Regulation of phosphatidylcholine biosynthesis

1H NMR visible lipids in the life and death of cells

A functionally and metabolically interesting class of cell lipid can be observed by 1H nuclear magnetic resonance (NMR) spectroscopy in situ. These prominent resonances are not only associated with malignancy and cell death, but also act as heralds of benign processes, such as cell activation and proliferation. Originally, these NMR observations were explained with a membrane lipid microdomain model. However, recent studies have identified intracellular droplets, so called lipid bodies, as important contributors to these resonances. This finding bears novel implications for our understanding and assessment of lipid biochemistry in the life and death of cells.

NMR spectroscopic analyses of liver phosphatidylcholine and phosphatidylethanolamine biosynthesis in rats exposed to peroxisome proliferators-A class of nongenotoxic hepatocarcinogens

Peroxisome proliferators (PPs) are commercial/industrial chemicals that display tumor promoter activity in rodents. The mechanism is not completely understood, and our ability to predict tumorigenicity a priori is even less developed. Wy-14,643, perfluorooctanoic acid (PFOA), and di(2-ethylhexyl)phthalate (DEHP) are strong, moderate, and weak tumor promoters, respectively, while perfluorodecanoic acid (PFDA) lacks promoter activity. This investigation examined the effects of these PPs on the biosyntheses of phosphatidylcholine (PtdC) and phosphatidylethanolamine (PtdE) in rat liver. After exposure to PPs, rats were administered [1-(13)C]choline + [2-(13)C]ethanolamine and liver extracts were analyzed by (31)P and (13)C NMR. The ratio of choline-derived to ethanolamine-derived phospholipids, R(c/e), was significantly affected by all PPs (p < 0. 05). R(c/e) values were in the order Wy-14,643 > PFOA > DEHP > control > PFDA. The amounts of PtdC derived via the CDP-choline pathway versus PtdE-N-methyltransferase (PEMT) activity was 71 vs 29% in controls. This distribution was significantly affected by treatments with Wy-14,643 (95 vs 5%), DEHP (87 vs 13%), and PFDA (39 vs 61%) (p < 0.02). Data suggest that Wy-14,643, PFOA, and DEHP cause a preference for choline and the CDP-choline pathway for biosynthesis of PtdC. Additionally, Wy-14,643 and DEHP inhibited the PEMT pathway. In contrast, PFDA-treated rats showed a preference for ethanolamine, and PtdC was predominately synthesized through the PEMT pathway. These data corroborate studies by Vance and co-workers which suggest that the pathways for PtdC biosynthesis are important for hepatocarcinogenesis. Further studies to evaluate the potential of these measurements as a biomarker for PP-associated tumorigenesis is warranted.

Increased choline kinase activity in 1,2-dimethylhydrazine-induced rat colon cancer

Cancer cells acquire particular characteristics that benefit their proliferation. We previously reported that human colon cancers examined had increased choline kinase activity and phosphocholine levels. The elevated phosphocholine levels were in part due to both activation of choline kinase and increased choline kinase alpha protein levels. In this report, we analyzed choline kinase, which catalyzes the phosphorylation of choline to produce phosphocholine, in rat 1,2-dimethylhydrazine (DMH)-induced colon cancer. This study is the first to demonstrate increased choline kinase alpha enzymatic activity, protein levels, and mRNA levels in DMH-induced colon cancer as well as human colon cancer, although phosphocholine was not increased in DMH-induced rat cancer. The increase in the mRNA level was partly due to an increase in the transcription of the choline kinase alpha gene. The increased choline kinase activity may be a specific characteristic acquired by cancer cells that benefits their proliferation.

Hormonal regulation and cellular localization of fatty acid synthase in human fetal lung

Fatty acid synthase (FAS; EC 2.3.1.85) supplies de novo fatty acids for pulmonary surfactant synthesis, and FAS gene expression is both developmentally and hormonally regulated in the fetal lung. To further examine hormonal regulation of FAS mRNA and to determine the cellular localization of FAS gene expression, we cultured human fetal lungs (18-22 wk gestation) as explants for 1-4 days in the absence (control) or presence of glucocorticoid [dexamethasone (Dex), 10 nM] and/or cAMP agents (8-bromo-cAMP, 0.1 mM and IBMX, 0.1 mM). FAS protein content and activity increased similarly in the presence of Dex (109 and 83%, respectively) or cAMP (87 and 111%, respectively), and responses were additive in the presence of both hormones (230 and 203%, respectively). With a rabbit anti-rat FAS antibody, FAS immunoreactivity was not detected in preculture lung specimens but appeared in epithelial cells lining the tubules with time in culture. Dex and/or cAMP markedly increased staining of epithelial cells, identified as type II cells, whereas staining of mesenchymal fibroblasts was very low under all conditions. With in situ hybridization, FAS mRNA was found to be enriched in epithelial cells lining the alveolar spaces, and the reaction product increased in these cells when the explants were cultured with the hormones. The increased FAS mRNA content in the presence of Dex and/or cAMP is primarily due to increased stabilization of mRNA, although Dex alone increased the transcription rate by approximately 30%. We conclude that hormonal treatment of cultured human fetal lungs increases FAS gene expression primarily by increasing stability of the message. The induction of FAS during explant culture and by hormones occurs selectively in type II epithelial cells, consistent with the regulatory role of this enzyme in de novo synthesis of fatty acid substrate for surfactant synthesis in perinatal lungs.

Regulation of phosphatidylcholine homeostasis by calcium-independent phospholipase A2

Phosphatidylcholine (PtdCho) is the most abundant phospholipid in mammalian cell membranes and is essential for cell viability. The levels of this lipid must be tightly controlled to maintain homeostasis. Therefore, changes in the rate of PtdCho synthesis are generally balanced by changes in PtdCho catabolism and vice versa. It is commonly accepted that the rate of PtdCho synthesis is regulated by CTP:phosphocholine cytidylyltransferase (CT). However, it is not certain if PtdCho mass is regulated by specific catabolic enzyme(s). Our goal is to determine if PtdCho homeostasis is regulated by a phospholipase A2 (PLA2). To this end, we have prepared Chinese hamster ovary (CHO) cell lines that overexpress CT. CT activity is 7-10-fold higher in the transfected cells than in parental CHO cells. This increase in CT activity is associated with increases in both PtdCho synthesis and PtdCho catabolism. Glycerophosphocholine is the PtdCho catabolite that accumulates in the transfected cells, which suggests that PtdCho turnover is mediated by a phospholipase A2 (PLA2). Indeed, higher levels of calcium-independent PLA2 activity are measured in the cytosols of the CHO cells that overexpress CT, compared to parental CHO cells. The elevated calcium-independent PLA2 activity is associated with increases in the expression of the 80-kDa calcium-independent PLA2 (iPLA2). Together, these data suggest that the 80-kDa iPLA2 may be modulated in response to changes in PtdCho levels and therefore is involved in the regulation of PtdCho homeostasis in CHO cells.

Increased choline kinase activity and elevated phosphocholine levels in human colon cancer

Nuclear magnetic resonance spectroscopy has detected elevated phosphocholine levels in human tumor tissues and cells, and in cells that were transformed with the activated Ha-ras gene and stimulated in vitro with growth-promoting factors such as platelet-derived growth factor, epidermal growth factor, and phorbol ester. However, the mechanism of the elevation and the function of the increased phosphocholine levels have not been clearly demonstrated. We studied phosphocholine levels enzymatically and analyzed the activity of choline kinase, which catalyzes the phosphorylation of choline to produce phosphocholine, in human colon cancer and adenoma. Both choline kinase activity and phosphocholine levels were increased in colon cancer and adenoma tissue. The activation of choline kinase and the increased levels of choline kinase alpha were partly responsible for the elevated phosphocholine levels. This study suggests that choline kinase might play a role in growth promotion or signal transduction in carcinogenesis.

Characterization of choline compounds with in vitro 1H magnetic resonance spectroscopy for the discrimination of primary brain tumors

RATIONALE AND OBJECTIVES

The authors sought to compare 1H magnetic resonance spectroscopy (MRS) spectra from extracts of low-grade and high-grade gliomas, especially with respect to the signals of choline-containing compounds.

METHODS

Perchloric acid extracts of six high-grade and six low-grade gliomas were analyzed by 1H MRS at 9.4 Tesla.

RESULTS

The signals of glycerophosphocholine (GPC) at 3.23 ppm, phosphocholine (PC) at 3.22 ppm, and choline (Cho) at 3.21 ppm were identified in both types of tumors. The absolute concentrations of all Cho-containing compounds (GPC + PC + Cho) in high-grade and low-grade gliomas were significantly different. The relative contributions of each of the Cho-containing compounds to the total choline signal were also statistically different. For high-grade gliomas, the choline signal is composed of GPC, PC, and Cho in a well-balanced contribution, whereas in low-grade gliomas, the signal is largely due to GPC with a small involvement of PC and Cho.

CONCLUSIONS

The differences in the concentration and the repartition of Cho-containing compounds seem to be a marker of high-grade gliomas. They could also help to discriminate between high- and low-grade gliomas in some difficult cases, especially if there is histologic uncertainty between anaplastic astrocytomas and low-grade oligodendrogliomas.

Regulation of mitogenesis by water-soluble phospholipid intermediates

Many recent observations implicate choline and ethanolamine kinases as well as phosphatidylcholine-specific phospholipase C in the regulation of mitogenesis and carcinogenesis. For example, human cancers generally contain high concentrations of phosphoethanolamine and phosphocholine, and in different cell lines various growth factors, cytokines, oncogenes and chemical carcinogens were all shown to stimulate the formation of phosphocholine and phosphoethanolamine. In addition, other reports have appeared showing that both extracellular and intracellular phosphocholine as well as ethanolamine and its derivatives can regulate cell growth. This area of research has clearly arrived at a stage when it becomes important to examine critically the feasibility of water-soluble phospholipid intermediates serving as potential regulators of cell growth in vivo. Accordingly, the goal of this review is to summarise available information relating to the formation and mitogenic actions of intracellular and extracellular phosphocholine as well as ethanolamine and its derivatives.

Choline derived from the phosphatidylcholine specific phospholipase D is not directly available for the CDP choline pathway in phorbol ester-treated C3H10T1/2 Cl 8 fibroblasts

We have shown that 12-O-tetradecanoylphorbol 13-acetate (TPA) increases protein kinase C (PKC)-mediated choline transport, incorporation of choline into phosphatidylcholine (PtdCho) and PtdCho degradation by phospholipase D (PLD) in C3H10T1/2 Cl 8 cells. Dual prelabeling experiment using [3H]/[14C]choline indicated that intracellular choline generated from the PLD reaction was not directly recycled to PtdCho synthesis within the cell, and that a large fraction of the choline was transported out of the TPA-treated cells. In contrast, medium derived choline was preferably channeled to PtdCho synthesis. These results indicate that in TPA-treated cells, the choline derived from the PKC-mediated increased PLD activity and the choline newly taken up by the cell behave as two distinctly different metabolic pools.

Galactosemic cataractogenesis disrupts intracellular interactions and changes the substrate specificity of choline/ethanolamine kinase

The functional characteristics of enzymes depends upon their environment, and within physiologically intact cells, many metabolic pathways are thought to involve multienzyme complexes and other enzyme-enzyme interactions that increase efficiency and specificity by mechanisms such as channeling of intermediates. A disease such as cataract may change the intracellular environment, but the effects of these changes on enzyme-enzyme interactions can be observed only in relatively intact cells, and in enzymes that have unambiguously different properties in different environments. In intact rat lenses, choline and ethanolamine are phosphorylated independently, with no competition between the two compounds, as the first step of phospholipid biosynthesis. However, disruption of lens structure and intracellular interactions by homogenization leads to a paradoxical change in enzymic properties, causing choline and ethanolamine to become competing alternative substrates of a single enzyme that resembles the purified choline/ethanolamine kinase from liver and other tissues. The properties of ethanolamine kinase in intact cataractous lenses from rats fed a 50% galactose diet for 7-14 days were intermediate between those of intact control lenses and those in lens homogenates. In monolayers of human and dog lens epithelial cells and human retinal pigment epithelial cells ethanolamine kinase was similar to that in intact tissue, showing that the kinetic differences between intact lenses and homogenates result from the intracellular environment, not from artifacts of diffusion, and that they are not exclusive to rats or to lens cells. Results with intact lenses from monkeys, rabbits, pigs and dogs showed some differences between species, but in every case, choline had little or no effect on the phosphorylation of radiolabeled ethanolamine. Further studies will be necessary to determine how the changes in intracellular environment during cataractogenesis affect other enzymes and whether other model systems for cataractogenesis cause similar changes.

Activation of CTP:phosphocholine cytidylyltransferase by hypochlorite-oxidized phosphatidylcholines

CTP:phosphocholine cytidylyltransferase (CT) catalyzes a rate-limiting, regulatory step in mammalian biosynthesis of phosphocholine (PC). Anionic phospholipids, fatty acids and diacylglycerol activate CT and promote its intercalation into the lipid bilayer, whereas zwitterionic phospholipids such as phosphatidylcholines do not. We investigated the effectiveness of polyunsaturated phosphatidylcholines as CT activators after hypochlorite oxidation. Detection and quantitation of oxidized PCs were evaluated by thin layer chromatography, high performance liquid chromatography, and conjugated dienes. Purified CT was assayed in the presence of multilamellar vesicles, containing variable concentrations of oxidized and parent PCs. The results demonstrate that particular species of oxidized PCs activate CT as potently as anionic lipids. The greater the number of double bonds available for oxidation in the fatty acid at the sn-2 position of the PC, the more effective was the oxidized PC as an activator of CT. Oxidized phospholipids at 1:1 bleach/lipid activated CT in the following order: PAPC>PL3PC>PL2PC compared to unoxidized controls. Since oxidized phospholipids decrease bilayer order (M.L. Wratten et al., Biochemistry 31 (1992) 10901-10907) these results are consistent with the activation of CT by perturbations of lipid bilayer packing.

Phorbol ester stimulation of phosphatidylcholine synthesis requires expression of both protein kinase C-alpha and phospholipase D

The protein kinase C (PKC) activator phorbol 12-myristate 13-acetate (PMA) stimulates both the synthesis and phospholipase D (PLD)-mediated hydrolysis of phosphatidylcholine (PtdCho). Here, attached and suspended NIH 3T3 fibroblasts as well as variants of the MCF-7 human breast carcinoma cell line expressing PKC-alpha and a PtdCho-specific PLD activity at widely different levels were used to determine the possible role of PKC-alpha, PtdCho hydrolysis, and choline uptake in the mediation of PMA effect on PtdCho synthesis. In wild-type MCF-7 cells, which express both PKC-alpha and PLD activities at very low levels, PMA had little effects on the uptake or incorporation [14C]choline into PtdCho. In multidrug resistant MCF-7/MDR1 cells, which highly express PKC-alpha but lack the PtdCho-specific PLD activity, 100-nM PMA had relatively small stimulatory effects on the uptake of [14C]choline (approximately 1.5-fold) and [14C]PtdCho synthesis (1.5- to 2-fold). In NIH 3T3 fibroblasts and MCF-7/PKC-alpha cells, both expressing PKC-alpha and PLD activities at high levels, 10-100-nM PMA enhanced [14C]choline uptake only slightly (1.7- to 2.2-fold), while it had much greater (approximately 4-9-fold) stimulatory effects on PtdCho synthesis. PMA significantly enhanced the formation of phosphatidic acid (PtdOH) in MCF-7/PKC-alpha cells (2.8-fold increase), but not in MCF-7/MDR1 cells (1.4-fold increase), while in both cell lines it had only small (1.3-1.5-fold) stimulatory effects on 1,2-diacylglycerol (1, 2-DAG) formation. In suspended NIH 3T3 cells, 200-300-mM ethanol blocked the stimulatory effect of PMA on PtdOH formation without affecting PtdCho synthesis indicating that neither PtdOH nor 1,2-DAG derived from it is a mediator of PMA effect on PtdCho synthesis. In attached NIH 3T3 cells, dimethylbenz[a]anthracene enhanced phosphocholine formation and, thus, choline uptake without increasing PtdCho synthesis or modifying the effect of PMA. While the results indicate that the stimulatory effect of PMA on PtdCho synthesis requires the expression of both PKC-alpha and a PtdCho-specific PLD, they do not support a role for 1,2-DAG, PtdOH or choline in the mediation of PMA effect.

Studies on the regulation of CTP:phosphocholine cytidylyltransferase using permeabilized HEP G2 cells: evidence that both active and inactive enzyme are membrane-bound

To obtain more insight into the mechanisms regulating CTP:phosphocholine cytidylyltransferase (CT), we determined the effect of oleate treatment on the rate of CT release from permeabilized Hep G2 cells and the distribution of the CT remaining in the permeabilized cells. When we permeabilized untreated cells in pH 7.5 buffer containing 0.15 M KCl, the rate of CT release was much slower than the release of lactate dehydrogenase. Oleate treatment caused a further decrease in CT release from cells. In untreated cells, 70-80% of the CT remaining in cells 10 min after permeabilization was recovered as soluble CT. Oleate treatment increased the amount of bound CT but over 50% of the CT in cells 10 min after permeabilization was recovered as soluble CT. In both control and oleate-treated cells, the increase in CT release with time correlated with a decrease in the amount of CT recovered from permeabilized cells as soluble CT. These results suggested that CT existed in a form that was not immediately available for release from permeabilized cells, but was recovered in the soluble fraction after cell disruption. When cells were permeabilized in 10 mM imidazole-20% glycerol-5 mM Mg2+ pH 6.5, over 80% of CT in control and over 90% of CT in oleate-treated cells was recovered bound to the particulate fraction. Essentially no CT was released from the cells. The recovery of CT in the particulate fraction required Mg2+ to be present when permeabilization was initiated. The addition of Mg2+, after cells were disrupted, did not increase CT in the particulate fraction. In untreated cells, 50% of bound CT was active. Oleate treatment increased the amount of active CT in the particulate fraction to over 70% of total. About 50% of particulate CT in untreated cells but only 15% in oleate-treated cells was extracted with 0.15 M KCl. Inactive CT was preferentially extracted by KCl. The bound CT was recovered in isolated nuclei. Overall, the results suggested that both inactive and active CT are bound to nuclear membranes, and that the activation of CT involves conversion of CT loosely bound to membrane to a form more tightly bound to membranes perhaps by hydrophobic interaction with phospholipids. This model does not involve translocation from a soluble pool.

Low activity of key phospholipid catabolic and anabolic enzymes in human substantia nigra: possible implications for Parkinson's disease

To determine whether increased oxidative stress in substantia nigra of patients with idiopathic Parkinson's disease might be related to decreased ability of nigral cells to detoxify oxidized membrane phospholipids, we compared levels of the major phospholipid metabolizing enzymes in autopsied substantia nigra with those in non-nigral (n = 11) brain areas of the normal human brain. Whereas most enzymes possessed a relatively homogeneous distribution, the activity of the major phospholipid catabolizing enzyme phospholipase A2, assayed in the presence of calcium ions, varied amongst different regions, with substantia nigra possessing the lowest activity. Similarly, calcium-independent phospholipase A2 activity, although possessing a relatively homogeneous regional distribution, was also low in the substantia nigra. This, coupled with low activity of phosphoethanolamine- and phosphocholine-cytidylyltransferases, major regulatory enzymes of phospholipid synthesis, in this brain region, suggest that the rate of phospholipid turnover is low in the substantia nigra. Low activity of key phospholipid catabolic and anabolic enzymes in human substantia nigra might result in reduced ability to repair oxidative membrane damage, as may occur in Parkinson's disease.

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.

Apoptosis triggered by 1-O-octadecyl-2-O-methyl-rac-glycero-3-phosphocholine is prevented by increased expression of CTP:phosphocholine cytidylyltransferase

A HeLa cell line was constructed for the regulation of CTP:phosphocholine cytidylyltransferase (CCT) expression via a tetracycline-responsive promoter to test the role of CCT in apoptosis triggered by exposure of cells to the antineoplastic phospholipid 1-O-octadecyl-2-O-methyl-rac-glycero-3-phosphocholine (ET-18-OCH3). Basal CCT expression in the engineered HeLa cell line was the same as in control HeLa cells lines, and CCT activity and protein were elevated 25-fold following 48 h of induction with doxycycline. Increased CCT expression prevented ET-18-OCH3-induced apoptosis. Acylation of exogenous lysophosphatidylcholine circumvented the requirement for CCT activity by providing an alternate route to phosphatidylcholine, and heightened CCT expression and lysophosphatidylcholine supplementation were equally effective in reversing the cytotoxic effect of ET-18-OCH3. Neither CCT overexpression nor lysophosphatidylcholine supplementation allowed the HeLa cells to proliferate in the presence of ET-18-OCH3, indicating that the cytostatic property of ET-18-OCH3 was independent of its effect on membrane phospholipid synthesis. These data provide compelling genetic evidence to support the conclusion that the interruption of phosphatidylcholine synthesis at the CCT step by ET-18-OCH3 is the primary physiological imbalance that accounts for the cytotoxic action of the drug.

The antiproliferative effect of hexadecylphosphocholine toward HL60 cells is prevented by exogenous lysophosphatidylcholine

The mechanisms that account for the anti-proliferative properties of the biologically active lysophospholipid analog hexadecylphosphocholine (HexPC) were investigated in HL60 cells. HexPC inhibited the incorporation of choline into phosphatidylcholine and the pattern of accumulation of soluble choline-derived metabolites pinpointed CTP:phosphocholine cytidylyltransferase (CT) as the inhibited step in vivo. HexPC also inhibited recombinant CT in vitro. HexPC treatment led to accumulation of cells in G2/M phase, triggered DNA fragmentation and caused morphological changes associated with apoptosis. The supplementation of HexPC-treated cells with exogenous lysophosphatidylcholine (LPC) completely reversed the cytotoxic effects of HexPC and restored HL60 cell proliferation in the presence of the drug. LPC provided an alternate pathway for phosphatidylcholine synthesis via the acylation of exogenous LPC. This result contrasted with the response of HL60 cells to 1-O-octadecyl-2-O-methyl-rac-glycero-3-phosphocholine (ET-18-OCH3) where LPC overcame the cytotoxic effects but did not support continued cell proliferation. Morphological integrity, DNA stability and cell viability were maintained in cells treated with LPC plus either antineoplastic agent. Thus the inhibition of phosphatidylcholine biosynthesis at the CT step accounts for the cytotoxicity of both HexPC and ET-18-OCH3 which is overridden by providing an alternate pathway for phosphatidylcholine synthesis via the acylation of exogenous LPC.

Cytidylyltransferase-binding protein is identical to transcytosis-associated protein (TAP/p115) and enhances the lipid activation of cytidylyltransferase

We previously identified a protein from rat liver that binds CTP:phosphocholine cytidylyltransferase (CT). We have now purified this protein (cytidylyltransferase-binding protein (CTBP)) from rat liver. The purification involved precipitation at pH 5 and extraction of the precipitate with buffer, followed by sequential chromatography on DEAE-Sepharose and butyl-agarose. Final purification was accomplished by either preparative electrophoresis or hydroxylapatite chromatography. Amino acid sequences from six peptides derived from pure CTBP matched sequences in transcytosis-associated protein (TAP) with 98% identity. Thus, CTBP was positively identified to be TAP. Purified CTBP increased the activity of purified CT measured with phosphatidylcholine (PC)/oleic acid. In the absence of PC/oleic acid, CTBP did not stimulate CT activity. Dilution of CT to reduce the Triton X-100 concentration produced a loss of CT activity. The lost activity was recovered by the addition of CTBP plus PC/oleic acid to the assay, but not by the addition of either PC/oleic acid or CTBP alone. Removal of CTBP from purified preparations by immunoprecipitation with CTBP antibodies eliminated the activation of CT. Both CT and CTBP were shown to bind to PC/oleic acid liposomes. The formation of complexes between CT and CTBP in the absence of PC/oleic acid liposomes could not be demonstrated. These results suggest that CTBP functions to modify the interaction of CT with PC/oleic acid liposomes, resulting in an increase in the catalytic activity perhaps by the formation of a ternary complex between CT, CTBP, and lipid. Overall, these results suggest that CTBP (TAP) may function to coordinate the biosynthesis of phosphatidylcholine with vesicle transport.

Ethanolamine, but not phosphoethanolamine, potentiates the effects of insulin, phosphocholine, and ATP on DNA synthesis in NIH 3T3 cells--role of mitogen-activated protein-kinase-dependent and protein-kinase-independent mechanisms

NIH 3T3 fibroblasts express a phospholipase D activity hydrolyzing phosphatidylethanolamine (PtdEtn) which produces ethanolamine (Etn) in response to a variety of growth regulating agents. The main objective of this work was to evaluate the effects of Etn on mitogenesis and to determine whether these effects require its metabolism to phosphoethanolamine (PEtn) or PtdEtn. To increase conversion of Etn to PEtn, an Etn-specific kinase derived from Drosophila was highly expressed in NIH 3T3 cells. Overexpression of this Etn kinase resulted in large (10-12.5-fold) increases in PEtn formation, but only in modest (1.2-1.7-fold) increases in PtdEtn synthesis. In both vector control and Etn kinase overexpressor cells, Etn had biphasic effects on insulin-induced DNA synthesis with maximal (approximately 2-fold) potentiating effects being observed at 0.5-1 mM concentrations, followed by an inhibitory phase at higher Etn concentrations. In the Etn kinase overexpressor lines, the inhibitory phase was elicited by lower Etn concentrations and it was partially blocked by 5 mM choline due to decreased formation of PEtn. In both vector control and Etn kinase overexpressor cells, phosphocholine (PCho) and insulin synergistically stimulated DNA synthesis; their effects were further enhanced by physiologically relevant (5-60 microM) concentrations of Etn by a mechanism independent of mitogen-activated protein (MAP) kinase. Concentrations of Etn >50 microM also enhanced the effects of both PCho and the synergistic effects of PCho plus ATP; however, in the latter case 20 microM Etn was inhibitory. The magnitude of both the potentiating and inhibitory effects of Etn on PCho-induced as well as PCho + ATP-induced DNA synthesis were similar in the vector control and Etn kinase overexpressor cells; they were associated with stimulation and inhibition, respectively, of p42 MAP kinase activity. The results indicate that in NIH 3T3 cells Etn exerts significant effects on DNA synthesis which, except inhibition of insulin-induced DNA synthesis by higher concentrations of Etn, do not correlate with the metabolism of Etn to PEtn or PtdEtn.

Disruption of the murine gene encoding phosphatidylethanolamine N-methyltransferase

All nucleated cells make phosphatidylcholine via the CDP-choline pathway. Liver has an alternative pathway in which phosphatidylcholine is made by methylation of phosphatidylethanolamine catalyzed by phosphatidylethanolamine N-methyltransferase (PEMT). We investigated the function of PEMT and its role in animal physiology by targeted disruption of its gene, Pempt2. A targeting vector that interrupts exon 2 was constructed and introduced into mice yielding three genotypes: normal (+/+), heterozygotes (+/-), and homozygotes (-/-) for the disrupted PEMT gene. Only a trace of PE methylation activity remained in Pempt2(-/-) mice. Antibody to one form of the enzyme, PEMT2, indicated complete loss of this protein from Pempt2(-/-) mice and a decrease in Pempt2(+/-) mice, compared with Pempt2(+/+) mice. The levels of hepatic phosphatidylethanolamine and phosphatidylcholine were minimally affected. The active form of CTP:phosphocholine cytidylyltransferase, the regulated enzyme in the CDP-choline pathway, was increased 60% in the PEMT-deficient mice. Injection of [L-methyl-3H]methionine demonstrated that the in vivo PEMT activity was eliminated in the Pempt2(-/-) mice and markedly decreased in the Pempt2(+/-) mice. This experiment also demonstrated that the choline moiety derived from PEMT in the liver can be distributed via the plasma throughout the mouse where it is found as phosphatidylcholine, lysophosphatidylcholine, and sphingomyelin. Mice homozygous for the disrupted Pempt2 gene displayed no abnormal phenotype, normal hepatocyte morphology, normal plasma lipid levels and no differences in bile composition. This is the first application of the "knockout mouse" technique to a gene for phospholipid biosynthesis.

Expression of protein kinase C-beta promotes the stimulatory effect of phorbol ester on phosphatidylethanolamine synthesis

Stimulation of phosphatidylethanolamine (PtdEtn) synthesis by the protein kinase C (PKC) activator phorbol 12-myristate 13-acetate (PMA) has reportedly been found only in hepatocytes expressing the alpha-, betaII-, epsilon-, and zeta-PKC isozymes. In contrast, stimulation of phosphatidylcholine synthesis by PKC activators, known to be mediated by PKC-alpha, is widespread in mammalian cells. In this work, various cell lines exhibiting characteristic differences in their PKC systems were used to determine the role of specific PKC isozymes in the mediation of PMA effect on PtdEtn synthesis. In NIH 3T3 fibroblasts, which express high levels of PKC-alpha but none of the beta (betaI or betaII) isoforms, PMA did not stimulate PtEtn synthesis. In contrast, in Rat-6 fibroblasts overexpressing PKC-betaI, 10-100 nM PMA considerably (1.7- to 2.6-fold) enhanced PtdEtn synthesis. In wild-type or multidrug resistant MCF-7 human breast carcinoma cells, which express PKC-alpha and PKC-betaII (to varying extents) but not PKC-betaI, PMA had only small or no effects on PtdEtn synthesis. In contrast, in MCF-7 cells overexpressing PKC-alpha, and as a consequence also expressing the betaI- and betaII-PKC isoforms, PMA effectively stimulated the synthesis of PtdEtn. Finally, in HL60 human leukemia cells, which contains PKC-betaII as the major PKC isoform, PMA again stimulated PtdEtn synthesis. The results establish that while stimulation of PtdEtn synthesis by PMA occurs only in selected cell lines, this phenomenon is not restricted to hepatocytes. Furthermore, the data indicate that expression of either PKC-betaI or PKC-betaII, but not PKC-alpha, correlates with the effect of PMA on PtdEtn synthesis. Overall, these observations strongly suggest that regulation of PtdEtn and PtdCho synthesis by PMA involves separate PKC isozymes, i.e., PKC-beta and PKC-alpha, respectively.

Effects of 2',3'-dideoxycytidine and 2',3'-dideoxycytidine 5'-triphosphate on phospholipid metabolism in permeabilized rat hepatocytes

Both 2',3'-dideoxycytidine (ddC) and 2',3'-dideoxycytidine 5'-triphosphate (ddCTP) inhibit the synthesis of the major phospholipids phosphatidylcholine (PC) and phosphatidylethanolamine (PE) in permeabilized rat hepatocytes. For PC, this appears to be based on competitive inhibition of cholinephosphotransferase (CDPcholine:1,2-diacylglycerol cholinephosphotransferase; EC 2.7.8.2). The study was based on short-term incubations (6-12 min) of the nucleoside/nucleotide analogs with alpha-toxin permeabilized rat hepatocytes. At a concentration of 1 mM, ddC and ddCTP decreased the incorporation of radiolabelled glycerol-3-phosphate into PC by approximately 50% as compared with control. This was accompanied by a significant increase in diacylglycerol labelling. In the presence of 1 mM CDP-ethanolamine and increasing concentrations of ddC(TP) (0.01-1 mM), the incorporation of radiolabelled glycerol-3-phosphate into PE was decreased to approximately 60% of the control value. When both PC and PE synthesis were operative, the inhibition by ddC(TP) was restricted to PC synthesis. ddC and ddCTP were found to have inhibition constants (K(i)) of 496 microM and 452 microM, respectively, for the inhibition of PC synthesis from CDP-choline. Although the inhibitory concentrations of the nucleoside analog and its triphosphate ester are much higher than the in vivo plasma concentrations, the possibility is raised that the peripheral neuropathy, seen as a dose-dependent adverse effect of ddC treatment in acquired immunodeficiency syndrome therapy is, at least partly, caused by a perturbation of the phospholipid constitution of neuronal membranes.

CTP:phosphocholine cytidylyltransferase

CTP:phosphocholine cytidylyltransferase (CCT) catalyzes the synthesis of CDP-choline and is regulatory for phosphatidylcholine biosynthesis. This review focuses on recent developments in understanding the catalytic and regulatory mechanisms of this enzyme. Evidence for the nuclear localization of the enzyme is discussed, as well as evidence suggesting cytoplasmic localization. A comparison of the catalytic domains of CCTs from a wide variety of organisms is presented, highlighting a large number of completely conserved residues. Work implying a role for the conserved HXGH sequence in catalysis is described. The membrane-binding domain in rat CCT has been defined, and the role of lipids in activating the enzyme is discussed. The identification of the phosphorylation domain is described, as well as approaches to understand the role of phosphorylation in enzyme activity. Other possible control mechanisms such as enzyme degradation and gene expression are presented.

Identification of an 11-residue portion of CTP-phosphocholine cytidylyltransferase that is required for enzyme-membrane interactions

CTP-phosphocholine cytidylyltransferase (CT) is a key regulatory enzyme in the biosynthesis of phosphatidylcholine (PC) in many cells. Enzyme-membrane interactions appear to play an important role in CT activation. A putative membrane-binding domain appears to be located between residues 236 and 293 from the N-terminus. To map the membrane-binding domain more precisely, glutathione S-transferase fusion proteins were prepared that contained deletions of various domains in this putative lipid-binding region. The fusion proteins were assessed for their binding of [3H]PC/oleic acid vesicles. Fusion proteins encompassing residues 267-277 bound to PC/oleic acid vesicles, whereas fragments lacking this region exhibited no specific binding to the lipid vesicles. The membrane-binding characteristics of the CT fusion proteins were also examined using intact lung microsomes. Only fragments encompassing residues 267-277 competed with full-length 125I-labelled CT, expressed in recombinant Sf9 insect cells, for microsomal membrane binding. To investigate the role of this region in PC biosynthesis, A549 and L2 cells were transfected with cDNA for CT mutants under the control of a glucocorticoid-inducible long terminal repeat (LTR) promoter. Induction of CT mutants containing residues 267-277 in transfectants resulted in reduced PC synthesis. The decrease in PC synthesis was accompanied by a shift in endogenous CT activity from the particulate to the soluble fraction. Expression of CT mutants lacking this region in A549 and L2 cells did not affect PC formation and subcellular distribution of CT activity. These results suggest that the CT region located between residues 267 and 277 from the N-terminus is required for the interaction of CT with membranes.

Identification of functional conserved residues of CTP:glycerol-3-phosphate cytidylyltransferase. Role of histidines in the conserved HXGH in catalysis

The CTP:glycerol-3-phosphate cytidylyltransferase (GCT) of Bacillus subtilis has been shown to be similar in primary structure to the CTP:phosphocholine cytidylyltransferases of several organisms. To identify the residues of this cytidylyltransferase family that function in catalysis, the conserved hydrophilic amino acid residues plus a conserved tryptophan of the GCT were mutated to alanine. The most dramatic losses in activity occurred with H14A and H17A; these histidine residues are part of an HXGH sequence similar to that found in class I aminoacyl-tRNA synthetases. The kcat values for H14A and H17A were decreased by factors of 5 x 10(-5) and 4 x 10(-4), respectively, with no significant change in Km values. Asp-11, which is found near the HXGH sequence in the cytidylyltransferases but not aminoacyl-tRNA synthetases, was also important for activity, with the D11A mutation decreasing activity by a factor of 2 x 10(-3). Several residues found in the sequence RTEGISTT, a signature sequence for this cytidylyltransferase family, as well as other isolated residues were also shown to be important for activity, with kcat values decreasing by factors of 0.14-4 x 10(-4). The Km values of three mutant enzymes, D38A, W74A, and D94A, for both CTP and glycerol-3-phosphate were 6-130-fold higher than that of the wild-type enzyme. Mutant enzymes were analyzed by two-dimensional NMR to determine if the overall structures of the enzymes were intact. One of the mutant enzymes, D66A, was defective in overall structure, but several of the others, including H14A and H17A, were not. These results indicate that His-14 and His-17 play a role in catalysis and suggest that their role is similar to the role of the His residues in the HXGH sequence in class I aminoacyl-tRNA synthetases, i.e. to stabilize a pentacoordinate transition state.

Regulation of phospholipid biosynthesis by Ca(2+)-calmodulin-dependent protein kinase inhibitors

Inhibitors of Ca(2+)-calmodulin (CaM)-dependent protein kinases strongly modify phospholipid metabolism. Two compounds, KN62 and KT5926 recognized as blockers of Ca(2+)-CaM-dependent protein kinase II, induced a specific increase in phosphatidylserine (PtdSer) synthesis without noticeable changes in phosphatidylcholine (PtdCho) and phosphatidylethanolamine (PtdEtn) biosynthesis. The increase of PtdSer synthesis was dependent on the presence of Ca2+ in the incubation medium and was impaired in cells whose Ca2+ stores were depleted by pretreatment with CD3 mAb, thapsigargin or EGTA. The mechanism of the stimulation of PtdSer synthesis by these two compounds seems to involve an accumulation of Ca2+ into the endoplasmic reticulum, possibly due to an increased activity of the endoplasmic reticulum Ca(2+)-ATPase. By contrast, ML-7 and ML-9, two inhibitors of the myosin light chain kinase (MLCK), another Ca(2+)-CaM-dependent kinase, were both capable of increasing PtdSer synthesis and decreasing PtdCho and PtdEtn synthesis, reproducing the effect previously described with CaM-antagonists. The increase of PtdSer caused by ML-7 and ML-9 was Ca(2+)-dependent while the inhibition of PtdCho and PtdEtn synthesis was not. The use of these four protein kinase inhibitors thus suggests the possible existence of two CaM-dependent pathways that differentially regulates phospholipid metabolism in T cells.

Cloning of a human cDNA for CTP-phosphoethanolamine cytidylyltransferase by complementation in vivo of a yeast mutant

CTP-phosphoethanolamine cytidylyltransferase (ET) is the enzyme that catalyzes the formation of CDP-ethanolamine in the phosphatidylethanolamine biosynthetic pathway from ethanolamine. We constructed a Saccharomyces cerevisiae mutant of which the ECT1 gene, putatively encoding ET, was disrupted. This mutant showed a growth defect on ethanolamine-containing medium and a decrease of ET activity. A cDNA clone was isolated from a human glioblastoma cDNA expression library by complementation of the yeast mutant. Introduction of this cDNA into the yeast mutant clearly restored the formation of CDP-ethanolamine and phosphatidylethanolamine in cells. ET activity in transformants was higher than that in wild-type cells. The deduced protein sequence exhibited homology with the yeast, rat, and human CTP-phosphocholine cytidylyltransferases, as well as yeast ET. The cDNA gene product was expressed as a fusion with glutathione S-transferase in Escherichia coli and shown to have ET activity. These results clearly indicate that the cDNA obtained here encodes human ET.

Phospholipid biosynthetic enzymes in human brain

Growing evidence suggests an involvement of brain membrane phospholipid metabolism in a variety of neurodegenerative and psychiatric conditions. This has prompted the use of drugs (e.g., CDPcholine) aimed at elevating the rate of neural membrane synthesis. However, no information is available regarding the human brain enzymes of phospholipid synthesis which these drugs affect. Thus, the objective of our study was to characterize the enzymes involved, in particular, whether differences existed in the relative affinity of substrates for the enzymes of phosphatidylethanolamine (PE) compared to those of phosphatidylcholine (PC) synthesis. The concentration of choline in rapidly frozen human brain biopsies ranged from 32-186 nmol/g tissue, a concentration similar to that determined previously for ethanolamine. Since human brain ethanolamine kinase possessed a much lower affinity for ethanolamine (Km = 460 microM) than choline kinase did for choline (Km = 17 microM), the activity of ethanolamine kinase in vivo may be more dependent on substrate availability than that of choline kinase. In addition, whereas ethanolamine kinase was inhibited by choline, and to a lesser extent by phosphocholine, choline kinase activity was unaffected by the presence of ethanolamine, or phosphoethanolamine, and only weakly inhibited by phosphocholine. Phosphoethanolamine cytidylyltransferase (PECT) and phosphocholine cytidylyltransferase (PCCT) also displayed dissimilar characteristics, with PECT and PCCT being located predominantly in the cytosolic and particulate fractions, respectively. Both PECT and PCCT exhibited a low affinity for CTP (Km approximately 1.2 mM), suggesting that the activities of these enzymes, and by implication, the rate of phospholipid synthesis, are highly dependent upon the cellular concentration of CTP. In conclusion our data indicate different regulatory properties of PE and PC synthesis in human brain, and suggest that the rate of PE synthesis may be more dependent upon substrate (ethanolamine) availability than that of PC synthesis.

Phosphatidylcholine signaling in response to CSF-1

The formation of cell membrane following CSF-1 stimulation of a macrophage cell line is coordinated with cell cycle progression. The majority of membrane phospholipid accumulates during the S phase and results from cell-cycle dependent oscillations in the rates of phosphatidylcholine biosynthesis and degradation. Both synthesis and degradation are enhanced during the G1 phase, resulting in a high rate of phosphatidylcholine turnover. Degradation of phosphatidylcholine after CSF-1 stimulation is mediated by a phospholipase C, and the release of diacylglycerol during G1 phase is biphasic. The degradation essentially stops during the S phase, thus allowing biosynthesis to supply the necessary membrane for cell division and doubling. The degradation of phosphatidylcholine during G1 signals the downstream activation of c-fos and junB transcription and can be mimicked by incubation of the macrophage cells with exogenous bacterial phospholipase C. In contrast, the expression of c-myc transcripts normally associated with CSF-1 stimulation is severely compromised in phospholipase C-treated cells, indicating that the diacylglycerol signals a pathway distinct from the pathway that governs c-myc activation. Constitutive expression of c-myc complements phospholipase C activity and permits the growth of cells in the presence of exogenous bacterial enzyme and the absence of CSF-1. Protein kinase C is not required to mediate the diacylglycerol signal that supports cell growth. GTP exchange on Ras is not enhanced, and MAP kinase activity is not stimulated in response to phosphatidylcholine degradation by exogenous phospholipase C. The 85 kDa cytoplasmic phospholipase A2 is activated, however, as well as a novel protein we have called p96. Rapid serine phosphorylation of p96 follows stimulation of cells with either CSF-1 or exogenous phospholipase C. Analysis of the murine cDNA encoding p96 reveals an amino-terminal domain with significant similarity to the amino-terminal domain of the Drosophila-disabled gene product and a carboxy-terminal domain containing proline-rich sequences characteristic of SH3 binding regions. The sequence of p96 suggests an interactive role for this unique protein in the CSF-1 signal transduction cascade.

Progesterone triggers the rapid activation of phospholipase D in the amphibian oocyte plasma membrane when initiating the G2/M transition

Previous reports indicate that, in the Rana pipiens oocyte, progesterone triggers a rapid rise in 1,2-diacylglycerol (DAG) derived from phosphatidylcholine (PC) in the plasma membranes. This DAG transient, which appears and is terminated within 60-90 s, is derived both from a phospholipase which we assumed to be phospholipase C and from sphingomyelin (SM) synthase. We now find that progesterone stimulates PC and DAG turnover primarily via the phospholipase D (PLD) and phosphatidic acid phosphohydrolase (PAP) pathways as well as via the SM-ceramide pathway. Rana oocytes were prelabeled with [3H]choline chloride under conditions in which about 70% is incorporated into PC of the plasma membrane of the intact oocyte or with [3H]lysoplatelet activating factor (1-O-octadecyl-sn-glycero-3-phosphocholine, lysoPAF) which is selectively incorporated into plasma membrane PC. Progesterone induced the release of [3H]choline from intact oocytes into the medium within 60-90 s. This choline release was dose-dependent and was not inhibited by a putative PC-specific phospholipase C inhibitor, D609. Progesterone also induced a transient rise in [3H]lysoPAF-derived [3H]DAG within 1-2 min followed by a rise in [3H]PA. In the presence of 20 mM ethanol, progesterone stimulated formation of [3H]lysoPAF-derived phosphatidylethanol, indicating progesterone activation of PC-specific PLD and concomitant formation of PA. A DGK inhibitor (D102) reduced the level of [3H]PA, produced a sustained rise in [3H]DAG and was a weak inducer of meiosis in oocytes not exposed to progesterone. A PA phosphohydrolase inhibitor (propranolol) elevated [3H]PA and completely inhibited the progesterone-induced rise in DAG. Progesterone thus acts at oocyte plasma membrane receptors to release PC-derived DAG via both SM synthase and PC-PLD. The duration of the DAG signal is regulated by the coordinate action of DGK and PAP.

Radioautographic study of the incorporation of (3H)-choline into the phospholipids of secretory ameloblasts and enamel of normal and essential-fatty-acid-deficient rats

(3H)-choline, a precursor for phosphatidylcholine (PC) and sphingomyelin (SM), was injected into rats killed after 4, 24, 48, and 96 hrs. Radioautography carried out on malachite-green/aldehyde-fixed tissues demonstrated that labeled choline was incorporated into cells and further released into the extracellular matrix. In predentin, labeling decreased rapidly, whereas in dentin, silver grains formed a stable band. In contrast, labeling was still high at 48 and 96 hrs in secretory ameloblasts as well as in the forming enamel. This indicates that ameloblasts are actively involved in the synthesis of membranes. Membrane remnants of the ameloblasts could be released into the forming enamel. In rats fed with an essential fatty-acid-deficient (EFAD) diet for 42 days, (3H)-choline uptake was delayed and reduced in pulp cells and odontoblasts, and consequently the migration of labeled phospholipids into dentin. The influence of the EFAD diet on secretory ameloblasts was limited. No difference was detected between normally fed and EFAD-fed rats in the forming enamel.

Substitution of serine for glycine-91 in the HXGH motif of CTP:phosphocholine cytidylyltransferase implicates this motif in CTP binding

The effect of mutations in the proposed catalytic domain of CTP:phosphocholine cytidylyltransferase was investigated by constructing the single mutants CT-S91 and CT-C114 from the double mutant CT-S91C114, previously shown to have 4-fold lower than wild-type activity [Walkey, C.R., Kalmar, G. B., & Cornell, R. B. (1994) J. Biol. Chem. 269, 5742-5749]. The constructs were overexpressed in COS cells. The mutation Gly-91 to Ser-91 was found to be responsible for the decreased activity, whereas Ser-114 to Cys-114 had no effect. An alanine substitution at position 91, CT-A91, had a lesser effect on cytidylyltransferase activity. CT-S91 and CT-WT were purified from COS cells, and their kinetic constants were determined. CT-S91 had a 4-fold lower Vmax, and a K(m) for CTP 25-fold higher than the wild-type enzyme, suggesting that substitution of Gly-91 with serine interferes with CTP binding. The K(m) for phosphocholine was not affected in the CT-S91 mutant. There was no difference in the chymotrypsin sensitivities of CT-S91 and CT-WT, indicating that the mutation did not cause a global change in protein structure. However, the CT-S91 activity was more susceptible to inhibition by the denaturant urea than that of CT-WT, indicative of a perturbation of the active site folding. Gly-91 resides in the local sequence HSGH, which has been proposed to be a CTP-binding motif in the novel cytidylyltransferase superfamily [Bork, P., Holm, L., Koonin, E.V., & Sander, C. (1995) Proteins: Struct., Funct., Genet. 22, 259-266]. Our results represent the first experimental validation of this hypothesis.

c-Ha-ras oncogene expression increases choline uptake, CTP: phosphocholine cytidylyltransferase activity and phosphatidylcholine biosynthesis in the immortalized human keratinocyte cell line HaCaT

The effect of c-Ha-ras transfection on phosphatidylcholine biosynthesis of the keratinocyte cell line HaCaT was investigated. It was shown that ras-transfection caused a 3-fold increase of choline incorporation into phosphatidylcholine. By investigating the mechanisms underlying this phenomenon, two targets were obtained. First, the choline uptake was elevated by 2-fold in ras-transfected HaCaT cells as compared with untransfected HaCaT cells, and second, the activity of the rate-limiting enzyme of phosphatidylcholine biosynthesis, CTP:phosphocholine cytidylyltransferase, was increased by 43%. Stimulation of HaCaT cells and ras-transfected HaCaT cells with oleate revealed that the increased activity of cytidylyltransferase might be due to a higher level of enzyme. In these experiments, a 75% increase of the specific activity of fully stimulated, membrane-bound cytidylyltransferase was found in ras-transfected HaCaT cells. Choline kinase which has been previously described as a target of ras-transfection in fibroblasts was unaffected.

Expression of phosphatidylethanolamine N-methyltransferase-2 in McArdle-RH7777 hepatoma cells inhibits the CDP-choline pathway for phosphatidylcholine biosynthesis via decreased gene expression of CTP:phosphocholine cytidylyltransferase

Phosphatidylethanolamine N-methyltransferase-2 (PEMT2) of rat liver was expressed in McArdle-RH7777 rat hepatoma cells, which lack endogenous PEMT activity. Expression of the enzyme was confirmed by assay of PEMT activity and immunoblotting. There was no change in the amount of phosphatidylcholine in the transfected cells [Cu, Houweling and Vance (1994) J. Biol. Chem. 269, 24531-24533], even though the expression of PEMT2 caused an increased incorporation of [methyl-3H]methionine and [3H]ethanolamine into phosphatidylcholine. In contrast, [3H]serine incorporation into phosphatidylcholine was only marginally enhanced by PEMT2 expression. Incorporation of [methyl-3H]choline into phosphatidylcholine was decreased by greater than 60%, suggesting that the CDP-choline pathway was inhibited as a result of PEMT2 expression. CTP:phosphocholine cytidylyltransferase (CT) activities in transfected cell lines were decreased in proportion to the level of expression of PEMT2. Immunoblot analyses showed a decrease in CT mass as a function of PEMT2 expression. In contrast, there was no change in the mass of protein disulphide-isomerase or the relative amounts of most proteins expressed in the PEMT2-transfected, compared with control, cells. Similarly, the expression of CT mRNA was decreased in PEMT2-expressing cells, whereas the mRNAs for protein disulphide-isomerase and actin were unchanged. When cell growth was slowed by incubating McArdle-RH7777 cells at 25 degrees C, compared with 37 degrees C, there was no difference in the specific activity of the CT. These results argue that PEMT2 expression down-regulates the CDP-choline pathway by decreasing the expression of the gene for the CT. The decreased activity of the CDP-choline pathway might contribute to the slower rate of cell division in PEMT2-transfected hepatoma cells.

The enhancement of phosphatidylcholine biosynthesis by angiotensin II in H9c2 cells

The effect of angiotensin II on the biosynthesis of phosphatidylcholine in rat heart myoblastic (H9c2) cells was investigated. Cells were incubated with [methyl-3H]choline, and the labelling of phosphatidylcholine at different time intervals was examined. When cells were pretreated with angiotensin II, a significant increase in the labelling of phosphatidylcholine was observed. Analysis of the labelled phosphatidylcholine precursors indicated that the conversion of phosphocholine to CDP-choline was enhanced by angiotensin II treatment. Determination of enzyme activities in the CDP-choline pathway revealed that the activities of choline kinase or CDP-choline: diacylglycerol cholinephosphotransferase were not changed, but the activities of CTP:phosphocholine cytidylyltransferase were stimulated in both the particulate and soluble fractions. The stimulation of the cytidylyltransferase by angiotensin II was not abolished by okadaic acid, indicating that the activation of the enzyme was not mediated via the okadaic-sensitive dephosphorylation mechanism. Alternatively, the stimulation of the cytidylyltransferase activity was completely abolished by protein kinase C inhibitors. Immunoblotting studies revealed that levels of the cytidylyltransferase in the soluble and particulate fractions were not affected by angiotensin II treatment. We conclude that the increase in phosphatidylcholine biosynthesis by angiotensin II was a direct result of the enhancement of the cytidylyltransferase activity. The enhancement of enzyme activity was not mediated via enzyme translocation, but by a mechanism which was intimately associated with the protein kinase C cascade.

Stimulation of phosphatidylcholine biosynthesis in mouse MLE-12 type-II cells by conditioned medium from cortisol-treated rat fetal lung fibroblasts

Treatment of murine adult MLE-12 type-II and fetal-rat type-II cells with fetal-rat-fibroblast-conditioned medium (FFCM) resulted in a 2-fold stimulation of [14C]choline incorporation into phosphatidylcholine. Soluble CTP:phosphocholine cytidylyltransferase (CT) activity was increased approx. 3-fold in FFCM-treated fetal-rat type-II cells but was not changed in MLE-12 cells. Neither choline kinase nor cholinephosphotransferase activities were affected by treatment of MLE-12 cells with FFCM. Long-term labelling of MLE-12 cells with [14C]choline, followed by a 14-18 h chase with FFCM, resulted in a 2.5-fold decrease in [14C]phosphocholine levels relative to controls, suggesting that CT was being activated. In contrast, oleate treatment increased CT activity in the particulate fraction in both cells. Western blots indicate that soluble CT undergoes dephosphorylation in response to FFCM, but no translocation to the particulate fraction was noted. Treatment with oleate stimulated a marked translocation. Tryptic phosphopeptide maps from FFCM-treated cells revealed only minor alterations in the phosphorylation pattern. It is concluded that FFCM and oleate activate CT through different mechanisms. The results are consistent with FFCM activating CT in MLE-12 as well as fetal type-II cells. However, the reason why this activation cannot be detected in vitro is not known.

Two new sphingomyelin analogues inhibit phosphatidylcholine biosynthesis by decreasing membrane-bound CTP: phosphocholine cytidylyltransferase levels in HaCaT cells

The effects of two newly synthesized sphingomyelin analogues on phosphatidylcholine biosynthesis were investigated in the immortalized human keratinocyte cell line HaCaT. N-Acetyl-erythro-sphingosine-1-phosphocholine (AcSM) and N-octanoyl-erythro-sphingosine-1-phosphocholine (OcSM) inhibited the incorporation of choline into phosphatidylcholine with half-inhibitory concentrations (IC50) of 6 micrograms/ml and 10 micrograms/ml respectively. Further experiments revealed that AcSM and OcSM interfered with the translocation of the rate-limiting enzyme of phosphatidylcholine biosynthesis, CTP:phosphocholine cytidylyltransferase (EC 2.7.7.15), in HaCaT cells and inhibited cytidylyltransferase activity in vitro. Despite the fact that OcSM was a potent inhibitor of cytidylyltransferase in vitro, its effects on phosphatidylcholine biosynthesis and translocation of cytidylyltransferase in HaCaT cells were less pronounced as compared with AcSM. Finally, we showed that the comparatively strong effects of AcSM in cell culture experiments were due to the uptake of large amounts of this sphingomyelin analogue into the cells. The results presented demonstrate that the activity of cytidylyltransferase may be negatively regulated by a high ratio of choline head group-containing sphingolipids.

The association of lipid activators with the amphipathic helical domain of CTP:phosphocholine cytidylyltransferase accelerates catalysis by increasing the affinity of the enzyme for CTP

The biochemical mechanism for the regulation of enzyme activity by lipid modulators and the role of the amphipathic alpha-helical domain of CTP:phosphocholine cytidylyltransferase (CT) was investigated by analyzing the kinetic properties of the wild-type protein and two truncation mutants isolated from a baculovirus expression system. The CT[delta 312-367] mutant protein lacked the carboxyl-terminal phosphorylation domain and retained high catalytic activity along with both positive and negative regulation by lipid modulators. The CT[delta 257-367] deletion removed in addition the region containing three consecutive amphipathic alpha-helical repeats. The CT[delta 257-367] mutant protein exhibited a significantly lower specific activity compared to CT or CT[delta 312-367] when expressed in either insect or mammalian cells; however, CT[delta 257-367] activity was refractory to either stimulation or inhibition by lipid regulators. Lipid activators accelerated CT activity by decreasing the Km for CTP from 24.7 mM in their absence to 0.7 mM in their presence. The Km for phosphocholine was not affected by lipid activators. The activity of CT[delta 257-367] was comparable to the activity of wild-type CT in the absence of lipid activators and the CTP Km for CT[delta 257-367] was 13.9 mM. The enzymatic properties of the CT[delta 231-367] mutant were comparable to those exhibited by the CT[257-367] mutant indicating that removal of residues 231 through 257 did not have any additional influence on the lipid regulation of the enzyme. Thus, the region between residues 257 and 312 was required to confer lipid regulation on CT, and the association of activating lipids with this region of the protein stimulated catalysis by increasing the affinity of the enzyme for CTP.

Molecular cloning of CTP:phosphocholine cytidylyltransferase from Plasmodium falciparum

CTP:phosphocholine cytidylyltransferase (CCT) is the rate-limiting and regulatory enzyme in the synthesis of phosphatidylcholine, the major membrane phospholipid, in Plasmodium. The structural gene encoding CCT was isolated from the human malaria parasite Plasmodium falciparum. This was achieved using the PCR to amplify genomic DNA with degenerate primers constructed on the basis of conserved regions identified within yeast and rat liver CCT molecules, and using the PCR product to screen a genomic library. The P. falciparum CCT gene encodes a protein of 370 amino acids (42. 6 kDa) and displays 41-43% similarity (28-29% identity) to CCT molecules of the other organisms cloned to date. The central domain of CCT, proposed as the catalytic domain of the CTP-transfer reaction, shows 68-72% similarity and 48-55% identity among P. falciparum, human, rat and yeast enzymes. This gene is present in a single copy, as determined by Southern-blotting of genomic DNA, and located on chromosome 13 of P. falciparum. Large transcripts were detected by Northern analysis and indicate that this gene is expressed in the asexual intraerythrocytic stages. The coding region of the P. falciparum CCT gene was inserted into an Escherichia coli expression vector to confirm the function of the CCT product. The recombinant CCT expressed in E. coli is catalytically active, as evidenced by the conversion of phosphocholine to CDP-choline.

Partial purification of a phosphoethanolamine methyltransferase from rat brain cytosol

The conversion of phosphoethanolamine to phosphocholine requires 3 separate N-methyltransferases. We had previously purified the enzyme catalyzing the last methylation, phosphodimethylethanolamine N-methyltransferase. We have successfully purified the enzyme catalyzing the initial methylation of phosphoethanolamine. A 434 fold purified enzyme from rat brain was obtained by the sequential use of ammonium sulfate fractionation, Q-Sepharose fast flow column chromatography and a omega-aminoethyl agarose column chromatography. The pH optimum was 11 or greater, the Km value for phosphoethanolamine was 167.8 +/- 41.7 microM and the Vmax was 487.3 +/- 85 mmoles/mg/hr. The kinetics for S-adenosyl-methionine, the methyldonor, has characteristics of cooperative binding with a Km of 1.805 +/- 0.59 mM and a Vmax of 16.9 +/- 3.6 mumoles/mg/hr. The activity was stimulated 6 fold by 2.5 mM MnCl2 and inhibited by DZA and S-adenosylhomocysteine. These results reinforce the early in vivo observations which had provided suggestive evidence for the existence of a pathway for the methylation of phosphoethanolamine to phosphocholine in rat brain.

Identification of an inhibitory domain of CTP:phosphocholine cytidylyltransferase

The function of the putative amphipathic helices between residues 236 and 314 of CTP:phosphocholine cytidylyltransferase was examined by constructing two truncation mutants; CT314 was missing the entire phosphorylation segment, whereas CT236 was missing both the region with the putative amphipathic helices and the phosphorylation segment. Stable cells lines expressing these truncation mutants in Chinese hamster ovary 58 cells were isolated and characterized. CT314 was predominantly soluble in control cells but became membrane-associated in cells treated with oleate, which also causes translocation of wild-type cytidylyltransferase. CT236 was found to be soluble both in control cells and in cells treated to cause translocation. These results strongly suggest that the membrane-binding site is located within residues 237-314. When assayed for activity in vitro, the mutant forms were catalytically active in the presence of exogenous lipids. CT236, moreover, was as active in the absence of lipids as in their presence, whereas CT314 required lipids for activity. The rate of phosphatidylcholine synthesis in cells expressing CT236 was considerably higher than in wild-type cells, consistent with the enzyme being constitutively active in the cells. These results indicate that residues 237-314 constitute an inhibitory segment; when this segment is removed from the catalytic domain by truncation or by binding to membranes, an inhibitory constraint is removed and cytidylyltransferase is activated.