INHIBITION OF UROKINASE-TYPE PLASMINOGEN-ACTIVATOR (UPA) ABROGATES MYOGENESIS IN-VITRO

Urokinase-type plasminogen activator (uPA) is one of the components of blood's fibrinolytic cascade, uPA nets as a broad spectrum proteolyti c enzyme involved in different physio-pathological processes including cellular fibrinolysis, adhesion, migration, invasion and remodeling. Here. we present evidence that uPA participates in myogenesis, a proce ss which requires drastic cell membrane reorganization, leading to the plurinucleated myotube from the progenitor myoblast. We have dissecte d the expression of uPA throughout the different myogenic compartments and found an increase in uPA enzymatic activity associated with myotu be formation in C2C12 myoblast cells, with uPA mRNA increasing prior t he onset of fusion and differentiation, When both fusion and different iation were blocked by specific inhibitors (DMSO, cytochalasin B) the levels of uPA were strongly downregulated. This process was reversible and specific: the removal of the inhibitors immediately restored the Levels of uPA mRNA while the specific inhibition of uPA enzymatic acti vity by an anti-uPA antibody resulted in a 50% reduction of the extent of fusion and in the abrogation of muscle-specific gene products, suc h as alpha-actin and MyoD. Moreover, the conversion of fibroblasts to muscle-like cells upon acquisition of MyoD resulted in a dramatic incr ease of uPA mRNA. which was partially due to transcriptional activatio n of the uPA gene. These results indicate that the increase in uPA exp ression prior to fusion and differentiation occurs via a MyoD-mediated mechanism whereas the normal MyoD expression requires the plasminogen activation-dependent activity of this protease. Therefore. these stud ies extend the sphere of influence of myogenic factors to fibrinolysis . an intrinsic component of the hematological system. Taken together, one mechanism used by the myoblast cell to become a differentiated myo tube, involving the inductive extracellular proteolysis of urokinase, is proposed.