ANTIGENIC AND FUNCTIONAL EXPRESSION OF TISSUE FACTOR IN ENDOTOXIN-STIMULATED U937 CELLS - REGULATION OF ACTIVITY BY CALCIUM IONOPHORE A23187

Tissue Factor (TF) is a transmembrane glycoprotein that serves as cofa ctor for Factor VII (FVII) in the initiation of blood coagulation and that is differentially expressed in a number of cell types. being cons titutively expressed in some and inducible in others. We studied the l ocalization and the functional activity of TF in monocytic leukemia U9 37 cells at different rime intervals after lipopolysaccharides (LPS) s timulation, and the effect of calcium ionophore on the surface express ed TF. Exposure of U937 cells to 10 mu g/ml LPS resulted in a time dep endent increase of TF expression that reached a maximum at 12 h for TF antigen and at 24 h for TF activity. Blocking of surface TF with inhi bitory anti-TF antibody abolished >93% of the activity of lysed cells stimulated for 24 h, while it blocked only 80% of the activity in lyse d cells stimulated for 12 h suggesting that at that time about 20% of TF is not accessible for the antibody. Even at 24 h when the specific activity of surface expressed TF is 5.5 times higher than at 12 h, thi s specific activity is still 10 fold lower than that of TF in lysed ce lls. Addition of Ca++ ionophore A23187 to LPS stimulated cells resulte d in a fast increase of TF activity that was dependent on the dose of ionophore, on the extracellular Ca++ concentration and on the time tha t the cells had been incubated with LPS. Kinetic analysis of Factor X hydrolysis by the TF/FVII complex demonstrated that the addition of io nophore resulted in an increase of both the Vmax and the apparent Km, however, without affecting the catalytic efficiency of the reaction. T hese data suggest that in U937 cells part of the TF induced by LPS is cryptic, that the specific activity of surface expressed tissue factor increases with time and that intracellular changes in Ca++ concentrat ion can be important in the regulation of the expression of TF activit y on the cell surface.