Background-Tissue factor (TF) is the main initiator of coagulation in vivo. Recently, however, a role for TF as a cell receptor involved in signal transduction has been suggested. The aim of the present study was to assess whether activated factor VII (FVIIa) binding to TF could induce smooth muscle cell (SMC) proliferation and to clarify the possible intracellular mechanism(s) responsible for this proliferation. Methods and Results-Cell proliferation was induced by FVIIa in a dose-dependent manner, as assessed by [3 H]thymidine incorporation and direct cell counting, whereas no response was observed with active site-inhibited FVIIa (FVIIai), which is identical to FVIIa but is devoid of enzymatic activity. Similarly, no proliferation was observed when binding of FVIIa to TF was prevented by the monoclonal anti-TF antibody AP-1. Activation of the p44/42 mitogen-activated protein (MAP) kinase (extracellular signal-regulated kinases 1 and 2 [ERK 1/2]) pathway on binding of FVIIa to TF was demonstrated by transient ERK phosphorylation in Western blots and by suppression of proliferation with the specific MEK (MAP kinase/ERK kinase) inhibitor UO126. ERK phosphorylation was not observed with FVIIai or when cells were pretreated with AP-1. Conclusions-These data indicate a specific effect by which binding of FVIIa to TF on the surface of SMCs induces proliferation via a coagulation-independent mechanism and possibly indicate a new link between coagulation, inflammation, and atherosclerosis. Key Words: coagulants Ⅲ signal transduction Ⅲ cell division T issue factor (TF) is a cell surface-anchored glycoprotein that binds both the zymogen factor VII (FVII) and the active serine protease factor VIIa (FVIIa). 1 TF/FVIIa complex activates coagulation by cleaving its natural substrates, factors IX and X, ultimately leading to thrombin generation, fibrin deposition, and platelet activation. 1,2 On these grounds, TF has been considered to function as a major initiator of blood coagulation in vivo. Recently, however, some evidence has suggested that TF may mediate important coagulationindependent biological phenomena, including embryonic and oncogenic angiogenesis, 3,4 and inflammation. 5 In addition, recent data indicate that TF/FVIIa interaction may activate different intracellular signals, culminating in a variety of cell responses, thus suggesting a role for TF as a "true" cell membrane receptor. For example, TF/FVIIa interaction has been shown to be a strong chemotactic stimulus for smooth muscle cells (SMCs), mediating their migration in vitro. 6 FVIIa has been also shown to induce phosphorylation of the extracellular signalregulated kinases 1 and 2 (ERK 1/2) 7 and to upregulate poly(A) polymerase, 8 urokinase-type plasminogen activator receptor, 9 and a host of genes identified with the use of cDNA microarrays. 10 However, despite evidence suggesting close links between TF, activation of coagulation, and intracellular signaling, no direct effects of TF/FVIIa on SMC proliferation have yet been demonstrated. Thus, in the pr...