Influence of pravastatin, a specific inhibitor of HMG-CoA reductase, on hepatic metabolism of cholesterol

BACKGROUND

Inhibitors of the rate-limiting enzyme of cholesterol biosynthesis, 3-hydroxy-3-methyl-glutaryl coenzyme A (HMG-CoA) reductase, are now used frequently to treat hypercholesterolemia. We studied the effects of specific inhibition of cholesterol synthesis by one of these agents (pravastatin) on the hepatic metabolism of cholesterol in patients with gallstone disease who were scheduled to undergo cholecystectomy.

METHODS

Ten patients were treated with pravastatin (20 mg twice a day) for three weeks before cholecystectomy; 20 patients not treated served as controls. A liver specimen was obtained from each patient at operation, and the activities of rate-determining enzymes in cholesterol metabolism as well as low-density-lipoprotein (LDL)-receptor binding activity were determined.

RESULTS

Pravastatin therapy reduced plasma total cholesterol by 26 percent and LDL cholesterol by 39 percent (P less than 0.005). Serum levels of free lathosterol, a precursor of cholesterol whose concentration reflects the rate of cholesterol synthesis in vivo, decreased by 63 percent (P less than 0.005), indicating reduced de novo biosynthesis of cholesterol. Microsomal HMG-CoA reductase activity, when analyzed in vitro in the absence of the inhibitor, was increased 11.8-fold (1344 +/- 311 vs. 105 +/- 14 pmol per minute per milligram of protein in the controls; P less than 0.001). The expression of LDL receptors was increased by 180 percent (P less than 0.005), whereas the activities of cholesterol 7 alpha-hydroxylase (which governs bile acid synthesis) and of acyl-coenzyme A:cholesterol O-acyltransferase (which regulates cholesterol esterification) were unaffected by treatment.

CONCLUSIONS

Inhibition of hepatic HMG-CoA reductase by pravastatin results in an increased expression of hepatic LDL receptors, which explains the lowered plasma levels of LDL cholesterol.

Immunological quantitation and localization of ACAT-1 and ACAT-2 in human liver and small intestine

By using specific anti-ACAT-1 antibodies in immunodepletion studies, we previously found that ACAT-1, a 50-kDa protein, plays a major catalytic role in the adult human liver, adrenal glands, macrophages, and kidneys but not in the intestine. Acyl-coenzyme A:cholesterol acyltransferase (ACAT) activity in the intestine may be largely derived from a different ACAT protein. To test this hypothesis, we produced specific polyclonal anti-ACAT-2 antibodies that quantitatively immunodepleted human ACAT-2, a 46-kDa protein expressed in Chinese hamster ovary cells. In hepatocyte-like HepG2 cells, ACAT-1 comprises 85-90% of the total ACAT activity, with the remainder attributed to ACAT-2. In adult intestines, most of the ACAT activity can be immunodepleted by anti-ACAT-2. ACAT-1 and ACAT-2 do not form hetero-oligomeric complexes. In differentiating intestinal enterocyte-like Caco-2 cells, ACAT-2 protein content increases by 5-10-fold in 6 days, whereas ACAT-1 protein content remains relatively constant. In the small intestine, ACAT-2 is concentrated at the apices of the villi, whereas ACAT-1 is uniformly distributed along the villus-crypt axis. In the human liver, ACAT-1 is present in both fetal and adult hepatocytes. In contrast, ACAT-2 is evident in fetal but not adult hepatocytes. Our results collectively suggest that in humans, ACAT-2 performs significant catalytic roles in the fetal liver and in intestinal enterocytes.

ACAT2 is localized to hepatocytes and is the major cholesterol-esterifying enzyme in human liver

BACKGROUND

Two acyl-coenzyme A:cholesterol acyltransferase (ACAT) genes, ACAT1 and ACAT2, have been identified that encode 2 proteins responsible for intracellular cholesterol esterification.

METHODS AND RESULTS

In this study, immunohistology was used to establish their cellular localization in human liver biopsies. ACAT2 protein expression was confined to hepatocytes, whereas ACAT1 protein was found in Kupffer cells only. Studies with a highly specific ACAT2 inhibitor, pyripyropene A, in microsomal activity assays demonstrated that ACAT2 activity was highly variable among individual human liver samples, whereas ACAT1 activity was more similar in all specimens. ACAT2 provided the major cholesterol-esterifying activity in 3 of 4 human liver samples examined.

CONCLUSIONS

The data suggest that in diseases in which dysregulation of cholesterol metabolism occurs, such as hypercholesterolemia and atherosclerosis, ACAT2 should be considered a target for prevention and treatment.

Regulation and immunolocalization of acyl-coenzyme A: cholesterol acyltransferase in mammalian cells as studied with specific antibodies

Acyl-coenzyme A:cholesterol acyltransferase (ACAT) catalyzes the formation of intracellular cholesterol esters in various tissues. We recently reported the cloning and expression of human macrophage ACAT cDNA. In the current study, we report the production of specific polyclonal antibodies against ACAT by immunizing rabbits with the recombinant fusion protein composed of glutathione S-transferase and the first 131 amino acids of ACAT protein. Immunoblot analysis showed that the antibodies cross-reacted with a 50-kDa protein band from a variety of human cell lines. These antibodies immunodepleted more than 90% of detergent-solubilized ACAT activities from six different human cell types, demonstrating that the 50-kDa protein is the major ACAT catalytic component in these cells. In multiple human tissues examined, the antibodies recognized protein bands with various molecular weights. These antibodies also cross-reacted with the ACAT protein in Chinese hamster ovary cells. Immunoblot analysis showed that the ACAT protein contents in human fibroblast cells, HepG2 cells, or Chinese hamster ovary cells were not affected by sterol in the medium, demonstrating that the main mechanism for sterol-dependent regulation of ACAT activity in these cells is not change in ACAT protein content. As revealed by indirect immunofluorescent microscopy, the ACAT protein in tissue culture cells was located in the endoplasmic reticulum. This finding, along with earlier studies, suggests that cholesterol concentration in the endoplasmic reticulum may be the major determinant for regulating ACAT activity in the intact cells.

Quantitation of the pool of cholesterol associated with acyl-CoA:cholesterol acyltransferase in human fibroblasts

The esterification of cholesterol in homogenates of human fibroblasts was explored as a means of estimating the size of the pool of cholesterol associated with the endoplasmic reticulum (ER) in vivo. The rationale was that the acyl-coenzyme A:cholesterol acyltransferase (ACAT) in homogenates should have access only to cholesterol associated with the (rough) ER membrane fragments in which it resides. Reacting whole homogenates to completion with an excess of [14C]oleoyl-CoA converted approximately 0.1-2% of total cell-free cholesterol to [14C]cholesteryl esters. Control studies indicated that membranes not associated with ACAT did not contribute cholesterol to this reaction. The extent of in vitro cholesterol esterification varied with pretreatment of the cells. Exposing intact cells to serum lipoproteins, oxysterols, or sphingomyelinase increased cholesterol esterification in homogenates severalfold; exposing the cells to mevinolin or cholesterol oxidase had the opposite effect. The variation in cholesterol esterification did not correlate with either the total cellular cholesterol or the intrinsic activity of ACAT, neither of which was changed significantly by the pretreatments. Rather, the total amount of cholesterol esterified in homogenates paralleled the rate of cholesterol esterification in the corresponding intact cells. The pool of cholesterol esterified in vitro therefore appears to reflect that associated with the ER in vivo. Since several of the mechanisms keeping cell cholesterol under tight feedback control are themselves located in the ER, this pool might not only be regulated physiologically, but could, in turn, help to regulate homeostatic effector pathways.

Hepatic ACAT activity in African green monkeys is highly correlated to plasma LDL cholesteryl ester enrichment and coronary artery atherosclerosis

Previous studies and this study of African green monkeys show a strong positive correlation between plasma low density lipoprotein (LDL) size and the extent of coronary artery atherosclerosis (CAA). Increased LDL size was principally due to the accumulation of cholesteryl oleate molecules within the particle core, suggesting that many of these cholesteryl esters were of tissue origin, i.e., from the acyl-coenzyme A:cholesterol acyltransferase (ACAT) reaction instead of the lecithin:cholesterol acyl-transferase (LCAT) reaction. The current study was conducted to test the hypothesis that ACAT in the liver is the source of the increased numbers of cholesteryl oleate molecules in plasma LDL particles that appear to increase the atherogenic potential of LDL. Monkeys were fed diets rich in fat (lard, safflower oil, or fish oil) and cholesterol for 3-6 years before liver perfusion, ACAT assay, and evaluation of CAA. Hepatic ACAT activity was positively correlated with hepatic cholesteryl ester secretion (r = 0.61, p < 0.001), plasma LDL cholesteryl ester content (r = 0.60, p < 0.0001), and the extent of CAA (r = 0.62, p < 0.0001). The number of cholesteryl oleate molecules within LDL increased proportionally with LDL size in each of the diet groups. Hepatic cholesteryl oleate concentration was correlated with the accumulation of cholesteryl oleate in liver perfusate (r = 0.72, p < 0.01) and with plasma LDL cholesterol oleate content (r = 0.73, p < 0.0001). Our interpretation is that these data, obtained in a relevant primate model of CAA, suggest that hepatic ACAT increases the atherogenicity of LDL by augmenting both the secretion by the liver and accumulation in plasma LDL of cholesteryl oleate.

Hepatic cholesterol metabolism in estrogen-treated men

Operative liver biopsies were obtained from two male patients who developed gallstone disease during estrogen treatment of metastatic prostatic carcinoma. The heparin-sensitive binding of 125I-low-density lipoprotein (LDL) to liver homogenates (reflecting the expression of the LDL receptor) was determined, together with the activities of the rate-limiting enzymes in cholesterol synthesis [3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase], bile acid production (cholesterol 7 alpha-hydroxylase), and cholesterol esterification (acyl CoA:cholesterol acyl transferase). The results were related to data available in 18 patients (5 male, 13 female) who underwent cholecystectomy because of gallstone disease. The hepatic 125I-LDL-binding activity was increased threefold compared with five controls, and the activity of HMG-CoA reductase was increased twofold. There was no major difference in the activities of cholesterol 7 alpha-hydroxylase or acyl CoA:cholesterol acyl transferase. The concentration of free and total cholesterol in liver microsomes was approximately 30% lower in the estrogen-treated men than in 11 controls. The results indicate that estrogen at pharmacological doses stimulates hepatic LDL-receptor expression and HMG-CoA reductase activity in men. The increased LDL-receptor expression could in part explain the enhanced plasma clearance of injected 125I-LDL and hence the reduction in plasma LDL cholesterol previously shown to occur in estrogen-treated men.

Cytokine regulation of macrophage apo E secretion: opposing effects of GM-CSF and TGF-beta

Biosynthesis of apolipoprotein (apo) E has been previously demonstrated to be regulated in macrophages by intracellular free cholesterol levels as well as by macrophage activating factors. In this report, the regulation of apo E secretion by cytokines detected within atherosclerotic lesions has been investigated. Granulocyte macrophage-colony stimulating factor (GM-CSF) stimulated macrophages had a 3-5-fold reduction in apo E secretion, comparable to that observed for gamma interferon (IFN gamma), while tumor necrosis factor alpha (TNF alpha) and interleukin 1 beta (IL-1 beta) resulted in a 2-fold decrease. In contrast to the reduction in apo E secretion by these cytokines, transforming growth factor beta (TGF-beta) stimulated macrophages secreted 3-fold greater amounts of apo E than controls. The reduced secretion of apo E by GM-CSF was reversible, heat labile, dose dependent, maximal 48 h after cytokine exposure and was coincident with an increase in fibronectin secretion. The opposing effects of GM-CSF and TGF-beta on apo E secretion were consistent with similar changes detected in apo E mRNA levels. Cytokine effects on apo E secretion in cholesterol loaded macrophages were also investigated and found to be similar to the non-loaded cells with GM-CSF decreasing and TGF-beta increasing apo E secretion. The observed differences in apo E secretion did not correlate with any significant changes in either cellular cholesterol distribution in the non-cholesterol loaded macrophages or in basal ACAT activity. In addition to changes in apo E secretion, cytokine treated macrophages pulsed with [14C]oleate and acetylated LDL for 2-6 h had a 2-fold increase (GM-CSF) or decrease (TGF-beta) in cholesterol esterification. Therefore, GM-CSF and TGF-beta mediated changes in apo E secretion may occur through a mechanism independent of changes in cellular free cholesterol levels. These results suggest that cytokines expressed within an atheroma may play an important role in the modulation of macrophage mediated reverse cholesterol transport.

Citrus pectin and cholesterol interact to regulate hepatic cholesterol homeostasis and lipoprotein metabolism: a dose-response study in guinea pigs

Guinea pigs were fed increasing concentrations of citrus pectin (CP) (0-12.5%, wt/wt) with low (LC, 0.04%) or high (HC, 0.25%) cholesterol. Animals fed LC diets had reduced plasma LDL concentrations with 10% and 12.5% CP and hepatic membrane apolipoprotein B/E receptor expression increased with high dosages of CP. Hepatic 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase activity increased with 12.5% CP whereas hepatic cholesterol concentrations and acyl-CoA:cholesterol acyltransferase (ACAT) activity were not different. In contrast, with HC diets, plasma LDL concentrations were reduced in a dose-response manner by 29%, 30%, and 67% with 7.5%, 10%, and 12.5% CP intake (P < 0.001) and apolipoprotein B/E receptor number was increased and inversely correlated with plasma LDL in the HC group (r = -0.81, P < 0.005). Animals fed HC diets had a dose-dependent decrease in hepatic cholesterol and ACAT activity, with intake of 12.5% CP having the major effect. Hepatic HMG-CoA reductase activity was suppressed by HC diets and only intake of 12.5% CP reversed this suppression. The most significant effects of CP on hepatic cholesterol, enzymes of hepatic cholesterol homeostasis, and the apolipoprotein B/E receptor were in animals fed the HC diets. These metabolic alterations partially explain the reduced plasma LDL of guinea pigs fed large amounts of CP.

Altered pattern of lipid metabolism in patients with lung cancer

Cholesterol distribution in tumoral tissues and lipid composition in the plasma compartment were determined in patients affected by different histologic types of lung cancer. The results showed that tumoral lung tissues contained 2-fold more total cholesterol and 3.5-fold more esterified cholesterol than normal lung tissues. In the patients the alterations in intracellular cholesterol were also associated with peculiar changes in cholesterol distribution in the plasma compartment. Serum high-density lipoprotein (HDL) cholesterol levels were markedly lower in than in controls. No significant changes in other lipid parameters were observed in these patients. We suggest that the reduced levels of serum HDL cholesterol observed in patients with lung tumors may be a consequence of the disease, probably mediated by the greater utilization of cholesterol for new membrane biogenesis and by the accumulation of esterified cholesterol in tumoral tissues.

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.

Tissue-specific changes in lipid composition and lipoprotein lipase activity during the development of the chick embryo

Lipoprotein lipase was present at a high specific activity in adipose tissue and heart of the chick embryo at the 14th day of development. The enzyme was also present in skeletal muscle but was absent from brain and liver. Major increases in the activity of lipoprotein lipase in adipose tissue and heart occurred from day 12 of development, concomitant with the beginning of the period of lipid uptake from the yolk. These results suggest that lipoprotein lipase may be involved in the utilisation of yolk-derived lipid by the tissues of the embryo. Relatively high levels of docosahexaenoic acid (22:6(n--3)) were present in the triacylglycerol isolated from plasma, adipose tissue, heart and liver. The relative proportions of this fatty acid in the triacylglycerol of the different tissues may be explicable in terms of the substrate specificity of lipoprotein lipase.

Cytotoxic cellular cholesterol is selectively removed by apoA-I via ABCA1

Excess intracellular free cholesterol (FC) is cytotoxic. This study examines prevention of FC-induced cytotoxicity in J774 macrophage foam cells by incubation with apolipoprotein AI (apoA-I). J774 were cholesterol enriched using acetylated low-density lipoprotein and FC/phospholipid (PL) dispersions. Treatment with an acyl coenzyme-A:cholesterol acyltransferase (ACAT) inhibitor, in the absence of extracellular acceptors, produced hydrolysis of stored esterified cholesterol (EC) and FC-induced cytotoxicity. Incubation of cells with ACAT inhibitor plus apoA-I resulted in FC efflux (0.39 +/- 0.02%/h) along with a reduction in cytotoxicity (26.30 +/- 5.80%), measured by adenine release. Small unilamellar vesicles (SUV) caused greater FC efflux (0.53 +/- 0.02%/h, P = 0.001), but a modest reduction in cytotoxicity (8.40 +/- 2.70%, P = 0.008). Co-incubation of ACAT inhibitor plus the cholesterol transport inhibitor U18666A or the antioxidant Probucol reduced efflux to apoA-I, but not to SUV. Pre-treatment of J774 foam cells with CTP-cAMP upregulates hormone sensitive lipase (HSL) and further upregulates ATP binding cassette A1 (ABCA1). Using mouse serum as a cholesterol acceptor, CTP-cAMP caused greater protection against FC-induced cytotoxicity compared to cells without pre-treatment, suggesting a role of ABCA1 in removal of cytotoxic FC. We conclude that a cytotoxic pool of FC is located in the plasma membrane, is readily available for efflux to apoA-I, and removal of cytotoxic cholesterol may involve ABCA1.

Enrichment of acyl coenzyme A:cholesterol O-acyltransferase near trans-golgi network and endocytic recycling compartment

Acyl coenzyme A:cholesterol O-acyltransferase (ACAT) is the enzyme responsible for cholesterol esterification in macrophages leading to foam cell formation. The determination of its localization is a critical step in understanding its regulation by cholesterol. Using immunofluorescence and confocal microscopy, we previously showed that the enzyme colocalized with markers of the endoplasmic reticulum, but in addition, ACAT was found in an unidentified paranuclear site. In the present study, we further define the localization of paranuclear ACAT. First, we found that ACAT does not colocalize with sorting endosomes or late endosomes labeled with fluorescent alpha(2)-macroglobulin. The paranuclear ACAT is close to the endocytic recycling compartment labeled with fluorescent transferrin. We also show that the paranuclear structure containing ACAT is very close to TGN38, a membrane protein of the trans-Golgi network (TGN), but farther from Gos28, a marker of cis, medial, and trans Golgi. After treatment with nocodazole, the central localization of ACAT did not colocalize with markers of the TGN. These data indicate that a significant fraction of ACAT resides in membranes that may be a subcompartment of the endoplasmic reticulum in proximity to the TGN and the endocytic recycling compartment. Because the TGN and the endocytic recycling compartment are engaged in extensive membrane traffic with the plasma membrane, esterification of cholesterol in these membranes may play an important role in macrophage foam cell formation during atherogenesis.

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

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

Dehydroepiandrosterone, an adrenal androgen, increases human foam cell formation: a potentially pro-atherogenic effect

OBJECTIVES

We studied the effects of dehydroepiandrosterone (DHEA), an abundant adrenal androgen on two key early events of atherogenenis: 1) human monocyte adhesion to vascular endothelium, and 2) human foam cell formation.

BACKGROUND

In the U.S., where DHEA is available without prescription, there has recently been a rapid increase in unsupervised self-administration of DHEA. The vascular biologic effects of DHEA are largely unknown, however.

METHODS

Regarding adhesion, human umbilical vein endothelial cells (HUVECs), exposed to either DHEA (42 or 420 nmol/l) or control, were incubated with human monocytes, and adhesion was measured by hemocytometry. Surface expression of endothelial cell adhesion molecules was measured by ELISA. Regarding foam cell formation, studies of lipid loading were performed on macrophages treated with DHEA or control and/or the androgen receptor antagonist hydroxyflutamide (HF) (4 micromol/l). Intracellular cholesterol and cholesteryl esters (CE) were quantified by high-performance liquid chromatography. Expression of foam cell formation-related genes was measured by reverse-transcription polymerase chain reaction.

RESULTS

DHEA produced a dose-dependent receptor-mediated increase in the male macrophage CE content (up to 120 +/- 4% of control values, p = 0.015). DHEA upregulated messenger ribonucleic acid expression of the lipoprotein-processing enzymes acyl coenzyme A:cholesterol acyltransferase I and lysosomal acid lipase by 3.4- and 5.3-fold, respectively (p < 0.05 vs. control), but had no effect on scavenger receptor expression (p > 0.2). There was no significant effect of DHEA on monocyte-endothelial adhesion (<10% change in values, p = 0.56) or endothelial cell expression of cell adhesion molecules (p > 0.1).

CONCLUSIONS

DHEA increases human macrophage foam cell formation, a potentially pro-atherogenic effect. This effect appears to be mediated via the androgen receptor and involves the upregulation of lipoprotein-processing enzymes.

Dietary cholesterol supplementation improves growth and behavioral response of pigs selected for genetically high and low serum cholesterol

We hypothesized that, in pigs selected for low (L) or high (H) serum cholesterol for four generations, neonatal endogenous cholesterol synthesis would be sufficient to meet requirements for brain and body growth. In Experiment 1, eight 16-wk-old L pigs received a diet with or without 200 mg cholesterol/100 g diet for 35 d. Supplemented pigs grew approximately 25% faster and had a significantly greater concentration of free cholesterol in the cerebrum. In Experiment 2, 16 H and 16 L newborn pigs were fed a milk replacer with or without 200 mg cholesterol/100 g diet for 28 d. Pigs fed cholesterol had greater average daily gain (P < or = 0.09), significantly reduced liver 3-hydroxy-3-methylglutaryl-coenzyme A reductase activity, and significantly increased cerebral cholesterol content than pigs not fed cholesterol. One of three indices of exploratory behavior was significantly greater in the L pigs that received cholesterol compared with L pigs that did not receive cholesterol. These data suggest that these neonatal pigs are unable to produce sufficient cholesterol to meet requirements for normal growth and brain development and are dependent on dietary cholesterol in milk.

The influence of dietary saturated and unsaturated fat on hepatic cholesterol metabolism and the biliary excretion of chylomicron cholesterol in the rat

The biliary excretion of [3H] cholesterol carried in chylomicrons derived from palm oil (rich in long chain saturated fatty acids), olive oil (rich in monounsaturated fatty acids) or corn oil (rich in n-6 polyunsaturated fatty acids was studied in vivo in rats fed the corresponding oil in the diet for 21 days. The secretion of radioactivity into bile as both bile acids and unesterified cholesterol was significantly slower in the animals fed palm oil as compared to those given olive or corn oil, indicating that dietary saturated fat retards the excretion of cholesterol from the diet as compared to mono- or n-6 polyunsaturated fat. In order to investigate the mechanisms underlying these differences, the influence of the three high fat diets on cholesterol esterification, cholesteryl ester hydrolysis and bile acid synthesis in the liver and on biliary lipid output were also measured. The ratio of cholesterol esterification to cholesteryl ester hydrolysis was markedly raised in the olive and corn oil-fed as compared to palm oil-fed animals. Biliary cholesterol secretion was higher in corn oil-fed rats than in those fed olive or palm oil or a low fat diet, and this was associated with a markedly increased lithogenic index in these animals. The activity of cholesterol 7alpha hydroxylase was higher in the olive and corn oil-fed than in the palm oil-fed animals, although the expression of mRNA for the enzyme was increased only in the olive oil diet group. After 20 h biliary drainage, the rate of bile acid secretion into bile was increased in the rats fed olive and corn oil rather than to palm oil. These findings indicate that feeding rats mono- or n-6 polyunsaturated as compared to saturated fat in the diet promotes the storage of cholesteryl ester in the liver and leads to increased bile acid synthesis, resulting in the more rapid excretion of cholesterol originating from the diet via the bile.

Effects of perturbations in hepatic free and esterified cholesterol pools on bile acid synthesis, cholesterol 7 alpha-hydroxylase, HMG-CoA reductase, acyl-CoA:cholesterol acyltransferase and cytosolic cholesteryl ester hydrolase

Effects of expansion of the hepatic free cholesterol pool on bile acid and cholesterol metabolism and homeostasis were examined in rats fed cholesterol in high-fat diets or treated with oleyl-p-(n-decyl)-benzenesulfonate (ODS) or progesterone. Cholesterol feeding for 10-16 days, which increased free (33%) and esterified (6-fold) cholesterol, had no effect on cholate synthesis, total bile acid synthesis, or cholate turnover, whereas these activities were increased 60-80% by ODS and progesterone, which produced only small increases (19%) in free cholesterol. Cholesterol feeding reduced beta-hydroxy-beta-methylglutaryl (HMG)-CoA reductase (72%) and cholesteryl ester hydrolase (48%) and increased acyl-CoA:cholesterol acyltransferase (184%), whereas ODS and progesterone reversed these compensatory responses in cholesterol-fed rats. Cholesterol 7 alpha-hydroxylase was changed no more than 22% by any treatment. A bolus of ODS elevated biliary cholesterol output 41% and shifted biliary bile acid synthesis and composition toward 12-deoxy bile acids. These effects were not seen in ODS-fed or progesterone-treated rats, in which cholesteryl ester stores were depleted. It is concluded that effects of free cholesterol on bile acid synthesis and biliary cholesterol are probably mediated by specific precursor or regulatory pools which can be independently regulated and which represent a relatively small fraction of hepatic free cholesterol.

Expression of SOAT1 in Adrenocortical Carcinoma and Response to Mitotane Monotherapy: An ENSAT Multicenter Study

CONTEXT

Objective response rate to mitotane in advanced adrenocortical carcinoma (ACC) is approximately 20%, and adverse drug effects are frequent. To date, there is no marker established that predicts treatment response. Mitotane has been shown to inhibit sterol-O-acyl transferase 1 (SOAT1), which leads to endoplasmic reticulum stress and cell death in ACC cells.

OBJECTIVE

To investigate SOAT1 protein expression as a marker of treatment response to mitotane.

PATIENTS

A total of 231 ACC patients treated with single-agent mitotane as adjuvant (n = 158) or advanced disease therapy (n = 73) from 12 ENSAT centers were included. SOAT1 protein expression was determined by immunohistochemistry on formalin-fixed paraffin-embedded specimens.

SETTING

Retrospective study at 12 ACC referral centers.

MAIN OUTCOME MEASURE

Recurrence-free survival (RFS), progression-free survival (PFS), and disease-specific survival (DSS).

RESULTS

Sixty-one of 135 patients (45%) with adjuvant mitotane treatment had recurrences and 45/68 patients (66%) with mitotane treatment for advanced disease had progressive disease. After multivariate adjustment for sex, age, hormone secretion, tumor stage, and Ki67 index, RFS (hazard ratio [HR] = 1.07; 95% confidence interval [CI], 0.61-1.85; P = 0.82), and DSS (HR = 1.30; 95% CI, 0.58-2.93; P = 0.53) in adjuvantly treated ACC patients did not differ significantly between tumors with high and low SOAT1 expression. Similarly, in the advanced stage setting, PFS (HR = 1.34; 95% CI, 0.63-2.84; P = 0.45) and DSS (HR = 0.72; 95% CI, 0.31-1.70; P = 0.45) were comparable and response rates not significantly different.

CONCLUSIONS

SOAT1 expression was not correlated with clinical endpoints RFS, PFS, and DSS in ACC patients with mitotane monotherapy. Other factors appear to be relevant for mitotane treatment response and ACC patient survival.

Acyl-coenzyme A: cholesterol acyltransferase assay: silica gel column separation of reaction products

Acyl-coenzyme A:cholesterol acyltransferase (ACAT) assays are usually performed by incubation of the enzyme with a labeled substrate followed by thin-layer chromatography separation and subsequent quantification of cholesteryl esters (CE) formed. Herein, a method is described for rapid separation of CE from other lipids, by elution from a silica gel column with a solvent mixture of petroleum ether/diethyl ether (98:2, v/v). Silica gel column chromatography is reliable and more rapid and safer than TLC. The best results were obtained when the reaction was stopped by Dole extraction followed by CE separation on a silica gel column. Assays for ACAT from rat intestinal microsomes showed that the specific activity values obtained using this method were reproducible and in good agreement with those obtained by conventional TLC method.

Regulation of low-density-lipoprotein receptors in the human hepatoma cell line Hep G2. Effect of phorbol 12-myristate 13-acetate and low-density lipoprotein

The regulation of low-density-lipoprotein(LDL)-receptor activity by 4 beta-phorbol 12-myristate 13-acetate (PMA) and LDL was investigated in the human hepatoma cell line Hep G2. Treatment of Hep G2 cells for 22h with PMA results in an 18.6-fold increase in the amount of LDL-binding sites on the cell surface. The rate of turnover of LDL receptors was not significantly altered upon PMA treatment. Treatment of cultured rat parenchymal cells and human parenchymal cells with PMA did not lead to increased binding of LDL to these cells, suggesting that protein-kinase-C-mediated regulation of LDL-receptor activity is specific for Hep G2 cells and that in this aspect of regulation of LDL receptors, Hep G2 cells do not reflect human hepatocytes. The down-regulation of LDL receptors by a 22-h prior incubation with LDL in PMA-treated Hep G2 cells, in which LDL receptors are upregulated, is more effective than in non-treated cells. Prior incubation of control Hep G2 cells with an excess of LDL caused a partial down-regulation to 33% of the initial level of receptor binding. In PMA-treated Hep G2 cells an excess of non-labeled LDL, led to a down-regulation to 13% of the PMA-induced level. Prior incubation of Hep G2 cells with LDL in the presence of PMA led to a 2.3-fold increase of intracellular cholesteryl esters and a 9.1-fold increase in acyl-CoA:cholesterol-acyltransferase (ACAT) activity. In control cells, LDL prior incubation led to a 1.6-fold increase in intracellular cholesteryl esters and a 1.8-fold increase of ACAT activity. It is concluded that in Hep G2 cells LDL itself can be an effective suppressor of the expression of LDL receptors, provided that the initial amount of receptor allows an adequate intracellular delivery of cholesterol to its sterol-regulatory site.

Analysis of exotoxins produced by atypical isolates of Aeromonas salmonicida, by enzymatic and serological methods

In this study, exotoxins produced by 62 Aeromonas salmonicida strains and the bacterium Haemophilus piscium were analysed. Enzymatic assays, zymograms and serological detection were used to monitor secretion by bacterial strains of the previously described exotoxins P1, GCAT and AsaP1 and also the extracellular P2 metallo-gelatinase and a serine caseinase, which is different from the P1 protease and has not yet been characterized. Based on the results, the strains were divided into five groups. One comprised the type strains for A. salmonicida ssp. masoucida, H. piscium and 36% of the atypical isolates, and another, a type strain for A. salmonicida ssp. smithia together with 14% of the atypical isolates. A second type strain of A. salmonicida ssp. smithia was grouped with 8% of the atypical isolates. The largest group contained the type strains for A. salmonicida ssp. achromogenes and 38% of the atypical isolates. The type strains for A. salmonicida ssp. salmonicida were in the last group with all the four typical strains and 4% of the atypical isolates. The combination of zymogram and serological detection used is recommended as the most reliable method for characterizing A. salmonicida strains according to their exotoxin secretion.

Fish oil and oat bran in combination effectively lower plasma cholesterol in the rat

Male rats were fed a semi-purified diet containing oat bran or wheat bran with or without a marine fish oil to investigate the effects of such combinations on lipid metabolism. Oat bran alone and wheat bran plus fish oil gave lower plasma cholesterol concentrations than wheat bran alone while oat bran plus fish oil gave the lowest. Oat bran increased plasma triacylglycerols compared with wheat bran but oat bran plus fish oil gave concentrations similar to those seen with wheat bran plus fish oil. Oat bran gave higher hepatic cholesterol synthesis rates and a higher activity of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase compared to wheat bran. The addition of fish oil to either bran diet decreased cholesterol synthesis but HMG CoA reductase activity was not reduced. Oat bran increased hepatic acyl coenzyme A:cholesterol acyl transferase (ACAT) activity and increased the ratio of esterified to unesterified cholesterol in hepatic microsomal membranes compared with wheat bran. Fish oil decreased hepatic LDL receptor activity and increased HDL binding activity when added to the wheat bran diet but these effects were not seen with oat bran. Oat bran also had no effect on hepatic lipoprotein receptor activity compared with wheat bran. These results show that fish oil and oat bran have complementary cholesterol lowering effects in the rat.