The Kennedy pathway generates phosphocoline and phosphoethanolamine through its two branches. Choline Kinase (ChoK) is the first enzyme of the Kennedy branch of synthesis of phosphocholine, the major component of the plasma membrane. ChoK family of proteins is composed by ChoKα and ChoKβ isoforms, the first one with two different variants of splicing. Recently ChoKα has been implicated in the carcinogenic process, since it is over-expressed in a variety of human cancers. However, no evidence for a role of ChoKβ in carcinogenesis has been reported.
Here we compare the in vitro and in vivo properties of ChoKα1 and ChoKβ in lipid metabolism, and their potential role in carcinogenesis. Both ChoKα1 and ChoKβ showed choline and ethanolamine kinase activities when assayed in cell extracts, though with different affinity for their substrates. However, they behave differentially when overexpressed in whole cells. Whereas ChoKβ display an ethanolamine kinase role, ChoKα1 present a dual choline/ethanolamine kinase role, suggesting the involvement of each ChoK isoform in distinct biochemical pathways under in vivo conditions. In addition, while overexpression of ChoKα1 is oncogenic when overexpressed in HEK293T or MDCK cells, ChoKβ overexpression is not sufficient to induce in vitro cell transformation nor in vivo tumor growth. Furthermore, a significant upregulation of ChoKα1 mRNA levels in a panel of breast and lung cancer cell lines was found, but no changes in ChoKβ mRNA levels were observed. Finally, MN58b, a previously described potent inhibitor of ChoK with in vivo antitumoral activity, shows more than 20-fold higher efficiency towards ChoKα1 than ChoKβ.
This study represents the first evidence of the distinct metabolic role of ChoKα and ChoKβ isoforms, suggesting different physiological roles and implications in human carcinogenesis. These findings constitute a step forward in the design of an antitumoral strategy based on ChoK inhibition.
Phosphatidylethanolamine (GPEtn), a major phospholipid component of trypanosome membranes, is synthesized de novo from ethanolamine through the Kennedy pathway. Here the composition of the GPEtn molecular species in the bloodstream form of Trypanosoma brucei is determined, along with new insights into phospholipid metabolism, by in vitro and in vivo characterization of a key enzyme of the Kennedy pathway, the cytosolic ethanolamine-phosphate cytidylyltransferase (TbECT). Gene knockout indicates that TbECT is essential for growth and survival, thus highlighting the importance of the Kennedy pathway for the pathogenic stage of the African trypanosome. Phosphatiylserine decarboxylation, a potential salvage pathway, does not appear to be active in cultured bloodstream form T. brucei, and it is not upregulated even when the Kennedy pathway is disrupted. In vivo metabolic labelling and phospholipid composition analysis by ESI-MS/MS of the knockout cells confirmed a significant decrease in GPEtn species, as well as changes in the relative abundance of other phospholipid species. Reduction in GPEtn levels had a profound influence on the morphology of the mutants and it compromised mitochondrial structure and function, as well as glycosylphosphatidylinositol anchor biosynthesis. TbECT is therefore genetically validated as a potential drug target against the African trypanosome.
It has been established that yeast membrane phospholipid content is responsive to the inositol and choline content of the growth medium. Alterations in the levels of transcription of phospholipid biosynthetic enzymes contribute significantly to this response. We now describe conditions under which ethanolamine can exert significant influence on yeast membrane phospholipid composition. We demonstrate that mutations which block a defined subset of the reactions required for the biosynthesis of phosphatidylcholine (PC) via the CDP-choline pathway cause ethanolamine-dependent effects on the steady-state levels of bulk PC in yeast membranes. Such an ethanolamine-dependent reduction in bulk membrane PC content was observed for both choline kinase (cki) and choline phosphotransferase (cpt1) mutants, but it was not observed for mutants defective in cholinephosphate cytidylyltransferase, the enzyme that catalyzes the penultimate reaction of the CDP-choline pathway for PC biosynthesis. Moreover, the ethanolamine effect observed for cki and cpt1 mutants was independent of the choline content of the growth medium. Finally, we found that haploid yeast strains defective in the activity of both the choline and ethanolamine phosphotransferases experienced an ethanolamine-insensitive reduction in steady-state PC content, an effect which was not observed in strains defective in either one of these activities alone. The collective data indicate that specific enzymes of the CDP-ethanolamine pathway for phosphatidylethanolamine biosynthesis, while able to contribute to PC synthesis when yeast cells are grown under conditions of ethanolamine deprivation, do not do so when yeast cells are presented with this phospholipid headgroup precursor.
Ethanolamine kinase catalyzes the committed step in the synthesis of phosphatidylethanolamine via the CDP-ethanolamine branch of the Kennedy pathway. Regulation of the EKI1-encoded ethanolamine kinase by the essential nutrient zinc was examined in Saccharomyces cerevisiae. The level of ethanolamine kinase activity increased when zinc was depleted from the growth medium. This regulation correlated with increases in the CDP-ethanolamine pathway intermediates phosphoethanolamine and CDP-ethanolamine, and an increase in the methylated derivative of phosphatidylethanolamine, phosphatidylcholine. The β-galactosidase activity driven by the PEKI1-lacZ reporter gene was elevated in zinc-depleted cells, indicating that the increase in ethanolamine kinase activity was attributed to a transcriptional mechanism. The expression level of PEKI1-lacZ reporter gene activity in the zrt1Δzrt2Δ mutant (defective in plasma membrane zinc transport) cells grown with zinc was similar to the activity expressed in wild-type cells grown without zinc. This indicated that EKI1 expression was sensitive to intracellular zinc. The zinc-mediated regulation of EKI1 expression was attenuated in the zap1Δ mutant defective in the zinc-regulated transcription factor Zap1p. Direct interactions between Zap1p and putative zinc-responsive elements in the EKI1 promoter were demonstrated by electrophoretic mobility shift assays. Mutations of these elements to a nonconsensus sequence abolished Zap1p-DNA interactions. Taken together, this work demonstrated that the zinc-mediated regulation of ethanolamine kinase and the synthesis of phospholipids via the CDP-ethanolamine branch of the Kennedy pathway were controlled in part by Zap1p.
Choline-containing teichoic acid seems to be essential for the adsorption of bacteriophage Dp-1 to pneumococci. This conclusion is based on the following observations: In contrast to pneumococci grown in choline-containing medium, cells grown in medium containing ethanolamine or other submethylated aminoalcohols instead of choline were found to be resistant to infection by Dp-1. Live choline-grown bacteria and heat- or UV-inactivated cells and purified cell walls prepared from these cells were capable of adsorbing phage Dp-1; ethanolamine-grown pneumococci or cell wall preparations were unable to do so. Adsorption of Dp-1 to choline-containing cell walls was competitively inhibited by phosphorylcholine and by several choline-containing soluble cell surface components, such as the Forssman antigen and the teichoic acid-glycan complexes formed by autolytic cell wall degradation. Cell walls prepared from pneumococci grown in ethanolamine or phosphorylethanolamine were inactive. Electron microscopic studies with pneumococci that had segments of choline-containing cell wall material amid ethanolamine-containing regions indicated that the Dp-1 phage particles adsorbed exclusively to the choline-containing surface areas. We suggest that the choline residues of the pneumococcal teichoic acid are essential components of the Dp-1 phage receptors in this bacterium.
A fast and quantitative 2D high-resolution magic angle spinning (HR-MAS) total correlation spectroscopy (TOCSY) experiment was developed to resolve and quantify the choline- and ethanolamine-containing metabolites in human prostate tissues in ≈1 hr prior to pathologic analysis. At a 40-ms mixing time, magnetization transfer efficiency constants were empirically determined in solution and used to calculate metabolite concentrations in tissue. Phosphocholine (PC) was observed in 11/15 (73%) cancer tissues but only 6/32 (19%) benign tissues. PC was significantly higher (0.39 ± 0.40 mmol/kg vs. 0.02 ± 0.07 mmol/kg, z = 3.5), while ethanolamine (Eth) was significantly lower in cancer versus benign prostate tissues (1.0 ± 0.8 mmol/kg vs. 2.3 ± 1.9 mmol/kg, z = 3.3). Glycerophosphocholine (GPC) (0.57± 0.87 mmol/kg vs. 0.29± 0.26 mmol/kg, z = 1.2), phosphoethanolamine (PE) (4.4± 2.2 mmol/kg vs. 3.4± 2.6 mmol/kg, z = 1.4), and glycerophosphoethanolamine (GPE) (0.54± 0.82 mmol/kg vs. 0.15± 0.15 mmol/kg, z = 1.8) were higher in cancer versus benign prostate tissues. The ratios of PC/GPC (3.5± 4.5 vs. 0.32± 1.4, z = 2.6), PC/PE (0.08± 0.08 vs. 0.01± 0.03, z = 3.5), PE/Eth (16± 22 vs. 2.2± 2.0, z = 2.4), and GPE/Eth (0.41± 0.51 vs. 0.06± 0.06, z = 2.6) were also significantly higher in cancer versus benign tissues. All samples were pathologically interpretable following HR-MAS analysis; however, degradation experiments showed that PC, GPC, PE, and GPE decreased 7.7± 2.2%, while Cho+mI and Eth increased 18% in 1 hr at 1°C and a 2250 Hz spin rate.
phospholipid metabolism; phosphocholine; rotor synchronization; adiabatic mixing scheme
Pluripotent embryonic stem (ES) cells, a potential source of somatic precursors for cell therapies, cause tumors after transplantation. Studies of mammalian carcinogenesis using nuclear magnetic resonance (NMR) spectroscopy have revealed changes in the choline region, particularly increased phosphocholine (PCho) content. High PCho levels in murine ES (mES) cells have recently been attributed to cell pluripotency. The phosphoinositide 3-kinase (PI3K)/Akt pathway has been implicated in tumor-like properties of mES cells. This study aimed to examine a potential link between the metabolic profile associated with choline metabolism of pluripotent mES cells and PI3K/Akt signaling. We used mES (ES-D3) and murine embryonal carcinoma cells (EC-F9) and compared the metabolic profiles of 1) pluripotent mES (ESD0), 2) differentiated mES (ESD14), and 3) pluripotent F9 cells. Involvement of the PI3K/Akt pathway was assessed using LY294002, a selective PI3K inhibitor. Metabolic profiles were characterized in the extracted polar fraction by 1H NMR spectroscopy. Similarities were found between the levels of choline phospholipid metabolites (PCho/total choline and PCho/glycerophosphocholine [GPCho]) in ESD0 and F9 cell spectra and a greater-than five-fold decrease of the PCho/GPCho ratio associated with mES cell differentiation. LY294002 caused no significant change in relative PCho levels but led to a greater-than two-fold increase in PCho/GPCho ratios. These results suggest that the PCho/GPCho ratio is a metabolic trait shared by pluripotent and malignant cells and that PI3K does not underlie its development. It is likely that the signature identified here in a mouse model may be relevant for safe therapeutic applications of human ES cells.
Altered metabolism of membrane phospholipids has been implicated in bipolar disorder. In humans, uridine is an important precursor of cytidine diphosphate (CDP)-choline, which plays a critical role in phospholipid synthesis and is currently being evaluated as a potential treatment for bipolar depression.
A total of 17 healthy males (mean age ± SD: 32.73 ± 7.2 years; range: 21.8- 46.4 years) were enrolled in this study. Subjects underwent a 31-phosphorus magnetic resonance spectroscopy (31P-MRS) acquisition at baseline and then again after seven days of either 2 g of uridine or placebo administration. A two-dimensional chemical shift imaging 31P-MRS acquisition collected spectral data from a 4 × 4 cluster of voxels acquired in the axial plane encompassing the subcortical structures as well as frontaltemporal cortical gray and white matter. The slab thickness was 3 cm and the approximate total volume of brain sampled was 432 cm3. The spectra obtained were analyzed using a fully automated in-house fitting algorithm. A population-averaged generalized estimating equation was used to evaluate changes both in phosphomonoesters (PME) [phosphocholine (PCho) and phosphoethanolamine (PEtn)] and phosphodiesters (PDE) [glycerophosphocholine (GPCho) and glycerophosphethanolamine (GPEtn)]. Metabolite ratios were reported with respect to the total integrated 31P resonance area.
The uridine group had significantly increased total PME and PEtn levels over the one-week period [6.32% and 7.17% for PME and PEtn, respectively (p < 0.001)]. Other metabolite levels such as PCho, PDE, GPEtn and GPCho showed no significant changes following either uridine or placebo (all p > 0.05).
This is the first study to report a direct effect of uridine on membrane phospholipid precursors in healthy adults using 31P-MRS. Sustained administration of uridine appears to increase PME in healthy subjects. Further investigation is required to clarify the effects of uridine in disorders with altered phospholipid metabolism such as bipolar disorder.
bipolar disorder; epilepsy; magnetic resonance spectroscopy; phospholipids; uridine
Phosphatidylcholine and phosphatidylethanolamine are the two main phospholipids in eukaryotic cells comprising ∼50 and 25% of phospholipid mass, respectively. Phosphatidylcholine is synthesized almost exclusively through the CDP-choline pathway in essentially all mammalian cells. Phosphatidylethanolamine is synthesized through either the CDP-ethanolamine pathway or by the decarboxylation of phosphatidylserine, with the contribution of each pathway being cell type dependent. Two human genes, CEPT1 and CPT1, code for the total compliment of activities that directly synthesize phosphatidylcholine and phosphatidylethanolamine through the CDP-alcohol pathways. CEPT1 transfers a phosphobase from either CDP-choline or CDP-ethanolamine to diacylglycerol to synthesize both phosphatidylcholine and phosphatidylethanolamine, whereas CPT1 synthesizes phosphatidylcholine exclusively. We show through immunofluorescence that brefeldin A treatment relocalizes CPT1, but not CEPT1, implying CPT1 is found in the Golgi. A combination of coimmunofluorescence and subcellular fractionation experiments with various endoplasmic reticulum, Golgi, and nuclear markers confirmed that CPT1 was found in the Golgi and CEPT1 was found in both the endoplasmic reticulum and nuclear membranes. The rate-limiting step for phosphatidylcholine synthesis is catalyzed by the amphitropic CTP:phosphocholine cytidylyltransferase α, which is found in the nucleus in most cell types. CTP:phosphocholine cytidylyltransferase α is found immediately upstream cholinephosphotransferase, and it translocates from a soluble nuclear location to the nuclear membrane in response to activators of the CDP-choline pathway. Thus, substrate channeling of the CDP-choline produced by CTP:phosphocholine cytidylyltransferase α to nuclear located CEPT1 is the mechanism by which upregulation of the CDP-choline pathway increases de novo phosphatidylcholine biosynthesis. In addition, a series of CEPT1 site-directed mutants was generated that allowed for the assignment of specific amino acid residues as structural requirements that directly alter either phospholipid head group or fatty acyl composition. This pinpointed glycine 156 within the catalytic motif as being responsible for the dual CDP-alcohol specificity of CEPT1, whereas mutations within helix 214–228 allowed for the orientation of transmembrane helices surrounding the catalytic site to be definitively positioned.
The substrate selectivity of four Trypanosoma brucei sphingolipid synthases was examined. TbSLS1, an inositol phosphorylceramide (IPC) synthase and TbSLS4, a bi-functional sphingomyelin (SM)/ethanolamine phosphorylceramide (EPC) synthase, were inactivated by Ala substitutions of a conserved triad of residues His210, His253 and Asp257 thought to form part of the active site. TbSLS4 also catalyzed the reverse reaction, production of ceramide from sphingomyelin, but none of the Ala substitutions of the catalytic triad in TbSLS4 were able to do so. Site-directed mutagenesis identified residues proximal to the conserved triad that were responsible for the discrimination between charge and size of the different head groups. For discrimination between anionic (phosphoinositol) and zwitterionic (phosphocholine, phosphoethanolamine) head groups, doubly mutated V172D/S252F TbSLS1 and D172V/F252S TbSLS3 showed reciprocal conversion between IPC and bi-functional SM/EPC synthases. For differentiation of zwitterionic head group size, N170A TbSLS1 and A170N/N187D TbSLS4 showed reciprocal conversion between EPC and bi-functional SM/EPC synthases. These studies provide a mapping of the SLS active site and demonstrate that differences in catalytic specificity of the T. brucei enzyme family are controlled by natural variations in as few as three residue positions.
Choline kinase is the first enzyme in the CDP-choline pathway that synthesizes phosphatidylcholine, the major phospholipid in eukaryotic cell membranes. In humans, choline kinase exists as three isoforms (CKα1, α2, and β). Specific inhibition of CKα has been reported to selectively kill tumoral cells. Monoclonal and polyclonal antibodies against CKα used in previous studies to detect the level of this isozyme in different cellular or biochemical contexts were able to detect either the α1 or the α2 isoform.
In this study, an antiserum against CKα was produced by immunizing rabbits with denatured, purified recombinant CKα2 full-length protein. This antiserum was highly specific for CKα when tested with extracts from different cell lines, and there was no cross reactivity with purified CKβ and other related proteins like human ethanolamine kinases (EK) and yeast choline or ethanolamine kinases. The antiserum simultaneously detected both CKα1 and α2 isoforms in MCF-7 and HepG2 cell extracts, but not in HeLa, HCT-116, and mouse embryonic stem cell extracts. Subsequent protein dot blot assay of total CKα in a human normal/tumor protein array of 30 tissue samples by using the antiserum showed that CKα was not overexpressed in all tumor tissues when compared to their normal counterparts. Most striking differences between tumor and normal CKα expression levels were observed in kidney (11-fold higher in tumor) and liver (15-fold lower in tumor) samples.
Apart from its high sensitivity and specificity, the antiserum produced in this work, which does not require further purification, has the advantage of co-detecting both α1 and α2 isoforms in cell extracts for direct comparison of their expression levels.
Parathyroid hormone (PTH) and phorbol-12,13-dibutyrate (PDBu) stimulate phospholipase D (PLD) activity and phosphatidylcholine (PC) hydrolysis in UMR-106 osteoblastic cells . The current studies were designed to determine whether ethanolamine-containing phospholipids, and specifically phosphatidylethanolamine (PE), could also be substrates. In cells labeled with 14C-ethanolamine PTH and PDBu treatment decreased 14C-phosphatidylethanolamine. In cells co-labeled with 3H-choline and 14C-ethanolamine, PTH and PDBu treatment increased both 3H-choline and 14C-ethanolamine release from the cells. Choline and ethanolamine phospholipid hydrolysis was increased within 5 min, and responses were sustained for at least 60 min. Maximal effects were obtained with 10 nM PTH and 50 nM PDBu. Dominant negative PLD1 and PLD2 constructs inhibited the effects of PTH on the phospholipid hydrolysis. The results suggest that both PC and PE are substrates for phospholipase D in UMR-106 osteoblastic cells and could therefore be sources of phospholipid hydrolysis products for downstream signaling in osteoblasts.
Phosphatidylethanolamine; phosphatidylcholine; phospholipase D; parathyroid hormone; osteoblast; UMR-106
Normal mammary epithelial cells (ethanolamine responsive) require ethanolamine to enable them to grow in defined culture medium because they cannot synthesize de novo a sufficient amount of phosphatidylethanolamine. Mammary tumor cells which retain properties of the normal tissue are also likely to be ethanolamine responsive, whereas dedifferentiated, highly tumorigenic mammary tumor cells are ethanolamine nonresponsive. The nonresponsive tumor cells are able to synthesize the necessary amount of phosphatidylethanolamine to sustain growth. Therefore, the progression of malignancy seems to convert ethanolamine-responsive mammary cells to ethanolamine-nonresponsive ones. In an attempt to prove the above assumption and to understand the mechanism responsible for the conversion during the progression of malignant transformation, mammary tumor cell line 64-24, which is typically ethanolamine responsive, was transfected with simian virus 40, polyomavirus, EJ-ras, or v-myc oncogenes, and the resulting transfectants were examined for their growth response to ethanolamine. Many of the transfectants exhibited typical transformed phenotypes; however, none of the transfectants converted to ethanolamine-nonresponsive cells. Some of the SV40 and polyomavirus transformants were able to grow in the absence of ethanolamine, although they grew better in the presence of ethanolamine, unlike typical ethanolamine-nonresponsive cells. These cells could grow in the absence of ethanolamine, even though their membrane phospholipid was phosphatidylethanolamine deficient. The present study indicates that the expression of any one of the four oncogenes tested, which allows the cells to exhibit transformed phenotypes in 64-24 cells, is not sufficient for the conversion of ethanolamine-responsive cells to -nonresponsive cells.
Phosphatidyl ethanolamine and lipopolysaccharide were extracted and purified from the cell envelope fractions of Escherichia coli and Salmonella typhimurium. The two components were studied separately and after recombination, by use of electron microscopy and monolayer techniques, and by measuring their ability to participate in the enzyme-catalyzed uridine diphosphate-galactose:lipopolysaccharide α, 3 galactosyl transferase reaction, which requires a lipopolysaccharide-phospholipid complex as substrate. Electron microscopy of purified lipopolysaccharide showed a uniform population of hollow spheres, with each sphere bounded by a continuous leaflet. The diameter of the spheres was approximately 500 to 1,000 A, and the thickness of the enveloping leaflet was approximately 30 A. Phosphatidyl ethanolamine showed a regular lamellar structure. When lipopolysaccharide and phosphatidyl ethanolamine were mixed under conditions of heating and slow-cooling, the leaflet of the lipopolysaccharide spheroids appeared to extend directly into the phosphatidyl ethanolamine structure, with continuity between the two leaflets. Various stages of penetration were seen. At high concentrations of lipopolysaccharide, there were disruptive changes in phosphatidyl ethanolamine leaflets similar to those seen when saponin acts on cholesterol-lecithin leaflets. Monolayer experiments indicated that lipopolysaccharide penetrated a monomolecular film of phosphatidyl ethanolamine at an air-water interface, as revealed by an increase in surface pressure. The results indicate that a common leaflet structure containing lipopolysaccharide and phosphatidyl ethanolamine may be formed in vitro, and suggest that a similar leaflet may exist in the intact bacterial cell envelope.
The bacterium Pelobacter carbinolicus is able to grow by fermentation, syntrophic hydrogen/formate transfer, or electron transfer to sulfur from short-chain alcohols, hydrogen or formate; it does not oxidize acetate and is not known to ferment any sugars or grow autotrophically. The genome of P. carbinolicus was sequenced in order to understand its metabolic capabilities and physiological features in comparison with its relatives, acetate-oxidizing Geobacter species.
Pathways were predicted for catabolism of known substrates: 2,3-butanediol, acetoin, glycerol, 1,2-ethanediol, ethanolamine, choline and ethanol. Multiple isozymes of 2,3-butanediol dehydrogenase, ATP synthase and [FeFe]-hydrogenase were differentiated and assigned roles according to their structural properties and genomic contexts. The absence of asparagine synthetase and the presence of a mutant tRNA for asparagine encoded among RNA-active enzymes suggest that P. carbinolicus may make asparaginyl-tRNA in a novel way. Catabolic glutamate dehydrogenases were discovered, implying that the tricarboxylic acid (TCA) cycle can function catabolically. A phosphotransferase system for uptake of sugars was discovered, along with enzymes that function in 2,3-butanediol production. Pyruvate:ferredoxin/flavodoxin oxidoreductase was identified as a potential bottleneck in both the supply of oxaloacetate for oxidation of acetate by the TCA cycle and the connection of glycolysis to production of ethanol. The P. carbinolicus genome was found to encode autotransporters and various appendages, including three proteins with similarity to the geopilin of electroconductive nanowires.
Several surprising metabolic capabilities and physiological features were predicted from the genome of P. carbinolicus, suggesting that it is more versatile than anticipated.
Pelobacter; Genome; Metabolism; Physiology; Geobacter; 2,3-butanediol
A mutant (JY2190) of Streptococcus pneumoniae Rx1 which had acquired the ability to grow in the absence of choline and analogs was isolated. Lipoteichoic acid (LTA) and wall teichoic acid (TA) isolated from the mutant were free of phosphocholine and other phosphorylated amino alcohols. Both polymers showed an unaltered chain structure and, in the case of LTA, an unchanged glycolipid anchor. The cell wall composition was also not altered except that, due to the lack of phosphocholine, the phosphate content of cell walls was half that of the parent strain. Isolated cell walls of the mutant were resistant to hydrolysis by pneumococcal autolysin (N-acetylmuramyl-l-alanine amidase) but were cleaved by the muramidases CPL and cellosyl. The lack of active autolysin in the mutant cells became apparent by impaired cell separation at the end of cell division and by resistance against stationary-phase and penicillin-induced lysis. As a result of the absence of choline in the LTA, pneumococcal surface protein A (PspA) was no longer retained on the cytoplasmic membrane. During growth in the presence of choline, which was incorporated as phosphocholine into LTA and TA, the mutant cells separated normally, did not release PspA, and became penicillin sensitive. However, even under these conditions, they did not lyse in the stationary phase, and they showed poor reactivity with antibody to phosphocholine and an increased release of C-polysaccharide from the cell. In contrast to ethanolamine-grown parent cells (A. Tomasz, Proc. Natl. Acad. Sci. USA 59:86–93, 1968), the choline-free mutant cells retained the capability to undergo genetic transformation but, compared to Rx1, with lower frequency and at an earlier stage of growth. The properties of the mutant could be transferred to the parent strain by DNA of the mutant.
Washed human platelets were incubated with radioactive glycerol; the platelets were able to synthesize de novo the major phosphoglycerides including phosphatidic acid, phosphatidylinositol, phosphatidyl choline, phosphatidyl ethanolamine, and phosphatidyl serine. The specific activities of the phosphoglycerides obtained after glycerol incorporation indicate that phosphatidic acid, phosphatidylinositol, and phosphatidyl choline are metabolically active relative to phosphatidyl ethanolamine and that formation of phosphatidyl serine occurs to a much more limited extent. When platelets were incubated with bovine thrombin, 1 U/ml, the pattern of glycerol incorporation into phospholipid was changed. There was a 3-fold decrease in the total incorporation into lipid in 30 min with a relative 5-fold decreased incorporation into phosphatidyl choline and phosphatidyl ethanolamine and a 5-fold increased incorporation into phosphatidyl serine. The increased incorporation into phosphatidyl serine. The increased incorporation into phosphatidyl serine was maximal within the first 2 min but was transient, since within 20 minutes, the rate returned to that seen in platelets incubated with glycerol alone. Purified human thrombin also produced this same effect on phospholipid synthesis in platelets. Trypsin produced effects on phosphoglyceride formation similar to those seen with thrombin, and the trypsin-induced effect was inhibited by prior incubation of trypsin with soybean trypsin inhibitor, suggesting that proteolysis may be required for the observed effects on phospholipid synthesis.
Dysregulated choline metabolism is a well-known feature of breast cancer, but the underlying mechanisms are not fully understood. In this study, the metabolomic and transcriptomic characteristics of a large panel of human breast cancer xenograft models were mapped, with focus on choline metabolism.
Tumor specimens from 34 patient-derived xenograft models were collected and divided in two. One part was examined using high-resolution magic angle spinning (HR-MAS) MR spectroscopy while another part was analyzed using gene expression microarrays. Expression data of genes encoding proteins in the choline metabolism pathway were analyzed and correlated to the levels of choline (Cho), phosphocholine (PCho) and glycerophosphocholine (GPC) using Pearson’s correlation analysis. For comparison purposes, metabolic and gene expression data were collected from human breast tumors belonging to corresponding molecular subgroups.
Most of the xenograft models were classified as basal-like (N = 19) or luminal B (N = 7). These two subgroups showed significantly different choline metabolic and gene expression profiles. The luminal B xenografts were characterized by a high PCho/GPC ratio while the basal-like xenografts were characterized by highly variable PCho/GPC ratio. Also, Cho, PCho and GPC levels were correlated to expression of several genes encoding proteins in the choline metabolism pathway, including choline kinase alpha (CHKA) and glycerophosphodiester phosphodiesterase domain containing 5 (GDPD5). These characteristics were similar to those found in human tumor samples.
The higher PCho/GPC ratio found in luminal B compared with most basal-like breast cancer xenograft models and human tissue samples do not correspond to results observed from in vitro studies. It is likely that microenvironmental factors play a role in the in vivo regulation of choline metabolism. Cho, PCho and GPC were correlated to different choline pathway-encoding genes in luminal B compared with basal-like xenografts, suggesting that regulation of choline metabolism may vary between different breast cancer subgroups. The concordance between the metabolic and gene expression profiles from xenograft models with breast cancer tissue samples from patients indicates that these xenografts are representative models of human breast cancer and represent relevant models to study tumor metabolism in vivo.
The Saccharomyces cerevisiae CKI1-encoded choline kinase catalyzes the committed step in phosphatidylcholine synthesis via the Kennedy pathway. The enzyme is phosphorylated on multiple serine residues, and some of this phosphorylation is mediated by protein kinase A. In this work, we examined the hypothesis that choline kinase is also phosphorylated by protein kinase C. Using choline kinase as a substrate, protein kinase C activity was dose- and time-dependent, and dependent on the concentrations of choline kinase (Km = 27 μg/ml) and ATP (Km = 15 μM). This phosphorylation, which occurred on a serine residue, was accompanied by a 1.6-fold stimulation of choline kinase activity. The synthetic peptide SRSSS25QRRHS (Vmax/Km = 17.5 mM-1 μmol min-1 mg-1) that contains the protein kinase C motif for Ser25 was a substrate for protein kinase C. A Ser25 to Ala (S25A) mutation in choline kinase resulted in a 60% decrease in protein kinase C phosphorylation of the enzyme. Phosphopeptide mapping analysis of the S25A mutant enzyme confirmed that Ser25 was a protein kinase C target site. In vivo, the S25A mutation correlated with a decrease (55%) in phosphatidylcholine synthesis via the Kennedy pathway whereas an S25D phosphorylation site mimic correlated with an increase (44%) in phosphatidylcholine synthesis. Whereas the S25A (protein kinase C site) mutation did not affect the phosphorylation of choline kinase by protein kinase A, the S30A (protein kinase A site) mutation caused a 46% reduction in enzyme phosphorylation by protein kinase C. A choline kinase synthetic peptide (SQRRHS30LTRQ) containing Ser30 was a substrate (Vmax/Km = 3.0 mM−1 μmol min−1 mg−1) for protein kinase C. Comparison of phosphopeptide maps of the wild type and S30A mutant choline kinase enzymes phosphorylated by protein kinase C confirmed that Ser30 was also a target site for protein kinase C.
Choline transport of Saccharomyces cerevisiae was measured by the filtration method with the use of glass microfiber paper. The uptake was time and temperature dependent. The kinetics of choline transport showed Michaelis behavior; an appearent Km for choline was 0.56 microM. N-Methylethanolamine, N,N-dimethylethanolamine, and beta-methylcholine were competitive inhibitors of choline transport, with Ki values of 40.1, 3.1, and 6.9 microM, respectively. Ethanolamine, phosphorylcholine, and various amino acids examined had no effect. Choline transport required metabolic energy; removal of glucose resulted in a great loss of transport activity, and the remaining activity was abolished by 2,4-dinitrophenol, carbonyl cyanide p-trifluoromethoxyphenyl hydrazone, arsenate, and cyanide. External Na+ was not required, and the transport was not effected by ionophores, valinomycin, and gramicidin D. These results indicate that S. cerevisiae possess an active choline transport system mediated by a specific carrier. This view is further supported by the isolation and characterization of a choline transport mutant. The choline transport activity in this mutant was very low, whereas the transport of L-leucine, L-methionine, D-glucose, and myo-inositol was normal. Together with the choline transport mutant, mutants defective in choline kinase were also isolated.
Choline-O-sulfate uptake by Penicillium notatum showed the following characteristics. (i) Transport was mediated by a permease which is highly specific for choline-O-sulfate. No significant inhibition of transport was caused by choline, choline-O-phosphate, acetylcholine, ethanolamine-O-phosphate, ethanolamine-O-sulfate, methanesulfonyl choline, 2-aminoethane thiosulfate, or the monomethyl or dimethyl analogues of choline-O-sulfate. Similarly, no significant inhibition was caused by any common sulfur amino acid or inorganic sulfur compound. Mutants lacking the inorganic sulfate permease possessed the choline-O-sulfate permease at wild-type levels. (ii) Choline-O-sulfate transport obeyed saturation kinetics (Km = 10−4 to 3 × 10−4m; Vmax = 1 to 6 μmoles per g per min). The kinetics of transport between 10−9 and 10−1m external choline-O-sulfate showed that only one saturable mechanism is present. (iii) Transport was sensitive to 2,4-dinitrophenol, azide, N-ethylmaleimide, p-chloromercuribenzoate, and cyanide. Ouabain, phloridzin, and eserine had no effect. (iv) Transport was pH-dependent with an optimum at pH 6. Variations in the ionic strength of the incubation medium had no effect. (v) Transport was temperature-dependent with a Q10 of greater than 2 between 3 and 40 C. Transport decreased rapidly above 40 C. (vi) Ethylenediaminetetraacetate (sodium salts, pH 6) had no effect, nor was there any stimulation by metal or nonmetal ions. Cu++, Ag+, and Hg++ were inhibitory. (vii) The initial rate at which the ester is transported was independent of intracellular hydrolysis. After long periods of incubation (> 10 min), a significant proportion of the transported choline-O-sulfate was hydrolyzed intracellulary. In the presence of 5 × 10−3m external choline-O-sulfate, the mycelia accumulated choline-O-sulfate to an apparent intracellular concentration of 0.075 m by 3 hr. Transport was unidirectional. No efflux or exchange of 35S-choline-O-sulfate was observed when preloaded mycelia were suspended in buffer alone or in buffer containing a large excess of unlabeled choline-O-sulfate. (viii) The specific transport activity of the mycelium depended on the sulfur source used for growth. (ix) Sulfur starvation of sulfur-sufficient mycelium resulted in an increase in the specific transport activity of the mycelium. This increase was prevented by cycloheximide, occurred only when a metabolizable carbon source was present, and resulted from an increase in the Vmax of the permease, rather than from a decrease in Km. The increase could be partially reversed by refeeding the mycelia with unlabeled choline-O-sulfate, sulfide, sulfite, l-homocysteine, l-cysteine, or compounds easily converted to cysteine. The results strongly suggested that the choline-O-sulfate permease is regulated primarily by repression-derepression, but that intracellular choline-O-sulfate and cysteine can act as feedback inhibitors.
CTP synthetase is a cytosolic-associated glutamine amidotransferase enzyme that catalyzes the ATP-dependent transfer of the amide nitrogen from glutamine to the C-4 position of UTP to form CTP. In the yeast Saccharomyces cerevisiae, the reaction product CTP is an essential precursor of all membrane phospholipids that are synthesized via the Kennedy (CDP-choline and CDP-ethanolamine branches) and CDP-diacylglycerol pathways. The URA7 and URA8 genes encode CTP synthetase in S. cerevisiae, and the URA7 gene is responsible for the majority of CTP synthesized in vivo. The CTP synthetase enzymes are allosterically regulated by CTP product inhibition. Mutations that alleviate this regulation result in an elevated cellular level of CTP and an increase in phospholipid synthesis via the Kennedy pathway. The URA7-encoded enzyme is phosphorylated by protein kinases A and C, and these phosphorylations stimulate CTP synthetase activity and increase cellular CTP levels and the utilization of the Kennedy pathway. The CTPS1 and CTPS2 genes that encode human CTP synthetase enzymes are functionally expressed in S. cerevisiae, and rescue the lethal phenotype of the ura7Δ ura8Δ double mutant that lacks CTP synthetase activity. The expression in yeast has revealed that the human CTPS1-encoded enzyme is also phosphorylated and regulated by protein kinases A and C.
CTP; CTP synthetase; CDP-diacylglycerol; CDP-choline; CDP-ethanolamine; phospholipid synthesis; phosphorylation
Altered choline phospholipid metabolism is a hallmark of cancer, leading to malignant choline metabolite profiles consisting of low glycerophosphocholine (GPC) and high phosphocholine (PC) in human breast cancers. Glycerophosphocholine phosphodiesterase (GPC-PDE) catalyzes the degradation of GPC to free choline and glycerol-3-phosphate. The gene(s) encoding for the GPC-PDE(s) responsible for GPC degradation in breast cancers have not yet been identified. Here we have demonstrated for the first time that the GPC-PDE encoded by glycerophosphodiester phosphodiesterase domain containing 5 (GDPD5) is associated with breast cancer malignancy. Two human breast cancer cell lines (n=8 and 10) and primary human breast tumor samples (n=19) were studied with combined magnetic resonance spectroscopy (MRS) and qRT-PCR to investigate several isoforms of GDPD expression with respect to choline phospholipid metabolite levels. Out of five GDPDs tested, GDPD5 was found to be significantly overexpressed in highly malignant estrogen receptor negative (ER−) compared to weakly malignant estrogen receptor positive (ER+) human breast cancer cells (P=0.027) and breast tumors from patients (P=0.015). GDPD5 showed significantly positive correlations with PC (P<0.001), total choline (tCho) (P=0.007) and PC/GPC (P<0.001) levels in human breast tumors. GDPD5 showed a trend towards negative correlation with GPC levels (P=0.130). Human breast cancers with malignant choline metabolite profiles consisting of low GPC and high PC levels highly co-expressed GDPD5, choline kinase alpha (CHKA), and phosphatidylcholine-specific phospholipase D1 (PLD1), while cancers containing high GPC and relatively low PC levels displayed low co-expression of GDPD5, CHKA, and PLD1. GDPD5, CHKA and PLD1 were significantly overexpressed in highly malignant ER− tumors in our patient cohort. Our study identified GDPD5 as a GPC-PDE that likely participates in regulating choline phospholipid metabolism in breast cancer, which possibly occurs in cooperation with CHKA and PLD1.
choline phospholipid metabolism; glycerophosphodiester phosphodiesterase domain containing 5; choline kinase; magnetic resonance spectroscopy; breast cancer
To investigate the anti-kinetoplastid activity of choline-derived analogues with previously reported antimalarial efficacy.
From an existing choline analogue library, seven antimalarial compounds, representative of the first-, second- and third-generation analogues previously developed, were assessed for activity against Trypanosoma and Leishmania spp. Using a variety of techniques, the effects of choline analogue exposure on the parasites were documented and a preliminary investigation of their mode of action was performed.
The activities of choline-derived compounds against Trypanosoma brucei and Leishmania mexicana were determined. The compounds displayed promising anti-kinetoplastid activity, particularly against T. brucei, to which 4/7 displayed submicromolar EC50 values for the wild-type strain. Low micromolar concentrations of most compounds cleared trypanosome cultures within 24–48 h. The compounds inhibit a choline transporter in Leishmania, but their entry may not depend only on this carrier; T. b. brucei lacks a choline carrier and the mode of uptake remains unclear. The compounds had no effect on the overall lipid composition of the cells, cell cycle progression or cyclic adenosine monophosphate production or short-term effects on intracellular calcium levels. However, several of the compounds, displayed pronounced effects on the mitochondrial membrane potential; this action was not associated with production of reactive oxygen species but rather with a slow rise of intracellular calcium levels and DNA fragmentation.
The choline analogues displayed strong activity against kinetoplastid parasites, particularly against T. b. brucei. In contrast to their antimalarial activity, they did not act on trypanosomes by disrupting choline salvage or phospholipid metabolism, instead disrupting mitochondrial function, leading to chromosomal fragmentation.
Trypanosoma brucei; leishmaniasis; protozoan parasite; lipid metabolism; choline
Choline kinase is the first enzyme in the Kennedy pathway (CDP-choline pathway) for the biosynthesis of the most essential phospholipid, phosphatidylcholine, in Plasmodium falciparum. In addition, choline kinase also plays a pivotal role in trapping essential polar head group choline inside the malaria parasite. Recently, Plasmodium falciparum choline kinase (PfCK) has been cloned, overexpressed, and purified. However, the function of this enzyme in parasite growth and survival has not been evaluated owing to the lack of a suitable inhibitor. Purified recombinant PfCK enabled us to identify an inhibitor of PfCK, hexadecyltrimethylammonium bromide (HDTAB), which has a very close structural resemblance to hexadecylphosphocholine (miltefosin), the well-known antiproliferative and antileishmanial drug. HDTAB inhibited PfCK in a dose-dependent manner and offered very potent antimalarial activity in vitro against Plasmodium falciparum. Moreover, HDTAB exhibited profound antimalarial activity in vivo against the rodent malaria parasite Plasmodium yoelii (N-67 strain). Interestingly, parasites at the trophozoite and schizont stages were found to be particularly sensitive to HDTAB. The stage-specific antimalarial effect of HDTAB correlated well with the expression pattern of PfCK in P. falciparum, which was observed by reverse transcription-PCR and immunofluorescence microscopy. Furthermore, the antimalarial activity of HDTAB paralleled the decrease in phosphatidylcholine content, which was found to correlate with the decreased phosphocholine generation. These results suggest that inhibition of choline kinase by HDTAB leads to decreased phosphocholine, which in turn causes a decrease in phosphatidylcholine biosynthesis, resulting in death of the parasite.