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)