α‐Synuclein is a synaptic modulatory protein implicated in the pathogenesis of Parkinson disease. The precise functions of this small cytosolic protein are still under investigation. α‐Synuclein has been proposed to regulate soluble N ‐ethylmaleimide‐sensitive factor attachment protein receptor (SNARE) proteins involved in vesicle fusion. Interestingly, α‐synuclein fails to interact with SNARE proteins in conventional protein‐binding assays, thus suggesting an indirect mode of action. As the structural and functional properties of both α‐synuclein and the SNARE proteins can be modified by arachidonic acid, a common lipid regulator, we analysed this possible tripartite link in detail. Here, we show that the ability of arachidonic acid to stimulate SNARE complex formation and exocytosis can be controlled by α‐synuclein, both in vitro and in vivo . α‐Synuclein sequesters arachidonic acid and thereby blocks the activation of SNAREs. Our data provide mechanistic insights into the action of α‐synuclein in the modulation of neurotransmission.

Macrophages can be activated through TLRs for a variety of innate immune responses. In contrast with the wealth of data existing on TLR-dependent gene expression and resultant cytokine production, very little is known on the mechanisms governing TLR-mediated arachidonic acid (AA) mobilization and subsequent eicosanoid production. We have previously reported the involvement of both cytosolic group IVA phospholipase A₂ (cPLA₂) and secreted group V phospholipase A₂ (sPLA₂-V) in regulating the AA mobilization response of macrophages exposed to bacterial LPS, a TLR4 agonist. In the present study, we have used multiple TLR agonists to define the role of various PLA₂s in macrophage AA release via TLRs. Activation of P388D₁ and RAW2647.1 macrophage-like cells via TLR1/2, TLR2, TLR3, TLR4, TLR6/2, and TLR7, but not TLR5 or TLR9, resulted in AA mobilization that appears to involve the activation of both cPLA₂ and sPLA₂ but not of calcium-independent phospholipase A₂. Furthermore, inhibition of sPLA₂-V by RNA interference or by two cell-permeable compounds, namely scalaradial and manoalide, resulted in a marked reduction of the phosphorylation of ERK1/2 and cPLA₂ via TLR1/2, TLR2, TLR3, and TLR4, leading to attenuated AA mobilization. Collectively, the results suggest a model whereby sPLA₂-V contributes to the macrophage AA mobilization response via various TLRs by amplifying cPLA₂ activation through the ERK1/2 phosphorylation cascade.

Cellular availability of free arachidonic acid (AA) is an important step in the production of pro- and anti-inflammatory eicosanoids. Control of free AA levels in cells is carried out by the action of phospholipase A₂s and lysophospholipid acyltransferases, which are responsible for the reactions of deacylation and incorporation of AA from and into the sn-2 position of phospholipids, respectively. In this work, we have examined the pathways for AA incorporation into phospholipids in human monocytes stimulated by zymosan. Our data show that stimulated cells exhibit an enhanced incorporation of AA into phospholipids that is not secondary to an increased availability of lysophospholipid acceptors due to phospholipase A₂ activation but rather reflects the receptor-regulated nature of the AA reacylation pathway. In vitro activity measurements indicate that the receptor-sensitive step of the AA reacylation pathway is the acyltransferase using lysophosphatidylcholine (lysoPC) as acceptor, and inhibition of the enzyme lysoPC acyltransferase 3 by specific small interfering RNA results in inhibition of the stimulated incorporation of AA into phospholipids. Collectively, these results define lysoPC acyltransferase 3 as a novel-signal–regulated enzyme that is centrally implicated in limiting free AA levels in activated cells.

Activation of macrophages and macrophage cell lines by bacterial LPS elicits a delayed phase of PG biosynthesis that appears to be entirely mediated by cyclooxygenase-2 (COX-2). In previous work, we found that a catalytically active group V secreted phospholipase A₂ (sPLA₂-V) was required for COX-2 induction, but the nature of the sPLA₂-V metabolite involved was not defined. In this study, we identify lysophosphatidylcholine (lysoPC) as the sPLA₂-V downstream mediator involved in COX-2 induction by LPS-stimulated macrophages. Inhibition of sPLA₂-V by RNA interference or by the cell-permeable compound scalaradial blocked LPS-induced COX-2 expression, and this inhibition was overcome by incubating the cells with a nonhydrolyzable lysoPC analog, but not by arachidonic acid or oleic acid. Moreover, inhibition of sPLA₂-V by scalaradial also prevented the activation of the transcription factor c-Rel, and such an inhibition was also selectively overcome by the lysoPC analog. Collectively, these results support a model whereby sPLA₂-V hydrolysis of phospholipids upon LPS stimulation results in lysoPC generation, which in turn regulates COX-2 expression by a mechanism involving the transcriptional activity of c-Rel.