VERY-LOW-DENSITY LIPOPROTEIN RECEPTOR BINDS AND MEDIATES ENDOCYTOSIS OF UROKINASE-TYPE PLASMINOGEN ACTIVATOR-TYPE-1 PLASMINOGEN-ACTIVATOR INHIBITOR COMPLEX

Very low density lipoprotein receptor (VLDL-R) was found to be expressed in bovine mammary gland and the human breast carcinoma cell line MCF-7 as an M(r) 105,000 variant, and in Chinese hamster ovary (CHO) cells transfected with human VLDL-R cDNA as an M(r) 130,000 variant. The receptor was purified by ligand affinity chromatography with immobilized M(r) 40,000 receptor-associated protein (RAP). The purified receptor was found to bind urokinase-type plasminogen activator-type-1 plasminogen activator inhibitor complex (u-PA . PAI-1), while there was no or very weak binding of active site blocked u-PA (DFP-u-PA), PAI-1 or u-PA-type-2 plasminogen activator inhibitor complex. The binding of u-PA . PAI-1 was blocked by RAP. The transfected CHO cells had an efficient, RAP sensitive endocy tosis of u-PA . PAI-1, severalfold higher than non-transfected parental CHO cells. u-PA . PAI-1 endocytosis was partially inhibited by DFP-u-PA, which blocks binding of the complex to the u-PA receptor. RAP and DFP-u-PA sensitive u-PA . PAI-1 endocytosis was also observed in MCF-7 cells, which were without detectable levels of other RAP-binding endocytosis receptors. These results show that VLDL-R represents a novel endocytosis mechanism for u-PA receptor-bound u-PA . PAI-1.