Differential expression of thromboxane A synthase and prostaglandin H synthase in megakaryocytic cell line.

We determined the expression of isoforms of prostaglandin H synthase (PGHS) and thromboxane A synthase (TXAS) in a human megakaryocyte cell line (MEG-01. The basal levels of full-length TXAS mRNA and the 60 kDa TXAS protein were high when compared to those of PGHS-1 and PGHS-2 in uninduced cells. Despite a high TXAS level, uninduced MEG-01 cells synthesized only a small amount of thromboxane A2 (TXA2) due to limited PGHS-1 or PGHS-2 expressions. Following PMA induction there was little change in TXAS. PGHS-2 mRNA was significantly increased at only 3 h of PMA treatment and the level declined rapidly, whereas PGHS-1 mRNA and protein levels were concordantly stimulated. Induction of PGHS-1 reached plateau on day 3 of PMA treatment. Analysis of arachidonate metabolism in cells induced by PMA for 3 and 5 days showed a high level of PGH2 synthesis which exceeded the TXAS capacity for TXA2 synthesis. Only traces of PGHS-2 mRNA and alternate-spliced TXAS mRNA were detected in human platelets. We conclude that TXAS and PGHS are differentially expressed in MEG-01 during PMA-induced differentiation.