Endurance Runners with Intramyocellular Lipid Accumulation and High Insulin Sensitivity Have Enhanced Expression of Genes Related to Lipid Metabolism in Muscle

Abstract

Context: Endurance-trained athletes have high oxidative capacities, enhanced insulin sensitivities, and high intracellular lipid accumulation in muscle. These characteristics are likely due to altered gene expression levels in muscle.

Design and setting: We compared intramyocellular lipid (IMCL), insulin sensitivity, and gene expression levels of the muscle in eight nonobese healthy men (control group) and seven male endurance athletes (athlete group). Their IMCL levels were measured by proton-magnetic resonance spectroscopy, and their insulin sensitivity was evaluated by glucose infusion rate (GIR) during a euglycemic-hyperinsulinemic clamp. Gene expression levels in the vastus lateralis were evaluated by quantitative RT-PCR (qRT-PCR) and microarray analysis.

Results: IMCL levels in the tibialis anterior muscle were approximately 2.5 times higher in the athlete group compared to the control group, while the IMCL levels in the soleus muscle and GIR were comparable. In the microarray hierarchical clustering analysis, gene expression patterns were not clearly divided into control and athlete groups. In a gene set enrichment analysis with Gene Ontology gene sets, "RESPONSE TO LIPID" was significantly upregulated in the athlete group compared with the control group. Indeed, qRT-PCR analysis revealed that, compared to the control group, the athlete group had 2-3 times higher expressions of proliferator-activated receptor gamma coactivator-1 alpha (PGC1A), adiponectin receptors (AdipoRs), and fatty acid transporters including fatty acid transporter-1, plasma membrane-associated fatty acid binding protein, and lipoprotein lipase.

Conclusions: Endurance runners with higher IMCL levels have higher expression levels of genes related to lipid metabolism such as PGC1A, AdipoRs, and fatty acid transporters in muscle.

Keywords: adiponectin receptor; athlete’s paradox; insulin resistance; intramyocellular lipid.

Figure 1

Absolute intramyocellular lipid (IMCL) and glucose infusion rate (GIR) levels in the control and athlete groups. Absolute values of GIR and IMCL in the control group (Con) and athlete group (Ath). The average in each group is shown as closed circle. * p < 0.05 Intramyocellular lipid, IMCL; tibialis anterior, TA; soleus, SOL; methylene signal intensity, S-fat; creatine signal, Cre.

Figure 2

Hierarchical clustering of microarray data in the control and athlete groups. Euclidean distance was used as the metric. The tree was constructed using Ward’s method. Ath1–Ath5, Endurance athletes (n = 5); Con1–Con8 Control subjects (n = 8).

Figure 3

Microarray findings in the control and athlete groups. (A) Volcano plot comparing gene expression in the control versus athlete group. The x-axis indicates differential expression profiles plotted in fold-induction ratios on the log₂ scale. The y-axis indicates the statistical significance of the difference in expression on the log₁₀ scale. Genes upregulated in the athlete group are shown in red. Genes downregulated in the athlete group are shown in blue. (B) The top 20 significantly upregulated Gene Ontology gene sets (c5.all.v2.5) based on GSEA in the control group. (C) The top 20 significantly upregulated Gene Ontology gene sets (c5.all.v2.5) based on GSEA in the athlete group. Gene set enrichment analysis; GSEA, Normalized enrichment score; NES.

Figure 3

Microarray findings in the control and athlete groups. (A) Volcano plot comparing gene expression in the control versus athlete group. The x-axis indicates differential expression profiles plotted in fold-induction ratios on the log₂ scale. The y-axis indicates the statistical significance of the difference in expression on the log₁₀ scale. Genes upregulated in the athlete group are shown in red. Genes downregulated in the athlete group are shown in blue. (B) The top 20 significantly upregulated Gene Ontology gene sets (c5.all.v2.5) based on GSEA in the control group. (C) The top 20 significantly upregulated Gene Ontology gene sets (c5.all.v2.5) based on GSEA in the athlete group. Gene set enrichment analysis; GSEA, Normalized enrichment score; NES.

Figure 4

Gene expression analysis of skeletal muscle in the control and athlete groups using qRT-PCR. (A) Expression levels of fatty acid transporter in the control group (white bars) and the athlete group (black bars). Data are relative to the expression level in the control group, which was set to 1. (B) Expression levels of genes related to fatty acid β-oxidation in the control group (white bars) and the athlete group (black bars). Data are relative to the expression level in the control group, which was set to 1. * p < 0.05, ** p < 0.01. Values were obtained by normalization to a housekeeping gene (ACTB). Data are presented as means ± SD. CD36: CD36, FATP1: fatty acid transporter protein 1, FABPpm: plasma membrane-associated fatty acid binding protein, LPL: lipoprotein lipase, PPARA: peroxisome proliferator-activated receptor-α, PGC1A: proliferator-activated receptor gamma coactivator-1α, PDHA: pyruvate dehydrogenase-α; HADHB: hydroxyacyl-CoA dehydrogenase-β, SDHB succinate dehydrogenase subunit B, CPT1: carnitine palmitoyltransferase 1, ACSL: long chain acyl-CoA synthetase, ADIPOR1: adiponectin receptor 1, ADIPOR2: adiponectin receptor 2.