LOW-DENSITY-LIPOPROTEIN RECEPTOR-RELATED PROTEIN MODULATES THE EXPRESSION OF TISSUE-TYPE PLASMINOGEN-ACTIVATOR IN HUMAN COLON FIBROBLASTS

Human colon fibroblasts (HCF) produce tissue type plasminogen activato r (t-PA) in culture, but after 24-48 h, t-PA ceases to accumulate in t he medium, Here, we report negative feedback regulation of t-PA expres sion, exerted by t-PA or complexes of t-PA with its physiological inhi bitor, plasminogen activator inhibitor type 1 (PAI-1), Inhibition of t -PA expression could be induced by addition of exogenous t-PA or t-PA . PAI-1 complexes and reversed by monoclonal antibody directed against the active site of t-PA. Analysis of metabolically radiolabeled prote in and cellular mRNA showed that both t-PA protein and mRNA levels dec lined considerably after 24 h, When I-125-labeled t-PA or t-PA . PAI-1 complexes were incubated with HCF, monensin-inhibitable endocytosis a nd catabolism were observed, The low density lipoprotein receptor-rela ted protein (LRP) was found to be expressed by HCF and to mediate thes e events, Addition of the 39-kDa receptor associated protein (RAP), an antagonist for ligand interactions with LRP, removed the block to t-P A expression and restored its accumulation in the medium, Moreover, RA P completely prevented the degradation of exogenous I-125-labeled t-PA by HCF, suggesting that LRP is the endocytic receptor for t-PA in the se cells, These results demonstrate that cellular modulation of t-PA e xpression in HCF involves LRP receptor-mediated clearance of t-PA This LRP receptor-mediated event results in down-regulation of t-PA expres sion at the mRNA level.