In vivo studies on pathways for the biosynthesis of lecithin in the rat

Germ-free status but not subacute polychlorinated biphenyl (PCB) exposure altered hepatic phosphatidylcholine and ether-phosphatidylcholine levels in mice

Polychlorinated biphenyls (PCBs) are persistent organic pollutants that pose a current ecosystem and human health concern. PCB exposure impacts the gut microbiome in animal models, suggesting a mechanistic link between PCB exposure and adverse health outcomes. The presence and absence of the microbiome and exposure to PCBs independently affect the lipid composition in the liver, which in turn affects the PCB disposition in target tissues, such as the liver. Here, we investigated microbiome × subacute PCB effects on the hepatic lipid composition of conventional and germ-free female mice exposed to 0, 6, or 30 mg/kg body weight of an environmental PCB mixture in sterile corn oil once daily for 3 consecutive days. Hepatic triacylglyceride and polar lipid levels were quantified using mass spectrometric methods following the subacute PCB exposure. The lipidomic analysis revealed no PCB effect on the hepatic levels. No microbiome effect was observed on levels of triacylglyceride and most polar lipid classes. The total hepatic levels of phosphatidylcholine (PC) and ether-phosphatidylcholine (ePC) lipids were lower in germ-free mice than the conventional mice from the same exposure group. Moreover, levels of several unsaturated PCs, such as PC(36:5) and PC(42:10), and ePCs, such as ePC(36:2) and ePC(4:2), were lower in germ-free than conventional female mice. Based on a KEGG pathway meta-analysis of RNA sequencing data, the ether lipid metabolism pathway is altered in the germ-free mouse liver. In contrast to the liver, extractable lipid levels, determined gravimetrically, differed in several tissues from naïve conventional vs. germ-free mice. Overall, microbiome × subacute PCB exposure effects on hepatic lipid composition are unlikely to affect PCB distribution into the mouse liver. Further studies are needed to assess how the different extractable lipid levels in other tissues alter PCB distribution in conventional vs. germ-free mice.

8 Final Report on the Safety Assessment of Triethanolamine, Diethanolamine, and Monoethanolamine

Triethanolamine (TEA), Diethanolamine (DEA), and Monoethanolamine (MEA) are amino alcohols used in cosmetic formulations as emulsifiers, thickeners, wetting agents, detergents, and alkalizing agents. The nitrosation of the etha-nolamines may result in the formation of N-nitrosodiethanolamine (NDELA) which is carcinogenic in laboratory animals.

In single-dose oral toxicity for rats, TEA was practically nontoxic to slightly toxic, and DEA and MEA were slightly toxic. Long-term oral ingestion of the ethanolamines by rats and guinea pigs produced lesions limited mainly to the liver and kidney. Long-term cutaneous applications to animals of the ethanolamines also produced evidence of hepatic and renal damage. TEA and DEA showed little potential for rabbit skin irritation in acute and subchronic skin irritation tests. MEA was corrosive to rabbit skin at a 30% concentration in a single semioccluded patch application and at a >10% concentration in 10 open applications over a period of 14 days.

The ethanolamines were nonmutagenic in the Ames test and TEA is also nonmutagenic to Bacillus subtilis. TEA did not cause DNA-damage inducible repair in an unscheduled DNA synthesis test. TEA had no carcinogenic or cocarcinogenic activity when dermally applied to mice for 18 months.

Clinical skin testing of TEA and cosmetic products containing TEA and DEA showed mild skin irritation in concentrations above 5%. There was very little skin sensitization. There was no phototoxicity or photosensitization reactions with products containing up to 20.04% TEA. A formulation containing 11.47% MEA and a formulation containing 1.6% DEA and 5.9% MEA were irritating to human skin in patch tests.

The Panel concludes that TEA, DEA, and MEA are safe for use in cosmetic formulations designed for discontinuous, brief use followed by thorough rinsing from the surface of the skin. In products intended for prolonged contact with the skin, the concentration of ethanolamines should not exceed 5%. MEA should be used only in rinse-off products. TEA and DEA should not be used in products containing N-nitrosating agents.

Polymorphism of the PEMT gene and susceptibility to nonalcoholic fatty liver disease (NAFLD)

Phosphatidylethanolamine N-methyltransferase (PEMT) catalyzes phosphatidylcholine synthesis. PEMT knockout mice have fatty livers, and it is possible that, in humans, nonalcoholic fatty liver disease (NAFLD) might be associated with PEMT gene polymorphisms. DNA samples from 59 humans without fatty liver and from 28 humans with NAFLD were genotyped for a single nucleotide polymorphism in exon 8 of PEMT, which leads to a V175M substitution. V175M is a loss of function mutation, as determined by transiently transfecting McArdle-RH7777 cells with constructs of wild-type PEMT open reading frame or the V175M mutant. Met/Met at residue 175 (loss of function SNP) occurred in 67.9% of the NAFLD subjects and in only 40.7% of control subjects (P<0.03). For the first time we report that a polymorphism of the human PEMT gene (V175M) is associated with diminished activity and may confer susceptibility to NAFLD.

Physiological regulation of phospholipid methylation alters plasma homocysteine in mice

Biological methylation reactions and homocysteine (Hcy) metabolism are intimately linked. In previous work, we have shown that phosphatidylethanolamine N-methyltransferase, an enzyme that methylates phosphatidylethanolamine to form phosphatidylcholine, plays a significant role in the regulation of plasma Hcy levels through an effect on methylation demand (Noga, A. A., Stead, L. M., Zhao, Y., Brosnan, M. E., Brosnan, J. T., and Vance, D. E. (2003) J. Biol. Chem. 278, 5952-5955). We have further investigated methylation demand and Hcy metabolism in liver-specific CTP:phosphocholine cytidylyltransferase-alpha (CTalpha) knockout mice, since flux through the phosphatidylethanolamine N-methyltransferase pathway is increased 2-fold to meet hepatic demand for phosphatidylcholine. Our data show that plasma Hcy is elevated by 20-40% in mice lacking hepatic CTalpha. CTalpha-deficient hepatocytes secrete 40% more Hcy into the medium than do control hepatocytes. Liver activity of betaine:homocysteine methyltransferase and methionine adenosyltransferase are elevated in the knockout mice as a mechanism for maintaining normal hepatic S-adenosylmethionine and S-adenosylhomocysteine levels. These data suggest that phospholipid methylation in the liver is a major consumer of AdoMet and a significant source of plasma Hcy.

Insights into the requirement of phosphatidylcholine synthesis for liver function in mice

Phosphatidylcholine (PC) is made in the liver by the CDP-choline pathway and via phosphatidylethanolamine N-methyltransferase (PEMT), which catalyzes the conversion of phosphatidylethanolamine to PC. Unexpectedly, hepatic apolipoprotein B-100 secretion is inhibited in male, but not female, Pemt-/- mice (Noga, A. A., Y. Zhao, and D. E. Vance. 2002. J. Biol. Chem. 277: 42358-42365; Noga, A. A., and D. E. Vance. 2003. J. Biol. Chem. 278: 21851-21859). To gain further insight into this process, we compared PC metabolism in male and female mice fed chow or a high-fat/high-cholesterol (HF/HC) diet. Immunoblot analyses demonstrated that twice as much PEMT2 was present in livers from female compared with male mice. In contrast, assays of CTP:phosphocholine cytidylyltransferase from livers of Pemt+/+ mice demonstrated more active cytidylyltransferase in male than in female mice. Secretion of PEMT-derived PC into lipoproteins was examined in vivo by injection of mice with [methyl-3H]methionine in the presence of Triton WR1339. The PEMT-derived PC shifts to smaller-sized particles in response to a HF/HC diet, but only in male mice. Secretion of PEMT-derived PC into bile was enhanced in mice fed a HF/HC diet. These results demonstrate that the synthesis and targeting of PC produced by the PEMT pathway in the livers of mice differs in a gender- and diet-specific manner.

A gender-specific role for phosphatidylethanolamine N-methyltransferase-derived phosphatidylcholine in the regulation of plasma high density and very low density lipoproteins in mice

Phosphatidylethanolamine N-methyltransferase (PEMT)is involved in a secondary pathway for production of phosphatidylcholine (PC) in liver. We fed Pemt-/-mice a high fat/high cholesterol diet for 3 weeks to determine whether or not PC derived from PEMT is required for very low density lipoprotein secretion. Lipid analyses of plasma and liver indicated that male Pemt-/- mice accumulated triacylglycerols in their livers and were unable to secrete the same amount of triacylglycerols from the liver as did Pemt+/+ mice. Plasma levels of triacylglycerol and both apolipoproteins B100 and B48 were significantly decreased only in male Pemt-/- mice. Experiments in which mice were injected with Triton WR1339 showed that, whereas hepatic apoB100 secretion was decreased in male Pemt-/- mice, the decrease in plasma apoB48 in male Pemt-/- mice was not due to reduced secretion. Moreover, female and, to a lesser extent, male Pemt-/- mice showed a striking 40% decrease in plasma PC and cholesterol in high density lipoproteins. These results suggest that, even though the content of hepatic PC was normal in PEMT-deficient mice, plasma lipoprotein levels were profoundly altered in a gender-specific manner.

Development of hepatocellular adenomas and carcinomas associated with fibrosis in C57BL/6J male mice given a choline-deficient, L-amino acid-defined diet

Development of hepatocellular carcinomas in rats caused by a choline-deficient, L-amino acid-defined (CDAA) diet, usually associated with fatty liver, fibrosis, cirrhosis and oxidative DNA damage, has been recognized as a useful model of hepatocarcinogenesis caused by endogenous factors. In the present study, in order to further explore involved factors and genes, we established an equivalent model in spontaneous liver tumor-resistant C57BL/6J mice. Six-week-old males and females were continuously fed the CDAA diet and histological liver lesions and oxidative DNA damage due to 8-hydroxydeoxyguanosine (8-OHdG) were examined after 22, 65 and 84 weeks. In male mice, fatty change and fibrosis were evident at 22 weeks, and preneoplastic foci of altered hepatocytes were seen at an incidence of 8/8 (100%) and a multiplicity of 6.6 +/- 4.0 per mouse at 65 weeks. Hepatocellular adenomas and carcinomas developed at incidences of 16/24 (66.7%) and 5/24 (20.8%), and multiplicities of 1.42 +/- 1.32 and 0.29 +/- 0.62, respectively, at 84 weeks. The female mice exhibited resistance to development of these lesions. The CDAA diet also increased 8-OHdG levels in male but not female mice. These results indicate that a CDAA diet causes hepatocellular preneoplastic foci, adenomas and carcinomas associated with fibrosis and oxidative DNA damage in mice, as in rats, providing a hepatocarcinogenesis model caused by endogenous factors in mice.

Final report on the safety assessment of Lecithin and Hydrogenated Lecithin

Lecithin is a naturally occurring mixture of the diglycerides of stearic, palmitic, and oleic acids, linked to the choline ester of phosphoric acid, commonly called phosphatidylcholine. Hydrogenated Lecithin is the product of controlled hydrogenation of Lecithin. Bilayers of these phospholipids in water may form liposomes, a spherical structure in which the acyl chains are inside and not exposed to the aqueous phase. Lecithin and Hydrogenated Lecithin are used in a large number of cosmetic formulations as skin conditioning agents-miscellaneous and as surfactant-emulsifying agents. Hydrogenated Lecithin is also used as a suspending agent-nonsurfactant. Historical data on concentration of use of Lecithin reveals that 0.1% to 1.0% is the concentration range most frequently seen, with concentrations up to 50% reported for two moisturizing products. A solution of 65% Lecithin is currently reported to be used at concentrations up to 3% in cosmetics. Nonocclusive application of Lecithin-containing liposomes to murine skin resulted in 30% penetration to the subdermis. In piglet skin, the same application resulted in 99% accumulating in the stratum corneum. In general, liposomes are considered effective in capturing other compounds inside their spherical structure and delivering any such captured compound through the skin barrier. As a result, caution should be exhibited in formulating cosmetic products that contain these ingredients in combination with other ingredients whose safety is based on their lack of absorption or where dermal absorption is a concern. Lecithin is virtually nontoxic in acute oral studies, short-term oral studies, and subchronic dermal studies in animals. Lecithin is not a reproductive toxicant, nor is it mutagenic in several assays. In an oral carcinogenicity study, brain neoplasms were found in mice exposed to Lecithin. In a subcutaneous carcinogenicity study, no neoplasms were found in mice and rats exposed to Lecithin. Adverse reactions to Lecithin in a metered-dose inhaler have been reported. Lecithin and Hydrogenated Lecithin were generally nonirritating and nonsensitizing in animal and human skin. Based on the available data, Lecithin and Hydrogenated Lecithin are safe as used in rinse-off cosmetic products; they may be safely used in leave-on products at concentrations up to 15%, the highest concentration tested in clinical irritation and sensitization studies; but the safety of use could not be substantiated in cosmetic products likely to be inhaled. Because of the possibility of formation of nitrosamines, these ingredients should not be used in cosmetic products in which N-nitroso compounds may be formed.

1-Methyl-4-phenyl-pyridinium increases S-adenosyl-L-methionine dependent phospholipid methylation

1-Methyl-4-phenyl-pyridinium (MPP(+)) and S-adenosyl-L-methionine (SAM) cause Parkinson's disease (PD)-like changes. SAM and MPP(+) require their charged S-methyl and N-methyl groups, so the PD-like symptoms may be related to their ability to modulate the methylation process. The SAM-dependent methylation of phosphatidylethanolamine (PTE) to produce phosphatidylcholine (PTC), via phosphatidylethanolamine-N-methyltransferase (PEMT), and the hydrolysis of PTC to form lyso-PTC, a cytotoxic agent, are potential loci for the action of MPP(+). In this study, the effects of MPP(+) on the methylation of PTE to PTC and the production of lyso-PTC were determined. The results showed that SAM increased PTC and lyso-PTC. The rat striatum showed the highest PEMT activity and lyso-PTC formation, which substantiate with the fact that the striatum is the major structure that is affected in PD. MPP(+) significantly enhanced PEMT activity and the formation of lyso-PTC in the rat liver and brain. MPP(+) increased the affinity and the V(max) of PEMT for SAM. 1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) effect was lesser and inhibited by deprenyl (MAO-B inhibitor). The nor-methyl analogs of MPP(+) were inactive, but some of the charged analogs of MPP(+) showed comparable effects to those of MPP(+). Lyso-PTC that can be increased by SAM and MPP(+) caused severe impairments of locomotor activities in rats. These results indicate that SAM and MPP(+) have complementary effects on phospholipid methylation. Thus, SAM-induced hypermethylation could be involved in the etiology of PD and an increase of phospholipid methylation could be one of the mechanisms by which MPP(+) causes parkinsonism.

The biochemistry of hypo- and hyperlipidemic fatty acid derivatives: metabolism and metabolic effects

A selection of amphipatic hyper- and hypolipidemic fatty acid derivatives (fibrates, thia- and branched chain fatty acids) are reviewed. They are probably all ligands for the peroxisome proliferation activation receptor (PPARalpha) which has a low selectivity for its ligands. These compounds give hyper- or hypolipidemic responses depending on their ability to inhibit or stimulate mitochondrial fatty acid oxidation in the liver. The hypolipidemic response is explained by the following metabolic effects: Lipoprotein lipase is induced in liver where it is normally not expressed. Apolipoprotein CIII is downregulated. These two effects in liver lead to a facilitated (re)uptake of chylomicrons and VLDL, thus creating a direct transport of fatty acids from the gut to the liver. Fatty acid metabolizing enzymes in the liver (CPT-I and II, peroxisomal and mitochondrial beta-oxidation enzymes, enzymes of ketogenesis, and omega-oxidation enzymes) are induced and create an increased capacity for fatty acid oxidation. The increased oxidation of fatty acids "drains" fatty acids from the body, reduces VLDL formation, and ultimately explains the antiadiposity and improved insulin sensitivity observed after administration of peroxisome proliferators.

Pulsatile growth hormone secretion decreases S-adenosylmethionine synthetase in rat liver

S-adenosylmethionine synthetase (AdoMet synthetase) is responsible for the synthesis of the major methyl donor S-adenosylmethionine. The AdoMet synthetase gene was identified by subtractive suppressive hybridization as being expressed at higher levels in the liver of rats continuously exposed to growth hormone (GH) than in rats intermittently exposed to the hormone. Further studies on the regulation of AdoMet synthetase showed that the activity and mRNA levels were higher in female than in male rats. Hypophysectomy increased AdoMet synthetase mRNA in both male and female rats. Combined thyroxine and cortisol treatment of hypophysectomized rats had no effect on AdoMet synthetase mRNA levels. Two daily injections of GH for 7 days, mimicking the male secretory pattern of GH, decreased AdoMet synthetase activity and mRNA levels. A continuous infusion of GH, mimicking the female secretory pattern of GH, had small or no effects on AdoMet synthetase activity and decreased the mRNA levels to a lesser degree than two daily injections. It is concluded that the lower AdoMet synthetase activity in male rats is due to an inhibitory effect of the male characteristic pulsatile secretory pattern of GH on AdoMet synthetase mRNA expression.

Interactions among pathways for phosphatidylcholine metabolism, CTP synthesis and secretion through the Golgi apparatus

Phosphatidylcholine is the major phospholipid in eukaryotic cells. It serves as a structural component of cell membranes and a reservoir of several lipid messengers. Recent studies in yeast and mammalian systems have revealed interrelationships between the two pathways of phosphatidylcholine metabolism, and between these pathways and those for CTP synthesis and secretion via the Golgi. These processes involve the regulation of the CDP-choline and phosphatidylethanolamine-methylation pathways of phosphatidylcholine synthesis, CTP synthetase, phospholipase D and the phospholipid-transfer protein Sec14p.

Selected pharmacodynamic and biochemical properties of selenonium choline

1. Five minutes after intravenous administration of 50 mg/kg of the novel choline analogue selenonium choline [(CH3)2Se + CH2CH2OH, SeCh], only 8% of the administered dose was accounted for in blood, brain, liver, heart, and kidney tissues. 2. SeCh was acetylated in vivo to acetylselenonium choline (ASeCh) in all of the tissues examined. 3. During postmortem incubation, brain concentrations of SeCh and ASeCh increased to 535% and to 425%, respectively. 4. K(m) and Vmax values for the phosphorylation of SeCh by choline kinase were higher and lower, respectively, compared to the phosphorylation of choline. 5. Acetylation of SeCh was described with K(m) and Vmax values that were both higher than the values for Ch. 6. The data suggest that SeCh is phosphorylated and incorporated into various lipids in brain tissue, and is acetylated to ASeCh by both nonneural and neural tissues.

The role of phosphatidylcholine biosynthesis in the regulation of the INO1 gene of yeast

In yeast, as in other eukaryotes, phosphatidylcholine (PC) can be synthesized via methylation of phosphatidylethanolamine or from free choline via the CDP-choline pathway. In yeast, PC biosynthesis is required for the repression of the phospholipid biosynthetic genes, including the INO1 gene, in response to inositol. In this study, we analyzed the effect of mutations in genes encoding enzymes involved in PC biosynthesis on the transcriptional regulation of phospholipid biosynthetic genes. We report that repression of INO1 transcription in response to inositol is clearly dependent on ongoing PC biosynthesis, but it is independent of the route of synthesis. Our results also suggest that intermediates in the phosphatidylethanolamine methylation and CDP-choline pathways are not responsible for generating the regulatory signal that results in repression of INO1 and other coregulated genes of phospholipid biosynthesis. Furthermore, repression of INO1 is not tightly correlated to the proportion of PC in the total cellular phospholipids. Rather, we report that when the rate of synthesis of PC becomes growth limiting, the addition of inositol fails to repress the phospholipid biosynthetic genes, but when the rate of PC synthesis is sufficient to sustain normal growth, the addition of inositol to the growth medium has the effect of repressing INO1 and other phospholipid biosynthetic genes.

Sexual dimorphism in the regulation of cell turnover during liver hyperplasia

A sexual dimorphism occurs in liver cell proliferation following partial hepatectomy, female liver regenerating faster than male, while a continuous excess of choline to females shifts their growth pattern toward that of males (L. Tessitore, P. Pani and M.U. Dianzani, Carcinogenesis, 13 (1992) 1929). In this study we have investigated (a) if the same sexual modulation occurs in a different type of liver growth, hyperplasia induced by a direct mitogen and (b) if the pre-administration of choline to females is able to modulate this dimorphism. Liver hyperplasia induced by lead nitrate, a potent mitogen, has also shown a peculiar sexual dimorphism in all phases of the proliferative process. In contrast with liver regeneration after partial hepatectomy, the mitogenic action of lead nitrate was less effective and was delayed in females as compared with males, by evaluating liver weight, protein accumulation, DNA synthesis and mitotic index. These results were also confirmed by the trend of liver regression by apoptosis. The apoptotic index was higher in males than in females. A prolonged administration of an excess of choline has partially filled these sexual differences, since choline has moved, in females, all the observed parameters (liver weight, protein accumulation, DNA synthesis, mitotic and apoptotic indexes) to values closer to those observed in males.

Deoxycytidine kinase is phosphorylated in vitro by protein kinase C alpha

Deoxycytidine (dCyd) kinase was effectively phosphorylated by protein kinase C. The reaction was rapid, occurring at 4 degrees C as well as at 37 degrees C and approximately 0.7 mol of phosphate could be incorporated per mol of deoxycytidine kinase. Phosphoserine was the primary amino acid to be phosphorylated. Phosphorylation of deoxycytidine kinase resulted in a 100% increase in the Vmax using dCyd as a substrate (52.16 +/- 1.3 versus 104.47 +/- 11.4 nmol/min/mg protein), and an increase in the apparent Km (2.0 +/- 0.2 microM versus 6.9 +/- 1.2 microM). The inactive antimetabolite, ara-C, is activated within a cell by deoxycytidine kinase phosphorylation of the prodrug. Recent studies have shown that ara-C activates protein kinase C in vivo [1]. Furthermore, ara-C has been shown to be metabolized to ara-CDP-choline via reversal of the cholinephosphotransferase [2] producing diglyceride, a cellular activator of protein kinase C. Thus, in situ, deoxycytidine kinase may be phosphorylated by protein kinase C with the result that self-potentiation of ara-C toxicity may occur via increased activity of deoxycytidine kinase.

Phospholipid methylation in brain membrane preparations: kinetic mechanism

The methylation reactions which convert phosphatidylethanolamine (PE) to phosphatidylcholine (PC) have been studied kinetically using exogenously added intermediates and crude membrane preparations from brain. The addition of exogenous PE resulted in no change in the methylation rates compared to that of endogenous PE. The addition of the two intermediates, monomethylphosphatidylethanolamine (PMME) and dimethylphosphatidylethanolamine (PDME), resulted in significantly increased rates of methylation and allowed the kinetic analysis of these latter two methylation reactions. The mechanism for this enzyme appears to be similar to human RBC (Reitz et al. (1989) J. Biol. Chem. 264, 8097-8106) which was a rapid-equilibrium random Bi-Bi sequential mechanism. There were some slight differences between the brain enzyme and that from the RBC, but there is little reason to suggest a fundamentally different mechanism. It is more likely that the differences may relate to an additional dead-end complex for the enzyme from brain such that saturation with AdoMet cannot eliminate AdoHcy inhibition. The KM values for the two phospholipid substrates were 41-44 microM and 39 microM for the methylation of PMME and PDME, respectively. The KM for S-adenosylmethionine (AdoMet) was 7-9 microM with PMME and 4 microM with PDME as the other substrates. The Ki(lipid) varied from 54 microM with PMME to 225 microM with PDME, and the Ki(AdoMet) was 11 microM with PMME and 21 microM with PDME. The product from the use of AdoMet, S-adenosylhomocysteine (AdoHcy), was shown to be a noncompetitive inhibitor of both lipid substrates as well as AdoMet. The methylation of PMME was somewhat higher in cerebellum and brain stem compared to cortex and striatum, but the methylation of PDME was similar in cerebellum, brain stem and cortex.

Reversibility of the reactions catalyzed by cholinephosphotransferase and ethanolaminephosphotransferase solubilized from rat-brain microsomes

The incorporation of CMP into CDP-ethanolamine and CDP-choline, catalyzed by ethanolaminephosphotransferase (EC 2.7.8.1) and cholinephosphotransferase (EC 2.7.8.2), respectively, has been studied in solubilized preparations of rat-brain microsomes. Mn2+ ions were required for the maximal activity of both enzymes. The CMP concentration needed to reach the half-maximal reaction rate was 1.6 microM for both activities. The rate of incorporation of CMP into CDP-choline and CDP-ethanolamine was increased by increasing the concentration of phosphatidylcholine and phosphatidylethanolamine, respectively, in detergent-phospholipid micellar systems. The rate of the reaction at pH 6.5 was comparable with that measured at pH 8.5, whereas the rate of synthesis of phosphatidylcholine and phosphatidylethanolamine, catalyzed by the same enzymes, increased with pH. Ethanolaminephosphotransferase, which catalyzes the synthesis of phosphatidylethanolamine from CDP-ethanolamine and diacylglycerol, was co-eluted with the enzyme activity catalyzing the reverse reaction, when solubilized microsomes were submitted to anion exchange chromatography on DEAE Bio-Gel A. Cholinephosphotransferase was inactivated during the chromatographic procedure.

Choline: an important nutrient in brain development, liver function and carcinogenesis

Choline is required to make certain phospholipids which are essential components of all membranes. It is a precursor for biosynthesis of the neurotransmitter acetylcholine and also is an important source of labile methyl groups. Much attention has been given to the effect of supplemental choline upon brain function, i.e., enhancement of acetylcholine synthesis and release. In addition, choline supplements administered to rats in utero or shortly after birth permanently after brain function. The mechanisms for this effect is unknown and under investigation at this time. Healthy humans fed diets deficient in choline, and humans fed parenterally have decreased plasma choline concentrations and develop liver dysfunction that is similar to that seen in choline-deficient animals. In experimental animals, fatty liver occurs in choline deficiency because phosphatidylcholine synthesis is required for very low-density lipoprotein secretion. This accumulation of lipids in liver may explain why choline-deficient rats spontaneously develop hepatocarcinoma. We found that choline deficiency was associated with the accumulation of 1,2-diacylglycerol, an activator of protein kinase C. Several lines of evidence indicate that cancers might develop secondary to abnormalities in protein kinase C-mediated signal transduction.

Sexual dimorphism in the preferential secretion of unsaturated lysophosphatidylcholine by rat hepatocytes but no secretion by sheep hepatocytes

(1) Rat and ovine hepatocytes were incubated in monolayer culture with various fatty acids to determine their effects on the composition of the lysophosphatidylcholine that was secreted. (2) No lysophosphatidylcholine was detected in the medium from the ovine hepatocytes even though these cells were hormonally responsive and they secreted phosphatidylcholine and triacylglycerol in very-low-density lipoprotein. (3) Lysophosphatidylcholine was readily detected in the incubation medium of rat hepatocytes. The predominant fatty acids in this lipid were unsaturated. Stearate and arachidonate contributed 15 and 34%, and 24 and 26% of the total fatty acids when hepatocytes from male and female rats were used, respectively. The relative proportions of stearate and arachidonate in the phosphatidylcholine secreted from the hepatocytes were 20 and 14%, and 28 and 21% for the males and females, respectively. The equivalent values for stearate and arachidonate for phosphatidylcholine in the hepatocytes were 18 and 17% and 33 and 22% for male and female rats. These results provide further indications of sex differences in hepatic phospholipid metabolism and extend this to the secretion of phosphatidylcholine and lysophosphatidylcholine. (4) The addition of 1 mM stearate to the incubation medium did not significantly decrease the proportion of arachidonate in the lysophosphatidylcholine obtained from the hepatocytes of the male rats. However, the relative proportion of arachidonate was decreased in incubations that contained 1 mM oleate or linoleate. (5) The results provide evidence that the preferential secretion of unsaturated lysophosphatidylcholine by the liver may provide a system for transporting unsaturated fatty acids and choline to other organs in non-ruminant animals. However, this mechanism may not operate for ruminants.

Global in vivo replacement of choline by N-aminodeanol. Testing a hypothesis about progressive degenerative dementia: I. Dynamics of choline replacement

Severe disruption of certain cholinergic pathways is a characteristic feature of Alzheimer's disease. Attempts to establish animal models by interfering with cholinergic function have not been very successful. We now present data which show a substantial and progressive replacement of free and phospholipid-bound choline by the novel choline isostere N-amino-N,N-dimethylaminoethanol during its dietary administration in place of choline. Free choline in blood fell to approximately 20% of controls after 10 to 30 days on diet. Phospholipid-bound choline in plasma was reduced to less than 15%, and in erythrocytes to about 22%. After 120 days of diet free and bound choline were reduced in most tissues to approximately 30% of controls. Only liver retained more than 80% of free choline. Acetylcholine was decreased to 33 to 50% of control. Total true and false transmitter in experimental animals was in all tissues less that acetylcholine in controls, suggesting that muscarinic transmission would be impaired. Moderate reduction of choline acetyltransferase activity was seen in striatum and myenteric plexus, and of QNB-binding in hippocampus, striatum and myenteric plexus.

Phosphatidylcholine synthesis in the rat: the substrate for methylation and regulation by choline

Two lines of evidence led us to reexamine the possibility that methylation of phosphoethanolamine and its partially methylated derivatives, in addition to methylation of the corresponding phosphatidyl derivatives, plays a role in mammalian phosphatidylcholine biosynthesis: (a) Results obtained by Salerno and Beeler with rat [Salerno, D.M. and Beeler, D.A. (1973) Biochim. Biophys. Acta 326, 325-338] appear to strongly support such a role for methylation of phosphobases; (b) Such reactions have recently been shown to play major roles in phosphatidylcholine synthesis by higher plants [see Datko, A.H. and Mudd, S.H. (1988) Plant Physiol. 88, 854-861 and references therein]. We found that, following continuous labeling of rat liver with L-[methyl-3H]methionine for 10.4 min (intraperitoneal administration) or for 0.75 min (intraportal administration), virtually no 3H was detected in methylated derivatives of phosphoethanolamine, but readily detectable amounts of 3H were present in the base moiety of each methylated derivative of phosphatidylethanolamine. Thus, there was no indication that phospho-base methylation makes a significant contribution. Studies of cultured rat hepatoma cells showed definitively for the first time in a mammalian system that choline deprivation up-regulates the rate of flow of methyl groups originating in methionine into phosphatidylethanolamine and derivatives. Even under these conditions, methylation of phosphoethanolamine bases appeared to play a negligible role.

Sex differences in fatty acid composition of rat liver phosphatidylcholine are regulated by the plasma pattern of growth hormone

The effects of continuous and intermittent (at 12 h intervals) administration of growth hormone (GH), and the effects of gonadal steroids on the regulation of the fatty acid composition of liver phosphatidylcholine were studied in gonadectomized and hypophysectomized adult female Sprague-Dawley rats. Gonadal steroids have been shown to influence the fatty acid composition of liver phosphatidylcholine in the rat. It is shown in the present study that neither testosterone nor estradiol had any effects on liver phosphatidylcholine in hypophysectomized rats. There was a 'masculinizing' effect of hypophysectomy of female rats on the fatty acid composition of liver phosphatidylcholine (i.e., an increase in the proportion of palmitic, oleic and linoleic acids and a decrease in the proportion of stearic and arachidonic acids). Continuous infusion of human GH and bovine GH partly reversed the 'masculinizing' effect of hypophysectomy. In contrast, there were no effects of intermittent administration of human GH. Also, there was no effect of prolactin infusion. It is concluded that the sexually dimorphic secretory pattern of GH may be involved in the regulation of the sexual differentiation of the fatty acid composition of liver phosphatidylcholine in the rat.

In vivo synthesis of phosphatidylcholine in rat brain via the phospholipid methylation pathway

We examined the in vivo synthesis of brain phosphatidylcholine (PC) by the methylation of phosphatidylethanolamine (PE). [3H-methyl]methionine was infused i.c.v., by indwelling cannula, and brain samples were taken 0.5-18 h thereafter and assayed for [3H]PC, as well as for its biosynthetic intermediates [3H]phosphatidylmonomethylethanolamine ([3H]PMME) and [3H]phosphatidyldimethylethanolamine ([3H]PDME), and for [3H]lysophosphatidylcholine ([3H]LPC) and S-[3H]adenosylmethionine ([3H]SAM). Most of the [3H]PC (79-94%) was present ipsilateral to the infusion site, indicating that the radioactivity in the [3H]PC was primarily of intracerebral origin, and not taken up from the blood. Moreover, only very low levels of [3H]PC were attained in brains of animals receiving [3H]methionine i.p. and these levels were symmetrically distributed. [3H]PMME and [3H]PDME turned over with apparent half-lives of 2.2 h and 2.4 h. In contrast, the accumulation of brain [3H]PC was biphasic, suggesting the existence of two pools, the more labile of which turned over rapidly (t1/2 = 5 h) and was formed for as long as [3H]PMME and [3H]PDME are present in the brain, and another, which was distinguishable only at 18 h after the [3H]methionine infusion. (The latter pool may have been synthesized from [3H]choline that was released via the hydrolysis of some of the brain [3H]PC previously formed by the methylation of PE.) Subcellular fractionation of brain tissue obtained after in vivo labelling with [3H]methionine revealed that mitochondrial PC had the highest specific radioactivity (dpm per mumol total lipid phosphorus), and myelin the least. These observations affirm that rat brain does synthesize PC in vivo by methylating PE, and the technique provides an experimental system which may be useful for examining the physiological regulation of this process.

Alterations in molecular species of cholesterol esters formed via plasma lecithin-cholesterol acyltransferase in human subjects consuming fish oil

The influence of a dietary supplement of n-3 polyunsaturated fatty acids containing eicosapentaenoic acid (EPA, 20:5n-3) and docosahexaenoic acid (DHA, 22:6n-3) on the molecular species of cholesteryl esters (CE) formed via the plasma lecithin (phosphatidylcholine)-cholesterol acyltransferase (LCAT; EC 2.3.1.43) reaction was evaluated. For this purpose, one group of eight subjects received an encapsulated fish lipid concentrate (MaxEPA) and another group of eight volunteers in the control group received encapsulated olive oil for 22 days. Plasma lipid profiles and fatty acid compositions of plasma phosphatidylcholine (PC) and CE were measured at day 0 and day 22 in all subjects. A decrease in plasma triglyceride (by 34%) and a moderate rise in high-density lipoprotein (HDL)-cholesterol (by 13%) was observed in the MaxEPA group. For characterization of the plasma LCAT-derived reaction products formed in vitro, [14C]cholesterol was used as the substrate and the newly formed molecular species of [14C]CE were separated by argentation thin-layer chromatography. Marked shifts were found in the abundance of the various classes of LCAT-derived products in the MaxEPA group whereas no significant changes were observed in the controls. The proportion of the [14C]CE as pentaenoic (EPA) species rose by 9-fold (from 1.5% at day 0 to 14.4% at day 22) as the dienoic (linoleate) species fell (from 50.6 to 39.2%); a moderate rise in the hexaenoic (DHA) species (from 1.7 to 2.4%) with no significant change in the tetraenoic (arachidonate) (AA) species was observed. The LCAT results were in the order of the observed shifts in the fatty acid patterns of the plasma CE.(ABSTRACT TRUNCATED AT 250 WORDS)

The influence of gonadal steroids and the pituitary on the levels and composition of plasma phospholipids in the rat

Gonadal steroids have been shown to influence plasma phospholipids. In the present study, the possible interaction between gonadal steroids and the pituitary in the regulation of plasma phospholipids was studied in rats. The total phospholipid concentration (higher in females) and the fatty acid composition of plasma lecithin was different in male compared to female rats. Gonadectomy resulted in a "feminization" of plasma phospholipids (total concentration and fatty acids in lecithin) in male rats but had no effect in females. Testosterone treatment of gonadectomized males or intact females resulted in a "masculinization" of plasma phospholipids, whereas estrogen treatment of intact males resulted in a "feminization." Hypophysectomy resulted in a marked decrease in plasma phospholipid concentration and the fatty acid composition of lecithin showed a "masculine" pattern in both males and females. Neither testosterone nor estrogen treatment had any effects on plasma phospholipids in hypophysectomized male and female rats, respectively. It is concluded that gonadal steroids and the hypothalamic-pituitary axis interact in the regulation of the synthetic and perhaps also degradative pathways controlling plasma phospholipids.

In vitro effects of alpha-bromopalmitate on metabolism of essential fatty acids studied in isolated rat hepatocytes: sex differences

alpha-Bromopalmitate was shown to have a far more pronounced effect on metabolism of labelled linoleic acid (18:2, n-6) and arachidonic acid (20:4, n-6) in isolated liver cells from female rats than in those from males. alpha-Bromopalmitate decreased triacylglycerol synthesis with a concomitant accumulation of fatty acid in diacylglycerol, indicating that the acylation of diacylglycerol is affected by alpha-bromopalmitate.

A comparison of the reversibility of phosphoethanolamine transferase and phosphocholine transferase in rat brain microsomes

The reversibility of phosphoethanolamine transferase (EC 2.7.8.1) in rat brain is demonstrated in this paper. Microsomal ethanolamine glycerophospholipids were prelabeled with an intracerebral injection of [3H]ethanolamine 4 h before killing young rats. Labeled CDPethanolamine was produced by incubation of the microsomes with CMP, although to a lesser extent than for the previously observed release of CDPcholine. Ethanolamine and choline glycerophospholipids were labeled with [2-3H]glycerol by incubation with primary cultures of rat brain. Microsomes from rat brains, with diisopropyl phosphofluoridate for inhibition of lipases, were incubated with the labeled glycerophospholipids separately, and labeled diacylglycerols were produced. The kinetic parameters of phosphoethanolamine transferase and phosphocholine transferase (EC 2.7.8.2) were compared by incubating rat brain microsomes with [3H]CMP. Inclusion of AMP in the reaction mixture was necessary in order to inhibit the hydrolysis of CMP by an enzyme with the properties of 5'-nucleotidase (EC 3.1.3.5). For phosphoethanolamine transferase and phosphocholine transferase respectively, the Km values for CMP were 40 and 125 microM and the V values were 2.3 and 21.6 nmol/h per mg protein. The reversibility of both enzymes permits the interconversion of the diacylglycerol moieties of choline and ethanolamine glycerophospholipids. During brain ischemia, a principal pathway for degradation of ethanolamine glycerophospholipids may be by reversal of phosphoethanolamine transferase followed by hydrolysis of diacylglycerols by the lipase.

Developmental pattern for phosphatidylserine decarboxylase in rat brain

In adult rats, a significant portion of brain ethanolamine glycerophospholipids are synthesized by a pathway involving phosphatidylserine decarboxylase, a mitochondrial enzyme. We have now examined whether this enzyme plays a particularly prominent role during development. Activities for both phosphatidylserine decarboxylase and succinate dehydrogenase (another mitochondrial enzyme) were determined in brain homogenates from rats 5 days of age to adulthood. Succinate dehydrogenase activity, expressed on a per unit brain protein basis, increased markedly during development. This pattern has been reported previously and is as expected from the postnatal increase in oxidative metabolism. In contrast, phosphatidylserine decarboxylase activity decreased 40% from 5 to 30 days of age. The apparent Km for brain phosphatidylserine decarboxylase was 85 microM in both young (8- and 20-day-old) and adult animals. Parallel studies in vivo were carried out to determine the contribution of the phosphatidylserine decarboxylase pathway, relative to pathways utilizing ethanolamine directly, to the synthesis of brain ethanolamine glycerophospholipids. Animals were injected intracranially with a mixture of L-[G-3H]serine and [2-14C]ethanolamine and incorporation into the base moieties of the phospholipids determined. The 3H/14C ratio of ethanolamine glycerophospholipids decreased about 50% during development. Our studies in vitro and in vivo both suggest that phosphatidylserine decarboxylase plays a significant role in the synthesis of brain ethanolamine glycerophospholipids at all ages, although it is relatively more prominent early in development.

Sex differences in the metabolism of essential fatty acids studied in isolated rat liver cells

The triacylglycerol synthesis from exogenous linoleic acid (18:2(n-6], linolenic acid (18:3(n-3], dihomogammalinolenic acid (20:3(n-6], eicosapentaenoic acid (20:5(n-3] and oleic acid (18:1(n-9] was observed to be significantly increased in isolated liver cells from female rats compared with males. The rate of fatty acid oxidation and phospholipid biosynthesis was concomitantly more important in male cells. With the C22-polyenoic fatty acids, adrenic acid (22:4(n-6] and docosahexaenoic acid (22:6(n-3), only a minor sex-related difference in fatty acid metabolism was found.

Phospholipid methylation in rabbit aorta

The formation of phosphatidylcholine by successive methylations of phosphatidylethanolamine using S-adenosylmethionine as the methyl donor was studied in homogenates of rabbit aorta. Addition of phosphatidylmonomethylethanolamine and phosphatidyldimethylethanolamine, but not phosphatidylethanolamine, stimulated methyltransferase activity and this activity was further stimulated when the phospholipids were dispersed in taurocholate prior to addition to the assay system. No incorporation of radiolabel into sphingomyelin or lysolecithin was detected indicating minimal metabolism of newly formed phosphatidylcholine. The majority of methyltransferase activity was detected in the high-speed pellet of the aortic homogenate; however, since activity was also detected in the high-speed supernatant, the low-speed supernatant preparation was used as the source of enzyme. Methyltransferase activity was characterized in cultured arterial smooth muscle cells using methionine as the radiolabeled precursor. The major product formed was phosphatidylcholine. No difference in enzyme activity was seen as a function of the length of time that cells were in culture or anatomic location of the aortic explant used as a source of cells. Treatment of the cells with cycloheximide did not affect methyltransferase activity. The ability of catecholamine agonists and vasoactive peptides to influence methyltransferase activity was investigated both in the cell-free preparation and in cultured cells. These compounds did not appear to alter methyltransferase activity in rabbit aortic smooth muscle cells.

Stimulation of the methylation pathway for phosphatidylcholine synthesis in rat lungs by choline deficiency

The methylation of phosphatidylethanolamine (PE) to form phosphatidylcholine (PC) was investigated using the isolated rat lung perfused with radiolabeled ethanolamine. Lungs from choline-deficient rats showed increased incorporation of radiolabel into PC at 2 h of perfusion. Increased PC synthesis from PE was also observed with lungs from rats fed a lipotrophic (choline plus methionine deficient) diet when methionine was added to the lung perfusate. These results indicate increased activity of the methylation pathway for lung PC synthesis during choline deficiency.

Phosphatidylethanolamine methyltransferase activity in chick liver microsomes

The synthesis of phosphatidylcholine from phosphatidylethanolamine is carried out by chick liver microsomes (Gallus domesticus). Different concentrations of PE, NPE and NNPE were used as exogenous substrates. Saturation of the S-adenosylmethionine has been found for the three different reactions with or without exogenous substrate. Kinetic parameters have been determined for this enzyme system in chick liver microsomes. The three methyl reactions had a similar pH profile with an optimum at pH = 8. Divalent ions such as Ca2+ or Mg2+ did not stimulate the enzyme activity. The results suggest that the synthesis of phosphatidylcholine from phosphatidylethanolamine by chick liver microsomes exhibits a kinetic pattern with different aspects than that described for other animal or human preparations.

Polar head group decarboxylation and methylation of phospholipids: an alternate route for phosphatidylcholine formation in cultured neuronal cells

Decarboxylation of phosphatidylserine (SPG) and methionine-dependent, stepwise methylation of phosphatidylethanolamine (EPG) to form phosphatidylcholine (CPG) were examined in monolayer cultures of rat cerebral cells. Ethanolamine, monomethylaminoethanol, or dimethylaminoethanol nitrogenous bases (N-bases) added to culture medium at millimolar level result each in synthesis of the corresponding phospholipid via a de novo pathway at initial rates of 0.18, 0.30, and 0.36 nmol/h/micrograms DNA, respectively. Addition of methyl-labeled methionine to culture medium at tracer levels or at millimolar concentration enabled measurements of the rates of phospholipid methylation from EPG phosphatidylmonomethylaminoethanol (Me1EPG) and phosphatidyldimethylaminoethanol (Me2EPG) precursors. At tracer doses, the rates of methylation from the above respective phospholipids are 0.45, 1.17, and 1.70 pmol/h/micrograms DNA. At 1 mM methionine, synthesis of CPG proceeds from [14C]EPG or [14C]Me2EPG at initial rates of 8 and 17 pmol/h/micrograms DNA, respectively. Although the latter phospholipid analog can be generated from its monomethyl precursor, methylation of EPG does not result in the accumulation of Me2EPG, suggesting two segregated and metabolically distinct pathways. In the presence of N-bases, of the total [3H]serine incorporated into cellular phospholipids 30-36.5% of labelled SPG is converted into decarboxylation products. The decarboxylation and methylation routes contribute a significant portion of choline from endogenous sources, most likely through conversion of SPG.

The biosynthesis of phosphatidylserine and phosphatidylethanolamine from L-[3-14C]serine in isolated rat hepatocytes

Incorporation of L-[3-14C]serine into phosphatidylserine (PS) and phosphatidylethanolamine (PE) has been studied in isolated rat hepatocytes. Ethanolamine inhibited the incorporation, indicating competition with serine in the base-exchange reaction. Choline, monomethylethanolamine, dimethylethanolamine and dimethyl-3-aminopropan-1-ol had no such effect. The observed rate of PS biosynthesis corresponded to 7-17 nmol/min per liver at 0.55 mM L-serine. The results indicate that only a small fraction (1/25 to 1/70) of the PS pool equilibrates with the base-exchange enzyme, and that decarboxylation to PE occurs preferentially from this pool. The rate of PS synthesis and decarboxylation can therefore not be calculated by methods which assume random, homogeneous labelling of the total PS pool. The apparent rate of PS decarboxylation increased approx. 4-fold when L-serine increased from 0.5 to 2.25 mM, suggesting that decarboxylation of PS to PE might be regulated by the concentration of L-serine or by the amount of PS present in the hepatocyte cell membranes. Lauric, palmitic, stearic, oleic and linoleic acid decreased the rate of PS synthesis. At 0.5 mM, lauric and palmitic acid were most inhibitory. At 1.0 mM, linoleic acid was the least inhibitory fatty acid. The saturated hexaenoic and saturated tetraenoic species of PS contained 51 and 29%, respectively, of the incorporated L-[3-14C]serine. The combined monoene dienoic/diene dienoic fraction had the highest rate of synthesis judged by its relative specific activity. At 0.9 mM concentration, linoleic acid doubled the relative specific activity of the combined monoene dienoic/diene dienoic fraction of PS. Incorporation of L-[3-14C]serine into molecular species of PE resembled that into PS, both in the absence and presence of linoleic acid, suggesting that the phosphatidylserine decarboxylase (EC 4.1.1.65) has a low specificity towards the fatty acid composition of PS. The results indicate that biosynthesis of PS from L-serine occurs mainly by the base-exchange with only negligible contribution from direct incorporation of phosphatidic acid or diacylglycerol. Furthermore, the deacylation-reacylation pathway seem to contribute only little to the determination of the fatty acid composition of hepatocyte PS. Active PS turnover seems to be confined to a small fraction of the PS pool.

Activation of partially purified rat liver lipid methyltransferase by phosphorylation

Incubation of partially purified rat liver lipid methyltransferase with MgATP and the catalytic subunit of the cyclic AMP dependent protein kinase results in up to 4-fold activation of the methylation reaction. When (gamma-32p) MgATP is included in the assay mixture, the analysis of the phosphoprotein products by electrophoresis shows the incorporation of 32p into a single protein band of about 50K and pI 4.75. It is concluded that rat liver lipid methyltransferase can be converted from a low activity dephosphorylated form to a high activity phosphorylated form.