Inositol 1,4,5-trisphosphate (IP3) induced rapid formation of thromboxane B2 in saponin-permeabilised human platelets: mechanism of IP3 action

Evidence that inositol 1,4,5-trisphosphate 3-kinase and inositol 1,3,4,5-tetrakisphosphate are negative regulators of platelet function

Background

Inositol 1,3,4,5-tetrakisphosphate (IP4) is formed from inositol 1,4,5-trisphosphate (IP3) by IP3 3-kinase (ITPK) in most cells. Its function is unknown but has been suggested to be involved in Ca2+ entry, IP3 regulation, and phosphoinositide 3-kinase antagonism.

Objectives

To better elucidate a function for IP4, we tested a specific inhibitor of ITPK (GNF362) on platelets, the effects of IP4 directly in permeabilized platelets and its effect on phosphatidylinositol 3,4,5-trisphosphate (PIP3) binding to pleckstrin-homology (PH) domain-containing proteins in platelets.

Methods

Human platelets were utilized in whole blood for thrombus formation, in platelet-rich plasma and washed suspensions for aggregation, and for Ca2+ studies, or resuspended in high K+ and low Na+ buffers for permeabilization experiments. Phosphorylation of AKT-Ser473 and Rap1-GTP formation were measured by Western blotting and PIP3 binding using PIP3 beads.

Results

GNF362-enhanced platelet aggregation stimulated by low concentrations of ADP, collagen, thrombin, U46619, and thrombus formation in collagen-coated capillaries. GNF362 induced a transient elevation of Ca2+ concentration, elevated basal levels of IP3, and enhanced the peak height of Ca2+ elevated by agonists. In permeabilized platelets, IP4 inhibited GTPγS induced formation of AKT-Ser473 phosphorylation and platelet aggregation. IP4 reduced GTPγS-stimulated Rap1-GTP levels and potently reduced extraction of RASA3 and BTK by PIP3 beads.

Conclusion

ITPK and IP4 are negative regulators of platelet function. IP4 regulation of PH domain-containing proteins may represent a pathway by which platelet activation may be controlled during thrombosis.

Phosphatidylinositol-3,4,5-trisphosphate stimulates Ca(2+) elevation and Akt phosphorylation to constitute a major mechanism of thromboxane A2 formation in human platelets

Phosphatidylinositol trisphosphate (PIP3) has been implicated in many platelet functions however many of the mechanisms need clarification. We have used cell permeable analogues of PIP3,1-O-(1,2-di-palmitoyl-sn-glyero-3-O-phosphoryl)-D-myo-inositol-3,4,5-trisphosphate (DiC16-PIP3) or 1-O-(1,2-di-octanoyl-sn-glyero-3-O-phosphoryl)-D-myo-inositol-3,4,5-trisphosphate (DiC8-PIP3) to study their effects on activation on washed human platelets. Addition of either DiC8- or DiC16-PIP3 to human platelets induced aggregation in the presence of extracellular Ca(2+). This was reduced by the presence of indomethacin, the phospholipase C inhibitor U73122 and apyrase. DiC8-PIP3 induced the phosphorylation of Akt-Ser(473) which was reduced by the Akt inhibitor IV, wortmannin and EGTA (suggesting a dependence on Ca(2+) entry). In Fura2 loaded platelets DiC8-PIP3 was effective at increasing intracellular Ca(2+) in a distinct and transient manner that was reduced in the presence of indomethacin, U73122 and 2-aminoethyl diphenylborinate (2APB). Ca(2+) elevation was reduced by the non-SOCE inhibitor LOE908 and also by the SOCE inhibitor BTP2. DiC8-PIP3 induced the release of Ca(2+) from stores which was not affected by the proton dissipating agent bafilomycin A1 and was more potent than the two-pore channel agonist DiC8-PI[3,5]P2 suggesting release from an endoplasmic reticulum type store. DiC8-PIP3 weakly induced the tyrosine phosphorylation of Syk but not of PLCγ2. Finally like thrombin DiC8-PIP3 induced the formation of thromboxane B2 that was inhibited by the Akt inhibitor IV. These studies suggest that PIP3 via Ca(2+) elevation and Akt phosphorylation forms a central role in thromboxane A2 formation and the amplification of platelet activation.

Platelet prostanoid receptors

Prostaglandins (PGs) and thromboxanes are important modulators of platelet activation, and there is strong evidence to support the existence of distinct thromboxane, prostacyclin, PGD2 and PGE2 receptors on the platelet plasma membrane. In this review, each of these platelet prostanoid receptors is discussed in detail, with respect to their receptor pharmacology, molecular biology and signal transduction, and as to any therapeutic implications of the development of specific agonists and/or antagonists. In addition, it considers the possibility that there are separate vascular receptors for 8-epi PGF2 alpha, which are not present on the platelet.

Aspects of the thromboxane receptor system

1. The aim of this review is to establish what is known about the thromboxane (TP) receptor, and to identify where future research is headed. In addition, the impact of the recent advances at the molecular level on resolving pharmacological controversies, such as possible subtypes of the TP receptor, is discussed and what molecular information is known about the TP receptor presented. 2. The clinical status of TP receptor antagonists is considered particularly in relation to the potential role of epi prostaglandins. 3. Basic information about TP agonists, antagonists and signal transduction pathways is also given.

Receptor occupancy regulates Ca2+ entry and intracellular Ca2+ redistribution in activated human platelets

Fura-2-loaded human platelets were used to study Ca2+ release from intracellular compartments, as well as Ca2+ influx from the extracellular space. We investigated the response towards the endoperoxide/thromboxane-receptor agonist. U46619, and the inhibitor of the endoplasmic-reticulum Ca(2+)-ATPase, thapsigargin. U46619 dose-dependently depleted intracellular Ca2+ stores, followed by active sequestration of released Ca2+. Ca2+ influx induced by U46619 largely relies on receptor occupancy. Removing the thromboxane analogue from its receptor by using the endoperoxide/thromboxane-receptor antagonist BM 13177 largely blunted U46619-mediated Ca2+ influx. The Ca(2+)-ATPase inhibitor thapsigargin evoked a gradual rise in intracellular Ca2+, which was potentiated by a preceding activation of platelets with the receptor agonist U46619. This agonist-sensitizing effect also depends on receptor occupancy. Removing U46619 from its receptor by addition of the endoperoxide/thromboxane-receptor antagonist BM13177 suppressed the sensitizing effect completely. Furthermore, interrupting downstream receptor signalling events by raising intracellular levels of cyclic nucleotides (cyclic AMP, cyclic GMP) again suppressed the U46619-sensitizing effect on thapsigargin-induced Ca2+ release. This study indicates that the process of Ca2+ release followed by resequestration in response to a platelet agonist by its own is not sufficient to produce the sensitizing effect. Rather, a continuously occupied receptor triggering sustained downstream signalling events seems to be required for sensitization. The presence of a receptor agonist may induce an increased cycling of Ca2+ between the agonist-responsive and the thapsigargin-dischargeable compartment, leading to faster and more intense accumulation of Ca2+ in the cytosolic compartment after inhibition of the Ca(2+) ATPase. Suggestively, receptor occupancy increases the Ca(2+)-releasing potency of thapsigargin by coupling the thapsigargin-sensitive Ca(2+)-storing compartments with an agonist-responsive compartment that exhibits a high leakage rate in stimulated platelets.

Preventive effect of nicardipine on hyperplastic changes in venous bypass grafts

Nicardipine is a relatively new calcium channel blocker with important properties that could result in attenuation of the adverse proliferative changes in autogenous vein bypass grafts. In this experimental, randomized, controlled study, the effect of nicardipine on the pathologic findings in aortoaortic bypass graft was assessed. Forty-two male rabbits (Orycytolagus cuniculus) were randomized to three groups: group 1 received nicardipine and groups 2 and 3 placebo for 4 weeks, after which an aortaortic bypass was realized with an autogenous inferior vena cava segment. During the following 4 weeks, groups 1 and 2 received nicardipine, and placebo was continued in group 3. The animals were sacrificed at the end of the study to permit removal and evaluation of the bypass grafts. The mean intimal and medial thickness values for groups 1 and 2 were lower than those for group 3, indicating that nicardipine has a significant preventive effect on the hyperplastic changes in venous bypass grafts compared to placebo. The mean intimal and medial thickness values of group 1 were also lower than those of group 2, and the differences carried statistical relevance, suggesting that the use of nicardipine before grafting could potentiate its protective effect. To provide stimulus for further research, an attempt is made to relate the hyperplasia-preventing effect of nicardipine to possible mechanisms.

Formation and contraction of a microfilamentous shell in saponin-permeabilized platelets

To study the mechanism of granule centralization in platelets, we permeabilized with saponin in either EGTA (5 mM) or calcium (1 or 10 microM). Under all conditions, platelets retained 40-50% of their total actin and greater than 70% of their actin-binding protein (ABP) but lost greater than 80% of talin and myosin to the supernatant. Thin sections of platelets permeabilized in EGTA showed a microfilament network under the residual plasma membrane and throughout the cytoplasm. Platelets permeabilized in calcium contained a microfilament shell partly separated from the residual membrane. The shell stained brightly for F-actin. A less dense microfilament shell was also seen in sections of ADP-stimulated intact platelets subsequently permeabilized in EGTA. In the presence of 1 mM ATP gamma S and calcium, myosin was retained (70%) and was localized by indirect immunofluorescence in bright central spots that also stained intensely for F-actin. Electron micrographs showed centralized granules surrounded by a closely packed mass of microfilaments much like the structures seen in thrombin-stimulated intact platelets subsequently permeabilized in EGTA. Permeabilization in calcium, ATP, and okadaic acid, produced the same configuration of centralized granules and packed microfilaments; myosin was retained and the myosin regulatory light chain became phosphorylated. Microtubule coil disassembly before permeabilization did not inhibit granule centralization. These results suggest a possible mechanism for granule centralization in these models. The cytoskeletal network first separates from some of its connections to the plasma membrane by a calcium-dependent mechanism not involving ABP proteolysis. Phosphorylated myosin interacts with the microfilaments to contract the shell moving the granules to the platelet's center.

Activation of phospholipase C is not correlated to the formation of prostaglandins and superoxide in cultured rat liver macrophages

This study evaluates the role of inositol phosphates as possible mediators of the activation of phospholipase A2 and NADPH oxidase in cultured rat liver macrophages. Inositol phosphate formation was achieved by zymosan, immune complexes, latex particles and calcium ionophore while the release of arachidonic acid and the formation of prostaglandin E2 was also elicited by phorbol ester and NaF, but not by latex particles; generation of superoxide was obtained by zymosan and phorbol ester only. The kinetics of the formation of inositol phosphates revealed that within the first few minutes after zymosan addition inositol trisphosphate was formed, followed by inositol bisphosphate and inositol monophosphate. Pre-treatment of the cells with dexamethasone or removal of extracellular calcium led to an inhibition of the zymosan-induced formation of inositol phosphates and prostaglandin E2 but had no effect on the generation of superoxide; inhibition of the Na+/H+ exchanger by removal of extracellular sodium ions led to a decrease of the zymosan-induced synthesis of prostaglandin E2, but did not affect the formation of inositol phosphates and superoxide. Pre-treatment of the cells with phorbol ester decreased the zymosan-induced synthesis of prostaglandin E2 and superoxide, but even enhanced the zymosan-induced formation of inositol phosphates. These data indicate that in cultured rat liver macrophages the formation of prostaglandins and superoxide cannot be correlated to an activation of phospholipase C.

Electrokinetic, analytical and functional heterogeneity of circulating human platelets: separation of subpopulations by continuous flow electrophoresis after taxol stabilization

A continuous flow electrophoresis procedure has been developed to study platelet subpopulation heterogeneity with separations based upon surface electrical charge differences. Taxol at low concentrations has been used to transiently stabilize the cells during the separations. At a concentration of 10(-5) M taxol has no effect upon a wide range of physical, analytical and enzymatic properties and does not compromise agonist-induced activation responses (aggregation and secretion). A typical normal platelet subpopulation profile extends over 15-20 fractions with mobilities from -0.97 to -0.78 microns per s per volt per cm. Platelet size (resistive particle counter volumes) differed significantly across the profile, the most electronegative cells being the larger, and the least electronegative the smaller platelets. Total platelet sialic acid content and surface neuraminidase-labile sialic acid correlated positively with electronegativity, but the surface -SH group status had an inverse relationship with the least electronegative smaller platelets, having twice as many surface DTNB-titratable - SH groups as the most electrophoretically mobile and larger cells. Normalisation of analytical and enzymatic data to cell volumes revealed that the smaller less electronegative platelets were substantially richer in all constituents and properties than the larger more electronegative platelets. These smaller cells showed higher activities for lysosomal enzymes, and their functions (capacity to transport 5-hydroxytryptamine and adenosine across the plasma membrane and responsiveness to thrombin expressed by synthesis of thromboxane B2 (TXB2) or release of 5HT) were greater than the larger more electronegative cells. No significant differences were observed, however, in the subpopulations by optical aggregometry using six different agonists each at three different concentrations. This free flow electrophoresis separation of platelets, which can be carried out on a preparative scale, may have some advantages over the conventional density gradient separations of subpopulations for investigating clinical states affecting thrombopoietic regulation or platelet losses from the circulation due to vessel wall disease, prosthetic implants or during extracorporeal circuitry.

Inhibitory effect of 8-bromo cyclic GMP on an extracellular Ca2+-dependent arachidonic acid liberation in collagen-stimulated rabbit platelets

The inhibitory effect of cyclic GMP on collagen-induced platelet activation was studied using 8-bromo cyclic GMP (8brcGMP) in washed rabbit platelets. Addition of collagen (1 micrograms/ml) to platelet suspension caused shape change and aggregation associated with thromboxane (TX) A2 formation. 8brcGMP (10-1000 microM) inhibited collagen-induced platelet aggregation and TXA2 formation in a concentration-dependent manner. 8brcGMP did not affect platelet cyclooxygenase pathways, but markedly inhibited collagen-induced arachidonic acid (AA) liberation from membrane phospholipids in [3H]AA-prelabeled platelets, indicating that the inhibitory effect of 8brcGMP on collagen-induced aggregation is due to an inhibition of AA liberation. In [32P]orthophosphate-labeled platelets, collagen stimulated phosphorylation of a 20,000 dalton (20-kD) and 40-kD proteins. 8BrcGMP stimulated phosphorylation of a specific protein having molecular weight of 46-kD and inhibited collagen-induced both 20- and 40-kD protein phosphorylation. Collagen could stimulate the AA liberation without activation of phospholipase C or Na+-H+ exchange, but could not in the absence of extracellular Ca2+. These findings suggest that cyclic GMP inhibits collagen-induced AA liberation which is mediated by an extracellular Ca2+-dependent phospholipase A2. However, cyclic GMP seems to inhibit the Ca2+-activated phospholipase A2 indirectly, since 8brcGMP had no effect on Ca2+ ionophore A23187-induced platelet aggregation or AA liberation. It is therefore suggested that cyclic GMP may regulate collagen-induced increase in an availability of extracellular Ca2+ which is responsible for phospholipase A2 activation in rabbit platelets.

Action of guanosine 5'-[beta-thio]diphosphate on thrombin-induced activation and Ca2+ mobilization in saponin-permeabilized and intact human platelets

The non-hydrolysable guanine analogues guanosine 5'-[gamma-thio]triphosphate (GTP[S]) and guanosine 5'-[beta-thio]diphosphate (GDP[S]) have been used extensively (as promoters and inhibitors respectively) to probe the importance of G-protein function. We report on the use of GDP[S] in permeabilized and intact platelets. The stimulatory analogue GTP[S] (9-60 microM) induces shape change, aggregation and 5-hydroxy[14C]-tryptamine secretion when added to saponin (12-14 micrograms/ml)-permeabilized platelets, but not to intact platelets. In line with the activation responses in permeabilized cells, GTP[S] induces an increase in [32P]-phosphatidic acid, which is indicative of phospholipase C activity. GDP[S] (greater than 400 microM) totally inhibits GTP[S] (90 microM)-stimulated phospholipase C activity and functional responses in saponized platelets. GDP[S] (1 mM) was also effective at inhibiting low-dose thrombin (0.1 unit/ml)-induced aggregation and secretion responses (without affecting shape change) in permeabilized platelets with inhibition of [32P]-phosphatidic acid formation. At higher doses of thrombin (greater than 0.5 unit/ml), both functional responses and [32P]phosphatidic acid formation are restored in the presence of GDP[S]. Studies on intact cells revealed that GDP[S] was as effective at inhibiting low-dose thrombin-induced functional responses as in the permeabilized cells, but there was no inhibition of [32P]phosphatidic acid formation, indicating that the agent is nonmembrane-penetrating. This reflected the fact that GDP[S] has additional inhibitory sites on the surface of platelets. In Fura-2-loaded cells GDP[S] inhibited thrombin-induced Ca2+ mobilization, as measured by Fura-2 fluorescence, in a dose-dependent manner. In studies with and without Ca2+ present on the outside, the effect of GDP[S] was to block Ca2+ influx. These studies indicate that, although GDP[S] is a valuable tool in studying G-protein function in permeabilized cells, it also has inhibitory activities on the surface of platelets, and one of these has been identified as an effect on the Ca2+-influx channel after agonist stimulation.

Does protein kinase C activation mediate thrombin-induced arachidonate release in human platelets?

Thrombin stimulated rapid formation of diacylglycerol, inositol 1,4,5-trisphosphate (IP3) and thromboxane B2 (TXB2) in human platelets. Formation of diacylglycerol and IP3 appeared to precede that of TXB2. Activation of protein kinase C by diacylglycerol combining with Ca+2 mobilization by IP3 has been implicated in mediating arachidonate release. However, addition of the protein kinase C inhibitor 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine (H-7) to platelet suspension did not inhibit thrombin-stimulated arachidonate release and TXB2 synthesis, whereas addition of the Ca+2 antagonist, 3,4,5-trimethoxybenzoic acid 8-(diethylamino) octyl ester (TMB-8) or the calmodulin antagonist N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide (W-7) abolished arachidonate release. The correlation of IP3 production with arachidonate release on increasing the concentrations of thrombin was further examined. IP3 production reached near maximum at 0.2 U/ml, whereas TXB2 synthesis continued to increase at 1 U/ml. These results suggest that protein kinase C activation may not mediate arachidonate release and that Ca+2 mobilization by IP3 may only partially account for arachidonate release in platelets stimulated with relatively high concentrations of thrombin.

Ca2+ requirement of prostanoid but not of superoxide production by rat Kupffer cells

The release of the prostanoids prostaglandin D2 (PGD2), prostaglandin E2 (PGE2) and thromboxane induced by zymosan and phorbol ester in cultured rat Kupffer cells was found to depend on the extracellular concentration of Ca2+ to some extent. Prostanoid formation following the addition of the calcium ionophore A 23187 was totally inhibited when calcium ions were withdrawn from the medium whereas the prostanoid synthesis from added arachidonic acid was independent of Ca2+. A half-maximal rate of PGE2 release by cells treated with zymosan, phorbol ester or A23187 was obtained at 0.6-0.7 microM free extracellular Ca2+ and greater than or equal to 100 microM free Ca2+ was required to stimulate PGE2 formation maximally. The calmodulin antagonist R24571 partially inhibited the release of PGE2 elicited by zymosan and A23187 but not by phorbol ester or arachidonic acid. Verapamil and nifedipine, two calcium channel blockers, had no effect on the formation of PGE2 irrespective of the stimulus. TMB 8 [3,4,5-trimethoxybenzoic acid 8-(diethylamino)-octyl ester] an intracellular calcium antagonist, inhibited the synthesis of PGE2 induced by zymosan and phorbol ester. The superoxide formation following the addition of zymosan and phorbol ester was not influenced by removal of calcium ions from the medium or by addition of the various calcium antagonists. The data presented here suggest that Ca2+-dependent reactions are involved in the synthesis of prostanoids induced by zymosan and phorbol ester and that both extracellular Ca2+ and mobilization of Ca2+ from intracellular stores are needed to induce maximally the production of prostanoids in cultured rat Kupffer cells.

Introduction of antibody (PL/IM 430) to a 100 kDa protein into permeabilised platelets inhibits intracellular sequestration of Ca2+

A monoclonal antibody (PL/IM 430), previously found to inhibit the uptake of Ca2+ into highly purified platelet intracellular membrane vesicles (Hack, N., Wilkinson, J.M. and Crawford, N. 1988, Biochem. J. 250, 355-361) has been introduced into saponin-permeabilised platelets. At a saponin concentration (20-25 micrograms/ml) commensurate with total LDH release, sequestration of Ca2+ into intracellular non-mitochondrial stores is inhibited by the antibody (approximately 50% inhibition at 20 micrograms/ml IgG). At higher saponin concentrations when intracellular binding of 125I-labelled mAb is maximum, inhibition of Ca2+ sequestration approaches 70%. The inhibition is specific, control studies with non-platelet directed mouse IgG and mAbs which immunoblot platelet antigens other than the 100 kDa protein did not affect the Ca2+ sequestration. No effect of the antibody were observed against IP3-induced release of prestored Ca2+, either in permeabilised platelets or with isolated intracellular membrane vesicles. The mAb PL/IM 430 appears to bind only to the Ca2+ translocating channel protein associated with the intracellular membrane (Ca2+ + Mg2+) ATPase and not to Ca2+ channels responsive to IP3.