Effects of acyl-CoA: cholesterol O-acyltransferase inhibition on cholesterol absorption and plasma lipoprotein composition in hamsters

Multiple mechanisms of hypocholesterolemic action of pactimibe, a novel acyl-coenzyme A:cholesterol acyltransferase inhibitor

Novel acyl-coenzyme A:cholesterol acyltransferase inhibitor pactimibe has been evaluated in vivo; it exhibited significant serum cholesterol lowering activities in hamsters and monkeys without affecting non-high density lipoprotein cholesterol levels. The mechanism of the hypocholesterolemic action of pactimibe was examined in normocholesterolemic hamsters in this study. Results with the dual-isotope plasma ratio method indicated that pactimibe inhibits cholesterol absorption from the intestine, reduces cholesteryl ester formation in the liver, and enhances its elimination from the body. The Triton WR-1339 experiment showed that pactimibe inhibited secretion of very low density lipoprotein cholesterol from the liver. These results suggest that pactimibe is likely to have multiple mechanisms of action responsible for its effectiveness in reducing serum cholesterol.

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.

Synthesis of iodinated biochemical tools related to the 2-azetidinone class of cholesterol absorption inhibitors

The discoveries of Sch 48461 and Sch 58235 and their novel pharmacology of inhibition of cholesterol absorption have prompted efforts to determine their biological mechanism of action (MOA). To this end, a series of radioiodinated analogues with good to excellent in vivo activity have been designed and synthesized as single enantiomers. They are structurally consistent with the allowable SAR of the 2-azetidinone class of cholesterol absorption inhibitors.

Cholesterol-lowering activity of naringenin via inhibition of 3-hydroxy-3-methylglutaryl coenzyme A reductase and acyl coenzyme A:cholesterol acyltransferase in rats

The effects of dietary supplementation of a citrus bioflavonoid, naringenin, on the cholesterol metabolism were studied. For 42 days male rats were fed a 1% (wt/wt) high-cholesterol diet with or without a naringenin supplementation (0.1%, wt/wt) to study its effect on plasma lipid levels, hepatic lipid contents, activities of hepatic acyl coenzyme A:cholesterol O-acyltransferase (ACAT) and 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase, and the excretion of fecal neutral sterols. Naringenin did not significantly alter the concentration of plasma triglycerides, but lowered the plasma cholesterol (3.80 vs. 3.12 mmol/l) concentration and the hepatic cholesterol content (70.3 vs. 54.0 mg/g) significantly (p < 0.05) compared to those of the controls. HMG-CoA reductase (1,879.0 vs. 1,715.0 pmol/min/mg) and ACAT activities (806.0 vs. 563.0 pmol/min/mg) were significantly lower in the naringenin-supplemented group than in controls. Naringenin supplementation caused a marked decrease in the excretion of fecal neutral sterols (242.9 mg/day) compared to the controls (521.9 mg/day). These results show that naringenin lowers the plasma and hepatic cholesterol concentrations by suppressing HMG-CoA reductase and ACAT in rats fed a high-cholesterol diet.

Effects of F-1394, an acyl-CoA:cholesterol acyltransferase (ACAT) inhibitor, on ACAT activity in HepG2 cells and on hepatic secretion of lipids in Triton WR-1339-induced hyperlipidemic rats: possible role of hepatic ACAT in very low density lipoprotein secretion

We examined the inhibitory potency of F-1394 ((1S,2S)-2-[3-(2,2-dimethylpropyl)-3-nonylureido]cyclohexane -1-yl 3-[(4R)-N-(2,2,5,5-tetramethyl-1,3-dioxane-4-carbonyl)amino]propionate), an acyl-CoA:cholesterol acyltransferase (ACAT) inhibitor, on ACAT activity and its hypolipidemic effect. F-1394 inhibited whole-cell ACAT activity in HepG2 cells with an IC50 value of 42 nM. The potency of F-1394 was greater than that of the five other ACAT inhibitors tested (YM-17E, CI-976, 57-118, CL-277,082 and DL-melinamide). In rats made hyperlipidemic by Triton WR-1339, F-1394 caused a reduction in the hepatic secretion rate of cholesterol. These data suggest that inhibition of hepatic ACAT activity helps to reduce very low density lipoprotein secretion from the liver into the circulation.

Effect of chronic central administration of glucagon-like peptide-1 (7-36) amide on food consumption and body weight in normal and obese rats

Glucagon-like peptide (7-36) amide (GLP-1) acutely inhibits food and water consumption in rats after intracerebroventricular (icv) administration. To assess the potential for desensitization of these effects, we investigated the effects of chronic icv administration of GLP-1 on food consumption and body weight in Sprague-Dawley (SD) rats and Zucker (fa/fa) obese rats. In vitro functional densensitization of the GLP-1 receptor was not observed after overnight exposure of Rin m5F insulinoma cells to GLP-1 at concentrations up to 10 nM. Administration of GLP-1 to SD rats (30 microg icv twice a day for 6 days) resulted in significant reductions in 24-hour food consumption each day (25 +/- 1%). Continuous icv infusion of GLP-1 for 7 and 14 days significantly inhibited cumulative food consumption and reduced body weight in SD rats. In the genetically obese Zucker rat, chronic dosing with GLP-1 (30 microg icv) once a day for 6 days caused significant reductions in food consumption each day and a reduction in body weight. These results indicate that the GLP-1 pathways in the central nervous system controlling food consumption do not desensitize after chronic exposure to GLP-1 and suggest that agonists of the central GLP-1 receptor may be effective agents for the treatment of obesity.

Reduced cholesterol absorption in hamsters by crilvastatin, a new 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitor

Crilvastatin, a new drug from the pyrrolidone family, has been previously shown to inhibit the activity of 3-hydroxy-3-methylglutaryl coenzyme A reductase, in vitro and in vivo, to reduce the absorption of dietary cholesterol and to stimulate the activity of cholesterol 7 alpha-hydroxylase in the rat. The aim of this study was to evaluate the effects of crilvastatin on cholesterol and bile acid metabolism in the hamster. In hamsters fed on a lithogenic diet for 8 weeks, crilvastatin treatment (200 mg/day per kg body weight) did not change plasma lipid levels, failed to improve bile parameters and did not prevent gallstone formation. In hamsters fed on a basal cholesterol-rich (0.2%) diet for 8 weeks, crilvastatin at the same dose reduced the cholesterol level in the plasma by 20%, with a decrease of both low-density and high-density lipoprotein cholesterol. The drug did not significantly stimulate the biliary secretion of bile acids but significantly decreased the activity of acyl coenzyme A:cholesterol acyltransferase in the small intestine by 64%. This effect was enhanced when cholestyramine, a bile acid-sequestering resin, was given in combination with crilvastatin. Crilvastatin alone did not change the activity of cholesterol 7 alpha-hydroxylase in the liver, despite the marked reduction in both hepatic cholesterogenesis and intestinal absorption of dietary cholesterol (the absorption coefficient was 44 +/- 2% in treated hamsters vs. 61 +/- 7% in controls).

Cholesterol esterification is not essential for secretion of lipoprotein components by HepG2 cells

Hepatic acyl CoA:cholesterol acyltransferase (ACAT) activity may determine storage of cholesterol and supply of cholesteryl esters for the neutral lipid core of very low density lipoprotein. Inhibition of cholesterol esterification in HepG2 cells, by the ACAT inhibitor 447C88, partially reduced the secretion of labelled total cholesterol, but the secretion of apoprotein B mass, and of radiolabelled triacylglycerol and phosphatidylcholine were unaffected. Furthermore, this compound was shown to substantially deplete the intracellular cholesteryl ester mass without affecting secretion of lipoprotein components. In contrast, the less potent ACAT inhibitor, CL277,082, significantly decreased secretion of labelled triacylglycerol, phosphatidylcholine and total cholesterol, in a manner which mirrored the decreases in secretion of apoB. This study clearly illustrates that ACAT inhibitors can exert differential effects on secretion of apoB-containing lipoproteins, which do not correlate with their efficacy in inhibiting ACAT, arguing that cholesterol esterification is not essential for lipoprotein secretion from these cells.

Amides of piperidine, morpholine and piperazine substituted 1-phenylethylamines: inhibitors of acylCoA:cholesterol acyltransferase (ACAT) activity in vitro and in vivo

Amides of some substituted 1,2-diarylethylamines have been shown to exhibit potent acylCoA:cholesterol acyltransferase (ACAT, EC 2.3.1.26) inhibitory activity in vitro in microsomal ACAT assays but show poor in vivo activity in a cholesterol-fed hamster model. In an effort to design ACAT inhibitors that are potent in both our in vitro and in vivo assays a series of amides of piperidine, morpholine and piperazine substituted 1-phenylethylamines were synthesized. Compounds of this series were found to be very potent inhibitors of ACAT in a microsomal ACAT assay and also exhibited potent activity in a cholesterol-fed hamster model.

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.

Hypocholesterolemic activity of a novel inhibitor of cholesterol absorption, SCH 48461

The amount of cholesterol that circulates in the plasma as lipoproteins can be affected by the balance of cholesterol metabolism within and between the intestines and liver. In the present report, we describe a novel hypocholesterolemic agent and document its pharmacological effects in animal models of hypercholesterolemia. The oral administration of (3R,4S)-1,4-bis-(4-methoxyphenyl)-3-(3-phenylpropyl)-2-azetidinone (SCH 48461) reduced plasma cholesterol concentrations in cholesterol-fed hamsters, rats and rhesus monkeys with ED50s of 1, 2 and 0.2 mg/kg per day, respectively, SCH 48461 was also highly effective in reducing hepatic cholesteryl ester accumulation in cholesterol-fed hamsters and rats after 7 days of treatment. In one 3 week study, rhesus monkeys were fed a 0.25% cholesterol/22% saturated fat diet with or without SCH 48461. At the end of the 3 week period the control group's VLDL + LDL-cholesterol increased to 180 Mg/dl from a baseline of approximately 65 mg/dl while plasma apolipoprotein B levels had doubled. Animals treated daily with 1 mg/kg SCH 48461 maintained their baseline levels of VLDL + LDL-cholesterol, HDL-cholesterol, and plasma apolipoproteins B and A-I. After 3 weeks the diets of the two groups were switched. Within 1 week SCH 48461 (1 mg/kg per day) rapidly reversed the elevated VLDL + LDL-cholesterol levels of the previous control group to near baseline values. SCH 48461 exerted its hypocholesterolemic effect through the inhibition of cholesterol absorption. A dose of 10 mg/kg per day inhibited cholesterol absorption in cholesterol-fed hamsters by 68% while a similar reduction was achieved in chow-fed monkeys with 3 mg/kg per day. This latter dose inhibited cholesterol absorption in cholesterol-fed monkeys by 95%. Treatment of cholesterol-fed monkeys with 10 mg/kg per day SCH 48461 significantly increased fecal neutral sterol excretion (52 vs. 32 mg/kg) but had no effect on acidic sterol excretion. Using a 2-h absorption model in cholesterol-fed hamsters, SCH 48461 caused a 46% inhibition of unesterified [14C]cholesterol accumulation in the intestinal wall and a 90% inhibition of cholesteryl ester formation at a dose of 10 mg/kg. Similar data were observed when the plasma radioactivity was assessed, indicating inhibition of both free (61%) and esterified (85%) cholesterol appearance. In contrast, CI-976, a potent acyl-CoA:cholesterol acyltransferase (ACAT) inhibitor, did not affect the uptake of free cholesterol into the intestines while inhibiting cholesterol esterification (98% inhibition).(ABSTRACT TRUNCATED AT 400 WORDS)

Demonstration of a direct effect on hepatic acyl CoA: cholesterol acyl transferase (ACAT) activity by an orally administered enzyme inhibitor in the hamster

Orally active inhibitors of acyl CoA:cholesterol acyl transferase (ACAT), such as Lederle CL277082 (LE), are known to reduce plasma and hepatic cholesteryl ester levels, although the mechanisms are not well understood. Several groups have reported the inhibition of cholesterol absorption upon oral ACAT inhibitor administration. In this study, we used 7-day dietary and drug treatments of hamsters to examine the possible effects of LE on hepatic ACAT. ACAT assays were performed using liver homogenates in the absence and presence of a saturating level of exogenously added cholesterol. LE (100 mg/kg/day) treatment of chow or 0.5% cholesterol-fed animals caused reductions in ACAT activity without additional cholesterol as compared with non-treated animals. When a saturating level of cholesterol was added to the assays, reductions in ACAT activity upon LE treatment of chow- or cholesterol-fed animals were also observed. Treatment of cholesterol-fed animals with cholestyramine in the diet reduced ACAT activity in the absence of added cholesterol. However, ACAT activities similar to those of non-treated animals were observed at a saturating level of cholesterol. This latter effect demonstrates that inhibition of cholesterol absorption reduces cholesterol delivery to the liver but does not reduce cholesterol esterifying capacity since cholestyramine is not absorbed and has no direct effect on the liver. The decreased ACAT activity in homogenates from LE-treated animals could also be mimicked in a dose-dependent manner by the addition of exogenous LE to liver homogenates from non-treated animals. These results indicate that hepatic ACAT activity is regulated by the availability of free cholesterol, and that orally administered LE has a direct effect on hepatic ACAT activity in the liver. In addition, the data are consistent with LE activity in the liver as being responsible, in part, for the reduced hepatic and plasma cholesteryl esters in treated animals.

Imidazolylbenzopyrane derivatives: a new class of acyl-CoA: cholesterol acyltransferase (ACAT) inhibitors

Inhibitors of the enzyme Acyl-CoA: Cholesterol Acyltransferase are regarded as potentially useful agents in the treatment of hypercholesterolemia and atherosclerosis. We report here a novel series of 2, 6-disubstituted-3-imidazolylbenzopyrane derivatives with significant in vitro ACAT inhibitory activity (IC50 range 0.05-0.5 microM). Compounds of this series such as 26 are examples of a new, structurally distinct class of potent ACAT inhibitors with high specificity for the aortic subtype of the enzyme. The structure-activity relationships of the 3-imidazolylbenzopyrane ACAT inhibitors were investigated by systematic manipulation of two regions of the parent compound 1 and the inhibitory activity resulted linked to the substituent in position 6 of the benzopyrane ring and modulated by the size of lipophilic substituents in position 2. Investigation of the mechanism of the inhibitory effect leads to the conclusion that these compounds act in a non-competitive fashion.

Effect of the ACAT inhibitor CI-976 on plasma cholesterol concentrations and distribution in hamsters fed zero- and low-cholesterol diets

The overall objective of the present study was to determine if the ACAT inhibitor CI-976 can lower plasma cholesterol in hamsters fed zero or low, "human-like" levels of cholesterol. With a purified diet containing zero dietary cholesterol, CI-976 significantly lowered VLDL cholesterol (VLDL-C), but not total plasma cholesterol (TPC). When 0.06% cholesterol was added to this diet, reductions in both VLDL and LDL cholesterol (LDL-C) lowered TPC. Efficacy was still greater with 0.2% dietary cholesterol, but not potency. Mixing CI-976 into the purified diet resulted in greater decreases in VLDL-C compared to gavage administration, but LDL-C reductions with 0.2% cholesterol were optimal with gavage. With nonpurified, chow-based diets efficacy was markedly greater with diet-admix administration, regardless of the amount of dietary cholesterol. CI-976 inhibited cholesterol absorption with chow-based diets more potently compared to nonabsorbable agents (e.g., beta-sitosterol, tigogenin cellobioside), and the lowering of LDL-C was greatest when inhibition of cholesterol absorption was maximal. We conclude that the ACAT inhibitor CI-976 is efficacious in hamster models which utilize human-like levels of dietary cholesterol. Moreover, the data suggest that the pharmacologic responses to lipophilic ACAT inhibitors in the hamster, or even other lipid-regulating drugs, are likely to depend not only on the type of basal diet but also on the mode of drug administration.