Esterification of cholesterol by subcellular fractions from swine arteries, and inhibition by amphipathic and polyanionic compounds

Regulation of cholesteryl ester hydrolases

Recent developments in understanding the biochemical and molecular nature of the CE hydrolases and their impact on cellular cholesterol trafficking have further defined the enzyme's mechanism of action with reasonable clarity. The availability of the cDNA probe for the human lysosomal acid lipase/CE hydrolase and the hormone-sensitive lipase now makes it possible to study CE hydrolase gene regulation and expression in human tissue; and it can now be stated with more assurance that the cytoplasmic CE hydrolase (NCEH) is most likely activated through phosphorylation by the cyclic AMP-dependent protein kinase. Evidence also shows that the NCEH is most likely identical to the hormone-sensitive lipase and that it plays an important role in cholesterol efflux properties of the cell. Recent advances in the discovery of the role of the eicosanoid/cytokine network in the regulation of CE hydrolysis, highlighted in Figure 10, further emphasize the interesting but complex nature of the cholesterol trafficking processes in cells, particularly under pathophysiological conditions such as cell injury, repair, and inflammation. It can be speculated that in several years, when the crystal structure of the CE hydrolase is known, the structure-function properties of this enzyme's catalytic domain, as it relates to the physical state of the CE substrates, should further clarify the precise role of this enzyme in intracellular cholesterol mobilization and trafficking under a variety of cellular conditions.

Dietary induction of pancreatic cholesterol esterase: a regulatory cycle for the intestinal absorption of cholesterol

Atherosclerosis has a strong dietary basis without a proven molecular mechanism for cholesterol absorption. To investigate the potential role of pancreas in this process and its interaction with the two dietary forms of cholesterol (free and esterified), we undertook to study the role of pancreatic cholesterol esterase in cholesterol absorption. The results showed that (i) cholesterol esters contribute a disproportionately high fraction of absorbed dietary cholesterol, (ii) rates of intestinal cholesterol absorption are related to pancreatic cholesterol esterase activity, (iii) mRNA specific for pancreatic cholesterol esterase is induced 15-fold by dietary sterol esters and 10-fold by free sterol, (iv) the induction of cholesterol esterase mRNA is reversible, and (v) free cholesterol transport into cultured human intestinal cells is enhanced 300% by pancreatic cholesterol esterase. These data implicate pancreatic cholesterol esterase as pivotal in a metabolic loop under positive feedback control for the absorption of dietary cholesterol, whether free or esterified.

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

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

Acyl coenzyme A:cholesterol acyltransferase in neonatal chick brain

An acyl coenzyme A:cholesterol acyltransferase activity which directly incorporates palmitoyl coenzyme A into cholesterol esters using endogenous cholesterol as substrate was demonstrated in microsomal preparations from neonatal chick brain. The enzyme showed, at pH 7.4, about 2-fold greater activity than that observed at pH 5.6. Nearly 10-times higher esterifying activity was found in brain microsomes using palmitoyl coenzyme A than that with palmitic acid. The acyltransferase activity was clearly different from the other cholesterol-esterifying enzymes previously found in brain, which incorporated free fatty acids into cholesterol esters and did not require ATP or coenzyme A as cofactors. Chick brain microsomes also incorporated palmitoyl coenzyme A into phospholipids and triacylglycerols. However, most of the radioactivity from this substrate was found in the fatty acid fraction, due to the presence of an acyl coenzyme A hydrolase activity in the enzyme preparations. Therefore, the formation of palmitate was tested during all the experiments. The brain acyltransferase assay conditions were optimized with respect to protein concentration, incubation time and palmitoyl coenzyme A concentration. Microsomal activity was independent of the presence of dithiothreitol in the incubation medium and microsomes can be stored at -40 degrees C for several weeks without losing activity. Addition of fatty acid-free bovine serum albumin to brain microsomal preparations produced a considerable increase in the acyltransferase activity, while acyl coenzyme A hydrolase was clearly inhibited. Results obtained show the existence in neonatal chick brain of an acyl coenzyme A:cholesterol acyltransferase activity similar to that found in a variety of tissues from different species but not previously reported in brain.

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

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

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

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

Esterification of cholesterol in human small intestine: the importance of acyl-CoA:cholesterol acyltransferase

Human intestinal mucosa contains acyl-CoA:cholesterol acyltransferase activity. The enzyme has been studied by using oleylcarnitine, CoA and carnitine palmitoyltransferase as an oleyl-CoA regenerating system. The enzyme was found in the particulate fraction of the cells, it had a pH optimum between 7.2 and 8.2, and was inhibited by taurocholate. The specific enzymic activity in biopsies from intestinal mucosa of normal men was found to be 3.6 +/- 1.37 nmol cholesteryl ester formed mg protein-1 h-1, an activity which can account for all cholesteryl esters in intestinal lymph. Low enzymic activity was found in biopsies from patients with small intestinal disorders. Two pancreatectomized patients had values within the normal range.

Properties and subcellular distribution of acyl-CoA: cholesterol acyltransferase (ACAT) in guinea-pig liver

The optimal in vivo conditions for measuring liver acyl-CoA:cholesterol acyltransferase, EC.2.2.1.26 (ACAT) activity in cytoplasmic extract from guinea-pigs have been examined. ACAT in liver from ordinarily fed guinea-pigs is very low. The effect of different compounds on ACAT activity was evaluated. Sodium fluoride, oligomycin, divalent cations, oleyl-CoA, and bile acids were all shown to inhibit ACAT activity markedly. Albumin stimulated the cholesterol esterification independently of the presence of fatty acids. Subcellular distribution of liver ACAT in relation to different marker enzymes shows that the bulk of ACAT activity is present in the microsomal fraction. By comparing the esterification of exogenous and endogenous cholesterol, the results indicate that these two substrates are not esterified to the same extent. Both the 'enzyme" level and substrate quality are of importance for the high ACAT activity found in rat-liver homogenates compared to guinea-pig.

Inhibition of cholesterol esterification in rabbit aorta by prostaglandin E2

Cholesterol esterification in the arterial wall was investigated with cell-free preparations of intima-media from control rabbits and rabbits rendered atherosclerotic by feeding a diet containing 1% cholesterol. In the presence of 2 mM ATP and 0.1 mM CoA, the major activity for esterification of [4-14C] cholesterol added in vitro was found in the 12,000 g and 105,000 g pellets. In control animals, the activity in the latter pellet was twice that in the former. After cholesterol-feeding for 6 months, the activity increased 5-fold in the 105,000 g pellet and 2-fold in the 12,000 g pellet of the atherosclerotic intima-media. Prostaglandin E2 (PGE2) in concentrations between 2 and 12 X 10(-7) M exhibited a dose-dependent inhibition of the esterifying activity in both particulate preparations. The inhibition was 97% at PGE2 concentrations greater than 1.2 X 10(-6) M in preparations from control animals. Inhibition by PGE2 in preparations from atherosclerotic rabbits was also observed. These results suggest a possible regulatory role of PGE2 in cholesterol esterification in the arterial wall.

Study of drugs affecting cholesterol-induced atherosclerosis in rabbits

1. Effects of various drugs on cholesterol-induced atherosclerosis in rabbits during the progression phase have been studied. The drugs tested are antimetabolites (mercaptopurine, hydroxyurea), surface active agents (sodiumdodecyl sulfate), inhibitor of adrenocoritcal steroid synthesis (o, p'-DDD), lysosome stablizers (chloroquine, acetylsalicylic acid) with antihistaminic (chlorpheniramine) and cholesterol binder (nystatin). 2. Mercaptopurine treatment showed marekd reduction in both atherosclerotic lesions and cholesterol concentrations of the serum and aorta. 3. Hydroxyurea reduced both the aortic cholesterol concentration and the lesions, but the serum cholesterol concentration remained high. 4. Sodiumdodecyl sulfate and o, o'-DDD showed slight inhibition of the development of atherosclerosis. 5. Pyridinocarbamate showed a slight beneficial effect on the prevention of atherosclerosis only when it was administered prior to the meal. 6. Nystatin, chloroquine and acetylsalicylic acid + chlorpheniramine showed little effect.

Effects of administration of hypolipidemic agent, 2,2'''- [ (1-methyl-4, 4-diphenylbutylidene) bis (p-phenyleneoxy)] bistriethylamine oxalate (SQ 10,591) upon cholesterol esterification by aorta, adrenal, and testes of cholesterol-fed rabbits]

Administration of 2,2'''-([1-methyl-4, 4-diphenylbutylidene] bis(p-phenyl-eneoxy]) bistriethylamine oxalate (SQ 10,591) at 20 mg/Kg daily for 5 days to cholesterol-fed rabbits resulted in no change in aortic microsomal cholesterol esterification with a palmitoyl coenzyme A substrate or of aortic mitochondrial cholesterol esterification with a palmitate substrate. Esterification by both reactions in the adrenal was much higher than in either aorta or testes. Adrenal and testicular mitochondrial esterification and testicular microsomal esterification were inhibited significantly after SQ 10,591 administration. In vitro addition of 0.0001 M SQ 10,591 significantly inhibited both microsomal and mitochondrial cholesterol esterification in aorta, adrenal, and testes.