CHARACTERIZATION OF THROMBOMODULIN EXPRESSION IN RESPONSE TO RETINOICACID AND IDENTIFICATION OF A RETINOIC ACID RESPONSE ELEMENT IN THE HUMAN THROMBOMODULIN GENE

Thrombomodulin (TM) is an essential cofactor for the physiologic activ ation of the anticoagulant protein C by thrombin. We have observed tha t the expression of TM mRNA in response to retinoic acid was markedly increased in human U937 monoblast-like cells, and human MEG01 megakary ocyte-like cells, but not in human umbilical vein cells, murine hemang ioma cells, human K562 erythroblast-like cells, and murine HSD fibrobl ast-like cells. TM activity in U937 cells and MEG01 cells was not dete ctable in untreated cells, but developed rapidly after treatment with retinoic acid. In endothelial cells there was minimal change in TM act ivity in response to retinoic acid treatment. We have isolated clones for the genes for murine and human TM and have identified potential re tinoic acid response elements in the 5'-flanking region of the human g ene. In U937 cells the increase in mRNA levels was associated with inc reased transcription, and transient transfection studies with reporter plasmids demonstrate functional retinoic acid response elements prese nt in the 5'-flanking region of the gene. Deletion of, and mutations i ntroduced into, the potential retinoic acid response element confirm t he functional response in transient transfections.