Nuclear factors AF-1 and C/EBP bind to the human ApoB gene promoter and modulate its transcriptional activity in hepatic cells

Quercetin represses apolipoprotein B expression by inhibiting the transcriptional activity of C/EBPβ

Quercetin is one of the most abundant polyphenolic flavonoids found in fruits and vegetables and has anti-oxidative and anti-obesity effects. Because the small intestine is a major absorptive organ of dietary nutrients, it is likely that highly concentrated food constituents, including polyphenols, are present in the small intestinal epithelial cells, suggesting that food factors may have a profound effect in this tissue. To identify novel targets of quercetin in the intestinal enterocytes, mRNA profiling using human intestinal epithelial Caco-2 cells was performed. We found that mRNA levels of some apolipoproteins, particularly apolipoprotein B (apoB), are downregulated in the presence of quercetin. On the exposure of Caco-2 cells to quercetin, both mRNA and protein levels of apoB were decreased. Promoter analysis of the human apoB revealed that quercetin response element is localized at the 5′-proximal promoter region, which contains a conserved CCAAT enhancer-binding protein (C/EBP)-response element. We found that quercetin reduces the promoter activity of apoB, driven by the enforced expression of C/EBPβ. Quercetin had no effect on either mRNA or protein levels of C/EBPβ. In contrast, we found that quercetin inhibits the transcriptional activity of C/EBPβ but not its recruitment to the apoB promoter. On the exposure of Caco-2 cells to quercetin 3-O-glucuronide, which is in a cell-impermeable form, no notable change in apoB mRNA was observed, suggesting an intracellular action of quercetin. In vitro interaction experiments using quercetin-conjugated beads revealed that quercetin binds to C/EBPβ. Our results describe a novel regulatory mechanism of transcription of apolipoprotein genes by quercetin in the intestinal enterocytes.

Purified apolipoprotein B gene regulatory factor-3 is DNA topoisomerase I

Hepatic cell-specific expression of the human apolipoprotein B (apoB) gene is controlled by at least four cis-acting elements located between positions -128 and +122 [Chuang, S. S., & Das, H. K. (1996) Biochem. Biophys. Res. Commun. 220, 553-562]. A negative cis-acting element (+20 to +40) is located in the first nontranslated exon of the human apoB gene, and apoB regulatory factor-3 (BRF-3) interacts with this. In this paper, we report the purification and characterization of BRF-3 from rat liver nuclear extracts. BRF-3 has been purified to apparent homogeneity by DEAE-cellulose, heparin-agarose, and DNA-specific affinity chromatography. Purified BRF-3 produced two polypeptide bands with apparent molecular masses of 70 kDa and 67 kDa in SDS/PAGE as detected by silver staining. Both 70-kDa and 67-kDa proteins have been found to hybridize specifically with labeled double-stranded oligonucleotide containing BRF-3 binding site in a South-Western blot. Double-stranded oligonucleotide containing mutations in the BRF-3 binding site was found to abolish DNA binding by these two proteins. Amino acid sequences of tryptic peptides derived from affinity purified 70-kDa and 67-kDa rat BRF-3 proteins were found to have 100% sequence homologies with DNA topoisomerase I. These data suggest that the 70-kDa and 67-kDa forms of BRF-3 are derived by proteolytic cleavage of topoisomerase I, and therefore, topoisomerase I may play an important role in transcriptional regulation of apoB.

Induction of the human CYP1A2 enhancer by phorbol ester

Induction of cytochrome (CYP) P4501A2 by such polycyclic aromatic hydrocarbons as 3-methylcholanthrene (3MC) can lead to the bioactivation of carcinogenic aromatic amines and heterocyclic amines. A 3MC response element was recently identified approximately 2.2 kb upstream of the transcription start site of the human CYP1A2 gene. Sequence analysis of this enhancer identified, in addition to a binding site for the aryl hydrocarbon receptor, two other sequences, referred to as 5'AP1 and 3'AP1, each with complete homology to the phorbol 12-O-tetradecanoate 13-acetate (TPA) response element consensus sequence. Nuclear extracts from TPA-treated HepG2 cells protected both the 5'AP1 and 3'AP1 sequences against digestion with DNase I. Gel mobility shift and supershift assays revealed that TPA treatment of HepG2 results in increased binding activity of the AP-1 proteins, c-Jun, JunD, and c-Fos, to both sites. We transiently expressed, in HepG2, either a fragment containing both the 5'AP1 and 3'AP1 sites (-2.3pT81Luc) or only the 3'AP1 site (-2.2pT81Luc) cloned into a plasmid containing the luciferase gene under transcriptional control of the thymidine kinase promoter. TPA treatment of cells transfected with -2.3pT81Luc resulted in an approximately threefold induction of luciferase activity over untreated control cells, while the -2.2pT81Luc construction containing only the 3'AP1 site displayed an approximately sixfold induction. These studies suggest that the human CYP1A2 gene may be regulated by tumor promoters in addition to polycyclic aromatic hydrocarbons.

Apolipoprotein B gene regulatory factor-2 (BRF-2) is structurally and immunologically highly related to hepatitis B virus X associated protein-1 (XAP-1)

Hepatic cell-specific expression of the human apolipoprotein B (apoB) gene is controlled by at least four cis-acting elements located between positions -128 and +122 [Chuang, S. S., & Das, H. K. (1996) Biochem. Biophys. Res. Commun. 220, 553-562]. The distal element (-128 to -85) appears to be liver specific because it shows positive activity in HepG2 cells and negative activity in HeLa cells. ApoB gene regulatory factor-2 (BRF-2) interacts with the sequence (-104 to -85). BRF-2 has been purified from rat liver nuclear extract, and its molecular weight has been determined to be approximately 120 kDa [Zhuang et al. (1992) Mol. Cell. Biol. 12, 3183-3191]. In this paper we report the isolation of two isoforms of BRF-2 by further purification using high-performance liquid chromatography. Both isoforms produced a single approximately 120-kDa band in sodium dodecyl sulfate polyacrylamide gel electrophoresis detected by silver stain. The amino acid sequences of two tryptic peptides derived from HPLC-purified heavier BRF-2 isoform were determined to be YLAIAPPIIK and ALYYLQIHPQELR. These two peptides were found to share 100% sequence homology with human hepatitis B virus X associated protein-1 (XAP-1) and monkey UV-damaged DNA-binding protein (UV-DDB). Anti-peptide antisera raised against two synthetic peptides of XAP-1 recognized a approximately 120-kDa polypeptide band in both BRF-2 isoforms in a western blot analysis. By using apoB promoter fragments containing various internal deletions and a substitution mutation as templates for gel mobility shift assays, we identified the region between -104 and -85 as crucial for binding by the high-molecular weight form. In contrast, the lower molecular weight isoform bound to all apoB mutants tested. Anti-peptide 2 antiserum directed against XAP-1 was found to inhibit in vitro transcription of the apoB gene in rat liver nuclear extracts by 50%. These results suggest that BRF-2 and XAP-1 are structurally and immunologically highly related trans-activators of the apoB gene. We propose that BRF-2 exists both as a monomer (BRF-2M) and as a homooligomer. probably a homodimer (BRF-2D), in solution; oligomerization appears to be an essential step for imparting sequence-specificity to BRF-2 protein and thereby facilitating its role as a trans-activator of the apoB gene.

The effect of individual amino acids on ApoB100 and Lp(a) secretion by HepG2 cells

The rate at which HepG2 cells secrete apoB100 lipoproteins is inversely related to the concentration of amino acids in the medium (Zhang, Z., Sniderman, A. D., Kalant, D., Vu, H., Monge, J. C., Tao, Y., and Cianflone, K. (1993) J. Biol. Chem. 268, 26920-26926). The purpose of the present study was to determine the effect of individual amino acids on apoB100 and lipoprotein secretion. Asparagine was associated with modestly increased secretion. The branched chain amino acids (leucine, isoleucine, and valine) and lysine had minor inhibitory effects. The other amino acids, by contrast, decreased apoB secretion, although the magnitude of the effect varied considerably, the most potent being tyrosine, cysteine, phenylalanine, tryptophan, methionine, and glutamine. Although the effect on Lp(a) generally paralleled that on apoB100, it was usually much less pronounced. No amino acid caused a marked decrease in albumin, apoAI, or total protein secreted from the HepG2 cells. The amino acid effect on apoB was paralleled by similar decreases in secreted cholesterol ester (CE) primarily in the low density lipoprotein density range (d < 1.006-1.063 g/ml), although there was no significant change in intracellular CE. Neither intracellular nor secreted triglycerides (TG) or free cholesterol changed, resulting in a slightly larger TG-enriched particle being secreted. The effect was confirmed in cultured primary hamster hepatocytes, where a mixture of amino acids also caused a decrease in apoB secretion (up to 40%). ApoAI appeared to increase as with the HepG2 cells. Secreted CE paralleled apoB . There was no change in intracellular or secreted TG or free cholesterol, resulting in a substantially larger TG-rich particle being secreted. mRNA for apoB100 increased with asparagine, decreased moderately with branched chain amino acids, and decreased further with glutamine, as shown by dot blot and Northern blotting. Pulse-chase studies indicated that there was no change in apoB secretion efficiency under any condition. These results extend our previous observations by demonstrating specificity of the amino acid effect on apoB100 secretion. Although an effect on transcription is the likely mechanism, the exact basis for this remains to be determined.

In vivo footprinting analysis of the hepatic control region of the human apolipoprotein E/C-I/C-IV/C-II gene locus

Expression of both the apolipoprotein (apo)E and apoC-I genes in the liver is specified by a 319-nucleotide hepatic control region (HCR-1) that is located 15 kilobase pairs downstream of the apoE gene and 5 kilobase pairs downstream of the apoC-I gene. In vivo footprint analysis of HCR-1 in intact nuclei revealed several liver-specific protein-binding sites that were not detectable by in vitro methods. In addition to three previously identified in vitro footprints, four in vivo footprints were identified in a region of HCR-1 that is required for directing gene expression to hepatocytes. Prominent liver-specific DNase I-hypersensitive sites were associated with these footprints. Liver-specific nuclear protein binding to these sites was confirmed by oligonucleotide gel-retention assays. The in vivo analysis also identified a cluster of nuclear protein-binding sites in the Alu family repeat segment adjacent to the domain required for liver expression. Micrococcal nuclease digestion indicated the presence of a nucleosome in the central domain of HCR-1 in liver chromatin that was in phase with the nucleosome location in tissues that did not express the transgene. These results suggest that HCR-1 functions in a highly structured chromatin environment requiring a complex interaction of liver-enriched transcription factors.

Chylomicron assembly and catabolism: role of apolipoproteins and receptors

Chylomicrons are lipoproteins synthesized exclusively by the intestine to transport dietary fat and fat-soluble vitamins. Synthesis of apoB48, a translational product of the apob gene, is required for the assembly of chylomicrons. The apob gene transcription in the intestine results in 14 and 7 kb mRNAs. These mRNAs are post-transcriptionally edited creating a stop codon. The edited mRNAs chylomicrons from the shorter apoB48 peptide remains to be elucidated. In addition, the roles of proteins involved in the assembly pathway, e.g. apobec-1, MTP and apoA-IV, needs to be studied. Cloning of enzymes involved in the intestinal biosynthesis of triglycerides will be crucial to fully appreciate the assembly of chylomicrons. There is a need for cell culture and transgenic animal models that can be used for intestinal lipoprotein assembly. The catabolism of chylomicrons is far more complex and efficient than the catabolism of VLDL. Even though the major steps involved in the catabolism of chylomicrons are now known, the determinants for apolipoprotein exchange, processing of remnants in the space of Disse, as well as the mechanism of uptake of these particles by extra-hepatic tissue needs further exploration.

Structure of the hepatic control region of the human apolipoprotein E/C-I gene locus

The specificity of expression in the liver of the human apolipoprotein (apo) E/C-I gene locus is determined by a hepatic control region (HCR) that is located 15 kilobases downstream of the apoE gene. DNase I footprint studies of this sequence using nuclear extracts identified a region of the HCR that is enriched in nuclear protein-binding sites. Nuclease analysis of chromatin revealed liver-specific DNase I-hypersensitive sites that were associated with this region, and additional liver-specific nuclease-sensitive sites associated with the apoE gene were identified. The HCR domain has a limited binding affinity for the nuclear scaffold. The specific domain required for liver expression was tested by ligating subfragments of the HCR to the apoE gene and examining their activity in transgenic mice. A segment of 319 nucleotides that contained several potential regulatory sequences was required for full activity of liver-specific transcription with shorter segments yielding much lower levels of expression in the liver. All constructs that contained a fully active HCR were expressed in approximately a copy-dependent manner, suggesting that transgene expression was independent of integration position. Taken together, the properties of the HCR are consistent with its function as a locus control region for the liver-specific expression of the apoE gene.

Multisubunit receptors in the immune system and their association with the cytoskeleton: in search of functional significance

Various multisubunit receptors of the immune system share similarities in structure and induce closely related signal transduction pathways upon ligand binding. Examples include the T cell antigen receptor (TCR), the B cell antigen receptor (BCR), and the high-affinity receptor for immunoglobulin E (Fc epsilon RI). Although these receptors are devoid of intrinsic kinase activity, they can associate with a similar array of intracellular kinases, phosphatases and other signaling molecules. Furthermore, these receptor complexes all form an association with the cytoskeletal matrix. In this review, we compare the structural and functional characteristics of the TCR, BCR and Fc epsilon RI. We examine the role of the cytoskeleton in regulating receptor-mediated signal transduction, as analyzed in other well-characterized receptors, including the epidermal growth factor receptor and integrin receptors. On the basis of this evidence, we review the current data depicting a cytoskeletal association for multisubunit immune system receptors and explore the potential bearing of this interaction on signaling function.

Regulation of apolipoprotein B biogenesis in human hepatocytes: posttranscriptional control mechanisms that determine the hepatic production of apolipoprotein B-containing lipoproteins

OBJECTIVES

Hepatic overproduction of apolipoprotein B (apoB)-containing lipoproteins appears to be a common cause of hyperlipoproteinemia in humans. Patients with overproduction states secrete denser cholesterol ester-rich lipoprotein particles which are highly atherogenic. The formation of apoB particles involves a very complex process that requires the coordinated synthesis and assembly of apoB, triglycerides, cholesterol esters, phospholipids, and other components. ApoB expression is an important prerequisite for the assembly and secretion of apoB particles. Evidence to date appears to suggest that apoB expression is regulated posttranscriptionally. ApoB secretion rate is determined at the levels of apoB translocation into the endoplasmic reticulum (ER) as well as degradation within the ER.

RESULTS AND HYPOTHESIS

Based on available data, we postulate that the rate of apoB particle secretion is determined at the critical point where newly-synthesized apoB interacts with core lipids, particularly triglycerides. The supply of these lipids determines the rate of translocation of the apoB molecule across the ER membrane and into the ER lumen. Lipidation of apoB facilitates its proper folding, its assembly into a lipoprotein particle, and its extracellular secretion. In the absence of lipids, apoB is misfolded resulting in the abortion of ER translocation and subsequent degradation by an apoB specific protease.

CONCLUSIONS

The balance between intracellular degradation and extracellular secretion determines the rate at which the human liver secretes apoB particles.

Synthesis and secretion of hepatic apolipoprotein B-containing lipoproteins

Human apolipoprotein (apo) B-100 is required for the synthesis and secretion of hepatic triacyglycerol-rich lipoproteins. This review summarizes recent developments in understanding the interaction of cis-acting DNA sequences and trans-acting protein factors in regulation of apo B gene expression and apo B mRNA editing, and the role of structural determinants of apo B-100 in the assembly and secretion of hepatic lipoproteins. In particular, experimental results obtained from cell culture studies using techniques of molecular and cellular biology are described and discussed. The relationship between apo B length and its ability to recruit lipids is presented, and the involvement of factors other than apo B in hepatic triacylglycerol-rich lipoprotein production is discussed.

Characterization of the chicken apolipoprotein A-I gene 5'-flanking region

Apolipoprotein A-I (apoA-I) is a major protein component of plasma high-density lipoprotein in all species studied, and plays an important role in cholesterol homeostasis. In an earlier study, we cloned and structurally characterized the chicken apoA-I gene. In this study, the 5'-flanking region of the chicken apoA-I gene was sequenced and functionally characterized. Sequence analysis of the 510-nucleotide 5' upstream region revealed the presence of TATA and CCAAT boxes. In addition, we identified binding sites for several transcription factors such as Sp1, AP1, and NFI.2. When the 5' fragment was ligated into a promoterless CAT vector and transfected into a chicken hepatocarcinoma cell line (LMH), the bacterial chloramphenicol acetyl transferase (CAT) gene was expressed, suggesting transcriptional regulation is associated with this region. Transfection studies with other 5' deletion constructs revealed that the sequence spanning the region -82 to +87 contained the major transcriptional activity. DNase I footprinting, gel retardation, and Southwestern blot analyses showed that the fragment interacts with nuclear proteins.

Comparative analysis of sequences at the 5' end of the human and mouse apolipoprotein B genes

A comparison was made between the DNA sequences in two regions of the mouse and the human apolipoprotein B genes: the 5'-flanking sequence and the region between the first exon and the second intron. Considerable homology was observed, particularly in the immediate 5' region and in the second intron. Because promoter and enhancer elements have been previously localized to these regions in the human apolipoprotein B gene, it is proposed that regions of conserved base sequence delineate binding regions for regulatory proteins. In some cases, contiguous regions of homology are longer than expected for regions designed as recognition sites for individual nuclear proteins, and may define regions recognizable by a cluster of interacting proteins. Both the human and mouse genes contain repetitive elements and a hypervariable dinucleotide repeat.

Apolipoprotein B upstream suppressor site: identification of an element which can decrease apolipoprotein B transcription

Elevated plasma levels of apolipoprotein B (apoB) may predispose to development of premature coronary atherosclerosis. We have identified the first well localized domain of the apoB gene which can effect negative regulation of its transcription. This region binds trans-activating factors present only in apoB producing cell lines. Mutagenesis of this region causes up-regulation of its transcriptional activity. We have termed this element apoB upstream suppressor site (aBUSS) and its trans-activators the apoB repressor proteins (ARP). aBUSS and ARP may play important roles in the transcriptional modulation of apoB.