Platelet size distribution measurements as indicators of shear stress-induced platelet aggregation

Urokinase exerts antimetastatic effects by dissociating clusters of circulating tumor cells

Clusters of circulating tumor cells (CTC) exhibit more robust metastatic properties than single CTC. Thus, understanding the distinct behaviors of CTC clusters and how CTC clustering is regulated may offer new insights into how to limit metastasis. In this study, we utilized an in vivo confocal system to observe the clustering behavior of CTC in real time, finding that the number of clusters increased proportionally with the growth of the primary tumor. Our experiments also indicated that the flow rate of the CTC clusters in blood vessels was relatively slower than single CTC due to increased vessel wall adhesion. Depending on disease stage, 5% to 10% of total CTC in circulation were in clusters, with this proportion increasing to >24% within lung metastases examined. Notably, in the 4T1 mouse model of breast cancer metastasis, we found that injecting host animals with urokinase-type plasminogen activator, a clinical thrombolytic agent, was effective at preventing the assembly of CTC clusters and prolonging overall host survival by approximately 20% relative to control animals. Our results suggest a tractable approach to limit metastasis by suppressing the formation or stability of CTC clusters circulating in the blood of cancer patients.

Inhibition of urokinase activity reduces primary tumor growth and metastasis formation in a murine lung carcinoma model

RATIONALE

Lung cancer is the most common malignancy in humans. Urokinase (uPA) plays a crucial role in carcinogenesis by facilitating tumor cell invasion and metastasis.

OBJECTIVES

We investigated the effect of the highly specific urokinase inhibitor CJ-463 (benzylsulfonyl-D-Ser-Ser-4-amidinobenzylamide) on tumor growth, metastasis formation, and tumor vascularization in the murine Lewis lung carcinoma (LLC) and a human small lung cancer model.

METHODS

A quantity of 3 x 10(6) LLC cells were subcutaneously injected into the right flank of C57Bl6/N mice, uPA knock out, and uPA receptor knockout mice. Seven days later mice were randomized to receive intraperitoneally either saline (control group), CJ-463 (10 and 100 mg/kg, twice a day), or its ineffective stereoisomer (10 mg/kg, twice a day). Tumor volume was measured every second day and metastasis formation was monitored by volumetric-computed tomography. Twelve days after onset of treatment mice were killed and tumors were prepared for histologic examination.

MEASUREMENTS AND MAIN RESULTS

Treatment with CJ-463 resulted in a significant inhibition of primary tumor growth, with the highest efficacy seen in the 100 mg/kg group. In addition, histological analysis of the lung revealed a significant reduction in lung micrometastasis in the 100 mg/kg group. Similarly, a reduced seeding of tumor cells into the lung after intravenous injection of LLC cells was observed in inhibitor-treated mice. In these mice, treatment with CJ-463 appeared not to significantly alter the relative extent of tumor vascularization. In vitro, proliferation of LLC cells remained unchanged upon inhibitor treatment. CJ-463 was found to similarly reduce tumor growth in uPA receptor knockout mice, but was ineffective in uPA knockout mice.

CONCLUSIONS

Our results suggest that synthetic low-molecular-weight uPA-inhibitors offer as novel agents for treatment of lung cancer.

Platelets undergo phosphorylation of Syk at Y525/526 and Y352 in response to pathophysiological shear stress

Atherosclerotic plaques can lead to partial vascular occlusions that produce abnormally high levels of arterial wall shear stress. Such pathophysiological shear stress can promote shear-induced platelet aggregation (SIPA), which has been linked to acute myocardial infarction, unstable angina, and stroke. This study investigated the role of the tyrosine kinase Syk in shear-induced human platelet signaling. The extent of Syk tyrosine phosphorylation induced by pathophysiological levels of shear stress (100 dyn/cm(2)) was significantly greater than that resulting from physiological shear stress (10 dyn/cm(2)). With the use of phospho-Syk specific antibodies, these data are the first to show that key regulatory sites of Syk at tyrosines 525/526 (Y525/526) and tyrosine 352 (Y352) were phosphorylated in response to pathophysiological shear stress. Increased phosphorylation at both sites was attenuated by pharmacological inhibition of Syk using two different Syk inhibitors, piceatannol and 3-(1-methyl-1H-indol-3-yl-methylene)-2-oxo-2,3-dihydro-1H-indole-5-sulfonamide (OXSI-2), and by inhibition of upstream Src-family kinases (SFKs). Shear-induced response at the Syk 525/526 site was ADP dependent but not contingent on glycoprotein (GP) IIb-IIIa ligation or the generation of thromboxane (Tx) A(2). Pretreatment with Syk inhibitors not only reduced SIPA and Syk phosphorylation in isolated platelets, but also diminished, up to 50%, the platelet-mediated thrombus formation when whole blood was perfused over type-III collagen. In summary, this study demonstrated that Syk is a key molecule in both SIPA and thrombus formation under flow. Pharmacological regulation of Syk may prove efficacious in treating occlusive vascular disease.

Spatio-temporal flow analysis in bileaflet heart valve hinge regions: potential analysis for blood element damage

Point-wise velocity measurements have been traditionally acquired to estimate blood damage potential induced by prosthetic heart valves with emphasis on peak values of velocity magnitude and Reynolds stresses. However, the inherently Lagrangian nature of platelet activation and hemolysis makes such measurements of limited predictive value. This study provides a refined fluid mechanical analysis, including blood element paths and stress exposure times, of the hinge flows of a CarboMedics bileaflet mechanical heart valve placed under both mitral and aortic conditions and a St Jude Medical bileaflet valve placed under aortic conditions. The hinge area was partitioned into characteristic regions based on dominant flow structures and spatio-temporal averaging was performed on the measured velocities and Reynolds shear stresses to estimate the average bulk stresses acting on blood elements transiting through the hinge. A first-order estimate of viscous stress levels and exposure times were computed. Both forward and leakage flow phases were characterized in each partition by dynamic flows dependent on subtle leaflet movements and transvalvular pressure fluctuations. Blood elements trapped in recirculation regions may experience exposure times as long as the entire forward flow phase duration. Most calculated stresses were below the accepted blood damage threshold. Estimates of the stress levels indicate that the flow conditions within the boundary layers near the hinge and leaflet walls may be more detrimental to blood cells than bulk flow conditions, while recirculation regions may promote thrombus buildup.

Bruton tyrosine kinase is essential for botrocetin/VWF-induced signaling and GPIb-dependent thrombus formation in vivo

Botrocetin (bt)-facilitated binding of von Willebrand factor (VWF) to the platelet membrane glycoprotein (GP) Ib-IX-V complex on platelets in suspension initiates a signaling cascade that causes alphaIIbbeta3 activation and platelet aggregation. Previous work has demonstrated that bt/VWF-mediated agglutination activates alphaIIbbeta3 and elicits ATP secretion in a thromboxane A2 (TxA2)-dependent manner. The signaling that results in TxA2 production was shown to be initiated by Lyn, enhanced by Src, and propagated through Syk, SLP-76, PI3K, PLCgamma2, and PKC. Here, we demonstrate that the signaling elicited by GPIb-mediated agglutination that results in TxA2 production is dependent on Bruton tyrosine kinase (Btk). The results demonstrate that Btk is downstream of Lyn, Syk, SLP-76, and PI3K; upstream of ERK1/2, PLCgamma2, and PKC; and greatly enhances Akt phosphorylation. The relationship(s), if any, between ERK1/2, PLCgamma2, and PKC were not elucidated. The requirement for Btk and TxA2 receptor function in GPIb-dependent arterial thrombosis was confirmed in vivo by characterizing blood flow in ferric chloride-treated mouse carotid arteries. These results demonstrate that the Btk family kinase, Tec, cannot provide the function(s) missing because of the absence of Btk and that Btk is essential for both bt/VWF-mediated agglutination-induced TxA2 production and GPIb-dependent stable arterial thrombus formation in vivo.

A comparison of flow field structures of two tri-leaflet polymeric heart valves

Polymeric heart valves have the potential to reduce thrombogenic complications associated with current mechanical valves and overcome fatigue-related problems experienced by bioprosthetic valves. In this in vitro study, the velocity fields inside and downstream of two different prototype tri-lealfet polymeric heart valves were studied. Experiments were conducted on two 23 mm prototype polymeric valves, provided by AorTech Europe, having open or closed commissure designs and leaflet thickness of 120 and 80 microm, respectively. A two-dimensional LDV system was used to measure the velocity fields in the vicinity of the two valves under simulated physiological conditions. Both commissural design and leaflet thickness were found to affect the flow characteristics. In particular, very high levels of Reynolds shear stress of 13,000 dynes/cm2 were found in the leakage flow of the open commisure design. Maximum leakage velocities in the open and closed designs were 3.6 m/s and 0.5 m/s respectively; the peak forward flow velocities were 2.0 m/s and 2.6 m/s, respectively. In both valve designs, shear stress levels exceeding 4,000 dyne/cm2 were observed at the trailing edge of the leaflets and in the leakage and central orifice jets during peak systole. Additionally, regions of low velocity flow conducive to thrombus formation were observed in diastole. The flow structures measured in these experiments are consistent with the location of thrombus formation observed in preliminary animal experiments.

The roles of ADP and TXA in botrocetin/VWF-induced aggregation of washed platelets

BACKGROUND

Binding of von Willebrand factor (VWF) to the platelet membrane glycoprotein (GP) Ib-IX-V complex initiates a cascade of events leading to alphaIIbbeta3 activation and platelet aggregation. The roles of ADP and thromboxane A2 (TXA2) in agglutination-induced GPIbalpha-mediated platelet activation have not been fully described.

METHODS

Botrocetin and human VWF were used to stimulate washed mouse platelets. Platelets deficient in TXA2 receptors, Galphaq, or alphaIIbbeta3, and inhibitors and chelating agents were used to investigate the roles of TXA2, ADP, alphaIIbbeta3 and Ca2+ in botrocetin/VWF-induced signaling.

RESULTS

Our data demonstrate that botrocetin/VWF/GPIbalpha-mediated agglutination results in calcium-independent protein kinase C (PKC) and phospholipase A2 (PLA2) activities required for GPIbalpha-elicited TXA2 production that in turn causes dense granule secretion. Aggregation of washed platelets requires TXA2-induced alphaIIbbeta3 activation and ADP signaling. TXA2 or ADP can activate alphaIIbbeta3, but both are required for alpha-granule secretion and aggregation. Botrocetin/VWF-induced dense granule secretion is Galphaq-dependent. alpha-Granule secretion requires initial ADP signaling through P2Y1 and subsequent signaling through P2Y12. Signaling initiated by agglutination is propagated and amplified in an alphaIIbbeta3-dependent manner.

CONCLUSIONS

In contrast to adhesion or shear stress-induced GPIb-elicited signaling, agglutination-elicited GPIb signaling that activates alphaIIbbeta3 requires TXA2. Agglutination-elicited TXA2 production is independent of Ca2+ influx and mobilization of internal Ca2+ stores. Therefore, our results demonstrate that agglutination-elicited GPIb signaling causes alphaIIbbeta3 activation by a mechanism that is distinct from those used by adhesion, or shear stress-induced GPIb signaling.

Cleavage of platelet endothelial cell adhesion molecule-1 (PECAM-1) in platelets exposed to high shear stress

Platelet endothelial cell adhesion molecule-1 (PECAM-1, CD31) is a 130 kDa transmembrane glycoprotein that belongs to the immunoglobulin superfamily and is expressed on the surface of endothelial cells, platelets, and other blood cells. Although the importance of this adhesion molecule in various cell-cell interactions is established, its functional role in platelets remains to be elucidated. In this study, we examined whether PECAM-1 underwent changes in platelets exposed to high shear stress. Platelet PECAM-1 was cleaved under high shear stress and was released into the extracellular fluid as a fragment with an approximate molecular weight of 118 kDa. The cleavage was inhibited by an anti-VWF MoAb, but not by recombinant VWF A1 domains. These findings suggest that the GPIb-VWF interaction is involved in PECAM-1 cleavage under high shear stress, and that the cleavage is independent of GPIb clustering by VWF multimers. Furthermore, EGTA or calpeptin inhibited PECAM-1 cleavage. This finding provides evidence for the involvement of calpain in PECAM-1 cleavage. Flow-cytometric analysis revealed that PECAM-1 expression on the platelet surface was decreased under high shear stress. This reduction occurred exclusively in a specific population of platelets, which corresponded to platelet-derived microparticles (PMP). In conclusion, PECAM-1 cleavage under high shear stress is closely related to the activation of calpain and the process of PMP formation mediated by the GPIb-VWF interaction.

Comparison of shear stress-induced platelet microparticle formation and phosphatidylserine expression in presence of alphaIIbbeta3 antagonists

The use of platelet glycoprotein IIb-IIIa (alphaIIbbeta3) antagonists is an accepted practice in the treatment of acute coronary syndromes. Recent studies have demonstrated that alpha beta receptor antagonists are effective in inhibiting the procoagulant activity of platelets under static conditions. No investigation, however, has compared the ability of these platelet antagonists to inhibit platelet procoagulant activity, defined as an increase in phosphatidylserine (PS) expression, under conditions of shear stress. Thus, the goal of this study was to quantify the amount of microparticle formation and PS expression of platelets exposed to physiologic and pathophysiologic levels of shear stress in the absence and presence of three clinically approved parenteral alpha beta antagonists (abciximab, eptifibatide, and tirofiban). Flow cytometric results demonstrated that although microparticle formation was significantly inhibited by all three antagonists, PS expression by sheared platelets was affected differently depending on the antagonist present. Specifically, abciximab suppressed PS expression compared with the saline control; both abciximab and eptifibatide significantly reduced PS expression compared with tirofiban; and tirofiban potentiated PS expression relative to the saline control at the highest shear stress. This is the first demonstration of differential regulation of platelet PS expression and, by inference, procoagulant activity in the presence of alpha receptor antagonists under shear stress. The current results may have future importance in improving the design of platelet antagonists as well as defining the general role of fluid shear stress in platelet thrombus formation.

Cocaine activates platelets and increases the formation of circulating platelet containing microaggregates in humans

OBJECTIVE

To determine whether there is evidence of platelet activation following in vivo cocaine administration in humans, as cocaine abuse is associated with myocardial infarction and stroke, and platelet activation leading to thrombosis is a possible mechanism.

SETTING

University hospital.

DESIGN AND SUBJECTS

Following a randomised, double blind crossover design, 14 healthy volunteers were studied twice, receiving cocaine (2 mg/kg intranasally) once and placebo once. Flow cytometric analysis of P-selectin expression (an alpha granule membrane protein found on the surface of activated platelets), quantification of the platelet specific proteins platelet factor 4 and beta thromboglobulin, and measurement of platelet containing microaggregate and platelet microparticle (fragment) formation were used to assess platelet activation. Circulating von Willebrand factor antigen (vWF) was measured to evaluate a possible role of endothelial stimulation concurrent with platelet activation.

RESULTS

There was an increase in both platelet factor 4 (mean (SD), 16 (7) to 39 (22) IU/ml, p = 0. 04) and beta thromboglobulin (70 (20) to 98 (26) IU/ml, p < 0.01) at 120 minutes following cocaine administration. Platelet containing microaggregate formation was increased at 40 minutes (from 47 (3.2)% to 54 (2.0)%, p < 0.001) and 80 minutes (55 (2.5)%, p = 0.04). Bleeding time decreased following cocaine from 10 (1) to 9 (1) minutes (p = 0.07). No changes in any of the measured variables were noted following placebo administration.

CONCLUSIONS

Cocaine exposure causes platelet activation, alpha granule release, and platelet containing microaggregate formation. These data support the view that cocaine, even at the relatively low doses commonly self administered by occasional abusers, may promote thrombosis and predispose healthy individuals to ischaemic events. Platelet inhibitors should be considered early in any patient with suspected cocaine related ischaemia.

Mechanical factors affecting hemostasis and thrombosis

Both physical and chemical factors can influence the activity of platelets and coagulation factors responsible for the formation of thrombotic and hemostatic masses in the vicinity of an injured vessel wall. Studies performed in controlled shear devices (viscometers) have indicated that physical factors alone can induce platelet aggregation, even in the absence of exogenous chemical factors. The physical considerations which appear to be important for the local activation of hemostatic/thrombotic mechanisms appear to be related to the magnitude of the shear rate/stress, the duration of the applied physical force and the local geometry. Blood flow alone has multiple influences on platelet and coagulative mechanisms. It has been well established that at physiologically encountered shear conditions, increases in the local shear rate enhance the attachment of platelets to the vessel wall and the growth of platelet aggregates on adherent platelets. In contrast, increases in local shear conditions inhibit the production of fibrin formation on surfaces where tissue factor (TF) is exposed. At levels of shear rate/stress high as compared to normal physiological conditions, but comparable to those observed at the apex of severely stenosed vessels, platelet aggregate formation is dependent on the duration of the exposure time. Considerable advances in our understanding of flow-related mechanisms have evolved from the use of well-defined perfusion chambers employing parallel flow streamlines. However, processes leading to hemostasis and thrombosis generally occur in more complicated flow situations where flow streamlines are not parallel and in which abnormally high, as well as abnormally low, shear rates and shear stress levels may be encountered in close proximity to each other.

Platelets and cardiopulmonary bypass

Exposure of blood to an extracorporeal circulation, such as CPB, causes a variety of physiological responses. Haematological derangements are just one of many potential dangers to the patient who undergoes CPB. The paradox of CPB-related problems with the haematological system is that there are some factors tipping the balance towards a bleeding tendency, and others that favour a prothrombotic state. Both of these issues must be dealt with independently to create the safest environment for surgery. It has been demonstrated that platelets play a key role in both haemostatic dysfunction and thrombotic complications of CPB. Much has been achieved, both clinically and in the laboratory, in the understanding of the precise role platelets play in these events, but the exact mechanisms involved have yet to be completely identified. As research progresses, our understanding will increase, but until then clinical practice must be dictated by the current evidence available.

Experimental fat embolism induces urine 2,3-dinor-6-ketoprostaglandin F1alpha and 11-dehydrothromboxane B2 excretion in pigs

OBJECTIVE

To evaluate the in vivo production of prostacyclin and thromboxane A2 during the initial phase of experimental fat embolism as assessed, respectively, by determinations of urine 2,3-dinor-6-ketoprostaglandin F1alpha and 11-dehydrothromboxane B2 excretion.

DESIGN

Randomized, controlled trial.

SETTING

Animal laboratory.

SUBJECTS

Twenty seven domestic pigs, weighing 24 to 31 kg.

INTERVENTIONS

All pigs were anesthetized and mechanically ventilated during the experiment. Eighteen pigs were subjected to an intracaval infusion of 10% allogeneic bone marrow suspension at a dose of 100 mg/kg over 5 mins. Nine pigs received only bone marrow suspension (fat embolism group). Nine pigs were given an intravenous bolus of aspirin (300 mg) 1 hr before the bone marrow suspension infusion. After the induction of fat embolism, intravenous aspirin was administered at a dose of 150 mg/hr for 2 hrs (aspirin-treated group). Nine pigs were infused with saline (control group).

MEASUREMENTS AND MAIN RESULTS

In the fat embolism group, cardiac index decreased within 30 mins, while mean arterial pressure remained unchanged. Central venous pressure and pulmonary artery occlusion pressure remained relatively stable over time in the animals with fat embolism. Mean pulmonary arterial pressure and pulmonary vascular resistance increased immediately after the bone marrow suspension infusion from 23 +/- 0.8 (SEM) to 34 +/- 1.3 mm Hg and from 305 +/- 28 to 585 +/- 45 dyne x sec/cm5, respectively; these variables remained increased throughout the study period. Simultaneously, pulmonary shunt in the fat embolism group increased persistently from the baseline of 12.3 +/- 2.8%, and reached its maximum of 26.1 +/- 4.8% at the end of the experiment. Instant and gradual decreases in PaO2 (from 95 +/- 4 to 67 +/- 5 torr [12.6 +/- 0.5 to 8.9 +/- 0.7 kPa]), hemoglobin oxygen saturation (from 97.2 +/- 0.4 to 91.8 +/- 1.8%), and oxygen delivery (from 16.3 +/- 1.0 to 12.6 +/- 0.4 mL/min/kg) were observed in the fat embolism group. In the bone marrow suspension-infused animals, urine 2,3-dinor-6-ketoprostaglandin F1alpha excretion increased transiently from 451 +/- 63 up to 1466 +/- 499 pg/micromol creatinine, while urine 11-dehydrothromboxane B2 excretion increased transiently from 385 +/- 36 up to 2307 +/- 685 pg/micromol creatinine. In the aspirin-treated animals, urinary excretion of these prostanoid metabolites was reduced by 81% and 88%, respectively. The changes in mean pulmonary arterial pressure and PaO2 were ameliorated, and the alterations in pulmonary shunt and SaO2 were abolished in the animals with aspirin treatment.

CONCLUSIONS

Pulmonary hypertension, increased pulmonary vascular tone, and increased pulmonary shunt are hallmarks of the present fat embolism model. These hemodynamic responses may, at least partly, be related to the changed balance between prostacyclin and thromboxane A2 production.

A numerical analysis of factor X activation in the presence of tissue factor--factor VIIa complex in a flow reactor

A mathematical model has been developed to investigate previously obtained experimental findings relating to the activation of factor X by surface-bound tissue factor--factor VIIa (TF:VIIa) in a tubular flow reactor. In those experiments, factor X was perfused through a microcapillary tube over a range of flow (shear) conditions and the activated product, factor Xa, was measured at the outlet of the tube using a chromogenic assay. In the present study, the steady-state convection-diffusion equation with Michaelis-Menten kinetics used to describe the reaction at the wall was numerically integrated using an implicit method based on linear systems of ordinary differential equations. The results from the numerical analysis indicated that shear rate directly affects both Km and Vmax. Values of Km decreased from 151 to 16 nM as the shear rate increased from 25 to 2400 sec-1. Additionally, there was a twofold increase in Vmax from 1.4 to 3.0 pmol/cm2/min as the shear rate increased from 25 to 300 sec-1. These findings are in contrast with classical enzyme behavior and imply a direct effect of fluid flow on the kinetics of factor X activation.