Selak M A
Department of Biochemistry and Molecular Biology, University of New Hampshire, Durham 03824.
Biochem J. 1994 Jan 15;297 ( Pt 2)(Pt 2):269-75. doi: 10.1042/bj2970269.
Neutrophil cathepsin G and thrombin, the only platelet agonists that are proteases, exhibit a mandatory requirement for catalytic activity to induce platelet aggregation and signal transduction. The thrombin receptor is a G-protein-coupled receptor which undergoes proteolysis to generate a tethered ligand that causes self-activation. Since cathepsin G strongly resembles thrombin in its ability to activate platelets, we have attempted to determine whether cathepsin G and thrombin function through the same or different receptors. Evidence that thrombin and cathepsin G act at different receptors was as follows: (a) an antibody directed against the thrombin receptor blocked thrombin-induced but not cathepsin G-induced platelet responses; (b) human fibroblasts responded to thrombin and to a synthetic thrombin receptor peptide (comprising residues 42-55 of the thrombin receptor) by exhibiting an elevation in cytosolic Ca2+ concentration but did not respond to cathepsin G; and (c) platelets pretreated with neutrophil elastase failed to respond to thrombin but responded when rechallenged by cathepsin G. Thrombin and cathepsin G exhibit heterologous desensitization that is potentiated by okadaic acid and is attenuated by staurosporine, indicating that phosphorylation of serine/threonine residues is important for desensitization and that protein kinase C may be involved. Since catalytic activity of cathepsin G is required for platelet stimulation, it is probable that platelet activation by cathepsin G requires receptor proteolysis and that a tethered ligand mechanism is involved, suggesting that platelets may possess a family of protease receptors.
中性粒细胞组织蛋白酶G和凝血酶是仅有的两种作为蛋白酶的血小板激动剂，它们对诱导血小板聚集和信号转导的催化活性有强制性需求。凝血酶受体是一种G蛋白偶联受体，它会经历蛋白水解以产生一种引发自身激活的拴系配体。由于组织蛋白酶G在激活血小板的能力上与凝血酶极为相似，我们试图确定组织蛋白酶G和凝血酶是通过相同还是不同的受体发挥作用。凝血酶和组织蛋白酶G作用于不同受体的证据如下：(a) 一种针对凝血酶受体的抗体可阻断凝血酶诱导的血小板反应，但不能阻断组织蛋白酶G诱导的血小板反应；(b) 人成纤维细胞对凝血酶和一种合成的凝血酶受体肽（包含凝血酶受体的42 - 55位残基）有反应，表现为胞质Ca2+浓度升高，但对组织蛋白酶G无反应；(c) 用中性粒细胞弹性蛋白酶预处理的血小板对凝血酶无反应，但在用组织蛋白酶G再次刺激时会有反应。凝血酶和组织蛋白酶G表现出异源脱敏，冈田酸可增强这种脱敏，而星形孢菌素可减弱这种脱敏，这表明丝氨酸/苏氨酸残基的磷酸化对脱敏很重要，并且蛋白激酶C可能参与其中。由于组织蛋白酶G的催化活性是血小板刺激所必需的，因此组织蛋白酶G激活血小板很可能需要受体蛋白水解，并且涉及拴系配体机制，这表明血小板可能拥有一族蛋白酶受体。
