Lipoxins are a series of anti-inflammatory mediators. Lipoxins are short lived endogenously produced nonclassic eicosanoids whose appearance in inflammation signals the resolution of inflammation. They are abbreviated as LX, an acronym for lipoxygenase (LO) interaction products. At present two lipoxins have been identified; lipoxin A4 (LXA4) and lipoxin B4 (LXB4).
Lipoxins, as well as certain peptides, are high affinity ligands for the lipoxin A4 receptor (LXA4R), which was first identified based on sequence homology as the formyl peptide receptor like receptor (FPRL1). Lipoxin signaling through the LXA4R inhibits chemotaxis, transmigration, superoxide generation and NF-κB activation.
Conversely, peptide signaling through the same receptor, in vitro, has been shown to stimulate chemotaxis of polymorphonuclear cells (PMNs) and calcium mobilization. The peptides that have ALXR affinity tend to be signals for leukocyte migration and subsequent phagocytosis such as acute phase proteins, bacterial peptides, HIV envelope proteins and neurotoxic peptides.
Similarly to the leukotrienes, LXA4 will form the cysteinyl-lipoxins LXC4, LXD4 and LXE4. At subnanomolar concentrations, LXA4 and LXB4 inhibit leukotriene-stimulated interactions of human neutrophils and endothelial cells.
Lipoxins are high affinity antagonists to the cysteinyl leukotriene receptor type 1 (CysLT1) to which several leukotrienes (LTC4, LTD4 and LTE4) mediate their smooth muscle contraction and eosinophil chemotactic effects. The CysLT1 receptor is also the site of action for the asthma drug montelukast (Singulair).