The long-term objective of this proposal is to understand the mechanisms by which the interaction of plasminogen with specific cellular receptors functions to enhance fibrinolysis and proteolysis. Activation of plasminogen results in generation of plasmin, a broad spectrum serine protease and the primary enzyme which degrades fibrin in vivo. A functional fibrinolytic system is essential for restoring blood vessel patency following blood coagulation and is also required for processes involving cell migration such as wound healing, the inflammatory response and metastasis. Disturbances in the balance of the fibrinolytic system can have pathologic consequences ranging from thrombosis to bleeding disorders. Thrombotic diseases are the leading cause of death in the United States. This proposal is based upon data establishing that plasminogen interacts with cells in a structurally specific manner and that this interaction promotes plasminogen activation and establishes a local nidus of protected plasmin activity. The high capacity of cells for plasminogen suggests that several cell surface molecules may interact with plasminogen. However, a specific subclass of plasminogen receptors with carboxyl terminal lysyl residues is predominantly responsible for promoting plasminogen activation. Using 2-dimensional gel analysis we identified a previously unrecognized candidate plasminogen receptor of monocytes, TIP49. We obtained protein and cDNA sequence information for TIP49 and the cDNA sequence predicted a carboxyl terminal lysine. Therefore, we will 1) functionally characterize TIP49 by testing the hypothesis that TIP49 binds plasminogen and enhances plasminogen activation in its purified form and when present on cell surfaces. We found that another candidate plasminogen receptor, alpha-enolase, is present on cell surfaces but exposes a carboxyl terminal lysine on some cell types, but not on others. Therefore, we will 2) address the basis for differential surface exposure of the carboxyl terminal lysine of alpha-enolase. We will use a panel of monoclonal antibodies that recognize receptor induced binding sites in plasminogen to 3) to test the hypothesis that conformational changes in plasminogen upon binding to cells expose latent epitopes in the ligand and that these changes enhance plasminogen activation. Using receptor- identification techniques in our laboratory, we will 4) test the hypothesis that antiangiogenic plasminogen and apo(a) fragments exert their suppressive activities by binding to specific cellular receptors. Accomplishment of these aims should provide broad insights into the interactions of plasminogen and plasminogen domains with cells, an essential feature of fibrinolysis and angiogenesis.
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