Sambrano G R, Huang W, Faruqi T, Mahrus S, Craik C, Coughlin S R
Cardiovascular Research Institute, University of California, San Francisco, San Francisco, California 94143-0130, USA.
J Biol Chem. 2000 Mar 10;275(10):6819-23. doi: 10.1074/jbc.275.10.6819.
Of the four known protease-activated receptors (PARs), PAR1 and PAR4 are expressed by human platelets and mediate thrombin signaling. Whether these receptors are redundant, interact, or play at least partially distinct roles is unknown. It is possible that PAR1 and/or PAR4 might confer responsiveness to proteases other than thrombin. The neutrophil granule protease, cathepsin G, is known to cause platelet secretion and aggregation. We now report that this action of cathepsin G is mediated by PAR4. Cathepsin G triggered calcium mobilization in PAR4-transfected fibroblasts, PAR4-expressing Xenopus oocytes, and washed human platelets. An antibody raised against the PAR4 thrombin cleavage site blocked platelet activation by cathepsin G but not other agonists. Desensitization with a PAR4 activating peptide had a similar effect. By contrast, inhibition of PAR1 function had no effect on platelet responses to cathepsin G. When neutrophils were present, the neutrophil agonist fMet-Leu-Phe triggered calcium signaling in Fura-2-loaded platelets. Strikingly, this neutrophil-dependent platelet activation was blocked by the PAR4 antibody. These data show that PAR4 mediates platelet responses to cathepsin G and support the hypothesis that cathepsin G might mediate neutrophil-platelet interactions at sites of vascular injury or inflammation.
在四种已知的蛋白酶激活受体（PARs）中，PAR1和PAR4在人血小板中表达并介导凝血酶信号传导。这些受体是冗余的、相互作用的，还是至少发挥部分不同的作用尚不清楚。PAR1和/或PAR4有可能赋予对凝血酶以外的蛋白酶的反应性。已知中性粒细胞颗粒蛋白酶组织蛋白酶G可引起血小板分泌和聚集。我们现在报告，组织蛋白酶G的这一作用是由PAR4介导的。组织蛋白酶G在转染了PAR4的成纤维细胞、表达PAR4的非洲爪蟾卵母细胞和洗涤过的人血小板中触发了钙动员。一种针对PAR4凝血酶切割位点产生的抗体可阻断组织蛋白酶G对血小板的激活，但对其他激动剂无此作用。用PAR4激活肽进行脱敏处理也有类似效果。相比之下，抑制PAR1功能对血小板对组织蛋白酶G的反应没有影响。当中性粒细胞存在时，中性粒细胞激动剂甲硫氨酰-亮氨酰-苯丙氨酸（fMet-Leu-Phe）在负载Fura-2的血小板中触发钙信号传导。引人注目的是，这种依赖中性粒细胞的血小板激活被PAR4抗体阻断。这些数据表明PAR4介导血小板对组织蛋白酶G的反应，并支持组织蛋白酶G可能在血管损伤或炎症部位介导中性粒细胞与血小板相互作用的假说。
