Ceramide metabolism in oxidative and glycolytic muscle: Significance for lipid-induced insulin resistance

Altered ceramide accumulation contributes to skeletal muscle insulin resistance, but mechanisms underlying fibre-type-specific susceptibility remain unclear. We hypothesized that fibre-type-specific ceramide metabolism governs vulnerability to lipid-induced insulin resistance. Lipidomics and quantification of ceramide-pathway enzymes were performed in mouse skeletal muscles with distinct fibre-type composition (oxidative, mixed and glycolytic) from control-diet (n = 12) and high-fat-diet (HFD; n = 12) mice. In humans, lipidomics and enzyme profiling were done in vastus lateralis biopsies from 36 adults stratified into oxidative or glycolytic phenotypes; insulin sensitivity was determined by glucose tolerance testing. siRNA-mediated silencing of SGMS1 and SGMS2 followed by lipidomics probed sphingomyelin-ceramide cycling in human myoblasts. In mouse muscle, ceramide composition rather than total content, differed by fibre type: oxidative muscle was enriched in very-long-chain ceramides, whereas glycolytic and mixed muscles contained higher C18-ceramides, paralleled by fibre-type-specific expression of enzymes involved in de novo synthesis and sphingomyelin-ceramide cycling. HFD induced ceramide remodelling, with C18-ceramides accumulating in oxidative and mixed muscles and very-long-chain species decreasing in glycolytic muscle; among all assessed enzymes, only SGMS2 was significantly downregulated in oxidative muscle. In humans, an oxidative phenotype associated with higher very-long-chain ceramides and insulin sensitivity, whereas a glycolytic phenotype displayed higher C16-18 ceramides, higher SGMS1 and SMPD2 expression, and lower insulin sensitivity. Elastic net regression identified C16-18 ceramides and galactosylceramides as negative predictors of insulin sensitivity. SGMS2 silencing caused broader ceramide accumulation than SGMS1 silencing, supporting a central role for SGMS2-mediated sphingomyelin-ceramide cycling in limiting ceramide burden.