Glycerolipid biosynthesis in eukaryotes

6-Benzylaminopurine causes lipid dyshomeostasis via disruption of glycerophospholipid metabolism in zebrafish

6-Benzylaminopurine (6-BA) is ubiquitous in agricultural production and is accessible to humans through diets. The modulation of lipid metabolism by 6-BA has been previously demonstrated in plants and oleaginous microorganisms. Therefore, whether it alters lipid homeostasis in other living organisms requires further investigation. In this study, doses ≥10 mg 6-BA/L caused malformation of the yolk sac, steatosis, and other hepatopathies in zebrafish larvae. Exposure to 25 mg 6-BA/L resulted in increased levels of triglyceride and total cholesterol. Results of transcriptomic analysis indicated that 6-BA alters genes associated with fatty acid and glycerophospholipid metabolism. Among them, the expression levels of hmgcra, elovl7b, and apobb.2 were downregulated, whereas those of lpcat3, bco1l, cyp7al, fabp1b.1, elp6, pde6ha, apoa4b.2_2, sgk1, dgkaa, and mogat2 were upregulated. Correspondingly, a study of the metabolome identified lysophosphatidylcholine (LPC) as the major differentially expressed metabolite in response to 6-BA treatment. Therefore, abnormal accumulation of LPCs and dyshomeostasis of glycerophospholipid metabolism were identified as potential mechanisms causing the toxicity of 6-BA, which should be assessed to understand the risks of 6-BA and the products contaminated by it. ENVIRONMENTAL IMPLICATION: 6-Benzylaminopurine (6-BA), an important residue in "toxic bean sprouts," is ubiquitous in agricultural production and is common in typical diets. Its regulation of lipid metabolism has been demonstrated in plants and oleaginous microorganisms. Whether it alters lipid homeostasis in other organisms and the underlying mechanisms remain largely unknown. The worldwide use of 6-BA and the potential exposure of humans have aroused public attention owing to its hazardous effects; thus, its hazardous effects, particularly those on lipid homeostasis, deserve careful clarification.

Unveiling Glycerolipid Fragmentation by Cryogenic Infrared Spectroscopy

Mass spectrometry is routinely employed for structure elucidation of molecules. Structural information can be retrieved from intact molecular ions by fragmentation; however, the interpretation of fragment spectra is often hampered by poor understanding of the underlying dissociation mechanisms. For example, neutral headgroup loss from protonated glycerolipids has been postulated to proceed via an intramolecular ring closure but the mechanism and resulting ring size have never been experimentally confirmed. Here we use cryogenic gas-phase infrared (IR) spectroscopy in combination with computational chemistry to unravel the structures of fragment ions and thereby shed light on elusive dissociation mechanisms. Using the example of glycerolipid fragmentation, we study the formation of protonated five-membered dioxolane and six-membered dioxane rings and show that dioxolane rings are predominant throughout different glycerolipid classes and fragmentation channels. For comparison, pure dioxolane and dioxane ions were generated from tailor-made dehydroxyl derivatives inspired by natural 1,2- and 1,3-diacylglycerols and subsequently interrogated using IR spectroscopy. Furthermore, the cyclic structure of an intermediate fragment occurring in the phosphatidylcholine fragmentation pathway was spectroscopically confirmed. Overall, the results contribute substantially to the understanding of glycerolipid fragmentation and showcase the value of vibrational ion spectroscopy to mechanistically elucidate crucial fragmentation pathways in lipidomics.

Casein kinase II-mediated phosphorylation of lipin 1β phosphatidate phosphatase at Ser-285 and Ser-287 regulates its interaction with 14-3-3β protein

The mammalian lipin 1 phosphatidate phosphatase is a key regulatory enzyme in lipid metabolism. By catalyzing phosphatidate dephosphorylation, which produces diacylglycerol, the enzyme plays a major role in the synthesis of triacylglycerol and membrane phospholipids. The importance of lipin 1 to lipid metabolism is exemplified by cellular defects and lipid-based diseases associated with its loss or overexpression. Phosphorylation of lipin 1 governs whether it is associated with the cytoplasm apart from its substrate or with the endoplasmic reticulum membrane where its enzyme reaction occurs. Lipin 1β is phosphorylated on multiple sites, but less than 10% of them are ascribed to a specific protein kinase. Here, we demonstrate that lipin 1β is a bona fide substrate for casein kinase II (CKII), a protein kinase that is essential to viability and cell cycle progression. Phosphoamino acid analysis and phosphopeptide mapping revealed that lipin 1β is phosphorylated by CKII on multiple serine and threonine residues, with the former being major sites. Mutational analysis of lipin 1β and its peptides indicated that Ser-285 and Ser-287 are both phosphorylated by CKII. Substitutions of Ser-285 and Ser-287 with nonphosphorylatable alanine attenuated the interaction of lipin 1β with 14-3-3β protein, a regulatory hub that facilitates the cytoplasmic localization of phosphorylated lipin 1. These findings advance our understanding of how phosphorylation of lipin 1β phosphatidate phosphatase regulates its interaction with 14-3-3β protein and intracellular localization and uncover a mechanism by which CKII regulates cellular physiology.

Lipid production and characterization by Mortierella (Umbelopsis) isabellina cultivated on lignocellulosic sugars

AIMS

To study and characterize the lipids produced by Mortierella (Umbelopsis) isabellina, during its growth on mixtures of glucose and xylose.

METHODS AND RESULTS

Glucose and xylose were utilized as carbon sources, solely or in blends, under nitrogen-limited conditions, in batch-flask trials (initial sugars at 80 g l^-1 ). Significant lipid production (maximum lipid 17·8 g l^-1 ; lipid in DCW 61·0% w/w; lipid on glucose consumed 0·23 g g^-1 ) occurred on glucose employed solely, while xylose concentration in the growth medium was conversely correlated with lipid accumulation. With increasing xylose concentrations into the blend, lipid storage decreased while xylitol in significant concentrations (up to 24 g l^-1 ) was produced. Irrespective of the sugar blend employed, significant quantities of endopolysaccharides were detected in the first growth steps (in the presence of nitrogen into the medium or barely after its disappearance) while lipids were stored thereafter. Neutral lipids, mainly composed of triacylglycerols, were the main microbial lipid fraction. Phospholipids were quantified both through fractionation and subsequent gravimetric determination and also through determination of phosphorus, and it seemed that the second method was more accurate. Phospholipids were mainly composed of phosphatidylcholine and another nonidentified compound presumably being phosphatidyldimethylethanolamine.

CONCLUSIONS

Mortierella isabellina is suitable to convert lignocellulosic sugars into lipids.

SIGNIFICANCE AND IMPACT OF THE STUDY

Differentiations between metabolism on xylose and glucose were reported. Moreover, this is one of the first reports indicating extensive analysis of microbial lipids produced by M. isabellina.

Effect of leptin infusion on insulin sensitivity and lipid metabolism in diet-induced lipodystrophy model mice

BACKGROUND

Lipodystrophies are rare acquired and genetic disorders characterized by the complete or partial absence of body fat with a line of metabolic disorders. Previous studies demonstrated that dietary conjugated linoleic acid (CLA) induces hepatic steatosis and hyperinsulinemia through the drastic reduction of adipocytokine levels due to a paucity of adipose tissue in mice and the pathogenesis of these metabolic abnormalities in CLA-fed mice is similar to that in human lipodystrophy. The present study explores the effect of leptin infusion on the pathogenesis of diet-induced lipodystrophy in mice. C57BL/6N mice were assigned to three groups: (1) mice were fed a semisynthetic diet supplemented with 6% corn oil and infused PBS intraperitoneally (normal group), (2) mice were fed a semisynthetic diet supplemented with 4% corn oil plus 2% CLA and infused PBS intraperitoneally (lipodystrophy-control group), and (3) mice were fed a semisynthetic diet supplemented with 4% corn oil plus 2% CLA and infused recombinant murine leptin intraperitoneally (lipodystrophy-leptin group). All mice were fed normal or lipodystrophy model diets for 4 weeks and were infused intrapeneally 0 or 5 mug of leptin per day from third week of the feeding period for 1 week.

RESULTS

The results indicate that leptin infusion can attenuate hepatic steatosis and hyperinsulinemia through the reduction of hepatic triglyceride synthesis and the improvement of insulin sensitivity in diet-induced lipodystrophy model mice.

CONCLUSION

We expect the use of this model for clarifying the pathophysiology of lipodystrophy-induced metabolic abnormalities and evaluating the efficacy and safety of drug and dietary treatment.

Unique molecular signatures of glycerophospholipid species in different rat tissues analyzed by tandem mass spectrometry

Glycerophospholipids (GPL) in animal tissues are composed of a large array of molecular species that mainly differ in the fatty acyl composition. In order to further understand the roles of GPL at the molecular level, it is necessary to have comprehensive, accurate accounts of the molecular makeup for these molecules in animal tissues. However, this task was difficult simply because the conventional technologies of profiling GPL species depended heavily on technical skill for accuracy and reliability and were extremely labor-intensive. In recent years, tandem mass spectrometry (MS/MS) proved to be a highly reliable and sensitive technology for profiling small molecules, including GPL, in biological samples. In this study, we used this technology to perform simultaneous comparative analyses for phosphatidylcholine (PC), phosphatidylethanolamine (PE), phosphatidylserine (PS) and phosphatidylinositol (PI) in the same lipid preparations of liver, lung, kidney, heart, pancreas, stomach, small intestine, spleen, skeleton muscle and brain of an adult rat. We produced molecular profiles of these 4 GPL classes in these 10 different tissues that are highly reproducible between different scans of the same sample and between samples from different animals. It is intriguing that each tissue was found to possess a unique signature of GPL profile that may be used to identify unknown tissues. More importantly, these profiles may also set reference points for studying changes of GPL metabolism in different physiological and pathological conditions.

Lipid analysis of the sex pheromone gland of the moth Heliothis virescens

The sex pheromone gland of female Heliothis virescens was analyzed for fatty acid and lipid content. Base methanolysis of the gland showed a large amount of methyl (Z)-11-hexadecenoate (Z11-16:Acyl), the fatty acyl analog of the major pheromone component, (Z)-11-hexadecenal, as well as a small amount of methyl (Z)-11-octadecenoate. Methyl esters of various common fatty acids were also observed. HPTLC analysis of the glandular lipids revealed large quantities of triacylglycerols (TGs), and lesser amounts of 1,2-diacylglycerols (1,2-DGs), 2-monoacylglycerols (2-MGs), phosphatidyl ethanolamines, and phosphatidyl cholines. The greatest amount of Z11-16:Acyl in these lipids was in the TGs, with lesser amounts in the two phospholipid classes and only trace amounts in the other neutral lipids. The glands of females at various ages and photoperiodic times were extracted, fractionated into neutral and polar fractions by silica SPE, and fatty acid titers in these fractions determined. All fatty acids, but notably Z11-16:Acyl, showed significant total and neutral lipid fraction peaks at mid scotophase for 2-day-old females; a less dramatic, but significant, Z11-16:Acyl peak in the polar fraction was also observed. However, only a relatively small proportion (<50%) of this acid was recovered from the silica at all times. This "non-recoverable" Z11-16:Acyl showed a dramatic and significant peak at mid scotophase for 2-day females, corresponding roughly with maximal pheromone titer. All other acids in the gland were recovered in high proportions, and their respective "non-recoverable" titers were not different at any of the times analyzed. Based on previous work, this non-recoverable Z11-16:Acyl is likely the CoA ester. Therefore, it appears that the pheromone gland of H. virescens maintains pools of Z11-16:Acyl in both CoA ester and TG forms, which are available for biosynthesis of pheromone. These pools are greatest during maximal pheromone production when the biosynthetic enzymes, possibly the fatty acid reductase, are unable to utilize rapidly enough the quantities of Z11-16:Acyl biosynthesized.

Trimetazidine effect on phospholipid synthesis in ventricular myocytes: consequences in alpha-adrenergic signaling

The anti-anginal drug trimetazidine (TMZ) has been shown to increase the synthesis of phospholipids in ventricular myocytes, including phosphatidyl-inositol (PI). This study focused on the consequences of increasing PI metabolism on alpha-adrenergic signaling pathway in cultured rat cardiomyocytes. In the cells treated with TMZ, the synthesis of PI from inositol was largely increased as compared with the control (+55% in 60 min). The stimulation of alpha-adrenergic receptors by phenylephrine (PE) induced a dose-dependent production of inositide phosphates (IPs) by phospholipase C (PLC) activation. However, the amount of available IPs was significantly lower in TMZ-treated cells, in a dose-dependent manner. This effect was observed in the presence and absence of the IP1-phosphatase inhibitor LiCl. The in vitro determination of PLC activity revealed that this effect could not be attributed to the direct inhibition of the enzyme by TMZ. The TMZ-induced reduction of IPs in the PE-stimulated cardiomyocytes should be attributed to the increase of inositol recycling and incorporation in membrane structures, elicited by increased phospholipid synthesis. The consequences of this reduction in IPs availability were investigated on the cardiomyocyte hypertrophy induced by alpha-adrenergic chronic stimulation. Acute stimulation with PE increased protein synthesis (+50%), but this increase was largely prevented by TMZ. In conclusion, TMZ reduces cell available IPs, by accelerating their recycling in membranes as PI. This effect results in a cytoprotection in the pathological process of hypertrophy elicited by chronic alpha-adrenergic stimulation.

Biochemistry and genetics of interorganelle aminoglycerophospholipid transport

The organelle specific reactions that constitute the biosynthetic pathway for aminoglycerophospholipid synthesis provide an important means for examining the biochemistry and genetics of intracellular lipid transport. Biochemical studies with intact and permeabilized cells, and isolated organelles have defined some of the essential features of lipid transport between the endoplasmic reticulum and mitochondria and Golgi/vacuole. Genetic screens have now also identified mutations and genes that are involved in aminoglycerophospholipid traffic between different membranes in mammalian cells, yeast and bacteria. Increasingly, studies focused upon intermembrane lipid movement are revealing important new information about this essential aspect of membrane biogenesis.

Overexpression of PEMT2 downregulates the PI3K/Akt signaling pathway in rat hepatoma cells

Phosphatidylethanolamine N-methyltransferase 2 (PEMT2) is an isoform of PEMT that converts phosphatidylethanolamine to phosphatidylcholine in mammalian liver. Overexpression of PEMT2 led to inhibition of proliferation of hepatoma cells [J. Biol. Chem. 269 (1994) 24531]. The present study aims to unravel the molecular mechanism of the reduced proliferation, especially the signaling transducer proteins involved in this process. Thus, we chose PI3K/Akt pathway that is initiated by growth factors and leads to cell survival and proliferation. Rat hepatoma CBRH-7919 cells transfected with pemt2-cDNA showed that: (1) signaling proteins including c-Met, PDGF receptor, PI3K, Akt and Bcl-2 all had reduced expression as shown by Western blotting studies; (2) flow cytometric and DNA ladder assays showed that 22.9% of the pemt2-transfected cells were undergoing apoptosis; (3) the activity of Akt was decreased as shown by Western blotting using antibody directed against p-Akt (Thr308); (4) wortmannin and PD98059, inhibitors of PI3K and MEK, respectively, both inhibited Akt activity, indicating that PI3K and MAPK pathways were merging at Akt in CBRH-7919 cells. The above results suggest that overexpression of PEMT2 strongly downregulated the PI3K/Akt signaling pathway at multiple sites and induced apoptosis. This, at least partly, explains the molecular mechanism of impaired proliferation induced by pemt2 transfection.

Regulation of vesicle trafficking, transcription, and meiosis: lessons learned from yeast regarding the disparate biologies of phosphatidylcholine

Phosphatidylcholine (PtdCho) is the major phospholipid present in eukaryotic cell membranes generally comprising 50% of the phospholipid mass of most cells and their requisite organelles. PtdCho has a major structural role in maintaining cell and organelle integrity, and thus its synthesis must be tightly monitored to ensure appropriate PtdCho levels are present to allow for its coordination with cell growth regulatory mechanisms. One would also expect that there needs to be coordinated regulation of PtdCho synthesis with its transport from its site of synthesis to cellular organelles to ensure organellar structures and functions are maintained. Each of these processes need to be intimately coordinated with cellular growth decision making processes. To this end, it has recently been revealed that ongoing PtdCho synthesis is required for global transcriptional regulation of phospholipid synthesis. PtdCho is also a major component of intracellular transport vesicles and the synthesis of PtdCho is intimately involved in the regulation of vesicle transport from the Golgi apparatus to the cell surface and the vacuole (yeast equivalent of the mammalian lysosome). This review details some of the more recent advances in our knowledge concerning the role of PtdCho in the regulation of global lipid homeostasis through (i) its restriction of the trafficking of intracellular vesicles that distribute lipids and proteins from their sites of synthesis to their ultimate cellular destinations, (ii) its regulation of specific transcriptional processes that coordinate lipid biosynthetic pathways, and (iii) the role of PtdCho catabolism in the regulation of meiosis. Combined, these regulatory roles for PtdCho ensure vesicular, organellar, and cellular membrane biogenesis occur in a coordinated manner.

Measurement of the length of the a helical section of a peptide directly using atomic force microscopy

Using atomic force microscopy (AFM), the length of the alpha-helix structure of poly-L-lysine was investigated by stretching the peptide directly, one molecule at a time. In the absence of urea, many rupturing points that seemed to be due to the breaking of some hydrogen bonds were observed in force-extension curves, while these points were never observed in the presence of 8 M urea. In the presence of 0.4 or 1.6 M urea, both force-extension curve types were observed. Total peptide elongation for each condition was calculated from force-extension curves reflecting the alpha-helix rupturing process. The experimental value of total elongation divided by the theoretical value of total alpha-helix elongation yields the alpha-helix content. This value was compatible with circular dichroism (CD) measurement results. This suggests that peptide conformation and content of the alpha-helix on a single molecule scale can be investigated by direct mechanical measurement using atomic force microscopy.

Influence of trimetazidine on the synthesis of complex lipids in the heart and other target organs

Trimetazidine exerts antianginal properties at the cellular level, without haemodynamic effect in clinical and experimental conditions. This cytoprotection was attributed to a decreased utilization of fatty acids for energy production, balanced by an increased incorporation in structural lipids. This study evaluated the influence of Trimetazidine on complex lipid synthesis from [2-(3)H] glycerol, in ventricular myocytes, isolated rat hearts and in vivo in the myocardium and several other tissues. In cardiomyocytes, Trimetazidine increased the synthesis of phosphatidyl-choline (+ 80%), phosphatidyl-ethanolamine (+ 210%), phosphatidyl-inositol (+ 250%) and cardiolipid (+ 100%). The common precursor diacylglycerol was also increased (+ 40%) whereas triacylglycerol was decreased (-70%). Similar results were obtained in isolated hearts with 10 microm Trimetazidine (phosphatidyl-choline + 60%, phosphatidyl-ethanolamine + 60%, phosphatidyl-inositol + 100% and cardiolipid + 50%), the last two phospholipids containing 85% of the radioactivity. At 1 microm, Trimetazidine still stimulated the phospholipid synthesis although the difference was found significant only in phosphatidyl-inositol and cardiolipid. In vivo studies (10 mg/kg per day for 7 days and 5 mg/kg, i.p. before the experiment) revealed significant changes in the intracellular lipid biosynthesis, with increased labelling of phospholipids and reduced incorporation of glycerol in nonphosphorous lipids. Trimetazidine increased the glycerol uptake from plasma to the other tissues (liver, cochlea, retina), resulting in an altered lipid synthesis. The anti-anginal properties of Trimetazidine involve a reorganisation of the glycerol-based lipid synthesis balance in cardiomyocytes, associated with an increased uptake of plasma glycerol that may contribute to explain the pharmacological properties reported in other organs.

Structure, expression profile and alternative processing of the human phosphatidylethanolamine N-methyltransferase (PEMT) gene

Phosphatidylethanolamine N-methyltransferase (PEMT) catalyzes the conversion of phosphatidylethanolamine (PE) to phosphatidylcholine (PC) in a series of three methylation reactions. Preliminary studies of PEMT in humans led to the cloning of three cDNAs each of which has a different 5' untranslated region (5'UTR). To determine the origin of PEMT splice variants and to investigate expression of the gene in human liver, we isolated a bacterial artificial chromosome (BAC) clone containing the full-length human gene. Each of the three unique untranslated first exons is present in a contiguous array in the gene, confirming the integrity of the cDNAs and alternative processing of PEMT transcripts. Human liver, heart and testis contain the highest levels of PEMT transcripts and of these, liver has the greatest PEMT expression. Furthermore, each of the three PEMT transcripts is present in varying abundance in liver whereas heart and testis contain only one and two transcripts, respectively. Thus, differential promoter usage in the human PEMT gene generates three unique transcripts and confers a tissue-specific expression pattern.

Unique phospholipid metabolism in mouse heart in response to dietary docosahexaenoic or alpha-linolenic acids

Diet and fatty acid metabolism interact in yet unknown ways to modulate membrane fatty acid composition and certain cellular functions. For example, dietary precursors or metabolic products of n-3 fatty acid metabolism differ in their ability to modify specific membrane components. In the present study, the effect of dietary 22:6n-3 or its metabolic precursor, 18:3n-3, on the selective accumulation of 22:6n-3 by heart was investigated. The mass and fatty acid compositions of individual phospholipids (PL) in heart and liver were quantified in mice fed either 22:6n-3 (from crocodile oil) or 18:3n-3 (from soybean oil) for 13 wk. This study was conducted to determine if the selective accumulation of 22:6n-3 in heart was due to the incorporation of 22:6n-3 into cardiolipin (CL), a PL most prevalent in heart and known to accumulate 22:6n-3. Although heart was significantly enriched with 22:6n-3 relative to liver, the accumulation of 22:6n-3 by CL in heart could not quantitatively account for this difference. CL from heart did accumulate 22:6n-3, but only in mice fed preformed 22:6n-3. Diets rich in non-22:6n-3 fatty acids result in a fatty acid composition of phosphatidylcholine (PC) in heart that is unusually enriched with 22:6n-3. In this study, the mass of PC in heart was positively correlated with the enrichment of 22:6n-3 into PC. The increased mass of PC was coincident with a decrease in the mass of phosphatidylethanolamine, suggesting that 22:6n-3 induced PC synthesis by increasing phosphatidylethanolamine-N-methyltransferase activity in the heart.

Interorganelle transport of aminoglycerophospholipids

The aminoglycerophospholipids of eukaryotic cells, phosphatidylserine (PtdSer), phosphatidylethanolamine (PtdEtn), and phosphatidylcholine (PtdCho), can be synthesized by multiple pathways. The PtdSer pathway encompasses the synthesis of PtdSer, its decarboxylation to PtdEtn and subsequent methylation reactions to form PtdCho. The Kennedy pathways consist of the synthesis of PtdEtn and PtdCho from Etn and Cho precursors via CDP-Etn and CDP-Cho intermediates. The reactions along the PtdSer pathway are spatially segregated with PtdSer synthesis occurring in the endoplasmic reticulum or mitochondria-associated membrane (MAM), PtdEtn formation occurring in the mitochondria and Golgi/vacuole compartments and PtdCho formation occurring in the endoplasmic reticulum or MAM. The organelle-specific metabolism of the different lipids in the PtdSer pathway has provided a convenient biochemical means for defining events in the interorganelle transport of the aminoglycerophospholipids in intact cells, isolated organelles and permeabilized cells. Studies with both mammalian cells and yeast demonstrate many significant similarities in lipid transport processes between the two systems. Genetic experiments in yeast now provide the tools to create new strains with mutations along the PtdSer pathway that can be conditionally rescued by the Kennedy pathway reactions. The genetic studies in yeast indicate that it is now possible to begin to define genes that participate in the interorganelle transport of the aminoglycerophospholipids.

Addition of arginine but not glycine to lysine plus methionine-enriched diets modulates serum cholesterol and liver phospholipids in rabbits

Previous experiments from our laboratory showed that in rabbits fed an amino acid diet corresponding to 30% casein, enrichment of the diet with L-lysine and L-methionine caused a marked increase in serum total and LDL cholesterol levels as well as a substantial body weight loss. Both effects were partially prevented by supplementation with L-arginine. The present studies were designed to extend this earlier observation by assessing the role of different dietary amino acids in modulation of cholesterolemic responses and body weights. In the first experiment, the original lysine and methionine-enriched diet was supplemented with glycine in an attempt to modify methionine metabolism, and thus to reduce body weight loss. In addition, the mechanism of action of lysine and methionine was investigated by quantitation of major liver phospholipids. The results showed that glycine addition had no effect on weight loss or hypercholesterolemia, nor did it alter plasma levels of homocyst(e)ine, an intermediate in methionine metabolism. However, enrichment of the diet with lysine and methionine (with or without glycine) significantly increased liver levels of phosphatidylcholine and the ratio of phosphatidylcholine to phosphatidylethanolamine, apparently through increased enzymatic conversion. These changes were consistent with higher lipoprotein levels and thus may explain the hypercholesterolemia. A second experiment showed that similar effects on body weights and serum cholesterol could be obtained by adding lysine and methionine to a diet containing amino acids equivalent to only 15% casein, or 15% intact casein. This approach is more physiologic and also reduces the expense of experiments designed to study the effects of lysine and methionine in more detail.

The unique acyl chain specificity of biliary phosphatidylcholines in mice is independent of their biosynthetic origin in the liver

The liver synthesizes phosphatidylcholine (PC) de novo from choline via the CDP-choline pathway, and from phosphatidylethanolamine (PE) via the phosphatidylethanolamine N-methyltransferase (PEMT) pathway. Significant amounts of PC, which are highly specific in their acyl chain composition, are secreted into bile by the liver. To determine whether either of the 2 PC biosynthetic routes is sufficient to provide physiological PC concentrations in bile, or is responsible for the unique acyl chain composition of bile PC, we analyzed gallbladder bile composition in mice that synthesized PC either via the PEMT pathway (induced by feeding a choline-deficient diet) or the CDP-choline pathway (based on genetic PEMT-deficiency). The PC concentration in gallbladder bile of mice that synthesize PC mainly via the CDP-choline pathway was comparable with control mice that synthesize PC via both pathways, whereas it was reduced by approximately 40% in mice that synthesize PC via the PEMT pathway. The acyl chain composition of bile PC was similar irrespective of the active PC biosynthetic pathway in the liver. These data demonstrate that the CDP-choline pathway alone, but not the PEMT pathway alone, can account for physiological concentrations of PC in gallbladder bile. Moreover, the specificity of biliary PC fatty acyl composition is determined independently from the synthetic origin of PC.

Phospholipid biosynthesis in the yeast Saccharomyces cerevisiae and interrelationship with other metabolic processes

In this review, we have discussed recent progress in the study of the regulation that controls phospholipid metabolism in S. cerevisiae. This regulation occurs on multiple levels and is tightly integrated with a large number of other cellular processes and related metabolic and signal transduction pathways. Progress in deciphering this complex regulation has been very rapid in the last few years, aided by the availability of the sequence of the entire Saccharomyces genome. The assignment of functions to the remaining unassigned open reading frames, as well as ascertainment of remaining gene-enzyme relationships in phospholipid biosynthesis in yeast, promises to provide detailed understanding of the genetic regulation of a crucial area of metabolism in a key eukaryotic model system. Since the processes of lipid metabolism, secretion, and signal transduction show fundamental similarities in all eukaryotes, the dissection of this regulation in yeast promises to have wide application to our understanding of metabolic control in all eukaryotes.

Inhibition of phosphatidylcholine and phosphatidylethanolamine biosynthesis in rat-2 fibroblasts by cell-permeable ceramides

Phospholipids and sphingolipids are important precursors of lipid-derived second messengers such as diacylglycerol and ceramide, which participate in several signal transduction pathways and in that way mediate the effects of various agonists. The cross-talk between glycerophospholipid and sphingolipid metabolism was investigated by examining the effects of cell-permeable ceramides on phosphatidylcholine (PtdCho) and phosphatidylethanolamine (PtdEtn) synthesis in Rat-2 fibroblasts. Addition of short-chain C6-ceramide to the cells resulted in a dose- and time-dependent inhibition of the CDP-pathways for PtdCho and PtdEtn synthesis. Treatment of cells for 4 h with 50 microM C6-ceramide caused an 83% and a 56% decrease in incorporation of radiolabelled choline and ethanolamine into PtdCho and PtdEtn, respectively. Exposure of the cells for longer time-periods (>/= 16 h) to 50 microM C6-ceramide resulted in apoptosis. The structural analogue dihydro-C6-ceramide did not affect PtdCho and PtdEtn synthesis. In pulse-chase experiments, radioactive choline and ethanolamine accumulated in CDP-choline and CDP-ethanolamine under the influence of C6-ceramide, suggesting that synthesis of both PtdCho and PtdEtn were inhibited at the final step in the CDP-pathways. Indeed, cholinephosphotransferase and ethanolaminephosphotransferase activities in membrane fractions from C6-ceramide-treated cells were reduced by 64% and 43%, respectively, when compared with control cells. No changes in diacylglycerol mass levels or synthesis of diacylglycerol from radiolabelled palmitate were observed. It was concluded that C6-ceramide affected glycerophospholipid synthesis predominantly by inhibition of the step in the CDP-pathways catalysed by cholinephosphotransferase and ethanolaminephosphotransferase.

Enhancement of free fatty acid incorporation into phospholipids by choline plus cytidine

Cytidine and choline, present in cytidine 5'-diphosphate choline (CDP-choline), are major precursors of the phosphatidylcholine found in cell membranes and important regulatory elements in phosphatide biosynthesis. Administration of CDP-choline to rats increases blood and brain cytidine and choline levels; this enhances the production of endogenous CDP-choline which then combines with fatty acids (as diacylglycerol), to yield phosphatidylcholine. We examined the effect of providing cytidine and choline on incorporation of free fatty acids into phosphatidylcholine and other major phospholipids in PC12 cells. Addition of equimolar cytidine and choline (100-500 microM) to [3H]-arachidonic acid (50 microM, 0.2 microCi, bound to bovine serum albumin) dose-dependently increased the accumulations of [3H]-phosphatidylcholine (PtdCho), [3H]-phosphatidylinositol (PtdIno) and [3H]-phosphatidylethanolamine (PtdEtn) (by up to 27+/-3%, 16+/-3% and 11+/-3%, respectively, means+/-S.E.M.). This effect was seen with 8-18 h of incubation. The incorporation of [3H]-oleic acid into [3H]-PtdCho was even more enhanced (by up to 42+/-3%) as were the incorporations of [14C]-choline and [3H]-glycerol. The effects of choline and cytidine were enhanced by 12-O-tetradecanoylphorbol-13-acetate (TPA, 1 microM), which activates CTP:phosphocholine cytidylyltransferase (CT) and facilitates choline uptake. Replacing choline by ethanolamine also enhanced the incorporation of [3H]-arachidonic acid into [3H]-PtdEtn, [3H]-PtdIno and [3H]-PtdCho. Arachidonic acid (10-200 microM) alone failed to affect the incorporation of [14C]-choline into phosphatidylcholine. We suggest that the increases in phospholipid synthesis caused by concurrent cytidine and choline supplementation enhance the incorporation of arachidonic acid and certain other fatty acids into the major glycerophospholipids. Removing these fatty acids as source of potentially toxic oxidation products could contribute to the beneficial effects of CDP-choline in treating stroke or other brain damage.

Lysophosphatidylcholine acyltransferase activity in Saccharomyces cerevisiae: regulation by a high-affinity Zn2+ binding site

Saccharomyces cerevisiae cells were demonstrated to contain lysophosphatidylcholine (lysoPtdCho) acyltransferase (E.C. 2.3.1.23) activity. The enzyme displayed Km(app) of 69 microM for lysoPtdCho and 152 microM for oleoyl CoA. Enzyme activity was not affected by the addition of 1 mM Mg2+, Mn2+, Ca2+, or 200 mM EDTA. However, Zn2+ inhibited lysoPtdCho acyltransferase activity to 33% control values at 0.1 mM and to 7% at 1.0 mM Zn2+. To further explore the possibility that lysoPtdCho acyltransferase may contain a high-affinity Zn2+ binding site, we tested the strong Zn2+ chelator o-phenanthroline for its ability to inhibit enzyme activity. LysoPtdCho acyltransferase activity was inhibited to 18 and 27%, respectively, those of control values in the presence of 2 and 1 mM o-phenanthroline, implying that a high-affinity Zn2+ binding site exists in lysoPtdCho acyltransferase or in an accessory protein that is essential for protein stability and/or activity. Saccharomyces cerevisiae lysoPtdCho acyltransferase activity displayed a broad lysoPtdCho fatty acyl chain substrate specificity utilizing lysoPtdCho molecules ranging in length from C10-C20 (the entire range tested). In addition, the enzyme was capable of using the ether-linked analog of lysoPtdCho, 1-O-alkyl-2-hydroxy-sn-3-glycerophosphocholine, as a substrate. The ability of S. cerevisiae to incorporate radiolabeled 1-O-alkyl-2-hydroxy-sn-3-glycerophosphocholine into phosphatidylcholine in vitro was exploited to demonstrate a direct precursor-product relationship between lysoPtdCho molecules and their incorporation into phosphatidylcholine in vivo. Identical labeling results were obtained in S. cerevisiae cells disrupted for their major transacylase activity, PLB1, demonstrating that the incorporation of lysolipid was via acyltransferase, and not transacylase, activity.

Agonist-stimulated glycerophospholipid acyl turnover in alveolar macrophages

The inflammatory compounds beta-glucan, a particulate agonist, and tannin, a soluble agonist, are present in cotton dust and, when inhaled, cause massive arachidonic acid release from alveolar macrophages. Earlier work had shown that these agonists exhibit different effects on arachidonate liberation and release, and that only tannin inhibits the uptake and incorporation of exogenous arachidonic acid, suggesting inhibition of reacylation. Here we have used the time-dependent incorporation of 18O from H218O-containing media into glycerophospholipid acyl groups as an indicator of acyl turnover in resting and agonist-treated rabbit alveolar macrophages. Highest turnover rates were seen in phosphatidylinositol ( approximately 30% per hour) and in choline phospholipids (10-20% per hour). Both beta-glucan and tannin stimulated acyl turnover, especially arachidonic acid turnover, in these and other lipid classes by a factor of 2 or more. We conclude that neither agonist promotes arachidonic acid accumulation in and release from alveolar macrophages by inhibiting reacylation.

Reversible translocation of CTP:phosphocholine cytidylyltransferase from cytosol to membranes in the adult bovine liver around parturition

The key regulatory enzyme of phosphatidylcholine (PC) synthesis, CTP:phosphocholine cytidylyltransferase (CT), is known to be activated in vitro by translocation from soluble to particulate fractions of the cell. In the present study the periparturient cow was chosen as a model to investigate whether translocation of CT can contribute to the regulation of PC synthesis in vivo. Between parturition and 1.5 weeks post-partum, the cytosolic CT activity in the liver of the adult animal decreased 1.9-fold, and this correlated with a 1.8-fold increase in microsomal CT activity. At that time, microsomal CT activity started to decline again whereas the cytosolic activity rose concomitantly until both activities reached their pre-partum values at 8 weeks post-partum. The activities of soluble and membrane-bound CTP:phosphoethanolamine cytidylyltransferase (ET), the analogous enzyme in the CDP-ethanolamine pathway, did not change significantly throughout this period. Whereas hepatic PC concentrations declined until about 2 weeks post-partum and thereafter gradually returned to pre-partum levels, the PC levels in very-low-density-lipoproteins, started to rise 2 weeks after the partus reaching a maximum of 219% of the original value at 8 weeks post-partum. These results strongly suggest that there is a reversible redistribution of CT between cytosol and membranes in a physiologically relevant animal model, supporting the concept that translocation of CT is occurring in vivo.

Control of membrane phosphatidylcholine biosynthesis by diacylglycerol levels in neuronal cells undergoing neurite outgrowth

Phospholipids are the major components of cell membranes and are required for cellular growth. We studied membrane phosphatidylcholine (PtdCho) biosynthesis in neuronal cells undergoing neurite outgrowth, by using PC12 cells as a model system. When neurite outgrowth was induced by exposing PC12 cells to nerve growth factor for 2 and 4 days, the amounts of [14C]choline incorporated into [14C]phosphatidylcholine per cell (i.e., per DNA) increased approximately 5- and 10-fold, respectively, as compared with control cells, reflecting increases in the rate of PtdCho biosynthesis. [14C]choline uptake was not affected. Analysis of the three major PtdCho biosynthetic enzymes showed that the activity of CDPcholine:1,2-diacylglycerol cholinephosphotransferase was increased by approximately 50% after nerve growth factor treatment, but the activities of choline kinase or choline-phosphate cytidylyltransferase were unaltered; the cholinephosphotransferase displayed a high Km value ( approximately 1,200 microM) for diacylglycerol. Moreover, free cellular diacylglycerol levels increased by approximately 1.5- and 4-fold on the second and fourth days, respectively. These data indicate that PtdCho biosynthesis is enhanced when PC12 cells sprout neurites, and the enhancement is mediated primarily by changes in cholinephosphotransferase activity and its saturation with diacylglycerol. This suggests a novel regulatory role for diacylglycerol in membrane phospholipid biosynthesis.

CDP-choline:1,2-diacylglycerol cholinephosphotransferase

Cholinephosphotransferase transfers a phosphocholine moiety from CDP-choline to diacylglycerol thus forming phosphatidylcholine (PtdCho) and CMP. This reaction defines the ultimate step in the Kennedy pathway for the genesis of de novo synthesized PtdCho. Hence, the intracellular location of cholinephosphotransferase identifies both the site from which de novo synthesized PtdCho is transported to other organelles and the site from which it is assembled with proteins and other lipids for secretion from the cell during the generation of lung surfactant, lipoproteins, and bile. Most subcellular fractionation studies observed the majority of cholinephosphotransferase activity in the endoplasmic reticulum, although the method of subcellular fractionation was found to grossly affect these results with activity alternately dispersed within Golgi, nuclear, and mitochondrial fractions. Coupling subcellular fractionation results with immunofluorescence or electron microscopy studies would resolve the issue of the site of PtdCho synthesis. However, antibodies have yet to be generated to cholinephosphotransferase since its integral membrane-bound nature has prevented its purification from any source and a mammalian cholinephosphotransferase cDNA has also yet to be isolated. However, cholinephosphotransferase genes have recently been isolated from the yeast Saccharomyces cerevisiae. Structure/function analysis of the S. cerevisiae cholinephosphotransferase has allowed for an in depth molecular examination resulting in the identification of the catalytic site. In addition, this analysis has generated the predicted amino acid data necessary to produce antibodies to pursue the site of PtdCho synthesis in this organism, as well as to provide information that should allow for the isolation of mammalian cholinephosphotransferase cDNA(s).

The phospholipid methyltransferases in yeast

In fungal microorganisms including fission yeast, Schizosaccharomyces pombe and baker's yeast, Saccharomyces cerevisiae, two enzymes are required to catalyze the synthesis of phosphatidylcholine (PC) from phosphatidylethanolamine (PE). The genes encoding the class I and class II phospholipid N-methyltransferases (PLMTs) have been cloned from both yeasts. The class II PLMTs catalyze the first methylation step from PE to phosphatidyl-monomethylethanolamine (PMME). Representatives of the class II type enzymes have been isolated only from yeast and the amino acid sequence of these enzymes contain regions of internal duplication. The class I PLMTs catalyze the last two methylation steps from PMME to PC. The class I PLMTs from both yeasts are homologous to the products of the phosphatidylethanolamine methyltransferase (PEMT) genes isolated from mouse and rat (described in the article by Vance et al. in this volume). Like the mammalian PEMT gene products, the S. cerevisiae class I enzyme can catalyze all three methylation steps to PC biosynthesis. S. cerevisiae strains, in which either the class II or class I enzyme is deleted, grow slowly in the absence of choline and exhibit low levels of PC. However, in S. pombe, mutants lacking either one of the two PLMTs are choline auxotrophs. Thus, both enzymes are required in S. pombe for maximal growth in the absence of exogenous choline. The S. cerevisiae methyltransferase genes are regulated at the level of transcription in response to the soluble precursors, inositol and choline as well as to growth phase. The mechanism of regulation of the S. pombe methyltransferases is not yet understood but appears to occur post-transcriptionally in response to choline availability. In addition, the S. pombe PLMT genes are regulated transcriptionally in response to growth phase.

CTP:phosphoethanolamine cytidylyltransferase

CTP:phosphoethanolamine cytidylyltransferase (ET) catalyzes the conversion of phosphoethanolamine into CDP-ethanolamine. Immunogold electron microscopy studies have demonstrated that, in hepatocytes, ET is localized predominantly in areas of the cytoplasm that are rich in rough endoplasmic reticulum (RER). Within these areas the enzyme shows a bimodal distribution between the cisternae of the RER and the cytosolic space. Studies on the substrate specificity of ET have shown that it can utilize both CTP and dCTP as substrates, but not other trinucleotides. In addition, the enzyme shows a very pronounced specificity for phosphoethanolamine. Under most conditions ET contributes significantly to the overall regulation of the CDP-ethanolamine pathway. Reversible binding of the enzyme to the endoplasmic reticulum could potentially play a key-role in metabolic channeling of phosphatidylethanolamine synthesis. ET has been purified from rat liver. Convincing evidence has been provided that ET and CTP:phosphocholine cytidylyltransferase (CT), the analogous enzyme in the CDP-choline pathway, are separate activities that reside on different proteins. The gene coding for yeast ET has been cloned. The deduced amino acid sequence contained a region in the N-terminal half with significant similarities to the conserved catalytic domain of both yeast and rat CT. The human cDNA for ET was also cloned recently. The predicted amino acid sequence of human ET shows a high degree of similarity (36% identity) to that of yeast ET, but the human protein is longer than the yeast protein, especially at the C-terminal region. Interestingly, both yeast and human ET have a large repetitive sequence in their N-terminal and C-terminal half.

CDP-choline:alkylacetylglycerol cholinephosphotransferase catalyzes the final step in the de novo synthesis of platelet-activating factor

Platelet-activating factor (PAF) can be synthesized de novo or by a remodeling mechanism involving the sn-2 acyl moiety of alkylacylglycerophosphocholines, a membrane-bound precursor. The final step in the de novo pathway is catalyzed by a dithiothreitol-insensitive cholinephosphotransferase that utilizes 1-alkyl-2-acetyl-sn-glycerol and CDP-choline as substrates. This article reviews various studies concerning the occurrence, assay, subcellular location, biochemical properties, substrate specificity, and regulatory controls of the PAF-related cholinephosphotransferase. Alkylacetylglycerol cholinephosphotransferase, which is located on the cytoplasmic surface of the endoplasmic reticulum, is widely distributed among mammalian tissues. Both the alkyl and acyl analogs of radylacetylglycerol are utilized at equivalent rates. Optimal enzyme activity occurs at pH 8.0 and Mg2+ is required, whereas calcium, deoxycholate, ethanol, and centrophenoxine are inhibitory. Formation of CDP-choline by cytidylyltransferase appears to play a crucial role in the regulation of PAF produced via the cholinephosphotransferase route. Significant differences exist in the behavior of the cholinephosphotransferase activities responsible for the synthesis of PAF and phosphatidylcholine. However, neither enzyme activity has been purified or cloned and, therefore, it is unknown whether a single or two separate proteins are responsible for the observed catalytic activities that form these two distinctly different classes of phospholipids.