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Objective-Cyclooxygenase-1 (COX-1), but not COX-2, is expressed in human platelets, and thromboxane A 2 (TXA 2 ) produced via COX-1 induces platelet aggregation. The objectives of this study were to investigate the expression of COX-1 and COX-2 during platelet differentiation and to determine whether these enzymes are involved in the differentiation. Methods and Results-CD34ϩ progenitor cells isolated from human cord blood were cultured with thrombopoietin and c-kit ligand. The cells differentiated into megakaryocytes (CD34 Ϫ /CD41 ϩ ) after 8 days of culture and into platelets (CD41 ϩ /prodium iodide Ϫ ) after 14 days of culture. The CD34 ϩ cells expressed a trace of COX-1 gene and no COX-2 gene. On day 5, COX-2 gene expression was observed and continued throughout the remainder of the culture. COX-1 gene expression increased after 8 days of culture. The treatment of this liquid culture with indomethacin, a dual inhibitor of COX-1 and COX-2, and NS-398, a COX-2-specific inhibitor, suppressed megakaryocyte differentiation. In contrast, at a dose of 10 Ϫ7 M, mofezolac, which is a highly selective inhibitor of COX-1, did not affect differentiation. NS-398 -induced suppression of megakaryocyte differentiation was partly abrogated by stable analogues of TXA 2 . Conclusions-We report here that COX-2 and COX-1 are constitutively expressed in megakaryocytes, and TXA 2 produced by COX-2 plays an important role in megakaryocytopoiesis.

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