One of the phenotypic hallmarks of migrating endothelial cells, both in vivo and in vitro, is expression of the urokinase-type plasminogen activator (u-PA), a key mediator of extracellular proteolysis. In the study reported here, we have used an in vitro model of endothelial cell migration to explore the mechanism of this phenomenon. We have found that wounding of an endothelial cell monolayer triggers a marked, rapid and sustained increase in expression of a specific high-affinity receptor for u-PA (u-PAr) on the surface of migrating cells. Migrating cells displayed an increase in the levels of u-PA and u-PAr mRNAs, and this increase was mediated by endogenous basic fibroblast growth factor (bFGF). We also show that the increase in u-PA activity on migrating cells can be accounted for by an increase in receptor-bound u-PA, and that the increase in activity is also dependent on endogenous bFGF. These results demonstrate that the expression of plasmin-mediated proteolytic activity by migrating endothelial cells is a consequence of increased production of both u-PA and its receptor, and that this in turn is mediated by endogenous bFGF. This suggests that u-PA, produced at increased levels by migrating cells, binds to u-PAr whose expression is upregulated on the same cells. These observations are in accord with the postulated role of u-PAr in mediating efficient and spatially restricted extracellular proteolysis, particularly in the context of cell migration.

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