Reduced Skeletal Muscle Oxidative Capacity and Elevated Ceramide but not Diacylglycerol Content in Severe Obesity

Objective: The link between a reduced capacity for skeletal muscle mitochondrial fatty acid oxidation (FAO) and lipotoxicity in human insulin resistance has been the subject of intense debate. The objective of this study was to investigate whether reduced FAO is associated with elevated acyl CoA, ceramide, and diacylglycerol (DAG) in severely obese insulin resistant subjects. Design and Methods: Muscle biopsies were conducted in lean (L, 22.6 +/- 0.5 kg/m(2), n = 8), Class I (CI, 32.1 +/- 0.4 kg/m(2), n = 7) and Class II&III obese (CII&III, 45.6 +/- 1.1 kg/m(2), n = 15) women for acyl CoA, sphingolipid and DAG profiling. Intramyocellular triglyceride (IMTG) content was determined by histology. FAO was assessed by incubating muscle homogenates with [1-14C] palmitate and measuring 14CO(2) production. Cardiolipin content was quantified as an index of mitochondrial content. Lipid metabolism proteins, DGAT1, PLIN5, and PNPLA2 were quantified in biopsy samples by western blot. Results: CII&III were more insulin resistant (HOMA-IR: 4.5 +/- 0.5 vs. 1.1 +/- 0.1, P < 0.001), and had lower FAO (similar to 58%, P = 0.007) and cardiolipin content (similar to 31%, P = 0.013) compared to L. IMTG was elevated in CI (P = 0.04) and CII&III (P = 0.04) compared to L. Sphingolipid content was higher in CII&III compared to L (13.6 +/- 1.1 vs. 10.3 +/- 0.5 pmol/mg, P = 0.031) whereas DAG content was not different among groups. DGAT1 was elevated in CII&III, and PLIN5 was elevated in CI compared to L. Conclusions: Severe obesity is associated with reduced muscle oxidative capacity and occurs concomitantly with elevated IMTG, ceramide and insulin resistance.