Synthesis of phosphatidylethanolamine and ethanolamine plasmalogen by the CDP-ethanolamine and decarboxylase pathways in rat heart, kidney and liver

Phosphatidylethanolamine homeostasis under conditions of impaired CDP-ethanolamine pathway or phosphatidylserine decarboxylation

Phosphatidylethanolamine is the major inner-membrane lipid in the plasma and mitochondrial membranes. It is synthesized in the endoplasmic reticulum from ethanolamine and diacylglycerol (DAG) by the CDP-ethanolamine pathway and from phosphatidylserine by decarboxylation in the mitochondria. Recently, multiple genetic disorders that impact these pathways have been identified, including hereditary spastic paraplegia 81 and 82, Liberfarb syndrome, and a new type of childhood-onset neurodegeneration-CONATOC. Individuals with these diseases suffer from multisystem disorders mainly affecting neuronal function. This indicates the importance of maintaining proper phospholipid homeostasis when major biosynthetic pathways are impaired. This study summarizes the current knowledge of phosphatidylethanolamine metabolism in order to identify areas of future research that might lead to the development of treatment options.

Mutations in PCYT2 disrupt etherlipid biosynthesis and cause a complex hereditary spastic paraplegia

CTP:phosphoethanolamine cytidylyltransferase (ET), encoded by PCYT2, is the rate-limiting enzyme for phosphatidylethanolamine synthesis via the CDP-ethanolamine pathway. Phosphatidylethanolamine is one of the most abundant membrane lipids and is particularly enriched in the brain. We identified five individuals with biallelic PCYT2 variants clinically characterized by global developmental delay with regression, spastic para- or tetraparesis, epilepsy and progressive cerebral and cerebellar atrophy. Using patient fibroblasts we demonstrated that these variants are hypomorphic, result in altered but residual ET protein levels and concomitant reduced enzyme activity without affecting mRNA levels. The significantly better survival of hypomorphic CRISPR-Cas9 generated pcyt2 zebrafish knockout compared to a complete knockout, in conjunction with previously described data on the Pcyt2 mouse model, indicates that complete loss of ET function may be incompatible with life in vertebrates. Lipidomic analysis revealed profound lipid abnormalities in patient fibroblasts impacting both neutral etherlipid and etherphospholipid metabolism. Plasma lipidomics studies also identified changes in etherlipids that have the potential to be used as biomarkers for ET deficiency. In conclusion, our data establish PCYT2 as a disease gene for a new complex hereditary spastic paraplegia and confirm that etherlipid homeostasis is important for the development and function of the brain.

Altered ethanolamine plasmalogen and phosphatidylethanolamine levels in blood plasma of patients with bipolar disorder

AIM

Ethanolamine-containing phospholipids are synthesized in endoplasmic reticulum (ER) and mitochondria. ER stress and mitochondrial dysfunction have been implicated in bipolar disorder (BP). In this study, we aimed to examine the relationship of ethanolamine plasmalogen (PLE) and phosphatidylethanolamine (PTE) levels in blood plasma with BP.

METHODS

Plasma PLE and PTE levels were compared between 34 patients with BP (DSM-IV) and 38 healthy control participants matched for age, sex, and ethnicity (Japanese). Furthermore, the relationships of plasma PLE and PTE levels with clinical variables were explored.

RESULTS

Plasma PLE levels were significantly lower in patients with BP than in healthy controls (P = 0.0033). In subgroup analyses, plasma PLE levels were significantly lower in patients with BP type I (BP I) than in healthy controls (P = 0.0047); furthermore, plasma PTE levels were significantly lower in patients with BP I than in controls (P = 0.016) and patients with BP type II (BP II) (P = 0.010). Receiver-operating characteristic curve analysis revealed that the discriminatory power of plasma PTE levels for distinguishing between BP I and II was fair (area under the curve = 0.78; P = 0.0095). There were no significant correlations of plasma PLE or PTE levels with depression or manic symptoms in patients.

CONCLUSIONS

Plasma PLE and PTE levels were associated with BP I, but not with BP II. Moreover, plasma PTE levels differed between patients with BP I and II. Our findings highlight the importance of ethanolamine phospholipids in the pathophysiology of BP, especially BP I.

Serum Metabolomic Alterations Associated with Proteinuria in CKD

BACKGROUND AND OBJECTIVES

Data are scarce on blood metabolite associations with proteinuria, a strong risk factor for adverse kidney outcomes. We sought to investigate associations of proteinuria with serum metabolites identified using untargeted profiling in populations with CKD.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

Using stored serum samples from the African American Study of Kidney Disease and Hypertension (AASK; n=962) and the Modification of Diet in Renal Disease (MDRD) study (n=620), two rigorously conducted clinical trials with per-protocol measures of 24-hour proteinuria and GFR, we evaluated cross-sectional associations between urine protein-to-creatinine ratio and 637 known, nondrug metabolites, adjusting for key clinical covariables. Metabolites significantly associated with proteinuria were tested for associations with CKD progression.

RESULTS

In the AASK and the MDRD study, respectively, the median urine protein-to-creatinine ratio was 80 (interquartile range [IQR], 28-359) and 188 (IQR, 54-894) mg/g, mean age was 56 and 52 years, 39% and 38% were women, 100% and 7% were black, and median measured GFR was 48 (IQR, 35-57) and 28 (IQR, 18-39) ml/min per 1.73 m2. Linear regression identified 66 serum metabolites associated with proteinuria in one or both studies after Bonferroni correction (P<7.8×10-5), 58 of which were statistically significant in a meta-analysis (P<7.8×10-4). The metabolites with the lowest P values (P<10-27) were 4-hydroxychlorthalonil and 1,5-anhydroglucitol; all six quantified metabolites in the phosphatidylethanolamine pathway were also significant. Of the 58 metabolites associated with proteinuria, four were associated with ESKD in both the AASK and the MDRD study.

CONCLUSIONS

We identified 58 serum metabolites with cross-sectional associations with proteinuria, some of which were also associated with CKD progression.

PODCAST

This article contains a podcast at https://www.asn-online.org/media/podcast/CJASN/2019_02_07_CJASNPodcast_19_03_.mp3.

A mutation of EPT1 (SELENOI) underlies a new disorder of Kennedy pathway phospholipid biosynthesis

Mutations in genes involved in lipid metabolism have increasingly been associated with various subtypes of hereditary spastic paraplegia, a highly heterogeneous group of neurodegenerative motor neuron disorders characterized by spastic paraparesis. Here, we report an unusual autosomal recessive neurodegenerative condition, best classified as a complicated form of hereditary spastic paraplegia, associated with mutation in the ethanolaminephosphotransferase 1 (EPT1) gene (now known as SELENOI), responsible for the final step in Kennedy pathway forming phosphatidylethanolamine from CDP-ethanolamine. Phosphatidylethanolamine is a glycerophospholipid that, together with phosphatidylcholine, constitutes more than half of the total phospholipids in eukaryotic cell membranes. We determined that the mutation defined dramatically reduces the enzymatic activity of EPT1, thereby hindering the final step in phosphatidylethanolamine synthesis. Additionally, due to central nervous system inaccessibility we undertook quantification of phosphatidylethanolamine levels and species in patient and control blood samples as an indication of liver phosphatidylethanolamine biosynthesis. Although this revealed alteration to levels of specific phosphatidylethanolamine fatty acyl species in patients, overall phosphatidylethanolamine levels were broadly unaffected indicating that in blood EPT1 inactivity may be compensated for, in part, via alternate biochemical pathways. These studies define the first human disorder arising due to defective CDP-ethanolamine biosynthesis and provide new insight into the role of Kennedy pathway components in human neurological function.

Analysis of Plasmalogen Synthesis in Cultured Cells

Plasmalogen synthesis can be analyzed by metabolic labeling, followed by the separation of ethanolamine plasmalogens from glycerophospholipids on one-dimensional thin-layer chromatography. The vinyl-ether bond of plasmalogens is acid-labile, which allows separating plasmalogens as 2-acyl-glycerophospholipids from diacyl-glycerophospholipids.

Homeostasis of phospholipids - The level of phosphatidylethanolamine tightly adapts to changes in ethanolamine plasmalogens

Ethanolamine plasmalogens constitute a group of ether glycerophospholipids that, due to their unique biophysical and biochemical properties, are essential components of mammalian cellular membranes. Their importance is emphasized by the consequences of defects in plasmalogen biosynthesis, which in humans cause the fatal disease rhizomelic chondrodysplasia punctata (RCDP). In the present lipidomic study, we used fibroblasts derived from RCDP patients, as well as brain tissue from plasmalogen-deficient mice, to examine the compensatory mechanisms of lipid homeostasis in response to plasmalogen deficiency. Our results show that phosphatidylethanolamine (PE), a diacyl glycerophospholipid, which like ethanolamine plasmalogens carries the head group ethanolamine, is the main player in the adaptation to plasmalogen insufficiency. PE levels were tightly adjusted to the amount of ethanolamine plasmalogens so that their combined levels were kept constant. Similarly, the total amount of polyunsaturated fatty acids (PUFAs) in ethanolamine phospholipids was maintained upon plasmalogen deficiency. However, we found an increased incorporation of arachidonic acid at the expense of docosahexaenoic acid in the PE fraction of plasmalogen-deficient tissues. These data show that under conditions of reduced plasmalogen levels, the amount of total ethanolamine phospholipids is precisely maintained by a rise in PE. At the same time, a shift in the ratio between ω-6 and ω-3 PUFAs occurs, which might have unfavorable, long-term biological consequences. Therefore, our findings are not only of interest for RCDP but may have more widespread implications also for other disease conditions, as for example Alzheimer's disease, that have been associated with a decline in plasmalogens.

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PE accurately compensates for the lack of plasmalogens in vitro and in vivo.

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PE levels decrease to adapt to excess of ethanolamine plasmalogens (PlsEtn).

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Plasmalogen deficiency favors incorporation of arachidonic acid into PE.

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Docosahexaenoic acid in ethanolamine phospholipids decreases upon PlsEtn depletion.

Breast cancer cells adapt to metabolic stress by increasing ethanolamine phospholipid synthesis and CTP:ethanolaminephosphate cytidylyltransferase-Pcyt2 activity

The significance of phosphatidylethanolamine (PE) in breast cancer cell metabolism was investigated under stress conditions caused by serum deficiency. Serum deficient MCF-7 cells adapt to stress conditions by increasing synthesis and content of PE and diacylglycerol (DAG). The biosynthesis of PE from DAG and ethanolamine was regulated at the level of formation of CDP-ethanolamine, the metabolic step catalyzed by Pcyt2. The catalytic activity of Pcyt2 was elevated 2-3-fold, yet the enzyme remained rate-limiting in serum-deficient cells. Contributions to the elevated Pcyt2 activity included transcriptional and translational components. The mRNA levels of two splice variants, Pcyt2α and Pcyt2β, were 1.5-3-fold higher in deficient cells. The total amounts of Pcyt2 and Pcyt2α proteins were similarly elevated 1.5-2.5-fold. In vivo [γ(32)Pi] radiolabeling revealed that Pcyt2 was additionally regulated by phosphorylation. Under unfavorable metabolic conditions, both endogenous and His/Myc-tagged Pcyt2 were increasingly phosphorylated at Ser residues. The results established that elevated DAG formation and the increased activity of the rate-regulatory enzyme Pcyt2 were critical modulators of the PE Kennedy pathway, and total PE content in serum deprived breast cancer cells. Therefore, as an essential gene sensitive to nutritional microenvironment, Pcyt2 could represent a legitimate target in novel metabolic strategies for cancer.

Alterations in phosphatidylethanolamine levels affect the generation of Aβ

Several studies suggest that the generation of Aβ is highly dependent on the levels of cholesterol within membranes' detergent-resistant microdomains (DRM). Indeed, the β-amyloid precursor protein (APP) cleaving machinery, namely β- and γ-secretases, has been shown to be present in DRM and its activity depends on membrane cholesterol levels. Counterintuitive to the localization of the cleavage machinery, the substrate, APP, localizes to membranes' detergent-soluble microdomains enriched in phospholipids (PL), indicating that Aβ generation is highly dependent on the capacity of enzyme and substrate to diffuse along the lateral plane of the membrane and therefore on the internal equilibrium of the different lipids of DRM and non-DRM domains. Here, we studied to which extent changes in the content of a main non-DRM lipid might affect the proteolytic processing of APP. As phosphatidylethanolamine (PE) accounts for the majority of PL, we focused on its impact on the regulation of APP proteolysis. In mammalian cells, siRNA-mediated knock-down of PE synthesis resulted in decreased Aβ owing to a dual effect: promoted α-secretase cleavage and decreased γ-secretase processing of APP. In vivo, in Drosophila melanogaster, genetic reduction in PL synthesis results in decreased γ-secretase-dependent cleavage of APP. These results suggest that modulation of the membrane-soluble domains could be a valuable alternative to reduce excessive Aβ generation.

Functional analysis of two isoforms of phosphatidylethanolamine N-methyltransferase

The enzyme catalysing the conversion of PE (phosphatidylethanolamine) into PC (phosphatidylcholine), PEMT (PE N-methyltransferase), exists as two isoforms, PEMT-L (longer isoform of PEMT) and PEMT-S (shorter isoform of PEMT). In the present study, to compare the functions of the two isoforms of PEMT, we established HEK (human embryonic kidney)-293 cell lines stably expressing PEMT-L and PEMT-S. Both PEMT-L and PEMT-S were localized in the ER (endoplasmic reticulum). PEMT-L, but not PEMT-S, was N-glycosylated with high-mannose oligosaccharides. The enzymatic activity of PEMT-S was much higher than that of PEMT-L. By using novel enzymatic assays for measuring PC and PE, we showed that PEMT-L and PEMT-S expression remarkably increased the cellular PC content, whereas the PE content was decreased by PEMT-S expression, but was hardly affected by PEMT-L expression. The cellular content of phosphatidylserine was also reduced by the expression of PEMT-L or PEMT-S. MS analyses demonstrated that the expression of PEMT-S led to more increases in the molecular species of PC and PC-O (ether-linked PC) with longer polyunsaturated chains than that of PEMT-L, whereas the PC-O species with shorter chains were increased more by PEMT-L expression than by PEMT-S expression, suggesting a difference in the substrate specificity of PEMT-L and PEMT-S. On the other hand, various PE and PE-O species were decreased by PEMT-S expression. In addition, PEMT-L and PEMT-S expression promoted the proliferation of HEK-293 cells. Based upon these findings, we propose a model in which the enzymatic activity and substrate specificity are regulated by the glycosylated N-terminal region of PEMT-L localized in the ER lumen.

The development of a metabolic disease phenotype in CTP:phosphoethanolamine cytidylyltransferase-deficient mice

Phosphatidylethanolamine (PE) is an important inner membrane phospholipid mostly synthesized de novo via the PE-Kennedy pathway and by the decarboxylation of phosphatidylserine. CTP:phosphoethanolamine cytidylyltransferase (Pcyt2) catalyzes the formation of CDP-ethanolamine, which is often the rate regulatory step in the PE-Kennedy pathway. In the current investigation, we show that the reduced CDP-ethanolamine formation in Pcyt2(+/-) mice limits the rate of PE synthesis and increases the availability of diacylglycerol. This results in the increased formation of triglycerides, which is facilitated by stimulated de novo fatty acid synthesis and increased uptake of pre-existing fatty acids. Pcyt2(+/-) mice progressively accumulate more diacylglycerol and triglycerides with age and have modified fatty acid composition, predominantly in PE and triglycerides. Pcyt2(+/-) additionally have an inherent blockage in fatty acid utilization as energy substrate and develop impaired tolerance to glucose and insulin at an older age. Accordingly, gene expression analyses demonstrated the up-regulation of the main lipogenic genes and down-regulation of mitochondrial fatty acid beta-oxidation genes. These data demonstrate for the first time that to preserve membrane PE phospholipids, Pcyt2 deficiency generates compensatory changes in triglyceride and energy substrate metabolism, resulting in a progressive development of liver steatosis, hypertriglyceridemia, obesity, and insulin resistance, the main features of the metabolic syndrome.

Phosphatidylserine and phosphatidylethanolamine in mammalian cells: two metabolically related aminophospholipids

Phosphatidylserine (PS) and phosphatidylethanolamine (PE) are two aminophospholipids whose metabolism is interrelated. Both phospholipids are components of mammalian cell membranes and play important roles in biological processes such as apoptosis and cell signaling. PS is synthesized in mammalian cells by base-exchange reactions in which polar head groups of preexisting phospholipids are replaced by serine. PS synthase activity resides primarily on mitochondria-associated membranes and is encoded by two distinct genes. Studies in mice in which each gene has been individually disrupted are beginning to elucidate the importance of these two synthases for biological functions in intact animals. PE is made in mammalian cells by two completely independent major pathways. In one pathway, PS is converted into PE by the mitochondrial enzyme PS decarboxylase. In addition, PE is made via the CDP-ethanolamine pathway, in which the final reaction occurs on the endoplasmic reticulum and nuclear envelope. The relative importance of these two pathways of PE synthesis has been investigated in knockout mice. Elimination of either pathway is embryonically lethal, despite the normal activity of the other pathway. PE can also be generated from a base-exchange reaction and by the acylation of lyso-PE. Cellular levels of PS and PE are tightly regulated by the implementation of multiple compensatory mechanisms.

Alternative splicing of CTP:phosphoethanolamine cytidylyltransferase produces two isoforms that differ in catalytic properties

CTP:phosphoethanolamine cytidylyltransferase (Pcyt2) catalyzes the rate-controlling reaction of the CDP-ethanolamine (Kennedy) pathway. We have previously established that Pcyt2 is encoded by a single gene that can be alternatively spliced from an internal exon into two transcripts, designated Pcyt2alpha and Pcyt2beta. Little is currently known about the regulation of Pcyt2. Here, we functionally express both murine Pcyt2 (mPcyt2) transcripts and investigate the roles of the two proteins in the regulation of mPcyt2 activity. We demonstrate that the tagged and purified alpha and beta proteins differ significantly in their kinetic properties. The K(m) of mPcyt2alpha for phosphoethanolamine was 318.4 microM, compared with 140.3 microM for mPcyt2beta. The maximal velocities of the alpha and beta isoforms at saturating conditions for both substrates were 138.0 and 114.4 nmol/min/mumol enzyme, respectively. When phosphoethanolamine was used at a fixed concentration of 1 mM, the K(m) of mPcyt2alpha for CTP was 102.0 microM and that of mPcyt2beta was 84.09 microM. Using a combination of nondenaturing PAGE, gel filtration chromatography, and immunoprecipitation, we provide evidence that mPcyt2alpha and mPcyt2beta proteins can form both homodimeric and heterodimeric complexes. We show that alternative splicing of the mPcyt2 transcript is ubiquitous but could also be regulated in a tissue-specific manner, producing a variable ratio of mPcyt2alpha/mPcyt2beta mRNAs. The expression of two distinct protein isoforms maybe an important mechanism by which Pcyt2 activity is regulated.

Differences in membrane acyl phospholipid composition between an endothermic mammal and an ectothermic reptile are not limited to any phospholipid class

This study examined questions concerning differences in the acyl composition of membrane phospholipids that have been linked to the faster rates of metabolic processes in endotherms versus ectotherms. In liver, kidney, heart and brain of the ectothermic reptile, Trachydosaurus rugosus, and the endothermic mammal, Rattus norvegicus, previous findings of fewer unsaturates but a greater unsaturation index (UI) in membranes of the mammal versus those of the reptile were confirmed. Moreover, the study showed that the distribution of phospholipid head-group classes was similar in the same tissues of the reptile and mammal and that the differences in acyl composition were present in all phospholipid classes analysed, suggesting a role for the physical over the chemical properties of membranes in determining the faster rates of metabolic processes in endotherms. The most common phosphatidylcholine (PC) molecules present in all tissues (except brain) of the reptile were 16:0/18:1, 16:0/18:2, 18:0/18:2,18:1/18:1 and 18:1/18:2, whereas arachidonic acid (20:4), containing PCs 16:0/20:4, 18:0/20:4, were the common molecules in the mammal. The most abundant phosphatidylethanolamines (PE) used in the tissue of the reptile were 18:0/18:2, 18:0/20:4, 18:1/18:1, 18:1/18:2 and 18:1/20:4, compared to 16:0/18:2, 16:0/20:4, 16:0/22:6, 18:0/20:4, 18:0/22:6 and 18:1/20:4 in the mammal. UI differences were primarily due to arachidonic acid found in both PC and PEs, whereas docosahexaenoic acid (22:6) was a lesser contributor mainly within PEs and essentially absent in the kidney. The phospholipid composition of brain was more similar in the reptile and mammal compared to those of other tissues.

Metabolic and molecular aspects of ethanolamine phospholipid biosynthesis: the role of CTP:phosphoethanolamine cytidylyltransferase (Pcyt2)

The CDP-ethanolamine branch of the Kennedy pathway is the major route for the formation of ethanolamine-derived phospholipids, including diacyl phosphatidylethanolamine and alkenylacyl phosphatidylethanolamine derivatives, known as plasmalogens. Ethanolamine phospholipids are essential structural components of the cell membranes and play regulatory roles in cell division, cell signaling, activation, autophagy, and phagocytosis. The physiological importance of plasmalogens has not been not fully elucidated, although they are known for their antioxidant properties and deficiencies in a number of inherited peroxisomal disorders. This review highlights important aspects of ethanolamine phospholipid metabolism and reports current molecular information on 1 of the regulatory enzymes in their synthesis, CTP:phosphoethanolamine cytidylyltransferase (Pcyt2). Pcyt2 is encoded by a single, nonredundant gene in animal species that could be alternatively spliced into 2 potential protein products. We describe properties of the mouse and human Pcyt2 genes and their regulatory promoters and provide molecular evidence for the existence of 2 distinct Pcyt2 proteins. The goal is to obtain more insight into Pcyt2 catalytic function and regulation to facilitate a better understanding of the production of ethanolamine phospholipids via the CDP-ethanolamine branch of the Kennedy pathway.

The CDP-ethanolamine pathway and phosphatidylserine decarboxylation generate different phosphatidylethanolamine molecular species

In mammalian cells, phosphatidylethanolamine (PtdEtn) is mainly synthesized via the CDP-ethanolamine (Kennedy) pathway and by decarboxylation of phosphatidylserine (PtdSer). However, the extent to which these two pathways contribute to overall PtdEtn synthesis both quantitatively and qualitatively is still not clear. To assess their contributions, PtdEtn species synthesized by the two routes were labeled with pathway-specific stable isotope precursors, d(3)-serine and d(4)-ethanolamine, and analyzed by high performance liquid chromatography-mass spectrometry. The major conclusions from this study are that (i) in both McA-RH7777 and Chinese hamster ovary K1 cells, the CDP-ethanolamine pathway was favored over PtdSer decarboxylation, and (ii) both pathways for PtdEtn synthesis are able to produce all diacyl-PtdEtn species, but most of these species were preferentially made by one pathway. For example, the CDP-ethanolamine pathway preferentially synthesized phospholipids with mono- or di-unsaturated fatty acids on the sn-2 position (e.g. (16:0-18:2)PtdEtn and (18:1-18:2)PtdEtn), whereas PtdSer decarboxylation generated species with mainly polyunsaturated fatty acids on the sn-2 position (e.g. (18:0-20:4)PtdEtn and (18:0-20:5)PtdEtn in McArdle and (18: 0-20:4)PtdEtn and (18:0-22:6)PtdEtn in Chinese hamster ovary K1 cells). (iii) The main PtdEtn species newly synthesized from the Kennedy pathway in the microsomal fraction appeared to equilibrate rapidly between the endoplasmic reticulum and mitochondria. (iv) Newly synthesized PtdEtn species preferably formed in the mitochondria, which is at least in part due to the substrate specificity of the phosphatidylserine decarboxylase, seemed to be retained in this organelle. Our data suggest a potentially essential role of the PtdSer decarboxylation pathway in mitochondrial functioning.

Developmental and metabolic effects of disruption of the mouse CTP:phosphoethanolamine cytidylyltransferase gene (Pcyt2)

The CDP-ethanolamine pathway is responsible for the de novo biosynthesis of ethanolamine phospholipids, where CDP-ethanolamine is coupled with diacylglycerols to form phosphatidylethanolamine. We have disrupted the mouse gene encoding CTP:phosphoethanolamine cytidylyltransferase, Pcyt2, the main regulatory enzyme in this pathway. Intercrossings of Pcyt2(+/-) animals resulted in small litter sizes and unexpected Mendelian frequencies, with no null mice genotyped. The Pcyt2(-/-) embryos die after implantation, prior to embryonic day 8.5. Examination of mRNA expression, protein content, and enzyme activity in Pcyt2(+/-) animals revealed the anticipated 50% decrease due to the gene dosage effect but rather a 20 to 35% decrease. [(14)C]ethanolamine radiolabeling of hepatocytes, liver, heart, and brain corroborated Pcyt2 gene expression and activity data and showed a decreased rate of phosphatidylethanolamine biosynthesis in heterozygotes. Total phospholipid content was maintained in Pcyt2(+/-) tissues; however, this was not due to compensatory increases in the decarboxylation of phosphatidylserine. These results establish the necessity of Pcyt2 for murine development and demonstrate that a single Pcyt2 allele in heterozygotes can maintain phospholipid homeostasis.

Crystallization and preliminary X-ray analysis of CTP:phosphoethanolamine cytidylyltransferase (ECT) from Saccharomyces cerevisiae

CTP:phosphoethanolamine cytidylyltransferase (ECT) is the enzyme that catalyzes the conversion of phosphoethanolamine to CDP-ethanolamine in the phosphatidylethanolamine-biosynthetic pathway (Kennedy pathway). ECT from Saccharomyces cerevisiae was crystallized by the sitting-drop vapour-diffusion method using PEG 4000 as precipitant. The crystals diffracted X-rays from a synchrotron-radiation source to 1.88 A resolution. The space group was assigned as primitive tetragonal, P4(1)2(1)2 or P4(3)2(1)2, with unit-cell parameters a = b = 66.3, c = 150.8 A. The crystals contain one ECT molecule in the asymmetric unit (V(M) = 2.2 A(3) Da(-1)), with a solvent content of 43%.

Placental thrombosis and spontaneous fetal death in mice deficient in ethanolamine kinase 2

Ethanolamine kinase catalyzes the first step in the CDP-ethanolamine pathway for the formation of the major membrane phospholipid phosphatidylethanolamine (PtdEtn). In this work, the predicted Etnk2 cDNA was established as a soluble protein with ethanolamine-specific kinase activity that was most highly expressed in liver. Mice with an inactivated Etnk2 gene were derived, and its absence reduced the rate of PtdEtn synthesis from exogenous ethanolamine in hepatocytes. PtdEtn is a major precursor to phosphatidylcholine in liver; however, Etnk2(-/-) mice did not have reduced amounts of either PtdEtn or phosphatidylcholine or an altered phospholipid molecular species distribution. The knock-out animals were able to adapt to a choline-deficient diet. The Etnk2(-/-) mice exhibited a maternal-specific intrauterine growth retardation phenotype that resulted in a 33% reduction in litter size and frequent perinatal death. Histological analysis of pregnant Etnk2(-/-) females showed that fetal development failed at the late stage of pregnancy in a significant percentage of embryos because of the appearance of extensive placental thrombosis. These results illustrate a non-redundant role for EtnK2 expression in regulating placental hemostasis.

Control of the CDPethanolamine pathway in mammalian cells: effect of CTP:phosphoethanolamine cytidylyltransferase overexpression and the amount of intracellular diacylglycerol

For an insight regarding the control of PtdEtn (phosphatidylethanolamine) synthesis via the CDPethanolamine pathway, rat liver cDNA encoding ECT (CTP:phosphoethanolamine cytidylyltransferase) was transiently or stably transfected in Chinese-hamster ovary cells and a rat liver-derived cell line (McA-RH7777), resulting in a maximum of 26- and 4-fold increase in specific activity of ECT respectively. However, no effect of ECT overexpression on the rate of [3H]ethanolamine incorporation into PtdEtn was detected in both cell lines. This was explored further in cells overexpressing four times ECT activity (McA-ECT1). The rate of PtdEtn breakdown and PtdEtn mass were not changed in McA-ECT1 cells in comparison with control-transfected cells. Instead, an accumulation of CDPethanolamine (label and mass) was observed, suggesting that in McA-ECT1 cells the ethanolaminephosphotransferase-catalysed reaction became rate-limiting. However, overexpression of the human choline/ethanolaminephosphotransferase in McA-ECT1 and control-transfected cells had no effect on PtdEtn synthesis. To investigate whether the availability of DAG (diacylglycerol) limited PtdEtn synthesis in these cells, intracellular DAG levels were increased using PMA or phospholipase C. Exposure of cells to PMA or phospholipase C stimulated PtdEtn synthesis and this effect was much more pronounced in McA-ECT1 than in control-transfected cells. In line with this, the DAG produced after PMA exposure was consumed more rapidly in McA-ECT1 cells and the CDPethanolamine level decreased accordingly. In conclusion, our results suggest that the supply of CDPethanolamine, via the expression level of ECT, is an important factor governing the rate of PtdEtn biosynthesis in mammalian cells, under the condition that the amount of DAG is not limiting.

Genomic organization and differential splicing of the mouse and human Pcyt2 genes

CTP: ethanolaminephosphate cytidylyltransferase (Pcyt2) is an important regulatory enzyme in phosphatidylethanolamine and plasmalogen biosynthesis. We cloned the mouse gene mPcyt2 and established its relationship with the human homolog PCYT2. The two genes share similar size and contain two conserved catalytic domains but exhibit different exon/intron organization. An internal region could be alternatively spliced producing a longer mouse transcript, mPcyt2 alpha, and a shorter human transcript, PCYT2 beta. The spliced region is entirely made from mPcyt2 Exon 7 and encodes the peptide PPHPTPAGDTLSSEVSSQ, located upstream of the second catalytic motif HIGH. Mouse and human proteins also differ in amino acid composition at the C-terminus due to an additional splicing between Exons 13 and 14 in PCYT2. The 5' RACE analyses and subsequent cloning of the promoter regions demonstrated that the mPcyt2 and PCYT2 promoters are located immediately upstream of the first exon. There is no sequence homology between the two promoters but they are both TATA-less, have conserved CAAT boxes at a matching distance (-85/-70 bp) from the transcription start site and contain cis-elements for transcription factors of the CAAT, Sp1 and NF1 family, all in accordance with ubiquitous expression of both genes. The mPcyt2 gene is highly expressed in liver, brain, adipose tissues, heart, skeletal muscle, spleen, lungs and kidney. In THP-1 and U937 cells, PCYT2 expression could vary with the stage of cell differentiation. Luciferase reporter analyses show that the Pcyt2 and PCYT2 promoters are strong promoters similar to other ubiquitous promoters, such as those of Pcyt1 and SV-40.

Biogenesis and cellular dynamics of aminoglycerophospholipids

Aminoglycerophospholipids phosphatidylserine (PtdSer), phosphatidylethanolamine (PtdEtn), and phosphatidylcholine (PtdCho) comprise about 80% of total cellular phospholipids in most cell types. While the major function of PtdCho in eukaryotes and PtdEtn in prokaryotes is that of bulk membrane lipids, PtdSer is a minor component and appears to play a more specialized role in the plasma membrane of eukaryotes, e.g., in cell recognition processes. All three aminoglycerophospholipid classes are essential in mammals, whereas prokaryotes and lower eukaryotes such as yeast appear to be more flexible regarding their aminoglycerophospholipid requirement. Since different subcellular compartments of eukaryotes, namely the endoplasmic reticulum and mitochondria, contribute to the biosynthetic sequence of aminoglycerophospholipid formation, intracellular transport, sorting, and specific function of these lipids in different organelles are of special interest.

Translocation of phosphatidylthreonine and -serine to mitochondria diminishes exponentially with increasing molecular hydrophobicity

Some cultured cells contain significant amounts of a rarely recognized phospholipid, phosphatidylthreonine. Since phosphatidylthreonine is a structural analog of phosphatidylserine, the question rises whether it is transported to mitochondria and decarboxylated to phosphatidylisopropanolamine therein. We studied this issue with hamster kidney cell-line using a novel approach, i.e. electrospray mass-spectrometry and stable isotope-labeled precursors. Scanning for a neutral loss of 155, which is characteristic for phosphatidylisopropanolamine, indicated that this lipid is indeed present. The identity of phosphatidylisopropanolamine was supported by the following: (i) it co-chromatographed with phosphatidylethanolamine; (ii) its molecular species profile was similar to that of phosphatidylethanolamine; (iii) its head group was labeled from 13C-threonine; and (iv) its concentration increased in parallel with phosphatidylthreonine. Tests with solubilized decarboxylase and subcellular fractionation studies indicated that the low cellular content of phosphatidylisopropanolamine is due to inefficient decarboxylation, rather than poor translocation of phosphatidylthreonine to mitochondria. Importantly, the average hydrophobicity of phosphatidylisopropanolamine molecular species was significantly less than that of phosphatidylthreonine species, indicating that hydrophilic phosphatidylthreonine species translocate to mitochondria far more rapidly than hydrophobic ones. Parallel results were obtained for phosphatidylserine. These findings imply that efflux from the ER membrane could be the rate-limiting step in the phosphatidylthreonine and -serine translocation to mitochondria.

Regulation of phosphatidylcholine and phosphatidylethanolamine synthesis in rat hepatocytes by 5-aminoimidazole-4-carboxamide ribonucleoside (AICAR)

The present study was undertaken to study the role of AMP-activated kinase (AMPK) in the biosynthesis of two major membrane phospholipids, phosphatidylcholine (PC) and phosphatidylethanolamine (PE). Incubation of rat hepatocytes with 5-aminoimidazole-4-carboxamide ribonucleoside (AICAR), an activator of AMPK, produced dose-dependent inhibition of the incorporation of [(3)H]choline and [(3)H]ethanolamine into PC and PE, respectively. Determination of the cellular uptake of choline and ethanolamine showed that the reduced synthesis of PC and PE did not result from impaired uptake of these two precursors. The decreased synthesis of PC was not mirrored by a reduction in the activities of the enzymes of the CDP-choline pathway. The diminution of PE biosynthesis, however, was paralleled by a depressed activity of CTP:phosphoethanolamine cytidylyltransferase (ET), the pace-setting enzyme of the CDP-ethanolamine pathway. AICAR treatment of hepatocytes stimulated the conversion of choline into betaine, indicating that reduced PC synthesis most probably resulted from a decrease in the availability of choline. In addition, AICAR induced a 50% reduction in the cellular level of diacylglycerols, which may further impair the synthesis of PC and PE. The results thus indicate that AICAR inhibits the biosynthesis of PC and PE and that the effect is exerted at different sites in the two pathways. Increased oxidation of choline to betaine is the main target of AICAR in the PC pathway, whereas inhibition of ET activity is the locus of AICAR action in the PE pathway.

Overexpression of a mammalian ethanolamine-specific kinase accelerates the CDP-ethanolamine pathway

Ethanolamine kinase (EKI) is the first committed step in phosphatidylethanolamine (PtdEtn) biosynthesis via the CDP-ethanolamine pathway. We identify a human cDNA encoding an ethanolamine-specific kinase EKI1 and the structure of the EKI1 gene located on chromosome 12. EKI1 overexpression in COS-7 cells results in a 170-fold increase in ethanolamine kinase-specific activity and accelerates the rate of [3H]ethanolamine incorporation into PtdEtn as a function of the ethanolamine concentration in the culture medium. Acceleration of the CDP-ethanolamine pathway does not result in elevated cellular PtdEtn levels, but rather the excess PtdEtn is degraded to glycerophosphoethanolamine. EKI1 has negligible choline kinase activity in vitro and does not influence phosphatidylcholine biosynthesis. Acceleration of the CDP-ethanolamine pathway also does not change the rate of PtdEtn formation via the decarboxylation of phosphatidylserine. The data demonstrate the existence of separate ethanolamine and choline kinases in mammals and show that ethanolamine kinase can be a rate-controlling step in PtdEtn biosynthesis.

Cloning and expression of CTP:phosphoethanolamine cytidylyltransferase cDNA from rat liver

CTP:phosphoethanolamine cytidylyltransferase (ET) is a key regulatory enzyme in the CDP-ethanolamine pathway for phosphatidylethanolamine synthesis. As a first step in the elucidation of the structure-function relationship and the regulation of ET, an ET cDNA was cloned from rat liver. The cloned cDNA encodes a protein of 404 amino acid residues with a calculated molecular mass of 45.2 kDa. The deduced amino acid sequence is very similar to that of human ET (89% identity). Furthermore, it shows less, but significant, similarity to yeast ET as well as to other cytidylyltransferases, including rat CTP:phosphocholine cytidylyltransferase and Bacillus subtilis glycerol-3-phosphate cytidylyltransferase. Like human and yeast ET, rat ET has a large repetitive internal sequence in the N- and C-terminal halves of the protein. Both parts of the repeat contain the HXGH motif, the most conserved region in the N-terminal active domain of other cytidylyltransferases, indicating the existence of two catalytic domains in ET. The hydropathy profile revealed that rat ET is largely hydrophilic and lacks a hydrophobic stretch long enough to span a bilayer membrane. There was no prediction for an amphipathic alpha-helix. Transfection of COS cells with the cDNA clone resulted in an 11-fold increase in ET activity, corresponding to an increase in the amount of ET protein as detected on a Western blot. Determination of the ET activity during liver development showed a 2. 5-fold increase between day 17 of gestation and birth (day 22) and the amount of ET protein changed accordingly. Northern blot analysis showed that this was accompanied by an increase in the amount of ET mRNA. Between day 17 of gestation and birth, the amount of mRNA in fetal rat liver increased approx. 6-fold, suggesting the regulation of ET at both pretranslational and post-translational levels during rat liver development.

Inhibition of phosphatidylcholine and phosphatidylethanolamine biosynthesis in rat-2 fibroblasts by cell-permeable ceramides

Phospholipids and sphingolipids are important precursors of lipid-derived second messengers such as diacylglycerol and ceramide, which participate in several signal transduction pathways and in that way mediate the effects of various agonists. The cross-talk between glycerophospholipid and sphingolipid metabolism was investigated by examining the effects of cell-permeable ceramides on phosphatidylcholine (PtdCho) and phosphatidylethanolamine (PtdEtn) synthesis in Rat-2 fibroblasts. Addition of short-chain C6-ceramide to the cells resulted in a dose- and time-dependent inhibition of the CDP-pathways for PtdCho and PtdEtn synthesis. Treatment of cells for 4 h with 50 microM C6-ceramide caused an 83% and a 56% decrease in incorporation of radiolabelled choline and ethanolamine into PtdCho and PtdEtn, respectively. Exposure of the cells for longer time-periods (>/= 16 h) to 50 microM C6-ceramide resulted in apoptosis. The structural analogue dihydro-C6-ceramide did not affect PtdCho and PtdEtn synthesis. In pulse-chase experiments, radioactive choline and ethanolamine accumulated in CDP-choline and CDP-ethanolamine under the influence of C6-ceramide, suggesting that synthesis of both PtdCho and PtdEtn were inhibited at the final step in the CDP-pathways. Indeed, cholinephosphotransferase and ethanolaminephosphotransferase activities in membrane fractions from C6-ceramide-treated cells were reduced by 64% and 43%, respectively, when compared with control cells. No changes in diacylglycerol mass levels or synthesis of diacylglycerol from radiolabelled palmitate were observed. It was concluded that C6-ceramide affected glycerophospholipid synthesis predominantly by inhibition of the step in the CDP-pathways catalysed by cholinephosphotransferase and ethanolaminephosphotransferase.

Channelling of intermediates in the biosynthesis of phosphatidylcholine and phosphatidylethanolamine in mammalian cells

Previous studies with electropermeabilized cells have suggested the occurrence of metabolic compartmentation and Ca2+-dependent channeling of intermediates of phosphatidylcholine (PC) biosynthesis in C6 rat glioma cells. With a more accessible permeabilization technique, we investigated whether this is a more general phenomenon also occurring in other cell types and whether channeling is involved in phosphatidylethanolamine (PE) synthesis as well. C6 rat glioma cells, C3H10T12 fibroblasts and rat hepatocytes were permeabilized with Staphylococcus aureus alpha-toxin, and the incorporation of the radiolabelled precursors choline, phosphocholine (P-choline), ethanolamine and phosphoethanolamine (P-EA) into PC and PE were measured both at high and low Ca2+ concentrations. In glioma cells, permeabilization at high Ca2+ concentration did not affect [14C]choline or [14C]P-choline incorporation into PC. However, reduction of free Ca2+ in the medium from 1.8 mM to <1 nM resulted in a dramatic increase in [14C]P-choline incorporation into permeabilized cells, whereas [14C]choline incorporation remained unaffected. Also, in fibroblasts, reduction of extracellular Ca2+ increased [14C]P-choline and [14C]P-EA incorporation into PC and PE respectively. In hepatocytes, a combination of alpha-toxin and low Ca2+ concentration severely impaired [14C]choline incorporation into PC. Therefore, alpha-toxin-permeabilized hepatocytes are not a good model in which to study channeling of intermediates in PC biosynthesis. In conclusion, our results indicate that channeling is involved in PC synthesis in glioma cells and fibroblasts. PE synthesis in fibroblasts is also at least partly dependent on channeling.

Reversible translocation of CTP:phosphocholine cytidylyltransferase from cytosol to membranes in the adult bovine liver around parturition

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.

Enzymes of the Primary Phosphatidylethanolamine Biosynthetic Pathway in Postgermination Castor Bean Endosperm (Developmental Profiles and Partial Purification of the Mitochondrial CTP:Ethanolaminephosphate Cytidylyltransferase)

Ethanolamine kinase, CTP:ethanolaminephosphate cytidylyltransferase (ECT), and ethanolaminephosphotransferase, which sequentially catalyze the primary pathway for phosphatidylethanolamine synthesis, were measured in castor bean (Ricinus communis L. var Hale) endosperm for 6 d after the onset of imbibition. Ethanolamine kinase (EC 2.7.1.82) activity was cytosolic, increasing slowly during the first 5 d and then declining. Total ECT (EC 2.7.7.14) activity increased until the 4th d, but the endoplasmic reticulum fraction of the activity peaked at d 3, and the mitochondrial activity peaked at d 4. Diacylglycerol:CDPethanolamine ethanolaminephosphotransferase (EC 2.7.8.1) increased during the first 2 d after imbibition began, after which it declined. The lowest activity of ethanolamine kinase during postgermination was more than 5-fold higher than the maximum activity of ECT, and the total activity of diacylglycerol:CDPethanolamine ethanolaminephosphotransferase at d 2 was at least triple that of ECT of the endoplasmic reticulum. We have partially purified ECT from mitochondrial fractions of postgermination castor bean endosperm starting with mitochondria purified by sucrose (Suc) density gradient centrifugation and broken by osmotic shock and homogenization. The membrane-bound ECT was solubilized with 1.5% 3-[(3-cholamidopropyl)dimethylammonio]-2-hydroxy-1-propanesulfonate and purified approximately 118-fold by polyethylene glycol precipitation, chromatography on Sephacryl S-200, and then Suc gradient centrifugation. The continuous presence of both salt (0.5 M NaCl) and detergent (1% [w/v] 3-[(3-cholamidopropyl)dimethylammonio]-2-hydroxy-1-propanesulfonate) was necessary to prevent aggregation. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the final activity peak resulted in a prominent protein band at 35 kD, which correlated with bands from peak ECT activity fractions after both Suc gradient centrifugation and gel filtration on Sephacryl S-200. The activity of this enzyme was enhanced by the addition of several phospholipids.

Effects of maturation on the phospholipid and phospholipid fatty acid compositions in primary rat cortical astrocyte cell cultures

Phospholipid and phospholipid fatty acid compositional changes were studied in rat cortical astrocytes during dibutyryl cyclic adenosine monophosphate (dBcAMP, 0.25 mM) treatment starting after 14 days in culture (DIC). After 15 DIC, ethanolamine- and choline glycerophospholipid levels were increased 1.2- and 1.3-fold, respectively in treated compared to control cells. However, after 21 and 28 DIC, these levels were not significantly different between groups. Both groups had an increase in phosphatidylserine levels with increasing time in culture. Similarly, ethanolamine plasmalogen levels were transiently elevated after 21 DIC, but returned to previous levels after 28 DIC. The phospholipid fatty acid compositions for the acid stable and labile ethanolamine- and choline glycerophospholipids indicated that in dBcAMP treated cells, 20:4 n-6 and 22:6 n-3 proportions were elevated with increasing time in culture relative to control cells. As 20:4 n-6 proportions increased, there was a concomitant decrease in 20:3 n-9 proportions, suggesting an up regulation of n-6 series elongation and desaturation. In contrast, in control cells, the 20:4 n-6 proportions decreased with a corresponding increase in the 20:3 n-9 proportions. Thus, in treated cells, the cellular phospholipid fatty acid composition was dramatically different than control cells, suggesting that dBcAMP treatment may act to increase fatty acid elongation and desaturation.

Ethanolamine modulates the rate of rat hepatocyte proliferation in vitro and in vivo

A low molecular weight, heat-resistant hepatotrophic factor in an extract from the bovine intestinal mucosa was purified and identified as ethanolamine by structural analyses. The mode of action of ethanolamine in vitro and in vivo coincided with that of the crude extract of the tissue, indicating that ethanolamine is the active component. Ethanolamine synergistically elevated the stimulation of DNA synthesis in hepatocytes in primary culture when added together with a growth factor, such as epidermal growth factor, with the ED50 being 20 microM, although it showed little stimulatory effect by itself. Contrary to these in vitro results, the intraperitoneal administration of ethanolamine hydrochloride (24 mg of ethanolamine per kg of body weight) enhanced hepatocyte proliferation in regenerating rat livers after two-thirds hepatectomy without the administration of any growth factors. In the regenerating liver, hepatocyte proliferation may be initiated by an endogenous growth factor, but the supply of ethanolamine in circulation may not be sufficient for optimal hepatocyte proliferation; thus, the exogenous administration of ethanolamine may further enhance hepatocyte proliferation. Ethanolamine in circulation may be a humoral hepatotrophic factor.

Evidence for receptor and G-protein regulation of a phosphatidylethanolamine-hydrolysing phospholipase A1 in guinea-pig heart microsomes: stimulation of phospholipase A1 activity by DL-isoprenaline and guanine nucleotides

While evidence has been presented for the receptor-mediated activation of phospholipases A2, C and D, the activation of phospholipase A1 subsequent to receptor activation has not been established. Phospholipase A1-catalysed hydrolysis of 1-palmitoyl-2-linoleoyl-glycerophosphoethanolamine (GPE) by guinea-pig heart microsomes was stimulated 40-60% by isoprenaline. This isoprenaline-mediated increase in activity was blocked by propranolol and butoxamine, a specific beta 2-adrenergic antagonist, but not by atenolol, a specific beta 1-adrenergic antagonist. Neither clonidine nor phenylephrine, alpha 1- and alpha 2-adrenergic agonists respectively, had a stimulatory effect on the hydrolysis of the PE substrate. Guanosine 5'(-)[gamma-thio]triphosphate (GTP[S]) and guanosine 5'(-)[beta,gamma-imido]triphosphate, but not guanosine 5'(-)[beta-thio]diphosphate (GDP[S]) or adenosine 5'(-)[gamma-thio]triphosphate, stimulated the hydrolysis of 1-palmitoyl-2-linoleoyl-GPE by phospholipase A1. GDP[S] inhibited the isoprenaline-mediated stimulation of phospholipase A1 activity. Phospholipase A1 hydrolysis of 1-palmitoyl-2-linoleoyl-GPE was not dependent on cations; however, the stimulatory effects of isoprenaline and GTP[S] on the hydrolytic activity were abolished by cation chelators. The above data suggest that phospholipase A1 activity in guinea-pig heart microsomes is activated by the binding of isoprenaline to beta 2-adrenergic receptors. Furthermore the stimulation of phospholipase A1 activity by the agonist may be mediated via activation of G-proteins.

In vitro and in vivo 13C and 31P NMR analyses of phosphocholine metabolism in rat glioma cells

In vivo magnetic resonance spectroscopy (MRS) has revealed that phosphomonoesters (PME) such as phosphocholine (PCho) and phosphoethanolamine (PEth) are elevated in tumors and rapidly proliferating tissues. The regulation of PME levels and their relationship to proliferation are not well known. In the present study, we investigated the regulation of PCho and PEth levels in rat glioma cells grown in vivo and in vitro using 31P and 13C MRS. However, the ability of cells to produce choline endogenously is variable. To fully understand regulation of PCho levels, it is necessary to characterize the activity of the endogenous pathway, if it exists. This was first investigated by following the metabolic fate of 13C-labeled methionine of 9L glioma tumors in vivo. Our results indicate that there is a significant amount of de novo choline synthesis in vivo. However, similar experiments performed in vitro using cells cultured in bioreactors indicated that glioma cells themselves are unable to synthesize choline de novo, suggesting that the in vivo results were due to the involvement of extra-tumoral organs, e.g., liver. Further in vitro experiments demonstrated that the uptake and phosphorylation of physiologically relevant concentrations of exogenous choline is very active in these systems. Thus, it appears that the exogenous pathway for PCho biosynthesis predominates and regulates PCho levels in glioma cells. Our results also demonstrate that PCho levels are lowest, and PEth levels are highest, in non-proliferating cells. These observations indicate that there is a decrease in the biosynthesis of PCho concomitant with a reduction in culture growth. The source of the increased PEth is, as yet, undefined.

Serine and ethanolamine incorporation into different plasmalogen pools: subcellular analyses of phosphoglyceride synthesis in cultured glioma cells

In cultured glioma cells, plasma membrane (PM) is enriched in phosphatidylserine (PtdSer) and plasmalogens (1-O-alk-1'-enyl-2-acyl-sn-glycero-3-phosphoethanolamine). Serine can be a precursor of headgroups of both PtdSer and ethanolamine phosphoglycerides (PE) including plasmalogens and non-plasmalogen PE (NP-PE). Synthesis of phospholipids was investigated at the subcellular level using established fractionation procedures and incorporation of [3H(G)]L-serine and [1,2-14C]ethanolamine. Specific radioactivity of PtdSer from [3H]serine was 2-fold greater in PM than in microsomes, reaching maximum by 2-4 h. Labeled plasmalogen from [3H]serine appeared in PM by 4 h and increased to 48 h, whereas almost no plasmalogen accumulated in microsomes within 12 h. In contrast, labeled plasmalogen from [1,2-14C]ethanolamine appeared in both PM and microsomes at early incubation times and became enriched in PM beyond 12 h. Thus, in glioma cells: (1) greater and faster accumulation of labeled PtdSer in PM may reflect direct synthesis from serine within PM; (2) PM is a major source of PtdSer for decarboxylation and PE synthesis; (3) NP-PE in both PM and microsome provides headgroup for synthesis of plasmalogen; and, (4) plasmalogen synthesis may involve different intracellular pools depending on headgroup origin.

3-Deazaadenosine and MDL29350 differentially affect the methylation of serine-and ethanolamine-derived phosphatidylethanolamine in Hep G2 cells

The effect of 3-deazaadenosine (DZA) and the hypolipidemic drug MDL29350 (2-[3,5-di(t-butyl-4-hydroxyphenyl)thio]hexanoic acid) on the synthesis and methylation of phosphatidylethanolamine (PE) originating from the cytidine diphosphate (CDP) ethanolamine pathway and PE originating from decarboxylation of phosphatidylserine (PS) was investigated. DZA and MDL29350 did not affect the synthesis of PE by either pathway; however, methylation of ethanolamine-derived PE was inhibited by 80% and methylation of serine-derived PE was inhibited by 36% by 20 mumol/LDZA or MDL29350. The differential inhibition of the methylation of PE synthesized via serine or ethanolamine suggests that in Hep G2 cells PE-N-methyltransferase (PENMT) may be segregated into distinct compartments that are differentially accessible to the drugs.

The ethanolamine requirement of keratinocytes for growth is not due to defective synthesis of ethanolamine phosphoacylglycerols by the decarboxylation pathway

We have investigated whether the growth requirement of keratinocytes for ethanolamine is due to defective synthesis of ethanolamine phosphoacylglycerols (EPG) via decarboxylation of serine phosphoacylglycerols. Proliferating keratinocytes readily incorporated [3H]ethanolamine into phosphatidylethanolamine (PE) and [3H]serine into phosphatidylserine (PS) and PE. Non-proliferating keratinocytes in ethanolamine-free medium incorporated [3H]glycerol into phosphatidylcholine (PC), PS and PE in decreasing order of label incorporated. The order of decreasing incorporation of glycerol after addition of ethanolamine to the medium was PC > PE > PS. Incubation of non-proliferating keratinocytes with [3H]serine resulted in incorporation of label into PS and PE. The extent of incorporation of [3H]serine into PS in non-proliferating keratinocytes was not less than that in proliferating cells. Addition of ethanolamine to the medium of non-proliferating keratinocytes did not change the quantity of label incorporated into PS, but resulted in a decrease of label incorporated into PE. When cells were prelabelled overnight with [3H]serine and subsequently incubated in medium containing ethanolamine, the loss of label from PS was inhibited relative to that of control cells incubated in medium without ethanolamine. The activity of PS decarboxylase activity in keratinocyte mitochondria was inhibited by phosphoethanolamine and PE, but not by ethanolamine or CDP-ethanolamine. Both proliferating and non-proliferating keratinocytes incorporated [3H]serine into ether-linked ethanolamine phospholipids. Taken together, the above results suggest that (1) both proliferating and non-proliferating keratinocytes are able to synthesize PE and ether-linked ethanolamine phospholipids from serine, and therefore the ethanolamine-requirement of the cells is not due to a defective decarboxylase pathway; (2) any inability of the decarboxylase pathway to meet cellular EPG requirement is not due to decreased synthesis of serine phospholipids; (3) synthesis of PE via decarboxylation, the major route in nonproliferating keratinocytes, appears to decrease when ethanolamine is made available and the CDP-ethanolamine pathway is functioning; (4) phosphoethanolamine and increased PE produced from the CDP-ethanolamine pathway may inhibit PS decarboxylase activity in the cells and provide a means of coordinating the synthesis of PE by the two pathways to prevent excess production.

Limited metabolic interaction of serine with ethanolamine and choline in the turnover of phosphatidylserine, phosphatidylethanolamine and plasmalogens in cultured glioma cells

Modulation of choline phosphoglyceride turnover has been investigated extensively but less is known about regulation of serine and ethanolamine phosphoglyceride synthesis and turnover. We investigated incorporation and interactions of [3H(G)]L-serine, [1,2-14C]ethanolamine and [methyl-3H]choline in cultured glioma cells. Exogenous serine did not compete with ethanolamine or choline incorporation and did not chase labeled headgroup from ethanolamine phosphoglycerides (PE); serine displaced headgroup of prelabeled phosphatidylserine (PtdSer) resulting in less labeled PtdSer for decarboxylation. In contrast, exogenous ethanolamine markedly chased labeled headgroup of non-plasmenylethanolamine phosphoglycerides (NP-PE) with less effect on plasmalogen (1-O-alk-1'-enyl-2-acyl-sn-glycero-3-phosphoethanolamine) whether headgroup was derived from [3H]serine or [14C]ethanolamine. Label in chase medium was mainly ethanolamine to 12 h; phosphoethanolamine was present with longer chase (12-48 h). Choline did not compete with serine incorporation and had little chase effect on PtdSer and PE. Choline and ethanolamine competitively interacted with preference for choline. These data suggest that (1) PtdSer synthesis in cultured glioma cells may involve more than headgroup exchange; (2) PE turnover with metabolite release to medium may involve both phospholipase D and phospholipase C; (3) acceleration of PE turnover by exogenous ethanolamine primarily involves NP-PE with lesser involvement of plasmalogen; and (4) in contrast to lack of interaction between serine and other headgroup precursors, choline and ethanolamine compete primarily at uptake.

Effect of delta 9-tetrahydrocannabinol and merthiolate on acyltransferase activities in guinea pig liver microsomes

delta 9-Tetrahydrocannabinol (THC) and merthiolate have been utilized as lysophospholipid acyltransferase inhibitors in metabolic studies. However, their effects on acyltransferases other than lysophosphatidylcholine:acyl-CoA acyltransferase (LPCAT) are not known. We have therefore investigated the effectiveness of THC and merthiolate in inhibiting the acylation of lysophosphatidylcholine, lysophosphatidylethanolamine, lysophosphatidylserine, lysophosphatidylinositol (LPI) and lysophosphatidic acid (LPA) in guinea pig liver microsomes using oleoyl-CoA and arachidonoyl-CoA as acyl donors. THC inhibited LPCAT and lysophosphatidylethanolamine:acyl-CoA acyltransferase (LPEAT) by 40-50%, but had no effect or only slightly increased the activities of the other acyltransferases when assayed with oleoyl-CoA as the acyl donor. The results obtained with arachidonoyl-CoA were similar to those with oleoyl-CoA, with the exception of a 40% inhibition of lysophosphatidylserine:acyl-CoA acyltransferase (LPSAT) at concentrations of 50 microM or higher. At similar concentrations, merthiolate was more effective than THC in inhibiting the acyltransferases examined. Selective effects on the acyltransferases were observed at low concentrations of merthiolate (20 microM or less). Thus, LPCAT was most susceptible, followed by LPI acyltransferases, LPSAT, LPEAT and lysophosphatidic acid:acyl-CoA acyltransferases (LPAAT). The presence of LPA did not affect the inhibition of LPCAT by merthiolate. Thus the resilience of LPAAT to merthiolate inhibition was not due to chelation of the compound by the acidic lysolipid. Thiol reagents including N-ethyl-maleiamide, 5,5'-dithio-bis-nitrobenzoic acid, iodoacetate, beta-mercaptoethanol and dithiothreitol had little or no effect on the acyltransferases relative to equimolar concentrations of merthiolate.(ABSTRACT TRUNCATED AT 250 WORDS)

Modulation of phosphatidylethanolamine biosynthesis by exogenous ethanolamine and analogues in the hamster heart

In the hamster heart, exogenous ethanolamine is taken up by the heart and utilized for the biosynthesis of phosphatidylethanolamine. The role of the exogenous supply of ethanolamine on phosphatidylethanolamine biosynthesis was examined by perfusing hamster heart with various concentrations of labelled ethanolamine. Analysis of the radioactivity distributed in the ethanolamine-containing metabolites indicated that at low exogenous ethanolamine concentrations (< or = 0.1 microM), the conversion of phosphoethanolamine to CDP-ethanolamine was rate-limiting for phosphatidylethanolamine biosynthesis. However, perfusion with higher concentrations of ethanolamine (> or = 0.4 microM) resulted in the phosphorylation of ethanolamine becoming rate-limiting. Since the intracellular ethanolamine levels remained unchanged, the alterations in radioactivity distribution suggested that the newly imported ethanolamine was preferentially utilized for phosphatidylethanolamine biosynthesis. The effects of ethanolamine analogues on ethanolamine uptake and subsequent conversion to phosphatidylethanolamine at physiological concentrations of exogenous ethanolamine were examined. Monomethylethanolamine was found to inhibit ethanolamine uptake, the conversion of ethanolamine to phosphoethanolamine and incorporation of radioactivity into phosphatidylethanolamine. The accumulation of radioactivity in the ethanolamine fraction by monomethylethanolamine, despite of the inhibition of ethanolamine uptake, further confirms the rate-limiting role of ethanolamine kinase in the biosynthesis of phosphatidylethanolamine.

The determination of tissue ethanolamine levels by reverse-phase high-performance liquid chromatography

A rapid and sensitive procedure for the determination of ethanolamine levels in mammalian tissues is reported. Ethanolamine was extracted from the tissue with a chloroform/methanol mixture, followed by phase separation. The aqueous phase was subjected to charcoal chromatography and the eluant was derivatized with phenylisothiocyanate. The amount of phenylthiocarbamyl (PTC) ethanolamine in the tissue extract was determined by reverse-phase high-performance liquid chromatography. Quantitation of PTC ethanolamine was linear between 0.1-1.0 nmol. The pool sizes of ethanolamine in hamster heart, liver and kidney were found to be 1.07, 0.92 and 1.11 mumol/g wet weight, respectively. The sensitivity of the method would allow the determination of ethanolamine in very small tissue samples.

Phosphatidylethanolamine metabolism in rat liver after partial hepatectomy. Control of biosynthesis of phosphatidylethanolamine by the availability of ethanolamine

The effect of partial (70%) hepatectomy on phosphatidylethanolamine (PE) synthesis was studied in rat liver during the first 4 post-operative days. Between 4 and 96 h after partial hepatectomy, the mass of PE increased from 30% to 80% of sham-operation values. In line with the increase in PE mass, the rate of PE synthesis in vivo from [14C]ethanolamine was stimulated 1.6- and 1.3-fold at 22 and 48 h after partial hepatectomy respectively. Surprisingly, the activity of CTP:phosphoethanolamine cytidylyltransferase (EC 2.7.7.14) was virtually unchanged after partial hepatectomy. In addition, neither ethanolamine kinase (EC 2.7.1.82) nor ethanolaminephosphotransferase (EC 2.7.8.1) showed any changes in activity over the time period studied. Hepatic levels of ethanolamine and phosphoethanolamine were drastically increased after partial hepatectomy, as compared with sham operation, whereas levels of CDP-ethanolamine and microsomal diacylglycerol were not affected. Interestingly, partial hepatectomy caused the concentration of free ethanolamine in serum to increase from 29 microM to approx. 50 microM during the first day after surgery. In hepatocytes isolated from non-operated animals, incorporation of [3H]ethanolamine into PE was stimulated by increasing the ethanolamine concentration from 10 up to 50 microM, whereas the radioactivity associated with phosphoethanolamine only increased at ethanolamine concentrations higher than 30 microM. Taken together, our results indicate that the observed increase in serum ethanolamine concentration after partial hepatectomy is probably responsible for both the increase in PE biosynthesis and the accumulation of ethanolamine and phosphoethanolamine in regenerating liver.