Prostacyclin receptor-independent inhibition of phospholipase C activity by non-prostanoid prostacyclin mimetics

A comparison of vasodilation mode among selexipag (NS-304; [2-{4-[(5,6-diphenylpyrazin-2-yl)(isopropyl)amino]butoxy}-N-(methylsulfonyl)acetamide]), its active metabolite MRE-269 and various prostacyclin receptor agonists in rat, porcine and human pulmonary arteries

Selexipag (NS-304; [2-{4-[(5,6-diphenylpyrazin-2-yl)(isopropyl)amino]butoxy}-N- (methylsulfonyl)acetamide]) is a novel, orally available non-prostanoid prostacyclin receptor (IP receptor) agonist that has recently been approved for the treatment of pulmonary arterial hypertension (PAH). We examined the effect of the active metabolite of selexipag, MRE-269, and IP receptor agonists that are currently available as PAH therapeutic drugs on the relaxation of rat, porcine and human pulmonary artery. cAMP formation in human pulmonary artery smooth muscle cells was induced by all test compounds (MRE-269, epoprostenol, iloprost, treprostinil and beraprost sodium) and suppressed by IP receptor antagonists (CAY10441 and 2-[4-(1H-indol-4-yloxymethyl)-benzyloxycarbonylamino]-3-phenyl-propionic acid). MRE-269 induced endothelium-independent vasodilation of rat extralobar pulmonary artery (EPA). In contrast, endothelial denudation or the addition of a nitric oxide synthase inhibitor markedly attenuated the vasodilation of EPA induced by epoprostenol, treprostinil and beraprost sodium but not iloprost. The vasorelaxant effects of MRE-269 on rat small intralobar pulmonary artery (SIPA) and EPA were the same, while the other IP receptor agonists induced less vasodilation in SIPA than in EPA. Furthermore, a prostaglandin E receptor 3 antagonist enhanced the vasodilation induced by all IP receptor agonists tested except MRE-269. We also investigated the relaxation induced by IP receptor agonists in pulmonary arteries from non-rodent species and found similar vasodilation modes in porcine and human as in rat preparations. These results suggest that MRE-269, in contrast to other IP receptor agonists, works as a selective IP receptor agonist, thus leading to pronounced vasorelaxation of rat, porcine and human pulmonary artery.

Novel long-acting prostacyclin agonist (ONO-1301) with an angiogenic effect: promoting synthesis of hepatocyte growth factor and increasing cyclic AMP concentration via IP-receptor signaling

The purpose of this study was to evaluate the angiogenic potency of ONO-1301, a novel prostacyclin agonist, using a murine sponge model. Solutions of ONO-1301 or hepatocyte growth factor (HGF), as a positive control, were injected into sponges in the backs of mice, daily for 14 days. Hemoglobin and HGF levels in the sponge were increased for up to 14 days on daily treatment with ONO-1301 while on HGF treatment, they peaked on day 7 and had decreased again by day 14. ONO-1301 also upregulated c-Met expression for 14 days in a dose-dependent manner. When the mice were pretreated with an antibody to HGF or the prostaglandin I (IP)-receptor antagonist CAY10441, the angiogenic effect of ONO-1301 was dramatically reduced. Plasma concentrations of cyclic adenosine monophosphate (cAMP) were increased in a dose-dependent manner by once daily treatment with ONO-1301 for 14 days. This effect was reduced by pretreatment with the IP-receptor antagonist. In conclusion, hemoglobin level was increased by repeated treatment with ONO-1301 for 14 days. It is suggested that ONO-1301 induced angiogenesis by promoting the synthesis of HGF and upregulated c-Met expression, followed by an increase in cAMP concentrations mediated by IP-receptor signaling.

Endothelin-1 activation of ETB receptors leads to a reduced cellular proliferative rate and an increased cellular footprint

Endothelin-1 (ET-1) is a vasoactive peptide which signals through two G-protein coupled receptors, endothelin receptor A (ETA) and B (ETB). We determined that ET-1 activation of its ETB receptor in stably cDNA transfected CHO cells leads to a 55% reduction in cell number by end-point cell counting and a 35% decrease in cell growth by a real-time cell-substrate impedance-based assay after 24h of cell growth. When CHO ETB cells were synchronized in the late G1 cell cycle phase, ET-1 delayed their S phase progression compared to control by 30% as determined by [(3)H]-thymidine incorporation. On the other hand, no such delay was observed during late G2/M to G1 transit when cells were treated with ET-1 after release from mitotic arrest. Using the cell-substrate impedance-based assay, we observed that ET-1 induces opposing morphological changes in CHO ETA and CHO ETB cells with ETB causing an increase in the cell footprint and ETA a decrease. Likewise, in pulmonary artery smooth muscle cells, which express both ETA and ETB receptors, ET-1 induces an ETA-dependent contraction and an ETB dependent dilation. These results are shedding light on a possible beneficial role for ETB in diseases involving ET-1 dysfunction such as pulmonary hypertension.

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.

The action of prostaglandins on ion channels

Prostaglandins, in particular PGE(2) and prostacyclin PGI(2) have diverse biological effects. Most importantly, they are involved in inflammation and pain. Prostaglandins in nano- and micromolar concentrations sensitize nerve cells, i.e. make them more sensitive to electrical or chemical stimuli. Sensitization arises from the effect of prostaglandins on ion channels and occurs both at the peripheral terminal of nociceptors at the site of tissue injury (peripheral sensitization) and at the synapses in the spinal cord (central sensitization). The first step is the binding of prostaglandins to receptors in the cell membrane, mainly EP and IP receptors. The receptors couple via G proteins to enzymes such as adenylate cyclase and phospholipase C (PLC). Activation of adenylate cyclase leads to increase of cAMP and subsequent activation of protein kinase A (PKA) or PKA-independent effects of cAMP, e.g. mediated by Epac (=exchange protein activated by cAMP). Activation of PLC causes increase of inositol phosphates and increase of cytosolic calcium. This article summarizes the effects of PGE(2), PGE(1), PGI2 and its stable analogues on non-selective cation channels and sodium, potassium, calcium and chloride channels. It describes the mechanism responsible for the facilitatory or inhibitory prostaglandin effects on ion channels. Understanding these mechanisms is essential for the development of useful new analgesics.

Prostanoid receptor antagonists: development strategies and therapeutic applications

Identification of the primary products of cyclo-oxygenase (COX)/prostaglandin synthase(s), which occurred between 1958 and 1976, was followed by a classification system for prostanoid receptors (DP, EP(1), EP(2) ...) based mainly on the pharmacological actions of natural and synthetic agonists and a few antagonists. The design of potent selective antagonists was rapid for certain prostanoid receptors (EP(1), TP), slow for others (FP, IP) and has yet to be achieved in certain cases (EP(2)). While some antagonists are structurally related to the natural agonist, most recent compounds are 'non-prostanoid' (often acyl-sulphonamides) and have emerged from high-throughput screening of compound libraries, made possible by the development of (functional) assays involving single recombinant prostanoid receptors. Selective antagonists have been crucial to defining the roles of PGD(2) (acting on DP(1) and DP(2) receptors) and PGE(2) (on EP(1) and EP(4) receptors) in various inflammatory conditions; there are clear opportunities for therapeutic intervention. The vast endeavour on TP (thromboxane) antagonists is considered in relation to their limited pharmaceutical success in the cardiovascular area. Correspondingly, the clinical utility of IP (prostacyclin) antagonists is assessed in relation to the cloud hanging over the long-term safety of selective COX-2 inhibitors. Aspirin apart, COX inhibitors broadly suppress all prostanoid pathways, while high selectivity has been a major goal in receptor antagonist development; more targeted therapy may require an intermediate position with defined antagonist selectivity profiles. This review is intended to provide overviews of each antagonist class (including prostamide antagonists), covering major development strategies and current and potential clinical usage.

Gi-coupled prostanoid receptors are the likely targets for COX-1-generated prostanoids in rat pheochromocytoma (PC12) cells

Cyclooxygenase-1 (COX-1) behaves as a delayed response gene in rat pheochromocytoma (PC12) cells exposed to nerve growth factor (NGF). To investigate the possible targets for COX-1 generated prostanoids in the early stages of neuronal differentiation, we have examined the expression of prostanoid receptors by PC12 cells using functional assays. Prostanoid receptor-specific agonists failed to activate adenylyl cyclase in undifferentiated and NGF-treated PC12 cells; neither did they stimulate phospholipase C activity. EP3 receptor agonists and PGF(2alpha) were the only active ligands, able to inhibit forskolin-stimulated adenylyl cyclase activity. PC12 cells expressed EP3 and FP receptor mRNA, but only the responses to EP3 receptor agonists were inhibited by the EP3 receptor antagonist ONO-AE3-240. The functional role of NGF-stimulated COX-1 remains to be determined since we found no strong evidence of a role for EP3 receptors in the morphological changes induced by NGF during the early stages of differentiation of PC12 cells.

The constitutive activity of the ghrelin receptor attenuates apoptosis via a protein kinase C-dependent pathway

The ghrelin receptor (GHS-R1a) displays a high level of constitutive signaling through a phospholipase C/protein kinase C-dependent pathway. Therefore, we have investigated the role of agonist-dependent and agonist-independent signaling of GHS-R1a in apoptosis using the seabream GHS-R1a stably expressed in human embryonic kidney 293 cells (HEK-sbGHS-R1a cells). Cadmium-induced activation of caspase-3 was significantly attenuated in HEK-sbGHS-R1a cells compared to wild-type HEK293 cells, while the apoptotic responses to the protein kinase C inhibitor staurosporine were similar. GHS-R1a ligands had no effect on caspase-3 activation or on cell proliferation. Concentrations of the inverse agonist [d-Arg(1),d-Phe(5),d-Trp(7,9),Leu(11)]-substance P sufficient to inhibit constitutive inositol phosphate generation did not enhance caspase-3 activity, suggesting a possible role of phosphatidylcholine-specific phospholipase C in the anti-apoptotic activity of GHS-R1a. In conclusion, our data suggests that the constitutive activity of sbGHS-R1a may be sufficient alone to attenuate apoptosis via a protein kinase C-dependent pathway.

2-{4-[(5,6-Diphenylpyrazin-2-yl)(isopropyl)amino]butoxy}-N-(methylsulfonyl)acetamide (NS-304), an Orally Available and Long-Acting Prostacyclin Receptor Agonist Prodrug

Prostacyclin (PGI(2)) and its analogs are useful for the treatment of various vascular disorders, but their half-lives are too short for widespread clinical application. To overcome this drawback, we have synthesized a novel diphenylpyrazine derivative, 2-[4-[(5,6-diphenylpyrazin-2-yl)(isopropyl)amino]butoxy]-N-(methylsulfonyl)acetamide (NS-304), a prodrug of the active form [4-[(5,6-diphenylpyrazin-2-yl)(isopropyl)amino]butoxy]acetic acid (MRE-269). NS-304 is an orally available and potent agonist for the PGI(2) receptor (IP receptor). The inhibition constant (K(i)) of MRE-269 for the human IP receptor was 20 nM; in contrast, the K(i) values for other prostanoid receptors were >2.6 microM. MRE-269 was therefore a highly selective agonist for the IP receptor. The plasma concentrations of MRE-269 remained near peak levels for more than 8 h after oral administration of NS-304 to rats and dogs, and NS-304 increased femoral skin blood flow in rats in a long-lasting manner without affecting the hemodynamics. These findings indicate that NS-304 acts as a long-acting IP receptor agonist in vivo. The continuous vasodilation evoked by NS-304 was not attenuated by repeated treatment, indicating that NS-304 is unlikely to cause severe desensitization of the IP receptor in rats. Moreover, a microdose pharmacokinetic study in which NS-304 was orally administered to healthy male volunteers showed conversion of NS-304 to MRE-269 and a long plasma elimination half-life for MRE-269 (7.9 h). In conclusion, NS-304 is an orally available and long-acting IP receptor agonist prodrug, and its active form, MRE-269, is highly selective for the IP receptor. Therefore, NS-304 is a promising drug candidate for various vascular diseases, especially pulmonary arterial hypertension and arteriosclerosis obliterans.

The truncated ghrelin receptor polypeptide (GHS-R1b) acts as a dominant-negative mutant of the ghrelin receptor

The dimerization properties of the ghrelin receptor (GRLN-R) and its non-signalling, naturally occurring, truncated splice variant (GHS-R1b) have been investigated in human embryonic kidney 293 cells heterologously expressing these proteins. Using the techniques of bioluminescence resonance energy transfer and co-immunoprecipitation, we detected the formation of GRLN-R homodimers and GRLN-R/GHS-R1b heterodimers, but ghrelin-induced conformational changes were only detected in the GRLN-R homodimers. When the expression of GHS-R1b exceeded that of GRLN-R, there was a decrease in the cell surface expression of GRLN-R with a consequent decrease in constitutive activation of phosphatidylinositol-specific phospholipase C (PI-PLC). Furthermore, there was no change in ghrelin affinity, and the efficacy of cell signalling as measured by stimulation of PI-PLC and extracellular signal-regulated kinase 1/2 was unchanged. Cellular localization studies suggest that GRLN-R is normally distributed between the plasma membrane and cytosolic fractions, but in the presence of GHS-R1b, GRLN-R is localized to the nucleus. Therefore, we propose that the decrease in GRLN-R constitutive signalling was due to translocation of GRLN-R to the nucleus due to the formation of GRLN-R/GHS-R1b heterodimers. Therefore, GHS-R1b appears to act as a dominant-negative mutant of the full-length GRLN-R.

Over-expression of the truncated ghrelin receptor polypeptide attenuates the constitutive activation of phosphatidylinositol-specific phospholipase C by ghrelin receptors but has no effect on ghrelin-stimulated extracellular signal-regulated kinase 1/2 activity

In addition to regulating growth hormone release from the pituitary, ghrelin receptors also influence cell proliferation and apoptosis. By studying mitogen-activated protein kinase activity in human embryonic kidney 293 cells over-expressing ghrelin receptors, we aimed to identify the specific cell signalling pathways used by ghrelin receptors, and to determine if the truncated ghrelin receptor polypeptide had any influence on the functional activity of ghrelin receptors. We found that ghrelin activated extracellular signal-regulated kinases 1/2 with an EC50 value of 10 nM, and that this response was inhibited by the ghrelin receptor antagonists D-Lys3-GHRP-6 and [D-Arg1,D-Phe5,D-Trp(7,9),Leu11]-substance P. Ghrelin had little or no effect on the activity of c-Jun N-terminal kinase, p38 kinase or Akt. Ghrelin appeared to activate extracellular signal-regulated kinases 1/2 through a calcium-independent novel protein kinase C isoform which may utilize diacylglycerol derived from hydrolysis of phosphatidylcholine rather than from phosphatidylinositol. Ghrelin-stimulated extracellular signal-regulated kinases 1/2 activity was independent of transactivation of epidermal growth factor receptors, and even when ghrelin receptor internalization was blocked by concanavalin A or a beta-arrestin mutant, there was no decrease in phosphorylated extracellular signal-regulated kinases 1/2, suggesting this is a G protein-dependent process. The truncated ghrelin receptor polypeptide had no effect on ghrelin receptor signalling to extracellular signal-regulated kinases 1/2, but decreased the constitutive activation of phosphatidylinositol-specific phospholipase C by ghrelin receptors. In conclusion, our results suggest that any up-regulation of the truncated ghrelin receptor polypeptide might preferentially attenuate functional activity dependent on the constitutive activation of ghrelin receptors, while leaving ghrelin-dependent signalling unaffected.

Identification and characterization of surrogate peptide ligand for orphan G protein-coupled receptor mas using phage-displayed peptide library

In the present study, a phage-displayed random peptide library was used to identify surrogate peptide ligands for orphan GPCR mas. Sequence analysis of the isolated phage clones indicated a selective enrichment of some peptide sequences. Moreover, multiple alignments of the isolated phage clones gave two conserved peptide motifs from which we synthesized peptide MBP7 for further evaluation. Characterization of the representative phage clones and the synthetic peptide MBP7 by immunocytochemistry revealed a strong punctate cell surface staining in CHO cells expressing mas-GFP fusion protein. The isolated phage clones and synthetic peptide MBP7 induced mas internalization in a stable CHO cell clone (MC0M80) over-expressing mas. In addition, MBP7-stimulated phospholipase C activity and intracellular calcium mobilization in these same cells. In summary, we have demonstrated a systematic approach to derive surrogate peptide ligands for orphan GPCRs. With this technique, we have identified two conserved peptide motifs which allow us to identify potential protein partners for mas, and have generated a peptide agonist MBP7 which will be invaluable for functional characterization of the mas oncogene.

Signal transduction mechanism of the seabream growth hormone secretagogue receptor

We have recently cloned the full-length cDNAs of the two growth hormone secretagogue receptor (GHSR) subtypes from a teleost species, the black seabream (Acanthopagrus schlegeli) [Mol. Cell. Endocrinol. 214 (2004) 81], namely sbGHSR-1a and sbGHSR-1b. Functional expression of these two receptor constructs in human embryonic kidney 293 (HEK293) cells indicated that stimulation of sbGHSR-1a by growth hormone secretagogues (GHS) could evoke increases in intracellular Ca2+ concentration ([Ca2+]i), whereas sbGHSR-1b appeared to play an inhibitory role on the signal transduction activity of sbGHSR-1a. In the present study, we have further investigated the signal transduction mechanism of sbGHSR-1a. The peptide GHS GHRP-6 and the non-peptide GHS L163,540 were able to trigger a receptor specific and phospholipase C (PLC)-dependent elevation of [Ca2+]i in HEK293 cells stably expressing sbGHSR-1a. This GHS-induced calcium mobilization was also dependent on protein kinase C activated L-type calcium channel opening. It was found that sbGHSR-1a could function in an agonist-independent manner as it exhibited a high basal activity of inositol phosphate production in the absence of GHS, indicating that the fish receptor is constitutively active. In addition, the extracellular signal-regulated kinases 1 and 2 (ERK1/2) were found to be activated upon stimulation of sbGHSR-1a by GHRP-6. This observation provides direct evidence in the coupling of sbGHSR-1a to ERK1/2 activation. Neither Gs nor Gi proteins are coupled to the receptor, as GHS did not induce cAMP production nor inhibit forskolin-stimulated cAMP accumulation in the sbGHSR-1a bearing cells. Furthermore, the ability of the GHSR antagonist D-Lys3-GHRP-6 to inhibit basal PLC and basal ERK1/2 activity suggests that this compound is an inverse agonist. In summary, the sbGHSR-1a appears to couple through the G(q/11)-mediated pathway to activate PLC, resulting in increased IP3 production and Ca2+ mobilization from both intracellular and extracellular stores. Moreover, sbGHSR-1a may trigger multiple signal transduction cascades to exert its physiological functions.

Prostacyclin receptor-mediated activation of extracellular signal-regulated kinases 1 and 2

The prostacyclin mimetic cicaprost increased phosphorylation of extracellular signal-regulated kinases 1 and 2 (ERK1/2) in Chinese hamster ovary cells transiently expressing human (hIP-CHO) or mouse prostacyclin (mIP-CHO) receptors, but not in human neuroblastoma SK-N-SH cells or rat/mouse neuroblastoma-glioma NG108-15 cells which endogenously express IP receptors. Cicaprost stimulated ERK1/2 activity in hIP-CHO and mIP-CHO cells with EC50 values of 60 and 83 nM, respectively, and this response was significantly inhibited by protein kinase C inhibitors and agents which elevate cyclic AMP. A poor correlation was discovered between the level of ERK1/2 activity and the ability of agents to increase or decrease cyclic AMP production. The potent inhibitory effect of 3-isobutyl-1-methyl xanthine on cicaprost-stimulated phospho-ERK1/2 may be due to inhibition of phosphoinositide 3-kinase. Therefore, IP receptor-mediated activation of ERK1/2 in CHO cells occurs through a Gq/11/protein kinase C-dependent and a phosphoinoside 3-kinase-dependent process which is insensitive to IP receptor-generated cyclic AMP.

Partial Agonism of Taprostene at Prostanoid IP Receptors in Vascular Preparations from Guinea-Pig, Rat, and Mouse

This study investigates whether incomplete relaxation of vascular smooth muscle preparations induced by the prostacyclin analogue taprostene is due to partial agonism at prostanoid IP receptors. In the presence of the prostanoid EP4 receptor antagonist AH 23848, 3 microM taprostene induced 45% relaxation of phenylephrine-contracted guinea-pig saphenous vein rings and displaced log concentration-response curves for the prostacyclin analogues AFP-07, TEI-9063, and cicaprost to the right, parallel to their predicted addition curves. In contrast, taprostene interacted additively with prostaglandin E2 (PGE2), ONO-AE1-259 (selective EP2 agonist), and acetylcholine. Similarly, on rat tail artery contracted with phenylephrine, 3 microM taprostene (20% relaxation) opposed AFP-07- but not PGE2-induced relaxation. However, under U-46619-induced tone (AH 23848 absent), taprostene antagonized AFP-07 and cicaprost more than TEI-9063, suggesting that the latter has more than one relaxation mechanism. The presence of a sensitive EP3 contractile system in mouse aorta interfered with IP receptor-mediated relaxation. By generating tone with phenylephrine and the potent EP3 agonist sulprostone, it was possible to show that 3 microM taprostene (15% relaxation) selectively opposed relaxations induced by AFP-07, TEI-9063, and cicaprost. Our experiments indicate that taprostene is a partial agonist at prostanoid IP receptors, and may be a lead to an IP receptor antagonist.

Agonists can discriminate between cloned human and mouse prostacyclin receptors

The ability of prostacyclin analogues to stimulate adenylyl cyclase (AC) and phospholipase C (PLC) in Chinese hamster ovary (CHO) cells expressing cloned human (hIP) or cloned mouse (mIP) prostacyclin receptors has been compared. For hIP, the order of potency (pEC(50)) for stimulating AC and PLC pathways was similar: AFP-07 (9.3, 8.4)>cicaprost (8.3, 6.9), iloprost (7.9, 6.8)>taprostene (7.4, 6.8)>carbacyclin (6.9, 6.6), PGE(1) (6.6, 5.1). Although the standard IP agonists cicaprost and iloprost behaved similarly in both hIP and mIP receptor-expressing cells, carbacyclin and PGE(1) showed significantly higher potency at the mIP receptor, suggesting that the agonist recognition sites on hIP and mIP receptors are not identical. A further distinction between hIP and mIP receptors was found with taprostene, which had greater efficacy at hIP receptors (AC 94%, PLC 14%) than at mIP receptors (AC 77%, PLC 0%) (cicaprost=100% in each assay).

Prostaglandin El induces an inward current in voltage-clamped NG108-15 cells

The aim of this study was to explore the effect of prostaglandin E1 (PGE1) on the membrane current of whole-cell voltage-clamped NG108-15 neuroblastoma x glioma hybrid cells. Perforated patch was used. The membrane current at -70 mV (leakage current) and the current-voltage curve produced by ramp pulses from -70 to 0 mV were recorded; from the I-V curve, the conductance of the leakage current and its reversal potential were determined. Bath application of PGE1 (22 nM-3 microM) produced an inward current accompanied by a reversible conductance increase. The PGE1 effect varied greatly from cell to cell. In a group of 11 differentiated cells, the inward current induced by 0.2 microM PGE1 was on average 171.1 +/- 49.8 pA, the conductance increased 2.66 +/- 0.50-fold and the reversal potential shifted by + 13.2 +/- 4.0 mV. The average values observed with 22 nM and 3 microM PGE1 were similar. The cell-permeable cAMP analog CPT-cAMP (0.5 mM) acted like PGE1. In 9 out of 16 cells, the PGE1 effect did not disappear and was not even noticeably reduced when the NaCl in the bath was replaced by N-methyl-D-glucamine (NMDG). The PGE1 effect was also seen in Ca2(+)-free NMDG bath but vanished when NMDG was replaced by glucose. It is concluded that PGE1, probably acting via intracellular cAMP, opens non-selective cation channels with large pore diameters which allow the passage of big organic cations.

Protein kinase A-dependent coupling of mouse prostacyclin receptors to Gi is cell-type dependent

The ability of the prostacyclin (IP) receptor agonist cicaprost to activate Gs-, Gq/11- and Gi-mediated cell signalling pathways has been examined in Chinese hamster ovary (CHO) cells and human embryonic kidney 293 (HEK 293) cells expressing the cloned human (hIP) or mouse (mIP) prostacyclin receptor, and compared with data from NG108-15 and SK-N-SH cells that endogenously express rat/mouse and human IP receptors, respectively. Cicaprost stimulated [3H]cyclic AMP production with EC50 values of 1.5-22 nM, and stimulated [3H]inositol phosphate production (EC50 values 49-457 nM) in all but the SK-N-SH cells. Cicaprost failed to inhibit forskolin-stimulated [3H]cyclic AMP production in any of these cell lines. Therefore, although both human and mouse IP receptors couple to Gs and Gq/11-mediated signalling pathways in a cell type-dependent manner, we could find no evidence for IP receptor coupling to Gi.

Properties of chimeric prostacyclin/prostaglandin D2 receptors: site-directed mutagenesis reveals the significance of the isoleucine residue at position 323

Mouse prostacyclin (mIP) receptors transiently expressed in Chinese hamster ovary (CHO) cells activated both adenylyl cyclase and phospholipase C, with a 33-fold preference for signaling through Gs. The prostacyclin (IP) receptor agonists cicaprost, iloprost, carbacyclin, and prostaglandin E1 showed a similar order of potency for activation of both signaling pathways in cells transiently transfected with the mIP and the chimeric prostacyclin/prostaglandin D2 (IPN-VII/DPC and IPN-V/DPVI-C) receptors. Substitution of the carboxyl-terminal tail of the prostacyclin receptor with the corresponding region of the mDP receptor (IPN-VII/DPC) produced a receptor with increased coupling to both Gs and Gq. However, this increased G-protein coupling was lost in the IPN-V/DPVI-C receptor. The observation that both these chimeric receptors can activate phospholipase C indicates that the carboxyl-terminal tail of the IP receptor is not entirely responsible for its ability to couple to Gq. Site-directed mutagenesis studies suggest that isoleucine at position 323 in the IPN-VII/DPC receptor plays an important role in mediating the increased potency of this chimeric receptor.

Prostacyclin receptor-independent inhibition of phospholipase C activity by non-prostanoid prostacyclin mimetics

1. Chinese hamster ovary (CHO) cells were transiently transfected with the mouse prostacyclin (mIP) receptor to examine IP agonist-mediated stimulation of [(3)H]-cyclic AMP and [(3)H]-inositol phosphate production. 2. The prostacyclin analogues, cicaprost, iloprost, carbacyclin and prostaglandin E(1), stimulated adenylyl cyclase activity with EC(50) values of 5, 6, 25 and 95 nM, respectively. These IP agonists also stimulated the phospholipase C pathway with 10 - 40 fold lower potency than stimulation of adenylyl cyclase. 3. The non-prostanoid prostacyclin mimetics, octimibate, BMY 42393 and BMY 45778, also stimulated adenylyl cyclase activity, with EC(50) values of 219, 166 and 398 nM, respectively, but failed to stimulate [(3)H]-inositol phosphate production. 4. Octimibate, BMY 42393 and BMY 45778 inhibited iloprost-stimulated [(3)H]-inositol phosphate production in a non-competitive manner. 5. Activation of the endogenously-expressed P(2) purinergic receptor by ATP led to an increase in [(3)H]-inositol phosphate production which was inhibited by the non-prostanoid prostacyclin mimetics in non-transfected CHO cells. Prostacyclin analogues and other prostanoid receptor ligands failed to inhibit ATP-stimulated [(3)H]-inositol phosphate production. 6. A comparison between the IP receptor-specific non-prostanoid ONO-1310 and the structurally-related EP(3) receptor-specific agonist ONO-AP-324, indicated that the inhibitory effect of non-prostanoids was specific for those compounds known to activate IP receptors. 7. The non-prostanoid prostacyclin mimetics also inhibited phospholipase C activity when stimulated by constitutively-active mutant Galpha(q)RC, Galpha(14)RC and Galpha(16)QL transiently expressed in CHO cells. These drugs did not inhibit adenylyl cyclase activity when stimulated by the constitutively-active mutant Galpha(s)QL. 8. These results suggest that non-prostanoid prostacyclin mimetics can specifically inhibit [(3)H]-inositol phosphate production by targeting G(q/11) and/or phospholipase C in CHO cells, and that this effect is independent of IP receptors.