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

Joseph J Senn

doi:10.1074/jbc.m513304200

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 IkappaBalpha, and increased NF-kappaB 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.

Highlights

A slight increase in the absolute protein level of Toll like receptor 2 (TLR2) is detectable after 15 min of exposure to palmitate, but this increase is does not continue and myeloid differentiation factor 88 (MyD88) protein levels do not change throughout the duration of this experiment, indicating that the observed association is most probably due to activation of the receptor pathway and not a significant increase in protein expression
Several hypotheses on the role of fatty acids in the development of insulin resistance suggest the primary effect of fatty acids is to alter cellular metabolism or function leading to secondary events that effect insulin signal transduction
It is quite possible that several mechanisms play roles in this process, recent research has strongly implied that an inflammatory process may be the initiating event in the development of fatty acid induced-insulin resistance [8, 32]

Summary

Introduction

Exposure to elevated NEFAs caused an elevation in reactive oxygen species, activation of proinflammatory pathways such as the transcription factor NF-␬B, the production of cytokines, and mitochondrial dysfunction All of these observations have been correlated with the effects on the insulin receptor signal transduction pathway, leading to decreased activation of signal transduction events (tyrosine phosphorylation of insulin receptor substrate 1 or phosphorylation of AKT) and Glut translocation, resulting in inhibited glucose uptake and utilization. Several mechanisms have previously been proposed for NEFA-induced inflammatory signals, new research in the field on innate immunity has suggested that saturated fatty acids may serve as a ligand for several members of the Toll-like receptor family [9, 10]. Subsequent downstream signal transduction events lead to the activation of multiple members of the mitogen-activated protein kinase family, in some cell types atypical PKCs and eventually the activation of transcription factors including NF-␬B, AP-1, and interferon regulatory factor, leading to the transcription of multiple proinflammatory cytokines [13]

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