Expression of Prostacyclin Receptor in Human Megakaryocytes

Abnormal megakaryopoiesis and platelet function in cyclooxygenase-2-deficient mice

Previous studies suggest that cyclooxygenase-2 (COX-2) might influence megakaryocyte (MK) maturation and platelet production in vitro. Using a gene deletion model, we analysed the effect of COX-2 deficiency on megakaryopoiesis and platelet function. COX-2-/- mice (10-12 weeks old) have hyper-responsive platelets as suggested by their enhanced aggregation, TXA2 biosynthesis, CD62P and CD41/CD61 expression, platelet-fibrinogen binding, and increased thromboembolic death after collagen/epinephrine injection compared to wild-type (WT). Moreover, increased platelet COX-1 expression and reticulated platelet fraction were observed in COX-2-/- mice while platelet count was similar to WT. MKs were significantly reduced in COX-2-/- bone marrows (BMs), with high nuclear/cytoplasmic ratios, low ploidy and poor expression of lineage markers of maturation (CD42d, CD49b). However, MKs were significantly increased in COX-2-/- spleens, with features of MK maturation markers which were not observed in MKs of WT spleens. Interestingly, the expression of COX-1, prostacyclin and PGE2 synthases and prostanoid pattern were modified in BMs and spleens of COX-2-/- mice. Moreover, COX-2 ablation reduced the percentage of CD49b+ cells, the platelet formation and the haematopoietic stem cells in bone marrow and increased their accumulation in the spleen. Splenectomy decreased peripheral platelet number, reverted their hyper-responsive phenotype and protected COX-2-/- mice from thromboembolism. Interestingly, fibrosis was observed in spleens of old COX-2-/- mice (28 weeks old). In conclusion, COX-2 deletion delays BM megakaryopoiesis promoting a compensatory splenic MK hyperplasia, with a release of hyper-responsive platelets and increased thrombogenicity in vivo. COX-2 seems to contribute to physiological MK maturation and pro-platelet formation.

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.

Intermolecular cross-talk between the prostaglandin E2 receptor (EP)3 of subtype and thromboxane A(2) receptor signalling in human erythroleukaemic cells

BACKGROUND AND PURPOSE

In previous studies investigating cross-talk of signalling between prostaglandin (PG)E(2) receptor (EP) and the TPalpha and TPbeta isoforms of the human thromboxane (TX)A(2) receptor (TP), 17-phenyl trinor PGE(2)-induced desensitization of TP receptor signalling through activation of the AH6809 and SC19220-sensitive EP(1) subtype of the EP receptor family, in a cell-specific manner. Here, we sought to further investigate that cross-talk in human erythroleukaemic (HEL) 92.1.7 cells.

EXPERIMENTAL APPROACH

Specificity of 17-phenyl trinor PGE(2) signalling and its possible cross-talk with signalling by TPalpha/TPbeta receptors endogenously expressed in HEL cells was examined through assessment of agonist-induced inositol 1,4,5-trisphosphate (IP)(3) generation and intracellular calcium ([Ca(2+)](i)) mobilization.

KEY RESULTS

While 17-Phenyl trinor PGE(2) led to activation of phospholipase (PL)Cbeta to yield increases in IP(3) generation and [Ca(2+)](i), it did not desensitize but rather augmented that signalling in response to subsequent stimulation with the TXA(2) mimetic U46619. Furthermore, the augmentation was reciprocal. Signalling by 17-phenyl trinor PGE(2) was found to occur through AH6809- and SC19920-insensitive, Pertussis toxin-sensitive, G(i)/G(betagamma)-dependent activation of PLCbeta. Further pharmacological investigation using selective EP receptor subtype agonists and antagonists confirmed that 17-phenyl trinor PGE(2)-mediated signalling and reciprocal cross-talk with the TP receptors occurred through the EP(3), rather than the EP(1), EP(2) or EP(4) receptor subtype in HEL cells.

CONCLUSIONS AND IMPLICATIONS

The EP(1) and EP(3) subtypes of the EP receptor family mediated intermolecular cross-talk to differentially regulate TP receptor-mediated signalling whereby activation of EP(1) receptors impaired or desensitized, while that of EP(3) receptors augmented signalling through TPalpha/TPbeta receptors, in a cell type-specific manner.

Hemodynamics and epoprostenol use are associated with thrombocytopenia in pulmonary arterial hypertension

BACKGROUND

Thrombocytopenia develops in some patients with advanced pulmonary arterial hypertension (PAH) while receiving IV epoprostenol therapy. In this study, we evaluate whether epoprostenol use, other PAH medication use, hemodynamics, or PAH etiology are associated with thrombocytopenia in PAH.

METHODS

Platelet counts were evaluated in 47 PAH patients receiving IV epoprostenol, and in 44 patients with an inadequate response to initial therapy with oral agents in a cross-sectional study. Associations between thrombocytopenia (platelet count < 150,000/mL) and epoprostenol use, hemodynamics, PAH etiology, and use of other PAH medications were evaluated in univariable and multivariable analyses.

RESULTS

PAH subtypes included idiopathic (69%), fenfluramine (18%), connective tissue disease (10%), and congenital heart disease (2%)-associated PAH. Thrombocytopenia was observed in 34% of patients treated with epoprostenol, compared with 15% of patients receiving oral therapy (odds ratio [OR], 2.9; p < 0.05), and the association between epoprostenol and thrombocytopenia remained significant after adjustment for differences in hemodynamics (OR, 5.0; p < 0.05). Right atrial pressure (OR, 1.12 per mm Hg; p < 0.05) and mixed venous oxygen saturation (Svo(2)) [OR, 0.92 per percentage; p < 0.05] were also associated with thrombocytopenia in univariable analyses; after logistic regression analysis, both the use of epoprostenol and Svo(2) were independently associated with thrombocytopenia. In a separate analysis including only patients with current or prior epoprostenol use, epoprostenol dose and right atrial pressure were inversely associated with platelet count.

CONCLUSION

Epoprostenol use and severity of hemodynamic abnormalities are associated with thrombocytopenia in PAH, and these effects appear to be independent and additive.

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 the PGI(2)/IP system contributes to the development of circulatory failure in a rat model of endotoxic shock

Prostacyclin levels are increased in septic patients and several animal models of septic shock, and selective inhibition of cyclooxygenase-2 improved cardiovascular dysfunction in rats treated with lipopolysaccharide (LPS). Here, we examine the specific role of prostacyclin and of the receptor for prostacyclin (IP) in the development of LPS-induced circulatory failure. Intravenous injection of LPS (10 mg/kg) into male Sprague-Dawley rats caused a strong increase in plasma prostacyclin levels, which was paralleled by a decrease in blood pressure and an increase in heart rate. Moreover, LPS injection increased the mRNA expression of the IP receptor in the heart, aorta, lung, liver, adrenal glands, and kidneys. Cotreatment with the IP antagonist CAY-10441 (1, 10, 30, and 100 mg/kg) dose-dependently moderated the LPS-induced changes in mean arterial blood pressure, heart rate, cardiac output, and systemic vascular resistance. The development of cardiovascular failure was ameliorated by CAY-10441 in spite of the typical LPS-induced increases in plasma levels of cytokines and NO. In vitro, cytokines dose- and time-dependently induced IP expression in rat vascular smooth muscle cells. Incubation of cells with the stable IP agonist iloprost in the presence of the phosphodiesterase inhibitor 3-isobutyl-1-mehylxanthine resulted in higher cAMP levels in cytokine-treated cells compared with untreated cells. Taken together, our data demonstrate a prominent role of the prostacyclin/IP system in the development of LPS-induced cardiovascular failure.

Prostacyclin prevents nitric oxide-induced megakaryocyte apoptosis

1 We have previously demonstrated that nitric oxide (NO) triggers CD34(+)-derived megakaryocyte apoptosis. We here show that prostacyclin (PGI(2)) inhibits PAPA/NO-induced megakaryocyte death detected by fluorescent microscopy and flow cytometry. 2 The cAMP-specific phosphodiesterase inhibitor, Ro 20-1724, and the permeable analog dibutyryl-cAMP also delayed apoptosis. PGI(2) effect was fully prevented when adenylyl cyclase activity was suppressed by SQ 22536, and partially reversed by the permeable protein kinase A inhibitor PKI 14-22 amide. ELISA showed that while both PGI(2) and NO alone or synergistically raised cAMP, only NO was able to increase intracellular cGMP levels. 3 Treatment of megakaryocytes with PGI(2) abolished both basal and NO-raised cGMP levels. Addition of 8-pCPT-cGMP or activation of soluble guanylyl cyclase by BAY 41-2272 induced cell death in a concentration-dependent manner, and ODQ, an inhibitor of guanylyl cyclase, prevented both PAPA/NO- or BAY 41-2272-induced apoptosis. Specific cGMP phosphodiesterase inhibition by Zaprinast or suppression of adenylyl cyclase by SQ 22536 enhanced the PAPA/NO proapoptotic effect. 4 PGI(2) completely inhibited NO-mediated generation and the increased activity of the cleaved form of caspase-3. 5 In conclusion, our results demonstrate that contrary to their well-known direct and synergistic inhibitory effects on platelets, PGI(2) and NO regulate opposite megakaryocyte survival responses through a delicate balance between intracellular cyclic nucleotide levels and caspase-3 activity control.

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.

Regulation of the human thromboxane A2 receptor gene in human megakaryoblastic MEG-01 cells

Thromboxane A(2) (TXA(2)) is an important mediator for platelet aggregation and blood vessel constriction. TXA(2) receptor (TP receptor) is expressed in different cell types including smooth muscle cells, endothelial cells and platelets. Expression level of TP receptor may modulate the action of TXA(2) on target cells. In megakaryoblastic MEG-01 cells, a cell line representing a model for platelet precursor cells, addition of phorbolester 12-O-tetradecanoylphorbol-13-acetate (TPA) caused an increase in transcriptional activity of TP receptor gene promoter. Within 20 h a rise in expression of TP receptor mRNA and protein was observed. The effect of TPA was concentration-dependent and was blocked by specific inhibitors of protein kinase C. Flow cytometry analysis indicated that the increase in TP receptor expression appeared to be one of the earliest events in the course of TPA-induced maturation of MEG-01 cells. Stimulation of the protein kinase A pathway by incubation with forskolin or IBMX caused a decrease in transcriptional activity. Promoter deletion experiments indicated that the responsive elements for protein kinase A and C are located upstream and downstream, respectively, of -700 bp of the TP receptor gene. These experiments indicate that the expression of the human thromboxane receptor is differently regulated in platelet precursor cells by the protein kinase A and C pathway.

Human prostacyclin receptor

Prostacyclin, a member of the eicosanoid family of lipid mediators, is the major product of arachidonic acid metabolism formed in the marcovascular endothelium. It is a potent vasodilator, antithrombotic, and antiplatelet agent that mediates it effects through a membrane-associated receptor termed the IP. Cloning of the cDNA for IP, from human and other species, indicated its membership of the G protein-coupled receptor superfamily and has allowed detailed examination of the signaling and regulatory pathways utilized by this receptor. This article examines the current state of knowledge of the IP, its signaling and regulation, and its biological role in vivo and examines the possible existence of multiple PGI2 receptor sites.

Development of platelet inhibition by cAMP during megakaryocytopoiesis

Prostacyclin is a potent inhibitor of agonist-induced Ca2+ increases in platelets, but in the megakaryocytic cell line MEG-01 this inhibition is absent. Using human megakaryocytic cell lines representing different stages in megakaryocyte (Mk) maturation as well as stem cells and immature and mature megakaryocytes, we show that the inhibition by prostacyclin develops at a late maturation stage shortly before platelets are formed. This late appearance is not caused by insufficient cAMP formation or absent protein kinase A (PKA) activity in immature cells. Instead, the appearance of Ca2+ inhibition by prostacyclin is accompanied by a sharp increase in the expression of the catalytic subunit of PKA (PKA-C) but not by changes in the expression of the PKA-regulatory subunits Ialpha/beta, IIalpha, and IIbeta. Overexpression of PKA-C in the megakaryocytic cell line CHRF-288-11 potentiates the Ca2+ inhibition by prostacyclin. Thus, up-regulation of PKA-C appears to be a key step in the development of Ca2+ inhibition by prostacyclin in platelets.

Cyclic AMP raises intracellular Ca(2+) in human megakaryocytes independent of protein kinase A

The immature megakaryoblastic cell line MEG-01 responds to iloprost with an increase in cytosolic Ca(2+) and cAMP. The Ca(2+) response is almost absent in CHRF-288-11 cells, but cAMP formation is preserved in this more mature megakaryoblastic cell line. Also, in human hematopoietic stem cells, iloprost induces a Ca(2+) response and cAMP formation. The Ca(2+) response is downregulated during megakaryocytopoiesis, but cAMP formation remains unchanged. The Ca(2+) increase may be caused by cAMP-mediated inhibition of Ca(2+) sequestration, because it is (1) independent of Ca(2+) entry; (2) mimicked by forskolin, an activator of adenylyl cyclase, and isobutylmethylxanthine, an inhibitor of phosphodiesterases; and (3) preserved in the presence of inhibitors of protein kinase A and inositol-1,4,5-triphosphate receptors. The small GTPase Rap1 has been implicated in the control of Ca(2+) sequestration. Indeed, Rap1 activation parallels the iloprost- and forskolin-induced Ca(2+) increase and is unaffected by the calcium chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N',-tetraacetic acid-AM. These findings reveal a novel mechanism for elevating cytosolic Ca(2+) by cAMP, possibly via GTP-Rap1.

Is prostacyclin responsible for producing the hyperdynamic response during early sepsis?

OBJECTIVE

Although polymicrobial sepsis is characterized by an early hyperdynamic phase (2-10 hrs after cecal ligation and puncture [CLP]), followed by a late hypodynamic phase (20 hrs after CLP), it remains unknown whether prostacyclin or prostaglandin I2 (PGI2) plays a significant role in modulating the hyperdynamic state during early sepsis. The aim of this study was to determine whether inhibition of PGI2 synthesis prevents the occurrence of the hyperdynamic response during early sepsis.

DESIGN

Prospective, controlled animal study.

SETTING

A university research laboratory.

SUBJECTS

Adult male Sprague-Dawley rats were subjected to sepsis by CLP.

INTERVENTIONS AND MEASUREMENTS

Blood samples were collected at 2, 5, 10, or 20 hrs after CLP, and plasma concentrations of PGI2, in the form of its stable product 6-keto-PGF1alpha, were measured by radioimmunoassay. In additional studies, a PGI2 synthase inhibitor, tranylcypromine, was administered subcutaneously at the time of CLP and again at 3 hrs after CLP. At 5 hrs after the onset of sepsis, the maximal rates of the left ventricular pressure rise (+dP/dtmax) and fall (-dP/dtmax) were determined by an in vivo heart performance analyzer. Microvascular blood flow in the liver, small intestine, and spleen was assessed by laser Doppler flowmetry.

MAIN RESULTS

Plasma concentrations of 6-keto-PGF1alpha increased significantly at 2-20 hrs after CLP. At 5 hrs after the onset of sepsis, +/-dP/dt(max) and microvascular blood flow in the tested tissues increased significantly. Inhibition of PGI2 synthase activity did not prevent the occurrence of hypercardiovascular responses under such conditions. Moreover, the administration of tranylcypromine significantly reduced circulating concentrations of 6-keto-PGF1alpha at 5 hrs after CLP.

CONCLUSIONS

Because inhibition of PGI2 production did not prevent the occurrence of the hyperdynamic and hypercardiovascular response during the early stage of sepsis, mediators other than PGI2 appear to play a major role in producing the hyperdynamic response under such conditions.

Establishment and characterization of a new human megakaryoblastic cell line (SET-2) that spontaneously matures to megakaryocytes and produces platelet-like particles

A new factor-independent megakaryoblastic cell line, designated SET-2, was established from the peripheral blood of a patient with leukemic transformation of essential thrombocythemia (ET). SET-2 expressed CD 4, 7, 13, 33, 34, 36, 38, 41, 61, 71, 117, 126, 130 and c-mpl. In addition, it spontaneously produced numerous platelet-like particles in liquid culture. These particles were shown to be the same size as normal platelets, and to express CD 36, 38, 41, 61 and 71. Proliferation of SET-2 was not influenced by thrombopoietin (TPO) and other hemopoietic cytokines. SET-2 was found to express the platelet-specific proteins such as platelet factor 4 and beta-thromboglobulin. The levels of expression were not altered by TPO. SET-2 also secreted interleukin-6 into the supernatants, as well as normal megakaryocytes. These results suggest that SET-2 spontaneously matures to megakaryocytes and produces platelet-like particles. These findings indicate that SET-2 may be useful for investigating the proliferation and differentiation mechanisms of leukemia cells and the role of c-mpl on megakaryoblasts, megakaryocytes, and platelets in ET. Leukemia (2000) 14, 142-152.

In vivo methotrexate selection of murine hemopoietic cells transduced with a retroviral vector for Gaucher disease

The studies described were performed to investigate whether in vivo selection of retrovirus-transduced hemopoietic cells is feasible starting from a low percentage of transduced hemopoietic stem cells (PHSCs). The vector used is an amphotropic bicistronic retroviral vector carrying a cDNA for human lysosomal glucocerebrosidase (hGC) for treatment of Gaucher disease and a methotrexate (MTX) resistant mutant cDNA encoding human dihydrofolate reductase (DHFR). We tested the effect of MTX selection in mice that were either myeloablated or not before infusion of transduced cells. In addition, we determined whether repeated administration of transduced bone marrow cells has an additional effect on the percentage of hGC expressing cells. The results obtained have shown that, in myeloablated mice transplanted once with transduced bone marrow and treated twice weekly with 10 mg/kg of MTX for a total of 6 months, a two- to three-fold increased numbers of hGC expressing cells could be detected in both peripheral blood and bone marrow as compared with non-MTX treated mice. In mice transplanted with transduced bone marrow once every 2 weeks for a total of four times, percentages of hGC expressing cells were not significantly increased as compared with mice transplanted once. In non-ablated mice neither MTX selection nor multiple infusions of transduced bone marrow resulted in detection of hGC expressing cells 6 months after transplantation, indicating that the success of in vivo selection using MTX is highly dependent on the ratio of transduced hemopoietic stem cells transplanted versus residing and untransduced stem cells.

Developmental Expression of Plasminogen Activator Inhibitor-1 Associated With Thrombopoietin-Dependent Megakaryocytic Differentiation

Plasminogen activator inhibitor-1 (PAI-1) is present in the platelet -granule and is released on activation. However, there is some debate as to whether the megakaryocyte and platelet synthesize PAI-1, take it up from plasma, or both. We examined the expression of PAI-1 in differentiating megakaryocytic progenitor cells (UT-7) and in CD34+/CD41− cells from cord blood. UT-7 cells differentiated with thrombopoietin (TPO) resembled megakaryocytes (UT-7/TPO) with respect to morphology, ploidy, and the expression of glycoprotein IIb-IIIa. PAI-1 messenger RNA (mRNA) expression was upregulated and PAI-1 protein synthesized in the UT-7/TPO cells accumulated in the cytoplasm without being released spontaneously. In contrast, erythropoietin (EPO)-stimulated UT-7 cells (UT-7/EPO) did not express PAI-1 mRNA after stimulation with TPO because they do not have endogenous c-Mpl. After cotransfection with human wild-typec-mpl, the cells (UT-7/EPO-MPL) responded to phorbol 12-myristate 13-acetate (PMA), tumor necrosis factor- (TNF-), and interleukin-1β (IL-1β) with enhanced PAI-1 mRNA expression within 24 to 48 hours. However, induction of PAI-1 mRNA in UT-7/EPO-MPL cells by TPO required at least 14-days stimulation. UT-7/EPO cells expressing c-Mpl changed their morphology and the other characteristics similar to the UT-7/TPO cells. TPO also differentiated human cord blood CD34+/CD41− cells to CD34−/CD41+ cells, generated morphologically mature megakaryocytes, and induced the expression of PAI-1 mRNA. These results suggest that both PAI-1 mRNA and de novo PAI-1 protein synthesis is induced after differentiation of immature progenitor cells into megakaryocytes by TPO.