Tissue expression studies on the mouse acyl-CoA: cholesterol acyltransferase gene (Acact): findings supporting the existence of multiple cholesterol esterification enzymes in mice

Stealth Liposomes Encapsulating a Potent ACAT1/SOAT1 Inhibitor F12511: Pharmacokinetic, Biodistribution, and Toxicity Studies in Wild-Type Mice and Efficacy Studies in Triple Transgenic Alzheimer's Disease Mice

Cholesterol is essential for cellular function and is stored as cholesteryl esters (CEs). CEs biosynthesis is catalyzed by the enzymes acyl-CoA:cholesterol acyltransferase 1 and 2 (ACAT1 and ACAT2), with ACAT1 being the primary isoenzyme in most cells in humans. In Alzheimer's Disease, CEs accumulate in vulnerable brain regions. Therefore, ACATs may be promising targets for treating AD. F12511 is a high-affinity ACAT1 inhibitor that has passed phase 1 safety tests for antiatherosclerosis. Previously, we developed a nanoparticle system to encapsulate a large concentration of F12511 into a stealth liposome (DSPE-PEG2000 with phosphatidylcholine). Here, we injected the nanoparticle encapsulated F12511 (nanoparticle F) intravenously (IV) in wild-type mice and performed an HPLC/MS/MS analysis and ACAT enzyme activity measurement. The results demonstrated that F12511 was present within the mouse brain after a single IV but did not overaccumulate in the brain or other tissues after repeated IVs. A histological examination showed that F12511 did not cause overt neurological or systemic toxicity. We then showed that a 2-week IV delivery of nanoparticle F to aging 3xTg AD mice ameliorated amyloidopathy, reduced hyperphosphorylated tau and nonphosphorylated tau, and reduced neuroinflammation. This work lays the foundation for nanoparticle F to be used as a possible therapy for AD and other neurodegenerative diseases.

Inhibition of ACAT as a Therapeutic Target for Alzheimer's Disease Is Independent of ApoE4 Lipidation

APOE4, encoding apolipoprotein E4 (apoE4), is the greatest genetic risk factor for Alzheimer's disease (AD), compared to the common APOE3. While the mechanism(s) underlying APOE4-induced AD risk remains unclear, increasing the lipidation of apoE4 is an important therapeutic target as apoE4-lipoproteins are poorly lipidated compared to apoE3-lipoproteins. ACAT (acyl-CoA: cholesterol-acyltransferase) catalyzes the formation of intracellular cholesteryl-ester droplets, reducing the intracellular free cholesterol (FC) pool. Thus, inhibiting ACAT increases the FC pool and facilitates lipid secretion to extracellular apoE-containing lipoproteins. Previous studies using commercial ACAT inhibitors, including avasimibe (AVAS), as well as ACAT-knock out (KO) mice, exhibit reduced AD-like pathology and amyloid precursor protein (APP) processing in familial AD (FAD)-transgenic (Tg) mice. However, the effects of AVAS with human apoE4 remain unknown. In vitro, AVAS induced apoE efflux at concentrations of AVAS measured in the brains of treated mice. AVAS treatment of male E4FAD-Tg mice (5xFAD+/-APOE4+/+) at 6-8 months had no effect on plasma cholesterol levels or distribution, the original mechanism for AVAS treatment of CVD. In the CNS, AVAS reduced intracellular lipid droplets, indirectly demonstrating target engagement. Surrogate efficacy was demonstrated by an increase in Morris water maze measures of memory and postsynaptic protein levels. Amyloid-beta peptide (Aβ) solubility/deposition and neuroinflammation were reduced, critical components of APOE4-modulated pathology. However, there was no increase in apoE4 levels or apoE4 lipidation, while amyloidogenic and non-amyloidogenic processing of APP were significantly reduced. This suggests that the AVAS-induced reduction in Aβ via reduced APP processing was sufficient to reduce AD pathology, as apoE4-lipoproteins remained poorly lipidated.

Pharmacological inhibition of acyl-coenzyme A:cholesterol acyltransferase alleviates obesity and insulin resistance in diet-induced obese mice by regulating food intake

BACKGROUND/OBJECTIVES

Acyl-coenzyme A:cholesterol acyltransferases (ACATs) catalyze the formation of cholesteryl ester (CE) from free cholesterol to regulate intracellular cholesterol homeostasis. Despite the well-documented role of ACATs in hypercholesterolemia and their emerging role in cancer and Alzheimer's disease, the role of ACATs in adipose lipid metabolism and obesity is poorly understood. Herein, we investigated the therapeutic potential of pharmacological inhibition of ACATs in obesity.

METHODS

We administrated avasimibe, an ACAT inhibitor, or vehicle to high-fat diet-induced obese (DIO) mice via intraperitoneal injection and evaluated adiposity, food intake, energy expenditure, and glucose homeostasis. Moreover, we examined the effect of avasimibe on the expressions of the genes in adipogenesis, lipogenesis, inflammation and adipose pathology in adipose tissue by real-time PCR. We also performed a pair feeding study to determine the mechanism for body weight lowering effect of avasimibe.

RESULTS

Avasimibe treatment markedly decreased body weight, body fat content and food intake with increased energy expenditure in DIO mice. Avasimibe treatment significantly lowered blood levels of glucose and insulin, and improved glucose tolerance in obese mice. The beneficial effects of avasimibe were associated with lower levels of adipocyte-specific genes in adipose tissue and the suppression of food intake. Using a pair-feeding study, we further demonstrated that avasimibe-promoted weight loss is attributed mainly to the reduction of food intake.

CONCLUSIONS

These results indicate that avasimibe ameliorates obesity and its-related insulin resistance in DIO mice through, at least in part, suppression of food intake.

Regulation of intestinal lipid metabolism: current concepts and relevance to disease

Lipids entering the gastrointestinal tract include dietary lipids (triacylglycerols, cholesteryl esters and phospholipids) and endogenous lipids from bile (phospholipids and cholesterol) and from shed intestinal epithelial cells (enterocytes). Here, we comprehensively review the digestion, uptake and intracellular re-synthesis of intestinal lipids as well as their packaging into pre-chylomicrons in the endoplasmic reticulum, their modification in the Golgi apparatus and the exocytosis of the chylomicrons into the lamina propria and subsequently to lymph. We also discuss other fates of intestinal lipids, including intestinal HDL and VLDL secretion, cytosolic lipid droplets and fatty acid oxidation. In addition, we highlight the applicability of these findings to human disease and the development of therapeutics targeting lipid metabolism. Finally, we explore the emerging role of the gut microbiota in modulating intestinal lipid metabolism and outline key questions for future research.

ATR-101, a Selective and Potent Inhibitor of Acyl-CoA Acyltransferase 1, Induces Apoptosis in H295R Adrenocortical Cells and in the Adrenal Cortex of Dogs

ATR-101 is a novel, oral drug candidate currently in development for the treatment of adrenocortical cancer. ATR-101 is a selective and potent inhibitor of acyl-coenzyme A:cholesterol O-acyltransferase 1 (ACAT1), an enzyme located in the endoplasmic reticulum (ER) membrane that catalyzes esterification of intracellular free cholesterol (FC). We aimed to identify mechanisms by which ATR-101 induces adrenocortical cell death. In H295R human adrenocortical carcinoma cells, ATR-101 decreases the formation of cholesteryl esters and increases FC levels, demonstrating potent inhibition of ACAT1 activity. Caspase-3/7 levels and terminal deoxynucleotidyl transferase 2'-deoxyuridine 5'-triphosphate nick end labeled-positive cells are increased by ATR-101 treatment, indicating activation of apoptosis. Exogenous cholesterol markedly potentiates the activity of ATR-101, suggesting that excess FC that cannot be adequately esterified increases caspase-3/7 activation and subsequent cell death. Inhibition of calcium release from the ER or the subsequent uptake of calcium by mitochondria reverses apoptosis induced by ATR-101. ATR-101 also activates multiple components of the unfolded protein response, an indicator of ER stress. Targeted knockdown of ACAT1 in an adrenocortical cell line mimicked the effects of ATR-101, suggesting that ACAT1 mediates the cytotoxic effects of ATR-101. Finally, in vivo treatment of dogs with ATR-101 decreased adrenocortical steroid production and induced cellular apoptosis that was restricted to the adrenal cortex. Together, these studies demonstrate that inhibition of ACAT1 by ATR-101 increases FC, resulting in dysregulation of ER calcium stores that result in ER stress, the unfolded protein response, and ultimately apoptosis.

Genetics of gallstone disease

Gallstone disease (GSD) is one of the most common biliary tract disorders worldwide. The prevalence, however, varies from 5.9-21.9% in Western society to 3.1-10.7% in Asia. Most gallstones (75%) are silent. Approximately half of symptomatic gallstone carriers experience a second episode of biliary pain within 1 year. These individuals are at increased risk of developing acute cholecystitis, acute cholangitis, and biliary pancreatitis. As can be expected, these complications burden health care systems because of their invasive nature and surgical cost. Factors that contribute to gallstone formation include supersaturation of cholesterol in bile, gallbladder hypomotility, destabilization of bile by kinetic protein factors, and abnormal mucins. Epidemiologic studies have implicated multiple environmental factors and some common genetic elements in gallstone formation. Genetic factors that influence gallstone formation have been elaborated from linkage studies of twins, families, and ethnicities. Accumulating evidence suggests that genetic factors play a role in GSD.

ACAT-selective and Nonselective DGAT1 Inhibition

Acyl-coenzyme A: cholesterol O-Acyltransferase (ACAT) and Acyl-coenzyme A: diacylglycerol O-acyltransferase (DGAT) enzymes play important roles in synthesizing neutral lipids, and inhibitors of these enzymes have been investigated as potential treatments for diabetes and other metabolic diseases. Administration of a Acyl-coenzyme A: diacylglycerol O-acyltransferase 1 (DGAT1) inhibitor with very limited cellular selectivity over ACAT resulted in significant adrenocortical degenerative changes in dogs. These changes included macrosteatotic vacuolation associated with adrenocyte cell death in the zonae glomerulosa and fasciculata and minimal to substantial mixed inflammatory cell infiltration and were similar to those described previously for some ACAT inhibitors in dogs. In the mouse, similar but only transient adrenocortical degenerative changes were seen as well as a distinctive nondegenerative reduction in cortical fine vacuolation. In the marmoset, only the distinctive nondegenerative reduction in cortical fine vacuolation was observed, suggesting that the dog, followed by the mouse, is the most sensitive species for cortical degeneration. Biochemical analysis of adrenal cholesterol and cholesteryl ester indicated that the distinctive reduction in cortical fine vacuolation correlated with a significant reduction in cholesteryl ester in the mouse and marmoset, whereas no significant reduction in cholestryl ester, but an increase in free cholesterol was observed in dogs. Administration of a DGAT1 inhibitor with markedly improved selectivity over ACAT to the marmoset and the mouse resulted in no adrenal pathology at exposures sufficient to cause substantial DGAT1 but not ACAT inhibition, thereby implicating ACAT rather than DGAT1 inhibition as the probable cause of the observed adrenal changes. Recognizing that the distinctive nondegenerative reduction in cortical fine vacuolation in the mouse could be used as a histopathological biomarker for an in vivo model of the more severe changes observed in dogs, the mouse has subsequently been used as a model to select DGAT1 inhibitors free of adrenocortical toxicity.

ACAT1 gene ablation increases 24(S)-hydroxycholesterol content in the brain and ameliorates amyloid pathology in mice with AD

Cholesterol metabolism has been implicated in the pathogenesis of several neurodegenerative diseases, including the abnormal accumulation of amyloid-beta, one of the pathological hallmarks of Alzheimer disease (AD). Acyl-CoA:cholesterol acyltransferases (ACAT1 and ACAT2) are two enzymes that convert free cholesterol to cholesteryl esters. ACAT inhibitors have recently emerged as promising drug candidates for AD therapy. However, how ACAT inhibitors act in the brain has so far remained unclear. Here we show that ACAT1 is the major functional isoenzyme in the mouse brain. ACAT1 gene ablation (A1-) in triple transgenic (i.e., 3XTg-AD) mice leads to more than 60% reduction in full-length human APPswe as well as its proteolytic fragments, and ameliorates cognitive deficits. At 4 months of age, A1- causes a 32% content increase in 24-hydroxycholesterol (24SOH), the major oxysterol in the brain. It also causes a 65% protein content decrease in HMG-CoA reductase (HMGR) and a 28% decrease in sterol synthesis rate in AD mouse brains. In hippocampal neurons, A1- causes an increase in the 24SOH synthesis rate; treating hippocampal neuronal cells with 24SOH causes rapid declines in hAPP and in HMGR protein levels. A model is provided to explain our findings: in neurons, A1- causes increases in cholesterol and 24SOH contents in the endoplasmic reticulum, which cause reductions in hAPP and HMGR protein contents and lead to amelioration of amyloid pathology. Our study supports the potential of ACAT1 as a therapeutic target for treating certain forms of AD.

Retinol Esterification by DGAT1 Is Essential for Retinoid Homeostasis in Murine Skin

Retinoic acid (RA) is a potent signaling molecule that is essential for many biological processes, and its levels are tightly regulated by mechanisms that are only partially understood. The synthesis of RA from its precursor retinol (vitamin A) is an important regulatory mechanism. Therefore, the esterification of retinol with fatty acyl moieties to generate retinyl esters, the main storage form of retinol, may also regulate RA levels. Here we show that the neutral lipid synthesis enzyme acyl-CoA:diacylglycerol acyltransferase 1 (DGAT1) functions as the major acyl-CoA:retinol acyltransferase (ARAT) in murine skin. When dietary retinol is abundant, DGAT1 deficiency results in elevated levels of RA in skin and cyclical hair loss; both are prevented by dietary retinol deprivation. Further, DGAT1-deficient skin exhibits enhanced sensitivity to topically administered retinol. Deletion of the enzyme specifically in the epidermis causes alopecia, indicating that the regulation of RA homeostasis by DGAT1 is autonomous in the epidermis. These findings show that DGAT1 functions as an ARAT in the skin, where it acts to maintain retinoid homeostasis and prevent retinoid toxicity. Our findings may have implications for human skin or hair disorders treated with agents that modulate RA signaling.

Expression levels of ACAT1 and ACAT2 genes in the liver and intestine of baboons with high and low lipemic responses to dietary lipids

Acyl-coenzyme A:cholesterol acyltransferase (ACAT) 1 and ACAT2 play an important role in cellular cholesterol esterification and thus modulate intestinal cholesterol absorption and hepatic lipoprotein secretion. The relative expression levels of ACAT1 and ACAT2 in human tissues differ from those in other animals, including nonhuman primates. The present study compared the relative expression levels of ACAT1 and ACAT2 in baboons with high and low lipemic responses to dietary lipids. We isolated RNA and prepared cDNA from frozen liver and small intestine from high- and low-responding pedigreed baboons necropsied after consuming a high-cholesterol and high-fat diet for 18 months. The expression of ACAT1 and ACAT2 was measured by TaqMan real-time quantitative PCR normalized to 18s ribosomal RNA. The expression of ACAT1 was higher than that of ACAT2 in the liver, whereas the expression of ACAT2 was higher than that of ACAT1 in the duodenum and jejunum. There was no difference in the expression of ACAT1 or ACAT2 in the liver and intestine between high- and low-responding baboons except that the expression of ACAT1 was higher in the duodenum of high responders than in that of low responders. Western blot analysis also showed a higher level of ACAT1 protein in the duodenum of high responders than in that of low responders. There was a significant correlation between duodenal ACAT expression levels and total plasma cholesterol concentration in baboons. These results suggest that differences in ACAT1 expression may affect plasma cholesterol concentration and partly affect diet-induced hyperlipidemia.

Potential role of acyl-coenzyme A:cholesterol transferase (ACAT) Inhibitors as hypolipidemic and antiatherosclerosis drugs

Acyl-coenzyme A:cholesterol transferase (ACAT) is an integral membrane protein localized in the endoplasmic reticulum. ACAT catalyzes the formation of cholesteryl esters from cholesterol and fatty acyl coenzyme A. The cholesteryl esters are stored as cytoplasmic lipid droplets inside the cell. This process is very important to the organism as high cholesterol levels have been associated with cardiovascular disease. In mammals, two ACAT genes have been identified, ACAT1 and ACAT2. ACAT1 is ubiquitous and is responsible for cholesteryl ester formation in brain, adrenal glands, macrophages, and kidneys. ACAT2 is expressed in the liver and intestine. The inhibition of ACAT activity has been associated with decreased plasma cholesterol levels by suppressing cholesterol absorption and by diminishing the assembly and secretion of apolipoprotein B-containing lipoproteins such as very low density lipoprotein (VLDL). ACAT inhibition also prevents the conversion of macrophages into foam cells in the arterial walls, a critical event in the development of atherosclerosis. This review paper will focus on the role of ACAT in cholesterol metabolism, in particular as a target to develop novel therapeutic agents to control hypercholesterolemia, atherosclerosis, and Alzheimer's disease.

A human skin multifunctional O-acyltransferase that catalyzes the synthesis of acylglycerols, waxes, and retinyl esters

Acyl-CoA-dependent O-acyltransferases catalyze reactions in which fatty acyl-CoAs are joined to acyl acceptors containing free hydroxyl groups to produce neutral lipids. In this report, we characterize a human multifunctional O-acyltransferase (designated MFAT) that belongs to the acyl-CoA:diacylglycerol acyltransferase 2/acyl-CoA:monoacylglycerol acyltransferase (MGAT) gene family and is highly expressed in the skin. Membranes of insect cells and homogenates of mammalian cells overexpressing MFAT exhibited significantly increased MGAT, acyl-CoA:fatty acyl alcohol acyltransferase (wax synthase), and acyl-CoA:retinol acyltransferase (ARAT) activities, which catalyze the synthesis of diacylglycerols, wax monoesters, and retinyl esters, respectively. Furthermore, when provided with the appropriate substrates, intact mammalian cells overexpressing MFAT accumulated more waxes and retinyl esters than control cells. We conclude that MFAT is a multifunctional acyltransferase that likely plays an important role in lipid metabolism in human skin.

Acyl-coenzyme A:cholesterol acyltransferase promotes oxidized LDL/oxysterol-induced apoptosis in macrophages

7-Ketocholesterol (7KC) is a cytotoxic component of oxidized low density lipoproteins (OxLDLs) and induces apoptosis in macrophages by a mechanism involving the activation of cytosolic phospholipase A2 (cPLA2). In the current study, we examined the role of ACAT in 7KC-induced and OxLDL-induced apoptosis in murine macrophages. An ACAT inhibitor, Sandoz 58-035, suppressed 7KC-induced apoptosis in P388D1 cells and both 7KC-induced and OxLDL-induced apoptosis in mouse peritoneal macrophages (MPMs). Furthermore, compared with wild-type MPMs, ACAT-1-deficient MPMs demonstrated significant resistance to both 7KC-induced and OxLDL-induced apoptosis. Macrophages treated with 7KC accumulated ACAT-derived [14C]cholesteryl and [3H]7-ketocholesteryl esters. Tandem LC-MS revealed that the 7KC esters contained primarily saturated and monounsaturated fatty acids. An inhibitor of cPLA2, arachidonyl trifluoromethyl ketone, prevented the accumulation of 7KC esters and inhibited 7KC-induced apoptosis in P388D1 cells. The decrease in 7KC ester accumulation produced by the inhibition of cPLA2 was reversed by supplementing with either oleic or arachidonic acid (AA); however, only AA supplementation restored the induction of apoptosis by 7KC. These results suggest that 7KC not only initiates the apoptosis pathway by activating cPLA2, as we have reported previously, but also participates in the downstream signaling pathway when esterified by ACAT to form 7KC-arachidonate.

Regulation of Macrophage Cholesterol Efflux through Hydroxymethylglutaryl-CoA Reductase Inhibition

The cholesterol biosynthetic pathway produces numerous signaling molecules. Oxysterols through liver X receptor (LXR) activation regulate cholesterol efflux, whereas the non-sterol mevalonate metabolite, geranylgeranyl pyrophosphate (GGPP), was recently demonstrated to inhibit ABCA1 expression directly, through antagonism of LXR and indirectly through enhanced RhoA geranylgeranylation. We used HMG-CoA reductase inhibitors (statins) to test the hypothesis that reduced synthesis of mevalonate metabolites would enhance cholesterol efflux and attenuate foam cell formation. Preincubation of THP-1 macrophages with atorvastatin, dose dependently (1-10 microm) stimulated cholesterol efflux to apolipoprotein AI (apoAI, 10-60%, p < 0.05) and high density lipoprotein (HDL(3)) (2-50%, p < 0.05), despite a significant decrease in cholesterol synthesis (2-90%). Atorvastatin also increased ABCA1 and ABCG1 mRNA abundance (30 and 35%, p < 0.05). Addition of mevalonate, GGPP or farnesyl pyrophosphate completely blocked the statin-induced increase in ABCA1 expression and apoAI-mediated cholesterol efflux. A role for RhoA was established, because two inhibitors of Rho protein activity, a geranylgeranyl transferase inhibitor and C3 exoenzyme, increased cholesterol efflux to apoAI (20-35%, p < 0.05), and macrophage expression of dominant-negative RhoA enhanced cholesterol efflux to apoAI (20%, p < 0.05). In addition, atorvastatin increased the RhoA levels in the cytosol fraction and decreased the membrane localization of RhoA. Atorvastatin treatment activated peroxisome proliferator activated receptor gamma and increased LXR-mediated gene expression suggesting that atorvastatin induces cholesterol efflux through a molecular cascade involving inhibition of RhoA signaling, leading to increased peroxisome proliferator activated receptor gamma activity, enhanced LXR activation, increased ABCA1 expression, and cholesterol efflux. Finally, statin treatment inhibited cholesteryl ester accumulation in macrophages challenged with atherogenic hypertriglyceridemic very low density lipoproteins indicating that statins can regulate foam cell formation.

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.

pX Gene Causes Hypercholesterolemia in Hypercholesterolemia-Resistant BALB/c Mice

To investigate the high incidence of atherosclerosis in the patients affected with rheumatoid arthritis, we examined the effect of feeding a cholesterol-enriched diet on the development of hypercholesterolemia in pX transgenic mice, which spontaneously develop chronic inflammatory arthritis. Cholesterol feeding to pX transgenic mice induced a striking elevation in serum total cholesterol (ca. 500 mg/dl) compared with their littermates, BALB/c mice used as controls. The pX transgenic mice exhibited elevated mRNA levels of ACAT1, and ABCG5 in the small intestine compared with their littermates, and furthermore, apoA1, ABCA1, ABCG5, ACAT1, and ACAT2 mRNAs were induced more easily by a cholesterol-enriched diet in pX transgenic mice than their littermates. As ACAT1 mRNA in the small intestine is known not to be induced by feeding a cholesterol-enriched diet, a possibility was inferred that interferon-gamma induced by Tax, a pX gene product, might play an important role in the induction of ACAT1 mRNA and the following hypercholesterolemia. These findings suggest that pX gene plays an important role in inducing hypercholesterolemia in BALB/c mice, which are genetically less susceptible to hypercholesterolemia and atherosclerosis and that RA patients carrying HTLV-1 virus have a predilection for hypercholesterolemia, a main risk factor for cardiovascular diseases.

Evaluation of 3-hydroxy-3-methylglutaryl-COA-reductase, cholesterol-7?-hydroxylase and acyl-COA:cholesterol acyltransferase activities: alternative chromatographic methods to separate metabolites

Alternative HPLC and solid-phase extraction column methods were developed to separate metabolites of enzymes involved in cholesterol metabolism in rabbit liver microsomes: hydroxyl-methylglutaryl-CoA reductase, cholesterol-7alpha-hydroxylase and acyl-CoA:cholesterol acyltransferase. A comparison method of thin-layer chromatography and solid-phase extraction column were assayed to separate substrate and metabolite of hydroxy-methylglutaryl-CoA reductase, whereas for cholesterol-7alpha-hydroxylase and acyl-CoA:cholesterol acyltransferase, this comparison was done between thin layer chromatography and HPLC. The results obtained by the new analytical chromatographic methods are not significantly different than those observed in literature. Moreover a larger percentage recovery was obtained for analysed metabolites. Our results demonstrate the reliability of these alternative chromatographic techniques and showed that they are valuable tools to precisely and rapidly measure the activity of those enzymes.

Lipid lowering activity of drugs affecting cholesterol absorption

AIM

Dietary cholesterol absorption, endogenous cholesterol synthesis and biliary cholesterol excretion regulate whole body cholesterol balance as a result of biotransformation into bile acids or direct cholesterol excretion. Recent studies have significantly advanced our understanding of intestinal sterol absorption at molecular level. This review concentrates on two major issues: the mechanisms of sterol absorption, and the currently available or experimental drugs that affect this pathway.

DATA SYNTHESIS

Nuclear hormone receptors, such as the liver X, farnesoid X and retinoid X receptors, regulate the absorption of dietary sterols by modulating the transcription of several genes involved in cholesterol metabolism, The ABC proteins transport dietary cholesterol from enterocytes back to the intestinal lumen, thus limiting the amount of absorbed cholesterol. By means of the same mechanism, ABC transporters also provide an efficient barrier against the absorption of plant sterols. Phytosterols, bile acid sequestrants, ezetimibe and ACAT inhibitors are possible means of affecting these pathways.

CONCLUSION

In addition to providing an insight into the molecular mechanisms of sterol absorption, these recent findings may lead to new therapeutic options for the treatment of hypercholesterolemia. This is particularly true in the case of patients at high risk of coronary artery disease requiring aggressive lipid-lowering therapy combining a statin with drugs affecting cholesterol absorption in order to ensure the optimal management of dyslipidemia.

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.

Deficiency of acyl CoA:cholesterol acyltransferase 2 prevents atherosclerosis in apolipoprotein E-deficient mice

Deficiency of acyl CoA:cholesterol acyltransferase 2 (ACAT2) in mice results in a reduction in cholesterol ester synthesis in the small intestine and liver, which in turn limits intestinal cholesterol absorption, hepatic cholesterol gallstone formation, and the accumulation of cholesterol esters in the plasma lipoproteins. Here we examined the contribution of ACAT2-derived cholesterol esters to atherosclerosis by crossing ACAT2-deficient (ACAT2(-/-)) mice with apolipoprotein (apo) E-deficient (ApoE(-/-)) mice, an atherosclerosis-susceptible strain that has impaired apoE-mediated clearance of apoB-containing lipoproteins. ACAT2(-/-) ApoE(-/-) mice and ACAT2(+/+) ApoE(-/-) (control) mice had similar elevations of plasma apoB and total plasma lipids; however, the lipid cores of the apoB-containing lipoproteins in ACAT2(-/-) ApoE(-/-) mice contained primarily triglycerides rather than cholesterol esters. At 30 wk of age, only the control mice had significant atherosclerosis, which was nearly absent in ACAT2(-/-) ApoE(-/-) mice. ACAT2 deficiency in the apoE-deficient background also led to a compensatory increase in the activity of lecithincholesterol acyltransferase, the major plasma cholesterol esterification enzyme, which increased high-density lipoprotein cholesterol esters. Our results demonstrate the crucial role of ACAT2-derived cholesterol esters in the development of atherosclerosis in mice and suggest that triglyceride-rich apoB-containing lipoproteins are not as atherogenic as those containing cholesterol esters. Our results also support the rationale of pharmacological inhibition of ACAT2 as a therapy for atherosclerosis.

Genetic background of cholesterol gallstone disease

Cholesterol gallstone formation is a multifactorial process involving a multitude of metabolic pathways. The primary pathogenic factor is hypersecretion of free cholesterol into bile. For people living in the Western Hemisphere, this is almost a normal condition, certainly in the elderly, which explains the very high incidence of gallstone disease. It is probably because the multifactorial background genes responsible for the high incidence have not yet been identified, despite the fact that genetic factors clearly play a role. Analysis of the many pathways involved in biliary cholesterol secretion reveals many potential candidates and considering the progress in unraveling the regulatory mechanisms of the responsible genes, identification of the primary gallstone genes will be successful in the near future.

Fatty acids differentially regulate hepatic cholesteryl ester formation and incorporation into lipoproteins in the liver of the mouse

These experiments tested the hypothesis that fatty acids (FAs) that drive cholesterol esterification also enhance sterol secretion and were undertaken using a mouse model where lipoprotein-cholesterol output by the liver could be assessed in vivo. The turnover of sterol in the animals was kept constant ( approximately 160 mg/d per kg) while the liver was enriched with the single FAs 8:0, 14:0, 18:1, or 18:2. Under these conditions, the steady-state concentration of cholesteryl ester in the liver varied 6-fold, from 1.2 to 7.9 mg/g, and the expansion of this pool was directly related to the specific FA enriching the liver (FA 18:1>18:2>8:0> 14:0). Secretion of lipoprotein-cholesterol varied 5-fold and was a linear function of the concentration of cholesteryl ester in the liver. These studies demonstrate that unsaturated FAs drive the esterification reaction and enhance lipoprotein cholesterol secretion by the liver under conditions where cholesterol balance across this organ is constant. Thus, individual FAs interact with cholesterol to profoundly regulate both the output and uptake of sterol by the liver, and these effects are articulated through the esterification reaction.

DGAT1 is not essential for intestinal triacylglycerol absorption or chylomicron synthesis

Dietary triacylglycerols are a major source of energy for animals. The absorption of dietary triacylglycerols involves their hydrolysis to free fatty acids and monoacylglycerols in the intestinal lumen, the uptake of these products into enterocytes, the resynthesis of triacylgylcerols, and the incorporation of newly synthesized triacylglycerols into nascent chylomicrons for secretion. In enterocytes, the final step in triacylglycerol synthesis is believed to be catalyzed primarily through the actions of acyl-CoA:diacylglycerol acyltransferase (DGAT) enzymes. In this study, we analyzed intestinal triacylglycerol absorption and chylomicron synthesis and secretion in DGAT1-deficient (Dgat1(-/-)) mice. Surprisingly, DGAT1 was not essential for quantitative dietary triacylglycerol absorption, even in mice fed a high fat diet, or for the synthesis of chylomicrons. However, Dgat1(-/-) mice had reduced postabsorptive chylomicronemia (1 h after a high fat challenge) and accumulated neutral-lipid droplets in the cytoplasm of enterocytes when chronically fed a high fat diet. These results suggest a reduced rate of triacylglycerol absorption in Dgat1(-/-) mice. Analysis of intestine from Dgat1(-/-) mice revealed activity for two other enzymes, DGAT2 and diacylglycerol transacylase, that catalyze triacylglycerol synthesis and apparently help to compensate for the absence of DGAT1. Our findings indicate that multiple mechanisms for triacylglycerol synthesis in the intestine facilitate triacylglycerol absorption.

Molecular mechanisms of sterol absorption

Recent studies have significantly advanced our understanding of intestinal sterol absorption at the molecular level. Nuclear hormone receptors (such as liver X receptor, farnesoid X receptor and retinoid X receptor) regulate the absorption of dietary sterols by modulating the transcription of several important genes involved in cholesterol metabolism. One of these genes encodes a molecule [adenosine triphosphate-binding cassette (ABC) transporter] that transports dietary cholesterol from enterocytes back out to the intestinal lumen, thereby limiting the amount of cholesterol absorbed. ABC transporters also provide an efficient barrier against the absorption of plant sterols. Another key process that affects intestinal sterol absorption is the synthesis of cholesterol esters. Mice lacking the enzyme for cholesterol esterification in the small intestine have a reduced capacity to absorb dietary cholesterol and are protected against diet-induced hypercholesterolemia and gallstone formation. In addition to elucidating some of the molecular mechanisms of sterol absorption, these recent findings may lead to new therapeutic options to treat hypercholesterolemia.

Cholesterol esterification by host and parasite is essential for optimal proliferation of Toxoplasma gondii

Upon host cell invasion the apicomplexan parasite Toxoplasma gondii resides in a specialized compartment termed the parasitophorous vacuole that is derived from the host cell membrane but modified by the parasite. Despite the segregation of the parasitophorous vacuole from the host endocytic network, the intravacuolar parasite has been shown to acquire cholesterol from the host cell. In order to characterize further the role of sterol metabolism in T. gondii biology, we focused our studies on the activity of acyl-CoA:cholesterol acyltransferase (ACAT), a key enzyme for maintaining the intracellular homeostasis of cholesterol through the formation of cholesterol esters. In this study, we demonstrate that ACAT and cholesterol esters play a crucial role in the optimal replication of T. gondii. Moreover, we identified ACAT activity in T. gondii that can be modulated by pharmacological ACAT inhibitors with a consequent detrimental effect on parasite replication.

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.

Increased atherosclerosis in LDL receptor-null mice lacking ACAT1 in macrophages

During atherogenesis, circulating macrophages migrate into the subendothelial space, internalize cholesterol-rich lipoproteins, and become foam cells by progressively accumulating cholesterol esters. The inhibition of macrophage acyl coenzyme A:cholesterol acyltransferase (ACAT), which catalyzes the formation of cholesterol esters, has been proposed as a strategy to reduce foam cell formation and to treat atherosclerosis. We show here, however, that hypercholesterolemic LDL receptor-deficient (LDLR(-/-)) mice reconstituted with ACAT1-deficient macrophages unexpectedly develop larger atherosclerotic lesions than control LDLR(-/-) mice. The ACAT1-deficient lesions have reduced macrophage immunostaining and more free cholesterol than control lesions. Our findings suggest that selective inhibition of ACAT1 in lesion macrophages in the setting of hyperlipidemia can lead to the accumulation of free cholesterol in the artery wall, and that this promotes, rather than inhibits, lesion development.

Resistance to diet-induced hypercholesterolemia and gallstone formation in ACAT2-deficient mice

The importance of cholesterol ester synthesis by acyl CoA:cholesterol acyltransferase (ACAT) enzymes in intestinal and hepatic cholesterol metabolism has been unclear. We now demonstrate that ACAT2 is the major ACAT in mouse small intestine and liver, and suggest that ACAT2 deficiency has profound effects on cholesterol metabolism in mice fed a cholesterol-rich diet, including complete resistance to diet-induced hypercholesterolemia and cholesterol gallstone formation. The underlying mechanism involves the lack of cholesterol ester synthesis in the intestine and a resultant reduced capacity to absorb cholesterol. Our results indicate that ACAT2 has an important role in the response to dietary cholesterol, and suggest that ACAT2 inhibition may be a useful strategy for treating hypercholesterolemia or cholesterol gallstones.

Absence of ACAT-1 attenuates atherosclerosis but causes dry eye and cutaneous xanthomatosis in mice with congenital hyperlipidemia

Acyl-CoA:cholesterol acyltransferase (ACAT) catalyzes esterification of cellular cholesterol. To investigate the role of ACAT-1 in atherosclerosis, we have generated ACAT-1 null (ACAT-1-/-) mice. ACAT activities were present in the liver and intestine but were completely absent in adrenal, testes, ovaries, and peritoneal macrophages in our ACAT-1-/- mice. The ACAT-1-/- mice had decreased openings of the eyes because of atrophy of the meibomian glands, a modified form of sebaceous glands normally expressing high ACAT activities. This phenotype is similar to dry eye syndrome in humans. To determine the role of ACAT-1 in atherogenesis, we crossed the ACAT-1-/- mice with mice lacking apolipoprotein (apo) E or the low density lipoprotein receptor (LDLR), hyperlipidemic models susceptible to atherosclerosis. High fat feeding resulted in extensive cutaneous xanthomatosis with loss of hair in both ACAT-1-/-:apo E-/- and ACAT-1-/-:LDLR-/- mice. Free cholesterol content was significantly increased in their skin. Aortic fatty streak lesion size as well as cholesteryl ester content were moderately reduced in both double mutant mice compared with their respective controls. These results indicate that the local inhibition of ACAT activity in tissue macrophages is protective against cholesteryl ester accumulation but causes cutaneous xanthomatosis in mice that lack apo E or LDLR.

Enrichment of acyl coenzyme A:cholesterol O-acyltransferase near trans-golgi network and endocytic recycling compartment

Acyl coenzyme A:cholesterol O-acyltransferase (ACAT) is the enzyme responsible for cholesterol esterification in macrophages leading to foam cell formation. The determination of its localization is a critical step in understanding its regulation by cholesterol. Using immunofluorescence and confocal microscopy, we previously showed that the enzyme colocalized with markers of the endoplasmic reticulum, but in addition, ACAT was found in an unidentified paranuclear site. In the present study, we further define the localization of paranuclear ACAT. First, we found that ACAT does not colocalize with sorting endosomes or late endosomes labeled with fluorescent alpha(2)-macroglobulin. The paranuclear ACAT is close to the endocytic recycling compartment labeled with fluorescent transferrin. We also show that the paranuclear structure containing ACAT is very close to TGN38, a membrane protein of the trans-Golgi network (TGN), but farther from Gos28, a marker of cis, medial, and trans Golgi. After treatment with nocodazole, the central localization of ACAT did not colocalize with markers of the TGN. These data indicate that a significant fraction of ACAT resides in membranes that may be a subcompartment of the endoplasmic reticulum in proximity to the TGN and the endocytic recycling compartment. Because the TGN and the endocytic recycling compartment are engaged in extensive membrane traffic with the plasma membrane, esterification of cholesterol in these membranes may play an important role in macrophage foam cell formation during atherogenesis.

Massive xanthomatosis and altered composition of atherosclerotic lesions in hyperlipidemic mice lacking acyl CoA:cholesterol acyltransferase 1

Inhibitors of acyl CoA:cholesterol acyltransferase (ACAT) have attracted considerable interest as a potential treatment for atherosclerosis. Currently available inhibitors probably act nonselectively against the two known ACATs. One of these enzymes, ACAT1, is highly expressed in macrophages in atherosclerotic lesions, where it contributes to foam-cell formation. In this study, we examined the effects of selective ACAT1 deficiency in two mouse models of atherosclerosis. In the setting of severe hypercholesterolemia caused by deficiency in apoE or the LDL receptor (LDLR), total ACAT1 deficiency led to marked alterations in cholesterol homeostasis and extensive deposition of unesterified cholesterol in the skin and brain. Bone marrow transplantation experiments demonstrated that ACAT1 deficiency in macrophages was sufficient to cause dermal xanthomas in hyperlipidemic LDLR-deficient mice. ACAT1 deficiency did not prevent the development of atherosclerotic lesions in either apoE-deficient or LDLR-deficient mice, despite causing relatively lower serum cholesterol levels. However, the lesions in ACAT1-deficient mice were atypical in composition, with reduced amounts of neutral lipids and a paucity of macrophages in advanced lesions. Although the latter findings may be associated with increased lesion stability, the marked alterations in cholesterol homeostasis indicate that selectively inhibiting ACAT1 in the setting of severe hyperlipidemia may have detrimental consequences.

Localization of human acyl-coenzyme A: cholesterol acyltransferase-1 (ACAT-1) in macrophages and in various tissues

To investigate the distribution of acyl-coenzyme A:cholesterol acyltransferase-1 (ACAT-1) in various human tissues, we examined tissues of autopsy cases immunohistochemically. ACAT-1 was demonstrated in macrophages, antigen-presenting cells, steroid hormone-producing cells, neurons, cardiomyocytes, smooth muscle cells, mesothelial cells, epithelial cells of the urinary tracts, thyroid follicles, renal tubules, pituitary, prostatic, and bronchial glands, alveolar and intestinal epithelial cells, pancreatic acinar cells, and hepatocytes. These findings showed that ACAT-1 is present in a variety of human tissues examined. The immunoreactivities are particularly prominent in the macrophages, steroid hormone-producing cells, followed by hepatocytes, and intestinal epithelia. In cultured human macrophages, immunoelectron microscopy revealed that ACAT-1 was located mainly in the tubular rough endoplasmic reticulum; immunoblot analysis showed that the ACAT-1 protein content did not change with or without cholesterol loading; however, on cholesterol loading, about 30 to 40% of the total immunoreactivity appeared in small-sized vesicles. These vesicles were also enriched in 78-kd glucose-regulated protein (GRP 78), a specific marker for the endoplasmic reticulum. Immunofluorescent microscopy demonstrated extensive colocalization of ACAT-1 and GRP 78 signals in both the tubular and vesicular endoplasmic reticulum before and after cholesterol loading. These results raise the possibility that foam cell formation may activate an endoplasmic reticulum vesiculation process, producing vesicles enriched in the ACAT-1 protein.

Oxysterols: modulators of cholesterol metabolism and other processes

Oxygenated derivatives of cholesterol (oxysterols) present a remarkably diverse profile of biological activities, including effects on sphingolipid metabolism, platelet aggregation, apoptosis, and protein prenylation. The most notable oxysterol activities center around the regulation of cholesterol homeostasis, which appears to be controlled in part by a complex series of interactions of oxysterol ligands with various receptors, such as the oxysterol binding protein, the cellular nucleic acid binding protein, the sterol regulatory element binding protein, the LXR nuclear orphan receptors, and the low-density lipoprotein receptor. Identification of the endogenous oxysterol ligands and elucidation of their enzymatic origins are topics of active investigation. Except for 24, 25-epoxysterols, most oxysterols arise from cholesterol by autoxidation or by specific microsomal or mitochondrial oxidations, usually involving cytochrome P-450 species. Oxysterols are variously metabolized to esters, bile acids, steroid hormones, cholesterol, or other sterols through pathways that may differ according to the type of cell and mode of experimentation (in vitro, in vivo, cell culture). Reliable measurements of oxysterol levels and activities are hampered by low physiological concentrations (approximately 0.01-0.1 microM plasma) relative to cholesterol (approximately 5,000 microM) and by the susceptibility of cholesterol to autoxidation, which produces artifactual oxysterols that may also have potent activities. Reports describing the occurrence and levels of oxysterols in plasma, low-density lipoproteins, various tissues, and food products include many unrealistic data resulting from inattention to autoxidation and to limitations of the analytical methodology. Because of the widespread lack of appreciation for the technical difficulties involved in oxysterol research, a rigorous evaluation of the chromatographic and spectroscopic methods used in the isolation, characterization, and quantitation of oxysterols has been included. This review comprises a detailed and critical assessment of current knowledge regarding the formation, occurrence, metabolism, regulatory properties, and other activities of oxysterols in mammalian systems.

Acyl coenzyme A: cholesterol acyltransferase inhibition and hepatic apolipoprotein B secretion

Acyl coenzyme A: cholesterol acyltransferase (ACAT) is postulated to play a role in hepatic and intestinal lipoprotein secretion. There is accumulating evidence, both in vitro and in vivo, that cholesterol and/or cholesteryl ester availability can regulate hepatic VLDL secretion. How ACAT inhibition regulates the assembly and secretion of apolipoprotein (apo) B containing lipoproteins within the hepatocyte has not been clearly established. ApoB kinetic studies performed in animals indicate that reduction in VLDL apoB secretion is an important mechanism whereby ACAT inhibitors decrease the plasma concentrations of these lipoproteins. However, in cultured hepatocytes, the effect of ACAT inhibition on apoB secretion has been inconsistent. Recent evidence has suggested the existence of more than one ACAT enzyme in mammals, which has culminated in the recent cloning of ACAT2. ACAT1 and ACAT2 respond differently to ACAT inhibitors of differing structures and classes. ACAT2 is present in the liver and intestine, the sites of apoB containing lipoprotein secretion and may represent the enzyme responsible for generating cholesteryl esters destined for lipoprotein assembly and secretion.

Inhibitors of acyl-CoA:cholesterol O-acyltransferase. 3. Discovery of a novel series of N-alkyl-N-[(fluorophenoxy)benzyl]-N'-arylureas with weak toxicological effects on adrenal glands

A series of N-alkyl-N-[(fluorophenoxy)benzyl]-N'-arylureas were prepared and evaluated for their ability to inhibit intestinal acyl-CoA:cholesterol O-acyltransferase and to inhibit accumulation of cholesteryl esters in macrophages in vitro. In vivo hypocholesterolemic activity was assessed in cholesterol-fed rats by oral administration as a dietary admixture and/or by gavage in a PEG400 vehicle. Modification of the alkyl substituent on the N'-aryl moiety and on the urea nitrogen significantly influenced macrophage assay in vitro. Toxicological study revealed a distinct relationship between macrophage assay and the toxicity observed in adrenal glands of rabbits treated with representatives of this series of compounds. Investigations utilizing the macrophage assay as an indicator for adrenal toxicity led to the identification of compounds 1g (FR190809) and 1k (FR186485, or FR195249 as its hydrochloride salt) as potent, nonadrenotoxic, orally efficacious ACAT inhibitors irrespective of the administration method.

ACAT-2, a second mammalian acyl-CoA:cholesterol acyltransferase. Its cloning, expression, and characterization

The synthesis of cholesterol esters by acyl-CoA:cholesterol acyltransferase (ACAT, EC 2.3.1.26) is an important component of cellular cholesterol homeostasis. Cholesterol ester formation also is hypothesized to be important in several physiologic processes, including intestinal cholesterol absorption, hepatic lipoprotein production, and macrophage foam cell formation in atherosclerotic lesions. Mouse tissue expression studies and the disruption of the mouse ACAT gene (Acact) have indicated that more than one ACAT exists in mammals and specifically that another enzyme is important in mouse liver and intestine. We now describe a second mammalian ACAT enzyme, designated ACAT-2, that is 44% identical to the first cloned mouse ACAT (henceforth designated ACAT-1). Infection of H5 insect cells with an ACAT-2 recombinant baculovirus resulted in expression of a approximately 46-kDa protein in cell membranes that was associated with high levels of cholesterol esterification activity. Both ACAT-1 and ACAT-2 also catalyzed the esterification of the 3beta-hydroxyl group of a variety of oxysterols. Cholesterol esterification activities for ACAT-1 and ACAT-2 exhibited different IC50 values when assayed in the presence of several ACAT-specific inhibitors, demonstrating that ACAT inhibitors can selectively target specific forms of ACAT. ACAT-2 was expressed primarily in mouse liver and small intestine, supporting the hypothesis that ACAT-2 contributes to cholesterol esterification in these tissues. The mouse ACAT-2 gene (Acact2) maps to chromosome 15 in a region containing a quantitative trait locus influencing plasma cholesterol levels. The identification and cloning of ACAT-2 will facilitate molecular approaches to understanding the role of ACAT enzymes in mammalian biology.

Immunolocalization of acyl-coenzyme A:cholesterol O-acyltransferase in macrophages

Macrophages in atherosclerotic lesions accumulate large amounts of cholesteryl-fatty acyl esters ("foam cell" formation) through the intracellular esterification of cholesterol by acyl-coenzyme A:cholesterol O-acyltransferase (ACAT). In this study, we sought to determine the subcellular localization of ACAT in macrophages. Using mouse peritoneal macrophages and immunofluorescence microscopy, we found that a major portion of ACAT was in a dense reticular cytoplasmic network and in the nuclear membrane that colocalized with the luminal endoplasmic reticulum marker protein-disulfide isomerase (PDI) and that was in a similar distribution as the membrane-bound endoplasmic reticulum marker ribophorin. Remarkably, another portion of the macrophage ACAT pattern did not overlap with PDI or ribophorin, but was found in as yet unidentified cytoplasmic structures that were juxtaposed to the nucleus. Compartments containing labeled beta-very low density lipoprotein, an atherogenic lipoprotein, did not overlap with the ACAT label, but rather were embedded in the dense reticular network of ACAT. Furthermore, cell-surface biotinylation experiments revealed that freshly harvested, non-attached macrophages, but not those attached to tissue culture dishes, contained approximately 10-15% of ACAT on the cell surface. In summary, ACAT was found in several sites in macrophages: a cytoplasmic reticular/nuclear membrane site that overlaps with PDI and ribophorin and has the characteristics of the endoplasmic reticulum, a perinuclear cytoplasmic site that does not overlap with PDI or ribophorin and may be another cytoplasmic structure or possibly a unique subcompartment of the endoplasmic reticulum, and a cell-surface site in non-attached macrophages. Understanding possible physiological differences of ACAT in these locations may reveal an important component of ACAT regulation and macrophage foam cell formation.

Acyl CoA:cholesterol acyltransferase genes and knockout mice

Acyl coenzyme A:cholesterol acyltransferase (ACAT) (EC 2.3.1.26) is an enzyme, located in the endoplasmic reticulum of many types of cells, that catalyzes cholesterol ester formation from cholesterol and fatty acyl CoA substrates. Sterol esterification by ACAT or homologous enzymes is conserved in evolution dating back to yeast. The recent cloning of a human cDNA for ACAT, together with genome sequencing projects, has led to the identification of an ACAT gene family and provided molecular tools for determining ACAT's functions in vivo. Summarized here is the current knowledge concerning the molecular genetics of ACAT.

Molecular cloning, functional expression and tissue distribution of rat acyl-coenzyme A:cholesterol acyltransferase

Acyl-coenzyme A:cholesterol acyltransferase (ACAT) is an enzyme catalyzing the intracellular formation of cholesteryl esters from free cholesterol and fatty acyl-CoA. In the present study, we cloned rat ACAT cDNA and determined its tissue distribution. Rat ACAT cDNA, having a coding region of 1635 bp with its deduced protein sequence of 545 amino acids and two typical motifs such as signature sequences and leucine heptad motif, showed 83, 92 and 90% identity with human, mouse, and hamster ACAT, respectively. Expression of rat ACAT cDNA in A293 cells and CHO cells resulted in a 3.0 to 3.5-fold increase in the enzyme activity. Among twelve tissues examined, ACAT activity was highest in adrenal followed by liver and intestine while that of aorta was extremely low. The mRNA level was also the highest in adrenal among four tissues examined. However, in contrast to its high ACAT activity, the liver mRNA level was extremely low (adrenal >> intestine > aorta >> liver). Consistent with mRNA levels, immunohistochemical analyses with a specific ACAT antibody detected significant ACAT signals in adrenal and intestine but a negligible signal in liver. These results indicate that adrenal most abundantly expresses ACAT in rat. Furthermore, rat liver showed a high ACAT activity but an extremely low ACAT mRNA and negligible immunohistochemical reactivity, suggesting the presence of a structurally different ACAT protein(s) in rat liver.