Thrombin-induced Human Platelet Aggregation Research Articles

Small-Molecule Disruptors of the Interaction between Calcium- and Integrin-Binding Protein 1 and Integrin αIIbβ3 as Novel Antiplatelet Agents.

Thrombosis, a key factor in most cardiovascular diseases, is a major contributor to human mortality. Existing antithrombotic agents carry a risk of bleeding. Consequently, there is a keen interest in discovering innovative antithrombotic agents that can prevent thrombosis without negatively impacting hemostasis. Platelets play crucial roles in both hemostasis and thrombosis. We have previously characterized calcium- and integrin-binding protein 1 (CIB1) as a key regulatory molecule that regulates platelet function. CIB1 interacts with several platelet proteins including integrin αIIbβ3, the major glycoprotein receptor for fibrinogen on platelets. Given that CIB1 regulates platelet function through its interaction with αIIbβ3, we developed a fluorescence polarization (FP) assay to screen for potential inhibitors. The assay was miniaturized to 1536-well and screened in quantitative high-throughput screening (qHTS) format against a diverse compound library of 14,782 compounds. After validation and selectivity testing using the FP assay, we identified 19 candidate inhibitors and validated them using an in-gel binding assay that monitors the interaction of CIB1 with αIIb cytoplasmic tail peptide, followed by testing of top hits by intrinsic tryptophan fluorescence (ITF) and microscale thermophoresis (MST) to ascertain their interaction with CIB1. Two of the validated hits shared similar chemical structures, suggesting a common mechanism of action. Docking studies further revealed promising interactions within the hydrophobic binding pocket of the target protein, particularly forming key hydrogen bonds with Ser180. The compounds exhibited a potent antiplatelet activity based on their inhibition of thrombin-induced human platelet aggregation, thus indicating that disruptors of the CIB1- αIIbβ3 interaction could carry a translational potential as antithrombotic agents.

In vitro assessment and phase I randomized clinical trial\xa0of anfibatide a snake venom derived anti-thrombotic agent targeting human platelet GPIb\u03b1

The interaction of platelet GPIbα with von Willebrand factor (VWF) is essential to initiate platelet adhesion and thrombosis, particularly under high shear stress conditions. However, no drug targeting GPIbα has been developed for clinical practice. Here we characterized anfibatide, a GPIbα antagonist purified from snake (Deinagkistrodon acutus) venom, and evaluated its interaction with GPIbα by surface plasmon resonance and in silico modeling. We demonstrated that anfibatide interferds with both VWF and thrombin binding, inhibited ristocetin/botrocetin- and low-dose thrombin-induced human platelet aggregation, and decreased thrombus volume and stability in blood flowing over collagen. In a single-center, randomized, and open-label phase I clinical trial, anfibatide was administered intravenously to 94 healthy volunteers either as a single dose bolus, or a bolus followed by a constant rate infusion of anfibatide for 24 h. Anfibatide inhibited VWF-mediated platelet aggregation without significantly altering bleeding time or coagulation. The inhibitory effects disappeared within 8 h after drug withdrawal. No thrombocytopenia or anti-anfibatide antibodies were detected, and no serious adverse events or allergic reactions were observed during the studies. Therefore, anfibatide was well-tolerated among healthy subjects. Interestingly, anfibatide exhibited pharmacologic effects in vivo at concentrations thousand-fold lower than in vitro, a phenomenon which deserves further investigation.Trial registration: Clinicaltrials.gov NCT01588132.

Total, Neutral, and Polar Lipids of Brewing Ingredients, By-Products and Beer: Evaluation of Antithrombotic Activities.

The in vitro antithrombotic properties of polar lipid constituents of malted grain (MG), pelleted hops (PH), brewer’s spent grain (BSG), spent hops (SH), wort, and bottled beer from the same production line were assessed in human platelets. The total lipids (TL) were extracted according to the Bligh and Dyer method and further separated into the total neutral lipids (TNL) and total polar lipids (TPL) extracts by counter-current distribution. The TL, TNL, and TPL extracts of all samples were assessed for their ability to inhibit platelet-activating factor (PAF) and thrombin-induced human platelet aggregation. The raw materials, by-products, wort, and beer lipid extracts all exhibited antithrombotic properties against PAF and thrombin. However, the beer TPL exhibited the lowest IC50 values against PAF-induced (7.8 ± 3.9 µg) and thrombin-induced (4.3 ± 3.0 µg) platelet aggregation indicating that these polar lipids were the most antithrombotic. The lipid extracts tended to be more bioactive against the thrombin pathway. The fatty acid content of all the TPL extracts were assessed using GC-MS. The fatty acid composition of the most bioactive TPL extracts, the wort and the beer, shared similar fatty acid profiles. Indeed, it was noted that fermentation seems to play a role in increasing the antithrombotic properties of polar lipids against PAF and thrombin by moderately altering the polar lipid fatty acid composition. Furthermore, the use of brewing by-products as a source of functional cardioprotective lipids warrants further investigation and valorisation.

Adrenoceptor α2A signalling countervails the taming effects of synchronous cyclic nucleotide-elevation on thrombin-induced human platelet activation and aggregation

The healthy vascular endothelium constantly releases autacoids which cause an increase of intracellular cyclic nucleotides to tame platelets from inappropriate activation. Elevating cGMP and cAMP, in line with previous reports, cooperated in the inhibition of isolated human platelet intracellular calcium-mobilization, dense granules secretion, and aggregation provoked by thrombin. Further, platelet alpha granules secretion and, most relevant, integrin αIIaβ3 activation in response to thrombin are shown to be prominently affected by the combined elevation of cGMP and cAMP. Since stress-related sympathetic nervous activity is associated with an increase in thrombotic events, we investigated the impact of epinephrine in this setting. We found that the assessed signalling events and functional consequences were to various extents restored by epinephrine, resulting in full and sustained aggregation of isolated platelets. The restoring effects of epinephrine were abolished by either interfering with intracellular calcium-elevation or with PI3-K signalling. Finally, we show that in our experimental setting epinephrine likewise reconstitutes platelet aggregation in heparinized whole blood, which may indicate that this mechanism could also apply in vivo.

Inhibitory Effect of 20(S)-Ginsenoside Rg3 on Human Platelet Aggregation and Intracellular Ca2+ Levels via Cyclic Adenosine Monophosphate Dependent Manner.

Intracellular Ca2+ ([Ca2+]i) induces platelet aggregation, and influences the activation of aggregation associated-molecules. The increased [Ca2+]i activates both the Ca2+/calmodulin-dependent phosphorylation of the myosin light chain and the diacylglycerol-dependent phosphorylation of pleckstrin to trigger granule secretion (i.e., dense body and α-granule) and platelet aggregation. This study was carried out to elucidate the antagonistic effect of 20(S)-ginsenoside Rg3 (G-Rg3) present in Panax ginseng Mayer on Ca2+. G-Rg3 inhibited thrombin-induced human platelet aggregation in a dose-dependent manner and suppressed thrombin-induced elevation of [Ca2+]i mobilization. G-Rg3 increased the levels of cAMP, and subsequently, elevated the phosphorylation of inositol 1,4,5-triphosphate receptor I (Ser1756) during thrombin-induced human platelet aggregation. Moreover, G-Rg3 inhibited thapsigargin-induced Ca2+ influx and the thrombin-induced elevation of extracellular signal-regulated kinase 2 phosphorylation. G-Rg3 exhibited an inhibitory effect on [Ca2+]i levels leading to granule release and thus a therapeutic potential against platelet-mediated thrombotic disease is suggested.

The heterotrimeric G protein Gβ1 interacts with the catalytic subunit of protein phosphatase 1 and modulates G protein–coupled receptor signaling in platelets

Thrombosis is caused by the activation of platelets at the site of ruptured atherosclerotic plaques. This activation involves engagement of G protein-coupled receptors (GPCR) on platelets that promote their aggregation. Although it is known that protein kinases and phosphatases modulate GPCR signaling, how serine/threonine phosphatases integrate with G protein signaling pathways is less understood. Because the subcellular localization and substrate specificity of the catalytic subunit of protein phosphatase 1 (PP1c) is dictated by PP1c-interacting proteins, here we sought to identify new PP1c interactors. GPCRs signal via the canonical heterotrimeric Gα and Gβγ subunits. Using a yeast two-hybrid screen, we discovered an interaction between PP1cα and the heterotrimeric G protein Gβ1 subunit. Co-immunoprecipitation studies with epitope-tagged PP1c and Gβ1 revealed that Gβ1 interacts with the PP1c α, β, and γ1 isoforms. Purified PP1c bound to recombinant Gβ1-GST protein, and PP1c co-immunoprecipitated with Gβ1 in unstimulated platelets. Thrombin stimulation of platelets induced the dissociation of the PP1c-Gβ1 complex, which correlated with an association of PP1c with phospholipase C β3 (PLCβ3), along with a concomitant dephosphorylation of the inhibitory Ser1105 residue in PLCβ3. siRNA-mediated depletion of GNB1 (encoding Gβ1) in murine megakaryocytes reduced protease-activated receptor 4, activating peptide-induced soluble fibrinogen binding. Thrombin-induced aggregation was decreased in PP1cα-/- murine platelets and in human platelets treated with a small-molecule inhibitor of Gβγ. Finally, disruption of PP1c-Gβ1 complexes with myristoylated Gβ1 peptides containing the PP1c binding site moderately decreased thrombin-induced human platelet aggregation. These findings suggest that Gβ1 protein enlists PP1c to modulate GPCR signaling in platelets.

Synergistic effect of peptide inhibitors derived from the extracellular and intracellular domain of αIIb subunit of integrin αIIbβ3 on platelet activation and aggregation

αIIbβ3, the major platelet integrin, plays a central role in hemostasis and thrombosis. Upon platelet activation, conformation of αIIbβ3 changes and allows fibrinogen binding and, subsequently, platelet aggregation. It was previously shown that a lipid-modified platelet permeable peptide, which corresponds to the intracellular acidic membrane distal sequence 1000LEEDDEEGE1008 of αIIb (pal-K-LEEDDEEGE or pal-K-1000-1008), inhibits thrombin-induced human platelet aggregation, by inhibiting talin association with the integrin. YMESRADR, a peptide corresponding to the extracellular sequence 313–320 of αIIb, is also a potent platelet aggregation inhibitor by mimicking the effect of a clasp between the head domains of αIIb and β3. The aim of the present study was to investigate the synergistic effect of the intra- and extracellular- peptide inhibitors on platelet aggregation, as well as on the phosphorylation of two signaling proteins, focal adhesion kinase (FAK) and extracellular signal-regulated kinase (ERK). Platelet preincubation with Pal-K-LEEDDEGE followed by YMESRADR showed a synergistic inhibitory activity on platelet aggregation. Platelet incubation with threshold inhibitory concentrations of both peptides provoked almost the total inhibition of aggregation, PAC-1 binding, and fibrinogen binding, but not P-selectin exposure on activated platelets’ surface. Like RGDS peptide, this mixture inhibits FAK phosphorylation whose phosphorylation is well known to be consecutive to the aggregation (postoccupancy events). However, in contrast to RGDS peptide that enhances ERK phosphorylation and activation, the mixture of threshold inhibitory concentrations of Pal-K-LEEDDEEGE and YMESRADR inhibits ERK phosphorylation. We suggest that the use of the intracellular in combination with the extracellular peptide inhibitor, acting with a non-RGD-like mechanism, may provide an alternative way to antagonize integrin αIIbβ3 activation.

Abstract 140: Magnesium Lithospermate B Inhibits Blood Coagulation and Platelet Aggregation: Novel Mechanism of a Traditional Drug for Cardiovascular Diseases

Magnesium lithospermate B (MLB) is one of the major components of Salvia miltiorrhiza root (Danshen). Danshen extracts have been used to control cardiovascular disease for centuries. In 2005, intravenous injection of Danshen depside salt was approved in China for treatment of chronic angina. Although clinical observations have suggested that Danshen extracts inhibited thrombosis, the exact mechanism has not been adequately explored. Using an in vitro whole blood clotting assay, we observed that MLB (250 μM) significantly reduced clot size. Both the clot wet and dry weights were decreased following treatment (108.3 mg vs. 63.5 mg, and 32.8 mg vs. 18.5 mg, p<0.05, respectively). Using thromboelastography, we found that MLB markedly decreased the mechanical strength of the clot and modestly delayed initiation of coagulation in cell-free blood plasma prepared by centrifugation (10,000 хg, 10 min). Under confocal microscopy, we further observed that MLB significantly reduced the density of the fibrin network formed in plasma following thrombin treatment, suggesting that MLB targets coagulation factors to inhibit coagulation. Recent network pharmacology analyses predict that MLB may interact with VWF, factor XIII (FXIII), or thrombin in the coagulation cascade. We found that MLB did not inhibit VWF-dependent platelet agglutination induced by botrocetin. ELISA revealed that MLB also did not significantly alter the binding of activated FXIII (FXIIIa) to fibrinogen. However, when native FXIII from blood plasma was used for the same assay, MLB significantly reduced the binding of FXIIIa to fibrinogen. Since generation of FXIIIa from FXIII is thrombin-dependent, these data suggest that MLB inhibits thrombin activity or thrombin generation. Indeed, we found that MLB markedly inhibited thrombin-induced gel-filtered human and mouse platelet aggregation. These data demonstrated a novel role for MLB in the inhibition of blood coagulation and platelet aggregation, likely through direct inhibition of thrombin function, although we cannot exclude its additional anti-thrombotic activities. Thus, purified MLB may represent an efficient, low-cost agent for treatment of artery and deep vein thrombosis.

Targeting the anionic region of human protease‐activated receptor 4 inhibits platelet aggregation and thrombosis without interfering with hemostasis

Human platelet activation and aggregation is a complex process. To date, many therapies have been developed targeting proteins that mediate this process to prevent unwanted activation. However, the current standard of care for acute coronary syndromes still has limitations, including bleeding risk. To evaluate the protease-activated receptor 4 (PAR4) anionic cluster as a viable antiplatelet target by using a polyclonal antibody (CAN12). We used western blotting, aggregation and secretion ex vivo to evaluate the ability of CAN12 to interact with PAR4 and inhibit platelet activation. The effects of CAN12 in vivo were evaluated with the Rose Bengal arterial thrombosis model and two models of hemostasis. CAN12 was able to interact with human PAR4 and delay PAR4 cleavage. In addition, CAN12 inhibited thrombin-induced human platelet aggregation and secretion in a dose-dependent manner. The specificity of CAN12 was agonist-dependent. In vivo, we determined that CAN12 was able to inhibit arterial thrombosis, and, using two independent methods, we found that CAN12 did not influence hemostasis. Targeting the extracellular anionic cluster on PAR4 is a viable novel strategy as an antiplatelet therapy.

Abstract 499: Development of a Novel Antithrombotic Therapy Targeting Platelet GPIbα: Assessment of Anfibatide In Vitro and in Phase I Clinical Trial

Platelet adhesion and aggregation are critical for hemostasis and atherothrombosis. Interaction between the GPIb complex and VWF initiates platelet adhesion and contributes to complete vessel occlusion, particularly at sites of stenosis. However, no therapeutic targeting this pathway has been developed. Previously, we demonstrated in murine thrombosis models that Anfibatide, a snake venom-derived GPIbα antagonist, inhibited botrocetin-induced VWF-GPIbα binding and inhibited thrombus formation in vitro and in vivo, without significantly activating platelets or changing bleeding. We subsequently hypothesized that Anfibatide would be a safe and potent anti-thrombotic agent in humans. We tested Anfibatide with human platelets in vitro and conducted the first anti-GPIbα phase I clinical trial with 94 healthy volunteers. In vitro, Anfibatide inhibited ristocetin-induced and low dose thrombin-induced human platelet aggregation. Surface plasmon resonance studies showed that Anfibatide binds to GPIbα and blocks binding of both VWF A1 domain and thrombin. In ex vivo perfusion chambers, Anfibatide strongly inhibited human platelet adhesion, aggregation, and thrombus formation on a collagen-coated surface, especially at high shear flow conditions. Importantly, Anfibatide effectively dissolved the preformed thrombi in perfusion chambers. Anfibatide incubation with human whole blood did not significantly change coagulation parameters measured by thromboelastography. After treating volunteers with Anfibatide, no serious adverse events, premature discontinuations due to adverse events, or deaths occurred during the study. Anfibatide bound to 95% of GPIb and inhibited 90% of platelet aggregation without prolonging activated partial thromboplastin time, prothrombin time, thrombin time, or bleeding time. There was no bleeding tendency, no alteration of platelet count, and no detectable anti-Anfibatide antibodies. In conclusion, Anfibatide provides anti-thrombotic effects through inhibition of both GPIbα-VWF and GPIbα-thrombin interactions. These results demonstrate that Anfibatide is a safe and potent anti-platelet reagent with great potential for future anti-thrombotic therapy.

An Antibody Targeted To The Anionic Region Of Human Protease Activated Receptor 4 Inhibits Thrombosis Without Influencing Bleeding

Protease activated receptors (PARs) are G-protein coupled receptors which are activated by cleavage of their N-terminus by thrombin. This generates a tethered ligand which is then able to activate the corresponding receptor. Human platelets express PAR1 and PAR4, which both have crucial roles in mediating the response of platelets to injury. Our hypothesis is that PAR4 is an ideal target for new anti-platelet therapies because it is required for stable clot formation and has limited tissue distribution. We have previously determined a region on PAR4 that is required for efficient activation by thrombin. A polyclonal antibody (CAN12) targeted to this region of the PAR4 exodomain does not cross react to PAR1. Initial studies determined that CAN12 is able to block thrombin-induced human platelet aggregation with an IC50 of 10 ng/ml. Control IgG does not inhibit aggregation at 2 mg/ml. In mouse platelets, CAN12 inhibits P-selectin expression and integrin activation. In the Rose-Bengal mouse model of carotid artery thrombosis, CAN12 (1 mg/kg) administered 10 minutes prior to injury was able to completely inhibit the formation of a thrombus in a dose dependent manner. The antibody delayed thrombosis to greater than 90 min; the experiment was terminated at 90 minutes. In contrast, control treatment (2 mg/kg IgG or saline) resulted in complete occlusion at ∼40 minutes. Further, the minimal dose of CAN12 required for complete inhibition of thrombosis (0.5 mg/kg) administered fifteen minutes after injury also delayed thrombosis from ∼50 minutes to ∼80 minutes. This indicates that CAN12 is able to disrupt a thrombus after it has been initiated. Preliminary evidence indicates that CAN12 is able to delay the cleavage of PAR4. Importantly, CAN12 (2 mg/kg) treatment does not increase bleeding time or blood loss in the tail clip assay compared to control IgG (2 mg/kg) treatment. There was also no significant increase in bleeding in the saphenous vein assay. The mice treated with CAN12 (2 mg/kg) had an average bleeding time of 102 seconds for 12 clot formations in 20 minutes compared to the control mice (IgG 2 mg/kg) which had an average bleeding time of 143 seconds for 11 clot formations. These data demonstrate that we are able to inhibit platelet aggregation in vitro and thrombosis in vivo without influencing bleeding time. Overall, these studies provide insight towards the development of new anti-platelet therapies and, specifically, PAR4 as an antiplatelet therapy target. Disclosures:No relevant conflicts of interest to declare.

Gβ1 a Component Of The Heterotrimeric G Protein Is a New Protein Phosphatase 1c Interacting Protein That Regulates Platelet Activation

Platelet activation at the site of injury is tied to signal transduction events that are mediated by protein kinases and phosphatases. Reversible tyrosine, serine/threonine (Ser/Thr) phosphorylation-dependent assembly and/or disassembly of effector (cytoskeletal, signaling and adaptor) protein complexes propagate signaling downstream of G protein coupled receptors (GPCRs). Compared to kinases, the contribution of Ser/Thr phosphatases and its effectors in GPCR signaling studies is not well explored. Our previous studies had revealed that the catalytic subunit of protein phosphatase 1γ (PP1cγ) support GPCR signaling and thrombus formation. Since cell signaling networks are dependent on protein-protein interactions, we sought to identify the potential effectors of PP1cγ. We employed yeast two-hybrid interaction studies with the full length PP1cγ fused to GAL4 activating domain as bait and screened human bone marrow library. A novel interaction of PP1cγ with a protein called Gβ1 (GNB1) was identified. Gβ1 is a component of the heterotrimeric G proteins like the Gα and couple to GPCR. However, unlike Gα subunits, Gβ1 is unexplored in platelets. Co-immunoprecipitation (co-IP) studies validated PP1cγ-Gβ1 interaction in 293 cells expressing PP1cγ-HA and Gβ1-FLAG. Importantly, Gβ1 interacted with all the PP1c isoforms, suggesting that Gβ1 could target all PP1c isoforms to the GPCR complex. Purified PP1c bound to recombinant Gβ1-GST protein but not to GST protein, indicating that the in vitro interaction of PP1c with Gβ1 was direct and independent of Gα and Gγ subunits. A small molecule inhibitor of G protein βγ, gallein decreased thrombin-induced human platelet aggregation and adhesion to immobilized fibrinogen. There is a paucity of Gβ1-/- platelets because Gβ1-/- mice die within 2 days of birth due to microencephaly. siRNA mediated depletion of Gβ1 in murine megakaryocytes reduced PAR4-activating peptide induced soluble fibrinogen binding to αIIbβ3. These studies suggest a functional role for Gβ1 in GPCR signaling. PP1c co-immunoprecipitated with Gβ1 in resting platelets and agonist (thrombin and ADP) treatment under non-stirring conditions induced dissociation of PP1c from Gβ1. These studies indicate that Gβ1-PP1c complex in platelets is responsive to agonist. Furthermore, PP1c and Gβ1 associated with P2Y12 receptor in resting but not agonist activated platelets in a co-IP assay, suggesting a role for this complex in G protein signaling. Finally, agonist induced dissociation of PP1c from Gβ1 correlated with the association of PP1c with the downstream GPCR effector phospholipase C β3 (PLCβ3) with a concomitant dephosphorylation of PLCβ3 at Ser1105. Since previous studies have revealed that PLCβ3 activity is inhibited by Ser1105 phosphorylation, our observation suggest that agonist-induced association of PP1c with PLCβ3 facilitates dephosphorylation and activation of PLCβ3. These studies highlight a coupling of GPCR signaling with the phosphatase driven signal transduction during platelet activation. Disclosures:No relevant conflicts of interest to declare.

Abstract 18777: Glucagon-like Peptide-1 Inhibits Thrombin-induced Human Platelet Aggregation

Background: Studies of glycemic control in diabetics receiving glucagon-like peptide-1 (GLP-1)-targeted drugs have suggested reduced incidences of cardiovascular events over 6-12 months. Mechanisms underlying this surprisingly rapid effect are not known. Methods & Results: To investigate potential anti-thrombotic effects of GLP-1 and related analogs, we tested GLP-1(7-36)NH2, the most abundant endogenous GLP-1, and the commercially available GLP-1 analog exenatide in thrombin-induced platelet aggregation assays using freshly isolated gel-filtered human platelets (3x108/ml) in normoglycemic conditions (5.5mM glucose). Incubation with GLP-1 (10-10 to 10-6 M) or exenatide (5x10-10 to 10-8 M) for 15 min at 37°C delayed and reduced platelet aggregation as compared to PBS-treated controls. GLP-1 (10^-9M) resulted in a 60% decrease in aggregation 1 min after thrombin stimulation. To investigate whether this effect may be mediated by a GLP-1 receptor (R), whole cell lysates (40 µg) from gel-filtered human platelets and the human megakaryocyte (MGK) cell line MEG-01 were probed with a polyclonal anti- GLP-1R Ab (1:1250; Santa Cruz). Although Western blot showed bands (∼56 kD) consistent with the known GLP-1R in both platelets and MEG-01 cells, GAPDH-normalized expression levels of GLP-1R were ∼13 and ∼45 fold lower in these cells than in human pancreas. Given the limited specificity of Abs raised against GPCRs, RT-PCR was performed on RNA from MEG-01 cells with primers designed to yield full-length human GLP-1R cDNA. PCR products were cloned and sequenced, revealing inserts identical to GLP-1R. As this receptor is known to be adenylate cyclase-coupled, cAMP assays were also performed (EIA kit, Cayman Chemicals). MEG-01 cells incubated for 15 min with 10-7 M GLP-1 showed a ∼60% increase in intracellular cAMP, an inhibitor of platelet activation, as compared to untreated controls; while the cAMP response to prostacyclin PGI2 was much greater (positive control; 16 fold). Conclusion: Human platelets and MGK express low levels of GLP-1R. While GLP-1 induced only small increases in cAMP in MGK, both GLP-1 and exenatide inhibited thrombin-induced platelet aggregation. These data suggest that GLP-1-targeted drugs may possess anti-platelet effects.

Baupain, A Plant Cysteine Proteinase That Hinders Thrombin-Induced Human Platelet Aggregation

Bauninia forficata is trivially known as cow paw, and popularly used in Brazil for treatment of diabetes mellitus. Denominated baupain a cysteine proteinase was purified from B. forficata leaves. In this study, we investigated the baupain effect on aggregation of isolated human platelets in vitro and the results show that baupain hinders thrombin - but not ADP- and collagen- induced platelet aggregation. With synthetic quenched-fluorescent peptides, the kinetics of the cleavage site of human proteinase-activated receptor 1 / 2 / 3 and 4 [PAR-1 / 2 / 3 and 4] by baupain was determined. In conclusion, similar to bromelain and papain, baupain hinders human platelets aggregation, probably through an unspecific cleavage in the Phe-Leu bond of PAR1.

A review of the use of bromelain in cardiovascular diseases

In 2004 an estimated 17.1 million people died from cardiovascular diseases (CVDs) worldwide, representing 29% of all global deaths. According to the American Heart Association, heart disease and stroke are the main cause of death and disability among people with type 2 diabetes. Additional safe and effective approaches are needed for the prevention and management of CVDs which may include nutritional supplements. To identify the potential of bromelain (a food supplement) on the risk factors associated with CVDs. An electronic and manual search was conducted during November 2009 to March 2010. The databases searched included: Ovid MEDLINE; All EBM Reviews-Cochrane Database of Systematic Reviews (Cochrane DSR), American College of Physicians (ACP) Journal Club, Database of Abstracts of Reviews of Effects (DARE), Cochrane Central Register of Controlled Trials (CCTR), Cochrane Methodology Register (CMR), Health Technology Assessment (HTA) and National Health Service Economic Evaluation Database (NHSEED); Allied and Complementary Medicine (AMED); British Nursing Index and Archive; EMBASE; Health Management Information Consortium (HMIC); Science Direct and Electronic Thesis Online Services (ETHOS). Only papers in the English language were included. Randomised controlled trials (RCTs), human studies, animal studies and experimental studies related to bromelain for CVDs. The quality assessment of all the selected studies was conducted by the authors. Data from 3 animal trials and 3 human trials were included in the review. Data collected included: type of trial, drug dosage, duration, outcome measures, characteristics of bromelain used, significance of results and conclusion. Out of 223 papers retrieved, 6 papers met the inclusion criteria and could be included in the review. These comprised of 3 animal and 3 human trials, each of which investigated the use of bromelain for CVDs. Results suggested that bromelain could be used for treating acute thrombophlebitis, as it decreases aggregation of blood platelets, has a cardio-protective effect, ameliorates rejection-induced arterial wall remodelling, prevents thrombin-induced human platelet aggregation as well as reduces thrombus formation. No substantive study of bromelain and clinical CVDs has been carried out in human populations. Only a few studies on bromelain and CVDs were published from 1948 to 2010. This may be an area worthy to be explored in future CVDs research.

Characteristics of a New DNA Aptamer, Direct Inhibitor of Thrombin

Characteristics of a new antithrombin DNA-aptamer RE31 were studied. This aptamer inhibited thrombin formation in human plasma catalyzed by exogenous (lengthening of thrombin time) and endogenous thrombin (lengthening of partial prothrombin time and activated partial thromboplastin time). In addition, the aptamer completely suppressed thrombin-induced aggregation of human platelets. On the other hand, RE31 did not reduce amidolytic activity of thrombin towards the short peptide substrate, in other words, did not modify the state of enzyme active center. By the capacity to inhibit clotting reactions, RE31 was superior to the previously described highly effective 31-component antithrombin aptamer 31TBA (thrombin binding aptamer, TBA). The effect of RE31 was species-specific: it inhibited human thrombin activity more effectively than activities of rat and rabbit thrombins.

Inhibition of Thrombin Activity with DNA-Aptamers

The effects of two DNA aptamers (oligonucleotides) 15TBA and 31TBA (15- and 31-mer thrombin-binding aptamers, respectively) on thrombin activity were studied. Both aptamers added to human plasma dose-dependently increased thrombin time (fibrin formation upon exposure to exogenous thrombin), prothrombin time (clotting activation by the extrinsic pathway), and activated partial thromboplastin time (clotting activation by the intrinsic pathway). At the same time, these aptamers did not modify amidolytic activity of thrombin evaluated by cleavage of synthetic chromogenic substrate. Aptamers also inhibited thrombin-induced human platelet aggregation. The inhibitory effects of 31TBA manifested at lower concentrations than those of 15TBA in all tests. These data indicate that the studied antithrombin DNA aptamers effectively suppress its two key reactions, fibrin formation and stimulation of platelet aggregation, without modifying active center of the thrombin molecule.

Sensitivity of thrombin-induced platelet aggregation to inhibition by ethanol

Background The purpose of this study was to determine the concentration of ethanol required for inhibition of thrombin-induced aggregation of human platelets in vitro. Methods Three methods, light transmission method, particle counting method using light scattering and screen filtration pressure method using whole blood, were used for evaluation of platelet aggregation. Results Thrombin (0.2 U/ml)-induced platelet aggregation was significantly inhibited by ethanol, and the threshold ethanol concentrations for inhibition determined by measurements using the light transmission method, particle counting method and screen filtration pressure method were 0.5%, 0.125% and 2%, respectively. Platelet aggregation induced by a lower concentration (0.05 U/ml) of thrombin was detected by the screen filtration pressure method but not by the light transmission method or particle counting method. The threshold ethanol concentration for the inhibition of thrombin (0.05 U/ml)-induced platelet aggregation was 0.125% according to results obtained by using the screen filtration pressure method. Conclusions Sensitivities for detection of thrombin-induced platelet aggregation and for detection of inhibition of platelet aggregation by ethanol were high in the screen filtration pressure method and the particle counting method, respectively. The threshold concentration of ethanol to inhibit thrombin-induced platelet aggregation is about 0.125% (approximately 21 mmol/l), which is easily attainable after drinking alcohol.

Effect of low and high dose melagatran and other antithrombotic drugs on platelet aggregation

Thrombin-induced aggregation of human platelets can be completely inhibited by melagatran, the bioactive form of ximelagatran, an oral direct thrombin inhibitor. The potential of melagatran to differentially inhibit alpha- and gamma-thrombins was tested with a synthetic thrombin substrate. Washed human platelets were also employed to determine if melagatran differentially inhibited alpha- and gamma-thrombin-induced platelet aggregation at distinct platelet thrombin receptors. In vitro studies were conducted with washed human platelets to determine thrombin-induced aggregation responses in the presence of varying doses of the anti-thrombotic drugs: melagatran, argatroban, heparin, and hirudin. Melagatran rapidly inhibits the hydrolysis of a thrombin chromogenic substrate within 0-1 min with alpha-, beta- and gamma-thrombin being equally inhibited by high dose melagatran while alpha-thrombin was significantly more sensitive at low doses. The maximum level of melagatran inhibition of alpha- and gamma-thrombin-induced platelet aggregation requires platelets to be pre-incubated with the drug for 10-30 min. Melagatran appears to have no direct effect on the PAR-1 receptor. It does appear to have a direct effect on the GPIbalpha thrombin receptor activity as well as the PAR-4 receptor. Inhibition of platelet aggregation is dose dependent, however, at low melagatran doses (0.01-0.04 nM) platelets aggregate at significantly (P < 0.05) higher levels. The lower the level of thrombin-induced aggregation that was observed with control samples (aggregations from 10% to 39%), corresponded with a higher observed melagatran-induced stimulation with drug-treated platelets. The range of stimulation varies between several hundred percent at approximately 10% aggregation to around 20% at about 20-39% aggregation. Preliminary studies indicate that this in vitro stimulatory effect is abrogated in platelets derived from volunteers who took aspirin (81 mg/day) for 7 days. Three other anti-thrombotic drugs, argatroban, heparin and hirudin, were tested with low drug levels but none were found to consistently stimulate the reaction. These results indicate that melagatran acts as both a direct thrombin inhibitor and indirectly by some interaction with the platelet membrane. While melagatran has been withdrawn from clinical use, its ability to differentially inhibit gamma-thrombin/PAR-4 versus alpha-thrombin/PAR-1 at low doses may warrant it, or less toxic analogs to be used in the future for as yet unknown disease states involving PAR-4.

Expression of functional recombinant mosquito salivary apyrase: A potential therapeutic platelet aggregation inhibitor

Excessive platelet activation and accumulation can lead to vessel occlusion and thus present major therapeutic challenges in cardiovascular medicine. Apyrase, an ecto-enzyme with ADPase and ATPase activities, rapidly metabolizes ADP and ATP released from platelets and endothelial cells, thereby reducing platelet activation and recruitment. In the present study, we expressed a 68-kDa recombinant mosquito (Aedes aegypti) salivary apyrase using a baculovirus/insect cell expression system and purified it to homogeneity using anion-exchange chromatography on a large scale. A yield of 18 mg of purified recombinant apyrase was obtained from 1 litre of the medium. Kinetic analysis indicated that the recombinant apyrase had a Km of 12.5 µM for ADP and a Km of 15.0 µM for ATP. The recombinant apyrase inhibited ADP-, collagen- and thrombin-induced human platelet aggregation in a dose-dependent manner, indicating that the recombinant protein retained nucleotidase activity in a whole cell system, which suggests that it may serve as a therapeutic agent for inhibition of platelet-mediated thrombosis.