New families of bioactive oxidized phospholipids generated by immune cells: identification and signaling actions

Abstract

Phospholipids are of critical importance in mammalian cell biology, both through providing a permeability barrier and acting as substrates for synthesis of lipid mediators. Recently, several new families of bioactive lipids were identified that form through the enzymatic oxidation of membrane phospholipids in circulating innate immune cells and platelets. These comprise eicosanoids attached to phosphatidylethanolamine and phosphatidylcholine and form within 2-5 minutes of cell activation by pathophysiologic agonists, via the coordinated action of receptors and enzymes. In this review, we summarize what is currently known regarding their structures, mechanisms of formation, cell biology, and signaling actions. We show that phospholipid oxidation by acutely activated immune cells is a controlled event, and we propose a central role in regulating membrane biology and innate immune function during health and disease. We also review the mass spectrometry methods used for identification of the lipids and describe how these approaches can be used for discovery of new lipid mediators in complex biologic samples.

Figure 1

Structures of esterified eicosanoids acutely generated by human and murine immune cells. (A) Lipids generated by human monocytes and bronchial epithelial cells. (B) Lipids generated by human platelets and murine peritoneal macrophages. (C) Lipids generated by human macrophages. (D) Lipids generated by human neutrophils. (E-F) Lipids generated by human platelets. Note that additional lipids not shown here include esterified HpODEs generated by monocytes and macrophages, and PGE₂/D₂-PEs generated by human platelets.

Figure 2

Summary of mechanism of formation and action of 12- and 15-HETE-PEs generated by human monocytes and murine peritoneal macrophages. (A) HETE-PEs and KETE-PEs are already present in the membranes of IL-4–cultured monocytes and peritoneal macrophages, but their levels are elevated ∼ 2-fold on ionophore activation. Generation involves direct oxidation of membrane phospholipids. (B) In vitro, HETE-PEs inhibit TLR4 signaling, activate PPAR-γ transcriptional activity, and stimulate dissociation of PEBP1 from Raf. PHGPx indicates, phospholipid hydroperoxide glutathione peroxidase; PGDH, prostaglandin dehydrogenase; PEBP1, PE-binding protein-1; and AA, arachidonic acid.

Figure 3

Summary of mechanism of formation and action of 5-HETE-PLs by human neutrophils. (A) Generation of the lipids is stimulated via receptor-dependent stimuli, including bacterial peptides, and intracellular signaling mediators. Hydrolysis of arachidonate by cPLA₂ is required, then oxidation by 5-LOX, reduction by GPX, and re-esterification into the phospholipid membrane. (B) HETE-PEs enhance superoxide generation and IL-8 release while inhibiting NET formation. fMLP indicates N-formyl-methionine-leucine-phenylalanine; PLC, phospholipase C; cPLA₂, cytosolic phospholipase A₂; FACL, fatty acyl CoA ligase; and GPX, glutathione peroxidase.

Figure 4

Summary of mechanism of formation and action of 12-HETE-PLs in human platelets. (A) The 12-HETE-PLs are generated in response to thrombin activation of PAR1 and PAR4, via several signaling intermediates. Hydrolysis of arachidonate by cPLA2 is required. (B) Some HETE-PEs translocate to the outside of the plasma membrane and can enhance tissue factor-dependent thrombin generation. sPLA₂ indicates secretory phospholipase A₂; FACL, fatty acyl CoA ligase; and PLC, phospholipase C.