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 activator (t-PA) in culture, but after 24-48 h, t-PA ceases to accumulate in the medium. Here, we report negative feedback regulation of t-PA expression, exerted by t-PA or complexes of t-PA with its physiological inhibitor, 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 protein and cellular mRNA showed that both t-PA protein and mRNA levels declined considerably after 24 h. When ¹²⁵I-labeled t-PA or t-PA·PAI-1 complexes were incubated with HCF, monensin-inhibitable endocytosis and catabolism were observed. The low density lipoprotein receptor-related protein (LRP) was found to be expressed by HCF and to mediate these events. Addition of the 39-kDa receptor- associated protein (RAP), an antagonist for ligand interactions with LRP, removed the block to t-PA expression and restored its accumulation in the medium. Moreover, RAP completely prevented the degradation of exogenous ¹²⁵I-labeled t-PA by HCF, suggesting that LRP is the endocytic receptor for t-PA in these cells. These results demonstrate that cellular modulation of t-PA expression in HCF involves LRP receptor-mediated clearance of t-PA. This LRP receptor-mediated event results in down-regulation of t-PA expression at the mRNA level.