Control of Low Density Lipoprotein Receptor Gene Promoter Activity

Inhibition of drug metabolism by blocking the activation of nuclear receptors by ketoconazole

Individual variation in drug metabolism is a major cause of unpredictable side effects during therapy. Drug metabolism is controlled by a class of orphan nuclear receptors (NRs), which regulate expression of genes such as CYP (cytochrome)3A4 and MDR-1 (multi-drug resistance-1), that are involved in this process. We have found that xenobiotic-mediated induction of CYP3A4 and MDR-1 gene transcription was inhibited by ketoconazole, a commonly used antifungal drug. Ketoconazole mediated its effect by inhibiting the activation of NRs, human pregnenolone X receptor and constitutive androstene receptor, involved in regulation of CYP3A4 and MDR-1. The effect of ketoconazole was specific to the group of NRs that control xenobiotic metabolism. Ketoconazole disrupted the interaction of the xenobiotic receptor PXR with the co-activator steroid receptor co-activator-1. Ketoconazole treatment resulted in delayed metabolism of tribromoethanol anesthetic in mice, which was correlated to the inhibition of PXR activation and downmodulation of cyp3a11 and mdr-1 genes and proteins. These studies demonstrate for the first time that ketoconazole represses the coordinated activation of genes involved in drug metabolism, by blocking activation of a specific subset of NRs. Our results suggest that ketoconazole can be used as a pan-antagonist of NRs involved in xenobiotic metabolism in vivo, which may lead to novel strategies that improve drug effect and tolerance.

A Novel Functional Interaction between the Sp1-like Protein KLF13 and SREBP-Sp1 Activation Complex Underlies Regulation of Low Density Lipoprotein Receptor Promoter Function

Cholesterol homeostasis is regulated by a family of transcription factors designated sterol regulatory element-binding proteins (SREBPs). Precise control of SREBP-targeted genes requires additional interactions with co-regulatory transcription factors. In the case of the low density lipoprotein receptor (LDLR), SREBP cooperates with the specificity protein Sp1 to activate the promoter. In this report, we describe a novel pathway in LDLR transcriptional regulation distinct from the SREBP-Sp1 activation complex involving the Sp1-like protein Krueppel-like factor 13 (KLF13). Using a combination of RNA interference, electrophoretic mobility shift, chromatin immunoprecipitation, and reporter assays, deletion, and site-directed mutagenesis, we demonstrated that KLF13 mediates repression in a DNA context-selective manner. KLF13 repression of LDLR promoter activity appears to be needed to keep the receptor silent, a state that can be antagonized by Sp1, SREBP, and inhibitors of histone deacetylase activity. Chromatin immunoprecipitation assay confirmed that KLF13 binds proximal LDLR DNA sequences in vivo and that exogenous oxysterol up-regulates such binding. Together these studies identify a novel regulatory pathway in which gene repression by KLF13 must be overcome by the Sp1-SREBP complex to activate the LDLR promoter. Therefore, these data should replace a pre-existent and more simple paradigm that takes into consideration only the induction of the activator proteins Sp1-SREBP as necessary for LDLR promoter drive without including default repression, such as that by KLF13, of the LDLR gene.

Oxysterols: friends, foes, or just fellow passengers?

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.

Lipoproteins regulate expression of the steroidogenic acute regulatory protein (StAR) in mouse adrenocortical cells

The steroidogenic acute regulatory protein (StAR) is required for the movement of cholesterol from the outer to the inner mitochondrial membrane, the site of cholesterol side chain cleavage. Here we describe a novel form of regulation of StAR gene expression in steroidogenic cells. Treatment of Y-1 BS1 adrenocortical cells with either low density lipoprotein (LDL) or high density lipoprotein (HDL) increases expression of endogenous StAR mRNA and protein in a dose-dependent manner. Induction of StAR mRNA by lipoprotein requires basal cAMP-dependent protein kinase, since the inhibitor, R(p)-8-Br-cAMP, inhibited induction of StAR protein by LDL. Likewise, basal StAR expression or LDL induction of StAR protein was not detectable in Y-1 kin-8 cells which are deficient in cAMP-dependent protein kinase. Aminoglutethimide and ketoconazole were used to determine if side chain cleavage of lipoprotein-derived cholesterol is required for induction of StAR mRNA. Treatment with either drug alone induced StAR mRNA expression 1.5-3-fold, while induction of StAR in cells treated with either drug plus LDL, was equal to, or greater than, induction seen with either agent alone, suggesting that lipoprotein does not regulate StAR via generation of an oxysterol intermediate. Both LDL and HDL increased expression of a mouse -966 StAR promoter-reporter construct 1.5-2.5-fold, indicating that regulation occurs at the level of transcription. In contrast, neither lipoprotein was able to induce transcription from a -966 StAR promoter in which the steroidogenic factor-1 site at -135 was abolished, indicating that regulation of StAR transcription by lipoproteins requires steroidogenic factor-1. The regulation of StAR gene expression by lipoproteins may represent a positive feedback circuit which links cholesterol availability with steroidogenic output.

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.

Effects of oxysterols upon macrophage and lymphocyte functions in vitro

Oxygenated derivatives of cholesterol (oxysterols), found in high concentrations in atherosclerotic lesions, are potent immunosuppressive agents inhibiting T-cell responses to different stimuli. The action of oxysterols on macrophage functions and macrophage-lymphocyte interaction has been poorly investigated. In this work, the effects of 25-hydroxycholesterol (25-OHCh) and 7-ketocholesterol (7-KCh) upon some functions of murine peritoneal macrophage (PM), such as generation of reactive oxygen intermediates (ROI), secretion of neopterin and interleukin-1 (IL-1)-like activity, Fc-receptor (FcR) activity, and murine and human lymphocyte functions, participating in lymphocyte-macrophage interactions, such as macrophage-activating factor (MAF) and Ia-inducing factor (IaIF) secretion, were studied in vitro. 7-KCh in concentration of 5 micrograms/mL culture medium only, but not 25-OHCh, significantly inhibited ROI generation by zymosan-stimulated PM. Pretreatment of PM for 22 h with 25-OHCh and 7-KCh led to the decrease of IL-1-like activity secretion. 25-OHCh and 7-KCh inhibited both FcR-dependent binding and phagocytosis of sheep red blood cells (SRBC). Oxysterols did not change both spontaneous and lipopolysaccharide-stimulated secretion of neopterin by PM. 25-OHCh dose-dependently and more efficiently than 7-KCh inhibited murine splenocyte secretion of MAF, which activity was determined by the ability of splenocyte-conditioned medium to stimulate ROI generation in PM. Both 25-OHCh and 7-KCh inhibited significantly proliferative activity of human mixed lymphocyte culture (MLC), as well as lymphocyte secretion of IaIF, which stimulates the expression of HLA antigens in cultured human monocytes. Purified Ch did not alter these parameters. These data showed, that some inflammatory functions of macrophages and lymphocytes may be modified by such environmental conditions as the presence of oxysterols.

Reduction of oxysterol levels up-regulates HMG-CoA reductase activity in rat liver

Cholesterol regulates hepatic 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase activity by feedback inhibition. It has been suggested that oxidized derivatives of cholesterol (oxysterols) play an important role, as an intracellular mediator, in the feedback inhibition of cholesterol biosynthesis. We, therefore, investigated the role of intracellular oxysterols in the regulation of HMG-CoA reductase activity. Rats were fed with food (control), cholesterol, clofibrate as a potentiator of the microsomal monooxygenase cytochrome P-450 enzyme system, ketoconazole as a strong inhibitor of the system, or butylated hydroxytoluene (BHT) as an antioxidant. We analyzed and compared hepatic microsomal oxysterol levels among the groups. The results of this study indicated that the oxysterol level, especially 7beta-hydroxycholesterol and 7-ketocholestrol, in the liver was lowered by the administration of ketoconazole and BHT, and HMG-CoA reductase activity was increased in response to these agents. However, there was no change in the HMG-CoA reductase activity, after the administration of clofibrate. We conclude that reduced levels of oxysterol may release the inhibitory effect on the HMG-CoA reductase enzyme and lead to up-regulation of the enzyme.

The effect of oxidizing cholesterol on gastrointestinal absorption, plasma clearance, tissue distribution, and processing by endothelial cells

Little is known about the absorption or metabolism of oxysterols. Toward better appreciating the metabolic consequences of oxidizing cholesterol, we compared labeled cholesterol with the labeled oxysterols 7 alpha-hydroxycholesterol, 7 beta-hydroxycholesterol, and 7-ketocholesterol prepared from [4-14C]cholesterol, [26,26,26,27,27,27-2H6]cholesterol, and [23,24,25,26,27-13C5] cholesterol. Gastrointestinal absorption of oxysterols in rats was 91.5 +/- 0.3% compared with 75 +/- 1.1% for cholesterol, determined by fecal collection (P < .001). When injected intravenously and followed by gas chromatography/mass spectrometry, 7 alpha-hydroxycholesterol was cleared at 23 times the rate of cholesterol. After 5 minutes, only 1.2 +/- 0.2% of 7 alpha-hydroxycholesterol remained in the plasma, whereas 28.0 +/- 1.7% of cholesterol and 40.0 +/- 2.5% of a triglyceride emulsion injected simultaneously were still present. [14C]7 alpha-Hydroxycholesterol injected intravenously was also rapidly cleared from plasma, was widely distributed in tissues and organs, and showed evidence of extensive metabolism at 5 minutes. The fractional rate of uptake of radiolabeled oxysterols by cultured endothelial cells was 15.7 times that of cholesterol (P < .001), and the fractional rate of efflux was 3.4 times that of cholesterol (P < .001). Oxysterols passed through endothelial cells grown on transwell membranes at a rate 4.3 times that of cholesterol (P < .001). Fractional oxysterol transport across the endothelial cell monolayer was increased 62 +/- 17% when HDL was added to the medium in the lower chamber (P = .003). Oxysterols were extensively metabolized to even more polar metabolites during endothelial cell transit. These properties of oxysterols potentially provide a mechanism for enhancing transport of cholesterol through tissues and preventing accumulation of cholesterol in those cells that can oxidize it.

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.

Transcriptional modulators affect in vivo protein binding to the low density lipoprotein receptor and 3-hydroxy-3-methylglutaryl coenzyme A reductase promoters

Treatment of HepG2 cells with known effectors of low density lipoprotein receptor (LDLR) gene expression altered the in vivo pattern of protein-DNA interactions in the promoter. The observed changes are consistent with proteins binding in vivo to the sterol regulatory element (SRE), to Sp1-like sites, as well as to other regions. Protein bound to the SRE in all conditions, but the nature of the dimethyl sulfate reactivity changed depending on the physiological state of the cell. Hypermethylation within the SRE of the low density lipoprotein receptor promoter was observed when cells were treated with cholesterol synthesis inhibitors, insulin, or phorbol 12-myristate 13-acetate, suggesting that the SRE regulates this promoter through sterol-independent as well as sterol-dependent mechanisms. No significant changes were observed in binding to the Sp1-like sites, suggesting that differential binding to these sites does not play a role in altered transcription levels. Analysis of the 3-hydroxy-3-methylglutaryl coenzyme A reductase promoter also revealed protections that varied in a cell type-specific manner. Binding to the 3-hydroxy-3-methylglutaryl coenzyme A reductase SRE and putative nuclear factor 1 sites could be observed but varied little in different physiological conditions.

A novel nonhepatic hydroxycholesterol 7 alpha-hydroxylase that is markedly stimulated by interleukin-1 beta. Characterization in the immature rat ovary

During studies on the regulation of rat ovarian steroidogenic enzymes by interleukin-1 beta (IL-1 beta), we observed substantial metabolism of 25-hydroxycholesterol to two unusual polar products. This unexpected effect was observed both in isolated granulosa cells and in whole ovarian dispersates and was also induced by tumor necrosis factor alpha, but not by insulin-like growth factor I or follicle-stimulating hormone. The effect was dependent on time and the dose of IL-1 beta and was blocked by and IL-1 receptor antagonist. The formation of the polar metabolites was inhibited by ketoconazole and trilostane, but not by aminoglutethimide. Subsequent purification of these novel metabolites and analysis by gas chromatography/ mass spectrometry, NMR, and high performance liquid chromatography revealed them to be related 7 alpha-hydroxylated hydroxycholesterols (cholest-4-ene-7 alpha,25-diol-3-one and cholest-5-ene-3 beta,7 alpha,25-triol). IL-1 beta-stimulated ovarian 7 alpha-hydroxylase activity (3-10 pmol/min/mg of cellular protein) was nearly 4-fold that of control levels using 25-hydroxycholesterol as substrate. Activities at or below control levels were observed when IL-1 beta-treated cell sonicates were boiled or assayed in the presence of NADH (rather than NADPH), indicating that involvement of a nonenzymatic process was unlikely. IL-1 beta-stimulated 7 alpha-hydroxylase activity was inhibited to basal levels by a 10-fold excess of unlabeled 25- or 27-hydroxycholesterol, but not by cholesterol, pregnenolone, progesterone, testosterone, or dehydroepiandrosterone, suggesting that ovarian 7 alpha-hydroxylase is specific for hydroxycholesterols. Furthermore, when IL-1 beta-treated ovarian cultures were incubated with radiolabeled cholesterol or testosterone, no 7 alpha-hydroxylated products were observed. We were also unable to detect any mRNA transcripts for liver cholesterol 7 alpha-hydroxylase in IL-1 beta-stimulated ovarian cultures. This study describes an ovarian hydroxycholesterol 7 alpha-hydroxylase that differs from liver cholesterol 7 alpha-hydroxylase and from other nonhepatic progestin/ androgen 7 alpha-hydroxylases. The novel finding of the regulation of a 7 alpha-hydroxylase by IL-1 beta (and tumor necrosis factor alpha) suggests a unique role for cytokines in the regulation of cholesterol metabolism in the ovary and possibly other tissues.

Pharmacological inhibition of the intracellular transport of low-density lipoprotein-derived cholesterol in Chinese hamster ovary cells

Mammalian cells, cultured in the presence of serum lipoproteins, acquire cholesterol necessary for growth from the uptake and lysosomal hydrolysis of low-density lipoproteins (LDL). The mechanism(s) of intracellular transport of LDL-derived cholesterol from lysosomes to other cellular sites is unknown. In this study, various pharmacological agents were assessed for their ability to inhibit the movement of LDL-cholesterol from lysosomes to the plasma membrane. The only pharmacological agent tested in these experiments that specifically inhibited LDL-cholesterol movement was U18666A. Ketoconazole impaired the intracellular transport of LDL-cholesterol; however, ketoconazole also had a general effect on cholesterol movement, since it impeded the desorption of endogenously synthesized cholesterol into the medium. Other drugs that affected cholesterol movement appeared to be nonspecific. Cholesterol transport from lysosomes to plasma membranes was not significantly altered by agents that affect lysosomal function or cytoskeletal organization, as well as energy poisons and cycloheximide.