Characteristics of the cholesterol-esterifying activity in normal and atherosclerotic rabbit aortas

Role of platelets in the development of atherosclerotic disease and possible interference with platelet inhibitor drugs

During the last two decades, significant advances have been made in the understanding of atherosclerotic disease. The pathogenesis of atherosclerosis appears to depend on a precise sequence of critical events based on the interaction of blood elements and lipids with the arterial wall. The major critical events and their sequence appears to be as follows: hemodynamic stress and endothelial injury; arterial wall-platelet interaction; smooth muscle cell proliferation; lipid entry and accumulation; significant arterial narrowing with fibrosis and development of thrombi; and complications in the form of calcification, ulceration, aneurysm, acute thrombotic occlusion and embolization. This sequence of critical events starts at a young age and in all geographic racial groups. Their evolution into advanced symptomatic lesions takes many years and varies in incidence and extent among different geographic and ethnic groups. It appears that in promoting and accelerating this process into the advanced stage of the disease, the presence at a young age of the so-called risk factors of atherosclerotic disease is most important. The recent advances in the understanding of the atherosclerotic process will be highlighted in this chapter with particular attention being focused on the role of platelets and thrombosis in the development of the disease and the possible role of platelet inhibitor drugs on the prevention of coronary atherosclerotic disease.

HL-004, the ACAT inhibitor, prevents the progression of atherosclerosis in cholesterol-fed rabbits

HL-004, N-(2,6-diisopropylphenyl) tetradecylthioacetamide, a novel acyl-CoA:cholesterol acyltransferase (ACAT) inhibitor, was evaluated concerning the possible prevention of hyperlipidemia and atherosclerosis in 1% cholesterol-fed rabbits. HL-004 (0.2, 5 and 25 mg/kg) was orally administered once a day for 12 weeks. HL-004 inhibited the rise of total serum cholesterol at a dose of 5 mg/kg and over. In the thoracic aorta, HL-004 at the doses of 5 mg/kg and 25 mg/kg reduced the total cholesterol content by 56.3% and 84.2% compared with control, and decreased ACAT activity, dose-dependently. HL-004 also attenuated the development of aortic lesions. The area of atherosclerotic lesions was reduced by 30.3% with 5 mg/kg of HL-004 and 100% with 25 mg/kg. In this study, we suggest that the main reason for HL-004 preventing the progression of atherosclerosis is its hypocholesterolemic effect due to the inhibition of cholesterol absorption in the intestine.

The hypolipidemic action of the ACAT inhibitor HL-004 in hamsters fed normal chow

1. A novel ACAT (acyl-CoA: cholesterol acyltransferase) inhibitor, HL-004, exhibited a strong inhibitory effect on the hepatic and intestinal ACAT, but was less effective on the adrenal ACAT in vitro. 2. HL-004 selectively decreased serum VLDL cholesterol, and inhibited hepatic ACAT activity in hamsters fed normal chow. 3. These results suggest that the cholesterol-lowering effect of HL-004 can be attributed to a decrease in hepatic VLDL secretion via inhibition of ACAT.

1,25-Dihydroxyvitamin D3-induced HL-60 macrophages: regulation of cholesterol and LDL metabolism

Differentiation of human promyelocytic leukemic HL-60 cells with 1,25-dihydroxyvitamin D3 (D3) results in macrophages which exhibit specific and saturable receptor-mediated processing of both native and modified low density lipoprotein (LDL). Analysis of binding kinetics demonstrated that macrophages bind LDL and acetyl-LDL with similar affinities, yet possess significantly different numbers of receptors (55 +/- 6 x 10(3) LDL receptors/cell vs 79 +/- 7 x 10(3) acetyl-LDL receptors/cell). D3-induced HL-60 macrophages challenged with LDL or acetyl-LDL exhibited suppression of HMG-CoA reductase activity as well as a significant induction in the incorporation of [14C]oleate into cholesteryl ester compared with macrophages incubated with lipoprotein depleted serum. Maximum increases in ACAT activity were obtained in macrophages incubated with 25-hydroxycholesterol plus LDL or acetyl-LDL. The increase in ACAT activity in macrophages challenged with acetyl-LDL paralleled the increase in cellular cholesterol content and the increase of oil red O lipid stainable material, imparting the macrophages with a foamy appearance. The data indicate that D3-induced HL-60 macrophages are a useful model for the study of lipoprotein--macrophage interactions as related to foam cell development and atherogenesis.

Effects of HL-004, a novel ACAT inhibitor, on cholesterol accumulation and removal in cultured smooth muscle cells from stroke-prone spontaneously hypertensive rats (SHRSP)

The cholesterol metabolism of cultured smooth muscle cells (SMC) from the thoracic aorta of SMC from stroke-prone spontaneously hypertensive rats (SHRSP) and Wistar-Kyoto rats (WKY) was compared. SMC from SHRSP had a higher acylCoA:cholesterol acyltransferase (ACAT) activity and accumulated more cholesterol than those from WKY. By using SMC from SHRSP, the effects of a novel ACAT inhibitor, HL-004, on the accumulation and removal of cholesterol were investigated. HL-004 inhibited microsomal ACAT activity from rabbit liver, intestine, aorta, and cultured SMC of SHRSP with 50% inhibition (IC50) values of 2.2, 1.7, 7.9, and 20 nM, respectively. HL-004 suppressed the accumulation of the intracellular cholesteryl ester (CE), but did not affect the intracellular free cholesterol (FC) content. Removal of cholesterol from the lipid-loaded SMC was accelerated by HL-004. These effects of HL-004 on cholesterol levels showed a good parallel to ACAT inhibition. It would thus appear that the suppression of cholesterol accumulation and the removal of cholesterol in SMC by HL-004 can be attributed to its ACAT inhibition in the cell, which reduces the content of intracellular CE.

Age-related changes in blood and liver lipids of male wistar rats

The results of this study indicate that the age-dependent plasma cholesterol increase observed in male Wistar rats is correlated with changes in both the distribution of high-density lipoprotein fractions and the storage of hepatic cholesterol. Specifically, the lipoprotein distribution showed a significant increase in the proportion of HDL(1) and a symmetrical decrease in both the HDL(2) and HDL(3) fractions during the 3 month to 18 month age period. There were no significant changes in the very-low density and low-density lipoprotein fractions. The chemical composition of lipoproteins showed many age-related variations, especially in the proportion of cholesteryl ester and in the distribution of HDL subfractions. A study of fatty acyl composition of the major lipid classes showed that, within cholesteryl ester found in liver, there was an increase in the proportion of saturated fatty acids. Polyunsaturated fatty acids increased in the cholesteryl esters found in high-density lipoproteins of older rats. These observations suggest that the age-dependent accumulation of body cholesterol occurs by a reduced catabolism of HDL(1) fraction, and modifications in plasma and liver lipids.

Inhibition of acyl-CoA cholesterol O-acyltransferase reduces the cholesteryl ester enrichment of atherosclerotic lesions in the Yucatan micropig

Atherosclerotic lesion development may be altered indirectly by regulating plasma cholesterol or directly by inhibition of acyl-CoA cholesterol O-acyltransferase (ACAT) within cells of the artery. Yucatan micropigs were meal-fed a 2% cholesterol, 8% peanut oil, 8% coconut oil purified diet for 1 month prior to administration of the potent, bioavailable ACAT inhibitor CI-976, and induction of atherosclerotic lesions by chronic endothelial damage. After 84-108 days of therapy, CI-976 decreased mean plasma VLDL-cholesterol 85-91% and cumulative VLDL-exposure (area under VLDL-time curve) by 65%. However, overall plasma total, LDL and HDL cholesterol and triglyceride levels were unchanged. CI-976 decreased liver cholesteryl ester (CE) content 65% without significantly affecting adrenal CE content. The CE content of the injured left femoral, left iliac and abdominal aorta and uninjured right femoral and iliac arteries and thoracic aorta was reduced 62-78% by CI-976. Systemic plasma CI-976 levels measured 24 h post-dose ranged from 2.26 to 4.05 micrograms/ml and significantly correlated with the reduction in both VLDL and vessel CE content. Thus, we conclude that inhibition of ACAT can blunt the cholesteryl ester enrichment of developing atherosclerotic lesions by preventing reesterification and storage of lipoprotein cholesterol within vascular cells and by reducing the plasma level and delivery to the arterial wall of such atherogenic lipoproteins as VLDL.

Regulation of ACAT activity by a cholesterol substrate pool during the progression and regression phases of atherosclerosis: implications for drug discovery

The regulation of aortic ACAT by a cholesterol substrate pool (CSP) was investigated in a rabbit progression/regression model of dietary-induced atherosclerosis. ACAT activity increased 25-fold during the 10-week progression phase of the study. ACAT activity decreased 8-fold during the 24-week regression phase of the study, however, it was still 14-fold greater than in normal aortas. ACAT activity assayed in the absence vs. the presence of exogenous cholesterol was used as a qualitative measure of the amount of cholesterol in the CSP. The CSP was filled to 28% of capacity in normal aortas, this increased to 75% during the progression phase. By the end of the regression phase, the CSP was filled to 100% of capacity even though serum cholesterol levels had returned to normal. The data are discussed in terms of emerging concepts of intracellular cholesterol trafficking, ACAT inhibitors, and the types of atherosclerotic lesions which may be subject to amelioration by ACAT inhibitors.

Rapid development of atherosclerotic lesions in the rabbit carotid artery induced by perivascular manipulation

A new rabbit model of atherosclerosis is described in which several of the features seen in early human atherosclerosis are generated within a period of 7 days. The positioning of a hollow silastic collar around the carotid artery of a cholesterol-fed rabbit results in macrophage and smooth muscle cell infiltration into the arterial subendothelium, foam cell formation and the deposition of extracellular lipid. A time-dependent accumulation of extracellular cholesteryl ester occurs within the arterial wall. Each of these changes occurs in the presence of a morphologically intact endothelium as assessed using light microscopy, scanning and transmission electron microscopy. A high cholesterol diet did not affect the extent of proliferation but exacerbated cholesteryl ester accumulation. It is proposed that the changes induced by the collar may be mediated by obstruction of the adventitial vasa vasorum with the creation of a localised ischaemic region.

Effects of cholesterol oxidation derivatives on cholesterol esterifying and cholesteryl ester hydrolytic enzyme activity of cultured rabbit aortic smooth muscle cells

The effects of 5 micrograms/ml of 25-hydroxycholesterol; cholestane-3 beta,5 alpha,6 beta-triol; and cholesterol on acyl CoA cholesterol acyltransferase, acid cholesteryl ester hydrolase and neutral cholesteryl ester hydrolase was studied in cultured rabbit aortic smooth muscle cells. After 1 hour incubation, 25-hydroxycholesterol resulted in a fourfold stimulation of acyl CoA cholesterol acyltransferase activity. No stimulation by 25-hydroxycholesterol was noted before 15 minutes or after 5 hours of incubation. Neither cholestane-3 beta,5 alpha,6 beta-triol nor cholesterol influenced acyl CoA cholesterol acyltransferase activity at any time interval. No significant effects of any of the sterols were noted on acid cholesteryl ester hydrolase or neutral cholesteryl ester hydrolase activity. The imbalance between acyl CoA cholesterol acyl transferase and hydrolase activities induced by 25-hydroxycholesterol could result in cholesteryl ester accumulation by arterial smooth muscle cells, which may be associated with atherosclerosis.

Chemical modification of acyl-CoA:cholesterol O-acyltransferase. 1. Identification of acyl-CoA:cholesterol O-acyltransferase subtypes by differential diethyl pyrocarbonate sensitivity

Acyl-CoA:cholesterol O-acyltransferase (EC 2.3.1.26) (ACAT) catalyzes the intracellular synthesis of cholesteryl esters from cholesterol and fatty acyl-CoA at neutral pH. Despite the probable pathophysiologic role of ACAT in vascular cholesteryl ester accumulation during atherogenesis, its mechanism of action and its regulation remain to be elucidated because the enzyme polypeptide has never been identified or purified. Present chemical modification results identify two distinct tissue types of ACAT, based on marked differences in reactivity of an active-site histidine residue toward diethyl pyrocarbonate (DEP) and acetic anhydride. The apparent Ki of the DEP-sensitive ACAT subtype, typified by aortic ACAT, was 40 microM, but the apparent Ki of the DEP-resistant ACAT subtype, typified by liver ACAT, was 1500 microM, indicating a 38-fold difference in sensitivity to DEP. Apparent Ki's of aortic and liver ACAT for inhibition by acetic anhydride were also discordant (less than 500 microM and greater than 5 mM, respectively). On the basis of the reversibility of inhibition by hydroxylamine, a neutral pKa for maximal modification, and acetic anhydride protection against DEP inactivation, DEP and acetic anhydride appear to modify a common histidine residue. Oleoyl-CoA provided partial protection against inactivation by DEP and acetic anhydride, suggesting that the modified histidine is at or near the active site of ACAT. Systematic investigation of ACAT activity from 14 different organs confirmed the existence of 2 subtypes of ACAT on the basis of their different reactivities toward DEP and acetic anhydride.(ABSTRACT TRUNCATED AT 250 WORDS)

Influence of 5-tridecylpyrazole-3-carboxylic acid, a new hypolipidaemic agent, on cholesteryl ester formation in rabbit intestinal mucosa

The comparative effects of 5-tridecylpyrazole-3-carboxylic acid (TDPC), beta-sitosterol and melinamide on the esterification of cholesterol (CH) have been investigated in rabbit intestinal microsomes and cytosol in-vitro. The three agents did not show an effect on cholesteryl ester formation by cholesterol esterase (CEase). TDPC and beta-sitosterol did not affect cholesteryl oleate formation from oleoyl CoA by microsomal acyl CoA:cholesterol acyltransferase (ACAT), whereas melinamide significantly inhibited cholesteryl oleate formation. TDPC significantly inhibited the incorporation of oleic acid into cholesteryl oleate, which is associated with acyl CoA synthetase (ACS) plus ACAT in mucosal microsomes, at a concentration of 20-100 microM. On the other hand, 5-tridecylpyrazole-3-carbinol (TDPC-OH) a congener of TDPC, and beta-sitosterol did not show any effect. From these results, it is demonstrated that carboxylic moiety of TDPC is necessary to inhibit ACS in-vitro. According to the kinetic analytical results, it is suggested that TDPC acts as a competitive inhibitor of ACS. These results suggest that the inhibitory effect of TDPC on cholesteryl ester formation may be mediated by an inhibition of ACS activity. It is apparent from the data presented that there are substantial differences between TDPC, beta-sitosterol and melinamide with respect to their action on cholesteryl ester formation in rabbit intestinal mucosa.

Studies on aorta during development. II. Differences in ontogeny of the key enzymes involved in cholesteryl ester synthesis and hydrolysis in rabbit aorta

It is well known that cholesteryl ester accumulation is dramatically increased in the atherosclerotic artery. The enzymes acyl-CoA: cholesterol acyltransferase (ACAT), acid cholesteryl esterase (ACE) and neutral cholesteryl esterase (NCE) may play key roles in the accumulation of cholesteryl esters in the arterial wall. However, very little is known regarding the developmental pattern of the key enzymes involved in cholesteryl ester synthesis and hydrolysis. The total activities of ACAT, ACE and NCE were measured by radioassay using liposomal substrates in rabbit aortic homogenates. Our results indicate that ACAT activity decreases as a quadratic function with age (P less than 0.05). ACAT activity (pmol/100 mg protein/min) decreased from a high value in the fetus at term (63.3 +/- 7.4) to gradually lower values with increasing age. On the other hand, ACE activity (pmol/mg protein/min) was low in the fetus at term, and changed as a quadratic function with age (P less than 0.05) increasing gradually to higher activities with age up to a maximum at 12 weeks then decreased at 21 weeks. NCE activity (pmol/mg protein/min) increased dramatically from a low value in the fetus at term (3.34 +/- 0.48) to a maximum value at 1.5 weeks (14.65 +/- 2.73) then decreased as a linear function with increasing age up to 21 weeks (P less than 0.05). Plasma total cholesterol (mg/dl) also increased sharply from the fetal value at term of 98.5 +/- 5.2 to a maximum value at 1.5 weeks of 666.4 +/- 33.4, then decreased as a quadratic function with increasing age up to 21 weeks (40.8 +/- 6.7) (P less than 0.05). The free cholesterol content (microgram/mg protein) of the aortic tissue was initially high in the fetus (24.8 +/- 5.9) then increased with age. Examination of the ratio of synthesis to hydrolysis of cholesteryl esters as an index of enzyme activity units demonstrated a very high index in the fetus of 6.1 that rapidly decreased with increasing age in the young adult rabbit down to a value of 0.4 by 21 weeks of age. Correlation coefficients between enzyme activities, plasma cholesterol levels and aortic cholesterol levels indicated (a) a positive correlation of NCE activity with plasma cholesterol, (b) a negative correlation of NCE and ACE with aortic-cholesteryl ester content, and (c) no significant correlation of ACAT activity with either plasma cholesterol or aortic cholesterol content, indicating other factors are involved.(ABSTRACT TRUNCATED AT 400 WORDS)

A high cholesterol/cholate diet induced fatty liver in spontaneously hypertensive rats

A high cholesterol diet was found to induce fatty liver in spontaneously hypertensive rats. Although cholesterol ester and triacylglycerol accumulated in large amounts in liver, the increases of these lipids in plasma were relatively small and no increase in cholesterol and cholesterol ester was observed in aorta. In rats fed normal diet, plasma cholesterol ester mainly consisted of arachidonate species; however, oleate and linoleate esters became the most prominent species in rats fed a high-cholesterol diet. The amounts of oleate and linoleate at the 2-position of phosphatidylcholine in both plasma and liver were increased slightly, but the fatty acids of aorta lipids changed little by feeding a high cholesterol diet. These results indicate that the livers of rats fed the high cholesterol diet do not secrete cholesterol ester and triacylglycerol with altered fatty acids as rapidly as they are synthesized and that the increased levels of cholesterol oleate in liver and plasma are not directly correlated with atherogenic lesions under these conditions.

Regulation of acyl-CoA:cholesterol acyltransferase activity in normal and atherosclerotic rabbit aortas: role of a cholesterol substrate pool

A new substrate optimized assay for acyl-CoA:cholesterol acyltransferase (ACAT) was developed that permits the accurate measurement of ACAT activity in normal arterial microsomes. The apparent Km and Vmax of ACAT with respect to oleoyl-CoA were determined to be 3 microM and 17.7 pmole min-1 mg-1. While the Km value is similar to other values reported in the literature, the Vmax is 5- to 8-fold higher. The higher Vmax is attributable to the saturation of ACAT with not only oleoyl-CoA, but also cholesterol. The observation that exogenous cholesterol was necessary for the determination of maximal ACAT activity indicates that under normal conditions the endogenous level of microsomal cholesterol does not saturate ACAT. Assay of ACAT in the presence and absence of exogenous cholesterol permits a qualitative assessment of the amount of cholesterol in the cholesterol substrate pool of ACAT. Using this approach, it was found that hypercholesterolemia results in the expansion of the cholesterol substrate pool of ACAT. Of the 21-fold increase in ACAT activity in atherosclerotic aortas observed in this study. 80% of the increase was attributable to expansion of the cholesterol substrate pool, while 20% was attributable to more enzyme. Notably, the increase in the amount of ACAT was observed after only 2 weeks of hypercholesterolemia.

Role of acid lipase in cholesteryl ester accumulation during atherogenesis. Correlation of enzyme activity with acid lipase-containing macrophages in rabbit and human lesions

Purified acid lipase was previously shown to hydrolyze the artificial substrate, alpha-naphthyl palmitate, as well as triglycerides and cholesteryl esters and to form cholesteryl esters. To determine to what extent these activities are associated with acid lipase-containing cells in atherosclerotic plaques, we examined rabbit aortas at different stages of experimental lesion induction and human atherosclerotic arteries. Assays of cholesteryl ester formation, and alpha-naphthyl palmitate and cholesteryl ester hydrolysis were performed on homogenates of lesions and the hydrolysis of the artificial fatty acid ester was used as a histochemical marker to identify acid lipase positive foam cells in sections of the same lesions. The volume of lesions occupied by cells stained for acid lipase correlated strongly with the enzyme activities of the arterial homogenates. These results suggest that acid lipase-containing cells may mediate the accumulation of cholesteryl ester during atherogenesis. Since acid lipase activity marks macrophages, these methods may be useful for relating macrophage distribution and function to lesion progression, regression, and complication.

Conditions that may result in (de-)phosphorylation of hepatic acyl-CoA:cholesterol acyltransferase result also in modulation of substrate supply in vitro

The present experiments were designed to study intervesicular transfer of cholesterol in rat liver microsomal fraction and modulation of the activity of acyl-CoA:cholesterol acyltransferase (ACAT) under conditions that are expected to result in the covalent modification (phosphorylation/dephosphorylation) of the enzyme. Preincubation of rat liver microsomal fraction followed by assay of ACAT showed a time-dependent increase in activity. This rate was temperature-dependent. Preincubation in the presence of cholesterol/phospholipid liposomes resulted in a time-dependent transfer of cholesterol from liposomal to the microsomal vesicles and in an increase in the rate of ACAT change owing to the preincubation. Both these rates were dependent on liposomal cholesterol concentration and on temperature. The presence of cytosol in the preincubation mixture increased the rate of change of ACAT activity in the absence or in the presence of cholesterol/phospholipid liposomes. In the latter case the presence of cytosol also increased the rate of transfer of cholesterol from liposomal to the microsomal vesicles. Activation energies of the rate of this transfer and of the rate of increase of ACAT activity were similar in the presence and in the absence of cytosol. Both in the absence and in the presence of cytosol, the presence of NaF (50 mM) in the preincubation mixture considerably decreased the rate of transfer of cholesterol from liposomal to microsomal vesicles and the rate of increase of ACAT activity. The presence of Mg2+ in the preincubation mixture produced no effect on the rate of transfer of cholesterol from liposomal to the microsomal vesicles, although under most conditions it decreased the rate of increase of ACAT activity caused by the preincubation. These results are discussed in relation to the molecular mechanism involved in this intervesicular transfer of cholesterol and to the modulation of ACAT activity by substrate supply, and also in relation to the hypothesis that ACAT activity can be modulated by a mechanism involving the phosphorylation/dephosphorylation of the enzyme.

The role of plasma membranes in the transfer of non-esterified cholesterol to the acyl-CoA:cholesterol acyltransferase substrate pool in liver microsomal fraction

The incubation at 37 degrees C of rat-liver microsomal fraction followed by re-isolation of the treated microsomal vesicles results in a time-dependent increase in the activity of acyl-CoA:cholesterol acyltransferase. The rate of this increase was higher in the microsomal fraction from rats fed cholesterol-supplemented diet or starved overnight as compared with that in the microsomal fraction from rats fed standard diet. The presence of a plasma membrane preparation in the incubation mixture also resulted in a time-dependent increase in acyl-CoA:cholesterol acyltransferase activity at a rate that was dependent on the concentration of plasma membranes. During the incubation of the microsomal fraction in the presence of phosphatidylcholine liposomes, cholesterol is transferred from the microsomal to liposomal vesicles. This transfer followed first-order kinetics with respect to cholesterol concentration in the donor with a rate that increased with the concentration of liposomes in the incubation mixture. The presence of phospholipid was also associated with a decrease in the activity of the acyltransferase that was related to the concentration of phospholipid in the incubation mixture. The incubation of the microsomal fraction in the presence of phosphatidylcholine-cholesterol liposomes resulted in a time-dependent and concentration-dependent transfer of liposomal cholesterol to the microsomal fraction and the acyltransferase substrate pool. The measurement of the rate of transfer of liposomal cholesterol to the microsomal vesicles and to the acyltransferase substrate pool at various temperatures showed that activation energies for the two processes are similar. Similar to these various was also the activation energy for the increase in acyl-CoA:cholesterol acyltransferase activity due to preincubation in the absence of artificial membrane vesicles. The present results suggest that there is, under the present conditions, a time-dependent and temperature-dependent flow of cholesterol from plasma membranes to the acyltransferase substrate pool and that this flow is either diverted in the presence of phospholipid liposomes or increased in the presence of cholesterol-phospholipid liposomes.

Carnitine ester hydrolysis in arteries from normal and cholesterol-fed rabbits and the effects of carnitine esters on arterial microsomal ACAT

Carnitine ester hydrolysis was observed in homogenates of normal rabbit (Oryctolagus cuniculus) aortas and in intact aortas from normal and cholesterol-fed rabbits using [14C]palmitoylcarnitine as a substrate. Hydrolytic activity was decreased approximately 50% in arterial tissue from cholesterol-fed rabbits and may account for the observation that carnitine esters accumulate in arteries of animals fed atherogenic diets. Long-chain acylcarnitines (C14-C18) were found to be moderate inhibitors of microsomal acylCoA:cholesterol acyltransferase (ACAT, EC 2.3.1.26); short-chain acylcarnitine (C2-C10) and carnitine itself were not inhibitors. The data suggest that the increase in activity of arterial ACAT that characteristically parallels the development of atherosclerosis does not occur as a result of carnitine ester accumulation.

Lipids and cholesterol esterifying enzyme changes by Anna Pavala Sindhooram therapy in experimental rat hyperlipaemia

The effect of Anna Pavala Sindhooram (APS), an indigenous drug showing lipid lowering action was tested in experimental rat atherosclerosis induced by feeding an atherogenic diet. APS was found to decrease the levels of serum cholesterol and phospholipids while triglycerides remained unaffected in atherogenic diet fed rats. Lipid levels in the aorta, liver and intestine were also increased by atherogenic diet feeding, and APS administration with diet restriction reversed this trend. Cholesterol ester was lowered. Both cholesteryl ester hydrolase (CEH) and synthetase (CES) activities in the tissues were elevated while the CEH/CES ratio was lowered in atherosclerosis. APS administration led to a decrease in enzyme activities and an increase in the CEH/CES ratio. APS in vitro inhibited both enzyme activities. NMR spectroscopic studies showed that the soluble components of APS bind or modify cholesterol. Iron, copper, magnesium and calcium present in APS may play a role in the removal of cholesterol ester from the aorta and its disposal.

Acyl-coenzyme A: cholesterol acyltransferase. Transfer of cholesterol to its substrate pool and modulation of activity

The preincubation at 37 degrees C of rat liver microsomal fraction, followed by re-isolation of the treated vesicles, results in a time-dependent increase in the activity of acyl-CoA: cholesterol acyltransferase. The presence of cholesterol-phospholipid (1:1, mol/mol) liposomes results in higher rate of increase in activity and under these conditions the rate of increase is liposomal cholesterol concentration-dependent. The preincubation of the microsomal fraction in the presence of [3H]cholesterol-phospholipid liposomes results in transfer of [3H]cholesterol to the re-isolated microsomal vesicles and this transfer follows first-order kinetics in respect to the donor concentration. These preincubations result also in a time-dependent and liposomal cholesterol concentration-dependent increase in the incorporation of [3H]cholesterol into the cholesteryl oleate produced on assay of cholesterol acyltransferase activity. From specific radioactivity data of the cholesteryl esters synthesised on assay of cholesterol acyltransferase in treated microsomal preparations, the rate of liposomal [3H]cholesterol equilibration with the cholesterol acyltransferase substrate pool can be calculated. The half-time of this transfer decreased with the concentration of liposomal cholesterol present during the preincubation. The activation energy for the transfer of liposomal cholesterol to the cholesterol acyltransferase substrate pool was 87.9 kJ/mol and was independent of the concentration of liposomal cholesterol. The activation energy for the rate of increase of total cholesteryl oleate was similar to this value for low concentrations of liposomal cholesterol and progressively decreased with increasing concentrations of liposomal cholesterol. The data suggest that under the present conditions, the time-dependent and temperature-dependent increase in cholesterol acyltransferase activity is due to the transfer of non-esterified cholesterol from other microsomal and/or liposomal vesicles to the vesicles that contain the enzyme and therefore to increased availability of substrate.

Phospholipid fatty acid modification of rat liver microsomes affects acylcoenzyme A:cholesterol acyltransferase activity

The effect of phospholipid fatty acyl composition on the activity of acylcoenzyme A:cholesterol acyltransferase was investigated in rat liver microsomes. Specific phosphatidylcholine replacements were produced by incubating the microsomes with liposomes and bovine liver phospholipid-exchange protein. Although the fatty acid composition of the microsomes was modified appreciably, there was no change in the microsomal phospholipid or cholesterol content. As compared to microsomes enriched for 2 h with dioleoylphosphatidylcholine, those enriched with dipalmitoylphosphatidylcholine exhibited 30-45% less acyl-CoA:cholesterol acyltransferase activity. Enrichment with 1-palmitoyl-2-linoleoylphosphatidylcholine increased acyl-CoA:cholesterol acyltransferase activity by 20%. By contrast, dilinoleoylphosphatidylcholine abolished microsomal acyl-CoA:cholesterol acyltransferase activity almost completely. Addition of cofactors that stimulated microsomal lipid peroxidation inhibited acyl-CoA:cholesterol acyltransferase activity by only 10%, however, and did not increase the inhibition produced by submaximal amounts of dilinoleoylphosphatidylcholine. Certain of the phosphatidylcholine replacements produced changes in palmitoyl-CoA hydrolase, NADPH-dependent lipid peroxidase, glucose-6-phosphatase and UDPglucuronyl transferase activities, but they did not closely correlate with the alterations in acyl-CoA:cholesterol acyltransferase activity. Electron spin resonance measurements with the 5-nitroxystearate probe indicated that microsomal lipid ordering was reduced to a roughly similar extent by dioleoyl- or by dilinoleoylphosphatidylcholine enrichment. Since these enrichments produce widely different effects on acyl-CoA:cholesterol acyltransferase activity, changes in bulk membrane lipid fluidity cannot be the only factor responsible for phospholipid fatty acid compositional effect on acyl-CoA:cholesterol acyltransferase. The present results are more consistent with a modulation resulting from either changes in the lipid microenvironment of acyl-CoA:cholesterol acyltransferase or a direct interaction between specific phosphatidylcholine fatty acyl groups and acyl-CoA:cholesterol acyltransferase.

Lipid composition and cholesterol esterifying activity in microsomal preparations of porcine coronary arteries and heart tissue

Lipid composition and cholesterol esterifying activity were determined in microsomal preparations from coronary arteries and heart tissues of swine. There was a higher concentration of free cholesterol in coronary arteries than in the heart, whereas phospholipid was more concentrated in the heart compared to the coronary arteries. Esterified cholesterol was a minor form of cholesterol in both tissues. Individual classes of microsomal lipids possessed characteristic fatty acid spectra and a number of differences were noted between coronary arteries and heart tissue. The portion of microsomal polyunsaturated fatty acids, particularly linoleic acid, was notably higher in the cholesteryl ester, free fatty acid, and phospholipid fractions of heart tissue compared to the corresponding lipid fractions of the coronary arteries. Cholesterol esterifying activity, measured with 14C-labeled fatty acids, was fairly low in coronary arteries, but considerable activity was present in heart tissue. Oleic acid substrate esterified cholesterol most effectively, followed by linoleic and elaidic acid. Under the incubation conditions for cholesterol esterifying activity, however, the bulk of the fatty acid was actively incorporated into phospholipid rather than cholesteryl ester or triglyceride. Among the fatty acids tested, linoleic acid was the most preferential substrate for phospholipid synthesis and phospholipid synthesizing activity was much greater in heart tissue than in coronary arteries.

Enzymatic formation of cholesteryl ester from cholesterol by gallbladder mucosa

The formation of cholesteryl ester from cholesterol and acyl CoA catalyzed by the enzyme acyl CoA:cholesterol acyltransferase (EC 2.3.1.26) was studied in guinea pig gallbladder mucosa homogenate and the subcellular fractions. The enzymatic activity was enriched in the microsomal fraction. Highest activity was observed with palmitoyl, stearoyl or oleoyl CoA as substrate. Lowest activity was observed with linoleoyl CoA. These data elucidate one mechanism for the formation of cholesteryl ester from cholesterol by the gallbladder wall.

Studies on the activity of acyl-CoA:cholesterol O-acyltransferase and acid cholesterol ester synthetase in rat aortas

Acyl-CoA: cholesterol O-acyltransferase (ACAT) activity was measured in microsomal preparations from rat aorta (intima-media) with [14C]oleoyl CoA and endogenous cholesterol as substrates. The specific activity of ACAT in liver and adrenal microsomal preparations was 10--20-times greater than ACAT activity in aortic microsomes; no ACAT activity could be detected in fat pad microsomes. ACAT activity in liver and adrenal microsomes was enhanced by the addition of exogenous cholesterol. In contrast, exogenous cholesterol did not increase ACAT activity in rat aortic microsomes. Levels of endogenous cholesterol and ACAT activity in microsomal preparations from rat aorta were not reduced when circulating plasma cholesterol levels were decreased by the administration of 4-aminopyrazolopyrimidine to rats. Acid cholesterol ester synthetase activity was not detectable in high-speed supernatant fractions from rat aorta; low levels of activity could be measured in rat aorta microsomal preparations but this was less than 10% of ACAT activity. Thus, ACAT would seem to be the principal enzymatic route for the synthesis of cholesterol esters in aorta.

Modulation of cytoplasmic cholesteryl ester of smooth muscle cells in culture derived from rat, rabbit and bovine aorta

Esterification of cholesterol in smooth muscle cells, isolated from rat, rabbit and bovine aorta, was achieved by incubation with cholesterol enriched medium containing [7(n)-3H]cholesterol. The newly formed cholesteryl ester was readily hydrolyzed when the cells were post-incubated with medium containing lipoprotein deficient serum. The rate of loss of labeled cholesteryl ester was not inhibited by the presence of 100 microM chloroquine. Addition of LDL to the post-incubation medium retarded the decrease in labeled cellular cholesteryl ester in rat smooth muscle cells and this effect of LDL was abolished by chloroquine. In bovine and rabbit smooth muscle cells, enriched in cholesteryl ester, addition of LDL to post-incubation medium resulted in an increase in labeled cholesteryl ester and in cholesteryl ester mass. Retardation in the loss of labeled cellular cholesteryl ester occurred also on addition of oleic acid to the post-incubation medium. In the presence of HDL and especially of high density apolipoprotein-sphingomyelin liposomes there was an efflux of cellular free cholesterol and a reduction in cholesteryl ester. These findings indicate that the catabolism of cytoplasmic cholesteryl ester in aortic smooth muscle cells is catalyzed by extralysosomal enzymes. The cytoplasmic cholesteryl ester hydrolase is apparently not activated by cyclic AMP. The intracellular availability of unesterified cholesterol, which can be modulated by plasma lipoproteins, may determine the residence time of cellular cholesteryl ester. Thus under pathological conditions an increase in extracellular LDL accompanied by a reduction in HDL would prolong the residence time of cholesteryl esters and thus promote their intracellular accretion.

Acyl-coenzyme A:cholesterol acyltransferase in human liver. In vitro detection and some characteristics of the enzyme

The enzyme, acyl-coenzyme A:cholesterol acyltransferase (ACAT), is responsible for the intracellular esterification of cholesterol. Although it has been detected in the liver from a variety of animals and in human skin fibroblasts and human intestine, it has been reported to be absent from human liver. Since this enzyme may play an important role in cholesterol homeostasis, evidence for its presence in human liver was again sought. Using labeled oleoyl CoA and the endogenous cholesterol as reactants, ACAT was detected in fresh samples of human liver obtained from patients undergoing staging laparotomy for Hodgkin's disease. The enzyme is present almost exclusively in membrane fractions with little activity detected in cytosol. Microsomal ACAT activity was linear with incubation time for up to 10 min. After this, the rate of cholesterol esterification remained constant despite the fact that adequate acyl CoA was present as judged by the continued incorporation of oleate into triglyceride. ACAT activity is destroyed by heating at 100 degrees C for 10 min. It was inhibited only up to 20%-30% by 1 mM 5,5'-dithiobis-(2-nitrobenzoic acid), which completely inactivates the serum cholesterol esterifying enzyme, lecithin:cholesterol acyltransferase (LCAT). Like ACAT in human skin fibroblasts, human liver ACAT was also inhibited by progesterone in vitro. ACAT activity averaged 10.3 +/- 5.1 pmole cholesteryl oleate/min/mg microsomal protein for 3 normal livers and 39.0 +/- 12.5 for 2 fatty livers. Thus, the level of ACAT activity estimated for the whole liver was 2.1-35.8 mumol/hr in the fasting state. This activity may account for some portion of the cholesterol esters present in plasma VLDL in fasting normolipidemic individuals. However, it is likely that the major role of hepatic ACAT is in the regulation and maintenance of hepatic cholesterol homeostasis.

Properties of acyl-CoA:cholesterol O-acyltransferase in aortic microsomes from atherosclerotic rabbits

Acyl-CoA:cholesterol O-acyltransferase (EC 2.3.1.26) was in microsomal fractions from atherosclerotic rabbit aortic tissue. Activity was increased over 70-fold following 8-11 weeks of cholesterol feeding. Comparison of the lipid composition of aortic microsomal fractions from control and cholesterol-fed animals showed a 2-fold increase in the molar ratio of unesterified cholesterol to phospholipid in the cholesterol-fed group, although no change in phospholipid content or composition was found. Aortic microsomes were fractionated by sucrose density gradient centrifugation. Acyl-CoA:cholesterol O-acyltransferase was localized in fractions containing neutral alpha-glucoside activity but was clearly separated from 5'-nucleotidase activity. The cholesteryl ester formed during in vitro incubation of incubations of microsomes with either [1-14C]-oleoyl CoA or [7-3H]cholesterol was localized in the same region of the density gradient as acyl-CoA:cholesterol O-acyltransferase. The studies indicate that the increased acyl-CoA:cholesterol O-acyltransferase activity found in cells from aortic atherosclerotic lesions is due to both an increased amount of enzyme and to an expanded pool of microsomal cholesterol which is available for esterification.

Stimulation of cholesterol esterification in hepatic microsomes by lipoproteins from normal and hypercholesterolemic rabbit serum

Incubation of plasma lipoproteins with rabbit hepatic microsomes enriched the microsomes with free cholesterol and stimulated cholesterol esterification. The rate of cholesterol esterification correlated well (r = 0.96) with the concentration of microsomal free cholesterol. Lipoproteins from normal and hypercholesterolemic serum varied in their propensity to stimulate cholesterol esterification. Among the normal lipoproteins, low density lipoproteins was more stimulatory than either high density lipoproteins or intermediate density lipoproteins. However, the intermediate density lipoproteins fraction from hypercholesterolemic serum was consistently more stimulatory than any of the normal lipoproteins. The augmentation of cholesterol content, when microsomes were exposed to mixed hyperlipidemic lipoproteins, was proportionately much greater than augementation of phospholipid or protein concentration.