Coordinate Regulation of TLR-Mediated Arachidonic Acid Mobilization in Macrophages by Group IVA and Group V Phospholipase A2s

Abstract

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.

Maximal T Cell–Mediated Antitumor Responses Rely upon CCR5 Expression in Both CD4+ and CD8+ T Cells

“…This differential role of CCR5 in TLR9-and TLR3-mediated immunostimulation suggests integration of local and systemic signaling pathways that affect CCR5 and the TLR. The mechanism underlying this specificity is unknown; some reports pinpoint differences in TLR9-and TLR3-mediated signaling to explain the specific counter regulation of TLR9-mediated inflammatory responses by the glucocorticoid receptor (49) and the differential effects of TLR9 and TLR3 agonists on age-associated antitumor responses (41) or on arachidonic acid mobilization (50). TLR9 and TLR3 are thus not wholly equivalent in reactivating the immune system and suggest future studies to understand the combinatorial control of CCR5 responses by TLR.…”

A new era of secreted phospholipase A2

“…Although these phenotypes in Pla2g5 Ϫ / Ϫ mice are often accompanied by reduced levels of eicosanoids, it is unclear whether sPLA 2 -V indeed drives AA metabolism by itself in vivo because of its fatty acid selectivity as noted above. Considering that the infl ammatory responses are often accompanied by activation of cytosolic PLA 2 ␣ (cPLA 2 ␣ ), a major AA-releasing PLA 2 ( 87 ), the observed alterations in eicosanoid levels in Pla2g5 Ϫ / Ϫ mice might merely refl ect the disease-associated changes in cPLA 2 ␣ activation, rather than hydrolytic liberation of AA by sPLA 2 -V. In relation to this, there is evidence suggesting that sPLA 2 -V regulates cPLA 2 ␣ phosphorylation ( 88,89 ). Moreover, transgenic overexpression of sPLA 2 -V leads to respiratory distress and neonatal death with no or only a modest increase in pulmonary eicosanoid levels ( 34 ).…”

Signaling Role for Lysophosphatidylcholine Acyltransferase 3 in Receptor-Regulated Arachidonic Acid Reacylation Reactions in Human Monocytes

“…Zymosan aliquots were diluted in serum-free medium and sonicated before addition to the cells. No PLA 2 activity was detected in the zymosan batches used in this study, as assessed by in vitro activity assay (29)(30)(31)(32)). …”