Check out our New Publishers' Select for Free Articles

BBA - Molecular and Cell Biology of Lipids (v.1791, #12)

Editorial Board (pp. i).

Control of free arachidonic acid levels by phospholipases A₂ and lysophospholipid acyltransferases by Gema Pérez-Chacón; Alma M. Astudillo; David Balgoma; María A. Balboa; Jesús Balsinde (pp. 1103-1113).

Arachidonic acid (AA) and its oxygenated derivatives, collectively known as the eicosanoids, are key mediators of a wide variety of physiological and pathophysiological states. AA, obtained from the diet or synthesized from linoleic acid, is rapidly incorporated into cellular phospholipids by the concerted action of arachidonoyl-CoA synthetase and lysophospholipid acyltransferases. Under the appropriate conditions, AA is liberated from its phospholipid storage sites by the action of one or various phospholipase A₂ enzymes. Thus, cellular availability of AA, and hence the amount of eicosanoids produced, depends on an exquisite balance between phospholipid reacylation and hydrolysis reactions. This review focuses on the enzyme families that are involved in these reactions in resting and stimulated cells.

Keywords: Abbreviations; AA; arachidonic acid; ACS; acyl-CoA synthetase; PA; phosphatidic acid; PC; choline glycerophospholipids; PE; ethanolamine glycerophospholipids; PI; phosphatidylinositol; PIP; ₂; phosphatidylinositol 4,5-bisphosphate; PS; phosphatidylserine; PG; phosphatidylglycerol; PLA; ₂; phospholipase A; ₂; cPLA; ₂; α; group IVA cytosolic phospholipase A; ₂; α; iPLA; ₂; calcium-independent phospholipase A; ₂; iPLA; ₂; -VIA; group VIA calcium-independent phospholipase A; ₂; sPLA; ₂; secreted phospholipase A; ₂; LPAAT; lysoPA:acyl-CoA acyltransferase; LPCAT; lysoPC:acyl-CoA acyltransferase; LPEAT; lysoPE:acyl-CoA acyltransferase; LPIAT; lysoPI:acyl-CoA acyltransferase; MBOAT; membrane bound; O; -acyltransferase; AGPAT; acyl-glycerol phosphate acyltransferaseArachidonic acid; Phospholipase A; ₂; Free fatty acid; Eicosanoids; Acyltransferase; Phospholipid remodeling

Characterization of the human tumor suppressors TIG3 and HRASLS2 as phospholipid-metabolizing enzymes by Toru Uyama; Xing-Hua Jin; Kazuhito Tsuboi; Takeharu Tonai; Natsuo Ueda ⁎ (pp. 1114-1124).

Tazarotene-induced protein 3 (TIG3) and HRAS-like suppressor family 2 (HRASLS2) exhibit tumor-suppressing activities and belong to the lecithin retinol acyltransferase (LRAT) protein family. Since Ca²⁺-independent N-acyltransferase and H-rev107 (another tumor suppressor), both of which are members of the LRAT family, have been recently reported to possess catalytic activities related to phospholipid metabolism, we examined possible enzyme activities of human TIG3 and HRASLS2 together with human H-rev107. The purified recombinant proteins of TIG3, HRASLS2, and H-rev107 functioned as phospholipase (PL) A_1/2 in a Ca²⁺-independent manner with maximal activities of 0.53, 0.67, and 2.57 μmol/min/mg of protein, respectively. The proteins were active with various phosphatidylcholines (PCs) and phosphatidylethanolamines (PEs), and for most of substrates the PLA₁ activity was much higher than the PLA₂ activity. In addition, HRASLS2 catalyzed N-acylation of PE to form N-acyl-PE and O-acylation of lyso PC to form PC. TIG3 and H-rev107 catalyzed the N-acylation and O-acylation at relatively low rates. Moreover, these three proteins showed different expression profiles in human tissues. These results suggest that the tumor suppressors TIG3, HRASLS2 and H-rev107 are involved in the phospholipid metabolism with different physiological roles.

Keywords: N; -acylphosphatidylethanolamine; Acyltransferase; LRAT family; Phospholipase A; ₁; Phospholipase A; ₂; Tumor suppressor

Apolipoproteins A-I, A-II and E are independently distributed among intracellular and newly secreted HDL of human hepatoma cells by Baiba K. Gillard; Hu-Yu Alice Lin; John B. Massey; Henry J. Pownall (pp. 1125-1132).

Whereas hepatocytes secrete the major human plasma high density lipoproteins (HDL)-protein, apo A-I, as lipid-free and lipidated species, the biogenic itineraries of apo A-II and apo E are unknown. Human plasma and HepG2 cell-derived apo A-II and apo E occur as monomers, homodimers and heterodimers. Dimerization of apo A-II, which is more lipophilic than apo A-I, is catalyzed by lipid surfaces. Thus, we hypothesized that lipidation of intracellular and secreted apo A-II exceeds that of apo A-I, and once lipidated, apo A-II dimerizes. Fractionation of HepG2 cell lysate and media by size exclusion chromatography showed that intracellular apo A-II and apo E are fully lipidated and occur on nascent HDL and VLDL respectively, while only 45% of intracellular apo A-I is lipidated. Secreted apo A-II and apo E occur on small HDL and on LDL and large HDL respectively. HDL particles containing both apo A-II and apo A-I form only after secretion from both HepG2 and Huh7 hepatoma cells. Apo A-II dimerizes intracellularly while intracellular apo E is monomeric but after secretion associates with HDL and subsequently dimerizes. Thus, HDL apolipoproteins A-I, A-II and E have distinct intracellular and post-secretory pathways of hepatic lipidation and dimerization in the process of HDL formation. These early forms of HDL are expected to follow different apolipoprotein-specific pathways through plasma remodeling and reverse cholesterol transport.

Keywords: Abbreviations; βMSH; β-mercaptoethanol; LpA-I and LpA-I/A-II; HDL containing only apo A-I or both apo A-I and apo A-II respectively; NEM; N-ethyl maleimide; NP-40; nonidet P40; RCT; reverse cholesterol transport; SEC; size exclusion chromatography; TPS; thiopropyl sepharoseapo A-I; apo A-II; apo E; HDL formation; HepG2; Huh7

Es-x/Ces1 prevents triacylglycerol accumulation in McArdle-RH7777 hepatocytes by Kerry W.S. Ko; Bruce Erickson; Richard Lehner (pp. 1133-1143).

Mouse esterase-x/carboxylesterase 1 (Es-x/Ces1) is a close homolog of triacylglycerol hydrolase/carboxylesterase 3 (TGH/Ces3). Es-x possesses a conserved esterase/lipase active site motif, suggesting that like TGH it could play a role in hepatic triacylglycerol (TG) metabolism. McArdle-RH7777 cells stably transfected with Es-x cDNA accumulated significantly less TG and had increased production of acid-soluble metabolites (an indicator of β-oxidation) during incubations with 0.4mM oleic acid when compared to empty vector or TGH cDNA transfected cells. Reduction of cellular TG persisted in the presence of esterase/lipase inhibitor E600 indicating that Es-x-mediated TG lowering can be largely explained by reduced partitioning of exogenous fatty acids to TG and increased redirection to β-oxidation, rather than by increased TG turnover. Glycerol supplementation increased TG synthesis in both control and Es-x expressing cells to similar extent suggesting that Es-x expression did not reduce flux of metabolic intermediates through the glycerol-3-phosphate pathway. While Es-x expression reduced cellular TG levels, secretion of TG and apolipoprotein B remained unchanged when compared to control cells. Overall, these results suggest that Es-x limits hepatic TG accumulation by promoting β-oxidation.

Keywords: Abbreviations; ADRP; adipose differentiation-related protein; ApoB; apolipoprotein B; ASM; acid-soluble metabolites; ATGL; adipose triglyceride lipase; DG; diacylglycerol; ECL; enhanced chemiluminescence; ER; endoplasmic reticulum; Es; esterase; OA; oleic acid; PDI; protein disulfide isomerase; FP; fluorophosphonate; G-3-P; glycerol-3-phosphate; HRP; horseradish peroxidase; McA; McArdle-RH7777; MUH; methyl umbelliferyl heptanoate; PA; phosphatidic acid; PC; phosphatidylcholine; PE; phosphatidylethanolamine; pNP; p; -nitrophenyl; PNS; post-nuclear supernatant; PS; phosphatidylserine; TG; triacylglycerolCarboxylesterase; Triacylglycerol; Hepatocyte; Lipase; Cholesterol

Fibrillar collagen type I stimulation of apolipoprotein B secretion in Caco-2 cells is mediated by β1 integrin by Don R. Ratcliffe; Jahangir Iqbal; M. Mahmood Hussain; Eva B. Cramer (pp. 1144-1154).

Caco-2 cells spontaneously differentiate into enterocyte-like cells and secrete apolipoprotein B (apoB) lipoproteins. We evaluated the effect of different extracellular matrix proteins on lipoprotein secretion by these cells. Caco-2 cells grown on human amnion connective tissue (HACT) secreted twice as much apoB as control cells on Transwells, but secreted similar amounts of apoA1. Cells cultured on fibrillar collagen type I secreted increased amounts of apoB similar to the cells cultured on HACT, but cells cultured on non-fibrillar collagen type I, type IV collagen or laminin-1 did not. The increased secretion was nullified by a function inhibiting anti-integrin β1 monoclonal antibody. Therefore, interactions between type I collagen and β1 integrins augment apoB secretion by Caco-2 cells. Cells on HACT formed a more uniform columnar epithelium with lipid droplets polarized to the basolateral membrane. We also studied the effect of extracellular matrix proteins on transepithelial resistance (TER) of differentiated Caco-2 cells. TER in cells cultured on HACT was similar to that on Transwells, but cells on laminin-1 and collagen IV exhibited higher TER. Thus, various extracellular matrix proteins regulate apoB secretion and TER differently. This new observation that extracellular matrix proteins can enhance apoB secretion in Caco-2 cells could be useful to explore the modulation of lipid transport by these proteins.

Keywords: Abbreviations; HACT; human amnion connective tissue; MTP; microsomal triglyceride transfer protein; apoAI; apolipoprotein AI; apoB; apolipoprotein B; CMF-PBS; calcium and magnesium free phosphate buffered saline; FBS; fetal bovine serum; ELISA; enzyme linked immunoassay; TER; transepithelial electrical resistance; DMEM; Dulbecco's modified Eagle's mediumApoB; Caco-2 cell; Collagen; Amnion; Integrin; Transepithelial resistance; Differentiation; Extracellular matrix protein

Chemical screen to reduce sterol accumulation in Niemann–Pick C disease cells identifies novel lysosomal acid lipase inhibitors by Anton I. Rosenbaum; Madalina Rujoi; Amy Y. Huang; Hong Du; Gregory A. Grabowski; Frederick R. Maxfield (pp. 1155-1165).

Niemann–Pick C disease (NPC) is a lysosomal storage disorder causing abnormal accumulation of unesterified free cholesterol in lysosomal storage organelles. High content phenotypic microscopy chemical screens in both human and hamster NPC-deficient cells have identified several compounds that partially revert the NPC phenotype. Cell biological and biochemical studies show that several of these molecules inhibit lysosomal acid lipase, the enzyme that hydrolyzes LDL-derived triacylglycerol and cholesteryl esters. The effects of reduced lysosomal acid lipase activity in lowering cholesterol accumulation in NPC mutant cells were verified by RNAi-mediated knockdown of lysosomal acid lipase in NPC1-deficient human fibroblasts. This work demonstrates the utility of phenotypic cellular screens as a means to identify molecular targets for altering a complex process such as intracellular cholesterol trafficking and metabolism.

Keywords: Cholesterol accumulation; Lysosomal storage organelles; Lysosomal acid lipase; Orlistat; NPC

Absence of stearoyl-CoA desaturase-1 does not promote DSS-induced acute colitis by Marcia L.E. MacDonald; Nagat Bissada; Bruce A. Vallance; Michael R. Hayden (pp. 1166-1172).

Absence of stearoyl-CoA desaturase-1 (SCD1) in mice leads to chronic inflammation of the skin and increased susceptibility to atherosclerosis, while also increasing plasma inflammatory markers. A recent report suggested that SCD1 deficiency also increases disease severity in a mouse model of inflammatory bowel disease, induced by dextran sulfate sodium (DSS). However, SCD1-deficient mice are known to consume increased amounts of water, which would also be expected to increase the intake of DSS-treated water. The aim of this study was to determine the effect of SCD1 deficiency on DSS-induced acute colitis with DSS dosing adjusted to account for genotype differences in fluid consumption. Wild-type controls were treated with 3.5% DSS for 5days to induce moderately severe colitis, while the concentration of DSS given to SCD1-deficient mice was lowered to 2.5% to control for increased fluid consumption. Colonic inflammation was assessed by clinical and histological scoring. Although SCD1-deficient mice consumed a total intake of DSS that was greater than that of wild-type controls, colonic inflammation, colon length and fecal blood were not altered by SCD1-deficiency in DSS-induced colitis, while diarrhea and total weight loss were modestly improved. Despite SCD1 deficiency leading to chronic inflammation of the skin and increased susceptibility to atherosclerosis, it does not accelerate inflammation in the DSS-induced model of acute colitis when DSS intake is controlled. These observations suggest that SCD1 deficiency does not play a significant role in colonic inflammation in this model.

Keywords: Abbreviations; SCD; stearoyl-CoA desaturase; DSS; dextran sulfate sodium; H&E; hematoxylin and eosin; IBD; inflammatory bowel disease; LPC; lysophosphatidylcholineInflammation; Dextran sulfate; Lipid; Dose–response relationship

Differential association of adipophilin and TIP47 proteins with cytoplasmic lipid droplets in mouse enterocytes during dietary fat absorption by Bonggi Lee; Jiabin Zhu; Nathan E. Wolins; Ji-Xin Cheng; Kimberly K. Buhman (pp. 1173-1180).

Recently, we found that enterocytes dynamically store triglycerides (TGs) in cytoplasmic lipid droplets (CLDs) during dietary fat absorption. A dynamic pool of TG in the form of CLDs which expands and depletes relative to time post dietary fat challenge is present in the absorptive cells of the small intestine, enterocytes. To identify cellular factors which may play a role in the regulation of this dynamic process we investigated the expression and localization of a lipid droplet associated protein family, PAT proteins, in enterocytes of mice chronically and acutely challenged by dietary fat. We found that adipophilin and Tip47 are the only PAT genes present in mouse intestinal mucosa and both genes are present at higher levels after high-fat challenges. We found TIP47 protein present in the intestine from chow and high-fat challenged mice; however, adipophilin protein was only present after high-fat challenges. In addition, TIP47 protein level was higher after an acute than a chronic high-fat challenge whereas adipophilin protein level was higher after a chronic than an acute high-fat challenge. We co-imaged TG in CLDs using CARS microscopy and TIP47 or adipophilin using immunocytochemistry in isolated enterocytes from mice challenged chronically and acutely by high levels of dietary fat. TIP47, but not adipophilin, coats CLDs in enterocytes after an acute high-fat challenge suggesting that TIP47 plays a role in the synthesis of CLDs from newly synthesized TG at the beginning of the process of dietary fat absorption in enterocytes. Adipophilin, on the other hand, coats CLDs only in enterocytes of chronic high-fat fed mice suggesting that adipophilin may play a role in the stabilization of TG stored in CLDs in longer term. These results suggest distinct roles for TIP47 and adipophilin in dietary fat absorption.

Keywords: Abbreviations; TG; triglyceride; CLD; cytoplasmic lipid droplet; TIP47; tail interacting protein of 47; kDa; CARS; Coherent anti-Stokes Raman scattering; INT; intestinal mucosa; WAT; white adipose tissue; NTC; no template controlDietary fat absorption; Cytoplasmic lipid droplets; Triglyceride; Intestine; Coherent anti-Stokes Raman scattering microscopy

Suppression of VLDL secretion by cultured hepatocytes incubated with chylomicron remnants enriched in n−3 polyunsaturated fatty acids is regulated by hepatic nuclear factor-4α by Lopez-Soldado Iliana López-Soldado; Michael Avella; Kathleen M. Botham (pp. 1181-1189).

Dietary n−3 polyunsaturated fatty acids (PUFA) suppress the secretion of very low density lipoprotein (VLDL) directly when delivered to the liver in chylomicron remnants (CMR). The role of sterol regulatory element-binding proteins (SREBPs) and hepatic nuclear factor-4α (HNF-4α) in the regulation of this effect was investigated. Chylomicron remnant-like particles (CRLPs) containing triacylglycerol (TG) from palm (rich in saturated fatty acids (SFA)) or fish (rich in n−3 PUFA) oil were incubated with cultured rat hepatocytes (24h) and the expression of protein and mRNA for SREBP-1, SREBP-2 and HNF-4α, and levels of mRNA for their target genes were determined. SREBP-1 and -2 protein expression in the membrane and nuclear fractions was unaffected by either type of CRLPs. mRNA abundance for SREBP-1c and -2 was also unchanged by CRLP-treatment, as were levels of mRNA for target genes of SREBP-1, including steroyl CoA desaturase, acetyl CoA carboxylase, fatty acid synthase and ATP citrate lyase, and SREBP-2 (3-hydroxy-3-methylglutaryl CoA reductase). In contrast, HNF-4α protein and mRNA levels were significantly decreased by CRLPs enriched in n−3 PUFA, but not SFA, and the expression of mRNA for HNF-4α target genes, including HNF-1α, apolipoprotein B and the microsomal TG transfer protein, was also lowered by n−3 PUFA-, but not SFA-enriched CRLPs. These findings suggest that the direct suppression of VLDL secretion by dietary n−3 PUFA delivered to the liver in CMR is mediated via decreased expression of HNF-4α.

Keywords: Chylomicron remnant; Very low density lipoprotein; Hepatic nuclear factor-4α; Sterol regulatory element-binding protein; n; −; 3 polyunsaturated fatty acid; Rat hepatocyte

N-3 polyunsaturated fatty acids suppress insulin-induced SREBP-1c transcription via reduced trans-activating capacity of LXRα by George Howell III; Xiong Deng; Chandrahassa Yellaturu; Edwards A. Park; Henry G. Wilcox; Rajendra Raghow; Marshall B. Elam (pp. 1190-1196).

Insulin coordinately up-regulates lipogenic gene transcription via induction of sterol regulatory element binding protein-1c (SREBP-1c). Conversely, polyunsaturated fatty acids (PUFA) decrease lipogenic gene transcription via suppression of SREBP-1c. We therefore examined the ability of n-3 PUFA to mitigate induction of SREBP-1c and its downstream lipogenic targets by insulin in primary rat hepatocyte cultures. Insulin induced expression of SREBP-1c mRNA 5-6 fold as well as rat SREBP-1c promoter activity. These effects were prevented by the n-3 fatty acids eicosapentaenoic acid (20:5 n-3; EPA) and docosahexaenoic acid (22:6 n-3, DHA), but not by the monounsaturated fatty acid oleic acid (18:1 n-6, OLA). N-3 fatty acids also effectively prevented insulin induction of the downstream lipogenic enzyme targets fatty acid synthase (FAS) and acetyl carboxyl coenzyme acetyltransferase-1 (ACC-1), and reduced de novo lipogenesis. The SREBP-1c promoter contains an insulin response unit consisting of tandem LXRα response elements (LXREs) as well as sites for NF-Y, Sp1, and SREBP-1c itself. The LXREs were identified as a primary site mediating suppression of SREBP-1c transcription by n-3 PUFA. DHA effectively prevented LXRα-dependent activation of both the wild type SREBP-1c promoter and the synthetic LXRE-driven promoter, and significantly blunted LXRα-dependent activation of a Gal4-LXRα chimeric protein thus demonstrating that n-3 PUFA effectively mitigate induction of SREBP-1c by insulin via reduced trans-activation of LXRα.

Keywords: Sterol response element binding protein-1c; Insulin; Polyunsaturated fatty acid; Docosahexaenoic acid; Eicosapentaenoic acid; Liver x receptor alpha

Cholesterol homeostasis in ABCA1/LCAT double-deficient mouse by Mohammad Anwar Hossain; Maki Tsujita; Nobukatsu Akita; Fumihiko Kobayashi; Shinji Yokoyama (pp. 1197-1205).

We examined cholesterol homeostasis in mice with the two major cholesterol transport pathways for catabolism interrupted by disrupting abca1, lcat, or both. Plasma HDL markedly decreased in these genotype but LDL/VLDL decreased only in the double deficiency. Fractional catabolic rate of HDL increased in the order of wild type< abca1(−/−)= lcat(−/−)< abca1(−/−)lcat(−/−). Cholesterol accumulated in the liver by disrupting either gene and more by the double disruption. HDL biogenesis by primary-cultured hepatocytes was negligible in the abca1 deficiency and substantially reduced in the lcat deficiency. Secretion of LDL/VLDL was also decreased in these cells but to a less extent. Cholesterol content in the hepatocytes was in a reciprocal order to lipoprotein generation. Expression of hepatic mRNA of the sterol-related genes reflected the cellular cholesterol increase, such as decrease in SREBP2 and HMG-CoA reductase and increase in apoA-I, apoE, and ABCG1. Cholesterol decreased in the steroidogenic organs by disruption of either gene resulting from low-plasma HDL. Cholesterol in other peripheral tissues generally decreased under normal chow feeding, and interestingly, it was recovered by high-cholesterol feeding, including the cholesterol content in the brain. No apparent vascular lipid deposition was observed in any genotype. Deletion of the two major factors in “reverse cholesterol transport” may not directly result in severe cholesterol transport stagnation in the body of mouse. Other compensatory pathways may back up cholesterol transport among the organs and tissues even when these pathways are impaired.

Keywords: Cholesterol; HDL; ABCA1; LCAT; Atherosclerosis

CYP7B1-mediated metabolism of 5α-androstane-3α,17β-diol (3α-Adiol): A novel pathway for potential regulation of the cellular levels of androgens and neurosteroids by Hanna Pettersson; Johan Lundqvist; Ernst Oliw; Maria Norlin (pp. 1206-1215).

The current study presents data indicating that 5α-androstane-3α,17β-diol (3α-Adiol) undergoes a previously unknown metabolism into hydroxymetabolites, catalyzed by CYP7B1. 3α-Adiol is an androgenic steroid which serves as a source for the potent androgen dihydrotestosterone and also can modulate gamma-amino butyric acid A (GABA_A) receptor function in the brain. The steroid hydroxylase CYP7B1 is known to metabolize cholesterol derivatives, sex hormone precursors and certain estrogens, but has previously not been thought to act on androgens or 3α-hydroxylated steroids. 3α-Adiol was found to undergo NADPH-dependent metabolism into 6- and 7-hydroxymetabolites in incubations with porcine microsomes and human kidney-derived HEK293 cells, which are high in CYP7B1 content. This metabolism was suppressed by addition of steroids known to be metabolized by CYP7B1. In addition, 3α-Adiol significantly suppressed CYP7B1-mediated catalytic reactions, in a way as would be expected for substrates that compete for the same enzyme. Recombinant expression of human CYP7B1 in HEK293 cells significantly increased the rate of 3α-Adiol hydroxylation. Furthermore, the observed hydroxylase activity towards 3α-Adiol was very low or undetectable in livers of Cyp7b1(−/−) knockout mice. The present results indicate that CYP7B1-mediated catalysis may play a role for control of the cellular levels of androgens, not only of estrogens. These findings suggest a previously unknown mechanism for metabolic elimination of 3α-Adiol which may impact intracellular levels of dihydrotestosterone and GABA_A-modulating steroids.

Keywords: Abbreviations; 3α-Adiol; 5α-androstane-3α,17β-diol; 3β-Adiol; 5α-androstane-3β,17β-diol; AR; androgen receptor; ARE; androgen response element; CYP; cytochrome P450; DHEA; dehydroepiandrosterone; DHT; dihydrotestosterone; ER; estrogen receptor; ERE; estrogen response element; GABA; _A; gamma-amino butyric acid A; HEK; human embryonic kidney; RLU; relative light unitsCYP7B1; 5α-Androstane-3α,17β-diol; Steroid metabolism; Androgen; Neurosteroid

Sections

Personal tools

BBA - Molecular and Cell Biology of Lipids (v.1791, #12)

Sections

Personal tools

Local resources

BBA - Molecular and Cell Biology of Lipids (v.1791, #12)