Summary
The low density lipoprotein receptor-related protein (LRP) is a multiligand clearance
receptor that removes free tissue-type plasminogen activator (t-PA) or complexes of
t-PA with plasminogen activator inhibitor type 1 (PAI-1) from the blood circulation
or the pericellular space. Co-receptors are essential for LRP-mediated clearance of
several ligands (e.g. glycosaminoglycans for thrombin/protease nexin and lipoprotein
lipase, and the urokinase receptor for urokinase/PAI-1 complexes). The present study
was undertaken to investigate whether LRP-mediated t-PA clearance requires a co-receptor
as well.
In five cell lines from different organs and species degradation of t-PA and t-PA/PAI-1
was mediated by LRP (or LRP-like receptors). No degradation of t-PA and t-PA/PAI-1
occurred in THP-1 or U-937 human monocyte-like cells, despite the presence of functional
LRP. As glycosaminoglycans can bind t-PA and PAI-1 we investigated whether they are
involved in t-PA/PAI-1 degradation. Pre-treatment of COS cells or HT1080 cells with
chlorate, an inhibitor of glycosaminoglycan sulfation, did not decrease t-PA/PAI-1
degradation. Furthermore, CHO cells genetically deficient in glycosaminoglycans efficiently
degraded t-PA/PAI-1. Thus it is unlikely that glycosaminoglycans are co-receptors
for degradation of t-PA or t-PA/PAI-1.
This study indicates that THP-1 and U-937 cells lack a critical component (co-receptor?)
for the LRP-mediated degradation of t-PA.
Abbreviations: LRP, low density lipoprotein receptor-related protein; PAI-1, plasminogen activator
inhibitor type 1; RAP, receptor-associated protein; t-PA, tissue-type plasminogen
activator; u-PA, urokinase; u-PAR, urokinase receptor.
Keywords
Clearance - t-PA - PAI-1 - LRP - thrombin