Mechanisms of factor VIIa‐catalyzed activation of factor VIII

Summary. Background: Factor (F)VIIa, complexed with tissue factor (TF), is a primary trigger of blood coagulation, and has extremely restricted substrate specificity. The complex catalyzes limited proteolysis of FVIII, but these mechanisms are poorly understood. Objectives: In the present study, we investigated the precise mechanisms of FVIIa/TF‐catalyzed FVIII activation. Results: FVIII activity increased ～4‐fold within 30 s in the presence of FVIIa/TF, and then decreased to initial levels within 20 min. FVIIa (0.1 nm ), at concentrations present physiologically in plasma, activated FVIII in the presence of TF, and this activation was more rapid than that induced by thrombin. The heavy chain (HCh) of FVIII was proteolyzed at Arg⁷⁴⁰ and Arg³⁷² more rapidly by FVIIa/TF than by thrombin, consistent with the enhanced activation of FVIII. Cleavage at Arg³³⁶ was evident at ～1 min, whilst little cleavage of the light chain (LCh) was observed. Cleavage of the HCh by FVIIa/TF was governed by the presence of the LCh. FVIII bound to Glu‐Gly‐Arg‐active‐site‐modified FVIIa (K_d, ～0.8 nm ) with a higher affinity for the HCh than for the LCh (K_d, 5.9 and 18.9 nm ). Binding to the A2 domain was particularly evident. Von Willebrand factor (VWF) modestly inhibited FVIIa/TF‐catalyzed FVIII activation, in keeping with the concept that VWF could moderate FVIIa/TF‐mediated reactions. Conclusions: The results demonstrated that this activation mechanism was distinct from those mediated by thrombin, and indicated that FVIIa/TF functions through a ‘priming’ mechanism for the activation of FVIII in the initiation phase of coagulation.     

Heme binds to factor VIII and inhibits its interaction with activated factor IX

Summary. Background: Heme is a redox active macrocyclic compound that is released upon tissue damage or hemorrhages. The extracellular release of large amounts of heme saturates scavenging heme‐binding proteins. Free heme has been proposed to affect coagulation and has been co‐purified with the factor VIII (FVIII)‐von Willebrand factor (VWF) complex. The sites from which heme is released upon injury overlap with the sites to which FVIII is targeted for performing its hemostatic functions. Objectives: To investigate the interaction of heme with FVIII and the consequence for the procoagulant activity of FVIII in vitro. Methods and results: Heme bound to several sites on FVIII with high apparent affinity. Heme‐binding inhibited FVIII procoagulant activity in a dose‐dependent manner. FVIII inactivation in the presence of saturating amounts of heme implicated a reduced interaction of FVIII with activated FIX, as shown by ELISA, surface plasmon resonance and fluorescence quenching. Heme‐mediated inactivation of FVIII was prevented by VWF, but not by human serum albumin, a heme‐binding protein known for its protective activity in hemolytic conditions. Conclusions: Our data identify FVIII as a novel heme‐binding protein. Occupation of high affinity heme‐binding sites on FVIII at low concentrations of free heme did not inactivate FVIII. Conversely, large molar excesses of heme over FVIII, which correspond to conditions of extensive heme release, inhibited FVIII activity in vitro. It remains to be demonstrated whether, under such conditions, heme‐mediated modulation of the activity of FVIII plays some role in the regulation of coagulation.     

肥厚腰椎黄韧带中成纤维细胞生长因子、转化生长因子及结缔组织生长因子的表达

背景：腰椎黄韧带肥厚是临床上引起腰椎管狭窄的主要因素之一,但其分子机制仍不是非常清楚。目的：分析纤维化相关细胞因子碱性成纤维细胞生长因子、转化生长因子β1和结缔组织生长因子在腰椎黄韧带肥厚过程中的作用。方法：取临床手术所取黄韧带,对照组6例（椎管内占位且无腰椎不稳患者黄韧带）、突出组（单纯腰椎间盘突出症患者黄韧带）6例、腰椎管狭窄症组6例。采用实时定量RT-PCR的方法检测各组黄韧带中碱性成纤维细胞生长因子、转化生长因子β1、结缔组织生长因子及Ⅰ、Ⅲ、Ⅴ型胶原蛋白的mRNA含量,分析3个细胞因子在黄韧带肥厚过程中的作用。结果与结论：腰椎管狭窄症组碱性成纤维细胞生长因子mRNA表达均明显高于突出组和对照组（均P 〈0.05）;腰椎管狭窄症组转化生长因子β1mRNA在3组中的表达明显高于对照组和突出组（均P 〈0.01）;结缔组织生长因子 mRNA 3组间差异无显著性意义（P 〉0.05）。腰椎管狭窄症组Ⅰ型胶原蛋白mRNA表达明显高于突出组和对照组（均P 〈0.05）;Ⅲ型胶原蛋白、Ⅴ型胶原蛋白mRNA表达3组之间差异无显著性意义（P 〉0.05）。结果说明碱性成纤维细胞生长因子、转化生长因子β1在腰椎黄韧带肥厚形成过程中有重要作用,引起黄韧带肥厚的主要胶原产物为Ⅰ型胶原蛋白。     

The first epidermal growth factor-like domains of factor Xa and factor IXa are important for the activation of the factor VII--tissue factor complex.

During tissue factor (TF)-induced coagulation, the factor (F)VIIa-TF complex activates factor (F)X and factor (F)IX. Through positive feedback, the generated FXa and FIXa activate FVII-TF. The first epidermal growth factor-like (EGF1) domains of FX and FIX serve as important TF-recognition motifs when FVIIa-TF activates FX or FIX. Here, we investigated the role of EGF1 domains of FXa and FIXa during the activation of FVII-TF and inhibition by tissue factor pathway inhibitor (TFPI). FXaPCEGF1 (EGF1 domain of FXa replaced with that of protein C), and FXaQ49P (EGF1 domain mutant with impaired calcium-binding), and the corresponding FIXa mutants were generated, and their abilities to activate FVII-TF were compared with the wild-type (WT) enzymes. In the absence of TF, the rates of FVII activation were similar between WT enzymes and mutant FXa and FIXa proteases. In the presence of either soluble TF (sTF) or relipidated TF, each mutant of FXa or FIXa activated FVII-TF at a slower rate than the corresponding WT enzyme. Kinetics of inhibition of the amidolytic activity of WT and the mutant FXa proteases by either two-domain or full-length TFPI were similar. However, compared with the complex of TFPI-FXaWT, the abilities of the complexes of TFPI-FXa mutants to inhibit FVIIa-TF were impaired. We conclude that the EGF1 domains of FXa and FIXa are important for the activation of FVII-TF and for the formation of FVIIa-TF-FXa-TFPI complex.     

转化生长因子-β、碱性成纤维生长因子和血小板衍生生长因子在骨折愈合中的表达

目的:观察转化生长因子-β(TGF-β)、碱性成纤维细胞生长因子(bFGF)和血小板衍生生长因子(PDGF)在骨折愈合中的表达和分布情况,进而探讨其作用机制。方法:选用SD大鼠制作胫骨骨折愈合模型,伤后不同时期处死取材,分别进行组织学和TGF-β、bFGF和PDGF免疫组化染色观察。结果:(1)伤后3d开始形成原始骨痂。1周时肉芽组织中的间质细胞开始分化为软骨细胞,软骨形成后再进行软骨内化骨。4周时形成连接骨折端的桥接骨痂。(2)伤后早期血肿中炎性细胞表达bFGF、PDGF。伤后1周骨膜增殖细胞、肉芽组织中的成纤维细胞、内皮细胞、骨端骨细胞以及原始骨痂成骨细胞表达TGF-β、bFGF和PDGF。伤后2周软骨细胞表达TGF-β、bFGF和PDGF。结论:TGF-β、bFGF和PDGF有着各自的表达和分布特点,并共同调解骨原细胞的增殖和成骨细胞、软骨细胞的分化,最终完成骨折愈合。     

急性心肌梗死患者组织因子及组织因子途径激活抑制物水平的变化

目的探讨急性心肌梗死(AMI)患者组织因子(TF)及组织因子途径激活抑制物(TF-PI)的变化及临床意义。方法用ELISA法分别检测50例AMI患者及20例健康对照的外周血TF及TFPI的抗原水平。比较15例支架手术患者冠状窦内及外周血的TFPI水平。比较并发糖尿病和/或高脂血症AMI患者与无并发症者的TFAg及TFPIAg水平。结果AMI患者的TFAg及TFPIAg较正常人均明显升高(P<0.01);支架患者冠状窦内的TFPI水平明显低于其外周血(P<0.01);有并发症患者的TF及TFPI水平均明显高于无并发症患者(分别为P<0.01和P<0.05)。结论AMI患者外周血凝血活性增高,并发糖尿病和/或高脂血症AMI患者的凝血活性较无并发症患者高,血栓形成使冠状动脉腔内的TFPI消耗而降低。     

The relationship between ABO histo-blood group, factor VIII and von Willebrand factor

ABO histo-blood group is a major determinant of plasma levels of factor VIII (FVIII) and von Willebrand factor (vWF). Blood group O individuals have significantly (approximately 25%) lower plasma levels of both glycoproteins. This association is of clinical significance. Low plasma levels of either FVIII or vWF have long been established as causes of excess bleeding. Conversely, there is accumulating evidence that elevated FVIII-vWF levels may represent an important risk factor for ischaemic heart disease and venous thromboembolic disease. In spite of the well-documented association between ABO blood group and FVIII-vWF levels, the underlying mechanism remains unknown. However, it has been established that the ABO effect is primarily mediated through a direct functional effect of the ABO locus on plasma vWF levels. Theoretically, ABO blood group may alter the rate of vWF synthesis or secretion within endothelial cells. Alternatively, ABO group may affect vWF plasma clearance rates. ABH antigenic determinants have been identified on the N-linked oligosaccharide chains of circulating vWF and FVIII, according to the blood group of the individual. It remains unclear whether these carbohydrate structures are responsible for mediating the effect of ABO blood group on plasma vWF levels.     

Structural and functional features of factor XI

Summary.  Factor XI (FXI) has structural and mechanistic features that distinguish it from other coagulation proteases. A relatively recent addition to vertebrate plasma coagulation, FXI is a homodimer, with each subunit containing four apple domains and a protease domain. The apple domains form a disk structure with binding sites for platelets, high molecular weight kininogen, and the substrate factor IX (FIX). FXI is converted to the active protease FXIa by cleavage of the Arg³⁶⁹−Ile³⁷⁰ bond on each subunit. This converts the catalytic domains to the active forms, and unmasks exosites on the apple domains required for FIX binding. FXI activation by factor XIIa or thrombin proceeds through an intermediate with only one activated submit (1/2-FXIa). 1/2-FXIa activates FIX in a similar manner to FXIa. While the importance of the homodimeric structure of FXI is not certain, it may represent a strategy for binding to FIX and a platelet surface simultaneously.     

Evolutionary conservation of the allosteric activation of factor VIIa by tissue factor in lamprey

Essentials

• Tissue factor (TF) enhances factor VIIa (FVIIa) activity through structural and dynamic changes.
• We analyzed conservation of TF‐activated FVIIa allosteric networks in extant vertebrate lamprey.
• Lamprey Tf/FVIIa molecular dynamics show conserved Tf‐induced structural/dynamic FVIIa changes.
• Lamprey Tf activation of FVIIa allosteric networks follows molecular pathways similar to human.

Summary

Background

Previous studies have provided insight into the molecular basis of human tissue factor (TF) activation of activated factor VII (FVIIa). TF‐induced allosteric networks of FVIIa activation have been rationalized through analysis of the dynamic changes and residue connectivities in the human soluble TF (sTF)/FVIIa complex structure during molecular dynamics (MD) simulation. Evolutionary conservation of the molecular mechanisms for TF‐induced allosteric FVIIa activation between humans and extant vertebrate jawless fish (lampreys), where blood coagulation emerged more than 500 million years ago, is unknown and of considerable interest.

Objective

To model the sTf/FVIIa complex from cloned Petromyzon marinus lamprey sequences, and with comparisons to human sTF/FVlla investigate conservation of allosteric mechanisms of FVIIa activity enhancement by soluble TF using MD simulations.

Methods

Full‐length cDNAs of lamprey tf and f7 were cloned and characterized. Comparative models of lamprey sTf/FVIIa complex and free FVIIa were determined based on constructed human sTF/FVIIa complex and free FVIIa models, used in full‐atomic MD simulations, and characterized using dynamic network analysis approaches.

Results

Allosteric paths of correlated motion from Tf contact points in lamprey sTf/FVIIa to the FVIIa active site were determined and quantified, and were found to encompass residue–residue interactions along significantly similar paths compared with human.

Conclusions

Despite low conservation of residues between lamprey and human proteins, 30% TF and 39% FVII, the structural and protein dynamic effects of TF activation of FVIIa appear conserved and, moreover, present in extant vertebrate proteins from 500 million years ago when TF/FVIIa‐initiated extrinsic pathway blood coagulation emerged.

     

Expression of hepatocyte growth factor, keratinocyte growth factor and their receptors in experimental chronic pancreatitis

BACKGROUND: Hepatocyte (HGF) and Keratinocyte growth factors (KGF) are key factors of tissue organization and regeneration. These peptide growth factors and their receptors c-met and keratinocyte growth factor receptor (KGFR) are overexpressed in pancreatic cancer. AIM: Expression and localization of ligands and receptors were investigated during the development of experimental chronic pancreatitis. METHODS: Chronic pancreatitis was induced in rats by intravenous injection of dibutyltin dichloride. One to 60 days after treatment, the expression of growth factors and receptors was analysed by competitive polymerase chain reaction, Western blot analysis and immunohistochemistry. RESULTS: HGF mRNA expression increased (10-fold) until days 7-14 followed by a decrease to control level. Expression of c-met mRNA constantly increased (15-fold). KGF and KGFR mRNA expression were increased after 14-28 days (5-fold) and then returned to control levels. mRNA expression patterns correlated with changes in the protein expression, whereas protein levels of KGF remained unchanged. Ligands were localized in mesenchymal cells and their receptors on epithelial cells. CONCLUSIONS: The significant increase of HGF and c-met expression suggests an essential role of this growth factor in the morphological changes during the development of chronic pancreatitis. Changes in the expression of KGF and KGFR are less pronounced.     

Plasminogen activator inhibitor‐1 inhibits factor VIIa bound to tissue factor

Summary. Background and objective: A growing body of experimental evidence supports broad inhibitory and regulatory activity of plasminogen activator inhibitor 1 (PAI‐1). The present study was designed to investigate whether PAI‐1 inhibits factor (F) VIIa complexed with tissue factor (TF), a well‐known procoagulant risk factor. Methods and results: The ability of PAI‐1 to inhibit FVIIa‐TF activity was evaluated in both clotting and factor X (FX) activation assays. PAI‐1 and its complex with vitronectin inhibit: (i) clotting activity of FVIIa‐TF (PAI‐1_IC50, 817 and 125 nm , respectively); (ii) FVIIa‐TF‐mediated FX activation (PAI‐1_IC50, 260 and 50 nm , respectively); and (iii) FVIIa bound to TF expressed on the surface of stimulated endothelial cells (PAI‐1_IC50, 260 and 120 nm , respectively). The association rate constant (k_a) for PAI‐1 inhibition of FVIIa‐TF was determined using a chromogenic assay. K_a for PAI‐1 inhibition of FVIIa bound to relipidated TF is 3.3‐fold higher than that for FVIIa bound to soluble TF (k_a = 0.09 ± 0.01 and 0.027 ± 0.03 μm ^?1 min^?1, respectively). Vitronectin increases k_a for both soluble and relipidated TF by 3.5‐ and 30‐fold, respectively (to 0.094 ± 0.020 and 2.7 ± 0.2 μm ^?1 min^?1). However, only a 3.5‐ to 5.0‐fold increase in the acylated FVIIa was observed on SDS PAGE in the presence of vitronectin for both relipidated and soluble TF, indicating fast formation of PAI‐1/vitronectin/FVIIa/relipidated TF non‐covalent complex. Conclusions: Our results demonstrate potential anticoagulant activity of PAI‐1 in the presence of vitronectin, which could contribute to regulation of hemostasis under pathological conditions such as severe sepsis, acute lung injury and pleural injury, where PAI‐1 and TF are overexpressed.     

Recombinant human factor VIIa (rFVIIa) can activate factor FIX on activated platelets

The studies reported here show that factor (F)VIIa can activate factor (F)IX on activated platelets in the absence of tissue factor. Both FIX and FIXa bind to the activated platelet surface with a K(d) of 8 nM and 2 nM, respectively. With factor (F)VIIIa, FIXa binds more tightly to platelets (K(d) 0.6 nM). At rFVIIa concentrations < 100 nm, no direct binding to the activated platelet surface can be detected with electrophoretic light scattering. However, in the presence of FIX, rFVIIa binding to platelets at concentrations as low as 10 nm rFVIIa can be detected. This is reflected by a decrease in the FIX K(d) from 8 to 1.6 nM. When rFVIIa is added to activated platelets in the presence of both FIX and FVIIIa, the K(d) for FIX decreases to 0.6, suggesting that rFVIIa activates FIX on the surface of activated platelets in the absence of tissue factor. The activation of FIX by FVIIa on activated platelets can also be demonstrated by a functional assay for FIXa. These data show that pharmacological doses of rFVIIa result in the direct activation of FIX by rFVIIa to form additional tenase complexes ultimately resulting in improved thrombin generation. These results may explain, at least in part, the mechanism of action of rFVIIa in hemorrhagic conditions seen in otherwise normal patients who develop an acquired coagulopathy due to trauma, surgery or a variety of other events in which rFVIIa has been found to be effective.     

腔隙性脑梗死患者血浆组织因子和组织因子途径抑制物抗原水平的测定

目的探讨腔隙性脑梗死(lacunarinfarct,LI)患者血浆组织因子(tissue factor,TF)和组织因子途径抑制物(tissuefactor pathwayinhibitor,TFPI)测定的临床意义以及组织因子途径在LI发病中的作用.方法择确诊的LI患者63例,采用酶联免疫吸附的方法测定血浆TF和TFPI相关指标抗原水平,与正常对照组比较并对不同危险因素患者组之间的结果进行分析.结果①与正常对照组比较,LI患者组血浆TF抗原水平显著增高(217.4±101.3pg/ml对140.9±27.1pg/ml,P=0.0003)、游离TFPI抗原水平降低(41.4±16.7 ng/ml对30.0±18.6 ng/ml,P=0.005);②合并高血压、糖尿病和血脂异常LI患者血浆TFPI相关指标的改变不同;③LI患者血浆t-TFPI和tr-TFPI抗原水平与血浆TF抗原水平相关.结论LI患者血浆组织因子途径改变表现为凝血活性增高和抗凝活性减低.     

Linkage analysis of factor VIII and von Willebrand factor loci as quantitative trait loci

Summary.   Elevated factor (F)VIII levels contribute to venous thrombotic risk. FVIII levels are determined to a large extent by levels of von Willebrand factor (VWF), its carrier protein which protects FVIII against proteolysis. VWF levels are largely dependent on ABO blood group. Subjects with blood group non-O have higher VWF and FVIII levels than individuals with blood group O. Apart from ABO blood group no genetic determinants of high FVIII levels have been identified, whereas clustering of FVIII levels has been reported within families even after adjustment for ABO blood group and VWF levels. We investigated the FVIII and VWF loci as possible quantitative trait loci (QTL) influencing FVIII and VWF levels. Two sequence repeats in the FVIII gene and three repeats in the VWF gene were typed in 52 FV Leiden families. Multipoint sib-pair linkage analysis was performed with the MAPMAKER/SIBS program. FVIII levels adjusted for VWF levels and age, and VWF levels adjusted for ABO blood group and age, were used for this linkage analysis. No linkage of FVIII levels to the FVIII locus was found, whereas we found evidence that the VWF locus contains a QTL for VWF levels [maximum likelihood no dominance variance lod score = 0.70 ( P  = 0.04) and non-parametric Z-score = 1.92 ( P  = 0.03)]. About 20% of the total variation in VWF levels may be attributed to this VWF locus.     

Activated factor X cleaves factor VIII at arginine 562, limiting its cofactor efficiency

Summary. Background: Factor VIII (FVIII) and its activated form (FVIIIa) are subject to proteolysis that dampens their cofactor function. Among the proteases that attack FVIII (activated factor X (FXa), activated protein C (APC) and plasmin), only APC cleaves within the FVIII A2 domain at R562 to fully abolish FVIII activity. Objectives: We investigated the possible involvement of the FXa cleavage at R562 within the A2 domain in the process of FVIII inactivation. Methods: An antibody (GMA012/R8B12) that recognizes the carboxy‐terminus extremity of the A2 domain (A2C) was used to evaluate FXa action. A molecule mutated at R562 was also generated to assess the functional role of this particular residue. Results and Conclusions: The appearance of the A2C domain as a function of time evidenced the identical cleavage within the A2 domain of FVIII and FVIIIa by FXa. This cleavage required phospholipids and occurred within minutes. In contrast, the isolated A2 domain was not cleaved by FXa. Von Willebrand factor and activated FIX inhibited the cleavage in a dose‐dependent manner. Mutation R562K increased both the FVIII specific activity and the generation of FXa due to an increase in FVIII catalytic efficiency. Moreover, A2C fragment could not be identified from FVIII‐R562K cleavage. In summary, this study defines a new mechanism for A2 domain‐mediated FVIII degradation by FXa and implicates the bisecting of the A2 domain at R562.     

Comparative thrombotic event incidence after infusion of recombinant factor VIIa versus factor VIII inhibitor bypass activity

Thrombosis is a rare but well-recognized potential complication of Factor VIII Inhibitor Bypass Activity (FEIBA) infusion. Recombinant factor VIIa (rFVIIa) is increasingly used as an alternative to FEIBA; however, the thrombotic safety profile of rFVIIa remains incompletely characterized. To determine the incidence rates of thrombotic adverse events (AEs) after infusion of rFVIIa and FEIBA. Data from the MedWatch pharmacovigilance program of the US Food and Drug Administration, as supplemented by published case reports, were used in conjunction with estimated numbers of infusions available from manufacturers to assess comparative incidence of thrombotic AEs in patients receiving rFVIIa or FEIBA in the period from April 1999 through June 2002. Reported thrombotic AEs were rare, with incidence rates of 24.6 per 10(5) infusions (CI, 19.1-31.2 per 10(5) infusions) for rFVIIa and 8.24 per 10(5) infusions (CI, 4.71-13.4 per 10(5) infusions) for FEIBA. Thrombotic AEs were significantly more frequent in rFVIIa than FEIBA recipients (incidence rate ratio, 2.98; CI, 1.71-5.52). The most commonly documented single type of thrombotic AE after rFVIIa infusion was cerebrovascular thrombosis, while myocardial infarction was the most frequent type in patients receiving FEIBA. Contrasting AE reporting patterns between rFVIIa and FEIBA may have contributed to the observed difference in thrombotic event incidence. Nevertheless, this comprehensive pharmacovigilance assessment does not support superior thrombotic safety of rFVIIa and suggests that thrombotic AE risk may be higher in rFVIIa than FEIBA recipients.     

重组人表皮生长因子和碱性成纤维生长因子促进人角膜上皮细胞的增殖

背景：重组人表皮生长因子和碱性成纤维生长因子的制剂临床上已经用于眼表创伤的修复,但是对于使用何种浓度的生长因子就能够最大程度的促进愈合以及两种生长因子促进创面愈合的效果比较一直存在争议。 目的：初步观察重组人表皮生长因子、碱性成纤维生长因子对培养的人角膜上皮细胞克隆的影响。 方法：用不同浓度的重组人表皮生长因子和碱性成纤维生长因子作用于体外培养的人角膜上皮细胞,应用四甲基偶氮唑盐比色法分别在不同浓度的生长因子作用人角膜上皮细胞3,5,7 d后检测人角膜上皮细胞的增殖能力;平板克隆形成实验法观察细胞克隆形态及计算细胞克隆形成率。 结果与结论：不同质量浓度的重组人表皮生长因子和碱性成纤维生长因子分别对人角膜上皮细胞干预5d后,重组人表皮生长因子和碱性成纤维生长因子在10μg/L质量浓度时MTT值最大;质量浓度10μg/L重组人表皮生长因子促进人角膜上皮细胞克隆形成率高于质量浓度10μg/L碱性成纤维生长因子（P=0.02）。结果证实,重组人表皮生长因子和碱性成纤维生长因子均能促进人角膜上皮细胞增殖并增加其克隆形成能力。质量浓度10μg/L 重组人表皮生长因子干预5 d促进人角膜上皮细胞克隆形成效果最好。