Phospholipid metabolism in the mammalian heart

Formation and function of phosphatidylserine and phosphatidylethanolamine in mammalian cells

Phosphatidylserine (PS) and phosphatidylethanolamine (PE) are metabolically related membrane aminophospholipids. In mammalian cells, PS is required for targeting and function of several intracellular signaling proteins. Moreover, PS is asymmetrically distributed in the plasma membrane. Although PS is highly enriched in the cytoplasmic leaflet of plasma membranes, PS exposure on the cell surface initiates blood clotting and removal of apoptotic cells. PS is synthesized in mammalian cells by two distinct PS synthases that exchange serine for choline or ethanolamine in phosphatidylcholine (PC) or PE, respectively. Targeted disruption of each PS synthase individually in mice demonstrated that neither enzyme is required for viability whereas elimination of both synthases was embryonic lethal. Thus, mammalian cells require a threshold amount of PS. PE is synthesized in mammalian cells by four different pathways, the quantitatively most important of which are the CDP-ethanolamine pathway that produces PE in the ER, and PS decarboxylation that occurs in mitochondria. PS is made in ER membranes and is imported into mitochondria for decarboxylation to PE via a domain of the ER [mitochondria-associated membranes (MAM)] that transiently associates with mitochondria. Elimination of PS decarboxylase in mice caused mitochondrial defects and embryonic lethality. Global elimination of the CDP-ethanolamine pathway was also incompatible with mouse survival. Thus, PE made by each of these pathways has independent and necessary functions. In mammals PE is a substrate for methylation to PC in the liver, a substrate for anandamide synthesis, and supplies ethanolamine for glycosylphosphatidylinositol anchors of cell-surface signaling proteins. Thus, PS and PE participate in many previously unanticipated facets of mammalian cell biology. This article is part of a Special Issue entitled Phospholipids and Phospholipid Metabolism.

31P-MR spectroscopic imaging in hypertensive heart disease

Hypertensive heart disease (HHD) causes structural changes (e.g., fibrosis) that result in diastolic and systolic myocardial dysfunction. Alterations of (31)P metabolism and cardiac energy impairments were assessed in patients with HHD by MR spectroscopy (MRS) and correlated with left ventricular systolic function. Thirty-six patients with HHD and 20 healthy controls (mean age 35.2+/-10.7 years) were examined with (31)P-MRS at 1.5 T by using an ECG-gated CSI sequence. Twenty-five patients (mean age 64.3+/-9.3 years) had diastolic dysfunction, but preserved systolic function (HHD-D), whereas 11 patients (62.3+/-11.4 years) suffered from additional impaired systolic function (HHD-S). In both patient groups, the PCr/gamma-ATP ratio was lower than in the controls (controls: 2.07+/-0.17; P<0.001), and in HHD-S was lower than in HHD-D (1.43+/-0.21 vs. 1.65+/-0.25; P=0.012). PCr/gamma-ATP ratios were linearly correlated with LVEF (Pearson's r: 0.39; P=0.025). In the HHD-S group, the PDE/gamma-ATP ratio was significantly lower (0.56+/-0.36) than in the controls (1.14+/-0.42; P=0.001). In contrast to the group of HHD-D patients, whose slightly decreased PCr/gamma-ATP ratios compared to controls may be explained by age differences, the more distinct changes observed in HHD-S patients indicate an altered energy metabolism. The observed metabolic changes were related to functional impairments, as indicated by a reduced LVEF. Reduced PDE/ATP ratios indicate changes in the phospholipid metabolism.

Effects of norepinephrine and cardiotrophin-1 on phospholipase D activity and incorporation of myristic acid into phosphatidylcholine in rat heart

The present study is part of a project on phospholipase D (PLD) in cardiac hypertrophy and analyzed effects on PLD activity of two growth stimuli, norepinephrine (NE) and cardiotrophin-1 (CT-1), in incubated rat heart. Phosphatidylcholine (PC) was labeled by (3)H-myristic acid. PLD produced (3)H-phosphatidylethanol ((3)H-PEth) from (3)H-PC in the presence of ethanol and maintained a basal formation of (3)H-PEth. Short-term and long-term exposure to NE for 2 or 13 h, respectively, enhanced the formation of (3)H-PEth, which was blocked by prazosin. Long-term pretreatment with NE or CT-1 increased the incorporation of (3)H-myristic acid into PC, which was blocked by atenolol. When the (3)H-PEth formation was expressed as a fraction of (3)H-PC, PLD activity seemingly was unchanged (NE) or markedly reduced (CT-1); the true effects, namely, stimulation by NE and nonresponsiveness towards CT-1, were unraveled by atenolol (NE) or when PLD activity was expressed as (3)H-PEth per ng protein. In conclusion, alpha-adrenoceptor activation increased PLD activity. Long-term treatment with NE (via beta-receptors) or CT-1 enhanced the (3)H-myristic acid incorporation into a PC compartment, that was not available for the alpha-receptor-mediated PLD activation. These results were discussed in regard to cellular mechanisms of cardiac hypertrophy and to the transphosphatidylation assay of PLD.

Potential mechanisms for the enhancement of HERG K+ channel function by phospholipid metabolites

1. Phospholipid metabolites lysophospholipids cause extracellular K(+) accumulation and action potential shortening with increased risk of arrhythmias during myocardial ischemia. Here we studied effects of several lysophospholipids with different lengths of hydrocarbon chains and charged headgroups on HERG K(+) currents (I(HERG)) expressed in HEK293 cells and the potential mechanisms using whole-cell patch-clamp techniques. 2. Only the lipids with 16 hydrocarbons such as 1-palmitoyl-lysophosphatidylcholine (LPC-16) and 1-palmitoyl-lysophosphatidylglycerol (LPG-16) were found to produce significant enhancement of I(HERG) and negative shifts of HERG activation, although the voltage dependence of the effects was different between LPC-16 and LPG-16 which have differently charged headgroups. The lipid with 18 hydrocarbons modestly increased I(HERG). The lipids with 6 or 24 hydrocarbons had no effect or slightly decreased I(HERG). 3. Inhibition or activation of protein kinase C did not alter the effects of LPC-16 and LPG-16. Participation of phosphatidylinositol-4,5-bisphosphate in I(HERG) enhancement by LPC-16/LPG-16 was also excluded. 4. Vitamin E augmented the effects of LPC-16/LPG-16 whereas xanthine/xanthine oxidase reduced I(HERG): indicating that LPC-16/LPG-16 produced dual effects on I(HERG): direct enhancement of I(HERG) and indirect suppression via production of superoxide anion. 5. We conclude that enhancement of HERG function by lysophospholipids is specific to the lipids with 16-hydrocarbon chain structure and the pattern of voltage dependence is determined by the polar headgroups. The increase in I(HERG) is best described by direct interactions between lipid molecules and HERG proteins, which is consistent with lack of effects via membrane destabilization or modulation by intracellular signaling pathways.

N-3 polyunsaturated fatty acid supplementation alters inositol phosphate metabolism and protein kinase C activity in adult porcine cardiac myocytes

Several mechanisms have been proposed to explain the anti-arrhythmic effects of n-3 polyunsaturated fatty acids. One mechanism is the effect of modifying cell membrane phospholipid and their subsequent effect on intracellular cell signaling via the second messengers, Ins(1,4,5)P(3) and diacylglycerol. Isolated cardiac myocytes from adult pig hearts were used to investigate the effect of n-3 polyunsaturated fatty acids, eicosapentaenoic acid and docosahexaenoic acid, on the inositol phosphate metabolism and protein kinase C activity. Adult porcine cardiac myocytes were grown in media supplemented with 400 µM arachidonic acid, eicosapentaenoic acid and docosahexaenoic acid. After 24 hr, fatty acid analyses of total lipids by TLC in supplemented cells showed that eicosapentaenoic acid and docosahexaenoic acid were selectively incorporated into the phosphatidylinositol fraction. In the diacylglycerol fraction, there was a small incorporation of both eicosapentaenoic acid and docosahexaenoic acid but it was not significantly different from that of controls. To study the effect of membrane phospholipid modification on the phospholipase C mediated inositol lipid cycle, cardiac myocytes were labeled with 4µCi/ml myo-[2-(3)H]Ins for 48 hr. After stimulation with epinephrine and phenylephrine (alpha-receptor agonist) the water soluble [(3)H]Ins products were separated by chromatography on Dowex AG 1-X8 and measured by scintillation counting. After stimulation, the levels of [(3)H]Ins(1,4,5)P(3) and [(3)H]Ins(1,3,4,5)P(4) in eicosapentaenoic acid and docosahexaenoic acid supplemented myocytes were significantly reduced (P < 0.05) compared to arachidonic acid supplemented myocytes. Similarly, eicosapentaenoic acid and docosahexaenoic acid supplemented cells had reduced levels of protein kinase C activity after stimulation compared to arachidonic acid supplemented cells. From these experiments, it is evident that n-3 PUFA supplementation modulates intracellular cell signaling suggesting a possible anti-arrhythmic mechanism.

Suppression of inositol phosphate release by cardiac myocytes isolated from fish oil-fed pigs

Fatty acid composition of cardiac myocytes and release of inositol phosphates in pigs fed a fish oil supplemented diet was examined. Two groups of female pigs were fed diets supplemented with either 50 g/kg diet beef tallow (as control) or 50 g/kg diet fish oil (MaxEPA) rich in n-3 fatty acids. After 6 weeks of supplementation, the pigs were anesthetized and hearts were removed. Cardiac myocytes were isolated, lipid extracted and separated into non-polar and polar lipids by thin-layer chromatography. Fatty acid composition of individual neutral and polar lipid classes were examined by gas chromatography. To study the effect of membrane phospholipid modification on the phospholipase C (PLC) mediated release of inositol phosphates, cardiac myocytes were labelled with 4 microCi/mL myo-[2-(3)H]inositol for 48 h. After stimulation with epinephrine and phenylephrine, the water soluble [3H]inositol products were extracted, separated from [3H]inositol and [3H]glycerophosphoinositol by chromatography on Dowex AG 1-X8 and quantitated by scintillation counting. Cardiac myocytes isolated from fish oil-fed pigs had higher levels of n-3 polyunsaturated fatty acid in the non-esterified fatty acid and phospholipid fraction. Similarly, these cardiac myocytes had increased level of n-3 fatty and decreased n-6 fatty acids in all the phospholipid fractions, PE, PC, P1 and PS (p < 0.05). After stimulation, the levels of [3H]inositol trisphosphate (IP3) and [3H]inositol tetrakisphosphate (IP4) in cardiac myocytes isolated from fish oil-fed pigs were significantly reduced (p < 0.05) compared to myocytes isolated from beef tallow fed-pigs. This study for the first time has utilised adult cardiac myocytes to demonstrate the effect of n-3 PUFA supplementation on cardiac myocyte phospholipid fatty acid composition and release of second messengers.

Separation and quantitation of phospholipids and lysophospholipids by high-performance liquid chromatography

We describe a comprehensive approach to the separation, quantitation, and characterization of phospholipids and lysophospholipids present in complex biological samples. The central feature is a normal-phase HPLC separation of individual phospholipid and lysophospholipid classes. In this single chromatographic step, phospholipids and lysophospholipids are separated and recovered for quantitation by organic phosphate assay and characterization by acyl-group composition. Recovery of phospholipids and lysophospholipids from HPLC averages 80-90%. Isolated phospholipid and lysophospholipid fractions are available for separation of individual molecular species by second-dimension reverse-phase HPLC and characterization of individual molecular species by mass spectrometry.

Incorporation of fatty acids into phosphatidylcholine is reduced during storage of human erythrocytes: evidence for distinct lysophosphatidylcholine acyltransferases

The incorporation of [1-14C]palmitic or [1-14C]oleic acid into phosphatidylcholine and the effect on blood group antigen expression were examined in human erythrocytes stored at 4 degrees C for 0-3 weeks. Blood drawn into EDTA was obtained by venepuncture from healthy volunteers. A 50% suspension of washed erythrocytes was incubated in buffer containing [1-14C]fatty acid for up to 60 min at 37 degrees C with moderate shaking. Phosphatidylcholine was extracted and analyzed for uptake of radiolabelled fatty acid and phospholipid phosphorus content. Incorporation of [1-14C]palmitic or [1-14C]oleic acid into phosphatidylcholine was reduced during storage. The mechanism for the reduction in radiolabelled fatty acid incorporation into phosphatidylcholine was a 64% (p < 0.05) reduction in membrane phospholipase A2 activity. Although human erythrocyte membranes isolated from freshly drawn blood are capable of reacylating lysophosphatidylcholine to phosphatidylcholine, with storage, a markedly different substrate preference between palmitoyl-Coenzyme A and oleoyl-Coenzyme A was observed. Lysophosphatidylcholine acyltransferase activity assayed with oleoyl-Coenzyme A was unaltered with storage. In contrast, lysophosphatidylcholine acyltransferase activity assayed with palmitoyl-Coenzyme A was elevated 5.5-fold (p < 0.05). Despite these changes, storage of erythrocytes for up to 3 weeks did not result in altered expression of the various blood group antigens investigated. We conclude that the incorporation of palmitate and oleate into phosphatidylcholine is dramatically reduced during storage of human erythrocytes. The observed differential in vitro substrate utilization suggests that distinct acyltransferases are involved in the acylation of lysophosphatidylcholine to phosphatidylcholine in human erythrocytes.

HNE-derived 2-pentylpyrroles are generated during oxidation of LDL, are more prevalent in blood plasma from patients with renal disease or atherosclerosis, and are present in atherosclerotic plaques

Free radical oxidation of human plasma low-density lipoprotein (LDL) produces 2-pentylpyrrole epitopes that are generated by reaction of 4-hydroxy-2-nonenal (HNE), a product of lipid oxidation, with protein lysyl residues. The HNE-derived 2-pentylpyrrole ("HNE-pyrrole") epitopes were detected with an enzyme-linked immunosorbent assay (ELISA) using antibodies (ON-KLH) raised against protein-bound 2-pentylpyrrole obtained by the reaction of 2-oxononanal (ON) with keyhole limpet hemocyanin (KLH). HNE-pyrrole epitopes in human plasma are not associated primarily with LDL protein, apolipoprotein (apo) B, since only 15% of the total HNE-pyrrole immunoreactivity is removed by immunoprecipitation of apo B. The levels of ON-KLH immunoreactivity detected in human plasma were found to be significantly elevated in renal failure and atherosclerosis patients when compared to those in healthy volunteers. HNE-pyrrole immunoreactivity was also detected in atherosclerotic plaques. The highest levels were associated with extracellular connective tissue. Levels of ON-KLH immunoreactivity in human plasma far exceed levels of free HNE, presumably because of the rapid clearance of free relative to protein-bound HNE. Therefore, HNE-pyrrole epitopes provide a more indelible marker of oxidative injury than levels of free HNE.

Lysophosphatidylethanolamine acyltransferase activity is elevated during cardiac cell differentiation

We examined if elevation in lysophosphatidylethanolamine acyltransferase activity was associated with elevation in phosphatidylethanolamine content during differentiation of P19 teratocarcinoma cells into cardiac myocytes. P19 cells were induced to undergo differentiation into cardiac myocytes by the addition of 1% dimethylsulfoxide to the medium. Immunofluorescence microscopy revealed the presence of striated myosin at 8 days post-dimethylsulfoxide addition confirming differentiation into cardiac cells. The content of phosphatidylethanolamine was increased 2.1-fold (P<0.05) in differentiated cells compared to undifferentiated cells, whereas the content of phosphatidylcholine was reduced 29% (P<0.05). There were no alterations in the pool sizes of other phospholipids, including cardiolipin. The relative abundance of fatty acids in phospholipids of P19 cells was 18:1 > 18:0 > 16:1 = 18:2 > 16:0 = 14:0 > 20:4 and differentiation did not affect the relative amounts of these fatty acids within individual phospholipids. When cells were incubated with [1,3-(3)H]glycerol, radioactivity incorporated into phosphatidylethanolamine was elevated 5.8-fold, whereas radioactivity incorporated into phosphatidylcholine was unaltered. Ethanolaminephosphotransferase, cholinephosphotransferase and membrane CTP:phosphocholine cytidylyltransferase activities were elevated in differentiated cells compared to undifferentiated cells, whereas membrane and cytosolic phospholipase A2 activities were unaltered. Lysophosphatidylethanolamine acyltransferase activities were elevated 2.4-fold (P<0.05). Lysophosphatidylcholine acyltransferase, monolysocardiolipin acyltransferase, acyl-Coenzyme A synthetase and acyl-Coenzyme A hydrolase activities were unaltered in differentiated cells compared to undifferentiated cells. We postulate that during cardiac cell differentiation, the observed elevation in lysophosphatidylethanolamine acyltransferase activity accompanies the elevation in phosphatidylethanolamine mass, possibly to maintain the fatty acyl composition of this phospholipid within the membrane.

Phosphatidylcholine metabolism in human endothelial cells: modulation by phosphocholine

Phosphatidylcholine is the principal phospholipid in mammalian tissues, and a major source for the production of arachidonic acid. In this study, the effect of exogenous phosphocholine, a precursor of phosphatidylcholine biosynthesis, on the metabolism of phosphatidylcholine in human umbilical vein endothelial cells was investigated. Incubation of endothelial cells with exogenous phosphocholine at concentrations of 1 to 5 mM was found to inhibit choline uptake and its subsequent incorporation into phosphatidylcholine. Phosphocholine appeared to inhibit choline uptake in a competitive manner. Since phosphatidylcholine is metabolized mainly by the action of phospholipase A2, with the release of arachidonic acid and other fatty acids, the effect of phosphocholine on arachidonic acid release in endothelial cells was also examined. The induction of arachidonic acid release by ATP was enhanced in cells treated with 1 mM phosphocholine. In vitro assays of phospholipase A2 activity in cells incubated with phosphocholine, however, did not produced any significant change in the activity of this enzyme. The results of this study show that phosphocholine modulates the biosynthesis and catabolism of phosphatidylcholine in an indirect manner.

Specific modifications of phosphatidylinositol and nonesterified fatty acid fractions in cultured porcine cardiomyocytes supplemented with n-3 polyunsaturated fatty acids

Mechanisms for the antiarrhythmic effect of n-3 polyunsaturated fatty acids (PUFA) are currently being investigated using isolated cardiac myocytes. It is still not known whether the incorporation of n-3 PUFA into membrane phospholipids is a prerequisite for its protective action or if n-3 PUFA exert antiarrhythmic effects in their nonesterified form as demonstrated by recent studies. Adult porcine cardiomyocytes were grown in media supplemented with arachidonic acid (AA), eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA). After 24 h, analysis of total lipids showed that the myocytes were enriched with the respective fatty acids compared to control cells. Large proportions of all three fatty acids supplemented (69% AA, 72% DHA, and 66% EPA) remained unesterified. Fatty acid analyses of total phospholipids (PL) revealed that the incorporation of EPA and DHA, though small, was significantly different (P<0.05) from that of the control cells. The PL fraction was further separated into phosphatidylinositol (PI), phosphatidylethanolamine, phosphatidylcholine, and phosphatidylserine to study the pattern of incorporation of the fatty acids in these fractions. It became apparent that EPA and DHA were selectively incorporated into the PI fraction. This study demonstrates that in adult porcine cardiomyocytes, the n-3 PUFA supplementation selectively modulates two important lipid fractions, nonesterified fatty acid and PI, which were implicated in the mechanisms of prevention of cardiac arrhythmias.