The influence of amines on various platelet responses

The interaction of integrin αIIbβ3 with fibrin occurs through multiple binding sites in the αIIb β-propeller domain

The currently available antithrombotic agents target the interaction of platelet integrin αIIbβ3 (GPIIb-IIIa) with fibrinogen during platelet aggregation. Platelets also bind fibrin formed early during thrombus growth. It was proposed that inhibition of platelet-fibrin interactions may be a necessary and important property of αIIbβ3 antagonists; however, the mechanisms by which αIIbβ3 binds fibrin are uncertain. We have previously identified the γ370-381 sequence (P3) in the γC domain of fibrinogen as the fibrin-specific binding site for αIIbβ3 involved in platelet adhesion and platelet-mediated fibrin clot retraction. In the present study, we have demonstrated that P3 can bind to several discontinuous segments within the αIIb β-propeller domain of αIIbβ3 enriched with negatively charged and aromatic residues. By screening peptide libraries spanning the sequence of the αIIb β-propeller, several sequences were identified as candidate contact sites for P3. Synthetic peptides duplicating these segments inhibited platelet adhesion and clot retraction but not platelet aggregation, supporting the role of these regions in fibrin recognition. Mutant αIIbβ3 receptors in which residues identified as critical for P3 binding were substituted for homologous residues in the I-less integrin αMβ2 exhibited reduced cell adhesion and clot retraction. These residues are different from those that are involved in the coordination of the fibrinogen γ404-411 sequence and from auxiliary sites implicated in binding of soluble fibrinogen. These results map the binding of fibrin to multiple sites in the αIIb β-propeller and further indicate that recognition specificity of αIIbβ3 for fibrin differs from that for soluble fibrinogen.

Fructose 1,6-bisphosphate prevented endotoxemia, macrophage activation, and liver injury induced by D-galactosamine in rats

OBJECTIVE

Fructose 1,6-bisphosphate (F1,6BP) protects organs against a wide range of challenges involving inflammation. We hypothesized that the primary action of F1,6BP is to prevent macrophage activation and cytokine release. Our aim was to determine the tissue and cellular targets for this bisphosphorylated sugar and to provide new insights into its mechanisms of action.

DESIGN

Prospective, controlled laboratory study.

SETTING

Animal resource facilities and research laboratory.

SUBJECTS

Male Sprague-Dawley rats (200-250 g body weight).

INTERVENTIONS

The protective action of F1,6BP was analyzed in galactosamine (GalN)-induced hepatitis in rats. The in vivo effects of F1,6BP were evaluated by changes in transaminase activities, blood endotoxins, and tumor necrosis factor (TNF)-alpha production in GalN-challenged rats. The targets of F1,6BP to reduce macrophage response to lipopolysaccharide (LPS) were determined by correlation between changes in TNF-alpha production and K+ fluxes through cell membrane in primary cultures of Kupffer cells.

MEASUREMENTS AND MAIN RESULTS

The in vivo results indicate that F1,6BP treatment prevented GalN-induced injury in liver parenchymal cells. This protection was mainly associated with a reduction of the inflammatory response. F1,6BP prevention of GalN-induced endotoxemia correlated with preclusion of mast cell degranulation and histamine release that preceded the increased plasma endotoxins and liver production of TNF-alpha. In addition, F1,6BP treatment decreased sensitivity to LPS, which reduced the GalN-induced increase in TNF-alpha. The in vitro results show that F1,6BP inhibited Kupffer cell response and reduced TNF-alpha production by preventing LPS-induced K+ channel activation.

CONCLUSIONS

The role of exogenous F1,6BP as a K+ channel modulator underlies its antihistaminic and anti-inflammatory action and increases its interest as a protective compound.

Polyamines inhibit both platelet aggregation and glycoprotein IIb/IIIa activation

Platelet aggregation is inhibited by the polyamines putrescine, spermidine, and spermine. To date, the mechanism of action has not been clearly identified. Evidence suggests that polyamines may interact with the fibrinogen receptor (GP IIb/IIIa), interfering with platelet-platelet attachment. The effect of polyamines on human platelet aggregation and GP IIb/IIIa activation was evaluated. For the aggregation experiments, platelets were obtained from heparin- or citrate-collected blood. Our results indicate that the polyamines putrescine, spermidine, and spermine cause a dose-dependent inhibition of ADP- or collagen-mediated platelet aggregation with an order of potency spermine>spermidine>putrescine. In addition, spermine arrests or inhibits thrombin-, epinephrine-, arachidonate-, or ristocetin-induced platelet aggregation. Expression of platelet membrane glycoproteins IIb, IIIa, and IX is not reduced by polyamines. However, spermine inhibits the ADP- or thrombin-induced activation of GP IIb/IIIa. It is concluded that the final step in aggregation, common to all agonists, ie, fibrinogen binding to GP IIb/IIIa, is inhibited by spermine through inhibition of the agonist-induced activation of GP IIb/IIIa that precedes fibrinogen-ligand binding.

Inhibition of arterial thrombosis by polyamines in a canine coronary artery injury model

Polyamines are polycations present in all living organisms and have been shown to play an important role in various physiological functions. Previous studies have shown that various amines including polyamines inhibit platelet activation. Among the amines tested tetra-amine, spermine is the potent inhibitor of platelet aggregation. In spite of vast literature on the anti-aggregatory effect of amines, there are no definitive studies testing their efficacy in an in vivo thrombosis model. In the present study, we investigated if polyamines could inhibit in-vivo thrombosis. A partially occlusive thrombus was generated by application of electric current in canine coronary artery. In control animals, the artery was completely in 76+/-14 min after the current was discontinued. When 40 mg/kg (1.44 mM) spermine was given immediately after stopping the current blood flow remained patent for >240 min. At equimolar concentration, triamine, spermidine and diamine putrescine are also equally effective in preventing thrombus development. The anti thrombic effect of polyamines was not associated with increased bleeding tendency, as judged by the amount of blood adsorbed by a gauge pad placed in a surgical incision extending to the muscle tissue and by a standard template bleeding. These results indicate that apart from inhibiting in-vitro platelet aggregation polyamines can also inhibit in-vivo thrombus formation.

Effect of polyamines on in vitro platelet aggregation and in vivo thrombus formation

INTRODUCTION

Polyamines are polycations present in all living organisms and have been shown to play an important role in various physiological functions. Previous studies have shown that various amines including polyamines inhibited platelet activation, but there were no definitive studies testing their efficacy in an in vivo thrombosis model. We carried out detailed in vitro platelet aggregation studies using various concentrations of polyamines as well as agonists.

METHODS

Platelet aggregation was measured by a turbidimetric method. Electric current induced in vivo thrombosis model is used for assessing antithrombotic effect. Incidence of bleeding was evaluated by template bleeding and incisional bleeding.

RESULTS

Polyamines inhibited agonist-induced platelet aggregation in a dose-dependent manner. The inhibitory effect of polyamines is inversely proportional to the concentration of the agonist used. Among the polyamines, spermine is the potent inhibitor of platelet aggregation. A partially occlusive thrombus was generated by application of electric current in canine coronary artery. In control animals, the artery was completely occluded in 70 +/- 11 min after the current was discontinued. Blood flow remained patent for >240 min when 2 mg/kg spermine was given immediately after stopping the current. The antithrombotic effect of spermine was not associated with increased bleeding tendency.

CONCLUSION

These results indicate that apart from inhibiting in vitro platelet aggregation polyamines can also inhibit in vivo thrombus formation. To our knowledge, this is the first study demonstrating this phenomenon.

On the mechanism of the spermine-exerted inhibition on alpha-thrombin-induced platelet activation

Previous reports have shown that various amines inhibited platelet activation, but no definitive conclusions on their action mechanism were drawn. We have further investigated the action of spermine on platelet responses evoked by alpha-thrombin and other agonists. Spermine inhibited in a concentration-dependent manner (1-10 mM), and more efficiently than spermidine and putrescine, the alpha-thrombin-induced (1.5 nM) platelet activation. Spermine added at a concentration that inhibited completely aggregation only partially affected the thrombin-induced increase in cytosolic Ca(2+) concentration, protein phosphorylation, and ATP secretion. The polyamine had little effect on the morphology of resting platelets, as measured by electron microscopy, thrombin hydrolytic activity, and fibrinogen clotting capacity but decreased the thrombin binding to platelets and isolated glycocalicin. Spermine partially inhibited the aggregation elicited by ADP, vasopressin, platelet-activating factor, thrombin receptor-activating peptide, fluoroaluminate, ionomycin, and dioctanoylglycerol but did not affect the cytosolic Ca(2+) increase induced by these agonists. The polyamine bound to both glycocalicin and platelets, and it inhibited the fibrinogen binding to stimulated platelets. The amount of 14C-spermine bound to resting cells decreased in the presence of the glycoprotein GPIb-antibody LJIB1, whereas the polyamine bound to activated platelets, which was higher than that tied to resting cells, was markedly reduced by LJCP8 or decorsin, a GPIIb/IIIa antibody and antagonist-peptide, respectively. These results indicate that spermine specifically inhibits the thrombin binding to GPIb of resting platelets and the fibrinogen binding to GPIIb/IIIa (integrin alpha(IIb)beta(3)) of activated platelets.

Microtiter plate immunoassay for the evaluation of platelet adhesion to fibronectin

Investigations of platelet adhesion to adhesive proteins have been pursued to understand the basic mechanisms of hemostasis and thrombosis. Most assays used to determine platelet adhesion under stasis conditions rely on radiolabeled platelets. We describe a new microtiter immunoassay to study platelet adhesion to adhesive proteins under stasis conditions. Direct comparison of platelet adhesion to fibronectin using a standard platelet adhesion assay based on 51Cr-labeled platelets and the new immunoassay showed that the optical density values obtained with the immunoassay are directly proportional to the number of platelets bound. The choice of platelet suspension buffer crucial for the design of such experiments, because the adhesion of resting platelets to fibronectin is increased in response to thrombin stimulation. This increase buffer rather than Tris buffer. Platelet adhesion to fibronectin is increased in response to thrombin stimulation. This increase can be inhibited by synthetic RGD peptides. The thrombin-induced increase of platelet adhesion to fibronectin could be detected with antibodies against actin and glycoprotein IIb-IIIa, but not against the alpha-granule constituent platelet factor 4 (PF4). This assay is very versatile, because it avoids the use of radioactivity, and allows the parallel processing of a large number of samples. In addition, the parallel use of antibodies against different platelet antigens allows the screening for platelet activation events associated with the measured platelet adhesion.

Expression of constitutive endothelial nitric oxide synthase in human blood platelets

Nitric oxide (NO) activates the soluble isoform of guanylate cyclase in platelets and inhibits platelet function. Several studies suggest the existence of a pathway for NO synthesis in platelets as a form of feedback inhibition, but the identity of the NO synthase (NOS) isoform present within platelets is unknown. We isolated human platelets, and synthesized cDNA from platelet RNA for analysis by PCR. Primers for human neuronal or inducible NOS failed to yield a PCR signal. However, primers specific for endothelial NOS (ecNOS) amplified a DNA band of the expected size. Analysis of nucleotide sequence revealed that the amplified DNA is ecNOS. NOS enzyme activity was detected in the platelet particulate subcellular fraction, as previously demonstrated for ecNOS in other cells. Thus, ecNOS is present in human platelets, and may play a role in the regulation of platelet function by an endogenous NO pathway.

Histamine as an intracellular messenger in human platelets

The results of investigations in platelets provide evidence for an intracellular messenger role for histamine. Studies of neutrophils and of cellular proliferation suggest that there may be a wider role for histamine as an intracellular messenger modulating activation processes in cells.

Investigation of the existence and biological role of L-arginine/nitric oxide pathway in human platelets by spin-trapping/EPR studies

The aim of the present study was to apply spin trapping/EPR spectroscopy to investigate the existence and biological role of the L-arginine/nitric oxide pathway in human platelet aggregation. Three different spin traps were used: two nitroso, 3,5-dibromo-4-nitrosobenzenesulfonate (DBNBS) and 2-methyl-2-nitrosopropane (MNP), and a nitrone, 5,5-dimethyl-1-pyrroline N-oxide (DMPO). The effect of spin-trap concentration on the collagen-induced human platelet aggregation was compared to the anti-aggregatory effect caused by L-arginine. The results show that the nitroso spin traps (DBNBS and MNP) are more effective than L-arginine in preventing platelet aggregation. DMPO has virtually no effect on the collagen-induced aggregation except at a high concentration (300 mM). Furthermore, activation of platelets with a low concentration of collagen (17 micrograms/ml) and in the presence of DBNBS or MNP yields several EPR-detectable spin adducts. Some of the observed spin adducts do not correspond to those originating from the interaction of a free radical, nitric oxide (NO.) gas, with the spin traps [Arroyo, C.M. & Kohno, M. (1991) Free Radical Res. Commun. 14, 145-155]. Only one adduct of DBNBS, with a relative intensity of 0.1, observed in the washed-platelet experiment and in the presence of superoxide dismutase, is similar to the EPR spectrum obtained following a reaction of pure NO. gas with DBNBS. This suggests that the EPR spectrum of the DBNBS adduct consisting of a triplet may originate from the production of NO. by these cells. Additional DBNBS and MNP spin adducts were generated during platelet activation in the presence of Ca2+ and of a cytosol-depleted L-arginine preparation from washed platelets to which L-arginine was subsequently added. The formation of these DBNBS and MNP spin adducts were inhibited by N omega-methyl-L-arginine (MeArg, 100 microM), suggesting that these originated from a product of NO synthase. Furthermore, the formation of DBNBS and MNP spin adducts in platelet suspensions was enhanced by the presence of superoxide dismutase; however, their formation was prevented by the endothelial-derived relaxing factor (EDRF) inhibitors methylene blue and hemoglobin. The results from the MeArg and EDRF inhibitor experiments support the existence of the L-arginine/NO pathway in platelets. In addition, the prevention of spin-adduct formation by EDRF inhibitors, suggests that the mechanisms of EDRF formation and the L-arginine/NO pathway in endothelial cells and platelets are similar.(ABSTRACT TRUNCATED AT 400 WORDS)

An L-arginine/nitric oxide pathway present in human platelets regulates aggregation

Aggregation of human washed platelets with collagen is accompanied by a concentration-dependent increase in cyclic GMP but not cyclic AMP. NG-Monomethyl-L-arginine (L-MeArg), a selective inhibitor of nitric oxide (NO) synthesis from L-arginine, reduces this increase and enhances aggregation. L-Arginine, which has no effect on the basal levels of cyclic GMP, augments the increase in this nucleotide induced by collagen and also inhibits aggregation. Both of these effects of L-arginine are attenuated by L-MeArg. The anti-aggregatory action of L-arginine is potentiated by prostacyclin and by M&B22948, a selective inhibitor of the cyclic GMP phosphodiesterase, but not by HL725, a selective inhibitor of the cyclic AMP phosphodiesterase. L-Arginine also inhibits platelet aggregation in whole blood in a similar manner, although the concentrations required are considerably higher. L-Arginine stimulates the soluble guanylate cyclase and increases cyclic GMP in platelet cytosol. This stimulation is dependent on NADPH and Ca2+ and is associated with the formation of NO. Both the formation of NO and the stimulation of the soluble guanylate cyclase induced by L-arginine are enantiomer specific and abolished by L-MeArg. Thus, human platelets contain an NO synthase which is activated when platelets are stimulated. The consequent generation of NO modulates platelet reactivity by increasing cyclic GMP. Changes in the activity of this pathway in platelets may have physiological, pathophysiological, and therapeutic significance.

Inhibition of platelet aggregation by a whole cell extract from strains of group B streptococcus

A whole cell extract (HCl-Ext) from strains of group B streptococci (GBS) possessing fibrinogen binding activity prevented the platelet aggregation induced with adenosine 5'-diphosphate (ADP), collagen and thrombin, while aggregation by epinephrine and ristocetin was slightly inhibited and arachidonic acid-induced platelet aggregation was not affected whatsoever. When the HCl-Ext was added after commencement of the aggregation, deaggregation was observed in cases induced by ADP, collagen, and thrombin. By precoating the washed platelets with HCl-Ext, both of ADP- and collagen-induced platelet aggregation were suppressed. The active factor in the HCl-Ext seemed to be undialyzable, trypsin-susceptible, and proteinaceous substance, unlike GBS polysaccharide type antigen.

Aggregating human platelets cause direct contraction and endothelium-dependent relaxation of isolated canine coronary arteries. Role of serotonin, thromboxane A2, and adenine nucleotides

Aggregating human platelets contract isolated rings of canine coronary artery without endothelium, but relax rings with intact endothelium. We performed experiments to identify the substances released from platelets responsible for these effects. The contraction in rings without endothelium was reduced by treating the platelets with thromboxane synthetase inhibitor, dazoxiben, or treating the vessels with the thromboxane-receptor antagonist, SQ 29548. The serotonergic antagonist, methiothepin, also reduced the platelet-induced contraction. The combination of methiothepin plus dazoxiben or SQ 29548 caused a further inhibition. The endothelium-dependent relaxation to platelets during contractions evoked by prostaglandin F2 alpha was nearly abolished by the ADP- and ATP-scavenger, apyrase. It was not inhibited by methiothepin, which antagonizes endothelium-dependent relaxations to serotonin. Thus, both serotonin and thromboxane A2 contribute to the direct activation of coronary smooth muscle by aggregating human platelets, whereas adenine nucleotides are the principal mediators of the endothelium-dependent relaxation.

Thrombin enhanced adhesion of platelets to von Willebrand factor substrates

When exposed to thrombin, the adhesion of platelets to a von Willebrand factor (vWf) substrate, relative to a control substrate, is selectively increased. Adhesion to a vWf substrate is dependent upon the concentration of vWf, the duration of the adhesion assay, the concentration of thrombin, and the presence of divalent cations. The enhanced adhesion results from an action of thrombin on the platelets; no effect on the vWf substrate is involved. Once adherent to the substrate, the platelets undergo a profound change in morphology from the spiny sphere phenotype characteristic of activated platelets to a flattened and highly spread state. The adhesion of activated platelets to solid phase vWf is not inhibited by physiological concentrations of fibrinogen.