Articles de revues : « Apc protein »

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Brugge, Jeroen, Guido Tans, Jan Rosing et Elisabetta Castoldi. « Protein S levels modulate the activated protein C resistance phenotype induced by elevated prothrombin levels ». Thrombosis and Haemostasis 95, n^o 02 (2006) : 236–42. http://dx.doi.org/10.1160/th05-08-0582.

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SummaryElevated plasma prothrombin levels, due to the prothrombin 20210 G/A mutation or to acquired causes, area risk factor for venous thrombosis,partly because of prothrombin-mediated inhibition of the protein C anticoagulant pathway and consequent activated proteinC (APC) resistance. We determined the effect of plasma prothrombin concentration on the APC resistance phenotype and evaluated the role of protein S levels asa modulating variable. The effect of prothrombin and protein S levels on APC resistance was investigated in reconstituted plasma systems and in a population of healthy individuals using both the aPTT-based and the thrombin generation-based APC resistance tests. In reconstituted plasma, APC resistance increased at increasing prothrombin concentration in both assays. Enhanced APC resistance was caused by the effect of prothrombin on the clotting time in the absence of APC in the aPTT-based test, and on thrombin formation in the presence of APC in the thrombin generation-based test. In plasma from healthy individuals prothrombin levels were highly correlated to protein S levels. Since prothrombin and proteinS had opposite effects on the APC resistance phenotype, the prothrombin/protein S ratio was a better predictor of APC resistance than the levels of either protein alone. Prothrombin titrations in plasmas containing different amounts of proteinS confirmed that proteinS levels modulate the ability of prothrombin to induce APC resistance. These findings suggest that carriers of the prothrombin 20210 G/A mutation, who have a high prothrombin/protein S ratio, may experience a higher thrombosis risk than non-carriers with comparable prothrombin levels.

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Sen, Prosenjit, Sanghamitra Sahoo, Usha Pendurthi et L. Vijaya Mohan Rao. « Zinc Binding to Protein C and Activated Protein C Modulates Their Interaction with Endothelial Cell Protein C Receptor. » Blood 114, n^o 22 (20 novembre 2009) : 331. http://dx.doi.org/10.1182/blood.v114.22.331.331.

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Abstract Abstract 331 Introduction/background: Zinc is a multi-functional element that is essential for life and the second most abundant metal ion, after iron in eukaryotic organisms. Zinc deficiency has been associated with bleeding disorders and with defective platelet aggregation, suggesting it may play an important role in hemostasis. Zinc ions have been shown to enhance activation of the intrinsic pathway of coagulation but to down-regulate the extrinsic pathway of coagulation. All vitamin K-dependent coagulation proteins have calcium binding sites and may therefore to some extent, interact with other divalent metal ions, including zinc, through these sites. Recent crystallography studies identified a pair of Zn2+ binding sites in FVIIa protease domain, and with the exception of Glu220, all the side chains involved in both the Zn1 and Zn2 coordination in FVIIa are unique to FVIIa and are not present in other vitamin K-dependent clotting factors (Bajaj et al., J Biol Chem 2006; 281:24873-88). Nonetheless, Zn2+ may bind to other vitamin K-dependent clotting factors at sites different from those identified in FVIIa. Objective: The aim of the present study is to investigate the effect of zinc ions on the protein C pathway, particularly on protein C/APC binding to EPCR, protein C activation and APC catalytic activity. Methods: Protein C and APC binding to EPCR on endothelial cells was examined by radioligand binding studies. Protein C activation and APC catalytic activity were evaluated in chromogenic assays. Equilibrium dialysis was used to measure zinc binding to protein C/APC. Conformational changes in protein C/APC were monitored by intrinsic fluorescence quenching. Results: Zn2+ does not replace the Ca2+ as a mandatory cofactor for protein C/APC binding to EPCR but Zn2+ at physiologically relevant concentrations (10 to 25 μM) markedly increased Ca2+-dependent protein C and APC binding to EPCR (∼2 to 5-fold). The kinetic analysis of protein C and APC binding to EPCR suggested that Zn2+ enhanced protein C/APC binding to EPCR by increasing the binding affinity of protein C/APC to its receptor (Kd for APC: – Zn2+, 117 ± 27 nM; + Zn2+, 9.3 ± 3.3 nM; Kd, for protein C: – Zn2+, 96 ± 26 nM; + Zn2+, 21.4 ± 6.6 nM). The enhancing effect of Zn2+ on APC binding to EPCR was also observed in the presence of physiological concentrations of Mg2+, which itself increased the APC binding to EPCR, two-fold. Zn2+-mediated increased protein C binding to EPCR resulted in increased APC generation. The effect of Zn2+ was not limited to enhancing protein C and APC binding to EPCR but also affected the catalytic activity of APC. Zn2+ inhibited the amidolytic activity of APC half-maximally at 50 to 100 μM. Zn2+ also inhibited the amidolytic activity of Gla domain deleted (GD)-APC in a similar fashion. The inhibitory effect of Zn2+ was partially reversed by physiological concentrations of calcium. Addition of Zn2+ to protein C or APC quenched the intrinsic fluorescence of both APC and GD-APC. Data from the equilibrium binding studies performed with 65Zn2+ revealed that Zn2+ binds to both GD-APC and APC, but that the amount of Zn2+ bound to APC was 3 to 4-fold higher than the amount bound to GD-APC. Kinetic analysis of equilibrium binding studies suggested that two Zn2+ atoms bind to APC outside the Gla domain with relatively high affinity (∼70 μM). At least one of the Zn2+ sites may overlap with the Ca2+ binding site as the Zn2+ binding to GD-APC was inhibited by approximately 50% by saturating concentrations of Ca2+. The substantially increased Zn2+ binding to the APC compared to GD-APC suggested that the N-terminus of the Gla domain of protein C contains multiple Zn2+ binding sites. Interestingly, Zn2+ bound to APC and GD-APC with a similar high affinity suggesting that the Gla domain, as well as the protease domain, may contain high affinity binding sites for Zn2+. A majority of the Zn2+ binding sites in the Gla domain appear to be distinct from the Ca2+ binding sites as less than 40% of the maximal Zn2+ binding could be blocked by Ca2+. The putative zinc binding sites in protein C/APC appeared to be unique as no consensus canonical zinc binding sequences homologous to other known zinc binding proteins were found in protein C. Conclusions: Our present data show that Zn2+ binds to protein C/APC and induces a conformational change in these proteins, which in turn leads to higher affinity binding to their cellular receptor EPCR. Overall our results suggest that zinc ions may play an important regulatory role in the protein C pathway. Disclosures: No relevant conflicts of interest to declare.

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Preston, Roger JS, Shona Harmon, Fionnuala B. Ni Ainle, Jennifer A. Johnson, Moya Cunningham, O. Smith, Barry White et James S. O’Donnell. « Dissociation of Activated Protein C Functions by Elimination of Protein S Cofactor Enhancement ». Blood 112, n^o 11 (16 novembre 2008) : 21. http://dx.doi.org/10.1182/blood.v112.11.21.21.

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Abstract Activated protein C (APC) plays a critical anticoagulant role by inactivating factor Va (FVa) and factor VIIIa (FVIIIa) and thus down-regulating thrombin generation. In addition, APC bound to the endothelial cell protein C receptor (EPCR) can initiate PAR-1 mediated cytoprotective signalling. Although protein S constitutes a critical cofactor for APC anticoagulant function, the molecular basis through which protein S interacts with APC is not fully understood. In this study, we employed a site-directed mutagenesis strategy to characterise the effects of four single amino acid substitutions (D35T, D36A, L38D and A39V) within a region of the APC Gla domain important for protein S cofactor enhancement. To maintain Gla domain structural integrity, each residue was substituted with the corresponding residue of the human prothrombin Gla domain. Protein C variants were expressed in HEK 293 cells and purified by ion-exchange chromatography. Upon activation, the amidolytic activity of each recombinant APC variant was identical to that of wild type APC. The anticoagulant function of recombinant wild type and variant APC was compared in a tissue factor-initiated thrombin generation assay using protein C-deficient plasma. Wild type APC diminished thrombin generation in a concentration-dependent manner as expected. Variants APC-D35T, APC-D36A and APC-A39V exhibited only mildly impaired (<2-fold) anticoagulant activity compared to wild type APC. The anticoagulant activity of APC-L38D, however, was severely impaired. APC-L38D was unable to achieve half-maximal inhibition of endogenous thrombin potential (ETP) at APC concentrations as high as 150nM, compared to wild type APC, which achieved half-maximal inhibition at 7.2nM APC. To clarify the role of Leu-38 in facilitating APC anticoagulant function, we further studied the ability of APC-L38D to be stimulated in protein S-deficient plasma reconstituted with plasma-purified protein S. Co-incubation of wild type APC with increasing protein S concentration (12.5–200nM) caused a corresponding reduction in ETP (IC50 = 24nM protein S). In contrast, APC-L38D was unresponsive to protein S. In the presence of APC-L38D, ETP was reduced only 22% at 1.5μM protein S (10-fold higher than plasma free protein S). In a phospholipid-dependent FVa proteolysis time course assay, both wild type APC and APC-L38D rapidly reduced FVa cofactor activity, indicating that the observed impaired plasma anticoagulant activity of APC-L38D is not mediated by impaired interaction with anionic phospholipids or FVa. In a modified version of this assay, wild type APC-mediated FVa proteolysis was rapidly enhanced by added protein S, with half-maximal inhibition observed at 5nM protein S. In contrast, APC-L38D exhibited no protein S-enhanced FVa proteolysis. Cumulatively, these data confirm that Leu-38 mediates APC anticoagulant function in plasma by facilitating critical protein S cofactor enhancement of FVa proteolysis. Previous studies have shown that APC Gla domain mutations can influence EPCR binding, a pre-requisite for PAR-1 mediated cytoprotective signalling. Consequently, we assessed APC binding to sEPCR using surface plasmon resonance. Binding affinities of APC-L38D and wild type APC were very similar (KD 112±25nM versus 117±36nM). Furthermore, the ability of APC-L38D to protect EAhy926 cells from staurosporine-induced apoptosis was also investigated using RT-PCR quantification of pro- (bax) and anti- (bcl-2) apoptotic gene expression. Pre-incubation with APC-L38D significantly reduced the bax/bcl-2 ratio to the same extent as wild type APC. The EPCR-dependence of these anti-apoptotic activities was confirmed using RCR-252, (an inhibitory anti-EPCR antibody) which ablated the cytoprotective effect of both APC species. In conclusion, we demonstrate that a single amino acid substitution (L38D) can significantly impair APC anticoagulant activity due to elimination of protein S cofactor enhancement. However, despite the location of Leu-38 in the Gla domain, APC-L38D retains its ability to bind EPCR, and trigger PAR-1 mediated cytoprotective signalling in a manner indistinguishable from that of wild type APC. Consequently, elimination of protein S cofactor enhancement of APC anticoagulant function represents a novel and effective strategy by which to dissociate the anticoagulant and cytoprotective functions of APC for potential therapeutic gain.

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Marx, Antje, Hans Weiler, Volker Liebe, Siegfried Lang, Jens Kaden, Wolfgang Zieger, Martin Borggrefe, Guenter Huhle, Karl Haase et Martina Brueckmann. « Stabilization of monocyte chemoattractant protein-1-mRNA by activated protein C ». Thrombosis and Haemostasis 89, n^o 01 (2003) : 149–60. http://dx.doi.org/10.1055/s-0037-1613554.

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SummaryThe activated protein C (APC) pathway has been suggested to be a common link between coagulation and inflammation. APC may function to restore hemostasis via modulation of cytokine expression. We investigated the effect of APC on the endothelial expression of monocyte chemoattractant protein-1 (MCP-1), a chemokine that is controlled by the activation of central proinflammatory transcription factors, such as nuclear factor-kappa B (NF-κ B).We found that human APC (2.5-10 μ g/ml) upregulated the amount of MCP-1-mRNA in human umbilical vein endothelial cells (HUVEC) and caused a time- and dose-dependent increase in MCP-1 protein production (p <0.001 for APC 2.5 μg/ ml at 4 up to 24 h). In this cell culture model MCP-1 induced an improvement of cell migration and wound repair after injury to endothelial monolayers. After stimulation of MCP-1-mRNA-transcription with TNF-α (0.1-1 ng/ml), HUVEC’s were washed and an inhibitor of gene transcription, Actinomycin D (1 μg/ml), was added in the presence or absence of APC. HUVEC’s receiving APC contained more MCP-1-mRNA than controls after one hour and up to eight hours suggesting an inhibitory effect of APC on MCP-1-mRNA degradation (with APC: 753 ± 56 atto mol of MCP-1-mRNA per ml of cell lysate vs. 263 ± 60 atto mol/ml without APC at t =4 h; p <0.001). Electrophoretic mobility shift assays revealed that APC attenuated NF- κB DNA-binding capacity implying that NF- B may not be involved in the upregulatory effect of APC on MCP-1 production.The ability of APC to upregulate the production of MCP-1, most likely by increasing the stability of MCP-1-mRNA rather than by transcriptional activation via NF- B, identifies a novel immunomodulatory pathway, by which APC may control the local inflammatory reaction, thereby initiating wound repair and modulating the extent of endothelial injury.

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Maurissen, Lisbeth F. A., M. Christella L. G. D. Thomassen, Gerry A. F. Nicolaes, Björn Dahlbäck, Guido Tans, Jan Rosing et Tilman M. Hackeng. « Re-evaluation of the role of the protein S-C4b binding protein complex in activated protein C-catalyzed factor Va-inactivation ». Blood 111, n^o 6 (15 mars 2008) : 3034–41. http://dx.doi.org/10.1182/blood-2007-06-089987.

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AbstractProtein S expresses cofactor activity for activated protein C (APC) by enhancing the APC-catalyzed proteolysis at R306 in factor Va. It is generally accepted that only free protein S is active and that complex formation with C4b-binding protein (C4BP) inhibits the APC-cofactor activity of protein S. However, the present study shows that protein S-C4BP expresses APC-cofactor activity and stimulates APC-catalyzed proteolysis at R306 more than 10-fold, but instead inhibits proteolysis at R506 by APC 3- to 4-fold. Free protein S stimulates APC-catalyzed cleavage at R306 approximately 20-fold and has no effect on cleavage at R506. The resulting net effect of protein S-C4BP complex formation on APC-catalyzed factor Va inactivation is a 6- to 8-fold reduction in factor Va inactivation when compared with free protein S, which is not explained by inhibition of APC-cofactor activity of protein S at R306, but by generation of a specific inhibitor for APCcatalyzed proteolysis at R506 of factor Va. These results are of interest for carriers of the factor VLeiden mutation (R506Q), as protein S-C4BP effectively enhances APC-catalyzed factor Va (R306) inactivation in plasma containing factor VLeiden.

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Nishioka, Junji, Masaru Ido, Tatsuya Hayashi et Koji Suzuki. « The Gla26 Residue of Protein C Is Required for the Binding of Protein C to Thrombomodulin and Endothelial Cell Protein C Receptor, but not to Protein S and Factor Va ». Thrombosis and Haemostasis 75, n^o 02 (1996) : 275–82. http://dx.doi.org/10.1055/s-0038-1650260.

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SummaryA functionally defective protein C (PC)-Mie, detected in the plasma of a patient with hereditary thrombophilia, has Lys substituted for γ-carboxyglutamic acid (Gla)26 residue. The activation rate of PC-Mie by Protac or thrombin in the absence of Ca2+ and that by thrombin with native thrombomodulin (TM), recombinant soluble truncated TM or on cultured endothelial cells in the presence of Ca2+ were all apparently lower than that of normal PC. The anticoagulant activity of Protac-activated PC (APC)-Mie on the plasma clotting time and the rate of inactivation of factor Va by APC-Mie in the presence of phospholipids were lower than those seen with normal APC. APC-Mie and normal APC bound equally to protein S and to biotinyl-factor Va. However, neither PC-Mie nor APC-Mie bound to phospholipids and to cultured human endothelial cells. It was similar to that observed with Gla-domainless PC/APC, but different from that seen with normal PC/APC. These results suggest that Gla26-dependent conformation is required for the binding of PC/APC to phospholipids, TM and the surface of endothelial cell PC/APC receptor, but not to protein S and factor Va.

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Pérez-Casal, Margarita, Colin Downey, Kenji Fukudome, Gernot Marx et Cheng Hock Toh. « Activated protein C induces the release of microparticle-associated endothelial protein C receptor ». Blood 105, n^o 4 (15 février 2005) : 1515–22. http://dx.doi.org/10.1182/blood-2004-05-1896.

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Abstract Activated protein C (APC) treatment is now used for patients with severe sepsis. We investigated its effect in vitro on primary, physiologically relevant cells and demonstrate a novel mechanism of endothelial protein C receptor (EPCR) release that is not inhibited by metalloproteinase inhibitors. Exposure of human umbilical vein endothelial cells or monocytes to APC (6.25-100 nM) results in the release of EPCR-containing microparticles, as demonstrated by confocal microscopy and characterized through flow cytometry, enzyme-linked immunosorbent assay quantitation of isolated microparticles, and Western blotting. The phenomenon is time- and concentration-dependent and requires the APC active site, EPCR, and protease activated receptor 1 (PAR1) on endothelial cells. Neither protein C nor boiled or d-Phe-Pro-Arg-chloromethylketone–blocked APC can induce microparticle formation and antibody blockade of EPCR or PAR1 cleavage and activation abrogates this APC action. Coincubation with hirudin does not alter the APC effect. The released microparticle bound is full-length EPCR (49 kDa) and APC retains factor V–inactivating activity. Although tumor necrosis factor-α (10 ng/mL) can also induce microparticle-associated EPCR release to a similar extent as APC (100 nM), it is only APC-induced microparticles that contain bound APC. This novel observation could provide new insights into the consequences of APC therapy in the septic patient.

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Preston, Roger JS, Jennifer A. Johnson, Fionnuala Ni Ainle, Shona Harmon, Owen P. Smith, Barry White et James S. O’Donnell. « Platelet Factor 4 Mediates Activated Protein C Resistance by Impairment of Protein S Cofactor Enhancement ». Blood 112, n^o 11 (16 novembre 2008) : 20. http://dx.doi.org/10.1182/blood.v112.11.20.20.

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Abstract Platelet factor 4 (PF4) is an abundant platelet α-granule chemokine released following platelet activation. PF4 interacts with thrombomodulin and the γ-carboxyglutamic acid (Gla) domain of protein C to significantly enhance activated protein C (APC) generation by the thrombin-thrombomodulin complex on the surface of endothelial cells. However, the protein C Gla domain not only mediates protein C activation in vivo, but also plays a critical role in modulating the diverse functional properties of APC once generated. The functional consequences of the interaction between the APC Gla domain and PF4 in relation to APC anticoagulant, anti-inflammatory and anti-apoptotic functions have not previously been fully defined. In a tissue factor-initiated thrombin generation assay, APC impaired thrombin generation as previously described. However PF4 inhibited APC anticoagulant activity in a concentration-dependent manner (IC50 for PF4 inhibition of APC anticoagulant function, 11μg/ml). In contrast, addition of two other cationic polypeptides protamine and polybrene, both significantly enhanced APC anticoagulant activity in plasma. To elucidate the mechanism through which PF4 inhibits APC anticoagulant activity, we utilized a phospholipid-dependent FVa proteolysis time course assay. In the absence of protein S, PF4 had no effect upon FVa proteolysis by APC, indicating that PF4 does not influence the ability of APC to interact with either anionic phospholipids or FVa. However, in the presence of protein S, PF4 significantly inhibited APC-mediated FVa proteolysis (3–5 fold). Collectively, these findings demonstrate that in addition to enhancing APC generation, PF4 also significantly attenuates APC anticoagulant activity in plasma by impairing critical protein S cofactor enhancement of FVa proteolysis, and suggest that PF4 contributes to the poorly-understood APC resistance phenotype associated with activated platelets. APC bound to the endothelial cell protein C receptor (EPCR) via its Gla domain can activate PAR-1 on endothelial cells, triggering complex intracellular signaling that result in anti-inflammatory and anti-apoptotic cellular responses. To ascertain whether PF4 interaction with the protein C/APC Gla domain might impair APC-EPCR-PAR-1 cytoprotective signaling, APC protection against thrombin-induced endothelial barrier permeability and staurosporine-induced apoptosis in the presence of PF4 was determined. APC significantly attenuated thrombin-induced endothelial cell barrier permeability, as expected. PF4 alone (up to 1μM) had no independent effect upon endothelial barrier permeability, and did not protect against thrombin-mediated increased permeability. In contrast to its inhibition of APC anticoagulant activity, PF4 did not significantly inhibit the endothelial barrier protective properties of APC. To determine whether PF4 might interfere with APC-mediated cytoprotection, staurosporine-induced apoptosis in EAhy926 cells was assessed by RT-PCR quantification of pro-apoptotic (Bax) to anti-apoptotic (Bcl-2) gene expression. Pre-treatment of EAhy926 cells with APC decreased the Bax/Bcl-2 ratio close to that determined for untreated EAhy926 cells. PF4 alone, or in combination with APC, had no effect upon apoptosis-related gene expression as determined by alteration of Bax/Bcl-2 expression ratios in response to staurosporine. In summary, PF4 inhibits APC anticoagulant function via inhibition of essential protein S cofactor enhancement in plasma, whilst retaining EPCR/PAR-1 mediated cytoprotective signalling on endothelial cells. This provides a rationale for how PF4 can exert prothrombotic effects in vivo, but also mediate enhanced APC generation on the surface of endothelial cells to induce both anti-inflammatory and anti-apoptotic events. Based on these observations, we propose that PF4 acts as a critical regulator of APC generation in vivo, but also targets APC towards cytoprotective, rather than anticoagulant functions at sites of vascular injury with concurrent platelet activation.

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de Fouw, N. J., Y. F. de Jong, F. Haverkate et R. M. Bertina. « Activated Protein C Increases Fibrin Clot Lysis by Neutralization of Plasminogen Activator Inhibitor No Evidence for a Cofactor Role of Protein S ». Thrombosis and Haemostasis 60, n^o 02 (1988) : 328–33. http://dx.doi.org/10.1055/s-0038-1647055.

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summaryThe effect of purified human activated protein G (APC) on fibrinolysis was studied using a clot iysis system consisting of purified glu-plasminogen, tissue-type plasminogen activator, plasminogen activator inhibitor (released from endothelial cells or blood platelets), fibrinogen, 125T-fibrinogen and thrombin. All proteins were of human origin.In this system APC could increase fibrinolysis in a dose dependent way, without affecting fibrin formation or fibrin crosslinking. However, this profibrinolytic effect of APC could only be observed when plasminogen activator inhibitor (PAI-l) was present. The effect of APC was completely quenched by pretreatment of APC with anti-protein C IgG or di-isopropylfluorophosphate. Addition of the cofactors of APC:protein S, Ca2+-ions and phospholipid-alone or in combination did not enhance the profibrinolytic effect of APC. These observations indicate that human APC can accelerate in vitro clot lysis by the inactivation of PAI-1 activity. However, the neutralization of PAI-1 by APC is independent of the presence or absence of protein S, phospholipid and Ca2+-ions.

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Deane, Rashid, Barbra LaRue, Abhay P. Sagare, Francis J. Castellino, Zhihui Zhong et Berislav V. Zlokovic. « Endothelial Protein C Receptor-Assisted Transport of Activated Protein C across the Mouse Blood—Brain Barrier ». Journal of Cerebral Blood Flow & ; Metabolism 29, n^o 1 (8 octobre 2008) : 25–33. http://dx.doi.org/10.1038/jcbfm.2008.117.

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Activated protein C (APC), a serine-protease with anticoagulant, anti-inflammatory, and cytoprotective activities, is neuroprotective and holds potential to treat different neurologic disorders. It is unknown whether APC crosses the blood—brain barrier (BBB) to reach its therapeutic targets in the brain. By using a brain vascular perfusion technique, we show that 125I-labeled plasma-derived mouse APC enters the brain from cerebrovascular circulation by a concentration-dependent mechanism. The permeability surface area product of 125I-APC (0.1 nmol/L) in different forebrain regions ranged from 3.11 to 4.13 μL/min/g brain. This was approximately 80- to 110-fold greater than for 14C-inulin, a simultaneously infused reference tracer. The Km value for APC BBB cortical transport was 1.6±0.2 nmol/L. Recombinant APC variants with reduced anticoagulant activity, 5A-APC and 3K3A-APC, but not protein C, exhibited high affinity for the APC BBB transport system. Blockade of APC-binding site on endothelial protein C receptor (EPCR), but not blockade of its protease-activated receptor-1 (PAR1) catalytic site, inhibited by > 85% APC entry into the brain. APC brain uptake was reduced by 64% in severely deficient EPCR mice, but not in PAR1 null mice. These data suggest that APC and its variants with reduced anticoagulant activity cross the BBB via EPCR-mediated saturable transport.

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Van Walderveen, Maria Christina, Leslie R. Berry et Anthony Chan. « Covalent Antithrombin-Heparin Complex Catalysis of Activated Protein C Inhibition by Protein C Inhibitor. » Blood 114, n^o 22 (20 novembre 2009) : 3167. http://dx.doi.org/10.1182/blood.v114.22.3167.3167.

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Abstract Abstract 3167 Poster Board III-107 Introduction Thrombin (IIa), when bound to thrombomodulin (TM), readily converts protein C (PC) into activated PC (APC). APC functions as an anticoagulant by inactivating activated factors V (FVa) and VIII (FVIIIa), and indirectly reduces IIa generation. Once formed, APC activity is controlled through inhibition by PC inhibitor (PCI), a reaction that is catalyzed by heparin (H). Chan et al. developed a covalent antithrombin-heparin (ATH) complex with increased anticoagulant activity compared to H. The current investigation looked at the role and mechanism by which ATH affects the inhibition of APC by PCI. Methods Discontinous second order rate constant inhibition assays of APC+PCI, in the presence of ATH or unfractionated H (UFH), were performed. These experiments were repeated in the presence of low molecular weight H (LMWH) or ATH (LMWATH), or a high molecular weight ATH (HMWATH). The affinity of ATH or UFH for APC or PCI was assessed using native electrophoresis and APC or PCI immobilized onto agarose beads. Results Second order rate experiments revealed that, at peak values, ATH (k2=2.0 × 107 ± 1.2 × 106) was a significantly slower catalyst of APC inhibition by PCI than UFH (k2= 3.0 × 107 ± 2.0 × 106; p=0.005). The peak reaction rate value for UFH occurred at a higher catalyst concentration (300nM) compared to ATH (60nM). LMWH was a poor catalyst of APC inhibition by PCI (k2= 4.2 × 105 ± 8.2 × 104). However, both LMWATH and HMWATH had high catalytic function (k2= 4.1 × 107 ± 2.1 × 106 and k2= 2.7 × 107 ± 4.8 × 105, respectively). Although UFH and ATH were able to bind to either APC or PCI at a pH of 7.3, increasing the pH to 8.8 eliminated all binding affinity, except that of ATH for APC. Conclusion UFH functions mechanistically through a template-mediated effect by bridging APC with PCI. The catalytic function of UFH is dependent upon H chain length, as LMWH was ineffective at enhancing APC inhibition by PCI. On the other hand, ATH may act mainly through conformational APC activation. There was no H chain length dependence for ATH, as LMWATH was able to significantly catalyze PCI inhibition of APC. We speculate that ATH's AT moiety may assist interaction with APC but prevents the H component of ATH from bridging the enzyme with the inhibitor. Compared to UFH, ATH is a more potent inhibitor of IIa, but is less effective at inhibiting APC and thereby decreasing IIa generation. These results add to the growing evidence that ATH is a superior anticoagulant to UFH. Disclosures No relevant conflicts of interest to declare.

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Brinkman, Herm Jan, Erica Sellink, Bas de Laat et Koen Mertens. « Differential Anticoagulant Effects of Protein S on Vascular Cells and Platelets. » Blood 112, n^o 11 (16 novembre 2008) : 2026. http://dx.doi.org/10.1182/blood.v112.11.2026.2026.

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Abstract Background: Protein S is a vitamin K-dependent plasma protein and involved in down-regulation of the coagulation process. Protein S serves as a cofactor of activated protein C (APC) in the proteolytic inactivation of activated factor V and VIII. Protein S is also able to exert its anticoagulant activity independent of APC, e.g. by supporting the anticoagulant activity of tissue factor pathway inhibitor (TFPI). The anticoagulant properties of protein S have been thoroughly characterized by in vitro methods. However, fewer studies focus on protein S function on vascular cells. These studies are limited to model systems employing purified coagulation factors. The aim of this study was to investigate the role of protein S in plasma that is in contact with natural cell membranes, including endothelial cells, smooth muscle cells and platelets. Method: We employed thrombography to evaluate protein S function in 50 % v/v recalcified citrated plasma in the presence of washed platelets, cultured umbilical vein endothelial cells or cultured umbilical artery smooth muscle cells. Since we aimed at a comparison between different cellular membranes, micro-particle free plasma was used. As a reference, we also examined synthetic phospholipid membranes composed of phosphatidyl serine, phosphatidyl choline and phosphatidyl ethanolamine in a 2/6/2 molar ratio. Thrombin activity was measured employing the fluorogenic substrate z-Gly-Gly- Arg-AMC. Protein S activity was probed with CLB-PS13, an antibody directed against the protein S Gla-domain. The APC-independent activity of protein S was assayed in the presence of an inhibitory antibody against protein C. In studies employing phospholipids, thrombin generation was triggered with relipidated tissue factor (TF). Expression of TF on endothelial cells was induced during a 6-hour preincubation with PMA. Results: In the presence of CLB-PS13, the APC-independent activity of protein S became apparent as an increase in peak height in the thrombogram. Lag time, time to peak and area under the curve remained essentially unaffected. Peak height was increased two-fold when examining phospholipids at standard conditions (4 μM lipids and 1 pM TF). This increase in peak height by CLB-PS13 was concentration dependent and complete at 10 μg/ml IgG. Increasing the TF concentration from 1 to 5 pM resulted in loss of the APC-independent activity of protein S on these membranes. APC cofactor activity was assessed in the presence of APC. Addition of APC resulted in inhibition of the thrombin formation on phospholipids with an IC50 of 0.4 nM. CLB-PS13 completely abolished this decrease in thrombin generation up to 50 nM APC, irrespective whether 1 or 5 pM TF was present. Our results are compatible with the view that at high procoagulant stimuli the TFPI-cofactor activity of protein S is abolished. Furthermore, these results show that in plasma APC is completely dependent on protein S. Protein S activity on platelets was studied in the presence of 1 pM TF. As for synthetic lipid membranes and in the absence of APC, CLB-PS13 increased the peak height in the thrombogram. APC inhibited platelet mediated thrombin generation (IC50 = 19 nM), and this inhibition was completely abolished by CLB-PS13. These observations suggest that platelets support both APC-dependent and APC-independent anticoagulant activities of protein S. Thrombography on TF-expressing endothelial cells and smooth muscle cells revealed massive thrombin generation that could not be enhanced with CLB-PS13, indicating that protein S does not contribute to regulation of thrombin formation under these conditions. The APC-dependent activity of protein S became apparent as an abolished inhibition by APC in the presence of CLB-PS13. However, APC proved relatively inefficient in inhibiting thrombin generation on TF-expressing vascular cells (IC50 > 50 nM). Inhibition of TF restored the APC-independent protein S activity. Conclusion: Our results indicate that, in plasma, vascular cells and platelets are able to support the APC-dependent as well as the APC-independent anticoagulant activities of protein S. The APC-independent activity on vascular cells is abolished upon increasing TF expression, while the APC-dependent activity of protein S is limited by the relatively low anticoagulant activity of APC on these cell surfaces. We conclude that protein S activity on cells require relatively high levels of APC or low expression of TF.

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Jörgensen, Pia-Marie, Eva Brundell, Maria Starborg et Christer Höög. « A Subunit of the Anaphase-Promoting Complex Is a Centromere-Associated Protein in Mammalian Cells ». Molecular and Cellular Biology 18, n^o 1 (1 janvier 1998) : 468–76. http://dx.doi.org/10.1128/mcb.18.1.468.

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ABSTRACT Sister chromatids in early mitotic cells are held together mainly by interactions between centromeres. The separation of sister chromatids at the transition between the metaphase and the anaphase stages of mitosis depends on the anaphase-promoting complex (APC), a 20S ubiquitin-ligase complex that targets proteins for destruction. A subunit of the APC, called APC-α in Xenopus (and whose homologs are APC-1, Cut4, BIME, and Tsg24), has recently been identified and shown to be required for entry into anaphase. We now show that the mammalian APC-α homolog, Tsg24, is a centromere-associated protein. While this protein is detected only during the prophase to the anaphase stages of mitosis in Chinese hamster cells, it is constitutively associated with the centromeres in murine cells. We show that there are two forms of this protein in mammalian cells, a soluble form associated with other components of the APC and a centromere-bound form. We also show that both the Tsg24 protein and the Cdc27 protein, another APC component, are bound to isolated mitotic chromosomes. These results therefore support a model in which the APC by ubiquitination of a centromere protein regulates the sister chromatid separation process.

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Bolte, Melanie, Patrick Dieckhoff, Cindy Krause, Gerhard H. Braus et Stefan Irniger. « Synergistic inhibition of APC/C by glucose and activated Ras proteins can be mediated by each of the Tpk1–3 proteins in Saccharomyces cerevisiae ». Microbiology 149, n^o 5 (1 mai 2003) : 1205–16. http://dx.doi.org/10.1099/mic.0.26062-0.

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Proteolysis triggered by the anaphase-promoting complex/cyclosome (APC/C) is essential for the progression through mitosis. APC/C is a highly conserved ubiquitin ligase whose activity is regulated during the cell cycle by various factors, including spindle checkpoint components and protein kinases. The cAMP-dependent protein kinase (PKA) was identified as negative regulator of APC/C in yeast and mammalian cells. In the yeast Saccharomyces cerevisiae, PKA activity is induced upon glucose addition or by activated Ras proteins. This study shows that glucose and the activated Ras2Val19 protein synergistically inhibit APC/C function via the cAMP/PKA pathway in yeast. Remarkably, Ras2 proteins defective in the interaction with adenylate cyclase fail to influence APC/C, implying that its function is regulated exclusively by PKA, but not by alternative Ras pathways. Furthermore, it is shown that the three PKAs in yeast, Tpk1, Tpk2 and Tpk3, have redundant functions in regulating APC/C in response to glucose medium. Single or double deletions of TPK genes did not prevent inhibition of APC/C, suggesting that each of the Tpk proteins can take over this function. However, Tpk2 seems to inhibit APC/C function more efficiently than Tpk1 and Tpk3. Finally, evidence is provided that Cdc20 is involved in APC/C regulation by the cAMP/PKA pathway.

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Joslyn, G., D. S. Richardson, R. White et T. Alber. « Dimer formation by an N-terminal coiled coil in the APC protein ». Proceedings of the National Academy of Sciences 90, n^o 23 (1 décembre 1993) : 11109–13. http://dx.doi.org/10.1073/pnas.90.23.11109.

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Mutations in the human APC gene are associated with an inherited predisposition to colon cancer. APC codes for polypeptides of approximately 2800 amino acids, with sequence homologies to coiled-coil proteins in the first 900 residues. To determine the oligomerization properties of the APC protein, we used genetic and biochemical approaches to examine the ability of APC fragments to self-associate. A subdomain comprising the first 55 amino acids of APC was found to form a stable, parallel, helical dimer, as expected for a coiled coil. The location of a key dimerization element at the N terminus of the protein supports models in which mutations in APC exert effects through dimerization of the mutant gene products.

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Wijnen, Merel van, Jeanette G. Stam, Cornells van't Veer, Joost C. M. Meijers, Pieter H. Reitsma, Rogier M. Bertina et Bonno N. Bouma. « The Interaction of Protein S with the Phospholipid Surface Is Essential for the Activated Protein C-independent Activity of Protein S ». Thrombosis and Haemostasis 76, n^o 03 (1996) : 397–403. http://dx.doi.org/10.1055/s-0038-1650590.

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SummaryProtein S is a vitamin-K dependent glycoprotein involved in the regulation of the anticoagulant activity of activated protein C (APC). Recent data showed a direct anticoagulant role of protein S independent of APC, as demonstrated by the inhibition of prothrombinase and tenase activity both in plasma and in purified systems. This anticoagulant effect of protein S can be explained either by a direct interaction of protein S with one of the components of the complexes and/or by the interference with the binding of these components to phospholipid surfaces.During our investigation we noted that protein S preparations purified in different ways and derived from different sources, expressed discrepant APC cofactor and direct anticoagulant activity. In order to elucidate these differences and to study the mechanism of the APC-inde-pendent activity of protein S, we compared the protein S preparations in phospholipid-binding properties and anticoagulant activity. The dissociation constant for the binding of protein S to immobilized phospholipids ranged from 7 to 74 nM for the different protein S preparations. APC-independent inhibition of both prothrombinase and tenase activity performed on phospholipid vesicles and in plasma showed a strong correlation with the affinity for phospholipids. The APC-independent activity could be abolished by monoclonal antibodies that were either calcium-dependent and/or directed against epitopes in the Gla-region of protein S, suggesting that the protein S-phospholipid interaction is crucial for the APC-independent anticoagulant function of protein S. Protein S preparations with a low APC-independent activity expressed a high APC-cofactor activity suggesting that the affinity of protein S for phospholipids is of less importance in the expression of APC-cofactor activity of protein S.We conclude that high affinity interactions of protein S with the membrane surface are essential for the direct anticoagulant activity of protein S and we suggest that inhibition of the prothrombinase and the tenase complex by protein S is a consequence of the occupation of the phospholipid surface by protein S molecules.

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Perez-Casal, Margarita, Kenji Fukudome et Cheng Hock Toh. « A Novel Mechanism of Endothelial Protein C Receptor Release, in Microparticulate Form, by Activated Protein C. » Blood 104, n^o 11 (16 novembre 2004) : 1923. http://dx.doi.org/10.1182/blood.v104.11.1923.1923.

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Abstract Activated protein C (APC) administration is now used for treating patients with severe sepsis. We investigated its effect on primary, physiologically relevant cells and demonstrate a novel mechanism of endothelial protein C receptor (EPCR) release from the cell surface. Exposure of human umbilical vein endothelial cells or monocytes to APC (from physiological levels of 0.5 up to 100nM) resulted in the increasing release of EPCR-containing microparticles (EPCR-MP), as demonstrated by confocal microscopy. Further characterisation through flow cytometry showed a concomitant fall in EPCR levels from the cell surface. This release of EPCR could not be inhibited by the metalloproteinase inhibitors 1, 10-phenanthroline or Ro31-9790, unlike soluble EPCR (sEPCR) that is metalloproteinase cleaved at the cell surface following thrombin or pro- inflammatory cytokine stimulation. Western blotting confirmed the molecular weight of EPCR-MP to be identical to the full-length membrane form (49 kD) and different from sEPCR (45 kDa). APC was also bound to EPCR-MP and could be quantified by ELISA using EPCR capture and APC detection by chromogenic substrate, S2366. Using an initial factor Va incubation step followed by a prothrombinase assay, the APC bound to EPCR-MP could significantly reduce thrombin generation. This was abrogated in the presence of excess α1-antitrypsin, an APC inhibitor. By contrast, APC bound to sEPCR could no longer inactivate factor Va. Further characterisation showed the APC induction of EPCR-MP to be time dependent with increasing release over 24 hours, as quantified by ELISA. The phenomenon also required the active site of APC. Neither protein C, heat-inactivated or D-Phe-Pro-Arg-chloromethylketone-blocked APC could induce EPCR-MP formation. Co-incubation with hirudin (6mM) did not alter the APC effect and excluded any role of contaminating thrombin. This novel observation provides new insights into the consequences of APC therapy in the septic patient as well as demonstrating for the first time that there can be 2 circulating forms of EPCR. Unlike sEPCR however, EPCR-MP can facilitate and potentially disseminate the anticoagulant activity of bound APC.

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HACKENG, Tilman M., Subramanian YEGNESWARAN, Arthur E. JOHNSON et John H. GRIFFIN. « Conformational changes in activated protein C caused by binding of the first epidermal growth factor-like module of protein S ». Biochemical Journal 349, n^o 3 (25 juillet 2000) : 757–64. http://dx.doi.org/10.1042/bj3490757.

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The first epidermal growth factor-like module of human plasma protein S (EGF1, residues 76–116) was chemically synthesized and tested for its ability to inhibit the anticoagulant cofactor activity of protein S for the anticoagulant protease, activated protein C (APC). EGF1 completely inhibited the stimulation of APC activity by protein S in plasma coagulation assays, with 50% inhibition at approx. 1µM EGF1, suggesting direct binding of EGF1 to APC. To investigate a direct interaction between EGF1 and APC, fluorescence resonance energy transfer (FRET) experiments were employed. APC labelled in the active site with fluorescein as the donor, and phospholipid vesicles containing octadecylrhodamine as the acceptor, showed that EGF1 association with APC caused an increase in energy transfer consistent with a relocation of the active site of APC from 94Å (9.4nm) to 85Å above the phospholipid surface (assuming κ2 = 2/3). An identical increase in energy transfer between the APC active site-bound fluorescein and phospholipid-bound rhodamine was obtained upon association of protein S or protein S–C4b-binding protein complex with APC. The latter suggests the presence of a ternary complex of protein S–C4b-binding protein with APC on the phospholipid surface. To confirm a direct interaction of EGF1 with APC, rhodamine was covalently attached to the α-N-terminus of EGF1, and binding of the labelled EGF1 to APC was directly demonstrated using FRET. The data suggested a separation between the active site of APC and the N-terminus of EGF1 of 76Å (κ2 = 2/3), placing the APC-bound protein S-EGF1 close to, but above, the phospholipid surface and near the two EGF domains of APC. Thus we provide direct evidence for binding of protein S-EGF1 to APC and show that it induces a conformational change in APC.

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van der Meer, Felix J. M., Nico H. van Tilburg, Aat van Wijngaarden, Irma K. van der Linden, Ernest Briët et Rogier M. Bertina. « A Second Plasma Inhibitor of Activated Protein C : α1-Antitrypsin ». Thrombosis and Haemostasis 62, n^o 02 (1989) : 756–62. http://dx.doi.org/10.1055/s-0038-1646897.

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SummaryInactivation of activated protein C (APC) in normal human plasma was studied in the absence and presence of heparin. In the absence of heparin APC inactivation followed pseudo-first order kinetics. In the presence of heparin the neutralization of APC was found to be biphasic. Up to 500 nM APC could be readily inactivated in normal plasma, indicating that the concentration of the APC inhibitor must be higher than previously assumed. Plasma deficient in the protein C inhibitor (PCI-I, as described by Suzuki and coworkers) and deficient in p2-glycoprotein I still possessed APC neutralizing capacity, presumably through the formation of complexes of APC with another plasma protein as was demonstrated by immunoblotting with anti-protein C antibodies. Together these data made us to conclude that a second inhibitor of APC (PCI-II) must be present in normal human plasma. This second inhibitor should be heparin independent, have a relatively high plasma concentration and form complexes with APC.Subsequently, we purified this PCI-II by isolating APC-PCI-II complexes from plasma deficient of vitamin K dependent proteins, PCI-I and β2-glycoprotein-I, to which purified human APC had been added. Purified PCI-II has a molecular weight of 50,000 daltons and aminoacid analysis revealed that PCI-II is identical with α1-antitrypsin (α1-AT). The second order rate constant for the reaction between purified α1-AT and APC was found to be 269 M−1 min−1 in the absence of calcium and 602 M−1 min−1 in the presence of calcium.Finally, the analysis of the kinetics of APC inactivation in the plasma of a patient congenitally deficient in α1-AT (3% α1-AT antigen) revealed that there probably is a third inhibitor of APC in plasma, different from PCI-I and from α1-AT.

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Gruber, A., E. Mori, GJ del Zoppo, L. Waxman et JH Griffin. « Alteration of fibrin network by activated protein C ». Blood 83, n^o 9 (1 mai 1994) : 2541–48. http://dx.doi.org/10.1182/blood.v83.9.2541.2541.

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Abstract The antithrombotic plasma enzyme, activated protein C (APC), may play a role in thrombolysis. In vitro, acceleration of clot lysis by APC depends on its ability to inhibit the activation of prothrombin. The effect of APC on the assembly and dispersion of fibrin network was studied using turbidimetry, plasmin digestion of fibrin, and electron microscopy of plasma clots. The addition of APC before clotting but not after clotting accelerated clot lysis. The rate of increase in the turbidity of clotting plasma was reduced by APC. The turbidity of plasma clots containing APC was directly related to the clot lysis time. Fibrin from plasma clots that were formed in the presence of APC yielded less fibrin degradation products than fibrin from clots without added APC. Furthermore, APC reduced the diameter and relative number of fibrin fibers in plasma clots during gel assembly. We propose that APC may enhance the efficacy of thrombolysis by reducing the relative mass of fibrin within maturing thrombi.

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Gruber, A., E. Mori, GJ del Zoppo, L. Waxman et JH Griffin. « Alteration of fibrin network by activated protein C ». Blood 83, n^o 9 (1 mai 1994) : 2541–48. http://dx.doi.org/10.1182/blood.v83.9.2541.bloodjournal8392541.

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The antithrombotic plasma enzyme, activated protein C (APC), may play a role in thrombolysis. In vitro, acceleration of clot lysis by APC depends on its ability to inhibit the activation of prothrombin. The effect of APC on the assembly and dispersion of fibrin network was studied using turbidimetry, plasmin digestion of fibrin, and electron microscopy of plasma clots. The addition of APC before clotting but not after clotting accelerated clot lysis. The rate of increase in the turbidity of clotting plasma was reduced by APC. The turbidity of plasma clots containing APC was directly related to the clot lysis time. Fibrin from plasma clots that were formed in the presence of APC yielded less fibrin degradation products than fibrin from clots without added APC. Furthermore, APC reduced the diameter and relative number of fibrin fibers in plasma clots during gel assembly. We propose that APC may enhance the efficacy of thrombolysis by reducing the relative mass of fibrin within maturing thrombi.

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Mosnier, Laurent O., Jose A. Fernandez, Antonella Zampolli, Xia V. Yang, Zaverio M. Ruggeri et John H. Griffin. « In Vivo Anti-Thrombotic Potency of Engineered Activated Protein C Variants. » Blood 110, n^o 11 (16 novembre 2007) : 2704. http://dx.doi.org/10.1182/blood.v110.11.2704.2704.

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Abstract Activated protein C (APC) has both anticoagulant activity via inactivation of factors Va and VIIIa and cytoprotective activities on cells that include anti-apoptotic and anti-inflammatory activities, alterations of gene expression profiles and protection of endothelial barrier function. The relative importance of APC’s anticoagulant activity vs. APC’s direct cytoprotective effects on cells for reduction of mortality in severe sepsis patients and protective effects in animal injury models is not entirely clear. In this current study, genetically engineered APC variants with different activity spectra were tested for in vivo anti-thrombotic potency. Recently we made a non-anticoagulant APC variant, 5A-APC (RR229/230AA and KKK191-193AAA), that retains normal in vitro cytoprotective effects and an ability to reduce mortality in murine sepsis models (Kerschen et al, ASH2006, J Exper Med, 2007). In contrast to 5A-APC, mutation of E149 to A in APC increased anticoagulant activity in clotting assays while diminishing cytoprotective effects on cells. Murine APC variants, E149A-APC and 5A-APC (KKK192-194AAA + RR230/231AA) were used to determine in vivo anti-thrombotic potency in an acute carotid artery thrombosis model in mice, using FeCl3-induced injury. Under the conditions employed, first occlusion occurred within 3.5 min (mean: 171 sec; range 150-200 sec) in the absence of APC. Murine wild type (wt)-APC effectively delayed time to first occlusion in a dose-dependent manner (0 to 1.8 mg/kg wt-APC; mean: 561 sec; range 400-960 sec). The E149A-APC variant exhibited potent in vivo anti-thrombotic activity (1.8 mg/kg; mean: 1020 sec; range 540- >1600 sec) and was superior to wt-APC as evident by the absence of appreciable occlusion in 2/6 E149A-APC vs. 0/6 wt-APC treated animals. Thus E149A-APC was hyperactive in plasma clotting assays as well as hyperactive in an acute FeCl3-induced arterial thrombosis model. To test the hypothesis that an increased protein S cofactor activity contributed to its enhanced anticoagulant activity, E149A-APC anticoagulant activity was tested in normal and protein S deficient plasma. Compared to wt-APC, E149A-APC showed 3-fold increased anticoagulant activity in normal plasma but not in protein S deficient plasma. In studies with purified proteins, protein S concentrations required for half-maximal stimulation of factor Va inactivation by E149A-APC were 3-fold lower compared to wt-APC, whereas factor Va inactivation rates were indistinguishable in the absence of protein S. These data support our hypothesis that increased protein S cofactor activity is, at least partially, responsible for the observed hyper anticoagulant and anti-thrombotic potency in vitro and in vivo. In contrast to E149A-APC, 5A-APC was severely deficient in anti-thrombotic activity in vivo. Even at concentrations up to 8 mg/kg, 5A-APC (mean: 245 sec; range 172-300 sec) failed to delay significantly time to first occlusion compared to no APC. These data highlight important distinctions between structural requirements for APC’s anticoagulant, anti-thrombotic and cytoprotective functions. Engineered APC variants with differentially altered activities (e.g. cytoprotective vs. anticoagulant) may lead to safer or better therapeutic APC variants for a variety of indications including sepsis, ischemic stroke or other pathologies.

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Yegneswaran, Subramanian, Phuong Nguyen, John Griffin et Andrew Gale. « Prothrombin amino terminal region helps protect coagulation factor Va from proteolytic inactivation by activated protein C ». Thrombosis and Haemostasis 101, n^o 01 (2009) : 55–61. http://dx.doi.org/10.1160/th08-07-0491.

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SummaryThe hypothesis that prothrombin (FII) protects coagulation factor Va (FVa) from proteolytic inactivation by activated protein C (APC) was tested using purified proteins. FII dose-dependently protected FVa from APC proteolysis under conditions where competition of proteins for binding to negatively-charged phospholipid surface was not relevant (i.e. either at high phospholipid vesicle concentrations or using soluble dicaproylphosphatidylserine at levels below its critical micellar concentration). Cleavages in FVa at both Arg506 and Arg306 by APC were inhibited by FII. FII did not alter the amidolytic activity of APC towards chromogenic oligopeptide substrates or inhibit FVIIIa inactivation by APC, implying that the FII-mediated protection of FVa from APC proteolysis was due to the ability of FII to inhibit protein-protein interactions between FVa and APC. FII also protected FVa from inactivation by Gla-domainless APC, ruling out a role for the APC Gla domain for these observations. To identify domains of FII responsible for the observed phenomenon, various forms or fragments of FII were employed. Biotin-PheProArg-CMK-inhibited meizothrombin and fII-fragment 12 protected FVa from proteolysis by APC. In contrast, no significant protection of FVa from APC cleavage was observed for Gladomainless-FII, prethrombin-1, prethrombin-2, FII fragment 1 or active site inhibited-thrombin (DEGR-thrombin). Overall, these data demonstrate that the Gla domain of FII linked to kringle 1 and 2 is necessary for the ability of FII to protect FVa from APC cleavage and support the general concept that assembly of the FII activation complex (FXaFVaFIIlipid surface) protects FVa from APC inactivation so that the procoagulant, thrombin generating pathway can act unhindered by APC. Only following FII activation and dissociation of the FII Gla domain fragments from the FII-ase complex, can APC inactivate FVa and down-regulate thrombin generation.

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Feistritzer, Clemens, Laurent O. Mosnier, Enrico Di Cera, John H. Griffin et Matthias Riewald. « Efficient Barrier Protective Signaling by Activated Protein C Is Mechanistically Linked to Protein C Activation on Endothelial Cells. » Blood 106, n^o 11 (16 novembre 2005) : 28. http://dx.doi.org/10.1182/blood.v106.11.28.28.

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Abstract Protein C (PC) is activated by thrombomodulin-bound thrombin on the endothelial cell surface and activated protein C (APC) inhibits blood coagulation in a negative feedback loop. Endothelial PC receptor (EPCR) can bind PC/APC and activation of EPCR-bound PC is enhanced. Exogenous APC has barrier protective effects on endothelial cells that depend on EPCR binding and protease activated receptor-1 (PAR1) cleavage and that may contribute to the anti-inflammatory effects of APC. Plasma APC concentrations in vivo are low compared to the substrate PC and in order to induce protective signaling exogenous APC has to compete with PC for EPCR binding. In this study we investigated whether the endogenous PC activation pathway may be linked to efficient protective responses analyzing endothelial barrier permeability in a dual chamber system. When endothelial EA.hy926 cells were incubated for 3 h in the presence of 80 nM purified PC and different concentrations of thrombin a dose-dependent linear increase of APC activity in the cell medium was observed over time. APC generation was detectable upon incubation with 20 pM thrombin or higher and a significant barrier protective response to 20 pM thrombin was found only in the presence of PC. 40 pM thrombin enhanced barrier integrity in the presence and absence of PC, consistent with our previous results. To exclude direct thrombin effects on endothelial permeability and to compare protective effects of exogenous and endogenously generated APC, we used the anticoagulant double mutant thrombin W215A/E217A (WE). WE was about 10 times less active than wildtype thrombin for PC activation in our system. However, PAR1-dependent induction of MAP kinase phosphorylation required more than 1000-fold higher concentrations of the thrombin mutant. Thus, 1–10 nM WE leads to APC generation without directly inducing PAR1-dependent signaling. When cells were incubated with various concentrations of exogenous APC or WE+80 nM PC, barrier protective effects of 5 nM exogenous APC and 2 nM WE+80 nM PC (1.3 nM APC generated after 3 h) were similar. Because APC is generated at a constant rate during the incubation period, the average concentration of generated APC in the cell medium was only about 0.65 nM, suggesting that signaling by endogenously generated APC was significantly more efficient. To conclusively demonstrate that protective effects in response to WE are mediated by APC generation, we used recombinant zymogen wildtype PC and a PC variant with a substitution of the active site serine with alanine (PC S360A). Cells were incubated with control or 80 nM wildtype PC and PC S360A, in the presence or absence of WE (4 nM) and exogenous APC (3.3 nM). WE induced protective signaling only in the presence of wildtype PC but not PC S360A. Barrier protective effects of exogenous APC were blocked by both wildtype PC and PC S360A, consistent with their expected role as competitive inhibitors for APC binding to EPCR. These data demonstrate that efficient barrier enhancement by APC is indeed mechanistically coupled to the PC activation pathway. Signaling by endogenously generated APC may play an important role in the regulation of inflammation.

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Fernandez, Jose A., Hiroshi Deguchi, Natalie M. Pecheniuk, Subramanian Yegneswaran, Carole L. Banka et John H. Griffin. « Plasma High Density Lipoprotein and Anticoagulant Response to Activated Protein C (APC) and Protein S ». Blood 118, n^o 21 (18 novembre 2011) : 2249. http://dx.doi.org/10.1182/blood.v118.21.2249.2249.

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Abstract Abstract 2249 Previously we reported that plasma high density lipoprotein (HDL) enhances activated protein C (APC)/protein S (PS) anticoagulant action in plasma clotting and factor Va inactivation assays (APC/PS enhancement) and that lower HDL levels are found in male venous thrombosis patients or in patients with recurrent venous thrombosis versus controls, giving rise to our hypothesis that HDL helps protect against venous thrombosis. In this study, we sought (1) to identify which HDL particles enhance APC/PS activity, (2) to assess correlations between this activity and HDL particle size, and (3) to evaluate the recent challenge to our HDL anticoagulant activity hypothesis (Oslakovic et al, J Clin Invest, 2010). To identify HDL subfractions with APC/PS enhancing activity, we subfractionated HDL by sequential density gradient ultracentrifugation over the HDL density range of 1.063–1.21 mg/dl. When the anticoagulant property of these HDL subtractions was studied, we found that the less dense HDL subfractions corresponding to HDL2 particles enhanced APC/PS anticoagulant activity much more than other HDL fractions. Thus, we identified larger HDL particles as the key subfraction for this anticoagulant property of HDL. Hence, we hypothesized that plasma levels of large HDL particles would correlate with plasma sensitivity of APC/protein S. To test this hypothesis, we used proton NMR to quantify the levels of large, medium and small HDL particles and to determine the average size of HDL particles in plasmas from 39 normal adults. We performed dilute tissue factor-induced clotting assays in the absence or presence of APC/protein S to define APC/PS sensitivity and assessed correlations. There was a positive correlation between large HDL particle concentration and plasma sensitivity to APC/protein S (r=0.40, p=0.02). The size of HDL particles was also positively correlated with plasma sensitivity to APC/protein S (r=0.42, p=0.01). As previously reported, apoAI concentrations which is major apolipoprotein in HDL correlated with plasma sensitivity to APC/protein S (r=0.52, p=0.0007). Thus, as hypothesized, apoAI-containing large HDL particle concentrations in plasma correlate very well with the anticoagulant response to APC/PS. Recently Oslakovic et al purported to show that APC/PS enhancement was not an intrinsic property of HDL and claimed that this activity was due to negatively charged phospholipid contaminants of HDL based on gel filtration (Superose 6) analysis of their frozen HDL preparation. Therefore, we applied Superose 6 gel filtration analyses to four different fresh, never-frozen HDL preparations coming either from commercial sources or from in-house preparations. Our studies showed that when each HDL prep was subjected to Superose 6 fractionation, the column fractions containing large HDL (apoAI positive fractions) enhanced APC/protein S anticoagulant activity in plasma clotting assays and also for the inactivation of factor Va in purified systems. Moreover, immobilized anti-apoAI-antibodies removed the APC/PS enhancing effect, further establishing the fact that apoAI-containing HDL particles enhance APC/PS activity. When we analyzed HDL preps stored at 4 °C over successive weeks, we found that HDL fractions lost the ability to enhance APC/PS, showing the relatively labile nature of this activity. Freezing and thawing HDL was also deleterious for this activity. Thus, the APC/PS enhancing activity of fresh, never-frozen HDL preps is primarily due to HDL particles. All of our new findings confirm our previous conclusion that HDL enhances APC/PS anticoagulant actions. Our extensive data set strongly contradicts the conclusions from the recent HDL study by Oslakovic et al who unfortunately performed their bioassays using frozen HDL made from previously frozen lipidmic plasma. In summary, freshly purified, non-frozen large HDL particles made from fresh plasma enhance APC/PS activity. The sensitivity of plasma of healthy adults to APC/PS anticoagulant effects is significantly correlated with the plasma levels of large HDL particles, suggesting the physiological importance of large HDL particles as enhancing the anticoagulant APC/PS system and supporting our hypothesis that large HDL particles may be protective against venous thrombosis, at least in part, via this activity. Disclosures: No relevant conflicts of interest to declare.

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Mosnier, Laurent O., Antonella Zampolli, Edward J. Kerschen, Reto A. Schuepbach, Yajnavalka Banerjee, José A. Fernández, Xia V. Yang et al. « Hyperantithrombotic, noncytoprotective Glu149Ala-activated protein C mutant ». Blood 113, n^o 23 (4 juin 2009) : 5970–78. http://dx.doi.org/10.1182/blood-2008-10-183327.

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Abstract Activated protein C (APC) reduces mortality in severe sepsis patients. APC exerts anticoagulant activities via inactivation of factors Va and VIIIa and cytoprotective activities via endothelial protein C receptor and protease-activated receptor-1. APC mutants with selectively altered and opposite activity profiles, that is, greatly reduced anticoagulant activity or greatly reduced cytoprotective activities, are compared here. Glu149Ala-APC exhibited enhanced in vitro anticoagulant and in vivo antithrombotic activity, but greatly diminished in vitro cytoprotective effects and in vivo reduction of endotoxin-induced murine mortality. Thus, residue Glu149 and the C-terminal region of APC's light chain are identified as functionally important for expression of multiple APC activities. In contrast to Glu149Ala-APC, 5A-APC (Lys191-193Ala + Arg229/230Ala) with protease domain mutations lacked in vivo antithrombotic activity, although it was potent in reducing endotoxin-induced mortality, as previously shown. These data imply that APC molecular species with potent antithrombotic activity, but without robust cytoprotective activity, are not sufficient to reduce mortality in endotoxemia, emphasizing the need for APC's cytoprotective actions, but not anticoagulant actions, to reduce endotoxin-induced mortality. Protein engineering can provide APC mutants that permit definitive mechanism of action studies for APC's multiple activities, and may also provide safer and more effective second-generation APC mutants with reduced bleeding risk.

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Mosnier, Laurent O., Berislav V. Zlokovic et John H. Griffin. « The cytoprotective protein C pathway ». Blood 109, n^o 8 (16 novembre 2006) : 3161–72. http://dx.doi.org/10.1182/blood-2006-09-003004.

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Abstract Protein C is best known for its mild deficiency associated with venous thrombosis risk and severe deficiency associated with neonatal purpura fulminans. Activated protein C (APC) anticoagulant activity involves proteolytic inactivation of factors Va and VIIIa, and APC resistance is often caused by factor V Leiden. Less known is the clinical success of APC in reducing mortality in severe sepsis patients (PROWESS trial) that gave impetus to new directions for basic and preclinical research on APC. This review summarizes insights gleaned from recent in vitro and in vivo studies of the direct cytoprotective effects of APC that include beneficial alterations in gene expression profiles, anti-inflammatory actions, antiapoptotic activities, and stabilization of endothelial barriers. APC's cytoprotection requires its receptor, endothelial cell protein C receptor, and protease-activated receptor-1. Because of its pleiotropic activities, APC has potential roles in the treatment of complex disorders, including sepsis, thrombosis, and ischemic stroke. Although much about molecular mechanisms for APC's effects on cells remains unclear, it is clear that APC's structural features mediating anticoagulant actions and related bleeding risks are distinct from those mediating cytoprotective actions, suggesting the possibility of developing APC variants with an improved profile for the ratio of cytoprotective to anticoagulant actions.

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Li, Xian, Sara J. Bidarian, Martha Sim, Xiaohong Song et Jeremy P. Wood. « Protein S Coordinates the Inhibition of Prothrombinase By Tfpiα and Activated Protein C ». Blood 134, Supplement_1 (13 novembre 2019) : 2386. http://dx.doi.org/10.1182/blood-2019-130799.

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Background: Protein S (PS), a vitamin K-dependent plasma glycoprotein, functions as a cofactor for the anticoagulants activated protein C (APC) and tissue factor (TF) pathway inhibitor alpha (TFPIa), which inhibit factors Va (FVa) and Xa (FXa), respectively. Although it is unclear which of these functions is/are important in vivo, homozygous deficiency of PS is associated with life-threatening thrombosis shortly after birth. FVa and FXa form the prothrombinase complex, which generates thrombin, suggesting that PS has a role in the direct inhibition of thrombin production. However, neither the PS/APC nor PS/TFPIα system alone is effective at inhibiting thrombin generation by prothrombinase. In addition to its role in regulating coagulation, PS also functions as an inhibitor of the complement system. Approximately 60% of plasma PS circulates bound to complement factor C4bp, which blocks its anticoagulant activity. We sought to determine the impact of PS/APC on TFPIα function, and vice versa, using purified protein and plasma-based systems. Methods and Results: To assess the effect of the combined PS/APC and PS/TFPIα systems on thrombin generation, we supplemented plasma with thrombomodulin, which promotes APC activation. In the absence of thrombomodulin, 5nM TFPIα decreased peak thrombin by 55.1% (33.1±1.9 nM in the presence of TFPIα vs. 73.7±39.9 nM in the absence) and endogenous thrombin potential (ETP) by 35.4% (475±42 nM*min vs. 735±189nM*min). In the presence of thrombomodulin, TFPIα decreased these parameters by 65.7% (11.4 ± 2.6 nM) and 77.5% (107±22 nM*min), respectively, suggesting that APC makes TFPIα a more potent inhibitor of thrombin generation. We next sought to study each of these PS functions in a purified protein system and in plasma. To study the effect of PS/APC on TFPIα function, we produced a recombinant protein consisting of the first two epidermal growth factor-like domains of PS (EGF1-2), which contain the putative APC binding sites. In a purified protein assay, APC inhibited the rate of thrombin activation by prothrombinase by 19.62±0.01% in the absence of PS and by 34.96±0.02% in the presence of 50nM PS. EGF1-2 dose-dependently reversed the effect of PS, with 75% reversal achieved with the addition of 200nM EGF1-2. Unexpectedly, EGF1-2 had the opposite effect in plasma thrombin generation assays and potently inhibited TF-initiated thrombin generation either in the presence or absence of thrombomodulin. We hypothesize that the EGF1-2 domains also form part of the C4bp binding site, and that addition of EGF1-2 protein resulted in release of PS from C4bp and an increase in the anticoagulant PS pool. In support of this hypothesis, EGF1-2 had no effect on thrombin generation in PS-depleted plasma, which is also depleted of C4bp. When the PS-depleted plasma was supplemented with 150nM PS, EGF1-2 had the expected procoagulant activity (increasing peak thrombin from 50.4±19.9 nM to 90.4±6.0 nM). Notably, even with a saturating concentration of EGF1-2, thrombomodulin and PS significantly decreased thrombin generation, suggesting that PS-TFPIα-mediated FXa inhibition promotes APC-mediated FVa degradation, even if PS cannot directly bind APC. We similarly assessed the impact of the PS-TFPI function, using a protein from the saliva of black flies, "black fly protease inhibitor" (BFPI), which contains the TFPIα domain that inhibits FXa but lacks the domain that binds PS. BFPI inhibits free FXa similarly to TFPIα, but PS does not promote this inhibition. Like TFPIα, BFPI is a poor inhibitor of thrombin generation by prothrombinase containing thrombin-activated FVa (5 nM BFPI had no impact on thrombin generation in the presence or absence of PS). However, in the presence of APC and PS, 5nM BFPI decreased the maximum rate of thrombin generation by 17.3±3.3%. These data suggest that PS/APC-mediated degradation of FVa promotes TFPIα-mediated inhibition of FXa, regardless of whether PS is able to bind TFPIα. Conclusions: Our data suggest that the PS-APC and PS-TFPIα systems cooperatively regulate thrombin generation by prothrombinase. While maximal inhibition requires that PS act as a cofactor for both APC and TFPIα, PS-APC independently promotes TFPIα function, and PS-TFPIα separately promotes APC. Based on these data, we propose a model in which PS-APC-mediated inhibition of FVa renders FXa susceptible to TFPIα and vice versa. Disclosures No relevant conflicts of interest to declare.

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Barth, Angela I. M., Anne L. Pollack, Yoram Altschuler, Keith E. Mostov et W. James Nelson. « NH2-terminal Deletion of β-Catenin Results in Stable Colocalization of Mutant β-Catenin with Adenomatous Polyposis Coli Protein and Altered MDCK Cell Adhesion ». Journal of Cell Biology 136, n^o 3 (10 février 1997) : 693–706. http://dx.doi.org/10.1083/jcb.136.3.693.

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β-Catenin is essential for the function of cadherins, a family of Ca2+-dependent cell–cell adhesion molecules, by linking them to α-catenin and the actin cytoskeleton. β-Catenin also binds to adenomatous polyposis coli (APC) protein, a cytosolic protein that is the product of a tumor suppressor gene mutated in colorectal adenomas. We have expressed mutant β-catenins in MDCK epithelial cells to gain insights into the regulation of β-catenin distribution between cadherin and APC protein complexes and the functions of these complexes. Full-length β-catenin, β-catenin mutant proteins with NH2-terminal deletions before (ΔN90) or after (ΔN131, ΔN151) the α-catenin binding site, or a mutant β-catenin with a COOH-terminal deletion (ΔC) were expressed in MDCK cells under the control of the tetracycline-repressible transactivator. All β-catenin mutant proteins form complexes and colocalize with E-cadherin at cell–cell contacts; ΔN90, but neither ΔN131 nor ΔN151, bind α-catenin. However, β-catenin mutant proteins containing NH2-terminal deletions also colocalize prominently with APC protein in clusters at the tips of plasma membrane protrusions; in contrast, full-length and COOH-terminal– deleted β-catenin poorly colocalize with APC protein. NH2-terminal deletions result in increased stability of β-catenin bound to APC protein and E-cadherin, compared with full-length β-catenin. At low density, MDCK cells expressing NH2-terminal–deleted β-catenin mutants are dispersed, more fibroblastic in morphology, and less efficient in forming colonies than parental MDCK cells. These results show that the NH2 terminus, but not the COOH terminus of β-catenin, regulates the dynamics of β-catenin binding to APC protein and E-cadherin. Changes in β-catenin binding to cadherin or APC protein, and the ensuing effects on cell morphology and adhesion, are independent of β-catenin binding to α-catenin. These results demonstrate that regulation of β-catenin binding to E-cadherin and APC protein is important in controlling epithelial cell adhesion.

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Griffin, John H., Berislav V. Zlokovic et Laurent O. Mosnier. « Activated protein C : biased for translation ». Blood 125, n^o 19 (7 mai 2015) : 2898–907. http://dx.doi.org/10.1182/blood-2015-02-355974.

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Abstract The homeostatic blood protease, activated protein C (APC), can function as (1) an antithrombotic on the basis of inactivation of clotting factors Va and VIIIa; (2) a cytoprotective on the basis of endothelial barrier stabilization and anti-inflammatory and antiapoptotic actions; and (3) a regenerative on the basis of stimulation of neurogenesis, angiogenesis, and wound healing. Pharmacologic therapies using recombinant human and murine APCs indicate that APC provides effective acute or chronic therapies for a strikingly diverse range of preclinical injury models. APC reduces the damage caused by the following: ischemia/reperfusion in brain, heart, and kidney; pulmonary, kidney, and gastrointestinal inflammation; sepsis; Ebola virus; diabetes; and total lethal body radiation. For these beneficial effects, APC alters cell signaling networks and gene expression profiles by activating protease-activated receptors 1 and 3. APC’s activation of these G protein–coupled receptors differs completely from thrombin’s activation mechanism due to biased signaling via either G proteins or β-arrestin-2. To reduce APC-associated bleeding risk, APC variants were engineered to lack >90% anticoagulant activity but retain normal cell signaling. Such a neuroprotective variant, 3K3A-APC (Lys191-193Ala), has advanced to clinical trials for ischemic stroke. A rich data set of preclinical knowledge provides a solid foundation for potential translation of APC variants to future novel therapies.

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Okajima, Kenji, Shin Koga, Megumi Kaji, Masayasu Inoue, Tomohiro Nakagaki, Akinobu Funatsu, Hiroaki Okabe, Kiyoshi Takatsuki et Nobuo Aoki. « Effect of Protein C and Activated Protein C on Coagulation and Fibrinolysis in Normal Human Subjects ». Thrombosis and Haemostasis 63, n^o 01 (1990) : 048–53. http://dx.doi.org/10.1055/s-0038-1645685.

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SummaryAlthough protein C (PC) and activated protein C (APC) have been postulated to be useful for treating patients with thrombosis, their critical effect remains to be studied in human subjects. To examine whether purified PC or APC are useful for treating patients with thrombosis without showing any adverse effect, wc studied effects on coagulation and fibrinolysis in normal human subjects.When highly purified human PC was administered intravenously to healthy subjects, plasma levels of immunoreactive PC decreased with a half-life of 10.9 h. Intravenously administered APC decreased with a half-life of 23 min as measured by prolongation of activated partial thromboplastin time (APTT). However, 1.7 h was obtained for the plasma half-life of APC when it was measured immunologically. These findings suggested that a significant fraction of the administered APC was rapidly inhibited by plasma inhibitor. Upon administration of APC, APTT was prolonged and plasma levels of clotting factor VIII (FVIII) decreased transiently as measured by clotting assay. However, when determined by a chromogenic assay method in which 120-fold diluted plasma samples were used, plasma levels of FVIII remained unchanged. Plasma levels of F-V did not decrease after APC administration. These findings suggested that prolongation of APTT and apparent decrease in plasma F-VIII clotting activity might be due to the in vitro-effect of APC present in plasma samples used. Diurnal fluctuation of plasminogen activator inhibitor in normal subjects was not affected by administration of APC.Thus, PC or APC seems to function selectively at the site of thrombin-formation without lowering plasma levels of coagulation factors.

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Sun, Yong-Hui, Lei Shen et Björn Dahlbäck. « Gla domain–mutated human protein C exhibiting enhanced anticoagulant activity and increased phospholipid binding ». Blood 101, n^o 6 (15 mars 2003) : 2277–84. http://dx.doi.org/10.1182/blood-2002-06-1691.

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Protein C is a member of the vitamin K– dependent protein family. Proteins in this family have similar γ-carboxyglutamic acid (Gla)–rich domains, but their affinities for negatively charged phospholipid membranes vary more than 1000-fold. We have shown that it is possible to enhance anticoagulant activity and membrane affinity of protein C by selective mutagenesis of the Gla domain. In this study, 3 new mutants, Q10G11N12 (QGN), S23E32D33Y44 (SEDY), and Q10G11N12S23E32D33Y44 (QGNSEDY), were created. In plasma-based coagulation assays, the activated form of QGNSEDY (QGNSEDY-APC) demonstrated approximately 20-fold higher anticoagulant activity than wild-type activated protein C (WT APC), while QGN-APC and SEDY-APC did not. Both normal activated factor V (FVa) and FVa Leiden (Arg506Gln) were degraded much more efficiently by QGNSEDY-APC than by WT APC in the presence as well as in the absence of protein S. Binding of protein C variants to negatively charged phospholipid membranes was investigated using light scattering and the BIAcore technique. QGNSEDY demonstrated 3- to 7-fold enhanced binding as compared with WT protein C, suggesting the membrane affinity to be influenced by several residues located at different parts of the Gla domain. The anticoagulant activity as well as phospholipid binding ability was only enhanced when multiple regions of the Gla domain were modified. The results provide insights into the molecular mechanisms that are involved in determining the binding affinity of the interaction between Gla domains and phospholipid membranes. The unique properties of QGNSEDY-APC suggest this APC variant possibly to have greater therapeutic potential than WT APC.

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Ahnström, Josefin, Helena M. Andersson, Kevin Canis, Eva Norstrøm, Yao Yu, Björn Dahlbäck, Maria Panico, Howard R. Morris, James T. B. Crawley et David A. Lane. « Activated protein C cofactor function of protein S : a novel role for a γ-carboxyglutamic acid residue ». Blood 117, n^o 24 (16 juin 2011) : 6685–93. http://dx.doi.org/10.1182/blood-2010-11-317099.

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Abstract Protein S has an important anticoagulant function by acting as a cofactor for activated protein C (APC). We recently reported that the EGF1 domain residue Asp95 is critical for APC cofactor function. In the present study, we examined whether additional interaction sites within the Gla domain of protein S might contribute to its APC cofactor function. We examined 4 residues, composing the previously reported “Face1” (N33S/P35T/E36A/Y39V) variant, as single point substitutions. Of these protein S variants, protein S E36A was found to be almost completely inactive using calibrated automated thrombography. In factor Va inactivation assays, protein S E36A had 89% reduced cofactor activity compared with wild-type protein S and was almost completely inactive in factor VIIIa inactivation; phospholipid binding was, however, normal. Glu36 lies outside the ω-loop that mediates Ca2+-dependent phospholipid binding. Using mass spectrometry, it was nevertheless confirmed that Glu36 is γ-carboxylated. Our finding that Gla36 is important for APC cofactor function, but not for phospholipid binding, defines a novel function (other than Ca2+ coordination/phospholipid binding) for a Gla residue in vitamin K–dependent proteins. It also suggests that residues within the Gla and EGF1 domains of protein S act cooperatively for its APC cofactor function.

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Lee, Yong-Seok, Sun-Lim Choi, Heejung Jun, Se-Jeong Yim, Jin-A. Lee, Hyoung F. Kim, Seung-Hee Lee et al. « AU-rich element-binding protein negatively regulates CCAAT enhancer-binding protein mRNA stability during long-term synaptic plasticity in Aplysia ». Proceedings of the National Academy of Sciences 109, n^o 38 (4 septembre 2012) : 15520–25. http://dx.doi.org/10.1073/pnas.1116224109.

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The consolidation of long-term memory for sensitization and synaptic facilitation in Aplysia requires synthesis of new mRNA including the immediate early gene Aplysia CCAAT enhancer-binding protein (ApC/EBP). After the rapid induction of ApC/EBP expression in response to repeated treatments of 5-hydroxytryptamine (5-HT), ApC/EBP mRNA is temporarily expressed in sensory neurons of sensory-to-motor synapses. However, the molecular mechanism underlying the rapid degradation of ApC/EBP transcript is not known. Here, we cloned an AU-rich element (ARE)-binding protein, ApAUF1, which functions as a destabilizing factor for ApC/EBP mRNA. ApAUF1 was found to bind to the 3′ UTR of ApC/EBP mRNA that contains AREs and subsequently reduces the expression of ApC/EBP 3′ UTR-containing reporter genes. Moreover, overexpression of ApAUF1 inhibited the induction of ApC/EBP mRNA in sensory neurons and also impaired long-term facilitation of sensory-to-motor synapses by repetitive 5-HT treatments. These results provide evidence for a critical role of the posttranscriptional modification of ApC/EBP mRNA during the consolidation of synaptic plasticity.

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Vorlaufer, Elisabeth, et Jan-Michael Peters. « Regulation of the Cyclin B Degradation System by an Inhibitor of Mitotic Proteolysis ». Molecular Biology of the Cell 9, n^o 7 (juillet 1998) : 1817–31. http://dx.doi.org/10.1091/mbc.9.7.1817.

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The initiation of anaphase and exit from mitosis depend on the anaphase-promoting complex (APC), which mediates the ubiquitin-dependent proteolysis of anaphase-inhibiting proteins and mitotic cyclins. We have analyzed whether protein phosphatases are required for mitotic APC activation. In Xenopus egg extracts APC activation occurs normally in the presence of protein phosphatase 1 inhibitors, suggesting that the anaphase defects caused by protein phosphatase 1 mutation in several organisms are not due to a failure to activate the APC. Contrary to this, the initiation of mitotic cyclin B proteolysis is prevented by inhibitors of protein phosphatase 2A such as okadaic acid. Okadaic acid induces an activity that inhibits cyclin B ubiquitination. We refer to this activity as inhibitor of mitotic proteolysis because it also prevents the degradation of other APC substrates. A similar activity exists in extracts of Xenopus eggs that are arrested at the second meiotic metaphase by the cytostatic factor activity of the protein kinase mos. In Xenopus eggs, the initiation of anaphase II may therefore be prevented by an inhibitor of APC-dependent ubiquitination.

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Shea, Lei, Xuhua He et Björn Dahlbäck. « Synergistic Cofactor Function of Factor V and Protein S to Activated Protein C in the Inactivation of the Factor Villa – Factor IXa Complex ». Thrombosis and Haemostasis 78, n^o 03 (1997) : 1030–36. http://dx.doi.org/10.1055/s-0038-1657682.

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SummaryHuman factor V has been shown not only to be a precursor to procoagulant factor Va but also to express anticoagulant properties. Thus, factor V was recently found to potentiate the effect of protein S as cofactor to activated protein C (APC) in the inactivation of the factor VIIIa-factor IXa complex. The purpose of this study was to determine whether the APC-cofactor function of factor V was also expressed in the bovine protein C system and to elucidate the molecular background for the species specificity of APC. For this purpose, the effects of protein S and factor V on APC-mediated inactivation of factor VIIIa were studied using purified APC, protein S and factor V of human and bovine.origin. The factor VIIIa investigated here was part of a Xase complex (i.e. factor IXa, factor VIIIa, phospholipid and calcium) and the APC-mediated inhibition of factor VIIIa was monitored by the ability of the Xase complex to activate factor X. Synergistic APC-cofactor function of factor V and protein S was demonstrated in the bovine system. The effect of bovine APC was potentiated by bovine protein S but not by human protein S, whereas both human or bovine protein S stimulated the function of human APC. Factor V did not express species specificity in its APC-cofactor activity even though bovine factor V was more potent than its human counterpart. Recombinant human/bovine protein S chimeras were used to demonstrate that the thrombin sensitive region and first epidermal growth factor-like module of protein S determine the species specificity of the APC-protein S interaction. In conclusion, both human and bovine factor V were found to express APC-cofactor activity which depends on the presence of protein S. The species specificity of APC was shown to be caused by the interaction between APC and protein S.

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Ostapenko, Denis, Janet L. Burton, Ruiwen Wang et Mark J. Solomon. « Pseudosubstrate Inhibition of the Anaphase-Promoting Complex by Acm1 : Regulation by Proteolysis and Cdc28 Phosphorylation ». Molecular and Cellular Biology 28, n^o 15 (2 juin 2008) : 4653–64. http://dx.doi.org/10.1128/mcb.00055-08.

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ABSTRACT The ubiquitin ligase activity of the anaphase-promoting complex (APC)/cyclosome needs to be tightly regulated for proper cell cycle progression. Substrates are recruited to the APC by the Cdc20 and Cdh1 accessory proteins. The Cdh1-APC interaction is inhibited through phosphorylation of Cdh1 by Cdc28, the major cyclin-dependent protein kinase in budding yeast. More recently, Acm1 was reported to be a Cdh1-binding and -inhibitory protein in budding yeast. We found that although Acm1 is an unstable protein and contains the KEN-box and D-box motifs typically found in APC substrates, Acm1 itself is not an APC substrate. Rather, it uses these motifs to compete with substrates for Cdh1 binding, thereby inhibiting their recruitment to the APC. Mutation of these motifs prevented Acm1-Cdh1 binding in vivo and rendered Acm1 inactive both in vitro and in vivo. Acm1 stability was critically dependent on phosphorylation by Cdc28, as Acm1 was destabilized following inhibition of Cdc28, mutation of consensus Cdc28 phosphorylation sites in Acm1, or deletion of the Bmh1 and Bmh2 phosphoprotein-binding proteins. Thus, Cdc28 serves dual roles in inhibiting Cdh1-dependent APC activity during the cell cycle: stabilization of the Cdh1 inhibitor Acm1 and direct phosphorylation of Cdh1 to prevent its association with the APC.

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KOYAMA, Takatoshi, et Shinsaku HIROSAWA. « Activated Protein C (APC) Resistance ». Japanese Journal of Thrombosis and Hemostasis 5, n^o 6 (1994) : 442–46. http://dx.doi.org/10.2491/jjsth.5.442.

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Kulkarni, Kiran, Ziguo Zhang, Leifu Chang, Jing Yang, Paula C. A. da Fonseca et David Barford. « Building a pseudo-atomic model of the anaphase-promoting complex ». Acta Crystallographica Section D Biological Crystallography 69, n^o 11 (12 octobre 2013) : 2236–43. http://dx.doi.org/10.1107/s0907444913018593.

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The anaphase-promoting complex (APC/C) is a large E3 ubiquitin ligase that regulates progression through specific stages of the cell cycle by coordinating the ubiquitin-dependent degradation of cell-cycle regulatory proteins. Depending on the species, the active form of the APC/C consists of 14–15 different proteins that assemble into a 20-subunit complex with a mass of approximately 1.3 MDa. A hybrid approach of single-particle electron microscopy and protein crystallography of individual APC/C subunits has been applied to generate pseudo-atomic models of various functional states of the complex. Three approaches for assigning regions of the EM-derived APC/C density map to specific APC/C subunits are described. This information was used to dock atomic models of APC/C subunits, determined either by protein crystallography or homology modelling, to specific regions of the APC/C EM map, allowing the generation of a pseudo-atomic model corresponding to 80% of the entire complex.

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D'Angelo, A., MS Lockhart, SV D'Angelo et FB Jr Taylor. « Protein S is a cofactor for activated protein C neutralization of an inhibitor of plasminogen activation released from platelets ». Blood 69, n^o 1 (1 janvier 1987) : 231–37. http://dx.doi.org/10.1182/blood.v69.1.231.231.

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Abstract Platelets stimulated with thrombin release an inhibitor of plasminogen activator (PAI), which has been shown previously to be neutralized by activated protein C (APC). The requirements for optimal neutralization of PAI activity were investigated. The releasate of gel-filtered human platelets stimulated with thrombin served as a source of PAI. When 6 X 10(8) platelets/mL were incubated with thrombin (1 IU/mL), the releasate contained 18 to 26 ng/mL PAI as determined by incubation of the releasate with urokinase and measurement of residual urokinase activity on plasminogen (S2251). Preincubation of PAI with up to 4 micrograms/mL APC for two hours yielded less than 20% neutralization of PAI activity. In the presence of protein S, phospholipid, and Ca2+, neutralization of PAI activity was time-dependent with 50% neutralization occurring in two hours with 1 microgram/mL APC. The cofactor effects of protein S and phospholipid were concentration- dependent with half-maximal acceleration at approximately 3 micrograms/mL protein S and 10 micrograms/mL phospholipid when the experiments were performed at 1 microgram/mL APC. Diisopropylfluorophosphate-inactivated APC, gla-domainless APC, and thrombin-cleaved protein S had no effect on PAI activity, indicating requirement for preservation of the APC active site and of the Ca2+ binding ability of both APC and protein S. These results suggest coordinate binding of APC and protein S onto phospholipid membrane as a prerequisite for optimal expression of PAI neutralized by APC.

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D'Angelo, A., MS Lockhart, SV D'Angelo et FB Jr Taylor. « Protein S is a cofactor for activated protein C neutralization of an inhibitor of plasminogen activation released from platelets ». Blood 69, n^o 1 (1 janvier 1987) : 231–37. http://dx.doi.org/10.1182/blood.v69.1.231.bloodjournal691231.

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Platelets stimulated with thrombin release an inhibitor of plasminogen activator (PAI), which has been shown previously to be neutralized by activated protein C (APC). The requirements for optimal neutralization of PAI activity were investigated. The releasate of gel-filtered human platelets stimulated with thrombin served as a source of PAI. When 6 X 10(8) platelets/mL were incubated with thrombin (1 IU/mL), the releasate contained 18 to 26 ng/mL PAI as determined by incubation of the releasate with urokinase and measurement of residual urokinase activity on plasminogen (S2251). Preincubation of PAI with up to 4 micrograms/mL APC for two hours yielded less than 20% neutralization of PAI activity. In the presence of protein S, phospholipid, and Ca2+, neutralization of PAI activity was time-dependent with 50% neutralization occurring in two hours with 1 microgram/mL APC. The cofactor effects of protein S and phospholipid were concentration- dependent with half-maximal acceleration at approximately 3 micrograms/mL protein S and 10 micrograms/mL phospholipid when the experiments were performed at 1 microgram/mL APC. Diisopropylfluorophosphate-inactivated APC, gla-domainless APC, and thrombin-cleaved protein S had no effect on PAI activity, indicating requirement for preservation of the APC active site and of the Ca2+ binding ability of both APC and protein S. These results suggest coordinate binding of APC and protein S onto phospholipid membrane as a prerequisite for optimal expression of PAI neutralized by APC.

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Wang, Yang, Yoshiaki Azuma, David Moore, Neil Osheroff et Kristi L. Neufeld. « Interaction between Tumor Suppressor Adenomatous Polyposis Coli and Topoisomerase IIα : Implication for the G2/M Transition ». Molecular Biology of the Cell 19, n^o 10 (octobre 2008) : 4076–85. http://dx.doi.org/10.1091/mbc.e07-12-1296.

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The tumor suppressor adenomatous polyposis coli (APC) is implicated in regulating multiple stages of the cell cycle. APC participation in G1/S is attributed to its recognized role in Wnt signaling. APC function in the G2/M transition is less well established. To identify novel protein partners of APC that regulate the G2/M transition, APC was immunoprecipitated from colon cell lysates and associated proteins were analyzed by matrix-assisted laser desorption ionization/time of flight (MALDI-TOF). Topoisomerase IIα (topo IIα) was identified as a potential binding partner of APC. Topo IIα is a critical regulator of G2/M transition. Evidence supporting an interaction between endogenous APC and topo IIα was obtained by coimmunoprecipitation, colocalization, and Förster resonance energy transfer (FRET). The 15-amino acid repeat region of APC (M2-APC) interacted with topo IIα when expressed as a green fluorescent protein (GFP)-fusion protein in vivo. Although lacking defined nuclear localization signals (NLS) M2-APC predominantly localized to the nucleus. Furthermore, cells expressing M2-APC displayed condensed or fragmented nuclei, and they were arrested in the G2 phase of the cell cycle. Although M2-APC contains a β-catenin binding domain, biochemical studies failed to implicate β-catenin in the observed phenotype. Finally, purified recombinant M2-APC enhanced topo IIα activity in vitro. Together, these data support a novel role for APC in the G2/M transition, potentially through association with topo IIα.

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Barford, David. « Structure, function and mechanism of the anaphase promoting complex (APC/C) ». Quarterly Reviews of Biophysics 44, n^o 2 (22 novembre 2010) : 153–90. http://dx.doi.org/10.1017/s0033583510000259.

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AbstractThe complex molecular events responsible for coordinating chromosome replication and segregation with cell division and growth are collectively known as the cell cycle. Progression through the cell cycle is orchestrated by the interplay between controlled protein synthesis and degradation and protein phosphorylation. Protein degradation is primarily regulated through the ubiquitin proteasome system, mediated by two related E3 protein ubiquitin ligases, the Skp1 cullin F-box (SCF) and the anaphase promoting complex (also known as the cyclosome) (APC/C). The APC/C is a multi-subunit cullin-RING E3 ubiquitin ligase that regulates progression through the mitotic phase of the cell cycle and controls entry into S phase by catalysing the ubiquitylation of cyclins and other cell cycle regulatory proteins. Selection of APC/C targets is controlled through recognition of short destruction motifs, predominantly the D-box and KEN-box. APC/C-mediated coordination of cell cycle progression is achieved through the temporal regulation of APC/C activity and substrate specificity, exerted through a combination of co-activator subunits, reversible phosphorylation and inhibitory proteins and complexes. The aim of this article is to discuss the APC/C from a structural and mechanistic perspective. Although an atomic structure of the APC/C is still lacking, a combination of genetic, biochemical, electron microscopy studies of intact APC/C and crystallographic analysis of individual subunits, together with analogies to evolutionarily related E3 ligases of the RING family, has provided deep insights into the molecular mechanisms of catalysis and substrate recognition, and structural organisation of the APC/C.

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Yasuda, Kyota, Huaye Zhang, David Loiselle, Timothy Haystead, Ian G. Macara et Stavroula Mili. « The RNA-binding protein Fus directs translation of localized mRNAs in APC-RNP granules ». Journal of Cell Biology 203, n^o 5 (2 décembre 2013) : 737–46. http://dx.doi.org/10.1083/jcb.201306058.

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RNA localization pathways direct numerous mRNAs to distinct subcellular regions and affect many physiological processes. In one such pathway the tumor-suppressor protein adenomatous polyposis coli (APC) targets RNAs to cell protrusions, forming APC-containing ribonucleoprotein complexes (APC-RNPs). Here, we show that APC-RNPs associate with the RNA-binding protein Fus/TLS (fused in sarcoma/translocated in liposarcoma). Fus is not required for APC-RNP localization but is required for efficient translation of associated transcripts. Labeling of newly synthesized proteins revealed that Fus promotes translation preferentially within protrusions. Mutations in Fus cause amyotrophic lateral sclerosis (ALS) and the mutant protein forms inclusions that appear to correspond to stress granules. We show that overexpression or mutation of Fus results in formation of granules, which preferentially recruit APC-RNPs. Remarkably, these granules are not translationally silent. Instead, APC-RNP transcripts are translated within cytoplasmic Fus granules. These results unexpectedly show that translation can occur within stress-like granules. Importantly, they identify a new local function for cytoplasmic Fus with implications for ALS pathology.

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Berger, H. Jr, CG Kirstein et CL Orthner. « Pharmacokinetics of activated protein C in guinea pigs ». Blood 77, n^o 10 (15 mai 1991) : 2174–84. http://dx.doi.org/10.1182/blood.v77.10.2174.2174.

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Abstract Protein C is a vitamin K-dependent zymogen of the serine protease, activated protein C (APC), an important regulatory enzyme in hemostasis. In view of the potential of human APC as an anticoagulant and profibrinolytic agent, the pharmacokinetics and tissue distribution of APC were studied in guinea pigs. The plasma elimination of a trace dose of 125I-APC was biphasic following an initial rapid elimination of approximately 15% of the injected dose within 1 to 2 minutes. This rapid removal of 125I-APC from the circulation was found to be a result of an association with the liver regardless of the route of injection. Essentially identical results were obtained with active site-blocked forms of APC generated with either diisopropylfluorophosphate or D- phenylalanyl-L-prolyl-L-arginine chloromethyl ketone, which indicates that the active site was not essential for the liver association. Accumulation of all three forms of APC in the liver peaked at 30 minutes and then declined as increasing amounts of degraded radiolabeled material appeared in the gastrointestinal tract and urine. Removal of the gamma-carboxyglutamic acid (gla) domain of diisopropylphosphoryl-APC resulted in a 50% reduction in the association with liver and an accumulation in the kidneys. Protein C and protein S were cleared from the circulation at rates approximately one-half and one-fourth, respectively, that of APC. Both in vitro and in vivo, APC was found to form complexes with protease inhibitors present in guinea pig plasma. Complex formation resulted in a more rapid disappearance of the enzymatic activity of APC than elimination of the protein moiety. These findings indicate two distinct mechanisms for the elimination of APC. One mechanism involves reaction with plasma protease inhibitors and subsequent elimination by specific hepatic receptors. The other mechanism involves the direct catabolism of APC by the liver via a pathway that is nonsaturable over a substantial dose range and independent of the active site. This pattern of elimination is distinctly different from that observed with the homologous coagulation enzymes thrombin, factor IXa, and factor Xa.

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Berger, H. Jr, CG Kirstein et CL Orthner. « Pharmacokinetics of activated protein C in guinea pigs ». Blood 77, n^o 10 (15 mai 1991) : 2174–84. http://dx.doi.org/10.1182/blood.v77.10.2174.bloodjournal77102174.

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Protein C is a vitamin K-dependent zymogen of the serine protease, activated protein C (APC), an important regulatory enzyme in hemostasis. In view of the potential of human APC as an anticoagulant and profibrinolytic agent, the pharmacokinetics and tissue distribution of APC were studied in guinea pigs. The plasma elimination of a trace dose of 125I-APC was biphasic following an initial rapid elimination of approximately 15% of the injected dose within 1 to 2 minutes. This rapid removal of 125I-APC from the circulation was found to be a result of an association with the liver regardless of the route of injection. Essentially identical results were obtained with active site-blocked forms of APC generated with either diisopropylfluorophosphate or D- phenylalanyl-L-prolyl-L-arginine chloromethyl ketone, which indicates that the active site was not essential for the liver association. Accumulation of all three forms of APC in the liver peaked at 30 minutes and then declined as increasing amounts of degraded radiolabeled material appeared in the gastrointestinal tract and urine. Removal of the gamma-carboxyglutamic acid (gla) domain of diisopropylphosphoryl-APC resulted in a 50% reduction in the association with liver and an accumulation in the kidneys. Protein C and protein S were cleared from the circulation at rates approximately one-half and one-fourth, respectively, that of APC. Both in vitro and in vivo, APC was found to form complexes with protease inhibitors present in guinea pig plasma. Complex formation resulted in a more rapid disappearance of the enzymatic activity of APC than elimination of the protein moiety. These findings indicate two distinct mechanisms for the elimination of APC. One mechanism involves reaction with plasma protease inhibitors and subsequent elimination by specific hepatic receptors. The other mechanism involves the direct catabolism of APC by the liver via a pathway that is nonsaturable over a substantial dose range and independent of the active site. This pattern of elimination is distinctly different from that observed with the homologous coagulation enzymes thrombin, factor IXa, and factor Xa.

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Quinn, Louise M., Clive Drakeford, James S. O’Donnell et Roger J. S. Preston. « Engineering activated protein C to maximize therapeutic efficacy ». Biochemical Society Transactions 43, n^o 4 (1 août 2015) : 691–95. http://dx.doi.org/10.1042/bst20140312.

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The anticoagulant-activated protein C (APC) acts not solely as a crucial regulator of thrombus formation following vascular injury, but also as a potent signalling enzyme with important functions in the control of both acute and chronic inflammatory disease. These properties have been exploited to therapeutic effect in diverse animal models of inflammatory disease, wherein recombinant APC administration has proven to effectively limit disease progression. Subsequent clinical trials led to the use of recombinant APC (Xigris) for the treatment of severe sepsis. Although originally deemed successful, Xigris was ultimately withdrawn due to lack of efficacy and an unacceptable bleeding risk. Despite this apparent failure, the problems that beset Xigris usage may be tractable using protein engineering approaches. In this review, we detail the protein engineering approaches that have been utilized to improve the therapeutic characteristics of recombinant APC, from early studies in which the distinct anti-coagulant and signalling activities of APC were separated to reduce bleeding risk, to current attempts to enhance APC cytoprotective signalling output for increased therapeutic efficacy at lower APC dosage. These novel engineered variants represent the next stage in the development of safer, more efficacious APC therapy in disease settings in which APC plays a protective role.

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Spek, C. Arnold, et Valder Arruda. « The Role of Activated Protein C in Cancer ». Blood 118, n^o 21 (18 novembre 2011) : SCI—18—SCI—18. http://dx.doi.org/10.1182/blood.v118.21.sci-18.sci-18.

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Abstract Abstract SCI-18 Activated protein C (APC) is a natural anticoagulant that blocks the amplification of the coagulation cascade via inactivation of factors Va and VIIIa. The APC/PC pathway is initiated by complex formation of thrombin, thrombomodulin (TM), and the endothelial protein C receptor (EPCR) allowing the conversion of zymogen protein C into its activated form. Based on the well-accepted view that coagulation contributes to cancer progression and that anticoagulant treatment may benefit some cancer patient groups, it has been hypothesized that the natural anticoagulant protein C pathway may also play a role in cancer progression. Interestingly, it has recently been shown that endogenous APC limits experimental metastasis of B16 melanoma cells in mouse lungs. Notably, an APC-blocking antibody dramatically increased the number of experimental lung metastasis although not due to diminished anticoagulant activity of APC but largely due to reduced APC-driven S1P1-mediated VE-cadherin-dependent vascular barrier enhancement. In line with these findings repeated administration of recombinant human APC as well as transgenic overexpression of EPCR also diminished experimental metastasis of B16 melanoma cells. It may thus be tempting to speculate that recombinant APC could be a novel treatment strategy to limit cancer progression. However, APC has however a short half-life, needs intravenous administration, and is associated with severe bleeding complications, complicating the potential clinical application of these findings. In contrast to APC, zymogen PC has a longer half-life and is associated with significantly less bleeding complications. To prove or refute the hypothesis that zymogen PC may be an attractive alternative treatment option for APC in cancer patients, we recently compared the effect of continuous overexpression of murine APC or zymogen PC in the liver by viral-mediated gene transfer in experimental metastasis. Interestingly, both APC and zymogen PC overexpression was highly effective in limiting experimental metastasis. An APC variant (APC-5A) with reduced anticoagulant but normal signaling properties did not limit experimental metastasis, whereas the protective effect of zymogen PC remained even in the absence of protease activated receptor-1 (PAR-1), which is the main mediator of APCs cytoprotective effect. Zymogen PC may thus be a novel therapeutic target to limit cancer progression. In conclusion, the natural anticoagulant APC pathway may play an important role in limiting cancer cell extravasation and interventions seeking to modulate the PC system may ultimately benefit the cancer patient. The challenge is however to confirm these findings in alternative preclinical cancer models and eventually to translate our findings into a clinical setting. Disclosures: No relevant conflicts of interest to declare.

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Bangalore, Neelesh, William N. Drohan et Carolyn L. Orthner. « High Affinity Binding Sites for Activated Protein C and Protein C on Cultured Human Umbilical Vein Endothelial Cells ». Thrombosis and Haemostasis 72, n^o 03 (1994) : 465–74. http://dx.doi.org/10.1055/s-0038-1648890.

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SummaryActivated protein C (APC) is an antithrombotic serine proteinase having anticoagulant, profibrinolytic and anti-inflammatory activities. Despite its potential clinical utility, relatively little is known about its clearance mechanisms. In the present study we have characterized the interaction of APC and its active site blocked forms with human umbilical vein endothelial cells (HUVEC). At 4° C 125I-APC bound to HUVEC in a specific, time dependent, saturable and reversible manner. Scatchard analysis of the binding isotherm demonstrated a Kd value of 6.8 nM and total number of binding sites per cell of 359,000. Similar binding isotherms were obtained using radiolabeled protein C (PC) zymogen as well as D-phe-pro-arg-chloromethylketone (PPACK) inhibited APC indicating that a functional active site was not required. Competition studies showed that the binding of APC, PPACK-APC and PC were mutually exclusive suggesting that they bound to the same site(s). Proteolytic removal of the N-terminal γ-carboxyglutamic acid (gla) domain of PC abolished its ability to compete indicating that the gla-domain was essential for cell binding. Surprisingly, APC binding to these cells appeared to be independent of protein S, a cofactor of APC generally thought to be required for its high affinity binding to cell surfaces. The identity of the cell binding site(s), for the most part, appeared to be distinct from other known APC ligands which are associated with cell membranes or extracellular matrix including phospholipid, thrombomodulin, factor V, plasminogen activator inhibitor type 1 (PAI-1) and heparin. Pretreatment of HUVEC with antifactor VIII antibody caused partial inhibition of 125I-APC binding indicating that factor VIII or a homolog accounted for ∼30% of APC binding. Studies of the properties of surface bound 125I-APC or 125I-PC and their fate at 4°C compared to 37 °C were consistent with association of ∼25% of the initially bound radioligand with an endocytic receptor. However, most of the radioligand appeared not to be bound to an endocytic receptor and dissociated rapidly at 37° C in an intact and functional state. These data indicate the presence of specific, high affinity binding sites for APC and PC on the surface of HUVEC. While a minor proportion of binding sites may be involved in endocytosis, the identity and function of the major proportion is presently unknown. It is speculated that this putative receptor may be a further mechanisms of localizing the PC antithrombotic system to the vascular endothelium.

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Váradi, K., J. Rosing, G. Tans, I. Pabinger, B. Keil et H. P. Schwarz. « Factor V Enhances the Cofactor Function of Protein S in the APC-Mediated Inactivation of Factor VIII : Influence of the Factor VR506Q Mutation ». Thrombosis and Haemostasis 76, n^o 02 (1996) : 208–14. http://dx.doi.org/10.1055/s-0038-1650556.

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SummaryFactor V and protein S are cofactors of activated protein C (APC) which accelerate APC-mediated factor VIII inactivation. The effects of factor V and protein S were quantitated in a reaction system in which plasma factor VIII was inactivated by APC and the loss of factor VIII activity was monitored in a factor X-activating system in which a chro-mogenic substrate was used to probe factor Xa formation. Factor V increased the rate of APC-mediated factor VIII inactivation in a dose-dependent manner in representative plasma samples with protein S or factor V deficiency, abnormal factor V (heterozygous or homozygous for factor VR506Q), or a combination of heterozygous protein S deficiency and heterozygous factor VR506Q. This effect was much less pronounced in the plasma samples with a decreased protein S level, but the impaired response in these plasmas was corrected by addition of protein S, indicating that both factor V and protein S are required for optimal inactivation of factor VIII by APC. The effects of factor V and protein S were also studied in a reaction system with purified proteins. APC-catalysed factor VIII inactivation was enhanced 3.7-fold in the presence of 1.1 nM factor V and 1.5-fold in the presence of 2.4 nM protein S. When both 1.1 nM factor V and 2.4 nM protein were present the rate enhancement was 11-fold. Factor V is a more potent cofactor than protein S, as can be concluded from the fact that 0.04 nM factor V gave the same stimulation as 2.4 nM protein S. Protein S lost its cofactor function after complexation with C4b binding protein, which indicates that it is free protein S that acts as a cofactor. To investigate the effect of the R506Q mutation in factor V on APC-mediated factor VIII inactivation, factor V was purified from the plasma of patients homozygous for factor VR506Q. In the absence of protein S, factor VR506Q did not enhance factor VIII inactivation by APC, but in the presence of 2.4 nM protein S a slight enhancement was observed. The APC cofactor activity of factor V was lost when factor V was activated with thrombin or with the factor V activator from Russell’s viper venom. These data indicate that optimal inactivation of factor VIII by APC requires the presence of an intact factor V molecule and free protein S.

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