Three direct repeats and a TATA-like sequence are required for regulated expression of the human low density lipoprotein receptor gene

Macrophage-Derived 25-Hydroxycholesterol Promotes Vascular Inflammation, Atherogenesis, and Lesion Remodeling

BACKGROUND

Cross-talk between sterol metabolism and inflammatory pathways has been demonstrated to significantly affect the development of atherosclerosis. Cholesterol biosynthetic intermediates and derivatives are increasingly recognized as key immune regulators of macrophages in response to innate immune activation and lipid overloading. 25-Hydroxycholesterol (25-HC) is produced as an oxidation product of cholesterol by the enzyme cholesterol 25-hydroxylase (CH25H) and belongs to a family of bioactive cholesterol derivatives produced by cells in response to fluctuating cholesterol levels and immune activation. Despite the major role of 25-HC as a mediator of innate and adaptive immune responses, its contribution during the progression of atherosclerosis remains unclear.

METHODS

The levels of 25-HC were analyzed by liquid chromatography-mass spectrometry, and the expression of CH25H in different macrophage populations of human or mouse atherosclerotic plaques, respectively. The effect of CH25H on atherosclerosis progression was analyzed by bone marrow adoptive transfer of cells from wild-type or Ch25h-/- mice to lethally irradiated Ldlr-/- mice, followed by a Western diet feeding for 12 weeks. Lipidomic, transcriptomic analysis and effects on macrophage function and signaling were analyzed in vitro from lipid-loaded macrophage isolated from Ldlr-/- or Ch25h-/-;Ldlr-/- mice. The contribution of secreted 25-HC to fibrous cap formation was analyzed using a smooth muscle cell lineage-tracing mouse model, Myh11ERT2CREmT/mG;Ldlr-/-, adoptively transferred with wild-type or Ch25h-/- mice bone marrow followed by 12 weeks of Western diet feeding.

RESULTS

We found that 25-HC accumulated in human coronary atherosclerotic lesions and that macrophage-derived 25-HC accelerated atherosclerosis progression, promoting plaque instability through autocrine and paracrine actions. 25-HC amplified the inflammatory response of lipid-loaded macrophages and inhibited the migration of smooth muscle cells within the plaque. 25-HC intensified inflammatory responses of lipid-laden macrophages by modifying the pool of accessible cholesterol in the plasma membrane, which altered Toll-like receptor 4 signaling, promoted nuclear factor-κB-mediated proinflammatory gene expression, and increased apoptosis susceptibility. These effects were independent of 25-HC-mediated modulation of liver X receptor or SREBP (sterol regulatory element-binding protein) transcriptional activity.

CONCLUSIONS

Production of 25-HC by activated macrophages amplifies their inflammatory phenotype, thus promoting atherogenesis.

Low density lipoprotein receptor endocytosis in cardiovascular disease and the factors affecting LDL levels

Cardiovascular disease (CVD) is the one of major global health issues with approximately 30% of the mortality reported in the mid-income population. Low-density lipoprotein (LDL) plays a crucial role in development of CVD. High LDL along with others forms a plaque and blocks arteries, resulting in CVD. The present chapter deals with the mechanism of receptor-mediated endocytosis of LDL and its management by drugs such as statins and PCSK9 inhibitors along with dietary supplementation for health improvements.

Tilianin improves lipid profile and alleviates atherosclerosis in ApoE-/- mice through up-regulation of SREBP2-mediated LDLR expression

BACKGROUND

The huge global burden of atherosclerotic cardiovascular diseases (CVDs) represents an urgent unmet need for the development of novel therapeutics. Dracocephalum moldavica L. has been used as a traditional Uygur medicine to treat various CVDs for centuries. Tilianin is a major flavonoid component of D. moldavica L. and has potential for preventing atherosclerosis. However, the molecular mechanisms that tilianin attenuate atherosclerosis are far from fully understood.

PURPOSES

The purpose of this study is to investigate the efficiency and underlying mechanisms of tilianin in controlling lipid profile and preventing atherogenesis.

METHODS

The lipid-lowering effect of tilianin was evaluated in C57BL/6 and ApoE^-/- mice by systematically determining serum biochemical parameters. The effects of tilianin on the atherosclerotic lesion were observed in aortic roots and whole aortas of ApoE^-/- mice with oil red O staining. Caecal content from ApoE^-/- mice were collected for 16S rRNA gene sequence analysis to assess the structure of the gut microbiota. The inhibition of hepatosteatosis was verified by histological examination, and a liver transcriptome analysis was performed to elucidate the tilianin-induced hepatic transcriptional alterations. Effects of tilianin on the expression and function of LDLR were examined in HepG2 cells and ApoE^-/- mice. Further mechanisms underlying the efficacy of tilianin were investigated in HepG2 cells.

RESULTS

Tilianin treatment improved lipid profiles in C57BL/6 and dyslipidemic ApoE^-/- mice, especially reducing the serum LDL-cholesterol (LDL-C) level. Significant reductions of atherosclerotic lesion area and hepatosteatosis were observed in tilianin-treated ApoE^-/- mice. The altered gut microbial composition in tilianin groups was associated with lipid metabolism and atherosclerosis. The liver transcriptome revealed that tilianin regulated the transcription of lipid metabolism-related genes. Then both in vitro and in vivo analyses revealed the potent effect of tilianin to enhance hepatic LDLR expression and its mediated LDL-C uptake. Further studies confirmed a critical role of SREBP2 in hepatic LDLR up-regulation by tilianin via increasing precursor and thus mature nuclear SREBP2 level.

CONCLUSION

This study demonstrated the lipid-lowering effect of tilianin through SREBP2-mediated transcriptional activation of LDLR. Our findings reveal a novel anti-atherosclerotic mechanism of tilianin and underlie its potential clinical use in modulating CVDs with good availability and affordability.

Euphornin L promotes lipid clearance by dual regulation of LDLR and PCSK9

Our previous study identified euphornin L as an active lipid-lowering compound in high-fat diet-fed Golden Syrian hamsters. The aim of the present study was to investigate the mechanisms underlying the lipid-lowering effects of euphornin L. Euphornin L in HepG2 cells was assessed via DiI-LDL update assays and found to increase LDL-update and LDLR protein levels. RNA interference assays demonstrated that its LDL-update effects were LDLR-dependent. Dual luciferase reporter and mRNA stability assays revealed that euphornin L had little effect on LDLR mRNA transcription but lengthened the half-life of LDLR mRNA by activating ERK protein in cells. Euphornin L decreased the secretion of PCSK9 protein and alleviated PCSK9-mediated LDLR protein degradation. In vivo experiments in hamsters, which were treated with euphornin L (30 mg/kg/day) for 3 weeks, confirmed these findings. LDLR protein levels in liver tissue were upregulated, while PCSK9 protein levels in serum were downregulated. Altogether, the present study demonstrated that euphornin L increased LDLR protein levels by dual regulation of LDLR mRNA and PCSK9 protein, and represented an active compound for lipid-lowering drug development.

Interaction of galectin-7 with HMGCS1 in vitro may facilitate cholesterol deposition in cultured keratinocytes

Three-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) synthase 1 (HMGCS1) was identified to interact with Gal-7, a pro-apoptotic β-galactoside binding protein, by yeast two-hybrid system. Their interaction was confirmed by in vitro β-galactosidase, BIA core and immunoprecipitation assays. Distinct interactive site of HMGCS1was found to reside at Phe-26. The expression of HMGCS1 in cultured keratinocytes was up-regulated by exogenous Gal-7 and down-regulated in Gal-7 siRNA transfected cells. HMGCS1-overexpressing cells were found to induce Gal-7 expression, which suggests that Gal-7 and HMGCS1 expressions are both stimulated by a positive feedback regulation. The amount of cholesterol, a final biosynthetic product of HMGCS1-involved pathway, was increased in Gal-7 treated cells, and was significantly reduced in Gal-7 siRNA transfected cells. The increase of cholesterol level in Gal-7 treated cells was inhibited by wild type HMGCS1 peptide but not by Phe-26 mutated peptide, suggesting that the interaction of Gal-7/HMGCS1 is related to cellular cholesterol level. Foam cells in granulomatous tissues of the specimens from normolipemic cutaneous xanthoma showed positive reactions with the antibodies for Gal-7 and HMGCS1 as well as lipid markers. These results are likely to indicate that Gal-7 induction in epidermal keratinocytes causes both apoptotic cell death and HMGCS1-mediated cholesterol accumulation which will be phagocytized by macrophages. This mechanism may explain the pathogenesis of normolipemic cutaneous xanthoma.

Retrospective on Cholesterol Homeostasis: The Central Role of Scap

Scap is a polytopic membrane protein that functions as a molecular machine to control the cholesterol content of membranes in mammalian cells. In the 21 years since our laboratory discovered Scap, we have learned how it binds sterol regulatory element-binding proteins (SREBPs) and transports them from the endoplasmic reticulum (ER) to the Golgi for proteolytic processing. Proteolysis releases the SREBP transcription factor domains, which enter the nucleus to promote cholesterol synthesis and uptake. When cholesterol in ER membranes exceeds a threshold, the sterol binds to Scap, triggering several conformational changes that prevent the Scap-SREBP complex from leaving the ER. As a result, SREBPs are no longer processed, cholesterol synthesis and uptake are repressed, and cholesterol homeostasis is restored. This review focuses on the four domains of Scap that undergo concerted conformational changes in response to cholesterol binding. The data provide a molecular mechanism for the control of lipids in cell membranes.

Molecular Genetic Background of an Autosomal Dominant Hypercholesterolemia in the Czech Republic

Autosomal dominant hypercholesterolemia (ADH), more known as familial hypercholesterolemia (FH), is a lipid metabolism disorder characterized by an elevation in low-density lipoprotein cholesterol (LDL-C) and increased risk for cardiovascular disease. In this study, we assessed a spectrum of mutations causing ADH in 3914 unrelated Czech patients with clinical diagnosis of hypercholesterolemia. Samples have been collected within the framework of the MedPed project running in the Czech Republic since 1998. So far we have found 432 patients (11.0 %) with the APOB gene mutation p.(Arg3527Gln) and 864 patients (22.1 %) with the LDLR gene mutation. In 864 probands carrying the LDLR gene mutation, 182 unique allelic variants were detected. We have identified 14 patients homozygous for mutations in the LDLR or APOB genes. We performed function analyses of p.(Leu15Pro) and p.(Gly20Arg) sequence variations.

Construction of minicircle DNA vectors capable of correcting familial hypercholesterolemia phenotype in a LDLR-deficient mouse model

Familial hypercholesterolemia (FH) caused by defect in low-density lipoprotein receptor (LDLR) is a life-threatening disease with poor response to conventional treatments. Earlier gene therapy studies have generated promising results, but further development is hampered because the cells harboring the viral vectors were eliminated by host immune system soon after delivery, whereas the nonviral vectors were too bulky to be delivered to target cells. To overcome these problems, we constructed multiple minicircle (MC) DNA vectors to express the therapeutic LDLR. MC is an optimized nonviral vector that is capable of expressing high level of transgene product persistently. We found that among the seven MCs tested, the best is MC5 with multiple advanced features. First, the LDLr gene was placed under the control of sterol regulatory element (SRE) using LDLr gene promoter or apoprotein E (ApoE) promoter, allowing the transcription of the LDLr gene to be regulated by serum low-density lipoprotein (LDL) cholesterol as its functional gene counterpart. Second, a hepatic control region (HCR) was placed upstream of the promoter that serves as a controller to ensure liver-specific expression. Third, the modified Kozak sequence was placed in front of the LDLr gene start codon to enhance its translation efficiency. MC5 was 5.23 kb in size, and was capable of tight physiological control in intracellular LDL cholesterol level even when challenged with high dose of sterols in vitro. Importantly, it was able to correct the phenotype of LDLR-deficient mice C57BL/6 LDLR(-/-) for more than 105 days without detectable toxicity. Therefore, this MC has the clinical application potential for treating FH.

A novel peroxisome proliferator response element modulates hepatic low-density lipoprotein receptor gene transcription in response to PPARδ activation

The hepatic expression of low-density lipoprotein (LDL) receptor (LDLR) gene is regulated primarily at the transcriptional level by a sterol-regulatory element (SRE) in its proximal promoter region which is the site of action of SRE-binding protein 2 (SREBP2). However whether additional cis-regulatory elements contribute to LDLR transcription has not been fully explored. We investigated the function of a putative peroxisome proliferator-activated receptor (PPAR)-response element (PPRE) sequence motif located at -768 to -752 bases upstream of the transcription start site of human LDLR gene in response to PPARδ activation. Promoter luciferase reporter analyses showed that treating HepG2 cells with PPARδ agonist L165041 markedly increased the activity of a full-length LDLR promoter construct (pLDLR-1192) without any effects on the shorter promoter reporter pLDLR-234 that contains only the core regulatory elements SRE-1 and SP1 sites. Importantly, mutation of the PPRE sequence greatly attenuated the induction of the full-length LDLR promoter activity by L165041 without affecting rosuvastatin (RSV)-mediated transactivation. EMSA and ChIP assay further confirmed the binding of PPARδ to the LDLR-PPRE site. Treating HepG2 cells with L165041 elevated the mRNA and protein expressions of LDLR without affecting the LDLR mRNA decay rate. The induction of LDLR expression by PPARδ agonist was further observed in liver tissue of mice and hamsters treated with L165041. Altogether, our studies identify a novel PPRE-mediated regulatory mechanism for LDLR transcription and suggest that combined treatment of statin with PPARδ agonists may have advantageous effects on LDLR expression.

Nicotine-induced expression of low-density lipoprotein receptor in oral epithelial cells

Background

Nicotine use is one of the most important risk factors for the development of cardiovascular and periodontal diseases. Numerous reports have suggested the possible contribution of disturbed lipid metabolism for the development of both disease groups. Despite these observations, little is known about the relationship between tobacco smoking and the development of these diseases. Our previous microarray data revealed that nicotine induced low-density lipoprotein receptor (LDLR) expression in oral epithelial cells (OECs). The aim of the present study was to confirm nicotine-mediated LDLR induction and to elucidate the signaling mechanisms leading to the augmented expression of LDLR in OECs.

Methods and Results

LDLR and nicotinic acetylcholine receptor (nAChR) subunit expression was detected by real-time PCR. The production of LDLR was demonstrated by immunofluorescence staining. nAChR-mediated LDLR induction was examined by pre-incubation of the cells with its specific inhibitor, α-bungarotoxin (α-BTX). The functional importance of transcription factor specific protein 1 (Sp1) was examined by luciferase assay, mithramycin pre-incubation or by small interfering RNA (siRNA) transfection. The specific binding of Sp1 to R3 region of LDLR 5’-untranslated region was demonstrated with electrophoretic mobility shift assay (EMSA) and streptavidin-agarose precipitation assay followed by western blotting. The results confirmed that nicotine induced LDLR expression at the transcriptional level. Nicotine was sensed by nAChR and the signal was transduced by Sp1 which bound to the R3 region of LDLR gene. Augmented production of LDLR in the gingival epithelial cells was further demonstrated by immunofluorescence staining using the gingival tissues obtained from the smoking patients.

Conclusions

Taken together, the results suggested that nicotine might contribute to the development of both cardiovascular and periodontal diseases by inducing the LDLR in OECs thereby disturbing lipid metabolism.

Molecular pathology of familial hypercholesterolemia, related dyslipidemias and therapies beyond the statins

The development of the statin class of cholesterol-lowering drugs is one of the most significant success stories of modern pharmacotherapy. World-wide there are an estimated 150 million people on statins, with the emerging economies of India and China predicted to contribute significantly to that number. Notwithstanding their success, a significant number of people cannot tolerate statins because of serious side effects; of equal concern, a substantial proportion of high risk patients fail to reach cholesterol-lowering targets. For these subjects there is an urgent need for new cholesterol-lowering agents to be used alone or in combination with statins. The success of statins has been largely underpinned by knowledge of cholesterol homeostasis at a molecular level, knowledge that was first gleaned in the 1980s from Brown and Goldstein's pioneering studies of familial hypercholesterolemia (FH, OMIM 143890). Follow-up work that has identified a number of intracellular and circulating factors, all capable of disrupting LDL clearance, has revealed that the low-density lipoprotein receptor- (LDLR) mediated clearance pathway is substantially more complex than previously thought. These factors were discovered in studies of individuals with very rare inherited conditions that lead to either hypo- or hypercholesterolemia. These investigations, besides providing clearer insight into the molecular mechanisms regulating plasma LDL concentrations, have also revealed a number of novel therapeutic targets independent from statins. Consequently, a number of novel therapeutic approaches that are based on small interfering bio-molecules, including antisense oligonucleotides, are now in clinical development. These are aimed at impairing the assembly, synthesis and secretion of apolipoprotein B-containing lipoproteins and/or accelerating their hepatic catabolism. The aim of this article is to focus on these recent advances in the understanding of the molecular basis of cholesterol metabolism that should herald novel cholesterol-lowering agents beyond the statins.

Functional analysis of LDLR promoter and 5' UTR mutations in subjects with clinical diagnosis of familial hypercholesterolemia

Familial hypercholesterolemia (FH) is a dominant disorder due to mutations in the LDLR gene. Several mutations in the LDLR promoter are associated with FH. Screening of 3,705 Spanish FH patients identified 10 variants in the promoter and 5' UTR. Here, we analyse the functionality of six newly identified LDLR variants. Mutations located in the LDLR promoter regulatory elements R2 and R3 (c.-155_-150delACCCCinsTTCTGCAAACTCCTCCC, c.-136C>G, c.-140C>G, and c.-140C>T) resulted in 6 to 15% residual activity in reporter expression experiments and changes in nuclear protein binding affinity compared to wild type. No reduction was observed when cells were transfected with c.-208T, c.-88A, and c.-36G mutant fragments. Our results indicate that mutations localized in R2 and R3 are associated with hypercholesterolemia, whereas mutations outside the LDLR response elements are not a cause of FH. This data emphasizes the importance of functional analysis of variants in the LDLR promoter to determine their association with the FH phenotype.

Transcription factor Sp1 is essential for the regulation of the porcine caveolin-1 gene

Caveolin-1 (CAV-1) is a key structural component of caveolae that regulates cholesterol. Employing transgenic techniques to regulate the cholesterol content of pork through CAV-1 is hindered by our lack of knowledge about its regulation. To investigate the regulatory mechanism of porcine CAV-1, a DNA segment containing the 5'-flanking region of CAV-1 was isolated from porcine genomic DNA and sequenced. The luciferase reporter assay detected five cis-acting elements for efficient expression of the CAV-1 gene at the region spanning nucleotides -213 to -20 with serially deleted 5'-flanking sequences and site-directed mutants, -123 to -114 was the core promoter. The electrophoretic mobility shift assay demonstrated potential binding of Sp1 protein to this core promoter. The purpose of this study is to systematically elucidate the transcriptional regulation mechanism of porcine CAV-1 and to contribute to the investigation of the interaction between CAV-1 and cholesterol.

Organelle stress-induced activating transcription factor-3 downregulates low-density lipoprotein receptor expression in Sk-Hep1 human liver cells

The low-density lipoprotein receptor (LDLR) plays a central role in cholesterol homeostasis. Here, we provide evidence that an increase in endoplasmic reticulum (ER) stress response or a disturbance of mitochondrial function inhibits LDLR expression in human liver Sk-Hep1 cells. Both organelle stresses triggered activation of activating transcription factor-3 (ATF3), which subsequently reduced LDLR expression. Serial deletion studies revealed that the LDLR promoter region within -234 bp was involved in the repression of LDLR by ATF3. In addition, we identified the region between -8 and -3 of LDLR promoter region as a putative binding site for ATF3 by using deletion construct lacking 6 bp nucleotide corresponding to this region. Transfection of ATF3-specific siRNA rescued LDLR expression under organelle stress, indicating that ATF3 was mainly responsible for the repression of LDLR by these stressors. Additionally, chromatin immunoprecipitation revealed that ATF3 directly binds to the LDLR promoter in a stress-dependent manner. The unique sterol-independent LDLR repression by organelle stress via ATF3 demonstrated here could be involved in obesity-related hypercholesterolemia, which can lead to insulin resistance and type 2 diabetes.

Activation of LDL receptor expression by small RNAs complementary to a noncoding transcript that overlaps the LDLR promoter

Low-density lipoprotein receptor (LDLR) is a cell-surface receptor that plays a central role in regulating cholesterol levels. Increased levels of LDLR would lead to reduced cholesterol levels and contribute to strategies designed to treat hypercholesterolemia. We have previously shown that duplex RNAs complementary to transcription start sites can associate with noncoding transcripts and activate gene expression. Here we show that duplex RNAs complementary to the promoter of LDLR activate expression of LDLR and increase the display of LDLR on the surface of liver cells. Activation requires complementarity to the LDLR promoter and can be achieved by chemically modified duplex RNAs. Promoter-targeted duplex RNAs can overcome repression of LDLR expression by 25-hydroxycholesterol and do not interfere with activation of LDLR expression by lovastatin. These data demonstrate that small RNAs can activate LDLR expression and affect LDLR function.

Elucidation of an SRE-1/SREBP-independent cellular pathway for LDL-receptor regulation: from the cell surface to the nucleus

Reduction in blood levels of low-density lipoprotein (LDL) cholesterol lowers the risk of coronary heart disease. The elucidation of cellular pathways that control LDL-receptor expression through a cholesterol-mediated negative feedback mechanism has provided a crucial molecular basis for the development and clinical applications of statins in the treatment of hypercholesterolemia. The characterization of signaling transduction pathways elicited by cytokine oncostatin M (OM) in liver cells has revealed a novel cellular pathway that activates LDL-receptor transcription independent of intracellular levels of cholesterol and sterol-regulatory element binding proteins. This transcriptional activation is achieved through interactions of the sterol-independent regulatory element of LDL-receptor promoter and transcription factors Egr1 and c/EBPbeta, and is dependent upon the activation of the extracellular signal-regulated kinase signaling cascade by OM. In vivo OM administration in hyperlipidemic animals reduces circulating cholesterol and prevents lipid accumulation in the liver. Exploring this sterol-independent cellular pathway may lead to new therapeutic advances.

Long-term physiologically regulated expression of the low-density lipoprotein receptor in vivo using genomic DNA mini-gene constructs

Familial hypercholesterolemia (FH) is a condition caused by mutations in the low-density lipoprotein receptor (LDLR) gene. Expression of LDLR is highly regulated and excess receptor expression is cytotoxic. To incorporate essential gene regulation into a gene therapy vector for FH, we generated vectors in which the expression of therapeutic human LDLR gene, or luciferase reporter gene, is driven by 10 kb of human LDLR genomic DNA encompassing the promoter region including elements essential for physiologically regulated expression. Using luciferase expression and specific LDL binding and internalization assays, we have shown in vitro that the genomic promoter element confers long-term, physiologically regulated gene expression and complementation of receptor deficiency in culture for 240 cell-generations. This was demonstrated in the presence of sterols or statins, modifiers of LDLR promoter activity. In vivo, we demonstrate efficient liver-specific delivery and expression of luciferase following hydrodynamic tail-vein injection and confirm that expression from the LDLR promoter element is sensitive to statin administration. We also demonstrate long-term LDLR expression from the 10-kb promoter element up to 9 months following delivery. The vector system that we describe provides the efficient delivery, long-term expression, and physiological regulation required for a successful gene therapy intervention for FH.

Transcriptional regulation of 17beta-hydroxysteroid dehydrogenase type 12 by SREBP-1

17beta-hydroxysteroid dehydrogenase type 12 (17beta-HSD12) has been demonstrated to be involved in enzymatic conversion of weak estrogen, estrone to more potent one, estradiol. However, this enzyme was also reported to be involved in an elongation of very long chain fatty acid (VLCFA). Many genes involved in lipid metabolism are regulated by the transcription factor termed sterol regulatory element-binding proteins (SREBPs). Results of our present study demonstrated that the existence of putative SRE sequence which is recognized as responsive element for SREBPs in 5'-flanking region of 17beta-HSD12 gene. Results of luciferase assay demonstrated that the transcriptional activity of this SRE sequence depends on the activation of SREBP-1 in HepG2 (hepatocellular carcinoma cell line, human) and SK-BR-3 (breast carcinoma cell line, human). 17beta-HSD12 expression was also induced in the HepG2 cells treated with the absence of sterols in which SREBPs were activated. All these results obtained in this study clearly indicate that SREBP-1 represents one of the transcriptional regulators of human 17beta-HSD12.

Novel genes in cell cycle control and lipid metabolism with dynamically regulated binding sites for sterol regulatory element-binding protein 1 and RNA polymerase II in HepG2 cells detected by chromatin immunoprecipitation with microarray detection

Sterol regulatory element-binding proteins 1 and 2 (SREBP-1 and SREBP-2) are important regulators of genes involved in cholesterol and fatty acid metabolism, but have also been implicated in the regulation of the cell cycle and have been associated with the pathogenesis of type 2 diabetes, atherosclerosis and obesity, among others. In this study, we aimed to characterize the binding sites of SREBP-1 and RNA polymerase II through chromatin immunoprecipitation and microarray analysis in 1% of the human genome, as defined by the Encyclopaedia of DNA Elements consortium, in a hepatocellular carcinoma cell line (HepG2). Our data identified novel binding sites for SREBP-1 in genes directly or indirectly involved in cholesterol metabolism, e.g. apolipoprotein C-III (APOC3). The most interesting biological findings were the binding sites for SREBP-1 in genes for host cell factor C1 (HCFC1), involved in cell cycle regulation, and for filamin A (FLNA). For RNA polymerase II, we found binding sites at classical promoters, but also in intergenic and intragenic regions. Furthermore, we found evidence of sterol-regulated binding of SREBP-1 and RNA polymerase II to HCFC1 and FLNA. From the results of this work, we infer that SREBP-1 may be involved in processes other than lipid metabolism.

Ikaros modulates cholesterol uptake: a link between tumor suppression and differentiation

Ikaros is a transcription factor that directs lymphoid lineage commitment and pituitary neuroendocrine cell expansion and function. Here, we show that Ikaros regulates the low-density lipoprotein receptor (LDL-R) to alter metabolism in pituitary corticotroph cells. The DNA-binding Ikaros isoform Ik1 binds and enhances activity of the LDL-R promoter. Ik1 decreases methylation and increases acetylation of histone H3 (Lys(9)) at the LDL-R promoter. Confocal microscopy and quantitative fluorometry show enhanced LDL endocytosis in Ik1-transfected cells that exhibit abundant endoplasmic reticulum, large Golgi complexes, and prominent secretory granule formation, consistent with more robust cholesterol incorporation into functionally relevant membrane-rich organelles. Consistent with these data, LDL-R(-/-) mice, like Ik(-/-) mice, have decreased circulating levels of adrenocorticotropic hormone. These findings expand the repertoire of Ikaros actions to include regulation of the cholesterol uptake metabolic pathway with therapeutic implications for lipid-modifying drugs in Ikaros-associated cancers.

Delivery and long-term expression of a 135 kb LDLR genomic DNA locus in vivo by hydrodynamic tail vein injection

BACKGROUND

The delivery of a complete genomic DNA locus in vivo may prove advantageous for complementation gene therapy, especially when physiological regulation of gene expression is desirable. Hydrodynamic tail vein injection has been shown to be a highly efficient means of non-viral delivery of plasmid DNA to the liver. Here, we apply hydrodynamic tail vein injection to deliver and express large genomic DNA inserts > 100 kb in vivo.

METHODS

Firstly, a size series (12-172 kb) of bacterial artificial chromosome (BAC) plasmids, carrying human genomic DNA inserts, episomal retention elements, and the enhanced green fluorescent protein (EGFP) reporter gene, was delivered to mice by hydrodynamic tail vein injection. Secondly, an episomal BAC vector carrying the whole genomic DNA locus of the human low-density lipoprotein receptor (LDLR) gene, and an expression cassette for the LacZ reporter gene, was delivered by the same method.

RESULTS

We show that the efficiency of delivery is independent of vector size, when an equal number of plasmid molecules are used. We also show, by LacZ reporter gene analysis, that BAC delivery within the liver is widespread. Finally, BAC-end PCR, RT-PCR and immunohistochemistry demonstrate plasmid retention and long-term expression (4 months) of human LDLR in transfected hepatocytes.

CONCLUSION

This is the first demonstration of somatic delivery and long-term expression of a genomic DNA transgene > 100 kb in vivo and shows that hydrodynamic tail vein injection can be used to deliver and express large genomic DNA transgenes in the liver.

Identification of two herbal compounds with potential cholesterol-lowering activity

Low-density lipoprotein receptor (LDLR) plays a pivotal role in the control of plasma LDL-cholesterol level. This occurs predominantly at the transcriptional level through two gene regulation elements, named SRE: sterol-responsive element and SIRE: sterol-independent responsive element. We have developed a high-throughput screening using LDLR promoter activation-based assay to search for cholesterol-lowering compounds from a Chinese herb-based natural compound library. With this approach, we identified two compounds, named Daphnetoxin and Gniditrin, from Chinese herb Daphne giraldii Nitsche, which could activate LDLR promoter. Characterization of these compounds showed that they increased the level of LDLR mRNA and consequently up-regulate LDLR expression. The structures of these compounds are different from well-known LDLR promoter activating compounds such as GW707. The results suggested that these herbal compounds could represent good candidates for development of new classes of cholesterol-lowering drugs.

Berberine-induced LDLR up-regulation involves JNK pathway

Berberine, an herbal alkaloid, has been reported to have a lipid lowering effect by stabilizing hepatic LDLR mRNA in an ERK-dependent manner rather than promoting transcriptional activity. However, considering the complexity of interconnected signal pathways in biological processes, it is highly possible that there exist signal pathway(s) other than ERK pathway which contribute to the berberine-induced up-regulation of LDLR. In the present study, we examined possible involvement of other signal pathways in berberine-induced hepatic LDLR up-regulation. As evidenced by RT-PCR, berberine-induced LDLR mRNA expression was inhibited by JNK inhibitor SP600125 pretreatment. Furthermore, we demonstrate that putative c-jun binding site of LDLR promoter is important in berberine-induced LDLR transcription using luciferase assay. The result of EMSA also shows that berberine induces c-jun binding to LDLR promoter and this is decreased by SP600125 pretreatment. The present study demonstrates that berberine increases transcriptional activity of LDLR promoter and this involves JNK pathway.

Quantitative effects of common genetic variations in the 3′UTR of the human LDL-receptor gene and their associations with plasma lipid levels in the Atherosclerosis Risk in Communities study

The low-density lipoprotein receptor (LDLR) plays a pivotal role in cholesterol homeostasis. However, the role of genetic variations in the 3'UTR of the LDLR in relation to plasma cholesterol has been largely understudied. Six SNPs, G44243A, G44332A, C44506G, G44695A, C44857T and A44964G, within the 5' region of the 3'UTR fall into three common haplotypes, GGCGCA, AGCACG, and GGCGTA, occurring at frequencies of 0.45, 0.31 and 0.17, respectively, in Caucasians (n = 29) and 0.13, 0.13 and 0.38, respectively, in African Americans (n = 32), with three other haplotypes occurring at lesser frequencies. In a tissue culture based system, expression of a reporter gene carrying a 3'UTR that includes the 1 kb nucleotide sequences corresponding to the AGCACG or GGCGTA was 70 or 63%, respectively, of the same sequence with GGCGCA. Genotyping of two "haplotype tagging" SNPs, C44857T and A44964G, in the Atherosclerosis Risk in Communities (ARIC) study population showed that in Caucasians, but not in African Americans, the inferred TA haplotype had a significant LDL-cholesterol lowering effect. The adjusted LDL-cholesterol levels in the TA/TA diplotypes were lower by 6.10 mg/dl in men (P < 0.001) and by 4.63 mg/dl in women (P < 0.01) than in individuals with other diplotypes. Caucasian men homozygous for CA, in contrast, showed significantly higher LDL-cholesterol (P < 0.04), lower HDL-cholesterol (P < 0.02) and higher LDL/HDL ratios (P < 0.001). Thus our data shows that 3'UTR sequences that cause higher reporter gene expression in vitro are associated in Caucasians with plasma lipid profiles indicative of higher cardiovascular risk, suggesting that further studies of quantitative variants in the LDLR gene will be valuable.

Human low-density lipoprotein receptor gene and its regulation

The low-density lipoprotein (LDL) receptor is a transmembrane glycoprotein that mediates the binding and endocytosis of lipoproteins containing apolipoprotein B and E, especially the cholesterol-rich LDL. Mutations in the LDL receptor gene can produce dysfunctional LDL receptors and cause familial hypercholesterolemia. The expression of the LDL receptor gene is under an intriguing regulation by sterol and nonsterol mediators either at the transcriptional level or at the posttranscriptional level, both of which are linked to cell signaling pathways. Upregulation of liver LDL receptor expression is effective in treating hypercholesterolemia. In this review, we focus on the latest progress on the mechanisms and regulation of the LDL receptor gene expression.

Seamless cloning and gene fusion

Gene fusion technology is a key tool in facilitating gene function studies. Hybrid molecules in which all the components are joined precisely, without the presence of intervening and unwanted extraneous sequences, enable accurate studies of molecules and the characterization of individual components. This article reviews situations in which seamlessly fused genes and proteins are required or desired and describes molecular approaches that are available for generating these hybrid molecules.

KLF11-mediated repression antagonizes Sp1/sterol-responsive element-binding protein-induced transcriptional activation of caveolin-1 in response to cholesterol signaling

Cholesterol is a potent regulator of gene expression via a canonical pathway co-regulated by SREBP and Sp1. Here we establish the caveolin-1 gene promoter as a cell type-specific model for SREBP/Sp1 regulation whereby lipoprotein cholesterol depletion activates caveolin-1 transcription in endothelial type cells, but not in fibroblasts, both in vitro and in vivo. By extending this model, we describe a novel pathway distinct from the prototypical SREBP/Sp1 regulatory loop involving the Sp1-like protein, KLF11. Through a combination of RNA interference, chromatin immunoprecipitation assays, electrophoretic mobility shift assays, and reporter assays, we demonstrate that in the presence of cholesterol, KLF11 acts as a dominant repressor of the caveolin-1 gene. Mechanistically, cholesterol depletion results in displacement of KLF11 from an Sp1 site flanking an SRE, indicating that activation by SREBP/Sp1 requires antagonism of KLF11 repression. The displacement of KLF11 results from both a down-regulation of its expression and competition by Sp1 for DNA binding. Therefore, these studies identify a novel pathway whereby KLF11 repression is coordinated with Sp1/SREBP activation of cholesterol-dependent gene expression in a cell type-specific manner and outline the mechanisms by which these functions are achieved.

Molecular Cloning and Functional Characterization of the Transcription Factor YY2

YY1 is a ubiquitous zinc finger transcription factor that binds to and regulates promoters and enhancers of many cellular and viral genes. Here we report the isolation of a human cDNA encoding a DNA sequence-specific binding protein with significant homology to the transcription factor YY1. A sequence analysis of this novel protein, YY2, revealed an overall 65% identity in the DNA sequence and a 56% identity in protein sequence compared with human YY1. The most pronounced similarity between YY1 and YY2 exists within the zinc finger regions of the two proteins, and consistent with this observation, YY2 can bind to and regulate some promoters known to be controlled by YY1. Similar to YY1, YY2 contains both transcriptional activation and repression functions. The finding of a protein with structure and function similar to YY1 provides a new opportunity to explore additional mechanisms by which YY1-responsive genes can be regulated and suggests that gene regulation by YY1 is far more complicated than previously assumed.

Hyperlipidaemic effect of fish oil in Bio F1B hamsters

We investigated the dietary influence of low and high levels of fish oil, supplemented with or without dietary cholesterol, on the plasma lipoprotein profile in Bio F1B hamsters, a model susceptible to diet-induced hyperlipidaemia. The MIX diet, a diet supplemented with a mixture of lard and safflower-seed oil, was used as the control diet to maintain the saturated MUFA and PUFA levels similar to the fish-oil diet. The animals were fed the specific diets for 2 weeks and fasted for 14 h before killing. The plasma from the animals fed high levels of fish oil was milky and rich in chylomicron-like particles. The plasma total cholesterol, VLDL- and LDL-cholesterol and -triacylglycerol concentrations were significantly higher, whereas HDL-cholesterol was lower in hamsters fed fish oil compared with the MIX-diet-fed hamsters. Increasing the amount of fat in the diet increased plasma lipids in both the fish-oil- and the MIX-diet-fed hamsters; however, this hyperlipidaemic effect of dietary fat level was greater in the hamsters fed the fish-oil diet. The hepatic lipid concentrations were not dramatically different between the fish-oil-fed and the MIX-diet-fed hamsters. However, the hepatic LDL-receptor mRNA levels were significantly low in the fish-oil-fed hamsters compared with the MIX-diet-fed hamsters. Increasing the amount of fish oil in the diet further decreased the hepatic LDL-receptor mRNA expression. It is concluded that F1B hamsters are susceptible to fish-oil-induced hyperlipidaemia, especially at high fat levels, and this increase is partially explained by the inhibition of hepatic LDL-receptor mRNA expression.

New promoter mutations in the low-density lipoprotein receptor gene which induce familial hypercholesterolaemia phenotype: molecular and functional analysis

Low-density lipoprotein receptor (LDLR) is a cell-surface glycoprotein that mediates specific uptake and catabolism of plasma LDL. Mutations located in the coding region of the LDLR gene affect the structure and function of the protein and cause familial hypercholesterolaemia (FH). Mutations in the regulatory regions of the gene are rare, but in some cases have been shown to alter the transcriptional activity of the gene and cause the FH phenotype as well. Adult heterozygous FH individuals have a markedly raised plasma cholesterol that is associated with accelerated atherosclerosis and premature coronary heart disease. The aim of this study was the functional characterization of a promoter mutation in the LDLR gene in one family from the register of Czech FH subjects. Molecular screening revealed that three members of this family carried a -27C > T nucleotide transition in the promoter sequence (calculated from the start of transcription). All three manifested a heterozygous FH phenotype. This new mutation is located between the TATA box and sterol-dependent regulatory element repeat 3. Using a luciferase reporter assay system, we analysed the transcriptional efficiency of the normal and mutant alleles. The mutation reduced promoter activity to background level. Another new promoter mutation -60C > T was identified in an unrelated patient in the conserved nucleotide sequence of the sterol-dependent regulation element repeat 2 which virtually abolished the promoter activity. We assume a causal effect of this -60C > T transition on the basis of its position in the promoter sequence.

Subcellular localization of soybean 7S globulin in HepG2 cells and LDL receptor up-regulation by its alpha' constituent subunit

The aims of this work were to monitor the subcellular localization of soybean 7S globulin in HepG2 cells and determine its interaction with cell protein components, by using laser-induced fluorescence capillary electrophoresis (LIF-CE). Furthermore, we evaluated in the same cell line the involvement of the alpha' constituent subunit from 7S globulin in the modulation of LDL catabolism. The results indicated a main fluorescein isothiocyanate-tagged 7S globulin (FITC-7S) component in the cytosolic fraction, that was not present in the nuclear compartment. The electrophoretic mobility of this tagged component suggested either a dissociation of the 7S oligomer or its partial intracellular degradation. Interactions of soybean 7S globulin with FITC-thioredoxin 1 and FITC-cyclophilin B, HepG2 cell membrane proteins, were demonstrated in in vitro assays. In a separate experiment with HepG2 cells, the ability of the alpha' subunit purified from soybean 7S globulin to modulate the activity of the LDL receptors was evaluated by tracking the uptake and degradation of labeled LDL. The up-regulation of LDL receptors by the alpha' subunit, as further confirmed by a LDL receptor promoter assay, was significantly greater than that found in the control cells. In conclusion, this study, while confirming our previous indirect evidence of the key role of alpha' subunit on the cell cholesterol homeostasis, reveals a potentially interesting association of soybean 7S globulin with proteins, such as thioredoxin 1 and cyclophilin B, that are involved in cell protection against oxidative stress.

Infectious delivery of a 135-kb LDLR genomic locus leads to regulated complementation of low-density lipoprotein receptor deficiency in human cells

The ability to deliver efficiently a complete genomic DNA locus to human and rodent cells will likely find widespread application in functional genomic studies and novel gene therapy protocols. In contrast to a cDNA expression cassette, the use of a complete genomic DNA locus delivers a transgene intact with its native promoter, the exons, all the intervening introns, and the regulatory regions. The presence of flanking, noncoding genomic DNA sequences could prove critical for prolonged and appropriate gene expression. We have recently developed a technology for the rapid conversion of bacterial artificial chromosome (BAC) clones into high-capacity herpes simplex virus-based amplicon vectors. Here, we express the human low-density lipoprotein receptor (LDLR), mutated in familial hypercholesterolemia (FH), from a 135-kb BAC insert. The infectious LDLR genomic locus vectors were shown to express at physiologically appropriate levels in three contexts. First, the LDLR locus was expressed appropriately in the ldl(-/-)a7 Chinese hamster ovary (CHO) cell line immediately following infectious delivery; second, the locus was maintained within a replicating episomal vector and expressed at broadly physiological levels in CHO cells for 3 months following infectious delivery; and third, the locus was efficiently expressed in human fibroblasts derived from FH patients. Finally, we show that the infectious LDLR locus retains classical expression regulation by sterol levels in human cells. This long-term expression and physiological regulation of LDLR prepares the way for in vivo functional studies of infectious delivery of BAC inserts.

Synergistic activation of human LDL receptor expression by SCAP ligand and cytokine oncostatin M

OBJECTIVE

A recent study identified a new class of compounds designated as the sterol-regulatory element binding protein (SREBP) cleavage-activating protein (SCAP) ligands that putatively bind to SCAP, leading to increased LDL receptor (LDLR) expression. In this study, we examined the effects of SCAP ligand GW707 in comparison with lovastatin and cytokine oncostatin M (OM) on the regulation of LDLR expression in cultured HepG2 cells.

METHODS AND RESULTS

Our studies uncovered several new features that distinguish SCAP ligand from lovastatin, a classic 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitor, and from OM, which utilize an SREBP-independent regulatory pathway. We show that the induction of LDLR mRNA expression by GW707 is not affected by intracellular cholesterol but is completely abolished by blocking de novo protein synthesis. Moreover, the effects of GW707 but not lovastatin on LDLR promoter activity, mRNA expression, and uptake of 1,1'-dioctadecyl-3,3,3',3'-tetramethyl-indocarbocyanin perchlorate-LDL are markedly enhanced by OM. We further demonstrate that the amounts of the mature form of SREBP-2 translocated to the nucleus under GW707 treatment are increased by costimulating cells with OM.

CONCLUSIONS

Our studies provide the first evidence that higher levels of LDLR expression and function can be achieved through simultaneous stimulation of the SREBP-dependent and SREBP-independent pathways, suggesting a strategy to develop an adjunct therapeutic intervention utilizing both pathways.

Identification of Egr1 as the oncostatin M-induced transcription activator that binds to sterol-independent regulatory element of human LDL receptor promoter

Previously, we identified the low density lipoprotein receptor (LDLR) promoter region -17 to -1 as a novel sterol-independent regulatory element (SIRE) that mediates the stimulating effect of oncostatin M (OM). The goal of this study was to identify the OM-induced transcription activator that binds to the SIRE sequence. By conducting a electrophoretic mobility shift assay (EMSA) followed by UV crosslinking and SDS-PAGE, we show that a protein with a molecular mass of 85 kDa was present in the OM-induced SIRE DNA-protein complex. Western blotting and supershift assays reveal that the 85 kDa factor is early growth response gene 1 (Egr1). The interaction of Egr1 with the SIRE sequence was further confirmed in vivo by chromatin immunoprecipitation assays. The functional role of Egr1 in LDLR transcription was assessed by cotransfection of an Egr1 expression vector with an LDLR promoter reporter construct. We show that overexpression of Egr1 significantly increases LDLR promoter activity when cotransfected with CCAAT/enhancer binding protein beta (c/EBPbeta) or with cAMP-responsive element binding protein (CREB) expression vectors. Our studies clearly demonstrate that Egr1 is the OM-induced transcription factor that binds to the SIRE sequence of the LDLR promoter and also suggest that Egr1 may have a functional role in OM-induced upregulation of LDLR transcription through interaction with other SIRE binding proteins such as c/EBPbeta or CREB.

Requirement of Sp1 and estrogen receptor alpha interaction in 17beta-estradiol-mediated transcriptional activation of the low density lipoprotein receptor gene expression

Estrogen is one of the most important physiological regulators of low density lipoprotein receptor (LDLR) expression. Despite many studies conducted in animals and humans showing increased expressions of LDLR messenger RNA by hormone treatment, the molecular basis of the effect of estrogen on LDLR transcription has not been clearly elucidated. By using HepG2 cells that transiently express functional estrogen receptor alpha (ERalpha) and LDLR promoter constructs, we show that the specific interaction of ERalpha with the transcription factor Sp1 bound to the LDLR promoter is responsible for the activation of LDLR transcription by estrogen. We demonstrate that 1) mutations to abrogate the binding of Sp1 to its recognition sequences present in repeat 1 and repeat 3 elements of the LDLR promoter completely abolish the ERalpha-mediated activation of the LDLR promoter activity; 2) mutations that abolish the selective DNA-binding activity or inactivate the C-terminal transcription activation function (AF2) of ERalpha had no effect on the ability of ERalpha to activate LDLR transcription; however, transcriptional activation was completely lost by deletion of the N-terminal transcription activation region (AF1); 3) a subregion of AF1 (amino acids 67-139) was further identified to be important for ERalpha to activate the LDLR promoter; and 4) ERalpha enhanced the formation of Sp1-repeat 3 DNA complexes. We also show that mutation at the sterol-responsive element-1 site diminishes the activity of ERalpha on LDLR transcription, thereby suggesting that the sterol-responsive element-1-binding protein may interact with the Sp1-ERalpha complex to trans-activate LDLR gene transcription. This study for the first time provides a molecular basis for an understanding of the regulation of LDLR transcription by estrogens.

Expression of the alpha 5 integrin subunit gene promoter is positively regulated by the extracellular matrix component fibronectin through the transcription factor Sp1 in corneal epithelial cells in vitro

The accumulation of fibronectin (FN) in response to corneal epithelium injury has been postulated to turn on expression of the FN-binding integrin alpha(5)beta(1). In this work, we determined whether the activity directed by the alpha(5) gene promoter can be modulated by FN in rabbit corneal epithelial cells (RCEC). The activity driven by chloramphenicol acetyltransferase/alpha(5) promoter-bearing plasmids was drastically increased when transfected into RCEC grown on FN-coated culture dishes. The promoter sequence mediating FN responsiveness was shown to bear a perfect inverted repeat that we designated the fibronectin-responsive element (FRE). Analyses in electrophoretic mobility shift assays provided evidence that Sp1 is the predominant transcription factor binding the FRE. Its DNA binding affinity was found to be increased when RCEC are grown on FN-coated dishes. The addition of the MEK kinase inhibitor PD98059 abolished FN responsiveness suggesting that alteration in the state of phosphorylation of Sp1 likely accounts for its increased binding to the alpha(5) FRE. The FRE also proved sufficient to confer FN responsiveness to an otherwise unresponsive heterologous promoter. However, site-directed mutagenesis indicated that only the 3' half-site of the FRE was required to direct FN responsiveness. Collectively, binding of FN to its alpha(5)beta(1) integrin activates a signal transduction pathway that results in the transcriptional activation of the alpha(5) gene likely through altering the phosphorylation state of Sp1.

Interleukin-6 stimulates LDL receptor gene expression via activation of sterol-responsive and Sp1 binding elements

Inflammatory or malignant diseases are associated with elevated levels of cytokines and abnormal low density lipoprotein (LDL) cholesterol metabolism. In the acute-phase response to myocardial injury or other trauma or surgery, total and LDL cholesterol levels are markedly decreased. We investigated the effects of the proinflammatory cytokine interleukin (IL)-6 on LDL receptor (LDL-R) function and gene expression in HepG2 cells. IL-6 dose-dependently increased the binding, internalization, and degradation of (125)I-LDL. IL-6-stimulated HepG2 cells revealed increased steady-state levels of LDL-R mRNA. In HepG2 cells transiently transfected with reporter gene constructs harboring the sequence of the LDL-R promoter extending from nucleotide -1563 (or from nucleotide -234) through -58 relative to the translation start site, IL-6 dose-dependently increased promoter activity. In the presence of LDL, a similar relative stimulatory effect of IL-6 was observed. Studies using a reporter plasmid with a functionally disrupted sterol-responsive element (SRE)-1 revealed a reduced stimulatory response to IL-6. In gel-shift assays, nuclear extracts of IL-6-treated HepG2 cells showed an induced binding of SRE binding protein (SREBP)-1a and SRE binding protein(SREBP)-2 to the SRE-1 that was independent of the cellular sterol content and an induced binding of Sp1 and Sp3 to repeat 3 of the LDL-R promoter. Our data indicate that IL-6 induces stimulation of the LDL-R gene, resulting in enhanced gene transcription and LDL-R activity. This effect is sterol independent and involves, on the molecular level, activation of nuclear factors binding to SRE-1 and the Sp1 binding site in repeat 2 and repeat 3 of the LDL-R promoter, respectively.

Nutrient regulation of gene expression by the sterol regulatory element binding proteins: increased recruitment of gene-specific coregulatory factors and selective hyperacetylation of histone H3 in vivo

We have evaluated the mechanism for sterol-regulated gene expression by the sterol regulatory element binding proteins (SREBPs) in intact cells. We show that activation of SREBPs by sterol depletion results in the increased binding of Sp1 to a site adjacent to SREBP in the promoter for the low density lipoprotein (LDL) receptor gene in vivo. Similarly, sterol depletion resulted in the increased recruitment of two distinct SREBP coregulatory factors, NF-Y and CREB, to the promoter for hydroxymethyl glutaryl CoA reductase, another key gene of intracellular cholesterol homeostasis. Furthermore, increased acetylation of histone H3 but not H4 was also detected in chromatin from both promoters on SREBP activation. Thus, SREBP activation results in the similar selective recruitment of different coregulatory generic transcription factors to two separate cholesterol-regulated promoters. These studies demonstrate the utility of the chromatin immunoprecipitation technique for analyzing the differential action of low-abundance transcription factors in fundamental regulatory events in intact cells. Our results also provide key in vivo support for the mechanism proposed from cell-free experiments, where SREBP increased the binding of Sp1 to the LDL receptor promoter. Finally, our findings also indicate that subtle differences in the pattern of core histone acetylation play a role in selective gene activation.

Identification of a novel sterol-independent regulatory element in the human low density lipoprotein receptor promoter

The cytokine oncostatin M (OM) activates human low density lipoprotein receptor (LDLR) gene transcription through a sterol-independent mechanism. Previous studies conducted in our laboratory have narrowed the OM-responsive element to promoter region -52 to +13, which contains the repeat 3 and two TATA-like sequences. We now identify LDLR promoter region -17 to -1 as a sterol-independent regulatory element (SIRE) that is critically involved in OM-, transcription factor CCAAT/enhancer-binding protein (C/EBP)-, and second messenger cAMP-mediated activation of LDLR transcription. The SIRE sequence overlaps the previously described TATA-like element and consists of an active C/EBP-binding site (-17 to -9) and a functional cAMP-responsive element (CRE) (-8 to -1). We demonstrate that (a) mutations within either the C/EBP or CRE site have no impact on basal or cholesterol-mediated repression of LDLR transcription, but they completely abolish OM-mediated activation of LDLR transcription; (b) replacing the repeat 3 sequence that contains the Sp1-binding site with a yeast transcription factor GAL4-binding site in the LDLR promoter construct does not affect OM inducibility, thereby demonstrating that OM induction is mediated through the SIRE sequence in conjunction with a strong activator bound to the repeat 3 sequence; (c) electrophoretic mobility shift and supershift assays confirm the specific binding of transcription factors C/EBP and cAMP-responsive element-binding protein to the SIRE; (d) cotransfection of a human C/EBPbeta expression vector (pEF-NFIL6) with the LDLR promoter construct pLDLR234 increases LDLR promoter activity; and (e) OM and dibutyryl cAMP synergistically activate LDLR transcription through this regulatory element. This study identifies, for the first time, a cis-acting regulatory element in the LDLR promoter that is responsible for sterol-independent regulation of LDLR transcription.

Signal transduction from the endoplasmic reticulum to the cell nucleus

The endoplasmic reticulum (ER) serves several important functions. Cholesterol, an essential component of cellular membranes, is synthesized on the ER surface. Inside the organelle, proteins destined for secretion or transport to the cell surface are folded and become glycosylated. Because these processes are essential for cell viability, a disturbance in ER function presents significant stress to the cell. In response to ER stress, three distinct signal transduction pathways can be activated. Two of these, the unfolded protein response and the ER-overload response, respond to disturbances in protein processing. The third, the sterol regulatory cascade, is activated by depletion of cholesterol. This review summarizes the recent advances in our understanding of these ER-nuclear signal transduction pathways. In addition, it points to novel regulatory mechanisms discovered in these pathways, which may be widely used in other systems.

Co-stimulation of promoter for low density lipoprotein receptor gene by sterol regulatory element-binding protein and Sp1 is specifically disrupted by the yin yang 1 protein

Sterol regulation of gene expression in mammalian cells is mediated by an interaction between the cholesterol-sensitive sterol regulatory element-binding proteins (SREBPs) and promoter-specific but generic co-regulatory transcription factors such as Sp1 and NF-Y/CBF. Thus, sterol-regulated promoters that require different co-regulatory factors could be regulated independently through targeting the specific interaction between the SREBPs and the individual co-regulatory proteins. In the present studies we demonstrate that transiently expressed yin yang 1 protein (YY1) inhibits the SREBP-mediated activation of the low density lipoprotein (LDL) receptor in a sensitive and dose-dependent manner. The inhibition is independent of YY1 binding directly to the LDL receptor promoter, and we show that the same region of YY1 that interacts in solution with Sp1 also interacts with SREBP. Furthermore, other SREBP-regulated genes that are not co-regulated by Sp1 are either not affected at all or are not as sensitive to the repression. Thus, the specific interaction that occurs between SREBPs and Sp1 to stimulate the LDL receptor promoter is a specific target for inhibition by the YY1 protein, and we provide evidence that the mechanism can be at least partially explained by the ability of YY1 to inhibit the interaction between SREBP and Sp1 in solution in vitro. The LDL receptor is the key gene of cholesterol uptake, and the rate-controlling genes of cholesterol synthesis are stimulated by the concerted action of SREBPs along with coregulators that are distinct from Sp1. Therefore, repression of gene expression through specifically targeting the interaction between SREBP and Sp1 would provide a molecular mechanism to explain how cholesterol uptake can be regulated independently from cholesterol biosynthesis in mammalian cells.

Transcriptional activation of the murine CTP:phosphocholine cytidylyltransferase gene (Ctpct): combined action of upstream stimulatory and inhibitory cis-acting elements

CTP:phosphocholine cytidylyltransferase plays a key role in regulating the rate of phosphatidylcholine biosynthesis. However, the proximal regulatory elements for the gene (Ctpct) that encode this enzyme and the cognate transcription factors involved have not been characterized. Ctpct promoter activities were deduced from promoter deletion constructs linked to a luciferase reporter and transiently transfected into C3H10T1/2 and McArdle RH7777 cells. Positive regulatory elements were located between -130 and -52 bp from the transcription start site. Basal expression resided downstream between -52 and +38 bp. DNase I protection and electromobility-shift assays indicated that Sp1-related nuclear factors bind to a stimulatory, a possible inhibitory and minimal promoter element. Gel-shift assays confirmed that all three regulatory regions bound Sp1. Sp1 was further implicated when Sp1-deficient Drosophila cells were co-transfected with promoter-reporter constructs and an Sp1 construct. DNase I assays also indicated that the Ap1 binding elements could be occupied in the proximal activator and minimal promoter regions. Gel-shift assays demonstrated that the distal activator region could bind Ap1 and an unknown transcription factor. We conclude that Sp1, Ap1 and an unknown transcription factor have important roles in regulating expression of the Ctpct gene.

Induction of low density lipoprotein receptor (LDLR) transcription by oncostatin M is mediated by the extracellular signal-regulated kinase signaling pathway and the repeat 3 element of the LDLR promoter

Oncostatin M (OM) activates the transcription of the human low density lipoprotein receptor (LDLR) in HepG2 cells through a sterol-independent mechanism. Our previous studies showed that mutations within the repeat 3 sequence of the LDLR promoter significantly decreased OM activity on LDLR promoter luciferase reporter constructs that contain the sterol responsive element-1 (repeat 2) and Sp1 binding sites (repeats 1 and 3). In this study, we investigated the signal transduction pathways that are involved in OM-induced LDLR transcription. In HepG2 cells, OM induced a rapid increase in LDLR mRNA expression, with increases detected at 30 min and maximal induction at 1 h. This OM effect was not blocked by protein synthesis inhibitors, inhibitors of p38 kinase, phosphatidylinositol 3-kinase, or c-Jun N-terminal kinase, but OM activity was completely abolished by pretreating cells with inhibitors of the extracellular signal-regulated kinase (ERK) kinase (mitogen/ERK kinase (MEK)). To investigate whether the repeat 3 sequence of the LDLR promoter is the OM-responsive element that converts ERK activation at the promoter level, three luciferase reporters, pLDLR-TATA containing only the TATA-like elements of the promoter, pLDLR-R3 containing repeat 3 and the TATA-like elements, and pLDLR-234 containing repeats 1, 2, 3 and the TATA-like elements were constructed and transiently transfected into HepG2 cells. OM had no effect on the basal promoter construct pLDLR-TATA; however, including a single copy of repeat 3 sequence in the TATA vector (pLDLR-R3) resulted in a full OM response. The activity of OM on pLDLR-R3 was identical to that of pLDLR-234. Importantly, the ability of OM to increase luciferase activities in both pLDLR-R3- and pLDLR-234-transfected cells was blocked in a dose-dependent manner by inhibition of MEK. These results demonstrate that the mitogen-activated protein kinase MEK/ERK cascade is the essential signaling pathway by which OM activates LDLR gene transcription and provide the first evidence that the repeat 3 element is a new downstream target of ERK activation.

A critical role for cAMP response element-binding protein (CREB) as a Co-activator in sterol-regulated transcription of 3-hydroxy-3-methylglutaryl coenzyme A synthase promoter

3-Hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) synthase, a key regulatory enzyme in the pathway for endogenous cholesterol synthesis, is a target for negative feedback regulation by cholesterol. When cellular sterol levels are low, the sterol regulatory element-binding proteins (SREBPs) are released from the endoplasmic reticulum membrane, allowing them to translocate to the nucleus and activate SREBP target genes. However, in all SREBP-regulated promoters studied to date, additional co-regulatory transcription factors are required for sterol-regulated activation of transcription. We have previously shown that, in addition to SREBPs, NF-Y/CBF is required for sterol-regulated transcription of HMG-CoA synthase. This heterotrimeric transcription factor has recently been shown to function as a co-regulator in several other SREBP-regulated promoters, as well. In addition to cis-acting sites for both SREBP and NF-Y/CBF, the sterol regulatory region of the synthase promoter also contains a consensus cAMP response element (CRE), an element that binds members of the CREB/ATF family of transcription factors. Here, we show that this consensus CRE is essential for sterol-regulated transcription of the synthase promoter. Using in vitro binding assays, we also demonstrate that CREB binds to this CRE, and mutations within the CRE that result in a loss of CREB binding also result in a loss of sterol-regulated transcription. We further show that efficient activation of the synthase promoter in Drosophila SL2 cells requires the simultaneous expression of all three factors: SREBPs, NF-Y/CBF, and CREB. To date this is the first promoter shown to require CREB for efficient sterol-regulated transcription, and to require two different co-regulatory factors in addition to SREBPs for maximal activation.

Chromatin, TAFs, and a novel multiprotein coactivator are required for synergistic activation by Sp1 and SREBP-1a in vitro

The promoter selectivity factor Sp1 often cooperates with other enhancer-binding proteins to activate transcription. To study the molecular underpinnings of these regulatory events, we have reconstituted in vitro the synergy observed in vivo between Sp1 and the sterol-regulated factor SREBP-1a at the low density lipoprotein receptor (LDLR) promoter. Using a highly purified human transcription system, we found that chromatin, TAFs, and a novel SREBP-binding coactivator activity, which includes CBP, are all required to mediate full synergistic activation by Sp1 and SREBP-1a. The SREBP-binding domain of CBP inhibits activation by SREBP-1a and Sp1 in a dominant-negative fashion that is both chromatin- and activator-specific. Whereas recombinant CBP alone is not sufficient to mediate activation, a human cellular fraction containing CBP can support high levels of chromatin-dependent synergistic activation. Purification of this activity to near homogeneity resulted in the identification of a multiprotein coactivator, including CBP, that selectively binds to the SREBP-1a activation domain and is capable of mediating high levels of synergistic activation by SREBP/Sp1 on chromatin templates. The development of a reconstituted chromatin transcription system has allowed us to isolate a novel coactivator that is recruited by the SREBP-1a activation domain and that functions in concert with TFIID to coordinate the action of multiple activators at complex promoters in the context of chromatin.

Oleate potentiates oxysterol inhibition of transcription from sterol regulatory element-1-regulated promoters and maturation of sterol regulatory element-binding proteins

Activation of genes containing SRE-1 (sterol regulatory element 1) sequences is known to be under the regulation of sterols through modulation of the proteolytic maturation of SREBPs (SRE-1-binding proteins). Previous work has demonstrated SREBP-mediated transcriptional activation of genes encoding enzymes of sterol and fatty acid biosynthesis. Because synthesis of both sterols and C18 fatty acids are required for cell growth, in the absence of exogenous supplements of these lipids, we examined the hypothesis that fatty acid can also be regulatory in SREBP maturation. Our data indicate that C18 fatty acids can potentiate the biological activities of a typical, regulatory sterol: 25-hydroxycholesterol. Inhibition of C18 fatty acid synthesis in cells cultured in serum-free medium renders them resistant to killing by 25-hydroxycholesterol. Repression of expression of reporter constructs driven by promoters bearing SRE-1 element(s) by 25-hydroxycholesterol is increased by C18 fatty acid supplementation. C18 fatty acids also increase the inhibitory effect of 25-hydroxycholesterol on proteolytic maturation and nuclear localization of SREBPs. Furthermore, we also show that C18 fatty acid supplementation can enhance the inhibitory effect of 25-hydroxycholesterol on sterol and fatty acid biosynthesis. These results demonstrate that maximal down-regulation of SREBP maturation and the consequent repression of SRE-1 promoters occurs in response to both a regulatory sterol and fatty acid.