Extracellular and cell-associated localizations of plasminogen activators and plasminogen activator inhibitor-1 in cultured endothelium.

The extracellular localizations of urokinase-type plasminogen activator (uPA), tissue-type plasminogen activator (tPA), and plasminogen activator inhibitor-1 (PAI-1) were examined in cultured bovine capillary endothelial cells (BCEs) by an immunofluorescence method using BCEs treated with or without saponin and focal contact preparations. The specific immunofluorescence of cell surface uPA showed a patchy or strand-like distribution and was colocalized with vinculin strands indicating that uPA secreted from BCEs was mainly deposited at the cell surface of focal contacts. BCEs at a subconfluent density showed a higher intensity of specific immunofluorescence for uPA than when they were at a confluent density. tPA was observed over the dorsal surface of cultured BCEs and accentuated at their margins, suggesting that tPA was diffusely distributed on the luminal surface of BCEs in vivo. PAI-1 was distributed in the extracellular matrix under cultured BCEs. These findings suggest that uPA and PAI-1 are located under BCEs participating in the regulation of proteolytic activities provoked by plasminogen-PAs-plasmin system in vivo. The localization of tPA appears to be consistent with its function, which is to maintain the fluidity of the blood and to initiate thrombolysis in vivo.