Muscle satellite cell activation following injury is essential for muscle repair, and hepatocyte growth factor/scatter factor (HGF) was the first growth factor shown to be able to stimulate activation and early division of adult satellite cells in culture and in muscle tissue. In addition, HGF was shown to be present in uninjured and injured skeletal muscle. Experiments in this report demonstrate that cultured satellite cells also synthesize and secrete HGF. Reverse transcription-polymerase chain reaction (RT-PCR) was used to demonstrate the presence of HGF mRNA in cultured adult satellite cells as early as 12 h from the time of plating. Message content was detectable at early times in culture and appeared to increase between 36 and 48 h. HGF protein expression was demonstrated during this time period by immunofluorescence localization; HGF was localized to mononucleated cells and multinucleated myotubes. HGF message was not detectable in muscle-derived fibroblast clones, and fibroblast-like cells in satellite cell cultures were negative for HGF by immunofluorescence analysis. Furthermore, Western blot analysis revealed the presence of HGF in satellite cell culture conditioned medium, associated with the cell surface and inside cells. Finally, the addition of neutralizing HGF antibodies during the proliferation phase in culture (42-90 h) significantly reduced cell proliferation. These experiments indicate that HGF is expressed by cultured satellite cells and that endogenous HGF from satellite cells can act in an autocrine fashion.
Because HGF plays a central role in satellite cell activation, it is likely that direct administration of HGF into damaged muscle may represent a potentially useful approach for stimulating muscle repair. This approach may also be useful in enhancing the efficiency of myoblast transplantation in vivo.