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

Formulative Study and Intracellular Fate Evaluation of Ethosomes and Transethosomes for Vitamin D3 Delivery

In this pilot study, ethosomes and transethosomes were investigated as potential delivery systems for cholecalciferol (vitamin D3), whose deficiency has been correlated to many disorders such as dermatological diseases, systemic infections, cancer and sarcopenia. A formulative study on the influence of pharmaceutically acceptable ionic and non-ionic surfactants allowed the preparation of different transethosomes. In vitro cytotoxicity was evaluated in different cell types representative of epithelial, connective and muscle tissue. Then, the selected nanocarriers were further investigated at light and transmission electron microscopy to evaluate their uptake and intracellular fate. Both ethosomes and transethosomes proven to have physicochemical properties optimal for transdermal penetration and efficient vitamin D3 loading; moreover, nanocarriers were easily internalized by all cell types, although they followed distinct intracellular fates: ethosomes persisted for long times inside the cytoplasm, without inducing subcellular alteration, while transethosomes underwent rapid degradation giving rise to an intracellular accumulation of lipids. These basic results provide a solid scientific background to in vivo investigations aimed at exploring the efficacy of vitamin D3 transdermal administration in different experimental and pathological conditions.

Phosphatidylcholine: Greasing the Cholesterol Transport Machinery

Negative feedback regulation of cholesterol metabolism in mammalian cells ensures a proper balance of cholesterol with other membrane lipids, principal among these being the major phospholipid phosphatidylcholine (PC). Processes such as cholesterol biosynthesis and efflux, cholesteryl ester storage in lipid droplets, and uptake of plasma lipoproteins are tuned to the cholesterol/PC ratio. Cholesterol-loaded macrophages in atherosclerotic lesions display increased PC biosynthesis that buffers against elevated cholesterol levels and may also facilitate cholesterol trafficking to enhance cholesterol sensing and efflux. These same mechanisms could play a generic role in homeostatic responses to acute changes in membrane free cholesterol levels. Here, I discuss the established and emerging roles of PC metabolism in promoting intracellular cholesterol trafficking and membrane lipid homeostasis.

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.

Activated α2-macroglobulin binding to human prostate cancer cells triggers insulin-like responses

Ligation of cell surface GRP78 by activated α2-macroglobulin (α2M*) promotes cell proliferation and suppresses apoptosis. α2M*-treated human prostate cancer cells exhibit a 2-3-fold increase in glucose uptake and lactate secretion, an effect similar to insulin treatment. In both α2M* and insulin-treated cells, the mRNA levels of SREBP1-c, SREBP2, fatty-acid synthase, acetyl-CoA carboxylase, ATP citrate lyase, and Glut-1 were significantly increased together with their protein levels, except for SREBP2. Pretreatment of cells with α2M* antagonist antibody directed against the carboxyl-terminal domain of GRP78 blocks these α2M*-mediated effects, and silencing GRP78 expression by RNAi inhibits up-regulation of ATP citrate lyase and fatty-acid synthase. α2M* induces a 2-3-fold increase in lipogenesis as determined by 6-[(14)C]glucose or 1-[(14)C]acetate incorporation into free cholesterol, cholesterol esters, triglycerides, free fatty acids, and phosphatidylcholine, which is blocked by inhibitors of fatty-acid synthase, PI 3-kinase, mTORC, or an antibody against the carboxyl-terminal domain of GRP78. We also assessed the incorporation of [(14)CH3]choline into phosphatidylcholine and observed similar effects. Lipogenesis is significantly affected by pretreatment of prostate cancer cells with fatostatin A, which blocks sterol regulatory element-binding protein proteolytic cleavage and activation. This study demonstrates that α2M* functions as a growth factor, leading to proliferation of prostate cancer cells by promoting insulin-like responses. An antibody against the carboxyl-terminal domain of GRP78 may have important applications in prostate cancer therapy.

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.

The role of phospholipids in the biological activity and structure of the endoplasmic reticulum

The endoplasmic reticulum (ER) is an interconnected network of tubular and planar membranes that supports the synthesis and export of proteins, carbohydrates and lipids. Phospholipids, in particular phosphatidylcholine (PC), are synthesized in the ER where they have essential functions including provision of membranes required for protein synthesis and export, cholesterol homeostasis, and triacylglycerol storage and secretion. Coordination of these biological processes is essential, as highlighted by findings that link phospholipid metabolism in the ER with perturbations in lipid storage/secretion and stress responses, ultimately contributing to obesity/diabetes, atherosclerosis and neurological disorders. Phospholipid synthesis is not uniformly distributed in the ER but is localized at membrane interfaces or contact zones with other organelles, and in dynamic, proliferating ER membranes. The topology of phospholipid synthesis is an important consideration when establishing the etiology of diseases that arise from ER dysfunction. This review will highlight our current understanding of the contribution of phospholipid synthesis to proper ER function, and how alterations contribute to aberrant stress responses and disease. This article is part of a Special Issue entitled: Functional and structural diversity of endoplasmic reticulum.

Choline deprivation: an overview of the major hepatic metabolic response pathways

Choline (Ch) is an important nutrient that is involved in many physiological functions. Deprivation of Ch (CD) may lead to hepatocellular modifications and/or even hepatic tumorigenesis and it can be a frequent problem in clinical settings; it can accompany various common pathological (alcoholism and malnutrition) or physiological states (pregnancy and lactation). The aim of this review is to provide an up-to-date overview of the major metabolic pathways involved in the hepatic response toward the experimentally or clinically induced CD, and to shed more light on the implicated (and probably interrelated) mechanisms responsible for the observed hepatocellular modifications and/or carcinogenesis.

The intrinsically disordered nuclear localization signal and phosphorylation segments distinguish the membrane affinity of two cytidylyltransferase isoforms

Membrane phosphatidylcholine homeostasis is maintained in part by a sensing device in the key regulatory enzyme, CTP:phosphocholine cytidylyltransferase (CCT). CCT responds to decreases in membrane phosphatidylcholine content by reversible membrane binding and activation. Two prominent isoforms, CCTα and -β2, have nearly identical catalytic domains and very similar membrane binding amphipathic helical (M) domains but have divergent and structurally disordered N-terminal (N) and C-terminal phosphorylation (P) regions. We found that the binding affinity of purified CCTβ2 for anionic membranes was weaker than CCTα by more than an order of magnitude. Using chimeric CCTs, insertion/deletion mutants, and truncated CCTs, we show that the stronger affinity of CCTα can be attributed in large part to the electrostatic membrane binding function of the polybasic nuclear localization signal (NLS) motif, present in the unstructured N-terminal segment of CCTα but lacking in CCTβ2. The membrane partitioning of CCTβ2 in cells enriched with the lipid activator, oleic acid, was also weaker than that of CCTα and was elevated by incorporation of the NLS motif. Thus, the polybasic NLS can function as a secondary membrane binding motif not only in vitro but in the context of cell membranes. A comparison of phosphorylated, dephosphorylated, and region P-truncated forms showed that the in vitro membrane affinity of CCTβ2 is more sensitive than CCTα to phosphorylation status, which antagonizes membrane binding of both isoforms. These data provide a model wherein the primary membrane binding motif, an amphipathic helical domain, works in collaboration with other intrinsically disordered segments that modulate membrane binding strength. The NLS reinforces, whereas the phosphorylated tail antagonizes the attraction of domain M for anionic membranes.

Oxysterol activation of phosphatidylcholine synthesis involves CTP:phosphocholine cytidylyltransferase α translocation to the nuclear envelope

In addition to suppressing cholesterol synthesis and uptake, oxysterols also activate glycerophospholipid and SM (sphingomyelin) synthesis, possibly to buffer cells from excess sterol accumulation. In the present study, we investigated the effects of oxysterols on the CDP-choline pathway for PtdCho (phosphatidylcholine) synthesis using wild-type and sterol-resistant CHO (Chinese-hamster ovary) cells expressing a mutant of SCAP [SREBP (sterol-regulatory-element-binding protein) cleavage-activating protein] (CHO-SCAP D443N). [3H]Choline-labelling experiments showed that 25OH (25-hydroxycholesterol), 22OH (22-hydroxycholesterol) and 27OH (27-hydroxycholesterol) increased PtdCho synthesis in CHO cells as a result of CCTα (CTP:phosphocholine cytidylyltransferase α) translocation and activation at the NE (nuclear envelope). These oxysterols also activate PtdCho synthesis in J774 macrophages. in vitro, CCTα activity was stimulated 2- to 2.5-fold by liposomes containing 5 mol% 25OH, 22OH or 27OH. Inclusion of up to 5 mol% cholesterol did not further activate CCTα. 25OH activated CCTα in CHO-SCAP D443N cells leading to a transient increase in PtdCho synthesis and accumulation of CDP-choline. CCTα translocation to the NE and intranuclear tubules in CHO-SCAP D443N cells was complete after 1 h exposure to 25OH compared with only partial translocation by 4–6 h in CHO-Mock cells. These enhanced responses in CHO-D443N cells were sterol-dependent since depletion with cyclodextrin or lovastatin resulted in reduced sensitivity to 25OH. However, the lack of effect of cholesterol on in vitro CCT activity indicates an indirect relationship or involvement of other sterols or oxysterol. We conclude that translocation and activation of CCTα at nuclear membranes by side-chain hydroxylated sterols are regulated by the cholesterol status of the cell.

Transcriptional regulation of phosphatidylcholine biosynthesis

Phosphatidylcholine biosynthesis in animal cells is primarily regulated by the rapid translocation of CTP:phosphocholine cytidylyltransferase alpha between a soluble form that is inactive and a membrane-associated form that is activated. Until less than 10 years ago there was no information on the transcriptional regulation of phosphatidylcholine biosynthesis. Research has identified the transcription factors Sp1, Rb, TEF4, Ets-1 and E2F as enhancing the expression of the cytidylyltransferase and Net as a factor that represses cytidylyltransferase expression. Key transcription factors involved in cholesterol or fatty acid metabolism (SREBPs, LXRs, PPARs) do not have a major role in transcriptional regulation of the cytidylyltransferase. Rather than being linked to cholesterol or energy metabolism, regulation of the cytidylyltransferase is linked to the cell cycle, cell growth and differentiation. Transcriptional regulation of phospholipid biosynthesis is more elegantly understood in yeast and involves responses to inositol, choline and zinc in the culture medium.

SFH2 regulates fatty acid synthase activity in the yeast Saccharomyces cerevisiae and is critical to prevent saturated fatty acid accumulation in response to haem and oleic acid depletion

The yeast Saccharomyces cerevisiae is a facultative anaerobic organism. Under anaerobiosis, sustained growth relies on the presence of exogenously supplied unsaturated fatty acids and ergosterol that yeast is unable to synthesize in the absence of oxygen or upon haem depletion. In the absence of exogenous supplementation with unsaturated fatty acid, a net accumulation of SFA (saturated fatty acid) is observed that induces significant modification of phospholipid profile [Ferreira, Régnacq, Alimardani, Moreau-Vauzelle and Bergès (2004) Biochem. J. 378, 899–908]. In the present paper, we focus on the role of SFH2/CSR1, a hypoxic gene related to SEC14 and its involvement in lipid metabolism upon haem depletion in the absence of oleic acid supplementation. We observed that inactivation of SFH2 results in enhanced accumulation of SFA and phospholipid metabolism alterations. It results in premature growth arrest and leads to an exacerbated sensitivity to exogenous SFA. This phenotype is suppressed in the presence of exogenous oleic acid, or by a controlled expression of FAS1, one of the two genes encoding FAS. We present several lines of evidence to suggest that Sfh2p and oleic acid regulate SFA synthase in yeast at different levels: whereas oleic acid acts on FAS2 at the transcriptional level, we show that Sfh2p inhibits fatty acid synthase activity in response to haem depletion.

Expansion of the nucleoplasmic reticulum requires the coordinated activity of lamins and CTP:phosphocholine cytidylyltransferase alpha

The nucleoplasmic reticulum (NR), a nuclear membrane network implicated in signaling and transport, is formed by the biosynthetic and membrane curvature-inducing properties of the rate-limiting enzyme in phosphatidylcholine synthesis, CTP:phosphocholine cytidylyltransferase (CCT) alpha. The NR is formed by invagination of the nuclear envelope and has an underlying lamina that may contribute to membrane tubule formation or stability. In this study we investigated the role of lamins A and B in NR formation in response to expression and activation of endogenous and fluorescent protein-tagged CCTalpha. Similarly to endogenous CCTalpha, CCT-green fluorescent protein (GFP) reversibly translocated to nuclear tubules projecting from the NE in response to oleate, a lipid promoter of CCT membrane binding. Coexpression and RNA interference experiments revealed that both CCTalpha and lamin A and B were necessary for NR proliferation. Expression of CCT-GFP mutants with compromised membrane-binding affinity produced fewer nuclear tubules, indicating that the membrane-binding function of CCTalpha promotes the expansion of the NR. Proliferation of atypical bundles of nuclear membrane tubules by a CCTalpha mutant that constitutively associated with membranes revealed that expansion of the double-bilayer NR requires the coordinated assembly of an underlying lamin scaffold and induction of membrane curvature by CCTalpha.

A choline-deficient diet exacerbates fatty liver but attenuates insulin resistance and glucose intolerance in mice fed a high-fat diet

Liver fat accumulation is proposed to link obesity and insulin resistance. To dissect the role of liver fat in the insulin resistance of diet-induced obesity, we altered liver fat using a choline-deficient diet. C57Bl/6 mice were fed a low-fat (10% of calories) or high-fat (45% of calories) diet for 8 weeks; during the final 4 weeks, diets were either choline deficient or choline supplemented. In choline replete animals, high-fat feeding induced weight gain, elevated liver triglycerides (171%), hyperinsulinemia, and glucose intolerance. Choline deficiency did not affect body or adipose depot weights but amplified liver fat accumulation with high-fat diet (281%, P < 0.01). However, choline deficiency lowered fasting plasma insulin (from 983 +/- 175 to 433 +/- 36 pmol/l, P < 0.01) and improved glucose tolerance on a high-fat diet. In mice on 30% fat diet, choline deficiency increased liver mRNA levels of the rate-limiting enzyme in phosphatidylcholine synthesis and of enzymes involved in free fatty acid esterification, without affecting those of de novo lipogenesis or fatty acid oxidation. We conclude that liver fat accumulation per se does not cause insulin resistance during high-fat feeding and that choline deficiency may shunt potentially toxic free fatty acids toward innocuous storage triglyceride in the liver.

Oxysterol-binding protein and vesicle-associated membrane protein-associated protein are required for sterol-dependent activation of the ceramide transport protein

Sphingomyelin (SM) and cholesterol are coregulated metabolically and associate physically in membrane microdomains involved in cargo sorting and signaling. One mechanism for regulation of this metabolic interface involves oxysterol binding protein (OSBP) via high-affinity binding to oxysterol regulators of cholesterol homeostasis and activation of SM synthesis at the Golgi apparatus. Here, we show that OSBP regulation of SM synthesis involves the endoplasmic reticulum (ER)-to-Golgi ceramide transport protein (CERT). RNA interference (RNAi) experiments in Chinese hamster ovary (CHO)-K1 cells revealed that OSBP and vesicle-associated membrane protein-associated protein (VAP) were required for stimulation of CERT-dependent ceramide transport and SM synthesis by 25-hydroxycholesterol and cholesterol depletion in response to cyclodextrin. Additional RNAi experiments in human embryonic kidney 293 cells supported OSBP involvement in oxysterol-activated SM synthesis and also revealed a role for OSBP in basal SM synthesis. Activation of ER-to-Golgi ceramide transport in CHO-K1 cells required interaction of OSBP with the ER and Golgi apparatus, OSBP-dependent Golgi translocation of CERT, and enhanced CERT-VAP interaction. Regulation of CERT by OSBP, sterols, and VAP reveals a novel mechanism for integrating sterol regulatory signals with ceramide transport and SM synthesis in the Golgi apparatus.

CTP:phosphocholine cytidylyltransferase binds anionic phospholipid vesicles in a cross-bridging mode

CTP:phosphocholine cytidylyltransferase (CCT) catalyzes the rate-limiting step in phosphatidylcholine (PC) synthesis, and its activity is regulated by reversible association with membranes, mediated by an amphipathic helical domain M. Here we describe a new feature of the CCTalpha isoform, vesicle tethering. We show, using dynamic light scattering and transmission electron microscopy, that dimers of CCTalpha can cross-bridge separate vesicles to promote vesicle aggregation. The vesicles contained either class I activators (anionic phospholipids) or the less potent class II activators, which favor nonlamellar phase formation. CCT increased the apparent hydrodynamic radius and polydispersity of anionic phospholipid vesicles even at low CCT concentrations corresponding to only one or two dimers per vesicle. Electron micrographs of negatively stained phosphatidylglycerol (PG) vesicles confirmed CCT-mediated vesicle aggregation. CCT conjugated to colloidal gold accumulated on the vesicle surfaces and in areas of vesicle-vesicle contact. PG vesicle aggregation required both the membrane-binding domain and the intact CCT dimer, suggesting binding of CCT to apposed membranes via the two M domains situated on opposite sides of the dimerization domain. In contrast to the effects on anionic phospholipid vesicles, CCT did not induce aggregation of PC vesicles containing the class II lipids, oleic acid, diacylglycerol, or phosphatidylethanolamine. The different behavior of the two lipid classes reflected differences in measured binding affinity, with only strongly binding phospholipid vesicles being susceptible to CCT-induced aggregation. Our findings suggest a new model for CCTalpha domain organization and membrane interaction, and a potential involvement of the enzyme in cellular events that implicate close apposition of membranes.

Valproic acid inhibits leptin secretion and reduces leptin messenger ribonucleic acid levels in adipocytes

Treatment of epilepsy or bipolar disorder with valproic acid (VPA) induces weight gain and increased serum levels for the satiety hormone, leptin, through an unidentified mechanism. In this study we tested the effects of VPA, a short-chain branched fatty acid (C8:0), on leptin biology and fatty acid metabolism in 3T3-L1 adipocytes. VPA significantly reduced leptin secretion in a dose-dependent manner. Because fatty acid accumulation has been hypothesized to block leptin secretion, we tested the effect of VPA on fatty acid metabolism. Using 14C-radiolabeled VPA, we found that the 14C was mainly incorporated into triacylglycerol. VPA did not alter lipogenesis from acetate, nor did it change the amount of intracellular free fatty acids available for triacylglycerol synthesis. Decreased leptin secretion was accompanied by a reduction in leptin mRNA, even though VPA treatment did not alter the protein levels for known transcription factors affecting leptin transcription including: CCAAT/enhancer binding protein-alpha, peroxisome proliferator-activated receptor-gamma, or steroid regulatory element binding protein 1a. VPA altered levels of leptin mRNA independent of de novo protein synthesis without affecting leptin mRNA degradation. This report demonstrates that VPA decreases leptin secretion and mRNA levels in adipocytes in vitro, suggesting that VPA therapy may be associated with altered leptin homeostasis contributing to weight gain in vivo.

Phospholipid biosynthesis in mammalian cells

Identification of the genes and gene products involved in the biosynthesis of phosphatidylcholine, phosphatidylethanolamine, and phosphatidylserine has lagged behind that in many other fields because of difficulties encountered in purifying the respective proteins. Nevertheless, most of these genes have now been identified. In this review article, we have highlighted important new findings on the individual enzymes and the corresponding genes of phosphatidylcholine synthesis via its two major biosynthetic pathways: the CDP-choline pathway and the methylation pathway. We also review recent studies on phosphatidylethanolamine biosynthesis by two pathways: the CDP-ethanolamine pathway, which is active in the endoplasmic reticulum, and the phosphatidylserine decarboxylase pathway, which operates in mitochondria. Finally, the two base-exchange enzymes, phosphatidylserine synthase-1 and phosphatidylserine synthase-2, that synthesize phosphatidylserine in mammalian cells are also discussed.

Regulation of group VIA phospholipase A2 expression by sterol availability

Several lines of evidence suggest that glycerophospholipid mass is maintained through the coordinate regulation of CTP:phosphocholine cytidylyltransferase-alpha (CTalpha) and the group VIA calcium-independent phospholipase A2 (iPLA2). CTalpha expression is modulated by sterol and this is mediated in part through sterol regulatory element binding proteins (SREBP). In this report, we investigate the possibility that iPLA2 expression is controlled in a similar manner. When Chinese hamster ovary (CHO) cells were cultured under sterol-depleted conditions, iPLA2 catalytic activity, mRNA, and protein were induced by between two- and threefold. These inductions were suppressed when the cells were supplemented with exogenous sterols. Luciferase reporter assays indicated that sterol depletion induced transcription of iPLA2, an analysis of the 5' flanking region suggested that the iPLA2 gene contained a putative sterol regulatory element (SRE), and electrophoretic mobility shift assay (EMSA) analysis indicated that this element can bind SREBP-2. Notably, a mutant CHO cell line (SRD4) that constitutively generates mature SREBP proteins exhibited increased iPLA2 activity and expression compared to wild-type cells. These data suggest that iPLA2 expression is regulated in a manner consistent with other important genes in sterol and glycerophospholipid metabolism. Such coordinate regulation may be essential for maintaining the lipid composition of cell membranes.

Phosphatidylcholine deficiency upregulates enzymes of triacylglycerol metabolism in CHO cells

We studied the regulation of triacylglycerol (TAG) metabolism by phosphatidylcholine (PC) in CHO MT58 cells, which are deficient in PC synthesis because of a temperature-sensitive CTP:phosphocholine cytidylyltransferase. At the permissive growth temperature (34 degrees C), these cells contained 49% less TAG and 30% less PC than wild-type CHO K1 cells. Treatment with dipalmitoylphosphatidylcholine normalized both the PC and TAG levels. Despite low TAG levels, the incorporation of [14C]oleate into TAG was increased in CHO MT58 cells. The in vitro de novo synthesis of TAG and the activity of diacylglycerol acyltransferase were 90% and 34% higher, respectively. Two other key enzyme activities in TAG synthesis, acyl-CoA synthetase and mitochondrial glycerol-3-phosphate acyltransferase (GPAT), increased by 48% and 2-fold, respectively, and mitochondrial GPAT mRNA increased by approximately 4-fold. Additionally, TAG hydrolysis was accelerated in CHO MT58 cells, and in vitro lipolytic activity increased by 68%. These studies suggest that a homeostatic mechanism increases TAG synthesis and recycling in response to PC deficiency. TAG recycling produces diacylglycerol and fatty acids that can be substrates for de novo PC synthesis and for lysophosphatidylcholine (lysoPC) acylation. In CHO MT58 cells, in which de novo PC synthesis is blocked, lysoPC acylation with fatty acid originating from TAG may represent the main pathway for generating PC.

Oleic acid modulates the post-translational glycosylation of macrophage ApoE to increase its secretion

There has been increasing interest in a potential role for fatty acids in adversely affecting organismal substrate utilization and contributing to the cardiovascular complications in insulin resistance. Fatty acids have already been implicated in regulating the expression of a number of genes in resident cells of the vessel wall. In the current studies, we evaluated a potential role for fatty acids in the regulation of macrophage apoE expression. Incubation in oleic acid increased the synthesis and secretion of apoE by human monocyte-derived macrophages. Part of this stimulation was mediated at a post-translational locus. Oleic acid increased the secretion of apoE from macrophages that constitutively expressed a human apoE3 cDNA. Incubation in palmitic acid decreased apoE secretion from these cells. The effect of oleic acid on apoE secretion could not be accounted for by the known effect of fatty acid on cellular sterol, because incubation in oleic acid did not suppress the degradation of nascent apoE. Incubation in oleic acid for at least 6 h was required to observe an effect on apoE secretion. Oleic acid altered the glycosylation pattern of cellular and secreted apoE, with a loss of the most heavily sialylated isoform. Oleic acid had no effect on the glycosylation of interleukin 6 secreted from macrophages. Elimination of apoE glycosylation, by substitution of threonine 194 with alanine, eliminated oleic acid-mediated stimulation of apoE secretion. These results indicate that oleic acid increases apoE secretion from macrophages at a locus involving post-translational glycosylation.

Biogenesis and cellular dynamics of aminoglycerophospholipids

Aminoglycerophospholipids phosphatidylserine (PtdSer), phosphatidylethanolamine (PtdEtn), and phosphatidylcholine (PtdCho) comprise about 80% of total cellular phospholipids in most cell types. While the major function of PtdCho in eukaryotes and PtdEtn in prokaryotes is that of bulk membrane lipids, PtdSer is a minor component and appears to play a more specialized role in the plasma membrane of eukaryotes, e.g., in cell recognition processes. All three aminoglycerophospholipid classes are essential in mammals, whereas prokaryotes and lower eukaryotes such as yeast appear to be more flexible regarding their aminoglycerophospholipid requirement. Since different subcellular compartments of eukaryotes, namely the endoplasmic reticulum and mitochondria, contribute to the biosynthetic sequence of aminoglycerophospholipid formation, intracellular transport, sorting, and specific function of these lipids in different organelles are of special interest.

Insights into the requirement of phosphatidylcholine synthesis for liver function in mice

Phosphatidylcholine (PC) is made in the liver by the CDP-choline pathway and via phosphatidylethanolamine N-methyltransferase (PEMT), which catalyzes the conversion of phosphatidylethanolamine to PC. Unexpectedly, hepatic apolipoprotein B-100 secretion is inhibited in male, but not female, Pemt-/- mice (Noga, A. A., Y. Zhao, and D. E. Vance. 2002. J. Biol. Chem. 277: 42358-42365; Noga, A. A., and D. E. Vance. 2003. J. Biol. Chem. 278: 21851-21859). To gain further insight into this process, we compared PC metabolism in male and female mice fed chow or a high-fat/high-cholesterol (HF/HC) diet. Immunoblot analyses demonstrated that twice as much PEMT2 was present in livers from female compared with male mice. In contrast, assays of CTP:phosphocholine cytidylyltransferase from livers of Pemt+/+ mice demonstrated more active cytidylyltransferase in male than in female mice. Secretion of PEMT-derived PC into lipoproteins was examined in vivo by injection of mice with [methyl-3H]methionine in the presence of Triton WR1339. The PEMT-derived PC shifts to smaller-sized particles in response to a HF/HC diet, but only in male mice. Secretion of PEMT-derived PC into bile was enhanced in mice fed a HF/HC diet. These results demonstrate that the synthesis and targeting of PC produced by the PEMT pathway in the livers of mice differs in a gender- and diet-specific manner.

Activation of CTP:phosphocholine cytidylyltransferase alpha expression during the S phase of the cell cycle is mediated by the transcription factor Sp1

An essential step during cell division is induction of phosphatidylcholine biosynthesis. In this pathway, CTP:phosphocholine cytidylyltransferase alpha (CT alpha) plays an important regulatory role. Previous studies (Golfman, L. S., Bakovic, M., and Vance, D. E. (2001) J. Biol. Chem. 276, 43688-43692) demonstrated that CT alpha mRNA accumulates during S phase in preparation for cellular mitosis. We now demonstrate that increased binding of the transcription factor Sp1 to the proximal promoter of CT alpha is responsible for increased transcription during the S phase. The Sp1 binding element present in position -67/-62 is essential for activation, and the Sp1 site in position -31/-9 is required to enhance transcription. Inhibition of Sp1 expression by RNA interference abolished the enhanced expression of CT alpha. Immunoprecipitation studies demonstrated that Sp1 interacts with cyclin E, cyclin A, and cyclin-dependent kinase 2 during the S phase. We conclude that Sp1 binding to the CT alpha proximal promoter is necessary to enhance transcription during the S phase. This is the first elucidation of a mechanism by which expression of a key enzyme in phospholipid biosynthesis is regulated during the cell cycle.

The SREBP pathway--insights from Insigs and insects

Animal cells coordinate lipid homeostasis by end-product feedback regulation of transcription. The control occurs through the proteolytic release of transcriptionally active sterol regulatory element binding proteins (SREBPs) from intracellular membranes. This feedback system has unexpected features that are found in all cells. Here, we consider recently discovered components of the regulatory machinery that govern SREBP processing, as well as studies in Drosophila that indicate an ancient role for the SREBP pathway in integrating membrane composition and lipid biosynthesis.

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.

Regulation of the CDP-choline pathway by sterol regulatory element binding proteins involves transcriptional and post-transcriptional mechanisms

The synthesis of phosphatidylcholine (PtdCho) by the CDP-choline pathway is under the control of the rate-limiting enzyme CTP:phosphocholine cytidylyltransferase (CCT). Sterol regulatory element binding proteins (SREBPs) have been proposed to regulate CCT at the transcriptional level, or via the synthesis of lipid activators or substrates of the CDP-choline pathway. To assess the contributions of these two mechanisms, we examined CCTalpha expression and PtdCho synthesis by the CDP-choline pathway in cholesterol and fatty acid auxotrophic CHO M19 cells inducibly expressing constitutively active nuclear forms of SREBP1a or SREBP2. Induction of either SREBP resulted in increased expression of mRNAs for sterol-regulated genes, elevated fatty acid and cholesterol synthesis (>10-50-fold) and increased PtdCho synthesis (2-fold). CCTalpha mRNA was increased 2-fold by enforced expression of SREBP1a or SREBP2. The resultant increase in CCTalpha protein and activity (2-fold) was restricted primarily to the soluble fraction of cells, and increased CCTalpha activity in vivo was not detected. Inhibition of the synthesis of fatty acids or their CoA esters by cerulenin or triacsin C respectively following SREBP induction effectively blocked the accompanying elevation in PtdCho synthesis. Thus PtdCho synthesis was driven by increased synthesis of fatty acids or a product thereof. These data show that transcriptional activation of CCTalpha is modest relative to that of other SREBP-regulated genes, and that stimulation of PtdCho synthesis by SREBPs in CHO cells is due primarily to increased fatty acid synthesis.

Identification of Ets-1 as an important transcriptional activator of CTP:phosphocholine cytidylyltransferase alpha in COS-7 cells and co-activation with transcriptional enhancer factor-4

Phosphatidylcholine biosynthesis via the CDP-choline pathway is primarily regulated by CTP:phosphocholine cytidylyltransferase (CT). Transcriptional enhancer factor-4 (TEF-4) enhances the transcription of CTalpha in COS-7 cells by interactions with the basal transcription machinery (Sugimoto, H., Bakovic, M., Yamashita, S., and Vance, D.E. (2001) J. Biol. Chem. 276,12338-12344). To identify the most important transcription factor involved in basal CTalpha transcription, we made CTalpha promoter-deletion and -mutated constructs linked to a luciferase reporter and transfected them into COS-7 cells. The results indicate that an important site regulating basal CTalpha transcription is -53/-47 (GACTTCC), which is a putative consensus-binding site of Ets transcription factors (GGAA) in the opposite orientation. Gel shift analyses indicated the existence of a binding protein for -53/-47 (GACTTCC) in nuclear extracts of COS-7 cells. When anti-Ets-1 antibody was incubated with the probe in gel shift analyses, the intensity of the binding protein was decreased. The binding of endogenous Ets-1 to the promoter probe was increased when TEF-4 was expressed; however, the amount of Ets-1 detected by immunoblotting was unchanged. When cells were transfected with Ets-1 cDNA, the luciferase activity of CTalpha promoter constructs was greatly enhanced. Co-transfection experiments with Ets-1 and TEF-4 showed enhanced expression of reporter constructs as well as CTalpha mRNA. These results suggest that Ets-1 is an important transcriptional activator of the CTalpha gene and that Ets-1 activity is enhanced by TEF-4.

Oncogenic Ha-Ras transformation modulates the transcription of the CTP:phosphocholine cytidylyltransferase alpha gene via p42/44MAPK and transcription factor Sp3

We have shown previously that expression of the murine CTP:phosphocholine cytidylyltransferase (CT) alpha gene is regulated during cell proliferation (Golfman, L. S., Bakovic, M., and Vance, D. E. (2001) J. Biol. Chem. 276, 43688-43692). We have now characterized the role of Ha-Ras in the transcriptional regulation of the CTalpha gene. The expression of CTalpha and CTbeta2 proteins and mRNAs was stimulated in C3H10T1/2 murine fibroblasts expressing oncogenic Ha-Ras. Incubation of cells with the specific inhibitor (PD98059) of p42/44(MAPK) decreased the expression of both CT isoforms. Transfection of fibroblasts with CTalpha promoter-luciferase constructs resulted in an approximately 2-fold enhanced luciferase expression in Ha-Ras-transformed, compared with nontransformed, fibroblasts. Electromobility shift assays indicated enhanced binding of the Sp3 transcription factor to the CTalpha promoter in Ha-Ras-transformed cells. Expression of several forms of Sp3 was increased in nuclear extracts of Ha-Ras-transformed fibroblasts compared with nontransformed cells. Tyrosine phosphorylation of one Sp3 form was decreased, whereas phosphorylation of two other forms of Sp3 was increased in nuclear extracts of Ha-Ras-transformed cells. When control fibroblasts were transfected with a Sp3-expressing plasmid, an enhanced expression of CTalpha and CTbeta was observed. However, the expression of CTalpha or CTbeta was not increased in Ha-Ras-transformed cells transfected with a Sp3 plasmid presumably because expression was already maximally enhanced. The results suggest that Sp3 is a downstream effector of a Ras/p42/44(MAPK) signaling pathway which increases CTalpha gene transcription.

Regulation of fatty acid synthase expression in breast cancer by sterol regulatory element binding protein-1c

Activation of fatty acid synthase (FAS) expression and fatty acid synthesis is a common event in human breast cancer. Sterol regulatory element binding proteins (SREBPs) are a family of transcription factors that regulate genes involved in lipid metabolism, including FAS. SREBP-1c expression is induced in liver and adipose tissue by insulin and by fasting/refeeding and is critical for nutritional regulation of lipogenic gene expression. In contrast, upregulation of fatty acid metabolism during in vitro transformation of human mammary epithelial cells and in breast cancer cells was driven by increased MAP kinase and PI 3-kinase signaling, which increased SREBP-1 levels. SREBP-1a was more abundant than SREBP-1c in many proliferative tissues and cultured cells and was thus a candidate to regulate lipogenesis for support of membrane synthesis during cell growth. We now show that SREBP-1c and FAS mRNA were both increased by H-ras transformation of MCF-10a breast epithelial cells and were both reduced by exposure of MCF-7 breast cancer cells to the MAP kinase inhibitor, PD98059, or the PI 3-kinase inhibitor, wortmannin, while SREBP-1a and SREBP-2 showed less variation. Similarly, the mRNA levels for FAS and SREBP-1c in a panel of primary human breast cancer samples showed much greater increases than did those for SREBP-1a and SREBP-2 and were significantly correlated with each other, suggesting coordinate regulation of SREBP-1c and FAS in clinical breast cancer. We conclude that regulation of FAS expression in breast cancer is achieved through modulation of SREBP-1c, similar to the regulation in liver and adipose tissue, although the upstream regulation of liopgenesis differs in these tissues.

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.

Caspase processing and nuclear export of CTP:phosphocholine cytidylyltransferase alpha during farnesol-induced apoptosis

CTP:phosphocholine cytidylyltransferase alpha (CCT alpha) is a nuclear enzyme that catalyzes the rate-limiting step in the CDP-choline pathway, the primary route for synthesis of phosphatidylcholine (PtdCho) in eukaryotic cells. Induction of apoptosis by farnesol (FOH) and other cytotoxic drugs has been shown to alter PtdCho synthesis via the CDP-choline pathway. Here we report that FOH-induced apoptosis in CHO cells caused a dose-dependent activation of CCT alpha and inhibition of the final step in the pathway, resulting in a biphasic effect on PtdCho synthesis. Activation of CCT alpha was accompanied by enzyme translocation to the nuclear envelope within 30 min of FOH addition to cells. Following translocation to membranes, CCT alpha was exported from the nucleus and underwent caspase-mediated proteolysis that coincided with poly(ADP-ribose) polymerase cleavage. Site-directed mutagenesis and in vivo and in vitro expression studies mapped a caspase 6 and/or 8 cleavage site to TEED(28 downward arrow)G, the final residue in the CCT alpha nuclear localization signal. Nuclear export of CCT alpha appeared to be an active process in FOH-treated CHO cells that was independent of caspase removal of the nuclear localization signal. Caspase cleavage of CCT alpha occurred during UV or chelerythrine-induced apoptosis; however, nuclear membrane translocation and nuclear export were not evident under these conditions. Thus, caspase cleavage of CCT alpha was a late feature of several apoptotic programs that occurred in the nucleus or at the nuclear envelope. Activation and nuclear export of CCT alpha were early events in FOH-induced apoptosis that contributed to altered PtdCho synthesis and, in conjunction with caspase cleavage, excluded CCT alpha from the nucleus.

Transcription of the CTP:phosphocholine cytidylyltransferase alpha gene is enhanced during the S phase of the cell cycle

We have studied the transcription of the CTP:phosphocholine cytidylyltransferase alpha (CTalpha) gene in C3H10T1/2 fibroblasts as a function of the cell cycle. The cells were incubated for 48 h with 0.5% fetal bovine serum. The cells were induced into the G(1) phase of the cell cycle by the addition of medium with 10% fetal bovine serum. The cells began the synthesis of DNA after 12 h. At 16 and 20 h there was an increased amount of CTalpha mRNA that coincided with an increase in the expression of CTalpha proximal promoter-luciferase constructs (-201/+38 and -130/+38). Luciferase constructs with the basal promoter (-52/+38) showed no change in activity during the cell cycle. Incorporation of [(3)H]choline into phosphatidylcholine began to increase by 8 h after the addition of serum and peaked at 18 h. The mass of phosphatidylcholine nearly doubled between 8 and 26 h after addition of serum. CT activity increased by 6 h after serum addition and was maintained until 22 h. Thus, the increase of phosphatidylcholine biosynthesis in the G(1) phase of the cell cycle is not due to enhanced transcription of the CTalpha gene. Instead increased transcription of the CTalpha gene occurred during the S phase of the cell cycle in preparation for mitosis.

Novel members of the human oxysterol-binding protein family bind phospholipids and regulate vesicle transport

Oxysterol-binding proteins (OSBPs) are a family of eukaryotic intracellular lipid receptors. Mammalian OSBP1 binds oxygenated derivatives of cholesterol and mediates sterol and phospholipid synthesis through as yet poorly undefined mechanisms. The precise cellular roles for the remaining members of the oxysterol-binding protein family remain to be elucidated. In yeast, a family of OSBPs has been identified based on primary sequence similarity to the ligand binding domain of mammalian OSBP1. Yeast Kes1p, an oxysterol-binding protein family member that consists of only the ligand binding domain, has been demonstrated to regulate the Sec14p pathway for Golgi-derived vesicle transport. Specifically, inactivation of the KES1 gene resulted in the ability of yeast to survive in the absence of Sec14p, a phosphatidylinositol/phosphatidylcholine transfer protein that is normally required for cell viability due to its essential requirement in transporting vesicles from the Golgi. We cloned the two human members of the OSBP family, ORP1 and ORP2, with the highest degree of similarity to yeast Kes1p. We expressed ORP1 and ORP2 in yeast lacking Sec14p and Kes1p function and found that ORP1 complemented Kes1p function with respect to cell growth and Golgi vesicle transport, whereas ORP2 was unable to do so. Phenotypes associated with overexpression of ORP2 in yeast were a dramatic decrease in cell growth and a block in Golgi-derived vesicle transport distinct from that of ORP1. Purification of ORP1 and ORP2 for ligand binding studies demonstrated ORP1 and ORP2 did not bind 25-hydroxycholesterol but instead bound phospholipids with both proteins exhibiting strong binding to phosphatidic acid and weak binding to phosphatidylinositol 3-phosphate. In Chinese hamster ovary cells, ORP1 localized to a cytosolic location, whereas ORP2 was associated with the Golgi apparatus, consistent with our vesicle transport studies that indicated ORP1 and ORP2 function at different steps in the regulation of vesicle transport.