Role of secretory and cytosolic phospholipase A(2) enzymes in lysophosphatidylcholine-stimulated monocyte arachidonic acid release

Metabolomic profiling revealed altered lipid metabolite levels in childhood food allergy

BACKGROUND

The pathophysiology of childhood food allergy (FA) and its natural history are poorly understood. Clarification of the underlying mechanism may help identify novel biomarkers and strategies for clinical intervention in children with FA.

OBJECTIVE

This study aimed to identify metabolites associated with the development and resolution of FA.

METHODS

The metabolomic profiles of 20 children with FA and 20 healthy controls were assessed by liquid chromatography-tandem mass spectrometry. Comparative analysis was performed to identify metabolites associated with FA and FA resolution. For subjects with FA, serum samples were collected at the time of diagnosis and after resolution to identify the changes in metabolite levels. The selected metabolites were then quantified in a quantification cohort to validate the results. Finally, genome-wide association analysis of the metabolite levels was performed.

RESULTS

The study demonstrated a significantly higher level of sphingolipid metabolites and a lower level of acylcarnitine metabolites in children with FA than those in healthy controls. At diagnosis, subjects with resolving FA had a significantly high level of omega-3 metabolites and a low level of platelet-activating factors compared to persistent FA. However, the level of omega-3 metabolites decreased in children with resolving FA but increased in children with persistent FA during the same time. The quantification data of omega-3-derived resolvins, platelet-activating factor, and platelet-activating factor acetylhydrolase activity further supported these results.

CONCLUSION

The lipid metabolite profile is closely related to childhood FA and FA resolution. This study suggests potential predictive biomarkers and provides insight into the mechanisms underlying childhood FA.

cPLA2α Enzyme Inhibition Attenuates Inflammation and Keratinocyte Proliferation

As a regulator of cellular inflammation and proliferation, cytosolic phospholipase A₂ α (cPLA₂α) is a promising therapeutic target for psoriasis; indeed, the cPLA₂α inhibitor AVX001 has shown efficacy against plaque psoriasis in a phase I/IIa clinical trial. To improve our understanding of the anti-psoriatic properties of AVX001, we sought to determine how the compound modulates inflammation and keratinocyte hyperproliferation, key characteristics of the psoriatic epidermis. We measured eicosanoid release from human peripheral blood mononuclear cells (PBMC) and immortalized keratinocytes (HaCaT) and studied proliferation in HaCaT grown as monolayers and stratified cultures. We demonstrated that inhibition of cPLA₂α using AVX001 produced a balanced reduction of prostaglandins and leukotrienes; significantly limited prostaglandin E₂ (PGE₂) release from both PBMC and HaCaT in response to pro-inflammatory stimuli; attenuated growth factor-induced arachidonic acid and PGE₂ release from HaCaT; and inhibited keratinocyte proliferation in the absence and presence of exogenous growth factors, as well as in stratified cultures. These data suggest that the anti-psoriatic properties of AVX001 could result from a combination of anti-inflammatory and anti-proliferative effects, probably due to reduced local eicosanoid availability.

Uncovering pathophysiological changes in frontotemporal dementia using serum lipids

Blood serum is enriched in lipids and has provided a platform to understand the pathogenesis of a number of human diseases with improved diagnosis and development of biomarkers. Understanding lipid changes in neurodegenerative diseases is particularly important because of the fact that lipids make up >50% of brain tissues. Frontotemporal dementia (FTD) is a common cause of early onset dementia, characterized by brain atrophy in the frontal and temporal regions, concomitant loss of lipids and dyslipidemia. However, little is known about the link between dyslipidemia and FTD pathophysiology. Here, we utilized an innovative approach – lipidomics based on mass spectrometry – to investigate three key aspects of FTD pathophysiology – mitochondrial dysfunction, inflammation, and oxidative stress. We analyzed the lipids that are intrinsically linked to neurodegeneration in serum collected from FTD patients and controls. We found that cardiolipin, acylcarnitine, lysophosphatidylcholine, platelet-activating factor, o-acyl-ω-hydroxy fatty acid and acrolein were specifically altered in FTD with strong correlation between the lipids, signifying pathophysiological changes in FTD. The lipid changes were verified by measurement of the common disease markers (e.g. ATP, cytokine, calcium) using conventional assays. When put together, these results support the use of lipidomics technology to detect pathophysiological changes in FTD.

Fatty acyl composition of lysophosphatidylcholine is important in atherosclerosis

Atherosclerosis is a major cause of death for mankind. Although the pathophysiology of atherosclerosis is a complex and multifactorial process, growing body of evidence has identified phospholipids-mediated signaling as an important factor in the induction and progression of atherosclerosis. Lysophosphatidylcholine (LPC) is a major phospholipid in oxidized low-density lipoprotein, and is generally considered to be atherogenic. However, some studies have shown anti-atherogenic properties of LPC. The controversial findings surrounding the pro- or anti-atherogenic properties of LPC appear to be due to the chain length and the degree of saturation of the fatty acyl moiety of LPC. Studies have suggested that the presence of omega (n)-polyunsaturated fatty acids (PUFA) at the sn-1 position of LPC modulates the inflammatory response thereby making LPC anti-atherogenic. We have recently shown that feeding a diet high in n-3 PUFA resulted in the enrichment of LPC in both plasma and liver of C57BL/6 mice with n-3 PUFA. Others have also shown that supplementation with fish oil leads to preferential incorporation of n-3 PUFA into LPC. We also found that plasma obtained from mice fed a diet high in n-3 PUFA showed higher cholesterol efflux capacity compared to animals fed a low n-3 PUFA diet, despite no changes in high-density lipoprotein concentrations. We are therefore hypothesizing that n-3 PUFA enriched LPC has anti-atherogenic properties by promoting cholesterol efflux from macrophages and by reducing inflammation. Our anticipated long term objective is to establish that the fatty acyl moiety of LPC can be used as a potential biomarker for the risk of developing atherosclerosis. Validating this hypothesis would have a substantial impact on the public health with respect to early diagnosis of cardiovascular risks, and designing dietary based therapeutic strategies for the prevention and management of atherosclerosis and other heart related diseases.

Modified lipoproteins provide lipids that modulate dendritic cell immune function

Both physiological and pathological situations can result in biochemical changes of low-density lipoproteins (LDL). Because they can deliver signals to dendritic cells (DC), these modified lipoproteins now appear as regulators of the immune response. Among these modified lipoproteins, oxidized LDL (oxLDL) that accumulate during inflammatory conditions have been extensively studied. Numerous studies have shown that oxLDL induce the maturation of DC, enhancing their ability to activate IFNγ secretion by T cells. LDL treated by secreted phospholipase A(2) also promote DC maturation. Among the bioactive lipids generated by oxidation or phospholipase treatment of LDL, lysophosphatidylcholine (LPC) and some saturated fatty acids induce DC maturation whereas some unsaturated fatty acids or oxidized derivatives have opposite effects. Among other factors, the nuclear receptor peroxisome-proliferator activated receptor γ (PPARγ) plays a crucial role in this regulation. Non-modified lipoproteins also contribute to the regulation of DC function, suggesting that the balance between native and modified lipoproteins, as well as the biochemical nature of the LDL modifications, can regulate the activation threshold of DC. Here we discuss two pathological situations in which the impact of LDL modifications on inflammation and immunity could play an important role. During atherosclerosis, modified LDL accumulating in the arterial intima may interfere with DC maturation and function, promoting a Th1 immune response and a local inflammation favoring the development of the pathology. In patients chronically infected, the hepatitis C virus (HCV) interferes with lipoprotein metabolism resulting in the production of infectious modified lipoproteins. These lipo-viral-particles (LVP) are modified low-density lipoproteins containing viral material that can alter DC maturation and affect specific toll-like receptor signaling. In conclusion, lipoprotein modifications play an important role in the regulation of immunity by delivering signals of danger to DC and modulating their function.

LysoPC and PAF Trigger Arachidonic Acid Release by Divergent Signaling Mechanisms in Monocytes

Oxidized low-density lipoproteins (LDLs) play an important role during the development of atherosclerosis characterized by intimal inflammation and macrophage accumulation. A key component of LDL is lysophosphatidylcholine (lysoPC). LysoPC is a strong proinflammatory mediator, and its mechanism is uncertain, but it has been suggested to be mediated via the platelet activating factor (PAF) receptor. Here, we report that PAF triggers a pertussis toxin- (PTX-) sensitive intracellular signaling pathway leading to sequential activation of sPLA(2), PLD, cPLA(2), and AA release in human-derived monocytes. In contrast, lysoPC initiates two signaling pathways, one sequentially activating PLD and cPLA(2), and a second parallel PTX-sensitive pathway activating cPLA(2) with concomitant activation of sPLA(2), all leading to AA release. In conclusion, lysoPC and PAF stimulate AA release by divergent pathways suggesting involvement of independent receptors. Elucidation of monocyte lysoPC-specific signaling mechanisms will aid in the development of novel strategies for atherosclerosis prevention, diagnosis, and therapy.

Multiple phospholipase A2 enzymes participate in the inflammatory process in osteoarthritic cartilage

OBJECTIVE

The aim of this study was to determine the involvement of pro-inflammatory phospholipase A2 (PLA2) enzymes in human chondrocytes from patients with osteoarthritis (OA).

METHODS

PLA2 involvement in OA chondrocytes was analysed by (a) arachidonic acid (AA) and oleic acid release, (b) PLA2 mRNA analysis, and (c) prostaglandin E2 (PGE2) production in cultured OA chondrocytes in response to various cytokines and platelet activating factor (PAF).

RESULTS

Pro-inflammatory cytokines and PAF stimulation led to increased AA release, interleukin (IL)-1β and tumour necrosis factor (TNF) being the strongest inducers. The pattern of oleic acid release was similar to but less prominent than AA release, suggesting that predominantly arachidonyl selective enzymes were activated. IL-1β, TNF, IL-6, and IL-8 upregulated secretory group IIA, IID, and V phospholipase A2 (sPLA2-IIA, -IID, -V) and cytosolic group IVA phospholipase A2 (cPLA2-IVA) expression, where induction of chondrocyte sPLA2-IID is a novel finding. Furthermore, IL-1β, TNF, and IL-6 also induced COX2 expression. PAF induced expression of group IIA, IID and IVA PLA2, and COX2. In line with its anti-inflammatory properties, IL-4 was unable to induce either AA release or expression of PLA2s or COX2. IL-1β and TNF strongly increased PGE2 production, with IL-1β as the most prominent inducer.

CONCLUSION

Multiple PLA2 isoforms are expressed and influenced by pro-inflammatory stimuli in OA chondrocytes. Hence, several PLA2 enzymes may contribute to chondrocyte function by their upregulation and activation, and increased AA release and PGE2 production may therefore be important effectors in OA pathophysiology. PLA2 enzymes and cPLA2-IVA in particular are thus possible therapeutic targets in OA.

Platelet activating factor stimulates arachidonic acid release in differentiated keratinocytes via arachidonyl non-selective phospholipase A2

Platelet activating factor (PAF, 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine) is known to be present in excess in psoriatic skin, but its exact role is uncertain. In the present study we demonstrate for the first time the role of group VI PLA₂ in PAF-induced arachidonic acid release in highly differentiated human keratinocytes. The group IVα PLA₂ also participates in the release, while secretory PLA₂s play a minor role. Two anti-inflammatory synthetic fatty acids, tetradecylthioacetic acid and tetradecylselenoacetic acid, are shown to interfere with signalling events upstream of group IVα PLA₂ activation. In summary, our major novel finding is the involvement of the arachidonyl non-selective group VI PLA₂ in PAF-induced inflammatory responses.

Lysophosphatidylcholine activates a novel PKD2-mediated signaling pathway that controls monocyte migration

OBJECTIVE

Monocyte activation and migration are crucial events in the development of atherosclerosis and other inflammatory diseases. This study examined the role of protein kinase D (PKD) in monocyte migration. Method and Results- PKD2 is the predominant isoform of PKD expressed in monocytic THP-1 cells and primary human monocytes. Lysophosphatidylcholine (lysoPC), a prominent component of oxidized low-density lipoprotein, induces rapid and marked PKD activation in these cells. Using multiple approaches, including dominant-negative mutants and small interfering RNA knock-down, we found that lysoPC-induced PKD2 activation was required for the activation of both ERK and p38 MAPK. p38 MAPK mediation of lysoPC-induced monocytic cell migration was reported previously; our results reveal that the lysoPC-induced PKD2-p38 pathway controls monocyte migration.

CONCLUSIONS

This study provides the first evidence that (1) lysoPC activates PKD, (2) PKD2 has a novel role in p38 activation, and (3) the PKD2-activated p38 pathway is responsible for lysoPC-induced migration of THP-1 cells and human monocytes. Thus, PKD is a novel and functional intracellular regulator in both lysoPC signaling and monocyte migration. These results suggest a new role for PKD2 in the development of atherosclerosis and other inflammatory diseases.

Lipoprotein-associated phospholipase A2: a cardiovascular risk predictor and a potential therapeutic target

Lipoprotein-associated phospholipase A2 (Lp-PLA2), present in the circulation and in atherosclerotic plaque, is an inflammatory marker with potential use as a predictor of cardiovascular risk and as a therapeutic target. Although Lp-PLA2 is associated with both LDL and HDL, it is important to determine whether Lp-PLA2 has a predominantly pro- or anti-atherogenic effect. Increasing evidence suggests a proatherogenic role for Lp-PLA2. ©iEpidemiologic and clinical evidence suggests Lp-PLA2 is an independent predictor of risk and may be superior to other inflammatory markers owing to its specificity and minimal biovariation. Lp-PLA2 inhibitors currently being investigated in clinical trials are promising novel anti-inflammatory agents with a specificity for the vascular bed and a potential for decreasing plaque vulnerability.

Dietary n-3 polyunsaturated fatty acids and endothelium dysfunction induced by lysophosphatidylcholine in Syrian hamster aorta

This study investigated the influence of an eicosapentaenoic acid (EPA)- or a docosahexaenoic acid (DHA)-supplemented diet on the deleterious effects of lysophosphatidylcholine (LPC) on endothelium-dependent vasorelaxation of Golden Syrian hamster thoracic aorta. In a second step, LPC-modulated phospholipase A(2) (PLA(2))-derived ways of relaxation were investigated. Golden Syrian hamsters were fed for 6 weeks with a control diet or an EPA- or DHA-supplemented diet. Aortic fatty acid composition was analyzed by gas chromatography. Aortic rings were incubated for 20 minutes with LPC before constructing cumulative concentration-response curves for acetylcholine (ACh; 3 nmol/L-30 micromol/L) or sodium nitroprusside (3 nmol/L-30 micromol/L). The EPA- or DHA-supplemented diet increased n-3 polyunsaturated fatty acids in aortic fatty acids content because of the increase of EPA or DHA content, respectively, and decreased arachidonic acid aortic content. Lysophosphatidylcholine (1, 10, 15, and 20 micromol/L) induced a concentration-dependent inhibition of ACh-induced relaxation of preconstricted aortic rings in the control group, but did not influence sodium nitroprusside-induced aortic relaxation. The DHA- or EPA-supplemented diet worsened LPC (20 micromol/L) inhibitory effects on ACh-induced vasorelaxation. In the control diet group, ACh-induced relaxation was abolished by the nitric oxide synthase inhibitor (l-N(G)-nitro-arginine methyl ester; 100 micromol/L), whether LPC was added or not. The ACh-induced vasorelaxation was partially inhibited by PLA(2) inhibitors methyl arachidonyl fluorophosphonate (25 micromol/L) and arachidonyl trifluoromethyl ketone (20 micromol/L) as well as by the combination of 2 Ca(2+)-dependent potassium (K(Ca)) channel inhibitors charybdotoxin (0.1 micromol/L) plus apamin (0.3 micromol/L). In the presence of LPC (20 micromol/L), ACh-induced vasorelaxation was abolished by these inhibitors. These effects were not influenced by DHA or EPA diet. Our results suggested that EPA- or DHA-supplemented diet did not exhibit any beneficial effect against LPC-induced inhibition of endothelium-dependent aortic relaxation in Golden Syrian hamsters. These LPC effects were associated in our study not only with an inhibition of nitric oxide-dependent vasorelaxation, but also with a concomitant activation of a compensatory vasorelaxant pathway depending both on PLA(2) metabolites and on K(Ca) channel opening.

PhospholipaseA2: A key regulator of inflammatory signalling and a connector to fibrosis development in atherosclerosis

Atherosclerosis is a progressive inflammatory disease that takes place in the intima of the arterial wall. It is characterized by activation of endothelial cells, proliferation of smooth muscle cells and macrophages, accumulation of lipoproteins, deposition of extracellular matrix components and enhanced lipolytic enzyme activity. Phospholipase A(2) (PLA(2)) has been postulated to play an important role in the inflammatory process of atherosclerosis, but its molecular mechanism is uncertain. The secretory PLA(2) is expressed at increased levels in an atherosclerotic plaque and may hydrolyze low-density lipoproteins (LDL). This action promotes the production of pro-inflammatory lipids such as lysophospholipids, unsaturated fatty acids and eicosanoids. The current review highlights recent findings on how LDL-derived lipid mediators, generated by sPLA_2 modification of LDL, regulate pro-inflammatory activation and intracellular signaling in macrophages. Moreover, the review discusses how PLA_2 enzymes regulate signalling that promotes collagen accumulation and fibrotic plaque development. PLA_2 could therefore function as a connector between inflammation and fibrosis, the latter being an endpoint of chronic inflammation.

Lysophosphatidylcholine (LPC) attenuates macrophage-mediated oxidation of LDL

We have previously shown that paraoxonase 1 action on macrophages produced lysophosphatidylcholine (LPC) and significantly decreased cell-mediated LDL oxidation. Thus, in the present study, we questioned whether LPC can directly inhibit macrophage-mediated oxidation of LDL. Addition of increasing LPC concentrations (0-5 microM) to J774A.1 macrophages, mouse peritoneal macrophages (MPM), or to human monocytes-derived macrophages (HMDM) resulted in up to 83%, 67%, and 75% inhibition in cell-mediated oxidation of LDL, respectively. The mechanism for this LPC effect involves up to 60% inhibition of superoxide anion release from MPM in response to phorbol ester (PMA), 26% inhibition of PMA-induced NADPH oxidase activation (p47phox translocation from the cytosol to the plasma membrane), and a 2-fold stimulation of the macrophage paraoxonase 2 (PON2) lactonase activity. We thus conclude that inhibition of macrophage-mediated oxidation of LDL by LPC can contribute to attenuation of macrophage foam cell formation and atherosclerotic lesion development.

Lipoprotein-associated phospholipase A2: a novel marker of cardiovascular risk and potential therapeutic target

Although the clinical benefit of statins is well established, these agents reduce the risk of cardiovascular events by only 20 - 40%, and the residual risk for high-risk patients is considerable. The recognition of atherosclerosis as an inflammatory disease has opened the door to numerous complementary therapeutic approaches to further reduce risk and the overall burden of cardiovascular disease. Lipoprotein-associated phospholipase A(2) (Lp-PLA(2)) is a novel inflammatory marker of cardiovascular risk that is being evaluated as a potential therapeutic target. The biological function of this enzyme in atherosclerosis has been controversial but recent evidence supports its pro-atherogenic role. The enzyme is predominantly bound to low-density lipoprotein cholesterol particles in humans, and its activity produces bioactive lipid mediators that promote inflammatory processes present at every stage of atherogenesis, from atheroma initiation to plaque destabilisation and rupture. Initial clinical studies suggest that the inhibitors of Lp-PLA(2) can block enzyme activity in plasma and within atherosclerotic plaques. However, more studies are needed to determine the potential clinical benefits of inhibiting Lp-PLA(2).

Role of lipoprotein-associated phospholipase A2 in atherosclerosis: biology, epidemiology, and possible therapeutic target

The development of atherosclerotic vascular disease is invariably linked to the formation of bioactive lipid mediators and accompanying vascular inflammation. Lipoprotein-associated phospholipase A2 (Lp-PLA2) is an enzyme that is produced by inflammatory cells, co-travels with circulating low-density lipoprotein (LDL), and hydrolyzes oxidized phospholipids in LDL. Its biological role has been controversial with initial reports purporting atheroprotective effects of Lp-PLA2 thought to be a consequence of degrading platelet-activating factor and removing polar phospholipids in modified LDL. Recent studies, however, focused on pro-inflammatory role of Lp-PLA2 mediated by products of the Lp-PLA2 reaction (lysophosphatidylcholine and oxidized nonesterified fatty acids). These bioactive lipid mediators, which are generated in lesion-prone vasculature and to a lesser extent in the circulation (eg, in electronegative LDL), are known to elicit several inflammatory responses. The proinflammatory action of Lp-PLA2 is also supported by a number of epidemiology studies suggesting that the circulating level of the enzyme is an independent predictor of cardiovascular events, despite some attenuation of the effect by inclusion of LDL, the primary carrier of Lp-PLA2, in the analysis. These observations provide a rationale to explore whether inhibiting Lp-PLA2 activity and consequent interference with the formation of bioactive lipid mediators will abrogate inflammation associated with atherosclerosis, produce favorable changes in intermediate cardiovascular end points (eg, biomarkers, imaging, and endothelial function), and ultimately reduce cardiovascular events in high-risk patients.

Modification of LDL with human secretory phospholipase A(2) or sphingomyelinase promotes its arachidonic acid-releasing propensity

Oxidation and lipolytic remodeling of LDL are believed to stimulate LDL entrapment in the arterial wall, expanding the inflammatory response and promoting atherosclerosis. However, the cellular responses and molecular mechanisms underlying the atherogenic effects of lipolytically modified LDL are incompletely understood. Human THP-1 monocytes were prelabeled with [(3)H]arachidonic acid (AA) before incubation with LDL or LDL lipolytically modified by secretory PLA(2) (sPLA(2)) or bacterial sphingomyelinase (SMase). LDL elicited rapid and dose-dependent extracellular release of AA in monocytes. Interestingly, LDL modified by sPLA(2) or SMase displayed a marked increase in AA mobilization relative to native LDL, and this increase correlated with enhanced activity of cytosolic PLA(2) (cPLA(2)) assayed in vitro as well as increased monocyte tumor necrosis factor-alpha secretion. The AA liberation was attenuated by inhibitors toward cPLA(2) and sPLA(2), indicating that both PLA(2) enzymes participate in LDL-induced AA release. In conclusion, these results demonstrate that LDL lipolytically modified by sPLA(2) or SMase potentiates cellular AA release and cPLA(2) activation in human monocytes. From our results, we suggest novel atherogenic properties for LDL modified by sPLA(2) and SMase in AA release and signaling, which could contribute to the inflammatory gene expression observed in atherosclerosis.