Binding of tissue plasminogen activator to human monocytes and monocytoid cells

Monocytes and monocytoid cell lines previously have been shown to express receptors for plasminogen and urokinase (u-PA). In the present study, the monocytoid cell lines, U937 and THP-1, are shown to bind tissue plasminogen activator (t-PA) in a specific, saturable, and reversible manner. These cells bound t-PA with low affinity (kd = 0.67 to 0.97 mumol/L) and high capacity (0.71 to 3.3 x 10(6) receptors/cell). Human peripheral blood monocytes bound t-PA with a kd (0.9 mumol/L) similar to that of the monocytoid cells but with a lower capacity (0.17 x 10(6) sites/cell). These binding parameters also were similar to the low-affinity interaction of t-PA with endothelial cells as measured with the cells in suspension (kd = 0.73 mumol/L and 1.1 x 10(6) sites/cell). Lysine analogues and active or diisopropylfluorophosphate-inactivated u-PA inhibited t-PA binding to monocytes, monocytoid cells, and endothelial cells with similar IC50 (concentration producing 50% inhibition) values, suggesting that the same recognition specificity mediates t-PA binding to all of these cell types. The existence of a high-affinity binding site for t-PA on monocytoid cells was also explored in detail. Unlike endothelial cells where plasminogen activator inhibitor-1 has been implicated in mediating a high-affinity interaction of t-PA with the cells, no evidence for a role of this inhibitor in ligand binding to the monocytoid cells was found. Furthermore, using both high and low 125I-t-PA concentrations, competition analyses with lysine analogues or u-PA, or treatment of the cells with carboxypeptidase B, failed to indicate the presence of distinguishable classes of t-PA binding sites. In sum, low-affinity receptors for t-PA are expressed at high density on monocytes and monocytoid cells, identifying a new element in the fibrinolytic arsenal of these cells.