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Abstract
Three pulmonary disease conditions result from the accumulation of phospholipids in the lung. These conditions are the human lung disease known as pulmonary alveolar proteinosis, the lipoproteinosis that arises in the lungs of rats during acute silicosis, and the phospholipidoses induced by numerous cationic amphiphilic therapeutic agents. In this paper, the status of phospholipid metabolism in the lungs during the process of each of these lung conditions has been reviewed and possible mechanisms for their establishment are discussed. Pulmonary alveolar proteinosis is characterized by the accumulation of tubular myelin-like multilamellated structures in the alveoli and distal airways of patients. These structures appear to be formed by a process of spontaneous assembly involving surfactant protein A and surfactant phospholipids. Structures similar to tubular myelin-like multilamellated structures can be seen in the alveoli of rats during acute silicosis and, as with the human condition, both surfactant protein A and surfactant phospholipids accumulate in the alveoli. Excessive accumulation of surfactant protein A and surfactant phospholipids in the alveoli could arise from their overproduction and hypersecretion by a subpopulation of Type II cells that are activated by silica, and possibly other agents. Phospholipidoses caused by cationic amphiphilic therapeutic agents arise as a result of their inhibition of phospholipid catabolism. Inhibition of phospholipases results in the accumulation of phospholipids in the cytoplasm of alveolar macrophages and other cells. While inhibition of phospholipases by these agents undoubtedly occurs, there are many anomalous features, such as the accumulation of extracellular phospholipids and surfactant protein A, that cannot be accounted for by this simplistic hypothesis.
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Affiliation(s)
- Gary E. R. Hook
- Biochemical Pathology Group, Laboratory of Pulmonary Pathobiology, National Institute of Environmental Health Sciences, P.O. Box 12233, Research Triangle Park, North Carolina 27709
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2
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Kuang Y, Salem N, Tian H, Kolthammer JA, Corn DJ, Wu C, Wang F, Wang Y, Lee Z. Imaging lipid synthesis in hepatocellular carcinoma with [methyl-11c]choline: correlation with in vivo metabolic studies. J Nucl Med 2010; 52:98-106. [PMID: 21149484 DOI: 10.2967/jnumed.110.080366] [Citation(s) in RCA: 25] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/03/2023] Open
Abstract
UNLABELLED PET with [methyl-(11)C]-choline (11C-choline) can be useful for detecting well-differentiated hepatocellular carcinoma (HCC) that is not 18F-FDG-avid. This study was designed to examine the relationship between choline metabolism and choline tracer uptake in HCC for PET with 11C-choline. METHODS Dynamic PET scans of 11C-choline were acquired using the woodchuck models of HCC. After imaging, [methyl-(14)C]-choline was injected, and metabolites from both HCC tissue samples and the surrounding hepatic tissues were extracted and analyzed by radio-high-performance liquid chromatography. The enzymatic activities of choline kinase and choline-phosphate cytidylyltransferase were assayed for correlation with the imaging and metabolism data. RESULTS PET with 11C-choline showed an HCC detection rate of 9 of 10. The tumor-to-liver ratio for the 9 detected HCCs was 1.89±0.55. Hematoxylin-eosin staining confirmed that all spots with high tracer uptake were well-differentiated HCCs. Variation of radioactivity distribution within HCCs indicated a heterogeneous uptake of choline. The activities of both choline kinase and choline-phosphate cytidylyltransferase were found to be significantly higher in HCC than in the surrounding hepatic tissues. The major metabolites of 11C-choline were phosphocholine in HCC and betaine and choline in the surrounding hepatic tissues at 12 min after injection; in HCC, phosphocholine rapidly converted to phosphatidylcholine at 30 min after injection. CONCLUSION HCCs display enhanced uptake of radiolabeled choline despite a moderate degree of physiologic uptake in the surrounding hepatic tissues. Initially, increased radiolabeled choline uptake in HCCs is associated with the transport and phosphorylation of choline; as time passes, the increased uptake of radiolabeled choline reflects increased phosphatidylcholine synthesis derived from radiolabeled cytidine 5'-diphosphocholine (CDP-choline) in HCCs. In contrast, the surrounding hepatic tissues exhibit extensive oxidation of radiolabeled choline via the phosphatidylethanolamine methylation pathway, a major contributor to the observed physiologic uptake.
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Affiliation(s)
- Yu Kuang
- Department of Biomedical Engineering, Case Western Reserve University, and Department of Radiology, University Hospitals Case Medical Center, Cleveland, Ohio 44106, USA
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3
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Ridsdale R, Tseu I, Wang J, Post M. Functions of membrane binding domain of CTP:phosphocholine cytidylyltransferase in alveolar type II cells. Am J Respir Cell Mol Biol 2009; 43:74-87. [PMID: 19684306 DOI: 10.1165/rcmb.2009-0231oc] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/24/2022] Open
Abstract
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.
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Affiliation(s)
- Ross Ridsdale
- Physiology and Experimental Medicine Program, Hospital for Sick Children Research Institute, 555 University Avenue, Toronto, ON, M5G 1X8 Canada
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4
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Taneva S, Dennis MK, Ding Z, Smith JL, Cornell RB. Contribution of each membrane binding domain of the CTP:phosphocholine cytidylyltransferase-alpha dimer to its activation, membrane binding, and membrane cross-bridging. J Biol Chem 2008; 283:28137-48. [PMID: 18694933 DOI: 10.1074/jbc.m802595200] [Citation(s) in RCA: 27] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
CTP:phosphocholine cytidylyltransferase (CCT), a rate-limiting enzyme in phosphatidylcholine synthesis, is regulated by reversible membrane interactions mediated by an amphipathic helical domain (M) that binds selectively to anionic lipids. CCT is a dimer; thus the functional unit has two M domains. To probe the functional contribution of each domain M we prepared a CCT heterodimer composed of one full-length subunit paired with a CCT subunit truncated before domain M that was also catalytically dead. We compared this heterodimer to the full-length homodimer with respect to activation by anionic vesicles, vesicle binding affinities, and promotion of vesicle aggregation. Surprisingly for all three functions the dimer with just one domain M behaved similarly to the dimer with two M domains. Full activation of the wild-type subunit was not impaired by loss of one domain M in its partner. Membrane binding affinities were the same for dimers with one versus two M domains, suggesting that the two M domains of the dimer do not engage a single bilayer simultaneously. Vesicle cross-bridging was also unhindered by loss of one domain M, suggesting that another motif couples with domain M for cross-bridging anionic membranes. Mutagenesis revealed that the positively charged nuclear localization signal sequence constitutes that second motif for membrane cross-bridging. We propose that the two M domains of the CCT dimer engage a single bilayer via an alternating binding mechanism. The tethering function involves the cooperation of domain M and the nuclear localization signal sequence, each engaging separate membranes. Membrane binding of a single M domain is sufficient to fully activate the enzymatic activity of the CCT dimer while sustaining the low affinity, reversible membrane interaction required for regulation of CCT activity.
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Affiliation(s)
- Svetla Taneva
- Department of Molecular Biology and Biochemistry, Simon Fraser University, Burnaby, British Columbia V5A-1S6, Canada
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5
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McCoy DM, Fisher K, Robichaud J, Ryan AJ, Mallampalli RK. Transcriptional regulation of lung cytidylyltransferase in developing transgenic mice. Am J Respir Cell Mol Biol 2006; 35:394-402. [PMID: 16645180 PMCID: PMC2643291 DOI: 10.1165/rcmb.2005-0401oc] [Citation(s) in RCA: 13] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/25/2005] [Accepted: 04/17/2006] [Indexed: 11/24/2022] Open
Abstract
Lung development is associated with a surge in surfactant phosphatidylcholine (PC) production to prepare the newborn for extrauterine breathing. This process is associated with a marked increase in the activity of the rate-regulatory surfactant enzyme, CTP:phosphocholine cytidylyltransferase (CCTalpha). To investigate the molecular basis for developmental activation of CCTalpha, we analyzed expression of endogenous CCTalpha and a reporter gene, beta-galactosidase, in fetal, newborn, and adult promoter-reporter transgenic mice. Transgenics harboring approximately 2 kb of the CCTalpha promoter linked upstream of a beta-galactosidase reporter gene displayed relatively high expression in distal lung epithelia. Endogenous lung CCTalpha and beta-galactosidase activities, protein content, and transcript levels displayed maximal expression within the newborn period. CCTalpha and beta-galactosidase activities and enzyme levels increased with time in cultured fetal lung explants isolated from transgenics. Transfectional analysis using CCTalpha promoter-reporter constructs in developing rat type II cells revealed that a region encompassing -169/+71 contained the DNA elements required for perinatal activation. The studies demonstrate that developmental induction of surfactant phospholipid is due, at least in part, to transcriptional activation of the CCTalpha gene.
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Affiliation(s)
- Diann M McCoy
- Department of Internal Medicine, University of Iowa Carver College of Medicine, Iowa City, Iowa 52242, USA
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6
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Ryan A, Fisher K, Thomas C, Mallampalli R. Transcriptional repression of the CTP:phosphocholine cytidylyltransferase gene by sphingosine. Biochem J 2005; 382:741-50. [PMID: 15139854 PMCID: PMC1133833 DOI: 10.1042/bj20040105] [Citation(s) in RCA: 13] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/19/2004] [Revised: 04/19/2004] [Accepted: 05/13/2004] [Indexed: 11/17/2022]
Abstract
We examined the effects of the bioactive lipid, sphingosine, on the expression of the rate-limiting enzyme involved in surfactant phosphatidylcholine synthesis, CCTalpha (CTP:phosphocholine cytidylyltransferase alpha). Sphingosine decreased phosphatidylcholine synthesis by inhibiting CCT activity in primary alveolar type II epithelia. Sphingosine decreased CCTalpha protein and mRNA levels by approx. 50% compared with control. The bioactive lipid did not alter CCTalpha mRNA stability, but significantly inhibited its transcriptional rate. In murine lung epithelia, sphingosine selectively reduced CCTalpha promoter-reporter activity when transfected with a 2 kb CCTalpha promoter/luciferase gene construct. Sphingosine also decreased transgene expression in murine type II epithelia isolated from CCTalpha promoter-reporter transgenic mice harbouring this 2 kb proximal 5'-flanking sequence. Deletional analysis revealed that sphingosine responsiveness was mapped to a negative regulatory element contained within 814 bp upstream of the coding region. The results indicate that bioactive sphingolipid metabolites suppress surfactant lipid synthesis by inhibiting gene transcription of a key surfactant biosynthetic enzyme.
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Affiliation(s)
- Alan J. Ryan
- *Department of Veterans Affairs Medical Center, University of Iowa College of Medicine, Iowa City, IA 52242, U.S.A
| | - Kurt Fisher
- †Departments of Internal Medicine and Biochemistry, University of Iowa College of Medicine, Iowa City, IA 52242, U.S.A
| | - Christie P. Thomas
- †Departments of Internal Medicine and Biochemistry, University of Iowa College of Medicine, Iowa City, IA 52242, U.S.A
| | - Rama K. Mallampalli
- *Department of Veterans Affairs Medical Center, University of Iowa College of Medicine, Iowa City, IA 52242, U.S.A
- †Departments of Internal Medicine and Biochemistry, University of Iowa College of Medicine, Iowa City, IA 52242, U.S.A
- To whom correspondence should be addressed, at Pulmonary and Critical Care Division, C-33K, GH, Departments of Internal Medicine and Biochemistry (email )
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7
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Kent C. Regulatory enzymes of phosphatidylcholine biosynthesis: a personal perspective. Biochim Biophys Acta Mol Cell Biol Lipids 2005; 1733:53-66. [PMID: 15749057 DOI: 10.1016/j.bbalip.2004.12.008] [Citation(s) in RCA: 80] [Impact Index Per Article: 4.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/30/2004] [Revised: 12/14/2004] [Accepted: 12/17/2004] [Indexed: 12/22/2022]
Abstract
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.
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Rodríguez-González A, Ramirez de Molina A, Fernández F, Lacal JC. Choline kinase inhibition induces the increase in ceramides resulting in a highly specific and selective cytotoxic antitumoral strategy as a potential mechanism of action. Oncogene 2004; 23:8247-59. [PMID: 15378008 DOI: 10.1038/sj.onc.1208045] [Citation(s) in RCA: 67] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
Abstract
Choline kinase (ChoK, E.C. 2.7.1.32) is involved in the synthesis of phosphatidylcholine (PC), and has been found to be increased in human tumors and tumor-derived cell lines. Furthermore, ChoK inhibitors have been reported to show a potent and selective antitumoral activity both in vitro and in vivo. Here, we provide the basis for a rational understanding of the antitumoral activity of ChoK inhibitors. In normal cells, blockage of de novo phosphorylcholine (PCho) synthesis by inhibition of ChoK promotes the dephosphorylation of pRb, resulting in a reversible cell cycle arrest at G0/G1 phase. In contrast, ChoK inhibition in tumor cells renders cells unable to arrest in G0/G1 as manifested by a lack of pRb dephosphorylation. Furthermore, tumor cells specifically suffer a drastic wobble in the metabolism of main membrane lipids PC and sphingomyelin (SM). This lipid disruption results in the enlargement of the intracellular levels of ceramides. As a consequence, normal cells remain unaffected, but tumor cells are promoted to apoptosis. Thus, we provide in this study the rationale for the potential clinical use of ChoK inhibitors.
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Affiliation(s)
- Agustín Rodríguez-González
- Translational Oncology Unit, Department of Molecular and Cellular Biology of Cancer, Instituto de Investigaciones Biomédicas, CSIC, Arturo Duperier 4, 28029 Madrid, Spain
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9
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Xie M, Smith JL, Ding Z, Zhang D, Cornell RB. Membrane Binding Modulates the Quaternary Structure of CTP:Phosphocholine Cytidylyltransferase. J Biol Chem 2004; 279:28817-25. [PMID: 15069071 DOI: 10.1074/jbc.m403311200] [Citation(s) in RCA: 22] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
CTP:phosphocholine cytidylyltransferase (CCT), a key enzyme that controls phosphatidylcholine synthesis, is regulated by reversible interactions with membranes containing anionic lipids. Previous work demonstrated that CCT is a homodimer. In this work we show that the structure of the dimer interface is altered upon encountering membranes that activate CCT. Chemical cross-linking reactions were established which captured intradimeric interactions but not random CCT dimer collisions. The efficiency of capturing covalent cross-links with four different reagents was diminished markedly upon presentation of activating anionic lipid vesicles but not zwitterionic vesicles. Experiments were conducted to show that the anionic vesicles did not interfere with the chemistry of the cross-linking reactions and did not sequester available cysteine sites on CCT for reaction with the cysteine-directed cross-linking reagent. Thus, the loss of cross-linking efficiency suggested that contact sites at the dimer interface had increased distance or reduced flexibility upon binding of CCT to membranes. The regions of the enzyme involved in dimerization were mapped using three approaches: 1) limited proteolysis followed by cross-linking of fragments, 2) yeast two-hybrid analysis of interactions between select domains, and 3) disulfide bonding potential of CCTs with individual cysteine to serine substitutions for the seven native cysteines. We found that the N-terminal domain (amino acids 1-72) is an important participant in forming the dimer interface, in addition to the catalytic domain (amino acids 73-236). We mapped the intersubunit disulfide bond to the cystine 37 pair in domain N and showed that this disulfide is sensitive to anionic vesicles, implicating this specific region in the membrane-sensitive dimer interface.
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Affiliation(s)
- Mingtang Xie
- Department of Molecular Biology and Biochemistry, Simon Fraser University, 8888 University Drive, Burnaby, British Columbia V5A 1S6, Canada
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10
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Zhou J, You Y, Zabner J, Ryan AJ, Mallampalli RK. The CCT promoter directs high-level transgene expression in distal lung epithelial cell lines. Am J Respir Cell Mol Biol 2004; 30:61-8. [PMID: 12829450 DOI: 10.1165/rcmb.2003-0020oc] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/24/2022] Open
Abstract
Gene therapy requires the presence of a robust and yet small promoter to drive high-level expression of desired proteins. In comparative analysis, we investigated the promoter strength of the CTP:phosphocholine cytidylyltransferase promoter (CCT alpha) with other commonly used promoters, which were all cloned into a similar background vector (PGL3 basic). Transient promoter-reporter assays in murine lung epithelial (MLE-12) cells revealed that the core CCT alpha promoter (240 bp) was observed to exhibit a 40-fold, 8-fold, and 3-fold higher level of activity compared with the simian virus 40, human cytomegalovirus, and Rous sarcoma virus promoters, respectively. The CCT alpha promoter was significantly more active than the Clara cell 10, thymidine kinase, and phosphoglycerate kinase promoters. This pattern of high-level expression for CCT alpha was detected primarily in cell lines of distal lung epithelial origin (MLE-12, RLE, H441) and was reduced in other cell lines (A549, CHO, HepG 2). CCT alpha promoter-reporter activity, CCT alpha transcript levels, and immunoreactive protein levels increased significantly in the presence of all-trans retinoic acid. The CCT alpha promoter, in a retinoic acid-inducible manner, efficiently directed expression of murine erythropoietin in MLE-12 cells. Collectively, these observations suggest that the CCT alpha construct might be useful to drive high-level, regulatable expression of heterologous proteins in alveolar epithelia.
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Affiliation(s)
- Jiming Zhou
- Pulmonary Division, C-33K, GH, Department of Internal Medicine, University of Iowa College of Medicine, Iowa City, IA 52242, USA.
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11
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Spragg RG, Li J. Effect of phosphocholine cytidylyltransferase overexpression on phosphatidylcholine synthesis in alveolar type II cells and related cell lines. Am J Respir Cell Mol Biol 2000; 22:116-24. [PMID: 10615073 DOI: 10.1165/ajrcmb.22.1.3295] [Citation(s) in RCA: 24] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/24/2022] Open
Abstract
Disaturated phosphatidylcholine (DSPC) is the predominate phospholipid component of lung surfactant. In the alveolar type II cell, the cytidine diphosphocholine (CDP-choline) pathway is the major biosynthetic pathway for DSPC. To investigate the hypothesis that phosphocholine cytidylyltransferase (CT) is the rate-limiting enzyme in the CDP-choline pathway, rat alveolar type II cells or lung tumor-derived cell lines (A549 or H441) with type II cell features were transfected with CT complementary DNA (cDNA). Cell fractions were subsequently assayed for CT protein and activity, and cell rates of DSPC synthesis were determined. In all cases, cell CT protein and activity were increased after transfection with CT cDNA but not after control transfection. Rat type II cells, but not A549 or H441 cells, increased the rate of DSPC synthesis after transfection with CT cDNA. Exposure of type II cells transfected with CT cDNA to palmitic acid resulted in a further increase in CT protein and activity. Exposure to dexamethasone resulted in increased CT protein and activity and increased synthesis of DSPC. The results confirm that CT has a rate-limiting and regulatory role in the synthesis of type II cell DSPC, and raise possibilities for novel therapeutic interventions.
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Affiliation(s)
- R G Spragg
- VA Medical Center, San Diego, California 92161, USA.
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12
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Weber CH, Park YS, Sanker S, Kent C, Ludwig ML. A prototypical cytidylyltransferase: CTP:glycerol-3-phosphate cytidylyltransferase from bacillus subtilis. Structure 1999; 7:1113-24. [PMID: 10508782 DOI: 10.1016/s0969-2126(99)80178-6] [Citation(s) in RCA: 81] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
BACKGROUND The formation of critical intermediates in the biosynthesis of lipids and complex carbohydrates is carried out by cytidylyltransferases, which utilize CTP to form activated CDP-alcohols or CMP-acid sugars plus inorganic pyrophosphate. Several cytidylyltransferases are related and constitute a conserved family of enzymes. The eukaryotic members of the family are complex enzymes with multiple regulatory regions or repeated catalytic domains, whereas the bacterial enzyme, CTP:glycerol-3-phosphate cytidylyltransferase (GCT), contains only the catalytic domain. Thus, GCT provides an excellent model for the study of catalysis by the eukaryotic cytidylyltransferases. RESULTS The crystal structure of GCT from Bacillus subtilis has been determined by multiwavelength anomalous diffraction using a mercury derivative and refined to 2.0 A resolution (R(factor) 0.196; R(free) 0.255). GCT is a homodimer; each monomer comprises an alpha/beta fold with a central 3-2-1-4-5 parallel beta sheet. Additional helices and loops extending from the alpha/beta core form a bowl that binds substrates. CTP, bound at each active site of the homodimer, interacts with the conserved (14)HXGH and (113)RTXGISTT motifs. The dimer interface incorporates part of a third motif, (63)RYVDEVI, and includes hydrophobic residues adjoining the HXGH sequence. CONCLUSIONS Structure superpositions relate GCT to the catalytic domains from class I aminoacyl-tRNA synthetases, and thus expand the tRNA synthetase family of folds to include the catalytic domains of the family of cytidylyltransferases. GCT and aminoacyl-tRNA synthetases catalyze analogous reactions, bind nucleotides in similar U-shaped conformations, and depend on histidines from analogous HXGH motifs for activity. The structural and other similarities support proposals that GCT, like the synthetases, catalyzes nucleotidyl transfer by stabilizing a pentavalent transition state at the alpha-phosphate of CTP.
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Affiliation(s)
- C H Weber
- Biophysics Research Division Department of Biological Chemistry University of Michigan Pathology Department University of Michigan Medical School Ann Arbor, MI 48109, USA
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Carrella M, Feldman D, Cogoi S, Csillaghy A, Weinhold PA. Enhancement of mdr2 gene transcription mediates the biliary transfer of phosphatidylcholine supplied by an increased biosynthesis in the pravastatin-treated rat. Hepatology 1999; 29:1825-32. [PMID: 10347126 DOI: 10.1002/hep.510290620] [Citation(s) in RCA: 28] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 12/07/2022]
Abstract
An increase of biliary lipid secretion is known to occur in the rat under sustained administration of statin-type 3-hydroxy-3-methylglutaryl (HMG) coenzyme A (CoA) reductase inhibitors. The present study has addressed critical mechanisms of hepatic lipid synthesis and phosphatidylcholine (PC) biliary transport in the rat fed with a 0.075% pravastatin diet for 3 weeks. After treatment, biliary secretion of PC and cholesterol increased to 233% and 249% of controls, while that of bile salts was unchanged. Activity of cytidylyltransferase (CT), a major regulatory enzyme in the CDP-choline pathway of PC synthesis, was raised in both microsomal and cytosolic fractions (226% and 150% of controls), and there was an increase to 187% in the mass of active enzyme as determined by Western blot of microsomal protein using an antibody specific to CT. Cytosolic activity of choline kinase, another enzyme of the CDP-choline pathway, also increased to 175% of controls. In addition, there was an over eightfold increase in the HMG CoA reductase activity and mRNA. Thus, an increased PC and cholesterol synthetic supply to hepatocytes appeared as a basic mechanism for the biliary hypersecretion of these lipids. Notwithstanding the increased synthesis, hepatic PC content was unchanged, suggesting an enhanced transfer of this lipid into bile. Indeed, there was a sevenfold increase of multidrug resistance gene 2 (mdr2) gene mRNA coding for a main PC canalicular translocase. Thus, hypersecretion of biliary PC in the model studied can be explained by an up-regulation of mdr2 gene transcription and its P-glycoprotein product mediating the biliary transfer of PC supplied by an increased biosynthesis.
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Affiliation(s)
- M Carrella
- Cattedra di Gastroenterologia, Facoltà di Medicina e Chirurgia, Università degli Studi di Udine, Italy
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14
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Friesen JA, Campbell HA, Kent C. Enzymatic and cellular characterization of a catalytic fragment of CTP:phosphocholine cytidylyltransferase alpha. J Biol Chem 1999; 274:13384-9. [PMID: 10224101 DOI: 10.1074/jbc.274.19.13384] [Citation(s) in RCA: 58] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
To probe the mechanism of lipid activation of CTP:phosphocholine cytidylyltransferase (CCTalpha), we have characterized a catalytic fragment of the enzyme that lacks the membrane-binding segment. The kinetic properties of the purified fragment, CCTalpha236, were characterized, as well as the effects of expressing the fragment in cultured cells. CCTalpha236 was truncated after residue 236, which corresponds to the end of the highly conserved catalytic domain. The activity of purified CCTalpha236 was independent of lipids and about 50-fold higher than the activity of wild-type CCTalpha assayed in the absence of lipids, supporting a model in which the membrane-binding segment functions as an inhibitor of the catalytic domain. The kcat/Km values for CCTalpha236 were only slightly lower than those for lipid-activated CCTalpha. The importance of the membrane-binding segment in vivo was tested by expression of CCTalpha236 in CHO58 cells, a cell line that is temperature-sensitive for growth and CCTalpha activity. Expression of wild-type CCTalpha in these cells complemented the defective growth phenotype when the cells were cultured in complete or delipidated fetal bovine serum. Expression of CCTalpha236, however, did not complement the growth phenotype in the absence of serum lipids. These cells were capable of making phosphatidylcholine in the delipidated medium, so the inability of the cells to grow was not due to defective phosphatidylcholine synthesis. Supplementation of the delipidated medium with an unsaturated fatty acid allowed growth of CHO58 cells expressing CCTalpha236. These results indicate that the membrane-binding segment of CCTalpha has an important role in cellular lipid metabolism.
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Affiliation(s)
- J A Friesen
- Department of Biological Chemistry, University of Michigan Medical Center, Ann Arbor, Michigan 48109-0606, USA
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15
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Clement JM, Kent C. CTP:phosphocholine cytidylyltransferase: insights into regulatory mechanisms and novel functions. Biochem Biophys Res Commun 1999; 257:643-50. [PMID: 10208837 DOI: 10.1006/bbrc.1999.0512] [Citation(s) in RCA: 48] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/09/2023]
Abstract
A key regulatory enzyme in phosphatidylcholine biosynthesis, CTP:cholinephosphate cytidylyltransferase (CCT), catalyzes the formation of CDP-choline. This review discusses the essential features of CCT and addresses intriguing new insights into the catalytic and regulatory properties of this complex enzyme. Characterization of a lipid-binding segment in rat CCT is described and the role of lipids in CCT activation is discussed. An analysis of the phosphorylation domain is presented and possible physiological rationales for reversible phosphorylation of CCT are discussed. The nuclear localization of CCT is examined in the context of multiple CCT isoforms, as is recent evidence establishing a potential link between CCT activity and vesicular transport.
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Affiliation(s)
- J M Clement
- Department of Biological Chemistry, University of Michigan Medical School, Ann Arbor, Michigan, 48109, USA
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16
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Batenburg JJ, Haagsman HP. The lipids of pulmonary surfactant: dynamics and interactions with proteins. Prog Lipid Res 1998; 37:235-76. [PMID: 10193527 DOI: 10.1016/s0163-7827(98)00011-3] [Citation(s) in RCA: 77] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/20/2022]
Affiliation(s)
- J J Batenburg
- Laboratory of Veterinary Biochemistry, Graduate School of Animal Health, Utrecht University, The Netherlands.
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17
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Weinhold PA, Barrett D. Studies on the regulation of CTP:phosphocholine cytidylyltransferase using permeabilized HEP G2 cells: evidence that both active and inactive enzyme are membrane-bound. BIOCHIMICA ET BIOPHYSICA ACTA 1998; 1391:307-19. [PMID: 9555069 DOI: 10.1016/s0005-2760(97)00206-3] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/07/2023]
Abstract
To obtain more insight into the mechanisms regulating CTP:phosphocholine cytidylyltransferase (CT), we determined the effect of oleate treatment on the rate of CT release from permeabilized Hep G2 cells and the distribution of the CT remaining in the permeabilized cells. When we permeabilized untreated cells in pH 7.5 buffer containing 0.15 M KCl, the rate of CT release was much slower than the release of lactate dehydrogenase. Oleate treatment caused a further decrease in CT release from cells. In untreated cells, 70-80% of the CT remaining in cells 10 min after permeabilization was recovered as soluble CT. Oleate treatment increased the amount of bound CT but over 50% of the CT in cells 10 min after permeabilization was recovered as soluble CT. In both control and oleate-treated cells, the increase in CT release with time correlated with a decrease in the amount of CT recovered from permeabilized cells as soluble CT. These results suggested that CT existed in a form that was not immediately available for release from permeabilized cells, but was recovered in the soluble fraction after cell disruption. When cells were permeabilized in 10 mM imidazole-20% glycerol-5 mM Mg2+ pH 6.5, over 80% of CT in control and over 90% of CT in oleate-treated cells was recovered bound to the particulate fraction. Essentially no CT was released from the cells. The recovery of CT in the particulate fraction required Mg2+ to be present when permeabilization was initiated. The addition of Mg2+, after cells were disrupted, did not increase CT in the particulate fraction. In untreated cells, 50% of bound CT was active. Oleate treatment increased the amount of active CT in the particulate fraction to over 70% of total. About 50% of particulate CT in untreated cells but only 15% in oleate-treated cells was extracted with 0.15 M KCl. Inactive CT was preferentially extracted by KCl. The bound CT was recovered in isolated nuclei. Overall, the results suggested that both inactive and active CT are bound to nuclear membranes, and that the activation of CT involves conversion of CT loosely bound to membrane to a form more tightly bound to membranes perhaps by hydrophobic interaction with phospholipids. This model does not involve translocation from a soluble pool.
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Affiliation(s)
- P A Weinhold
- Veterans Affairs Medical Center and Department of Biological Chemistry, University of Michigan Medical School, Ann Arbor, MI, USA.
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18
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Viscardi RM, Strauss K, Hasday JD. Oleic acid stimulates rapid translocation of cholinephosphate cytidylyltransferase in type II cells. BIOCHIMICA ET BIOPHYSICA ACTA 1997; 1349:157-70. [PMID: 9421188 DOI: 10.1016/s0005-2760(97)00124-0] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/05/2023]
Abstract
Activity of cholinephosphate cytidylyltransferase, the rate-limiting enzyme in phosphatidylcholine synthesis, increases rapidly in the lung after birth predominantly due to an increase in membrane-associated activity. Although there is strong evidence that enzyme translocation is a major regulatory mechanism in other cells, this mechanism has not been conclusively demonstrated in intact alveolar type II cells. In this study, we show that oleic acid stimulates rapid translocation of cytidylyltransferase activity and protein from cytosol to microsomes in both primary cultured fetal and adult type II cells and MLE12 cells, a cell line derived from murine distal respiratory epithelial cells. Shifts in subcellular distribution occurred within 5 min of exposure to 200 microM oleic acid. The magnitude of the increases in microsomal enzyme activity and immunoreactive protein levels was several-fold greater in d21 fetal cells than adult type II cells. Oleic acid-induced translocation was confirmed in in vitro translocation experiments. After incubating MLE12 cell postmitochondrial supernatants at 37 degrees C with oleic acid and separation of enzyme isoforms on glycerol density gradients, enzyme activity was decreased in gradient fractions corresponding to both cytosolic isoforms and microsomal activity increased 7.9-fold compared to the distribution of enzyme activity in postmitochondrial supernatants incubated at 4 degrees C without oleic acid. The increase in microsomal activity was associated with an increased incorporation of [14C]oleic acid in the membrane free fatty acid fraction. Developmental changes in type II cell membrane lipid composition may induce the rapid translocation/activation of cytidylyltransferase in the lung after birth.
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Affiliation(s)
- R M Viscardi
- Department of Pediatrics, University of Maryland Hospital, Baltimore 21201, USA.
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19
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Abstract
CTP:phosphocholine cytidylyltransferase (CCT) catalyzes the synthesis of CDP-choline and is regulatory for phosphatidylcholine biosynthesis. This review focuses on recent developments in understanding the catalytic and regulatory mechanisms of this enzyme. Evidence for the nuclear localization of the enzyme is discussed, as well as evidence suggesting cytoplasmic localization. A comparison of the catalytic domains of CCTs from a wide variety of organisms is presented, highlighting a large number of completely conserved residues. Work implying a role for the conserved HXGH sequence in catalysis is described. The membrane-binding domain in rat CCT has been defined, and the role of lipids in activating the enzyme is discussed. The identification of the phosphorylation domain is described, as well as approaches to understand the role of phosphorylation in enzyme activity. Other possible control mechanisms such as enzyme degradation and gene expression are presented.
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Affiliation(s)
- C Kent
- Department of Biological Chemistry, University of Michigan Medical Center, Ann Arbor 48109-0606, USA.
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20
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Vazquez JM, Roldan ER. Diacylglycerol species as messengers and substrates for phosphatidylcholine re-synthesis during Ca2+-dependent exocytosis in boar spermatozoa. Mol Reprod Dev 1997; 48:95-105. [PMID: 9266766 DOI: 10.1002/(sici)1098-2795(199709)48:1<95::aid-mrd12>3.0.co;2-v] [Citation(s) in RCA: 16] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/05/2023]
Abstract
We have investigated pathways of lipid metabolism in spermatozoa and generation of various metabolites with potential messenger functions during exocytosis stimulated with A23187/Ca2+. Stimulation of boar spermatozoa resulted in a considerable rapid increase in saturated/unsaturated 1,2-diacylglycerol (1,2-SU-DAG) and, concomitantly, a substantial reduction in disaturated 1,2-diacylglycerol (1,2-DS-DAG), and in phosphatidylcholine (PC). These changes preceded the onset of exocytosis. Phosphatidic acid was sometimes generated in parallel, but usually rose later, suggesting that 1,2-SU-DAG may be formed directly by phospholipase C action. Lipid changes observed in stimulated spermatozoa that have been prelabelled with several lipid precursors ([14C]palmitic acid, [14C]glycerol, [14C]choline, or [14C]arachidonic acid) suggested the existence of a unique process involving the utilization of the high basal levels of 1,2-DS-DAG to form 1,2-SU-DAG, with the latter being subsequently employed to replenish the PC pool. An ensuing generation of lysoPC and arachidonic acid, which paralleled the occurrence of exocytosis, revealed that the newly synthesized PC was hydrolyzed by phospholipase A2. The highest levels of 1,2-SU-DAG, minimum levels of 1,2-DS-DAG, and the regeneration of the PC pool were tightly coupled to the beginning of visible exocytosis. These results suggest that changes in these lipid metabolites may be fundamental processes during acrosomal exocytosis occurring in response to physiological agonists.
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Affiliation(s)
- J M Vazquez
- Department of Development and Signalling, The Babraham Institute, Cambridge, U.K
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21
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Yang J, Wang J, Tseu I, Kuliszewski M, Lee W, Post M. Identification of an 11-residue portion of CTP-phosphocholine cytidylyltransferase that is required for enzyme-membrane interactions. Biochem J 1997; 325 ( Pt 1):29-38. [PMID: 9224626 PMCID: PMC1218525 DOI: 10.1042/bj3250029] [Citation(s) in RCA: 21] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/04/2023]
Abstract
CTP-phosphocholine cytidylyltransferase (CT) is a key regulatory enzyme in the biosynthesis of phosphatidylcholine (PC) in many cells. Enzyme-membrane interactions appear to play an important role in CT activation. A putative membrane-binding domain appears to be located between residues 236 and 293 from the N-terminus. To map the membrane-binding domain more precisely, glutathione S-transferase fusion proteins were prepared that contained deletions of various domains in this putative lipid-binding region. The fusion proteins were assessed for their binding of [3H]PC/oleic acid vesicles. Fusion proteins encompassing residues 267-277 bound to PC/oleic acid vesicles, whereas fragments lacking this region exhibited no specific binding to the lipid vesicles. The membrane-binding characteristics of the CT fusion proteins were also examined using intact lung microsomes. Only fragments encompassing residues 267-277 competed with full-length 125I-labelled CT, expressed in recombinant Sf9 insect cells, for microsomal membrane binding. To investigate the role of this region in PC biosynthesis, A549 and L2 cells were transfected with cDNA for CT mutants under the control of a glucocorticoid-inducible long terminal repeat (LTR) promoter. Induction of CT mutants containing residues 267-277 in transfectants resulted in reduced PC synthesis. The decrease in PC synthesis was accompanied by a shift in endogenous CT activity from the particulate to the soluble fraction. Expression of CT mutants lacking this region in A549 and L2 cells did not affect PC formation and subcellular distribution of CT activity. These results suggest that the CT region located between residues 267 and 277 from the N-terminus is required for the interaction of CT with membranes.
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Affiliation(s)
- J Yang
- The Medical Research Council Group in Lung Development and the Neonatal Research Division, Hospital for Sick Children, Department of Pediatrics, University of Toronto, Toronto, Ontario M5G 1X8, Canada
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22
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The CDP-ethanolamine pathway in mammalian cells. ACTA ACUST UNITED AC 1997. [DOI: 10.1016/s1874-5245(97)80013-4] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register]
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23
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Abstract
The gas-liquid interface within the alveolus is completely lined with a complex mixture of lipids and unique proteins termed pulmonary surfactant, which both reduces surface tension and permits it to vary directly with the radius of curvature. In this way it minimizes the work of breathing and permits alveoli of different sizes to exist in equilibrium. However, surfactant does far more in that it also controls fluid balance in the lung and appears to play a key role in host defence. Either a deficiency in surfactant or an aberrant surfactant results in atelectasis and oedema. The surfactant system is very dynamic: alveolar surfactant phosphatidylcholine, the principal component, having a half life of only a few hours, with as much as 85% being recycled. Although distortion of the alveolar type II cell is now accepted as the principal stimulus for release, much remains to be discovered of modulating factors and intracellular signalling in the control of surfactant homeostasis. Likewise, many questions remain concerning the control of synthesis of the surfactant phospholipids, neutral lipids and proteins and their assembly into the tubular myelin form of alveolar surfactant, the refining of the monolayer with breathing, the control of re-uptake of different components into the type II cells and the roles of the proteins.
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Affiliation(s)
- T E Nicholas
- Department of Human Physiology, School of Medicine, Flinders University, Adelaide, Australia.
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24
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Regulation of mammalian CTP. ACTA ACUST UNITED AC 1996. [DOI: 10.1016/s1874-5245(96)80003-6] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register]
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25
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MacDonald JI, Possmayer F. Stimulation of phosphatidylcholine biosynthesis in mouse MLE-12 type-II cells by conditioned medium from cortisol-treated rat fetal lung fibroblasts. Biochem J 1995; 312 ( Pt 2):425-31. [PMID: 8526851 PMCID: PMC1136279 DOI: 10.1042/bj3120425] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/31/2023]
Abstract
Treatment of murine adult MLE-12 type-II and fetal-rat type-II cells with fetal-rat-fibroblast-conditioned medium (FFCM) resulted in a 2-fold stimulation of [14C]choline incorporation into phosphatidylcholine. Soluble CTP:phosphocholine cytidylyltransferase (CT) activity was increased approx. 3-fold in FFCM-treated fetal-rat type-II cells but was not changed in MLE-12 cells. Neither choline kinase nor cholinephosphotransferase activities were affected by treatment of MLE-12 cells with FFCM. Long-term labelling of MLE-12 cells with [14C]choline, followed by a 14-18 h chase with FFCM, resulted in a 2.5-fold decrease in [14C]phosphocholine levels relative to controls, suggesting that CT was being activated. In contrast, oleate treatment increased CT activity in the particulate fraction in both cells. Western blots indicate that soluble CT undergoes dephosphorylation in response to FFCM, but no translocation to the particulate fraction was noted. Treatment with oleate stimulated a marked translocation. Tryptic phosphopeptide maps from FFCM-treated cells revealed only minor alterations in the phosphorylation pattern. It is concluded that FFCM and oleate activate CT through different mechanisms. The results are consistent with FFCM activating CT in MLE-12 as well as fetal type-II cells. However, the reason why this activation cannot be detected in vitro is not known.
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Affiliation(s)
- J I MacDonald
- Department of Obstetrics and Gynaecology, University of Western Ontario, London, Canada
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26
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Wang Y, Kent C. Identification of an inhibitory domain of CTP:phosphocholine cytidylyltransferase. J Biol Chem 1995; 270:18948-52. [PMID: 7642553 DOI: 10.1074/jbc.270.32.18948] [Citation(s) in RCA: 54] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/26/2023] Open
Abstract
The function of the putative amphipathic helices between residues 236 and 314 of CTP:phosphocholine cytidylyltransferase was examined by constructing two truncation mutants; CT314 was missing the entire phosphorylation segment, whereas CT236 was missing both the region with the putative amphipathic helices and the phosphorylation segment. Stable cells lines expressing these truncation mutants in Chinese hamster ovary 58 cells were isolated and characterized. CT314 was predominantly soluble in control cells but became membrane-associated in cells treated with oleate, which also causes translocation of wild-type cytidylyltransferase. CT236 was found to be soluble both in control cells and in cells treated to cause translocation. These results strongly suggest that the membrane-binding site is located within residues 237-314. When assayed for activity in vitro, the mutant forms were catalytically active in the presence of exogenous lipids. CT236, moreover, was as active in the absence of lipids as in their presence, whereas CT314 required lipids for activity. The rate of phosphatidylcholine synthesis in cells expressing CT236 was considerably higher than in wild-type cells, consistent with the enzyme being constitutively active in the cells. These results indicate that residues 237-314 constitute an inhibitory segment; when this segment is removed from the catalytic domain by truncation or by binding to membranes, an inhibitory constraint is removed and cytidylyltransferase is activated.
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Affiliation(s)
- Y Wang
- Department of Biological Chemistry, University of Michigan Medical Center, Ann Arbor 48109-0606, USA
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27
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Wang Y, Kent C. Effects of altered phosphorylation sites on the properties of CTP:phosphocholine cytidylyltransferase. J Biol Chem 1995; 270:17843-9. [PMID: 7629086 DOI: 10.1074/jbc.270.30.17843] [Citation(s) in RCA: 56] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/26/2023] Open
Abstract
To investigate the role of phosphorylation and dephosphorylation in modulating the activity and location of CTP:phosphocholine cytidylyltransferase, we used site-directed mutagenesis to construct four mutant forms of cytidylyltransferase. These forms were 5SP-->AP, in which five of the seven Ser-Pro sequences were converted to Ala-Pro; 7SP-->AP, in which all of the seven Ser-Pro sequences converted to Ala-Pro; 16S-->A, in which all sixteen Ser residues that can be phosphorylated in wild type cytidylyltransferase were converted to Ala; and 16S-->E, in which all sixteen Ser residues were converted to Glu. The mutant enzymes were expressed in the strain 58 Chinese hamster ovary cell line, which is temperature-sensitive for growth and cytidylyltransferase activity. All mutant enzyme forms were enzymatically as active as the wild type when assayed under optimal conditions. In untreated cells, more of the Ser-->Ala mutants were membrane-associated than in cells expressing wild type enzyme, consistent with the phosphorylation state of the enzyme affecting its affinity for membranes. About half of the 16S-->A mutant remained soluble, however, indicating that dephosphorylation alone does not trigger membrane association. Although the amount of membrane-associated enzyme in the 16S-->A mutant was about 10-fold greater than that of wild type, phosphatidylcholine synthesis was increased by only about 75%, suggesting that membrane association does not necessarily cause full activation. All mutant forms, including the 16S-->E mutant, translocated to the particulate fraction upon oleate treatment, indicating that a high negative charge in the phosphorylation region does not preclude association of cytidylyltransferase with membranes. All mutant enzymes were able to support growth of strain 58 at 40 degrees C, and the rate of phosphatidylcholine synthesis was not greatly altered in the cell lines expressing mutant cytidylyltransferase forms. These results are consistent with a role for phosphorylation in the equilibrium distribution of cytidylyltransferase but suggest that changes in enzyme activity and location are not triggered exclusively by changes in the phosphorylation state.
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Affiliation(s)
- Y Wang
- Department of Biological Chemistry, University of Michigan Medical Center, Ann Arbor 48109, USA
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28
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Wang Y, MacDonald JI, Kent C. Identification of the nuclear localization signal of rat liver CTP:phosphocholine cytidylyltransferase. J Biol Chem 1995; 270:354-60. [PMID: 7814396 DOI: 10.1074/jbc.270.1.354] [Citation(s) in RCA: 81] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/27/2023] Open
Abstract
CTP:phosphocholine cytidylyltransferase (CT) is a major regulatory enzyme in phosphatidylcholine synthesis in mammalian cells. CT is found in both soluble and particulate forms, both of which are nuclear. We report here the identification of a 21-residue sequence at the amino terminus of CT, 8KVNSRKRRKEVPGPNGATEED28, which was sufficient to direct beta-galactosidase into the cell nucleus. Further deletions from either end of this sequence greatly reduced the nuclear localization of beta-galactosidase. Deletions of amino acids within the nuclear localization signal or of the entire signal disrupted CT nuclear localization, but CT was not completely excluded from the nucleus. Clones of stable transfectants of the nuclear localization signal-deficient CT expressed in Chinese hamster ovary (CHO) 58 cells, which is temperature-sensitive for growth and CT activity, were isolated and characterized. The deletion mutants were active under the same conditions as the wild-type enzyme. Despite the difference in subcellular location from wild-type CT, the nuclear localization mutants were fully able to complement the CT-deficient cell line CHO 58 for both growth and choline incorporation into phosphatidylcholine at the nonpermissive temperature. The mobility of the mutant enzymes on SDS gels was altered relative to the mobility of wild-type CT; however, the extent of phosphorylation of the mutant enzymes was decreased only slightly. Thus, the distribution of CT in both cytoplasm and nucleus, rather than exclusively nucleus, has little effect on the ability of CT to function in growing CHO cells.
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Affiliation(s)
- Y Wang
- Department of Biological Chemistry, University of Michigan Medical Center, Ann Arbor 48109-0606
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29
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Kalmar GB, Kay RJ, LaChance AC, Cornell RB. Primary structure and expression of a human CTP:phosphocholine cytidylyltransferase. BIOCHIMICA ET BIOPHYSICA ACTA 1994; 1219:328-34. [PMID: 7918629 DOI: 10.1016/0167-4781(94)90056-6] [Citation(s) in RCA: 40] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/27/2023]
Abstract
Human CTP:phosphocholine cytidylyltransferase (CT) cDNAs were isolated by PCR amplification of a human erythroleukemic K562 cell library. Initially two degenerate oligonucleotide primers derived from the sequence of the rat liver CT cDNA were used to amplify a centrally located 230 bp fragment. Subsequently overlapping 5' and 3' fragments were amplified, each using one human CT primer and one vector-specific primer. Two cDNAs encoding the entire translated domain were also amplified. The human CT (HCT) has close homology at the nucleotide and amino acid level with other mammalian CTs (from rat liver, mouse testis or mouse B6SutA hemopoietic cells and Chinese hamster ovary). The region which deviates most from the rat liver CT sequence is near the C-terminus, where 7 changes are clustered within 34 residues (345-359), of the putative phosphorylation domain. The region of the proposed catalytic domain (residues 75-235) is 100% identical with the rat liver sequence. Significant homology was observed between the proposed catalytic domain of CT and the Saccharomyces cerevisiae MUQ1 gene product, and between the proposed amphipathic alpha-helical membrane binding domains of CT and soybean oleosin, a phospholipid-binding protein. There are several shared characteristics of these amphipathic helices. An approx. 42,000 Da protein was over-expressed in COS cells using a pAX142 expression vector containing one of the full-length HCT cDNA clones. The specific activity of the HCT in COS cell homogenates was the same as that of analogously expressed rat liver CT. The activity of HCT was lipid dependent. The soluble form was activated 3 to 4-fold by anionic phospholipids and by oleic acid or diacylglycerol-containing PC vesicles.
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Affiliation(s)
- G B Kalmar
- Institute of Molecular Biology and Biochemistry, Simon Fraser University, Burnaby, BC, Canada
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30
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Tronchère H, Record M, Tercé F, Chap H. Phosphatidylcholine cycle and regulation of phosphatidylcholine biosynthesis by enzyme translocation. BIOCHIMICA ET BIOPHYSICA ACTA 1994; 1212:137-51. [PMID: 8180240 DOI: 10.1016/0005-2760(94)90248-8] [Citation(s) in RCA: 87] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/29/2023]
Affiliation(s)
- H Tronchère
- INSERM Unité 326, Hôpital Purpan, Toulouse, France
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31
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MacDonald J, Kent C. Identification of phosphorylation sites in rat liver CTP: phosphocholine cytidylyltransferase. J Biol Chem 1994. [DOI: 10.1016/s0021-9258(17)34092-9] [Citation(s) in RCA: 87] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022] Open
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32
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Craig L, Johnson J, Cornell R. Identification of the membrane-binding domain of rat liver CTP:phosphocholine cytidylyltransferase using chymotrypsin proteolysis. J Biol Chem 1994. [DOI: 10.1016/s0021-9258(17)41864-3] [Citation(s) in RCA: 43] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/22/2022] Open
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33
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Weinhold PA, Charles L, Feldman DA. Regulation of CTP: phosphocholine cytidylyltransferase in HepG2 cells: effect of choline depletion on phosphorylation, translocation and phosphatidylcholine levels. BIOCHIMICA ET BIOPHYSICA ACTA 1994; 1210:335-47. [PMID: 8305489 DOI: 10.1016/0005-2760(94)90238-0] [Citation(s) in RCA: 23] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/29/2023]
Abstract
We studied the effect of choline depletion on the biosynthesis of phosphatidylcholine (PC) and the distribution and phosphorylation of cytidylyltransferase (CT) in HepG2 cells. Phosphocholine concentrations decreased within 24 h of choline depletion to values less than 2% of controls. The incorporation of [3H]glycerol into PC was reduced in choline-depleted (CD) cells. The apparent turnover of PC was similar in CD and choline-supplemented (CS) cells (T1/2 = 20 h). The methylation pathway for PC synthesis increased nearly 10-fold in CD cells. Cell growth was similar in CD and CS cells. Over 95% of CT activity in CS cells was in the soluble pool. Choline depletion resulted in a progressive decrease in CT activity and immunodetected enzyme in the soluble pool and a corresponding increase in membrane CT over a 48-h period. Choline supplementation of CD cells caused a rapid release of membrane CT (complete release by 3 h). Two phosphorylated forms of CT were identified. One form contained a higher level of phosphorylation (HPCT) than the other form (LPCT). HPCT migrated slightly slower than LPCT on SDS gels. CD cells contained only LPCT in both soluble and membrane pools. CS cells contained only HPCT. During choline depletion PC content decreased nearly 20% but CT binding did not occur until LPCT was generated in cytosol. Conversely, choline supplementation released LPCT into cytosol and HPCT was formed only after the release. We conclude that both the induction of binding sites, perhaps by depletion of PC and dephosphorylation of HPCT to LPCT, are required for CT translocation to membranes. The release of CT from membranes is initiated by changes in membrane binding sites followed by trapping of the CT in the soluble pool by phosphorylation of LPCT to HPCT.
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34
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Sharma A, Gonzales LW, Ballard PL. Hormonal regulation of cholinephosphate cytidylyltransferase in human fetal lung. BIOCHIMICA ET BIOPHYSICA ACTA 1993; 1170:237-44. [PMID: 8218341 DOI: 10.1016/0005-2760(93)90005-t] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/29/2023]
Abstract
Cytidylyltransferase (CTP: cholinephosphate cytidylyltransferase, EC 2.7.7.15, CYT) is a regulatory enzyme for synthesis of pulmonary surfactant phosphatidylcholine (PC). The effects of glucocorticoid, T3, and cAMP on CYT activity were studied in explants of human fetal lung (18-22 weeks gestation) cultured for 1-6 days in serum-free medium. Dexamethasone (Dex, 10 nM) treatment for 5 days increased homogenate CYT activity (+115%, P < 0.02) when assayed in the presence of added lipid co-factor (L-alpha-phosphatidylglycerol, PG, 1.1 mM) and tended to increase activity in its absence (+77%, P = 0.12). Cytosolic activity was also significantly elevated in the presence of added co-factor (+124%, P < 0.01), but there was no effect of Dex on microsomal specific activity. Dex increased the recovery of CYT activity in the cytosolic fraction (75% vs. 43% (control) of the homogenate activity), but not in the microsomal, nuclear or mitochondrial fractions. Assayed in the presence of added co-factor, stimulation of CYT by Dex was apparent after 48 h exposure and maximal by 5-6 days exposure to < or = 30 nM concentration. T3 or agents that increase endogenous cAMP stimulated cytosolic activity by 40% and 36-74%, respectively, after 4-6 days exposure, but none produced an additive increase in the presence of Dex. We conclude that stimulation of CYT activity contributes to hormonal induction of surfactant lipids by each of these hormones. Glucocorticoids may increase the amount of CYT enzyme as well as activate the enzyme via increased synthesis of lipid co-factor.
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Affiliation(s)
- A Sharma
- Department of Pediatrics, University of California, San Francisco 94143
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35
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Houweling M, Tijburg LB, Vaartjes WJ, Batenburg JJ, Kalmar GB, Cornell RB, Van Golde LM. Evidence that CTP:choline-phosphate cytidylyltransferase is regulated at a pretranslational level in rat liver after partial hepatectomy. EUROPEAN JOURNAL OF BIOCHEMISTRY 1993; 214:927-33. [PMID: 8391439 DOI: 10.1111/j.1432-1033.1993.tb17996.x] [Citation(s) in RCA: 27] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/30/2023]
Abstract
Regulation of CTP:choline-phosphate cytidylyltransferase activity was studied in regenerating rat liver. The formation of phosphatidylcholine from [14C]choline in hepatocytes isolated from regenerating liver at 22 h after surgery was increased 1.9-fold when compared with hepatocytes from sham-operated animals. This effect was accompanied by a 1.4-fold increase in cytosolic cytidylyltransferase activity as well as by a 1.5-fold increase in the amount of immunoreactive cytidylyltransferase protein, and a 1.7-fold increase in [35S]methionine incorporation into cytidylyltransferase protein. Northern blot analysis of cytidylyltransferase mRNA showed two signals at 1.5 and 5.0 kb. Partial hepatectomy caused a significant 2-3-fold increase in the 1.5-kb and 5.0-kb messengers at 12 h after surgery. During the next 10 h after partial hepatectomy cytidylyltransferase mRNA levels slightly decreased, although they were still elevated in comparison with sham-operated rats 20-22 h after surgery. In contrast to the elevated cytidylyltransferase mRNA levels, the amount of acetyl-CoA carboxylase mRNA did not increase between 12 and 22 h after surgery, which is in line with the unchanged activity of this enzyme. In conclusion, our data demonstrate that in regenerating liver phosphatidylcholine biosynthesis and cytidylyltransferase activity are regulated at a pretranslational level.
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Affiliation(s)
- M Houweling
- Laboratory of Veterinary Biochemistry, Utrecht University, The Netherlands
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36
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Wieder T, Geilen CC, Reutter W. Antagonism of phorbol-ester-stimulated phosphatidylcholine biosynthesis by the phospholipid analogue hexadecylphosphocholine. Biochem J 1993; 291 ( Pt 2):561-7. [PMID: 8387273 PMCID: PMC1132561 DOI: 10.1042/bj2910561] [Citation(s) in RCA: 27] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/30/2023]
Abstract
The antagonization of phorbol 12-myristate 13-acetate (PMA)-stimulated phosphatidylcholine (PtdCho) biosynthesis by the phospholipid analogue hexadecylphosphocholine (HePC) in MDCK cells was investigated and compared with the corresponding influence in HeLa cells. In both cell lines, PMA-stimulated PtdCho biosynthesis was antagonized by 50 microM HePC. However, subsequent experiments provided evidence that PMA enhances PtdCho biosynthesis by at least two mechanisms: (i) by stimulation of choline uptake and (ii) by translocation of CTP:choline phosphate cytidylyltransferase to membranes. In MDCK cells, 5 nM PMA caused a 4-fold increase in [methyl-3H]choline incorporation into PtdCho, which was paralleled by an approx. 2-fold stimulation of choline uptake. These data indicate that choline uptake might play an important role in the regulation of PtdCho biosynthesis in this cell line, especially since we could not detect any significant increase in membrane-bound cytidyltransferase activity in PMA-treated MDCK cells. In contrast, enhanced PtdCho biosynthesis in HeLa cells is achieved by a 2-fold increase in particulate cytidylyltransferase activity after PMA stimulation. Translocation of cytidylyltransferase from the cytosol to membranes is therefore important in HeLa cells. Nevertheless, in both cell lines, the main target of HePC seems to be the translocation process. In MDCK cells, addition of 50 microM HePC decreases membrane-bound cytidylyltransferase activity by about 45%, compared with control cells and PMA-treated cells. In HeLa cells, PMA-induced translocation of cytidylyltransferase to membranes is totally abolished by HePC.
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Affiliation(s)
- T Wieder
- Institut für Molekularbiologie und Biochemie, Freie Universität Berlin, Germany
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Wang Y, MacDonald J, Kent C. Regulation of CTP:phosphocholine cytidylyltransferase in HeLa cells. Effect of oleate on phosphorylation and intracellular localization. J Biol Chem 1993. [DOI: 10.1016/s0021-9258(18)53350-0] [Citation(s) in RCA: 75] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/16/2022] Open
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Feldman D, Weinhold P. Identification of a protein complex between choline-phosphate cytidylyltransferase and a 112-kDa protein in rat liver. J Biol Chem 1993. [DOI: 10.1016/s0021-9258(18)53668-1] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022] Open
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Jamil H, Utal AK, Vance DE. Evidence that cyclic AMP-induced inhibition of phosphatidylcholine biosynthesis is caused by a decrease in cellular diacylglycerol levels in cultured rat hepatocytes. J Biol Chem 1992. [DOI: 10.1016/s0021-9258(18)46010-3] [Citation(s) in RCA: 71] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022] Open
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Utal A, Jamil H, Vance D. Diacylglycerol signals the translocation of CTP:choline-phosphate cytidylyltransferase in HeLa cells treated with 12-O-tetradecanoylphorbol-13-acetate. J Biol Chem 1991. [DOI: 10.1016/s0021-9258(18)54396-9] [Citation(s) in RCA: 52] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/22/2022] Open
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Weinhold PA, Charles LG, Feldman DA. Microsomal CTP:choline phosphate cytidylyltransferase: kinetic mechanism of fatty acid stimulation. BIOCHIMICA ET BIOPHYSICA ACTA 1991; 1086:57-62. [PMID: 1659454 DOI: 10.1016/0005-2760(91)90154-a] [Citation(s) in RCA: 17] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/28/2022]
Abstract
Fatty acids are known to cause an increase in the incorporation of radioactive choline into phosphatidylcholine. A coincident increase in membrane cytidylyltransferase activity is well documented. The purpose of the present studies was to determine the direct effects of oleic acid on the kinetic properties of membrane cytidylyltransferase. An examination of the reaction characteristics of membrane cytidylyltransferase revealed that membranes from adult rat lung contained high CTPase activity. This activity prevented the determination of reaction velocities at low CTP concentrations. The CTPase activity was blocked by the addition of ADP or ATP to the reaction. The addition of 6.0 mM ADP to the assay mixture enabled us to determine the effect of oleate on the CTP Km. Oleate (122 microM) caused a significant decrease in CTP Km for microsomal cytidylyltransferase (0.99 mM to 0.33 mM) and H-Form cytidylyltransferase (1.04 mM to 0.27 mM). Oleate did not decrease the CTP Km for L-Form cytidylyltransferase. Oleate had no effect on the choline phosphate Km in microsomal, H-Form or L-Form cytidylyltransferase. Oleate also increased the Vmax for cytidylyltransferase. The increase was dependent upon the concentration of oleate with a maximal increase of 50-60% at 100-130 microM oleate. We conclude that oleate has a direct stimulatory effect on cytidylyltransferase when it is in the active form (membrane bound or H-Form lipoprotein complex). We suggest that the kinetic effects operate synergistically with other regulatory mechanisms such as translocation or conversion of inactive to active species. The direct effect of oleate on the cytidylyltransferase may be an important regulatory mechanism when CTP concentrations are limiting.
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Weinhold PA, Charles L, Rounsifer ME, Feldman DA. Control of phosphatidylcholine synthesis in Hep G2 cells. Effect of fatty acids on the activity and immunoreactive content of choline phosphate cytidylyltransferase. J Biol Chem 1991. [DOI: 10.1016/s0021-9258(18)38088-8] [Citation(s) in RCA: 33] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/22/2022] Open
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Kalmar GB, Kay RJ, Lachance A, Aebersold R, Cornell RB. Cloning and expression of rat liver CTP: phosphocholine cytidylyltransferase: an amphipathic protein that controls phosphatidylcholine synthesis. Proc Natl Acad Sci U S A 1990; 87:6029-33. [PMID: 2166941 PMCID: PMC54465 DOI: 10.1073/pnas.87.16.6029] [Citation(s) in RCA: 116] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/30/2022] Open
Abstract
CTP:phosphocholine cytidylyltransferase (EC 2.7.7.15) is a key regulatory enzyme in the synthesis of phosphatidylcholine in higher eukaryotes. This enzyme can interconvert between an inactive cytosolic form and an active membrane-bound form. To unravel the structure of the transferase and the mechanism of its interaction with membranes, we have cloned a cytidylyltransferase cDNA from rat liver by the oligonucleotide-directed polymerase chain reaction. Transfection of the rat clone into COS cells resulted in a 10-fold increase in cytidylyltransferase activity and content. The activity of the transfected transferase was lipid-dependent. The central portion of the derived protein sequence of the rat clone is highly homologous to the previously determined yeast cytidylyltransferase sequence [Tsukagoshi, Y., Nikawa, J. & Yamashita, S. (1987) Eur. J. Biochem. 169, 477-486]. The rat protein sequence lacks any signals for covalent lipid attachment and lacks a hydrophobic domain long enough to span a bilayer. However, it does contain a potential 58-residue amphipathic alpha-helix, encompassing three homologous 11-residue repeats. We propose that the interaction of cytidylyltransferase with membranes is mediated by this amphipathic helix lying on the surface with its axis parallel to the plane of the membrane such that its hydrophobic residues intercalate the phospholipids.
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Affiliation(s)
- G B Kalmar
- Department of Chemistry/Biochemistry, Simon Fraser University, Burnaby, BC, Canada
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Feldman DA, Rounsifer ME, Charles L, Weinhold PA. CTP:phosphocholine cytidylyltransferase in rat lung: relationship between cytosolic and membrane forms. BIOCHIMICA ET BIOPHYSICA ACTA 1990; 1045:49-57. [PMID: 2164421 DOI: 10.1016/0005-2760(90)90202-9] [Citation(s) in RCA: 20] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/30/2022]
Abstract
The purpose of these studies was to determine the properties of the membrane-bound cytidylyltransferase in adult lung and to assess the relationship between the microsomal enzyme and the two forms of cytidylyltransferase in cytosol. Microsomes, isolated by glycerol density centrifugation, contained significantly less cytidylyltransferase than microsomes isolated by differential centrifugation (11.6 +/- 3.2 vs. 30 +/- 11 nmol/min per g lung). The released activity was recovered as H-form cytidylyltransferase. Cytidylyltransferase activity was not removed from microsomes by washing of the microsomal pellet with homogenizing buffer. Triton X 100 extracted all of the cytidylyltransferase from microsomes. The extracted activity was similar to H-form. Chlorpromazine dissociated microsomal enzyme to L-form. Chlorpromazine has been shown previously to dissociate H-form to L-form. These results suggested that microsomal cytidylyltransferase existed in a form similar if not identical to cytosolic H-form. In vitro translocation experiments demonstrated that the L-form of cytidylyltransferase was the species which binds to microsomal membranes. Triton X 100 extraction of microsomes from translocations experiments removed the bound enzyme activity. Glycerol density fractionation indicated that the activity in the Triton extract was H-form cytidylyltransferase. We concluded that the active lipoprotein form of cytidylyltransferase (H-form) is the membrane-associated form of cytidylyltransferase in adult lung; that it is formed after the L-form binds to microsomal membranes and that cytosolic H-form is released from the membrane.
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Rooney SA, Smart DA, Weinhold PA, Feldman DA. Dexamethasone increases the activity but not the amount of choline-phosphate cytidylyltransferase in fetal rat lung. BIOCHIMICA ET BIOPHYSICA ACTA 1990; 1044:385-9. [PMID: 2163682 DOI: 10.1016/0005-2760(90)90085-c] [Citation(s) in RCA: 26] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/30/2022]
Abstract
The activity of choline-phosphate cytidylyltransferase is increased by glucocorticoids in late gestation fetal lung in association with increased phosphatidylcholine biosynthesis. Previous indirect data had suggested that the stimulatory effect of the hormone was due to activation of existing enzyme rather than synthesis of new cytidylyltransferase protein. Using a rabbit antibody raised against purified rat liver choline-phosphate cytidylyltransferase, we have now quantitated the amount of the enzyme in fetal rat lung explants cultured with and without dexamethasone. Our results show that the hormone increased the activity of the enzyme but not the amount of cytidylyltransferase protein. Thus the stimulatory effect of dexamethasone on cytidylyltransferase is due to activation of existing enzyme rather than induction of enzyme synthesis.
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Affiliation(s)
- S A Rooney
- Department of Pediatrics, Yale University School of Medicine, New Haven, CT
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Affiliation(s)
- C Kent
- Department of Biochemistry, Purdue University, West Lafayette, IN 47907-6799
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