EP2 and EP4 prostanoid receptor signaling

Polymorphisms of prostaglandin-endoperoxide synthase 2 gene, and prostaglandin-E receptor 2 gene, C-reactive protein concentrations and risk of atherothrombosis: a nested case-control approach

OBJECTIVE

Recent data have shown an association between polymorphisms of prostaglandin-endoperoxide synthase-2 gene (PTGS2; alias COX-2), and prostaglandin-E receptor-2 gene (PTGER2) and risk of atherothrombotic disorders.

METHODS

We evaluated two PTGS2 (rs20417, rs689470), and three PTGER2 (rs708494/uS5, rs708495/uS7, and chr14: 50 764 013/uS10) gene polymorphisms among 600 Caucasian male participants of the Physicians' Health Study with incident myocardial infarction (MI) or ischemic stroke and 600 age- and smoking-matched controls who remained free of all reported cardiovascular disease.

RESULTS

Genotype distributions were in Hardy-Weinberg equilibrium in the control groups. Genotype and allele distribution were similar between cases and controls. The polymorphisms tested were in linkage disequilibrium. Results from the adjusted haplotype-based conditional logistic regression analysis showed a modest association of the PTGER2 2-1-1 haplotype with reduced risk of MI (odds ratio = 0.50, 95% CI; CI = 0.26-0.97, P = 0.04), and the 2-2-1 haplotype with reduced risk of ischemic stroke (odds ratio = 0.68, 95% CI = 0.47-0.99, P = 0.048). In contrast to prior data, we found no evidence for an association of the PTGS2 polymorphisms/haplotypes tested with risk of incident MI nor with ischemic stroke. However, we found suggestive evidence for an association of specific PTGER2 haplotypes with reduced risk of these outcomes.

CONCLUSION

Although these prospective data implicate the potential involvement of prostaglandin-E receptor-2 gene variation in atherothrombosis, external validation of our findings is needed.

Interaction between prostaglandin E2 and l-cis-diltiazem, a specific blocker of cyclic nucleotide gated channels in bovine aortic endothelial cells

Prostaglandins are known to transduce their signals via 7 transmembrane prostanoid receptors, which typically signal through coupling to G proteins and downstream second messenger molecules and protein kinase activation. Recently we have shown that cyclic nucleotides affect prostaglandins binding to bovine aortic endothelial cells independent of protein kinases. Here we show that incubation of bovine aortic endothelial cells with permeable analogs of cAMP or cGMP leads to a rapid and reversible reduction in PGE(2) binding to the cells. Since cyclic nucleotides are known modulators of cyclic nucleotide gated channels, we examined the effect of a specific cyclic nucleotide gated channel blocker l-cis-diltiazem on prostaglandin E(2) (PGE(2)) binding to bovine aortic endothelial cells. L-cis-diltiazem is shown to displace PGE(2) binding to bovine aortic endothelial cells in a dose dependent manner. In addition the effect of PGE(2) and l-cis-diltiazem on thapsigargin induced calcium elevation in the cells was compared. Both agents reduced in bovine aortic endothelial cells the thapsigargin induced calcium elevation by about half. PGE(2) also retarded the time course of the response to thapsigargin. Simultaneous treatment of the cells with both PGE(2) and l-cis-diltiazem did not yield an inhibitory effect beyond that observed with l-cis-diltiazem alone. Together our data point at the cyclic nucleotide gated channels as a feasible candidate for association with the PGE(2) binding site in bovine aortic endothelial cells.

Characteristics of acid extrusion from Chinese hamster ovary cells expressing different prostaglandin EP receptors

Acid extrusion responses to prostaglandin E2 were investigated in Chinese hamster ovary (CHO) cells heterologously expressing human EP1, EP2, and EP3I receptors (hEP1, hEP2 and hEP3I) by using a microphysiometer that detected small pH changes in the extracellular microenvironment. In the cells expressing hEP1, which is known to increase intracellular Ca2+, prostaglandin E2 (1 and 10 nM) slowly accelerated acid extrusion, but at higher concentrations an initial transient phase (approximately 5 times greater than the basal acidification) overlapped the slowly developing phase. In contrast, the cells expressing hEP2, which evokes cAMP production, showed dual responses to prostaglandin E2: an initial reduction followed by an acceleration of acid extrusion. In the cells expressing hEP3I, which is known to produce both a decrease in cAMP and a modest increase in intracellular Ca2+, acid extrusion was gradually accelerated by prostaglandin E2 and reached a plateau at around 2 min. Elimination of extracellular Ca2+ diminished the responses to prostaglandin E2 in hEP1 cells, but had little effect on the responses in hEP2 and hEP3I cells. Forskolin mimicked the dual effects of prostaglandin E2 observed in the hEP2 cells. Pretreatment with pertussis toxin inhibited the response to prostaglandin E2 in hEP3I cells, but the responses in hEP1 and hEP2 cells were not affected. Na+/H+ exchanger (NHE) inhibitors (EIPA and HOE642) suppressed all the responses induced by prostaglandin E2 in hEP1, hEP2, and hEP3I cells. These results suggest that EP receptor subtypes regulate acid extrusion mainly via NHE-1 through distinct signal transduction pathways in CHO cells.

Paracrine and autocrine regulation of epidermal growth factor-like factors in cumulus oocyte complexes and granulosa cells: key roles for prostaglandin synthase 2 and progesterone receptor

The molecular bridges that link the LH surge with functional changes in cumulus cells that possess few LH receptors are being unraveled. Herein we document that epidermal growth factor (EGF)-like factors amphiregulin (Areg), epiregulin (Ereg), and betacellulin (Btc) are induced in cumulus oocyte complexes (COCs) by autocrine and paracrine mechanisms that involve the actions of prostaglandins (PGs) and progesterone receptor (PGR). Areg and Ereg mRNA and protein levels were reduced significantly in COCs and ovaries collected from prostaglandin synthase 2 (Ptgs2) null mice and Pgr null (PRKO) mice at 4 h and 8 h after human chorionic gonadotropin, respectively. In cultured COCs, FSH/forskolin induced Areg mRNA within 0.5 h that peaked at 4 h, a process blocked by inhibitors of p38MAPK (SB203580), MAPK kinase (MEK) 1 (PD98059), and PTGS2 (NS398) but not protein kinase A (PKA) (KT5720). Conversely, AREG but not FSH induced Ptsg2 mRNA at 0.5 h with peak expression of Ptgs2 and Areg mRNAs at 4 h, processes blocked by the EGF receptor tyrosine kinase inhibitor AG1478 (AG), PD98059, and NS398. PGE2 reversed the inhibitory effects of AG on AREG-induced expression of Areg but not Ptgs2, placing Ptgs2 downstream of EGF-R signaling. Phorbol 12-myristate 13-acetate (PMA) and adenovirally expressed PGRA synergistically induced Areg mRNA in granulosa cells. In COCs, AREG not only induced genes that impact matrix formation but also genes involved in steroidogenesis (StAR, Cyp11a1) and immune cell-like functions (Pdcd1, Runx1, Cd52). Collectively, FSH-mediated induction of Areg mRNA via p38MAPK precedes AREG induction of Ptgs2 mRNA via ERK1/2. PGs acting via PTGER2 in cumulus cells provide a secondary, autocrine pathway to regulate expression of Areg in COCs showing critical functional links between G protein-coupled receptor and growth factor receptor pathways in ovulating follicles.

Microsomal prostaglandin E2 synthase-1 in breast cancer: a potential target for therapy

The anti-tumour actions of cyclooxygenases (COX) are thought to be mediated by inhibition of prostaglandin E(2) (PGE(2)) synthesis. However, COX-2 inhibition also alters cellular production of other prostaglandins such as prostacyclin (PGI(2)). The latter action is believed to be important for the development of adverse cardio-vascular events. Microsomal PGES (mPGES-1) is an enzyme downstream to COX-2 and affects PGE(2) production only. It is possible that targeting mPGES-1 could decrease PGE(2) production without affecting PGI(2) production. In order to assess the potential of mPGES-1 as a target for therapy, we analysed its expression in breast cell lines and normal and malignant breast tissues. The expression of mPGES-1 and COX-2 was correlated in tumour cells and vascular endothelium, and with prognostic parameters in breast cancer. Although not detectable in normal epithelial cells, expression was noted in areas of fibrocystic change and in situ carcinoma. mPGES-1 expression was noted in 79% of breast cancer tissues. Its expression did not correlate with COX-2 overexpression or with prognostic markers of breast cancer. Endothelial cells did not show mPGES-1 expression. Upregulation of mPGES-1 is therefore frequent in pre-malignant and malignant breast disease. In this study, coordinate over-expression of COX-2 and mPGES-1 was not observed, particularly in the endothelial cells of blood vessels. Targeting mPGES-1 might prove to be an alternative therapeutic strategy to inhibit PGE2 production.

Co-localization of COX-2, CYP4F8, and mPGES-1 in epidermis with prominent expression of CYP4F8 mRNA in psoriatic lesions

Cyclooxygenase-2 (COX-2), cytochrome P450 4F8 (CYP4F8), and microsomal PGE synthase-1 (mPGES-1) form PGE and 19-hydroxy-PGE in human seminal vesicles. We have examined COX-2, CYP4F8, and mPGES-1 in normal skin and in psoriasis. All three enzymes were detected in epidermis by immunofluorescence and co-localized in the suprabasal cell layers. In lesional psoriasis the enzymes were also co-localized in the basal cell layers. Real-time RT-PCR analysis suggested that CYP4F8 mRNA was induced 15-fold in lesional compared to non-lesional epidermis. mRNA of all enzymes were present in cultured HEK and HaCaT cells, but the prominent induction of CYP4F8 mRNA in psoriasis could not be mimicked by treatment of these keratinocytes with a mixture of inflammatory cytokines or with phorbol 12-myristate-13-acetate. The function of CYP4F8 in epidermis might be related to lipid oxidation and keratinocyte proliferation.

COX-2-derived prostacyclin protects against bleomycin-induced pulmonary fibrosis

Prostacyclin is one of a number of lipid mediators elaborated from the metabolism of arachidonic acid by the cyclooxygenase (COX) enzymes. This prostanoid is a potent inhibitor of platelet aggregation, and its production by endothelial cells and protective role in the vasculature are well established. In contrast, much less is known regarding the function of this prostanoid in other disease processes. We show here that COX-2-dependent production of prostacyclin plays an important role in the development of fibrotic lung disease, limiting both the development of fibrosis and the consequential alterations in lung mechanics. In stark contrast, loss of prostaglandin E(2) synthesis and signaling through the G(s)-coupled EP2 and EP4 receptors had no effect on the development of disease. These findings suggest that prostacyclin analogs will protect against bleomycin-induced pulmonary fibrosis in COX-2(-/-) mice. If such protection is observed, investigation of these agents as a novel therapeutic approach to pulmonary fibrosis in humans may be warranted.

Anti-inflammatory and relaxatory effects of prostaglandin E2 in myometrial smooth muscle

The onset of human labour is complex and involves multiple mediators, prostaglandins, cytokines and chemokines. However, whilst prostaglandins are routinely used for labour induction and inhibitors of prostaglandin synthesis are used to prevent pre-term labour, these practices are not invariably successful, and the rationale for their use is equivocal. As COX-2 and prostaglandin E(2) (PGE(2)) production is increased towards term, we have investigated the effect of PGE(2) and other cAMP-elevating agents on events associated with labour induction. Time-dependent increases in granulocyte-macrophage colony-stimulating factor (GM-CSF) and interleukin-8 (IL-8) release were observed following treatment of primary human myometrial smooth muscle (HMSM) cells with IL-1beta, via mechanisms that required de novo transcription and translation. Prior treatment with PGE(2) (1 microM) produced 86 and 80% decreases in GM-CSF and IL-8 release, respectively. Similarly, the cAMP analogue, 8-bromo-cAMP (8Br-cAMP) and the phosphodiesterase-4 (PDE(4)) inhibitor, rolipram, also repressed GM-CSF and IL-8 release. In addition, PGE(2), 8Br-cAMP, rolipram and salbutamol all had a dose-dependent inhibitory effect on spontaneous myometrial contractions in vitro. In this study, PGE(2) reduced the release of factors associated with cervical ripening and attenuated force development in myometrial smooth muscle, raising the possibility that in myometrium, PGE(2) may act to down-regulate some of the processes that contribute to the onset of human labour and may be beneficial in helping to maintain pregnancy towards term.

Effects of prostagladin E(2) on gel-forming mucin secretion in normal human nasal epithelial cells

CONCLUSION

The findings of this study indicate that prostaglandin E(2) (PGE(2)) induces MUC5AC gene expression and mucin secretion via the EP4 receptor in cultured normal human nasal epithelial cells.

OBJECTIVES

Recently, PGE(2) was found to induce MUC5AC production via an agonist of EP2/EP4, but not EP1/EP3, in normal human airway epithelium. However, the receptor that mediates MUC5AC has not been determined. This study aimed to investigate the MUC5AC mucin gene and mucin secretion by PGE(2) and its receptors in cultured normal human nasal epithelial cells.

METHODS

After treatment with PGE(2) and/or PGE(2) antagonist, gel-forming mucin mRNA expression was determined by reverse transcription-polymerase chain reaction. Total mucin and MUC5AC mucin levels were measured using an immuno-dot blotting assay.

RESULTS

PGE(2) increased only MUC5AC gene expression and MUC5AC mucin, but not expression of other gel-forming mucin genes. An EP2 receptor antagonist (AH 6809) did not suppress the PGE(2)-induced MUC5AC gene expression or MUC5AC mucin. However, an EP4 receptor antagonist (AH23848) significantly suppressed the level of PGE(2)-induced MUC5AC gene expression and MUC5AC mucin.

Involvement of EP1 and EP2 receptors in the regulation of the Na,K-ATPase by prostaglandins in MDCK cells

Prostaglandins are key regulators of ion transport in the kidney. In MDCK cells, which model distal tubule cells, the transcription of the Na,K-ATPase beta1 subunit is regulated by PGE1 and PGE2. To identify the EP receptors that mediate transcriptional regulation, transient transfection studies are conducted using the human beta1promoter/luciferase construct, pHbeta1-1141 Luc. The involvement of EP1 and EP2 receptors is indicated by studies with the EP1 selective agonist 17-phenyl trinor PGE2, and the EP2 selective agonist butaprost (which stimulate), as well as by studies with the antagonists SC-51089 (EP1 specific) and AH 6809 (EP1 and EP2 specific). Consistent with the involvement of Gs coupled EP2 receptors, is that the PGE1 stimulation is inhibited by the PKAI expression vector (encoding the protein kinase A (PKA) inhibitory protein), as well as by the myristolated PKA inhibitory peptide PKI. In addition to this evidence (for the involvement of EP2 receptors), evidence for the involvement of EP1 receptors in the PGE1 mediated stimulation of Na,K-ATPase beta subunit gene transcription includes the stimulatory effect of 17-phenyl trinor PGE2, as well as the inhibitory effects of SC-51089. Also consistent with the involvement of Gq coupled EP1 receptors, the PGE1 stimulation is inhibited by the PKCI vector (encoding the PKC inhibitory domain), the PKC inhibitor Go 6976, thapsigargin, as well as the calmodulin antagonists W7 and W13.

Role of beta-arrestin 1 in the metastatic progression of colorectal cancer

G protein-coupled receptor ligand-dependent transactivation of growth factor receptors has been implicated in human cancer cell proliferation, migration, and cell survival. For example, prostaglandin E(2) (PGE(2))-induced transactivation of the EGF receptor (EGFR) in colorectal carcinoma cells is mediated by means of a c-Src-dependent mechanism and regulates cell proliferation and migration. Recent evidence indicates that beta-arrestin 1 may act as an important mediator in G protein-coupled receptor-induced activation of c-Src. Whether beta-arrestin 1 serves a functional role in these events is, however, unknown. We investigated the effects of PGE(2) on colorectal cancer cells expressing WT and mutant beta-arrestin 1. Here we report that PGE(2) induces the association of a prostaglandin E receptor 4/beta-arrestin 1/c-Src signaling complex resulting in the transactivation of the EGFR and downstream Akt (PKB) signaling. The interaction of beta-arrestin 1 and c-Src is critical for the regulation of colorectal carcinoma cell migration in vitro as well as metastatic spread of disease to the liver in vivo. These results show that the prostaglandin E/beta-arrestin 1/c-Src signaling complex is a crucial step in PGE(2)-mediated transactivation of the EGFR and may play a pivotal role in tumor metastasis. Furthermore, our data implicate a functional role for beta-arrestin 1 as a mediator of cellular migration and metastasis.

PGE2 stimulates human brain natriuretic peptide expression via EP4 and p42/44 MAPK

Brain natriuretic peptide (BNP) produced by cardiac myocytes has antifibrotic and antigrowth properties and is a marker of cardiac hypertrophy. We previously showed that prostaglandin E2 (PGE2) is the main prostaglandin produced in myocytes treated with proinflammatory stimuli and stimulates protein synthesis by binding to its EP4 receptor. We hypothesized that PGE2, acting through EP4, also regulates BNP gene expression. We transfected neonatal ventricular myocytes with a plasmid encoding the human BNP (hBNP) promoter driving expression of a luciferase reporter gene. PGE2 increased hBNP promoter activity 3.5-fold. An EP4 antagonist reduced the stimulatory effect of PGE2 but not an EP1 antagonist. Because EP4 signaling can involve adenylate cyclase, cAMP, and protein kinase A (PKA), we tested the effect of H-89, a PKA inhibitor, on PGE2 stimulation of the hBNP promoter. H-89 at 5 muM decreased PGE2 stimulation of BNP promoter activity by 100%. Because p42/44 MAPK mediates the effect of PGE2 on protein synthesis, we also examined the role of MAPKs in the regulation of BNP promoter activity. PGE2 stimulation of the hBNP promoter was inhibited by a MEK1/2 inhibitor and a dominant-negative mutant of Raf, indicating that p42/44 MAPK was involved. In contrast, neither a p38 MAPK inhibitor nor a JNK inhibitor reduced the stimulatory effect of PGE2. Involvement of small GTPases was also studied. Dominant-negative Rap inhibited PGE2 stimulation of the hBNP promoter, but dominant-negative Ras did not. We concluded that PGE2 stimulates the BNP promoter mainly via EP4, PKA, Rap, and p42/44 MAPK.

A novel prostaglandin E receptor 4-associated protein participates in antiinflammatory signaling

Prostaglandin E2 exerts an antiinflammatory action by ligation of the heptahelical receptor EP4 in human macrophages. Because the mechanism by which EP4 receptor stimulation suppresses inflammatory activation in macrophages remains undefined, we sought interactors with the carboxyl-terminal cytoplasmic domain of the EP4 receptor. Yeast 2-hybrid screening of the human bone marrow cDNA library with the EP4 receptor as a bait identified a cDNA clone encoding a 669-amino acid protein, designated here as EP4 receptor-associated protein (EPRAP), which contains 8 ankyrin motifs that might recruit other signaling molecules. EPRAP bound to the full-length EP4 receptor in HEK293 cells cotransfected with V5-tagged EPRAP and FLAG-tagged EP4 receptor cDNA, as anti-FLAG antibody coimmunoprecipitated EPRAP with the EP4 receptor from the lysates of cotransfected cells. Human macrophages derived from peripheral blood monocytes expressed an approximately 70-kDa protein detected by Western blotting with a polyclonal anti-EPRAP antibody. Fluorescence immunohistochemistry colocalized EPRAP with the EP4 receptor in human atheromata. Interference with EPRAP function by small interference RNA limited prostaglandin E2-mediated suppression of chemokine expression in macrophages activated with lipopolysaccharide and tumor necrosis factor alpha. In conclusion, the antiinflammatory action of prostaglandin E2 in macrophages involves EPRAP that associates directly with the cytoplasmic tail of EP4 receptor.

Lack of expression of the EP2 but not EP3 receptor for prostaglandin E2 results in suppression of skin tumor development

The EP2 receptor for prostaglandin E2 (PGE2) is a membrane receptor that mediates at least part of the action of PGE2. It has been shown that EP2 plays a critical role in tumorigenesis in mouse mammary gland and colon. However, the possibility that the EP2 receptor is involved in the development of skin tumors was unknown. The purpose of this study was to investigate the role of the EP2 receptor in mouse skin carcinogenesis. Unlike EP3 knockout mice, the EP2 knockout mice produced significantly fewer tumors and reduced tumor incidence compared with wild type (WT) mice in a 7,12-dimethylbenz(a)anthracene (DMBA)/12-O-tetradecanoylphorbol-13-acetate (TPA) two-stage carcinogenesis protocol. EP2 knockout mice had significantly reduced cellular proliferation of mouse skin keratinocytes in vivo and in vitro compared with that in WT mice. In addition, the epidermis of EP2 knockout mice 48 hours after topical TPA treatment was significantly thinner compared with that of WT mice. The inflammatory response to TPA was reduced in EP2 knockout mice, based on a reduced number of macrophages in the dermis and a reduced level of interleukin-1alpha mRNA expression, compared with WT mice. EP2 knockout mice also had significantly reduced epidermal cyclic AMP levels after PGE2 treatment compared with WT mice. Tumors from WT mice produced more blood vessels and fewer apoptotic cells than those of EP2 knockout mice as determined by immunohistochemical staining. Our data suggest that the EP2 receptor plays a significant role in the protumorigenic action of PGE2 in skin tumor development.

Targeting prostaglandin E2 receptors as an alternative strategy to block cyclooxygenase-2-dependent extracellular matrix-induced matrix metalloproteinase-9 expression by macrophages

COX-2-dependent prostaglandin (PG) E2 synthesis regulates macrophage MMP expression, which is thought to destabilize atherosclerotic plaques. However, the administration of selective COX-2 inhibitors paradoxically increases the frequency of adverse cardiovascular events potentially through the loss of anti-inflammatory prostanoids and/or disturbance in the balance of pro- and anti-thrombotic prostanoids. To avoid these collateral effects of COX-2 inhibition, a strategy to identify and block specific prostanoid-receptor interactions may be required. We previously reported that macrophage engagement of vascular extracellular matrix (ECM) triggers proteinase expression through a MAPKerk1/2-dependent increase in COX-2 expression and PGE2 synthesis. Here we demonstrate that elicited macrophages express the PGE2 receptors EP1-4. When plated on ECM, their expression of EP2 and EP4, receptors linked to PGE2-induced activation of adenylyl cyclase, is strongly stimulated. Forskolin and dibutryl cyclic-AMP stimulate macrophage matrix metalloproteinase (MMP)-9 expression in a dose-dependent manner. However, an EP2 agonist (butaprost) has no effect on MMP-9 expression, and macrophages from EP2 null mice exhibited enhanced COX-2 and MMP-9 expression when plated on ECM. In contrast, the EP4 agonist (PGE1-OH) stimulated macrophage MMP-9 expression, which was inhibited by the EP4 antagonist ONO-AE3-208. When compared with COX-2 silencing by small interfering RNA or inhibition by celecoxib, the EP4 antagonist was as effective in inhibiting ECM-induced proteinase expression. In addition, ECM-induced MMP-9 expression was blocked in macrophages in which EP4 was silenced by small interfering RNA. Thus, COX-2-dependent ECM-induced proteinase expression is effectively blocked by selective inhibition of EP4, a member of the PGE2 family of receptors.

Cutting edge: proangiogenic properties of alternatively activated dendritic cells

Angiogenesis plays an important role in tissue remodeling and repair during the late phase of inflammation. In the present study, we show that human dendritic cells (DC) that matured in the presence of anti-inflammatory molecules such as calcitriol, PGE2, or IL-10 (alternatively activated DC) selectively secrete the potent angiogenic cytokine vascular endothelial growth factor (VEGF) isoforms VEGF165 and VEGF121. No VEGF production was observed in immature or classically activated DC. Also, the capacity to produce VEGF was restricted to the myeloid DC subset. When implanted in the chick embryo chorioallantoic membrane, alternatively activated DC elicit a marked angiogenic response, which is inhibited by neutralizing anti-VEGF Abs and by the VEGFR-2 inhibitor SU5416. Therefore, alternatively activated DC may contribute to the resolution of the inflammatory reaction by promoting VEGF-induced angiogenesis.

EP(4) prostanoid receptor coupling to a pertussis toxin-sensitive inhibitory G protein

The EP(2) and EP(4) prostanoid receptor subtypes are G-protein-coupled receptors for prostaglandin E(2) (PGE(2)). Both receptor subtypes are known to couple to the stimulatory guanine nucleotide binding protein (Galpha(s)) and, after stimulation with PGE(2), can increase the formation of intracellular cAMP. In addition, PGE(2) stimulation of the EP(4) receptor can activate phosphatidylinositol 3-kinase (PI3K) leading to phosphorylation of the extracellular signal-regulated kinases (ERKs) and induction of early growth response factor-1 (EGR-1). We now report that the PGE(2)-mediated phosphorylation of the ERKs and induction of EGR-1 can be blocked by pretreatment of EP(4)-expressing cells with pertussis toxin (PTX). Furthermore, pretreatment with PTX increased the amount of PGE(2)-stimulated intracellular cAMP formation in EP(4)-expressing cells but not in EP(2)-expressing cells. These data indicate that the EP(4) prostanoid receptor subtype, but not the EP(2), couples to a PTX-sensitive inhibitory G-protein (Galpha(i)) that can inhibit cAMP-dependent signaling and activate PI3K/ERK-dependent signaling.

Overexpression of COX-2, Prostaglandin E synthase-1 and prostaglandin E receptors in blood mononuclear cells and plaque of patients with carotid atherosclerosis: regulation by nuclear factor-kappaB

BACKGROUND AND OBJECTIVE

Prostaglandin E2 (PGE(2), a product of the cyclooxygenase 2 (COX-2) and membrane-associated Prostaglandin E Synthase (mPGES-1) pathway, has been implicated in the instability of atherosclerotic plaques. We have studied COX-2, mPGES-1 and PGE2 receptors (EPs) expression in peripheral blood mononuclear cells (PBMC) and atherosclerotic plaques of 29 patients with carotid stenosis as well as the effect of different nuclear factor-kappaB (NF-kappaB) inhibitors on COX-2, mPGES-1 and EPs expression in cultured monocytic cells (THP-1).

METHODS

COX-2, mPGES-1 and EP expression was analyzed by RT-PCR (PBMC), immunohistochemistry (plaques) and Western blot (THP-1). PGE2 levels were determined by ELISA (plasma and cell supernatants).

RESULTS

In relation to healthy controls, COX-2, mPGES-1 and EP-3/EP-4 mRNA expression was increased in PBMC from patients. In the inflammatory region of atherosclerotic plaques, an increase of COX-2, mPGES-1 and EPs expression was also observed. Activated NF-kappaB and COX-2, mPGES-1 and EPs proteins were colocalized in the plaque's cells. In cytokine-treated cultured THP-1, the NF-kappaB inhibitors parthenolide, Bay 11-7082 and PDTC reduced COX-2, mPGES-1 and EP-1/EP-3/EP-4 expression as well as PGE2 levels. By employing specific agonists and antagonists, we noted that the cytokine- and PGE2-induced metalloproteinase 9 (MMP-9) expression and activity occurs through EP-1/EP-3/EP-4, an effect downregulated by NF-kappaB inhibitors.

CONCLUSIONS

Patients with carotid atherosclerosis depict an overexpression of COX-2, mPGES-1 and EPs simultaneously in the PBMC as well as in the vulnerable region of plaques. The studies in cultured monocytic cells suggest that NF-kappaB inhibitors and/or EPs antagonists could represent a novel therapeutic approach to the treatment of plaque instability and rupture.

Duodenal secretion in humans mediated by the EP4 receptor subtype

AIM

Assessment of functional EP receptor subtypes involved in PGE2-induced secretion in human duodenum. The spectrum of activities by PGE2 in mammals, including cytoprotective bicarbonate secretion in duodenum, is mediated through four G protein-coupled receptor subtypes (EP1-EP4).

METHODS

Biopsies from the second part of duodenum from patients undergoing endoscopy were mounted in modified Ussing chambers. Basal and stimulated short circuit current (SCC) and slope conductance (SG) were measured. Dose-response relations for PGE2 and subtype receptors EP1/EP3 (sulprostone), EP2 (butaprost), and EP4 (1-OH PGE1) were assessed by cumulated doses of single agonists.

RESULTS

PGE2 caused a dose-dependent increase in SCC, maximum 101 +/- 20 microA cm(-2) with an EC50 of 35.6 +/- 5.8 nm (n = 3). Likewise 1-OH PGE1 caused a dose-dependent SCC increase, maximum 63.3 +/- 28.6 microA cm(-2) with an EC50 of 56.7 +/- 7.2 nm (n = 3). 1-OH PGE1 at 500 nm increased SCC by 18.0 +/- 3.0 microA cm(-2) (n = 10) and SG by 2.9 +/- 0.4 mS cm(-2) (n = 6). Sulprostone (n = 6) and butaprost (n = 6) had no effects on SCC or SG. SCC was inhibited 31.4 +/- 13.2% (n = 10) by bumetanide (25 microM serosa) and 18.6 +/- 5.8% (n = 10) by acetazolamide (250 microM lumen). Diphenylamine-2-carboxylate (DPC) (250 microM mucosa) and SITS (10 microM mucosa) had almost no effect.

CONCLUSIONS

Effects of PGE2 on secretion in the second part of human duodenum is mediated through the EP4 receptor and not through EP1, EP2, or EP3.

Prostaglandin E2 induces the expression of functional inhibitory CD94/NKG2A receptors in human CD8+ T lymphocytes by a cAMP-dependent protein kinase A type I pathway

The CD94/NKG2A heterodimer is a natural killer receptor (NKR), which inhibits cell-mediated cytotoxicity upon interaction with MHC class I gene products. It is expressed by NK cells and by a small fraction of activated T cells, predominantly of CD8+ phenotype. Abnormal upregulation of the CD94/NKG2A inhibitory NKR on cytotoxic T cells (CTLs) could be responsible for a failure of immunosurveillance in cancer or HIV infection. In an attempt to identify the mechanisms leading to inhibitory NKR upregulation on T cells, we analyzed the expression of the CD94/NKG2A heterodimer on human CTLs activated with anti-CD3 mAb in the presence of PGE2 or with 8-CPT-cAMP, an analogue of cyclic AMP. As previously described, anti-CD3 mAb-mediated activation induced the expression of CD94/NKG2A on a small fraction of CD8+ T cells. Interestingly, when low concentrations of PGE2 or 8-CPT-cAMP were present during the culture, the proportion of CD8+ T cells expressing CD94/NKG2A was two- to five-fold higher. This upregulation was partially prevented by PKA inhibitors, such as KT5720 and Rp-8-Br-cAMP (type I selective). We also report that cAMP induces upregulation of NKG2A at the mRNA level. We further demonstrated that cross-linking of CD94 on CD8+ T cells expressing the CD94/NKG2A heterodimer inhibits their cytotoxic activity in a bispecific antibody redirected lysis assay. Our findings clearly demonstrate that the PGE2/cAMP/PKA type I axis is involved in the expression of CD94/NKG2A receptor on human CD8+ T lymphocytes.

A short guide to the nomenclature of seven-transmembrane spanning receptors for lipid mediators

The International Union of Pharmacology (IUPHAR) has established a Nomenclature Committee comprised of sub-committees of experts to evaluate types and subtypes of receptors and ion channels in an effort to establish universally accepted nomenclature [Vanhoutte, P.M., Barnard, E.A., Cosmides, G.J., Humphrey, P.P., Spedding, M., Godfraind, T., 1994. International Union of Pharmacology Committee on Receptor Nomenclature and Drug Classification. Pharmacological Reviews 46, 111-116]. This overview cites the reports of the IUPHAR subcommittees and other prominent review articles in an effort to compile receptors for lipid mediators that bind to and evoke their pharmacological responses via seven-transmembrane spanning, G-protein-coupled receptors.

Prostanoids Secreted by Alveolar Macrophages Enhance Ionic Currents in Swine Tracheal Submucosal Gland Cells

We examined the effect of substances released by swine alveolar macrophages (AMs) on ionic currents in airway submucosal gland cells (SGCs). AMs obtained by lavage were activated by 24-h zymosan exposure (0.1 mg/ml). Supernatant was collected and used to stimulate short-circuit current changes (DeltaI(SC)) in SGC monolayers in Ussing chambers. Dexamethasone (1 microM) or indomethacin (5 muM) during zymosan exposure of AMs reduced or abolished the supernatant-induced DeltaI(SC). Zymosan exposure induced a 5-fold increase in cyclooxygenase (COX)-2 but not COX-1 protein levels in AMs. Prostaglandin E(2) (PGE(2)) concentration in the supernatant from zymosan-activated AMs was 550 +/- 10 nM (n = 3) compared with 28 +/- 3 nM for unstimulated AMs (n = 3). PGE(2), applied serosally, induced DeltaI(SC) with an EC(50) of 15.5 +/- 1.3 nM (n = 4) and 3.6 +/- 1.8 microM (n = 3) when applied apically. Four types of endoprostanoid receptors (EP(1-4)) were detected in SGCs using Western blot. PGE(2)-induced DeltaI(SC) were inhibited by AH6809 (6-isopropoxy-9-oxoxanthene-2-carboxylic acid) but not by SC19220 (8-chloro-dibenzo[b,f][1,4]oxazepine-10(11H)-carboxylic acid, 2-acetylhydrazide), suggesting that endoprostanoid (EP)(2) but not EP(1) receptors were activated by PGE(2). Pretreatment of SGCs with supernatant from zymosan-activated AMs, PGE(2), or forskolin enhanced the sensitivity to acetylcholine (ACh)-induced DeltaI(SC). PGE(2)-induced DeltaI(SC) were blocked by charybdotoxin (ChTX), chromanol 293B, or glibenclamide. ACh-induced DeltaI(SC) were only blocked by ChTX or glibenclamide. None of these blockers altered PGE(2) pretreatment-induced sensitization of ACh-induced DeltaI(SC). These results demonstrate that prostanoids released from activated AMs directly increase cystic fibrosis transmembrane conductance regulator and K(+) channel activity. ACh-induced DeltaI(SC) are also enhanced due to enhanced activation of Ca(2+)-activated K(+) channels (K(Ca)).

Fibroblasts express RANKL and support osteoclastogenesis in a COX-2-dependent manner after stimulation with titanium particles

Synovial fibroblasts are possible mediators of osteolysis. Fibroblasts respond directly to titanium particles and increase RANKL expression through a COX-2/PGE2/EP4/PKA signaling pathway. Fibroblasts pretreated with titanium or PGE2 stimulated osteoclast formation, showing the functional importance of RANKL induction. Synovial fibroblasts and their activation pathways are potential targets to prevent osteolysis.

INTRODUCTION

Bone loss adjacent to the implant is a major cause of joint arthroplasty failure. Although the cellular and molecular response to microscopic wear debris particles is recognized as causative, little is known concerning role of synovial fibroblasts in these events.

MATERIALS AND METHODS

Murine embryonic fibroblasts and knee synovial fibroblasts in culture stimulated with titanium particles were examined by FACS, real time RT-PCR, Northern blot, and Western blot for expressions of vascular cell adhesion molecule (VCAM)1, RANKL, cyclooxygenase (COX)-1, and COX-2, and the four prostaglandin E2 (PGE2) receptor isoforms. Experiments were performed in the presence and absence of COX inhibitors, protein kinase A (PKA) and protein kinase C (PKC) inhibitors, and various EP receptor agonists. Osteoclast formation was examined in co-cultures of pretreated glutaraldehyde-fixed fibroblasts and primary murine spleen cells treated with macrophage-colony stimulating factor (M-CSF) for 7-days.

RESULTS

TNF-alpha stimulated VCAM1 expression, consistent with a synovial fibroblast phenotype. Titanium particles stimulated RANKL gene and protein expressions in fibroblasts in a dose-dependent manner. Gene expression was increased 5-fold by 4 h, and protein levels reached a maximum after 48 h. Within 1 h, titanium particles also induced COX-2 mRNA and protein levels, whereas both indomethacin and celecoxib blocked the stimulation of RANKL, suggesting a COX-2-mediated event. Furthermore, PGE2 induced RANKL gene and protein expression and rescued RANKL expression in titanium-treated cultures containing COX-2 inhibitors. Fibroblast cultures pretreated with either PGE2 or titanium particles enhanced osteoclast formation, indicating the functional importance of RANKL induction. EP4 was the most abundant PGE2 receptor isoform, EP1 and EP2 were expressed at low levels, and EP3 was absent. The EP1 selective agonist iloprost and the EP2 selective agonist butaprost minimally stimulated RANKL. In contrast, the EP2 and EP4 agonist misoprostol induced RANKL to a magnitude similar to PGE2. Finally, PKA antagonism strongly repressed RANKL stimulation by PGE2.

CONCLUSION

Fibroblasts respond directly to titanium particles and increase RANKL expression through a COX-2/PGE2/EP4/PKA signaling pathway. Thus, the synovial fibroblast is important mediator of osteolysis and target for therapeutic strategies.

Analysis of the effects of different alcohols on MCF-7 human breast cancer cells

Alcohol consumption is known to be an increased risk factor for breast cancer, but the underlying molecular mechanisms are not well understood. We have recently shown that the exposure of MCF-7 breast cancer cells to 0.1% ethanol enhanced their proliferation and increased their content in both estrogen receptor-alpha (ERalpha) and aromatase. The aim of the present work was to determine if the effects of ethanol could be mimicked by other short-chain aliphatic alcohols such as methanol and 1-butanol. Our results show that these compounds do not stimulate MCF-7 cell proliferation. An increase in ERalpha content was observed by Western blot in methanol-treated cells, but this parameter was not affected in butanol-treated cells. Neither of these two alcohols induced an increase in aromatase mRNA level. So despite a similarity in molecular structure, these primary alcohols do not exert the same effects. Taken together, these results suggest that the increase in aromatase expression might be a key event required for the enhanced proliferation observed in the presence of ethanol.

Prostaglandin E2 Stimulates the beta-catenin/T cell factor-dependent transcription in colon cancer

Cyclooxygenase and its derived prostaglandin E2 (PGE2) have been shown to stimulate the growth of cancer cells and promote tumor angiogenesis. Here, we show that PGE2 activated the beta-catenin/T cell factor-dependent transcription in colon cancer cells through the cAMP/protein kinase A pathway. The expression of cyclin D1 and vascular endothelial growth factor was induced by PGE2 in LS-174T cells. Moreover, PGE2 and mutated beta-catenin stimulated the transcription of cyclin D1 and vascular endothelial growth factor in a synergistic fashion. Mechanistically, PGE2 increased the phosphorylation of glycogen synthase kinase-3 and consequently accumulated beta-catenin. In addition, PGE2 induced the expression of T cell factor-4 transcription factor, which formed transcriptionally active complex with beta-catenin. In animal experiments, administration of 16,16-dimethyl PGE2 strongly increased the expression of cyclin D1 and vascular endothelial growth factor in APC(min/+) mouse polyps. Thus, our results provide a novel mechanism, suggesting that cyclooxygenase-2/PGE2 may exert pro-oncogenic actions through stimulating the beta-catenin/T cell factor-mediated transcription, which plays critical roles in colorectal carcinogenesis.

PGE2-induced hypertrophy of cardiac myocytes involves EP4 receptor-dependent activation of p42/44 MAPK and EGFR transactivation

Upon induction of cyclooxygenase-2 (COX-2), neonatal ventricular myocytes (VMs) mainly synthesize prostaglandin E2 (PGE2). The biological effects of PGE2 are mediated through four different G protein-coupled receptor (GPCR) subtypes (EP1–4). We have previously shown that PGE2 stimulates cAMP production and induces hypertrophy of VMs. Because the EP4 receptor is coupled to adenylate cyclase and increases in cAMP, we hypothesized that PGE2 induces hypertrophic growth of cardiac myocytes through a signaling cascade that involves EP4-cAMP and activation of protein kinase A (PKA). To test this, we used primary cultures of VMs and measured [3H]leucine incorporation into total protein. An EP4 antagonist was able to partially block PGE2 induction of protein synthesis and prevent PGE2-dependent increases in cell surface area and activity of the atrial natriuretic factor promoter, which are two other indicators of hypertrophic growth. Surprisingly, a PKA inhibitor had no effect. In other cell types, G protein-coupled receptor activation has been shown to transactivate the epidermal growth factor receptor (EGFR) and result in p42/44 mitogen-activated protein kinase (MAPK) activation and cell growth. Immunoprecipitation of myocyte lysates demonstrated that the EGFR was rapidly phosphorylated by PGE2 in VMs, and the EP4 antagonist blocked this. In addition, the selective EGFR inhibitor AG-1478 completely blocked PGE2-induced protein synthesis. We also found that PGE2 rapidly phosphorylated p42/44 MAPK, which was inhibited by the EP4 antagonist and by AG-1478. Finally, the p42/44 MAPK inhibitor PD-98053 (25 μmol/l) blocked PGE2-induced protein synthesis. Altogether, we believe these are the first data to suggest that PGE2 induces protein synthesis in cardiac myocytes in part via activation of the EP4 receptor and subsequent activation of p42/44 MAPK. Activation of p42/44 MAPK is independent of the common cAMP-PKA pathway and involves EP4-dependent transactivation of EGFR.

Differential regulation of phosphorylation of the cAMP response element-binding protein after activation of EP2 and EP4 prostanoid receptors by prostaglandin E2

The EP2 and EP4 prostanoid receptors are G-protein-coupled receptors whose activation by their endogenous ligand, prostaglandin (PG) E2, stimulates the formation of intracellular cAMP. We have previously reported that the stimulation of cAMP formation in EP4-expressing cells is significantly less than in EP2-expressing cells, despite nearly identical levels of receptor expression (J Biol Chem 277:2614-2619, 2002). In addition, a component of EP4 receptor signaling, but not of EP2 receptor signaling, was found to involve the activation of phosphatidylinositol 3-kinase (PI3K). In this study, we report that PGE2 stimulation of cells expressing either the EP2 or EP4 receptor results in the phosphorylation of the cAMP response element binding protein (CREB) at serine-133. Pretreatment of cells with N-[2-(4-bromocinnamylamino)ethyl]-5-isoquinoline (H-89), an inhibitor of protein kinase A (PKA), attenuated the PGE2-mediated phosphorylation of CREB in EP2-expressing cells, but not in EP4-expressing cells. Pretreatment of cells with wortmannin, an inhibitor of PI3K, had no effects on the PGE2-mediated phosphorylation of CREB in either EP2- or EP4-expressing cells, although it significantly increased the PGE2-mediated activation of PKA in EP4-expressing cells. However, combined pretreatment with H-89 and wortmannin blocked PGE2-mediated phosphorylation in EP2-expressing cells as well as in EP2-expressing cells. PGE2-mediated intracellular cAMP formation was not affected by pretreatment with wortmannin, or combined treatment with wortmannin and H-89, in either the EP2- or EP4-expressing cells. These findings suggest that PGE2 stimulation of EP4 receptors, but not EP2 receptors, results in the activation of a PI3K signaling pathway that inhibits the activity of PKA and that the PGE2-mediated phosphorylation of CREB by these receptors occurs through different signaling pathways

Regulation of vascular endothelial cell growth factor expression in mouse mammary tumor cells by the EP2 subtype of the prostaglandin E2 receptor

Prostaglandin E(2) (PGE(2)), a major metabolite of the cyclooxygenase pathway in the mammary gland, induces angiogenesis during mammary tumor progression. To better define the molecular mechanisms involved, we examined the role of the G protein-coupled receptors (GPCR) for PGE(2) in mammary tumor cell lines isolated from MMTV-cyclooxygenase-2 (COX-2) transgenic mice. Expression of the EP2 subtype of the PGE(2) receptor was correlated with the tumorigenic phenotype and the ability to induce vascular endothelial growth factor (VEGF). Overexpression of EP2 by adenoviral transduction into EP2-null cells resulted in the induction of VEGF expression in response to PGE(2) and CAY10399, an EP2 receptor agonist. The induction of VEGF by the EP2 receptor did not require the hypoxia inducible factor (HIF)-1alpha pathway, MAP kinase pathway, or phosphoinositide-3-kinase/Akt pathway, but required the cAMP/protein kinase A pathway. These results suggest that EP2 receptor is a critical element for PGE(2) mediated VEGF induction in mouse mammary tumor cells.

Prostaglandin E2 down-regulation of cytochrome P-450 2B1 expression induced by phenobarbital is through EP2 receptor in rat hepatocytes

Cytochrome P-450 is an important bioactivation-detoxification system in vivo. Its expression is regulated by foreign chemicals and dietary factors, and lipids have been found to regulate its gene expression. We showed previously that prostaglandin E(2) (PGE(2)), a fatty acid metabolite, down-regulates cytochrome P-450 2B1 (CYP 2B1) expression induced by phenobarbital. The objective of the present study was to determine whether PGE(2) type 2 receptor (EP(2))-which is coupled to Gs-protein when bound by PGE(2), leading to cAMP production-is involved in this down-regulation. We also determined the possible roles of EP(2) downstream pathways in this down-regulation. We used a primary rat hepatocyte culture model in which EP(2) was shown to be present to study this question. The intracellular cAMP concentration in primary rat hepatocytes was significantly higher after treatment with 1microM PGE(2) than after treatment with 0, 0.01, or 0.1microM PGE(2). Butaprost, an EP(2) agonist, down-regulated CYP 2B1 expression in a dose-dependent manner. SQ22536, an adenylate cyclase inhibitor, reversed the down-regulation by PGE(2) as did H-89, a protein kinase A inhibitor. These results suggest that EP(2) and the downstream pathways of cAMP and protein kinase A are involved in the down-regulation of CYP 2B1 expression by PGE(2) in the presence of phenobarbital.

Prostaglandin E2 reduces radiation-induced epithelial apoptosis through a mechanism involving AKT activation and bax translocation

Prostaglandin E2 (PGE2) synthesis modulates the response to radiation injury in the mouse intestinal epithelium through effects on crypt survival and apoptosis; however, the downstream signaling events have not been elucidated. WT mice receiving 16,16-dimethyl PGE2 (dmPGE2) had fewer apoptotic cells per crypt than untreated mice. Apoptosis in Bax(-/-) mice receiving 12 Gy was approximately 50% less than in WT mice, and the ability of dmPGE2 to attenuate apoptosis was lost in Bax(-/-) mice. Positional analysis revealed that apoptosis in the Bax(-/-) mice was diminished only in the bax-expressing cells of the lower crypts and that in WT mice, dmPGE2 decreased apoptosis only in the bax-expressing cells. The HCT-116 intestinal cell line and Bax(-/-) HCT-116 recapitulated the apoptotic response of the mouse small intestine with regard to irradiation and dmPGE2. Irradiation of HCT-116 cells resulted in phosphorylation of AKT that was enhanced by dmPGE2 through transactivation of the EGFR. Inhibition of AKT phosphorylation prevented the reduction of apoptosis by dmPGE2 following radiation. Transfection of HCT-116 cells with a constitutively active AKT reduced apoptosis in irradiated cells to the same extent as in nontransfected cells treated with dmPGE2. Treatment with dmPGE2 did not alter bax or bcl-x expression but suppressed bax translocation to the mitochondrial membrane. Our in vivo studies indicate that there are bax-dependent and bax-independent radiation-induced apoptosis in the intestine but that only the bax-dependent apoptosis is reduced by dmPGE2. The in vitro studies indicate that dmPGE2, most likely by signaling through the E prostaglandin receptor EP2, reduces radiation-induced apoptosis through transactivation of the EGFR and enhanced activation of AKT and that this results in reduced bax translocation to the mitochondria.

Prostaglandin E2 reduces radiation-induced epithelial apoptosis through a mechanism involving AKT activation and bax translocation

Prostaglandin E2 (PGE2) synthesis modulates the response to radiation injury in the mouse intestinal epithelium through effects on crypt survival and apoptosis; however, the downstream signaling events have not been elucidated. WT mice receiving 16,16-dimethyl PGE2 (dmPGE2) had fewer apoptotic cells per crypt than untreated mice. Apoptosis in Bax(-/-) mice receiving 12 Gy was approximately 50% less than in WT mice, and the ability of dmPGE2 to attenuate apoptosis was lost in Bax(-/-) mice. Positional analysis revealed that apoptosis in the Bax(-/-) mice was diminished only in the bax-expressing cells of the lower crypts and that in WT mice, dmPGE2 decreased apoptosis only in the bax-expressing cells. The HCT-116 intestinal cell line and Bax(-/-) HCT-116 recapitulated the apoptotic response of the mouse small intestine with regard to irradiation and dmPGE2. Irradiation of HCT-116 cells resulted in phosphorylation of AKT that was enhanced by dmPGE2 through transactivation of the EGFR. Inhibition of AKT phosphorylation prevented the reduction of apoptosis by dmPGE2 following radiation. Transfection of HCT-116 cells with a constitutively active AKT reduced apoptosis in irradiated cells to the same extent as in nontransfected cells treated with dmPGE2. Treatment with dmPGE2 did not alter bax or bcl-x expression but suppressed bax translocation to the mitochondrial membrane. Our in vivo studies indicate that there are bax-dependent and bax-independent radiation-induced apoptosis in the intestine but that only the bax-dependent apoptosis is reduced by dmPGE2. The in vitro studies indicate that dmPGE2, most likely by signaling through the E prostaglandin receptor EP2, reduces radiation-induced apoptosis through transactivation of the EGFR and enhanced activation of AKT and that this results in reduced bax translocation to the mitochondria.

Prostaglandin E2 promotes the survival of bone marrow-derived dendritic cells

Since dendritic cells (DC) participate in both innate and adaptive immunity, their survival and expansion is tightly controlled. Little is known about the mechanisms of DC apoptosis. PGE(2), an arachidonic acid metabolite, plays an essential role in DC migration. We propose a novel function for PGE(2) as a DC survival factor. Our studies demonstrate that PGE(2) protects DC in vitro against apoptosis induced by withdrawal of growth factors or ceramide. DC matured in conditions that inhibit endogenous PGE(2) release are highly susceptible to apoptosis and exogenous PGE(2) re-establishes the more resistant phenotype. The antiapoptotic effect is mediated through EP-2/EP-4 receptors and involves the PI3K --> Akt pathway. PGE(2) leads to increased phosphorylation of Akt, protection against mitochondrial membrane compromise, and decreased caspase 3 activity. Macroarray data indicate that PGE(2) leads to the down-regulation of a number of proapoptotic molecules, i.e., BAD, several caspases, and granzyme B. In vivo, higher numbers of immature and Ag-loaded CFSE-labeled DC are present in the draining lymph nodes of mice inoculated with PGE(2) receptor agonists, compared with animals treated with ibuprofen or controls injected with PBS. This suggests that PGE(2) acts as an endogenous antiapoptotic factor for DC and raises the possibility of using PGE(2) agonists to increase the survival of Ag-loaded DC following in vivo administration.

cAMP-mediated inhibition of DNA replication and S phase progression: involvement of Rb, p21Cip1, and PCNA

cAMP exerts an antiproliferative effect on a number of cell types including lymphocytes. This effect of cAMP is proposed to be mediated by its ability to inhibit G1/S transition. In this report, we provide evidence for a new mechanism whereby cAMP might inhibit cellular proliferation. We show that elevation of intracellular levels of cAMP inhibits DNA replication and arrests the cells in S phase. The cAMP-induced inhibition of DNA synthesis was associated with the increased binding of p21Cip1 to Cdk2-cyclin complexes, inhibition of Cdk2 kinase activity, dephosphorylation of Rb, and dissociation of PCNA from chromatin in S phase cells. The ability of cAMP to inhibit DNA replication and trigger release of PCNA from chromatin required Rb and p21Cip1 proteins, since both processes were only marginally affected by increased levels of cAMP in Rb-/- and p21Cip1-/- 3T3 fibroblasts. Importantly, the implications of cAMP-induced inhibition of DNA synthesis in cancer treatment was demonstrated by the ability of cAMP to reduce apoptosis induced by S phase-specific cytotoxic drugs. Taken together, these results demonstrate a novel role for cAMP in regulation of DNA synthesis and support a model in which activation of cAMP-dependent signaling protects cells from the effect of S phase-specific antitumor agents.

Matrix metalloproteinase secretion by gastric epithelial cells is regulated by E prostaglandins and MAPKs

Because matrix metalloproteinases (MMPs) play roles in inflammatory tissue injury, we asked whether MMP secretion by gastric epithelial cells may contribute to gastric injury in response to signals involved in Helicobacter pylori-induced inflammation and/or cyclooxygenase inhibition. Tumor necrosis factor (TNF)-alpha, interleukin (IL)-1beta, and epidermal growth factor (EGF) stimulated gastric cell MMP-1 secretion, indicating that MMP-1 secretion occurs in inflammatory as well as non-inflammatory situations. MMP-1 secretion required activation of the MAPK Erk and subsequent protein synthesis but was down-regulated by the alternate MAPK, p38. In contrast, secretion of MMP-13 was stimulated by TNF-alpha/IL-1beta but not EGF and was Erk-independent and mediated by p38. MMP-13 secretion was more rapid (peak, 6 h) than MMP-1 (peak > or =30 h) and only partly depended on protein synthesis, suggesting initial release of a pre-existing MMP-13 pool. Therefore, MMP-1 and MMP-13 secretion are differentially regulated by MAPKs. MMP-1 secretion was regulated by E prostaglandins (PGEs) in an Erk-dependent manner. PGEs enhanced Erk activation and MMP-1 secretion in response to EGF but inhibited Erk and MMP-1 when TNF-alpha and IL-1beta were the stimuli, indicating that the effects of PGEs on gastric cell responses are context-dependent. These data show that secretion of MMPs is differentially regulated by MAPKs and suggest mechanisms through which H. pylori infection and/or cyclooxygenase inhibition may induce epithelial cell signaling to contribute to gastric ulcerogenesis.

Antiviral therapy with non-selective β-blockers: preliminary experimental and clinical corroboration

Treatment with non-selective beta-blockers has been proposed to have an indirect antiviral activity acting via an enhanced performance of the immune system, and the mechanism of this activity has been laid out earlier. Experimental and clinical findings are presented that corroborate the hypothesis that inhibiting the immunosuppressive and stress-related cAMP-PKA pathway will enhance the immune system's ability to recognize foreign antigen and to access its vast repertory in an improved way, resulting in an indirect antiviral activity. Other drugs having an inhibitory effect on the cAMP-PKA pathway in cells of the immune system and therefore expected to have a comparable activity spectrum with different specific side-effects are presented, for example aspirin. Additionally the so far unexplained anticancer activity of aspirin is related to the same mechanism of an enhanced performance of the immune system.

PGE2 inhibits chondrocyte differentiation through PKA and PKC signaling

Prostaglandins are ubiquitous metabolites of arachidonic acid, and cyclooxygenase inhibitors prevent their production and secretion. Animals with loss of cyclooxygenase-2 function have reduced reparative bone formation, but the role of prostaglandins during endochondral bone formation is not defined. The role of PGE2 as a regulator of chondrocyte differentiation in chick growth plate chondrocytes (GPCs) was examined. While PGE2, PGD2, PGF2alpha, and PGJ2 all inhibited colX expression, approximately 80% at 10(-6) M, PGE2 was the most potent activator of cAMP response element (CRE)-mediated transcription. PGE2 dose-dependently inhibited the expression of the differentiation-related genes, colX, VEGF, MMP-13, and alkaline phosphatase gene, and enzyme activity with significant effects at concentrations as low as 10(-10) M. PGE2 induced cyclic AMP response element binding protein (CREB) phosphorylation and increased c-Fos protein levels by 5 min, and activated transcription at CRE-Luc, AP-1-Luc, and c-Fos promoter constructs. The protein kinase A (PKA) inhibitor, H-89, completely blocked PGE2-mediated induction of CRE-Luc and c-Fos promoter-Luc promoters, and partially inhibited induction of AP-1-Luc, while the protein kinase C (PKC) inhibitor Go-6976 partially inhibited all three promoters, demonstrating substantial cross-talk between these signaling pathways. PGE2 inhibition of colX gene expression was dependent upon both PKA and PKC signaling. These observations demonstrate potent prostaglandin regulatory effects on chondrocyte maturation and show a role for both PKA and PKC signaling in PGE2 regulatory events.

Prostaglandin receptor signalling and function in human endometrial pathology

Prostaglandins are bioactive lipids that exert an autocrine or paracrine function by binding to specific G-protein-coupled receptors (GPCRs) to activate intracellular signalling and gene transcription. Prostaglandins are key regulators of reproductive processes, including ovulation, implantation and menstruation. Prostaglandins have been ascertained to have a role in various pathological changes of the reproductive tract including menorrhagia, dysmenorrhea, endometriosis and cancer. Although the mechanism by which prostaglandins modulate these changes remains unclear, much evidence suggests that prostaglandins and their receptors and downstream signalling pathways are involved in angiogenesis and in alterations in cell adhesion, morphology, motility, invasion and metastases. The potential role of prostaglandin receptors in pathological changes of the endometrium has significance for the future development of therapeutic interventions.

Prostaglandin EP receptors: Targets for treatment and prevention of colorectal cancer?

The importance of the prostaglandin (PG) synthesis pathway, particularly the rate-limiting enzymatic step catalyzed by cyclooxygenase, to colorectal carcinogenesis and development of novel anticolorectal cancer therapy is well established. The predominant PG species in benign and malignant colorectal tumors is PGE2. PGE2 acts via four EP receptors termed EP1 to EP4. Recently, EP receptors have been identified as potential targets for treatment and/or prevention of colorectal cancer. This review summarizes existing knowledge of the expression and function of the EP receptor subtypes in human and rodent intestine during tumorigenic progression and describes the current literature on targeting EP receptor signaling during intestinal tumorigenesis.

Prostaglandin E2 selectively antagonizes prostaglandin F2alpha-stimulated T-cell factor/beta-catenin signaling pathway by the FPB prostanoid receptor

FP prostanoid receptors are G-protein-coupled receptors that consist of two isoforms named FPA and FPB. Both isoforms activate inositol phosphate second messenger signaling pathways by their endogenous ligand prostaglandin F2alpha (PGF2alpha). Previously we have shown that both isoforms undergo Rho-mediated cell rounding following treatment with PGF2alpha. Following the removal of PGF2alpha, however, FPA-expressing cells return to their original morphology, whereas FPB-expressing cells do not. It was also found that PGF2alpha-could activate T-cell factor (Tcf)/beta-catenin signaling in cells expressing the FPB isoform but not in cells expressing the FPA isoform. We now show that prostaglandin E2 (PGE2) can induce cell rounding and stimulate the formation of inositol phosphates to the same extent as PGF2alpha in cells expressing either the FPA or FPB isoforms. However, PGE2 has much lower efficacy as compared with PGF2alpha for the activation of Tcf/beta-catenin signaling in FPB-expressing cells, and the cell rounding is reversible. Interestingly, pretreatment of FPB-expressing cells with PGE2-attenuated PGF2alpha-stimulated Tcf/beta-catenin signaling in a dose-dependent manner while having no effect on PGF2alpha-stimulated inositol phosphates formation. Thus, the ratio of endogenous PGE2 and PGF2alpha has the potential to selectively regulate one signaling pathway over another. This represents a novel mechanism for the regulation of cell signaling that is distinct from regulation occurring at the level of the receptor and its effector pathways.