Disulfide isomerization switches tissue factor from coagulation to cell signaling

Cell-surface tissue factor (TF) binds the serine protease factor Vlla to activate coagulation or, alternatively, to trigger signaling through the G protein-coupled, protease-activated receptor 2 (PAR2) relevant to inflammation and angiogenesis. Here we demonstrate that TF*Vlla-mediated coagulation and cell signaling involve distinct cellular pools of TF. The surface-accessible, extracellular [Cys.sup.186]-[Cys.sup.209] disulfide bond of TF is critical for coagulation, and protein disulfide isomerase (PDI) disables coagulation by targeting this disulfide. A TF mutant (TF C209A) with an unpaired [Cys.sup.186] retains TF*Vlla signaling activity, and it has reduced affinity for Vlla, a characteristic of signaling TF on cells with constitutive TF expression. We further show that PDI suppresses TF coagulant activity in a nitric oxide-dependent pathway, linking the regulation of TF thrombogenicity to oxidative stress in the vasculature. Furthermore, a unique monoclonal antibody recognizes only the noncoagulant, cryptic conformation of TF. This antibody inhibits formation of the TF-PAR2 complex and TF.Vlla signaling, but it does not prevent coagulation activation. These experiments delineate an upstream regulatory mechanism that controls TF function, and they provide initial evidence that TF*Vlla signaling can be specifically inhibited with minimal effects on coagulation.