title: Mechanistic coupling of protease signaling and initiation of coagulation by tissue factor.
abstract: The crucial role of cell signaling in hemostasis is clearly established by the action of the downstream coagulation protease thrombin that cleaves platelet-expressed G-protein-coupled protease activated receptors (PARs). Certain PARs are cleaved by the upstream coagulation proteases factor Xa (Xa) and the tissue factor (TF)--factor VIIa (VIIa) complex, but these enzymes are required at high nonphysiological concentrations and show limited recognition specificity for the scissile bond of target PARs. However, defining a physiological mechanism of PAR activation by upstream proteases is highly relevant because of the potent anti-inflammatory in vivo effects of inhibitors of the TF initiation complex. Activation of substrate factor X (X) by the TF--VIIa complex is here shown to produce enhanced cell signaling in comparison to the TF--VIIa complex alone, free Xa, or Xa that is generated in situ by the intrinsic activation complex. Macromolecular assembly of X into a ternary complex of TF--VIIa--X is required for proteolytic conversion to Xa, and product Xa remains transiently associated in a TF--VIIa--Xa complex. By trapping this complex with a unique inhibitor that preserves Xa activity, we directly show that Xa in this ternary complex efficiently activates PAR-1 and -2. These experiments support the concept that proinflammatory upstream coagulation protease signaling is mechanistically coupled and thus an integrated part of the TF--VIIa-initiated coagulation pathway, rather than a late event during excessive activation of coagulation and systemic generation of proteolytic activity.
authors: Department of Immunology, The Scripps Research Institute, La Jolla, CA 92037, USA.
authors: M
authors: M
authors: Riewald
authors: W
authors: W
authors: Ruf
mesh_terms: D000818:Animals; D001777:Blood Coagulation; D016466:CHO Cells; D006224:Cricetinae; D011485:Protein Binding; D011487:Protein Conformation; D015398:Signal Transduction; D013917:Thrombin / Q000737:chemistry / Q000378:metabolism*; D013925:Thromboplastin / Q000737:chemistry / Q000378:metabolism*
chemical_list: D013925:Thromboplastin; D013917:Thrombin