Mediator complex cooperatively regulates transcription of retinoic acid target genes with Polycomb Repressive Complex 2 during neuronal differentiation

Pharmacological EZH2 inhibition combined with retinoic acid treatment promotes differentiation and apoptosis in rhabdomyosarcoma cells

BACKGROUND

Rhabdomyosarcomas (RMS) are predominantly paediatric sarcomas thought to originate from muscle precursor cells due to impaired myogenic differentiation. Despite intensive treatment, 5-year survival for patients with advanced disease remains low (< 30%), highlighting a need for novel therapies to improve outcomes. Differentiation therapeutics are agents that induce differentiation of cancer cells from malignant to benign. The histone methyltransferase, Enhancer of Zeste Homolog 2 (EZH2) suppresses normal skeletal muscle differentiation and is highly expressed in RMS tumours.

RESULTS

We demonstrate combining inhibition of the epigenetic modulator EZH2 with the differentiating agent retinoic acid (RA) is more effective at reducing cell proliferation in RMS cell lines than single agents alone. In PAX3-FOXO1 positive RMS cells this is due to an RA-driven induction of the interferon pathway resulting in apoptosis. In fusion negative RMS, combination therapy led to an EZH2i-driven upregulation of myogenic signalling resulting in differentiation. In both subtypes, EZH2 is significantly associated with enrichment of trimethylated lysine 27 on histone 3 (H3K27me3) in genes that are downregulated in untreated RMS cells and upregulated with EZH2 inhibitor treatment. These results provide insight into the mechanism that drives the anti-cancer effect of the EZH2/RA single agent and combination treatment and indicate that the reduction of EZH2 activity combined with the induction of RA signalling represents a potential novel therapeutic strategy to treat both subtypes of RMS.

CONCLUSIONS

The results of this study demonstrate the potential utility of combining EZH2 inhibitors with differentiation agents for the treatment of paediatric rhabdomyosarcomas. As EZH2 inhibitors are currently undergoing clinical trials for adult and paediatric solid tumours and retinoic acid differentiation agents are already in clinical use this presents a readily translatable potential therapeutic strategy. Moreover, as inhibition of EZH2 in the poor prognosis FPRMS subtype results in an inflammatory response, it is conceivable that this strategy may also synergise with immunotherapies for a more effective treatment in these patients.

Distinct roles for CKM-Mediator in controlling Polycomb-dependent chromosomal interactions and priming genes for induction

Precise control of gene expression underpins normal development. This relies on mechanisms that enable communication between gene promoters and other regulatory elements. In embryonic stem cells (ESCs), the cyclin-dependent kinase module Mediator complex (CKM-Mediator) has been reported to physically link gene regulatory elements to enable gene expression and also prime genes for induction during differentiation. Here, we show that CKM-Mediator contributes little to three-dimensional genome organization in ESCs, but it has a specific and essential role in controlling interactions between inactive gene regulatory elements bound by Polycomb repressive complexes (PRCs). These interactions are established by the canonical PRC1 (cPRC1) complex but rely on CKM-Mediator, which facilitates binding of cPRC1 to its target sites. Importantly, through separation-of-function experiments, we reveal that this collaboration between CKM-Mediator and cPRC1 in creating long-range interactions does not function to prime genes for induction during differentiation. Instead, we discover that priming relies on an interaction-independent mechanism whereby the CKM supports core Mediator engagement with gene promoters during differentiation to enable gene activation.

The Mediator Complex: A Central Coordinator of Plant Adaptive Responses to Environmental Stresses

As sessile organisms, plants are constantly exposed to a variety of environmental stresses and have evolved adaptive mechanisms, including transcriptional reprogramming, in order to survive or acclimate under adverse conditions. Over the past several decades, a large number of gene-specific transcription factors have been identified in the transcriptional regulation of plant adaptive responses. The Mediator complex plays a key role in transducing signals from gene-specific transcription factors to the transcription machinery to activate or repress target gene expression. Since its first purification about 15 years ago, plant Mediator complex has been extensively analyzed for its composition and biological functions. Mutants of many plant Mediator subunits are not lethal but are compromised in growth, development and response to biotic and abiotic stress, underscoring a particularly important role in plant adaptive responses. Plant Mediator subunits also interact with partners other than transcription factors and components of the transcription machinery, indicating the complexity of the regulation of gene expression by plant Mediator complex. Here, we present a comprehensive discussion of recent analyses of the structure and function of plant Mediator complex, with a particular focus on its roles in plant adaptive responses to a wide spectrum of environmental stresses and associated biological processes.

The role and mechanisms of polycomb repressive complex 2 on the regulation of osteogenic and neurogenic differentiation of stem cells

The stem cells differentiate into osteoblasts or neurocytes is the key process for treatment of bone‐ or neural tissue‐related diseases which is caused by ageing, fracture, injury, inflammation, etc Polycomb group complexes (PcGs), especially the polycomb repressive complex 2 (PRC2), act as pivotal epigenetic regulators by modifying key developmental regulatory genes during stem cells differentiation. In this review, we summarize the core subunits, the variants and the potential functions of PRC2. We also highlight the underlying mechanisms of PRC2 associated with the osteogenic and neurogenic differentiation of stem cells, including its interaction with non‐coding RNAs, histone acetyltransferases, histone demethylase, DNA methyltransferase and polycomb repressive complex 1. This review provided a substantial information of epigenetic regulation mediated by PRC2 which leads to the osteogenic and neurogenic differentiation of stem cells.

Expansion and Functional Diversification of TFIIB-Like Factors in Plants

As sessile organisms, plants have evolved unique patterns of growth and development, elaborate metabolism and special perception and signaling mechanisms to environmental cues. Likewise, plants have complex and highly special programs for transcriptional control of gene expression. A case study for the special transcription control in plants is the expansion of general transcription factors, particularly the family of Transcription Factor IIB (TFIIB)-like factors with 15 members in Arabidopsis. For more than a decade, molecular and genetic analysis has revealed important functions of these TFIIB-like factors in specific biological processes including gametogenesis, pollen tube growth guidance, embryogenesis, endosperm development, and plant-microbe interactions. The redundant, specialized, and diversified roles of these TFIIB-like factors challenge the traditional definition of general transcription factors established in other eukaryotes. In this review, we discuss general transcription factors in plants with a focus on the expansion and functional analysis of plant TFIIB-like proteins to highlight unique aspects of plant transcription programs that can be highly valuable for understanding the molecular basis of plant growth, development and responses to stress conditions.

Angel or Devil ? - CDK8 as the new drug target

Cyclin-dependent kinase 8 (CDK8) plays an momentous role in transcription regulation by forming kinase module or transcription factor phosphorylation. A large number of evidences have identified CDK8 as an important factor in cancer occurrence and development. In addition, CDK8 also participates in the regulation of cancer cell stress response to radiotherapy and chemotherapy, assists tumor cell invasion, metastasis, and drug resistance. Therefore, CDK8 is regarded as a promising target for cancer therapy. Most studies in recent years supported the role of CDK8 as a carcinogen, however, under certain conditions, CDK8 exists as a tumor suppressor. The functional diversity of CDK8 and its exceptional role in different types of cancer have aroused great interest from scientists but even more controversy during the discovery of CDK8 inhibitors. In addition, CDK8 appears to be an effective target for inflammation diseases and immune system disorders. Therefore, we summarized the research results of CDK8, involving physiological/pathogenic mechanisms and the development status of compounds targeting CDK8, provide a reference for the feasibility evaluation of CDK8 as a therapeutic target, and guidance for researchers who are involved in this field for the first time.

The TGFB2-AS1 lncRNA Regulates TGF-β Signaling by Modulating Corepressor Activity

Molecular processes involving lncRNAs regulate cell function. By applying transcriptomics, we identify lncRNAs whose expression is regulated by transforming growth factor β (TGF-β). Upon silencing individual lncRNAs, we identify several that regulate TGF-β signaling. Among these lncRNAs, TGFB2-antisense RNA1 (TGFB2-AS1) is induced by TGF-β through Smad and protein kinase pathways and resides in the nucleus. Depleting TGFB2-AS1 enhances TGF-β/Smad-mediated transcription and expression of hallmark TGF-β-target genes. Increased dose of TGFB2-AS1 reduces expression of these genes, attenuates TGF-β-induced cell growth arrest, and alters BMP and Wnt pathway gene profiles. Mechanistically, TGFB2-AS1, mainly via its 3′ terminal region, binds to the EED adaptor of the Polycomb repressor complex 2 (PRC2), promoting repressive histone H3K27me³ modifications at TGF-β-target gene promoters. Silencing EED or inhibiting PRC2 methylation activity partially rescues TGFB2-AS1-mediated gene repression. Thus, the TGF-β-induced TGFB2-AS1 lncRNA exerts inhibitory functions on TGF-β/BMP signaling output, supporting auto-regulatory negative feedback that balances TGF-β/BMP-mediated responses.

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TGF-β signaling transcriptionally regulates lncRNAs that regulate TGF-β signaling

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TGFB2-AS1 is induced by TGF-β to negatively regulate Smad transcriptional output

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TGFB2-AS1 associates with EED, the Polycomb repressor complex 2 adaptor

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TGFB2-AS1 promotes repressive histone modifications at TGF-β-target genes

Cdk8 is required for establishment of H3K27me3 and gene repression by Xist and mouse development

We previously identified the cyclin dependent kinase Cdk8 as a putative silencing factor for Xist To investigate its role in X inactivation, we engineered a Cdk8 mutation in mouse embryonic stem cells (ESCs) carrying an inducible system for studying Xist function. We found that Xist repressed X-linked genes at half of the expression level in Cdk8 mutant cells, whereas they were almost completely silenced in the controls. Lack of Cdk8 impaired Ezh2 recruitment and the establishment of histone H3 lysine 27 tri-methylation but not PRC1 recruitment by Xist Transgenic expression of wild-type but not catalytically inactive Cdk8 restored efficient gene repression and PRC2 recruitment. Mutation of the paralogous kinase Cdk19 did not affect Xist function, and combined mutations of Cdk8 and Cdk19 resembled the Cdk8 mutation. In mice, a Cdk8 mutation caused post-implantation lethality. We observed that homozygous Cdk8 mutant female embryos showed a greater developmental delay than males on day 10.5. Together with the inefficient repression of X-linked genes in differentiating Cdk8 mutant female ESCs, these data show a requirement for Cdk8 in the initiation of X inactivation.

Diversity and Emerging Roles of Enhancer RNA in Regulation of Gene Expression and Cell Fate

Enhancers are cis-regulatory elements in the genome that cooperate with promoters to control target gene transcription. Unlike promoters, enhancers are not necessarily adjacent to target genes and can exert their functions regardless of enhancer orientations, positions and spatial segregations from target genes. Thus, for a long time, the question as to how enhancers act in a temporal and spatial manner attracted considerable attention. The recent discovery that enhancers are also abundantly transcribed raises interesting questions about the exact roles of enhancer RNA (eRNA) in gene regulation. In this review, we highlight the process of enhancer transcription and the diverse features of eRNA. We review eRNA functions, which include enhancer-promoter looping, chromatin modifying, and transcription regulating. As eRNA are transcribed from active enhancers, they exhibit tissue and lineage specificity, and serve as markers of cell state and function. Finally, we discuss the unique relationship between eRNA and super enhancers in phase separation wherein eRNA may contribute significantly to cell fate decisions.

Components from the Human c-myb Transcriptional Regulation System Reactivate Epigenetically Repressed Transgenes

A persistent challenge for mammalian cell engineering is the undesirable epigenetic silencing of transgenes. Foreign DNA can be incorporated into closed chromatin before and after it has been integrated into a host cell's genome. To identify elements that mitigate epigenetic silencing, we tested components from the c-myb and NF-kB transcriptional regulation systems in transiently transfected DNA and at chromosomally integrated transgenes in PC-3 and HEK 293 cells. DNA binding sites for MYB (c-myb) placed upstream of a minimal promoter enhanced expression from transiently transfected plasmid DNA. We targeted p65 and MYB fusion proteins to a chromosomal transgene, UAS-Tk-luciferase, that was silenced by ectopic Polycomb chromatin complexes. Transient expression of Gal4-MYB induced an activated state that resisted complete re-silencing. We used custom guide RNAs and dCas9-MYB to target MYB to different positions relative to the promoter and observed that transgene activation within ectopic Polycomb chromatin required proximity of dCas9-MYB to the transcriptional start site. Our report demonstrates the use of MYB in the context of the CRISPR-activation system, showing that DNA elements and fusion proteins derived from c-myb can mitigate epigenetic silencing to improve transgene expression in engineered cell lines.

Transcriptome analysis of Xenopus orofacial tissues deficient in retinoic acid receptor function

BACKGROUND

Development of the face and mouth is orchestrated by a large number of transcription factors, signaling pathways and epigenetic regulators. While we know many of these regulators, our understanding of how they interact with each other and implement changes in gene expression during orofacial development is still in its infancy. Therefore, this study focuses on uncovering potential cooperation between transcriptional regulators and one important signaling pathway, retinoic acid, during development of the midface.

RESULTS

Transcriptome analyses was performed on facial tissues deficient for retinoic acid receptor function at two time points in development; early (35 hpf) just after the neural crest migrates and facial tissues are specified and later (60 hpf) when the mouth has formed and facial structures begin to differentiate. Functional and network analyses revealed that retinoic acid signaling could cooperate with novel epigenetic factors and calcium-NFAT signaling during early orofacial development. At the later stage, retinoic acid may work with WNT and BMP and regulate homeobox containing transcription factors. Finally, there is an overlap in genes dysregulated in Xenopus embryos with median clefts with human genes associated with similar orofacial defects.

CONCLUSIONS

This study uncovers novel signaling pathways required for orofacial development as well as pathways that could interact with retinoic acid signaling during the formation of the face. We show that frog faces are an important tool for studying orofacial development and birth defects.

Structure and Functions of the Mediator Complex

Mediator is a key factor in the regulation of expression of RNA polymerase II-transcribed genes. Recent studies have shown that Mediator acts as a coordinator of transcription activation and participates in maintaining chromatin architecture in the cell nucleus. In this review, we present current concepts on the structure and functions of Mediator.

Polycomb Repressive Complex 2: Emerging Roles in the Central Nervous System

The polycomb repressive complex 2 (PRC2) is responsible for catalyzing both di- and trimethylation of histone H3 at lysine 27 (H3K27me2/3). The subunits of PRC2 are widely expressed in the central nervous system (CNS). PRC2 as well as H3K27me2/3, play distinct roles in neuronal identity, proliferation and differentiation of neural stem/progenitor cells, neuronal morphology, and gliogenesis. Mutations or dysregulations of PRC2 subunits often cause neurological diseases. Therefore, PRC2 might represent a common target of different pathological processes that drive neurodegenerative diseases. A better understanding of the intricate and complex regulatory networks mediated by PRC2 in CNS will help to develop new therapeutic approaches and to generate specific brain cell types for treating neurological diseases.

Mediator cyclin-dependent kinases upregulate transcription of inflammatory genes in cooperation with NF-κB and C/EBPβ on stimulation of Toll-like receptor 9

In eukaryotes, the Mediator complex has important roles in regulation of transcription by RNA polymerase II. Mediator is a large complex with more than 20 subunits that form head, middle, tail and CDK/cyclin modules. Among them, CDK8 and/or CDK19 (CDK8/19), and their counterpart cyclin C, form the CDK/cyclin module together with Mediator subunits MED12 and MED13. Despite evidences of both activation and repression, the precise functional roles of CDK8/19 in transcription are still elusive. Our previous results indicate that CDK8/19 recruits epigenetic regulators to repress immunoresponse genes. Here, this study focused on Toll-like receptors (TLRs), which exert innate immune responses through recognition of pathogen-associated molecular patterns and examined the functional roles of CDK8/19. As a result, CDK8/19 regulated transcription of inflammatory genes on stimulation of TLR9 in myeloma-derived RPMI8226 cells, which led to expression of inflammation-associated genes such as IL8, IL10, PTX3 and CCL2. Mediator subunits CDK8/19 and MED1, inflammation-related transcriptional activator NF-κB and C/EBPβ, and general transcription factors TFIIE and TFIIB colocalized at the promoter regions of these genes under this condition. Our results show that CDK8/19 positively regulates inflammatory gene transcription in cooperation with NF-κB and C/EBPβ on stimulation of TLR9.