DIFFERENTIAL REGULATION OF UROKINASE-TYPE PLASMINOGEN-ACTIVATOR EXPRESSION BY BASIC FIBROBLAST GROWTH-FACTOR AND SERUM IN MYOGENESIS - REQUIREMENT OF A COMMON MITOGEN-ACTIVATED PROTEIN-KINASE PATHWAY

The broad spectrum protease urokinase-type plasminogen activator (uPA) has been implicated in muscle regeneration in vivo as well as in myog enic proliferation and differentiation in vitro. These processes are k nown to be modulated by basic fibroblast growth factor (FGF-P) and ser um. We therefore investigated the mechanism(s) underlying the regulati on of uPA expression by these two stimuli in proliferating and differe ntiating myoblasts. The expression of uPA mRNA and the activity of the uPA gene product were induced by FGF-2 and serum in proliferating myo blasts. uPA induction occurred at the level of transcription and requi red the uPA-PEA3/AP1 enhancer element, since deletion of this site in the full promoter abrogated induction by FGF-S and serum. Using L6E9 s keletal myoblasts, devoid of endogenous FGF receptors, which have been engineered to express either FGF receptor-1 (FGFR1) or FGF receptor-4 (FGFR4), we have demonstrated that both receptors, known to be expres sed in skeletal muscle cell precursors, were able to mediate uPA induc tion by FGF-2, whereas serum stimulation was FGF receptor-independent. The induction of uPA by FGF-2 and serum in FGFR1- and in FGFR4-expres sing myoblasts required the mitogen-activated protein kinase pathway, since treatment of cells with a specific inhibitor of the mitogen-acti vated protein kinase/extracellular signal-regulated kinase-2 kinase, P D98059, blocked uPA promoter induction. Although FGF-S and serum induc ed uPA in proliferating myoblasts, their actions on cell-cell contact- induced differentiating myoblasts differed dramatically. FGF-S, but no t serum, repressed uPA expression in differentiation-committed myoblas ts, and these effects were also shown to occur at the level of uPA tra nscription. Altogether, these results indicate a dual regulation of th e uPA gene by FGF-P and serum, which ensures uPA expression throughout the whole myogenic process in different myoblastic lineages. The effe cts of FGF-2 and serum on uPA expression may contribute to the proteol ytic activity required during myoblast migration and fusion, as well a s in muscle regeneration.