Elevated TLR4 Expression and Signaling in Muscle from Insulin Resistant Subjects

Abstract

Objectiv: TLR4 has been implicated in the pathogenesis of free fatty acid (FFA)-induced insulin resistance by activating inflammatory pathways, including IκB/NFκB. However, it is not known whether insulin resistant subjects have abnormal TLR4 signaling. We examined whether insulin resistant subjects have abnormal TLR4 expression and TLR4-driven (IκB/NFκB) signaling in skeletal muscle.

Methods: TLR4 gene expression and protein content were measured in muscle biopsies in 7 lean, 8 obese, and 14 type 2 diabetic (T2DM) subjects. A primary human myotube culture system was employed to examine whether FFAs stimulate IκB/NFκB via TLR4, and whether FFAs increase TLR4 expression/content in muscle.

Results: Obese and T2DM subjects had significantly elevated TLR4 gene expression and protein content in muscle. TLR4 muscle protein content correlated with the severity of insulin resistance. Obese and T2DM subjects also had lower IκBα content, an indication of elevated IκB/NFκB signaling. The increase in TLR4 and NFκB signaling was accompanied by elevated expression of the NFκB-regulated genes, IL-6 and SOD2. In primary human myotubes, acute palmitate treatment stimulated IκB/NFκB, and blockade of TLR4 prevented the ability of palmitate to stimulate the IκB/NFκB pathway. Increased TLR4 content and gene expression observed in muscle from insulin-resistant subjects were reproduced by treating myotubes from lean, normal-glucose-tolerant subjects with palmitate. Palmitate also increased IL-6 and SOD2 gene expression, and this effect was prevented by inhibiting NFκB.

Conclusion: Abnormal TLR4 expression and signaling, possibly caused by elevated plasma FFAs levels, may contribute to the pathogenesis of insulin resistance in humans.