Role of Urokinase Plasminogen Activator Receptor in Thrombospondin 1-Mediated Tumor Cell Invasion

We previously showed that thrombospondin 1 (TSP-1) upregulates the plasminogen/plasmin system and promotes breast tumor cell invasion. Preliminary data from our laboratory using neutralizing antibodies suggested that the upregulation in breast tumor cell invasion seen in response to TSP-1 involved the urokinase plasminogen activator receptor (uPAR). To confirm these findings in MDA-MB-231 breast cancer cells, we developed three other strategies to study the role of uPAR in tumor cell adhesion and TSP-1-mediated tumor cell invasion: (a) enzymatic cleavage of uPAR with glycosylphosphatidylinositol-specific phospholipase C; (b) inhibition at the mRNA level with a uPAR antisense construct (cells named LKAS-MDA); (c) inhibition of plasminogen binding with the lysine analogue ϵ-aminocaproic acid. Adhesion to laminin and type I and type IV collagen with and without the addition of ϵ-aminocaproic acid was studied. Tumor cell invasion was studied in a modified Boyden chamber collagen invasion assay. Antisense uPAR inhibition decreased uPAR expression by 48–66% and cell-associated urokinase plasminogen activator (uPA) by 30–68%. Additionally, antisense uPAR inhibition induced a 68–70% reduction in uPA and plasmin activities. Antisense uPAR transfection increased tumor cell adhesion by 46–53%. A similar effect was observed in ϵ-aminocaproic acid-treated MDA-MB-231 cells. TSP-1-mediated tumor cell invasion was almost completely inhibited by either antisense uPAR inhibition or treatment with phospholipase C or ϵ-aminocaproic acid. We conclude that uPAR plays a crucial role in the regulation of tumor cell adhesion and TSP-1-mediated tumor cell invasion.