Role of diacylglycerol activation of PKCθ in lipid-induced muscle insulin resistance in humans

Abstract

Muscle insulin resistance is a key feature of obesity and type 2 diabetes and is strongly associated with increased intramyocellular lipid content and inflammation. However, the cellular and molecular mechanisms responsible for causing muscle insulin resistance in humans are still unclear. To address this question, we performed serial muscle biopsies in healthy, lean subjects before and during a lipid infusion to induce acute muscle insulin resistance and assessed lipid and inflammatory parameters that have been previously implicated in causing muscle insulin resistance. We found that acute induction of muscle insulin resistance was associated with a transient increase in total and cytosolic diacylglycerol (DAG) content that was temporally associated with protein kinase (PKC)θ activation, increased insulin receptor substrate (IRS)-1 serine 1101 phosphorylation, and inhibition of insulin-stimulated IRS-1 tyrosine phosphorylation and AKT2 phosphorylation. In contrast, there were no associations between insulin resistance and alterations in muscle ceramide, acylcarnitine content, or adipocytokines (interleukin-6, adiponectin, retinol-binding protein 4) or soluble intercellular adhesion molecule-1. Similar associations between muscle DAG content, PKCθ activation, and muscle insulin resistance were observed in healthy insulin-resistant obese subjects and obese type 2 diabetic subjects. Taken together, these data support a key role for DAG activation of PKCθ in the pathogenesis of lipid-induced muscle insulin resistance in obese and type 2 diabetic individuals.

Keywords: insulin signaling; lipotoxicity.

Fig. 1.

(A) Myocellular DAG concentrations in the membrane and cytosolic fraction and myocellular ceramide concentrations during lipid infusion in young lean healthy controls (CON) (n = 10). (B) Activation of myocellular PKCθ, -δ, and -ε during lipid infusion in CON (n = 10–14). (C) Phosphorylation of serine 1101 residue at IRS1-Ser1101-Px at baseline and its relative increase after 4 h glycerol (white and light gray columns) or lipid (dark gray and black columns) infusion in young lean healthy controls (CON) (n = 7). (D and E) PI3K-Px (E) and membrane/cytosolic ratio of Akt-Ser473 phosphorylation (Akt-Ser473-Px) (D) at baseline and after 4.5 h of glycerol (light gray column) or lipid (black column) infusion during insulin stimulation for 30 min in CON (n = 7). Data are given as means ± SEM. *P < 0.05; ^#P < 0.01; ^§P < 0.001. (F) Increased plasma FAs lead to myocellular accumulation of DAGs and consequent IRS1-Ser1101-Px, impaired PI3K-Px, and blunted insulin stimulation of Akt-Ser473-Px.

Fig. 2.

Concentration of membrane (A) and cytosolic (B) DAG species in healthy, lean controls (CON) (n = 16) at baseline (white columns), after 2.5 h (light gray columns), and after 4 h (dark gray columns) of lipid infusion. (C and D) Membrane (C) and cytosolic (D) DAG species concentrations in CON (white columns; n = 16), in young obese humans (OBE) (light gray columns; n = 10), and in elderly obese patients with T2D (black columns; n = 10). Data are given as means ± SEM. *P < 0.05; ^#P < 0.01; ^§P < 0.001 (2.5 or 4 h of lipid infusion vs. 0 h and OBE and T2D vs. CON).

Fig. 3.

Myocellular concentration of membrane and cytosolic DAGs and myocellular total ceramide concentrations (A) and activation of PKCθ, -δ, and -ε (B) in young lean healthy controls (CON) (white columns; n = 16), in young obese humans (OBE) (dark gray columns; n = 10), and in elderly obese patients with T2D (black column; n = 10). Data are given as means ± SEM. *P < 0.05; ^#P < 0.01; ^§P < 0.001.