Coagulation of the blood is regulated by a member of the cytokine receptor family known as tissue factor (TF). Tissue factor associates with the protease factor VIIa and leads to activation of a proteolytic cascade that results in cleavage of fibrinogen and blood coagulation. But the TF-VIIa complex also initiates signaling through the protease-activated receptor 2 (PAR2), which influences a range of biological processes, including inflammation, angiogenesis, and vascular function. Ahamed et al. now report that a disulfide bond between two cysteine residues on the extracellular portion of TF acts as a switch to promote one or the other of these two actions of TF--coagulation or cell signaling through PAR2. The authors used a set of antibodies that recognized distinct epitopes on TF and dose-response experiments to show that two distinct pools of TF regulated coagulation (measured as proteolytic generation of factor Xa) or PAR2 signaling [measured by monitoring phosphorylation of ERKs (extracellular signal–regulated kinases)]. Studies with mutant TF supported the authors’ suspicion that a conformational change in TF might result from formation or breakage of the Cys186-Cys209 disulfide bond. Furthermore, TF was found to associate with the enzyme protein disulfide isomerase (PDI). Reduction of the disulfide bond by PDI appeared to suppress coagulation, but TF retained its signaling activity through PAR2. Conversely, an antibody that inhibited interaction of TF with PAR2 inhibited signaling but allowed TF to function in control of coagulation. Pharmacological inhibition of PDI inhibited signaling through PAR2 initiated by the TF-VIIa complex, but not activation of the coagulation cascade. The findings may have therapeutic implications in that they demonstrate that it is possible to inhibit signaling initiated by TF-VIIa without compromising regulation of coagulation.
J. Ahamed, H. H. Versteeg, M. Kerver, V. M. Chen, B. M. Mueller, P. J. Hogg, W. Ruf, Disulfide isomerization switches tissue factor from coagulation to cell signaling. Proc. Natl. Acad. Sci. U.S.A. 103, 13932-13937 (2006). [Abstract] [Full Text]