Separation and characterization of arachidonate-containing phosphoglycerides

Influence of iron-deficient diets during gestation and lactation on cerebral fatty acids and eicosanoids in guinea pig offspring-Comparison of studies with different sources of dietary lipids

Previous studies showed that mild iron deficiency anaemia (IDA) induced by feeding an iron deficient (ID) diet to female guinea pigs during gestation and lactation to alters the auditory functions of the offspring when corn oil is the only source of dietary lipids. Conversely, feeding an ID diet with a dietary fatty acid composition similar to that of typical human western diets induced minor impairments. Since tissue fatty acid metabolism is affected by dietary iron, the current study measured the impacts of these ID diets (ID-corn and ID-west) compared to the corresponding iron-sufficient control diets (IS-corn and IS-west) on encephalum fatty acid metabolism in the offspring at post-natal day 24. IDA induced by the ID-corn diet resulted in significant increases in encephalum n-6 PUFA content, but IDA induced by the ID-west diet had little impact on fatty acid profiles compared to the IS-west group. Brain COX II protein expression and FADS2 mRNA expression were statistically unaffected in both experiments, but encephalum PGE₂ concentrations were significantly reduced in ID-west pups. These results suggest IDA studies during prenatal development should consider dietary lipid compositions.

On the cellular metabolism of the click chemistry probe 19-alkyne arachidonic acid

Alkyne and azide analogs of natural compounds that can be coupled to sensitive tags by click chemistry are powerful tools to study biological processes. Arachidonic acid (AA) is a FA precursor to biologically active compounds. 19-Alkyne-AA (AA-alk) is a sensitive clickable AA analog; however, its use as a surrogate to study AA metabolism requires further evaluation. In this study, AA-alk metabolism was compared with that of AA in human cells. Jurkat cell uptake of AA was 2-fold greater than that of AA-alk, but significantly more AA-Alk was elongated to 22:4. AA and AA-alk incorporation into and remodeling between phospholipid (PL) classes was identical indicating equivalent CoA-independent AA-PL remodeling. Platelets stimulated in the pre-sence of AA-alk synthesized significantly less 12-lipoxygenase (12-LOX) and cyclooxygenase products than in the presence of AA. Ionophore-stimulated neutrophils produced significantly more 5-LOX products in the presence of AA-alk than AA. Neutrophils stimulated with only exogenous AA-alk produced significantly less 5-LOX products compared with AA, and leukotriene B₄ (LTB₄₎-alk was 12-fold less potent at stimulating neutrophil migration than LTB₄, collectively indicative of weaker leukotriene B₄ receptor 1 agonist activity of LTB₄-alk. Overall, these results suggest that the use of AA-alk as a surrogate for the study of AA metabolism should be carried out with caution.

Fatty acid remodeling in cellular glycerophospholipids following the activation of human T cells

Changes in fatty acid (FA) and glycerophospholipid (GPL) metabolism associated with cell cycle entry are not fully understood. In this study FA-GPL remodeling was investigated in resting and proliferating primary human T cells. Significant changes were measured in the composition and distribution of FAs in GPLs following receptor activation of human T cells. The FA distribution of proliferating T cells was very similar to that of the human Jurkat T cell line and when the stimulus was removed from proliferating T cells, they stopped proliferating and the FA distribution largely reverted back to that of resting T cells. The cellular content of saturated and monounsaturated FAs was significantly increased in proliferating cells, which was associated with an induction of FA synthase and stearoyl-CoA desaturase-1 gene expression. Additionally, cellular arachidonate was redistributed in GPLs in a distinct pattern that was unlike any other FAs. This redistribution was associated with an induction of CoA-dependent and CoA-independent remodeling. Accordingly, significant changes in the expression of several acyl-CoA synthetases, lysophospholipid acyltransferases, and phospholipase A2 were measured. Overall, these results suggest that metabolic pathways are activated in proliferating T cells that may represent fundamental changes associated with human cell proliferation.

Anti-CD3 and concanavalin A-induced human T cell proliferation is associated with an increased rate of arachidonate-phospholipid remodeling. Lack of involvement of group IV and group VI phospholipase A2 in remodeling and increased susceptibility of proliferating T cells to CoA-independent transacyclase inhibitor-induced apoptosis

In this study arachidonate-phospholipid remodeling was investigated in resting and proliferating human T lymphocytes. Lymphocytes induced to proliferate with either the mitogen concanavalin A or with anti-CD3 (OKT3) in combination with interleukin 2 (OKT3/IL-2) showed a greatly accelerated rate of [3H]arachidonate-phospholipid remodeling compared with resting lymphocytes or with lymphocytes stimulated with OKT3 or IL-2 alone. The concanavalin A-stimulated cells showed a 2-fold increase in the specific activity of CoA-independent transacylase compared with unstimulated cells, indicating that this enzyme is inducible. Stimulation with OKT3 resulted in greatly increased quantities of the group VI calcium-independent phospholipase A2 but not of the quantity of group IV cytosolic phospholipase A2. However, group IV phospholipase A2 became phosphorylated in OKT3-stimulated cells, as determined by decreased electrophoretic mobility. Incubation of cells with the group VI phospholipase A2 inhibitor, bromoenol lactone, or the dual group IV/group VI phospholipase A2 inhibitor, methyl arachidonyl fluorophosphonate, did not block arachidonate-phospholipid remodeling resting or proliferating T cells, suggesting that these phospholipases A2 were not involved in arachidonate-phospholipid remodeling. The incubation of nonproliferating human lymphocytes with inhibitors of CoA-independent transacylase had little impact on cell survival. In contrast, OKT3/IL-2-stimulated T lymphocytes were very sensitive to apoptosis induced by CoA-independent transacylase inhibitors. Altogether these results indicate that increased arachidonate-phospholipid remodeling is associated with T cell proliferation and that CoA-independent transacylase may be a novel therapeutic target for proliferative disorders.

A decrease in remodeling accounts for the accumulation of arachidonic acid in murine mast cells undergoing apoptosis

The goal of this study was to examine arachidonic acid (AA) metabolism by murine bone marrow-derived mast cells (BMMC) during apoptosis induced by cytokine depletion. BMMC deprived of cytokines for 12-48 h displayed apoptotic characteristics. During apoptosis, levels of AA, but not other unsaturated fatty acids, correlated with the percentage of apoptotic cells. A decrease in both cytosolic phospholipase A(2) expression and activity indicated that cytosolic phospholipase A(2) did not account for AA mobilization during apoptosis. Free AA accumulation is also unlikely to be due to decreases in 5-lipoxygenase and/or cyclooxygenase activities, since BMMC undergoing apoptosis produced similar amounts of leukotriene B(4) and significantly greater amounts of PGD(2) than control cells. Arachidonoyl-CoA synthetase and CoA-dependent transferase activities responsible for incorporating AA into phospholipids were not altered during apoptosis. However, there was an increase in arachidonate in phosphatidylcholine (PC) and neutral lipids concomitant with a 40.7 +/- 8.1% decrease in arachidonate content in phosphatidylethanolamine (PE), suggesting a diminished capacity of mast cells to remodel arachidonate from PC to PE pools. Further evidence of a decrease in AA remodeling was shown by a significant decrease in microsomal CoA-independent transacylase activity. Levels of lyso-PC and lyso-PE were not altered in cells undergoing apoptosis, suggesting that the accumulation of lysophospholipids did not account for the decrease in CoA-independent transacylase activity or the induction of apoptosis. Together, these data suggest that the mole quantities of free AA closely correlated with apoptosis and that the accumulation of AA in BMMC during apoptosis was mediated by a decreased capacity of these cells to remodel AA from PC to PE.

The distribution and metabolism of arachidonate-containing phospholipids in cellular nuclei

The cell nucleus has been identified as a location to which several arachidonic acid-metabolizing enzymes are located in stimulated cells. However, little information exists describing the distribution of arachidonate-containing phospholipids associated with the nucleus or the control of their composition. In this study, nuclei isolated from human monocyte-like THP-1 cells were found to have a distribution of arachidonyl-phospholipids which is markedly different from that of other cellular membranes. THP-1 nuclei which contained 22% of total cellular arachidonate, showed a near equal distribution of arachidonate in 1-acyl-2-arachidonoyl-glycero-3-phosphocholine, 1-acyl-2-arachidonyl-glycero-3-phosphoethanolamine, 1-acyl-2-arachidonoyl-glycero-3-phosphoinositol and 1-alk-1-enyl-2-arachidonoyl-glycero-3-phosphoethanolamine molecular species. In contrast in non-nuclear membranes, arachidonate was located primarily in 1-alk-1-enyl-2-arachidonoyl-glycero-3-phosphoethanolamine molecular species which accounted for approximately half of the arachidonate in all non-nuclear phospholipids. Isolated nuclei were incapable of initially acylating arachidonic acid into their phospholipids in the absence of cellular cytosol. However, they were capable of efficiently remodelling existing arachidonate between phospholipid classes and subclasses. Isolated nuclei contained 25-30% of the cellular activity of CoA-independent transacylase, the key enzyme responsible for arachidonate-phospholipid remodelling. This enzyme is also critical in the control of arachidonate availability following cell stimulation. Given that the cellular distribution of arachidonate is such that nuclei are enriched in donor substrates for the CoA-independent transacylase reaction, that non-nuclear membranes are enriched in acceptor substrates and that nuclei have the enzymatic machinery to remodel arachidonate efficiently, these results suggest that CoA-independent transacylase may be responsible for the remodelling of arachidonate not only between different phospholipid species within the same organelles but also between different sub-cellular compartments.

Relationship between arachidonate--phospholipid remodeling and apoptosis

Our previous studies reveal that three structurally distinct inhibitors of the enzyme CoA-independent transacylase, including the antiproliferative alkyllysophospholipid ET-18-O-CH3, induce programmed cell death (apoptosis) in the promyelocytic cell line HL-60. The objective of the current study was to better elucidate the mechanism responsible for apoptosis. CoA-IT is an enzyme believed to be responsible for the remodeling of long chain polyunsaturated fatty acids like arachidonate between the phospholipids of mammalian cells. The chronic (24-48 h) treatment of HL-60 cells with all three CoA-IT inhibitors resulted in the inhibition of the remodeling of labeled arachidonate from choline- into ethanolamine-containing phospholipid molecular species. GC-MS analysis of the fatty acids in phospholipids revealed that CoA-IT inhibitor treatment induced a marked loss of arachidonate-containing phosphatidylethanolamine and an increase in arachidonate-containing phosphatidylcholine. This redistribution was specific to arachidonate since the mass distribution of linoleic acid in glycerolipids was not affected. In spite of the dramatic redistribution of arachidonate, the total cellular arachidonate content was not altered nor was the relative distribution of total phospholipid classes. The increase of arachidonate in phosphatidylcholine was specifically due to an increase in 1-acyl-2-arachidonoyl-sn-glycero-3-phosphocholine species, whereas the loss of arachidonate in PE was from both 1-acyl- and 1-alk-1-enyl-2-arachidonoyl-sn-glycero-3-phosphoethanolamine species. The incubation of cells with exogenous arachidonic acid or ethanolamine did not reverse the inhibition of proliferation induced by CoA-IT inhibitor treatment. Incubation with CoA-IT inhibitors also induced the characteristic cytoplasmic and nuclear changes associated with apoptosis as assessed by transmission electron microscopy and DNA fragmentation as determined by flow cytometry. Taken together, these data show that apoptosis in HL-60 cells, induced by blocking arachidonate-phospholipid remodeling, is correlated with a redistribution of arachidonate in membrane phospholipids and suggest that such alterations represent a signal which controls the capacity of cells to proliferate.

Fatty acid and phospholipid selectivity of different phospholipase A2 enzymes studied by using a mammalian membrane as substrate

Previous studies using phospholipid mixed vesicles have demonstrated that several types of phospholipase A2 (PLA2) enzymes exhibit different selectivity for fatty acids at the sn-2 position, for the type of chemical bond at the sn-1 position or for the phosphobase moiety at the sn-3 position of phospholipids. In the present study, we have utilized natural mammalian membranes from U937 monocytes to determine whether two purified 14 kDa PLA2 isoenzymes (Type I, Type II) and a partially purified 110 kDa PLA2 exhibit substrate selectivity for certain fatty acids or phospholipids. In these studies, arachidonic acid (AA) release from membranes was measured under conditions where the remodelling of AA mediated by CoA-independent transacylase (CoA-IT) activity has been eliminated. In agreement with the mixed-vesicle models, AA was the major unsaturated fatty acid hydrolysed from membranes by the 110 kDa PLA2, suggesting that this PLA2 is selective in releasing AA from natural membranes. By contrast, Type I and Type II PLA2s were less selective in releasing AA from phospholipids and released a variety of unsaturated fatty acids at molar ratios that were proportional to the ratios of these fatty acids in U937 microsomal membranes. Examination of AA release from phospholipid classes indicated that all three enzymes released AA from the major AA-containing phospholipid classes (phosphatidylethanolamine, phosphatidylcholine, and phosphatidylinositol) of U937 membranes. The 110 kDa PLA2 released AA from phospholipid subclasses in ratios that were proportional to the AA content within phospholipid classes and subclasses of U937 membranes. These data suggested that the 110 kDa PLA2 shows no preference either for the sn-1 linkage or for the sn-3 phosphobase moiety of phospholipids. By contrast, Type I and Type II PLA2s preferentially released AA from ethanolamine-containing phospholipids and appeared to prefer the 1-acyl-linked subclass. Taken together, these data indicate that the 110 kDa PLA2 selectively releases AA from U937 membranes, whereas Type I and Type II PLA2 release a variety of unsaturated fatty acids. Furthermore, the 110 kDa PLA2 releases the same molar ratios of AA from all major phospholipid subclasses, whereas Type I and Type II PLA2s show some specificity for phosphatidylethanolamine when these enzymes are incubated with a complex mammalian membrane substrate.

Arachidonic acid metabolism during antigen and ionophore activation of the mouse bone marrow derived mast cell

This study has examined the metabolism of arachidonic acid in the mouse bone marrow-derived mast cell (BMMC) during immunologic and nonimmunologic activation. The predominant pools of endogenous arachidonate in the mast cells were found in ethanolamine (46%), choline (39%) and inositol (14%) containing glycerolipids. Initial studies established conditions where equilibrium labelling of these major phospholipids in the BMMC could be reached. Upon challenge, arachidonate was lost from all major phospholipid classes (phosphatidylethanolamine greater than phosphatidylcholine greater than phosphatidylinositol). There was a small but significant increase in the amount of label associated with phosphatidic acid during cell activation. Arachidonate was distributed among 1-acyl, 1-alkyl and 1-alk-1-enyl-linked subclasses of PC and PE. The rank order of loss of labelled arachidonate from the major PE and PC subclasses during antigen and ionophore activation was 1-alk-enyl-2-arachidonoyl-GPE greater than 1-acyl-2-arachidonoyl-GPC greater than 1-acyl-2-arachidonoyl-GPE greater than 1-alkyl-2-arachidonoyl-GPC. Labelled products released into the supernatant fluids and free arachidonic acid within the cell accounted for the bulk of arachidonate lost from phospholipids. Labelled products in the supernatant fluids were composed of LTB4, LTC4, PGD2 and free arachidonic acid. BMMC phospholipids were also labelled for 24 hr with [3H]choline, [3H]myoinositol or [14H]ethanolamine and labelled 2-lyso phospholipids were measured after cell activation. Radioactivity in lysophospholipids from PC, PE and PI increased significantly between 30 s and 2 min after antigen activation and then declined. Taken together, these studies suggest that arachidonate is mobilized predominantly from PE and in particular 1-alk-1-enyl-2-arachidonoyl-GPE by the direct removal of arachidonate from the sn-2 position of the molecule. Most of this arachidonate is then released from cells as eicosanoids or free fatty acid.