Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “Esterification”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 55 records · Page 3Linked to original sources
PubMedDate not supplied

[Effect of ethynyl estradiol and estradiol dipropionate on the esterification of saturated and unsaturated fatty acids in the rat liver].

Effect of estradiol ethynyl and estradiol propionate on esterification of saturated and non-saturated fatty acids in liver tissue, as one of possible mechanisms in development of estrogen-induced hypertriglyceridemia, was studied using labelled precursors 3H-palmitic and 14C-linoleic acids. Both these estrogens stimulated the incorporation of exogenous fatty acids into triglycerides. After administration of estradiol dipropionate relative esterification of 14C-linoleic acid into triglycerides was increased as compared with controls. Estradiol ethynyl increased and estradiol dipropionate decreased the incorporation of exogenous fatty acids into liver phospholipids. Both estrogens activated the relative esterification of linoleic acid into phospholipids. A decrease in exogenous fatty acids esterification into fraction of cholesterol esters, caused by both these estrogens, occurred due to an increase in relative incorporation of linoleic acid into these lipids. As shown in experiments of relative esterification of saturated and non-saturated fatty acids, estradiol propionate stimulated synthesis of more non-saturated forms of triglycerides, phospholipids and cholesterol esters in rat liver tissue as compared with the estradiol ethynyl action. The data obtained and the data of literature suggest that activation of exogenous fatty acids esterification into triglycerides and alterations in relative esterification of saturated and non-saturated fatty acids into triglycerides and phospholipids of liver tissue are of importance in pathogenesis of estrogen-induced hypertriglyceridemia.

Animals↗
PubMed1992

Cultured fibroblasts from patients with Niemann-Pick disease type C and type D exhibit distinct defects in cholesterol esterification.

The Niemann-Pick group of diseases can be broadly classified into two types based on clinical and biochemical characteristics. Type I is characterized by a primary deficiency of lysosomal sphingomyelinase while Type II may have a defect in the regulation of intracellular cholesterol metabolism. We have studied cholesterol esterification in cultured fibroblasts from patients with two phenotypes of Type II disease: an Acadian population of southwestern Nova Scotia (Canada) with a form of the disease known as Niemann-Pick type D (NPD) and a group of panethnic origin with Niemann-Pick type C (NPC). Addition of whole serum to normal fibroblasts grown initially in lipoprotein-deficient serum caused a rapid (within 6 h) increase in cholesterol esterification, reaching maximum values at around 24 h, while NPC fibroblasts showed little increase (less than 10% of normal). In contrast, cholesterol esterification in NPD fibroblasts increased slowly during the first 6-12 h and reached 50% of normal values by 24 h. 25-Hydroxycholesterol, a non-lipoprotein stimulator of cholesterol esterification, caused a similar stimulation of cholesterol esterification in NPC, NPD and normal cells. This was inhibited by addition of serum in mutant but not in normal cells. Within 24 h of serum addition, free cholesterol accumulated in all cell types with NPC greater than NPD greater than normal. These observations indicate that (a) regulation of cholesterol esterification in response to serum lipoproteins (but not 25-hydroxycholesterol) is abnormal in both NPC and NPD fibroblasts, and (b) the biochemical phenotypes of fibroblasts from NPC and NPD patients are distinct.

Cells, Cultured↗
PubMed1997

Regulation of cholesterol synthesis and esterification in primary cultures of macrophages following uptake of chylomicron remnants.

The effects of chylomicron remnants (CR), beta-very-low-density-lipoproteins (beta-VLDL) and low-density-lipoproteins (LDL) on intracellular cholesterol synthesis and esterification in primary rabbit macrophages was determined by assaying for HMG-CoA reductase activity and cholesterol esterification. At physiological cholesterol concentrations, both CR and LDL inhibited cholesterol synthesis by almost 60% while beta-VLDL was less potent achieving only 30% inhibition. Cholesterol esterification rates were increased four-fold by CR and LDL, whereas beta-VLDL increased esterification 14 times above controls. Qualitatively, the effect of CR on cholesterol synthesis and esterification in rabbit macrophages differs from observations in transformed macrophage cells. Quantitatively, the enhanced rates of cholesterol esterification and weak inhibition of cholesterol synthesis following beta-VLDL uptake may explain why this lipoprotein rapidly induces foam cell formation in vitro.

Animals↗
PubMed1981

Intestinal fatty acid esterification activity in jejunoileal bypass patients.

In four patients undergoing reversal of jejunoileal bypass we compared functional (in continuity) with bypassed intestine in order to determined the effects of luminal contents. Total mucosal thickness, villus height, and crypt depth, as well as in vitro fatty acid esterification activity were determined. Morphological studies in segments exposed to luminal contents revealed that the ileum had a greater mucosal thickness than the jejunum (p less than 0.001) and that the difference was reflected in both villus height and crypt depth (p less than 0.001). The functioning segments of both jejunum and ileum had a greater mucosal thickness than corresponding bypassed segments consequent to a difference in villus height (p less than 0.001) but not crypt depth. Despite similar exposure to luminal contents, total fatty acid esterification was significantly higher (p less than 0.001) in the functioning jejunum than in the ileum. Jejunum incontinuity possessed higher esterification activity than bypassed jejunum. These results indicate that 1) luminal contents are the most important modulator of intestinal fatty acid esterification activity and the absence of luminal contents in bypassed intestine leads to a significant reduction in esterification activity, 2) the jejunum and ileum possess intrinsic differences in esterification activity even when both are exposed to an identical luminal environment.

Adult↗
PubMedDate not supplied

Cholesterol esterification in macrophages. Stimulation by lipoproteins containing apo B isolated from human aortas.

A lipoprotein fraction possessing many of the characteristics of plasma low density lipoproteins (P-LDL) was isolated from homogenates of lesioned human aortas by affinity chromatography. In contrast to P-LDL, this fraction, termed A-LP, was found to be more electronegative than P-LDL and to stimulate cholesterol esterification in mouse peritoneal macrophages (MPM). This stimulation tended toward saturation at approximately 100 micrograms/ml lipoprotein cholesterol. Cholesterol esterification was partially inhibited by fucoidin, a competitive inhibitor of the scavenger receptor on MPM. These results suggest that A-LP is recognized by a high affinity binding site on MPM. Stimulation of cholesterol esterification in MPM by A-lP was inhibited by the lysosomotropic agent, chloroquine, indicating that degradation in lysosomes was a prerequisite for cholesterol esterification. Substantial degradation of apo B within the intact A-LP was demonstrated by SDS-PAGE and immunoblotting. This degradation could be responsible for the increased negative charge on A-LP, and the enhanced recognition of A-LP by a receptor on MPM. Stimulation of cholesterol esterification increased linearly over a 48-hour time period, suggesting that the receptor on MPM recognizing A-LP was not down-regulated. This unabated increase in cholesterol esterification resulted in massive accumulation of intracellular cholesteryl esters and a gradual increase in oil red O-positive droplets, giving the MPM the characteristic morphology of foam cells. If these mechanisms demonstrated in vitro also occur in the intact artery, they could explain how foam cells are formed within the fatty streak lesion.

Animals↗
PubMed1979

Small intestinal mucosal fatty acid uptake and esterification in infants and children.

Oleic acid uptake and esterification in intact intestinal mucosa were studied in 14 infants and children with chronic non-specific diarrhea, but histologically normal small intestinal mucosal biopsies, using an in vitro technique. The uptake rate was 5.876 +/- 1.942 nmol fatty acid/mg Nigrogen/minute and the esterification rate was 4.060 +/- 1.010 nmol fatty acid/mg Nitrogen/minute, comparable to previous adult esterification studies. No effect of age on either esterification or uptake was present. Mucosal injury resulted in significant reductions in esterification (p less than 0.001) and uptake (p less than 0.05) compared to controls. Bile acid deficiencies led to reductions in mucosal esterification (p less than 0.05) but not uptake.

Bile Acids and Salts↗
PubMed1999

Effect of overfeeding during the dry period on the rate of esterification in adipose tissue of dairy cows during the periparturient period.

The in vitro rate of esterification of fatty acids in adipose tissue was compared between cows that were fed at restricted energy intake and cows that were overfed during the dry period. Subcutaneous adipose tissue was biopsied at -1, 0.5, 1, and 3 wk from parturition. The basal in vitro rate of esterification was quantified, as well as the rate of esterification after the addition of glucose or glucose plus insulin. The basal rate in adipose tissue from overfed cows at -1 wk was higher than in adipose tissue from cows that were fed at restricted energy intake and indicated enhanced storage of triacylglycerols in adipose tissue of overfed cows at that time. The rate of esterification after the addition of glucose or glucose plus insulin was increased in both groups at each sampling time, but the mean rates, expressed as a percentage of the basal rates, were lower for overfed cows than for cows that were fed at restricted energy intake at 0.5 and 1 wk. Although the addition of glucose or glucose plus insulin increased esterification rates in adipose tissue from both groups of cows, adipose tissue from overfed cows was less sensitive to the addition of these compounds. In conclusion, overfeeding during the dry period predisposed cows to accumulate fat in adipose tissue during the prepartum period. The smaller increase in esterification rate after the addition of glucose or glucose plus insulin in adipose tissue of overfed cows indicates a lower ability of the adipose tissue to esterify circulating fatty acids or to reesterify mobilized fatty acids, which, combined with higher rates of lipolysis postpartum, contributes to continuously elevated concentrations of circulating nonesterified fatty acids postpartum, leading to a more severe hepatic lipidosis in overfed cows.

Adipose Tissue↗
PubMed1997

Tissue expression studies on the mouse acyl-CoA: cholesterol acyltransferase gene (Acact): findings supporting the existence of multiple cholesterol esterification enzymes in mice.

Cholesterol esterification is involved in the regulation of cellular cholesterol content and has been hypothesized to play a role in important physiologic processes including intestinal cholesterol absorption, hepatic lipoprotein production, and macrophage foam cell formation in atherosclerotic lesions. Although initial studies of the mouse acyl CoA:cholesterol acyltransferase gene (Acact) suggested that its gene product was responsible for cholesterol esterification in most tissues, we observed recently that Acact-/- mice have only tissue-specific reductions in cholesterol esterification. To better understand the role of Acact in cholesterol esterification, we used in situ hybridization and immunoblotting to perform tissue expression studies in wild-type mice. We found high levels of Acact expression in steroidogenic tissues, sebaceous glands, and atherosclerotic lesions, but not in the liver or the small intestine. These data support the hypothesis that multiple cholesterol esterification enzymes exist in mammals and that another enzyme is likely to be responsible for cholesterol esterification activity in mouse liver and intestine.

Adrenal Cortex↗
PubMed1973

Dependence of rates of lipolysis, esterification, and free fatty acid release in isolated fat cells on age, cell size, and nutritional state.

Rates of lipolysis, esterification, and free fatty acid release were estimated in isolated epididymal fat cells prepared from rats fed either ad lib. or with a restricted caloric intake. Basal and epinephrine- or theophylline-stimulated rates of lipolysis correlated positively with cell size in the ad lib.-fed group only. Rates of esterification, both basal and epinephrine-stimulated, correlated positively with cell size in the ad lib.-fed group but negatively in the caloric-restricted group. These findings indicate that nutritional factors can modify any possible influence of adipose cell size on lipolysis and esterification. On the other hand, in both groups of rats, epinephrine- and theophylline-stimulated rates of lipolysis correlated positively with the basal rates of lipolysis. Also, rates of esterification in the presence of epinephrine correlated positively with the basal rates of esterification, suggesting that stimulated rates of lipolysis and esterification are at least partly determined by the basal rates regardless of nutritional status. The activity of glycerokinase measured in homogenates of isolated fat cells, if applicable to intact fat cells, was sufficient to cause considerable underestimations of the basal rates of lipolysis (using glycerol production as an index). When lipolysis was stimulated, the potential errors of estimating lipolysis by glycerol production alone were negligible.

Adipocytes↗
PubMed1975

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.

Adrenal Glands↗
PubMed1982

The use of in vivo-in vitro labeling techniques to study phospholipid fatty acid turnover and fatty acid esterification into triglycerides in adipose tissue of aging mice.

We are interested in membrane phospholipid and triglyceride synthesis and turnover in aging cells. As a preliminary, short-term feasibility study we have used an established in vivo-in vitro technique to estimate the initial rates of [1-14C] palmitate (complexed to albumin) esterification to triglycerides and phospholipids in adipocytes and non-adipocytes in the epididymal fat pads of aging mice (8-92 weeks). We have expressed our data in terms of unit cell, unit triglyceride mass and unit (membrane) phospholipid mass. Fat pad and adipocyte size, cell surface area, and adipocyte volume changes were measured and found to follow the same relations as reported in the literature, with some exceptions in very old mice (retired breeders). Rates of fatty acid esterification to triglycerides were about 100 times faster than those to phospholipids in adipocytes. Aging caused a marked fall in the rates of triglyceride fatty acid formation from added palmitate; thus, the rate of fatty acid esterification to triglycerides fell from 0.75 to 0.13 nequiv. fatty acid per min per fat pad (youngest most active group, cf. oldest group). Esterification of fatty acids into phospholipids in adipocytes of the oldest mice was significantly lower than in those of the young and middle-aged groups. Contamination of adipocytes by non-adipocytes was observed in fat pads from old, but not from young, mice. The non-adipocytes accounted for about half of the phospholipid fatty acid esterification. The rate of phospholipid esterification was so slow in adipocytes (all ages) and so relatively fast in non-adipocytes that further studies of phospholipid fatty acid turnover in adipocytes using this system are not considered feasible, especially as a means for studying removal rates of autoxidized fatty acids from membrane phospholipids in vivo during aging.

Adipose Tissue↗
PubMed1967

Effect of glucocorticoid hormones on fatty acid mobilization and re-esterification in rat adipose tissue.

1. The effect of dexamethasone and cortisol on fatty acid mobilization and re-esterification has been studied in intact adipose tissue and isolated fat cells of the rat. 2. Dexamethasone added in vitro inhibited both the re-esterification of mobilized free fatty acids and the esterification of palmitate in the medium. 3. Under several conditions (low concentrations of dexamethasone, cortisol at a high concentration, with tissue from starved animals), steroid-induced release of free fatty acids could be accounted for by decreased re-esterification only, overall lipolytic activity remaining unmodified. At higher concentrations of dexamethasone, however, stimulation of lipolytic activity also occurred. 4. Decreased re-esterification produced by dexamethasone was observed in the total absence of glucose from the incubation medium. Further, dexamethasone stimulated the disappearance of prelabelled [(14)C]glycogen from the tissue. 5. The evidence presented suggests that mobilization of free fatty acids induced by glucocorticoid hormones under physiological conditions is primarily due to a decrease of the re-esterification rate rather than to lipase activation.

Adipose Tissue↗
PubMed1976

The esterification of exogenous fatty acids by adipose tissue of hypertriglyceridaemic subjects with or without diabetes mellitus.

1. The esterification of exogenous palmitate to diglyceride and triglyceride in adipocytes was studied in obese and diabetic patients with and without hypertriglyceridaemia. The rate of esterification correlated significantly with the triglyceride content of adipocytes. 2. In diabetic patients with hypertriglyceridaemia, the rate of esterification to triglyceride was significantly greater than in diabetic patients with normotriglyceridaemia. This difference could not be attributed to differences in glucose tolerance or to the degree of obesity. 3. Fasting plasma insulin levels were greater in the hypertriglyceridaemic group than in the normotriglyceridaemic group. The difference in esterification rates could have been due to differences in adipocyte size. 4. The esterification of fatty acid in adipose tissue of diabetic patients was lower than in non-diabetic subjects and this difference could not be accounted for by differences in adipocyte size or differences in the intracellular pools of fatty acid in adipose tissue. 5. The role of esterification of exogenous fatty acids in adipose tissue as a possible determinant of the uptake of glyceride fatty acids from plasma is discussed.

Adipose Tissue↗
PubMed2004

Effects of 25-hydroxycholesterol on cholesterol esterification and sterol regulatory element-binding protein processing are dissociable: implications for cholesterol movement to the regulatory pool in the endoplasmic reticulum.

The regulatory pool of cholesterol is located in the endoplasmic reticulum (ER) and is key to how mammalian cells sense and respond to changes in cellular cholesterol levels. The extent of cholesterol esterification by the ER-resident protein, acyl-coenzyme A:cholesterol acyl-transferase (ACAT), has become the standard method for monitoring cholesterol transport to the ER and is assumed to reflect the regulatory pool of ER cholesterol. The oxysterol, 25-hydroxycholesterol (25HC), is thought to trigger intracellular cholesterol transport to the ER. In support of this contention, we confirmed previous reports that 25HC activates cholesterol esterification and is a potent suppressor of the sterol regulatory element-binding protein (SREBP) pathway. Processing of the ER membrane-bound SREBP into a soluble transcription factor is controlled by cholesterol levels in the ER. In this study, we addressed whether or not cholesterol esterification necessarily reflects cholesterol movement to the cholesterol homeostatic machinery in the ER as determined by SREBP processing. We found that three agents that inhibited the ability of 25HC to induce cholesterol esterification (progesterone, nigericin, and monensin) did not have a corresponding effect on 25HC suppression of SREBP processing. Moreover, ACAT inhibition did not alter the sensitivity of SREBP processing to 25HC. Therefore, cholesterol esterification by the ER-resident protein ACAT is dissociable from cholesterol transport to the cholesterol homeostatic machinery in the ER. In light of our results, we question the security of previous work that has inferred cholesterol transport to the ER regulatory pool based solely on cholesterol esterification.

Animals↗
PubMed1985

The role of cholesterol esterification in ovarian steroidogenesis: studies in cultured swine granulosa cells using a novel inhibitor of acyl coenzyme A: cholesterol acyltransferase.

We have used a novel competitive inhibitor of acyl coenzyme A:cholesterol acyltransferase (ACAT), Sandoz compound 58-035 [3-(decyldimethyl-silyl)N-[2-(4-methyl-phenyl)1-phenylethyl propanamide], to assess the importance of the cholesterol esterification reaction in ovarian steroidogenesis. Compound 58-035 markedly (greater than or equal to 96%) inhibited ACAT activity of swine ovarian microsomes in a dose-dependent (0.1-3.5 micrograms/ml) fashion. In addition, treatment of cultured granulosa cells with this fatty acylamide effectively (greater than or equal to 98%) suppressed hormonally stimulated cholesterol esterification, as assessed by the incorporation of [3H]oleic acid into cholesteryl ester. Accordingly, we used this inhibitor to test the role of cholesterol esterification in ovarian cells. In cultures with limited or no serum supplementation, long term (2- to 6-day) treatment of granulosa cells with compound 58-035 significantly increased basal progesterone production and amplified by 2- to 10-fold the stimulatory actions of trophic hormones, such as estradiol, FSH, estradiol combined with FSH, or insulin. The amplifying effect of ACAT inhibition on hormone-stimulated progesterone production could be mimicked by providing exogenous cholesterol substrate in the form of low density lipoprotein (LDL). Cotreatment with compound 58-035 and LDL resulted in no further augmentation of steroidogenesis. In contrast to the facilitative effects of compound 58-035 in longer term cultures, this ACAT inhibitor did not alter progesterone biosynthesis acutely (2-20 h) in swine or hamster ovarian cells. These observations suggest that there is an obligatory partitioning of some sterol into the ester pool in granulosa cells. In times of diminished availability of cholesterol, inhibition of the esterification pathway can make additional cholesterol available for use in steroid hormone biosynthesis. Thus, in the intact Graafian follicle, where LDL cholesterol delivery to granulosa cells and intracellular cholesteryl ester stores are limited, regulation of the ACAT reaction may significantly modulate rates of progesterone biosynthesis. The present results indicate that the use of a selective inhibitor of cholesterol esterification can permit one to probe the functional significance of the esterification reaction in steroidogenic cells.

Amides↗
PubMed2005

Lipase-catalyzed esterification of selected phenolic acids with linolenyl alcohols in organic solvent media.

Lipase-catalyzed esterification of selected phenolic acids with linolenyl alcohols was investigated in selected organic solvent media. The enzyme activity for the esterification of dihydrocaffeic acid with linolenyl alcohol in solvent mixtures of hexane/2-butanone of 75:25 (v/v) and 65:35 (v/v) was 0.88 and 0.47 micromol of esterified dihydrocaffeic acid/(g of solid enzyme.min), respectively, with a corresponding esterification yield of 76 and 58%, respectively. However, the esterification of ferulic acid with linolenyl alcohol in the reaction medium of hexane/2-butanone of 65:35 (v/v) resulted in a low yield (16%). Using the reaction medium of hexane/2-butanone of 75:25 (v/v), an increase in linolenyl alcohol concentration with a concomitant use of a constant amount of dihydrocaffeic acid resulted in an increase in esterification yield. The highest esterification yield of 99% was obtained with a ratio of dihydrocaffeic acid to linolenyl alcohol of 1:8 after 7 d of reaction. Biosynthesis of the end product, linolenyl dihydrocaffeate, was confirmed by electrospray ionization mass spectroscopy structural analysis; the ester product demonstrated an antiradical activity close to that of alpha-tocopherol.

Antioxidants↗
PubMed1990

Monocyte colony-stimulating factor enhances uptake and degradation of acetylated low density lipoproteins and cholesterol esterification in human monocyte-derived macrophages.

We have investigated effects of monocyte colony-stimulating factor (M-CSF) on the uptake of acetylated low density lipoproteins (acetyl-LDL) and the activity of cholesterol esterification in human monocyte-derived macrophage. The cells were cultured with M-CSF for 10 days and then incubated with acetyl-LDL for 24 h. M-CSF (128 ng/ml) enhanced the uptake and degradation of 10 micrograms/ml of 125I-acetyl LDL 7.5-fold (n = 6) and the effect of M-CSF was dose-dependent at the concentrations of 0.5-32 ng/ml. The binding experiments at 4 degrees C demonstrated that the number of acetyl-LDL receptor was increased by the addition of M-CSF. Supporting this, ligand blotting analysis revealed a significant increase in a receptor protein for acetyl-LDL (240 kDa). Binding of LDL was also enhanced by M-CSF but less significantly than that of acetyl-LDL. Cellular cholesterol esterification in the presence of 10 micrograms/ml acetyl-LDL was enhanced 24.1-fold (n = 13) by 128 ng/ml M-CSF. It was evident that M-CSF enhanced cholesterol esterification to a greater extent than the cellular uptake of acetyl-LDL (24.1- versus 7.5-fold). Cholesterol esterification was also enhanced by the addition of granulocyte-macrophage colony-stimulating factor and interleukin 1. We conclude that M-CSF enhances the uptake of both acetyl-LDL and LDL by increasing their receptor number, and further enhances the process of cholesterol esterification, resulting in a remarkable increase in cholesterol esterification in macrophages. These findings strongly suggest the significant involvement of cytokines such as M-CSF in cholesterol metabolism of macrophages.

Biological Transport↗
PubMed1980

Regulation of cholesterol esterification and biosynthesis in monolayer cultures of normal adult rat hepatocytes.

Adult rat parenchymal liver cells were isolated and cultured in monolayers. Cholesterol esterification in the intact cultured cells was determined by measuring incorporation of tritiated oleic acid into cell cholesterol ester. Addition of 10 microgram/ml of 25-hydroxycholesterol to the medium gave a 3- to 6-fold increase in cholesterol esterification, while the incorporation of oleic acid into phospholipids and triglycerides remained unaltered. Similar stimulation of cholesterol esterification by 25-hydroxycholesterol was also found if [14C]mevalonolactone or [3H]cholesterol (the latter presented to the cells in high density lipoproteins) were used as precursors. The stimulatory effect of 25-hydroxycholesterol was maximal after only 15-min incubation and was independent of protein synthesis. After 4 to 6 h of incubation with 25-hydroxycholesterol, its stimulatory effect was reduced significantly, and after 18 h of incubation no stimulation was observed. Thus, the liver cells in some fashion adapt to the continuing presence of 25-hydroxycholesterol. Isolated microsomes prepared from cells previously incubated with 25-hydroxycholesterol showed acyl-CoA:cholesterol acyltransferase activity 3 times that of microsomes from control cells. Incubation of isolated microsomes with 25-hydroxycholesterol increased acyl-CoA:cholesterol acyltransferase activity 2-fold. The net cellular content of ester cholesterol increased after 2 to 6 h incubation of hepatocytes with 25-hydroxycholesterol; there was a net decrease in cellular free cholesterol. Mevalonolactone (10 mM) also stimulated cholesterol esterification and inreased the cellular content of ester cholesterol (3- to 4-fold). The effectiveness of mevalonolactone did not diminish with longer periods of preincubation. Furthermore, the stimulatory effects of 25-hydroxycholesterol and mevalonolactone added together were at least 50 to 100% greater than the effects of either agent alone, suggesting that the mechanisms by which they increase cellular cholesterol esterification are different. Both 25-hydroxycholesterol and mevalonolactone rapidly inhibited microsomal 3-hydroxy-3-methylglutaryl-CoA reductase activity. Pure cholesterol had no effect on cellular cholesterol esterification or on 3-hydroxy-3-methylglutaryl-CoA reductase activity at concentrations up to 10 microgram/ml.

Animals↗