Thrombin is primarily known for its role in homeostasis and thrombosis. However, this enzyme also plays important roles in wound healing and pathologic situations such as inflammation and tumorigenesis. Among the molecules stimulated by thrombin in these latter processes are the stress response proteins, chemokines. Chemokines are also known for their roles in inflammatory responses and tumor development. These correlative observations strongly suggest that chemokines may be mediators of some of thrombin's functions in these processes. Elucidation of the molecular mechanisms of stimulation of chemokines by thrombin may help to unravel the ways in which their expression can be modulated. Up-regulation of the chemokine 9E3/cCAF by thrombin occurs via its proteolytically activated receptor with subsequent transactivation of the epidermal growth factor receptor tyrosine kinase. This study shows that stimulation by thrombin very rapidly activates this chemokine at the transcriptional level, that 2 Elk1 binding elements located between -534 and -483 bp of the promoter are major thrombin response elements, that activation occurs via the Elk1 transcription factor, and that the latter is directly activated by MEK1/ERK2. The common occurrence of Elk1 binding domains in the promoters of immediate early response genes suggests that it may be characteristically involved in gene activation by stress-inducing agents. (Blood. 2000;96:3696-3706)