Epigenetics of gene expression in human hepatoma cells: expression profiling the response to inhibition of DNA methylation and histone deacetylation.

BACKGROUND: DNA methylation and histone deacetylation are epigenetic mechanisms that play major roles in eukaryotic gene regulation. We hypothesize that many genes in the human hepatoma cell line HepG2 are regulated by DNA methylation and histone deacetylation. Treatment with 5-aza-2'-deoxycytidine (5-aza-dC) to inhibit DNA methylation with and/or Trichostatin A (TSA) to inhibit histone deacetylation should allow us to identify genes that are regulated epigenetically in hepatoma cells. RESULTS: 5-aza-dC had a much larger effect on gene expression in HepG2 cells than did TSA, as measured using Affymetrix HG-U133 Plus 2.0 microarrays. The expression of 1504 probe sets was affected by 5-aza-dC (at p < 0.01), 535 probe sets by TSA, and 1929 probe sets by the combination of 5-aza-dC and TSA. 5-aza-dC treatment turned on the expression of 211 probe sets that were not detectably expressed in its absence. Expression of imprinted genes regulated by DNA methylation, such as H19 and NNAT, was turned on or greatly increased in response to 5-aza-dC. Genes involved in liver processes such as xenobiotic metabolism (CYP3A4, CYP3A5, and CYP3A7) and steroid biosynthesis (CYP17A1 and CYP19A1), and genes encoding CCAAT element-binding proteins (C/EBPalpha, C/EBPbeta, and C/EBPgamma) were affected by 5-aza-dC or the combination. Many of the genes that fall within these groups are also expressed in the developing fetal liver and adult liver. Quantitative real-time RT-PCR assays confirmed selected gene expression changes seen in microarray analyses. CONCLUSION: Epigenetics play a role in regulating the expression of several genes involved in essential liver processes such as xenobiotic metabolism and steroid biosynthesis in HepG2 cells. Many genes whose expression is normally silenced in these hepatoma cells were re-expressed by 5-aza-dC treatment. DNA methylation may be a factor in restricting the expression of fetal genes during liver development and in shutting down expression in hepatoma cells.

Effect of SWI/SNF chromatin remodeling complex on HIV-1 Tat activated transcription.

BACKGROUND: Human immunodeficiency virus type 1 (HIV-1) is the etiologic agent of acquired immunodeficiency virus (AIDS). Following entry into the host cell, the viral RNA is reverse transcribed into DNA and subsequently integrated into the host genome as a chromatin template. The integrated proviral DNA, along with the specific chromatinized environment in which integration takes place allows for the coordinated regulation of viral transcription and replication. While the specific roles of and interplay between viral and host proteins have not been fully elucidated, numerous reports indicate that HIV-1 retains the ability for self-regulation via the pleiotropic effects of its viral proteins. Though viral transcription is fully dependent upon host cellular factors and the state of host activation, recent findings indicate a complex interplay between viral proteins and host transcription regulatory machineries including histone deacetylases (HDACs), histone acetyltransferases (HATs), cyclin dependent kinases (CDKs), and histone methyltransferases (HMTs). RESULTS: Here, we describe the effect of Tat activated transcription at the G1/S border of the cell cycle and analyze the interaction of modified Tat with the chromatin remodeling complex, SWI/SNF. HIV-1 LTR DNA reconstituted into nucleosomes can be activated in vitro using various Tat expressing extracts. Optimally activated transcription was observed at the G1/S border of the cell cycle both in vitro and in vivo, where chromatin remodeling complex, SWI/SNF, was present on the immobilized LTR DNA. Using a number of in vitro binding as well as in vivo chromatin immunoprecipitation (ChIP) assays, we detected the presence of both BRG1 and acetylated Tat in the same complex. Finally, we demonstrate that activated transcription resulted in partial or complete removal of the nucleosome from the start site of the LTR as evidenced by a restriction enzyme accessibility assay. CONCLUSION: We propose a model where unmodified Tat is involved in binding to the CBP/p300 and cdk9/cyclin T1 complexes facilitating transcription initiation. Acetylated Tat dissociates from the TAR RNA structure and recruits bromodomain-binding chromatin modifying complexes such as p/CAF and SWI/SNF to possibly facilitate transcription elongation.

GATA-2 and HNF-3beta regulate the human alcohol dehydrogenase 1A (ADH1A) gene.

In this paper, we have identified several distal cis-acting elements that contribute to the regulation and tissue- specificity of ADH1A, which encodes an alcohol dehydrogenase (ADH) that metabolizes ethanol. A negative element from bp -1873 to -1558, relative to the translational start site, decreased transcriptional activity to 52% in H4IIE-C3 cells and 70% in CV-1 cells. A positive element from bp -2459 to -2173 increased transcriptional activity twofold in H4IIE-C3 cells and 1.7-fold in CV-1 cells. Gel mobility shift and supershift assays demonstrated that GATA-2 bound a region within this positive element. A tissue-specific regulatory element from bp -6380 to -5403 increased transcription twofold in H4IIE-C3 cells while decreasing transcription to 86% in CV-1 cells. Within this tissue-specific fragment, the region from bp -5668 to -5403 increased transcription 1.7-fold in H4IIE-C3 cells and 1.3-fold in CV-1 cells. Hepatocyte nuclear factor-3beta (HNF- 3beta) bound a region of the tissue-specific element in CV-1 cells, but not in H4IIE-C3 cells. Positive regulation of the ADH1A gene may be influenced by GATA-2 binding, while differences in HNF-3beta binding in cells/tissues may contribute to tissue specificity.

Integrative analysis of the Caenorhabditis elegans genome by the modENCODE project.

We systematically generated large-scale data sets to improve genome annotation for the nematode Caenorhabditis elegans, a key model organism. These data sets include transcriptome profiling across a developmental time course, genome-wide identification of transcription factor-binding sites, and maps of chromatin organization. From this, we created more complete and accurate gene models, including alternative splice forms and candidate noncoding RNAs. We constructed hierarchical networks of transcription factor-binding and microRNA interactions and discovered chromosomal locations bound by an unusually large number of transcription factors. Different patterns of chromatin composition and histone modification were revealed between chromosome arms and centers, with similarly prominent differences between autosomes and the X chromosome. Integrating data types, we built statistical models relating chromatin, transcription factor binding, and gene expression. Overall, our analyses ascribed putative functions to most of the conserved genome.

Differential regulation of the alcohol dehydrogenase 1B (ADH1B) and ADH1C genes by DNA methylation and histone deacetylation.

BACKGROUND: The human class I alcohol dehydrogenase (ADH) genes (ADH1A, ADH1B, and ADH1C) differ in expression during development and in various tissues. They are repressed in the HepG2 human hepatoma cell line. We hypothesized that epigenetic modifications play a role in this repression and that class I ADH gene expression would be enhanced upon global inhibition of DNA methylation and histone deacetylation. METHODS: Southern blotting was used to assess the methylation status of each class I ADH gene. HepG2 and HeLa cells were treated with either the DNA methylation inhibitor 5-aza-2'-deoxycytidine (5-aza-dC), the histone deacetylase inhibitor Trichostatin A (TSA), or both in combination, and class I ADH gene expression was analyzed. Chromatin immunoprecipitation assays were performed to analyze histone H3 acetylation. Transient transfections and gel mobility shift assays were used to analyze the role that methylation plays in inhibiting transcription factor binding and promoter function. RESULTS: We show that the upstream regions of ADH1A, ADH1B, and ADH1C are methylated in HepG2 cells. 5-Aza-2'-deoxycytidine treatment enhanced expression of both ADH1B and ADH1C. Trichostatin A treatment elevated expression of ADH1C. ADH1A expression was not stimulated by either 5-aza-dC or TSA. H3 histones associated with a methylated upstream region of ADH1B were hyperacetylated in TSA-treated, but not in 5-aza-dC-treated, HepG2 cells. A methylated upstream region of ADH1C achieved histone H3 hyperacetylation upon either 5-aza-dC or TSA treatment. Methylation of the ADH1B proximal promoter in vitro decreased its activity to 54% and inhibited the binding of the upstream stimulatory factor. CONCLUSIONS: These findings suggest that the class I ADH genes are regulated by epigenetic mechanisms in human hepatoma cells. The temporal and tissue-specific expression of these genes may in part result from differences in epigenetic modifications and the availability of key transcription factors.