INHIBITION OF UROKINASE ACTIVITY BY THE ANTIANGIOGENIC FACTOR 16K PROLACTIN - ACTIVATION OF PLASMINOGEN-ACTIVATOR INHIBITOR-1 EXPRESSION

The N-terminal fragment of PRL (16K PRL) is an antiangiogenic factor t hat, in vitro, inhibits several components of angiogenesis including b asic fibroblast growth factor (bFGF)-induced cell division, migration, and organization of capillary endothelial cells. An essential step in the regulation of angiogenesis is the activation of urokinase (urokin ase type plasminogen activator, uPA), which in turn activates a cascad e of proteases that play essential roles in endothelial cell migration and tissue remodeling. Treatment of bovine capillary endothelial cell s (BBEC) with 16K PRL inhibited bFGF-stimulated urokinase activity in BBEC as detected by plasminogen substrate gel assay. 16K PRL did not a ppear to be acting via an effect on uPA expression because no change i n messenger RNA levels were observed. However, protein levels of plasm inogen activator inhibitor-1 (PAI-1), a specific inhibitor of urokinas e, were increased by 16K PRL independent of the action of bFGF. The 16 K PRL-induced increase in PAI-1 protein levels appear to be the result of increased expression of the PAI-1 gene. Increased production of PA I-1 induced by 16K PRL results in the formation of inactive PAI-1/uPA complexes, consistent with the observed decrease in uPA activity.