CL 277,082: a novel inhibitor of ACAT-catalyzed cholesterol esterification and cholesterol absorption

CL 277,082 (I) was found to be a potent inhibitor of acyl CoA:cholesterol acyltransferase (ACAT, EC 2.3.1.26) in microsomes from a variety of tissues with IC50 values of 0.14 microM for intestinal mucosal microsomes, 0.74 microM for liver, and 1.18 microM for rat adrenal. I was also shown to inhibit ACAT in cultured smooth muscle cells (IC50 = 0.8 microM) and was found to be specific in inhibiting cholesterol esterification since it did not inhibit fatty acid incorporation into triglycerides or phospholipids. Also, other cholesterol esterifying enzymes such as lecithin:cholesterol acyltransferase (LCAT) and pancreatic cholesterol esterase were not inhibited by I, nor was esterification of retinol by acyl CoA:retinol acyltransferase (ARAT) from intestinal mucosal microsomes inhibited. I was a potent inhibitor of cholesterol absorption in cholesterol-fed rats by markedly inhibiting increases in liver and serum cholesterol concentration (ED50 = 5.2 mg/kg per day) while increasing the excretion of neutral 14C-labeled sterol in the feces.     

Normal cholesterol absorption in rats deficient in intestinal acyl coenzyme A:cholesterol acyltransferase activity

Acyl coenzyme A:cholesterol acyl transferase and/or cholesterol esterase may regulate the esterification and absorption of exogenous cholesterol. To assess this, mucosal acyl coenzyme A:cholesterol acyl transferase activity was inhibited selectively with three different drugs [Sandoz #58-035, inhibitor 1; Lederle inhibitor 2 and inhibitor 3] and the effect upon the absorption of a [4-14C]cholesterol meal was studied in the lymph fistula rat. Compared to control rats, ACAT activity measured in mucosal homogenates from the drug-treated rats was reduced 80-90%, 40%, and 30%, respectively, during the predicted time-frame for maximum mucosal esterification of cholesterol (i.e., after cholesterol is fed and before it appears in lymph). In contrast, [14C]cholesterol absorption in the drug-treated animals was unchanged from controls [5.7 +/- 1.2 (inhibitor 1) vs. 5.4 +/- 1.6 mumol/6 hr (control); 6.1 +/- 2.1 (inhibitor 2) and 5.2 +/- 1.5 (inhibitor 3) vs. 4.1 +/- 1.3 mumol/6 hr (control)]. Of the absorbed [14C]cholesterol, approximately 75% was esterified in all groups. Cholesterol esterase activity measured in the drug-treated rats was unchanged compared to controls nor did the drugs inhibit this enzyme in vitro. Under the conditions of this study, drugs causing substantial inhibition of acyl coenzyme A:cholesterol acyl transferase activity had no effect on the absorption of exogenous cholesterol.     

Intestine-specific MTP and global ACAT2 deficiency lowers acute cholesterol absorption with chylomicrons and HDLs

Intestinal cholesterol absorption involves the chylomicron and HDL pathways and is dependent on microsomal triglyceride transfer protein (MTP) and ABCA1, respectively. Chylomicrons transport free and esterified cholesterol, whereas HDLs transport free cholesterol. ACAT2 esterifies cholesterol for secretion with chylomicrons. We hypothesized that free cholesterol accumulated during ACAT2 deficiency may be secreted with HDLs when chylomicron assembly is blocked. To test this, we studied cholesterol absorption in mice deficient in intestinal MTP, global ACAT2, and both intestinal MTP and global ACAT2. Intestinal MTP ablation significantly increased intestinal triglyceride and cholesterol levels and reduced their transport with chylomicrons. In contrast, global ACAT2 deficiency had no effect on triglyceride absorption but significantly reduced cholesterol absorption with chylomicrons and increased cellular free cholesterol. Their combined deficiency reduced cholesterol secretion with both chylomicrons and HDLs. Thus, contrary to our hypothesis, free cholesterol accumulated in the absence of MTP and ACAT2 is unavailable for secretion with HDLs. Global ACAT2 deficiency causes mild hypertriglyceridemia and reduces hepatosteatosis in mice fed high cholesterol diets by increasing hepatic lipoprotein production by unknown mechanisms. We show that this phenotype is preserved in the absence of intestinal MTP in global ACAT2-deficient mice fed a Western diet. Further, we observed increases in hepatic MTP activity in these mice. Thus, ACAT2 deficiency might increase MTP expression to avoid hepatosteatosis in cholesterol-fed animals. Therefore, ACAT2 inhibition might avert hepatosteatosis associated with high cholesterol diets by increasing hepatic MTP expression and lipoprotein production.     

Absorption of intestinal free cholesterol is lowered by supplementation of Areca catechu L. extract in rats

Areca extracts exhibiting a strong inhibitory activity against pancreatic cholesterol esterase (pCEase) in vitro were previously found to lower the absorption of dietary cholesteryl ester. Therefore, to determine whether a combined Areca extract also affects the absorption of intestinal free cholesterol, male rats were fed a diet containing free cholesterol (1%, w/w) either with or without an Areca nut extract supplement (0.5%, w/w). The Areca extract supplement significantly lowered the plasma cholesterol concentration by 25% without any change in the plasma triglyceride concentration, when compared to the control group. The supplement also significantly lowered the small intestinal pCEase activity by 39.1% compared to that of the control group. As regards the hepatic and intestinal ACAT activities, only the intestinal enzyme activity was significantly lowered by the supplement, when compared to the control group. The absorbed cholesterol that appeared in the blood after an oral dose of [1,2(n)-3H] free cholesterol was significantly lower in the rats supplemented with the Areca nut extract, compared with the control group. These results suggest that the inhibition of intestinal ACAT and possibly pCEase may facilitate the metabolic efficiency of the Areca nut extract as regards the absorption of intestinal free cholesterol. The structure and chemical properties of the active compound in the water-soluble Areca extract remain to be elucidated.     

Effect of dietary n-3 fatty acids on HMG-CoA reductase and ACAT activities in liver and intestine of the rabbit

The regulation of hepatic and intestinal 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase and acyl-CoA; cholesterol acyltransferase (ACAT) activities by dietary fish oil was examined in the rabbit. Rabbits were fed 10% menhaden oil or menhaden oil plus 1% cholesterol for 14 days. They were compared with animals fed a control diet or one enriched with long-chain saturated fats consisting of 10% cocoa butter oil or cocoa butter oil plus 1% cholesterol. Plasma cholesterol was increased in rabbits fed the fish oil and the two cholesterol-containing diets. In the liver, ACAT activity was increased and HMG-CoA reductase activity was decreased in rabbits ingesting the fish oil. The same was true for animals ingesting both cholesterol-containing diets. In the intestine, ACAT activity was not affected by the ingestion of the fish oil compared to control rabbits; however, it was significantly higher in animals fed the fish oil compared to animals ingesting the cocoa butter. HMG-CoA reductase activity was decreased in the distal two-thirds of the intestine in animals fed the menhaden oil compared to activities observed in controls. In animals ingesting the cholesterol diets, intestinal reductase was significantly decreased, whereas intestinal ACAT activity was increased in rabbits ingesting the cocoa butter and cholesterol diet when compared to their controls. Lipid analysis of hepatic and intestinal microsomes demonstrated an enrichment of n-3 polyunsaturated fatty acids in membranes from rabbits ingesting the menhaden oil.(ABSTRACT TRUNCATED AT 250 WORDS)     

Regulation of hepatic cholesterol and lipoprotein metabolism in ethinyl estradiol-treated rats

The regulation of hepatic cholesterol and lipoprotein metabolism was studied in the ethinyl estradiol-treated rat in which low density lipoprotein (LDL) receptors are increased many fold. Cholesterol synthesis was reduced at both its diurnal peak and trough by ethinyl estradiol. The diurnal variation in 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase was abolished, whereas that for acyl coenzyme A: cholesterol acyltransferase (ACAT) was retained. LDL receptor number did not vary diurnally. Feeding these animals a cholesterol-rich diet for 48 h suppressed cholesterol synthesis and reductase activities to levels similar to those found in cholesterol-fed control animals, but ACAT activity was unaffected. LDL receptors were reduced about 50%. Intravenously administered cholesterol-rich lipoproteins suppressed HMG-CoA reductase and LDL receptors in 2 h but had a variable effect on ACAT activity. Intragastric administration of mevalonolactone reduced reductase and increased acyltransferase activity but had little effect on LDL receptors when given 2 or 4 h before death. Although animals fed a cholesterol-rich diet before and during ethinyl estradiol treatment became hypocholesterolemic, free and esterified cholesterol concentrations in liver were high as was ACAT activity. HMG-CoA reductase was inhibited to levels found in control animals fed the cholesterol-rich diet. LDL receptors were increased to a level about 50% of that reached in animals receiving a control diet and ethinyl estradiol. These data demonstrate that key enzymes of hepatic cholesterol metabolism and hepatic LDL receptors respond rapidly to cholesterol in the ethinyl estradiol-treated rat. Furthermore, estradiol increases LDL receptor activity several fold in cholesterol-loaded livers.     

Comparative effects of simvastatin (MK-733) and pravastatin (CS-514) on hypercholesterolemia induced by cholesterol feeding in rabbits

The preventive effects of simvastatin (MK-733) and pravastatin (CS-514), 3-hydroxy-3-methylglutarylcoenzyme A (HMG-CoA) reductase inhibitors, on hypercholesterolemia induced by 0.25% cholesterol feeding were compared in rabbits. MK-733 (6, 2 and 0.7 mg/kg) was found to prevent the increase in serum total cholesterol levels dose-dependently. High dose CS-514 (18 mg/kg) also limited the increase in the cholesterol levels, but medium (6 mg/kg) and low doses (2 mg/kg) of CS-514 were ineffective in preventing it. MK-733 inhibited the increase in VLDL and LDL cholesterol levels dose-dependently. MK-733 suppressed the increase in serum phospholipid levels. MK-733 inhibited the accumulation of cholesterol in the liver. The high dose of CS-514 also limited it. High dose MK-733 (6 mg/kg) reduced the cholesterol concentration in gallbladder bile. Neither MK-733 nor CS-514 affected bile acid excretion in the gallbladder bile. High dose MK-733 decreased the lithogenic index. MK-733 increased the number of LDL receptors, and high dose CS-514 also increased it. The suppressive effect of CS-514 on serum cholesterol levels at 18 mg/kg was found to be less than that of MK-733 at 0.7 mg/kg.     

Role of acyl CoA:cholesterol acyltransferase in cholesterol absorption and its inhibition by 57-118 in the rabbit

Esterification of cholesterol in rabbit small intestine mucosal microsomes by acyl CoA:cholesterol acyltransferase (ACAT, Ec 2.3.1.26) and mucosal cytosol by cholesterol esterase (EC 3.1.1.13) was studied. Compound 57-118. N-(1-oxo-9-octadecenyl)-DL-tryptophan(Z)ethyl ester, an inhibitor of cholesterol absorption, was found to inhibit in vitro ACAT in mucosal microsomes at concentrations of 2-20 nmol/0.5 ml incubation mixture, but had no effect on cholesterol esterase in the cytosol at similar concentrations. A kinetic analysis using a Lineweaver-Burk plot indicates that 57-118 acts as a competitive inhibitor of ACAT. An ex vivo study in the rabbit where 57-118 was given by gavage at a dose of 200 mg/kg also showed inhibition of ACAT but not of cholesterol esterase. High performance liquid chromatography determination of 57-118 in various subcellular fractions demonstrated the presence of this substance after oral administration in concentrations in mucosal microsomes equivalent to those required to show inhibition of ACAT in vitro. These data support the work of Norum et al. (1979, Eur. J. Clin. Invest. 9: 55-62) indicating mucosal ACAT plays a significant role in cholesterol absorption.     

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

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

12-[(5-iodo-4-azido-2-hydroxybenzoyl)amino]dodecanoic acid: biological recognition by cholesterol esterase and acyl-CoA:cholesterol O-acyltransferase

Potential probes of protein cholesterol and fatty acid binding sites, namely, 12-[(5-iodo-4-azido-2-hydroxybenzoyl)amino]dodecanoate (IFA) and its coenzyme A (IFA:CoA) and cholesteryl (IFA:CEA) esters, were synthesized. These radioactive, photoreactive lipid analogues were recognized as substrates and inhibitors of acyl-CoA:cholesterol O-acyltransferase (ACAT) and cholesterol esterase, neutral lipid binding enzymes which are key elements in the regulation of cellular cholesterol metabolism. In the dark, IFA reversibly inhibited cholesteryl [14C]oleate hydrolysis by purified bovine pancreatic cholesterol esterase with an apparent Ki of 150 microM. Cholesterol esterase inhibition by IFA became irreversible after photolysis with UV light and oleic acid (1 mM) provided 50% protection against inactivation. Incubation of homogeneous bovine pancreatic cholesterol esterase with IFA:CEA resulted in its hydrolysis to IFA and cholesterol, indicating recognition of IFA:CEA as a substrate by cholesterol esterase. The coenzyme A ester, IFA:CoA, was a reversible inhibitor of microsomal ACAT activity under dark conditions (apparent Ki = 20 microM), and photolysis resulted in irreversible inhibition of enzyme activity with 87% efficiency. IFA:CoA was also recognized as a substrate by both liver and aortic microsomal ACATs, with resultant synthesis of 125IFA:CEA. IFA and its derivatives, IFA:CEA and IFA:CoA, are thus inhibitors and substrates for cholesterol esterase and ACAT. Biological recognition of these photoaffinity lipid analogues will facilitate the identification and structural analysis of hitherto uncharacterized protein lipid binding sites.     

3-Hydroxy-3-methylglutaryl coenzyme A reductase activity in the human gastrointestinal tract

Activity of 3-hydroxy-3-methylglutaryl coenzyme A reductase (EC 1.1.1.34) was measured in intestinal mucosa of the human gastrointestinal tract. Activity was highest in gastric mucosa (18.2 pmol per mg per min) and there was a constant low level in the small bowel and colon (approximately 10 pmol per mg per min). Phosphorylation/dephosphorylation modulation of intestinal reductase activity was demonstrated in normal mucosa. Expressed jejunal reductase activity was significantly higher in celiac sprue mucosa and mucosa from defunctionalized intestine of jejunoileal bypass patients. Enzyme activity also increased during 24-hr mucosal organ culture in the absence of exogenous cholesterol. Addition to the culture medium of pure cholesterol or 25-hydroxycholesterol dissolved in a small volume of ethanol suppressed the culture-induced increase to 86 +/- 3% and 69 +/- 5% of paired controls, respectively. This evidence suggests that a moderate degree of feedback regulation of intestinal cholesterol synthesis by luminal sterol occurs in man. Mucosal HMG-CoA reductase activity was also measured in patients with hyperlipoproteinemia. Patients with either predominant hypercholesterolemia or predominant hypertriglyceridemia lipid profiles had "normal" expressed reductase activity, but feedback regulation by free cholesterol could not be demonstrated in either group under these conditions.     

Inhibition of lymphatic absorption of cholesterol by cholestane-3 beta, 5 alpha, 6 beta-triol

The effect of cholestane-3,5alpha,6-triol (CT) on the intestinal absorption of cholesterol and oleic acid, as well as the absorption of labeled CT, was studied in lymph ductcannulated rats. Intragastric administration of 50 mg of CT in an emulsion with cholesterol-7alpha-(3)H and oleic acid-1-(14)C resulted in 50% inhibition of sterol transfer into lymph but only 8% depression of fatty acid absorption over an 8 hr period. The absorption of labeled CT into lymph was only 2-3% compared with 50% absorption of cholesterol when each was fed alone. 10% of the fed CT was recovered in the intestinal mucosa, and of this, one-half was associated with the brush border fraction. In rats fed CT 6 days prior to cholesterol and fatty acid administration, there was no effect on fatty acid absorption, while cholesterol absorption was reduced by almost 30%. When the intestinal mucosa from these animals were investigated by electron microscopy, it appeared that CT feeding resulted in numerous enlarged mitochondria and a marked increase in length of the microvilli. If animals were allowed to recover for 6 days from the CT prefeeding regime, the intestinal mucosa appeared normal, and the absorption of cholesterol approached that in controls. A possible mechanism for CT inhibition of cholesterol absorption was shown to be competition for the enzyme cholesterol esterase which esterifies cholesterol prior to entrance into the lymphatic system. CT itself is poorly esterified and poorly absorbed, but it is effective in inhibiting esterification of cholesterol in vitro.     

The influence of estradiol on cholesterol processing by the corpus luteum

Recent studies from our laboratory have suggested that estradiol or androgen precursor may stimulate steroidogenesis in the luteal cell by modulating intracellular cholesterol metabolism including mobilization of cholesteryl esters, stimulation of lipoprotein receptor activity and induction of 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMG-CoA reductase) activity. To test the functionality of cholesteryl ester turnover per se, we measured the activities of acyl CoA:cholesterol acyltransferase (ACAT) and cholesteryl esterase, the enzymes involved in cholesteryl ester synthesis and hydrolysis, respectively; we also measured de novo synthesis of cholesterol, cholesteryl esters, and steroids. Pregnant rats, hypophysectomized and hysterectomized on Day 12, were treated for 72 h with either estradiol or testosterone, and luteal microsomal and cytosolic fractions were utilized to measure ACAT and cholesteryl esterase activity, respectively. Intact corpora luteal were employed for [14C]acetate incorporation experiments. Basal ACAT activity (expressed as pmol.min-1.CL-1 increased from a mean of 78 +/- 16 in vehicle-treated rats to 119 +/- 18 and 197 +/- 16 in the estradiol- and testosterone-treated rats, respectively. Similarly, total ACAT activity (measured in the presence of exogenous cholesterol) was also increased in estradiol- and testosterone-treated groups. On the other hand, cholesterol esterase activity (expressed either pmol.min-1.CL-1 or pmol.min-1.mg protein-1) was similar in all three groups and comparable to corpora lutea from intact pregnant rats. Hypophysectomy and hysterectomy caused a 50-60% reduction in [14C]acetate incorporation into sterols when compared with intact pregnant rat. Treatment with either estradiol or testosterone not only restored the cholesterol biosynthetic capacity but also enhanced the overall rate of [14C]acetate incorporation into steroids as compared to intact pregnant rats. The major (-80%), newly synthesized steroid was identified as progesterone. In conclusion, the present studies suggest that the major function of luteal estradiol is to induce de novo cholesterol biosynthesis, regulate ACAT activity, and channel available free cholesterol (derived from both endogenous and exogenous sources) for steroidogenesis.     

Anti-atherogenic effect of citrus flavonoids, naringin and naringenin, associated with hepatic ACAT and aortic VCAM-1 and MCP-1 in high cholesterol-fed rabbits

The anti-atherogenic effects of the citrus flavonoids, naringin and naringenin, were evaluated in high cholesterol-fed rabbits. At 3 months of age, 30 male New Zealand White (NZW) rabbits were divided into three groups (n = 10 per group). The rabbits were fed a 1% cholesterol diet alone (control group) or a diet supplemented with either 0.1% naringin or 0.05% naringenin for 8 weeks. The plasma lipoprotein levels, total cholesterol, triglyceride, and high-density lipoprotein showed no significant differences in the control and experimental groups. Hepatic acyl-CoA:cholesterol acyltransferase (ACAT) activity was slightly low in naringin (5.0%)- and naringenin (15.0%)-fed rabbits, compared to control group. The aortic fatty streak areas were significantly lower in both the naringin (19.2 +/- 5.6%)- and naringenin (18.1 +/- 6.5%)-supplemented groups than in the control group (60.4 +/- 14.0%). The expression levels of vascular cell adhesion molecule-1 (VCAM-1) and monocyte chemotactic protein-1 (MCP-1), by semiquantitative RT-PCR analysis of the thoracic aorta, were significantly lower in the flavonoids supplemented groups than in the control group. These results suggest that the anti-atherogenic effect of the citrus flavonoids, naringin and naringenin, is involved with a decreased hepatic ACAT activity and with the downregulation of VCAM-1 and MCP-1 gene expression.     

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

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

A simple method for reconstitution of CHO cell and human fibroblast acyl coenzyme A: cholesterol acyltransferase activity into liposomes

A new method for reconstituting acyl coenzyme A: cholesterol acyltransferase (ACAT) activity from either Chinese hamster ovary (CHO) or human fibroblast cell extracts into cholesterol-phosphatidylcholine liposomes is described. The method is rapid (less than 60 min) and easy to perform. The procedure involves solubilizing the cell extracts with deoxycholate followed by dilution into preformed liposomes. Ficoll gradient analysis demonstrated that, after reconstitution, almost all of the detectable ACAT activity co-migrated with the liposomes. Exogenous cholesterol in the liposomes was absolutely necessary for providing ACAT activity, but not for incorporation of the ACAT enzyme into the vesicle bilayer. Human fibroblast cell extracts prepared from cells grown in medium containing 10% fetal calf serum were found to contain a 10-fold higher microsomal ACAT activity compared to extracts from cells grown in 10% delipidated fetal calf serum. In contrast, when the ACAT activity from these extracts was measured using the reconstitution assay, there was no difference in the specific activities. These results support our previous work (Doolittle, G. M., and T. Y. Chang. 1982. Biochim. Biophys. Acta. 713: 529-537; and Chang, C. C. Y., et al. 1986. Biochemistry. 25: 1693-1699), and suggest that cholesterol regulates ACAT activity in CHO cells and human fibroblasts by mechanism(s) other than modulation of the amount of enzyme.     

Intestinal cholesterol esterase: intracellular enzyme or contamination of cytosol by pancreatic enzymes?

The location of cholesterol esterase in rabbit intestine was re-evaluated. In three different experiments that were designed to eliminate contaminating mucus and pancreatic enzymes from the lumen of the small intestine, it was observed that the activities of cholesterol esterase and amylase in intestinal cytosol and whole homogenate decreased in parallel fashion. After the mucus was carefully wiped from the intestinal mucosa prior to the preparation of cytosol, amylase and cholesterol esterase activities decreased sevenfold. The recovery of the total activity of both enzymes in the cytosol was approximately 15%. When the lumen of the small intestine was filled with phosphate buffer and incubated at 37 degrees C for 20 min, cholesterol esterase and amylase activities in the cytosol prepared from this segment were further decreased. Moreover, the activities of amylase and cholesterol esterase were completely recovered from the lumen. Amylase and cholesterol esterase activities in the cytosol were eliminated if dithiothreitol was used as a mucolytic agent to prepare intestinal mucosa for the isolation of intestinal cells. In whole homogenates prepared from these intestinal segments, approximately 10-15% of the total cholesterol esterase activity remained. This activity, which could not be accounted for by pancreatic contamination, was associated with intestinal nuclei and cellular debris. Progesterone, ethinyl estradiol, and 25-hydroxycholesterol regulated microsomal acyl CoA:cholesterol acyltransferase activity and caused similar directional changes in the rate of cholesteryl ester synthesis in isolated intestinal cells. These same sterols, however, failed to affect cytosolic cholesterol esterase activity in vitro.     

Impact of an enriched-cholesterol diet on enzymatic cholesterol metabolism during rabbit gestation

An appropriate cholesterol homeostasis is vital for the maintenance and the optimal fetal development. The cholesterol is essential for the synthesis of progesterone and 17beta-estradiol, hormones that actively participate to sustain gestation. However, the administration of 0.2% enriched cholesterol diet (ECD) during rabbit gestation significantly increased the cholesterol blood profile (total-cholesterol, LDL, HDL, esterified-cholesterol and free-cholesterol) of dams and offspring, and induced a reduction of the offspring weight of 15% as compared to the control group. Enzymes involved in cholesterol metabolism (ACAT, HMG-CoA-reductase and cholesterol-7alpha-hydroxylase) are greatly influenced by cholesterol profile. We hypothesized that the administration of an ECD during rabbit gestation modifies the activity of those enzymes. Female rabbits (pregnant or not) were fed with a standard diet or an ECD. At term, livers (dams and offspring) and placentas were collected and ACAT, HMG-CoA-reductase and cholesterol-7alpha-hydroxylase activities were assayed. Our results demonstrate that gestation induced a reduction of ACAT activity (48.9%) in dam's liver and, an augmentation of HMG-CoA-reductase activity (142.4%) whereas it has no effect on cholesterol-7alpha-hydroxylase activity. The administration of the ECD has no additive effect on ACAT, but significantly reduced the HMG-CoA-reductase activity and cholesterol-7alpha-hydroxylase activity as compared with the pregnant control group. In placentas the ECD supplementation has an influence for HMG-CoA-reductase activity, where a 43% increased in observed. Any ACAT activity was detected in placenta and the ECD has no influence on the cholesterol-7alpha-hydroxylase activity. Whereas their offspring's liver present a reduction of ACAT and HMG-CoA-reductase activity. Gestation associated with ECD reduces significantly the HMG-CoA-reductase activity, decreasing the cholesterol synthesis, but placenta seems to compensate this effect by increasing its HMG-CoA-reductase activity.     

Upregulation of acyl-CoA: cholesterol acyltransferase in chronic renal failure

Chronic renal failure (CRF) is associated with profound abnormalities of lipid metabolism and accelerated arteriosclerotic cardiovascular disease. In a recent study, we found marked downregulation of hepatic lecithin-cholesterol acyltransferase, or LCAT, expression, which can account for impaired HDL maturation and depressed HDL cholesterol concentration in CRF. Here, we report on the effect of CRF on acyl-CoA:cholesterol acyltransferase (ACAT) expression. ACAT is an intracellular enzyme that catalyzes esterification of free cholesterol to cholesterol ester for storage or secretion. ACAT plays a major role in hepatic production and release of VLDL, intestinal absorption of cholesterol, foam cell formation, and atherogenesis. We examined hepatic expression of ACAT-1 and ACAT-2 mRNA (Northern blot) and protein (Western blot) abundance and total ACAT activity in male CRF rats (6 wk after 5/6 nephrectomy) and sham-operated controls. The CRF animals showed a significant reduction in creatinine clearance, marked hypertriglyceridemia, modest hypercholesterolemia, and significant upregulation of hepatic tissue ACAT-2 protein and mRNA abundance. In contrast, hepatic ACAT-1 mRNA and protein abundance were unaffected by CRF. Upregulation of ACAT-2 expression was accompanied by a significant increase in hepatic ACAT activity and a significant decrease in hepatic microsomal and whole liver free cholesterol concentration. Thus CRF results in significant upregulation of hepatic ACAT-2 (but not ACAT-1) expression and ACAT activity, which may, in part, contribute to the associated lipid disorders.     

beta-sitosterol: esterification by intestinal acylcoenzyme A: cholesterol acyltransferase (ACAT) and its effect on cholesterol esterification

Rabbits were fed either 10% coconut oil, 10% coconut oil and 1% beta-sitosterol, 10% coconut oil and 1% cholesterol, or 10% coconut oil and 1% beta-sitosterol plus 1% cholesterol for 4 weeks. Microsomal membranes from intestines of animals fed the 1% beta-sitosterol diet had 48% less cholesterol and were enriched twofold in beta-sitosterol compared to membranes from animals fed the coconut oil diet alone. Acylcoenzyme A:cholesterol acyltransferase (ACAT) activity in jejunum and ileum was decreased significantly in animals fed the plant sterol alone. In membranes from animals fed 1% beta-sitosterol and 1% cholesterol, beta-sitosterol content increased 50% whereas cholesterol was modestly decreased compared to their controls fed only cholesterol. Intestinal ACAT was unchanged in the animals fed both sterols when compared to their controls. beta-Sitosterol esterification was determined by incubating intestinal microsomal membranes with either [(14)C]beta-sitosterol-albumin emulsion or [(14)C]beta-sitosterol:dipalmitoyl phosphatidylcholine (DPPC) liposomes to radiolabel the endogenous sterol pool. Oleoyl-CoA was then added. The CoA-dependent esterification rate of beta-sitosterol was very slow compared to that of cholesterol using both techniques. An increased amount of endogenous microsomal beta-sitosterol, which occurs in animals fed 1% beta-sitosterol, did not interfere with the stimulation of ACAT activity secondary to cholesterol enrichment of the membranes. Enriching microsomal membranes three- to five-fold with beta-sitosterol did not affect ACAT activity. Freshly isolated intestinal cells were incubated for 1 hour with [(3)H]oleic acid and beta-sitosterol:DPPC or 25-hydroxycholesterol:DPPC. Incorporation of oleic acid into cholesteryl esters did not change in the presence of beta-sitosterol but increased fourfold after the addition of 25-hydroxycholesterol. We conclude that the CoA-dependent esterification rate of cholesterol is at least 60 times greater than that of beta-sitosterol. Membrane beta-sitosterol does not interfere with nor compete with cholesterol esterification. Inadequate esterification of this plant sterol may play a role in the poor absorption of beta-sitosterol by the gut.-Field, F. J., and S. N. Mathur. beta-Sitosterol: esterification by intestinal acylcoenzyme A:cholesterol acyltransferase (ACAT) and its effect on cholesterol esterification.