Is palmitate truly pro-inflammatory? Experimental confounders and context-specificity.

Based primarily on cell culture results, saturated fatty acids (SFAs) are proposed to promote inflammation and contribute to metabolic dysfunction through toll-like receptor activation. Studies are often complicated by a requirement for carriers (e.g. BSA) or solvents (e.g. ethanol) to increase SFA solubility. To ascertain if these factors influence interpretations of SFA-associated inflammation activity, we measured responses of RAW264.7 monocyte/macrophages and C2C12 myotubes to various BSA, ethanol and cyclodextrin (alternative FA carrier) conditions. Fatty acid free low-endotoxin BSA preparations (0.33% to 2% wt/vol) activated, whereas 0.5-1.0% ethanol inhibited, RAW264.7 TNF- release. Ethanol modestly increased IL-6 secretion in C2C12 myotubes. Cyclodextrins (0.3 - 6.0 mM) were tested as alternative carriers of palmitate, but their usefulness was limited due to toxicity and solubility issues. Using a lower-inflammation BSA source and no ethanol, ~24 hr sodium palmitate treatment (up to 600 µM) failed to trigger RAW264.7 TNF- release, and in fact significantly dampened BSA-induced inflammation by >50%. In C2C12 myotubes, only high palmitate concentrations (500-600 µM) elicited IL-6 secretion (> 2.5-fold increase). Acute palmitate (200 or 500 µM) treatment did not activate MAP-kinase pathways above that of fresh BSA-containing media alone in either cell type. These results highlight the importance of experimental conditions in studies exploring SFA inflammation effects. The limited (or even anti-inflammatory) effects of palmitate that we observed indicate that immunomodulatory effects of SFAs are context-specific. Thus, caution is needed when interpreting the literature related to putative pro-inflammatory effects of SFA.