THE EFFECT OF THROMBOSPONDIN ON INVASION OF FIBRIN GELS BY HUMAN A549LUNG-CARCINOMA

Thrombospondin (TSP) was evaluated for its effect on the capacity of h uman A549 lung adenocarcinoma cells to invade and degrade fibrin gels. Cells suspended in DMEM containing 0.01 units/mL plasminogen were add ed to a 2.5 mm diameter well in a 2 mm thick fibrin gel. Various conce ntrations of TSP were added either to the cells or to the gel. After 1 8 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, causi ng the application well to increase in diameter several-fold. In contr ast, addition of either TSP to the gel or to the cell suspension inhib ited fibrinolysis in a dose-dependent manner and promoted attachment a nd spreading of cells in the fibrin matrix. In contrast, fibronectin h ad no effect. The effect of TSP on both tumor cell-associated fibrinol ysis and cell adhesion was inhibited with an anti-TSP monoclonal antib ody. The cell adhesive peptides CSVTCG, derived from the type 1 repeat s of TSP, and GRGDS also had no effect on tumor cell-associated fibrin olysis. TSP inhibited fibrinolysis by inhibiting tumor cell-secreted u rokinase activity, but had no effect on total urokinase secreted- anti gen. In contrast, cell-associated urokinase activity was protected fro m inhibition by TSP. These results suggest that TSP may promote tumor cell metastasis not only by promoting cell-attachment but also by prot ecting tumor cell-fibrin emboli from degradation by tumor secreted- an d host fibrinolytic enzymes. Additionally, cell-associated enzymes imp ortant in matrix degradation are protected from inhibition by TSP.