Binding of tissue plasminogen activator to human monocytes and monocytoid cells

Abstract

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.

Plasminogen on the surfaces of fibrin clots prevents adhesion of leukocytes and platelets

“…Furthermore, plasma levels of t-PA are low (1-9 ng mL )1 ) under normal physiologic conditions [33,34] and become high only after venous occlusion or after the administration of agents that induce its acute secretion from endothelial cells [34]. As isolated blood monocytes and platelets bind t-PA with low affinity (K d values in the range 0.3-0.9 lM) [35,36]) these cells are unlikely to bind t-PA under basal conditions and/or retain it after cell isolation. It has been proposed that acute release of t-PA from endothelial cells near the forming thrombus may result in a local concentration that is much higher than the basal level of t-PA and also than that of its inhibitor plasminogen activator inhibitor-1 (PAI-1), and will therefore accelerate thrombolysis [34].…”

Regulation of plasminogen activator inhibitor‐1 secretion by urokinase and tissue plasminogen activator in rat epithelioid‐type smooth muscle cells

“…tPA, unlike uPA, appears to have not just a single receptor type. Instead, it binds to a variety of cell surface molecules, all of which are not known for their signal transducing capacity (Felez et al , 1991; Hajjar, 1991; Cesarman et al , 1994; Ellis & Whawell, 1997). In spite of the absence of a specific receptor similar to uPA receptor, tPA has been found to act as a growth factor for VSMCs and fibroblasts (De Petro et al , 1994; Herbert et al , 1994), although these studies did not identify any tPA signalling mechanisms.…”