Aims/hypothesis: Substantial evidence suggests a link between elevated inflammation and development of insulin resistance. Toll-like receptor 2 (TLR2) recognises a large number of lipid-containing molecules and transduces inflammatory signalling in a variety of cell types, including insulin-responsive cells. Considering the contribution of the fatty acid composition in TLR2-depedent signalling, we hypothesised that the inflammatory signals transduced by TLR2 contribute to insulin resistance.
Methods: Mice deficient in TLR2 were used to investigate the in vivo roles of TLR2 in initiating and maintaining inflammation-associated insulin resistance and energy homeostasis.
Results: We first recapitulated the observation with elevated expression of TLR2 and inflammatory cytokines in white adipose tissue and liver of ob/ob mice. Aged or high-fat-fed TLR2-deficient mice were protected from obesity and adipocyte hypertrophy compared with wild-type mice. Moreover, mice lacking TLR2 exhibited improved glucose tolerance and insulin sensitivity regardless of feeding them regular chow or a high-fat diet. This is accompanied by reductions in expression of inflammatory cytokines and activation of extracellular signal-regulated kinase (ERK) in a liver-specific manner. The attenuated hepatic inflammatory cytokine expression and related signalling are correlated with increased insulin action specifically in the liver in TLR2-deficient mice, reflected by increased insulin-stimulated protein kinase B (Akt) phosphorylation and IRS1 tyrosine phosphorylation and increased insulin-suppressed hepatocyte glucose production.
Conclusions/interpretation: The absence of TLR2 attenuates local inflammatory cytokine expression and related signalling and increases insulin action specifically in the liver. Thus, our work has identified TLR2 as a key mediator of hepatic inflammation-related signalling and insulin resistance.