Elevated toll-like receptor 4 expression and signaling in muscle from insulin-resistant subjects

OBJECTIVE-Tall-like receptor (TLR)4 has been implicated in the pathogenesis of free fatty acid (FFA)-induced insulin resistance by activating inflammatory pathways, including inhibitor of κB(IκB)/nuclear factor κB (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. RESEARCH DESIGN AND METHODS-TLR4 gene expression and protein content were measured in muscle biopsies in 7 lean, 8 obese, and 14 type 2 diabetic subjects. A primary human myotube culture system was used to examine whether FFAs stimulate IκB/NFκB via TLR4 and whether FFAs increase TLR4 expression/content in muscle. RESULTS-Obese and type 2 diabetic 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 type 2 diabetic 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 interleukin (IL)-6 and superoxide dismutase (SOD)2. 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. CONCLUSIONS-Abnormal TLR4 expression and signaling, possibly caused by elevated plasma FFA levels, may contribute to the pathogenesis of insulin resistance in humans.