Font J, Azula F J, Marino A, Nieva N, Trueba M, Macarulla J M
Department of Biochemistry and Molecular Biology, Faculty of Sciences, University of Basque Country, Bilbao, Spain.
Prostaglandins. 1992 Apr;43(4):383-95. doi: 10.1016/0090-6980(92)90038-u.
Phorbol esters, potent activators of protein kinase C (PKC), greatly enhance the release of arachidonic acid and its metabolites (TXA2, HETES, HHT) by Ca2+ ionophores in human platelets. In this paper, we report the relationship between intracellular Ca2+ mobilization and external calcium influx into platelets and the ability of PMA plus A23187 to promote thromboxane A2 (TXA2) synthesis. The enhanced levels of TXA2 due to the synergistic stimulation of the platelets with A23187 and phorbol esters are not affected significantly by the presence of external Ca2+ or the calcium-chelator EGTA. PKC inhibitors, staurosporine and sphingosine, abolished phorbol myristate acetate (PMA) potentiation of TXA2 production which strongly supports the role of PKC in the synergism. Platelet aggregation is more sensitive to PMA and external calcium than TXA2 formation. PMA increased TXA2 production as much as 4-fold at low ionophore concentrations. The A23187-induced rise in [Ca2+]i was reduced by pretreatment of human platelets with phorbol esters, both in the presence and absence of EGTA, and staurosporine reversed this inhibitory effect. These results indicate that the synergistic stimulation of TXA2 production by A23187 and phorbol esters is promoted by intracellular Ca2+ mobilization and not by external calcium influx. Our data also suggest that PKC is involved in the regulation of Ca2+ mobilization from some specific intracellular stores and that PKC may also stimulate the Ca(2+)-dependent phospholipase A2 at suboptimal Ca2+i concentrations.
佛波酯是蛋白激酶C（PKC）的强效激活剂，能极大地增强钙离子载体在人血小板中促使花生四烯酸及其代谢产物（血栓素A2、羟二十碳四烯酸、12-羟基-5,8,10,14-二十碳四烯酸）释放的能力。在本文中，我们报告了细胞内钙离子动员与血小板外部钙内流之间的关系，以及佛波醇酯（PMA）加A23187促进血栓素A2（TXA2）合成的能力。由A23187和佛波酯协同刺激血小板导致的TXA2水平升高，不受外部钙离子或钙螯合剂乙二醇双四乙酸（EGTA）存在的显著影响。PKC抑制剂星形孢菌素和鞘氨醇消除了十四酰佛波醇乙酯（PMA）对TXA2生成的增强作用，这有力地支持了PKC在协同作用中的作用。血小板聚集对PMA和外部钙比对TXA2形成更敏感。在低离子载体浓度下，PMA使TXA2生成增加多达4倍。无论有无EGTA，用佛波酯预处理人血小板都会降低A23187诱导的细胞内钙离子浓度升高，而星形孢菌素可逆转这种抑制作用。这些结果表明，A23187和佛波酯对TXA2生成的协同刺激是由细胞内钙离子动员而非外部钙内流促进的。我们的数据还表明，PKC参与了从某些特定细胞内储存中动员钙离子的调节，并且PKC也可能在低于最佳细胞内钙离子浓度时刺激钙离子依赖性磷脂酶A2。
