Sphingolipid Involvement in Insulin Sensitivity in C2C12 Myotubes

Elevated plasma free fatty acids (FFA) have been shown to contribute to insulin insensitivity in skeletal muscle partially via their intracellular conversion to sphingolipids. To investigate the mechanisms by which FFA mediate insulin insensitivity in skeletal muscle, C2C12 myotubes were grown in the presence of 75mM palmitic acid, leading to increased cellular sphingolipid content over 5 days. Interestingly, inhibition of sphingolipid production decreased glycogen synthase 1 mRNA levels in the myotubes by over 50%, indicating a role for sphingolipids in mediating a key component of insulin action. On the other hand, FFA treatment significantly reduced insulin‐induced ERK phosphorylation at early time points (24h), and after 5 days, reduced insulin‐stimulated glycogen synthesis. Importantly, both of these effects were ameliorated by inhibition of sphingolipid production. These results indicate that sphingolipid involvement in skeletal muscle function is multiplex and can act both positively and negatively to mediate insulin responsiveness. Recently developed LC/MS techniques for sphingolipid quantification are being used in conjunction with microarrays to decipher the complex roles of sphingolipids in mediating cellular responses to insulin.