On the mechanism of the modulation in vitro of acyl-CoA:cholesterol acyltransferase by progesterone

Progesterone-receptor antagonists and statins decrease de novo cholesterol synthesis and increase apoptosis in rat and human periovulatory granulosa cells in vitro

Progesterone-receptor (PR) stimulation promotes survival in rat and human periovulatory granulosa cells. To investigate the mechanisms involved, periovulatory rat granulosa cells were incubated in vitro with or without the PR-antagonist Org 31710. Org 31710 caused the expected increase in apoptosis, and expression profiling using cDNA microarray analysis revealed regulation of several groups of genes with functional and/or metabolic connections. This regulation included decreased expression of genes involved in follicular rupture, increased stress responses, decreased angiogenesis, and decreased cholesterol synthesis. A decreased cholesterol synthesis was verified in experiments with both rat and human periovulatory granulosa cells treated with the PR-antagonists Org 31710 or RU 486 by measuring incorporation of [14C]acetate into cholesterol, cholesterol ester, and progesterone. Correspondingly, specific inhibition of cholesterol synthesis in periovulatory rat granulosa cells using 3-hydroxy-3-methylglutaryl-coenzyme A reductase inhibitors (lovastatin, mevastatin, or simvastatin) increased apoptosis, measured as DNA fragmentation and caspase-3/7 activity. The increase in apoptosis caused by simvastatin was reversed by addition of the cholesterol synthesis-intermediary mevalonic acid. These results show that PR antagonists reduce cholesterol synthesis in periovulatory granulosa cells and that cholesterol synthesis is important for granulosa cell survival.

Progesterone metabolism in human fibroblasts is independent of P-glycoprotein levels and Niemann-Pick type C disease

Progesterone inhibits intracellular transport of lysosomal cholesterol in cultured cells, and thus at least in part mimics the biochemical phenotype of Niemann-Pick type C disease (NPC) in human fibroblasts. The goal of this study was to determine whether metabolism of progesterone to other steroids is affected by the NPC mutation or by P-glycoprotein (a known progesterone target). We found that human fibroblasts metabolize progesterone in three steps: rapid conversion to 5alpha-pregnane-3,20-dione, which is then reduced to 5alpha-pregnane-3beta(alpha)-ol-20-one with subsequent 6alpha-hydroxylation. The pattern and rates of progesterone metabolism were not significantly different in a variety of fibroblasts from normal individuals, NPC patients, and obligate heterozygotes. Inhibition of steroid 5alpha-reductase with finasteride completely blocked metabolism of progesterone but had no effect on inhibition of LDL-stimulated cholesterol esterification (IC50 = 10 microM). Progesterone also partially inhibited 25-hydroxycholesterol-induced cholesterol esterification, with similar dose-dependence in normal and NPC fibroblasts. P-glycoprotein levels varied significantly among the various fibroblasts tested, but no correlation with NPC phenotype or rate of progesterone metabolism was noted, and P-glycoprotein inhibitors did not affect conversion of progesterone to products. These results indicate that metabolism of progesterone in human fibroblasts is largely independent of its ability to interfere with cholesterol traffic and P-glycoprotein function.

Crystallization of free cholesterol in model macrophage foam cells

-The present study examined free cholesterol (FC) crystallization in macrophage foam cells. Model foam cells (J774 or mouse peritoneal macrophages [MPMs]) were incubated with acetylated low density lipoprotein and FC/phospholipid dispersions for 48 hours, resulting in the deposition of large stores of cytoplasmic cholesteryl esters (CEs). The model foam cells were then incubated for up to 5 days with an acyl-coenzyme A:cholesterol acyltransferase (ACAT) inhibitor (CP-113,818) in the absence of an extracellular FC acceptor to allow intracellular accumulation of FC. FC crystals of various shapes and sizes formed in the MPMs but not in the J774 macrophages. Examination of the MPM monolayers by microscopy indicated that the crystals were externalized rapidly after formation and thereafter continued to increase in size. Incubating J774 macrophages with 8-(4-chlorophenylthio)adenosine 3':5'-cyclic monophosphate (CPT-cAMP) in addition to CP-113,818 caused FC crystal formation as a consequence of CPT-cAMP stimulation of CE hydrolysis and inhibition of cell growth. In addition, 2 separate cholesterol phases (liquid-crystalline and cholesterol monohydrate) in the plane of the membrane bilayer were detected after 31 hours of ACAT inhibition by the use of small-angle x-ray diffraction of J774 macrophage foam cells treated with CPT-cAMP. Other compounds reported to inhibit ACAT, namely progesterone (20 microgram/mL) and N-acetyl-D-sphingosine (c(2)-ceramide, 10 microgram/mL), induced cellular toxicity in J774 macrophage foam cells and FC crystallization when coincubated with CPT-cAMP. Addition of the extracellular FC acceptors apolipoproteins (apo) E and A-I (50 microgram/mL) reduced FC crystal formation. In MPMs, lower cell density and frequent changes of medium were conducive to crystal formation. This may be due to "dilution" of apoE secreted by the MPMs and is consistent with our observation that the addition of exogenous apoE or apoA-I inhibits FC crystal formation in J774 macrophage foam cells cotreated with CP-113,818 plus CPT-cAMP. These data demonstrate that FC crystals can form from the hydrolysis of cytoplasmic stores of CEs in model foam cells. FC crystal formation can be modulated by the addition of extracellular FC acceptors or by affecting the cellular rate of CE hydrolysis. This process may contribute to the formation of FC crystals in atherosclerotic plaques.

Plasma lipoproteins and cholesterol metabolism in Yoshida rats: an animal model of spontaneous hyperlipemia

The purpose of this study was to characterize the lipoprotein profile and cholesterol metabolism in Yoshida rats, a strain of inbred genetically hyperlipemic animals. For comparison, Brown Norway rats were used as control animals. Plasma cholesterol and triglycerides were higher in Yoshida as compared to Brown Norway, the elevation of cholesterol being due to a rise in HDL fraction. Triglyceride distribution among lipoproteins showed an increase in VLDL fraction. Hyperlipemia was not related to diabetes, hypothyroidism or nephropathy. Plasma triglycerides production was increased in Yoshida rats, while lipoprotein and hepatic lipases were similar in the two groups. Hypercholesterolemia was associated with a defect of lipoprotein receptor activity and with elevated HMG-CoA reductase and cholesterol 7 alpha - hydroxylase; conversely ACAT activity was lower in Yoshida as compared to Brown Norway rats. Sterol fecal excretion was comparable in the two groups and hypercholesterolemia in Yoshida rats was not associated to an increase of cholesterol saturation of the bile. We suggest that lipoprotein overproduction is the main cause for hyperlipidemia in this strain of rats.

Effect of simvastatin (MK-733) on the regulation of cholesterol synthesis in Hep G2 cells

A new antihypercholesterolemic drug, simvastatin (MK-733), which is a prodrug of a potent 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitor, inhibited cholesterol synthesis from [14C]acetate concentration dependently without inhibiting it from [3H]mevalonate in Hep G2 cells. Therefore, MK-733 is thought to be converted to L-654,969, the active beta-hydroxy acid form of MK-733 in the cells and/or medium. MK-733 inhibited cholesterol ester synthesis, but did not affect phospholipid, free fatty acid and triacylglycerol synthesis. This compound increased HMG-CoA reductase activity concentration dependently and raised the specific binding, internalization and degradation of 125I-labeled low density lipoprotein by Hep G2 cells. Another HMG-CoA reductase inhibitor, pravastatin (CS-514), also behaved like MK-733. However, its potency was far less than that of MK-733.

Effect of natural and structurally modified bile acids on cholesterol metabolizing enzymes in rat liver microsomes

The effect of chenodeoxycholic (CDCA), ursodeoxycholic (UDCA), tauroursodeoxycholic (TUDCA), cholic (CA), ursocholic (UCA) acids, analogues of CDCA and UDCA with a cyclopropyl ring at C22, C23 (cypro-CDCA and cypro-UDCA) and 23-methylursodeoxycholic acid (MUDCA) on cholesterol 7 alpha-hydroxylase was studied in rat liver microsomes. Cypro-analogues consisted of a mixture of four diasteroisomers, while MUDCA was the racemic mixture of two enantiomers. Each steroid was added to liver microsomes at concentrations ranging from 10 to 200 microM. With the exception of UCA and CA, all the bile acids inhibited cholesterol 7 alpha-hydroxylase activity. The inhibition shown by cypro-CDCA and cypro-UDCA was stronger than that observed with the corresponding natural compounds. 22S,23S cypro-UDCA exhibited an inhibitory effect which was more pronounced than that of the diasteroisomer mixture. The isomer 22R,23S was less effective and decreased cholesterol 7 alpha-hydroxylase activity in a manner comparable to that of UDCA. The effect of CDCA, UDCA and the cyclopropyl analogues was also tested with respect to HMG-CoA reductase and acylCoA cholesterol acyltransferase (ACAT) activities. ACAT was stimulated by the isomer 22S,23S cypro-UDCA but not affected by the other bile acids. No effect was observed as regards HMG-CoA reductase.

The effect of etofibrate on cholesterol and bile acid metabolism in the hamster

Hamsters were given etofibrate at a dose of 300 mg/kg body wt, by gavage for 5 days, while being fed a chow diet. After treatment, serum cholesterol levels were 27% lower compared to those of the control animals. A similar trend was observed for triglyceride levels. Hepatic lipid levels were unchanged by the treatment. HMG-CoA reductase and acylCoA cholesterol acyltransferase were decreased while cholesterol 7 alpha-hydroxylase was not significantly modified by etofibrate. A choleretic effect and an increase of cholesterol excretion into hepatic bile was observed in treated animals. Nevertheless, composition and cholesterol saturation index of gallbladder bile were similar in control and treated animals. With respect to controls, hepatic bile of treated hamsters contained a lesser amount of cholic and deoxycholic acid and a greater amount of ursodeoxycholic acid.

Inhibition of cellular cholesterol esterification can decrease low density lipoprotein receptor number in human fibroblasts

In fibroblasts deprived of exogenous cholesterol to induce low density lipoprotein receptors there is a continuing flux of cholesterol esterification. The structurally unrelated inhibitors of acyl-CoA; cholesterol acyl-transferase, progesterone, trimethylcyclohexanyl mandelate and 3-[decyldimethylsilyl]-N-[2-(4-methylphenyl)-1-phenylethyl] propanamide, (58035), could all inhibit this basal rate of esterification within 1h of addition. Exposure of cholesterol-deprived fibroblasts for 17h to progesterone or trimethylcyclohexanyl mandelate caused decreased specific binding and metabolism of low density lipoprotein. The effect was not a direct inhibition of lipoprotein binding; it was time dependent and followed from the reversible inhibition of cholesterol esterification by these two compounds. The irreversible inhibition of esterification by 58035 left the receptor number unaffected. The results indicate that down regulation of low density lipoprotein receptors is initiated by accumulation of cholesterol in a specific intracellular pool. Inhibition of cholesterol esterification by progesterone and trimethylcyclohexanyl mandelate causes accumulation of cholesterol in this pool but 58035 does not.

Synthesis and hydrolysis of cholesteryl esters by isolated rat-liver lysosomes and cell-free extracts of human lung fibroblasts

The objective of this study was to examine and characterize the cholesteryl ester synthesizing [S] and hydrolyzing [H] properties of the acid cholesteryl ester hydrolase (acid cholesteryl ester hydrolase), both in isolated rat liver lysosomes and in cell-free extracts from cultured fibroblasts. For both liver lysosomes and fibroblasts extracts, the major synthesizing activity was found around pH 4 and did not require exogenous ATP. The rate of hydrolysis was measured at pH 4.5. Several different inhibitors were used in order to characterize the reactions. Ammonium chloride did not markedly affect the activity of acid cholesteryl ester hydrolase at pH 4 [S] or 4.5 [H], whereas chloroquine was a potent inhibitor of acid CEase in both liver lysosomes and fibroblast extracts. The [S] activity of the acid cholesteryl ester hydrolase in either material was not affected by the acylCoA:cholesterol acyltransferase inhibitor Compound 58-035 from Sandoz. Progesterone, on the other hand, which is an often used acylCoA:cholesterol acyltransferase inhibitor, markedly blocked both activities of the acid CEase. Our results indicate that the lysosomal compartment of both studied tissues, in addition to hydrolysis activity, also have a significant esterification activity. It appears that both activities are carried out by the same enzyme.

Induction of intracellular acyl-CoA:cholesterol acyltransferase activity in glioblastoma cells by lidocaine

The perturbation of cellular cholesteryl ester biosynthesis in glioblastoma C-6 cells by lidocaine was investigated. Lidocaine specifically inhibited the incorporation of radioactive oleic acid into cellular cholesteryl ester but had no significant effect on the incorporation of oleic acid into phosphatidylcholine. Oxygenated cholesterol-enhanced cholesteryl ester formation was less sensitive to lidocaine inhibition. Several other local anesthetics were compared. Lidocaine altered cholesteryl ester formation in time- and dose-dependent manners. Lidocaine was a powerful inhibitor initially and its potency declined with time. Lidocaine was capable of directly inhibiting acyl-CoA:cholesterol acyltransferase (ACAT) activity in broken cell homogenates. The lidocaine-mediated inhibition of cellular cholesteryl ester formation triggered an enhanced intracellular ACAT activity that was not fully expressed in the presence of lidocaine. The activation of ACAT activity by lidocaine might represent a compensatory mechanism by which the inhibitory effect of lidocaine was partially overcome with time.

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

The present experiments were designed to study intervesicular transfer of cholesterol in rat liver microsomal fraction and modulation of the activity of acyl-CoA:cholesterol acyltransferase (ACAT) under conditions that are expected to result in the covalent modification (phosphorylation/dephosphorylation) of the enzyme. Preincubation of rat liver microsomal fraction followed by assay of ACAT showed a time-dependent increase in activity. This rate was temperature-dependent. Preincubation in the presence of cholesterol/phospholipid liposomes resulted in a time-dependent transfer of cholesterol from liposomal to the microsomal vesicles and in an increase in the rate of ACAT change owing to the preincubation. Both these rates were dependent on liposomal cholesterol concentration and on temperature. The presence of cytosol in the preincubation mixture increased the rate of change of ACAT activity in the absence or in the presence of cholesterol/phospholipid liposomes. In the latter case the presence of cytosol also increased the rate of transfer of cholesterol from liposomal to the microsomal vesicles. Activation energies of the rate of this transfer and of the rate of increase of ACAT activity were similar in the presence and in the absence of cytosol. Both in the absence and in the presence of cytosol, the presence of NaF (50 mM) in the preincubation mixture considerably decreased the rate of transfer of cholesterol from liposomal to microsomal vesicles and the rate of increase of ACAT activity. The presence of Mg2+ in the preincubation mixture produced no effect on the rate of transfer of cholesterol from liposomal to the microsomal vesicles, although under most conditions it decreased the rate of increase of ACAT activity caused by the preincubation. These results are discussed in relation to the molecular mechanism involved in this intervesicular transfer of cholesterol and to the modulation of ACAT activity by substrate supply, and also in relation to the hypothesis that ACAT activity can be modulated by a mechanism involving the phosphorylation/dephosphorylation of the enzyme.