healing was restored by adenoviral reintroduction of mPGES-1. The other skeletal disorders were not affected by the mPGES-1 deficiency. In vivo and ex vivo analyses revealed an impaired proliferation of chondrocytes in cartilage with the mPGES-1 deficiency, at an early stage of fracture healing.

CONCLUSION

In these mouse models of skeletal disorders, mPGES-1 was indispensable for bone repair through chondrocyte proliferation, but was not essential for the skeleton under normal physiologic conditions, nor did it play a role in the pathophysiologic conditions of bone loss due to ovariectomy, bone loss due to unloading, or stress-induced OA.

Inflammation and breast cancer. Cyclooxygenase/prostaglandin signaling and breast cancer

Many human cancers exhibit elevated prostaglandin (PG) levels due to upregulation of cyclooxygenase-2 (COX-2), a key enzyme in eicosanoid biosynthesis. COX-2 over-expression has been observed in about 40% of cases of invasive breast carcinoma and at a higher frequency in preinvasive ductal carcinoma in situ tumors, Extensive pharmacologic and genetic evidence implicates COX enzymes in neoplasia. Epidemiologic analyses demonstrate a protective effect of COX-inhibiting nonsteroidal anti-inflammatory drugs with respect to human cancer. Complementary experimental studies have established that both conventional nonsteroidal anti-inflammatory drugs and selective COX-2 inhibitors suppress mammary tumor formation in rodent breast cancer models. Furthermore, knocking out Cox-2 reduces mammary tumorigenesis and angiogenesis, and, conversely, transgenic COX-2 over-expression induces tumor formation. The utility of COX/PG signaling as a target for chemoprevention has been established by randomized controlled clinical trials. However, these studies also identified increased cardiovascular risk associated with use of selective COX-2 inhibitors. Thus, current efforts are directed toward identifying safer approaches to antagonizing COX/PG signaling for cancer prevention and treatment, with a particular focus on PGE₂ regulation and signaling, because PGE₂ is a key protumorigenic prostanoid.

Microsomal prostaglandin E synthase-1 deletion suppresses oxidative stress and angiotensin II-induced abdominal aortic aneurysm formation

BACKGROUND

Microsomal prostaglandin (PG) E(2) synthase-1 (mPGES-1) catalyzes isomerization of the cyclooxygenase product PGH(2) into PGE(2). Deletion of mPGES-1 modulates experimentally evoked pain and inflammation and retards atherogenesis. The role of mPGES-1 in abdominal aortic aneurysm is unknown.

METHODS AND RESULTS

The impact of mPGES-1 deletion on formation of angiotensin II-induced abdominal aortic aneurysm was studied in mice lacking low-density lipoprotein receptor (LDLR(-/-)). Male mice deficient in both mPGES-1 and LDLR (mPGES-1(-/-) LDLR(-/-)) and littermate LDLR(-/-) mice were initiated on a high-fat diet at 6 months of age, followed 1 week later by continuous infusion of angiotensin II (1 microg/kg per minute) for an additional 4 weeks. Angiotensin II infusion upregulated aortic expression of cyclooxygenase-2 and mPGES-1, increased aortic macrophage recruitment and vascular nitrotyrosine staining (which reflects local oxidative stress), and augmented urinary excretion of the isoprostane 8,12-iso-iPF(2alpha)-VI (which reflects lipid peroxidation in vivo) and the major metabolite of PGE(2) (PGE-M). Deletion of mPGES-1 decreased both the incidence (87.5% versus 27.3%; P=0.02) and the severity of abdominal aortic aneurysm and depressed the aortic and systemic indices of oxidative stress. Deletion of mPGES-1 also depressed urinary PGE-M, whereas it augmented excretion of PGD(2) and PGI(2) metabolites, reflecting rediversion of the accumulated PGH(2) substrate in the double knockouts.

CONCLUSIONS

Deletion of mPGES-1 protects against abdominal aortic aneurysm formation induced by angiotensin II in hyperlipidemic mice, coincident with a reduction in oxidative stress. The potential efficacy of selective inhibition of mPGES-1 in preventing or retarding aneurysm formation warrants further investigation.

Hyperexpression of cyclooxygenase 2 in the lupus immune system and effect of cyclooxygenase 2 inhibitor diet therapy in a murine model of systemic lupus erythematosus

OBJECTIVE

To investigate the role of cyclooxygenase 2 (COX-2) in the functioning of different cell types involved in the lupus autoimmune response, and to examine the therapeutic effect of COX-2 inhibitors in mice prone to spontaneously develop systemic lupus erythematosus (SLE).

METHODS

Lupus-prone (SWR x NZB)F(1) mice were fed with a diet containing different doses of the COX-2-specific inhibitor celecoxib or the nonspecific inhibitor aspirin, or a combination of both, and the effects of the therapy on autoantibody production, development of lupus nephritis, and mortality were determined. Expression of COX-2 by different cells of the lupus immune system and the effect of COX-2 inhibitors on the function of these cells in vitro and in vivo were assessed.

RESULTS

The immune cells of mice with SLE spontaneously hyperexpressed COX-2, and COX-2 inhibitors could cause cell apoptosis. Treatment with COX-2 inhibitors resulted in decreased autoantibody production and inhibition of the T cell response to the major lupus autoantigen, nucleosome, and its presentation by antigen-presenting cells. Surprisingly, a significant increase in survival occurred only in mice receiving intermittent therapy with the lowest dose of celecoxib (500 parts per million), approximating <100 mg of celecoxib/day in humans. A continuous diet, but not intermittent feeding, with the combination of celecoxib and aspirin delayed development of nephritis temporarily, but failed to prolong survival. Indeed, treatment with aspirin alone increased mortality.

CONCLUSION

The contributions of the major players in the pathogenic autoimmune response, namely, T cells, B cells, dendritic cells, and macrophages that are abnormally hyperactive in lupus, depend on the increased expression and activity of COX-2, similar to inflammatory cells in target organs. Intermittent pulse therapy with low doses of select COX-2 inhibitors would be of value in the treatment of lupus.

Elevated microsomal prostaglandin-E synthase-1 in Alzheimer's disease

BACKGROUND

The proinflammatory prostaglandin E(2) (PGE(2)) fluctuates over time in the cerebrospinal fluid of patients with Alzheimer's disease (AD), but the cerebral distribution and expression patterns of microsomal prostaglandin-E synthase (mPGES)-1 have not been compared with those of normal human brains.

METHODS

Middle frontal gyrus tissue from AD and age-matched control brains was analyzed by Western blot, immunofluorescence, and immunohistochemistry with mPGES-1-specific antibodies.

RESULTS

Western blotting revealed that mPGES-1 expression was significantly elevated in AD tissue. Furthermore, immunofluorescence of mPGES-1 was observed in neurons, microglia, and endothelial cells of control and AD tissue. Although mPGES-1 was consistently present in astrocytes of control tissue, it was present in only some astrocytes of AD tissue. Immunohistochemical staining suggested that mPGES-1 was elevated in pyramidal neurons of AD tissue when compared with controls.

CONCLUSIONS

The results suggest that mPGES-1 is normally expressed constitutively in human neurons, microglia, astrocytes, and endothelial cells but is up-regulated in AD.

Human microsomal prostaglandin E synthase-1 (mPGES-1) binding with inhibitors and the quantitative structure-activity correlation

The detailed structures of microsomal prostaglandin E synthase-1 (mPGES-1) binding with inhibitors have been studied, for the first time, by using a newly developed computational three-dimensional (3D) structural model of mPGES-1 along with a 3D-quantitative structure--activity relationship (3D-QSAR) analysis. The obtained satisfactory binding structures and 3D-QSAR models strongly suggest that the 3D structural model of mPGES-1 is reasonable for study of mPGES-1 binding with inhibitors and for future design of novel mPGES-1 inhibitors.

Membrane prostaglandin E synthase-1: a novel therapeutic target

Prostaglandin E(2) (PGE(2)) is the most abundant prostaglandin in the human body. It has a large number of biological actions that it exerts via four types of receptors, EP1-4. PGE(2) is formed from arachidonic acid by cyclooxygenase (COX-1 and COX-2)-catalyzed formation of prostaglandin H(2) (PGH(2)) and further transformation by PGE synthases. The isomerization of the endoperoxide PGH(2) to PGE(2) is catalyzed by three different PGE synthases, viz. cytosolic PGE synthase (cPGES) and two membrane-bound PGE synthases, mPGES-1 and mPGES-2. Of these isomerases, cPGES and mPGES-2 are constitutive enzymes, whereas mPGES-1 is mainly an induced isomerase. cPGES uses PGH(2) produced by COX-1 whereas mPGES-1 uses COX-2-derived endoperoxide. mPGES-2 can use both sources of PGH(2). mPGES-1 is a member of the membrane associated proteins involved in eicosanoid and glutathione metabolism (MAPEG) superfamily. It requires glutathione as an essential cofactor for its activity. mPGES-1 is up-regulated in response to various proinflammatory stimuli with a concomitant increased expression of COX-2. The coordinate increased expression of COX-2 and mPGES-1 is reversed by glucocorticoids. Differences in the kinetics of the expression of the two enzymes suggest distinct regulatory mechanisms for their expression. Studies, mainly from disruption of the mPGES-1 gene in mice, indicate key roles of mPGES-1-generated PGE(2) in female reproduction and in pathological conditions such as inflammation, pain, fever, anorexia, atherosclerosis, stroke, and tumorigenesis. These findings indicate that mPGES-1 is a potential target for the development of therapeutic agents for treatment of several diseases.