High-density lipoprotein enhancement of anticoagulant activities of plasma protein S and activated protein C

Low-density lipoprotein (LDL) and high-density lipoprotein (HDL) cholesterol levels are associated, respectively, with either increased risk or apparent protective effects for atherothrombosis. The ability of purified LDL and HDL to downregulate thrombin formation, a contributor to atherothrombotic processes, was assessed. Purified HDL, but not LDL, significantly enhanced inactivation of coagulation factor Va by activated protein C (APC) and protein S, and HDL stimulated protein S–dependent proteolytic inactivation of Va by APC, apparently due to cleavage at Arg306 in Va. In normal plasma, added HDL enhanced APC/protein S anticoagulant activity in modified prothrombin-time clotting assays. When the anticoagulant potency of HDL was compared with phospholipid (PL) vesicles of well-defined composition using this assay, HDL appeared qualitatively different from PL vesicles because HDL showed only good anticoagulant activity, whereas PL vesicles were rather procoagulant. When 20 normal plasmas were tested using this clotting assay, apoA-I levels correlated with anticoagulant response to APC/protein S (r = 0.47, P = 0.035), but not with activated partial thromboplastin time–based APC resistance ratios. Because HDL enhances the anticoagulant protein C pathway in vitro, we speculate that HDL may help downregulate thrombin generation in vivo and that this anticoagulant action is one of HDL's beneficial activities.     

Cholesterol enhances phospholipid-dependent activated protein C anticoagulant activity

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

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.     

Peptidomimetic inhibitors for activated protein C: implications for hemophilia management

BACKGROUND: Several clinical studies and experiments with transgenic mice have suggested that the severity of the bleeding phenotype in hemophilic patients is substantially reduced in association with impaired inactivation of factor (F) Va by activated protein C (APC) in the presence of the FV Leiden mutation. Experiments using a synthetic coagulation proteome model showed that the presence of FV Leiden significantly increased thrombin generation in the absence of FVIII or FIX. OBJECTIVE: To test the effect of APC inhibition on thrombin generation in hemophilia. METHODS: Prothrombinase and a synthetic coagulation proteome model of tissue factor-triggered thrombin generation were used. RESULTS: Peptide-based APC inhibitors, which mimic the P4-P4' residues surrounding the APC cleavage site at Arg306 of FVa, were synthesized. These compounds are specific and reversible inhibitors of APC, with Ki values as low as 1-2 microM; most have insignificant affinity for FXa or thrombin. The affinity for APC is dependent upon the location and character of the protecting groups. Representatives of this group of compounds inhibit FVa inactivation by APC and prolong FVa functional activity in the prothrombinase complex. When evaluated in a synthetic coagulation proteome model, one inhibitor partially compensated for the absence of FVIII. CONCLUSIONS: Synthetic APC inhibitors may be useful as adjuvants for hemophilia treatment.     

Does activated protein C‐resistant factor V contribute to thrombin generation in hemophilic plasma?

In this study we assessed the role of factor V (FV) inactivation in hemophilic plasma with particular reference to the activated protein C (APC)-resistant variants FV-R506Q (FV Leiden) and FV-R306T (FV Cambridge). Purified recombinant full-length FV carrying these single substitutions and FV-R306T/R506Q were used in thrombin generation experiments. Plasma was first immunodepleted of FV, and subsequently of factors VIII, IX, or combinations thereof. Thrombin generation was initiated by low concentrations of recombinant tissue factor. Recombinant soluble thrombomodulin (TM) was used to trigger the APC system. Surprisingly, TM concentrations that reduced thrombin generation in normal plasma by no more than 50% virtually abolished thrombin formation in plasma deficient in the factor VIII/IX complex. This was already apparent at TM levels as low as 0.1 nmol L(-1). By varying the concentrations of purified (activated) protein C to plasma that was additionally depleted of protein C, we confirmed that impaired thrombin generation indeed was the result of the action of APC. In contrast, this did not occur when FV-depleted plasma had been reconstituted with FV-R306T/R506Q. Addition of FV-R306T or FV-R506Q partially reduced prothrombin activation, demonstrating the involvement of both APC cleavage sites. FV inactivation also occurred on the surface of human microvascular endothelial cells. Apparently, these cells express sufficient TM to down-regulate thrombin production via the APC pathway. We further conclude that in hemophilic plasma this pathway can induce a secondary defect because of premature FV inactivation. It therefore seems conceivable that APC-resistant FV has the potential of alleviating hemophilic bleeding.     

Inactivation of human factor VIII by activated protein C. Cofactor activity of protein S and protective effect of von Willebrand factor.

Activated protein C (APC) acts as a potent anticoagulant enzyme by inactivating Factor V and Factor VIII. In this study, protein S was shown to increase the inactivation of purified Factor VIII by APC ninefold. The reaction rate was saturated with respect to the concentration of protein S when protein S was present in a 10-fold molar excess over APC. The heavy chain of Factor VIII was cleaved by APC and protein S did not alter the degradation pattern. Factor VIII circulates in a complex with the adhesive protein von Willebrand factor. When purified Factor VIII was recombined with von Willebrand factor, the inactivation of Factor VIII by APC proceeded at a 10-20-fold slower rate as compared with Factor VIII in the absence of von Willebrand factor. Protein S had no effect on the inactivation of the Factor VIII-von Willebrand factor complex by APC. After treatment of this complex with thrombin, however, the actions of APC and protein S towards Factor VIII were completely restored. In hemophilia A plasma, purified Factor VIII associated with endogenous von Willebrand factor, resulting in a complete protection against APC (4 nM). By mixing hemophilic plasma with plasma from a patient with severe von Willebrand's disease, we could vary the amount of von Willebrand factor. 1 U of von Willebrand factor was needed to provide protection of 1 U Factor VIII. Also in plasma from patients with the IIA-type variant of von Willebrand's disease, Factor VIII was protected. In von Willebrand's disease plasma, which was depleted of protein S, APC did not inactivate Factor VIII. These results indicate that protein S serves as a cofactor in the inactivation of Factor VIII and Factor VIIIa by APC and that von Willebrand factor can regulate the action of these two anticoagulant proteins.     

Modulation of heparin cofactor II activity by histidine-rich glycoprotein and platelet factor 4.

Heparin cofactor II is a plasma protein that inhibits thrombin rapidly in the presence of either heparin or dermatan sulfate. We have determined the effects of two glycosaminoglycan-binding proteins, i.e., histidine-rich glycoprotein and platelet factor 4, on these reactions. Inhibition of thrombin by heparin cofactor II and heparin was completely prevented by purified histidine-rich glycoprotein at the ratio of 13 micrograms histidine-rich glycoprotein/microgram heparin. In contrast, histidine-rich glycoprotein had no effect on inhibition of thrombin by heparin cofactor II and dermatan sulfate at ratios of less than or equal to 128 micrograms histidine-rich glycoprotein/microgram dermatan sulfate. Removal of 85-90% of the histidine-rich glycoprotein from plasma resulted in a fourfold reduction in the amount of heparin required to prolong the thrombin clotting time from 14 s to greater than 180 s but had no effect on the amount of dermatan sulfate required for similar anti-coagulant activity. In contrast to histidine-rich glycoprotein, purified platelet factor 4 prevented inhibition of thrombin by heparin cofactor II in the presence of either heparin or dermatan sulfate at the ratio of 2 micrograms platelet factor 4/micrograms glycosaminoglycan. Furthermore, the supernatant medium from platelets treated with arachidonic acid to cause secretion of platelet factor 4 prevented inhibition of thrombin by heparin cofactor II in the presence of heparin or dermatan sulfate. We conclude that histidine-rich glycoprotein and platelet factor 4 can regulate the antithrombin activity of heparin cofactor II.     

Identification of functional endothelial protein C receptor in human plasma.

The endothelial cell protein C receptor (EPCR) binds protein C and facilitates activation by the thrombin-thrombomodulin complex. EPCR also binds activated protein C (APC) and inhibits APC anticoagulant activity. In this study, we detected a soluble form of EPCR in normal human plasma. Plasma EPCR appears to be approximately 43, 000 D, and circulates at approximately 100 ng/ml (98.4+/-27.8 ng/ml, n = 22). Plasma EPCR was purified from human citrated plasma using ion exchange, immunoaffinity, and protein C affinity chromatography. Flow cytometry experiments demonstrated that plasma EPCR bound APC with an affinity similar to that previously determined for recombinant soluble EPCR (Kdapp = 30 nM). Furthermore, plasma EPCR inhibited both protein C activation on an endothelial cell line and APC anticoagulant activity in a one-stage Factor Xa clotting assay. The physiological function of plasma EPCR is uncertain, but if the local concentrations are sufficiently high, particularly in disease states, the present data suggest that the soluble plasma EPCR could attenuate the membrane-bound EPCR augmentation of protein C activation and the anticoagulant function of APC.     

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.     

Inhibitory effect of activated protein C and heparin on arterial thrombosis]

Activated protein C (APC) exerts an anticoagulant effect by inactivating coagulation factors Va and VIIIa, preferentially on the surface of the cell membrane, and may enhance fibrinolysis by inhibiting PAI-1. APC inhibits venous and microvascular thrombosis, as well as, platelet-rich thrombus formation on the Dacron graft inserted in the arteriovenous shunt. We examined the effect of APC and heparin on arterial thrombosis using small mesenteric arteries of the rat, in which platelet-rich thrombus formation was induced by segmental deep vessel injury and the changes were monitored using a video-microscope system. Both APC (0.9 and 3.0 mg/kg, iv) and heparin (300 and 1000 U/kg) inhibited thrombus formation similarly. APC did not prolong APTT compared with heparin. APC may inhibit arterial thrombosis after vascular damage without serious bleeding side effects.     

The measurement of thrombin in clotting blood by radioimmunoassay.

We have developed a radioimmunoassay for human thrombin using rabbit anti-human thrombin IgG. The assay can measure 2 ng thrombin/ml plasma, 500-fold more sensitive than clotting assays. Human prothrombin is less reactive in the assay than thrombin by at least four orders of magnitude, and there is no demonstrable cross-reactivity with human factor Xa, the clotting factor structurally most similar to thrombin. The assay does not detect thrombin bound to anthithrombin III. Using the assay, we have demonstrated that plasma from 20 normal subjects does not contain detectable thrombin. We measured thrombin generation in clotting blood in polypropylene tubes and observed that thrombin appears (approximately equal to 3 ng/ml) within 45 S-5 min after venipuncture. This material is thrombin, not intermediates of prothrombin activation, since it disappears after addition of heparin, which promotes thrombin antithrombin III complex formation. After a plateau of 2-10 min, there is further thrombin generation, which results in clotting after 15-27 min at a level of 40-50 ng thrombin/ml. The thrombin generated 9-25 min before clotting may activate factors V and VIII and stimulate platelet aggregation and release. In contrast, the cascade hypothesis assigns a role for thrombin only late in blood clotting. Radioimmunoassay of thrombin and other clotting factors will be useful for clinical and physiological studies of blood clotting especially since the assay seems specific for thrombin and is independent of other activities that affect bioassays.     

Coagulation protein function: the influence of acetaldehyde-modified heparin on thrombin activity.

BACKGROUND: The affect of acetaldehyde-treated heparin on thrombin activity has been investigated using factor II-deficient human plasma. METHODS: It was observed that 0.021 units of heparin exerts a marked inhibition of thrombin activity (1.03 units) as measured by clotting times, prolonging the clotting times from 9.6 +/- 0.1 seconds to 24.8 +/- 0.1 seconds. However, when the heparin is preincubated with 447 mmol/L acetaldehyde at RT for 30 minutes prior to mixing with thrombin, a clotting time in excess of 200 seconds is observed. Clotting times remain elevated with heparin-acetaldehyde mixtures of 89.4, 17.9, 3.6, and 0.72 mmol/L acetaldehyde, with corresponding clotting times of > 200, 156.0 +/- 2.1, 81.6 +/- 1.0, 38.8 +/- 0.6 seconds, respectively. At 140 mumol/L acetaldehyde-heparin mixtures, the clotting time was 17.0 +/- 2.0 seconds. RESULTS: These data support the hypothesis from this laboratory that acetaldehyde-modified heparin enhances coagulation time. They further indicate that thrombin is targeted by the acetaldehyde-treated heparin. Heparin-acetaldehyde mixtures also reacted with plasma prior to the addition of thrombin to modestly prolong coagulation time. Similarly, but more effectively, thrombin/heparin mixtures increased the clotting time of acetaldehyde-exposed plasma. These data further suggest the possibility that reactions of acetaldehyde and heparin are not restricted to those with thrombin, and that they may extend to other blood factors/proteins. CONCLUSIONS: The amount of heparin (0.021 units) required to substantially affect clotting time of thrombin (1.03 units) is substantially lower than that required to prolong clotting of 0.1 mL of whole plasma (0.36 units), by an order of magnitude. It is inferred that heparin may interact with numerous cationic proteins or proteins with cationic domains in blood plasma, among them being the clotting factors.     

Inhibition of coagulation and inflammation by activated protein C or antithrombin reduces intestinal ischemia/reperfusion injury in rats

OBJECTIVE: To examine whether administration of activated protein C or antithrombin reduces local splanchnic derangement of coagulation and inflammation and attenuates intestinal dysfunction and injury following intestinal ischemia/reperfusion. DESIGN: Randomized prospective animal study. SETTING: University research institute. SUBJECTS: Adult male Wistar rats, weighing 300-325 g (n = 72). INTERVENTIONS: Rats were subjected to superior mesenteric artery occlusion consisting of 20 or 40 mins of ischemia and 3 hrs of reperfusion. A randomized intravenous administration of vehicle (0.9% NaCl), heparin, antithrombin, or activated protein C was performed during ischemia, 15 mins before reperfusion. Coagulation and fibrinolysis variables obtained from portal blood were correlated with mucosal fibrin deposition (determined by anti-rat fibrin antibody staining), intestinal function (glucose/water clearance), and intestinal injury (histologic evaluation by Park/Chiu score). MEASUREMENTS AND MAIN RESULTS: Activated protein C- or antithrombin-treated animals demonstrated less ischemia/reperfusion-induced intestinal dysfunction and histologic changes compared with control animals, whereas intravenous administration of heparin only showed less histologic derangement. Activated protein C- or antithrombin-treated animals showed less thrombin generation, fibrin degradation products, and fibrin deposition compared with control animals, as confirmed by histologic examination, whereas heparin administration showed only a limited reduction of portal fibrin degradation product concentrations. Furthermore, activated protein C or antithrombin administration markedly inhibited the inflammatory response, as reflected by reduced interleukin-6 plasma concentrations to baseline values, whereas heparin had no effect. CONCLUSIONS: Administration of activated protein C or antithrombin inhibited local and systemic derangement of coagulation and inflammation following intestinal ischemia/reperfusion, diminished mucosal fibrin deposition, and attenuated ischemia/reperfusion-induced intestinal injury. These observations suggest that activated protein C or antithrombin reduces ischemia/reperfusion-induced intestinal injury, both through their anticoagulant and anti-inflammatory effects.     

Pharmacologic properties of an unfractionated heparin butyryl derivative with long-lasting effects.

This article reports on the pharmacologic properties of an O-acylated butyryl derivative (C4-UH) of unfractionated heparin (UH). In a purified system, the ability of C4-UH to catalyze the inhibition of thrombin and of factor Xa in the presence of antithrombin III was similar to that of UH. Addition of albumin (10 mg/ml) to the reagents reduced the antithrombin and antifactor Xa catalytic potency of C4-UH 68-fold and 20-fold, respectively, and did not alter those of UH. As judged from the prolongation of the activated partial thromboplastin time and the thrombin clotting time, the anticoagulant activities of C4-UH were two times weaker than those of UH. After calibration against UH, the antifactor Xa-specific and antithrombin-specific activities were two and 6.6 times lower, respectively. After bolus intravenous injection into rabbits, the apparent clearances of C4-UH were reduced 2.4 (antifactor Xa activity) and 3.2 times (antithrombin activity) in comparison with those of UH. This property accounted for the higher plasma concentrations generated during a constant infusion of the same dose. In the Wessler thromboplastin model, the minimum doses providing the maximum antithrombotic effect after bolus injection were equivalent for both compounds when expressed as antifactor Xa units; the duration of the antithrombotic effect of this derivative was prolonged, whereas the hemorrhagic potential was unaffected. This study opens a new concept for heparin derivatives having lower clearances and long-lasting effects. These properties could be linked to nonspecific binding of C4-UH to plasma proteins, thereby reducing the amount of free compound available to interact with antithrombin III.     

Coagulation factors and the protein C system as determinants of thrombin generation in a normal population.

BACKGROUND: Thrombin generation is a powerful tool to probe overall plasma coagulability. OBJECTIVE: To determine which plasma factors influence the various parameters of the thrombin generation curve, for example lag time, peak height and endogenous thrombin potential (ETP), under different experimental conditions. PATIENTS AND METHODS: Plasma levels of coagulation factors and inhibitors, as well as thrombin generation at 1 pm tissue factor (TF) +/- thrombomodulin (TM) and at 13.6 pm TF +/- activated protein C (APC), were determined in plasma from 140 healthy individuals. Data were analysed by multiple regression models. RESULTS: Thrombin generation increased with age and was higher in females than in males. Under all conditions, the lag time was mainly dependent on the levels of free tissue factor pathway inhibitor (TFPI), free protein S (PS), factor VII (FVII), FIX and fibrinogen. The major determinants of thrombin generation (ETP and peak height) at 1 pm TF were fibrinogen, FXII (despite inhibition of contact activation), free TFPI and antithrombin (AT), both in the absence and in the presence of TM. Thrombin generation in the presence of TM was also dependent on protein C levels. At 13.6 pm TF, thrombin generation was determined by prothrombin, AT, fibrinogen, free TFPI and FV levels in the absence of APC, and by free TFPI, free PS and FX levels in the presence of APC. CONCLUSIONS: The lag time, ETP and peak height of thrombin generation depend on the levels of multiple coagulation factors and inhibitors. The specific assay determinants vary with the experimental conditions.     

Elevated thrombin-forming capacity of tissue factor-activated cord compared with adult plasma

Summary.   Clinically observed excellent hemostasis in neonates despite low levels of clotting factors is not completely understood so far. Therefore, we investigated whether physiological low levels of the inhibitor protein C (PC) facilitate thrombin formation in tissue factor (TF)-activated plasma samples. PC was activated by endogenously generated thrombin after addition of soluble thrombomodulin (TM). The capability of activated PC (APC) to suppress thrombin formation was significantly more pronounced in adult than in cord plasma. Addition of 4 n m of TM decreased the thrombin potential (TP) in cord plasma by 10%, and in adult plasma by 52% in the presence of 5 p m TF. We demonstrate that this low anticoagulant action of PC is attributable to the low levels of tissue factor pathway inhibitor (TFPI) and antithrombin (AT) physiologically present in cord plasma. Addition of 4 n m TM decreased the TP by 58% in cord plasma adjusted to contain TFPI and AT at adult levels in the presence of 5 p m TF. Thus, the combined low anticoagulant action of the three inhibitors APC, TFPI, and AT in cord plasma allows enhanced thrombin formation associated with shorter clotting times compared with adult plasma when low amounts of TF are applied to initiate clot formation. Although our laboratory experiments do not allow definite conclusions for various clinical situations, our data might contribute to explain excellent hemostasis in neonates despite low levels of procoagulants.     

Activated protein C

Summary.  Protein C is a vitamin K-dependent plasma protein zymogen whose genetic mild or severe deficiencies are linked with risk for venous thrombosis or neonatal purpura fulminans, respectively. Studies over past decades showed that activated protein C (APC) inactivates factors (F) Va and VIIIa to down-regulate thrombin generation. More recent basic and preclinical research on APC has characterized the direct cytoprotective effects of APC that involve gene expression profile alterations, anti-inflammatory and anti-apoptotic activities and endothelial barrier stabilization. These actions generally require endothelial cell protein C receptor (EPCR) and protease activated receptor-1. Because of these direct cytoprotective actions, APC reduces mortality in murine endotoxemia and severe sepsis models and provides neuroprotective benefits in murine ischemic stroke models. Furthermore, APC reduces mortality in patients with severe sepsis (PROWESS clinical trial). Although much remains to be clarified about mechanisms for APC's direct effects on various cell types, it is clear that APC's molecular features that determine its antithrombotic action are partially distinct from those providing cytoprotective actions because we have engineered recombinant APC variants with selective reduction or retention of either anticoagulant or cytoprotective activities. Such APC variants can provide relatively enhanced levels of either cytoprotective or anticoagulant activities for various therapeutic applications. We speculate that APC variants with reduced anticoagulant action but normal cytoprotective actions hold the promise of reducing bleeding risk because of attenuated anticoagulant activity while reducing mortality based on direct cytoprotective effects on cells.     

Protein C inhibitor directly and potently inhibits activated hepatocyte growth factor activator

Background:  Hepatocyte growth factor (HGF) plays an important role in tissue repair and regeneration. HGF activator (HGFA), a factor XIIa-like serine protease, activates HGF precursor to HGF. The precursor of HGFA, proHGFA, is activated by thrombin generated at sites of tissue injury. It is known that protein C inhibitor (PCI), an inhibitor of activated protein C (APC), also inhibits thrombin–thrombomodulin (TM) complex. Objectives:  In the present study we evaluated the effect of PCI on thrombin-catalyzed proHGFA activation in the presence of TM, and on HGFA activity. Results:  PCI did not inhibit thrombin-TM-mediated proHGFA activation, but it directly inhibited activated HGFA by forming an enzyme inhibitor complex. The second-order rate constants ( m ⁻¹ min⁻¹) of the reaction between HGFA and PCI in the presence or absence of heparin (10 U mL⁻¹) were 4.3 × 10⁶ and 4.0 × 10⁶, respectively. The inhibition of HGFA by PCI resulted in a significant decrease of HGFA-catalyzed activation of HGF precursor. Exogenous HGFA added to normal human plasma formed a complex with plasma PCI, and this complex formation was competitively inhibited by APC in the presence of heparin, but very weakly in the absence of heparin. We also demonstrated using recombinant R362A-PCI that Arg362 residue of PCI is important for HGFA inhibition by PCI as judged from the three-dimensional structures constructed using docking models of PCI and HGFA or APC. Conclusion:  These observations indicate that PCI is a potent inhibitor of activated HGFA, suggesting a novel function for PCI in the regulation of tissue repair and regeneration.     

Regulation of inflammatory responses by activated protein C: the molecular mechanism(s) and therapeutic implications.

Activated protein C (APC), a natural anticoagulant, is formed from protein C by the action of the thrombin-thrombomodulin (TM) complex on the endothelial cell surface. Endothelial protein C receptor augments the activation of protein C by the thrombin/TM system. APC inactivates the activated form of coagulation factors V and VIII in the presence of protein S. Administration of APC reduced the pulmonary vascular injury and hypotension as well as the coagulation abnormalities by inhibiting production of the tumor necrosis factor-alpha (TNF-alpha) in rats given endotoxin (ET). These therapeutic effects of APC could not be attributed to its anticoagulant effects. APC inhibited ET-induced TNF-alpha production in human monocytes by inhibiting activation of nuclear factor K-B and activator protein-1 in vitro. Administration of the human plasma-derived APC ameliorated coagulation abnormalities without any adverse effects in patients with disseminated intravascular coagulation (DIC). Recombinant APC was reported to reduce the mortality of patients with severe sepsis, and the therapeutic effect was more marked in such patients with overt DIC than those without it. These observations strongly suggest that APC plays important roles in the regulation of inflammation as well as coagulation. Both anti-inflammatory and anticoagulant properties of APC might contribute to the therapeutic usefulness in patients with severe sepsis.     

Laboratory assessment of Activated Protein C Resistance/Factor V-Leiden and performance characteristics of a new quantitative assay

Activated Protein C Resistance is mainly associated to a factor V mutation (RQ506), which induces a deficient inactivation of activated factor V by activated protein C, and is associated to an increased risk of venous and arterial thrombosis in affected individuals, caused by the prolonged activated factor V survival. Its prevalence is mainly in Caucasians (about 5%), and this mutation is absent in Africans and Asians. Presence of Factor V-Leiden is usually evidenced with clotting methods, using a two-step APTT assay performed without or with APC: prolongation of blood coagulation time is decreased if this factor is present. The R506Q Factor V-Leiden mutation is now usually characterized using molecular biology, and this technique tends to become the first intention assay for characterization of patients. Both techniques are qualitative, and allow classifying tested individuals as heterozygotes or homozygotes for the mutation, when present. A new quantitative assay for Factor V-Leiden, using a one-step clotting method, has been developed, and designed with highly purified human coagulation proteins. Clotting is triggered with human Factor Xa, in presence of calcium and phospholipids (mixture which favours APC action over clotting process). Diluted tested plasma, is supplemented with a clotting mixture containing human fibrinogen, prothrombin, and protein S at a constant concentration. APC is added, and clotting is initiated with calcium. Calibration is performed with a pool of plasmas from patients carrying the R506Q Factor V mutation, and its mixtures with normal plasma. Homozygous patients have clotting times of about <40 sec; heterozygous patients have clotting times of about 40–60 sec and normal individuals yield clotting times >70 sec. Factor V-Leiden concentration is usually >75% in homozygous patients, 30-60% in heterozygous patients and below 5% in normal. The assay is insensitive to clotting factor deficiencies (II, VII, VIII: C, IX, X), dicoumarol or heparin therapies, and has no interference with lupus anticoagulant (LA).This new assay for Factor V-Leiden can be easily used in any coagulation laboratory, is performed as a single test, and is quantitative. This assay has a high robustness, is accurate and presents a good intra- (<3%) and inter-assay (<5%) variability. It contributes solving most of the laboratory issues faced when testing factor V-Leiden. Quantitation of Factor V-L could contribute to a better assessment of thrombotic risk in affected patients, as this complication is first associated to and caused by the presence of a defined amount of FVa.