The effect of thrombospondin on invasion of fibrin gels by human A549 lung carcinoma

: Thrombospondin (TSP) was evaluated for its effect on the capacity of human A549 lung adenocarcinoma cells to invade and degrade fibrin gels. Cells suspended in DMEM containing 0.01 units/mL plasminogen were added to a 2.5 mm diameter well in a 2 mm thick fibrin gel. Various concentrations of TSP were added either to the cells or to the gel. After 18 hours, the number of spread and gel-adherent cells were counted and the diameter of the well was measured to determine the extent of tumor-induced fibrinolysis. In the absence of TSP, the tumor cells were non-adherent but catalyzed the rapid degradation of the fibrin gel, causing the application well to increase in diameter several-fold. In contrast, addition of either TSP to the gel or to the cell suspension inhibited fibrinolysis in a dose-dependent manner and promoted attachment and spreading of cells in the fibrin matrix. In contrast, fibronectin had no effect. The effect of TSP on both tumor cell-associated fibrinolysis and cell adhesion was inhibited with an anti-TSP monoclonal antibody. The cell adhesive peptides CSVTCG, derived from the type 1 repeats of TSP, and GRGDS also had no effect on tumor cell-associated fibrinolysis. TSP inhibited fibrinolysis by inhibiting tumor cell-secreted urokinase activity, but had no effect on total urokinase secreted-antigen. In contrast, cell-associated urokinase activity was protected from inhibition by TSP. These results suggest that TSP may promote tumor cell metastasis not only by promoting cell-attachment but also by protecting tumor cell-fibrin emboli from degradation by tumor secreted- and host fibrinolytic enzymes.(ABSTRACT TRUNCATED AT 250 WORDS)