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The Journal of Immunology, 2002, 168: 5811-5816.
Copyright © 2002 by The American Association of Immunologists

Lipopolysaccharide-Dependent Prostaglandin E2 Production Is Regulated by the Glutathione-Dependent Prostaglandin E2 Synthase Gene Induced by the Toll-Like Receptor 4/MyD88/NF-IL6 Pathway1

Satoshi Uematsu*,{dagger}, Makoto Matsumoto*, Kiyoshi Takeda*,{dagger} and Shizuo Akira2,*,{dagger}

* Department of Host Defense, Research Institute for Microbial Diseases, Osaka University, and {dagger} Solution-Oriented Research for Science and Technology of Japan Science and Technology Corp., Suita, Osaka, Japan


    Abstract
 Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 
Macrophages produce a large amount of PGE2 during inflammation. This lipid mediator modulates various immune responses. PGE2 acts on macrophages and inhibits production of cytokines such as TNF-{alpha} and IL-12. Membrane-bound glutathione-dependent PGE2 synthase (mPGES) has been shown to be a terminal enzyme of the cyclooxygenase-2-mediated PGE2 biosynthesis. Here we identified mPGES as a molecule that is induced by LPS in macrophages. The expression of mPGES was not induced by LPS in mice lacking Toll-like receptor 4 or MyD88. Furthermore, mice deficient in NF-IL6 showed neither induction of mPGES nor biosynthesis of PGE2 in response to LPS, indicating that mPGES expression in response to LPS is regulated by a Toll-like receptor 4/MyD88/NF-IL6-dependent signaling pathway. We generated mPGES-deficient mice and investigated the role of mPGES in vivo. The mice showed no augmentation of the PGE2 production in response to LPS. However, they were not impaired in the LPS-induced production of inflammatory cytokines and showed normal response to the LPS-induced shock. Thus, mPGES is critically involved in the biosynthesis of PGE2 induced by LPS, but is dispensable for the modulation of inflammatory responses.


    Introduction
 Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 
Prostaglandin E2 is a lipid mediator that has an important role in multiple physiologic processes, including kidney function, vascular homeostasis, bone remodeling, fever generation, gastrointestinal function, pregnancy, and acute inflammatory responses (1). PGE2 is synthesized in three enzymatic steps via the cyclooxygenase (Cox) pathway. Arachidonic acid is released from membrane glycerophospholipids by phospholipase A2 (PLA2)3 and is a substrate of the PGH synthase (PGHS) (2). Different forms of PLA2 including a cytosolic PLA2 and several secreted PLA2s are involved in supplying arachidonic acid to the PGHS. PGHS has distinctive Cox and peroxidase activities. Two isoforms of Cox, Cox-1, and Cox-2, have been identified. Cox-1 is constitutively expressed in most tissues and mediates the immediate biosynthesis of PGE2 within several minutes of stimulation with Ca2+ mobilizers. Cox-2 is induced by inflammatory stimuli and mediates a delayed biosynthesis of PGE2 (3, 4). To produce biologically active prostanoid, PGE2 synthase (PGES), a terminal prostanoid synthase, catalyzes the conversion of Cox-derived PGH2 to PGE2. PGES requires glutathione for optimal catalytic activity. PGES activity has been detected in both cytosolic- and membrane-associated fractions of various cells and tissues (5, 6, 7). Recently, the glutathione-dependent cytosolic PGES (cPGES/p23) was identified. This enzyme, which is expressed constitutively and ubiquitously, converts Cox-1-derived, but not Cox-2-derived, PGH2 to PGE2 (6). PGES activity has also been shown to be strongly induced by proinflammatory stimuli in macrophages (7, 8). Microsomal GST1-like 1 (MGST1-L1), a member of the MAPEG (membrane-associated proteins involved in eicosanoid and glutathione metabolism) superfamily, has been shown to exhibit significant PGES activity (9, 10). More recently, membrane-associated PGES (mPGES) has been detected in LPS-stimulated macrophages and was shown to be identical with MGST1-L1. Membrane-associated PGES/MGST1-L1 is proposed to be preferentially coupled with the inducible Cox-2 to promote the delayed biosynthesis of PGE2 (11).

In this study we identified mPGES/MGST1-L1 as a LPS-inducible gene in macrophages by a suppression subtractive hybridization technique. We showed that LPS-induced mPGES expression is regulated through a Toll-like receptor 4 (TLR4)/MyD88-dependent pathway and entirely depends upon the NF-IL6 transcription factor. Furthermore, we generated mice deficient in mPGES and demonstrated that mPGES/MGST1-L1 is essential for delayed PGE2 synthesis by activated macrophages in response to LPS.


    Materials and Methods
 Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 
Reagents

LPS from Salmonella minnesota Re-595 prepared by a phenol-chloroform-petroleum ether extraction procedure was purchased from Sigma-Aldrich (St. Louis, MO). Thioglycolate broth (Brewer’s formula) was purchased from Difco (Detroit, MI). IFN-{gamma} was obtained from Genzyme (Cambridge, MA). Restriction and DNA modification enzymes were products of TOYOBO (Tsuruga, Japan).

Subtractive hybridization

RAW264.7 (1 x 108) cells were stimulated with LPS (100 ng/ml) for 4 h. RNA was obtained from the cells with an RNA easy kit (Qiagen, Hilden, Germany) following poly(A)+ RNA selection using Oligotex-dT30 latex beads (Takara, Otsu, Japan). Then all procedures were performed according to the instructions of the PCR-select cDNA subtraction kit (Clontech Laboratories, Palo Alto, CA).

Preparation of peritoneal macrophages

Peritoneal macrophages from wild-type, TLR4-mutant, MyD88-mutant, NF-IL6-mutant, and mPGES-mutant mice were isolated as described previously (12, 13, 14).

Northern blot analysis

Total RNA was extracted from peritoneal macrophages (5 x 106) using the TRIzol reagent (Invitrogen, Carlsbad, CA). Then total RNA (5 µg) was electrophoresed, transferred to a nylon membrane, and hybridized with 32P-labeled cDNA probe specific for mPGES, Cox-1, Cox-2, and cPGES/p23 as described previously (14). The same membrane was rehybridized with cDNA specific for {beta}-actin.

Western blot analysis

The cell lysates were separated on SDS-PAGE, transferred onto a nitrocellulose membrane, and incubated with the blocking buffer containing 5.0% skim milk. The membrane was incubated with the indicated Ab and then visualized with ECL system (NEN Life Science, Boston, MA).

RT-PCR

Total RNA was isolated from an adherent monolayer of peritoneal macrophages stimulated with LPS (100 ng/ml). cDNA synthesis was synthesized using Superscript II Moloney murine leukemia virus reverse transcriptase (Invitrogen). The cDNA product was amplified by primers for mPGES (5'-AGCACACTGCTGGTCATCAAGATGTAC-3' and 5'-CCTGAGAGGACAACGAGGAAATGTATC-3').

Generation of mPGES-mutant mice

Membrane-associated PGES genomic DNA was screened from a 129/SvJ mouse genomic library (Stratagene, La Jolla, CA), subcloned into pBluescript vector (Stratagene), and characterized by restriction enzyme mapping and DNA sequencing. A targeting vector was constructed to replace the 1.2-kb genomic fragment containing the second exon with the neomycin resistance gene from pMC1-neo (Stratagene). The targeting vector was flanked by the 5.5-kb 5' genomic fragment and the 1.2-kb 3' fragment and contained an HSV-thymidine kinase cassette at the 5' end of the vector. The targeting vector was linearized with SalI and electroporated into E14.1 embryonic stem (ES) cells. The clones resistant to G418 and gancycrovir were screened for homologous recombination by PCR and confirmed by Southern blot analysis using the probe indicated in Fig. 2GoA. The mutant mice were essentially generated as described previously (12).



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FIGURE 2. Induction of mPGES mRNA and production of PGE2 in response to LPS in NF-IL6-deficient macrophages. A, Thioglycolate-elicited peritoneal macrophages from wild-type and NF-IL6-deficient mice were cultured with 100 ng/ml LPS for 0.5, 2, 8, and 16 h. Total RNA (5.0 µg) was electrophoresed, transferred, and hybridized with 32P-labeled probe specific for mPGES. B, Thioglycolate-elicited peritoneal macrophages from wild-type and NF-IL6-deficient mice were cultured with 100 ng/ml LPS for 8 h and analyzed for expression of Cox-1 and cPGES/p23 by Northern blotting. Two representatives are shown from four independent experiments. C, Thioglycolate-elicited peritoneal macrophages from wild-type and NF-IL6-deficient mice were cultured with 100 ng/ml LPS for the indicated period and analyzed for the expression of Cox-2 by Northern blotting. D, Thioglycolate-elicited peritoneal macrophages from wild-type and NF-IL6-deficient mice were cultured with 100 ng/ml LPS for the indicated period, followed by Western blot analysis with anti-Cox-1- or anti-Cox-2-specific Ab. E, Thioglycolate-elicited peritoneal macrophages from wild-type and NF-IL6-deficient mice were cultured with various concentrations of LPS for 24 h. The concentrations of PGE2 in the culture supernatants were measured by ELISA. Experiments were independently performed three times with similar results.

 
ELISA

To evaluate the production of cytokines by macrophages in vitro, thioglycolate-elicited peritoneal cells were seeded onto 96-well plates (2 x 105 cells/well) and stimulated with the indicated reagents for 24 h. Concentrations of TNF-{alpha}, IL-12p40, and IL-6 were measured by ELISA according to the manufacturer’s instructions (Genzyme, Cambridge, MA). Production of PGE2 was measured with a PGE2 monoclonal enzyme immunoassay kit (Cayman Chemicals, Ann Arbor, MI). For the in vivo experiments wild-type and mPGES-mutant mice were i.p. injected with 0.1 ml PBS containing 1 mg LPS, and sera were taken at the indicated time points. Serum levels of TNF-{alpha}, IL-12p40, and PGE2 were determined by ELISA.


    Results
 Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 
Membrane-associated PGES is induced by LPS in macrophages

To identify the LPS-inducible genes responsible for immune and inflammatory responses in activated macrophages we prepared a cDNA library from LPS-stimulated cells of the mouse macrophage line, RAW264.7, and screened the library by a suppression-subtracted hybridization technique. Using this technique we previously identified novel LPS-inducible genes such as the LPS-inducible CCR and inducible I{kappa}B kinase genes (15, 16). In addition we obtained a gene encoding membrane-bound glutathione-dependent PGE2 synthase (mPGES). We examined the expression of mPGES in response to LPS in mouse peritoneal macrophages. Thioglycolate-elicited peritoneal macrophages were treated with 100 ng/ml LPS for 0.5, 2, 8, and 16 h, and the mRNA expression of mPGES was examined by Northern blot analysis. As shown in Fig. 1GoA, mRNA for mPGES was not detected in nonstimulated macrophages, but was markedly expressed in response to LPS. Four major transcripts were detected, and the shortest one matched the mouse mPGES cDNA (accession no. AB041997) in length. The mRNA expression of mPGES was induced at 2 h, reached a peak at 8 h, and lasted until 16 h.



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FIGURE 1. Expression of mPGES mRNA in response to LPS in macrophages. A, Thioglycolate-elicited peritoneal macrophages from wild-type mice were cultured with 100 ng/ml LPS for 0.5, 2, 8, and 16 h. Total RNA (5.0 µg) was electrophoresed, transferred, and hybridized with 32P-labeled probe specific for mPGES. B, Thioglycolate-elicited peritoneal macrophages from wild-type, MyD88-deficient, and TLR4-deficient mice were cultured with 100 ng/ml LPS for 8 h and analyzed for mPGES expression by Northern blotting.

 
We examined the LPS-induced expression of mPGES in macrophages from mice lacking TLR4 or MyD88, both of which mediate LPS signaling. LPS stimulation did not induce mPGES expression in TLR4- and MyD88-deficient mice, showing that mPGES is induced in response to LPS via a TLR4-MyD88-dependent pathway (Fig. 1GoB).

NF-IL6-deficient macrophages are defective in the LPS-induced expression of mPGES

When macrophages are activated by various inflammatory stimuli, NF-IL6 exhibits the augmented transcriptional activity and plays an important role in inflammatory responses of the macrophages (17). NF-IL6 binding motifs have been identified in the promoter regions of several LPS-inducible genes, including Cox-2 (18, 19, 20). Therefore, we analyzed the LPS-induced expression of mPGES in NF-IL6-deficient macrophages. Peritoneal macrophages from wild-type and NF-IL6-deficient mice were stimulated with LPS, and the expression of mPGES was examined by Northern blotting. In NF-IL6-deficient macrophages, LPS-induced expression of mPGES was almost completely abolished (Fig. 2GoA). We next examined the expression of Cox-1, Cox-2, and cPGES/p23 in NF-IL6-deficient macrophages. In wild-type macrophages Cox-1 mRNA was detected before stimulation with LPS and was down-regulated after the stimulation (Fig. 2GoB). In contrast, Cox-2 mRNA was induced, reached a peak at 2 h, and then declined after 8 h in wild-type macrophages stimulated by LPS (Fig. 2GoC). The expression of cPGES/p23 mRNA was detected before and augmented after the stimulation (Fig. 2GoB). In NF-IL6-deficient macrophages, down-regulation of Cox-1 and up-regulation of cPGES/p23 were similarly observed (Fig. 2GoB). However, the LPS-induced expression of Cox-2 was compromised in NF-IL6-deficient cells. At 2 h the expression of Cox-2 mRNA was significantly reduced compared with that of wild-type cells, although the expression of Cox-2 mRNA was increased at 8 h in NF-IL6-deficient cells (Fig. 2GoC). We further analyzed Cox-1 and Cox-2 protein expression in LPS-stimulated NF-IL6-deficient peritoneal macrophages. Despite reduced expression of Cox-1 mRNA after LPS stimulation, the protein level was not altered in wild-type or NF-IL6-deficient macrophages (Fig. 2GoD, upper panel). The expression of Cox-2 protein was induced at 12 h and declined at 24 h in wild-type macrophages, whereas the induction of Cox-2 protein in NF-IL6-deficient macrophages was observed with delayed kinetics compared with wild-type macrophages (Fig. 2GoD, lower panel). Thus, severe impairment in mPGES induction and delayed induction of Cox-2 were observed in LPS-stimulated NF-IL6-deficient macrophages. We next analyzed the production of PGE2 in macrophages from NF-IL6-deficient mice. Thioglycolate-elicited peritoneal macrophages were cultured with various concentrations of LPS for 24 h. Nonstimulated cells secreted small amounts of PGE2, and stimulation with LPS induced the production of PGE2 in a dose-dependent manner in wild-type mice. In NF-IL6-deficient macrophages, LPS did not induce the production of PGE2, although nonstimulated cells produced small amounts. These results indicate that NF-IL6 is involved in the LPS-induced production of PGE2 by mediating the gene expression of mPGES.

Generation of mPGES-deficient mice

To elucidate the role of mPGES in vivo, we generated mPGES-mutant mice by gene targeting. The mouse mPGES gene was disrupted by introducing a targeted mutation into E14.1 ES cells. A targeting vector was designed to replace the second exon with the neomycin resistance gene (Fig. 3GoA). This region of the mPGES gene shows high homology with rat and human mPGES (15). Homologous recombination was achieved in seven of 120 ES cell clones resistant to neomycin and ganciclovir. Three ES cell lines containing a mutant mPGES allele were microinjected into C57BL/6 blastocysts. One line of these chimeric mice successfully transmitted the disrupted mPGES gene through the germline. Homozygous mice carrying the mutant allele were born at the expected Mendelian ratio (Fig. 3GoB). They were healthy and did not show any obvious abnormalities until 20 wk. We investigated the expression of mPGES mRNA in peritoneal macrophages stimulated with 100 ng/ml LPS for 8 h by Northern blot analysis (Fig. 3GoC). We detected in the mutant mice mPGES transcripts of almost the same size. We next conducted RT-PCR using mRNA of LPS-stimulated macrophages from the mutant mice (Fig. 3GoD). RT-PCR with primers that flank the cDNA coded by the second exon resulted in the production of a 270-bp band in wild-type mice and a 190-bp band in the mutant mice. Sequence analysis of these products showed that second exon was completely deleted in mRNA from the mutant mice, indicating that normal mPGES protein was not produced in these animals (Fig. 3GoE).



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FIGURE 3. Targeted disruption of the mPGES gene. A, Schematic drawing of the targeting procedure. The mPGES wild-type genome, the targeting vector, and the predicted disrupted gene are shown. A solid bar denotes the second exon. The neo box represents the MC-1 neo poly(A)+ gene. The HSV-thymidine kinase box represents the HSV-thymidine kinase gene. Restriction enzymes: B, BamHI; E, EcoRI; S, SalI. B, Southern blotting of genomic DNA from offspring of the heterozygous intercrosses. Genomic DNA was extracted from mouse tails, digested with BamHI, and hybridized with the radiolabeled probe indicated in A. Southern blotting gave a single 2.4-kb band for wild-type mice (+/+), a 1.8-kb band for homozygotes (-/-), and both bands for heterozygotes (+/-). C, Northern blot analysis of peritoneal macrophages. Wild-type (+/+) and homozygous (-/-) mice were i.p. injected with 2 ml 4% thioglycolate. Three days later, peritoneal exudate cells were harvested and cultured with or without 100 ng/ml LPS for 8 h. Total RNA was extracted from adherent cells, electrophoresed, transferred to a nylon membrane, and hybridized with the mouse mPGES cDNA probe. The same membrane was rehybridized with a {beta}-actin probe. D and E, RT-PCR analysis of total RNA from peritoneal macrophages of wild-type (+/+) and homozygous (-/-) mice were stimulated with 100 ng/ml LPS for 8 h. Primers were established to flank the cDNA coded by the second exon. PCR with this set of primers yielded a shorter band in mPGES-/- mice, due to a 90-bp deletion of the second exon.

 
Membrane-associated PGES is indispensable for PGE2 production in response to LPS

We first examined the production of PGE2 in response to LPS in macrophages from mPGES-mutant mice. Thioglycolate-elicited peritoneal macrophages were cultured with various concentrations of LPS. Macrophages from mPGES-mutant mice produced small amounts of PGE2, similar to wild-type cells in the unstimulated condition (Fig. 4GoA). However, no LPS-induced augmentation of PGE2 production was observed in mPGES-mutant macrophages. This indicates that the targeted deletion of the mPGES gene leads to a loss of functional protein production, and that mPGES is essential for LPS-induced PGE2 production in macrophages.



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FIGURE 4. Impaired PGE2 production in response to LPS in mPGES-mutant mice. A, Thioglycolate-elicited peritoneal macrophages from wild-type and mPGES-mutant mice were cultured with 100 ng/ml LPS for 24 h. Concentrations of PGE2 in the culture supernatants were measured by ELISA. Experiments were independently performed three times with similar results. B, Wild-type (n = 10) and mPGES-mutant (n = 9) mice were i.p. injected with 0.1 ml PBS containing 1 mg LPS. Serum was collected before and 6 h after the injection. Serum levels of PGE2 were measured by ELISA. Data are represented as the mean serum level ± SD.

 
We further analyzed the LPS-induced PGE2 production in vivo. Wild-type mice (n = 10) and mPGES-mutant mice (n = 9) were i.p. injected with 0.1 ml PBS containing 1 mg LPS (Fig. 4GoB). The serum level of PGE2 was measured 6 h after injection. In wild-type mice, the serum level of PGE2 increased by 3-fold in response to LPS, whereas it remained at the basal level after administration in mPGES-mutant mice. Taken together these results demonstrate that mPGES is an indispensable enzyme for the production of PGE2 in response to LPS.

Normal production of proinflammatory cytokines in response to LPS in mPGES-mutant mice

PGE2 has been shown to modulate the production of proinflammatory cytokines (21). Therefore, we analyzed the LPS-induced production of proinflammatory cytokines in mPGES-mutant mice. Thioglycolate-elicited peritoneal macrophages were cultured with 100 ng/ml LPS in the presence or the absence of IFN-{gamma} (50 U/ml) for 24 h, and the production of TNF-{alpha} IL-12p40 and IL-6 was examined by ELISA (Fig. 5GoA). The production of these cytokines in mPGES-mutant macrophages was comparable to that observed in wild-type macrophages. We further examined in vivo the response to LPS in wild-type and mPGES-deficient mice. Wild-type (n = 3) or mPGES-mutant (n = 3) mice were i.p. injected with 1 mg LPS. Sera were taken at the indicated time points, and concentrations of TNF-{alpha} and IL-12p40 were determined by ELISA (Fig. 5GoB). Both mice had significantly elevated serum concentrations of these cytokines, and there was no difference in the level of cytokine production between wild-type and mPGES-mutant mice. We monitored the survival rate of mice after LPS administration. Almost all wild-type and mPGES-mutant mice died within 5 days of the LPS challenge (Fig. 5GoC). Thus, mPGES-mutant mice succumbed to endotoxin shock despite showing no elevation of PGE2.



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FIGURE 5. Normal responsiveness of mPGES-mutant mice to LPS. A, Thioglycolate-elicited peritoneal macrophages from wild-type and mPGES-mutant mice were cultured with 100 ng/ml LPS in the presence or the absence of IFN-{gamma} (50 U/ml) for 24 h. Concentrations of TNF-{alpha} IL-12p40 and IL-6 in the culture supernatants were measured by ELISA. ND, not detected. B, Wild-type (n = 3) or mPGES-mutant (n = 3) mice were i.p. injected with 1.0 mg LPS. Serum levels of TNF-{alpha} and IL-12p40 were measured at the indicated time points by ELISA. Experiments were independently performed three times with similar results. C, Age-matched wild-type (n = 10) and mPGES-deficient (n = 10) mice were i.p. injected with 1.0 mg LPS. Mortality was assessed daily for 6 days.

 

    Discussion
 Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 
A previous study demonstrated that the expression of mPGES is increased by LPS in macrophages (11). We also identified mPGES from the screening of an LPS-stimulated macrophage cDNA library. LPS-stimulated expression of mPGES was not observed in TLR4-deficient macrophages. TLR4 triggers intracytoplasmic signaling by recruiting the adaptor molecule MyD88 to the membrane (13, 14). Recent studies indicated the existence of MyD88-dependent and -independent pathways in TLR4-mediated signaling (22, 23). LPS-induced Cox-2 expression has been shown to be mediated by a MyD88-dependent pathway (23). Similarly, LPS induction of mPGES completely depends on MyD88. Furthermore, LPS-induced mPGES expression was abolished in NF-IL6-deficient macrophages. NF-IL6 is a member of the C/EBP family of transcription factors involved in the expression of various genes induced by inflammatory stimuli (24). We tried to identify NF-IL6 binding sites in the promoter region of the mPGES gene, but could not find a cis-acting element responsible for LPS-induced transcriptional activation within the 2.5-kb fragment upstream of the transcription start site (data not shown). This may indicate that NF-IL6 regulates mPGES gene expression via the cis-acting element present outside the proximal promoter.

LPS-induced Cox-2 expression was observed with delayed kinetics in NF-IL6-deficient macrophages. Transcriptional activation of the Cox-2 gene induced by LPS has been shown to partly depend on NF-IL6 in macrophages. Indeed, NF-IL6 binding sites have been identified in the promoter region of the Cox-2 gene (4, 25, 26, 27, 28). A recent study showed that Cox-2 mRNA induction and promoter activity were defective in NF-IL6-deficient macrophages, and the defect could be rescued by the expression of NF-IL6 (26). Furthermore, it has been demonstrated that Cox-2 mRNA induction is biphasic. The initial phase of Cox-2 induction depends on NF-IL6, and the second phase requires coordination of NF-IL6 and NF-IL6{beta} in activated macrophages (29). NF-{kappa}B and cAMP response element CRE have also been shown to regulate the expression of the Cox-2 gene (25, 26, 27, 28). From these findings we speculate that the reduced expression of Cox-2 during the early time period is due to the absence of NF-IL6, but enhanced Cox-2 expression in the late period may be due to compensation by other transcription factors. Although Cox-2 protein was significantly induced at the late time period of LPS stimulation, PGE2 production was severely reduced in NF-IL6-deficient macrophages. Thus, NF-IL6 is critically involved in LPS-induced production of PGE2 through modulating the expression of mPGES. In contrast, Cox-1 and cPGES/p23 induction was not altered in NF-IL6-deficient macrophages. After LPS stimulation, down-regulation of Cox-1 mRNA and up-regulation of cPGES/p23 mRNA were observed. However, the protein level of Cox-1 was not changed after LPS stimulation. At present, it remains unknown how LPS stimulation down-regulates the expression of Cox-1 mRNA.

Generation of mPGES-mutant mice revealed an essential role for mPGES in the LPS-induced production of PGE2. A recent study showed that the kinetic change in PGE2 production correlated well with that in mPGES mRNA and protein expression and that dexamethasone reduced both PGE2 synthesis and mPGES expression in LPS-stimulated macrophages. This implies that the PGE2 production activity detected in LPS-stimulated macrophages is largely dependent on mPGES. PGE2 production in response to LPS was completely abolished in mPGES-mutant mice, demonstrating that mPGES is essential for LPS-induced biosynthesis of PGE2. Although LPS-induced PGE2 production was abolished in mPGES-mutant mice, the basal PGE2 level in mPGES-mutant mice was comparable to that in wild-type mice. This indicates that production of PGE2 at the basal level occurs independent of mPGES and mainly depends on Cox-1 and cPGES/p23, which are constitutively expressed. Indeed, mice lacking Cox-1 showed reduced PGE2 levels in the stomach (30). Thus, the present study also established that the Cox-2/mPGES pathway is indispensable for PGE2 production induced by proinflammatory stimuli, whereas Cox-1/cPGES is required for the basal PGE2 production responsible for maintenance of homeostasis.

There are several reports that PGE2 contributes to immune suppression and that PGE2 secreted from activated macrophages in response to proinflammatory stimuli acts on the macrophages themselves and exhibits an inhibitory function in a negative feedback loop (21). Addition of exogenous PGE2 has been shown to reduce LPS-induced IL-6 and TNF-{alpha} production, but not IL-1{beta} production, in macrophages (31). Another study showed that PGE2 induces the production of IL-6 (32). Thus, it remains unclear how PGE2 regulates the production of inflammatory cytokines. Membrane-associated PGES-mutant mice showed no LPS-induced elevation of PGE2, providing a good model to analyze the involvement of PGE2 in the production of inflammatory cytokines. The LPS-induced production of TNF-{alpha} and IL-12 was not altered at various time points, even 24 h after LPS injection in mPGES-mutant mice, indicating that endogenous PGE2 expressed in response to LPS is not essential for the regulation of inflammatory cytokine production.

To date, four PGE2 receptors, designated EP1, EP2, EP3, and EP4, have been identified, and their physiological roles demonstrated (1). Mice deficient in EP4 show an increased incidence of patent ductus arteriosus with high neonatal mortality. EP2-deficient female mice consistently deliver fewer pups than their wild-type counterparts due to slightly impaired ovulation and a marked reduction in fertilization. EP3-deficient mice show an impaired febrile response (1). However, mPGES-deficient mice do not show these abnormalities. This may be due to the basally produced PGE2, probably through the Cox-1/cPGES pathway.

In this study we have demonstrated that LPS-induced mPGES expression is essential for LPS-dependent PGE2 production, but not for inflammatory cytokine production. It should be analyzed whether cPGES/p23 is involved in the basal PGE2 production. The generation of mice lacking both mPGES and cPGES should provide new insight into the role of PGE2.


    Acknowledgments
 
We thank E. Horita for excellent secretarial assistance, N. Okita and N. Iwami for technical assistance, and all members of our laboratory for their helpful advice during the preparation of this manuscript.


    Footnotes
 
1 This work was supported by Special Coordination of Funds for Promoting Science and Technology from the Ministry of Education, Culture, Sports, Science and Technology of the Japanese Government. Back

2 Address correspondence and reprint requests to Dr. Shizuo Akira, Department of Host Defense, Research Institute for Microbial Diseases, Osaka University, 3-1 Yamada-oka, Suita, Osaka 565-0871, Japan. E-mail address: sakira{at}biken.osaka-u.ac.jp Back

3 Abbreviations used in this paper: PLA2, phospholipase A2; PGES, PGE2 synthase; PGHS, PGH synthase; Cox, cyclooxygenase; cPGES, cytosolic PGES; ES, embryonic stem; MAPEG, membrane-associated proteins involved in eicosanoid and glutathione metabolism; MGST1-L1, microsomal glutathione-S-transferase-1-like-1; mPGES, membrane-bound PGES; TLR, Toll-like receptor. Back

Received for publication December 27, 2001. Accepted for publication April 2, 2002.


    References
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 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 

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