Fc gamma II receptor-mediated platelet activation induced by anti-CD9 monoclonal antibody opens Ca2+ channels which are distinct from those associated with Ca2+ store depletion

Anti-human platelet CD9 mAb, NNKY1-19, induced platelet activation in a Fc gamma RII-dependent manner in terms of aggregation and secretion of intracellular granule contents. These responses were considerably suppressed by aspirin. [Ca2+]i elevation in the absence of extracellular Ca2+ ([Ca2+]e), which represents the amount of Ca2+ released from intracellular Ca2+ ([Ca2+]i) stores, was also greatly reduced, whereas Ca2+ influx was sustained at similar levels. We thus investigated the mechanism that leads to the opening of Ca2+ channels in platelets incubated with aspirin. IP3 production and Ca2+ efflux were below detectable levels. 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid loading of platelets to chelate [Ca2+]i did not reduce Ca2+ influx, as assessed by 45Ca2+ measurement. These findings suggested that NNKY1-19 induces Ca2+ channels to open without [Ca2+]i mobilization or by depleting the [Ca2+]i stores. The magnitude of Ca2+ influx was evaluated by adding [Ca2+]e to a platelet suspension activated by various agonists in the absence of [Ca2+]e. The dose dependence of the Ca2+ influx on [Ca2+]e concentrations differed according to the mode of activation. The ED50 value of Ca2+ after thrombin or thapsigargin stimulation was 0.6 mM, whereas that of NNKY1-19 activation was about 3 mM. The addition of anti-Fc gamma RII mAb, IV.3, even 10 min after the initiation of platelet activation induced by NNKY1-19, inhibited the Ca2+ influx. These findings suggest that the Fc gamma RII-dependent activation of platelets induced by NNKY1-19 directly opens Ca2+ channels, which are distinct from those opened by thrombin or thapsigargin.

Phosphoinositide metabolism triggered by wheat germ agglutinin in human platelets

The lectin wheat germ agglutinin (WGA) has been shown to elicit platelet functional responses such as aggregation and release reaction as a consequence of binding several cell surface glycoproteins in platelets, whereas physiological activators act on their specific receptors. To examine the mechanism of WGA-induced platelet activation, we analyzed phosphoinositide metabolism triggered by this lectin. The binding of WGA to human platelets induced a rapid and dose-dependent surge in phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2) hydrolysis, as monitored by an increase in [Ca2+]i and phosphatidic acid production. Furthermore, it similarly elicited the synthesis of PtdIns(3,4)P2, which is a major 3-phosphorylated phosphoinositide in platelets. This 3-phosphorylated lipid generation induced by WGA was very strong and did not appear to be a consequence of the classical PtdIns(4,5)P2 hydrolysis. WGA also induced a marked increase in the degree of tyrosine phosphorylation of proteins in platelets, and this increase was correlated with the phosphoinositide metabolism, i.e. PtdIns(4,5)P2 breakdown and PtdIns(3,4)P2 synthesis. The non-selective inhibitor of protein kinases, staurosporine, inhibited similarly both the increase in tyrosine phosphorylation of several platelet proteins and the phosphoinositide changes in a dose-dependent manner. These findings suggest that both PtdIns(4,5)P2 hydrolysis and PtdIns(3,4)P2 synthesis are related to protein-tyrosine phosphorylation in platelets stimulated with WGA.

Quantitative measurement of sphingosine 1-phosphate in biological samples by acylation with radioactive acetic anhydride

We describe here in detail the development of a method to quantitatively measure sphingosine 1-phosphate (Sph-1-P), a bioactive sphingolipid. Sph-1-P was first extracted from cells into the upper aqueous phase under alkaline conditions by Folch's phase separation and then reextracted into the lower chloroform phase under acidic conditions. This phosphorylated sphingoid base extracted was quantitatively converted to N-[3H]-acetylated Sph-1-P, that is [3H]C2-ceramide 1-phosphate (C2-Cer-1-P), by N-acylation with [3H]acetic anhydride. The [3H]C2-Cer-1-P formed with the acylation was resolved by thin-layer chromatography, detected with autoradiography, and quantitated by scraping the corresponding band and counting its radioactivity with a scintillation counter. This assay allows quantification of Sph-1-P over a range from at least 100 pmol (often 30 pmol) to 10 nmol (the highest level tested). The utility and validity of our assay were demonstrated using human platelets. The amount of Sph-1-P in platelet extracts was proportional to the cell number and calculated as 141 +/- 4 pmol/10(8) cells (mean +/- SD, n = 3), which was about four times higher than that of sphingosine. The potent agonist thrombin did not affect the total Sph-1-P amounts in platelet suspensions but induced the release of Sph-1-P stored in the cells into the medium.

Sphingosine-1-phosphate inhibits PDGF-induced chemotaxis of human arterial smooth muscle cells: spatial and temporal modulation of PDGF chemotactic signal transduction

Activation of the PDGF receptor on human arterial smooth muscle cells (SMC) induces migration and proliferation via separable signal transduction pathways. Sphingosine-1-phosphate (Sph-1-P) can be formed following PDGF receptor activation and therefore may be implicated in PDGF-receptor signal transduction. Here we show that Sph-1-P does not significantly affect PDGF-induced DNA synthesis, proliferation, or activation of mitogenic signal transduction pathways, such as the mitogen-activated protein (MAP) kinase cascade and PI 3-kinase, in human arterial SMC. On the other hand, Sph-1-P strongly mimics PDGF receptor-induced chemotactic signal transduction favoring actin filament disassembly. Although Sph-1-P mimics PDGF, exogenously added Sph-1-P induces more prolonged and quantitatively greater PIP2 hydrolysis compared to PDGF-BB, a markedly stronger calcium mobilization and a subsequent increase in cyclic AMP levels and activation of cAMP-dependent protein kinase. This excessive and prolonged signaling favors actin filament disassembly by Sph-1-P, and results in inhibition of actin nucleation, actin filament assembly and formation of focal adhesion sites. Sph-1-P-induced interference with the dynamics of PDGF-stimulated actin filament disassembly and assembly results in a marked inhibition of cell spreading, of extension of the leading lamellae toward PDGF, and of chemotaxis toward PDGF. The results suggest that spatial and temporal changes in phosphatidylinositol turnover, calcium mobilization and actin filament disassembly may be critical to PDGF-induced chemotaxis and suggest a possible role for endogenous Sph-1-P in the regulation of PDGF receptor chemotactic signal transduction.

Sphingosine-1-phosphate: a platelet-activating sphingolipid released from agonist-stimulated human platelets

Sphingosine-1-phosphate (Sph-1-P) is the initial product of catabolism of sphingosine by sphingosine kinase and is cleaved by Sph-1-P lyase to a fatty aldehyde and ethanolamine phosphate. This phosphorylated sphingoid base is not only an intermediary catabolite, but also a bioactive lipid with important functions, including stimulation of cell proliferation in Swiss 3T3 fibroblasts and inhibition of tumor cell motility. In the present study, we examined functional roles of Sph-1-P in human platelets. Sph-1-P induced platelet shape change and aggregation reactions, although it failed to elicit secretion. Sphingosine, ceramide, sphingomyelin, and N,N-dimethylsphingosine did not mimic the positive effects of Sph-1-P on platelets. Subthreshold concentrations of Sph-1-P and weak platelet agonists such as adenosine diphosphate (ADP) and epinephrine synergistically elicited aggregation, which may be important for efficient amplification of platelet activation. Sph-1-P induced intracellular Ca2+ mobilization and the dose-response for Ca2+ release correlated closely with the concentration required for induction of shape change. On addition of [3H]sphingosine to intact platelets, the label was rapidly converted to Sph-1-P, and subsequently to ceramide and sphingomyelin. Interestingly, the Sph-1-P formed was specifically released into medium on stimulation of platelets with physiologic agonists. The amount of Sph-1-P in platelets, as measured by its conversion into radiolabeled N-acetyl-Sph-1-P, was 1.4 nmol/10(9) cells and was about four times higher than the mass of Sph present. When compared by mole percent Sph-1-P/phospholipid, the value for platelets is over 10 times higher than that for neutrophils. Our results suggest that Sph-1-P, rapidly converted from sphingosine, abundantly stored in platelets, and released on the cell activation, may play a physiologic role in thrombosis, hemostasis, and the natural wound-healing processes.

Anti-CD9 monoclonal antibody activates p72syk in human platelets

NNKY 1-19, anti-CD9 monoclonal antibody (MoAb), induced protein tyrosine phosphorylation of 125-, 97-, 75-, 64-, and 40-kDa proteins in human platelets, whereas F(ab')2 fragments of NNKY 1-19 did not, suggesting that the stimulation of Fc gamma II receptors is required for the induction of protein tyrosine phosphorylation. Tyrosine-phosphorylated proteins of 97 and 125 kDa were associated with aggregation, while NNKY 1-19-induced protein tyrosine phosphorylation was completely inhibited by prostaglandin I2 (PGI2). The activity of p72syk was assessed in immunoprecipitation kinase assays to determine at which step the signal transduction pathway leading to protein tyrosine phosphorylation was suspended. NNKY 1-19 induced a rapid and transient increase in the p72syk-associated tyrosine kinase activity that peaked at 10 s and subsided to the original level 2 min after stimulation. Coinciding with this time course, p60c-src transiently associated with p72syk. In platelets preexposed to GRGDS peptides or PGI2, NNKY 1-19 also increased the p72syk-associated tyrosine kinase activity and led to the association of p60c-src with p72syk. However, in contrast to the control without any inhibitor, the elevated tyrosine kinase activity and the associated state of the two tyrosine kinases persisted as long as 5 min after stimulation. F(ab')2 fragments of NNKY 1-19 induced changes similar to those observed with the effects of GRGDS peptides or PGI2 treatment on intact IgG NNKY 1-19 stimulation. F(ab')2 fragments of another CD9 MoAb, PMA2, had effects on p72syk essentially similar to those of NNKY 1-19. These findings suggest that the binding of anti-CD9 MoAb to CD9 on the platelet membrane per se induces an increase in the p72syk-associated tyrosine kinase activity but that Fc gamma II receptor-mediated signal(s) is required for the full activation of platelets and the appearance of tyrosine-phosphorylated proteins. The elevated intracellular cAMP level induced by PGI2 acts at a step distal to the activation of p72syk and inhibited the signal transduction pathway leading to protein tyrosine phosphorylation and aggregation. p72syk activation occurs in the absence of aggregation, but aggregation appears to reduce the elevated p72syk activity induced by anti-CD9 MoAb.

Sulphonylurea agents inhibit platelet aggregation and [Ca2+]i elevation induced by arachidonic acid

The effects of three hypoglycaemic agents--glimepiride, glibenclamide and gliclazide--were evaluated on platelet aggregation and intracellular Ca2+ elevation induced by arachidonic acid. Platelet aggregation was assessed both by the conventional method using changes in light transmission and by a newly-developed procedure using light scattering which allows the detection of small as well as large aggregates. Glimepiride and glibenclamide inhibited the formation of small and large aggregates induced by optimal concentrations of arachidonic acid in a dose-dependent manner. The ID50 values for the inhibition of platelet aggregation were approximately one third of those for arachidonic acid metabolism, suggesting that both agents have certain direct inhibitory effects on platelet aggregation unrelated to arachidonic acid metabolism. Gliclazide inhibited the formation of small aggregates induced by low concentrations of arachidonic acid to a limited extent. However, it inhibited the formation of large aggregates but not small aggregates when higher concentrations of arachidonic acid were used. Glimepiride and glibenclamide inhibited [Ca2+]i elevation induced by arachidonic acid in a dose-dependent manner, whereas gliclazide had no inhibitory effect. Taken together, these suggest that gliclazide does not inhibit arachidonic acid metabolism but does have certain direct inhibitory effects on platelet aggregation.

Protein tyrosine phosphorylation in human platelets induced by interaction between glycoprotein Ib and von Willebrand factor

The interaction between von Willebrand factor (vWF) and glycoprotein Ib (GPIb) induced by ristocetin or botrocetin resulted in associated platelet aggregation, protein tyrosine phosphorylation (PTP) of a 64 kDa protein, as detected by a monoclonal antibody against phosphotyrosine (PY-20), and intracellular Ca2+ elevation that is largely dependent upon Ca2+ influx in human platelets. It is of interest that 75-80, 97 and 125 kDa proteins which are strongly tyrosine-phosphorylated in platelet activation induced by thrombin and other agonists were not detected. Neither vWF nor a coaggregating agent (ristocetin or botrocetin) alone induced aggregation, [Ca2+]i elevation or the 64 kDa PTP. NMC-4, an antibody which inhibits both ristocetin- or botrocetin-induced vWF binding to GPIb, abolished the appearance of the 64 kDa PTP as well as other responses, suggesting that it is specifically induced by the GPIb-vWF interaction. Aspirin, or ONO-3708, a competitive inhibitor of thromboxane A2, did not modify the 64 kDa PTP, while [Ca2+]i elevation was moderately suppressed. Depletion of extracellular Ca2+ or RGD peptides suppressed neither the 64 kDa PTP nor aggregation. H-7, a protein kinase C inhibitor, did not inhibit the 64 kDa PTP, while staurosporine, a potent protein kinase inhibitor, inhibited the 64 kDa PTP and Ca2+ influx, but not aggregation, in a dose-dependent manner. It is suggested that the 64 kDa PTP is associated with platelet aggregation induced by the interaction between GPIb and vWF.

Induction of apoptosis by sphingosine in human leukemic HL-60 cells: a possible endogenous modulator of apoptotic DNA fragmentation occurring during phorbol ester-induced differentiation

The present studies were undertaken to characterize the potential role of sphingosine in the regulation of apoptosis in HL-60 promyelocytic leukemia cells. A 6-h exposure of HL-60 cells to sphingosine or its methylated derivative, N,N-dimethylsphingosine, caused internucleosomal DNA fragmentation and stereotypical morphological changes characteristic of apoptosis (i.e., cell shrinkage, nuclear condensation, and the formation of apoptotic bodies), as well as that to pharmacological inhibitors of protein kinase C such as 1-(5-isoquinolinesulfonyl)-2-methylpiperazine and staurosporine. Apoptosis by sphingosine and N,N-dimethylsphingosine was measured using a flow cytometric method. The percentages of apoptotic cells in cultures treated with sphingosine (10 microM) and N,N-dimethylsphingosine (10 microM) for 6 h were 55.6 +/- 7.8% and 84.2 +/- 11.6%, respectively. HL-60 cells were induced to differentiate toward macrophages by treatment with 5 nM 4 beta-phorbol 12-myristate 13-acetate (PMA). Internucleosomal DNA fragmentation, which was a hallmark of apoptosis, was first detected after 10-h exposure to PMA and increased with longer treatment. Sphingosine concentrations in the cells increased concomitantly with the increasing proportion of apoptotic cells during cell differentiation. Sphingosine level in HL-60 cells differentiated by treatment with PMA for 48 h was about 3.3-fold greater than that in untreated cells. Differentiated HL-60 cells exhibited a markedly increased conversion of exogenously added [3H]ceramide to [3H]sphingosine, indicating elevation of ceramidase activity. Moreover, exposure to sphingosine resulted in down-regulation of c-myc mRNA. These observations suggest the possible role of sphingosine in induction of apoptotic DNA fragmentation during PMA-induced differentiation in myeloid leukemia cells. Sphingosine may function as an endogenous modulator mediating the apoptotic signal.

A possible role of sphingosine in induction of apoptosis by tumor necrosis factor-alpha in human neutrophils

Treatment of human neutrophils with tumor necrosis factor-alpha (TNF-alpha) resulted in an increase in concentration of ceramide and its catabolite, sphingosine. Sphingosine, a potent endogenous protein kinase C (PKC) inhibitor, as well as TNF-alpha, induced internucleosomal DNA fragmentation and morphological changes characteristic of apoptotic cells. Ceramide and sphingosine-1-phosphate, the initial product of sphingosine catabolism, did not cause apoptosis under our experimental conditions. In addition, 1-(5-isoquinolinesulfonyl)-2-methylpiperazine (H7) and N,N-dimethylsphingosine (DMS), known as PKC inhibitors, also induced apoptosis, suggesting that induction of apoptosis by sphingosine may be related to inhibition of PKC activity. These results indicate that sphingosine deacylated from ceramide may be an endogenous modulator mediating apoptotic signals by TNF-alpha in neutrophils.

Effects of sulfonylurea agents on platelet arachidonic acid metabolism; study on platelet homogenates

Effects of three sulfonylurea agents on arachidonic acid metabolism of platelet homogenates were evaluated using HPLC. Gliclazide had no significant inhibitory effects on arachidonic acid metabolism. Glibenclamide and glimepiride both inhibited the production of cyclooxygenase-related metabolites, thromboxane B2 (TXB2) and 12-hydroxy 5,8,10-heptadecatrienoic acid (HHT), whereas 12-hydroxy 5,8,10,14-eicosatetraenoic acid (12-HETE), a 12-lipoxygenase-related product, was unaffected. These findings confirmed part of our previous report using intact platelets, except that we found in the present study that glibenclamide had no inhibitory effect on 12-lipoxygenase.

Synthesis of phosphatidylinositol 3,4-bisphosphate is regulated by protein-tyrosine phosphorylation but the p85 alpha subunit of phosphatidylinositol 3-kinase may not be a target for tyrosine kinases in thrombin-stimulated human platelets

To elucidate the mechanism involving synthesis of phosphatidylinositol 3,4-bisphosphate (PtdIns(3,4)P2), which is the main species of 3-phosphorylated phosphoinositides in activated blood platelets, we observed a correlation among protein-tyrosine phosphorylation, protein kinase C (PKC) activation, and PtdIns(3,4)P2 synthesis in these anucleate cells. Thrombin (1 U/ml) elicited marked protein-tyrosine phosphorylation, PKC activation, and PtdIns(3,4)P2 synthesis. In contrast, 1 microM 12-O-tetrade-canoylphorbol 13-acetate barely induced tyrosine phosphorylation and PtdIns(3,4)P2 synthesis although it strongly activated PKC. A variety of kinase inhibitors were tested for their ability to inhibit the thrombin effects. Both staurosporine and tyrphostin inhibited thrombin-stimulated tyrosine phosphorylation and PtdIns(3,4)P2 synthesis. H-7, which specifically, although weakly, inhibited PKC activation, had no effect on tyrosine phosphorylation and PtdIns(3,4)P2 production. Among the various kinase inhibitors tested, staurosporine was the most potent inhibitor of protein tyrosine phosphorylation and PtdIns(3,4)P2 synthesis, and there was a good correlation of the inhibition between these two parameters, although it also inhibited PKC activation. To examine the involvement of PtdIns 3-kinase, which is believed to play an important role in 3-phosphorylated phosphoinositide synthesis, we studied tyrosine phosphorylation and the association with tyrosine-phosphorylated proteins of the p85 alpha subunit of PtdIns 3-kinase in thrombin-stimulated platelets. We did not detect tyrosine-phosphorylated protein by Western blotting where p85 alpha was located. Similarly, when platelet lysates were precipitated with anti-p85 alpha antibodies and then blotted with anti-phosphotyrosine antibodies, tyrosine-phosphorylated p85 alpha was undetectable. Furthermore, when the cell lysates were precipitated with anti-phosphotyrosine antibodies, no p85 alpha was found in the immunoprecipitates. These results show that PtdIns(3,4)P2 synthesis in stimulated platelets is mediated by tyrosine phosphorylation, as it is in proliferating cells, but the p85 alpha subunit of PtdIns 3-kinase may not be a target for tyrosine kinases and that staurosporine, though non-specific, would be a useful tool for elucidating signal transduction involving D-3-phosphorylated phosphoinositide generation and protein-tyrosine phosphorylation in blood platelets.

Detection of platelet aggregates with a particle counting method using light scattering

A novel method to detect platelet aggregation by means of the particle counting technique using light scattering has been developed. An optical device designed to focus on a limited area of platelet-rich plasma measured the intensity of light scattered by particles passing through the area, minimizing multiple light scattering. The use of polystyrene spheres of different diameters confirmed that the light scattering intensity increases in proportion to the particle size in a suspension. Platelet activation induced by various agonists resulted in light scattering of higher intensities, which correlated well with the number and size of aggregates as observed under a microscope. These findings confirmed that the intensity of light scattering detected by the new device provides information on the number and size of aggregates in a suspension. The new method was compared with conventional platelet aggregometry using overall light scattering or changes in light transmission (optical density). The new device appeared to be particularly sensitive to small aggregates such as those formed in platelet activation induced by low concentrations of agonists. Furthermore, the new method has an advantage over the conventional aggregometry, in that it allows the aggregate size distribution and the extent of aggregation to be estimated.

Phosphatase inhibitors suppress Ca2+ influx induced by receptor-mediated intracellular Ca2+ store depletion in human platelets

The effects of three phosphatase inhibitors including okadaic acid, calyculin A and tautomycin were evaluated on platelet Ca2+ mobilization. Calyculin A and tautomycin at appropriate concentrations appeared to have a selective inhibitory effect on thrombin-induced Ca2+ influx, but not on [Ca2+]i release from intracellular Ca2+ storage sites. In contrast, pretreatment with okadaic acid at concentrations that effectively lowered Ca2+ influx also suppressed Ca2+ release from intracellular Ca2+ stores. In a system that specifically evaluates the effects of agents on Ca2+ influx induced by the Ca(2+)-depleted state of intracellular Ca2+ storage sites, the three phosphatase inhibitors attenuated Ca2+ influx in a dose dependent manner and showed complete inhibition at appropriate concentrations. These findings suggest that protein phosphorylation/dephosphorylation plays an important role in mediating signals to open Ca2+ channels when Ca2+ depletion in intracellular Ca2+ stores is caused by thrombin. In contrast, Ca2+ influx induced by thapsigargin, a Ca(2+)-ATPase inhibitor, was only partially suppressed by pretreatment with each of the three phosphatase inhibitors. Based on these findings, we suggest that the Ca(2+)-depleted state of intracellular Ca2+ stores by thapsigargin induces the opening of Ca2+ channels via phosphatase inhibitor-insensitive pathways. All the phosphatase inhibitors, at the highest concentrations tested in the present study, only partially inhibited Mn2+ entry induced by thrombin. These findings suggest that there are at least two types of divalent ion channels on platelet plasma membranes and that one of them, that preferentially allows Mn2+ entry, is resistant to the inhibitory effects of phosphatase inhibitors.

Effects of the prior activation of protein kinase C on human platelet activation induced by thrombin

Effects of the prior activation of protein kinase C (PKC) on the responses induced by thrombin were studied using human blood platelets, in which PKC is abundantly expressed. At a concentration of 25 nM, 12-O-tetradecanoyl-phorbol 13-acetate (TPA) induced little aggregation or release by itself but selectively elicited PKC activation in a time-dependent manner, as monitored by 47-kDa protein phosphorylation. Increases in the intracellular Ca2+ concentration of human platelets caused by thrombin, at any concentration, were markedly inhibited by the prior addition of 25 nM TPA in a time-dependent manner. However, the effects of TPA on platelet aggregation and secretion induced by thrombin varied, depending upon the agonist concentration; the PKC activator markedly enhanced the aggregation and secretion induced by lower concentrations of thrombin but had a tendency to weakly inhibit those induced by higher concentrations of thrombin. TPA enhanced the ionomycin effect upon aggregation and release without obvious effects on the ionophore-induced Ca2+ mobilization, suggesting that PKC potentiates platelet function by increasing intracellular sensitivity to Ca2+. There was a good time-dependent correlation of the TPA effects among the three parameters, namely, the phosphorylation of the 47-kDa protein, inhibition of the [Ca2+]i increase induced by thrombin, and enhanced ionomycin effects in aggregation and release. It is most likely that the final responses of agonist-activated platelets depend upon the balance of two factors induced by PKC; inhibition of Ca2+ signals and enhancement of intracellular sensitivity to Ca2+.

A case of POEMS syndrome with thrombocytopenia and biliverdinaemia

This report describes a case of POEMS syndrome with extremely rare complications of thrombocytopenia and biliverdinaemia. The thrombocytopenia appeared to be due to immunopathogenesis. Steroid treatment resulted in complete disappearance of the biliverdinaemia in this patient as well as symptoms and signs of POEMS syndrome, suggesting that the biliverdinaemia was related to POEMS syndrome.

Effects of genistein, a tyrosine kinase inhibitor, on platelet functions. Genistein attenuates thrombin-induced Ca2+ mobilization in human platelets by affecting polyphosphoinositide turnover

Genistein, a tyrosine kinase inhibitor, had no or only slight inhibitory effects on platelet aggregation or serotonin release induced by thrombin, while intracellular Ca2+ concentration ([Ca2+]i) elevation was substantially attenuated. It also inhibited the cyclooxygenase pathway, but this effect was not directly related to the suppressive effect of genistein on [Ca2+]i elevation. In order to clarify the mechanism by which genistein suppresses Ca2+ mobilization, its effect was examined on inositol phospholipid metabolism. The production of inositol-1,4,5-trisphosphate was inhibited by genistein in a dose-dependent manner, while 47 kDa protein phosphorylation or phosphatidic acid formation was not affected, suggesting that genistein does not inhibit phospholipase C activity. Pretreatment of unstimulated platelets with genistein increased the amount of phosphatidylinositol-4-monophosphate [PI(4)P], while that of phosphatidylinositol-4,5-bisphosphate [PI(4,5)P2] was reduced. Thrombin stimulation of genistein-pretreated cells intensified this tendency, i.e. a further increase in the amount of PI(4)P and a decrease in the amount of PI(4,5)P2 in an inversely proportional manner. Taken together, these findings imply that genistein acted at the step of PI(4)P 5-kinase which produces PI(4,5)P2 from PI(4)P. Protein tyrosine phosphorylation induced by thrombin was not affected by genistein, suggesting that the inhibitory effect of genistein on polyphosphoinositides was unrelated to tyrosine kinase inhibition.

Anti-CD9 monoclonal antibody elicits staurosporine inhibitable phosphatidylinositol 4,5-bisphosphate hydrolysis, phosphatidylinositol 3,4-bisphosphate synthesis, and protein-tyrosine phosphorylation in human platelets

Phosphoinositide metabolism elicited by anti-CD9 monoclonal antibody, a well-characterized platelet activator, was studied using acetylsalicylic acid-treated human platelets. TP82, which is an anti-CD9 monoclonal antibody, induced classical phosphatidylinositol 4,5-bisphosphate hydrolysis, as monitored by intracellular Ca2+ mobilization and phosphatidic acid production, and synthesis of phosphatidylinositol 3,4-bisphosphate, which is a major component of newly-described 3-phosphorylated inositol phospholipids produced during platelet activation. These changes were severely inhibited by 1 microM staurosporine, a potent, though non-selective, protein kinase inhibitor, which also abolished TP82 induction of tyrosine phosphorylation of multiple platelet proteins. Protein-tyrosine phosphorylation appears necessary to initiate both the classical phosphoinositide turnover and synthesis of the newly-described 3-phosphorylated inositol phospholipids in anti-CD9 monoclonal antibody-induced platelet activation.

Effects of dilazep on platelet functions and volume change in patients with diabetes mellitus

Dilazep (CAS 35898-87-4), a vasodilating agent, was administered to patients with diabetes mellitus (300 mg/day for a minimum of two weeks). Various parameters of platelet function were evaluated both pre- and post-therapy. While dilazep affected neither aggregation nor intracellular Ca++ mobilization, it had a tendency to attenuate arachidonic acid metabolism induced by 0.1 U/ml thrombin (p = 0.1). The rate of platelet volume increase induced by weak acid loading was significantly higher post-therapy than pre-therapy. Since platelet volume change is generally suppressed in diabetic patients, dilazep administration appeared to normalize volume change induced by acid loading. It is implicated that dilazep reduced the membrane rigidity of platelets from diabetic patients, thereby allowing easy expansion of platelet volume.

Protein kinase C inhibitors suppress disc-sphere changes of human platelets, as assessed with the shape-change parameter

Changes in the shape of human platelets and the biochemical mechanism responsible were evaluated, using the shape-change parameter. Neither the Na+/H+ exchanger, nor intracellular or extracellular calcium ions (Ca2+) affected disc-sphere changes induced by low doses of thrombin. Treatment with inhibitors of Ca2+ mobilization, calmodulin or mysoin light-chain kinase had no significant effect on the shape change of platelets. Staurosporine and H-7, both of which are inhibitors of protein kinase C, inhibited disc-sphere changes at low concentrations. Moreover, calphostin C, a specific inhibitor of protein kinase C, effectively inhibited thrombin-induced shape change, as assessed by the shape-change parameter, in a dose-dependent manner. 1-Oleoyl-2-acetyl-glycerol and 1,2-dioctanoyl-glycerol, which are synthetic protein kinase C activators, induced shape changes similar to those induced by thrombin. A decrease in the surface area of platelet images on scanning electron micrographs was used to quantify the disc-sphere transformation. The mean platelet areas was significantly decreased after stimulation with thrombin or 1-oleoyl-2-acetyl-glycerol. Pretreatment with H-7 inhibited the thrombin-induced disc-sphere change, as assessed by changes in the platelet surface area. Our results obtained with various inhibitors suggest that thrombin-induced platelet change, as assessed by the shape-change parameter, are associated with activation of protein kinase C.

Low concentrations of sodium fluoride inhibit Ca2+ influx induced by receptor-mediated platelet activation

Sodium fluoride (NaF) alone below the concentration of 10 mM had no effect on platelet intracellular Ca2+ ([Ca2+]i). When platelets were incubated with low concentrations of NaF (< 10 mM) prior to thrombin stimulation, the second phase of [Ca2+]i elevation which is attributable to Ca2+ influx was suppressed, while the initial rapid peak of [Ca2+]i which is attributable to internal Ca2+ release was unaffected. Ca2+ influx assessed by the addition of extracellular Ca2+ to cells preactivated by thrombin in the absence of extracellular Ca2+ was also inhibited by NaF in a dose-dependent manner. NaF was also effective in inhibiting thrombin- or U-46619-induced Mn2+ entry. This inhibitory effect of NaF on Ca2+ influx occurred after a lag of at least 30 s. However, Ca2+ influx induced by ionomycin-induced Ca2+ depletion or by thapsigargin, a Ca(2+)-ATPase inhibitor, was only partially suppressed by NaF treatment. It is suggested that Ca2+ entry induced by receptor-mediated activation is NaF-sensitive and that the depletion of Ca2+ storage sites by artificial procedures facilitates the opening of Ca2+ channels via NaF-insensitive pathways.

Wheat germ agglutinin-induced intracellular calcium mobilization in human platelets: suppression by staurosporine and resistance to cyclic AMP inhibition

The lectin wheat germ agglutinin (WGA) elicited a prompt and sharp increase in intracellular Ca2+ concentration in human platelets. The WGA-induced Ca2+ mobilization was markedly inhibited by a protein kinase inhibitor staurosporine, whereas Ca2+ mobilization by receptor-mediated agonists, including thrombin, platelet-activating factor, and arginine-vasopressin, was not. In contrast, the lectin-induced Ca2+ mobilization was resistant to cyclic AMP inhibition, compared with that induced by receptor-mediated agonists. These findings indicate that the mechanism of intracellular Ca2+ mobilization, or possibly phospholipase C activation, induced by WGA is different from that induced by receptor-mediated agonists in human platelets.

Effects of oral hypoglycaemic agents on platelet functions

The in vitro effects of three oral hypoglycaemic agents, gliclazide (1-(4-methylbenzensulfonyl)-3-[3-azabicylo(3,3,0)octyl]urea) , glibenclamide (1-[4-[2-(chloro-2-methoxybenzamide)-ethyl]-phenyl- sulfonyl]-3-cyclohexyl-urea) and glimepiride (1-[4-[2-(3-ethyl-4-methyl-2-oxo-3-pyrroline-carboxamide)- ethyl]-phenylsulphonyl]3-(4-methylcyclohexyl)-urea), on functions of human platelets were evaluated. None of these agents up to a concentration of 40 microM inhibited platelet aggregation induced by thrombin. Glibenclamide and glimepiride in the range of 20-40 microM suppressed Ca2+ release from internal Ca2+ stores induced by thrombin. Gliclazide showed no effect on arachidonic acid metabolism of human platelets. Glimepiride selectively inhibited the cyclooxygenase pathway, while the activities of 12-lipoxygenase and phospholipase A2 were unaffected. Glibenclamide inhibited both the cyclooxygenase and 12-lipoxygenase pathways. It also attenuated arachidonic acid release from phospholipase A2. Oral hypoglycaemic agents with inhibitory effects on arachidonic acid metabolism may prove useful for the treatment of diabetic patients with enhanced platelet functions.

Synthesis of phosphatidylinositol 3,4-bisphosphate but not phosphatidylinositol 3,4,5-trisphosphate is closely correlated with protein-tyrosine phosphorylation in thrombin-activated human platelets

Synthesis of D-3-phosphorylated phosphoinositides and its correlation with protein-tyrosine phosphorylation were examined using human platelets. Thrombin stimulation of platelets resulted in time- and dose-dependent production of phosphatidylinositol 3,4-bisphosphate (PtdIns(3,4)P2), which is absent from resting platelets. In contrast, phosphatidylinositol 3,4,5-trisphosphate (PtdIns(3,4,5)P3) was detected in resting platelets, but remained unaffected by thrombin treatment. The production of PtdIns(3,4)P2 but not PtdIns(3,4,5)P3 was inhibited by pretreatment with staurosporine or dibutyryl cyclic adenosine monophosphate (dbcAMP). Protein-tyrosine phosphorylation, which is reportedly involved in generation of 3-phosphorylated phosphoinositides, was elicited in thrombin-activated platelets. The tyrosine phosphorylation was suppressed by pretreatment with staurosporine or dbcAMP. These observations suggest that synthesis of PtdIns(3,4)P2 but not PtdIns(3,4,5) P3 is closely correlated with protein-tyrosine phosphorylation in human platelets.

Suppression by wortmannin of platelet responses to stimuli due to inhibition of pleckstrin phosphorylation

Studies were made of inhibition by wortmannin, a fungal metabolite, of human platelet responses to various stimuli. Wortmannin at concentrations as low as 1-100 nM inhibited several receptor-agonist-induced 5-hydroxytryptamine release from platelets, without affecting agonist-induced increases in the intracellular concentration of Ca2+. Phorbol 12-myristate 13-acetate (PMA), an active tumour promoter, caused 5-hydroxytryptamine release when combined with a low concentration of ionomycin, and platelet aggregation by itself; these effects of the phorbol ester were also inhibited by wortmannin as well as by staurosporine, a potent, although non-specific, protein kinase C (PKC) inhibitor, in a similar molar concentration range. The platelet responses to the receptor agonists or PMA were accompanied by increased incorporation of [32P]Pi into pleckstrin, a protein selectively expressed in platelets and other blood cells arising from haematopoietic stem cells, as a result of PKC activation in the intact cells. The pleckstrin phosphorylation was inhibited by wortmannin in ways mostly similar to those in which it inhibited the 5-hydroxytryptamine-release responses. Nevertheless, wortmannin failed to inhibit PKC activity measurable in a cell-free assay system which is highly susceptible to staurosporine. Nor did it inhibit the translocation of cytosolic PKC to membranes induced by addition of PMA to platelet cells. Thus wortmannin, which is not a direct inhibitor of PKC, could interfere with the kinase-dependent phosphorylation of pleckstrin, which may play an important role in the cellular responses to receptor stimulation.