Isolation of very low density lipoprotein phospholipids enriched in ethanolamine phospholipids from rats injected with Triton WR 1339

The effects of exposure to and timing of a choline-deficient diet during pregnancy and early postnatal life on the skeletal muscle transcriptome of the offspring

BACKGROUND & AIMS

Nonalcoholic fatty liver disease (NAFLD) is related to muscle loss, but the precise mechanism underlying this association remains unclear. The aim of the present study was thus to determine the influence of maternal fatty liver and dietary choline deficiency during pregnancy and/or lactation periods on the skeletal muscle gene expression profile among 24-day-old male rat offspring.

METHODS

Histological examination of skeletal muscle tissue specimens obtained from offspring of dams suffering from fatty liver, provided with proper choline intake during pregnancy and lactation (NN), fed a choline-deficient diet during both periods (DD), deprived of choline only during pregnancy (DN), or only during lactation (ND), was performed. The global transcriptome pattern was assessed using a microarray approach (Affymetrix® Rat Gene 2.1 ST Array Strip). The relative expression of selected genes was validated by real-time PCR (qPCR).

RESULTS

Morphological differences in fat accumulation in skeletal muscle related to choline supply were observed. The global gene expression profile was consistent with abnormal morphological changes. Mettl21c gene was overexpressed in all choline-deficient groups compared to the NN group, while two genes, Cdkn1a and S100a4, were downregulated. Processes of protein biosynthesis were upregulated, and processes related to cell proliferation and lipid metabolism were inhibited in DD, DN, and ND groups compared to the NN group.

CONCLUSIONS

Prenatal and early postnatal exposure to fatty liver and dietary choline deficiency leads to changes in the transcriptome profile in skeletal muscle of 24-day old male rat offspring and is associated with muscle damage, but the mechanism of it seems to be different at different developmental stages of life. Adequate choline intake during pregnancy and lactation can prevent severe muscle disturbance in the progeny of females suffering from fatty liver.

A Physiologically Relevant Dose of 50% Egg-Phosphatidylcholine Is Sufficient in Improving Gut Permeability while Attenuating Immune Cell Dysfunction Induced by a High-Fat Diet in Male Wistar Rats

BACKGROUND

Obesity is associated with increased intestinal permeability and a diminished immune response. Phosphatidylcholine (PC), a form of choline found in eggs, has been shown to beneficially modulate T-cell response in the context of obesity when provided as the sole form of choline in the diet.

OBJECTIVE

This study aimed to determine the impact of varying doses of PC as part of a high-fat diet (HFD) on immune cell function and intestinal permeability.

METHODS

Male Wistar rats 4 wk of age were randomly assigned to consume 1 of 6 diets for 12 wk containing the same amount of total choline but differing in the forms of choline: 1-control low-fat (CLF, 20% fat, 100% free choline [FC]); 2-control high-fat (CHF, 50% fat, 100% FC); 3-100% PC (100PC, 50% fat, 100% egg-PC); 4-75% PC (75PC, 50% fat, 75% egg-PC+25% FC); 5-50% PC (50PC, 50% fat, 50% egg-PC+50% FC); and 6-25% PC (25PC; 50% fat, 25% egg-PC+75% FC). Intestinal permeability was measured by fluorescein isothiocyanate-dextran. Immune function was assessed by ex vivo cytokine production of splenocytes and cells isolated from the mesenteric lymph node (MLN) after stimulation with different mitogens.

RESULTS

Feeding the CHF diet increased intestinal permeability compared with the CLF diet, and doses of PC 50% or greater returned permeability to levels similar to that of the CLF diet. Feeding the CHF diet lowered splenocyte production of interleukin (IL)-1β, IL-2, IL-10, and tumor necrosis factor-alpha, and MLN production of IL-2 compared with the CLF group. The 50PC diet most consistently significantly improved cytokine levels (IL-2, IL-10, tumor necrosis factor-alpha) compared with the CHF diet.

CONCLUSIONS

Our results show that a dose of 50% of total choline derived from egg-PC can ameliorate HFD-induced intestinal permeability and immune cell dysfunction.

Protein-metabolite association studies identify novel proteomic determinants of metabolite levels in human plasma

Although many novel gene-metabolite and gene-protein associations have been identified using high-throughput biochemical profiling, systematic studies that leverage human genetics to illuminate causal relationships between circulating proteins and metabolites are lacking. Here, we performed protein-metabolite association studies in 3,626 plasma samples from three human cohorts. We detected 171,800 significant protein-metabolite pairwise correlations between 1,265 proteins and 365 metabolites, including established relationships in metabolic and signaling pathways such as the protein thyroxine-binding globulin and the metabolite thyroxine, as well as thousands of new findings. In Mendelian randomization (MR) analyses, we identified putative causal protein-to-metabolite associations. We experimentally validated top MR associations in proof-of-concept plasma metabolomics studies in three murine knockout strains of key protein regulators. These analyses identified previously unrecognized associations between bioactive proteins and metabolites in human plasma. We provide publicly available data to be leveraged for studies in human metabolism and disease.

Plasma Metabolite Profiles Following Consumption of Animal Protein and Soybean-Based Diet in Hypercholesterolemic Postmenopausal Women

Subjective reporting of food intake can be unreliable. No objective method is available to distinguish between diets differing in protein type. To address this gap, a secondary analysis of a randomized controlled cross-over feeding trial was conducted. Assessed were fasting plasma metabolite profiles and their associations with cardiometabolic risk factors (CMRFs). Hypercholesterolemic post-menopausal women (N = 11) were provided with diets containing predominantly animal protein (AP) and soy protein (SP). Untargeted metabolomics were used to determine the plasma metabolite profiles at the end of each diet phase. Concentrations of identified metabolites (N = 829) were compared using paired t-tests adjusted for false discovery rate, partial least square-discrimination analysis (PLS-DA) and receiver operating characteristics (ROC). Among the identified metabolites, 58 differed significantly between the AP and SP diets; the majority were phospholipids (n = 36), then amino acids (n = 10), xenobiotics (n = 7), vitamin/vitamin-related (n = 3) and lipids (n = 2). Of the top 10 metabolites, amino acid-derived metabolites, phospholipids and xenobiotics comprised the main categories differing due to dietary protein type. ROC curves confirmed that the top 10 metabolites were potential discriminating biomarkers for AP- and SP-rich diets. In conclusion, amino acid-derived metabolites, phosphatidylethanolamine-derived metabolites and isoflavones were identified as potential metabolite biomarkers distinguishing between dietary protein type.

The Complex Tail of Circulating Sphingolipids in Atherosclerosis and Cardiovascular Disease

Sphingolipids (SLs) are critical players in a number of cellular processes and have recently been implicated in a large number of human diseases, including atherosclerosis and cardiovascular disease (CVD). SLs are generated intracellularly in a stepwise manner, starting with the generation of the sphingoid long chain base (LCB), followed by N-acylation of the LCB to form ceramide, which can be subsequently metabolized to sphingomyelin and glycosphingolipids. Fatty acids, which are taken up by cells prior to their activation to fatty acyl-CoAs, are used in 2 of these enzymatic steps, including by ceramide synthases, which use fatty acyl-CoAs of different chain lengths to generate ceramides with different N-acyl chain lengths. Recently, alterations in plasma ceramides with specific N-acyl chain lengths and degrees of saturation have emerged as novel biomarkers for the prediction of atherosclerosis and overall cardiovascular risk in the general population. We briefly review the sources of plasma SLs in atherosclerosis, the roles of SLs in CVD, and the possible use of the "ceramide score" as a prognostic marker for CVD.

Perspectives on Mitochondria-ER and Mitochondria-Lipid Droplet Contact in Hepatocytes and Hepatic Lipid Metabolism

Emerging evidence suggests that mitochondrion–endoplasmic reticulum (ER) and mitochondrion–lipid droplet (LD) contact sites are critical in regulating lipid metabolism in cells. It is well established that intracellular organelles communicate with each other continuously through membrane contact sites to maintain organelle function and cellular homeostasis. The accumulation of LDs in hepatocytes is an early indicator of non-alcoholic fatty liver disease (NAFLD) and alcohol-related liver disease (ALD), which may indicate a breakdown in proper inter-organelle communication. In this review, we discuss previous findings in mitochondrion–ER and mitochondrion–LD contact, focusing on their roles in lipid metabolism in hepatocytes. We also present evidence of a unique mitochondrion–LD contact structure in hepatocytes under various physiological and pathological conditions and propose a working hypothesis to speculate about the role of these structures in regulating the functions of mitochondria and LDs and their implications in NAFLD and ALD.

Comparative Lipidomics of Chick Yolk Sac during the Embryogenesis Provides Insight into Understanding the Development-Related Lipid Supply

Yolk sac (YS, include the yolk content) at different chick embryogenesis stages possesses varying lipid distributions, which are nutrition-influencing factors for the health of an early embryo and a later adult. YS lipids can substantially influence embryogenesis metabolism, but a comprehensive understanding of lipid's influence remains unknown. Herein, the effects of embryogenesis on lipid profiling of chick YS were investigated by UHPLC-MS/MS-based lipidomics. A total of 2231 lipid species across 57 subclasses were identified in the YS, and 1011 lipids were significantly different (P < 0.05) at the incubation days of 0, 7, 13, and 18. Specifically, phosphocholine and phosphatidylglycerol in late-stage embryogenesis potentially assist with prehatching gas exchange and infection resistance in the environment after lung respiration. In addition, the accumulated lysophosphatidylcholine at day 18 may induce apoptosis and disturb the membrane structure of YS to enable better absorption by the embryo abdomen. The decreased cardiolipin in late embryogenesis may be due to transportation to the embryo and integration into the mitochondrial membrane to accelerate energy metabolism for the rapidly developing embryo after day 13. Therefore, this study demonstrated the lipid profile alteration of the developing YS, providing theoretical guidance for researching the developmental origins of health and disease.

Characterization of the Plasma Lipidome in Dairy Cattle Transitioning from Gestation to Lactation: Identifying Novel Biomarkers of Metabolic Impairment

The discovery of novel biomarkers for peripartal diseases in dairy cows can improve our understanding of normal and dysfunctional metabolism, and lead to nutritional interventions that improve health and milk production. Our objectives were to characterize the plasma lipidome and identify metabolites associated with common markers of metabolic disease in peripartal dairy cattle. Multiparous Holstein cows (n = 27) were enrolled 30 d prior to expected parturition. Blood and liver samples were routinely collected through to d 14 postpartum. Untargeted lipidomics was performed using quadrupole time-of-flight mass spectrometry. Based on postpartum measures, cows were categorized into low or high total fatty acid area under the curve (total FAAUC; d 1-14 postpartum; 4915 ± 1369 vs. 12,501 ± 2761 (μmol/L × 14 d); n = 18), β-hydroxybutyrate AUC (BHBAAUC; d 1-14 postpartum; 4583 ± 459 vs. 7901 ± 1206 (μmol/L × 14 d); n = 18), or liver lipid content (d 5 and 14 postpartum; 5 ± 1 vs. 12 ± 2% of wet weight; n = 18). Cows displayed decreases in plasma triacylglycerols and monoalkyl-diacylglycerols, and the majority of phospholipids reached a nadir at parturition. Phosphatidylcholines (PC) 32:3, 35:5, and 37:5 were specific for high total FAAUC, PC 31:3, 32:3, 35:5, and 37:5 were specific for high BHBAAUC, and PC 31:2, 31:3, and 32:3 were specific for high liver lipid content. PC 32:3 was specific for elevated total FA, BHBA, and liver lipid content. Lipidomics revealed a dynamic peripartal lipidome remodeling, and lipid markers associated with elevated total FA, BHBA, and liver lipid content. The effectiveness of nutrition to impact these lipid biomarkers for preventing excess lipolysis and fatty liver warrants evaluation.

Folic acid supplementation prevents high fructose-induced non-alcoholic fatty liver disease by activating the AMPK and LKB1 signaling pathways

BACKGROUND/OBJECTIVES

The present study aimed to evaluate the effects of folic acid supplementation in high-fructose-induced hepatic steatosis and clarify the underlying mechanism of folic acid supplementation.

MATERIALS/METHODS

Male SD rats were fed control, 64% high-fructose diet, or 64% high-fructose diet with folic acid for eight weeks. Plasma glutamate-pyruvate transaminase, glutamate-oxaloacetate transaminase, lipid profiles, hepatic lipid content, S-adenosylmethionine (SAM), and S-adenosylhomocysteine (SAH) were measured.

RESULTS

The HF diet significantly increased hepatic total lipid and triglyceride (TG) and decreased hepatic SAM, SAH, and SAM:SAH ratio. In rats fed a high fructose diet, folic acid supplementation significantly reduced hepatic TG, increased hepatic SAM, and alleviated hepatic steatosis. Moreover, folic acid supplementation in rats fed high fructose enhanced the levels of phosphorylated AMP-activated protein kinase (AMPK) and liver kinase B (LKB1) and inhibited phosphorylation of acetyl coenzyme A carboxylase (ACC) in the liver.

CONCLUSIONS

These results suggest that the protective effect of folic acid supplementation in rats fed high fructose may include the activation of LKB1/AMPK/ACC and increased SAM in the liver, which inhibit hepatic lipogenesis, thus ameliorating hepatic steatosis. The present study may provide evidence for the beneficial effects of folic acid supplementation in the treatment of non-alcoholic fatty liver disease.

The Disruption of Liver Metabolic Circadian Rhythms by a Cafeteria Diet Is Sex-Dependent in Fischer 344 Rats

Circadian rhythms are ~24 h fluctuations of different biological processes that are regulated by the circadian clock system. They exert a major influence on most of the metabolism, such as the hepatic metabolism. This rhythmicity can be disrupted by obesogenic diets, fact that is considered to be a risk factor for the development of metabolic diseases. Nevertheless, obesogenic diets do not affect both genders in the same manner. We hypothesized that the circadian rhythms disruption of the hepatic metabolism, caused by obesogenic diets, is gender-dependent. Male and female Fischer 344 rats were fed either a standard diet or a cafeteria diet and sacrificed at two different moments, at zeitgeber 3 and 15. Only female rats maintained the circadian variations of the hepatic metabolism under a cafeteria diet. Most of those metabolites were related with the very low density lipoprotein (VLDL) synthesis, such as choline, betaine or phosphatidylcholine. Most of these metabolites were found to be increased at the beginning of the dark period. On the other hand, male animals did not show these time differences. These findings suggest that females might be more protected against the circadian disruption of the hepatic metabolism caused by a cafeteria diet through the increase of the VLDL synthesis at the beginning of the feeding time.

Obesity and Cage Environment Modulate Metabolism in the Zucker Rat: A Multiple Biological Matrix Approach to Characterizing Metabolic Phenomena

Obesity and its comorbidities are increasing worldwide imposing a heavy socioeconomic burden. The effects of obesity on the metabolic profiles of tissues (liver, kidney, pancreas), urine, and the systemic circulation were investigated in the Zucker rat model using ¹H NMR spectroscopy coupled to multivariate statistical analysis. The metabolic profiles of the obese ( fa/ fa) animals were clearly differentiated from the two phenotypically lean phenotypes, ((+/+) and ( fa/+)) within each biological compartment studied, and across all matrices combined. No significant differences were observed between the metabolic profiles of the genotypically distinct lean strains. Obese Zucker rats were characterized by higher relative concentrations of blood lipid species, cross-compartmental amino acids (particularly BCAAs), urinary and liver metabolites relating to the TCA cycle and glucose metabolism; and lower amounts of urinary gut microbial-host cometabolites, and intermatrix metabolites associated with creatine metabolism. Further to this, the obese Zucker rat metabotype was defined by significant metabolic alterations relating to disruptions in the metabolism of choline across all compartments analyzed. The cage environment was found to have a significant effect on urinary metabolites related to gut-microbial metabolism, with additional cage-microenvironment trends also observed in liver, kidney, and pancreas. This study emphasizes the value in metabotyping multiple biological matrices simultaneously to gain a better understanding of systemic perturbations in metabolism, and also underscores the need for control or evaluation of cage environment when designing and interpreting data from metabonomic studies in animal models.

An Updated Review of Lysophosphatidylcholine Metabolism in Human Diseases

Lysophosphatidylcholine (LPC) is increasingly recognized as a key marker/factor positively associated with cardiovascular and neurodegenerative diseases. However, findings from recent clinical lipidomic studies of LPC have been controversial. A key issue is the complexity of the enzymatic cascade involved in LPC metabolism. Here, we address the coordination of these enzymes and the derangement that may disrupt LPC homeostasis, leading to metabolic disorders. LPC is mainly derived from the turnover of phosphatidylcholine (PC) in the circulation by phospholipase A₂ (PLA₂). In the presence of Acyl-CoA, lysophosphatidylcholine acyltransferase (LPCAT) converts LPC to PC, which rapidly gets recycled by the Lands cycle. However, overexpression or enhanced activity of PLA₂ increases the LPC content in modified low-density lipoprotein (LDL) and oxidized LDL, which play significant roles in the development of atherosclerotic plaques and endothelial dysfunction. The intracellular enzyme LPCAT cannot directly remove LPC from circulation. Hydrolysis of LPC by autotaxin, an enzyme with lysophospholipase D activity, generates lysophosphatidic acid, which is highly associated with cancers. Although enzymes with lysophospholipase A₁ activity could theoretically degrade LPC into harmless metabolites, they have not been found in the circulation. In conclusion, understanding enzyme kinetics and LPC metabolism may help identify novel therapeutic targets in LPC-associated diseases.

Loss of miR-141/200c ameliorates hepatic steatosis and inflammation by reprogramming multiple signaling pathways in NASH

Accumulation of lipid droplets and inflammatory cell infiltration is the hallmark of nonalcoholic steatohepatitis (NASH). The roles of noncoding RNAs in NASH are less known. We aim to elucidate the function of miR-141/200c in diet-induced NASH. WT and miR-141/200c-/- mice were fed a methionine and choline deficient (MCD) diet for 2 weeks to assess markers of steatosis, liver injury, and inflammation. Hepatic miR-141 and miR-200c RNA levels were highly induced in human patients with NASH fatty liver and in WT MCD mice. miR-141/200c-/- MCD mice had reduced liver weights and triglyceride (TG) levels, which was associated with increased microsomal TG transfer protein (MTTP) and PPARα but reduced SREBP1c and FAS expression. Inflammation was attenuated and F4/80 macrophage activation was suppressed in miR-141/200c-/- mice, as evidenced by decreased serum aminotransferases and IL-6 and reduced hepatic proinflammatory, neutrophil, and profibrotic genes. Treatment with LPS in BM-derived macrophages isolated from miR-200c/141-/- mice polarized macrophages toward the M2 antiinflammatory state by increasing Arg1 and IL-10 levels while decreasing the M1 marker iNOS. In addition, elevated phosphorylated AMPK (p-AMPK), p-AKT, and p-GSK3β and diminished TLR4 and p-mTOR/p-4EBP1 proteins were observed. Lipidomics and metabolomics revealed alterations of TG and phosphatidylcholine (PC) lipid species by miR-141/200c deficiency. In summary, miR-141/200c deficiency diminished NASH-associated hepatic steatosis and inflammation by reprogramming lipid and inflammation signaling pathways.

The critical role of phosphatidylcholine and phosphatidylethanolamine metabolism in health and disease

Phosphatidylcholine (PC) and phosphatidylethanolamine (PE) are the most abundant phospholipids in all mammalian cell membranes. In the 1950s, Eugene Kennedy and co-workers performed groundbreaking research that established the general outline of many of the pathways of phospholipid biosynthesis. In recent years, the importance of phospholipid metabolism in regulating lipid, lipoprotein and whole-body energy metabolism has been demonstrated in numerous dietary studies and knockout animal models. The purpose of this review is to highlight the unappreciated impact of phospholipid metabolism on health and disease. Abnormally high, and abnormally low, cellular PC/PE molar ratios in various tissues can influence energy metabolism and have been linked to disease progression. For example, inhibition of hepatic PC synthesis impairs very low density lipoprotein secretion and changes in hepatic phospholipid composition have been linked to fatty liver disease and impaired liver regeneration after surgery. The relative abundance of PC and PE regulates the size and dynamics of lipid droplets. In mitochondria, changes in the PC/PE molar ratio affect energy production. We highlight data showing that changes in the PC and/or PE content of various tissues are implicated in metabolic disorders such as atherosclerosis, insulin resistance and obesity. This article is part of a Special Issue entitled: Membrane Lipid Therapy: Drugs Targeting Biomembranes edited by Pablo V. Escribá.

The Long Term Kinetic of Plasma Lipids and Lipoproteins in Tyloxapol Injected Rats

INTRODUCTION

The level of plasma triglyceride is balanced by the rate of secretion into and clearance from the plasma. Tyloxapol (Triton WR1339) is a nonionic detergent that inhibits lipoprotein lipase and hence clearance of triglyceride from the plasma.

AIM

To determine the kinetic of plasma lipids and lipoproteins following injection of tyloxapol over a period of two weeks.

MATERIALS AND METHODS

Fifteen male rats were starved over-night and injected intravenously with tyloxapol (400mg/kg). Blood samples were taken in three steps as, the early (1-6 hours), the middle (1-2 days) and the third (3-9 days) phase. Plasma total cholesterol and triglyceride were measured by enzymatic methods and total phospholipids were analysed as molybdenum blue. Serum lipoproteins were fractionated by electrophoresis on agarose gel (Sebia Inc).

RESULTS

The changes of plasma lipids following tyloxapol injection showed three distinctive phases. The early phase lasts at least 6 hours, and the concentrations of triglyceride, total cholesterol and phospholipids increased linearly. The rate of triglyceride secretion was 259.7 ± 8.1 mg/h.dl in this phase, which was comparable to the mean rate of 250.6 ± 37.0 mg/h.dl or 102.8 ± 15.2 mg/h.kg body in starved male rat. During the next 48 hour the lipids continued to accumulate but at a lower rate, and the levels of triglyceride, cholesterol and total phospholipids rose up to about 3200, 586 and 715 mg/dl respectively. In the last phase, the levels of plasma lipids decreased toward the basal levels after 5 days. In serum lipoprotein electrophoresis, the VLDL and LDL increased and HDL fraction disappeared simultaneously during the initial 2 hours of tyloxapol injection. The VLDL fell down toward the normal range, preceded to the reappearance of HDL during 5 days.

CONCLUSION

A single intravenous injection of tyloxapol shows three distinctive phases. In the early phase, triglyceride accumulates linearly and the rate of its increment in plasma is a good estimate of the rate of VLDL secretion from the liver.

Lpcat3-dependent production of arachidonoyl phospholipids is a key determinant of triglyceride secretion

The role of specific phospholipids (PLs) in lipid transport has been difficult to assess due to an inability to selectively manipulate membrane composition in vivo. Here we show that the phospholipid remodeling enzyme lysophosphatidylcholine acyltransferase 3 (Lpcat3) is a critical determinant of triglyceride (TG) secretion due to its unique ability to catalyze the incorporation of arachidonate into membranes. Mice lacking Lpcat3 in the intestine fail to thrive during weaning and exhibit enterocyte lipid accumulation and reduced plasma TGs. Mice lacking Lpcat3 in the liver show reduced plasma TGs, hepatosteatosis, and secrete lipid-poor very low-density lipoprotein (VLDL) lacking arachidonoyl PLs. Mechanistic studies indicate that Lpcat3 activity impacts membrane lipid mobility in living cells, suggesting a biophysical basis for the requirement of arachidonoyl PLs in lipidating lipoprotein particles. These data identify Lpcat3 as a key factor in lipoprotein production and illustrate how manipulation of membrane composition can be used as a regulatory mechanism to control metabolic pathways.

Aging-related Changes in Mouse Serum Glycerophospholipid Profiles

Objectives

Metabolic dysfunction is a common hallmark of the aging process and aging-related pathogenesis. Blood metabolites have been used as biomarkers for many diseases, including cancers, complex chronic diseases, and neurodegenerative diseases.

Methods

In order to identify aging-related biomarkers from blood metabolites, we investigated the specific metabolite profiles of mouse sera from 4-month-old and 21-month-old mice by using a combined flow injection analysis–tandem mass spectrometry and liquid chromatography–tandem mass spectrometry.

Results

Among the 156 metabolites detected, serum levels of nine individual metabolites were found to vary with aging. Specifically, lysophosphatidylcholine (LPC) acyl (a) C24:0 levels in aged mice were decreased compared to that in young mice, whereas phosphatidylcholine (PC) acyl-alkyl (ae) C38:4, PC ae C40:4, and PC ae C42:1 levels were increased. Three classes of metabolites (amino acids, LPCs, and PCs) differed in intraclass correlation patterns of the individual metabolites between sera from young and aged mice. Additionally, the ratio of LPC a C24:0 to PC ae C38:4 was decreased in the aged mice, whereas the ratio of PC ae C40:4 to LPC a C24:0 was increased, supporting the aging-related metabolic changes of glycerophospholipids.

Conclusion

The ratios of the individual metabolites PC and LPC could serve as potential biomarkers for aging and aging-related diseases.

Antihypercholesterolemic and Antioxidative Potential of an Extract of the Plant, Piper betle, and Its Active Constituent, Eugenol, in Triton WR-1339-Induced Hypercholesterolemia in Experimental Rats

Hypercholesterolemia is a dominant risk factor for atherosclerosis and cardiovascular diseases. In the present study, the putative antihypercholesterolemic and antioxidative properties of an ethanolic extract of Piper betle and of its active constituent, eugenol, were evaluated in experimental hypercholesterolemia induced by a single intraperitoneal injection of Triton WR-1339 (300 mg/kg b.wt) in Wistar rats. Saline-treated hypercholesterolemic rats revealed significantly higher mean blood/serum levels of glucose, total cholesterol, triglycerides, low density and very low density lipoprotein cholesterol, and of serum hepatic marker enzymes; in addition, significantly lower mean serum levels of high density lipoprotein cholesterol and significantly lower mean activities of enzymatic antioxidants and nonenzymatic antioxidants were noted in hepatic tissue samples from saline-treated hypercholesterolemic rats, compared to controls. However, in hypercholesterolemic rats receiving the Piper betle extract (500 mg/kg b.wt) or eugenol (5 mg/kg b.wt) for seven days orally, all these parameters were significantly better than those in saline-treated hypercholesterolemic rats. The hypercholesterolemia-ameliorating effect was better defined in eugenol-treated than in Piper betle extract-treated rats, being as effective as that of the standard lipid-lowering drug, lovastatin (10 mg/kg b.wt). These results suggest that eugenol, an active constituent of the Piper betle extract, possesses antihypercholesterolemic and other activities in experimental hypercholesterolemic Wistar rats.

Characterization of in vitro modified human very low-density lipoprotein particles and phospholipids by capillary electrophoresis

A simple capillary zone electrophoresis (CZE) method was used to characterize human very low-density lipoprotein (VLDL) particles for four healthy donors. One major peak was observed for native, in vitro oxidized and glycated VLDL particles. The effective mobilities and peak areas of the capillary electrophoresis (CE) profiles showed good reproducibility and precision. The mobility of the oxidized VLDL peak was higher than that of the native VLDL. The mobility of the glycated VLDL peak was similar to that of the native VLDL. Phospholipids isolated from VLDL particles were analyzed by our recently developed micellar electrokinetic chromatography (MEKC) with a high-salt stacking method. At absorbance 200 nm, the native VLDL phospholipids showed a major peak and a minor peak for each donor. For oxidized VLDL phospholipids, the area of the major peak reduced for three donors, possibly due to phospholipid decomposition. For glycated VLDL phospholipids, the peak mobilities were more positive than native VLDL phospholipids for two donors, possibly due to phospholipid-linked advanced glycation end products (AGEs). Very interestingly, at absorbance 234 nm, the major peak of oxidized VLDL phospholipids was resolved as two peaks for each donor, possibly due to conjugated dienes formed upon oxidation.

Lipidomic analysis of the liver identifies changes of major and minor lipid species in adiponectin deficient mice

Adiponectin protects from hepatic fat storage but adiponectin deficient mice (APN-/-) fed a standard chow do not develop liver steatosis. This indicates that other pathways might be activated to compensate for adiponectin deficiency. An unbiased and comprehensive screen was performed to identify hepatic alterations of lipid classes in these mice. APN-/- mice had decreased hepatic cholesteryl esters while active SREBP2 and systemic total cholesterol were not altered. Upregulation of cytochromes for bile acid synthesis suggests enhanced biliary cholesterol excretion. Analysis of 37 individual fatty acid species showed reduced stearate whereas total fatty acids were not altered. Total amount of triglycerides and phospholipids were equally abundant. A selective increase of monounsaturated phosphatidylcholine and phosphatidylethanolamine which positively correlate with hepatic and systemic triglycerides with the latter being elevated in APN-/- mice, was identified. Stearoyl-CoA desaturase 1 (SCD1) is involved in the synthesis of monounsaturated fatty acids and despite higher mRNA expression enzyme activity was not enhanced. Glucosylceramide postulated to contribute to liver damage was decreased. This study demonstrates that adiponectin deficiency is associated with hepatic changes in lipid classes in mice fed a standard chow which may protect from liver steatosis.

A phospholipidomic analysis of all defined human plasma lipoproteins

Since plasma lipoproteins contain both protein and phospholipid components, either may be involved in processes such as atherosclerosis. In this study the identification of plasma lipoprotein-associated phospholipids, which is essential for understanding these processes at the molecular level, are performed. LC-ESI/MS, LC-ESI-MS/MS and High Performance Thin Layer Chromatography (HPTLC) analysis of different lipoprotein fractions collected from pooled plasma revealed the presence of phosphatidylethanolamine (PE), phosphatidylinositol (PI), and sphingomyeline (SM) only on lipoproteins and phosphatidylcholine (PC), Lyso-PC on both lipoproteins and plasma lipoprotein free fraction (PLFF). Cardiolipin, phosphatidylglycerol (PG) and Phosphatidylserine (PS) were observed neither in the lipoprotein fractions nor in PLFF. All three approaches led to the same results regarding phospholipids occurrence in plasma lipoproteins and PLFF. A high abundancy of PE and SM was observed in VLDL and LDL fractions respectively. This study provides for the first time the knowledge about the phospholipid composition of all defined plasma lipoproteins.

Phosphatidylcholine biosynthesis and lipoprotein metabolism

Phosphatidylcholine (PC) is the major phospholipid component of all plasma lipoprotein classes. PC is the only phospholipid which is currently known to be required for lipoprotein assembly and secretion. Impaired hepatic PC biosynthesis significantly reduces the levels of circulating very low density lipoproteins (VLDLs) and high density lipoproteins (HDLs). The reduction in plasma VLDLs is due in part to impaired hepatic secretion of VLDLs. Less PC within the hepatic secretory pathway results in nascent VLDL particles with reduced levels of PC. These particles are recognized as being defective and are degraded within the secretory system by an incompletely defined process that occurs in a post-endoplasmic reticulum compartment, consistent with degradation directed by the low-density lipoprotein receptor and/or autophagy. Moreover, VLDL particles are taken up more readily from the circulation when the PC content of the VLDLs is reduced, likely due to a preference of cell surface receptors and/or enzymes for lipoproteins that contain less PC. Impaired PC biosynthesis also reduces plasma HDLs by inhibiting hepatic HDL formation and by increasing HDL uptake from the circulation. These effects are mediated by elevated expression of ATP-binding cassette transporter A1 and hepatic scavenger receptor class B type 1, respectively. Hepatic PC availability has recently been linked to the progression of liver and heart disease. These findings demonstrate that hepatic PC biosynthesis can regulate the amount of circulating lipoproteins and suggest that hepatic PC biosynthesis may represent an important pharmaceutical target. This article is part of a Special Issue entitled Triglyceride Metabolism and Disease.

Elimination of the CDP-ethanolamine pathway disrupts hepatic lipid homeostasis

Phosphoethanolamine cytidylyltransferase (ECT) catalyzes the rate-controlling step in a major pathway for the synthesis of phosphatidylethanolamine (PtdEtn). Hepatocyte-specific deletion of the ECT gene in mice resulted in normal appearing animals without overt signs of liver injury or inflammation. The molecular species of PtdEtn in the ECT-deficient livers were significantly altered compared with controls and matched the composition of the phosphatidylserine (PtdSer) pool, illustrating the complete reliance on the PtdSer decarboxylase pathway for PtdEtn synthesis. PtdSer structure was controlled by the substrate specificity of PtdSer synthase that selectively converted phosphatidylcholine molecular species containing stearate paired with a polyunsaturated fatty acid to PtdSer. There was no evidence for fatty acid remodeling of PtdEtn. The elimination of diacylglycerol utilization by the CDP-ethanolamine pathway led to a 10-fold increase in triacylglycerols in the ECT-deficient hepatocytes that became engorged with lipid droplets. Triacylglycerol accumulation was associated with a significant elevation in the expression of the transcription factors and target genes that drive de novo lipogenesis. The absence of the ECT pathway for diacylglycerol utilization at the endoplasmic reticulum triggers increased fatty acid synthesis to support the formation of triacylglycerols leading to liver steatosis.

Lack of phosphatidylethanolamine N-methyltransferase alters plasma VLDL phospholipids and attenuates atherosclerosis in mice

OBJECTIVE

Impaired hepatic phosphatidylcholine (PC) synthesis lowers plasma lipids. We, therefore, tested the hypothesis that lack of phosphatidylethanolamine N-methyltransferase (PEMT), a hepatic enzyme catalyzing PC biosynthesis, attenuates the development of atherosclerosis.

METHODS AND RESULTS

Mice deficient in both PEMT and low-density lipoprotein receptors (Pemt(-/-)/Ldlr(-/-) mice) were fed a high-fat/high-cholesterol diet for 16 weeks. Atherosclerotic lesion area was approximately 80% lower (P<0.01) in Pemt(-/-)/Ldlr(-/-) mice than in Pemt(+/+)/Ldlr(-/-) mice, consistent with the atheroprotective plasma lipoprotein profile (ie, significant reduction in very low-density lipoprotein [VLDL]/intermediate-density lipoprotein/low-density lipoprotein-associated phospholipids [approximately 45%], triacylglycerols [approximately 65%], cholesterol [approximately 58%], and cholesteryl esters [approximately 68%]). Plasma apoB was decreased by 40% to 60%, whereas high-density lipoprotein levels were not altered. In addition, PEMT deficiency reduced plasma homocysteine by 34% to 52% in Pemt(-/-)/Ldlr(-/-) mice. The molar ratio of PC/phosphatidylethanolamine in nascent VLDLs produced by Pemt(-/-)/Ldlr(-/-) mice was lower than in VLDLs in Pemt(+/+)/Ldlr(-/-) mice. Furthermore, deletion of PEMT modestly reduced hepatic VLDL secretion in Ldlr(-/-) mice and altered the rate of VLDL clearance from plasma.

CONCLUSIONS

This is the first report showing that inhibition of hepatic phospholipid biosynthesis attenuates atherosclerosis.

Role of phosphatidylcholine biosynthesis in the regulation of lipoprotein homeostasis

PURPOSE OF REVIEW

This review summarizes the role of phosphatidylcholine metabolism in plasma lipoprotein homeostasis.

RECENT FINDINGS

While it was previously known that phosphatidylcholine biosynthesis was required for normal hepatic VLDL secretion, recent studies have shown that both phosphatidylcholine biosynthetic pathways (the cytidine 5'-diphosphocholine and the phosphatidylethanolamine methylation pathways) are required. In addition, a requirement of acyl-coenzyme A synthetase 3, but not acyl-coenzyme A synthetase 1 or 4, for phosphatidylcholine synthesis and VLDL secretion is now documented. ABCA1 has been implicated in the transfer of phosphatidylcholine to apolipoproteinA-1 both during and after secretion of apolipoproteinA-1. Other studies have introduced the concept of reverse phosphatidylcholine transport in which both HDL and LDL supply phosphatidylcholine to the liver. An unexpected finding is that half of the phosphatidylcholine delivered to liver from lipoproteins is converted into triacylglycerol.

SUMMARY

The liver is both a donor of phosphatidylcholine during the assembly and secretion of lipoproteins as well as a recipient of phosphatidylcholine from plasma lipoproteins.

Phosphatidylserine and phosphatidylethanolamine in mammalian cells: two metabolically related aminophospholipids

Phosphatidylserine (PS) and phosphatidylethanolamine (PE) are two aminophospholipids whose metabolism is interrelated. Both phospholipids are components of mammalian cell membranes and play important roles in biological processes such as apoptosis and cell signaling. PS is synthesized in mammalian cells by base-exchange reactions in which polar head groups of preexisting phospholipids are replaced by serine. PS synthase activity resides primarily on mitochondria-associated membranes and is encoded by two distinct genes. Studies in mice in which each gene has been individually disrupted are beginning to elucidate the importance of these two synthases for biological functions in intact animals. PE is made in mammalian cells by two completely independent major pathways. In one pathway, PS is converted into PE by the mitochondrial enzyme PS decarboxylase. In addition, PE is made via the CDP-ethanolamine pathway, in which the final reaction occurs on the endoplasmic reticulum and nuclear envelope. The relative importance of these two pathways of PE synthesis has been investigated in knockout mice. Elimination of either pathway is embryonically lethal, despite the normal activity of the other pathway. PE can also be generated from a base-exchange reaction and by the acylation of lyso-PE. Cellular levels of PS and PE are tightly regulated by the implementation of multiple compensatory mechanisms.

Hydrolysis of minor glycerophospholipids of plasma lipoproteins by human group IIA, V and X secretory phospholipases A2

We investigated the hydrolysis of the minor glycerophospholipids of human HDL(3), total HDL and LDL using human group IIA, V and X secretory phospholipases A(2) (sPLA(2)s). For this purpose we employed the enzyme and substrate concentrations and incubation times optimized for hydrolysis of phosphatidylcholine (PtdCho), the major glycerophospholipid of plasma lipoproteins. In contrast to PtdCho, which was readily hydrolyzed by group V and X sPLA(2)s, and to a lesser extent by group IIA sPLA(2), the minor ethanolamine, inositol and serine glycerophospholipids exhibited marked resistance to hydrolysis by all three sPLA(2)s. Thus, when PtdCho was hydrolyzed about 80%, the ethanolamine and inositol glycerophospholipids reached a maximum of 40% hydrolysis. The hydrolysis of phosphatidylserine (PtdSer), which was examined to a more limited extent, showed similar resistance to group IIA, V and X sPLA(2)s, although the group V sPLA(2) attacked it more readily than group X sPLA(2) (52% versus 39% hydrolysis, respectively). Surprisingly, the group IIA sPLA(2) hydrolysis remained minimal at 10-15% for all minor glycerophospholipids, and was of the order seen for the PtdCho hydrolysis by group IIA sPLA(2) at the 4-h digestion time. All three enzymes attacked the oligo- and polyenoic species in proportion to their mole percentage in the lipoproteins, although there were exceptions. There was evidence of a more rapid destruction of the palmitoyl compared to the stearoyl arachidonoyl glycerophospholipids. Overall, the characteristics of hydrolysis of the molecular species of the lipoprotein-bound diradyl GroPEtn, GroPIns and GroPSer by group V and X sPLA(2)s differed significantly from those observed with lipoprotein-bound PtdCho. As a result, the acidic inositol and serine glycerophospholipids accumulated in the digestion residues of both LDL and HDL, and presumably increased the acidity of the residual particles. An accumulation of the ethanolamine glycerophospholipids in the sPLA(2) digestion residues also had not been previously reported. These results further emphasize the diversity in the enzymatic activity of the group IIA, V and X sPLA(2)s. Since these sPLA(2)s possess comparable tissue distribution, their combined activity may exacerbate their known proinflammatory and proatherosclerotic function.

Absorption and lipoprotein transport of sphingomyelin

Dietary sphingomyelin (SM) is hydrolyzed by intestinal alkaline sphingomyelinase and neutral ceramidase to sphingosine, which is absorbed and converted to palmitic acid and acylated into chylomicron triglycerides (TGs). SM digestion is slow and is affected by luminal factors such as bile salt, cholesterol, and other lipids. In the gut, SM and its metabolites may influence TG hydrolysis, cholesterol absorption, lipoprotein formation, and mucosal growth. SM accounts for approximately 20% of the phospholipids in human plasma lipoproteins, of which two-thirds are in LDL and VLDL. It is secreted in chylomicrons and VLDL and transferred into HDL via the ABCA1 transporter. Plasma SM increases after periods of large lipid loads, during suckling, and in type II hypercholesterolemia, cholesterol-fed animals, and apolipoprotein E-deficient mice. SM is thus an important amphiphilic component when plasma lipoprotein pools expand in response to large lipid loads or metabolic abnormalities. It inhibits lipoprotein lipase and LCAT as well as the interaction of lipoproteins with receptors and counteracts LDL oxidation. The turnover of plasma SM is greater than can be accounted for by the turnover of LDL and HDL particles. Some SM must be degraded via receptor-mediated catabolism of chylomicron and VLDL remnants and by scavenger receptor class B type I receptor-mediated transfer into cells.