Constitutive expression and involvement of cyclooxygenase-2 in human megakaryocytopoiesis

Cyclooxygenase-1 (COX-1), but not COX-2, is expressed in human platelets, and thromboxane A2 (TXA2) 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.

CD34+ progenitor cells isolated from human cord blood were cultured with thrombopoietin and c-kit ligand. The cells differentiated into megakaryocytes (CD34-/CD41+) after 8 days of culture and into platelets (CD41+/prodium iodide-) after 14 days of culture. The CD34+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(-7) 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 TXA2.

We report here that COX-2 and COX-1 are constitutively expressed in megakaryocytes, and TXA2 produced by COX-2 plays an important role in megakaryocytopoiesis.