The P2Y1 receptor mediates ADP-induced p38 kinase-activating factor generation in human platelets

Autophagy plays a positive role in induction of epidermal proliferation

Autophagy is a multistage catabolic process that mediates stress responses. However, the role of autophagy in epidermal proliferation, particularly under conditions when the epidermis becomes "activated" (hyperproliferative), remains unclear. We have shown that inhibition of Beclin 1, a key activator in the initiation phase of autophagy, attenuates imiquimod (IMQ)-induced epidermal hyperplasia in adult mice as well as naturally occurring hyperproliferation in neonatal mouse epidermis. Inhibition of Beclin 1 did not change the levels of several key inflammatory molecules or the numbers of immune cells in lesional skins. This indicates that autophagy does not affect inflammatory regulators in IMQ-treated mouse skin. Bioinformatic analysis combined with gene expression quantitative assays, revealed that a deficiency in autophagy decreases the expression of PDZ Binding Kinase (PBK), a regulator of the cell cycle, in mouse epidermis and human epidermal keratinocytes (HEKs). Interestingly, the decrease in PBK results in inhibition of proliferation in HEKs and such reduced proliferation can be rescued by activation of p38, the downstream signaling of PBK. Collectively, autophagy plays a positive role in epidermal proliferation, which is in part via regulating PBK expression.

Carbamylated Low-Density Lipoproteins Induce a Prothrombotic State Via LOX-1: Impact on Arterial Thrombus Formation In Vivo

BACKGROUND

Carbamylation alters low-density lipoprotein (LDL) structure and is thought to promote vascular inflammation and dysfunction in patients with chronic kidney disease (CKD).

OBJECTIVES

This study sought to determine whether carbamylated LDL (cLDL) exerts prothrombotic effects in vascular cells and platelets and whether cLDL enhances arterial thrombus formation in vivo.

METHODS

LDL was isolated from healthy subjects or patients with CKD by sequential ultracentrifugation. Ex vivo carbamylation of LDL from healthy subjects was induced with potassium cyanate. Arterial thrombus formation was analyzed in a murine carotid artery photochemical injury model. Protein expression and mRNA levels were analyzed by Western blotting, flow cytometry, and real-time PCR. Platelet aggregation was measured by impedance aggregometry.

RESULTS

Intravenous administration of cLDL in mice accelerated arterial thrombus formation compared to treatment with native LDL (nLDL) or vehicle. Tissue lysates of mouse carotid arteries revealed that cLDL induced the expression of TF, PAI-1, and LOX-1 mRNA in vascular cells. In human aortic smooth muscle and endothelial cells, cLDL induced TF and PAI-1 expression. In contrast, nLDL had no effect on either cell type. While nLDL and cLDL had no aggregatory effect on resting platelets, cLDL enhanced platelet aggregation in response to different agonists. This effect was mediated by mitogen-activated protein kinase p38 phosphorylation and LOX-1 translocation to the surface. LDL isolated from patients with CKD mimicked the prothrombotic effects of cLDL on vascular cells, platelets, and thrombus formation in vivo.

CONCLUSIONS

We found that cLDL induces prothrombotic effects in vascular cells and platelets by activation of the LOX-1 receptor and enhances thrombus formation in vivo. This observation reveals a new mechanism underlying the increased incidence of acute thrombotic events observed in patients with CKD and may lead to the development of new lipid-targeting therapies in this population.

Melanoma cell metastasis via P-selectin-mediated activation of acid sphingomyelinase in platelets

Metastatic dissemination of cancer cells is one of the hallmarks of malignancy and accounts for approximately 90 % of human cancer deaths. Within the blood vasculature, tumor cells may aggregate with platelets to form clots, adhere to and spread onto endothelial cells, and finally extravasate to form metastatic colonies. We have previously shown that sphingolipids play a central role in the interaction of tumor cells with platelets; this interaction is a prerequisite for hematogenous tumor metastasis in at least some tumor models. Here we show that the interaction between melanoma cells and platelets results in rapid and transient activation and secretion of acid sphingomyelinase (Asm) in WT but not in P-selectin-deficient platelets. Stimulation of P-selectin resulted in activation of p38 MAPK, and inhibition of p38 MAPK in platelets prevented the secretion of Asm after interaction with tumor cells. Intravenous injection of melanoma cells into WT mice resulted in multiple lung metastases, while in P-selectin-deficient mice pulmonary tumor metastasis and trapping of tumor cells in the lung was significantly reduced. Pre-incubation of tumor cells with recombinant ASM restored trapping of B16F10 melanoma cells in the lung in P-selectin-deficient mice. These findings indicate a novel pathway in tumor metastasis, i.e., tumor cell mediated activation of P-selectin in platelets, followed by activation and secretion of Asm and in turn release of ceramide and tumor metastasis. The data suggest that p38 MAPK acts downstream from P-selectin and is necessary for the secretion of Asm.

Selective novel inverse agonists for human GPR43 augment GLP-1 secretion

GPR43/Free Fatty Acid Receptor 2 (FFAR2) is known to be activated by short-chain fatty acids and be coupled to Gi and Gq family of heterotrimeric G proteins. GPR43 is mainly expressed in neutrophils, adipocytes and enteroendocrine cells, implicated to be involved in inflammation, obesity and type 2 diabetes. However, several groups have reported the contradictory data about the physiological functions of GPR43, so that its roles in vivo remain unclear. Here, we demonstrate that a novel compound of pyrimidinecarboxamide class named as BTI-A-404 is a selective and potent competitive inverse agonist of human GPR43, but not the murine ortholog. Through structure-activity relationship (SAR), we also found active compound named as BTI-A-292. These regulators increased the cyclic AMP level and reduced acetate-induced cytoplasmic Ca(2+) level. Furthermore, we show that they modulated the downstream signaling pathways of GPR43, such as ERK, p38 MAPK, and NF-κB. It was surprising that two compounds augmented the secretion of glucagon-like peptide 1 (GLP-1) in NCI-H716 cell line. Collectively, these novel and specific competitive inhibitors regulate all aspects of GPR43 signaling and the results underscore the therapeutic potential of them.

Inhibitory effect of caffeic acid on ADP-induced thrombus formation and platelet activation involves mitogen-activated protein kinases

Caffeic acid (CA), one of the active constituents of Radix Salvia miltiorrhizae, exhibits antioxidant and anti-inflammatory activities. However, few studies have assessed the ability of CA to inhibit platelet mediated thrombus generation in vivo. In this study, we investigated the antithrombotic effect of CA in mouse cerebral arterioles and venules using intravital microscopy. The antiplatelet activity of CA in ADP stimulated mouse platelets in vitro was also examined in attempt to explore the underlying mechanism. Our results demonstrated that CA (1.25–5 mg/kg) significantly inhibited thrombus formation in vivo. In vitro, CA (25–100 μM) inhibited ADP-induced platelet aggregation, P-selectin expression, ATP release, Ca²⁺ mobilization, and integrin αIIbβ3 activation. Additionally, CA attenuated p38, ERK, and JNK activation, and enhanced cAMP levels. Taken together, these data provide evidence for the inhibition of CA on platelet-mediated thrombosis in vivo, which is, at least partly, mediated by interference in phosphorylation of ERK, p38, and JNK leading to elevation of cAMP and down-regulation of P-selectin expression and αIIbβ3 activation. These results suggest that CA may have potential for the treatment of aberrant platelet activation-related diseases.

p38MAPK is involved in apoptosis development in apheresis platelet concentrates after riboflavin and ultraviolet light treatment

BACKGROUND

Pathogen inactivation (PI) accelerates the platelet (PLT) storage lesion, including apoptotic-like changes. Proteomic studies have shown that phosphorylation levels of several kinases increase in PLTs after riboflavin and UV light (RF-PI) treatment. Inhibition of p38MAPK improved in vitro PLT quality, but the biochemical basis of this kinase's contribution to PLT damage requires further analysis.

STUDY DESIGN AND METHODS

In a pool-and-split design, apheresis PLT concentrates were either treated or kept untreated with or without selected kinase inhibitors. Samples were analyzed throughout 7 days of storage, monitoring in vitro quality variables including phosphatidylserine exposure, degranulation, and glucose metabolism. Changes in the protein expression of Bax, Bak, and Bcl-xL and the activities of caspase-3 and -9 were determined by immunoblot analysis and flow cytometry, respectively.

RESULTS

The expression levels of the proapoptotic proteins Bax and Bak, but not the antiapoptotic protein Bcl-xL, were significantly increased after the RF-PI treatment. This trend was reversed in the presence of p38MAPK inhibitor SB203580. As a result of increasing proapoptotic protein levels, caspase-3 and -9 activities were significantly increased in RF-PI treatment during storage compared with control (p < 0.05). Similarly, p38MAPK inhibition significantly reduced these caspase activities compared with vehicle control after RF-PI treatment (p < 0.05).

CONCLUSION

These findings revealed that p38MAPK is involved in signaling leading to apoptosis triggered by RF-PI. Elucidation of the biochemical processes influenced by PI is a necessary step in the development of strategies to improve the PLT quality and ameliorate the negative effects of PI treatment.

P2X1-initiated p38 signalling enhances thromboxane A2-induced platelet secretion and aggregation

ATP released by activated platelets can serve as a positive feedback machinery to amplify platelet responses by activating P2X1 receptors. It has, however, not been defined how P2X1 activities influence thromboxane A2 (TXA2)-stimulated platelet functional responses. Our aim was to elaborate the molecular mechanisms of P2X1 engagements in TXA2-induced platelet secretion and aggregation. P2X1 inhibition by 1 µM NF449 inhibited platelet P-selectin expression induced by a low concentration of the TXA2 analogue U46619 (0.3 µM) (32.0 ± 2.0% vs 43.4 ± 3.0%; n=5; p<0.05). p38 inhibition by SB203580, but not ERK inhibition by U0126, elicited a similar inhibition by NF499. The combination of NF449 and SB203580 provided, however, no additive effects. U46619-induced platelet aggregation was similarly decreased by NF449 and SB203580 alone or in combination, and by P2X1 pre-desensitisation with α,β-Me-ATP. U46619 caused rapid and reversible P2X1-dependent p38 phosphorylation. However, the P2X1-p38 pathway mainly enhanced mild platelet activation by U46619, because α,β-Me-ATP supplementation or p38 blockade had no effect on intense platelet activation induced by a higher concentration of U46619 (3 µM). In conclusion, P2X1 activation, via p38 signalling, potentiates platelet activation initiated by low doses of U46619. Hence, the P2X1-induced p38 signalling promotes more robust platelet activation in response to mild platelet stimuli.

Thrombopoietin amplifies ADP-induced HSP27 phosphorylation in human platelets: importance of pre-treatment

It has been shown that thrombopoietin (TPO) amplifies agonist-induced platelet activation. However, the precise mechanism of action of TPO has not yet been fully elucidated. We have previously reported that the adenosine diphosphate (ADP)‑induced phosphorylation of heat shock protein 27 (HSP27) via the p38 mitogen-activated protein (MAP) kinase pathway correlates with the ADP-induced platelet-derived growth factor (PDGF)-AB secretion and the release of soluble CD40 ligand (sCD40L) from human platelets. In the present study, we investigated the effects of TPO on platelet activation induced by ADP. We examined the effects of TPO on ADP-induced platelet activation under different treatments: TPO was administered 15 min prior to stimulation with ADP (pre-treatment); TPO and ADP were simultaneously administered (simultaneous treatment); and TPO was administered 2 min following stimulation with ADP (post-treatment). TPO, which alone had no effect on platelet aggregation, synergistically enhanced the ADP (1 mM)-induced platelet aggregation only when it was administered prior to stimulation with ADP. Pre-treatment with TPO significantly increased the secretion of PDGF-AB and the release of sCD40L, and markedly enhanced the ADP-induced phosphorylation of p38 MAP kinase and HSP27 in the platelets. However, simultaneous treatment with TPO or TPO post-treatment failed to affect the ADP-induced platelet aggregation, the secretion of PDGF-AB, the release of sCD40L and the phosphorylation p38 MAP kinase or HSP27. These results strongly suggest that pre-treatment with TPO significantly amplifies ADP-induced HSP27 phosphorylation via the p38 MAP kinase pathway in human platelets.

Major contribution of the P2Y₁receptor in purinergic regulation of TNFα-induced vascular inflammation

BACKGROUND

Atherosclerosis is an inflammatory disease, and extracellular nucleotides are one of the factors possibly involved in vascular inflammation. The P2Y(1) receptor for adenosine 5'-diphosphate has been shown to be involved in the development of atherosclerosis in apolipoprotein E--deficient mice. Our aim is to determine whether the endothelial P2Y(1) receptor plays a role in leukocyte recruitment during vascular inflammation and characterize underlying mechanisms.

METHODS AND RESULTS

We show here that the P2Y(1) receptor plays a role in leukocyte recruitment in inflamed mouse femoral arteries. Moreover, in wild-type bone marrow--transplanted chimeric P2Y(1)-deficient mice with an apolipoprotein E--deficient background, a strong reduction of adhesion molecule--dependent leukocyte recruitment was observed after local injection of tumor necrosis factor and interleukin 1β, excluding a role for the platelet or other hematopoietic cell type P2Y(1) in these events. Similarly, the in vitro adhesion of isolated mouse monocytes to tumor necrosis factor α--stimulated murine endothelial cell monolayers and their migration across the cell layers were strongly reduced in P2Y(1)-deficient compared with wild-type endothelial cells, as was the expression of the adhesion molecules P-selectin, Vascular cell adhesion molecule 1, and intercellular adhesion molecule 1. Pharmacological inhibition using the selective antagonist MRS2500 also resulted in decreased expression of adhesion molecules. These events are related to the p38 mitogen-activated protein kinase and activating transcription factor 2 pathway. Finally, in vivo administration of MRS2500 resulted in strong reduction of leukocyte recruitment in inflamed femoral arteries of apolipoprotein E--deficient mice.

CONCLUSIONS

The data highlight a key role of the endothelial P2Y(1) receptor in acute vascular inflammation. Pharmacological targeting the P2Y(1) receptor could represent a promising approach for the treatment of vascular inflammation.

Involvement of P2Y6 receptor in p38 MAPK-mediated COX-2 expression in response to UVB irradiation of human keratinocytes

Ultraviolet B (UVB) radiation induces inflammation in human skin. Extracellular nucleotides are released from cells in response to various stimuli and act as intercellular signaling molecules through activation of P2 receptors. In this study, we investigated the involvement of extracellular nucleotides and P2 receptors in UVB-radiation-induced inflammation using human keratinocyte-derived HaCaT cells. UVB radiation induced rapid ATP release from HaCaT cells; this was inhibited by pretreatment with anion transporter blockers or maxi-anion channel blockers. In addition, the radiation-induced activation of p38 MAPK was significantly blocked by pretreatment with ecto-nucleotidase (apyrase) or P2Y6 receptor antagonist (MRS2578). Expression of COX-2, mediated by activation of p38 MAPK, was also induced by UVB radiation. Both pretreatment with MRS2578 and knockdown of the P2Y6 receptor by siRNA transfection attenuated the induction of COX-2 in HaCaT cells exposed to UVB radiation. Our results indicate that UVB radiation evokes ATP release from human keratinocytes and also that activation of P2Y6 receptor mediates the UVB-radiation-induced activation of p38 MAPK and expression of COX-2. Thus P2Y6 receptor is a mediator of UVB-radiation-induced inflammatory responses in keratinocytes.

Platelet kainate receptor signaling promotes thrombosis by stimulating cyclooxygenase activation

RATIONALE

Glutamate is a major signaling molecule that binds to glutamate receptors including the ionotropic glutamate receptors; kainate (KA) receptor (KAR), the N-methyl-d-aspartate receptor, and the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor. Each is well characterized in the central nervous system, but glutamate has important signaling roles in peripheral tissues as well, including a role in regulating platelet function.

OBJECTIVE

Our previous work has demonstrated that glutamate is released by platelets in high concentrations within a developing thrombus and increases platelet activation and thrombosis. We now show that platelets express a functional KAR that drives increased agonist induced platelet activation.

METHODS AND RESULTS

KAR induced increase in platelet activation is in part the result of activation of platelet cyclooxygenase in a mitogen-activated protein kinase-dependent manner. Platelets derived from KAR subunit knockout mice (GluR6(-/-)) are resistant to KA effects and have a prolonged time to thrombosis in vivo. Importantly, we have also identified polymorphisms in KAR subunits that are associated with phenotypic changes in platelet function in a large group of whites and blacks.

CONCLUSIONS

Our data demonstrate that glutamate regulation of platelet activation is in part cyclooxygenase-dependent and suggest that the KAR is a novel antithrombotic target.

Antithrombin III suppresses ADP-induced platelet granule secretion: inhibition of HSP27 phosphorylation

Antithrombin III (AT-III), an anti-coagulant, has recently been reported to directly affect human platelet functions. However, the exact mechanism of AT-III in platelets remains to be clarified. We have previously shown that adenosine diphosphate (ADP)-induced phosphorylation of heat shock protein 27 (HSP27) via p44/p42 mitogen-activated protein kinase (MAPK) and p38 MAPK is correlated with platelet granule secretion. In the present study, we investigated the relationship between AT-III and the ADP-induced platelet granule secretion. The ADP-induced secretion of platelet-derived growth factor (PDGF)-AB and serotonin (5-HT) were significantly suppressed by AT-III. The ADP-induced soluble CD40 ligand (sCD40L) release was inhibited by either PD98059, a MEK inhibitor, or SB203580, a p38 MAPK inhibitor. AT-III also inhibited the sCD40L release. AT-III markedly attenuated the ADP-induced phosphorylation levels of p44/p42 MAPK and p38 MAPK. Furthermore, the ADP-induced HSP27 phosphorylation was suppressed by AT-III. These results strongly suggest that AT-III directly acts on platelets and suppresses ADP-induced platelet granule secretion due to inhibiting HSP27 phosphorylation via p44/p42 MAPK and p38 MAPK.

alphaB-crystallin extracellularly suppresses ADP-induced granule secretion from human platelets

alphaB-crystallin, a low-molecular-weight heat shock protein (HSP), has binding sites on platelets. However, the exact role of alphaB-crystallin is not clarified. In this study, we investigated the effect of alphaB-crystallin on platelet granule secretion. alphaB-crystallin attenuated the adenosine diphosphate (ADP)-induced phosphorylation of p44/p42 mitogen-activated protein kinase (MAPK) and p38 MAPK. The ADP-stimulated HSP27 phosphorylation was markedly reduced by alphaB-crystallin. alphaB-crystallin significantly suppressed the ADP-induced secretions of both platelet-derived growth factor (PDGF)-AB and serotonin. Therefore, our results strongly suggest that alphaB-crystallin extracellularly suppresses platelet granule secretion by inhibition of HSP27 phosphorylation via p44/p42 MAPK and p38 MAPK.

Mechanism of activation and functional role of protein kinase Ceta in human platelets

The novel class of protein kinase C (nPKC) isoform eta is expressed in platelets, but not much is known about its activation and function. In this study, we investigated the mechanism of activation and functional implications of nPKCeta using pharmacological and gene knock-out approaches. nPKCeta was phosphorylated (at Thr-512) in a time- and concentration-dependent manner by 2MeSADP. Pretreatment of platelets with MRS-2179, a P2Y1 receptor antagonist, or YM-254890, a G(q) blocker, abolished 2MeSADP-induced phosphorylation of nPKCeta. Similarly, ADP failed to activate nPKCeta in platelets isolated from P2Y1 and G(q) knock-out mice. However, pretreatment of platelets with P2Y12 receptor antagonist, AR-C69331MX did not interfere with ADP-induced nPKCeta phosphorylation. In addition, when platelets were activated with 2MeSADP under stirring conditions, although nPKCeta was phosphorylated within 30 s by ADP receptors, it was also dephosphorylated by activated integrin alpha(IIb)beta3 mediated outside-in signaling. Moreover, in the presence of SC-57101, a alpha(IIb)beta3 receptor antagonist, nPKCeta dephosphorylation was inhibited. Furthermore, in murine platelets lacking PP1cgamma, a catalytic subunit of serine/threonine phosphatase, alpha(IIb)beta3 failed to dephosphorylate nPKCeta. Thus, we conclude that ADP activates nPKCeta via P2Y1 receptor and is subsequently dephosphorylated by PP1gamma phosphatase activated by alpha(IIb)beta3 integrin. In addition, pretreatment of platelets with eta-RACK antagonistic peptides, a specific inhibitor of nPKCeta, inhibited ADP-induced thromboxane generation. However, these peptides had no affect on ADP-induced aggregation when thromboxane generation was blocked. In summary, nPKCeta positively regulates agonist-induced thromboxane generation with no effects on platelet aggregation.

Mitogen-activated protein kinases in hemostasis and thrombosis

The mitogen-activated protein (MAP) kinases ERK2, p38 and JNK1 are present in platelets and are activated by various stimuli, such as thrombin, collagen, von Willebrand factor (VWF) and ADP. Until recently, MAP kinases were only studied in the conventional model of agonist-induced platelet aggregation mediated by fibrinogen and integrin alphaIIbbeta3. However, this approach is likely to be too limited for a physiological understanding of platelet MAP kinases and their signaling pathways. Recent studies with varying blood-flow conditions and animal models of thrombosis have provided deeper insight into the role of MAP kinases in thrombus formation and the dependence of these kinases on shear conditions. This review summarizes and discusses the physiological functions of these kinases in hemostasis and thrombosis as revealed by various technical approaches.

A specific CD36-dependent signaling pathway is required for platelet activation by oxidized low-density lipoprotein

Platelet hyperactivity associated with hyperlipidemia may contribute to development of a prothrombotic state. We previously showed that oxidized low-density lipoprotein (oxLDL) formed in the setting of hyperlipidemia and atherosclerosis activated platelets in a CD36-dependent manner. We now show that mitogen-activated protein kinase c-Jun N-terminal kinase (JNK)2 and its upstream activator MKK4 were phosphorylated in platelets exposed to oxLDL. Using apoE(-/-) mice as a model of hyperlipidemia, we showed that JNK was constitutively phosphorylated in platelets in a CD36-dependent manner. Inhibition of src kinase activity reduced JNK phosphorylation by oxLDL. Immunoprecipitations revealed that active phosphorylated forms of src kinases Fyn and Lyn were recruited to CD36 in platelets exposed to oxLDL. Pharmacological inhibition of the mitogen-activated protein kinase JNK or src family kinases abolished platelet activation by oxLDL in vitro. Using a murine carotid artery thrombosis model we demonstrated CD36-dependent phosphorylation of platelet JNK within thrombi. Furthermore, pharmacological inhibition of JNK prolonged thrombosis times in wild-type but not cd36-null mice in vivo. These findings suggest that a specific CD36-dependent signaling pathway is required for platelet activation by oxLDL and may provide insights related to development of novel antiplatelet therapies more relevant to atherothrombosis than to normal hemostasis.

HSP27 phosphorylation is correlated with ADP-induced platelet granule secretion

Adenosine diphosphate (ADP) plays a crucial role in hemostasis and thrombosis by activating platelets. ADP has been reported to induce heat-shock protein (HSP) 27 phosphorylation in human platelets. However, the exact role of HSP27 phosphorylation in human platelets has not yet been clarified. In the present study, we investigated the mechanisms and the roles of ADP-induced HSP27 phosphorylation in human platelets. We showed for the first time that both of decreased phosphorylation levels of HSP27 by PD98059, a MEK1/2 inhibitor and SB203580, a p38 MAPK inhibitor were correlated with the suppressed levels of platelet granule secretion but not with platelet aggregation. Furthermore, the inhibition of either the p44/p42 MAPK or p38 MAPK pathways had no effect on ADP-induced platelet aggregation. These results strongly suggest that the ADP-induced phosphorylation of HSP27 via p44/p42 MAPK and/or p38 MAPK is therefore sufficient for platelet granule secretion but not for platelet aggregation in humans.

Regulation and functional consequences of ADP receptor-mediated ERK2 activation in platelets

We have previously shown that ADP-induced thromboxane generation in platelets requires signalling events from the G(q)-coupled P2Y1 receptor (platelet ADP receptor coupled to stimulation of phospholipase C) and the G(i)-coupled P2Y12 receptor (platelet ADP receptor coupled to inhibition of adenylate cyclase) in addition to outside-in signalling. While it is also known that extracellular calcium negatively regulates ADP-induced thromboxane A2 generation, the underlying mechanism remains unclear. In the present study we sought to elucidate the signalling mechanisms and regulation by extracellular calcium of ADP-induced thromboxane A2 generation in platelets. ERK (extracllular-signal-regulated kinase) 2 activation occurred when outside-in signalling was blocked, indicating that it is a downstream event from the P2Y receptors. However, blockade of either P2Y1 or the P2Y12 receptors with corresponding antagonists completely abolished ERK phosphorylation, indicating that both P2Y receptors are required for ADP-induced ERK activation. Inhibitors of Src family kinases or the ERK upstream kinase MEK [MAPK (mitogen-activated protein kinase)/ERK kinase] abrogated ADP-induced ERK phosphorylation and thromboxane A2 generation. Finally ADP- or G(i)+G(z)-induced ERK phosphorylation was blocked in the presence of extracellular calcium. The present studies show that ERK2 is activated downstream of P2Y receptors through a complex mechanism involving Src kinases and this plays an important role in ADP-induced thromboxane A2 generation. We also conclude that extracellular calcium blocks ADP-induced thromboxane A2 generation through the inhibition of ERK activation.

Activation of ERK1/2 by extracellular nucleotides in macrophages is mediated by multiple P2 receptors independently of P2X7-associated pore or channel formation

Macrophages express several P2X and P2Y nucleotide receptors and display the phenomenon of ATP-induced P2X7-dependent membrane permeabilization, which occurs through a poorly understood mechanism. Several P2 receptors are known to be coupled to the activation of mitogen-activated protein kinases (MAPKs) and Ca2+ signaling. Here, we use macrophages to investigate the phosphorylation of extracellular signal-regulated kinases 1 and 2 (ERK1/2) by nucleotides and the involvement of MAPKs and intracellular Ca2+ concentration in ATP-induced membrane permeabilization. Short-term (5 min) pre-exposure to oxidized ATP (oATP), a P2X7 antagonist that does not inhibit P2X7-associated inward currents or membrane permeabilization, inhibits the activation of ERK1/2 by ATP, ADP, the P2X7 agonist 2'-3'-O-(4-benzoylbenzoyl)-ATP (BzATP), but not by UTP and UDP. We conclude that macrophages display several P2Y receptors coupled to the ERK1/2 pathway and that oATP antagonizes the action of purine nucleotides, possibly binding to P2X7 and/or other purine-binding P2Y receptors. We also show that BzATP and ATP activate ERK1/2 by two different pathways since ERK1/2 activation by BzATP, but not by ATP, is blocked by the tryrosine kinase inhibitor, genistein, and the Src protein kinase inhibitor, tyrphostin. However, the activation of ERK1/2 by ATP is blocked by the protein kinase C (PKC) inhibitor, chelerythrine chloride. Under the same conditions, membrane permeabilization is not blocked by genistein, tyrphostin, or chelerythrine chloride, indicating that tyrosine kinase, Src protein kinase, and PKC are not required for pore opening. Membrane permeabilization is independent of ERK1/2 activation since chelerythrine, or short-term exposure to oATP or PD98059, efficiently block ERK1/2 activation without inhibiting membrane permeabilization. In addition, membrane permeabilization is not inhibited by SB203580 and SB202190, two inhibitors of p38 MAPK, nor by intracellular BAPTA, which blocks ATP-induced Ca2+ signals. These results suggest that multiple P2 receptors lead to ERK1/2 activation, that ligation of the same receptors by agonists with different affinities can lead to differential stimulation of separate pathways, and that MAPKs and intracellular Ca2+ fluxes are independent of P2X7-associated pore formation.

Nucleotide receptor signaling in platelets

Upon injury to a vessel wall the exposure of subendothelial collagen results in the activation of platelets. Platelet activation culminates in shape change, aggregation, release of granule contents and generation of lipid mediators. These secreted and generated mediators trigger a positive feedback mechanism potentiating the platelet activation induced by physiological agonists such as collagen and thrombin. Adenine nucleotides, adenosine diphosphate (ADP) and adenosine triphosphate (ATP), released from damaged cells and that are secreted from platelet-dense granules, contribute to the positive feedback mechanism by acting through nucleotide receptors on the platelet surface. ADP acts through two G protein-coupled receptors, the Gq-coupled P2Y1 receptor, and the Gi-coupled P2Y12 receptor. ATP, on the other hand, acts through the ligand-gated channel P2X1. Stimulation of platelets by ADP leads to shape change, aggregation and thromboxane A2 generation. ADP-induced dense granule release depends on generated thromboxane A2. Furthermore, costimulation of both P2Y1 and P2Y12 receptors is required for ADP-induced platelet aggregation. ATP stimulation of P2X1 is involved in platelet shape change and helps to amplify platelet responses mediated by agonists such as collagen. Activation of each of these nucleotide receptors results in unique signal transduction pathways that are important in the regulation of thrombosis and hemostasis.

Thrombin stimulation of p38 MAP kinase in human platelets is mediated by ADP and thromboxane A2 and inhibited by cGMP/cGMP-dependent protein kinase

p38 MAP kinase in human platelets is activated by platelet agonists including thrombin, thromboxane A2 (TxA2), ADP, and others. However, both upstream mechanisms of p38 MAP kinase activation, and their downstream sequelae, are presently controversial and essentially unclear. Certain studies report sequential activation of cGMP-dependent protein kinase (PKG) and p38/ERK pathways by platelet agonists, leading to integrin activation and secretion, whereas others establish an essential role of Src/ERK-mediated TxA2 generation for fibrinogen receptor activation in human platelets. Here, we show that ADP secreted from platelet-dense granules, and subsequent activation of P2Y12 receptors, as well as TxA2 release are important upstream mediators of p38 MAP kinase activation by thrombin. However, p38 MAP kinase activation did not significantly contribute to calcium mobilization, P-selectin expression, alphaIIbbeta3 integrin activation, and aggregation of human platelets in response to thrombin. Finally, PKG activation did not stimulate, but rather inhibited, p38 MAP kinase in human platelets.

LDL oxidized by hypochlorous acid causes irreversible platelet aggregation when combined with low levels of ADP, thrombin, epinephrine, or macrophage-derived chemokine (CCL22)

The in vitro oxidation of low-density lipoprotein (LDL) by hypochlorous acid produces a modified form (HOCl-LDL) capable of stimulating platelet function. We now report that HOCl-LDL is highly effective at inducing platelet function, causing stable aggregation and alpha-granule secretion. Such stimulation depended on the presence of low levels of primary agonists such as adenosine diphosphate (ADP) and thrombin, or others like epinephrine (EPI) and macrophage-derived chemokine (MDC, CCL22). Agonist levels, which by themselves induced little or reversible aggregation, caused strong stable aggregation when combined with low levels of HOCl-LDL. Platelet activation by HOCl-LDL and ADP (1 microM) caused P-selectin (CD62P) exposure, without serotonin or adenosine triphosphate (ATP) secretion. Intracellular calcium levels rose slowly (from 100 to 200 nM) in response to HOCl-LDL alone and rapidly when combined with ADP to about 300 nM. p38 mitogen-activated protein kinase (MAPK) became phosphorylated in response to HOCl-LDL alone. This phosphorylation was not blocked by the protein kinase C (PKC) inhibitor bisindolylmaleimide, which reduced the extent of aggregation and calcium increase. However, the p38 MAPK inhibitor SB203580 blocked platelet aggregation and phosphorylation of p38 MAPK. These findings suggest that HOCl-LDL exposed during atherosclerotic plaque rupture, coupled with low levels of primary agonists, can rapidly induce extensive and stable thrombus formation.

Gi-dependent and -independent mechanisms downstream of the P2Y12 ADP-receptor

The P2Y12 ADP receptor is one of the major regulators of platelet activation and the target of antithrombotic thienopyridines (ticlopidine and clopidogrel). It has been recently cloned but the signaling pathways triggered by this receptor are still poorly documented. Here, we show that stimulation of the human P2Y12 receptor stably expressed in Chinese hamster ovary cells activates two major intracellular signaling mechanisms leading either to cell proliferation or to actin cytoskeleton reorganization. Both effects were blocked by the active metabolite of clopidogrel, a specific antagonist of P2Y12. The P2Y12-mediated stimulation of proliferation required the pertussis toxin-sensitive activation of PI3-kinase/Akt upstream of MAP-kinases. A partial contribution of a transactivation mechanism, through the tyrosine kinase receptor platelet-derived growth factor (PDGF)-R-beta, was also observed. Conversely, the P2Y12-mediated reorganization of the actin cytoskeleton was Gi-independent, requiring activation of RhoA and Rho-kinase. Our results provide new insights into the molecular basis of P2Y12-mediated intracellular signaling. These data may prove to be useful for a better understanding of the physiological role of P2Y12, particularly in platelets and glial cells which express this important therapeutic target.

Purification and functional reconstitution of the human P2Y12 receptor

The human P2Y12 receptor (P2Y12-R) is a member of the G protein coupled P2Y receptor family, which is intimately involved in platelet physiology. We describe here the purification and functional characterization of recombinant P2Y12-R after high-level expression from a baculovirus in Sf9 insect cells. Purified P2Y12-R, Gbeta1gamma2, and various Galpha-subunits were reconstituted in lipid vesicles, and steady-state GTPase activity was quantified. GTP hydrolysis in proteoliposomes formed with purified P2Y12-R and Galphai2beta1gamma2 was stimulated by addition of either 2-methylthio-ADP (2MeSADP) or RGS4 and was markedly enhanced by their combined presence. 2MeSADP was the most potent agonist (EC50 = 80 nM) examined, whereas ADP, the cognate agonist of the P2Y12-R, was 3 orders of magnitude less potent. ATP had no effect alone but inhibited the action of 2MeSADP; therefore, ATP is a relatively low-affinity antagonist of the P2Y12-R. The G protein selectivity of the P2Y12-R was examined by reconstitution with various G protein alpha-subunits in heterotrimeric form with Gbeta1gamma2. The most robust coupling of the P2Y12-R was to Galphai2, but effective coupling also occurred to Galphai1 and Galphai3. In contrast, little or no coupling occurred to Galphao or Galphaq. These results illustrate that the signaling properties of the P2Y12-R can be studied as a purified protein under conditions that circumvent the complications that occur in vivo because of nucleotide metabolism and interconversion as well as nucleotide release.

Thiamylal and Pentobarbital Have Opposite Effects on Human Platelet Aggregation In Vitro

The effects of barbiturates on human platelet function are not fully understood. We designed the present study to clarify the effects of thiamylal and pentobarbital on human platelet aggregation and to elucidate the underlying mechanisms in vitro. Human platelet aggregation induced by adenosine diphosphate (ADP), epinephrine, arachidonic acid (AA), and (+)-9,11-epithia-11,12-methano-thromboxane A(2) (STA(2)), measured with an 8-channel light transmission aggregometer, was compared in the absence and presence of thiamylal or pentobarbital. To estimate thromboxane A(2) (TXA(2)) receptor binding affinity, Scatchard analysis was done using [(3)H]-S145, a specific TXA(2) receptor antagonist. STA(2)-TXA(2) receptor binding assay was also examined. The release of AA was determined in platelets preincubated with [(3)H]-AA and stimulated by ADP, using a liquid scintillation analyzer. Cytosolic free calcium concentration ([Ca(2+)](i)) was measured in fluo-3/AM-loaded platelets using a fluorometer. Thiamylal enhanced, but pentobarbital suppressed, ADP- and epinephrine-induced platelet aggregation, but they did not affect AA- or STA(2)-induced platelet aggregation. They had no effect on TXA(2) receptor binding affinity. Although thiamylal increased and pentobarbital decreased release of [(3)H]-AA from ADP-stimulated platelets, both barbiturates had no effect on ADP-induced [Ca(2+)](i) increase. We conclude that thiamylal enhances but pentobarbital suppresses human platelet aggregation in vitro. These effects of barbiturates are mediated by altered AA release without affecting [Ca(2+)](i) increase.

IMPLICATIONS

Thiamylal enhances but pentobarbital suppresses human platelet aggregation in vitro. These effects are attributed to altered arachidonic acid release from platelets, possibly by the effects of phospholipase A(2), but not secondary to altered cytosolic free calcium concentration.

Concomitant recruitment of ERK1/2 and p38 MAPK signalling pathway is required for activation of cytoplasmic phospholipase A2 via ATP in articular chondrocytes

Extracellular ATP is a pro-inflammatory mediator involved in the release of prostaglandin from articular chondrocytes, but little is known about its effects on intracellular signaling. ATP triggered the rapid release of prostaglandin E(2) (PGE(2)) by acting on P2Y(2) receptors in rabbit articular chondrocytes. We have explored the signaling events involved in this synthesis. ATP significantly increased arachidonic acid production, which involved the activation of the 85-kDa cytosolic phospholipase A(2) (cPLA(2)) but not a secreted form of PLA(2), as demonstrated by various PLA(2) inhibitors and translocation experiments. We also showed that ATP induced the phosphorylation of p38 and ERK1/2 mitogen-activated-protein kinases (MAPKs). Both PD98059, an inhibitor of the ERK pathway, and SB203580, an inhibitor of p38 MAPK, completely inhibited the ATP-induced release of PGE(2). Finally, dominant-negative plasmids encoding p38 and ERK transfected alone into the cells impaired the ATP-induced release of PGE(2) to about the same extent as both plasmids transfected together. These results suggest that PGE(2) production induced by ATP requires the activation of both ERK1/2 and p38 MAPKs. Thus, ATP acts via P2Y(2)-purine receptors to recruit cPLA(2) by activating both ERK1/2 and p38 MAPKs and stimulates the release of PGE(2) from articular chondrocytes.

P2X(1)-mediated activation of extracellular signal-regulated kinase 2 contributes to platelet secretion and aggregation induced by collagen

Adenosine triphosphate (ATP) and its stable analog, alpha,beta-methylene ATP, activate the platelet P2X(1) ion channel, causing a rapid Ca(++) influx. Here, we show that, in washed apyrase-treated platelets, alpha,beta-methylene ATP elicits reversible extracellular signal-regulated kinase 2 (ERK2) phosphorylation through a Ca(++)- and protein kinase C-dependent pathway. In contrast, high-performance liquid chromatography-purified adenosine diphosphate (ADP) did not trigger ERK2 phosphorylation. alpha,beta-Methylene ATP also activated the ERK2 pathway in P2X(1)-transfected HEK293 cells but not in cells expressing mutated P2X(1)delL nonfunctional channels. Because ATP released from the dense granules during platelet activation contributes to platelet aggregation elicited by low doses of collagen, and because collagen causes ERK2 phosphorylation, we have investigated the role of P2X(1)-mediated ERK2 activation in these platelet responses. We found that the antagonism of P2X(1) with ADP or desensitization of this ion channel with alpha,beta-methylene ATP both resulted in impaired ERK2 phosphorylation, ATP secretion, and platelet aggregation induced by low concentrations of collagen (< or = 1 microg/mL) without affecting the minor early dense granule release. Selective MEK1/2 inhibition by U-0126 and Ca(++) chelation with EGTA (ethyleneglycoltetraacetic acid) behaved similarly, whereas the PKC inhibitor GF109203-X totally prevented collagen-induced secretion and ERK2 activation. In contrast, when elicited by high collagen concentrations (2 microg/mL), platelet aggregation and secretion no longer depended on P2X(1) or ERK2 activation, as shown by the lack of their inhibition by alpha,beta-methylene ATP or U-0126. We thus conclude that mild platelet stimulation with collagen rapidly releases ATP, which activates the P2X(1)-PKC-ERK2 pathway. This process enhances further degranulation of the collagen-primed granules allowing platelet aggregation to be completed.

Thiopental enhances human platelet aggregation by increasing arachidonic acid release

Conflicting results have been reported regarding the effect of thiopental on aggregation and cytosolic calcium levels in platelets. The present study attempted to clarify these phenomena. Using platelet-rich plasma or washed suspensions, platelet aggregation, thromboxane (TX) B2 formation, arachidonic acid (AA) release, and cytosolic free calcium concentrations ([Ca2+]i) were measured in the presence or absence of thiopental (30–300 µM). Platelet activation was induced by adenosine diphosphate (ADP, 0.5–15 µM), epinephrine (0.1–20 µM) arachidonic acid (0.5–1.5 mM), or (+)-9,11-epithia-11,12-methano-TXA2 (STA2, 30–500 nM). Measurements of primary aggregation were performed in the presence of indomethacin (10 µM). Low concentrations of ADP and epinephrine, which did not induce secondary aggregation in a control study, induced strong secondary aggregation in the presence of thiopental ([Formula: see text]100 µM). Thiopental ([Formula: see text]100 µM) also increased the TXB2 formation induced by ADP and epinephrine. Thiopental (300 µM) increased ADP- and epinephrine-induced 3H-AA release. Thiopental (300 µM) also augmented the ADP- and epinephrine-induced increases in [Ca2+]i in the presence of indomethacin. Thiopental appears to enhance ADP- and epinephrine-induced secondary platelet aggregation by increasing AA release during primary aggregation, possibly by the activation of phospholipase A2.Key words: barbiturates, anesthetics, eicosanoids, phospholipase.

P2y1 and P2y2 receptor-operated Ca2+ signals in primary cultures of cardiac microvascular endothelial cells

Intracellular Ca2+ signals elicited by nucleotide agonists were investigated in primary cultures of rat cardiac microvascular endothelial cells using the fura-2 technique. UTP increased the intracellular [Ca2+] in 94% of the cells, whereas 2MeSATP was active in 32%. The rank order of potency was ATP = UTP > 2MeSATP and the maximal response to 2MeSATP was lower compared to UTP and ATP. ATP and UTP showed strong homologous and heterologous desensitization. ATP fully inhibited the 2MeSATP response, while UTP abolished 2MeSATP-elicited transients in 25% of cells. 2MeSATP did not desensitize the UTP or ATP response. Adenosine 2',5'-diphosphate inhibited the response to 2MeSATP, while it did not modify the response to ATP and UTP. 2MeSATP was more sensitive to suramin than UTP and ATP. These results indicate that P(2Y1) and P(2Y2) receptors may be coexpressed in CMEC. Nucleotide-induced Ca2+ signals lacked a sustained plateau and were almost independent from extracellular Ca2+. ATP and UTP elicited Ca2+ transients longer than 2MeSATP-evoked transients. The kinetics of Ca2+ responses was not affected by bath solution stirring or ectonucleotidase inhibition. Furthermore, the nonhydrolyzable ATP analogue AMP-PNP induced Ca2+ signals similar to those elicited by ATP and UTP. These results suggest that the distinct kinetics of nucleotide-evoked Ca2+ responses do not depend on the activity of ectonucleotidases or ATP autocrine stimulation. The possibility that Ca2+ signals with different time courses may modulate different cellular responses is discussed.