Abstract

The fibrinolysis proteinase, tissue-type plasminogen activator (tPA), triggers cell-signaling and regulates cell physiology. In PC12 cells, Schwann cells, and macrophages, the N-methyl-D-aspartate receptor (NMDA-R) mediates tPA-signaling. Plasminogen activator Inhibitor-1 (PAI1) is a rapid inhibitor of tPA enzyme activity. Although tPA-initiated cell-signaling is not dependent on its enzyme active site, we show that tPA-signaling is neutralized by PAI1. In PC12 cells, PAI1 blocked ERK1/2 activation by tPA and neurite outgrowth. In Schwann cells, PAI1 blocked tPA-mediated ERK1/2 activation and cell migration. In macrophages, PAI1 blocked the ability of tPA to inhibit IκBα phosphorylation and cytokine expression. The cell-signaling activity of tPA-PAI1 complex was rescued by forming the complex with PAI1^R76→E, which binds to LRP1 with decreased affinity, by pre-treating cells with the LRP1 antagonist, Receptor-associated Protein, or by LRP1 gene-silencing. The inhibitory activity of LRP1 in tPA-PAI1 complex-initiated cell-signaling was unanticipated given the reported role of LRP1 as an NMDA-R co-receptor in signaling responses elicited by free tPA or α₂-macroglobulin. We conclude that PAI1 functions as an inhibitor not only of the enzyme activity of tPA but also tPA receptor-mediated activities.