Toll-like Receptor-2 Is Essential for the Development of Palmitate-induced Insulin Resistance in Myotubes

Abstract

Fatty acids can activate proinflammatory pathways leading to the development of insulin resistance, but the mechanism is undiscovered. Toll like receptor 2 (TLR2) recognizes lipids, activates proinflammatory pathways, and is genetically associated with inflammatory diseases. This study aimed to examine the role of TLR2 in palmitate-induced insulin resistance in C2C12 myotubes. Treatment with palmitate rapidly induced the association of myeloid differentiation factor 88 (MyD88) with the TLR2 receptor, activated the stress-linked kinases p38, JNK, and protein kinase C, induced degradation of IB␣, and increased NF-B DNA binding. The activation of these pathways by palmitate was sensitive and temporally regulated and occurred within the upper physiologic range of saturated fatty acid concentrations in vivo, suggesting a receptor-mediated event and not simple lipotoxicity. When compared with an equimolar concentration of palmitate, fibroblast-stimulating lipopeptide-1, a known TLR2 ligand, was a slightly more potent activator of signal transduction and interleukin (IL)-6 production. Palmitate inhibited insulin signal transduction in C2C12 cells beginning 1-2 h after exposure and reached a maximum at 12-16 h. An antagonist TLR2 antibody, mAb 2.5, led to a 50 -60% decrease in palmitate-induced IL-6 production and partially restored insulin signal transduction, whereas an isotype-matched control antibody had no effect. RNA interference-mediated inhibition of TLR2 and MyD88 expression in C2C12 muscle cells resulted in a near complete inhibition of palmitate-induced insulin resistance and IL-6 production. This study provides strong evidence that TLR2 mediates the initial events of fatty acid-induced insulin resistance in muscle.

Toll-like receptor 2 deficiency improves insulin sensitivity and hepatic insulin signalling in the mouse

“…For example, palmitate induced IL-6 production and inflammatory signalling, leading to inhibition of insulin-activated signal transduction through TLR2 in myotubes [21]. In addition, inhibition of TLR2 expression by TLR2 antisense oligonucleotide improves insulin sensitivity and signalling in muscle and WAT from mice fed the HF diet [38].…”

“…These results suggest that the fatty acid composition plays a central role in ligand recognition and receptor activation for TLR2. Furthermore, other studies demonstrated that TLR2 is involved in NEFA-induced insulin resistance [21][22][23]. It is reasonable to postulate that the fatty acid moiety from nutrients could potentially activate TLR2 and transduce the inflammatory signals.…”

Toll-like receptor 2-deficient mice are protected from insulin resistance and beta cell dysfunction induced by a high-fat diet

“…Intriguingly, both TLR2 (which heterodimers with TLR1, or TLR6) and TLR4 recognise lipid-based structures; classically bacterial lipopeptides and lipopolysaccharide, respectively. Recent studies have shown that NEFAs, including palmitate and oleate, can activate both TLR2 and TLR4 signalling to induce proinflammatory cytokine production in various tissues, leading to an impairment of tissue-specific effects [15][16][17][18][19]. In addition, both Tlr2 −/− and Tlr4 −/− mice show improvements in the metabolic syndrome associated with HFD-induced obesity [17,[20][21][22].…”

Inhibition or deletion of the lipopolysaccharide receptor Toll-like receptor-4 confers partial protection against lipid-induced insulin resistance in rodent skeletal muscle

“…Indeed, diacylglycerol, but not ceramide, is increased in L6 myotubes after a 6 h incubation with palmitate (data not shown). Furthermore, a role for TLR-2 has recently been implicated in lipidinduced insulin resistance in skeletal muscle [47].…”