Studies on the characterisation of prostanoid receptors: a proposed classification

Prostaglandins for Postpartum Hemorrhage: Pharmacology, Application, and Current Opinion

BACKGROUND

Postpartum hemorrhage (PPH) remains a common cause of maternal mortality worldwide. Medical intervention plays an important role in the prevention and treatment of PPH. Prostaglandins (PGs) are currently recommended as second-line uterotonics, which are applied in cases of persistent bleeding despite oxytocin treatment.

SUMMARY

PG agents that are constantly used in clinical practice include carboprost, sulprostone, and misoprostol, representing the analogs of PGF2α, PGE2, and PGE1, respectively. Injectable PGs, when used to treat PPH, are effective in reducing blood loss but probably induce cardiovascular or respiratory side effects. Misoprostol is characterized by oral administration, low cost, stability in storage, broad availability, and minimal side effects. It remains a treatment option for uterine atony in low-resource settings, but its effectiveness as a uterotonic for independent application may be limited. Key Messages: The present review article discusses the physiological roles of various natural PGs, evaluates the existing evidence of PG analogs in the prevention and treatment of PPH, and finally provides a reference to assist obstetricians in selecting appropriate uterotonics.

Prostaglandin E2 receptors and their role in gastrointestinal motility - Potential therapeutic targets

Prostaglandin E2 (PGE2) is found throughout the gastrointestinal tract in a diverse variety of functions and roles. The recent discovery of four PGE2 receptor subtypes in intestinal muscle layers as well as in the enteric plexus has led to much interest in the study of their roles in gut motility. Gut dysmotility has been implicated in functional disease processes including irritable bowel syndrome (IBS) and slow transit constipation, and lubiprostone, a PGE2 derivative, has recently been licensed to treat both conditions. The diversity of actions of PGE2 in the intestinal tract is attributed to its differing effects on its downstream receptor types, as well as their varied distribution in the gut, in both health and disease. This review aims to identify the role and distribution of PGE2 receptors in the intestinal tract, and aims to elucidate their distinct role in gut motor function, with a specific focus on functional intestinal pathologies.

PGE2 vs PGF2α in human parturition

Prostaglandin E2 (PGE2) and F2α (PGF2α) are the two most prominent prostanoids in parturition. They are involved in cervical ripening, membrane rupture, myometrial contraction and inflammation in gestational tissues. Because multiple receptor subtypes for PGE2 and PGF2α exist, coupled with diverse signaling pathways, the effects of PGE2 and PGF2α depend largely on the spatial and temporal expression of these receptors in intrauterine tissues. It appears that PGE2 and PGF2α play different roles in parturition. PGE2 is probably more important for labor onset, while PGF2α may play a more important role in labor accomplishment, which may be attributed to the differential effects of PGE2 and PGF2α in gestational tissues. PGE2 is more powerful than PGF2α in the induction of cervical ripening. In terms of myometrial contraction, PGE2 produces a biphasic effect with an initial contraction and a following relaxation, while PGF2α consistently stimulates myometrial contraction. In the fetal membranes, both PGE2 and PGF2α appear to be involved in the process of membrane rupture. In addition, PGE2 and PGF2α may also participate in the inflammatory process of intrauterine tissues at parturition by stimulating not only neutrophil influx and cytokine production but also cyclooxygenase-2 expression thereby intensifying their own production. This review summarizes the differential roles of PGE2 and PGF2α in parturition with respect to their production and expression of receptor subtypes in gestational tissues. Dissecting the specific mechanisms underlying the effects of PGE2 and PGF2α in parturition may assist in developing specific therapeutic targets for preterm and post-term birth.

Effect of Prostanoids on Human Platelet Function: An Overview

Prostanoids are bioactive lipid mediators and take part in many physiological and pathophysiological processes in practically every organ, tissue and cell, including the vascular, renal, gastrointestinal and reproductive systems. In this review, we focus on their influence on platelets, which are key elements in thrombosis and hemostasis. The function of platelets is influenced by mediators in the blood and the vascular wall. Activated platelets aggregate and release bioactive substances, thereby activating further neighbored platelets, which finally can lead to the formation of thrombi. Prostanoids regulate the function of blood platelets by both activating or inhibiting and so are involved in hemostasis. Each prostanoid has a unique activity profile and, thus, a specific profile of action. This article reviews the effects of the following prostanoids: prostaglandin-D₂ (PGD₂), prostaglandin-E₁, -E₂ and E₃ (PGE₁, PGE₂, PGE₃), prostaglandin F_2α (PGF_2α), prostacyclin (PGI₂) and thromboxane-A₂ (TXA₂) on platelet activation and aggregation via their respective receptors.

Heteromerization fingerprints between bradykinin B2 and thromboxane TP receptors in native cells

Bradykinin (BK) and thromboxane-A2 (TX-A2) are two vasoactive mediators that modulate vascular tone and inflammation via binding to their cognate "class A" G-protein coupled receptors (GPCRs), BK-B2 receptors (B2R) and TX-prostanoid receptors (TP), respectively. Both BK and TX-A2 lead to ERK1/2-mediated vascular smooth muscle cell (VSMC) proliferation and/or hypertrophy. While each of B2R and TP could form functional dimers with various GPCRs, the likelihood that B2R-TP heteromerization could contribute to their co-regulation has never been investigated. The main objective of this study was to investigate the mode of B2R and TP interaction in VSMC, and its possible impact on downstream signaling. Our findings revealed synergistically activated ERK1/2 following co-stimulation of rat VSMC with a subthreshold dose of BK and effective doses of the TP stable agonist, IBOP, possibly involving biased agonist signaling. Single detection of each of B2R and TP in VSMC, using in-situ proximity ligation assay (PLA), provided evidence of the constitutive expression of nuclear and extranuclear B2R and TP. Moreover, inspection of B2R-TP PLA signals in VSMC revealed agonist-modulated nuclear and extranuclear proximity between B2R and TP, whose quantification varied substantially following single versus dual agonist stimulations. B2R-TP interaction was further verified by the findings of co-immunoprecipitation (co-IP) analysis of VSMC lysates. To our knowledge, this is the first study that provides evidence supporting the existence of B2R-TP heteromerization fingerprints in primary cultured VSMC.

Periaqueductal Grey EP3 Receptors Facilitate Spinal Nociception in Arthritic Secondary Hypersensitivity

Descending controls on spinal nociceptive processing play a pivotal role in shaping the pain experience after tissue injury. Secondary hypersensitivity develops within undamaged tissue adjacent and distant to damaged sites. Spinal neuronal pools innervating regions of secondary hypersensitivity are dominated by descending facilitation that amplifies spinal inputs from unsensitized peripheral nociceptors. Cyclooxygenase-prostaglandin (PG) E2 signaling within the ventrolateral periaqueductal gray (vlPAG) is pronociceptive in naive and acutely inflamed animals, but its contributions in more prolonged inflammation and, importantly, secondary hypersensitivity remain unknown. In naive rats, PG EP3 receptor (EP3R) antagonism in vlPAG modulated noxious withdrawal reflex (EMG) thresholds to preferential C-nociceptor, but not A-nociceptor, activation and raised thermal withdrawal thresholds in awake animals. In rats with inflammatory arthritis, secondary mechanical and thermal hypersensitivity of the hindpaw developed and was associated with spinal sensitization to A-nociceptor inputs alone. In arthritic rats, blockade of vlPAG EP3R raised EMG thresholds to C-nociceptor activation in the area of secondary hypersensitivity to a degree equivalent to that evoked by the same manipulation in naive rats. Importantly, vlPAG EP3R blockade also affected responses to A-nociceptor activation, but only in arthritic animals. We conclude that vlPAG EP3R activity exerts an equivalent facilitation on the spinal processing of C-nociceptor inputs in naive and arthritic animals, but gains in effects on spinal A-nociceptor processing from a region of secondary hypersensitivity. Therefore, the spinal sensitization to A-nociceptor inputs associated with secondary hypersensitivity is likely to be at least partly dependent on descending prostanergic facilitation from the vlPAG.

SIGNIFICANCE STATEMENT

After tissue damage, sensitivity to painful stimulation develops in undamaged areas (secondary hypersensitivity). This is found in many painful conditions, particularly arthritis. The periaqueductal gray (PAG) is an important center that controls spinal nociceptive processing, on which secondary hypersensitivity depends. Prostaglandins (PGs) are mediators of inflammation with pronociceptive actions within the PAG under normal conditions. We find that secondary hindpaw hypersensitivity in arthritic rats results from spinal sensitization to peripheral A-nociceptor inputs. In the PAG of arthritic, but not naive, rats, there is enhanced control of spinal A-nociceptor processing through PG EP3 receptors. The descending facilitatory actions of intra-PAG PGs play a direct and central role in the maintenance of inflammatory secondary hypersensitivity, particularly relating to the processing of A-fiber nociceptive information.

The Promise of Prostaglandins: Have They Fulfilled Their Potential as Therapeutic Targets for the Delay of Preterm Birth?

OBJECTIVE

The elucidation some 30 years ago by Sir Mont Liggins that the activation of the hypothalamic-pituitary-adrenal-placental axis in fetal sheep led to elevated maternal prostaglandin (PG) concentrations and the initiation of labor provided hope that targeting PG synthesis or action would lead to effective tocolysis and lowering of the human preterm birth rate. This was the "promise of PGs."

METHODS AND RESULTS

Although early trials showed that nonsteroidal anti-inflammatory drugs (NSAIDs), which inhibit PG H synthase (PGHS), delayed preterm birth by 48 hours, other trials revealed an association between NSAIDs and adverse fetal effects, including oligohydramnios, patent ductus arteriosus, necrotizing enterocolitis, intraventricular hemorrhage, and persistent pulmonary hypertension of the newborn (PPHN). Hope was revived when studies in the mid 1990s demonstrated that much of the PGs synthesized by intrauterine tissues at preterm labor were derived from the inducible isoenzyme PGHS-2. Unfortunately, administration of specific PGHS-2 inhibitors led to the same adverse fetal effects displayed by the mixed PGHS-1 and -2 NSAIDs, causing interest in the promise of PGs to wane. This led to the development of new strategies for specific PG inhibition or antagonism. One of these is the application of a specific PGF2alpha receptor blocker, Theratechnologies (THG)113.31. THG113.31 decreases the in vitro contractile activity of mouse, sheep, and human myometrium in response to exogenous PGF2alpha, delays lipopolysaccharide (LPS)-induced preterm birth in mice, and lowers uterine electromyographic activity and delays preterm birth in sheep administered RU486. There have been no observable maternal or fetal side effects with its use.

CONCLUSION

By developing new strategies based on other therapeutic targets, the promise of PGs may once again offer hope for delaying preterm birth.

Structural features of subtype-selective EP receptor modulators

Prostaglandin E2 is a potent endogenous molecule that binds to four different G-protein-coupled receptors: EP1-4. Each of these receptors is a valuable drug target, with distinct tissue localisation and signalling pathways. We review the structural features of EP modulators required for subtype-selective activity, as well as the structural requirements for improved pharmacokinetic parameters. Novel EP receptor subtype selective agonists and antagonists appear to be valuable drug candidates in the therapy of many pathophysiological states, including ulcerative colitis, glaucoma, bone healing, B cell lymphoma, neurological diseases, among others, which have been studied in vitro, in vivo and in early phase clinical trials.

Mapping PTGERs to the Ovulatory Follicle: Regional Responses to the Ovulatory PGE2 Signal

Prostaglandin E2 (PGE2) is a key intrafollicular mediator of ovulation in many, if not all, mammalian species. PGE2 acts at follicular cells via four distinct PGE2 receptors (PTGERs). Within the ovulatory follicle, each cell type (e.g., oocyte, cumulus granulosa cell, mural granulosa cell, theca cell, endothelial cell) expresses a different subset of the four PTGERs. Expression of a subset of PTGERs has consequences for the generation of intracellular signals and ultimately the unique functions of follicular cells that respond to PGE2. Just as the ovulatory LH surge regulates PGE2 synthesis, the LH surge also regulates expression of the four PTGERs. The pattern of expression of the four PTGERs among follicular cells before and after the LH surge forms a spatial and temporal map of PGE2 responses. Differential PTGER expression, coupled with activation of cell-specific intracellular signals, may explain how a single paracrine mediator can have pleotropic actions within the ovulatory follicle. Understanding the role of each PTGER in ovulation may point to previously unappreciated opportunities to both promote and prevent fertility.

The mechanistic basis of prostacyclin and its stable analogues in pulmonary arterial hypertension: Role of membrane versus nuclear receptors

Pulmonary arterial hypertension (PAH) is a progressive disease of distal pulmonary arteries in which patients suffer from elevated pulmonary arterial pressure, extensive vascular remodelling and right ventricular failure. To date prostacyclin (PGI2) therapy remains the most efficacious treatment for PAH and is the only approved monotherapy to have a positive impact on long-term survival. A key thing to note is that improvement exceeds that predicted from vasodilator testing strongly suggesting that additional mechanisms contribute to the therapeutic benefit of prostacyclins in PAH. Given these agents have potent antiproliferative, anti-inflammatory and endothelial regenerating properties suggests therapeutic benefit might result from a slowing, stabilization or even some reversal of vascular remodelling in vivo. This review discusses evidence that the pharmacology of each prostacyclin (IP) receptor agonist so far developed is distinct, with non-IP receptor targets clearly contributing to the therapeutic and side effect profile of PGI2 (EP3), iloprost (EP1), treprostinil (EP2, DP1) along with a family of nuclear receptors known as peroxisome proliferator-activated receptors (PPARs), to which PGI2 and some analogues directly bind. These targets are functionally expressed to varying degrees in arteries, veins, platelets, fibroblasts and inflammatory cells and are likely to be involved in the biological actions of prostacylins. Recently, a highly selective IP agonist, selexipag has been developed for PAH. This agent should prove useful in distinguishing IP from other prostanoid receptors or PPAR binding effects in human tissue. It remains to be determined whether selectivity for the IP receptor gives rise to a superior or inferior clinical benefit in PAH.

Aspirin-intolerant asthma: a comprehensive review of biomarkers and pathophysiology

Aspirin-exacerbated respiratory disease is a tetrad of nasal polyps, chronic hypertrophic eosinophilic sinusitis, asthma, and sensitivity to aspirin. Unawareness of this clinical condition by patients and physicians may have grave consequences because of its association with near-fatal asthma. The pathogenesis of aspirin-intolerant asthma is not related with an immunoglobin E mechanism, but with an abnormal metabolism of the lipoxygenase (LO) and cyclooxygenase (COX) pathways. At present, a diagnosis of aspirin sensitivity can be established only by provocative aspirin challenge, which represents a health risk for the patient. This circumstance has encouraged the search for aspirin intolerance-specific biomarkers. Major attempts have focused on mediators related with inflammation and eicosanoid regulation. The use of modern laboratory techniques including high-throughput methods has facilitated the detection of dozens of biological metabolites associated with aspirin-intolerant asthma disease. Not surprisingly, the majority of these is implicated in the LO and COX pathways. However, substantial amounts of data reveal the participation of many genes deriving from different ontologies. Biomarkers may represent a powerful, noninvasive tool in the diagnosis of aspirin sensitivity; moreover, they could provide a new way to classify asthma phenotypes.

Prostaglandin E2 mediates connecting tubule glomerular feedback

Connecting tubule glomerular feedback (CTGF) is a mechanism in which Na reabsorption in the connecting tubule (CNT) causes afferent arteriole (Af-Art) dilation. CTGF is mediated by eicosanoids, including prostaglandins and epoxyeicosatrienoic acids; however, their exact nature and source remain unknown. We hypothesized that during CTGF, the CNT releases prostaglandin E2, which binds its type 4 receptor (EP4) and dilates the Af-Art. Rabbit Af-Arts with the adherent CNT intact were microdissected, perfused, and preconstricted with norepinephrine. CTGF was elicited by increasing luminal NaCl in the CNT from 10 to 80 mmol/L. We induced CTGF with or without the EP4 receptor blocker ONO-AE3-208 added to the bath in the presence of the epoxyeicosatrienoic acid synthesis inhibitor MS-PPOH. ONO-AE3-208 abolished CTGF (control, 9.4 ± 0.5; MS-PPOH+ONO-AE3-208, -0.6 ± 0.2 μm; P<0.001; n=6). To confirm these results, we used a different, specific EP4 blocker, L161982 (10(-5) mol/L), that also abolished CTGF (control, 8.5 ± 0.9; MS-PPOH+L161982, 0.8 ± 0.4 μm; P<0.001; n=6). To confirm that the eicosanoids that mediate CTGF are released from the CNT rather than the Af-Art, we first disrupted the Af-Art endothelium with an antibody and complement. Endothelial disruption did not affect CTGF (7.9 ± 0.9 versus 8.6 ± 0.6 μm; P=NS; n=7). We then added arachidonic acid to the lumen of the CNT while maintaining zero NaCl in the perfusate. Arachidonic acid caused dose-dependent dilation of the attached Af-Art (from 8.6 ± 1.2 to 15.3 ± 0.7 μm; P<0.001; n=6), and this effect was blocked by ONO-AE3-208 (10(-7) mol/L). We conclude that during CTGF, the CNT releases prostaglandin E2, which acts on EP4 on the Af-Art inducing endothelium-independent dilation.

PGE2 maintains the tone of the guinea pig trachea through a balance between activation of contractile EP1 receptors and relaxant EP2 receptors

BACKGROUND AND PURPOSE

The guinea pig trachea (GPT) is commonly used in airway pharmacology. The aim of this study was to define the expression and function of EP receptors for PGE(2) in GPT as there has been ambiguity concerning their role.

EXPERIMENTAL APPROACH

Expression of mRNA for EP receptors and key enzymes in the PGE(2) pathway were assessed by real-time PCR using species-specific primers. Functional studies of GPT were performed in tissue organ baths.

KEY RESULTS

Expression of mRNA for the four EP receptors was found in airway smooth muscle. PGE(2) displayed a bell-shaped concentration-response curve, where the initial contraction was inhibited by the EP(1) receptor antagonist ONO-8130 and the subsequent relaxation by the EP(2) receptor antagonist PF-04418948. Neither EP(3) (ONO-AE5-599) nor EP(4) (ONO-AE3-208) selective receptor antagonists affected the response to PGE(2). Expression of COX-2 was greater than COX-1 in GPT, and the spontaneous tone was most effectively abolished by selective COX-2 inhibitors. Furthermore, ONO-8130 and a specific PGE(2) antibody eliminated the spontaneous tone, whereas the EP(2) antagonist PF-04418948 increased it. Antagonists of other prostanoid receptors had no effect on basal tension. The relaxant EP(2) response to PGE(2) was maintained after long-term culture, whereas the contractile EP(1) response showed homologous desensitization to PGE(2), which was prevented by COX-inhibitors.

CONCLUSIONS AND IMPLICATIONS

Endogenous PGE(2), synthesized predominantly by COX-2, maintains the spontaneous tone of GPT by a balance between contractile EP(1) receptors and relaxant EP(2) receptors. The model may be used to study interactions between EP receptors.

Inactivation of the E-prostanoid 3 receptor attenuates the angiotensin II pressor response via decreasing arterial contractility

OBJECTIVE

The present studies aimed at elucidating the role of prostaglandin E(2) receptor subtype 3 (E-prostanoid [EP] 3) in regulating blood pressure.

METHODS AND RESULTS

Mice bearing a genetic disruption of the EP3 gene (EP(3)(-/-)) exhibited reduced baseline mean arterial pressure monitored by both tail-cuff and carotid arterial catheterization. The pressor responses induced by EP3 agonists M&B28767 and sulprostone were markedly attenuated in EP3(-/-) mice, whereas the reduction of blood pressure induced by prostaglandin E(2) was comparable in both genotypes. Vasopressor effect of acute or chronic infusion of angiotensin II (Ang II) was attenuated in EP3(-/-) mice. Ang II-induced vasoconstriction in mesenteric arteries decreased in EP3(-/-) group. In mesenteric arteries from wild-type mice, Ang II-induced vasoconstriction was inhibited by EP3 selective antagonist DG-041 or L798106. The expression of Arhgef-1 is attenuated in EP3 deficient mesenteric arteries. EP3 antagonist DG-041 diminished Ang II-induced phosphorylation of myosin light chain 20 and myosin phosphatase target subunit 1 in isolated mesenteric arteries. Furthermore, in vascular smooth muscle cells, Ang II-induced intracellular Ca(2+) increase was potentiated by EP3 agonist sulprostone but inhibited by DG-041.

CONCLUSIONS

Activation of the EP3 receptor raises baseline blood pressure and contributes to Ang II-dependent hypertension at least partially via enhancing Ca(2+) sensitivity and intracellular calcium concentration in vascular smooth muscle cells. Selective targeting of the EP3 receptor may represent a potential therapeutic target for the treatment of hypertension.

Prostanoid receptors involved in regulation of the beating rate of neonatal rat cardiomyocytes

Although prostanoids are known to be involved in regulation of the spontaneous beating rate of cultured neonatal rat cardiomyocytes, the various subtypes of prostanoid receptors have not been investigated in detail. In our experiments, prostaglandin (PG)F_2α and prostanoid FP receptor agonists (fluprostenol, latanoprost and cloprostenol) produced a decrease in the beating rate. Two prostanoid IP receptor agonists (iloprost and beraprost) induced first a marked drop in the beating rate and then definitive abrogation of beating. In contrast, the prostanoid DP receptor agonists (PGD₂ and BW245C) and TP receptor agonists (U-46619) produced increases in the beating rate. Sulprostone (a prostanoid EP₁ and EP₃ receptor agonist) induced marked increases in the beating rate, which were suppressed by SC-19220 (a selective prostanoid EP₁ antagonist). Butaprost (a selective prostanoid EP₂ receptor agonist), misoprostol (a prostanoid EP₂ and EP₃ receptor agonist), 11-deoxy-PGE₁ (a prostanoid EP₂, EP₃ and EP₄ receptor agonist) did not alter the beating rate. Our results strongly suggest that prostanoid EP₁ receptors are involved in positive regulation of the beating rate. Prostanoid EP₁ receptor expression was confirmed by western blotting with a selective antibody. Hence, neonatal rat cardiomyocytes express both prostanoid IP and FP receptors (which negatively regulate the spontaneous beating rate) and prostanoid TP, DP₁ and EP₁ receptors (which positively regulate the spontaneous beating rate).

Acute effects of prostaglandin E1 and E2 on vascular reactivity and blood flow in situ in the chick chorioallantoic membrane

The chick chorioallantoic membrane (CAM) subserves gas exchange in the developing embryo and shell-less culture affords a unique opportunity for direct observations over time of individual blood vessels to pharmacologic interventions. We tested a number of lipids including prostaglandins PGE(1&2) for vascular effects and signaling in the CAM. Application of PGE(1&2) induced a decrease in the diameter of large blood vessels and a concentration-dependent, localized, reversible loss of blood flow through small vessels. The loss of flow was also mimicked by misoprostol, an agonist for 3 of 4 known PGE receptors, EP(2-4), and by U46619, a thromboxane mimetic. Selective receptor antagonists for EP(3) and thromboxane each partially blocked the response. This is a first report of the effects of prostaglandins on vasoreactivity in the CAM. Our model allows the unique ability to examine simultaneous responses of large and small vessels in real time and in vivo.

Binding and activity of the prostacyclin receptor (IP) agonists, treprostinil and iloprost, at human prostanoid receptors: treprostinil is a potent DP1 and EP2 agonist

The prostacyclin analogues, iloprost and treprostinil are extensively used in treating pulmonary hypertension. Their binding profile and corresponding biochemical cellular responses on human prostanoid receptors expressed in cell lines, have now been compared. Iloprost had high binding affinity for EP1 and IP receptors (Ki 1.1 and 3.9 nM, respectively), low affinity for FP, EP3 or EP4 receptors, and very low affinity for EP2, DP1 or TP receptors. By contrast, treprostinil had high affinity for the DP1, EP2 and IP receptors (Ki 4.4, 3.6 and 32 nM, respectively), low affinity for EP1 and EP4 receptors and even lower affinity for EP3, FP and TP receptors. In functional assays, iloprost had similar high activity in elevating cyclic AMP levels in cells expressing the human IP receptor and stimulating calcium influx in cells expressing EP1 receptors (EC50 0.37 and 0.3 nM, respectively) with the rank order of activity on the other receptors comparable to the binding assays. As with binding studies, treprostinil elevated cyclic AMP with a similar high potency in cells expressing DP1, IP and EP2 receptors (EC50 0.6, 1.9 and 6.2 nM, respectively), but had low activity at the other receptors. Activation of IP, DP1 and EP2 receptors, as with treprostinil, can all result in vasodilatation of human pulmonary arteries. However, activation of EP1 receptors can provoke vasoconstriction, and hence may offset the IP-receptor mediated vasodilator effects of iloprost. Treprostinil may therefore differ from iloprost in its overall beneficial pulmonary vasorelaxant profile and other pharmacological actions, especially in diseases where the IP receptor is down-regulated.

International Union of Basic and Clinical Pharmacology. LXXXIII: classification of prostanoid receptors, updating 15 years of progress

It is now more than 15 years since the molecular structures of the major prostanoid receptors were elucidated. Since then, substantial progress has been achieved with respect to distribution and function, signal transduction mechanisms, and the design of agonists and antagonists (http://www.iuphar-db.org/DATABASE/FamilyIntroductionForward?familyId=58). This review systematically details these advances. More recent developments in prostanoid receptor research are included. The DP(2) receptor, also termed CRTH2, has little structural resemblance to DP(1) and other receptors described in the original prostanoid receptor classification. DP(2) receptors are more closely related to chemoattractant receptors. Prostanoid receptors have also been found to heterodimerize with other prostanoid receptor subtypes and nonprostanoids. This may extend signal transduction pathways and create new ligand recognition sites: prostacyclin/thromboxane A(2) heterodimeric receptors for 8-epi-prostaglandin E(2), wild-type/alternative (alt4) heterodimers for the prostaglandin FP receptor for bimatoprost and the prostamides. It is anticipated that the 15 years of research progress described herein will lead to novel therapeutic entities.

High-resolution mapping of prostaglandin E2-dependent signaling networks identifies a constitutively active PKA signaling node in CD8+CD45RO+ T cells

To analyze prostaglandin E(2) (PGE(2)) signaling in lymphoid cells, we introduce a multipronged strategy, combining temporal quantitative phosphoproteomics and phospho flow cytometry. We describe the PGE(2)-induced phosphoproteome by simultaneous monitoring of approximately 250 regulated phospho-epitopes, which, according to kinase prediction algorithms, originate from a limited number of kinase networks. Assessing these signaling pathways by phospho flow cytometry provided higher temporal resolution at various PGE(2) concentrations in multiple lymphoid cell subsets. This showed elevated levels of protein kinase A (PKA) signaling in unstimulated CD8(+)CD45RO(+) T cells, which correlated with suppressed proximal T-cell receptor signaling, indicating that PKA sets the threshold for activation. The combination of phosphoproteomics and high throughput phospho flow cytometry applied here provides a comprehensive generic framework for the analysis of signaling networks in mixed cell populations.

Iloprost for additional anticoagulation in continuous renal replacement therapy--a pilot study

PURPOSE

The aim of this pilot study was to compare the effect of heparin anticoagulation with and without iloprost administration during continuous renal replacement therapy (CRRT) in critically ill patients.

MATERIAL AND METHODS

In a prospective, randomized, controlled pilot study at an intensive care unit at a university hospital, 20 patients requiring CRRT were investigated. Patients were allocated into two groups: group 1, the heparin group; and group 2, the heparin plus 1 ng/kg/min iloprost. In both groups, activated partial thromboplastin time (aPTT) was adjusted to 40-50 sec. Observation time was a maximum of 7 days.

RESULTS

Median filter run time was significantly prolonged by iloprost administration to a median of 14 h (13-26 h) compared to 10 h (4-12 h) in the heparin group (p = 0.004). A decrease in platelet count was attenuated by iloprost administration (p = 0.012). There were no bleeding complications in either group. Hemofiltration efficiency did not differ significantly between the groups.

CONCLUSION

Additional administration of iloprost prolonged the filter run time of continuous veno-venous hemofiltration (CVVH) in this setting and attenuated the fall in platelet count during CRRT.

Effects of gestational age on prostaglandin EP receptor expression and functional involvement during in vitro contraction of the guinea pig uterus

Prostaglandin E(2) (PGE(2)) exerts diverse biological effects through four G-protein-coupled cell surface receptor subtypes, EP1-4. This study's objective was to characterize EP1-4 receptor mRNA expression within pregnant guinea pig myometrium during early implantation stage (gestation day [GD] 6) and late stage gestation (GD 50) and evaluate in vitro contractile activity of receptor subtype selective agonists. Using RT-PCR, qualitative gene expression patterns of EP2, EP3, and EP4 mRNA were detected in the myometrium and remained unchanged between the gestational ages. EP1 mRNA remained undetected in pregnant tissue. In vitro contractile activity was evaluated in GD 6 and GD 50 myometrium using vehicle and EP agonists PGE(2), 17-phenyl trinor PGE(2), sulprostone, misoprostol, and CP-533,536. All spasmogens in pregnant myometrium were EP1/EP3 selective agonists, though likely acting via EP3 receptors in this test model. CP-533,536--a highly selective EP2 receptor agonist--and the vehicle failed to induce myometrial contraction at both gestational ages.

Schwann cell-derived factors modulate synaptic activities at developing neuromuscular synapses

Glial cells are active participants in the function, formation, and maintenance of the chemical synapse. To investigate the molecular basis of neuron-glia interactions at the peripheral synapse, we examined whether and how Schwann cell-derived factors modulate synaptic function at developing neuromuscular junctions (NMJs). Schwann cell-conditioned medium (SC-CM) from Xenopus Schwann cell cultures was collected and applied to Xenopus nerve-muscle cocultures. We found that SC-CM increased the frequency of spontaneous synaptic currents (SSCs) within 3-15 min by an average of approximately 150-fold at developing neuromuscular synapses. The increase in SSC frequency by SC-CM is a presynaptic effect independent of neuronal excitability and requires the influx of Ca2+. In contrast to its potentiating effect on spontaneous transmitter release, SC-CM suppressed the evoked transmitter release. The SC-CM effect required the presence of motoneuron soma but not protein synthesis. Using molecular weight cutoff filters and dialysis membranes, we found that the molecular weight of functional factor(s) in SC-CM was within 500 and 5000 Da. The SC-CM effect was not attributable to currently known factors that modulate synaptic efficacy, including neurotrophins, glutamate, and ATP. SC-CM also enhanced spontaneous synaptic release at developing NMJs in Xenopus tadpoles in situ. Our results suggest that Schwann cells release small molecules that enhance spontaneous synaptic activities acutely and potently at developing neuromuscular synapses, and the glial cell-enhanced spontaneous neurotransmission may contribute to synaptogenesis.

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.

Localization and steroid regulation of prostaglandin E2 receptor protein expression in ovine cervix

Although prostaglandin E2 (PGE2) has been identified as a central mediator of the cervical ripening process, the mechanisms responsible for PGE2 ripening are still poorly understood, partly because of the lack of information concerning the precise cellular localization and regulation of PGE2 (EP) receptors in the cervix. To provide new insights into the mechanisms of cervical ripening, we used indirect immunofluorescence to localize cervical EP receptor protein expression in ovariectomized ewes and examined the effect of administration of progesterone or estradiol. EP receptors were widely distributed in cervical blood vessels, epithelium of the cervical canal, circular and longitudinal muscles, and stroma. Estradiol replacement decreased EP1 and EP3 receptor protein in blood vessel media (by 23 and 31% respectively, P < 0.05) and decreased EP1 receptor protein expression in the longitudinal muscle layer (by 27%, P < 0.05). Stromal EP1 and EP3 receptor protein expression was also reduced by estradiol (by 29 and 20% respectively, P < 0.05). Progesterone replacement had no significant effect on EP receptor protein expression. The arterial changes would favor PGE2-induced vasodilatation, subsequent edema and leukocyte infiltration during the cervical ripening process whereas the muscular alterations would facilitate smooth muscle relaxation and cervical dilatation. Furthermore, estradiol provoked perinuclear localization of EP3 receptor protein in the longitudinal muscle layer. This latter result suggests that cellular EP receptor localization is regulated by estradiol and that PGE2 may also control smooth muscle contraction and regulate ovine cervical dilatation in an intracrine manner via EP3 receptors.

Delay of preterm birth in sheep by THG113.31, a prostaglandin F2alpha receptor antagonist

OBJECTIVE

A novel prostaglandin F2alpha receptor antagonist, THG113.31, was tested for the suppression of uterine contractility and delay of preterm labor in sheep.

STUDY DESIGN

We determined the tocolytic effectiveness of THG113.31 on contractions that were stimulated in vitro by prostaglandin F2alpha and E2 in longitudinal and circular myometrial strips. We also tested the ability of THG113.31 in vivo to lower uterine electromyographic activity that was induced by the progesterone receptor blocker, RU486, and to delay preterm birth.

RESULTS

THG113.31 suppressed the amplitude of prostaglandin F2alpha, but not prostaglandin E2-induced contractions of both circular and longitudinal myometrium (P<.01). The times to delivery after RU486 were 34.8+/-1.1 hours (saline solution) and 41.9+/-0.5 hours (THG113.31; P<.001) or an average delay of 7.1 hours. There were no changes in fetal blood gases (PaO2 , PaCO2 , pH, or SaO2) because of THG113.31. Fetal cortisol levels rose in each group, and fetal and maternal prostaglandin E2 and F2alpha metabolite concentrations rose similarly in both groups.

CONCLUSION

THG113.31 specifically suppresses prostaglandin F2alpha-induced myometrial contractility and delays delivery.

Functional pharmacology of human prostanoid EP2 and EP4 receptors

Prostanoid EP(2) and EP(4) receptor-mediated responses are difficult to distinguish pharmacologically because of the lack of potent, selective antagonists. We describe systematic agonist fingerprints for recombinant human prostanoid EP(2) and EP(4) receptors expressed in CHO and HEK293 cells, respectively. The rank orders of potency of endogenous prostaglandins were: prostanoid EP(2) receptors: prostaglandin E(2)>>prostaglandin D(2)=prostaglandin F(2alpha)>prostaglandin I(2); prostanoid EP(4) receptors: prostaglandin E(2)>>prostaglandin I(2)>prostaglandin D(2)=prostaglandin F(2alpha). Butaprost free acid (9-oxo-11alpha,16R-dihydroxy-17-cyclobutyl-prost-13E-en-1-oic acid) behaved as a highly selective partial agonist at prostanoid EP(2) receptors while butaprost methyl ester elicited small, low potency responses. The prostanoid EP(1) and EP(3) receptor agonists misoprostol (9-oxo-11alpha,16-dihydroxy-16-methyl-prost-13E-en-1-oic acid, methyl ester), sulprostone (N-(methylsulphonyl)-9-oxo-11alpha,15R-dihydroxy-16-phenoxy-17,18,19,20-tetranor-prosta-5Z,13E-dien-1-amide), and GR63799X ([1R-[1alpha(Z),2beta(R*),3alpha]-(-)-4-benzoylamino)phenyl-7-[3-hydroxy-3-phenoxy-propoxy)-5-oxocyclopentyl]-4-heptenoate), and the prostanoid DP receptor agonist BW245C ((4S)-(3-[(3R,S)-3-cyclohexyl-3-hydropropyl]-2,5-dioxo)-4-imidazolidineheptanoic acid), activated both prostanoid EP(2) and EP(4) receptors. Prostaglandin I(2), iloprost (6,9alpha-methylene-11alpha,15S-dihydroxy-16-methyl-prosta-5E,13E-dien-18-yn-1-oic acid, trometamol salt) and cicaprost (5-[(E)-(1S, 5S, 6S, 7R)-7-hydroxy-6-[(3S, 4S)-3-hydroxy-4-methylnona-1,6-diinyl]-bicyclo[3.3.0]octan-3-yliden]-3-oxapentanoic acid; ZK96480) were full agonists at prostanoid EP(4) receptors. Key differentiating agonists are: butaprost FA, 16,16-dimethyl-prostaglandin E(2), 19-(R)-hydroxy prostaglandin E(2), misoprostol, BW245C, prostaglandin F(2alpha) and prostaglandin D(2).

The immunomodulatory actions of E-type prostaglandins

Prostaglandins (PGs) have been recognised as modulators of immune responses. This has been proved by both in vitro studies and from observations in animals and humans. Administration of prostaglandins for therapeutic purposes, however, has been hampered by their limited bioavailability and their pleiotropic effects, with resultant toxicological profile. Despite this, some success has been demonstrated in the clinic for the control of graft rejection, especially when used as part of a broader immunosuppressant regimen. Full realisation of the therapeutic potential of prostaglandins will depend on a better understanding of their mechanism of action at the cellular level. Recently, it has been appreciated that prostaglandins do not merely inhibit T-cell function, but appear to modulate the profile of lymphocyte sub-populations through regulation of cytokine synthesis and release. Recent efforts have also begun to focus on identifying prostaglandin receptor subtypes important for immune regulation and offer a means, together with targeted delivery, of utilising the immunosuppressant/anti-inflammatory effects of E-type prostaglandins in a safe and effective manner.

COX-2 Inhibitors at The 8th International Conference of the Inflammation Research Association

An intensive search is underway for novel selective inhibitors of cyclo-oxygenase (COX)-2. These compounds promise to be potent anti-inflammatory agents with little gastrointestinal intolerance. Meloxicam, with some selectivity for COX-2, is already marketed, and at least two companies are carrying out clinical studies with selective inhibitors. A variety of potential successor compounds were presented at the 8th International Conference of the Inflammation Research Association (IRA) in Hershey, PA on 27-31 October 1996.

Prostanoid receptor expression by human airway smooth muscle cells and regulation of the secretion of granulocyte colony-stimulating factor

The prostanoid receptors on human airway smooth muscle cells (HASMC) that augment the release by IL-1β of granulocyte colony-stimulating factor (G-CSF) have been characterized and the signaling pathway elucidated. PCR of HASM cDNA identified products corresponding to EP2, EP3, and EP4receptor subtypes. These findings were corroborated at the protein level by immunocytochemistry. IL-1β promoted the elaboration of G-CSF, which was augmented by PGE2. Cicaprost (IP receptor agonist) was approximately equiactive with PGE2, whereas PGD2, PGF2α, and U-46619 (TP receptor agonist) were over 10-fold less potent. Neither SQ 29,548 nor BW A868C (TP and DP1receptor antagonists, respectively) attenuated the enhancement of G-CSF release evoking any of the prostanoids studied. With respect to PGE2, the EP receptor agonists 16,16-dimethyl PGE2(nonselective), misoprostol (EP2/EP3selective), 17-phenyl-ω-trinor PGE2(EP1selective), ONO-AE1-259, and butaprost (both EP2selective) were full agonists at enhancing G-CSF release. AH 6809 (10 μM) and L-161,982 (2 μM), which can be used in HASMC as selective EP2and EP4receptor antagonists, respectively, failed to displace to the right the PGE2concentration-response curve that described the augmented G-CSF release. In contrast, AH 6809 and L-161,982 in combination competitively antagonized PGE2-induced G-CSF release. Augmentation of G-CSF release by PGE2was mimicked by 8-BrcAMP and abolished in cells infected with an adenovirus vector encoding an inhibitor protein of cAMP-dependent protein kinase (PKA). These data demonstrate that PGE2facilitates G-CSF secretion from HASMC through a PKA-dependent mechanism by acting through EP2and EP4prostanoid receptors and that effective antagonism is realized only when both subtypes are blocked concurrently.

Species differences in the effects of prostanoids on MAP kinase phosphorylation, myosin light chain phosphorylation and contraction in bovine and cat iris sphincter smooth muscle

There is evidence from our own laboratory and that of others that EP-receptor ligands are strong contractile agonists in bovine iris sphincter and that FP-receptor agonists are strong contractile agonists in cat iris sphincter. Here, we have investigated the effects of prostaglandin (PG) receptor agonists of the FP-, EP-, TP- and DP-class on myosin light chain (MLC) phosphorylation, p42/p44 MAP kinase phosphorylation and contraction in the iris sphincter of bovine and cat. Using three signal transduction mechanism assays, namely MLC phosphorylation, MAP kinase phosphorylation and contraction, we demonstrated that in bovine iris sphincter the rank order of potency of the PG agonists in the contractile and MLC phosphorylation assays is as follows: E2>U46619>F2alpha>D2, and in cat F2alpha>D2>E2>U46619. In the MAP kinase assay, in bovine iris sphincter the rank order of potency is E2>F2alpha and in cat F2alpha>E2. These conclusions are supported by the following findings: (1) In the contractile assay, in the bovine sphincter the EC50s for PGF2alpha, PGE2, U46619 and PGD2 were found to be 1.4x10(-7), 5.0x10(-9), 9.0x10(-9) and 1.3x10(-6)M, respectively, and the corresponding values in the cat were 1.9x10(-8), 2.3x10(-7), 1.5x10(-6) and 6.9x10(-8)M, respectively. (2) In the MLC phophorylation assay, in the bovine sphincter PGF2alpha, PGE2, U46619 and PGD2 increased MLC phophorylation by 118%, 165%, 153% and 72%, respectively, and the corresponding values in cat were 175%, 99%, 90% and 95%, respectively. (3) In the MAP kinase assay, in the bovine iris sphincter PGF2alpha and PGE2, increased MAP kinase phosphorylation by 276% and 328%, respectively, and the corresponding values in cat were 308% and 245%, respectively. The data presented demonstrate pronounced species differences in the effects of the prostanoids on the MLC kinase signaling pathway in bovine and cat irides and furthermore confirm the existence of FP-receptors in that of the bovine.

The role of prostaglandins in the initiation of parturition

Parturition is composed of five separate but integrated physiological events: fetal membrane rupture, cervical dilatation, myometrial contractility, placental separation and uterine involution. Prostaglandins (PGs) have central roles in each of these, but the most studied is myometrial contraction. Elevated uterine PGs or the enhanced sensitivity of the myometrium to PGs leads to contractions and labour. The regulator of PG synthesis is the mRNA expression of PGHS-2. Cytokines are important stimulators of this gene expression, and cortisol and other factors may be as well. This enzyme is an important therapeutic target in the prevention of preterm labour. Some preterm births occur without an elevation of uterine PGs, even though they are delayed by non-steroidal anti-inflammatory drugs (NSAIDs), suggesting enhanced myometrial sensitivity to PGs. The PGF(2alpha) receptor, FP, is emerging as a central component of uterine sensitivity and may prove to be involved with preterm birth and a reasonable target for tocolysis.

Myometrial activation and preterm labour: evidence supporting a role for the prostaglandin F receptor--a review

An increase in the myometrial expression of the prostaglandin (PG) receptors, and especially the PGF(2alpha) receptor (FP), may be an important component of the process initiating preterm labour. In this review of the literature and presentation of new possibilities, evidence will be discussed that demonstrates an increase in mouse uterine FP mRNA occurs at preterm birth whereas uterine PGF(2alpha) concentrations do not increase, suggesting elevated uterine receptor expression and sensitivity is a mechanism for preterm labour initiation. The first examination of the complete human myometrial FP promoter will be described and evidence presented that demonstrates the pro-inflammatory cytokine, interleukin-1beta, stimulates FP mRNA expression. Finally new data showing that administration of a specific FP antagonist delays preterm birth in sheep will be presented.

Prostanoid EP3 and TP receptors-mediated inhibition of noradrenaline release from the isolated rat stomach

The postganglionic sympathetic nerves of the isolated rat stomach were electrically stimulated twice at 1 Hz for 1 min. Prostaglandin E(2) and ONO-AE-248 (16S-9-deoxy-9beta-chloro-15-deoxy-16-hyfroxy-17,17-trimethylene-19,20-didehydro prostaglandin F(2)) (an EP(3) receptor agonist) reduced the evoked noradrenaline release, while ONO-DI-004 (17S-2,5-ethano-6-oxo-17,20-dimethyl prostaglandin E(1)) (an EP(1) receptor agonist), ONO-AE1-259-01 (11,15-O-dimethyl prostaglandin E(2)) (an EP(2) receptor agonist) and ONO-AE1-329 [16-(3-methoxymethyl)phenyl-omega-tetranor-3,7-dithia prostaglandin E(1)] (an EP(4) receptor agonist) had no effect. U-46619 (9,11-dideoxy-9alpha,11alpha-methanoepoxy prostaglandin F(2alpha)) and I-BOP (7-[3-[3-hydroxy-4-(4-iodophenoxy)-1-butenyl]-7-oxabicyclo[2,2,1] hept-2-yl]-,[1S[1alpha,2alpha(Z),3beta(1E,3S)4alpha]]-5-heptenoic acid) (TP receptor agonists) also reduced the noradrenaline release and these inhibitory effects were abolished by SQ-29548 (7-[3-[[2-[(phenylamino) carbonyl] hydrazino]methyl]-7-oxabicyclo[2,2,1]hept-2-yl][1S(1alpha,2alpha(Z), 3alpha,4alpha]-5-heptenoic acid) (a TP receptor antagonist). The inhibitory effect of U-46619, but not ONO-AE-248, was abolished by pertussis toxin. These results suggest that the prostanoid EP(3) and TP receptors mediate the inhibition of gastric noradrenaline release; TP, but not EP(3), receptor-mediated inhibition is mediated by a pertussis toxin-sensitive mechanism in rats.

Vascular activities of prostaglandins and selective prostanoid receptor agonists in human retinal microvessels

Prostanoid analogs have recently been introduced into clinical use for the management of increased intraocular pressure (IOP). This class of compounds is known to exert effects on vascular components and some endogenous parent prostaglandins have been shown to alter regional ocular blood flow and exhibit significant vasoactive properties in isolated ocular blood vessels, so the possibility exists that prostanoids could affect the ocular microcirculation either by absorption into the systemic circulation or by direct localized activity on the retinal microvasculature. Thus, the aim of this study was to examine systematically the effects of a broad variety of agonists that exhibit preferential activity at EP(1)-, EP(2)-, EP(3)-, FP-, DP-, IP-, and TP-prostanoid receptor sites on microvessel caliber in the microvasculature associated with human retinal tissues grafted into the hamster cheek pouch membrane. The selective DP-receptor agonist, BW245C and the selective TP-receptor agonist, U-46619, were the only compounds tested that exhibited significant vasoactive effects relative to baseline resting diameters in retinal microvessels. A dose-dependent increase in arteriolar caliber was elicited by BW245C over a concentration range of 10(-8)-10(-4)M at the tested 5- and 10-min timepoints. U-46619 evoked a sharp decrease in microvessel diameter within a 10(-7)-10(-4)M gamut, with the dose-response profiles at 5- and 10-min timepoints remaining essentially parallel over the tested range of concentrations. In contrast to the vasoconstriction induced by U-46619, retinal microvessel calibers were not markedly affected by AGN 192093, a thromboxane-like agonist with additional unique properties. No significant changes in human retinal arteriolar diameters relative to baseline were observed in response to a broad panel of parent and derived compounds known to be selective for EP-, FP- and IP-prostanoid receptors.