INDUCTION OF CYCLOOXYGENASE-1 IN A HUMAN MEGAKARYOBLASTIC CELL-LINE (CMK) DIFFERENTIATED BY PHORBOL ESTER

Human megakaryoblastic cells (CMK line) are known to differentiate to mature megakaryocyte-like cells by treatment with 12-O-tetradecanoylph orbol 13-acetate (TPA). There are two isozymes of prostaglandin-formin g cyclooxygenase enzyme. Constitutive cyclooxygenase-1 and inducible c yclooxygenase-2 were followed during differentiation of CMK cells. Tre atment of the cells with 0.1 mu M TPA for 4 days resulted in a 5-20-fo ld increase in cyclooxygenase activity. Northern and Western blot anal yses revealed that cyclooxygenase-1 mRNA and protein increased in para llel with the enzyme activity. In contrast, cyclooxygenase-2 mRNA was detected only at 3 h. Furthermore, most of the increased cyclooxygenas e activity was immunoprecipitated with anti-cyclooxygenase-1 antibody, and was not affected by a cyclooxygenase-2-specific inhibitor, NS-398 . These results indicated that cyclooxygenase-1 rather than cyclooxyge nase-2 was predominantly induced depending on TPA. The enzyme thus ind uced was localized by immunoelectron microscopy in nuclear envelope an d endoplasmic reticulum of the CMK cells.