Role of Acyl-Coenzyme A:Cholesterol Acyltransferase-1 in the Control of Hepatic Very Low Density Lipoprotein Secretion and Low Density Lipoprotein Receptor Expression in the Mouse and Hamster

The role of KLF2 in regulating hepatic lipogenesis and blood cholesterol homeostasis via the SCAP/SREBP pathway

Liver steatosis is a common metabolic disorder resulting from imbalanced lipid metabolism, which involves various processes such as de novo lipogenesis, fatty acid uptake, fatty acid oxidation, and VLDL secretion. In this study, we discovered that KLF2, a transcription factor, plays a crucial role in regulating lipid metabolism in the liver. Overexpression of KLF2 in the liver of db/db mice, C57BL/6J mice, and Cd36-/- mice fed on a normal diet resulted in increased lipid content in the liver. Additionally, transgenic mice (ALB-Klf2) that overexpressed Klf2 in the liver developed liver steatosis after being fed a normal diet. We found that KLF2 promotes lipogenesis by increasing the expression of SCAP, a chaperone that facilitates the activation of SREBP, the master transcription factor for lipogenic gene expression. Our mechanism studies revealed that KLF2 enhances lipogenesis in the liver by binding to the promoter of SCAP and increasing the expression of genes involved in fatty acid synthesis. Reduction of KLF2 expression led to a decrease in SCAP expression and a reduction in the expression of SREBP1 target genes involved in lipogenesis. Overexpression of KLF2 also increased the activation of SREBP2 and the mRNA levels of its downstream target SOAT1. In C57BL/6J mice fed a high-fat diet, overexpression of Klf2 increased blood VLDL secretion, while reducing its expression decreased blood cholesterol levels. Our study emphasizes the novelty that hepatic KLF2 plays a critical role in regulating lipid metabolism through the KLF2/SCAP/SREBPs pathway, which is essential for hepatic lipogenesis and maintaining blood cholesterol homeostasis.

HCV Interplay with Lipoproteins: Inside or Outside the Cells?

Hepatitis C virus (HCV) infection is a major public health issue leading to chronic liver diseases. HCV particles are unique owing to their particular lipid composition, namely the incorporation of neutral lipids and apolipoproteins. The mechanism of association between HCV virion components and these lipoproteins factors remains poorly understood as well as its impact in subsequent steps of the viral life cycle, such as entry into cells. It was proposed that the lipoprotein biogenesis pathway is involved in HCV morphogenesis; yet, recent evidence indicated that HCV particles can mature and evolve biochemically in the extracellular medium after egress. In addition, several viral, cellular and blood components have been shown to influence and regulate this specific association. Finally, this specific structure and composition of HCV particles was found to influence entry into cells as well as their stability and sensitivity to neutralizing antibodies. Due to its specific particle composition, studying the association of HCV particles with lipoproteins remains an important goal towards the rational design of a protective vaccine.

Sterol O-Acyltransferase 2 Contributes to the Yolk Cholesterol Trafficking during Zebrafish Embryogenesis

To elucidate whether Sterol O-acyltransferase (Soat) mediates the absorption and transportation of yolk lipids to the developing embryo, zebrafish soat1 and soat2 were cloned and studied. In the adult zebrafish, soat1 was detected ubiquitously while soat2 mRNA was detected specifically in the liver, intestine, brain and testis. Whole mount in situ hybridization demonstrated that both soat1 and soat2 expressed in the yolk syncytial layer, hatching gland and developing cardiovascular as well as digestive systems, suggesting that Soats may play important roles in the lipid trafficking and utilization during embryonic development. The enzymatic activity of zebrafish Soat2 was confirmed by Oil Red O staining in the HEK293 cells overexpressing this gene, and could be quenched by Soat2 inhibitor Pyripyropene A (PPPA). The zebrafish embryos injected with PPPA or morpholino oligo against soat2 in the yolk showed significantly larger yolk when compared with wild-type embryos, especially at 72 hpf, indicating a slower rate of yolk consumption. Our result indicated that zebrafish Soat2 is catalytically active in synthesizing cholesteryl esters and contributes to the yolk cholesterol trafficking during zebrafish embryogenesis.

Hepatic overexpression of methionine sulfoxide reductase A reduces atherosclerosis in apolipoprotein E-deficient mice

Methionine sulfoxide reductase A (MsrA), a specific enzyme that converts methionine-S-sulfoxide to methionine, plays an important role in the regulation of protein function and the maintenance of redox homeostasis. In this study, we examined the impact of hepatic MsrA overexpression on lipid metabolism and atherosclerosis in apoE-deficient (apoE^−/−) mice. In vitro study showed that in HepG2 cells, lentivirus-mediated human MsrA (hMsrA) overexpression upregulated the expression levels of several key lipoprotein-metabolism-related genes such as liver X receptor α, scavenger receptor class B type I, and ABCA1. ApoE^−/− mice were intravenously injected with lentivirus to achieve high-level hMsrA expression predominantly in the liver. We found that hepatic hMsrA expression significantly reduced plasma VLDL/LDL levels, improved plasma superoxide dismutase, and paraoxonase-1 activities, and decreased plasma serum amyloid A level in apoE^−/− mice fed a Western diet, by significantly altering the expression of several genes in the liver involving cholesterol selective uptake, conversion and excretion into bile, TG biosynthesis, and inflammation. Moreover, overexpression of hMsrA resulted in reduced hepatic steatosis and aortic atherosclerosis. These results suggest that hepatic MsrA may be an effective therapeutic target for ameliorating dyslipidemia and reducing atherosclerosis-related cardiovascular diseases.

Identification of putative active site residues of ACAT enzymes

In this report, we sought to determine the putative active site residues of ACAT enzymes. For experimental purposes, a particular region of the C-terminal end of the ACAT protein was selected as the putative active site domain due to its high degree of sequence conservation from yeast to humans. Because ACAT enzymes have an intrinsic thioesterase activity, we hypothesized that by analogy with the thioesterase domain of fatty acid synthase, the active site of ACAT enzymes may comprise a catalytic triad of ser-his-asp (S-H-D) amino acid residues. Mutagenesis studies revealed that in ACAT1, S456, H460, and D400 were essential for activity. In ACAT2, H438 was required for enzymatic activity. However, mutation of D378 destabilized the enzyme. Surprisingly, we were unable to identify any S mutations of ACAT2 that abolished catalytic activity. Moreover, ACAT2 was insensitive to serine-modifying reagents, whereas ACAT1 was not. Further studies indicated that tyrosine residues may be important for ACAT activity. Mutational analysis showed that the tyrosine residue of the highly conserved FYXDWWN motif was important for ACAT activity. Furthermore, Y518 was necessary for ACAT1 activity, whereas the analogous residue in ACAT2, Y496, was not. The available data suggest that the amino acid requirement for ACAT activity may be different for the two ACAT isozymes.

Apolipoprotein B and triacylglycerol secretion in human triacylglycerol hydrolase transgenic mice

Apolipoprotein B (apoB)-containing lipoproteins play a critical role in whole body lipid homeostasis and the pathogenesis of atherosclerosis. The assembly of hepatic apoB-containing lipoproteins, VLDL, is governed by the availability of lipids, including triacylglycerol (TG). The majority of TG associated with VLDL is derived from the hepatic cytoplasmic lipid stores by a process involving lipolysis followed by reesterification. Microsomal triacylglycerol hydrolase (TGH) has been demonstrated to play a role in the lipolysis/reesterification process. To evaluate the potential regulatory role of TGH in hepatic VLDL assembly, we developed inducible transgenic mice expressing a human TGH minigene under the control of the mouse metallothionein promoter. Induction of human TGH by zinc resulted in liver-specific expression of the enzyme associated with 3- to 4-fold increases in lipolytic activity that could be attenuated with a TGH-specific inhibitor. Augmented TGH activity led to increased secretion of newly synthesized apoB and plasma TG levels. These results suggest that increased hepatic expression of TGH leads to a more proatherogenic plasma lipid and apoB profile.

ACAT2 stimulates cholesteryl ester secretion in apoB-containing lipoproteins

Previous studies in nonhuman primates revealed a striking positive correlation between liver cholesteryl ester (CE) secretion rate and the development of coronary artery atherosclerosis. CE incorporated into hepatic VLDL is necessarily synthesized by ACAT2, the cholesterol-esterifying enzyme in hepatocytes. We tested the hypothesis that the level of ACAT2 expression, in concert with cellular cholesterol availability, affects the CE content of apolipoprotein B (apoB)-containing lipoproteins. In a model system of lipoprotein secretion using COS cells cotransfected with microsomal triglyceride transfer protein and truncated forms of apoB, ACAT2 expression resulted in a 3-fold increase in microsomal ACAT activity and a 4-fold increase in the radiolabeled CE content of apoB-lipoproteins. After cholesterol-cyclodextrin (Chol-CD) treatment, CE secretion was increased by 27-fold in ACAT2-transfected cells but by only 7-fold in control cells. Chol-CD treatment also caused the percentage of CE in the apoB-lipoproteins to increase from 3% to 33% in control cells and from 16% to 54% in ACAT2-transfected cells. In addition, ACAT2-transfected cells secreted 3-fold more apoB than control cells. These results indicate that under all conditions of cellular cholesterol availability tested, the relative level of ACAT2 expression affects the CE content and, hence, the potential atherogenicity, of nascent apoB-containing lipoproteins.

Overexpression of rat long chain acyl-coa synthetase 1 alters fatty acid metabolism in rat primary hepatocytes

Long chain acyl-CoA synthetases (ACSL) activate fatty acids (FA) and provide substrates for both anabolic and catabolic pathways. We have hypothesized that each of the five ACSL isoforms partitions FA toward specific downstream pathways. Acsl1 mRNA is increased in cells under both lipogenic and oxidative conditions. To elucidate the role of ACSL1 in hepatic lipid metabolism, we overexpressed an Acsl1 adenovirus construct (Ad-Acsl1) in rat primary hepatocytes. Ad-ACSL1, located on the endoplasmic reticulum but not on mitochondria or plasma membrane, increased ACS specific activity 3.7-fold. With 100 or 750 mum [1-(14)C]oleate, Ad-Acsl1 increased oleate incorporation into diacylglycerol and phospholipids, particularly phosphatidylethanolamine and phosphatidylinositol, and decreased incorporation into cholesterol esters and secreted triacylglycerol. Ad-Acsl1 did not alter oleate incorporation into triacylglycerol, beta-oxidation products, or total amount of FA metabolized. In pulse-chase experiments to examine the effects of Ad-Acsl1 on lipid turnover, more labeled triacylglycerol and phospholipid, but less labeled diacylglycerol, remained in Ad-Acsl1 cells, suggesting that ACSL1 increased reacylation of hydrolyzed oleate derived from triacylglycerol and diacylglycerol. In addition, less hydrolyzed oleate was used for cholesterol ester synthesis and beta-oxidation. The increase in [1,2,3-(3)H]glycerol incorporation into diacylglycerol and phospholipid was similar to the increase with [(14)C]oleate labeling suggesting that ACSL1 increased de novo synthesis. Labeling Ad-Acsl1 cells with [(14)C]acetate increased triacylglycerol synthesis but did not channel endogenous FA away from cholesterol ester synthesis. Thus, consistent with the hypothesis that individual ACSLs partition FA, Ad-Acsl1 increased FA reacylation and channeled FA toward diacylglycerol and phospholipid synthesis and away from cholesterol ester synthesis.

Increased cholesterol biosynthesis and hypercholesterolemia in mice overexpressing squalene synthase in the liver

Squalene synthase (SS) is the first committed enzyme for cholesterol biosynthesis, located at a branch point in the mevalonate pathway. To examine the role of SS in the overall cholesterol metabolism, we transiently overexpressed mouse SS in the livers of mice using adenovirus-mediated gene transfer. Overexpression of SS increased de novo cholesterol biosynthesis with increased 3-hydroxy-3-methyglutaryl-CoA (HMG-CoA) reductase activity, in spite of the downregulation of its own mRNA expression. Furthermore, overexpression of SS increased plasma concentrations of LDL, irrespective of the presence of functional LDL receptor (LDLR). Thus, the hypercholesterolemia is primarily caused by increased hepatic production of cholesterol-rich VLDL, as demonstrated by the increases in plasma cholesterol levels after intravenous injection of Triton WR1339. mRNA expression of LDLR was decreased, suggesting that defective LDL clearance contributed to the development of hypercholesterolemia. Curiously, the liver was enlarged, with a larger number of Ki-67-positive cells. These results demonstrate that transient upregulation of SS stimulates cholesterol biosynthesis as well as lipoprotein production, providing the first in vivo evidence that SS plays a regulatory role in cholesterol metabolism through modulation of HMG-CoA reductase activity and cholesterol biosynthesis.

Dietary fatty acids differentially modulate messenger RNA abundance of low-density lipoprotein receptor, 3-hydroxy-3-methylglutaryl coenzyme A reductase, and microsomal triglyceride transfer protein in Golden-Syrian hamsters

Dietary fatty acids modulate plasma and intracellular cholesterol concentrations. Circulating non-high-density lipoprotein cholesterol (nHDL-C) concentration is determined by rates of hepatic very low-density lipoprotein assembly and secretion, and clearance of subsequent metabolic products. The effect of dietary fat (butter, traditional margarine, soybean oil, and canola oil) was assessed with respect to plasma lipids, hepatic lipid composition, and messenger RNA (mRNA) abundance of low-density lipoprotein (LDL) receptor, 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase, sterol regulatory element-binding protein (SREBP) 2, and microsomal triglyceride transfer protein (MTP) in the Golden-Syrian hamster (Charles River Laboratories, Wilmington, MA). Hamsters were fed with a nonpurified diet (6.25 fat g/100 g) with 0.1 g cholesterol/100 g (control diet) or control diet with an additional 10 g experimental fat/100 g for 12 weeks. Hamsters fed with the control diet, unsaturated fats (canola and soybean oils), and margarine, relative to butter, had significantly lower total cholesterol and nHDL-C and triglyceride concentrations. Additional dietary fat, regardless of fatty acid profile, resulted in higher hepatic cholesterol concentrations. In contrast, relative to the control diet-, butter-, or margarine-fed hamsters, these changes were associated with a 4- and 8-fold higher LDL receptor and 5- and 9-fold higher SREBP mRNA abundance, in hamsters fed with canola and soybean oils, respectively. MTP mRNA, a marker of very low-density lipoprotein particle formation, was higher in canola- and soybean oil-fed hamsters relative to the control diet-fed hamsters, although differences were modest. These results suggest that the substitution of canola and soybean oils for butter results in lower nHDL-C concentrations that may be related to increased mRNA abundance of the LDL receptor, SREBP-2, and MTP genes.

Liver-directed overexpression of mitochondrial glycerol-3-phosphate acyltransferase results in hepatic steatosis, increased triacylglycerol secretion and reduced fatty acid oxidation

Glycerol-3-phosphate acyltransferase (GPAT) catalyzes the first committed step in triacylglycerol (TAG) and phospholipid biosynthesis. GPAT activity has been identified in both ER and mitochondrial subcellular fractions. The ER activity dominates in most tissues except in liver, where the mitochondrial isoform (mtGPAT) can constitute up to 50% of the total activity. To study the in vivo effects of hepatic mtGPAT overexpression, mice were transduced with adenoviruses expressing either murine mtGPAT or a catalytically inactive variant of the enzyme. Overexpressing mtGPAT resulted in massive 12- and 7-fold accumulation of liver TAG and diacylglycerol, respectively but had no effect on phospholipid or cholesterol ester content. Histological analysis showed extensive lipid accumulation in hepatocytes. Furthermore, mtGPAT transduction markedly increased adipocyte differentiation-related protein and stearoyl-CoA desaturase-1 (SCD-1) in the liver. In line with increased SCD-1 expression, 18:1 and 16:1 in the hepatic TAG fraction increased. In addition, mtGPAT overexpression decreased ex vivo fatty acid oxidation, increased liver TAG secretion rate 2-fold, and increased plasma TAG and cholesterol levels. These results support the hypothesis that increased hepatic mtGPAT activity associated with obesity and insulin resistance contributes to increased TAG biosynthesis and inhibition of fatty acid oxidation, responses that would promote hepatic steatosis and dyslipidemia.

Human acyl-CoA:cholesterol acyltransferase (ACAT) and its potential as a target for pharmaceutical intervention against atherosclerosis

Acyl-CoA:cholesterol acyltransferase (ACAT) catalyzes the formation of cholesteryl esters from cholesterol and long-chain fatty-acyl-coenzyme A. At the single-cell level, ACAT serves as a regulator of intracellular cholesterol homeostasis. In addition, ACAT supplies cholesteryl esters for lipoprotein assembly in the liver and small intestine. Under pathological conditions, the accumulation of cholesteryl esters produced by ACAT in macrophages contributes to foam cell formation, a hallmark of the early stage of atherosclerosis. Several reviews addressing various aspects of ACAT and ACAT inhibitors are available. This review briefly outlines the current knowledge on the biochemical properties of human ACATs, and then focuses on discussing the merit of ACAT as a drug target for pharmaceutical interventions against atherosclerosis.

Cholic acid as key regulator of cholesterol synthesis, intestinal absorption and hepatic storage in mice

To study the effects of cholic acid (CA) feeding on hepatic cholesterol metabolism, male sterol 12alpha-hydroxylase (CYP8B1) knockout (-/-) mice and wildtype controls (+/+) were fed either a control diet or the same diet supplemented with CA (0.1% or 0.5% w/w) or cholesterol (1% w/w). During feeding of the control diet, cholesterol synthesis was increased in CYP8B1-/- compared to +/+ mice. Both cholesterol and CA feeding down regulated mRNA expression of cholesterogenic genes and hepatic de novo cholesterol synthesis as also reflected by a concomitant decrease in the nuclear factor SREBP-2 precursor protein and increased hepatic free cholesterol levels. Mice with an intact CYP8B1 gene (CYP8B1+/+ and C57Bl/6 mice) accumulated higher concentrations of cholesteryl esters (24- and 25-fold, respectively) in their livers compared to CYP8B1-/- mice (8-fold). Feeding of CA increased intestinal cholesterol absorption in CYP8B1+/+ mice by 23% and in CYP8B1-/- mice by 50%. While plasma cholesterol did not differ between CYP8B1+/+ and -/- mice under control conditions and cholesterol feeding a decrease was seen in CYP8B1-/- but not CYP8B1+/+ mice fed CA. This study indicates that CA is an important determinant for intestinal cholesterol absorption and that the levels of the transcription factor SREBP-2 in the liver are dependent upon the combined effect of CA on intestinal cholesterol absorption and CYP7A1. The possibility is discussed that inhibition of CYP8B1 and thus CA synthesis may be beneficial for the treatment of hyperlipidemic disorders.

The active site His-460 of human acyl-coenzyme A:cholesterol acyltransferase 1 resides in a hitherto undisclosed transmembrane domain

Human acyl-coenzyme A:cholesterol acyltransferase 1 (hACAT1) esterifies cholesterol at the endoplasmic reticulum (ER). We had previously reported that hACAT1 contains seven transmembrane domains (TMD) (Lin, S., Cheng, D., Liu, M. S., Chen, J., and Chang, T. Y. (1999) J. Biol. Chem. 274, 23276-23285) and nine cysteines. The Cys near the N-terminal is located at the cytoplasm; the two cysteines near the C-terminal form a disulfide bond and are located in the ER lumen. The other six free cysteines are located in buried region(s) of the enzyme (Guo, Z.-Y., Chang, C. C. Y., Lu, X., Chen, J., Li, B.-L., and Chang, T.-Y. (2005) Biochemistry 44, 6537-6548). In the current study, we show that the conserved His-460 is a key active site residue for hACAT1. We next performed Cys-scanning mutagenesis within the region of amino acids 354-493, expressed these mutants in Chinese hamster ovary cells lacking ACAT1, and prepared microsomes from transfected cells. The microsomes are either left intact or permeabilized with detergent. The accessibility of the engineered cysteines of microsomal hACAT1 to various maleimide derivatives, including mPEG(5000)-maleimide (large, hydrophilic, and membrane-impermeant), N-ethylmaleimide, 4-acetamido-4'-maleimidylstilbene-2,2'-disulfonic acid (small, hydrophilic, and ER membrane-permeant), and N-phenylmaleimide (small, hydrophobic, and ER membrane-permeant), were monitored by Western blot analysis. The results led us to construct a revised, nine-TMD model, with the active site His-460 located within a hitherto undisclosed transmembrane domain, between Arg-443 and Tyr-462.

ACAT2 is a target for treatment of coronary heart disease associated with hypercholesterolemia

The inhibition of intracellular cholesterol esterification as a means to prevent atherosclerosis has been considered to have potential for many years. Two different ACAT enzymes were discovered about 7 years ago, and it has become clear that the two enzymes provide separate physiologic functions. Much has been learned from mice with gene deletions for either ACAT1 or ACAT2. Deletion of ACAT2 has consistently been atheroprotective whereas deletion of ACAT1 has been varyingly problematic. ACAT1 functions in converting cellular cholesterol into cholesteryl ester in response to cholesterol abundance inside the cells. In atherosclerotic lesions, where macrophages ingest excess cholesterol, the ability to esterify the newly-acquired cholesterol seems important for cell survival. Inhibition of ACAT1 may bring undesired consequences with destabilization of cellular membrane function upon cholesterol accumulation leading to macrophage cell death. In contrast, ACAT2 is expressed only in hepatocytes and enterocytes, where ACAT1 is silent, and appears to provide cholesteryl esters for transport in lipoproteins. These two cell types have an abundance of additional mechanisms for disposing of cholesterol so that depletion of ACAT2 does not signal apoptosis. At the present time, the bulk of the available data suggest that the strategy seeming to bear the most potential for treatment of coronary heart disease associated with hypercholesterolemia would be to specifically inhibit ACAT2.

Host cell lipids control cholesteryl ester synthesis and storage in intracellular Toxoplasma

The intracellular protozoan Toxoplasma gondii lacks a de novo mechanism for cholesterol synthesis and therefore must scavenge this essential lipid from the host environment. In this study, we demonstrated that T. gondii diverts cholesterol from low-density lipoproteins for cholesteryl ester synthesis and storage in lipid bodies. We identified and characterized two isoforms of acyl-CoA:cholesterol acyltransferase (ACAT)-related enzymes, designated TgACAT1alpha and TgACAT1beta in T. gondii. Both proteins are coexpressed in the parasite, localized to the endoplasmic reticulum and participate in cholesteryl ester synthesis. In contrast to mammalian ACAT, TgACAT1alpha and TgACAT1beta preferentially incorporate palmitate into cholesteryl esters and present a broad sterol substrate affinity. Mammalian ACAT-deficient cells transfected with either TgACAT1alpha or TgACAT1beta are restored in their capability of cholesterol esterification. TgACAT1alpha produces steryl esters and forms lipid bodies after transformation in a Saccharomyces cerevisiae mutant strain lacking neutral lipids. In addition to their role as ACAT substrates, host fatty acids and low-density lipoproteins directly serve as Toxoplasma ACAT activators by stimulating cholesteryl ester synthesis and lipid droplet biogenesis. Free fatty acids significantly increase TgACAT1alpha mRNA levels. Selected cholesterol esterification inhibitors impair parasite growth by rapid disruption of plasma membrane. Altogether, these studies indicate that host lipids govern neutral lipid synthesis in Toxoplasma and that interference with mechanisms of host lipid storage is detrimental to parasite survival in mammalian cells.

Insights into unique physiological features of neutral lipids in Apicomplexa: from storage to potential mediation in parasite metabolic activities

The fast intracellular multiplication of apicomplexan parasites including Toxoplasma and Plasmodium, requires large amounts of lipids necessary for the membrane biogenesis of new progenies. Hence, the study of lipids is fundamental in order to understand the biology and pathogenesis of these deadly organisms. Much has been reported on the importance of polar lipids, e.g. phospholipids in Plasmodium. Comparatively, little attention has been paid to the metabolism of neutral lipids, including sterols, steryl esters and acylglycerols. In eukaryotic cells, free sterols are membrane components whereas steryl esters and acylglycerols are stored in cytosolic lipid inclusions. The first part of this review describes the recent advances in neutral lipid synthesis and storage in Toxoplasma and Plasmodium. New potential pharmacological targets in the pathways producing neutral lipids are outlined. In addition to lipid bodies, Apicomplexa contain unique secretory organelles involved in parasite invasion named rhoptries. These compartments appear to sequester most of the cholesterol found in the exocytic pathway. The second part of the review focuses on rhoptry cholesterol and its potential roles in the biogenesis, structural organisation and function of these unique organelles among eukaryotes.

Overexpression of human diacylglycerol acyltransferase 1, acyl-coa:cholesterol acyltransferase 1, or acyl-CoA:cholesterol acyltransferase 2 stimulates secretion of apolipoprotein B-containing lipoproteins in McA-RH7777 cells

The relative importance of each core lipid in the assembly and secretion of very low density lipoproteins (VLDL) has been of interest over the past decade. The isolation of genes encoding diacylglycerol acyltransferase (DGAT) and acyl-CoA:cholesterol acyltransferases (ACAT1 and ACAT2) provided the opportunity to investigate the effects of isolated increases in triglycerides (TG) or cholesteryl esters (CE) on apolipoprotein B (apoB) lipoprotein biogenesis. Overexpression of human DGAT1 in rat hepatoma McA-RH7777 cells resulted in increased synthesis, cellular accumulation, and secretion of TG. These effects were associated with decreased intracellular degradation and increased secretion of newly synthesized apoB as VLDL. Similarly, overexpression of human ACAT1 or ACAT2 in McA-RH7777 cells resulted in increased synthesis, cellular accumulation, and secretion of CE. This led to decreased intracellular degradation and increased secretion of VLDL apoB. Overexpression of ACAT2 had a significantly greater impact upon assembly and secretion of VLDL from liver cells than did overexpression of ACAT1. The addition of oleic acid (OA) to media resulted in a further increase in VLDL secretion from cells expressing DGAT1, ACAT1, or ACAT2. VLDL secreted from DGAT1-expressing cells incubated in OA had a higher TG:CE ratio than VLDL secreted from ACAT1- and ACAT2-expressing cells treated with OA. These studies indicate that increasing DGAT1, ACAT1, or ACAT2 expression in McA-RH7777 cells stimulates the assembly and secretion of VLDL from liver cells and that the core composition of the secreted VLDL reflects the enzymatic activity that is elevated.

Endogenous cholesterol synthesis is associated with VLDL-2 apoB-100 production in healthy humans

Subjects with high plasma cholesterol levels exhibit a high production of VLDL apolipoprotein B-100 (apoB-100), suggesting that cholesterol is a mediator for VLDL production. The objective of the study was to examine whether endogenous cholesterol synthesis, reflected by the lathosterol-cholesterol ratio (L-C ratio), affects the secretory rates of different VLDL subfractions. Ten healthy subjects were studied after overnight fasting. During a 10 h primed, constant infusion of 13C-valine (15 micromol/kg/h), enrichment was determined in apoB-100 from ultracentrifugally isolated VLDL-1 and VLDL-2 by gas chromatography mass spectrometry. The synthesis rates of VLDL-1 apoB-100 and VLDL-2 apoB-100, catabolism, and transfer were estimated by compartmental analysis. Mean VLDL-1 apoB-100 pool size was 90 +/- 15 mg, and mean VLDL-2 apoB-100 pool size was 111 +/- 14 mg. Absolute synthesis rate of VLDL-1 apoB-100 was 649 +/- 127 mg/day and 353 +/- 59 mg/day for VLDL-2 apoB-100. There was a strong association between the absolute synthesis rate of VLDL-2 apoB-100 and L-C ratio (r 2 = 0.61, P < 0.01). In contrast, no correlation was observed between L-C ratio and absolute synthesis rate of VLDL-1 apoB-100 (r 2 = 0.302, P = 0.09). In conclusion, these data provide additional support for an independent regulation of VLDL-1 apoB-100 and VLDL-2 apoB-100 production. Endogenous cholesterol synthesis is correlated only with the VLDL-2 apoB-100 production.

Human acyl-coenzyme A:cholesterol acyltransferase expressed in chinese hamster ovary cells: membrane topology and active site location

Acyl-CoA:cholesterol acyltransferase (ACAT) is a membrane-bound enzyme that produces cholesteryl esters intracellularly. Two ACAT genes (ACAT1 and ACAT2) have been identified. The expression of ACAT1 is ubiquitous, whereas that of ACAT2 is tissue restricted. Previous research indicates that ACAT1 may contain seven transmembrane domains (TMDs). To study ACAT2 topology, we inserted two different antigenic tags (hemagglutinin, monoclonal antibody Mab1) at various hydrophilic regions flanking each of its predicted TMDs, and expressed the recombinant proteins in mutant Chinese hamster ovary cells lacking endogenous ACAT. Each tagged ACAT2 was expressed in the endoplasmic reticulum as a single undegraded protein band and was at least partially active enzymatically. We then used cytoimmunofluorescence and protease protection assays to monitor the sidedness of the hemagglutinin and Mab1 tags along the ER membranes. The results indicated that ACAT2 contains only two detectable TMDs, located near the N terminal region. We also show that a conserved serine (S245), a candidate active site residue, is not essential for ACAT catalysis. Instead, a conserved histidine (H434) present within a hydrophobic peptide segment, may be essential for ACAT catalysis. H434 may be located at the cytoplasmic side of the membrane.

Corn fiber oil lowers plasma cholesterol by altering hepatic cholesterol metabolism and up-regulating LDL receptors in guinea pigs

To evaluate some of the mechanisms involved in the hypocholesterolemic effects of corn fiber oil (CFO), male Hartley guinea pigs were fed diets containing increasing doses of CFO [0 (control), 5, 10 or 15 g/100 g]. Total fat was adjusted to 15 g/100 g in all diets with regular corn oil. Diets contained 0.25 g/100 g cholesterol. A positive control group (LC) with low dietary cholesterol (0.04 g/100 g) was also included. Plasma LDL cholesterol concentrations were 32, 55 and 57% (P < 0.0005) lower with increasing doses of CFO. Compared with controls, intake of CFO resulted in 27-32% lower hepatic microsomal cholesterol (P < 0.0001), the regulatory pool of LDL receptor (LDL-R) expression. CFO intake resulted in favorable plasma and hepatic cholesterol concentrations, similar to those in guinea pigs fed the LC diet. Hepatic cholesterol 7alpha-hydroxylase (CYP7) activity was approximately 88% higher in guinea pigs fed the two higher dosages of CFO (P < 0.05). In parallel, CYP7 mRNA abundance was approximately 88% higher in guinea pigs fed all three CFO diets. CFO treatment also induced hepatic LDLR mRNA by 66-150% with significant differences at the highest CFO dose. These results suggest that CFO, as a result of decreased bile acid absorption, increased mRNA abundance and activity of CYP7. Because hepatic cholesterol is the substrate for CYP7, a lowering of cholesterol concentrations in the total and microsomal pools was observed. As a response to the depleted microsomal free cholesterol pool, the LDL receptor was up-regulated, drawing more cholesterol from plasma, thus leading to the observed decrease in plasma LDL cholesterol concentrations.

Inhibition of acyl coenzyme A-cholesterol acyltransferase: a possible treatment of atherosclerosis?

Our full understanding of atherosclerosis and our ability to prevent its sequellae are incomplete. As a result, further investigation of novel antiatherosclerotic mechanisms and agents continues. Acyl coenzyme A-cholesterol acyltransferase (ACAT) inhibition has been evaluated as a potential mechanism by which the current treatment arsenal may be expanded. ACAT is present in a variety of tissues and is responsible for catalyzing the conversion of free cholesterol to the more readily stored cholesteryl esters. Impressive lipid effects demonstrated in animals have not generally been demonstrated in human clinical trials. Partial ACAT inhibition with specific agents has resulted in lesion regression and decreased progression, whereas complete ACAT inhibition via genetic alterations has led to an exacerbation of cholesterol deposition in tissues in animal models. No ACAT inhibitor has yet been fully evaluated in human clinical trials for its impact on atherosclerotic disease progression. Several hurdles, such as sample size requirements needed to detect effect over background therapy and lack of sensitive surrogate efficacy markers, have served as a deterrent to the development of this class of investigational drug. However, with recent technologic advancements, more sensitive methods of measuring disease progression may be available. Human clinical trials are currently underway, with several agents reported in Phase II clinical trials. Within the next few years, results from these trials may determine whether or not ACAT inhibitors will be added to the list of treatment options for the prevention of atherosclerotic disease progression.

Roles of acyl-coenzyme A:cholesterol acyltransferase-1 and -2

Acyl-coenzyme A:cholesterol acyltransferase (ACAT) is an intracellular enzyme that produces cholesteryl esters in various tissues. In mammals, two ACAT genes (ACAT1 and ACAT2) have been identified. Together, these two enzymes are involved in storing cholesteryl esters as lipid droplets, in macrophage foam-cell formation, in absorbing dietary cholesterol, and in supplying cholesteryl esters as part of the core lipid for lipoprotein synthesis and assembly. The key difference in tissue distribution of ACAT1 and ACAT2 between humans, mice and monkeys is that, in adult human liver (including hepatocytes and bile duct cells), the major enzyme is ACAT1, rather than ACAT2. There is compelling evidence implicating a role for ACAT1 in macrophage foam-cell formation, and for ACAT2 in intestinal cholesterol absorption. However, further studies at the biochemical and cell biological levels are needed in order to clarify the functional roles of ACAT1 and ACAT2 in the VLDL or chylomicron synthesis/assembly process.

Very-low-density lipoprotein assembly and secretion

The assembly of apolipoprotein B (apoB) into VLDL is broadly divided into two steps. The first involves transfer of lipid by the microsomal triglyceride transfer protein (MTP) to apoB during translation. The second involves fusion of apoB-containing precursor particles with triglyceride droplets to form mature VLDL. ApoB and MTP are homologs of the egg yolk storage protein, lipovitellin. Homodimerization surfaces in lipovitellin are reutilized in apoB and MTP to achieve apoB-MTP interactions necessary for first step assembly. Structural modeling predicts a small lipovitellin-like lipid binding cavity in MTP and a transient lipovitellin-like cavity in apoB important for nucleation of lipid sequestration. The formation of triglyceride droplets in the endoplasmic reticulum requires MTP however, their fusion with apoB may be MTP-independent. Second step assembly is modulated by phospholipase D and A2. Phospholipases may prime membrane transport steps required for second step fusion and/or channel phospholipids into a pathway for VLDL triglyceride production. The enzymology of VLDL triglyceride synthesis is still poorly understood; however, it appears that ACAT2 is the sole source of cholesterol esters for VLDL and chylomicron assembly. VLDL production is controlled primarily at the level of presecretory degradation. Recently, it was discovered that the LDL receptor modulates VLDL production through its interactions with nascent VLDL in the secretory pathway.

Acyl coenzyme A: cholesterol acyltransferase types 1 and 2: structure and function in atherosclerosis

Two enzymes are responsible for cholesterol ester formation in tissues, acyl coenzyme A:cholesterol acyltransferase types 1 and 2 (ACAT1 and ACAT2). The available evidence suggests different cell locations, membrane orientations, and metabolic functions for each enzyme. ACAT1 and ACAT2 gene disruption experiments in mice have shown complementary results, with ACAT1 being responsible for cholesterol homeostasis in the brain, skin, adrenal, and macrophages. ACAT1 -/- mice have less atherosclerosis than their ACAT1 +/+ counterparts, presumably because of the decreased ACAT activity in the macrophages. By contrast, ACAT2 -/- mice have limited cholesterol absorption in the intestine, and decreased cholesterol ester content in the liver and plasma lipoproteins. Almost no cholesterol esterification was found when liver and intestinal microsomes from ACAT2 -/- mice were assayed. Studies in non-human primates have shown the presence of ACAT1 primarily in the Kupffer cells of the liver, in non-mucosal cell types in the intestine, and in kidney and adrenal cortical cells, whereas ACAT2 is present only in hepatocytes and in intestinal mucosal cells. The membrane topology for ACAT1 and ACAT2 is also apparently different, with ACAT1 having a serine essential for activity on the cytoplasmic side of the endoplasmic reticulum membrane, whereas the analogous serine is present on the lumenal side of the endoplasmic reticulum for ACAT2. Taken together, the data suggest that cholesterol ester formation by ACAT1 supports separate functions compared with cholesterol esterification by ACAT2. The latter enzyme appears to be responsible for cholesterol ester formation and secretion in lipoproteins, whereas ACAT1 appears to function to maintain appropriate cholesterol availability in cell membranes.