Low density lipoprotein receptor-related protein/alpha 2-macroglobulin receptor mediates cellular uptake of pro-urokinase.
The low density lipoprotein receptor-related protein/alpha 2-macroglobulin receptor (LRP) is a large cell surface receptor consisting of a 515-kDa heavy chain and an 85-kDa light chain proteolytically derived from a 600-kDa precursor. Previous work has shown that LRP is responsible for mediating the internalization of urinary-type plasminogen activator (uPA) complexed to plasminogen activator inhibitor type I (PAI-1) (Nykjaer et al., 1992; Herz et al., 1992). The current study indicates that pro-urokinase (pro-uPA) and two chain urokinase (tc-uPA) bind directly to purified LRP, and that LRP mediates their internalization and degradation in Hep G2 cells. In vitro binding assays demonstrated that pro-uPA and tc-uPA bind to purified LRP with affinities (Kd = 45 and 60 nM, respectively) that are approximately 15 to 20-fold weaker than the affinity of uPA.PAI-1 complex for LRP (Kd = 3 nM). Competitive binding experiments revealed that pro-uPA and tc-uPA completely inhibit binding of uPA.PAI-1 complexes to purified LRP. The binding of 125I-pro-uPA to LRP is blocked by the 39-kDa receptor-associated protein, but not by an amino-terminal fragment of uPA, which is known to block binding of uPA to the urokinase receptor. 125I-Pro-uPA can be internalized and degraded by Hep G2 cells independent of PAI-1. Both the internalization and degradation are completely blocked by receptor-associated protein or affinity-purified LRP antibodies, indicating that LRP is mediating this process. These processes are also blocked by the amino-terminal fragment, which suggests that the favored pathway for uPA metabolism is initial binding to the urokinase receptor, followed by ligand transfer to LRP, then internalization leading to degradation.
Kounnas
M Z
MZ
Biochemistry Laboratory, American Red Cross, Rockville, Maryland 20855.
Henkin
J
J
Argraves
W S
WS
Strickland
D K
DK
D001667:Binding, Competitive; D004792:Enzyme Precursors / Q000378:metabolism*; D006801:Humans; D007457:Iodine Radioisotopes; D026503:Low Density Lipoprotein Receptor-Related Protein-1; D011971:Receptors, Immunologic / Q000378:metabolism*; D011994:Recombinant Proteins / Q000378:metabolism; D014407:Tumor Cells, Cultured; D014568:Urokinase-Type Plasminogen Activator / Q000378:metabolism*; D000511:alpha-Macroglobulins / Q000378:metabolism*
D004792:Enzyme Precursors; D007457:Iodine Radioisotopes; D026503:Low Density Lipoprotein Receptor-Related Protein-1; D011971:Receptors, Immunologic; D011994:Recombinant Proteins; D000511:alpha-Macroglobulins; D014568:Urokinase-Type Plasminogen Activator; C067551:saruplase