Prostanoid receptors: structures, properties, and functions

The selective cyclooxygenase-2 inhibitor rofecoxib reduces kainate-induced cell death in the rat hippocampus

Treatment of male Sprague-Dawley rats with kainic acid (10 mg/kg, i.p.) triggered limbic seizures in 60% of the animals starting within 30 min and lasting for about 6 h. Cyclooxygenase-2 (COX-2) mRNA was strongly induced in the pyramidal cells of the hippocampus, in the amygdala and the piriform cortex after 8 h, as shown by in situ hybridization, and returned to control levels after 72 h. At this time marked cell loss occurred in the CA1-CA3 areas of the hippocampus. We hypothesize that rofecoxib, a selective COX-2 inhibitor, might abbreviate the late neurotoxicity, possibly associated with COX-2 induction. Animals which developed seizures were treated for 3 days with rofecoxib (10 mg/kg, i.p., n = 12) starting 6 or 8 h after kainic acid injection. Histological staining of viable cells confirmed that rofecoxib treatment selectively diminished cell loss in the hippocampus. The TdT-mediated dUTP nick end labelling (TUNEL) technique was used to estimate delayed cell death. Abundant TUNEL-positive cells were detected in seizure rats 72 h after kainic acid injection in pyramidal cells of the hippocampus (CA1-CA3), in cells of the thalamus, the amygdala and the piriform cortex. Treatment with rofecoxib selectively and significantly (P < 0.05) attenuated the number of TUNEL-positive cells in the hippocampus, whereas the cells of the thalamus, amygdala and piriform cortex were not protected. Therefore we conclude that COX-2 might contribute to cell death of pyramidal cells of the hippocampus as a consequence of limbic seizures.

Mechanisms involved in prostaglandin-induced increase in bone resorption in neonatal mouse calvaria

Prostaglandins (PG) E1, E2 and F2alpha induce bone resorption in isolated neonatal parietal bone cultures, and an associated increase in interleukin-6 (IL-6) production. Indomethacin had little effect on the response to PGE2, or the relatively non-selective EP receptor agonists 11-deoxy PGE1 and misoprostol, but blocked the effects of PGF2alpha and the F receptor agonist fluprostenol, indicating an indirect action via release of other prostaglandins. It is more likely that there is positive autoregulation of prostaglandins production in this preparation mediated via stimulation of F receptors. The effects of selective EP receptor agonists sulprostone (EP1,3) and 17-phenyl trinor PGE2(EP1), indicated the involvement of EP2 and/or EP4 receptors, which signal via cAMP. The relatively weak increase in IL-6 production by misoprostol (with respect to resorption) suggests that these responses are controlled by different combination of EP2 and EP4 receptors. The PKA activator, forskolin, induced small increases in bone resorption at lower concentrations (50-500 ng/ml) but a reversal of this effect, and inhibition of resorption induced by other stimuli (PTH, PGE2), at higher concentrations (0.5-5 microg/ml). IL-6 production was markedly increased only at the higher concentrations. The inhibitory effect of forskolin may be a calcitonin-mimetic effect. PMA induced both resorption and IL-6 production which were both blocked by indomethacin, indicating a role for PKC in the control of prostaglandin production.

The prostaglandin E2 EP1 receptor mediates pain perception and regulates blood pressure

The lipid mediator prostaglandin E2 (PGE2) has diverse biological activity in a variety of tissues. Four different receptor subtypes (EP1-4) mediate these wide-ranging effects. The EP-receptor subtypes differ in tissue distribution, ligand-binding affinity, and coupling to intracellular signaling pathways. To identify the physiological roles for one of these receptors, the EP1 receptor, we generated EP1-deficient (EP1-/-) mice using homologous recombination in embryonic stem cells derived from the DBA/1lacJ strain of mice. The EP1-/- mice are healthy and fertile, without any overt physical defects. However, their pain-sensitivity responses, tested in two acute prostaglandin-dependent models, were reduced by approximately 50%. This reduction in the perception of pain was virtually identical to that achieved through pharmacological inhibition of prostaglandin synthesis in wild-type mice using a cyclooxygenase inhibitor. In addition, systolic blood pressure is significantly reduced in EP1 receptor-deficient mice and accompanied by increased renin-angiotensin activity, especially in males, suggesting a role for this receptor in cardiovascular homeostasis. Thus, the EP1 receptor for PGE2 plays a direct role in mediating algesia and in regulation of blood pressure.

Molecular and biochemical characterization of prostacyclin receptors in rat kidney

The prostacyclin (IP) message was detected by RT-PCR in the renal cortex, outer (OM) and inner medulla (IM), and in freshly isolated (IMCD-f) and cultured inner medullary collecting duct (IMCD-c), and also the E-prostanoid (EP)1,3,4 receptor subtypes, but not EP2. Digoxigenin in situ hybridization localized IP mRNA in the tubules of the OM and IM, and the vasculature, and also in the glomeruli, arteries, and tubules of the cortex. IP splice variants or subtypes could not be detected by RT-PCR followed by TA cloning, though several nonfunctional point mutations or single base pair deletions were observed. Iloprost (ILP), cicaprost (CCP), PGE2, and arginine vasopressin (AVP) stimulated cAMP in both IMCD preparations. In addition, AVP-stimulated cAMP in IMCD-f was inhibited by all three prostanoids, but not in IMCD-c. Calcium experiments were performed on IMCD-c or microdissected IMCD (IMCD-m). CCP, ILP, and PGE2 did not alter intracellular calcium concentration ([Ca2+]i) in IMCD-c. However, on IMCD-m, both PGE2 and ILP increased [Ca2+]i levels equipotently and CCP had no effect. Pretreatment with the EP1 antagonist AH-6809 indicates that the response to ILP and PGE2 is mediated via EP1. These results suggest that IP receptors in the rat IMCD mediate the cAMP but not calcium signaling linked to PGI2; to date no subtypes or splice variants have been identified.

Prostaglandin D2 selectively induces chemotaxis in T helper type 2 cells, eosinophils, and basophils via seven-transmembrane receptor CRTH2

Prostaglandin (PG)D2, which has long been implicated in allergic diseases, is currently considered to elicit its biological actions through the DP receptor (DP). Involvement of DP in the formation of allergic asthma was recently demonstrated with DP-deficient mice. However, proinflammatory functions of PGD2 cannot be explained by DP alone. We show here that a seven-transmembrane receptor, CRTH2, which is preferentially expressed in T helper type 2 (Th2) cells, eosinophils, and basophils in humans, serves as the novel receptor for PGD2. In response to PGD2, CRTH2 induces intracellular Ca2+mobilization and chemotaxis in Th2 cells in a Galphai-dependent manner. In addition, CRTH2, but not DP, mediates PGD2-dependent cell migration of blood eosinophils and basophils. Thus, PGD2 is likely involved in multiple aspects of allergic inflammation through its dual receptor systems, DP and CRTH2.

Role of EP(2) and EP(3) PGE(2) receptors in control of murine renal hemodynamics

The kidney plays a central role in long-term regulation of arterial blood pressure and salt and water homeostasis. This is achieved in part by the local actions of paracrine and autacoid mediators such as the arachidonic acid-prostanoid system. The present study tested the role of specific PGE(2) E-prostanoid (EP) receptors in the regulation of renal hemodynamics and vascular reactivity to PGE(2). Specifically, we determined the extent to which the EP(2) and EP(3) receptor subtypes mediate the actions of PGE(2) on renal vascular tone. Renal blood flow (RBF) was measured by ultrasonic flowmetry, whereas vasoactive agents were injected directly into the renal artery of male mice. Studies were performed on two independent mouse lines lacking either EP(2) or EP(3) (-/-) receptors and the results were compared with wild-type controls (+/+). Our results do not support a unique role of the EP(2) receptor in regulating overall renal hemodynamics. Baseline renal hemodynamics in EP(2)-/- mice [RBF EP(2)-/-: 5.3 +/- 0.8 ml. min(-1). 100 g kidney wt(-1); renal vascular resistance (RVR) 19.7 +/- 3.6 mmHg. ml(-1). min. g kidney wt] did not differ statistically from control mice (RBF +/+: 4.0 +/- 0.5 ml. min(-1). 100 g kidney wt(-1); RVR +/+: 25.4 +/- 4.9 mmHg. ml(-1). min. 100 g kidney wt(-1)). This was also the case for the peak RBF increase after local PGE(2) (500 ng) injection into the renal artery (EP(2)-/-: 116 +/- 4 vs. +/+: 112 +/- 2% baseline RBF). In contrast, we found that the absence of EP(3) receptors in EP(3)-/- mice caused a significant increase (43%) in basal RBF (7.9 +/- 0.8 ml. min(-1). g kidney wt(-1), P < 0.05 vs. +/+) and a significant decrease (41%) in resting RVR (11.6 +/- 1.4 mmHg. ml(-1). min. g kidney wt(-1), P < 0.05 vs. +/+). Local administration of 500 ng of PGE(2) into the renal artery caused more pronounced renal vasodilation in EP(3)-/- mice (128 +/- 2% of basal RBF, P < 0.05 vs. +/+). We conclude that EP(3 )receptors mediate vasoconstriction in the kidney of male mice and its actions are tonically active in the basal state. Furthermore, EP(3) receptors are capable of buffering PGE(2)-mediated renal vasodilation.

A novel 7-transmembrane receptor expressed in nerve growth factor-dependent sensory neurons

This study reports on the full-length cDNA cloning of a gene identified on the basis of its preferential expression in nerve growth factor, compared with neurotrophin-3-dependent neurons. It encodes a putative 7-transmembrane polypeptide that is distantly related to other members of the G protein-coupled receptor superfamily. Unique features of this receptor include a very long carboxy-terminal tail of 360 amino acids and a specific expression pattern in the chick peripheral nervous system, including nerve growth factor-dependent sensory and sympathetic neurons, as well as enteric neurons. In the central nervous system, the receptor is strongly developmentally regulated and is expressed at high levels in the external granule cell layer of the cerebellum, as well as in motoneurons of the spinal cord, and in retinal ganglion cells.

Expression and regulation of prostaglandin E receptor subtype mRNAs in rat sensory ganglia and spinal cord in response to peripheral inflammation

Prostaglandins are known to act via seven transmembrane domain receptors to exert actions on both peripheral and central neurons resulting in changes in neuronal excitability. Prostaglandin E2, the prostaglandin most often associated with inflammation, itself acts on a family of closely related receptors, the EP receptors. Using semi-quantitative reverse transcription polymerase chain reaction (RT-PCR), we have shown that rat primary afferent neurons express the mRNA for all EP receptor subtypes, and that some, but not all EP receptor subtype mRNAs are down-regulated in sensory neurons in response to an acute peripheral inflammation. We also show for the first time that all EP receptor subtype mRNAs are expressed in rat lumbar spinal cord. Spinal cord EP receptor subtype mRNAs are also regulated in acute inflammation in a pattern distinct from the changes seen in sensory ganglia in response to the same inflammatory stimulus.

The lung HETEs (and EETs) up

Arachidonic acid metabolites of the cyclooxygenase and lipoxygenase pathways have a variety of important lung functions. Recent observations indicate that cytochrome P-450 (P-450) monooxygenases are also expressed in the lung, localized to specific pulmonary cell types (e.g., epithelium, endothelium, and smooth muscle), and may modulate critical lung functions. This review summarizes recent data on the presence and biological activity of P-450-derived eicosanoids in the pulmonary vasculature and airways, including effects on pulmonary vascular and bronchial smooth muscle tone and airway epithelial ion transport. We hypothesize a number of potential functions of P-450-derived arachidonate metabolites in the lungs such as contribution to hypoxic pulmonary vasoconstriction, regulation of bronchomotor tone, control of the composition of airway lining fluid, and limitation of pulmonary inflammation. Finally, we describe a number of emerging technologies, including congenic and transgenic strains of experimental animals, P-450 isoform-specific inhibitors and inhibitory antibodies, eicosanoid analogs, and vectors for delivery of P-450 cDNAs and antisense oligonucleotides. These tools will facilitate further studies on the contribution of endogenously formed P-450 eicosanoid metabolites to lung function, under both normal and pathological conditions.

NSAID-induced nephrotoxicity from the fetus to the child

In this review we report data available from the literature on the use of nonsteroidal anti-inflammatory drugs (NSAIDs) and the development of nephrotoxicity in the fetus, neonates and children. Up to the present day, several cases of severe and sometimes irreversible renal insufficiency have been described in neonates exposed to indomethacin prenatally or in the first days of life for treatment of patent ductus arteriosus (PDA). Until now, very few studies have been carried out on alternative treatments for PDA in preterm infants; ibuprofen has been shown to be as effective as indomethacin in closing the ductus in this patient group without affecting renal function. In children, NSAID-induced renal failure is a rare event and is usually reversible after discontinuation of the drug. However, caution should be taken when NSAIDs are administered to individuals with preexisting renal problems or with other potentially nephrotoxic drugs. In these situations, new approaches such as cyclo-oxygenase-2 selective inhibitors or prostanoid receptor selective antagonists could lead to alternative therapies for use in paediatrics.

Vasoactive receptors in abdominal blood vessels of the dogfish shark, Squalus acanthias

Previous studies have demonstrated that the ventral aorta of the dogfish shark, Squalus acanthias, responds to a variety of cell-signaling agents. To investigate the generality of vasoactive receptors in the shark vasculature, in particular a conductance artery (anterior mesenteric) and vein (posterior intestinal), I measured the effect of acetylcholine, endothelin, nitric oxide, natriuretic peptides, and prostaglandins on tension in isolated rings from these vessels. Both vessels responded to these agents, and responses to receptor-specific ligands for endothelin and natriuretic peptide receptors suggest that B-type endothelin receptors are expressed in both vessels and that the artery expresses both A- and B-type natriuretic peptide receptors; however, the vein (like the ventral aorta) expresses only the B-type natriuretic peptide receptor. My data suggest that a suite of signaling systems is ubiquitous in both arteries and veins in at least this elasmobranch species. Their role in hemodynamics and osmoregulation (perfusion of gill and rectal gland) remains to be determined.

Prostaglandin F2alpha-induced expression of 20alpha-hydroxysteroid dehydrogenase involves the transcription factor NUR77

Prostaglandin F(2)alpha (PGF(2)alpha) binding to its receptor on the rat corpus luteum triggers various signal transduction pathways that lead to the activation of a steroidogenic enzyme, 20alpha-hydroxysteroid dehydrogenase (20alpha-HSD), which in turn catabolizes progesterone. The molecular mechanism underlying PGF(2)alpha-induced 20alpha-HSD enzyme activity has not yet been explored. In this report we show, using mice lacking PGF(2)alpha receptor and pregnant rats, that PGF(2)alpha is responsible for the rapid and massive expression of the 20alpha-HSD gene at the end of pregnancy leading to a decrease in progesterone secretion. We also present evidence that PGF(2)alpha enhances 20alpha-HSD promoter activity. We have determined a region upstream of the -1590 position in the 20alpha-HSD promoter that confers regulation by PGF(2)alpha in ovarian primary cells. This region encompasses a unique transcription factor-binding site with a sequence of a NUR77 response element. Deletion of this motif or overexpression of a NUR77 dominant negative protein caused a complete loss of 20alpha-HSD promoter activation by PGF(2)alpha. NUR77 also transactivated the 20alpha-HSD promoter in transient transfection experiments in corpus luteum-derived cells (GG-CL). This induction required the NUR77-transactivating domain. We also show that PGF(2)alpha induces a very rapid expression of NUR77 that binds to a distal response element located at -1599/-1606 but does not interact with another proximal putative NUR77 response element located downstream in the promoter. A rapid increase in NUR77 mRNA was observed in mice corpora lutea just before parturition at a time when 20alpha-HSD becomes expressed. This increase in the expression of both genes was not seen in PGF(2)alpha receptor knockout mice. By using cyclosporin A and PGF(2)alpha treatment, we established that inhibition of NUR77 DNA binding in vivo prevents PGF(2)alpha induction of the 20alpha-HSD gene in the corpus luteum. Taken together, our results demonstrate, for the first time, that PGF(2)alpha induces in the corpus luteum the expression of the nuclear orphan receptor and transcription factor, NUR77, which in turn leads to the transcriptional stimulation of 20alpha-HSD, triggering the decrease in serum progesterone essential for parturition.

Agonist-induced long-term desensitization of the human prostacyclin receptor

Phosphorylation of the human prostacyclin (PGI(2)) receptor (hIP-R) by diacylglycerol-regulated protein kinase C (PKC) has been reported to be responsible for its rapid desensitization in HEK293 cells. In this study we demonstrate, that human fibroblasts reveal a much slower hIP-R desensitization kinetics, which was neither affected by stimulation nor inhibition of PKC by either phorbol 12-myristate-13-acetate or GF-109203X suggesting a different cellular mechanism. Although agonist-promoted sequestration of a C-terminally green fluorescent protein-tagged hIP-R was demonstrated, it did not account for the long-term desensitization. Concanavalin A did not abolish, but accelerated receptor desensitization kinetics. Resensitization of hIP-R involved receptor recycling and/or de novo synthesis of receptor protein, depending on the duration of prior desensitization. This is the first study investigating the mechanisms of hIP-R desensitization in intact human cells naturally expressing hIP-R. Our data suggest, that a hitherto unknown mechanism of hIP-R long-term desensitization, which is independent of receptor phosphorylation by conventional and novel type PKC isoforms or endocytosis, is a key event in regulating the cellular responsiveness to PGI(2).

Localization and regulation of cytosolic phospholipase A(2)

Liberation of arachidonic acid by cytosolic phospholipase A(2) (cPLA(2)) upon cell activation is often the initial and rate-limiting step in leukotriene and prostaglandin biosynthesis. This review discusses the essential features of cPLA(2) isoforms and addresses intriguing insights into the catalytic and regulatory mechanisms. Gene expression, posttranslational modification and subcellular localization can regulate these isoforms. Translocation of cPLA(2)alpha from the cytosol to the perinuclear region in response to calcium transients is critical for the immediate arachidonic acid release. Therefore, particular emphasis is placed on the mechanism of the translocation and the role of the proteins and lipids implicated in this process. The regional distribution and cellular localization of cPLA(2) may help to better understand its function as an arachidonic acid supplier to downstream enzymes and as a regulator of specific cellular processes.

Isoprostanes: generation, pharmacology, and roles in free-radical-mediated effects in the lung

Isoprostanes are produced during peroxidation of membrane lipids by free radicals and reactive oxygen species, and are currently used as markers of many disease states and experimental conditions in which oxidative stress is a prominent feature. A small number of reports have described the ability of some isoprostanes to evoke important biological effects in smooth muscle and other cell types. However, most of these studies were done using rat tissues, and only two specific isoprostanes - 8-iso-PGE(2)and 8-iso-PGF(2alpha)- were tested. In this review, we describe the generation of isoprostanes during oxidative stress, and their effects on smooth muscle, including our novel findings of their effects on human airway, pulmonary artery and pulmonary vein smooth muscles. Collectively, the data suggest that isoprostanes may not only be markers, but may in fact mediate the effects of free radicals and reactive oxygen species.

Characterization of prostanoid receptors mediating contraction of the gastric fundus and ileum: studies using mice deficient in prostanoid receptors

Receptors mediating prostanoid-induced contractions of longitudinal sections of gastric fundus and ileum were characterized by using tissues obtained from mice deficient in each type and subtype of prostanoid receptors. The fundus and ileum from mice deficient in either EP(3) (EP(3)(-/-) mice), EP(1) (EP(1)(-/-) mice) and FP (FP(-/-) mice) all showed decreased contraction to PGE(2) compared to the tissues from wild-type mice, whereas contraction of the fundus slightly increased in EP(4)(-/-) mice. 17-phenyl-PGE(2) also showed decreased contraction of the fundus from EP(3)(-/-), EP(1)(-/-) and FP(-/-) mice. Sulprostone showed decreased contraction of the fundus from EP(3)(-/-) and FP(-/-) mice, and decreased contraction of the ileum to this compound was seen in tissues from EP(3)(-/-), EP(1)(-/-) and FP(-/-) mice. In DP(-/-) mice, sulprostone showed increased contraction. DI-004 and AE-248 caused the small but concentration-dependent contraction of both tissues, and these contractions were abolished in tissues obtained from EP(1)(-/-) and EP(3)(-/-) mice, respectively, but not affected in other mice. Contractions of both fundus and ileum to PGF(2)alpha was absent at lower concentrations (10(-9) to 10(-7) M), and suppressed at higher concentrations (10(-6) to 10(-5) M) of the agonist in the FP(-/-) mice. Suppression of the contractions at the higher PGF(2)alpha concentrations was also seen in the fundus from EP(3)(-/-), EP(1)(-/-) and TP(-/-) mice and in the ileum from EP(3)(-/-) and TP(-/-) mice. Contraction of the fundus to PGD(2) was significantly enhanced in DP(-/-) mice, and contractions of the fundus and ileum to this PG decreased in FP(-/-) and EP(3)(-/-) mice. Contractions of both tissues to I-BOP was absent at 10(-9) to 10(-7) M and much suppressed at higher concentrations in TP(-/-) mice. Slight suppression to this agonist was also observed in the tissues from EP(3)(-/-) mice. PGI(2) induced small relaxation of both tissues from wild-type mice. These relaxation reactions were much potentiated in EP(3)(-/-) mice. On the other hand, significant contraction to PGI(2) was observed in both tissues obtained from IP(-/-) mice. These results show that contractions of the fundus and ileum induced by each prostanoid agonist are mediated by actions of this agonist on multiple types of prostanoid receptors and in some cases modified by its action on relaxant receptors.

Regulation of the human prostanoid TPalpha and TPbeta receptor isoforms mediated through activation of the EP(1) and IP receptors

The intermolecular cross-regulation mediated by the prostanoid IP-receptor (IP)/EP(1) receptor (EP(1)) agonists PGI(2) and 17 phenyl trinor PGE(2) on TP receptor (TP) signalling within platelets was compared to that which occurs to the individual TPalpha and TPbeta receptors over-expressed in human embryonic kidney (HEK) 293 cells. Ligand mediated TP receptor activation was monitored by analysing mobilization of intracellular calcium ([Ca(2+)](i)) following stimulation with the selective thromboxane (TX) A(2) mimetic U46619. Consistent with previous studies, in platelets, PGI(2) acting through endogenous IP receptors completely inhibited U46619-mediated TP receptor signalling in a protein kinase (PK) A-dependent, PKC-independent manner. In HEK 293 cells, PGI(2), acting through endogenous AH6809 sensitive EP(1) rather than IP receptors, and the selective EP(1) receptor agonist 17 phenyl trinor PGE(2) antagonized U46619-mediated signalling by both TPalpha and TPbeta receptors in a PKC-dependent, PKA-independent manner. The maximum response induced by either ligand was significantly (P<0.005) greater for the TPalpha receptor than the TPbeta receptor, pointing to possible physiologic differences between the TP isoforms, although the potency of each ligand was similar for both TP receptors. TP(Delta328), a truncated variant of TP receptor lacking the C-tail sequences unique to TPalpha or TPbeta receptors, was not sensitive to EP(1) receptor-mediated regulation by PGI(2) or 17 phenyl trinor PGE(2) In conclusion, these data confirm that TPalpha and TPbeta receptors are subject to cross regulation by EP(1) receptor signalling in HEK 293 cells mediated by PKC at sites unique to the individual TP receptors and that TPalpha receptor responses are significantly more reduced by EP(1) receptor regulation than those of the TPbeta receptor.

Pharmacology of oxytocin and prostaglandins

Significant advances have been made in the last 20 years in our knowledge about the subcellular events occurring during myometrial contractions and cervical ripening and in the mechanism of action of oxytocin and prostaglandins. These advances have been instrumental in furthering our understanding of the mechanism of action of inhibitors of uterine contractility and have opened the door to clinical trials of agents such as specific COX-II inhibitors that may have the potential to inhibit labor without serious maternal or fetal side effects. There is still a long way to go, however, before all the complex actions of oxytocin and prostaglandins can be understood at a subcellular level, particularly the mechanism of action of prostaglandins in the process of cervical ripening.

Cyclooxygenase-2 mediates the cardioprotective effects of the late phase of ischemic preconditioning in conscious rabbits

We examined the role of cyclooxygenase-2 (COX-2) in the late phase of ischemic preconditioning (PC). A total of 176 conscious rabbits were used. Ischemic PC (six cycles of 4-min coronary occlusions/4-min reperfusions) resulted in a rapid increase in myocardial COX-2 mRNA levels (+231 +/- 64% at 1 h; RNase protection assay) followed 24 h later by an increase in COX-2 protein expression (+216 +/- 79%; Western blotting) and in the myocardial content of prostaglandin (PG)E(2) and 6-keto-PGF(1alpha) (+250 +/- 85% and +259 +/- 107%, respectively; enzyme immunoassay). Administration of two unrelated COX-2 selective inhibitors (NS-398 and celecoxib) 24 h after ischemic PC abolished the ischemic PC-induced increase in tissue levels of PGE(2) and 6-keto-PGF(1alpha). The same doses of NS-398 and celecoxib, given 24 h after ischemic PC, completely blocked the cardioprotective effects of late PC against both myocardial stunning and myocardial infarction, indicating that COX-2 activity is necessary for this phenomenon to occur. Neither NS-398 nor celecoxib lowered PGE(2) or 6-keto-PGF(1alpha) levels in the nonischemic region of preconditioned rabbits, indicating that constitutive COX-1 activity was unaffected. Taken together, these results demonstrate that, in conscious rabbits, up-regulation of COX-2 plays an essential role in the cardioprotection afforded by the late phase of ischemic PC. Therefore, this study identifies COX-2 as a cardioprotective protein. The analysis of arachidonic acid metabolites strongly points to PGE(2) and/or PGI(2) as the likely effectors of COX-2-dependent protection. The recognition that COX-2 mediates the antistunning and antiinfarct effects of late PC impels a reassessment of current views regarding this enzyme, which is generally regarded as detrimental.

Amino acid residues conferring ligand binding properties of prostaglandin I and prostaglandin D receptors. Identification by site-directed mutagenesis

Using chimeras of the mouse prostaglandin (PG) I receptor (mIP) and the mouse PGD receptor (mDP), we previously revealed that the cyclopentane ring recognition by these receptors is specified by a region from the first to third transmembrane domain of each receptor; recognition by this region of mIP is broad, accommodating the D, E, and I types of cyclopentane rings, whereas that of mDP binds the D type of PGs alone (Kobayashi, T., Kiriyama, M., Hirata, T., Hirata, M., Ushikubi, F., and Narumiya, S. (1997) J. Biol. Chem. 272, 15154-15160). In the present study, we performed a more detailed chimera analysis, and narrowed the domain for the ring recognition to a region from the first transmembrane domain to the first extracellular loop. One chimera with the replacement of the second transmembrane domain and the first extracellular loop of mDP with that of mIP bound only iloprost. The amino acid substitutions in this chimera suggest that Ser(50) in the first transmembrane domain of mIP confers the broad ligand recognition of mIP and that Lys(75) and Leu(83) in the second transmembrane domain of mDP confer the high affinity to PGD(2) and the strict specificity of ligand binding of mDP, respectively.

Inhibitory effect of a prostaglandin E receptor subtype EP(1) selective antagonist, ONO-8713, on development of azoxymethane-induced aberrant crypt foci in mice

We previously reported that prostaglandin E(2) contributes to colon carcinogenesis through its binding to the prostaglandin E receptor subtype EP(1) using a genetic approach in EP(1)-knockout mice and a pharmacological approach with the EP(1) selective antagonist, ONO-8711. In the present study, we examined the effects of another more selective EP(1) receptor antagonist, ONO-8713, on development of azoxymethane (AOM)-induced aberrant crypt foci (ACFs) in male C57BL/6J mice treated i.p. with 10mg/kg body weight AOM once a week for 3weeks. Administration of ONO-8713 at doses of 250, 500 and 1000ppm in diet during and post-AOM treatment for 5weeks resulted in a dose-dependent reduction of ACF formation, being 15, 30 and 36% inhibition of the control value, respectively. The level of inhibition was similar to that with ONO-8711. Moreover, ONO-8713 suppressed the development of ACF when administered at post-AOM, as in the case of ONO-8711. The data confirm EP(1) receptor involvement in colon carcinogenesis.

Prostaglandin E(2) enhances chemical and mechanical sensitivities of pulmonary C fibers in the rat

It has been recently reported that pulmonary reflex responses to injection or inhalation challenge of capsaicin are enhanced by exogenous Prostaglandin E(2) (PGE(2)). The present study was carried out to determine whether PGE(2) enhances the stimulatory effects of chemical stimulants and lung inflation on vagal pulmonary C fibers, and if so, whether the excitabilities of other types of lung afferents are also augmented by PGE(2). In anesthetized, open-chest rats, administration of PGE(2) (1.5 microgram/kg/min for 2 min) did not significantly change the baseline activity of vagal pulmonary C fibers, but it markedly enhanced the stimulatory effects of both low (0.25 microgram/kg) and high doses (0.5 microgram/kg) of capsaicin on these fibers. Similarly, potentiating effects of PGE(2) were found on the pulmonary C-fiber responses to injections of lactic acid and adenosine, although considerable variability existed in the degrees of potentiation between the different stimulants. Furthermore, PGE(2) infusion also significantly enhanced the C-fiber response to constant-pressure lung inflation (tracheal pressure [Pt] = 30 cm H(2)O). In contrast, PGE(2) did not alter the responses of either slowly adapting pulmonary receptors or rapidly adapting pulmonary receptors to lung inflation. In summary, these results show that the sensitivity of pulmonary C-fiber afferents to both mechanical and chemical stimuli is enhanced by PGE(2), suggesting that endogenous release of this autocoid may play a part in the airway irritation and dyspneic sensation associated with airway inflammation.

Identification of CYP4F8 in human seminal vesicles as a prominent 19-hydroxylase of prostaglandin endoperoxides

A novel cytochrome P450, CYP4F8, was recently cloned from human seminal vesicles. CYP4F8 was expressed in yeast. Recombinant CYP4F8 oxygenated arachidonic acid to (18R)-hydroxyarachidonate, whereas prostaglandin (PG) D(2), PGE(1), PGE(2), PGF(2alpha), and leukotriene B(4) appeared to be poor substrates. Three stable PGH(2) analogues, 9,11-epoxymethano-PGH(2) (U-44069), 11, 9-epoxymethano-PGH(2) (U-46619), and 9,11-diazo-15-deoxy-PGH(2) (U-51605) were rapidly metabolized by omega2- and omega3-hydroxylation. U-44069 was oxygenated with a V(max) of approximately 260 pmol min(-)(1) pmol P450(-1) and a K(m) of approximately 7 micrometer. PGH(2) decomposes mainly to PGE(2) in buffer and to PGF(2alpha) by reduction with SnCl(2). CYP4F8 metabolized PGH(2) to 19-hydroxy-PGH(2), which decomposed to 19-hydroxy-PGE(2) in buffer and could be reduced to 19-hydroxy-PGF(2alpha) with SnCl(2). 18-Hydroxy metabolites were also formed (approximately 17%). PGH(1) was metabolized to 19- and 18-hydroxy-PGH(1) in the same way. Microsomes of human seminal vesicles oxygenated arachidonate, U-44069, U-46619, U-51605, and PGH(2), similar to CYP4F8. (19R)-Hydroxy-PGE(1) and (19R)-hydroxy-PGE(2) are the main prostaglandins of human seminal fluid. We propose that they are formed by CYP4F8-catalyzed omega2-hydroxylation of PGH(1) and PGH(2) in the seminal vesicles and isomerization to (19R)-hydroxy-PGE by PGE synthase. CYP4F8 is the first described hydroxylase with specificity and catalytic competence for prostaglandin endoperoxides.

Microvascular effects of selective prostaglandin analogues in the eye with special reference to latanoprost and glaucoma treatment

Prostaglandin F(2alpha) analogues have recently been introduced on the market for glaucoma treatment. While these drugs have a well-documented intraocular pressure reducing effect only a limited number of studies have been published regarding their effects on the microvasculature in the eye. Since many naturally occurring prostaglandins have marked effects on the cardiovascular system it is conceivable that synthetic prostaglandins used as glaucoma drugs may exert microvascular effects in the eye, even if they exhibit receptor selectivity. Latanoprost, the active principle of Xalatan((R)) eye drops, is a selective FP prostanoid receptor agonist, and much of the paper is focused on the microvascular effects of latanoprost and some closely related prostaglandin analogues. The purpose of the paper is to review the literature on the microvascular effects of prostaglandins in the eye, and to present some unpublished data on the effects of selective prostaglandin analogues. Most of the prostaglandin analogues studied exhibit selectivity for the FP prostanoid receptor. Results from studies with the following prostaglandin analogues are presented in the paper: PGF(2alpha)-isopropyl ester (PGF(2alpha)-IE), 17-phenyl-18,19,20-trinor-PGF(2alpha)-isopropyl ester (17-phenyl-PGF(2a)-IE), 15-keto-17-phenyl-18,19, 20-trinor-PGF(2alpha)-isopropyl ester (15-keto-17-phenyl-PGF(2a)-IE), 13,14-dihydro-17-phenyl-18,19,20-trinor-PGF(2alpha)-isopropy l ester (latanoprost), 13,14-dihydro-15R,S-17-phenyl-18,19, 20-trinor-PGF(2alpha)-isopropyl ester (PhXA34), 17-phenyl-18,19, 20-trinor-PGE(2)-isopropyl ester (17-phenyl-PGE(2)-IE), and 19R-hydroxy-PGE(2) (19R-OH-PGE(2)). The regional blood flow has been determined with radioactively labelled microspheres, the blood volume with (51)Cr labelled erythrocytes and the capillary permeability to albumin with (125)I and (131)I labelled albumin. PGF(2alpha)-IE has been shown to exert marked microvascular effects in the rabbit anterior segment including vasodilatation, increased capillary permeability, and a breakdown of the blood-aqueous barrier. 17-phenyl-PGF(2alpha)-IE, 15-keto-17-phenyl-PGF(2alpha)-IE, and PhXA34/latanoprost exerted significantly less vasodilatory effect, and little effect on capillary permeability was seen with the FP receptor agonists when studied with Evans blue. Intravenous administration of PhXA34 at a dose range of 1-100 microg/kg b.w. had no consistent effect on the regional blood flow in the eye indicating that FP receptors in the ocular blood vessels are not expressed in the rabbit, or alternatively are not functionally coupled to regulation of vascular tone. In cats topical application of PGF(2alpha)-IE had no significant effect the on the regional blood flow in cannulated eyes. No blood flow experiments were performed in intact eyes with PGF(2alpha)-IE. 17-phenyl-PGF(2alpha)-IE and latanoprost caused some vasodilation in the anterior segment. None of the analogues had any significant effect on the blood volume in the ocular tissues, but an increase in capillary permeability to albumin was seen in several tissues of the eye. However, in the eyelid, nictitating membrane and conjunctiva exposed to high concentrations of the prostaglandins no or only little leakage of albumin was detected. It appears that the intraocular microvasculature in the cat exhibits some sensitivity to FP prostanoid receptor agonists. (ABSTRACT TRUNCATED)

A functional analysis of EP4 receptor-expressing neurons in mediating the action of prostaglandin E2 within specific nuclei of the brain in response to circulating interleukin-1beta

Proinflammatory cytokines released by cells of myeloid lineage have the ability to stimulate different populations of neurons through intermediate molecules released by cells of the blood-brain barrier. The aim of the present study was to verify the hypothesis that prostaglandins (PGs) play a site-specific role in activating selective groups of neurons via a privileged interaction between PG of the E2 type and its EP4 receptor. In a first set of experiments, animals were treated with the inhibitor of PG synthesis ketorolac to determine the endogenous contribution of PG in mediating the neuronal activation and EP4 expression in response to circulating interleukin-1beta(IL-1beta). The subsequent experiment consisted of evaluating the role of PGE2 in activating EP4-expressing neurons in the rat brain. Ketorolac completely abolished the endogenous release of PGE2 in the liver and prevented the induction of immediate-early genes and up-regulation of EP4 mRNA in specific groups of neurons, such as the parvocellular paraventricular nucleus and the A1 catecholaminergic population of cells. This effect was, however, not generalized throughout the brain as PGE2 inhibition failed to abolish IL-1beta-induced c-fos transcription in the nucleus of the solitary tract, parabrachial nucleus, bed nucleus of the stria terminalis, and the circumventricular organs. Of interest are the data that central PGE2 injection activated EP4 gene transcription in neurons that no longer responded to the intravenous IL-1beta bolus when the animals were pretreated with ketorolac. Site-specific interaction between the ligand and its receptor was further supported by the induction of c-fos-immunoreactive nuclei within EP4-expressing neurons in response to intracerebroventricular PGE2 infusion. Both intracerebroventricular PGE2 and intravenous IL-1beta injection provoked a sharp and rapid increase in plasma corticosterone levels, an effect that was completely prevented in inhibiting PG production in IL-1beta-challenged rats. These data provide the evidence that EP4 is expressed in numerous nuclei involved in autonomic and neuroendocrine control, although a privileged interaction seems to take place in specific nuclei and areas, including the endocrine hypothalamus and the A1 cell group of the ventrolateral medulla. It is quite possible that EP4 acts as the functional receptor for PGE2 to activate the neuronal circuit involved in the activation of the glucocorticoid axis, as an essential neuroendocrine response for the appropriate control of systemic inflammation.

COX-2 and iNOS, good targets for chemoprevention of colon cancer

Cyclooxygenase (COX)-2 has been suggested to play an important role in colon carcinogenesis. We found that the COX-2 selective inhibitor, nimesulide, reduces azoxymethane (AOM)-induced aberrant crypt foci (ACF) in rats and colon carcinogenesis in mice, as well as formation of intestinal polyps in Min mice. Thus, selective inhibitors of COX-2, which catalyzes the synthesis of prostanoids, could be good candidates as chemopreventive agents against colon cancer. Examination of the effect of prostanoid receptor deficiency and a selective antagonist of prostanoid receptor on the development of AOM-induced ACF in mice revealed the involvement of the EP1 receptor. Moreover, a selective EP1 antagonist reduced the number of intestinal polyps in Min mice. These results suggest that PGE2 contributes to colon carcinogenesis through binding to the EP1 receptor. Nitric oxide synthase (NOS) is known to be overexpressed in colon cancers of humans and rats, and a NOS inhibitor, L-NG-nitroarginine methyl ester, was found to inhibit the development of AOM-induced ACF in rats. Thus, NOS including iNOS could also be a good target for chemoprevention of colon cancer, as in the COX-2 case.

Prostaglandin receptors: their role in regulating renal function

Renal cyclooxygenase-1 and cyclooxygenase-2 actively metabolize arachidonate to metabolism five primary prostanoids: prostaglandin E2, prostaglandin F2a, prostaglandin I2, thromboxane A2, and prostaglandin D2. These lipid mediators interact with a family of distinct G-protein-coupled prostanoid receptors designated EP, FP, IP, TP, and DP, respectively, which exert important regulatory effects on renal function. The intrarenal distribution of these prostanoid receptors has been mapped and the consequences their activation are being characterized. The FP, TP, and EP1 receptors preferentially couple to increased cell Ca2+. EP2, EP4, DP, and IP receptors stimulate cyclic adenosine monophosphate, whereas the EP3 receptor preferentially couples to Gi, inhibiting cyclic adenosine monophosphate generation. EP1 and EP3 messenger RNA expression predominate in the collecting duct and thick limb, respectively, where their stimulation reduces sodium chloride and water absorption, promoting natriuresis and diuresis. Interestingly, only a mild change in renal water handling is seen in the EP3 receptor knockout mouse. Although only low levels EP2 receptor messenger RNA are detected in kidney and its precise intrarenal localization is uncertain, mice with targeted disruption of the EP2 receptor display salt-sensitive hypertension, suggesting it also plays an important role in salt excretion. In contrast, EP4 messenger RNA is readily detected in the glomerulus where it may contribute to the regulation of renin release and decrease glomerular resistance. TP receptors are also highly expressed in the glomerulus, where they may increase glomerular vascular resistance. The IP receptor messenger RNA is most highly expressed in the afferent arteriole and it may also modulate renal arterial resistance and renin release. At present there is little evidence for DP receptor expression in the kidney. Together these receptors act as physiologic buffers that protect the kidney from excessive functional changes during periods of physiologic stress. Loss of the combined effects of these receptors contributes to the side effects seen in the setting of nonsteroidal anti-inflammatory drug administration, whereas selective antagonists for these receptors may provide new therapeutic approaches in disease.