Inhibitors of sterol synthesis. Effects of dietary 5 alpha-cholest-8(14)-en-3 beta-ol-15-one on early enzymes in hepatic cholesterol biosynthesis

Oxysterols: modulators of cholesterol metabolism and other processes

Oxygenated derivatives of cholesterol (oxysterols) present a remarkably diverse profile of biological activities, including effects on sphingolipid metabolism, platelet aggregation, apoptosis, and protein prenylation. The most notable oxysterol activities center around the regulation of cholesterol homeostasis, which appears to be controlled in part by a complex series of interactions of oxysterol ligands with various receptors, such as the oxysterol binding protein, the cellular nucleic acid binding protein, the sterol regulatory element binding protein, the LXR nuclear orphan receptors, and the low-density lipoprotein receptor. Identification of the endogenous oxysterol ligands and elucidation of their enzymatic origins are topics of active investigation. Except for 24, 25-epoxysterols, most oxysterols arise from cholesterol by autoxidation or by specific microsomal or mitochondrial oxidations, usually involving cytochrome P-450 species. Oxysterols are variously metabolized to esters, bile acids, steroid hormones, cholesterol, or other sterols through pathways that may differ according to the type of cell and mode of experimentation (in vitro, in vivo, cell culture). Reliable measurements of oxysterol levels and activities are hampered by low physiological concentrations (approximately 0.01-0.1 microM plasma) relative to cholesterol (approximately 5,000 microM) and by the susceptibility of cholesterol to autoxidation, which produces artifactual oxysterols that may also have potent activities. Reports describing the occurrence and levels of oxysterols in plasma, low-density lipoproteins, various tissues, and food products include many unrealistic data resulting from inattention to autoxidation and to limitations of the analytical methodology. Because of the widespread lack of appreciation for the technical difficulties involved in oxysterol research, a rigorous evaluation of the chromatographic and spectroscopic methods used in the isolation, characterization, and quantitation of oxysterols has been included. This review comprises a detailed and critical assessment of current knowledge regarding the formation, occurrence, metabolism, regulatory properties, and other activities of oxysterols in mammalian systems.

Sterol synthesis. Synthesis of 3 beta-hydroxy-25,26,26,26,27,27,27-heptafluorocholest-5-en-7-one and its effects on HMG-CoA reductase activity in Chinese hamster ovary cells, on ACAT activity in rat jejunal microsomes, and serum cholesterol levels in rats

3 beta-Hydroxycholest-5-en-7-one (I; 7-ketocholesterol) is an oxysterol of continuing interest in biology and medicine. In the present study, we have prepared a side-chain fluorinated analog, 3 beta-hydroxy-25,26,26,26,27,27,27-heptafluorocholest-5-en-7-one (VI), with the anticipation that the F7 substitution would block major metabolism of the 7-ketosterol, and thereby enhance its potential in vivo effects on serum cholesterol levels and other parameters. Chromium trioxide/dimethyl pyrazole oxidation of the acetate derivative of the previously described 25,26,26,26,27,27,27-heptafluorocholest-5-en-3 beta-ol (Swaminathan et al., 1993. J. Lipid Res. 34, 1805-1823) followed by mild alkaline hydrolysis gave VI. The effects of VI on 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase activity in Chinese hamster ovary (CHO-K1) cells, on acyl coenzyme A-cholesterol acyltransferase (ACAT) activity in rat jejunal microsomes, and on serum cholesterol levels and other parameters in male Sprague-Dawley rats were determined and compared with those obtained with I and with another alpha, beta-unsaturated ketosterol, i.e. 3 beta-hydroxy-5 alpha-cholest-8(14)-en-15-one (II). I and VI showed essentially the same potency, considerably less than that of II, in lowering the levels of HMG-CoA reductase activity in CHO-K1 cells. Whereas addition of II to rat jejunal microsomes inhibited ACAT activity (IC50 approximately 3 microM), I and VI had no effect under the conditions studied (from 1 to 16 microM). Dietary administration of I, at levels of 0.1 and 0.15%, had no effect on food consumption, gain in body weight, or serum cholesterol levels. At 0.2%, I caused a modest decrease in body weight gain and a slight decrease in serum cholesterol levels (relative to ad libitum but not pair-fed control animals). The F7-7-ketosterol VI, at 0.26% in diet (the molar equivalent of 0.2% I), had no effect on food consumption, body weight, or serum cholesterol levels. Administration of I (0.1, 0.15 or 0.2% in diet) caused increases in the weight of small intestine. In contrast, no effect of VI (0.26% in diet) on small intestinal weight was observed.

Review of progress in sterol oxidations: 1987-1995

Material dealing with the chemistry, biochemistry, and biological activities of oxysterols is reviewed for the period 1987-1995. Particular attention is paid to the presence of oxysterols in tissues and foods and to their physiological relevance.

Irreversible inhibition of 3-hydroxy-3-methylglutaryl-coenzyme A synthase from yeast by F-244 and (RS)-β-butyrolactone

The inhibition of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) synthase from yeast was compared for both F-244 (1) and (RS)-β -butyrolactone (5). F-244 exhibits irreversible inhibition with an IC50of 8 nM, similar to that reported for the rat liver enzyme, while the binding constant (1/K1) and inactivation rate constant (kinact) similar to values reported for the human cytoplasmic enzyme. (RS)-β-Butyrolactone (5) also irreversibly inhibits the enzyme, but with much lower efficiency (IC502 mM). The values for K1(9 mM) and kinact(0.0078 s−1) for 5 were determined. The results show that kinactfor 5 and 1 differ by a factor of only 2.5, while K1for 5 is higher by a factor of 1.8 × 105. Hence, the β-lactone ring is shown to be the sole essential structural feature in 1 for irreversible inactivation of HMG-CoA synthase; however, the remaining functionality enhances the binding of 1 to the enzyme relative to 5. Key words: HMG-CoA synthase, F-244, butyrolactone, irreversible, inhibitor.

Inhibitors of sterol synthesis. An improved chemical synthesis of 26-oxygenated delta 8(14)-15-ketosterols having the 25R configuration

(25R)-3 beta,26-Dihydroxy-5 alpha-cholest-8(14)-en-15-one (I) was synthesized in four steps from (25R)-3 beta,26-diacetoxycholesta-5,7-diene (III) in 30% overall yield. Isomerization of III with HCl in chloroform-dichloromethane at -60 degrees C gave (25R)-3 beta,26-diacetoxy-5 alpha-cholesta-7,14-diene together with the 5 alpha-delta 8,14 and 5 beta-delta 8,14 isomers in a 5:1:1 ratio. Epoxidation of the crude diene mixture with m-chloroperbenzoic acid, followed by hydrolysis in acetone containing concentrated HClO4 (0.1%) gave (25R)-3 beta,26-diacetoxy-5 alpha-cholest-8(14)-en-15-one (VIII), accompanied by numerous minor byproducts, including the 5 alpha,14 beta-delta 7, 5 alpha, 14 beta-delta 8 and 5 beta,14 beta-delta 8 isomers of VIII. All four 15-ketosterol esters were isolated by chromatography and fully characterized by mass spectrometry and 1H and 13C nuclear magnetic resonance. Treatment of VIII with potassium carbonate in degassed methanol gave I.

Inhibitors of sterol synthesis: effects of a 7 alpha-alkyl analog of 3 beta-hydroxy-5 alpha-cholest-8(14)-en-15-one on 3-hydroxy-3-methylglutaryl coenzyme A reductase activity in cultured mammalian cells and on serum cholesterol levels and other parameters in rats

The 7 alpha-methyl analog (II) of 3 beta-hydroxy-5 alpha-cholest-8(14)-en-15- one (I) was prepared by chemical synthesis and evaluated with respect to its effects on HMG-CoA reductase activity in CHO-K1 cells and on serum cholesterol levels in rats. The 7 alpha-methyl substitution had no detectable effect on the potency of I in lowering HMG-CoA reductase activity in the cultured cells. In contrast, the 7 alpha-methyl substitution had a marked effect on the action of I in the suppression of food consumption in rats. Whereas II was less potent than I in lowering serum cholesterol levels in rats, it did so at dosage levels at which only slight or moderate effects on food consumption were observed. Full 1H and 13C-NMR assignments for II and intermediates in its synthesis have been presented. Conformational analysis, based on 1H-1H coupling constants, NMR shieldings and force-field calculations, indicated that the 7 alpha-methyl substitution had virtually no effect on the conformation of the 15-ketosterol apart from minor distortions of ring B.

Inhibitors of sterol synthesis. Chemical syntheses and spectral properties of 26-oxygenated derivatives of 3 beta-hydroxy-5 alpha-cholest-8(14)-en-15-one and their effects on 3-hydroxy-3-methylglutaryl coenzyme A reductase activity in CHO-K1 cells

26-Oxygenated derivatives of delta 8(14)-15-ketosterols have been synthesized from (25R)-3 beta,26-diacetoxy-5 alpha-cholest-8(14)-en-15-one (IX) as part of a program to prepare potential metabolites and analogs of 3 beta-hydroxy-5 alpha-cholest-8(14)-en-15-one (I), a potent regulator of cholesterol metabolism. Partial hydrolysis of IX gave a mixture, from which the 3 beta,26-diol II and the 26-acetate (XI) and 3 beta-acetate (X) monoesters were isolated. Mitsunobu reaction of XI followed by hydrolysis gave (25R)-3 alpha,26-dihydroxy-5 alpha-cholest-8(14)-en-15-one (VI). Oxidation of XI with pyridinium chlorochromate followed by hydrolysis of the acetate gave (25R)-26-hydroxy-5 alpha-cholest-8(14)-ene-3,15-dione (VII). Oxidation of X with Jones reagent followed by hydrolysis of the acetate gave (25R)-3 beta-hydroxy-15-keto-5 alpha-cholest-8(14)-en-26-oic acid (IVa). Jones oxidation of II gave (25R)-3,15-diketo-5 alpha-cholest-8(14)-en-26-oic acid (VII). 1H and 13C nuclear magnetic resonance assignments and analyses of mass spectral fragmentation data are presented for each of the new compounds and their derivatives. The 3,15-diketone VII was found to be highly active in lowering the levels of 3-hydroxy-3-methylglutaryl coenzyme A reductase activity in CHO-K1 cells, with a potency comparable to that of I. In contrast, 3 alpha,26-diol VI was less potent than I or VII. The two carboxylic acid analogs IVa and VIII were considerably less potent than VI in lowering the levels of HMG-CoA reductase activity.

Inhibitors of sterol synthesis. A major role of chylomicrons in the metabolism of 5 alpha-cholest-8(14)-en-3 beta-ol-15-one in the rat

The metabolism of 5 alpha-cholest-8(14)-en-3 beta-ol-15-one (I), a potent regulator of cholesterol (Chol) metabolism which has significant hypocholesterolemic activity upon oral administration to animals, has been investigated in male rats. After intragastric administration of [2,4-3H] I and [4-14C]Chol in triolein to intestinal lymph duct-cannulated rats, most of the 3H of the lymph was associated with chylomicrons. Most of the 3H in the chylomicrons was associated with fatty acid esters of I and the oleate ester represented the major species of the esters of I. After intravenous injection of the isolated doubly-labeled chylomicrons to intact rats, rapid clearance of 3H and 14C from blood was observed which was associated with a rapid and selective uptake of 3H and 14C by liver. The rate of disappearance of 3H from blood and the rate of uptake of 3H by liver were similar, if not identical, to those for 14C. In contrast, the disappearance of 3H from the liver was much more rapid than that of 14C. Studies of the distribution of 3H in liver demonstrated rapid formation of free I and the formation of [3H]Chol. In addition, significant amounts of the 3H in liver were associated with polar materials, a finding which was not observed in the case of 14C. After intravenous administration of the doubly-labeled chylomicrons to bile duct-cannulated rats, very rapid and substantial metabolism of the administered 3H to polar biliary metabolites was observed. The bulk of the 3H not recovered in bile at 49 h after the injection of the labeled chylomicrons was recovered in blood and tissues and almost all (integral of 94%) of this material was associated with Chol and Chol esters. The combined results indicate an important role for chylomicrons in the overall metabolism of I. The selective delivery of I to liver as its oleate ester in chylomicrons (or, more probably, as chylomicron remnants) and the subsequent metabolism of the oleate ester of I in liver has important consequences with respect to the actions of I which are discussed herein.