Several FGF family members are expressed in skeletal muscle; however, the roles of these factors in skeletal muscle development are unclear. We examined the RNA expression, protein levels, and biological activities of the FGF family in the MM14 mouse skeletal muscle cell line. Proliferating skeletal muscle cells express FGF-1, FGF-2, FGF-6, and FGF-7 mRNA. Differentiated myofibers express FGF-5, FGF-7, and reduced levels of FGF-6 mRNA. FGF-3, FGF-4, and FGF-8 were not detectable by RT-PCR in either proliferating or differentiated skeletal muscle cells. FGF-I and FGF-2 proteins were present in proliferating skeletal muscle cells, but undetectable after terminal differentiation. We show that transfection of expression constructs encoding FGF-1 or FGF-2 mimics the effects of exogenously applied FGFs, inhibiting skeletal muscle cell differentiation and stimulating DNA synthesis. These effects require activation of an FGF tyrosine kinase receptor as they are blocked by transfection of a dominant negative mutant FGF receptor. Transient transfection of cells with FGF-1 or FGF-2 expression constructs exerted a global effect on myoblast DNA synthesis, as greater than 50% of the nontransfected cells responded by initiating DNA synthesis. The global effect of cultures transfected with FGF-2 expression vectors was blocked by an anti-FGF-2 monoclonal antibody, suggesting that FGF-2 was exported from the transfected cells. Despite the fact that both FGF-l and FGF-2 lack secretory signal sequences, when expressed intracellularly, they regulate skeletal muscle development. Thus, production of FGF-1 and FGF-2 by skeletal muscle cells may act as a paracrine and autocrine regulator of skeletal muscle development in vivo.

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