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.