Lipotoxicity caused by excessive fat deposition in skeletal muscle cells is a characteristic of type II diabetes, heart disease and obesity. Defining how glucose stimulates this is therefore an important goal. Isabelle Guillet-Deniau and co-workers have now dissected the signalling mechanisms involved, using contracting myotubes derived from cultured muscle satellite cells (see ). They show that glucose treatment stimulates cells to take up glucose and upregulate expression of lipogenic enzymes. The authors then demonstrate that prior to this the cells synthesize and activate sterol-regulatory-element-binding protein 1c (SREBP-1c), a transcription factor that regulates cholesterol and fatty acid metabolism. Moreover, they show that knocking down SREBP-1c by RNAi blocks glucose-induced upregulation of lipogenic enzymes. Guillet-Deniau and co-workers go on to demonstrate that stimulation of SREBP-1c requires the JAK2/STAT3 signalling pathway but is independent of insulin, which is significant given that insulin can also activate SREBP-1c. Their results thus implicate SREBP-1c-dependent stimulation of lipogenesis by glucose in lipotoxicity. They also emphasize the effectiveness of RNAi in a biologically relevant muscle model.
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