Nuclear localization of soluble CTP:phosphocholine cytidylyltransferase

Phosphatidylserine enrichment in the nuclear membrane regulates key enzymes of phosphatidylcholine synthesis

Phosphatidylserine (PS) is an important anionic phospholipid that is synthesized within the endoplasmic reticulum (ER). While PS shows the highest enrichment and serves important functional roles in the plasma membrane (PM) but its role in the nucleus is poorly explored. Using three orthogonal approaches, we found that PS is also uniquely enriched in the inner nuclear membrane (INM) and the nuclear reticulum (NR). Nuclear PS is critical for supporting the translocation of CCTα and Lipin1α, two key enzymes important for phosphatidylcholine (PC) biosynthesis, from the nuclear matrix to the INM and NR in response to oleic acid treatment. We identified the PS-interacting regions within the M-domain of CCTα and M-Lip domain of Lipin1α, and show that lipid droplet formation is altered by manipulations of nuclear PS availability. Our studies reveal an unrecognized regulatory role of nuclear PS levels in the regulation of key PC synthesizing enzymes within the nucleus.

Lipid transfer proteins and a PI 4-kinase initiate nuclear phosphoinositide signaling

Phosphoinositide (PIP n ) messengers are present in non-membranous regions of nuclei, where they are assembled into a phosphatidylinositol (PI) 3-kinase (PI3K)/Akt pathway that is distinct from the cytosolic membrane-localized pathway. In the nuclear pathway, PI kinases/phosphatases bind the p53 tumor suppressor protein (wild-type and mutant) to generate p53-PIP n complexes that regulate Akt activation. However, this pathway is dependent on poorly characterized nuclear PIP n pools. Here we report that PI transfer proteins (PITPs), which transport PI between membranes to enable membrane-localized PIP n synthesis, accumulate in the nucleoplasm in response to stress and supply nuclear PIP n pools. PITPα/β and the PI 4-kinase PI4KIIα bind p53 and are required to generate p53-PI4P, which is further phosphorylated to synthesize p53-PIP n complexes that regulate nuclear Akt activation and stress-resistance. Remarkably, PITPα/β and PI4KIIα initiate PIP n -linkage to multiple proteins that are detectable by immunoblotting and [ 3 H] myo -inositol metabolic labeling and are resistant to denaturation, suggesting a posttranslational modification.

In brief

Phosphatidylinositol transfer proteins initiate the nuclear PIP n -linked protein network in membrane-free regions.

Sp1 mediated the inhibitory effect of glutamate on pulmonary surfactant synthesis

BACKGROUND

Studies have shown that the release of endogenous glutamate (Glu) participates in lung injury by activating N-methyl-D-aspartate receptor (NMDAR), but the mechanism is still unclear. This study was to investigate the effects and related mechanisms of Glu on the lipid synthesis of pulmonary surfactant (PS) in isolated rat lung tissues.

METHODS

The cultured lung tissues of adult SD rats were treated with Glu. The amount of [3H]-choline incorporation into phosphatidylcholine (PC) was detected. RT-PCR and Western blot were used to detect the changes of mRNA and protein expression of cytidine triphosphate: phosphocholine cytidylyltransferase alpha (CCTα), a key regulatory enzyme in PC biosynthesis. Western blot was used to detect the expression of NMDAR1, which is a functional subunit of NMDAR. Specific protein 1 (Sp1) expression plasmids were used. After transfected with Sp1 expression plasmids, the mRNA and protein levels of CCTα were detected by RT-PCR and Western blot in A549 cells. After treated with NMDA and MK-801, the mRNA and protein levels of Sp1 were detected by RT-PCR and Western blot in A549 cells.

RESULTS

Glu decreased the incorporation of [3H]-choline into PC in a concentration- and time- dependent manner. Glu treatment significantly reduced the mRNA and protein levels of CCTα in lungs. Glu treatment up-regulated NMDAR1 protein expression, and the NMDAR blocker MK-801 could partially reverse the reduction of [3H]-choline incorporation induced by Glu (10-4 mol/L) in lungs. After transfected with Sp1 plasmid for 30 h, the mRNA and protein expression levels of CCTα were increased and the protein expression of Sp1 was also up-regulated. After A549 cells were treated with NMDA, the level of Sp1 mRNA did not change significantly, but the expression of nucleus protein in Sp1 was significantly decreased, while the expression of cytoplasmic protein was significantly increased. However, MK-801could reverse these changes.

CONCLUSIONS

Glu reduced the biosynthesis of the main lipid PC in PS and inhibited CCTα expression by activating NMDAR, which were mediated by the inhibition of the nuclear translocation of Sp1 and the promoter activity of CCTα. In conclusion, NMDAR-mediated Glu toxicity leading to impaired PS synthesis may be a potential pathogenesis of lung injury.

Lipodistrophy: a paradigm for understanding the consequences of "overloading" adipose tissue

Lipodystrophies have been recognized since at least the nineteenth century and, despite their rarity, tended to attract considerable medical attention because of the severity and somewhat paradoxical nature of the associated metabolic disease that so closely mimics that of obesity. Within the last 20 yr most of the monogenic subtypes have been characterized, facilitating family genetic screening and earlier disease detection as well as providing important insights into adipocyte biology and the systemic consequences of impaired adipocyte function. Even more recently, compelling genetic studies have suggested that subtle partial lipodystrophy is likely to be a major factor in prevalent insulin-resistant type 2 diabetes mellitus (T2DM), justifying the longstanding interest in these disorders. This progress has also underpinned novel approaches to treatment that, in at least some patients, can be of considerable therapeutic benefit.

Genetic diseases of the Kennedy pathways for membrane synthesis

The two branches of the Kennedy pathways (CDP-choline and CDP-ethanolamine) are the predominant pathways responsible for the synthesis of the most abundant phospholipids, phosphatidylcholine and phosphatidylethanolamine, respectively, in mammalian membranes. Recently, hereditary diseases associated with single gene mutations in the Kennedy pathways have been identified. Interestingly, genetic diseases within the same pathway vary greatly, ranging from muscular dystrophy to spastic paraplegia to a childhood blinding disorder to bone deformations. Indeed, different point mutations in the same gene (PCYT1; CCTα) result in at least three distinct diseases. In this review, we will summarize and review the genetic diseases associated with mutations in genes of the Kennedy pathway for phospholipid synthesis. These single-gene disorders provide insight, indeed direct genotype-phenotype relationships, into the biological functions of specific enzymes of the Kennedy pathway. We discuss potential mechanisms of how mutations within the same pathway can cause disparate disease.

Lipids and Membrane Microdomains: The Glycerolipid and Alkylphosphocholine Class of Cancer Chemotherapeutic Drugs

Synthetic antitumor lipids are metabolically stable lysophosphatidylcholine derivatives, encompassing a class of non-mutagenic drugs that selectively target cancerous cells. In this chapter we review the literature as relates to the clinical efficacy of these antitumor lipid drugs and how our understanding of their mode of action has evolved alongside key advances in our knowledge of membrane structure, organization, and function. First, the history of the development of this class of drugs is described, providing a summary of clinical outcomes of key members including edelfosine, miltefosine, perifosine, erufosine, and erucylphosphocholine. A detailed description of the biophysical properties of these drugs and specific drug-lipid interactions which may contribute to the selectivity of the antitumor lipids for cancer cells follows. An updated model on the mode of action of these lipid drugs as membrane disorganizing agents is presented. Membrane domain organization as opposed to targeting specific proteins on membranes is discussed. By altering membranes, these antitumor lipids inhibit many survival pathways while activating pro-apoptotic signals leading to cell demise.

CCT α is a novel biomarker for diagnosis of laryngeal squamous cell cancer

Choline phosphate-based delivery systems can target the acidic tumor microenvironment. In this study, we set out to evaluate the diagnostic value of Choline phosphate cytidylyltransferase-α (CCTα) in laryngeal squamous cell cancer (LSCC). The expression of CCTα was detected using immunohistochemistry in 50 LSCC patients’ tissues and 16 vocal polyps as control group. Then, clinical data was collected and we used receiver operating characteristic curve (ROC) to estimate the potential of CCTα as diagnostic biomarker. We found CCTα levels to be significantly high in the tissues derived from LSCC patients, (p < 0.001). Further, we observed a positive correlation of CCTα with tumor size (p < 0.001), TNM stage (p < 0.001), lymph node metastasis (p < 0.001) as well as the grade of LSCC malignancy (p < 0.001). Furthermore, AUC was determined to be 0.939 by ROC, and the optimal cutoff value 3.100, with 76.0% sensitivity and 100% specificity. We also found an epigenetic basis of CCTα over-expression in LSCC tissues with significantly reduced methylation of CCTα in LSCC tissues, compared to vocal polyps (p < 0.001). These results support epigenetically-induced over-expression of CCTα as a potential diagnostic marker for LSCC.

De novo lipogenesis at the mitotic exit is used for nuclear envelope reassembly/expansion. Implications for combined chemotherapy

Mitosis has been traditionally considered a metabolically inactive phase. We have previously shown, however, that extensive alterations in lipids occur as the cells traverse mitosis, including increased de novo fatty acid (FA) and phosphatidylcholine (PtdCho) synthesis and decreased lysophospholipid content. Given the diverse structural and functional properties of these lipids, we sought to study their metabolic fate and their importance for cell cycle completion. Here we show that FA and PtdCho synthesized at the mitotic exit are destined to the nuclear envelope. Importantly, FA and PtdCho synthesis, but not the decrease in lysophospholipid content, are necessary for cell cycle completion beyond G₂/M. Moreover, the presence of alternative pathways for PtdCho synthesis renders the cells less sensitive to its inhibition than to the impairment of FA synthesis. FA synthesis, thus, represents a cell cycle-related metabolic vulnerability that could be exploited for combined chemotherapy. We explored the combination of fatty acid synthase (FASN) inhibition with agents that act at different phases of the cell cycle. Our results show that the effect of FASN inhibition may be enhanced under some drug combinations.

Phospholipid synthesis fueled by lipid droplets drives the structural development of poliovirus replication organelles

Rapid development of complex membranous replication structures is a hallmark of picornavirus infections. However, neither the mechanisms underlying such dramatic reorganization of the cellular membrane architecture, nor the specific role of these membranes in the viral life cycle are sufficiently understood. Here we demonstrate that the cellular enzyme CCTα, responsible for the rate-limiting step in phosphatidylcholine synthesis, translocates from the nuclei to the cytoplasm upon infection and associates with the replication membranes, resulting in the rerouting of lipid synthesis from predominantly neutral lipids to phospholipids. The bulk supply of long chain fatty acids necessary to support the activated phospholipid synthesis in infected cells is provided by the hydrolysis of neutral lipids stored in lipid droplets. Such activation of phospholipid synthesis drives the massive membrane remodeling in infected cells. We also show that complex membranous scaffold of replication organelles is not essential for viral RNA replication but is required for protection of virus propagation from the cellular anti-viral response, especially during multi-cycle replication conditions. Inhibition of infection-specific phospholipid synthesis provides a new paradigm for controlling infection not by suppressing viral replication but by making it more visible to the immune system.

Phosphatidylcholine synthesis regulates triglyceride storage and chylomicron secretion by Caco2 cells

Intracellular lipid droplets (LDs) supply fatty acids for energy, membrane biogenesis, and lipoprotein secretion. The surface monolayer of LDs is composed of phospholipids, primarily phosphatidylcholine (PC), that stabilize the neutral lipid core of triglyceride (TG). To determine the relationship between PC synthesis and TG storage and secretion in chylomicrons, we used a model of intestinal-derived human epithelial colorectal adenocarcinoma (Caco2) cells with knockout of PCYT1A, which encodes the rate-limiting enzyme CTP:phosphocholine cytidylyltransferase (CCT)α in the CDP-choline pathway, that were treated with the fatty acid oleate. CRISPR/Cas9 knockout of CCTα in Caco2 cells (Caco2-KO cells) reduced PC synthesis by 50%. Compared with Caco2 cells, Caco2-KO cells exposed to oleate had fewer and larger LDs and greater TG accumulation as a result. The addition of exogenous lysophosphatidylcholine to Caco2-KO cells reversed the LD morphology defect. Caco2-KO cells, differentiated into epithelial monolayers, accumulated intracellular TG and had deficient TG and chylomicron-associated apoB48 secretion; apoB100 secretion was unaffected by CCTα knockout or oleate. Metabolic-labeling and LD imaging of Caco2-KO cells indicated preferential shuttling of de novo synthesized TG into larger LDs rather than into chylomicrons. Thus, reduced de novo PC synthesis in Caco2 cells enhances TG storage in large LDs and inhibits apoB48 chylomicron secretion.

From yeast to humans - roles of the Kennedy pathway for phosphatidylcholine synthesis

The major phospholipid present in most eukaryotic membranes is phosphatidylcholine (PC), comprising ~ 50% of phospholipid content. PC metabolic pathways are highly conserved from yeast to humans. The main pathway for the synthesis of PC is the Kennedy (CDP-choline) pathway. In this pathway, choline is converted to phosphocholine by choline kinase, phosphocholine is metabolized to CDP-choline by the rate-determining enzyme for this pathway, CTP:phosphocholine cytidylyltransferase, and cholinephosphotransferase condenses CDP-choline with diacylglycerol to produce PC. This Review discusses how PC synthesis via the Kennedy pathway is regulated, its role in cellular and biological processes, as well as diseases known to be associated with defects in PC synthesis. Finally, we present the first model for the making of a membrane via PC synthesis.

From masochistic enzymology to mechanistic physiology and disease

The pioneering work of Eugene Kennedy in the 1950s established the choline pathway for phosphatidylcholine (PC) biosynthesis. However, the regulation of PC biosynthesis was poorly understood at that time. When I started my lab at the University of British Columbia in the 1970s, this was the focus of my research. This article provides my reflections on these studies that began with enzymology and the use of cultured mammalian cells, and progressed to utilize the techniques of molecular biology and gene-targeted mice. The research in my lab and others demonstrated that the regulated and rate-limiting step in the choline pathway for PC biosynthesis was catalyzed by CTP:phosphocholine cytidylyltransferase. This enzyme is regulated by its movement from a soluble form (largely in the nucleus) to a membrane-associated form where the enzyme becomes activated. Gene targeting in mice subsequently demonstrated that this gene is essential for development of mouse embryos. The other mammalian pathway for PC biosynthesis is catalyzed by phosphatidylethanolamine N-methyltransferase (PEMT) that converts phosphatidylethanolamine to PC. Understanding of the regulation and function of the integral membrane protein PEMT was improved when the enzyme was purified (a masochistic endeavor) in 1987, leading to the cloning of the Pemt cDNA. Generation of knock-out mice that lacked PEMT showed that they were protected from atherosclerosis, diet-induced obesity, and insulin resistance. The protection from atherosclerosis appears to be due to decreased secretion of lipoproteins from the liver. We continue to investigate the mechanism(s) by which Pemt^-/- mice are protected from weight gain and insulin resistance.

The critical role of phosphatidylcholine and phosphatidylethanolamine metabolism in health and disease

Phosphatidylcholine (PC) and phosphatidylethanolamine (PE) are the most abundant phospholipids in all mammalian cell membranes. In the 1950s, Eugene Kennedy and co-workers performed groundbreaking research that established the general outline of many of the pathways of phospholipid biosynthesis. In recent years, the importance of phospholipid metabolism in regulating lipid, lipoprotein and whole-body energy metabolism has been demonstrated in numerous dietary studies and knockout animal models. The purpose of this review is to highlight the unappreciated impact of phospholipid metabolism on health and disease. Abnormally high, and abnormally low, cellular PC/PE molar ratios in various tissues can influence energy metabolism and have been linked to disease progression. For example, inhibition of hepatic PC synthesis impairs very low density lipoprotein secretion and changes in hepatic phospholipid composition have been linked to fatty liver disease and impaired liver regeneration after surgery. The relative abundance of PC and PE regulates the size and dynamics of lipid droplets. In mitochondria, changes in the PC/PE molar ratio affect energy production. We highlight data showing that changes in the PC and/or PE content of various tissues are implicated in metabolic disorders such as atherosclerosis, insulin resistance and obesity. This article is part of a Special Issue entitled: Membrane Lipid Therapy: Drugs Targeting Biomembranes edited by Pablo V. Escribá.

Torsins Are Essential Regulators of Cellular Lipid Metabolism

Torsins are developmentally essential AAA+ proteins, and mutation of human torsinA causes the neurological disease DYT1 dystonia. They localize in the ER membranes, but their cellular function remains unclear. We now show that dTorsin is required in Drosophila adipose tissue, where it suppresses triglyceride levels, promotes cell growth, and elevates membrane lipid content. We also see that human torsinA at the inner nuclear membrane is associated with membrane expansion and elevated cellular lipid content. Furthermore, the key lipid metabolizing enzyme, lipin, is mislocalized in dTorsin-KO cells, and dTorsin increases levels of the lipin substrate, phosphatidate, and reduces the product, diacylglycerol. Finally, genetic suppression of dLipin rescues dTorsin-KO defects, including adipose cell size, animal growth, and survival. These findings identify that torsins are essential regulators of cellular lipid metabolism and implicate disturbed lipid biology in childhood-onset DYT1 dystonia.

Quantitative profiling of the endonuclear glycerophospholipidome of murine embryonic fibroblasts

A reliable method for purifying envelope-stripped nuclei from immortalized murine embryonic fibroblasts (iMEFs) was established. Quantitative profiling of the glycerophospholipids (GPLs) in envelope-free iMEF nuclei yields several conclusions. First, we find the endonuclear glycerophospholipidome differs from that of bulk membranes, and phosphatidylcholine (PtdCho) and phosphatidylethanolamine species are the most abundant endonuclear GPLs by mass. By contrast, phosphatidylinositol (PtdIns) represents a minor species. We also find only a slight enrichment of saturated versus unsaturated GPL species in iMEF endonuclear fractions. Moreover, much lower values for GPL mass were measured in the iMEF nuclear matrix than those reported for envelope-stripped IMF-32 nuclei. The collective results indicate that the nuclear matrix in these cells is a GPL-poor environment where GPL occupies only approximately 0.1% of the total nuclear matrix volume. This value suggests GPL accommodation in this compartment can be satisfied by binding to resident proteins. Finally, we find no significant role for the PtdIns/PtdCho-transfer protein, PITPα, in shuttling PtdIns into the iMEF nuclear matrix.

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.

Participation of prostaglandin D2 in the mobilization of the nuclear-localized CTP:phosphocholine cytidylyltransferase alpha in renal epithelial cells

Phosphatidylcholine (PC) is the main constituent of mammalian cell membranes. Consequently, preservation of membrane PC content and composition - PC homeostasis - is crucial to maintain cellular life. PC biosynthetic pathway is generally controlled by CTP:phosphocholine cytidylyltransferase (CCT), which is considered the rate-limiting enzyme. CCTα is an amphitropic protein, whose enzymatic activity is commonly associated with endoplasmic reticulum redistribution. However, most of the enzyme is located inside the nuclei. Here, we demonstrate that CCTα is the most abundant isoform in renal collecting duct cells, and its redistribution is dependent on endogenous prostaglandins. Previously we have demonstrated that PC synthesis was inhibited by indomethacin (Indo) treatment, and this effect was reverted by exogenous PGD(2). In this work we found that Indo induced CCTα distribution into intranuclear Lamin A/C foci. Exogenous PGD(2) reverted this effect by inducing CCTα redistribution to nuclear envelope, suggesting that PGD(2) maintains PC synthesis by CCTα mobilization. Interestingly, we found that the effect of PGD(2) was dependent on ERK1/2 activation. In conclusion, our previous observations and the present results lead us to suggest that papillary cells possess the ability to maintain their structural integrity through the synthesis of their own survival molecule, PGD(2), by modulating CCTα intracellular location.

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.

Molecular Mechanism for the Thermo-Sensitive Phenotype of CHO-MT58 Cell Line Harbouring a Mutant CTP:Phosphocholine Cytidylyltransferase

Control and elimination of malaria still represents a major public health challenge. Emerging parasite resistance to current therapies urges development of antimalarials with novel mechanism of action. Phospholipid biosynthesis of the Plasmodium parasite has been validated as promising candidate antimalarial target. The most prevalent de novo pathway for synthesis of phosphatidylcholine is the Kennedy pathway. Its regulatory and often also rate limiting step is catalyzed by CTP:phosphocholine cytidylyltransferase (CCT). The CHO-MT58 cell line expresses a mutant variant of CCT, and displays a thermo-sensitive phenotype. At non-permissive temperature (40°C), the endogenous CCT activity decreases dramatically, blocking membrane synthesis and ultimately leading to apoptosis. In the present study we investigated the impact of the analogous mutation in a catalytic domain construct of Plasmodium falciparum CCT in order to explore the underlying molecular mechanism that explains this phenotype. We used temperature dependent enzyme activity measurements and modeling to investigate the functionality of the mutant enzyme. Furthermore, MS measurements were performed to determine the oligomerization state of the protein, and MD simulations to assess the inter-subunit interactions in the dimer. Our results demonstrate that the R681H mutation does not directly influence enzyme catalytic activity. Instead, it provokes increased heat-sensitivity by destabilizing the CCT dimer. This can possibly explain the significance of the PfCCT pseudoheterodimer organization in ensuring proper enzymatic function. This also provide an explanation for the observed thermo-sensitive phenotype of CHO-MT58 cell line.

Choline phosphate cytidylyltransferase-α is a novel antigen detected by the anti-ERCC1 antibody 8F1 with biomarker value in patients with lung and head and neck squamous cell carcinomas

BACKGROUND

The determination of in situ protein levels of ERCC1 with the 8F1 monoclonal antibody is prognostic of survival in patients with non-small cell lung cancer (NSCLC). The authors previously demonstrated that 8F1 recognizes a second nuclear antigen. This antigen was identified and its value as a biomarker of clinical outcomes analyzed.

METHODS

The second antigen was identified by mass spectrometry. Protein identity and antibody specificity were confirmed through knockdown and overexpression experiments. Immunohistochemistry of 187 early-stage NSCLC samples and 60 head and neck squamous cell carcinomas (HNSCCs) was used to examine the influence of the second antigen on 8F1 immunoreactivity and its association with patient outcomes.

RESULTS

Choline phosphate cytidylyltransferase-α (CCTα, also known as phosphate cytidylyltransferase 1 choline alpha [PCYT1A], a phospholipid synthesis enzyme regulated by RAS) is the second antigen recognized by 8F1. In NSCLC samples, CCTα contributed (rho, 0.38) to 8F1 immunoreactivity. In samples of squamous cell carcinomas of the lung, CCTα was found to be the dominant determinant of 8F1 immunoreactivity, whereas its contribution in other subtypes of lung cancer was negligible. High expression of CCTα, but not ERCC1, was found to be prognostic of longer disease-free survival (log-rank P = .002) and overall survival (log-rank P = .056). Similarly, in patients with HNSCC, CCTα contributed strongly to 8F1 immunoreactivity (rho, 0.74), and high CCTα expression was found to be prognostic of survival (log-rank P = .022 for disease-free survival and P = .027 for overall survival).

CONCLUSIONS

CCTα is the second antigen detected by 8F1. High CCTα expression appears to be prognostic of survival in patients with NSCLC who are treated by surgery alone and patients with HNSCC. CCTα is a promising biomarker of patient survival and deserves further study.

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.

On the future of mass-spectrometry-based lipidomics

Lipids have highly diverse functions that go beyond cellular membrane structure and energy storage. One of the great challenges in lipid research will be to understand how the enormous complexity of lipid homeostasis is maintained. Genetic approaches combined with mass spectrometry-based lipidomics will help to elucidate how cells create and maintain their nonrandom lipid distribution within tissues, cells, organelles and lipid bilayers. Lipid homeostasis is crucial for many cellular processes and we are currently only beginning to understand the specific functions of lipids and the local environment that they create.

ras-Induced up-regulation of CTP:phosphocholine cytidylyltransferase α contributes to malignant transformation of intestinal epithelial cells

Cancer cells have enhanced lipogenic capacity characterized by increased synthesis of fatty acids and complex lipids, including phosphatidylcholine (PC). As the rate-limiting enzyme in the CDP-choline pathway for PC synthesis, CTP:phosphocholine cytidylyltransferase α (CCTα) is implicated in the provision of membranes and bioactive lipids necessary of cell proliferation. In this study, we assessed the role of CCTα in malignant intestinal epithelial cells transformed with activated H-ras (IEC-ras). Three IEC-ras clones had significant up-regulation CCTα expression, but PC synthesis and in vitro activity of CCTα were similar to control IEC. RNA interference of CCTα in adherent IEC-ras did not affect PC synthesis, confirming that the enzyme was relatively inactive. However, CCTα silencing in ras-transformed IEC reduced anchorage-independent growth, a criterion for malignant transformation, as well as tumorigenicity in mice. Relative to their adherent counterparts, detached IEC-ras had increased PC synthesis that was attenuated by inducible CCTα silencing. Detachment of IEC-ras was accompanied by increased CCTα phosphorylation and cytosolic enzyme activity. We conclude that the expanded pool of CCTα in IEC-ras is activated by detachment. This provides the increased PC biosynthetic capacity that contributes to malignant transformation of intestinal epithelial cells when detached from the extracellular matrix.

Phosphatidylcholine and the CDP-choline cycle

The CDP-choline pathway of phosphatidylcholine (PtdCho) biosynthesis was first described more than 50 years ago. Investigation of the CDP-choline pathway in yeast provides a basis for understanding the CDP-choline pathway in mammals. PtdCho is considered as an intermediate in a cycle of synthesis and degradation, and the activity of a CDP-choline cycle is linked to subcellular membrane lipid movement. The components of the mammalian CDP-choline pathway include choline transport, choline kinase, phosphocholine cytidylyltransferase, and choline phosphotransferase activities. The protein isoforms and biochemical mechanisms of regulation of the pathway enzymes are related to their cell- and tissue-specific functions. Regulated PtdCho turnover mediated by phospholipases or neuropathy target esterase participates in the mammalian CDP-choline cycle. Knockout mouse models define the biological functions of the CDP-choline cycle in mammalian cells and tissues. This article is part of a Special Issue entitled Phospholipids and Phospholipid Metabolism.

CTP:phosphocholine cytidylyltransferase α (CCTα) and lamins alter nuclear membrane structure without affecting phosphatidylcholine synthesis

CTP:phosphocholine cytidylyltransferase α (CCTα) is a nuclear enzyme that catalyzes the rate-limiting step in the CDP-choline pathway for phosphatidylcholine (PC) synthesis. Lipid activation of CCTα results in its translocation to the nuclear envelope and expansion of an intranuclear membrane network termed the nucleoplasmic reticulum (NR) by a mechanism involving membrane deformation. Nuclear lamins are also required for stability and proliferation of the NR, but whether this unique structure, or the nuclear lamina in general, is required for PC synthesis is not known. To examine this relationship, the nuclear lamina was depleted by RNAi or disrupted by expression of the Hutchinson-Gilford progeria syndrome (HGPS) mutant lamin A (progerin), and the effect on CCTα and choline metabolism was analyzed. siRNA-mediated silencing of lamin A/C or lamin B1 in CHO cells to diminish the NR had no effect on PC synthesis, while double knockdown non-specifically inhibited the pathway. Confirming this minor role in PC synthesis, only 10% of transiently overexpressed choline/ethanolamine phosphotransferase was detected in the NR. In CHO cells, CCTα was nucleoplasmic and co-localized with GFP-progerin in nuclear folds and invaginations; however, HGPS fibroblasts displayed an abnormal distribution of CCTα in the cytoplasm and nuclear envelope that was accompanied by a 2-fold reduction in PC synthesis. In spite of its altered localization, choline-labeling experiments showed that CCT activity was unaffected, and inhibition of PC synthesis was traced to reduced activity of a hemicholinium-sensitive choline transporter. We conclude that CCTα and lamins specifically cooperate to form the NR, but the overall structure of the nuclear envelope has a minimal impact on CCT activity and PC synthesis.

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.

The rate-limiting enzyme in phosphatidylcholine synthesis is associated with nuclear speckles under stress conditions

Phosphatidylcholine (PtdCho) is the most abundant phospholipid in eukaryotic membranes and its biosynthetic pathway is generally controlled by CTP:Phosphocholine Cytidylyltransferase (CCT), which is considered the rate-limiting enzyme. CCT is an amphitropic protein, whose enzymatic activity is commonly associated with endoplasmic reticulum (ER) translocation; however, most of the enzyme is intranuclearly located. Here we demonstrate that CCTα is concentrated in the nucleoplasm of MDCK cells. Confocal immunofluorescence revealed that extracellular hypertonicity shifted the diffuse intranuclear distribution of the enzyme to intranuclear domains in a foci pattern. One population of CCTα foci colocalised and interacted with lamin A/C speckles, which also contained the pre-mRNA processing factor SC-35, and was resistant to detergent and salt extraction. The lamin A/C silencing allowed us to visualise a second more labile population of CCTα foci that consisted of lamin A/C-independent foci non-resistant to extraction. We demonstrated that CCTα translocation is not restricted to its redistribution from the nucleus to the ER and that intranuclear redistribution must thus be considered. We suggest that the intranuclear organelle distribution of CCTα is a novel mechanism for the regulation of enzyme activity.

Specific interaction between E2F1 and Sp1 regulates the expression of murine CTP:phosphocholine cytidylyltransferase alpha during the S phase

CTP:phosphocholine cytidylyltransferase alpha (CCTalpha) is a key enzyme for phosphatidylcholine biosynthesis in mammalian cells. This enzyme plays an essential role in all processes that require membrane biosynthesis such as cell proliferation and viability. Thus, CCTalpha activity and expression fluctuate during the cell cycle to achieve PtdCho requirements. We demonstrated, for the first time, that CCTalpha is localized in the nucleus in cells transiting the S phase, whereas it is localized in the cytoplasm of G(0)-arrested cells, suggesting a specific role of nuclear CCTalpha during the S phase. We also investigated how E2F1 influences the regulation of the CCTalpha-promoter during the S phase; we demonstrated that E2F1 is necessary, but not sufficient, to activate CCTalpha expression when this factor is over-expressed. However, when E2F1 and Sp1 were over-expressed, the transcription from the CCTalpha-promoter reporter construct was super-activated. Transient transfection studies demonstrated that E2F1 could super-activate Sp1-dependent transcription in a promoter containing only the Sp1 binding sites "B" or "C", and that Sp1 could activate Sp1-dependent transcription in a promoter containing the E2F site, thus, further demonstrating a functional interaction of these factors. In conclusion, the present results allowed us to portray the clearest picture of the CCTalpha-gene expression in proliferating cells, and understand the mechanism by which cells coordinate cell cycle progression with the requirement for phosphatidylcholine.

14-3-3zeta escorts CCTalpha for calcium-activated nuclear import in lung epithelia

Integrity of animal biomembranes is critical to preserve normal cellular functions and viability. Phosphatidylcholine, an indispensible membrane component, requires the enzyme CCTalpha for its biosynthesis. Nuclear expression of CCTalpha is needed for expansion of the nuclear membrane network, but mechanisms for CCTalpha nuclear import are unknown. Herein, we show that in epithelia, extracellular Ca(2+) triggers CCTalpha cytoplasmic-nuclear translocation. CCTalpha nuclear import was associated with binding to 14-3-3zeta, a key regulator of protein trafficking. 14-3-3zeta was both sufficient and required for CCTalpha nuclear import. Helix G within the 14-3-3zeta binding groove interacts with a putative molecular signature within the CCTalpha carboxyl-terminal phosphoserine motif (residues 328-343). 14-3-3zeta was critically involved in preserving phosphatidylcholine synthesis and cell viability in a model of Pseudomonas aeruginosa infection where Ca(2+) concentrations increase within epithelia. Thus, 14-3-3zeta controls CCTalpha nuclear import in response to calcium signals, thereby regulating mammalian phospholipid synthesis. Agassandian, M., Chen, B. B., Schuster, C. C., Houtman, J. C. D., Mallampalli, R. K. 14-3-3zeta escorts CCTalpha for calcium-activated nuclear import in lung epithelia.

Functions of membrane binding domain of CTP:phosphocholine cytidylyltransferase in alveolar type II cells

CTP:phosphocholine cytidylyltransferase (CCTalpha) plays a key role in the biosynthesis of surfactant phosphatidylcholine. In this study, we investigated the role of its membrane-binding (M) domain in modulating its structure, function, and cellular distribution. Multiple enhanced green fluorescent protein-CCTalpha constructs were generated to evaluate the subcellular distribution in A549 cells. The M domain targeted CCTalpha to the perinuclear (membrane-rich) region. Microinjections with glutathione-S-transferase fusion protein containing the M domain corroborated the perinuclear targeting. Deletion of the M domain or substitutions of the hydrophobic residues with arginine/serine in the VEEKS(267-277) motif of the M domain resulted in a nuclear appearance and indented nuclei. Membrane binding of CCTalpha decreased gradually as the number of positively charged arginine residues increased in the VEEKS motif. To identify whether membrane-protein interactions cause structural alterations in CCTalpha, we visualized the protein in the absence and presence of lipids by transmission electron microscopy. These studies revealed that CCTalpha forms a dimer-like complex that condenses upon binding to lipid vesicles, but not lipid monolayers. The influence of the M domain on CCTalpha activity was assessed in transgenic mice overexpressing the N-terminal catalytic domain (CCTalpha(1-239)), N-terminal catalytic plus M domain (CCTalpha(1-290)), or full-length CCTalpha(1-367) in fetal type II cells by using the surfactant protein C promoter. Only overexpression of CCTalpha(1-367) increased surfactant phosphatidylcholine synthesis. Thus, the M domain influences membrane binding, cellular distribution, and topology of CCTalpha, but the domain alone is not sufficient to confer CCT activity in alveolar type II cells in vivo.

Molecular mechanisms involved in farnesol-induced apoptosis

The isoprenoid alcohol farnesol is an effective inducer of cell cycle arrest and apoptosis in a variety of carcinoma cell types. In addition, farnesol has been reported to inhibit tumorigenesis in several animal models suggesting that it functions as a chemopreventative and anti-tumor agent in vivo. A number of different biochemical and cellular processes have been implicated in the growth-inhibitory and apoptosis-inducing effects of farnesol. These include regulation of 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase and CTP:phosphocholine cytidylyltransferase alpha (CCTalpha), rate-limiting enzymes in the mevalonate pathway and phosphatidylcholine biosynthesis, respectively, and the generation of reactive oxygen species. In some cell types the action of farnesol is mediated through nuclear receptors, including activation of farnesoid X receptor (FXR) and peroxisome proliferator-activated receptors (PPARs). Recent studies have revealed that induction of endoplasmic reticulum (ER) stress and the subsequent activation of the unfolded protein response (UPR) play a critical role in the induction of apoptosis by farnesol in lung carcinoma cells. This induction was found to be dependent on the activation of the MEK1/2-ERK1/2 pathway. In addition, farnesol induces activation of the NF-kappaB signaling pathway and a number of NF-kappaB target genes. Optimal activation of NF-kappaB was reported to depend on the phosphorylation of p65/RelA by the MEK1/2-MSK1 signaling pathway. In a number of cells farnesol-induced apoptosis was found to be linked to activation of the apoptosome. This review provides an overview of the biochemical and cellular processes regulated by farnesol in relationship to its growth-inhibitory, apoptosis-promoting, and anti-tumor effects.

The Kap60-Kap95 karyopherin complex directly regulates phosphatidylcholine synthesis

Phosphatidylcholine is the major phospholipid in eukaryotic cells. There are two main pathways for the synthesis of phosphatidylcholine: the CDP-choline pathway present in all eukaryotes and the phosphatidylethanolamine methylation pathway present in mammalian hepatocytes and some single celled eukaryotes, including the yeast Saccharomyces cerevisiae. In S. cerevisiae, the rate-determining step in the synthesis of phosphatidylcholine via the CDP-choline pathway is catalyzed by Pct1. Pct1 converts phosphocholine and CTP to CDP-choline and pyrophosphate. In this study, we determined that Pct1 is in the nucleoplasm and at endoplasmic reticulum and nuclear membranes. Pct1 directly interacts with the alpha-importin Kap60 via a bipartite basic region in Pct1, and this region of Pct1 was required for its entry into the nucleus. Pct1 also interacted with the beta-importin Kap95 in cell extracts, implying a model whereby Pct1 interacts with Kap60 and Kap95 with this tripartite complex transiting the nuclear pore. Exclusion of Pct1 from the nucleus by elimination of its nuclear localization signal or by decreasing Kap60 function did not affect the level of phosphatidylcholine synthesis. Diminution of Kap95 function resulted in almost complete ablation of phosphatidylcholine synthesis under conditions where Pct1 was extranuclear. The beta-importin Kap95 is a direct regulator membrane synthesis.

Nuclear export of the rate-limiting enzyme in phosphatidylcholine synthesis is mediated by its membrane binding domain

CTP:phosphocholine cytidylyltransferase alpha (CCTalpha), the rate-limiting enzyme in the CDP-choline pathway for phosphatidylcholine (PtdCho) synthesis, is activated by translocation to nuclear membranes. However, CCTalpha is cytoplasmic in cells with increased capacity for PtdCho synthesis and following acute activation, suggesting that nuclear export is linked to activation. The objective of this study was to identify which CCTalpha domains were involved in nuclear export in response to the lipid activators farnesol (FOH) and oleate. Imaging of CCT-green fluorescent protein (GFP) mutants expressed in CCTalpha-deficient CHO58 cells showed that FOH-mediated translocation to nuclear membranes and export to the cytoplasm required the membrane binding amphipathic helix (domain M). Nuclear export was reduced by a mutation that mimics constitutive phosphorylation of the CCT phosphorylation (P) domain. However, domain M alone was sufficient to promote translocation to the nuclear envelope and export of a nuclear-localized GFP construct in FOH- or oleate-treated CHO58 cells. In the context of acute activation with lipid mediators, nuclear export of CCT-GFP mutants correlated with in vitro activity but not PtdCho synthesis. This study describes a nuclear export pathway that is dependent on membrane interaction of an amphipathic helix, thus linking lipid-dependent activation to the nuclear/cytoplasmic distribution of CCTalpha.

Physiological consequences of disruption of mammalian phospholipid biosynthetic genes

By 1959, Eugene Kennedy and coworkers had outlined most pathways of phospholipid biosynthesis. In the next four decades, the emphasis was on enzymology and regulation of these pathways. In the last 12 years, several lines of mice with disrupted genes of phospholipid biosynthesis were generated. From this research, we have learned that embryonic lethality occurs in mice that lack choline kinase (CK) alpha, CTP:phosphocholine cytidylyltransferase alpha, CTP:phosphoethanolamine cytidylyltransferase, or phosphatidylserine decarboxylase. Whereas mice that lack CK beta are viable but develop hindlimb muscular dystrophy and neonatal bone deformity. Mice that lack CTP:phosphocholine cytidylytransferase beta have gonadal dysfunction and defective axon branching. Mice that lack phosphatidylethanolamine N-methyltransferase exhibit no phenotype until fed a choline-deficient diet, which leads to rapid liver failure. Future research should extend our knowledge about the function of these and other enzymes of phospholipid biosynthesis.

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.

Phosphatidylcholine biosynthesis via CTP:phosphocholine cytidylyltransferase 2 facilitates neurite outgrowth and branching

Hallmarks of neuronal differentiation are neurite sprouting, extension, and branching. We previously showed that increased expression of CTP:phosphocholine cytidylyltransferase beta2 (CTbeta2), an isoform of a key phosphatidylcholine (PC) biosynthetic enzyme, accompanies neurite outgrowth (Carter, J. M., Waite, K. A., Campenot, R. B., Vance, J. E., and Vance, D. E. (2003) J. Biol. Chem. 278, 44988-44994). CTbeta2 mRNA is highly expressed in the brain. We show that CTbeta2 is abundant in axons of rat sympathetic neurons and retinal ganglion cells. We used RNA silencing to decrease CTbeta2 expression in PC12 cells differentiated by nerve growth factor. In CTbeta2-silenced cells, numbers of primary and secondary neurites were markedly reduced, suggesting that CTbeta2 facilitates neurite outgrowth and branching. However, the length of individual neurites was significantly increased, and the total amount of neuronal membrane was unchanged. Neurite branching of PC12 cells is known to be inhibited by activation of Akt and promoted by the Akt inhibitor LY294002. Our experiments showed that LY294002 increases neurite sprouting and branching in control PC12 cells but not in CTbeta2-deficient cells. CTbeta2 was not phosphorylated in vitro by Akt. However, inhibition of Cdk5 by roscovitine blocked CTbeta2 phosphorylation and reduced neurite outgrowth and branching. These results highlight the importance of CTbeta2 in neurons for promoting neurite outgrowth and branching and represent the first identification of a lipid biosynthetic enzyme that facilitates these functions.

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.

Role of phosphocholine cytidylyltransferase alpha in lung development

Lung development depends upon the differentiation and expansion of a variety of specialized epithelial cell types, including distal type I and type II pneumocytes in the late term. Previous studies have shown a strict dependence on the choline cytidylyltransferase alpha isoform (CCTalpha) to mediate membrane phospholipid formation in cultured cells and during preimplantation embryogenesis. CCTalpha expression is highest in lung, and there has long been speculation about its precise role, due to the dual requirement for phospholipid in proliferating cell membranes and for lung surfactant production from alveolar type II cells. We investigated the function of CCTalpha in lung development, using an inducible, epithelial cell-specific CCTalpha knockout mouse line. Deletion of CCTalpha beginning at embryonic day 7.5 did not restrict lung development but resulted in severe respiratory failure at birth. Alveolar lavage and lung lipid analyses showed significant decreases in the major surfactant phospholipid, dipalmitoyl-phosphatidylcholine. The fatty acids destined for the surfactant phospholipid were redirected to an expanded triglyceride pool. Transcripts encoding type II cell-specific markers were expressed in the knockout mice, indicating the expected progression of differentiation in lung epithelia. However, surfactant protein levels were reduced, with the exception of that for surfactant protein B, which was elevated. Ultrastructural analysis of the type II cells showed Golgi complex abnormalities and aberrant lamellar bodies, which deliver surfactant lipid and protein to the alveolar lumen. Thus, CCTalpha was not required for the proliferation or differentiation of lung epithelia but was essential for the secretory component of phospholipid synthesis and critical for the proper formation of lamellar bodies and surfactant protein homeostasis.

Subcellular localization of phosphatidylinositol synthesis

It is well-established that the endoplasmic reticulum is the major site of phosphatidylinositol (PtdIns) synthesis. The PtdIns synthetic ability of other organelles, such as plasma membrane and nucleus, remains controversial. In the present study, we re-examine this question by comparing PtdIns synthesis in isolated cytoplasts (enucleated cells) with that in corresponding karyoplasts (nuclei surrounded by plasma membrane but lacking most cytoplasmic components). We report that cytoplasts are competent to carry out both basal and stimulated PtdIns synthesis as well as polyphosphoinositide hydrolysis, while karyoplasts can neither synthesize PtdIns nor hydrolyze phosphoinositides in response to agonists. The karyoplasts are, however, capable of synthesizing phosphatidylcholine (PtdCho), as previously reported. From these data, we conclude that PtdIns synthesis is limited to cytoplasmic components, and cannot be sustained by either plasma membrane or nucleus under conditions that permit robust PtdCho synthesis.

Fundamental research is the basis for understanding and treatment of many human diseases

There are numerous examples of how fundamental research has been required to understand and treat human disease. This article focuses on three human diseases of lipid metabolism in which advancements in understanding and treatment would not have been possible without basic research. Fabry disease is an inherited metabolic disorder caused by the lack of a specific enzyme in glycosphingolipid catabolism. Cardiovascular disease is a complex and multifactorial disease but as many as half of the cases can be attributed to abnormal levels of plasma cholesterol. The incidence of liver disease is increasing due to the current epidemic of obesity. It is only recently that curiosity-driven research has yielded valuable insight into the mechanism by which liver disease evolves.

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

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

Dynamic lipidomics of the nucleus

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

Early embryonic lethality in mice with targeted deletion of the CTP:phosphocholine cytidylyltransferase alpha gene (Pcyt1a)

CTP:phosphocholine cytidylyltransferase (CCT) catalyzes a rate-controlling step in the biosynthesis of phosphatidylcholine (PtdCho). Multiple CCT isoforms, CCTalpha, CCTbeta2, and CCTbeta3, are encoded by two genes, Pcyt1a and Pcyt1b. The importance of CCTalpha in mice was investigated by deleting exons 5 and 6 in the Pcyt1a gene using the Cre-lox system. Pcyt1a-/- zygotes failed to form blastocysts, did not develop past embryonic day 3.5 (E3.5), and failed to implant. In situ hybridization in E11.5 embryos showed that Pcyt1a is expressed ubiquitously, with the highest level in fetal liver, and CCTalpha transcripts are significantly more abundant than transcripts encoding CCTbeta or phosphatidylethanolamine (PtdEtn) N-methyl transferase, two other enzymes capable of producing PtdCho. Reduction of the CCTalpha transcripts in heterozygous E11.5 embryos was accompanied by upregulation of CCTbeta and PtdEtn N-methyltransferase transcripts. In contrast, enzymatic and real-time PCR data revealed that CCTbeta (Pcyt1b) expression is not upregulated to compensate for the reduction in CCTalpha expression in adult liver and other tissues from Pcyt1a+/- heterozygous mice. PtdCho biosynthesis measured by choline incorporation into isolated hepatocytes was not compromised in the Pcyt1a+/- mice. Liver PtdCho mass was the same in Pcyt1a+/+ and Pcyt1a+/- adult animals, but lung PtdCho mass decreased in the heterozygous mice. These data show that CCTalpha expression is required for early embryonic development, but that a 50% reduction in enzyme activity has little detectable impact on the operation of the CDP-choline metabolic pathway in adult tissues.

Regulatory enzymes of phosphatidylcholine biosynthesis: a personal perspective

Phosphatidylcholine is a prominent constituent of eukaryotic and some prokaryotic membranes. This Perspective focuses on the two enzymes that regulate its biosynthesis, choline kinase and CTP:phosphocholine cytidylyltransferase. These enzymes are discussed with respect to their molecular properties, isoforms, enzymatic activities, and structures, and the possible molecular mechanisms by which they participate in regulation of phosphatidylcholine levels in the cell.

Increased Phosphatidylcholine Production but Disrupted Glycogen Metabolism in Fetal Type II Cells of Mice That Overexpress CTP:Phosphocholine Cytidylyltransferase

CTP:phosphocholine cytidylyltransferase (CCT) is a rate-determining enzyme in the de novo synthesis of phosphatidylcholine (PtdCho). Alveolar type II cells synthesize large quantities of disaturated PtdCho, the surface-active agent of pulmonary surfactant, particularly at late gestation when the lung prepares itself for postnatal air breathing. To clarify the role of CCTalpha in lung surfactant maturation, we overexpressed CCTalpha(1-367) using the surfactant protein-C promoter. Lungs of transgenic mice were analyzed at day 18 of gestation (term = 19 days). Overexpression of CCTalpha(1-367) increased the synthesis and content of PtdCho in fetal type II cells isolated from the transgenic mice. Also, PtdCho content of fetal lung fluid was increased. No changes in surfactant protein content were detected. Interestingly, fetal type II cells of transgenic mice contained more glycogen than control cells. Incorporation studies with [U-(14)C]glucose demonstrated that overexpression of CCTalpha(1-367) in fetal type II cells increased glycogen synthesis without affecting glycogen breakdown. To determine which domain contributes to this glycogen phenotype, two additional transgenes were created overexpressing either CCTalpha(1-239) or CCTalpha(239-367). Glycogen synthesis and content were increased in fetal type II cells expressing CCTalpha(239-367) but not CCTalpha(1-239)(.) We conclude that overexpression of CCTalpha increases surfactant PtdCho synthesis without affecting surfactant protein levels but that it disrupts glycogen metabolism in differentiating type II cells via its regulatory domain.