Differential metabolism of diradyl glycerol molecular subclasses and molecular species by rabbit brain diglyceride kinase

The foundations and development of lipidomics

For over a century, the importance of lipid metabolism in biology was recognized but difficult to mechanistically understand due to the lack of sensitive and robust technologies for identification and quantification of lipid molecular species. The enabling technological breakthroughs emerged in the 1980s with the development of soft ionization methods (Electrospray Ionization and Matrix Assisted Laser Desorption/Ionization) that could identify and quantify intact individual lipid molecular species. These soft ionization technologies laid the foundations for what was to be later named the field of lipidomics. Further innovative advances in multistage fragmentation, dramatic improvements in resolution and mass accuracy, and multiplexed sample analysis fueled the early growth of lipidomics through the early 1990s. The field exponentially grew through the use of a variety of strategic approaches, which included direct infusion, chromatographic separation, and charge-switch derivatization, which facilitated access to the low abundance species of the lipidome. In this Thematic Review, we provide a broad perspective of the foundations, enabling advances, and predicted future directions of growth of the lipidomics field.

Analysis of cholesteryl esters and diacylglycerols using lithiated adducts and electrospray ionization-tandem mass spectrometry

Cholesteryl ester (CE) and diacylglycerol (DAG) molecular species are important lipid storage and signaling molecules. Mass spectrometric analyses of these lipids are complicated by the presence of isobaric molecular ions shared by these lipid classes and by relatively poor electrospray ionization, which is a consequence of an inherently weak dipole moment in these lipid classes. The current study demonstrates that lithiated adducts of CE and DAG molecular ions have enhanced ionization and lipid class-specific fragmentation in tandem mass spectrometry (MS/MS) scan modes, thereby allowing the implementation of strategies capable of lipid class-specific detection. Using neutral loss (NL) mode for the loss of cholestane from cholesterol esters (NL 368.5) and specific selected reaction monitoring for DAG molecular species, the response of specific molecular species to that of internal standards was determined. CE and DAG molecular species were quantified in human coronary artery endothelial cells (HCAECs) incubated with both palmitic acid and oleic acid. Furthermore, NL 368.5 spectra revealed the oxidation of the aliphatic fatty acid residues of CE molecular species. Taken together, these studies demonstrate a new analytical approach to assessing CE and DAG molecular species that exploits the utility of lithiated adducts in conjunction with MS/MS approaches.

D609 blocks cell survival and induces apoptosis in neural stem cells

In order to investigate the effects of tricyclodecane-9-yl-xanthogenate (D609) on the survival of neural stem cells (NSCs), which were isolated from rat forebrain, we treated the NSCs with D609 in the presence of basic fibroblast growth factor (bFGF). We found that when NSCs were exposed to 18.76-56.29 microM D609, the viability of the cells remarkably declined and apoptosis occurred. At the same time, the ROS level in NSCs was depressed. The data suggested that D609 was a powerful growth inhibitor and apoptosis inducer in NSCs.

Diacylglycerols activate mitochondrial cationic channel(s) and release sequestered Ca(2+)

Mitochondria contribute to cytosolic Ca(2+) homeostasis through several uptake and release pathways. Here we report that 1,2-sn-diacylglycerols (DAG's) induce Ca(2+) release from Ca(2+)-loaded mammalian mitochondria. Release is not mediated by the uni-porter or the Na(+)/Ca(2+) exchanger, nor is it attributed to putative catabolites. DAG's-induced Ca(2+) efflux is biphasic. Initial release is rapid and transient, insensitive to permeability transition inhibitors, and not accompanied by mitochondrial swelling. Following initial rapid release of Ca(2+) and relatively slow reuptake, a secondary progressive release of Ca(2+) occurs, associated with swelling, and mitigated by permeability transition inhibitors. The initial peak of DAG's-induced Ca(2+) efflux is abolished by La(3+) (1 mM) and potentiated by protein kinase C inhibitors. Phorbol esters, 1,3-diacylglycerols and 1-monoacylglycerols do not induce mitochondrial Ca(2+) efflux. Ca(2+)-loaded mitoplasts devoid of outer mitochondrial membrane also exhibit DAG's-induced Ca(2+) release, indicating that this mechanism resides at the inner mitochondrial membrane. Patch clamping brain mitoplasts reveal DAG's-induced slightly cation-selective channel activity that is insensitive to bongkrekic acid and abolished by La(3+). The presence of a second messenger-sensitive Ca(2+) release mechanism in mitochondria could have an important impact on intracellular Ca(2+) homeostasis.

Separate myocardial ethanolamine phosphotransferase activities responsible for plasmenylethanolamine and phosphatidylethanolamine synthesis

Ethanolamine phosphotransferase (EPT) is a key enzyme responsible for the synthesis of ethanolamine glycerophospholipids. Plasmenylethanolamine is a predominant molecular subclass of ethanolamine glycerophospholipids in the heart. The present study was designed to identify the selective use of 1-O-alk-1'-enyl-2-acyl-sn-glycerol as a substrate for EPT as a mechanism responsible for the predominance of plasmenylethanolamine in the rabbit heart. EPT activity in rabbit myocardial membranes using 1,2-diacyl-sn-glycerol as substrate is activated by Mn2+, inhibited by dithiobisnitrobenzoic acid (DTNB) and is unaffected by Ca2+. In contrast, ethanolamine phosphotransferase activity using 1-O-alk-1'-enyl-2-acyl-sn-glycerol as substrate is inhibited by Mn2+ and Ca2+, but is activated by DTNB. Additionally, ethanolamine phosphotransferase activity using 1-O-alk-1'-enyl-2-acyl-sn-glycerol substrate was more sensitive to thermal denaturation compared with that of 1,2-diacyl-sn-glycerol. Taken together, these results suggest that separate ethanolamine phosphotransferase activities are present in heart membranes that are responsible for the synthesis of phosphatidylethanolamine and plasmenylethanolamine.

Relationships between phosphatidylcholine-specific phospholipase C and integrins in cell-substratum adhesion and apoptosis in vascular endothelial cells

In order to understand the mechanism by which VEC control cell-substratum adhesion and apoptosis, we investigated relationships between PC-PLC and the integrins that are normally expressed in VEC. We found that promotion of cell-substratum adhesion by suppression of PC-PLC was almost completely blocked by a monoclonal antibody (mAb) against integrin beta1, and was partially blocked by a mAb against intergrin beta3. The production of diacylglycerol (DAG) which was inhibited by suppression of PC-PLC activity, was increased by mAbs against intergrin beta1 and beta3. When the mAb against integrin beta4 was added to the seeding medium, cell-substratum adhesion and spreading of cells were triggered, but the activity of PC-PLC was unaffected by this mAb. Furthermore, when both the mAb against integrin beta4 and a specific inhibitor (D609) of PC-PLC were present in the seeding medium, cell-substratum adhesion and spreading were promoted to a greater extent than when either of these agents was present alone. These data suggest that integrins beta1 and beta3 might regulate cell-substratum adhesion and apoptosis via a PC-PLC-dependent pathway, while integrin beta4 might regulate these phenomena via PC-PLC-independent pathway. These findings provide the first evidence of relationships between PC-PLC and integrins in cell-substratum adhesion and apoptosis.

The selective activation of the cardiac sarcolemmal sodium-calcium exchanger by plasmalogenic phosphatidic acid produced by phospholipase D

Since plasmalogens are the predominant phospholipid of cardiac sarcolemma, the activation of the sodium-calcium exchanger by either plasmenylethanolamine or plasmalogenic phosphatidic acid generated by phospholipase D was explored. Sodium-calcium exchange activity was 7-fold greater in proteoliposomes comprised of plasmenylethanolamine compared to proteoliposomes comprised of only plasmenylcholine. Phospholipase D treatment of proteoliposomes resulted in 1 mol % conversion of plasmenylcholine or phosphatidylcholine to their respective phosphatidic acid molecular species with a concomitant 8-fold or 2-fold activation of sodium-calcium exchange activity, respectfully. Thus, phospholipase D-mediated hydrolysis of plasmalogens to phosphatidic acid may be an important mechanism for the regulation of the sodium-calcium exchanger.

An animal cell mutant with a deficiency in acyl/alkyl-dihydroxyacetone-phosphate reductase activity. Effects on the biosynthesis of ether-linked and diacyl glycerolipids

In the accompanying paper (James, P. F., and Zoeller, R. A. (1997) J. Biol. Chem. 272, 23532-23539), we reported the isolation of a series of mutants from the fibroblast-like cell line, CHO-K1, that are deficient in the incorporation of the long chain fatty alcohol, hexadecanol, into complex lipids. All but one of these mutants, FAA. K1B, were deficient in long-chain-fatty alcohol oxidase (FAO) activity. We have further characterized this FAO+ isolate. FAA.K1B cells displayed a 40% decrease in [9,10-3H]hexadecanol uptake when compared with the parent strain. Although incorporation of hexadecanol into the phospholipid fraction was decreased by 52%, the cells accumulated label in alkylglycerol (20-fold over wild type). The increase in 1-alkylglycerol labeling corresponded to a 4-fold increase in alkylglycerol mass. Short term labeling with 32Pi showed a 45-50% decrease in overall phospholipid biosynthesis in FAA.K1B. Both diacyl- and ether-linked species were affected, suggesting a general defect in phospholipid biosynthesis. Mutant cells were able to partially compensate for the decreased biosynthesis by decreasing the turnover of the phospholipid pools. The primary lesion in FAA. K1B was identified as a 95% reduction in acyl/alkyl-dihydroxyacetone-phosphate reductase activity. Whole cell homogenates from FAA.K1B were unable to reduce either acyl-dihydroxyacetone phosphate (DHAP) or alkyl-DHAP, supporting the notion that the reduction of these two compounds is catalyzed by a single enzyme. These data suggest that the biosynthesis of diacyl phospholipids, in Chinese hamster ovary cells, begins with the acylation of dihydroxyacetone phosphate as well as glycero-3-phosphate and that the "DHAP pathway" contributes significantly to diacyl glycerolipid biosynthesis. Also, the severe reduction in acyl/alkyl-DHAP reductase activity in FAA.K1B resulted in only a moderate decrease in ether lipid biosynthesis. These latter data together with the observed increase in alkylglycerol levels support the existence of a shunt pathway that is able to partially bypass the enzymatic lesion.

Tissue distribution of the cytofectin component of a plasmid-DNA/cationic lipid complex following intravenous administration in mice

Allovectin-7 is a gene therapy agent that consists of plasmid DNA (pDNA) encoding the human HLA-B7 class I and beta2-microglobulin genes (VCL-1005), complexed with the cationic lipid DMRIE Br and DOPE. A tritiated version of the cytofectin component, DMRIE Br, was synthesized by regiospecific isotope incorporation to a very high specific activity. The 3H-labeled DMRIE/DOPE mixture was complexed with VCL-1005 to produce a radiolabeled version of Allovectin-7. The VCL-1005/3H-DMRIE/DOPE complex was administered intravenously to mice, and the tissue distribution of radioactivity was analyzed 24 hr later. Excretion of radioisotope was monitored for 96 hr post dosing. At 24 hr post administration, a tissue distribution for the radioisotope of liver >> spleen > lung >> heart > brain approximately muscle approximately blood was observed. During the 96-hr period post dose, very little administered radioactivity (<17%) was excreted and the majority of the isotope (83%) remained in the animal. This is the first report on the biodistribution of the cytofectin component of a pDNA-cationic lipid complex for which the distribution of the plasmid component has also been reported.

Signal transduction in vascular smooth muscle: diacylglycerol second messengers and PKC action

Agonist-stimulated phospholipid turnover can generate diacylglycerol (DAG), an intracellular second messenger that activates protein kinase C (PKC). DAG can be produced from the hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP2) by a phosphoinositide-specific phospholipase C and by the degradation of phosphatidylcholine (PC) by a phospholipase C or the concerted actions of phospholipase D and phosphatidate phosphohydrolase. In vascular smooth muscle, agonist-stimulated DAG accumulation is biphasic; PIP2 hydrolysis produces a transient increase in DAG, which is followed by a sustained phase of DAG accumulation from PC degradation. Metabolism of DAG attenuates PKC activation and thus results in signal termination. The metabolic fates for DAG include 1) ATP-dependent phosphorylation to form phosphatidic acid (DAG kinase), 2) hydrolysis to release fatty acids and glycerol (DAG and monoacylglycerol lipases), 3) synthesis of triacylglycerol (DAG acyltransferase), and 4) synthesis of PC (choline phosphotransferase). Hydrolysis through the lipase pathway is the predominant metabolic fate of DAG in vascular smooth muscle. Activation of PKC in vascular smooth muscle modulates agonist-stimulated phospholipid turnover, produces an increase in contractile force, and regulates cell growth and proliferation. Further research is required to investigate cross talk between signal transduction mechanisms involving lipid second messengers. In addition, spatial considerations such as nuclear PKC activation and the influence of diradylglycerol generation on the duration of PKC activation are important issues.

Vitamin E suppresses diacylglycerol (DAG) level in thrombin-stimulated endothelial cells through an increase of DAG kinase activity

The present study has examined the role of vitamin E, a natural lipid antioxidant, in the production of diacylglycerol (DAG) and phosphatidic acid (PA) in thrombin-stimulated human endothelial cells. Cells were labelled with [3H]myristate and the incorporation and distribution of [3H]myristate into cellular lipids was not affected by vitamin E. However, in response to thrombin stimulation, considerably more PA and less DAG were formed in cells enriched with vitamin E. The time-course of thrombin stimulation indicated that vitamin E attenuated the accumulation of sustained DAG levels with a concomitant increase in PA. Direct determination of DAG mass further confirmed that vitamin E suppresses the accumulation of DAG induced by thrombin. In the presence of ethanol, the formation of [3H]phosphatidylethanol (PEt) in [3H]myristate-labelled cells stimulated by thrombin was unaffected by vitamin E enrichment. DL-Propranolol, a PA phosphohydrolase inhibitor, caused an accumulation of PA, without affecting DAG formation in either vitamin E-treated and untreated cells. This indicated that the increase in PA and decrease in DAG in vitamin E-treated cells was not due to a stimulation of phospholipase D or an inhibition of PA phosphohydrolase. Determination of inositol phosphates formation in response to thrombin showed that the change of DAG levels elicited by vitamin E was independent of phospholipase C-induced hydrolysis of inositol phospholipids. In contrast, analysis of DAG kinase activity revealed that vitamin E enrichment enhanced the activity of the enzyme in both basal and thrombin-stimulated cells. Taken together, these data indicated that vitamin E caused an increased conversion of DAG to PA by activating DAG kinase activity without causing any change in the activities of phospholipase D, PA phosphohydrolase or phospholipase C.

Phospholipase D and exocytosis of the ram sperm acrosome

We have investigated whether phospholipase D (PLD) is involved in events leading to acrosomal exocytosis. Ram spermatozoa pre-labelled with [3H]alkyl-lysophosphatidylcholine and stimulated with the ionophore A23187 (1 microM) and Ca2+ (3 mM) in the presence of ethanol, showed a slow time-dependent increase in [3H]phosphatidic acid and [3H]phosphatidylethanol (PEt), the latter being clear evidence of PLD activity. Unlabelled cells similarly treated underwent acrosomal exocytosis. However, [3H]PEt formation was inhibited by high Ca2+ concentrations, although such conditions result in maximal acrosomal exocytosis. Treatment with A23187/Ca2+ led to a fast generation of [3H]alkyl-diglyceride and an increase in 1,2-diacylglycerol mass, which preceded [3H]PEt formation. The rises in [3H]alkyl-diglyceride and 1,2-diacylglycerol mass took place regardless of the presence or absence of ethanol. Inclusion of propranolol, a phosphatidic acid phosphohydrolase inhibitor, did not affect the early rise of labelled or unlabelled 1,2-diglycerides either. Stimulation of spermatozoa with A23187/Ca2+ in the presence of either ethanol or propranolol did not affect the occurrence of acrosomal exocytosis. Taken together, these results indicate that although Ca2+ entry triggers a late activation of PLD, this enzyme is not involved in the early generation of diglycerides. Moreover, they suggest that PLD does not make a substantial contribution in events leading to exocytosis of the sperm acrosome. Therefore, generation of diglycerides may take place primarily via phospholipase C.

Intracellular signalling mediated by protein-tyrosine kinases: networking through phospholipid metabolism

In recent years, it has become apparent that receptor-mediated intracellular signals are not linear cascades beginning at the plasma membrane and terminating with the production of a needed metabolite or the induction of gene expression. Instead, complex networks of interactive intracellular signals are activated in response to extracellular stimuli. Many responses to extracellular stimuli are mediated by protein-tyrosine kinases (PTKs). Activating PTKs leads to the recruitment of a variety of intracellular signalling molecules that execute a complex set of instructions. The response to PTK activity is dependent upon which PTK is activated and the cellular context in which the PTK exists. Several signalling molecules recruited by PTKs are involved in the metabolism of phospholipids. In this Mini Review, intracellular signalling networks activated by PTKs are discussed with an emphasis on the potential for generating highly specific and sophisticated responses to PTK activity through phospholipid metabolism.

Regulation of the metabolism of 1,2-diacylglycerols and inositol phosphates that respond to receptor activation

This review assimilates information on the regulation of the metabolism of those inositol phosphates and diacylglycerols that respond to receptor activation. Particular emphasis is placed on the regulation of specific enzymes, the occurrence of isoenzymes, and metabolic compartmentalization; the overall aim is to demonstrate the significance of these activities in relation to the physiological impact of the various cell signalling processes.