Presence of oxidized cholesterol in caveolae uncouples active platelet-derived growth factor receptors from tyrosine kinase substrates.
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Platelet-derived growth factor receptor beta (PDGFRbeta) in fibroblasts is concentrated in caveolae where it controls the tyrosine phosphorylation of multiple proteins. Caveolae are enriched in cholesterol and sphingolipids, but the role of these lipids in PDGFR signal transduction is unknown. We report that introduction of cholest-4-en-3-one into caveolae membranes uncouples PDGFR autophosphorylation from tyrosine phosphorylation of neighboring proteins. Cholest-4-en-3-one appears to interfere with the normal interaction between PDGFR and its partners. The results suggest that tightly packed caveolae lipids form a membrane platform that functions as a lipid scaffold for organizing the molecular interactions of multiple signaling pathways. (+info)
Microsomal 12alpha-hydroxylation of 7alpha-[12alpha, 12beta-2H2]hydroxy-4-cholesten-3-one.
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The synthesis of 7alpha-[12alpha, 12beta-2 H2]hydroxy-4-cholesten-3-one is described. It was shown with different techniques that this compound was 12alpha-hydroxylated by the microsomal fraction of a rat liver homogenate without marked isotope effect, indicating that cleavage of the C--H bond is not the rate-limiting step in this hydroxylation. The rate of 12alpha-hydroxylation was decreased by about 20% when performed in a medium containing deuterated water. The findings were discussed with reference to the specific properties of the 12alpha-hydroxylating system and to the results of previous studies on rate-limiting step in microsomal hydroxylation of steroids. (+info)
Determinants of variable response to statin treatment in patients with refractory familial hypercholesterolemia.
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Interindividual variability in low density lipoprotein (LDL) cholesterol (LDL-C) response during treatment with statins is well documented but poorly understood. To investigate potential metabolic and genetic determinants of statin responsiveness, 19 patients with refractory heterozygous familial hypercholesterolemia were sequentially treated with placebo, atorvastatin (10 mg/d), bile acid sequestrant, and the 2 combined, each for 4 weeks. Levels of LDL-C, mevalonic acid (MVA), 7-alpha-OH-4-cholesten-3-one, and leukocyte LDL receptor and hydroxymethylglutaryl coenzyme A reductase mRNA were determined after each treatment period. Atorvastatin (10 mg/d) reduced LDL-C by an overall mean of 32.5%. Above-average responders (LDL-C -39.5%) had higher basal MVA levels (34.4+/-6.1 micromol/L) than did below-average responders (LDL-C -23.6%, P<0.02; basal MVA 26.3+/-6.1 micromol/L, P<0.01). Fewer good responders compared with the poor responders had an apolipoprotein E4 allele (3 of 11 versus 6 of 8, respectively; P<0.05). There were no baseline differences between them in 7-alpha-OH-4-cholesten-3-one, hydroxymethylglutaryl coenzyme A reductase mRNA, or LDL receptor mRNA, but the latter increased in the good responders on combination therapy (P<0.05). Severe mutations were not more common in poor than in good responders. We conclude that poor responders to statins have a low basal rate of cholesterol synthesis that may be secondary to a genetically determined increase in cholesterol absorption, possibly mediated by apolipoprotein E4. If so, statin responsiveness could be enhanced by reducing dietary cholesterol intake or inhibiting absorption. (+info)
27-hydroxycholesterol is an endogenous ligand for liver X receptor in cholesterol-loaded cells.
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The nuclear receptors liver X receptor alpha (LXRalpha) (NR1H3) and LXRbeta (NR1H2) are important regulators of genes involved in lipid metabolism, including ABCA1, ABCG1, and sterol regulatory element-binding protein-1c (SREBP-1c). Although it has been demonstrated that oxysterols are LXR ligands, little is known about the identity of the physiological activators of these receptors. Here we confirm earlier studies demonstrating a dose-dependent induction of ABCA1 and ABCG1 in human monocyte-derived macrophages by cholesterol loading. In addition, we show that formation of 27-hydroxycholesterol and cholestenoic acid, products of CYP27 action on cholesterol, is dependent on the dose of cholesterol used to load the cells. Other proposed LXR ligands, including 20(S)-hydroxycholesterol, 22(R)-hydroxycholesterol, and 24(S),25-epoxycholesterol, could not be detected under these conditions. A role for CYP27 in regulation of cholesterol-induced genes was demonstrated by the following findings. 1) Introduction of CYP27 into HEK-293 cells conferred an induction of ABCG1 and SREBP-1c; 2) upon cholesterol loading, CYP27-expressing cells induce these genes to a greater extent than in control cells; 3) in CYP27-deficient human skin fibroblasts, the induction of ABCA1 in response to cholesterol loading was ablated; and 4) in a coactivator association assay, 27-hydroxycholesterol functionally activated LXR. We conclude that 27-hydroxylation of cholesterol is an important pathway for LXR activation in response to cholesterol overload. (+info)
Characterization of two steroidal ketones and two isoprenoid alcohols in dairy products.
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Two steroidal ketones, delta-4-cholesten-3-one and delta-3,-5-cholestadiene-7-one, were isolated and identified for the first time in anhydrous milk fat and in nonfat dry milk. Together with these, two isoprenoid alcohols, phytol and dihydrophytol, were identified in anhydrous milk fat. Their identities were established on the basis of chromatographic and mass spectral data and confirmed by comparison with authentic materials. (+info)
Oxysterols: friends, foes, or just fellow passengers?
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Oxysterols are oxygenated derivatives of cholesterol that are intermediates or even end products in cholesterol excretion pathways. Because of their ability to pass cell membranes and the blood-brain barrier at a faster rate than cholesterol itself, they are also important as transport forms of cholesterol. In addition, oxysterols have been ascribed a number of important roles in connection with cholesterol turnover, atherosclerosis, apoptosis, necrosis, inflammation, immunosuppression, and the development of gallstones. According to current concepts, oxysterols are physiological mediators in connection with a number of cholesterol-induced metabolic effects. However, most of the evidence for this is still indirect, and there is a discrepancy between the documented potent effects of oxysterols under in vitro conditions and the studies demonstrating that they are of physiological importance in vivo. Oxysterol-binding proteins, such as liver X receptor-alpha (a nuclear receptor), do have a regulatory role in cholesterol turnover, but the physiological ligand of the protein has not yet been defined with certainty. Recently developed genetically engineered mouse models with markedly reduced or increased concentration of some of the oxysterols have exhibited surprisingly small changes in cholesterol turnover and homeostasis. The present review is a critical evaluation of the literature on oxysterols, in particular, the in vivo evidence for a role of oxysterols as physiological regulators of cholesterol homeostasis and as atherogenic factors. (+info)
Oat bran stimulates bile acid synthesis within 8 h as measured by 7alpha-hydroxy-4-cholesten-3-one.
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BACKGROUND: Oat bran contains soluble fibers, such as beta-glucan, that increase bile acid excretion and thus decrease serum cholesterol. Bile acid synthesis correlates with serum concentrations of the metabolite 7alpha-hydroxy-4-cholesten-3-one (alpha-HC). OBJECTIVE: The objective was to investigate whether consumption of beta-glucan from oat bran increases bile acid synthesis, as measured by the serum alpha-HC concentration, within hours after consumption in response to the loss of bile acids from the liver. DESIGN: In a randomized, single-blind, wheat bran-controlled study with crossover design, 8 subjects were served a controlled diet during 2 periods of 3 d each, with an 11-d washout between the periods. Breakfast included either 75 g extruded oat bran, of which 11 g was beta-glucan, or 75 g wheat bran, of which 1 g was beta-glucan. Alpha-HC was measured by HPLC on each day at 0, 12, and 24 h after breakfast and also at 8 h after breakfast on the first day. RESULTS: After 8 and 12 h of the oat bran diet period, the serum alpha-HC concentration was 84% (P = 0.012) and 92% (P = 0.017) higher, respectively, than that before breakfast. Serum concentrations returned to the baseline value after 24 h. Wheat bran did not influence serum alpha-HC concentrations. CONCLUSIONS: Consumption of beta-glucan from oat bran nearly doubled the serum alpha-HC concentration within 8 h, indicating increased bile acid synthesis. alpha-HC in serum could be used as a marker of increased bile acid excretion induced by the diet. (+info)
Monitoring hepatic cholesterol 7alpha-hydroxylase activity by assay of the stable bile acid intermediate 7alpha-hydroxy-4-cholesten-3-one in peripheral blood.
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We describe an accurate method for monitoring the enzymatic activity of hepatic cholesterol 7alpha-hydroxylase (C7alphaOH; CYP7A1), the rate-limiting and major regulatory enzyme in the synthesis of bile acids. Assay of 7alpha-hydroxy-4-cholesten-3-one (C4), an intermediate in bile acid synthesis, revealed that the level of C4 in peripheral blood serum or plasma showed a strong correlation to the enzymatic activity of hepatic C7alphaOH, both at steady-state conditions (r = 0.929) as well as during the rapid changes that occur during the diurnal phases. This assay should be of value in clarifying the regulation of bile acid synthesis in vivo in laboratory animals and humans since it allows for the monitoring of hepatic C7alphaOH activity using peripheral blood samples. (+info)