Cytochalasin B, the blood platelet release reaction and cyclic GMP

The Role of NO/sGC/cGMP/PKG Signaling Pathway in Regulation of Platelet Function

Circulating blood platelets are controlled by stimulatory and inhibitory factors, and a tightly regulated equilibrium between these two opposing processes is essential for normal platelet and vascular function. NO/cGMP/ Protein Kinase G (PKG) pathways play a highly significant role in platelet inhibition, which is supported by a large body of studies and data. This review focused on inconsistent and controversial data of NO/sGC/cGMP/PKG signaling in platelets including sources of NO that activate sGC in platelets, the role of sGC/PKG in platelet inhibition/activation, and the complexity of the regulation of platelet inhibitory mechanisms by cGMP/PKG pathways. In conclusion, we suggest that the recently developed quantitative phosphoproteomic method will be a powerful tool for the analysis of PKG-mediated effects. Analysis of phosphoproteins in PKG-activated platelets will reveal many new PKG substrates. A future detailed analysis of these substrates and their involvement in different platelet inhibitory pathways could be a basis for the development of new antiplatelet drugs that may target only specific aspects of platelet functions.

A phosphoinositide 3-kinase-AKT-nitric oxide-cGMP signaling pathway in stimulating platelet secretion and aggregation

Phosphoinositide 3-kinase (PI3K) and Akt play important roles in platelet activation. However, the downstream mechanisms mediating their functions are unclear. We have recently shown that nitric-oxide (NO) synthase 3 and cGMP-dependent protein kinase stimulate platelet secretion and aggregation. Here we show that PI3K-mediated Akt activation plays an important role in agonist-stimulated platelet NO synthesis and cGMP elevation. Agonist-induced elevation of NO and cGMP was inhibited by Akt inhibitors and reduced in Akt-1 knock-out platelets. Akt-1 knock-out or Akt inhibitor-treated platelets showed reduced platelet secretion and aggregation in response to low concentrations of agonists, which can be reversed by low concentrations of 8-bromo-cGMP or sodium nitroprusside (an NO donor). Similarly, PI3K inhibitors diminished elevation of cGMP and inhibited platelet secretion and the second wave platelet aggregation, which was also partially reversed by 8-bromo-cGMP. These results indicate that the NO-cGMP pathway is an important downstream mechanism mediating PI3K and Akt signals leading to platelet secretion and aggregation. Conversely, the PI3K-Akt pathway is the major upstream mechanism responsible for activating the NO-cGMP pathway in platelets. Thus, this study delineates a novel platelet activation pathway involving sequential activation of PI3K, Akt, nitric-oxide synthase 3, sGC, and cGMP-dependent protein kinase.

The actin cytoskeleton differentially regulates platelet α-granule and dense-granule secretion

Stimulation of platelets with strong agonists results in centralization of cytoplasmic organelles and secretion of granules. These observations have led to the supposition that cytoskeletal contraction facilitates granule release by promoting the interaction of granules with one another and with membranes of the open canalicular system. Yet, the influence of the actin cytoskeleton in controlling the membrane fusion events that mediate granule secretion remains largely unknown. To evaluate the role of the actin cytoskeleton in platelet granule secretion, we have assessed the effects of latrunculin A and cytochalasin E on granule secretion. Exposure of platelets to low concentrations of these reagents resulted in acceleration and augmentation of agonist-induced α-granule secretion with comparatively modest effects on dense granule secretion. In contrast, exposure of platelets to high concentrations of latrunculin A inhibited agonist-induced α-granule secretion but stimulated dense granule secretion. Incubation of permeabilized platelets with low concentrations of latrunculin A primed platelets for Ca2+- or guanosine triphosphate (GTP)-γ-S-induced α-granule secretion. Latrunculin A-dependent α-granule secretion was inhibited by antibodies directed at vesicle-associated membrane protein (VAMP), demonstrating that latrunculin A supports soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) protein-dependent membrane fusion. These results indicate that the actin cytoskeleton interferes with platelet exocytosis and differentially regulates α-granule and dense granule secretion.

A platelet secretion pathway mediated by cGMP-dependent protein kinase

Platelet secretion (exocytosis) is critical in amplifying platelet activation, in stabilizing thrombi, and in arteriosclerosis and vascular remodeling. The signaling mechanisms leading to secretion have not been well defined. We have shown previously that cGMP-dependent protein kinase (PKG) plays a stimulatory role in platelet activation via the glycoprotein Ib-IX pathway. Here we show that PKG also plays an important stimulatory role in mediating aggregation-dependent platelet secretion and secretion-dependent second wave platelet aggregation, particularly those induced via Gq-coupled agonist receptors, the thromboxane A2 (TXA2) receptor, and protease-activated receptors (PARs). PKG I knock-out mouse platelets and PKG inhibitor-treated human platelets showed diminished aggregation-dependent secretion and also showed a diminished secondary wave of platelet aggregation induced by a TXA2 analog and thrombin receptor-activating peptides that were rescued by the granule content ADP. Low dose collagen-induced platelet secretion and aggregation were also reduced by PKG inhibitors. Furthermore PKG I knockout and PKG inhibitors significantly attenuated activation of the Gi pathway that is mediated by secreted ADP. These data unveil a novel PKG-dependent platelet secretion pathway and a mechanism by which PKG promotes platelet activation.

A predominant role for cAMP-dependent protein kinase in the cGMP-induced phosphorylation of vasodilator-stimulated phosphoprotein and platelet inhibition in humans

The vasodilator-stimulated phosphoprotein (VASP) plays an important role in cGMP-induced platelet inhibition. Since VASP is an in vitro substrate for cGMP-dependent protein kinase (PKG), it has been presumed that VASP phosphorylation induced by cGMP is mediated by PKG. Here we show that, in human platelets, phosphorylation of VASP at Ser239 induced by either cGMP analogs or nitric oxide (NO) donor glyco-SNAP1 is inhibited by PKA inhibitors KT5720, PKI, Rp-Br-cAMPS, and H89, but not by PKG inhibitors KT5823 or Rp-pCPT-cGMPS. Unlike human platelets, cGMP analog-induced phosphorylation of VASP in mouse platelets is inhibited by both PKG and PKA inhibitors. Ineffectiveness of PKG inhibitors in inhibiting VASP phosphorylation in human platelets is not due to an inability to inhibit PKG, as these PKG inhibitors but not PKA inhibitors inhibit a different cGMP-induced intracellular signaling event: phosphorylation of extracellular signal-responsive kinase. Furthermore, PKA inhibitors reverse cGMP-induced inhibition of thrombin-induced platelet aggregation, whereas PKG inhibitors further enhance the inhibitory effect of cGMP analogs. Thus, PKA plays a predominant role in the cGMP-induced phosphorylation of VASP and platelet inhibition in human platelets.

Roles for both cyclic GMP and cyclic AMP in the inhibition of collagen-induced platelet aggregation by nitroprusside

In studies on human platelets, nitroprusside (NP) alone at 1-10 micromol/l increased platelet cyclic AMP (cAMP) by 40-70%, whereas increases in cyclic GMP (cGMP) were much larger in percentage though not in concentration terms. Collagen enhanced these increases in cAMP up to fourfold, without affecting cGMP. This effect was partly prevented by indomethacin or aspirin, indicating that platelet cyclo-oxygenase products acted synergistically with NP to increase cAMP. ADP released from the platelets by collagen tended to restrict this cAMP accumulation. Addition of 2',5'-dideoxyadenosine (DDA), an inhibitor of adenylyl cyclase, decreased both the inhibition of collagen-induced platelet aggregation by NP and the associated accumulation of cAMP without affecting cGMP, indicating that cAMP mediates part of the inhibitory effect of NP. Unlike DDA, 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ), an inhibitor of guanylyl cyclase, blocked all increases in both cGMP and cAMP caused by NP, as well as the inhibition of platelet aggregation, suggesting that cAMP accumulation was secondary to that of cGMP. Human platelet cGMP-dependent protein kinase (PKG) coelectrophoresed with the purified bovine type Ibeta isoenzyme. An inhibitor of this enzyme (Rp)-beta-phenyl-1,N2-etheno-8-bromoguanosine 3',5'-cyclic-monophosphorothioate, diminished the inhibition of collagen-induced platelet aggregation by NP, but had little additional effect when DDA was present. This showed that both PKG and cAMP participate in the inhibition of collagen-induced platelet aggregation by NP. Moreover, selective activators of PKG and cAMP-dependent protein kinases had supra-additive inhibitory effects, suggesting that an optimal inhibitory effect of NP requires simultaneous activation of both enzymes.

Antiplatelet mechanisms of TA-993 and its metabolite MB3 in ADP-induced platelet aggregation

We investigated the antiplatelet mechanisms of TA-993 [(-)-cis-3-acetoxy-5-(2-(dimethylamino)ethyl)-2, 3-dihydro-8-methyl-2-(4-methylphenyl)-1,5-benzothiazepin-4(5H)-one maleate] and its metabolite MB3 (deacetyl and N-monomethyl TA-993) in human platelets stimulated by ADP in vitro. TA-993 and MB3 concentration-dependently inhibited fibrinogen binding to the ADP-stimulated platelets as well as inhibiting platelet aggregation. The antiplatelet effect of MB3 was about 300 times more potent than those of TA-993 and a glycoprotein IIb/IIIa receptor antagonist, Arg-Gly-Asp-Ser (RGDS). Aggregation of ADP-treated fixed platelets caused by the addition of fibrinogen was inhibited by RGDS but not by TA-993 and MB3. TA-993 and MB3 inhibited ADP-induced polymerization of actin filaments. Neither TA-993 nor MB3 affected cyclic AMP and cyclic GMP levels in resting platelets, and nor suppressed the increase in intracellular Ca(2+) concentration induced by ADP. These results suggest that the antiplatelet mechanisms of TA-993 and MB3 may involve inactivation of glycoprotein IIb/IIIa receptors via inhibition of the polymerization of actin.

Cytoskeletal assembly and vinculin-cytoskeleton interaction in different phases of the activation of bovine platelets

Vinculin is an Mr 130 kDa protein that has been implicated in membrane-cytoskeleton interaction in various cell types. It has been demonstrated that vinculin is not a cytoskeletal component in resting platelets, but part of it becomes associated with the cytoskeleton during thrombin-induced activation. In this study, using a quantitative immunoblotting technique, the relation of vinculin to the cytoskeleton in different phases of activation of bovine platelets was explored, and the process of incorporation of vinculin into the cytoskeleton was related to that of cytoskeletal assembly. The assembly of cytoskeleton proceeded at a significantly faster rate than the association of vinculin with it, which shows that the latter process is not due to passive trapping of vinculin into the Triton-insoluble residue, but certain biochemical changes had to occur before such an interaction became possible. When the formation of pseudopodia was prevented by cytochalasin B, but neither aggregation nor the release reaction induced by thrombin were inhibited, the recovery of vinculin in the Triton-insoluble residue even increased. In both time- and thrombin-concentration-dependent studies, poor correlation was found between vinculin-cytoskeleton association and the extent of aggregation. Activation with phorbol-myristate-acetate, which is a strong stimulus for aggregation but produces only a slight release in the granular content, resulted in the association of only a negligible amount of vinculin with the cytoskeletal fraction. The incorporation of vinculin into the cytoskeletal fraction of thrombin activated platelets started with the release reaction but still proceeded, and the greatest part of the reaction occurred after secretion had gone to completion.(ABSTRACT TRUNCATED AT 250 WORDS)

Separable function of platelet release reaction and clot retraction

Amiloride, a known Na+/H+ exchange inhibitor, inhibited platelet serotonin release in a dose-dependent manner (100 microM for 50% inhibition, and 1mM for the nearly complete inhibition), although amiloride (1mM) accelerated clot retraction when it was measured at decreased platelet concentration. On the contrary, cytochalasin B (10 micrograms/ml) accelerated platelet serotonin release, but it inhibited clot retraction. These results demonstrate that release reaction and clot retraction, both of which are important processes involved in platelet activation, can be functionally separated.

The platelet activation induced by wheat germ agglutinin

In human platelets, wheat germ agglutinin (WGA) induced serotonin release without cell agglutination. WGA induced the phosphorylation of both 40-kDa and 20-kDa proteins in a parallel manner, and at least, the phosphorylation of 40-kDa protein was preceded by transient formation of endogenous diacylglycerol (DG) accompanied by a decrease in phosphatidylinositol (PI). Both phosphorylation of these two proteins and serotonin release were inhibited by prior treatment of platelets with dibutyryl cyclic AMP, W-7, or TMB-8. These results suggest that both phosphatidylinositol turnover and Ca2+ mobilization play an essential role in WGA-induced platelet activation.

Lysosomotropic agents selectively potentiate thrombin-induced acid hydrolase secretion from platelets

Thrombin induces partial secretion (up to 60%) of beta-N-acetyl-D-hexosaminidase (EC 3.2.1.52) from untreated platelets. Preincubation of platelets with 10 mM NH4Cl for up to 2 hr resulted in a time-dependent and marked stimulation of thrombin-induced secretion of both this enzyme and other acid glycosidases from platelets. The enhancement of the thrombin-induced secretion was not due to cell lysis, and NH4Cl alone did not cause leakage of lysosomal enzymes into the medium. The effect could be reversed by reincubating the platelets in NH4Cl-free medium. Stimulation of thrombin-induced secretion also was produced by a series of aliphatic primary amines from methylamine to butylamine, and by micromolar concentrations of chloroquine. The effect of weak bases on platelets appeared to be quite specific for enhancing lysosomal enzyme secretion. Thrombin-induced secretion of adenine nucleotides from dense granules and of beta-thromboglobulin from alpha granules was slightly enhanced by NH4Cl but was slightly inhibited by methylamine. The only direct effect of the weak bases on platelets was the displacement of serotonin from dense granules. Accumulation of weak bases in acidic pools in the platelets (e.g., lysosomes) might, therefore, be responsible for the enhanced secretion of lysosomal enzymes. By using controlled digitonin-induced platelet lysis, it was found that preincubation of platelets with NH4Cl lowered the digitonin concentration required for enzyme solubilization. We suggest that loading of lysosomes with weak bases dissociates already bound enzyme inside the lysosomes, resulting in a more effective discharge upon stimulation by thrombin.

Interaction of Solanum tuberosum agglutinin with human platelets

Solanum tuberosum agglutinin (STA) binds to the surface of human platelets and leads to their agglutination. Lectin staining shows that 125I-STA most intensely labels a major platelet membrane glycoprotein identified as GPIIIa followed by GPIV . STA does not induce release reaction, TXB2 formation or platelet protein phosphorylation. Since STA-induced agglutination is independent of intracellular metabolism of platelets, STA may prove to be a useful tool to explore the clinical condition in which the composition of platelet membrane protein is altered.

Inhibitory action of guanosine 3',5'-monophosphate on thrombin-induced calcium mobilization in human platelets

During thrombin-induced serotonin release from platelets, phosphorylation of myosin light chain was increased. This protein phosphorylation, which is initiated by Ca2+ mobilization, and serotonin release were inhibited progressively by increasing concentrations of sodium nitroprusside, a potent stimulator of platelet guanylate cyclase, or 8-bromo-cyclic GMP. Thrombin-induced increase in cytoplasmic free Ca2+, which was also inhibited by these drugs in a dose-dependent manner. These results provide evidence that an increase in platelet cyclic GMP inhibits thrombin-induced Ca2+ mobilization and prevents platelet activation.

Elevation of cytoplasmic free calcium concentration by stable thromboxane A2 analogue in human platelets

9, 11-Epithio-11, 12-methano-thromboxane A2 (STA2), a stable analogue of thromboxane A2, caused a rapid rise in cytoplasmic free Ca2+ concentration ([Ca2+]i) in human platelets as measured with the fluorescent Ca2+ indicator quin2. Concomitantly, this compound induced phosphorylation of myosin light chain which is catalyzed by Ca2+, calmodulin-dependent protein kinase. These reactions were fast enough to trigger serotonin release. 13-Azaprostanoic acid, a receptor level antagonist of thromboxane A2 inhibited STA2-induced elevation of [Ca2+]i, phosphorylation of myosin light chain and serotonin release. These results provide evidence that STA2 interacts with a thromboxane A2 receptor which leads to elevation of [Ca2+]i.

Effect of cyclic AMP on cytoplasmic free calcium in human platelets stimulated by thrombin: direct measurement with quin2

With the fluorescent Ca2+ indicator quin2, we measured directly cytoplasmic free Ca2+ [( Ca2+]i) in washed human platelets stimulated by thrombin and examined the effect of cyclic AMP on [Ca2+]i levels and 14C-serotonin release. Thrombin (0.2 U/ml) evoked a rise in [Ca2+]i from the basal level of about 100 nM to integral of 3 microM which was fast enough to trigger serotonin release. This rise was inhibited in a dose-dependent manner by preincubation with prostaglandin E1 (PGE1) (0.1-10 microM) or dibutyryl cyclic AMP (0.01 -1 mM). Parallel to this, serotonin release was also inhibited by these drugs. When added to platelets after stimulation by thrombin, PGE1 caused the rapid decrease of elevated [Ca2+]i. These results provide direct evidence that [Ca2+]i levels in platelets are regulated by cyclic AMP.

Protein phosphorylation and diglyceride production during serotonin release induced by epinephrine plus ADP in human platelets

Epinephrine (1 microM) plus ADP (5 microM) induced serotonin release from human platelets although neither epinephrine nor ADP alone brought about such a release. During this release reaction, the phosphorylation of 40K-dalton protein was induced to an extent similar to that induced by 0.5 unit/ml thrombin. The amount of diglyceride (DG) produced by epinephrine plus ADP, however, was much smaller than that produced by thrombin. The reaction velocities of these reactions induced by epinephrine plus ADP were slower than those induced by thrombin. Epinephrine or ADP alone hardly produced any DG and induced 40K-dalton protein phosphorylation only slightly. Indomethacin (1 microgram/ml), a cyclooxygenase inhibitor, remarkably inhibited epinephrine plus ADP induced serotonin release, 40K-dalton protein phosphorylation and DG production although this agent had little effect on the same reactions induced by thrombin. These results suggest that prostaglandin endoperoxides or thromboxane A2 may be involved in serotonin release, 40K-dalton protein phosphorylation and DG production induced by epinephrine plus ADP.

Cytochalasins inhibit arachidonic acid metabolism in thrombin-stimulated platelets

Low concentrations (0.5-1 microM) of cytochalasins inhibit the thrombin-stimulated polymerization of monomeric actin to filamentous actin in platelets. Similar concentrations of cytochalasin B inhibit the formation and metabolism of arachidonic acid in horse platelets stimulated by low concentrations of thrombin (0.1-0.5 unit/ml). However, the release of serotonin is not inhibited by cytochalasin B. Cytochalasins B and D (0.5-1 microM) markedly reduce, in thrombin-stimulated human or horse platelets, the metabolism of the liberated arachidonic acid by cyclooxygenase activity to thromboxane B2 and 12-hydroxy-5,8,10-heptadecatrienoic acid and the conversion of arachidonic acid by lipoxygenase activity to 12-hydroxy-5,8,10,14-icosatetraenoic acid. The generation of arachidonic acid from platelet phospholipids and the formation of phosphatidic acid are much less affected by cytochalasin B or D. Cytochalasins do not directly inhibit platelet cyclooxygenase, lipoxygenase, phospholipase A2, or phosphatidyl-inositol-specific phospholipase C. In addition, the metabolism of exogenously added arachidonic acid by intact platelets is not inhibited by cytochalasins B and D. The results indicate that polymerization of actin in platelets stimulated by thrombin may be required for the effective metabolism of arachidonic acid released from platelet phospholipids.

Mechanisms of cellular interaction with monosodium urate crystals. IgG-dependent and IgG-independent platelet stimulation by urate crystals

Monosodium urate crystals (MSU) stimulate suspensions of washed platelets or neutrophils. When MSU crystals are coated with IgG, as occurs in plasma, stimulation is markedly enhanced. These studies which use MSU-induced human platelet serotonin secretion as a model examine the nature of cellular recognition mechanisms for MSU crystals and IgG-coated MSU crystals. F(ab')2 fragments of specific anti-Fc antibody blocked and the lipopolysaccharide of Salmonella minnesota R595 enhanced human platelet secretion induced by IgG-coated urate crystals. These agents had little effect on stimulation by uncoated crystals. This indicated that urate crystals stimulate platelets independently of fluid phase IgG. Urate crystals directly stimulated suspensions of washed rabbit platelets which lack Fc receptors. In contrast to human cells, stimulation was blocked by IgG. This again demonstrated IgG-independent cell stimulation by urate crystals. Calcium pyrophosphate dihyrate crystals could trigger human platelet secretion only when coated with IgG. This suggests that when crystals are coated with IgG, the surface-bound IgG alone may be the stimulus to the cell. This was supported by the finding that polyvinylpyridine-N-oxide, a hydrogen acceptor, blocked human platelet stimulation by uncoated, but not IgG-coated, urate crystals. These data indicate that urate crystals (and potentially other surface or particles) can stimulate a mediator cell by at least two mechanisms: by direct stimulation without the mediation of adsorbed IgG or, when coated with IgG, by triggering the cell via immunoglobulin receptors.

On the interrelationship of prostaglandin endoperoxide G2 and cyclic nucleotides in platelet function

The prostaglandin endoperoxide G2 caused rapid aggregation and relase of ADP and [14C]serotonin in human platelets. Since the presence of the ADP phosphorylating system creatine phosphate/creatine phosphokinase markedly inhibited the aggregation caused by the endoperoxide, this effect seemed to be mediated mainly by ADP, which is instantaneously released by the endoperoxide. The prostaglandin G2 counteracted the increasing effect of prostaglandin E1 on the adenosine 3':5'-monophosphate (cAMP) levels in platelet-rich plasma. This effect of prostaglandin G2 was only observed when ADP was released by the endoperoxide. This finding indicates that the effect of prostaglandin G2 on the cAMP levels in platelet-rich plasma is principally mediated by ADP. The rapid release of ADP by prostaglandin G2 and the time courses for the effects of the endoperoxide and ADP on the level of cAMP give further evidence for this hypothesis. ADP also caused primary aggregation in the presence of indomethacin, and prostaglandin synthesis inhibitors did not influence the decreasing effect of ADP on the cAMP levels. N2,O2-Dibutyrylguanosine 3':5'-monophosphate did not influence the aggregation and release-reaction caused by ADP and no changes of the cGMP levels were observed after addition of prostaglandin G2.

Cyclic nucleotides and platelet aggregation. Effect of aggregating agents on the activity of cyclic nucleotide-metabolizing enzymes

The activities of adenylate and guanylate cyclase and cyclic nucleotide 3':5'-phosphodiesterase were determined during the aggregation of human blood platelets with thrombin, ADP, arachidonic acid and epinephrine. The activity of guanylate cyclase is altered to a much larger degree than adenylate cyclase, while cyclic nucleotide phosphodiesterease activity remains unchanged. During the early phases of thrombin-and ADP-induced platelet aggregation a marked activation of the guanylate cyclase occurs whereas aggregation induced by arachidonic acid or epinephrine results in a rapid diminution of this activity. In all four cases, the adenylate cyclase activity is only slightly decreased when examined under identical conditions. Platelet aggregation induced by a wide variety of aggregating agents including collagen and platelet isoantibodies results in the "release" of only small amounts (1-3%) of guanylate cyclase and cyclic nucleotide phosphodiesterase and no adenylate cyclase. The guanylate cyclase and cyclic nucleotide phosphodiesterase activities are associated almost entirely with the soluble cytoplasmic fraction of the platelet, while the adenylate cyclase if found exclusively in a membrane bound form. ADP and epinephrine moderately inhibit guanylate and adenylate cyclase in subcellular preparations, while arachidonic and other unsaturated fatty acids moderately stimulate (2-4-fold) the former. It is concluded that (1) the activity of platelet guanylate cyclase during aggregation depends on the nature and mode of action of the inducing agent, (2) the activity of the membrnae adenylate cyclase during aggregation is independent of the aggregating agent and is associated with a reduction of activity and (3) cyclic nucleotide phosphodiesterase remains unchanged during the process of platelet aggregation and release. Furthermore, these observations suggest a role for unsaturated fatty acids in the control of intracellular cyclic GMP levels.