Activation of acyl-CoA: cholesterol acyltransferase in rat liver microsomes by 25-hydroxycholesterol

25-Hydroxycholesterol stimulated acyl-CoA:cholesterol acyltransferase (ACAT) activity in rat liver microsomes in vitro with half-maximal stimulation at 16.8 microM oxysterol and a maximal activity that was three times that in its absence. The current study was conducted to determine the effect of 25-hydroxycholesterol on rates and extent of intervesicular cholesterol transfers within microsomes and to determine whether this activation of ACAT could be accounted for on the basis of increased cholesterol availability for the enzyme. Cholesterol transfer kinetics were assessed in systems that either enriched or depleted microsomal cholesterol. Incubation of microsomes at 37 degrees C with phosphatidylcholine:cholesterol liposomes or purified plasma membranes resulted in enrichment of microsomal cholesterol. Incubation of microsomes with just phosphatidylcholine liposomes resulted in depletion of cholesterol. The extent of cholesterol enrichment or depletion depended on incubation time and the initial concentration of cholesterol in donor and acceptor vesicles. The rate and extent of cholesterol transfer from liposomes to microsomes were slightly increased when 25-hydroxycholesterol was present during the transfer process. Irrespective of the treatment, 25-hydroxycholesterol continued to stimulate the ACAT activity of the treated microsomes. Microsomes that were enriched or depleted of cholesterol in the absence of 25-hydroxycholesterol yielded as much enzyme activities when assayed in the presence of 25-hydroxycholesterol as with the systems that contained 25-hydroxycholesterol during both the transfer process and enzyme assays. The results suggest that a major part of the activation of microsomal ACAT by 25-hydroxycholesterol is not ascribable to increased substrate availability for the enzyme.

Regulation of cholesterol synthesis and esterification in primary cultures of macrophages following uptake of chylomicron remnants

The effects of chylomicron remnants (CR), beta-very-low-density-lipoproteins (beta-VLDL) and low-density-lipoproteins (LDL) on intracellular cholesterol synthesis and esterification in primary rabbit macrophages was determined by assaying for HMG-CoA reductase activity and cholesterol esterification. At physiological cholesterol concentrations, both CR and LDL inhibited cholesterol synthesis by almost 60% while beta-VLDL was less potent achieving only 30% inhibition. Cholesterol esterification rates were increased four-fold by CR and LDL, whereas beta-VLDL increased esterification 14 times above controls. Qualitatively, the effect of CR on cholesterol synthesis and esterification in rabbit macrophages differs from observations in transformed macrophage cells. Quantitatively, the enhanced rates of cholesterol esterification and weak inhibition of cholesterol synthesis following beta-VLDL uptake may explain why this lipoprotein rapidly induces foam cell formation in vitro.

Estrogen stimulation of P450 cholesterol side-chain cleavage activity in cultures of human placental syncytiotrophoblasts

The present study determined whether estrogen has a role in regulating the P450 cholesterol side-chain cleavage enzyme (P450scc) and/or de novo/deesterification cholesterol pathways involved in progesterone biosynthesis within human syncytiotrophoblasts. Human placental syncytiotrophoblasts were cultured for 48 h with estradiol, and P450scc activity was determined by the formation of progesterone from 25-hydroxycholesterol. Estradiol at 10(-7) or 10(-6) M and 25-hydroxycholesterol increased mean (+/- SE) progesterone production by syncytiotrophoblasts (ng/0.5 x 10(6) cells) to a value (19.2 +/- 1.1) that was 104% (p < 0.001) higher than that of the untreated controls (9.4 +/- 0.8) and 52% higher (p < 0.001) than with 25-hydroxycholesterol alone (12.6 +/- 0.9). The stimulation of progesterone secretion apparently was not the result of a change in progesterone metabolism to its principal metabolite, because 20alpha-hydroxypregn-4-en-3-one represented a minor secretory component (0.7-1.7 ng/0.5 x 10(6) cells) under these conditions, and levels were not substantially altered by estrogen. In contrast to the stimulatory effect of estradiol on P450scc activity, estrogen did not alter either the P450scc mRNA levels or the activities of 3-hydroxy-3-methylglutaryl coenzyme-A reductase and cholesterol ester hydrolase-rate-limiting enzymes for the de novo and deesterification pathways, respectively, for cholesterol formation in syncytiotrophoblasts in culture. Collectively, these results indicate that estrogen regulates the P450scc component of the progesterone biosynthetic pathway, which we suggest signals functional/biochemical differentiation of syncytiotrophoblasts during primate pregnancy.

Acyl-coenzyme A:cholesterol acyltransferase

Due to its presumed role in regulating cellular cholesterol homeostasis, and in various pathophysiological conditions, acyl-coenzyme A:cholesterol acyltransferase (ACAT) has attracted much attention. Cloning the ACAT gene provides the necessary tool to advance molecular studies of this enzyme. The topics reviewed in this chapter include the pathophysiological roles of ACAT, the biochemistry and molecular biology of the ACAT protein and the ACAT gene, and the mode of regulation by sterol or nonsterol agents in mammalian cells. In addition, we present a working model linking the presumed allosteric property of ACAT with cholesterol trafficking into and out of the endoplasmic reticulum.

Intestinal resection and ursodeoxycholic acid: effect on hepatic 3-hydroxy-3-methylglutaryl coenzyme A reductase and acyl-CoA:cholesterol acyltransferase activities in the rat

The effect of distal small bowel resection (DSBR) and/or ursodeoxycholic-acid (UDCA) ingestion on hepatic activities of 3-hydroxy-3-methylglutaryl Coenzyme A (HMG-CoA) reductase and acyl-CoA:cholesterol acyltransferase (ACAT) was studied in rats. The total activity and the activity state of the enzyme HMG-CoA reductase were also determined. DSBR significantly increased the total activity of HMG-CoA reductase. This increase was enhanced when rats were treated with UDCA. The active form of the enzyme was increased by DSBR. However, the ingestion of UDCA did not modify these results. Therefore, the observed increase in the total activity of HMG-CoA reductase in resected-rats after the administration of UDCA was due to an increase in the inactive form of the enzyme. The activity of ACAT was decreased in resected animals. This decrease was greater after the treatment of UDCA in resected rats when compared with the corresponding group of untreated rats with UDCA. We conclude that UDCA ingestion increases the effect of intestinal resection on hepatic HMG-CoA reductase and ACAT activities.

Effects of FR145237, an acyl-CoA:cholesterol acyltransferase inhibitor, on diet-induced hypercholesterolemia in diabetic rats

Recent studies have shown that acyl-CoA:cholesterol acyltransferase (ACAT) plays an important role in the initiation of diabetes-associated hypercholesterolemia. To confirm this hypothesis, effects of a potent ACAT inhibitor, FR145237, on diet-induced hypercholesterolemia were examined in streptozotocin (STZ)-induced diabetic rats. One-week feeding of 1% cholesterol and 0.5% cholic acid to normal rats and STZ-induced diabetic rats increased plasma cholesterol levels in both groups, and the response was more remarkable in the STZ rats than in the normal ones (1266 +/- 476 mg/dl and 146 +/- 7 mg/dl, respectively). FR145237 dose-dependently reduced the rise in plasma cholesterol levels in the STZ rats and the levels were almost normalized by treatment with 1 mg/kg/day of the compound. These results suggest that hyperresponse to dietary cholesterol was induced in the STZ rats and that ACAT is involved in the hyperresponse. The effects of FR145237 on other plasma lipids such as high density lipoprotein (HDL) cholesterol and triglyceride (TG) levels were also examined.

Effect of exogenous cholesterol and dithiothreitol on the activity of human liver microsomal acyl-coenzyme A:cholesterol acyltransferase (ACAT)

Acyl-coenzyme A:cholesterol acyltransferase (ACAT) is the intracellular enzyme responsible for the esterification of cholesterol with long-chain fatty acyl-CoA derivatives and has been implicated in atherosclerosis and gallstone disease. The effects of exogenous cholesterol and dithiothreitol (DTT) on the ACAT activity of human liver microsomes have been determined. Pre-incubation of microsomes with exogenous cholesterol gave a marked stimulation of activity. Experiments with [3H]cholesterol and [14C]oleoyl-CoA indicated the time course of equilibration of exogenous with endogenous cholesterol as ACAT substrates, and showed that ACAT activity could be accurately measured using [3H]cholesterol/Tween 80, providing that the concentration of endogenous microsomal cholesterol was also determined. Pre-incubation of liver microsomes for 90 min in the presence of 2 mmol/l DTT and exogenous cholesterol/Tween 80 resulted in a 60% reduction in ACAT activity, compared with the corresponding activity when DTT was omitted. If microsomes were pre-incubated with DTT prior to the pre-incubation with exogenous cholesterol/Tween 80, an 85-90% reduction in ACAT activity occurred. In contrast, pre-incubation of microsomes with DTT in the absence of exogenous cholesterol/Tween 80 (only endogenous cholesterol present) resulted, initially in a stimulation of ACAT activity; on further pre-incubation, activity returned to control levels. These results indicate that the supply of cholesterol to the enzyme active site is an important factor in ACAT assays in vitro and that DTT has a major effect on this process, suggesting that these factors may be important in controlling ACAT activity in vivo.

Oxidative tyrosylation of HDL enhances the depletion of cellular cholesteryl esters by a mechanism independent of passive sterol desorption

It is believed that HDL protects against atherosclerosis by removing excess cholesteryl esters from cells of the artery wall. Previous studies have suggested that HDL depletes cells of cholesteryl esters both by stimulating cholesterol efflux from the plasma membrane and by activating transport processes that divert cholesterol from the cholesteryl ester cycle, but it is unknown if these are independent processes. We previously found that HDL oxidized by tyrosyl radical has a markedly enhanced ability to promote the removal of cholesterol from cultured cells [Francis, G. A., et al. (1993) Proc. Natl. Acad. Sci. U.S.A. 90, 6631-6635]. Here we show that incubation of cholesterol-loaded human fibroblasts with low concentrations of tyrosylated HDL depleted cells of cholesteryl esters and increased cellular free cholesterol without increasing efflux of cholesterol into the medium as compared to incubation with untreated HDL. Cells preincubated with tyrosylated HDL and then exposed to a variety of cholesterol acceptors exhibited significantly higher rates of free cholesterol efflux than did cells preincubated with HDL. This effect was observed in the presence or absence of an inhibitor of acyl CoA:cholesterol acyltransferase (ACAT) and was independent of cholesteryl ester hydrolysis, suggesting that alterations in cholesteryl ester cycle enzymes were not responsible for the loss of cholesteryl esters. In contrast to the reduction of cholesteryl esters, the rates of cholesterol and phospholipid efflux from the plasma membranes of cells exposed to tyrosylated HDL and HDL were identical. These results suggest for the first time that a mechanism exists to deplete cellular cholesteryl esters and the cholesterol substrate pool for esterification by ACAT prior to the removal of cholesterol from the plasma membrane. Identification of products in tyrosylated HDL responsible for this redistribution of cellular cholesterol may provide important insights into mechanisms of intracellular cholesterol trafficking and the ability of modified forms of HDL to protect the artery against wall pathological cholesterol accumulation.

Acyl-coenzyme A: cholesterol- acyl-coenzyme A:1,2-diacylglycerol acyltransferase and phosphatidate phosphorylase activities in liver microsomes from nephrotic rats

The lipid components of very low density lipoproteins (VLDL) were significantly elevated in the nephrotic rats. Also the nephrotic VLDL particles had a significantly higher ratio of surface lipids (FC + PL) to core lipids (TG + CE) probably indicating production of smaller sized VLDL in the nephrotic rats. Electron microscopy showed VLDL particles with a reduced mean size in the nephrotic rats. The activities of VLDL core lipid synthesizing enzymes were evaluated in experimental nephrotic syndrome. In addition, the effects of exogeneous cholesterol, 25-OH-cholesterol and low density lipoprotein (LDL) on the acyl-coenzyme A: cholesterol acyltransferase (ACAT) activity were investigated as well. ACAT activity in nephrosis was normal, but stimulated to varying extents in the presence of these factors. On the other hand, the acylcoenzyme A:1,2-diacylglycerol acyltransferase (ADGAT) and phosphatidate phosphorylase-activities were significantly increased in the nephrotic rats. The microsomal cholesterol (free and ester) and phospholipid concentrations were normal but the triglyceride level was significantly reduced in the experimental group. We speculate that an excess production of smaller-sized VLDL particles due to altered activities of microsomal lipid synthesizing enzymes may occur in puromycin amino nucleoside induced-nephrotic rats.

Chemical modification and structure-activity relationships of pyripyropenes. 2. 1,11-Cyclic analogs

A series of 1,11-cyclic analogs of pyripyropene A were prepared. Replacement of the 1,11-acyl groups of pyripyropenes with 1,11-cyclic acetals effectively improved in vitro acyl CoA:cholesterol acyltransferase (ACAT) inhibitory activity. Especially noteworthy is benzylidene acetal analog 35, the most potent inhibitor (IC50 = 5.6 nM) among the derivatives prepared so far, which showed 16 times more potent inhibitory activity than pyripyropene A.

Lithogenic diet and gallstone formation in mice: integrated response of activities of regulatory enzymes in hepatic cholesterol metabolism

Supersaturation of bile with cholesterol is a prerequisite of the development of gallstones. With the intention to study the integrated response of enzymes regulating hepatic cholesterol metabolism during gallstone formation we used an established model for the induction of cholesterol gallstone disease in mice. Ten mice were fed on a lithogenic diet containing 10 g cholesterol/kg and 5 g cholic acid/kg for 8 weeks and were compared with ten mice fed on a standard pellet diet. Cholesterol crystals or gallstones developed in 90% of gallbladders in treated mice. The lithogenic diet had an inhibitory effect on the rate-limiting enzyme of cholesterol biosynthesis, hepatic 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase (EC 1.1.1.88) activity, 39.6 (SEM 2.8) v. 171.0 (SEM 47.3) pmol/min per mg protein. Cholesterol 7 alpha-hydroxylase (EC 1.14.13.17) activity, regulating bile acid synthesis, was decreased by 80%, and this was assumed to be due to cholic acid in the diet. The cholesterol-enriched diet also induced a tenfold increase in cholesterol esterification rate in the liver, i.e. acyl-CoA:cholesterol acyl transferase (ACAT; EC 2.3.1.26) activity. The total, as well as esterified, cholesterol contents of liver homogenates were significantly higher in cholesterol- and cholic acid-treated mice and correlated well with the ACAT activity (rs 0.72 (P < 0.005), and rs 0.68 (P < 0.01) respectively). A significantly higher ACAT activity was obtained in mice given cholesterol and cholic acid even when the enzyme was saturated with exogenous cholesterol, thus indicating an increased amount of the enzyme. The formation of gallstones is dependent on a delicate balance between lithogenic factors (increased absorption of cholesterol and reduced secretion of bile acids) and defence mechanisms (decreased synthesis and increased esterification of cholesterol). In the specific animal model studied here the two defence mechanisms cannot compensate for the increased absorption of cholesterol and the reduced synthesis of bile acids.

Effectiveness of resistant starch, compared to guar gum, in depressing plasma cholesterol and enhancing fecal steroid excretion

Amylase-resistant starch (RS) represents a substrate that can be administered in substantial amounts in the diet, in contrast to gel-forming polysaccharides, such as guar gum (GG). The aim of this work was thus to compare the effects of GG and RS on cholesterol metabolism in rats adapted to 0.4% cholesterol diets, using dietary GG or RS levels (8 or 20%, respectively) that led to a similar development of fermentations, as assessed by the degree of enlargement of the cecum. The RS diet elicited a marked rise in the cecal pool of short-chain fatty acids, especially acetic and butyric acid, whereas the GG diet favored high-propionic acid fermentations. Both polysaccharides markedly altered the cholesterol excretion, from 50% of ingested cholesterol in controls, up to about 70% in rats adapted to the RS or GG diets. With these diets, the fecal excretion of bile acids was enhanced (67 and 144% with the RS and GG diets, respectively). RS and GG diets were effective in lowering plasma cholesterol (about -40%) and triglycerides (-36%). There was practically no effect of the diets on cholesterol in d > 1.040 lipoproteins (high density lipoproteins), whereas RS (and to a larger extent, GG) were very effective to depress cholesterol in d < 1.040 lipoproteins (especially in triglyceride-rich lipoproteins). Fermentable polysaccharides counteracted the accumulation of cholesterol in the liver, especially cholesterol esters. In parallel, liver acyl CoA:cholesterol acyltransferase was depressed in rats fed the RS or GG diets, whereas only the GG diet counteracted the downregulation of 3-hydroxy-3-methylglutaryl-CoA by cholesterol. These data suggest that RS may be practically as effective as a gel-forming gum, such as GG, on steroid excretion and on cholesterol metabolism.

Molecular cloning and characterization of two isoforms of Saccharomyces cerevisiae acyl-CoA:sterol acyltransferase

Esterification of cholesterol by acyl-CoA:cholesterol acyltransferase (ACAT) is a key element in maintaining cholesterol homeostasis in cells of higher animals. In the budding yeast, Saccharomyces cerevisiae, accumulation of ergosteryl esters accompanies entry into stationary phase and sporulation. We have determined that two genes in yeast, SAT1 and SAT2, encode isozymes of acyl-CoA:sterol acyltransferase (ASAT) which are functionally related to ACAT. The SAT1 isozyme is the major catalytic isoform, accounting for at least 65-75% of total ASAT activity. Targeted deletions of one or both genes do not compromise mitotic cell growth or spore germination. However, diploids that are homozygous for a SAT1 null mutation exhibit significantly reduced sporulation efficiency. Furthermore, a larger fraction of the sporulating diploids arrest after the first meiotic division. Human ACAT expressed in sat1 sat2 mutant cells can catalyze esterification of cholesterol and, to a lesser extent, ergosterol in vitro, but restores ergosteryl oleate formation in vivo to only approximately 8% of that catalyzed by yeast ASAT in wild-type cells.

Effects of novel synthetic sterol probes on enzymes of cholesterol metabolism in cell-free and cellular systems

A series of novel sterols was synthesised as probes for the enzymatic and cellular functions of two important enzymes of intracellular cholesterol metabolism, acyl-CoA:cholesterol acyltransferase (ACAT) and cholesterol 7 alpha-hydroxylase. The compounds were 6-fluoro-5-cholesten-3 beta-ol (6-fluorocholesterol), 7-cholesten-3 beta-ol (7-cholestenol), 6 beta-fluorocholestan-3 beta-ol (6 beta-fluorocholestanol), 3-acetoxy-6-fluorocholestan-3 beta-ol (3-acetoxy-6-fluorocholestanol) and 7-methoxy-5-cholesten-3 beta-ol (7-methoxycholesterol). They were designed to reveal the effect of small changes in sterol structure, particularly reactivity of certain parts of the ring structure and polarity, on enzyme activity and intracellular cholesterol metabolism. The 3 beta-hydroxy group was essential for interaction with both enzymes since 3-acetoxy-6-fluorocholestanol did not affect any of the enzyme-catalysed reactions. 6-Fluorocholesterol and 7-cholestenol had no effect on cholesterol esterification but did inhibit the hydroxylation of cholesterol, as did the other compounds with groups that could influence the 7 position, namely 6 beta-fluorocholestanol and 7-methoxycholesterol. The fluorocholestanols were all competitive substrates for cholesterol esterification in cell-free and cellular assays of ACAT activity. 7-Methoxycholesterol was a surprisingly effective inhibitor of ACAT for a simple sterol. However, 6-fluorocholesterol did not have any effect on ACAT, suggesting that interactions between the enzyme and the region around C-6 and C-7 of the sterol are important. These results show that it is possible to dissect components of cholesterol metabolism using simple, specifically substituted sterols and thus define a new approach to studying the relationships between the various enzymes that catalyse intracellular cholesterol metabolism.

Reduced cholesterol esterification in CaCo-2 cells by indirect action of pravastatin

In microsomal preparations of CaCo-2 cells pravastatin decreased cholesterol esterifying activity at 25 micrograms/ml to 82.5% and at 800 micrograms/ml to 56.2% of controls. Pravastatin reduced cholesteryl ester formation dose-dependently also in viable CaCo-2 cells. However, the maximal inhibition was by 90.4% at pravastatin concentration of 25 micrograms/ml, half maximal inhibition occurred between concentrations of 5 and 10 micrograms/ml. Addition of mevalonolactone, which serves as endogenous source of cholesterol, antagonized this effect. At 10 mM mevalonolactone (MVL) even doses up to 200 micrograms/ml of pravastatin were ineffective. On the other hand, pravastatin suppressed cholesteryl ester formation when acyl-CoA cholesterol acyltransferase (ACAT) (E.C. 2.3.1.26) activity was stimulated by addition of exogenous liposomal or Low Density Lipoprotein (LDL)-derived cholesterol. This inhibition was refractory to increasing amounts of exogenous cholesterol up to 400 micrograms/ml. Therefore we conclude that only excessive doses of pravastatin suppress ACAT activity directly. In viable cells the observed inhibition of cholesteryl ester formation is due to the block in de novo synthesis of cholesterol, causing a lack of substrate for ACAT and of non-sterol products of mevalonic acid. Furthermore pravastatin interferes with the esterification and/or intracellular transport only of exogenous cholesterol, confirming former results of a compartmentalized cholesterol metabolism in the enterocyte.

Anti-atherosclerotic effect of E5324, an inhibitor of acyl-CoA:cholesterol acyltransferase, in Watanabe heritable hyperlipidemic rabbits

E5324, n-butyl-N'-[2-[3-(5-ethyl-4-phenyl-1H-imidazol-1-yl)propoxy]-6- methylphenyl]urea, a novel and potent inhibitor of acyl-CoA:cholesterol acyltransferase (ACAT), was evaluated for its anti-atherosclerotic and lipid-lowering effects in Watanabe heritable hyperlipidemic (WHHL) rabbits. At 3 months of age, 40 male WHHL rabbits were divided into 4 groups. The rabbits were fed a standard rabbit chow (control group), or standard rabbit chow containing E5324 (0.1% or 0.02%) or 1% probucol for 16 weeks. Even the high dose of E5324 did not lower the plasma total cholesterol levels throughout the experiment. Probucol slightly reduced the plasma cholesterol levels, and showed anti-atherosclerotic activity, i.e., reductions of atherosclerotic plaque formation and cholesterol content in the aorta. Although E5324 did not lower plasma cholesterol, atherosclerotic plaque formation in the aortic arch and thoracic aorta was reduced (by about 34% and 41%, respectively, at the high dose; P < 0.05). Cholesterol content in the aortic arch and thoracic aorta was also reduced (by about 59% and 62% at the high dose, respectively) compared with the control. These results suggest that E5324 acts directly on the arterial wall through ACAT inhibition, and prevents the progression of atherosclerosis in WHHL rabbits.

Altered lipid metabolism in apolipoprotein E-deficient mice does not affect cholesterol balance across the liver

Adaptation of cholesterol and bile acid synthesis and of biliary cholesterol secretion represent key metabolic responses to maintain cholesterol homeostasis and have been suggested to be influenced by apolipoprotein E (apoE) phenotype in humans. We have investigated hepatic metabolism and secretion of cholesterol into bile in homozygous apoE-deficient (apoE -/-) mice fed normal lab chow. Plasma cholesterol levels were 10 times higher in apoE (-/-) mice than in controls (+/+); triacylglycerol levels were only minimally affected. Hepatic cholesterol (+56%) and triacylglycerol (+232%) contents were significantly increased in apoE (-/-) mice, whereas those of cholesteryl ester and of phospholipids were similar in both groups. Lipid accumulated predominantly in periportal areas of apoE (-/-) livers. Hepatic 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMG CoA reductase) messenger RNA (mRNA) level and activity were reduced by 45% and 50%, respectively, in apoE (-/-) mice. In contrast, plasma lathosterol/cholesterol ratios, indicative for whole-body cholesterol synthesis, were fourfold increased in these mice. Acyl-coenzyme A:cholesterol acyltransferase (ACAT) activity was similar in livers of both groups. Despite the marked changes in hepatic cholesterol metabolism, neither hepatic bile acid synthesis, bile acid pool size and composition, nor hepatic cholesterol 7alpha-hydroxylase and sterol 27-hydroxylase mRNA levels differed between apoE (-/-) and (+/+) mice. In addition, biliary cholesterol secretion was unaffected in the knock-out mice. Our results show that lack of apoE leads to marked changes in hepatic cholesterol metabolism without altering cholesterol balance across the liver. The data are compatible with increased peripheral cholesterol biosynthesis in apoE-deficient mice.

The distal pathway of lipoprotein-induced cholesterol esterification, but not sphingomyelinase-induced cholesterol esterification, is energy-dependent

The stimulation of the intracellular cholesterol esterification pathway by atherogenic lipoproteins in macrophages is a key step in the development of atheroma foam cells. The esterification pathway can also be stimulated by hydrolysis of cell-surface sphingomyelin by the enzyme sphingomyelinase (SMase). In both cases, intracellular cholesterol transport to the cholesterol esterifying enzyme, acyl-CoA:cholesterol O-acyltransferase (ACAT), is thought to be critical, although the mechanism of cholesterol transport is not known. In this report, we explore two fundamental properties of the cholesterol esterification pathway, namely its dependence on energy and the effect of other treatments that block membrane vesicle trafficking. After the atherogenic lipoprotein, beta-very low density lipoprotein (beta-VLDL), was internalized by macrophages and hydrolyzed in lysosomes, the cells were depleted of energy by treatment with sodium azide and 2-deoxyglucose or by permeabilization. Under these conditions, which allowed equal beta-VLDL-cholesteryl ester hydrolysis, cholesterol esterification was markedly decreased in the energy-depleted cells. This effect was not due to blockage of lysosomal cholesterol export. In the permeabilized cell system, energy repletion restored beta-VLDL-induced cholesterol esterification. Remarkably, stimulation of cholesterol esterification by SMase was not inhibited by energy depletion. Energy depletion also inhibited beta-VLDL-induced, but not SMase-induced, cholesterol esterification in Chinese hamster ovary cells. Similar experiments were carried out using N-ethylmaleimide, low potassium medium, or inhibitors of phosphatidylinositol 3-kinase, each of which blocks intracellular membrane vesicle trafficking. These treatments also inhibited beta-VLDL-induced, but not SMase-induced, cholesterol esterification. Finally, we show here that SMase treatment of cells leads to an increase in plasma membrane vesiculation that is relatively resistant to energy depletion. In summary, the stimulation of cholesterol esterification by lipoproteins, but not by SMase, is energy-dependent, N-ethylmaleimide-sensitive, and blocked by both low potassium and phosphatidylinositol 3-kinase inhibitors. The affected step or steps are distal to cholesterol export from lysosomes and not due to direct inhibition of the ACAT enzyme. Thus, the mechanisms involved in lipoprotein-induced versus SMase-induced cholesterol esterification are different, perhaps due to the involvement of energy-dependent vesicular cholesterol transport in the lipoprotein pathway and a novel, energy-independent vesicular transport mechanism in the SMase pathway.

Gene expression of acyl-coenzyme-A:cholesterol-acyltransferase is upregulated in human monocytes during differentiation and foam cell formation

The gene expression and enzyme kinetics of acyl coenzyme A:cholesterol acyltransferase (ACAT) were investigated in human monocytes, macrophages, and foam cells. Northern blot analysis using a 1.65-kb coding region of human ACAT cDNA as the probe showed that each of the cell types exhibited four mRNA transcripts. The levels of the 4.2- and 3.7-kb ACAT transcripts were three- and sixfold higher, respectively, in macrophages than monocytes. These transcripts were expressed at the same high levels after conversion of macrophages to foam cells. In contrast, the 6.3- and 4.4-kb transcripts for ACAT were expressed at a relatively constant level in all three cell types. The expression of mRNA for glyceraldehyde phosphate dehydrogenase, the control gene in this study, was also expressed at a constant level in each of the cell types. The increase in ACAT mRNA was accompanied by changes in the kinetic properties of the enzyme. Specifically, there was a 14-fold increase in Vmax and a 71% decrease in Km with respect to oleoyl coenzyme A. Although not definitive, the concomitant changes in mRNA and Vmax strongly suggest that the amount of ACAT protein increases upon conversion of monocytes to macrophages. The data show that ACAT in monocytes can be regulated by both substrate and gene expression.

Activation of acyl-coenzyme A:cholesterol acyltransferase activity by cholesterol is not due to altered mRNA levels in HepG2 cells

Many studies have shown that sterols can stimulate acyl-coenzyme A:cholesterol acyltransferase (ACAT) activity in cells. To elucidate this mechanism, effects of sterol-mediated induction on both the enzyme activity of ACAT and its mRNA levels were studied in human hepatoblastoma cell line, HepG2 cells. When HepG2 cells were loaded with cholesterol and 25-hydroxycholesterol, both the whole-cell ACAT activity and the microsomal ACAT activity were increased by 85.1% and 41.3%. In contrast, cholesterol depletion of HepG2 cells with compactin, a 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitor, resulted in a decrease in both the whole-cell and the microsomal ACAT activity by 46.4% and 58.3%. Under identical conditions, RT-PCR and Northern blotting analyses revealed that neither cholesterol loading nor cholesterol depletion of HepG2 cells altered the amounts of ACAT mRNA. Moreover, these treatments had no effect on the enzymatic ACAT activity determined by the reconstituted assay in which HepG2 cell homogenate had been supplemented in vitro with a saturating level of exogenous cholesterol. These results indicate that cholesterol-induced up-regulation of ACAT activity in HepG2 cells does not occur at the level of transcription, but rather at a posttranscriptional level.

Sterol esterification in yeast: a two-gene process

Unesterified sterol modulates the function of eukaryotic membranes. In human cells, sterol is esterified to a storage form by acyl-coenzyme A (CoA): cholesterol acyl transferase (ACAT). Here, two genes are identified, ARE1 and ARE2, that encode ACAT-related enzymes in yeast. The yeast enzymes are 49 percent identical to each other and exhibit 23 percent identity to human ACAT. Deletion of ARE2 reduced sterol ester levels to approximately 25 percent of normal levels, whereas disruption of ARE1 did not affect sterol ester biosynthesis. Deletion of both genes resulted in a viable cell with undetectable esterified sterol. Measurements of [14C]acetate incorporation into saponified lipids indicated down-regulation of sterol biosynthesis in the are1 are2 mutant cells. With the use of a consensus sequence to the yeast and human genes, an additional number of the ACAT gene family was identified in humans.

Hypolipidemic effect of taurine in stroke-prone spontaneously hypertensive rats

The hypolipidemic and antiatherosclerotic effects of taurine were investigated in genetically hypertensive rats: strokeprone spontaneously hypertensive rats (SHRSP). SHRSP were fed a hypercholesterolemic (HC) diet supplemented with 3% taurine for 50 days, and serum cholesterol was monitored. Cholesterol content and enzymatic activity responsible for cholesterol synthesis and metabolism were also determined in the liver, aorta, and intestine. Taurine prevented increases in the cholesterol level of the serum, liver, and aorta induced by a HC diet. Severe fat deposits of the mesenteric arteries induced by a HC diet were improved by the taurine treatment, showing the hypolipidemic and antiatherosclerotic effects of taurine. Taurine enhanced the activity of cholesterol 7 alpha-hydroxylase, a rate-limiting enzyme of bile acid synthesis, and stimulated bile acid production. These results suggest that taurine stimulates bile acid synthesis, which is closely related to the enhancement of cholesterol 7 alpha-hydroxylase activity, and thereby reduces serum cholesterol. In addition, a decrease in the intestinal acyl CoA:cholesterol acyltransferase activity by taurine suggests that the inhibition of cholesterol absorption may also be related to the hypolipidemic effect of taurine, in part.

Impact of essential fatty acid deficiency on hepatic sterol metabolism in rats

The major aim of the current investigation was to define whether essential fatty acid (EFA) deficiency modifies the intrahepatic metabolism and biliary output of sterols in rats. EFA-deficient diet caused an impoverishment in linoleic, arachidonic, and docosahexaenoic acids, and a marked enrichment in the eicosatrienoic acid of the plasma, liver, and hepatic microsomes. During a short term of biliary drainage, a significant decline of the pool size of biliary sterols was noted in EFA-deficient rats compared with control rats. To assess the biosynthesis of biliary components, the common bile duct was cannulated and the pool size depleted (24 hours). Subsequently, a 6-hour bile collection disclosed a significant decrease (nmoles/min/g liver) in bile acids (4.8 +/- 0.3 vs. 8.4 +/- 0.7, P < .005), cholesterol (0.26 +/- 0.01 vs. 0.34 +/- 0.02, P < .05), and phospholipids (1.49 +/- 0.11 vs. 2.82 +/- 0.32, P < .005) in EFA-deficient rats compared with controls (n = 6/group). When cholesterogenesis was measured by the incorporation of [14C]acetate and 3H20 into cholesterol, using liver slices, it was also found to be significantly (P < .001) reduced in EFA-deficient rats. The activity of hepatic 3-hydroxy-3-methylglutaryl coenzyme A reductase, the rate-limiting enzyme in cholesterol biosynthesis, was consistently lower (80 percent, P < .001) in EFA-deficient rats. In parallel experiments, the direct measurement of microsomal acyl-CoA:cholesterol acyl-transferase (ACAT) showed a decrease averaging 52 percent (P < .001). This is in striking contrast to the elevated activity (157 percent, P < .005) of cholesterol 7 alpha-hydroxylase, the rate-limiting enzyme in bile acid biosynthesis. Current experiments also suggest that the enzyme regulation involving phosphorylation and dephosphorylation is modified by EFA deficiency. Among the structural alterations observed in the morphology of hepatocytes in EFA-deficient rats, the lumen of bile canaliculi was reduced in size. These results underline the importance of EFA in the major mechanisms involved in the maintenance of hepatocyte sterol balance.

Taurine accelerates the regression of hypercholesterolemia in stroke-prone spontaneously hypertensive rats

The effects of taurine on the regression of pre-established hypercholesterolemia were examined in stroke-prone spontaneously hypertensive rats (SHRSP). Hypercholesterolemia was induced by feeding a hypercholesterolemic diet to SHRSP for 30 days. Then, the diet was switched to normal chow with or without 3% taurine, and the effects were followed up for another 30 days. During regression serum cholesterol level was rapidly decreased, and was accelerated by taurine. A similar accelerated decrease in cholesterol content by taurine was seen also in tissues including the liver, intestine, and aorta. In the liver, acyl-CoA:cholesterol acyltransferase (ACAT) activity was significantly low in the taurine-supplemented group, parallel with the hepatic cholesteryl ester content. On the other hand, hepatic cholesterol 7 alpha-hydoxylase activity maintained a higher level in the taurine-supplemented group. These results showed that taurine accelerates the regression of hypercholesterolemia, and suggested that this effect is related to the increase in cholesterol catabolism to bile acid through the enhancement of 7 alpha-hydoxylase activity.