| The zymogen plasminogen is present in the plasma at a concentration of 2 muM and is converted to enzymatically active plasmin by plasminogen activators (PAs) such as tissue-type PA (tPA) or urokinase-type PA (uPA). The central serine proteinase, plasmin, by virtue of its ability to degrade fibrin and extracellular matrix proteins and to activate a variety of proteinases, plays a pivotal role in physiological processes such as fibrinolysis, inflammation and wound healing. Plasmin is also involved in several pathophysiological processes, such as tumor cell invasion and metastasis. The conversion of plasminogen to plasmin is dramatically accelerated on the fibrin clot or on the cell surface by its candidate receptor, annexin II heterotetramer (Allt). Al It consists of two copies each of an annexin II (p36) subunit and an S100A10 (p11) subunit. In this thesis, the regulation of plasmin activity by annexin II tetramer (Allt) is characterized. Allt binds fibrin with a Kd of 270 nM and also binds plasminogen with a Kd of 1.26 muM via its p11 subunit, resulting in remarkable acceleration of tPA-dependent plasminogen activation on the fibrin surface. Allt also stimulates plasmin autodegradation on the fibrin surface, suggesting possible regulation of plasmin activity. Transfection of human HT1080 cells with the human p11 gene in the antisense orientation resulted in a loss of p11 from the cell surface, and concomitant decreases in plasminogen binding capacity, plasmin production, extracellular matrix degradation, and cellular invasiveness as well as the reduced development of lung metastatic foci in SCID mice. In contrast, HT1080 cells transfected with the p11 gene in the sense orientation displayed increased cell surface p11, and concomitant increases in plasminogen binding, plasmin activity, extracellular matrix degradation, and cellular invasiveness as well as enhanced development of lung metastatic foci. The p11-overexpressing cells displayed accelerated plasmin formation and enhanced plasmin autocatalysis in a time-dependent manner. This thesis establishes that the dual role of Alit in plasmin formation and destruction may generate a transient pulse of proteolytic activity, resulting in the spatial and temporal regulation of plasmin activity on the fibrin clot or on the cell surface. |
