Leukocyte accumulation promoting fibrin deposition is mediated in vivo by P-selectin on adherent platelets

The glycoprotein P-selectin is a cell adhesion molecule of stimulated platelets and endothelial cells, which mediates the interaction of these cells with neutrophils and monocytes. It is a membrane component of cell storage granules, and is a member of the selectin family which includes E-selectin and L-selectin. P-selectin recognizes both lineage-specific carbohydrate ligands on monocytes and neutrophils, including the Lewis x antigen, sialic acid, and a protein component. In inflammation and thrombosis, P-selectin may mediate the interaction of leukocytes with platelets bound in the region of tissue injury and with stimulated endothelium. To evaluate the role of P-selectin in platelet-leukocyte adhesion in vivo, the accumulation of leukocytes within an experimental thrombus was explored in an arteriovenous shunt model in baboons. A Dacron graft implanted within an arteriovenous shunt is thrombogenic, accumulating platelets and fibrin within its lumen. These bound platelets express P-selectin. Here we show that antibody inhibition of leukocyte binding to P-selectin expressed on platelets immobilized on the graft blocks leukocyte accumulation and inhibits the deposition of fibrin within the thrombus. These results indicate that P-selectin is an important adhesion molecule on platelets, mediating platelet-leukocyte binding in vivo, that the presence of leukocytes in thrombi is mediated by P-selectin, and that these leukocytes promote fibrin deposition.

N-bromoacetyl-peptide substrate affinity labeling of vitamin K dependent carboxylase

Vitamin K dependent carboxylase (carboxylase) is a membrane-associated endoplasmic reticular enzyme that catalyzes the conversion of certain glutamate residues of essential blood coagulation proteins to gamma-carboxyglutamyl (Gla) residues. A series of N-bromoacetyl-peptide substrate affinity labels based on the Gla domain of these blood-clotting proteins was synthesized, and the substrate and inactivator kinetic parameters were assessed. The most promising of these affinity peptides, N-bromoacetyl-FLEELY, was both substrate for carboxylase and an irreversible time-dependent inactivator of the enzyme, inactivating 80% of carboxylase under pseudo-first-order conditions. Addition of saturating amounts of a competing peptide substrate completely abolished the inhibitory properties of N-bromoacetyl-FLEELY, consistent with inactivation occurring at the active site. The partition ratio of inactivation/carboxylation was 1/30. The 94-kDa carboxylase was purified to 15-50% purity by a modification of a recent protocol [Wu, S.-M., Morris, D. P., & Stafford, D. W. (1991) Proc. Natl. Acad. Sci. U.S.A. 88, 2236-2240] and covalently labeled with N-bromoacetyl-FLEEL[125I]Y. On silver-stained 10% sodium dodecyl sulfate-polyacrylamide gels, the predominant radiolabeled band was the 94,000 molecular weight species. This result independently validates that the 94-kDa protein is a carboxylase.

Dioxygen transfer during vitamin K dependent carboxylase catalysis

The vitamin K dependent carboxylase of liver microsomes is involved in the posttranslational modification of certain serine protease zymogens which are critical components of the blood clotting cascade. During coupled carboxylation/oxygenation this carboxylase converts glutamate residues, dihydrovitamin K, CO2, and O2 to a gamma-carboxyglutamyl (Gla) residue, vitamin K (2R,3S)-epoxide, and H2O with a stoichiometry of 1:1 for all substrates and products. In this paper we investigate the role of molecular oxygen in the reaction by following the course of the oxygen atoms using 18O2. Two different mass spectroscopic techniques, electron ionization positive ion mass spectrometry and supercritical fluid chromatography-negative ion chemical ionization mass spectrometry, were used to quantitate the amount of 18O incorporation into the various oxygens of the vitamin K epoxide product. We found that 0.95 mol atoms of oxygen were incorporated into the epoxide oxygen, 0.05 mol atoms of oxygen were incorporated into the quinone oxygen of vitamin K epoxide, and the remaining ca. 1.0 mol atoms of oxygen were incorporated into H2O. No incorporation of oxygen into vitamin K epoxide from 50% H2(18)O was observed. Thus, the carboxylase operates as a dioxygenase 5% of the time during carboxylation/oxygenation. The relevance of these findings with respect to the nonenzymic "basicity enhancement" model proposed by Ham and Dowd [(1990) J. Am. Chem. Soc. 112, 1660-1661] is discussed.

Construction, expression, and characterization of a chimera of factor IX and factor X. The role of the second epidermal growth factor domain and serine protease domain in factor Va binding

The prothrombinase complex, which catalyzes the conversion of prothrombin to thrombin, consists of activated Factor X, Factor Va, a membrane surface and Ca2+. To examine the structures that support Factor Va binding to Factor X, we used in vitro mutagenesis to construct a chimeric molecule that includes regions of Factor IX and Factor X. This chimera (IXGla,E1XE2,SP) was prepared from cDNA encoding the second epidermal growth factor (EGF) and serine protease domains of Factor X linked downstream from the cDNA encoding the signal peptide, propeptide, Gla domain, and first EGF domain of Factor IX. The cDNAs encoding the Factor IX/X chimera and wild-type Factor X were each expressed in Chinese hamster ovary cells and the secreted proteins purified by affinity chromatography using polyclonal anti-Factor X antibodies. The chimera migrated as a single major band corresponding to a molecular weight of 68,000. By Western blotting, the chimeric protein stained with both polyclonal anti-Factor X and anti-Factor IX antibodies. gamma-Carboxyglutamic acid analysis demonstrated near complete carboxylation of both the wild-type Factor X and the Factor IX/X chimera. Compared with Factor X, the rate of zymogen activation of the Factor IX/X chimera was about 50% that of Factor X when activated by Factor IXa, Factor VIIIa, phospholipid, and Ca2+. The enzyme form of the Factor IX/X chimera, activated Factor IX/X, generated using the coagulant protein of Russell's viper venom, expressed full amidolytic activity compared with Factor Xa. The activated Factor IX/X chimera had about 14% of the activity of Factor Xa when employed in a prothrombinase assay; this activity reached 100% with increasing concentrations of Factor Va. A binding assay was employed to test the ability of the active site-inactivated Factor IX/Xa chimera to inhibit the binding of Factor Xa to the Factor Va-phospholipid complex, thus inhibiting the activation of prothrombin to thrombin. In this assay the active site-inactivated form of the chimera competed with Factor Xa completely but with decreased affinity for the Factor Va-phospholipid complex. These data indicate that the second EGF domain and the serine protease domain of Factor Xa are sufficient to interact with Factor Va. The Factor IX/X chimera is a good substrate for the tenase complex; the defective enzymatic activity of the activated Factor IX/X chimera can be accounted for by its decreased affinity for Factor Va relative to Factor Xa.

P-selectin and E-selectin. Distinct but overlapping leukocyte ligand specificities

P-selectin on platelets and endothelial cells and E-selectin on endothelial cells are leukocyte receptors that recognize lineage-specific carbohydrates on neutrophils and monocytes. The proposed ligands for these receptors contain the Le(x) core and sialic acid. Since other investigators have shown that both E-selectin and P-selectin bind to sialylated Le(x), we evaluated whether E-selectin and P-selectin recognize the same counter-receptor on leukocytes. The interaction of HL60 cells with Chinese hamster ovary (CHO) cells expressing P-selectin or E-selectin was studied. To determine whether a protein component is required in addition to sialyl Le(x) for either P-selectin or E-selectin recognition, HL60 cells or neutrophils were digested with proteases, including chymotrypsin, elastase, proteinase Glu-C, ficin, papain, or thermolysin. Cells treated with these proteases bound E-selectin but not P-selectin. Fucosidase or neuraminidase treatment of HL60 cells markedly decreased binding to both E-selectin- and P-selectin-expressing CHO cells. Growth of HL60 cells in tunicamycin inhibited the ability of these cells to support P-selectin-mediated binding and, to a lesser extent, E-selectin-mediated binding. Purified P-selectin inhibited CHO:P-selectin binding to HL60 cells, but incompletely inhibited CHO:E-selectin binding to HL60 cells. However, purified soluble E-selectin inhibited CHO:P-selectin and CHO:E-selectin binding to HL60 cells equivalently and completely. COS cells, unable to bind to E-selectin or P-selectin, bound E-selectin but not P-selectin upon transfection with alpha-1,3-fucosyltransferase or alpha-1,3/1,4-fucosyltransferase. Similarly, LEC 11 cells expressing sialyl Le(x) bound E-selectin- but not P-selectin-expressing CHO cells. Sambucus nigra lectin, specific for the sialyl-2,6 beta Gal/GalNAc linkage, inhibited P-selectin but not E-selectin binding to HL60 cells. Although sialic acid and Le(x) are components of the P-selectin ligand and the E-selectin ligand, these results indicate that the ligands are related, having overlapping specificities, but are structurally distinct. A protein component containing sialyl Le(x) in proximity to sialyl-2,6 beta Gal structures on the P-selectin ligand may contribute to its specificity for P-selectin.

Detection of distinct isoform patterns of the beta-amyloid precursor protein in human platelets and lymphocytes

Cerebral deposition of the amyloid beta-protein (A beta P), approximately 40 residue fragment of the integral membrane protein, beta-amyloid precursor protein (beta APP), has been implicated as the probable cause of some cases of familial Alzheimer's disease (AD). The parallels between A beta P deposition in AD and the deposition of certain plasma proteins in systemic amyloid diseases has heightened interest in the analysis of beta APP in circulating cells and plasma. Here, we describe distinct isoform patterns of beta APP in peripheral platelets and lymphocytes. PCR-mediated amplification of mRNA from purified platelets demonstrated the expression of all three major beta APP transcripts (beta APP770,751,695). The full-length, approximately 140 kDa form of beta APP751,770 was detected in membranes of resting and activated platelets but very little immature, approximately 122 kDa beta APP751,770 was found, suggesting a different processing of beta APP in platelets than that described in a variety of cultured cells and tissues. Platelets stimulated with thrombin, calcium ionophore, or collagen released the soluble, carboxyl-truncated form of beta APP (protease nexin-II), but no evidence for the shedding of full-length beta APP associated with platelet microparticles was found, in contrast to previous reports. As a positive control marker for microparticles, the fibrinogen receptor subunit, GPIIIa, was readily detected in platelet releasates. Resting and activated platelets contained similar amounts of the approximately 10 kDa carboxyl terminal beta APP fragment that is retained in platelet membranes following the constitutive cleavage of protease nexin-II. Nonstimulated peripheral B and T lymphocytes contained small amounts of membrane-associated mature and immature beta APP751,770. The potentially amyloidogenic full-length beta APP molecules present in circulating platelets and lymphocytes but not in microparticles could serve as a source of the microvascular A beta P deposited during aging and particularly in AD.

P-selectin, a granule membrane protein of platelets and endothelial cells, follows the regulated secretory pathway in AtT-20 cells

P-selectin (PADGEM, GMP-140, CD62) is a transmembrane protein specific to alpha granules of platelets and Weibel-Palade bodies of endotheial cells. Upon stimulation of these cells, P-selectin is translocated to the plasma membrane where it functions as a receptor for monocytes and neutrophils. To investigate whether the mechanism of targeting of P- selectin to granules is specific for megakaryocytes and endothelial cells and/or dependent on von Willebrand factor, a soluble adhesive protein that is stored in the same granules, we have expressed the cDNA for P-selectin in AtT-20 cells. AtT-20 cells are a mouse pituitary cell line that can store proteins in a regulated fashion. By double-label immunofluorescence, P-selectin was visible as a punctate pattern at the tips of cell processes. This pattern closely resembled the localization of ACTH, the endogenous hormone produced and stored by the AtT-20 cells. Fractionation of the transfected cells resulted in the codistribution of P-selectin and ACTH in cellular compartments of the same density. Immunoelectron microscopy using a polyclonal anti-P- selectin antibody demonstrated immunogold localization in dense granules, morphologically indistinguishable from the ACTH granules. Binding experiments with radiolabeled monoclonal antibody to P-selectin indicated that there was also surface expression of P-selectin on the AtT-20 cells. After stimulation with the secretagogue 8-Bromo-cAMP the surface expression increased twofold, concomitant with the release of ACTH. In contrast, the surface expression of P-selectin transfected into CHO cells, which do not have a regulated pathway of secretion, did not change with 8-Br-cAMP treatment. In conclusion, we provide evidence for the regulated secretion of a transmembrane protein (P-selectin) in a heterologous cell line, which indicates that P-selectin contains an independent sorting signal directing it to storage granules.

Structure of the propeptide of prothrombin containing the gamma-carboxylation recognition site determined by two-dimensional NMR spectroscopy

The propeptides of the vitamin K dependent blood clotting and regulatory proteins contain a gamma-carboxylation recognition site that directs precursor forms of these proteins for posttranslational gamma-carboxylation. Peptides corresponding to the propeptide of prothrombin were synthesized and examined by circular dichroism (CD) and nuclear magnetic resonance spectroscopy (NMR). CD spectra indicate that these peptides have little or no secondary structure in aqueous solutions but that the addition of trifluoroethanol induces or stabilizes a structure containing alpha-helical character. The maximum helical content occurs at 35-40% trifluoroethanol. This trifluoroethanol-stabilized structure was solved by two-dimensional NMR spectroscopy. The NMR results demonstrate that residues -13 to -3 form an amphipathic alpha-helix. NMR spectra indicate that a similar structure is present at 5 degrees C, in the absence of trifluoroethanol. Of the residues previously implicated in defining the gamma-carboxylation recognition site, four residues (-18, -17, -16, and -15) are adjacent to the helical region and one residue (-10) is located within the helix. The potential role of the amphipathic alpha-helix in the gamma-carboxylation recognition site is discussed.

Platelet-derived microparticles express high affinity receptors for factor VIII

Factor VIII is a cofactor in the tenase enzyme complex which assembles on the membrane of activated platelets. A critical step in tenase assembly is membrane binding of factor VIII. Platelet membrane factor VIII-binding sites were characterized by flow cytometry using either fluorescein maleimide-labeled recombinant factor VIII or a fluorescein-labeled monoclonal antibody against factor VIII. Following activation by thrombin, most platelets bound factor VIII within 90 s. In addition, over the course of several minutes, membranous vesicles (microparticles) were shed from the platelet plasma membrane and each microparticle bound as much factor VIII as a stimulated platelet. Over 30 min, stimulated platelets (but not microparticles) lost the capacity to bind factor VIII. Factor VIII bound saturably to microparticles from platelets stimulated with thrombin, thrombin plus collagen, or the complement proteins C5b-9. The binding of factor VIII was compared to factor V, a structurally homologous coagulation cofactor. Analysis of microparticle binding kinetics yielded similar on and off rates for factor VIII and factor Va and KD values of 2-10 nM. In the presence of 20 nM factor Va, the binding of factor VIII to microparticles was increased, and there was a comparable increase in platelet tenase activity. At higher factor Va concentrations, factor VIII binding and tenase activity were inhibited. Conversely, factor VIII had a similar dose-dependent effect on factor Va binding and platelet prothrombinase activity. Synthetic phospholipid vesicles containing phosphatidylserine competed with microparticles for binding of factor VIII and factor Va. These studies indicate that activated platelets express a transient increase in high affinity receptors for factor VIII, whereas platelet-derived microparticles express a sustained increase in receptors. The binding characteristics of platelet membrane receptors for factor VIII are similar to those for factor Va.

Sulfated glycolipids are the platelet autoantigens for human platelet-binding monoclonal anti-DNA autoantibodies

The human monoclonal autoantibody HF2-1/17, produced by a human-human hybridoma derived from lymphocytes of a lupus patient with thrombocytopenia, reacts with single stranded DNA and platelets. To determine the chemical nature of the autoantigen against which this antibody is directed on platelets, this platelet antigen was purified by the lipid extraction of sonicated platelets, DEAE-Sephadex chromatography, and high performance liquid chromatography. The purified glycolipids, a trace component in platelets, demonstrated high reactivity with the HF2-1/17 antibody using a competition enzyme-linked immunosorbent assay system or immunostaining of thin layer chromatograms. The purified glycolipids co-migrated with bovine sulfatides by thin layer chromatography. The purified glycolipids contain sulfate and galactose but not sialic acid or phosphate. Fast atom bombardment-mass spectrometry revealed these sulfatides to be sulfated monohexyl ceramides. The dominant species has a molecular weight of 794 while a minor form has a molecular weight of 812 due to an extra hydroxyl group and loss of a double bond. These results indicate that the platelet autoantigen against which the human monoclonal anti-DNA antibody is directed represents a family of novel monogalactosyl sulfatides.

Surreptitious ingestion of a long-acting vitamin K antagonist/rodenticide, brodifacoum: clinical and metabolic studies of three cases

The vitamin K metabolism of three patients with factitious purpura due to brodifacoum ingestion was studied. These patients, who presented with bleeding disorders due to deficiency of the vitamin K-dependent blood clotting proteins, were refractory to vitamin K1 at standard doses and required fresh frozen plasma to control bleeding until large doses of vitamin K1 were used. Metabolic studies demonstrated a blockade in vitamin K utilization, consistent with the presence of a vitamin K antagonist, but the patients denied use of anticoagulants. Warfarin assays were negative. We show that the factitious purpura in each patient was due to the surreptitious ingestion of brodifacoum, a potent second generation long-acting vitamin K antagonist used as a rodenticide. The coagulopathies responded to long-term therapy with large doses of vitamin K1. The serum elimination half-time for brodifacoum ranged from 16 to 36 days in these patients. The anticoagulant effect is of long duration, requiring chronic vitamin K treatment. With increasing availability of new rodenticides, factitious purpura due to surreptitious ingestion of these potent vitamin K antagonists is emerging as a new problem, previously associated with warfarin, with important implications for diagnosis and treatment.

PADGEM-dependent adhesion of platelets to monocytes and neutrophils is mediated by a lineage-specific carbohydrate, LNF III (CD15)

PADGEM (platelet activation-dependent granule-external membrane protein) is a leukocyte receptor of activated platelets that mediates cellular adhesion of platelets to neutrophils and monocytes. To identify the natural ligand on neutrophils and monocytes that interacts with PADGEM, we have evaluated anti-leukocyte antibodies for their ability to block leukocyte-PADGEM binding. Only anti-CD15 antibodies were able to inhibit the binding of neutrophils, monocytes, HL60 cells, and U937 cells to platelets. Anti-CD15 antibodies inhibited the binding of U937 cells to PADGEM-expressing COS cells and to purified PADGEM incorporated into phospholipid vesicles. The CD15 antigen, lacto-N-fucopentaose III (Gal beta 1----4[Fuc alpha 1----3]NAcGlc beta 1----3Gal-beta 1----4Glc), inhibited the interaction of neutrophils or HL60 cells with platelets, whereas lacto-N-fucopentaose I did not; lacto-N-fucopentaose II demonstrated minimal inhibition. Lacto-N-fucopentaose III, and to a lesser extent lacto-N-fucopentaose II, but not lacto-N-fucopentaose I, inhibited the interaction of HL60 cells with COS cells transfected with PADGEM cDNA. CD15, lacto-N-fucopentaose III or Lex, is a component of the PADGEM ligand on neutrophils and monocytes.

Identification of amino acids in the gamma-carboxylation recognition site on the propeptide of prothrombin

A gamma-carboxylation recognition site on the propeptide of the vitamin K-dependent blood coagulation proteins directs the carboxylation of glutamic acid residues by binding to the vitamin K-dependent carboxylase. To determine residues that define this site, we evaluated the effect of mutation of certain residues in the prothrombin propeptide on the extent of carboxylation. The prothrombin cDNA modified by site-specific mutagenesis was expressed in Chinese hamster ovary cells using a system that yields functional fully carboxylated prothrombin. The cell supernatants containing recombinant prothrombin were evaluated for the extent of gamma-carboxylation by immunoassay. Conformation-specific anti-prothrombin:Ca(II)-specific antibodies measure native completely carboxylated prothrombin; anti-prothrombin:total antibodies measure all forms of prothrombin, regardless of gamma-carboxyglutamic acid content. Mutation of His-18 to Gly, Val-17 to Ser, Leu-15 to Gly or Asp, or Ala-10 to Asp was associated with a partial (30-65%) inhibition of gamma-carboxylation. Mutation of Ala-14 to Ser or Ser-8 to Val did not inhibit gamma-carboxylation. From this and earlier work, residues whose mutation leads to a significant impairment of carboxylation include His-18, Val-17, Phe-16, Leu-15, and Ala-10. Residues whose mutation does not alter the carboxylation recognition site include Ala-14, Ser-8, Arg-4, and Arg-1. To determine the size of the recognition site, the in vitro carboxylation of propeptide-containing synthetic peptides was compared. A 28-residue peptide, based upon residues -18 to +10 of prothrombin, and a 54-residue peptide, based upon residues -18 to +36 of prothrombin, were carboxylated by partially purified bovine carboxylase with similar Km values of 2-5 microM. These results indicate that the gamma-carboxyglutamic acid-rich region of prothrombin makes a minimal contribution to carboxylase binding. A molecular surface of about five amino acids located within the propeptide appears to define the carboxylation recognition site on the precursor forms of the vitamin K-dependent proteins.

Randomized prospective trial comparing the native prothrombin antigen with the prothrombin time for monitoring oral anticoagulant therapy

The dosage of the anticoagulant warfarin sodium is based upon the prolongation of the prothrombin time into an optimal therapeutic range. We have developed a new assay for the native prothrombin antigen that measures the fully gamma-carboxylated prothrombin using a radioimmunoassay. Based on preliminary data that indicated that the native prothrombin antigen predicted both bleeding and thrombotic complications more accurately than the prothrombin time in patients anticoagulated with warfarin sodium, we have performed a randomized prospective trial comparing the complication rate in warfarin-treated patients monitored with the native prothrombin antigen or the prothrombin time. Patients with indications for anticoagulation were randomized to be monitored by the native prothrombin antigen (therapeutic range, 12 to 24 micrograms/mL) or the prothrombin time index (therapeutic range, 1.5 to 2.0). Of the prothrombin time group (N = 80), seven (8.8%) had bleeding or thrombotic complications, with a complication rate of 9.5%/patient-year. In the native prothrombin antigen group (N = 76), one subject (1.3%) had a bleeding complication. The complication rate per patient-year was 1.5%. These results indicate an 85% reduction in the complication rate of the native prothrombin antigen group compared with the complication rate of the prothrombin time group. This difference is statistically significant by the Fisher exact test (P = .037) and by Kaplan Meier survival analysis (P = .040). This study suggests that the use of the native prothrombin antigen assay has the potential to decrease the complications associated with anticoagulation therapy with warfarin sodium.