Chromosomal protein HMGN1 enhances the acetylation of lysine 14 in histone H3

KAT7/HMGN1 signaling epigenetically induces tyrosine phosphorylation-regulated kinase 1A expression to ameliorate insulin resistance in Alzheimer's disease

BACKGROUND

Epidemiological studies have revealed a correlation between Alzheimer's disease (AD) and type 2 diabetes mellitus (T2D). Insulin resistance in the brain is a common feature in patients with T2D and AD. KAT7 is a histone acetyltransferase that participates in the modulation of various genes.

AIM

To determine the effects of KAT7 on insulin patients with AD.

METHODS

APPswe/PS1-dE9 double-transgenic and db/db mice were used to mimic AD and diabetes, respectively. An in vitro model of AD was established by Aβ stimulation. Insulin resistance was induced by chronic stimulation with high insulin levels. The expression of microtubule-associated protein 2 (MAP2) was assessed using immunofluorescence. The protein levels of MAP2, Aβ, dual-specificity tyrosine phosphorylation-regulated kinase-1A (DYRK1A), IRS-1, p-AKT, total AKT, p-GSK3β, total GSK3β, DYRK1A, and KAT7 were measured via western blotting. Accumulation of reactive oxygen species (ROS), malondialdehyde (MDA), and SOD activity was measured to determine cellular oxidative stress. Flow cytometry and CCK-8 assay were performed to evaluate neuronal cell death and proliferation, respectively. Relative RNA levels of KAT7 and DYRK1A were examined using quantitative PCR. A chromatin immunoprecipitation assay was conducted to detect H3K14ac in DYRK1A.

RESULTS

KAT7 expression was suppressed in the AD mice. Overexpression of KAT7 decreased Aβ accumulation and MAP2 expression in AD brains. KAT7 overexpression decreased ROS and MDA levels, elevated SOD activity in brain tissues and neurons, and simultaneously suppressed neuronal apoptosis. KAT7 upregulated levels of p-AKT and p-GSK3β to alleviate insulin resistance, along with elevated expression of DYRK1A. KAT7 depletion suppressed DYRK1A expression and impaired H3K14ac of DYRK1A. HMGN1 overexpression recovered DYRK1A levels and reversed insulin resistance caused by KAT7 depletion.

CONCLUSION

We determined that KAT7 overexpression recovered insulin sensitivity in AD by recruiting HMGN1 to enhance DYRK1A acetylation. Our findings suggest that KAT7 is a novel and promising therapeutic target for the resistance in AD.

Chromatin binding protein HMGN1 promotes HBV cccDNA transcription and replication by regulating the phosphorylation of histone 3

BACKGROUND AND AIMS

Direct elimination of cccDNA remains a formidable obstacle due to the persistent and stable presence of cccDNA in hepatocyte nuclei. The silencing of cccDNA transcription enduringly is one of alternative strategies in the treatment of hepatitis B. Protein binding to cccDNA plays an important role in its transcriptional regulation; thus, the identification of key factors involved in this process is of great importance.

APPROACHES AND RESULTS

In the present study, high mobility group nucleosome binding domain 1 (HMGN1) was screened out based on our biotin-avidin enrichment system. First, chromatin immunoprecipitation and fluorescent in situ hybridization assays confirmed the binding of HMGN1 with cccDNA in the nucleus. Second, functional experiments in HBV-infected cells showed that the promoting effect of HMGN1 on HBV transcription and replication depended on the functional region of the nucleosomal binding domain, while transfection of the HMGN1 mutant showed no influence on HBV compared with the vector. Third, further mechanistic exploration revealed that the silencing of HMGN1 increased the level of phosphorylase CLK2 and promoted H3 phosphorylation causing the reduced accessibility of cccDNA. Moreover, silenced HMGN1 was mimicked in HBV (r) cccDNA mouse model of HBV infection in vivo. The results showed that silencing HMGN1 inhibited HBV replication in vivo.

CONCLUSIONS

In summary, our study identified that a host protein can bind to cccDNA and promote its transcription, providing a candidate strategy for anti-HBV targeting to interfere with the transcriptional activity of cccDNA microchromosomes.

The growth hormone receptor interacts with transcriptional regulator HMGN1 upon GH-induced nuclear translocation

Growth hormone (GH) actions are mediated through binding to its cell-surface receptor, the GH receptor (GHR), with consequent activation of downstream signalling. However, nuclear GHR localisation has also been observed and is associated with increased cancer cell proliferation. Here we investigated the functional implications of nuclear translocation of the GHR in the human endometrial cancer cell-line, RL95-2, and human mammary epithelial cell-line, MCF-10A. We found that following GH treatment, the GHR rapidly translocates to the nucleus, with maximal localisation at 5-10 min. Combined immunoprecipitation-mass spectrometry analysis of RL95-2 whole cell lysates identified 40 novel GHR binding partners, including the transcriptional regulator, HMGN1. Moreover, microarray analysis demonstrated that the gene targets of HMGN1 were differentially expressed following GH treatment, and co-immunoprecipitation showed that HMGN1 associates with the GHR in the nucleus. Therefore, our results suggest that GHR nuclear translocation might mediate GH actions via interaction with chromatin factors that then drive changes in specific downstream transcriptional programs.

linc-mipep and linc-wrb encode micropeptides that regulate chromatin accessibility in vertebrate-specific neural cells

Thousands of long intergenic non-coding RNAs (lincRNAs) are transcribed throughout the vertebrate genome. A subset of lincRNAs enriched in developing brains have recently been found to contain cryptic open-reading frames and are speculated to encode micropeptides. However, systematic identification and functional assessment of these transcripts have been hindered by technical challenges caused by their small size. Here, we show that two putative lincRNAs (linc-mipep, also called lnc-rps25, and linc-wrb) encode micropeptides with homology to the vertebrate-specific chromatin architectural protein, Hmgn1, and demonstrate that they are required for development of vertebrate-specific brain cell types. Specifically, we show that NMDA receptor-mediated pathways are dysregulated in zebrafish lacking these micropeptides and that their loss preferentially alters the gene regulatory networks that establish cerebellar cells and oligodendrocytes - evolutionarily newer cell types that develop postnatally in humans. These findings reveal a key missing link in the evolution of vertebrate brain cell development and illustrate a genetic basis for how some neural cell types are more susceptible to chromatin disruptions, with implications for neurodevelopmental disorders and disease.

H2A Ubiquitination Alters H3-tail Dynamics on Linker-DNA to Enhance H3K27 Methylation

Polycomb repressive complex 1 (PRC1) and PRC2 are responsible for epigenetic gene regulation. PRC1 ubiquitinates histone H2A (H2Aub), which subsequently promotes PRC2 to introduce the H3 lysine 27 tri-methyl (H3K27me3) repressive chromatin mark. Although this mechanism provides a link between the two key transcriptional repressors, PRC1 and PRC2, it is unknown how histone-tail dynamics contribute to this process. Here, we have examined the effect of H2A ubiquitination and linker-DNA on H3-tail dynamics and H3K27 methylation by PRC2. In naïve nucleosomes, the H3-tail dynamically contacts linker DNA in addition to core DNA, and the linker-DNA is as important for H3K27 methylation as H2A ubiquitination. H2A ubiquitination alters contacts between the H3-tail and DNA to improve the methyltransferase activity of the PRC2-AEBP2-JARID2 complex. Collectively, our data support a model in which H2A ubiquitination by PRC1 synergizes with linker-DNA to hold H3 histone tails poised for their methylation by PRC2-AEBP2-JARID2.

Metformin Induces Apoptosis in Human Pancreatic Cancer (PC) Cells Accompanied by Changes in the Levels of Histone Acetyltransferases (Particularly, p300/CBP-Associated Factor (PCAF) Protein Levels)

Accumulating evidence (mainly from experimental research) suggests that metformin possesses anticancer properties through the induction of apoptosis and inhibition of the growth and proliferation of cancer cells. However, its effect on the enzymes responsible for histone acetylation status, which plays a key role in carcinogenesis, remains unclear. Therefore, the aim of our study was to evaluate the impact of metformin on histone acetyltransferases (HATs) (i.e., p300/CBP-associated factor (PCAF), p300, and CBP) and on histone deacetylases (HDACs) (i.e., SIRT-1 in human pancreatic cancer (PC) cell lines, 1.2B4, and PANC-1). The cells were exposed to metformin, an HAT inhibitor (HATi), or a combination of an HATi with metformin for 24, 48, or 72 h. Cell viability was determined using an MTT assay, and the percentage of early apoptotic cells was determined with an Annexin V-Cy3 Apoptosis Detection Assay Kit. Caspase-9 activity was also assessed. SIRT-1, PCAF, p300, and CBP expression were determined at the mRNA and protein levels using RT-PCR and Western blotting methods, respectively. Our results reveal an increase in caspase-9 in response to the metformin, indicating that it induced the apoptotic death of both 1.2B4 and PANC-1 cells. The number of cells in early apoptosis and the activity of caspase-9 decreased when treated with an HATi alone or a combination of an HATi with metformin, as compared to metformin alone. Moreover, metformin, an HATi, and a combination of an HATi with metformin also modified the mRNA expression of SIRT-1, PCAF, CBP, and p300. However, metformin did not change the expression of the studied genes in 1.2B4 cells. The results of the Western blot analysis showed that metformin diminished the protein expression of PCAF in both the 1.2B4 and PANC-1 cells. Hence, it appears possible that PCAF may be involved in the metformin-mediated apoptosis of PC cells.

Eugenol attenuates ischemia-mediated oxidative stress in cardiomyocytes via acetylation of histone at H3K27

Despite clinical advances, ischemia-induced cardiac diseases remain an underlying cause of death worldwide. Epigenetic modifications, especially alterations in the acetylation of histone proteins play a pivotal role in counteracting stressful conditions, including ischemia. In our study, we found that histone active mark H3K27ac was significantly reduced and histone repressive mark H3K27me3 was significantly upregulated in the cardiomyocytes exposed to the ischemic condition. Then, we performed a high throughput drug screening assay using rat ventricular cardiomyocytes during the ischemic condition and screened an antioxidant compound library comprising of 84 drugs for H3K27ac by fluorescence microscopy. Our data revealed that most of the phenolic compounds like eugenol, apigenin, resveratrol, bis-demethoxy curcumin, D-gamma-tocopherol, ambroxol, and non-phenolic compounds like l-Ergothioneine, ciclopirox ethanolamine, and Tanshinone IIA have a crucial role in maintaining the cellular H3K27ac histone marks during the ischemic condition. Further, we tested the role of eugenol on cellular protection during ischemia. Our study shows that ischemia significantly reduces cellular viability and increases total reactive oxygen species (ROS), and mitochondrial ROS in the cells. Interestingly, eugenol treatment significantly restores the cellular acetylation at H3K27, decreases cellular ROS, and improves cellular viability. To explore the mechanism of eugenol-medicated inhibition of deacetylation, we performed a RNAseq experiment. Analysis of transcriptome data using IPA indicated that eugenol regulates several cellular functions associated with cardiovascular diseases, and metabolic processes. Further, we found that eugenol regulates the expression of HMGN1, CD151 and Ppp2ca genes during ischemia. Furthermore, we found that eugenol might protect the cells from ischemia through modulation of HMGN1 protein expression, which plays an active role in regulation of histone acetylation and cellular protection during stress. Thus, our study indicated that eugenol can be exploited as an agent to protect the ischemic cells and also could be used to develop a novel drug for treating cardiac disease.

Expression, tumor immune infiltration, and prognostic impact of HMGs in gastric cancer

Background

In the past decade, considerable research efforts on gastric cancer (GC) have been expended, however, little advancement has been made owing to the lack of effective biomarkers and treatment options. Herein, we aimed to examine the levels of expression, mutations, and clinical relevance of HMGs in GC to provide sufficient scientific evidence for clinical decision-making and risk management.

Methods

GC samples were obtained from The Cancer Genome Atlas (TCGA). University of California Santa Cruz (UCSC) XENA, Human Protein Atlas (HPA), Gene Expression Profiling Interactive Analysis (GEPIA), Kaplan-Meier Plotter, cBioPortal, GeneMANIA, STRING, LinkedOmics, and DAVID databases were employed. The "ggplot2" package in the R software (×64 3.6.3) was used to thoroughly analyze the effects of HMGs. qRT-PCR was performed to assess HMG levels in GC cell lines.

Results

A total of 375 GC tissues and 32 paraneoplastic tissues were analyzed. The levels of HMGA1, HMGA2, HMGB1, HMGB2, HMGB3, HMGN1, HMGN2, and HMGN4 expression were increased in GC tissues relative to normal gastric tissues. HMGA1, HMGA2, HMGB1, HMGB2, and HMGB3 were highly expressed in GC cell lines. The OS was significantly different in the group showing low expressions of HMGA1, HMGA2, HMGB1, HMGB2, HMGB3, HMGN2, HMGN3, and HMGN5. There was a significant difference in RFS between the groups with low HMGA2, HMGB3, and high HMGN2 expression. The levels of HMGA2, HMGB3, and HMGN1 had a higher accuracy for prediction to distinguish GC from normal tissues (AUC value > 0.9). HMGs were tightly associated with immune infiltration and tumor immune escape and antitumor immunity most likely participates in HMG-mediated oncogenesis in GC. GO and KEGG enrichment analyses showed that HMGs played a vital role in the cell cycle pathway.

Conclusions

Our results strongly suggest a vital role of HMGs in GC. HMGA2 and HMGB3 could be potential markers for prognostic prediction and treatment targets for GC by interrupting the cell cycle pathway. Our findings might provide renewed perspectives for the selection of prognostic biomarkers among HMGs in GC and may contribute to the determination of the optimal strategy for the treatment of these patients.

HMGN2 represses gene transcription via interaction with transcription factors Lef-1 and Pitx2 during amelogenesis

The chromatin-associated high mobility group protein N2 (HMGN2) cofactor regulates transcription factor activity through both chromatin and protein interactions. Hmgn2 expression is known to be developmentally regulated, but the post-transcriptional mechanisms that regulate Hmgn2 expression and its precise roles in tooth development remain unclear. Here, we demonstrate that HMGN2 inhibits the activity of multiple transcription factors as a general mechanism to regulate early development. Bimolecular fluorescence complementation, pull-down, and coimmunoprecipitation assays show that HMGN2 interacts with the transcription factor Lef-1 through its HMG-box domain as well as with other early development transcription factors, Dlx2, FoxJ1, and Pitx2. Furthermore, EMSAs demonstrate that HMGN2 binding to Lef-1 inhibits its DNA-binding activity. We found that Pitx2 and Hmgn2 associate with H4K5ac and H3K4me2 chromatin marks in the proximal Dlx2 promoter, demonstrating Hmgn2 association with open chromatin. In addition, we demonstrate that microRNAs (miRs) mir-23a and miR-23b directly target Hmgn2, promoting transcriptional activation at several gene promoters, including the amelogenin promoter. In vivo, we found that decreased Hmgn2 expression correlates with increased miR-23 expression in craniofacial tissues as the murine embryo develops. Finally, we show that ablation of Hmgn2 in mice results in increased amelogenin expression because of increased Pitx2, Dlx2, Lef-1, and FoxJ1 transcriptional activity. Taken together, our results demonstrate both post-transcriptional regulation of Hmgn2 by miR-23a/b and post-translational regulation of gene expression by Hmgn2–transcription factor interactions. We conclude that HMGN2 regulates tooth development through its interaction with multiple transcription factors.

HMGA1, Moonlighting Protein Function, and Cellular Real Estate: Location, Location, Location!

The gene encoding the High Mobility Group A1 (HMGA1) chromatin remodeling protein is upregulated in diverse cancers where high levels portend adverse clinical outcomes. Until recently, HMGA1 was assumed to be a nuclear protein exerting its role in cancer by transcriptionally modulating gene expression and downstream signaling pathways. However, the discovery of an extracellular HMGA1-RAGE autocrine loop in invasive triple-negative breast cancer (TNBC) cell lines implicates HMGA1 as a "moonlighting protein" with different functions depending upon cellular location. Here, we review the role of HMGA1, not only as a chromatin regulator in cancer and stem cells, but also as a potential secreted factor that drives tumor progression. Prior work found that HMGA1 is secreted from TNBC cell lines where it signals through the receptor for advanced glycation end products (RAGE) to foster phenotypes involved in tumor invasion and metastatic progression. Studies in primary TNBC tumors also suggest that HMGA1 secretion associates with distant metastasis in TNBC. Given the therapeutic potential to target extracellular proteins, further work to confirm this role in other contexts is warranted. Indeed, crosstalk between nuclear and secreted HMGA1 could change our understanding of tumor development and reveal novel therapeutic opportunities relevant to diverse human cancers overexpressing HMGA1.

The regulatory effect of acetylation of HMGN2 and H3K27 on pyocyanin-induced autophagy in macrophages by affecting Ulk1 transcription

Pyocyanin (PYO) is a major virulence factor secreted by Pseudomonas aeruginosa, and autophagy is a crucial homeostatic mechanism for the interaction between the pathogens and the host. It remains unknown whether PYO leads to autophagy in macrophages by regulating histone acetylation. The high mobility group nucleosomal binding domain 2 (HMGN2) has been reported to regulate the PYO‐induced autophagy and oxidative stress in the epithelial cells; however, the underlying molecular mechanism has not been fully elucidated. In this study, PYO was found to induce autophagy in macrophages, and the mechanism might be correlated with the up‐regulation of HMGN2 acetylation (HMGN2ac) and the down‐regulation of H3K27 acetylation (H3K27ac) by modulation of the activities of acetyltransferases and deacetylases. Moreover, we further demonstrated that the up‐regulated HMGN2ac enhances its recruitment to the Ulk1 promoter, while the down‐regulation of H3K27ac reduces its recruitment to the Ulk1 promoter, thereby promoting or inhibiting the transcription of Ulk1. In conclusion, HMGN2ac and H3K27ac play regulatory roles in the PYO‐induced autophagy in macrophages.

Site-specific modification and segmental isotope labelling of HMGN1 reveals long-range conformational perturbations caused by posttranslational modifications

Interactions between histones, which package DNA in eukaryotes, and nuclear proteins such as the high mobility group nucleosome-binding protein HMGN1 are important for regulating access to DNA. HMGN1 is a highly charged and intrinsically disordered protein (IDP) that is modified at several sites by posttranslational modifications (PTMs) - acetylation, phosphorylation and ADP-ribosylation. These PTMs are thought to affect cellular localisation of HMGN1 and its ability to bind nucleosomes; however, little is known about how these PTMs regulate the structure and function of HMGN1 at a molecular level. Here, we combine the chemical biology tools of protein semi-synthesis and site-specific modification to generate a series of unique HMGN1 variants bearing precise PTMs at their N- or C-termini with segmental isotope labelling for NMR spectroscopy. With access to these precisely-defined variants, we show that PTMs in both the N- and C-termini cause changes in the chemical shifts and conformational populations in regions distant from the PTM sites; up to 50-60 residues upstream of the PTM site. The PTMs investigated had only minor effects on binding of HMGN1 to nucleosome core particles, suggesting that they have other regulatory roles. This study demonstrates the power of combining protein semi-synthesis for introduction of site-specific PTMs with segmental isotope labelling for structural biology, allowing us to understand the role of PTMs with atomic precision, from both structural and functional perspectives.

Chromatin accessibility promotes hematopoietic and leukemia stem cell activity

Chromatin organization is a highly orchestrated process that influences gene expression, in part by modulating access of regulatory factors to DNA and nucleosomes. Here, we report that the chromatin accessibility regulator HMGN1, a target of recurrent DNA copy gains in leukemia, controls myeloid differentiation. HMGN1 amplification is associated with increased accessibility, expression, and histone H3K27 acetylation of loci important for hematopoietic stem cells (HSCs) and leukemia, such as HoxA cluster genes. In vivo, HMGN1 overexpression is linked to decreased quiescence and increased HSC activity in bone marrow transplantation. HMGN1 overexpression also cooperates with the AML-ETO9a fusion oncoprotein to impair myeloid differentiation and enhance leukemia stem cell (LSC) activity. Inhibition of histone acetyltransferases CBP/p300 relieves the HMGN1-associated differentiation block. These data nominate factors that modulate chromatin accessibility as regulators of HSCs and LSCs, and suggest that targeting HMGN1 or its downstream effects on histone acetylation could be therapeutically active in AML.

Maintenance of active chromatin states by HMGN2 is required for stem cell identity in a pluripotent stem cell model

Background

Members of the HMGN protein family modulate chromatin structure and influence epigenetic modifications. HMGN1 and HMGN2 are highly expressed during early development and in the neural stem/progenitor cells of the developing and adult brain. Here, we investigate whether HMGN proteins contribute to the chromatin plasticity and epigenetic regulation that is essential for maintaining pluripotency in stem cells.

Results

We show that loss of Hmgn1 or Hmgn2 in pluripotent embryonal carcinoma cells leads to increased levels of spontaneous neuronal differentiation. This is accompanied by the loss of pluripotency markers Nanog and Ssea1, and increased expression of the pro-neural transcription factors Neurog1 and Ascl1. Neural stem cells derived from these Hmgn-knockout lines also show increased spontaneous neuronal differentiation and Neurog1 expression. The loss of HMGN2 leads to a global reduction in H3K9 acetylation, and disrupts the profile of H3K4me3, H3K9ac, H3K27ac and H3K122ac at the Nanog and Oct4 loci. At endodermal/mesodermal genes, Hmgn2-knockout cells show a switch from a bivalent to a repressive chromatin configuration. However, at neuronal lineage genes whose expression is increased, no epigenetic changes are observed and their bivalent states are retained following the loss of HMGN2.

Conclusions

We conclude that HMGN1 and HMGN2 maintain the identity of pluripotent embryonal carcinoma cells by optimising the pluripotency transcription factor network and protecting the cells from precocious differentiation. Our evidence suggests that HMGN2 regulates active and bivalent genes by promoting an epigenetic landscape of active histone modifications at promoters and enhancers.

Trisomy of a Down Syndrome Critical Region Globally Amplifies Transcription via HMGN1 Overexpression

Down syndrome (DS, trisomy 21) is associated with developmental abnormalities and increased leukemia risk. To reconcile chromatin alterations with transcriptome changes, we performed paired exogenous spike-in normalized RNA and chromatin immunoprecipitation sequencing in DS models. Absolute normalization unmasks global amplification of gene expression associated with trisomy 21. Overexpression of the nucleosome binding protein HMGN1 (encoded on chr21q22) recapitulates transcriptional changes seen with triplication of a Down syndrome critical region on distal chromosome 21, and HMGN1 is necessary for B cell phenotypes in DS models. Absolute exogenous-normalized chromatin immunoprecipitation sequencing (ChIP-Rx) also reveals a global increase in histone H3K27 acetylation caused by HMGN1. Transcriptional amplification downstream of HMGN1 is enriched for stage-specific programs of B cells and B cell acute lymphoblastic leukemia, dependent on the developmental cellular context. These data offer a mechanistic explanation for DS transcriptional patterns and suggest that further study of HMGN1 and RNA amplification in diverse DS phenotypes is warranted.

How trisomy 21 contributes to Down syndrome phenotypes, including increased leukemia risk, is not well understood. Mowery et al. use per-cell normalization approaches to reveal global transcriptional amplification in Down syndrome models. HMGN1 overexpression is sufficient to induce these alterations and promotes lineage-associated transcriptional programs, signaling, and B cell progenitor phenotypes.

First proteomic analysis of diestrus and anestrus canine oocytes at the germinal vesicle reveals candidate proteins involved in oocyte meiotic competence

In domestic dogs, oocyte maturation rates are low and the percentage of oocytes that remain in the stage of germinal vesicle (GV) regardless of culture conditions is high. The present study was conducted to characterize the proteome of canine oocyte at the germinal vesicle stage using label-free mass spectrometry. Ovaries were collected from 415 adult domestic dogs and oocytes were divided anestrus and diestrus group. Protein lysates were subjected to quantitative proteomic analysis to identify differentially expressed proteins in different status reproductive. All runs for each sample were performed on an Easy nLC1000 nano-LC chromatograph system directly connected to a quadrupole-type Orbitrap mass spectrometer. For identification of peptides and proteins, raw data of the spectra were loaded into MaxQuant software version 1.5.2.8. Proteomic data were analysed according to gene ontology and a protein-protein interaction network. 312 proteins were identified and grouped according to their biological processes, molecular functions and cellular component. Forty-six differentially expressed proteins among diestrus and control group were associated with at least one GO term in the biological process database. Several proteins involved in the cell cycle, fertilization, regulation of transcription and signalling pathways that are essential for the full development of oocytes and fertilization were expressed. This study identified proteins that were absent, and more or less expressed in different status reproductive. These differentially expressed proteins revealed a framework of molecular reorganization within a GV that renders its competency. This knowledge will enable the identification of target competence biomarkers and thus the establishment of more adequate means of cultivation to improve the M-I and II indexes in this species and also to better understand the physiology of the domestic dog, promoting the development of new reproduction biotechniques.

Sex-specific differences in the intracellular proteome of human endothelial cells from dizygotic twins

Differences between men and women are being continuously identified in many human diseases. The underlying reasons are not yet fully understood. Beside the influence of endogenous hormones and life style, intrinsic sex-specific dimorphisms at the cellular level may also play a role. HUVECs from twin pairs of opposite sex provide an excellent tool to address the question of sex-specific differences at the molecular level. We compared for the first time protein levels of male and female HUVECs from dizygotic twins using a proteomic approach. To investigate differences under basal and stress conditions, cells were either left untreated or wounded and serum starved for different time points. Approximately 10% of all proteins monitored showed significant sexual dimorphisms in their level under the different conditions tested. The majority of the proteins displayed a higher abundance in female cells. The magnitude of the difference in protein levels between male and female cells was rather small. The most prominent differences throughout all conditions were observed for several X-chromosome encoded proteins with higher levels in female (UBA1, HDHD1) or in male cells (G6PD). Proteins involved in basic cellular processes, such as gene expression and translation (e.g. HMGN1, SRP54) displayed sex-specific levels in particular conditions only. SIGNIFICANCE: This study provides novel insights into sexual dimorphic protein levels in HUVECs from twin pairs of the opposite sex. The findings identify proteins with sex-specific differences in their levels under different cell culture conditions. The study also highlights the presence of X-chromosome encoded proteins escaping X-chromosomal inactivation. The results emphasize the need to consider the cellular sex of male and female HUVECs in in vitro experiments.

Proteomic analysis of germinal vesicles in the domestic cat model reveals candidate nuclear proteins involved in oocyte competence acquisition

STUDY QUESTION

Do nuclear proteins in the germinal vesicle (GV) contribute to oocyte competence acquisition during folliculogenesis?

SUMMARY ANSWER

Proteomic analysis of GVs identified candidate proteins for oocyte competence acquisition, including a key RNA processing protein-heterogeneous nuclear ribonucleoprotein A2/B1 (hnRNPA2B1).

WHAT IS KNOWN ALREADY

The domestic cat GV, which is physiologically similar to the human GV, gains the intrinsic ability to resume meiosis and support early embryo development during the pre-antral-to-antral follicle transition. However, little is known about nuclear proteins that contribute to this developmental process.

STUDY DESIGN SIZE, DURATION

GVs were enriched from pre-antral (incompetent) and antral (competent) follicles from 802 cat ovaries. Protein lysates were subjected to quantitative proteomic analysis to identify differentially expressed proteins in GVs from the two follicular categories.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Two biological replicates (from independent pools of ovaries) of pre-antral versus antral samples were labeled by tandem mass tags and then assessed by liquid chromatography-tandem mass spectrometry. Proteomic data were analyzed according to gene ontology and a protein-protein interaction network. Immunofluorescent staining and protein inhibition assays were used for validation.

MAIN RESULTS AND THE ROLE OF CHANCE

A total of 174 nuclear proteins was identified, with 54 being up-regulated and 22 down-regulated (≥1.5-fold) after antrum formation. Functional protein analysis through gene ontology over-representation tests revealed that changes in molecular network within the GVs during this transitional phase were related to chromatin reorganization, gene transcription, and maternal RNA processing and storage. Protein inhibition assays verified that hnRNPA2B1, a key nuclear protein identified, was required for oocyte meiotic maturation and subsequent blastocyst formation.

LARGE SCALE DATA

Data are available via ProteomeXchange with identifier PXD007211.

LIMITATIONS REASONS FOR CAUTION

Proteins identified by proteomic comparison may (i) be involved in processes other than competence acquisition during the pre-antral-to-antral transition or (ii) be co-expressed in other macrostructures besides the GV. Expressional and functional validations should be performed for candidate proteins before downstream application.

WIDER IMPLICATIONS OF THE FINDINGS

Collective results generated a blueprint to better understand the molecular mechanisms involved in GV competence acquisition and identified potential nuclear competence markers for human fertility preservation.

STUDY FUNDING AND COMPETING INTEREST(S)

Funded by the National Center for Research Resources (R01 RR026064), a component of the National Institutes of Health (NIH) and currently by the Office of Research Infrastructure Programs/Office of the Director (R01 OD010948). The authors declare that there is no conflict of interest.

Precision medicine approaches may be the future for CRLF2 rearranged Down Syndrome Acute Lymphoblastic Leukaemia patients

Breakthrough studies over the past decade have uncovered unique gene fusions implicated in acute lymphoblastic leukaemia (ALL). The critical gene, cytokine receptor-like factor 2 (CRLF2), is rearranged in 5-16% of B-ALL, comprising 50% of Philadelphia-like ALL and cooperates with genomic lesions in the Jak, Mapk and Ras signalling pathways. Children with Down Syndrome (DS) have a predisposition to developing CRLF2 rearranged-ALL which is observed in 60% of DS-ALL patients. These patients experience a poor survival outcome. Mutations of genes involved in epigenetic regulation are more prevalent in DS-ALL patients than non-DS ALL patients, highlighting the potential for alternative treatment strategies. DS-ALL patients also suffer greater treatment-related toxicity from current ALL treatment regimens compared to non-DS-ALL patients. An increased gene dosage of critical genes on chromosome 21 which have roles in purine synthesis and folate transport may contribute. As the genomic landscape of DS-ALL patients is different to non-DS-ALL patients, targeted therapies for individual lesions may improve outcomes. Therapeutically targeting each rearrangement with targeted or combination therapy that will perturb the transforming signalling pathways will likely improve the poor survival rates of this subset of patients.

What happens at the lesion does not stay at the lesion: Transcription-coupled nucleotide excision repair and the effects of DNA damage on transcription in cis and trans

Unperturbed transcription of eukaryotic genes by RNA polymerase II (Pol II) is crucial for proper cell function and tissue homeostasis. However, the DNA template of Pol II is continuously challenged by damaging agents that can result in transcription impediment. Stalling of Pol II on transcription-blocking lesions triggers a highly orchestrated cellular response to cope with these cytotoxic lesions. One of the first lines of defense is the transcription-coupled nucleotide excision repair (TC-NER) pathway that specifically removes transcription-blocking lesions thereby safeguarding unperturbed gene expression. In this perspective, we outline recent data on how lesion-stalled Pol II initiates TC-NER and we discuss new mechanistic insights in the TC-NER reaction, which have resulted in a better understanding of the causative-linked Cockayne syndrome and UV-sensitive syndrome. In addition to these direct effects on lesion-stalled Pol II (effects in cis), accumulating evidence shows that transcription, and particularly Pol II, is also affected in a genome-wide manner (effects in trans). We will summarize the diverse consequences of DNA damage on transcription, including transcription inhibition, induction of specific transcriptional programs and regulation of alternative splicing. Finally, we will discuss the function of these diverse cellular responses to transcription-blocking lesions and their consequences on the process of transcription restart. This resumption of transcription, which takes place either directly at the lesion or is reinitiated from the transcription start site, is crucial to maintain proper gene expression following removal of the DNA damage.

High-mobility group nucleosome binding domain 1 (HMGN1) functions as a Th1-polarizing alarmin

High-mobility group (HMG) nucleosome binding domain 1 (HMGN1), which previously was thought to function only as a nucleosome-binding protein that regulates chromatin structure, histone modifications, and gene expression, was recently discovered to be an alarmin that contributes extracellularly to the generation of innate and adaptive immune responses. HMGN1 promotes DC recruitment through interacting with a Gαi protein-coupled receptor (GiPCR) and activates DCs predominantly through triggering TLR4. HMGN1 preferentially promotes Th1-type immunity, which makes it relevant for the fields of vaccinology, autoimmunity, and oncoimmunology. Here, we discuss the alarmin properties of HMGN1 and update recent advances on its roles in immunity and potential applications for immunotherapy of tumors.

Lessons from the Crypt: HMGA1-Amping up Wnt for Stem Cells and Tumor Progression

High mobility group A1 (HMGA1) chromatin remodeling proteins are enriched in aggressive cancers and stem cells, although their common function in these settings has remained elusive until now. Recent work in murine intestinal stem cells (ISC) revealed a novel role for Hmga1 in enhancing self-renewal by amplifying Wnt signaling, both by inducing genes expressing Wnt agonist receptors and Wnt effectors. Surprisingly, Hmga1 also "builds" a stem cell niche by upregulating Sox9, a factor required for differentiation to Paneth cells; these cells constitute an epithelial niche by secreting Wnt and other factors to support ISCs. HMGA1 is also highly upregulated in colon cancer compared with nonmalignant epithelium and SOX9 becomes overexpressed during colon carcinogenesis. Intriguingly, HMGA1 is overexpressed in diverse cancers with poor outcomes, where it regulates developmental genes. Similarly, HMGA1 induces genes responsible for pluripotency and self-renewal in embryonic stem cells. These findings demonstrate that HMGA1 maintains Wnt and other developmental transcriptional networks and suggest that HMGA1 overexpression fosters carcinogenesis and tumor progression through dysregulation of these pathways. Studies are now needed to determine more precisely how HMGA1 modulates chromatin structure to amplify developmental genes and how to disrupt this process in cancer therapy. Cancer Res; 78(8); 1890-7. ©2018 AACR.

High-Mobility Group Nucleosome-Binding Protein 1 as Endogenous Ligand Induces Innate Immune Tolerance in a TLR4-Sirtuin-1 Dependent Manner in Human Blood Peripheral Mononuclear Cells

High-mobility group nucleosome-binding protein 1 (HMGN1) functions as a non-histone chromatin-binding protein in the cell nucleus. However, extracellular HMGN1 acts as an endogenous danger-associated inflammatory mediator (also called alarmin). We demonstrated that HMGN1 not only directly stimulated cytokine production but also had the capacity to induce immune tolerance by a TLR4-dependent pathway, similar to lipopolysaccharide (LPS)-induced tolerance. HMGN1-induced tolerance was accompanied by a metabolic shift associated with the inhibition of the induction of Warburg effect (aerobic glycolysis) and histone deacetylation via Sirtuin-1. In addition, HMGN1 pre-challenge of mice also downregulated TNF production similar to LPS-induced tolerance in vivo. In conclusion, HMGN1 is an endogenous TLR4 ligand that can induce both acute stimulation of cytokine production and long-term tolerance, and thus it might play a modulatory role in sterile inflammatory processes such as those induced by infection, trauma, or ischemia.

HMGN1 and 2 remodel core and linker histone tail domains within chromatin

The structure of the nucleosome, the basic building block of the chromatin fiber, plays a key role in epigenetic regulatory processes that affect DNA-dependent processes in the context of chromatin. Members of the HMGN family of proteins bind specifically to nucleosomes and affect chromatin structure and function, including transcription and DNA repair. To better understand the mechanisms by which HMGN 1 and 2 alter chromatin, we analyzed their effect on the organization of histone tails and linker histone H1 in nucleosomes. We find that HMGNs counteract linker histone (H1)-dependent stabilization of higher order ‘tertiary’ chromatin structures but do not alter the intrinsic ability of nucleosome arrays to undergo salt-induced compaction and self-association. Surprisingly, HMGNs do not displace H1s from nucleosomes; rather these proteins bind nucleosomes simultaneously with H1s without disturbing specific contacts between the H1 globular domain and nucleosomal DNA. However, HMGNs do alter the nucleosome-dependent condensation of the linker histone C-terminal domain, which is critical for stabilizing higher-order chromatin structures. Moreover, HMGNs affect the interactions of the core histone tail domains with nucleosomal DNA, redirecting the tails to more interior positions within the nucleosome. Our studies provide new insights into the molecular mechanisms whereby HMGNs affect chromatin structure.

Elevated HMGN4 expression potentiates thyroid tumorigenesis

Thyroid cancer originates from genetic and epigenetic changes that alter gene expression and cellular signaling pathways. Here, we report that altered expression of the nucleosome-binding protein HMGN4 potentiates thyroid tumorigenesis. Bioinformatics analyses reveal increased HMGN4 expression in thyroid cancer. We find that upregulation of HMGN4 expression in mouse and human cells, and in the thyroid of transgenic mice, alters the cellular transcription profile, downregulates the expression of the tumor suppressors Atm, Atrx and Brca2, and elevates the levels of the DNA damage marker γH2AX. Mouse and human cells overexpressing HMGN4 show increased tumorigenicity as measured by colony formation, by tumor generation in nude mice, and by the formation of preneoplastic lesions in the thyroid of transgenic mice. Our study identifies a novel epigenetic factor that potentiates thyroid oncogenesis and raises the possibility that HMGN4 may serve as an additional diagnostic marker, or therapeutic target in certain thyroid cancers.

HDAC6 Deacetylates HMGN2 to Regulate Stat5a Activity and Breast Cancer Growth

Stat5a is a transcription factor utilized by several cytokine/hormone receptor signaling pathways that promotes transcription of genes associated with proliferation, differentiation, and survival of cancer cells. However, there are currently no clinically approved therapies that directly target Stat5a, despite ample evidence that it contributes to breast cancer pathogenesis. Here, deacetylation of the Stat5a coactivator and chromatin-remodeling protein HMGN2 on lysine residue K2 by HDAC6 promotes Stat5a-mediated transcription and breast cancer growth. HDAC6 inhibition both in vitro and in vivo enhances HMGN2 acetylation with a concomitant reduction in Stat5a-mediated signaling, resulting in an inhibition of breast cancer growth. Furthermore, HMGN2 is highly acetylated at K2 in normal human breast tissue, but is deacetylated in primary breast tumors and lymph node metastases, suggesting that targeting HMGN2 deacetylation is a viable treatment for breast cancer. Together, these results reveal a novel mechanism by which HDAC6 activity promotes the transcription of Stat5a target genes and demonstrate utility of HDAC6 inhibition for breast cancer therapy.

IMPLICATIONS

HMGN2 deacetylation enhances Stat5a transcriptional activity, thereby regulating prolactin-induced gene transcription and breast cancer growth. Mol Cancer Res; 14(10); 994-1008. ©2016 AACR.

The biology, pathogenesis and clinical aspects of acute lymphoblastic leukemia in children with Down syndrome

Children with Down syndrome (DS) are at a 20-fold increased risk for acute lymphoblastic leukemia (DS-ALL). Although the etiology of this higher risk of developing leukemia remains largely unclear, the recent identification of CRLF2 (cytokine receptor like factor 2) and JAK2 mutations and study of the effect of trisomy of Hmgn1 and Dyrk1a (dual-specificity tyrosine phosphorylation-regulated kinase 1A) on B-cell development have shed significant new light on the disease process. Here we focus on the clinical features, biology and genetics of ALL in children with DS. We review the unique characteristics of DS-ALL on both the clinical and molecular levels and discuss the differences in treatments and outcomes in ALL in children with DS compared with those without DS. The identification of new biological insights is expected to pave the way for novel targeted therapies.

HMGN proteins modulate chromatin regulatory sites and gene expression during activation of naïve B cells

The activation of naïve B lymphocyte involves rapid and major changes in chromatin organization and gene expression; however, the complete repertoire of nuclear factors affecting these genomic changes is not known. We report that HMGN proteins, which bind to nucleosomes and affect chromatin structure and function, co-localize with, and maintain the intensity of DNase I hypersensitive sites genome wide, in resting but not in activated B cells. Transcription analyses of resting and activated B cells from wild-type and Hmgn^−/− mice, show that loss of HMGNs dampens the magnitude of the transcriptional response and alters the pattern of gene expression during the course of B-cell activation; defense response genes are most affected at the onset of activation. Our study provides insights into the biological function of the ubiquitous HMGN chromatin binding proteins and into epigenetic processes that affect the fidelity of the transcriptional response during the activation of B cell lymphocytes.

Functional interplay between histone H1 and HMG proteins in chromatin

The dynamic interaction of nucleosome binding proteins with their chromatin targets is an important element in regulating the structure and function of chromatin. Histone H1 variants and High Mobility Group (HMG) proteins are ubiquitously expressed in all vertebrate cells, bind dynamically to chromatin, and are known to affect chromatin condensation and the ability of regulatory factors to access their genomic binding sites. Here, we review the studies that focus on the interactions between H1 and HMGs and highlight the functional consequences of the interplay between these architectural chromatin binding proteins. H1 and HMG proteins are mobile molecules that bind to nucleosomes as members of a dynamic protein network. All HMGs compete with H1 for chromatin binding sites, in a dose dependent fashion, but each HMG family has specific effects on the interaction of H1 with chromatin. The interplay between H1 and HMGs affects chromatin organization and plays a role in epigenetic regulation.

High mobility group (HMG) proteins: Modulators of chromatin structure and DNA repair in mammalian cells

It has been almost a decade since the last review appeared comparing and contrasting the influences that the different families of High Mobility Group proteins (HMGA, HMGB and HMGN) have on the various DNA repair pathways in mammalian cells. During that time considerable progress has been made in our understanding of how these non-histone proteins modulate the efficiency of DNA repair by all of the major cellular pathways: nucleotide excision repair, base excision repair, double-stand break repair and mismatch repair. Although there are often similar and over-lapping biological activities shared by all HMG proteins, members of each of the different families appear to have a somewhat 'individualistic' impact on various DNA repair pathways. This review will focus on what is currently known about the roles that different HMG proteins play in DNA repair processes and discuss possible future research areas in this rapidly evolving field.

High mobility group nucleosome-binding family proteins promote astrocyte differentiation of neural precursor cells

Astrocytes are the most abundant cell type in the mammalian brain and are important for the functions of the central nervous system. Although previous studies have shown that the STAT signaling pathway or its regulators promote the generation of astrocytes from multipotent neural precursor cells (NPCs) in the developing mammalian brain, the molecular mechanisms that regulate the astrocytic fate decision have still remained largely unclear. Here, we show that the high mobility group nucleosome-binding (HMGN) family proteins, HMGN1, 2, and 3, promote astrocyte differentiation of NPCs during brain development. HMGN proteins were expressed in NPCs, Sox9(+) glial progenitors, and GFAP(+) astrocytes in perinatal and adult brains. Forced expression of either HMGN1, 2, or 3 in NPCs in cultures or in the late embryonic neocortex increased the generation of astrocytes at the expense of neurons. Conversely, knockdown of either HMGN1, 2, or 3 in NPCs suppressed astrocyte differentiation and promoted neuronal differentiation. Importantly, overexpression of HMGN proteins did not induce the phosphorylation of STAT3 or activate STAT reporter genes. In addition, HMGN family proteins did not enhance DNA demethylation and acetylation of histone H3 around the STAT-binding site of the gfap promoter. Moreover, knockdown of HMGN family proteins significantly reduced astrocyte differentiation induced by gliogenic signal ciliary neurotrophic factor, which activates the JAK-STAT pathway. Therefore, we propose that HMGN family proteins are novel chromatin regulatory factors that control astrocyte fate decision/differentiation in parallel with or downstream of the JAK-STAT pathway through modulation of the responsiveness to gliogenic signals.

Functional compensation among HMGN variants modulates the DNase I hypersensitive sites at enhancers

DNase I hypersensitive sites (DHSs) are a hallmark of chromatin regions containing regulatory DNA such as enhancers and promoters; however, the factors affecting the establishment and maintenance of these sites are not fully understood. We now show that HMGN1 and HMGN2, nucleosome-binding proteins that are ubiquitously expressed in vertebrate cells, maintain the DHS landscape of mouse embryonic fibroblasts (MEFs) synergistically. Loss of one of these HMGN variants led to a compensatory increase of binding of the remaining variant. Genome-wide mapping of the DHSs in Hmgn1(-/-), Hmgn2(-/-), and Hmgn1(-/-)n2(-/-) MEFs reveals that loss of both, but not a single HMGN variant, leads to significant remodeling of the DHS landscape, especially at enhancer regions marked by H3K4me1 and H3K27ac. Loss of HMGN variants affects the induced expression of stress-responsive genes in MEFs, the transcription profiles of several mouse tissues, and leads to altered phenotypes that are not seen in mice lacking only one variant. We conclude that the compensatory binding of HMGN variants to chromatin maintains the DHS landscape, and the transcription fidelity and is necessary to retain wild-type phenotypes. Our study provides insight into mechanisms that maintain regulatory sites in chromatin and into functional compensation among nucleosome binding architectural proteins.

HMGB1 in health and disease

Complex genetic and physiological variations as well as environmental factors that drive emergence of chromosomal instability, development of unscheduled cell death, skewed differentiation, and altered metabolism are central to the pathogenesis of human diseases and disorders. Understanding the molecular bases for these processes is important for the development of new diagnostic biomarkers, and for identifying new therapeutic targets. In 1973, a group of non-histone nuclear proteins with high electrophoretic mobility was discovered and termed high-mobility group (HMG) proteins. The HMG proteins include three superfamilies termed HMGB, HMGN, and HMGA. High-mobility group box 1 (HMGB1), the most abundant and well-studied HMG protein, senses and coordinates the cellular stress response and plays a critical role not only inside of the cell as a DNA chaperone, chromosome guardian, autophagy sustainer, and protector from apoptotic cell death, but also outside the cell as the prototypic damage associated molecular pattern molecule (DAMP). This DAMP, in conjunction with other factors, thus has cytokine, chemokine, and growth factor activity, orchestrating the inflammatory and immune response. All of these characteristics make HMGB1 a critical molecular target in multiple human diseases including infectious diseases, ischemia, immune disorders, neurodegenerative diseases, metabolic disorders, and cancer. Indeed, a number of emergent strategies have been used to inhibit HMGB1 expression, release, and activity in vitro and in vivo. These include antibodies, peptide inhibitors, RNAi, anti-coagulants, endogenous hormones, various chemical compounds, HMGB1-receptor and signaling pathway inhibition, artificial DNAs, physical strategies including vagus nerve stimulation and other surgical approaches. Future work further investigating the details of HMGB1 localization, structure, post-translational modification, and identification of additional partners will undoubtedly uncover additional secrets regarding HMGB1's multiple functions.

Triplication of a 21q22 region contributes to B cell transformation through HMGN1 overexpression and loss of histone H3 Lys27 trimethylation

Down syndrome confers a 20-fold increased risk of B cell acute lymphoblastic leukemia (B-ALL)¹ and polysomy 21 is the most frequent somatic aneuploidy amongst all B-ALLs². Yet, the mechanistic links between chr.21 triplication and B-ALL remain undefined. Here we show that germline triplication of only 31 genes orthologous to human chr.21q22 confers murine progenitor B cell self-renewal in vitro, maturation defects in vivo, and B-ALL with either BCR-ABL or CRLF2 with activated JAK2. Chr.21q22 triplication suppresses H3K27me3 in progenitor B cells and B-ALLs, and “bivalent” genes with both H3K27me3 and H3K4me3 at their promoters in wild-type progenitor B cells are preferentially overexpressed in triplicated cells. Strikingly, human B-ALLs with polysomy 21 are distinguished by their overexpression of genes marked with H3K27me3 in multiple cell types. Finally, overexpression of HMGN1, a nucleosome remodeling protein encoded on chr.21q22^3–5, suppresses H3K27me3 and promotes both B cell proliferation in vitro and B-ALL in vivo.

The transcription factor GLI1 interacts with SMAD proteins to modulate transforming growth factor β-induced gene expression in a p300/CREB-binding protein-associated factor (PCAF)-dependent manner

The biological role of the transcription factor GLI1 in the regulation of tumor growth is well established; however, the molecular events modulating this phenomenon remain elusive. Here, we demonstrate a novel mechanism underlying the role of GLI1 as an effector of TGFβ signaling in the regulation of gene expression in cancer cells. TGFβ stimulates GLI1 activity in cancer cells and requires its transcriptional activity to induce BCL2 expression. Analysis of the mechanism regulating this interplay identified a new transcriptional complex including GLI1 and the TGFβ-regulated transcription factor, SMAD4. We demonstrate that SMAD4 physically interacts with GLI1 for concerted regulation of gene expression and cellular survival. Activation of the TGFβ pathway induces GLI1-SMAD4 complex binding to the BCL2 promoter whereas disruption of the complex through SMAD4 RNAi depletion impairs GLI1-mediated transcription of BCL2 and cellular survival. Further characterization demonstrated that SMAD2 and the histone acetyltransferase, PCAF, participate in this regulatory mechanism. Both proteins bind to the BCL2 promoter and are required for TGFβ- and GLI1-stimulated gene expression. Moreover, SMAD2/4 RNAi experiments showed that these factors are required for the recruitment of GLI1 to the BCL2 promoter. Finally, we determined whether this novel GLI1 transcriptional pathway could regulate other TGFβ targets. We found that two additional TGFβ-stimulated genes, INTERLEUKIN-7 and CYCLIN D1, are dependent upon the intact GLI1-SMAD-PCAF complex for transcriptional activation. Collectively, these results define a novel epigenetic mechanism that uses the transcription factor GLI1 and its associated complex as a central effector to regulate gene expression in cancer cells.

Loss of the nucleosome-binding protein HMGN1 affects the rate of N-nitrosodiethylamine-induced hepatocarcinogenesis in mice

We report that HMGN1, a nucleosome-binding protein that affects chromatin structure and function, affects the growth of N-nitrosodiethylamine (DEN)-induced liver tumors. Following a single DEN injection at 2 weeks of age, Hmgn1(tm1/tm1) mice, lacking the nucleosome-binding domain of HMGN1, had earlier signs of liver tumorigenesis than their Hmgn1(+/+) littermates. Detailed gene expression profiling revealed significant differences between DEN-injected and control saline-injected mice, but only minor differences between the injected Hmgn1(tm1/tm1) mice and their Hmgn1(+/+) littermates. Pathway analysis revealed that the most significant process affected by loss of HMGN1 involves the lipid/sterol metabolic pathway. Our study indicates that in mice, loss of HMGN1 leads to transcription changes that accelerate the progression of DEN-induced hepatocarcinogenesis, without affecting the type of tumors or the final total tumor burden of these mice.

IMPLICATIONS

Loss of HMGN1 leads to accelerated progression of DEN-induced hepatocarcinogenesis in mice.

Mammalian transcription-coupled excision repair

Transcriptional arrest caused by DNA damage is detrimental for cells and organisms as it impinges on gene expression and thereby on cell growth and survival. To alleviate transcriptional arrest, cells trigger a transcription-dependent genome surveillance pathway, termed transcription-coupled nucleotide excision repair (TC-NER) that ensures rapid removal of such transcription-impeding DNA lesions and prevents persistent stalling of transcription. Defective TC-NER is causatively linked to Cockayne syndrome, a rare severe genetic disorder with multisystem abnormalities that results in patients' death in early adulthood. Here we review recent data on how damage-arrested transcription is actively coupled to TC-NER in mammals and discuss new emerging models concerning the role of TC-NER-specific factors in this process.

High mobility group N proteins modulate the fidelity of the cellular transcriptional profile in a tissue- and variant-specific manner

The nuclei of most vertebrate cells contain members of the high mobility group N (HMGN) protein family, which bind specifically to nucleosome core particles and affect chromatin structure and function, including transcription. Here, we study the biological role of this protein family by systematic analysis of phenotypes and tissue transcription profiles in mice lacking functional HMGN variants. Phenotypic analysis of Hmgn1(tm1/tm1), Hmgn3(tm1/tm1), and Hmgn5(tm1/tm1) mice and their wild type littermates with a battery of standardized tests uncovered variant-specific abnormalities. Gene expression analysis of four different tissues in each of the Hmgn(tm1/tm1) lines reveals very little overlap between genes affected by specific variants in different tissues. Pathway analysis reveals that loss of an HMGN variant subtly affects expression of numerous genes in specific biological processes. We conclude that within the biological framework of an entire organism, HMGNs modulate the fidelity of the cellular transcriptional profile in a tissue- and HMGN variant-specific manner.

Basic concepts of epigenetics: impact of environmental signals on gene expression

Through epigenetic modifications, specific long-term phenotypic consequences can arise from environmental influence on slowly evolving genomic DNA. Heritable epigenetic information regulates nucleosomal arrangement around DNA and determines patterns of gene silencing or active transcription. One of the greatest challenges in the study of epigenetics as it relates to disease is the enormous diversity of proteins, histone modifications and DNA methylation patterns associated with each unique maladaptive phenotype. This is further complicated by a limitless combination of environmental cues that could alter the epigenome of specific cell types, tissues, organs and systems. In addition, complexities arise from the interpretation of studies describing analogous but not identical processes in flies, plants, worms, yeast, ciliated protozoans, tumor cells and mammals. This review integrates fundamental basic concepts of epigenetics with specific focus on how the epigenetic machinery interacts and operates in continuity to silence or activate gene expression. Topics covered include the connection between DNA methylation, methyl-CpG-binding proteins, transcriptional repression complexes, histone residues, histone modifications that mediate gene repression or relaxation, histone core variant stability, H1 histone linker flexibility, FACT complex, nucleosomal remodeling complexes, HP1 and nuclear lamins.

Radiation-induced alterations in histone modification patterns and their potential impact on short-term radiation effects

Detection and repair of radiation-induced DNA damage occur in the context of chromatin. An intricate network of mechanisms defines chromatin structure, including DNA methylation, incorporation of histone variants, histone modifications, and chromatin remodeling. In the last years it became clear that the cellular response to radiation-induced DNA damage involves all of these mechanisms. Here we focus on the current knowledge on radiation-induced alterations in post-translational histone modification patterns and their effect on the chromatin accessibility, transcriptional regulation and chromosomal stability.

The chromatin-binding protein HMGN3 stimulates histone acetylation and transcription across the Glyt1 gene

HMGNs are nucleosome-binding proteins that alter the pattern of histone modifications and modulate the binding of linker histones to chromatin. The HMGN3 family member exists as two splice forms, HMGN3a which is full-length and HMGN3b which lacks the C-terminal RD (regulatory domain). In the present study, we have used the Glyt1 (glycine transporter 1) gene as a model system to investigate where HMGN proteins are bound across the locus in vivo, and to study how the two HMGN3 splice variants affect histone modifications and gene expression. We demonstrate that HMGN1, HMGN2, HMGN3a and HMGN3b are bound across the Glyt1 gene locus and surrounding regions, and are not enriched more highly at the promoter or putative enhancer. We conclude that the peaks of H3K4me3 (trimethylated Lys(4) of histone H3) and H3K9ac (acetylated Lys(9) of histone H3) at the active Glyt1a promoter do not play a major role in recruiting HMGN proteins. HMGN3a/b binding leads to increased H3K14 (Lys(14) of histone H3) acetylation and stimulates Glyt1a expression, but does not alter the levels of H3K4me3 or H3K9ac enrichment. Acetylation assays show that HMGN3a stimulates the ability of PCAF [p300/CREB (cAMP-response-element-binding protein)-binding protein-associated factor] to acetylate nucleosomal H3 in vitro, whereas HMGN3b does not. We propose a model where HMGN3a/b-stimulated H3K14 acetylation across the bodies of large genes such as Glyt1 can lead to more efficient transcription elongation and increased mRNA production.

The HMGN family of chromatin-binding proteins: dynamic modulators of epigenetic processes

The HMGN family of proteins binds to nucleosomes without any specificity for the underlying DNA sequence. They affect the global and local structure of chromatin, as well as the levels of histone modifications and thus play a role in epigenetic regulation of gene expression. This review focuses on the recent studies that provide new insights on the interactions between HMGN proteins, nucleosomes, and chromatin, and the effects of these interactions on epigenetic and transcriptional regulation. This article is part of a Special Issue entitled: Chromatin in time and space.

Barrier-to-Autointegration Factor influences specific histone modifications

Defects in the nuclear envelope or nuclear 'lamina' networks cause disease and can perturb histone posttranslational (epigenetic) regulation. Barrier-to-Autointegration Factor (BAF) is an essential but enigmatic lamina component that binds lamins, LEM-domain proteins, DNA and histone H3 directly. We report that BAF copurified with nuclease-digested mononucleosomes and associated with modified histones in vivo. BAF overexpression significantly reduced global histone H3 acetylation by 18%. In cells that stably overexpressed BAF 3-fold, silencing mark H3-K27-Me1/3 and active marks H4-K16-Ac and H4-Ac5 decreased significantly. Significant increases were also seen for silencing mark H3-K9-Me3, active marks H3-K4-Me2, H3-K9/K14-Ac and H4-K5-Ac and a mark (H3-K79-Me2) associated with both active and silent chromatin. Other increases (H3-S10-P, H3-S28-P and silencing mark H3-K9-Me2) did not reach statistical significance. BAF overexpression also significantly influenced cell cycle distribution. Moreover, BAF associated in vivo with SET/I2PP2A (protein phosphatase 2A inhibitor; blocks H3 dephosphorylation) and G9a (H3-K9 methyltransferase), but showed no detectable association with HDAC1 or HATs. These findings reveal BAF as a novel epigenetic regulator and are discussed in relation to BAF deficiency phenotypes, which include a hereditary progeria syndrome and loss of pluripotency in embryonic stem cells.

Uncovering combinatorial interactions in chromatin

The DNA in our bodies is wrapped around octamers of histone proteins to form nucleosomes. This structural organization facilitates packaging of the entire genome into a single nucleus but is also a platform for post-translational modifications which have functional roles within the cell. Over the last few years, modifications of histone residues have been identified and potential roles of individual modifications in processes such as DNA repair, replication and gene transcription have been uncovered. However, we know much less about the combinatorial action of the individual marks and how one modification impacts on the function of another. Recent developments in quantitative proteomics methodology and increasing amounts of genomic data generated using high-throughput techniques are allowing insights into how multiple modifications are interpreted by the cell.

Distinct properties of human HMGN5 reveal a rapidly evolving but functionally conserved nucleosome binding protein

The HMGN family is a family of nucleosome-binding architectural proteins that affect the structure and function of chromatin in vertebrates. We report that the HMGN5 variant, encoded by a gene located on chromosome X, is a rapidly evolving protein with an acidic C-terminal domain that differs among vertebrate species. We found that the intranuclear organization and nucleosome interactions of human HMGN5 are distinct from those of mouse HMGN5 and that the C-terminal region of the protein is the main determinant of the chromatin interaction properties. Despite their apparent differences, both mouse and human HMGN5 proteins interact with histone H1, reduce its chromatin residence time, and can induce large-scale chromatin decompaction in living cells. Analysis of HMGN5 mutants suggests that distinct domains in HMGN5 affect specific steps in the interaction of H1 with chromatin. Elevated levels of either human or mouse HMGN5 affect the transcription of numerous genes, most in a variant-specific manner. Our study identifies HMGN5 as a rapidly evolving vertebrate nuclear protein with species-specific properties. HMGN5 has a highly disordered structure, binds dynamically to nucleosome core particles, modulates the binding of H1 to chromatin, reduces the compaction of the chromatin fiber, and affects transcription.