Insufficient mobilization of calcium by early breakdown of phosphatidylinositol 4,5-bisphosphate for aggregation of human platelets by collagen

Cyclic nucleotides inhibit MAP kinase activity in low-dose collagen-stimulated platelets

Collagen-induced platelet activation is a complex process involving multiple signaling pathways. The role(s) of MAP kinases (ERKs and p38(MAPK)) are unclear, although at high, but not low, collagen concentrations p38(MAPK) is involved in cPLA(2)-mediated arachidonic acid release, prior to thromboxane generation. Cyclic nucleotides are conventionally regarded as mediators of platelet inhibition. However recent studies suggested a role for cGMP early in a MAP kinase pathway in platelet activation. In the current study the roles and relationships of MAP kinases, cyclic nucleotides and cPLA(2) in platelet activation by low-dose collagen and a thromboxane analogue (U46619) have been evaluated. Stimulants of neither adenylate cyclase (PGI(2)) nor guanylate cyclase (NaNP) alone had any effect on the basal phosphorylation of either MAP kinase. PGI(2) inhibited ERK/p38(MAPK) phosphorylation in response to both agonists which was unaffected by a cPLA(2) inhibitor (AACOCF(3)). NaNP inhibited collagen-induced ERK/p38(MAPK) phosphorylation, which was enhanced by AACOCF(3) and reversed by a guanylate cyclase inhibitor (ODQ). However NaNP had no effect on U46619-induced p38(MAPK) phosphorylation. Thus adenylate cyclase activation inhibits low-dose collagen-induced MAP kinase phosphorylation both prior, and distal, to thromboxane release. The study also supports an inhibitory, rather than stimulatory, role for guanylate cyclase in platelet signaling.

Inhibition of the MEK/ERK pathway has no effect on agonist-induced aggregation of human platelets

The activation of human platelets by a variety of agonists is accompanied by the phosphorylation of the extracellular signal-regulated kinase (ERK) isoforms of mitogen-activated protein (MAP) kinases. However, the role(s) of, and the substrate(s) for, these enzymes in platelet function remain unclear. Studies on ERKs in platelets have relied on pharmacological tools, including an inhibitor of ERK activation, U0126 [1,4-diamino-2,3-dicyano-1,4-bis(2-aminophenylthio)butadiene]. In the present study, the effects of U0126 and its "inactive" analogue, U0125 [1,4-diamino-2,3-dicyano-1,4-bis(phenylthio)butadiene], on human platelet aggregation and MAP kinase activity were examined. Several agonists with a variety of signaling pathways were studied including thrombin, a thromboxane analogue, arachidonic acid, collagen, calcium ionophores, and the phorbol ester phorbol myristate acetate (PMA). U0126, at concentrations consistent with inhibition of the isolated enzyme, inhibited ERK phosphorylation, and therefore MEK activation, in response to each agonist. Under such conditions, U0126 did not affect the phosphorylation of a second MAP kinase, p38(MAPK); however, platelet aggregation was also unaffected. Higher concentrations of U0126, and of U0125, inhibited platelet aggregation in response to collagen and PMA with no effect on that induced by the other agonists. These results dissociate ERK activation from platelet aggregation, suggesting an alternative role for ERKs in platelet function. In addition, the effects of higher concentrations of U0126 are likely due to an action on protein kinase C, likely unrelated to ERK inhibition, suggesting that the inhibitor concentration is crucial to the interpretation of such studies.

Platelet lysis and functional perturbation by 13-methyl myristate. The major fatty acid in Flavobacterium ranacida

Flavobacterium ranacida consisted of 75% of 13-methyl myristate in total fatty acids. The acid at > 60 microM caused the lysis of gel-filtered platelets (GFP) in both time- and concentration-dependent manners. Scanning electron microscopy showed that: 1). GFP in 40 microM of the acid changed the morphology to speculate discoid shape at 15 sec, and to ellipsoids after 30 sec; and 2), the cells gradually swelled to spherical forms as the concentration of the acid increased. At nonlytic concentration, the acid inhibited platelet responses to various agonists with differential concentrations. The order of inhibitory potency was U46619 > low dose collagen > ADP-fibrinogen > phorbol ester > high dose collagen. The results demonstrated that 13-methyl myristate exhibited both cell lytic activity and perturbation on membrane function.

Effect of non-saponin fraction from Panax ginseng on cGMP and thromboxane A2 in human platelet aggregation

The non-saponin fraction (NSF; lipophilic fraction) from the roots of Panax ginseng inhibited the aggregation of human platelets induced by thrombin (0.1 units/ml) in a dose-dependent manner. NSF induced the elevation of cGMP concentration in human platelets in a similar manner to molsidomine, a known vasodilator. NSF also inhibited Ca(2+)-influx into platelets. While verapamil, a Ca(2+)-antagonist, increased the cAMP level in platelets stimulated by thrombin, NSF had little effect on cAMP formation. Instead, NSF potently inhibited the thromboxane A2 (TXA2) production. The results suggest that NSF may regulate the levels of cGMP and TXA2 to inhibit platelet aggregation induced by thrombin.

Platelet shape change and Ca2+ mobilization induced by collagen, but not thrombin or ADP, are inhibited by phenylarsine oxide

In this report we have examined the effects of the protein tyrosine phosphatase inhibitor phenylarsine oxide (PAO) on receptor-mediated platelet shape change, secretion and aggregation. PAO was found to inhibit platelet aggregation induced by collagen, thrombin, ADP and epinephrine at IC50 values of 0.35 mumol/l, 2.5 mumol/l, 0.2 mumol/l and 0.3 mumol/l, respectively. Agonist-induced secretion of ATP was inhibited at similar or lower concentrations of PAO. The specificity of the interaction of PAO with platelet proteins was demonstrated by the ability of the disulfhydryl compound 2,3-dimercaptopropanol, which abstracts PAO from proteins to form a stable cyclic adduct, to reverse PAO inhibition of both agonist-induced platelet secretion and aggregation. Dimercaptopropanesulphonic acid, a membrane-impermeable analogue of dimercaptopropanol, did not reverse inhibition of collagen-induced shape change or aggregation by PAO, thereby demonstrating that PAO acted intracellularly. PAO inhibited collagen-induced shape change and internal Ca2+ mobilization but had no effect on these two phenomena when induced by thrombin or ADP. PAO was also unable to prevent arachidonic acid-induced shape change, indicating that PAO acts at a site prior to the phospholipase A2-mediated release of arachidonic acid to inhibit collagen-induced shape change. PAO induced the accumulation of a number of phosphotyrosine-containing proteins and inhibited the collagen-induced phosphorylation of a 40 kD protein. The potency and agonist-specific effects of PAO on platelet activation suggest that this inhibitor will be of value in elucidation of signal transduction pathways involved in receptor-mediated platelet function.

Eicosanoid/thromboxane A2-independent and -dependent generation of lysoplasmenylethanolamine via phospholipase A2 in collagen-stimulated human platelets

Collagen-induced human platelet stimulation is dependent on the release of arachidonic acid (AA) from membrane phospholipid and the formation of thromboxane A2 (TxA2) for TxA2-induced platelet activation. Since plasmenylethanolamine represents the single major phospholipid reservoir of AA in resting human platelets, we assessed its hydrolysis via phospholipase A2 upon platelet stimulation with low levels of collagen by determining the generation of [3H]lysoplasmenylethanolamine via eicosanoid/TxA2-independent and -dependent processes. Ethanolamine phospholipids in platelets were prelabelled with [3H]ethanolamine before stimulation with either collagen or the TxA2 mimetic U46619, in the presence or absence of BW755C, a dual inhibitor of the cyclooxygenase/lipoxygenase activities, or GR32191B, a TxA2-receptor antagonist. Collagen stimulation promoted a marked generation of [3H]lysoplasmenylethanolamine, which was only moderately decreased when TxA2 synthesis or TxA2 receptors were blocked by BW755C or GR32191B respectively. The moderate rise in [3H]lysoplasmenylethanolamine formation with U46619 as the agonist was only slightly affected by BW755C and blocked by GR32191B. Evidence for eicosanoid/TxA2-independent and -dependent generation of [3H]lysophosphatidylethanolamine was also obtained. A significant quantitative loss of AA from plasmenylethanolamine was also demonstrated in collagen-stimulated platelets. The present findings indicate the activation of plasmenylethanolamine cleavage via phospholipase A2 in collagen-stimulated human platelets, which, to a considerable extent, does not depend on eicosanoid/TxA2 synthesis. This may represent an important source of releasable AA for TxA2 generation and the promotion of further liberation of AA and phospholipid-mediated signalling pathways.

BW755C or staurosporine inhibits collagen-stimulated phosphoinositide phosphorylation in platelets

Stimulation of platelets by collagen results in increased formation of the polyphosphoinositides, phosphatidylinositol phosphate (PtdInsP) and phosphatidylinositol bisphosphate (PtdInsP2) through stimulation of phosphoinositide kinase activities. We investigated a possible regulatory role of endogenous thromboxane formation and protein kinase C (PKC) activation in the induction of phosphoinositide phosphorylation following collagen stimulation, as well as following stimulation by the thromboxane mimetic, U-46619. Human platelets were prelabeled with [3H]inositol and stimulated with collagen (2 micrograms/mL) or U-46619 (1 microM), in the absence or presence of either the cyclo-oxygenase/lipoxygenase inhibitor, BW755C, or staurosporine, a putative inhibitor or PKC. Collagen stimulation resulted in a time-dependent increase in [3H]inositol-labeled PtdInsP and PtdInsP2 which was completely inhibited in the presence of BW755C. Addition of U-46619 to BW755C-treated, collagen-stimulated platelets restored the increased polyphosphoinositide formation. Stimulation of platelets with U-46619 alone also resulted in increased formation of [3H]PtdInsP and [3H]PtdInsP2, but this was not affected by the presence of BW755C. These results suggest that the collagen-induced activation of phosphoinositide kinases was dependent upon thromboxane formation, but that U-46619-induced phosphoinositide formation was rather independent of further thromboxane production. Pretreatment of platelets with staurosporine, prior to agonist addition, completely blocked the collagen-stimulated rise in radiolabeled PtdInsP and the U-46619-induced PtdInsP and PtdInsP2 generations, suggesting that protein kinase, possibly PKC, may play a role in the activation of phosphoinositide kinases by these agonists.

d-Indobufen inhibits collagen-induced intracellular calcium mobilization and inositol phosphates formation in human platelets

We obtained evidence that the cyclooxygenase inhibitor d-indobufen, (+)2[p-(1-oxo-2-isoindolinyl)phenyl] buthylic acid has a potent and specific inhibitory effect on collagen-induced aggregation and 40K-protein phosphorylation (Mamiya, S., Hagiwara, M., Ishikawa, T., and Hidaka, H. Thromb. Res. 54, 447, 1989). In Fura-2 loaded platelets, d-indobufen inhibited collagen-induced intracellular calcium mobilization in a dose dependent manner and this inhibitory effect on calcium mobilization paralleled that on aggregation, either in the presence or absence of extracellular free calcium ions. This compound inhibited inositol phosphates (IPs) formation in collagen-stimulated platelets. In arachidonic acid-stimulated platelets, d-indobufen caused a lag on calcium mobilization, as observed with arachidonic acid-induced aggregation. There was no significant effect on thrombin- or A23187-induced calcium mobilization or on aggregation. These observations suggest that the collagen receptor couples to a distinct intracellular calcium mobilization system possibly via inositol phospholipid metabolism and that d-indobufen blocks the collagen-induced aggregation by arresting mobilization of intracellular calcium.

Effect of albumin-bound DHA on phosphoinositide phosphorylation in collagen stimulated human platelets

The effect of exogenous albumin-bound docosahexaenoic acid (22:6n-3) (DHA), arachidonic acid (20:4n-6) (AA), and eicosapendaenoic acid (20:5n-3) (EPA) on phosphoinositide metabolism following collagen stimulation was studied using [3H]inositol prelabelled platelets. Collagen stimulation (3 min, 1.8 micrograms/ml) increased the labelling of both phosphatidylinositol 4-monophosphate (PIP), and phosphatidylinositol 4,5-biphosphate (PIP2). Of the fatty acids tested, only pre-incubation (2 min) with DHA (20 microM) significantly attenuated the collagen-induced increased PIP and PIP2 labelling; EPA was without effect, while AA enhanced PIP labelling. Forty microM DHA was less effective at attenuating the increased PIP and PIP2 labelling even though this concentration of DHA resulted in greater inhibition of platelet aggregation. Neither concentration of DHA attenuated the increased polyphosphoinositide labelling resulting from stimulation by the endoperoxide analogue U46619, or the phorbol ester, PMA. These data suggest that the effect of DHA on attenuating the increased PIP and PIP2 labelling following collagen stimulation likely occurs before thromboxane receptor occupancy, may not occur at the level of protein kinase C activation, and could be mediated in part via a lessened synthesis of thromboxane A2.

Phorbol ester stimulates platelet spreading and thrombi-like aggregate formation on the surface of immobilized type V collagen

We have studied the effect of the tumor-promoting phorbol ester, 4 beta-phorbol-12 beta-myristate-13 a-acetate (PMA), and of the stable prostaglandin endoperoxide analogue U46619 on the interaction of human blood platelets with surfaces coated with monomeric human type V collagen (CV) and on free calcium concentration in platelet cytoplasm. It was shown by scanning electron microscopy that native resting platelets sparingly attach to CV and fail to spread or aggregate on the collagenous substrate in the absence of PMA and U46619. Addition of 0.15-1.5 nM PMA or 1.5 microM U46619 stimulates platelet spreading and formation of multilayer (thrombi-like) platelet aggregates on the per se non-thrombogenic type V collagen substrate. It was further demonstrated using the fluorescent indicator quin2 that U46619 (0.1 microM) increases cytoplasmic free calcium concentration from basal level (100-120 nM) up to 600 nM, whereas PMA (0.75-15 nM) exerts only a minor effect, increasing free calcium level by 30-40 nM. These results indicate that the tumor-promoting phorbol ester PMA induces massive platelet spreading and aggregation on surfaces coated with non-thrombogenic type V collagen via activation of protein kinase C with little or no apparent change in free cytoplasmic calcium.

Increased Ca2+ influx into platelets induced by thromboxane A2 analog in patients with ischemic heart disease

We compared platelet aggregation and intracellular free calcium concentrations [( Ca2+]i) following stimulation with STA2, an analog of thromboxane A2 between patients with ischemic heart disease (IHD) showing significant stenosis in coronary angiograms and controls. In the presence of extracellular calcium, platelet aggregation and intracellular Ca2+ increase were enhanced by STA2 stimulation in a dose-dependent fashion and were higher in the IHD patients than in the controls. However, in the absence of extracellular calcium, no difference in intracellular Ca2+ increase was observed due to its total dependence on release from intracellular stores. These results suggest that thromboxane A2 increases platelet intracellular Ca2+-inducing aggregation, and this increase and aggregation, which is enhanced by thromboxane A2 in IHD patients, is due to promotion of Ca2+ influx by thromboxane A2. IHD patients appear to have an abnormality in the platelet membrane which may explain such thromboxane A2-dependent increased permeability to Ca2+.

Effect of phorbol 12-myristate 13-acetate on collagen-induced signal transduction in rabbit platelet

Investigations were made on the inhibitory effect of phorbol 12-myristate 13-acetate (PMA), a powerful activator on protein kinase C, on collagen-induced signal transduction in washed rabbit platelets. Upon activation of the platelets with a low-dose of collagen (5 micrograms/ml), which was suppressed by 10 microM indomethacin, pretreatment of the platelets with 2 nM PMA caused prolongation of lag phase (2 min) before the onsets of the aggregation and ATP secretion as compared with PMA-untreated platelets (30 sec). Under this condition, appearance of the cell responses including the phosphatidic acid formation, thromboxane (Tx) generation and Ca2+-influx was similarly retarded for 2-3 min, whereas arachidonic acid liberation from the membrane phospholipids was not significantly affected by the PMA pretreatment. After such lag phase, every response appeared rapidly and reached almost the control value (without PMA). Upon activation of the same platelets with a high-dose of collagen (50 micrograms/ml), which was only half suppressible by indomethacin, PMA in the presence of indomethacin almost completely suppressed the phosphatidic acid formation as well as the aggregation and ATP secretion. Thus, our results suggest that collagen-platelet interaction may elicit direct activation of phospholipase A2 and C, and that the latter enzyme activation may be regulated by a negative effect of protein kinase C. However, the phospholipase A2 activation may be regulated by a mechanism independent of such effect. In PMA-pretreated platelets in response to a low-dose of collagen, the prolonged lag phase for aggregation appears to be due to impaired conversion of liberated arachidonic acid to TxA2.

Differential effects of 15-HPETE on arachidonic acid metabolism in collagen-stimulated human platelets

The 15-hydroperoxyeicosatetraenoic acid (15-HPETE) has been shown to affect platelet aggregation induced by collagen, arachidonic acid (AA), and PGH2-analogue. Furthermore, it also inhibits the platelet cyclooxygenase and lipoxygenase enzymes, and prostacyclin synthase. The present study was designed to test the effect of 15-HPETE on the mobilization of endogenous AA in collagen-stimulated human platelets. For this purpose, human platelets pretreated with BW755C (a dual inhibitor of cyclooxygenase and lipoxygenase) were stimulated with collagen in the presence of varied concentrations of 15-HPETE. We observed a significant inhibition of oxygenases at all concentrations of 15-HPETE. In contrast, our results indicate that 15-HPETE at lower concentrations (10 microM and 30 microM) significantly stimulated the collagen-induced release of AA from phospholipid sources. Although higher concentrations of 15-HPETE (50 microM and 100 microM) caused some inhibition of AA accumulation in the free fatty acid fraction (25% and 60%), the degree of inhibition was significantly lower than the inhibition observed for the oxygenases (65% and 88% for cyclooxygenase and 77% and 94% for lipoxygenase respectively). These results provide support that hydroperoxides also regulate phospholipases presumably by a different mechanism, which may be important in the detoxification of phospholipid peroxides.

Two phasic generation of thromboxane A2 by the action of collagen on rat platelets

Two-phase thromboxane A2 (TXA2) generation was observed in washed rat platelets stimulated by collagen. One coincided with shape change and the other with aggregation, and both reactions were inhibited by indomethacin. The collagen-induced first phase was not affected by treatment of the platelets with TXA2 receptor antagonist BM13177, while the shape change, the second phase TXA2 generation and aggregation were completely suppressed. U46619, a stable TXA2 mimetic, induced only platelet shape change and not aggregation or endogenous icosanoid mobilization. However, upon addition of U46619 to platelets previously stimulated by collagen in the presence of indomethacin, icosanoids formation was induced together with aggregation. These results suggest that both the collagen-induced initial TXA2 and the occupation of the receptor by collagen might be a trigger for the second-phase TXA2 formation in concert with platelet aggregation.

Quantitative analysis of polyenoic phospholipid molecular species by high performance liquid chromatography

The quantitative analysis of phospholipid molecular species containing polyenoic fatty acids is described. Dinitrobenzoyl derivatives of diacylglycerols prepared from phospholipids were separated into individual molecular species by reversed-phase high performance liquid chromatography (HPLC) using a combination of two solvent systems and were quantified at 254 nm. Thirty-six molecular species were resolved from the phosphatidylcholines of rat hearts, human platelets and Chinese hamster V79-R cells. The derivatives of alkenylacyl molecular species from platelet phosphatidylethanolamine were resolved concomitantly with diacyl molecular species.