Plasma contains protein S monomers and multimers with similar direct anticoagulant activity

Protein S-Leu17Pro disrupts the hydrophobicity of its signal peptide causing a proteasome-dependent degradation

INTRODUCTION

Protein S is a vitamin K-dependent glycoprotein with important anticoagulant, fibrinolytic, anti-inflammatory, anti-apoptotic, and cytoprotective functions. Congenital protein S deficiency is an autosomal dominant thrombophilia due to protein S gene (PROS1) variations. Our group identified a variation in PROS1 that translates into protein S deficiency: c.50 T > C (p.Leu17Pro). Here, we investigated the mechanisms by which this variation results in protein S deficiency.

MATERIALS AND METHODS

The effect of L17P substitution on protein S signal peptide was predicted by in silico (a computational prediction technique) analysis of hydrophobicity and signal peptide cleavage. Recombinant protein S was overexpressed in HEK293 and COS-7 cells. Intracellular kinetics and extracellular secretion of recombinant protein S-L17P were analyzed by western blotting and immunocytochemistry.

RESULTS

In silico hydrophobicity analysis showed that protein S-L17P had disrupted hydrophobic status in the h-region of its signal peptide. Under normal culture conditions, recombinant protein S -L17P was not detected in either transfectant cell lysates or medium. Upon treatment with a proteasome inhibitor, recombinant protein S-L17P was clearly detected in the cell lysate, but not in the culture medium. Recombinant protein S-L17P did not undergo post-translational modification with N-glycosylation, suggesting that the nascent polypeptide of recombinant protein S-L17P is not transported to the endoplasmic reticulum lumen, but is mislocalized to the cytosol.

CONCLUSION

PROS1-L17P variation translates into protein S deficiency. Protein S-L17P causes its cytosolic mislocalization resulting in its proteasome-dependent degradation.

Platelets compensate for poor thrombin generation in type 3 von Willebrand disease

In type 3 von Willebrand disease (VWD3), the most severe form with absent von Willebrand factor (VWF), the bleeding phenotype is variable. Platelet contribution to the hemostatic defect in VWD3 calls upon further studies. We investigated the contribution of platelets to in vitro thrombin generation (TG) and platelet procoagulant activity in VWD3. TG was assessed by calibrated automated thrombogram (CAT) in platelet-poor (PPP) and -rich plasma (PRP) from 9 patients before and in 6 patients also 30 min after receiving their regular VWF therapy. Responsiveness of PPP to FVIII and protein S was also investigated. TG data were compared with routine laboratory variables, rotational thromboelastometry (ROTEM) and platelet expression of P-selectin and phosphatidylserine in flow cytometry. Compared with healthy controls, TG was markedly decreased in VWD3 PPP (peak thrombin was 16% of normal median), but not in PRP (77% of normal median) (p = 0.002). Six out of nine patients (67%) were high responders in their platelet P-selectin, and 5/9 (56%) in phosphatidylserine expression. Replacement therapy improved TG in PPP, while in PRP TG only modestly increased or was unaffected. In PPP, FVIII levels associated with TG and in vitro FVIII-supplemented TG inclined up to threefold. Conversely, a FVIII inhibitory antibody reduced plasma TG in all, but especially in patients with remnant FVIII levels. Inhibition of protein S improved plasma TG, particularly at low FVIII levels. ROTEM failed to detect VWD3.In VWD3, TG is reduced in PPP and regulated by FVIII and protein S, but TG is close to normal in PRP. VWD3 platelets seem to compensate for the FVIII-associated reduction in TG by their exposure of P-selectin and phosphatidylserine.

Multifocal avascular necrosis in a patient with refractory immune thrombocytopenia and antiphospholipid antibodies; case report and review of literature

Avascular necrosis (AVN) is a devastating condition that is rarely reported in patients with immune thrombocytopenia (ITP). Treatment with steroids remains a major risk factor for developing AVN. However, the incidence of AVN in patients with ITP requiring corticosteroid therapy is much less than that observed with other clinical conditions requiring corticosteroids. ITP is a bleeding disorder but can be also be a pro-thrombotic state via different mechanisms and thus could result in AVN. Among the possible causes of this pro-thrombotic state is the presence of antiphospholipid antibodies (aPLs). In this case, we report a patient with refractory ITP who developed multifocal AVN around the time she acquired new aPLs. We also discuss different mechanisms by which risk of thrombosis is increased in ITP and the relationship between ITP, aPLs and antiphospholipid syndrome.

Mouse macrophages show different requirements for phosphatidylserine receptor Tim4 in efferocytosis

Protein S (ProS) and growth arrest-specific 6 (Gas6) bind to phosphatidylserine (PtdSer) and induce efferocytosis upon binding TAM-family receptors (Tyro3, Axl, and Mer). Here, we produced mouse ProS, Gas6, and TAM-receptor extracellular region fused to IgG fragment crystallizable region in HEK293T cells. ProS and Gas6 bound Ca²⁺ dependently to PtdSer (K_d 20-40 nM), Mer, and Tyro3 (K_d 15-50 nM). Gas6 bound Axl strongly (K_d < 1.0 nM), but ProS did not bind Axl. Using NIH 3T3-based cell lines expressing a single TAM receptor, we showed that TAM-mediated efferocytosis was determined by the receptor-binding ability of ProS and Gas6. Tim4 is a membrane protein that strongly binds PtdSer. Tim4 alone did not support efferocytosis, but enhanced TAM-dependent efferocytosis. Resident peritoneal macrophages, Kupffer cells, and CD169⁺ skin macrophages required Tim4 for TAM-stimulated efferocytosis, whereas efferocytosis by thioglycollate-elicited peritoneal macrophages or primary cultured microglia was TAM dependent, but not Tim4 dependent. These results indicate that TAM and Tim4 collaborate for efficient efferocytosis in certain macrophage populations.

Mer receptor tyrosine kinase mediates both tethering and phagocytosis of apoptotic cells

Billions of inflammatory leukocytes die and are phagocytically cleared each day. This regular renewal facilitates the normal termination of inflammatory responses, suppressing pro-inflammatory mediators and inducing their anti-inflammatory counterparts. Here we investigate the role of the receptor tyrosine kinase (RTK) Mer and its ligands Protein S and Gas6 in the initial recognition and capture of apoptotic cells (ACs) by macrophages. We demonstrate extremely rapid binding kinetics of both ligands to phosphatidylserine (PtdSer)-displaying ACs, and show that ACs can be co-opsonized with multiple PtdSer opsonins. We further show that macrophage phagocytosis of ACs opsonized with Mer ligands can occur independently of a requirement for αV integrins. Finally, we demonstrate a novel role for Mer in the tethering of ACs to the macrophage surface, and show that Mer-mediated tethering and subsequent AC engulfment can be distinguished by their requirement for Mer kinase activity. Our results identify Mer as a receptor uniquely capable of both tethering ACs to the macrophage surface and driving their subsequent internalization.

TAM receptors, Gas6, and protein S: roles in inflammation and hemostasis

TAM receptors (Tyro3, Axl, and Mer) belong to a family of receptor tyrosine kinases that have important effects on hemostasis and inflammation. Also, they affect cell proliferation, survival, adhesion, and migration. TAM receptors can be activated by the vitamin K–dependent proteins Gas6 and protein S. Protein S is more commonly known as an important cofactor for protein C as well as a direct inhibitor of multiple coagulation factors. To our knowledge, the functions of Gas6 are limited to TAM receptor activation. When activated, the TAM receptors have effects on primary hemostasis and coagulation and display an anti-inflammatory or a proinflammatory effect, depending on cell type. To comprehend the effects that the TAM receptors and their ligands have on hemostasis and inflammation, we compare studies that report the different phenotypes displayed by mice with deficiencies in the genes of this receptor family and its ligands (protein S+/−, Gas6−/−, TAM−/−, and variations of these). In this manner, we aim to display which features are attributable to the different ligands. Because of the effects TAM receptors have on hemostasis, inflammation, and cancer growth, their modulation could make interesting therapeutic targets in thromboembolic disease, atherosclerosis, sepsis, autoimmune disease, and cancer.

Plasma protein S residues 37-50 mediate its binding to factor Va and inhibition of blood coagulation

Protein S (PS) is an anticoagulant plasma protein whose deficiency is associated with increased risk of venous thrombosis. PS directly inhibits thrombin generation by the blood coagulation pathways by several mechanisms, including by binding coagulation factors (F) Va and Xa. To identify PS sequences that mediate inhibition of FVa activity, antibodies and synthetic peptides based on PS sequence were prepared and employed in plasma coagulation assays, purified component prothrombinase assays, binding assays, and immunoblots. In the absence of activated protein C, monoclonal antibody (Mab) S4 shortened FXa-induced clotting in normal plasma but not in PS-depleted plasma. Mab S4 also blocked PS inhibition of FVa-dependent prothrombinase activity in purified component assays in the absence or presence of phospholipids and inhibited binding of PS to immobilised FVa. Epitope mapping identified N-terminal region residues 37-67 of PS as this antibody's epitope. A peptide representing PS residues 37-50 inhibited FVa-dependent prothrombinase activity in a non-competitive manner, with 50% inhibition observed at 11 µM peptide, whereas a peptide with a D-amino acid sequence of 37-50 was ineffective. FVa, but not FXa, bound specifically to the immobilised peptide representing residues 37-50, and the peptide inhibited binding of FVa to immobilised PS. These data implicate PS residues 37-50 as a binding site for FVa that mediates, at least in part, the direct inhibition of FVa-dependent procoagulant activity by PS.

Antithrombotic activity of protein S infused without activated protein C in a baboon thrombosis model

Protein S (ProS) is an essential plasma protein that enhances the anticoagulant activity of activated protein C (APC). In vitro , purified native human Zn2+-containing ProS also exerts direct anticoagulant activity by inhibiting prothrombinase and extrinsic FXase activities independently of APC. We investigated antithrombotic effects of ProS infused without APC in a baboon shunt model of thrombogenesis that employs a device consisting of arterial and venous shear flow segments. In in vitro experiments, the Zn2+-containing human ProS used for the studies displayed >10-fold higher prothrombinase inhibitory activity and anticoagulant activity in tissue factor-stimulated plasma, and four-fold higher inhibition of the intrinsic pathway than the Zn2+-deficient ProS used. In the thrombosis model, ProS (33 μg/minute for 1 hour) or saline was infused locally; platelet and fibrin deposition in the shunt were measured over 2 hours. During experiments performed at 50 ml/minute blood flow, Zn2+-containing ProS inhibited platelet deposition 73-96% in arterial-type flow segments and 90-99% in venous-type flow segments; Zn2+-deficient ProS inhibited platelet deposition 52% in arterial-type flow segments and 65-73% in venous-type flow segments. At 100 ml/min blood flow rate, Zn2+-containing ProS inhibited platelet deposition by 39% and 73% in the respective segments; Zn2+-deficient ProS inhibited platelet deposition by 5% and 0% in the respective segments. Zn2+-containing ProS suppressed fibrin deposition by 67-90%. Systemic APC-independent ProS activity was significantly increased and thrombin-antithrombin complex levels were significantly decreased after infusion of ProS. Thus, infused human Zn2+-containing ProS is antithrombotic in primates, and may have therapeutic potential even in protein C-deficient human patients.

Zn²(+) -containing protein S inhibits extrinsic factor X-activating complex independently of tissue factor pathway inhibitor

BACKGROUND

Protein S (PS) has direct anticoagulant activity, independently of activated protein C (APC). The mechanisms underlying this activity remain unclear, because PS preparations differ in activity, giving rise to conflicting results. Some purification procedures result in loss of intramolecular Zn²(+) , which is essential for inhibition of prothrombinase.

OBJECTIVE

To investigate the inhibition of extrinsic factor (F)Xase by Zn²(+) -containing PS.

METHODS

Purified component extrinsic FXase assays were used to determine FXa generation in the presence and absence of PS and/or tissue factor pathway inhibitor (TFPI). Binding assays, immunoblots and thrombin generation assays in plasma supported the FXase data.

RESULTS

Zn²(+) -containing PS potently inhibited extrinsic FXase in the presence of saturating phospholipids, independently of TFPI, whereas inhibition of extrinsic FXase by Zn²(+) -deficient PS required TFPI. Immunoblots for FXa and functional assays showed that Zn²(+) -containing PS inhibited primarily the quantity of FXa formed by tissue factor (TF)-FVIIa, rather than FXa amidolytic activity. Zn²(+) -containing PS, but not Zn²(+) -deficient PS, bound to TF with high affinity (K(dapp) = 41 nm) and targeted TF function. Binding of PS to FVIIa was negligible, whereas PS showed appreciable binding to FX. Increasing FX concentrations 10-fold reduced PS inhibition five-fold, suggesting that PS inhibition of FXase is FX-dependent. PS also exhibited TFPI-independent and APC-independent anticoagulant activity during TF-initiated thrombin generation in plasma.

CONCLUSIONS

PS that retains native Zn²(+) also retains anticoagulant functions independently of APC and TFPI. Inhibition of extrinsic FXase by PS at saturating levels of phospholipids depends on PS retention of intramolecular Zn²(+) , interaction with FX, and, particularly, interaction with TF.

The Kunitz-3 domain of TFPI-alpha is required for protein S-dependent enhancement of factor Xa inhibition

Protein S (PS) enhances the inhibition of factor Xa (FXa) by tissue factor pathway inhibitor-alpha (TFPI-alpha) in the presence of Ca(2+) and phospholipids. Altered forms of recombinant TFPI-alpha were used to determine the structures within TFPI-alpha that may be involved in this PS-dependent effect. Wild-type TFPI-alpha (TFPI(WT)), TFPI-alpha lacking the K3 domain (TFPI-(DeltaK3)), and TFPI-alpha containing a single amino acid change at the putative P1 residue of K3 (R199L, TFPI(K3P1)) produced equivalent FXa inhibition in the absence of PS, whereas the response in FXa inhibition produced by PS was reduced with TFPI(K3P1) (EC(50) 61.8 +/- 13.4nM vs 8.0 +/- 0.4nM for TFPI(WT)) and not detectable with TFPI-(DeltaK3). Ligand blotting and surface plasmon resonance experiments demonstrated that FXa bound TFPI(WT) and TFPI-(DeltaK3) but not the isolated K3 domain, whereas PS bound TFPI(WT) and the K3 domain but not TFPI-(DeltaK3). Addition of TFPI(WT), TFPI(K3P1), or TFPI-(DeltaK3) produced comparable prolongation of FXa-induced coagulation in PS-deficient plasma, but the anticoagulant effect of TFPI(WT) was substantially greater than that of TFPI(K3P1) > TFPI-(DeltaK3) in normal plasma and PS-deficient plasma reconstituted with PS. We conclude that the PS-mediated enhancement of FXa inhibition by TFPI-alpha involves an interaction between PS and TFPI-alpha, which requires the K3 domain of TFPI-alpha.

Protein S as cofactor for TFPI

In the last decades evidence was obtained that protein S not only acts as cofactor of activated protein C (APC) in the downregulation of coagulation, but also expresses anticoagulant activity in the absence of APC. The search for the mechanism(s) underlying the APC-independent anticoagulant activity of protein S was hampered by the fact that protein S exhibited 2 seemingly identical anticoagulant activities in model systems and in plasma. Later it was shown that the anticoagulant activity of purified protein S in model systems was dependent on the concentration of phospholipid vesicles and was explained by low amounts of protein S multimers generated during purification that effectively inhibited phospholipid-dependent coagulation reactions via competition for phospholipid binding sites. Plasma does not contain multimers, and the anticoagulant activity of protein S in plasma was not affected by the phospholipid concentration but was dependent on the amount of tissue factor (TF) used for initiation of thrombin generation. This led to the discovery that protein S acts as cofactor of tissue factor pathway inhibitor (TFPI) which stimulates the inhibition of factor Xa by TFPI approximately 10-fold. The current review describes the background of the TFPI-cofactor activity of protein S as well as the rationale for the observation that the TFPI/protein S system particularly inhibits the TF pathway at low procoagulant stimuli.

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.

The thrombin-sensitive region of protein S mediates phospholipid-dependent interaction with factor Xa

To test the hypothesis that factor Xa (fXa) interacts with protein S, fXa was labeled active-site specifically with a dansyl (D) dye via a Glu-Gly-Arg (EGR) tether to yield DEGR-fXa(i). When protein S was added to phosphatidylcholine/phosphatidylserine (PC/PS, 4:1) vesicle-bound DEGR-fXa(i), the anisotropy of the dansyl moiety was altered from 0.219 +/- 0.002 to 0.245 +/- 0.003. This change in dansyl anisotropy was not observed when DEGR-Xa(i) was titrated with protein S in the absence of PC/PS vesicles, or in the presence of 100% PC vesicles, or when PC/PS vesicle-bound DEGR-fXa(i) was titrated with thrombin-cleaved protein S. The protein S-dependent dansyl fluorescence change was specific for fXa because it was not observed for two homologous and similarly labeled DEGR-fIXa(i) and DEGR-fVIIa(i). Furthermore, protein S specifically and saturably altered the fluorescence anisotropy of PC/PS-bound active site-labeled LWB-FPR-fXa(i) (Kd = 33 nm) and was photocross-linked to PC/PS-bound LWB-FPR-fXa(i) analog, independently confirming the above results. Chemically synthesized microprotein S, comprising residues 1-116 of protein S and including the gamma-carboxyglutamic-rich domain, the thrombin-sensitive region (TSR), and the first epidermal growth factor-like domain (EGF1) of protein S, altered the anisotropy of PC/PS-bound DEGR-fXa(i) from 0.219 to 0.242, similar to the effect of the protein S titration (Kd = 303 nm), suggesting that microprotein S binds to DEGR-fXa(i). To identify individual protein S domain(s) that binds DEGR-fXa(i), the EGF1 and TSR domains were chemically synthesized and studied. The TSR altered the anisotropy of DEGR-fXa(i) by approximately 16% (Kd = 3.9 microm), but the EGF1 domain had no effect on the signal. In controls, the TSR domain did not alter the anisotropy of DEGR-fIXa(i) and DEGR-fVIIa(i), respectively. These data demonstrate that membrane-bound fXa binding to protein S involves the TSR of protein S.