Urokinase-type plasminogen activator up-regulates its own expression by endothelial cells and monocytes via the u-PAR pathway

Signal transduction by urokinase-type plasminogen activator (u-PA) bound to its cell receptor has been well established. In the present study, we foun d, for the first time to our knowledge, that u-PA promotes its own synthesi s by endothelial cells and monocytes. This phenomenon was characterized and shown to involve the u-PA receptor (u-PAR) pathway. The finding may be of general importance, since most cells that express u-PAR also produce u-PA. Human umbilical vein endothelial cells (HUVECs), U937 monocytes, and human peripheral blood monocytes (PFMCs) were incubated with diisopropylfluoropho sphate (DFP)-pretreated u-PA, the amino-terminal fragment (ATF) of u-PA, or the kringle domain. A threefold up-regulation of u-PA secretion and synthe sis by u-PA or ATF was found. The predominant effect was expressed in HUVEC s, in which u-PA mRNA was also up-regulated. The u-PA kringle domain had no effect on u-PA synthesis, leading to the conclusion that the EGF domain wa s responsible. This was also consistent with the additional finding that th e u-PAR, to which the EGF domain binds, was necessary for the up-regulation . The results indicate that u-PA up-regulates itself via its EGF domain and u-PAR. The possibilities that the results were related to displacement of receptor-bound u-PA or the blocking of u-PA incorporation into the cells we re excluded. A modest up-regulation of u-PAR was also associated with this phenomenon. (C) 2001 Elsevier Science Ltd. All rights reserved.