Synthesis and release of factor VIII by cultured human endothelial cells

Evaluation of D-dimer and factor VIII in cirrhotic patients with asymptomatic portal venous thrombosis

D-dimer and factor VIII levels raise in advanced cirrhosis. We investigated the behavior and the diagnostic usefulness of D-dimer and factor VIII in cirrhotic patients with asymptomatic portal venous thrombosis. Factor VIII coagulant and D-dimer values were measured in 136 consecutive outpatients with stable cirrhosis divided according to Child-Pugh (CP) classification, who underwent color/power ultrasonography to detect portal thrombosis. Portal thrombosis was found in 33 patients (24.2%). In patients without thrombosis, factor VIII was significantly higher in CP class C compared with class A and B. Conversely, class C patients with portal thrombosis had lower factor VIII levels than those without thrombosis. In both groups, D-dimer was significantly increased in class C compared with class A and B. In class C, thrombotic patients showed higher D-dimer values than did patients without thrombosis. In class C, a D-dimer value > or = 0.55 microg/mL provided a sensitivity and a negative predictive value for portal thrombosis of 100%, and a factor VIII coagulant level < or = 80% showed a specificity and a negative predictive value of 76% and 84%, respectively. In class B, a D-dimer value > or = 0.225 microg/mL had a sensitivity of 89% and a negative predictive value of 82%. In conclusion, our study shows that factor VIII values increase in severe cirrhosis but significantly decrease in the presence of concomitant portal thrombosis, likely because of consumption during thrombosis; D-dimer is enhanced by both liver failure and portal thrombosis; in severe cirrhosis, normal D-dimer and factor VIII values may safely exclude the presence of asymptomatic portal thrombosis.

Von Willebrand factor antigen in assessment of vasculitis in patients with connective tissue diseases

Von Willebrand factor antigen (vWF:Ag) is synthesized and secreted by endothelial cells. In the present study we tried to assess the relationship between plasma level of vWF:Ag and vascular damage in patients with vasculitis. The study was carried out on 59 patients with connective tissue diseases. Vasculitis was diagnosed by biopsies of the skin. The patients with vasculitis had a significantly elevated level of vWF:Ag; however, no significant correlation between the amount of plasma vWF:Ag and the degree of vasculitis was found. The obtained results show that the plasma level of vWF:Ag may reflect the presence of vascular, especially endothelial, damage in patients with connective tissue diseases.

Gray platelet syndrome. Dissociation between abnormal sorting in megakaryocyte alpha-granules and normal sorting in Weibel-Palade bodies of endothelial cells

The gray platelet syndrome (GPS) is a rare congenital bleeding disorder in which megakaryocytes and platelets are deficient in alpha-granule secretory proteins. Since the Weibel-Palade bodies (WPB) of endothelial cells as well as the alpha-granules contain the von Willebrand Factor (vWF) and P-selectin, we examined by transmission electron microscopy the dermis capillary network of two patients with GPS. Endothelial cells showed the presence of normal WPB with typical internal tubules. Using single and double immunogold labeling for vWF and P-selectin, we detected vWF within WPB, where it was codistributed with the tubules, whereas P-selectin delineated the outline of WPB. Therefore, the fundamental targeting defect in GPS is specific to the megakaryocytic cell line.

Abnormal plasma fibrinolysis in patients with rheumatoid arthritis and impaired endothelial fibrinolytic response in those complicated by vasculitis

OBJECTIVES

(a) To assess plasma fibrinolytic parameters in patients with rheumatoid arthritis (RA) and to determine whether there are differences between patients with RA alone and those with RA complicated by vasculitis. (b) To determine if patients with RA respond differently to venous occlusion compared with normal subjects and to assess whether such a response differs in patients with RA alone and those with rheumatoid vasculitis. (c) To determine the extent of vascular damage in patients with rheumatoid vasculitis and if this correlates with the levels of plasma fibrinolytic parameters.

METHODS

Sixty three patients with RA (38 had RA only and 25 had evidence of rheumatoid vasculitis) were recruited. Plasma levels of tissue plasminogen activator antigen (t-PA Ag), plasminogen activator inhibitor (PAI) activity, and factor VIII von Willebrand factor (vWF) were measured before and 10 minutes after venous occlusion.

RESULTS

Patients with RA, with or without rheumatoid vasculitis, had higher baseline PAI levels than control subjects. The difference was statistically significant for patients with RA alone but was not statistically significant for patients with rheumatoid vasculitis. After venous occlusion, t-PA Ag levels increased significantly in normal subjects and patients with RA alone, but not in patients with rheumatoid vasculitis. Plasma levels of vWF were significantly higher in patients with rheumatoid vasculitis than in normal subjects and those with RA alone. In patients with RA alone, baseline vWF correlated positively with t-PA Ag levels, whereas a negative correlation was found between these two parameters in patients with rheumatoid vasculitis. A negative correlation between vWF and t-PA Ag levels after venous occlusion was also found in patients with rheumatoid vasculitis.

CONCLUSIONS

Patients with rheumatoid vasculitis showed evidence of vascular damage with increased levels of vWF and impaired t-PA Ag release after venous occlusion, a useful measurement of endothelial reserve to remove fibrin. This may be of pathophysiological importance in the development of vasculitis in these patients.

Platelet-activating factor secreted by DDAVP-treated monocytes mediates von Willebrand factor release from endothelial cells

We have previously shown that although DDAVP (1-deamino-8-D-arginine vasopressin), a synthetic analogue of the natural hormone arginine vasopressin, does not directly promote release of vWf from human umbilical vein endothelial cells (ECs), enhanced release does occur when ECs were exposed to either monocytes or to supernatants recovered from DDAVP-treated monocytes. In the present study, we have found that exposure of monocytes to DDAVP did not increase secretion of interleukins (IL)-1 beta, IL-6, IL-8, tumor necrosis factor (TNF-alpha), growth factors G-CSF (granulocyte-), GM-CSF (granulocyte, monocyte-colony stimulating factor), prostaglandins (PG) E2, PGF2 alpha, or PGI2 or purine nucleotides such as ATP and ADP. However, increased levels of platelet-activating factor (PAF) were secreted by DDAVP-treated monocytes in a time- and dose-dependent manner that positively correlated with the enhancement in vWf release from ECs. Moreover, this effect could also be elicited when lipid extracts of these supernatants or purified PAF were added directly to ECs. This response could be inhibited with (+/-)-trans-2,5-Bis(3,4,5-trimethoxyphenyl)-1,3-dioxolane, a specific PAF receptor antagonist, when the ECs were exposed to supernatants from DDAVP-treated monocytes or to pure PAF. The present data indicate that enhanced secretion of PAF from monocytes is one mechanism whereby DDAVP can provoke release of vWf from ECs.

Haemostasis in hypothyroidism

Abnormalities that have been reported for platelet indices and function, coagulation factors and tests, and the fibrinolytic system in hypothyroidism are reviewed. These abnormalities, although usually of limited importance clinically, may occasionally lead to major bleeding episodes and to diagnostic confusion.

Anti-endothelial cell antibodies in patients with Wegener's granulomatosis and micropolyarteritis

Anti-endothelial cell antibodies (AECA) have been detected by cell surface radioimmunoassay in nine out of 15 patients with micropolyarteritis (MPA) and in two out of five patients with Wegener's granulomatosis. AECA mostly belonged to the IgG isotype and were present in the active phase of the diseases. These antibodies were not detectable in 10 sera from patients with essential mixed cryoglobulinaemia, suggesting that they were not a mere epiphenomenon consequent to the inflammatory vascular injury. The binding activity was not related to ABH antigens or to HLA class I antigens displayed by resting human endothelial cells in culture and was not influenced by removing immune complexes. Absorption of the anti-neutrophil cytoplasmic antibodies (ANCA), present in MPA and Wegener's granulomatosis sera, did not affect the endothelial binding. AECA-positive sera did not display lytic activity against endothelial cells, neither alone nor after addition of fresh complement or normal human peripheral blood mononuclear cells. Although AECA are not cytolytic for endothelial cell monolayers in vitro, the reactivity against intact endothelial cells suggests their possible involvement in in vivo pathological processes affecting vascular structures in small vessel primary vasculitides.

von Willebrand factor and its disorders: an overview of recent molecular studies

von Willebrand factor (vWF) is a plasma protein with multiple functions in haemostasis. The vWF gene, located on chromosome 12p2.1, encodes a primary gene product of 2813 amino acids. Post-translational modification, assembly and secretion of vWF are highly complex. The pro vWF promoter is covalently linked by intermolecular disulphide bonds to form a dimer of MW approximately 440 kDa. This then polymerises to form multimers ranging in MW from 1-20 x 10(6). Simultaneously the pro piece of vWF is cleaved, releasing a 741 amino acid peptide known as vW Ag II from the polymerised protomers. Two distinct secretion pathways are found in the endothelial cell, a regulated pathway with storage in Weibl-Palade bodies and a constitutive pathway. Platelets store vWF in their alpha-granules. Mature vWF participates in platelet adhesion, spreading and aggregation and is a carrier of factor VIII, protecting the latter from degradation. Disorders of vWF are highly diverse. At least 20 subtypes of von Willebrand's disease have been described to date, based on features of the vWF present in or absent from patients plasma and platelets. Some patients have reduced amounts of apparently normal vWF whilst others have clearly abnormal vWF with aberrant structure and function. Rare patients virtually or completely lack vWF. The genetic and structural basis of some of these abnormalities is just beginning to emerge. This article outlines the molecular biology and physiology of vWF, and reviews some recent progress on the molecular pathology and genetics of von Willebrand's disease.

Co-distribution of von Willebrand factor and fibronectin in cultured rhesus endothelial cells

The synthesis and secretion of von Willebrand factor (VWF, or Factor VIII-related antigen) and fibronectin by cultured endothelial cells from rhesus monkey choroid retina were demonstrated by immunofluorescence, immunoperoxidase and single radial immunodiffusion techniques. Both VWF and fibronectin are localized in intracellular granules and extracellular fibrils. The results of double immunofluorescence staining and post-embedding immunoelectron microscopy showed that there was a co-distribution of VWF and fibronectin not only in pericellular fibrils where they co-aligned with each other to be the components of extracellular matrix, but also in intracellular granules, suggesting they were synthesized or translocated in the same compartment.

Enhanced release of von Willebrand factor by human endothelial cells in culture in the presence of phorbol myristate acetate and interleukin 1

Human umbilical vein endothelial cells (EC) were cultured in the presence of 4 beta-phorbol 12-myristate 13-acetate (PMA), interleukin 1 (IL-1) and interleukin 2 (IL-2), and secretion of von Willebrand factor activity (vWF, Ristocetin Co-factor) and von Willebrand factor antigen (vWFAg) measured at intervals. The levels of both vWF and vWFAg in cultures containing IL-1 were significantly higher than those in control cultures after longer than 5-6 days growth. Similarly, the levels of vWF and vWFAg were significantly increased in cultures containing PMA, but in these instances the rate of cell growth appeared to be enhanced, and confluence was observed after 6 days compared to 9-10 days in control cultures. vWF and vWFAg also increased significantly in the supernatant of confluent control EC incubated further with IL-1. Moreover, the immuno-fluorescence pattern of vWFAg in these treated cells was markedly less granular than that of control cells. Immuno-fluorescence of PMA-treated cells suggested that vWFAg might be less granular than in control EC but the mean levels of vWF and vWFAg in the supernatant of cells incubated with PMA were not significantly different from control values. The results of all assays in the presence of IL-2 were not significantly different from those of control cells. In all instances no morphological evidence of endothelial injury was observed, and more than 90% of cells remained viable at the termination of cultures. The results indicated that the synthesis and release of vWF were increased in the presence of PMA, and secretion of vWF was stimulated by IL-1.(ABSTRACT TRUNCATED AT 250 WORDS)

Factor VIII related antigen in the assessment of vasculitis

Factor VIII related antigen, an endothelial cell product, was markedly raised in systemic necrotising arteritis, reflecting disease activity, but was not raised in active cutaneous vasculitis. In rheumatoid arthritis high levels were only found in systemic vasculitis or Felty's syndrome, but in other connective tissue diseases increased levels were more frequently detected and usually related to disease activity. It did not correlate with C reactive protein. It was also raised in non-inflammatory peripheral vascular disease and after surgery and was not specific for vasculitic endothelial damage. Factor VIII related antigen may be useful in identifying and monitoring systemic necrotising and large vessel arteritis.

Composition of the von Willebrand factor storage organelle (Weibel-Palade body) isolated from cultured human umbilical vein endothelial cells

von Willebrand factor (VWF) is a large, adhesive glycoprotein that is biosynthesized and secreted by cultured endothelial cells (EC). Although these cells constitutively release VWF, they also contain a storage pool of this protein that can be rapidly mobilized. In this study, a dense organelle fraction was isolated from cultured umbilical vein endothelial cells by centrifugation on a self-generated Percoll gradient. Stimulation of EC by 4-phorbol 12-myristate 13-acetate (PMA) resulted in the disappearance of this organelle fraction and the synchronous loss of Weibel-Palade bodies as judged by immunoelectron microscopy. Electrophoretic and serologic analyses of biosynthetically labeled dense organelle fraction revealed that it is comprised almost exclusively of VWF and its cleaved pro sequence. These two polypeptides were similarly localized exclusively to Weibel-Palade bodies by ultrastructural immunocytochemistry. The identity of the dense organelle as the Weibel-Palade body was further established by direct morphological examination of the dense organelle fraction. The VWF derived from this organelle is distributed among unusually high molecular weight multimers composed of fully processed monomeric subunits and is rapidly and quantitatively secreted in unmodified form after PMA stimulation. These studies: establish that the Weibel-Palade body is the endothelial-specific storage organelle for regulated VWF secretion; demonstrate that in cultured EC, the VWF concentrated in secretory organelles is of unusually high molecular weight and that this material may be rapidly mobilized in unmodified form; imply that proteolytic processing of VWF involved in regulated secretion takes place after translocation to the secretory organelle; provide a basis for further studies of intracellular protein trafficking in EC.

von Willebrand disease

von Willebrand disease (vWD) is a bleeding disorder characterized by a complex hemostatic defect. Abnormal platelet function, usually reflected by a prolonged bleeding time, is the result of a quantitative or qualitative defect of von Willebrand factor (vWF). A secondary deficiency of factor VIII procoagulant protein (factor VIII) may occur leading to a coagulation defect as well. These two glycoprotein macromolecules circulate as a complex in plasma. This article will review current understanding of structures and functions of vWF factor and factor VIII as they relate to the pathogenesis, diagnosis, classification and therapy of vWD.

Molecular pathology and immunology of factor VIII (hemophilia A and factor VIII inhibitors)

Factor VIII is a large procoagulant glycoprotein that circulates in plasma in a noncovalent complex with von Willebrand factor. It is essential for the efficient cleavage of coagulation factor X by factor IXa, and its absence causes a severe bleeding disorder. Plasma factor VIII is reduced from the normal range of approximately 100 to 200 ng/ml in patients with the hereditary coagulation defect, hemophilia A, as well as in patients who develop autoantibodies that inactivate factor VIII. The understanding of factor VIII structure has been enhanced by recent studies that have characterized the X chromosome gene responsible for its synthesis, and preliminary information is now available about specific genetic defects. The basis for antibody formation in approximately 15 per cent of repeatedly transfused hemophilic patients is less clear at this time, however, for these individuals appear to have a variety of genetic defects that are not characteristically different from the patients who do not develop inhibitors. Although the antibodies cause a serious problem for affected individuals, they have been very useful in characterizing normal factor VIII and nonfunctional factor VIII-like protein that is found in the plasmas of 10 per cent of patients with mild hemophilia. Moreover, they are very useful reagents that can be used for immunoassay of factor VIII that has been modified in ways that have destroyed its procoagulant function.

Immunocytochemical localization of factor VIII-related antigen in tumors of the human central nervous system

Using both immunohisto- and immunocytochemical techniques with periodate-lysine-paraformaldehyde (PLP) fixation, we have studied the distribution of Factor VIII-related antigen (FVIIIR:Ag) in 12 cases of tumors of the human central nervous system (CNS) and one sample of non-tumor brain tissue. FVIIIR:Ag was found both extracellularly and intracellularly. It was localized in the vascular lumen, between endothelial cells, and in the endothelial cell basement membrane. In the endothelial cell cytoplasm, FVIIIR:Ag was found in the endoplasmic reticulum, perinuclear space, and in intracytoplasmic vacuoles and vesicles. Characteristic of malignant tumors (six out of seven) was a strongly-positive dilated endoplasmic reticulum. This may reflect increased FVIIIR:Ag synthesis in the endothelial cells of malignant tumors. Only one of five benign tumors showed such staining. Six of 12 tumors and the non-tumor brain showed perinuclear FVIIIR:Ag. Both ad- and abluminal vesicles in the tumor endothelial cells contained FVIIIR:Ag suggesting that endocytosis, transcellular transport, and/or endocytosis, as well as FVIIIR:Ag synthesis occurs. The non-tumor brain showed normal capillary structure and very little FVIIIR:Ag immunoreactivity. The relationship of these FVIIIR:Ag abnormalities to the hypercoagulable state seen in some malignant brain tumor patients remains to be clarified.

Transplantation of normal bone marrow into a pig with severe von Willebrand's disease

Bone marrow from a normal male pig was transplanted into a related female pig with severe homozygous von Willebrand's disease (vWd). After engraftment the circulating leukocytes were of the male karyotype, and the platelets were strongly positive for von Willebrand factor (vWF) by indirect immunofluorescence. The average level of vWF was 1.96 U/dl and of ristocetin cofactor was 2.8 U/dl. The ear immersion bleeding time before transplantation was consistently more than 15 min and afterwards varied between 5 min and more than 15 min. Transfused vWF corrected the bleeding time at a level of 10 U/dl, which is lower than that required for a von Willebrand pig. We concluded that: the plasmatic compartment is only minimally replenished by the vWF from platelets and megakaryocytes; and the platelet vWF alone only partially corrects the abnormal tests of the hemostatic mechanism in severe vWd.

The mechanism of fibrin-induced disorganization of cultured human endothelial cell monolayers

Deposition of polymerizing fibrin on the vascular endothelium is the final event in intravascular coagulation. Exposure of fibrin clots to confluent monolayers of cultured human endothelial cells for 4 to 24 hours resulted in the disappearance of their normal cobblestone morphology and in the formation of endothelial cell aggregates. The present study was designed to evaluate the conditions and structural requirements of the fibrin clot for the induction of disorganization. Even after harsh treatment with denaturing agents or loading with large amounts of fibrinogen antibodies, polymerized fibrin always induced disorganization of the monolayers. In contrast, soluble fibrin that was kept in solution by either fibrinogen, fragment D-cate, or the tetrapeptide Gly-Pro-Arg-Pro did not cause any alteration of the monolayers. The fibrinogen degradation product D-cate (Mr 94,000) itself had no microscopically detectable influence on the monolayer structure. In the absence of fibrin, the effect of thrombin on endothelial cells was found to be distinct from that induced by fibrin; however, the exposure of pieces of glass coverslips caused alterations in morphology indistinguishable from the fibrin-induced disorganization of the monolayer. Experiments using protein-coated polyester films indicated that the ability of the endothelial cells to attach to the overlying material, independent of its chemical structure, is the prerequisite for the induction of disorganization, but not a defined component of the fibrin molecule. Disorganization of vascular endothelium in vivo might be important for the organization and revascularization of an occluding thrombus.

Increase in factor VIII clotting activity in the perfusate of isolated dog hind leg and heart by components of kallikrein-kinin system

Transient increase in Factor VIII clotting (F.VIIIc) activity was observed by infusion of kallikrein or bradykinin (BK) to the isolated dog leg or heart preparation which was perfused with physiological solution, but the increase in Factor IX activity was not observed, suggesting that the increase in F.VIIIc activity was not due to contaminated plasma components of the perfusate. The increase coincided with the increase in prostacyclin (PGI2) which was determined by immunoassay of 6-keto-PGF1 alpha and by platelet aggregation inhibiting activity. Although increase in PGI2 by BK was inhibited by the pretreatment of the preparations with indomethacin or tranylcypromine, the increase in F.VIIIc activity was not inhibited, suggesting that the increase in F.VIIIc activity was not mediated by PGI2.

[Significance of the endothelium of the vascular wall for maintaining hemostasis]

The endothelial lining contributes in many respects to the patency of the vasculature. The production of heparan sulphate, release of prostacyclin and expression of the membrane cofactor thrombomodulin that is essentially required for the activation of protein C represent important mechanisms that warrant thromboresistance. If the integrity of the vessel wall is lost, the exposed subendothelium that has been built up by the endothelial cells serves as a highly reactive surface for platelets whose adherence is facilitated by another endothelial cell product, the von Willebrand Factor. Induction of tissue factor production after exposure to endotoxin also emphasizes an important role für the endothelium in the pathogenesis of disseminated intravascular coagulation. Once thrombosis has occurred the release of plasminogen activator of tissue-type from the endothelium leads to dissolution of the clot and a functional restoration of the blood vessel.

Distribution of factor VIII mRNA and antigen in human liver and other tissues

The cellular site of synthesis of factor VIII (FVIII:C; anti-haemophilic factor) has long been sought. Previous studies suggested the liver as a major site of synthesis, but extrahepatic sources such as spleen and lung have been implicated. Using an immunoradiometric assay (IRMA), we recently localized factor VIII antigen (FVIII:Ag, formerly FVIII:CAg), to whole perfused guinea pig liver and spleen, and to isolated hepatocytes, with lesser or trace amounts in other tissues. Using an immunohistological technique, Stel et al. detected FVIII:Ag in normal human liver sinusoidal endothelial cells, while Exner et al. detected FVIII:Ag by IRMA in extracts of human lymph nodes, lung, liver and spleen. The localization of antigen in tissues does not, however, distinguish sites of factor VIII synthesis from those of storage, and such experiments are subject to misinterpretation due to entrapment of plasma factor VIII in tissues. The recent cloning of the human factor VIII gene provides hybridization probes for the detection of factor VIII messenger RNA in cells, thus directly determining sites of synthesis. During complementary DNA cloning, we detected factor VIII mRNA in liver, and it has been localized by others in liver and placenta and in liver and kidney. In the present study, we detected factor VIII mRNA in isolated human hepatocytes, in spleen and in numerous tissues including lymph nodes and kidney, but not in white blood cells or cultured endothelial cells. We also found that the factor VIII, factor VII, factor IX and protein C antigens in liver are predominantly localized in hepatocytes, while very little von Willebrand factor antigen (vWF:Ag, formerly FVIIIR Ag) is detectable in this organ.

Factor VIII related antigen in gestational trophoblastic disease

Serum factor VIII related antigen (factor VIII R:Ag) levels were determined in 34 patients with hydatidiform mole at diagnosis and at 2, 4 and 6 weeks after evacuation of uterus and in 272 normal pregnant women. Serum factor VIII R:Ag levels in molar pregnancies before evacuation of the uterus were significantly higher than those in normal pregnancies of the same gestation age. Serum factor VIII R:Ag levels in the group of patients with residual trophoblastic disease were significantly higher than those in the group without residual trophoblastic disease before evacuation and at 6 weeks after evacuation of uterus, but there is considerable overlap in the levels of factor VIII R:Ag between the two groups of patients especially before evacuation of uterus. Factor VIII R:Ag level does not appear to be an useful marker in predicting the outcome of hydatidiform mole.

Factor VIII-related antigen levels in normal pregnancy and puerperium

The changes in serum Factor VIII-related antigen (Factor VIII RAg) levels in normal pregnancy and puerperium in Chinese women were studied by the Laurell 'rocket' immunoelectrophoresis. At 6 wk of gestation, the serum Factor VIII RAg levels were already higher than those of non-pregnant women and gradually increased through term pregnancy. Parturition did not have immediate effects on the circulating levels of Factor VIII RAg. The levels of Factor VIII RAg observed during labour and days 1-5 postpartum were similar to those of term pregnancy. By the 6th wk postpartum, the serum Factor VIII RAg declined to basal levels, similar to those of non-pregnant control samples.

Platelet factor 4 does not promote von Willebrand factor binding to human platelets

Ristocetin, protamine and Polybrene promote factor VIII:vWF binding and agglutination of formalinized platelets. It has been suggested that these polycations neutralize platelet negative surface charges and promote the attachment of VIII:vWF to platelets. Platelet factor 4 (PF4), protamine, and Polybrene inhibit heparin activity by neutralizing heparin negative charges. We tested the hypothesis that PF4, which is bound to the platelet surface after platelet activation and secretion, could promote the binding of VIII:vWF and subsequent platelet agglutination. Purified PF4 in concentrations comparable to those of ristocetin did not agglutinate formalinized platelets or induce the disappearance of VIII:vWF from the suspending plasma. Platelets were thrombin-treated in order to induce the release of PF4, and then formalinized and resuspended in normal plasma. These platelets did not agglutinate spontaneously, or at lower ristocetin concentrations than platelets that were not treated with thrombin before formalin fixation. Platelets were also activated by thrombin in the presence of EDTA to prevent surface binding of VIII:vWF or secreted PF4, and then formalinized. These platelets did not bind VIII:vWF in the presence of purified PF4. We conclude that even though PF4 binds to both polyanions and the platelet membrane, it does not promote the attachment of VIII:vWF.

Hybridoma antibodies to human von Willebrand factor. II. Relative role of intramolecular loci in mediation of platelet adhesion to the subendothelium

Hybridoma antibodies specific for seven independent topographical sites were used to characterize von Willebrand factor (vWF) and to relate the epitopes to functional loci required for vWF-mediated adhesion of platelets to subendothelium and ristocetin-induced platelet aggregation. The capacity of antibodies to influence the adhesion of human platelets to rabbit aortic subendothelium was analysed in annular perfusion chambers. At a high shear rate similar to that of the microcirculation, four monoclonal antibodies inhibited adhesion. In contrast, no inhibition was observed at low shear. Only one of the four antibodies that inhibited platelet adhesion also attenuated ristocetin-cofactor activity (VIIIR:RCo). Conversely, one antibody that inhibited VIIIR:RCo had no effect upon platelet adhesion. These data support the hypothesis that the molecular loci involved in the two biological functions of vWF are not identical. When these conclusions are considered within the context of a spatial map of the vWF protein surface developed by competitive displacement analysis, the epitopes related to platelet adhesion appear to be spaced and differ from those involved in ristocetin-induced platelet-platelet interaction.

The effect of calcium on the secretion of factor VIII-related antigen by cultured human endothelial cells

Cultured human endothelial cells derived from the umbilical cord vein are able to release factor VIII-related antigen into the culture medium. The experiments described in this paper show the presence of two pathways for the secretion of factor VIII-related antigen from endothelial cells. There is a basal release of this antigen, independent of the presence of extracellular calcium ions. This release can be inhibited by cycloheximide and is therefore directly related to de novo protein synthesis. Besides this basal release, there is an extra release of factor VIII-related antigen that can be stimulated by thrombin, the Ca2+-ionophore A23187 or phorbol myristate acetate. As demonstrated by immunofluorescence, the stimulus-inducible release originates from storage granules in the cells. This stimulus-inducible release is dependent on extracellular Ca2+ but independent of intracellular cAMP.

Biosynthesis of the subunits of factor VIIIR by bovine aortic endothelial cells

The biosynthesis of the subunit of factor VIIIR was studied in bovine aortic endothelial cells by the techniques of immunoprecipitation and NaDodSO4/polyacrylamide gel electrophoresis. It was determined that the subunit is first produced as a Mr 240,000 glycoprotein precursor, which appears to undergo proteolytic cleavage at or about the time of secretion into the medium with a resultant change in apparent Mr to 225,000, the size of the mature subunit found in plasma. The Mr 240,000 species was detected within 10 min of the start of labeling of cells, but factor VIIIR was not detected in the culture medium until approximately equal to 50 min. Treatment of the cells with tunicamycin resulted in a decrease in the apparent Mr of both species but did not grossly inhibit processing of precursor to product or the secretion of the latter. Thus, much or all N-linked glycosylation of factor VIIIR is not essential for these steps. Accumulation of factor VIIIR in the medium continued over a 24-hr period of cell labeling with [35S]-methionine, without significant net intracellular accumulation of the precursor, suggesting that a large storage pool of factor VIIIR is not present in endothelial cells under these conditions.

Characterization of marrow-derived adherent cells. Evidence against an endothelial subpopulation

Cultured, marrow-derived, adherent cells (MDAC) provide a microenvironment which supports the proliferation of haemopoietic stem cells (HSC) for extended periods in vitro. Morphological characterization suggested that MDAC populations consisted of a variety of cell types, including mononuclear phagocytes, fibroblastoid cells, fat cells and vascular endothelial cells. Recently performed functional characterization studies suggest that they consist largely of collagen-producing, fibroblastic cells. MDAC were not, however, examined systematically for endothelial cell characteristics. Unrecharged cultures of MDAC, shown in parallel studies to support in vitro haemopoiesis, were examined for endothelial cell markers. These included the presence of Weibel-Palade bodies and synthesis of factor VII related antigen. They were also examined biochemically for synthesis of basement membrane (type IV) collagen. The results of these investigations were negative in all cultures examined. It is thus concluded that vascular endothelial cells are not present as a significant component of the unrecharged MDAC population and do not, therefore, contribute to the functional haemopoietic microenvironment in vitro or in vitro.

Regulation of human megakaryocytopoiesis. An in vito analysis

We have recently described an assay system for human peripheral blood megakaryocyte colony-forming unit cells (CFU-M) using an anti-platelet glycoprotein antiserum probe to define megakaryocyte colonies grown in vitro. This system was applied to study the nature and regulation of human bone marrow CFU-M. In the absence of a specific megakaryocyte growth-promoting factor, 12.4 +/- 3.0 (means +/- SEM) megakaryocyte colonies were cloned per 5 X 10(5) cells cultured. Colonies were present after 6 d of incubation reaching peak numbers between days 10 and 14 and slowly decreasing thereafter. Erythropoietin in concentrations of up to 4 U/ml failed to augment colony numbers. Also failing to enhance megakaryocyte colony plating efficiency were media containing burst-promoting activity and colony-stimulating activity. A medium conditioned by human embryonic kidney cells, which has been previously demonstrated to contain thrombopoietin, also had no effect on megakaryocyte colony numbers. In contrast, sera from three patients with severe aplastic anemia produced significant enhancement of CFU-M-derived colony formation in vitro. Both the number of megakaryocyte colonies present and the number of megakaryocytes per colony were increased in proportion to the final concentration of aplastic anemia serum. In the presence of 10% aplastic anemia serum, cultured megakaryocyte colony numbers were linear with respect to the number of bone marrow mononuclear cells plated suggesting a clonal origin of each of the colonies. This in vitro assay for bone marrow CFU-M is a reliable means by which to study the regulation of human megakaryocytopoiesis. Initial data suggest that megakaryocyte production is stimulated by a factor detectable in aplastic anemia serum that may be distinct from other known hematopoietic stem cell regulators.