ACAT inhibition decreases LDL cholesterol in rabbits fed a cholesterol-free diet. Marked changes in LDL cholesterol without changes in LDL receptor mRNA abundance

The effects of hesperidin supplementation on cardiovascular risk factors in adults: a systematic review and dose-response meta-analysis

Hesperidin is a naturally occurring bioactive compound that may have an impact on cardiovascular disease risks, but the evidence is not conclusive. To investigate further, this study aimed to explore the effects of hesperidin supplementation on cardiovascular risk factors in adults. A comprehensive search was conducted up to August 2022 using relevant keywords in databases such as Scopus, PubMed, Embase, Cochrane Library, and ISI Web of Science for all randomized controlled trials (RCTs). The results showed that hesperidin supplementation had a significant effect on reducing serum triglyceride (TG), total cholesterol (TC), low-density cholesterol (LDL), tumor necrosis factor-alpha (TNF-α), and systolic blood pressure (SBP), whereas weight was increased. However, no significant effect was observed on high-density cholesterol (HDL), waist circumference (WC), fasting blood glucose (FBG), insulin, homeostatic model assessment for insulin resistance (HOMA-IR), C-reactive protein (CRP), interleukin-6 (IL-6), body mass index (BMI), and diastolic blood pressure (DBP). The study also found that an effective dosage of hesperidin supplementation was around 1,000 mg/d, and a more effective duration of supplementation was more than eight weeks to decrease insulin levels. Furthermore, the duration of intervention of more than six weeks was effective in decreasing FBG levels.

Acyl-Coenzyme A: Cholesterol Acyltransferase (ACAT) in Cholesterol Metabolism: From Its Discovery to Clinical Trials and the Genomics Era

The purification and cloning of the acyl-coenzyme A: cholesterol acyltransferase (ACAT) enzymes and the sterol O-acyltransferase (SOAT) genes has opened new areas of interest in cholesterol metabolism given their profound effects on foam cell biology and intestinal lipid absorption. The generation of mouse models deficient in Soat1 or Soat2 confirmed the importance of their gene products on cholesterol esterification and lipoprotein physiology. Although these studies supported clinical trials which used non-selective ACAT inhibitors, these trials did not report benefits, and one showed an increased risk. Early genetic studies have implicated common variants in both genes with human traits, including lipoprotein levels, coronary artery disease, and Alzheimer’s disease; however, modern genome-wide association studies have not replicated these associations. In contrast, the common SOAT1 variants are most reproducibly associated with testosterone levels.

Corn silk extract improves cholesterol metabolism in C57BL/6J mouse fed high-fat diets

BACKGROUND/OBJECTIVES

Corn silk (CS) extract contains large amounts of maysin, which is a major flavonoid in CS. However, studies regarding the effect of CS extract on cholesterol metabolism is limited. Therefore, the purpose of this study was to determine the effect of CS extract on cholesterol metabolism in C57BL/6J mouse fed high-fat diets.

MATERIALS/METHODS

Normal-fat group fed 7% fat diet, high-fat (HF) group fed 25% fat diet, and high-fat with corn silk (HFCS) group were orally administered CS extract (100 mg/kg body weight) daily. Serum and hepatic levels of total lipids, triglycerides, and total cholesterol as well as serum free fatty acid, glucose, and insulin levels were determined. The mRNA expression levels of acyl-CoA: cholesterol acyltransferase (ACAT), cholesterol 7-alpha hydroxylase (CYP7A1), farnesoid X receptor (FXR), lecithin cholesterol acyltransferase (LCAT), low-density lipoprotein receptor, 3-hyroxy-3-methylglutaryl-coenzyme A reductase (HMG-CoA reductase), adiponectin, leptin, and tumor necrosis factor α were determined.

RESULTS

Oral administration of CS extract with HF improved serum glucose and insulin levels as well as attenuated HF-induced fatty liver. CS extracts significantly elevated mRNA expression levels of adipocytokines and reduced mRNA expression levels of HMG-CoA reductase, ACAT, and FXR. The mRNA expression levels of CYP7A1 and LCAT between the HF group and HFCS group were not statistically different.

CONCLUSIONS

CS extract supplementation with a high-fat diet improves levels of adipocytokine secretion and glucose homeostasis. CS extract is also effective in decreasing the regulatory pool of hepatic cholesterol, in line with decreased blood and hepatic levels of cholesterol though modulation of mRNA expression levels of HMG-CoA reductase, ACAT, and FXR.

Pharmacological profile of SMP-797, a novel acyl-coenzyme a: cholesterol acyltransferase inhibitor with inducible effect on the expression of low-density lipoprotein receptor

We investigated the pharmacological profile of SMP-797, a novel hypocholesterolemic agent. SMP-797 showed inhibitory effects on acyl-coenzyme A: cholesterol acyltransferase (ACAT) activities in various microsomes and in human cell lines, and hypocholesterolemic effects in rabbits fed a cholesterol-rich diet and hamsters fed a normal diet. In hamsters, the reduction of total cholesterol level by SMP-797 was mainly due to the decrease of low-density lipoprotein (LDL) cholesterol level rather than that of very low-density lipoprotein (VLDL) cholesterol level. Interestingly, SMP-797 increased the hepatic low-density lipoprotein receptor expression in vivo when it decreased the low-density lipoprotein cholesterol level. SMP-797 also increased low-density lipoprotein receptor expression in HepG2 cells like atorvastatin, an HMG-CoA reductase inhibitor, although other acyl-coenzyme A: cholesterol acyltransferase inhibitor had no effect. In addition, SMP-797 had no effect on cholesterol synthesis in HepG2 cells. These results suggested that the increase of low-density lipoprotein receptor expression by SMP-797 was independent of its acyl-coenzyme A: cholesterol acyltransferase inhibitory action and did not result from the inhibition of hepatic cholesterol synthesis. In conclusion, these results suggest that SMP-797 is a novel hypocholesterolemic agent showing a cholesterol-lowering effect in which the increase of hepatic low-density lipoprotein receptor expression as well as the inhibition of acyl-coenzyme A: cholesterol acyltransferase is involved.

Cholesterol absorption and hepatic acyl-coenzyme A:cholesterol acyltransferase activity play major roles in lipemic response to dietary cholesterol and fat in laboratory opossums

Partially inbred lines of laboratory opossums differ considerably in their low-density lipoprotein (LDL) cholesterol responses to dietary cholesterol and fat. Genetic analysis suggested that a single major gene is responsible for the variation in LDL cholesterol on the high cholesterol and high fat (HCHF) diet. We measured cholesterol absorption and acyl-coenzyme A:cholesterol acyltransferase (ACAT) activity in intestine and liver to narrow the search for the major gene. We measured plasma lipoproteins and percent cholesterol absorption by the fecal isotope ratio method in high and low responding lines of opossums on basal and HCHF diets. We also measured lipids in liver and ACAT activity in liver and intestine on the HCHF diet. High and low lines exhibited no differences in percent cholesterol absorption on the basal diet. However, high responding opossums had significantly higher percent cholesterol absorption, hepatic free and esterified cholesterol, and hepatic ACAT activity than low responding opossums on the HCHF diet. Hepatic ACAT activity but not the intestinal ACAT activity was associated with hepatic cholesterol concentration and percent cholesterol absorption. Cholesterol absorption is a major determinant of diet-induced hyperlipidemia in opossums. Hepatic ACAT activity but not the intestinal ACAT may also play a role in diet-induced hyperlipidemia in opossums.

Oral administration of D-003, a mixture of very long chain fatty acids prevents casein-induced endogenous hypercholesterolemia in rabbits

D-003 is a mixture of very long chain saturated fatty acids (VLCSFA) purified from sugar cane wax with cholesterol-lowering effects proven in animal models and healthy volunteers. D-003 inhibits cholesterol biosynthesis through the regulation of HMG-CoA reductase activity. Rabbits fed diets enriched with casein develop endogenous hypercholesterolemia (EH), making them a very useful model for determining the mechanism of action of drugs affecting lipids. We examined whether D-003 prevented EH. Rabbits were fed a casein diet for 4 weeks, administered simultaneously with D-003 (5, 50, and 100 mg·kg–1·day–1). As expected, nontreated rabbits became hipercholesterolemic; however, as early as 15 days following administration, the treated group (50 and 100 mg·kg–1·day–1) had significantly decreased total cholesterol and low-density lipoprotein cholesterol (LDL-C). Triglycerides were not affected; however, at study completion, HDL-C levels significantly increased at all the doses assayed. D-003 inhibited de novo synthesis of cholesterol, since the incorporation of3H2O into sterols in the liver and proximal small bowel was significantly depressed. Also, D-003 significantly raised the rate of removal of [125I]-LDL from serum and significantly elevated [125I]-LDL binding activity to liver homogenates. Taken together, these results show that the efficacy of D-003 in reducing casein-derived hypercholesteromeia could involve, at least partially, an inhibition of hepatic cholesterol bio synthesis, which may elicit a decreased cholesterol concentration in hepatocytes, preventing the loss of hepatic LDL receptors induced by casein administration. However, since casein-induced hypercholesterolemia is also a consequence of a stimulation of cholesterol absorption in the lumen and an increase of the output of cholesterol associated with LDL, the effect of D-003 on cholesterol absorption and LDL synthesis by the liver should be investigated.Key words: D-003, very long chain saturated fatty acids, casein-fed rabbits, LDL-C, cholesterol biosynthesis, LDL clearance, LDL receptor.

Inhibitory effects of Polygonum cuspidatum water extract (PCWE) and its component rasveratrol on acyl-coenzyme A–cholesterol acyltransferase activity for cholesteryl ester synthesis in HepG2 cells

The pharmacological effects of Polygonum cuspidatum water extract (PCWE) on lipid biosynthesis were investigated in cultured human hepatocyte HepG2 cells. The addition of PCWE (5 and 20 microg/ml), which had no effect on cell proliferation and cellular protein content, caused a marked decrease in the cellular cholesterol content, particularly, the cholesteryl ester content following 24 h of incubation. The incorporation of (14)C-oleate into the cellular cholesteryl ester fraction was also reduced remarkably during incubation for 6 and 24 h. The effect of PCWE on acyl-coenzyme A-cholesterol acyltransferase (ACAT) activity were studied in vitro to explore the mechanism by which PCWE inhibits cholesterol ester formation. The data confirmed that PCWE, in a dose dependent manner, remarkably inhibits ACAT activity. Among the main active chemicals of P. cuspidatum, resveratrol, a kind of flavonoid, decreased ACAT activity in a dose-dependent manner from the level of 10(-3) M. Theses results strongly suggest that PCWE reduces the cholesteryl ester formation in human hepatocytes by inhibiting ACAT.

Anti-atherosclerotic properties of the acyl-coenzyme A:cholesterol acyltransferase inhibitor F 12511 in casein-fed New Zealand rabbits

SUMMARY

The anti-atherosclerotic properties of F 12511, a novel acyl-coenzyme A:cholesterol acyltransferase (ACAT) inhibitor, were studied in rabbits that were fed a cholesterol-free casein-rich diet and developed endogenous hypercholesterolemia and fibrofatty preatheroma lesions. After 6 weeks of casein feeding, an endothelial abrasion was performed in the abdominal aorta; at week 8, a control group was maintained on this diet while F 12511 (8 mg/kg/d) was administered as a diet admixture for the subsequent 24 weeks. Total plasma cholesterol level rose to 250-300 mg/dl in both groups before starting the treatment; F 12511 time-dependently reduced total plasma cholesterol by 50%, and also decreased by 50% the incidence of lesions and macrophage accumulation in uninjured aorta (thoracic arch, celiac bifurcation). Residual lesions in the treated group were characterized by few macrophages, essentially under the endothelium, and by a larger content of smooth muscle cells. Quantitative image analysis of serial sections of mechanically injured abdominal aorta revealed a 20% surface covered by preatheroma lesions in the placebo group; F 12511 significantly reduced this surface. These data suggest that the combination of endogenous hypercholesterolemia with endothelial injury in the rabbit may offer a useful model to study atherosclerosis; lipid lowering by F 12511 reduces the incidence of vascular lesions and macrophage infiltration and may reinforce the fibrous skeleton of the atheroma.

Cholesterol-fed and transgenic rabbit models for the study of atherosclerosis

The rabbit has been extensively utilized as an ideal model of atherosclerosis because of its size, easy manipulation, and extraordinary response to dietary cholesterol. The availability of spontaneously hypercholesterolemic model, Watanabe heritable hyperlipidemic rabbits (WHHL) and St. Thomas rabbits, has also provided insights into understanding human familiar hypercholesterolemia and atherosclerosis. With the advent of genetically engineered rabbits, transgenic rabbits have become a novel means to explore a number of proteins that are associated with cardiovascular diseases including atherosclerosis. To date, transgenes for human apo(a), apoA-I, apoB, apoE2, apoE3, hepatic lipase, lecithin: cholesterol acyltransferase (LCAT), lipoprotein lipase, 15-lipoxygenase, as well as for rabbit apolipoprotein B mRNA-editing enzyme catalytic polypeptide 1 (APOBEC-1), have been expressed in rabbits. In addition, human apoA-I, LCAT and apo(a) have been introduced into WHHL rabbits which have deficient LDL receptor function. All of these transgenes have been found to have significant effects on plasma lipoprotein metabolism or/and atherosclerosis. These studies have revealed new insights into the mechanisms responsible for the development of atherosclerosis. In this article, we provide a brief review on the rabbit model for the study of atherosclerosis with emphasis on transgenic rabbit models developed during the past few years.

F 12511, a novel ACAT inhibitor, and atorvastatin regulate endogenous hypercholesterolemia in a synergistic manner in New Zealand rabbits fed a casein-enriched diet

F 12511, a novel ACAT inhibitor, lowers plasma cholesterol levels in New Zealand rabbits fed a cholesterol-free casein-rich diet. In rabbits endogenous hypercholesterolemia pre-established for 8 weeks was used to compare treatments with F 12511 and atorvastatin for a further 8-week period, and to determine whether both agents act synergistically. F 12511 appears to be 3-4-fold more potent than atorvastatin in reducing total plasma cholesterol (active doses ranging from 0.16 to 2.5 and from 1.25 to 10 mg/kg per day, respectively) while the hypocholesterolemic efficacy of both compounds at 2.5 mg/kg per day amounted to 70 and 45%, respectively. A reduction by as much as 75% of esterified cholesterol in liver mediated by F 12511 could account for the decrease of plasma VLDL, LDL and apo B-100, whereas a reduction of the LDL production rate has been described as the main mechanism underlying the atorvastatin effect. F 12511 modified adrenal cholesterol balance only at the largest dose studied. In a further experiment the co-administration of threshold doses of F 12511 and atorvastatin (0.63 and 1.25 mg/kg per day, respectively) lowered plasma total cholesterol and apo B-100 containing lipoproteins to a greater extent and more rapidly than either agent alone. In the liver a decrease by atorvastatin in free cholesterol substrate for ACAT may amplify the effect of F 12511 on cholesteryl ester content leading to a diminution, in at least an additive manner, of the assembly and secretion of atherogenic lipoproteins in New Zealand rabbits which have developed an endogenous hypercholesterolemia. Thus, the combination of the ACAT inhibitor F 12511 with atorvastatin can represent a better approach than either agent alone to regulate lipoprotein metabolism in certain pathophysiological situations.

Abnormalities in apo B-containing lipoproteins in diabetes and atherosclerosis

Atherosclerosis is the major cause of death in patients with diabetes. Low-density lipoprotein (LDL) being the most important cholesterol-carrying lipoprotein has been studied extensively in both diabetes and non-diabetes. This paper reviews the literature but also focuses on the precursors of LDL and in particular the postprandial apo B-containing lipoproteins. Abnormalities in the postprandial lipoproteins and alteration in chylomicron assembly and clearance are discussed and the evidence presented suggesting the importance of dysregulation of these lipoproteins in atherosclerotic progression. The relationship between chylomicron production in the intestine and hepatic release of very low-density lipoproteins (VLDL) is explored, as is the interrelationship between clearance rates of these lipoproteins. The size of LDL influences its atherogenicity. VLDL composition and size in relation to its influence on LDL is discussed. The effect of diet on the composition of lipoproteins and the relationship between fatty acid composition and clearance is reviewed. Evidence that diabetic control beneficially alters lipoprotein composition is presented suggesting how improved diabetic control may reduce atherosclerosis. The review concludes with a discussion on the effect of the apo B-containing lipoproteins and their modification through glycation and oxidation on macrophage and endothelial function.

Pharmacological profile of F 12511, (S)-2',3', 5'-trimethyl-4'-hydroxy-alpha-dodecylthioacetanilide a powerful and systemic acylcoenzyme A: cholesterol acyltransferase inhibitor

The pharmacological profile of F 12511 (S)-2',3', 5'-trimethyl-4'-hydroxy-alpha-dodecylthio-phenylacetanilide, a new inhibitor of acyl-CoA: cholesterol acyltransferase (EC 2.3.1.26; ACAT), was evaluated by using different in vitro and in vivo models. In vitro, F 12511 was shown to be a highly potent inhibitor of ACAT activity in microsomal preparations from various animal species as well as of cholesterol esterification in relevant human cell lines in culture. The concentrations of F 12511 required to produce 50% inhibition of ACAT activity (IC(50) values) in microsomal preparations ranged from 41nM for hypercholesterolemic rabbit intestine to 223 nM for normocholesterolemic hamster liver. In whole cell assays using hepatic (Hep G2), intestinal (CaCo-2) and macrophagic (THP-1) cell lines, F 12511 inhibited ACAT activity with IC(50) values of 3, 7, and 71 nM, respectively. In vivo, orally administered F 12511 displayed high potency and efficacy as an antihypercholesterolemic compound in different cholesterol-fed animals (rat, guinea-pig, rabbit). For instance, in guinea-pigs the dose required to reduce plasma cholesterol levels by 30% (ED(30) value) was 0.008 mg.kg(-1.) In rabbits, an animal species prone to atherosclerosis, the hypocholesterolemic effect was accompanied by a dose-related reduction in the incidence of aortic fatty streaks that reached asymptote at 2.5 mg.kg(-1) and by an improvement of the impaired endothelial function. When given orally to chow-fed hamsters, F 12511 elicited a dose-related decrease in plasma cholesterol from 9% at 0.63 mg.kg(-1) up to 31% at 40 mg.kg(-1) associated with a preferential reduction in atherogenic lipoproteins, very low density lipoproteins (VLDL) and low density lipoproteins (LDL). Moreover, in the same dose range, F 12511 decreased hepatic cholesteryl ester concentrations and reduced liver ex vivo ACAT activity. By using a bioassay, ACAT inhibitory activity was present in plasma of treated hamsters 1 hr after oral administration of F 12511. Hence, the results in chow-fed hamsters are consistent with systemic and direct hepatic effects of F 12511. In guinea-pigs, an adreno-sensitive species, F 12511 did not impair the adrenal function (adrenocorticotrophic hormone challenge) at doses up to 2.5 mg.kg(-1,) far higher than those eliciting hypocholesterolemic effects in the same species. In conclusion, the results suggest that F 12511, a powerful and systemic ACAT inhibitor, constitutes an appropriate tool to determine whether the inhibition of ACAT constitutes an effective therapy for the treatment of hypercholesterolemia and of atherosclerosis in man.

Current, new and future treatments in dyslipidaemia and atherosclerosis

The new therapeutic options available to clinicians treating dyslipidaemia in the last decade have enabled effective treatment for many patients. The development of the HMG-CoA reductase inhibitors (statins) have been a major advance in that they possess multiple pharmacological effects (pleiotropic effects) resulting in potent reductions of low density lipoproteins (LDL) and prevention of the atherosclerotic process. More recently, the newer fibric acid derivatives have also reduced LDL to levels comparable to those achieved with statins, have reduced triglycerides, and gemfibrozil has been shown to increase high density lipoprotein (HDL) levels. Nicotinic acid has been made tolerable with sustained-release formulations, and is still considered an excellent choice in elevating HDL cholesterol and is potentially effective in reducing lipoprotein(a) [Lp(a)] levels, an emerging risk factor for coronary heart disease (CHD). Furthermore, recent studies have reported positive lipid-lowering effects from estrogen and/or progestogen in postmenopausal women but there are still conflicting reports on the use of these agents in dyslipidaemia and in females at risk for CHD. In addition to lowering lipid levels, these antihyperlipidaemic agents may have directly or indirectly targeted thrombogenic, fibrinolytic and atherosclerotic processes which may have been unaccounted for in their overall success in clinical trials. Although LDL cholesterol is still the major target for therapy, it is likely that over the next several years other lipid/lipoprotein and nonlipid parameters will become more generally accepted targets for specific therapeutic interventions. Some important emerging lipid/lipoprotein parameters that have been associated with CHD include elevated triglyceride, oxidised LDL cholesterol and Lp(a) levels, and low HDL levels. The nonlipid parameters include elevated homocysteine and fibrinogen, and decreased endothelial-derived nitric oxide production. Among the new investigational agents are inhibitors of squalene synthetase, acylCoA: cholesterol acyltransferase, cholesteryl ester transfer protein, monocyte-macrophages and LDL cholesterol oxidation. Future applications may include thyromimetic therapy, cholesterol vaccination, somatic gene therapy, and recombinant proteins, in particular, apolipoproteins A-I and E. Non-LDL-related targets such as peroxisome proliferator-activating receptors, matrix metalloproteinases and scavenger receptor class B type I may also have clinical significance in the treatment of atherosclerosis in the near future. Before lipid-lowering therapy, dietary and lifestyle modification is and should be the first therapeutic intervention in the management of dyslipidaemia. Although current recommendations from the US and Europe are slightly different, adherence to these recommendations is essential to lower the risk of atherosclerotic vascular disease, more specifically CHD. New guidelines that are expected in the near future will encompass global opinions from the expert scientific community addressing the issue of target LDL goal (aggressive versus moderate lowering) and the application of therapy for newer emerging CHD risk factors.

Oxysterols: modulators of cholesterol metabolism and other processes

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

Metabolism of cholesterol is altered in the liver of C3H mice fed fats enriched with different C-18 fatty acids

We examined whether the degree of saturation of C-18 fatty acids influenced hepatic cholesterol metabolism in C3H mice. The mice were fed diets containing 20 g/100 g fat, enriched in stearic (18:0), oleic (18:1) or linoleic acid (18:2) with or without 1 g/100 g cholesterol. Plasma total cholesterol concentration was lower in mice fed the 18:0 diet relative to those fed the 18:1- or 18:2-enriched diets (P < 0.05) regardless of dietary cholesterol supplementation. Dietary cholesterol significantly raised hepatic total cholesterol concentration (P < 0.05) in those fed the 18:1- and 18:2-enriched diets, but not in mice fed the 18:0-enriched diet. Dietary cholesterol raised biliary cholesterol concentration (P < 0. 05) in mice fed the 18:1- and 18:2-enriched diets, but not in mice fed the 18:0-enriched diet. The cholesterol saturation index was variably affected by the fat diets. Feeding diets containing cholesterol suppressed the hepatic 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGR) activity and induced acyl coenzyme A:cholesterol acyl transferase (ACAT) activity compared with feeding diets without cholesterol (P < 0.05), indicating that the liver was exposed to dietary cholesterol. Hepatic ACAT activity was lower in mice fed the 18:0-enriched diet compared with those fed the 18:1- or 18:2-enriched diets (P < 0.05). Addition of cholesterol to the 18:1 diet induced the largest increase of hepatic ACAT activity, and this was associated with the enrichment of VLDL with cholesterol. Varying the degree of saturation of C-18 fatty acids influences the metabolism and disposition of hepatic cholesterol.

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

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

Effects of NTE-122, a novel acyl-CoA:cholesterol acyltransferase inhibitor, on cholesterol esterification and secretions of apolipoprotein B-containing lipoprotein and bile acids in HepG2

We studied the effect of NTE-122 (trans-1,4-bis[[1-cyclohexyl-3-(4-dimethylamino phenyl) ureido]methyl]cyclohexane), a novel acyl-CoA:cholesterol acyltransferase (ACAT) inhibitor, on intracellular cholesterol esterification and the secretion of apolipoprotein B100 (apoB)-containing lipoprotein and bile acids in the human hepatoma cell line HepG2. NTE-122 markably inhibited [3H]oleate incorporation into cholesteryl esters in HepG2 cells incubated with 5 microg/ml 25-hydroxycholesterol as a stimulus for ACAT (IC50=6.0 nM). On the other hand, NTE-122 did not affect [3H]oleate incorporation into triglycerides and phospholipids and [14C]acetate incorporation into cholesterol. The stimulation of ACAT by 25-hydroxycholesterol caused significant increases in the secretion of radiolabeled cholesteryl esters, radiolabeled triglycerides and apoB mass. NTE-122 pronouncedly inhibited the secretion of radiolabeled cholesteryl esters in proportion to the inhibition of cellular cholesterol esterification, and it significantly reduced the secretion of radiolabeled triglycerides and apoB mass in HepG2 cells incubated with 25-hydroxycholesterol. Furthermore, NTE-122 increased the secretion of bile acids synthesized from [14C]-cholesterol. These results suggest that NTE-122 is capable of exhibiting anti-hyperlipidemic effects by reducing both the cholesterol content and the amount of secreted very low-density lipoprotein and enhancing the excretion of bile acid from the liver.

Lack of predictability of classical animal models for hypolipidemic activity: a good time for mice?

Hypolipidemic drugs that are efficacious in man are not always active in classical animal models of dyslipidemia. Inhibitors of HMG-CoA reductase (statins) do not lower plasma cholesterol in rats, but yet this species was alone in providing activity for fibrate-type drugs. Nicotinic acid possesses many desirable features with regard to clinical use, but most of these actions are lacking in rats and monkeys. The metabolism of low density lipoproteins in hamsters is widely thought to be similar to that in humans, yet neither statins or fibrates lower plasma lipids in these species. With the advent of mouse models expressing specific human genes (or disruption of genes) it is now possible to re-examine the effect of established drugs and to characterize new hypolipidemic compounds with respect to site and mechanism of action. Drug responses observed in humans are now being seen in such mouse models (e.g. HDL elevation with fenofibrate in mice with the human apo A-I gene). Moreover, mice are now being screened for compounds that lower plasma (human) Lp(a), or lower plasma cholesterol in the absence of LDL receptors. It is proposed that these new genetic mouse models may afford a more focused examination of drug action and provide, for new compounds, better prediction of the human response.

Inhibition of cholesterol esterification by DuP 128 decreases hepatic apolipoprotein B secretion in vivo: effect of dietary fat and cholesterol

To further test the hypothesis that newly synthesized cholesteryl esters regulate hepatic apolipoprotein B (apoB) secretion into plasma, apoB kinetic studies were carried out in seven control miniature pigs and in seven animals after 21 days intravenous administration of the acyl coenzyme A:cholesterol acyltransferase (ACAT) inhibitor DuP 128 (2.2 mg/kg/day). Pigs were fed a fat (34% of calories; polyunsaturated/monounsaturated/saturated ratio, 1:1:1) and cholesterol (400 mg/day; 0.1%; 0.2 mg/kcal) containing pig chow based diet. DuP 128 significantly reduced total plasma triglyceride and very low density lipoprotein (VLDL) triglyceride concentrations by 36 and 31%, respectively (P<0.05). Autologous 131I-VLDL and 125I-LDL were injected simultaneously into each pig and apoB kinetic data was analyzed using multicompartmental analysis (SAAM II). The VLDL apoB pool size decreased by 26% (0.443 vs. 0.599 mg/kg; P<0. 001) which was due entirely to a 28% reduction in VLDL apoB production or secretion rate (1.831 vs. 2.548 mg/kg/h; P=0.006). The fractional catabolic rate (FCR) for VLDL apoB was unchanged. The LDL apoB pool size and production rate were unaffected by DuP 128 treatment. Hepatic microsomal ACAT activity decreased by 51% (0.44 vs. 0.90 nmol/min/mg; P<0.001). Although an increase in hepatic free cholesterol and subsequent decrease in both LDL receptor expression and LDL apoB FCR might be expected, this did not occur. The concentration of hepatic free cholesterol decreased 12% (P=0.008) and the LDL apoB FCR were unaffected by DuP 128 treatment. In addition, DuP 128 treatment did not alter the concentration of hepatic triglyceride or the activity of diacylglycerol acyltransferase, indicating a lack of effect of DuP 128 on hepatic triglyceride metabolism. In our previous studies, DuP 128 treatment of miniature pigs fed a low fat, cholesterol free diet, decreased VLDL apoB secretion by 65% resulting in a reduction in plasma apoB of 60%. We conclude that in miniature pigs fed a high fat, cholesterol containing diet, the inhibition of hepatic cholesteryl ester synthesis by DuP 128 decreases apoB secretion into plasma, but the effect is attenuated relative to a low fat, cholesterol free diet.

HMG-CoA reductase and ACAT inhibitors act synergistically to lower plasma cholesterol and limit atherosclerotic lesion development in the cholesterol-fed rabbit

Given the beneficial effects of HMG-CoA reductase and ACAT inhibitors on hypercholesterolemia and atherosclerosis, we hypothesized that coadministration would improve the hypolipidemic response and not only limit lesion development but also alter the cellular composition of atherosclerotic lesions so as to induce a stable atherosclerotic lesion morphology. Plasma total cholesterol exposure was reduced 29 and 39% with atorvastatin (2.5 mg/kg) and CI-976 (5 mg/kg), respectively, and 60% upon coadministration due primarily to reductions in VLDL-cholesterol. Modest changes in liver cholesterol ester (CE) content were observed with atorvastatin or CI-976; however, a striking 48% reduction was noted upon coadministration. Liver HMG-CoA reductase mRNA levels were reduced 73% by cholesterol feeding and drug treatment did not prevent the reduction; however, atorvastatin alone and upon coadministration blunted the decrease in LDL receptor mRNA levels. The CE content of the iliac-femoral was unaffected by atorvastatin but was reduced 35% by CI-976 and 53% upon coadministration. Thoracic aortic CE content was reduced 38% by atorvastatin, 48% by CI-976 and 80% upon coadministration. Iliac-femoral lesion and macrophage area were reduced 48 and 67% by atorvastatin, respectively, and 68 and 81% by CI-976 but upon coadministration only an 85% reduction in macrophage area was noted. Aortic arch cross-sectional lesion and macrophage area were unaffected by atorvastatin, decreased 72-80% by CI-976 and reduced 87-92% upon coadministration. We conclude that inhibition of HMG-CoA reductase and ACAT acts synergistically to lower plasma total and lipoprotein cholesterol levels and to limit the development of atherosclerotic lesions in the cholesterol-fed rabbit by presumably regulating cholesterol trafficking pathways within liver and vascular cells.

Structural and metabolic consequences of liposome-lipoprotein interactions

The two major proposed uses for liposomes, i.e., drug delivery and mobilization of peripheral deposits of cholesterol, each impose requirements and restrictions on liposomal structure, particularly as it affects interactions with lipoproteins. This chapter focuses on the role of lipoproteins and apolipoproteins in (1) disrupting membrane structure and causing the leakage of liposomal contents by inducing disc formation and (2) marking liposomes for whole-particle uptake by receptors involved in lipoprotein metabolism. Control of membrane stability and whole-particle half-life can be achieved by several strategies, such as membrane stiffening, shielding the membrane surface, and increasing the dose or predosing with "empty" liposomes. The rationales and applicabilities of these strategies are discussed in the contexts of liposomes as drug delivery vehicles and as antiatherogenic particles. Directions for further basic and applied research are also presented.

Relationship between bioavailability and hypocholesterolemic activity of YM17E, an inhibitor of ACAT, in cholesterol-fed rats

The relationship between bioavailability and the serum cholesterol-lowering effect of YM17E, an ACAT inhibitor was investigated. Serum cholesterol levels in cholesterol-fed rats decreased after both oral and intravenous administration of YM17E. Marked inhibition of cholesterol absorption was observed after oral administration, but not after intravenous administration. YM17E and its five active metabolites were primarily distributed in the liver after intravenous administration, but in small intestine and liver after oral administration. Hepatic ACAT activity in cholesterol-fed rats was inhibited by intravenous administration. Cholesteryl ester input into plasma by Triton WR-1339 treatment to the rats was inhibited by intravenous administration of YM17E. Plasma clearance of 125I-LDL in cholesterol-fed rats increased after YM17E treatment suggesting a decrease in LDL production. These results indicate that the hypocholesterolemic effect of intravenous YM17E was due to hepatic ACAT inhibition, not an inhibition of intestinal cholesterol absorption. The contribution of ACAT inhibition in small intestine and liver on the pharmacological effect could be explained by plasma inhibitor concentration after oral or intravenous administration of YM17E. From these results, it is concluded that the change in bioavailability of ACAT inhibitors change the mechanism of hypocholesterolemic effects, shifting the relative contributions of small intestinal and hepatic ACAT inhibition.

Hypolipidaemic properties of a potent and bioavailable alkylsulphinyl-diphenylimidazole ACAT inhibitor (RP 73163) in animals fed diets low in cholesterol

RP 73163 ((S)-2-[5-(3,5-dimethyl-l-pyrazolyl)pent-l-yl)-sulphinyl]-5, 6-diphenylimidazole) has been shown to be a potent and specific inhibitor of acyl-CoA:cholesterol acyltransferase (EC 2.3.1.26; ACAT) in vitro using the tissues of experimental animals as sources of the enzyme. The concentrations of RP 73163 required to produce 50% inhibition of ACAT activity (IC50 values) in microsomal preparations ranged from 86 nM for rat liver to 370 nM for rabbit intestine. In whole cell assays using human hepatic (HepG2), intestinal (Caco2), and monocytic (THP-1) cell lines, RP 73163 inhibited ACAT activity with IC50 values of 266, 158, and 314 nM, respectively. The addition of RP 73163 (0.03-1.0 microM) to the medium of cultured HepG2 cells produced a concentration-dependent decrease in apolipoprotein B (apoB) secretion. The compound has high systemic bioavailability. Using a bioassay, a concentration of active inhibitor equivalent to 29 microM of parent compound was present in plasma 1 hr after oral administration of RP 73163 (50 mg.kg-1). In rats that had been fed a basal diet ad libitum or starved for 18 hr prior to blood sampling, the administration of RP 73163 (50 mg.kg-1 b.i.d. for 7 days) reduced plasma triglyceride levels by 50% without affecting the concentration of cholesterol. This hypotriglyceridaemic effect was associated with reductions in plasma very-low-density-lipoprotein (VLDL) and low-density-lipoprotein (LDL) levels. RP 73163 decreased the rate of VLDL secretion by 24% in Triton WR-1339-treated rats that had been fasted overnight but did not affect the secretion rate in animals fed ad libitum, indicating that ACAT was only important in regulating VLDL secretion under certain nutritional conditions. RP 73163 reduced the accumulation of intraperitoneally administered [3H]leucine into the plasma VLDL-apoB pool in both fed and fasted states. The results suggest that, in fed animals at least, an increase in the clearance of VLDL from the bloodstream may contribute to the hypolipidaemic activity of the compound. In rabbits with casein-induced endogenous hypercholesterolaemia, RP 73163 specifically reduced the levels of cholesterol carried by LDL. In conclusion, the hypolipidaemic actions of RP 73163, a potent and systemically bioavailable ACAT inhibitor, are consistent with a reduction in the secretion of apoB containing lipoproteins by hepatic tissue and possibly with an increase in the clearance of these particles.

Effect of FR145237, a novel ACAT inhibitor, on atherogenesis in cholesterol-fed and WHHL rabbits. Evidence for a direct effect on the arterial wall

The hypocholesterolemic and antiatherosclerotic activities of FR145237, a novel acyl-CoA:cholesterol acyltransferase (ACAT) inhibitor, were evaluated in cholesterol-fed and Watanabe heritable hyperlipidemic (WHHL) rabbits. In the first experiment, rabbits were fed a high cholesterol (1% cholesterol) diet for 2 weeks and further fed a high cholesterol diet containing FR145237 for 8 weeks. FR145237 (0.1, 0.32 and 1.0 mg/kg) dose-dependently lowered the plasma total cholestrol levels by 80%, 96% and 97%, respectively. and reduced aortic atherosclerosis by 44%, 90% and 90%, respectively. To clarify a direct effect of FR145237 at the aortic wall, a second experiment was performed. Rabbits were fed a high-cholesterol diet for 8 weeks to establish aortic atherosclerosis and then fed a normal diet with or without FR145237 for 8 weeks. Cholesterol content in the aorta and the liver was significantly reduced in the FR145237 group (10 mg/kg) by 50% and 43%, respectively, though plasma total cholesterol level did not differ from that in the control group. In the WHHL rabbits, FR145237 (10 mg/kg) did not affect plasma cholesterol level but significantly reduced the atherosclerotic lesion in the coronary arteries by 61%. These results suggest that FR145237 potently lowers the plasma cholesterol level in hypercholesterolemia induced by dietary cholesterol but not that by LDL receptor deficiency, and that FR145237 has a direct antiatherosclerotic activity on the arterial wall independent of its hypocholesterolemic activity.

Lipid-lowering activity of atorvastatin and lovastatin in rodent species: triglyceride-lowering in rats correlates with efficacy in LDL animal models

Since inhibitors of HMG-CoA reductase lower plasma triglycerides rather than cholesterol in rats, we compared the triglyceride-lowering activity of lovastatin in rats to that of atorvastatin, a more potent synthetic inhibitor, prior to evaluating these drugs in established animal models in which low density lipoproteins (LDL) rather than high density lipoproteins (HDL) are the major transporters of plasma cholesterol. Atorvastatin was more efficacious than lovastatin in normal, chow-fed rats, and more potent in rats with endogenous hypertriglyceridemia (sucrose-fed). In hypertriglyceridemic rats plasma apoB concentrations decreased only with atorvastatin (30 mg/kg), and VLDL-triglyceride secretion (Triton method) was also decreased more by atorvastatin. The inactive enantiomer of atorvastatin did not lower plasma triglycerides. Thus, triglyceride-lowering was dependent upon inhibition of HMG-CoA reductase. Liver unesterified cholesterol and cholesteryl esters (mg/g) were increased by both drugs in normal rats but remained unchanged in hypertriglyceridemic rats. In normal, chow-fed guinea pigs atorvastatin was a more potent cholesterol-lowering drug, and unlike lovastatin, lowered plasma triglycerides and VLDL-cholesterol. In casein-fed rabbits with endogenous hypercholesterolemia and in chow-fed rabbits atorvastatin lowered LDL-cholesterol more potently than lovastatin, but in chow-fed rabbits neither drug had an effect on the in vivo rate of VLDL-lipid secretion, suggesting that efficacy was due to inhibition of direct LDL production and/or enhanced LDL clearance. We conclude that normal rats can be used as a preclinical tool to assess the efficacy of HMG-CoA reductase inhibitors since triglyceride-lowering correlates with cholesterol-lowering in LDL animal models. In this regard atorvastatin is a more potent hypolipidemic agent than lovastatin in animals. A common but not sole mechanism for these drugs may be direct inhibition of the hepatic production of the major apoB-containing lipoprotein in a given species, e.g. VLDL in rats and LDL in guinea pigs and rabbits.

Hypocholesterolemic action and prevention of cholesterol absorption via the gut by F-1394, a potent acyl-CoA:cholesterol acyltransferase (ACAT) inhibitor, in cholesterol diet-fed rats

In the present study, we investigated the hypocholesterolemic effect of F-1394 ((1s,2s)-2-[3-(2,2-dimethylpropyl)-3-nonylureido]aminocycloh exane-1-yl 3-[N-(2,2,5,5-tetramethyl-1,3-dioxane-4-carbonyl)amino] propionate), a potent and selective inhibitor of acyl-CoA:cholesterol acyltransferase (ACAT), and the effect on cholesterol absorption via the gut in rats fed a 1% cholesterol diet. Single administration of F-1394 to the cholesterol diet-fed rats at the doses of 3-30 mg/kg, p.o. decreased the serum cholesterol levels by 16-54% 3 hr after the administration. The ACAT activity in the small intestinal mucosa of the rats given orally F-1394 (30 mg/kg) was significantly inhibited 3 hr after the administration. The hypocholesterolemic action of F-1394 had a faster onset than that of DL-melinamide or CL-277,083. The study by the dual isotope ratio method showed that F-1394 (30 mg/kg, p.o.) significantly suppressed the dietary cholesterol absorption. Furthermore, in the determination of cholesterol absorption by using 14C-cholesterol as the oral tracer, the administration of F-1394 (30 mg/kg, p.o.) 1 or 2 hr before or immediately after the application of the oral tracer significantly prevented the appearance of the radioactivity in the circulation by around 90%. These results indicate that oral administration of F-1394 inhibits the ACAT activity in the small intestinal mucosa and subsequently contributes much to the prevention of cholesterol absorption via the gut, resulting in the decrease in serum cholesterol levels in the cholesterol diet-fed rats. Furthermore, the effect of F-1394 appears immediately after its administration in contrast to that of DL-melinamide or CL-277,082.

Comparative effects of HMG-CoA reductase inhibitors on apo B production in the casein-fed rabbit: atorvastatin versus lovastatin

Rabbits fed a diet enriched in casein develop an endogenous hypercholesterolemia (EH) due both to an increased low density lipoprotein (LDL) synthetic rate and decreased LDL receptor activity. Pre-established EH in this model was used to assess the ability and mechanism by which atorvastatin lowers total plasma cholesterol (TPC) compared to the reference agent lovastatin. Rabbits were fed a casein diet for 6 weeks, obtaining average TPC levels above 200 mg/dl. To ensure equivalent mean cholesterol concentrations, animals were randomized into treatment groups based on the 6-week TPC levels, and fed the casein diet alone or in combination with either atorvastatin or lovastatin for an additional 6 weeks. Under these conditions, new steady-state cholesterol values were established. Lipoprotein concentrations and distributions were determined at this point. Compared to pretreatment values, TPC were similar in untreated animals. Atorvastatin, however, significantly reduced TPC by 38%, 45%, and 54% at the 1, 3, and 10 mg/kg doses, respectively. Statistically significant lowering of TPC (35%) by lovastatin was only achieved at the 10 mg/kg dose. To determine the mechanism by which atorvastatin lowered TPC in the EH rabbits, kinetic studies using human [125I]-LDL were performed in a subset of animals maintained on the casein diet alone (n = 5), or those treated with 3 mg/kg of atorvastatin (n = 5) or lovastatin (n = 7). In this set of studies, atorvastatin significantly lowered TPC compared to control and lovastatin-treated rabbits by 57% and 46%, respectively. Lovastatin treatment resulted in a 20% decrease in TPC as compared to untreated controls.(ABSTRACT TRUNCATED AT 250 WORDS)

Unexpected inhibition of cholesterol 7 alpha-hydroxylase by cholesterol in New Zealand white and Watanabe heritable hyperlipidemic rabbits

We investigated the effect of cholesterol feeding on plasma cholesterol concentrations, hepatic activities and mRNA levels of HMG-CoA reductase and cholesterol 7 alpha-hydroxylase and hepatic LDL receptor function and mRNA levels in 23 New Zealand White (NZW) and 17 Watanabe heritable hyperlipidemic (WHHL) rabbits. Plasma cholesterol concentrations were 9.9 times greater in WHHL than NZW rabbits and rose significantly in both groups when cholesterol was fed. Baseline liver cholesterol levels were 50% higher but rose only 26% in WHHL as compared with 3.6-fold increase with the cholesterol diet in NZW rabbits. In both rabbit groups, hepatic total HMG-CoA reductase activity was similar and declined > 60% without changing enzyme mRNA levels after cholesterol was fed. In NZW rabbits, cholesterol feeding inhibited LDL receptor function but not mRNA levels. As expected, receptor-mediated LDL binding was reduced in WHHL rabbits. Hepatic cholesterol 7 alpha-hydroxylase activity and mRNA levels were 2.8 and 10.4 times greater in NZW than WHHL rabbits. Unexpectedly, cholesterol 7 alpha-hydroxylase activity was reduced 53% and mRNA levels were reduced 79% in NZW rabbits with 2% cholesterol feeding. These results demonstrate that WHHL as compared with NZW rabbits have markedly elevated plasma and higher liver cholesterol concentrations, less hepatic LDL receptor function, and very low hepatic cholesterol 7 alpha-hydroxylase activity and mRNA levels. Feeding cholesterol to NZW rabbits increased plasma and hepatic concentrations greatly, inhibited LDL receptor-mediated binding, and unexpectedly suppressed cholesterol 7 alpha-hydroxylase activity and mRNA to minimum levels similar to WHHL rabbits. Dietary cholesterol accumulates in the plasma of NZW rabbits, and WHHL rabbits are hypercholesterolemic because reduced LDL receptor function is combined with decreased catabolism of cholesterol to bile acids.

Studies on acyl-CoA: cholesterol acyltransferase (ACAT) inhibitory effects and enzyme selectivity of F-1394, a pantotheic acid derivative

(1s,2s)-2-[3-(2,2-Dimethylpropyl)-3-nonylureido]aminocyclohe xane-1-yl 3-[N-(2,2,5,5-tetramethyl-1,3-dioxane-4-carbonyl)amino]propionate (F-1394), a pantotheic acid derivative, is a newly synthesized inhibitor of acyl-CoA:cholesterol acyltransferase (ACAT). In the present study, we investigated the inhibitory effects of F-1394 on the activities of ACAT. F-1394 reduced the ACAT activities in rat liver microsomes, homogenate of rabbit small intestinal mucosa and lysate of J774 macrophages with IC50 values of 6.4 nM, 10.7 nM and 32 nM, respectively. The kinetic studies showed that F-1394 exerted competitive-type inhibition, and the Ki values in liver and small intestinal ACAT were 4.0 nM and 9.9 nM, respectively. The inhibitory effects of F-1394 on the activity of ACAT were more potent than that of other ACAT inhibitors or hypolipidemic agents. The study on enzyme selectivity indicated that F-1394 did not affect 3-hydroxy-3-methylglutaryl CoA reductase, acyl-CoA synthetase and cholesterol esterase. F-1394 weakly inhibited the activity of lecithine:cholesterol acyltransferase (LCAT) originating from rat plasma. The inhibitory potency of F-1394 for the activity of liver microsomal ACAT was 4,690-fold stronger than that for the activity of LCAT. These findings indicate that F-1394 is a potent and selective inhibitor of ACAT, and its inhibition manner is the competitive type.