Platelet shape change induced by thrombin receptor activation. Rapid stimulation of tyrosine phosphorylation of novel protein substrates through an integrin- and Ca(2+)-independent mechanism

Negative regulation of Gq-mediated pathways in platelets by G(12/13) pathways through Fyn kinase

Platelets contain high levels of Src family kinases (SFKs), but their functional role downstream of G protein pathways has not been completely understood. We found that platelet shape change induced by selective G(12/13) stimulation was potentiated by SFK inhibitors, which was abolished by intracellular calcium chelation. Platelet aggregation, secretion, and intracellular Ca(2+) mobilization mediated by low concentrations of SFLLRN or YFLLRNP were potentiated by SFK inhibitors. However, 2-methylthio-ADP-induced intracellular Ca(2+) mobilization and platelet aggregation were not affected by PP2, suggesting the contribution of SFKs downstream of G(12/13), but not G(q)/G(i), as a negative regulator to platelet activation. Moreover, PP2 potentiated YFLLRNP- and AYPGKF-induced PKC activation, indicating that SFKs downstream of G(12/13) regulate platelet responses through the negative regulation of PKC activation as well as calcium response. SFK inhibitors failed to potentiate platelet responses in the presence of G(q)-selective inhibitor YM254890 or in G(q)-deficient platelets, indicating that SFKs negatively regulate platelet responses through modulation of G(q) pathways. Importantly, AYPGKF-induced platelet aggregation and PKC activation were potentiated in Fyn-deficient but not in Lyn-deficient mice compared with wild-type littermates. We conclude that SFKs, especially Fyn, activated downstream of G(12/13) negatively regulate platelet responses by inhibiting intracellular calcium mobilization and PKC activation through G(q) pathways.

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.

Unique ligand selectivity of the GPR92/LPA5 lysophosphatidate receptor indicates role in human platelet activation

Lysophosphatidic acid (LPA) is a ligand for LPA(1-3) of the endothelial differentiation gene family G-protein-coupled receptors, and LPA(4-8) is related to the purinergic family G-protein-coupled receptor. Because the structure-activity relationship (SAR) of GPR92/LPA(5) is limited and whether LPA is its preferred endogenous ligand has been questioned in the literature, in this study we applied a combination of computational and experimental site-directed mutagenesis of LPA(5) residues predicted to interact with the headgroup of LPA. Four residues involved in ligand recognition in LPA(5) were identified as follows: R2.60N mutant abolished receptor activation, whereas H4.64E, R6.62A, and R7.32A greatly reduced receptor activation. We also investigated the SAR of LPA(5) using LPA analogs and other non-lysophospholipid ligands. SAR revealed that the rank order of agonists is alkyl glycerol phosphate > LPA > farnesyl phosphates >> N-arachidonoylglycine. These results confirm LPA(5) to be a bona fide lysophospholipid receptor. We also evaluated several compounds with previously established selectivity for the endothelial differentiation gene receptors and found several that are LPA(5) agonists. A pharmacophore model of LPA(5) binding requirements was developed for in silico screening, which identified two non-lipid LPA(5) antagonists. Because LPA(5) transcripts are abundant in human platelets, we tested its antagonists on platelet activation and found that these non-lipid LPA(5) antagonists inhibit platelet activation. The present results suggest that selective inhibition of LPA(5) may provide a basis for future anti-thrombotic therapies.

The effect of tirofiban on fibrinogen/agonist-induced platelet shape change and aggregation

There is evidence linking raised plasma fibrinogen (fib) and platelet hyperactivity with vascular events. One way to inhibit platelets is to block the platelet membrane glycoprotein (GP) IIb/IIIa receptor, which binds circulating fib or von Willebrand factor and cross-links platelets at the final common pathway to platelet aggregation. Tirofiban is a potent and specific fib receptor antagonist, used in the treatment of unstable angina. The authors assessed the effect of tirofiban on spontaneous platelet aggregation (SPA), fib-induced, serotonin (5HT)-induced, and adenosine diphosphate (ADP)-induced aggregation in whole blood by calculating the percentage free platelet count. These various agonists were used alone and in combination. The authors also measured the effect of tirofiban on agonists-induced (ADP, 5HT) platelet shape change (PSC). The effect of fib on PSC was also evaluated in platelet-rich plasma using a high-resolution (0.07 fL) channelyzer. Tirofiban significantly inhibited SPA, fib (2, 4, 8 g/L), ADP, ADP + fib combination, and 5HT-induced aggregation. Tirofiban had no effect on agonist-induced PSC. There was no apparent change in platelet volume with fib. In conclusion, tirofiban does not appear to have an effect on PSC, an early phase of platelet activation. Tirofiban seems to be a nonspecific and an effective inhibitor of platelet aggregation (a later phase of platelet activation) in whole blood. The clinical significance of these findings remains to be established.

Optical biosensor differentiates signaling of endogenous PAR1 and PAR2 in A431 cells

Background

Protease activated receptors (PARs) consist of a family of four G protein-coupled receptors. Many types of cells express several PARs, whose physiological significance is mostly unknown.

Results

Here, we show that non-invasive resonant waveguide grating (RWG) biosensor differentiates signaling of endogenous protease activated receptor subtype 1 (PAR₁) and 2 (PAR₂) in human epidermoid carcinoma A431 cells. The biosensor directly measures dynamic mass redistribution (DMR) resulted from ligand-induced receptor activation in adherent cells. In A431, both PAR₁ and PAR₂ agonists, but neither PAR₃ nor PAR₄ agonists, trigger dose-dependent Ca²⁺ mobilization as well as G_q-type DMR signals. Both Ca²⁺ flux and DMR signals display comparable desensitization patterns upon repeated stimulation with different combinations of agonists. However, PAR₁ and PAR₂ exhibit distinct kinetics of receptor re-sensitization. Furthermore, both trypsin- and thrombin-induced Ca²⁺ flux signals show almost identical dependence on cell surface cholesterol level, but their corresponding DMR signals present different sensitivities.

Conclusion

Optical biosensor provides an alternative readout for examining receptor activation under physiologically relevant conditions, and differentiates the signaling of endogenous PAR₁ and PAR₂ in A431.

Lysophosphatidic acid stimulation of platelets rapidly induces Ca2+-dependent dephosphorylation of cofilin that is independent of dense granule secretion and aggregation

Cofilin is an actin dynamizing protein and inactivated after Ser3 phosphorylation by LIM-kinases (LIMKs). We studied whether in platelets stimulated by lysophosphatidic acid (LPA), Rho-kinase or p21-activated kinase (PAK) mediates LIMK-1 activation leading to subsequent phosphorylation and inactivation of cofilin and the increase of F-actin. During LPA (0.1 microM)-induced shape change, a rapid Rho-kinase activation and a slower activation of PAK were observed. Rho-kinase activation led to rapid LIMK-1 (Thr508) phosphorylation. Despite of LIMK-1 activation, cofilin net phosphorylation was not increased. Cofilin rapidly associated with F-actin and preceded the F-actin increase. Pretreatment with the Rho-kinase inhibitor Y-27632 inhibited LIMK-1 phosphorylation, unmasked cofilin dephosphorylation and inhibited the reversible F-actin increase during shape change. In the presence of fibrinogen, LPA (10 microM) induced ATP-secretion from dense granules and aggregation, and cofilin was rapidly dephosphorylated and then rephosphorylated in a Rho-kinase/LIMK-1-dependent manner. In the absence of fibrinogen, cofilin de- and rephosphorylation after LPA (10 microM) was unchanged, but secretion and aggregation were absent. Cofilin dephosphorylation was completely blocked by BAPTA-AM indicating that it was mediated by an increase of cytosolic Ca(2+). We conclude that in LPA-stimulated platelets, Rho-kinase-dependent LIMK-1 activation mediates the F-actin increase during shape change without enhancing cofilin net phosphorylation. However, a rapid dephosphorylation of cofilin occurs during secretion and aggregation, which is Ca(2+)-dependent, upstream of secretion and aggregation and might regulate these platelet responses.

Activation of platelet function through G protein-coupled receptors

Because of their ability to become rapidly activated at places of vascular injury, platelets are important players in primary hemostasis as well as in arterial thrombosis. In addition, they are also involved in chronic pathological processes including the atherosclerotic remodeling of the vascular system. Although primary adhesion of platelets to the vessel wall is largely independent of G protein-mediated signaling, the subsequent recruitment of additional platelets into a growing platelet thrombus requires mediators such as ADP, thromboxane A(2), or thrombin, which act through G protein-coupled receptors. Platelet activation via G protein-coupled receptors involves 3 major G protein-mediated signaling pathways that are initiated by the activation of the G proteins G(q), G(13), and G(i). This review summarizes recent progress in understanding the mechanisms underlying platelet activation and thrombus extension via G protein-mediated signaling pathways.

Rapid stimulation of tyrosine phosphorylation signals downstream of G-protein-coupled receptors for thromboxane A2 in human platelets

Signals ensuing from trimeric G-protein-coupled receptors synergize to induce platelet activation. At low doses, the thromboxane A2 analogue U46619 does not activate integrin alphaIIbbeta3 or trigger platelet aggregation, but it induces shape changes. In the present study, we addressed whether low doses of U46619 trigger tyrosine phosphorylation independently of integrin alphaIIbbeta3 activation and ADP secretion, and synergize with adrenaline (epinephrine) to induce aggregation in acetylsalicylic acid (aspirin)-treated platelets. Low doses of U46619 triggered tyrosine phosphorylation of different proteins, including FAK (focal adhesion kinase), Src and Syk, independently of signals ensuing from integrin alphaIIbbeta3 or ADP receptors engaged by secreted ADP. The G(12/13)-mediated Rho/Rho-kinase pathway was also increased by low doses of U46619; however, this pathway was not upstream of tyrosine phosphorylation, because this occurred in the presence of the Rho-kinase inhibitor Y-27632. Although low doses of U46619 or adrenaline alone were unable to trigger platelet aggregation and integrin alphaIIbbeta3 activation, the combination of the two stimuli effectively induced these responses. PP2, a tyrosine kinase inhibitor, and Y-27632 inhibited platelet activation induced by low doses of U46619 plus adrenaline and, when used in combination, totally suppressed this platelet response. In addition, the two inhibitors selectively blocked tyrosine kinases and the Rho/Rho-kinase pathway respectively. These findings suggest that both tyrosine phosphorylation and the Rho/Rho-kinase pathway are required to activate platelet aggregation via G(12/13) plus G(z) signalling.

Regulation of LIM-kinase 1 and cofilin in thrombin-stimulated platelets

Cofilin is a regulator of actin filament dynamics. We studied whether during platelet activation Rho kinase stimulates LIM kinase (LIMK) leading to subsequent phosphorylation and inactivation of cofilin. Platelet shape change and aggregation/secretion were induced by low and high concentrations of thrombin, respectively. We found that during these platelet responses Rho kinase activation was responsible for mediating rapid Thr508 phosphorylation and activation of LIMK-1 and for the F-actin increase during shape change and, in part, during secretion. Surprisingly, during shape change cofilin phosphorylation was unaltered, and during aggregation/secretion cofilin was first rapidly dephosphorylated by an okadaic acid-insensitive phosphatase and then slowly rephosphorylated by LIMK-1. LIMK-1 phosphorylation and cofilin dephosphorylation and rephosphorylation during aggregation were independent of integrin alpha(IIb)beta(3) engagement. Cofilin phosphorylation did not regulate cofilin association with F-actin and was unrelated to the F-actin increase in thrombin-activated platelets. Our study identifies LIMK-1 as being activated by Rho kinase in thrombin-stimulated platelets. Two counteracting pathways, a cofilin phosphatase and LIMK-1, are activated during platelet aggregation/secretion regulating cofilin phosphorylation sequentially and independently of integrin alpha(IIb)beta(3) engagement. Rho kinase-mediated F-actin increase during platelet shape change and secretion involves a mechanism other than LIMK-1-mediated cofilin phosphorylation, raising the possibility of another LIMK substrate regulating platelet actin assembly.

Synthesis, Structure−Activity Relationships, and Biological Evaluation of Fatty Alcohol Phosphates as Lysophosphatidic Acid Receptor Ligands, Activators of PPARγ, and Inhibitors of Autotaxin†

We previously reported that fatty alcohol phosphates (FAP) represent a minimal pharmacophore required to interact with lysophosphatidic acid (LPA) receptors. To improve the activity of the first-generation saturated FAP series, a structure-activity relationship (SAR) study was carried out that includes modifications to the headgroup and alkyl side chain of the FAP pharmacophore. A series of unsaturated (C(10)-C(18)) FAP, headgroup-modified hydrolytically stable saturated (C(10)-C(18)) alkyl phosphonates, and saturated and unsaturated (C(10)-C(18)) thiophosphate analogues were synthesized and evaluated for activity in RH7777 cells transfected with individual LPA(1)(-3) receptors, in PC-3 cells and in human platelets that endogenously express all three isoforms. In this series we identified several LPA(1)- and LPA(3)-selective antagonists with IC(50) values in the nanomolar range. Oleoyl-thiophosphate (15g) was shown to be a pan-agonist, whereas tetradecyl-phosphonate (16c) was identified as a pan-antagonist. These compounds were also tested for the ability to activate the transcription factor PPARgamma, an intracellular receptor for LPA, in CV1 cells transfected with the PPRE-Acox-Rluc reporter gene. All the FAP tested, along with the previously reported LPA GPCR antagonists dioctanoyl glycerol pyrophosphate (2), Ki16425 (6), and the agonist OMPT (3), were activators of PPARgamma. The pan-agonist oleoyl-thiophosphate (15g) and pan-antagonist tetradecyl-phosphonate (16c) mimicked LPA in inhibiting autotaxin, a secreted lysophospholipase D that produces LPA in biological fluids.

Functional selectivity of G protein signaling by agonist peptides and thrombin for the protease-activated receptor-1

Thrombin activates protease-activated receptor-1 (PAR-1) by cleavage of the amino terminus to unmask a tethered ligand. Although peptide analogs can activate PAR-1, we show that the functional responses mediated via PAR-1 differ between the agonists. Thrombin caused endothelial monolayer permeability and mobilized intracellular calcium with EC(50) values of 0.1 and 1.7 nm, respectively. The opposite order of activation was observed for agonist peptide (SFLLRN-CONH(2) or TFLLRNKPDK) activation. The addition of inactivated thrombin did not affect agonist peptide signaling, suggesting that the differences in activation mechanisms are intramolecular in origin. Although activation of PAR-1 or PAR-2 by agonist peptides induced calcium mobilization, only PAR-1 activation affected barrier function. Induced barrier permeability is likely to be Galpha(12/13)-mediated as chelation of Galpha(q)-mediated intracellular calcium with BAPTA-AM, pertussis toxin inhibition of Galpha(i/o), or GM6001 inhibition of matrix metalloproteinase had no effect, whereas Y-27632 inhibition of the Galpha(12/13)-mediated Rho kinase abrogated the response. Similarly, calcium mobilization is Galpha(q)-mediated and independent of Galpha(i/o) and Galpha(12/13) because pertussis toxin Y-27632 and had no effect, whereas U-73122 inhibition of phospholipase C-beta blocked the response. It is therefore likely that changes in permeability reflect Galpha(12/13) activation, and changes in calcium reflect Galpha(q) activation, implying that the pharmacological differences between agonists are likely caused by the ability of the receptor to activate Galpha(12/13) or Galpha(q). This functional selectivity was characterized quantitatively by a mathematical model describing each step leading to Rho activation and/or calcium mobilization. This model provides an estimate that peptide activation alters receptor/G protein binding to favor Galpha(q) activation over Galpha(12/13) by approximately 800-fold.

Structural identification of oxidized acyl-phosphatidylcholines that induce platelet activation

Oxidation of low-density lipoprotein (LDL) generates proinflammatory and prothrombotic mediators that may play a crucial role in cardiovascular and inflammatory diseases. In order to study platelet-activating components of oxidized LDL 1-stearoyl-2-arachidonoyl-sn-glycero-3-phosphocholine, a representative of the major phospholipid species in LDL, the 1-acyl-phosphatidylcholines (PC), was oxidized by CuCl(2) and H(2)O(2). After separation by high-performance liquid chromatography, three compounds were detected which induced platelet shape change at low micromolar concentrations. Platelet activation by these compounds was distinct from the pathways stimulated by platelet-activating factor, lyso-phosphatidic acid, lyso-PC and thromboxane A(2), as evidenced by the use of specific receptor antagonists. Further analyses of the oxidized phospholipids by electrospray ionization mass spectrometry structurally identified them as 1-stearoyl-2-azelaoyl-sn-glycero-3-phosphocholine (m/z 694; SAzPC), 1-stearoyl-2-glutaroyl-sn- glycero-3-phosphocholine (m/z 638; SGPC), and 1-stearoyl-2-(5-oxovaleroyl)-sn-glycero-3-phosphocholine (m/z 622; SOVPC). These observations demonstrate that novel 1-acyl-PC which had previously been found to stimulate interaction of monocytes with endothelial cells also induce platelet activation, a central step in acute thrombogenic and atherogenic processes.

Inhibitory effect of high concentration of glucose on relaxations to activation of ATP-sensitive K+ channels in human omental artery

OBJECTIVE

The present study was designed to examine in the human omental artery whether high concentrations of D-glucose inhibit the activity of ATP-sensitive K+ channels in the vascular smooth muscle and whether this inhibitory effect is mediated by the production of superoxide.

METHODS AND RESULTS

Human omental arteries without endothelium were suspended for isometric force recording. Changes in membrane potentials were recorded and production of superoxide was evaluated. Glibenclamide abolished vasorelaxation and hyperpolarization in response to levcromakalim. D-glucose (10 to 20 mmol/L) but not l-glucose (20 mmol/L) reduced these vasorelaxation and hyperpolarization. Tiron and diphenyleneiodonium, but not catalase, restored vasorelaxation and hyperpolarization in response to levcromakalim in arteries treated with D-glucose. Calphostin C and Gö6976 simultaneously recovered these vasorelaxation and hyperpolarization in arteries treated with D-glucose. Phorbol 12-myristate 13 acetate (PMA) inhibited the vasorelaxation and hyperpolarization, which are recovered by calphostin C as well as Gö6976. D-glucose and PMA, but not l-glucose, significantly increased superoxide production from the arteries, whereas such increased production was reversed by Tiron.

CONCLUSIONS

These results suggest that in the human visceral artery, acute hyperglycemia modulates vasodilation mediated by ATP-sensitive K+ channels via the production of superoxide possibly mediated by the activation of protein kinase C.

Subtype-selective antagonists of lysophosphatidic Acid receptors inhibit platelet activation triggered by the lipid core of atherosclerotic plaques

BACKGROUND

Lysophosphatidic acid (LPA) is a platelet-activating component of mildly oxidized LDL (mox-LDL) and lipids isolated from human atherosclerotic plaques. Specific antagonists of platelet LPA receptors could be useful inhibitors of thrombus formation in patients with cardiovascular disease.

METHODS AND RESULTS

Short-chain analogs of phosphatidic acid (PA) were examined for their effect on two initial platelet responses, platelet shape change and Ca2+ mobilization. Dioctylglycerol pyrophosphate [DGPP(8:0)] and dioctylphosphatidic acid [PA(8:0)], recently described selective antagonists of the LPA1 and LPA3 receptors, inhibited platelet activation evoked by LPA but not by other platelet stimuli. DGPP(8:0) was more potent than PA(8:0). DGPP(8:0) also inhibited platelet shape change induced by mox-LDL and lipid extracts from human atherosclerotic plaques. Notably, we demonstrate for the first time that the lipid-rich core isolated from soft plaques was able to directly induce shape change. This effect was completely abrogated by prior incubation of platelets with DGPP(8:0). Moreover, coapplication of the lipid-rich core or LPA together with subthreshold concentrations of ADP or epinephrine synergistically induced platelet aggregation; this effect was inhibited by DGPP(8:0). Analysis by liquid chromatography-mass spectrometry revealed the presence of LPA alkyl- and acyl-molecular species with high platelet-activating potency (16:0-alkyl-LPA, 20:4-acyl-LPA).

CONCLUSIONS

LPA molecules present in the core region of atherosclerotic plaques trigger rapid platelet activation through the stimulation of LPA1 and LPA3 receptors. Antagonists of platelet LPA receptors might provide a new strategy to prevent thrombus formation in patients with cardiovascular diseases.

Convulxin induces platelet shape change through myosin light chain kinase and Rho kinase

Once platelets are activated, the first event to occur is a rapid change in shape, associated with Ca2+/calmodulin-dependent myosin light chain (MLC) phosphorylation and with Rho kinase activation. The purpose of this study was to investigate which is the biochemical pathway that leads to platelet shape change in response to convulxin, a selective GpVI activator, and to verify whether MLC phosphorylation is essential for this process. The inhibition of the Ca2+-dependent pathway by means of the Ca2+ chelator BAPTA, the Ca2+/calmodulin inhibitor W-7 or the cAMP enhancing drug iloprost reduced about 50% of platelet shape change in response to convulxin. The treatment with either the Rho kinase inhibitors Y27632 or HA 1077 had no effect on platelet shape change induced by convulxin. When both Ca2+/calmodulin-dependent and Rho kinase-dependent pathways were concomitantly inhibited by the combined use of Y27632 plus BAPTA, W-7 or iloprost, platelet shape change was completely abolished. Our findings suggest that convulxin-induced platelet shape change occurs via both pathways, the Ca2+/calmodulin-dependent, which appears to be more important, and the Rho kinase-dependent one. The pattern of MLC phosphorylation was not modified by Rho kinase inhibitors. Conversely, the inhibition of the Ca2+-dependent pathway caused a strong reduction of MLC phosphorylation in BAPTA-treated platelets, and a total inhibition in W-7 or iloprost-treated platelets. Our results demonstrate that following Rho kinase-dependent pathway platelet shape change can occur without the involvement of MLC phosphorylation.

EPA, but not DHA, decreases mean platelet volume in normal subjects

The first indication of platelet activation is an increase in mean platelet volume (MPV). n-3 FA are known to inhibit platelet function and to reduce the risk for coronary heart disease. The purpose of this study was to determine the effects of EPA and DHA on MPV. Healthy subjects received olive oil placebo for 4 wk and then were randomly assigned to receive 4 g of ethyl esters of either safflower oil (n = 11), EPA (n = 10), or DHA (n = 12) for 4 wk. At the end of placebo run-in and treatment periods, MPV (fL; mean +/- SEM) and platelet count (PLT-CT; 10(3)/microL blood) were measured in the basal state and after ex vivo stimulation with collagen (10 microg/mL), cold (4 degrees C), and heat (37 degrees C). Unlike DHA, EPA lowered MPV as compared with safflower oil (7.2 +/- 0.1 vs. 7.5 +/- 0.1 fL; P < 0.05) and raised PLT-CT (211 +/- 18 vs.192 +/- 18 10(3)/microL; P < 0.05) in the fasting state. Collagen and cold significantly increased MPV whereas heat lowered MPV regardless of treatment. All stimuli decreased PLT-CT. EPA significantly increased platelet EPA (0.2 +/- 0.1 vs. 3.3 +/- 0.4%) and docosapentaenoic acid (DPA; 2.2 +/- 0.3 vs. 2.9 +/- 0.3%) concentrations, but not DHA. DHA treatment significantly increased DHA (1.4 +/- 0.2 vs. 4.1 +/- 0.5%) and DPA (2.0 +/- 0.4 vs. 3.0 +/- 0.4%) concentrations, but not EPA. In conclusion, EPA, but not DHA, reduces platelet activation, an early step in platelet aggregation.

Reduction of post injury neointima formation due to 17beta-estradiol and phytoestrogen treatment is not influenced by the pure synthetic estrogen receptor antagonist ICI 182,780 in vitro

BACKGROUND

Animal and organ culture experiments have shown beneficial inhibitory estrogen effects on post injury neointima development. The purpose of this study was to investigate whether such estrogen effects are influenced by the estrogen receptor antagonist ICI 182,780. Different concentrations of 17beta-estradiol and the phytoestrogens genistein and daidzein were tested.

METHODS

Female New Zealand White rabbits were benumbed. In situ vascular injury of the thoracic and abdominal aorta was performed by a 3F Fogarty catheter. Segments of 5 mm were randomised and held in culture for 21 days. Three test series were performed: 1) control group--20 microM ICI--30 microM ICI--40 microM ICI. 2) control group--20 microM ICI--40 microM 17beta-estradiol--40 microM 17beta-estradiol + 20 microM ICI. 3) control group--20 microM ICI--40 microM daidzein--40 microM daidzein + 20 microM ICI--20 microM genistein--20 microM genistein + 20 microM ICI. After 21 days the neointima-media-ratio was evaluated.

RESULTS

1) Treatment with ICI 182,780 did not reduce neointima formation significantly (p = 0.05). 2) 40 microM 17beta-estradiol alone (p < 0.0001) and in combination with 20 microM ICI (p < 0.0001) reduced neointima formation significantly. 3) 20 microM genistein alone (p = 0.0083) and combined with 20 microM ICI (p = 0.0053) reduced neointima formation significantly. 40 microM daidzein did not have a significant (p = 0.0637) effect.

CONCLUSIONS

The estrogen receptor antagonist ICI 182,780 did not modulate the inhibitory estrogen effects on post injury neointima formation. These results do not support the idea that such effects are mediated by vascular estrogen receptors.

Rac, a small guanosine triphosphate-binding protein, and p21-activated kinase are activated during platelet spreading on collagen-coated surfaces: roles of integrin alpha(2)beta(1)

In this study, the receptors and signals involved in collagen-induced platelet spreading were examined. It was found that platelet spreading on collagen (presenting a polygon shape with a number of filopodialike projections) was inhibited by the anti-integrin alpha(2) antibody, suggesting the involvement of integrin alpha(2)beta(1) in this process. Studies with a glutathione-S-transferase fusion protein that binds specifically to activated Rac and in vitro p21-activated kinase (PAK) kinase assays revealed that Rac and PAK were activated during this collagen-activated process. Platelet spreading on collagen-coated surfaces was inhibited strongly by PP1 (a Src family kinase inhibitor) or weakly by wortmannin (a phosphatidylinositol 3-kinase [PI3-kinase] inhibitor) but not at all by Y-27632 (a Rho kinase inhibitor). The surfaces coated with anti-integrin alpha(2)beta(1) antibodies also induced platelet spreading (presenting an almost complete round shape) and activation of Rac and PAK, although more slowly than collagen-coated surfaces. The antibody-induced responses were strongly inhibited by PP1 or wortmannin but not by Y-27632. The same concentration of Y-27632 inhibited collagen-induced shape change of platelets in suspension. These findings suggest that Rac and/or PAK activation, but not Rho, may play certain roles in platelet spreading via integrin alpha(2)beta(1) and that Src family kinases and PI3-kinase participate in these processes. Furthermore, the difference between spreading on collagen and the anti-integrin antibody suggests the involvement of other receptor(s) (in addition to the integrin alpha(2)beta(1)) for collagen-induced spreading, the most likely candidate being glycoprotein VI.

Myristoylated alanine-rich C kinase substrate phosphorylation is involved in thrombin-induced serotonin release from platelets

Stimulation of platelets by thrombin induces protein kinase C (PKC) activation, phosphorylation of pleckstrin, aggregation and serotonin release. Here, we demonstrate that, in human platelets, thrombin stimulation also induced phosphorylation of the myristoylated alanine-rich C kinase substrate (MARCKS) and serotonin release in intact and digitonin-permeabilized platelets. MARCKS is known to bind actin and cross-link actin filaments, and this is inhibited by PKC-evoked MARCKS phosphorylation. MARCKS phosphorylation and serotonin release in response to increasing concentrations of thrombin have a similar EC50 and time course and, in permeabilized platelets, peptide MPSD, with an amino acid sequence corresponding to the phosphorylation site domain of MARCKS, blocked both responses. However, pleckstrin and myosin light chain phosphorylations were not modified. Ala-MPSD, in which the four serine residues of MPSD were substituted by alanines was ineffective. The results suggest a role for MARCKS in platelet secretion. The fact that pleckstrin phosphorylation has a different time course and was not modified in the presence of MPSD when MARCKS phosphorylation and serotonin release were inhibited would suggest either that pleckstrin phosphorylation is unrelated to secretion or that it might only be involved upstream in the events leading to secretion.

Human CLP36, a PDZ-domain and LIM-domain protein, binds to α-actinin-1 and associates with actin filaments and stress fibers in activated platelets and endothelial cells

A 38-kd protein that associates with F-actin structures in activated platelets and endothelial cells was purified, cloned, and characterized. The protein contains an N-terminal PDZ motif, a large intervening sequence, and a C-terminal LIM domain and was identified as the human homolog of rat CLP36. The study showed that CLP36 associates with actin filaments and stress fibers that are formed during shape change and spreading of platelets and during migration and contraction of endothelial cells. CLP36 binds to α-actinin-1 as shown by coimmunoprecipitation, pull-down experiments, yeast 2-hybrid analysis, and blot overlay assays and colocalizes with α-actinin-1 along endothelial actin stress fibers. In contrast to α-actinin-1, CLP36 was absent from focal adhesions in both activated platelets and endothelial cells. The N-terminal part of CLP36 containing the PDZ domain and the intervening region, but not the LIM domain, targeted enhanced green fluorescent protein fusion proteins to stress fibers in endothelial cells. Yeast 2-hybrid analysis demonstrated that the intervening sequence, but not the PDZ or the LIM domain of CLP36, binds to the spectrinlike repeats 2 and 3 of α-actinin-1. The study further shows that CLP36 binds to α-actinin in resting platelets and translocates as a CLP36/α-actinin complex to the newly formed actin cytoskeleton in activated platelets. The results indicate that CLP36 binds via α-actinin-1 to actin filaments and stress fibers in activated human platelets and endothelial cells. The study suggests that CLP36 may direct α-actinin-1 to specific actin structures and at this position might modulate the function of α-actinin-1.

Human CLP36, a PDZ-domain and LIM-domain protein, binds to α-actinin-1 and associates with actin filaments and stress fibers in activated platelets and endothelial cells

A 38-kd protein that associates with F-actin structures in activated platelets and endothelial cells was purified, cloned, and characterized. The protein contains an N-terminal PDZ motif, a large intervening sequence, and a C-terminal LIM domain and was identified as the human homolog of rat CLP36. The study showed that CLP36 associates with actin filaments and stress fibers that are formed during shape change and spreading of platelets and during migration and contraction of endothelial cells. CLP36 binds to α-actinin-1 as shown by coimmunoprecipitation, pull-down experiments, yeast 2-hybrid analysis, and blot overlay assays and colocalizes with α-actinin-1 along endothelial actin stress fibers. In contrast to α-actinin-1, CLP36 was absent from focal adhesions in both activated platelets and endothelial cells. The N-terminal part of CLP36 containing the PDZ domain and the intervening region, but not the LIM domain, targeted enhanced green fluorescent protein fusion proteins to stress fibers in endothelial cells. Yeast 2-hybrid analysis demonstrated that the intervening sequence, but not the PDZ or the LIM domain of CLP36, binds to the spectrinlike repeats 2 and 3 of α-actinin-1. The study further shows that CLP36 binds to α-actinin in resting platelets and translocates as a CLP36/α-actinin complex to the newly formed actin cytoskeleton in activated platelets. The results indicate that CLP36 binds via α-actinin-1 to actin filaments and stress fibers in activated human platelets and endothelial cells. The study suggests that CLP36 may direct α-actinin-1 to specific actin structures and at this position might modulate the function of α-actinin-1.

The GPIb thrombin-binding site is essential for thrombin-induced platelet procoagulant activity

The role of the platelet glycoprotein (GP) Ib-V-IX receptor in thrombin activation of platelets has remained controversial although good evidence suggests that blocking this receptor affects platelet responses to this agonist. The mechanism of expression of procoagulant activity in response to platelet agonists is also still obscure. Here, the binding site for thrombin on GPIb is shown to have a key role in the exposure of negatively charged phospholipids on the platelet surface and thrombin generation, in response to thrombin, which also requires protease-activated receptor-1, GPIIb-IIIa, and platelet-platelet contact. Von Willebrand factor binding to GPIb is not essential to initiate development of platelet procoagulant activity. Inhibition of fibrinogen binding to GPIIb-IIIa also failed to block platelet procoagulant activity. Both heparin and low molecular weight heparin block thrombin-induced platelet procoagulant activity, which may account for part of their clinical efficacy. This study demonstrates a new, critical role for platelet GPIb in hemostasis, showing that platelet activation and coagulation are tightly interwoven, which may have implications for alternative therapies for thrombotic diseases.

The GPIb thrombin-binding site is essential for thrombin-induced platelet procoagulant activity

The role of the platelet glycoprotein (GP) Ib-V-IX receptor in thrombin activation of platelets has remained controversial although good evidence suggests that blocking this receptor affects platelet responses to this agonist. The mechanism of expression of procoagulant activity in response to platelet agonists is also still obscure. Here, the binding site for thrombin on GPIb is shown to have a key role in the exposure of negatively charged phospholipids on the platelet surface and thrombin generation, in response to thrombin, which also requires protease-activated receptor-1, GPIIb-IIIa, and platelet-platelet contact. Von Willebrand factor binding to GPIb is not essential to initiate development of platelet procoagulant activity. Inhibition of fibrinogen binding to GPIIb-IIIa also failed to block platelet procoagulant activity. Both heparin and low molecular weight heparin block thrombin-induced platelet procoagulant activity, which may account for part of their clinical efficacy. This study demonstrates a new, critical role for platelet GPIb in hemostasis, showing that platelet activation and coagulation are tightly interwoven, which may have implications for alternative therapies for thrombotic diseases.

Mildly oxidized low density lipoprotein rapidly stimulates via activation of the lysophosphatidic acid receptor Src family and Syk tyrosine kinases and Ca2+ influx in human platelets

In contrast to native low density lipoprotein (LDL), mildly oxidized LDL (mox-LDL) induced platelet shape change and stimulated during shape change the tyrosine phosphorylation of specific proteins including Syk; the translocation of Src, Fyn, and Syk to the cytoskeleton; and the increase of cytosolic Ca(2+) due to mainly Ca(2+) entry. The stimulation of these early signal pathways by mox-LDL was inhibited by desensitization of the lysophosphatidic acid (LPA) receptor and specific LPA receptor antagonists, was independent of the alpha(IIb)beta(3)-integrin, and was mimicked by LPA. Stimulation of tyrosine phosphorylation and Syk activation were independent of the increase of cytosolic Ca(2+) and were suppressed by genistein and two specific inhibitors of the Src family tyrosine kinases, PP1 and PD173956. In contrast to PP1 and PD 173956, genistein prevented shape change by mox-LDL. The results indicate that mox-LDL, through activation of the LPA receptor, stimulates two separate early signal pathways, (a) Src family and Syk tyrosine kinases, and (b) Ca(2+) entry. The activation of these early signaling pathways by mox-LDL probably plays a role in platelet responses subsequent to shape change. The inhibition of mox-LDL-induced platelet activation by LPA receptor antagonists or dietary isoflavonoids such as genistein could have implications in the prevention and therapy of cardiovascular diseases.

Tyrosine dephosphorylation, but not phosphorylation, of p130Cas is dependent on integrin alpha IIb beta 3-mediated aggregation in platelets: implication of p130Cas involvement in pathways unrelated to cytoskeletal reorganization

The newly described adapter molecule p130 Crk-associated substrate (Cas) has been reported to contribute to cytoskeletal organization through assembly of actin filaments and to be pivotal in embryonic development and in oncogene-mediated transformation. We characterized the regulation of Cas tyrosine phosphorylation in highly differentiated, anucleate platelets. Phospholipase C-activating receptor agonists, including collagen, thrombin receptor-activating peptide (TRAP), and U46619 (a thromboxane A2 analogue), and A23187 (a Ca2+ ionophore) induced rapid Cas tyrosine phosphorylation in platelets. 12-O-Tetradecanoylphorbol 13-acetate and 1-oleoyl-2-acetyl-sn-glycerol, protein kinase C (PKC) activators, also induced Cas tyrosine phosphorylation, albeit sluggishly. Cas tyrosine phosphorylation induced by collagen or TRAP was transient in aggregating platelets; Cas became dephosphorylated in a manner dependent on integrin alpha IIb beta 3-mediated aggregation. While BAPTA-AM (an intracellular Ca2+ chelator) inhibited Cas phosphorylation induced by collagen or TRAP, Ro31-8220 (a PKC inhibitor) rather prolonged it. Under the conditions, this PKC inhibitor suppressed platelet aggregation but not intracellular Ca2+ mobilization. In contrast to Cas involvement in focal adhesions in other cells, platelet Cas phosphorylation preceded the activation of focal adhesion kinase (FAK), and blockage of alpha IIb beta 3-mediated platelet aggregation with a GRGDS peptide resulted in prolongation of stimulation-dependent Cas tyrosine phosphorylation but in suppression of FAK tyrosine phosphorylation. Furthermore, TRAP-induced Cas phosphorylation was insensitive to cytochalasin D, an actin polymerization inhibitor. The failure of FAK to associate with Cas in immunoprecipitation studies also suggests that Cas tyrosine phosphorylation is independent of FAK activation. Of the signaling molecules investigated in this study, Src seemed to associate with Cas. Finally, Cas existed mainly in cytosol and membrane cytoskeleton fractions in the resting state, and remained unchanged during platelet aggregation, when FAK translocated to the cytoskeletal fraction. Our findings on platelet Cas suggest that (i) rapid Cas tyrosine phosphorylation occurs following phosphoinositide turnover by receptor-mediated agonists and may be mediated by intracellular Ca2+ mobilization; (ii) PKC activation, by itself, may elicit sluggish Cas phosphorylation; (iii) Cas tyrosine dephosphorylation, but not phosphorylation, is dependent on integrin alpha IIb beta 3-mediated aggregation; and (iv) Cas is not involved in cytoskeletal reorganization. Anucleate platelets seem to provide a unique model system to fully elucidate the functional role(s) of Cas.

The role of heterotrimeric G proteins in platelet activation

Activation of platelets plays a central role in hemostasis as well as in various thromboembolic diseases like myocardial infarction or stroke. Most platelet activating stimuli function through receptors which couple to heterotrimeric G proteins of the Gi, Gq and G12 families. Recent studies have elucidated the roles of individual G proteins in the regulation of platelet functions like shape change, aggregation and granule secretion. The signaling pathways mediated by heterotrimeric G proteins operate synergistically to induce a full activation of platelets. This review summarizes recent progress in the understanding of upstream regulation of platelet activation through G protein-coupled receptors.

Dichotomous Regulation of Myosin Phosphorylation and Shape Change by Rho-Kinase and Calcium in Intact Human Platelets

Both Rho-kinase and the Ca2+/calmodulin-dependent myosin light chain (MLC) kinase increase the phosphorylation of MLC. We show that upon thrombin receptor stimulation by low-dose thrombin or the peptide ligand YFLLRNP, or upon thromboxane receptor activation by U46619, shape change and MLC phosphorylation in human platelets proceed through a pathway that does not involve an increase in cytosolic Ca2+. Under these conditions, Y-27632, a specific Rho-kinase inhibitor, prevented shape change and reduced the stimulation of MLC-phosphorylation. In contrast, Y-27632 barely affected shape change and MLC-phosphorylation by adenosine diphosphate (ADP), collagen-related peptide, and ionomycin that were associated with an increase in cytosolic Ca2+ and inhibited by BAPTA-AM/EGTA treatment. Furthermore, C3 exoenzyme, which inactivates Rho, inhibited preferentially the shape change induced by YFLLRNP compared with ADP and ionomycin. The results indicate that the Rho/Rho-kinase pathway is pivotal in mediating the MLC phosphorylation and platelet shape change by low concentrations of certain G protein–coupled platelet receptors, independent of an increase in cytosolic Ca2+. Our study defines 2 alternate pathways, Rho/Rho-kinase and Ca2+/calmodulin-regulated MLC-kinase, that lead independently of each other through stimulation of MLC-phosphorylation to the same physiological response in human platelets (ie, shape change).

Dichotomous Regulation of Myosin Phosphorylation and Shape Change by Rho-Kinase and Calcium in Intact Human Platelets

Both Rho-kinase and the Ca2+/calmodulin-dependent myosin light chain (MLC) kinase increase the phosphorylation of MLC. We show that upon thrombin receptor stimulation by low-dose thrombin or the peptide ligand YFLLRNP, or upon thromboxane receptor activation by U46619, shape change and MLC phosphorylation in human platelets proceed through a pathway that does not involve an increase in cytosolic Ca2+. Under these conditions, Y-27632, a specific Rho-kinase inhibitor, prevented shape change and reduced the stimulation of MLC-phosphorylation. In contrast, Y-27632 barely affected shape change and MLC-phosphorylation by adenosine diphosphate (ADP), collagen-related peptide, and ionomycin that were associated with an increase in cytosolic Ca2+ and inhibited by BAPTA-AM/EGTA treatment. Furthermore, C3 exoenzyme, which inactivates Rho, inhibited preferentially the shape change induced by YFLLRNP compared with ADP and ionomycin. The results indicate that the Rho/Rho-kinase pathway is pivotal in mediating the MLC phosphorylation and platelet shape change by low concentrations of certain G protein–coupled platelet receptors, independent of an increase in cytosolic Ca2+. Our study defines 2 alternate pathways, Rho/Rho-kinase and Ca2+/calmodulin-regulated MLC-kinase, that lead independently of each other through stimulation of MLC-phosphorylation to the same physiological response in human platelets (ie, shape change).

Differential effects of ganodermic acid S on the thromboxane A2-signaling pathways in human platelets

Ganodermic acid S (GAS) [lanosta-7,9(11),24-triene-3beta,15alpha-diacetoxy-26-oic acid], isolated from the Chinese medicinal fungus Ganoderma lucidum (Fr.) Karst (Polyporaceae), exerted a concentration-dependent inhibition on the response of human gel-filtered platelets (GFP) to U46619 (9,11-dideoxy-9alpha,11alpha-methanoepoxyprostaglandin F2alpha), a thromboxane (TX) A2 mimetic. GAS at 2 microM inhibited 50% of cell aggregation. GAS at 7.5 microM inhibited 80% of Ca2+ mobilization, 40% of phosphorylation of myosin light chain and pleckstrin, 80% of alpha-granule secretion, and over 95% of aggregation. GAS also strongly inhibited U46619-induced diacylglycerol formation, arachidonic acid release, and TXB2 formation. An immunoblotting study of protein-tyrosine phosphorylation showed that GAS inhibited the formation of phosphotyrosine proteins at the steps involving the engagement of integrin alphaIIbbeta3 and aggregation. However, GAS did not inhibit U46619-induced platelet shape change or the inhibitory effect of U46619 on the prostaglandin E1-evoked cyclic AMP level in GFP. It is concluded that GAS inhibits platelet response to TXA2 on the receptor-Gq-phospholipase Cbeta1 pathway, but not on the receptor-G1 pathway.

Tyrosine phosphorylation of cortactin associated with Syk accompanies thromboxane analogue-induced platelet shape change

Thromboxane A(2) (TxA(2)) is a potent vasoconstrictor and platelet agonist. Pharmacological studies have defined two classes of thromboxane receptors (TPs) in human platelets; sites that bind the agonist 1S-(1,2(5Z),3-(1E,3S),4)-7- 3-(3-hydroxy-4-(4'-iodophenoxy)-1-butenyl)-7-oxabicyclo-2.2. 1-heptan-2-yl-5-heptenoic acid (I-BOP) with high affinity support platelet shape change, whereas low affinity sites that bind irreversibly the antagonist GR 32191 transduce platelet aggregation. As the mechanisms of signal transduction involved in platelet aggregation begin to be elucidated, few results concern those involved in platelet shape change, which is independent of the engagement of GPIIb/IIIa. To elucidate the respective role of the two classes of pharmacological binding sites of TPs in shape change, platelets were incubated with I-BOP at low concentrations or stimulated by I-BOP at high concentrations after pretreatment with GR 32191 or activated with low concentrations of 8-epi-prostaglandin F(2)alpha. Under these three conditions, there is a rapid stimulation of protein tyrosine phosphorylation of the 80/85-kDa doublet identified as the cytoskeletal protein cortactin. Tyrosine phosphorylation of cortactin is kinetically correlated with the occurrence of shape change. These biochemical and morphological events are both inhibited by SQ 29548, a TP antagonist, indicating the specificity of the signal. Since tyrosine kinase Syk was activated early during platelet activation, we examined the possibility that cortactin is a potential substrate of Syk in TxA(2)-induced platelet shape change. p72 Syk phosphorylation and kinase activity took place during the period when platelets were changing shape upon low concentrations of I-BOP stimulation. Furthermore, cortactin was associated with Syk, and this association increases along with the level of phosphorylation. These data suggest a novel pathway for a G protein-coupled TxA(2) high affinity receptor to the protein-tyrosine kinase Syk, which is associated with cortactin in the very early steps of platelet activation.

Lysophosphatidic acid mediates the rapid activation of platelets and endothelial cells by mildly oxidized low density lipoprotein and accumulates in human atherosclerotic lesions

Oxidized low density lipoprotein (LDL) is a key factor in the pathogenesis of atherosclerosis and its thrombotic complications, such as stroke and myocardial infarction. It activates endothelial cells and platelets through mechanisms that are largely unknown. Here, we show that lysophosphatidic acid (LPA) was formed during mild oxidation of LDL and was the active compound in mildly oxidized LDL and minimally modified LDL, initiating platelet activation and stimulating endothelial cell stress-fiber and gap formation. Antagonists of the LPA receptor prevented platelet and endothelial cell activation by mildly oxidized LDL. We also found that LPA accumulated in and was the primary platelet-activating lipid of atherosclerotic plaques. Notably, the amount of LPA within the human carotid atherosclerotic lesion was highest in the lipid-rich core, the region most thrombogenic and most prone to rupture. Given the potent biological activity of LPA on platelets and on cells of the vessel wall, our study identifies LPA as an atherothrombogenic molecule and suggests a possible strategy to prevent and treat atherosclerosis and cardiocerebrovascular diseases.

A novel molecular design of thrombin receptor antagonist

In a computer modeling of transmembrane domains of human thrombin receptor, Lys-158 was found near the ligand binding site. To capture this basic residue, analogs of peptide ligand containing a series of acidic amino acids were synthesized and assayed for human platelet aggregation, and Ser-(p-F)Phe-Aad(= alphaaminoadipic acid)-Leu-Arg-Asn-Pro-NH2 was found to be a potent antagonist.

Increased adhesion and aggregation of platelets lacking cyclic guanosine 3',5'-monophosphate kinase I

Atherosclerotic vascular lesions are considered to be a major cause of ischemic diseases, including myocardial infarction and stroke. Platelet adhesion and aggregation during ischemia-reperfusion are thought to be the initial steps leading to remodeling and reocclusion of the postischemic vasculature. Nitric oxide (NO) inhibits platelet aggregation and smooth muscle proliferation. A major downstream target of NO is cyclic guanosine 3', 5'-monophosphate kinase I (cGKI). To test the intravascular significance of the NO/cGKI signaling pathway in vivo, we have studied platelet-endothelial cell and platelet-platelet interactions during ischemia/reperfusion using cGKI-deficient (cGKI-/-) mice. Platelet cGKI but not endothelial or smooth muscle cGKI is essential to prevent intravascular adhesion and aggregation of platelets after ischemia. The defect in platelet cGKI is not compensated by the cAMP/cAMP kinase pathway supporting the essential role of cGKI in prevention of ischemia-induced platelet adhesion and aggregation.

Activation of G12/G13 results in shape change and Rho/Rho-kinase-mediated myosin light chain phosphorylation in mouse platelets

Platelets respond to various stimuli with rapid changes in shape followed by aggregation and secretion of their granule contents. Platelets lacking the alpha-subunit of the heterotrimeric G protein Gq do not aggregate and degranulate but still undergo shape change after activation through thromboxane-A2 (TXA2) or thrombin receptors. In contrast to thrombin, the TXA2 mimetic U46619 led to the selective activation of G12 and G13 in Galphaq-deficient platelets indicating that these G proteins mediate TXA2 receptor-induced shape change. TXA2 receptor-mediated activation of G12/G13 resulted in tyrosine phosphorylation of pp72(syk) and stimulation of pp60(c-src) as well as in phosphorylation of myosin light chain (MLC) in Galphaq-deficient platelets. Both MLC phosphorylation and shape change induced through G12/G13 in the absence of Galphaq were inhibited by the C3 exoenzyme from Clostridium botulinum, by the Rho-kinase inhibitor Y-27632 and by cAMP-analogue Sp-5,6-DCl-cBIMPS. These data indicate that G12/G13 couple receptors to tyrosine kinases as well as to the Rho/Rho-kinase-mediated regulation of MLC phosphorylation. We provide evidence that G12/G13-mediated Rho/Rho-kinase-dependent regulation of MLC phosphorylation participates in receptor-induced platelet shape change.

Relationship between intracellular calcium-dependent process and protein-tyrosine phosphorylation in human platelets: studies of platelets from a patient with defective A23187-induced platelet aggregation

We had postulated that in a patient with defective calcium ionophore (A23187)-induced platelet aggregation, whose platelets showed normal intracellular Ca2+ mobilization in either the presence or absence of extracellular Ca2+ in response to A23187. A defect was present in an intracellular calcium-dependent process. We have now investigated whether the agonist-induced protein-tyrosine phosphorylation (PTP) was altered. Protein-tyrosine phosphorylation (PTP)-induced by A23187 in the patient's platelets was greatly diminished but that induced by thrombin was almost normal. These results suggest that an intracellular calcium-dependent process plays a fundamental role in A23187-induced PTP, whereas it does not in thrombin-induced PTP.

Shape change is independent of tyrosine phosphorylation of p130 in human platelets

It has been previously suggested that tyrosine phosphorylation of p62, p68, and p130 might be necessary for the platelet shape change to occur. In preliminary studies we observed that high concentrations (30 microM) of a protein kinase C inhibitor, Ro 31-8220, selectively suppressed p130 tyrosine phosphorylation induced by thrombin, the thromboxane synthetic analogue (U46619) and ADP. Therefore, we have investigated the correlation, if any, between p130 tyrosine phosphorylation and platelet shape change induced by the same agonists in the presence of Ro 31-8220. Our results demonstrated that high concentrations of this compound almost completely abolished p130 tyrosine phosphorylation, whereas they had no effect on platelet shape change, thus proving a dissociation between these two phenomena. Our data support the hypothesis that a role in platelet shape change might be played by tyrosine phosphorylation of proteins other than p130.

Inhibition of thrombin-induced contractile responses by protein kinase inhibitors in porcine pulmonary arteries

The clotting enzyme thrombin is known to cause receptor-mediated contractile effects in isolated blood vessels. In the present studies the influence of protein kinase inhibitors on the contractile response of porcine pulmonary arteries to thrombin (3 U/ml) was investigated. Endothelium-denuded rings (2-3 mm) from small arteries were placed in organ baths for isometric tension recording. The vessels were preincubated for 30 min with the inhibitors before inducing contractions. In the presence of the protein kinase C (PKC)-inhibitors staurosporine, BIM I (bisindolyl-maleimide I), chelerythrine and Ro 31-8220 (1 microM each), the contractile responses to the PKC activator phorbol 12,13-dibutyrate (PDBu; 50 nM) were diminished by 70-100%. However, for inhibition of thrombin-induced contractions generally higher concentrations of the inhibitors were required. Only staurosporine at 1 microM inhibited the thrombin effect by about 75%. The tyrosine kinase inhibitor erbstatin (30 microM) did not significantly alter the thrombin effect, whereas genistein at 10 microM caused a significant inhibition of contractile responses to both thrombin and PGF2alpha. At 100 microM genistein also inhibited the contractile effects of PdBu and KCl. These studies suggest that activation of both PKC and non-receptor tyrosine kinases seems to be involved in the signal transduction pathways of thrombin-induced contractile effects in isolated vessels.

Thrombin per se does not induce tyrosine protein phosphorylation in human platelets as judged by western blotting, while collagen does: the significance of synergistic, autocrine stimulation

Thrombin elicits responses in platelets such as shape change, aggregation, arachidonate liberation and secretion of the contents of three storage granules, processes that coincide with serine/threonine and tyrosine phosphorylation of numerous proteins, hydrolysis of polyphosphoinositides and mobilisation of Ca2+ within the cell. However, the significance of these parallel signal transduction processes has not been clearly elucidated in the light of the prevalent autocrine stimulation in platelets: a great amplification of the thrombin signal through secreted ADP, by production of thromboxane A2 from the liberated arachidonic acid, by the close cell contact produced by aggregation caused by exposure of integrin receptors that become ligated by fibrinogen and other platelet-produced factors. In the present communication five pathways of autocrine stimulation have been prevented by appropriate inhibitors. Under these conditions thrombin stimulated platelet secretion with little tyrosine phosphorylation, except for a 125-130 kDa protein that was tyrosine-phosphorylated in response to one of the inhibitors, the peptide Arg-Gly-Asp-Ser (RGDS) used to block aggregation. In sharp contrast, collagen elicits massive tyrosine phosphorylation and platelet secretion in the absence of autocrine stimulation. When the thrombin-induced tyrosine phosphorylations was corrected for RGDS-induced phosphorylation, the presence of inhibitors of autocrine stimulation reduced the thrombin-induced phosphorylation by 97%. Our results strongly suggests that tyrosine phosphorylation is not part of the signal transduction pathway initiated by thrombin per se, but it represents an integral part of signal transduction initiated by collagen.

Mitogen-activated protein and tyrosine kinases in the activation of astrocyte volume-activated chloride current

Astrocytes swell during neuronal activity as they accumulate K+ to buffer the increase in external K+ released from neurons. This swelling activates volume-sensitive Cl- channels, which are thought to be important in regulatory volume decrease and in the response of the CNS to trauma and excitotoxicity. Mitogen-activated protein (MAP) kinases also are activated by cell volume changes, but their roles in volume regulation are unknown. We have investigated the role of tyrosine and MAP kinases in the activation of volume-activated Cl- channels in cultured astrocytes, using whole-cell patch-clamp recording and Western immunoblots. As previously described, hypo-osmotic solution induced an outwardly rectifying Cl- current, which was blocked by NPPB and SITS. This Cl- current did not depend on [Ca2+ ]i because it was still observed when 20 mM BAPTA was included in the pipette, but it did exhibit rundown when ATP was omitted. Inhibition of tyrosine kinases with genistein or tyrphostin A23 (but not the inactive agents daidzein and tyrphostin A1) blocked the Cl- current. The MAP kinase kinase (MEK) inhibitor PD 98059 reversibly inhibited activation of the Cl- current by hypo-osmotic solution. Western immunoblots showed that genistein or PD 98059 blocked activation of Erk-1 and Erk-2 by hypo-osmotic solution in astrocytes. Therefore, activation of tyrosine and MAP kinases by swelling is a critical step in the opening of volume-sensitive Cl- channels.

Effects of tyrphostins and genistein on the circulatory failure and organ dysfunction caused by endotoxin in the rat: a possible role for protein tyrosine kinase

1 Here we compared the effects of various inhibitors of the activity of protein tyrosine kinase on (i) the expression of the activity of the inducible isoform of nitric oxide (NO) synthase (iNOS) caused by endotoxin (lipopolysaccharide, LPS) in cultured macrophages, (ii) the induction of iNOS and cyclooxygenase 2 (COX-2) protein and activity in rats with endotoxaemia, and (iii) the circulatory failure and organ dysfunction caused by LPS in the anesthetized rat. 2 Activation of murine cultured macrophages with LPS (1 microgram ml-1) resulted, within 24 h, in a significant increase in nitrite (an indicator of the formation of NO) in the cell supernatant. This increase in nitrate was attenuated by the tyrphostins AG126, AG556, AG490 or AG1641 or by genistein in a dose-dependent fashion (IC50: approximately 15 microM). In contrast, tyrphostin A1 (an analogue of tyrphostin AG126) or daidzein (an analogue of genistein) had no effect on the rise in nitrite caused by LPS. 3 Administration of LPS (E. coli, 10 mg kg-1, i.v.) caused hypotension and a reduction of the pressor responses elicited by noradrenaline (NA, 1 microgram kg-1, i.v.). Pretreatment of rats with the tyrphostins AG126, AG490, AG556, AG1641 or A1 attenuated the circulatory failure caused by LPS. Although genistein attenuated the vascular hyporeactivity to NA, it did not affect the hypotension caused by LPS. Daidzein did not affect the circulatory failure caused by LPS. 4 Endotoxaemia for 360 min resulted in rises in the serum levels of (i) urea and creatinine (indicators of renal failure), (ii) alanine aminotransferase (ALT), aspartate aminotransferase (AST), bilirubin and gamma-glutamyl transferase (gamma GT) (indicators of liver injury/dysfunction), lipase (an indicator of pancreatic injury) as well as lactate (an indicator of tissue hypoxia). None of the tyrosine kinase inhibitors tested had a significant effect on the rise i the serum levels of urea, but the tyrphostins AG126, AG556 or A1 significantly attenuated the rises in the serum level of creatinine caused by LPS. In addition, all tyrphostins and genistein attenuated the liver injury/failure, the pancreatic injury, the hypoglycaemia and the lactic acidosis caused by LPS. In contrast, daidzein did not reduce the organ injury/dysfunction or the lactic acidosis caused by LPS. 5 Injection of LPS resulted (within 90 min) in a substantial increase in the serum level of tumor necrosis factor alpha (TNF alpha), which was attenuated by pretreatment of LPS-rats with any of the tyrphostins used. Genistein, but not daidzein, also reduced the rise in the serum levels of TNF alpha caused by LPS. Endotoxaemia for 6 h also resulted in a substantial increase in the expression of iNOS and COX-2 protein and activity in the lung, which was attenuated by pretreatment of LPS-rats with the tyrphostins AG126, AG556 or genistein, but not by daidzein. 6 Thus, tyrphostins (AG126, AG556, AG1641 or A1) and genistein, but not daidzein (inactive analogue of genistein), prevent the (i) circulatory failure, (ii) the multiple organ dysfunction (liver and pancreatic dysfunction/injury lactacidosis, hypoglycaemia), as well as (iii) the induction of iNOS and COX-2 protein and activity in rats with endotoxic shock.

A 27-kDa matrix receptor from rat brain synaptosomes: selective recognition of the Arg-Gly-Asp-Ser domain and unique resistance to calcium-dependent proteolysis

A 27-kDa protein from adult rat brain synaptosomes was purified by matrix-affinity chromatography. The matrix receptor interacted with the Arg-Gly-Asp-Ser sequence recognized by integrin-type adhesion molecules, and was labeled by integrin antibodies. Levels of the 27-kDa species in brain membranes were unaffected by proteolysis, however, conventional integrin subunits exhibited robust degradation. This unique resistance to proteolysis may allow the new matrix receptor to contribute to the stability of synaptic contacts.

Protein tyrosine kinase involvement in the production of superoxide anion by neutrophils exposed to Aroclor 1242, a mixture of polychlorinated biphenyls

Neutrophils produce superoxide anion (O2-) when exposed in vitro to Aroclor 1242, a mixture of polychlorinated biphenyls (PCBs). The mechanism for this effect shares some similarities with the mechanism by which the physiologic agonist f-Met-Leu-Phe (fMLP) activates neutrophils. Since production of O2- in response to fMLP involves GTP-binding proteins and protein tyrosine kinases (PTKs), the current study was undertaken to determine whether these signalling pathways are involved in PCB-induced neutrophil activation. Neutrophils exposed to Aroclor 1242 or fMLP produced significant O2-. Pretreatment of intact neutrophils with pertussis toxin or cholera toxin or exposure of permeabilized cells to GDPbetaS significantly inhibited O2- production in fMLP-treated neutrophils but did not alter the response to Aroclor 1242. Pretreatment with genistein, an inhibitor of PTKs, significantly inhibited O2- production in both Aroclor 1242- and fMLP-treated neutrophils; however, daidzein, a structural analogue of genistein which lacks activity against PTKs, was without effect. Exposure of neutrophils to Aroclor 1242 resulted in an increase within 1 min in tyrosine phosphorylation of proteins in the 40 and 60 kDa molecular mass ranges which persisted for up to 10 min. Similar results were obtained with 2,2',4,4'-tetrachlorobiphenyl (2,2',4,4'-TCB), a PCB congener that stimulates O2- production. In contrast, 3,3',4,4',5-pentachlorobiphenyl (3,3',4,4',5-PeCB), a congener that does not generate O2-, caused only a transient increase in tyrosine phosphorylation of proteins in the 40 kDa range with no effect on 60 kDa proteins. These data suggest that Aroclor 1242 activates neutrophils to produce O2- by a mechanism that requires tyrosine kinase activity; however, heterotrimeric G-proteins are not likely to be involved.

Regulation of platelet plasma membrane Ca2+-ATPase by cAMP-dependent and tyrosine phosphorylation

As a consequence of its central role in the regulation of calcium metabolism in the platelet, the plasma membrane Ca2+-ATPase (PMCA) was assessed for cAMP-dependent and tyrosine phosphorylation. Addition of forskolin or prostaglandin E1, agents known to elevate platelet cAMP and calcium efflux, to platelets pre-labeled with [32P]PO4 resulted in the direct phosphorylation of platelet PMCA. Similarly, addition of the catalytic subunit of protein kinase A to platelet plasma membranes resulted in a 1.4-fold stimulation of activity. Thus, the previously reported inhibition of platelet activation by elevated intracellular cAMP may be accomplished in part by stimulation of PMCA, likely resulting in a decrease in intracellular calcium. Treatment with thrombin evoked tyrosine phosphorylation of platelet PMCA, while PMCA from resting platelets exhibited little tyrosine phosphorylation. Phosphorylation of platelet plasma membranes by pp60(src) resulted in 75% inhibition of PMCA activity within 15 min. Similarly, membranes isolated from thrombin-treated platelets exhibited 40% lower PMCA activity than those from resting platelets. Phosphorylation of erythrocyte ghosts and purified PMCA by pp60(src) also resulted in up to 75% inhibition of Ca2+-ATPase activity, and inhibition was correlated with tyrosine phosphorylation. Sequencing of a peptide obtained after 32P labeling of purified erythrocyte PMCA in vitro showed that tyrosine 1176 of PMCA4b is phosphorylated by pp60(src). These results indicate that tyrosine phosphorylation of platelet PMCA may serve as positive feedback to inhibit PMCA and increase intracellular calcium during platelet activation.

Arg-Gly-Asp-Ser-selective adhesion and the stabilization of long-term potentiation: pharmacological studies and the characterization of a candidate matrix receptor

Peptides known to block the extracellular interactions of adhesion receptors belonging to a subclass of the integrin family were tested for their effects on the stabilization of long-term potentiation (LTP) in hippocampal slices. Theta burst stimulation delivered after infusions of Gly-Ala-Val-Ser-Thr-Ala (GAVSTA) resulted in a potentiation effect that decayed steadily over a period of 40 min; LTP elicited in the presence of inactive control peptides remained stable over this time period. GAVSTA had no detectible influence on baseline responses, induction processes, or the initial degree of potentiation. Infusions of integrin antagonists after application of theta bursts also resulted in the occurrence of a decremental form of LTP. Affinity chromatography was then used in an effort to identify targets of the structurally dissimilar integrin blockers that disrupt LTP stabilization. Both integrin antagonists Gly-Arg-Gly-Asp-Ser-Pro and GAVSTA eluted a major species of 55 kDa (synaptegrin-1) from GRGDSP-affinity columns that had been loaded with solubilized synaptic membranes; lesser concentrations of three polypeptides of approximately 20, 27, and 30 kDa were also collected. Synaptegrin-1 was labeled by antibodies to the RGDS-binding integrin alpha5beta1. In addition, the synaptegrin, as well as the 27 kDa, protein was found to copurify with pre- and postsynaptic markers during the isolation of forebrain synaptosomes. These results indicate that a matrix recognition event occurring several minutes after induction of LTP is a necessary step in the stabilization of potentiated synapses; they also identify an integrin-like matrix receptor of 55 kDa that may contribute to this event.

Tyrphostin 23 blocks phosphorylation of p42 but not p38 mitogen-activated protein kinase by zooxanthellatoxin-A

Zooxanthellatoxin-A isolated from a symbiotic dinoflagellate, caused aggregation in rabbit platelets that was inhibited by genistein (50 microM) and tyrphostin 23 (500 microM). Zooxanthellatoxin-A increased tyrosine phosphorylation of 42-kDa proteins which were identified as p42 and p38 mitogen-activated protein kinase (MAPK) by immunoprecipitation. Tyrphostin 23 inhibited the tyrosine phosphorylation of p42 MAPK but not p38 MAPK. In contrast, genistein abolished zooxanthellatoxin-A-induced tyrosine phosphorylation of both p42 and p38 MAPK. The results suggest that tyrphostin 23 selectively inhibits tyrosine phosphorylation of p42 MAPK. The p38 MAPK tyrosine phosphorylation is not involved in zooxanthellatoxin-A-induced platelet aggregation.