Peptidoglycan and mannose-based molecular patterns trigger the arachidonic acid cascade in human polymorphonuclear leukocytes

Abstract

The release of arachidonic acid (AA) in response to microorganism-derived products acting on pattern recognition receptors (PRR) was assayed in human polymorphonuclear leukocytes (PMN). Peptidoglycan (PGN) and mannan were found to be strong inducers of AA metabolism, as they produced the release of AA at a similar extent to that produced by agonists of pathophysiological relevance such as complement-coated zymosan particles and IgG immune complexes. In sharp contrast, lipoteichoic acid, LPS, muramyldipeptide, and the bacterial lipoprotein mimic palmitoyl-3-cysteine-serine-lysine-4 failed to do so. Leukotriene B4 and PGE2 were synthesized in response to mannan and PGN, thus suggesting that the lipoxygenase and the cyclooxygenase routes are operative in human PMN in response to pathogen-associated molecular patterns (PAMP). Analysis of the lipid extracts of supernatants and cell pellets as well as pharmacological studies with the calpain inhibitor calpeptin and the cytosolic phospholipase A2 (PLA2) inhibitor pyrrolidine-1 showed the dependence of AA release on cytosolic PLA2-catalyzed reactions. The effect of PGN was not inhibited by previous treatment with anti-TLR2 mAb, thus suggesting a nonarchetypal involvement of the TLR2 signaling route and/or participation of other receptors. Because of the abundance of mannose-based and PGN-containing PAMP in fungi and bacteria and the wide array of PRR in human PMN, these finding disclose a role of prime importance for PAMP and PRR in AA metabolism in the inflammatory response mediated by PMN.

Mannan and peptidoglycan induce COX‐2 protein in human PMN via the mammalian target of rapamycin

“…The present results extend the functional relevance of receptors for a variety of PAMP in human PMN regarding their effect on AA metabolism and, in combination with the results obtained in another study [22], unveil a close association between AA release and the induction of COX-2. This indicates that the supply of unesterified AA generated under these conditions is coincidental with the enhanced expression of COX-2 protein.…”

“…formylated peptides and PAF, where the activation of their cognate G-protein coupled receptors is largely linked to the 5-lipoxygenase pathway due to the constitutive status of this enzyme and its post-translational regulation by Ca 2+ [34] and phosphorylation [35]. Noteworthy, the stimuli used in this study elicited the production of both LTB 4 and PGE 2 [22], thus fitting well with the existence of two programs for AA metabolism in PMN: (i) the constitutive metabolic route conveying AA into the 5-lipoxygenase pathway, and (ii) the inducible route linked to COX-2 induction expression. The response to PAMP, although showing some features of the first program, fits best with the second program.…”

“…However, our findings cannot be uniquely explained on this basis, since COX-2 mRNA is detected in resting PMN at a level not found in monocytes and macrophages, and the induction of COX-2 protein in PMN shows a more rapid time course than that observed in monocytes under identical conditions [41]. Unlike actinomycin D, all of the drugs acting on the cascade involving PI3K, mTOR, and the translation machinery showed an almost complete inhibition of COX-2 protein expression, thus suggesting that COX-2 is another element of the array of constitutive messenger RNA translated by proinflammatory stimuli in human PMN [19].The MR seems to be the most likely PRR for mannosecontaining molecular patterns [22], but regarding PGN, three different types of receptors could be taken into account. In a previous study, we have confirmed the expression of TLR2, TLR1, TLR6, NOD2, and PGNrecognizing protein (PGRP)-S [22], all of which have some binding capacity for PGN or its elementary building blocks.…”

“…In a previous study, we have observed that peptidoglycan (PGN) and mannan are potent stimuli for AA release and PGE 2 production in human PMN [22]. Since PGE 2 can be produced both by COX-1, the constitutive isoform of COX, and by COX-2, the inducible isoform, we have addressed the effect of a set of PAMP signatures on the expression of COX-2 in human PMN.…”

“…The possibility that this mechanism could be operative in PAMP-dependent responses and might influence AA metabolism through the expression of COX-2, the inducible isoform of COX, is a challenging hypothesis. It could provide a rationale for pharmacological intervention with the macrolide antibiotic rapamycin, an inhibitor of the mammalian target of rapamycin (mTOR) [21], or alternatively, an explanation for some of the untoward effects of rapamycin.In a previous study, we have observed that peptidoglycan (PGN) and mannan are potent stimuli for AA release and PGE 2 production in human PMN [22]. Since PGE 2 can be produced both by COX-1, the constitutive isoform of COX, and by COX-2, the inducible isoform, we have addressed the effect of a set of PAMP signatures on the expression of COX-2 in human PMN.…”