Identification of a Novel Noninflammatory Biosynthetic Pathway of Platelet-activating Factor*

Abstract

Platelet-activating factor (PAF) is a potent lipid mediator playing various inflammatory and physiological roles. PAF is biosynthesized through two independent pathways called the de novo and remodeling pathways. Lyso-PAF acetyltransferase (lyso-PAF AT) was believed to biosynthesize PAF under inflammatory conditions, through the remodeling pathway. The first isolated lyso-PAF AT (LysoPAFAT/LPCAT2) had consistent properties. However, we show in this study the finding of a second lyso-PAF AT working under noninflammatory conditions. We partially purified a Ca 2؉ -independent lyso-PAF AT from mouse lung. Immunoreactivity for lysophosphatidylcholine acyltransferase 1 (LPCAT1) was detected in the active fraction. Lpcat1-transfected Chinese hamster ovary cells exhibited both LPCAT and lyso-PAF AT activities. We confirmed that LPCAT1 transfers acetate from acetyl-CoA to lyso-PAF by the identification of an acetyl-CoA (and other acyl-CoAs) interacting site in LPCAT1. We further showed that LPCAT1 activity and expression are independent of inflammatory signals. Therefore, these results suggest the molecular diversity of lyso-PAF ATs is as follows: one (LysoPAFAT/LPCAT2) is inducible and activated by inflammatory stimulation, and the other (LPCAT1) is constitutively expressed. Each lyso-PAF AT biosynthesizes inflammatory and physiological amounts of PAF, depending on the cell type. These findings provide important knowledge for the understanding of the diverse pathological and physiological roles of PAF.

Acyl-CoA:Lysophospholipid Acyltransferases

“…This relationship is similar to cyclooxygenases 1 and 2, which are constitutively expressed and inducible enzymes, respectively (41). This finding indicates that there are two kinds of PAF remodeling pathways: the inflammatory/inducible (LPCAT2) and non-inflammatory/ constitutive (LPCAT1) remodeling pathways (40).…”

Phosphorylation of Lysophosphatidylcholine Acyltransferase 2 at Ser34 Enhances Platelet-activating Factor Production in Endotoxin-stimulated Macrophages

“…However, neither the lyso-PAFAT cDNA sequence nor the mechanism of lyso-PAFAT activation had been elucidated. Recently, we identified two types of lyso-PAFATs: LPCAT2, which is an inducible lyso-PAFAT (9), and LPCAT1, which has constitutive lyso-PAFAT activity (8). LPCAT2 mRNA in macrophages is also up-regulated by LPS treatment for 16 h (9).…”

“…To detect the band shift, which represents phosphorylated protein, an SDS-polyacrylamide gel containing 50 M Phos-tag acrylamide with 100 M Mn 2ϩ was used. Assay of Lyso-PAF Acetyltransferase and LPCAT-Lyso-PAF acetyltransferase and LPCAT assays were performed as described previously (8,9).…”

“…Recently, we identified two molecular entities of lyso-PAFATs: a constitutively expressed lyso-PAFAT, LPCAT1 (lysophosphatidylcholine acyltransferase 1) (8), and an inducible lyso-PAFAT, LPCAT2 (9). In these previous reports, endogenous lyso-PAFAT in inflammatory cells was activated by prophlogistic stimuli (6) and LPCAT2 in mouse peritoneal macrophages was indeed activated by lipopolysaccharide (LPS) stimulation (9).…”

Amyloid-β42signals tau hyperphosphorylation and compromises neuronal viability by disrupting alkylacylglycerophosphocholine metabolism

“…LPCAT activity also increases in AD (9), notably in the posterior-temporal entorhinal cortex, a region characterized by the earliest tau pathology (2). Transfer of a long-chain acyl group to the sn-2 position by LPCAT1 and LPCAT2 regenerates structural membrane lipids, whereas addition of a small acetyl group produces a family of powerful lipid second messengers known as platelet activating factors (PAFs) (10,11).…”