Aggretin, a novel platelet-aggregation inducer from snake (Calloselasma rhodostoma) venom, activates phospholipase C by acting as a glycoprotein Ia/IIa agonist

A potent platelet aggregation inducer, aggretin, was purified from Malayan-pit-viper (Calloselasma rhodostoma) venom by ionic-exchange chromatography, gel-filtration chromatography and HPLC. It is a heterodimeric protein (29 kDa) devoid of esterase, phospholipase A and thrombin-like activity. Aggretin (> 5 nM) elicited platelet aggregation with a lag period in both human platelet-rich plasma and washed platelet suspension. EDTA (5 mM), prostaglandin E1 (1 microM) and 3,4,5-trimethoxybenzoic acid 8-(diethylamino)octyl ester ('TMB-8'; 100 microM) abolished its aggregating activity, indicating that exogenous bivalent cations and intracellular Ca2+ mobilization are essential for aggretin-induced platelet aggregation. Neomycin (4 mM) and mepacrine (50 microM) completely inhibited aggretin (33 nM)-induced aggregation; however, creatine phosphate/creatine phosphokinase (5 mM, 5 units/ml) and indomethacin (50 microM) did not significantly affect its aggregating activity. Aggretin caused a significant increase of [3H]InsP formation in [3H]Ins-loaded platelets, intracellular Ca2+ mobilization and thromboxane B2 formation. Neomycin, a phospholipase C inhibitor, completely inhibited both the increase of [3H]InsP and intracellular Ca2+ mobilization of platelets stimulated by aggretin. A monoclonal antibody (6F1) directed against glycoprotein Ia/IIa inhibited platelet shape change and aggregation induced by aggretin. 125I-aggretin bound to platelets with a high affinity (Kd = 4.0 +/- 1.1 nM), and the number of binding sites was estimated to be 2119 +/- 203 per platelet. It is concluded that aggretin may act as a glycoprotein Ia/IIa agonist to elicit platelet aggregation through the activation of endogenous phospholipase C, leading to hydrolysis of phosphoinositides and subsequent intracellular Ca2+ mobilization.

The antiplatelet activity of ancrod on administration to rabbits

Ancrod, a thrombin-like enzyme purified from the venom of Calloselasma rhodostoma, was administered to rabbits intravenously, and blood samples were obtained at 1, 3, 6, 10, and 24 hours after infusion. Ancrod caused a rapid and sustained defibrinogenation within the first 6 hours, with production of fibrinogen degradation products (FDPs) peaking at 1 hour and declining to background level at 6 hours. No significant changes in platelet count, white cell count, or hematocrit was observed. Citrated PRP prepared 1, 3, and 6 hours after ancrod infusion showed diminished aggregation, adenosine triphosphate (ATP) release, and thromboxane B2 formation on the addition of collagen. Although platelet suspension prepared from defibrinogenated platelet-rich plasma (PRP) at 3 hours showed no significant change in aggregation and ATP-releasing activity, the latent period of platelet aggregation was prolonged. When the remaining platelet-poor plasma obtained from defibrinogenated PRP at 3 hours was used to suspend the normal washed platelets prepared from PRP before ancrod infusion, the platelets showed a similar defect in aggregation and release action. Addition of fibrinogen (200 micrograms/ml to 2 mg/ml) to the above preparation partially restored aggregation but not capacity for secretion and thromboxane formation. When normal washed platelets were suspended with the defibrinogenated plasma, prepared by mixing ancrod with normal plasma in vitro and removing the formed fibrin, the platelet suspension showed impaired platelet aggregability, and the aggregability could be restored to the normal level by the addition of exogenous fibrinogen to this preparation.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of platelet aggregation induced by human breast carcinoma and its inhibition by snake venom peptides, trigramin and rhodostomin

MCF-7 cells, a metastatic human breast carcinoma line, caused dose-dependent platelet aggregation in heparinized human platelet-rich plasma (PRP). MCF-7 tumor cell-induced platelet aggregation (TCIPA) was almost blocked by apyrase (0.5 U/ml) and completely inhibited by hirudin (5 U/ml). This TCIPA was unaffected by cysteine proteinase inhibition with E-64 (10 microM), but was limited by cell pretreatment with phospholipase A2. MCF-7 cell suspension caused marked, dose-dependent decrease in plasma recalcification times using normal, Factor VIII-deficient, and Factor IX-deficient human plasma. This effect was potentiated in cell lysates but was inhibited in intact cells preincubated with sphingosine. MCF-7 cell suspension did not affect the recalcification time of Factor VII-deficient plasma. Taken together, these data suggest that MCF-7 TCIPA arises from MCF-7 tissue factor activity expression. Trigramim and rhodostomin, RGD-containing snake venom peptides which antagonized the binding of fibrinogen to platelet membrane glycoprotein IIb/IIIa, prevented MCF-7 TCIPA. Likewise, synthetic peptide GRGDS as well as monoclonal antibodies against human tissue factor, platelet membrane glycoprotein IIb/IIIa and Ib prevented MCF-7 TCIPA, which was unaffected by control peptide GRGES. On a molar basis, trigramin (IC50, 0.1 microM) and rhodostomin (IC50, 0.03 microM), were about 5,000 and 18,000 times, respectively, more potent than GRGDS (IC50, 0.54 mM).

The Arg-Gly-Asp-containing peptide, rhodostomin, inhibits in vitro cell adhesion to extracellular matrices and platelet aggregation caused by saos-2 human osteosarcoma cells

Saos-2 cells, derived from a primary human osteosarcoma, caused dose-dependent platelet aggregation in heparinised human platelet-rich plasma. Saos-2 tumour cell-induced platelet aggregation (TCIPA) was completely inhibited by hirudin but unaffected by apyrase. The cell suspension shortened the plasma recalcification times of normal, factor VIII-deficient and factor IX-deficient human plasmas in a dose-dependent manner. However, the cell suspension did not affect the recalcification time of factor VII-deficient plasma. Moreover, a monoclonal antibody (MAb) against human tissue factor completely abolished TCIPA. Flow cytometric analysis using anti-integrin MAbs as the primary binding ligands demonstrated that the integrin receptors alpha v beta 3, alpha 5 beta 1 and alpha 6 beta 1 were present of Saos-2 cells, which might mediate tumour cell adhesion to extracellular matrix. Rhodostomin, an Arg-Gly-Asp (RGD)-containing snake venom peptide which antagonises the binding of fibrinogen to platelet membrane glycoprotein IIb/IIIa, prevented Saos-2 TCIPA as well as tumour cell adhesion to vitronectin, fibronectin and collagen type I. Likewise, the synthetic peptide Gly-Arg-Gly-Asp-Ser (GRGDS) showed a similar effect. On a molar basis, rhodostomin was about 18,000 and 1000 times, respectively, more potent than GRGDS in inhibiting TCIPA and tumour cell adhesion.

Determination of the structure of two novel echistatin variants and comparison of the ability of echistatin variants to inhibit aggregation of platelets from different species

Two new variants of short disintegrins were purified from the venom of Echis carinatus leakeyi and named echistatin beta and gamma. These proteins were found to be about 85% similar in amino acid sequence to echistatin alpha which has been well studied. The disulphide pattern of echistatin gamma appeared to be identical with that of echistatin alpha. They all contain the adhesive recognition sequence Arg-Gly-Asp (RGD) but inhibit the aggregation of platelets from human and other mammals with different potencies. Echistatin beta and alpha are far more effective on platelets from humans and guinea pigs than those from rabbits and rats whereas echistatin gamma is less discriminating of the platelets of the species tested. This species-dependent platelet sensitivity to echistatin beta and gamma could be attributed to the variations in residues 15, 21, 22 and 27, which are close to or within the RGD loop, rather than to the C-terminal variations after residue 46. Taking advantage of the presence of methionine residues flanking both sides of the ARGDDM motif in echistatin gamma, we deleted this hexapeptide by CNBr cleavage to produce des-(23-28)-echistatin gamma. The modified protein showed c.d. and fluorescent spectra grossly similar to the intact echistatin but its antiplatelet potency decreased more than 200-fold. We thus propose that a favourable conformation of the RGD region is responsible mainly for the high-affinity binding of echistatin to the platelet glycoprotein IIb-IIIa as shown previously for the binding of medium-size disintegrin.

Disintegrin modulates rat glomerular mesangial cell behavior

Disintegrins are a group of molecules containing Arg-Gly-Asp (RGD) sequence which can interfere with cell-matrix interaction. Rat mesangial cells are known to express an RGD-sensitive integrin receptor. We investigate the effect of a potent disintegrin, triflavin, on rat mesangial cell adhesion and proliferation. While synthetic RGD-containing peptide (GRGDS) inhibits mesangial cell adhesion to fibronectin, type I and type III collagen, triflavin has a similar but much more potent effect. Triflavin, when added to the medium, inhibits serum-stimulated rat mesangial cell proliferation in both growing and growth-arrested cells. In serum-free medium, platelet-derived growth factor promoted mesangial cell growth is also inhibited by triflavin. When coated as a substratum, both GRGDS and triflavin inhibit mesangial cell growth. The inhibition of proliferation by exogenously added triflavin and GRGDS is, at least in part, due to their antiadhesive effect. We conclude that triflavin can modulate both adhesion and proliferation of rat mesangial cells.

Characterization of integrin expression and regulation on SW-480 human colon adenocarcinoma cells and the effect of rhodostomin on basal and upregulated tumor cell adhesion

Integrins are a superfamily of cell surface glycoproteins that mediate cell-extracellular matrix (ECM) and cell-cell adhesion. Immunofluorescence microscopy and flow cytometric analysis using anti-integrin mAbs as the primary binding ligands demonstrated that the platelet integrin receptor alpha IIb beta 3, as well as alpha v beta 3, alpha 5 beta 1 and alpha 6 beta 1, are present on the surface of SW-480 human colon adenocarcinoma cells. Monoclonal antibodies (mAbs) against alpha IIb beta 3 and alpha 5 beta 1 inhibited unstimulated basal adhesion to fibronectin by approximately 30% and 40%, respectively. The surface immunoreactivity of tumor cells for alpha IIb beta 3 was enhanced by pretreatment (5 min) with a phorbol ester (12-O-tetradecanoylphorbol-13-acetate (TPA)) or a lipoxygenase metabolite of arachidonic acid, 12-hydroxyeicosatetraenoic acid (12-HETE) in a dose- and time-dependent manner. SW-480 cells possess a large intracellular pool of alpha IIb beta 3, from which the receptor complex translocates to the cell surface following pretreatment with TPA or 12(S)-HETE. This pretreatment enhances adhesion to fibronectin, which is mediated exclusively by alpha IIb beta 3 integrins. Staurosporine was found to block alpha IIb beta 3 up-regulation and enhanced-adhesion. TPA and 12(S)-HETE also facilitated the redistribution of alpha IIb beta 3 during the enhanced-spreading process. Rhodostomin, an Arg-Gly-Asp- (RGD) containing antiplatelet snake venom peptide, was about 400-times more potent than RGDS at inhibiting control, TPA- or 12(S)-HETE-enhanced adhesion of SW-480 cells to fibronectin. The binding of mAbs against alpha IIb beta 3, alpha v beta 3 and alpha 5 beta 1 was inhibited by pretreatment with rhodostomin, suggesting that rhodostomin binds via its RGD sequence to multiple integrin receptors (i.e., alpha IIb beta 3, alpha v beta 3, alpha 5 beta 1) expressed on the SW-480 cell surface, inhibiting cell adhesion to ECM.

Analysis of human platelet glycoprotein IIb-IIIa by fluorescein isothiocyanate-conjugated disintegrins with flow cytometry

Disintegrins are a group of snake venom peptides which inhibit human platelet aggregation by acting as glycoprotein IIb-IIIa (GPIIb-IIIa) antagonists. They are cysteine-rich, Arg-Gly-Asp (RGD)-containing peptides, and bind to GPIIb-IIIa complex on platelet membrane with a very high affinity (Kd, 10(-7)-10(-8) M). In this study, we analyzed GPIIb-IIIa complex on platelet membrane by flow cytometry using fluorescein isothiocyanate (FITC)-conjugated disintegrins as probes. Of these FITC-conjugated disintegrins, FITC-Rhodostomin is the most sensitive probe because Rhodostomin was conjugated with more FITC molecules than Trigramin and Halysin were. The binding fluorescence intensity of FITC-Trigramin (FITC-Tg), FITC-Halysin (FITC-Hy) and FITC-Rhodostomin (FITC-Rn) was measured in both resting and ADP-activated platelets of diluted human platelet-rich plasma. The binding fluorescence of FITC-disintegrins was abolished by EDTA and 7E3, a monoclonal antibody against GPIIb-IIIa. ADP markedly increased the fluorescence intensity of FITC-Tg and FITC-Hy bound on platelets especially when lower doses of these probes were used, whereas it had little effect on that of FITC-Rn. Therefore, FITC-Tg and FITC-Hy can be used for the detection of the activated platelets as noted by a higher ratio of fluorescence intensity (approx. 2-4) between ADP-activated and resting platelets as compared with that (approx. 1-1.3) in the case of FITC-Rn as the probe. The platelets from three patients with Glanzmann's thrombasthenia were probed with FITC-disintegrins.(ABSTRACT TRUNCATED AT 250 WORDS)

In vivo antithrombotic effect of triflavin, an Arg-Gly-Asp containing peptide on platelet plug formation in mesenteric microvessels of mice

Triflavin, an Arg-Gly-Asp-containing snake venom peptide, inhibits platelet aggregation through the blockade of fibrinogen binding to the activated platelets. In this study, platelet thrombus formation was induced by irradiation of the mesenteric venules with filtered light in mice pretreated intravenously with fluorescein sodium. Electron microscopy reveals moderately damaged endothelial cells, as well as aggregates consisting almost exclusively of platelets with pseudopod formation, and degranulated appearance. Triflavin (10-20 micrograms/g) significantly prolonged the lag period of inducing platelet plug formation in mesenteric venules when it was intravenously infused. Triflavin (20 micrograms/g) prolonged the occlusion time about 2-fold (from control 112 +/- 23 to 240 +/- 47 s). Furthermore, PGE1 briefly prolonged the occlusion time about 1.5-fold (from 105 +/- 21 to 168 +/- 20 s) when it was given by continuous infusion (40 micrograms/kg/min). On the other hand, triflavin was also effective in reducing the mortality of ADP-induced acute pulmonary thromboembolism in mice when administered intravenously at dose of 2-4 micrograms/g. Heparin (1.5 U/g) and indomethacin (200 micrograms/g) had no significant effect in prolonging the occlusion time or in reducing ADP-induced pulmonary embolism in mice. Therefore, triflavin is an effective antithrombotic agent in preventing the thromboembolism in these two in vivo models.

Ancrod-formed fibrin stimulates prostacyclin production of human umbilical vein endothelial cells via de novo synthesis of cyclooxygenase

Ancrod, a thrombin-like enzyme, has been used as defibrinogenating agent in prevention of venous thrombosis. This study showed ancrod in citrated plasma elevated 6-keto PGF1 alpha production of human umbilical vein endothelial cells; this increment of 6-keto PGF1 alpha was completely inhibited by Gly-Pro-Arg-Pro, an inhibitor of fibrin polymerization. The enhanced prostacyclin production, but not basal level of prostacyclin, was inhibited by actinomycin D and cycloheximide. In washed aspirin-pretreated cells, ancrod-formed fibrin induced restoration of prostacyclin production by a cycloheximide- and actinomycin D-sensitive process. Ancrod-formed fibrin stimulated synthesis of cyclooxygenase as probed by Western blotting and this enhancement was blocked by actinomycin D and cycloheximide. In conclusion, we first report that ancrod-formed fibrin stimulates prostacyclin production of human endothelial cells and this event is dependent on de novo synthesis of cyclooxygenase.

Characterization of platelet aggregation induced by human colon adenocarcinoma cells and its inhibition by snake venom peptides, trigramin and rhodostomin

SW-480 cells, derived from a primary human colon adenocarcinoma, caused dose-dependent platelet aggregation in heparinized human platelet-rich plasma. SW-480 tumour cell-induced platelet aggregation (TCIPA) was completely inhibited by hirudin (5 U/ml) but unaffected by apyrase (10 U/ml). This TCIPA was also unaffected by cysteine proteinase inhibition with E-64 (10 microM) but was limited by cell pretreatment with phospholipase A2. SW-480 cell suspension caused marked dose-dependent decreases in plasma recalcification times using normal, factor VIII-deficient and factor IX-deficient human plasma. This effect was potentiated with cell lysates but inhibited in intact cells pretreated with sphingosine. SW-480 cell suspension did not affect the recalcification time of factor VII-deficient plasma. Moreover, monoclonal antibody against human tissue factor completely abolished SW-480 TCIPA. Taken together, these data suggest that SW-480 TCIPA arises from SW-480 tissue factor activity expression. Trigramin and rhodostomin, RGD-containing snake venom peptides, which antagonize the binding of fibrinogen to platelet membrane glycogen IIb/IIIa, prevented SW-480 TCIPA. Likewise, synthetic peptide GRGDS as well as monoclonal antibodies against platelet membrane glycoprotein IIb/IIIa and Ib prevent SW-480 TCIPA, which was unaffected by control peptide GRGES. On a molar basis, trigramin (IC50 0.09 microM) and rhodostomin (IC50 0.03 microM) were about 6000 and 18,000 times, respectively, more potent than GRGDS (IC50 0.56 mM).

Antiplatelet and vasorelaxing actions of the acetoxy derivative of cedranediol isolated from Juniperus squamata

The antiplatelet and vasorelaxing actions of 14-acetoxycedrol, an acetyl derivative of the sesquiterpene 8,14-cedranediol isolated from Juniperus squamata Hayata, were investigated in washed rabbit platelets and rat aorta, respectively. 14-Acetoxycedrol inhibited the aggregation and ATP release of rabbit platelets induced by ADP, arachidonic acid, platelet-activating factor (PAF), collagen, and thrombin. Prolongation of the incubation time of 14-acetoxycedrol with platelets did not cause further inhibition and the aggregability of the treated platelets could be restored after washing of the platelets. It inhibited thromboxane B2 formation of washed platelets caused by arachidonic acid, collagen, and thrombin in a concentration-dependent manner. The formation of inositol phosphate caused by collagen and PAF was inhibited by 14-acetoxycedrol, while that caused by thrombin was not affected. 14-Acetoxycedrol markedly inhibited the intracellular calcium rise caused by PAF, and slightly inhibited that caused by thrombin in quin-2/AM-load platelets. In rat thoracic aortae, 14-acetoxycedrol inhibited the high K+ (60 mM) and Ca2+ (0.03-3 mM) induced cumulative contractions in a concentration-dependent manner, while it did not affect the phasic and tonic contractions elicited by norepinephrine. The tonic contractions elicited by KCl (60 mM) and Bay K 8644 were also relaxed by 14-acetoxycedrol. It is concluded that the antiplatelet effect of 14-acetoxycedrol is due to the inhibition of thromboxane formation and phosphoinositides breakdown and the vasorelaxing action of 14-acetoxycedrol is due to inhibition of Ca2+ influx through the voltage-dependent Ca2+ channel.

Effect on human platelet aggregation of phospholipase A2 purified from Heloderma horridum (beaded lizard) venom

By means of gel filtration, ionic exchange chromatography and DEAE-column HPLC, an acidic phospholipase A2 (PLA2) was purified from beaded lizard (Heloderma horridum) venom. The purified PLA is a single-chain polypeptide, consisting of about 163 amino acid residues with a molecular mass of 19,000 Da as calculated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and amino acid analysis. HHV-PLA showed a rather specific inhibitory effect on platelet aggregation induced by U46619 and epinephrine in human platelet-rich plasma in a dose- and time-dependent manner, whereas it had little effect on collagen- and ADP-induced aggregation. ATP-release reaction induced by various agonists were dose- and time-dependently inhibited by HHV-PLA, even though platelet aggregation was apparently not affected in human washed platelets. When HHV-PLA was chemically modified with p-bromophenacyl bromide, both of its enzymatic activity and antiplatelet activity were lost. Furthermore, exogenous lysophosphatidylcholine and HHV-PLA treated phosphatidylcholine inhibited platelet aggregation induced by U46619 in human washed platelets. In conclusion, PLA enzyme from H. horridum venom inhibits exclusively U46619- or thromboxane-induced platelet aggregation of human platelet-rich plasma probably by virtue of their PLA enzymatic activity on plasma phospholipids, converting phospholipids (e.g., phosphatidylcholine) into lysophospholipids, which in turn interfere with the coupling of TXA2 receptor and its signalling transduction system.

Triflavin, an antiplatelet peptide, inhibits tumor cell-extracellular matrix adhesion through an arginine-glycine-aspartic acid-dependent mechanism

The interaction of tumor cells with extracellular matrix components such as laminin, fibronectin, and collagen has been shown to be mediated through a family of cell-surface receptors that specifically recognize an arginine-glycine-aspartic acid amino acid sequence within each protein. Triflavin, a 7.5 kDa cysteine-rich polypeptide purified from Trimeresurus flavoviridis snake venom, belongs to a family of arginine-glycine-aspartic acid-containing peptides termed disintegrins that have been isolated from the venoms of various vipers and shown to be potent inhibitors of platelet aggregation. In this study, we showed that triflavin inhibited adhesion of human hepatoma J-5 cells to extracellular matrices (fibronectin, vitronectin, fibrinogen, and collagen type I) in a dose-dependent manner. On the other hand, triflavin exerted a limited inhibitory effect on cell attachment to collagen type IV and laminin (< or = 40%). Triflavin is approximately 1000 times more potent than glycine-arginine-glycine-aspartic acid-serine at inhibiting cell adhesion. When immobilized on plate, triflavin promoted J-5 cell attachment; this attachment was inhibited by glycine-arginine-glycine-aspartic acid-serine. In addition, triflavin labeled with iodine 125 binds to J-5 cells in a saturable manner and its binding was also inhibited by glycine-arginine-glycine-aspartic acid-serine. Its Kd value was estimated to be 3.9 x 10(-7) mol/L and the number of binding sites was around 60,000 per cell. Furthermore, triflavin did not affect tritiated thymidine uptake during a 3-day incubation.(ABSTRACT TRUNCATED AT 250 WORDS)

The relaxant action of osthole isolated from Angelica pubescens in guinea-pig trachea

The effect of osthole, isolated from Angelica pubescens, on the contraction of guinea-pig trachea was studied. Osthole (25-100 mumol/l), theophylline (10-1000 mumol/l) and higher concentrations of nifedipine (0.1-100 mumol/l) suppressed the contraction response curves of tracheal smooth muscle caused by carbachol, prostaglandin F2 alpha (PGF2 alpha), U46619 (thromboxane A2 analogue) and leukotriene C4 (LTC4) in a concentration-dependent manner. The contraction caused by high K+ (120 mmol/l) and cumulative concentrations of CaCl2 (0.03-3 mmol/l) was also inhibited concentration-dependently by osthole (25-100 mumol/l), theophylline (10-1000 mumol/l) and lower concentrations of nifedipine (0.01-0.1 mumol/l). The relaxant actions of osthole were not affected by propranolol (1 mumol/l), glibenclamide (10 mumol/l) or removal of tracheal epithelium. Osthole (100 mumol/l) was still effective in causing tracheal relaxation in the presence of nifedipine (1 mumol/l). In Ca(2+)-free- and EGTA (0.2 mmol/l)-containing medium, the relaxing effect of osthole was more potent than in normal Krebs solution. Osthole (25 and 50 mumol/l) caused 2.9 and 6.5, or 3.0 and 5.6 fold, respectively, increase in potency of forskolin or sodium nitroprusside in causing tracheal relaxation but did not affect that by cromakalim. Osthole (50 mumol/l) enhanced the increase in tissue cAMP and cGMP levels induced by forskolin and sodium nitroprusside, respectively, and in higher concentrations (100 and 250 mumol/l), itself increased markedly tissue cAMP and cGMP contents. Osthole (10-250 mol/l) inhibited the activity of cAMP and cGMP phosphodiesterases in a concentration-dependent manner. It is concluded that osthole exerts a non-specific relaxant effect on the trachealis by inhibiting the cAMP and cGMP phosphodiesterases.

Ex-vivo and in-vivo antithrombotic effect of triflavin, an RGD-containing peptide

Triflavin, an Arg-Gly-Asp-containing snake venom peptide, inhibits platelet aggregation through the blockade of fibrinogen binding to the activated platelets. It binds to fibrinogen receptors associated with the glycoprotein IIb/IIIa complex with a Kd value of 7 x 10(-8) M. In this study, we found that 125I-triflavin reached the maximal binding to human platelets within 5 min at 25 degrees C. In addition, when triflavin was intravenously administered at 1.0 mg kg-1 to rabbits, it reversibly impaired the platelet aggregation of platelet-rich plasma caused by ADP (20 microM) ex-vivo over 30 min. The platelet counts of the experimental rabbits remained unchanged. Triflavin was effective in reducing the mortality of ADP-induced acute pulmonary thromboembolism in mice when administered intravenously at a dose of 2 micrograms g-1. Therefore, triflavin was proven to be an effective antithrombotic agent in preventing ADP-induced acute pulmonary thromboembolism in mice and impairing reversibly the platelet function of rabbits when given intravenously.

In vivo effect of a thrombin-like enzyme on platelet plug formation induced in mesenteric microvessels of mice

Ancrod caused defibrinogenation and exhibited ex vivo antiplatelet activity in experimental rabbits. In this study, platelet thrombus was induced by irradiation of the mesenteric microvessels with filtered light in mice pretreated with fluorescein sodium intravenously. Ancrod (0.5-2 U/kg) dose-dependently, significantly prolonged the time lapse of inducing platelet plug formation in mesenteric venules when it was intravenously infused. At these doses, ancrod depleted plasma fibrinogen and displayed ex vivo antiplatelet aggregation induced by collagen. Ancrod (1 U/kg) prolonged the occlusion time about 2.1 folds (from control 103.2 +/- 17.0 to 211.2 +/- 26.3 seconds) with a duration longer than 60 min. On the other hand, PGI2 briefly prolonged the occlusion time about 1.5 folds when it was given by continuous infusion (250-500 ng/kg/min). Heparin (100-250 U/kg) had no significant effect in this model. Therefore, ancrod may be used as a therapeutic agent not only in treatment of venous thrombosis and possibly in prevention of arterial thrombosis.

Triflavin, an Arg-Gly-Asp-containing peptide, inhibits human cervical carcinoma (HeLa) cell-substratum adhesion through an RGD-dependent mechanism

Triflavin, a 7.5-kDa cysteine-rich polypeptide purified from Trimeresurus flavoviridis snake venom, belongs to a family of RGD-containing peptides, termed disintegrins, that have been isolated from the venoms of various vipers and shown to be potent inhibitors of platelet aggregation. The interaction of tumor cells with extracellular matrices such as fibronectin, vitronectin, and collagen has been shown to be mediated through a family of cell surface receptors that specifically recognize an arginine-glycine-aspartic acid (RGD) sequence within each adhesive protein. In this study, we show that triflavin dose-dependently inhibited adhesion of human cervical carcinoma (HeLa) cells to extracellular matrices (ECMs; i.e., fibronectin, fibrinogen, and vitronectin). On the other hand, triflavin exerted a limited inhibitory effect on cell adhesion to laminin and collagen (type I and IV). On a molar basis, triflavin is approximately 800 times more potent than Gly-Arg-Gly-Asp-Ser (GRGDS) at inhibiting cell adhesion. When immobilized on plate, triflavin significantly promoted HeLa cell adhesion, and this attachment was inhibited by GRGDS. Furthermore, FITC-conjugated triflavin bound to cells in a saturable manner and its binding was inhibited by GRGDS. In addition, triflavin did not affect [3H]thymidine uptake of HeLa cells during a 3-day incubation. These results suggest that triflavin probably binds to integrin receptors expressed on HeLa cell surface via its RGD sequence within its molecule, thereby inhibiting the adhesion of extracellular matrices to HeLa cells.

Frangulin B, an antagonist of collagen-induced platelet aggregation and adhesion, isolated from Rhamnus formosana

Emodin and its glycoside frangulin B were isolated from the plant Rhamnus formosana. Emodin inhibited the aggregation of rabbit platelets induced by arachidonic acid and collagen, without affecting that by ADP or PAF, while emodin acetate had no any antiplatelet effect. Frangulin B inhibited selectively and concentration-dependently collagen-induced aggregation and ATP release in rabbit platelets, without affecting those induced by arachidonic acid, ADP, PAF and thrombin. Frangulin B also inhibited the platelet aggregation induced by trimucytin which was reported to be a collagen receptor agonist isolated from Trimeresurus muscrosquamatus snake venom. The aggregability of platelets inhibited by frangulin B could be recovered after washing the platelets. Frangulin B also selectively suppressed the thromboxane B2 formation caused by collagen, but not those by arachidonic acid and thrombin. Similarly, the formation of inositol phosphate caused by collagen was also suppressed by frangulin B, while that of PAF or thrombin was not affected. In the presence of PGE1, frangulin B also decreased Mg(2+)-dependent platelet adhesion to collagen. It is concluded that frangulin B may be an antagonist of collagen receptor in platelet membrane.

Antiplatelet protease, kistomin, selectively cleaves human platelet glycoprotein Ib

Kistomin, a metalloprotease purified from venom of Calloselasma rhodostoma, dose- and time-dependently prolonged the latent period of aggregation and inhibited ATP secretion of human washed platelets stimulated by thrombin. It inhibited aggregation induced by low concentrations of thrombin (< or = 0.2 U/ml) whereas it had only slight effect on aggregation induced by high concentrations of thrombin (> or = 0.5 U/ml). Meanwhile it also inhibited ristocetin-induced platelet aggregation in a dose- and time-dependent manner. It significantly inhibited cytosolic calcium rise of Quin 2--loaded platelets, completely blocked thromboxane B2 formation, and blocked [3H]inositol phosphates formation of [3H]myoinositol loaded platelets stimulated by 0.1 U/ml of thrombin. Kistomin inhibited significantly thromboxane but not [3H]inositol phosphates formation of platelets stimulated by a high concentration of thrombin (1 U/ml). Incubation of platelets with kistomin resulted in a selective cleavage of platelet membrane glycoprotein Ib as revealed by SDS/PAGE stained by periodic acid/Schiff reagent. These results suggested that thrombin activates platelets at least through two receptors/or effectors-mediated events. In addition to glycoprotein Ib, other surface membrane component(s) (e.g., the seven transmembrane domain thrombin receptor) may also be important in regulating the biochemical events of human platelets in response to thrombin. However, the extent and rate of platelet aggregation stimulated by low concentrations of thrombin ( < or = 0.2 U/ml) are closely related with the intactness of glycoprotein Ib.

Triflavin, an Arg-Gly-Asp-containing peptide, inhibits tumor cell-induced platelet aggregation

In this study, we examined the effect of triflavin, an Arg-Gly-Asp (RGD)-containing snake venom peptide, on human cervical carcinoma (HeLa) cell- and B16-F10 mouse melanoma cell-induced platelet aggregation (TCIPA) in heparinized platelet-rich plasma. TCIPA appears to play an important role in the development of certain experimental tumor metastases. Two ADP-scavenging agents, apyrase (10 U/ml) and creatine phosphate (CP) (5 mM)/creatine phosphokinase (CPK) (5 U/ml) completely inhibited B16-F10 TCIPA, but hirudin (5 U/ml) had no effect. In contrast, apyrase and CP/CPK did not inhibit HeLa TCIPA while hirudin completely inhibited it. Furthermore, HeLa cells initially induced platelet aggregation and then blood coagulation at a later stage. In addition, HeLa cells shortened, in a concentration-dependent manner, the recalcification time of normal as well as factor VIII- and IX-deficient human plasma, but did not affect the recalcification time of factor VII-deficient plasma. This suggests that HeLa TCIPA occurs via activation of the extrinsic pathway, probably owing to tumor cell expression of tissue factor-like activity. HeLa cell-induced thrombin generation was confirmed by detection of amidolytic activity towards a chromogenic substrate, S-2238 (H-D-Phe-Pip-Arg-p-NA). Triflavin and GRGDS inhibited, in a dose-dependent manner, TCIPA caused by either cell line. On a molar basis, triflavin was 10,000-30,000 times more potent than GRGDS in this regard. Moreover, monoclonal antibodies raised against glycoprotein (GP) IIb/IIIa complex (i.e., 7E3 and AP2) and against GP Ib (i.e., AP1) completely inhibited HeLa TCIPA. 7E3 and AP2 inhibited B16-F10 TCIPA by up to 80% whereas AP1 showed only 30% inhibition of B16-F10 TCIPA. In conclusion, the inhibitory effect of triflavin on HeLa and B16-F10 TCIPA may be mediated principally by the binding of triflavin to the fibrinogen receptor associated with GP IIb/IIIa complex on the platelet surface. However, GP Ib is also involved in HeLa TCIPA as thrombin formation is the key factor in triggering platelet aggregation caused by HeLa cells.

Vasoconstricting effect in rat aorta caused by thaliporphine isolated from the plant Neolitsea konishii K

Thaliporphine (0.1-100 microM) produced sustained, concentration-dependent contraction in isolated rings of rat aorta. Thaliporphine (ED50 = 1.5 +/- 0.5 microM) was less potent than endothelin (ED50 = 3.9 +/- 0.4 nM), but was more potent than Bay K 8644 (ED50 = 5.5 +/- 0.6 microM). Thaliporphine also contracted guinea-pig trachea and ileum preparations, and increased the force of beating of rat left atria, but was less potent on these tissues than on rat aorta. Thaliporphine-induced contraction of rat aorta was not affected by prazosin (0.3 microM), atropine (1 microM), saralasin (10 microM) or ketanserin (10 microM). However, the contraction was slightly potentiated by the removal of endothelium. Preincubation of the rat aorta in Ca(2+)-free Krebs solution (containing 1 mM EGTA) for 15 min completely abolished thaliporphine-induced contractions, and the subsequent addition of 3 mM CaCl2 restored thaliporphine-induced contractions to the control level. In rat aorta, thaliporphine-induced contractions were significantly reduced by nifedipine (1-30 nM) or verapamil (0.01-0.3 microM), and significantly increased by Bay K 8644 (0.1 microM) or KCl (20 mM). The pA2 values for nifedipine and verapamil against thaliporphine-induced contractions were 9.4 +/- 0.1 and 8.4 +/- 0.2, respectively. The results indicate that thaliporphine exerts its vasoconstrictor effect on rat aorta mainly by promoting Ca2+ entry.

Trimucytin: a collagen-like aggregating inducer isolated from Trimeresurus mucrosquamatus snake venom

Trimucytin is a potent platelet aggregation inducer isolated from Trimeresurus mucrosquamatus snake venom. Similar to collagen, trimucytin has a run of (Gly-Pro-X) repeats at the N-terminal amino acids sequence. It induced platelet aggregation, ATP release and thromboxane formation in rabbit platelets in a concentration-dependent manner. The aggregation was not due to released ADP since it was not suppressed by creatine phosphate/creatine phosphokinase. It was not either due to thromboxane A2 formation because indomethacin and BW755C did not have any effect on the aggregation even thromboxane B2 formation was completely abolished by indomethacin. Platelet-activating factor (PAF) was not involved in the aggregation since a PAF antagonist, kadsurenone, did not affect. However, RGD-containing peptide triflavin inhibited the aggregation, but not the release of ATP, of platelets induced by trimucytin. Indomethacin, mepacrine, prostaglandin E1 and tetracaine inhibited the thromboxane B2 formation of platelets caused by collagen and trimucytin. Forskolin and sodium nitroprusside inhibited both platelet aggregation and ATP release, but not the shape change induced by trimucytin. In quin-2 loaded platelets, the rise of intracellular calcium concentration caused by trimucytin was decreased by 12-O-tetradecanoyl phorbol-13 acetate, imipramine, TMB-8 and indomethacin. In the absence of extracellular calcium, both collagen and trimucytin caused no thromboxane B2 formation, but still induced ATP release which was completely blocked by R 59022. Inositol phosphate formation in platelets was markedly enhanced by trimucytin and collagen. MAB1988, an antibody against platelet membrane glycoprotein Ia, inhibited trimucytin- and collagen-induced platelet aggregation and ATP release.(ABSTRACT TRUNCATED AT 250 WORDS)

CIS-19, a novel platelet activating factor receptor antagonist: in vitro and in vivo studies

The effects of CIS-19 (cis-2-(3,4-dimethoxyphenyl)-6-isopropoxy-7-methoxyl-1-(N-methylforma mido)-1,2, 3,4-tetrahydronaphthalene) was determined in vitro in rabbit platelets and in vivo in rats and guinea-pigs. CIS-19 inhibited in a selective and concentration-dependent manner the aggregation and ATP release reaction of rabbit platelets induced by PAF (4 nM). The IC50 values of CIS-19 on PAF-induced aggregation of washed platelets and platelet-rich plasma were 11.3 +/- 2.7 and 16.8 +/- 3.0 microM respectively. BN52021 also inhibited PAF-induced aggregation of washed platelets with an IC50 value of 11.7 +/- 2.8 microM. CIS-19 inhibited [3H]PAF (4 nM) binding to washed rabbit platelets with an IC50 value of 1.5 +/- 0.2 microM. The concentration-response curve of PAF-induced aggregation of washed platelets was shifted rightwards by CIS-19 with pA2 and pA10 values of 7.1 (6.8-7.3 for 95% confidence limit) and 6.1 (5.8-6.2) respectively. The thromboxane B2 formation of washed platelets caused by AA, collagen or thrombin was not affected by CIS-19 of concentrations below 400 microM. CIS-19 (25 microM) completely blocked PAF-induced, but not collagen- or thrombin-induced [3H]inositol monophosphate formation of washed platelets. When CIS-19 (2.5 and 5 mg/kg) was injected i.v. into the femoral vein, it did not affect the blood pressure of rats, but antagonized PAF (2.5 micrograms/kg, i.v.)-induced hypotensive shock either preventively or curatively. CIS-19 (2.5 and 5 mg/kg) also blocked PAF (50 ng/kg)-induced, but not AA (50 micrograms/kg)-induced, bronchoconstriction in guinea-pigs. It is concluded that CIS-19 is an effective PAF receptor antagonist not only in vitro, but also in vivo.

Rhodostomin, an RGD-containing peptide expressed from a synthetic gene in Escherichia coli, facilitates the attachment of human hepatoma cells

Rhodostomin (Rho) from snake venom, a potent inhibitor of platelet aggregation, contains 68 amino acids having an RGD sequence and 12 cysteine residues. A chemically synthesized Rho gene was cloned and expressed in Escherichia coli. The expression of Rho gene fused with the glutathione S-transferase (GST) gene was about 10-30% of total cell proteins. The Rho-fusion protein could be recognized by antibodies raised against either a native Rho peptide or a synthetic peptide. The purified GST-Rho coated on culture plates facilitated the attachment of human hepatoma cells, which was inhibitable by co-incubation with a synthetic hexapeptide GRGDSP but not with a related peptide of GRGESP, suggesting that the E. coli-expressed Rho-fusion protein was properly folded and biologically functional.