Evidence that remodeling of the fatty acids of phosphatidylcholine is regulated in isolated rat hepatocytes and involves both the sn-1 and sn-2 positions

The phospholipase A2 superfamily as a central hub of bioactive lipids and beyond

In essence, "phospholipase A₂" (PLA₂) means a group of enzymes that release fatty acids and lysophospholipids by hydrolyzing the sn-2 position of glycerophospholipids. To date, more than 50 enzymes possessing PLA₂ or related lipid-metabolizing activities have been identified in mammals, and these are subdivided into several families in terms of their structures, catalytic mechanisms, tissue/cellular localizations, and evolutionary relationships. From a general viewpoint, the PLA₂ superfamily has mainly been implicated in signal transduction, driving the production of a wide variety of bioactive lipid mediators. However, a growing body of evidence indicates that PLA₂s also contribute to phospholipid remodeling or recycling for membrane homeostasis, fatty acid β-oxidation for energy production, and barrier lipid formation on the body surface. Accordingly, PLA₂ enzymes are considered one of the key regulators of a broad range of lipid metabolism, and perturbation of specific PLA₂-driven lipid pathways often disrupts tissue and cellular homeostasis and may be associated with a variety of diseases. This review covers current understanding of the physiological functions of the PLA₂ superfamily, focusing particularly on the two major intracellular PLA₂ families (Ca²⁺-dependent cytosolic PLA₂s and Ca²⁺-independent patatin-like PLA₂s) as well as other PLA₂ families, based on studies using gene-manipulated mice and human diseases in combination with comprehensive lipidomics.

LPGAT1 controls the stearate/palmitate ratio of phosphatidylethanolamine and phosphatidylcholine in sn-1 specific remodeling

Most mammalian phospholipids contain a saturated fatty acid at the sn-1 carbon atom and an unsaturated fatty acid at the sn-2 carbon atom of the glycerol backbone group. While the sn-2 linked chains undergo extensive remodeling by deacylation and reacylation (Lands cycle), it is not known how the composition of saturated fatty acids is controlled at the sn-1 position. Here, we demonstrate that lysophosphatidylglycerol acyltransferase 1 (LPGAT1) is an sn-1 specific acyltransferase that controls the stearate/palmitate ratio of phosphatidylethanolamine (PE) and phosphatidylcholine. Bacterially expressed murine LPGAT1 transferred saturated acyl-CoAs specifically into the sn-1 position of lysophosphatidylethanolamine (LPE) rather than lysophosphatidylglycerol and preferred stearoyl-CoA over palmitoyl-CoA as the substrate. In addition, genetic ablation of LPGAT1 in mice abolished 1-LPE:stearoyl-CoA acyltransferase activity and caused a shift from stearate to palmitate species in PE, dimethyl-PE, and phosphatidylcholine. Lysophosphatidylglycerol acyltransferase 1 KO mice were leaner and had a shorter life span than their littermate controls. Finally, we show that total lipid synthesis was reduced in isolated hepatocytes of LPGAT1 knockout mice. Thus, we conclude that LPGAT1 is an sn-1 specific LPE acyltransferase that controls the stearate/palmitate homeostasis of PE and the metabolites of the PE methylation pathway and that LPGAT1 plays a central role in the regulation of lipid biosynthesis with implications for body fat content and longevity.

Abnormal liver phosphatidylcholine synthesis revealed in patients with acute respiratory distress syndrome

Acute respiratory distress syndrome (ARDS) is associated with a severe pro-inflammatory response; although decreased plasma cholesterol concentration has been linked to systemic inflammation, any association of phospholipid metabolic pathways with ARDS has not been characterized. Plasma phosphatidylcholine (PC), the major phospholipid of circulating lipoproteins, is synthesized in human liver by two biologically diverse pathways: the cytidine diphosphocholine (CDP):choline and phosphatidylethanolamine N-methyltransferase (PEMT) pathways. Here, we used ESI-MS/MS both to characterize plasma PC compositions and to quantify metabolic fluxes of both pathways using stable isotopes in patients with severe ARDS and in healthy controls. Direct incorporation of methyl-D9-choline estimated CDP:choline pathway flux, while PEMT flux was determined from incorporations of one and two methyl-D3 groups derived from methyl-D9-choline. The results of MS/MS analysis showed significant alterations in plasma PC composition in patients with ARDS versus healthy controls. In particular, the increased overall methyl-D9-PC enrichment and, most importantly, the much lower methyl-D3-PC and methyl-D6-PC enrichments suggest increased flux through the CDP:choline pathway and reduced flux through the PEMT pathway in ARDS. To our knowledge, this study is the first to demonstrate significant plasma PC molecular compositional changes combined with associated alterations in the dynamics of PC synthetic pathways in patients with ARDS.

Introduction of phospholipids to cultured cells with cyclodextrin

Previous studies indicate that methyl-β-cyclodextrin (meβ-CD) can greatly enhance translocation of long-chain phospholipids from vesicles to cells in culture, which is very useful when studying, e.g., phospholipid metabolism and trafficking. However, the parameters affecting the transfer have not been systematically studied. Therefore, we studied the relevant parameters including meβ-CD and vesicle concentration, incubation time, phospholipid structure, and cell type. Because meβ-CD can extract cholesterol and other lipids from cells, thereby potentially altering cell growth or viability, these issues were studied as well. The results show that efficient incorporation of phospholipid species with hydrophobicity similar to that of natural species can be obtained without significantly compromising cell growth or viability. Cellular content of phosphatidyl-serine, -ethanolamine, and -choline could be increased dramatically, i.e., 400, 125, and 25%, respectively. Depletion of cellular cholesterol could be prevented or alleviated by inclusion of the proper amount of cholesterol in the donor vesicles. In summary, meβ-CD mediates efficient transfer of long-chain (phospho) lipids from vesicles to cells without significantly compromising their growth or viability. This lays a basis for detailed studies of phospholipid metabolism and trafficking as well as enables extensive manipulation of cellular phospholipid composition, which is particularly useful when investigating mechanisms underlying phospholipid homeostasis.

Sex, but not maternal protein or folic acid intake, determines the fatty acid composition of hepatic phospholipids, but not of triacylglycerol, in adult rats

The aim of the study was to investigate whether the protein and folic acid content of the maternal diet and the sex of the offspring alter the polyunsaturated fatty acid content of hepatic phospholipids and triacylglycerol (TAG). Pregnant rats were fed diets containing 18% or 9% protein with either 1 or 5mg/kg folic acid. Maternal diet did not alter hepatic lipid composition in the adult offspring. Data from each maternal dietary group were combined and reanalysed. The proportion of 18:0, 20:4n-6 and 22:6n-3 in liver phospholipids was higher in females than in males, while hepatic TAG composition did not differ between sexes. Delta5 Desaturase expression was higher in females than in males. Neither Delta5 nor Delta6 desaturase expression was related to polyunsaturated fatty acid concentrations. These results suggest that sex differences in liver phospholipid fatty acid composition may reflect primary differences in the specificity of phospholipid biosynthesis.

Mitochondrial dysfunction in rat with nonalcoholic fatty liver Involvement of complex I, reactive oxygen species and cardiolipin

Mitochondrial dysfunction and oxidative stress play a central role in the pathophysiology of nonalcoholic fatty liver disease (NAFLD). This study aimed to elucidate the mechanism(s) responsible for mitochondrial dysfunction in nonalcoholic fatty liver. Fatty liver was induced in rats with a choline-deficient (CD) diet for 30 days. We examined the effect of CD diet on various parameters related to mitochondrial function such as complex I activity, oxygen consumption, reactive oxygen species (ROS) generation and cardiolipin content and oxidation. The activity of complex I was reduced by 35% in mitochondria isolated from CD livers compared with the controls. These changes in complex I activity were associated with parallel changes in state 3 respiration. Hydrogen peroxide (H(2)O(2)) generation was significantly increased in mitochondria isolated from CD livers. The mitochondrial content of cardiolipin, a phospholipid required for optimal activity of complex I, decreased by 38% as function of CD diet, while there was a significantly increase in the level of peroxidized cardiolipin. The lower complex I activity in mitochondria from CD livers could be completely restored to the level of control livers by exogenously added cardiolipin. This effect of cardiolipin could not be replaced by other phospholipids nor by peroxidized cardiolipin. It is concluded that CD diet causes mitochondrial complex I dysfunction which can be attributed to ROS-induced cardiolipin oxidation. These findings provide new insights into the alterations underlying mitochondrial dysfunction in NAFLD.

Probing phospholipid dynamics by electrospray ionisation mass spectrometry

Recent advances in electrospray ionisation mass spectrometry (ESI-MS) have greatly facilitated the analysis of phospholipid molecular species in a growing diversity of biological and clinical settings. The combination of ESI-MS and metabolic labelling employing substrates labelled with stable isotopes is especially exciting, permitting studies of phospholipid synthesis and turnover in vivo. This review will first describe the methodology involved and will then detail dynamic lipidomic studies that have applied the stable isotope incorporation approach. Finally, it will summarise the increasing number of studies that have used ESI-MS to characterise structural and signalling phospholipid molecular species in development and disease.

Effect of reduced maternal protein intake in pregnancy in the rat on the fatty acid composition of brain, liver, plasma, heart and lung phospholipids of the offspring after weaning

Reduced protein intake during pregnancy decreased maternal hepatic and plasma docosahexaenoic acid concentrations and impaired docosahexaenoic acid accumulation into fetal brain in the rat. The present study investigated whether restriction of maternal protein intake during pregnancy in the rat alters membrane phospholipid fatty acid composition in the offspring after weaning. Female rats (six per group) were mated and fed diets containing either 180 or 90 g protein/kg throughout pregnancy. Mothers were transferred to standard chow after delivery and the litters reduced to eight pups. Weaning was at 28 d and pups were killed 5 to 6 d later. Tissue weights or membrane total phosphatidylcholine (PC) and phosphatidylethanolamine (PE) concentrations in the offspring did not differ between dietary groups. There were significant differences between the 180 and 90 g/kg groups in liver, brain, lung and heart fatty acid composition that differed between tissues and phospholipid classes. For example, docosahexaenoic and arachidonic acid concentrations were 23 and 10 % lower respectively in hepatic PC, but not PE, in the 90 g/kg group. In brain, docosahexaenoic acid concentration was 17 % lower in PC, but not PE, while arachidonic acid content was 21 % greater in PE but unchanged in PC. The greatest differences were in unsaturated fatty acids, which suggests alterations to desaturase activities and/or the specificity of phospholipid biosynthesis. These results suggest that restricted maternal protein intake during pregnancy results in persistent alterations to membrane fatty acid content.

Eritadenine-induced alterations of plasma lipoprotein lipid concentrations and phosphatidylcholine molecular species profile in rats fed cholesterol-free and cholesterol-enriched diets

The effects of dietary eritadenine on the concentration of plasma lipoprotein lipids and the molecular species profile of plasma lipoprotein phosphatidylcholine (PC) were investigated in rats fed cholesterol-free and cholesterol-enriched diets to obtain insights into the relationship between the changes in PC molecular species profile and the hypocholesterolemic action of eritadenine. The effect of eritadenine on the secretion rate of very low density lipoprotein (VLDL) from the liver was also estimated. Rats were fed the control or eritadenine-supplemented (50 mg/kg) diets with or without exogenous cholesterol for 14 d. Eritadenine supplementation significantly decreased the cholesterol of major plasma lipoproteins, high density lipoprotein and VLDL, in rats fed cholesterol-free and cholesterol-enriched diets, respectively. The ratio of PC to phosphatidylethanolamine, delta6-desaturase activity, and the ratio of arachidonic acid to linoleic acid in liver microsomes were markedly decreased by eritadenine irrespective of the presence or absence of exogenous cholesterol. Dietary eritadenine increased the proportion of 16:0-18:2 molecular species with a decrease in 18:0-20:4 in plasma lipoprotein PC in both rats fed cholesterol-free and cholesterol-enriched diets. Eritadenine did not depress the secretion rate of VLDL in rats fed a cholesterol-free diet containing a high level of choline. The results indicate that dietary eritadenine elicits its hypocholesterolemic action with modulations of the fatty acid and molecular species profiles of PC irrespective of the presence or absence of exogenous cholesterol. The eritadenine-induced alteration of PC molecular species profile is discussed in relation to the hypocholesterolemic action of eritadenine.

Dietary eritadenine and ethanolamine depress fatty acid desaturase activities by increasing liver microsomal phosphatidylethanolamine in rats

The effects of eritadenine, a constituent of the Lentinus edodes mushroom, and ethanolamine, the base constituent of phosphatidylethanolamine (PE), on fatty acid desaturase activities and lipid profiles were investigated comparatively in rats. Rats were fed a control diet or a diet supplemented with either eritadenine (0.05 g/kg) or ethanolamine (8 g/kg) for 14 d. Eritadenine and ethanolamine had marked hypocholesterolemic effects. The concentration of liver microsomal PE was significantly increased and the ratio of phosphatidylcholine (PC) to PE was significantly decreased by both eritadenine and ethanolamine. These changes in phospholipid profile were also observed in the mitochondria and plasma membranes in the liver. The activities of the Delta5-, Delta6- and Delta9-desaturases in liver microsomes were significantly decreased by eritadenine and ethanolamine; there was a significant correlation between the activity of Delta5- or Delta6-desaturase and the proportion of PE in the total phospholipids or the PC/PE ratio. Reflecting decreased Delta5- and Delta6-desaturase activities, the 20:4(n-6)/18:2(n-6) ratio was significantly decreased by eritadenine and ethanolamine in PC of the liver microsomes, mitochondria and plasma membranes. Although the 20:4(n-6)/18:2(n-6) ratio of liver microsomal PE was also significantly decreased by eritadenine and ethanolamine, the fatty acid composition of phosphatidylinositol and phosphatidylserine was less affected by these compounds. Eritadenine and ethanolamine increased the proportion of 16:0-18:2 and decreased the proportion of 18:0-20:4 in liver PC. The results suggest that dietary eritadenine and ethanolamine might lead to decreases in desaturase activities and changes in fatty acid and molecular species composition of PC through an increase in liver microsomal PE.

Electrospray ionisation mass spectrometric analysis of lipid restructuring in the carp (Cyprinus carpio L.) during cold acclimation

Cold acclimation of carp from 30°C to 10°C causes a restructuring of liver microsomal phospholipids characterised by increased proportions of monounsaturated fatty acid in phosphatidylcholine (PC) and phosphatidylethanolamine (PE). Here, we have used electrospray ionisation mass spectrometry (ESI-MS) to determine the patterns of alteration to individual molecular species compositions of PC, PE and phosphatidylinositol (PI) in response to gradually decreasing temperature. The results demonstrate that cold induces precise changes to a limited number of phospholipid species, and that these changes are distinct and different for each phospholipid class. The major change for PC was increased 16:1/22:6, but for PE the species that increased was 18:1/22:6. By contrast, the PI species that increased during cold acclimation were characterised by an sn-1 monounsaturated fatty acid in combination with arachidonoyl or eicosapentaenoyl fatty acid at the sn-2 position. Analysis of acyl distribution indicates that cold only caused the accumulation of monounsaturated fatty acids at the sn-1 and not at the sn-2 position of phospholipids. These results highlight the tight and restricted range of modifications that membranes make to their phospholipid composition in response to thermal stress.

Chlorpromazine and human platelet glycerolipid metabolism: precursor specificity and significance of drug-platelet interaction time

Chlorpromazine is known to have a number of effects on glycerolipid metabolism in a variety of cell types, and in some cases reports are contradictory. To investigate the basis for some of these discrepancies, we reinvestigated the effects of chlorpromazine on some aspects of platelet glycerolipid metabolism. Time-courses conducted with [3H]glycerol or [3H]palmitic acid showed that the effects of chlorpromazine on the labelling of phosphatidylcholine, diacylglycerol, and triacyglycerol were highly dependent upon platelet-drug interaction time. The time-dependent changes in labelling patterns were independent of the presence of radiolabel during incubation, and were not the results of time-dependent changes in the platelets per se. The effects of chlorpromazine on the labelling of platelet glycerolipids by [3H]glycerol, [3H]palmitic acid, [32P]P(i) ([32P]phosphatase), and [14C]choline were compared. Dose-response curves conducted at 30-min incubation time showed that chlorpromazine potently inhibited labelling of diacylglycerol and diacyglycerol-derived lipids (triacyglycerol and phosphatidylcholine) by the 3H-labelled precursors. Labelling of phosphatidylcholine by [32P]P(i) or [14C]choline was, however, not affected at all by the drug. We conclude that the effects of chlorpromazine on platelets are highly time-dependent, and that the prolonged effects are most likely to be of biological significance. Furthermore, in platelets the effects of the drug on the labelling of phosphatidylcholine by isotope-labelled precursors are highly dependent on the route of incorporation of the specific precursor chosen.

Pregnancy-associated adaptations to hepatic phosphatidylcholine biosynthesis in the guinea-pig

Pregnancy is associated with increased phosphatidylcholine (PC) 16:0/22:6 and PC16:0/20:4 concentrations in rat liver and plasma, guinea-pig liver, and in plasma in women. These changes may be related to supply of polyunsaturated fatty acids (PUFA) to the fetus. For the rat, these adaptations to hepatic PC composition are regulated by modifications to synthesis de novo from choline. However, it is not known whether these adaptations are applicable to other species. Consequently, we have determined biochemical mechanisms for regulation of hepatic PC synthesis in the pregnant guinea pig. The PUFA content of guinea-pig liver PC synthesised de novo did not change significantly during pregnancy. [Methyl-14C]methionine incorporation into PC in vivo, however, showed increased PC16:0/22:6 and PC16:0/20:4 contents. [Methyl-14C]methionine incorporation into PC over 6 hr in vivo increased during early pregnancy, while PC synthesis de novo did not change. In contrast to the rat, modulation of PE N-methylation is a primary mechanism for regulating the PUFA content of hepatic PC in the pregnant guinea-pig. The use of distinct metabolic strategies to achieve comparable pregnancy-associated adaptations to hepatic PC composition between these animal species suggests both evolutionary convergence and a fundamental the role for PC16:0/22:6 and PC16:0/20:4 in PUFA metabolism during gestation.

Generation and remodeling of highly polyunsaturated molecular species of rat hepatocyte phospholipids

Freshly isolated rat hepatocytes were incubated for 20 min with [U-14C]glycerol in the presence or absence of unlabeled linoleic (18:2n-6), arachidonic (20:4n-6), or docosahexaenoic (22:6n-3) acid, added as albumin complex in 10% ethanol. Most of the radioactivity (approximately 95%) recovered in hepatocyte lipids was present in phosphatidylcholine (PC), phosphatidylethanolamine (PE), and triacylglycerol (TAG). The presence of exogenous fatty acids resulted in (i) higher incorporation of [U-14C]glycerol, (ii) higher percentage of label in TAG, and (iii) enhanced formation of PC and PE molecular species bearing the exogenous fatty acid at both the sn-1 and sn-2 positions of glycerol. In each case, these molecular species contained 60 to 70% of the label in that lipid class. Further incubation of the cells for 40 and 80 min in the absence of labeled substrate and exogenous fatty acids resulted in a redistribution of label among PC and PE molecular species due to deacylation-reacylation at the sn-1 position of glycerol.

Role of molecular species catabolism in the temperature-induced restructuring of phosphatidylcholines in liver microsomes of thermally-acclimated rainbow trout (Oncorhynchus mykiss)

Most previous studies of the temperature-induced restructuring of phospholipid molecular species composition have examined steps in the biosynthesis of phospholipids to explain the accumulation of unsaturated fatty acids in membranes of cold-acclimated poikilotherms. In contrast, the present study explores the role of phospholipases in this restructuring process by determining the rates of degradation of specific molecular species of phosphatidylcholine, using enzymes (microsomes) freshly isolated from the liver of rainbow trout. (Oncorhynchus mykiss) acclimated to either 5° or 20°C. The substrate preparation employed to assay phospholipase activity possessed a range of molecular species, all radiolabeled with 1-(14)C-palmitic acid at thesn-1 position, similar to that present in native trout liver microsomes. After defined periods of incubation (120 and 240 min at 5°C; 60 and 120 min at 20°C), phospholipids were extracted from the reaction mixture and the distribution of radioactivity among the molecular species of phosphatidylcholine was determined by HPLC/liquid scintillation counting. In general, molecular species catabolism was not significantly influenced by either assay or acclimation temperature. Only in 20°C-acclimated fish did a reduction in assay temperature (from to 20 to 5°C) result in significantly increased proportions of radioactivity being recovered in one polyunsaturated fatty acid-containing species (16:0/22:6-PC). It is concluded: 1) that phospholipase specificity, assayed under conditions approximating thosein situ, is not significantly influenced by temperature; and 2), that the increased proportions of unsaturated fatty acid-containing molecular species of phosphatidylcholine observed at low temperatures must reflect the specificity of biosynthetic rather than degradative processes.

Cholesterol-lowering effect of soyabean lecithin in normolipidaemic rats by stimulation of biliary lipid secretion

The purpose of the present study was to assess the role of the liver in the plasma-cholesterol-lowering effect of soyabean lecithin. Normolipidaemic rats were fed on lecithin-enriched or control diets with the same amount of protein. The lecithin diets contained 200 g/kg high-fat commercial semi-purified soyabean lecithin (230 g/kg total lipids as soyabean phosphatidylcholine) or 200 g/kg high-fat purified soyabean lecithin (930 g/kg total lipids as soyabean phosphatidylcholine). The control diets were a lowfat diet (40 g fat/kg) and a high-fat triacylglycerol-rich diet (200 g fat/kg). The high-fat diets were isoenergetic. The cholesterol-lowering effect of the lecithin-enriched diets was associated with significantly lower levels of plasma total- and HDL-cholesterol and significantly higher levels of bile phosphatidylcholine (PC), bile salts and cholesterol. These findings suggest that the liver plays a major role in the reduction of plasma cholesterol, the increased biliary lipid being provided by both HDL and the hepatic microsomal pools of PC and cholesterol.

Mammalian secreted and cytosolic phospholipase A2 show different specificities for phospholipid molecular species

Previous studies using phospholipid vesicles containing single molecular species have shown cytosolic phospholipase (85 kDa) (PL) A2 to possess a marked preference for arachidonic acid (20:4n-6)-containing species, while secreted PLA2 (14 kDa) exhibited little acyl chain selectivity. In this study, we have defined the molecular specificity of cytosolic PLA2 using phospholipid vesicles derived from rat liver which contain complex mixtures of molecular species. Phosphatidylcholine (PC) and phosphatidylethanolamine (PE) were isolated from rat liver by chloroform and methanol extraction, and solid-phase separation. PC and PE vesicles were hydrolysed by either human recombinant cytosolic or porcine pancreatic PLA2. Molecular species compositions were determined by reverse phase high performance liquid chromatography (HPLC) with post-column fluorescence derivitisation. HPLC analysis after limited hydrolysis demonstrated that the secreted phospholipase A2 showed no significant acyl chain specificity using these phospholipid mixtures. However, the cytosolic enzyme demonstrated a high degree of preference for arachidonic acid-containing species such that there was no hydrolysis of other molecular species. The extent of hydrolysis of PC16:0/20:4 was 1.4-fold greater (P < 0.05, n = 3) than PC18:0/20:4, while PE16:0/20:4 and PE18:0/20:4 were hydrolysed to a similar degree. Under these assay conditions, the cytosolic enzyme showed a preference for PE as compared with PC. This study confirms that cytosolic PLA2 is highly selective for sn-2 20:4n-6-containing phospholipid molecular species even when presented with a complex natural species mixture. This specificity is consistent with the cytosolic enzyme having a primary role in the process of arachidonic release within cells.(ABSTRACT TRUNCATED AT 250 WORDS)

The composition of individual molecular species of plasma phosphatidylcholine in human pregnancy

The molecular species composition of plasma phosphatidylcholine (PC) was measured in sequential blood samples from 13 pregnant women from 16 weeks of gestation to delivery at term. The increased total plasma PC concentration at term was due solely to increased concentrations of individual species containing palmitate (16:0) rather than stearate (18:0) at the sn-1 position. The specific increase of PC16:0/22:6 concentration in mid-gestation suggests that adaptations to maternal hepatic PC metabolism may provide a mechanism to ensure adequate supply of 22:6(n-3) to the fetus. While cord plasma PC was comparable to liver PC composition from three stillborn term infants, the compositions of these tissues differed from maternal plasma PC, which contained significantly more PC16:0/18:2 and PC18:0/18:2. These results suggest that, although fetal acquisition of polyunsaturated fatty acids (PUFA) is dependent on the maternal lipid supply, the detailed composition of fetal plasma PC may be regulated largely by intrinsic fetal mechanisms such as placental and liver PC metabolism. Similarly, the specific alterations to maternal plasma PC composition in pregnancy, which we postulate are associated with the supply of PUFA to the fetus, were substantially independent of variations in maternal dietary lipid nutrition.

Mechanisms of hepatic phosphatidylcholine synthesis in adult rat: effects of pregnancy

Late pregnancy in the rat (gestational ages 16-21 days) was accompanied by a specific increase in hepatic phosphatidylcholine (PC) and phosphatidylethanolamine (PE) molecular species containing C16:0 at the sn-1 position and polyunsaturated essential fatty acids (PUFA), in particular C22:6(n-3), at the sn-2 position. Incorporation of either CDP:[Me-14C]choline or CDP:[1,2-14C]-ethanolamine into hepatic microsomal sn-1 C16:0 PC or PE molecular species in vitro was greater at term than in non-pregnant animals, suggesting modifications to the composition of specific diacylglycerol (DAG) pools destined for synthesis of either PC or PE. Also, incorporation of [Me-14C]choline or [Me-14C]methionine into hepatic PC in vivo over 6 h in term pregnant rats was consistent with decreased phospholipase A1-dependent acyl remodelling of sn-1 C16:0 to sn-1 C18:0 molecular species. There was, however, no evidence to support any change to the specificity of acyl remodelling. The rate of PC synthesis by the de novo pathway in vivo was increased in term liver compared with non-pregnant animals, accompanied by increased choline-phosphotransferase activity in vitro in d21 liver microsomes. The rate of PC synthesis by PE N-methylation did not appear to change during pregnancy. Changes in composition of plasma PC species at term reflected those of newly synthesized hepatic PC. Our data suggest supply of PUFA to the developing fetal rat is the result of specific adaptations to maternal hepatic phospholipid biosynthesis rather than passive transfer from the maternal diet.

Biosynthesis of phospholipid molecular species in isolated liver cells studied by combining fatty acid substrates esterified in the sn-1 and sn-2 positions

The simultaneous incorporation of a saturated fatty acid in the sn-1 position and an unsaturated fatty acid in the sn-2 position in phosphatidylcholine (PC) and ethanolamine (PE) was studied in isolated liver cells. We combined a saturated fatty acid, 16:0 or 18:0 and an unsaturated fatty acid substrate, 18:2,n-6 or 20:4,n-6. In this situation the saturated fatty acids were preferentially oxidized and the unsaturated fatty acids were preferentially esterified in PL and TG. Addition of unlabelled 16:0 increased the incorporation of [14C]18:2 in 16:0-18:2 in PC and PE, reduced the incorporation in 18:2-18:2 but did not reduce the incorporation in 18:0-18:2. 18:0 increased the esterification of [14C]18:2 in 18:0-18:2, reduced the incorporation in 18:2-18:2 but did not reduce the incorporation in 16:0-18:2. The latter is the dominating 14C-labelled species formed from [14C]18:2 also in the presence of unlabelled 18:0. Addition of 20:4 stimulated the incorporation of [14C]16:0 in 16:0-20:4 and markedly reduced the formation of 16:0-18:2, 16:0-18:1 and 16:0-22:6. Addition of 18:2 increased the incorporation of [14C]16:0 in 16:0-18:2 and reduced the formation of 16:0-20:4 and 16:0-18:1. It is concluded that the unsaturated fatty acids 18:2 or 20:4 have a stronger impact on the synthesis of phospholipid molecular species than the saturated fatty acids 16:0 or 18:0 have. Thus 20:4,n-6 and 18:2,n-6 are able to direct available [14C]16:0 or [14C]18:0 to the sn-1 position. 16:0 and 18:0 are not in the same way able to direct [14C]18:2,n-6 to the synthesis of 16:0-18:2 or 18:0-18:2 at the expense of other 14C-labelled molecular species.

Incorporation of stearic acid (18:0) and palmitic acid (16:0) in phospholipid molecular species studied in isolated rat liver cells

The incorporation of [1-14C]16:0 and [1-14C]18:0 in the molecular species of PC and PE in isolated rat liver cells was studied. More [14C]18:0 than [14C]16:0 was esterified both in PC and PE. Also the chain elongated and desaturated products (16:1, 18:0 and 18:1) were incorporated. The main molecular phospholipid species formed from [14C]18:0 were 18:0-18:2, 18:0-20:4 and 18:0-22:6. 18:0-18:0 species was not detected, independent of the substrate concentration (0.1-0.9 mM). With [14C]16:0 at low substrate concentration (0.1 mM) the dominating species are 16:0-18:2, 16:0-20:4 and 16:0-22:6. These species were detected already after 10 min. The same main species are formed both in PC and PE, but the relative amounts differ. In PC the combination with 18:2 is most abundant for both saturated fatty acid substrates. In PE 18:0-20:4 dominates when 18:0 is the substrate, and 16:0-22:6 when 16:0 is. At higher substrate concentrations (0.4-0.9 mM) 16:0 is also esterified in 16:0-16:0. This molecular species is efficiently degraded in the cells within 2-3 h, in contrast to the other species formed. The results suggest that 16:0 and 18:0 are directly incorporated in the sn-1 position in physiologically important phospholipid molecular species. With an excess of 16:0, 16:0-16:0 is also formed in substantial amounts, but this uncommon species is thereafter removed.

Phosphatidylethanolamine derived from phosphatidylserine is deacylated and reacylated in rat hepatocytes

The metabolism of phosphatidylserine (PS), phosphatidylethanolamine (PE) and phosphatidylcholine (PC), derived from [3H]serine, has been studied in rat hepatocytes. After an initial pulse with radioactivity for 10 min and a chase for up to 240 min, cells were harvested and PS, PE and PC isolated. At the end of the pulse, greater than 90% of [3H]serine derived phospholipid radioactivity was associated with PS. In the subsequent chase, newly-made PS was degraded rapidly with less than 25% of the label lost from PS appearing in the PE and PC pools. In contrast, [3H]serine-labeled PE turnover was not detectable. Very little newly-made PS was converted to PC. PE and PC were further fractionated into molecular species by high-performance liquid chromatography. We report that [3H]serine-labeled PE is deacylated/reacylated with the major product of remodeling being 18:0-20:4 PE. In contrast, [3H]serine-labeled PC is not significantly remodeled.

Mechanisms of hepatic phosphatidylcholine synthesis in the developing guinea pig: contributions of acyl remodelling and of N-methylation of phosphatidylethanolamine

Hepatic phosphatidylcholine (PC) from the immature fetal guinea pig at day 55 of gestation comprised mainly unsaturated molecular species containing C18:2(n-6) and C22:6(n-3) at the sn-2 position, reflecting placental permeability to essential fatty acids. At both day 55 and term (day 68), [Me-14C]choline was incorporated in utero over 3 h largely into sn-1-C16:0 PC species, with incorporation into sn-1-C18:0 PC species increasing by 18 h of incubation. Comparison of specific radioactivities after 3 h and 18 h suggests PC acyl remodelling by phospholipase A1. No incorporation into C20:4(n-6)-containing PC species could be detected of either [Me-14C]choline in vivo or CDP-[Me-14C]choline in isolated microsomes. The major phosphatidylethanolamine (PE) species were 16:0/22:6 and 18:0/22:6. Although [14C]ethanolamine was initially incorporated mainly into sn-1-C16:0 species, specific-radioactivity analysis suggested differential turnover rather than acyl remodelling. [1,2-14C]Ethanolamine and [Me-14C]methionine incorporation into PC molecular species indicated that both newly synthesized and total PE pools were available for N-methylation. Since the PC pool synthesized from PE included C20:4- and C22:6-containing species, N-methylation may provide a mechanism for supplying essential long-chain fatty acids to developing tissues that can be regulated independently from bulk PC synthesis.