Characterization of two human genes encoding acyl coenzyme A:cholesterol acyltransferase-related enzymes

Thematic review series: glycerolipids. DGAT enzymes and triacylglycerol biosynthesis

Triacylglycerols (triglycerides) (TGs) are the major storage molecules of metabolic energy and FAs in most living organisms. Excessive accumulation of TGs, however, is associated with human diseases, such as obesity, diabetes mellitus, and steatohepatitis. The final and the only committed step in the biosynthesis of TGs is catalyzed by acyl-CoA:diacylglycerol acyltransferase (DGAT) enzymes. The genes encoding two DGAT enzymes, DGAT1 and DGAT2, were identified in the past decade, and the use of molecular tools, including mice deficient in either enzyme, has shed light on their functions. Although DGAT enzymes are involved in TG synthesis, they have distinct protein sequences and differ in their biochemical, cellular, and physiological functions. Both enzymes may be useful as therapeutic targets for diseases. Here we review the current knowledge of DGAT enzymes, focusing on new advances since the cloning of their genes, including possible roles in human health and diseases.

Cloning and characterization of an acyl-CoA-dependent diacylglycerol acyltransferase 1 (DGAT1) gene from Tropaeolum majus, and a study of the functional motifs of the DGAT protein using site-directed mutagenesis to modify enzyme activity and oil content

A full-length cDNA encoding a putative diacylglycerol acyltransferase 1 (DGAT1, EC 2.3.1.20) was obtained from Tropaeolum majus (garden nasturtium). The 1557-bp open reading frame of this cDNA, designated TmDGAT1, encodes a protein of 518 amino acids showing high homology to other plant DGAT1s. The TmDGAT1 gene was expressed exclusively in developing seeds. Expression of recombinant TmDGAT1 in the yeast H1246MATalpha quadruple mutant (DGA1, LRO1, ARE1, ARE2) restored the capability of the mutant host to produce triacylglycerols (TAGs). The recombinant TmDGAT1 protein was capable of utilizing a range of (14)C-labelled fatty acyl-CoA donors and diacylglycerol acceptors, and could synthesize (14)C-trierucin. Collectively, these findings confirm that the TmDGAT1 gene encodes an acyl-CoA-dependent DGAT1. In plant transformation studies, seed-specific expression of TmDGAT1 was able to complement the low TAG/unusual fatty acid phenotype of the Arabidopsis AS11 (DGAT1) mutant. Over-expression of TmDGAT1 in wild-type Arabidopsis and high-erucic-acid rapeseed (HEAR) and canola Brassica napus resulted in an increase in oil content (3.5%-10% on a dry weight basis, or a net increase of 11%-30%). Site-directed mutagenesis was conducted on six putative functional regions/motifs of the TmDGAT1 enzyme. Mutagenesis of a serine residue in a putative SnRK1 target site resulted in a 38%-80% increase in DGAT1 activity, and over-expression of the mutated TmDGAT1 in Arabidopsis resulted in a 20%-50% increase in oil content on a per seed basis. Thus, alteration of this putative serine/threonine protein kinase site can be exploited to enhance DGAT1 activity, and expression of mutated DGAT1 can be used to enhance oil content.

Dyslipidemia in insulin resistance: clinical challenges and adipocentric therapeutic frontiers

The ever-increasing rates of obesity and diabetes worldwide have the potential to further fuel the epidemic of cardiovascular disease that we are experiencing today. To slow this epidemic successfully, insulin resistance and associated lipid abnormalities that frequently accompany it are key clinical targets. Yet, we are still challenged to reach the mandated clinical goals for lipids that would minimize the development and progression of cardiovascular disease. Adoption of a comprehensive approach by clinicians, in line with recent recommendations for stricter treatment goals for the at-risk patient, is essential to achieving cardiovascular risk reduction. The challenge for clinicians is integrating strategies, approaches and treatments that address the multiple metabolic defects in patients with insulin resistance and dyslipidemia. New perspectives can help effectively meet this ongoing challenge. Emerging evidence suggests that adipose tissue is intimately involved in the inter-relationships between insulin resistance and dyslipidemia. The future probably involves therapeutic strategies that directly target adipose tissue to optimally reduce cardiometabolic risk.

Lysophospholipid acyltransferases and arachidonate recycling in human neutrophils

The cycle of deacylation and reacylation of phospholipids plays a critical role in regulating availability of arachidonic acid for eicosanoid production. The major yeast lysophospholipid acyltransferase, Ale1p, is related to mammalian membrane-bound O-acyltransferase (MBOAT) proteins. We expressed four human MBOATs in yeast strains lacking Ale1p and studied their acyl-CoA and lysophospholipid specificities using novel mass spectrometry-based enzyme assays. MBOAT1 is a lysophosphatidylserine (lyso-PS) acyltransferase with preference for oleoyl-CoA. MBOAT2 also prefers oleoyl-CoA, using lysophosphatidic acid and lysophosphatidylethanolamine as acyl acceptors. MBOAT5 prefers lysophosphatidylcholine and lyso-PS to incorporate linoleoyl and arachidonoyl chains. MBOAT7 is a lysophosphatidylinositol acyltransferase with remarkable specificity for arachidonoyl-CoA. MBOAT5 and MBOAT7 are particularly susceptible to inhibition by thimerosal. Human neutrophils express mRNA for these four enzymes, and neutrophil microsomes incorporate arachidonoyl chains into phosphatidylinositol, phosphatidylcholine, PS, and phosphatidylethanolamine in a thimerosal-sensitive manner. These results strongly implicate MBOAT5 and MBOAT7 in arachidonate recycling, thus regulating free arachidonic acid levels and leukotriene synthesis in neutrophils.

Selective inhibition of acyl-CoA:cholesterol acyltransferase 2 isozyme by flavasperone and sterigmatocystin from Aspergillus species

Five known fungal metabolites, aurasperone A, aurasperone D, averufanin, flavasperone and sterigmatocystin, were isolated from the culture broths of Aspergillus species as inhibitors of acyl-CoA:cholesterol acyltransferase (ACAT) in the cell-based assay using ACAT1- and ACAT2-expressing CHO cells. These compounds share a similar polycyclic skeleton. Among them, flavasperone and sterigmatocystin, having an angular skeleton, showed selective inhibition toward ACAT2 isozyme, while the others having a linear one had no selectivity in inhibition.

Selectivity of pyripyropene derivatives in inhibition toward acyl-CoA:cholesterol acyltransferase 2 isozyme

Selectivity of 96 semisynthetic derivatives prepared from fungal pyripyropene A, originally isolated as a potent inhibitor of acyl-CoA:cholesterol acyltransferase (ACAT), toward ACAT1 and ACAT2 isozymes was investigated in the cell-based assay using ACAT1- and ACAT2-expressing CHO cells. Eighteen derivatives including PR-71 (7-O-isocaproyl derivative) showed much more potent ACAT2 inhibition (IC50: 6.0 to 62 nM) than pyripyropene A (IC50: 70 nM). Among them, however, natural pyripyropene A showed the highest selectivity toward ACAT2 with a selectivity index (SI) of >1000, followed by PR-71 (SI, 667).

An N-terminal fragment of mouse DGAT1 binds different acyl-CoAs with varying affinity

A histidine-tagged recombinant N-terminal fragment of type-1 mouse liver diacylglycerol acyltransferase (DGAT; EC 2.3.1.20), MmDGAT1(1-95)His6, was expressed in Escherichia coli, and used to investigate possible acyl-CoA-binding properties. Analysis of the purified fragment by MALDI-TOF mass spectrometry revealed a polypeptide with molecular mass of about 11 kDa which was consistent with the calculated molecular mass based on the deduced amino acid sequence. Lipidex-1000 binding assays indicated that MmDGAT1(1-95)His(6) interacted with long chain fatty acyl-CoAs similar to observations on DGAT1 from oilseed rape (Brassica napus). Binding, as a function of acyl-CoA concentration, differed for palmitoyl (16:0), stearoyl (18:0), and erucoyl (cisDelta(13)22:1)-CoA. Binding of stearoyl- or erucoyl-CoA to MmDGAT1(1-95)His(6) as a function of acyl-CoA concentration, however, was sigmoid and displayed positive cooperativity suggesting that MmDGAT1 may be subject to allosteric modulation by acyl-CoAs. An intra-polypeptide segment within the N-terminal region of MmDGAT1 contained remnants of an acyl-CoA-binding signature initially identified in plant DGAT1. The acyl-CoA-binding site in mammalian DGAT1 could represent a potential target for therapeutic interventions for disorders such as type-2 diabetes and obesity.

Microsomal triglyceride transfer protein enhances cellular cholesteryl esterification by relieving product inhibition

Cholesteryl ester synthesis by the acyl-CoA:cholesterol acyltransferase enzymes ACAT1 and ACAT2 is, in part, a cellular homeostatic mechanism to avoid toxicity associated with high free cholesterol levels. In hepatocytes and enterocytes, cholesteryl esters are secreted as part of apoB lipoproteins, the assembly of which is critically dependent on microsomal triglyceride transfer protein (MTP). Conditional genetic ablation of MTP reduces cholesteryl esters and enhances free cholesterol in the liver and intestine without diminishing ACAT1 and ACAT2 mRNA levels. As expected, increases in hepatic free cholesterol are associated with decreases in 3-hydroxy-3-methylglutaryl-CoA reductase and increases in ATP-binding cassette transporter 1 mRNA levels. Chemical inhibition of MTP also decreases esterification of cholesterol in Caco-2 and HepG2 cells. Conversely, coexpression of MTP and apoB in AC29 cells stably transfected with ACAT1 and ACAT2 increases cholesteryl ester synthesis. Liver and enterocyte microsomes from MTP-deficient animals synthesize lesser amounts of cholesteryl esters in vitro, but addition of purified MTP and low density lipoprotein corrects this deficiency. Enrichment of microsomes with cholesteryl esters also inhibits cholesterol ester synthesis. Thus, MTP enhances cellular cholesterol esterification by removing cholesteryl esters from their site of synthesis and depositing them into nascent apoB lipoproteins. Therefore, MTP plays a novel role in regulating cholesteryl ester biosynthesis in cells that produce lipoproteins. We speculate that non-lipoprotein-producing cells may use different mechanisms to alleviate product inhibition and modulate cholesteryl ester biosynthesis.

Downregulation of hepatic lipoprotein assembly in rats by fermented products of Monascus pilosus

OBJECTIVE

Hypercholesterolemia is a major risk factor for atherosclerosis. The fermented products of Monascus sp. have been known for their antihypercholesterolemic effect; however, the studies mostly have focused on the inhibition of cholesterol biosynthesis in liver. In this study, we examined whether fermented products of Monascus pilosus have regulatory effects on the hepatic lipoprotein assembly.

METHODS

Male Wistar rats were fed 1% cholesterol diet for 2 wk. After hypercholesterolemia was induced, the animals were maintained on this cholesterol diet supplemented with M. pilosus-fermented products grown in regular medium/garlic-containing medium or garlic powder for another 6 wk. The concentration of blood lipids and the expression of proteins involved in lipoprotein assembly and hepatic antioxidation were assayed.

RESULTS

Maintenance on a 1% cholesterol diet for 2 wk significantly (P < 0.05) raised animals' blood lipid levels and increased the expression of intestinal microsomal triacylglycerol transfer protein, hepatic apolipoprotein B-100, and acyl-coenzyme A:cholesterol acyltransferase. Supplementation of M. pilosus-fermented products or garlic powder significantly (P < 0.05) lowered animals' blood lipid levels and inhibited the expression of intestinal microsomal triacylglycerol transfer protein and hepatic apolipoprotein B-100. The 3-hydroxy-3-methylglutaryl-coenzyme A reductase was downregulated by the M. pilosus-fermented product grown in regular medium but not in garlic-containing medium. The expression of antioxidant enzymes was significantly upregulated by the M. pilosus-fermented product grown in garlic-containing medium.

CONCLUSION

Monascus sp.-fermented products exert the hypocholesterolemic effect by mechanisms other than the inhibition of cholesterol biosynthesis.

Nonalcoholic fatty liver disease: emerging mechanisms and consequences

PURPOSE OF REVIEW

One of the critical complications of obesity and diabetes is nonalcoholic fatty liver disease, a disorder of triacylglycerol accumulation in the liver that has potential to develop into end stage liver failure. In this review, the recent progress in understanding the role of hepatic triacylglycerol synthesis in the development of nonalcoholic fatty liver disease is discussed.

RECENT FINDINGS

It has become apparent that the development of hepatic steatosis is a complex, multifactorial process. Although the molecular pathways underlying its development have been described, there are no established therapies for nonalcoholic fatty liver disease. Recently, however, DGAT1 and DGAT2, the enzymes responsible for the final step in triacylglycerol synthesis, have been characterized as playing a vital role in hepatic triacylglycerol metabolism. Cellular and murine models in which diacylglycerol acyltransferase expression is altered suggest that these enzymes may play a role in the development hepatic steatosis, are feasible targets in the treatment of nonalcoholic fatty liver disease, but also function as lipotoxic buffers.

SUMMARY

Hepatic steatosis remains the watershed event in the progression of nonalcoholic fatty liver disease. The diacylglycerol acyltransferases are emerging as important mediators of hepatic triacylglycerol accumulation. Therefore, these enzymes are attractive targets in the development of therapies to prevent liver triacylglycerol accumulation and the consequences of nonalcoholic fatty liver disease.

Knockdown of acyl-CoA:diacylglycerol acyltransferase 2 with antisense oligonucleotide reduces VLDL TG and ApoB secretion in mice

Acyl-CoA:diacylglycerol acyltransferases (DGATs) are enzymes that catalyze the formation of triglyceride (TG) from acyl-CoA and diacylglycerol. Two DGATs have been identified which belong to two distinct gene families and both are ubiquitously expressed. DGAT2 knockout mice are lipopenic and die shortly after birth. In the current study, wild type mice were treated with increasing doses (25-60 mg/kg twice weekly) of a DGAT2 gene-specific antisense oligonucleotide (ASO). Treatment resulted in a dose dependent decrease in hepatic DGAT2 gene expression (up to 80%) which was associated with a 40% decrease in hepatic DGAT2 activity and a 45% decrease in hepatic TG. Decreased levels of DGAT2 resulted in a significant dose dependent decrease in VLDL TG secretion (up to 52%) and reduced plasma TG, total cholesterol, and ApoB. Similar results were obtained when DGAT1 KO mice were treated with the DGAT2 ASO. Treatment of ob/ob mice with the DGAT2 ASO resulted in significant decreases in weight gain (10%), adipose weight (25%) and hepatic TG content (80%). Our findings indicate that the majority of TG destined for secretion by liver is synthesized by DGAT2 and suggests that DGAT2 may be a therapeutic target for treatment of hypertriglyceridemia, hepatic steatosis and obesity.

Discovery and combinatorial synthesis of fungal metabolites beauveriolides, novel antiatherosclerotic agents

For discovery of a new type of antiatherosclerotic agents, a cell-based assay of lipid droplet accumulation using primary mouse peritoneal macrophages was conducted as a model of macrophage-derived foam cell accumulation, which occurs in the early stage of atherosclerogenesis. During the screening of microbial metabolites for inhibitors of lipid droplet accumulation, 13-membered cyclodepsipeptides, known beauveriolide I and new beauveriolide III, were isolated from the culture broth of fungal Beauveria sp. FO-6979, a soil isolate, by solvent extraction, ODS column chromatography, silica gel column chromatography, and preparative HPLC. The structure including the absolute stereochemistry of beauveriolide III was elucidated as cyclo-[(3 S,4 S)-3-hydroxy-4-methyloctanoyl- l-phenylalanyl- l-alanyl- d-alloisoleucyl] by spectral analyses, amino acid analyses, and synthetic methods. Furthermore, the absolute stereochemistry was confirmed by the total synthesis of beauveriolides. Study on the mechanism of action revealed that beauveriolides inhibited macrophage acyl-CoA:cholesterol acyltransferase (ACAT) activity to block the synthesis of cholesteryl ester (CE), leading to a reduction of lipid droplets in macrophages. There are two ACAT isozymes in mammals, ACAT1 and ACAT2. ACAT1 is ubiquitously expressed in most tissues and cells including macrophages, while ACAT2 is expressed predominantly in the liver (hepatocytes) and the intestine (enterocytes). Interestingly, beauveriolides inhibited both ACAT1 and ACAT2 to a similar extent in an enzyme assay that utilized microsomes but inhibited ACAT1 selectively in intact cell-based assays. Beauveriolides proved orally active in both low-density lipoprotein receptor and apolipoprotein E knockout mice, reducing the atheroma lesion of heart and aorta without any side effects such as diarrhea or cytotoxicity to adrenal tissues as observed for many synthetic ACAT inhibitors. To obtain more potent inhibitors, a focused library of beauveriolide analogues was prepared by combinatorial chemistry in which solid-phase assembly of linear depsipeptides was carried out using a 2-chlorotrityl linker, followed by solution-phase cyclization, yielding 104 beauveriolide analogues. Among them, diphenyl derivatives were found to show 10 times more potent inhibition of CE synthesis in macrophages than beauveriolide III. Furthermore, most analogues showed selective ACAT1 inhibition or inhibition of both ACAT1 and ACAT2, but interestingly certain analogues gave selective ACAT2 inhibition. These data indicated that subtle structural differences of the inhibitors could discriminate the active sites of the ACAT1 and ACAT2 isozymes. Efforts of further analogue synthesis would make it possible to obtain highly selective ACAT1/ACAT2 inhibitors.

Esculeogenin A, a new tomato sapogenol, ameliorates hyperlipidemia and atherosclerosis in ApoE-deficient mice by inhibiting ACAT

OBJECTIVE

We recently identified esculeoside A, a new spirosolane-type glycoside, with a content in tomatoes that is 4-fold higher than that of lycopene. In the present study, we examined the effects of esculeoside A and esculeogenin A, a new aglycon of esculeoside A, on foam cell formation in vitro and atherogenesis in apoE-deficient mice.

METHODS AND RESULTS

Esculeogenin A significantly inhibited the accumulation of cholesterol ester (CE) induced by acetylated low density lipoprotein (acetyl-LDL) in human monocyte-derived macrophages (HMDM) in a dose-dependent manner without inhibiting triglyceride accumulation, however, it did not inhibit the association of acetyl-LDL to the cells. Esculeogenin A also inhibited CE formation in Chinese hamster ovary cells overexpressing acyl-coenzymeA (CoA): cholesterol acyl-transferase (ACAT)-1 or ACAT-2, suggesting that esculeogenin A suppresses the activity of both ACAT-1 and ACAT-2. Furthermore, esculeogenin A prevented the expression of ACAT-1 protein, whereas that of SR-A and SR-BI was not suppressed. Oral administration of esculeoside A to apoE-deficient mice significantly reduced the levels of serum cholesterol, triglycerides, LDL-cholesterol, and the areas of atherosclerotic lesions without any detectable side effects.

CONCLUSIONS

Our study provides the first evidence that purified esculeogenin A significantly suppresses the activity of ACAT protein and leads to reduction of atherogenesis.

Potential therapeutics for obesity and atherosclerosis: Inhibitors of neutral lipid metabolism from microorganisms

Diacylglycerol acyltransferase (DGAT) and acyl-CoA: cholesterol acyltransferase (ACAT) are the enzymes that catalyze the final reactions of triacylgycerol (TG) and cholesteryl ester (CE) synthesis, and accumulation of TG and CE in adipocytes and arteries causes obesity and atherosclerosis, respectively. Therefore, DGAT and ACAT have been viewed as potential therapeutic targets for these diseases. From the screening program for DGAT inhibitors, new compounds were discovered from fungal and plant extracts, and are expected to provide leads for drug development. From the screening programs for ACAT inhibitors and lipid droplet synthesis inhibitors, new compounds with chemical structures different from those of known synthetic inhibitors were discovered from the cultures of fungal and actinomycete strains. Among them, fungal beauveriolide III rather selectively inhibited ACAT1 isozyme, while fungal pyripyropene A was found to be a highly selective inhibitor of ACAT2 isozyme. Both inhibitors proved orally active in in vivo models. Furthermore, a library of beauveriolide and pyripyropene analogs was prepared by combinatorial and semisynthetic methods, respectively. The future prospects of these inhibitors are discussed.

Upregulation of myocellular DGAT1 augments triglyceride synthesis in skeletal muscle and protects against fat-induced insulin resistance

Increased fat deposition in skeletal muscle is associated with insulin resistance. However, exercise increases both intramyocellular fat stores and insulin sensitivity, a phenomenon referred to as "the athlete's paradox". In this study, we provide evidence that augmenting triglyceride synthesis in skeletal muscle is intrinsically connected with increased insulin sensitivity. Exercise increased diacylglycerol (DAG) acyltransferase (DGAT) activity in skeletal muscle. Channeling fatty acid substrates into TG resulted in decreased DAG and ceramide levels. Transgenic overexpression of DGAT1 in mouse skeletal muscle replicated these findings and protected mice against high-fat diet-induced insulin resistance. Moreover, in isolated muscle, DGAT1 deficiency exacerbated insulin resistance caused by fatty acids, whereas DGAT1 overexpression mitigated the detrimental effect of fatty acids. The heightened insulin sensitivity in the transgenic mice was associated with attenuated fat-induced activation of DAG-responsive PKCs and the stress mediator JNK1. Consistent with these changes, serine phosphorylation of insulin receptor substrate 1 was reduced, and Akt activation and glucose 4 membrane translocation were increased. In conclusion, upregulation of DGAT1 in skeletal muscle is sufficient to recreate the athlete's paradox and illustrates a mechanism of exercise-induced enhancement of muscle insulin sensitivity. Thus, increasing muscle DGAT activity may offer a new approach to prevent and treat insulin resistance and type 2 diabetes mellitus.

A selective ACAT-1 inhibitor, K-604, suppresses fatty streak lesions in fat-fed hamsters without affecting plasma cholesterol levels

BACKGROUND

Acyl-coenzyme A:cholesterol O-acyltransferase-1 (ACAT-1), a major ACAT isozyme in macrophages, plays an essential role in foam cell formation in atherosclerotic lesions. However, whether pharmacological inhibition of macrophage ACAT-1 causes exacerbation or suppression of atherosclerosis is controversial.

METHODS AND RESULTS

We developed and characterized a novel ACAT inhibitor, K-604. The IC(50) values of K-604 for human ACAT-1 and ACAT-2 were 0.45 and 102.85 micromol/L, respectively, indicating that K-604 is 229-fold more selective for ACAT-1. Kinetic analysis indicated that the inhibition was competitive with respect to oleoyl-coenzyme A with a K(i) value of 0.378 micromol/L. Exposure of human monocyte-derived macrophages to K-604 inhibited cholesterol esterification with IC(50) of 68.0 nmol/L. Furthermore, cholesterol efflux from THP-1 macrophages to HDL(3) or apolipoprotein A-I was enhanced by K-604. Interestingly, administration of K-604 to F1B hamsters on a high-fat diet at a dose of >or=1mg/kg suppressed fatty streak lesions without affecting plasma cholesterol levels.

CONCLUSIONS

K-604, a potent and selective inhibitor of ACAT-1, suppressed the development of atherosclerosis in an animal model without affecting plasma cholesterol levels, providing direct evidence that pharmacological inhibition of ACAT-1 in the arterial walls leads to suppression of atherosclerosis.

Regulation of triglyceride metabolism. I. Eukaryotic neutral lipid synthesis: "Many ways to skin ACAT or a DGAT"

Esterification of sterols, fatty acids and other alcohols into biologically inert forms conserves lipid resources for many cellular functions. Paradoxically, the accumulation of neutral lipids such as cholesteryl ester or triglyceride, is linked to several major disease pathologies. In a remarkable example of genetic expansion, there are at least eleven acyltransferase reactions that lead to neutral lipid production. In this review, we speculate that the complexity and apparent redundancy of neutral lipid synthesis may actually hasten rather than impede the development of novel, isoform-specific, therapeutic interventions for acne, type 2 diabetes, obesity, hyperlipidemia, fatty liver disease, and atherosclerosis.

Monounsaturated fatty acyl-coenzyme A is predictive of atherosclerosis in human apoB-100 transgenic, LDLr-/- mice

ACAT2, the enzyme responsible for the formation of cholesteryl esters incorporated into apolipoprotein B-containing lipoproteins by the small intestine and liver, forms predominantly cholesteryl oleate from acyl-CoA and free cholesterol. The accumulation of cholesteryl oleate in plasma lipoproteins has been found to be predictive of atherosclerosis. Accordingly, a method was developed in which fatty acyl-CoA subspecies could be extracted from mouse liver and quantified. Analyses were performed on liver tissue from mice fed one of four diets enriched with one particular type of dietary fatty acid: saturated, monounsaturated, n-3 polyunsaturated, or n-6 polyunsaturated. We found that the hepatic fatty acyl-CoA pools reflected the fatty acid composition of the diet fed. The highest percentage of fatty acyl-CoAs across all diet groups was in monoacyl-CoAs, and values were 36% and 46% for the n-3 and n-6 polyunsaturated diet groups and 55% and 62% in the saturated and monounsaturated diet groups, respectively. The percentage of hepatic acyl-CoA as oleoyl-CoA was also highly correlated to liver cholesteryl ester, plasma cholesterol, LDL molecular weight, and atherosclerosis extent. These data suggest that replacing monounsaturated with polyunsaturated fat can benefit coronary heart disease by reducing the availability of oleoyl-CoA in the substrate pool of hepatic ACAT2, thereby reducing cholesteryl oleate secretion and accumulation in plasma lipoproteins.

Synthesis and biological evaluation of unprecedented classes of spiro-beta-lactams and azido-beta-lactams as acyl-CoA:cholesterol acyltransferase inhibitors

Unprecedented classes of four- and five-membered hydroxyl-spiro-beta-lactams and hydroxyl-azido-beta-lactams were prepared via regioselective ring opening of hydroxyl-epoxides. The potential of these particular beta-lactams as biologically active compounds has been confirmed by the results obtained in ACAT inhibition assays.

Selectivity of microbial acyl-CoA: cholesterol acyltransferase inhibitors toward isozymes

The selectivity of microbial inhibitors of acyl-CoA: cholesterol acyltransferase (ACAT) toward the two isozymes, ACAT1 and ACAT2, was assessed in cell-based assays. Purpactin A (IC50 values of ACAT1 vs. IC50 values of ACAT2; 2.5 microM vs. 1.5 microM), terpendole C (10 microM vs. 10 microM), glisoprenin A (4.3 microM vs. 10 microM), spylidone (25 microM vs. 5.0 microM) and synthetic CL-283,546 (0.1 microM vs. 0.09 microM) inhibited ACAT1 and ACAT2 to similar extents. Beauveriolides I (0.6 microM vs. 20 microM) and III (0.9 microM vs. >20 microM) inhibited ACAT1 rather selectively, while pyripyropenes A (>80 microM vs. 0.07 microM), B (48 microM vs. 2.0 microM), C (32 microM vs. 0.36 microM) and D (38 microM vs. 1.5 microM) showed selective inhibition against ACAT2. In particular, pyripyropene A was found to be the most selective ACAT2 inhibitor with a selective index of more than 1,000.

Cholesterol absorption inhibitors as a therapeutic option for hypercholesterolaemia

The development of cholesterol-lowering drugs (including a variety of statins, bile acid-binding resins and recently discovered inhibitors of cholesterol absorption) has expanded the options for cardiovascular prevention. Recent treatment guidelines emphasise that individuals at substantial risk for atherosclerotic coronary heart disease should meet defined targets for LDL cholesterol concentrations. Combination therapy with drugs that have different or complementary mechanisms of action is often needed to achieve lipid goals. Existing approaches to the treatment of hypercholesterolaemia are still ineffective in halting the progression of coronary artery disease in some patients despite combination therapies. Other patients are resistant to conventional drug treatment and remain at high risk for the development and progression of atherosclerotic cardiovascular disease and alternative approaches are needed. The discovery and development of ezetimibe (a novel, selective and potent cholesterol absorption inhibitor) has advanced the treatment of hypercholesterolaemia. New agents including the phytostanol preparation FM-VP4 and inhibitors of acyl coenzyme A:cholesterol acyltransferase, the apical Na(+)-dependent bile acid transporter and microsomal triglyceride transfer protein may also play a future role in combination therapy. This review focuses on the recent progress in the molecular mechanisms of intestinal cholesterol absorption and transport, and novel therapeutic approaches to inhibit the cholesterol absorption process.

Tung tree DGAT1 and DGAT2 have nonredundant functions in triacylglycerol biosynthesis and are localized to different subdomains of the endoplasmic reticulum

Seeds of the tung tree (Vernicia fordii) produce large quantities of triacylglycerols (TAGs) containing approximately 80% eleostearic acid, an unusual conjugated fatty acid. We present a comparative analysis of the genetic, functional, and cellular properties of tung type 1 and type 2 diacylglycerol acyltransferases (DGAT1 and DGAT2), two unrelated enzymes that catalyze the committed step in TAG biosynthesis. We show that both enzymes are encoded by single genes and that DGAT1 is expressed at similar levels in various organs, whereas DGAT2 is strongly induced in developing seeds at the onset of oil biosynthesis. Expression of DGAT1 and DGAT2 in yeast produced different types and proportions of TAGs containing eleostearic acid, with DGAT2 possessing an enhanced propensity for the synthesis of trieleostearin, the main component of tung oil. Both DGAT1 and DGAT2 are located in distinct, dynamic regions of the endoplasmic reticulum (ER), and surprisingly, these regions do not overlap. Furthermore, although both DGAT1 and DGAT2 contain a similar C-terminal pentapeptide ER retrieval motif, this motif alone is not sufficient for their localization to specific regions of the ER. These data suggest that DGAT1 and DGAT2 have nonredundant functions in plants and that the production of storage oils, including those containing unusual fatty acids, occurs in distinct ER subdomains.

Direct Effect of F12511, A Systemic Inhibitor of Acyl-CoA Cholesterol Acyltransferase on Bovine Aortic Endothelial Cells

F12511(S)-2',3',5'-trimethyl-4'-hydroxy-alpha-dodecylthio-alpha-phenylacetanilide (F12511) is a new Acyl-CoA cholesterol acyltransferase (ACAT) inhibitor that not only reduces the plasma cholesterol levels but also has anti-atherosclerotic actions in animals models. The study's aim was to analyze if F12511 may directly modify the ability of tumor necrosis factor--alpha (TNF-alpha)-incubated bovine aortic endothelial cells (BAEC) to express endothelial nitric oxide synthase (eNOS) protein and inflammatory-related proteins such as platelet endothelial cell adhesion molecule (PECAM) and CD40 ligand (CD40L). The addition of increasing concentrations of F12511 (10 to 10 mol/L) failed to modify the level of eNOS protein expressed in control BAEC. TNF-alpha (10 ng/mL) reduced the expression of eNOS protein. In TNF-alpha--incubated BAEC, F12511 protected eNOS expression in a concentration-dependent manner. TNF-alpha stimulated the expression of both CD40L and PECAM in cultured BAEC. F12511 (10 mol/L) failed to modify the expression of CD40L and PECAM in control and TNF-alpha-incubated BAEC. Reverse transcriptase polymerase chain reaction showed a marked expression of the ACAT-2 isoform and absent of expression of the ACAT-1 isoform in BAEC. The presence of ACAT-2 isoform in BAEC was further confirmed by Western blot. F12511 failed to modify the expression of the proinflammatory associated proteins PECAM and CD40L in the endothelium but protected eNOS expression in the endothelial cells exposed to inflammatory conditions.

Absolute Stereochemistry of Fungal Beauveriolide III and ACAT Inhibitory Activity of Four Stereoisomers

Fungal beauveriolide III (BeauIII, 1b), a cyclodepsipeptide inhibiting acyl-CoA:cholesterol acyltransferase (ACAT) and showing antiatherogenic activity in mouse models, consists of L-Phe, L-Ala, D-allo-Ile, and 3-hydroxy-4-methyloctanoic acid (HMA) moieties, but the stereochemistry of the HMA part has not until now been fully defined. To determine it, four HMA stereoisomers were synthesized and labeled with (S)-(+)-2-(anthracene-2,3-dicarboximido)-1-propyl trifluoromethane sulfonate (AP-OTf), a chiral fluorescent reagent. The derivatives were separated by HPLC and compared with the natural HMA derivative, which was thereby identified as (3S,4S)HMA in BeauIII. Furthermore, the four beauveriolide III isomers ((3S,4S)BeauIII (23a), (3R,4R)BeauIII (23b), (3R,4S)BeauIII (23c), and (3S,4R)BeauIII (23d)) were synthesized, and it was shown that all the spectral data for 23a were identical with those for natural 1b. Isomers 23a and 23d showed potent inhibitory activity of lipid droplet accumulation in macrophages, while the other two isomers caused weak inhibition. Thus, the 3S configuration of BeauIII is important for this activity. Furthermore, 23a and 23d showed rather specific inhibition against the ACAT1 isozyme.

Importance of acyl-coenzyme A:cholesterol acyltransferase 1/2 dual inhibition for anti-atherosclerotic potency of pactimibe

Pactimibe sulfate, [7-(2,2-dimethylpropanamido)-4,6-dimethyl-1-octylindolin-5-yl]acetic acid hemisulfate, a novel Acyl-coenzyme A:cholesterol acyltransferase (ACAT) inhibitor, was investigated in vitro and in vivo to characterize its potential. Pactimibe exhibited dual inhibition for ACAT1 and ACAT2 (concentrations inhibiting 50% [IC50s] at micromolar levels) more potently than avasimibe. Kinetic analysis revealed pactimibe is a noncompetitive inhibitor of oleoyl-CoA (Ki value: 5.6 microM). Furthermore, pactimibe markedly inhibited cholesteryl ester formation (IC50: 6.7 microM) in human monocyte-derived macrophages, and inhibited copper-induced oxidation of low density lipoprotein more potently than probucol. Pactimibe exerted potent lipid-lowering and anti-atherosclerotic effects in atherogenic diet-fed hamsters. At doses of 3 and 10 mg/kg for 90 days, pactimibe decreased serum total cholesterol by 70% and 72%, and aortic fatty streak area by 79% and 95%, respectively. Despite similar cholesterol lowering, fatty streak area reduction was greater by 10 mg/kg. These results suggest that ACAT1/2 dual inhibitor pactimibe has anti-atherosclerotic potential beyond its plasma cholesterol-lowering activity.

A critical role for the histidine residues in the catalytic function of acyl-CoA:cholesterol acyltransferase catalysis: evidence for catalytic difference between ACAT1 and ACAT2

To investigate a role for histidine residues in the expression of normal acyl-CoA:cholesterol acyltransferase (ACAT) activity, the histidine residues located at five different positions in two isoenzymes were substituted by alanine, based on the sequence homology between ACAT1 and ACAT2. Among the 10 mutants generated by baculovirus expression technology, H386A-ACAT1, H460A-ACAT1, H360A-ACAT2, and H399A-ACAT2 lost their enzymatic activity completely. A reduction in catalytic activity is unlikely to result from structural changes in the substrate-binding pocket, because their substrate-binding affinities were normal. However, the enzymatic activity of H386A-ACAT1 was restored to <37% of the level of the wild-type activity when cholesterol was replaced by 25-hydroxycholesterol as substrate. H527A-ACAT1 and H501A-ACAT2, termed carboxyl end mutants, exhibit activities of approximately 96% and approximately 75% of that of the wild-type. Interestingly, H425A-ACAT1 showed 59% of the wild-type activity, in contrast to its equivalent mutant, H399A-ACAT2. These results demonstrate that the histidine residues located at the active site are very crucial both for the catalytic activity of the enzyme and for distinguishing ACAT1 from ACAT2 with respect to enzyme catalysis and substrate specificity.

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.

Molecular Target of Decursins in the Inhibition of Lipid Droplet Accumulation in Macrophages

During screening for inhibitors of lipid droplet accumulation in mouse peritoneal macrophages, two coumarins identified as decursin and decursinol angelate were isolated from the roots of Angelicae gigantis. The cellular molecular target of these inhibitors in macrophages was studied. Decursin and decursinol angelate inhibited cholesteryl ester (CE) synthesis with IC50 values of 9.7 and 10.1 microM, respectively, whereas they enhanced triacylglycerol (TG) synthesis. Lysosomal metabolism of cholesterol to CE was inhibited by the compounds, indicating that the site of inhibition is one of the steps between the exiting of cholesterol from the lysosomes and CE synthesis in the endoplasmic reticulum. Therefore, acyl-CoA:cholesterol acyltransferase (ACAT) activity in the microsomal fractions prepared from mouse macrophages was studied, and the results showed inhibition of this activity by decursin and decursinol angelate with IC50 values of 43 and 22 microM, respectively. Thus, it was concluded that the compounds inhibit macrophage ACAT activity to decrease CE synthesis, leading to a reduction of lipid droplets in macrophages.

Whole-body insulin resistance in the absence of obesity in FVB mice with overexpression of Dgat1 in adipose tissue

Insulin resistance is often associated with obesity. We tested whether augmentation of triglyceride synthesis in adipose tissue by transgenic overexpression of the diacylglycerol aclytransferase-1 (Dgat1) gene causes obesity and/or alters insulin sensitivity. Male FVB mice expressing the aP2-Dgat1 had threefold more Dgat1 mRNA and twofold greater DGAT activity levels in adipose tissue. After 30 weeks of age, these mice had hyperglycemia, hyperinsulinemia, and glucose intolerance on a high-fat diet but were not more obese than wild-type littermates. Compared with control littermates, Dgat1 transgenic mice were both insulin and leptin resistant and had markedly elevated plasma free fatty acid levels. Adipocytes from Dgat1 transgenic mice displayed increased basal and isoproterenol-stimulated lipolysis rates and decreased gene expression for fatty acid uptake. Muscle triglyceride content was unaffected, but liver mass and triglyceride content were increased by 20 and 300%, respectively. Hepatic insulin signaling was suppressed, as evidenced by decreased phosphorylation of insulin receptor-beta (Tyr(1,131)/Tyr(1,146)) and protein kinase B (Ser473). Gene expression data suggest that the gluconeogenic enzymes, glucose-6-phosphatase and phosphoenolpyruvate carboxykinase, were upregulated. Thus, adipose overexpression of Dgat1 gene in FVB mice leads to diet-inducible insulin resistance, which is secondary to redistribution of fat from adipose tissue to the liver in the absence of obesity.

Acyl-CoA: cholesterol acyltransferase-2 gene polymorphisms and their association with plasma lipids and coronary artery disease risks

Acyl-CoA: cholesterol acyltransferase-2 (ACAT2), an intracellular cholesterol esterification enzyme found only in the intestine and liver, has been demonstrated to be associated with hypercholesterolemia and atherosclerosis in mice. To explore the possible impact of ACAT2 gene variants on CAD susceptibility and plasma lipid levels, three polymorphisms, 41A>G (Glu>Gly), 734C>T (Thr>Ile), and IVS4-57_58 ins48 bp (D/I), were genotyped in 809 CAD patients (CAD+) and 1,304 controls (CAD-) from three distinct Singaporean ethnic groups (1,228 Chinese, 367 Malays and 518 Indians). The 734T allele frequency was significantly lower in CAD+ (0.20) than CAD- (0.26) in Chinese (P=0.003) and I allele of D/I was significantly higher in CAD+ (0.17) than CAD- (0.10) in Indians (P=0.011). The 41G allele was significantly more frequent among normolipidemic (0.19) than dyslipidemic (0.13) individuals in Chinese (P=0.008). In normolipidemic females, 734C>T was associated with apoA1, apoB and lipoprotein (a) in Indians, and with apoA1 in Malays, whereas 41A>G is associated with total cholesterol in Indians. The 734C>T polymorphism was in almost complete linkage disequilibrium (LD) with the IVS4-57_58 ins48 bp and in very strong LD with 41A>G in all the three ethnic groups. In the normolipidemic females, the AG/CT had much higher apoB than AA/CC in Indians. We found that the three ACAT2 polymorphisms studied are associated with CAD risk and plasma lipid levels but their effects are not consistent across genders and ethnic groups.

Two human ACAT2 mRNA variants produced by alternative splicing and coding for novel isoenzymes

Acyl coenzyme A:cholesterol acyltransferase 2 (ACAT2) plays an important role in cholesterol absorption. Human ACAT2 is highly expressed in small intestine and fetal liver, but its expression is greatly diminished in adult liver. The full-length human ACAT2 mRNA encodes a protein, designated ACAT2a, with 522 amino acids. We have previously reported the organization of the human ACAT2 gene and the differentiation-dependent promoter activity in intestinal Caco-2 cells. In the current work, two human ACAT2 mRNA variants produced by alternative splicing are cloned and predicted to encode two novel ACAT2 isoforms, named ACAT2b and ACAT2c, with 502 and 379 amino acids, respectively. These mRNA variants differ from ACAT2a mRNA by lack of the exon 4 (ACAT2b mRNA) and exons 4-5 plus 8-9-10 (ACAT2c mRNA). Significantly, comparable amounts of the alternatively spliced ACAT2 mRNA variants were detected by RT-PCR, and Western blot analysis confirmed the presence of their corresponding proteins in human liver and intestinecells. Furthermore, phosphorylation and enzymatic activity analyses demonstrated that the novel isoenzymes ACAT2b and ACAT2c lacked the phosphorylatable site SLLD, and their enzymatic activities reduced to 25%-35% of that of ACAT2a. These evidences indicate that alternative splicing produces two human ACAT2 mRNA variants that encode the novel ACAT2 isoenzymes. Our findings might help to understand the regulation of the ACAT2 gene expression under certain physiological and pathological conditions.

Cholesteryl esters in malignancy

Cholesteryl esters, formed by the esterification of cholesterol with long-chain fatty acids, on one hand, are the means by which cholesterol is transported through the blood by lipoproteins, on the other, the way cholesterol itself can be accumulated in the cells. Therefore, these important molecules play an active part in metabolic pathways that form the basis of cholesterol trafficking and homeostasis. The role of different regulatory mechanisms in cholesterol homeostasis in physiologic and neoplastic conditions with emphasis on intracellular content of cholesteryl esters is here reviewed. Numerous studies carried out on tumor cell lines, experimental tumors, and human tumors have shown an abnormal cholesterol metabolism that is reflected by an increase in intracellular cholesteryl esters due to an alteration in all the mechanisms that form the basis of regulation, in particular: cholesterol de novo biosynthesis; uptake of exogenous cholesterol LDL receptor mediated; cholesterol esterification mediated by the ACAT activity; cholesterol efflux HDL receptor mediated. The most recent analytic-spectroscopic applications that permit cholesteryl ester determination on tumor lipidic extracts and directly in vivo are also reported. This review gives an overview of cholesterol homeostasis in physiological and pathological conditions where cholesteryl esters are over-expressed.

Investigating the allosterism of acyl-CoA:cholesterol acyltransferase (ACAT) by using various sterols: in vitro and intact cell studies

ACAT1 (acyl-CoA:cholesterol acyltransferase 1) is thought to have two distinct sterol-binding sites: a substrate-binding site and an allosteric-activator site. In the present work, we investigated the structural features of various sterols as substrates and/or activators in vitro. The results show that without cholesterol, the plant sterol sitosterol is a poor substrate for ACAT. In the presence of cholesterol, ACAT1-mediated esterification of sitosterol is highly activated while ACAT2-mediated esterification of sitosterol is only moderately activated. For ACAT1, we show that the stereochemistry of the 3-hydroxy group at steroid ring A is a critical structural feature for a sterol to serve as a substrate, but less critical for activation. Additionally, enantiomeric cholesterol, which has the same biophysical properties as cholesterol in membranes, fails to activate ACAT1. Thus ACAT1 activation by cholesterol is the result of stereo-specific interactions between cholesterol and ACAT1, and is not related to the biophysical properties of phospholipid membranes. To demonstrate the relevance of the ACAT1 allosteric model in intact cells, we showed that sitosterol esterification in human macrophages is activated upon cholesterol loading. We further show that the activation is not due to an increase in ACAT1 protein content, but is partly due to an increase in the cholesterol content in the endoplasmic reticulum where ACAT1 resides. Together, our results support the existence of a distinct sterol-activator site in addition to the sterol-substrate site of ACAT1 and demonstrate the applicability of the ACAT1 allosteric model in intact cells.

Acyl coenzyme A dependent retinol esterification by acyl coenzyme A: diacylglycerol acyltransferase 1

We provide biochemical evidence that enzymes involved in the synthesis of triacylglycerol, namely acyl coenzyme A:diacylglycerol acyltransferase (DGAT) and acyl coenzyme A:monoacylglycerol acyltransferase (MGAT), are capable of carrying out the acyl coenzyme A:retinol acyltransferase (ARAT) reaction. Among them, DGAT1 appears to have the highest specific activity. The apparent K(m) values of recombinant DGAT1/ARAT for retinol and palmitoyl coenzyme A were determined to be 25.9+/-2.1 microM and 13.9+/-0.3 microM, respectively, both of which are similar to the values previously determined for ARAT in native tissues. A novel selective DGAT1 inhibitor, XP620, inhibits recombinant DGAT1/ARAT at the retinol recognition site. In the differentiated Caco-2 cell membranes, XP620 inhibits approximately 85% of the Caco-2/ARAT activity indicating that DGAT1/ARAT may be the major source of ARAT activity in these cells. Of the two most abundant fatty acyl retinyl esters present in the intact differentiated Caco-2 cells, XP620 selectively inhibits retinyl-oleate formation without influencing the retinyl-palmitate formation. Using this inhibitor, we estimate that approximately 64% of total retinyl ester formation occurs via DGAT1/ARAT. These studies suggest that DGAT1/ARAT is the major enzyme involved in retinyl ester synthesis in Caco-2 cells.

Gene expression profiling of the preclinical scrapie-infected hippocampus

The molecular events that underlie prion disease neuropathology remain poorly defined. Within the hippocampus of the ME7/CV mouse scrapie model, profound CA1 neuronal loss occurs between 160 and 180 days post-infection (dpi). To elucidate the molecular events that may contribute to this neuronal loss, we have applied Affymetrix high-density oligonucleotide probe arrays to the study of ME7-infected hippocampal gene expression at 170 dpi. The study has identified 78 genes that are differentially expressed greater than 1.5-fold within the preclinical ME7-infected hippocampus prior to the profound late stage glial cell activation. The results indicate oxidative and endoplasmic reticulum (ER) stress, activated ER and mitochondrial apoptosis pathways, and activated cholesterol biosynthesis within the scrapie-infected hippocampus, and offer insight into the molecular events which underlie the neuropathology.

Sterol homeostasis in the budding yeast, Saccharomyces cerevisiae

Lipid related diseases, such as obesity, type 2 diabetes, and atherosclerosis are epidemics in developed civilizations. A common underlying factor among these syndromes is excessive subcellular accumulation of lipids such as cholesterol and triglyceride. The homeostatic events that govern these metabolites are understood to varying degrees of sophistication. We describe here the utilization of a genetically powerful model organism, budding yeast, to identify and characterize novel aspects of sterol and lipid homeostasis.

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.

Increased Very Low Density Lipoprotein Secretion and Gonadal Fat Mass in Mice Overexpressing Liver DGAT1

Acyl-CoA:diacylglycerol acyltransferases (DGATs) catalyze the last step in triglyceride (TG) synthesis. The genes for two DGAT enzymes, DGAT1 and DGAT2, have been identified. To examine the roles of liver DGAT1 and DGAT2 in TG synthesis and very low density lipoprotein (VLDL) secretion, liver DGAT1- and DGAT2-overexpressing mice were created by adenovirus-mediated gene transfection. DGAT1-overexpressing mice had markedly increased DGAT activity in the presence of the permeabilizing agent alamethicin. This suggests that DGAT1 possesses latent DGAT activity on the lumen of the endoplasmic reticulum. DGAT1-overexpressing mice showed increased VLDL secretion, resulting in increased gonadal (epididymal or parametrial) fat mass but not subcutaneous fat mass. The VLDL-mediated increase in gonadal fat mass might be due to the 4-fold greater expression of the VLDL receptor protein in gonadal fat than in subcutaneous fat. DGAT2-overexpressing mice had increased liver TG content, but VLDL secretion was not affected. These results indicate that DGAT1 but not DGAT2 has a role in VLDL synthesis and that increased plasma VLDL concentrations may promote obesity, whereas increased DGAT2 activity has a role in steatosis.

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.

Reduced ABCA1-Mediated Cholesterol Efflux and Accelerated Atherosclerosis in Apolipoprotein E–Deficient Mice Lacking Macrophage-Derived ACAT1

Background— Macrophage acyl-coenzyme A:cholesterol acyltransferase 1 (ACAT1) and apolipoprotein E (apoE) have been implicated in regulating cellular cholesterol homeostasis and therefore play critical roles in foam cell formation. Deletion of either ACAT1 or apoE results in increased atherosclerosis in hyperlipidemic mice, possibly as a consequence of altered cholesterol processing. We have studied the effect of macrophage ACAT1 deletion on atherogenesis in apoE-deficient (apoE −/− ) mice with or without the restoration of macrophage apoE.

Methods and Results— We used bone marrow transplantation to generate apoE −/− mice with macrophages of 4 genotypes: apoE +/+ /ACAT1 +/+ (wild type), apoE +/+ /ACAT1 −/− (ACAT −/− ), apoE −/− /ACAT1 +/+ (apoE −/− ), and apoE −/− /ACAT1 −/− (2KO). When macrophage apoE was present, plasma cholesterol levels normalized, and ACAT1 deficiency did not have significant effects on atherogenesis. However, when macrophage apoE was absent, ACAT1 deficiency increased atherosclerosis and apoptosis in the proximal aorta. Cholesterol efflux to apoA-I was significantly reduced (30% to 40%; P <0.001) in ACAT1 −/− peritoneal macrophages compared with ACAT1 +/+ controls regardless of apoE expression. 2KO macrophages had a 3- to 4-fold increase in ABCA1 message levels but decreased ABCA1 protein levels relative to ACAT1 +/+ macrophages. Microarray analyses of ACAT1 −/− macrophages showed increases in proinflammatory and procollagen genes and decreases in genes regulating membrane integrity, protein biosynthesis, and apoptosis.

Conclusions— Deficiency of macrophage ACAT1 accelerates atherosclerosis in hypercholesterolemic apoE −/− mice but has no effect when the hypercholesterolemia is corrected by macrophage apoE expression. However, ACAT1 deletion impairs ABCA1-mediated cholesterol efflux in macrophages regardless of apoE expression. Changes in membrane stability, susceptibility to apoptosis, and inflammatory response may also be important in this process.

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.

The triacylglycerol synthesis enzyme DGAT1 also catalyzes the synthesis of diacylglycerols, waxes, and retinyl esters

The final step of triacylglycerol biosynthesis is catalyzed by acyl CoA:diacylglycerol acyltransferase (DGAT) enzymes. The two known DGATs, DGAT1 and DGAT2, are encoded by unrelated genes. Although both DGAT1 and DGAT2 knockout mice have reduced tissue triacylglycerol contents, they have disparate phenotypes, prompting us to investigate whether the two enzymes have unrecognized functional differences. We now report that DGAT1 exhibits additional acyltransferase activities in vitro, including those of acyl CoA:monoacylglycerol acyltransferase (MGAT), wax monoester and wax diester synthases, and acyl CoA:retinol acyltransferase (ARAT), which catalyze the synthesis of diacylglycerols, wax esters, and retinyl esters, respectively. These activities were demonstrated in in vitro assays with membranes from insect cells or homogenates from COS7 cells overexpressing DGAT1. Wax synthase and ARAT activities were also demonstrated in intact COS7 cells expressing DGAT1. Additionally, cells and tissues from DGAT1-deficient mice exhibited reduced ARAT activity, and the mice had increased levels of unesterified retinol in their livers on a high-retinol diet. Our findings indicate that DGAT1 can utilize a variety of acyl acceptors as substrates in vitro and suggest that these activities may be relevant to the in vivo functions of DGAT1.

Isolation and characterization of an Arabidopsis thaliana knockout line for phospholipid: diacylglycerol transacylase gene (At5g13640)

To more fully understand the function of phospholipid: diacylglycerol acyltransferase (PDAT, EC 2.3.1.158) in plants we have isolated and characterized a knockout mutant of Arabidopsis thaliana L. which has a T-DNA insertion in PDAT locus At5g13640. Lipid analysis was conducted on these plants to assess the contribution of the PDAT gene to lipid composition. The fatty acid content and composition in seeds do not show significant changes in the mutant. This is a contrary situation to yeast where PDAT is a major contributor to triacylglycerol (TAG) accumulation in exponential growth phase. The results indicate that PDAT activity encoded by At5g13640 is not a major determinant of TAG synthesis in Arabidopsis seeds.

Age-related alteration in hepatic acyl-CoA: cholesterol acyltransferase and its relation to LDL receptor and MAPK

The aim of this study was to evaluate changes in the regulation of lipid metabolism and mitogen-activated protein kinases (MAPK) in the liver of C57BL/6 mice as they age. This was done by assessing the status of total cholesterol content and its enzyme, acyl-CoA: cholesterol acyltransferase (ACAT), in liver microsomal preparations and the low-density lipoprotein receptor (LDLr) mRNA expression in the livers of 4-24-month-old C57B/6 mice, without exogenous cholesterol feeding. With aging, there was an increase in cholesterol content and ACAT activity in liver microsomes. Northern blot analysis and real-time quantitative polymerase chain reaction data showed that ACAT-2 mRNA increased with age as well. LDLr expression decreased significantly in an age-dependent manner. In addition, we studied the basal and activated forms of MAPK, e.g. extracellular regulatory kinase (ERK-1/2), c-jun NH2-terminal kinase (JNK-1/2) and p38 MAPK. During aging, there was a considerable decrease in phosphorylated ERK-1/2 level while JNK-1/2 and p38 MAPK levels increased with age. Our studies showed an altered LDLr expression and altered phosphorylated MAPK in the liver of C57BL/6 mice during aging. These alterations might contribute to the development of atherosclerosis, hypercholesterolemia and other cholesterol-related conditions.

HMG-CoA reductase inhibition reverses LCAT and LDL receptor deficiencies and improves HDL in rats with chronic renal failure

Dyslipidemia is a prominent feature of chronic renal failure (CRF) and a major risk factor for atherosclerosis and the progression of renal disease. CRF-induced dyslipidemia is marked by hypertriglyceridemia and a shift in plasma cholesterol from HDL to the ApoB-containing lipoproteins. Several studies have demonstrated a favorable response to administration of 3-hydroxy-3-methylglutaryl (HMG)-CoA reductase inhibitors (statins) in CRF. This study was intended to explore the effect of statin therapy on key enzymes and receptors involved in cholesterol metabolism. Accordingly, CRF ( nephrectomized) and sham-operated rats were randomized to untreated and statin-treated (rosuvastatin 20 mg·kg−1·day−1) groups and observed for 6 wk. The untreated CRF rats exhibited increased total cholesterol-to-HDL cholesterol ratio, diminished plasma lecithin:cholesterol acyltransferase (LCAT) and the hepatic LDL receptor, elevated hepatic acyl-CoA:cholesterol acyltransferase (ACAT), and no change in hepatic HMG-CoA reductase, cholesterol 7α-hydroxylase, or HDL receptor (SRB-1). Statin administration lowered HMG-CoA reductase activity, normalized plasma LCAT, total cholesterol-to-HDL cholesterol ratio, and hepatic LDL receptor but did not significantly change either plasma total cholesterol, hepatic cholesterol 7α-hydroxylase, total ACAT activity, or SRB-1 in the CRF animals. Statin administration to the normal control rats led to significant increases in plasma LCAT and hepatic LDL receptor, significant reductions of total cholesterol-to-HDL cholesterol ratio, hepatic HMG-CoA reductase activity, and cholesterol 7α-hydroxylase abundance with virtually no change in plasma cholesterol concentration. Thus administration of rosuvastatin reversed LCAT and LDL receptor deficiencies and promoted a shift in plasma cholesterol from ApoB-containing lipoproteins to HDL in CRF rats.

Cholesterol at the crossroads: Alzheimer's disease and lipid metabolism

Alzheimer's Disease (AD) is a devastating disease that affects millions of elderly persons. Despite years of intense investigations, genetic risk factors that affect the majority of AD cases have yet to be determined. Recent studies suggest that cholesterol metabolism has integral part in AD pathogenesis, suggesting that genes that regulate lipid metabolism may also play roles in AD. This review will first describe emerging evidence that links cholesterol to the mechanisms thought to underlie AD. Based on this rationale, candidate genes located in regions implicated in AD that have roles in lipid metabolism will then be discussed.