Signal transduction through trimeric G proteins in megakaryoblastic cell lines

Acute-phase concentrations of soluble fibrinogen inhibit neutrophil adhesion under flow conditions in vitro through interactions with ICAM-1 and MAC-1 (CD11b/CD18)

BACKGROUND

Immobilized fibrinogen and fibrin facilitate leukocyte adhesion, as they are potent ligands for leukocyte MAC-1 (CD11b/CD18). However, fibrinogen in its soluble form also binds to MAC-1, albeit with low affinity. The level of soluble fibrinogen is increased during chronic and acute inflammation, but the function of this increase is unknown.

OBJECTIVES

To study the effect of soluble fibrinogen in concentrations found in severe acute inflammation on leukocyte adhesion.

METHODS

Isolated leukocytes and soluble fibrinogen were studied in various in vitro settings under static and under flow conditions.

RESULTS

Soluble fibrinogen functioned as a natural antagonist of neutrophil functions that are dependent on MAC-1, such as the respiratory burst induced by unopsonized zymosan and adhesion to ICAM-1 and heparin. In addition, soluble fibrinogen inhibited lymphocyte function-associated antigen 1-dependent lymphocyte binding to ICAM-1 through a direct interaction with ICAM-1. Soluble fibrinogen reduced MAC-1-dependent binding of interleukin-8-activated neutrophils to ICAM-1-expressing cells under flow conditions. Importantly soluble fibrinogen in acute-phase concentrations (4-10 mg mL(-1) ) dose-dependently reduced neutrophil firm adhesion to tumor necrosis factor-α-activated endothelium to 40% under flow conditions.

CONCLUSIONS

We propose a model in which the increased circulating concentrations of soluble fibrinogen found during the acute-phase response can act as a natural antagonist of leukocyte recruitment, and therefore might contribute to the resolution of inflammation.

Induction of insulin resistance by the adipokines resistin, leptin, plasminogen activator inhibitor-1 and retinol binding protein 4 in human megakaryocytes

BACKGROUND

In normal platelets, insulin inhibits agonist-induced Ca(2+) mobilization by raising cyclic AMP. Platelet from patients with type 2 diabetes are resistant to insulin and show increased Ca(2+) mobilization, aggregation and procoagulant activity. We searched for the cause of this insulin resistance.

DESIGN AND METHODS

Platelets, the megakaryocytic cell line CHRF-288-11 and primary megakaryocytes were incubated with adipokines and with plasma from individuals with a disturbed adipokine profile. Thrombin-induced Ca(2+) mobilization and signaling through the insulin receptor and insulin receptor substrate 1 were measured. Abnormalities induced by adipokines were compared with abnormalities found in platelets from patients with type 2 diabetes.

RESULTS

Resistin, leptin, plasminogen activator inhibitor-1 and retinol binding protein 4 left platelets unchanged but induced insulin resistance in CHRF-288-11 cells. Interleukin-6, tumor necrosis factor-α and visfatin had no effect. These results were confirmed in primary megakaryocytes. Contact with adipokines for 2 hours disturbed insulin receptor substrate 1 Ser(307)-phosphorylation, while contact for 72 hours caused insulin receptor substrate 1 degradation. Plasma with a disturbed adipokine profile also made CHRF-288-11 cells insulin-resistant. Platelets from patients with type 2 diabetes showed decreased insulin receptor substrate 1 expression.

CONCLUSIONS

Adipokines resistin, leptin, plasminogen activator-1 and retinol binding protein 4 disturb insulin receptor substrate 1 activity and expression in megakaryocytes. This might be a cause of the insulin resistance observed in platelets from patients with type 2 diabetes.

Pituitary Adenylate Cyclase-Activating Polypeptide (PACAP) impairs the regulation of apoptosis in megakaryocytes by activating NF-κB: a proteomic study

We previously showed that the Pituitary Adenylate Cyclase-Activating Polypeptide (PACAP) and its receptor VPAC1 are negative regulators of megakaryopoiesis and platelet function, but their downstream signaling pathway that inhibits this process still remained unknown. A combined proteomic, transcriptomic, and bioinformatic approach was here used to elucidate the molecular mechanisms underlying PACAP signaling via VPAC1 in megakaryocytes. Two-dimensional difference gel electrophoresis and tandem MS were applied to detect differentially expressed proteins in megakaryocytic CHRF cells stimulated with PACAP. The majority of the 120 proteins modulated by PACAP belong to the class of "cell cycle and apoptosis" proteins. The up- or down-regulated expression of some proteins was confirmed by immunoblot and immunohistochemical analysis. A meta-analysis of our data and 12 other published studies was performed to evaluate signaling pathways involved in different cellular models of PACAP response. From 2384 differentially expressed genes/proteins, 83 were modulated by PACAP in at least three independent studies and Ingenuity Pathway Analysis further identified apoptosis as the highest scored network with NF-κB as a key-player. PACAP inhibited serum depletion-induced apoptosis of CHRF cells via VPAC1 stimulation. In addition, PACAP switched on NF-κB dependent gene expression since higher nuclear levels of the active NF-κB p50/p65 heterodimer were found in CHRF cells treated with PACAP. Finally, a quantitative real time PCR apoptosis array was used to study RNA from in vitro differentiated megakaryocytes from a PACAP overexpressing patient, leading to the identification of 15 apoptotic genes with a 4-fold change in expression and Ingenuity Pathway Analysis again revealed NF-κB as the central player. In conclusion, our findings suggest that PACAP interferes with the regulation of apoptosis in megakaryocytes, probably via stimulation of the NF-κB pathway.

GPR92/LPA₅ lysophosphatidate receptor mediates megakaryocytic cell shape change induced by human atherosclerotic plaques

Aims

Oxidative processes and vascular inflammation underlying atherosclerosis lead to an accumulation of lysophosphatidic acid (LPA) molecules in the atheromatous intima. LPA, a platelet-activating component of human atherosclerotic plaques, possibly contributes to atherothrombus formation after plaque rupture. Human platelets express mRNA for the G protein-coupled receptors LPA_1–7 that derive from megakaryocytes. The aim of our study was to identify the functional LPA receptor(s) in human platelets by silencing individual LPA receptors in megakaryocytic (MK) cells.

Methods and results

We studied shape change of two human MK cell lines (Meg-01, Dami) by turbidometry, phase-contrast and scanning electron microscopy. They showed upon LPA stimulation a rapid, Rho-kinase-mediated shape change similar to that of human platelets. By qRT–PCR analysis we found expression of LPA_1–7 in both cell lines; LPA₄ and LPA₅ were the most abundant receptor transcripts. In both Meg-01 and Dami cells, the rank order of activation by LPA species was similar to that found in platelets: alkyl-LPA 18:1 > alkyl-LPA 16:0 > acyl-LPA 18:1 >> alkyl-LPA 18:0. Knock-down of individual LPA receptors by siRNA showed that LPA-mediated activation of MK cells was mediated by LPA₅, but not by LPA_1–4,6,7. Importantly, we found that human atherosclerotic plaque and lipid-rich core induced shape change of Dami cells, and that this effect was inhibited after LPA₅ silencing.

Conclusions

Our findings indicate that LPA₅ mediates LPA-induced shape change of MK cells and support its involvement in atherosclerotic plaque and lipid-rich core-mediated platelet activation. This receptor could be an attractive novel target for antithrombotic therapy.

Co-expression of the collagen receptors leukocyte-associated immunoglobulin-like receptor-1 and glycoprotein VI on a subset of megakaryoblasts

BACKGROUND

The collagen receptor glycoprotein VI generates activating signals through an immunoreceptor tyrosine-based activating motif on the co-associated Fc receptor gamma chain. Leukocyte-associated immunoglobulin-like receptor-1 also ligates collagen but generates inhibitory signals through immunoreceptor tyrosine-based inhibitory motifs. Thus far, the cellular expression of glycoprotein VI and leukocyte-associated immunoglobulin-like receptor-1 appears mutually exclusive.

DESIGN AND METHODS

Using flow cytometry, we studied expression of collagen receptors on differentiating human megakaryocytes. CD34(+) cells were isolated from umbilical cord blood and matured to megakaryocytes in vitro. Freshly isolated bone marrow cells were used to study primary megakaryocytes. Upon cell sorting, cytospins were made to examine cytological characteristics of differentiation.

RESULTS

Megakaryocyte maturation is accompanied by up-regulation of glycoprotein VI and down-regulation of leukocyte-associated immunoglobulin-like receptor-1. Interestingly, both in cultures from hematopoietic stem cells and primary cells obtained directly from bone marrow, we identified a subset of morphologically distinct megakaryocytes which co-express glycoprotein VI and leukocyte-associated immunoglobulin-like receptor-1.

CONCLUSIONS

This is the first report of a primary cell that co-expresses these collagen receptors with opposite signaling properties. Since megakaryocytes mature in the collagen-rich environment of the bone marrow, these findings may point to a role for leukocyte-associated immunoglobulin-like receptor-1 in the control of megakaryocyte maturation/migration.

Involvement of aquaporin in thromboxane A2 receptor-mediated, G 12/13/RhoA/NHE-sensitive cell swelling in 1321N1 human astrocytoma cells

The physiological role of the thromboxane A(2) (TXA(2)) receptor expressed on glial cells remains unclear. We previously reported that 1321N1 human astrocytoma cells pretreated with dibutyryl cyclic AMP (dbcAMP) became swollen in response to U46619, a TXA(2) analogue. In the present study, we examined the detailed mechanisms of TXA(2) receptor-mediated cell swelling in 1321N1 cells. The cell swelling caused by U46619 was suppressed by expression of p115-RGS, an inhibitory peptide of G alpha(12/13) pathway and C3 toxin, an inhibitory protein for RhoA. The swelling was also inhibited by treatment with Y27632, a Rho kinase inhibitor and 5-(ethyl-N-isopropyl)amiloride (EIPA), a Na(+)/H(+)-exchanger inhibitor. Furthermore, cell swelling was suppressed by the pretreatment with aquaporin inhibitors mercury chloride or phloretin in a concentration-dependent manner, suggesting that aquaporins are involved in U46619-induced 1321N1 cell swelling. In fact, U46619 caused [(3)H]H(2)O influx into the cells, which was inhibited by p115-RGS, C3 toxin, EIPA, mercury chloride and phloretin. This is the first report that the TXA(2) receptor mediates water influx through aquaporins in astrocytoma cells via TXA(2) receptor-mediated activation of G alpha(12/13), Rho A, Rho kinase and Na(+)/H(+)-exchanger.

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.

Extreme thrombocytosis associated with transient myeloproliferative disorder with Down Syndrome with t(11;17)(q13;q21)

A female patient with Down Syndrome (DS) had neonatal thrombocytosis with platelet counts exceeding 2,000 x 10(3)/microL and transient myeloproliferative disorder (TMD). Platelet counts remained elevated the first 2 months of life. A translocation located between chromosomes 17 and 11 was observed. We describe a patient with thrombocytosis and TMD showing an 11q13 translocation. The leukocytosis and thrombocytosis improved after an exchange transfusion.

Thromboxane A2 receptor-mediated G12/13-dependent glial morphological change

Glial cells express thromboxane A(2) receptor, but its physiological role remains unknown. The present study was performed to examine thromboxane A(2) receptor-mediated morphological change in 1321N1 human astrocytoma cells. Thromboxane A(2) receptor agonists U46619 and STA(2) caused a rapid morphological change to spindle shape from stellate form of the cells pretreated with dibutyryl cyclic AMP, but neither carbachol nor histamine caused the change, suggesting that G(q) pathway may not mainly contribute to the change. Rho kinase inhibitor Y-27632 inhibited U46619-induced morphological change, and U46619 increased the GTP-bound form of RhoA accompanied with actin stress fiber formation. These responses were reduced by expression of p115-RGS that inhibits G(12)/(13) signaling pathway. U46619 also caused the phosphorylation of extracellular signal-regulated kinase (ERK) and [(3)H]thymidine incorporation mainly through G(12)/(13)-Rho pathway. These results suggest that stimulation of thromboxane A(2) receptor causes the morphological change with proliferation mainly through G(12)/(13) activation in glial cells.

Involvement of Actin in Agonist-induced Endocytosis of the G Protein-coupled Receptor for Thromboxane A2

The role of actin in endocytosis of G protein-coupled receptors is poorly defined. In the present study, we demonstrate that agents that depolymerize (latrunculin B and cytochalasin D) or stabilize (jasplakinolide) the actin cytoskeleton blocked agonist-induced endocytosis of the beta isoform of the thromboxane A(2) receptor (TPbeta) in HEK293 cells. This suggests that endocytosis of TPbeta requires active remodeling of the actin cytoskeleton. On the other hand, disruption of microtubules with colchicine did not affect endocytosis of the receptor. Expression of wild-type and mutant forms of the small GTPases RhoA and Cdc42 potently inhibited endocytosis of TPbeta, further indicating a role for the dynamic regulation of the actin cytoskeleton in this pathway. Agonist treatment of TPbeta in HEK293 cells resulted in the formation of actin stress fibers through Galpha(q/11) signaling. Because we previously showed that endocytosis of TPbeta is dependent on arrestins, we decided to explore the relation between arrestin-2 and -3 and actin in endocytosis of this receptor. Interestingly, we show that the inhibition of TPbeta endocytosis by the actin toxins in HEK293 cells was overcome by the overexpression of arrestin-3, but not of arrestin-2. These results indicate that the actin cytoskeleton is not essential in arrestin-3-mediated endocytosis of TPbeta. However, arrestin-3 could not promote endocytosis of the TPbetaY339A and TPbetaI343A carboxyl-terminal mutants when the actin cytoskeleton was disrupted. Our data provide new evidence that the actin cytoskeleton plays an essential role in TPbeta endocytosis. Furthermore, our work suggests the existence of actin-dependent and -independent arrestin-mediated pathways of endocytosis.

Dimerization of the human receptors for prostacyclin and thromboxane facilitates thromboxane receptor-mediated cAMP generation

Prostacyclin (PGI(2)) and thromboxane (TxA(2)) are biological opposites; PGI(2), a vasodilator and inhibitor of platelet aggregation, limits the deleterious actions of TxA(2), a vasoconstrictor and platelet activator. The molecular mechanisms involved in the counterregulation of PGI(2)/TxA(2) signaling are unclear. We examined the interaction of the receptors for PGI(2) (IP) and TxA(2) (TPalpha). IP-induced cAMP and TP-induced inositol phosphate generation were unaltered when the receptors were co-expressed in HEK 293 cells (IP/TPalpha-HEK). TP-cAMP generation, in response to TP agonists or a TP-dependent isoprostane, iPE(2)III, was evident in IP/TPalpha-HEK and in aortic smooth muscle cells, but not in cells expressing either receptor alone, or in IP-deficient aortic smooth muscle cells. Augmentation of TP-induced cAMP generation, with the IP agonist cicaprost, was ablated in IP-deficient cells and was independent of direct IP signaling. IP/TPalpha heterodimers were formed constitutively when the receptors were co-expressed, with no overt changes in ligand binding to the individual receptor sites. However, despite inefficient binding of iPE(2)III to either the IP or TPalpha, expressed alone or in combination, robust cAMP generation was evident in IP/TPalpha-HEK, suggesting the formation of an alternative receptor site. Thus, IP/TPalpha dimerization was coincident with TP-cAMP generation, promoting a "PGI(2)-like" cellular response to TP activation. This represents a previously unknown mechanism by which IP may limit the cellular effects of TP.

EP1- and FP-mediated cross-desensitization of the alpha (alpha) and beta (beta) isoforms of the human thromboxane A2 receptor

1. Heterologous desensitization or intermolecular cross-talk plays a critical role in regulating intracellular signalling by diverse members of the G-protein-coupled receptor superfamily. We have previously established that the alpha and beta isoforms of the human thromboxane A(2) receptor (TP) undergo differential desensitization of signalling in response to 17 phenyl trinor prostaglandin (PG)E(2), an agonist of the EP(1) subtype of the PGE(2) receptor (EP) family. 2. Herein, we investigated the molecular basis of TPalpha and TPbeta desensitization in human embryonic kidney (HEK) 293 cells and in renal mesangial cells in response to 17 phenyl trinor PGE(2) and in response to the PGF(2alpha) receptor (FP) agonist PGF(2alpha), and sought to identify the target site(s) of those desensitizations. 3. Our results demonstrated that TPalpha and TPbeta receptors are subject to desensitization in response to both EP(1) and FP receptor activation and that these effects are mediated by direct protein kinase (PK)C phosphorylation of the individual TP isoforms within their unique carboxyl-terminal (C)-tail domains. 4. Moreover, deletion/site-directed mutagenesis and metabolic labelling studies identified Thr(337), within TPalpha, and Thr(399), within TPbeta, as the specific target residues for PKC phosphorylation and EP(1)- and FP-mediated desensitization of TPalpha and TPbeta signalling, respectively. 5. Hence, in conclusion, while the TPalpha and TPbeta diverge within their C-tail domains, they have evolved to share a similar mechanism of PKC-induced phosphorylation and desensitization in response to EP(1) and FP receptor activation, though it occurs at sites unique to the individual TP isoforms.

Cell-to-cell variability in the differentiation program of human megakaryocytes

Differentiation of CD34(+) stem/progenitor cells into megakaryocytes is thought to be a uniform, unidirectional process, in which cells transform step by step from less differentiated precursor stages to more differentiated megakaryocytes. Here we propose the concept and present evidence based on single-cell analysis that differentiation occurs along multiple, partially asynchronous routes. In all CD34(+) cells cultured with thrombopoietin, surface appearance of glycoprotein IIIa (GPIIIa) preceded that of GPIb, indicating that the expression of these glycoproteins occurs in a timely ordered manner. Cellular F-actin content increased in parallel with GPIb expression. Only cells that expressed GPIb were polyploid, pointing to co-regulation of GPIb expression, actin cytoskeleton formation and polyploidization during megakaryocytopoiesis. On the other hand, most progenitor cells responded to thrombin but not to thromboxane A(2) analogue by rises in cytosolic [Ca(2+)](i). The appearance of thromboxane-induced responses during megakaryocytopoiesis was not strictly linked to glycoprotein expression, because cells showed responsiveness either before or after GPIb expression. The same non-strictly sequential pattern was observed for disappearance of the Ca(2+) response by prostacyclin mimetic; in some megakaryocytes it occurred before and in others after GPIb expression. Thus, megakaryocytic differentiation follows along independent routes that are either strictly sequential (GPIIIa and GPIb expression) or proceed at different velocities (Ca(2+) signal regulation).

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.

Investigation of the mechanisms of G protein: effector coupling by the human and mouse prostacyclin receptors. Identification of critical species-dependent differences

We recently identified a novel mechanism explaining how the mouse (m) prostacyclin receptor (IP) couples to Galpha(s), Galpha(i), and Galpha(q) (Lawler, O. A., Miggin, S. M., and Kinsella, B. T. (2001) J. Biol. Chem. 276, 33596-33607) whereby mIP coupling to Galpha(i) and Galpha(q) is dependent on its initial coupling to Galpha(s) and subsequent phosphorylation by cAMP-dependent protein kinase A (PKA) on Ser(357). In the current study, the generality of that mechanism was investigated by examining the G protein coupling specificity of the human (h) IP. The hIP efficiently coupled to Galpha(s)/adenylyl cyclase and to Galpha(q)/phospholipase C activation but failed to couple to Galpha(i). Coupling of the hIP to Galpha(q), or indeed to Galpha(s) or Galpha(i), was unaffected by the PKA or protein kinase C (PKC) inhibitors H-89 and GF 109203X, respectively. Thus, mIP and hIP exhibit essential differences in their coupling to Galpha(i) and in their dependence on PKA in regulating their coupling to Galpha(q). Analysis of their primary sequences revealed that the critical PKA phosphorylation site within the mIP, at Ser(357), is replaced by a PKC site within the hIP, at Ser(328). Conversion of the PKC site of the hIP to a PKA site generated hIP(QL325,326RP) that efficiently coupled to Galpha(s) and to Galpha(i) and Galpha(q); coupling of hIP(QL325,326RP) to Galpha(i) but not to Galpha(s) or Galpha(q) was inhibited by H-89. Abolition of the PKC site of the hIP generated hIP(S328A) that efficiently coupled to Galpha(s) and Galpha(q) but failed to couple to Galpha(i). Finally, conversion of the PKA site at Ser(357) within the mIP to a PKC site generated mIP(RP354,355QL) that efficiently coupled to Galpha(s) but not to Galpha(i) or Galpha(q). Collectively, our data highlight critical differences in signaling by the mIP and hIP that are regulated by their differential phosphorylation by PKA and PKC together with contextual sequence differences surrounding those sites.

Gα16Couples Chemoattractant Receptors to NF-κB Activation

The guanine nucleotide-binding regulatory protein alpha-subunit, Galpha(16), is primarily expressed in hemopoietic cells, and interacts with a large number of seven-membrane span receptors including chemoattractant receptors. We investigated the biological functions resulting from Galpha(16) coupling of chemoattractant receptors in a transfected cell model system. HeLa cells expressing a kappaB-driven luciferase reporter, Galpha(16), and the formyl peptide receptor responded to fMLP with a approximately 7- to 10-fold increase in luciferase activity. This response was accompanied by phosphorylation of IkappaBalpha and elevation of nuclear kappaB-DNA binding activity, indicating activation of NF-kappaB. In contrast to Galpha(16), expression of Galpha(q), Galpha(13), and Galpha(i2) resulted in a marginal increase in kappaB luciferase activity. A GTPase-deficient, constitutively active Galpha(16) mutant (Q212L) could replace agonist stimulation for activation of NF-kappaB. Furthermore, expression of Galpha(16) (Q212L) markedly enhanced TNF-alpha-induced kappaB reporter activity. The Galpha(16)-mediated NF-kappaB activation was paralleled by an increase in phospholipase C-beta activity, and was blocked by pharmacological inhibitors of protein kinase C (PKC) and by buffering of intracellular Ca(2+). The involvement of a conventional PKC isoform was confirmed by the finding that expression of PKCalpha enhanced the effect of Galpha(16), and a dominant negative PKCalpha partially blocked Galpha(16)-mediated NF-kappaB activation. In addition to formyl peptide receptor, Galpha(16) also enhanced NF-kappaB activation by the C5a and C3a receptors, and by CXC chemokine receptor 2 and CCR8. These results suggest a potential role of Galpha(16) in transcriptional regulation downstream of chemoattractant receptors.

The effects of the statins lovastatin and cerivastatin on signalling by the prostanoid IP-receptor

The prostanoid-IP receptor may be unique among G protein coupled receptors in that it is isoprenylated. In this study, we investigated the effects of the statins lovastatin and cerivastatin on signalling by the mouse (m) IP and the human (h) IP receptors, over-expressed in human embryonic kidney (HEK) 293 cells and by the hIP receptor, endogenously expressed in human erythroleukaemia cells. Both statins significantly reduced IP receptor-mediated cyclic AMP generation and intracellular calcium ([Ca(2+)](i)) mobilization in a time and concentration dependent manner but had no effect on signalling by the non-isoprenylated beta(2) adrenergic receptor or by the human prostanoid-TP receptor isoforms. Cerivastatin (IC(50), 50 - 90 nM) was significantly more potent than lovastatin (IC(50), 0.80 - 4.2 microM) in inhibiting IP receptor signalling. Whereas IC(50) values indicated that the hIP receptor was significantly more sensitive than the mIP receptor to the statins, the extent of inhibition of cyclic AMP generation by the mIP receptor was significantly greater than that of the hIP receptor to either statin, even at the highest concentrations used. Pretreatment with either statin significantly reduced IP receptor mediated desensitization of signalling by the h.TPalpha, but not by the h.TPbeta, receptor isoform. These data generated in whole cells point to the possibility that statin therapy may interfere with IP receptor signalling in vivo; such interference may be extenuated under conditions where circulating statin levels are elevated and may account, in part, for some of the pleiotropic affects of the statins not attributed solely to their lipid lowering properties.

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

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

The alpha, but not the beta, isoform of the human thromboxane A2 receptor is a target for prostacyclin-mediated desensitization

In this study, we examined the effects the prostacyclin receptor (IP) agonist cicaprost exhibited on U46619-mediated thromboxane A(2) receptor (TP) signaling in platelets and compared it to that which occurs in human embryonic kidney (HEK) 293 cells stably overexpressing the individual TPalpha or TPbeta isoforms. Consistent with previous studies, cicaprost abrogated U46619-mediated platelet aggregation and mobilization of intracellular calcium ([Ca(2+)](i)). In HEK 293 cells, signaling by TPalpha, but not TPbeta, was subject to IP-mediated desensitization in a protein kinase A-dependent, protein kinase C-independent manner. Desensitization of TPalpha signaling was independent of the nature of the IP agonist used, the level of IP expression, or the subtype of G(q) protein. Signaling by TP(Delta)(328), a truncated variant of TP devoid of the divergent residues of the TPs, or by TPalpha(S329A), a site-directed mutant of TPalpha, were insensitive to IP agonist activation. Whole cell phosphorylations established that TPalpha, but not TPbeta or TPalpha(S329A), is subject to IP-mediated phosphorylation and that TPalpha phosphorylation is inhibited by H-89. Thus, we conclude that TPalpha, but not TPbeta, is subject to cross-desensitization by IP mediated through direct protein kinase A phosphorylation at Ser(329) and propose that TPalpha may be the isoform physiologically relevant to TP:IP-mediated vascular hemostasis.

Investigation of the role of the carboxyl-terminal tails of the alpha and beta isoforms of the human thromboxane A(2) receptor (TP) in mediating receptor:effector coupling

We have investigated the functional coupling of alpha and beta isoforms of the human thromboxane A(2) receptor (TP) to Galpha(16) and Galpha(12) members of the G(q) and G(12) families of heterotrimeric G proteins in human embryonic kidney (HEK) 293 cell lines HEK.alpha10 or HEK.beta3, stably over-expressing TPalpha and TPbeta, respectively. Moreover, using HEK.TP(Delta328) cells which over-express a variant of TP truncated at the point of divergence of TPalpha and TPbeta, we investigated the requirement of the C-tail per se in mediating G protein coupling and effector activation. Both TPalpha and TPbeta couple similarly to Galpha(16) to affect increases in inositol 1,4,5-trisphosphate (IP(3)) and mobilisation of intracellular calcium ([Ca(2+)](i)) in response to the TP agonist U46619. Whilst both TP isoforms mediated [Ca(2+)](i) mobilisation in cells co-transfected with Galpha(12), neither receptor generated corresponding increases in IP(3), indicating that the Galpha(12)-mediated increases in [Ca(2+)](i) do not involve PLC activation. Verapamil, an inhibitor of voltage dependent Ca(2+) channels, reduced [Ca(2+)](i) mobilisation in TPalpha and TPbeta cells co-transfected with Galpha(12) to approximately 40% of that mobilised in its absence, whereas [8-(N,N-diethylamino)-octyl-3,4, 5-trimethoxybenzoate, hydrochloride] (TMB-8), an antagonist of intracellular Ca(2+) release, had no effect on [Ca(2+)](i) mobilisation by either receptor isoform co-transfected with Galpha(12). Despite the lack of differential coupling specificity by TPalpha and TPbeta, TP(Delta328) signalled more efficiently in the absence of a co-transfected G protein compared to the wild type receptors but, on the other hand, displayed an impaired ability to couple to co-transfected Galpha(11), Galpha(12) or Galpha(16) subunits. In studies investigating the role of the C-tail in influencing coupling to the effector adenylyl cyclase, similar to TPalpha but not TPbeta, TP(Delta328) coupled to Galpha(s), leading to increased adenosine 3',5'-cyclic monophosphate (cAMP), rather than to Galpha(i). Whereas TP(Delta328) signalled more efficiently in the absence of co-transfected G protein compared to the wild type TPalpha, co-transfection of Galpha(s) did not augment cAMP generation by TP(Delta328). Hence, from these studies involving the wild type TPalpha, TPbeta and TP(Delta328), we conclude that the C-tail sequences of TP are not a major determinant of G protein coupling specificity to Galpha(11) and Galpha(16) members of the G(q) family or to Galpha(12); it may play a role in determining G(s) versus G(i) coupling and may act as a determinant of coupling efficiency.

Internalization of the TXA2 receptor alpha and beta isoforms. Role of the differentially spliced cooh terminus in agonist-promoted receptor internalization

Thromboxane A2 (TXA2) potently stimulates platelet aggregation and smooth muscle constriction and is thought to play a role in myocardial infarction, atherosclerosis, and bronchial asthma. The TXA2 receptor (TXA2R) is a member of the G protein-coupled receptor family and is found as two alternatively spliced isoforms, alpha (343 residues) and beta (407 residues), which share the first 328 residues. In the present report, we demonstrate by enzyme-linked immunosorbent assay and immunofluorescence microscopy that the TXA2Rbeta, but not the TXA2Ralpha, undergoes agonist-induced internalization when expressed in HEK293 cells as well as several other cell types. Various dominant negative mutants were used to demonstrate that the internalization of the TXA2Rbeta is dynamin-, GRK-, and arrestin-dependent in HEK293 cells, suggesting the involvement of receptor phosphorylation and clathrin-coated pits in this process. Interestingly, the agonist-stimulated internalization of both the alpha and beta isoforms, but not of a mutant truncated after residue 328, can be promoted by overexpression of arrestin-3, identifying the C-tails of both receptors as necessary in arrestin-3 interaction. Simultaneous mutation of two dileucine motifs in the C-tail of TXA2Rbeta did not affect agonist-promoted internalization. Analysis of various C-tail deletion mutants revealed that a region between residues 355 and 366 of the TXA2Rbeta is essential for agonist-promoted internalization. These data demonstrate that alternative splicing of the TXA2R plays a critical role in regulating arrestin binding and subsequent receptor internalization.

Recruitment of the LIM protein hic-5 to focal contacts of human platelets

Platelets are anuclear, membrane-bounded fragments derived from megakaryocytes which, upon stimulation, assemble an actin skeleton including stress fibres and focal contacts. The focal contacts resemble those of tissue culture cells. However, they lack paxillin, a conspicuous component of these organelles. We found that instead of paxillin, platelets contain a related protein with a molecular mass of 55 kDa that crossreacts with a monoclonal antibody against paxillin. The gene for the 55 kDa protein was cloned from a bone marrow cDNA library and turned out to be identical to a recently discovered gene encoding hic-5. Like paxillin, hic-5 is a cytoskeletal protein containing four carboxy-terminal LIM domains and LD motifs in the amino-terminal half. The LIM domains of both hic-5 and paxillin are capable of targetting green fluorescent protein to focal contacts. In addition, GST-hic-5 precipitates the focal adhesion kinase pp125(FAK) and talin from platelet extracts. Only trace amounts of hic-5 occur in DAMI cells, a megakaryocytic cell line, and in megakaryocytes cultured from CD34+ cells obtained from umbilical cord blood. However, RT-polymerase chain reactions performed with RNA obtained from platelets gave a positive result when primers specific for hic-5 were used, but were negative with paxillin-specific primers, indicating that a switch from paxillin expression to hic-5 expression must occur late in the maturation of megakaryocytes into platelets.