1. Date of download: 6/3/2016 Copyright © American College of Chest Physicians. All rights reserved. Tissue Factor, Thrombin, and Cancer * Chest. 2003;124(3_suppl):58S-68S. doi:10.1378/chest.124.3_suppl.58S TF, thrombin, and tumor angiogenesis. Top: Clotting-independent mechanisms. TF can induce angiogenesis via mechanisms that are independent of thrombin generation and fibrin deposition. These mechanisms are primarily mediated by the constitutive and aberrant expression of TF observed in many tumor cells and associated vascular endothelial cells (VECs). The serine residues on the cytoplasmic tail of the TF receptor can be phosphorylated by PKC, independent of ligand binding. Phosphorylation of the receptor initiates downstream signaling cascades that result in the transcriptional activation or inactivation of different genes. VEGF, a proangiogenic factor, is up-regulated by TF activation, while thrombospondin (TSP)-1, an antiangiogenic factor, is down- regulated by TF activation. VEGF, in turn, up-regulates TF to continue the vicious cycle of tumor growth and clot formation. VEGF also increases vascular permeability that leads to plasma protein leakage and the deposition of a fibrin-rich proangiogenic matrix around tumor cells and vascular endothelial cells. Increased VEGF and decreased thrombospondin-1 induce the proliferation of endothelial cells that contributes to increased tumor angiogenesis. The binding of factor VII (FVII) to its cognate ligand TF results in the activation of factor VII (FVIIa) and an increase in intracellular calcium (Ca 2+ ). Intracellular Ca 2+ activates PKC that, in turn, phosphorylates the cytoplasmic tail of the TF receptor. Ligand binding also promotes the attachment of the ABP-280 to the cytoplasmic tail, resulting in the assembly of actin filaments. This association regulates MAPK signaling and phosphorylation of focal adhesion kinases (FAK) and downstream signaling cascades that promote increased endothelial cell adhesion and migration essential for tumor angiogenesis. Reproduced with permission. 9 Center: Thrombin and angiogenesis: clotting-independent mechanism. TF can induce angiogenesis via thrombin generation, independent of fibrin deposition and clot formation. Activation of the TF receptor occurs via binding of its cognate ligand, factor VII. Factor VII is activated (FVIIa) and the TF/activated factor VII complex in turn activates factor X (FXa). If the tissue factor pathway inhibitor (TFPI) does not bind and inactivate the TF/activated factor VII/activated factor X ternary complex, activated factor X dissociates from the complex and associates with another phospholipid membrane in the presence of Ca 2+ and activated factor V to form the prothrombinase complex that proteolytically converts prothrombin to thrombin. Thrombin can induce angiogenesis independent of clot formation by cleaving the cell membrane-bound PARs. Thrombin-generated cleavage of part of the N-terminal domain of the PARs exposes a neo-N-terminus that functions as a tethered ligand. This tethered ligand binds intramolecularly to the second transmembrane domain of this seven- transmembrane G-protein–coupled receptor. Thrombin cleaves PAR-1, PAR-3, and PAR-4, but not PAR-2. Other proteases, such as trypsin, tryptase, the TF/activated factor VII complex, or activated factor X, can activate PAR-2. Activation of the PARs causes a conformational change that results in the exchange of bound GDP for GTP on associated G proteins. The G proteins are comprised of an α subunit that contains the nucleotide binding site and a β and γ heterodimer. Tissue-specific expression of various G-protein subunits confers differential responses to thrombin. The specific signal transduction cascade induced by PAR activation depends on the type of G-protein subunit that is attached to the PAR. Signal transduction cascades, such as the MAPKs, can lead to the transcriptional activation of a number of genes that are involved in angiogenesis. Thrombin activation of PARs leads to the up- regulation of many angiogenesis-related genes, including VEGF, VEGF receptors, TF, bFGF, and MMP-2. These genes can lead to a number of pleiotropic responses, such as change in endothelial cell shape, increased vascular permeability, increased endothelial cell proliferation, and increased proteolysis, which all contribute to increased tumor angiogenesis. Reproduced with permission. 9 Bottom: Fibrin and angiogenesis: clotting-dependent mechanisms. TF can also induce angiogenesis by clotting- dependent mechanisms via thrombin generation and fibrin deposition. Generation of thrombin from the prothrombinase complex results in an active serine protease that cleaves fibrinopeptide A (FPA) and fibrinopeptide B (FPB) from the fibrinogen molecule, resulting in the eventual conversion of soluble fibrinogen to XLF. Elevated levels of plasma FPA have been correlated with tumor growth and progression. Thrombin also activates platelets. Deposition of activated platelets with XLF forms the clot. Clot formation and dissolution contributes to tumor growth and angiogenesis. Activated platelets release a number of proangiogenic factors from their α granules, including VEGF, bFGF, and platelet-derived growth factor (PDGF), that contribute to increased tumor and endothelial cell proliferation and migration. VEGF induces plasma protein leakage that results in an extravascular XLF matrix around tumor cells. This matrix serves as a supportive scaffold that facilitates endothelial cell migration and tubule formation. The fibrinolytic degradation of the fibrin matrix also contributes to angiogenesis since degradation results in the exposure of proangiogenic cryptic sites that facilitate cell adhesion and migration, and the release of small proangiogenic fragments and sequestered growth factors. Reproduced with permission from Fernandez et al. 9 Figure Legend: