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Phospholipase A₂ regulates eicosanoid class switching during inflammasome activation

Norris, Paul C., Gosselin, David, Reichart, Donna, Glass, Christopher K., Dennis, Edward A.
Proceedings of the National Academy of Sciences of the United States of America 2014 v.111 no.35 pp. 12746-12751
Toll-like receptor 4, adenosine triphosphate, arachidonic acid, aspirin, biosynthesis, endotoxins, esterification, humans, hydrolysis, inflammation, interleukin-1, macrophages, models, neutrophils, phospholipase A2, phospholipids, secretion
Initiation and resolution of inflammation are considered to be tightly connected processes. Lipoxins (LX) are proresolution lipid mediators that inhibit phlogistic neutrophil recruitment and promote wound-healing macrophage recruitment in humans via potent and specific signaling through the LXA ₄ receptor (ALX). One model of lipoxin biosynthesis involves sequential metabolism of arachidonic acid by two cell types expressing a combined transcellular metabolon. It is currently unclear how lipoxins are efficiently formed from precursors or if they are directly generated after receptor-mediated inflammatory commitment. Here, we provide evidence for a pathway by which lipoxins are generated in macrophages as a consequence of sequential activation of toll-like receptor 4 (TLR4), a receptor for endotoxin, and P2X ₇, a purinergic receptor for extracellular ATP. Initial activation of TLR4 results in accumulation of the cyclooxygenase-2–derived lipoxin precursor 15-hydroxyeicosatetraenoic acid (15-HETE) in esterified form within membrane phospholipids, which can be enhanced by aspirin (ASA) treatment. Subsequent activation of P2X ₇ results in efficient hydrolysis of 15-HETE from membrane phospholipids by group IVA cytosolic phospholipase A ₂, and its conversion to bioactive lipoxins by 5-lipoxygenase. Our results demonstrate how a single immune cell can store a proresolving lipid precursor and then release it for bioactive maturation and secretion, conceptually similar to the production and inflammasome-dependent maturation of the proinflammatory IL-1 family cytokines. These findings provide evidence for receptor-specific and combinatorial control of pro- and anti-inflammatory eicosanoid biosynthesis, and potential avenues to modulate inflammatory indices without inhibiting downstream eicosanoid pathways.