Ceramides Contained in LDL Are Elevated in Type 2 Diabetes and Promote Inflammation and Skeletal Muscle Insulin Resistance

Dysregulated lipid metabolism and inflammation are linked to the development of insulin resistance in obesity, and the intracellular accumulation of the sphingolipid ceramide has been implicated in these processes. Here, we explored the role of circulating ceramide on the pathogenesis of insulin resistance. Ceramide transported in LDL is elevated in the plasma of obese patients with type 2 diabetes and correlated with insulin resistance but not with the degree of obesity. Treating cultured myotubes with LDL containing ceramide promoted ceramide accrual in cells and was accompanied by reduced insulin-stimulated glucose uptake, Akt phosphorylation, and GLUT4 translocation compared with LDL deficient in ceramide. LDL-ceramide induced a proinflammatory response in cultured macrophages via toll-like receptor–dependent and –independent mechanisms. Finally, infusing LDL-ceramide into lean mice reduced insulin-stimulated glucose uptake, and this was due to impaired insulin action specifically in skeletal muscle. These newly identified roles of LDL-ceramide suggest that strategies aimed at reducing hepatic ceramide production or reducing ceramide packaging into lipoproteins may improve skeletal muscle insulin action.Obesity is associated with the development of chronic diseases, including dyslipidemia, nonalcoholic fatty liver disease, type 2 diabetes, and atherosclerosis. Insulin resistance is a central feature of the pathophysiology of these disorders and is defined as a subnormal response of tissues to the actions of insulin, resulting in decreased glucose uptake into skeletal muscle and impaired suppression of glucose production by the liver. Although the mechanisms responsible for the development of insulin resistance are not fully defined, there is compelling evidence that defective lipid metabolism (1) and consequent subclinical inflammation (2) plays a causative role.Dyslipidemia resulting from overnutrition and defective adipocyte lipolysis is postulated to be a major contributor to liver and skeletal muscle insulin resistance, at least in part by promoting the intracellular accumulation of lipid metabolites that impair insulin signal transduction (1). Ceramide has been postulated as a primary lipid mediator of skeletal muscle insulin resistance based on findings that intracellular ceramide is elevated in insulin-resistant states (3–5) and that pharmacological inhibition of de novo ceramide synthesis enhances insulin action in insulin-resistant rodents (6). Ceramide induces insulin resistance by inhibiting insulin signal transduction, principally at Akt (7), and possibly via activation of serine/threonine kinases such as Jun NH₂-terminal kinase (JNK) (8), which in turn inhibits activation of insulin receptor substrate proteins (9). Ceramide is also postulated to activate proinflammatory pathways in macrophages, perhaps via amplification of toll-like receptor 4 (TLR4)–mediated inflammation (10,11). Interestingly, activation of TLR4 can increase ceramide levels in macrophages (12), supporting a model whereby ceramide can both induce and amplify macrophage inflammation. Consequently, the activation of tissue-resident macrophages would be predicted to promote a proinflammatory milieu that impairs insulin action.Although the consequences of tissue ceramide accumulation have been extensively studied over the last decade, it is now apparent that ceramides are also increased in the plasma of obese, type 2 diabetic mice (6,13) and humans (14), and that weight loss induced by gastric bypass surgery (15) or lifestyle modification (16) reduces plasma ceramide. Clinical data indicate that circulating ceramides correlate with systemic insulin resistance and inflammation (14,17), and pharmacological inhibition of whole-body ceramide synthesis in obese mice decreases plasma ceramide, reduces inflammatory parameters, and improves insulin action (6,18). Notably, not all studies report an association between obesity/diabetes and elevated circulating ceramide levels (19–21), and to date, there is no evidence for a direct effect of circulating ceramide on peripheral insulin action and inflammation. In the current study, we show that ceramides contained in LDL are elevated in type 2 diabetes and establish a link between LDL-ceramide, skeletal muscle insulin resistance, inflammation, and impaired systemic insulin action.