BACKGROUND: The urokinase receptor (uPAR/CD87) is highly expressed in malignant 
tumours. uPAR, as a GPI anchored protein, is preferentially located at the cell 
surface, where it interacts with its ligands urokinase (uPA) and the 
extracellular matrix protein vitronectin, thus promoting plasmin generation, 
cell-matrix interactions and intracellular signalling events. Interaction with a 
complex formed by uPA and its inhibitor PAI-1 induces cell surface down 
regulation and recycling of the receptor via the clathrin-coated pathway, a 
process dependent on the association to LRP-1.
METHODOLOGY/PRINCIPAL FINDINGS: In this study, we have found that along with the 
ligand-induced down-regulation, uPAR also internalizes and recycles 
constitutively through a second pathway that is independent of LRP-1 and 
clathrin but shares some properties with macropinocytosis. The 
ligand-independent route is amiloride-sensitive, does not require uPAR 
partitioning into lipid rafts, is independent of the activity of small GTPases 
RhoA, Rac1 and Cdc42, and does not require PI3K activity. Constitutively 
endocytosed uPAR is found in EEA1 positive early/recycling endosomes but does 
not reach lysosomes in the absence of ligands. Electron microscopy analysis 
reveals the presence of uPAR in ruffling domains at the cell surface, in 
macropinosome-like vesicles and in endosomal compartments.
CONCLUSIONS/SIGNIFICANCE: These results indicate that, in addition to the 
ligand-induced endocytosis of uPAR, efficient surface expression and membrane 
trafficking might also be driven by an uncommon macropinocytic mechanism coupled 
with rapid recycling to the cell surface.