NFY transcription factor binds to regulatory element AIC and transactivates the human apolipoprotein A-I promoter in HEPG2 cells

Impact of Promoter Polymorphisms on the Transcriptional Regulation of the Organic Cation Transporter OCT1 (SLC22A1)

The organic cation transporter 1 (OCT1, SLC22A1) is strongly expressed in the human liver and facilitates the hepatic uptake of drugs such as morphine, metformin, tropisetron, sumatriptan and fenoterol and of endogenous substances such as thiamine. OCT1 expression is inter-individually highly variable. Here, we analyzed SNPs in the OCT1 promoter concerning their potential contribution to the variability in OCT1 expression. Using electrophoretic mobility shift and luciferase reporter gene assays in HepG2, Hep3B, and Huh7 cell lines, we identified the SNPs −1795G>A (rs6935207) and −201C>G (rs58812592) as having effects on transcription factor binding and/or promoter activity. The A-allele of the −1795G>A SNP showed allele-specific binding of the transcription factor NF-Y leading to 2.5-fold increased enhancer activity of the artificial SV40 promoter. However, the −1795G>A SNP showed no significant effects on the native OCT1 promoter activity. Furthermore, the −1795G>A SNP was not associated with the pharmacokinetics of metformin, fenoterol, sumatriptan and proguanil in healthy individuals or tropisetron efficacy in patients undergoing chemotherapy. Allele-dependent differences in USF1/2 binding and nearly total loss in OCT1 promoter activity were detected for the G-allele of −201C>G, but the SNP is apparently very rare. In conclusion, common OCT1 promoter SNPs have only minor effects on OCT1 expression.

Type 1 Deiodinase Regulates ApoA-I Gene Expression and ApoA-I Synthesis Independent of Thyroid Hormone Signaling

OBJECTIVE

Plasma levels of high-density lipoprotein cholesterol (HDL-C) and apolipoprotein A-I (ApoA-I) are reduced in individuals with defective insulin signaling. Initial studies using liver-specific insulin receptor (InsR) knockout mice identified reduced expression of type 1 deiodinase (Dio1) as a potentially novel link between defective hepatic insulin signaling and reduced expression of the ApoA-I gene. Our objective was to examine the regulation of ApoA-I expression by Dio1.

APPROACH AND RESULTS

Acute inactivation of InsR by adenoviral delivery of Cre recombinase to InsR floxed mice reduced HDL-C and expression of both ApoA-I and Dio1. Overexpression of Dio1 in InsR knockout mice restored HDL-C and ApoA-I levels and increased the expression of ApoA-I. Dio1 knockout mice had low expression of ApoA-I and reduced serum levels of HDL-C and ApoA-I. Treatment of C57BL/6J mice with antisense to Dio1 reduced ApoA-I mRNA, HDL-C, and serum ApoA-I. Hepatic 3,5,3'-triiodothyronine content was normal or elevated in InsR knockout mice or Dio1 knockout mice. Knockdown of either InsR or Dio1 by siRNA in HepG2 cells decreased the expression of ApoA-I and ApoA-I synthesis and secretion. siRNA knockdown of InsR or Dio1 decreased activity of a region of the ApoA-I promoter lacking thyroid hormone response elements (region B). Electrophoretic mobility shift assay demonstrated that reduced Dio1 expression decreased the binding of nuclear proteins to region B.

CONCLUSIONS

Reductions in Dio1 expression reduce the expression of ApoA-I in a 3,5,3'-triiodothyronine-/thyroid hormone response element-independent manner.

Regulation of HDL genes: transcriptional, posttranscriptional, and posttranslational

HDL regulation is exerted at multiple levels including regulation at the level of transcription initiation by transcription factors and signal transduction cascades; regulation at the posttranscriptional level by microRNAs and other noncoding RNAs which bind to the coding or noncoding regions of HDL genes regulating mRNA stability and translation; as well as regulation at the posttranslational level by protein modifications, intracellular trafficking, and degradation. The above mechanisms have drastic effects on several HDL-mediated processes including HDL biogenesis, remodeling, cholesterol efflux and uptake, as well as atheroprotective functions on the cells of the arterial wall. The emphasis is on mechanisms that operate in physiologically relevant tissues such as the liver (which accounts for 80% of the total HDL-C levels in the plasma), the macrophages, the adrenals, and the endothelium. Transcription factors that have a significant impact on HDL regulation such as hormone nuclear receptors and hepatocyte nuclear factors are extensively discussed both in terms of gene promoter recognition and regulation but also in terms of their impact on plasma HDL levels as was revealed by knockout studies. Understanding the different modes of regulation of this complex lipoprotein may provide useful insights for the development of novel HDL-raising therapies that could be used to fight against atherosclerosis which is the underlying cause of coronary heart disease.

Cocoa flavanol metabolites activate HNF-3β, Sp1, and NFY-mediated transcription of apolipoprotein AI in human cells

SCOPE

To identify the mechanisms by which cocoa induces HDL levels and since apolipoprotein AI (ApoAI) is the major protein in HDLs, we analyzed, upon incubation with cocoa metabolites, ApoAI mRNA levels, its transcriptional regulation, and the levels of the transcription factors involved in this process.

METHODS AND RESULTS

Epicatechin and cocoa metabolites caused an increase in ApoAI expression in HepG2 cells. Electrophoretic mobility shift assays revealed the involvement of Sites A and B of the ApoAI promoter in the induction of ApoAI mRNA upon incubation with cocoa metabolites. Using supershift assays, we demonstrated the binding of HNF-3β, HNF-4, ER-α, and RXR-α to Site A and the binding of HNF-3β, NFY, and Sp1 to Site B. Luciferase assays performed with a construct containing Site B confirmed its role in the upregulation of ApoAI by cocoa metabolites. Incubation with 3-methyl-epicatechin led to an increase in HNF-3β mRNA, HNF-3β, ER-α, Sp1, and NFY protein levels and the activation of ApoAI transcription mediated by NFY, Sp1, and ER-α.

CONCLUSION

The activation of ApoAI transcription through Site B by cocoa flavanol metabolites is mainly mediated by an increase in HNF-3β, with a significant contribution of Sp1 and NFY, as a mechanism for the protective role of these compounds in cardiovascular diseases.

Docosahexaenoic acid suppresses apolipoprotein A-I gene expression through hepatocyte nuclear factor-3β

BACKGROUND

Dietary fish-oil supplementation has been shown in human kinetic studies to lower the production rate of apolipoprotein (apo) A-I, the major protein component of HDL. The underlying mechanism responsible for this effect is not fully understood.

OBJECTIVE

We investigated the effect and the mechanism of action of the very-long-chain n-3 (omega-3) polyunsaturated fatty acid docosahexaenoic acid (DHA), relative to the saturated fatty acid palmitic acid (PA), on the hepatic expression of apo A-I in HepG2 cells.

DESIGN

HepG2 cells were treated with different doses of DHA and PA (0-200 μmol/L). mRNA expression levels of apo A-I were assessed by real-time polymerase chain reaction, and apo A-I protein concentrations were measured by immunoassay. DHA dose-dependently suppressed apo A-I mRNA levels and also lowered apo A-I protein concentrations in the media, with maximum effects at 200 μmol/L. This concentration of fatty acids was used in all subsequent experiments.

RESULTS

To elucidate the mechanism mediating the reduction in apo A-I expression by DHA, transfection experiments were conducted with plasmid constructs containing serial deletions of the apo A-I promoter. The DHA-responsive region was mapped to the -185 to -148 nucleotide region of the apo A-I promoter, which binds the hepatocyte nuclear factor (HNF)-3β. Nuclear extracts from cells treated with DHA or PA had a similar nuclear abundance of HNF-3β. However, electrophoresis mobility shift assays showed less binding of HNF-3β to the -180 to -140 sequence of the apo A-I promoter than did PA-treated cells. As shown by chromatin immunoprecipitation analysis, less HNF-3β was recruited to the apo A-I promoter in DHA-treated cells than in PA-treated cells, which supports the concept of an interference of DHA with the binding of HNF-3β to the apo A-I promoter.

CONCLUSION

These findings suggest that, in human hepatoma HepG2 cells, DHA inhibits the binding of HNF-3β to the apo A-I promoter, resulting in the repression of apo A-I promoter transactivity and thus a reduction in apo A-I expression.

Transcriptional regulatory mechanisms of the human apolipoprotein genes in vitro and in vivo

The present review summarizes recent advances in the transcriptional regulation of the human apolipoprotein genes, focusing mostly, but not exclusively, on in-vivo studies and signaling mechanisms that affect apolipoprotein gene transcription. An attempt is made to explain how interactions of transcription factors that bind to proximal promoters and distal enhancers may bring about gene transcription. The experimental approaches used and the transcriptional regulatory mechanisms that emerge from these studies may also be applicable in other gene systems that are associated with human disease. Understanding extracellular stimuli and the specific mechanisms that underlie apolipoprotein gene transcription may in the long run allow us to selectively switch on antiatherogenic genes, and switch off proatherogenic genes. This may have beneficial effects and may confer protection from atherosclerosis to humans.

Promoter variants of the human mannose-binding lectin gene show different binding

Mannose-binding lectin (MBL) levels in the plasma of humans are highly variable. The level is influenced by gene mutations in exon1 and the promoter. Here we describe the distribution of three point mutations linked with a deletion in the MBL gene promoter in populations of Central Africa, Thailand, and Papua New Guinea. Among African children we find 20% with the wild-type allele, 53% are heterozygous, and 27% are homozygous for the mutation. In Thailand we find 65% with the wild-type allele, 33% are heterozygous, and 2% are homozygous for the variant. In Papua New Guinea the polymorphism is not found. The occurrence of the mutation was associated with MBL levels in the plasma (P = 0.043). Oligonucleotides derived from the variant promoter regions bind proteins differently according to their DNA sequence. The binding of proteins can be influenced by induction with interleukin-6.

Association of hepatic nuclear factor-4 in the apolipoprotein B promoter: a preliminary report

Previous studies have examined the arrangement of regulatory elements along the apolipoprotein B (apoB) promoter region (-3067 to +940) and a promoter fragment extending from nucleotides -150 to +124 has been demonstrated to be essential for transcriptional activation of the apoB gene in hepatic and intestinal cells. It has also been shown that transcriptional activation of apoB requires a synergistic interaction between hepatic nuclear factor-4 (HNF-4) and CCAAT/enhancer-binding protein alpha (C/EBP alpha) transcription factors. Here, we have examined the hypothesis that HNF-4 factor binding to DNA may induce a DNA helix bend, thus facilitating the communication with a C/EBP alpha factor located one helix turn from this HNF-4 factor in the apoB promoter. A gel electrophoretic mobility shift assay using wild type double-stranded oligonucleotides or modified wild type duplex oligonucleotides with 10 nucleotides inserted between HNF-4 and C/EBP alpha factor motifs showed similar retarded complexes, indicating that HNF-4 and C/EBP alpha factors interact independently of the distance between binding sites. However, when only one base, a thymidine, was inserted at the -71 position of the apoB promoter, the complex shift was completely abolished. In conclusion, these results regarding the study of the mechanisms involving the interaction between HNF-4 and C/EBP alpha factors in the apoB promoter suggest that the perfect 5'-CCCTTTGGA-3' motif is needed in order to facilitate the interaction between the two factors.

A survey of 178 NF-Y binding CCAAT boxes

The CCAAT box is one of the most common elements in eukaryotic promoters, found in the forward or reverse orientation. Among the various DNA binding proteins that interact with this sequence, only NF-Y (CBF, HAP2/3/4/5) has been shown to absolutely require all 5 nt. Analysis of a database with 178 bona fide NF-Y binding sites in 96 unrelated promoters confirms this need and points to specific additional flanking nucleotides (C, Pu, Pu on the 5'-side and C/G, A/G, G,A/C, G on the 3'-side) required for efficient binding. The frequency of CCAAT boxes appears to be relatively higher in TATA-less promoters, particularly in the reverse ATTGG orientation. In TATA-containing promoters the CCAAT box is preferentially located in the -80/-100 region (mean position -89) and is not found nearer to the Start site than -50. In TATA-less promoters it is usually closer to the +1 signal (at -66 on average) and is sometimes present in proximity to the Cap site. The consensus and location of NF-Y binding sites parallel almost perfectly a previous general statistical study on CCAAT boxes in 502 unrelated promoters. This is an indication that NF-Y is the major, if not the sole, CCAAT box recognizing protein and that it might serve different roles in TATA-containing and TATA-less promoters.