Oxysterol regulators of 3-hydroxy-3-methylglutaryl-CoA reductase in liver. Effect of dietary cholesterol

Hepatic regulatory oxysterols were analyzed to determine which oxysterols were present in livers of mice fed a cholesterol-free diet and whether repression of 3-hydroxy-3-methylglutaryl-CoA reductase following cholesterol feeding was accompanied by an increase in one or more oxysterols. Analysis of free and esterified sterols from mice fed a cholesterol-free diet resulted in the identification and quantitation of six regulatory oxysterols: 24-hydroxycholesterol, 25-hydroxycholesterol, 26-hydroxycholesterol, 7 alpha-hydroxycholesterol, 7 beta-hydroxycholesterol, and 7-ketocholesterol. Following the addition of cholesterol to the diet for 1 or 2 nights, hepatic 3-hydroxy-3-methylglutaryl-CoA reductase activity declined and the levels of oxysterols, especially those of the side-chain-hydroxylated sterols, increased. Total 3-hydroxy-3-methylglutaryl-CoA reductase repressor units attributable to identified free oxysterols increased 2.5- and 6-fold after 1 and 2 nights, respectively, of cholesterol feeding. The amounts of esterified 24-, 25-, and 26-hydroxycholesterol also increased, with the increase in esterified 24-hydroxycholesterol being the greatest. The 24-hydroxycholesterol was predominantly the 24S epimer and the 26-hydroxycholesterol was predominantly the 25R epimer, indicating enzymatic catalysis of their formation. The observed correlation between increased levels of regulatory oxysterols and repression of 3-hydroxy-3-methylglutaryl-CoA reductase in cholesterol-fed mice is consistent with a hypothesis that intracellular oxysterol metabolites regulate the level of the reductase.     

Evaluation of 3-hydroxy-3-methylglutaryl-CoA reductase activity by multiple-selected ion monitoring

A new method for the evaluation of 3-hydroxy-3-methylglutaryl-CoA reductase activity is described, based on the multiple-selected ion monitoring of the amount of mevalonate formed in incubations of 3-hydroxy-3-methylglutaryl-CoA with microsomal proteins. Analysis is carried out on crude extracts using deuterated mevalonic acid lactone as internal standard. The sensitivity of the technique allows the quantitative evaluation of mevalonate in microassays (100 μg microsomal protein) of the enzyme activity at the minimum value of the diurnal rhythm.     

Dietary regulation of 3-hydroxy-3-methylglutaryl-CoA reductase from rat intestine

The effect of dietary cholesterol on rat intestinal 3-hydroxy-3-methylglutaryl-coenzyme A reductase (EC 1.1.1.34) varied depending upon whether animals received the dietary cholesterol with polyunsaturated or saturated fats. When cholesterol was fed with polyunsaturates, the enzyme activity in both the jejunum and ileum was significantly suppressed, whereas only the enzyme in the jejunum was significantly suppressed when cholesterol was given with saturated fats. It is concluded that dietary cholesterol has a negative feedback effect on intestinal cholesterol synthesis.     

Activity of acyl-CoA: cholesterol acyltransferase and 3-hydroxy-3-methylglutaryl-CoA reductase in subfractions of hepatic microsomes enriched with cholesterol

The influence of membrane cholesterol on the activities of acyl-CoA: cholesterol acyltransferase and 3-hydroxy-3-methylglutaryl-CoA reductase was examined in three microsomal subfractions (RNA-rich, RNA-poor, and smooth) that had been enriched with cholesterol by incubation with mixed lipoproteins from hypercholesterolemic rabbit serum. Acyl-CoA: cholesterol acyltransferase activity was significantly stimulated in the three subfractions, particularly in the RNA-rich microsomal component. 3-Hydroxy-3-methylglutaryl-CoA reductase, on the other hand, was suppressed (30%) in only one (RNA-poor) of the three microsomal subfractions, despite a 1.4-fold increase in the concentration of membrane cholesterol. An attempt was made to distinguish between an effect based exclusively on an increase in available cholesterol substrate and an activation of acyl-CoA: cholesterol acyltransferase in RNA-rich microsomes enriched with cholesterol. An experimental design was devised so that substrate cholesterol was provided in the form of heated smooth microsomes and acyl-CoA: cholesterol acyltransferase was provided as a separate preparation in the form of RNA-rich microsomes. Appropriate controls were carried out to test for transfer of cholesteryl ester between the two sets of particles. The results suggested that cholesterol enhanced acyl-CoA: cholesterol acyltransferase activity by serving both as a substrate and as a non-substrate modulator.     

A method for rapid microassay of 3-hydroxy-3-methylglutaryl-CoA reductase activity

A method is devised for the separation of mevalonolactone (MVL) from hydroxymethylglutarate (HMG) in the assay of HMG-CoA reductase activity. The main steps in the procedure consist of absorbing the reaction mixture on the bottom part of a rectangular filter paper and selectively transfering the MVL into the top part of the paper by upward elution with toluene. Under the experimental conditions deseribed, MVL is recovered in an yield of approximately 60%, with little contamination with HMG. Among the advantages of the method are that it involves simple and very few manipulations, no internal standard is required to calculate the recovery of MVL, and simultaneous analyses of a large number of samples are possible.     

Effectors of rapid homeostatic responses of endoplasmic reticulum cholesterol and 3-hydroxy-3-methylglutaryl-CoA reductase

The cholesterol content of the endoplasmic reticulum (ER) and the activity of 3-hydroxy-3-methylglutaryl-CoA reductase (HMGR) imbedded therein respond homeostatically within minutes to changes in the level of plasma membrane cholesterol. We have now examined the roles of sterol regulatory element-binding protein (SREBP)-dependent gene expression, side chain oxysterol biosynthesis, and cholesterol precursors in the short term regulation of ER cholesterol levels and HMGR activity. We found that SREBP-dependent gene expression is not required for the response to changes in cell cholesterol of either the pool of ER cholesterol or the rate of cholesterol esterification. It was also found that the acute proteolytic inactivation of HMGR triggered by cholesterol loading required the conversion of cholesterol to 27-hydroxycholesterol. High levels of exogenous 24,25-dihydrolanosterol drove the inactivation of HMGR; lanosterol did not. However, purging endogenous 24,25-dihydrolanosterol, lanosterol, and other biosynthetic sterol intermediates by treating cells with NB-598 did not greatly affect either the setting of their ER cholesterol pool or the inactivation of their HMGR. In summary, neither SREBP-regulated genes nor 27-hydroxycholesterol is involved in setting the ER cholesterol pool. On the other hand, 27-hydroxycholesterol, rather than cholesterol itself or biosynthetic precursors of cholesterol, stimulates the rapid inactivation of HMGR in response to high levels of cholesterol.     

The regulation of 3-hydroxy-3-methylglutaryl-CoA reductase activity, cholesterol esterification and the expression of low-density lipoprotein receptors in cultured monocyte-derived macrophages.

Human blood monocytes cultured in medium containing 20% whole serum showed the greatest activity of 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase and [14C]acetate incorporation into non-saponifiable lipids around the 7th day after seeding, the period of greatest growth. Although there was enough low-density lipoprotein (LDL) in the medium to saturate the LDL receptors that were expressed by normal cells at that time, HMG-CoA reductase activity and acetate incorporation were as high in normal cells as in cells from familial-hypercholesterolaemic (FH) patients. Both the addition of extra LDL, which interacted with the cells by non-saturable processes, and receptor-mediated uptake of acetylated LDL significantly reduced reductase activity and increased incorporation of [14C]oleate into cholesteryl esters in normal cells and cells from FH patients ('FH cells'), and reduced the expression of LDL receptors in normal cells. Pre-incubation for 20h in lipoprotein-deficient medium apparently increased the number of LDL receptors expressed by normal cells but reduced the activity of HMG-CoA reductase in both normal and FH cells. During subsequent incubations the same rate of degradation of acetylated LDL and of non-saturable degradation of LDL by FH cells was associated with the same reduction in HMG-CoA reductase activity, although LDL produced a much smaller stimulation of oleate incorporation into cholesteryl esters. In normal cells pre-incubated without lipoproteins, receptor-mediated uptake of LDL could abolish reductase activity and the expression of LDL receptors. The results suggested that in these cells, receptor-mediated uptake of LDL might have a greater effect on reductase activity and LDL receptors than the equivalent uptake of acetylated LDL. It is proposed that endogenous synthesis is an important source of cholesterol for growth of normal cells, and that the site at which cholesterol is deposited in the cells may determine the nature and extent of the metabolic events that follow.     

In situ determination of the functional size of hepatic 3-hydroxy-3-methylglutaryl-CoA reductase by radiation inactivation analysis

Radiation inactivation analysis of liver pieces yielded a target size of 210 kDa for hepatic 3-hydroxy-3-methylglutaryl CoA (HMG-CoA) reductase [S)-mevalonate:NADP+ oxidoreductase (CoA-acylating), EC 1.1.1.34) from rats fed a normal diet. Feeding a diet containing mevinolin and colestipol, which causes a marked increase in enzyme activity, resulted in a reduction of the target size to 120 kDa. These results are consistent with those obtained by radiation inactivation and immunoblotting analysis of isolated microsomes and suggest that the increase in HMG-CoA reductase activity caused by these dietary agents is accompanied by a change from a dimer to a monomer form of the enzyme.     

New substrates and inhibitors of 3-hydroxy-3-methylglutaryl-CoA reductase

Methyl (RS)-5-bromo-3-hydroxy-3-methyl-pentanoate was prepared by bromination of methyl mevalonate and used for the formation of 4-carboxy-3-hydroxy-3-methylbutyl thioether derivatives by reaction with N-octanoyl-cysteamine, pantetheine, phosphopantetheine and coenzyme A. These thiols were also converted to the (RS)-3-hydroxy-3-methylglutaryl thioester derivatives. The thioesters formed with pantetheine and phosphopantetheine are substrates of 3-hydroxy-3-methylglutaryl-CoA reductase; Km and V values are similar to those of the superior CoA-derivative. The corresponding thioether derivatives in which the oxygen next to sulfur of the substrates is replaced by hydrogen, are inhibitors of the reductase. The inhibition is competitive with 3-hydroxy-3-methylglutaryl-CoA varied, and noncompetitive with NADPH varied. For each of the corresponding pairs of thioester and thioether derivatives Km (substrate) is nearly identical with Ki (inhibitor). The specificity and stereospecificity of the inhibitor action are also shown.     

Regulation of 3-hydroxy-3-methylglutaryl coenzyme A reductase mRNA levels by L-triiodothyronine

In hypophysectomized rats, hepatic 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase activity, immunoreactive 97-kilodalton (97-kDa) protein, and mRNA were all reduced to undetectable levels. Administration of triiodothyronine (T3) resulted in large increases in all three after a 36-h lag period. HMG-CoA reductase activity, immunoreactive 97-kDa protein levels, and reductase mRNA levels were tightly correlated. Feeding hypophysectomized rats diets containing the bile acid sequestrant colestipol, together with the potent reductase inhibitor mevinolin, resulted in an increase in HMG-CoA reductase activity similar to that seen with T3 but a lesser stimulation of reductase mRNA levels. These results suggest that agents which cause depletion of mevalonate-derived products may share in part with T3 a common mechanism for increasing levels of HMG-CoA reductase activity in order to satisfy cellular needs for these products. Dexamethasone treatment, which is known to prevent the T3-mediated stimulation of reductase activity, caused a marked decrease in 97-kDa immunoreactive material but had little effect on reductase mRNA levels.     

New site-directed reversible inhibitors of 3-hydroxy-3-methylglutaryl-CoA reductase.

1. CoA-thioether analogues of 3-hydroxy-3-methylglutaryl-CoA containing an additional methyl group at positions 2, 6(methyl at C3) or 4 of the acyl residue were prepared. To probe for hydrophobic interaction, their inhibitory properties were determined with 3-hydroxy-3-methylglutaryl-CoA reductase purified from baker's yeast. The CoA-thioethers were purely competitive inhibitors whose affinity to the reductase was near to that of the physiological substrate. 2. CoA-sulfoxides derived from the CoA-thioethers displayed affinities to the reductase superior to that of the physiological substrate (Km = 7 microM). Depending on the degree of recognition of diastereomers by the enzyme, the inhibitor constants of the two best inhibitors vary from Ki = 200 nM and Ki = 80 nM (diastereomeric mixtures) to 25 nM and 20 nM, respectively (if only one diastereomer would interact with the enzyme).     

Sterol-independent regulation of 3-hydroxy-3-methylglutaryl-CoA reductase by mevalonate in Chinese hamster ovary cells. Magnitude and specificity

In this paper, we assess the relative degree of regulation of the rate-limiting enzyme of isoprenoid biosynthesis, 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase, by sterol and nonsterol products of mevalonate by utilizing cultured Chinese hamster ovary cells blocked in sterol synthesis. We also examine the two other enzymes of mevalonate biosynthesis, acetoacetyl-CoA thiolase and HMG-CoA synthase, for regulation by mevalonate supplements. These studies indicate that in proliferating fibroblasts, treatment with mevalonic acid can produce a suppression of HMG-CoA reductase activity similar to magnitude to that caused by oxygenated sterols. In contrast, HMG-CoA synthase and acetoacetyl-CoA thiolase are only weakly regulated by mevalonate when compared with 25-hydroxycholesterol. Furthermore, neither HMG-CoA synthase nor acetoacetyl-CoA thiolase exhibits the multivalent control response by sterol and mevalonate supplements in the absence of endogenous mevalonate synthesis which is characteristic of nonsterol regulation of HMG-CoA reductase. These observations suggest that nonsterol regulation of HMG-CoA reductase is specific to that enzyme in contrast to the pleiotropic regulation of enzymes of sterol biosynthesis observed with oxygenated sterols. In Chinese hamster ovary cells supplemented with mevalonate at concentrations that are inhibitory to reductase activity, at least 80% of the inhibition appears to be mediated by nonsterol products of mevalonate. In addition, feed-back regulation of HMG-CoA reductase by endogenously synthesized nonsterol isoprenoids in the absence of exogenous sterol or mevalonate supplements also produces a 70% inhibition of the enzyme activity.     

A relationship between the activities of hepatic lanosterol 14 alpha-demethylase and 3-hydroxy-3-methylglutaryl-CoA reductase.

At 1-2 h after intragastric administration of ketoconazole, a cytochrome P-450 inhibitor, to rats, there was a 50-60% decrease in the activity of hepatic 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase. Inhibition reached a maximum at 6-12 h after the drug was given, but after 24 h enzyme activity was stimulated by 60%. The rates of synthesis of hepatic non-saponifiable lipids in vivo showed a similar time-dependent pattern of change. During the first few hours after drug administration, the hepatic cytochrome P-450-dependent metabolism of lanosterol was suppressed in vivo. However, 24 h after treatment, this activity was stimulated, an effect which was also observed by pre-treatment of the rats with the drug for several days. Suppression of hepatic HMG-CoA reductase and lanosterol 14 alpha-demethylase activities was accompanied by a relative increase in the accumulation of labelled polar sterols in the liver in vivo. In the intestine, ketoconazole also resulted in a rapid decline in the rate of synthesis of non-saponifiable lipids and an inhibition of lanosterol 14 alpha-demethylation in vivo. However, in contrast with the liver, there was no stimulation of non-saponifiable lipid synthesis after 24 h.