EVIDENCE OF A NONCONVENTIONAL ROLE FOR THE UROKINASE TRIPARTITE COMPLEX (UPAR UPA/PAI-1) IN MYOGENIC CELL-FUSION/

Urokinase can form a tripartite complex binding urokinase receptor (uP AR) and plasminogen activator inhibitor type-1 (PAI-1), a component of the extracellular matrix (ECM). The components of the tripartite comp lex are modulated throughout the in vitro myogenic differentiation pro cess. A series of experiments aimed at elucidating the role of the uro kinase tripartite complex in the fusion of human myogenic cells were p erformed in vitro. Myogenic cell fusion was associated with increased cell-associated urokinase-type plasminogen activator (uPA) activity, c ell-associated uPAR, and uPAR occupancy. Incubation of cultures with e ither uPA anticatalytic antibodies, or the amino-terminal fragment of uPA (ATF), which inhibits competitively uPA binding to its receptor, o r anti-PAI-l antibodies, which inhibit uPA binding to PAI-1, resulted in a 30 to 47% decrease in fusion. Incubation of cultures with the pla smin inhibitor aprotinin did not affect fusion. Decreased fusion rates induced by interfering with uPAR/uPA/PAI-1 interactions were not asso ciated with significant changes in mRNA levels of both the myogenic re gulatory factor myogenin and its inhibitor of DNA binding, Id. Incubat ion of cultures with purified uPA resulted in a decrease in fusion, li kely due to a competitive inhibition of PAI-1 binding of endogenous uP A. We conclude that muscle cell fusion largely depends on interactions between the members of the urokinase complex (uPAR/uPA/PAI-1), but do es not require proteolytic activation of plasmin. Since the intrinsic muscle cell differentiation program appears poorly affected by the sta te of integrity of the urokinase complex, and since cell migration is a prerequisite for muscle cell fusion in vitro, it is likely that the urokinase system is instrumental in fusion through its connection with the cell migration process. Our results suggest that the urokinase tr ipartite complex may be involved in cell migration in a non convention al way, playing the role of an adhesion system bridging cell membrane to ECM.