Hepatocyte growth factor activates quiescent skeletal muscle satellite cells in vitro

The effect of hepatocyte growth factor (HGF) on the activation of quiescent rat skeletal muscle satellite cells was evaluated in vitro. Satellite cells from 9-month-old adult rats are quiescent in vivo and when cultured, display a protracted lag phase prior to division that is not present in satellite cells from neonatal or regenerating muscle. Under normal growth conditions, satellite cells divide for the first time between 42 and 60 hr. Hepatocyte growth factor increased proliferation in a dose-dependent fashion prior to 48 hr with half-maximal stimulation at approximately 3 ng/ml; in addition, heparin enhanced this activity. The time course of cyclin-D1 and proliferating cell nuclear antigen (PCNA) expression was accelerated in HGF-treated satellite cells, indicating that cells entered the cell cycle earlier. No significant effects on muscle-derived fibroblast proliferation was observed. The signalling receptor for HGF is the product of the c-met protooncogene, and rtPCR analysis of satellite cells 0-72 hr in culture demonstrated the presence of this message throughout this time period. The presence of c-met in quiescent satellite cells, the ability of HGF to stimulate precocious entry into the cell cycle, and the previously described localization of HGF message in regenerating muscle (Jennische et al., 1993) indicate that HGF could act as an activator of quiescent satellite cells in vivo.