Role of the plasminogen–plasmin system in tumoral angiogenesis and paradoxical functions of PAI-1
A contribution of the plasminogen–plasmin system to tumour progression is suggested by the following: (a) increased expression of uPA, uPAR, and PAI-1 in various tumours, (b) the use of antisense mRNA, and (c) the administration of natural or synthetic serine proteinase inhibitors, uPA antagonists, or antibodies.
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This is further supported by the delayed angiogenesis seen recently in plasminogen deficient mice in an in vitro model of aortic rings embedded in a collagen gel,
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in addition to an in vivo model of malignant keratinocyte transplantation.
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Accordingly, loss of either plasminogen activator or plasminogen was shown to reduce T241 fibrosarcoma
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and carcinoma
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tumour growth. In contrast, no difference in tumour growth was seen in a comparative study of control and plasminogen deficient mice using the PymT mouse mammary tumour model.
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Although angiogenic endothelial cells require uPA and plasmin to degrade extracellular matrix components and to migrate, plasmin proteolysis needs to be controlled by a physiological inhibitor, such as PAI-1, to allow the stabilisation of the surrounding matrix and the assembly of endothelial cells into channels.
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The importance of balanced proteolysis in the process of angiogenesis might provide an explanation for the dual role of proteinases and inhibitors in angiogenesis. Indeed, surprisingly, PAI-1 is a strong negative prognostic marker in different types of cancer.
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This could be explained by a simultaneous enhancement in uPA and PAI-1 expression, resulting in a net excess of proteolytic activity. Alternatively, this paradoxical clinical observation may be related to a potential direct role of PAI-1 in cancer cell migration and invasion. Studies in PAI-1 deficient mice have revealed that PAI-1 is essential for the vascularisation of skin tumours.
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The requirement for host PAI-1 during tumour angiogenesis has been confirmed in a fibrosarcoma model
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and in the mouse aortic ring model.
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However, it is likely that PAI-1 is not of general importance in tumour progression and its angiogenic effect might be dependent on the tumour type, its cellular source (tumour cells versus host cells) (personal unpublished data, 2003), its concentration, and the step of cancer progression that is being considered. Indeed, PAI-1 deficiency did not affect sarcoma development,
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melanoma cell metastasis,
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or primary tumour growth and dissemination in a genetic model of mammary adenocarcinoma.
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Recently, a dose dependent effect of PAI-1 has been demonstrated in the aortic ring assay,
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in bFGF induced angiogenesis,
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and in choroidal angiogenesis.
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Therefore, PAI-1 appears to be proangiogenic at physiological concentrations and antiangiogenic at high, pharmacological concentrations.
In addition to its ability to control pericellular proteolysis, PAI-1 could act through its capacity to interact with uPAR, integrins, and vitronectin, thereby controlling cell migration.
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Therefore, PAI-1 can competitively inhibit the uPAR dependent attachment of cells to vitronectin. Recently, PAI-1 has been shown to detach cells by disrupting uPAR–vitronectin and integrin–vitronectin interactions.
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To investigate the mechanism of action of PAI-1, adenoviruses were used to deliver two mutant forms of PAI-1 to PAI-1 deficient mice in the transplantation system of malignant keratinocytes
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and in the aortic ring assay.
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In both models, angiogenesis is restored only by the PAI-1 mutant that retains plasminogen activator inhibitory activity, but not by the mutant that can interact with vitronectin but has impaired proteolytic control. These findings demonstrate that, in these models, PAI-1 requirement is merely related to its capacity to inhibit excessive proteolysis, rather than to interact with vitronectin. In a separate study using PAI-1 mutants, the inhibition of bFGF induced angiogenesis in the CAM assay requires both the antiprotease activity of PAI-1 and its vitronectin binding capacity.
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Therefore, the mechanism of PAI-1 action will probably be dependent on the experimental setting, the in vivo situation, and the type of cancer.