Cellular prostaglandin E2 production by membrane-bound prostaglandin E synthase-2 via both cyclooxygenases-1 and -2

Expression and Contribution of Three Different Isoforms of Prostaglandin E Synthase in the Bovine Endometrium1

Prostaglandins are involved in the regulation of several reproductive processes such as ovulation, luteolysis, and establishment of pregnancy. Prostaglandin E(2) (PGE(2)) appears to favor establishment of pregnancy in most mammals studied so far. The primary enzymes involved in the production of PGE(2) from arachidonic acid are cyclooxygenases and prostaglandin E synthases (PGES). Three PGES have been identified in humans, but in the bovine, microsomal PGES2 and cytosolic PGES genes have neither been cloned nor associated to any physiological processes. The present study was undertaken to clone bovine MPGES2 and CPGES and to report on their regulation in the endometrium during the estrous cycle. CPGES mRNA expression declines progressively during the cycle; its protein is not modulated according to a precise pattern. MPGES2 mRNA and protein expression decrease from the beginning of the cycle until Days 13-15 and then increase until ovulation. Immunohistochemical analysis reveals that both enzymes are located in luminal epithelial and glandular epithelial cells and at a lower level in stromal cells. In addition, using the bovine endometrial cell line BEND, where higher accumulation of PGE(2) is observed following treatment with phorbol 12-myristate 13-actetate (PMA) and tumor necrosis factor-alpha (TNF-alpha), we have found an associated increase of MPGES1 and COX2 but not CPGES or MPGES2 protein expression. Together, our results suggest that MPGES1 is not the only PGES present in the bovine endometrium but is the main enzyme associated with increased PGE(2) production in vitro.

Despite transcriptional and functional coordination, cyclooxygenase-2 and microsomal prostaglandin E synthase-1 largely reside in distinct lipid microdomains in WISH epithelial cells

Cytokine-induced prostaglandin (PG)E(2) synthesis requires increased expression of cyclooxygenase-2 (COX-2) in human WISH epithelial cells. Recently, an inducible downstream PGE synthase (microsomal PGE synthase-1, mPGES-1) has been implicated in this inflammatory pathway. We evaluated cooperation between COX-2 and mPGES-1 as a potential mechanism for induced PGE(2) production in WISH cells. Cytokine stimulation led to increased expression of both enzymes. Selective pharmacological inhibition of these enzymes demonstrated that induced PGE(2) release occurred through a dominant COX-2/mPGES-1 pathway. Unexpectedly, immunofluorescent microscopy revealed that the expression of these enzymes was not tightly coordinated among cells after cytokine challenge. Within cells expressing high levels of both mPGES-1 and COX-2, immunolabeling of high-resolution semithin cryosections revealed that COX-2 and mPGES-1 were largely segregated to distinct regions within continuous intracellular membranes. Using biochemical means, it was further revealed that the majority of mPGES-1 resided within detergent-insoluble membrane fractions, whereas COX-2 was found only in detergent-soluble fractions. We conclude that although mPGES-1 and COX-2 show transcriptional and functional coordination in cytokine-induced PGE(2) synthesis, complementary morphological and biochemical data suggest that a majority of intracellular mPGES-1 and COX-2 are segregated to discrete lipid microdomains in WISH epithelial cells.

Regulation of prostaglandin E2 synthesis after brain irradiation

PURPOSE

A local tissue reaction, termed neuroinflammation, occurs after irradiation of brain tissue. Previous work suggested that cyclooxygenase (COX)-2 activity was important for changes in gene expression associated with neuroinflammation as well as increased prostaglandin E2 (PGE2) levels seen after radiation treatment.

METHODS AND MATERIALS

To begin to determine the contributions of other enzymes involved in PGE2 production, we examined protein levels of COX-1 and COX-2 as well as 2 PGE synthases (membrane and cytosolic PGES) 4 h after 35 Gy single dose irradiation to the brains of C3HeN mice. We also evaluated the effects of specific COX inhibitors on PGE2 production and PGES expression.

RESULTS

As expected, COX-2 expression increased after radiation exposure. Brain irradiation also increased tissue protein levels for both PGES isoforms. Specific COX-2 inhibition with NS398 lowered brain PGE2 levels by about 60%. Surprisingly, COX-1 inhibition with SC560 completely prevented the elevation of PGE2 seen after irradiation. Interestingly, NS398 reduced the membrane-associated PGES isoform, whereas SC560 treatment lowered cytosolic isoform levels below those seen in unirradiated controls.

CONCLUSIONS

Taken together, these data indicate that both cyclooxygenases contribute to PGE2 production in irradiated brain and reveal dependence of PGES isoforms expression on specific cyclooxygenase activities.

Microsomal prostaglandin E synthase-1: the inducible synthase for prostaglandin E2

Microsomal prostaglandin E synthase-1 (mPGES-1) is an inducible enzyme that catalyzes the conversion of prostaglandin (PG)H₂ to PGE₂. Proinflammatory stimuli markedly increase levels of mPGES-1 expression both in vivo and in vitro. mPGES-1 knockout studies and animal models of inflammatory arthritis also provide a strong basis for the contribution of mPGES-1 in the increased local production of PGE₂ observed in inflammatory arthritis. The focus of this article is to review some recent advances in our understanding of mechanisms specific to the regulation of inducible mPGES-1 in inflammatory arthritis.

Triclosan reduces microsomal prostaglandin E synthase-1 expression in human gingival fibroblasts

OBJECTIVE

The effect of triclosan (2,4,4'-trichloro-2'-hydroxydiphenyl ether) on the expression of cyclooxygenase-2 (COX-2) and microsomal prostaglandin E synthase-1 (mPGES-1) and on the translocation of the nuclear factor-kappaB (NF-kappaB) in relation to prostaglandin E2 (PGE2) production was investigated in human gingival fibroblasts challenged with tumor necrosis factor alpha (TNFalpha).

METHODS

Fibroblasts were established from gingival biopsies obtained from six children. COX-2 mRNA and protein expression was quantified using mRNA quantitation and enzyme immunometric assay kits. mPGES-1 mRNA was analysed by RT-PCR, mPGES-1 protein and NF-kappaB translocation by immunoblotting. PGE2 was determined by radioimmunoassay.

RESULTS

The cytokine TNFalpha enhanced the expression of mRNA as well as the protein levels of both COX-2 and mPGES-1 and subsequently the production of PGE2 in gingival fibroblasts. Treatment of gingival fibroblasts with triclosan (1 microg/ml) significantly reduced the stimulatory effect of TNFalpha (10 ng/ml) on the expression of mPGES-1 at both the mRNA and the protein level by an average of 21% and 43%, respectively, and subsequently the production of PGE2 (p<0.01). Triclosan did not, however, affect the translocation of NF-kappaB or the expression of COX-2 in TNFalpha-stimulated cells.

CONCLUSION

The results show that triclosan reduces the augmented biosynthesis of PGE2 by inhibiting the mRNA and the protein expression of mPGES-1 in gingival fibroblasts. This finding may partly explain the anti-inflammatory effect of the agent previously reported in clinical studies.

Various secretory phospholipase A2 enzymes are expressed in rheumatoid arthritis and augment prostaglandin production in cultured synovial cells

Although group IIA secretory phospholipase A2 (sPLA2-IIA) is known to be abundantly present in the joints of patients with rheumatoid arthritis (RA), expression of other sPLA2s in this disease has remained unknown. In this study, we examined the expression and localization of six sPLA2s (groups IIA, IID, IIE, IIF, V and X) in human RA. Immunohistochemistry of RA sections revealed that sPLA2-IIA was generally located in synovial lining and sublining cells and cartilage chondrocytes, sPLA2-IID in lymph follicles and capillary endothelium, sPLA2-IIE in vascular smooth muscle cells, and sPLA2-V in interstitial fibroblasts. Expression levels of these group II subfamily sPLA2s appeared to be higher in severe RA than in inactive RA. sPLA2-X was detected in synovial lining cells and interstitial fibers in both active and inactive RA sections. Expression of sPLA2-IIF was partially positive, yet its correlation with disease states was unclear. Expression of sPLA2 transcripts was also evident in cultured normal human synoviocytes, in which sPLA2-IIA and -V were induced by interleukin-1 and sPLA2-X was expressed constitutively. Adenovirus-mediated expression of sPLA2s in cultured synoviocytes resulted in increased prostaglandin E2 production at low ng x mL(-1) concentrations. Thus, multiple sPLA2s are expressed in human RA, in which they may play a role in the augmentation of arachidonate metabolism or exhibit other cell type-specific functions.

Cytosolic phospholipase A2alpha regulates induction of brain cyclooxygenase-2 in a mouse model of inflammation

The products of arachidonic acid metabolism are key mediators of inflammatory responses in the central nervous system, and yet we do not know the mechanisms of their regulation. The phospholipase A(2) enzymes are sources of cellular arachidonic acid, and the enzymes cyclooxygenase-2 (COX-2) and microsomal PGE synthase-1 (mPGES-1) are essential for the synthesis of inflammatory PGE(2) in the brain. These studies seek to determine the function of cytosolic phospholipase A(2)alpha (cPLA(2)alpha) in inflammatory PGE(2) production in the brain. We wondered whether cPLA(2)alpha functions in inflammation to produce arachidonic acid or to modulate levels of COX-2 or mPGES-1. We investigated these questions in the brains of wild-type mice and mice deficient in cPLA(2)alpha (cPLA(2)alpha(-/-)) after systemic administration of LPS. cPLA(2)alpha(-/-) mice had significantly less brain COX-2 mRNA and protein expression in response to LPS than wild-type mice. The reduction in COX-2 was most apparent in the cells of the cerebral blood vessels and the leptomeninges. The brain PGE(2) concentration of untreated cPLA(2)alpha(-/-) mice was equal to their wild-type littermates. After LPS treatment, however, the brain concentration of PGE(2) was significantly less in cPLA(2)alpha(-/-) than in cPLA(2)alpha(+/+) mice (24.4 +/- 3.8 vs. 49.3 +/- 11.6 ng/g). In contrast to COX-2, mPGES-1 RNA levels increased equally in both mouse genotypes, and mPGES-1 protein was unaltered 6 h after LPS. We conclude that cPLA(2)alpha regulates COX-2 levels and modulates inflammatory PGE(2) levels. These results indicate that cPLA(2)alpha inhibition is a novel anti-inflammatory strategy that modulates, but does not completely prevent, eicosanoid responses.

Localization of various secretory phospholipase A2 enzymes in male reproductive organs

Current evidence suggests the presence of transcripts for several secretory phospholipase A(2) (sPLA(2)) enzymes in male genital organs. In this study, we examined by immunohistochemistry the localization of group IIA, IIC, IID, IIE, IIF, V and X sPLA(2)s in male genital organs. In sPLA(2)-IIA-deficient C57BL/6 mouse testis, sPLA(2)-IIC, -IID, -IIE, -IIF, -V and -X were diversely expressed in spermatogenic cells within the seminiferous tubules. Immunoblotting revealed the presence of these sPLA(2)s in mouse spermatozoa. In addition, sPLA(2)-IIF, -V and -X were localized in the interstitial Leydig cells. The same set of sPLA(2)s was detected in a mouse cultured Leydig cell line, and adenovirus-mediated transfer of these sPLA(2)s into Leydig cells resulted in increased prostaglandin production. sPLA(2)-IIC, -IID, -IIE, -IIF, -V and -X were also detected diversely in the epithelium of the epididymis, vas deferens, seminal vesicles, and prostate. In a sPLA(2)-IIA-positive FVB strain, weak expression of sPLA(2)-IIA was detected in Leydig cells. Notable differences in the sPLA(2) expression profiles were found in the seminal vesicles and prostate between mice and humans. Taken together, individual sPLA(2)s exhibit distinct or partially overlapping localizations in male reproductive organs, suggesting both specific and redundant functions.

Group VIB Ca2+-independent phospholipase A2gamma promotes cellular membrane hydrolysis and prostaglandin production in a manner distinct from other intracellular phospholipases A2

Although group VIA Ca2+-independent phospholipase A2beta (iPLA2beta) has been implicated in various cellular events, the functions of other iPLA2 isozymes remain largely elusive. In this study, we examined the cellular functions of group VIB iPLA2gamma. Lentiviral transfection of iPLA2gamma into HEK293 cells resulted in marked increases in spontaneous, stimulus-coupled, and cell death-associated release of arachidonic acid (AA), which was converted to prostaglandin E2 with preferred cyclooxygenase (COX)-1 coupling. Conversely, treatment of HEK293 cells with iPLA2gamma small interfering RNA significantly reduced AA release, indicating the participation of endogenous iPLA2gamma. iPLA2gamma protein appeared in multiple sizes according to cell types, and a 63-kDa form was localized mainly in peroxisomes. Electrospray ionization mass spectrometry of cellular phospholipids revealed that iPLA2gamma and other intracellular PLA2 enzymes acted on different phospholipid subclasses. Transfection of iPLA2gamma into HCA-7 cells also led to increased AA release and prostaglandin E2 synthesis via both COX-1 and COX-2, with a concomitant increase in cell growth. Immunohistochemistry of human colorectal cancer tissues showed elevated expression of iPLA2gamma in adenocarcinoma cells. These results collectively suggest distinct roles for iPLA2beta and iPLA2gamma in cellular homeostasis and signaling, a functional link between peroxisomal AA release and eicosanoid generation, and a potential contribution of iPLA2gamma to tumorigenesis.

Prostaglandin E Synthases in Zebrafish

Objective— Prostaglandin E synthases (PGESs) are being explored as antiinflammtory drug targets as alternatives to cyclooxygenase (COX)-2. Located downstream of the cyclooxygenases, PGESs catalyze PGE 2 formation, and deletion of microsomal (m)-PGES-1 abrogates inflammation. We sought to characterize the developmental expression of COX and PGES in zebrafish.

Methods and Results— We cloned zebrafish cytosolic (c) and m-PGES orthologs and mapped them to syntenic regions of chromosomes 23 and 5. cPGES was widely expressed during development and was coordinately regulated with zCOX-1 in the inner ear, the pronephros, and intestine. COX-2 and mPGES-1 exhibited restricted expression, dominantly in the vasculature of the aortic arch. However, the enzymes were anatomically segregated within the vessel wall. Experiments with antisense morpholinos and with nonsteroidal antiinflammatory drugs suggest that these genes may not be critical for development.

Conclusions— mPGES-1 is developmentally coregulated with COX-2 in vasculature. Given the high fecundidity and translucency of the zebrafish, this model may afford a high throughput system for characterization of novel PGES inhibitors.

Microsomal prostaglandin E synthase (mPGES)-1, mPGES-2 and cytosolic PGES expression in human gastritis and gastric ulcer tissue

Recently, three different prostaglandin E2 synthases have been identified: microsomal prostaglandin E synthase (mPGES)-1, cytosolic PGES (cPGES), and mPGES-2; however, their role and connection to cyclooxygenase (COX)-2 in the gastric ulcer repair process remain unknown. Therefore, we examined mPGES-1, cPGES, and mPGES-2 expression and localization in the stomach in vitro and in vivo. Tissues were obtained from Helicobacter pylori (H. pylori)-infected patients and consisted of surgical resections of gastric ulcers, or biopsies of gastric ulcers or gastritis. mPGES-1 mRNA and protein expression levels were examined by real-time polymerase chain reaction (PCR) and Western blot analysis, respectively. mPGES-1, cPGES, and mPGES-2 localization were analyzed immunohistochemically. Induction of PGES expression in response to interleukin (IL)-1beta was examined in vitro in the cultured human gastric fibroblast line Hs262.St. Real-time PCR analysis of mPGES-1 mRNA expression in biopsy samples showed significantly higher expression levels in open than in closed gastric ulcer tissue. Western blot analysis showed mPGES-1 protein expression limited to open ulcer tissue, while mPGES-2 and cPGES immunoreactivities were seen in both open and closed ulcer tissue. Immunohistochemical analysis showed strong mPGES-1 expression in fibroblasts and macrophages of the ulcer bed, paralleling COX-2 expression. cPGES and mPGES-2 expression levels were seen in both fibroblasts of the ulcer bed and in epithelial cells. Furthermore, stronger cPGES and mPGES-2 immunoreactivities were seen in scattered mast cell-like cells and neuroendocrine-like cells, respectively. Induction of mPGES-1 expression in response to IL-1beta was seen in cultured gastric fibroblasts in vitro, and double immunostaining showed mPGES-1 coexpression with COX-2 in fibroblasts of the ulcer bed in vivo. In conclusion, mPGES-1, cPGES, and mPGES-2 are all expressed in gastric ulcer tissue, but only mPGES-1 parallels COX-2 expression in mesenchymal and inflammatory cells of the ulcer bed, suggesting a key role for this enzyme in the ulcer repair process.

Cyclooxygenase isoenzymes and patency of ductus arteriosus

Prenatal patency of the ductus arteriosus is maintained mainly by prostaglandin (PG) E(2). Accordingly, the vessel is endowed in its muscular component with a complete, cyclooxygenase (COX) and PGE synthase (PGES), system for the synthesis of the compound. COX1 is better expressed than COX2, particularly in the premature, but COX2 is more extensively coupled with microsomal PGES (mPGES). No evidence was obtained of either COX being coupled with cytosolic PGES (cPGES). Functionally, these data translate into a differential constrictor response of the ductus to dual, COX1/COX2, vs. COX2-specific inhibitors (indomethacin vs. L-745,337), with the latter being less effective specifically prior to term. This difference, however, subsides upon treatment with endotoxin and the attendant upregulation of COX2 and mPGES. Furthermore, when studied separately, COX1 and COX2 prove to be unevenly responsive to indomethacin, and an immediate and fast developing contraction of the vessel occurs only when COX2 is inhibited. Deletion of either COX gene results into upregulation of NO synthase, and a similar compensatory reaction is expected when enzymes are suppressed pharmacologically. We conclude that PGE(2) and NO can function synergistically in keeping the ductus patent. This arrangement provides a possible explanation for failures of indomethacin or ibuprofen treatment in the management of the prematurely born infant with persistent ductus. Coincidentally, it opens the way to new therapeutic possibilities being based on interference with the NO effector or a more selective disruption, possibly having mPGES as a target, of the PGE(2) synthetic cascade.

Induced microsomal PGE synthase-1 is involved in cyclooxygenase-2-dependent PGE2 production in gastric fibroblasts

We have previously shown that the cyclooxygenase (COX)-2/PGE2 pathway plays a key role in VEGF production in gastric fibroblasts. Recent studies have identified three PGE synthase (PGES) isozymes: cytosolic PGES (cPGES) and microsomal PGES (mPGES)-1 and -2, but little is known regarding the expression and roles of these enzymes in gastric fibroblasts. Thus we examined IL-1beta-stimulated mPGES-1 and cPGES mRNA and protein expression in gastric fibroblasts by quantitative PCR and Western blot analysis, respectively, and studied both their relationship to COX-1 and -2 and their roles in PGE2 and VEGF production in vitro. IL-1beta stimulated increases in both COX-2 and mPGES-1 mRNA and protein expression levels. However, COX-2 mRNA and protein expression were more rapidly induced than mPGES-1 mRNA and protein expression. Furthermore, MK-886, a nonselective mPGES-1 inhibitor, failed to inhibit IL-1beta-induced PGE2 release at the 8-h time point, while totally inhibiting PGE2 at the later stage. However, MK-886 did inhibit IL-1beta-stimulated PGES activity in vitro by 86.8%. N-(2-cyclohexyloxy-4-nitrophenyl)-methanesulfonamide (NS-398), a selective COX-2 inhibitor, totally inhibited PGE2 production at both the 8-h and 24-h time points, suggesting that COX-2-dependent PGE2 generation does not depend on mPGES-1 activity at the early stage. In contrast, NS-398 did not inhibit VEGF production at 8 h, and only partially at 24 h, whereas MK-886 totally inhibited VEGF production at each time point. These results suggest that IL-1beta-induced mPGES-1 protein expression preferentially coupled with COX-2 protein at late stages of PGE2 production and that IL-1beta-stimulated VEGF production was totally dependent on membrane-associated proteins involved in eicosanoid and glutathione metabolism (MAPEG) superfamily proteins, which includes mPGES-1, but was partially dependent on the COX-2/PGE2 pathway.

Prostaglandins in non-insectan invertebrates: recent insights and unsolved problems

Prostaglandins (PG) are oxygenated derivatives of C20 polyunsaturated fatty acids including arachidonic and eicosapentaenoic acids. In mammals, these compounds have been shown to play key roles in haemostasis, sleep-wake regulation, smooth muscle tone, and vaso-, temperature and immune regulation. In invertebrates, PGs have been reported to perform similar roles and are involved in the control of oogenesis and spermatogenesis, ion transport and defence. Although there is often a detailed understanding of the actions of these compounds in invertebrates such as insects, knowledge of their mechanism of biosynthesis is often lacking. This account provides a critical review of our current knowledge on the structure and modes of biosynthesis of PGs in invertebrates, with particular reference to aquatic invertebrates. It emphasises some of the most recent findings, which suggest that some PGs have been misidentified.

Prostaglandins in invertebrates can be categorised into two main types; the classical forms, such as PGE2 and PGD2 that are found in mammals, and novel forms including clavulones, bromo- and iodo-vulones and various PGA2 and PGE2 esters. A significant number of reports of PG identification in invertebrates have relied upon methods such as enzyme immunoassay that do not have the necessary specificity to ensure the validity of the identification. For example, in the barnacle Balanus amphitrite, although there are PG-like compounds that bind to antibodies raised against PGE2, mass spectrometric analysis failed to confirm the presence of this and other classical PGs. Therefore, care should be taken in drawing conclusions about what PGs are formed in invertebrates without employing appropriate analytical methods. Finally, the recent publication of the Ciona genome should facilitate studies on the nature and mode of biosynthesis of PGs in this advanced deuterostomate invertebrate.

Effects of antirheumatic treatments on the prostaglandin E2 biosynthetic pathway

OBJECTIVE

Microsomal prostaglandin E synthase 1 (mPGES-1) is up-regulated in experimental arthritis and markedly expressed in synovial tissue biopsy samples from patients with rheumatoid arthritis (RA). This study was carried out to determine the effects of tumor necrosis factor (TNF) blockers and glucocorticoids on mPGES-1 and cyclooxygenase (COX) expression, as well as biosynthesis of PGE(2) in rheumatoid joints.

METHODS

In vitro effects of TNF blockers and dexamethasone on the PGE(2) biosynthetic pathway were examined in RA synovial fluid mononuclear cells (SFMCs) by flow cytometry. PGE(2) levels in culture supernatants were measured by enzyme immunoassay. Expression of enzymes responsible for PGE(2) synthesis ex vivo was evaluated by immunohistochemistry in synovial biopsy samples obtained from 18 patients before and after treatment with TNF blockers and from 16 patients before and after intraarticular treatment with glucocorticoids. Double immunofluorescence was performed using antibodies against mPGES-1, COX-1, COX-2, and CD163.

RESULTS

Double immunofluorescence revealed that mPGES-1 and COX-2 were colocalized in SFMCs as well as in RA synovial tissue cells. The addition of either TNF blockers or dexamethasone suppressed lipopolysaccharide-induced mPGES-1 and COX-2 expression in synovial fluid monocyte/macrophages in vitro and decreased the production of PGE(2). Intraarticular treatment with glucocorticoids significantly reduced both mPGES-1 and COX-2 expression in arthritic synovial tissue ex vivo. The number of COX-1-expressing cells in synovial tissue was also significantly decreased by glucocorticoid treatment. In contrast, neither mPGES-1 nor COX-2 expression in synovial tissue was significantly suppressed by anti-TNF therapy.

CONCLUSION

These data are the first to demonstrate the effects of antirheumatic treatments on mPGES-1 expression in RA and suggest that the inhibition of PGE(2) biosynthesis, preferably by targeting mPGES-1, might complement anti-TNF treatment for optimal antiinflammatory results in RA.

Regulation of the membrane-localized prostaglandin E synthases mPGES-1 and mPGES-2 in cardiac myocytes and fibroblasts

The proinflammatory mediator cyclooxygenase (COX)-2 and its product PGE2 are induced in the ischemic heart, contributing to inflammatory cell infiltration, fibroblast proliferation, and cardiac hypertrophy. PGE2 synthesis coupled to COX-2 involves two membrane-localized PGE synthases, mPGES-1 and mPGES-2; however, it is not clear how these synthases are regulated in cardiac myocytes and fibroblasts. To study this, we used primary cultures of neonatal ventricular myocytes (VM) and fibroblasts (VF) treated with IL-1β for 24 h. To test for involvement of MAPKs in IL-1β regulation of mPGES-1 and-2, cells were pretreated with the pharmacological inhibitors of p42/44 MAPK, p38 MAPK, and c-Jun kinase (JNK). mRNA was analyzed by RT-PCR. Protein was analyzed by densitometry of Western blots. mPGES-1 was undetectable in untreated VF but induced by IL-1β; inhibition of either p42/44 MAPK or JNK, but not p38 MAPK, was almost completely inhibitory. In VM, inhibition of the three MAPKs reduced IL-1β-stimulated mPGES-1 protein by 70–90%. mPGES-2 was constitutively synthesized in both VM and VF and was not regulated by IL-1β or MAPKs. Confocal microscopy revealed colocalization of both mPGES-1 and mPGES-2 with COX-2 in the perinuclear area of both VF and VM. Finally, PGE2 production was higher in VM than VF. Our data show that 1) mPGES-1 is induced in both VF and VM, 2) regulation of mPGES-1 by MAPK family members is different in the two cell types, 3) mPGES-2 is constitutively synthesized in both VM and VF and is not regulated, and 4) mPGES-1 and mPGES-2 are colocalized with COX-2 in both cells. Thus differences in activity of mPGES-1 and COX-2 or coupling of COX-2 with mPGES-1 may contribute to differences in PGE2 production by myocytes and fibroblasts.

mPGES-1 as a novel target for arthritis

PURPOSE OF REVIEW

Prostaglandin E2 (PGE2) is by far the major prostanoid synthesized in the joint and plays an important role in inflammation and pathogenesis of arthritis. Moreover, increased levels of PGE2 have been detected in serum and synovial fluids from arthritic patients. Little was known about the enzyme(s) involved in the isomerization of PGH2 into PGE2 synthesis until recent identification of PGE synthase (PGES). Several isoforms were characterized, among which microsomal PGES-1 (mPGES-1) has received much attention, because this enzyme is inducible and functionally linked with cyclooxygenase-2. This review focuses on recent findings regarding the regulation of mPGES-1 expression and the possible role of this enzyme in arthritis.

RECENT FINDINGS

Various in vitro and in vivo studies demonstrated that proinflammatory stimuli, such as interleukin-1beta and tumor necrosis factor-alpha upregulate the expression of mPGES-1 at the protein and mRNA level. Promoter analysis indicates that the transcription factor Egr-1 is involved in the positive regulation of mPGES-1. Studies from mPGES-1-deficient mice and animal models of inflammatory arthritis strongly suggest a role of mPGES-1 in the production of PGE2 and the pathogenesis of arthritis.

SUMMARY

This article reviews the regulation of mPGES-1 expression and provides evidence for a role of mPGES-1 in inducible PGE2 production and arthritis. Future studies using selective inhibitors of mPGES-1 activity or expression would clarify the role of this enzyme in arthritis.

Regulation of cytosolic prostaglandin E synthase by phosphorylation

cPGES [cytosolic PG (prostaglandin) E synthase] is constitutively expressed in various cells and can regulate COX (cyclo-oxygenase)-1-dependent immediate PGE2 generation. In the present study, we found that cPGES underwent serine phosphorylation, which was accelerated transiently after cell activation. Several lines of evidence suggest that a cPGES-activating protein kinase is CK-II (casein kinase II). Recombinant cPGES was phosphorylated directly by and associated with CK-II in vitro, resulting in marked reduction of the K m for the substrate PGH2. In activated cells, cPGES phosphorylation occurred in parallel with increased cPGES enzymic activity and PGE2 production from exogenous and endogenous arachidonic acid, and these processes were facilitated by Hsp90 (heat-shock protein 90), a molecular chaperone that formed a tertiary complex with cPGES and CK-II. Treatment of cells with inhibitors of CK-II and Hsp90 and with a dominant-negative CK-II attenuated the formation of the cPGES-CK-II-Hsp90 complex and attendant cPGES phosphorylation and activation. Mutations of either of two predicted CK-II phosphorylation sites on cPGES (Ser113 and Ser118) abrogated its phosphorylation and activation both in vitro and in vivo. Moreover, the CK-II-Hsp90-mediated activation of cPGES was ameliorated by the p38 mitogen-activated protein kinase inhibitor SB20358 or by the anti-inflammatory glucocorticoid dexamethasone. Taken together, the results of the present study have provided the first evidence that the cellular function of this eicosanoid-biosynthetic enzyme is under the control of a molecular chaperone and its client protein kinase.

Expression of proteins related to prostaglandin E2 biosynthesis is increased in human gastric cancer and during gastric carcinogenesis

Prostaglandin E2 (PGE2) is related to carcinogenesis. Cyclooxygenase (COX) and prostaglandin E synthase (PGES) are involved in PGE2 synthesis. However, overall situation of COX and microsomal PGES (mPGES) expression in gastric cancer has not been studied in detail. The expression of COX and mPGES was evaluated in 45 cases of gastric cancer (22 intestinal type and 23 diffuse type), 13 gastric dysplasia, 15 intestinal metaplasia, 18 Helicobacter pylori associated gastritis, and 10 normal gastric tissues by performing immunohistochemistry and Western blot analysis. COX-1 expression was higher in intestinal type cancers than diffuse ones. COX-2 and mPGES-1 were expressed more in cancers than in paired nonneoplastic adjacent tissues, and intestinal type cancers showed higher expression of COX-2 than diffuse ones. The expression of COX and mPGES was gradually increased with progression of gastric lesions and the highest in dysplasia. mPGES-1 was expressed not only in epithelial cells but also in stromal cells, whose phenotype was myofibroblast, endothelial cells and others. In conclusion, proteins related to PGE2 biosynthesis affect both histogenesis and the carcinogenesis of human gastric cancer.

Expression of microsomal prostaglandin E synthase 1 in rheumatoid arthritis synovium

OBJECTIVE

Microsomal prostaglandin E synthase 1 (mPGES-1) catalyzes the formation of PGE(2) from cyclooxygenase-derived PGH(2). Microsomal PGES-1 is induced by proinflammatory cytokines and is strongly linked to conditions that result in high PGE(2) biosynthesis. PGE(2) contributes to the pathogenesis of rheumatoid arthritis (RA), acting as a mediator of inflammation and promoting bone destruction. Induction of mPGES-1 in rheumatoid synoviocytes by proinflammatory cytokines has been demonstrated in vitro, indicating an important role in RA pathogenesis. Recent studies using mPGES-1-deficient mice demonstrated the importance of this gene in chronic inflammation. The aim of this study was to investigate the expression and localization of mPGES-1 in synovial biopsy specimens obtained from patients with RA.

METHODS

Synovial tissue samples from 24 patients with RA were obtained, and immunohistologic analysis was performed using polyclonal antibodies against mPGES-1. Double immunofluorescence staining was performed with antibodies to CD3, CD19, CD20, CD68, CD163, and prolyl 4-hydroxylase.

RESULTS

Intracellular mPGES-1 staining was observed in synovial membranes from all of the RA patients studied. Specifically, strong expression of mPGES-1 was detected in synovial lining cells. In sublining mononuclear and fibroblast-like cells, the extent of mPGES-1 staining was less than that in the synovial lining cells. In some patients, positive staining was observed in endothelial cells. With the double immunofluorescence technique, mPGES-1 production was detected in synovial macrophages and fibroblasts, while mPGES-1 expression was not observed in lymphocytes.

CONCLUSION

The demonstration of mPGES-1 expression in synovial tissues from patients with RA suggests a role for mPGES-1 in the RA disease process. Microsomal PGES-1 might be a potential new target for treatment strategies to control PGE(2) synthesis in patients with RA, without the systemic side effects associated with cyclooxygenase inhibitors.

Cyclooxygenase-1-dependent prostaglandin synthesis modulates tumor necrosis factor-alpha secretion in lipopolysaccharide-challenged murine resident peritoneal macrophages

Comprehensive studies of prostaglandin (PG) synthesis in murine resident peritoneal macrophages (RPM) responding to bacterial lipopolysaccharide (LPS) revealed that the primary PGs produced by RPM were prostacyclin and PGE(2). Detectable increases in net PG formation occurred within the first hour, and maximal PG formation had occurred by 6-10 h after LPS addition. Free arachidonic acid levels rose and peaked at 1-2 h after LPS addition and then returned to baseline. Cyclooxygenase-2 (COX-2) and microsomal PGE synthase levels markedly increased upon exposure of RPM to LPS, with the most rapid increases in protein expression occurring 2-6 h after addition of the stimulus. RPM constitutively expressed high levels of COX-1. Studies using isoform-selective inhibitors and RPM from mice bearing targeted deletions of ptgs-1 and ptgs-2 demonstrated that COX-1 contributes significantly to PG synthesis in RPM, especially during the initial 1-2 h after LPS addition. Selective inhibition of either COX isoform resulted in increased secretion of tumor necrosis factor-alpha (TNF-alpha); however, this effect was much greater with the COX-1 than with the COX-2 inhibitor. These results demonstrate autocrine regulation of TNF-alpha secretion by endogenous PGs synthesized primarily by COX-1 in RPM and suggest that COX-1 may play a significant role in the regulation of the early response to endotoxemia.

Reduced pain hypersensitivity and inflammation in mice lacking microsomal prostaglandin e synthase-1

We examined the in vivo role of membrane-bound prostaglandin E synthase (mPGES)-1, a terminal enzyme in the PGE2-biosynthetic pathway, using mPGES-1 knockout (KO) mice. Comparison of PGES activity in the membrane fraction of tissues from mPGES-1 KO and wild-type (WT) mice indicated that mPGES-1 accounted for the majority of lipopolysaccharide (LPS)-inducible PGES in WT mice. LPS-stimulated production of PGE2, but not other PGs, was impaired markedly in mPGES-1-null macrophages, although a low level of cyclooxygenase-2-dependent PGE2 production still remained. Pain nociception, as assessed by the acetic acid writhing response, was reduced significantly in KO mice relative to WT mice. This phenotype was particularly evident when these mice were primed with LPS, where the stretching behavior and the peritoneal PGE2 level of KO mice were far less than those of WT mice. Formation of inflammatory granulation tissue and attendant angiogenesis in the dorsum induced by subcutaneous implantation of a cotton thread were reduced significantly in KO mice compared with WT mice. Moreover, collagen antibody-induced arthritis, a model for human rheumatoid arthritis, was milder in KO mice than in WT mice. Collectively, our present results provide unequivocal evidence that mPGES-1 contributes to the formation of PGE2 involved in pain hypersensitivity and inflammation.

Activation of Peroxisome Proliferator-activated Receptor γ Inhibits Interleukin-1β-induced Membrane-associated Prostaglandin E2 Synthase-1 Expression in Human Synovial Fibroblasts by Interfering with Egr-1

Membrane-associated prostaglandin (PG) E(2) synthase-1 (mPGES-1) catalyzes the conversion of PGH(2) to PGE(2), which contributes to many biological processes. Peroxisome proliferator-activated receptor gamma (PPARgamma) is a ligand-activated transcription factor and plays an important role in growth, differentiation, and inflammation in different tissues. Here, we examined the effect of PPARgamma ligands on interleukin-1beta (IL-1beta)-induced mPGES-1 expression in human synovial fibroblasts. PPARgamma ligands 15-deoxy-Delta(12,14) prostaglandin J(2) (15d-PGJ(2)) and the thiazolidinedione troglitazone (TRO), but not PPARalpha ligand Wy14643, dose-dependently suppressed IL-1beta-induced PGE(2) production, as well as mPGES-1 protein and mRNA expression. 15d-PGJ(2) and TRO suppressed IL-1beta-induced activation of the mPGES-1 promoter. Overexpression of wild-type PPARgamma further enhanced, whereas overexpression of a dominant negative PPARgamma alleviated, the suppressive effect of both PPARgamma ligands. Furthermore, pretreatment with an antagonist of PPARgamma, GW9662, relieves the suppressive effect of PPARgamma ligands on mPGES-1 protein expression, suggesting that the inhibition of mPGES-1 expression is mediated by PPARgamma. We demonstrated that PPARgamma ligands suppressed Egr-1-mediated induction of the activities of the mPGES-1 promoter and of a synthetic reporter construct containing three tandem repeats of an Egr-1 binding site. The suppressive effect of PPARgamma ligands was enhanced in the presence of a PPARgamma expression plasmid. Electrophoretic mobility shift and supershift assays for Egr-1 binding sites in the mPGES-1 promoter showed that both 15d-PGJ(2) and TRO suppressed IL-1beta-induced DNA-binding activity of Egr-1. These data define mPGES-1 and Egr-1 as novel targets of PPARgamma and suggest that inhibition of mPGES-1 gene transcription may be one of the mechanisms by which PPARgamma regulates inflammatory responses.

Membrane-associated PGE synthase-1 (mPGES-1) is coexpressed with both COX-1 and COX-2 in the kidney

BACKGROUND

Prostaglandin E2 (PGE2) plays an important role in many physiologic and pathophysiologic processes in the kidney. Multiple enzymes are involved in PGE2 biosynthesis, including phospholipases, cyclooxygenases (COX), and the PGE2 synthases (PGES). The present studies were aimed at determining the intrarenal localization of mPGES-1 and whether it is coexpressed with COX-1 or COX-2.

METHODS

Rabbit mPGES-1 and COX-1 cDNAs were cloned using reverse transcription-polymerase chain reaction (RT-PCR) and screening a cDNA library. RNase protection assay and immunoblotting were used to examine mPGES-1 expression levels. In situ hybridization and immunostaining were used to determine the intrarenal localization of mPGES-1 and cyclooxygenases.

RESULTS

Rabbit mPGES-1 shares high sequence similarity to the human homolog. Nuclease protection studies showed that the kidney expresses among the highest level of mPGES-1 of any rabbit tissue. In situ hybridization showed COX-1 and mPGES-1 mRNA was highly expressed in renal medullary collecting ducts (MCD), and to a lesser extent in cortical collecting ducts (CCD). Fainter mPGES-1 expression was also observed in macula densa (MD) and medullary interstitial cells (RMICs), where COX-2 is highly expressed. Double-labeling studies (immunostaining plus in situ hybridization) and immunohistochemistry of mouse tissues confirmed that mPGES-1 predominantly colocalizes with COX-1 in distal convoluted tubule (DCT), CCD, and MCD, and is coexpressed with COX-2 at lower levels in MD and RMICs.

CONCLUSION

Together, these studies suggest mPGES-1 colocalizes with both COX-1 and COX-2 to mediate the biosynthesis of PGE2 in the kidney.

Deletion of microsomal prostaglandin E2 (PGE2) synthase-1 reduces inducible and basal PGE2 production and alters the gastric prostanoid profile

Microsomal prostaglandin E synthase-1 (mPGES-1) is an inducible protein recently shown to be an important source of inflammatory PGE2. Here we have used mPGES-1 wild type, heterozygote, and null mice to assess the impact of reduction or absence mPGES-1 protein on the production of PGE2 and other prostaglandins in lipopolysaccharide (LPS)-treated macrophages and mice. Thioglycollate-elicited peritoneal macrophages with mPGES-1 deficiency were found to lose their ability to produce PGE2 upon LPS stimulation. Resident mPGES-1(-/-) peritoneal macrophages exhibited severely impaired PGE2-releasing activity but retained some LPS-inducible PGE2 production capacity. Both macrophage types showed a 50% decrease in PGE2 production with removal of one copy of the mPGES-1 gene. In vivo, mPGES-1 deletion abolished the LPS-stimulated production of PGE2 in spleen, kidney, and brain. Surprisingly, lack of mPGES-1 activity resulted in an 80-90% decrease in basal, cyclooxygenase-1 (COX-1)-dependent PGE2 production in stomach and spleen, and a 50% reduction in brain and kidney. Other prostaglandins (thromboxane B2, PGD2, PGF(2alpha), and 6-keto-PGF(1alpha)) were significantly elevated in stomachs of mPGES-1-null mice but not in other tissues. Examination of mRNA for several terminal prostaglandin synthases did not reveal changes in expression levels associated with mPGES-1 deficiency, indicating that gastric prostaglandin changes may be due to shunting of cyclooxygenase products to other terminal synthases. These data demonstrate for the first time a dual role for mPGES-1 in both inflammatory and COX-1-mediated PGE2 production and suggest an interdependence of prostanoid production with tissue-specific alterations of prostaglandin levels in the absence of mPGES-1.

Intraparenchymal administration of interleukin-1β induces cyclooxygenase-2-mediated expression of membrane- and cytosolic-associated prostaglandin E synthases in mouse brain

Interleukin (IL)-1beta is a proinflammatory cytokine expressed in neural tissue following injury and in neurodegenerative states. To understand the consequences of its presence in brain, we carried out intraparenchymal IL-1beta injections and found significant increases in prostaglandin (PG)E2, a critical factor in inflammatory and physiological processes. Elevated mRNA and protein expression of the PGE2-synthetic-related enzymes cyclooxygenase (COX)-2 and membrane-associated PGE synthase (PGES) accompanied local PGE2 production. In addition, IL-1beta stimulated protein expression of cytosolic PGES. Finally, we showed attenuation of these IL-1beta-inductions by COX-2 inhibition, suggesting in vivo regulation of both PGE synthase isoforms in the brain.

Microsomal Prostaglandin E2 Synthase-1 Is Induced by Conditional Expression of RET/PTC in Thyroid PCCL3 Cells through the Activation of the MEK-ERK Pathway

RET/PTC rearrangements are believed to be tumor-initiating events in papillary thyroid carcinomas. We identified microsomal prostaglandin E2 synthase-1 (mPGES-1) as a RET/PTC-inducible gene through subtraction hybridization cloning and expression profiling with custom microarrays. The inducible prostaglandin E2 (PGE2) biosynthetic enzymes cyclooxygenase-2 (COX-2) and mPGES-1 are up-regulated in many cancers. COX-2 is overexpressed in thyroid malignancies compared with benign nodules and normal thyroid tissues. Eicosanoids may promote tumorigenesis through effects on tumor cell growth, immune surveillance, and angiogenesis. Conditional RET/PTC1 or RET/PTC3 expression in PCCL3 thyroid cells markedly induced mPGES-1 and COX-2. PGE2 was the principal prostanoid and up-regulated (by approximately 60-fold), whereas hydroxyeicosatetraenoic acid metabolites were decreased, consistent with shunting of prostanoid biosynthesis toward PGE2 by coactivation of the two enzymes. RET/PTC activated mPGES-1 gene transcription. Based on experiments with kinase inhibitors, with PCCL3 cell lines with doxycycline-inducible expression of RET/PTC mutants with substitutions of critical tyrosine residues in the kinase domain, and lines with inducible expression of activated mutants of H-RAS and MEK1, RET/PTC was found to regulate mPGES-1 through Shc-RAS-MEK-ERK. These data show a direct relationship between activation of a tyrosine kinase receptor oncogene and regulation of PGE2 biosynthesis. As enzymes involved in prostanoid biosynthesis can be targeted with pharmacological inhibitors, these findings may have therapeutic implications.