A Role for Ceramides, but Not Sphingomyelins, as Antagonists of Insulin Signaling and Mitochondrial Metabolism in C2C12 Myotubes

The accumulation of sphingolipids in obesity leads to impairments in insulin sensitivity and mitochondrial metabolism, but the precise species driving these defects is unclear. We have modeled these obesity-induced effects in cultured C2C12 myotubes, using BSA-conjugated palmitate to increase synthesis of endogenous sphingolipids and to inhibit insulin signaling and oxidative phosphorylation. Palmitate (a) induced the accumulation of sphingomyelin (SM) precursors such as sphinganine, dihydroceramide, and ceramide; (b) inhibited insulin stimulation of a central modulator of anabolic metabolism, Akt/PKB; (c) inhibited insulin-stimulated glycogen synthesis; and (d) decreased oxygen consumption and ATP synthesis. Under these conditions, palmitate failed to alter levels of SMs, which are the most abundant sphingolipids, suggesting that they are not the primary intermediates accounting for the deleterious palmitate effects. Treating cells with a pharmacological inhibitor of SM synthase or using CRISPR to knock out the Sms2 gene recapitulated the palmitate effects by inducing the accumulation of SM precursors and impairing insulin signaling and mitochondrial metabolism. To profile the sphingolipids that accumulate in obesity, we performed lipidomics on quadriceps muscles from obese mice with impaired glucose tolerance. Like the cultured myotubes, these tissues accumulated ceramides but not SMs. Collectively, these data suggest that SM precursors such as ceramides, rather than SMs, are likely nutritional antagonists of metabolic function in skeletal muscle.