Toll-like Receptor Agonists Promote Prolonged Triglyceride Storage in Macrophages*

Background: How microbial agonists induce macrophages to store triglycerides (TAG) for prolonged periods was not known. Results: Toll-like receptor (TLR)1/2, TLR3, and TLR4 agonists increased fatty acid uptake, increased TAG synthesis, and decreased lipolysis, augmenting TAG storage for ≥72 h. Conclusion: TAG retention is an active, multifaceted, and long lasting response to sensing microbes. Significance: Prolonged TAG storage contributes to foam cell formation. Macrophages in infected tissues may sense microbial molecules that significantly alter their metabolism. In a seeming paradox, these critical host defense cells often respond by increasing glucose catabolism while simultaneously storing fatty acids (FA) as triglycerides (TAG) in lipid droplets. We used a load-chase strategy to study the mechanisms that promote long term retention of TAG in murine and human macrophages. Toll-like receptor (TLR)1/2, TLR3, and TLR4 agonists all induced the cells to retain TAG for ≥3 days. Prolonged TAG retention was accompanied by the following: (a) enhanced FA uptake and FA incorporation into TAG, with long lasting increases in acyl-CoA synthetase long 1 (ACSL1) and diacylglycerol acyltransferase-2 (DGAT2), and (b) decreases in lipolysis and FA β-oxidation that paralleled a prolonged drop in adipose triglyceride lipase (ATGL). TLR agonist-induced TAG storage is a multifaceted process that persists long after most early pro-inflammatory responses have subsided and may contribute to the formation of “lipid-laden” macrophages in infected tissues.