Immunohistochemical demonstration of cyclooxygenase-2 (COX-2) and prostaglandin receptors EP2 and FP expression in the bovine intercaruncular uterine wall around term

During parturition, uterine-derived prostaglandins (PG) play an outstanding role regarding the functional elimination of the corpus luteum and the promotion of uterine contraction. The rate-limiting enzyme cyclooxygenase-2 (COX-2), highly regulated in a cell-type and localization specific manner throughout pregnancy, is involved in uterine prostanoid production. Prostaglandins exert their effects via G-protein-coupled receptors. Distribution and cellular localization of these receptors are decisive factors for prostaglandin-mediated actions. Since both COX-2 and PG receptors have only been assessed during pregnancy in the cow, these parameters were localized immunohistochemically near term to evaluate their specific role at parturition. Thus, during two periods, segments of the intercaruncular uterine wall were collected from cows at slaughter being eight and nine months pregnant, from cattle during caesarean section, and after spontaneous calving. Results reveal that COX-2 was mainly localized in the cytoplasm of surface epithelial cells with a high expression in animals with induced parturition. The enzyme could also be found in lower concentrations within the glandular epithelium without any effect of gestational time or labour. In contrast to relaxant prostaglandin E receptor type 2 (EP2), not showing any change in all tissue layers observed, contractile prostaglandin F(2alpha) receptor (FP) was modulated during the peripartal period revealing a peak expression in animals with induced parturition. FP was localized in surface and glandular epithelial cells as well as in endometrial stroma and myometrial smooth muscle cells. Our study indicates that labour and induction of parturition may have an effect on amounts of immunohistochemically detectable COX-2 and FP. EP2 remains rather unchanged during the peripartal period. COX-2 and FP thus contribute via changes in amount and distribution to mechanisms associated with parturition.

Protective effect of prostaglandin E2 receptors EP2 and EP4 in alloimmune response in vivo

Prostaglandin E2 (PGE2) is produced during inflammatory responses mediating a variety of both innate and adaptive immune responses through 4 distinct receptors: EP1 to EP4. The use of gene-targeted mice and selective agonists/antagonists responsible for each receptor has gradually revealed that each receptor plays a unique and important role in various disease conditions. In addition, PGE2 is known to have some immunosuppressive properties. In this study, we investigated the role of PGE2 receptors by examining the therapeutic efficacy of highly selective receptor agonists on the alloimmune response in vivo. We used a fully major histocompatibility complex (MHC)-mismatched murine cardiac transplantation model. C57BL/6 cardiac allografts were heterotopically transplanted into BALB/c recipients. We treated mice with a highly selective agonist for each EP receptor. EP2 and EP4 agonists significantly prolonged allograft survival compared with controls. In particular, the EP4 agonist was more effective than the EP2 agonist in the inhibition of acute allograft rejection. In conclusion, PGE2 receptors merit further study as novel therapeutics for clinical transplantation.

Elevated expression of prostaglandin receptor and increased release of prostaglandin E2 maintain the survival of CD45RO+ T cells in the inflamed human pleural space

Throughout the body, the distribution and differentiation of T-cell subsets varies in a way that optimizes host responses. The role of activation-induced cell death (AICD) in altering the distribution of T-lymphocyte subsets at an immune or inflammatory sites has been unexplored. The objective of this study was to assess whether pleural macrophages modulate AICD of specific pleural T-lymphocyte subsets. We found that pleural T-lymphocytes spontaneously undergo apoptosis, which is associated to increased expression of both FAS and FAS ligand, to decreased expression of Bcl 2 and to caspase 8 and 3 activation. While pleural T lymphocytes were partly protected from apoptosis, autologous peripheral blood T lymphocytes increased their apoptosis when cultured with exudative pleural fluids. Pleural CD45RO(+) T cells, in comparison to pleural CD45RA(+) T cells, were more susceptible to apoptosis, but were preferentially protected by exudative pleural fluids. Pleural prostaglandin E 2 (PGE(2)) was implicated in protecting T-lymphocytes from apoptosis because exudative pleural T lymphocytes highly express PGE(2) receptors, and because exudative pleural fluid contained high concentrations of PGE(2). Activated pleural macrophages released PGE(2) and reduced the spontaneous apoptosis of pleural T lymphocytes and depletion of PGE(2) from pleural fluids decreased this protective effect. This study demonstrates that PGE(2), released in the pleural fluids following pleural macrophage activation, prolongs the survival of specific T-cell subsets, resulting in differentiation of the T-cell repertoire within the inflamed pleural space.

Cyclooxygenase-2 inhibits UVB-induced apoptosis in mouse skin by activating the prostaglandin E2 receptors, EP2 and EP4

Cyclooxygenase-2 (COX-2) is induced by UVB light and reduces UVB-induced epidermal apoptosis; however, the mechanism is unclear. Therefore, wild-type (WT) and COX-2-/- mice were acutely treated with UVB (5 kJ/m(2)), and apoptotic signaling pathways were compared. Following exposure, apoptosis was 2.5-fold higher in COX-2-/- compared with WT mice. Because prostaglandin E(2) (PGE(2)) is the major UV-induced prostaglandin and manifests its activity via four receptors, EP1 to EP4, possible differences in EP signaling were investigated in WT and COX-2-/- mice. Following UVB exposure, protein levels of EP1, EP2, and EP4 were elevated in WT mice, but EP2 and EP4 levels were 50% lower in COX-2-/- mice. Activated cyclic AMP-dependent protein kinase (PKA) and Akt are downstream in EP2 and EP4 signaling, and their levels were reduced in UVB-exposed COX-2-/- mice. Furthermore, p-Bad (Ser(136) and Ser(155)), antiapoptotic products of activated Akt and PKA, respectively, were significantly reduced in UVB-exposed COX-2-/- mice. To further study the roles of EP2 and EP4, UVB-exposed CD-1 mice were topically treated with indomethacin to block endogenous PGE(2) production, and PGE(2), the EP2 agonist (butaprost) or EP4 agonist (PGE(1) alcohol), was applied. Indomethacin reduced PKA and Akt activation by approximately 60%, but PGE(2) and the agonists restored their activities. Furthermore, both agonists decreased apoptosis in COX-2-/- mice by 50%. The data suggest that COX-2-generated PGE(2) has antiapoptotic roles in UVB-exposed mouse skin that involves EP2- and EP4-mediated signaling.

Exploration of prostanoid receptor subtype regulating estradiol and prostaglandin E2 induction of spinophilin in developing preoptic area neurons

The prostaglandin E2 (PGE2) mediates estradiol-induced masculinization of sexual behavior in the rat during a perinatal sensitive period. PGE2 induces formation of dendritic spines on preoptic area (POA) neurons and this synaptic pattern change is associated with the ability to express male sexual behavior as an adult. Whether PGE2 is released from astrocytes or neurons in the developing POA is unknown. To further understanding of how PGE2 induces dendritic spine formation at the cellular level, we have explored the PGE2 receptor subtype mediating this response. There are four receptors for PGE2, EP1, EP2, EP3 and EP4, each having unique but interacting signal transduction profiles. Treatment of newborn female rats with the EP receptor agonists iloprost, butaprost and sulprostone indicated that stimulation of both the EP2 and EP3 receptors significantly increased spinophilin, a protein whose levels positively correlate to the presence of dendritic spines and masculinization of the POA. Use of antisense oligonucleotides against the mRNA for each receptor reveals that either EP2 or EP3 receptor knockdown reduces spinophilin in PGE2- or estradiol-treated females, whereas reducing EP1 or EP4 receptor levels by the same means has a smaller but also significant effect. A developmental profile of EP receptor expression indicates EP1 in particular is elevated for the first few days of life, corresponding to the critical period for masculinization, whereas mRNA levels for the other three receptors remain relatively constant.

Spinal Prostaglandins Facilitate Exaggerated A- and C-fiber-mediated Reflex Responses and Are Critical to the Development of Allodynia Early after L5-L6 Spinal Nerve Ligation

Background

Spinal prostaglandins are important in the early pathogenesis of spinal nerve ligation (SNL)-induced allodynia. This study examined the effect of SNL on the expression of cyclooxygenase (COX)-1, COX-2, and prostaglandin E2 receptors in the rat lumbar spinal cord, and the temporal and pharmacologic relation of these changes to the exaggerated A- and C-fiber-mediated reflex responses and allodynia, 24 h after injury.

Methods

Male Sprague-Dawley rats, fitted with intrathecal catheters, underwent SNL or sham surgery. Paw withdrawal threshold, electromyographic analysis of the biceps femoris flexor reflex, and immunoblotting of the spinal cord were used.

Results

Both allodynia (paw withdrawal threshold of < or = 4 g) and exaggerated A- and C-fiber-mediated reflex responses (i.e., decrease in activation threshold, increase in evoked activity, including windup; P < 0.05) were evident 24 h after SNL but not sham surgery. Allodynic animals exhibited significant increases in prostaglandin E2 receptor (subtypes 1-3) and COX-1 (but not COX-2) expression in the ipsilateral lumbar dorsal horn. The corresponding ventral horns and contralateral dorsal horn were unchanged from sham controls. Exaggerated A- and C-fiber-mediated reflex responses were significantly attenuated by intrathecal SC-560 or SC-51322, but not SC-236, given 24 h after SNL.

Conclusion

These results provide further evidence that spinal prostaglandins, derived primarily from COX-1, are critical in the exaggeration of A- and C-fiber input and allodynia, 24 h after SNL.

The isoprostanes - unique products of arachidonate peroxidation: their role as mediators of oxidant stress

The isoprostanes (IsoPs) are a series of novel prostaglandin-like compounds formed in vivo in humans from the free radical-catalyzed peroxidation of arachidonate independent of the cyclooxygenase. While quantification of these compounds is a highly accurate measure of oxidant stress in vivo in humans, IsoPs also possess potent biological activity and likely mediate certain aspects of oxidative injury. The purpose of this review is to summarize selected aspects of our knowledge regarding the bioactivity of the IsoPs. I will first briefly highlight mechanisms involved in IsoP formation. Subsequently, I will discuss the biological activities of certain IsoPs that are formed in abundance in vivo and focus on two compounds, 15-F(2t)-IsoP and 15-E(2t)-IsoP, that have been studied in the greatest detail. This review will then examine, at a molecular level, mechanisms by which IsoPs exert their bioactivity. It has been shown that they are ligands for various eicosanoid receptors, in particular, the thromboxane receptor. In addition, I will discuss the controversial evidence that a unique IsoP receptor(s) exists. Finally, I will offer avenues for future research related to the development of pharmacological approaches to modulate IsoP formation and action in vivo and thus decrease the pathophysiological sequelae of oxidative injury.

Prostaglandins regulate transcription by means of prostaglandin response elements located in the promoters of mammalian Na,K-ATPase beta 1 subunit genes

Prostaglandins are potent products of arachidonic acid metabolism that play significant roles in regulating ion transport in the kidney. In the Madin Darby canine kidney (MDCK) cell line prostaglandin E(1) (PGE(1)) stimulates the activity of the Na,K-ATPase and regulates transcription. Transient transfection studies conducted in MDCK cells with a human Na,K-ATPase beta1 subunit promoter/luciferase construct, pHbeta1-1141 Luc, showed a PGE(1) stimulation. The PGE(1) stimulation was inhibited by the PGE receptor antagonists SC19220 and AH6809, indicating the involvement of EP1 receptors (coupled to phospholipase C) and EP2 receptors (coupled to adenylate cyclase), respectively. A prostaglandin-regulatory element (PGRE) within the beta1 subunit promoter (-110 to -92, AGTCCCTGC) is sufficient to elicit a PGE(1) stimulation in a heterologous promoter (in pLUC-MCS). Studies with promoter mutants indicated that in addition to the PGRE, an adjacent Sp1 site was also essential for regulation by PGE(1). Consistent with the involvement of Sp1 are the results of DNA affinity precipitation studies, which indicate that Sp1 as well as CREB, and Sp3 all bind to the PGRE. The involvement of this PGRE in transcriptional regulation of the Na,K-ATPase beta1 gene was examined in a number of species. Only human and chimpanzee promoters possessed an identical PGRE site, unlike dog, rat, and mouse, which possessed Sp1 sites in similar locations. Two alternative PGREs were subsequently identified. The sequence of the one of these PGREs (TGACCTTC, -445 to -438) was conserved throughout all species examined, suggesting its physiologic significance.

EP2 prostanoid receptor promotes squamous cell carcinoma growth through epidermal growth factor receptor transactivation and iNOS and ERK1/2 pathways

In squamous cell carcinoma, the levels of nitric oxide (NO) derived from inducible NO synthase (iNOS) and prostaglandin E2 (PGE2) derived from cyclooxygenase-2 (COX-2) originated from tumor cells or tumor-associated inflammatory cells have been reported to correlate with tumor growth, metastasis, and angiogenesis. The present study examined the role of the iNOS signaling pathway in PGE2-mediated tumor invasiveness and proliferation in squamous cell carcinoma, A431, and SCC-9 cells. Cell invasion and proliferation promoted by PGE2 were blocked by iNOS silencing RNA or iNOS/guanylate cyclase (GC) pharmacological inhibition. Consistently, iNOS-GC pathway inhibitors blocked mitogen-activated protein kinase-ERK1/2 phosphorylation, which was required to mediate PGE2 functions. In vivo, in A431 cells implanted in nude mice, GC inhibition also decreased the tumor proliferation index and ERK1/2 activation. PGE2 effects were confined to the selective stimulation of the EP2 receptor subtype, leading to epidermal growth factor receptor (EGFR) transactivation via protein kinase A (PKA) and c-Src activation. EP2-mediated ERK1/2 activation and cell functions were abolished by inhibitors of PKA, c-Src, and EGFR, as well as by inhibiting iNOS pathway. Silencing of iNOS also impaired EGFR-induced ERK1/2 phosphorylation. These results indicate that iNOS/GC signaling is a downstream player in the control of EP2/EGFR-mediated tumor cell proliferation and invasion.

Stimulation of prostaglandin EP2 receptors prevents NMDA-induced excitotoxicity

Prostaglandin E(2) (PGE(2)) plays an important role in inflammation and neurologic disorders. The neuromodulatory effects of PGE(2) are mediated through regulation of four G-protein-coupled receptors known as EP1, EP2, EP3, and EP4. The goal of the current study was to determine whether EP2 receptor activation protects neurons from acute NMDA-mediated excitotoxicity. To examine the effects of EP2 activation, mice were given an injection of the EP2 receptor-selective agonist butaprost (K (i) = 110 nM for EP2 receptor; K (i) > 10,000 for other prostaglandin receptors) in the cerebral ventricle and then an injection of NMDA in the right striatum. After 48 h, a significant reduction in NMDA-induced lesion volume was observed in groups pretreated with butaprost (1-300 nmol/L), with maximal protection at 100 nmol/L (p < 0.001). To determine if EP2-activated protection was specific to neurons, mouse neuronal cultures were treated with butaprost, and cell viability was analyzed after 24 h of NMDA excitotoxicity. The results showed that butaprost significantly increased neuron survival in a dose-dependent fashion. Furthermore, treatment of primary neurons with butaprost significantly increased cAMP levels (p < 0.001). Together, these data reveal that EP2 receptor stimulation mediates neuroprotection against NMDA excitotoxicity both in vivo and in vitro and that butaprost can limit acute brain damage. Development and testing of specific PGE(2) receptor mimetics could lead to a decrease in side effects associated with anti-inflammatory drugs and could help to fight acute and/or chronic neurologic disorders.

TNF-alpha modulates hepatic Na+-K+ ATPase activity via PGE2 and EP2 receptors

The effect of TNF-alpha on liver Na(+)-K(+) ATPase was studied in Sprague-Dawley rats and in HepG2 cells. TNF-alpha was injected intraperitoneally to rats and 4h later the liver was isolated and the activity and protein expression of hepatic Na(+)-K(+) ATPase studied. The cytokine caused a significant down-regulation of the ATPase and a decrease in its activity. This effect disappeared in presence of indomethacin, an inhibitor of COX enzymes, and PGE2 injected to the animals imitated the effect of TNF-alpha. The observed in vivo effects of TNF and PGE2 on the pump appeared again when HepG2 cells were treated with the cytokine or the prostaglandin. The application of different agonist and antagonist to EP receptors showed that the effect of PGE2 is mediated via EP2 receptors. It was concluded that TNF-alpha induces in hepatocytes, PGE2 production which in turn reduces the activity and protein expression of the Na(+)-K(+) ATPase by activating EP2 receptors.

Activating of ATP-dependent K+ channels comprised of K(ir) 6.2 and SUR 2B by PGE2 through EP2 receptor in cultured interstitial cells of Cajal from murine small intestine

The interstitial cells of Cajal (ICC) are pacemaker cells in gastrointestinal tract and generate an electrical rhythm in gastrointestinal muscles. We investigated the possibility that PGE(2) might affect the electrical properties of cultured ICC by activating ATP-dependent K(+) channels and, the EP receptor subtypes and the subunits of ATP-dependent K(+) channels involved in these activities were identified. In addition, the regulation of intracellular Ca(2+) ([Ca(2+)](i)) mobilization may be involved the action of PGE(2) on ICC. Treatments of ICC with PGE(2) inhibited electrical pacemaker activities in the same manner as pinacidil, an ATP-dependent K(+) channel opener and PGE(2) had only a dose-dependent effect. Using RT-PCR technique, we found that ATP-dependent K(+) channels exist in ICC and that these are composed of K(ir) 6.2 and SUR 2B subunits. To characterize the specific membrane EP receptor subtypes in ICC, EP receptor agonists and RT-PCR were used: Butaprost (an EP(2) receptor agonist) showed the actions on pacemaker currents in the same manner as PGE(2). However sulprostone (a mixed EP(1) and EP(3) agonist) had no effects. In addition, RT-PCR results indicated the presence of the EP(2) receptor in ICC. To investigate cAMP involvement in the effects of PGE(2) on ICCs, SQ-22536 (an inhibitor of adenylate cyclase) and cAMP assays were used. SQ-22536 did not affect the effect of PGE(2) on pacemaker currents, and PGE(2) did not stimulate cAMP production. Also, we found PGE(2) inhibited the spontaneous [Ca(2+)](i) oscillations in cultured ICC. These observations indicate that PGE(2) alters pacemaker currents by activating the ATP-dependent K(+) channels comprised of K(ir) 6.2-SUR 2B in ICC and this action of PGE(2) are through EP(2) receptor subtype and also the activation of ATP-dependent K(+) channels involves intracellular Ca(2+) mobilization.

Prostaglandin receptor EP2 is responsible for cyclooxygenase-2 induction by prostaglandin E2 in mouse skin

The EP2 prostanoid receptor is one of the four subtypes of receptors for prostaglandin E2 (PGE2). We previously reported that deletion of EP2 led to resistance to chemically induced mouse skin carcinogenesis, whereas overexpression of EP2 resulted in enhanced tumor development. The purpose of this study was to investigate the underlying molecular mechanisms. We found that EP2 knockout mice had reduced cyclooxygenase-2 (COX-2) expression after 12-O-tetradecanoylphorbol-13-acetate (TPA) treatment compared with wild-type (WT) mice. Further, primary keratinocytes from EP2 transgenic mice had increased COX-2 expression after either TPA or PGE2 treatment and COX-2 expression was blocked by 10 microM SQ 22,536, an adenylate cyclase inhibitor. EP2 knockout mice had significantly decreased, whereas EP2 transgenic mice had significantly increased PGE2 production in response to a single treatment of TPA. Cyclic AMP response element-binding protein (CREB) phosphorylation was elevated to a greater extent in keratinocytes from EP2 transgenic mice compared with those of WT mice following PGE2 treatment. A protein kinase A (PKA) inhibitor reduced PGE2-mediated CREB phosphorylation in keratinocytes from EP2 transgenic mice. Furthermore, we found that there was no CREB phosphorylation in EP2 knockout mice following PGE2 treatment. PGE2-induced DNA synthesis (cell proliferation) was significantly decreased in keratinocytes from EP2 knockout mice following pretreatment with 10 microM SQ 22,536. Taken together, EP2 activation of the PKA/CREB-signaling pathway is responsible for keratinocyte proliferation and our findings reveal a positive feedback loop between COX-2 and PGE2 that is mediated by the EP2 receptor.

Deletion of Prostaglandin E2 EP2 Receptor Protects against Ultraviolet-Induced Carcinogenesis, but Increases Tumor Aggressiveness

Ultraviolet (UV) light is a complete carcinogen inducing and promoting squamous-cell carcinoma (SCC) of the skin. Recent work has shown that SCC initiation and promotion are enhanced by prostaglandin E2 (PGE2). PGE2 interacts with specific EP receptors to regulate cellular functions. Previous work from our group has shown that the prostaglandin E2 EP2 receptor is a powerful regulator of keratinocyte growth. SKH-1 hairless mice lacking the EP2 receptor were therefore studied to understand how this growth signaling pathway contributes to photocarcinogenesis. Our data indicate that UV-irradiated mice lacking EP2 receptors exhibit decreased proliferation and a poor capacity for epidermal hypertrophy in response to UV injury. In a chronic irradiation model, these animals were protected from tumor formation, developing 50% fewer tumors than wild-type controls. Despite this capacity to protect against tumorigenesis, animals lacking EP2 receptors grew tumors that were larger in size, with a more aggressive phenotype. Further study suggested that this susceptibility may be associated with synthesis of active metalloproteinase enzymes in greater quantities than keratinocytes expressing the EP2 receptor, thereby enhancing the invasive potential of EP2-/- cells.

Cyclooxygenase-2-derived prostaglandin e2 protects mouse embryonic stem cells from apoptosis

Little is known about prostaglandin synthesis and function in embryonic stem cells. We postulated that mouse embryonic stem (mES) cells possess enzymes to synthesize protective prostaglandins. Compared with differentiated adult cells, mES cells were less susceptible to H(2)O(2)-induced apoptosis. However, their apoptosis was enhanced by indomethacin or SC-236, a selective inhibitor of cyclooxygenase (COX)-2. Analysis of COX pathway enzymes by Western blotting revealed expression of COX-2 and cytosolic and microsomal prostaglandin E(2) (PGE(2)) synthases. COX-1 and prostacyclin (PGI(2)) synthases were undetectable. mES cells produced PGE(2) but not PGI(2). Importantly, PGE(2) rescued mES cells from apoptosis. To elucidate the signaling mechanism by which PGE(2) inhibits apoptosis, we analyzed E-type prostaglandin (EP) receptors by Western blots. All EP isoforms were detected except EP4. Butaprost, a specific EP2 agonist, rescued mES cells from apoptosis, whereas sulprostone, an EP1/EP3 agonist, had no effect, suggesting selective interaction of PGE(2) with EP2. The antiapoptotic effect of PGE(2) was abrogated by Ly-294002 or wortmannin but not H-89 or a specific inhibitor of protein kinase A, suggesting signaling via phosphatidylinositol-3 kinase (PI-3K). Akt was constitutively active in mES cells, which were inhibited by indomethacin and rescued by PGE(2). The rescuing effect of PGE(2) was abrogated by Ly-294002. These results indicate that mES cells constitutively express COX-2 and PGE synthases and produce PGE(2), which confers resistance to apoptosis via EP2-mediated activation of PI-3K to the Akt pathway. Disclosure of potential conflicts of interest is found at the end of this article.

Relationship between prostaglandin E2 receptors and clinicopathologic features in human prostate cancer tissue

OBJECTIVES

Prostaglandin E2 is involved in the carcinogenic process and malignant aggressiveness. These effects are mediated through binding to four specific type E prostanoid (EP) receptors (EP1R to EP4R). Although EPRs are overexpressed in a variety of cancers, their expression pattern varies among different cancer types. The aim of this study was to clarify the clinical significance of EPRs in prostate cancer.

METHODS

We examined the expression of each EPR in 122 prostate cancer tissue samples by immunohistochemistry. We also investigated the relationship between EPRs and cancer cell proliferation.

RESULTS

The rate of immunopositivity for EP1R in cancer cells (36.3% +/- 14.3%) was significantly greater (P < 0.01) than in nontumor glands (7.1% +/- 4.8%) and correlated positively with the Gleason score (P < 0.01), T stage (P < 0.01), N stage (P = 0.03), M stage (P < 0.01), and cancer cell proliferation (r = 0.35, P < 0.01). The EP2R expression in cancer cells (38.9% +/- 11.6%) was significantly greater (P < 0.01) than in nontumor glands (30.6% +/- 8.6%), and correlated with cancer cell proliferation (P < 0.01). The EP4R expression in cancer cells was also significantly greater (P < 0.01) than in nontumor glands. However, the expression of EP2R and EP4R did not correlate with the clinicopathologic features and EP3R expression was not associated with any parameters.

CONCLUSIONS

Our results have indicated that EP1R, EP2R, and EP4R are associated with prostate carcinogenesis. In particular, the EP1R seems to play an important role in malignant aggressiveness and tumor development in patients with prostate cancer.

Cyclooxygenase-2 and colorectal cancer chemoprevention: the beta-catenin connection

Colorectal cancer poses a major clinical challenge in the developed world where this disease is common. Recent findings suggest that the prostaglandin E(2), the proinflammatory product of elevated cyclooxygenase-2 activity in colon cancer, stimulates cancer cell growth through a G protein-dependent signaling pathway coupling the prostaglandin EP2 receptor to beta-catenin control. These findings provide new insights into the molecular framework needed to evaluate chemopreventive strategies for colorectal cancer.

Prostaglandin E(2) protects human lung fibroblasts from cigarette smoke extract-induced apoptosis via EP(2) receptor activation

Prostaglandin E(2) (PGE(2)) has been shown to have a strong cytoprotective effect, inhibiting apoptosis. In the present study, we evaluated whether PGE(2) has a protective effect on cigarette smoke extract (CSE)-induced apoptosis in human lung fibroblasts. Apoptosis was assessed by various methods, including DNA content analysis. CSE (15%-20%) led to apoptosis and induced imbalance in favor of pro- over anti-apoptotic protein expression and activated caspases. PGE(2) blocked CSE-induced apoptosis and modulated the balance of pro- and anti-apoptotic proteins and decreased the activation of caspases. This anti-apoptotic effect was mediated via EP(2) receptor activation as the EP(2) agonist butaprost mimicked PGE(2) activity and siRNA for the EP(2) receptor blocked it. An adenylyl cyclase inhibitor was found to abolish the PGE(2)-mediated cytoprotective effect. Correspondingly, c-AMP analogs blocked CSE-induced apoptosis. Consistently, the protein kinase A (PKA) inhibitor KT-5720 abolished PGE(2)-mediated protection. PGE(2) and butaprost phosphorylated Bad and KT-5720 blocked phosphorylation. These results suggest that PGE(2) inhibits CSE-induced apoptosis via EP(2) receptor activation and activation of PKA, which leads to an alteration in the balance between pro- and anti-apoptotic factors. Through such a mechanism, PGE(2) may alter survival of cells in the smoke-exposed lungs, thus affecting the pathogenesis of cigarette smoke-induced disease.

Prostaglandin E2 enhances the sensitizing effect of hyperthermia on pulmonary C-fibers in rats

This study was carried out to investigate whether the pulmonary C-fiber hypersensitivity induced by hyperthermia is altered by prostaglandin E(2) (PGE(2)). Single-unit afferent activities of pulmonary C-fibers were recorded in anesthetized, artificially ventilated rats when the intrathoracic temperature (T(it)) was maintained at normal (N; approximately 36 degrees C) and hyperthermia levels (H; approximately 41 degrees C) by perfusion of heated saline into the thoracic chamber for 3min. After approximately 20min of recovery, the fiber activities were recorded again during infusion of PGE(2) at both N and H levels of T(it). Our study showed: (1) The baseline fiber activity and responses to lung inflation, right-atrial injection of capsaicin and adenosine were all increased by increasing T(it) from N to H, and these hyperthermia-induced increases in sensitivities were also significantly augmented by PGE(2). (2) These enhanced sensitivities induced by PGE(2) were abolished by pretreatment with AH6809 and AH23848, selective antagonists of EP(2) and EP(4) prostanoid receptors, respectively. In conclusion, the hyperthermia-induced hypersensitivity of vagal pulmonary C-fibers is potentiated by PGE(2), and this effect is mediated through activation of EP(2) and EP(4) prostanoid receptors.

Spinal prostaglandin E receptors of the EP2 subtype and the glycine receptor α3 subunit, which mediate central inflammatory hyperalgesia, do not contribute to pain after peripheral nerve injury or formalin injection

Inflammation, peripheral nerve injury and chemical irritants can cause central sensitization in pain pathways. Prostaglandins produced in the CNS induce central sensitization during inflammation mainly by relieving nociceptive neurons from glycinergic inhibition. We have recently identified spinal prostaglandin E receptors of the EP2 subtype (EP2 receptors) and the glycine receptor alpha3 subunit (GlyR alpha3) as signal transduction elements involved in the generation of central inflammatory hyperalgesia. It is however still unknown to what extent inhibition of glycine receptors by PGE2 contributes to neuropathic or chemically induced pain. To address this question, we have analyzed mice deficient in the EP2 receptor (EP2-/- mice) or in the GlyR alpha3 subunit (GlyR alpha3-/- mice) using the chronic constriction injury (CCI) model of neuropathic pain and the formalin test. We found that EP2-/- mice and GlyR alpha3-/- mice develop thermal and mechanical hyperalgesia in the CCI model indistinguishable from that seen in wild-type mice. In the formalin test, EP2-/- mice, but not GlyR alpha3-/- mice, exhibited reduced nocifensive behavior. The lack of a phenotype in GlyR alpha3-/- mice together with the absence of a facilitating effect of intrathecal PGE2 on formalin-induced nociception in wild-type mice suggests that peripheral rather than spinal EP2 receptors are involved. These results indicate that inhibition of glycinergic neurotransmission by EP2 receptor activation does not contribute to pain following peripheral nerve injury or chemical irritation with formalin. Our results thus provide further evidence that inflammatory hyperalgesia and neuropathic pain involve different mechanisms of central sensitization.

Effects of selective prostaglandins E2 receptor agonists on cultured calvarial murine osteoblastic cells

We compared the direct effects of selective EP4 and EP2 receptor agonists (EP4A and EP2A) with prostaglandin E(2) (PGE(2)) on the differentiation of cultured murine calvarial osteoblastic cells. EP4A increased alkaline phosphatase activity and osteocalcin mRNA levels in these cultures similar to PGE(2). This effect was seen with both "direct plating" immediately after isolating the cells, or "indirect plating" in which the cells were grown to confluence and replated. EP2A had a smaller effect, significant only in "indirect plating" experiments. All three agents decreased the DNA and protein content in indirect plating experiments, but not in direct plating experiments. We conclude that the anabolic effect of PGE(2) in calvarial osteoblastic cell cultures is largely mediated by activation of the EP4 receptor, while activation of the EP2 receptor is less effective.

EP2 receptor stimulation promotes calcium responses in astrocytes via activation of the adenylyl cyclase pathway

Astrocytes are a heterogeneous population of cells that are endowed with a great variety of receptors for neurotransmitters and neuromodulators. Recently prostaglandin E2 has attracted great interest since it is not only released by astrocytes but also activates receptors coupled to either phospholipase C or adenylyl cyclase. We report that EP2 receptor stimulation triggers cAMP production but also causes release of Ca2+ from intracellular stores. This effect is shared by other receptors similarly coupled to adenylyl cyclase and elicited by direct stimulation of the enzyme or application of cAMP analogues. However, the stimulation of the Ca2+ response by cAMP is not mediated by protein kinase A, since a specific antagonist of this kinase had no effect. Such a cross-talk between cAMP and Ca2+ was not observed in all astrocytes. It might therefore reflect a specific resource of either a subpopulation or astrocytes in a specific functional state.

Modulation of ocular inflammatory responses by EP1 receptors in mice

The purpose of the study was to investigate the role of EP1 receptors in intraocular inflammation and to determine possible interplay between EP1, EP2 and EP4 receptors. The eyes of separate groups of EP1 receptor knockout and wild type mice were: 1) treated topically with prostaglandin E2 (PGE2) or the EP2 receptor selective agonist, butaprost; 2) given intravitreal injection of LPS; or 3) paracentesis performed. Another group of knockout mice were pretreated topically with an EP4 receptor selective antagonist prior to paracentesis or LPS treatment. Results demonstrated a significant increase (50% or more) in the protein levels of aqueous humor of the EP1 knockout mice in response to PGE2, paracentesis or LPS. The leukocyte infiltration in the aqueous humor of the knockout mice was 47% higher when compared with that in the wild type controls in response to LPS injection. No significant change was observed in the protein levels in response to butaprost. Pretreating the knockout mice with an EP4 receptor antagonist prior to paracentesis and LPS treatment substantially reduced the aqueous humor protein levels. Also, the leukocyte count in the aqueous humor of the knockout mice in response to LPS was reduced 4 fold after pretreatment with EP4 receptor antagonist when compared with the findings in knockout mice receiving LPS only. We concluded that EP1 receptor has no modulatory effect on EP2 receptors but there is definitely cross-talk between EP1 and EP4 receptors.

Prostaglandin E(2) inhibits collagen expression and proliferation in patient-derived normal lung fibroblasts via E prostanoid 2 receptor and cAMP signaling

Uncontrolled fibroblast activation is one of the hallmarks of fibrotic lung disease. Prostaglandin E(2) (PGE(2)) has been shown to inhibit fibroblast migration, proliferation, collagen deposition, and myofibroblast differentiation in the lung. Understanding the mechanisms for these effects may provide insight into the pathogenesis of fibrotic lung disease. Previous work has focused on commercially available fibroblast cell lines derived from tissue whose precise origin and histopathology are often unknown. Here, we sought to define the mechanism of PGE(2) inhibition in patient-derived fibroblasts from peripheral lung verified to be histologically normal. Fibroblasts were grown from explants of resected lung, and proliferation and collagen I expression was determined following treatment with PGE(2) or modulators of its receptors and downstream signaling components. PGE(2) inhibited fibroblast proliferation by 33% and collagen I expression by 62%. PGE(2) resulted in a 15-fold increase in intracellular cAMP; other cAMP-elevating agents inhibited collagen I in a manner similar to PGE(2). These effects were reproduced by butaprost, a PGE(2) analog selective for the cAMP-coupled E prostanoid (EP) 2 receptor, but not by selective EP3 or EP4 agonists. Fibroblasts expressed both major cAMP effectors, protein kinase A (PKA) and exchange protein activated by cAMP-1 (Epac-1), but only a selective PKA agonist was able to appreciably inhibit collagen I expression. Treatment with okadaic acid, a phosphatase inhibitor, potentiated the effects of PGE(2). Our data indicate that PGE(2) inhibits fibroblast activation in primary lung fibroblasts via binding of EP2 receptor and production of cAMP; inhibition of collagen I proceeds via activation of PKA.

The prostanoid EP2 receptor agonist butaprost increases uveoscleral outflow in the cynomolgus monkey

PURPOSE

To investigate the ocular hypotensive effect of the prostanoid EP2 receptor agonist butaprost and to establish its mechanism of action.

METHODS

All experiments were performed in cynomolgus monkeys after topical application of butaprost (0.1%). The effects of butaprost on aqueous humor flow were determined by fluorophotometry. Total outflow facility was measured by the two-level, constant-pressure perfusion method, and uveoscleral outflow was determined by perfusion of FITC-labeled dextran through the anterior chamber. Effects on ocular morphology were studied after tissue fixation with transcardial perfusion by paraformaldehyde and immersion fixation of the globe, in animals subjected to long-term treatment with butaprost. Conscious ocular normotensive monkeys and monkeys with unilateral ocular hypertension were used for intraocular pressure (IOP) studies.

RESULTS

Butaprost had no significant effect on aqueous humor flow or total outflow facility in ocular normotensive monkeys. Uveoscleral outflow was significantly higher in the butaprost treated eyes than in vehicle treated eyes, 1.03 +/- 0.20 vs. 0.53 +/- 0.18 microL.min(-1). After a 1-year treatment with butaprost, the morphology of the ciliary muscle was changed, showing increased spaces between ciliary muscle bundles and the apparent formation of new outflow channels. In many instances, changes were observed in the trabecular meshwork as well. Butaprost, in a single 0.1% dose, decreased IOP significantly in ocular normotensive monkeys and reduced IOP in laser-induced glaucomatous monkey eyes to the same level as that in the ocular normotensive contralateral eyes.

CONCLUSIONS

The prostanoid EP2 receptor agonist butaprost appears to lower IOP by increasing uveoscleral outflow, according to both physiological and morphologic findings. Although the prostanoid EP2 receptor is structurally and functionally distinct from the FP receptor, the effects of EP2 and FP receptor stimulation on aqueous humor outflow are similar.