X-linked sideroblastic anaemia due to ALAS2 mutations in the Netherlands: a disease in disguise.

BACKGROUND: X-linked sideroblastic anaemia (XLSA; OMIM#300751) is the most common inherited form of sideroblastic anaemia and is associated with several mutations in the erythroid specific 5-aminolevulinate synthase gene (ALAS2). This gene encodes for aminolevulinic acid synthase 2 (ALAS2), the catalytic enzyme involved in the first en rate-limiting step of haem biosynthesis.1-3 The disorder is characterised by mostly mild hypochromic microcytic anaemia with bone marrow ring sideroblasts. Even untransfused patients with mild or no anaemia are at risk for severe systemic iron overload due to ineffective erythropoiesis. To date, 61 different ALAS2 mutations have been reported in 120 families with XLSA. Descriptions of molecularly confirmed case series from the Netherlands, however, are lacking. METHODS: We reviewed age of presentation, clinical and biochemical features, ALAS₋2 defects and treatment characteristics of 15 Dutch patients from 11 unrelated families diagnosed with XLSA. RESULTS AND CONCLUSIONS: In one family a novel pathogenic c.1412G>A (p.Cys471Tyr) mutation was found. All other families shared the previously described c.1355G>A (p.Arg452His) mutation. Haplotype analysis in seven probands with the p.Arg452His mutation strongly suggests that six of them were ancestrally related. Nevertheless, their phenotype was very different. Our patients illustrate the phenotypical heterogeneity in the presentation of XLSA patients, the effectiveness of treatment regimens and the various pitfalls associated with the diagnosis, follow-up and treatment of the disease. A timely diagnosis avoids unnecessary investigations and allows adequate treatment that can prevent systemic iron load with subsequent severe life-threatening complications. Therefore, we suggest considering XLSA in both male and female patients with unexplained iron overload and÷or (mild) microcytic anaemia, also at older age.

Low incidence of infectious complications after nonmyeloablative compared with myeloablative allogeneic stem cell transplantation.

Allogeneic stem cell transplantation (SCT) using a myeloablative (MA) conditioning regimen is limited to relatively young patients because of increased transplant-related mortality in elderly patients. Nonmyeloablative (NMA) conditioning regimens have been developed aiming to reduce transplant mortality. In this study, we set out to evaluate the post-transplant occurrence of infectious complications in recipients of grafts from human leukocyte antigen (HLA)-identical sibling donors treated with either NMA or MA conditioning regimens. Data of 78 consecutively treated patients were analyzed. An NMA conditioning regimen was used in 40 patients and an MA regimen in 38 patients. A significantly lower rate of episodes of febrile neutropenia (0% vs. 34%, P<0.01) and post-transplant Epstein-Barr virus reactivations (0% vs. 18%, P<0.05) was found in SCT recipients treated with an NMA conditioning regimen compared with an MA conditioning regimen. Furthermore, fewer invasive fungal infections (2% vs. 12%, not significant) were diagnosed in the NMA group. The incidence of cytomegalovirus (CMV) reactivations and bacterial infections was low in both groups (CMV reactivations: 13% in both groups; bacterial infections: 10% in the NMA group vs. 8% in the MA group), while CMV disease developed in only 1 patient. Overall, compared to our MA regimen, we found a very low rate of infectious complications after NMA SCT.

Recombinant factor VIIa reverses the in vitro and ex vivo anticoagulant and profibrinolytic effects of fondaparinux.

BACKGROUND: Fondaparinux is a synthetic pentasaccharide, which selectively inhibits coagulation factor (F) Xa, and is registered for prevention of venous thromboembolism following hip fracture, hip replacement, and knee replacement surgery. Recently, it was shown that recombinant FVIIa (rFVIIa) reverses anticoagulant effects of fondaparinux in healthy volunteers. OBJECTIVES: In this study, we have explored the in vitro and ex vivo effects of rFVIIa on clot formation and thrombin-activatable fibrinolysis inhibitor (TAFI)-mediated down-regulation of fibrinolysis after fondaparinux administration. METHODS: In vitro clot lysis assays were performed in pooled normal plasma from healthy volunteers to which fondaparinux was added, and in serial samples from healthy volunteers who received a single bolus dose of fondaparinux, a single bolus dose of rFVIIa, or both. RESULTS AND CONCLUSIONS: Fondaparinux significantly delayed clot formation, and clot lysis was significantly increased due to decreased activation of TAFI. Addition of recombinant FVIIa corrected the inhibited clot formation induced by fondaparinux, and the acceleration of clot lysis was partially reversed. In vivo administration of fondaparinux (10 mg) to healthy volunteers similarly resulted in accelerated plasma clot lysis. Subsequent administration of rFVIIa (90 microg kg(-1)) normalized the clot lysis time up to 6 h postadministration. rFVIIa might be a good therapeutic option in patients treated with fondaparinux who develop bleeding complications, since both clot formation as well as fibrinolytic resistance are improved.

The usefulness of five D-dimer assays in the exclusion of deep venous thrombosis.

D-Dimer measurement is a promising tool in the exclusion of venous thrombosis. New d-dimer assays have been introduced, but need clinical validation. Our objective was to evaluate the clinical usefulness of four relatively new d-dimer assays and a classical ELISA in outpatients suspected for deep venous thrombosis. In 537 patients, participants in a large prospective management study using a clinical probability score and a d-dimer measurement (Tina-quant), additional samples were taken for d-dimer measurement using the Asserachrom ELISA, the VIDAS New, the STA-LIA and the Miniquant assay. Performances of each test were calculated using clinical data during a 3-month follow-up. Thrombosis was detected in 224 patients (42%). The area under the ROC curve was significantly higher for the Tina-quant as compared to the other assays. Using standard cut-off values, sensitivity, negative predictive value (NPV) and specificity of the Asserachrom were 97, 94 and 33%, respectively. For the VIDAS New, values were 100, 96 and 8%, respectively. The Tina-quant showed values of 99, 98 and 41%, respectively, and the STA-LIA 98, 95 and 32%. Values for the Miniquant were 95, 94 and 52%. The d-dimer assays in our study all show a high sensitivity and negative predictive value, but none of the assays reached an NPV of > 98% at standard cut-off values. d-Dimer assays with a low specificity still necessitate additional diagnostic tests in the majority of the patients.

Combination of a normal D-dimer concentration and a non-high pretest clinical probability score is a safe strategy to exclude deep venous thrombosis.

BACKGROUND: Serial ultrasonography is reliable for the diagnosis of deep venous thrombosis in symptomatic patients, but the low prevalence of thrombosis in this group renders the approach costly and inconvenient to patients. We studied the clinical validity of the combination of a pretest clinical probability score and a D-dimer test in the initial evaluation of patients suspected of deep venous thrombosis. METHODS AND RESULTS: Patients with a normal D-dimer concentration (<500 fibrin equivalent units [FEU] microg/L) and a non-high probability score (<3) had no further testing. Patients with a normal D-dimer concentration and a high probability score (> or =3) underwent one ultrasonogram. Serial ultrasonography was performed in patients with an abnormal D-dimer concentration. Patients were followed for 3 months. A total of 812 patients were evaluable for efficacy. Only 1 of 176 patients (0.6%; 95% CI, 0.02% to 3.1%) with a normal D-dimer concentration and a non-high probability score developed thrombosis during follow-up. A normal D-dimer concentration and a high probability score were found in 39 patients; 3 of them (7.7%; 95% CI, 1.6% to 20.9%) had thrombosis at presentation, and one (2.8%; 95% CI, 0.07% to 14. 5%) developed pulmonary embolism during follow-up. In 306 of 597 patients (51.3%) with an abnormal D-dimer concentration, thrombosis was detected by serial ultrasonography. Six patients (2.1%; 95% CI, 0.8% to 4. 4%) developed thrombosis during follow-up. No deaths due to thromboembolism occurred during follow-up. The total need for ultrasonography was reduced by 29%. CONCLUSION: The combination of a non-high pretest clinical probability score and a normal D-dimer concentration is a safe strategy to rule out deep venous thrombosis and to withhold anticoagulation.

[Withdrawing the use of acetylsalicyclic acid prior to an operation usually not necessary]. / Staken van gebruik van acetylsalicylzuur vóór een operatie meestal niet nodig.

Acetylsalicylic acid (aspirin) induces an irreversible inactivation of cyclo-oxygenase in blood platelets which lasts for the entire period that the platelets remain in the circulatory system, 7 to 10 days. In order to prevent excessive bleeding, patients presenting for surgery are asked to stop using aspirin 10 days before the procedure. Some studies have found that aspirin causes increased peri-operative blood loss, whilst other studies have found that it does not. All effect studies found in Medline (January 1966-May 2002) on surgery and bleeding complications due to aspirin were analysed. The studies available were assessed for methodological quality and the results were summarised in an evidence table. No clinically relevant bleeding complications were reported for cardiovascular, vascular and orthopaedic surgery and epidural anaesthesia. Most studies reported an increase in clinically non-relevant bleeding induced by aspirin. The literature contains too little information on cataract surgery, dermatological surgery, gynaecological and abdominal surgery, ENT and dental surgery, urological surgery, lung biopsy and endoscopic biopsy. In those types of surgery in which even a minor bleeding leads to severe complications, e.g. neurosurgery, aspirin should be withdrawn 5-10 days in advance. Also in patients with coagulation disorders, aspirin should be withdrawn prior to the operation. There is no scientific evidence for the withdrawal of aspirin in all patients, 5-10 days prior to surgery. Indeed for heart patients in particular, the continued use of aspirin is recommended.

[From gene to disease; hereditary non-spherocytic hemolytic anemia caused by pyruvate kinase deficiency]. / Van gen naar ziekte; erfelijke niet-sferocytaire hemolytische anemie veroorzaakt door pyruvaatkinasedeficiëntie.

Pyruvate kinase (PK) deficiency is a common cause of hereditary non-spherocytic haemolytic anaemia. It is an autosomal recessive disorder caused by mutations in the gene coding for erythrocyte and liver-type pyruvate kinase (PKLR). So far, more than 130 mutations in this gene have been identified. Clinical symptoms, usually restricted to homozygous and compound-heterozygous individuals, are variable, ranging from neonatal jaundice requiring erythrocyte transfusions to a fully compensated haemolytic anaemia. The exact mechanism of erythrocyte destruction is unknown, however adenosine-triphosphate depletion and an increase in 2,3-disphosphoglycerate are thought to be important. The diagnosis of pyruvate kinase deficiency depends upon the demonstration of low PK enzyme activity. Due to the pitfalls in determining true PK activity, DNA testing is a valuable tool in the diagnosis of pyruvate kinase deficiency. By centralizing the molecular diagnostics of pyruvate kinase deficiency in Utrecht, more care can be provided for the diagnosis, treatment and support of patients.

The defective down regulation of fibrinolysis in haemophilia A can be restored by increasing the TAFI plasma concentration.

TAFI (thrombin activatable fibrinolysis inhibitor) down regulates fibrinolysis after activation by relatively high concentrations of thrombin generated during coagulation via thrombin mediated factor XI activation and subsequent activation of the intrinsic pathway. It is this secondary burst of thrombin that is severely diminished in haemophilia A, a deficiency of coagulation factor VIII. We therefore investigated the role of TAFI in haemophilia A by measuring the clot lysis times of tissue factor induced fibrin formation and tPA mediated fibrinolysis. In haemophilia A plasma clot lysis times were normal at relatively high tissue factor concentrations but severely decreased at moderate to low tissue factor concentrations, indicating that the thrombin generation via the extrinsic pathway was insufficient to activate TAFI. Addition of factor VIII, TAFI or thrombomodulin restored the clot lysis times at low tissue factor concentrations. This confirms the hypothesis that the bleeding disorder in haemophilia A is not merely a defect in the initial clot formation but is in fact a triple defect: reduced thrombin formation via the extrinsic pathway at low tissue factor concentrations, a reduced secondary burst of thrombin generation via the intrinsic pathway and a defective down regulation of the fibrinolytic system by the intrinsic pathway.

The 33-kDa platelet alpha-granule membrane protein (GMP-33) is an N-terminal proteolytic fragment of thrombospondin.

GMP-33 is a platelet membrane associated protein that is recognised by RUU-SP 1.77, an antibody raised against activated platelets. GMP-33 is predominantly associated with the membrane of platelet alpha-granules and it is translocated to the plasma membrane upon platelet activation (Metzelaar et al. Blood 1992; 79: 372-9). In this study we have isolated the protein by immunoaffinity chromatography. The N-terminus was sequenced and was identical to the N-terminal sequence of human thrombospondin. The protein was N-glycosylated and bound to heparin as would be expected of the N-terminal part of thrombospondin. RUU-SP 1.77 reacted only with reduced thrombospondin. Plasmin and trypsin digestion of thrombospondin yielded fragments of approximately the same size as GMP 33 that reacted with RUU-SP 1.77 after reduction. No evidence for alternative splicing was found. We postulate that GMP 33 is an N-terminal proteolytic fragment of thrombospondin that is membrane associated.

Relationship between graft-versus-host disease and graft-versus-leukaemia in partial T cell-depleted bone marrow transplantation.

The success of allogeneic bone marrow transplantation (BMT) is limited by the major complications, graft-versus-host disease (GVHD) and relapse. The very beneficial effect of maximal T-cell depletion of the graft for prevention of GVHD has been counterbalanced by an increase in graft failure and relapse of disease. Therefore, we started an approach of partial T-cell depletion of the graft. GVHD and graft-versus-leukaemia (GVL) are strongly correlated after non-T cell-depleted BMT. Here, we report whether the correlation between GVHD and GVL also exists in partial T cell-depleted BMT from sibling donors. We retrospectively studied 117 adult patients with early haematological malignancies. Our method of partial T-cell depletion gave a relapse rate in patients with acute leukaemias similar to that observed in non-T cell-depleted BMT. However, patients with chronic myeloid leukaemia had a relapse rate that was similar to that observed in maximal T cell-depleted BMT. We found a significant correlation between the presence of chronic GVHD and an improved disease-free survival. Nevertheless, overall survival was lower in patients with chronic GVHD. There was no correlation between the occurrence of acute GVHD and disease-free or overall survival.

Thrombin-activatable fibrinolysis inhibitor deficiency in cirrhosis is not associated with increased plasma fibrinolysis.

BACKGROUND AND AIMS: The bleeding tendency of patients suffering from cirrhosis is in part ascribed to accelerated fibrinolysis. In this study, the role of the recently discovered inhibitor of fibrinolysis, thrombin-activatable fibrinolysis inhibitor (TAFI) in cirrhosis was examined. METHODS: In 64 patients with cirrhosis of varying severity, TAFI antigen levels were measured by enzyme-linked immunosorbent assay and compared with TAFI levels in control subjects. Furthermore, a plasma-based fibrinolysis assay was performed in the presence and absence of a specific inhibitor of activated TAFI. RESULTS: TAFI levels were decreased in cirrhosis. Mean TAFI levels were 66% in Child's A, 55% in Child's B, 47% in Child's C cirrhosis, and 26% in acute liver failure. Decreased TAFI antigen levels were highly correlated with antithrombin and alpha(2)-antiplasmin activity levels. Clot lysis times and clot lysis ratio (defined as ratio between clot lysis time in the absence and presence of a specific inhibitor of activated TAFI) of cirrhotics were not significantly different from healthy controls. CONCLUSIONS: Despite decreased levels of TAFI and other components of the fibrinolytic system, no evidence of increased plasma fibrinolytic potential in cirrhosis is observed using the plasma-based assay of this study. The reduction of antifibrinolytic factors in cirrhosis is compensated by the concomitant reduction in profibrinolytics.

Long-term outcome of individualized prophylactic treatment of children with severe haemophilia.

The development of arthropathy is a serious complication of severe haemophilia. With the use of prophylaxis, bleeds can be prevented and arthropathy delayed. We investigated whether an individually tailored prophylactic regimen can prevent arthropathy and whether it had a similar effect on orthopaedic outcome compared with that of a high-dose regimen. Efficacy was determined clinically and by radiographs of six major joints. Prophylaxis was started in 70 patients at a mean age of 4.1 years. Mean follow-up was 15.6 years (range 8-24.5 years). The mean factor VIII consumption was 2319 IU/kg/year. The mean number of joint bleeds was 3.5/year and the mean clinical score (maximum score 90) was 1.0, with a mean Pettersson joint score (maximum score 78) of 3.0 at a mean age of 13.5 years. In conclusion, long-term, early-onset, individualized prophylaxis in haemophilia is feasible and prevents arthropathy.

Elastase mediated fibrinolysis in acute promyelocytic leukemia.

The bleeding syndrome of acute promyelocytic leukemia (APL) is complex and consists of disseminated intravascular coagulation (DIC) and hyperfibrinolysis. Elastase, derived from malignant promyelocytes, is believed to mediate the fibrinogeno- and fibrinolysis by aspecific proteolysis. In this study we measured the role of elastase in fifteen patients with APL by using an assay for elastase degraded fibrin(ogen) and the results were compared with those obtained in patients with sepsis induced DIC. High levels of elastase were observed in sepsis and APL. The levels of fibrinogen and fibrin degradation products were significantly higher in APL patients compared to patients with sepsis induced DIC. Nevertheless, the level of elastase degraded fibrin(ogen) was higher in the sepsis group (635.3 ng/ml, compared to 144.3 ng/ml in APL; p <0.0001). So, the enormous increase in fibrin and fibrinogen degradation products in APL cannot be explained by elastase activity. This study suggests a minor role for elastase mediated proteolysis in the hemorrhagic diathesis in APL patients.

Molecular analysis of the genotype-phenotype relationship in factor X deficiency.

Factor X deficiency is a rare haemorrhagic condition, normally inherited as an autosomal recessive trait, in which a variable clinical presentation correlates poorly with laboratory phenotype. The factor X (F10) genes of 14 unrelated individuals with factor X deficiency (12 familial and two sporadic cases) were sequenced yielding a total of 13 novel mutations. Family studies were performed in order to distinguish the contributions of individual mutant F10 alleles to the clinical and laboratory phenotypes. Missense mutations were studied by means of molecular modelling, whereas single basepair substitutions in splice sites and the 5' flanking region were examined by in vitro splicing assay and luciferase reporter gene assay respectively. The deletion allele of a novel hexanucleotide insertion/deletion polymorphism in the F10 gene promoter region was shown by reporter gene assay, to reduce promoter activity by approximately 20%. One family manifesting an autosomal dominant pattern of inheritance possessed three clinically affected members who were heterozygous for a splice-site mutation that was predicted to lead to the production of a truncated protein product. A model which accounts for the dominant negative effect of this lesion is presented. Variation in the antigen level of heterozygous relatives of probands was found to be significantly higher between families than within families, consistent with the view that the nature of the F10 lesion(s) segregating in a given family is a prime determinant of the laboratory phenotype. By contrast, no such relationship could be discerned between laboratory phenotype and polymorphism genotype.

Reduced activity of TAFI (thrombin-activatable fibrinolysis inhibitor) in acute promyelocytic leukaemia.

Acute promyelocytic leukaemia (APL) is a disease that is distinguished from other leukaemias by the high potential for early haemorrhagic death. Several processes are involved, such as disseminated intravascular coagulation and hyperfibrinolysis. Recently, TAFI (thrombin-activatable fibrinolysis inhibitor) was identified as a link between coagulation and fibrinolysis. TAFI can be activated by thrombin, and in its activated form potently attenuates fibrinolysis by removing C-terminal lysine and arginine residues that are important for the binding and activation of plasminogen. Activation of TAFI by the coagulation system results in a down-regulation of fibrinolytic activity and, thereby, prevents a rapid dissolution of the fibrin clot. To establish whether TAFI was involved in the severity of the bleeding complications in APL, the TAFI antigen and activity levels were determined in a group of 15 patients. The TAFI antigen concentration was normal, but the activity of TAFI was severely reduced in APL by approximately 60%. The reduction of TAFI activity was most probably caused by the action of plasmin on TAFI because in vitro experiments revealed that plasmin slightly reduced antigen levels but severely reduced TAFI activity. The acquired functional TAFI deficiency in APL may contribute to the severity of the haemorrhagic diathesis because of the impaired capacity of the coagulation system to protect the fibrin clot from fibrinolysis.

Sequential regulation of the small GTPase Rap1 in human platelets.

Rap1, a small GTPase of the Ras family, is ubiquitously expressed and particularly abundant in platelets. Previously we have shown that Rap1 is rapidly activated after stimulation of human platelets with alpha-thrombin. For this activation, a phospholipase C-mediated increase in intracellular calcium is necessary and sufficient. Here we show that thrombin induces a second phase of Rap1 activation, which is mediated by protein kinase C (PKC). Indeed, the PKC activator phorbol 12-myristate 13-acetate induced Rap1 activation, whereas the PKC-inhibitor bisindolylmaleimide inhibited the second, but not the first, phase of Rap1 activation. Activation of the integrin alpha(IIb)beta(3), a downstream target of PKC, with monoclonal antibody LIBS-6 also induced Rap1 activation. However, studies with alpha(IIb)beta(3)-deficient platelets from patients with Glanzmann's thrombasthenia type 1 show that alpha(IIb)beta(3) is not essential for Rap1 activation. Interestingly, induction of platelet aggregation by thrombin resulted in the inhibition of Rap1 activation. This downregulation correlated with the translocation of Rap1 to the Triton X-100-insoluble, cytoskeletal fraction. We conclude that in platelets, alpha-thrombin induces Rap1 activation first by a calcium-mediated pathway independently of PKC and then by a second activation phase mediated by PKC and, in part, integrin alpha(IIb)beta(3). Inactivation of Rap1 is mediated by an aggregation-dependent process that correlates with the translocation of Rap1 to the cytoskeletal fraction.

Induction therapy with vincristine, adriamycin, dexamethasone (VAD) and intermediate-dose melphalan (IDM) followed by autologous or allogeneic stem cell transplantation in newly diagnosed multiple myeloma.

We performed a phase II study to test the efficacy and feasibility of induction therapy with vincristine, adriamycin and dexamethasone (VAD) and intermediate-dose melphalan, 70 mg/m2 (IDM), to autologous or allogeneic stem cell transplantation in newly diagnosed multiple myeloma (MM). A total of 77 patients received two cycles of VAD (n = 62) and/or two cycles of i.v. IDM 70 mg/m2 (n = 15) combined with G-CSF. PBSC were harvested after the first IDM, successfully in 87% of patients. Patients with a response to induction received myeloablative therapy with PBSCT (n = 50) followed by IFN maintenance or allo-BMT (n = 11). Seventy-two per cent of patients achieved a response after VAD which increased to 85% after IDM. Of patients who received PBSCT and allo-BMT, 24% and 45% achieved CR, respectively. Toxicity of induction consisted mainly of bone marrow suppression after IDM (median 8 days) with prolonged aplasia in 11% of patients after the second IDM. Only six infections WHO grade 3 occurred during induction. Treatment-related mortality of PBSCT and allo-BMT was 6% and 18%, respectively. Median time of follow-up is 44 months, and 50% of patients after PBSCT and 60% of patients after allo-BMT are still in remission. Survival rates of all patients were 82%, 75% and 63%, and for transplanted patients 86%, 79% and 68% after 12, 24 and 36 months. Well known prognostic factors, including alpha-IFN maintenance after PBSCT, were not significant for response or survival although patients in CR after allo-BMT had a strong tendency for better outcome. VAD/IDM is an effective and safe induction therapy for autologous and allogeneic stem cell transplantation. Based on these observations a phase III trial was started in October 1995 comparing IFN maintenance with PBSCT and allo-BMT after response to induction with VAD and IDM.

Low-density lipoprotein enhances platelet secretion via integrin-alphaIIbbeta3-mediated signaling.

LDL is known to increase the sensitivity of human platelets for agonists and to induce aggregation and secretion independently at high concentrations, but its mechanism of action is largely obscure. To clarify how LDL increases platelet sensitivity, cells were incubated in lipoprotein-poor plasma and treated with collagen at a concentration that induced approximately 20% secretion of 14C-serotonin. Preincubation with LDL (30 minutes at 37 degreesC) enhanced secretion in a dose-dependent manner to 60+/-14% at a concentration of 2 g LDL protein/L. Similar stimulation by LDL was seen when secretion was induced by the thrombin receptor-activating peptide. This enhancement was strongly reduced (1) in the presence of monoclonal antibody PAC1 against activated alphaIIbbeta3, a polyclonal antibody against alphaIIb, and in the presence of the fibrinogen peptides GRGDS and HHLGGAKQAGDV; (2) in alphaIIbbeta3-deficient platelets; and (3) after dissociation of alphaIIbbeta3. In contrast, binding of 125I-LDL to normal platelets in the presence of PAC1, anti-alphaIIb, GRGDS, and HHLGGAKQAGDV, and to alphaIIbbeta3-deficient platelets was normal. LDL increased the surface expression of fibrinogen in lipoprotein-poor plasma and fibrinogen-free medium, suggesting that extracellular and granular fibrinogen bind to alphaIIbbeta3 after platelet-LDL interaction. Platelets deficient in fibrinogen (<0.5% of normal) or von Willebrand Factor (<1% of normal) but containing normal amounts of other ligands for alphaIIbbeta3 preserved responsiveness to LDL, indicating that occupancy of alphaIIbbeta3 was not restricted to fibrinogen. Inhibition of protein kinase C (bisindolylmaleimide) diminished fibrinogen binding and sensitization by LDL; inhibition of tyrosine kinases (herbimycin A) left fibrinogen binding unchanged but diminished sensitization by LDL. We conclude that an increased concentration of LDL, such as observed in homozygous familial hypercholesterolemia, sensitizes platelets to stimulation by collagen and thrombin receptor-activating peptide via ligand-induced outside-in signaling through integrin-alphaIIbbeta3.