Overexpression of rat liver CTP:phosphocholine cytidylyltransferase accelerates phosphatidylcholine synthesis and degradation

Hypothesis: Chemical activity regulates and coordinates the processes maintaining glycerophospholipid homeostasis in mammalian cells

Mammalian cells maintain the complex glycerophospholipid (GPL) class compositions of their various membranes within close limits because this is essential to their well‐being or viability. Surprisingly, however, it is still not understood how those compositions are maintained except that GPL synthesis and degradation are closely coordinated. Here, we hypothesize that abrupt changes in the chemical activity of the individual GPL classes coordinate synthesis and degradation as well other the homeostatic processes. We have previously proposed that only a limited number of “allowed” or “optimal” GPL class compositions exist in cellular membranes because those compositions are energetically more favorable than others, that is, they represent local free energy minima (Somerharju et al 2009, Biochim. Biophys. Acta 1788, 12‐23). This model, however, could not satisfactorily explain how the “optimal” compositions are sensed by the key homeostatic enzymes, that is, rate‐limiting synthetizing enzymes and homeostatic phospholipases. We now hypothesize that when the mole fraction of a GPL class exceeds an optimal value, its chemical activity abruptly increases which (a) increases its propensity to efflux from the membrane thus making it susceptible for hydrolysis by homeostatic phospholipases; (b) increases its potency to inhibit its own biosynthesis via a feedback mechanism; (c) enhances its conversion to another glycerophospholipid class via a novel process termed “head group remodeling” or (d) enhances its translocation to other subcellular membranes. In summary, abrupt change in the chemical activity of the individual GPL classes is proposed to regulate and coordinate those four processes maintaining GPL class homeostasis in mammalian cells.

Membrane lipid compositional sensing by the inducible amphipathic helix of CCT

The amphipathic helical (AH) membrane binding motif is recognized as a major device for lipid compositional sensing. We explore the function and mechanism of sensing by the lipid biosynthetic enzyme, CTP:phosphocholine cytidylyltransferase (CCT). As the regulatory enzyme in phosphatidylcholine (PC) synthesis, CCT contributes to membrane PC homeostasis. CCT directly binds and inserts into the surface of bilayers that are deficient in PC and therefore enriched in lipids that enhance surface charge and/or create lipid packing voids. These two membrane physical properties induce the folding of the CCT M domain into a ≥60 residue AH. Membrane binding activates catalysis by a mechanism that has been partially deciphered. We review the evidence for CCT compositional sensing, and the membrane and protein determinants for lipid selective membrane-interactions. We consider the factors that promote the binding of CCT isoforms to the membranes of the ER, nuclear envelope, or lipid droplets, but exclude CCT from other organelles and the plasma membrane. The CCT sensing mechanism is compared with several other proteins that use an AH motif for membrane compositional sensing. This article is part of a Special Issue entitled: The cellular lipid landscape edited by Tim P. Levine and Anant K. Menon.

CTP:phosphocholine cytidylyltransferase: Function, regulation, and structure of an amphitropic enzyme required for membrane biogenesis

CTP:phosphocholine cytidylyltransferase (CCT) catalyzes a rate-limiting and regulated step in the CDP-choline pathway for the synthesis of phosphatidylcholine (PC) and PC-derived lipids. Control of CCT activity is multi-layered, and includes direct regulation by reversible membrane binding involving a built-in lipid compositional sensor. Thus CCT contributes to phospholipid compositional homeostasis. CCT also modifies the curvature of its target membrane. Knowledge of CCT structure and regulation of its catalytic function are relatively advanced compared to many lipid metabolic enzymes, and are reviewed in detail. Recently the genetic origins of two human developmental and lipogenesis disorders have been traced to mutations in the gene for CCTα.

PHOSPHATIDIC ACID PHOSPHOHYDROLASE Regulates Phosphatidylcholine Biosynthesis in Arabidopsis by Phosphatidic Acid-Mediated Activation of CTP:PHOSPHOCHOLINE CYTIDYLYLTRANSFERASE Activity

Regulation of membrane lipid biosynthesis is critical for cell function. We previously reported that disruption of PHOSPHATIDIC ACID PHOSPHOHYDROLASE1 (PAH1) and PAH2 stimulates net phosphatidylcholine (PC) biosynthesis and proliferation of the endoplasmic reticulum (ER) in Arabidopsis thaliana. Here, we show that this response is caused specifically by a reduction in the catalytic activity of the protein and positively correlates with an accumulation of its substrate, phosphatidic acid (PA). The accumulation of PC in pah1 pah2 is suppressed by disruption of CTP:PHOSPHOCHOLINE CYTIDYLYLTRANSFERASE1 (CCT1), which encodes a key enzyme in the nucleotide pathway for PC biosynthesis. The activity of recombinant CCT1 is stimulated by lipid vesicles containing PA. Truncation of CCT1, to remove the predicted C-terminal amphipathic lipid binding domain, produced a constitutively active enzyme. Overexpression of native CCT1 in Arabidopsis has no significant effect on PC biosynthesis or ER morphology, but overexpression of the truncated constitutively active version largely replicates the pah1 pah2 phenotype. Our data establish that membrane homeostasis is regulated by lipid composition in Arabidopsis and reveal a mechanism through which the abundance of PA, mediated by PAH activity, modulates CCT activity to govern PC content.

Substrate efflux propensity is the key determinant of Ca2+-independent phospholipase A-β (iPLAβ)-mediated glycerophospholipid hydrolysis

The A-type phospholipases (PLAs) are key players in glycerophospholipid (GPL) homeostasis and in mammalian cells; Ca(2+)-independent PLA-β (iPLAβ) in particular has been implicated in this essential process. However, the regulation of this enzyme, which is necessary to avoid futile competition between synthesis and degradation, is not understood. Recently, we provided evidence that the efflux of the substrate molecules from the bilayer is the rate-limiting step in the hydrolysis of GPLs by some secretory (nonhomeostatic) PLAs. To study whether this is the case with iPLAβ as well, a mass spectrometric assay was employed to determine the rate of hydrolysis of multiple saturated and unsaturated GPL species in parallel using micelles or vesicle bilayers as the macrosubstrate. With micelles, the hydrolysis decreased with increasing acyl chain length independent of unsaturation, and modest discrimination between acyl positional isomers was observed, presumably due to the differences in the structure of the sn-1 and sn-2 acyl-binding sites of the protein. In striking contrast, no significant discrimination between positional isomers was observed with bilayers, and the rate of hydrolysis decreased with the acyl chain length logarithmically and far more than with micelles. These data provide compelling evidence that efflux of the substrate molecule from the bilayer, which also decreases monotonously with acyl chain length, is the rate-determining step in iPLAβ-mediated hydrolysis of GPLs in membranes. This finding is intriguing as it may help to understand how homeostatic PLAs are regulated and how degradation and biosynthesis are coordinated.

The role of phosphatidylcholine and choline metabolites to cell proliferation and survival

The reorganization of metabolic pathways in cancer facilitates the flux of carbon and reducing equivalents into anabolic pathways at the expense of oxidative phosphorylation. This provides rapidly dividing cells with the necessary precursors for membrane, protein and nucleic acid synthesis. A fundamental metabolic perturbation in cancer is the enhanced synthesis of fatty acids by channeling glucose and/or glutamine into cytosolic acetyl-CoA and upregulation of key biosynthetic genes. This lipogenic phenotype also extends to the production of complex lipids involved in membrane synthesis and lipid-based signaling. Cancer cells display sensitivity to ablation of fatty acid synthesis possibly as a result of diminished capacity to synthesize complex lipids involved in signaling or growth pathways. Evidence has accrued that phosphatidylcholine, the major phospholipid component of eukaryotic membranes, as well as choline metabolites derived from its synthesis and catabolism, contribute to both proliferative growth and programmed cell death. This review will detail our current understanding of how coordinated changes in substrate availability, gene expression and enzyme activity lead to altered phosphatidylcholine synthesis in cancer, and how these changes contribute directly or indirectly to malignant growth. Conversely, apoptosis targets key steps in phosphatidylcholine synthesis and degradation that are linked to disruption of cell cycle regulation, reinforcing the central role that phosphatidylcholine and its metabolites in determining cell fate.

An in vivo ratio control mechanism for phospholipid homeostasis: evidence from lipidomic studies

While it is widely accepted that the lipid composition of eukaryotic membranes is under homeostatic control, the mechanisms through which cells sense lipid composition are still the subject of debate. It has been postulated that membrane curvature elastic energy is the membrane property that is regulated by cells, and that lipid composition is maintained by a ratio control function derived from the concentrations of type II and type 0 lipids, weighted appropriately. We assess this proposal by seeking a signature of ratio control in quantified lipid composition data obtained by electrospray ionization mass spectrometry from over 40 independent asynchronous cell populations. Our approach revealed the existence of a universal 'pivot' lipid, which marks the boundary between type 0 lipids and type II lipids, and which is invariant between different cell types or cells grown under different conditions. The presence of such a pivot species is a distinctive signature of the operation in vivo, in human cell lines, of a control function that is consistent with the hypothesis that membrane elastic energy is homeostatically controlled.

A 22-mer segment in the structurally pliable regulatory domain of metazoan CTP: phosphocholine cytidylyltransferase facilitates both silencing and activating functions

CTP:phosphocholine cytidylyltransferase (CCT), an amphitropic enzyme that regulates phosphatidylcholine synthesis, is composed of a catalytic head domain and a regulatory tail. The tail region has dual functions as a regulator of membrane binding/enzyme activation and as an inhibitor of catalysis in the unbound form of the enzyme, suggesting conformational plasticity. These functions are well conserved in CCTs across diverse phyla, although the sequences of the tail regions are not. CCT regulatory tails of diverse origins are composed of a long membrane lipid-inducible amphipathic helix (m-AH) followed by a highly disordered segment, reminiscent of the Parkinson disease-linked protein, α-synuclein, which we show shares a novel sequence motif with vertebrate CCTs. To unravel features required for silencing, we created chimeric enzymes by fusing the catalytic domain of rat CCTα to the regulatory tail of CCTs from Drosophila, Caenorhabditis elegans, or Saccharomyces cerevisiae or to α-synuclein. Only the tail domains of the two invertebrate CCTs were competent for both suppression of catalytic activity and for activation by lipid vesicles. Thus, both silencing and activating functions of the m-AH can tolerate significant changes in length and sequence. We identified a highly amphipathic 22-residue segment in the m-AH with features conserved among animal CCTs but not yeast CCT or α-synuclein. Deletion of this segment from rat CCT increased the lipid-independent V(max) by 10-fold, equivalent to the effect of deleting the entire tail, and severely weakened membrane binding affinity. However, membrane binding was required for additional increases in catalytic efficiency. Thus, full activation of CCT may require not only loss of a silencing conformation in the m-AH but a gain of an activating conformation, promoted by membrane binding.

UPR-Mediated Membrane Biogenesis in B Cells

The unfolded protein response (UPR) can coordinate the regulation of gene transcription and protein translation to balance the load of client proteins with the protein folding and degradative capacities of the ER. Increasing evidence also implicates the UPR in the regulation of lipid synthesis and membrane biogenesis. The differentiation of B lymphocytes into antibody-secreting cells is marked by significant expansion of the ER, the site for antibody synthesis and assembly. In activated B cells, the demand for membrane protein and lipid components leads to activation of the UPR transcriptional activator XBP1(S) which, in turn, initiates a cascade of biochemical events that enhance supplies of phospholipid precursors and build machinery for the synthesis, maturation, and transport of secretory proteins. The alterations in lipid metabolism that occur during this developmental transition and the impact of membrane phospholipid restriction on B cell secretory characteristics are discussed in this paper.

DGAT enzymes are required for triacylglycerol synthesis and lipid droplets in adipocytes

The total contribution of the acyl CoA:diacylglycerol acyltransferase (DGAT) enzymes, DGAT1 and DGAT2, to mammalian triacylglycerol (TG) synthesis has not been determined. Similarly, whether DGAT enzymes are required for lipid droplet (LD) formation is unknown. In this study, we examined the requirement for DGAT enzymes in TG synthesis and LDs in differentiated adipocytes with genetic deletions of DGAT1 and DGAT2. Adipocytes with a single deletion of either enzyme were capable of TG synthesis and LD formation. In contrast, adipocytes with deletions of both DGATs were severely lacking in TG and did not have LDs, indicating that DGAT1 and DGAT2 account for nearly all TG synthesis in adipocytes and appear to be required for LD formation during adipogenesis. DGAT enzymes were not absolutely required for LD formation in mammalian cells, however; macrophages deficient in both DGAT enzymes were able to form LDs when incubated with cholesterol-rich lipoproteins. Although adipocytes lacking both DGATs had no TG or LDs, they were fully differentiated by multiple criteria. Our findings show that DGAT1 and DGAT2 account for the vast majority of TG synthesis in mice, and DGAT function is required for LDs in adipocytes, but not in all cell types.

Introduction of phospholipids to cultured cells with cyclodextrin

Previous studies indicate that methyl-β-cyclodextrin (meβ-CD) can greatly enhance translocation of long-chain phospholipids from vesicles to cells in culture, which is very useful when studying, e.g., phospholipid metabolism and trafficking. However, the parameters affecting the transfer have not been systematically studied. Therefore, we studied the relevant parameters including meβ-CD and vesicle concentration, incubation time, phospholipid structure, and cell type. Because meβ-CD can extract cholesterol and other lipids from cells, thereby potentially altering cell growth or viability, these issues were studied as well. The results show that efficient incorporation of phospholipid species with hydrophobicity similar to that of natural species can be obtained without significantly compromising cell growth or viability. Cellular content of phosphatidyl-serine, -ethanolamine, and -choline could be increased dramatically, i.e., 400, 125, and 25%, respectively. Depletion of cellular cholesterol could be prevented or alleviated by inclusion of the proper amount of cholesterol in the donor vesicles. In summary, meβ-CD mediates efficient transfer of long-chain (phospho) lipids from vesicles to cells without significantly compromising their growth or viability. This lays a basis for detailed studies of phospholipid metabolism and trafficking as well as enables extensive manipulation of cellular phospholipid composition, which is particularly useful when investigating mechanisms underlying phospholipid homeostasis.

Coordination of lipid metabolism in membrane biogenesis

Bilayer synthesis during membrane biogenesis involves the concerted assembly of multiple lipid species, requiring coordination of the level of lipid synthesis, uptake, turnover, and subcellular distribution. In this review, we discuss some of the salient conclusions regarding the coordination of lipid synthesis that have emerged from work in mammalian and yeast cells. The principal instruments of global control are a small number of transcription factors that target a wide range of genes encoding enzymes that operate in a given metabolic pathway. Critical in mammalian cells are sterol regulatory element binding proteins (SREBPs) that stimulate expression of genes for the uptake and synthesis of cholesterol and fatty acids. From work with Saccharomyces cerevisiae, much has been learned about glycerophospholipid and ergosterol regulation through Ino2p/Ino4p and Upc2p transcription factors, respectively. Lipid supply is fine-tuned through a multitude of negative feedback circuits initiated by both end products and intermediates of lipid synthesis pathways. Moreover, there is evidence that the diversity of membrane lipids is maintained through cross-regulatory effects, whereby classes of lipids activate the activity of enzymes operating in another metabolic branch.

Cross-talk between remodeling and de novo pathways maintains phospholipid balance through ubiquitination

Phosphatidylcholine (PtdCho), the major phospholipid of animal membranes, is generated by its remodeling and de novo synthesis. Overexpression of the remodeling enzyme, LPCAT1 (acyl-CoA:lysophosphatidylcholine acyltransferase) in epithelia decreased de novo PtdCho synthesis without significantly altering cellular PtdCho mass. Overexpression of LPCAT1 increased degradation of CPT1 (cholinephosphotransferase), a resident Golgi enzyme that catalyzes the terminal step for de novo PtdCho synthesis. CPT1 degradation involved its multiubiquitination and processing via the lysosomal pathway. CPT1 mutants harboring arginine substitutions at multiple carboxyl-terminal lysines exhibited proteolytic resistance to effects of LPCAT1 overexpression in cells and restored de novo PtdCho synthesis. Thus, cross-talk between phospholipid remodeling and de novo pathways involves ubiquitin-lysosomal processing of a key molecular target that mechanistically provides homeostatic control of cellular PtdCho content.

Lysophosphatidylcholine acyltransferase 1 (LPCAT1) overexpression in human colorectal cancer

The alteration of the choline metabolite profile is a well-established characteristic of cancer cells. In colorectal cancer (CRC), phosphatidylcholine is the most prominent phospholipid. In the present study, we report that lysophosphatidylcholine acyltransferase 1 (LPCAT1; NM_024830.3), the enzyme that converts lysophosphatidylcholine into phosphatidylcholine, was highly overexpressed in colorectal adenocarcinomas when compared to normal mucosas. Our microarray transcription profiling study showed a significant (p < 10(-8)) transcript overexpression in 168 colorectal adenocarcinomas when compared to ten normal mucosas. Immunohistochemical analysis of colon tumors with a polyclonal antibody to LPCAT1 confirmed the upregulation of the LPCAT1 protein. Overexpression of LPCAT1 in COS7 cells localized the protein to the endoplasmic reticulum and the mitochondria and increased LPCAT1 specific activity 38-fold. In cultured cells, overexpressed LPCAT1 enhanced the incorporation of [(14)C]palmitate into phosphatidylcholine. COS7 cells transfected with LPCAT1 showed no growth rate alteration, in contrast to the colon cancer cell line SW480, which significantly (p < 10(-5)) increased its growth rate by 17%. We conclude that LPCAT1 may contribute to total choline metabolite accumulation via phosphatidylcholine remodeling, thereby altering the CRC lipid profile, a characteristic of malignancy.

The superlattice model of lateral organization of membranes and its implications on membrane lipid homeostasis

Most biological membranes are extremely complex structures consisting of hundreds of different lipid and protein molecules. According to the famous fluid-mosaic model lipids and many proteins are free to diffuse very rapidly in the plane of the membrane. While such fast diffusion implies that different membrane lipids would be laterally randomly distributed, accumulating evidence indicates that in model and natural membranes the lipid components tend to adopt regular (superlattice-like) distributions. The superlattice model, put forward based on such evidence, is intriguing because it predicts that 1) there is a limited number of allowed compositions representing local minima in membrane free energy and 2) those energy minima could provide set-points for enzymes regulating membrane lipid compositions. Furthermore, the existence of a discrete number of allowed compositions could help to maintain organelle identity in the face of rapid inter-organelle membrane traffic.

Contribution of each membrane binding domain of the CTP:phosphocholine cytidylyltransferase-alpha dimer to its activation, membrane binding, and membrane cross-bridging

CTP:phosphocholine cytidylyltransferase (CCT), a rate-limiting enzyme in phosphatidylcholine synthesis, is regulated by reversible membrane interactions mediated by an amphipathic helical domain (M) that binds selectively to anionic lipids. CCT is a dimer; thus the functional unit has two M domains. To probe the functional contribution of each domain M we prepared a CCT heterodimer composed of one full-length subunit paired with a CCT subunit truncated before domain M that was also catalytically dead. We compared this heterodimer to the full-length homodimer with respect to activation by anionic vesicles, vesicle binding affinities, and promotion of vesicle aggregation. Surprisingly for all three functions the dimer with just one domain M behaved similarly to the dimer with two M domains. Full activation of the wild-type subunit was not impaired by loss of one domain M in its partner. Membrane binding affinities were the same for dimers with one versus two M domains, suggesting that the two M domains of the dimer do not engage a single bilayer simultaneously. Vesicle cross-bridging was also unhindered by loss of one domain M, suggesting that another motif couples with domain M for cross-bridging anionic membranes. Mutagenesis revealed that the positively charged nuclear localization signal sequence constitutes that second motif for membrane cross-bridging. We propose that the two M domains of the CCT dimer engage a single bilayer via an alternating binding mechanism. The tethering function involves the cooperation of domain M and the nuclear localization signal sequence, each engaging separate membranes. Membrane binding of a single M domain is sufficient to fully activate the enzymatic activity of the CCT dimer while sustaining the low affinity, reversible membrane interaction required for regulation of CCT activity.

Phosphatidylcholine catabolism in the MCF-7 cell cycle

The catabolism of phosphatidylcholine (PtdCho) appears to play a key role in regulating the net accumulation of the lipid in the cell cycle. Current protocols for measuring the degradation of PtdCho at specific cell-cycle phases require prolonged periods of incubation with radiolabelled choline. To measure the degradation of PtdCho at the S and G2 phases in the MCF-7 cell cycle, protocols were developed with radiolabelled lysophosphatidylcholine (lysoPtdCho), which reduces the labelling period and minimizes the recycling of labelled components. Although most of the incubated lysoPtdCho was hydrolyzed to glycerophosphocholine (GroPCho) in the medium, the kinetics of the incorporation of label into PtdCho suggests that the labelled GroPCho did not contribute significantly to cellular PtdCho formation. A protocol which involved exposing the cells twice to hydroxyurea, was also developed to produce highly synchronized MCF-7 cells with a profile of G1:S:G2/M of 90:5:5. An analysis of PtdCho catabolism in the synchronized cells following labelling with lysoPtdCho revealed that there was increased degradation of PtdCho in early to mid-S phase, which was attenuated in the G2/M phase. The results suggest that the net accumulation of PtdCho in MCF-7 cells may occur in the G2 phase of the cell cycle.

Coordinate Regulation of Phospholipid Biosynthesis and Secretory Pathway Gene Expression in XBP-1(S)-induced Endoplasmic Reticulum Biogenesis

Development of the expansive endoplasmic reticulum (ER) present in specialized secretory cell types requires X-box-binding protein-1 (Xbp-1). Enforced expression of XBP-1(S), a transcriptional activator generated by unfolded protein response-mediated splicing of Xbp-1 mRNA, is sufficient to induce proliferation of rough ER. We previously showed that XBP-1(S)-induced ER biogenesis in fibroblasts correlates with increased production of phosphatidylcholine (PtdCho), the primary phospholipid of the ER membrane, and enhanced activities of the choline cytidylyltransferase (CCT) and cholinephosphotransferase enzymes in the cytidine diphosphocholine (CDP-choline) pathway of PtdCho biosynthesis. Here, we report that the level and synthesis of CCT, the rate-limiting enzyme in the CDP-choline pathway, is elevated in fibroblasts overexpressing XBP-1(S). Furthermore, overexpression experiments demonstrated that raising the activity of CCT, but not cholinephosphotransferase, is sufficient to augment PtdCho biosynthesis in fibroblasts, indicating that XBP-1(S) increases the output of the CDP-choline pathway primarily via its effects on CCT. Finally, fibroblasts overexpressing CCT up-regulated PtdCho synthesis to a level similar to that in XBP-1(S)-transduced cells but exhibited only a small increase in rough ER and no induction of secretory pathway genes. The more robust XBP-1(S)-induced ER expansion was accompanied by induction of a wide array of genes encoding proteins that function either in the ER or at other steps in the secretory pathway. We propose that XBP-1(S) regulates ER abundance by coordinately increasing the supply of membrane phospholipids and ER proteins, the key ingredients for ER biogenesis.

Adenoviral gene transfer of a mutant surfactant enzyme ameliorates pseudomonas-induced lung injury

Surfactant deficiency is an important contributor to the acute respiratory distress syndrome, a disorder that commonly occurs after bacterial sepsis. CTP:phosphocholine cytidylyltransferase (CCTalpha) is the rate-limiting enzyme required for the biosynthesis of dipalmitoylphosphatidylcholine (DPPC), the major phospholipid of surfactant. In this study, a cDNA encoding a novel, calpain-resistant mutant CCTalpha enzyme was delivered intratracheally in mice using a replication-deficient adenovirus 5 CTP:phosphocholine cytidylyltransferase construct (Ad5-CCT(Penta)) in models of bacterial sepsis. Ad5-CCT(Penta) gene transfer produced high-level CCTalpha gene expression, increased alveolar surfactant (DPPC) levels and improved lung surface tension and pressure-volume relationships relative to control mice. Pseudomonas aeruginosa (PA103) decreased DPPC synthesis, in part, via calpain-mediated degradation of CCTalpha. Deleterious effects of Pseudomonas on surfactant were lessened after infection with a mutant strain lacking the type III exotoxin, Exo U. Replication-deficient adenovirus 5 CTP:phosphocholine cytidylyltransferase gene delivery improved lung biophysical properties by optimizing surface activity in this Pseudomonas model of proteinase-mediated lung injury. The studies are the first demonstration of in vivo gene transfer of a lipogenic enzyme resulting in improved lung mechanics. The studies suggest that augmentation of DPPC synthesis via gene delivery of CCTalpha can attenuate impaired lung function in surfactant-deficient states such as bacterial sepsis.

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.

Glycerophosphocholine-dependent growth requires Gde1p (YPL110c) and Git1p in Saccharomyces cerevisiae

Glycerophosphocholine is formed via the deacylation of the phospholipid phosphatidylcholine. The protein encoded by Saccharomyces cerevisiae open reading frame YPL110c effects glycerophosphocholine metabolism in vivo, most likely by acting as a glycerophosphocholine phosphodiesterase. Deletion of YPL110c causes an accumulation of glycerophosphocholine in cells prelabeled with [14C]choline. Correspondingly, overexpression of YPL110c results in reduced intracellular glycerophosphocholine in cells prelabeled with [14C]choline. Glycerophospho[3H]choline supplied in the growth medium accumulates to a much greater extent in the intracellular fraction of a YPL110Delta strain than in a wild type strain. Furthermore, glycerophospho[3H]choline accumulation requires the transporter encoded by GIT1, a known glycerophosphoinositol transporter. Growth on glycerophosphocholine as the sole phosphate source requires YPL110c and the Git1p permease. In contrast to glycerophosphocholine, glycerophosphoinositol metabolism is unaffected by deletion of YPL110c. The open reading frame YPL110c has been termed GDE1.

Enteropathogenic Escherichia coli infection triggers host phospholipid metabolism perturbations

Enteropathogenic Escherichia coli (EPEC) specifically recognizes phosphatidylethanolamine (PE) on the outer leaflet of host epithelial cells. EPEC also induces apoptosis in epithelial cells, which results in increased levels of outer leaflet PE and increased bacterial binding. Consequently, it is of interest to investigate whether EPEC infection perturbs host cell phospholipid metabolism and whether the changes play a role in the apoptotic signaling. Our findings indicate that EPEC infection results in a significant increase in the epithelial cell PE level and a corresponding decrease in the phosphatidylcholine (PC) level. PE synthesis via both the de novo pathway and the serine decarboxylation pathway was enhanced, and de novo synthesis of phosphatidylcholine via CDP-choline was reduced. The changes were transitory, and the maximum change was noted after 4 to 5 h of infection. Addition of exogenous PC or CDP-choline to epithelial cells prior to infection abrogated EPEC-induced apoptosis, suggesting that EPEC infection inhibits the CTP-phosphocholine cytidylyltransferase step in PC synthesis, which is reportedly inhibited during nonmicrobially induced apoptosis. On the other hand, incorporation of exogenous PE by the host cells enhanced EPEC-induced apoptosis and necrosis without increasing bacterial adhesion. This is the first report that pathogen-induced apoptosis is associated with significant changes in PE and PC metabolism, and the results suggest that EPEC adhesion to a host membrane phospholipid plays a role in disruption of host phospholipid metabolism.

Cell Cycle Dependence of Group VIA Calcium-independent Phospholipase A2 Activity

Homeostasis of phosphatidylcholine (PC) is regulated by the opposing actions between CTP:phosphocholine cytidylyltransferase (CT) and the group VIA Ca(2+)-independent phospholipase A(2) (iPLA(2)). We investigated this process during the cell cycle. PC mass doubles during late G(1) and early S phase when its rate of catabolism is lowest. We show that iPLA(2) activity is cell cycle-dependent with peak activity during G(2)/M and late S phase. iPLA(2) activity declines during G(1) and is lowest at the G(1)/S transition and early S phase. The accumulation of PC correlates with decreased iPLA(2) activity, suggesting that regulation of this enzyme contributes to phospholipid accumulation. The levels of 80 kDa iPLA(2) protein do not change and thus cannot account for changes in enzyme activity. Reverse transcriptase and real-time PCR experiments show that splice variant iPLA(2) mRNAs are preferentially expressed during G(2)/M. Immunoblot analyses with an antibody directed against the N terminus of iPLA(2) revealed a approximately 50 kDa protein that is of appropriate size to be the truncated protein encoded by the ankyrin-iPLA(2)-1 splice variant mRNA. The levels of truncated iPLA(2) protein were high in cells in late G(1) and S phase cells that had low iPLA(2) activity and low in G(2)/M cells that had high iPLA(2) activity. The truncated protein co-immunoprecipitated with full-length iPLA(2), indicating a physical interaction between the two proteins. Together, these data suggest that truncated iPLA(2) proteins associate with active iPLA(2) and down-regulate its activity during G(1). This down-regulation may contribute to phospholipid accumulation during the cell cycle.

Contribution of lipid second messengers to the regulation of phosphatidylcholine synthesis during cell cycle re-entry

During entry into the cell cycle a phosphatidylcholine (PC) metabolic cycle is activated. We have examined the hypothesis that PC synthesis during the G(0) to G(1) transition is controlled by one or more lipid products of PC turnover acting directly on the rate-limiting enzyme in the synthesis pathway, CTP: phosphocholine cytidylyltransferase (CCT). The acceleration of PC synthesis was two- to threefold during the first hour after addition of serum to quiescent IIC9 fibroblasts. The rate increased to approximately 15-fold above the basal rate during the second hour. The production of arachidonic acid, diacylglycerol (DAG), and phosphatidic acid (PA) preceded the second, rapid phase of PC synthesis. However, an increase in the cellular content of these lipid mediators was detected only for DAG. CCT activation and translocation to membranes accompanied the second phase of the PC synthesis acceleration. Bromoenol lactone (BEL), an inhibitor of calcium-independent phospholipase A(2) and PA phosphatase, blocked production of fatty acids and DAG, inhibited both phases of the PC synthesis response to serum, and reduced CCT activity and membrane affinity. The effect of BEL on PC synthesis was partially reversed by in situ generation of DAG via exogenous PC-specific phospholipase C to generate approximately 2-fold elevation in PC-derived DAG. Exogenous arachidonic acid also partially reversed the inhibition by BEL, but only at a concentration that generated a supra-physiological cellular content of free fatty acid. 1-Butanol, which blocks PA production, had no effect on DAG generation, or on PC synthesis. We conclude that fatty acids and DAG could contribute to the initial slow phase of the PC synthesis response. DAG is the most likely lipid regulator of CCT activity and the rapid phase of PC synthesis. However, processes other than direct activation of CCT by lipid mediators likely contribute to the highly accelerated phase during entry into the cell cycle.

Upregulation of surfactant synthesis triggers ABCA1-mediated basolateral phospholipid efflux

Alveolar type II lung epithelia produce surfactant, an essential surface-active material highly enriched with disaturated phosphatidylcholine (DSPC), which requires a key regulatory enzyme, CTP:phosphocholine cytidylyltransferase alpha (CCTalpha), for its synthesis before its export apically into the alveolus. In this study, we examined whether surfactant phosphatidylcholine (PC) synthesis and export are physiologically linked. Stable overexpression of CCTalpha in lung epithelial cell lines increased rates of PC synthesis and cellular DSPC mass without altering total cellular PC content. Overexpression of CCTalpha was associated with i) increased basolateral, rather than apical, PC export catalyzed by ABCA1; ii) basolateral export of significant levels of unsaturated (nonsurfactant) PC; and iii) transcriptional activation of the ABCA1 gene via a liver X receptor/retinoic acid receptor-independent pathway. Cells exposed to PC vesicles exhibited a dose-dependent increase in ABCA1 transcriptional activity. These data provide the first evidence that surfactant PC synthesis is linked to its export via a basolateral lipid efflux pathway. This pathway is mediated, in part, by a phospholipid sensor, ABCA1, that appears to partake in the autoregulation of both cellular content and composition of PC, thereby providing a potentially novel exit route for a newly synthesized pool of PC distinct from surfactant.

Disruption of CCTbeta2 expression leads to gonadal dysfunction

There are two mammalian genes that encode isoforms of CTP:phosphocholine cytidylyltransferase (CCT), a key rate-controlling step in membrane phospholipid biogenesis. Quantitative determination of the CCT transcripts reveals that CCTalpha is ubiquitously expressed and is found at the highest levels in the testis and lung, with lower levels in the liver and ovary. CCTbeta2 is a very minor isoform in most tissues but is significantly expressed in the brain, lung, and gonads. CCTbeta3 is the third isoform recently discovered in mice and is expressed in the same tissues as CCTbeta2, with its highest level in testes. We investigated the role(s) of CCTbeta2 by generating knockout mice. The brains and lungs of mice lacking CCTbeta2 expression did not exhibit any overt defects. On the other hand, a large percentage of the CCTbeta2(-/-) females were sterile and their ovaries exhibited defective ovarian follicle development. The proportion of female CCTbeta2(-/-) mice with defective ovaries increased as the animals aged. The rare litters born from CCTbeta2(-/-) x CCTbeta2(-/0) matings had the normal number of pups. The abnormal ovarian histopathology was characterized by disorganization of the tissue in young adult mice and absence of follicles and ova in older mice, along with interstitial stromal cell hyperplasia which culminated in the emergence of tubulostromal ovarian tumors by 16 months of age. Grossly defective CCTbeta2(-/-) ovaries were associated with high follicle-stimulating (FSH) and luteinizing (LH) hormone levels. Male CCTbeta2(-/0) mice exhibited progressive multifocal testicular degeneration and reduced fertility but had normal FSH and LH levels. Thus, the most notable phenotype of CCTbeta2 knockout mice was gonad degeneration and reproductive deficiency. The results indicate that although CCTbeta2 is expressed at very low levels compared to the alpha-isoform, loss of CCTbeta2 expression causes a breakdown in the gonadal response to hormonal stimulation.

Neuropathy Target Esterase and Its Yeast Homologue Degrade Phosphatidylcholine to Glycerophosphocholine in Living Cells

Eukaryotic cells control the levels of their major membrane lipid, phosphatidylcholine (PtdCho), by balancing synthesis with degradation via deacylation to glycerophosphocholine (GroPCho). Here we present evidence that in both yeast and mammalian cells this deacylation is catalyzed by neuropathy target esterase (NTE), a protein originally identified by its reaction with organophosphates, which cause nerve axon degeneration. YML059c, a Saccharomyces cerevisiae protein with sequence homology to NTE, had similar catalytic properties to the mammalian enzyme in assays of microsome preparations and, like NTE, was localized to the endoplasmic reticulum. Yeast lacking YML059c were viable under all conditions examined but, unlike the wild-type strain, did not convert PtdCho to GroPCho. Despite the absence of the deacylation pathway, the net rate of [(14)C]choline incorporation into PtdCho in YML059c-null yeast was not greater than that in the wild type; this was because, in the null strain diminished net uptake of extracellular choline and decreased formation of the rate-limiting intermediate, CDP-choline, resulted in a reduced rate of PtdCho synthesis. In [(14)C]choline labeling experiments with cultured mammalian cell lines, production of [(14)C]GroPCho was enhanced by overexpression of catalytically active NTE and was diminished by reduction of endogenous NTE activity mediated either by RNA interference or organophosphate treatment. We conclude that NTE and its homologues play a central role in membrane lipid homeostasis.

Externalization of phosphatidylserine during apoptosis does not specifically require either isoform of phosphatidylserine synthase

Phosphatidylserine (PtdSer) is made in mammalian cells by two PtdSer synthases, PSS1 and PSS2. In the plasma membrane PtdSer is normally localized on the inner leaflet but undergoes transbilayer movement during apoptosis and becomes exposed on the cell surface. We induced apoptosis with staurosporine in four Chinese hamster ovary (CHO) cell lines that are deficient in PSS1 and/or PSS2 to determine if PtdSer generated by either of these enzymes is required for externalization on the cell surface during apoptosis. The onset of apoptosis was confirmed by the appearance of morphological changes and DNA fragmentation while the plasma membrane remained largely intact. In all cell lines, regardless of their content of PSS1 and/or PSS2, apoptosis occurred to approximately the same extent, and within approximately the same time frame, as in parental CHO-K1 cells. The exposure of PtdSer on the cell surface was assessed by annexin V labeling and flow cytometry. Cells that were deficient in either PSS1 or PSS2, as well as cells that were deficient in both PSS1 and PSS2, externalized normal amounts of PtdSer. Our study demonstrates, that reduction of in vitro serine-exchange activity, even by 97%, does not restrict the externalization of PtdSer during apoptosis. Moreover, a normal level of expression of PSS1 and/or PSS2 is not required for generating the pool of PtdSer externalized during apoptosis.

Overexpression of diacylglycerol acyltransferase-1 reduces phospholipid synthesis, proliferation, and invasiveness in simian virus 40-transformed human lung fibroblasts

Diacylglycerol (DAG) is a versatile molecule that participates as substrate in the synthesis of structural and energetic lipids, and acts as the physiological signal that activates protein kinase C. Diacylglycerol acyltransferase (DGAT), the last committed enzyme in triacylglycerol synthesis, could potentially regulate the content and use of both signaling and glycerolipid substrate DAG by converting it into triacylglycerol. To test this hypothesis, we stably overexpressed the DGAT1 mouse gene in human lung SV40-transformed fibroblasts (DGAT cells), which contains high levels of DAG. DGAT cells exhibited a 3.9-fold higher DGAT activity and a 3.2-fold increase in triacylglycerol content, whereas DAG and phosphatidylcholine decreased by 70 and 20%, respectively, compared with empty vector-transfected SV40 cells (Control cells). Both acylation and de novo synthesis of phosphatidylcholine, phosphatidylethanolamine, and sphingomyelin were reduced by 30-40% in DGAT cells compared with controls, suggesting that DGAT used substrates for triacylglycerol synthesis that had originally been destined to produce phospholipids. The incorporation of [14C]DAG and [14C]fatty acids released from plasma membrane by additions of either phospholipase C or phospholipase A2 into triacylglycerol was increased by 6.2- and 2.8-fold, respectively, in DGAT cells compared with control cells, indicating that DGAT can attenuate signaling lipids. Finally, DGAT overexpression reversed the neoplastic phenotype because it dramatically reduced the cell growth rate and suppressed the anchorage-independent growth of the SV40 cells. These results strongly support the view that DGAT participates in the regulation of membrane lipid synthesis and lipid signaling, thereby playing an important role in modulating cell growth properties.

Gene structure, expression and identification of a new CTP:phosphocholine cytidylyltransferase beta isoform

CTP:phosphocholine cytidylyltransferase (CCT) is a key regulatory enzyme in phosphatidylcholine (PtdCho) biosynthesis, and in mammals, there are two distinct genes that encode enzymes that catalyze this reaction. This work defines the structures of both the murine CCT genes (Pcyt1a and Pcyt1b) and identifies a new CCT protein, CCTbeta3, with a unique amino terminus that arises from an alternate initiation exon. CCTalpha is expressed in all tissues, and is most abundant in liver, kidney and heart. A second CCTalpha transcript is described that initiates from a separate untranslated exon that is most highly expressed in testis. The CCTbeta isoforms are most highly expressed in brain and reproductive tissues. CCTbeta3 is not expressed in embryonic brain tissues, but is a significant transcript in the adult. These data suggest unique roles for the CCT protein isoforms in the differential regulation of PtdCho biosynthesis in specific tissues.

A gender-specific role for phosphatidylethanolamine N-methyltransferase-derived phosphatidylcholine in the regulation of plasma high density and very low density lipoproteins in mice

Phosphatidylethanolamine N-methyltransferase (PEMT)is involved in a secondary pathway for production of phosphatidylcholine (PC) in liver. We fed Pemt-/-mice a high fat/high cholesterol diet for 3 weeks to determine whether or not PC derived from PEMT is required for very low density lipoprotein secretion. Lipid analyses of plasma and liver indicated that male Pemt-/- mice accumulated triacylglycerols in their livers and were unable to secrete the same amount of triacylglycerols from the liver as did Pemt+/+ mice. Plasma levels of triacylglycerol and both apolipoproteins B100 and B48 were significantly decreased only in male Pemt-/- mice. Experiments in which mice were injected with Triton WR1339 showed that, whereas hepatic apoB100 secretion was decreased in male Pemt-/- mice, the decrease in plasma apoB48 in male Pemt-/- mice was not due to reduced secretion. Moreover, female and, to a lesser extent, male Pemt-/- mice showed a striking 40% decrease in plasma PC and cholesterol in high density lipoproteins. These results suggest that, even though the content of hepatic PC was normal in PEMT-deficient mice, plasma lipoprotein levels were profoundly altered in a gender-specific manner.

Both acidic and basic amino acids in an amphitropic enzyme, CTP:phosphocholine cytidylyltransferase, dictate its selectivity for anionic membranes

Amphitropic proteins are regulated by reversible membrane interaction. Anionic phospholipids generally promote membrane binding of such proteins via electrostatics between the negatively charged lipid headgroups and clusters of basic groups on the proteins. In this study of one amphitropic protein, a cytidylyltransferase (CT) that regulates phosphatidylcholine synthesis, we found that substitution of lysines to glutamine along both interfacial strips of the membrane-binding amphipathic helix eliminated electrostatic binding. Unexpectedly, three glutamates also participate in the selectivity for anionic membrane surfaces. These glutamates become protonated in the low pH milieu at the surface of anionic, but not zwitterionic membranes, increasing protein positive charge and hydrophobicity. The binding and insertion into lipid vesicles of a synthetic peptide containing the three glutamates was pH-dependent with an apparent pK(a) that varied with anionic lipid content. Glutamate to glutamine substitution eliminated the pH dependence of the membrane interaction, and reduced anionic membrane selectivity of both the peptide and the whole CT enzyme examined in cells. Thus anionic lipids, working via surface-localized pH effects, can promote membrane binding by modifying protein charge and hydrophobicity, and this novel mechanism contributes to the membrane selectivity of CT in vivo.

Molecular and cell biology of phosphatidylserine and phosphatidylethanolamine metabolism

In this review, the pathways for phosphatidylserine (PS) and phosphatidylethanolamine (PE) biosynthesis, as well as the genes and proteins involved in these pathways, are described in mammalian cells, yeast, and prokaryotes. In mammalian cells, PS is synthesized by a base-exchange reaction in which phosphatidylcholine or PE is substrate for PS synthase-1 or PS synthase-2, respectively. Isolation of Chinese hamster ovary cell mutants led to the cloning of cDNAs and genes encoding these two PS synthases. In yeast and prokaryotes PS is produced by a biosynthetic pathway completely different from that in mammals: from a reaction between CDP-diacylglycerol and serine. The major route for PE synthesis in cultured cells is from the mitochondrial decarboxylation of PS. Alternatively, PE can be synthesized in the endoplasmic reticulum (ER) from the CDP-ethanolamine pathway. Genes and/or cDNAs encoding all the enzymes in these two pathways for PE synthesis have been isolated and characterized. In mammalian cells, PS is synthesized on the ER and/or mitochondria-associated membranes (MAM). PS synthase-1 and -2 are highly enriched in MAM compared to the bulk of ER. Since MAM are a region of the ER that appears to be in close juxtaposition to the mitochondrial outer membrane, it has been proposed that MAM act as a conduit for the transfer of newly synthesized PS into mitochondria. A similar pathway appears to operate in yeast. The use of yeast mutants has led to identification of genes involved in the interorganelle transport of PS and PE in yeast, but so far none of the corresponding genes in mammalian cells has been identified. PS and PE do not act solely as structural components of membranes. Several specific functions have been ascribed to these two aminophospholipids. For example, cell-surface exposure of PS during apoptosis is thought to be the signal by which apoptotic cells are recognized and phagocytosed. Translocation of PS from the inner to outer leaflet of the plasma membrane of platelets initiates the blood-clotting cascade, and PS is an important activator of several enzymes, including protein kinase C. Recently, exposure of PE on the cell surface was identified as a regulator of cytokinesis. In addition, in Escherichia coli, PE appears to be involved in the correct folding of membrane proteins; and in Drosophila, PE regulates lipid homeostasis via the sterol response element-binding protein.

Maternal loading with very low-density lipoproteins stimulates fetal surfactant synthesis

We examined whether administration of very low-density lipoproteins (VLDL) to pregnant rats increases surfactant phosphatidylcholine (PtdCho) content in fetal pre-type II alveolar epithelial cells. VLDL-triglycerides are hydrolyzed to fatty acids by lipoprotein lipase (LPL), an enzyme activated by heparin. Fatty acids released by LPL can incorporate into the PtdCho molecule or activate the key biosynthetic enzyme cytidylyltransferase (CCT). Dams were given BSA, heparin, VLDL, or VLDL with heparin intravenously. Radiolabeled VLDL given to the pregnant rat crossed the placenta and was distributed systemically in the fetus and incorporated into disaturated PtdCho (DSPtdCho) in pre-type II cells. Maternal administration of VLDL with heparin increased DSPtdCho content in cells by 45% compared with control (P < 0.05). VLDL produced a dose-dependent, saturable, and selective increase in CCT activity. VLDL did not significantly alter immunoreactive CCT content but increased palmitic, stearic, and oleic acids in pre-type II cells. Furthermore, hypertriglyceridemic apolipoprotein E knockout mice contained significantly greater levels of DSPtdCho content in alveolar lavage and CCT activity compared with either LDL receptor knockout mice or wild-type controls that have normal serum triglycerides. Thus the nutritional or genetic modulation of serum VLDL-triglycerides provides specific fatty acids that stimulate PtdCho synthesis and CCT activity thereby increasing surfactant content.

Effect of CTP:phosphocholine cytidylyltransferase overexpression on the mouse lung surfactant system

CTP:phosphocholine cytidylyltransferase (CT) is the rate-limiting enzyme in the biosynthesis by type II pneumocytes of phosphatidylcholine (PC), the predominant phospholipid in lung surfactant. Augmentation of endogenous CT activity might therefore result in enhanced surfactant PC production. To test this hypothesis, transgenic mice were created in which rat CT (rCT) was expressed under control of the human surfactant protein C (SP-C) promoter. Transgenic mice were identified by tail-clip PCR analysis and studies of four founder lines were initiated. Lung CT gene expression was enhanced in two transgenic founder lines relative to wild-type controls. These two transgenic lines also exhibited significantly higher levels of immunoreactive CT protein and CT activity in whole-lung homogenates and in cultured type II cell extracts. Disaturated PC (DSPC) content in whole-lung homogenates and the rate of DSPC synthesis in cultured type II cells were significantly increased in one transgenic line. However, neither the incorporation of radiolabeled precursors (choline and palmitate) into DSPC in vivo nor the cellular metabolism of DSPC differed significantly between transgenic and control mice. This transgenic model provides opportunity for further study of factors controlling surfactant phospholipid production in vivo.

Role of calcium-independent phospholipases (iPLA(2)) in phosphatidylcholine metabolism

The proposed role of calcium-independent phospholipase A(2) (iPLA(2)) in membrane phospholipid homeostasis was tested by examining the perturbation of phosphatidylcholine metabolism by enzyme overexpression. There are alternatively spliced forms of murine iPLA(2) that were widely expressed in mouse tissues: a long form containing exon-9 that is membrane-associated and a short form lacking exon-9 that is distributed between the membrane and cytosolic fractions. Enforced expression of either iPLA(2) isoform led to a significant increase in intracellular free fatty acid, lysophosphatidylcholine, and GPC without a concomitant increase in the incorporation of either exogenous arachidonic acid or choline. The accumulation of lysophosphatidylcholine in iPLA(2)-expressing cells illustrates the limited capacity of cells for reacylation and degradation of lysophospholipids. Since iPLA(2) overexpression did not accelerate either phospholipid remodeling or phosphatidylcholine synthesis, this enzyme does play a determinant (rate-controlling?) role in either of these cellular processes.

Turnover of phosphatidylcholine in Saccharomyces cerevisiae. The role of the CDP-choline pathway

The regulation of phosphatidylcholine degradation as a function of the route of phosphatidylcholine (PC) synthesis and changing environmental conditions has been investigated in the yeast Saccharomyces cerevisiae. In the wild-type strains studied, deacylation of phosphatidylcholine to glycerophosphocholine is induced when choline is supplied to the culture medium and, also, when the culture temperature is raised from 30 to 37 degrees C. In strains bearing mutations in any of the genes encoding enzymes of the CDP-choline pathway for phosphatidylcholine biosynthesis (CKI1, choline kinase; CPT1, 1, 2-diacylglycerol choline phosphotransferase; PCT1, CTP:phosphocholine cytidylyltransferase), no induction of phosphatidylcholine turnover and glycerophosphocholine production is seen in response to choline availability or elevated temperature. In contrast, the induction of phosphatidylcholine deacylation does occur in a strain bearing mutations in genes encoding enzymes of the methylation pathway for phosphatidylcholine biosynthesis (i.e. CHO2/PEM1 and OPI3/PEM2). Whereas the synthesis of PC via CDP-choline is accelerated when shifted from 30 to 37 degrees C, synthesis of PC via the methylation pathway is largely unaffected by the temperature shift. These results suggest that the deacylation of PC to GroPC requires an active CDP-choline pathway for PC biosynthesis but not an active methylation pathway. Furthermore, the data indicate that the synthesis and turnover of CDP-choline-derived PC, but not methylation pathway-derived PC, are accelerated by the stress of elevated temperature.

Regulation of mammalian cell membrane biosynthesis

This review explores current information on the interrelationship between phospholipid biochemistry and cell biology. Phosphatidylcholine is the most abundant phospholipid and it biosynthesis has been studied extensively. The choline cytidylyltransferase regulates phosphatidylcholine production, and recent advances in our understanding of the mechanisms that govern cytidylyltransferase include the discovery of multiple isoforms and a more complete understanding of the lipid regulation of enzyme activity. Similarities between phosphatidylcholine formation and the phosphatidylethanolamine and phosphatidylinositol biosynthetic pathways are discussed, together with current insight into control mechanisms. Membrane phospholipid doubling during cell cycle progression is a function of periodic biosynthesis and degradation. Membrane homeostasis is maintained by a phospholipase A-mediated degradation of excess phospholipid, whereas insufficient phosphatidylcholine triggers apoptosis in cells.

Why expression of phosphatidylethanolamine N-methyltransferase does not rescue Chinese hamster ovary cells that have an impaired CDP-choline pathway

The mutant Chinese hamster ovary cell line (CHO), MT58, has a temperature-sensitive mutation in CTP:phosphocholine cytidylyltransferase (CT), preventing phosphatidylcholine (PC) synthesis at 40 degrees C which results in apoptosis. Previous studies (Houweling, M., Cui, Z., and Vance, D. E. (1995) J. Biol. Chem. 270, 16277-16282) showed that expression of wild-type CT-alpha rescued the cells at 40 degrees C, whereas expression of phosphatidylethanolamine N-methyltransferase-2 (PEMT2) did not, even though PC levels appeared to be maintained at wild-type levels after 24 h at the restrictive temperature. We report that the failure of PEMT2 to rescue the MT58 cell line is due to inadequate long term PC synthesis. We found that changing the medium every 24 h rescued the PEMT2-expressing MT58 cells grown at 40 degrees C. This was due to the uptake and utilization of lipids in the serum. At 40 degrees C, PC levels in the wild-type CHO cells and CT-expressing MT58 cells increased over time whereas PC levels did not change in both the MT58 and PEMT2-expressing MT58 cell lines. Further investigation found that both the PEMT2-expressing MT58 and MT58 cell lines accumulated triacylglycerol at 40 degrees C. Pulse-chase experiments indicated that lyso-PC accumulated to a higher degree at 40 degrees C in the PEMT2-expressing MT58 cells compared with CT-expressing MT58 cells. Transfection of the PEMT-expressing MT58 cells with additional PEMT2 cDNA partially rescued the growth of these cells at 40 degrees C. Inhibition of PC degradation, by inhibitors of phospholipases, also stimulated PEMT-expressing MT58 cell growth at 40 degrees C. Best results were observed using a calcium-independent phospholipase A(2) inhibitor, methyl arachidonyl fluorophosphonate. This inhibitor also increased PC mass in the PEMT2-expressing MT58 cells. When the cells are shifted to 40 degrees C, PC degradation by enzymes such as phospholipases is greater than PC synthesis in the mutant PEMT2-expressing MT58 cells. Taken together, these results indicate that PEMT2 expression fails to rescue the mutant cell line at 40 degrees C because it does not maintain PC levels required for cellular replication.

Regulation of phosphatidylcholine metabolism in Chinese hamster ovary cells by the sterol regulatory element-binding protein (SREBP)/SREBP cleavage-activating protein pathway

Sterol regulation-defective (SRD) 4 cells expressing a mutant sterol regulatory element-binding protein (SREBP) cleavage-activating protein (SCAP D443N) and Chinese hamster ovary (CHO) cells stably expressing SCAP (CHO-SCAP) and SCAP D443N (CHO-SCAP-D443N) have increased cholesterol and fatty acid synthesis because of constitutive processing of SREBPs. We assessed whether constitutive activation of SREBPs also influenced the CDP-choline pathway for phosphatidylcholine (PtdCho) biosynthesis. Relative to control CHO 7 cells, SRD 4 cells displayed increased PtdCho synthesis and degradation as indicated by a 4-6-fold increase in [(3)H]choline incorporation into PtdCho and 10-15-fold increase in intracellular [(3)H]glycerophosphocholine. [(3)H]Phosphocholine levels in SRD 4 cells were reduced by over 10-fold, suggesting enhanced activity of CTP:phosphocholine cytidylyltransferase alpha (CCTalpha). CHO-SCAP and CHO-SCAP D443N cells displayed modest increases in [(3)H]choline incorporation into PtdCho (2-fold) and only a 2-fold reduction in [(3)H]phosphocholine. Elevated PtdCho metabolism in SRD 4, compared with SCAP-overexpressing cells, was correlated with fatty acid synthesis. Inhibition of fatty acid synthesis by cerulenin resulted in almost complete normalization of PtdCho synthesis and choline metabolite profiles in SRD 4 cells, indicating that fatty acids or a fatty acid-derived metabolite was responsible for up-regulation of PtdCho synthesis. In contrast to apparent activation in vivo, CCTalpha protein, mRNA, and in vitro activity were reduced in SRD 4 cells and unchanged in SCAP transfected cells. Unlike control and SCAP transfected cells, CCTalpha in SRD 4 cells was localized by immunofluorescence to the nuclear envelope, suggesting that residual enzyme activity in these cells was in an active membrane-associated form. Translocation of CCTalpha to the nuclear envelope was reproduced by treatment of CHO 7 cells with exogenous oleate. We conclude that the SREBP/SCAP pathway regulates PtdCho synthesis via post-transcriptional activation of nuclear CCTalpha by fatty acids or a fatty acid-derived signal.

Effect of phosphocholine cytidylyltransferase overexpression on phosphatidylcholine synthesis in alveolar type II cells and related cell lines

Disaturated phosphatidylcholine (DSPC) is the predominate phospholipid component of lung surfactant. In the alveolar type II cell, the cytidine diphosphocholine (CDP-choline) pathway is the major biosynthetic pathway for DSPC. To investigate the hypothesis that phosphocholine cytidylyltransferase (CT) is the rate-limiting enzyme in the CDP-choline pathway, rat alveolar type II cells or lung tumor-derived cell lines (A549 or H441) with type II cell features were transfected with CT complementary DNA (cDNA). Cell fractions were subsequently assayed for CT protein and activity, and cell rates of DSPC synthesis were determined. In all cases, cell CT protein and activity were increased after transfection with CT cDNA but not after control transfection. Rat type II cells, but not A549 or H441 cells, increased the rate of DSPC synthesis after transfection with CT cDNA. Exposure of type II cells transfected with CT cDNA to palmitic acid resulted in a further increase in CT protein and activity. Exposure to dexamethasone resulted in increased CT protein and activity and increased synthesis of DSPC. The results confirm that CT has a rate-limiting and regulatory role in the synthesis of type II cell DSPC, and raise possibilities for novel therapeutic interventions.

Distribution of CTP:phosphocholine cytidylyltransferase (CCT) isoforms. Identification of a new CCTbeta splice variant

CTP:phosphocholine cytidylyltransferase is a major regulator of phosphatidylcholine biosynthesis. A single isoform, CCTalpha, has been studied extensively and a second isoform, CCTbeta, was recently identified. We identify and characterize a third cDNA, CCTbeta2, that differs from CCTbeta1 at the carboxyl-terminal end and is predicted to arise as a splice variant of the CCTbeta gene. Like CCTalpha, CCTbeta2 is heavily phosphorylated in vivo, in contrast to CCTbeta1. CCTbeta1 and CCTbeta2 mRNAs were differentially expressed by the human tissues examined, whereas CCTalpha was more uniformly represented. Using isoform-specific antibodies, both CCTbeta1 and CCTbeta2 localized to the endoplasmic reticulum of cells, in contrast to CCTalpha which resided in the nucleus in addition to associating with the endoplasmic reticulum. CCTbeta2 protein has enzymatic activity in vitro and was able to complement the temperature-sensitive cytidylyltransferase defect in CHO58 cells, just as CCTalpha and CCTbeta1 supporting proliferation at the nonpermissive conditions. Overexpression experiments did not reveal discrete physiological functions for the three isoforms that catalyze the same biochemical reaction; however, the differential cellular localization and tissue-specific distribution suggest that CCTbeta1 and CCTbeta2 may play a role that is distinct from ubiquitously expressed CCTalpha.

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.

Lateral organisation of membrane lipids. The superlattice view

Most biological membranes are extremely complex structures consisting of hundreds or even thousands of different lipid and protein molecules. The prevailing view regarding the organisation of these membranes is based on the fluid-mosaic model proposed by Singer and Nicholson in 1972. According to this model, phospholipids together with some other lipids form a fluid bilayer in which these lipids are diffusing very rapidly laterally. The idea of rapid lateral diffusion implies that, in general, the different lipid species would be randomly distributed in the plain of the membrane. However, there are recent data indicating that the components tend to adopt regular (superlattice-like) distributions in fluid, mixed bilayers. Based on this, a superlattice model of membranes has been proposed. This superlattice model is intriguing because it allows only a limited certain number of 'critical' compositions. These critical compositions could play a key role in the regulation of the lipid compositions of biological membranes. Furthermore, such putative critical compositions could explain how compositionally distinct organelles can exist despite of rapid inter-organelle membrane traffic. In this review, these intriguing predictions are discussed along with the basic principles of the model and the evidence supporting it.