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

Regulation of Phosphatidylethanolamine Homeostasis—The Critical Role of CTP:Phosphoethanolamine Cytidylyltransferase (Pcyt2)

Phosphatidylethanolamine (PE) is the most abundant lipid on the protoplasmatic leaflet of cellular membranes. It has a pivotal role in cellular processes such as membrane fusion, cell cycle regulation, autophagy, and apoptosis. CTP:phosphoethanolamine cytidylyltransferase (Pcyt2) is the main regulatory enzyme in de novo biosynthesis of PE from ethanolamine and diacylglycerol by the CDP-ethanolamine Kennedy pathway. The following is a summary of the current state of knowledge on Pcyt2 and how splicing and isoform specific differences could lead to variations in functional properties in this family of enzymes. Results from the most recent studies on Pcyt2 transcriptional regulation, promoter function, autophagy, and cell growth regulation are highlighted. Recent data obtained from Pcyt2 knockout mouse models is also presented, demonstrating the essentiality of this gene in embryonic development as well as the major physiological consequences of deletion of one Pcyt2 allele. Those include development of symptoms of the metabolic syndrome such as elevated lipogenesis and lipoprotein secretion, hypertriglyceridemia, liver steatosis, obesity, and insulin resistance. The objective of this review is to elucidate the nature of Pcyt2 regulation by linking its catalytic function with the regulation of lipid and energy homeostasis.

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.

Genomic organization, promoter activity, and expression of the human choline transporter-like protein 1

Choline transporter-like (CTL) proteins of the CTL1 family are novel transmembrane proteins implicated in choline transport for phospholipid synthesis. In this study, we characterized the 5'-flanking region of the human (h)CTL1 gene and examined some of the possible mechanisms of its regulation, including promoter activity, splicing, and expression. The transcription start site of the hCTL1 gene was mapped by 5'-rapid amplification of cDNA ends (RACE), and the presence of two splice variants, hCTL1a and hCTL1b, was investigated using isoform-specific PCR and 3'-RACE. The hCTL1 promoter region of approximately 900 bp was isolated from MCF-7 human breast cancer cells. The promoter was TATA-less and driven by a long stretch of GC-rich sequence in accordance with widespread expression of hCTL1 at both mRNA and protein levels. Deletion analyses demonstrated that a very strong promoter is contained within 500 bp of the transcription start site, and more upstream regions did not increase its activity. The core promoter that conferred the minimal transcription is within the -188/+27-bp region, and its activity varied in human breast cancer and mouse skeletal muscle cells. Multiple motifs within the promoter regulatory region bound nuclear factors from both cultured cells and normal human skeletal muscle. The motifs within the three regions [S1 (-92/-61 bp), S2 (-174/-145 bp), and S3 (-289/-260 bp)] contained overlapping binding sites for hematopoietic transcription factors and ubiquitous transcription factors, in line with the expected gene function. Genomic analyses demonstrated a high conservation of hCTL1 and mouse CTL1 proximal promoters. Accordingly, mRNA profiles demonstrated that human splice variants were expressed ubiquitously, as demonstrated for the mouse transcripts; however, they differed from the profiles of rat CTL1 transcripts, which were more restricted to neurons and intestinal tissues. The shorter hCTL1b variant contained the cytosolic COOH-terminal motif L651KKR654 for endoplasmic reticulum retrieval/retention. This retention signal was conserved in hCTL1b and rat and mouse CTL1b and is typical for transmembrane proteins of type 1 topology.

Completing the cycles; the dynamics of endonuclear lipidomics

Signal transductions via periodic generation and mobilisation of lipid second messengers within the nuclear matrix of eukaryotic cells have focused renewed attention on their precursor phospholipids' location, structure, form and function. The nuclear matrix contains and supports dynamic pools of phosphatidylcholine and phosphatidylinositol which serve as parent molecules of lipid second messengers but also of other phospholipids requiring cyclical replacement as cells proliferate. Applications of new, highly sensitive and specific analytical methodologies based on tandem electrospray ionisation mass spectrometry and the use of stable isotopes have allowed both static and dynamic lipidomic profiling of these endonuclear phospholipid pools. Together with more conventional enzymatic analyses and evaluation of the effect of specific "knock-out" of phospholipid transfer capacity, a number of important principles have been established. Specifically, a compartmental capacity to synthesise and remodel highly saturated phosphatidylcholine exists alongside transport mechanisms that facilitate the nuclear import of phosphatidylinositol and other phospholipids synthesised elsewhere within the cell. Subnuclear fractionation and the use of newly emerging techniques for sensitive lipidomic profiling of polyphosphoinositides, diacylglycerols and phosphatidate molecular species offer the potential for further significant advances in the near future.

The rate-limiting enzyme in phosphatidylcholine synthesis regulates proliferation of the nucleoplasmic reticulum

The nucleus contains a network of tubular invaginations of the nuclear envelope (NE), termed the nucleoplasmic reticulum (NR), implicated in transport, gene expression, and calcium homeostasis. Here, we show that proliferation of the NR, measured by the frequency of NE invaginations and tubules, is regulated by CTP:phosphocholine cytidylyltransferase-alpha (CCTalpha), the nuclear and rate-limiting enzyme in the CDP-choline pathway for phosphatidylcholine (PtdCho) synthesis. In Chinese hamster ovary (CHO)-K1 cells, fatty acids triggered activation and translocation of CCTalpha onto intranuclear tubules characteristic of the NR. This was accompanied by a twofold increase in NR tubules quantified by immunostaining for lamin A/C or the NE. CHO MT58 cells expressing a temperature-sensitive CCTalpha allele displayed reduced PtdCho synthesis and CCTalpha expression and minimal proliferation of the NR in response to oleate compared with CHO MT58 cells stably expressing CCTalpha. Expression of CCTalpha mutants in CHO58 cells revealed that both enzyme activity and membrane binding promoted NR proliferation. In support of a direct role for membrane binding in NR tubule formation, recombinant CCTalpha caused the deformation of liposomes into tubules in vitro. This demonstrates that a key nuclear enzyme in PtdCho synthesis coordinates lipid synthesis and membrane deformation to promote formation of a dynamic nuclear-cytoplasmic interface.

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.

The selective utilization of substrates in vivo by the phosphatidylethanolamine and phosphatidylcholine biosynthetic enzymes Ept1p and Cpt1p in yeast

In yeast, the aminoalcohol phosphotransferases Ept1p and Cpt1p catalyze the final steps in the CDP-ethanolamine and CDP-choline routes leading to phosphatidylethanolamine (PE) and phosphatidylcholine (PC), respectively. To determine how these enzymes contribute to the molecular species profiles of PE and PC in vivo, wild-type, cpt1Delta, and ept1Delta cells were pulse labeled with deuterated ethanolamine and choline. Analysis of newly synthesized PE and PC using electrospray ionization tandem mass spectrometry revealed that PE and PC produced by Ept1p and Cpt1p have different species compositions, demonstrating that the enzymes consume distinct sets of diacylglycerol species in vivo. Using the characteristic phospholipid species profiles produced by Ept1p and Cpt1p as molecular fingerprints, it was also shown that in vivo CDP-monomethylethanolamine is preferentially used as substrate by Ept1p, whereas CDP-dimethylethanolamine and CDP-propanolamine are converted by Cpt1p.

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.

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.

Biosynthesis of phosphatidylcholine from a phosphocholine precursor pool derived from the late endosomal/lysosomal degradation of sphingomyelin

Previous studies suggest that the steps of the CDP- choline pathway of phosphatidylcholine synthesis are tightly linked in a so-called metabolon. Evidence has been presented that only choline that enters cells through the choline transporter, and not phosphocholine administered to cells by membrane permeabilization, is incorporated into phosphatidylcholine. Here, we show that [(14)C]phosphocholine derived from the lysosomal degradation of [(14)C]choline-labeled sphingomyelin is incorporated as such into phosphatidylcholine in human and mouse fibroblasts. Low density lipoprotein receptor-mediated endocytosis was used to specifically direct [(14)C]sphingomyelin to the lysosomal degradation pathway. Free labeled choline was not found either intracellularly or in the medium, not even when the cells were energy-depleted. Deficiency of lysosomal acid phosphatases in mouse or alkaline phosphatase in human fibroblasts did not affect the incorporation of lysosomal [(14)C]sphingomyelin-derived [(14)C]phosphocholine into phosphatidylcholine, supporting our finding that phosphocholine is not degraded to choline prior to its incorporation into phosphatidylcholine. Inhibition studies and analysis of molecular species showed that exogenous [(3)H]choline and sphingomyelin-derived [(14)C]phosphocholine are incorporated into phosphatidylcholine via a common pathway of synthesis. Our findings provide evidence that, in fibroblasts, phosphocholine derived from sphingomyelin is transported out of the lysosome and subsequently incorporated into phosphatidylcholine without prior hydrolysis of phosphocholine to choline. The findings do not support the existence of a phosphatidylcholine synthesis metabolon in fibroblasts.

Substrate channeling

Substrate channeling is the process in which the intermediate produced by one enzyme is transferred to the next enzyme without complete mixing with the bulk phase. This process is equivalent to a microcompartmentation of the intermediate, although classic diffusion occurs simultaneously to varying extents in many of these cases. This microcompartmentation and other factors of channeling provide many potential biological advantages. Extensive examples of channeling can be found in the cited reviews. The choice of methods to detect and characterize substrate channeling depends extensively on the type of enzyme associations involved, the constants of the system, and, to some extent, the mechanism of channeling. Thus it is important to distinguish stable, dynamic, and catalytically induced enzyme associations as well as recognize different mechanisms of substrate channeling. We discuss the principles, experimental details, and limitations and precautions of five rather general methods. These use measurements of transient times, isotope dilution or enhancement, competing reaction effects, enzyme buffering kinetics, and transient-state kinetics. These encompass methods applicable to studies in vitro, in situ, and in vivo. None of these methods is applicable to all systems. They are also susceptible to artifacts without proper attention to precautions. Transient-state kinetic methods clearly excel in elucidating molecular mechanisms of channeling. However, they are often not the best method for initial detection and characterization of the process and they are not applicable to many complex systems. Several other methods that have been successful in indicating substrate channeling are briefly described.

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.