CTP:phosphorylcholine cytidylyltransferase from rat liver. Isolation and characterization of the catalytic subunit

From masochistic enzymology to mechanistic physiology and disease

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

Alveolar Proteinosis and Phospholipidoses of the Lungs

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.

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

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

Physiological consequences of disruption of mammalian phospholipid biosynthetic genes

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

Sp-1 Binds Promoter Elements That Are Regulated by Retinoblastoma and Regulate CTP:Phosphocholine Cytidylyltransferase-α Transcription

The retinoblastoma (Rb) protein is implicated in transcriptional regulation of at least five cellular genes. Co-transfection of Rb and truncated promoter constructs has defined a discrete element (retinoblastoma control element (RCE)) within the promoters of each of these genes as being necessary for Rb-mediated transcriptional control. In the present report we demonstrate that two RCEs identified within the CTP:phosphocholine cytidylyltransferase-alpha (CTalpha) proximal promoter are essential to promote transcription. Mutations that abolished each RCE markedly decreased CTalpha transcription. Co-transfection of Rb and truncated promoter constructs demonstrated that Rb regulates CTalpha expression by different mechanisms depending on the phase of the cell cycle. The regulation of CTalpha expression by Rb required both the Sp1 and the RCEs. Maximal expression occurred when both Rb and Sp1 were overexpressed. Moreover, RCEs were required for Rb association with the DNA. This regulatory mechanism alters CTalpha activity and thereafter changes PC availability and cell physiology. This is the first report demonstrating not only that surrounding Sp1 binding sites alter regulation mediated by Rb, but also that the expression of a gene involved in PC biosynthesis shares a common regulatory pathway with genes responsible for cell growth and differentiation.

Sp1 Is a Co-activator with Ets-1, and Net Is an Important Repressor of the Transcription of CTP:Phosphocholine Cytidylyltransferase α

Phosphatidylcholine biosynthesis via the CDP-choline pathway is primarily regulated by CTP:phosphocholine cytidylyltransferase (CT) encoded by the Pcyt1a and Pcyt1b genes. Previously, we identified an Ets-1-binding site located at -49/-47 in the promoter of Pcyt1a as an important transcriptional element involved in basal CTalpha transcription (Sugimoto, H., Sugimoto, S., Tatei, K., Obinata, H., Bakovic, M., Izumi, T., and Vance, D. E. (2003) J. Biol. Chem. 278, 19716-19722). In this study, we determined whether or not there were other important elements and binding proteins for basal CTalpha transcription in the Pcyt1a promoter, and if other Ets family proteins bind to the Ets-1-binding site. The results indicate the formation of a ternary complex with Ets-1 binding at -49/-47 and Sp1 binding at -58/-54 of the Pcyt1a promoter that is important for activating CTalpha transcription. When nuclear extracts of COS-7 cells expressing various Ets family repressors were incubated with DNA probes, binding of Net to the probes was observed. Net dose-dependently depressed the promoter-luciferase activity by 98%, even when co-expressed with Ets-1. RNA interference targeting Net caused an increase of endogenous CTalpha mRNA. After synchronizing the cell cycle in NIH3T3 cells, CTalpha mRNA increased at the S-M phase corresponding to an increase of Ets-1 mRNA and a decrease of Net mRNA. These results indicated that Net is an important endogenous repressor for CTalpha transcription.

Regulatory enzymes of phosphatidylcholine biosynthesis: a personal perspective

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

Phosphorylation of Sp1 by cyclin-dependent kinase 2 modulates the role of Sp1 in CTP:phosphocholine cytidylyltransferase alpha regulation during the S phase of the cell cycle

Phosphatidylcholine is the major lipid component in mammalian membranes. Phosphatidylcholine synthesis increases in C3H10T1/2 fibroblasts during the G(1) and S phases of the cell cycle. Previous studies demonstrated that the mRNA encoding CTP:phosphocholine cytidylyltransferase alpha (CTalpha) increases during S phase (Golfman, L. S., Bakovic, M., and Vance, D. E. (2001) J. Biol. Chem. 276, 43688-43692) and that this activation is driven by increased binding of Sp1 to the CTalpha promoter (Banchio, C., Schang, L. M., and Vance, D. E. (2003) J. Biol. Chem. 278, 32457-32464). We now demonstrate that cyclin-dependent kinase 2 (CDK2) phosphorylation of Sp1 activates CTalpha transcription during S phase. Sp1 binds in a phosphorylated state to the CTalpha promoter. Sp1 binding is enhanced by association with cyclin A/E and CDK2, both in vivo and in vitro. In cells that overexpress Sp1, co-expression of cyclin A and CDK2 induces a high and constant level of CTalpha expression, whereas reduction in the expression of cyclin A, cyclin E, and CDK2 eliminates the induction of CTalpha expression in S phase. Furthermore, CTalpha expression is decreased in cells overexpressing a dominant-negative form of CDK2 and in cells treated with the CDK2 kinase inhibitors roscovitine and olomoucine. These results enhance our understanding of the regulatory mechanisms involved in the expression of CTalpha in preparation for cell division.

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

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

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

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

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

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

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

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

Enzymatic and cellular characterization of a catalytic fragment of CTP:phosphocholine cytidylyltransferase alpha

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.

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.

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.

Primary structure and expression of a human CTP:phosphocholine cytidylyltransferase

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.

Developmental changes in cholinephosphate cytidylyltransferase activity and microsomal phospholipid fatty acid composition in alveolar type II cells

Cholinephosphate cytidylyltransferase is the rate-limiting enzyme in the choline pathway of phosphatidylcholine synthesis in fetal and adult lung. To examine the developmental changes in cytidylyltransferase activity, subcellular fractions were prepared from freshly isolated type II cells from 19 and 21 d gestation fetal rats, newborn (0 h, 3 h, 3-12 h, +1, +2, +7 postnatal day), and adult rats and the fractions assayed for cytidylyltransferase activity. Cytidylyltransferase activity per cell was low during late fetal gestation, but rose rapidly during the first 3 h after birth, predominantly due to an increase in microsomal enzyme activity. Microsomal and cytosolic enzyme specific activity increased 6 and 3.9 fold, respectively, from birth (0 h) to +1 postnatal day. The subcellular distribution of enzyme activity changed from 40% microsomal in fetal type II cells to 85% microsomal at 3-12 h, and 60% in this compartment in adult type II cells. Although evidence suggests that membrane lipid may affect enzyme activity, developmental changes in type II cell membrane phospholipid fatty acid composition have not been previously studied. Capillary gas chromatography analysis of phospholipid fatty acids extracted from microsome fractions revealed a 3-fold increase in both total saturated and unsaturated phospholipid fatty acids from day 19 to day 21 gestation. There was a further 62% increase in total saturated fatty acids during the first postnatal day, concomitant with the peak in microsomal cytidylyltransferase activity.

Evidence that CTP:choline-phosphate cytidylyltransferase is regulated at a pretranslational level in rat liver after partial hepatectomy

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.

CDPcholine:1,2-diacylglycerol cholinephosphotransferase from rat liver microsomes. I. Solubilization and characterization of the partially purified enzyme and the possible existence of an endogenous inhibitor

The solubilization and partial purification of cholinephosphotransferase (CDPcholine:1,2-diacylglycerol cholinephosphotransferase, EC 2.7.8.2) from rat liver microsomes were examined in the presence of ionic (sodium deoxycholate), nonionic (Triton X-100, n-octylglycoside), or zwitter ionic (CHAPS) detergents. Among the four detergents tested, only sodium deoxycholate was found to be an efficient solubilizer of cholinephosphotransferase activity from microsomal membranes, whereas the other three detergents caused irreversible inactivation of the enzyme at the solubilization step. Addition of phospholipids at the solubilization step, or after solubilization of the membrane proteins, could not preserve or reconstitute activity to any extent. The sodium deoxycholate-solubilized activity was partially purified by gel permeation chromatography (Superose 12HR). The partially purified preparation appeared to consist of a large aggregate containing phospholipids; further dissociation of the protein-phospholipid complex caused complete inactivation of the enzyme. The partially purified cholinephosphotransferase showed a specific activity of 100-130 nmol/min/mg protein, which is the highest activity reported to date from any tissue source; this amounts to a 4-fold enrichment of cholinephosphotransferase activity from the original KCl-washed rat liver microsomes. Ethanolaminephosphotransferase (CDPethanolamine:1,2-diacylglycerol ethanolaminephosphotransferase, EC 2.7.8.1) activity was copurified and 6-fold enriched with a total recovery of 60%. During the purification of cholinephosphotransferase activity, a putative endogenous inhibitor of cholinephosphotransferase was also solubilized and was isolated from the microsomal membranes. This heat-labile, nondialyzable inhibitor was shown to act specifically on cholinephosphotransferase and not on ethanolaminephosphotransferase. Further characterization of the inhibitory activity revealed that it may act at the binding step of the cholinephosphotransferase to its lipid substrate, diacylglycerol.

Biogenesis of endoplasmic reticulum transport vesicles transferring gastric apomucin from ER to Golgi

Rough endoplasmic reticulum (RER) transport vesicles were generated from gastric mucous cell RER microsomes in the presence of labeled precursors of phospholipids. The vesicles contained 7-10% of their proteins in the form of apomucin (cargo), and 80% of de novo synthesized phosphatidylcholine (PC) was incorporated into the vesicular membrane. In the absence of choline and ethanolamine precursors or in the presence of 3 mM N-ethylmaleimide (NEM), an inhibitor of CTP:phosphocholine cytidylyltransferase, formation of the transport vesicles, their enrichment in the newly synthesized PC, and the total synthesis of PC decreased by 86%, whereas in the presence of 3 mM Zn2+, complete blockage of vesicle formation and PC synthesis was observed. Analysis of the mucin-transporting vesicles indicated that the CTP:phosphocholine cytidylyltransferase and 1,2-diacyl-sn-glycerol:CDP-choline phosphotransferase remained associated with transport vesicles released from ER. The enzymes and other proteins separated from the vesicle surface prior to vesicle fusion with Golgi and the process was induced by phosphorylation. Based on the results of this study, it is proposed that the formation of the ER transport vesicles of gastric mucosal cells is in concert with synthesis of phospholipids and thus in part is regulated by phospholipid-synthesizing enzymes that reside on the membrane during its biogenesis and dissociate from its surface once the task is completed.

Microsomal CTP:choline phosphate cytidylyltransferase: kinetic mechanism of fatty acid stimulation

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.

Expression in Escherichia coli of the Saccharomyces cerevisiae CCT gene encoding cholinephosphate cytidylyltransferase

The coding region of the CCT gene from the yeast Saccharomyces cerevisiae was cloned into the pUC18 expression vector. The plasmid directed the synthesis of an active cholinephosphate cytidylyltransferase in Escherichia coli, confirming that CCT is the structural gene for this enzyme. The enzyme produced in E. coli efficiently utilized cholinephosphate and N,N-dimethylethanolaminephosphate, but N-methylethanolamine-phosphate and ethanolaminephosphate were poor substrates. Consistently, disruption of the CCT locus in the wild-type yeast cells resulted in a drastic decrease in activities with respect to the former two substrates. When activity was expressed in E. coli, over 90% was recovered in the cytosol, whereas most of the activity of yeast cells was associated with membranes, suggesting that yeast cells possess a mechanism that promotes membrane association of cytidylyltransferase.

CTP:phosphocholine cytidylyltransferase in rat lung: relationship between cytosolic and membrane forms

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.

Dexamethasone increases the activity but not the amount of choline-phosphate cytidylyltransferase in fetal rat lung

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.

Stimulation of CTP: phosphocholine cytidylyltransferase and phosphatidylcholine synthesis by incubation of rat hepatocytes with phospholipase A2

The effect of phospholipase A2 treatment of rat hepatocytes on CTP: phosphocholine cytidylyltransferase and phosphatidylcholine synthesis was investigated. Cytidylyltransferase is recovered from the cytosol and in a membrane-bound form with the microsomes. Digitonin treatment of cells causes rapid release into the medium of the cytosolic, but not the microsomal form of the cytidylyltransferase. Incubation of hepatocytes for 10 min with phospholipase A2 (0.9 units/dish) in the medium, resulted in a 33% decrease in the cytidylyltransferase activity released by digitonin treatment (2.5 +/- 0.15 nmol/min per mg compared to 3.9 +/- 0.10 nmol/min per mg in the control). In agreement with the digitonin experiments, incubation with 0.9 units/dish of phospholipase A2 resulted in a decrease in the cytidylyltransferase activity in the cytosol (from 4.3 +/- 0.10 nmol/min per mg to 2.6 +/- 0.14 nmol/min per mg) and a corresponding increase in the microsomal fraction (from 0.9 +/- 0.16 nmol/min per mg to 1.8 +/- 0.20 nmol/min per mg). The effect of phospholipase A2 on cytidylyltransferase translocation was concentration- and time-dependent. Incubation of hepatocytes in the presence of phospholipase A2 (0.9 units/dish) for 10 min prior to pulse-chase experiments resulted in an increase in radiolabel incorporation into phosphatidylcholine (from 2.4 +/- 0.02.10(-5) dpm/dish to 3.1 +/- 0.1.10(-5) dpm/dish) and a corresponding decrease in radiolabel associated with the choline (from 2.5 +/- 0.05.10(-5) to 1.4 +/- 0.03.10(-5) dpm) and phosphocholine fractions (from 8.5 +/- 0.07.10(-5) to 6.9 +/- 0.05.10(-5) dpm). We conclude that phospholipase A2 can cause a stimulation of CTP: phosphocholine cytidylyltransferase activity and phosphatidylcholine synthesis in cultured rat hepatocytes.

Synergistic activation of CTP:phosphocholine cytidylyltransferase by phosphatidylethanolamine and oleic acid

CTP:phosphocholine cytidylyltransferase present in rat liver cytosol was activated almost 30-fold when assayed in the presence of liposomes containing 60 mole % dioleoyl phosphatidylethanolamine (DOPE). During the assay, some of the DOPE was degraded to lysoPE and oleic acid. Whereas cytidylyltransferase activity was not affected when assayed in the presence of liposomes containing lysoPE, liposomes containing oleic acid activated the enzyme. Activation by oleic acid could be eliminated by the addition of fatty acid-free bovine serum albumin (BSA) to the assay. When cytidylyltransferase activity was measured in the presence of both BSA and liposomes containing DOPE, enzyme activity was increased almost 20-fold, as compared with assays performed in the absence of added lipid. The 1.5-fold difference in cytidylyltransferase activity observed when cytosol was assayed with DOPE containing liposomes in the absence or presence of BSA (30-fold stimulation vs 20-fold stimulation) cannot be explained by the loss of activation attributable to oleic acid alone. Activation of the enzyme in the presence of liposomes containing DOPE and oleic acid is several-fold greater than the sum of the activations caused by the individual compounds. These data suggest that PE and oleic acid act synergistically in activating the cytidylyltransferase.

Characterization of cytosolic forms of CTP: choline-phosphate cytidylyltransferase in lung, isolated alveolar type II cells, A549 cell and Hep G2 cells

The subcellular forms of cytidylyltransferase (EC 2.7.7.15) in rat lung, rat liver, Hep G2 cells, A549 cells and alveolar Type II cells from adult rats were separated by glycerol density centrifugation. Cytosol prepared from lung, Hep G2 cells, A549 cells and alveolar Type II cells contained two forms of the enzyme. These species were identical to the L-Form and H-Form isolated previously from lung cytosol by gel filtration. Liver cytosol contained only the L-Form. Rapid treatment of Hep G2 cells with digitonin released all of the cytoplasmic cytidylyltransferase activity. The released activity was present in both H-Form and L-Form. The molecular weight of L-Form was determined from sedimentation coefficients and Stokes radius values to be 97,690 +/- 10,175. Thus, the L-Form appears to be a dimer of the Mr 45,000 catalytic subunit. The f/f degrees value of 1.5 indicated that the protein molecule has an axial ratio of 10, assuming a prolate ellipsoid shape. The estimated molecular weight of the H-Form was 284,000 +/- 25,000. The H-Form was dissociated into L-Form by incubation of cytosol at 37 degrees C. Triton X-100 (0.1%) and chlorpromazine (1.0 mM) also dissociated the H-Form into L-Form. Western blot analysis indicated that both forms contained the catalytic subunit. An increase in Mr 45,000 subunit coincided with the increase in cytidylyltransferase activity in L-Form, which resulted from the dissociated of H-Form. The L-Form was dependent on phospholipid for activity. The H-Form was active without lipid. Phosphatidylinositol was present in the H-Form isolated from Hep G2 cells. The phosphatidylinositol dispersed when the H-Form was dissociated into L-Form. Phosphatidylinositol and phosphatidylglycerol cause L-Form to aggregate into a form similar to H-Form. Phosphatidylcholine/oleic acid (1:1 molar ratio) and oleic acid also aggregated the L-Form. Phosphatidylcholine did not produce aggregation. We conclude that the H-Form is the active form of cytidylyltransferase in cytoplasm. The H-Form appears to be a lipoprotein consisting of an apoprotein (L-Form dimer of the Mr 45,000 subunit) complexed with lipids. A change in the relative distribution of H-Form and L-Form in cytosol would alter the cellular activity and thus may be important in the regulation of phosphatidylcholine synthesis.

Partial purification and characterization of CTP:cholinephosphate cytidylyltransferase from castor bean endosperm

CTP: cholinephosphate cytidylyltransferase (EC 2.7.7.15) has been purified approximately 600-fold from postgermination endosperm of castor bean. The enzyme was solubilized with n-octyl beta-D-glucopyranoside and then subjected to ion exchange and gel filtration chromatography. The Km's of the purified enzymatic activity were 0.37 and 1.1 mM for CTP and choline phosphate, respectively. Magnesium was required for activity. The purified cytidylyltransferase activity was inhibited by both phosphate and ATP. The extent of ATP inhibition was dependent on preincubation time, temperature, and Mg2+ and Ca2+ concentrations. The possible regulation of cytidylyltransferase in castor bean endosperm by protein phosphorylation is discussed.