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

Abstract

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.

Intracellular localization of diacylglycerols and sphingolipids influences insulin sensitivity and mitochondrial function in human skeletal muscle

“…Significant correlations between C16:0 and C18:0 sphingomyelin and ceramide species suggest that sphingomyelin degradation is an important reservoir for sarcolemmal ceramide production. Strong inverse relationships between sarcolemmal sphingomyelin and insulin sensitivity suggest sphingomyelin itself may promote insulin resistance, although there is evidence to the contrary (41).…”

Ceramide dependent lipotoxicity in metabolic diseases

“…Indeed, this issue has been discussed further in a recently published Crosstalk "debate" sponsored by the Journal of Physiology where investigators took oppositional positions about the roles for ceramides as modulators of muscle function [81,82]. Despite this contention, the initial in vitro studies convincingly demonstrated that increasing ceramide content in skeletal muscle cells potently inhibits insulin signaling [42,83,84]. Conversely, pharmacological inhibition of ceramide synthesis or increased degradation of ceramides in myotubes, attenuates palmitate induced inhibition of insulin signaling [85].…”

Anti-Diabetic Effects of Zhuoduqing Formula, a Chinese Herbal Decoction, on a Rat Model of Type 2 Diabetes

“…Type 2 diabetes often associates with dyslipidemia (Tian et al 2015) which not only increases the risk of cardiovascular disorder and initiates atherosclerosis, but also aggravates insulin resistance through inhibiting insulin signaling in insulin-targeted tissues including skeletal muscles (Park et al 2016). In addition, chronic dyslipidemia causes lipid deposition and induces pancreatic lipotoxicity, thus impairing insulin secretion and promoting the progression of type 2 diabetes (Litwak et al 2015).…”