Subcellular localisation and composition of intramuscular triacylglycerol influence insulin sensitivity in humans

Abstract

Aims/hypothesis: Subcellular localisation is an important factor in the known impact of bioactive lipids, such as diacylglycerol and sphingolipids, on insulin sensitivity in skeletal muscle; yet, the role of localised intramuscular triacylglycerol (IMTG) is yet to be described. Excess accumulation of IMTG in skeletal muscle is associated with insulin resistance, and we hypothesised that differences in subcellular localisation and composition of IMTG would relate to metabolic health status in humans.

Methods: We evaluated subcellular localisation of IMTG in lean participants, endurance-trained athletes, individuals with obesity and individuals with type 2 diabetes using LC-MS/MS of fractionated muscle biopsies and insulin clamps.

Results: Insulin sensitivity was significantly different between each group (athletes>lean>obese>type 2 diabetes; p < 0.001). Sarcolemmal IMTG was significantly greater in individuals with obesity and type 2 diabetes compared with lean control participants and athletes, but individuals with type 2 diabetes were the only group with significantly increased saturated IMTG. Sarcolemmal IMTG was inversely related to insulin sensitivity. Nuclear IMTG was significantly greater in individuals with type 2 diabetes compared with lean control participants and athletes, and total and saturated IMTG localised in the nucleus had a significant inverse relationship with insulin sensitivity. Total cytosolic IMTG was not different between groups, but saturated cytosolic IMTG species were significantly increased in individuals with type 2 diabetes compared with all other groups. There were no significant differences between groups for IMTG concentration in the mitochondria/endoplasmic reticulum.

Conclusions/interpretation: These data reveal previously unknown differences in subcellular IMTG localisation based on metabolic health status and indicate the influence of sarcolemmal and nuclear IMTG on insulin sensitivity. Additionally, these studies suggest saturated IMTG may be uniquely deleterious for muscle insulin sensitivity. Graphical abstract.

Keywords: Athlete’s paradox; Diabetes; Insulin sensitivity; Intramuscular triacylglycerol; Lipid composition; Skeletal muscle.

Fig. 1

Whole-cell total IMTG content (a), saturated IMTG content (b), and relationship between saturated IMTG content and insulin sensitivity (c) in endurance-trained athletes, lean and obese individuals, and individuals with type 2 diabetes (T2D). Values are means±SEM. *p<0.05 vs athletes; ^†p<0.05 vs lean; ^‡p<0.05 vs obese

Fig. 2

Subcellular IMTG localisation normalised to protein content (a) and subcellular saturated IMTG localisation normalised to protein content (b) in endurance-trained athletes, lean and obese individuals, and individuals with type 2 diabetes (T2D). Values are means±SEM. *p<0.05, ^§p<0.001 vs athletes; ^†p<0.05, ^¶p<0.001 vs lean; ^¥p<0.001 vs obese. ER, endoplasmic reticulum

Fig. 3

Correlations between saturated IMTG localised to the sarcolemma (a), nucleus (b), and cytosol (c) with insulin sensitivity. T2D, type 2 diabetes

Fig. 4

IMTG species in sarcolemmal (a), mitochondria/endoplasmic reticulum (b), nuclear (c), and cytosolic (d) fractions from participants. Values are means±SEM. *p<0.05, ^§p<0.01 vs athletes; ^†p<0.05, ^¶p<0.01 vs lean; ^‡p<0.05, ^¥p<0.01 vs obese. T2D, type 2 diabetes; TAG, triacylglycerol

Fig. 5

Correlations between total nuclear IMTG and total nuclear ceramide (a) and 1,2-DAG (b). Correlations in the cytosolic compartment between total IMTG and total ceramide (c), and saturated IMTG and disaturated 1,2-DAG (d).T2D, type 2 diabetes