The gamma-carboxyglutamic acid domain of anticoagulant protein S is involved in activated protein C cofactor activity, independently of phospholipid binding

We expressed 2 chimeras between human protein S (PS) and human prothrombin (FII) in which the prothrombin gamma-carboxyglutamic acid (Gla) domain replaced the PS Gla domain in native PS (Gla(FII)-PS) or in PS deleted of the thrombin-sensitive region (TSR) (Gla(FII)-DeltaTSR-PS). Neither PS/FII chimera had activated protein C (APC) cofactor activity in plasma clotting assays or purified systems, but both bound efficiently to phospholipids. This pointed to a direct involvement of the PS Gla domain in APC cofactor activity through molecular interaction with APC. Using computational methods, we identified 2 opposite faces of solvent-exposed residues on the PS Gla domain (designated faces 1 and 2) as potentially involved in this interaction. Their importance was supported by functional characterization of a PS mutant in which the face 1 and face 2 PS residues were reintroduced into Gla(FII)-PS, leading to significant APC cofactor activity, likely through restored interaction with APC. Furthermore, by characterizing PS mutants in which PS face 1 and PS face 2 were individually replaced by the corresponding prothrombin faces, we found that face 1 was necessary for efficient phospholipid binding but that face 2 residues were not strictly required for phospholipid binding and were involved in the interaction with APC.     

Cardiolipin enhances protein C pathway anticoagulant activity

The anticoagulant activity of activated protein C (APC) was studied using factor Xa-1-stage assays of both the procoagulant and anticoagulant activities of phospholipid vesicles containing phosphatidylserine or cardiolipin as active phospholipids. In the absence of APC, phosphatidylserine vesicles showed higher procoagulant activity than cardiolipin vesicles whereas cardiolipin vesicles supported APC-dependent anticoagulant activity better than phosphatidylserine vesicles. Enhancement of APC anticoagulant activity in plasma by cardiolipin was markedly stimulated by the APC cofactor protein S. In purified reaction mixtures, cardiolipin in phospholipid vesicles dose-dependently enhanced APC anticoagulant activity. This effect of cardiolipin was partially dependent on protein S, and immunoblotting studies showed that cardiolipin enhanced the APC-mediated cleavage of the factor Va heavy chain at Arg506 and Arg306. In solid-phase binding assays, increasing amounts of cardiolipin in multicomponent phospholipid vesicles increased the affinity for protein S and to a lesser extent APC. These data are consistent with the hypothesis that cardiolipin stimulates the anticoagulant protein C pathway by increasing the affinity of phospholipid surfaces for protein S:APC and by enhancing inactivation of factor Va by APC due to cleavages at Arg506 and Arg306 in factor Va. Based on this, it is further hypothesized that anti-cardiolipin or anti-oxidized cardiolipin antibodies may be thrombogenic because they inhibit phospholipid-dependent expression of the anticoagulant protein C pathway.     

Structural model of human alpha1-microglobulin: proposed scheme for the interaction with the Gla domain of anticoagulant protein C.

Alpha1-microglobulin (alpha1m) is a small glycoprotein with immunomodulatory properties. It is a member of the lipocalin family, a group of proteins that exhibit a well-conserved three-dimensional structure despite low sequence identity and that are known to bind small hydrophobic ligands. The types of ligands carried by alpha1m are still unknown, but it is known that this protein has yellow-brown chromophores attached to three lysines at position 92, 118 and 130. Alpha1m has one unpaired cysteine residue (Cys 34) that can form a disulphide bond with other proteins that also possess an exposed free unpaired cysteine. For instance, alpha1m interacts with the protein C (PC) Gla domain containing the Arg9Cys or Ser12Cys substitution. In order to gain insights about the alpha1m molecule and analyze the intriguing alpha1m-Gla domain interaction, it was decided to use bioinformatics. The three-dimensional structures of alpha1m and PC Gla domain were predicted. Alpha1m Cys 34 is solvent exposed and located near the entrance of the ligand-binding pocket. The chromophore-carrying lysines are found buried into the pocket, and the area around the entrance of this cavity displays about 10 positively charged residues. This electropositive region in alpha1m complements the essentially electronegative Gla domain and may play a role during intermolecular interactions. In addition, a few hydrophobic residues surround alpha1m Cys 34 and could be of importance during its interaction with macromolecular ligands.     

Inhibition of activated protein C anticoagulant activity by prothrombin

In this study, we test the hypothesis that prothrombin levels may modulate activated protein C (APC) anticoagulant activity. Prothrombin in purified systems or plasma dramatically inhibited the ability of APC to inactivate factor Va and to anticoagulate plasma. This was not due solely to competition for binding to the membrane surface, as prothrombin also inhibited factor Va inactivation by APC in the absence of a membrane surface. Compared with normal factor Va, inactivation of factor Va Leiden by APC was much less sensitive to prothrombin inhibition. This may account for the observation that the Leiden mutation has less of an effect on plasma-based clotting assays than would be predicted from the purified system. Reduction of protein C levels to 20% of normal constitutes a significant risk of thrombosis, yet these levels are observed in neonates and patients on oral anticoagulant therapy. In both situations, the correspondingly low prothrombin levels would result in an increased effectiveness of the remaining functional APC of approximately 5-fold. Thus, while the protein C activation system is impaired by the reduction in protein C levels, the APC that is formed is a more effective anticoagulant, allowing protein C levels to be reduced without significant thrombotic risk. In situations where prothrombin is high and protein C levels are low, as in early stages of oral anticoagulant therapy, the reduction in protein C would result only in impaired function of the anticoagulant system, possibly explaining the tendency for warfarin-induced skin necrosis.     

Activated protein C variants with normal cytoprotective but reduced anticoagulant activity

Recombinant activated protein C (APC), a well-defined anticoagulant enzyme, reduced mortality in severe sepsis patients in a phase 3 trial. However, 2 potent anticoagulants, antithrombin III and recombinant tissue factor pathway inhibitor, failed to do so, implying the physiologic relevance of APC's less well-defined anti-inflammatory and antiapoptotic activities. Recombinant APC therapy conveys an increased risk of serious bleeding complications due to APC anticoagulant activity. To generate recombinant APC variants with reduced risk of bleeding due to reduced anticoagulant activity, we dissected APC's anticoagulant activity from its cytoprotective activity by site-directed mutagenesis. Using staurosporine-induced endothelial cell apoptosis assays, we show here that Ala mutations (RR229/230AA and KKK191_ 193AAA) in 2 APC surface loops that severely reduce anticoagulant activity result in 2 APC variants that retain normal antiapoptotic activity that requires protease activated receptor-1 and endothelial cell protein C receptor. Thus, it is possible to reduce anticoagulant activity while preserving antiapoptotic activity of recombinant APC variants. We suggest that therapeutic use of such APC variants may reduce serious bleeding risks while providing the beneficial effects of APC acting directly on cells.     

Crystal structure of an anticoagulant protein in complex with the Gla domain of factor X

The gamma-carboxyglutamic acid (Gla) domain of blood coagulation factors is responsible for Ca2+-dependent phospholipid membrane binding. Factor X-binding protein (X-bp), an anticoagulant protein from snake venom, specifically binds to the Gla domain of factor X. The crystal structure of X-bp in complex with the Gla domain peptide of factor X at 2.3-A resolution showed that the anticoagulation is based on the fact that two patches of the Gla domain essential for membrane binding are buried in the complex formation. The Gla domain thus is expected to be a new target of anticoagulant drugs, and X-bp provides a basis for designing them. This structure also provides a membrane-bound model of factor X.     

True anti-anionic phospholipid immunoglobulin M antibodies can exert lupus anticoagulant activity

True (cofactor-independent) anticardiolipin antibodies (aCL) are thought to lack lupus anticoagulant (LA) activity and pathogenic potential. A serum monoclonal immunoglobulin Mlambda (mIgMlambda) with aCL and LA activities found in a man with a splenicIgMlambda+ B-cell lymphoplasmacytic lymphoma (LPL) without thrombotic events has been characterized. LPL-derived hybridoma clones (designated HY-FRO) producing the serum mIgMlambda were obtained. mIgMlambda secreted by HY-FRO grown in protein-free culture medium, like that purified from serum, (i) showed binding, in a cofactor-free system, to solid-phase CL and phosphatidylserine (PS) and to the membrane of PS-expressing cells (apoptotic cells and activated platelets); (ii) failed to bind neutral phospholipids (PL), beta2Glycoprotein, histone, ssDNA, dsDNA, human IgG and umbilical vein endothelial cells. Absorption with apoptotic cells abolished its binding to anionic plate-bound CL and PS. IgMlambda-FRO used poorly mutated VH and Vlambda region genes, with a pattern that was inconsistent with an antigen-driven selection. Basic amino acids were present in the IgH complementarity determining region 3 (CDR3), which can be important for binding to anionic PL. These findings demonstrate unequivocally that true anti-anionic PL IgM antibodies can exert LA and indicate this anti-PL type does not involve thrombophilia.     

Inflammation and the activated protein C anticoagulant pathway

After a coagulation stimulus, the blood clotting cascade amplifies largely unchecked until very high levels of thrombin are generated. Natural anticoagulant mechanisms (for example, the protein C anticoagulant pathway) are amplified to prevent excessive thrombin generation. Thrombin binds to thrombomodulin (TM) and this complex and then activates protein C approximately 1000 times faster than free thrombin. Protein C activation is enhanced approximately 20-fold further by the endothelial cell protein C receptor (EPCR). Activated protein C proteolytically inactivates factor Va (FVa) and FVIIIa, thereby blocking the amplification of the coagulation system, a process that is accelerated by protein S. TM not only accelerates protein C activation, but also decreases endothelial cell activation by blocking high-mobility group protein-B1 inflammatory functions and suppressing both nuclear factor-kappa B nuclear translocation and the mitogen-activated protein kinase pathways. The thrombin-TM complex also activates thrombin-activatable fibrinolysis inhibitor, a procarboxypeptidase that renders fibrin resistant to clot lysis and neutralizes vasoactive molecules such as complement C5a. Activated protein C has a variety of antiinflammatory activities. It suppresses inflammatory cytokine elevation in animal models of severe sepsis, inhibits leukocyte adhesion, decreases leukocyte chemotaxis, reduces endothelial cell apoptosis, helps maintain endothelial cell barrier function through activation of the sphingosine-1 phosphate receptor, and minimizes the decrease in blood pressure associated with severe sepsis. Most of these functions are dependent on binding to EPCR. Overall this pathway is critical to both regulation of the blood coagulation process, and control of the innate inflammatory response and some of its associated downstream pathologies.     

Mechanism and effects of the binding of lupus anticoagulant IgG and prothrombin to surface phospholipid

We report here experiments on how lupus anticoagulant antibodies (LA IgG) that react with prothrombin bind to surface phospholipid and affect prothrombin's affinity for surface phospholipid and activation to thrombin. LA IgG was purified by protein A chromatography from the plasma of 16 patients of whom four had associated hypoprothrombinemia and 10 had experienced thrombosis. Many LA IgG bound, in the absence of phospholipid and calcium, not only to immobilized prothrombin but to both prothrombin 1 and fragment 1, which established at least an oligoclonal origin of LA IgG. No LA IgG bound to thrombin. Although prothrombin and Ca2+ were required to support binding of LA IgG to immobilized phosphatidylserine (PS), prothrombin at higher concentrations inhibited binding, presumably by competing with prothrombin/LA IgG complexes for PS binding sites. Prothrombin 1, which cannot bind to PS, also inhibited binding of many LA IgG to PS, presumably by forming competing soluble prothrombin 1/LA IgG complexes. Despite their ability to react with prothrombin independent of phospholipid, LA IgG enhanced binding of prothrombin to immobilized phospholipid and to cultured human umbilical vein endothelial cells. Prothrombin bound with LA IgG to the surface of endothelial cell monolayers could be activated to thrombin after supernatant prothrombin and LA IgG were washed away. The relation is discussed of these observations to a hypothesis that LA IgG mediated concentration of prothrombin on cell surface phospholipid represents a mechanism by which LA IgG could increase thrombotic risk.     

The protein C ω-loop substitution Asn2Ile is associated with reduced protein C anticoagulant activity

We report a kindred with heritable protein C (PC) deficiency in which two siblings with severe thrombosis showed a composite type I and IIb PC deficiency phenotype, identified using commercial PC assays (proband: PC antigen 42 u/dl, amidolytic activity 40 u/dl, anticoagulant activity 9 u/dl). The independent PROC nucleotide variations c.669C>A (predictive of Ser181Arg) and c.131C>T (predictive of Asn2Ile) segregated with the type I and type IIb PC deficiency phenotypes respectively, but co-segregated in the siblings with severe thrombosis. Soluble thrombomodulin (sTM)-mediated inhibition of plasma thrombin generation from an individual with PC-Asn2Ile was lower (endogenous thrombin potential (ETP) 56 ± 1% that of ETP determined without sTM) than control plasma (ETP 15 ± 2%) indicating reduced PC anticoagulant activity. Recombinant APC-Asn2Ile exhibited normal amidolytic activity but impaired anticoagulant activity. Protein S (PS)-dependent anticoagulant activity of recombinant APC-Asn2Ile and binding of recombinant APC-Asn2Ile to endothelial protein C receptor (EPCR) were reduced compared to recombinant wild-type APC. Asn2 lies within the ω-loop of the PC/APC Gla domain and this region is critical for calcium-induced folding and subsequent interactions with anionic phospholipids, EPCR and PS. The disruption of these interactions in this naturally-occurring PC variant highlights their collective importance in mediating APC anticoagulant activity in vivo.     

Role of individual gamma-carboxyglutamic acid residues of activated human protein C in defining its in vitro anticoagulant activity.

To evaluate the contributions of individual gamma-carboxyglutamic acid (gla) residues to the overall Ca(2+)-dependent anticoagulant activity of activated human protein C (APC), we used recombinant (r) DNA technology to generate protein C (PC) variants in which each of the gla precursor glutamic acid (E) residues (positions 6, 7, 14, 16, 19, 20, 25, 26, and 29) was separately altered to aspartic acid (D). In one case, a gla26V mutation ([gla26V]r-PC) was constructed because a patient with this particular substitution in coagulation factor IX had been previously identified. Two additional r-PC mutants were generated, viz, an r-PC variant containing a substitution at arginine (R) 15 ([R15]r-PC), because this particular R residue is conserved in all gla-containing blood coagulation proteins, as well as a variant r-PC with substitution of an E at position 32 ([F31L, Q32E]r-PC), because gla residues are found in other proteins at this sequence location. This latter protein did undergo gamma-carboxylation at the newly inserted E32 position. For each of the 11 recombinant variants, a subpopulation of PC molecules that were gamma-carboxylated at all nonmutated gla-precursor E residues has been purified by anion exchange chromatography and, where necessary, affinity chromatography on an antihuman PC column. The r-PC muteins were converted to their respective r-APC forms and assayed for their amidolytic activities and Ca(2+)-dependent anticoagulant properties. While no significant differences were found between wild-type (wt) r-APC and r-APC mutants in the amidolytic assays, lack of a single gla residue at any of the following locations, viz, 7, 16, 20, or 26, led to virtual complete disappearance of the Ca(2+)-dependent anticoagulant activity of the relevant r-APC mutant, as compared with its wt counterpart. On the other hand, single eliminations of any of the gla residues located at positions 6, 14, or 19 of r-APC resulted in variant recombinant molecules with substantial anticoagulant activity (80% to 92%), relative to wtr-APC. Mutation of gla residues at positions 25 and 29 resulted in r-APC variants with significant but low (24% and 9% of wtr-APC, respectively) levels of anticoagulant activity. The variant, [R15L]r-APC, possessed only 19% of the anticoagulant activity of wrt-APC, while inclusion of gla at position 32 in the variant, [F31L, Q32gla]r-APC, resulted in a recombinant enzyme with an anticoagulant activity equivalent to that of wtr-APC.     

Liberating R169 promotes anticoagulant protein C

In this issue of Blood, Pozzi et al demonstrate that removing an anionic cage promotes exposure of R169 thereby generating a protein C (PC) that is far more readily activated.     

Quantification of human plasma phospholipid transfer protein (PLTP): relationship between PLTP mass and phospholipid transfer activity

A sensitive sandwich-type enzyme-linked immunosorbent assay (ELISA) for human plasma phospholipid transfer protein (PLTP) has been developed using a monoclonal capture antibody and a polyclonal detection antibody. The ELISA allows for the accurate quantification of PLTP in the range of 25-250 ng PLTP/assay. Using the ELISA, the mean plasma PLTP concentration in a Finnish population sample (n = 159) was determined to be 15.6 +/- 5.1 mg/l, the values ranging from 2.30 to 33.4 mg/l. PLTP mass correlated positively with HDL-cholesterol (r = 0.36, P < 0.001), apoA-I (r = 0.37, P < 0.001), apoA-II (r = 0.20, P < 0.05), Lp(A-I) (r=0.26, P=0.001) and Lp(A-I/A-II) particles (r=0.34, P<0.001), and negatively with body mass index (BMI) (r = -0.28, P < 0.001) and serum triacylglycerol (TG) concentration (r = -0.34, P < 0.001). PLTP mass did not correlate with phospholipid transfer activity as measured with a radiometric assay. The specific activity of PLTP, i.e. phospholipid transfer activity divided by PLTP mass, correlated positively with plasma TG concentration (r=0.568, P<0.001), BMI (r=0.45, P<0.001), apoB (r = 0.45, P < 0.001). total cholesterol (r=0.42, P < 0.001), LDL-cholesterol (r = 0.34, P < 0.001) and age (r = 0.36, P < 0.001), and negatively with HDL-cholesterol (r= -0.33, P < 0.001), Lp(A-I) (r= -0.21, P < 0.01) as well as Lp(A-I/A-II) particles (r = -0.32, P < 0.001). When both PLTP mass and phospholipid transfer activity were adjusted for plasma TG concentration, a significant positive correlation was revealed (partial correlation, r = 0.31, P < 0.001). The results suggest that PLTP mass and phospholipid transfer activity are strongly modulated by plasma lipoprotein composition: PLTP mass correlates positively with parameters reflecting plasma high density lipoprotein (HDL) levels, but the protein appears to be most active in subjects displaying high TG concentration.     

Regulation and functions of the protein C anticoagulant pathway

The protein C pathway plays a critical role in the negative regulation of the blood clotting process. We recently identified an endothelial cell receptor for protein C/activated protein C (APC). The receptor is localized almost exclusively on endothelial cells of large vessels and is present at only trace levels or indeed absent from capillaries in most tissues. Patients with sepsis or lupus erythematosus exhibit elevated levels of plasma EPCR which migrates on gels as a single band and is fully capable of binding protein C/APC. There is no correlation with thrombomodulin levels, probably due to different vascular localizations and/or cellular release mechanisms. To understand the mechanisms by which EPCR plasma levels are elevated, we examined EPCR mRNA expression in a rat endotoxin shock model. The EPCR mRNA gene exhibited an early immediate gene response to endotoxin with the mRNA levels increasing nearly 4 fold in the first 3-6 hrs, before returning toward baseline. Plasma levels of EPCR also rose about 4 fold with little change in tissue EPCR levels. Both processes were markedly attenuated by hirudin suggesting that thrombin was responsible for increases in mRNA and plasma EPCR levels. At the level of mRNA, the induction is mediated by a thrombin response element in the 5' flanking region of the gene. Direct thrombin infusion and cell culture experiments support this contention. On endothelium, thrombin is capable of releasing cell surface EPCR and this process is blocked by the metalloproteinase inhibitor orthophenanthroline. Taken together these studies indicate that elevation in soluble plasma EPCR reflects endothelial cell activation in the larger vessels and is likely to be an indication of local thrombin generation near these vessel surfaces.     

Complexes of anti-prothrombin antibodies and prothrombin cause lupus anticoagulant activity by competing with the binding of clotting factors for catalytic phospholipid surfaces

We investigated the mechanism by which anti-prothrombin antibodies cause lupus anticoagulant (LAC) activity. Addition of affinity-purified anti-prothrombin antibodies from LAC-positive plasma samples (alpha-FII-LAC+) to normal plasma induced LAC activity. Upon increasing the phospholipid concentration, LAC activity was neutralized. Addition of purified alpha-FII-LAC+ to normal plasma strongly inhibited factor Xa formation. No inhibition was measured when alpha-FII-LAC+ were added to prothrombin-deficient plasma or when purified anti-prothrombin antibodies from LAC-negative plasma samples (alpha-FII-LAC-) were added. When a combination of prothrombin and alpha-FII-LAC+ was added to the purified clotting complex, a strong inhibition of factor Xa and IIa formation was seen. The alpha-FII-LAC+ alone or a combination of prothrombin and alpha-FII-LAC- did not show inhibition. Ellipsometry studies showed that, in the presence of alpha-FII-LAC+, the affinity of prothrombin for a phospholipid surface increased dramatically, whereas a much lower increase was observed with alpha-FII-LAC-. Our results show that complexes of prothrombin and anti-prothrombin antibodies with LAC activity inhibit both prothrombinase and tenase. The antibodies increase the affinity of prothrombin for the phospholipid surface, thereby competing with clotting factors for the available catalytic phospholipid surface, a mechanism similar to that of anti-beta2-glycoprotein I antibodies.     

Exosite-dependent regulation of the protein C anticoagulant pathway

Activated protein C (APC) is a vitamin K-dependent anticoagulant serine protease in plasma that downregulates the coagulation cascade by degrading cofactors Va and VIIIa by limited proteolysis. In addition to its anticoagulant function, APC also exhibits potent profibrinolytic and anti-inflammatory properties. The proteolytic activity of APC in plasma is slowly inhibited by three serpins: protein C inhibitor, plasminogen activator inhibitor-1, and alpha(1)-antitrypsin. Recent structural and mutagenesis data have indicated that basic residues of three exposed surface loops known as the 39-loop (Lys(37)-Lys(39)), 60-loop (Lys(62), Lys(63)), and 70-80-loop (Arg(74), Arg(75), and Lys(78)) (chymotrypsin numbering) constitute an anion-binding exosite in APC that interacts with these macromolecular substrates and inhibitors. Moreover, this exosite plays a critical role in the thrombomodulin-dependent activation of the zymogen protein C by thrombin. This article briefly reviews how the binding of physiological protein and polysaccharide cofactors on this exosite modulates the protein C anticoagulant pathway in plasma.     

Five novel mutations located in exons III and IX of the protein C gene in patients presenting with defective protein C anticoagulant activity

We describe five families presenting with type II hereditary protein C deficiency characterized by normal antigen and amidolytic activity levels but low anticoagulant activity. All the exons and intron/exon junctions of the protein C gene were studied using a strategy combining amplification by the polymerase chain reaction (PCR), denaturing gradient gel electrophoresis of the amplified fragments, and direct sequencing of fragments displaying altered melting behavior. We detected five novel mutations. Three were located in the C-terminal part of the propeptide encoded by exon III: Arginine (Arg)-5 to tryptophan (Trp), Arg-1 to histidine (His), and Arg-1 to cysteine (Cys) mutations. The two others, located in exon IX, affected Arg 229 and serine (Ser) 252, which were respectively replaced by glutamine (Gln) and asparagine (Asn). DNA studies of the other exons from affected individuals showed no other abnormalities. These novel mutations provide further insight into the importance of the affected amino acids located close to the active site, near Asp 257, one of the three amino acids of the catalytic triad. The low anticoagulant activity of the abnormal protein C indicated that Arg 229 and Ser 252 play a key role during the interaction between protein C and its cofactor protein S, phospholipids, or factors Va and VIIIa. The Arg-1 to Cys mutation led to the dimerization of protein C with another plasmatic component, as evidenced by the presence in the plasma of a high molecular weight form of protein C that disappeared after reduction. No molecular mass abnormalities were observed in heavy and light chains of all other protein C mutants. In the five families explored, 9 (64%) of the 14 subjects bearing the mutations reported thrombotic events. This suggests that the protein C amino acids affected by the mutations are very important for the in vivo expression of the antithrombotic properties of protein C.     

Evaluation of protein C and protein S levels during oral anticoagulant therapy

A number of workers have examined protein C in relation to other vitamin K dependent factors during warfarin therapy and successfully identified protein C deficient patients by ratio calculation. However, protein S deficiency has not been addressed in this manner. This study compares protein C and protein S by functional and antigenic determination with procoagulant factors of similar half life (Factors VII and II) in an attempt to identify protein C and protein S deficient patients whilst on oral anticoagulant therapy. Procoagulant and anticoagulant factors were compared by linear regression in a population of normal blood donors and patients on stabilized warfarin therapy to obtain expected values for protein C and protein S dependent upon FVII and FII levels, respectively. Observed over expected values for protein C and protein S were calculated for individual patients and normal ranges derived. Comparison of similarly calculated observed over expected protein C and protein S ratios with these normal ranges successfully identified known protein C and protein S deficient patients who were taking warfarin at time of testing.     

Monocytes, but not endothelial cells, downregulate the anticoagulant activity of activated protein C

Activated protein C (APC) is a natural anticoagulant and inhibits thrombin generation by degrading factors Va and VIIIa. We evaluated the ability of APC to inhibit blood coagulation triggered by lipopolysaccharide (LPS)-stimulated [tissue factor (TF)-expressing] human mononuclear cells (MNCs) or umbilical vein endothelial cells (HUVECs). Using a plasma recalcification assay, we found that APC (up to 53.3 nmol/l final concentration) had a poor anticoagulant effect in the presence of LPS-stimulated MNCs, whereas it caused a marked prolongation of clotting time in the presence of LPS-stimulated HUVECs. A poor response to APC was also observed when platelet-free MNCs, monocyte-enriched preparations or the monocytoid cell line U937 were tested. Using a TF-independent (FXa-induced) thrombin generation assay, we demonstrated that both LPS-stimulated and unstimulated MNCs negated the inhibitory activity of APC. Direct determination of FVa activity indicated that MNCs were less efficient than HUVECs in promoting FVa inactivation by APC. Together, our results suggest that MNCs, at variance with HUVECs, protect factor Va from inactivation by APC, probably through the expression of a membrane component not present on endothelial cells. These strengthen the importance of monocytes in fibrin deposition associated with pathological conditions characterized by monocyte recruitment and activation.     

Retinoic acid induces matrix Gla protein gene expression in human cells.

The objective of this study was to investigate the possible regulation of the vitamin K-dependent matrix Gla (gamma-carboxyglutamic acid) protein (MGP) by retinoic acid, a regulation suggested by the recent observation that the human MGP promoter has a perfect direct repeat which is nearly identical to the retinoic acid-responsive element in the retinoic acid receptor-beta gene. We report that retinoic acid strongly increases MGP mRNA levels in all human cells tested, including osteoblasts, articular cartilage chondrocytes, and fibroblasts. In osteoblastic cells, MGP mRNA levels are increased by 25-fold at 1 microM retinoic acid and achieve half-maximal levels at 0.1 microM hormone. MGP is a small secreted protein of unknown function that is synthesized in a wide variety of vertebrate tissues. The present results suggest that part of the known actions of retinoic acid on skin, bone, cartilage, and other tissues in the human may be mediated by the stimulation of MGP synthesis and the consequent effect of increased MGP secretion on nearby target cells.