Effects of excess dietary cholesterol on adrenal cholesterol accumulation and steroidogenesis

Reversal of cognitive deficit of apolipoprotein E knockout mice after repeated exposure to a common environmental experience

This study tests the hypothesis that a history of common stressful experiences further promotes the cognitive deficit of apolipoprotein E (apoE)-knockout mice, an animal model to study aspects of Alzheimer's disease. In experiment 1, apoE-knockout and wild-type mice were repeatedly subjected to an environmental challenge (i.e. exposure to rats) and the effect was monitored on Morris water maze performance. Naive apoE-knockout mice were impaired, but surprisingly after rat stress their water maze performance improved and switched to a goal-directed search strategy. Rat stress induced in wild-type mice spatial learning deficits and an inefficient search strategy. Swim ability was not affected by rat stress and under basal conditions measures for locomotion and anxiety were similar for both genotypes. In experiments 2 and 3, we found that the rat stress paradigm attenuated the elevation of basal and stress-induced corticosterone concentrations in the apoE-knockout mice towards concentrations observed in wild-type mice. The expression of hippocampal mineralocorticoid and glucocorticoid receptor mRNA was similar in both genotypes, but in response to rat stress, the level of glucocorticoid receptor mRNA increased selectively in the CA1 pyramidal field. In conclusion, repeated exposure to a common environmental experience did abolish and reverse the difference in cognitive performance and corticosterone concentrations of apoE-knockout and wild-type mice.

Acyl-CoA binding protein (ACBP) regulates acyl-CoA:cholesterol acyltransferase (ACAT) in human mononuclear phagocytes

It is demonstrated that the acyl-CoA:cholesterol acyltransferase (ACAT) enzyme activity in rough endoplasmatic reticulum membranes is regulated by the acyl-CoA binding protein (ACBP). The ACAT activity is strongly inhibited by different ACBP/oleoyl-CoA complexes depending from the molar ratio of protein and fatty acid-CoA. Other lipid binding proteins such as bovine serum albumin and the liver fatty acid binding protein do not show any effects on ACAT activity. In addition, we can show that cholesterol loading with acetylated low density lipoproteins does not lead to an increase of the ACBP mRNA level. Consequently, the increase of the intracellular concentration of fatty acids because of the cholesteryl ester accumulation renders ACAT more active for cholesterol esterification. In binding studies we have characterized binding sites on microsomal membranes for the ACAT substrate oleoyl-CoA and the ACAT inhibitor diazepam. Diazepam competes with oleoyl-CoA and vice versa for its binding to microsomal membranes. This common binding site is suggested to be responsible for the transfer from ACBP-bound oleoyl-CoA to ACAT and, therefore, to be essential for the microsomal cholesterol esterification.

Effect of ACTH suppression on adrenal 3-hydroxy-3-methylglutaryl coenzyme A reductase mRNA in 4-aminopyrazolopyrimidine-treated rats

4-Aminopyrazolopyrimidine (4-APP) treatments to rats for 3 days induced 2-fold increase of circulating ACTH and 11-fold increase of adrenal 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase mRNA compared to NaCl-treated controls. This in vivo model was used to study the effect of the suppression of ACTH secretion on the adrenal HMG-CoA reductase mRNA level. Dexamethasone (Dex) administration to 4-APP-treated rats caused a rapid and parallel decline of the levels of plasma ACTH and adrenal HMG-CoA reductase mRNA to 50% within 2.5 h, whereas the free and esterified cholesterol content was increased 5 and 9.4 times respectively. These changes could be counteracted by the co-administration of ACTH with Dex. Aminoglutethimide (AG) administration to 4-APP-treated rats, which increased the adrenal esterified cholesterol content (7.5 times), decreased the HMG-CoA reductase mRNA level (44%), despite plasma ACTH level remaining elevated. Moreover, the participation of newly synthesized protein(s) in the lowering of adrenal HMG-CoA reductase mRNA level induced by ACTH suppression is suggested by the fact that cycloheximide (Cyclo), when co-administered with AG, completely blocked the decrease of HMG-CoA reductase mRNA level, despite the plasma ACTH level decreasing by 68% and the free and esterified cholesterol content increasing 3.9 and 12.3 times, compared to 4-APP-treated rats. Furthermore, the specificity of these effects was established by the fact that the beta-actin mRNA level was not affected by the administration of either Dex, AG, Cyclo, or AG + Cyclo to 4-APP-treated rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Impaired corticosteroid production by isolated adrenocortical cells of hypercholesterolemic rabbits

Changes in the structure and function of rabbit adrenocortical cells were studied eight weeks after cessation of a cholesterol-enriched diet. The plasma cholesterol level of the treated animals was still modestly elevated. All three zones of the adrenal cortex contained a slightly increased number of intracellular lipid droplets, and greater number of lipid-laden macrophages were located in the inner layers, as revealed by light and electron microscopy. The basal and ACTH-stimulated hormone production by the isolated adrenocortical cells were significantly inhibited compared with those of the controls. A possible causal relationship between the inhibited adrenocortical hormone production and the increased number of macrophages is discussed.

Chemical modification of acyl-CoA:cholesterol O-acyltransferase. 1. Identification of acyl-CoA:cholesterol O-acyltransferase subtypes by differential diethyl pyrocarbonate sensitivity

Acyl-CoA:cholesterol O-acyltransferase (EC 2.3.1.26) (ACAT) catalyzes the intracellular synthesis of cholesteryl esters from cholesterol and fatty acyl-CoA at neutral pH. Despite the probable pathophysiologic role of ACAT in vascular cholesteryl ester accumulation during atherogenesis, its mechanism of action and its regulation remain to be elucidated because the enzyme polypeptide has never been identified or purified. Present chemical modification results identify two distinct tissue types of ACAT, based on marked differences in reactivity of an active-site histidine residue toward diethyl pyrocarbonate (DEP) and acetic anhydride. The apparent Ki of the DEP-sensitive ACAT subtype, typified by aortic ACAT, was 40 microM, but the apparent Ki of the DEP-resistant ACAT subtype, typified by liver ACAT, was 1500 microM, indicating a 38-fold difference in sensitivity to DEP. Apparent Ki's of aortic and liver ACAT for inhibition by acetic anhydride were also discordant (less than 500 microM and greater than 5 mM, respectively). On the basis of the reversibility of inhibition by hydroxylamine, a neutral pKa for maximal modification, and acetic anhydride protection against DEP inactivation, DEP and acetic anhydride appear to modify a common histidine residue. Oleoyl-CoA provided partial protection against inactivation by DEP and acetic anhydride, suggesting that the modified histidine is at or near the active site of ACAT. Systematic investigation of ACAT activity from 14 different organs confirmed the existence of 2 subtypes of ACAT on the basis of their different reactivities toward DEP and acetic anhydride.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of mevinolin, an inhibitor of cholesterol synthesis, on the morphological and functional responses of rat adrenal zona fasciculata to a prolonged treatment with 4-aminopyrazolo-pyrimidine

Rat adrenocortical cells are almost completely dependent upon the continuous supply of cholesterol derived from serum lipoproteins. However, a prolonged (5-day) administration of 4-aminopyrazolo-pyrimidine (4-APP), a potent hypocholesterolaemic drug, though provoking a notable decrease in the intra-adrenal concentration of esterified and free cholesterol, did not significantly affect basal plasma level of corticosterone. Morphometry showed a conspicuous hypertrophy of zona fasciculata cells, coupled with a striking proliferation of smooth endoplasmic reticulum (SER) and peroxisomes and with a profound lipid-droplet depletion. The secretory response of zona fasciculata cells to ACTH was still present, but reduced by half with respect to control rats. The simultaneous administration of mevinolin, an inhibitor of cholesterol synthesis, to 4-APP-treated rats caused an additional drop in the intracellular content of free cholesterol and notably lowered basal plasma corticosterone concentration. Mevinolin magnified the 4-APP-induced zona fasciculata cell hypertrophy, as well as SER and peroxisome proliferation. The secretory response to ACTH was completely suppressed. These data are compatible with the view that the morphological changes, which rat zona fasciculata cells undergo during prolonged hypocholesterolaemia, are the expression of the activation of the endogenous cholesterol synthesis. This compensatory response, enabling zona fasciculata cells to maintain a normal basal rate of hormonal output and to respond (though less efficiently) to their main physiological stimulus, seems to be completely independent of any activation of the hypothalamo-hyphophyseal axis, since dexamethasone/ACTH treated rats were used. The hypothesis is advanced that the mechanism underlying this response may involve the decrease of the intracellular free-cholesterol pool.

Effect of mevinolin on rat hepatocytes: a morphometric study

A short-term (12 h) infusion with mevinolin, a competitive inhibitor of 3-hydroxy-3-methylglutaryl coenzyme A reductase, significantly lowered plasma cholesterol concentration in rats. After 7 days of continuous treatment hypocholesterolaemia disappeared, and this was coupled with a notable increase in the average volume of hepatocytes. Liver-cell hypertrophy was associated with a striking proliferation of the membranes of both rough and smooth endoplasmic reticulum, as well as with a tremendous increase in the number of peroxisomes. These structural changes are interpreted as the morphologic counterpart of the compensatory response of rat liver to the prolonged inhibitory effect of mevinolin on cholesterol synthesis.

Effect of long-term inhibition of hydroxy-methylglutaryl coenzyme A reductase by mevinolin on the zona fasciculata of rat adrenal cortex. A combined morphometric and biochemical study

The effects of a 7-day infusion with mevinolin, a potent competitive inhibitor of hydroxy-methylglutaryl coenzyme A (HMG-CoA) reductase, on the adrenal zona fasciculata were examined in normal and dexamethasone/ACTH-treated rats. In both groups of animals, the drug caused a lowering in plasma and intra-adrenal cholesterol concentrations, as well as a slight decrease in the blood level of corticosterone. Morphometry of zona fasciculata cells showed that specific mevinolin-induced changes (i.e. those occurring in both groups of rats and therefore not due to enhanced release of ACTH following decrease in circulating corticosterone) are severe lipid-droplet depletion and a conspicuous increase in smooth endoplasmic reticulum (SER) and peroxisomes. The hypothesis is discussed that these morphological changes express a compensatory response of zona fasciculata cells to counteract the mevinolin-induced inhibition of cholesterol synthesis in both liver and adrenal cortex.

Effects of a prolonged treatment with aminoglutethimide on the zona fasciculata of rat adrenal cortex: a morphometric investigation

The effects of a 7-day administration of aminoglutethimide (AG) on the adrenal zona fasciculata were examined in "normal" and dexamethasone/ACTH-treated rats. There was a 70-74% decrease in the concentration of corticosterone in blood, but no conspicuous qualitative changes suggesting cell degeneration occurred. Morphometry showed that AG induced a significant hypertrophy of the zona fasciculata and its parenchymal cells only in "normal" animals, which was due to an increase in the volume of the mitochondrial compartment and to proliferation of the smooth endoplasmic reticulum. This response to AG was considered to be non-specific and mediated by the enhanced secretion of ACTH following the decrease in the blood level of corticosterone. AG administration significantly increased the volume of the lipid-droplet compartment and the number of intramitochondrial lipid-like inclusions in both groups of animals. These changes were interpreted as the morphological counterpart of the AG-induced block of cholesterol utilization in steroid synthesis.

Hypercholesterolemia and the regulation of adrenal steroidogenesis in 2,3,7,8-tetrachlorodibenzo-p-dioxin-treated rats

Plasma and adrenal cholesterol disposition have been examined to gain further insight into the mechanisms by which 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) treatment decreases the diurnal peak in plasma corticosterone concentrations. TCDD induces an increase in plasma cholesterol concentration that is nearly complete on Day 2, at least 2 days before the most pronounced increase in adrenal cholesterol concentration (Days 4-6). This adrenal increase involves both free cholesterol and cholesterol esters, in contrast to the response to dietary hypercholesterolemia where only cholesterol esters increase. Although adrenocorticotropin (ACTH) does not increase adrenal mitochondrial cholesterol in normal rats (cholesterol turnover is faster than cholesterol uptake), this response changes between Days 6 and 9 after TCDD treatment such that ACTH then stimulates accumulation of mitochondrial cholesterol. This additional cholesterol is fully available to cytochrome P-450SCC, as judged both by active cholesterol metabolism in isolated mitochondria and by increased cholesterol-P-450SCC complex formation. The accompanying in vivo suppression of the peak plasma corticosterone concentration suggests a TCDD-induced inhibition of cholesterol side-chain cleavage (SCC). Consistent with this hypothesis, similar effects on adrenal mitochondrial cholesterol were produced by in vivo administration of the cholesterol side-chain cleavage inhibitor, aminoglutethimide, to ACTH-stimulated rats. Although the putative TCDD-induced inhibitory factor is apparently readily lost from mitochondria during preparation, inhibition may be retained in isolated cells. TCDD, therefore, affects adrenal cholesterol regulation by at least two mechanisms. Adrenal cholesterol content increases in part as a consequence of elevated plasma cholesterol, and cholesterol side-chain cleavage becomes partially inhibited in vivo.