Differentiation-enhanced binding of the amino-terminal fragment of human urokinase plasminogen activator to a specific receptor on U937 monocytes

The purified amino-terminal fragment (ATF) of human urokinase plasminogen activator (residues 1-135), which is not required for activation of plasminogen, binds with high affinity to specific plasma membrane receptors on U937 monocytes. Intact urokinase efficiently competes for 125I-labeled ATF binding; 50% competition occurs with 1 nM urokinase. A large part of receptor-bound urokinase remains on the cell surface for at least 2 hr at 37 degrees C. Differentiation of U937 monocytes into macrophage-like cells specifically increases ATF binding 10- to 20-fold. These results suggest an important role for urokinase in monocyte/macrophage biology: the native enzyme binds to the cells with the amino-terminal domain; the catalytic, carboxyl-terminal domain remains exposed on the cell surface to stimulate localized proteolysis and facilitate cell migration.