Phosphatidylcholine metabolism in cultured cells: catabolism via glycerophosphocholine

Phosphatidylethanolamine N-Methyltransferase Knockout Modulates Metabolic Changes in Aging Mice

Phospholipid metabolism, including phosphatidylcholine (PC) biosynthesis, is crucial for various biological functions and is associated with longevity. Phosphatidylethanolamine N-methyltransferase (PEMT) is a protein that catalyzes the biosynthesis of PC, the levels of which change in various organs such as the brain and kidneys during aging. However, the role of PEMT for systemic PC supply is not fully understood. To address how PEMT affects aging-associated energy metabolism in tissues responsible for nutrient absorption, lipid storage, and energy consumption, we employed NMR-based metabolomics to study the liver, plasma, intestine (duodenum, jejunum, and ileum), brown/white adipose tissues (BAT and WAT), and skeletal muscle of young (9-10 weeks) and old (91-132 weeks) wild-type (WT) and PEMT knockout (KO) mice. We found that the effect of PEMT-knockout was tissue-specific and age-dependent. A deficiency of PEMT affected the metabolome of all tissues examined, among which the metabolome of BAT from both young and aged KO mice was dramatically changed in comparison to the WT mice, whereas the metabolome of the jejunum was only slightly affected. As for aging, the absence of PEMT increased the divergence of the metabolome during the aging of the liver, WAT, duodenum, and ileum and decreased the impact on skeletal muscle. Overall, our results suggest that PEMT plays a previously underexplored, critical role in both aging and energy metabolism.

Identification of diagnostic serum biomarkers for Hunner-type interstitial cystitis

OBJECTIVES

Diagnosis of Hunner-type interstitial cystitis (HIC) relies on the ability to identify Hunner lesions endoscopically, which can lead to storage symptom misdiagnosis. Here, we examined serum biomarkers for HIC and verified their utility.

METHODS

Based on the previous definition of the Japanese guidelines, which did not distinguish HIC and non-HIC diseases, we searched for serum biomarkers in 25 patients with interstitial cystitis (IC) and 25 control participants using metabolomics during 2013-2014. In 2019, we conducted a validation study in HIC and control groups. Serum samples were analyzed using liquid chromatography-tandem mass spectrometry, and candidate biomarker concentrations were compared between the groups using Mann-Whitney test.

RESULTS

Metabolomics targeted 678 metabolites and revealed that the levels of 14 lysolipids, seven γ-glutamyl amino acids, and two monoacylglycerols were significantly different between the IC and control groups. The following metabolites were selected from each metabolite category as candidates: 1-linoleoylglycerophosphocholine (1-linoleloyl-GPC [18:2]), γ-glutamylisoleucine (γ-Glu-Ile), and 1-arachidonylglycerol (1-AG). The serum concentrations of 1-linoleoyl-GPC (18:2) in the HIC and control groups were 27 920 ± 6261 and 40 360 ± 1514 ng/mL (P = 0.0003), respectively. The serum concentrations of γ-Glu-Ile and 1-AG were not significantly different between the groups. When the cut-off value of 1-linoleoyl-GPC (18:2) was set at 28 400 ng/mL, the sensitivity and specificity were 68% and 84%, respectively.

CONCLUSIONS

Serum 1-linoleoyl-GPC (18:2) is a candidate diagnostic biomarker for HIC. Additional studies on whether this biomarker can distinguish HIC from other diseases with high urination frequency are required for its clinical use.

Metabolic signature in nucleus accumbens for anti-depressant-like effects of acetyl-L-carnitine

Emerging evidence suggests that hierarchical status provides vulnerability to develop stress-induced depression. Energy metabolic changes in the nucleus accumbens (NAc) were recently related to hierarchical status and vulnerability to develop depression-like behavior. Acetyl-L-carnitine (LAC), a mitochondria-boosting supplement, has shown promising antidepressant-like effects opening therapeutic opportunities for restoring energy balance in depressed patients. We investigated the metabolic impact in the NAc of antidepressant LAC treatment in chronically-stressed mice using ¹H-magnetic resonance spectroscopy (¹H-MRS). High rank, but not low rank, mice, as assessed with the tube test, showed behavioral vulnerability to stress, supporting a higher susceptibility of high social rank mice to develop depressive-like behaviors. High rank mice also showed reduced levels of several energy-related metabolites in the NAc that were counteracted by LAC treatment. Therefore, we reveal a metabolic signature in the NAc for antidepressant-like effects of LAC in vulnerable mice characterized by restoration of stress-induced neuroenergetics alterations and lipid function.

Focus on the glycerophosphocholine pathway in choline phospholipid metabolism of cancer

Activated choline metabolism is a hallmark of carcinogenesis and tumor progression, which leads to elevated levels of phosphocholine and glycerophosphocholine in all types of cancer tested so far. Magnetic resonance spectroscopy applications have played a key role in detecting these elevated choline phospholipid metabolites. To date, the majority of cancer-related studies have focused on phosphocholine and the Kennedy pathway, which constitutes the biosynthesis pathway for membrane phosphatidylcholine. Fewer and more recent studies have reported on the importance of glycerophosphocholine in cancer. In this review article, we summarize the recent literature on glycerophosphocholine metabolism with respect to its cancer biology and its detection by magnetic resonance spectroscopy applications.

Membrane lipid compositional sensing by the inducible amphipathic helix of CCT

The amphipathic helical (AH) membrane binding motif is recognized as a major device for lipid compositional sensing. We explore the function and mechanism of sensing by the lipid biosynthetic enzyme, CTP:phosphocholine cytidylyltransferase (CCT). As the regulatory enzyme in phosphatidylcholine (PC) synthesis, CCT contributes to membrane PC homeostasis. CCT directly binds and inserts into the surface of bilayers that are deficient in PC and therefore enriched in lipids that enhance surface charge and/or create lipid packing voids. These two membrane physical properties induce the folding of the CCT M domain into a ≥60 residue AH. Membrane binding activates catalysis by a mechanism that has been partially deciphered. We review the evidence for CCT compositional sensing, and the membrane and protein determinants for lipid selective membrane-interactions. We consider the factors that promote the binding of CCT isoforms to the membranes of the ER, nuclear envelope, or lipid droplets, but exclude CCT from other organelles and the plasma membrane. The CCT sensing mechanism is compared with several other proteins that use an AH motif for membrane compositional sensing. This article is part of a Special Issue entitled: The cellular lipid landscape edited by Tim P. Levine and Anant K. Menon.

CTP:phosphocholine cytidylyltransferase: Function, regulation, and structure of an amphitropic enzyme required for membrane biogenesis

CTP:phosphocholine cytidylyltransferase (CCT) catalyzes a rate-limiting and regulated step in the CDP-choline pathway for the synthesis of phosphatidylcholine (PC) and PC-derived lipids. Control of CCT activity is multi-layered, and includes direct regulation by reversible membrane binding involving a built-in lipid compositional sensor. Thus CCT contributes to phospholipid compositional homeostasis. CCT also modifies the curvature of its target membrane. Knowledge of CCT structure and regulation of its catalytic function are relatively advanced compared to many lipid metabolic enzymes, and are reviewed in detail. Recently the genetic origins of two human developmental and lipogenesis disorders have been traced to mutations in the gene for CCTα.

Tissue-specific differences in brain phosphodiesters in late-life major depression

OBJECTIVE

Late-life depression has been hypothesized to have a neurodegenerative component that leads to impaired executive function and increases in subcortical white matter hyperintensities. Phosphorus magnetic resonance spectroscopy (MRS) can quantify several important phosphorus metabolites in the brain, particularly the anabolic precursors and catabolic metabolites of the constituents of cell membranes, which could be altered by neurodegenerative activity.

METHODS

Ten patients with late-life major depression who were medication free at time of study and 11 aged normal comparison subjects were studied using (31)P MRS three-dimensional chemical shift imaging at 4 Tesla. Phosphatidylcholine and phosphatidylethanolamine comprise 90% of cell membranes in brain but cannot be quantified precisely with (31)P MRS. We measured phosphocholine and phosphoethanolamine, which are anabolic precursors, as well as glycerophosphocholine and glycerophosphoethanolamine, which are catabolic metabolites of phosphatidylcholine and phosphatidylethanolamine.

RESULTS

In accordance with our hypotheses, glycerophosphoethanolamine was elevated in white matter of depressed subjects, suggesting enhanced breakdown of cell membranes in these subjects. Glycerophosphocholine did not show any significant difference between comparison and depressed subjects but both showed an enhancement in white matter compared with gray matter. Contrary to our hypotheses, neither phosphocholine nor phosphoethanolamine showed evidence for reduction in late-life depression.

CONCLUSION

These findings support the hypothesis that neurodegenerative processes occur in white matter in patients with late-life depression more than in the normal elderly population.

Formation and regulation of mitochondrial membranes

Mitochondrial membrane phospholipids are essential for the mitochondrial architecture, the activity of respiratory proteins, and the transport of proteins into the mitochondria. The accumulation of phospholipids within mitochondria depends on a coordinate synthesis, degradation, and trafficking of phospholipids between the endoplasmic reticulum (ER) and mitochondria as well as intramitochondrial lipid trafficking. Several studies highlight the contribution of dietary fatty acids to the remodeling of phospholipids and mitochondrial membrane homeostasis. Understanding the role of phospholipids in the mitochondrial membrane and their metabolism will shed light on the molecular mechanisms involved in the regulation of mitochondrial function and in the mitochondrial-related diseases.

Energetic and cell membrane metabolic products in patients with primary insomnia: a 31-phosphorus magnetic resonance spectroscopy study at 4 tesla

STUDY OBJECTIVES

Primary insomnia (PI) is a sleep disorder characterized by difficulty with sleep initiation, maintenance, and/or the experience of nonrestorative sleep combined with a subsequent impairment of daytime functioning. The hyperarousal hypothesis has emerged as the leading candidate to explain insomnia symptoms in the absence of specific mental, physical, or substance-related causes. We hypothesized that the cellular energetic metabolites, including beta nucleoside triphosphate, which in magnetic resonance spectroscopy approximates adenosine triphosphate (ATP), and phosphocreatine (PCr), would show changes in PI reflecting increased energy demand.

DESIGN AND SETTING

Matched-groups, cross-sectional study performed at two university-based hospitals.

PATIENTS

Sixteen medication-free individuals (eight males, eight females; mean ± standard deviation (SD) age = 37.2 ± 8.4 y) with PI and 16 good sleepers (nine males, seven females; mean ± SD age = 37.6 ± 4.7 y).

MEASUREMENTS

Diagnosis was established for all individuals by unstructured clinical interview, Structured Clinical Interview for Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition (SCID), sleep diary, and actigraphy. Polysomnography was collected in individuals with PI. Phosphorous magnetic resonance spectroscopy (31P MRS) data were collected on all individuals at 4 Tesla. We assessed cell membrane (anabolic precursors and catabolic metabolites) and bioenergetic (ATP, phosphocreatine) metabolites in gray matter and white matter to determine their relationship to the presence and severity of PI.

RESULTS

Individuals with PI showed lower phosphocreatine in gray matter and an unexpected decrease of phosphocholine, a precursor of the cell membrane compound phosphatidylcholine, in white matter. In addition, there was a trend toward a negative association between polysomnographically determined wake after sleep onset and gray matter beta-nucleoside triphosphate and white matter phosphocholine in the primary insomnia group.

CONCLUSIONS

These results support the hyperarousal hypothesis in PI based on lower phosphocreatine in gray matter in the PI group.

Enzymes involved in osmolyte synthesis: how does oxidative stress affect osmoregulation in renal cells?

Kidney medulla cells are exposed to a wide range of changes in the ionic and osmotic composition of their environment as a consequence of the urine concentrating mechanism. During antidiuresis NaCl and urea concentrations increase and an efficient urinary concentrating mechanism is accompanied by medullar hypoxia. Medullar hypotonicity increases reactive oxygen species, a byproduct of mitochondria during ATP production. High intracellular ionic strength, hypoxia and elevated ROS concentration would have deleterious effects on medulla cell function. Medulla cells respond to hypertonicity by accumulating organic osmolytes, such as glycine betaine, glycerophosphorylcholine, sorbitol, inositol, and taurine, the main functions of which are osmoregulation and osmoprotection. The accumulation of compatible osmolytes is thus crucial for the viability of renal medulla cells. Studies about the effects of reactive oxygen species (ROS) on the enzymes involved in the synthesis of osmolytes are scarce. In this review we summarize the information available on the effects of ROS on the enzymes involved in osmolyte synthesis in kidney.

Phosphatidylcholine catabolism in the MCF-7 cell cycle

The catabolism of phosphatidylcholine (PtdCho) appears to play a key role in regulating the net accumulation of the lipid in the cell cycle. Current protocols for measuring the degradation of PtdCho at specific cell-cycle phases require prolonged periods of incubation with radiolabelled choline. To measure the degradation of PtdCho at the S and G2 phases in the MCF-7 cell cycle, protocols were developed with radiolabelled lysophosphatidylcholine (lysoPtdCho), which reduces the labelling period and minimizes the recycling of labelled components. Although most of the incubated lysoPtdCho was hydrolyzed to glycerophosphocholine (GroPCho) in the medium, the kinetics of the incorporation of label into PtdCho suggests that the labelled GroPCho did not contribute significantly to cellular PtdCho formation. A protocol which involved exposing the cells twice to hydroxyurea, was also developed to produce highly synchronized MCF-7 cells with a profile of G1:S:G2/M of 90:5:5. An analysis of PtdCho catabolism in the synchronized cells following labelling with lysoPtdCho revealed that there was increased degradation of PtdCho in early to mid-S phase, which was attenuated in the G2/M phase. The results suggest that the net accumulation of PtdCho in MCF-7 cells may occur in the G2 phase of the cell cycle.

Regulation of phosphatidylcholine homeostasis by calcium-independent phospholipase A2

Phosphatidylcholine (PtdCho) is the most abundant phospholipid in mammalian cell membranes and is essential for cell viability. The levels of this lipid must be tightly controlled to maintain homeostasis. Therefore, changes in the rate of PtdCho synthesis are generally balanced by changes in PtdCho catabolism and vice versa. It is commonly accepted that the rate of PtdCho synthesis is regulated by CTP:phosphocholine cytidylyltransferase (CT). However, it is not certain if PtdCho mass is regulated by specific catabolic enzyme(s). Our goal is to determine if PtdCho homeostasis is regulated by a phospholipase A2 (PLA2). To this end, we have prepared Chinese hamster ovary (CHO) cell lines that overexpress CT. CT activity is 7-10-fold higher in the transfected cells than in parental CHO cells. This increase in CT activity is associated with increases in both PtdCho synthesis and PtdCho catabolism. Glycerophosphocholine is the PtdCho catabolite that accumulates in the transfected cells, which suggests that PtdCho turnover is mediated by a phospholipase A2 (PLA2). Indeed, higher levels of calcium-independent PLA2 activity are measured in the cytosols of the CHO cells that overexpress CT, compared to parental CHO cells. The elevated calcium-independent PLA2 activity is associated with increases in the expression of the 80-kDa calcium-independent PLA2 (iPLA2). Together, these data suggest that the 80-kDa iPLA2 may be modulated in response to changes in PtdCho levels and therefore is involved in the regulation of PtdCho homeostasis in CHO cells.

Metabolism of the 'organic osmolyte' glycerophosphorylcholine in isolated rat inner medullary collecting duct cells. II. Regulation by extracellular osmolality

In isolated inner medullary collecting duct (IMCD) cells requirements for the organic osmolyte glycerophosphorylcholine (GPC) vary with extracellular osmolality. To investigate mechanisms of osmotic adaptation GPC metabolism was studied under different osmotic conditions. In contrast to the GPC precursors choline and phosphatidylcholine (PC) cellular GPC was proportional to the osmolality. Hypotonic decrease in cellular GPC was mediated by fast initial release significantly exceeding the low hypertonic release. In long-term studies the total amount of GPC decreased significantly under hypotonic conditions but remained constant under hypertonic conditions resulting in a significant difference after 15 h. To investigate osmotic influences on GPC synthesis and GPC degradation studies with [methyl-3H]choline were performed. Pulse-chase experiments displayed no significant osmotic differences in PC synthesis or in PC degradation to GPC indicated by a similar specific activity of PC. This suggested that phospholipase A2 (PC degradation) was osmotically insensitive. A small and distinct metabolic PC pool may be responsible for high radioactive labeling of newly synthesized GPC which displayed a significantly higher specific activity under hypotonic conditions accompanied by a decrease in GPC amount. Therefore, a higher activity of glycerophosphorylcholine:choline phosphodiesterase (GPC:choline phosphodiesterase) (GPC degradation) under hypotonic conditions is proposed. Similar conclusions can be drawn from using phosphatidyl[methyl-3H]choline. As further evidence for osmotic regulation of GPC:choline phosphodiesterase the specific activity of choline displayed a significant hypotonic increase with chase time which may be equivalent to increased GPC degradation. Therefore, the in vitro experiments suggest that cellular GPC is regulated by an osmosensitive GPC:choline phosphodiesterase. Such a regulation also seems to be present during long-term in vivo experiments. No evidence was found for a genetic adaptation of GPC:choline phosphodiesterase in vivo.

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.

Metabolism of the 'organic osmolyte' glycerophosphorylcholine in isolated rat inner medullary collecting duct cells. I. Pathways for synthesis and degradation

In isolated inner medullary collecting duct (IMCD) cells the adaptation to changes in extracellular osmolarity involves alterations in intracellular content of organic osmolytes such as glycerophosphorylcholine (GPC), sorbitol and others. To elucidate the basis of such alterations, the metabolism of GPC in IMCD cells was investigated with the labeled GPC precursor [methyl-3H]choline. The lipids phosphatidylcholine (PC), lyso PC (LPC) and sphingomyelin (SM), as well as the non lipids phosphorylcholine (Pcholine), GPC and an unknown water-soluble compound could be identified as intermediates of choline metabolism. In pulse-chase experiments the radioactivity of PC expressed as specific activity was at a higher level than the other metabolites (> 10-fold after 1h). Extended chase incubations caused the specific activity of PC and LPC to decrease significantly. GPC was the only metabolite with a significant increase in specific activity under these conditions, suggesting that PC (via LPC) could be the precursor of GPC. In short-term pulse experiments the specific activity of PC and LPC was always significantly higher compared to the specific activity of GPC. Pulse chase incubations using phosphatidyl[methyl-3H]choline showed a significant decrease in specific activity of PC after 15 h accompanied by a significant increase in specific activity of LPC as well as GPC. Inhibition of the PC hydrolyzing enzyme phospholipase A2 revealed a significant increase in the specific activity of PC. For GPC, a significant decrease in the radioactive labeling could be detected. The total amount of PC decreased by 10% under these conditions whereas the amount of GPC decreased by 22% which was significantly higher because of GPC breakdown. GPC degradation was catalyzed by GPC: choline diesterase generating choline (and phosphoglycerol). Significant activity of GPC:phosphocholine diesterase could not be detected. Betaine synthesis from choline was also not present. The slowest, and probably rate-limiting reaction of GPC synthesis from choline may be the reaction of phosphocholine cytidylyltransferase generating CDP choline, since no radioactive CDP choline could be detected under any conditions. Thus, isolated IMCD cells possess the ability for the synthesis of GPC from choline via PC and LPC, as well as for the GPC degradation to choline (and phosphoglycerol). Significant experimental evidence for the occurrence of de-novo synthesis of GPC from choline or a precursor function of GPC for PC could not be detected. However, although the former possibility seems unlikely, a final proof is still lacking.

Ethanolamine is released from glial cells in primary culture on stimulation with foetal calf serum and phorbol ester

Ethanolamine is released to the external medium at increased rates over basal levels when [3H]ethanolamine-labelled glial cells in primary culture, subcultures of astrocytes and O-2A lineage glia or C6 glioma cells are treated with phorbol ester or foetal calf serum. On equilibrium labelling of mixed glial cultures 90-95% of incorporated [3H]ethanolamine was in ethanolamine-containing phospholipids and 2% in the phosphatidylcholine component. On stimulation of glia with phorbol ester or foetal calf serum ethanolamine, and not phosphoethanolamine or choline, was released to the medium suggesting that a phospholipase D-mediated turnover of lipid, perhaps partially linked to protein kinase C, is involved. The results show that cells in primary culture behave similarly to cell lines in that signal transduction pathways involve a stimulated turnover of ethanolamine-phospholipids with the external release of ethanolamine.

Evidence that the rate of phosphatidylcholine catabolism is regulated in cultured rat hepatocytes

The regulation of phosphatidylcholine (PC) catabolism has been studied in choline-deficient rat hepatocytes. Supplementation of choline-deficient hepatocytes, prelabeled with [3H]choline, with 100 microM choline increased the rate of PC catabolism by approx. 2-fold. The major product of PC degradation was glycerophosphocholine in both choline-deficient and choline-supplemented cells. Choline supplementation decreased the radioactivity recovered in lysoPC by 50%. This effect was accompanied by a 2-fold increase of labeled glycerophosphocholine. Comparable results were obtained when PC of the cells was prelabeled with [3H]methionine or [3H]glycerol. The activity of phospholipase A in cytosol, mitochondria and microsomes isolated from choline-deficient rat liver was similar to the activity in control liver, when determined with [3H]PC vesicles as the substrate. Measurement of the activity of phospholipase A with endogenously [3H]choline-labeled PC showed that the formation of lysoPC in mitochondria isolated form choline-supplemented cells was 40% lower than in choline-deficient cells. Alternatively, the formation of [3H]glycerophosphocholine and [3H]choline in microsomes from choline-supplemented cells was significantly higher (1.4-fold) than in microsomes from choline-deficient cells. These results suggest that the rate of PC catabolism is regulated in rat hepatocytes and that the concentration of PC might be an important regulatory factor.

Accumulation of glycerophosphocholine (GPC) by renal cells: osmotic regulation of GPC:choline phosphodiesterase

Although GPC has long been recognized as a degradation product of phosphatidylcholine, only recently is there wide appreciation of its role as a compatible and counteracting osmolyte that protects cells from osmotic stress. GPC is osmotically regulated in renal cells. Its level varies directly with extracellular osmolality. Cells in the kidney medulla in vivo and in renal epithelial cell cultures (MDCK) accumulate large amounts of GPC when exposed to high concentrations of NaCl and urea. Osmotic regulation of GPC requires choline in the medium, presumably as a precursor for synthesis of GPC. Choline transport into the cells, however, is not osmoregulated. The purpose of the present studies was to use MDCK cell cultures as a defined model to distinguish whether osmotically induced accumulation of GPC results from increased GPC synthesis or decreased GPC disappearance. The rate of incorporation of 14C from [14C]choline into GPC, the steady-state GPC synthesis rate, and the activity of phospholipase A2 (which can catalyze a step in the synthesis of GPC from phosphatidylcholine) are not increased by high NaCl and urea. In fact all are decreased by approximately one-third. Therefore, we find no evidence that high NaCl and urea increases the GPC synthesis rate. On the other hand, the rate coefficient for cellular GPC disappearance and the activity of GPC:choline phosphodiesterase (EC 3.1.4.2), which catalyzes degradation of GPC, are decreased by approximately two-thirds by high NaCl and urea. We conclude that high NaCl and urea increase the level of GPC by inhibiting its enzymatic degradation.

Inhibition of phosphatidylcholine and phosphatidylethanolamine biosynthesis by cytochalasin B in cultured glioma cells: potential regulation of biosynthesis by Ca(2+)-dependent mechanisms

The major route of phosphatidylcholine (PtdCho) biosynthesis in mammalian cells is the sequence: choline (Cho)----phosphocholine (PCho)----cytidinediphosphate choline (CDP-Cho)----PtdCho. Recently, we have found that intermediates of this pathway are not freely diffusible in cultured rat glioma (C6) cells but are channeled towards PtdCho biosynthesis (George et al. (1989). Biochim. Biophys. Acta. 1004, 283-291). Channeling of intermediates in other mammalian systems is thought to be mediated through adsorption of enzymes to membranes and cytoskeletal elements to form multienzyme complexes. In this study, agents which perturb the structure and function of cytoskeletal elements were tested for effects on phospholipid metabolism in glioma cells. The filament-disrupting agent cytochalasin B (CB), but not other cytochalasins or the microtubule depolymerizer colchicine inhibited PtdCho and phosphatidylethanolamine (PtdEtn) biosynthesis as judged by dose-dependent reduction of labeling from [3H]Cho and [14C]ethanolamine (Etn). 32Pi pulse-labeling indicated that CB selectively decreased PtdCho and PtdEtn biosynthesis without affecting synthesis of other phospholipids. Synthesis of water-soluble intermediates of PtdCho metabolism was unaffected but the conversion of phosphoethanolamine to CDP-ethanolamine was reduced by CB. Effects of CB on phospholipid biosynthesis were not due to inhibition of glucose uptake as shown by experiments with 2-deoxyglucose, glucose-starved cells and other cytochalasins. Experiments with Ca(2+)-EGTA buffers and digitonin-permeabilized cells, and the Ca(2+)-channel blocker verapamil suggest that effects of CB on PtdCho and PtdEtn biosynthesis are due to alteration of intracellular Ca2+. Taken together, these results suggest that CB acts at sites distinct from glucose transport and cellular microfilaments to specifically inhibit PtdCho and PtdEtn biosynthesis by mechanisms dependent on intracellular Ca2+.

Rat liver and lung mitochondria do not incorporate radioactivity from glycerol-3-phosphate or CDP-choline into glycero-3-phosphocholine

The proposed formation of glycero-3-phosphocholine (GPC) from glycerol-3-phosphate (GP) and CDP-choline catalysed by the enzyme GPC-synthetase has been examined in liver and lung subcellular fractions. Previous observations on the incorporation of radioactive GP into the GPC-spot on paper chromatograms have been interpreted as evidence for the GPC-dependent synthesis of phosphatidylcholine. Although we could reproduce this incorporation of GP, we could not detect any incorporation of radioactive CDP-choline into the GPC-spot using the same paper chromatographic system. TLC separation of the substrate and products showed no detectable formation of GPC with either radioactive substrate. These results strongly suggest that the previously reported formation of GPC in liver and lung was due to an inaccurate identification of the true radioactive products. We demonstrate that the major radioactive product formed in liver mitochondria is glucose. A small amount of radioactive glycerol was also detected. Lung mitochondria incorporate radioactive GP into glycerol and into another unidentified compound or compounds. It is concluded that the occurrence of the GPC dependent formation of phosphatidylcholine is unlikely.

Modulation of phospholipase A2 activity by epidermal growth factor (EGF) in CHO cells transfected with human EGF receptor. Role of receptor cytoplasmic subdomain

Activation of phospholipase A2 (PLA2) in response to external stimuli may play a pivotal role in signal-transduction pathways via the generation of important cellular intermediates, including prostaglandins. Epidermal growth factor (EGF) has been shown to modulate prostaglandin production, possibly via direct activation of PLA2 or indirectly via interaction with a PLA2-modifying protein such as lipocortin I. We have investigated these pathways with two CHO cell-lines, one (CHOwt) transfected with the full-length human EGF receptor and the second (CHO 11) with a deletion mutant, delta 990, that has lost the autophosphorylation sites and part of the internalization domain. CHOwt cells responded to EGF with a rapid rise in lysophosphatidylcholine and arachidonic acid release concomitant with an increase in prostaglandin production. However, in the non-internalizing CHO 11 cells no such activation of PLA2 was observed. This was not due to an intrinsic lack of PLA2 in these cells, as PLA2 activation was shown on melittin addition, nor was this difference due to a defect in intracellular pathways, as arachidonic acid was released from both cell types by Ca2+ and protein kinase C modulators. However, only in CHOwt cells were these responses potentiated by concomitant addition of EGF. Thus the cytoplasmic subdomain of the EGF receptor, containing the major sites of autophosphorylation and the internalization domain, seems to be involved in the activation of PLA2 by EGF. In addition, we have shown that phosphorylation of lipocortin I is unlikely to play a role in PLA2 activation. In CHOwt cells and a positive control cell line, A431, activation of PLA2 was complete by 10 min, at which time there was no evidence of lipocortin I phosphorylation.

NMR spectroscopy analysis of phosphorus metabolites and the effect of adriamycin on these metabolite levels in an adriamycin-sensitive and -resistant human small cell lung carcinoma cell line

31P nuclear magnetic resonance (NMR) spectra of cells and of cell extract revealed high levels of phosphorylcholine (PC) and phosphocreatine (PCr) in an adriamycin-resistant human small cell lung carcinoma cell line (GLC4/ADR) and the adriamycin-sensitive parental cell line (GLC4). PCr levels in extracts of GLC4/ADR were increased compared to extracts of GLC4. We estimated that 11% of the total intracellular ATP is not bound to Mg2+ in both cell lines. This value corresponded to an intracellular free Mg2+ of 0.30 mM. The effects of different adriamycin concentrations, 0.05, 1 and 30 microM for GLC4 and 1, 30 and 200 microM for GLC4/ADR, on the phosphorus metabolite levels in continuously perfused cells were monitored. Significant differences between GLC4 and GLC4/ADR included: (a) a strong increase in the beta ATP level in the presence of 30 microM adriamycin in GLC4 only, followed by a fast decrease after 5 h of perfusion. (b) a less dramatic increase in the PC level in GLC4/ADR and an unchanged ATP level in the presence of increasing adriamycin concentrations. (c) an increased GPC level in GLC4/ADR in the presence of adriamycin. The changes in PC and GPC levels in the presence of adriamycin suggested that the phospholipid turnover was increased in GLC4/ADR and could be stimulated in the presence of adriamycin. In both cell lines, PCr levels decreased faster than the ATP levels after adriamycin treatment. Thus, biochemical markers for adriamycin resistance can be detected with NMR spectroscopy. However, more studies are necessary to obtain parameters to distinguish drug-sensitive from drug-resistant tumours in patients by NMR spectroscopy.

The metabolic fate of [3H-methyl]choline in cultured human neuroblastoma cell lines, LA-N-1 and LA-N-2

The conversion of choline in cultures of the human neuroblastoma cell lines, LA-N-1 and LA-N-2 cells, was investigated in order to identify potential precursors in acetylcholine (AcCho) synthesis. LA-N-1, a catecholaminergic and LA-N-2, a cholinergic, cell line were incubated with [3H-methyl]choline (Cho) for varying periods of time up to 72 h. The radioactivity present in lipids and water-soluble metabolites increased linearly up to 24 h in both cell lines. Approximately 20% of the radioactivity associated with the water-soluble metabolites in both control (untreated) and retinoic acid-induced differentiated (RA-treated cells) LA-N-2 cells was present as Cho and AcCho. There was no detectable AcCho in the catecholaminergic cell line, LA-N-1. The untreated and RA-treated LA-N-1 and LA-N-2 cells were labeled for 24 h with [3H-methyl]Cho, followed by a chase in growth medium containing 100 microM unlabeled choline. The distribution of radioactivity in the LA-N-2 cells was 6-10% of AcCho, 84-89% as phosphocholine (PCho), 1-3% as glycerophosphocholine (GroPCho), and 2-4% as Cho. The distribution of radioactivity in the LA-N-1 cells was similar except for the absence of AcCho. The distribution of radioactivity in the culture medium of LA-N-1 cells was 70-80% as Cho, 20-30% as PCho, and 1-3% as GroPCho. In contrast, the radioactivity was equally distributed between Cho (50%) and PCho (50%), with only 1-3% as GroPCho in the medium of LA-N-2 cells.

Determination of metabolite and nucleotide concentrations in proliferating lymphocytes by 1H-NMR of acid extracts

Nuclear magnetic resonance (NMR) studies of extracts have proven to be a powerful window onto the intracellular machinery of cells and tissues. The major advantages of in vitro 1H-NMR, namely chemical preservation, simultaneous detection, identification, and quantitation of compounds, and sensitivity to a large variety of classes of compounds, are employed in this study to characterize the metabolic course of mitogen-stimulated proliferation of human peripheral lymphocytes. A reliable method to quantitate amino acids, metabolic intermediates, soluble membrane lipid precursors, and purine, pyridine and pyrimidine nucleotides is presented, using samples as small as 30 mg wet weight. A total of 53 substances were detected in lymphocytes and other blood cells. During the course of lymphocyte culture, changes in intracellular concentrations of lactate, taurine, inositol and nucleotides, including NAD, IMP and high-energy phosphates, were especially marked. 1H-NMR compares favorably to 31P-NMR and to HPLC, and is especially attractive in light of expectations for future in vivo application.

The role of sphingomyelin in phosphatidylcholine metabolism in cultured human fibroblasts from control and sphingomyelin lipidosis patients and in Chinese hamster ovary cells

Human fibroblasts in culture take up exogenous [choline-Me-3H,32P]sphingomyelin (SM) from the medium and incorporate it into cellular SM and phosphatidylcholine [Spence, Clarke & Cook (1983) J. Biol. Chem. 258, 8595-8600]. The ratio of [3H]choline/[32P]Pi is similar in SM and phosphatidylcholine, indicating that the phosphocholine (P-Cho) moiety is transferred intact. Similar results are obtained with Niemann-Pick (NP) cells which are deficient in lysosomal sphingomyelinase activity, suggesting that the P-Cho transfer may not be mediated by the lysosomal sphingomyelinase and that alternative pathways of sphingomyelin catabolism are present in cultured cells. In this study we have shown that: (1) the P-Cho pool in control and NP cells incubated with exogenous labelled SM has a specific radioactivity intermediate between that of SM and PtdCho; (2) expansion of the intracellular P-Cho pool by incubation with exogenous choline reduces the incorporation of [3H]choline from SM into PtdCho; and (3) incorporation of P-Cho from SM into PtdCho is decreased at the non-permissive temperature in Chinese hamster ovary cells with a temperature-sensitive mutation in the cytidylyltransferase reaction. These results suggest that incorporation of P-Cho from SM into PtdCho involves a reaction sequence in which P-Cho is hydrolysed from SM by a sphingomyelinase, followed by incorporation of P-Cho into PtdCho via the cytidine pathway of biosynthesis (SM----P-Cho----CDP-Cho----PtdCho). The appreciable incorporation of P-Cho from SM into PtdCho in sphingomyelinase-deficient NP cells suggests a more substantial or effective lysosomal sphingomyelinase activity in intact cells than is measured in vitro, and/or a significant contribution by other sphingomyelinase activities in these cells.

Examination of the potential role of the glycerophosphorylcholine (GPC) pathway in the biosynthesis of phosphatidylcholine by liver and lung

The potential involvement of the glycerophosphorylcholine (GPC) pathway for the synthesis of phosphatidylcholine (PC) has been examined in rat liver and lung and in a human line, the A549 cell which possesses characteristics representative of mature alveolar type II epithelial cells. Although mitochondrial and microsomal fractions from the above sources readily incorporated radioactive glycerophosphate into lipids, the only incorporation observed with radioactive GPC was a small variable labelling with the mitochondrial and microsomal fractions from rat lung. Even with these fractions, no radioactivity from GPC was incorporated into PC or lysoPC. Attempts to increase the incorporation of GPC into lipids by manipulating the incubation conditions were unsuccessful. It was concluded that the occurrence of the GPC pathway in liver and lung is unlikely.

Phosphatidylcholine biosynthesis in cultured glioma cells: evidence for channeling of intermediates

The major pathway of choline (Cho) incorporation into phosphatidylcholine (PtdCho) in mammalian cells is sequential conversion of Cho to phosphocholine (PCho), cytidinediphosphate choline (CDP-Cho) and PtdCho. In intact cells, this sequence is usually demonstrated using radiolabeled Cho since PCho and CDP-Cho do not enter the cell intact. We have studied the incorporation of radiolabeled Cho, PCho and CDP-Cho into rat glioma (C6) cells following electropermeabilization. C6 cells were permeable as judged by [U-14C]sucrose and Erythrosin B uptake and more rapid incorporation of [1,2,3-3H]glycerol into cell lipids, and viable as assessed by uptake and incorporation of [methyl-3H]Cho, [1-14C]oleate and [1,2,3-3H]glycerol into complex lipids. Despite rapid incorporation of [methyl-3H]Cho into PtdCho in permeabilized cells, there was no incorporation of [methyl-14C]PCho or CDP-[methyl-14C]Cho into PtdCho. PCho (300 microM) and CDP-Cho (300 microM) failed to significantly reduce incorporation of 28 microM [methyl-3H]Cho into PtdCho. Radioactivity in PtdCho of cells prelabeled with [methyl-3H]Cho prior to permeabilization could be chased with 4 mM Cho but not with 4 mM PCho or 4 mM CDP-Cho. The water-soluble products of Cho metabolism--PCho, CDP-Cho and glycerophosphocholine--were retained at 37 degrees C in permeabilized cells compared with controls while there was uniform leakage from permeabilized cells at 4 degrees C. Hemicholinium-3, an inhibitor of high-affinity Cho transport, decreased [methyl-3H]Cho incorporation into PtdCho in permeabilized cells, as in controls, suggesting that even in permeabilized cells, Cho incorporation into PtdCho is linked to the transport system. We propose that individual steps of the cytidine pathway of PtdCho biosynthesis are functionally linked and that reaction intermediates are not freely diffusible within the cell but are channeled to PtdCho biosynthesis.

Lysophosphatidylcholine as an intermediate in phosphatidylcholine metabolism and glycerophosphocholine synthesis in cultured cells: an evaluation of the roles of 1-acyl- and 2-acyl-lysophosphatidylcholine

Previous studies in our laboratory have shown that the principal pathway of phosphatidylcholine (PtdCho) degradation in cultured mouse N1E-115 neuroblastoma, C6 rat glioma, primary rat brain glia and human fibroblasts is PtdCho----lysophosphatidylcholine (lysoPtdCho)----glycerophosphocholine (GroPCho)----glycerophosphate plus choline (Morash, S.C. et al. (1988) Biochim. Biophys. Acta 961, 194-202). GroPCho is the first quantitatively major degradation product in this pathway, and could be formed by phospholipases A1 or A2, followed by lysophospholipase, or by a co-ordinated attack releasing both fatty acids by phospholipase B. The quality and quantities of lysoPtdCho present in cells reflect the nature of the initial hydrolysis step (A1 or A2), specificities of the lysophospholipases, and activities of acyltransferases that form PtdCho from lysoPtdCho. The present study was undertaken to elucidate the relative importance of these pathways by examining the fate of exogenous 1-acyl and 2-acyl-lysoPtdCho incubated with N1E-115 and C6 cells in culture. By fatty acid composition, endogenous lysoPtdCho was found to be mainly 1-acyl in both cell types based on a predominance of saturated acyl species; this suggested either preferential further deacylation or reacylation of 2-acyl-lysoPtdCho, or that 2-acyl-lysoPtdCho was not formed. Exogenous 1- and 2-acyl-lysoPtdCho specifically radiolabelled with choline and/or fatty acid were incubated either singly or as equimolar mixtures with cells. Cell association was rapid and not reversible by washing and both species were taken up at similar rates. The 2-acyl species was acylated to PtdCho faster than the 1-acyl species in both cell lines. Acylation of both lyso species was higher in C6 compared to N1E-115 cells. Hydrolysis of lysoPtdCho to GroPCho was higher in N1E-115 cells and with 1-acyl-lysoPtdCho. Transacylation between two molecules of lysoPtdCho was a minor pathway. These results document the variety and relative importance of reactions of lysoPtdCho metabolism; under similar conditions, 1- and 2-acyl-lysoPtdCho are handled differently. Both species turn over actively, but only the 1-acyl species accumulates while 2-acyl-lysoPtdCho is likely to be reacylated to form PtdCho.

Dideoxycytidine, an anti-HIV drug, selectively inhibits growth but not phosphatidylcholine metabolism in neuroblastoma and glioma cells

Dideoxycytidine (ddCyd), an inhibitor of AIDS-related HIV, has been examined for effects on cell proliferation and phosphatidylcholine synthesis in tumor lines of nervous system origin. Uptake and metabolism of [3H]ddCyd, observed in all cells, was greatest in one human neuroblastoma line, HTB-10. Growth of the HTB-10 line was markedly inhibited by 40 microM ddCyd, whereas growth of C6 glioma and N1E-115 or HTB-11 neuroblastoma cells was unaltered. Phosphatidylcholine synthesis in the presence or absence of stimulation by phorbol ester was not specifically altered by ddCyd. Thus, ddCyd was incorporated and inhibited growth in a cell-specific manner but had little effect on cytidine-dependent phospholipid synthesis. This suggests that some cells derived from the nervous system may be more susceptible than others with respect to the positive and negative effects of ddCyd as a potential antiviral drug.