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

Hexadecylphosphocholine disrupts cholesterol homeostasis and induces the accumulation of free cholesterol in HepG2 tumour cells

Hexadecylphosphocholine (HePC) is a synthetic lipid belonging to the alkylphosphocholines (APC), a new group of antiproliferative agents that are proving to be promising candidates in anticancer therapy. We reported in a previous study that HePC interferes with phosphatidylcholine (PC) synthesis in HepG2 cells via both CDP-choline and phosphatidylethanolamine (PE) methylation. We have subsequently extended our studies to show that HePC interferes with sphingolipid metabolism by hindering the formation of sphingomyelin (SM), an effect accompanied by a substantial increase in the incorporation of the exogenous lipogenic precursors into ceramides. Interestingly, we demonstrate for the first time that HePC strongly inhibits the esterification of free cholesterol (FC) by acting at the level of acyl CoA:cholesterol acyltransferase (ACAT) (EC 2.3.1.26) activity. This effect is accompanied by a considerable increase in the synthesis of cholesterol, which leads to a rise in the levels of FC in cells. We are left in no doubt that the imbalance in the metabolism of membrane-lipid components vital to cell survival may well be responsible for the observed DNA fragmentation and activation of caspase-3, an enzyme involved in the cell apoptosis found in this study.

Characterization of the lipid-binding domain of the Plasmodium falciparum CTP:phosphocholine cytidylyltransferase through synthetic-peptide studies

Phospholipid biosynthesis plays a key role in malarial infection and is regulated by CCT (CTP:phosphocholine cytidylyltransferase). This enzyme belongs to the group of amphitropic proteins which are regulated by reversible membrane interaction. To assess the role of the putative membrane-binding domain of Plasmodium falciparum CCT (PfCCT), we synthesized three peptides, K21, V20 and K54 corresponding to residues 274-294, 308-327 and 274-327 of PfCCT respectively. Conformational behaviour of the peptides, their ability to bind to liposomes and to destabilize lipid bilayers, and their insertion properties were investigated by different biophysical techniques. The intercalation mechanisms of the peptides were refined further by using surface-pressure measurements on various monolayers at the air/water interface. In the present study, we show that the three studied peptides are able to bind to anionic and neutral phospholipids, and that they present an alpha-helical conformation upon lipid binding. Peptides V20 and the full-length K54 intercalate their hydrophobic parts into an anionic bilayer and, to a lesser extent, a neutral one for V20. Peptide K21 interacts only superficially with both types of phospholipid vesicles. Adsorption experiments performed at the air/water interface revealed that peptide K54 is strongly surface-active in the absence of lipid. Peptide V20 presents an atypical behaviour in the presence of phosphatidylserine. Whatever the initial surface pressure of a phosphatidylserine film, peptide V20 and phosphatidylserine entities seem linked together in a special organization involving electrostatic and hydrophobic interactions. We showed that PfCCT presents different lipid-dependence properties from other studied CCTs. Although the lipid-binding domain seems to be located in the C-terminal region of the enzyme, as with the mammalian counterpart, the membrane anchorage, which plays a key role in the enzyme regulation, is driven by two alpha-helices, which behave differently from one another.

Phosphatidylcholine metabolism in nuclei of phorbol ester-activated LA-N-1 neuroblastoma cells

The agonist stimulation of a variety of cells results in the induction of specific lipid metabolism in nuclear membranes, supporting the hypothesis of an important role of the lipids in nuclear signal transduction. While the existence of a phosphatidylinositol cycle has been reported in cellular nuclei, little attention has been given to the metabolism of phosphatidylcholine in nuclear signaling. In the present study the metabolism of phosphatidylcholine in the nuclei of neuroblastoma cells LA-N-1 was investigated. The incubation of LA-N-1 nuclei with radioactive choline, phosphocholine or CDP-choline led to the production of labelled phosphatidylcholine. The incorporation of choline and phosphocholine but not CDP-choline was enhanced in nuclei of TPA treated cells. Moreover the presence of choline kinase, phosphocholine cytidylyltransferase and phosphocholine transferase activities were detected in the nuclei and the TPA treatment of the cells stimulated the activity of the phosphocholine cytidylyltransferase. When cells prelabelled with [3H]palmitic acid were stimulated with TPA in the presence of ethanol, an increase of labelled diacylglycerol and phosphatidylethanol in the nuclei was observed. Similarly, an increase of labelled diacylglycerol and phosphatidic acid but not of phosphatidylethanol occurred in [3H]palmitic acid prelabelled nuclei stimulated with TPA in the presence of ethanol. However the production of phosphatidylethanol was observed when the nuclei were treated with TPA in the presence of ATP and GTPgammaS. The stimulation of [3H]choline prelabelled nuclei with TPA also generated the release of free choline and phosphocholine. The results indicate the presence of PLD and probably PLC activities in LA-N-1 nuclei and the involvement of phosphatidylcholine in the production of nuclear lipid second messengers upon TPA stimulation of LA-N-1 cells. The correlation of the disappearance of phosphatidylcholine, the production of diacylglycerol and phosphatidic acid with the stimulation of phosphatidylcholine synthesis in nuclei of TPA treated LA-N-1 suggests the existence of a phosphatidylcholine cycle in these nuclei.

Shuttling of CTP:Phosphocholine cytidylyltransferase between the nucleus and endoplasmic reticulum accompanies the wave of phosphatidylcholine synthesis during the G(0) --> G(1) transition

The transition from quiescence (G(0)) into the cell division cycle is marked by accelerated phospholipid turnover. We examined the rates of phosphatidylcholine (PC) synthesis and the activity, membrane affinity, and intracellular localization of the rate-limiting enzyme in the synthesis of PC, CTP:phosphocholine cytidylyltransferase (CT) during this transition. The addition of serum to quiescent IIC9 fibroblasts resulted in a wave of PC synthesis beginning at approximately 10 min, peaking at approximately 3 h with a >10-fold increase in rate, and declining to near basal rates by 10 h. CT activity, monitored in situ, was elevated approximately 3-fold between 1 and 2 h postserum. Neither CT mass nor its phosphorylation state changed during the surge in PC synthesis and CT activity. On the other hand, the ratio of particulate/soluble CT surged and then receded in concert with the wave of PC synthesis. During quiescence, CT was confined to the nucleus, as assessed by indirect immunofluorescence. Within 10 min after serum stimulation, a portion of the CT fluorescence appeared in the cytoplasm, where it intensified until approximately 4 h postserum. Thereafter, the cytoplasmic CT signal waned, while the nuclear signal increased, and by 8 h CT was once again predominantly nuclear. The dynamics of CT's apparent translocation in and out of the nucleus paralleled the wave of PC synthesis and the solubility changes of CT. Cytoplasmic CT co-localized with BiP, a resident endoplasmic reticulum protein, in a double labeling experiment. These data suggest that the wave of PC synthesis that accompanies the G(0) --> G(1) transition is regulated by the coordinated changes in CT activity, membrane affinity, and intracellular distribution. We describe for the first time a redistribution of CT from the nucleus to the ER that correlates with an activation of the enzyme. We propose that this movement is required for the stimulation of PC synthesis during entry into the cell cycle.

CTP:phosphocholine cytidylyltransferase: insights into regulatory mechanisms and novel functions

A key regulatory enzyme in phosphatidylcholine biosynthesis, CTP:cholinephosphate cytidylyltransferase (CCT), catalyzes the formation of CDP-choline. This review discusses the essential features of CCT and addresses intriguing new insights into the catalytic and regulatory properties of this complex enzyme. Characterization of a lipid-binding segment in rat CCT is described and the role of lipids in CCT activation is discussed. An analysis of the phosphorylation domain is presented and possible physiological rationales for reversible phosphorylation of CCT are discussed. The nuclear localization of CCT is examined in the context of multiple CCT isoforms, as is recent evidence establishing a potential link between CCT activity and vesicular transport.

Macrophage-targeted CTP:phosphocholine cytidylyltransferase (1-314) transgenic mice

CTP:phosphocholine cytidylyltransferase (CT) is a rate-limiting and complexly regulated enzyme in phosphatidylcholine (PC) biosynthesis and is important in the adaptation of macrophages to cholesterol loading. The goal of the present study was to use transgenesis to study the CT reaction in differentiated macrophages in vivo. We successfully created macrophage-targeted transgenic mice that overexpress a truncated form of CT, called CT-314. Sonicated homogenates of peritoneal macrophages overexpressing CT-314 protein demonstrated a two-fold increase in CT activity in vitro compared with homogenates from nontransgenic macrophages. CT-314 macrophages, however, demonstrated no increase in CT activity or PC biosynthesis in vivo. This finding could not be explained simply by intracellular mistargeting of CT-314, by the inability of CT-314 to associate with cellular membranes, or by substrate limitation. To further probe the mechanism, an in vitro assay using intact nuclei was developed in an attempt to preserve interactions between CT, which is primarily a nuclear enzyme in macrophages, and other nuclear molecules. This intact-nuclei assay faithfully reproduced the situation observed in living macrophages, namely, no significant increase in CT activity despite increased CT-314 protein. In contrast, CT activity in sonicated nuclei from CT-314 macrophages was substantially higher than that from nontransgenic macrophages. Thus, a sonication-sensitive interaction between excess CT and one or more nuclear molecules may be responsible for the limitation of CT activity in CT-314 macrophages. These data represent the first report of a CT transgenic animal and the first study of a differentiated cell type with excess CT.

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

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