Dekkers D W, Comfurius P, Vuist W M, Billheimer J T, Dicker I, Weiss H J, Zwaal R F, Bevers E M
Department of Biochemistry, Cardiovascular Research Institute Maastricht, Maastricht University, Maastricht, The Netherlands.
Blood. 1998 Mar 15;91(6):2133-8.
Scott syndrome is an hereditary bleeding disorder characterized by a deficiency in platelet procoagulant activity. Unlike normal blood cells, Scott platelets, as well as erythrocytes and lymphocytes, are strongly impaired in their ability to scramble their membrane phospholipids when challenged with Ca2+. In normal cells this collapse of membrane asymmetry leads to surface exposure of phosphatidylserine. Here we report that Scott erythrocytes show an apparent defect in tyrosine phosphorylation on treatment with Ca2+-ionophore. Diminished tyrosine phosphorylation was also apparent in activated Scott platelets, but much less pronounced than observed in red blood cells. On the other hand, tyrosine phosphorylation profiles observed in Scott red blood cell ghosts after sealing in the presence of adenosine triphosphate (ATP) were indistinguishable from those obtained from normal ghosts. Several observations argue in favor of a mechanism in which tyrosine phosphorylation in red blood cells is facilitated by, rather than required for scrambling of membrane lipids. Staurosporin blocks tyrosine phosphorylation in normal red blood cells, but does not inhibit the lipid scrambling process. White ghosts from normal erythrocytes, resealed in the absence of ATP, exhibit Ca2+-induced lipid scrambling without tyrosine phosphorylation. A selective inhibitor of Ca2+-induced lipid scrambling also showed an apparent inhibition of tyrosine phosphorylation in ionophore-treated normal red blood cells, similar to that observed in Scott erythrocytes. While this inhibitor also suppressed Ca2+-induced lipid scrambling in ghosts that were sealed in the presence of ATP, it did not inhibit tyrosine kinase activity. We conclude that the apparent deficiency in tyrosine phosphorylation in Scott cells is an epiphenomenon, possibly associated with a defect in phospholipid scrambling, but not causal to this defect.
斯科特综合征是一种遗传性出血性疾病，其特征是血小板促凝活性缺乏。与正常血细胞不同，斯科特血小板以及红细胞和淋巴细胞在受到Ca2+刺激时，其膜磷脂翻转能力受到严重损害。在正常细胞中，这种膜不对称性的破坏会导致磷脂酰丝氨酸暴露于表面。在此我们报告，斯科特红细胞在用Ca2+离子载体处理后酪氨酸磷酸化出现明显缺陷。活化的斯科特血小板中酪氨酸磷酸化也明显减少，但比在红细胞中观察到的要轻得多。另一方面，在存在三磷酸腺苷（ATP）的情况下密封后，斯科特红细胞血影中观察到的酪氨酸磷酸化谱与正常血影中获得的谱没有区别。一些观察结果支持这样一种机制，即红细胞中的酪氨酸磷酸化是由膜脂翻转促进的，而不是膜脂翻转所必需的。星形孢菌素可阻断正常红细胞中的酪氨酸磷酸化，但不抑制脂质翻转过程。在没有ATP的情况下重新密封的正常红细胞白色血影，表现出Ca2+诱导的脂质翻转而没有酪氨酸磷酸化。一种Ca2+诱导的脂质翻转选择性抑制剂在离子载体处理的正常红细胞中也显示出对酪氨酸磷酸化的明显抑制，类似于在斯科特红细胞中观察到的情况。虽然这种抑制剂也抑制了在存在ATP的情况下密封的血影中Ca2+诱导的脂质翻转，但它不抑制酪氨酸激酶活性。我们得出结论，斯科特细胞中酪氨酸磷酸化的明显缺乏是一种副现象，可能与磷脂翻转缺陷有关，但不是该缺陷的原因。
