CHARACTERIZATION OF THE INTERACTION OF A COMPLEX OF TISSUE-TYPE PLASMINOGEN-ACTIVATOR AND PLASMINOGEN-ACTIVATOR INHIBITOR TYPE-1 WITH RAT-LIVER CELLS

The present study was undertaken in order to determine the recognition site for tissue-type plasminogen activator-plasminogen activator inhi bitor type 1 [t-PA-PAI-1] complexes in rat liver in vivo and in vitro. After intravenous injection into rats t-PA-PAI-1 complexes were rapid ly removed from the plasma and the liver took up 80% of the injected d ose. Within the liver parenchymal and endothelial liver cells contribu ted mainly to the uptake of t-PA-PAI-1, and were responsible for 62% a nd 24% of the liver uptake, respectively. The interaction of t-PA-PAI- 1 with isolated rat parenchymal liver cells was of high affinity (Kd 1 7 nM). A well-known antagonist of the alpha(2)-macroglobulin receptor (alpha(2)MR/low-density lipoprotein receptor-related protein (LRP), GS T-39kDa protein (GST-39kDaP) efficiently inhibited the binding (IC50 0 .7 nM) of t-PA-PAI-1 to rat parenchymal liver cells. The interaction o f t-PA-PAI-1 with LRP on rat parenchymal liver cells was not Ca2+-depe ndent and is most probably mediated by a specific determinant on PAI-I , since an anti-PAI-1 monoaclonal antibody inhibited the binding of t- PA-PAI-1, where as free t-PA did not. The binding of t-PA-PAI-1 to rat hepatocytes could not be inhibited by a complex of plasmin and alpha( 2)-antiplasmin nor by various other ligands of LRP like beta-VLDL and lactoferrin. Binding of t-PA-PAI-1 to rat parenchymal liver cells was followed by internalization and subsequent degradation in the lysosoma l compartment. It is concluded that parenchymal and endothelial liver cells mediate the removal of t-PA-PAI-1 complexes from the circulation . LRP on rat parenchymal liver cells is responsible for the uptake and degradation of t-PA-PAI-1 and may therefore be important for the regu lation of the t-PA levels in the circulation.