Two types of prostacyclin receptor coupling to stimulation of adenylate cyclase and phosphatidylinositol hydrolysis in a cultured mast cell line, BNu-2cl3 cells

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

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

Prostanoids and prostanoid receptors in signal transduction

Prostanoids are arachidonic acid metabolites and are generally accepted to play pivotal functions in amongst others inflammation, platelet aggregation, and vasoconstriction/relaxation. Inhibition of their production with, for instance, aspirin has been used for over a century to combat a large variety of pathophysiological processes, with great clinical success. Hence, the cellular changes induced by prostanoids have been subject to an intensive research effort and especially prostanoid-dependent signal transduction has been extensively studied. In this review, we discuss the impact of the five basic prostanoids, TxA(2), PGF(2alpha), PGE(2), PGI(2), and PGD(2), via their receptors on cellular physiology. These inflammatory lipids may stimulate serpentine plasma membrane-localized receptors, which in turn affect major signaling pathways, such as the MAP kinase pathway and the protein kinase A pathway, finally resulting in altered cellular physiology. In addition, prostanoids may activate the PPARgamma members of the steroid/thyroid family of nuclear hormone receptors, which act as transcription factors and may thus directly influence gene transcription. Finally, evidence exists that prostanoids act as second messengers downstream of mitogen receptor activation, mediating events, such as cytoskeletal changes, maybe via direct interaction with GTPase activating proteins. The final cellular reaction to prostaglandin stimulation will most likely depend on combined effects of the above-mentioned levels of interaction between prostaglandins and their cellular receptors.

Localization of IP in rabbit kidney and functional role of the PGI(2)/IP system in cortical collecting duct

To clarify the role of the PGI(2)/PGI(2) receptor (IP) system in rabbit cortical collecting duct (RCCD), we characterized the expression of IP receptors in the rabbit kidney. We show by Northern and Western blotting that IP mRNA and protein was detectable in all three regions of the kidney. To determine how PGI(2) signals, we compared the effects of different PGI(2) analogs [iloprost (ILP), carba-prostacyclin (c-PGI(2)), and cicaprost (CCP)] in the isolated perfused RCCD. PGI(2) analogs did not increase water flow (L(p)). Although PGI(2) analogs did not reduce an established L(p) response to 8-chlorophenylthio-cAMP, they equipotently inhibited AVP-stimulated L(p) by 45%. The inhibitory effect of ILP and c-PGI(2) on AVP-stimulated L(p) is partially reversed by the protein kinase C inhibitor staurosporine and abolished by pertussis toxin; no effect was obtained with CCP. In fura 2-loaded RCCD, CCP did not alter cytosolic Ca(2+) concentration ([Ca(2+)](i)), but, in the presence of CCP, individual infusion of ILP and PGE(2) increased [Ca(2+)](i), suggesting that CCP did not cause desensitization to either ILP or PGE(2). We concluded that ILP and c-PGI(2) activate PKC and the liberation of [Ca(2+)](i) but not CCP. This suggested an important role for phosphatidylinositol hydrolysis in mediating ILP and c-PGI(2) effects but not CCP in RCCD.

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.

Factors affecting prostacyclin receptor agonist efficacy in different cell types

Octimibate and related nonprostanoid prostacyclin mimetics are partial agonists displaying highly tissue-specific responses. Octimibate demonstrated considerably greater efficacy for stimulation of adenylyl cyclase activity in Chinese hamster ovary cells transiently expressing mouse prostacyclin receptors (mIP-CHO cells) when compared to human SK-N-SH neuroblastoma cells, which endogenously express prostacyclin (IP) receptors. Pretreatment of both cell types with pertussis toxin (PTx) failed to influence IP agonist efficacy or potency, indicating a lack of involvement of an agonist-stimulated inhibitory G(i)-coupled pathway. Although stimulation of mIP-CHO cells with the full agonist cicaprost increased both [3H]cyclic AMP and [3H]inositol phosphate ([3H]IP) accumulation (pEC(50) values of 8.35 and 6.82, respectively), IP receptor signalling through G(q) in SK-N-SH cells was absent. Inhibition of protein kinase C (PKC) in mIP-CHO cells increased [3H]IP accumulation but had no effect on [3H]cyclic AMP accumulation. Therefore, the poor coupling of the IP receptor in SK-N-SH cells to G(q) is unlikely to explain the relatively low efficacy of octimibate for stimulating adenylyl cyclase in these cells. Furthermore, protein kinase A (PKA) inhibition appears to enhance IP receptor signalling through both G(s) and G(q) in mIP-CHO cells.

Protein kinase A-mediated phosphorylation of serine 357 of the mouse prostacyclin receptor regulates its coupling to G(s)-, to G(i)-, and to G(q)-coupled effector signaling

The prostacyclin receptor (IP) is primarily coupled to G alpha(s)-dependent activation of adenylyl cyclase; however, a number of studies indicate that the IP may couple to other secondary effector systems perhaps in a species-specific manner. In the current study, we investigated the specificity of G protein:effector coupling by the mouse (m) IP overexpressed in human embryonic kidney 293 cells and endogenously expressed in murine erythroleukemia cells. The mIP exhibited efficient G alpha(s) coupling and concentration-dependent increases in cAMP generation in response to the IP agonist cicaprost; however, mIP also coupled to G alpha(i) decreasing the levels of cAMP in forskolin-treated cells. mIP coupling to G alpha(i) was pertussis toxin-sensitive and was dependent on protein kinase (PK) A activation status. In addition, the mIP coupled to phospholipase C (PLC) activation in a pertussis toxin-insensitive, G alpha(i)-, G beta gamma-, and PKC-independent but in a G alpha(q)- and PKA-dependent manner. Whole cell phosphorylation assays demonstrated that the mIP undergoes cicaprost-induced PKA phosphorylation. mIP(S357A), a site-directed mutant of mIP, efficiently coupled to G alpha(s) but failed to couple to G alpha(i) or to efficiently couple to G alpha(q):PLC. Moreover, mIP(S357A) did not undergo cicaprost-induced phosphorylation confirming that Ser(357) is the target residue for PKA-dependent phosphorylation. Finally, co-precipitation experiments permitted the detection of G alpha(s), G alpha(i), and G alpha(q) in the immunoprecipitates of mIP, whereas only G alpha(s) was co-precipitated with mIP(S357A) indicating that Ser(357) of mIP is essential for G alpha(i) and G alpha(q) interaction. Moreover, inhibition of PKA blocked co-precipitation of mIP with G alpha(i) or G alpha(q). Taken together our data indicate that the mIP, in addition to coupling to G alpha(s), couples to G alpha(i) and G alpha(q); however, G alpha(i) and G alpha(q) coupling is dependent on initial cicaprost-induced mIP:G alpha(s) coupling and phosphorylation of mIP by cAMP-dependent PKA where Ser(357) was identified as the target residue for PKA phosphorylation.

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.

Characterization of prostanoid receptors mediating inhibition of histamine release from anti-IgE-activated rat peritoneal mast cells

1. Prostanoid receptors mediating inhibition of anti-IgE induced histamine release from rat peritoneal mast cells have been characterized pharmacologically. PGD2 and the specific DP receptor agonists BW 245C and ZK 118182 were the most potent inhibitors with half-maximal concentrations of 0.26, 0.06 and 0.02 microM respectively. The maximum inhibition attainable was 60-65% with 10(-5) M BW 245C and ZK 118182. 2. Among several EP receptor agonists investigated, only PGE2 and the EP2/EP3 receptor agonist misoprostol induced significant inhibition (46.8 +/- 4.7% at 10(-4) M and 18.7 +/- 6.8% at 10(-5) M respectively). The IP receptor agonists cicaprost and iloprost were both less potent than the DP agonists in inhibiting histamine release (45.2 +/- 3.3% and 35.1 +/- 2.5% inhibition respectively at 10(-5) M), whereas PGF2 alpha and the TP receptor agonist U-46619 were only marginally effective. 3. The EP4/TP receptor antagonist AH 23848 failed to affect the inhibitory actions of PGD2 or PGE2 even at 10(-5) M, whereas the DP/EP1/EP2 receptor antagonist AH 6809 slightly enhanced the effect of PGD2 at 10(-6) M. 4. At concentrations of 3 x 10(-6) to 10(-5) M, the putative DP receptor antagonist ZK 138357 dose-dependently suppressed the inhibitory activities of the DP agonists, PGE2 and cicaprost. The antagonism of ZK 138357 against the DP receptor agonists appeared to be competitive with pA2 values of around six. 5. In conclusion, these data support our earlier proposal that an inhibitory DP receptor is the predominant prostanoid receptor in rat peritoneal mast cell. The properties of this receptor in relation to putative DP receptor subtypes reported in the literature are discussed.

Phospholipase C activation by prostacyclin receptor agonist in cerebral microvascular smooth muscle cells

The mechanism through which iloprost permits cerebral vasodilation induced by specific stimuli is incompletely understood. Previous study suggests there might be interplay between the adenylyl cyclase and phospholipase C (PLC) systems. Coupling of the prostacyclin receptor with the PLC pathway system was investigated. Iloprost, a stable prostacyclin analog, was used as a prostacyclin receptor agonist. We investigated the effects of iloprost (10-12-10-6 M) on inositol 1,4,5-trisphosphate (IP3) production by piglet cerebrovascular smooth muscle cells in primary culture. Iloprost caused concentration- and time-dependent increases in IP3 production in control cells and in cells pretreated with LiCl (to prevent further IP3 metabolism). Iloprost treatment (10-12 M) of cerebrovascular smooth muscle cells, in the absence and presence of 20 mM LiCl, resulted in 2-fold and 4-fold increases in the formation of IP3, respectively. In contrast, 10-10 M to 10-6 M iloprost, either in the presence or absence of LiCl, induced moderate or no increase in IP3 formation. Iloprost (10-10-10-12 M) strongly stimulated diacylglycerol (DAG) generation, whereas higher concentrations (10-8 M) did not induce an increase. In conclusion, the results suggest that prostacyclin receptors on cerebromicrovascular smooth muscle can couple to PLC, generating the second messengers, IP3 and DAG.

Prostanoid signaling, localization, and expression of IP receptors in rat thick ascending limb cells

It is widely held that only one prostacyclin (IP) receptor exists that can couple to guanine stimulatory nucleotide binding proteins (Gs) leading to activation of adenyl cyclase. Although IP receptor mRNA is expressed in vascular arterial smooth muscle cells and platelets, with lower level expression in mature thymocytes, splenic lymphocytes, and megakaryocytes, there is no molecular evidence for IP receptor expression in renal epithelial cells. The purpose of the present study was to obtain molecular evidence for the expression and localization of the IP receptor and to study the signaling pathways of IP receptor in rat medullary thick ascending limb (MTAL). Biochemical studies showed that IP prostanoids do not increase cAMP in rat MTAL. However, in the presence of vasopressin, inhibition of cAMP formation by prostacyclin (PGI2) analogs is pertussis toxin sensitive and does not activate protein kinase C. In situ hybridization studies localized IP receptor mRNA expression to MTAL in the rat kidney outer medulla. The results of RT-PCR of freshly isolated RNA from MTAL, with primers specific for the mouse IP receptor cDNA, produced an amplification product of the correct predicted size that contained an expected Nco I endonuclease restriction site. We conclude that rat renal epithelial cells express the IP receptor, coupled to inhibition of cAMP production.

Localization of the prostacyclin receptor in human kidney

BACKGROUND

Prostacyclin is an important mediator of renal hemodynamics. Furthermore, recent studies argue for a role of this arachidonic acid metabolite in the regulation of salt and water handling in the distal nephron. To gain insight into the network of prostacyclin signal transduction, we analyzed the intrarenal distribution of the prostacyclin receptor (IP receptor) in adult human kidney.

METHODS

Specific polyclonal antibodies against a synthetic peptide of the human IP receptor were generated. By means of immunohistology the localization of IP receptor protein was studied. The mRNA expression for IP receptor was analyzed by in situ hybridization using specific cRNA probes.

RESULTS

In human kidney sections both IP receptor-immunoreactive protein and mRNA were expressed in smooth muscle cells and endothelial cells. Expression of the IP receptor was observed in glomerular cells, namely mesangial cells, endothelial cells, and podocytes. Both mRNA and protein expression for IP receptor was observable in Tamm-Horsfall-negative distal tubules and collecting ducts.

CONCLUSIONS

The vascular expression of the IP receptor is consistent with the known vasodilatory effect of prostacyclin in vascular beds. Glomerular expression argues for a role of this autacoid in the regulation of glomerular hemodynamics. The tubular distribution might point towards the involvement of prostacyclin in renal salt and water handling.

Roles of vasodilatory prostaglandins in mitogenesis of vascular smooth muscle cells

Vasodilatory prostaglandins (PGI2, PGE1) and synthetic prostacyclin mimetics inhibit smooth muscle cell proliferation in vitro after stimulation by growth factors. Similar results are obtained in vivo after endothelial injury, suggesting that vasodilatory prostaglandins might also control smooth muscle cell proliferation in vivo. However, available data from clinical trials are conflicting and currently do not support the concept that these compounds might be successfully used to suppress excessive smooth muscle cell growth in response to tissue injury, specifically restenosis after PTCA. One possible explanation for these different results is an agonist-induced down-regulation of prostacyclin receptors in vascular smooth muscle cells. It is possible that enhanced endogenous prostacyclin biosynthesis, subsequent to induction of COX-2 and/or in relation to the formation of a neointima from media smooth muscle cells, might have a similar effect. There is still uncertainty regarding the cellular signal transduction pathways and their possibly complex interaction, although cAMP-dependent reactions are probably involved. In addition, vasodilatory prostaglandins might also interfere with the generation and action of other growth modulating factors, including PDGF, hepatocyte growth factor and nitric oxide. In conclusion, vasodilatory prostaglandins might be considered as growth modulating endogenous mediators in vascular smooth muscle cells.

Agonist-dependent phosphorylation of an epitope-tagged human prostacyclin receptor

An epitope-tagged human prostacyclin receptor (HAhIP) was constructed and stably transfected into human embryonic kidney 293 cells. The receptor exhibited high (Kd = 0.4 +/- 0.08 nM, Bmax = 0.7 +/- 0.2 pmol/mg protein; n = 4) and low (Kd = 75 +/- 27.4 nM, Bmax = 7.1 +/- 3.6 pmol/mg protein; n = 4) affinity for iloprost and coupled to both cAMP (EC50 = 0.1 +/- 0.03 nM) and inositol phosphate (EC50 = 43.1 +/- 10 nM) production. The receptor resolved on SDS-polyacrylamide gel electrophoresis as a broad complex with a molecular mass of 44-62 kDa and is glycosylated and phosphorylated. Stimulation of transfected cells with iloprost induced a rapid time- and concentration-dependent phosphorylation of HAhIP. Pretreatment of cells with a protein kinase C (PKC) inhibitor (GF109203X; 5 microM) abolished basal phosphorylation and dramatically reduced iloprost-induced HAhIP phosphorylation. A protein kinase A (PKA) inhibitor (H89) was largely ineffective under the same conditions. HAhIP phosphorylation was stimulated by receptor-dependent (thrombin, 2 units/ml) or receptor-independent (phorbol 12-myristate 13-acetate, 5 microM) PKC activation; both were abolished by pretreatment of cells with GF109203X. In contrast, receptor-independent (forskolin (5 microM) or dibutyryl cAMP (1 microM)) activation of PKA did not induce HAhIP phosphorylation. These results indicate that the human prostacyclin receptor may be regulated by agonist-dependent phosphorylation. This appears to be mediated, in part, by activation of PKC but not by PKA.

The inhibitory effects of non-prostanoid prostacyclin mimetics on rat neutrophil function

The effects of three non-prostanoid prostacyclin mimetics on rat peritoneal neutrophil activity have been investigated and compared with the effects of the prostacyclin analogues cicaprost and iloprost. Cicaprost, iloprost, BMY 22389 (octimibate), BMY 42393 and BMY 45778 inhibited N-formyl-methionyl-leucyl-phenylalanine (FMLP)-stimulated neutrophil aggregation with IC50 values of 2.1, 4.5, 286, 462 and 20 nM, respectively. Cicaprost and iloprost produced clear concentration-related increases in [3H]cyclic AMP accumulation; EC50 values were 20 and 44 nM, respectively. In contrast, the three BMY compounds showed low efficacy as activators of adenylyl cyclase. The inhibitory effect of prostacyclin mimetics does not appear to depend on effects on intracellular calcium concentration, or on KATP channels. Extensive studies using cyclic AMP mimetics and antagonists suggest that the anti-aggregatory activity of the non-prostanoid prostacyclin mimetics on rat neutrophils may involve highly localized increases in cyclic AMP.

Molecular mechanisms of diverse actions of prostanoid receptors

This review summarizes recent advances in the molecular characterization of prostanoid receptors. Prostanoids exert versatile actions in diverse tissues and cells through specific cell surface receptors. Molecular biological studies revealed the primary structure of eight types and subtypes of prostanoid receptor from various species. These include the thromboxane A2 receptor, prostacyclin receptor, prostaglandin (PG) F receptor, PGD receptor and four subtypes of PGE receptors. They are coupled to different signal transduction systems. In addition, multiple isoforms of PGE receptor EP3 subtype have been identified in various species. They are produced through alternative RNA splicing from a single gene and differ only in their carboxy-terminal tails. These isoforms differ in the efficiency of G protein activation, in the specificity of coupling to G proteins or in sensitivity to desensitization. This molecular characterization is useful for understanding the diverse physiological roles of prostanoids.

Differential activation of Gi and Gs proteins by E- and I-type prostaglandins in membranes from the human erythroleukaemia cell line, HEL

The group of prostaglandin (PG) E2- and prostacyclin receptors consists of different subtypes, which exhibit different affinities for prostaglandins and synthetic analogues. PGE2 activities the E-type PG receptor subtypes EP1, EP2 and EP3, whereas the PGE2 analogue, sulprostone, binds only to the EP1 and EP3 receptor subtypes. The stable PGI2 analogues, iloprost and cicaprost, both activate the PGI2 receptor (IP) and iloprost, additionally, bind to the EP1 subtype. Using these subtype-selective PG receptor agonists, we studied the interaction of PG receptor subtypes with Gs and Gi-type heterotrimeric guanine nucleotide-binding proteins (G proteins) in membranes from the human erythroleukaemia cell line, HEL. Sulprostone stimulated high-affinity GTPase in HEL membranes in a pertussis toxin (PTX)-sensitive manner. In contrast, the stimulations induced by PGE2, iloprost and cicaprost were only partially inhibited by PTX. PGE2, sulprostone, iloprost and cicaprost stimulated cholera toxin-catalysed ADP-ribosylation as well as labelling with GTP azidoanilide of membrane proteins comigrating with immunologically identified Gi protein alpha subunits. Furthermore, PGE2, iloprost and cicaprost enhanced GTP azidoanilide-labelling of Gs protein alpha subunits, whereas sulprostone did not. We suggest that in HEL cells (1) EP1 and EP3 receptor subtypes activate G1 proteins, that (2) the EP2 receptor subtype activates Gs proteins and that (3) the IP receptor activates both Gi and Gs proteins.