Urokinase-type plasminogen activator-induced monocyte adhesion requires a carboxyl-terminal lysine and cAMP-dependent signal transduction

Urokinase-type plasminogen activator (u-PA) or its amino-terminal fragment (ATF) containing the u-PA receptor (u-PAR) binding domain, is known to promote monocyte adhesion. In the present study, U937 monocyte adhesion to a plastic surface was used to investigate the mechanism of its promotion by u-PA and ATF. Adhesion was found to be inhibited by cycloheximide or actinomycin D, implicating protein synthesis and gene expression in u-PA-induced monocyte adhesion. Adhesion was prevented by 2'-deoxyadenosine 3'-monophosphate, indicating that a cAMP-dependent pathway of signal transduction was involved, This concept was supported by the complementary finding that u-PA-induced adhesion was greatly promoted by forskolin, cholera toxin, or 8-bromo-cAMP, which by themselves induced little adhesion. Furthermore, similar to many other cAMP-dependent activities, cGMP diminished u-PA-induced adhesion. When u-PA or ATF was treated with immobilized carboxypeptidase B, its proadhesive effect was abolished, implicating the involvement of carboxyl-terminal lysine residues (Lys(158) on u-PA and Lys(135) on ATF). Moreover, when a carboxyl-terminal lysine analog was added, the proadhesive effect of carboxypeptidase B-treated u-PA or ATF was restored. In conclusion, the present study indicates that u-PA or ATF-induced monocyte adhesion involves cAMP-dependent signal transduction, which is triggered by u-PAR binding. It is also critically dependent on the presence of a carboxyl-terminal lysine.