[in Japanese]

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[in Japanese]

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[in Japanese]

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[in Japanese]

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[in Japanese]

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[in Japanese]

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[in Japanese]

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[in Japanese]

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[in Japanese]

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[in Japanese]

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Two platelet receptors, the glycoprotein (GP) Ib-IX-V complex and GPVI/FcRγ-chain, are pivotal in initiating signals that propagate both haemostasis and thrombosis. In the arterial circulation when an atherosclerotic plaque ruptures, these receptors initiate platelet adhesion in response to exposed thrombogenic materials by binding vessel wall von Willebrand Factor (VWF) and collagen, respectively. While these adhesive processes and subsequent events in thrombus formation have been the subject of intense investigation, the mechanisms that positively and negatively regulate the function of these and other receptors in activated platelets, and thus act to determine thrombus formation and stability, are only beginning to be understood. Platelet adhesion through the GPIb-IX-V complex is regulated at three distinct levels. First, the binding of VWF to the α-chain of GPIb is regulated by shear. Recent data from our laboratory exploiting analysis of canine-human chimaeras of GPIbα indicate that a negative-charge cluster involving leucine-rich repeats 2-4 plays an important role in the molecular regulation of shear-dependent VWF binding. Second, VWF affinity for the GPIb-IX-V complex is regulated by its association with the scaffolding protein, 14-3-3ζ, and phosphorylation of the α- and β-subunits of the GPIb-IX-V complex. Finally, calmodulin association with GPIbβ and GPV regulates the platelet activation dependent cleavage of GPIbα and GPV by the platelet membrane-associated metalloproteinase, ADAM17. Platelet adhesion through GPVI is primarily regulated by post-ligation-induced shedding of the GPVI ectodomain by ADAM10. Ligand-induced shedding is signaling-dependent and blocked by inhibitors of GPVI-dependent-signaling, Src, phosphoinositide 3-kinase (PI3-kinase) , or the ITAM-related Syk kinase. The calmodulin inhibitor W7 also induces shedding of GPVI, but independently of platelet activation. We have found that, like GPVI, another ITAM-containing receptor, the platelet Fc receptor, FcγRIIa, is cleaved in response to ligands of FcγRIIa (VM58, 14A2 or heparin-induced antibodies) or GPVI (convulxin) , or by W7. Conversely, ligands of FcγRIIa also induce GPVI shedding. FcγRIIa cleavage is blocked not only by Syk inhibition but also by the calpain inhibitor, E64d, and a cytoplasmic calpain-cleavage site was identified upstream of the ITAM domain. Thus, regulation of ligand binding or signaling by these unique platelet receptors through distinct proteolytic mechanisms may play a key role in limiting thrombus size and stability in response to vessel damage or immunological insult.

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Under physiologic conditions, a hemostatic balance is achieved through the effects of natural procoagulant and anticoagulant factors which, in equilibrium with each other, provide hemostasis at the sites of vascular injury. Abnormalities of these hemostasis factors result in a tendency towards hemorrhagic or thrombotic events. In this review the influence of inherited prothrombotic risk factors-especially the frequent factor V Leiden and prothrombin gene mutations-on normal and abnormal hemostasis is analyzed from an evolutionary point of view. The effect of inherited bleeding disorders on the development of thrombotic or atherosclerotic processes is also discussed.

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Based on quantitative observations of platelets and leukocytes adhering to or aggregating on vascular endothelial cells activated by active oxygen generated from photochemical reactions, we quantitatively assessed interactions between endothelial and blood cells in the antecedent process to thrombus formation, and also evaluated the inhibitory effects of ibudilast (a phosphodiesterase inhibitor) on the adhesion and aggregation. Ibudilast inhibited the photochemically-induced adhesion between endothelial cells and platelets at concentrations of 10 μM or higher in a concentration-dependent manner, whereas aspirin did not inhibit the adhesion. Ibudilast also inhibited the adhesion of neutrophils to endothelial cells at concentrations of 1 μM or higher. Furthermore, we evaluated the inhibitory effects of ibudilast on morphological changes of endothelial cells and on subendothelial exposure, and the compound was shown to have inhibitory effects. These results suggest that ibudilast has composite antithrombotic activities, i.e., activities on endothelial cells in addition to previously reported antiplatelet activities. Future research should include the identification of interacting adhesion molecules and the assessment of endothelial cell-derived adhesion accelerators to elucidate the relationship between the substances and ibudilast activities.

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In order to assess the current status of patients with acquired inhibitors against coagulation factors in Japan, a questionnaire survey has been conducted for three years. Of 56 cases from 42 facilities, 55 were acquired hemophiliacs with anti-factor VIII antibodies aged 12 to 85 years (median 70) . Autoimmune disease and malignant tumors accounted for about one third of associated disorders. Subcutaneous and muscle bleeds were the predominant manifestations of the disease, although a certain degree of serious bleeding was observed. Maximum level of the inhibitor antibodies ranged from 1.1 to 758 Bethesda Units/ml (median 29.5) , and more than half of the antibodies were detected together with factor VIII activity. While bypassing agents were mainly used for hemostatic treatment, prednisolone alone or in combination with other immunosuppressants was used in most of cases for immunosuppressive therapy. Of 40 evaluable cases, inhibitor antibodies disappeared in 21 cases, whereas 9 cases did not achieve remission, and 10 cases died. Statistical analysis revealed that three factors, such as infectious complication, response to the hemostatic therapy, and response to the immunosuppressive therapy had a significant prognostic value on survival analysis. These data suggest that infection control might be important as well as the hemostatic control for managing the disease.

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