Cleavage of protein S by a platelet membrane protease

Platelet-mediated proteolytic down regulation of the anticoagulant activity of protein S in individuals with haematological malignancies

The natural anticoagulant protein S contains a so-called thrombin-sensitive region (TSR), which is susceptible to proteolytic cleavage. We have previously shown that a platelet-associated protease is able to cleave protein S under physiological plasma conditions in vitro. The aim of the present study was to investigate the relation between platelet-associated protein S cleaving activity and in vivo protein S cleavage, and to evaluate the impact of in vivo protein S cleavage on its anticoagulant activity. Protein S cleavage in healthy subjects and in thrombocytopenic and thrombocythaemic patients was evaluated by immunological techniques. Concentration of cleaved and intact protein S was correlated to levels of activated protein C (APC)-dependent and APC-independent protein S anticoagulant activity. In plasma from healthy volunteers 25% of protein S is cleaved in the TSR. While in plasma there was a clear positive correlation between levels of intact protein S and both APC-dependent and APC-independent protein S anticoagulant activities, these correlations were absent for cleaved protein S. Protein S cleavage was significantly increased in patients with essential thrombocythaemia (ET) and significantly reduced in patients with chemotherapy-induced thrombocytopenia. In ET patients on cytoreductive therapy, both platelet count and protein S cleavage returned to normal values. Accordingly, platelet transfusion restored cleavage of protein S to normal values in patients with chemotherapy-induced thrombocytopenia. In conclusion, proteases from platelets seem to contribute to the presence of cleaved protein S in the circulation and may enhance the coagulation response in vivo by down regulating the anticoagulant activity of protein S.

Protein S and factor V in regulation of coagulation on platelet microparticles by activated protein C

INTRODUCTION

Platelets are the main source of microparticles in plasma and the concentration of microparticles is increased in many diseases. As microparticles expose negatively charged phospholipids, they can bind and assemble the procoagulant enzyme-cofactor complexes. Our aim was to elucidate possible regulation of these complexes on microparticles by the anticoagulant protein C system.

MATERIALS AND METHODS

Platelets were activated with thrombin ± collagen or the calcium ionophore A23187 ± thrombin to generate microparticles. The microparticles were analyzed using flow cytometry and functional coagulation assays to characterize parameters with importance for the activated protein C system.

RESULTS

Activation with A23187+thrombin was most efficient, fully converting the platelets to microparticle-like vesicles, characterized by high lactadherin and protein S binding capacity. Suppression of thrombin generation by activated protein C in plasma spiked with these microparticles was dependent on the presence of plasma protein S. Experiments with purified components showed that activated protein C inhibited both factor Va and factor VIIIa on the microparticle surface. Inhibition of factor Va was stimulated by, but not fully dependent on, the presence of protein S. In the factor VIIIa-degradation, activated protein C was dependent on the addition of protein S, and exogenous factor V further increased the efficiency.

CONCLUSIONS

Protein S is crucial for activated protein C-mediated inhibition of thrombin generation on platelet-derived microparticles in plasma. Moreover, protein S and factor V are synergistic cofactors in the inhibition of factor VIIIa. The results demonstrate that the activated protein C system has the capacity to counterbalance the procoagulant ability of microparticles.

Phosphorylation of protein S by platelet kinases enhances its activated protein C cofactor activity

Protein S (PS) is a multifunctional plasma protein of the hemostatic and inflammatory pathways, although mechanisms for its regulation are poorly understood. Since certain plasma proteins are regulated through extracellular phosphorylation, we investigated whether the anticoagulant activity of PS is regulated through phosphorylation by platelet-secreted kinases. PS was phosphorylated on exposure to activated platelets or their releasates, as judged by immunoblotting for phospho-amino acids and PS. PS phosphorylation was reduced by specific inhibitors of casein kinase 1 (CK1) and casein kinase 2 (CK2) (10 μM D4476, 100 μM CK2-inhibitory peptide YNLKSKSSEDIDESS). Involvement of CKs in PS phosphorylation was confirmed using purified CK1/CK2. Phosphorylation of PS by purified CK1 did not affect its activated protein C (APC) cofactor activity in activated partial thromboplastin time assays in PS-depleted plasma. However, phosphorylation of PS by CK2 or by CK1/CK2 increased PS cofactor activity ∼1.5-fold (158.7±4.8%, P<0.01) or ∼2-fold (191.5±6.4%, P<0.0001), respectively. The APC cofactor activity of PS in PS-depleted plasma exposed to platelet-secreted kinases was enhanced, while CK2 but not CK1 inhibitors reduced APC cofactor activity. Mass spectrometry revealed a phosphorylated CK2 site at Thr37 within the N-terminal Gla-domain. Thus, platelet-mediated extracellular phosphorylation of PS is a potential mechanism by which its activity is regulated.

Platelet protein S directly inhibits procoagulant activity on platelets and microparticles

Anticoagulant plasma protein S (PS) is essential for maintaining haemostatic balance. About 2.5% of PS is stored in platelets and released upon platelet stimulation. So far, little is known about the functionality and importance of platelet (plt)PS. A platelet-associated protease cleaves plasma-derived (pd)PS and pltPS in the "thrombin-sensitive region", abolishing activated protein C (APC) cofactor activity. However, we showed that cleaved PS retains APC-independent anticoagulant activities ("PS-direct"). To investigate whether pltPS or pdPS exert PS-direct on platelets or platelet-shed microparticles, thrombin and factor (F)Xa generation on unstimulated or stimulated washed platelets and microparticles were measured. Western blotting revealed that pltPS and pdPS bound to washed, stimulated platelets and microparticles, and that pltPS had slower electrophoretic mobility than pdPS. Platelet stimulation in the presence of inhibitory anti-PS antibodies resulted in 2.6 ± 1.6-fold (p<0.0004, n=20) more thrombin generation upon addition of FXa and prothrombin. PltPS exerted PS-direct that was similar to or greater than that of Zn(2+)-containing pdPS and much greater than that of Zn(2+)-deficient pdPS. Findings were confirmed using purified pltPS. Platelet-bound pltPS and microparticle-bound pltPS had similar PS-direct. Finally, platelet stimulation in the presence of inhibitory anti-PS antibodies resulted in 1.5 ± 0.2-fold (p<0.0001, n=11) more FXa generation upon addition of TF/FVIIa and FX. Thus, pltPS inhibits both prothrombinase and extrinsic FXase activities. Neutralising antibodies against APC and TFPI had no effect on the PS-direct of pltPS or pdPS on platelets. This study indicates that pltPS may be an essential pool of PS that counterbalances procoagulant activities on platelets.

Thrombin generation and activated protein C resistance in patients with essential thrombocythemia and polycythemia vera

We used the thrombin generation assay to evaluate the hypercoagulable state according to JAK2V617F mutational status in essential thrombocythemia (ET) and polycythemia vera (PV) patients. Thrombin generation was determined in the presence and absence of activated protein C (APC), and APC resistance was expressed as normalized APC sensitivity ratio (nAPCsr). Tissue factor pathway inhibitor (TFPI), total and free protein S (PS), prothrombin (FII), factor V (FV), and neutrophil elastase were measured in plasma; CD11b was measured on neutrophils. Compared with normal controls, patients had a lower endogenous thrombin potential in the absence of APC but had a higher endogenous thrombin potential in the presence of APC, showing the occurrence of APC resistance. The nAPCsr increased in JAK2V617F carriers compared with noncarriers and was highest in JAK2V617F homozygous patients. FII, FV, free PS, and TFPI levels were reduced in patients, mainly in JAK2V617F carriers. Multiple regression analysis indicated the low free PS level as major determinant of the increased nAPCsr. Elastase was increased in patients and inversely correlated with free PS. In conclusion, these data indicate the occurrence of acquired APC resistance in ET and PV patients, probably because of a reduction in free PS levels. The APC-resistant phenotype is influenced by the JAK2V617F mutational load.

Proteolytic cleavage of protein S during the hemostatic response

BACKGROUND

Protein S is a vitamin K-dependent protein with anticoagulant properties. It contains a so-called thrombin-sensitive region (TSR), which is susceptible to cleavage by coagulation factor Xa (FXa) and thrombin. Upon cleavage, the anticoagulant activity of protein S is abolished.

OBJECTIVE

The aim of the present study was to determine whether protein S is cleaved within the TSR during activation of the coagulation system under near physiological conditions.

RESULTS

In a reconstituted coagulation system containing apart from protein S only procoagulant constituents and synthetic phospholipid vesicles, protein S was cleaved at Arg60 by the FXa generated (3 mol min(-1) mol(-1) enzyme). FXa-catalyzed cleavage of protein S, however, was inhibited by factor Va and prothrombin by more than 70%. During clotting of recalcified citrated plasma in the presence of a synthetic lipid membrane, no FXa-catalyzed proteolysis of protein S was observed. Substituting platelets for phospholipid vesicles resulted both in the reconstituted system and in plasma in cleavage of the TSR. Cleavage was at Arg60 and was observed upon platelet activation, irrespective of the presence of FXa (13 pmol min(-1) 10(-8) platelets). No cleavage by thrombin was observed in either the reconstituted coagulation system or clotting plasma.

CONCLUSION

These findings suggest that in vivo the anticoagulant activity of protein S is not down-regulated by FXa or thrombin during activation of coagulation. Our results rather suggest a role for a platelet protease in down-regulating the anticoagulant activity of protein S during the hemostatic response.

Activated protein C-dependent and -independent anticoagulant activities of protein S have different structural requirements

Plasma protein S exhibits multiple anticoagulant activities. About 20% of protein S normally circulates in a form that is cleaved in its thrombin-sensitive region (TSR, residues 47-72) and this cleaved protein S is inactive as a cofactor for activated protein C (APC). To clarify whether the same cleavage(s) in the TSR neutralizes both APC-cofactor and APC-independent direct anticoagulant activities, protein S was treated with several proteases, and activities and cleavages were monitored. Thrombin cleaved protein S first at Arg49, which abolished protein S APC-cofactor activity, but not APC-independent activity. A slower second thrombin cleavage at Arg70 abolished the direct prothrombinase inhibitory activity of protein S and its ability to bind phospholipids. Factor Xa cleaved protein S only at Arg60 and abolished APC-cofactor activity but not APC-independent anticoagulant activity. The snake venom enzyme Protac C efficiently cleaved protein S at two sites in the TSR, which impaired both types of protein S anticoagulant activity in the presence of phospholipids. Protac C-cleaved protein S did not compete with Factor Xa for limiting phospholipid surfaces but could still inhibit prothrombinase activity in the absence of phospholipids. Thus, the APC-cofactor activity protein S is significantly more sensitive to structural changes in the TSR than is the APC-independent activity of protein S.

Reduction of antithrombin III, protein C, and protein S levels and activated protein C resistance in polycythemia vera and essential thrombocythemia patients with thrombosis

Patients with polycythemia vera (PV) or essential thrombocythemia (ET) show a high frequency of thrombosis. The reduction of hematocrit after phlebotomy and normalization of platelet counts do not completely eliminate thrombotic risk. Some preliminary studies reported a reduction in the concentration of natural anticoagulants (NA) in this group of patients. For this reason we evaluated protein S (PS) total antigen, antithrombin III (AT III), and protein C (PC) activity in 81 patients with chronic myeloproliferative disorders (33 with PV and 48 with ET). Data were compared with those obtained in 70 healthy sex- and age-matched subjects. Fifty-seven percent of patients (46 out of 81) showed one or more thrombotic episodes at diagnosis or during follow-up. Interestingly, we found a NA deficit in 43.5% of patients with thrombosis versus only 5.7% in the group of patients without thrombosis. These results may suggest new interpretations about the pathogenesis of thrombosis in PV or ET patients.

Molecular cloning and expression of murine and bovine endothelial cell protein C/activated protein C receptor (EPCR). The structural and functional conservation in human, bovine, and murine EPCR

Recently, we identified and cloned a human endothelial cell protein C/activated protein C receptor (EPCR). EPCR was predicted to be a type 1 transmembrane glycoprotein and a novel member of the CD1/major histocompatibility complex superfamily with 28% identity with CD1d. Even greater homology (62% identity) was detected with the murine protein, CCD41, which was previously characterized as a centrosome-associated, cell cycle-dependent protein. This raised the possibility that CCD41 was the murine homologue of EPCR. To address this possibility, to better understand structure-function relationships, and to facilitate physiological experiments on EPCR function, we cloned and sequenced murine and bovine EPCR from endothelial cell cDNA libraries. The nucleotide sequence of murine EPCR and CCD41 exhibited five differences corresponding to one base change, three single-base insertions, and one base deletion in the protein coding region. As a result, the predicted structures of EPCR and CCD41 differed in their amino and carboxyl termini but were identical in the central portion of the coding sequence. Based on comparison of the murine, bovine, and human EPCR sequences and the regions where discrepancies between murine EPCR and CCD41 were detected, we believe that CCD41 is probably identical to murine EPCR and that the reported sequence differences are likely the result of compression on the sequencing gel. Compared with human EPCR, the murine and bovine sequences were 69 and 73% identical, respectively, and 57% of the residues were identical between all three species. Both bovine and murine EPCR could bind human activated protein C when the cDNA clones were transfected into 293T cells. Like human EPCR, of the cell lines tested, the murine EPCR message was restricted to endothelium. Cloning of the murine and bovine homologue of EPCR will facilitate in vivo and in vitro studies of the role of EPCR in the protein C pathway.

Interaction of annexin V and platelets: effects on platelet function and protein S binding

Intact annexin V but not its fragments bound specifically to platelets in the presence of calcium. Maximal binding observed was 0.87 pmole annexin V per 10(8) platelets with an apparent dissociation constant of 2.13 x 10(-11) M. This represents approximately 5000 binding sites per platelet. Platelet stimulation by thrombin increased the binding of annexin V by 40-fold. Addition of phospholipid vesicles or treatment with phospholipase C inhibited annexin V binding to platelets, suggesting that phospholipid is the binding site for annexin V and that phosphatidylcholine forms at least part of this site. Binding of annexin V did not cause platelet aggregation and did not affect thrombin or collagen induced aggregation. However annexin V significantly inhibited the binding of protein S to the platelet surface.

Protein S and C alterations in acutely ill patients

Protein C, a potent vitamin K-dependent protein activated by an endothelial cell cofactor, thrombomodulin, has anticoagulant and profibrinolytic activity. Free protein S, a cofactor for protein C, potentiates protein C activity at the endothelial cell surface. Pulmonary thromboemboli are a consistent finding in adult respiratory distress syndrome (ARDS). To determine if protein S or protein C were affected by widespread endothelial cell damage in ARDS, we measured bound and free protein S levels and protein C antigenic and functional levels in 18 patients with acute lung injury, 6 critically ill patients without lung history, and 22 normal subjects. Free (PS:F) and bound (PS:Ag) protein S and protein C antigen (PC:Ag) levels were measured using an enzyme-linked immunoassay and protein C function (PC:Fn) by measuring its anticoagulant activity. We found a significant decrease in bound and free protein S levels of both patient groups in comparison to normal and a shift toward the inactive, bound protein S form. In addition, a significant decrease in free protein S compared to bound protein S in both patient groups was observed. While both PC:Ag and PC:Fn were significantly reduced compared to normal, the PC:Fn was significantly and severely decreased out of proportion to the PC:Ag in both patient groups. There was no difference between those with and without lung injury for both protein S and protein C. Analyzed according to etiology of lung injury, there was no difference in the bound and free protein S, nor in PC:Ag and PC:Fn levels between patients with sepsis and trauma. However, there were significant decreases in both protein S and protein C levels compared with normal subjects. Levels of both PS and PC levels in patients who survived did not differ from those who died. In summary, our data show that both protein S and C are markedly deranged in acutely ill patients who suffered from either sepsis or trauma, and these changes are independent of lung injury. The marked reductions in functional activity of PS and PC may be contributing factors to the thromboembolic complications often observed in these patients.

Studies of natural anticoagulant proteins and anticardiolipin antibodies in patients with the lupus anticoagulant

Components of the natural anticoagulant system (NAS) and anticardiolipin antibodies were examined in 21 patients with lupus anticoagulant (LA), 13 of whom had past histories of thrombotic episodes. No relationship could be shown between the antigenic levels of protein C and S (PC, PS) and a history of thrombosis. Inhibition of the anticoagulant activity of activated protein C (APC) was observed using plasma from 20/21 patients when phospholipid vesicles were used as the surface for the coagulation reaction. This effect was not affected by the addition of PS. When platelet membranes were employed only 2/21 patients demonstrated inhibition of APC. Under the latter condition, PS functional activity was inhibited in 7/21 patients, six of whom had a past history of thrombosis. Reduced antithrombin III or heparin cofactor II levels were observed in a total of 4/21 patients and may have contributed to the development of thrombosis in three of these patients. Antibodies specifically directed against these proteins were not detected suggesting the possibility of an associated constitutional deficiency. Anticardiolipin antibodies, though elevated in 17/21 patients, did not serve as a useful marker for an increased risk of thrombosis, and the level did not correlate with inhibition of the activity of APC or PS. We conclude that the mechanism of thrombosis in patients with LA is multi-factorial. A subset of patients in whom LA specifically inhibits PS function may represent patients who are at significant risk from thrombosis.

Low protein S in essential thrombocythemia with thrombosis

Patients with essential thrombocythemia (ET) are at increased risk for large-vessel and microvascular thrombosis, presumably because of abnormal platelet number and function. To determine if another hemostatic abnormality might contribute to this thrombotic risk, we investigated protein C and protein S and the fibrinolytic system in four patients with ET. The patients segregated into two distinct groups. The first group consisted of two patients with moderate thrombocytosis who were without thrombotic symptoms and who had normal protein S and protein C and normal fibrinolysis. The second group consisted of two patients with mild to moderate thrombocytosis and thrombosis. Both these latter two patients had abnormalities in protein S. The first patient had only 31% free protein S and 67% total protein S, with normal protein C and normal fibrinolysis. Following treatment, vasoocclusive symptoms resolved and platelet count decreased, but protein S remained low (28% free protein S). The second patient in this group had only 35% total protein S at initial study. Other hemostatic abnormalities were also present. Following treatment, symptoms resolved and protein S returned to normal. Based on this limited series, it appears that, in some patients with ET, thrombotic phenomena may be due to the presence of a second hemostatic abnormality in addition to the high platelet count and abnormal platelet function. This abnormality may be acquired as part of the disease process. Some of these abnormalities may be corrected following treatment.

Inhibition of activated protein C by platelets

Activated protein C (APC), an anticoagulant that acts by inactivating Factors Va and VIIIa, is dependent on a suitable surface for its action. In this study we examined the ability of human platelets to provide this surface and support APC-mediated anticoagulant effects. The activity of APC was examined in three systems: the Factor Xa recalcification time of Al(OH)3 adsorbed plasma, studies of thrombin generation in recalcified plasma, and assessment of the rate of inactivation of purified Factor Va. In comparison with phospholipid, intact platelets required significantly greater concentrations of APC to achieve a similar degree of anticoagulation. When washed platelet membranes were substituted for intact platelets, adequate support of APC was observed and the anticoagulant effect was similar to that obtained with phospholipid. Platelet releasate obtained by stimulation of platelets with thrombin and epinephrine contained an inhibitor that interfered with the ability of phospholipid and washed platelet membranes to catalyze the anticoagulant effects of APC. A noncompetitive inhibition was suggested by Dixon plot analysis of the interaction between platelet releasate and APC. The activity of the platelet APC inhibitor was immediate and was not enhanced by heparin, distinguishing it from the circulating protein C inhibitor. The presence of this inhibitor in the platelet and its release with platelet stimulation emphasizes the procoagulant role of this cell.

Inhibition of the anticoagulant activity of protein S by prothrombin

Protein S is a vitamin K-dependent protein cofactor to the anticoagulant, activated protein C (APC). This study examines the inhibition of human protein S anticoagulant activity by prothrombin. In the absence of protein S, the anticoagulant activity of APC measured in a Factor Xa recalcification time, was comparable using normal or plasma adsorbed with Al(OH)3. Protein S was an effective cofactor to APC in Al(OH)3-adsorbed plasma, but was significantly less active in normal plasma. Analysis of the difference in the two plasmas revealed that normal plasma contained an inhibitor to the anticoagulant activity of protein S that was removed by Al(OH)3 adsorption. Purification of this inhibitory activity demonstrated that it was mediated by the vitamin K-dependent protein, prothrombin. Prothrombin purified by conventional techniques caused immediate, dose-dependent inhibition of the cofactor activity of protein S in the presence of phospholipids or platelets, but had no effect on the anticoagulant activity of APC. The inhibition was demonstrable using a Factor Xa recalcification time, and studies of the rates of inactivation of purified Factor Va. Increasing concentrations of protein S overcame the inhibition by prothrombin and kinetic analysis of the interaction demonstrated that prothrombin acted as a competitive inhibitor to protein S. Immunoabsorption of prothrombin from plasma using immobilized antiprothrombin antibodies was associated with the complete removal of the protein S inhibitory activity. We conclude that the anticoagulant activity of protein S is modulated by prothrombin and that this may represent another regulatory mechanism of the natural anticoagulant system.

Norepinephrine down-regulates the activity of protein S on endothelial cells

The adrenergic agonist norepinephrine is shown to stimulate endothelium to induce protein S release and degradation, leading to diminished anti-coagulant activity and to down-regulation of protein S cell surface-binding sites. Norepinephrine-induced release of intracellular protein S was blocked by the alpha 1-adrenergic antagonist prazosin (10(-7) M) but not by the alpha-adrenergic antagonist propranolol (10(-6) M) or the alpha 2-adrenergic antagonist yohimbine (10(-5) M) indicating that this response resulted from the specific interaction of norepinephrine with a class of alpha 1-adrenergic receptors not previously observed on endothelium. Attenuation of norepinephrine-induced release of protein S by pertussis toxin in association with the ADP-ribosylation of a 41,000-D membrane protein indicates that this intracellular transduction pathway involves a regulatory G protein. The observation that protein S was released from endothelium in response to maneuvers which elevate intracellular calcium or activate protein kinase C suggests that the response may be mediated via intermediates generated through the hydrolysis of phosphoinositides. Morphologic studies were consistent with a mechanism in which norepinephrine causes exocytosis of vesicles containing protein S. In addition to release of protein S, norepinephrine also induced loss of endothelial cell protein S-binding sites, thereby blocking effective activated protein C-protein S-mediated factor Va inactivation on the cell surface. Norepinephrine-mediated endothelial cell stimulation thus results in loss of intracellular protein S and suppression of cell surface-binding sites, modulating the anti-coagulant protein C pathway on the vessel wall. These studies define a new relationship between an anti-coagulant mechanism and the autonomic nervous system, and indicate a potential role for an heretofore unrecognized class of alpha 1-adrenergic receptors in the regulation of endothelial cell physiology.