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

Acyl-coenzyme A:cholesterol acyltransferase

Due to its presumed role in regulating cellular cholesterol homeostasis, and in various pathophysiological conditions, acyl-coenzyme A:cholesterol acyltransferase (ACAT) has attracted much attention. Cloning the ACAT gene provides the necessary tool to advance molecular studies of this enzyme. The topics reviewed in this chapter include the pathophysiological roles of ACAT, the biochemistry and molecular biology of the ACAT protein and the ACAT gene, and the mode of regulation by sterol or nonsterol agents in mammalian cells. In addition, we present a working model linking the presumed allosteric property of ACAT with cholesterol trafficking into and out of the endoplasmic reticulum.

Effects of hypothyroidism and high-fat feeding on mRNA concentrations for the low-density-lipoprotein receptor and on acyl-CoA:cholesterol acyltransferase activities in rat liver

1. Induction of hypothyroidism in rats by feeding propylthiouracil (PTU) significantly increased serum cholesterol concentrations, and the effect was more pronounced for cholesterol in low-density lipoproteins (LDL) rather than high-density lipoproteins (HDL). The concentrations of serum triacylglycerol were decreased in hypothyroidism. These effects on serum lipids were also seen when the normal rats were pair-fed with the PTU-treated group. 2. Feeding a diet rich in saturated fat and cholesterol further increased cholesterol concentrations in LDL and also elevated that in very-low-density lipoprotein (VLDL) of hypothyroid rats. In euthyroid rats such a diet resulted in a relatively small increase in VLDL cholesterol, whereas LDL cholesterol was decreased. 3. Steady-state concentrations of mRNA for the hepatic LDL receptor were significantly decreased in the livers of hypothyroid rats, but were not significantly changed by high-fat feeding in euthyroid or hypothyroid rats. 4. The expression of the LDL receptor in hepatocytes cultured from hypothyroid rats was decreased relative to the euthyroid controls. 5. Whereas the esterification of cholesterol with oleate in hepatocytes cultured from hypothyroid rats was decreased, the activity of acyl-CoA:cholesterol acyltransferase (ACAT) in the livers of these animals was not changed. 6. High-fat feeding increased the hepatic ACAT activity in normal and hypothyroid rats. 7. Incubation of rat hepatocytes with 10 nM-tri-iodothyronine for 4 h increased the relative concentration of the mRNA for the LDL receptor by 25%. 8. It is therefore concluded that thyroid hormones stimulate the synthesis and expression of the hepatic LDL receptor. Elevated cholesterol concentrations in LDL in hypothyroidism probably result from a primary defect in the expression of the hepatic receptor, rather than indirectly via changes in ACAT activity.

Effect of ischaemia on the activities of human hepatic acyl-CoA:cholesterol acyltransferase and other microsomal enzymes

The effect of ex vivo ischaemia at 37 degrees C on the activities of human hepatic acyl-CoA:cholesterol acyltransferase (ACAT), acyl-CoA hydrolase and carboxylesterase and on the microsomal cholesterol and total phospholipid concentrations was determined in liver tissue from two patients. ACAT activity decreased exponentially with ischaemia, corresponding to half-lives of 59 and 54 min in the two samples. Acyl-CoA hydrolase activity of microsomes was not affected by ischaemia, whereas carboxylesterase activity of microsomes decreased with a corresponding increase in cytosolic carboxylesterase activity. No changes in microsomal cholesterol and total phospholipid concentrations corresponding to the changes in ACAT or carboxylesterase activity were observed. ACAT activity was also determined in microsomes prepared from twenty human liver samples obtained at surgery with in vivo warm ischaemia times ranging from 5-120 min. The data obtained agree well with the ex vivo results, showing a half-life of 57 min for the loss of ACAT activity. Therefore, in comparing ACAT activities in liver samples with different ischaemia times, an appropriate correction must be made for warm ischaemia time.

Gender-related effects of chronic ethanol ingestion on rat hepatic acyl-CoA:cholesterol acyltransferase

The influence of chronic ethanol ingestion on hepatic acyl-CoA: cholesterol acyltransferase activity was investigated to determine the relationship between alcohol intake and cholesterol ester accumulation. Rats were given nutritionally complete liquid diets supplemented with 6.3% ethanol or an isocaloric equivalent of dextrin-maltose for 5 weeks. During this period, the hepatic acyl-CoA: cholesterol acyltransferase activity of ethanol-fed male rats remained constant, whereas the same activity in pair-fed controls as well as chow-fed rats exhibited a 30% decrease in activity. Unlike alcohol-fed male rats, the hepatic acyl-CoA: cholesterol acyltransferase activity of female rats decreased by approximately 30% by the fifth week of ethanol ingestion. Despite the fact that the gender of the animals led to disparate levels of acyl-CoA: cholesterol acyltransferase activity in response to ethanol ingestion, similar levels of cholesteryl ester accumulation were observed. The altered levels of acyl-CoA: cholesterol acyltransferase activity caused no significant change in the cholesterol concentration, cholesterol/phospholipid ratio, phospholipid fatty acid composition, or the membrane fluidity of the hepatic microsomes. We conclude that the altered hepatic acyl-CoA: cholesterol acyltransferase activity of ethanol-fed female rats cannot be directly responsible for ethanol-induced accumulation of cholesteryl esters.

Hepatic acyl-CoA:cholesterol acyltransferase. Development of a standard assay and determination in patients with cholesterol gallstones

A standard assay was developed for human liver acyl-CoA:cholesterol acyltransferase (ACAT, EC 2.3.2.26) which is more sensitive than previous methods and allows accurate activity determinations on crude microsomal fractions. ACAT activity was measured in microsomes from livers of four gallstone patients and five controls. Preincubation with exogenous cholesterol produced an increase in ACAT activity in all liver samples: gallstone samples showed a mean increase of 1.8-fold, whereas non-gallstone samples showed a mean increase of 8.2-fold. The mean ACAT activity measured in the presence of exogenous cholesterol was 52.8 +/- 22.8 (n = 4) pmol . min-1 . mg-1 for gallstone samples and 82.8 +/- 13.5 (n = 4) pmol . min-1 . mg-1 for non-gallstone samples. These results suggest that patients suffering from cholesterol gallstones have a reduced ability to esterify potentially harmful free cholesterol compared with controls. They support the proposition that cholesterol gallstone formation is related to altered hepatic cholesterol metabolism.

Cycloheximide sensitivity in regulation of acyl coenzyme A:cholesterol acyltransferase activity in Chinese hamster ovary cells. 1. Effect of exogenous sterols

Chinese hamster ovary cells grown in medium containing low-density lipoprotein (LDL) express high acyl coenzyme A:cholesterol acyltransferase (ACAT) activity as measured by an [3H]oleate pulse. Removal of LDL from the medium causes rapid inactivation of ACAT activity; the t1/2 for the initial inactivation rate is 0.8 h. Preincubation with protein synthesis inhibitors (cycloheximide or emetine) for 2 h or longer lengthens the t1/2 for the initial inactivation rate to approximately 2.1 h. When LDL is removed for more than 10 h, the cells contain only 3% of the original ACAT activity. Cycloheximide under this condition causes an 8-fold increase in ACAT activity; the increase approaches a maximum in 6-8 h. The extent of ACAT activation by cycloheximide inversely depends on exogenous sterol present in the medium; LDL diminishes the activation, while cationized LDL or 25-hydroxycholesterol completely abolishes the activation. Adding LDL back to the sterol-free medium causes a 40-70-fold increase in ACAT activity; however, the activation of LDL is not further augmented if the cells are pretreated with cycloheximide. The above observations are qualitatively confirmed by ACAT assays in vitro with cell homogenates. LDL or cycloheximide has no effect on the rates of 3H-labeled triglyceride and 3H-labeled polar lipid synthesis. Efflux of prelabeled cholesterol from cells is cycloheximide-insensitive. Rates of degradation of [3H]-leucine-pulse-labeled total protein in cells grown with or without LDL are identical. The above results imply the existence of at least one specific short-lived factor that directly or indirectly inhibits ACAT activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of acyl-CoA:cholesterol acyltransferase from neonatal chick liver

Endogenous cholesterol esterification in chick liver microsomes was catalyzed by acyl-CoA:cholesterol acyltransferase using palmitoyl-CoA as substrate. An acyl-CoA hydrolase activity was also found in our microsomal preparations. Acyltransferase activity was stable after microsomes storage at -40 degrees C for 6 weeks and increased linearly with the preincubation time between 0 and 45 min. In our assay conditions, cholesteryl ester formation was linear up to 0.3 mg of microsomal protein in the reaction vial and 10 min of incubation. Maximal activity was found in reactions carried out in the presence of 1-2 mM dithiothreitol and 1.2 mg of bovine serum albumin, while acyl-CoA hydrolase was clearly inhibited by increasing albumin amounts.

Regulation by thyrotropin of acyl-CoA: cholesterol acyltransferase in cultured thyroid cells

ACAT activity measured in microsomes of thyroid cells cultured for 4 days in the presence of TSH (1 mU/ml) was two or three times lower than that of the control cells cultured for the same period in the absence of TSH. The pool of cellular cholesterol which served as the ACAT substrate, was not exchangeable with exogenous cholesterol provided in the form of liposomes. However, the incubation of microsomes with liposomes made it possible to increase the cholesterol content in the microsomes by 35%, and this resulted in the activation of ACAT activity. Nevertheless, maximum activity measured after activation in the microsomes of the control cells remained higher than that of the microsomes of cells cultured in the presence of TSH. These findings would suggest that TSH acted by diminishing the cellular content in ACAT enzyme, as well as modifying the distribution of cholesterol in the intracellular membranes.