Identification of a protein complex between choline-phosphate cytidylyltransferase and a 112-kDa protein in rat liver

Enhancement of mdr2 gene transcription mediates the biliary transfer of phosphatidylcholine supplied by an increased biosynthesis in the pravastatin-treated rat

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.

CTP:phosphocholine cytidylyltransferase: insights into regulatory mechanisms and novel functions

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.

Interactions among pathways for phosphatidylcholine metabolism, CTP synthesis and secretion through the Golgi apparatus

Phosphatidylcholine is the major phospholipid in eukaryotic cells. It serves as a structural component of cell membranes and a reservoir of several lipid messengers. Recent studies in yeast and mammalian systems have revealed interrelationships between the two pathways of phosphatidylcholine metabolism, and between these pathways and those for CTP synthesis and secretion via the Golgi. These processes involve the regulation of the CDP-choline and phosphatidylethanolamine-methylation pathways of phosphatidylcholine synthesis, CTP synthetase, phospholipase D and the phospholipid-transfer protein Sec14p.

Studies on the regulation of CTP:phosphocholine cytidylyltransferase using permeabilized HEP G2 cells: evidence that both active and inactive enzyme are membrane-bound

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.

Cytidylyltransferase-binding protein is identical to transcytosis-associated protein (TAP/p115) and enhances the lipid activation of cytidylyltransferase

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.

CTP:phosphocholine cytidylyltransferase

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.

Identification of the nuclear localization signal of rat liver CTP:phosphocholine cytidylyltransferase

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.

Regulation of CTP: phosphocholine cytidylyltransferase in HepG2 cells: effect of choline depletion on phosphorylation, translocation and phosphatidylcholine levels

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.