1
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Caldo KMP, Xu Y, Falarz L, Jayawardhane K, Acedo JZ, Chen G. Arabidopsis CTP:phosphocholine cytidylyltransferase 1 is phosphorylated and inhibited by sucrose nonfermenting 1-related protein kinase 1 (SnRK1). J Biol Chem 2019; 294:15862-15874. [PMID: 31439667 PMCID: PMC6816107 DOI: 10.1074/jbc.ra119.008047] [Citation(s) in RCA: 13] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/15/2019] [Revised: 08/19/2019] [Indexed: 11/06/2022] Open
Abstract
De novo phosphatidylcholine (PC) biosynthesis via the Kennedy pathway involves highly endergonic biochemical reactions that must be fine-tuned with energy homeostasis. Previous studies have shown that CTP:phosphocholine cytidylyltransferase (CCT) is an important regulatory enzyme in this pathway and that its activity can be controlled at both transcriptional and posttranslational levels. Here we identified an important additional mechanism regulating plant CCT1 activity. Comparative analysis revealed that Arabidopsis CCT1 (AtCCT1) contains catalytic and membrane-binding domains that are homologous to those of rat CCT1. In contrast, the C-terminal phosphorylation domain important for stringent regulation of rat CCT1 was apparently missing in AtCCT1. Instead, we found that AtCCT1 contains a putative consensus site (Ser-187) for modification by sucrose nonfermenting 1-related protein kinase 1 (SnRK1 or KIN10/SnRK1.1), involved in energy homeostasis. Phos-tag SDS-PAGE coupled with MS analysis disclosed that SnRK1 indeed phosphorylates AtCCT1 at Ser-187, and we found that AtCCT1 phosphorylation substantially reduces its activity by as much as 70%. An S187A variant exhibited decreased activity, indicating the importance of Ser-187 in catalysis, and this variant was less susceptible to SnRK1-mediated inhibition. Protein truncation and liposome binding studies indicated that SnRK1-mediated AtCCT1 phosphorylation directly affects the catalytic domain rather than interfering with phosphatidate-mediated AtCCT1 activation. Overexpression of the AtCCT1 catalytic domain in Nicotiana benthamiana leaves increased PC content, and SnRK1 co-expression reduced this effect. Taken together, our results suggest that SnRK1 mediates the phosphorylation and concomitant inhibition of AtCCT1, revealing an additional mode of regulation for this key enzyme in plant PC biosynthesis.
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Affiliation(s)
- Kristian Mark P Caldo
- Department of Agricultural, Food, and Nutritional Science, University of Alberta, Edmonton, Alberta T6G 2G2, Canada
| | - Yang Xu
- Department of Agricultural, Food, and Nutritional Science, University of Alberta, Edmonton, Alberta T6G 2G2, Canada
| | - Lucas Falarz
- Department of Agricultural, Food, and Nutritional Science, University of Alberta, Edmonton, Alberta T6G 2G2, Canada
| | - Kethmi Jayawardhane
- Department of Agricultural, Food, and Nutritional Science, University of Alberta, Edmonton, Alberta T6G 2G2, Canada
| | - Jeella Z Acedo
- Department of Chemistry, University of Alberta, Edmonton, Alberta T6G 2G2, Canada
| | - Guanqun Chen
- Department of Agricultural, Food, and Nutritional Science, University of Alberta, Edmonton, Alberta T6G 2G2, Canada
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2
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Membrane lipid compositional sensing by the inducible amphipathic helix of CCT. Biochim Biophys Acta Mol Cell Biol Lipids 2015; 1861:847-861. [PMID: 26747646 DOI: 10.1016/j.bbalip.2015.12.022] [Citation(s) in RCA: 52] [Impact Index Per Article: 5.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/09/2015] [Revised: 12/22/2015] [Accepted: 12/29/2015] [Indexed: 11/21/2022]
Abstract
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.
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3
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Cornell RB, Ridgway ND. CTP:phosphocholine cytidylyltransferase: Function, regulation, and structure of an amphitropic enzyme required for membrane biogenesis. Prog Lipid Res 2015; 59:147-71. [PMID: 26165797 DOI: 10.1016/j.plipres.2015.07.001] [Citation(s) in RCA: 91] [Impact Index Per Article: 10.1] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/07/2015] [Revised: 07/07/2015] [Accepted: 07/07/2015] [Indexed: 12/12/2022]
Abstract
CTP:phosphocholine cytidylyltransferase (CCT) catalyzes a rate-limiting and regulated step in the CDP-choline pathway for the synthesis of phosphatidylcholine (PC) and PC-derived lipids. Control of CCT activity is multi-layered, and includes direct regulation by reversible membrane binding involving a built-in lipid compositional sensor. Thus CCT contributes to phospholipid compositional homeostasis. CCT also modifies the curvature of its target membrane. Knowledge of CCT structure and regulation of its catalytic function are relatively advanced compared to many lipid metabolic enzymes, and are reviewed in detail. Recently the genetic origins of two human developmental and lipogenesis disorders have been traced to mutations in the gene for CCTα.
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Affiliation(s)
- Rosemary B Cornell
- Department of Molecular Biology and Biochemistry and the Department of Chemistry, Simon Fraser University, Burnaby, B.C. V5A-1S6, Canada.
| | - Neale D Ridgway
- Departments of Pediatrics, and Biochemistry and Molecular Biology, Atlantic Research Centre, Dalhousie University, Halifax, Nova Scotia B3H-4H7, Canada
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4
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Chong SSY, Taneva SG, Lee JMC, Cornell RB. The Curvature Sensitivity of a Membrane-Binding Amphipathic Helix Can Be Modulated by the Charge on a Flanking Region. Biochemistry 2014; 53:450-61. [DOI: 10.1021/bi401457r] [Citation(s) in RCA: 39] [Impact Index Per Article: 3.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/17/2023]
Affiliation(s)
- Sharon S. Y. Chong
- Department
of Molecular Biology and Biochemistry, Simon Fraser University, Burnaby, British Columbia, Canada V5A 1S6
| | - Svetla G. Taneva
- Department
of Molecular Biology and Biochemistry, Simon Fraser University, Burnaby, British Columbia, Canada V5A 1S6
| | - Joseph M. C. Lee
- Department
of Molecular Biology and Biochemistry, Simon Fraser University, Burnaby, British Columbia, Canada V5A 1S6
| | - Rosemary B. Cornell
- Department
of Molecular Biology and Biochemistry, Simon Fraser University, Burnaby, British Columbia, Canada V5A 1S6
- Department
of Chemistry, Simon Fraser University, Burnaby, British Columbia, Canada V5A 1S6
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5
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Dennis MK, Taneva SG, Cornell RB. The intrinsically disordered nuclear localization signal and phosphorylation segments distinguish the membrane affinity of two cytidylyltransferase isoforms. J Biol Chem 2011; 286:12349-60. [PMID: 21303909 DOI: 10.1074/jbc.m110.201715] [Citation(s) in RCA: 28] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
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.
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Affiliation(s)
- Melissa K Dennis
- Department of Molecular Biology and Biochemistry, Simon Fraser University, Burnaby, British Columbia, Canada
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6
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Inatsugi R, Kawai H, Yamaoka Y, Yu Y, Sekiguchi A, Nakamura M, Nishida I. Isozyme-Specific Modes of Activation of CTP:Phosphorylcholine Cytidylyltransferase in Arabidopsis thaliana at Low Temperature. ACTA ACUST UNITED AC 2009; 50:1727-35. [DOI: 10.1093/pcp/pcp115] [Citation(s) in RCA: 28] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/13/2022]
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7
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Ng MNP, Kitos TE, Cornell RB. Contribution of lipid second messengers to the regulation of phosphatidylcholine synthesis during cell cycle re-entry. Biochim Biophys Acta Mol Cell Biol Lipids 2004; 1686:85-99. [PMID: 15522825 DOI: 10.1016/j.bbalip.2004.09.001] [Citation(s) in RCA: 18] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/09/2004] [Revised: 07/29/2004] [Accepted: 09/01/2004] [Indexed: 10/26/2022]
Abstract
During entry into the cell cycle a phosphatidylcholine (PC) metabolic cycle is activated. We have examined the hypothesis that PC synthesis during the G(0) to G(1) transition is controlled by one or more lipid products of PC turnover acting directly on the rate-limiting enzyme in the synthesis pathway, CTP: phosphocholine cytidylyltransferase (CCT). The acceleration of PC synthesis was two- to threefold during the first hour after addition of serum to quiescent IIC9 fibroblasts. The rate increased to approximately 15-fold above the basal rate during the second hour. The production of arachidonic acid, diacylglycerol (DAG), and phosphatidic acid (PA) preceded the second, rapid phase of PC synthesis. However, an increase in the cellular content of these lipid mediators was detected only for DAG. CCT activation and translocation to membranes accompanied the second phase of the PC synthesis acceleration. Bromoenol lactone (BEL), an inhibitor of calcium-independent phospholipase A(2) and PA phosphatase, blocked production of fatty acids and DAG, inhibited both phases of the PC synthesis response to serum, and reduced CCT activity and membrane affinity. The effect of BEL on PC synthesis was partially reversed by in situ generation of DAG via exogenous PC-specific phospholipase C to generate approximately 2-fold elevation in PC-derived DAG. Exogenous arachidonic acid also partially reversed the inhibition by BEL, but only at a concentration that generated a supra-physiological cellular content of free fatty acid. 1-Butanol, which blocks PA production, had no effect on DAG generation, or on PC synthesis. We conclude that fatty acids and DAG could contribute to the initial slow phase of the PC synthesis response. DAG is the most likely lipid regulator of CCT activity and the rapid phase of PC synthesis. However, processes other than direct activation of CCT by lipid mediators likely contribute to the highly accelerated phase during entry into the cell cycle.
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Affiliation(s)
- Michael N P Ng
- Department of Molecular Biology and Biochemistry, Simon Fraser University, Burnaby, B.C., Canada V5A 1S6
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8
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Jackowski S, Wang J, Baburina I. Activity of the phosphatidylcholine biosynthetic pathway modulates the distribution of fatty acids into glycerolipids in proliferating cells. BIOCHIMICA ET BIOPHYSICA ACTA 2000; 1483:301-15. [PMID: 10666565 DOI: 10.1016/s1388-1981(99)00203-6] [Citation(s) in RCA: 73] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
PtdCho accumulation is a periodic, S phase-specific event that is modulated in part by cell cycle-dependent fluctuations in CTP:phosphocholine cytidylyltransferase (CCT) activity. A supply of fatty acids is essential to generate the diacylglycerol (DG) precursors for phosphatidylcholine (PtdCho) biosynthesis but it is not known whether the DG supply is also coupled to the cell cycle. Although the rate of fatty acid synthesis in a macrophage cell line was dramatically stimulated in response to the growth factor, CSF-1, it was not regulated by the cell cycle. Increased fatty acid synthesis correlated with elevated acetyl-CoA carboxylase (ACC) and fatty acid synthase (FAS) steady-state mRNA levels. Cellular fatty acid synthesis was essential for membrane PL synthesis. Cerulenin inhibition of endogenous fatty acid synthesis also inhibited PtdCho synthesis, which was not relieved by exogenous fatty acids. Inhibition of CCT activity by the addition of lysophosphatidylcholine (lysoPtdCho) or temperature-shift of a conditionally defective CCT diverted newly synthesized DG to the TG pool where it accumulated. Enforced expression of CCT stimulated PtdCho biosynthesis and reduced TG synthesis. Thus, the cellular DG supply did not regulate PtdCho biosynthesis and CCT activity governs the partitioning of DG into either the PL or TG pools, thereby controlling both PtdCho and TG biosynthesis.
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Affiliation(s)
- S Jackowski
- Department of Biochemistry St. Jude Children's Research Hospital, 332 North Lauderdale, Memphis, TN 38105-2794, USA.
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9
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Northwood IC, Tong AH, Crawford B, Drobnies AE, Cornell RB. Shuttling of CTP:Phosphocholine cytidylyltransferase between the nucleus and endoplasmic reticulum accompanies the wave of phosphatidylcholine synthesis during the G(0) --> G(1) transition. J Biol Chem 1999; 274:26240-8. [PMID: 10473578 DOI: 10.1074/jbc.274.37.26240] [Citation(s) in RCA: 78] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The transition from quiescence (G(0)) into the cell division cycle is marked by accelerated phospholipid turnover. We examined the rates of phosphatidylcholine (PC) synthesis and the activity, membrane affinity, and intracellular localization of the rate-limiting enzyme in the synthesis of PC, CTP:phosphocholine cytidylyltransferase (CT) during this transition. The addition of serum to quiescent IIC9 fibroblasts resulted in a wave of PC synthesis beginning at approximately 10 min, peaking at approximately 3 h with a >10-fold increase in rate, and declining to near basal rates by 10 h. CT activity, monitored in situ, was elevated approximately 3-fold between 1 and 2 h postserum. Neither CT mass nor its phosphorylation state changed during the surge in PC synthesis and CT activity. On the other hand, the ratio of particulate/soluble CT surged and then receded in concert with the wave of PC synthesis. During quiescence, CT was confined to the nucleus, as assessed by indirect immunofluorescence. Within 10 min after serum stimulation, a portion of the CT fluorescence appeared in the cytoplasm, where it intensified until approximately 4 h postserum. Thereafter, the cytoplasmic CT signal waned, while the nuclear signal increased, and by 8 h CT was once again predominantly nuclear. The dynamics of CT's apparent translocation in and out of the nucleus paralleled the wave of PC synthesis and the solubility changes of CT. Cytoplasmic CT co-localized with BiP, a resident endoplasmic reticulum protein, in a double labeling experiment. These data suggest that the wave of PC synthesis that accompanies the G(0) --> G(1) transition is regulated by the coordinated changes in CT activity, membrane affinity, and intracellular distribution. We describe for the first time a redistribution of CT from the nucleus to the ER that correlates with an activation of the enzyme. We propose that this movement is required for the stimulation of PC synthesis during entry into the cell cycle.
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Affiliation(s)
- I C Northwood
- Institute of Molecular Biology and Biochemistry and the Biochemistry Program, Simon Fraser University, Burnaby, British Columbia V5A 1S6, Canada
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10
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Lau KF, Miller CC, Anderton BH, Shaw PC. Expression analysis of glycogen synthase kinase-3 in human tissues. THE JOURNAL OF PEPTIDE RESEARCH : OFFICIAL JOURNAL OF THE AMERICAN PEPTIDE SOCIETY 1999; 54:85-91. [PMID: 10448973 DOI: 10.1034/j.1399-3011.1999.00083.x] [Citation(s) in RCA: 92] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/23/2022]
Abstract
Human glycogen synthase kinase-3 (GSK-3) is a multisubstrate, proline-directed kinase that phosphorylates tau, beta-amyloid and neurofilaments. In this study, the expression levels of the two GSK-3 isoforms, alpha and beta, RNA and proteins in different human tissues were examined. Northern analysis demonstrated that GSK-3alpha is encoded by a 2.6-kb mRNA and GSK-3beta by 8.3- and 2.8-kb mRNAs. The two GSK-3beta mRNA species were variably expressed in different tissues. Northern and quantitative polymerase chain reaction demonstrated that both GSK-3alpha and GSK-3beta mRNA were prominently expressed in testis, thymus, prostate and ovary but were low in adult lung and kidney. Western blot analysis showed that the 51-kDa GSK-3alpha protein was highly expressed in lung, ovary, kidney and testis, whereas the 46-kDa GSK-3beta protein was highly expressed in lung, kidney and brain. The differential expression of GSK-3alpha and GSK-3beta mRNA and proteins and the lack of relationship between transcription and translation in some tissues indicate that GSK-3alpha and GSK-3beta are subject to different means of regulation.
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Affiliation(s)
- K F Lau
- Department of Biochemistry, The Chinese University of Hong Kong, Shatin
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11
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Mason TM. The role of factors that regulate the synthesis and secretion of very-low-density lipoprotein by hepatocytes. Crit Rev Clin Lab Sci 1998; 35:461-87. [PMID: 9885772 DOI: 10.1080/10408369891234246] [Citation(s) in RCA: 62] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/25/2022]
Abstract
Lipoproteins are particles that contribute to overall metabolic homeostasis by transporting hydrophobic lipids in the blood plasma to and from different tissues in the body. Very-low-density lipoprotein (VLDL) is the principal vehicle for the transport of endogenous triglyceride (TG), and, ultimately, through its metabolic product, low-density lipoprotein (LDL), of cholesterol as well. It is synthesized mainly in hepatocytes, with small amounts also being produced by enterocytes in the fasting state. The mechanism of VLDL assembly is complex and is regulated at different levels by a variety of factors. The main structural protein of VLDL is called apolipoprotein B-100 (Apo B). Apo B formation and degradation therefore represent two major points of regulation of VLDL secretion. Hepatic levels of lipids such as phosphatidylcholine (PC), cholesteryl ester (CE), fatty acids (FA), and TG also affect VLDL synthesis. There are different views as to the specific mechanism by which each lipid class affects VLDL particle formation. In general, PC appears to promote the translocation of apo B from the cytosol to the lumen of the endoplasmic reticulum, a step that is crucial in the early stages of VLDL assembly. Apo B degradation is suppressed, and therefore VLDL secretion is enhanced, in the presence of elevated CE levels. For TG to be incorporated into the lipoprotein, it requires the action of a protein called microsomal triglyceride transfer protein (MTP). MTP might have a preference for TG comprised of FA with a certain degree of saturation. It becomes apparent that changes in diet that are accompanied by variations in the type of fats that are ingested affect VLDL formation and secretion. Regulation also occurs post-prandially in response to elevations in plasma insulin levels. Acute elevations in insulin inhibit VLDL secretion by promoting the degradation of apo B. This action is consistent with insulin's anabolic properties as it allows for the hepatic storage of lipid rather than for its distribution in VLDL to other tissues for fuel. Many studies have attempted to unravel the mechanisms of VLDL formation and secretion. The fact that so many factors are involved complicates the issue. The purpose of this article is to describe the relationship between different factors involved in VLDL assembly and secretion so that a better understanding of its metabolic regulation may be achieved.
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Affiliation(s)
- T M Mason
- Department of Physiology, University of Toronto, Canada
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12
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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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13
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Bladergroen BA, Wensing T, Van Golde LM, Geelen MJ. Reversible translocation of CTP:phosphocholine cytidylyltransferase from cytosol to membranes in the adult bovine liver around parturition. BIOCHIMICA ET BIOPHYSICA ACTA 1998; 1391:233-40. [PMID: 9555031 DOI: 10.1016/s0005-2760(98)00006-x] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/07/2023]
Abstract
The key regulatory enzyme of phosphatidylcholine (PC) synthesis, CTP:phosphocholine cytidylyltransferase (CT), is known to be activated in vitro by translocation from soluble to particulate fractions of the cell. In the present study the periparturient cow was chosen as a model to investigate whether translocation of CT can contribute to the regulation of PC synthesis in vivo. Between parturition and 1.5 weeks post-partum, the cytosolic CT activity in the liver of the adult animal decreased 1.9-fold, and this correlated with a 1.8-fold increase in microsomal CT activity. At that time, microsomal CT activity started to decline again whereas the cytosolic activity rose concomitantly until both activities reached their pre-partum values at 8 weeks post-partum. The activities of soluble and membrane-bound CTP:phosphoethanolamine cytidylyltransferase (ET), the analogous enzyme in the CDP-ethanolamine pathway, did not change significantly throughout this period. Whereas hepatic PC concentrations declined until about 2 weeks post-partum and thereafter gradually returned to pre-partum levels, the PC levels in very-low-density-lipoproteins, started to rise 2 weeks after the partus reaching a maximum of 219% of the original value at 8 weeks post-partum. These results strongly suggest that there is a reversible redistribution of CT between cytosol and membranes in a physiologically relevant animal model, supporting the concept that translocation of CT is occurring in vivo.
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Affiliation(s)
- B A Bladergroen
- Laboratory of Veterinary Biochemistry, Graduate School Animal Health, Utrecht University, P.O. Box 80.176, 3508 TD Utrecht, Netherlands
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14
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Feldman DA, Weinhold PA. Cytidylyltransferase-binding protein is identical to transcytosis-associated protein (TAP/p115) and enhances the lipid activation of cytidylyltransferase. J Biol Chem 1998; 273:102-9. [PMID: 9417053 DOI: 10.1074/jbc.273.1.102] [Citation(s) in RCA: 13] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/05/2023] Open
Abstract
We previously identified a protein from rat liver that binds CTP:phosphocholine cytidylyltransferase (CT). We have now purified this protein (cytidylyltransferase-binding protein (CTBP)) from rat liver. The purification involved precipitation at pH 5 and extraction of the precipitate with buffer, followed by sequential chromatography on DEAE-Sepharose and butyl-agarose. Final purification was accomplished by either preparative electrophoresis or hydroxylapatite chromatography. Amino acid sequences from six peptides derived from pure CTBP matched sequences in transcytosis-associated protein (TAP) with 98% identity. Thus, CTBP was positively identified to be TAP. Purified CTBP increased the activity of purified CT measured with phosphatidylcholine (PC)/oleic acid. In the absence of PC/oleic acid, CTBP did not stimulate CT activity. Dilution of CT to reduce the Triton X-100 concentration produced a loss of CT activity. The lost activity was recovered by the addition of CTBP plus PC/oleic acid to the assay, but not by the addition of either PC/oleic acid or CTBP alone. Removal of CTBP from purified preparations by immunoprecipitation with CTBP antibodies eliminated the activation of CT. Both CT and CTBP were shown to bind to PC/oleic acid liposomes. The formation of complexes between CT and CTBP in the absence of PC/oleic acid liposomes could not be demonstrated. These results suggest that CTBP functions to modify the interaction of CT with PC/oleic acid liposomes, resulting in an increase in the catalytic activity perhaps by the formation of a ternary complex between CT, CTBP, and lipid. Overall, these results suggest that CTBP (TAP) may function to coordinate the biosynthesis of phosphatidylcholine with vesicle transport.
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Affiliation(s)
- D A Feldman
- Veterans Administration Medical Center and the Department of Biological Chemistry, University of Michigan, Ann Arbor, Michigan 48105, USA
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15
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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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16
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Tang W, Keesler GA, Tabas I. The structure of the gene for murine CTP:phosphocholine cytidylyltransferase, Ctpct. Relationship of exon structure to functional domains and identification of transcriptional start sites and potential upstream regulatory elements. J Biol Chem 1997; 272:13146-51. [PMID: 9148929 DOI: 10.1074/jbc.272.20.13146] [Citation(s) in RCA: 46] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/04/2023] Open
Abstract
Phosphatidylcholine (PC) is the most abundant eukaryotic phospholipid and serves critical structural and cell-signaling functions. CTP:phosphocholine cytidylyltransferase (CT) is the rate-limiting enzyme in the CDP-choline pathway of PC biosynthesis, which is utilized by all tissues and is the sole or major PC biosynthetic pathway in all non-hepatic cells. Herein, we present the complete structure of the murine CT (Ctpct) gene. One P1 genomic clone and six subsequent plasmid subclones were isolated and analyzed for the exon-intron organization of the Ctpct gene. The gene spans approximately 26 kilobases and is composed of 9 exons and 8 introns. The exons match the distinct functional domains of the CT enzyme: exon 1 is untranslated; exon 2 codes for the nuclear localization signal domain; exons 4-7 encompass the catalytic domain; exon 8 codes for the alpha-helical membrane-binding domain; and exon 9 includes the C-terminal phosphorylation domain. Two transcriptional initiation sites, spaced 35 nucleotides apart, were identified using 5'-rapid amplification of cDNA ends polymerase chain reaction. The 5' natural flanking region was found to lack TATA or CAAT boxes and to contain GC-rich regions, which are features typical of promoters of housekeeping genes. Several sites that have the potential to interact with transcription regulatory factors, such as Sp1, AP1, AP2, AP3, Y1, and TFIIIA, were identified in the 5'-region of the gene and found to be distributed in two distinct clusters. These data will provide the basis for future studies on the cis- and trans-acting factors involved in Ctpct gene transcription and for the creation of induced mutant mouse models of altered CT activity.
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Affiliation(s)
- W Tang
- Department of Medicine, Columbia University College of Physicians and Surgeons, New York, New York 10032, USA
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17
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Wieprecht M, Wieder T, Paul C, Geilen CC, Orfanos CE. Evidence for phosphorylation of CTP:phosphocholine cytidylyltransferase by multiple proline-directed protein kinases. J Biol Chem 1996; 271:9955-61. [PMID: 8626633 DOI: 10.1074/jbc.271.17.9955] [Citation(s) in RCA: 35] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/31/2023] Open
Abstract
Reversible phosphorylation of CTP:phosphocholine cytidylyltransferase, the rate-limiting enzyme of phosphatidylcholine biosynthesis, is thought to play a role in regulating its activity. In the present study, the hypothesis that proline-directed kinases play a major role in phosphorylating cytidylyltransferase is substantiated using a c-Ha-ras-transfected clone of the human keratinocyte cell line HaCaT. Cellular extracts from epidermal growth factor-stimulated HaCaT cells and from ras-transfected HaCaT cells phosphorylated cytidylyltransferase much stronger as compared with extracts from quiescent HaCaT cells. The tryptic phosphopeptide pattern of cytidylyltransferase phosphorylated by cell-free extracts from ras-transfected HaCaT cells was similar compared with the patterns of cytidylyltransferase phosphorylated by p44mpkmitogen-activated protein kinase and p34cdc2 kinase in vitro, whereas in the case of casein kinase II the pattern was different. Furthermore, in c-Ha-ras-transfected HaCaT cells the in vivo phosphorylation state of cytidylyltransferase was 2-fold higher as compared with untransfected HaCaT cells. This higher phosphorylation of cytidylyltransferase in the ras-transfected clone was reduced to a level below the phosphorylation of cytidylyltransferase in untransfected cells, using olomoucine, a specific inhibitor of proline-directed kinases. The reduced phosphorylation of cytidylyltransferase in olomoucine-treated cells correlated with an enhanced stimulation of enzyme activity by oleic acid.
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Affiliation(s)
- M Wieprecht
- Institute of Molecular Biology and Biochemistry, University Medical Center Benjamin Franklin, Free University of Berlin, Berlin D-12200, Germany
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18
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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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19
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Shiratori Y, Houweling M, Zha X, Tabas I. Stimulation of CTP:phosphocholine cytidylyltransferase by free cholesterol loading of macrophages involves signaling through protein dephosphorylation. J Biol Chem 1995; 270:29894-903. [PMID: 8530387 DOI: 10.1074/jbc.270.50.29894] [Citation(s) in RCA: 47] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/31/2023] Open
Abstract
Free cholesterol-loaded macrophages in atheromata synthesize excess phosphatidylcholine (PC), which may be an important adaptive response to the excess free cholesterol (FC) load. We have recently shown that FC loading of macrophages leads to 2-4-fold increases in PC mass and biosynthesis and to the post-translational activation of the membrane-bound form of CTP:phosphocholine cytidylyltransferase (CT), a key enzyme in PC biosynthesis. Herein, we explore further the mechanism of CT activation in FC-loaded macrophages. First, enrichment of membranes from control macrophages with FC in vitro did not increase CT activity, and PC biosynthesis in vivo is up-regulated by FC loading even when CT and FC appear to be mostly in different intracellular sites. These data imply that FC activates membrane-bound CT by a signaling mechanism. That the proposed signaling mechanism involves structural changes in the CT protein was suggested by data showing that two different antibodies against synthetic CT peptides showed increased recognition of membrane-bound CT from FC-loaded cells despite no increase in CT protein. Since CT is phosphorylated, two-dimensional maps of peptides from 32P-labeled control and FC-loaded macrophages were compared: six peptide spots from membrane-bound CT, but none from soluble CT, were dephosphorylated in the FC-loaded cells. Furthermore, incubation of FC-loaded macrophages with the phosphatase inhibitor, calyculin A, blocked increases in both PC biosynthesis and antipeptide-antibody recognition of CT. Last, treatment of membranes from control macrophages with lambda phage protein phosphatase in vitro increased both CT activity (2-fold) and antipeptide-antibody recognition of CT; soluble CT activity and antibody recognition were not substantially affected by phosphatase treatment. In summary, FC loading of macrophages leads to the partial dephosphorylation of membrane-bound CT, and possibly other cellular proteins, which appears to be important in CT activation. This novel regulatory action of FC may allow macrophages to adapt to FC loading in atheromata.
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Affiliation(s)
- Y Shiratori
- Department of Medicine, Columbia University College of Physicians and Surgeons, New York, New York 10032, USA
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20
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Tran K, Man RY, Choy PC. The enhancement of phosphatidylcholine biosynthesis by angiotensin II in H9c2 cells. BIOCHIMICA ET BIOPHYSICA ACTA 1995; 1259:283-90. [PMID: 8541336 DOI: 10.1016/0005-2760(95)00175-1] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/31/2023]
Abstract
The effect of angiotensin II on the biosynthesis of phosphatidylcholine in rat heart myoblastic (H9c2) cells was investigated. Cells were incubated with [methyl-3H]choline, and the labelling of phosphatidylcholine at different time intervals was examined. When cells were pretreated with angiotensin II, a significant increase in the labelling of phosphatidylcholine was observed. Analysis of the labelled phosphatidylcholine precursors indicated that the conversion of phosphocholine to CDP-choline was enhanced by angiotensin II treatment. Determination of enzyme activities in the CDP-choline pathway revealed that the activities of choline kinase or CDP-choline: diacylglycerol cholinephosphotransferase were not changed, but the activities of CTP:phosphocholine cytidylyltransferase were stimulated in both the particulate and soluble fractions. The stimulation of the cytidylyltransferase by angiotensin II was not abolished by okadaic acid, indicating that the activation of the enzyme was not mediated via the okadaic-sensitive dephosphorylation mechanism. Alternatively, the stimulation of the cytidylyltransferase activity was completely abolished by protein kinase C inhibitors. Immunoblotting studies revealed that levels of the cytidylyltransferase in the soluble and particulate fractions were not affected by angiotensin II treatment. We conclude that the increase in phosphatidylcholine biosynthesis by angiotensin II was a direct result of the enhancement of the cytidylyltransferase activity. The enhancement of enzyme activity was not mediated via enzyme translocation, but by a mechanism which was intimately associated with the protein kinase C cascade.
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Affiliation(s)
- K Tran
- Department of Biochemistry and Molecular Biology, University of Manitoba, Winnipeg, Canada
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21
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Sproull SA, Morash SC, Byers DM, Cook HW. Phorbol ester stimulation of phosphatidylcholine synthesis in four cultured neural cell lines: correlations with expression of protein kinase C isoforms. Neurochem Res 1995; 20:1397-407. [PMID: 8789601 DOI: 10.1007/bf00970587] [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: 02/02/2023]
Abstract
Phosphatidylcholine (PtdCho) can provide lipid second messengers involved in signal transduction pathways. As a measure of phospholipid turnover in response to extracellular stimulation, we investigated differential enhancement of [3H]choline incorporation into PtdCho by phorbol esters. In C6 rat glioma and SK-N-SH human neuroblastoma cells, [3H]PtdCho synthesis was 2-4 fold stimulated by beta-12-O-tetradecanoylphorbol-13-acetate (beta-TPA) when [3H]choline was incubated simultaneously with, or 15 min prior to, beta-TPA treatment. By contrast, in N1E-115 mouse and SK-N-MC human neuroblastoma cells, phorbol esters had no appreciable effect on [3H]choline incorporation; however, in all cells, 200 microM oleic acid enhanced PtdCho synthesis, indicating a stimulable process. Alterations by thymeleatoxin (TMT), an activator of conventional PKC isoforms (alpha, beta and gamma), were similar to beta-TPA. We investigated whether expression of specific PKC isoforms might correlate with these effects of phorbol esters on PtdCho synthesis. All cell lines bound phorbol esters, had PKC activity that was translocated by phorbol esters and differentially expressed isoforms of PKC. Northern and western blot analyses, using specific cDNA and antibodies for PKC-alpha, -beta, -gamma, -delta, -epsilon, and -zeta, revealed that expression of alpha-isoform predominated in C6 and SK-N-SH cells. In contrast, TPA-responsive beta-isoform predominated in SK-N-MC cells. gamma-PKC was not detected in any cells and only in C6 cells was PKC-delta present and translocated by beta-TPA treatment. PKC-epsilon was not detected in SK-N-MC cell lines but translocated with TPA treatment in the other three cell lines. PKC-zeta was present in all cells but was unaltered by TPA treatment. Accordingly, stimulation of PtdCho turnover by phorbol esters correlated only with expression of PKC-alpha; presence of PKC-beta alone was insufficient for a TPA response.
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Affiliation(s)
- S A Sproull
- Department of Pediatrics, Dalhousie University, Halifax, Nova Scotia, Canada
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22
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Yang W, Boggs KP, Jackowski S. The association of lipid activators with the amphipathic helical domain of CTP:phosphocholine cytidylyltransferase accelerates catalysis by increasing the affinity of the enzyme for CTP. J Biol Chem 1995; 270:23951-7. [PMID: 7592590 DOI: 10.1074/jbc.270.41.23951] [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
The biochemical mechanism for the regulation of enzyme activity by lipid modulators and the role of the amphipathic alpha-helical domain of CTP:phosphocholine cytidylyltransferase (CT) was investigated by analyzing the kinetic properties of the wild-type protein and two truncation mutants isolated from a baculovirus expression system. The CT[delta 312-367] mutant protein lacked the carboxyl-terminal phosphorylation domain and retained high catalytic activity along with both positive and negative regulation by lipid modulators. The CT[delta 257-367] deletion removed in addition the region containing three consecutive amphipathic alpha-helical repeats. The CT[delta 257-367] mutant protein exhibited a significantly lower specific activity compared to CT or CT[delta 312-367] when expressed in either insect or mammalian cells; however, CT[delta 257-367] activity was refractory to either stimulation or inhibition by lipid regulators. Lipid activators accelerated CT activity by decreasing the Km for CTP from 24.7 mM in their absence to 0.7 mM in their presence. The Km for phosphocholine was not affected by lipid activators. The activity of CT[delta 257-367] was comparable to the activity of wild-type CT in the absence of lipid activators and the CTP Km for CT[delta 257-367] was 13.9 mM. The enzymatic properties of the CT[delta 231-367] mutant were comparable to those exhibited by the CT[257-367] mutant indicating that removal of residues 231 through 257 did not have any additional influence on the lipid regulation of the enzyme. Thus, the region between residues 257 and 312 was required to confer lipid regulation on CT, and the association of activating lipids with this region of the protein stimulated catalysis by increasing the affinity of the enzyme for CTP.
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Affiliation(s)
- W Yang
- Department of Biochemistry, St. Jude Children's Research Hospital, Memphis, Tennessee 38101, USA
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23
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Yang W, Jackowski S. Lipid activation of CTP:phosphocholine cytidylyltransferase is regulated by the phosphorylated carboxyl-terminal domain. J Biol Chem 1995; 270:16503-6. [PMID: 7622451 DOI: 10.1074/jbc.270.28.16503] [Citation(s) in RCA: 57] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/26/2023] Open
Abstract
The role of the phosphorylated carboxyl-terminal domain of CTP:phosphocholine cytidylyltransferase (CT) in the regulation of enzyme activity was investigated by comparing the catalytic properties of wild-type CT to two mutant proteins with altered carboxyl-terminal phosphorylation domains. CT isolated from a baculovirus expression system was extensively phosphorylated at multiple sites in the carboxyl-terminal domain. The CT[S315A] mutant lacked a major CT phosphorylation site, and the carboxyl-terminal deletion mutant, CT[delta 312-367], was not phosphorylated. The higher activities of CT[delta 312-367] and CT[S315A] relative to CT were attributed to differences in the sensitivities of the enzymes to lipid activators. The rank order of the apparent Km values for activation by either phosphatidylcholine/oleic acid or phosphatidylcholine/diacylglycerol was CT > CT[S315A] > CT[delta 312-367]. In addition, CT exhibited negative cooperativity in its activation by phosphatidylcholine/oleic acid (nH = 0.64) and phosphatidylcholine/diacylglycerol (nH = 0.74) vesicles, whereas CT[delta 312-367] and CT[S315A] did not. These data support the concept that the phosphorylation of the CT carboxyl-terminal domain interferes with the activation of CT by lipid regulators.
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Affiliation(s)
- W Yang
- Department of Biochemistry, St. Jude Children's Research Hospital, Memphis, Tennessee 38101, USA
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24
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Sparks JD, Sparks CE. Insulin regulation of triacylglycerol-rich lipoprotein synthesis and secretion. BIOCHIMICA ET BIOPHYSICA ACTA 1994; 1215:9-32. [PMID: 7948013 DOI: 10.1016/0005-2760(94)90088-4] [Citation(s) in RCA: 154] [Impact Index Per Article: 5.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/28/2023]
Abstract
This review has considered a number of observations obtained from studies of insulin in perfused liver, hepatocytes, transformed liver cells and in vivo and each of the experimental systems offers advantages. The evaluation of insulin effects on component lipid synthesis suggests that overall, lipid synthesis is positively influenced by insulin. Short-term high levels of insulin through stimulation of intracellular degradation of freshly translated apo B and effects on synthesis limit the ability of hepatocytes to form and secrete TRL. The intracellular site of apo B degradation may involve membrane-bound apo B, cytoplasmic apo B and apo B which has entered the ER lumen. How insulin favors intracellular apo B degradation is not known. An area of recent investigation is in insulin-stimulated phosphorylation of intracellular substrates such as IRS-1 which activates insulin specific cellular signaling molecules [245]. Candidate molecules to study insulin action on apo B include IRS-1 and SH2-containing signaling molecules. Insulin dysregulation in carbohydrate metabolism occurs in non-insulin-dependent diabetes mellitus due to an imbalance between insulin sensitivity of tissue and pancreatic insulin secretion (reviewed in Refs. [307,308]). Insulin resistance in the liver results in the inability to suppress hepatic glucose production; in muscle, in impaired glucose uptake and oxidation and in adipose tissue, in the inability to suppress release of free FA. This lack of appropriate sensitivity towards insulin action leads to hyperglycemia which in turn stimulates compensatory insulin secretion by the pancreas leading to hyperinsulinemia. Ultimately, there may be failure of the pancreas to fully compensate, hyperglycemia worsens and diabetes develops. The etiology of insulin resistance is being intensively studied for the primary defect may be over secretion of insulin by the pancreas or tissue insulin resistance and both of these defects may be genetically predetermined. We suggest that, in addition to effects in carbohydrate metabolism, insulin resistance in liver results in the inability of first phase insulin to suppress hepatic TRL production which results in hypertriglyceridemia leading to high levels of plasma FA which accentuate insulin resistance in other target organs. As recently reviewed [17,254] the role of insulin as a stimulator of hepatic lipogenesis and TRL production has been long established. Several lines of evidence support that insulin is stimulatory to the production of hepatic TRL in vivo. First, population based studies support a positive relationship between plasma insulin and total TG and VLDL [253]. Second, there is a strong association between chronic hyperinsulinemia and VLDL overproduction [309].(ABSTRACT TRUNCATED AT 400 WORDS)
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Affiliation(s)
- J D Sparks
- Department of Pathology, University of Rochester, School of Medicine and Dentistry, NY 14642
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25
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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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26
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Cao SG, Hatch GM. Stimulation of phosphatidylglycerolphosphate phosphatase activity by unsaturated fatty acids in rat heart. Lipids 1994; 29:475-80. [PMID: 7968268 DOI: 10.1007/bf02578244] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/28/2023]
Abstract
Phosphatidylglycerolphosphate (PGP) synthase and PGP phosphatase catalyze the sequential synthesis of phosphatidylglycerol from cytidine-5'-diphosphate 1,2-diacyl-sn-glycerol (CDP-DG) and glycerol-3-phosphate. PGP synthase and PGP phosphatase activities were characterized in rat heart mitochondrial fractions, and the effect of fatty acids on the activity of these enzymes was determined. PGP synthase was observed to be a heat labile enzyme that exhibited apparent Km values for CDP-PG and glycerol-3-phosphate of 46 and 20 microM, respectively. The addition of exogenous oleic acid to the assay mixture did not affect PGP synthase activity. PGP phosphatase was observed to be a heat labile enzyme, and addition of oleic acid to the assay mixture caused a concentration-dependent stimulation of PGP phosphatase activity. Maximum stimulation (1.9-fold) of enzyme activity was observed in the presence of 0.5 mM oleic acid, but the stimulation was slightly attenuated by the presence of albumin in the assay. The presence of oleic acid in the assay mixture caused the inactivation of PGP phosphatase activity to be retarded at 55 degrees C. Stimulation of PGP phosphatase activity was also observed with arachidonic acid, whereas taurocholic, stearic and palmitic acids did not significantly affect PGP phosphatase activity. The activity of mitochondrial phosphatidic acid phosphohydrolase was not affected by inclusion of oleic acid in the incubation mixture. We postulate that unsaturated fatty acids stimulate PGP phosphatase activity in rat heart.
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Affiliation(s)
- S G Cao
- Department of Internal Medicine, University of Manitoba, Winnipeg, Canada
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27
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Kawashima Y, Mizuguchi H, Musoh K, Kozuka H. The mechanism for the increased supply of phosphatidylcholine for the proliferation of biological membranes by clofibric acid, a peroxisome proliferator. BIOCHIMICA ET BIOPHYSICA ACTA 1994; 1212:311-8. [PMID: 8199202 DOI: 10.1016/0005-2760(94)90205-4] [Citation(s) in RCA: 16] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/29/2023]
Abstract
The metabolic changes induced by p-chlorophenoxyisobutyric acid (clofibric acid), a peroxisome proliferator, in hepatic glycerolipids for the supply of membrane phospholipids were studied. The administration of clofibric acid to rats caused hepatomegaly and an increase in hepatic contents of phosphatidylcholine (PtdCho) (1.13-fold on the basis of g liver and 1.50-fold on the basis of whole liver). The administration of the drug enhanced the formation in vivo of PtdCho from [3H]glycerol, which seemed to be due to the increase in activity of CTP:phosphocholine cytidylyltransferase. On the other hand, clofibric acid depressed the activity of phosphatidylethanolamine N-methyltransferase. The in vivo study using [3H]glycerol revealed that clofibric acid slightly reduced the secretion of PtdCho into circulation. On the other hand, the drug did not affect the turnover of PtdCho. These results may elucidate the metabolic alterations by which clofibric acid increases hepatic mass of PtdCho. The facilitated biosynthesis of PtdCho by the drug seemed to lead to the increased formation of phosphatidylserine and subsequently phosphatidylethanolamine. Physiological significance of the alterations in glycerolipid metabolism by clofibric acid was discussed in relation to biological action of the drug.
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Affiliation(s)
- Y Kawashima
- Faculty of Pharmaceutical Sciences, Toyama Medical and Pharmaceutical University, Japan
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28
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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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29
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Houweling M, Jamil H, Hatch G, Vance D. Dephosphorylation of CTP-phosphocholine cytidylyltransferase is not required for binding to membranes. J Biol Chem 1994. [DOI: 10.1016/s0021-9258(17)37321-0] [Citation(s) in RCA: 51] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/16/2022] Open
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30
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31
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Chu AJ. Differential regulations of phosphatidylcholine biosynthesis in U937 cells by inhibitors of protein and tyrosine kinases. THE INTERNATIONAL JOURNAL OF BIOCHEMISTRY 1994; 26:189-93. [PMID: 8174753 DOI: 10.1016/0020-711x(94)90144-9] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/29/2023]
Abstract
1. The differential effects of inhibitors of protein kinase (PK) or tyrosine kinase (TK) on phosphatidylcholine (PC) biosynthesis in monocyte-like U937 cells were compared in pulse-chase-studies in which the cells prelabelled with [3H]choline for 30 min were chased in the absence or presence of kinase inhibitors. 2. PKA inhibitor (H-89) decreased the label incorporation into PC, while PKA activator (8-BrcAMP) had no effect. 3. PKC inhibitors (chelerythrine and hypericin) inhibited PC biosynthesis; on the other hand, PKC activator (SC-10) was stimulatory. 4. The inhibition of PC biosynthesis by H-89 and chelerythrine was accompanied by the inactivation of CTP: cholinephosphate cytidylyltransferase (CT). 5. In contrast, TK inhibitor (genistein) markedly stimulated CT and PC biosynthesis, while erbstatin and tyrphostin No. 25 showed no effect.
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Affiliation(s)
- A J Chu
- Miami Heart Research Institute, FL 33140-2999
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32
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Walkey C, Kalmar G, Cornell R. Overexpression of rat liver CTP:phosphocholine cytidylyltransferase accelerates phosphatidylcholine synthesis and degradation. J Biol Chem 1994. [DOI: 10.1016/s0021-9258(17)37524-5] [Citation(s) in RCA: 72] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022] Open
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33
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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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34
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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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35
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Chu AJ. Mechanism by which ethanol inhibits phosphatidylcholine biosynthesis in human leukemic monocyte-like U937 cells. Cell Biochem Funct 1994; 12:45-55. [PMID: 8168230 DOI: 10.1002/cbf.290120107] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/29/2023]
Abstract
A previous study showing that ethanol (ETOH) blocked [3H]choline incorporation into phosphatidylcholine (PC) suggested an inhibition of PC biosynthesis in human leukemic monocyte-like U937 cells. The mechanism of the inhibitory action of ETOH was investigated. Cells were pulsed with [3H]choline for 30 min and chased in the presence or absence of ETOH for up to 6 h. PC biosynthesis was inhibited drastically within 1 h after exposure to ETOH which increased intracellular cAMP appreciably. After a 3-h treatment, ETOH significantly inhibited both choline kinase (CK) and the cytosolic CTP: cholinephosphate cytidylyltransferase (CT). The inactivated CT was no longer stimulated by exogenous phosphatidylglycerol (PG). There was no evidence for redistribution of CT activity between cytosol and microsomes. When cells were exposed to 8-Bromo-cAMP ranging from 100 to 300 microM, PC biosynthesis remained unaffected despite the drastically elevated cAMP. These results seem to suggest that the raised cAMP is not a prerequisite for the inhibition of PC biosynthesis in U937 cells. Following pretreatment with protein kinase inhibitors (H-89 and K-252a), PC biosynthesis was decreased significantly and the inhibitory effect of ETOH was potentiated. Taken together, our results suggest that the inhibition of PC biosynthesis and the inhibitory effect of ETOH are independent of the activation of cAMP-dependent protein kinase. Unlike protein kinase inhibitors, pretreatment with tyrosine kinase inhibitors (erbstatin, genistein and tyrphostin 25) resulted in differential effects on PC biosynthesis and on the inhibitory action of ETOH. Genistein stimulated PC biosynthesis by 30 per cent as well as partially preventing/reversing the ETOH action, while tyrphostin 25 produced a synergistic inhibition. The relevance of tyrosine phosphorylation/dephosphorylation to the regulation of PC biosynthesis and ETOH action remains to be established.
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Affiliation(s)
- A J Chu
- Miami Heart Institute, Miami Beach, FL 33140-2999
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Affiliation(s)
- D Allan
- Department of Physiology, University College London Medical School, U.K
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