Protein S protects neurons from excitotoxic injury by activating the TAM receptor Tyro3-phosphatidylinositol 3-kinase-Akt pathway through its sex hormone-binding globulin-like region

The anticoagulant factor protein S (PS) protects neurons from hypoxic/ischemic injury. However, molecular mechanisms mediating PS protection in injured neurons remain unknown. Here, we show mouse recombinant PS protects dose-dependently mouse cortical neurons from excitotoxic NMDA-mediated neuritic bead formation and apoptosis by activating the phosphatidylinositol 3-kinase (PI3K)-Akt pathway (EC(50) = 26 ± 4 nm). PS stimulated phosphorylation of Bad and Mdm2, two downstream targets of Akt, which in neurons subjected to pathological overstimulation of NMDA receptors (NMDARs) increased the antiapoptotic Bcl-2 and Bcl-X(L) levels and reduced the proapoptotic p53 and Bax levels. Adenoviral transduction with a kinase-deficient Akt mutant (Ad.Akt(K179A)) resulted in loss of PS-mediated neuronal protection, Akt activation, and Bad and Mdm2 phosphorylation. Using the TAM receptors tyrosine kinases Tyro3-, Axl-, and Mer-deficient neurons, we showed that PS protected neurons lacking Axl and Mer, but not Tyro3, suggesting a requirement of Tyro3 for PS-mediated protection. Consistent with these results, PS dose-dependently phosphorylated Tyro3 on neurons (EC(50) = 25 ± 3 nm). In an in vivo model of NMDA-induced excitotoxic lesions in the striatum, PS dose-dependently reduced the lesion volume in control mice (EC(50) = 22 ± 2 nm) and protected Axl(-/-) and Mer(-/-) transgenic mice, but not Tyro3(-/-) transgenic mice. Using different structural PS analogs, we demonstrated that the C terminus sex hormone-binding globulin-like (SHBG) domain of PS is critical for neuronal protection in vitro and in vivo. Thus, our data show that PS protects neurons by activating the Tyro3-PI3K-Akt pathway via its SHGB domain, suggesting potentially a novel neuroprotective approach for acute brain injury and chronic neurodegenerative disorders associated with excessive activation of NMDARs.

Regulation of TFPI function by protein S

Protein S is an anticoagulant cofactor of full-length tissue factor pathway inhibitor (TFPI) that facilitates optimal factor Xa-inhibition and efficient down-regulation of thrombin generation in plasma. Protein S and TFPI are constitutively active in plasma and therefore provide an effective anticoagulant barrier against unwanted procoagulant activity in the circulation. In this review, we describe the current status on how TFPI-activity depends on protein S, and show that TFPI and protein S are major regulators of thrombin generation both in the absence and presence of activated protein C (APC). As there is covariation of plasma TFPI and protein S levels both in health and in disease, these findings suggest that the risk of venous thrombosis associated with protein S deficiency states might be in part explained by the accompanying low plasma TFPI levels.

Plasma protein S contains zinc essential for efficient activated protein C-independent anticoagulant activity and binding to factor Xa, but not for efficient binding to tissue factor pathway inhibitor

Protein S (PS) is a cofactor for activated protein C (APC), which inactivates coagulation factors (F) Va and VIIIa. Deficiency of protein C or PS is associated with risk of thrombosis. We found that PS also has APC-independent anticoagulant activity (PS-direct) and directly inhibits thrombin generated by FXa/FVa (prothrombinase complex). Here we report that PS contains Zn(2+) that is required for PS-direct and that is lost during certain purification procedures. Immunoaffinity-purified PS contained 1.4 +/- 0.6 Zn(2+)/mol, whereas MonoQ-purified and commercial PS contained 0.15 +/- 0.15 Zn(2+)/mol. This may explain the controversy regarding the validity of PS-direct. Zn(2+) content correlated positively with PS-direct in prothrombinase assays and clotting assays, but APC-cofactor activity of PS was independent of Zn(2+) content. PS-direct and Zn(2+) were restored to inactive PS under mildly denaturing conditions. Conversely, o-phenanthroline reversibly impaired the PS-direct of active PS. Zn(2+)-containing PS bound FXa more efficiently (K(d)(app)=9.3 nM) than Zn(2+)-deficient PS (K(d)(app)=110 nM). PS bound TFPI efficiently, independently of Zn(2+) content (K(d)(app)=21 nM). Antibodies that block PS-direct preferentially recognized Zn(2+)-containing PS, suggesting conformation differences at or near the interface of 2 laminin G-like domains near the PS C terminus. Thus, Zn(2+) is required for PS-direct and efficient FXa binding and may play a role in stabilizing PS conformation.

[Abnormality in blood coagulation because of protein S-K196E mutation]

We recently identified protein S-K196E mutation as a genetic risk factor for venous thromboembolism in the Japanese population. The E allele frequency was found to be 0.009. Therefore, a substantial proportion of the Japanese population may be assumed to be carrying the E allele of protein S and is at risk of developing venous thromboembolism.

Effects of Factor Xa and Protein S on the Individual Activated Protein C-mediated Cleavages of Coagulation Factor Va

Activated protein C inhibits the procoagulant function of activated factor V (FVa) through proteolytic cleavages at Arg-306, Arg-506, and Arg-679. The cleavage at Arg-506 is kinetically favored but protected by factor Xa (FXa). Protein S has been suggested to annihilate the inhibitory effect of FXa, a proposal that has been challenged. To elucidate the effects of FXa and protein S on the individual cleavage sites of FVa, we used recombinant FVa:Q306/Q679 and FVa:Q506/Q679 variants, which can only be cleaved at Arg-506 and Arg-306, respectively. In the presence of active site blocked FXa (FXa-1.5-dansyl-Glu-Gly-Arg), the FVa inactivation was followed over time, and apparent second order rate constants were calculated. Consistent with results on record, we observed that FXa-1.5-dansyl-Glu-Gly-Arg decreased the Arg-506 cleavage by 20-fold, with a half-maximum inhibition of approximately 2 nM. Interestingly and in contrast to the inhibitory effect of FXa on the 506 cleavage, FXa stimulated the Arg-306 cleavage. Protein S counteracted the inhibition by FXa of the Arg-506 cleavage, whereas protein S and FXa yielded additive stimulatory effect of the cleavage at Arg-306. This suggests that FXa and protein S interact with distinct sites on FVa, which is consistent with the observed lack of inhibitory effect on FXa binding to FVa by protein S. We propose that the apparent annihilation of the FXa protection of the Arg-506 cleavage by protein S is due to an enhanced rate of Arg-506 cleavage of FVa not bound to FXa, resulting in depletion of free FVa and dissociation of FXa-FVa complexes.

Natural killer cell differentiation driven by Tyro3 receptor tyrosine kinases

Although understanding of the function and specificity of many natural killer (NK) cell receptors is increasing, the molecular mechanisms regulating their expression during late development of NK cells remain unclear. Here we use representational difference analysis to identify molecules required for late NK cell differentiation. Axl protein tyrosine kinase, together with the structurally related receptors Tyro3 and Mer, were essential for NK cell functional maturation and normal expression of inhibitory and activating NK cell receptors. Also, all three receptors were expressed in maturing NK cells, the ligands of these receptors were produced by bone marrow stromal cells, and recombinant versions of these ligands drove NK cell differentiation in vitro. These results collectively suggest that Axl, Tyro3 and Mer transmit signals that are essential for the generation of a functional NK cell repertoire.

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.

Protein S attenuates the invasive potential of THP-1 cells by interfering with plasminogen binding on cell surface via a protein C-independent mechanism

Protein S, a cofactor for activated protein C (aPC) to inactivate coagulation factors, also plays a pivotal role in inflammation. Based on our recent findings that aPC and protein S modifies tissue plasminogen activator (tPA)-catalyzed activation of Glu-plasminogen (Glu-plg), we analyzed possible role of protein S in cell-associated plasminogen activation and invasive potential of inflammatory cells. Monocyte-like THP-1 cells, to which both plasminogen and tPA bind, enhanced tPA-catalyzed plasminogen activation, which was partially abolished by protein S but not by aPC. Protein S attenuated both the plasminogen binding to THP-1 cells and associated their invasive potential through Matrigel.

Direct anticoagulant activity of protein S-C4b binding protein complex in Heerlen heterozygotes and normals

BACKGROUND

Plasma protein S normally circulates free (40%) or complexed with C4b-binding protein (PS-C4BP); only free protein S is a cofactor for activated protein C during factor (F) Va inactivation. Protein S-Heerlen lacks a carbohydrate group, leading to low plasma free protein S levels, but normal levels of PS-C4BP.

OBJECTIVES

Because protein S-Heerlen is not associated with thrombosis, we investigated whether PS-C4BP is directly anticoagulant in plasma and whether PS-Heerlen-C4BP has enhanced direct anticoagulant activity.

METHODS

An assay for protein S direct activity was applied to Heerlen-heterozygous plasmas. Free and complexed protein S were repeatedly isolated from normal and Heerlen-heterozygous plasmas and tested for direct anticoagulant activity in prothrombinase assays and in plasma.

RESULTS

Heerlen-heterozygous plasmas were deficient in free and total protein S antigen but had normal to high protein S direct anticoagulant activity. Purified Heerlen-heterozygous PS-C4BP was 7-fold more potent than normal PS-C4BP in inhibiting full prothrombinase activity, and 22-fold more potent in inhibiting prothrombin activation in the absence of FVa; it also specifically prolonged plasma clotting times 14-fold more than normal PS-C4BP. Heerlen-heterozygous PS-C4BP did not compete for limiting phospholipids any better than normal PS-C4BP. However, ligand blots and surface plasmon resonance studies showed that Heerlen-heterozygous PS-C4BP bound more avidly to FXa than did normal PS-C4BP (apparent Kd = 4.3 nm vs. 82 nm).

CONCLUSIONS

Plasma-derived PS-C4BP has direct anticoagulant activity in plasma and in purified systems. Enhanced direct activity of PS-Heerlen-C4BP may compensate for low free protein S levels and low cofactor activity in individuals with protein S-Heerlen.

The C4b-binding Protein-Protein S Complex Inhibits the Phagocytosis of Apoptotic Cells

The phagocytosis of apoptotic cells is a complex process involving numerous interactions between the target cell and the macrophage. We have examined a role of the major soluble inhibitor of the classic and lectin complement pathways, C4b-binding protein (C4BP), in the clearance of apoptotic cells. The major form of C4BP present in blood is composed of seven alpha-chains and one beta-chain, which binds protein S (PS). Approximately 70% of all PS in human plasma is trapped in such a complex and is able to localize C4BP to the surface of apoptotic cells due to the high affinity to phosphatidylserine. Free PS has recently been shown to enhance phagocytosis of apoptotic cells by macrophages. We observed a stimulatory effect of free PS on the engulfment of apoptotic cells (BL-41 and Jurkat) by primary human macrophages or THP-1 cells and a decrease of activity in serum depleted of PS in agreement with previous results. However, we also show that the process is strongly inhibited in the presence of the C4BP-PS complex. Addition of the C4BP-PS complex to serum deficient in both molecules abolished the enhancing effect of serum on phagocytosis. The effect of both free PS and the C4BP-PS complex could be inhibited with monoclonal antibody directed against the Gla domain of PS. Although the presence of the C4BP-PS complex on apoptotic cells may lead to decreased phagocytosis, it may still be beneficial to the host, since it could prevent secondary necrosis because it inhibits further complement attack.

Protein S: A conduit between anticoagulation and inflammation

OBJECTIVE

To review the multifaceted roles of the anticoagulant protein S, facilitating a better comprehension of this protein's role in anticoagulation and inflammation pathways and the crosstalk between these pathologic states.

DATA SOURCES AND STUDY SELECTION

Original research and review articles published in English pertaining to protein S, sourced from PubMed, during the last 30 yrs.

DATA EXTRACTION AND SYNTHESIS

The protein C anticoagulant pathway is an essential mechanism for attenuating thrombin generation by the membrane-bound procoagulant complexes, tenase and prothrombinase. Protein S is a nonenzymatic protein. In the absence of activated protein C, it demonstrates anticoagulant activity; in the presence of activated protein C, it functions as a cofactor for activated protein C-dependent proteolytic inactivation of the coagulation cofactors factor Va and factor VIIIa. However, in plasma, these anticoagulant activities are limited by the concentration of free protein S (approximately 40% of the total protein S plasma concentration). The remaining protein S (approximately 60%) is found in a high-affinity, calcium-stabilized complex with C4b-binding protein, which renders this fraction devoid of anticoagulant function. Several recent investigations have attributed novel activated protein C-independent functions of protein S to the association of protein S with C4b-binding protein, thus establishing the importance of this fraction of plasma protein S.

CONCLUSIONS

Together, these data support a role for protein S in both anticoagulation and inflammation, facilitating a better understanding of the need for both free and C4b-binding protein-bound protein S. Although these physiologic roles are truly dichotomous in terms of functional end point, mechanistically, both involve high-affinity membrane binding to phosphatidylserine-bearing surfaces. This binding is mediated by the n-terminal gamma-carboxyglutamic acid-rich domain of this protein.

Natural Anticoagulant Proteins in the Regulation of Autoimmunity: Potential Role of Protein S

Autoimmunity results when the immune system fails to distinguish between self and non-self factors in the body. The cellular and biochemical mechanisms that underlie development of autoimmunity are only partly understood. One current theory is that autoimmunity can result when there is a failure to clear dying cells from a tissue before they undergo lysis of the plasma membrane. That is, cells that die by apoptosis are thought to be cleared from a tissue by neighboring phagocytic cells, such as macrophages, before the cells have lost their plasma membrane integrity. This rapid removal of early apoptotic cells is thought to prevent induction of an inflammatory response to intracellular macromolecules, thereby allowing for an immunologically silent removal of the dying cells. Hence, any factor or condition that inhibits phagocytosis of early apoptotic cells may trigger or promote an autoimmune response to intracellular components. Depletion of factors required for the efficient phagocytosis of dying cells would have a similar outcome. The recent discovery that the natural anticoagulant protein S is required for efficient uptake of apoptotic cells (Anderson, H.A., Maylock, C.A., Williams, J.A., Paweletz, C.P., Shu, H., and Shacter, E. (2003) Nature Immunology 4, 87-91) reveals a potential new linkage between autoimmunity and coagulation systems. This article will review the dual roles of protein S as an anticoagulant and in regulating phagocytosis of apoptotic cells, with emphasis on exposing a possible novel role in regulating autoimmunity.

Deletion or replacement of the second EGF-like domain of protein S results in loss of APC cofactor activity

Human protein S (PS), a cofactor of anticoagulant-activated protein C (APC), is a modular protein containing 4 epidermal growth factor (EGF)-like domains. EGF1 appears to mediate PS interaction with APC, but the roles of EGFs 2, 3, and 4 are less clear. We synthesized PS variants lacking single EGF domains (EGF2, 3, or 4) and assessed their APC cofactor activity in a factor Va inactivation assay. The variant lacking EGF2 (variant 134) showed the most dramatic loss of activity (approximately 10% of recombinant wild-type PS activity). Replacement of EGF2 by an additional EGF3 (variant 1334) resulted in a comparable loss of activity, suggesting that the loss of a specific rather than "spacer" function of EGF2 was responsible. We confirmed that the variant 134 had a functional gamma-carboxyglutamic acid (Gla) domain and that EGF1 was correctly folded. This is the first clear evidence that EGF2 is required for the expression of PS activity.

Posttranslational protein S-palmitoylation and the compartmentalization of signaling molecules in neurons

Protein domains play a fundamental role in the spatial and temporal organization of intracellular signaling systems. While protein phosphorylation has long been known to modify the interactions that underlie this organization, the dynamic cycling of lipids should now be included amongst the posttranslational processes determining specificity in signal transduction. The characteristics of this process are reminiscent of the properties of protein and lipid phosphorylation in determining compartmentalization through SH2 or PH domains. Recent studies have confirmed the functional importance of protein S-palmitoylation in the compartmentalization of signaling molecules that support normal physiological function in cell division and apoptosis, and synaptic transmission and neurite outgrowth. In neurons, S-palmitoylation and targeting of proteins to rafts are regulated differentially in development by a number of processes, including some related to synaptogenesis and synaptic plasticity. Alterations in the S-palmitoylation state of proteins substantially affect their cellular function, raising the possibility of new therapeutic targets in cancer and nervous system injury and disease.

Vitamin K-dependent protein S localizing complement regulator C4b-binding protein to the surface of apoptotic cells

Apoptosis is characterized by a lack of inflammatory reaction in surrounding tissues, suggesting local control of complement activation. During the initial stage of apoptosis, cells expose negatively charged phospholipid phosphatidylserine on their surfaces. The vitamin K-dependent protein S has a high affinity for this type of phospholipid. In human plasma, 60-70% of protein S circulates in complex with C4b-binding protein (C4BP). The reason why protein S and C4BP form a high-affinity complex in plasma is not known. However, C4BP is an important regulator of the classical pathway of the complement system where it acts as a cofactor in degradation of complement protein C4b. Using Jurkat cells as a model system for apoptosis, we now show protein S to bind to apoptotic cells. We further demonstrate protein S-mediated binding of C4BP to apoptotic cells. Binding of the C4BP-protein S complex to apoptotic cells was calcium-dependent and could be blocked with Abs directed against the phospholipid-binding domain in protein S. Annexin V, which binds to exposed phosphatidylserine on the apoptotic cell surface, could inhibit the binding of protein S. The C4BP that was bound via protein S to the apoptotic cells was able to interact with the complement protein C4b, supporting a physiological role of the C4BP/protein S complex in regulation of complement on the surface of apoptotic cells.

Factor V and thrombotic disease: description of a janus-faced protein

The generation of thrombin by the prothrombinase complex constitutes an essential step in hemostasis, with thrombin being crucial for the amplification of blood coagulation, fibrin formation, and platelet activation. In the prothrombinase complex, the activated form of coagulation factor V (FVa) is an essential cofactor to the enzyme-activated factor X (FXa), FXa being virtually ineffective in the absence of its cofactor. Besides its procoagulant potential, intact factor V (FV) has an anticoagulant cofactor capacity functioning in synergy with protein S and activated protein C (APC) in APC-catalyzed inactivation of the activated form of factor VIII. The expression of anticoagulant cofactor function of FV is dependent on APC-mediated proteolysis of intact FV. Thus, FV has the potential to function in procoagulant and anticoagulant pathways, with its functional properties being modulated by proteolysis exerted by procoagulant and anticoagulant enzymes. The procoagulant enzymes factor Xa and thrombin are both able to activate circulating FV to FVa. The activity of FVa is, in turn, regulated by APC together with its cofactor protein S. In fact, the regulation of thrombin formation proceeds primarily through the upregulation and downregulation of FVa cofactor activity, and failure to control FVa activity may result in either bleeding or thrombotic complications. A prime example is APC resistance, which is the most common genetic risk factor for thrombosis. It is caused by a single point mutation in the FV gene (factor V(Leiden)) that not only renders FVa less susceptible to the proteolytic inactivation by APC but also impairs the anticoagulant properties of FV. This review gives a description of the dualistic character of FV and describes the gene-gene and gene-environment interactions that are important for the involvement of FV in the etiology of venous thromboembolism.

Implication of protein S thrombin-sensitive region with membrane binding via conformational changes in the gamma-carboxyglutamic acid-rich domain

In the vitamin K-dependent protein family, only protein S (PS) contains a thrombin-sensitive region (TSR), located between the domain containing the gamma-carboxyglutamic acid and the first epidermal growth factor-like domain. To better define the role of TSR in the PS molecule, we expressed a recombinant human PS (rHPS) and its analogue lacking TSR (rTSR-less), and prepared factor Xa- and thrombin-cleaved rHPS. A peptide reproducing TSR (TSR-peptide) was also synthesized in an attempt to obtain direct evidence of the domain involvement in PS anticoagulant activity. In a coagulation assay, both rTSR-less and factor Xa-cleaved PS were devoid of activated protein C cofactor activity. The TSR-peptide did not inhibit rHPS activity, showing that TSR must be embedded in the native protein to promote interaction with activated protein C. The binding of rHPS to activated platelets and to phospholipid vesicles was not modified after factor Xa- or thrombin-mediated TSR cleavage, whereas the binding of rTSR-less was markedly reduced. This suggested a role for TSR in conferring to PS a strong affinity for phospholipid membranes. TSR-peptide did not directly bind to activated platelets or compete with rHPS for phospholipid binding. The results of the present study show that TSR may not interact directly with membranes, but probably constrains the gamma-carboxyglutamic acid-rich domain in a conformation allowing optimal interaction with phospholipids.

The role of protein S in the activation of thrombin activatable fibrinolysis inhibitor (TAFI) and regulation of fibrinolysis

Thrombin activatable fibrinolysis inhibitor (TAFI) is a carboxy-peptidase B-like proenzyme that after activation by thrombin downregulates fibrinolysis. Thrombomodulin stimulates the activation of both TAFI and protein C whereas activated protein C inhibits the activation of TAFI by downregulation of thrombin formation, a process in which protein S acts as a cofactor. Here we determined the role of protein S in the activation of TAFI and regulation of fibrinolysis. Depletion of protein S from plasma or inhibition of protein S by specific antibodies resulted in an increased rate of TAFI activation and in an increased maximum of TAFIa activity generated. The effect on the rate of TAFI activation could be attributed to the APC-independent anticoagulant function of protein S whereas the effect on the maximum activity could be attributed to the APC cofactor function of protein S. Therefore it is concluded that protein S inhibits TAFI activation in two ways. On one hand, protein S functions as a cofactor for APC which results in a reduction of the maximum induced TAFI activity and on the other hand protein S inhibits the initial thrombin formation independently of APC which results in a decreased rate of TAFI activation. The effect of the APC-independent anticoagulant activity of protein S on the activation of TAFI provides a new mechanism for the regulation of fibrinolysis in the early stages of clot formation.

Activated protein C versus protein C in severe sepsis

OBJECTIVE

To delineate critical differences between activated protein C (APC) and its precursor, protein C, with regard to plasma levels in health and in severe sepsis, and to discuss the implications of these differences as they relate to treatment strategies in patients with severe sepsis. DATA SOURCE/STUDY SELECTION: Published literature including abstracts, manuscripts, and review articles reporting studies in both experimental animal models and humans that provide an understanding of the relationship and the critical differences between circulating levels of APC and protein C.

DATA EXTRACTION AND SYNTHESIS

The protein C pathway represents one of the major regulatory systems of hemostasis, exhibiting antithrombotic, profibrinolytic and anti-inflammatory properties. This pathway also plays a critical role in the pathophysiology of severe sepsis. Central to this pathway is the vitamin K-dependent serine protease, APC, and its precursor, protein C. The conversion of protein C to APC is dependent on the complex of thrombin and thrombomodulin, an integral endothelial surface receptor. The conversion of protein C to APC is further augmented by another endothelial surface protein, the endothelial protein C receptor. There are limited published data on APC levels in health and disease, probably due to the complexity of the assay methodology for measuring APC and the absence of commercially available diagnostic kits. In animals and humans with normal functioning endothelium, circulating levels of APC (1-3 ng/mL) are positively correlated with protein C (4000-5000 ng/mL) concentration and the amount of thrombin generated. In patients with severe sepsis, there is a generalized endothelial dysfunction, contributing to multiple organ failure with increased morbidity and mortality. Persistently low protein C levels are related to poor prognosis. Key to understanding the treatment strategy with APC or protein C is knowledge of the functional status of the endothelium and, specifically, whether the microvasculature in patients with severe sepsis can support the conversion of protein C to APC. To date, only APC (drotrecogin alfa [activated]) has been shown to reduce mortality in severe sepsis in a large, phase 3, placebo-controlled, double-blind international trial. In contrast, no data, other than open-label case studies, are available for evaluation of the effects of protein C in the treatment of severe sepsis.

CONCLUSION

The limited data available indicate that lower levels of protein C in sepsis occur in the absence of appreciable conversion to APC. These observations indicate that treatment with APC may be more efficacious than protein C in severe sepsis, where generalized endothelial dysfunction may impair conversion of protein C to APC. Additional research is required to confirm these observations.

The role of protein C, protein S, and resistance to activated protein C in Legg-Perthes disease

OBJECTIVES

It has been hypothesized that Legg-Perthes disease is caused by repeated vascular interruptions of the blood supply to the proximal femur, which are precipitated by coagulation system abnormalities. To test this theory, we conducted a case-control study among 57 patients with Legg-Perthes disease and an equal number of community controls. We measured protein C and protein S and resistance to activated protein C (APC-R) from plasma.

STUDY DESIGN

Participants were placed into 1 of 3 mutually exclusive categories based on the control distribution: 1) normal, defined as either above or within 1 standard deviation below the expected mean; 2) low normal, defined as between 1 and 2 standard deviations below the expected mean; and 3) low, defined as >2 standard deviations below the expected mean. DNA was analyzed to determine the presence of a point mutation in the factor V gene that causes APC-R.

RESULTS

We observed a statistically significant increased risk of Legg-Perthes disease with decreasing levels of protein C and a nearly significant increased risk with decreasing levels of protein S. The factor V gene defect was present in 5 (9%) of 55 cases and 3 (5%) of 56 controls (odds ratio 1.8, 95% confidence interval: 0.4-7.7), but the mean level on the APC-R plasma test was similar for cases and controls. Nine cases and 1 control had 2 low normal or low test results (odds ratio 13.0, 95% confidence interval: 2.2-75).

CONCLUSIONS

Our results support the belief that abnormalities of the coagulation system leading to a thrombophilic state play a role in Legg-Perthes disease; however, larger studies are needed before definitive recommendations for coagulation testing can be made.

Plasma glucosylceramide deficiency as potential risk factor for venous thrombosis and modulator of anticoagulant protein C pathway

To assess the relationship between venous thrombosis and plasma glucosylceramide (GlcCer) or phosphatidylethanolamine (PE), plasma levels of GlcCer and PE were determined for 70 venous thrombosis patients referred for evaluation and 70 healthy blood donors. The mean GlcCer level, but not the PE level, was lower in patients versus controls (4.9 vs 6.5 microg/mL [P =.0007] and 66 vs 71 microg/mL [P =.48], respectively). As a measure of relative risk, the odds ratio for deep vein thrombosis in subjects with GlcCer levels below the 10th percentile of controls was 5.7 (95% CI, 2.3-14). To assess the influence of glycolipids on anticoagulant response to activated protein C (APC):protein S in modified prothrombin time assays, the effects of depleting endogenous plasma GlcCer by glucocerebrosidase treatment or of adding exogenous purified GlcCer or other neutral glycolipids to plasma were tested. Glucocerebrosidase treatment reduced plasma sensitivity to APC:protein S in parallel with GlcCer reduction. Exogenously added GlcCer and the homologous Glc-containing globotriaosylceramide (Gb3Cer), but not galactosylceramide, dose-dependently prolonged clotting times of normal plasma in the presence, but not absence, of APC:protein S, which suggests that GlcCer or Gb3Cer can enhance protein C pathway anticoagulant activity. In studies using purified proteins, inactivation of factor Va by APC:protein S was enhanced by GlcCer alone and by GlcCer in multicomponent vesicles containing phosphatidylserine and phosphatidylcholine. These results suggest that the neutral glycolipids GlcCer and Gb3Cer may directly contribute to the anticoagulant activity of the protein C pathway and that deficiency of plasma GlcCer may be a risk factor for venous thrombosis. (Blood. 2001;97:1907-1914)

Cellular effects and signalling pathways activated by the anti-coagulant factor, protein S, in vascular cells protein S cellular effects

The anticoagulant factor protein S is a secreted vitamin K-dependent gamma-carboxylated protein that is mainly synthesised in the liver but is also made by endothelial cells and megakaryocytes in culture. In previous studies we have shown that protein S acts as a mitogen for cultured human vascular smooth muscle cells. The synthesis and secretion of protein S by endothelial cells suggests that in addition to its role in the coagulation cascade, protein S may be an important autocrine factor implicated in the pathophysiology of the vascular system. The effects of protein S on hVSMC proliferation, migration and survival are discussed. The activation of the components of the MAP kinase pathway, ERK1/2, JNK/SAPK and p38 is also summarised. Binding and chemical cross-linking experiments provided evidence for the existence of a cell surface protein S receptor(s). By virtue of its many cellular effects, it is suggested here that the anticoagulant factor protein S plays an important role in the pathophysiology of the vasculature.

Activated protein C resistance in Turkish women with severe preeclampsia

The purpose of this study was to investigate the occurrence rate of APC resistance (APC-R) with severe preeclampsia in Turkish women. Thirty-two consecutive women having severe preeclampsia were included in the study. Thirty-two healthy pregnant women served as the control group. APC-R assays were performed in the third trimester of pregnancy, and 3 and 9 months after delivery. APC-R was demonstrated in the third trimester, 3 months and 9 months after delivery in 27 (84.4%), 23 (71.9%) and 5 (15.6%) of 32 preeclamptic patients, respectively. APC-R rates were significantly higher in preeclamptic group than in normal pregnant women in the third trimester of pregnancy (p < 0.05). Decreased mean APC activity and also increased APC-R rate was still persisting in preeclamptic group for 3 months after delivery. Nine months after delivery, the mean APC activity and also APC-R rates approached to the normal pregnant women; however, there was a significant difference between both groups (p < 0.05). Our results indicate that acquired APC-R may be a contributory factor in the pathogenesis of preeclampsia.

C4b-binding protein protects coagulation factor Va from inactivation by activated protein C

We investigated the effect of C4BP on APC-mediated inactivation of factor Va (FVa) in the absence and presence of protein S. FVa inactivation was biphasic (k(506) = 4.4 x 10(8) M(-)(1) s(-)(1), k(306) = 2.7 x 10(7) M(-)(1) s(-)(1)), and protein S accelerated Arg(306) cleavage approximately 10-fold. Preincubation of protein S with C4BP resulted in a total abrogation of protein S cofactor activity. C4BP also protected FVa from inactivation by APC in the absence of protein S. Control experiments with CLB-PS13, a monoclonal anti-protein S antibody, indicated that inhibition of FVa inactivation by C4BP was not mediated through contaminating traces of protein S in our reaction systems. Protection of FVa was prevented by a monoclonal antibody directed against the C4BP alpha-chain. Recombinant rC4BPalpha comprised of only alpha-chains also protected FVa, but in the presence of protein S, the level of protection was decreased, since rC4BPalpha lacks the beta-chain responsible for C4BP binding to protein S. A truncated C4BP beta-chain (SCR-1+2) inhibited protein S cofactor activity, but had no effect on FVa inactivation by APC in the absence of protein S. In conclusion, C4BP protects FVa from APC-catalyzed cleavage in a protein S-independent way through direct interactions of the alpha-chaims of C4BP with FVa and/or APC.

Blood coagulation

Under normal circumstances, the coagulation system is balanced in favour of anticoagulation. Thrombin is the key effector enzyme of the clotting cascade. Antagonists of vitamin K inhibit a vitamin-K-dependent post-translational modification of several coagulation proteins, which is required for these proteins to attain a phospholipid-binding conformation. Heparin stimulates the activity of antithrombin, a serine-protease inhibitor. Analysis of knock-out mice has shown the relative importance of the coagulation factors in vivo. Gene therapy may soon be a therapeutic option for inherited deficiencies of factors VIII and IX.

Regulation of factor VIIIa by human activated protein C and protein S: inactivation of cofactor in the intrinsic factor Xase

Factor VIIIa is a trimer of A1, A2, and A3-C1-C2 subunits. Inactivation of the cofactor by human activated protein C (APC) results from preferential cleavage at Arg336 within the A1 subunit, followed by cleavage at Arg562 bisecting the A2 subunit. In the presence of human protein S, the rate of APC-dependent factor VIIIa inactivation increased several-fold and correlated with an increased rate of cleavage at Arg562. (Active site-modified) factor IXa, blocked cleavage at the A2 site. However, APC-catalyzed inactivation of factor VIIIa proceeded at a similar rate independent of factor IXa, consistent with the location of the preferential cleavage site within the A1 subunit. Addition of protein S failed to increase the rate of cleavage at the A2 site when factor IXa was present. In the presence of factor X, cofactor inactivation was inhibited, due to a reduced rate of cleavage at Arg336. However, inclusion of protein S restored near original rates of factor VIIIa inactivation and cleavage at the A1 site, thus overcoming the factor X-dependent protective effect. These results suggest that in the human system, protein S stimulates APC-catalyzed factor VIIIa inactivation by facilitating cleavage of A2 subunit (an effect retarded in the presence of factor IXa), as well as abrogating protective interactions of the cofactor with factor X. (Blood. 2000;95:1714-1720)

Vitamin K-dependent proteins in the developing and aging nervous system

Although the warfarin embryopathy syndrome, with its neurologic and bone abnormalities, has been known for decades, the role of vitamin K in the brain has not been studied systematically. Recently, it was demonstrated that vitamin K-dependent carboxylase expression is temporally regulated in a tissue-specific manner with high expression in the nervous system during the early embryonic stages and with liver expression after birth and in adult animals. This finding, along with the discovery of wide distribution of the novel vitamin K-dependent growth factor, Gas6, in the central nervous system, provides compelling evidence of a biologic role of vitamin K during the development of the nervous system. In animals and bacteria, vitamin K was observed to influence the brain sulfatide concentration and the activity and synthesis of an important enzyme involved in brain sphingolipids biosynthesis. Taken together, previous research results point to a possible role of vitamin K in the nervous system, especially during its development. Hence, the knowledge of the biologic role of vitamin K in the brain may be important for unveiling the mechanisms of normal and pathologic development and aging of the nervous system. The role of the vitamin K-dependent protein Gas6 in activation of signal transduction events in the brain in light of the age-related changes in the nervous system is also discussed.

Clinically symptomatic central venous catheter-related deep venous thrombosis in newborns

The objective of this study was to evaluate the incidence of clinically symptomatic central venous catheter (CVC)-related deep venous thrombosis (DVT) in newborns and small infants and to try to identify clinical and genetic risk factors for catheter-related DVT among children with thrombotic complications. CVC was inserted in 44 consecutive infants (age range 0-90 d) during the period January 1990 to December 1995 in the neonatal intensive care unit (NICU) of Kuopio University Hospital in Kuopio. The symptoms of DVT were: syndrome of superior vena cava in 2, swelling at the CVC puncture site in 6 and repeated CVC obstructions in 2. The formation of DVT was verified by venography. Children with DVT (n = 10) had 26 (10-365, in total 623) catheter days compared with 9 d (1-155, in total 591) in patients without DVT (n = 26) (p < 0.005). The median (range) number of days from catheter insertion to diagnosis of DVT was 19 (7-210). CVC had to be removed from 11 (25%) children due to various complications. There was no DVT-related mortality. A positive family history with thromboembolic episodes at a young age was found in 3 of 10 families with a child suffering CVC-related DVT. The levels of coagulation inhibitors were evaluated at the age of 9-69 mo in all 10 (23%) children with CVC-related DVT. We detected no deficiencies in protein S, protein C or antithrombin III. One child was heterozygous for the point mutation (R506Q) in the factor V gene known to cause activated protein C resistance (APCR). We conclude that newborns with CVC are at great risk of DVT and that the aetiology of DVT can rarely be identified via measurements of coagulation inhibitors.

Interaction between protein S and complement C4b-binding protein (C4BP). Affinity studies using chimeras containing c4bp beta-chain short consensus repeats

Human C4b-binding protein (C4BP) is a regulator of the complement system and plays an important role in the regulation of the anticoagulant protein C pathway. C4BP can bind anticoagulant protein S, resulting in a decreased cofactor function of protein S for activated protein C. C4BP is a multimeric protein containing several identical alpha-chains and a single beta-chain (C4BPbeta), each chain being composed of short consensus repeats (SCRs). Previous studies have localized the protein S binding site to the NH2-terminal SCR (SCR-1) of C4BPbeta. To further localize the protein S binding site, we constructed chimeras containing C4BPbeta SCR-1, SCR-2, SCR-3, SCR-1+2, SCR-1+3, and SCR-2+3 fused to tissue-type plasminogen activator. Binding assays of protein S with these chimeras indicated that SCR-2 contributes to the interaction of protein S with SCR-1, since the affinity of protein S for SCR-1+2 was up to 5-fold higher compared with SCR-1 and SCR-1+3. Using an assay that measures protein S cofactor activity, we showed that cofactor activity was decreased due to binding to constructs that contain SCR-1. SCR-1+2 inhibited more potently than SCR-1 and SCR-1+3. SCR-3 had no additional effect on SCR-1, and therefore the effect of SCR-2 was specific. In conclusion, beta-chain SCR-2 contributes to the interaction of C4BP with protein S.

A plasma coagulation assay for an activated protein C-independent anticoagulant activity of protein S

To study the physiological importance of the activated protein C (APC)-independent anticoagulant activity of protein S, we developed an assay specific for this activity. The ability of protein S to prolong the clotting time in an APC-independent way was expressed as the ratio of the clotting time in a plasma sample divided by the clotting time in the presence of a polyclonal antibody against human protein S (both after 1:1 dilution in protein S-C4BP deficient plasma). The mean protein S-dependent anticoagulant ratio (PSdAR) was 1.53+/-0.18 in 34 healthy controls, and was significantly lower in 16 heterozygous protein S deficient patients (PSdAR=1.15+/-0.09). In plasmas from patients under oral anticoagulant therapy the mean PSdAR was within the range of the control population (1.50+/-0.18). The mean total protein S antigen level in these plasmas was 58%, suggesting a higher specific APC-independent anticoagulant activity of protein S in these patients than in normals. This functional protein S assay can be used for the laboratory diagnosis of protein S deficiency, and to study the mechanism of the APC-independent anticoagulant activity in plasma.

The SHBG-like region of protein S is crucial for factor V-dependent APC-cofactor function

Activated protein C (APC) regulates blood coagulation by degrading factor Va (FVa) and factor VIIIa (FVIIIa). Protein S is a cofactor to APC in the FVa degradation, whereas FVIIIa degradation is potentiated by the synergistic APC-cofactor activity of protein S and factor V (FV). To elucidate the importance of the sex-hormone-binding globulin (SHBG)-like region in protein S for expression of anticoagulant activity, a recombinant protein S/Gas6 chimera was constructed. It comprised the amino-terminal half of protein S and the SHBG-like region of Gas6, a structurally similar protein having no known anticoagulant properties. The protein S/Gas6 chimera expressed 40-50%, APC-cofactor activity in plasma as compared to wild-type protein S. In the degradation of FVa by APC, the protein S/Gas6 chimera was only slightly less efficient than wild-type protein S. In contrast, the protein S/Gas6 chimera expressed no FV-dependent APC-cofactor activity in a FVIIIa-degradation system. This demonstrates the SHBG-like region to be important for expression of APC-cofactor activity of protein S and suggests that the SHBG-like region of protein S interacts with FV during the APC-mediated inactivation of FVIIIa.

Activated protein C sensitivity, protein C, protein S and coagulation in normal pregnancy

A prospective study of activated protein C sensitivity, protein C, protein S, and other coagulation factors in 239 women during normal pregnancy was carried out. Protein C activity appeared unaffected by gestation, although an elevation of protein C activity was observed in the early puerperium. A fall in total and free protein S with increasing gestation was observed. Activated protein C sensitivity ratio (APC:SR) showed a progressive fall through pregnancy. This fall correlated with changes in factor VIIIc, factor Vc and protein S. 38% of subjects, with no evidence of Factor V Leiden or anticardiolipin antibodies, showed a low APC:SR (APC:SR <2.6) in the third trimester of pregnancy. Aside from a significant reduction in birth weight, no difference in pregnancy outcome was observed between these subjects and those with a normal APC:SR. Activated protein C sensitivity ratio, modified by pre-dilution of patient samples with factor V depleted plasma, showed no consistent trend with gestation.

The inherited basis of venous thrombosis

Venous thrombosis represents a manifestation of disordered hemostatic balance. The classical presentation is of pain and swelling of the lower limb, although clinical history and examination are notoriously misleading in reaching a diagnosis. A number of acquired predispositions have been associated with a tendency to thrombosis, such as immobilisation, surgery, malignancy and certain types of oral contraception, but in at least half of the instances no predisposition can be identified. A variety of genetic risk factors have also been identified. Mutations within the genes for antithrombin, protein C and protein S are associated with a venous thromboembolic phenotype. The commonest thrombophilic predisposition however is a variant of coagulation factor V, factor V Leiden, which results from a single amino acid substitution rendering the factor V molecule resistant to activated protein C. Factor V Leiden is present in approximately 5% of individuals of European origin, and is found in up to 40% of those with confirmed venous thrombosis. Increasingly it is recognised that venous thrombosis should be considered a polygenic disorder, with interactions between the various single gene defects which predispose to thrombosis, as well as normal genetic variation between individuals in the levels of both procoagulant and anticoagulant proteins, all determining which individuals will express the phenotype of venous thrombosis.

A laboratory approach to the evaluation of hereditary hypercoagulability

The concept of hypercoagulability and especially its evaluation in the clinical laboratory has changed dramatically during the last few years. The genetic basis and the mechanisms of the various factors responsible for hypercoagulability are briefly reviewed with emphasis on the most common genetic deficiencies. The major thrust of this review centers on the cost-effective approach to examining patients with a personal or family history of venous thrombosis. Several new concepts dealing with thrombotic risk are presented with a focus on the theory that multiple factors cause thrombosis in affected patients. A proposal for a cost-effective sequential testing scheme for the accurate diagnosis of hereditary hypercoagulability is discussed. The knowledge of thrombotic risk factors is evolving rapidly, requiring the clinical laboratory to remain flexible. Ultimately, the clinical laboratory must take a leading role in the diagnosis of hereditary thrombotic disease by serving as the consultant to the primary caregiver by providing an up-to-date and cost-effective evaluation.

Increased tissue factor-initiated prothrombin activation as a result of the Arg506 --> Gln mutation in factor VLEIDEN

The effect of the Arg506 --> Gln mutation in factor VLEIDEN on thrombin generation was evaluated in a reconstituted system using the purified components of the tissue factor (TF) pathway to thrombin and the components of the protein C pathway. Recombinant full-length tissue factor pathway inhibitor (RTFPI) was included in the system because of a previously observed synergistic inhibitory effect of TFPI and the protein C pathway on TF-initiated thrombin generation. Thrombin generation initiated by 1.25 pM factor VIIa.TF in the absence of the protein C pathway components occurs following an initiation phase, after which prothrombin is quantitatively converted to 1.4 microM thrombin. The factor VLEIDEN mutation did not influence thrombin generation in the reconstituted model in the absence of the protein C pathway. In the presence of 2.5 nM TFPI, 65 nM protein C, and 10 nM recombinant soluble thrombomodulin (Tm), thrombin generation catalyzed by normal factor V was abolished after the initial formation of 25 nM thrombin. In contrast, persistent thrombin generation was observed in the presence of factor VLEIDEN in the same system, although the rate of thrombin generation was slower compared with the reaction without protein C and Tm. The rate of thrombin generation with factor VLEIDEN increased with time and ultimately resulted in quantitative prothrombin activation. When the TFPI concentration was reduced to 1.25 nM, thrombin generation is still curtailed in the presence of normal factor V. In contrast, under similar conditions using factor VLEIDEN, the protein C pathway totally failed to down-regulate thrombin generation. The dramatic effect of a 50% reduction in TFPI concentration on the inhibitory potential of the protein C pathway on thrombin generation catalyzed by factor VLEIDEN suggests that the observed synergy between TFPI and the protein C pathway is directly governed by the TFPI concentration and by cleavage of the factor Va heavy chain at Arg506. This cleavage appears to have a dramatic regulatory effect in the presence of low concentrations of TFPI. Markedly increased thrombin generation in the presence of both 1.25 nM TFPI and factor VLEIDEN was also observed when antithrombin-III was added to the system to complete the natural set of coagulation inhibitors. Protein S (300 nM) had a minimal effect in the model on the inhibition of thrombin generation by protein C, Tm, and TFPI, with either normal factor V or factor VLEIDEN. Protein S also failed to significantly potentiate the action of the protein C pathway in the presence of antithrombin-III in reactions employing normal factor V or factor VLEIDEN. The absence of an effect of protein S in the model, which employs saturating concentrations of phospholipid, suggests that the reported interactions of protein S with coagulation factors are not decisive in the reaction. Altogether the data predict that TFPI levels in the lower range of normal values are a risk factor for thrombosis when combined with the Arg506 --> Gln mutation in factor VLEIDEN.

Sequence and functional relationships between androgen-binding protein/sex hormone-binding globulin and its homologs protein S, Gas6, laminin, and agrin

Androgen-binding protein/sex hormone-binding globulin (ABP/SHBG) is an extracellular binding protein that regulates the bioavailability of sex steroids. ABP/SHBG is closely related to the globular (G) domain of vitamin K-dependent protein S family of proteins and more distantly related to the G domains of several extracellular matrix proteins. ABP/SHBG appears to have evolved from the fusion of two ancestral G domains. Expanding evidence suggests that ABP/SHBG has other functions that are mediated through membrane binding, including signal transduction; however, the types of binding proteins (receptors) have not been identified. Sequence comparisons of ABP/SHBG with G domains of its homologs protein S, Gas6, laminin, and agrin have identified regions of ABP/SHBG that may bind receptors related to homolog receptors. These membrane receptors include beta-integrins, alpha-dystroglycan, and receptor tyrosine kinases. The G domains of laminin and related proteins have clearly evolved from a common ancestor to interact with specific receptors and binding proteins. It remains to be determined if ABP/SHBG followed this evolutionary pathway.

Protein S deficiency

The protein C (PC) pathway, with its cofactor protein S (PS), is an important natural antithrombotic mechanism. Patients with phenotypic PS deficiency may develop recurrent thrombosis during adulthood, with a probability of remaining free of thrombosis of about 50% at age 45. The molecular basis for hereditary PS deficiencies is highly heterogeneous, with a large spectrum of mutations that have various effects on the expression of the relevant allele.