Regulation of the human thromboxane A2 receptor gene in human megakaryoblastic MEG-01 cells.

Thromboxane A(2) (TXA(2)) is an important mediator for platelet aggregation and blood vessel constriction. TXA(2) receptor (TP receptor) is expressed in different cell types including smooth muscle cells, endothelial cells and platelets. Expression level of TP receptor may modulate the action of TXA(2) on target cells. In megakaryoblastic MEG-01 cells, a cell line representing a model for platelet precursor cells, addition of phorbolester 12-O-tetradecanoylphorbol-13-acetate (TPA) caused an increase in transcriptional activity of TP receptor gene promoter. Within 20 h a rise in expression of TP receptor mRNA and protein was observed. The effect of TPA was concentration-dependent and was blocked by specific inhibitors of protein kinase C. Flow cytometry analysis indicated that the increase in TP receptor expression appeared to be one of the earliest events in the course of TPA-induced maturation of MEG-01 cells. Stimulation of the protein kinase A pathway by incubation with forskolin or IBMX caused a decrease in transcriptional activity. Promoter deletion experiments indicated that the responsive elements for protein kinase A and C are located upstream and downstream, respectively, of -700 bp of the TP receptor gene. These experiments indicate that the expression of the human thromboxane receptor is differently regulated in platelet precursor cells by the protein kinase A and C pathway.