HEMOSTASIS, THROMBOSIS, AND VASCULAR BIOLOGY The proximal serum response element in the Egr-1 promoter mediates response to thrombin in primary human endothelial cells

Thrombin signaling in endothelial cells provides an important link between coagulation and inflammation. We report here that thrombin induces endogenous Egr-1 mRNA and Egr-1 promoter activity in primary human endothelial cells by approximately 6-fold and 3-fold, respectively. In transient transfection assays, deletion of the 3 cluster of serum response elements (SREs), but not the 5 cluster of SREs, resulted in a loss of thrombin response. When coupled to a heterologous core promoter, a region spanning the 3 SRE cluster contained information for thrombin response, whereas a region spanning the 5 SRE cluster had no such effect. A point mutation of the most proximal SRE (SRE-1), but not of the proximal Ets motif or upstream SREs, abrogated the response to thrombin. In electrophoretic mobility shift assays, nuclear extracts from thrombintreated cells displayed increased binding of total and phosphorylated serum response factor (SRF) to SRE-1. Thrombinmediated induction of Egr-1 was blocked by inhibitors of MEK1/2, but not by inhibitors of protein kinase C, phosphatidylinositol 3-kinase, or p38 mitogen-activated protein kinase (MAPK). Taken together, these data suggest that thrombin induces Egr-1 expression in endothelial cells by a MAPK-dependent mechanism that involves an interaction between SRF and SRE-1. (Blood. 2002;100:4454-4461)