Esterification of 12(S)-hydroxy-5,8,10,14-eicosatetraenoic acid into the phospholipids of human peripheral blood mononuclear cells: inhibition of the proliferative response

Acute Hypercapnia/Ischemia Alters the Esterification of Arachidonic Acid and Docosahexaenoic Acid Epoxide Metabolites in Rat Brain Neutral Lipids

In the brain, approximately 90% of oxylipins are esterified to lipids. However, the significance of this esterification process is not known. In the present study, we (1) validated an aminopropyl solid phase extraction (SPE) method for separating esterified lipids using 100 and 500 mg columns and (2) applied the method to quantify the distribution of esterified oxylipins within phospholipids (PL) and neutral lipids (NL) (i.e. triacylglycerol and cholesteryl ester) in rats subjected to head-focused microwave fixation (controls) or CO2 -induced hypercapnia/ischemia. We hypothesized that oxylipin esterification into these lipid pools will be altered following CO2 -induced hypercapnia/ischemia. Lipids were extracted from control (n = 8) and CO2 -asphyxiated (n = 8) rat brains and separated on aminopropyl cartridges to yield PL and NL. The separated lipid fractions were hydrolyzed, purified with hydrophobic-lipophilic-balanced SPE columns, and analyzed with ultra-high-pressure liquid chromatography coupled to tandem mass spectrometry. Method validation showed that the 500 mg (vs 100 mg) aminopropyl columns yielded acceptable separation and recovery of esterified fatty acid epoxides but not other oxylipins. Two epoxides of arachidonic acid (ARA) were significantly increased, and three epoxides of docosahexaenoic acid (DHA) were significantly decreased in brain NL of CO2 -asphyxiated rats compared to controls subjected to head-focused microwave fixation. PL-bound fatty acid epoxides were highly variable and did not differ significantly between the groups. This study demonstrates that hypercapnia/ischemia alters the concentration of ARA and DHA epoxides within NL, reflecting an active turnover process regulating brain fatty acid epoxide concentrations.

Redox (phospho)lipidomics of signaling in inflammation and programmed cell death

In addition to the known prominent role of polyunsaturated (phospho)lipids as structural blocks of biomembranes, there is an emerging understanding of another important function of these molecules as a highly diversified signaling language utilized for intra- and extracellular communications. Technological developments in high-resolution mass spectrometry facilitated the development of a new branch of metabolomics, redox lipidomics. Analysis of lipid peroxidation reactions has already identified specific enzymatic mechanisms responsible for the biosynthesis of several unique signals in response to inflammation and regulated cell death programs. Obtaining comprehensive information about millions of signals encoded by oxidized phospholipids, represented by thousands of interactive reactions and pleiotropic (patho)physiological effects, is a daunting task. However, there is still reasonable hope that significant discoveries, of at least some of the important contributors to the overall overwhelmingly complex network of interactions triggered by inflammation, will lead to the discovery of new small molecule regulators and therapeutic modalities. For example, suppression of the production of AA-derived pro-inflammatory mediators, HXA3 and LTB4, by an iPLA2 γ inhibitor, R-BEL, mitigated injury associated with the activation of pro-inflammatory processes in animals exposed to whole-body irradiation. Further, technological developments promise to make redox lipidomics a powerful approach in the arsenal of diagnostic and therapeutic instruments for personalized medicine of inflammatory diseases and conditions.

Biosynthesis, biological effects, and receptors of hydroxyeicosatetraenoic acids (HETEs) and oxoeicosatetraenoic acids (oxo-ETEs) derived from arachidonic acid

Arachidonic acid can be oxygenated by a variety of different enzymes, including lipoxygenases, cyclooxygenases, and cytochrome P450s, and can be converted to a complex mixture of oxygenated products as a result of lipid peroxidation. The initial products in these reactions are hydroperoxyeicosatetraenoic acids (HpETEs) and hydroxyeicosatetraenoic acids (HETEs). Oxoeicosatetraenoic acids (oxo-ETEs) can be formed by the actions of various dehydrogenases on HETEs or by dehydration of HpETEs. Although a large number of different HETEs and oxo-ETEs have been identified, this review will focus principally on 5-oxo-ETE, 5S-HETE, 12S-HETE, and 15S-HETE. Other related arachidonic acid metabolites will also be discussed in less detail. 5-Oxo-ETE is synthesized by oxidation of the 5-lipoxygenase product 5S-HETE by the selective enzyme, 5-hydroxyeicosanoid dehydrogenase. It actions are mediated by the selective OXE receptor, which is highly expressed on eosinophils, suggesting that it may be important in eosinophilic diseases such as asthma. 5-Oxo-ETE also appears to stimulate tumor cell proliferation and may also be involved in cancer. Highly selective and potent OXE receptor antagonists have recently become available and could help to clarify its pathophysiological role. The 12-lipoxygenase product 12S-HETE acts by the GPR31 receptor and promotes tumor cell proliferation and metastasis and could therefore be a promising target in cancer therapy. It may also be involved as a proinflammatory mediator in diabetes. In contrast, 15S-HETE may have a protective effect in cancer. In addition to GPCRs, higher concentration of HETEs and oxo-ETEs can activate peroxisome proliferator-activated receptors (PPARs) and could potentially regulate a variety of processes by this mechanism. This article is part of a Special Issue entitled "Oxygenated metabolism of PUFA: analysis and biological relevance".

New families of bioactive oxidized phospholipids generated by immune cells: identification and signaling actions

Phospholipids are of critical importance in mammalian cell biology, both through providing a permeability barrier and acting as substrates for synthesis of lipid mediators. Recently, several new families of bioactive lipids were identified that form through the enzymatic oxidation of membrane phospholipids in circulating innate immune cells and platelets. These comprise eicosanoids attached to phosphatidylethanolamine and phosphatidylcholine and form within 2-5 minutes of cell activation by pathophysiologic agonists, via the coordinated action of receptors and enzymes. In this review, we summarize what is currently known regarding their structures, mechanisms of formation, cell biology, and signaling actions. We show that phospholipid oxidation by acutely activated immune cells is a controlled event, and we propose a central role in regulating membrane biology and innate immune function during health and disease. We also review the mass spectrometry methods used for identification of the lipids and describe how these approaches can be used for discovery of new lipid mediators in complex biologic samples.

Esterified eicosanoids: generation, characterization and function

Eicosanoids are oxidation products of C20 polyunsaturated fatty acids (e.g. arachidonic acid) that include prostaglandins, thromboxanes, leukotrienes and hydroperoxy fatty acids. They have important biological roles in vivo, including regulation of renal, cardiovascular and gastrointestinal function. Historically, eicosanoids were thought to mediate their signaling actions exclusively as free acids, however evidence is now emerging that they may also be generated attached to other functional groups including phospholipids and glycerol, and that these more complex forms are pathophysiological signaling mediators in their own right. Early studies showed that exogenously added eicosanoids could become esterified into membrane phospholipids of cells, while more recently, it was uncovered that esterified eicosanoids are formed endogenously. This review summarizes our current knowledge of this area, starting with the early discoveries documenting what is known about eicosanoid generation and their esterification, and moving on to discuss the discovery that esterified eicosanoids are generated endogenously by a number of different cell types. Recent research that is highlighting new structures and functions of these important lipid mediators will be presented. This article is part of a Special Issue entitled: Oxidized phospholipids-their properties and interactions with proteins.

Monohydroxylated fatty acid content in peripheral blood mononuclear cells and immune status of people at long times after the Chernobyl accident

The monohydroxylated fatty acid content of peripheral blood mononuclear cells from 23 cleanup workers and 16 unexposed individuals was studied in relation to their immune status after the Chernobyl accident. Men with absorbed doses below 0.32 Gy showed higher levels of free and esterified 12-hydroxyeicosatetraenoic acid (12-HETE) than unexposed men, whereas 15-HETE and the 17-hydroxy derivative of C22 fatty acid (17-OH 22), either free or esterified in phospholipids, were increased in a dose-dependent manner. The percentage of CD4-positive cells was also increased significantly in heavily irradiated men, whereas the percentage of CD8-positive cells tended to decrease with dose. Furthermore, the absolute count of CD4-positive cells was correlated positively with the amount of esterified 15-HETE in the phospholipid fraction of the mononuclear cells and with the total 15-HETE. These results show for the first time that the accumulation of autoxidized/lipoxygenase products of polyunsaturated fatty acids in the mononuclear cells of irradiated individuals was associated with immune imbalance. This may be the basis for certain late effects of radiation such as autoimmune disorders, somatic and neoplastic diseases, and early aging.

The priming effect of 12(S)-hydroxyeicosatetraenoic acid on lymphocyte phospholipase D involves specific binding sites

We have previously shown that 12(S)-hydroxyeicosatetraenoic acid (12(S)-HETE)-enrichment primed human peripheral blood mononuclear cells for phospholipase D activation by mitogens. Given that 12(S)-HETE-enriched cells stimulated with concanavalin A released free 12(S)-HETE in the extracellular medium, and that the priming effect of 12(S)-HETE on phospholipase D was suppressed by the non-permeant drug, suramin, we hypothesized an extracellular mechanism for 12(S)-HETE-induced PLD activation. Using [3H]12(S)-HETE as a ligand and a rapid filtration technique, we have pointed out the presence of specific low-affinity 12(S)-HETE binding sites on intact human mononuclear cells and lymphocytes. [3H]12(S)-HETE binding was efficiently displaced by other monohydroxylated and n-3 fatty acids but not by oleate and arachidonate, and was also significantly inhibited by suramin and pertussis toxin. Furthermore, 12(S)-HETE-induced PLD activation was strongly inhibited by pertussis toxin and genistein, but was not PKC-dependent. In addition, 12(S)-HETE also potentiated the ConA-induced tyrosine phosphorylation of a 46-50 kDa protein, which was inhibited by genistein. Collectively, these results suggest that 12(S)-HETE binding sites on human lymphocytes may be coupled to phospholipase D through pertussis toxin sensitive G-proteins and tyrosine kinases.

Occurrence of oxidized metabolites of arachidonic acid esterified to phospholipids in murine lung tissue

Isolation and characterization of murine pulmonary phospholipids revealed the normal occurrence of 10 isobaric eicosanoids corresponding to the incorporation of one oxygen atom into the arachidonate esterified to glycerophospholipids. Lungs from mice were removed and lipids were extracted and then separated into free carboxylic acid and phospholipids. Phospholipids were hydrolyzed to yield the free carboxylic acids prior to analysis. Reverse-phase HPLC and electrospray tandem mass spectrometry were used to identify and quantitate six monohydroxyeicosatetraenoic (HETE) and four epoxyeicosatetraenoic (EET) acid regioisomers using d8-HETE as internal standard. HETEs esterified to phospholipids were found to increase following intratracheal administration of tBuOOH (36 mg/kg), but not the levels of esterified EETs. Chiral analysis of esterified 15-HETE revealed an R/S ratio of 0.96, suggesting operation of a free radical mechanism responsible for generation of this monohydroxy arachidonate phospholipid, and this enantiomeric ratio was 1.10 following treatment of the mouse lung with tBuOOH. These results are consistent with a free-radical-based mechanism of oxidation of pulmonary glycerophospholipids containing arachidonate.

Arachidonic acid supplementation enhances synthesis of eicosanoids without suppressing immune functions in young healthy men

This study was conducted to determine the effects of arachidonic acid (AA) supplementation on human immune response (IR) and on the secretion of prostaglandin E2 (PGE2) and leukotriene B4 (LTB4). Ten healthy men (20-38 yr) participated in the study and lived at the Metabolic Suite of the Western Human Nutrition Research Center. They were fed a basal diet (57, 27, and 16 energy percentage from carbohydrate, fat, and protein, respectively, and AA 200 mg/d) for the first 15 d of the study. Additional AA (1.5 g/d) was added to the diet of six men from day 16 to 65, while the remaining four subjects remained on the basal diet. The diets of the two groups were crossed-over from day 66 to 115. In vitro indices of IR were examined using blood drawn on days 15, 58, 65, 108, and 115. Influenza antibody titers were determined in the sera prepared from blood drawn on days 92 and 115 (23 d postimmunization). AA supplementation caused significant increases in the in vitro secretion of LTB4, and PGE2, but it did not alter the in vitro secretion of tumor necrosis factor alpha; interleukins 1 beta, 2, 6; and the receptor for interleukin 2. Nor did it change the number of circulating lymphocytes bearing markers for specific subsets (B, T, helper, suppressor, natural killer) and the serum antibody titers against influenza vaccine. The opposing effects of PGE2 and LTB4 may have led to the lack of change in immune functions tested.

Free radicals, oxidative stress, and antioxidants in human health and disease

Free radicals and other reactive oxygen species (ROS) are constantly formed in the human body. Free-radical mechanisms have been implicated in the pathology of several human diseases, including cancer, atherosclerosis, malaria, and rheumatoid arthritis and neurodegenerative diseases. For example, the superoxide radical (O2 ·-) and hydrogen peroxide (H2O2) are known to be generated in the brain and nervous system in vivo, and several areas of the human brain are rich in iron, which appears to be easily mobilizable in a form that can stimulate free-radical reactions. Antioxidant defenses to remove O2 ·- and H2O2 exist. Superoxide dismutases (SOD) remove O2 ·- by greatly accelerating its conversion to H2O2. Catalases in peroxisomes convert H2O2 into water and O2 and help to dispose of H2O2 generated by the action of the oxidase enzymes that are located in these organelles. Other important H2O2-removing enzymes in human cells are the glutathione peroxidases. When produced in excess, ROS can cause tissue injury. However, tissue injury can itself cause ROS generation (e.g., by causing activation of phagocytes or releasing transition metal ions from damaged cells), which may (or may not, depending on the situation) contribute to a worsening of the injury. Assessment of oxidative damage to biomolecules by means of emerging technologies based on products of oxidative damage to DNA (e.g., 8-hydroxydeoxyguanosine), lipids (e.g., isoprostanes), and proteins (altered amino acids) would not only advance our understanding of the underlying mechanisms but also facilitate supplementation and intervention studies designed and conducted to test antioxidant efficacy in human health and disease.

Lipoxygenase inhibitors counteract protein kinase C mediated events in human T lymphocyte proliferation

Four structurally unrelated inhibitors of lipoxygenase (LO), i.e. nordihydroguaiaretic acid (NDGA), Esculetin, AA861 and 5,8,11,14-eicosatetraynoic acid (ETYA) suppressed mitogen induced proliferation of human peripheral blood lymphocytes in a dose-dependent manner. The degree of suppression was influenced by the type of the mitogenic stimulus. Receptor mediated stimulation, i.e. through phytohemagglutinin or the anti-CD3 antibody OKT3, was overall less susceptible, whereas proliferation initiated by direct activation of protein kinase C (PKC), i.e. through phorbol myristate acetate or indolactam V, was profoundly suppressed (up to 90%). The effect of the LO inhibitors was not due to non-specific interference with intracellular radical intermediates, because AA861 and ETYA showed no radical scavenging activity. Two PKC inhibitors, H-7 and H-8, similarly suppressed lymphocyte proliferation and showed essentially the same suppressive pattern as LO inhibitors. The results clearly indicate that LO product(s) participate in signal transduction mechanisms in T lymphocytes, possibly via stimulation of PKC activity and cell proliferation.

Uptake of 12-HETE by human bronchial epithelial cells (HBEC): effects on HBEC cytokine production

12-HETE, the major lipoxygenase end-product of platelets and macrophages, may be released in contact of bronchial epithelium in inflammatory diseases of the lung. We have studied the outcome of 12-HETE in presence of human bronchial epithelial cells (HBEC). When HBEC were incubated with [3H]12-HETE for 30 minutes, 27.5% of total radioactivity was found in HBEC and 72.5% in supernatants. Unesterified 12-HETE accounted for 22.4% of total radioactivity, 4.5% being recovered in phospholipids, preferentially in phosphatidylcholine and phosphatidylethanolamine. No incorporation in neutral lipids was detected. 72.9% of the incubated radioactivity was recovered in un identified metabolites. As 12-HETE has been shown to modulate the expression and production of various proteins, the consequence of the 12-HETE uptake on the release of GM-CSF and IL8 by HBEC was assessed. HBEC from control subjects were cultured for 24 hours with 12-HETE (10(-9) to 10(-7)M) in the presence or absence of TNF alpha. Detectable amounts of both cytokines were released in the supernatant in basal conditions at 24hr, and TNF alpha increased significantly the release of GM-CSF. 12-HETE at 10(-7)M weakly but significantly decreased the TNF-induced release of GM-CSF from HBEC. Thus the uptake of 12-HETE could affect the epithelial cell function in some situations.

Triggering of a phospholipase D pathway upon mitogenic stimulation of human peripheral blood mononuclear cells enriched with 12(S)-hydroxyicosatetraenoic acid

The influence of 12(S)-hydroxyicosatetraenoic acid (12-HETE), that we have previously shown to decrease the proliferative response of human lymphocytes to mitogens, on diacylglycerol and phosphatidic acid (PtdOH) formation was investigated in stimulated human peripheral blood mononuclear cells (PBMC). When human PBMC were first enriched with 12-HETE, then stimulated by the mitogenic lectin concanavalin A (Con A), the production of PtdOH normally associated with Con A stimulation was markedly increased as compared with non-enriched cells. The Con-A-induced rise in the PtdOH mass was markedly decreased by 1% ethanol in 12-HETE-enriched cells, whereas it was unaffected in control cells stimulated by Con A alone. Furthermore, in [3H]arachidonic-acid-labelled cells previously enriched with 12-HETE, the formation of [3H]arachidonic-acid-labelled phosphatidylalcohol was significantly increased upon Con A stimulation, no phosphatidylalcohol being synthesized in non-enriched cells. Collectively, these results suggest that, in the presence of 12-HETE, Con A stimulates a phospholipase D activity which was not triggered by Con A alone. These data are consistent with the lack of effect of suramin, reported as a phospholipase D inhibitor, which we observed in cells stimulated by Con A alone and with the suramin-induced decrease of PtdOH mass in 12-HETE-plus-Con-A-treated cells. Moreover, 12-[3H]HETE-enriched PBMC produced a significant amount of 12-[3H]HETE-containing PtdOH (0.4% of the total PtdOH) in resting conditions. Upon mitogenic stimulation by Con A, the phorbol ester tetradecanoylphorbol acetate or the anti-CD3 mAb OKT3, this proportion was decreased to 0.1-0.2%, since the total PtdOH mass was more drastically increased than the 12-HETE-containing PtdOH species. Although present in relatively low amount in stimulated cells, 12-HETE-containing PtdOH species might have been generated in strategic compartments of the membrane bilayer so that the following events involved in the transduction of the mitogenic signal could be impaired. GC analyses have pointed out drastic variations in the fatty acid composition of PtdOH in non-enriched and in 12-HETE-enriched stimulated cells. Especially PtdOH synthesized in 12-HETE-enriched cells upon Con A stimulation contained a higher amount of saturated fatty acids and a lower amount of arachidonic acid than that formed in control cells stimulated with Con A alone. Such saturated PtdOH species with a low arachidonic acid content are very likely to have a low mitogenic potential.