Reactivity of Fibrinogen Derivatives with Antisera to Human Fibrin D-Dimer and Its γ-γ Chain Remnant

Highly purified D-dimer was obtained from plasmin digest of human cross-linked fibrin. After reduction of its disulfide bonds, the γ-γ chain remnant, containing cross-linking site, was then isolated by ion-exchange chromatography on CM-cellulose. Antisera obtained by immunizing rabbits with D-dimer and its γ-γ chain remnant contained a small population of antibodies which specifically reacted with D-dimer. Thus, a specific radioimmunoassay system allowing detection and quantitation of D-dimer in the presence of fibrinogen and monomeric fragment D was made possible.

The Intrinsic Fluorescence of Human Fibrinogen and Its Fragments D and E

The tryptophan fluorescence of fibrinogen and its final degradation products - fragment D and E - were compared. Fibrinogen and its derivatives exhibit identical emission and excitation spectra. Their fluorescence intensity is influenced to a different extent by pH titration and temperature.

Our studies showed that tryptophan residues of core fragments D and E are much more exposed to quenching effects of acrylamide and ions than intact fibrinogen, which may be caused by conformational changes occurring over the domains during plasmin digestion of fibrinogen molecule.

The Decreased Circulating Platelet Mass and Its Relation to Bleeding Time in Chronic Renal Failure

The mean platelet volume, measured optically and by the electrical impedance method, was analysed in the patients with chronic renal failure. All parameters describing the circulating platelet biomass except the platelet volume heterogeneity were decreased in these patients. There is a strong inverse relation between the Ivy bleeding time and the platelet mass in the patient group.

Our results indicate that the smaller platelets circulating in the patients with chronic renal failure may contribute to the uremic bleeding diathesis, and disturbed thrombocytopoiesis occurs in chronic renal failure.

Acute phase protein alpha 1-acid glycoprotein interacts with plasminogen activator inhibitor type 1 and stabilizes its inhibitory activity

alpha(1)-Acid glycoprotein, one of the major acute phase proteins, was found to interact with plasminogen activator inhibitor type 1 (PAI-1) and to stabilize its inhibitory activity toward plasminogen activators. This conclusion is based on the following observations: (a) alpha(1)-acid glycoprotein was identified to bind PAI-1 by a yeast two-hybrid system. Three of 10 positive clones identified by this method to interact with PAI-1 contained almost the entire sequence of alpha(1)-acid glycoprotein; (b) this protein formed complexes with PAI-1 that could be immunoprecipitated from both the incubation mixtures and blood plasma by specific antibodies to either PAI-1 or alpha(1)-acid glycoprotein. Such complexes could be also detected by a solid phase binding assay; and (c) the real-time bimolecular interactions monitored by surface plasmon resonance indicated that the complex of alpha(1)-acid glycoprotein with PAI-1 is less stable than that formed by vitronectin with PAI-1, but in both cases, the apparent K(D) values were in the range of strong interactions (4.51 + 1.33 and 0.58 + 0.07 nm, respectively). The on rate for binding of PAI-1 to alpha(1)-glycoprotein or vitronectin differed by 2-fold, indicating much faster complex formation by vitronectin than by alpha(1)-acid glycoprotein. On the other hand, dissociation of PAI-1 bound to vitronectin was much slower than that from the alpha(1)-acid glycoprotein, as indicated by 4-fold lower k(off) values. Furthermore, the PAI-1 activity toward urokinase-type plasminogen activator and tissue-type plasminogen activator was significantly prolonged in the presence of alpha(1)-acid glycoprotein. These observations suggest that the complex of PAI-1 with alpha(1)-acid glycoprotein can play a role as an alternative reservoir of the physiologically active form of the inhibitor, particularly during inflammation or other acute phase reactions.

AlphaIIbbeta3 and its antagonism at the new millennium

Because of its major role in regulating platelet functions and its prominence on the cell surface, integrin alphaIIbbeta3 has been the subject of intensive investigations. Such studies have provided substantial insights into its structure-function relationships and have led to the development of anti-thrombotic drugs that target the receptor. Nevertheless, recent findings have indicated that our understanding of the structure and function of alphaIIbbeta3 remains inadequate. This article addresses two aspects of still evolving alphaIIbbeta3 function: 1) the interface between alphaIIbbeta3 and the blood coagulation system, resulting from interaction of prothrombin with the receptor; and 2) the molecular basis for recognition of the RGD and the fibrinogen gamma-chain peptide ligands by alphaIIbbeta3. As illustrated by these two examples, there is still much to be learned about alphaIIbbeta3 if we are to fully appreciate its functions and its potential as a therapeutic target.

Similar PAI-1 expression in visceral and subcutaneous fat of postmenopausal women

Recent studies showed that intraabdominal visceral fat located in the mesenterium and omentum may significantly influence the circulating plasminogen activator inhibitor type 1 (PAI-1). To substantiate this link, we performed analysis of PAI-1 expression in human visceral and subcutaneous adipose tissues in peri- and postmenopausal women. The samples of both visceral and subcutaneous fat from 28 generally healthy women (aged 45-69 years) with a wide range of body mass index (BMI; 22.30-38.67 kg/m2), who underwent surgical operation due to benign ovary and uterine tumours, were obtained. In these samples, expression of mRNAs for PAI-1, tumour necrosis factor alpha (TNFalpha), acyl-CoA synthetase (ACS), and glucose transporter (GLUT-4) was analysed by relative quantitative RT PCR and correlated with plasma PAI-1 antigen. In addition, visceral fat area was measured with computer tomography. Both types of fat tissues contained similar quantities of PAI-1 mRNA, and there was no correlation between plasma PAI-1, measured both by antigen and activity, with either visceral or subcutaneous fat PAI-1 mRNA. Furthermore, there was no significant association between the expression of PAI-1 mRNA and TNFalpha mRNA in tested fat samples. PAI-1 mRNA in both fat tissues was significantly correlated with plasma levels of estradiol (positive correlation) and follicle-stimulating hormone (FSH; negative correlation). Finally, the expression of PAI-1 mRNA was negatively correlated with mRNA of ACS present in both fat tissues. In summary, this study directly indicates that PAI-1 mRNA is similarly expressed in both subcutaneous and visceral fat of peri- and postmenopausal women and its expression strongly depends upon lipid metabolism.

Regulation of PAI-1 concentration in platelets by systemic administration of antisense oligonucleotides to rats

In this report we tested the effect of oligodeoxyribonucleotides antisense to PAI-1 mRNA administered into rats on PAI-1 concentration in platelets. Low doses of the antisense oligonucleotide (MPO-16R) reduced PAI-1 activity, both in rat blood plasma and platelet lysates by 20.5% and 28.7%, respectively. There was no change in platelet count after treatment with MPO-16R but treated platelets showed lower aggregability as compared with controls (37 +/- 13% and 54 +/- 12%, respectively). In an experimental model of rat arterial thrombosis, low doses of MPO-16R caused a significant delay in the occlusion time (31.8%). These data further support for the role of PAI-1 as a major determinant of arterial thrombolysis resistance and for the first time demonstrate the possibility of reduction of platelet PAI-1 concentration by antisense approach.

Comparison of platelet aggregability and P-selectin surface expression on platelets isolated by different methods

Three methods commonly used for isolation of blood platelets from plasma were compared. Platelets were isolated by: 1) a washing method; 2) a method of metrizamide-gradient centrifugation; 3) a modified method of gel-filtration. The last method employed BSA-Sepharose gel instead of routinely used Sepharose gel saturated with BSA. BSA-Sepharose gel was prepared by covalent binding of thermally deactivated BSA to CNBr-activated Sepharose 2B. In contrast to platelets isolated by the other methods, an aggregability of the gel-filtered platelets and control platelets in plasma, both activated with ADP, were comparable. When expression of P-selectin on the surface of freshly isolated platelets was examined, the gel-filtered platelets exhibited the same extent of fluorescence signal as platelets in the citrated blood, whereas platelets isolated by the other methods exhibited twice the extent of the signal. The methods involving the centrifugation process cause a low but a significant platelet activation.

The immunosuppressory and adhesive miniregion of the human major histocompatibility protein, human leukocyte antigen DQ

Our previous studies showed that the nonapeptide fragment of human leukocyte antigen DQ with the TPQRGDVYT sequence strongly suppresses the immune response [Z. Szewczuk, I. Z. Siemion, and Z. Wieczorek (1996) Molecular Immunology, 33, 903-9081]. The fragment contains the RGDVY sequence, which is very similar to thymopentin (pentapeptide RKDVY, an active fragment (32-36) of thymopoietin, an immune system activator produced in thymi), and at the same time contains the RGD sequence, known as an inhibitor of adhesion processes. In the present study we tested an influence of the nonapeptide and its shorter fragments on binding the activated platelets and K562 cells to fibrinogen and fibronectin, respectively. We also designed and synthesized a cyclic thymopentin-like peptide. C*RGDVYC* (where C* indicates Cys participating in disulfide bridge) to restrict its conformation. The cyclization product strongly suppresses the humoral and cellular immune response and selectively inhibits the adhesion of K562 cells to fibronectin. The results are discussed in the light of CD conformational studies.

Inhibition of plasminogen activator inhibitor release in endothelial cell cultures by antisense oligodeoxyribonucleotides with a 5'-end lipophilic modification

A series of conjugates containing residues of lipophilic alcohols covalently bound to 5' end of oligodeoxyribonucleotides targeted against human plasminogen activator inhibitor (PAI-1) mRNA was synthesized via the oxathiaphospholane approach. The highest anti-PAI-1 activity in EA.hy 926 endothelial cell cultures was found for conjugates containing menthyl or heptadecanyl groups linked with an oligonucleotide complementary to a segment of human PAI-1 mRNA. The phosphodiester antisense oligonucleotides, which otherwise exhibit only limited anti-PAI-1 activity, were found to be more active than phosphorothioate oligonucleotides when conjugated to lipophilic alcohol residues. For menthyl conjugates an evidence of antisense mechanism of inhibition was found.

Interferon gamma bound to endothelial cells is phosphorylated by ecto-protein kinases

The presence of protein kinase activity and its phosphorylated products has been demonstrated on the outer surface of the plasma membrane of endothelial cells. Extracellular phosphorylation was detected by incubation of primary endothelial cells (HUVEC's) and endothelial cell line EA.hy 926 with [gamma-32P]ATP. The reaction products were subjected to SDS/PAGE, autoradiography and scanning densitometry. Under the experimental conditions, five proteins with apparent molecular masses of 19, 23, 55, 88, and 110 kDa were prominently phosphorylated in both types of cells. Phosphorylation of the 19 kDa protein was the most rapid reaching maximum after 60 s and then the protein became dephosphorylated. Ecto-protein kinases responsible for the surface labeling of membrane proteins were characterized by using (a) protein kinase C inhibitors: K-252b, chelerythrine chloride, and [Ala113] myelin basic protein (104-118), (b) protein kinase A inhibitor Kemptide 8334, and (c) casein kinase II inhibitor 5,6-dichloro-1-beta-D-ribofuranosyl benzimidazole (DRB). Stimulation of endothelial cells with tumor necrosis factor alpha (TNF alpha) and interferon gamma (IFN gamma) is associated with 20-80% reduction of extracellular phosphorylation of all membrane proteins. IFN gamma bound to membrane receptors becomes rapidly phosphorylated. Only in the case of IFN gamma it was associated with the appearance of a strongly phosphorylated band of 17 kDa corresponding to IFN gamma itself. Phosphorylation of this 17 kDa exogenous substrate was prevented by an ecto-kinase inhibitor K-252b. The existence of ecto-phosphoprotein phosphatase activity in endothelial cells was evidenced by testing the effect of microcystin LR--a membrane impermeable reagent that inhibits both PP-1 and PP-2a phosphoprotein phosphatases. The extent of phosphorylation of 19 kDa and 110 kDa phosphoproteins significantly increased in the presence of microcystin. Our results suggest the presence of at least two ecto-kinase activities on endothelial cells that may play a significant role(s) in the regulation of cytokines function.

Dual regulatory effects of nitric oxide on plasminogen activator inhibitor type 1 expression in endothelial cells

In this report we compared the mechanism by which nitric oxide (NO), generated exogenously and endogenously, affects the plasminogen activator inhibitor type 1 (PAI-1) expression in endothelial cells. For this purpose, we stimulated the endothelial cell line EA.hy 926 with tumour necrosis factor alpha (TNFalpha) in the presence of the exogenously NO-releasing donors, sodium nitroprusside (SNP) and S-nitroso-N-acetylpenicillamine, or regulators of nitric oxide synthase (NOS) inhibitor N-nitro-L-arginine-methyl ester hydrochloride and substrate L-Arg. Expression of PAI-1 in EA.hy 926 cells was determined by measuring the level of mRNA, using relative quantitative reverse transcriptase PCR, and protein, using ELISA. In addition, we estimated the level of activation of two mitogen-activated protein kinases (MAPKs), extracellular signal-regulated kinase (ERK1/2) and c-Jun N-terminal kinase (JNK1/2), in the cells before and after treatment with TNFalpha, in the presence or absence of NO donors and inhibitors. In contrast to exogenously released NO that significantly reduced mostly basal PAI-1 expression, endogenously generated NO by NOS potentiated TNFalpha-induced upregulation of PAI-1 expression. Exogenously and endogenously generated NO causes different effects on activation of the MAPKs ERK1/2 and JNK1/2. Specifically, the SNP-released NO activates only ERK1/2, while endogenously generated NO in a pathway induced by TNFalpha activates both MAPKs. Thus our data indicate that due to different cellular locations and mechanisms of generation, NO may participate in various signalling pathways leading to opposite effects on PAI-1 expression in endothelial cells.

Activity of pp60c-src and association of pp60c-src, pp54/58lyn, pp60fyn, and pp72syk with the cytoskeleton in platelets activated by collagen

Collagen stimulation of platelets induced an increase in the specific activity of pp60c-src immunoprecipitated from the Triton-soluble fraction. The earliest time after collagen stimulation that an increase in pp60c-src activity was observed was 30 s. However, the maximum activity of pp60c-src in the Triton-soluble fraction was observed 60 s after collagen stimulation. At this time an approximately twofold increase of pp60c-src activity towards phosphorylation of KVEKIGEGTYGVVKK specific peptide and enolase and a 4.5-fold increase towards phosphorylation of pp60c-src itself was measured. Furthermore, the majority of pp60c-src as well as pp54/58lyn, pp60fyn, and pp72syk were found in the Triton-soluble fraction in resting platelets. Collagen induced, to different extents and velocities, translocation of all of these proteins from the Triton-soluble fraction to the Triton-insoluble, cytoskeleton-rich, platelets fraction. These results provide direct evidence that collagen stimulation of platelets increases the tyrosine kinase activity of pp60c-src and suggest that the platelet cytoskeleton plays an important role in collagen-induced signal transduction by localizing signaling molecules.

Selective Sp1 and Sp3 binding is crucial for activity of the integrin alphaV promoter in cultured endothelial cells

Human integrin alphavbeta receptors are expressed in a number of cells and their expression is regulated at the level of transcription and by post-transcriptional mechanisms. A substantial body of research exists on the structure, function, molecular biology and physiological significance of alphav integrin receptors. However, the importance of particular cis-acting DNA elements or trans-acting nuclear factors in the regulation of the alphav gene promoter is still not adequately understood. Previous functional analysis of the alphav gene 5' flanking region in transfected cultured cells identified cis elements critical for alphav transcription within a 222-bp region. To define further the location of this enhancing element, we performed DNase I footprinting of the human alphav gene promoter between -522 and the translation initiation site. For this purpose, nuclear extracts of alphavbeta3-positive cells, human umbilical vein endothelial cells, were used. Nuclear proteins of endothelial cells strongly protected essentially one region corresponding to the sequence between -194 and -172 of the alphav promoter region. Electrophoretic mobility shift assays with different oligonucleotides, and competition analysis identified a CTCCTCCTC sequence that is directly involved in the transcriptional activity of the alphav promoter. Purified Sp1 alone produced an identical footprint, and DNA binding assays using anti-Sp1 and anti-Sp3 antibodies showed that transcription factors Sp1 and Sp3 were the major nuclear proteins bound to this region.

Inhibition of plasminogen activator inhibitor release in endothelial cell cultures by antisense oligodeoxyribonucleotides with a 5'-end lipophilic modification

A series of conjugates containing residues of lipophilic alcohols covalently bound to 5' end of oligodeoxyribonucleotides targeted against human plasminogen activator inhibitor (PAI-1) mRNA was synthesized via the oxathiaphospholane approach. The highest anti-PAI-1 activity in EA.hy 926 endothelial cell cultures was found for conjugates containing menthyl or heptadecanyl groups linked with an oligonucleotide complementary to a segment of human PAI-1 mRNA. The phosphodiester antisense oligonucleotides, which otherwise exhibit only limited anti-PAI-1 activity, were found to be more active than phosphorothioate oligonucleotides when conjugated to lipophilic alcohol residues. For menthyl conjugates an evidence of antisense mechanism of inhibition was found.

Interferon gamma bound to endothelial cells is phosphorylated by ecto-protein kinases

The presence of protein kinase activity and its phosphorylated products has been demonstrated on the outer surface of the plasma membrane of endothelial cells. Extracellular phosphorylation was detected by incubation of primary endothelial cells (HUVEC's) and endothelial cell line EA.hy 926 with [gamma-32P]ATP. The reaction products were subjected to SDS/PAGE, autoradiography and scanning densitometry. Under the experimental conditions, five proteins with apparent molecular masses of 19, 23, 55, 88, and 110 kDa were prominently phosphorylated in both types of cells. Phosphorylation of the 19 kDa protein was the most rapid reaching maximum after 60 s and then the protein became dephosphorylated. Ecto-protein kinases responsible for the surface labeling of membrane proteins were characterized by using (a) protein kinase C inhibitors: K-252b, chelerythrine chloride, and [Ala113] myelin basic protein (104-118), (b) protein kinase A inhibitor Kemptide 8334, and (c) casein kinase II inhibitor 5,6-dichloro-1-beta-D-ribofuranosyl benzimidazole (DRB). Stimulation of endothelial cells with tumor necrosis factor alpha (TNF alpha) and interferon gamma (IFN gamma) is associated with 20-80% reduction of extracellular phosphorylation of all membrane proteins. IFN gamma bound to membrane receptors becomes rapidly phosphorylated. Only in the case of IFN gamma it was associated with the appearance of a strongly phosphorylated band of 17 kDa corresponding to IFN gamma itself. Phosphorylation of this 17 kDa exogenous substrate was prevented by an ecto-kinase inhibitor K-252b. The existence of ecto-phosphoprotein phosphatase activity in endothelial cells was evidenced by testing the effect of microcystin LR--a membrane impermeable reagent that inhibits both PP-1 and PP-2a phosphoprotein phosphatases. The extent of phosphorylation of 19 kDa and 110 kDa phosphoproteins significantly increased in the presence of microcystin. Our results suggest the presence of at least two ecto-kinase activities on endothelial cells that may play a significant role(s) in the regulation of cytokines function.

Peptide ligands can bind to distinct sites in integrin alphaIIbbeta3 and elicit different functional responses

The spatial relationship between the binding sites for two cyclic peptides, cyclo(S,S)KYGCRGDWPC (cRGD) and cyclo(S,S)KYGCHarGDWPC (cHarGD), high affinity analogs for the RGD and HLGGAKQAGDV peptide ligands, in integrin alphaIIbbeta3 (GPIIb-IIIa) has been characterized. For this purpose, cRGD and cHarGD were labeled with fluorescein isothiocyanate and tetramethylrhodamine 5-isothiocyanate, respectively. Both cyclic peptides were potent inhibitors of fibrinogen binding to alphaIIbbeta3, particularly in the presence of Mn2+; IC50 values for cRGD and cHarGD were 1 and <0.1 nM in the presence of Mn2+. Direct binding experiments and fluorescence resonance energy transfer analysis using the purified receptor showed that both peptides interacted simultaneously with distinct sites in alphaIIbbeta3. The distance between these sites was estimated to be 6.1 +/- 0.5 nm. Although cRGD bound preferentially to one site and cHarGD to the other, the sites were not fully specific, and each cyclic peptide or its linear counterpart could displace the other to some extent. The binding affinity of the cHarGD site was dramatically affected by Mn2+. cRGD, but not cHarGD, bound to recombinant beta3-(95-373) in a cation-dependent manner, indicating that the cRGD site is located entirely within this fragment. With intact platelets, binding of c-RGD and cHarGD to alphaIIbbeta3 resulted in distinct conformational alterations in the receptor as indicated by the differential exposure of ligand-induced binding site epitopes and also induced the opposite on membrane fluidity as shown by electron paramagnetic resonance analyses using 5-doxylstearic acid as a spin probe. These data support the concept the two peptide ligands bind to distinct sites in alphaIIbbeta3 and initiate different functional consequences within the receptor itself and within platelets.

The cytotoxicity of anti-PAI-1 oligonucleotides and their conjugates

The cytotoxicity of anti-PAI-5 hexadecanucleotides (phosphodiesters and phosphorothioates) and their conjugates with lipophilic alcohols was tested in EA.hy 926 hybrid endothelial cells. Some cytotoxicity was found for cholesteryl and bornyl conjugates at concentrations higher than those used for antisense inhibition experiments.

Release of calcium and P-selectin from intraplatelet granules is hampered by procaine

The inhibition of platelets by some local anaesthetics has been related to the modulation of platelet membrane lipid fluidity, and one of these compounds, procaine, has been proven to be particularly effective inhibitor. In the present study, we examined the effect of procaine on the mobilization of intracellular granule contents in isolated washed platelets. We revealed that the presence of 10 mg/ml procaine significantly hampered platelet release reaction, as demonstrated by the significant reduction in the expression of platelet P-selectin (CD62) on one hand, and significantly enhanced expression of GPIb alpha (CD42b) antigen on the other, following either 1 hour incubation of washed platelets at room temperature (%CD62: 37.1+/-6.8% of control incubated without procaine, p<<0.0001; %CD42b: 116.2+/-6.3% of control, p<0.0001) or activation of whole blood platelets with ADP, TRAP, or thrombin. Procaine, which acted as a rigidizer, significantly decreased platelet membrane fluidity (ESR h(+1)/h0 ratio of 5-DOXYL-Ste reduced down to 93.1+/-3.7% of control, p<0.001). In washed Fura-2-loaded platelets procaine not only brought about the significantly reduced Ca2+ release from intraplatelet storage pools after platelet stimulation with 15 micromol/l ADP (25.3+/-12.5% of control, p<0.001), but also it significantly increased the reduction in Ca2+ concentration upon the addition of Ca2+ chelator, EDTAK2 (48.9+/-13.5% vs. 40.9+/-12.1% of initial [Ca2+]i concentration, p(1,alpha)<0.025). Overall, procaine considerably reduced calcium mobilization from intraplatelet storage pools and Ca2+ efflux across platelet membrane. Based on these data, we suggest that the preventive effects of procaine on platelet release reaction and calcium mobilization might relate to the changes in the organization of membrane components embedded into a lipid bilayer, which are crucial in triggering of platelet release reaction. Procaine-mediated dislocations of some membrane components and/or distortion of lipid-protein interactions could generate a steric hindrance, which might interfere with platelet signal transduction, thus leading to impaired mobilization of Ca2+ and other components from intraplatelet storage pools.

Tyrosine phosphorylation events during different stages of collagen-platelet activation

Three groups of phosphoproteins have been distinguished, basing on the velocity and extent of phosphorylation in platelets stimulated with collagen. pp60c-src constituted the first group; the increase in its phosphorylation was the highest and most rapid (maximal in 30 s after the addition of collagen). pp80/85 and non-identified protein of 65 kDa formed the second group; the increase in their phosphorylation was twice smaller than that of pp60c-src, and reached its maximum 60 s after the addition of collagen. pp120, pp72syk, and two non-identified phosphoproteins of 90 and 75 kDa constituted the third group; the increase in their phosphorylation was 4-10-fold lower than that of pp60c-src and reached its maximum after 180 s. We conclude that the phosphorylation of pp60c-src is important for the change of shape of platelets, the phosphorylation of pp80/85 and pp65 for the initiation of the formation of aggregates and the phosphorylation of the third group of phosphoproteins for the formation of massive aggregates. This conclusion was supported by using a monoclonal anti-GPIb antibody, which did not inhibit the shape change of platelets and did not inhibit pp60c-src phosphorylation. This antibody inhibited aggregate formation as well as tyrosine phosphorylation of proteins belonging to the second and the third group of phosphoproteins.

Synthesis, biochemical and biological studies on oligonucleotides bearing a lipophilic dimethoxytrityl group

Dimethoxytritylphosphono-oligonucleotide conjugates have been prepared. They are totally resistant to nucleases present in human serum and do not affect cleavage of a complementary oligoribonucleotide by RNase H. Conjugates possessing a phosphate backbone gave better antisense inhibition of expression of plasminogen activator inhibitor type-1 within endothelial cells as compared with unconjugated oligonucleotides.

Association of pp60c-src with alpha IIb beta 3 in resting platelets

To detect whether 125I-alpha IIb beta 3 is associated with tyrosine kinases in platelets, antibodies specific to pp60c-src, pp54/58lyn, and pp62Fyn were used to precipitate their homologous antigens. In contrast to Lyn and Fyn kinases, pp60c-src appears to be complexed with alpha IIb beta 3. Both proteins, pp60c-src and alpha IIb beta 3, coprecipitated when antibodies to pp60c-src were used in the immunoprecipitation experiments. This conclusion was further supported by immunoprecipitation of alpha IIb beta 3 from Triton X-100 extracts of nonlabelled platelets with P2 antibodies. There was no pp60c-src detectable in immunoprecipitates obtained with antibodies specific to alpha 2 beta 1 or GPIb. Since PGE1 was used to prevent platelet activation in buffers throughout all procedures and there was no phosphorylation of pp72syk we assume that the platelets were in the resting state. Therefore, we conclude that alpha IIb beta 3 and pp60c-src can form a complex in resting platelets suggesting that pp60c-src is directly involved in initiating the outside-in signaling cascades in blood platelets.

Synthesis, biochemical and biological studies on oligonucleotides bearing a lipophilic dimethoxytrityl group

Dimethoxytritylphosphono-oligonucleotide conjugates have been prepared. They are totally resistant to nucleases present in human serum and do not affect cleavage of a complementary oligoribonucleotide by RNase H. Conjugates possessing a phosphate backbone gave better antisense inhibition of expression of plasminogen activator inhibitor type-1 within endothelial cells as compared with unconjugated oligonucleotides.

Phosphorothioate oligodeoxyribonucleotides antisense to PAI-1 mRNA increase fibrinolysis and modify experimental thrombosis in rats

The effect of systemic inhibition of PAI-1 expression in rats by PS-16R, a phosphorothioate analogue of hexadecadeoxyribonucleotide complementary to a signal peptide coding sequence of rat PAI-1 mRNA, on PAI-1 activity in blood plasma and thrombus formation was studied in rat models for experimental thrombosis. In previous in vitro studies, oligonucleotides of PS-16R family have been shown to inhibit efficiently PAI-1 synthesis in endothelial cells by antisense mechanism. When PS-16R was administered intravenously as a single bolus injection (1 to 5 mg per rat), it produced a significant reduction in PAI-1 activity of blood plasma. This effect was both time- and concentration-dependent. Under the same conditions, three groups of rats were treated with control oligodeoxynucleotides such as PS-16R with double mismatches, with scrambled sequence, and an oligodeoxynucleotide with sense sequence (complementary to PS-16R), respectively. Based on these preliminary experiments, a low dose of 1.5 mg per rat was selected to produce approximately 20-30% reduction of PAI-1 activity in blood plasma and the effect of such a decrease in PAI-1 expression was tested on thrombus formation in two rat models for experimentally induced thrombosis. Such a limited decrease in PAI-1 activity produced a significant antithrombotic effect in the arterial thrombosis model. There was a profound delay in the occlusion time in rats treated with PS-16R when compared to control animals (80 +/- 3 and 55 +/- 3 h, respectively), although blood plasma activity of PAI-1 in the same groups of rats differed only by 20%. There was also a tendency to reduce both an incidence of venous thrombosis (58.33 and 68.11%, respectively) and thrombus weight (2.1 +/- 0.4 and 2.9 +/- 0.9 mg, respectively) in the animals treated with PS-16R. However, this effect was not significant. Thus, low dose of PS-16R through inhibition of PAI-1 synthesis in targeted cells in rats reduced PAI-1 activity in blood plasma and protected against arterial thrombus formation in the rat.

Contribution of Pro212-Ile276 sequence of human protein C to its anticoagulant and profibrinolytic activity

Activated protein C exhibits strong anticoagulant and profibrinolytic properties. The Pro212-Ile276 fragment of a heavy chain of the protein molecule is located in the nearest neighborhood of the catalytic domain. It was found that polyclonal antibodies against this fragment recognize the sequence in the native molecule. To determine the contribution of the fragment in the anticoagulant and profibrinolytic activities of the enzyme, competitive inhibition analyses were performed. We found that amidolytic activity of the enzyme was inhibited by the recombinant Pro212-Ile276 fragment in a dose-dependent manner. Also, the fusion protein prolonged the time of clot lysis in a micro-clot lysis assay. The presence of the recombinant protein fragment did not influence the reaction of activated protein C with factor Va, neither the reaction of the enzyme with its specific inhibitor. We conclude, that Pro212-Ile276 region of protein C can not be identified with the binding pocket of the enzyme, but might be a binding site for small, low molecular weight substrates like chromogenic substrate.

Stereodependent inhibition of plasminogen activator inhibitor type 1 by phosphorothioate oligonucleotides: proof of sequence specificity in cell culture and in vivo rat experiments

Hexadecadeoxyribonucleotides complementary to a fragment of human PAI-1 mRNA located upstream of the start codon and their phosphorothioate analogs were studied in cultured HUVECs as sequence-dependent inhibitors of PAI-1 expression. The activity of the random mixture of diastereomers of phosphorothioate hexadecanucleotide PS-16H has been compared with that of isosequential, stereoregular [All-Sp] and [All-Rp] isomers. The highest inhibitory effect on PAI-1 synthesis was observed with the [All-Sp] diastereomer. Stereorandom phosphorothioate oligonucleotide PS-16R complementary to the same region of rat PAI-1 mRNA, when injected into tail vein of rats, substantially decreased the level of PAI-1 in blood plasma.

Peptide-specific antibodies as probes of the topography of the Ca2+-binging motifs in alphaIIbbeta3

Antibodies against synthetic peptides corresponding to four Ca2+-binding motifs of the alphaIIb subunit have been obtained and used as molecular probes to analyze the topography of the alphaIIbeta3 complex. The specificity of the antibodies has been characterized by ELISA and Western immunoblotting in terms of binding capacity and affinity to the isolated alphaIIbbeta3 and its alphaIIb subunit. Our data suggest that: (a) all four Ca2+-binding motifs of the alphaIIb are partially exposed on the surface of the intact molecule and accessible to antipeptide antibodies. However, they are not in close vicinity to the ligand recognition domain since the antibodies do not produce complete inhibition of platelet aggregation. (b) The conformation of amino acid stretches which form the second Ca2+-binding motif of alphaIIb is particularly dependent upon the presence of cation, and this region undergoes significant conformational alterations upon Ca2+ expulsion.

Differential effects of the tyrosine kinase inhibitors on collagen type 1-induced platelet aggregation and adhesion to this protein

Herbimycin A, lavendustin A, and methyl 2,5-dihydroxycinnamate were used to study the role of protein tyrosine kinases in collagen-platelet interaction. All three compounds produced a concentration dependent inhibition of platelet aggregation induced by collagen type I, characterized by values of IC50 equaled to 0.9, 10.0, and 5.0 microM, respectively. This effect was accompanied by strong inhibition of phosphorylation of p125FAK, p90, p72syk, p60c-arc, and p56lyn. In the absence of the inhibitors, phosphorylation of these proteins is evoked by aggregation of platelets. In addition to the antiaggregatory effect, the tyrosine kinase inhibitors reduced adhesion of platelets to collagen although to much lower extent than aggregation. Platelets which adhered to collagen showed also the presence of phosphorylated p125FAK, p90, p72syk, p60c-arc, and p56lyn. Of these proteins, the extent of phosphorylation of p90 was particularly high. Adhesion of platelets was associated with inhibition of phosphorylation of p125FAK, p60c-arc, and p56lyn only when high concentration of lavendustin A and methyl 2,5-dihydroxycinnamate were used. Herbimycin A did not affect adhesion-evoked protein tyrosine phosphorylation. Phosphorylation of p90 and p72syk was not affected by inhibitors. This study indicates that collagen type I can induce different transmembrane signalling dependent upon whether platelet aggregates formation or adhesion of platelets to this protein occurs.

Platelet membrane fluidity and intraplatelet Ca2+ mobilization are affected in uraemia

In present investigations, platelet membrane fluidity and intraplatelet Ca2+ mobilization were analysed in uraemic platelets by fluorescence techniques. Thirteen non-dialyzed uraemic patients and 16 control subjects were examined. Anisotropy of DPH-probe, measured at 37 degrees C, was significantly higher in control (0.2236 +/- 0.0050) than in uraemic platelets (0.1969 +/- 0.0082; p < 0.01). There was no difference between control (109.8 +/- 6.0 nM) and uraemic platelets (100.0 +/- 7.3 nM) when the basal [Ca2+]i in resting platelets was determined. Activation of platelets by ADP (12.5 microM) or by thrombin (0.1 U/ml) resulted in an increase in [Ca2+]i. It was significantly higher (p* < 0.003 for ADP and p* < 0.009 for thrombin, respectively) in control platelets (383.6 +/- 56.3 nM and 2031.0 +/- 298.8 nM, respectively) than in uraemic ones (191.0 +/- 21.3 nM and 838.7 +/- 144.1 nM, respectively). The amount of released Ca2+ was higher in control platelets activated by both ADP and thrombin (157.6 +/- 21.4 nM and 409.3 +/- 71.0 nM, respectively) than in uraemic platelets (76.7 +/- 15.7 nM and 203.0 +/- 29.3 nM, respectively) and the differences were significant (p < 0.01 and p* < 0.01, respectively). These results indicate an abnormal intracellular Ca2+ mobilization in uraemic platelets. Both increased membrane fluidity and decreased Ca2+ mobilization should be considered as a possible reason of reduced fibrinogen receptor exposure on uraemic platelets.

Antigenic properties of fibrinogen component of Hemaseel HMN subjected to the antiviral severe dry heat treatment

The experiments in this work were focused on determination of the extent to which fibrinogen component of fibrin glue (Hemaseel HMN, Haemacure Biotech.) retained the conformation of the parent molecule after dry heat antiviral treatment (one hour at 100 degrees C). For this purpose antigenic structure of the fibrinogen component, heated and non-heated, was compared to that of control fibrinogens, i.e. the isolated one and the fibrinogen present in fresh human blood plasma. The analytic parameters CI50 and CIs were calculated from the competitive inhibition data obtained in ELISA systems using antisera to fibrinogen, plasmic fragments D and E, and to polypeptide chains A alpha, B beta, and gamma. These immunochemical analyses indicated that there was a modified expression of some fibrinogen epitopes probably resulting from unfolding of the A alpha chain and the better exposure of the E domain to the hydrated environment induced upon a heating procedure. Our data show that dry heat treatment of fibrinogen component is not associated with a significant overall conformational change of the molecule.

Changes in platelet response to adenosine 5'-diphosphate caused by verapamil

Verapamil is widely used in the treatment of patients with coronary artery disease. The effect of verapamil on vascular smooth muscle cells is well documented. This effect is mediated by the inhibition of calcium fluxes across plasma membranes. Some data suggest that verapamil may affect platelet functions in thrombosis, but those observations were made for much higher verapamil concentration than could be achieved in vivo. Our current investigations are focused on an influence of low doses of verapamil (0.1-1.0 microM) on platelet response to ADP. We have found that verapamil at concentration of 0.1 microM can inhibit platelet aggregation (by 10%) evoked in PRP by 1.0-1.5 microM ADP. Moreover, the inhibitory effect is potentiated by prolonged time of platelet preincubation with verapamil. On the other hand, we have found a significant reduction in the number of fibrinogen receptors exposed on the platelet surface of patients (n = 21) treated with therapeutic doses (240 mg/day) of verapamil during two weeks of drug administration. The mean number of exposed receptors was reduced from 75,000 to 40,000 per platelet, with significance p < 0.0001. In vitro platelet preincubation with verapamil, even in much higher concentrations, did not affect fibrinogen binding to ADP activated platelets. It suggests, that in vitro exposure of platelets to verapamil for a short time has no effect on the expression of fibrinogen receptors on platelets, but prolonged in vivo interaction of this drug with platelets results in reduction of the fibrinogen receptor exposition. Thus, observed inhibition of platelet aggregation does not relay on a simple reduction of the number of exposed receptors, but intraplatelet signalling has to be affected. In fact, we have observed, in platelets pretreated with low doses of verapamil, significantly reduced release of calcium ions upon activation by ADP, whereas the calcium influx under such conditions does not seem to be affected.

Platelet membrane fluidity and intraplatelet Ca2+ homeostasis are affected in uremia

Recently we have described a dependence of platelet disability in thrombosis upon the progression in renal failure and an elevated level of RGDS-containing degradation products in uremic plasma, which is also correlated with progression in renal failure. Based on fluorescence techniques, our present investigations concerned possible changes in platelet membrane fluidity and intraplatelet calcium homeostasis in uremic platelets. Washed platelets loaded with DPH or with Fura-2 were examined with LS-50 luminescence spectrometer. Light anisotropy of DPH measured at 37 degrees C was significantly higher in control platelet membranes than in uremic ones. It can be considered as more fluidic membranes of uremic platelets. No difference between the basal intraplatelet calcium level was found for uremic and control platelets, but in the presence of 5 mM EGTA, the basal level was reduced significantly deeper in uremic platelets. Activation of platelets by both ADP (12.5 microM) and thrombin (0.1 U/ml) resulted in rapid increase in the intraplatelet calcium level in the examined platelets, but this increase was significantly higher for control platelets. The results indicate an abnormal intracellular calcium homeostasis in uremic platelets, which is associated with an increased fluidity of platelet membranes in uremia.

Microenvironmental changes in platelet membranes induced by the interaction of fibrinogen-derived peptide ligands with platelet integrins

A few studies have confirmed the influence of peptides containing either the RGD or dodecapeptide H-12-V (HHLGGAKQAGDV) sequence on cell membrane structure and function. In order to consider previous findings and to explore microenvironmental changes associated with the interaction of these two fibrinogen-derived peptides with platelet membranes, we employed fluorescence quenching and electron paramagnetic resonance techniques to monitor the possible alterations in platelet membrane dynamics induced by RGDS and H-12-V. The interaction of RGDS with platelet membranes resulted in reduced values of the h+1/ho parameter in both 5-doxylstearic acid and 16-doxylstearic acid spectra indicating a significant rigidification of the membrane lipid bilayer. Otherwise, the fibrinogen-derived peptide that contained the gamma chain C-terminal sequence H-12-V had a fluidizing effect on the platelet membrane lipid bilayer. The labelling of platelet membranes with 1-anilino-8-naphthalenesulphonate (ANS) enabled us to estimate the energy transfer efficiency and the apparent interchromophore distance between membrane protein tryptophan and ANS embedded into the membrane lipid bilayer. As RGDS interacts with platelet membrane this distance decreases, resulting in the relevant increase of energy transfer efficiency. The opposite alterations were recorded upon interaction of platelet membranes with H-12-V. Furthermore, a small shift towards longer wavelengths, which accompanies the spectra of ANS in control platelet membranes, vanishes during the interaction with the peptide H-12-V. This observation can be accounted for by a decrease in the polarity of the ANS environment, and may suggest an enhanced contact of the membrane tryptophan with phospholipid fatty acids. Thus, the data indicate that after the action of H-12-V on platelet membrane receptors, the membrane tryptophan residues become exposed to the external environment and the quenchable fraction of membrane tryptophan becomes smaller. The increase (a) in the relative rotational correlation time (tau c) of 4-(ethoxyfluorophosphinyloxy)-2,2,6,6-tetramethylpiperidine- 1-oxyl (ethoxyfluorophosphinyloxy-TEMPO) and (b) in the hw/hs ratio in the spectra of 4-maleimido-2,2,6,6-tetramethylpiperidine-1-oxyl (maleimido-TEMPO) indicate that under these conditions there is an effective immobilization of some domains located on the hydrated surface of membrane proteins and mobilization of those domains buried inside the membrane protein molecules. The interaction of RGDS with platelet membrane integrins resulted in contrary effects, as compared to H-12-V. In conclusion, our spectroscopic data indicate that these two fibrinogen-derived peptides induce opposite effects in the dynamics of platelet membrane components.

Diabetes mellitus alters the effect of peptide and protein ligands on membrane fluidity of blood platelets

The increased nonenzymatic glycosylation of platelet membrane proteins has been suggested to underlie platelet hypersensitivity in diabetes and the relationship of this to the reduced membrane lipid fluidity has been reported. As the modulation in membrane fluidity may determine the degree of accessibility of membrane receptors, the consequent alterations in membrane lipid-protein interactions in diabetes mellitus may also underlie the differentiated effects of various thrombotic and fibrinolytic agents on platelet membrane lipid bilayer. In the present study we employed electron paramagnetic resonance and fluorescence spectroscopy to explore the ligand-induced platelet membrane fluidity changes in diabetic state, i.e. under conditions when the membrane architecture is considerably altered. The yield of the excimer formation of pyrenemaleimide (PM), which depends directly upon the collisional rate and distances between molecules, was elevated in diabetic platelet membranes, thus pointing to the occurrence of some constraints in the structure/conformation of platelet membrane proteins in diabetes mellitus. Such an immobilization of PM was accompanied by the significant elevation in membrane protein glycation in diabetic platelets. The effects of various interacting ligands on platelet membrane fluidity were significantly lower in diabetic platelets, and the differences were much more distinct at the lower depths of a lipid bilayer. Nevertheless, the alterations in membrane lipid fluidity observed upon the interaction of a given ligand occurred with an approximately equal frequency in control and diabetic platelets. Moreover, the probability that these alterations were less profound in diabetic platelets was the same for all types of ligands studied. In diabetic patients the interaction of RGDS and tissue-type plasminogen activator with platelet membranes resulted in much smaller reductions of the h+/h0 parameters in 5-DOXYL-Ste acid-labelled platelets, thus indicating a lesser rigidization of membrane lipid bilayer in diabetes. Likewise, the fluidizing effect of both fibrinogen itself and fibrinogen-derived peptides containing gamma-chain carboxy-terminal sequence H-12-V was less pronounced in diabetic platelet membranes.

Inhibition by modified oligodeoxynucleotides of the expression of type-1 plasminogen activator inhibitor in human endothelial cells

Antisense phosphodiester and phosphorothioate oligodeoxynucleotides (23-residue or 24-residue oligodeoxynucleotides) were constructed for sequences of type-1 plasminogen-activator-inhibitor mRNA to assess their capability to modulate type-1 plasminogen-activator-inhibitor-mediated fibrinolysis. Antisense oligodeoxynucleotides were targeted at the mRNA sequence coding a signal peptide, at a part of the reactive center Ile342-Pro349, and at an internally translated segment Asn265-Leu272. The effect of antisense oligonucleotides on the concentration of type-1 plasminogen activator inhibitor in conditioned media and human endothelial cells was determined by the activity test with fibrin as a substrate, and by immunoprecipitation after metabolic labelling of cells with [35S]methionine. Three phosphorothioate oligodeoxynucleotides were specifically inhibitory while phosphodiester oligodeoxynucleotides with the same sequence did not show any activity. Phosphorothioate oligodeoxynucleotides 2, 4 and 6 inhibited the synthesis of type-1 plasminogen activator inhibitor in endothelial cells in a time-dependent and concentration-dependent manner. These data suggest that antisense oligodeoxynucleotides may be useful in the design of antithrombolytic therapeutics.

Concentration of RGDS-containing degradation products in uremic plasma is correlated with progression in renal failure

A concentration of protein degradation products containing the RGDS sequence, which could contribute to a lower reactivity of uremic platelets, has been estimated in both uremic (n = 16) and control (n = 7) plasmas. Degradation products and other small molecules were separated from plasma by filtration through AMICON YM-10 filter. RGDS antigen was determined in filtered material using the radioimmunoassay method based on monospecific anti-RGDS rabbit polyclonal antibodies. The concentration of RGDS-containing degradation products in uremic plasma ranged from 0.8 to 353 nM with mean value 58.6 +/- 24.9 nM and was higher than in control (0.7 to 5.9 nM, mean value 2.1 +/- 0.9 nM). Moreover, the level of RGDS-antigen positively correlated with plasma creatinine concentration (R = 0.87, p < 0.001). The filtered material showed an inhibitory effect on fibrinogen binding to control platelets in respect to RGDS-antigen concentration. We conclude that the elevated concentration of RGDS-containing degradation products in uremic plasma is partially responsible for bleeding tendency in renal failure.

Characterization of cation-binding sequences in the platelet integrin GPIIb-IIIa (alpha IIb beta 3) by terbium luminescence

The binding of cations to purified GPIIb-IIIa (alpha IIb beta 3) and synthetic peptides corresponding to the potential cation-binding sites within this integrin has been assessed by terbium luminescence spectroscopy. Tb3+ supported fibrinogen binding to purified GPIIb-IIIa, at lower concentrations than Ca2+, consistent with its higher affinity for cation-binding motifs. Titration analyses indicated the presence of five Tb(3+)-binding sites of relatively high affinity in the receptor. These sites also could be filled by divalent cations. Six sequences within GPIIb-IIIa have the appropriate spacing of five of the usual six coordination sites for cations in functional Ca(2+)-binding EF-hand motifs. Peptides containing Tyr and/or Trp at selected positions as fluorescence energy donors were synthesized, and their Tb(3+)-binding capacity was assessed. The four potential Ca(2+)-binding sequences in the GPIIb subunit, GPIIb 242-255, 296-309, 364-377, and 425-438, were functional, despite lacking the usual Glu residue at the terminal coordination position. These peptides bound Tb3+ with the same affinity as typical Ca(2+)-binding loop peptides and also bound Ca2+ and other divalent cations without preference. Of the two candidate GPIIIa sequences, 118-131 and 208-221, the former bound Tb3+ and divalent cations with an affinity similar to that of the GPIIb peptides, whereas the latter peptide was not functional. This functional difference, as well as data obtained with substituted peptides, emphasizes the importance of the first coordination position for interaction of synthetic peptide loops with cations. Together, these data identify the five cation-binding sites within intact GPIIb-IIIa.

GPIIIa(90-102) and GPIIIa(631-653) epitopes as markers of conformational changes occurring during the activation of the glycoprotein IIb/IIIa complex

To evaluate the range of conformational changes in the GPIIIa subunit upon the interaction of GPIIb/IIIa complex with fibrinogen, we employed two polyclonal antipeptide antibodies against synthetic peptides spanning chain segments (90-102) and (631-653) of GPIIIa. The first peptide fragment derives from the region preceding the putative binding site for the RGD sequence, while the second one is located in the vicinity of membrane lipid bilayer. The resting platelets bound about 13,700 and 11,500 molecules of anti-[GPIIIa(90-102)] and anti-[GPIIIa(631-653)] antibodies, respectively. The anti-[GPIIIa(90-102)] antibodies preferentially bound to the active form of GPIIb/IIIa complex. This binding was significantly reduced after dissociation of the complex, indicating that the recognized epitopes became partially hidden in the GPIIIa subunit. In contrast to anti-[GPIIIa(90-102)], the anti-[GPIIIa(631-653)] antibodies reacted optimally with the EDTA-treated GPIIb/IIIa. Upon stimulation of platelets with ADP, the number of binding sites anti-[GPIIIa(631-653)] was reduced 2.8-fold. The binding affinity of anti-[GPIIIa(90-102)] antibodies to the resting and ADP-stimulated platelets differed 4.7-fold, indicating that the stimulation of the platelets with ADP resulted in better exposure of this epitope. This binding was inhibited almost totally by fibrinogen in a dose-dependent fashion. Similarly, fibrinogen inhibited binding of the anti-[GPIIIa(631-653)] antibodies to the platelets. In the reversed system, binding of fibrinogen to platelets was inhibited only to 60% by the anti-[GPIIIa(90-102)] antibodies. Such inhibition of fibrinogen binding was sufficient to block aggregation of ADP-stimulated platelets by the anti-[GPIIIa(90-102)] antibodies in platelet-rich plasma. In contrast, the anti-[GPIIIa(631-653)] potentiated fibrinogen binding by 20%, confirming that there were additional interacting sites for fibrinogen in other regions of the GPIIb/IIIa complex. The presented data indicate that the activation of the fibrinogen receptor is associated with extensive conformational changes in the GPIIIa subunit, detectable by use of antipeptide antibodies recognizing not only the epitopes located next to the putative ligand-binding site but also the epitopes in the vicinity of the platelet membrane lipid bilayer.

The epidermal growth factor-like domain from tissue plasminogen activator. Cloning in E. coli, purification and ESR studies of its interaction with human blood platelets

To examine whether the epidermal growth factor (EGF)-like domain Pro47-Asp87 is involved in the interaction of tissue plasminogen activator (t-PA) with platelets, we have expressed this domain in E. coli. The peptide fragment was produced from a plasmid expression vector as a fusion protein with beta-galactosidase Met1-Val444 at high yield in eight clones of E. coli. The fusion protein was purified and subjected to mild acid hydrolysis with formic acid, then the peptide Pro47-Asp87, identified by immunoblotting using specific antibodies to t-PA, was isolated by HPLC. After incubation with blood platelets spin labelled with 16-doxylstearic acid or 5-doxylstearic acid, the Pro47-Asp87 peptide fragment reduced fluidity of the membrane lipid bilayer to the same extent as did intact t-PA as indicated by ESR measurements. Our data suggest that the EGF-like domain of t-PA can directly interact with blood platelets and thus it seems to contain those sites of the t-PA molecule that bind the platelet membrane components.

Expression of fibrinogen receptors on platelets of uremic patients is correlated with the content of GPIIb and plasma level of creatinine

Platelets of uremic patients, activated with ADP, exposed less fibrinogen receptors than control platelets, i.e. 24612 +/- 5541 and 33400 +/- 4302 receptors per platelet, respectively. However, this difference was not statistically significant. When compared with the total number of GPIIb/IIIa complexes, quantified from platelet glycoprotein IIb (GPIIb) contents, active receptors on the platelet surface represented 13.6% and 35.1% of total pool of fibrinogen receptors in uremic and control platelets, respectively. The number of exposed fibrinogen receptors was positively correlated with the amount of GPIIb copies in both uremic and normal platelets. In uremic platelets, both the number of exposed receptors and the number of GPIIb copies were correlated with the plasma creatinine concentration suggesting, that binding of fibrinogen to uremic platelets depends upon the degree of renal failure. Uremic platelets contain similar amounts of fibrinogen as control ones i.e. 13.2 +/- 2.3 micrograms and 17.6 +/- 2.2 micrograms per 1 x 10(8) platelets, respectively. Whereas for beta-thromboglobulin (beta-TG) there was a significant difference of 392 +/- 102 ng and 803 +/- 202 ng per 1 x 10(8) platelets, respectively. Reduced beta-TG content in uremic platelets suggests limited platelet activation in vivo. These results support the concept that uremic platelets have impaired functions and indicate that there is a relationship between the progression in renal failure and disability of platelets in thrombosis.

[Progress in knowledge about fibrinolysis regulation]

Plasminogen activator inhibitor-1 (PAI-1) plays particularly important role in regulation of homeostasis of fibrinolytic system. It neutralizes active molecules of tissue-type and urokinase-type plasminogen activators. PAI-1 is synthesized mainly in endothelial cells but it is present also in other cells. It was found in vitro that elevated expression of PAI-1 gene and increase in PAI-1 concentration released from endothelium is caused by many different biologically active substances. Among them there are thrombin, lipopolysaccharides, cell growth factors, cytokines and also glucocorticoids and phorbol esters. In this work mechanisms of regulation of PAI-1 synthesis and role of this protein in homeostasis of fibrinolytic system are described.

The amino acids that constitute sequence gamma 268-282 of fibrinogen are not involved in fibrin monomer polymerization

Congenitally abnormal fibrinogens with impaired fibrin monomer polymerization have been described to contain single amino-acid substitutions localized in certain positions of the gamma 275-330 peptide region. To evaluate the role of the amino-acid sequence in the vicinity of Arg275 in fibrin monomer polymerization, the peptide fragment corresponding to gamma 268-282 was synthesized and used to obtain peptide-specific antibodies. These antibodies, when purified immunochemically on the immobilized peptide, bound to the intact fibrinogen and fibrin monomers with the same binding affinity. However, they did not recognize the gamma 268-282 epitopes on the denatured and reduced fibrinogen molecules. The lack of influence of antipeptide antibodies on fibrin monomer polymerization indicates that the gamma 268-282 peptide is not directly involved in the structure of the polymerization site in the D domain of fibrinogen. It is suggested that substitution of Arg275 either by His or Cys in abnormal fibrinogens results probably in conformational changes which disturb a proper orientation of the polymerization site and reduce its expression.

Localization of the cross-linking site of GPRVVERHK in the gamma-chain of human fibrinogen

The peptide alpha 17-24-Lys (GPRVVERHK) corresponding to the N-terminus of the alpha chain of fibrin was synthesized and used to localize its binding site in the fibrinogen molecule. The peptide was radioiodinated, incubated with fibrinogen, cross-linked with a bifunctional reagent disuccinimidyl suberate and the resulting product was analyzed in several ways, including plasmin digestion. The binding of the radioactive peptide was mainly to the gamma-chain and was inhibited by unlabelled GPRVVERHK and GPRP. After plasmin digestion, the radioactivity was present in fragment D1 and also in its gamma-chain remnant, but not in fragments D2, D3 or E3. Fragment D1 cross-linked with iodinated GPRVVERHK was purified by affinity chromatography on immobilized anti-fragment D IgG, further digested with plasmin in the presence of EGTA and the peptides were fractionated by reverse-phase HPLC. The amino acid sequence analysis of the radioactive peak revealed the presence of two peptides, gamma 357-373 and GPRVVERHK. It was concluded that the binding site for GPRVVERHK is in the sequence gamma 357-373 which is present in fragment D1 but absent in fragments D2 and D3.

Tissue-type plasminogen activator induces alterations in structure and conformation of membrane proteins upon its interaction with human platelets

In our very recent ESR study we reported that upon rt-PA binding to platelets the H+1/h0 ratios of 16-doxylstearate and 5-doxylstearate spin labels incorporated into the lipid bilayer of platelet membranes were significantly decreased. It corresponded to the increased rigidity of platelet lipid bilayer. In order to further explore this phenomenon we employed a fluorescence-quenching technique which enabled us to estimate the energy transfer efficiency and the apparent interchromophore distance between membrane protein tryptophan and 1-anilino-8-naphthalenesulphonate (ANS) molecules embedded in the membrane lipid bilayer. As t-PA interacts with the platelet membrane this distance decreases, resulting in the relevant increase of energy transfer efficiency. Thus, the data indicate that upon t-PA binding the membrane tryptophan residues are more exposed to the external environment and the quenchable fraction of membrane tryptophan becomes greater. Furthermore, the spectrum of ANS is slightly shifted towards longer wavelengths, which can be accounted for by an increase in the polarity of the environment. It suggests a diminished contact of membrane tryptophan with phospholipid fatty acids. Based on these observations we concluded that the interaction of rt-PA with platelet membranes might induce conformational changes in the membrane proteins, and consequently result in rearrangements of lipid matrix and the alterations in lipid-protein interactions in platelet membranes.

Increased platelet-fibrinogen interaction in patients with hypercholesterolemia and hypertriglyceridemia

Binding of fibrinogen to platelets washed from the blood of patients with hypercholesterolemia and hypertriglyceridemia (n = 25) and control donors (n = 12) was compared. In addition, the content of platelet glycoprotein IIb was determined by radioimmunoassay. Fibrinogen was bound in significantly higher amounts (P < 0.02) to hyperlipidaemic platelets activated by ADP than to control ones (107,112 +/- 16,371 and 45,612 +/- 6495 molecules per platelet, respectively). The mean content of GPIIb was the same in hyperlipidaemic and in control platelets (2.06 +/- 0.16 and 1.94 +/- 0.21 micrograms/10(8) platelets, respectively). The amount of fibrinogen bound to the activated hyperlipidaemic platelets showed a positive correlation with total plasma cholesterol and LDL (r = 0.45 and 0.47, respectively) whereas a negative correlation with plasma HDL was found (r = -0.50). The increased expression of fibrinogen binding sites similar to that of hyperlipidaemic platelets could be produced by preincubation of normal platelets with palmitic acid. This was evidenced by a significant increase of fibrinogen binding sites in control platelets. This suggests that either palmitoylation of the receptor or microenvironment changes in the membrane lipid bilayer may be responsible for the enhanced platelet receptor capacity to bind fibrinogen.

Do the spectra of maleimide spin-labelled whole blood platelets reflect the structure and conformation of membrane proteins?

The maleimide spin label (4-maleimido-2,2,6,6-tetramethylpiperidine-1-oxyl, MSL), the commonly used reagent specific for cysteine thiol groups in proteins, penetrates cell membranes and binds to both the membrane and cytoplasmic protein moieties. In order to differentiate the labelling of these two subpopulations of cell proteins, we developed three different methodological approaches varying in the consequent distribution of this label in platelets. (A) The labelling of platelet proteins was negligible when bovine serum albumin was used in the Tyrode's buffer for the isolation of platelets, as the majority of the spin label was bound to the albumin coated on the platelets. (B) Preblocking of the reactive thiol groups in albumin with non-spin maleimide analog, N-ethylmaleimide (NEM), caused a considerable amount of MSL to bind with whole platelets but the impartment of membrane component was below 50%. It suggests that the majority of the spin label penetrated platelets and was bound to the intrinsic platelet proteins. (C) In order to prevent labelling of intrinsic platelet proteins with MSL, platelets were preincubated with N-ethylmaleimide, which was able to penetrate platelets and block the reactive thiol groups inside the cells. Such a treatment resulted in a saturation of the intrinsic protein residues with this non-spin analog. The subsequent incubation of thus-treated albumin-free platelets with MSL was to enhance considerably the likelihood of the attachment of MSL molecules to the thiol groups available in platelet-membrane proteins.(ABSTRACT TRUNCATED AT 250 WORDS)