To test the hypothesis that platelet activation contributes to tumor dissemination, we studied metastasis in mice lacking G␣q, a G protein critical for platelet activation. Loss of platelet activation resulted in a profound diminution in both experimental and spontaneous metastases. IntroductionA persuasive body of evidence has accumulated associating hemostatic factors with tumor growth, stroma formation, and tumor dissemination. [1][2][3] Clinical studies have shown that expression of procoagulants by cancer cells is prognostic of poor outcome. [4][5][6] Furthermore, the expression of tissue factor by tumor cells has been shown to promote metastatic disease in experimental animals, 7 whereas inhibitors of thrombin and other coagulation factors diminished metastatic potential. 1,8 The available data support the general hypothesis that local thrombin generation enhances tumor dissemination. However, it is presently not clear which specific thrombin substrates are important mediators of the metastatic process. Recent studies revealed that fibrinogen deficiency significantly diminishes metastatic potential, suggesting that fibrinogen is at least one thrombin substrate important in metastasis. 9,10 However, it is likely that other thrombin substrates also contribute to tumor cell metastasis based on the finding that pharmacologic inhibition of thrombin resulted in decreased metastatic potential even in the absence of fibrinogen. 10 Several studies support the view that thrombin-mediated platelet activation may play a role in tumor biology. The elimination of circulating platelets with antiplatelet antibodies was shown to result in a significant diminution in metastases using several transplantable murine tumor models. 11,12 Competitive inhibition of the key platelet integrin, ␣ IIb ␤ 3 , either pharmacologically or with antibodies to ␤ 3 , also diminished metastatic potential. 13,14 Similarly, pharmacologic inhibitors of platelet activation have been shown to decrease the metastatic potential of circulating tumor cells. 15,16 More recently, the genetic loss of the integrin ␤ 3 subunit in mice was shown to diminish metastasis. 17 Platelets could influence metastatic potential via several mechanisms. Platelet granules contain a variety of cellular growth factors (eg, platelet-derived growth factor [PDGF], vascular endothelial growth factor [VEGF]), matrix proteins (eg, vitronectin, fibronectin), and inflammatory mediators (eg, platelet factor-4, interleukin-8, macrophage inflammatory protein 1␣ , RANTES [regulated on activation, normal T expressed, and secreted], CCL17, CCXL1, CXCL5) that might influence tumor cell behavior and stroma formation. [18][19][20] Platelets may also contribute to the physical interaction between circulating tumor cells and vascular endothelial cells by supporting the stable adhesion to endothelium and/or transmigration of tumor cells out of the vasculature. Local platelet activation could promote the migration of inflammatory cells, enhancing tumor stroma formation. Alternatively, tumor c...

Plasminogen (Plg)-deficient mice were generated to define the physiological roles of this key fibrinolytic protein and its proteolytic derivatives, plasmin and angiostatin, in development, hemostasis, and reproduction. Pig -/-mice complete embryonic development, survive to adulthood, and are fertile. There is no evidence of fetal loss of Pig -/-mice based on the Mendelian pattern of transmission of the mutant Pig allele. Furthermore, embryonic development continues to term in the absence of endogenous, sibling-derived, or maternal Pig. However, Pig -/-mice are predisposed to severe thrombosis, and young animals developed multiple spontaneous thrombotic lesions in liver, stomach, colon, rectum, lung, pancreas, and other tissues. Fibrin deposition in the liver was a uniform finding in 5-to 21-week-old mice, and ulcerated lesions in the gastrointestinal tract and rectal tissue were common. A remarkable finding, considering the well-established linkage between plasmin and the proteolytic activation of plasminogen activators, was that the level of active urokinase-type plasminogen activator in urine was unaffected in Pig -/-mice. Therefore, Pig plays a pivotal role in fibrinolysis and hemostasis but is not essential for urokinase proenzyme activation, development, or growth to sexual maturity.

Activation of plasminogen (Plg) has been proposed to play a role in proteolytic degradation of extracellular matrices in tissue remodeling events, including wound healing. However, there has been no definitive proof of involvement of Plg in such processes. We now report that healing of skin wounds is severely impaired in mice made deficient in Plg by targeted gene disruption. The results demonstrate that Plg is required for normal repair of skin wounds in mice and support the assumption that it also plays a central role in other disease processes involving extracellular matrix degradation, such as cancer invasion.

Perivascular microglia activation is a hallmark of inflammatory demyelination in multiple sclerosis (MS), but the mechanisms underlying microglia activation and specific strategies to attenuate their activation remain elusive. Here, we identify fibrinogen as a novel regulator of microglia activation and show that targeting of the interaction of fibrinogen with the microglia integrin receptor Mac-1 (αMβ2, CD11b/CD18) is sufficient to suppress experimental autoimmune encephalomyelitis in mice that retain full coagulation function. We show that fibrinogen, which is deposited perivascularly in MS plaques, signals through Mac-1 and induces the differentiation of microglia to phagocytes via activation of Akt and Rho. Genetic disruption of fibrinogen–Mac-1 interaction in fibrinogen-γ390-396A knock-in mice or pharmacologically impeding fibrinogen–Mac-1 interaction through intranasal delivery of a fibrinogen-derived inhibitory peptide (γ377-395) attenuates microglia activation and suppresses relapsing paralysis. Because blocking fibrinogen–Mac-1 interactions affects the proinflammatory but not the procoagulant properties of fibrinogen, targeting the γ377-395 fibrinogen epitope could represent a potential therapeutic strategy for MS and other neuroinflammatory diseases associated with blood-brain barrier disruption and microglia activation.

Plasmin(ogen) is an extracellular serine protease implicated in the activation of latent growth factors and procollagenase, degradation of extracellular matrix components, and fibrin clearance. Plasminogen (Plg) deficiency in mice results in high mortality, wasting, spontaneous gastrointestinal ulceration, rectal prolapse, and severe thrombosis. Furthermore, Plg-deficient mice display delayed wound healing following skin injury, a defect partly related to impaired keratinocyte migration. We generated mice deficient in Plg and fibrinogen (Fib) and show that removal of fibrin(ogen) from the extracellular environment alleviates the diverse spontaneous pathologies previously associated with Plg deficiency and corrects healing times. Mice deficient in Plg and Fib are phenotypically indistinguishable from Fib-deficient mice. These data suggest that the fundamental and possibly only essential physiological role of Plg is fibrinolysis.

The leukocyte integrin α M β 2 /Mac-1 appears to support the inflammatory response through multiple ligands, but local engagement of fibrin(ogen) may be particularly important for leukocyte function. To define the biological significance of fibrin(ogen)-α M β 2 interaction in vivo, gene-targeted mice were generated in which the α M β 2 -binding motif within the fibrinogen γ chain (N 390 RLSIGE 396 ) was converted to a series of alanine residues. Mice carrying the Fibγ 390-396A allele maintained normal levels of fibrinogen, retained normal clotting function, supported platelet aggregation, and never developed spontaneous hemorrhagic events. However, the mutant fibrinogen failed to support α M β 2 -mediated adhesion of primary neutrophils, macrophages, and α M β 2 -expressing cell lines. The elimination of the α M β 2 -binding motif on fibrin(ogen) severely compromised the inflammatory response in vivo as evidenced by a dramatic impediment in leukocyte clearance of Staphylococcus aureus inoculated into the peritoneal cavity. This defect in bacterial clearance was due not to diminished leukocyte trafficking but rather to a failure to fully implement antimicrobial functions. These studies definitively demonstrate that fibrin(ogen) is a physiologically relevant ligand for α M β 2 , integrin engagement of fibrin(ogen) is critical to leukocyte function and innate immunity in vivo, and the biological importance of fibrinogen in regulating the inflammatory response can be appreciated outside of any alteration in clotting function.

IntroductionTissue factor (TF) is the membrane-associated glycoprotein receptor for coagulation factors VIIa and X that serves as the primary physiologic initiator of blood coagulation. In addition to supporting proteolytic events that ultimately lead to local thrombin generation, TF is also proposed to directly contribute to intracellular signaling events through the TF cytoplasmic domain and TF/fVIIa/fXamediated activation of PAR-1 and PAR-2. 1-4 A significant body of evidence has accumulated linking tumor cell-associated procoagulant function to cancer biology. Multiple clinical studies have shown a correlation between TF expression by tumor cells and advanced disease stage and poor outcome. [5][6][7][8][9] Furthermore, experimental data generated using animal models of tumor metastasis strongly favor the view that TF expression by malignant cells supports metastatic success. [10][11][12][13][14][15] Similarly, thrombin-mediated proteolysis, 16-21 fibrin(ogen), 22,23 and PAR-mediated platelet activation 24 also appear to be significant determinants of metastatic potential. Both platelets and fibrinogen were shown to support metastatic potential by limiting the capacity of natural killer (NK) cells to clear newly established micrometastatic foci. 25,26 However, hemostatic factors are likely to influence tumor dissemination through multiple mechanisms and the precise pathways coupling TF to malignancy remain to be defined.The tandem importance of tumor cell-associated TF and circulating coagulation system components in malignancy is consistent with the hypothesis that TF supports metastasis by providing cancer cells a means of directing proteolytic events leading to local thrombin generation and the formation of tumor cell-associated microthrombi. However, an intriguing alternate possibility is that TF supports tumor cell dissemination by mechanism(s) uncoupled from "traditional" thrombin generation and subsequent thrombus formation. In this regard, significant attention has focused on potential intracellular signaling events coupled to the cytoplasmic portion of TF. This interest was driven in part by early studies indicating that tumor cells expressing a mutant form of TF lacking the cytoplasmic domain were far less metastatic than tumor cells expressing full-length TF. 11,12,14 However, interpretation of these early studies was made more complex by the use of nonmurine tumor lines in xenograft assays in mice, the use of tumor cells expressing human TF or human TF derivatives in a setting where all other factors were of murine origin, and the requisite use of immunocompromised mice. Nevertheless, many studies have provided provocative evidence for an important linkage between TF-mediated signaling events and several key cellular processes capable of influencing metastasis, including cytoskeletal organization, 27 cell adhesion/migration, 28-30 apoptosis, 31,32 and angiogenesis. 33,34 An important role for the TF cytoplasmic domain in cellular signaling is also supported by more recent studies of transgenic mic...

The canonical role of the hemostatic and fibrinolytic systems is to maintain vascular integrity. Perturbations in either system can prompt primary pathological end points of hemorrhage or thrombosis with vessel occlusion. However, fibrin(ogen) and proteases controlling its deposition and clearance, including (pro)thrombin and plasmin(ogen), have powerful roles in driving acute and reparative inflammatory pathways that affect the spectrum of tissue injury, remodeling, and repair. Indeed, fibrin(ogen) deposits are a near-universal feature of tissue injury, regardless of the nature of the inciting event, including injuries driven by mechanical insult, infection, or immunological derangements. Fibrin can modify multiple aspects of inflammatory cell function by engaging leukocytes through a variety of cellular receptors and mechanisms. Studies on the role of coagulation system activation and fibrin(ogen) deposition in models of inflammatory disease and tissue injury have revealed points of commonality, as well as context-dependent contributions of coagulation and fibrinolytic factors. However, there remains a critical need to define the precise temporal and spatial mechanisms by which fibrinogen-directed inflammatory events may dictate the severity of tissue injury and coordinate the remodeling and repair events essential to restore normal organ function. Current research trends suggest that future studies will give way to the identification of novel hemostatic factor-targeted therapies for a range of tissue injuries and disease.