Summary
In order to study the role of the asialoglycoprotein receptor (ASGPr) in the rapid
plasma clearance of urokinase-type plasminogen activator (u-PA), a microtiter plate
binding assay was developed using ASGPr purified from rat liver extracts. Urinary
two-chain u-PA bound to immobilized ASGPr in a saturable manner with an EC50 of 0.2 µM. Binding was inhibited by rabbit antibodies against the ASGPr. In line with the
known carbohydrate specificity of the ASGPr, GalNAc proved to be the most effective
inhibitor from a series of monosaccharides, followed by Gal and Fuc, whereas GlcNAc
was ineffective. The N-linked oligosaccharides of urinary u-PA do not terminate with
the common Gal-GlcNAc element, but with a GalNAc-GlcNAc element which is partially
sulfated. Sulfated forms of u-PA were separated from non-sulfated forms by using the
lectin Wisteria floribunda agglutinin. Only the non-sulfated forms of u-PA (30% of the total) appeared to bind
to the ASGPr. From different u-PA preparations used for thrombolytic therapy only
urinary u-PA and u-PA produced by kidney cell cultures strongly bound to the ASGPr,
whereas (recombinant) u-PA expressed in mouse myeloma cells, Chinese hamster ovary
cells or E. coli scarcely bound to the receptor. It is concluded that u-PA bearing non-sulfated GalNAc-GlcNAc
elements is specifically recognized by the ASGPr present on liver cells.
Keywords
Asialoglycoprotein receptor - plasma clearance - plasminogen activator - urokinase-type
plasminogen activator - thrombolytic therapy