Cellular signalling in the regulation of skeletal muscle growth
The aim of this study was to investigate the involvement of phospholipid-derived second messengers in protein synthesis and satellite cell proliferation and to assess the suitability of the mouse C2C12 satellite cell line as a model for skeletal muscle. The protein kinase C (PKC) activator, phorbol-12-myristate-13-acetate (PMA) increased translation and phospholipase D (PLD) activity in both C 2C12 myoblasts and isolated extensor digitorum longus (EDL) muscle. Exogenous PLD also stimulated translation implying a link between the two events. Insulin increased translation in both models. The response in EDL muscle was mediated through PKC and cyclo-oxygenase metabolites, whilst that in myoblasts was regulated by phosphatidylinositol-3-kinase (PI 3-kinase). However, insulin, PMA and PLD had no effect on the rate of translation (or transcription) in C2C12 cells after they had fused to form multinucleated myotubes. C2C12 myoblasts responded to basic Fibroblast Growth Factor (bFGF), Epidermal Growth Factor (EGF), PMA and exogenous PLD with increases in RNA accretion and DNA synthesis However, bFGF and EGF had no effect on PLD demonstrating the presence of both PLD-dependent and-independent pathways in these cells. The data suggest that although C2C12 cells may be of use in the study of satellite cell hyperplasia, they do not appear to be a good model to investigate the insulin mediated signalling pathways regulating protein synthesis in skeletal muscle.