Inhibition of sPLA2-IIA Prevents LPS-Induced Neuroinflammation by Suppressing ERK1/2-cPLA2α Pathway in Mice Cerebral Cortex

Neuroinflammation is involved in various central nervous system (CNS) disorders, including brain infections, ischemia, trauma, stroke, and degenerative CNS diseases. In the CNS inflammation, secretory phospholipase A₂-IIA (sPLA₂-IIA) acts as a mediator, resulting in the generation of the precursors of pro-inflammatory lipid mediators, such as prostaglandins (PGs) and leukotrienes (LTs). However, the role of sPLA₂-IIA in neuroinflammation is more complicated and remains unclear yet. In the present study, we investigated the effect of sPLA₂-IIA inhibition by specific inhibitor SC-215 on the inflammation in LPS-induced mice cerebral cortex and primary astrocytes. Our results showed that the inhibition of sPLA₂-IIA alleviated the release of PGE₂ by suppressing the activation of ERK1/2, cPLA₂α, COX-2 and mPGES-1. These findings demonstrated that sPLA₂-IIA showed the potential to regulate the neuroinflammation in vivo and in vitro, indicating that sPLA₂-IIA might be a novel target for the treatment of acute neuroinflammation.     

Phospholipase A(2) regulation of arachidonic acid mobilization

Phospholipase A(2) (PLA(2)) constitutes a growing superfamily of lipolytic enzymes, and to date, at least 19 distinct enzymes have been found in mammals. This class of enzymes has attracted considerable interest as a pharmacological target in view of its role in lipid signaling and its involvement in a variety of inflammatory conditions. PLA(2)s hydrolyze the sn-2 ester bond of cellular phospholipids, producing a free fatty acid and a lysophospholipid, both of which are lipid signaling molecules. The free fatty acid produced is frequently arachidonic acid (AA, 5,8,11,14-eicosatetraenoic acid), the precursor of the eicosanoid family of potent inflammatory mediators that includes prostaglandins, thromboxanes, leukotrienes and lipoxins. Multiple PLA(2) enzymes are active within and surrounding the cell and these enzymes have distinct, but interconnected roles in AA release.     

Omega-3 PUFA derived anti-inflammatory lipid mediator resolvin E1

Inflammation is a defensive response to injury and infection, but excessive or inappropriate inflammation contributes to a range of acute and chronic human diseases. Clinical assessment of dietary supplementation of omega-3 polyunsaturated fatty acids (PUFA) including eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) indicate their beneficial impact on human diseases in which inflammation is suspected as a key component of the pathogenesis. Although the mechanism of EPA and DHA action is still not fully defined in molecular terms, recent studies have revealed that, during the course of acute inflammation, omega-3 PUFA-derived mediators including resolvins and protectins with potent anti-inflammatory and pro-resolving properties are produced. In this review, we provide an overview of the formation and actions of EPA-derived anti-inflammatory lipid mediator resolvin E1.     

Ratio of Pro-Resolving and Pro-Inflammatory Lipid Mediator Precursors as Potential Markers for Aggressive Periodontitis

Aggressive periodontitis (AgP) is a rapidly progressing type of periodontal disease in otherwise healthy individuals which causes destruction of the supporting tissues of the teeth. The disease is initiated by pathogenic bacteria in the dental biofilm, and the severity of inflammation and attachment loss varies with the host response. Recently, there has been an increased interest in determining the role of lipid mediators in inflammatory events and the concept of pro-inflammatory and pro-resolution lipid mediators has been brought into focus also in periodontal disease. The present study aimed to determine the profile of omega-3 or n3- as well as omega-6 or n6- polyunsaturated fatty acids (PUFAs) and PUFA-metabolites of linoleic acid, arachidonic acid (AA), eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) in gingival crevicular fluid (GCF), saliva and serum in AgP patients and healthy controls. In total, 60 selected n3- and n6-PUFAs and various PUFA metabolites were measured using high performance liquid chromatography-tandem electrospray ionisation mass spectrometry (HPLC-ESI-MS-MS). Of these, 51 could be quantified in this study. The concentrations of the majority were low in saliva samples compared with serum and GCF, but were mainly higher in AgP patients compared with healthy controls in all three kinds of sample. Ratios of n3- to n6-PUFAs (DHA + EPA)/AA were significantly lower in the GCF of AgP patients than in the healthy controls. Furthermore, various ratios of the direct precursors of the pro-resolution lipid mediators (precursors of resolvins and protectins) were calculated against the precursors of mainly pro-inflammatory lipid mediators. These ratios were mainly lower in GCF and saliva of AgP patients, compared with healthy controls, but only reached significance in GCF (P<0.05). To conclude, the ratios of precursors of pro-resolution/pro-inflammatory lipid mediators seem to be more relevant for describing the disease status of AgP than the concentration of specific lipid mediators.     

The opposing effects of n-3 and n-6 fatty acids

Polyunsaturated fatty acids (PUFAs) can be classified in n-3 fatty acids and n-6 fatty acids, and in westernized diet the predominant dietary PUFAs are n-6 fatty acids. Both types of fatty acids are precursors of signaling molecules with opposing effects, that modulate membrane microdomain composition, receptor signaling and gene expression. The predominant n-6 fatty acid is arachidonic acid, which is converted to prostaglandins, leukotrienes and other lipoxygenase or cyclooxygenase products. These products are important regulators of cellular functions with inflammatory, atherogenic and prothrombotic effects. Typical n-3 fatty acids are docosahexaenoic acid and eicosapentaenoic acid, which are competitive substrates for the enzymes and products of arachidonic acid metabolism. Docosahexaenoic acid- and eicosapentaenoic acid-derived eicosanoids antagonize the pro-inflammatory effects of n-6 fatty acids. n-3 and n-6 fatty acids are ligands/modulators for the nuclear receptors NFkappaB, PPAR and SREBP-1c, which control various genes of inflammatory signaling and lipid metabolism. n-3 Fatty acids down-regulate inflammatory genes and lipid synthesis, and stimulate fatty acid degradation. In addition, the n-3/n-6 PUFA content of cell and organelle membranes, as well as membrane microdomains strongly influences membrane function and numerous cellular processes such as cell death and survival.     

Old and new generation lipid mediators in acute inflammation and resolution

Originally regarded as just membrane constituents and energy storing molecules, lipids are now recognised as potent signalling molecules that regulate a multitude of cellular responses via receptor-mediated pathways, including cell growth and death, and inflammation/infection. Derived from polyunsaturated fatty acids (PUFAs), such as arachidonic acid (AA), eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA), each lipid displays unique properties, thus making their role in inflammation distinct from that of other lipids derived from the same PUFA. The diversity of their actions arises because such metabolites are synthesised via discrete enzymatic pathways and because they elicit their response via different receptors. This review will collate the bioactive lipid research to date and summarise the findings in terms of the major pathways involved in their biosynthesis and their role in inflammation and its resolution. It will include lipids derived from AA (prostanoids, leukotrienes, 5-oxo-6,8,11,14-eicosatetraenoic acid, lipoxins and epoxyeicosatrienoic acids), EPA (E-series resolvins), and DHA (D-series resolvins, protectins and maresins).     

Targeting cyclooxygenases-1 and -2 in neuroinflammation: Therapeutic implications

Neuroinflammation has been implicated in the pathogenesis or the progression of a variety of acute and chronic neurological and neurodegenerative disorders, including Alzheimer’s disease. Prostaglandin H synthases or cyclooxygenases (COX -1 and COX-2) play a central role in the inflammatory cascade by converting arachidonic acid into bioactive prostanoids. In this review, we highlighted recent experimental data that challenge the classical view that the inducible isoform COX-2 is the most appropriate target to treat neuroinflammation. First, we discuss data showing that COX-2 activity is linked to anti-inflammatory and neuroprotective actions and is involved in the generation of novel lipid mediators with pro-resolution properties. Then, we review recent data demonstrating that COX-1, classically viewed as the homeostatic isoform, is actively involved in brain injury induced by pro-inflammatory stimuli including Aβ, lipopolysaccharide, IL-1β, and TNF-α. Overall, we suggest revisiting the traditional views on the roles of each COX during neuroinflammation and we propose COX-1 inhibition as a viable therapeutic approach to treat CNS diseases with a marked inflammatory component.     

Unesterified docosahexaenoic acid is protective in neuroinflammation

Docosahexaenoic acid (22:6n‐3) is the major brain n‐3 polyunsaturated fatty acid and it is possible that docosahexaenoic acid is anti‐inflammatory in the brain as it is known to be in other tissues. Using a combination of models including the fat‐1 transgenic mouse, chronic dietary n‐3 polyunsaturated fatty acid modulation in transgenic and wild‐type mice, and acute direct brain infusion, we demonstrated that unesterified docosahexaenoic acid attenuates neuroinflammation initiated by intracerebroventricular lipopolysaccharide. Hippocampal neuroinflammation was assessed by gene expression and immunohistochemistry. Furthermore, docosahexaenoic acid protected against lipopolysaccharide‐induced neuronal loss. Acute intracerebroventricular infusion of unesterified docosahexaenoic acid or its 12/15‐lipoxygenase product and precursor to protectins and resolvins, 17S‐hydroperoxy‐docosahexaenoic acid, mimics anti‐neuroinflammatory aspects of chronically increased unesterified docosahexaenoic acid. LC‐MS/MS revealed that neuroprotectin D1 and several other docosahexaenoic acid‐derived specialized pro‐resolving mediators are present in the hippocampus. Acute intracerebroventricular infusion of 17S‐hydroperoxy‐docosahexaenoic acid increases hippocampal neuroprotectin D1 levels concomitant to attenuating neuroinflammation. These results show that unesterified docosahexaenoic acid is protective in a lipopolysaccharide‐initiated mouse model of acute neuroinflammation, at least in part, via its conversion to specialized pro‐resolving mediators; these docosahexaenoic acid stores may provide novel targets for the prevention and treatment(s) of neurological disorders with a neuroinflammatory component.

     

Advances in eicosanoid research, novel therapeutic implications

Eicosanoids are a family of oxygenated metabolites of arachidonic acid, including the prostaglandins, thromboxanes, leukotrienes and lipoxins. These lipid mediators play essential roles in normal cellular homeostasis as well as in a number of disease states. This review will focus on recent advances in the field of eicosanoids and highlight specific discoveries and achievements. Emphasis will be placed on structure and receptor biology, which are of significant pharmacological and clinical relevance.     

Essential fatty acids: biochemistry, physiology and pathology

Essential fatty acids (EFAs), linoleic acid (LA), and alpha-linolenic acid (ALA) are essential for humans, and are freely available in the diet. Hence, EFA deficiency is extremely rare in humans. To derive the full benefits of EFAs, they need to be metabolized to their respective long-chain metabolites, i.e., dihomo-gamma-linolenic acid (DGLA), and arachidonic acid (AA) from LA; and eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) from ALA. Some of these long-chain metabolites not only form precursors to respective prostaglandins (PGs), thromboxanes (TXs), and leukotrienes (LTs), but also give rise to lipoxins (LXs) and resolvins that have potent anti-inflammatory actions. Furthermore, EFAs and their metabolites may function as endogenous angiotensin-converting enzyme and 3-hdroxy-3-methylglutaryl coenzyme A reductase inhibitors, nitric oxide (NO) enhancers, anti-hypertensives, and anti-atherosclerotic molecules. Recent studies revealed that EFAs react with NO to yield respective nitroalkene derivatives that exert cell-signaling actions via ligation and activation of peroxisome proliferator-activated receptors. The metabolism of EFAs is altered in several diseases such as obesity, hypertension, diabetes mellitus, coronary heart disease, schizophrenia, Alzheimer's disease, atherosclerosis, and cancer. Thus, EFAs and their derivatives have varied biological actions and seem to be involved in several physiological and pathological processes.     

Smooth muscle contraction as a model to study the mediator role of endogenous lipoxygenase products of arachidonic acid

In the lung, the contraction of smooth muscle, or bronchospasm, is generally caused by an immunologic insult resulting in mast cell degranulation and the release of histamine, slow reacting substances, and other mediators of inflammation (1). Although the immediate response is bronchospasm, continued activation of this sequence of events results in a chronic inflammatory disease. In the uterus, numerous conditions can result in smooth muscle contraction. One major pathophysiological syndrome associated with increased uterine tone and severe rhythmic contraction is primary dysmenorrhea (2). In this disease state, prostaglandins have been shown to play a major role in these contractions (3,4), and inhibitors of cyclooxygenase have proven beneficial in clinical practice (5). Both dysmenorrhea and cervical ripening have been likened to inflammatory reactions due to varying degrees of vasodilation, invasion by inflammatory cells, proliferation of fibroblasts and smooth muscle contraction (6,7). Metabolism of arachidonic acid (AA) via cyclooxygenase to prostaglandins and thromboxanes and via lipoxygenase to hydroxyeicosatetraenoic acids (HETEs) and leukotrienes is an integral part of both the acute and chronic inflammatory reaction in the lung or uterus. The material reviewed here examines the effect of endogenous leukotrienes on both the lung and uterus and suggests that other smooth muscles and pathophysiological states may be more involved with the lipoxygenase pathway of AA metabolism than previously believed.     

n-3脂肪酸代谢产物抗炎作用的研究进展

炎症反应是机体正常组织对感染和损伤的应答,然而过度的炎症反应往往会引起急性和慢性疾病的发生.最近研究发现,由n-3多不饱和脂肪酸二十碳五烯酸和二十二碳六烯酸代谢产生的resolvins和protectins两类化合物,具有很强的抗炎和炎症修复活性.综述了resolvins和protectin D1的来源、抗炎作用和抗炎机制,为进一步开展n-3多不饱和脂肪酸及其代谢产物的抗炎作用研究、为炎症的防治提供新的思路.     

Dietary docosahexaenoic acid but not arachidonic acid influences central nervous system fatty acid status in baboon neonates

The influence of dietary docosahexaenoic acid (DHA, 22:6n-3) and arachidonic acid (AA, 20:4n-6) on infant central nervous system (CNS) composition has implications for neural development, including vision, cognition, and motor function. We consider here combined results of three published studies of DHA/AA-containing formulas and breastfeeding to evaluate the CNS tissue response of baboon neonates with varied concentration and duration of DHA/AA consumption [G.Y. Diau, A.T. Hsieh, E.A. Sarkadi-Nagy, V. Wijendran, P.W. Nathanielsz, J.T. Brenna, The influence of long chain polyunsaturate supplementation on docosahexaenoic acid and arachidonic acid in baboon neonate central nervous system, BMC Med. 3 (2005) 11; A.T. Hsieh, J.C. Anthony, D.A. Diersen-Schade, et al., The influence of moderate and high dietary long chain polyunsaturated fatty acids (LCPUFA) on baboon neonate tissue fatty acids, Pediatr. Res. 61 (2007) 537–45; E. Sarkadi-Nagy, V. Wijendran, G.Y. Diau, et al., The influence of prematurity and long chain polyunsaturate supplementation in 4-week adjusted age baboon neonate brain and related tissues, Pediatr. Res. 54 (2003) 244–252]. A total of 43 neonates born spontaneously at term, or preterm by Cesarean section, consumed diets with DHA–AA (%w/w) at several levels: none (0,0), moderate (0.3, 0.6), or high (>0.6, 0.67 or 1.2). CNS fatty acids were analyzed at 4 and 12 weeks postpartum for term baboons and 7.5 weeks for preterm neonates. CNS DHA was consistently greater by 5–30% in neonates consuming DHA and nearer 30% for cortex. In contrast, CNS AA was unaffected by dietary AA and decreased in all structures with age. Dietary DHA consistently supports greater CNS DHA and maintenance of cortex DHA concentration with feeding duration, while CNS AA is not related to dietary supply. These data on structure-specific LCPUFA accretion may provide insight into neural mechanisms responsible for suboptimal functional outcomes in infants consuming diets that do not support the highest tissue DHA levels.     

Significance of myocardial eicosanoid production

Summary The precise role of eicosanoids in the development of myocardial injury during ischemia and reperfusion is still a matter of debate. Enhanced local production of these bioactive compounds appears to be a common response to tissue injury. Most likely, the cardiac tissue has the capacity to generate prostaglandins, thromboxanes as well as leukotrienes. Prostacyclin (PGI,) is the major eicosanoid produced by the jeopardized myocardium. In addition, at sites of tissue injury activation of platelets and infiltrating leukocytes results in the formation of considerable amounts of thromboxanes and leukotrienes. The production of eicosanoids requires prior release of arachidonic acid (AA) from phospholipids. Both ischemia and reperfusion are associated with a rise in the tissue level of AA. The absence of a proportional relationship between the tissue level of AA and the amounts of PGI, produced suggests that the sites of AA accumulation and PGI₂ formation are different. It is conceivable that AA accumulation is mainly confined to myocytes, whereas the capacity to synthesize PGI, mainly resides in vascular cells. Both beneficial and detrimental effects of eicosanoids on cardiac tissue have been described. Prostaglandins act as vasodilators. Besides, some of the prostaglandins, especially PGI,, are thought to possess cyto-protective properties. Thromboxanes and leukotrienes may impede blood supply by increasing smooth muscle tone. Besides, leukotrienes augment vascular permeability. Experimental studies, designed to evaluate the effect of pharmacological agents, like PGI₂-analogues and lipoxygenase and cyclo-oxygenase inhibitors, indicat that eicosanoids influence the outcome of myocardial injury. However, the delineation of the physiological significance of the locally produced eicosanoids is complicated by such factors as the wide variety of AA derivatives produced and the dose-dependency of their effects.     

Omega-3 fatty acids and their lipid mediators: towards an understanding of resolvin and protectin formation

Omega-3 polyunsaturated fatty acids (n-3 PUFAs) have long been associated with decreased inflammation and are also implicated in the prevention of tumorigenesis. Conventional thinking attributed this mainly to a suppressive effect of these fatty acids on the formation of arachidonic acid-derived prostaglandins and leukotrienes. Recent years have seen the discovery of a new class of inflammation-dampening and resolution-promoting n-3 PUFA-derived lipid mediators called resolvins and protectins. Chemically, these compounds are hydroxylated derivatives of the parent n-3 PUFA eicosapentaenoic acid (EPA) for the E-resolvins, and docosahexaenoic acid (DHA) for the D-resolvins and protectin D1. While a relatively large number of these compounds have been identified and characterized until now, with differences in the positions of the hydroxyl-groups as well as in the chirality at the different carbon atoms, all compounds share common precursor metabolites, 17-hydroperoxydocosahexaenoic acid (17-H(p)DHA) for the DHA-derived compounds and 18-hydroperoxyeicosapentaenoic acid (18-H(p)EPE) for the EPA-derived compounds. In this review we summarize the current knowledge about EPA- and DHA-derived resolvins and protectins and explore the potential use of the pro-resolvins 17-hydroxydocosahexaenoic acid (17-HDHA) and 18-hydroxyeicosapentaenoic acid (18-HEPE) as indicators of anti-inflammatory n-3 PUFA mediator formation.     

Brain arachidonic and docosahexaenoic acid cascades are selectively altered by drugs, diet and disease

Metabolic cascades involving arachidonic acid (AA) and docosahexaenoic acid (DHA) within brain can be independently targeted by drugs, diet and pathological conditions. Thus, AA turnover and brain expression of AA-selective cytosolic phospholipase A(2) (cPLA(2)), but not DHA turnover or expression of DHA-selective Ca(2+)-independent iPLA(2), are reduced in rats given agents effective against bipolar disorder mania, whereas experimental excitotoxicity and neuroinflammation selectively increase brain AA metabolism. Furthermore, the brain AA and DHA cascades are altered reciprocally by dietary n-3 polyunsaturated fatty acid (PUFA) deprivation in rats. DHA loss from brain is slowed and iPLA(2) expression is decreased, whereas cPLA(2) and COX-2 are upregulated, as are brain concentrations of AA and its elongation product, docosapentaenoic acid (DPA). Positron emission tomography (PET) has shown that the normal human brain consumes 17.8 and 4.6 mg/day, respectively, of AA and DHA, and that brain AA consumption is increased in Alzheimer disease patients. In the future, PET could help to determine how human brain AA or DHA consumption is influenced by diet, aging or disease.     

Preferential formation of 13-hydroxylinoleic acid by human peripheral blood eosinophils

Lipid mediators released by inflammatory and immune cells play an important role in inflammatory and immune processes. Most attention has been focussed on arachidonic-derived mediators, including prostaglandins, thromboxanes, leukotrienes, and lipoxins. Literature data, however, suggest that also metabolites of the unsaturated fatty acid linoleic acid may be important in this respect. We have studied the formation and release of 9-hydroxy- and 13-hydroxy-linoleic acid (9-HODE and 13-HODE) by enriched populations of human peripheral blood neutrophils, eosinophils, basophils, monocytes, and lymphocytes. We demonstrate that the eosinophil preferentially produces 13-HODE, whereas the other cell types produce equal amounts of 9-HODE and 13-NODE. The biological significance of these findings is discussed.     

Lipids in critical care medicine

While enteral nutrition is the basis for the critically ill, parenteral nutrition is often used when a sufficient enteral nutrition is not or not fully achievable. Lipids are a mainstay of caloric supply in both cases as they combine the provision of building blocks for the membranes and are precursors for function molecules including lipid mediators bearing the ability to influence immunity. Pro-inflammatory lipid mediators as prostaglandins and leukotrienes are generated from arachidonic acid (AA), a key member of the n-6 polyunsaturated fatty acids (PUFA). In contrast, lipid mediators derived from the n-3 fatty acids eicosapentaenoic acid (EPA) or docosahexaenoic acid (DHA) may exhibit less inflammatory properties compared to their AA-derived counterparts. Furthermore, intercellular mediators as resolvins and protectins are generated from n-3 fatty acids. They induce the resolution of inflammation, hence the name resolution phase interaction product—resolvin. Modulating the amount of PUFA and the n-6/n-3 ratio were investigated as means to change the inflammatory response and improve the outcome of patients. Experimental data showed that n-3 fatty acids may improve acute lung injury and sepsis in animal models. Studies in patients undergoing major surgery with application of n-3 fatty acids demonstrated beneficial effects in terms of reduction of length of stay and infectious complications. Clinical data hints that this concept may also improve outcome in critically ill patients. Additionally, experimental and clinical data suggest that a reduction in n-6 PUFA may change the immune response. In conclusion, modulating the amount of PUFA, the n-6/n-3 ratio and the composition of lipid emulsions may prove to be a useful means to improve the outcome of critically ill patients.