Expression of an anchored urokinase in the apical endothelial cell membrane. Preservation of enzymatic activity and enhancement of cell surface plasminogen activation.

Abstract

A mutant single chain urokinase plasminogen activator (scu-PA) was constructed by the addition of an apical membrane targeting signal from decay accelerating factor to the scu-PA carboxyl terminus. Bovine aortic endothelial cells (EC) were transduced with the mutant scu-PA. Metabolic labeling, immunoprecipitation, and gel electrophoresis revealed that the mutant scu-PA was present in a single-chain form at the EC surface. Immunohistochemistry and enzyme-linked immunosorbent assay before and after treatment of EC with phosphotidylinositol-specific phospholipase C confirmed that scu-PA was attached to the EC surface by a glycosyl-phosphotidylinositol anchor. Approximately 10(6) anchored scu-PA molecules/cell were present; however, anchoring was not 100% efficient, with scu-PA released into the medium as well. Selective biotinylation of the apical and basolateral surfaces revealed that anchored scu-PA was polarized to the apical surface. Apically anchored scu-PA could be converted by plasmin to two-chain urokinase, with a normal specific activity (140,000 IU/mg) as measured with the chromogenic substrate S-2444. Expression of anchored scu-PA resulted in an increase in EC surface plasminogen activator activity, as compared with the activity of either untransduced EC or EC transduced with a wild type scu-PA. These experiments demonstrate: 1) apical membrane targeting can be accomplished in EC; 2) scu-PA can be anchored to the EC surface with preservation of enzymatic activity; 3) EC surface plasminogen activator activity is significantly increased by the presence of anchored scu-PA. Cell surface targeted plasminogen activators may eventually be useful in the prevention and treatment of intravascular thrombosis.