The metabolism of phosphoinositide-derived messenger molecules

The phosphoinositides are minor phospholipids present in all eukaryotic cells. They are storage forms for messenger molecules that transmit signals across the cell membrane and evoke responses to extracellular agonists. The phosphoinositides break down to liberate messenger molecules or precursors of messenger molecules. Many different compounds are formed, although the functions of only a few are understood. Recent studies elaborating the pathways for formation of products from phosphoinositides and the factors controlling their metabolism are summarized here.

Platelet activation

This review article describes the different receptors, second-messengers and mechanisms involved in platelet activation. Several platelet agonists have well-defined receptors at the platelet membrane of which a number are single polypeptides with 7 hydrophobic transmembrane domains. These receptors are connected, via GTP regulatory proteins, with cytoplasmic second-messenger-generating enzymes. Phospholipase C and adenylate cyclase are the two best-known enzymes, generating inositol triphosphate (IP3) and diacyl glycerol from phosphatidylinositol biphosphate and cyclic AMP from ATP respectively. The intraplatelet free calcium level, which is critical for the activation status of the platelet, is increased by IP3 and is lowered in the presence of rising cyclic AMP concentrations. Shape-change occurs with small increases in intraplatelet calcium, while aggregation and secretion of granules take place at higher calcium, levels. The role of myosin and actin filaments and of transmembrane glycoproteins is further discussed.

Thromboembolic complications and haemostatic changes in cyclosporin-treated cadaveric kidney allograft recipients

The incidence of thromboembolic complications was compared retrospectively in 90 cadaveric kidney allograft recipients treated with cyclosporin and low-dose steroids and the same number of cadaveric kidney allograft recipients treated with azathioprine, antilymphocyte globulin, and high-dose steroids. In the cyclosporin group, 17 thromboembolic complications occurred in 13 patients: 10 pulmonary emboli, 1 renal vein thrombosis, 3 deep vein thromboses, and 3 haemorrhoidal thromboses. In the azathioprine group, the only thromboembolic complication was 1 episode of superficial thrombophlebitis. Haemostatic tests in cyclosporin-treated and azathioprine-treated patients and normal subjects (10 in each group) showed increased concentrations of factor VIII C, fibrinogen, antithrombin III, and protein C in the cyclosporin-treated patients. Adenosine-5'-diphosphate-induced platelet aggregation was also significantly enhanced in the cyclosporin group. The effect of cyclosporin on haemostasis may predispose to thromboembolic complications.

A thromboxane synthetase inhibitor reorients endoperoxide metabolism in whole blood towards prostacyclin and prostaglandin E2

During incubation of citrated blood at 37 degrees C the levels of 6-keto prostaglandin F1 alpha (6-keto PGF1 alpha) and prostaglandin E2 (PGE2) remain constant, but rise markedly within one minute after the addition of collagen, particularly when thromboxane synthetase is blocked. The amount of 6-keto PGF1 alpha formed is dose-dependent for both collagen and the thromboxane synthetase inhibitor (UK-37,248). Moreover, the number of platelets will determine the extent of the 6-keto PGF1 alpha jump, that does not occur when blood is drawn after aspirin ingestion. The production of 6-keto PGF1 alpha in function of time is composed of a fast platelet-related (intercept) and a slower probably leukocyte-dependent contribution (slope). In the absence of UK-37,248 the intercept is 115 +/- 85 pg/ml, the slope is 12.9 +/- 7.7 pg/min/ml whereas in the presence of the thromboxane synthetase inhibitor they are 411 +/- 177 pg/ml and 56.2 +/- 25 pg/min/ml respectively. The present findings indicate that a thromboxane synthetase inhibitor, by not only reducing thromboxane A2 production but also enhancing prostacyclin generation with blood is exposed to thrombogenic stimuli such as collagen, should be superior to aspirin as an antithrombotic agent, although possible interference by enhanced PGE2 production should be taken into account.

MK-383 (L-700,462), a selective nonpeptide platelet glycoprotein IIb/IIIa antagonist, is active in man

BACKGROUND

Fibrinogen-dependent cross-linking of glycoprotein (GP) IIb/IIIa on activated platelets is the final mechanism leading to platelet aggregation. Inhibition of this mechanism may result in a novel antithrombotic agent. We studied the activity of MK-383 (L-700,462), a new, nonpeptide GPIIb/IIIa antagonist, in vitro and in vivo, in man.

METHODS AND RESULTS

MK-383, a nonpeptide tyrosine derivative, dose-dependently inhibited fibrinogen-dependent platelet aggregation, in vitro. Binding of 125I-labeled fibrinogen to activated platelets was prevented in a competitive manner with an IC50 of 10 +/- 4.2 nmol/L. The activity and tolerability of MK-383 were evaluated in a two-part double-blind, placebo-controlled, dose-escalation study in healthy male subjects using 1- and 4-hour intravenous infusions. Effects on ADP- and collagen-induced ex vivo platelet aggregation (APA or CPA) and template bleeding time (TBT) were evaluated. Twenty-four subjects participated in the 1-hour part. Six received placebo and 18 MK-383 in doses ranging from 0.05 to 0.40 microgram.kg-1 x min-1. MK-383 inhibited platelet aggregation and prolonged bleeding time in a dose-dependent manner. APA and CPA were totally inhibited at the end of infusion of 0.4 microgram.kg-1 x min-1 and returned to 55% and 89% of baseline, respectively, at 3 hours after infusion. TBT was prolonged at this dose from 5.0 +/- 1.3 minutes predose to 22.7 +/- 6 minutes at the end of the infusion (P < .01) and was normalized by 3 hours after infusion. In the 4-hour infusion part, 15 subjects received MK-383 (0.1 to 0.2 microgram.kg-1 x min-1), and five received placebo. Complete inhibition of ex vivo platelet aggregation was seen at 0.15 and 0.2 microgram.kg-1 x min-1. At 0.2 microgram.kg-1 x min-1, TBT was prolonged from 4.4 +/- 1.2 to 23.9 +/- 4.3 minutes at the end of infusion (P < .01) and remained slightly prolonged 3 hours after infusion (7.2 +/- 1.8 minutes). No adverse effects were observed in any of the 33 subjects receiving MK-383.

CONCLUSIONS

The results from this study indicate that MK-383 appears to be well tolerated and active in man. It is the first nonpeptide GPIIb/IIIa antagonist that can be used to investigate the antithrombotic potential of this new class of antiplatelet agents.

Pharmacokinetics and pharmacodynamics of MK-383, a selective non-peptide platelet glycoprotein-IIb/IIIa receptor antagonist, in healthy men

MK-383 (L-tyrosine, N-(n-butylsulfonyl)-O-[4-butyl(4-piperidinyl)], monohydrochloride monohydrate) is a potent and specific platelet fibrinogen receptor antagonist that may be useful in preventing processes that lead to occlusive thrombus formation in the lumen of the blood vessel. Two placebo-controlled phase I trials were completed in 56 healthy volunteers to investigate the safety, tolerability, pharmacokinetics, and pharmacodynamics of MK-383 administered as 1- and 4-hour infusions in the presence and absence of aspirin. When administered to healthy male subjects by constant infusions up to 0.4 microgram/kg/min over 1 hour or up to 0.2 microgram/min over 4 hours, it provided a well-tolerated reversible means of inhibiting platelet function. At infusion rates of 0.25 and 0.15 microgram/kg/min for 1 and 4 hours, respectively, MK-383 extended baseline bleeding time by 2.0- to 2.5-fold and inhibited adenosine diphosphate (ADP)-induced platelet aggregation by at least 80%. The pharmacokinetics of MK-383 include a mean plasma clearance of 329 ml/min, steady-state volume of distribution of 76 L, and half-life of 1.6 hours. The percentage of dose excreted in the urine was 37%. Correlations between MK-383 plasma concentration (C) and inhibition of platelet aggregation were examined by fitting with a sigmoid maximum-effect model. The plasma concentration yielding 50% inhibition (C50) for MK-383 in healthy volunteers is approximately 13 ng/ml, with a Hill coefficient > 5. Based on a naive pooled analysis, an exponential empirical model best describes the MK-383 C-extension of template bleeding time (BTE) relationship. The model indicates that the MK-383 plasma concentration necessary to double BTE is approximately 30 ng/ml (i.e., 2.5-fold greater than the C50 for ADP-induced inhibition of platelet aggregation). The pharmacokinetics of MK-383 was unaffected by pretreatment with 325 mg aspirin 1 day before and 1 hour before infusion. Conversely, aspirin pretreatment reduced C50 and increased bleeding time extension, suggesting that aspirin may have an additive effect with respect to inhibition of platelet function. Based on the putative role of the fibrinogen receptor in thrombotic processes and an acceptable human pharmacokinetic-pharmacodynamic profile, MK-383 should be evaluated in patients with unstable angina.

Inhibition of integrin function by a cyclic RGD-containing peptide prevents neointima formation

BACKGROUND

RGD-containing peptides are able to prevent binding of ligands to certain integrins such as alpha IIb beta 3 (glycoprotein IIb/IIIa) and alpha v beta 3 and as such are inhibitors for platelet aggregation and smooth muscle cell migration, both of which are involved in neointima formation.

METHODS AND RESULTS

Hamster carotid arteries were damaged, and neointima formation was determined at different time points. G4120, a cyclic RGD-containing peptide, was administered continuously intravenously by an implanted osmotic pump. Neointima formation was inhibited dose dependently. The inhibition was strongest when treatment was started before the vascular injury and continued for the full observation period. Treatment started after the damage and maintained until neointima assessment or started before and stopped earlier was less effective.

CONCLUSIONS

Inhibition of integrin function by an RGD-containing peptide results in reduction of the development of a neointima. This effect is due both to an early event, which could be due to inhibition of secretion of PDGF by the platelets with blocked alpha IIb beta 3, and to a late event, possibly by interference with smooth muscle cell alpha v beta 3.

The hemostatic system

The hemostatic system comprises platelet aggregation, coagulation and fibrinolysis also termed primary, secondary and tertiary hemostasis. From the platelet transcriptome 6000 mRNA species and represent receptors, ion channels, signalling molecules, kinases, phosphatases, and structural, metabolic and regulatory proteins. This abundance of regulatory proteins points towards the importance of signal transduction in platelet function. First platelets adhere to collagen, this induces activation signals such as TXA(2) that induces further Ca(2+) increase. Consecutively, fibrinogen binds to the integrin alpha(IIb)beta(3) resulting in aggregation.This self-amplifying process is controlled by signals, from endothelial cells, to restrict the platelet plug to the site of vessel injury. Secondary hemostasis (coagulation) consists of an extrinsic and intrinsic pathway. Thrombin is generated via Factor Xa resulting from the extrinsic tenase reaction that is turned of by tissue factor pathway inhibitor. While thrombin generation is maintained via positive feedback mechanisms activating factors V, VIII and XI. Excess thrombin is inhibited by antithrombin or by autodownregulation via activation of protein C. Since minor injuries are common, platelets and plasma clotting factors constantly produce clots to stop bleeding. If clots remained after the tissue healing, the vascular bed would become obstructed with clots therefore this is regulated by fibrinolysis, tertiary hemostasis. Tissue-type plasminogen activator synthesised by the endothelium, converts plasminogen to plasmin, the clot lysis enzyme. Plasmin clears the blood vessels by degrading fibrin. Fibrinolysis is controlled by plasminogen activators inhibitor (PAI-1), alpha2-antiplasmin and alpha2-macroglobulin, and thrombin-activatable fibrinolysis inhibitor (TAFI).

Thromboxane synthase inhibitors, thromboxane receptor antagonists and dual blockers in thrombotic disorders

Thromboxane A2 (TXA2) plays a pivotal role in platelet activation and is involved in the development of thrombosis. Thromboxane synthase inhibitors suppress TXA2 formation and increase the synthesis of the antiaggregatory prostaglandins PGI2 and PGD2; however, accumulated PGH2 may interact with the platelet and vessel wall TXA2 receptor, thus reducing the antiplatelet effects of this class of drug. TXA2 receptor antagonists block the activity of both TXA2 and PGH2 on platelets and the vessel wall. Very recently, drugs possessing both thromboxane synthase-inhibitory and thromboxane receptor-antagonist properties have been developed. Paolo Gresele and colleagues explain here why these drugs can be more efficacious than traditional antiplatelet agents and review the available experimental studies involving these drugs.

Role of proaggregatory and antiaggregatory prostaglandins in hemostasis. Studies with combined thromboxane synthase inhibition and thromboxane receptor antagonism

Thromboxane synthase inhibition can lead to two opposing effects: accumulation of proaggregatory cyclic endoperoxides and increased formation of antiaggregatory PGI2 and PGD2. The elimination of the effects of the cyclic endoperoxides by an endoperoxide-thromboxane A2 receptor antagonist should enhance the inhibition of hemostasis by thromboxane synthase blockers. We have carried out a series of double-blind, placebo-controlled, crossover studies in healthy volunteers to check if this hypothesis may be operative in vivo in man. In a first study, in 10 healthy male volunteers, the combined administration of the thromboxane receptor antagonist BM 13.177 and the thromboxane synthase inhibitor dazoxiben gave stronger inhibition of platelet aggregation and prolonged the bleeding time more than either drug alone. In a second study, in 10 different healthy male volunteers, complete inhibition of cyclooxygenase with indomethacin reduced the prolongation of the bleeding time by the combination BM 13.177 plus dazoxiben. In a third study, in five volunteers, selective cumulative inhibition of platelet TXA2 synthesis by low-dose aspirin inhibited platelet aggregation and prolonged the bleeding time less than the combination BM 13.177 plus dazoxiben. In vitro, in human platelet-rich plasma stimulated with arachidonic acid, the combination of BM 13.177 and dazoxiben increased intraplatelet cAMP while the single drugs did not affect it. Our results indicate that prostaglandin endoperoxides can partly substitute for the activity of TXA2 in vivo in man and that an increased formation of endogenous antiaggregatory and vasodilatory prostaglandins, as obtained with selective thromboxane synthase inhibitors, may contribute to the impairment of hemostasis.

Serum albumin enhances the impairment of platelet aggregation with thromboxane synthase inhibition by increasing the formation of prostaglandin D2

Dazoxiben, a thromboxane synthase inhibitor, inhibits arachidonic acid induced aggregation in platelet-rich plasma from some donors only ("responders"). We have studied the effect of dazoxiben in vitro on platelet aggregation and prostaglandin (PG) metabolism and the influence of the incubation period and of exogenously added serum albumin (SA). SA, which increases the production of anti-aggregatory PGD2 from cyclic endoperoxides, induced "non-responder" human platelets to respond. With rabbit platelets, however, that are insensitive to PGD2, exogenous SA failed to potentiate dazoxiben-induced inhibition. The ratio between PGD2 and TXB2 + PGE2 formed was crucial in determining the response of human platelets to dazoxiben: whenever this ratio was high, platelet aggregation was inhibited. SQ 22536, an adenylate cyclase inhibitor, and NO164, a PGD2 antagonist, reversed the inhibition by dazoxiben in human platelet-rich plasma, stressing the importance of a PGD2 mediated rise of cyclic AMP for the effectiveness of a thromboxane synthase inhibitor.

Dipyridamole Inhibits Platelet Aggregation in Whole Blood

Dipyridamole possesses antithrombotic properties in the animal and in man but it does not inhibit platelet aggregation in plasma. We evaluated the effect of dipyridamole ex vivo and in vitro on platelet aggregation induced by collagen and adenosine- 5’-diphosphate (ADP) in human whole blood with an impedance aggregometer. Two hundred mg dipyridamole induced a significant inhibition of both ADP- and collagen-induced aggregation in human blood samples taken 2 hr after oral drug intake. Administration of the drug for four days, 400 mg/day, further increased the antiplatelet effect. A significant negative correlation was found between collagen-induced platelet aggregation in whole blood and dipyridamole levels in plasma (p <0.001). A statistically significant inhibition of both collagen (p <0.0025) and ADP-induced (p <0.005) platelet aggregation was also obtained by incubating whole blood in vitro for 2 min at 37° C with dipyridamole (3.9 μM). No such effects were seen in platelet-rich plasma, even after enrichment with leukocytes. Low-dose adenosine enhanced in vitro inhibition in whole blood.

Our results demonstrate that dipyridamole impedes platelet aggregation in whole blood by an interaction with red blood cells, probably involving adenosine.

Antithrombotic effect of platelet glycoprotein Ib-blocking monoclonal antibody Fab fragments in nonhuman primates

Platelet adhesion in arterial blood flow is mainly supported by the platelet receptor glycoprotein (GP) Ib, which interacts with von Willebrand factor (vWF) that is bound to collagen at the site of vessel wall injury. Antibody 6B4 is a monoclonal antibody (MoAb) raised against purified human GPIb. MoAb 6B4 inhibits both ristocetin- and botrocetin-induced, vWF-dependent human platelet agglutination. MoAb 6B4 furthermore blocks shear-induced adhesion of human platelets to collagen I. We studied the antithrombotic effect of this inhibitory murine MoAb 6B4 in a baboon model of arterial thrombosis. When injected into baboons, intact IgG and its F(ab')(2) fragments caused almost immediate thrombocytopenia, whereas injection of the Fab fragments alone did not. Fab fragments were subsequently used to investigate their in vivo effect on platelet deposition onto a thrombogenic device, consisting of collagen-rich, glutaraldehyde-fixed bovine pericardium (0.6 cm(2)), at a wall shear rate ranging from 700 to 1000 s(-1). Baboons were either pretreated with Fabs to study the effect of inhibition on platelet adhesion or treated 6 minutes after placement of the thrombogenic device to investigate the effect on interplatelet cohesion. Pretreatment of the animals with bolus doses ranging from 80 to 640 microgram/kg Fab fragments significantly reduced (111)In-labeled platelet deposition onto the collagen surface by approximately 43% to 65%. Only the highest dose caused a significant prolongation (doubling) of the bleeding time. Ex vivo ristocetin-induced platelet agglutination was equally reduced. Treatment with a bolus of 110 microgram/kg Fab fragments after a thrombus was allowed to form for 6 minutes had no effect on further platelet deposition. We therefore conclude that Fab fragments or derivatives of inhibitory anti-GPIb antibodies may be useful compounds to prevent thrombosis.

von Willebrand factor binds to native collagen VI primarily via its A1 domain

Collagen VI is abundant in the arterial subendothelium. To investigate its mechanism of interaction with von Willebrand factor (vWF), collagen VI was isolated from human placenta and from the extracellular matrix of the human lung fibroblast cell line MRC-5. Purified vWF bound to non-digested collagen VI with moderately high affinity (EC50 approximately 5 nM) and could be inhibited by the Hirudo medicinalis collagen inhibitor calin. The anti-(human vWF A1 domain) monoclonal antibody (AJvW-2), as well as aurin tricarboxylic acid (ATA), at concentrations that saturate the vWF A1 domain, also inhibited this binding. In contrast, the monoclonal anti-(human vWF A3 domain) antibody (82D6A3) inhibited vWF binding to collagens I, III and IV, but had no effect on vWF binding to collagen VI. Likewise, vWF binding to collagen VI was not inhibited by the recombinant vWF domain D4. Polyclonal anti-(collagen VI) antibodies, specifically neutralizing the binding of vWF to collagen VI, confirmed that in the intact endothelial cell extracellular matrix, collagen VI was accessible for interaction with vWF. This binding was only marginally affected by 82D6A3 but was dose-dependently inhibited by AJvW-2, ATA and the A1 domain analogue VCL (recombinant A1 domain of vWF), with IC50 values comparable to those found for the inhibition of vWF binding to isolated collagen VI. The weak interaction of isolated human platelets with collagen VI was mediated via the platelet collagen receptor (GPIa/IIa) and was competitively inhibited by vWF but not by VCL, suggesting that vWF and GPIa/IIa bind to neighbouring but distinct sites on collagen VI. We conclude that vWF binds to collagen VI primarily via its A1 domain, which distinguishes it from the vWF A3 domain-mediated binding to fibrillar collagens.

Multiple ways to switch platelet integrins on and off

In the classical concept of platelet integrin activation, it is considered that unidirectional conformational changes of alpha(IIb)beta(3) and alpha(2)beta(1) regulate the adhesiveness of platelets for fibrin(ogen) and collagen, respectively. Here, we summarize recent evidence that these conformational changes: (i) can also occur in the reverse direction; and (ii) are not independent events. Platelet stimulation through the P2Y(12) receptors provokes only transient alpha(IIb)beta(3) activation via signaling routes involving phosphoinositide 3-kinases and Rap1b. Furthermore, alpha(IIb)beta(3) can be secondarily inactivated in platelets with prolonged high Ca(2+) rises, which expose phosphatidylserine and bind coagulation factors. Thus, platelet stimulation with strong agonists (collagen and thrombin) also results in transient integrin activation. Integrin alpha(2)beta(1) is found to be activated by a mechanism that is directly linked to alpha(IIb)beta(3) activation. Integrin alpha(2)beta(1) can adopt different activation states, depending on the trigger. Conclusively, reversibility and synchrony of platelet integrin activation are newly identified mechanisms to restrict thrombus growth and to allow optimal coagulation factor binding. Back-shifting of activated integrins towards their resting state may be a novel goal of antithrombotic medication.

An open conformation of ADAMTS-13 is a hallmark of acute acquired thrombotic thrombocytopenic purpura

Essentials Conformational changes in ADAMTS-13 are part of its mode-of-action. The murine anti-ADAMTS-13 antibody 1C4 discriminates between folded and open ADAMTS-13. ADAMTS-13 conformation is open in acute acquired thrombotic thrombocytopenic purpura (TTP). Our study forms an important basis to fully elucidate the pathophysiology of TTP.

SUMMARY

Background Acquired thrombotic thrombocytopenic purpura (aTTP) is an autoimmune disorder characterized by absent ADAMTS-13 activity and the presence of anti-ADAMTS-13 autoantibodies. Recently, it was shown that ADAMTS-13 adopts a folded or an open conformation. Objectives As conformational changes in self-antigens play a role in the pathophysiology of different autoimmune diseases, we hypothesized that the conformation of ADAMTS-13 changes during acute aTTP. Methods Antibodies recognizing cryptic epitopes in the spacer domain were generated. Next, the conformation of ADAMTS-13 in 40 healthy donors (HDs), 99 aTTP patients (63 in the acute phase versus 36 in remission), 12 hemolytic-uremic syndrome (HUS) patients and 63 sepsis patients was determined with ELISA. Results The antibody 1C4 recognizes a cryptic epitope in ADAMTS-13. Therefore, we were able to discriminate between a folded and an open ADAMTS-13 conformation. We showed that ADAMTS-13 in HDs does not bind to 1C4, indicating that ADAMTS-13 circulates in a folded conformation. Similar results were obtained for HUS and sepsis patients. In contrast, ADAMTS-13 of acute aTTP patients bound to 1C4 in 92% of the cases, whereas, in most cases, this binding was abolished during remission, showing that the conformation of ADAMTS-13 is open during an acute aTTP episode. Conclusions Our study shows that, besides absent ADAMTS-13 activity and the presence of anti-ADAMTS-13 autoantibodies, an open ADAMTS-13 conformation is also a hallmark of acute aTTP. Demonstrating this altered ADAMTS-13 conformation in acute aTTP will help to further unravel the pathophysiology of aTTP and lead to improved therapy and diagnosis.

ADAMTS-13 plasma level determination uncovers antigen absence in acquired thrombotic thrombocytopenic purpura and ethnic differences

BACKGROUND

The recently discovered plasma enzyme ADAMTS-13 cleaves the A2-domain of von Willebrand factor (VWF). A defective cleaving protease results in unusually large VWF multimers, which cause thrombotic thrombocytopenic purpura (TTP).

AIM

Analysis of the ADAMTS-13 antigen levels in TTP patients compared with normal donors.

METHODS

An antigen ELISA test was built, based on high affinity anti-ADAMTS-13 monoclonal antibodies, which were generated using genetic immunization.

RESULTS

Specificity of the ADAMTS-13 antigen test was confirmed, as (i) plasma from a patient with acquired TTP but presenting without inhibitor did not contain antigen and (ii) the binding of recombinant ADAMTS-13 was inhibited by increasing amounts of normal plasma. The assay is sensitive as it can detect antigen levels as low as 1.6% of normal. The concentration in normal pooled human plasma was determined (1.03 +/- 0.15 microg mL(-1)) and arbitrarily set to 1 U mL(-1). The antigen levels in congenital TTP samples (34 +/- 21 mU mL(-1), n = 2), as well as in samples from patients with acquired TTP (231 +/- 287 mU mL(-1), n = 11), were clearly reduced when compared with normal Caucasian donors (951 +/- 206 mU mL(-1), n = 16). Remarkably, normal Chinese donors have a significantly lower antigen titer (601 +/- 129 mU mL(-1), n = 15), when compared with normal Caucasians.

CONCLUSIONS

Our results show that acquired TTP patients suffer mainly from ADAMTS-13 antigen depletion, thereby indicating the importance of ADAMTS-13 antigen determination in diagnosis and patient follow-up.

The platelet insulin receptor: detection, partial characterization, and search for a function

A direct demonstration and partial characterization of the insulin receptor on human platelets was obtained by immunoprecipitation with a monoclonal antibody against the human insulin receptor. Two subunits were detected, one with a MW of 95 KD, which dose-dependently was phosphorylated upon challenge of the platelets with insulin and another with a MW of 69 KD that does not become phosphorylated. Retention on wheat germ agglutinin indicates that at least part of the receptor protein is glycosylated. In a search for cellular effects provoked by insulin on platelets, no changes could be detected in cAMP formation or degradation, inositol phosphate formation or phosphorylation of proteins other than the receptor itself.

BM 13.177, a selective blocker of platelet and vessel wall thromboxane receptors, is active in man

BM 13.177, a sulphonamide derivative, prevented platelet aggregation by thromboxane A2 in vitro and selectively inhibited contraction of isolated rabbit femoral arteries induced by two stable endoperoxide analogues. In a double-blind placebo-controlled study, oral BM 13.177 inhibited platelet aggregation induced by arachidonic acid, low dose collagen, and the two stable endoperoxide analogues, and slightly prolonged the bleeding time. Generation of thromboxane or of other prostaglandins was not affected. No side-effects were seen. BM 13.177 appears selectively and safely to block platelet and vessel wall thromboxane receptors and should be useful in elucidating the role of thromboxane A2 in disease.

Epitope mapping of inhibitory antibodies against platelet glycoprotein Ibalpha reveals interaction between the leucine-rich repeat N-terminal and C-terminal flanking domains of glycoprotein Ibalpha

The interaction of von Willebrand factor (vWF) with the platelet receptor glycoprotein Ibalpha (GPIbalpha) is important for platelet adhesion at high shear stress. Two functionally important antigenic areas within GPIbalpha were identified through the characterization of 5 new inhibitory anti-GPIb monoclonal antibodies (mAbs). The binding sites of 3 of these anti-GPIb mAbs, which were intercompeting and potently inhibiting shear stress-induced binding of vWF, were mapped within the N-terminal amino acid (aa) 1-59 area by the use of canine-human chimeras. These antibodies, however, had little or no effect (approximately 40% inhibition) on the binding of vWF induced by either botrocetin or ristocetin. On the other hand, the anti-GPIb mAbs 24G10 and 6B4, which blocked GPIb-vWF binding under all conditions examined, bound to 2 different regions of GPIbalpha, aa 1-81 and aa 201-268, respectively. The epitope for 6B4 was further narrowed by phage display revealing 2 sets of peptide sequences aligning within aa 259-262 and aa 230-242. In the latter region of GPIbalpha, the gain-of-function platelet-type von Willebrand disease (PT-vWD) mutations have been identified. Alignment was partially confirmed because the binding of 6B4 to recombinant GPIbalpha fragments carrying either one of the PT-vWD mutations was considerably impaired but not completely abolished. In contrast, mAb 24G10 bound more strongly to mutant PT-vWD GPIbalpha. However, although 24G10 competed with 6B4 for binding to platelets, it bound to an epitope within aa 1-81 of GPIbalpha. In conclusion, 2 functionally important areas within GPIbalpha were identified: one localized within the leucine-rich repeat N-terminal aa 1-59 area and one composed of residues aa 1-81 in close contact with aa 201-268. Moreover, further support is provided for the existence of an intramolecular interaction between the N-terminal flanking (aa 1-81) and C-terminal flanking (aa 201-268) regions. (Blood. 2001;98:652-660)

Linker regions and flexibility around the metalloprotease domain account for conformational activation of ADAMTS-13

BACKGROUND

Recently, conformational activation of ADAMTS-13 was identified. This mechanism showed the evolution from a condensed conformation, in which the proximal MDTCS and distal T2-CUB2 domains are in close contact with each other, to an activated, open structure due to binding with von Willebrand factor (VWF).

OBJECTIVES

Identification of cryptic epitope/exosite exposure after conformational activation and of sites of flexibility in ADAMTS-13.

METHODS

The activating effect of 25 anti-T2-CUB2 antibodies was studied in the FRETS-VWF73 and the vortex assay. Cryptic epitope/exosite exposure was determined with ELISA and VWF binding assay. The molecular basis for flexibility was hypothesized through rapid automatic detection and alignment of repeats (RADAR) analysis, tested with ELISA using deletion variants and visualized using electron microscopy.

RESULTS

Eleven activating anti-ADAMTS-13 antibodies, directed against the T5-CUB2 domains, were identified in the FRETS-VWF73 assay. RADAR analysis identified three linker regions in the distal domains. Interestingly, identification of an antibody recognizing a cryptic epitope in the metalloprotease domain confirmed the contribution of these linker regions to conformational activation of the enzyme. The proof of flexibility around both the T2 and metalloprotease domains, as shown by by electron microscopy, further supported this contribution. In addition, cryptic epitope exposure was identified in the distal domains, because activating anti-T2-CUB2 antibodies increased the binding to folded VWF up to ~3-fold.

CONCLUSION

Conformational activation of ADAMTS-13 leads to cryptic epitope/exosite exposure in both proximal and distal domains, subsequently inducing increased activity. Furthermore, three linker regions in the distal domains are responsible for flexibility and enable the interaction between the proximal and the T8-CUB2 domains.

Leukotriene B4 production by stimulated whole blood: comparative studies with isolated polymorphonuclear cells

A new method was developed to study leukotriene B4 (LTB4) production by stimulated whole blood. The calcium ionophore A23187 and serum-treated zymosan induced LTB4 production, measured by radioimmunoassay, in a dose- and time-dependent manner. The pattern of LTB4 production by whole blood differed markedly from that observed with isolated, purified polymorphonuclear leukocytes. Higher levels of LTB4 were reached and maintained in whole blood. The system allowed to detect drug effects on LTB4 synthesis in vitro. This new method to study the synthesis of LTB4 takes into account the complex interactions between different cell types which can modulate LTB4 metabolism.

Characterization of N,N’-bis(3-Picolyl)-4-Methoxy-Isophtalamide (Picotamide) as a Dual Thromboxane Synthase Inhibitor/Thromboxane A2 Receptor Antagonist in Human Platelets

Picotamide (G137 or N,N’-bis[3-picolyl]-4-methoxy-iso- phtalamide), a drug which has shown platelet inhibitory effects in vitro and ex vivo, was investigated for its mechanism of action on human platelets in vitro. This compound suppresses the aggregation of human platelets induced by arachidonic acid (IC50: 1.8 × 1(T5 M), low-dose collagen (IC50: 3.5 × 10-4 M), U46619 (IC50: 1.4 × 10-4 M) and by authentic TxA2 (IC50: 1 × 10-4 M), without affecting the aggregation induced by A23187 or primary aggregation by ADP. Picotamide inhibits dose-dependently TxA2 synthesis by platelets (IC50: 1.5 × 10-4 M) and enhances the formation of PGE2. Picotamide-treated platelets also favour the formation of PGI2 by aspirinated endothelial cells; in addition, the drug appears to exert a direct stimulatory effect on PGI2- synthesis, at least at high concentrations. Finally, in platelet-rich plasma stimulated with arachidonic acid, picotamide increases intraplatelet cAMP while no effects on cAMP are detected in unstimulated platelets.

In conclusion, picotamide is a dual thromboxane-synthase inhibitor/thromboxane-receptor antagonist in human platelets and introduces a new class of agents potentially useful in antithrombotic therapy.

The von Willebrand factor self-association is modulated by a multiple domain interaction

BACKGROUND

Platelet adhesion and aggregation at sites of vascular injury exposed to rapid blood flow require von Willebrand factor (VWF). VWF becomes immobilized by binding to subendothelial components or by a self-association at the interface of soluble and surface-bound VWF.

OBJECTIVES

As this self-association has been demonstrated only under shear conditions, our first goal was to determine whether the same interaction could be observed under static conditions. Furthermore, we wanted to identify VWF domain(s) important for this self-association.

RESULTS

Biotinylated VWF (b-VWF) interacted dose-dependently and specifically with immobilized VWF in an enzyme-linked immunosorbent assay (ELISA) assay, showing that shear is not necessary to induce the VWF self-association. Whereas anti-VWF monoclonal antibodies (mAbs) had no effect on the self-association, the proteolytic VWF-fragments SpII(1366-2050) and SpIII(1-1365) inhibited the b-VWF-VWF interaction by 70 and 80%, respectively. Moreover, a specific binding of b-VWF to immobilized Sp-fragments was demonstrated. Finally, both biotinylated SpII and SpIII were able to bind specifically to both immobilized SpII and SpIII. Similar results were observed under flow conditions, which confirmed the functional relevance of our ELISA system.

CONCLUSION

We have developed an ELISA binding assay in which a specific VWF self-association under static conditions can be demonstrated. Our results suggest a multiple domain interaction between immobilized and soluble VWF.