Regulatory enzymes of phosphatidylcholine biosynthesis: a personal perspective

A metabonomic approach to chemosensitivity prediction of cisplatin plus 5-fluorouracil in a human xenograft model of gastric cancer

The prediction of chemosensitivity is a challenging problem in the management of cancer. In the present study, a metabonomic approach was proposed to assess the feasibility of chemosensitivity prediction in a human xenograft model of gastric cancer. BALB/c-nu/nu mice were transplanted with MKN-45 cell line to establish the xenograft model. The mice were then randomized into treatment group (cisplatin and 5-fluorouracil) and control group (0.9% sodium chloride), and their plasma were collected before treatment. Metabolic profiles of all plasma samples were acquired by using high-performance liquid chromatography coupled with a quadrupole time-of-flight mass spectrometer (HPLC/Q-TOF-MS). Based on the data of metabolic profiles and k-Nearest Neighbor algorithm, a prediction model for chemosensitivity was developed and an average accuracy of 90.4% was achieved. In addition, a series of endogenous metabolites, including 1-acyl-lysophosphatidycholines, polyunsaturated fatty acids and their derivatives, were determined as potential indicators of chemosensitivity. In conclusion, our results suggest that the proposed metabonomic approach allows effective chemosensitivity prediction in human xenograft model of gastric cancer. The approach presents a new concept in the chemosensitivtiy prediction of cancer and is expected to be developed as a powerful tool in the personalized cancer therapy.

The Phosphobase Methylation Pathway in Caernorhabditis elegans: A New Route to Phospholipids in Animals

Parasitic nematodes are a major cause of human health problems with an estimated 1 billion people infected worldwide by these organisms. Identifying biochemical targets that differ between the parasite and host species is essential for finding effective new anti-parasitic molecules. The free-living nematode Caenorhabditis elegans is a powerful model system for experiments in genetics and developmental biology needed to achieve this goal; however, in-depth understanding of metabolic processes in this organism is limited as it still contains unexplored biochemical pathways. Eukaryotes. including nematodes and humans, share many similar metabolic pathways, which makes specific targeting of nematode parasites challenging. Recent studies suggest that C. elegans and other nematodes may use a plant-like pathway as the major biosynthetic route to phosphatidylcholine. In this pathway, a pair of phosphoethanolamine methyltransferases (PMT) catalyze the sequential methylation of phosphoethanolamine to phosphocholine, which can be incorporated into phosphatidylcholine. RNAi experiments demonstrate that both PMT are required for normal growth and development of C. elegans. Because the PMT are highly conserved across nematode parasites of humans, livestock, and plants, as well as in protozoan parasites, understanding how these enzymes function and the identification of inhibitors will aid in the development of new anti-parasite compounds of potential medical, veterinary, and agricultural value.

Production of cytidine 5'-diphosphorylcholine with high utilization of ATP by whole cells of Saccharomyces cerevisiae

Cytidine 5'-diphosphorylcholine (CDP-choline) was produced using a high efficiency ATP regeneration system and the Kennedy pathway in whole cells of Saccharomyces cerevisiae As 2.398. Out of eight variables, KH(2)PO(4), glycerol and (NH(4))(2)SO(4) were considered to be the most significant factors by response surface methodology including a Plackett-Burman design, path of steepest accent and central composite design. The optimum levels of the three variables were 20.13g/L KH(2)PO(4), 12.35g/L glycerol and 0.49g/L (NH(4))(2)SO(4), respectively. Energy utilization efficiency increased from 10.59% to 16.72% and choline chloride conversion yields increased from 12.35% to 42.78%. A high efficiency ATP regeneration system improves CDP-choline production.

The Kennedy pathway--De novo synthesis of phosphatidylethanolamine and phosphatidylcholine

The glycerophospholipids phosphatidylcholine (PC) and phosphatidylethanolamine (PE) account for greater than 50% of the total phospholipid species in eukaryotic membranes and thus play major roles in the structure and function of those membranes. In most eukaryotic cells, PC and PE are synthesized by an aminoalcoholphosphotransferase reaction, which uses sn-1,2-diradylglycerol and either CDP-choline or CDP-ethanolamine, respectively. This is the last step in a biosynthetic pathway known as the Kennedy pathway, so named after Eugene Kennedy who elucidated it over 50 years ago. This review will cover various aspects of the Kennedy pathway including: each of the biosynthetic steps, the functions and roles of the phospholipid products PC and PE, and how the Kennedy pathway has the potential of being a chemotherapeutic target against cancer and various infectious diseases.

Pre- and postnatal health: evidence of increased choline needs

Choline, a micronutrient found in food, serves as the starting material for several important metabolites that play key roles in fetal development, particularly the brain. Although human beings' requirement for choline is unknown, an Adequate Intake level of 425 mg/day was established for women with upward adjustments to 450 and 550 mg/day during pregnancy and lactation, respectively. The importance of choline in human development is supported by observations that a human fetus receives a large supply of choline during gestation; pregnancy causes depletion of hepatic choline pools in rats consuming a normal diet; human neonates are born with blood levels that are three times higher than maternal blood concentrations; and large amounts of choline are present in human milk. The development of the central nervous system is particularly sensitive to choline availability with evidence of effects on neural tube closure and cognition. Existing data show that the majority of pregnant (and presumably lactating) women are not achieving the target intake levels and that certain common genetic variants may increase requirements for choline beyond current recommendations. Because choline is not found in most varieties of prenatal vitamins (or regular multivitamins), increased consumption of choline-rich foods may be needed to meet the high pre- and postnatal demands for choline.

Phosphatidic acid phosphohydrolase 1 and 2 regulate phospholipid synthesis at the endoplasmic reticulum in Arabidopsis

Phospholipid biosynthesis is essential for the construction of most eukaryotic cell membranes, but how this process is regulated in plants remains poorly understood. Here, we show that in Arabidopsis thaliana, two Mg(2+)-dependent phosphatidic acid phosphohydrolases called PAH1 and PAH2 act redundantly to repress phospholipid biosynthesis at the endoplasmic reticulum (ER). Leaves from pah1 pah2 double mutants contain ~1.8-fold more phospholipid than the wild type and exhibit gross changes in ER morphology, which are consistent with massive membrane overexpansion. The net rate of incorporation of [methyl-(14)C]choline into phosphatidylcholine (PC) is ~1.8-fold greater in the double mutant, and the transcript abundance of several key genes that encode enzymes involved in phospholipid synthesis is increased. In particular, we show that PHOSPHORYLETHANOLAMINE N-METHYLTRANSFERASE1 (PEAMT1) is upregulated at the level of transcription in pah1 pah2 leaves. PEAMT catalyzes the first committed step of choline synthesis in Arabidopsis and defines a variant pathway for PC synthesis not found in yeasts or mammals. Our data suggest that PAH1/2 play a regulatory role in phospholipid synthesis that is analogous to that described in Saccharomyces cerevisiae. However, the target enzymes differ, and key components of the signal transduction pathway do not appear to be conserved.

Glycerophospholipid acquisition in Plasmodium - a puzzling assembly of biosynthetic pathways

Throughout the Plasmodium life cycle, malaria parasites repeatedly undergo rapid cellular growth and prolific divisions, necessitating intense membrane neogenesis and, in particular, the acquisition of high amounts of phospholipids. At the intraerythrocytic stage, glycerophospholipids are the main parasite membrane constituents, which mostly originate from the Plasmodium-encoded enzymatic machinery. Several proteins and entire pathways have been characterized and their features reported, thereby generating a global view of glycerophospholipid synthesis across Plasmodium spp. The malaria parasite displays a panoply of pathways that are seldom found together in a single organism. The major glycerophospholipids are synthesized via ancestral prokaryotic CDP-diacylglycerol-dependent pathways and eukaryotic-type de novo pathways. The parasite exhibits additional reactions that bridge some of these routes and are otherwise restricted to some organisms, such as plants, while base-exchange mechanisms are largely unexplored in Plasmodium. Marked differences between Plasmodium spp. have also been reported in phosphatidylcholine and phosphatidylethanolamine synthesis. Little is currently known about glycerophospholipid acquisition at non-erythrocytic stages, but recent data reveal that intrahepatocytic parasites, oocysts and sporozoites import various host lipids, and that de novo fatty acid synthesis is only crucial at the late liver stage. More studies on the different Plasmodium developmental stages are needed, to further assemble the different pieces of this glycerophospholipid synthesis puzzle, which contains highly promising therapeutic targets.

The Kennedy phospholipid biosynthesis pathways are refractory to genetic disruption in Plasmodium berghei and therefore appear essential in blood stages

Phosphatidylcholine (PC) and phosphatidylethanolamine (PE) are the main membrane phospholipids (PLs) of Plasmodium parasites and can be generated by the de novo (Kennedy) CDP-choline and CDP-ethanolamine pathways and by the CDP-diacylglycerol dependent pathway. The Kennedy pathways initiate from exogenous choline and ethanolamine involving choline kinase (CK) and ethanolamine kinase (EK), followed by the choline-phosphate cytidylyltransferase (CCT) and ethanolamine-phosphate cytidylyltransferase (ECT) that catalyse the formation of CDP-choline and CDP-ethanolamine. Finally, in Plasmodium, PC and PE are apparently synthesized by a common choline/ethanolamine-phosphotransferase (CEPT). Here, we have studied the essential nature of the Kennedy pathways in Plasmodium berghei, a rodent malaria parasite. Sequence analysis of the P. berghei CEPT, CCT, ECT and CK enzymes revealed the presence of all catalytic domains and essential residues and motifs necessary for enzymatic activities. Constructs were designed for the generation of gene knockout and GFP-fusions of the cept, cct, ect and ck genes in P. berghei. We found that all four genes were consistently refractory to knockout attempts. At the same time, successful tagging of these proteins with GFP demonstrated that the loci were targetable and indicated that these genes are essential in P. berghei blood stage parasites. GFP-fusions of CCT, ECT and CK were found in the cytosol whereas the GFP-CEPT mainly localised in the endoplasmic reticulum. These results indicate that both CDP-choline and CDP-ethanolamine de novo pathways are essential for asexual P. berghei development and are non-redundant with other possible sources of PC and PE.

Crystal structure of a mammalian CTP: phosphocholine cytidylyltransferase catalytic domain reveals novel active site residues within a highly conserved nucleotidyltransferase fold

CTP:phosphocholine cytidylyltransferase (CCT) is the key regulatory enzyme in the synthesis of phosphatidylcholine, the most abundant phospholipid in eukaryotic cell membranes. The CCT-catalyzed transfer of a cytidylyl group from CTP to phosphocholine to form CDP-choline is regulated by a membrane lipid-dependent mechanism imparted by its C-terminal membrane binding domain. We present the first analysis of a crystal structure of a eukaryotic CCT. A deletion construct of rat CCTalpha spanning residues 1-236 (CCT236) lacks the regulatory domain and as a result displays constitutive activity. The 2.2-A structure reveals a CCT236 homodimer in complex with the reaction product, CDP-choline. Each chain is composed of a complete catalytic domain with an intimately associated N-terminal extension, which together with the catalytic domain contributes to the dimer interface. Although the CCT236 structure reveals elements involved in binding cytidine that are conserved with other members of the cytidylyltransferase superfamily, it also features nonconserved active site residues, His-168 and Tyr-173, that make key interactions with the beta-phosphate of CDP-choline. Mutagenesis and kinetic analyses confirmed their role in phosphocholine binding and catalysis. These results demonstrate structural and mechanistic differences in a broadly conserved protein fold across the cytidylyltransferase family. Comparison of the CCT236 structure with those of other nucleotidyltransferases provides evidence for substrate-induced active site loop movements and a disorder-to-order transition of a loop element in the catalytic mechanism.

Identification of hydrophobic amino acids required for lipid activation of C. elegans CTP:phosphocholine cytidylyltransferase

CTP:phosphocholine cytidylyltransferase (CCT), critical for phosphatidylcholine biosynthesis, is activated by translocation to the membrane surface. The lipid activation region of Caenorhabditis elegans CCT is between residues 246 and 266 of the 347 amino acid polypeptide, a region proposed to form an amphipathic alpha helix. When leucine 246, tryptophan 249, isoleucine 256, isoleucine 257, or phenylalanine 260, on the hydrophobic face of the helix, were changed individually to serine low activity was observed in the absence of lipid vesicles, similar to wild-type CCT, while lipid stimulated activity was reduced compared to wild-type CCT. Mutational analysis of phenylalanine 260 implicated this residue as a contributor to auto-inhibition of CCT while mutation of L246, W249, I256, and I257 simultaneously to serine resulted in significantly higher activity in the absence of lipid vesicles and an enzyme that was not lipid activated. These results support a concerted mechanism of lipid activation that requires multiple residues on the hydrophobic face of the putative amphipathic alpha helix.

Identification of a physiologically relevant endogenous ligand for PPARalpha in liver

The nuclear receptor PPARalpha is activated by drugs to treat human disorders of lipid metabolism. Its endogenous ligand is unknown. PPARalpha-dependent gene expression is impaired with inactivation of fatty acid synthase (FAS), suggesting that FAS is involved in generation of a PPARalpha ligand. Here we demonstrate the FAS-dependent presence of a phospholipid bound to PPARalpha isolated from mouse liver. Binding was increased under conditions that induce FAS activity and displaced by systemic injection of a PPARalpha agonist. Mass spectrometry identified the species as 1-palmitoyl-2-oleoyl-sn-glycerol-3-phosphocholine (16:0/18:1-GPC). Knockdown of Cept1, required for phosphatidylcholine synthesis, suppressed PPARalpha-dependent gene expression. Interaction of 16:0/18:1-GPC with the PPARalpha ligand-binding domain and coactivator peptide motifs was comparable to PPARalpha agonists, but interactions with PPARdelta were weak and none were detected with PPARgamma. Portal vein infusion of 16:0/18:1-GPC induced PPARalpha-dependent gene expression and decreased hepatic steatosis. These data suggest that 16:0/18:1-GPC is a physiologically relevant endogenous PPARalpha ligand.

Effect of inhibition of neuropathy target esterase in mouse nervous tissues in vitro on phosphatidylcholine and lysophosphatidylcholine homeostasis

Neuropathy target esterase has been shown to be a lysophospholipase in mouse. The authors investigate the effect of neuropathy target esterase inhibition in mouse nervous tissues in vitro on the homeostasis of phosphatidylcholine and lysophosphatidylcholine by treating the homogenates with tri-ortho-cresyl phosphate, paraoxon, paraoxon plus mipafox, and phenylmethylsulfonyl fluoride. The activity of neuropathy target esterase is significantly inhibited by phenylmethylsulfonyl fluoride and paraoxon plus mipafox but not by paraoxon alone. Tri-ortho-cresyl phosphate slightly but significantly inhibits neuropathy target esterase activity in brain. The levels of phosphatidylcholine and lysophosphatidylcholine in all 3 nervous tissues are not obviously altered after treatment with tri-ortho-cresyl phosphate, paraoxon, or paraoxon plus mipafox. However, phosphatidylcholine and lysophosphatidylcholine levels are clearly enhanced by phenylmethylsulfonyl fluoride. It is concluded that inhibition of neuropathy target esterase in mouse nervous tissues is not enough to disrupt the homeostasis of phosphatidylcholine and lysophosphatidylcholine and that the upregulation by phenylmethylsulfonyl fluoride may be the consequence of combined inhibition of neuropathy target esterase and other phospholipases.

Pharmacodynamic markers for choline kinase down-regulation in breast cancer cells

High levels of choline kinase (ChoK) expression and choline phospholipid metabolites are often associated with malignant transformation, invasion, and metastasis, particularly in breast cancer. These findings have led to the development of novel pharmacologic or gene therapeutic interventions for ChoK-targeted inhibition. To identify pharmacodynamic markers for the therapeutic evaluation of ChoK down-regulation, we investigated the uptake and efflux of [(3)H]choline, a natural substrate of ChoK, and two other important metabolic indicators of malignancy, namely, [(3)H]thymidine and [(3)H]fluorodeoxyglucose, which measure proliferation and glucose metabolic changes, respectively, in ChoK-downregulated cells. Choline uptake in nonmalignant and malignant breast epithelial cell lines expressing graded levels of ChoK showed a ChoK-dependent uptake, retention, and efflux of [(3)H]choline. Reduced proliferation observed because of ChoK down-regulation resulted in reduced [(3)H]thymidine uptake and incorporation into DNA within 48 hours of treatment. Reduced [(3)H]thymidine incorporation levels were consistent with a decreased cell cycle S-phase fraction. No change in [(3)H]fluorodeoxyglucose uptake was observed between ChoK-downregulated and control cells in any of the three cell lines tested. These results demonstrate the utility of radiolabeled choline or choline analogs and proliferation imaging agents as pharmacodynamic markers for ChoK-targeted therapies and suggest a ChoK-mediated mechanism for tumor sequestration of choline-based imaging agents.

ATF6alpha induces XBP1-independent expansion of the endoplasmic reticulum

A link exists between endoplasmic reticulum (ER) biogenesis and the unfolded protein response (UPR), a complex set of signaling mechanisms triggered by increased demands on the protein folding capacity of the ER. The UPR transcriptional activator X-box binding protein 1 (XBP1) regulates the expression of proteins that function throughout the secretory pathway and is necessary for development of an expansive ER network. We previously demonstrated that overexpression of XBP1(S), the active form of XBP1 generated by UPR-mediated splicing of Xbp1 mRNA, augments the activity of the cytidine diphosphocholine (CDP-choline) pathway for biosynthesis of phosphatidylcholine (PtdCho) and induces ER biogenesis. Another UPR transcriptional activator, activating transcription factor 6alpha (ATF6alpha), primarily regulates expression of ER resident proteins involved in the maturation and degradation of ER client proteins. Here, we demonstrate that enforced expression of a constitutively active form of ATF6alpha drives ER expansion and can do so in the absence of XBP1(S). Overexpression of active ATF6alpha induces PtdCho biosynthesis and modulates the CDP-choline pathway differently than does enforced expression of XBP1(S). These data indicate that ATF6alpha and XBP1(S) have the ability to regulate lipid biosynthesis and ER expansion by mechanisms that are at least partially distinct. These studies reveal further complexity in the potential relationships between UPR pathways, lipid production and ER biogenesis.

The homeostasis of phosphatidylcholine and lysophosphatidylcholine in nervous tissues of mice was not disrupted after administration of tri-o-cresyl phosphate

Neuropathy target esterase (NTE) is proven to act as a lysophospholipase (LysoPLA) in mice and phospholipase B (PLB) in cultured mammalian cells. In sensitive species, organophosphate (OP)-induced delayed neurotoxicity is initiated when NTE is inhibited by > 70% and then aged. It is hypothesized that homeostasis of phosphatidylcholine (PC) and/or lysophosphatidylcholine (LPC) in mice might be disrupted by the OPs since NTE and other phospholipases could be inhibited. To test this hypothesis, we treated mice using tri-o-cresyl phosphate (TOCP), which can inhibit and age NTE. Phenylmethylsulfonyl fluoride (PMSF), which inhibits NTE but cannot age, was used as a negative control. Effects on activity of NTE, LysoPLA, and PLB, the levels of PC, LPC, and glycerophosphocholine (GPC), and the aging of NTE in the brain, spinal cord, and sciatic nerve were examined. The results showed that the activities of NTE, NTE-LysoPLA, LysoPLA, NTE-PLB, and PLB were significantly inhibited in both TOCP- and PMSF-treated mice, and the inhibition of NTE and NTE-LysoPLA or NTE-PLB showed a high correlation coefficient. The NTE inhibited by TOCP was of the aged type, while nearly all NTE inhibited by PMSF was of the unaged type. Although the GPC level was remarkedly decreased, no significant change of PC and LPC levels was observed. However, the inhibition of these enzymes in mice by TOCP exhibited different characteristics from the TOCP-treated hens that we previously reported, which indicates that these enzymes were inhibited and then recovered more rapidly in mice than in hens. All results suggest that PC and LPC homeostasis was not disrupted in mice after exposure to TOCP. Differences in inhibition of NTE, LysoPLA, and PLB activities by TOCP between mice and hens may elucidate why these two species display different signs after exposure to the same neuropathic OPs.

Phosphatidylethanolamine synthesis is required for optimal virulence of Brucella abortus

The Brucella cell envelope contains the zwitterionic phospholipids phosphatidylcholine (PC) and phosphatidylethanolamine (PE). Synthesis of PC occurs exclusively via the PC synthase pathway, implying that the pathogen depends on the choline synthesized by the host cell to form PC. Notably, PC is necessary to sustain a chronic infection process, which suggests that the membrane lipid content is relevant for Brucella virulence. In this study we investigated the first step of PE biosynthesis in B. abortus, which is catalyzed by phosphatidylserine synthase (PssA). Disruption of pssA abrogated the synthesis of PE without affecting the growth in rich complex medium. In minimal medium, however, the mutant required choline supplementation for growth, suggesting that at least PE or PC is necessary for Brucella viability. The absence of PE altered cell surface properties, but most importantly, it impaired several virulence traits of B. abortus, such as intracellular survival in both macrophages and HeLa cells, the maturation of the replicative Brucella-containing vacuole, and mouse colonization. These results suggest that membrane phospholipid composition is critical for the interaction of B. abortus with the host cell.

Mn2+-dependent ADP-ribose/CDP-alcohol pyrophosphatase: a novel metallophosphoesterase family preferentially expressed in rodent immune cells

ADPRibase-Mn (Mn2+-dependent ADP-ribose/CDP-alcohol pyrophosphatase) was earlier isolated from rat liver supernatants after separation from ADPRibase-I and ADPRibase-II (Mg2+-activated ADP-ribose pyrophosphatases devoid of CDP-alcohol pyrophosphatase activity). The last mentioned are putative Nudix hydrolases, whereas the molecular identity of ADPRibase-Mn is unknown. MALDI (matrix-assisted laser-desorption ionization) MS data from rat ADPRibase-Mn pointed to a hypothetical protein that was cloned and expressed and showed the expected specificity. It is encoded by the RGD1309906 rat gene, which so far has been annotated simply as 'hydrolase'. ADPRibase-Mn is not a Nudix hydrolase, but it shows the sequence and structural features typical of the metallophosphoesterase superfamily. It may constitute a protein family of its own, the members of which appear to be specific to vertebrates, plants and algae. ADP-ribose was successfully docked to a model of rat ADPRibase-Mn, revealing its putative active centre. Microarray data from the GEO (Gene Expression Omnibus) database indicated that the mouse gene 2310004I24Rik, an orthologue of RGD1309906, is preferentially expressed in immune cells. This was confirmed by Northern-blot and activity assay of ADPRibase-Mn in rat tissues. A possible role of ADPRibase-Mn in immune cell signalling is suggested by the second-messenger role of ADP-ribose, which activates TRPM2 (transient receptor potential melastatin channel-2) ion channels as a mediator of oxidative/nitrosative stress, and by the signalling function assigned to many of the microarray profile neighbours of 2310004I24Rik. Furthermore, the influence of ADPRibase-Mn on the CDP-choline or CDP-ethanolamine pathways of phospholipid biosynthesis cannot be discounted.

Functional genomic screen reveals genes involved in lipid-droplet formation and utilization

Eukaryotic cells store neutral lipids in cytoplasmic lipid droplets enclosed in a monolayer of phospholipids and associated proteins. These dynamic organelles serve as the principal reservoirs for storing cellular energy and for the building blocks for membrane lipids. Excessive lipid accumulation in cells is a central feature of obesity, diabetes and atherosclerosis, yet remarkably little is known about lipid-droplet cell biology. Here we show, by means of a genome-wide RNA interference (RNAi) screen in Drosophila S2 cells that about 1.5% of all genes function in lipid-droplet formation and regulation. The phenotypes of the gene knockdowns sorted into five distinct phenotypic classes. Genes encoding enzymes of phospholipid biosynthesis proved to be determinants of lipid-droplet size and number, suggesting that the phospholipid composition of the monolayer profoundly affects droplet morphology and lipid utilization. A subset of the Arf1-COPI vesicular transport proteins also regulated droplet morphology and lipid utilization, thereby identifying a previously unrecognized function for this machinery. These phenotypes are conserved in mammalian cells, suggesting that insights from these studies are likely to be central to our understanding of human diseases involving excessive lipid storage.

Advanced MRI of adult brain tumors

This article is intended to provide clinical neurologists with an overview of the major techniques of advanced MRI of brain tumor: diffusion-weighted imaging, perfusion-weighted imaging, dynamic contrast-enhanced T1 permeability imaging, diffusion-tensor imaging, and magnetic resonance spectroscopy. These techniques represent a significant addition to conventional anatomic MRI T2-weighted images, fluid attenuated inversion recovery (FLAIR) T2-weighted images, and gadolinium-enhanced T1-weighted images for assessing tumor cellularity, white matter invasion, metabolic derangement including hypoxia and necrosis, neovascular capillary blood volume, and permeability. Although a brief introduction and more extensive references to the technical literature is provided, the major focus is to provide a summary of recent clinical experience in application of these major advanced MRI techniques to differential diagnosis, grading, surgical planning, and monitoring of therapeutic response of tumors.

Lipid signaling and the modulation of surface charge during phagocytosis

Phagocytosis is an important component of innate and adaptive immunity. The formation of phagosomes and the subsequent maturation that capacitates them for pathogen elimination and antigen presentation are complex processes that involve signal transduction, cytoskeletal reorganization, and membrane remodeling. Lipids are increasingly appreciated to play a crucial role in these events. Sphingolipids, cholesterol, and glycerophospholipids, notably the phosphoinositides, are required for the segregation of signaling microdomains and for the generation of second messengers. They are also instrumental in the remodeling of the actin cytoskeleton and in directing membrane traffic. They accomplish these feats by congregating into liquid-ordered domains, by generating active metabolites that activate receptors, and by recruiting and anchoring specific protein ligands to the membrane, often altering their conformation and catalytic activity. A less appreciated role of acidic phospholipids is their contribution to the negative surface charge of the inner leaflet of the plasmalemma. The unique negativity of the inner aspect of the plasma membrane serves to attract and anchor key signaling and effector molecules that are required to initiate phagosome formation. Conversely, the loss of charge that accompanies phospholipid metabolism as phagosomes seal facilitates the dissociation of proteins and the termination of signaling and cytoskeleton assembly. In this manner, lipids provide a binary electrostatic switch to control phagocytosis.

Phosphoethanolamine N-methyltransferase (PMT-1) catalyses the first reaction of a new pathway for phosphocholine biosynthesis in Caenorhabditis elegans

The development of nematicides targeting parasitic nematodes of animals and plants requires the identification of biochemical targets not found in host organisms. Recent studies suggest that Caenorhabditis elegans synthesizes phosphocholine through the action of PEAMT (S-adenosyl-L-methionine:phosphoethanolamine N-methyltransferases) that convert phosphoethanolamine into phosphocholine. Here, we examine the function of a PEAMT from C. elegans (gene: pmt-1; protein: PMT-1). Our analysis shows that PMT-1 only catalyses the conversion of phosphoethanolamine into phospho-monomethylethanolamine, which is the first step in the PEAMT pathway. This is in contrast with the multifunctional PEAMT from plants and Plasmodium that perform multiple methylations in the pathway using a single enzyme. Initial velocity and product inhibition studies indicate that PMT-1 uses a random sequential kinetic mechanism and is feedback inhibited by phosphocholine. To examine the effect of abrogating PMT-1 activity in C. elegans, RNAi (RNA interference) experiments demonstrate that pmt-1 is required for worm growth and development and validate PMT-1 as a potential target for inhibition. Moreover, providing pathway metabolites downstream of PMT-1 reverses the RNAi phenotype of pmt-1. Because PMT-1 is not found in mammals, is only distantly related to the plant PEAMT and is conserved in multiple parasitic nematodes of humans, animals and crop plants, inhibitors targeting it may prove valuable in human and veterinary medicine and agriculture.

Phosphocholine as a biomarker of breast cancer: Molecular and biochemical studies

The discovery of metabolic and molecular markers that help improving the detection and diagnosis of breast cancer is an important goal to be achieved. A high composite-choline signal in magnetic resonance spectra of breast lesions has been demonstrated to improve the accuracy of breast cancer diagnosis. In the present study we revealed the principal molecular and biochemical steps associated with the induction of choline metabolism and phosphocholine accumulation in human breast cancer cell-lines in comparison with normal human mammary epithelial cells. We found upregulation of the expression levels of specific choline transporters: organic cation transporter-2 and choline high affinity transporter-1, as well as of the enzyme choline kinase alpha in the cancerous cells in comparison with that in the normal mammary epithelial cells. The expression levels of choline transporter like-1, organic cation transporter-1 and choline kinase beta were similar in normal and cancerous cells. We further showed that choline transport rates and choline kinase activity indeed increased by several fold in the cancer cells leading to the elevation of phosphocholine. The results strongly suggest that phosphocholine can serve as a biomarker of breast cancer reflecting upregulation of specific choline transporters and choline kinase genes.

Choline transport for phospholipid synthesis

Choline is an essential nutrient for all cells because it plays a role in the synthesis of the membrane phospholipid components of the cell membranes, as a methyl-group donor in methionine metabolism as well as in the synthesis of the neurotransmitter acetylcholine. Choline deficiency affects the expression of genes involved in cell proliferation, differentiation, and apoptosis, and it has been associated with liver dysfunction and cancer. Abnormal choline transport and metabolism have been implicated in a number of neurodegenerative disorders such as Alzheimer's and Parkinson's disease. Therefore, the study of choline transport and the characteristics of choline transporters are of central importance to understanding the mechanisms that underlie membrane integrity and cell signaling in such disorders. Kinetic studies with radiolabeled choline and inhibitors distinguish three systems for choline transport: (i) low-affinity facilitated diffusion, (ii) high-affinity, Na+-dependent transport, and (iii) intermediate-affinity, Na+-independent transport. It is only recently, however, that the proteins having transport characteristics of at least one of these systems have been identified. They include (i) polyspecific organic cation transporters (OCTs) with low affinity for choline, (ii) high-affinity choline transporters (CHT1s), and (iii) intermediate-affinity choline transporter-like (CTL1) proteins. CHT1 and CTL1 but not OCT transporters are selectively inhibited with hemicholinium-3 and essentially display characteristics of specialized transporters for targeted choline metabolism. CHT1 is abundant in neurons and almost exclusively supplies choline for acetyl-choline synthesis. The focus here is more on newly-discovered CTL1 choline transporters. They are expressed in different organisms and cell types, apparently not for the biosynthesis of acetylcholine but for the production of the most abundant metabolite of choline, the membrane lipid phosphatidylcholine.

Genomic organization, promoter activity, and expression of the human choline transporter-like protein 1

Choline transporter-like (CTL) proteins of the CTL1 family are novel transmembrane proteins implicated in choline transport for phospholipid synthesis. In this study, we characterized the 5'-flanking region of the human (h)CTL1 gene and examined some of the possible mechanisms of its regulation, including promoter activity, splicing, and expression. The transcription start site of the hCTL1 gene was mapped by 5'-rapid amplification of cDNA ends (RACE), and the presence of two splice variants, hCTL1a and hCTL1b, was investigated using isoform-specific PCR and 3'-RACE. The hCTL1 promoter region of approximately 900 bp was isolated from MCF-7 human breast cancer cells. The promoter was TATA-less and driven by a long stretch of GC-rich sequence in accordance with widespread expression of hCTL1 at both mRNA and protein levels. Deletion analyses demonstrated that a very strong promoter is contained within 500 bp of the transcription start site, and more upstream regions did not increase its activity. The core promoter that conferred the minimal transcription is within the -188/+27-bp region, and its activity varied in human breast cancer and mouse skeletal muscle cells. Multiple motifs within the promoter regulatory region bound nuclear factors from both cultured cells and normal human skeletal muscle. The motifs within the three regions [S1 (-92/-61 bp), S2 (-174/-145 bp), and S3 (-289/-260 bp)] contained overlapping binding sites for hematopoietic transcription factors and ubiquitous transcription factors, in line with the expected gene function. Genomic analyses demonstrated a high conservation of hCTL1 and mouse CTL1 proximal promoters. Accordingly, mRNA profiles demonstrated that human splice variants were expressed ubiquitously, as demonstrated for the mouse transcripts; however, they differed from the profiles of rat CTL1 transcripts, which were more restricted to neurons and intestinal tissues. The shorter hCTL1b variant contained the cytosolic COOH-terminal motif L651KKR654 for endoplasmic reticulum retrieval/retention. This retention signal was conserved in hCTL1b and rat and mouse CTL1b and is typical for transmembrane proteins of type 1 topology.

Effects of hypoxia, glucose deprivation and acidosis on phosphatidylcholine synthesis in HL-1 cardiomyocytes. CTP:phosphocholine cytidylyltransferase activity correlates with sarcolemmal disruption

A decrease in [3H]Cho (choline) incorporation in to PtdCho (phos-phatidylcholine) preceded the onset of LDH (lactate dehydrogenase) release in HL-1 cardiomyocytes submitted to simulated ischaemia. This observation led us to examine the role of PtdCho synthesis in sarcolemmal disruption in HL-1 cardiomyocytes. To address this objective we analysed the individual effects of hypoxia, glucose deprivation and acidosis, three prominent components of ischaemia, on the different steps of the Kennedy pathway for the synthesis of PtdCho. Pulse and pulse-chase experiments with [3H]Cho, performed in whole HL-1 cells submitted to hypoxia or normoxia, in the presence or absence of glucose at different pHs indicated first, that CK (choline kinase) was inhibited by hypoxia and acidosis, whereas glucose deprivation exacerbated the inhibition caused by hypoxia. Second, the rate-limiting reaction in PtdCho synthesis, catalysed by CCT (CTP:phosphocholine cytidylyltransferase), was inhibited by hypoxia and glucose deprivation, but unexpectedly activated by acidosis. In cellfree system assays, acidosis inhibited both CK and CCT. In experiments performed in whole cells, the effect of acidosis was likely to be direct on CK, but indirect or intact-cell-dependent on CCT. Since hypoxia and glucose deprivation favoured membrane disruption, but acidosis prevented it, we hypothesized that the modulation of CCT could be an important determinant of cell survival. Supporting this hypothesis, we show that CCT activity in whole-cell experiments clearly correlated with LDH release, but not with ATP concentration. Altogether our results suggest a significant role for CCT activity in sarcolemmal disruption during ischaemia.

Completing the cycles; the dynamics of endonuclear lipidomics

Signal transductions via periodic generation and mobilisation of lipid second messengers within the nuclear matrix of eukaryotic cells have focused renewed attention on their precursor phospholipids' location, structure, form and function. The nuclear matrix contains and supports dynamic pools of phosphatidylcholine and phosphatidylinositol which serve as parent molecules of lipid second messengers but also of other phospholipids requiring cyclical replacement as cells proliferate. Applications of new, highly sensitive and specific analytical methodologies based on tandem electrospray ionisation mass spectrometry and the use of stable isotopes have allowed both static and dynamic lipidomic profiling of these endonuclear phospholipid pools. Together with more conventional enzymatic analyses and evaluation of the effect of specific "knock-out" of phospholipid transfer capacity, a number of important principles have been established. Specifically, a compartmental capacity to synthesise and remodel highly saturated phosphatidylcholine exists alongside transport mechanisms that facilitate the nuclear import of phosphatidylinositol and other phospholipids synthesised elsewhere within the cell. Subnuclear fractionation and the use of newly emerging techniques for sensitive lipidomic profiling of polyphosphoinositides, diacylglycerols and phosphatidate molecular species offer the potential for further significant advances in the near future.

Regulation of phosphatidylcholine homeostasis by Sec14This paper is one of a selection of papers published in this Special Issue, entitled Young Investigator's Forum

Phosphatidylcholine is the major phospholipid in eukaryotic cells and serves as both a permeability barrier as well as a modulator of a plethora of cellular and biological functions. This review touches on the importance of proper regulation of phosphatidylcholine metabolism on health, and discusses how yeast genetics has contributed to furthering our understanding of the precise molecular events regulated by alterations in phosphatidylcholine metabolism. Yeast studies have determined that the phosphatidylcholine and (or) phosphatidylinositol binding protein, Sec14, is a major regulator of phosphatidylcholine homeostasis. Sec14 itself regulates vesicular transport from the Golgi, and the interrelationship between phosphatidylcholine metabolism and membrane movement within the cell is described in detail. The recent convergence of the yeast genetic studies with that of mammalian cell biology in how cells maintain phosphatidylcholine homeostasis is highlighted.

Dynamic lipidomics of the nucleus

Once nuclear envelope membranes have been removed from isolated nuclei, around 6% of mammalian cell phospholipid is retained within the nuclear matrix, which calculations suggest may occupy 10% of the volume of this subcellular compartment. It is now acknowledged that endonuclear phospholipid, largely ignored for the past 40 years, provides substrate for lipid-mediated signaling events. However, given its abundance, it likely also has other as yet incompletely defined roles. Endonuclear phosphatidylcholine is the predominant phospholipid comprising distinct and highly saturated molecular species compared with that of the whole cell. Moreover, this unusual composition is subject to tight homeostatic maintenance even under conditions of extreme dietary manipulation, presumably reflecting a functional requirement for highly saturated endonuclear phosphatidylcholine. Recent application of new lipidomic technologies exploiting tandem electrospray ionization mass spectrometry in conjunction with deuterium stable isotope labeling have permitted us to probe not just molecular species compositions but endonuclear phospholipid acquisition and turnover with unparalleled sensitivity and specificity. What emerges is a picture of a dynamic pool of endonuclear phospholipid subject to autonomous regulation with respect to bulk cellular phospholipid metabolism. Compartmental biosynthesis de novo of endonuclear phosphatidylcholine contrasts with import of phosphatidylinositol synthesized elsewhere. However, irrespective of the precise temporal-spatial-dynamic relationships underpinning phospholipid acquisition, derangement of endonuclear lipid-mediated signaling from these parental phospholipids halts cell growth and division indicating a pivotal control point. This in turn defines the manipulation of compartmentalized endonuclear phospholipid acquisition and metabolism as intriguing potential targets for the development of future antiproliferative strategies.

Deletion and alanine mutation analyses for the formation of active homo- or hetero-dimer complexes of mouse choline kinase-α and -β

Choline kinase (CK) is the first-step regulatory enzyme for the biosynthesis of phosphatidylcholine in all mammalian cells. It exists as at least three isoforms (alpha1, alpha2 and beta) that are encoded by two separate genes termed ck-alpha and ck-beta. The active enzyme has been proposed to consist of either their homo- or hetero-dimeric forms. Here, we report on the identification of several essential domains and amino acid residues involved in their active dimer formation. Full-length cDNAs or their truncated or alanine-mutated versions for mouse CK-alpha1 and CK-beta tagged with either HA or Myc at their N-termini were expressed in COS-7 cells. Each dimer formation was analyzed by immuno-precipitation followed by Western blotting. Kinetic analysis for CK reaction was performed with different expression products. Both the N-terminal domain-1 and C-terminal portions (E424-K430 for CK-alpha1 and Q379-K385 for CK-beta) were shown to be critical for the formation of active homo- or hetero-dimer complex. Interestingly, D320 in the CK-motif of CK-alpha1 was found to be essential for alpha1/alpha1 homo-dimerization but not for alpha1/beta hetero-dimerization. A mutation of the corresponding D276 of CK-beta to A276 did not show any effect on either its homo- or hetero-dimerization but it caused a strong inhibition of CK activity in either case.

Neuropathy target esterase and phospholipid deacylation

Certain organophosphates react with the active site serine residue of neuropathy target esterase (NTE) and cause axonal degeneration and paralysis. Cloning of NTE revealed the presence of homologues in eukaryotes from yeast to man and that the protein has both a catalytic and a regulatory domain. The latter contains sequences similar to the regulatory subunit of protein kinase A, suggesting that NTE may bind cyclic AMP. NTE is tethered via an amino-terminal transmembrane segment to the cytoplasmic face of the endoplasmic reticulum. Unlike wild-type yeast, mutants lacking NTE activity cannot deacylate CDP-choline pathway-synthesized phosphatidylcholine (PtdCho) to glycerophosphocholine (GroPCho) and fatty acids. In cultured mammalian cells, GroPCho levels rise and fall, respectively, in response to experimental over-expression, and inhibition, of NTE. A complex of PtdCho and Sec14p, a yeast phospholipid-binding protein, both inhibits the rate-limiting step in PtdCho synthesis and enhances deacylation of PtdCho by NTE. While yeast can maintain PtdCho homeostasis in the absence of NTE, certain post-mitotic metazoan cells may not be able to, and some NTE-null animals have deleterious phenotypes. NTE is not required for cell division in the early mammalian embryo or in larval and pupal forms of Drosophila, but is essential for placenta formation and survival of neurons in the adult. In vertebrates, the relative importance of NTE and calcium-independent phospholipase A2 for homeostatic PtdCho deacylation in particular cell types, possible interactions of NTE with Sec14p homologues and cyclic AMP, and whether deranged phospholipid metabolism underlies organophosphate-induced neuropathy are areas which require further investigation.