Insulin resistance is associated with higher intramyocellular triglycerides in type I but not type II myocytes concomitant with higher ceramide content

Abstract

Objective: We tested the primary hypotheses that sphingolipid and diacylglycerol (DAG) content is higher within insulin-resistant muscle and that the association between intramyocellular triglycerides (IMTG) and insulin resistance is muscle fiber type specific.

Research design and methods: A nested case-control analysis was conducted in 22 obese (BMI >30 kg/m(2)) women who were classified as insulin-resistant (IR; n = 12) or insulin-sensitive (IS; n = 10), determined by hyperinsulinemic-euglycemic clamp (>30% greater in IS compared with IR, P < 0.01). Sphingolipid and DAG content was determined by high-performance liquid chromatography-tandem mass spectrometry. Fiber type-specific IMTG content was histologically determined. Gene expression was determined by quantitative PCR.

Results: Total (555 +/- 53 vs. 293 +/- 54 pmol/mg protein, P = 0.004), saturated (361 +/- 29 vs. 179 +/- 34 pmol/mg protein, P = 0.001), and unsaturated (198 +/- 29 vs. 114 +/- 21 pmol/mg protein, P = 0.034) ceramides were higher in IR compared with IS. DAG concentrations, however, were similar. IMTG content within type I myocytes, but not type II myocytes, was higher in IR compared with IS subjects (P = 0.005). Insulin sensitivity was negatively correlated with IMTG within type I myocytes (R = -0.51, P = 0.026), but not with IMTG within type II myocytes. The proportion of type I myocytes was lower (41 vs. 59%, P < 0.01) in IR subjects. Several genes involved in lipid droplet and fatty acid metabolism were differentially expressed in IR compared with IS subjects.

Conclusions: Human skeletal muscle insulin resistance is related to greater IMTG content in type I but not type II myocytes, to greater ceramide content, and to alterations in gene expression associated with lipid metabolism.

FIG. 1.

Sphingolipid (A) and diacylglycerol (B) content in vastus lateralis of IR and IS subjects. IR data are expressed relative to IS group, which was set to a value of 1. *Significantly different from IS group, P < 0.05. sat, saturated; unsat, unsaturated.

FIG. 2.

Myocyte-specific IMTG content in vastus lateralis of IR and IS groups. A: Representative image of Oil red O stain (20× objective). B: Representative image of immunostain for fiber type (20× objective). C: Pearson correlation of R_d with type I myocyte–specific IMTG content. White data points = IS subjects; black data points = IR subjects. D: Pearson correlation of R_d with type II myocyte–specific IMTG content. White data points = insulin sensitive; black data points = insulin resistant. E: IMTG content in type I myocytes, type II myocytes, and total IMTG in vastus lateralis from IR and IS groups. *Significantly different from IS, P < 0.05. AU, arbitrary units. (A high-quality color digital representation of this figure is available in the online issue.)

FIG. 3.

Gene expression in vastus lateralis of IR and IS subgroups. IR gene expression is expressed relative to IS group, which was set to a value of 1. ■, IS group; □, IR group. ABHD5, abhydrolase domain containing 5; ACAC, acetyl-CoA carboxylase; ASAH, N-acylsphingosine amidohydrolase; CERK, ceramide kinase; CPT, carnitine palmitoyltransferase; MLYCD, malonyl-CoA decarboxylase; NRF, nuclear respiratory factor; S3-12, plasma membrane associated protein KIAA1881; SPTLC, serine palmitoyltransferase, long-chain base. *Significantly different from IS group, P < 0.05.