TISSUE FACTOR GENE-TRANSCRIPTION IN SERUM-STIMULATED FIBROBLASTS IS MEDIATED BY RECRUITMENT OF C-FOS INTO SPECIFIC AP-1 DNA-BINDING COMPLEXES

Serum stimulation of quiescent mouse fibroblasts results in transcriptional activation of tissue factor (TF), the cellular initiator of the protease cascade leading to blood coagulation. In this study, we demonstrate that two AP-1 DNA-binding elements located 200-220 bp upstream of the transcription start site are both necessary and sufficient to confer serum inducibility to the TF gene promoter in fibroblasts. Analysis of AP-1 DNA-binding complexes indicates that the predominant form of AP-1 activity in quiescent cells consists of an unidentified Fos-related protein and JunD. While c-Fos is notably absent from these preexisting complexes, serum stimulation results in the rapid entry of c-Fos into the TF AP-1 DNA-binding complexes. A similar induction of c-Fos DNA-binding activity occurs in cells treated with recombinant growth factors such as platelet-derived growth factor (PDGF) and fibroblast growth factor (FGF). Importantly, overexpression of JunD and c-Fos abrogates the requirement for serum in the stimulation of TF promoter activity in fibroblasts. Together, these data indicate that the entry of c-Fos into heterodimeric AP-1 DNA-binding complexes with JunD is a key event underlying serum-stimulated transcription of the TF gene in fibroblasts.