Histone acetyltransferase HBO1 inhibits NF-kappaB activity by coactivator sequestration

HBO1/KAT7/MYST2 HAT complex regulates human adenovirus replicative cycle

Human adenoviruses (HAdV) belong to a small DNA tumor virus family that continues as valuable models in understanding the viral strategies of usurping cell growth regulation. A number of HAdV type 2/5 early viral gene products interact with a variety of cellular proteins to build a conducive environment that promotes viral replication. Here we show that HBO1 (Histone Acetyltransferase Binding to ORC1), a member of the MYST histone acetyltransferase (HAT) complex (also known as KAT7 and MYST2) that acetylates most of the histone H3 lysine 14, is essential for HAdV5 growth. HBO1/MYST2/KAT7 HAT complexes are critical for a variety of cellular processes including control of cell proliferation. In HBO1 downregulated human cells, HAdV5 infection results in reduced expression of E1A and other viral early genes, virus growth is also reduced significantly. Importantly, HBO1 downregulation reduced H3 lysine 14 acetylation at viral promoters during productive infection, likely driving reduced viral gene expression. HBO1 was also associated with viral promoters during infection and co-localized with viral replication centers in the nuclei of infected cells. In transiently transfected cells, overexpression of E1A along with HBO1 stimulated histone acetyltransferase activity of HBO1. E1A also co-immunoprecipitated with HBO1 in transiently transfected cells. In summary, our results demonstrate that HAdV recruits the HBO1 HAT complex to aid in viral replication.

Deciphering structure, function and mechanism of lysine acetyltransferase HBO1 in protein acetylation, transcription regulation, DNA replication and its oncogenic properties in cancer

HBO1 complexes are major acetyltransferase responsible for histone H4 acetylation in vivo, which belongs to the MYST family. As the core catalytic subunit, HBO1 consists of an N-terminal domain and a C-terminal MYST domain that are in charge of acetyl-CoA binding and acetylation reaction. HBO1 complexes are multimeric and normally consist of two native subunits MEAF6, ING4 or ING5 and two kinds of cofactors as chromatin reader: Jade-1/2/3 and BRPF1/2/3. The choices of subunits to form the HBO1 complexes provide a regulatory switch to potentiate its activity between histone H4 and H3 tails. Thus, HBO1 complexes present multiple functions in histone acetylation, gene transcription, DNA replication, protein ubiquitination, and immune regulation, etc. HBO1 is a co-activator for CDT1 to facilitate chromatin loading of MCM complexes and promotes DNA replication licensing. This process is regulated by mitotic kinases such as CDK1 and PLK1 by phosphorylating HBO1 and modulating its acetyltransferase activity, therefore, connecting histone acetylation to regulations of cell cycle and DNA replication. In addition, both gene amplification and protein overexpression of HBO1 confirmed its oncogenic role in cancers. In this paper, we review the recent advances and discuss our understanding of the multiple functions, activity regulation, and disease relationship of HBO1.

Biological Functions of the ING Proteins

The proteins belonging to the inhibitor of growth (ING) family of proteins serve as epigenetic readers of the H3K4Me3 histone mark of active gene transcription and target histone acetyltransferase (HAT) or histone deacetylase (HDAC) protein complexes, in order to alter local chromatin structure. These multidomain adaptor proteins interact with numerous other proteins to facilitate their localization and the regulation of numerous biochemical pathways that impinge upon biological functions. Knockout of some of the ING genes in murine models by various groups has verified their status as tumor suppressors, with ING1 knockout resulting in the formation of large clear-cell B-lymphomas and ING2 knockout increasing the frequency of ameloblastomas, among other phenotypic effects. ING4 knockout strongly affects innate immunity and angiogenesis, and INGs1, ING2, and ING4 have been reported to affect apoptosis in different cellular models. Although ING3 and ING5 knockouts have yet to be published, preliminary reports indicate that ING3 knockout results in embryonic lethality and that ING5 knockout may have postpartum effects on stem cell maintenance. In this review, we compile the known information on the domains of the INGs and the effects of altering ING protein expression, to better understand the functions of this adaptor protein family and its possible uses for targeted cancer therapy.

LPS promotes HBO1 stability via USP25 to modulate inflammatory gene transcription in THP-1 cells

The histone acetyltransferase HBO1 (Histone acetyltransferase binding to origin recognition complex 1, Myst2/Kat7) participates in a range of life processes including DNA replication and tumorigenesis. Recent studies revealed that HBO1 is involved in gene transcriptional activation. However, the molecular behavior of HBO1 in inflammation is yet to be studied. Here we report that endotoxin lipopolysaccharide (LPS) elevates HBO1 protein level via up-regulating UPS25 (ubiquitin specific peptidase 25) and alters inflammatory gene transcription in THP-1 monocytes and in human primary macrophages. LPS protects HBO1 from ubiquitin proteasomal degradation without significantly altering its transcription. By immunoprecipitation, we identified that HBO1 associates with a deubiquitinating enzyme USP25 in THP-1 cells. LPS increases protein level of USP25 resulting in accumulation of HBO1 by suppression of HBO1 ubiquitination. Stabilized-HBO1 modulates inflammatory gene transcription in THP-1 cells. These findings indicate that USP25 promotes stability of HBO1 in bacterial infection thereby enhances HBO1-mediated inflammatory gene transcription.

Transcriptional Regulation of the Warburg Effect in Cancer by SIX1

Aerobic glycolysis (the Warburg effect) facilitates tumor growth, and drugs targeting aerobic glycolysis are being developed. However, how the Warburg effect is directly regulated is largely unknown. Here we show that transcription factor SIX1 directly increases the expression of many glycolytic genes, promoting the Warburg effect and tumor growth in vitro and in vivo. SIX1 regulates glycolysis through HBO1 and AIB1 histone acetyltransferases. Cancer-related SIX1 mutation increases its ability to promote aerobic glycolysis and tumor growth. SIX1 glycolytic function is directly repressed by microRNA-548a-3p, which is downregulated, inversely correlates with SIX1, and is a good predictor of prognosis in breast cancer patients. Thus, the microRNA-548a-3p/SIX1 axis strongly links aerobic glycolysis to carcinogenesis and may become a promising cancer therapeutic target.

Histone acetyltransferase 7 (KAT7)-dependent intragenic histone acetylation regulates endothelial cell gene regulation

Although the functional role of chromatin marks at promoters in mediating cell-restricted gene expression has been well characterized, the role of intragenic chromatin marks is not well understood, especially in endothelial cell (EC) gene expression. Here, we characterized the histone H3 and H4 acetylation profiles of 19 genes with EC-enriched expression via locus-wide chromatin immunoprecipitation followed by ultra-high-resolution (5 bp) tiling array analysis in ECs versus non-ECs throughout their genomic loci. Importantly, these genes exhibit differential EC enrichment of H3 and H4 acetylation in their promoter in ECs versus non-ECs. Interestingly, VEGFR-2 and VEGFR-1 show EC-enriched acetylation across broad intragenic regions and are up-regulated in non-ECs by histone deacetylase inhibition. It is unclear which histone acetyltransferases (KATs) are key to EC physiology. Depletion of KAT7 reduced VEGFR-2 expression and disrupted angiogenic potential. Microarray analysis of KAT7-depleted ECs identified 263 differentially regulated genes, many of which are key for growth and angiogenic potential. KAT7 inhibition in zebrafish embryos disrupted vessel formation and caused loss of circulatory integrity, especially hemorrhage, all of which were rescued with human KAT7. Notably, perturbed EC-enriched gene expression, especially the VEGFR-2 homologs, contributed to these vascular defects. Mechanistically, KAT7 participates in VEGFR-2 transcription by mediating RNA polymerase II binding, H3 lysine 14, and H4 acetylation in its intragenic region. Collectively, our findings support the importance of differential histone acetylation at both promoter and intragenic regions of EC genes and reveal a previously underappreciated role of KAT7 and intragenic histone acetylation in regulating VEGFR-2 and endothelial function.

Functional analysis of HBO1 in tumor development and inhibitor screening

The aim of the present study was to explore the functions of histone acetyltransferase binding to origin recog-nition complex (ORC) 1 (HBO1) during tumor development and to screen for HBO1 inhibitors. The chromatin immuno-precipitation sequencing (ChIP-seq) data of HBO1 in the RKO human colon cancer cell line (GSE33007) were downloaded from the Gene Expression Omnibus (GEO) database. The reads were then mapped back to a reference genome hg19. The PCR duplicate reads were removed by using SAMtools software and the shift was calculated using SPP and MaSC software. The peak calling was carried out using MACS 1.4.0 software. Furthermore, the inhibitors of HBO1 were screened out from the Specs database using Dock 6.6 software. The binding sites of HBO1 were mainly distributed in the intergenic, intronic and 3'-end regions. Further analysis revealed that a total of 9,467 target genes was identified around HBO1 binding sites in the RKO cell lines and those genes mainly participated in the cell cycle, biosynthetic process, as well as other processes. Finally, 5 inhibitors with best binding affinity in the positively charged cavity of HBO1 were screened out: i) 5-[(2-hydroxybenzylidene)amino] -2-(2‑{4‑[(2‑hydroxy-benzylidene)amino]-2-sulfonatophenyl}vinyl)benzenesulfonate, ii) 3-[4-(3-bromo-4-{2-[4-(ethoxycarbonyl)anilino]-2-oxo-ethoxy}-5-methoxybenzylidene)‑3‑methyl‑5‑oxo -4,5-dihydro-1H-pyrazol-1-yl]benzoic acid, iii) 4-(4-{3-iodo‑5‑ methoxy‑4-[2-(2-methoxyanilino)-2-oxoethoxy]benzylidene}-3-methyl-5-oxo-4,5-dihydro-1H-pyrazol-1-yl)benzoic acid, iv) 5-chloro-1,3-bis{[3,5,6-trihydroxy-4-(octyloxy)tetrahydro-2H-pyran-2-yl]methyl}-1,3-dihydro-2H-benzimidazol-2-one and v) 4-{[4-(tetradecylamino)-1-naphthyl]diazenyl}benzoic acid. As a whole, in this study, we identified the possible binding sites and biological functions of HBO1. The potential inhibitors of HBO1 were also screened, which prove to be helpful for the inhibition of HBO1 during tumor development.

The histone acetyltransferase Myst2 regulates Nanog expression, and is involved in maintaining pluripotency and self-renewal of embryonic stem cells

The histone acetyltransferase Myst2 plays an important role in embryogenesis, but its function in undifferentiated ES cells remains poorly understood. Here, we show that Myst2 plays a role in pluripotency and self-renewal of ES cells. Myst2 deficiency results in loss of characteristic morphology, decreased alkaline phosphatase staining and reduced histone acetylation, as well as aberrant expression of pluripotency and differentiation markers. Our ChIP data reveal a direct association of Myst2 with the Nanog promoter and Myst2-dependent Oct4 binding on the Nanog promoter. Together our data suggest that Myst2-mediated histone acetylation may be required for recruitment of Oct4 to the Nanog promoter, thereby regulating Nanog transcription in ES cells.

Hbo1 is a cyclin E/CDK2 substrate that enriches breast cancer stem-like cells

Expression of cyclin E proteolytic cleavage products, low-molecular weight cyclin E (LMW-E), is associated with poor clinical outcome in patients with breast cancer and it enhances tumorigenecity in mouse models. Here we report that LMW-E expression in human mammary epithelial cells induces an epithelial-to-mesenchymal transition phenotype, increases the CD44(hi)/CD24(lo) population, enhances mammosphere formation, and upregulates aldehyde dehydrogenase expression and activity. We also report that breast tumors expressing LMW-E have a higher proportion of CD44(hi)/CD24(lo) tumor cells as compared with tumors expressing only full-length cyclin E. In order to explore how LMW-E enriches cancer stem cells in breast tumors, we conducted a protein microarray analysis that identified the histone acetyltransferase (HAT) Hbo1 as a novel cyclin E/CDK2 substrate. The LMW-E/CDK2 complex phosphorylated Hbo1 at T88 without affecting its HAT activity. When coexpressed with LMW-E/CDK2, wild-type Hbo1 promoted enrichment of cancer stem-like cells (CSC), whereas the T88 Hbo1 mutant reversed the CSC phenotype. Finally, doxorubicin and salinomycin (a CSC-selective cytotoxic agent) synergized to kill cells expressing LMW-E, but not full-length cyclin E. Collectively, our results suggest that the heightened oncogenecity of LMW-E relates to its ability to promote CSC properties, supporting the design of therapeutic strategies to target this unique function.

The MYST-containing protein Chameau is required for proper sensory organ specification during Drosophila thorax morphogenesis

The adult thorax of Drosophila melanogaster is covered by a stereotyped pattern of mechanosensory bristles called macrochaetes. Here, we report that the MYST containing protein Chameau (Chm) contributes to the establishment of this pattern in the most dorsal part of the thorax. Chm mutant pupae present extra-dorsocentral (DC) and scutellar (SC) macrochaetes, but a normal number of the other macrochaetes. We provide evidences that chm restricts the singling out of sensory organ precursors from proneural clusters and genetically interacts with transcriptional regulators involved in the regulation of achaete and scute in the DC and SC proneural cluster. This function of chm likely relies on chromatin structure regulation since a protein with a mutation in the conserved catalytic site fails to rescue the formation of supernumerary DC and SC bristles in chm mutant flies. This is further supported by the finding that mutations in genes encoding chromatin modifiers and remodeling factors, including Polycomb group (PcG) and Trithorax group (TrxG) members, dominantly modulate the penetrance of chm extra bristle phenotype. These data support a critical role for chromatin structure modulation in the establishment of the stereotyped sensory bristle pattern in the fly thorax.

HBO1 is required for H3K14 acetylation and normal transcriptional activity during embryonic development

We report here that the MYST histone acetyltransferase HBO1 (histone acetyltransferase bound to ORC; MYST2/KAT7) is essential for postgastrulation mammalian development. Lack of HBO1 led to a more than 90% reduction of histone 3 lysine 14 (H3K14) acetylation, whereas no reduction of acetylation was detected at other histone residues. The decrease in H3K14 acetylation was accompanied by a decrease in expression of the majority of genes studied. However, some genes, in particular genes regulating embryonic patterning, were more severely affected than "housekeeping" genes. Development of HBO1-deficient embryos was arrested at the 10-somite stage. Blood vessels, mesenchyme, and somites were disorganized. In contrast to previous studies that reported cell cycle arrest in HBO1-depleted cultured cells, no defects in DNA replication or cell proliferation were seen in Hbo1 mutant embryo primary fibroblasts or immortalized fibroblasts. Rather, a high rate of cell death and DNA fragmentation was observed in Hbo1 mutant embryos, resulting initially in the degeneration of mesenchymal tissues and ultimately in embryonic lethality. In conclusion, the primary role of HBO1 in development is that of a transcriptional activator, which is indispensable for H3K14 acetylation and for the normal expression of essential genes regulating embryonic development.

HBO1 histone acetylase activity is essential for DNA replication licensing and inhibited by Geminin

HBO1, an H4-specific histone acetylase, is a coactivator of the DNA replication licensing factor Cdt1. HBO1 acetylase activity is required for licensing, because a histone acetylase (HAT)-defective mutant of HBO1 bound at origins is unable to load the MCM complex. H4 acetylation at origins is cell-cycle regulated, with maximal activity at the G1/S transition, and coexpression of HBO1 and Jade-1 increases histone acetylation and MCM complex loading. Overexpression of the Set8 histone H4 tail-binding domain specifically inhibits MCM loading, suggesting that histones are a physiologically relevant target for licensing. Lastly, Geminin inhibits HBO1 acetylase activity in the context of a Cdt1-HBO1 complex, and it associates with origins and inhibits H4 acetylation and licensing in vivo. Thus, H4 acetylation at origins by HBO1 is critical for replication licensing by Cdt1, and negative regulation of licensing by Geminin is likely to involve inhibition of HBO1 histone acetylase activity.

Epigenetics in human melanoma

BACKGROUND

Recent technological advances have allowed us to examine the human genome in greater detail than ever before. This has opened the door to an improved understanding of the gene expression patterns involved with cancer.

METHODS

A review of the literature was performed to determine the role of epigenetic modifications in human melanoma. We focused the search on histone deacetylation, methylation of gene promoter regions, demethylation of CpG islands, and the role of microRNA. We examined the relationship between human melanoma epigenetics and their importance in tumorigenesis, tumor progression, and inhibition of metastasis. The development and clinical application of select pharmacologic agents are also discussed.

RESULTS

We identified several articles that have extensively studied the role of epigenetics in melanoma, further elucidating the complex processes involved in gene regulation and expression. Several new agents directly affect epigenetic mechanisms in melanoma, with divergent affects on the metastatic potential of melanoma.

CONCLUSIONS

Epigenetic mechanisms have emerged as having a central role in gene regulation of human melanoma, including the identification of several putative tumor suppressor genes and oncogenes. Further research will focus on the development of novel therapeutics that will likely target and alter such epigenetic changes.

Histone acetyltransferase Hbo1: catalytic activity, cellular abundance, and links to primary cancers

In addition to the well-characterized proteins that comprise the pre-replicative complex, recent studies suggest that chromatin structure plays an important role in DNA replication initiation. One of these chromatin factors is the histone acetyltransferase (HAT) Hbo1 which is unique among HAT enzymes in that it serves as a positive regulator of DNA replication. However, several of the basic properties of Hbo1 have not been previously examined, including its intrinsic catalytic activity, its molecular abundance in cells, and its pattern of expression in primary cancer cells. Here we show that recombinant Hbo1 can acetylate nucleosomal histone H4 in vitro, with a preference for lysines 5 and 12. Using semi-quantitative western blot analysis, we find that Hbo1 is approximately equimolar with the number of active replication origins in normal human fibroblasts but is an order of magnitude more abundant in both MCF7 and Saos-2 established cancer cell lines. Immunohistochemistry for Hbo1 in 11 primary human tumor types revealed strong Hbo1 protein expression in carcinomas of the testis, ovary, breast, stomach/esophagus, and bladder.

The ING gene family in the regulation of cell growth and tumorigenesis

The five members of the inhibitor of growth (ING) gene family have garnered significant interest due to their putative roles as tumor suppressors. However, the precise role(s) of these ING proteins in regulating cell growth and tumorigenesis remains uncertain. Biochemical and molecular biological analysis has revealed that all ING members encode a PHD finger motif proposed to bind methylated histones and phosphoinosital, and all ING proteins have been found as components of large chromatin remodeling complexes that also include histone acetyl transferase (HAT) and histone deacetylase (HDAC) enzymes, suggesting a role for ING proteins in regulating gene transcription. Additionally, the results of forced overexpression studies performed in tissue culture have indicated that several of the ING proteins can interact with the p53 tumor suppressor protein and/or the nuclear factor-kappa B (NF-kappaB) protein complex. As these ING-associated proteins play well-established roles in numerous cell processes, including DNA repair, cell growth and survival, inflammation, and tumor suppression, several models have been proposed that ING proteins act as key regulators of cell growth not only through their ability to modify gene transcription but also through their ability to alter p53 and NF-kappaB activity. However, these models have yet to be substantiated by in vivo experimentation. This review summarizes what is currently known about the biological functions of the five ING genes based upon in vitro experiments and recent mouse modeling efforts, and will highlight the potential impact of INGs on the development of cancer.

FAD24, a regulator of adipogenesis and DNA replication, inhibits H-RAS-mediated transformation by repressing NF-kappaB activity

We have previously reported that a novel gene, factor for adipocyte differentiation (fad) 24, promotes adipogenesis. Moreover, our recent findings indicated that this regulation involves the control of DNA replication by fad24 during the early stage in adipogenesis. Considering that abnormal regulation of DNA replication is linked to transformation, we examined whether the over-expression of fad24 leads to the formation of colonies in soft agarose. The over-expression itself generated no colonies. Rather, it inhibited oncogenic H-ras-mediated formation of colonies. The over-expression of histone acetyltransferase binding to ORC1 (HBO1), a partner of FAD24, also inhibited the H-ras-mediated colony-forming process. Furthermore, we revealed that FAD24 interacts with p65, a subunit of NF-kappaB which is known to be activated by H-ras. The over-expression of fad24 repressed NF-kappaB-mediated promoter activity and gene expression. Taken together, these results reveal a novel role for fad24 in the repression of NF-kappaB activity and H-ras-mediated transformation.

Androgen receptor (AR) coregulators: a diversity of functions converging on and regulating the AR transcriptional complex

Androgens, acting through the androgen receptor (AR), are responsible for the development of the male phenotype during embryogenesis, the achievement of sexual maturation at puberty, and the maintenance of male reproductive function and behavior in adulthood. In addition, androgens affect a wide variety of nonreproductive tissues. Moreover, aberrant androgen action plays a critical role in multiple pathologies, including prostate cancer and androgen insensitivity syndromes. The formation of a productive AR transcriptional complex requires the functional and structural interaction of the AR with its coregulators. In the last decade, an overwhelming and ever increasing number of proteins have been proposed to possess AR coactivating or corepressing characteristics. Intriguingly, a vast diversity of functions has been ascribed to these proteins, indicating that a multitude of cellular functions and signals converge on the AR to regulate its function. The current review aims to provide an overview of the AR coregulator proteins identified to date and to propose a classification of these AR coregulator proteins according to the function(s) ascribed to them. Taken together, this approach will increase our understanding of the cellular pathways that converge on the AR to ensure an appropriate transcriptional response to androgens.

The MYST family of histone acetyltransferases and their intimate links to cancer

The histone acetyltransferases (HATs) of the MYST family are highly conserved in eukaryotes and carry out a significant proportion of all nuclear acetylation. These enzymes function exclusively in multisubunit protein complexes whose composition is also evolutionarily conserved. MYST HATs are involved in a number of key nuclear processes and play critical roles in gene-specific transcription regulation, DNA damage response and repair, as well as DNA replication. This suggests that anomalous activity of these HATs or their associated complexes can easily lead to severe cellular malfunction, resulting in cell death or uncontrolled growth and malignancy. Indeed, the MYST family HATs have been implicated in several forms of human cancer. This review summarizes the current understanding of these enzymes and their normal function, as well as their established and putative links to oncogenesis.

Males absent on the first (MOF): from flies to humans

Histone modifications such as acetylation, methylation and phosphorylation have been implicated in fundamental cellular processes such as epigenetic regulation of gene expression, organization of chromatin structure, chromosome segregation, DNA replication and DNA repair. Males absent on the first (MOF) is responsible for acetylating histone H4 at lysine 16 (H4K16) and is a key component of the MSL complex required for dosage compensation in Drosophila. The human ortholog of MOF (hMOF) has the same substrate specificity and recent purification of the human and Drosophila MOF complexes showed that these complexes were also highly conserved through evolution. Several studies have shown that loss of hMOF in mammalian cells leads to a number of different phenotypes; a G2/M cell cycle arrest, nuclear morphological defects, spontaneous chromosomal aberrations, reduced transcription of certain genes and an impaired DNA repair response upon ionizing irradiation. Moreover, hMOF is involved in ATM activation in response to DNA damage and acetylation of p53 by hMOF influences the cell's decision to undergo apoptosis instead of a cell cycle arrest. These data, highlighting hMOF as an important component of many cellular processes, as well as links between hMOF and cancer will be discussed.