Specific contribution of different phospholipid surfaces to the activation of prothrombin by the fully assembled prothrombinase

Therapeutic Role of Sirtuins Targeting Unfolded Protein Response, Coagulation, and Inflammation in Hypoxia-Induced Thrombosis

Thrombosis remains one of the leading causes of morbidity and mortality across the world. Many pathological milieus in the body resulting from multiple risk factors escort thrombosis. Hypoxic condition is one such risk factor that disturbs the integrity of endothelial cells to cause an imbalance between anticoagulant and procoagulant proteins. Hypoxia generates reactive oxygen species (ROS) and triggers inflammatory pathways to augment the coagulation cascade. Hypoxia in cells also activates unfolded protein response (UPR) signaling pathways in the endoplasmic reticulum (ER), which tries to restore ER homeostasis and function. But the sustained UPR linked with inflammation, generation of ROS and apoptosis stimulates the severity of thrombosis in the body. Sirtuins, a group of seven proteins, play a vast role in bringing down inflammation, oxidative and ER stress and apoptosis. As a result, sirtuins might provide a therapeutic approach towards the treatment or prevention of hypoxia-induced thrombosis. Sirtuins modulate hypoxia-inducible factors (HIFs) and counteract ER stress-induced apoptosis by attenuating protein kinase RNA-like endoplasmic reticulum kinase (PERK)/Eukaryotic translation initiation factor 2α (eIF2α) pathway activation. It prevents ER-stress mediated inflammation by targeting X-Box Binding Protein 1 (XBP1) and inhibiting nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κβ) signaling through deacetylation. Sirtuins also obstruct nucleotide-binding domain, leucine-rich-containing family, pyrin domain containing 3 (NLRP3) inflammasome activation to reduce the expression of several pro-inflammatory molecules. It protects cells against oxidative stress by targeting nuclear factor erythroid 2-related factor 2 (Nrf2), glutathione (GSH), forkhead box O3 (FOXO3), superoxide dismutase (SOD), catalase (CAT), peroxisome proliferator-activated receptor gamma coactivator 1-α (PGC-1α), glucose-6-phosphate dehydrogenase (G6PD), phosphoglucomutase-2 (PGAM2), and NF-κB, to name few. This review, thus, discusses the potential role of sirtuins as a new treatment for hypoxia-induced thrombosis that involves an intersection of UPR and inflammatory pathways in its pathological manifestation.

Assembly of alternative prothrombinase by extracellular histones initiate and disseminate intravascular coagulation

Thrombin generation is pivotal to both physiological blood clot formation and pathological development of disseminated intravascular coagulation (DIC). In critical illness, extensive cell damage can release histones into the circulation, which can increase thrombin generation and cause DIC, but the molecular mechanism is not clear. Typically, thrombin is generated by the prothrombinase complex, comprising activated factor X (FXa), activated co-factor V (FVa) and phospholipids to cleave prothrombin in the presence of calcium. In this study, we found that in the presence of extracellular histones, an alternative prothrombinase could form without FVa and phospholipids. Histones directly bind to prothrombin fragments F1 and F2 specifically, to facilitate FXa cleavage of prothrombin to release active thrombin, unlike FVa which requires phospholipid surfaces to anchor the classical prothrombinase complex. In vivo, histone infusion into mice induced DIC, which was significantly abrogated when prothrombin fragments F1+F2 were infused prior to histones, to act as decoy. In a cohort of intensive care unit (ICU) patients with sepsis (n=144), circulating histone levels were significantly elevated in patients with DIC. These data suggest that histone-induced alternative prothrombinase without phospholipid anchorage may disseminate intravascular coagulation, and reveal a new molecular mechanism of thrombin generation and DIC development. In addition, histones significantly reduced the requirement for FXa in the coagulation cascade to enable clot formation in Factor VIII and IX-deficient plasma, as well as in Factor VIII-deficient mice. In conclusion, this study highlights a novel mechanism in coagulation with therapeutic potential in both targeting systemic coagulation activation as well as in correcting coagulation factor deficiency.

Endothelial microparticles as conveyors of information in atherosclerotic disease

Endothelial microparticles (EMPs) are complex submicron membrane-shed vesicles released into the circulation following endothelium cell activation or apoptosis. They are classified as either physiological or pathological, with anticoagulant or pro-inflammatory effects respectively. Endothelial dysfunction caused by inflammation is a key initiating event in atherosclerotic plaque formation. Athero-emboli, resulting from ruptured carotid plaques are a major cause of stroke. Current clinical techniques for arterial assessment, angiography and carotid ultrasound, give accurate information about stenosis but limited evidence on plaque composition, inflammation or vulnerability; as a result, patients with asymptomatic, or fragile carotid lesions, may not be identified and treated effectively. There is a need to discover novel biomarkers and develop more efficient diagnostic approaches in order to stratify patients at most risk of stroke, who would benefit from interventional surgery. Increasing evidence suggests that EMPs play an important role in the pathogenesis of cardiovascular disease, acting as a marker of damage, either exacerbating disease progression or triggering a repair response. In this regard, it has been suggested that EMPs have the potential to act as biomarkers of disease status. In this review, we will present the evidence to support this hypothesis and propose a novel concept for the development of a diagnostic device that could be implemented in the clinic.

Apoptotic Cell Death and Efferocytosis in Atherosclerosis

Apoptotic cell death is an important feature of atherosclerotic plaques, and it seems to exert both beneficial and detrimental effects depending on the cell type and plaque stage. Because late apoptotic cells can launch proatherogenic inflammatory responses, adequate engulfment of apoptotic cells (efferocytosis) by macrophages is important to withstand atherosclerosis progression. Several efferocytosis systems, composed of different phagocytic receptors, apoptotic ligands, and bridging molecules, can be distinguished. Because phagocytes in atherosclerotic plaques are very much solicited, a fully operative efferocytosis system seems to be an absolute requisite. Indeed, recent studies demonstrate that deletion of just 1 of the efferocytosis pathways aggravates atherosclerosis. This review discusses the role of apoptosis in atherosclerosis and general mechanisms of efferocytosis, to end with indirect and direct indications of the significance of effective efferocytosis in atherosclerosis.

Endothelial microparticles and platelet and leukocyte activation in patients with the metabolic syndrome

Accumulating evidence has shown a strong association between the metabolic syndrome (MS) and a chronic inflammatory state predisposing to atherosclerosis. We investigated leukocyte, platelet, and endothelial activation markers and cellular interactions in 33 patients with the MS and 25 healthy controls. Using flow cytometry, we measured: (1)P-selectin expression in platelets; (2) platelet microparticles identified by CD31 expression; (3) endothelial microparticles (EMPs) identified by expression of CD31 (EMP(31)), CD62E (EMP(62E)), and CD51 (EMP(51)); (4) conjugates of leukocytes with platelet microparticles/platelets and with EMPs identified by CD54 (EMP(54)); and (5) CD11b expression in leukocytes. Patients with the MS had markedly elevated EMP(31), although EMP(62E) levels were normal, suggesting that EMP(31) levels were increased because of endothelial cell apoptosis, rather than activation. EMP(51), EMP(54)-lymphocyte conjugates, platelet expression of P-selectin, CD11b expression in leukocytes, and platelet-lymphocyte conjugates were also increased in patients with the MS. Platelet-leukocyte conjugates correlated with leukocyte activation, suggesting that platelet binding to leukocytes regulates leukocyte activation in vivo. In conclusion, our data demonstrate endothelial cell microparticle release, platelet and leukocyte activation, and increased binding of EMPs and platelets to leukocytes in patients with the MS.

Digoxin use is associated with increased platelet and endothelial cell activation in patients with nonvalvular atrial fibrillation

OBJECTIVES

The purpose of this study was to determine whether digoxin use is associated with increased flow cytometric markers of endothelial cell and platelet activation in patients with nonvalvular atrial fibrillation (AF).

BACKGROUND

Increased intracellular calcium is a key event in platelet activation, and several studies have demonstrated that digitalis activates platelets in vitro. Intracellular calcium also is a key regulator of endothelial cell function, and endogenous digitalis-like substances have been shown to affect biologic processes in endothelial cells.

METHODS

We studied 30 patients with nonvalvular AF. We measured the levels of (1) platelet expression of P-selectin (CD62P), (2) platelet microparticles (PMP); and (3) endothelial microparticles (EMP) identified by anti-CD31 (EMP31) and by anti-E-selectin antibodies (EMP62E).

RESULTS

Patients who were taking digoxin (n = 16; mean digoxin level = 0.93 ng/dL) did not demonstrate any significant differences in clinical or echocardiographic characteristics compared with patients not taking digoxin (n = 14). Patients taking digoxin had significantly increased levels of CD62P expression in platelets and platelet-leukocyte conjugates and markedly increased markers of endothelial activation: EMP62E and EMP31. After adjusting for potential confounders (including age, congestive heart failure, coronary artery disease, ejection fraction, antiplatelet, beta-blocker, and calcium channel blocker use), the differences persisted.

CONCLUSIONS

Digoxin use in patients with AF is associated with increased levels of endothelial and platelet activation. If digitalis activates endothelial cells and platelets at pharmacologic doses, use of digitalis in conditions such as AF could predispose to thrombosis and vascular events.

Elevation of endothelial microparticles, platelets, and leukocyte activation in patients with venous thromboembolism

OBJECTIVES

The purpose of this research was to determine the levels of platelet, leukocyte, and endothelial activation and markers of cellular interactions in patients with venous thromboembolism (VTE).

BACKGROUND

The details of interactions between endothelium, platelets, and leukocytes in VTE are not well understood.

METHODS

We studied 25 patients with VTE and compared 25 healthy controls. We used flow cytometry to measure: 1) endothelial microparticles (EMP) identified by CD31+/CD42b- (EMP(31)) or E-selectin (EMP(62E)); 2) platelet microparticles (CD31+/CD42b+); 3) surface expression of P-selectin in platelets and CD11b in leukocytes; 4) EMP-monocyte conjugates (percentage of monocytes positive for E-selectin); and 5) platelet-leukocyte conjugates (PLC) expressed as percentage of leukocytes positive for CD41.

RESULTS

Patients with VTE had marked elevations of EMP(31) (2,193 vs. 383 counts/microl; p = 0.003), EMP(62E) (368 vs. 223 counts/microl; p = 0.001), and EMP-monocyte conjugates (3.3% vs. 2.5%; p = 0.002), as well as increased activation of platelets (35.2 vs. 5.0 fluorescence intensity units for P-selectin; p < 0.0001) and leukocytes (13.9 vs. 7.7 U for CD11b; p = 0.004). Also elevated in VTE were PLC (61.7% vs. 39.6%; p = 0.01). Expression of CD11b in leukocytes strongly correlated with PLC (r = 0.74; p < 0.0001).

CONCLUSIONS

Marked activation of endothelium, platelets, and leukocytes occurs in VTE, and VTE, or the accompanying inflammatory process, involves the release of EMP and formation of EMP-monocyte conjugates and PLC. These findings support prior studies suggesting that release of EMP and their binding to monocytes are key events in thrombogenesis. Our findings also support the concept that the formation of PLC regulates leukocyte activation and participates in linking thrombosis with inflammation.

Correlation between apoptotic endothelial microparticles and serum interleukin-6 and C-reactive protein in healthy men

Inflammation has been associated with increased cardiovascular risk, and endothelial cell (EC) apoptosis has been implicated in atherogenesis. The correlation between circulating concentrations of interleukin-6 (IL-6), C-reactive protein (CRP), and endothelial microparticles (EMPs) expressing an apoptotic (EMP31) or activation (EMP62E) phenotype in 20 middle-aged healthy men was investigated. IL-6 was significantly correlated with EMP31 (r = 0.6, p = 0.004), which persisted after adjusting for body mass index and CRP. CRP was significantly correlated with body mass index (r = 0.49, p = 0.02) but not with EMP31 or EMP62E. EC apoptosis is associated with IL-6 levels in men and might be partially responsible for the increased cardiovascular risk associated with subclinical inflammation.

Overexpression of MMP9 and Tissue Factor in Unstable Carotid Plaques Associated with Chlamydia pneumoniae, Inflammation, and Apoptosis

Tissue remodeling by matrix metalloproteinases (MMPs) and plasminogen activators such as tissue factor (TF) is postulated to be involved in the pathogenesis of atherosclerosis. The in situ expression of MMP9 and TF in unstable atherosclerotic plaques has not been examined in detail. Moreover, interference of tissue remodeling by vascular inflammation, apoptosis, and Chlamydia pneumoniae inside plaque subregions is unclear. A total of 40 autopsy carotid arteries (controls) and 20 atherosclerotic carotid endarterectomy specimens (with type VI lesions, according to the American Heart Association classification) from stroke patients were analyzed for expression of MMP9 and TF using in situ techniques. The data on tissue remodeling were correlated with the presence of inflammatory cells (T cells, B-cells, macrophages), apoptosis, and the presence of C. pneumoniae using immunohistochemistry and Western blot analyses. We found a significant overexpression of MMP9 and TF in progressive atherosclerotic carotid arteries, especially in the shoulder and cap subregions (both p < 0.05). Expression of MMP9 and TF correlated significantly with T-cell and macrophage infiltrates as well as with apoptosis (p < 0.05). C. pneumoniae infection was significantly associated with elevated TF expression (p < 0.01) but not with MMP9. MMP9 and TF are thus significantly overexpressed in progressive atherosclerotic plaques, and their relevant subregions (shoulder and cap) are involved in plaque instability. This process is associated with local inflammatory cell infiltrates and apoptosis, which might be influenced by infectious agents such as C. pneumoniae.

Cholesterol enhances phospholipid-dependent activated protein C anticoagulant activity

The influence of cholesterol on activated protein C (APC) anticoagulant activity in plasma and on factor Va inactivation was investigated. Anticoagulant and procoagulant activities of phosphatidylcholine/phosphatidylserine (PC/PS) vesicles containing cholesterol were assessed in the presence and absence of APC using factor Xa-1-stage clotting and factor Va inactivation assays. Cholesterol at approximate physiological membrane levels (30%) in PC/PS (60%/10% w/w) vesicles prolonged the factor Xa-1-stage clotting time dose-dependently in the presence of APC but not in the absence of APC. APC-mediated cleavage of purified recombinant factor Va variants that were modified at specific APC cleavage sites (Q306/Q679-factor Va; Q506/Q679-factor Va) was studied to define the effects of cholesterol on APC cleavage at R506 and R306. When compared to control PC/PS vesicles, cholesterol in PC/PS vesicles enhanced factor Va inactivation and the rate of APC cleavage at both R506 and R306. Cholesterol also enhanced APC cleavage rates at R306 in the presence of the APC cofactor, protein S. In summary, APC anticoagulant activity in plasma and factor Va inactivation as a result of cleavages at R506 and R306 by APC is markedly enhanced by cholesterol in phospholipid vesicles. These results suggest that cholesterol in a membrane surface may selectively enhance APC activities.

Endothelial cell apoptosis in systemic lupus erythematosus: a common pathway for abnormal vascular function and thrombosis propensity

Women with systemic lupus erythematosus (SLE) are at risk for premature atherothrombosis independent of Framingham risk factors. We investigated whether endothelial cell (EC) apoptosis predicts abnormal vasomotor tone and contributes to circulating tissue factor (TF) levels in this disease. Brachial artery flow-mediated dilation (FMD) and nitroglycerin-mediated dilation were determined in women with SLE, healthy control subjects, and subjects with coronary artery disease (CAD) (n = 43/group). Quantification of circulating apoptotic ECs was performed by flow cytometry (CD146+ cells that stained for Annexin V [CD146AnnV+]) and immunofluorescent microscopy. Plasma TF was measured by enzyme-linked immunosorbent assay (ELISA). Compared with healthy control and CAD subjects, patients with SLE had higher numbers of circulating CD146AnnV+ cells (10 ± 3, 18 ± 5, and 89 ± 32 cells/mL, respectively, mean ± SEM; P &lt; .01). Increased CD146AnnV+ cells correlated strongly with abnormal vascular function (P = .037). After adjusting for known predictors of endothelial function, CD146AnnV+ was the only variable that predicted FMD (β = –4.5, P &lt; .001). Increased CD146AnnV+ was strongly associated with elevated levels of circulating TF (r = .46, P = .002). Circulating apoptotic ECs are elevated in young women with SLE and strongly correlate with markedly abnormal vascular function and elevated TF levels. Heightened endothelial apoptosis may represent an important mechanism for development of atherothrombosis in SLE.

Proexosite-1 on prothrombin is a factor Va-dependent recognition site for the prothrombinase complex

Although the contribution of basic residues of exosite-1 to the catalytic function of thrombin has been studied extensively, their role in the specificity of prothrombin recognition by factor Xa in the prothrombinase complex (factor Xa, factor Va, phosphatidylcholine/phosphatidylserine vesicles, and Ca2+) has not been examined. In this study, we prepared several mutants of prethrombin-1 (prothrombin lacking Gla and Kringle-1 domains) in which basic residues of this site (Arg35, Lys36, Arg67, Lys70, Arg73, Arg75, and Arg77 in chymotrypsinogen numbering) were individually substituted with a Glu. Following expression in mammalian cells and purification to homogeneity, these mutants were characterized with respect to their ability to function as zymogens for both factor Xa and the prothrombinase complex. Factor Xa by itself exhibited similar catalytic activity toward both the wild type and mutant substrates; however, its activity in the prothrombinase complex toward most of mutants was severely impaired. Further kinetic studies in the presence of Tyr63-sulfated hirudin-(54-65) peptide suggested that although the peptide inhibits the prothrombinase activation of the wild type zymogen with a KD of 0.5-0.7 microm, it is ineffective in inhibiting the activation of mutant zymogens (KD = 2-30 microm). These results suggest that basic residues of proexosite-1 on prothrombin are factor Va-dependent recognition sites for factor Xa in the prothrombinase complex.

Glucosylceramide, a neutral glycosphingolipid anticoagulant cofactor, enhances the interaction of human- and bovine-activated protein C with negatively charged phospholipid vesicles

The effect of glucosylceramide (GlcCer) on activated protein C (APC)-phospholipid interactions was examined using fluorescence resonance energy transfer. Human APC, labeled with either fluorescein (Fl-APC) or dansyl (DEGR-APC) donor, bound to phosphatidylcholine/phosphatidylserine (PC/PS, 9:1 w/w) vesicles containing octadecylrhodamine (OR) acceptor with a K(d) (app) = 16 micro g/ml, whereas Fl-APC (or DEGR-APC) bound to PC/PS/GlcCer(OR) (8:1:1) vesicles with a K(d) (app) = 3 micro g/ml. This 5-fold increase in apparent affinity was not species-specific since bovine DEGR-APC also showed a similar GlcCer-dependent enhancement of binding of APC to PC/PS vesicles. From the efficiency of fluorescence resonance energy transfer, distances of closest approach of approximately 63 and approximately 64 A were estimated between the dansyl on DEGR-APC and rhodamine in PC/PS/GlcCer(OR) and PC/PS(OR), respectively, assuming kappa(2) = 2/3. DEGR-APC bound to short chain C8-GlcCer with an apparent K(d) of 460 nm. The presence of C8-GlcCer selectively enhanced the binding of C16,6-NBD-phosphatidylserine but not C16,6-7-nitrobenz-2-oxa-1,3-diazole (NBD)-phosphatidylcholine to coumarin-labeled APC. These data suggest that APC binds to GlcCer, that PC/PS/GlcCer vesicles like PC/PS vesicles bind to the N-terminal gamma-carboxyglutamic acid domain of APC, and that one mechanism by which GlcCer enhances the activity of APC is by increasing its affinity for membrane surfaces containing negatively charged phospholipids.

Cooperative roles of factor V(a) and phosphatidylserine-containing membranes as cofactors in prothrombin activation

Activation of prothrombin, as catalyzed by the prothrombinase complex (factor X(a), enzyme; factor V(a) and phosphatidylserine (PS)-containing membranes, cofactors), involves production and subsequent proteolysis of two possible intermediates, meizothrombin (MzII(a)) and prethrombin 2 plus fragment 1.2 (Pre2 & F1.2). V(max), K(m), or V(max)/K(m) for all four proteolytic steps was determined as a function of membrane-phospholipid concentration. Proteolysis was monitored using a fluorescent thrombin inhibitor, a chromogenic substrate, and SDS-PAGE. The kinetic constants for the conversion of MzII(a) and Pre2 & F1.2 to thrombin were determined directly. Pre2 & F1.2 conversion was linear in substrate concentration up to 4 microm, whereas MzII(a) proteolysis was saturable. First order rate constants for formation of MzII(a) and Pre2 & F1.2 could not be determined directly and were determined from global fitting of the data to a parallel, sequential model, each step of which was treated by the Michaelis-Menten formalism. The rate of direct conversion to thrombin without release of intermediates from the membrane-V(a)-X(a) complex (i.e. "channeling") also was adjusted because both the membranes and factor V(a) have been shown to cause channeling. k(cat), K(m), or k(cat)/K(m) values were reported for one lipid concentration, for which all X(a) was likely incorporated into a X(a)-V(a) complex on a PS membrane. Comparing previous results, which were obtained either with factor V(a) (Boskovic, D. S., Bajzar, L. S., and Nesheim, M. E. (2001) J. Biol. Chem. 276, 28686-28693) or with membranes individually (Wu, J. R., Zhou, C., Majumder, R., Powers, D. D., Weinreb, G., and Lentz, B. R. (2002) Biochemistry 41, 935-949), with results presented here we conclude that both factor V(a) and PS-containing membranes induce similar rate increases and pathway changes. Moreover, we have determined: 1) factor V(a) has the greatest effect in enhancing rates of individual proteolytic events; 2) PS-containing membranes have the greatest role in increasing the preference for the MzII(a) versus Pre2 pathway; and 3) PS membranes cause approximately 50% of the substrate to be activated via channeling at 50 microm membrane concentration, but factor V(a) extends the range of efficient channeling to much lower or higher membrane concentrations.

Neutral glycosphingolipid-dependent inactivation of coagulation factor Va by activated protein C and protein S

To test whether neutral glycosphingolipids can serve as anticoagulant cofactors, the effects of incorporation of neutral glycosphingolipids into phospholipid vesicles on anticoagulant and procoagulant reactions were studied. Glucosylceramide (GlcCer), lactosylceramide (LacCer), and globotriaosylceramide (Gb(3)Cer) in vesicles containing phosphatidylserine (PS) and phosphatidylcholine (PC) dose dependently enhanced factor Va inactivation by the anticoagulant factors, activated protein C (APC) and protein S. Addition of GlcCer to PC/PS vesicles enhanced protein S-dependent APC cleavage in factor Va at Arg-506 by 13-fold, whereas PC/PS vesicles alone minimally affected protein S enhancement of this reaction. Incorporation into PC/PS vesicles of GlcCer, LacCer, or Gb(3)Cer, but not galactosylceramide or globotetraosylceramide, dose dependently prolonged factor Xa-1-stage clotting times of normal plasma in the presence of added APC without affecting baseline clotting times in the absence of APC, showing that certain neutral glycosphingolipids enhance anticoagulant but not procoagulant reactions in plasma. Thus, certain neutral glycosphingolipids (e.g. GlcCer, LacCer, and Gb(3)Cer) can enhance anticoagulant activity of APC/protein S by mechanisms that are distinctly different from those of phospholipids alone. We speculate that under some circumstances certain neutral glycosphingolipids either in lipoprotein particles or in cell membranes may help form antithrombotic microdomains that might enhance down-regulation of thrombin by APC in vivo.

Current perspective on the role of apoptosis in atherothrombotic disease

Thrombus formation on a disrupted atherosclerotic plaque is a threatening event that leads to vessel occlusion and acute ischemia. In this current perspective, we present evidence for apoptosis as a major determinant of the thrombogenicity of the plaque lipid core and a potential contributor to plaque erosion and associated thrombosis. Moreover, apoptosis may directly affect blood thrombogenicity through the release of apoptotic cells and microparticles into the bloodstream.

Lipid oxidation enhances the function of activated protein C

Although lipid oxidation products are usually associated with tissue injury, it is now recognized that they can also contribute to cell activation and elicit anti-inflammatory lipid mediators. In this study, we report that membrane phospholipid oxidation can modulate the hemostatic balance. Oxidation of natural phospholipids results in an increased ability of the membrane surface to support the function of the natural anticoagulant, activated protein C (APC), without significantly altering the ability to support thrombin generation. Lipid oxidation also potentiated the ability of protein S to enhance APC-mediated factor Va inactivation. Phosphatidylethanolamine, phosphatidylserine, and polyunsaturation of the fatty acids were all required for the oxidation-dependent enhancement of APC function. A subgroup of thrombotic patients with anti-phospholipid antibodies specifically blocked the oxidation-dependent enhancement of APC function. Since leukocytes are recruited and activated at the thrombus or sites of vessel injury, our findings suggest that after the initial thrombus formation, lipid oxidation can remodel the membrane surface resulting in increased anticoagulant function, thereby reducing the thrombogenicity of the thrombus or injured vessel surface. Anti-phospholipid antibodies that block this process would therefore be expected to contribute to thrombus growth and disease.

Elevated levels of shed membrane microparticles with procoagulant potential in the peripheral circulating blood of patients with acute coronary syndromes

BACKGROUND

Apoptotic microparticles are responsible for almost all tissue factor activity of the plaque lipid core. We hypothesized that elevated levels of procoagulant microparticles could also circulate in the peripheral blood of patients with recent clinical signs of plaque disruption and thrombosis.

METHODS AND RESULTS

We studied 39 patients with coronary heart disease, including 12 patients with stable angina and 27 patients with acute coronary syndromes (ACS), and 12 patients with noncoronary heart disease. We isolated the circulating microparticles by capture with annexin V and determined their procoagulant potential with a prothrombinase assay. The cell origins of microparticles were determined in an additional 22 patients by antigenic capture with specific antibodies. The level of procoagulant microparticles did not differ between stable angina patients and noncoronary patients (10.1+/-1.6 nmol/L phosphatidylserine [PS] equivalent versus 9.9+/-1.6 nmol/L PS equivalent, respectively). However, procoagulant microparticles were significantly elevated in patients with ACS (22.2+/-2.7 nmol/L PS equivalent) compared with other coronary (P<0.01) or noncoronary (P<0.01) patients. Microparticles of endothelial origin were significantly elevated in patients with ACS (P<0.01), which suggests an important role for endothelial injury in inducing the procoagulant potential.

CONCLUSIONS

High levels of procoagulant endothelial microparticles are present in the circulating blood of patients with ACS and may contribute to the generation and perpetuation of intracoronary thrombi.

Cardiolipin is a normal component of human plasma lipoproteins

Anticardiolipin (anti-CL) antibodies, diagnostic for antiphospholipid antibody syndrome, are associated with increased risks of venous and arterial thrombosis. Because CL selectively enhances activated protein C/protein S-dependent anticoagulant activities in purified systems and because CL is not known to be a normal plasma component, we searched for CL in plasma. Plasma lipid extracts [chloroform/methanol (2:1, vol/vol)] were subjected to analyses by using TLC, analytical HPLC, and MS. A plasma lipid component was purified that was indistinguishable from reference CL (M:1448). When CL in 40 fasting plasma lipid extracts (20 males, 20 females) was quantitated by using HPLC, CL (mean +/- SD) was 14.9 +/- 3.7 microgram/ml (range 9.1 to 24.2) and CL was not correlated with phosphatidylserine (3.8 +/- 1.7 microgram/ml), phosphatidylethanolamine (64 +/- 20 microgram/ml), or choline-containing phospholipid (1,580 +/- 280 microgram/ml). Based on studies of fasting blood donors, CL (>/=94%) was recovered in very low density, low density, and high density lipoproteins (11 +/- 5.3%, 67 +/- 11.0%, and 17 +/- 10%, respectively), showing that the majority of plasma CL (67%) is in low density lipoprotein. Analysis of relative phospholipid contents of lipoproteins indicated that high density lipoprotein is selectively enriched in CL and phosphatidylethanolamine. These results shows that CL is a normal plasma component and suggest that the epitopes of antiphospholipid antibodies could include CL or oxidized CL in lipoproteins or in complexes with plasma proteins (e. g., beta(2)-glycoprotein I, prothrombin, protein C, or protein S) or with platelet or endothelial surface proteins.

The effect of membrane composition on the hemostatic balance

The phospholipid composition requirements for optimal prothrombin activation and factor Va inactivation by activated protein C (APC) anticoagulant were examined. Vesicles composed of phosphatidylethanolamine (PE) and phosphatidylcholine (PC) supported factor Va inactivation relatively well. However, optimal factor Va inactivation still required relatively high concentrations of phosphatidylserine (PS). In addition, at a fixed concentration of phospholipid, PS, and APC, vesicles devoid of PE never attained a rate of factor Va inactivation achievable with vesicles containing PE. Polyunsaturation of any vesicle component also contributed significantly to APC inactivation of factor Va. Thus, PE makes an important contribution to factor Va inactivation that cannot be mimicked by PS. In the absence of polyunsaturation in the other membrane constituents, this contribution was dependent upon the presence of both the PE headgroup per se and unsaturation of the 1,2 fatty acids. Although PE did not affect prothrombin activation rates at optimal PS concentrations, PE reduced the requirement for PS approximately 10-fold. The Km(app) for prothrombin and the Kd(app) for factor Xa-factor Va decreased as a function of increasing PS concentration, reaching optimal values at 10-15% PS in the absence of PE but only 1% PS in the presence of PE. Fatty acid polyunsaturation had minimal effects. A lupus anticoagulant immunoglobulin was more inhibitory to both prothrombinase and factor Va inactivation in the presence of PE. The degree of inhibition of APC was significantly greater and much more dependent on the phospholipid composition than that of prothrombinase. Thus, subtle changes in the phospholipid composition of cells may control procoagulant and anticoagulant reactions differentially under both normal and pathological conditions.

Shed membrane microparticles with procoagulant potential in human atherosclerotic plaques: a role for apoptosis in plaque thrombogenicity

BACKGROUND

The specific role of apoptosis in human atherosclerosis remains unknown. During apoptotic cell death, phosphatidylserine exposure on the cell surface confers a high tissue-factor (TF)-dependent procoagulant activity.

METHODS AND RESULTS

In this study, we examined the role of apoptotic cell death in the promotion of plaque thrombogenicity. TF expression and its relation to apoptosis was analyzed in 16 human atherosclerotic plaques by the use of immunohistochemical techniques. The presence of shed membrane apoptotic microparticles was analyzed in extracts from 6 human atherosclerotic plaques and 3 underlying arterial walls. The microparticles were captured by annexin V and their amounts estimated with respect to their phospholipid content by use of a prothrombinase assay. The prothrombogenic potential of the microparticles was further assessed by the measurement of total and microparticle-dependent TF activity in the extracts. The cell origin of the microparticles was determined after capture by specific antibodies. We were able to detect marked TF expression in the plaques in close proximity to apoptotic cells and debris, suggesting a potential interaction between TF and the apoptotic cell surfaces. High levels of shed membrane apoptotic microparticles were detected in extracts from atherosclerotic plaques but not in the underlying arterial walls (29.5+/-3.7 nmol/L phosphatidylserine equivalent versus 1.3+/-0.4 nmol/L, respectively, P<0.02). The microparticles were mainly of monocytic and lymphocytic origin and retained 97+/-2% of total TF activity, indicating a direct causal relationship between shed membrane microparticles and procoagulant activity of plaque extracts.

CONCLUSIONS

These results indicate that shed membrane microparticles with procoagulant potential are produced in human atherosclerotic plaques. Apoptosis could be a critical determinant of plaque thrombogenicity after plaque rupture.

Lipid-protein interactions in blood coagulation

It has long been appreciated that lipids, particularly anionic phospholipids, promote blood coagulation. The last two decades have seen an increasing insight into the kinetic and mechanistic aspects regarding the mode of action of phospholipids in blood coagulation. This essay attempts to review these developments with particular emphasis on the structure of lipid-binding domains of blood coagulation proteins, and the variable effect of phospholipid composition on the interaction with these proteins. Some examples are discussed of how lipid membranes direct the pathway of enzymatic conversions in blood coagulation complexes, also illustrating that the membrane lipid surface is more than an inert platform for the assembly of coagulation factors. Finally, the controlled exposure of procoagulant lipid on the surface of blood cells is shortly reviewed, and an example is discussed of how interference with lipid-protein interactions in blood coagulation may result in pathological phenomena.

Picosecond tryptophan fluorescence of membrane-bound prothrombin fragment 1

The wavelength-dependent tryptophan (Trp) fluorescence decays of Ca-prothrombin fragment 1 (Ca-BF1), which contains three tryptophan residues, in the presence of pure phosphatidylcholine (PC) small unilamellar vesicles (SUV) and PC-SUV containing either 25% phosphatidyl-l-serine (PS), and 25% or 40% phosphatidylglycerol (PG) are characterized, using fluorescence lifetime distribution, conventional multiexponential, and global analysis. In analogy to previous investigations on apo- and Ca-BF1 (M. Hof, G.R. Fleming, V. Fidler, Proteins Struct. Func. Genet. 24 (1996) 485-494), the analysis resulted in a five exponential decay model in all investigated systems, where the five fluorescence lifetimes (e.g. 0. 04+/-0.02 ns (component A), 0.24+/-0.02 ns (B), 0.66+/-0.03 ns (C), 2.4+/-0.3 ns (D), and 5.4+/-0.4 ns (E) for Ca-BF1 in the presence of PC-SUV) are wavelength-independent. The fluorescence lifetimes and the corresponding amplitudes of the 'Gla-Trp' (components D and E) and of the two 'kringle-Trp' (components B, C, and D) remain unchanged when bound to the PS-containing vesicles. Saturation binding to PG-containing membranes leads to a prolongation of the Gla component E from 5.3 in solution to 7.5 ns, indicating a change in the Gla-domain conformation. The results represent the first experimental evidence of a lipid-specific conformational change in the N-terminal 'Gla domain' of a vitamin K-dependent protein.

Binding of anticardiolipin antibodies to protein C via beta2-glycoprotein I (beta2-GPI): a possible mechanism in the inhibitory effect of antiphospholipid antibodies on the protein C system

It is known that antiphospholipid antibodies (aPL) hamper the anticoagulant activity of the protein C system, but the mechanism is still obscure. In this study, we demonstrate that anticardiolipin antibodies (not anti-protein C autoantibodies) can bind protein C via beta2-GPI, which bears their binding epitope, in a fashion dependent on negatively charged phospholipids. We studied the binding of IgG from aPL to protein C in the presence of beta2-GPI by ELISA (anti-'protein C' antibody ELISA), and compared their binding with those obtained in the absence of beta2-GPI. In the anti-'protein C' antibody ELISA system, 47% of 78 aPL+ patients had a positive titre in the presence of cardiolipin (CL) and beta2-GPI, but binding was not found in the absence of beta2-GPI. Highly significant correlations were found between the titre of anti-'protein C' antibody in the presence of beta2-GPI and that of anti-beta2-GPI antibody (r = 0.802, P = 0.0001). We further analysed the interaction between protein C, phospholipids, beta2-GPI and human aCL MoAbs established from patients with antiphospholipid syndrome. In a first set of experiments, the binding of beta2-GPI to protein C and its phospholipid dependency were investigated. Beta2-GPI bound to protein C in the presence of CL or phosphatidylserine, but not in the presence of phosphatidylcholine or phosphatidylethanolamine. In a second group of experiments, the binding of three human monoclonal aCL recognizing the cryptic epitope of beta2-GPI (virtually anti-beta2-GPI antibodies) was evaluated in the presence of cardiolipin and beta2-GPI. All three human monoclonal aCL bound to protein C in the presence of CL and beta2-GPI, whereas they did not in the absence of either beta2-GPI or CL. These data suggest that protein C could be a target of aCL by making a complex with CL and beta2-GPI, leading to protein C dysfunction.

A chimeric protein C containing the prothrombin Gla domain exhibits increased anticoagulant activity and altered phospholipid specificity

To determine the structural basis of phosphatidylethanolamine (PE)-dependent activated protein C (APC) activity, we prepared a chimeric molecule in which the Gla domain and hydrophobic stack of protein C were replaced with the corresponding region of prothrombin. APC inactivation of factor Va was enhanced 10-20-fold by PE. Protein S enhanced inactivation 2-fold and independently of PE. PE and protein S had little effect on the activity of the chimera. Factor Va inactivation by APC was approximately 5-fold less efficient than with the chimera on vesicles lacking PE and slightly more efficient on vesicles containing PE. The cleavage patterns of factor Va by APC and the chimera were similar, and PE enhanced the rate of Arg506 and Arg306 cleavage by APC but not the chimera. APC and the chimera bound to phosphatidylserine:phosphatidylcholine vesicles with similar affinity (Kd approximately 500 nM), and PE increased affinity 2-3-fold. Factor Va and protein S synergistically increased the affinity of APC on vesicles without PE to 140 nM and with PE to 14 nM, but they were less effective in enhancing chimera binding to either vesicle. In a factor Xa one-stage plasma clotting assay, the chimera had approximately 5 times more anticoagulant activity than APC on PE-containing vesicles. Unlike APC, which showed a 10 fold dependence on protein S, the chimera was insensitive to protein S. To map the site of the PE and protein S dependence further, we prepared a chimera in which residues 1-22 were derived from prothrombin and the remainder were derived from protein C. This protein exhibited PE and protein S dependence. Thus, these special properties of the protein C Gla domain are resident outside of the region normally hypothesized to be critical for membrane interaction. We conclude that the protein C Gla domain possesses unique properties allowing synergistic interaction with factor Va and protein S on PE-containing membranes.

Soluble phospholipids enhance factor Xa-catalyzed prothrombin activation in solution

Acidic phospholipids play an important but incompletely understood role in prothrombin activation. Here we report the effect of short-chain phosphatidylserine (dicaproylphosphatidylserine, C6PS) and the corresponding phosphatidylglycerol (C6PG) and phosphatidylcholine (C6PC) derivatives on the rate of prothrombin activation by factor Xa. The critical micellar concentrations of these short-chained phospholipids have been determined under a variety of conditions that we used for kinetic and structural studies. Under conditions for which these lipids exist in a soluble form, the results demonstrate that: (i) the rate of human prothrombin activation by human factor Xa was enhanced in a calcium-dependent fashion up to 60-fold by addition of C6PS, roughly 20% of the optimal enhancement seen with bovine phosphatidylserine/palmitoyloleoylphosphatidylcholine (25/75 PS/POPC) membranes; (ii) C6PS inhibited the rate of hydrolysis of synthetic factor Xa substrate (S-2765), an effect that was mimicked, but at much lower lipid concentrations, by PS/POPC membranes; (iii) there was no enhancement of prothrombin activation and much less inhibition of hydrolysis of S-2765 by factor Xa in the presence of C6PG or C6PC; and (iv) the thermal denaturation of prothrombin was altered in a calcium-independent but dose-dependent fashion by either C6PS or C6PG. These results have been interpreted in terms of the existence of (a) specific PS binding site(s) on factor Xa (Kd approximately 73 microM) that regulate(s) the activity of this serine protease. Our results do not rule out the possibility that the rate of prothrombin activation is also influenced by a weaker, calcium-independent, and less specific acidic lipid binding site on prothrombin, the occupancy of which results in conformational changes in this protein. The results clearly suggest that PS binding regulates the rate of prothrombin activation.

Prothrombin contributes to the assembly of the factor Va-factor Xa complex at phosphatidylserine-containing phospholipid membranes

The activation of prothrombin is catalyzed by prothrombinase, a complex of factor Xa and factor Va assembled on a negatively charged phospholipid membrane. We used a tubular flow reactor to identify the relative contributions of factor Va, prothrombin, and the negatively charged phosphatidylserine to the assembly of prothrombinase. Perfusion of phospholipid-coated capillaries with a mixture of factor Xa, factor Va, and prothrombin resulted in a steady-state rate of thrombin production that increased with (i) the phosphatidylserine content of the phospholipid bilayer, (ii) the factor Va concentration, and, most interestingly, (iii) the prothrombin concentration of the perfusion solution. Incorporation of 20 mol % phosphoatidylethanolamine, a phospholipid with poor ability to promote prothrombinase activity, into a 5 mol % phosphatidylserine membrane also increased the steady-state rate of thrombin production. Direct measurements of the amount of prothrombinase in the flow reactor demonstrated that increased catalytic activities were the result of an increased steady-state amount of membrane-associated prothrombinase. Thus, similar turnover numbers of prothrombin activation (3100 min-1) were calculated, irrespective of the phosphatidylserine content of the membrane. We established for membranes with low phosphatidylserine content (< 10 mol%) a linear relationship between the prothrombinase activity and the arithematical product of the factor Va concentration in the perfusion solution and the prothrombin concentration near the catalytic surface. Our results indicate that, in addition to factor Va, prothrombin also is essential to the assembly of prothrombinase at macroscopic surfaces with low phosphatidylserine content. The data further suggest that the prothrombin concentration near the surface, controlled by the prothrombinase activity and mass transfer, is an important regulator of the prothrombinase surface density.

Are acidic lipid domains induced by extrinsic protein binding to membranes?

A widely accepted model for the association of extrinsically bound proteins with acidic lipid-containing membranes has been that approach of the protein to the membrane induces a domain of acidic lipids that serves as the protein binding site. This model has been applied to a variety of membrane proteins including those that participate in the proteolytic complex that converts prothrombin to thrombin during the final stages of the blood coagulation cascade. The 'prothrombinase complex' consists of a serine protease (factor Xa), its protein co-factor (factor Va) and the substrate itself (prothrombin), all bound to phosphatidylserine (PS)-containing membranes derived from stimulated platelets. We have used three approaches to test the domain model as it applies to the proteins of this complex. First, phase diagrams describing the mixing of acidic and neutral lipids have failed to provide evidence for extensive acidic lipid domains (on the order of 50 or more lipid molecules) induced by protein biding. Second, pyrene-containing neutral and acidic phospholipids have been used to test for the occurrence of domains of as few as 20-30 lipids associated with binding of the membrane-binding fragment 1 region of prothrombin. Again, no evidence for domains was obtained. Finally, we have shown that binding of these proteins can be described in terms of a generalized model that presumes an acidic-lipid-independent surface adsorption combined with specific binding of acidic lipids to 'm' sites on a protein. Our results suggest that the concept of a protein-induced domain should not be applied indistriminately to explain binding of extrinsic membrane proteins such as the protein kinase C.(ABSTRACT TRUNCATED AT 250 WORDS)

Lipid dynamics and peripheral interactions of proteins with membrane surfaces

A large body of evidence strongly indicates biomembranes to be organized into compositionally and functionally specialized domains, supramolecular assemblies, existing on different time and length scales. For these domains and intimate coupling between their chemical composition, physical state, organization, and functions has been postulated. One important constituent of biomembranes are peripheral proteins whose activity can be controlled by non-covalent binding to lipids. Importantly, the physical chemistry of the lipid interface allows for a rapid and reversible control of peripheral interactions. In this review examples are provided on how membrane lipid (i) composition (i.e., specific lipid structures), (ii) organization, and (iii) physical state can each regulate peripheral binding of proteins to the lipid surface. In addition, a novel and efficient mechanism for the control of the lipid surface association of peripheral proteins by [Ca2+], lipid composition, and phase state is proposed. The phase state is, in turn, also dependent on factors such as temperature, lateral packing, presence of ions, metabolites and drugs. Confining reactions to interfaces allows for facile and cooperative large scale integration and control of metabolic pathways due to mechanisms which are not possible in bulk systems.

Phosphatidylserine-containing membranes alter the thermal stability of prothrombin's catalytic domain: a differential scanning calorimetric study

Denaturation profiles of bovine prothrombin and its isolated fragments were examined in the presence of Na2EDTA, 5 mM CaCl2, and CaCl2 plus membranes containing 1-palmitoyl-2-oleoyl-3-sn-phosphatidylcholine (POPC) in combination with bovine brain phosphatidylserine (PS). We have shown previously [Lentz, B. R., Wu, J. R., Sorrentino, A. M., & Carleton, J. A. (1991) Biophys. J. 60, 70] that binding to PS/POPC (25/75) large unilamellar vesicles resulted in an enthalpy loss in the main endotherm of prothrombin denaturation (Tm approximately 57-58 degrees C) and a comparable enthalpy gain in a minor endotherm (Tm approximately 59 degrees C) accompanying an upward shift in peak temperature (Tm approximately 73 degrees C). This minor endotherm was also responsive to Ca2+ binding and, in the absence of PS/POPC membranes, corresponded to melting of the N-terminal, Ca2+ and membrane binding domain (fragment 1). Peak deconvolution analysis of the prothrombin denaturation profile and extensive studies of the denaturation of isolated prothrombin domains in the presence and absence of PS/POPC vesicles suggested that membrane binding induced changes in the C-terminal catalytic domain of prothrombin (prethrombin 2) and in a domain that links fragment 1 with the catalytic domain (fragment 2). Specifically, the results have confirmed that the fragment 2 domain interacts with the stabilizes the prethrombin 2 domain and also have shown that fragment 2 interacts directly with the membrane. In addition, the results have demonstrated a heretofore unrecognized interaction between the catalytic and membrane binding domains. This interaction can account for another portion of the denaturation enthalpy that appears at high temperatures in the presence of membranes.(ABSTRACT TRUNCATED AT 250 WORDS)

Prothrombin activation on dioleoylphosphatidylcholine membranes

Factor-Xa-catalyzed prothrombin activation is greatly accelerated by negatively charged phospholipids plus calcium ions. In 1990, we reported that neutral phosphatidylcholine membranes also stimulated prothrombin activation [Gerads, I., Govers-Riemslag, J.W.P., Tans, G., Zwaal, R. F. A. & Rosing, J. (1990) Biochemistry 29, 7967-7974]. In the present study, we have performed a detailed analysis of the prothrombin-converting activity of phosphatidylcholine membranes. Stimulation of prothrombin activation by phosphatidylcholine vesicles was particularly observed (a) with phosphatidylcholine molecules that contained unsaturated hydrocarbon side chains, (b) in the presence of factor Va, (c) at low ionic strength and (d) when Ca2+ were present in the reaction medium. It is unlikely that the prothrombinase activity of phosphatidylcholine preparations was due to contaminating anionic phospholipids. This is concluded from the fact that thin-layer chromatographic analysis showed that dioleoylphosphatidylcholine [(Ole)2GroPCho] contained less than 0.1 mol/100 mol anionic phospholipid, and that incorporation of such amounts of anionic lipids in (Ole)2-GroPCho membranes hardly increased their prothrombin-converting activity. At low ionic strength and in the presence of factor Va and Ca2+ (Ole)2GroPCho membranes accelerated prothrombin activation about 100-fold. At ionic strength (I) 0.06, prothrombin activation on 100 microM (Ole)2-GroPCho was characterized by a Km for prothrombin of 2 microM, a Vmax of 3020 IIa min-1.Xa-1 and a Kd for factor XaVa complex formation at the membrane surface of 7.5 nM. Prothrombin activation on (Ole)2GroPCho membranes was drastically reduced when the ionic strength was increased. The inhibition at high ionic strength could be explained by an effect on the Kd for XaVa complex formation which increased from 7.5 nM at I = 0.06 to 100 nM at I = 0.22. Prothrombin activation on (Ole)2GroPCho required Ca2+ and was dependent on the presence of gamma-carboxyglutamic acid domains in prothrombin and factor Xa. This indicates that similar interactions may account for the assembly of prothrombinase complexes on phosphatidylcholine and an anionic lipid-containing membranes.

Interaction of phospholipids with proteins and peptides. New advances IV

1. The review deals with the newest achievements in the field of the various interactions between phospholipids and proteins and peptides. 2. Interactions are classified according to the hydrophobic, hydrophilic or mixed character of the interactive forces. 3. The effect of the interaction on the structure and biological activity of the interacting molecular assemblies is also discussed.