Thrombin cleavage of extracellular matrix proteins

Novel Direct Anticoagulants and Atherosclerosis

Coagulation factors can affect cellular processes that include inflammatory signaling by acting on endothelial protease activated receptors, vascular smooth muscle and inflammatory cells beyond the coagulation cascade. This is important in the pathogenesis of atherosclerosis. Accordingly, experimental data points to beneficial effects of coagulation protease inhibitors on the attenuation of atherosclerosis progression in animal models. However, available clinical data do not support the use of anticoagulants as an add-on treatment of atherosclerosis. New clinical studies are needed with a better selection of patients to clarify the role of novel direct anticoagulants in the management of atherosclerosis.

The collαgen III fibril has a "flexi-rod" structure of flexible sequences interspersed with rigid bioactive domains including two with hemostatic roles

Collagen III is critical to the integrity of blood vessels and distensible organs, and in hemostasis. Examination of the human collagen III interactome reveals a nearly identical structural arrangement and charge distribution pattern as for collagen I, with cell interaction domains, fibrillogenesis and enzyme cleavage domains, several major ligand-binding regions, and intermolecular crosslink sites at the same sites. These similarities allow heterotypic fibril formation with, and substitution by, collagen I in embryonic development and wound healing. The collagen III fibril assumes a "flexi-rod" structure with flexible zones interspersed with rod-like domains, which is consistent with the molecule's prominence in young, pliable tissues and distensible organs. Collagen III has two major hemostasis domains, with binding motifs for von Willebrand factor, α2β1 integrin, platelet binding octapeptide and glycoprotein VI, consistent with the bleeding tendency observed with COL3A1 disease-causing sequence variants.

Dabigatran abrogates brain endothelial cell permeability in response to thrombin

Atrial fibrillation (AF) increases the risk and severity of thromboembolic stroke. Generally, antithrombotic agents increase the hemorrhagic risk of thromboembolic stroke. However, significant reductions in thromboembolism and intracerebral hemorrhage have been shown with the antithrombin dabigatran compared with warfarin. As thrombin has been implicated in microvessel injury during cerebral ischemia, we hypothesized that dabigatran decreases the risk of intracerebral hemorrhage by direct inhibition of the thrombin-mediated increase in cerebral endothelial cell permeability. Primary murine brain endothelial cells (mBECs) were exposed to murine thrombin before measuring permeability to 4-kDa fluorescein isothiocyanate-dextran. Thrombin increased mBEC permeability in a concentration-dependent manner, without significant endothelial cell death. Pretreatment of mBECs with dabigatran completely abrogated the effect of thrombin on permeability. Neither the expressions of the endothelial cell β1-integrins nor the tight junction protein claudin-5 were affected by thrombin exposure. Oxygen-glucose deprivation (OGD) also increased permeability; this effect was abrogated by treatment with dabigatran, as was the additive effect of thrombin and OGD on permeability. Taken together, these results indicate that dabigatran could contribute to a lower risk of intracerebral hemorrhage during embolism-associated ischemia from AF by protection of the microvessel permeability barrier from local thrombin challenge.

Hemostasis and alterations of the central nervous system

Modulation of coagulation has been successfully applied to ischemic disorders of the central nervous system (CNS). Some components of the coagulation system have been identified in the CNS, yet with limited exception their functions have not been clearly defined. Little is known about how events within the cerebral tissues affect hemostasis. Nonetheless, the interaction between cerebral cells and vascular hemostasis and the possibility that endogenous coagulation factors can participate in functions within the neurovascular unit provide intriguing possibilities for deeper insight into CNS functions and the potential for treatment of CNS injuries. Here, we consider the expression of coagulation factors in the CNS, the coagulopathy associated with focal cerebral ischemia (and its relationship to hemorrhagic transformation), the use of recombinant tissue plasminogen activator (rt-PA) in ischemic stroke and its study in animal models, the impact of rt-PA on neuron and CNS structure and function, and matrix protease generation and matrix degradation and hemostasis. Interwoven among these topics is evidence for interactions of coagulation factors with and within the CNS. How activation of hemostasis occurs in the cerebral tissues and how the brain responds are difficult questions that offer many research possibilities.

Thrombin stimulates proinflammatory and proliferative responses in primary cultures of human proximal tubule cells

BACKGROUND

Fibrin deposition is frequently observed within the tubulointerstitium in various forms of chronic renal disease. This suggests the presence of active components of the coagulation pathway, which may contribute to the progressive deterioration in renal function. The aim of this study was to investigate the proinflammatory and fibroproliferative effects of the coagulation protease thrombin on human proximal tubular cells (PTC) in culture.

METHODS

Primary cultures of PTC were established from normal kidney tissue and grown under serum-free conditions with or without thrombin or the protease-activated receptor (PAR) activating peptides TFLLRN-NH(2), SLIGKV-NH(2), and SFLLRN-NH(2) (100 to 400 micromol/L). DNA synthesis (thymidine incorporation), intracellular Ca(2+) mobilization (fura-2 fluorimetry), fibronectin secretion [enzyme-linked immunosorbent assay (ELISA), immunoblotting], monocyte chemoattractant protein-1 (MCP-1) secretion (ELISA), and transforming growth factor-beta1 (TGF-beta1) secretion (ELISA) were measured. Reverse transcription-polymerase chain reaction (RT-PCR) was used to assess PAR mRNA expression in these cells.

RESULTS

Thrombin enhanced DNA synthesis, fibronectin secretion, MCP-1 secretion, and TGF-beta1 secretion in a concentration-dependent manner. Cell injury [lactate dehydrogenase (LDH) release] and cellular protein levels were unaffected. RT-PCR showed that cultures of PTC expressed mRNA transcripts for the thrombin receptors PAR-1 and PAR-3, but not PAR-4. Thrombin and each of the PAR activating peptides enhanced intracellular calcium mobilization. However, the other effects of thrombin were only fully reproduced by the PAR-2-specific peptide, SLIGKV-NH(2), only partially by SFLLRN-NH(2), (a PAR-1 peptide that can activate PAR-2), and not at all by the PAR-1-specific peptide, TFLLRN-NH(2). Thrombin-induced DNA synthesis, fibronectin, and MCP-1 secretion were unaffected by a TGF-beta neutralizing antibody, the matrix metalloproteinase (MMP) inhibitor, GM6001 and the epidermal growth factor (EGF) receptor kinase inhibitor AG1478.

CONCLUSION

Thrombin initiates both proinflammatory and fibroproliferative responses in human PTC. These responses which are dependent on its protease activity appear not to be mediated by PAR-1 activation, the autocrine action of thrombin-induced TGF-beta1 secretion, MMP activation, or EGF receptor transactivation. The proinflammatory and fibroproliferative actions of thrombin on human PTC may help explain the extent of tubulointerstitial fibrosis observed in kidney diseases where fibrin deposition is evident.

The co-crystal structure of unliganded bovine alpha-thrombin and prethrombin-2: movement of the Tyr-Pro-Pro-Trp segment and active site residues upon ligand binding

Unliganded bovine alpha-thrombin and prethrombin-2 have been co-crystallized, in space group P21212, using either ammonium sulfate or polyethylene glycol 2000 (PEG2K), and their structures determined at 2.2 A and 2.3 A, respectively. Initial phases were determined by molecular replacement and refined using XPLOR to final R factors of 0.187 (Rfree = 0.255) and 0.190 (Rfree = 0.282) for the salt and PEG2K models, respectively. The apo-enzyme form of bovine alpha-thrombin shows dramatic shifts in placement for the Tyr-Pro-Pro-Trp segment, for Glu-192, and for the catalytic residues His-57 and Ser-195, when compared to 4 thrombin complexes representing different states of catalysis, namely (1) the Michaelis complex (residues 7-19 of fibrinogen A alpha with a non-cleavable scissile bond), (2) enzyme-inhibitor complex (D-Phe-Pro-Arg chloromethylketone), (3) enzyme product complex (residues 7-16 of fibrinopeptide A), and (4) the exosite complex (residues 53-64 of hirudin). The structures of bovine and human prethrombin-2 are generally similar to one another (RMS deviation of 0.68 A) but differ significantly in the Arg-15/Ile-16 cleavage region and in the three activation domains, which are disordered in bovine prethrombin-2, analogous to that seen for trypsinogen.

Thrombin as a multifunctional protein: induction of cell adhesion and proliferation

The serine protease thrombin (E.C.3.4.21.5) is well recognized for its central role in hemostasis. In addition, thrombin is unique among the enzymes participating in the clotting cascade, by virtue of its cell activation effects induced via the enzymatic pocket or via functional domains located throughout the molecule. In this review, we elaborate on "nonhemostatic" activities of thrombin among which are interactions with vessel wall components. These activities include promotion of cellular adhesion and induction of smooth muscle cell proliferation. Thrombin can exert these effects when it is in a fluid phase and when it is immobilized to extracellular matrix.

An Arg-Gly-Asp sequence within thrombin promotes endothelial cell adhesion

Thrombin, in addition to its central role in hemostasis, possesses diverse cellular bioregulatory functions implicated in wound healing, inflammation, and atherosclerosis. In the present study we demonstrate that thrombin molecules modified either at the procoagulant or catalytic sites induce endothelial cell (EC) adhesion, spreading, and cytoskeletal reorganization. The most potent adhesive thrombin analogue (NO2-alpha-thrombin) was obtained by nitration of tyrosine residues. The cell adhesion promoting activity of NO2-alpha-thrombin was blocked upon the formation of thrombin-antithrombin III (ATIII) complexes and by antiprothrombin antibodies, but was unaffected by hirudin. Arg-Gly-Asp-containing peptides, fully inhibited EC adhesion to NO2-alpha-thrombin, while synthetic peptides corresponding to thrombin "Loop B" mitogenic site and the thrombin-derived chemotactic fragment "CB67-129", were uneffective. Immunofluorescence studies indicated that EC adhesion to NO2-alpha-thrombin was followed by cell spreading, actin microfilament assembly, and formation of focal contacts. By the use of specific antibodies, the vitronectin (vn) receptor (alpha v beta 3) was found to be localized in clusters upon cell adhesion to NO2-alpha-thrombin. An anti alpha v beta 3 antibody blocked EC adhesion and spreading while antifibronectin (fn) receptor (alpha 5 beta 1) antibodies were uneffective. While native thrombin exhibited a very low cell attachment activity, thrombin that was incubated at 37 degrees C before coating of plastic surfaces induced EC attachment and spreading. We propose that under certain conditions the naturally hindered RGD domain within thrombin is exposed for interaction with alpha v beta 3 on EC. This in turn promotes cell adhesion, spreading, and reorganization of cytoskeletal elements, which may altogether contribute to repair mechanisms in the disturbed vessel wall. This study defines a new biological role of thrombin and characterizes a new recognition mechanism on EC for this molecule.

Functional tissue factor is entirely cell surface expressed on lipopolysaccharide-stimulated human blood monocytes and a constitutively tissue factor-producing neoplastic cell line

Tissue factor (TF) is an integral membrane glycoprotein which, as the receptor and essential cofactor for coagulation factors VII and VIIa (FVII and FVIIa, respectively), is the primary cellular activator of the coagulation protease cascade. Previous studies on the procoagulant activity of a variety of cell types (either lysed or in the intact state) have variously been interpreted as showing that TF is either stored intracellularly or is present in a cryptic form in the surface membrane. Using mAbs to TF, we have directly investigated the subcellular localization and functional activity of TF in lipopolysaccharide-stimulated blood monocytes and J82 bladder carcinoma cells. Blocking of surface TF of viable cells with inhibitory anti-TF mAbs abolished greater than 90% of TF activity of the intact cells as well as of lysed cells. Furthermore, quantitative analysis of the binding of FVII and anti-TF mAb to J82 cells demonstrated that all surface-expressed TF molecules were capable of binding the ligand, FVII. By immunoelectron microscopy, TF was present only in the surface membrane of monocytes and J82 cells, although the latter also contained apparently inactive TF antigen in multivesicular bodies. On the intact cell surface the catalytic activity of the TF-FVIIa complex was investigated and found to be markedly less relative to cell lysates. Membrane alterations that affect the cofactor activity of TF may be a means of regulating the extent of initiation of the coagulation protease cascade in various cellular settings.