REGULATION OF MYOGENESIS BY FIBROBLAST GROWTH-FACTORS REQUIRES BETA-GAMMA-SUBUNITS OF PERTUSSIS-TOXIN-SENSITIVE G-PROTEINS

Terminal differentiation of skeletal muscle cells in culture is inhibi ted by a number of different growth factors whose subsequent intracell ular signaling events are poorly understood. In this study, we have in vestigated the role of heterotrimeric G proteins in mediating fibrobla st growth factor (FGF)-dependent signals that regulate myogenic differ entiation, Pertussis toxin, which ADP-ribosylates and inactivates susc eptible G proteins, promotes terminal differentiation in the presence of FGF-2, suggesting that G alpha or G beta gamma subunits or both are involved in transducing the FGF-dependent signal(s) that inhibits myo genesis. We found that G beta gamma subunits are likely to be involved since the expression of the C terminus of beta-adrenergic receptor ki nase 1, a G beta gamma subunit-sequestering agent, promotes differenti ation in the presence of FGF-2, and expression of the free G beta gamm a dimer can replace FGF-2, rescuing cells from pertussis toxin-induced differentiation, addition of pertussis toxin also blocked FGF-2-media ted activation of mitogen-activated protein kinases (MAPKs). Ectopic e xpression of dominant active mutants in the Ras/MAPK pathway rescued c ells from pertussis toxin-induced terminal differentiation, suggesting that the G beta gamma subunits act upstream of the Ras/MAPK pathway. It is unlikely that the pertussis toxin-sensitive pathway is activated by other, as Set unidentified FGF receptors since PDGF (platelet-deri ved growth factor)-stimulated MM14 cells expressing a chimeric recepto r containing the FGF receptor-1 intracellular domain and the PDGF rece ptor extracellular domain were sensitive to pertussis toxin, Our data suggest that FGF-mediated signals involved in repression of myogenic d ifferentiation are transduced by a pertussis toxin-sensitive G-protein -coupled mechanism. This signaling pathway requires the action of G be ta gamma subunits and activation of MAPKs to repress skeletal muscle d ifferentiation.