Investigating the role of toll-like receptor 4 in saturated fatty acid-induced inflammation

Current dogma stipulates that long chain saturated fatty-acids (lcSFA) are agonistic ligands for Toll-Like Receptor 4 (TLR4), driving low-grade inflammation and ultimately contributing to the pathogenesis of insulin resistance. While there is a substantial body of evidence which suggests that TLR4 modulates lipid-induced inflammation, there is little direct evidence that lcSFAs are agonistic ligands for TLR4. As such, we endeavoured to identify the role of TLR4 in lcSFA-induced inflammation. Using a combination of pharmacological and biochemical methods we provide compelling evidence that lcSFAs are not agonistic ligands for TLR4. Paradoxically, however, we have also confirmed and extended the results of others, showing that genetic ablation of TLR4 in tlr4-/- and C3H/HeJ bone marrow-derived macrophages (BMDMs) prevents lcSFA-induced inflammation. We hypothesized that loss of TLR4 function may alter the expression of specific genes, rendering tlr4-/- macrophages insensitive to lcSFAs. Remarkably, and in support of this hypothesis, when ‘primed’ with either TNFα, the TLR1/2 agonist pam₃ CSK₄ or the TLR3 agonist poly(I:C), the ability to sense lcSFAs, as measured by JNK activation, was restored in tlr4-/- BMDMs. The restoration of sensitivity to lcSFAs in ‘primed’ tlr4-/- BMDMs was associated with the re-expression of NOD-like receptor family pyrin domain containing 3 (NLRP3), an NF-κB dependent gene. Our data suggests that lcSFAs are not agonistic ligands for TLR4. Rather, TLR4 appears to indirectly regulate lcSFA-induced inflammation by controlling the expression of fatty acid sensitive pathways. Our results direct future efforts towards identifying the gene(s) which are upregulated via TLR4 activation and mediate SFA-induced inflammation, so that these genes can be specifically targeted therapeutically in the context of obesity in the absence of compromised innate immunity.