ly-295427 and 25-hydroxycholesterol

Side-chain oxysterols, such as 25-hydroxycholesterol (25-HC), are key regulators of cholesterol homeostasis. New evidence suggests that the alteration of membrane structure by 25-HC contributes to its regulatory effects. We have examined the role of oxysterol membrane effects on cholesterol accessibility within the membrane using perfringolysin O (PFO), a cholesterol-dependent cytolysin that selectively binds accessible cholesterol, as a sensor of membrane cholesterol accessibility. We show that 25-HC increases cholesterol accessibility in a manner dependent on the membrane lipid composition. Structural analysis of molecular dynamics simulations reveals that increased cholesterol accessibility is associated with membrane thinning, and that the effects of 25-HC on cholesterol accessibility are driven by these changes in membrane thickness. Further, we find that the 25-HC antagonist LY295427 (agisterol) abrogates the membrane effects of 25-HC in a nonenantioselective manner, suggesting that agisterol antagonizes the cholesterol-homeostatic effects of 25-HC indirectly through its membrane interactions. These studies demonstrate that oxysterols regulate cholesterol accessibility, and thus the availability of cholesterol to be sensed and transported throughout the cell, by modulating the membrane environment. This work provides new insights into how alterations in membrane structure can be used to relay cholesterol regulatory signals.

Topics: Bacterial Toxins; Cell Membrane; Cholestanols; Cholesterol; Hemolysin Proteins; Homeostasis; Hydroxycholesterols; Liposomes; Membrane Lipids; Molecular Dynamics Simulation; Structure-Activity Relationship

Oxysterols activating liver X receptors (LXRs) repress expression of pro-inflammatory genes and have anti-inflammatory effects. Here, we show for the first time that bone marrow-derived murine mast cells (BMMCs) predominantly express LXRbeta. 25-hydroxycholesterol, a representative LXR activating oxysterol, suppressed IL-6 production and degranulation response in BMMCs following engagement of high-affinity IgE receptor (FcepsilonRI). Interestingly, 25-hydroxycholesterol reduced cell-surface FcepsilonRI expression by inhibiting assembly of FcepsilonRIalpha and FcepsilonRIbeta. We demonstrate that LXR activation was involved in the suppression of IL-6 production in BMMCs, but that reduced FcepsilonRI expression and degranulation response was mediated in an LXR-independent manner.

Topics: Animals; Antibody Affinity; Bone Marrow Cells; Cell Degranulation; Cells, Cultured; Cholestanols; Down-Regulation; Hydroxycholesterols; Interleukin-6; Liver X Receptors; Mast Cells; Mice; Mice, Inbred C57BL; Orphan Nuclear Receptors; Receptors, IgE; Signal Transduction; Sterols

Oxysterols play a significant role in cholesterol homeostasis. 25-Hydroxycholesterol (25HC) in particular has been demonstrated to regulate cholesterol homeostasis via oxysterol-binding protein and oxysterol-related proteins, the sterol regulatory element binding protein, and the rate-limiting enzyme of cholesterol biosynthesis, hydroxymethylglutaryl coenzyme A reductase. We have examined the effect of 25HC on pigmentation of cultured murine melanocytes and demonstrated a decrease in pigmentation with an IC(50) of 0.34 microM and a significant diminution in levels of melanogenic protein tyrosinase. Pulse-chase studies of 25HC-treated cells demonstrated enhanced degradation of tyrosinase, the rate-limiting enzyme of melanin synthesis, following endoplasmic reticulum (ER) and Golgi maturation. Protein levels of GS28, a member of an ER/cis-Golgi SNARE protein complex, were also diminished in 25HC-treated melanocytes, however levels of the ER chaperone calnexin and the cis-Golgi matrix protein GM130 were unaffected. Effects of 25HC on tyrosinase were completely reversed by 4 alpha-allylcholestan-3 alpha-ol, a sterol identified by its ability to reverse effects of 25HC on cholesterol homeostasis. Finally, the addition of 25HC to lipid deficient serum inhibited correct processing of tyrosinase. We conclude that 25HC acts in the Golgi compartment to regulate pigmentation by a mechanism shared with cholesterol homeostasis.

Topics: Animals; Cell Line; Cholestanols; Cholesterol; Endoplasmic Reticulum; Golgi Apparatus; Hydroxycholesterols; Melanins; Melanocytes; Membrane Proteins; Mice; Monophenol Monooxygenase; Peptide Hydrolases; Proteasome Endopeptidase Complex; Qb-SNARE Proteins; Signal Transduction; Structure-Activity Relationship; Ubiquitin-Activating Enzymes

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.

Topics: Animals; Blotting, Western; CCAAT-Enhancer-Binding Proteins; Cell Line; CHO Cells; Cholestanols; Cholesterol; Cricetinae; DNA-Binding Proteins; Dose-Response Relationship, Drug; Endoplasmic Reticulum; Enzyme Inhibitors; Hydroxycholesterols; Models, Biological; Protein Binding; Sterol Regulatory Element Binding Protein 1; Time Factors; Transcription Factors

The sterol LY295427 reduces plasma cholesterol levels in animals by increasing the expression of hepatic low density lipoprotein (LDL) receptors. Here we trace the hypocholesterolemic activity of LY295427 to an ability to reverse oxysterol-mediated suppression of sterol regulatory element-binding protein (SREBP) processing. Micromolar concentrations of LY295427 induced the metabolism of LDL in oxysterol-treated cultured cells and inhibited the stimulation of cholesteryl ester synthesis mediated by oxysterols. cDNA microarray and RNA blotting experiments revealed that LY295427 increased levels of the LDL receptor mRNA and those of other SREBP target genes. The compound stimulated the accumulation of SREBPs in the nuclei of cells grown in the presence of oxysterols within 4-6 h of addition to the medium. Induction required components of the normal SREBP-processing pathway, including the SREBP cleavage-activating protein and the Site 1 protease. LY295427 overcame the suppression of SREBP processing mediated by several oxysterols but not by LDL-derived cholesterol. We conclude that LY295427 achieves a therapeutically desirable end point by an unique mechanism of action.

Topics: Animals; Anticholesteremic Agents; CCAAT-Enhancer-Binding Proteins; Cell Line; Cell Nucleus; Cells, Cultured; CHO Cells; Cholestanols; Cholesterol; Cricetinae; DNA-Binding Proteins; DNA, Complementary; Fibroblasts; Humans; Hydroxycholesterols; Immunoblotting; Lipoproteins, LDL; Models, Chemical; Oligonucleotide Array Sequence Analysis; Protein Binding; RNA; Sphingomyelin Phosphodiesterase; Steroids; Sterol Regulatory Element Binding Protein 1; Time Factors; Transcription Factors; Triglycerides

4 alpha-(2-Propenyl)-5 alpha-cholestan-3 alpha-ol (LY295427) was previously identified from a CHO cell-based assay to be a potent LDL receptor up-regulator and had demonstrated to be an effective agent in lowering plasma cholesterol levels in hypercholesterolemic hamsters. In order to investigate the effect of flexibility of the 3 alpha-hydroxy-bearing A-ring on the activity, 4 alpha-(2-propenyl)-5,6-secocholestan-3 alpha-ol (11), a B-ring seco analog of LY295427, is thus synthesized from cholest-4-en-3-one. Test results indicate that 11 is not active in the CHO cell-based LDL receptor/luciferase assay at concentrations up to 20 micrograms/mL. The result underlines the importance of maintaining the A-B-C-D ring rigidity of the 3 alpha-sterols in terms of binding to the putative oxysterol receptor.

Topics: Animals; Anticholesteremic Agents; CHO Cells; Cholestanol; Cholestanols; Cholestenones; Cricetinae; Gene Expression Regulation; Genes, Reporter; Hydroxycholesterols; Luciferases; Promoter Regions, Genetic; Receptors, LDL; Structure-Activity Relationship

LY295427, (3 alpha,4 alpha,5 alpha)-4-(2-propenylcholestan-3-ol), acts through an unknown mechanism to derepress the transcription of the low density lipoprotein (LDL) receptor in the presence of 25-hydroxycholesterol (25-OH chol). Preincubation with LY295427 in Chinese hamster ovary (CHO) cells increased uptake of 25-OH chol in a time-dependent manner, suggesting that the drug interfered with the negative feedback mechanism of 25-OH chol on LDL receptor expression. To explore the mechanism by which LY295427 inhibited the suppressive actions of 25-OH chol, the radioactive ligand [3H]25-OH chol and specific antibodies to the oxysterol binding protein (OSBP) were used to identify possible drug:protein interactions. After separation by anion exchange chromatography, protein fractions from hamster liver cytosol were found to selectively bind [3H]25-OH chol with high affinity. In fractions in which 25-OH chol binding was evident, and in other distinct fractions that lacked specific binding, addition of LY295427 increased [3H]25-OH chol binding 2- to 5-fold. LY306039, the 3 beta-isomer of LY295427, failed to derepress the LDL receptor in CHO cells, and it had no effect on [3H]25-OH chol binding. Analysis of Western blots using polyclonal antibodies to OSBP showed that specific [3H]25-OH chol binding in the absence of LY295427 was present only in fractions containing OSBP. However, enhanced [3H]25-OH chol binding in the presence of LY295427 was evident in distinct fractions after immunodepletion of both the 90-100 kDa form of OSBP and a 170 kDa protein; and specific binding of a radioiodinated analog of LY295427 was detected in select fractions lacking [3H]25-OH chol binding in the absence of LY295427. Moreover, LY295427 did not displace or enhance [3H]25-OH chol binding to OSBP purified to near homogeneity. These data suggest that LY295427, while not dependent on the presence of oxysterol binding protein, binds to cytosolic protein(s) that interact with 25-hydroxycholesterol and other oxystcrols, thus preventing the repression of the LDL receptor.

Topics: Animals; Anticholesteremic Agents; Cholestanols; Cricetinae; Cytosol; Hydroxycholesterols; Liver; Male; Mesocricetus; Protein Binding; Proteins