Regulation of the biosynthesis of acyl analogs of platelet-activating factor by purinergic agonist in endothlial cells

Platelet activating-factor acetylhydrolase II: A member of phospholipase A2 family that hydrolyzes oxidized phospholipids

Intracellular platelet activating-factor acetylhydrolase type II (PAF-AH II) is a 40-kDa monomeric enzyme. It was originally identified as an enzyme that hydrolyzes the acetyl group of PAF (1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine). As a member of phospholipase A2 super family, PAF-AH II has broad substrate specificity. It can hydrolyze phospholipids with relatively short-length or oxidatively modified sn-2 chains which endows it with various functions such as protection against oxidative stress, transacetylase activity and producing lipid mediators. PAF-AH II has been proven to be involved in several diseases such as allergic diseases, oxidative stress-induced injury and ischemia injury, thus it has drawn more attention from researchers. In this paper, we outline an entire summary of PAF-AH II, including its structure, substrate specificity, activity assay, inhibitors and biological activities.

Identification and quantitation of changes in the platelet activating factor family of glycerophospholipids over the course of neuronal differentiation by high-performance liquid chromatography electrospray ionization tandem mass spectrometry

Glycerophospholipids are important structural lipids in membranes with changes associated with progressive neurodegenerative disorders such as Alzheimer disease. Synthesis of the platelet activating factor (PAF) glycerophospholipid subclass is implicated in the control of neuronal differentiation and death. In this article, we combine nanoflow HPLC and mass spectrometry to screen, identify, and quantitate changes in glycerophospholipid subspecies, specifically PAF family members, over the course of neuronal differentiation. Furthermore, precursor ion scans for fragments characteristic of PAF phosphocholine family members and the standard additions of PAF subspecies were combined to perform absolute quantitation of PAF lipids in undifferentiated and differentiated PC12 cells. Surprisingly, a marked asymmetry was detected in the two predominant PAF species (C16:0, C18:0) over the course of differentiation. These results describe a new technique for the sensitive analysis of lipids combining nanoflow HPLC, ESI-MS, and precursor ion scan. Limits of detection of as little as 2 pg of PAF and LPC were obtained, and analysis of the lipidome of as little as 70,000 cells was performed on this system. Furthermore, application to the PC12 model identified a quantifiable difference between PAF molecular species produced over the course of neuronal differentiation.

Lysophospholipid transacetylase in the regulation of PAF levels in human monocytes and macrophages

The transacetylase (TA), reported to be identical to platelet-activating factor (PAF) acetylhydrolase (II), is a multifunctional enzyme with three catalytic activities: lysophospholipid transacetylase (TA(L)), sphingosine transacetylase (TA(S)), and acetylhydrolase (AH). We report that TA(L) activity participates in the control of PAF levels in monocytes and macrophages and that its regulation differs in these two types of cells. In monocytes, LPS or granulocyte-macrophage colony-stimulating factor (GM-CSF) specifically increased the TA(L) activity. Western blot analysis and enzyme assays on immunoprecipitates revealed that the increased activity can be ascribed to PAF-AH (II) and that both translocation from cytosol to membranes and p38/ERKs-mediated phosphorylation regulate the enzyme activation. Instead, in macrophages differentiated in vitro from monocytes by incubation with FCS, an increase of both TA(L) and AH activities was observed. Moreover, activation of ERKs and p38 MAP kinase was not required for the up-regulation of PAF-AH (II) in differentiated macrophages. The differences observed in macrophages as compared to monocytes can be explained by 1) p38/ERKs-independent phosphorylation of PAF-AH (II) and 2) appearance of plasma PAF-AH in the course of macrophage differentiation.

Lycopene in association with alpha-tocopherol or tomato lipophilic extracts enhances acyl-platelet-activating factor biosynthesis in endothelial cells during oxidative stress

Lipophilic compounds contained in tomato can prevent cardiovascular diseases by modulating the atherogenic processes in vascular endothelium mediated by oxidized low-density lipoproteins (LDLs). We investigated the effects of lycopene on the metabolism of platelet-activating factor (PAF) and its much less biologically active acyl analog, acyl-PAF, known to prevent LDL oxidation. Lycopene, or lycopene in association with alpha-tocopherol, or whole tomato lipophilic extracts (containing more than 80% lycopene) were used in experiments in which endothelial cells (ECs) are known to synthesize PAF following H(2)O(2)-induced oxidative stress. The results indicated that in each case H(2)O(2)-stimulated PAF biosynthesis in ECs, which is catalyzed by acetyl-CoA acetyltransferase (AT), appeared strongly inhibited. However, acyl-PAF biosynthesis, which also occurs through the PAF-dependent transacetylase (TA), was significantly increased by lycopene only when it was in association with alpha-tocopherol or with the minor compounds present in the whole lipophilic tomato extract. These findings suggest that alpha-tocopherol or lipophilic compounds present in tomato juice potentiate the effects of lycopene on the modulation of PAF and acyl-PAF biosynthesis in ECs during oxidative stress.

Modulation by flavonoids of PAF and related phospholipids in endothelial cells during oxidative stress

PAF-dependent transacetylase (TA) modifies the functions of platelet-activating factor (PAF), a potent inflammatory lipid, either by transferring the acetyl group from PAF to lysophospholipids (TAL activity), or to sphingosine (TAS activity) or by hydrolyzing PAF (acetylhydrolase activity). In stimulated endothelial cells (EC), TAL activity contributes to the synthesis of acyl-PAF, an acyl analog of PAF, that antagonizes PAF functions and is regulated by the cellular redox state. In this study, we investigated the possible involvement of TA in the flavonoid antioxidant mechanism(s) during oxidative stress in EC induced by hydrogen peroxide. The treatment of EC with H2O2 resulted in 4-fold increase of the acetyl-CoA acetyltransferase activity (AT), that is responsible for PAF biosynthesis, while the TAL activity increased only by 53%. However, the preincubation of H2O2-treated EC with the flavonoids hesperedin, naringin, and quercetin strongly inhibited AT activity and activated TAL by 290%, 340%, and 250%, respectively. The induction of TAL activity resulted in enhanced biosynthesis of 1-acyl-2-[3H]acetyl-PAF in intact EC and was related to the flavonoid structure. These findings suggest that TAL is involved in the flavonoid anti-inflammatory action by enhancing the production of acyl-PAF.

Differential Regulation of Three Catalytic Activities of Platelet-Activating Factor (PAF)-Dependent Transacetylase

We have previously established that PAF-dependent transacetylase (TA) purified to apparent homogeneity from rat kidney membranes and cytosol contains three separate catalytic activities, namely PAF lysophospholipid transacetylase (TAL), PAF sphingosine transacetylase (TAS), and PAF acetylhydrolase (AH). In the present investigation, we studied the biochemical factors and mechanism(s) that differentially regulate these three TA activities of the purified enzymes. We found that only the TAS activity of the TA purified from the membranes was stimulated by phosphatidyl-serine (PS) with optimal concentration of activation occurring at 25 microM. Other acidic phospholipids, such as phosphatidylinositol (PI) and phosphatidylinositol 4-phosphate (PIP), are partially effective, while diacylglycerol and free fatty acids had no effect on the TAS activity. PS exerted its effect on the TAS activity through the increases of both Km and Vmax. In addition, N-ethylmalimide (NEM) and dithiobis-(2-nitro-5-thiobenzoic acid) (DTNB) strongly inhibited the TAS activity and partially decreased the TAL and AH activities of the purified membrane enzyme in a dose-dependent manner. The addition of PS, but not by its substrate, sphingosine, could prevented the inhibition by NEM on the basal level of TAS. On the other hand, the inhibition of TAL by NEM and DTNB were partially protected by the substrate, lysoplasmalogens. Furthermore, PAF fully protects the inhibition of AH, partially protects the inhibition of TAL, and does not protect the inhibition of TAS by NEM. These results suggested that the three individual catalytic activities of TA have different dependencies on the thiol-containing residue(s) of the enzyme, i.e., cysteine. Furthermore, the nonresponsiveness of the purified cytosolic TAS to PS activation is consistent with our previous notions that membrane and cytosolic TA are posttranslationally distinct.