A synthesis approach of mouse studies to identify genes and proteins in arterial thrombosis and bleeding

Antithrombotic therapies reduce cardiovascular diseases by preventing arterial thrombosis and thromboembolism, but at expense of increased bleeding risks. Arterial thrombosis studies using genetically modified mice have been invaluable for identification of new molecular targets. Because of low sample sizes and heterogeneity in approaches or methodologies, a formal meta-analysis to compare studies of mice with single-gene defects encountered major limitations. To overcome these, we developed a novel synthesis approach to quantitatively scale 1514 published studies of arterial thrombus formation (in vivo and in vitro), thromboembolism, and tail-bleeding of genetically modified mice. Using a newly defined consistency parameter (CP), indicating the strength of published data, comparisons were made of 431 mouse genes, of which 17 consistently contributed to thrombus formation without affecting hemostasis. Ranking analysis indicated high correlations between collagen-dependent thrombosis models in vivo (FeCl3 injury or ligation/compression) and in vitro. Integration of scores and CP values resulted in a network of protein interactions in thrombosis and hemostasis (PITH), which was combined with databases of genetically linked human bleeding and thrombotic disorders. The network contained 2946 nodes linked to modifying genes of thrombus formation, mostly with expression in megakaryocytes. Reactome pathway analysis and network characteristics revealed multiple novel genes with potential contribution to thrombosis/hemostasis. Studies with additional knockout mice revealed that 4 of 8 (Apoe, Fpr2, Ifnar1, Vps13a) new genes were modifying in thrombus formation. The PITH network further: (i) revealed a high similarity of murine and human hemostatic and thrombotic processes and (ii) identified multiple new candidate proteins regulating these processes.

Impaired Energy Metabolism in Platelets from Patients with Wiskott-Aldrich Syndrome

The platelet function defect seen in patients with Wiskott- Aldrich syndrome (WAS) has been ascribed to abnormal mitochondrial energy generation. The present study reveals a reduced energy content and low adenylate energy charge in platelets from two WAS-patients. Energy consumption in the resting platelets is slightly beyond the normal range, especially when ATP-resynthesis is primarily glycolytic. When platelets are stimulated with thrombin, the increase in energy consumption is 40-60% lower than in controls, both when energy is produced in glycolysis as when the mitochondria supply most of the energy. Analysis of the electron transport chain reveals no abnormalities. In contrast, the balance between glycolytic and mitochondrial ATP resynthesis is disturbed with a lowered contribution of oxidative ATP production. No such abnormalities are found in two WAS-carriers with the exception of a slight impairment in energy consumption during stimulation with thrombin. Thus, the platelet malfunction in WAS may be caused by a defect in the regulation of energy generation.

Blood Platelet Plasminogen Activator Inhibitor: Two Different Pools of Endothelial Cell Type Plasminogen Activator Inhibitor in Human Blood

An assay for plasminogen activator inhibitor in human platelets is described. With this assay we find an average value of 6.8 × 10−8 IU/platelet (S.D. = 3.0 × 10−8; n = 20) in a healthy population. We characterized the PA-inhibitor from platelets and identified it as endothelial cell type plasminogen activator inhibitor, by its immunologic and functional properties. Besides the plasma pool of plasminogen activator inhibitor with a very high turnover rate, platelets constitute a second pool of plasminogen activator inhibitor in the circulation of the same order of magnitude. The two different pools of plasminogen activator inhibitor might have a different physiologic function.

Evaluation of Platelet Tests for Measurement of Cell Integrity

Various tests were evaluated for their capacity to differentiate between platelet suspensions with different degrees of cell damage. Those suspensions were prepared by simultaneous isolation of platelets from the same platelet-rich plasma (PRP) using the following procedures:

1. centrifugation at 4°C with EDTA

2. gel filtration in Tangen’s buffer

3. gel filtration in Ca2+-free Tyrode’s solution

4. gel filtration in Ca2+-free Tyrode followed by dehydration against polyethylene glycol 20,000 and

5. albumin density gradient centrifugation.

In these suspensions and in the original PRP the following parameters were studied: 1. morphology; 2. aggregability upon ADP addition; 3. platelet factor 3 availability; 4. uptake of 14C-serotonin and 3H-adenine; 5. metabolism of 3H-adenine and adenylate energy charge; 6. endogenous total ATP, ADP and serotonin and 7. lactate dehydrogenase (LDH) activity.

Quantitation of pseudopod formation in the light or electron microscope and log dose response studies for ADP-induced aggregation proved to be the most sensitive and reproducible of the tests studied. Additional information could be obtained from measurement of the 3H-label in the ATP and hypoxanthine-inosine fractions and calculation of the adenylate energy charge. Determination of platelet factor 3 availability or uptake studies of 14C-serotonin and 3H-adenine were less suitable for discriminating between cell suspensions. Data for total ATP and serotonin concentrations and LDH activity differed between the cell suspensions but instead of detecting various degrees of cell damage they reflected alterations in platelet population caused by the isolation procedures.

Lateral Mobility of Integrin αIIbβ3 (Glycoprotein IIb/IIIa) in the Plasma Membrane of a Human Megakaryocyte

The migration of integrins to sites of cell-cell and cell-matrix contact is thought to be important for adhesion strengthening. We studied the lateral diffusion of integrin αIIbβ3 (glycoprotein Ilb/IIIa) in the plasma membrane of a cultured human megakaryocyte by fluorescence recovery after photobleaching of FITC-labelled monovalent Fab fragments directed against the P3 subunit. The diffusion of P3 on the unstimulated megakaryocyte showed a lateral diffusion coefficient (D) of 0.37 X10'9 cm2/s and a mobile fraction of about 50%. Stimulation with ADP (20 μM) or α-thrombin (10 U/ml) at 22° C induced transient decreases in both parameters reducing D to 0.21 X 10‘9 cm2/s and the mobile fraction to about 25%. The fall in D was observed within 1 min after stimulation but the fall in mobile fraction showed a lag phase of 5 min. The lag phase was absent in the presence of Calpain I inhibitor, whereas cytochalasin D completely abolished the decrease in mobile fraction. The data are compatible with the concept that cell activation induces anchorage of 50% of the mobile αIIbβ3 (25% of the whole population of receptor) to the cytoplasmic actin filaments, although, as discussed, other rationals are not ruled out.

Targeting of Porcine Pancreatic Phospholipase A2 to Human Platelets: Introduction of an RGD Sequence by Genetic Engineering

The possibility to induce specific disruption of activated platelets by binding of porcine pancreatic phospholipase A2 (PLA2) was tested by constructing a set of PLA2-mutants containing an Arg-Gly-Asp (RGD) sequence. One mutant was made with RGD as part of a surface-exposed loop (RGDloop). Four mutants were made with RGD as part of a C-terminal extension: one with RGD directly coupled to the C-terminus (RGDc) and three mutants (CRSx) with x = 22,42 and 82 hydrophylic non-charged amino acids between RGD and the enzyme. All mutants retained 20-80% activity of native PLA2 and showed little binding to resting platelets. The binding of the native enzyme and RGDloop was not increased following stimulation. In contrast, the mutants RGDc and CRSx showed stimulation-dependent binding to the platelet receptor GPIIb/IIIa, since GRGDS-peptide and a monoclonal antibody against the complex interfered with binding. In α-thrombin-stimulated platelets, CRS42 and CRS82 induced about 5% hydrolysis of [3H]-arachidonic acid-labeled phospholipids. Stimulation with a combination of a-thrombin and collagen (known to expose phosphatidylserine) increased hydrolysis to 11%. Despite the membrane disruption, the cells did not leak lactate dehydrogenase. We conclude that PLA2 can be targeted to activated platelets by introducing RGD in a C-terminal extension with a minimum distance (42 amino acids) between RGD and the enzyme. However, more hydrolytic activity is required to eliminate activated platelets among a suspension of resting platelets and other blood cells.

Abnormal Platelet Functions in a Patient with Abetalipoproteinemia

Studies with isolated lipoproteins and washed platelets suggest that lipoproteins may affect platelet functions. We investigated platelet-rich plasma (PRP) from a patient with abetalipoproteinemia (ABL), whose plasma lacks apo-B containing lipoproteins (VLDL, LDL and chylomicrons). ABL-PRP aggregated poorly with different agonists and failed to respond to arachidonate. Thromboxane. B2 (T×B2) formation was severely impaired. After gel-filtration most of the aggregation defects persisted in agreement with reduced metabolism of endogenous arachidonate. However, arachidonate-induced aggregation and T×B2 production partially normalized. Normal platelets suspended in ABL plasma showed similar defects in aggregation and T×B2 production but arachidonate-induced aggregation was much lower than expected on the basis of T×B2. We conclude that the abnormal platelet functions in ABL-PRP are caused by (i) an intrinsic platelet abnormality due to reduced arachidonate mobilization and (ii) a property in ABL plasma that inhibits aggregation partially by trapping the arachidonate and partially by an unidentified mechanism. The latter properties may be the result of the abnormal lipid composition of ABl-plasma.

Maturation of Megakaryoblastic Cells Is Accompanied by Upregulation of GSα-L Subtype and Increased cAMP Accumulation

In platelets and megakaryoblasts Gs, the trimeric G-protein that stimulates adenylyl cyclase, is present in a short, 45 kDa, and a long, 52 kDa isoform termed Gsα-S and Gsα-L, respectively. To assess the relative contribution of these isoforms in the cellular synthesis of cAMP, the ratio Gsα-S/Gsα-L was changed in the megakaryoblastic cell line DAMI by inducing cell maturation with recombinant human thrombopoietin (TPO) or the phorbol ester PMA. Flow cytometric analysis confirmed that this treatment induced a moderate (TPO) and extensive (PMA) increase in nuclear ploidy and expression of the glycoproteins-IIIa and -Ib. Northern blot analysis revealed down-regulation of total GS-mRNA after treatment of DAMI-cells with TPO and PMA. Western blot analysis showed significant (P <0.05) upregulation of Gs-L with respective amounts of 27 ± 4% of total G s in untreated cells, 35 ± 1% in TPO- and 41 ± 3% in PMA-treated DAMI cells (n = 3-4). DAMI cells contained 6 ± 1 pmol cAMP/106 cells, which was not changed by treatment with TPO or PMA. In untreated cells this level increased to 70 ± 9 pmol cAMP/106 cells after 10 min stimulation with 1 μmol/l of the stable prostacyclin analog iloprost. The same stimulation with iloprost resulted in 165 ± 32 pmol cAMP/106 in TPO-treated cells and in 588 ± 100 pmol cAMP/106 in cells treated with PMA. Thus, a shift from Gsα-S to Gsα-L during megakaryoblast maturation strongly potentiates the production of cAMP. A similar shift may occur during normal megakaryocyte maturation and may explain the extreme sensitivity to prostacyclin of platelets, which contain Gsα-S and Gsα-L in approximately equal amounts.

Platelet interaction with von Willebrand factor is enhanced by shear-induced clustering of glycoprotein Ibα

Initial platelet arrest at the exposed arterial vessel wall is mediated through glycoprotein Ibα binding to the A1 domain of von Willebrand factor. This interaction occurs at sites of elevated shear force, and strengthens upon increasing hydrodynamic drag. The increased interaction requires shear-dependent exposure of the von Willebrand factor A1 domain, but the contribution of glycoprotein Ibα remains ill defined. We have previously found that glycoprotein Ibα forms clusters upon platelet cooling and hypothesized that such a property enhances the interaction with von Willebrand factor under physiological conditions. We analyzed the distribution of glycoprotein Ibα with Förster resonance energy transfer using time-gated fluorescence lifetime imaging microscopy. Perfusion at a shear rate of 1,600 s(-1) induced glycoprotein Ibα clusters on platelets adhered to von Willebrand factor, while clustering did not require von Willebrand factor contact at 10,000 s(-1). Shear-induced clustering was reversible, not accompanied by granule release or αIIbβ3 activation and improved glycoprotein Ibα-dependent platelet interaction with von Willebrand factor. Clustering required glycoprotein Ibα translocation to lipid rafts and critically depended on arachidonic acid-mediated binding of 14-3-3ζ to its cytoplasmic tail. This newly identified mechanism emphasizes the ability of platelets to respond to mechanical force and provides new insights into how changes in hemodynamics influence arterial thrombus formation.

The platelet P2Y12 receptor contributes to granule secretion through Ephrin A4 receptor

The main responses of P2Y(1) ligation are platelet shape change and transient aggregation while P2Y(12) ligation amplifies P2Y(1)-induced aggregation and accelerates aggregation, secretion and thromboxane A(2) production induced by other agonist-receptor complexes. We searched for new targets of P2Y signalling using micro-arrays with 144 peptides representing known phosphosites of protein tyrosine kinases. ADP induced phosphorylation of peptides representing surface receptors, second messenger enzymes and cytoskeletal proteins. Strong phosphorylation was found in peptides representing Ephrin-receptor family members. Blockade of P2Y(1/12) inhibited phosphorylation of EphA4- and EphB1-peptides on micro-arrays. The EphA2/4 inhibitor 2,5-dimethylpyrrolyl benzoic acid derivative interfered with P2Y(1/12)-induced EphA4 phosphorylation, left P2Y(1)-induced aggregation unchanged but inhibited with P2Y(12)-induced secretion, second phase aggregation and thrombus formation on collagen at 1600 s(-1). These results show that platelet EphA4 is an important intermediate in P2Y(12)-induced granule secretion.

New gene functions in megakaryopoiesis and platelet formation

Platelets are the second most abundant cell type in blood and are essential for maintaining haemostasis. Their count and volume are tightly controlled within narrow physiological ranges, but there is only limited understanding of the molecular processes controlling both traits. Here we carried out a high-powered meta-analysis of genome-wide association studies (GWAS) in up to 66,867 individuals of European ancestry, followed by extensive biological and functional assessment. We identified 68 genomic loci reliably associated with platelet count and volume mapping to established and putative novel regulators of megakaryopoiesis and platelet formation. These genes show megakaryocyte-specific gene expression patterns and extensive network connectivity. Using gene silencing in Danio rerio and Drosophila melanogaster, we identified 11 of the genes as novel regulators of blood cell formation. Taken together, our findings advance understanding of novel gene functions controlling fate-determining events during megakaryopoiesis and platelet formation, providing a new example of successful translation of GWAS to function.

Deletion of the high-density lipoprotein receptor scavenger receptor BI in mice modulates thrombosis susceptibility and indirectly affects platelet function by elevation of plasma free cholesterol

OBJECTIVE

Scavenger receptor BI (SR-BI) is a cell surface receptor that promotes the selective uptake of cholesteryl esters from high-density lipoprotein (HDL) by the liver. In mice, SR-BI deficiency results in increased plasma HDL cholesterol levels and enhanced susceptibility to atherosclerosis. The aim of this study was to investigate the role of SR-BI deficiency on platelet function.

METHODS AND RESULTS

SR-BI-deficient mice were thrombocytopenic, and their platelets were abnormally large, probably because of an increased cholesterol content. The FeCl(3) acute injury model to study arterial thrombosis susceptibility showed that SR-BI wild-type mice developed total arterial occlusion after 24±2 minutes. In SR-BI-deficient mice, however, the time to occlusion was reduced to 13±1 minutes (P=0.02). Correspondingly, in SR-BI-deficient mice, platelets circulated in an activated state and showed increased adherence to immobilized fibrinogen. In contrast, platelet-specific disruption of SR-BI by bone marrow transplantation in wild-type mice did not alter plasma cholesterol levels or affect platelet count, size, cholesterol content, or reactivity, suggesting that changes in plasma cholesterol levels were responsible for the altered responsiveness of platelets in SR-BI-deficient mice.

CONCLUSIONS

The function of SR-BI in HDL cholesterol homeostasis and prevention of atherosclerosis is indirectly also essential for maintaining normal platelet function and prevention of thrombosis.

Platelet apoptosis by cold-induced glycoprotein Ibα clustering

BACKGROUND

Cold-storage of platelets followed by rewarming induces changes in Glycoprotein (GP) Ibα-distribution indicative of receptor clustering and initiates thromboxane A(2) -formation. GPIbα is associated with 14-3-3 proteins, which contribute to GPIbα-signaling and in nucleated cells take part in apoptosis regulation.

OBJECTIVES AND METHODS

We investigated whether GPIbα-clustering induces platelet apoptosis through 14-3-3 proteins during cold (4 h 0 °C)-rewarming (1 h 37 °C).

RESULTS

During cold-rewarming, 14-3-3 proteins associate with GPIbα and dissociate from Bad inducing Bad-dephosphorylation and activation. This initiates pro-apoptosis changes in Bax/Bcl-x(L) and Bax-translocation to the mitochondria, inducing cytochrome c release. The result is activation of caspase-9, which triggers phosphatidylserine exposure and platelet phagocytosis by macrophages. Responses are prevented by N-acetyl-D-glucosamine (GN), which blocks GPIbα-clustering, and by O-sialoglycoprotein endopeptidase, which removes extracellular GPIbα.

CONCLUSIONS

Cold-rewarming triggers apoptosis through a GN-sensitive GPIbα-change indicative of receptor clustering. Attempts to improve platelet transfusion by cold-storage should focus on prevention of the GPIbα-change.

Mlck1a is expressed in zebrafish thrombocytes and is an essential component of thrombus formation

BACKGROUND

We have used the advantages of the zebrafish model system to demonstrate which of the vertebrate myosin light chain kinase (MLCK) genes is expressed in thrombocytes and important for thrombus formation.

METHODS AND RESULTS

Here we report that Mlck1a is an essential component of thrombus formation. Phylogenetic data revealed four zebrafish orthologous for three human MLCK genes. To investigate expression of the zebrafish mlck genes in thrombocytes we compared GFP-tagged platelets with other cells by microarray analysis, and showed that mlck1a expression was 4.5-fold enriched in platelets. Furthermore, mlck1a mRNA and mRNA for the platelet-specific cd41 co-localized in thrombi. Expression of other mlck subtypes was lower in GFP-tagged platelets (mlck1b; 0.77-fold enriched) and absent in thrombi (mlck1b, -2, -3). To investigate the role of Mlck1a in thrombus formation, we knocked down mlck1a using two morpholinos. This resulted in impaired morphology changes of platelets adhering on fibrinogen. In a thrombosis model, in which thrombocytes adhere to the vessel wall damaged by laser irradiation, thrombus formation was slowed down in mlck1a-deficient embryos.

CONCLUSION

We conclude that Mlck1a is the subtype of MLCK that contributes to platelet shape change and thrombus formation.

Insulin inhibits tissue factor expression in monocytes

OBJECTIVES

Platelets from healthy subjects are inhibited by insulin but type 2 diabetes mellitus (T2DM) platelets have become insulin-resistant, which might explain their hyperactivity. In the present study we investigated whether monocytes are responsive to insulin.

METHODS AND RESULTS

LPS-induced tissue factor (TF) upregulation was measured in human monocytes and monocytic THP-1 cells in a factor Xa generation assay. Insulin (0.1-100 nmol L(-1)) induced a dose-dependent inhibition in both cell types and in monocytes 100 nmol L(-1) insulin inhibited cytosolic, membrane-bound and microparticle TF by 32 +/- 2, 27 +/- 3 and 52 +/- 4% (n = 3). Insulin induced Tyr phosphorylation of the insulin receptor (INS-R) and formation of an INS-R - G(i)alpha(2) complex, suggesting interference with LPS-induced cAMP control. Indeed, insulin interfered with LPS-induced cAMP decrease and TF upregulation in a manner similar to an inhibitor of G(i) (pertussis toxin) and agents that raise cAMP (iloprost, forskolin, IBMX) reduced TF upregulation. Although LPS failed to raise cytosolic Ca(2+), quenching of Ca(2+) increases (BAPTA-AM) reduced and induction of Ca(2+) entry (ionophore, P2X7 activation) enhanced upregulation of TF mRNA and procoagulant activity. Insulin interfered with MCP-1-induced Ca(2+) mobilization but not with ATP-induced Ca(2+) rises.

CONCLUSIONS

Insulin inhibits TF expression in monocytes and monocyte-derived microparticles through interference with G(i)alpha(2)-mediated cAMP suppression, which attenuates Ca(2+)-mediated TF synthesis.

Downregulation of platelet responsiveness upon contact with LDL by the protein-tyrosine phosphatases SHP-1 and SHP-2

OBJECTIVE

The sensitivity of platelets to aggregating agents increases when low-density lipoprotein (LDL) binds to apolipoprotein E receptor 2' (apoER2'), triggering activation of p38MAPK and formation of thromboxane A2. LDL signaling is terminated by PECAM-1 through recruitment and activation of the Ser/Thr protein phosphatase PP2A, but platelets remain unresponsive to LDL when PECAM-1 activation disappears. We report a second mechanism that halts LDL signaling and in addition lowers platelet responsiveness to aggregating agents.

METHODS AND RESULTS

After a first stimulation with LDL, platelets remain unresponsive to LDL for 60 minutes, despite normal apoER2' activation by a second dose of LDL. A possible cause is persistent activation of the tyrosine phosphatases SHP-1 and SHP-2, which may not only block a second activation of p38MAPK, PECAM-1, and PP2A by LDL but also seem to reduce aggregation by TRAP, collagen, and ADP.

CONCLUSION

These findings reveal that p38MAPK phosphorylation and platelet activation by LDL are suppressed by two mechanisms: (1) short activation of PECAM-1/PP2A, and (2) prolonged activation of SHP-1 and SHP-2. Activation of SHP-1 and SHP-2 is accompanied by reduced responsiveness to aggregating agents, which--if present in vivo--would make LDL an aggregation inhibitor during prolonged contact with platelets.

Role of membrane cholesterol in platelet calcium signalling in response to VWF and collagen under stasis and flow

Several studies have highlighted a specific role for membrane cholesterol domains in platelet signalling. Upon adhesion to von Willebrand factor (VWF) or collagen, cholesterol-rich domains (CRDs) accumulate in filopodial extensions and selectively harbour counterpart receptors (GPIb and GPVI) and associated signalling molecules. In the present study we have addressed the role of membrane cholesterol in Ca(2+) signalling and secretion during the interaction of platelets with VWF and collagen. VWF/ristocetin-induced platelet aggregation was delayed after treatment with methyl beta-cyclodextrin (mbCD), but the maximal aggregation response was not affected. Platelet spreading but not adhesion to immobilised VWF under flow was attenuated by cholesterol removal, and accompanied by moderate lowering in the spiking Ca(2+) response. On the other hand, platelet interaction with collagen was quite sensitive to cholesterol depletion. Platelet aggregation decreased after treatment with mbCD, and Ca(2+) responses were decreased, both under static and flow conditions. Cholesterol depletion affected the secondary feedback activation via release of thromboxane A(2) and ADP. The collagen-induced secretion of alpha granules and surface translocation of P-selectin and CD63 was also critically affected by cholesterol depletion. Confocal microscopy showed localization of p-Tyr at sites of contact with substrate and other platelets, where also CRDs accumulate. Our data thus reveal a more critical role for membrane cholesterol in collagen-induced than in VWF-induced Ca(2+) signalling, and furthermore support the concept that secondary activation responses are dependent on intact CRDs.

Hemostatic and signaling functions of transfused platelets

Metabolic studies have revealed a gradual impairment in platelet integrity during storage, a process termed the platelet storage lesion. Recent evidence shows that stored platelets also lose signaling responses to physiological agonists with impaired integrin activation, secretion, and aggregation of the cells. On the other hand, storage leads to a gain in platelet activation properties, such as release of microparticles and appearance of surface epitopes for their clearance by macrophages. New techniques for measuring flow-induced thrombus formation and platelet-dependent coagulation provide evidence that the hemostatic activity of platelets decreases during storage. Besides pharmacological inhibition, novel storage strategies, like metabolic suppression, should be considered to better preserve platelet functionality while limiting the expression of clearance markers. Understanding the changes that occur in association with the platelet storage lesion and the use of updated storage methods will help to generate platelets for transfusion with optimal hemostatic function and a long circulation time after transfusion.

Platelets take up the monoclonal antibody bevacizumab

PURPOSE

One of the key factors that promotes angiogenesis is vascular endothelial growth factor (VEGF). Platelets are the main source of VEGF in blood and contribute to angiogenesis by release of growth factors, including VEGF, from their alpha-granules on activation. The monoclonal antibody bevacizumab blocks VEGF in the blood of patients within hours after administration. Platelets are known to endocytose plasma proteins including immunoglobulins. We tested the hypothesis that platelets take up bevacizumab.

EXPERIMENTAL DESIGN

Fluorescence-activated cell sorting analysis, immunofluorescence imaging, and Western blotting were used to study uptake and release of bevacizumab by platelets in vitro and in vivo. The angiogenic activity of platelets preincubated with bevacizumab was studied in endothelial proliferation assays. Finally, we determined whether treatment with bevacizumab neutralizes VEGF in platelets from cancer patients.

RESULTS

We found that platelets are able to take up bevacizumab. Activation of platelets preincubated with bevacizumab resulted in release of the antibody and release of VEGF neutralized by bevacizumab. Immunofluorescence microscopy revealed that FITC-labeled bevacizumab and P-selectin colocalize, indicating alpha-granule localization. In addition, bevacizumab uptake inhibited platelet-induced human endothelial cell proliferation. In in vivo rabbit experiments, FITC-labeled bevacizumab was present in platelets after 2 h and up to 2 weeks following i.v. administration. Finally, we found that platelets take up bevacizumab in patients receiving bevacizumab treatment. Within 8 h after bevacizumab administration, platelet VEGF was almost completely neutralized due to this uptake.

CONCLUSION

These studies show that bevacizumab is taken up by platelets and may explain its clinical effect on wound healing and tumor growth.

Platelet activation by oxidized low density lipoprotein is mediated by CD36 and scavenger receptor-A

OBJECTIVE

The interaction of platelets with low density lipoprotein (LDL) contributes to the development of cardiovascular disease. Platelets are activated by native LDL (nLDL) through apoE Receptor 2' (apoER2')-mediated signaling to p38(MAPK) and by oxidized LDL (oxLDL) through lysophosphatidic acid (LPA) signaling to Rho A and Ca2+. Here we report a new mechanism for platelet activation by oxLDL.

METHODS AND RESULTS

Oxidation of nLDL increases p38(MAPK) activation through a mechanism that is (1) independent of LPA, and (2) unlike nLDL-signaling not desensitized by prolonged platelet-LDL contact or inhibited by receptor-associated protein or chondroitinase ABC. Antibodies against scavenger receptors CD36 and SR-A alone fail to block p38(MAPK) activation by oxLDL but combined blockade inhibits p38(MAPK) by >40% and platelet adhesion to fibrinogen under flow by >60%. Mouse platelets deficient in either CD36 or SR-A show normal p38(MAPK) activation by oxLDL but combined deficiency of CD36 and SR-A disrupts oxLDL-induced activation of p38(MAPK) by >70%.

CONCLUSION

These findings reveal a novel platelet-activating pathway stimulated by oxLDL that is initiated by the combined action of CD36 and SR-A.

Activation of Human Platelets by Misfolded Proteins

OBJECTIVE

Protein misfolding diseases result from the deposition of insoluble protein aggregates that often contain fibrils called amyloid. Amyloids are found in Alzheimer disease, atherosclerosis, diabetes mellitus, and systemic amyloidosis, which are diseases where platelet activation might be implicated.

METHODS AND RESULTS

We induced amyloid properties in 6 unrelated proteins and found that all induced platelet aggregation in contrast to fresh controls. Amyloid-induced platelet aggregation was independent of thromboxane A2 formation and ADP secretion but enhanced by feedback stimulation through these pathways. Treatments that raised cAMP (iloprost), sequestered Ca2+ (BAPTA-AM) or prevented amyloid-platelet interaction (sRAGE, tissue-type plasminogen activator [tPA]) induced almost complete inhibition. Modulation of the function of CD36 (CD36-/- mice), p38(MAPK) (SB203580), COX-1 (indomethacin), and glycoprotein Ib alpha (Nk-protease, 6D1 antibody) induced approximately 50% inhibition. Interference with fibrinogen binding (RGDS) revealed a major contribution of alphaIIb beta3-independent aggregation (agglutination).

CONCLUSIONS

Protein misfolding resulting in the appearance of amyloid induces platelet aggregation. Amyloid activates platelets through 2 pathways: one is through CD36, p38(MAPK), thromboxane A2-mediated induction of aggregation; the other is through glycoprotein Ib alpha-mediated aggregation and agglutination. The platelet stimulating properties of amyloid might explain the enhanced platelet activation observed in many diseases accompanied by the appearance of misfolded proteins with amyloid.

Polyphenolic grape extract inhibits platelet activation through PECAM-1: an explanation for the French paradox

BACKGROUND

Moderate and prolonged consumption of red wine is associated with decreased cardiovascular morbidity and mortality. Inhibition of platelet functions by ingredients in red wine is thought to be one of the causes. However, the molecular mechanism of this inhibition has remained unexplained.

MATERIALS AND METHODS

We measured aggregation, changes in cytosolic Ca(2+) and tyrosine phosphorylation of the inhibitory receptor platelet endothelial cell adhesion molecule-1 (PECAM-1) in platelets stimulated with thrombin receptor (PAR-1) activating peptide (TRAP) and ADP and investigated the effects of alcohol-free polyphenolic grape extract (PGE), alcohol, and the polyphenols catechin, epi-catechin, resveratrol, trans-resveratrol, and gallic acid.

RESULTS

Polyphenolic grape extract induced dose-dependent inhibition of TRAP-induced and ADP-induced platelet aggregation and Ca(2+) mobilization. Inhibition was accompanied by activation of PECAM-1. Apart from a slight inhibition by catechin, ethanol or other individual polyphenols failed to inhibit aggregation or activate PECAM-1.

CONCLUSIONS

Red wine inhibits platelet functions through its PGE content, which stimulates the inhibitory receptor PECAM-1, thereby attenuating platelet activation.

Role of glycoprotein Ibalpha in phagocytosis of platelets by macrophages

BACKGROUND

Platelet (PLT) storage at 0 to 4 degrees C suppresses bacterial multiplication, but induces clusters of glycoprotein (GP) Ibalpha that trigger their phagocytosis by macrophages and reduce their survival after transfusion. A method was sought that detects cold-induced changes in GPIbalpha involved in phagocytosis.

STUDY DESIGN AND METHODS

Human PLTs were isolated and stored for up to 48 hours at 0 degrees C. Binding of a phycoerythrin (PE)-labeled antibody directed against amino acids (AA) 1-35 on GPIbalpha (AN51-PE) was compared with phagocytosis of PLTs by matured monocytic THP-1 cells, analyzed by fluorescence-activated cell sorting.

RESULTS

Freshly isolated PLTs were detected as a single population of AN51-PE-positive particles and showed less than 5 percent phagocytosis. Cold storage led to a decrease in AN51-PE binding and an increase in phagocytosis. N-Acetylglucosamine, known to interfere with macrophage recognition of GPIbalpha clusters, restored normal AN51-PE binding to cold-stored PLTs and suppressed phagocytosis.

CONCLUSIONS

It is concluded that binding of an antibody against AA 1-35 on GPIbalpha reflects changes in GPIbalpha that make PLTs targets for phagocytosis by macrophages.

Metabolic energy reduction by glucose deprivation and low gas exchange preserves platelet function after 48 h storage at 4 °C

INTRODUCTION

We showed earlier that metabolically suppressed platelets (MSP) prepared by incubation in glucose-free, antimycin A medium at 37 degrees C better sustained storage at 4 degrees C than untreated controls at 22 degrees C. However, the use of the mitochondrial inhibitor antimycin A is incompatible with platelet transfusion.

OBJECTIVES

The aim of this study was to investigate how energy-reduced (ER) platelets could be prepared in the absence of antimycin A.

STUDY DESIGN AND METHODS

Platelets in gas-impermeable bags in glucose-free medium were kept at 22 degrees C for 4 h to reduce energy stores and thereafter stored at 4 degrees C (ER22-4). Controls were energy-reduced platelets without prior incubation at 22 degrees C (ER4), and MSPs in test tubes and untreated platelets in gas-permeable bags with glucose and stored at 22 degrees C (C22) and 4 degrees C (C4).

RESULTS

After 48 h storage, ER22-4 were superior to C22 with respect to pH preservation (6 x 4 +/- 0 x 4 vs. 5 x 0 +/- 0 x 4, n= 4), platelet count (800 +/- 225 vs. 650 +/- 150 x 10(9)), thrombin receptor-activating peptide-induced aggregation (50 +/- 15 vs. 10 +/- 5%) and glycoprotein (GP)Ib alpha expression (60 +/- 15% vs. 28 +/- 15). GPIb alpha expression was higher in ER22-4 than in ER4, indicating that energy suppression preserved GPIb alpha during cold storage.

CONCLUSION

Metabolic suppression without the use of antimycin A could be mimicked by storage of platelets in glucose-free medium in gas-impermeable bags. Energy suppression preserved GPIb alpha expression during storage at 4 degrees C.

Platelet activation by low density lipoprotein and high density lipoprotein

Cardiovascular disease is the main cause of death and disability in the Western society. Lipoproteins are important in the development of cardiovascular disease since they change the properties of different cells involved in atherosclerosis and thrombosis. The interaction of platelets with lipoproteins has been under intense investigation. Particularly the initiation of platelet signaling pathways by low density lipoprotein (LDL) has been studied thoroughly, since platelets of hypercholesterolemic patients, whose plasma contains elevated LDL levels due to absent or defective LDL receptors, show hyperaggregability in vitro and enhanced activity in vivo. These observations suggest that LDL enhances platelet responsiveness. Several signaling pathways induced by LDL have been revealed in vitro, such as signaling via p38 mitogen-activated protein kinase and p125 focal adhesion kinase. High density lipoprotein (HDL) consists of two subtypes, HDL(2) and HDL(3), which have opposing effects on platelet activation. This review provides a summary of the activation of signaling pathways after platelet-LDL and platelet-HDL interaction, with special emphasis on their role in the development of thrombosis and atherosclerosis.

Lipid rafts facilitate the interaction of PECAM-1 with the glycoprotein VI-FcR gamma-chain complex in human platelets

Glycoprotein (GP) VI, the main signaling receptor for collagen on platelets, is expressed in complex with the FcR gamma-chain. The latter contains an immunoreceptor tyrosine-based activation motif, which becomes phosphorylated, initiating a signaling cascade leading to the rapid activation and aggregation of platelets. Previous studies have shown that signaling by immunoreceptor tyrosine-based activation motif-containing receptors is counteracted by signals from receptors with immunoreceptor tyrosine-based inhibitory motifs. Here we show, by immunoprecipitation, that the GPVI-FcR gamma-chain complex associates with the immunoreceptor tyrosine-based inhibitory motif-containing receptor, PECAM-1. In platelets stimulated with collagen-related peptide (CRP-XL), tyrosine phosphorylation of PECAM-1 precedes that of the FcR gamma-chain, implying direct regulation of the former. The GPVI-FcR gamma-chain complex and PECAM-1 were present in both lipid raft and soluble fractions in human platelets; this distribution was unaltered by activation with CRP-XL. Their association occurred in lipid rafts and was lost after lipid raft depletion using methyl-beta-cyclodextrin. We propose that lipid raft clustering facilitates the interaction of PECAM-1 with the GPVI-FcR gamma-chain complex, leading to the down-regulation of the latter.

Platelet binding and phagocytosis by macrophages

BACKGROUND

Earlier it was reported that metabolic arrest followed by incubation at 4 degrees C reduces the platelet (PLT) storage defect. Here it is reported that this treatment also reduces binding and phagocytosis by macrophages.

STUDY DESIGN AND METHODS

Phagocytosis of mepacrine-labeled PLTs by macrophages changes the latter into bright fluorescent particles easily detected by fluorescence-activated cell sorting.

RESULTS

In combination with conventional binding analysis it was found that binding to phorbol 12-myristate 13-acetate-matured THP-1 cells is primarily regulated by PLT P-selectin expression and phagocytosis by combined phosphatidylserine (PS) exposure and glycoprotein (GP) Ibalpha clustering. It was found that trapping of PLT Ca2+ and raising cAMP reduces phagocytosis by lowering PS exposure. Chilling of PLTs leads to an increase in binding and PS- and GPIbalpha-mediated phagocytosis. Prior depletion of PLT energy stores prevents this increase by preserving low Ca2+ concentration, PS exposure, and PS-mediated phagocytosis.

CONCLUSION

These data characterize the individual factors that control PLT binding and phagocytosis and might help to define conditions that improve the survival of stored PLTs after transfusion.

Lipoprotein-associated proteins involved in platelet signaling

Platelets and lipoproteins are both key elements in the development of atherosclerosis and thrombosis. Based on their density, five classes of lipoproteins have been identified which all influence platelets via distinct mechanisms. The activation of platelets starts with binding of apolipoproteins to different platelet receptors and is followed by the activation of signaling pathways resulting in activation or inhibition of platelet functions like aggregation or secretion. In addition to apolipoproteins, lipoproteins are also associated to a large amount of proteins, enzymes and lipids that also can induce platelet activation or inhibition. This review provides a summary of the activation of signaling pathways after platelet-lipoprotein interactions initiated by lipoprotein-associated proteins and lipids.

Plasma ectonucleotidases prevent desensitization of purinergic receptors in stored platelets: importance for platelet activity during thrombus formation

BACKGROUND

Platelets (PLTs) contain purinergic receptors for ATP (P2X1) and ADP (P2Y1 and P2Y12) that rapidly desensitize upon stimulation with these nucleotides. In vivo, this is antagonized by ectonucleotidases on the surface of endothelial cells and white blood cells (WBCs). The receptor desensitization of ATP- and ADP-induced responses of PLTs stored in plasma without WBCs was investigated.

STUDY DESIGN AND METHODS

ATP- and ADP-induced PLT shape change (shear-induced) aggregation and Ca2+ signaling were measured in the presence or absence of plasma. Degradation of nucleotides in plasma was quantified by high-performance liquid chromatography.

RESULTS

Washed PLTs became refractory for ATP and ADP in shape change, aggregation, and Ca2+ responses during a 90-minute incubation at 37 degrees C. The PLT responses mediated by P2X1, P2Y1, and P2Y12 receptors gradually reduced or disappeared. When plasma was present, however, the PLTs persistently showed high responses to ATP and ADP. Heat treatment of plasma abolished this effect. Also under conditions of flow and high shear, PLTs in plasma kept high P2X1 activity, mediating aggregate formation. In isolated plasma, not containing WBCs, nucleotides were degraded in the order of ADP/UDP>ATP/UTP. Degradation of ATP was partly inhibited by blocking the ecto-NTPDase CD39, whereas degradation of both ATP and ADP was inhibited by blocking ectopyrophosphatase/phosphodiesterase activity. Part of the nucleotide-degrading activities appeared to be membrane-bound.

CONCLUSION

Ectonucleotidases in plasma preserve the functionality of P2X1 and P2Y receptors. Upon PLT storage, these plasma activities are essential to ensure adequate (shear-dependent) formation of aggregates and thrombi.

Platelet signaling induced by lipoproteins

Cardiovascular disease is the main cause of death and disability in the Western society. Lipoproteins play an important role in the development of this disease and affect different cell types involved in atherosclerosis and thrombosis. Based on their density, five classes of lipoproteins have been identified which all influence cells via distinct mechanisms. Modification turns lipoproteins into atherogenic particles with a prominent role in atherogenesis. The interaction of lipoproteins with platelets has been under investigation for a number of years. Especially the role of LDL in platelet signaling has been studied intensively as platelets of hypercholesterolemic patients are hyperreactive and show hyperaggregability in vitro and enhanced activity in vivo, suggesting that LDL enhances platelet responsiveness. Several signaling pathways induced by LDL have been revealed in vitro, such as signaling via p38 mitogen-activated protein kinase (p38MAPK) and p125 focal adhesion kinase (p125FAK). HDL opposes the activating properties of LDL on platelets, whereas the effects of chylomicrons, VLDL or IDL on platelet function are controversial. Modification of lipoproteins is associated with the generation of new constituents with new signaling properties. In particular, the platelet-activating properties of lysophosphatidic acid, which is a constituent of atherosclerotic plaques and is generated upon oxidation of LDL, have been investigated intensively. This review provides a summary of the activation of signaling pathways after platelet-lipoprotein interactions, with special emphasis on the role of these interactions in the development of thrombosis and atherosclerosis.

Platelet inhibition by insulin is absent in type 2 diabetes mellitus

OBJECTIVE

ADP-induced P2y12 signaling is crucial for formation and stabilization of an arterial thrombus. We demonstrated recently in platelets from healthy subjects that insulin interferes with Ca2+ increases induced by ADP-P2y1 contact through blockade of the G-protein Gi, and thereby with P2y12-mediated suppression of cAMP.

METHODS AND RESULTS

Here we show in patients with type 2 diabetes mellitus (DM2) that platelets have lost responsiveness to insulin leading to increased adhesion, aggregation, and procoagulant activity on contact with collagen. Using Ser473 phosphorylation of protein kinase B as output for insulin signaling, a 2-fold increase is found in insulin-stimulated normal platelets, but in DM platelets there is no significant response. In addition, DM2 platelets show increased P2y12-mediated suppression of cAMP and decreased P2y12 inhibition by the receptor antagonist AR-C69931MX.

CONCLUSIONS

The loss of responsiveness to insulin together with increased signaling through P2y12 might explain the hyperactivity of platelets in patients with DM2.

The phospholipid composition and cholesterol content of platelet-derived microparticles: a comparison with platelet membrane fractions

BACKGROUND

The processes that govern the distribution of molecules between platelets and the microparticles (MP) they release are unknown. Certain proteins are sorted selectively into MP, but lipid sorting has not been studied.

OBJECTIVES

To compare the phospholipid composition and cholesterol content of platelet-derived MP obtained with various stimuli with that of isolated platelet membrane fractions.

METHODS

Washed platelets from venous blood of healthy individuals (n = 6) were stimulated with collagen, thrombin, collagen plus thrombin, or A23187. Platelet activation, MP release and antigen exposure were assessed by flow cytometry. MPs were isolated by differential centrifugation. Platelet plasma-, granule- and intracellular membranes were isolated from platelet concentrates (n = 3; 10 donors each) by pressure homogenization and Percoll density gradient fractionation. The phospholipid composition and cholesterol content of MPs and membrane fractions were analyzed by high performance thin layer chromatography.

RESULTS

The phospholipid composition of MPs was intermediate compared with that of platelet plasma- and granule membranes, and differed significantly from that of intracellular membranes. There were small but significant differences in phospholipid composition between the MPs produced by the various agonists, which paralleled differences in P-selectin exposure in case of the physiological agonists collagen, thrombin, or collagen plus thrombin. The cholesterol content of MPs tended to be higher than that of the three-platelet membrane fractions.

CONCLUSIONS

Regarding its phospholipid content, the MP membrane is a composite of the platelet plasma- and granule membranes, showing subtle differences depending on the platelet agonist. The higher cholesterol content of MPs suggests their enrichment in lipid rafts.

Glucose Uptake via Glucose Transporter 3 by Human Platelets Is Regulated by Protein Kinase B

In insulin-responsive tissues, insulin is a potent activator of protein kinase B (PKB)-mediated glucose uptake through the facilitative glucose transporter GLUT4. In platelets, glucose uptake is mediated through GLUT3, which is present in plasma (15%) and intracellular alpha-granule (85%) membranes. Here we report the PKB-mediated glucose uptake by platelets by agents that do (thrombin) or do not (insulin) induce alpha-granule translocation to the plasma membrane. Both thrombin and insulin activate PKB and induce glucose uptake albeit with different kinetics. Inhibition of PKB by the pharmacological inhibitor ML-9 decreases thrombin-induced alpha-granule release and thrombin- and insulin-induced glucose uptake. At low glucose (0.1 mm), both agents stimulate glucose uptake by lowering the Km for glucose (thrombin and insulin) and increasing Vmax (thrombin). At high glucose (5 mm), stimulation of glucose uptake by insulin disappears, and insulin becomes an inhibitor of thrombin-induced glucose uptake via mechanisms independent of PKB. We conclude that in platelets glucose transport through GLUT3 is regulated by changes in surface expression and affinity modulation, which are both under control of PKB.

A tripeptide mimetic of von Willebrand factor residues 981-983 enhances platelet adhesion to fibrinogen by signaling through integrin alpha(IIb)beta3

BACKGROUND

RGD is a major recognition sequence for ligands of platelet alpha(IIb)beta3.

OBJECTIVE AND METHODS

To identify potential binding sites for alpha(IIb)beta3 apart from RGD, we screened phage display libraries by blocking the enrichment of RGD-containing phages with a GRGDS peptide and identified a novel integrin recognition tripeptide sequence, VPW.

RESULTS

Platelets adhered to an immobilized cyclic VPW containing peptide in a alpha(IIb)beta3-dependent manner; platelets and alpha(IIb)beta3-expressing CHO cells adhered faster to immobilized alpha(IIb)beta3-ligands in the presence of soluble VPW. In platelets adhering to fibrinogen, VPW accelerated the activation of the tyrosine kinase Syk which controls cytoskeletal rearrangements. In alpha(IIb)beta3-expressing CHO cells, VPW induced a faster formation of stress fibers. Sequence alignment positioned VPW to V980-P981-W982 in the von Willebrand factor (vWf) A-3 domain. In blood from a vWf-deficient individual, VPW increased platelet adhesion to fibrinogen but not to collagen under flow and rescued the impaired adhesion to vWf deficient in A-3.

CONCLUSION

These data reveal a VPW sequence that contributes to alpha(IIb)beta3 activation in in vitro experiments. Whether the V980-P981-W982 sequence in vWf shows similar properties under in vivo conditions remains to be established.

Effect of oxidation on the platelet-activating properties of low-density lipoprotein

OBJECTIVE

Because of the large variation in oxidizing procedures and susceptibility to oxidation of low-density lipoprotein (LDL) and the lack in quantification of LDL oxidation, the role of oxidation in LDL-platelet contact has remained elusive. This study aims to compare platelet activation by native LDL (nLDL) and oxidized LDL (oxLDL).

METHODS AND RESULTS

After isolation, nLDL was dialyzed against FeSO4 to obtain LDL oxidized to well-defined extents varying between 0% and >60%. The oxLDL preparations were characterized with respect to their platelet-activating properties. An increase in LDL oxidation enhances platelet activation via 2 independent pathways, 1 signaling via p38(MAPK) phosphorylation and 1 via Ca2+ mobilization. Between 0% and 15% oxidation, the p38(MAPK) route enhances fibrinogen binding induced by thrombin receptor (PAR-1)-activating peptide (TRAP), and signaling via Ca2+ is absent. At >30% oxidation, p38(MAPK) signaling increases further and is accompanied by Ca2+ mobilization and platelet aggregation in the absence of a second agonist. Despite the increase in p38(MAPK) signaling, synergism with TRAP disappears and oxLDL becomes an inhibitor of fibrinogen binding. Inhibition is accompanied by binding of oxLDL to the scavenger receptor CD36, which is associated with the fibrinogen receptor, alpha(IIb)beta3.

CONCLUSIONS

At >30% oxidation, LDL interferes with ligand binding to integrin alpha(IIb)beta3, thereby attenuating platelet functions.

Prolonged platelet preservation by transient metabolic suppression

BACKGROUND

In this study whether metabolic suppression can be used to preserve platelet (PLT) function during prolonged storage was investigated.

STUDY DESIGN AND METHODS

Washed human PLTs were incubated without glucose and with antimycin A to block energy generation. Metabolic suppressed PLTs (MSPs) were stored for 72 hours at different temperatures to find the optimal storage temperature. Controls were incubated with 5 mmol per L glucose and stored at 22 and 4 degrees C.

RESULTS

Following metabolic recovery with glucose, MSPs stored at 37, 22, and 4 degrees C showed an increase in basal P-selectin expression (PSE) reaching greater than 40 percent after about 2, 20, and 48 hours; a decrease in thrombin receptor-activating peptide SFLLRN (TRAP)-induced PSE inversely related to the increase in basal PSE; and a decrease in TRAP-induced aggregation reaching less than 30 percent after about 4, 24, and more than 72 hours. When compared with control suspensions, MSPs stored at 4 degrees C better preserved a low basal PSE and in addition showed a better adhesion to surface coated-von Willebrand factor and fibrinogen in a flow chamber.

CONCLUSION

Metabolic suppression before storage at 4 degrees C contributes to better preservation of PLT function.

Rapid intake of alcohol (binge drinking) inhibits platelet adhesion to fibrinogen under flow

BACKGROUND

Moderate alcohol consumption is associated with decreased mortality from cardiovascular disease. Drinking large amounts in a short period (binge drinking) is associated with increased cardiovascular morbidity. We tested whether rapid consumption of a large dose of alcohol affects platelet aggregation and adhesion.

METHODS

Healthy volunteers (n = 20) were asked to drink three glasses of alcohol or red wine in a 45-min period. Thereafter, another 45 min was allowed for absorption of alcohol. Ninety minutes after the start of the experiment, blood was collected. This entire cycle was repeated once, resulting in consumption of six alcohol-containing drinks in 3 hr. Adenosine-diphosphate (ADP)-induced aggregation was measured and platelet adhesion to fibrinogen and collagen was measured in a perfusion chamber at shear rates of 300/sec and 1600/sec. Platelet coverage and aggregate size were measured.

RESULTS

Acute alcohol intake significantly increased platelet aggregation in suspension when stimulated with low concentrations of ADP (0.1 and 0.5 microg/ml). This effect was not observed when consuming red wine. In contrast, adhesion to fibrinogen was significantly inhibited by alcohol but not red wine at high shear rate after six drinks (p = 0.025). The inhibition was accompanied by a reduction in aggregate size at 90 and 180 min after the start of the experiment. Adhesion to collagen was not altered by either alcohol or red wine.

CONCLUSIONS

Rapid intake of alcohol increases platelet aggregation, which might contribute to the increased mortality associated with binge drinking. Red wine does not show increased platelet aggregation, which might support the reduction of cardiovascular disease in red wine consumers. However, alcohol inhibits platelet adhesion to fibrinogen-coated surface under flow. The diminished adhesion might contribute to the cardioprotective effects of alcohol.

IRS-1 and Vascular Complications in Diabetes Mellitus

The expected explosive increase in the number of patients with diabetes mellitus will increase the stress on health care. Treatment is focused on preventing vascular complications associated with the disorder. In order to develop better treatment regimens, the field of research has made a great effort in understanding this disorder. This chapter summarizes the current views on the insulin signaling pathway with emphasis on intracellular signaling events associated with insulin resistance, which lead to the prothrombotic condition in the vasculature of patience with diabetes mellitus.

Alcohol and polyphenolic grape extract inhibit platelet adhesion in flowing blood

BACKGROUND

Moderate and prolonged alcohol consumption has been associated with decreased cardiovascular morbidity and mortality. Inhibition of platelet function in suspension attributes to these effects. Whether alcohol, red wine, or polyphenolic grape extracts (PGE) inhibit platelet adhesion is not known. We investigated platelet adhesion to fibrinogen and collagen in whole blood under standardised flow.

MATERIALS AND METHODS

Before perfusion was started, citrated whole blood from 95 volunteers was preincubated for five min with different alcohol concentrations, unfractioned red wine and PGE. Then, blood was perfused in a single-passage flow chamber over coverslips coated with human fibrinogen or collagen type III at shear rates of 300 s(-1) and 1600 s(-1).

RESULTS

Alcohol inhibited platelet adhesion to human fibrinogen at high shear rate (concentrations > or = 0.15 per thousand) and low shear rate (only at a concentration of 4.8 per thousand), whereas red wine (concentrations > or = 0.15 per thousand) inhibited platelet adhesion to human fibrinogen at both shear rates. In contrast, PGE (concentrations > or = 0.0225 g L(-1)) inhibited platelet adhesion to human fibrinogen only at low shear rate. None of these incubations affected adhesion to collagen.

CONCLUSIONS

Alcohol, red wine and PGE inhibit adhesion to fibrinogen but not to collagen. This inhibition might contribute to the cardioprotective effects of moderate alcohol consumption.

Colocalization of eNOS and the catalytic subunit of PKA in endothelial cell junctions: a clue for regulated NO production

Localization and coordinate phosphorylation/dephosphorylation of endothelial nitric oxide synthase (eNOS) are critical determinants for the basal and stimulated production of nitric oxide. Several phosphorylation sites in eNOS have been identified as targets of the cAMP-dependent protein kinase A (PKA). Basal eNOS activity is also regulated by interaction with caveolin-1, the major coat protein of caveolae. In the present study we have examined in rat aorta endothelium the subcellular steady-state distribution of eNOS, the catalytic subunit of PKA (PKA-c), and caveolin-1. Basal eNOS expression was found in two distinct locations, the endothelial cell surface and the Golgi complex. Cell surface eNOS was equally distributed over caveolar and non-caveolar membranes but was 2.5-fold enriched on luminal lamellipodia located at endothelial cell contacts. PKA-c colocalized with eNOS in the lamellipodia, whereas caveolin-1 was absent from these membrane domains. PKA-c was also found associated with cell surface caveolae and with tubulovesicular membranes of Golgi complex and endosomes. The topological proximity of eNOS with the catalytic subunit of PKA in restricted intracellular locations may provide mechanisms for differential PKA-mediated eNOS regulation.

Binding of low density lipoprotein to platelet apolipoprotein E receptor 2' results in phosphorylation of p38MAPK

Binding of low density lipoprotein (LDL) to platelets enhances platelet responsiveness to various aggregation-inducing agents. However, the identity of the platelet surface receptor for LDL is unknown. We have previously reported that binding of the LDL component apolipoprotein B100 to platelets induces rapid phosphorylation of p38 mitogen-activated protein kinase (p38MAPK). Here, we show that LDL-dependent activation of this kinase is inhibited by receptor-associated protein (RAP), an inhibitor of members of the LDL receptor family. Confocal microscopy revealed a high degree of co-localization of LDL and a splice variant of the LDL receptor family member apolipoprotein E receptor-2 (apoER2') at the platelet surface, suggesting that apoER2' may contribute to LDL-induced platelet signaling. Indeed, LDL was unable to induce p38MAPK activation in platelets of apoER2-deficient mice. Furthermore, LDL bound efficiently to soluble apoER2', and the transient LDL-induced activation of p38MAPK was mimicked by an anti-apoER2 antibody. Association of LDL to platelets resulted in tyrosine phosphorylation of apoER2', a process that was inhibited in the presence of PP1, an inhibitor of Src-like tyrosine kinases. Moreover, phosphorylated but not native apoER2' co-precipitated with the Src family member Fgr. This suggests that exposure of platelets to LDL induces association of apoER2' to Fgr, a kinase that is able to activate p38MAPK. In conclusion, our data indicate that apoER2' contributes to LDL-dependent sensitization of platelets.

Cell-to-cell variability in the differentiation program of human megakaryocytes

Differentiation of CD34(+) stem/progenitor cells into megakaryocytes is thought to be a uniform, unidirectional process, in which cells transform step by step from less differentiated precursor stages to more differentiated megakaryocytes. Here we propose the concept and present evidence based on single-cell analysis that differentiation occurs along multiple, partially asynchronous routes. In all CD34(+) cells cultured with thrombopoietin, surface appearance of glycoprotein IIIa (GPIIIa) preceded that of GPIb, indicating that the expression of these glycoproteins occurs in a timely ordered manner. Cellular F-actin content increased in parallel with GPIb expression. Only cells that expressed GPIb were polyploid, pointing to co-regulation of GPIb expression, actin cytoskeleton formation and polyploidization during megakaryocytopoiesis. On the other hand, most progenitor cells responded to thrombin but not to thromboxane A(2) analogue by rises in cytosolic [Ca(2+)](i). The appearance of thromboxane-induced responses during megakaryocytopoiesis was not strictly linked to glycoprotein expression, because cells showed responsiveness either before or after GPIb expression. The same non-strictly sequential pattern was observed for disappearance of the Ca(2+) response by prostacyclin mimetic; in some megakaryocytes it occurred before and in others after GPIb expression. Thus, megakaryocytic differentiation follows along independent routes that are either strictly sequential (GPIIIa and GPIb expression) or proceed at different velocities (Ca(2+) signal regulation).

IRS-1 mediates inhibition of Ca2+ mobilization by insulin via the inhibitory G-protein Gi

Platelet agonists initiate aggregation and secretion by activating receptors coupled to the G-protein G(q), thereby raising cytosolic Ca(2+), [Ca(2+)](i). The rise in [Ca(2+)](i) is facilitated via inhibition of cAMP formation by the inhibitory G-protein of adenylyl cyclase, G(i). Since insulin attenuates platelet activation, we investigated whether insulin interferes with cAMP regulation. Here we report that insulin (0.5-200 nmol/liter) interferes with agonist-induced increases in [Ca(2+)](i) (ADP, thrombin), cAMP suppression (thrombin), and aggregation (ADP). The effects of insulin are as follows: (i) independent of the P2Y(12) receptor, which mediates ADP-induced cAMP lowering; (ii) not observed during G(s)-mediated cAMP formation; (iii) unaffected by treatments that affect phosphodiesterases (3-isobutyl-1-methylxanthine); and (iv) not changed by interfering with NO-mediated regulation of cAMP degradation (N(G)-monomethyl-l-arginine). Hence, insulin might interfere with G(i). Indeed, insulin induces the following: (i) tyrosine phosphorylation of the insulin receptor, the insulin receptor substrate-1 (IRS-1) and G(i)alpha(2); (ii) co-precipitation of IRS-1 with G(i)alpha(2) but not with other G alpha subunits. Despite persistent receptor activation, the association of IRS-1 with G(i)alpha(2) is transient, being optimal at 5 min and 1 nmol/liter insulin, which is sufficient to suppress Ca(2+) signaling by ADP, and at 10 min and 100 nmol/liter insulin, which is required to suppress Ca(2+) signaling by thrombin. Epinephrine, a known platelet sensitizer and antagonist of insulin, abolishes the effect of insulin on [Ca(2+)](i), tyrosine phosphorylation of G(i)alpha(2), and aggregation by interfering with the phosphorylation of the insulin receptor beta subunit. We conclude that insulin attenuates platelet functions by interfering with cAMP suppression through IRS-1 and G(i).

Cytoplasmic regions of the beta3 subunit of integrin alphaIIbbeta3 involved in platelet adhesion on fibrinogen under flow conditions

Platelet adhesion to surface-bound fibrinogen depends on integrin alphaIIbbeta3. In the present study, we investigated the role of the regions 749EATSTFT756N and 755TNITYRG762T of the beta3 cytoplasmic tail in the regulation of platelet adhesion under flow conditions, by introducing peptide mimetics in platelets. Introduction of peptide EATSTFTN (E-N) increased surface coverage by 35%, an effect caused by 25% more adhesion. In contrast, peptide TNITYRGT (T-T) decreased surface coverage by 16%, as a result of 25% less adhesion. An S-->P substitution in the E-N peptide, thereby mimicking a mutation in Glanzmann's thrombasthenia, abolished the effect of E-N. A suboptimal concentration of cytochalasin D is known to enhance ligand binding to alphaIIbbeta3 in platelet suspensions. Under flow, cytochalasin D (1 micro mol L-1) induced 50% more platelet adhesion, with a strong reduction in platelet spreading. Both peptides opposed the increase in adhesion by cytochalasin D and partly (E-N) and completely (T-T) restored platelet spreading. Thus, the 749EATSTFT756N and 755TNITYRG762T regions of beta3 contribute to the regulation of alphaIIbbeta3 anchorage to the cytoskeleton and platelet spreading to an adhesive surface.

Platelet endothelial cell adhesion molecule-1 (PECAM-1) inhibits low density lipoprotein-induced signaling in platelets

At physiological concentrations, low density lipoprotein (LDL) increases the sensitivity of platelets to aggregation- and secretion-inducing agents without acting as an independent activator of platelet functions. LDL sensitizes platelets by inducing a transient activation of p38MAPK, a Ser/Thr kinase that is activated by the simultaneous phosphorylation of Thr180 and Tyr182 and is an upstream regulator of cytosolic phospholipase A2 (cPLA2). A similar transient phosphorylation of p38MAPK is induced by a peptide mimicking amino acids 3359-3369 in apoB100 called the B-site. Here we report that the transient nature of p38MAPK activation is caused by platelet endothelial cell adhesion molecule 1 (PECAM-1), a receptor with an immunoreceptor tyrosine-based inhibitory motif. PECAM-1 activation by cross-linking induces tyrosine phosphorylation of PECAM-1 and a fall in phosphorylated p38MAPK and cPLA2. Interestingly, LDL and the B-site peptide also induce tyrosine phosphorylation of PECAM-1, and studies with immunoprecipitates indicate the involvement of c-Src. Inhibition of the Ser/Thr phosphatases PP1/PP2A (okadaic acid) makes the transient p38MAPK activation by LDL and the B-site peptide persistent. Inhibition of Tyr-phosphatases (vanadate) increases Tyr-phosphorylated PECAM-1 and blocks the activation of p38MAPK. Together, these findings suggest that, following a first phase in which LDL, through its B-site, phosphorylates and thereby activates p38MAPK, a second phase is initiated in which LDL activates PECAM-1 and induces dephosphorylation of p38MAPK via activation of the Ser/Thr phosphatases PP1/PP2A.

The small GTPase Rap1 is activated by turbulence and is involved in integrin [alpha]IIb[beta]3-mediated cell adhesion in human megakaryocytes

The small GTPase Rap1, which is activated by a large variety of stimuli, functions in the control of integrin-mediated cell adhesion. Here we show that in human megakaryocytes and several other commonly used hematopoietic cell lines such as K562, Jurkat, and THP-1, stress induced by gentle tumbling of the samples resulted in rapid and strong activation of Rap1. This turbulence-induced activation could not be blocked by inhibitors previously shown to affect Rap1 activation in human platelets, such as the intracellular calcium chelator BAPTA-AM (1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid) and various protein kinase C inhibitors. Also inhibition of actin cytoskeleton dynamics did not influence this activation of Rap1, suggesting that this activation is mediated by cell surface receptors. Human platelets, however, were refractory to turbulence-induced activation of Rap1. To determine the consequences of Rap1 activation we measured adhesion of megakaryocytes to fibrinogen, which is mediated by the integrin alphaIIbbeta3, in the presence of inhibitors of Rap1 signaling. Introduction of both Rap1GAP and RalGDS-RBD in the megakaryoblastic cell line DAMI strongly reduced basal adhesion to immobilized fibrinogen. This inhibition was partially rescued by the phorbol ester 12-O-tetradecanoylphorbol-13-acetate but not by alpha-thrombin. From these results we conclude that in megakaryocytes turbulence induces Rap1 activation that controls alphaIIbbeta3-mediated cell adhesion.