Caspase-3, myogenic transcription factors and cell cycle inhibitors are regulated by leukemia inhibitory factor to mediate inhibition of myogenic differentiation

LIF dependent inhibition of differentiation appeared to involve several mechanisms. Differentiating myoblasts that were exposed to LIF displayed increased transcripts for c-fos. Transcripts for the cell cycle inhibitor p21 as well as muscle regulatory factors myoD and myogenin were decreased with LIF exposure. However, LIF did not directly induce a proliferative effect under differentiation conditions, but did prevent the proportion of myoblasts that were proliferating from decreasing as differentiation proceeded. LIF stimulation decreased the percentage of cells positive for active caspase-3 occurring during differentiation. Both the effect of LIF inhibiting caspase-3 activation and differentiation appeared dependent on mitogen activated protein kinase and extracellular signal regulated kinase kinase (MEK) signalling. The role of LIF in myogenic differentiation was further refined to demonstrate that myoblasts are unlikely to secrete LIF endogenously.Altogether this study provides a more comprehensive view of the role of LIF in myogenic differentiation including LIF and receptor regulation in myoblasts and myotubes, mechanisms of inhibition of differentiation and the link between caspase-3 activation, apoptosis and myogenic differentiation.Myogenic differentiation is a critical process for the development and homeostasis of muscle tissue. Myogenesis, the formation of muscle cell syncytia, occurs during embryonic development and in cases of muscle injury. When myofibers are damaged by stimuli such as mechanical stress, or loss of neurotrophic support, they regenerate by activation and proliferation of the normally quiescent resident satellite cell population [1]. Proliferating satellite cells, termed myoblasts, subsequently differentiate and fuse to create myotubes which can mature into functional myofibers. These mono-nucleated muscle progenitor cells differentiate by inducing the transcriptional activity of basic-helix-loop-helix transcription factors such as myoD