Factors affecting prostacyclin receptor agonist efficacy in different cell types

Prostacyclin receptor-dependent inhibition of human erythroleukemia cell differentiation is STAT3-dependent

We have previously demonstrated that activation of prostacyclin (IP) receptors in human erythroleukemia (HEL) cells phosphorylates the signal transducer and activator of transcription 3 (STAT3) via Gα(s) and Gα(16) hybrid signalling. This current study was designed to determine if functional responses to cicaprost in HEL cells were dependent on STAT3 phosphorylation. Cicaprost significantly enhanced the rapid change in HEL cell morphology induced by phorbol-12-myristate-13-acetate (PMA), and this effect was inhibited by the IP receptor antagonist RO1138452 and a STAT3 inhibitory peptide. Other indicators of PMA-induced HEL cell differentiation, such as increased expression of CD41/CD61 and an increase in cell complexity/granularity, were inhibited by cicaprost in an IP receptor-dependent and STAT3-dependent manner. Although thrombopoietic cytokines promote megakaryocytic differentiation and platelet production via activation of STAT3, the predominant STAT3-dependent effects of cicaprost in HEL cells were inhibitory towards the process of PMA-induced megakaryocytopoeisis.

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.

Novel signaling pathways promote a paracrine wave of prostacyclin-induced vascular smooth muscle differentiation

The important athero-protective role of prostacyclin is becoming increasingly evident as recent studies have revealed adverse cardiovascular effects in mice lacking the prostacyclin receptor, in patients taking selective COX-2 inhibitors, and in patients in the presence of a dysfunctional prostacyclin receptor genetic variant. We have recently reported that this protective mechanism includes the promotion of a quiescent differentiated phenotype in human vascular smooth muscle cells (VSMC). Herein, we address the intriguing question of how localized endothelial release of the very unstable eicosanoid, prostacyclin, exerts a profound effect on the vascular media, often 30 cell layers thick. We report a novel PKA-, Akt-1- and ERK1/2-dependent prostacyclin-induced prostacyclin release that appears to play an important role in propagation of the quiescent, differentiated phenotype through adjacent arterial smooth muscle cells in the vascular media. Treating VSMC with the prostacyclin analog iloprost induced differentiation (contractile protein expression and contractile morphology), and also up-regulated COX-2 expression, leading to prostacyclin release by VSMC. This paracrine prostacyclin release, in turn, promoted differentiation and COX-2 induction in neighboring VSMC that were not exposed to iloprost. Using siRNA and pharmacologic inhibitors, we report that this positive feedback mechanism, prostacyclin-induced prostacyclin release, is mediated by cAMP/PKA signaling, ERK1/2 activation, and a novel prostacyclin receptor signaling pathway, inhibition of Akt-1. Furthermore, these pathways appear to be regulated by the prostacyclin receptor independently of one another. We conclude that prevention of de-differentiation and proliferation through a paracrine positive feedback mechanism is a major cardioprotective function of prostacyclin.

Prostacyclin receptor-induced STAT3 phosphorylation in human erythroleukemia cells is mediated via Galpha(s) and Galpha(16) hybrid signaling

Human prostacyclin receptor (hIP) stimulates STAT3 via pertussis toxin-insensitive G proteins in human erythroleukemia (HEL) cells. Since hIP can utilize G(s) and G(q) proteins for signal transduction and that both G proteins can induce STAT3 phosphorylation and activation via complex signaling networks, we sought to determine if one of them is predominant in mediating the hIP signal. Stimulation of STAT3 Tyr(705) and Ser(727) phosphorylations by the IP-specific agonist, cicaprost, was sensitive to inhibition of protein kinase A, phospholipase Cbeta, protein kinase C, calmodulin-dependent protein kinase II and Janus kinase 2/3. Unlike Galpha(16)-mediated regulation of STAT3 in the same cells, cicaprost-induced STAT3 Tyr(705) phosphorylation was resistant to inhibition of Src and MEK while STAT3 Ser(727) phosphorylation distinctly required phosphatidylinositol-3 kinase. This unique inhibitor-sensitivity pattern of STAT3 phosphorylation was reproduced in HEL cells by stimulating the G(16)-coupled C5a receptor in the presence of dibutyryl-cAMP, suggesting that the change in inhibitor-sensitivity was due to activation of the G(s) pathway. This postulation was confirmed by expressing constitutively active Galpha(16)QL and Galpha(s)QL in human embryonic kidney 293 cells and the inhibitor-sensitivity of Galpha(16)QL-induced STAT3 phosphorylations could be converted by the mere presence of Galpha(s)QL to resemble that obtained with cicaprost in HEL cells. In addition, the restoration of the Galpha(16)-mediated inhibitor-sensitivity upon cicaprost induction in Galpha(s)-knocked down HEL cells again verified the pivotal role of G(s) signal. Taken together, our observations illustrate that co-stimulation of G(s) and G(q) can result in the fine-tuning of STAT3 activation status, and this may provide the basis for cell type-specific responses following activation of hIP.

Activation of STAT3 by Gαs Distinctively Requires Protein Kinase A, JNK, and Phosphatidylinositol 3-Kinase

Signal transducer and activator of transcription 3 (STAT3) can be stimulated by several G(s)-coupled receptors, but the precise mechanism of action has not yet been elucidated. We therefore examined the ability of Galpha(s)Q226L (Galpha(s)QL), a constitutively active mutant of Galpha(s), to stimulate STAT3 Tyr705 and Ser727 phosphorylations in human embryonic kidney 293 cells. Apart from Galpha(s)QL, the stimulation of Galpha(s) by cholera toxin or beta2-adrenergic receptor and the activation of adenylyl cyclase by forskolin, (Sp)-cAMP, or dibutyryl-cAMP all promoted both STAT3 Tyr705 and Ser727 phosphorylations. Moreover, the removal of Galpha(s) by RNA interference significantly reduced the beta2-adrenergic receptor-mediated STAT3 phosphorylations, denoting its capacity to regulate STAT3 activation by a G protein-coupled receptor. The possible downstream signaling molecules involved were assessed by using specific inhibitors and dominant negative mutants. Induction of STAT3 Tyr705 and Ser727 phosphorylations by Galpha(s)QL was suppressed by inhibition of protein kinase A, Janus kinase 2/3, Rac1, c-Jun N-terminal kinase (JNK), or phosphatidylinositol 3-kinase, and a similar profile was observed in response to beta2-adrenergic receptor stimulation. In contrast to the Galpha16-mediated regulation of STAT3 in HEK 293 cells (Lo, R. K., Cheung, H., and Wong, Y. H. (2003) J. Biol. Chem. 278, 52154-52165), the Galpha(s)-mediated responses, including STAT3-driven luciferase activation, were resistant to inhibition of phospholipase Cbeta. Surprisingly, Galpha(s)-mediated phosphorylation at Tyr705, but not at Ser727, was resistant to inhibition of c-Src, Raf-1, and MEK1/2 as well as to the expression of dominant negative Ras. Therefore, as with other Galpha-mediated activations of STAT3, the stimulatory signal arising from Galpha(s) is transduced via multiple signaling pathways. However, unlike the mechanisms employed by Galpha(i) and Galpha(14/16), Galpha(s) distinctively requires protein kinase A, JNK, and phosphatidylinositol 3-kinase for STAT3 activation.

Antimitogenic effects of prostacyclin on the G1 phase cyclin-dependent kinases

The prostanoid prostacyclin (PGI2) inhibits proliferation of cultured vascular SMCs by inhibiting cell cycle progression from G1 to S phase. Progression through G1 phase is regulated by the sequential activation of the G1 phase cyclin-dependent kinases (cdks). Recent studies have shown that PGI2-dependent activation of its receptor, IP, inhibits G1 phase progression by blocking the degradation of p27 and the activation of cyclin E-cdk2. High Density Lipoproteins (HDL) and its associated apolipoprotein, ApoE, also inhibit S phase entry of vascular SMCs, and the effects of HDL and ApoE are, at least in part, also mediated by the production of PGI2. The antimitogenic effects of hyaluronan may also be controlled by PGI2. This review summarizes the effects of PGI2 on the G1 phase cyclin-cdks and discusses the potential role of PGI2 as a common component of multiple extracellular signals that attenuate the proliferation of vascular SMCs.

Prostacyclin receptor induces STAT1 and STAT3 phosphorylations in human erythroleukemia cells: a mechanism requiring PTX-insensitive G proteins, ERK and JNK

The ability of the human prostacyclin receptor (hIP) to regulate the activities of signal transducers and activators of transcription (STATs) has not yet been documented. In the present study, we have delineated the mechanism by which hIP induces STAT3 phosphorylations in human erythroleukemia (HEL) cells. Stimulation of endogenous hIP by its specific agonist, cicaprost, resulted in STAT3 Tyr705 and Ser727 phosphorylations in a time- and concentration-dependent manner. Cicaprost-induced STAT3 Tyr705 and Ser727 phosphorylations were resistant to pertussis toxin (PTX) treatment, suggesting that these responses were mediated through PTX-insensitive G proteins. In addition, extracellular signal-regulated kinase (ERK) and c-Jun N-terminal kinase (JNK), but not p38 MAPK, were shown to be phosphorylated by cicaprost in a time- and concentration-dependent manner via PTX-insensitive G proteins. The levels of the interaction between STAT3, ERK and JNK were enhanced by cicaprost treatment. The involvement of Raf-1, MEK1/2 and JNK in cicaprost-induced phosphorylations of STAT3 was illustrated by the use of their selective inhibitors. In contrast, p38 MAPK did not appear to be required. Similar observations were obtained with STAT1 upon stimulation by cicaprost. Taken together, these results demonstrate for the first time that hIP activation by cicaprost can lead to STAT1 and STAT3 phosphorylations via signaling pathways involving PTX-insensitive G proteins, ERK and JNK.

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.

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.

Prostacylin receptor activation inhibits proliferation of aortic smooth muscle cells by regulating cAMP response element-binding protein- and pocket protein-dependent cyclin a gene expression

The prostanoid prostacyclin (PGI2) inhibits aortic smooth muscle cell proliferation by blocking cell cycle progression from G1-to S-phase. However, the mechanism of this inhibition is poorly understood. We report here that the PGI2 mimetic, cicaprost, inhibits the induction of cyclin A and activation of the cyclin A promoter in primary and established rodent aortic smooth muscle cells. The inhibition of cyclin A gene expression is associated with a block in cyclin E-cdk2 activity and phosphorylation of both the retinoblastoma protein and p107. Inactivation of pocket proteins with human papilloma virus protein E7 partially, but not completely, restored cyclin A promoter activity in cicaprost-treated cells. Complementary studies showed that occupancy of the cAMP response element (CRE) is required for efficient activation of the cyclin A promoter in aortic smooth muscle cells, that the CRE is primarily occupied by the CRE-binding protein (CREB) and phospho-CREB, and that cicaprost blocks the binding of CREB and phospho-CREB to the cyclin A promoter CRE. Treatment with pertussis toxin reversed the inhibitory effects of cicaprost on CRE occupancy, cyclin E-cdk2 activity, and S phase entry, suggesting the involvement of Gi signaling in cicaprost action. We conclude that PGI2 inhibits proliferation of aortic smooth muscle cells by coordinately blocking CRE- and pocket protein-dependent cyclin A gene expression.

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.

Impaired receptor binding and activation associated with a human prostacyclin receptor polymorphism

The human prostacyclin receptor (hIP) is a seven transmembrane-spanning G-protein-coupled receptor that plays an important role in vascular homeostasis. Recent genetic analyses (SNP database, NCBI) have revealed the first two polymorphisms within the coding sequence, V25M and R212H. Here we present structure-function characterizations of these polymorphisms at physiological pH (7.4) and at an acidic pH (6.8) that would be encountered during stress such as renal, respiratory, or heart failure. Through a series of competition binding and G-protein activation assays (measured by cAMP production), we determined that the V25M polymorph exhibited agonist binding and G-protein activation similar to wild-type receptor at normal pH (7.4). However, the R212H variant demonstrated a significant decrease in binding affinity at lower pH (R212H at pH 7.4, K(i) = 2.2 +/- 1.2 nm; pH 6.8 K(i) = 45.6 +/- 12.0 nm). The R212H polymorph also exhibited abnormal activation at both pH 7.4 and pH 6.8 (pH 7.4, R212H EC(50) = 2.8 +/- 0.5 nm versus wild-type hIP EC(50) = 0.5 +/- 0.1 nm; pH 6.8, R212H EC(50) = 3.2 +/- 1.6 nm versus wild-type hIP EC(50) = 0.5 +/- 0.2 nm). Polymorphisms of the human prostacyclin receptor potentially may be important predictors of disease progress during biological stressors such as acidosis in which urgent correction of bodily pH may be required to restore normal hemostasis and vasodilation. This study provides the mechanistic basis for further research into genetic risk factors and pharmacogenetics of cardiovascular disease associated with hIP.

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.