Inner workings and regulatory inputs that control Polycomb repressive complex 2

siRNA treatment targeting integrin α11 overexpressed via EZH2-driven axis inhibits drug-resistant breast cancer progression

BACKGROUND

Breast cancer, the most prevalent cancer in women worldwide, faces treatment challenges due to drug resistance, posing a serious threat to patient survival. The present study aimed to identify the key molecules that drive drug resistance and aggressiveness in breast cancer cells and validate them as therapeutic targets.

METHODS

Transcriptome microarray and analysis using PANTHER pathway and StemChecker were performed to identify the most significantly expressed genes in tamoxifen-resistant and adriamycin-resistant MCF-7 breast cancer cells. Clinical relevance of the key genes was determined using Kaplan-Meier survival analyses on The Cancer Genome Atlas dataset of breast cancer patients. Gene overexpression/knockdown, spheroid formation, flow cytometric analysis, chromatin immunoprecipitation, immunocytochemistry, wound healing/transwell migration assays, and cancer stem cell transcription factor activation profiling array were used to elucidate the regulatory mechanism of integrin α11 expression. Tumour-bearing xenograft models were used to demonstrate integrin α11 is a potential therapeutic target.

RESULTS

Integrin α11 was consistently upregulated in drug-resistant breast cancer cells, and its silencing inhibited cancer stem cells (CSCs) and epithelial-mesenchymal transition (EMT) while restoring sensitivity to anticancer drugs. HIF1α, GLI-1, and EZH2 contributed the most to the regulation of integrin α11 and EZH2 expression, with EZH2 being more necessary for EZH2 autoinduction than HIF1α and GLI-1. Additionally, unlike HIF1α or EZH2, GLI-1 was the sole transcription factor activated by integrin-linked focal adhesion kinase, indicating GLI-1 as a key driver of the EZH2-integrin α11 axis operating for cancer stem cell survival and EMT. Kaplan-Meier survival analysis using The Cancer Genome Atlas (TCGA) dataset also revealed both EZH2 and integrin α11 could be strong prognostic factors of relapse-free and overall survival in breast cancer patients. However, the superior efficacy of integrin α11 siRNA therapy over EZH2 siRNA treatment was demonstrated by enhanced inhibition of tumour growth and prolonged survival in murine models bearing tumours.

CONCLUSION

Our findings elucidate that integrin α11 is upregulated by EZH2, forming a positive feedback circuit involving FAK-GLI-1 and contributing to drug resistance, cancer stem cell survival and EMT. Taken together, the results suggest integrin α11 as a promising prognostic marker and a powerful therapeutic target for drug-resistant breast cancer.

A putative role for lncRNAs in epigenetic regulation of memory

The central dogma of molecular genetics is defined as encoded genetic information within DNA, transcribed into messenger RNA, which contain the instructions for protein synthesis, thus imparting cellular functionality and ultimately life. This molecular genetic theory has given birth to the field of neuroepigenetics, and it is now well established that epigenetic regulation of gene transcription is critical to the learning and memory process. In this review, we address a potential role for a relatively new player in the field of epigenetic crosstalk - long non-coding RNAs (lncRNAs). First, we briefly summarize epigenetic mechanisms in memory formation and examine what little is known about the emerging role of lncRNAs during this process. We then focus discussions on how lncRNAs interact with epigenetic mechanisms to control transcriptional programs under various conditions in the brain, and how this may be applied to regulation of gene expression necessary for memory formation. Next, we explore how epigenetic crosstalk in turn serves to regulate expression of various individual lncRNAs themselves. To highlight the importance of further exploring the role of lncRNA in epigenetic regulation of gene expression, we consider the significant relationship between lncRNA dysregulation and declining memory reserve with aging, Alzheimer's disease, and epilepsy, as well as the promise of novel therapeutic interventions. Finally, we conclude with a discussion of the critical questions that remain to be answered regarding a role for lncRNA in memory.

Enhancer of zeste homolog 2 contributes to apoptosis by inactivating janus kinase 2/ signal transducer and activator of transcription signaling in inflammatory bowel disease

BACKGROUND

Inflammatory bowel disease (IBD) is a prevalent worldwide health problem featured by relapsing, chronic gastrointestinal inflammation. Enhancer of zeste homolog 2 (EZH2) is a critical epigenetic regulator in different pathological models, such as cancer and inflammation. However, the role of EZH2 in the IBD development is still obscure.

AIM

To explore the effect of EZH2 on IBD progression and the underlying mechanism.

METHODS

The IBD mouse model was conducted by adding dextran sodium sulfate (DSS), and the effect of EZH2 on DSS-induced colitis was assessed in the model. The function of EZH2 in regulating apoptosis and permeability was evaluated by Annexin V-FITC Apoptosis Detection Kit, transepithelial electrical resistance analysis, and Western blot analysis of related markers, including Zona occludens 1, claudin-5, and occludin, in NCM460 and fetal human colon (FHC) cells. The mechanical investigation was performed by quantitative reverse transcription-polymerase chain reaction, Western blot analysis, and chromatin immunoprecipitation assays.

RESULTS

The colon length was inhibited in the DSS-treated mice and was enhanced by the EZH2 depletion in the system. DSS treatment caused a decreased histological score in the mice, which was reversed by EZH2 depletion. The inflammatory cytokines, such as tumor necrosis factor-α, interleukin-6, and interleukin-1β, were induced in the DSS-treated mice, in which the depletion of EZH2 could reverse this effect. Moreover, the tumor necrosis factor-α treatment induced the apoptosis of NCM460 and FHC cells, in which EZH2 depletion could reverse this effect in the cells. Moreover, the depletion of EZH2 attenuated permeability of colonic epithelial cells. Mechanically, the depletion of EZH2 or EZH2 inhibitor GSK343 was able to enhance the expression and the phosphorylation of janus kinase 2 (JK2) and signal transducer and activator of transcription in the NCM460 and FHC cells. Specifically, EZH2 inactivated JAK2 expression by regulating histone H3K27me3. JAK2 inhibitor TG101348 was able to reverse EZH2 knockdown-mediated colonic epithelial cell permeability and apoptosis.

CONCLUSION

Thus, we concluded that EZH2 contributed to apoptosis and inflammatory response by inactivating JAK2/ signal transducer and activator of transcription signaling in IBD. EZH2 may be applied as a potential target for IBD therapy.

The noncanonical role of EZH2 in cancer

Enhancer of zeste homolog 2 (EZH2) is the catalytic subunit of polycomb repressive complex 2 (PRC2). Dysregulation of EZH2 causes alteration of gene expression and functions, thereby promoting cancer development. The regulatory function of EZH2 varies across different tumor types. The canonical role of EZH2 is gene silencing through catalyzing the trimethylation of lysine 27 of histone H3 (H3K27me3) in a PRC2-dependent manner. Accumulating evidence indicates that EZH2 has an H3K27me3-independent function as a transcriptional coactivator and plays a critical role in cancer initiation, development, and progression. In this review, we summarize the regulation and function of EZH2 and focus on the current understanding of the noncanonical role of EZH2 in cancer.

Reprogramming Müller Glia to Regenerate Retinal Neurons

In humans, various genetic defects or age-related diseases, such as diabetic retinopathies, glaucoma, and macular degeneration, cause the death of retinal neurons and profound vision loss. One approach to treating these diseases is to utilize stem and progenitor cells to replace neurons in situ, with the expectation that new neurons will create new synaptic circuits or integrate into existing ones. Reprogramming non-neuronal cells in vivo into stem or progenitor cells is one strategy for replacing lost neurons. Zebrafish have become a valuable model for investigating cellular reprogramming and retinal regeneration. This review summarizes our current knowledge regarding spontaneous reprogramming of Müller glia in zebrafish and compares this knowledge to research efforts directed toward reprogramming Müller glia in mammals. Intensive research using these animal models has revealed shared molecular mechanisms that make Müller glia attractive targets for cellular reprogramming and highlighted the potential for curing degenerative retinal diseases from intrinsic cellular sources.

Proteasomal Degradation of Enhancer of Zeste Homologue 2 in Cholangiocytes Promotes Biliary Fibrosis

During biliary disease, cholangiocytes become activated by various pathological stimuli, including transforming growth factor β (TGF-β). The result is an epigenetically regulated transcriptional program leading to a pro-fibrogenic microenvironment, activation of hepatic stellate cells (HSCs), and progression of biliary fibrosis. This study evaluated how TGF-β signaling intersects with epigenetic machinery in cholangiocytes to support fibrogenic gene transcription. We performed RNA sequencing in cholangiocytes with or without TGF-β. Ingenuity pathway analysis identified "HSC Activation" as the highly up-regulated pathway, including overexpression of fibronectin 1 (FN), connective tissue growth factor, and other genes. Bioinformatics identified enhancer of zeste homologue 2 (EZH2) as an epigenetic regulator of the cholangiocyte TGF-β response. EZH2 overexpression suppressed TGF-β-induced FN protein in vitro, suggesting FN as a direct target of EZH2-based repression. Chromatin immunoprecipitation assays identified an FN promoter element in which EZH2-mediated tri-methylation of lysine 27 on histone 3 is diminished by TGF-β. TGF-β also caused a 50% reduction in EZH2 protein levels. Proteasome inhibition rescued EZH2 protein and led to reduced FN production. Immunoprecipitation followed by mass spectrometry identified ubiquitin protein ligase E3 component N-recognin 4 in complex with EZH2, which was validated by western blotting in vitro. Ubiquitin mutation studies suggested K63-based ubiquitin linkage and chain elongation on EZH2 in response to TGF-β. A deletion mutant of EZH2, lacking its N-terminal domain, abrogates both TGF-β-stimulated EZH2 degradation and FN release. In vivo, cholangiocyte-selective knockout of EZH2 exacerbates bile duct ligation-induced fibrosis whereas MDR2^-/- mice are protected from fibrosis by the proteasome inhibitor bortezomib. Conclusion: TGF-β regulates proteasomal degradation of EZH2 through N-terminal, K63-linked ubiquitination in cholangiocytes and activates transcription of a fibrogenic gene program that supports biliary fibrosis.

DZNep-mediated apoptosis in B-cell lymphoma is independent of the lymphoma type, EZH2 mutation status and MYC, BCL2 or BCL6 translocations

Enhancer of zeste homolog 2 (EZH2) tri-methylates histone 3 at position lysine 27 (H3K27me3). Overexpression and gain-of-function mutations in EZH2 are regarded as oncogenic drivers in lymphoma and other malignancies due to the silencing of tumor suppressors and differentiation genes. EZH2 inhibition is sought to represent a good strategy for tumor therapy. In this study, we treated Burkitt lymphoma (BL) and diffuse large B-cell lymphoma (DLBCL) cell lines with 3-deazaneplanocin—A (DZNep), an indirect EZH2 inhibitor which possesses anticancer properties both in-vitro and in-vivo. We aimed to address the impact of the lymphoma type, EZH2 mutation status, as well as MYC, BCL2 and BCL6 translocations on the sensitivity of the lymphoma cell lines to DZNep-mediated apoptosis. We show that DZNep inhibits proliferation and induces apoptosis of these cell lines independent of the type of lymphoma, the EZH2 mutation status and the MYC, BCL2 and BCL6 rearrangement status. Furthermore, DZNep induced a much stronger apoptosis in majority of these cell lines at a lower concentration, and within a shorter period when compared with EPZ-6438, a direct EZH2 inhibitor currently in phase II clinical trials. Apoptosis induction by DZNep was both concentration-dependent and time-dependent, and was associated with the inhibition of EZH2 and subsequent downregulation of H3K27me3 in DZNep-sensitive cell lines. Although EZH2, MYC, BCL2 and BCL6 are important prognostic biomarkers for lymphomas, our study shows that they poorly influence the sensitivity of lymphoma cell lines to DZNep-mediated apoptosis.

Three additional patients with EED-associated overgrowth: potential mutation hotspots identified?

Variants have been identified in the embryonic ectoderm development (EED) gene in seven patients with syndromic overgrowth similar to that observed in Weaver syndrome. Here, we present three additional patients with missense variants in the EED gene. All the missense variants reported to date (including the three presented here) have localized to one of seven WD40 domains of the EED protein, which are necessary for interaction with enhancer of zeste 2 polycomb repressive complex 2 subunit (EZH2). In addition, among the seven patients reported in the literature and the three new patients presented here, all of the reported pathogenic variants except one occurred at one of four amino acid residues in the EED protein. The recurrence of pathogenic variation at these loci suggests that these residues are functionally important (mutation hotspots). In silico modeling and calculations of the free energy changes resulting from these variants suggested that they not only destabilize the EED protein structure but also adversely affect interactions between EED, EZH2, and/or H3K27me3. These cases help demonstrate the mechanism(s) by which apparently deleterious variants in the EED gene might cause overgrowth and lend further support that amino acid residues in the WD40 domain region may be mutation hotspots.

Small Molecule Inhibitors of KDM5 Histone Demethylases Increase the Radiosensitivity of Breast Cancer Cells Overexpressing JARID1B

Background: KDM5 enzymes are H3K4 specific histone demethylases involved in transcriptional regulation and DNA repair. These proteins are overexpressed in different kinds of cancer, including breast, prostate and bladder carcinomas, with positive effects on cancer proliferation and chemoresistance. For these reasons, these enzymes are potential therapeutic targets. Methods: In the present study, we analyzed the effects of three different inhibitors of KDM5 enzymes in MCF-7 breast cancer cells over-expressing one of them, namely KDM5B/JARID1B. In particular we tested H3K4 demethylation (western blot); radio-sensitivity (cytoxicity and clonogenic assays) and damage accumulation (COMET assay and kinetics of H2AX phosphorylation). Results: we show that all three compounds with completely different chemical structures can selectively inhibit KDM5 enzymes and are capable of increasing sensitivity of breast cancer cells to ionizing radiation and radiation-induced damage. Conclusions: These findings confirm the involvement of H3K4 specific demethylases in the response to DNA damage, show a requirement of the catalytic function and suggest new strategies for the therapeutic use of their inhibitors.

Increased Extracellular Matrix Protein Production in Chronic Diabetic Complications: Implications of Non-Coding RNAs

Management of chronic diabetic complications remains a major medical challenge worldwide. One of the characteristic features of all chronic diabetic complications is augmented production of extracellular matrix (ECM) proteins. Such ECM proteins are deposited in all tissues affected by chronic complications, ultimately causing organ damage and dysfunction. A contributing factor to this pathogenetic process is glucose-induced endothelial damage, which involves phenotypic transformation of endothelial cells (ECs). This phenotypic transition of ECs, from a quiescent state to an activated dysfunctional state, can be mediated through alterations in the synthesis of cellular proteins. In this review, we discussed the roles of non-coding RNAs, specifically microRNAs (miRNAs) and long non-coding RNAs (lncRNAs), in such processes. We further outlined other epigenetic mechanisms regulating the biogenesis and/or function of non-coding RNAs. Overall, we believe that better understanding of such molecular processes may lead to the development of novel biomarkers and therapeutic strategies in the future.

Blocking EZH2 methylation transferase activity by GSK126 decreases stem cell-like myeloma cells

EZH2 is a critical epigenetic regulator that is deregulated in various types of cancers including multiple myeloma (MM). In the present study, we hypothesized that targeting EZH2 might induce apoptosis in myeloma cells including stem cell-like cells (CSCs). We investigated the effect of EZH2 inhibition on MM cells using a potent inhibitor (GSK126). The results showed that GSK126 effectively abrogated the methylated histone 3 (H3K27me3) level in MM.1S and LP1 cells, and inhibited the number of live cells and colony formation in soft agar of six MM cell lines. GSK126 induced massive apoptosis in MM.1S, LP1 and RPMI8226 cells. Progressive release of mitochondrial cytochrome c and AIF into the cytosol was detected in GSK126-treated MM cells. GSK126 treatment elicited caspase-3-dependent MCL-1 cleavage with accumulation of proapoptotic truncated MCL-1. These results suggested that GSK126 triggers the intrinsic mitochondrial apoptosis pathway. Enhanced apoptosis was observed in the combination of GSK126 with bortezomib. Using ALDH and side population (SP) assays to characterize CSCs, we found that GSK126 eliminated the stem-like myeloma cells by blocking the Wnt/β-catenin pathway. The in vivo anti-tumor effect of GSK126 was confirmed by using RPMI8226 cells in a xenograft mouse model. In conclusion, our findings suggest that EZH2 inactivation by GSK126 is effective in killing MM cells and CSCs as a single agent or in combination with bortezomib. Clinical trial of GSK126 in patients with MM may be warranted.

Epigenetic regulation of HIV-1 latency: focus on polycomb group (PcG) proteins

HIV-1 latency allows the virus to persist until reactivation, in a transcriptionally silent form in its cellular reservoirs despite the presence of effective cART. Such viral persistence represents a major barrier to HIV eradication since treatment interruption leads to rebound plasma viremia. Polycomb group (PcG) proteins have recently got a considerable attention in regulating HIV-1 post-integration latency as they are involved in the repression of proviral gene expression through the methylation of histones. This epigenetic regulation plays an important role in the establishment and maintenance of HIV-1 latency. In fact, PcG proteins act in complexes and modulate the epigenetic signatures of integrated HIV-1 promoter. Key role played by PcG proteins in the molecular control of HIV-1 latency has led to hypothesize that PcG proteins may represent a valuable target for future HIV-1 therapy in purging HIV-1 reservoirs. In this regard, various small molecules have been synthesized or explored to specifically block the epigenetic activity of PcG. In this review, we will highlight the possible therapeutic approaches to achieve either a functional or sterilizing cure of HIV-1 infection with special focus on histone methylation by PcG proteins together with current and novel pharmacological approaches to reactivate HIV-1 from latency that could ultimately lead towards a better clearance of viral latent reservoirs.

Inactivation of Ezh2 Upregulates Gfi1 and Drives Aggressive Myc-Driven Group 3 Medulloblastoma

The most aggressive of four medulloblastoma (MB) subgroups are cMyc-driven group 3 (G3) tumors, some of which overexpress EZH2, the histone H3K27 mono-, di-, and trimethylase of polycomb-repressive complex 2. Ezh2 has a context-dependent role in different cancers as an oncogene or tumor suppressor and retards tumor progression in a mouse model of G3 MB. Engineered deletions of Ezh2 in G3 MBs by gene editing nucleases accelerated tumorigenesis, whereas Ezh2 re-expression reversed attendant histone modifications and slowed tumor progression. Candidate oncogenic drivers suppressed by Ezh2 included Gfi1, a proto-oncogene frequently activated in human G3 MBs. Gfi1 disruption antagonized the tumor-promoting effects of Ezh2 loss; conversely, Gfi1 overexpression collaborated with Myc to bypass effects of Trp53 inactivation in driving MB progression in primary cerebellar neuronal progenitors. Although negative regulation of Gfi1 by Ezh2 may restrain MB development, Gfi1 activation can bypass these effects.

Polycomb Repressive Complex 2 Methylates Elongin A to Regulate Transcription

Polycomb repressive complex 2 (PRC2-EZH2) methylates histone H3 at lysine 27 (H3K27) and is required to maintain gene repression during development. Misregulation of PRC2 is linked to a range of neoplastic malignancies, which is believed to involve methylation of H3K27. However, the full spectrum of non-histone substrates of PRC2 that might also contribute to PRC2 function is not known. We characterized the target recognition specificity of the PRC2 active site and used the resultant data to screen for uncharacterized potential targets. The RNA polymerase II (Pol II) transcription elongation factor, Elongin A (EloA), is methylated by PRC2 in vivo. Mutation of the methylated EloA residue decreased repression of a subset of PRC2 target genes as measured by both steady-state and nascent RNA levels and perturbed embryonic stem cell differentiation. We propose that PRC2 modulates transcription of a subset of low expression target genes in part via methylation of EloA.

Polycomb and Trithorax Group Genes in Drosophila

Polycomb group (PcG) and Trithorax group (TrxG) genes encode important regulators of development and differentiation in metazoans. These two groups of genes were discovered in Drosophila by their opposing effects on homeotic gene (Hox) expression. PcG genes collectively behave as genetic repressors of Hox genes, while the TrxG genes are necessary for HOX gene expression or function. Biochemical studies showed that many PcG proteins are present in two protein complexes, Polycomb repressive complexes 1 and 2, which repress transcription via chromatin modifications. TrxG proteins activate transcription via a variety of mechanisms. Here we summarize the large body of genetic and biochemical experiments in Drosophila on these two important groups of genes.

Acetylation and methylation profiles of H3K27 in porcine embryos cultured in vitro

Methylation and acetylation of histone H3 at lysine 27 (H3K27) regulate chromatin structure and gene expression during early embryo development. While H3K27 acetylation (H3K27ac) is associated with active gene expression, H3K27 methylation (H3K27me) is linked to transcriptional repression. The aim of this study was to assess the profile of H3K27 acetylation and methylation (mono-, di- and trimethyl) during oocyte maturation and early development in vitro of porcine embryos. Oocytes/embryos were fixed at different developmental stages from germinal vesicle to day 8 blastocysts and submitted to an immunocytochemistry protocol to identify the presence and quantify the immunofluorescence intensity of H3K27ac, H3K27me1, H3K27me2 and H3K27me3. A strong fluorescent signal for H3K27ac was observed in all developmental stages. H3K27me1 and H3K27me2 were detected in oocytes, but the fluorescent signal decreased through the cleavage stages and rose again at the blastocyst stage. H3K27me3 was detected in oocytes, in only one pronucleus in zygotes, cleaved-stage embryos and blastocysts. The nuclear fluorescence signal for H3K27me3 increased from the 2-cell stage to 4-cell stage embryos, decreased at the 8-cell and morula stages and increased again in blastocysts. Different patterns of the H3K27me3 mark were observed at the blastocyst stage. Our results suggest that changes in the H3K27 methylation status regulate early porcine embryo development as previously shown in other species.

Structure of the PRC2 complex and application to drug discovery

The polycomb repressive complexes 2 (PRC2) complex catalyzes tri-methylation of histone H3 lysine 27 (H3K27), a repressive chromatin marker associated with gene silencing. Overexpression and mutations of PRC2 are found in a wide variety of cancers, making the catalytic activity of PRC2 an important target of cancer therapy. This review highlights recent structural breakthroughs of the human PRC2 complex bound to the H3K27 peptide and a small molecule inhibitor, which provide critically needed insight into PRC2-targeted drug discovery.

Inhibitors of Protein Methyltransferases and Demethylases

Post-translational modifications of histones by protein methyltransferases (PMTs) and histone demethylases (KDMs) play an important role in the regulation of gene expression and transcription and are implicated in cancer and many other diseases. Many of these enzymes also target various nonhistone proteins impacting numerous crucial biological pathways. Given their key biological functions and implications in human diseases, there has been a growing interest in assessing these enzymes as potential therapeutic targets. Consequently, discovering and developing inhibitors of these enzymes has become a very active and fast-growing research area over the past decade. In this review, we cover the discovery, characterization, and biological application of inhibitors of PMTs and KDMs with emphasis on key advancements in the field. We also discuss challenges, opportunities, and future directions in this emerging, exciting research field.

Split luciferase-based biosensors for characterizing EED binders

The EED (embryonic ectoderm development) subunit of the Polycomb repressive complex 2 (PRC2) plays an important role in the feed forward regulation of the PRC2 enzymatic activity. We recently identified a new class of allosteric PRC2 inhibitors that bind to the H3K27me3 pocket of EED. Multiple assays were developed and used to identify and characterize this type of PRC2 inhibitors. One of them is a genetically encoded EED biosensor based on the EED[G255D] mutant and the split firefly luciferase. This EED biosensor can detect the compound binding in the transfected cells and in the in vitro biochemical assays. Compared to other commonly used cellular assays, the EED biosensor assay has the advantage of shorter compound incubation with cells. The in vitro EED biosensor is much more sensitive than other label-free biophysical assays (e.g. DSF, ITC). Based on the crystal structure, the DSF data as well as the biosensor assay data, it's most likely that compound-induced increase in the luciferase activity of the EED[G255D] biosensor results from the decreased non-productive interactions between the EED subdomain and other subdomains within the biosensor construct. This new insight of the mechanism might help to broaden the use of the split luciferase based biosensors.

Repo-Man/PP1 regulates heterochromatin formation in interphase

Repo-Man is a protein phosphatase 1 (PP1) targeting subunit that regulates mitotic progression and chromatin remodelling. After mitosis, Repo-Man/PP1 remains associated with chromatin but its function in interphase is not known. Here we show that Repo-Man, via Nup153, is enriched on condensed chromatin at the nuclear periphery and at the edge of the nucleopore basket. Repo-Man/PP1 regulates the formation of heterochromatin, dephosphorylates H3S28 and it is necessary and sufficient for heterochromatin protein 1 binding and H3K27me3 recruitment. Using a novel proteogenomic approach, we show that Repo-Man is enriched at subtelomeric regions together with H2AZ and H3.3 and that depletion of Repo-Man alters the peripheral localization of a subset of these regions and alleviates repression of some polycomb telomeric genes. This study shows a role for a mitotic phosphatase in the regulation of the epigenetic landscape and gene expression in interphase.

The Role of the Histone Methyltransferase Enhancer of Zeste Homolog 2 (EZH2) in the Pathobiological Mechanisms Underlying Inflammatory Bowel Disease (IBD)

Regulatory T (Treg) cells expressing the transcription factor FOXP3 play a pivotal role in maintaining immunologic self-tolerance. We and others have shown previously that EZH2 is recruited to the FOXP3 promoter and its targets in Treg cells. To further address the role for EZH2 in Treg cellular function, we have now generated mice that lack EZH2 specifically in Treg cells (EZH2Δ/ΔFOXP3+). We find that EZH2 deficiency in FOXP3+ T cells results in lethal multiorgan autoimmunity. We further demonstrate that EZH2Δ/ΔFOXP3+ T cells lack a regulatory phenotype in vitro and secrete proinflammatory cytokines. Of special interest, EZH2Δ/ΔFOXP3+ mice develop spontaneous inflammatory bowel disease. Guided by these results, we assessed the FOXP3 and EZH2 gene networks by RNA sequencing in isolated intestinal CD4+ T cells from patients with Crohn's disease. Gene network analysis demonstrates that these CD4+ T cells display a Th1/Th17-like phenotype with an enrichment of gene targets shared by FOXP3 and EZH2. Combined, these results suggest that the inflammatory milieu found in Crohn's disease could lead to or result from deregulation of FOXP3/EZH2-enforced T cell gene networks contributing to the underlying intestinal inflammation.

Groth B, Hersey P. EZH2 as a mediator of treatment resistance in melanoma

Direct treatments of cancer such as chemotherapy, radiotherapy and targeted therapy have been shown to depend on recruitment of the immune system for their effectiveness. Recent studies have shown that development of resistance to direct therapies such as BRAF inhibitors in melanoma is associated with suppression of immune responses. We point to emerging data that implicate activation of the polycomb repressive complex 2 (PRC2) and its catalytic component-enhancer of zeste homolog 2 (EZH2)-in progression of melanoma and suppression of immune responses. EZH2 appears to have an important role in differentiation of CD4 T cells and particularly in the function of T regulatory cells, which suppress immune responses to melanoma. We review mechanisms of EZH2 activation at the genomic level and from activation of the MAP kinase, E2F or NF-kB2 pathways. These studies are consistent with activation of EZH2 as a common mechanism for induction of immune suppression in patients failing direct therapies and suggest EZH2 inhibitors may have a role in combination with immunotherapy and targeted therapies to prevent development of immunosuppression.

The inhibition of EZH2 ameliorates osteoarthritis development through the Wnt/β-catenin pathway

The purpose of our study was to elucidate the role of the histone methyltransferase enhancer of zeste homologue 2 (EZH2) in the pathophysiology of osteoarthritis (OA) and to develop a strategy to modulate EZH2 activity for OA treatment. The expression of EZH2 in normal and OA human cartilage was compared by western blotting. The effect of EZH2 overexpression and inhibition on chondrocyte hypertrophy related gene expression was examined by real-time PCR, and histone methylation on the promoter of the Wnt inhibitor SFRP1 was analyzed using a chromatin immunoprecipitation (ChIP) PCR. Histological assessment of OA mice joint was carried out to assess the in vivo effects of EZH2 inhibitor EPZ005687. We found EZH2 level was significantly increased in the chondrocytes of OA patients compared to normal humans. Overexpression of EZH2 promoted Indian Hedgehog, MMP-13, ADAMTS-5 and COLX expression, while inhibition of EZH2 reversed this trend. Furthermore, the induction of EZH2 led to β-catenin signaling activation by increasing H3K27me3 on the promoter of SFRP1, while the inhibition of EZH2 silenced β-catenin signaling. Finally, intraarticular injection of EPZ005687 delayed OA development in mice. These results implicated EZH2 activity in OA development. Pharmacological inhibition of EZH2 may be an effective therapeutic approach for osteoarthritis.

Aberrant differential expression of EZH1 and EZH2 in Polycomb repressive complex 2 among B- and T/NK-cell neoplasms

The Polycomb repressive complex-2 members (EZH2, EED, SUZ12 and EZH1) are important regulators of haematopoiesis, cell cycle and differentiation. Over-expression of EZH2 has been linked to cancer metastases and poor prognosis. Detailed information on the expression of other members in normal and neoplastic lymphoid tissue remains to be elucidated. Immunohistochemical and immunofluorescent analyses of 156 samples from haematopoietic neoplasms patients and 27 haematopoietic cell lines were used. B-cell neoplasms showed a significant over-expression of EZH2, EED and SUZ12 in the aggressive subtypes compared to the indolent subtypes and normal tissue (p = 0.000-0.046) while expression of EZH1 was decreased in mantle cell lymphoma compared to normal tissue (p = 0.011). T/NK-cell neoplasms also showed significant over-expression of EZH2, EED and SUZ12 (p = 0.000-0.002) and decreased expression of EZH1 (p = 0.001) compared to normal cells. EZH2 and EZH1 have opposite expression patterns both in normal and neoplastic lymphoid tissues as well as an opposite relation to Ki-67. These results were supported by western blotting analyses. Immunofluorescent staining revealed a difference in the intracellular localisation of EZH1 compared to other members. These evidences suggest that EZH2 and EZH1 are important in the counter-balancing mechanisms controlling proliferation/resting of lymphoid cells. The disruption of the balanced EZH2/EZH1 ratio may play important roles in the pathogenesis of lymphomas.

Spatial Organization of Epigenomes

The role of genome architecture in transcription regulation has become the focus of an increasing number of studies over the past decade. Chromatin organization can have a significant impact on gene expression by promoting or restricting the physical proximity between regulatory DNA elements. Given that any change in chromatin state has the potential to alter DNA folding and the proximity between control elements, the spatial organization of chromatin is inherently linked to its molecular composition. In this review, we explore how modulators of chromatin state and organization might keep gene expression in check. We discuss recent findings and present some of the less well-studied aspects of spatial genome organization such as chromatin dynamics and regulation by non-coding RNAs.

Homodimeric PHD Domain-containing Rco1 Subunit Constitutes a Critical Interaction Hub within the Rpd3S Histone Deacetylase Complex

Recognition of histone post-translational modifications is pivotal for directing chromatin-modifying enzymes to specific genomic regions and regulating their activities. Emerging evidence suggests that other structural features of nucleosomes also contribute to precise targeting of downstream chromatin complexes, such as linker DNA, the histone globular domain, and nucleosome spacing. However, how chromatin complexes coordinate individual interactions to achieve high affinity and specificity remains unclear. The Rpd3S histone deacetylase utilizes the chromodomain-containing Eaf3 subunit and the PHD domain-containing Rco1 subunit to recognize nucleosomes that are methylated at lysine 36 of histone H3 (H3K36me). We showed previously that the binding of Eaf3 to H3K36me can be allosterically activated by Rco1. To investigate how this chromatin recognition module is regulated in the context of the Rpd3S complex, we first determined the subunit interaction network of Rpd3S. Interestingly, we found that Rpd3S contains two copies of the essential subunit Rco1, and both copies of Rco1 are required for full functionality of Rpd3S. Our functional dissection of Rco1 revealed that besides its known chromatin-recognition interfaces, other regions of Rco1 are also critical for Rpd3S to recognize its nucleosomal substrates and functionin vivo. This unexpected result uncovered an important and understudied aspect of chromatin recognition. It suggests that precisely reading modified chromatin may not only need the combined actions of reader domains but also require an internal signaling circuit that coordinates the individual actions in a productive way.

Post-Translational Modifications of Histones in Vertebrate Neurogenesis

The process of neurogenesis, through which the entire nervous system of an organism is formed, has attracted immense scientific attention for decades. How can a single neural stem cell give rise to astrocytes, oligodendrocytes, and neurons? Furthermore, how is a neuron led to choose between the hundreds of different neuronal subtypes that the vertebrate CNS contains? Traditionally, niche signals and transcription factors have been on the spotlight. Recent research is increasingly demonstrating that the answer may partially lie in epigenetic regulation of gene expression. In this article, we comprehensively review the role of post-translational histone modifications in neurogenesis in both the embryonic and adult CNS.

Trithorax and Polycomb group-dependent regulation: a tale of opposing activities

Intricate layers of regulation determine the unique gene expression profiles of a given cell and, therefore, underlie the immense phenotypic diversity observed among cell types. Understanding the mechanisms that govern which genes are expressed and which genes are silenced is a fundamental focus in biology. The Polycomb and Trithorax group chromatin proteins play important roles promoting the stable and heritable repression and activation of gene expression, respectively. These proteins, which are conserved across metazoans, modulate post-translational modifications on histone tails and regulate nucleosomal structures. Here, we review recent advances that have shed light on the mechanisms by which these two classes of proteins act to maintain epigenetic memory and allow dynamic switches in gene expression during development.

The Arabidopsis GAGA-Binding Factor BASIC PENTACYSTEINE6 Recruits the POLYCOMB-REPRESSIVE COMPLEX1 Component LIKE HETEROCHROMATIN PROTEIN1 to GAGA DNA Motifs

Polycomb-repressive complexes (PRCs) play key roles in development by repressing a large number of genes involved in various functions. Much, however, remains to be discovered about PRC-silencing mechanisms as well as their targeting to specific genomic regions. Besides other mechanisms, GAGA-binding factors in animals can guide PRC members in a sequence-specific manner to Polycomb-responsive DNA elements. Here, we show that the Arabidopsis (Arabidopsis thaliana) GAGA-motif binding factor protein basic pentacysteine6 (BPC6) interacts with like heterochromatin protein1 (LHP1), a PRC1 component, and associates with vernalization2 (VRN2), a PRC2 component, in vivo. By using a modified DNA-protein interaction enzyme-linked immunosorbant assay, we could show that BPC6 was required and sufficient to recruit LHP1 to GAGA motif-containing DNA probes in vitro. We also found that LHP1 interacts with VRN2 and, therefore, can function as a possible scaffold between BPC6 and VRN2. The lhp1-4 bpc4 bpc6 triple mutant displayed a pleiotropic phenotype, extreme dwarfism and early flowering, which disclosed synergistic functions of LHP1 and group II plant BPC members. Transcriptome analyses supported this synergy and suggested a possible function in the concerted repression of homeotic genes, probably through histone H3 lysine-27 trimethylation. Hence, our findings suggest striking similarities between animal and plant GAGA-binding factors in the recruitment of PRC1 and PRC2 components to Polycomb-responsive DNA element-like GAGA motifs, which must have evolved through convergent evolution.

Directed targeting of chromatin to the nuclear lamina is mediated by chromatin state and A-type lamins

Nuclear organization has been implicated in regulating gene activity. Recently, large developmentally regulated regions of the genome dynamically associated with the nuclear lamina have been identified. However, little is known about how these lamina-associated domains (LADs) are directed to the nuclear lamina. We use our tagged chromosomal insertion site system to identify small sequences from borders of fibroblast-specific variable LADs that are sufficient to target these ectopic sites to the nuclear periphery. We identify YY1 (Ying-Yang1) binding sites as enriched in relocating sequences. Knockdown of YY1 or lamin A/C, but not lamin A, led to a loss of lamina association. In addition, targeted recruitment of YY1 proteins facilitated ectopic LAD formation dependent on histone H3 lysine 27 trimethylation and histone H3 lysine di- and trimethylation. Our results also reveal that endogenous loci appear to be dependent on lamin A/C, YY1, H3K27me3, and H3K9me2/3 for maintenance of lamina-proximal positioning.

Chemical probes of histone lysine methyltransferases

Growing evidence suggests that histone methyltransferases (HMTs, also known as protein methyltransferases (PMTs)) play an important role in diverse biological processes and human diseases by regulating gene expression and the chromatin state. Therefore, HMTs have been increasingly recognized by the biomedical community as a class of potential therapeutic targets. High quality chemical probes of HMTs, as tools for deciphering their physiological functions and roles in human diseases and testing therapeutic hypotheses, are critical for advancing this promising field. In this review, we focus on the discovery, characterization, and biological applications of chemical probes for HMTs.

Product binding enforces the genomic specificity of a yeast polycomb repressive complex

We characterize the Polycomb system that assembles repressive subtelomeric domains of H3K27 methylation (H3K27me) in the yeast Cryptococcus neoformans. Purification of this PRC2-like protein complex reveals orthologs of animal PRC2 components as well as a chromodomain-containing subunit, Ccc1, which recognizes H3K27me. Whereas removal of either the EZH or EED ortholog eliminates H3K27me, disruption of mark recognition by Ccc1 causes H3K27me to redistribute. Strikingly, the resulting pattern of H3K27me coincides with domains of heterochromatin marked by H3K9me. Indeed, additional removal of the C. neoformans H3K9 methyltransferase Clr4 results in loss of both H3K9me and the redistributed H3K27me marks. These findings indicate that the anchoring of a chromatin-modifying complex to its product suppresses its attraction to a different chromatin type, explaining how enzymes that act on histones, which often harbor product recognition modules, may deposit distinct chromatin domains despite sharing a highly abundant and largely identical substrate-the nucleosome.

Selective inhibitors of protein methyltransferases

Mounting evidence suggests that protein methyltransferases (PMTs), which catalyze methylation of histone and nonhistone proteins, play a crucial role in diverse biological processes and human diseases. In particular, PMTs have been recognized as major players in regulating gene expression and chromatin state. PMTs are divided into two categories: protein lysine methyltransferases (PKMTs) and protein arginine methyltransferases (PRMTs). There has been a steadily growing interest in these enzymes as potential therapeutic targets and therefore discovery of PMT inhibitors has also been pursued increasingly over the past decade. Here, we present a perspective on selective, small-molecule inhibitors of PMTs with an emphasis on their discovery, characterization, and applicability as chemical tools for deciphering the target PMTs' physiological functions and involvement in human diseases. We highlight the current state of PMT inhibitors and discuss future directions and opportunities for PMT inhibitor discovery.

Epigenetic control of Ccr7 expression in distinct lineages of lung dendritic cells

Adaptive immune responses to inhaled allergens are induced following CCR7-dependent migration of precursor of dendritic cell (pre-DC)-derived conventional DCs (cDCs) from the lung to regional lymph nodes. However, monocyte-derived (moDCs) in the lung express very low levels of Ccr7 and consequently do not migrate efficiently to LN. To investigate the molecular mechanisms that underlie this dichotomy, we studied epigenetic modifications at the Ccr7 locus of murine cDCs and moDCs. When expanded from bone marrow precursors, moDCs were enriched at the Ccr7 locus for trimethylation of histone 3 lysine 27 (H3K27me3), a modification associated with transcriptional repression. Similarly, moDCs prepared from the lung also displayed increased levels of H3K27me3 at the Ccr7 promoter compared with migratory cDCs from that organ. Analysis of DC progenitors revealed that epigenetic modification of Ccr7 does not occur early during DC lineage commitment because monocytes and pre-DCs both had low levels of Ccr7-associated H3K27me3. Rather, Ccr7 is gradually silenced during the differentiation of monocytes to moDCs. Thus, epigenetic modifications of the Ccr7 locus control the migration and therefore the function of DCs in vivo. These findings suggest that manipulating epigenetic mechanisms might be a novel approach to control DC migration and thereby improve DC-based vaccines and treat inflammatory diseases of the lung.

The prognostic role of EZH2 expression in rectal cancer patients treated with neoadjuvant chemoradiotherapy

BACKGROUND

Neoadjuvant chemoradiotherapy (nCRT) combined with surgery has been implemented as a standard treatment strategy in locally advanced rectal cancer (LARC). However, there is a wide spectrum of response to nCRT. The aim of this study was to determine whether enhancer of zeste homologue 2 (EZH2 ) expression could predict response to nCRT and outcomes for patients in LARC.

METHOD

The study examined the EZH2 expression in 112 biopsies by immohistochemistry. The associations between EZH2 and clinical characters were analyzed.

RESULTS

EZH2 expression in biopsy tissue was significantly related to increased tumor cell proliferation, as assessed by Ki-67 expression with a cutoff value of 37% (p <0.001). High EZH2 expression was correlated closely with low differentiation (p = 0.029), high CEA level (p = 0.041), T4 status (p = 0.011) and node metastasis (p =0.045). By univariate and multivariate analysis, we observed low EZH2 expression could reliably and independently predict the good response to nCRT ( p = 0.026 and p = 0.023) and down-staging ( p = 0.021 and p = 0.027). In univariate analysis, high EZH2 expression was significantly associated with poor 5-year disease-free survival (p = 0.025) and 5-year overall survival (p = 0.032). In multivariate analysis, EZH2 was a prognostic factor for 5-year DFS (HR = 2.287; 95% CI 1.137-4.602, p = 0.020) but not for 5-year OS (HR = 2.182; 95% CI 0.940-5.364, p = 0.069).

CONCLUSION

Our study revealed that low EZH2 expression in biopsy tissue might be a useful predictive factor of good tumor response to nCRT and longer 5-year DFS in patients with LARC. However this is a relatively small retrospective study, to further validate the role of EZH2 in rectal cancer, large consistent cohort studies are needed.

Regulation and Role of EZH2 in Cancer

Polycomb repressive complex 2 (PRC2) is the epigenetic regulator that induces histone H3 lysine 27 methylation (H3K27me3) and silences specific gene transcription. Enhancer of zeste homolog 2 (EZH2) is an enzymatic subunit of PRC2, and evidence shows that EZH2 plays an essential role in cancer initiation, development, progression, metastasis, and drug resistance. EZH2 expression is indeed regulated by various oncogenic transcription factors, tumor suppressor miRNAs, and cancer-associated non-coding RNA. EZH2 activity is also controlled by post-translational modifications, which are deregulated in cancer. The canonical role of EZH2 is gene silencing through H3K27me3, but accumulating evidence shows that EZH2 methlyates substrates other than histone and has methylase-independent functions. These non-canonical functions of EZH2 are shown to play a role in cancer progression. In this review, we summarize current information on the regulation and roles of EZH2 in cancer. We also discuss various therapeutic approaches to targeting EZH2.

EZH2: an emerging role in melanoma biology and strategies for targeted therapy

Histone modifications are increasingly being recognized as important epigenetic mechanisms that govern chromatin structure and gene expression. EZH2 is the catalytic subunit of the polycomb repressive complex 2 (PRC2), responsible for tri-methylation of lysine 27 on histone 3 (H3K27me3) that leads to gene silencing. This highly conserved histone methyltransferase is found to be overexpressed in many different types of cancers including melanoma, where it is postulated to abnormally repress tumor suppressor genes. Somatic mutations have been identified in approximately 3% of melanomas, and activating mutations described within the catalytic SET domain of EZH2 confer its oncogenic activity. In the following review, we discuss the evidence that EZH2 is an important driver of melanoma progression and we summarize the progress of EZH2 inhibitors against this promising therapeutic target.

What are memories made of? How Polycomb and Trithorax proteins mediate epigenetic memory

In any biological system with memory, the state of the system depends on its history. Epigenetic memory maintains gene expression states through cell generations without a change in DNA sequence and in the absence of initiating signals. It is immensely powerful in biological systems - it adds long-term stability to gene expression states and increases the robustness of gene regulatory networks. The Polycomb group (PcG) and Trithorax group (TrxG) proteins can confer long-term, mitotically heritable memory by sustaining silent and active gene expression states, respectively. Several recent studies have advanced our understanding of the molecular mechanisms of this epigenetic memory during DNA replication and mitosis.

The histone methyltransferase EZH2, an oncogene common to benign and malignant parathyroid tumors

Primary hyperparathyroidism (pHPT) resulting from parathyroid tumors is a common endocrine disorder with incompletely understood etiology. In renal failure, secondary hyperparathyroidism (sHPT) occurs with multiple tumor development as a result of calcium and vitamin D regulatory disturbance. The aim of this study was to investigate a potential role of the histone 3 lysine 27 methyltransferase EZH2 in parathyroid tumorigenesis. Parathyroid tumors from patients with pHPT included adenomas and carcinomas. Hyperplastic parathyroid glands from patients with HPT secondary to uremia and normal parathyroid tissue specimens were included in this study. Quantitative RT-PCR, western blotting, bisulfite pyrosequencing, colony formation assay, and RNA interference were used. EZH2 was overexpressed in a subset of the benign and in all malignant parathyroid tumors as determined by quantitative RT-PCR and western blotting analyses. Overexpression was explained by EZH2 gene amplification in a large fraction of tumors. EZH2 depletion by RNA interference inhibited sHPT-1 parathyroid cell line proliferation as determined by tritium-thymidine incorporation and colony formation assays. EZH2 depletion also interfered with the Wnt/β-catenin signaling pathway by increased expression of growth-suppressive AXIN2, a negative regulator of β-catenin stability. Indeed, EZH2 contributed to the total level of aberrantly accumulated transcriptionally active (nonphosphoylated) β-catenin in the parathyroid tumor cells. To our knowledge EZH2 gene amplification presents the first genetic aberration common to parathyroid adenomas, secondary hyperplastic parathyroid glands, and parathyroid carcinomas. This supports the possibility of a common pathway in parathyroid tumor development.

Selective inhibition of EZH2 by EPZ-6438 leads to potent antitumor activity in EZH2-mutant non-Hodgkin lymphoma

Mutations within the catalytic domain of the histone methyltransferase EZH2 have been identified in subsets of patients with non-Hodgkin lymphoma (NHL). These genetic alterations are hypothesized to confer an oncogenic dependency on EZH2 enzymatic activity in these cancers. We have previously reported the discovery of EPZ005678 and EPZ-6438, potent and selective S-adenosyl-methionine-competitive small molecule inhibitors of EZH2. Although both compounds are similar with respect to their mechanism of action and selectivity, EPZ-6438 possesses superior potency and drug-like properties, including good oral bioavailability in animals. Here, we characterize the activity of EPZ-6438 in preclinical models of NHL. EPZ-6438 selectively inhibits intracellular lysine 27 of histone H3 (H3K27) methylation in a concentration- and time-dependent manner in both EZH2 wild-type and mutant lymphoma cells. Inhibition of H3K27 trimethylation (H3K27Me3) leads to selective cell killing of human lymphoma cell lines bearing EZH2 catalytic domain point mutations. Treatment of EZH2-mutant NHL xenograft-bearing mice with EPZ-6438 causes dose-dependent tumor growth inhibition, including complete and sustained tumor regressions with correlative diminution of H3K27Me3 levels in tumors and selected normal tissues. Mice dosed orally with EPZ-6438 for 28 days remained tumor free for up to 63 days after stopping compound treatment in two EZH2-mutant xenograft models. These data confirm the dependency of EZH2-mutant NHL on EZH2 activity and portend the utility of EPZ-6438 as a potential treatment for these genetically defined cancers.

CBP loss cooperates with PTEN haploinsufficiency to drive prostate cancer: implications for epigenetic therapy

Despite the high incidence and mortality of prostate cancer, the etiology of this disease is not fully understood. In this study, we develop functional evidence for CBP and PTEN interaction in prostate cancer based on findings of their correlate expression in the human disease. Cbp(pc-/-);Pten(pc+/-) mice exhibited higher cell proliferation in the prostate and an early onset of high-grade prostatic intraepithelial neoplasia. Levels of EZH2 methyltransferase were increased along with its Thr350 phosphorylation in both mouse Cbp(-/-); Pten(+/-) and human prostate cancer cells. CBP loss and PTEN deficiency cooperated to trigger a switch from K27-acetylated histone H3 to K27-trimethylated bulk histones in a manner associated with decreased expression of the growth inhibitory EZH2 target genes DAB2IP, p27(KIP1), and p21(CIP1). Conversely, treatment with the histone deacetylase inhibitor panobinostat reversed this switch, in a manner associated with tumor suppression in Cbp(pc-/-);Pten(pc+/-) mice. Our findings show how CBP and PTEN interact to mediate tumor suppression in the prostate, establishing a central role for histone modification in the etiology of prostate cancer and providing a rationale for clinical evaluation of epigenetic-targeted therapy in patients with prostate cancer.

EZH2: biology, disease, and structure-based drug discovery

EZH2 is the catalytic subunit of the polycomb repressive complex 2 (PRC2), which is a highly conserved histone methyltransferase that methylates lysine 27 of histone 3. Overexpression of EZH2 has been found in a wide range of cancers, including those of the prostate and breast. In this review, we address the current understanding of the oncogenic role of EZH2, including its PRC2-dependent transcriptional repression and PRC2-independent gene activation. We also discuss the connections between EZH2 and other silencing enzymes, such as DNA methyltransferase and histone deacetylase. We comprehensively address the architecture of the PRC2 complex and the crucial roles of each subunit. Finally, we summarize new progress in developing EZH2 inhibitors, which could be a new epigenetic therapy for cancers.

High fat diet and in utero exposure to maternal obesity disrupts circadian rhythm and leads to metabolic programming of liver in rat offspring

The risk of obesity in adulthood is subject to programming beginning at conception. In animal models, exposure to maternal obesity and high fat diets influences the risk of obesity in the offspring. Among other long-term changes, offspring from obese rats develop hyperinsulinemia, hepatic steatosis, and lipogenic gene expression in the liver at weaning. However, the precise underlying mechanisms leading to metabolic dysregulation in the offspring remains unclear. Using a rat model of overfeeding-induced obesity, we previously demonstrated that exposure to maternal obesity from pre-conception to birth, is sufficient to program increased obesity risk in the offspring. Offspring of obese rat dams gain greater body weight and fat mass when fed high fat diet (HFD) as compared to lean dam. Since, disruptions of diurnal circadian rhythm are known to detrimentally impact metabolically active tissues such as liver, we examined the hypothesis that maternal obesity leads to perturbations of core clock components and thus energy metabolism in offspring liver. Offspring from lean and obese dams were examined at post-natal day 35, following a short (2 wk) HFD challenge. Hepatic mRNA expression of circadian (CLOCK, BMAL1, REV-ERBα, CRY, PER) and metabolic (PPARα, SIRT1) genes were strongly suppressed in offspring exposed to both maternal obesity and HFD. Using a mathematical model, we identified two distinct biological mechanisms that modulate PPARα mRNA expression: i) decreased mRNA synthesis rates; and ii) increased non-specific mRNA degradation rate. Moreover, our findings demonstrate that changes in PPARα transcription were associated with epigenomic alterations in H3K4me3 and H3K27me3 histone marks near the PPARα transcription start site. Our findings indicated that offspring from obese rat dams have detrimental alternations to circadian machinery that may contribute to impaired liver metabolism in response to HFD, specifically via reduced PPARα expression prior to obesity development.