Intracellular PAF catabolism by PAF acetylhydrolase counteracts continual PAF synthesis

Jiawei Chen,Lili Yang,Jason M Foulks,Andrew S Weyrich,Gopal K Marathe,Thomas M Mcintyre

doi:10.1194/jlr.m700325-jlr200

Abstract

Stimulated inflammatory cells synthesize platelet-activating factor (PAF), but lysates of these cells show little enhancement in PAF synthase activity. We show that human neutrophils contain intracellular plasma PAF acetylhydrolase (PLA2G7), an enzyme normally secreted by monocytes. The esterase inhibitors methyl arachidonoylfluorophosphonate (MAFP), its linoleoyl homolog, and Pefabloc inhibit plasma PAF acetylhydrolase. All of these inhibitors induced PAF accumulation by quiescent neutrophils and monocytes that was equivalent to agonist stimulation. Agonist stimulation after esterase inhibition did not further increase PAF accumulation. PAF acetylhydrolase activity in intact neutrophils was reduced, but not abolished, by agonist stimulation. Erythrocytes, which do not participate in the acute inflammatory response, inexplicably express the type I PAF acetylhydrolase, whose only known substrate is PAF. Inhibition of this enzyme by MAFP caused PAF accumulation by erythrocytes, which was hemolytic in the absence of PAF acetylhydrolase activity. We propose that PAF is continuously synthesized by a nonselective acyltransferase activity(ies) found even in noninflammatory cells as a component of membrane remodeling, which is then selectively and continually degraded by intracellular PAF acetylhydrolase activity to modulate PAF production.

Highlights

Stimulated inflammatory cells synthesize plateletactivating factor (PAF), but lysates of these cells show little enhancement in PAF synthase activity
Stimulated PAF accumulation does not correlate with PAF synthase activity Quiescent human neutrophils do not contain PAF, but extracts of cells stimulated by tumor necrosis factor-a (TNF-a), LPS, or the Ca21 ionophore A23187 all contain sufficient PAF to activate other neutrophils (Fig. 1A)
The amount of PAF generated by A23187-stimulated neutrophils was sufficient to induce a sustained increase in intracellular Ca21 in the test cells, whereas the PAF produced by LPS or TNF-a-stimulated cells was significantly less because it caused only transient stimulation when added to naıve cells

Summary

Introduction

Stimulated inflammatory cells synthesize plateletactivating factor (PAF), but lysates of these cells show little enhancement in PAF synthase activity. Acetyl-CoA:lyso-PAF acetyltransferase activity Neutrophils (108 in 5 ml of HBSS/A at 37jC) were stimulated with LPS, TNF-a, or A23187 for 10 min before adding chilled HEPES buffer (10 mM, pH 7.4). Human neutrophils express the intracellular type I PAF acetylhydrolase as an a2/a2 homodimer [41], which we confirmed by Western blot (Fig. 2A).

Results

Conclusion