Repression of Six3 by a corepressor regulates rhodopsin expression

Metastasis-associated protein 1 participates in regulating luminal acidification of the epididymis via repressing estrogen receptor alpha transcription

BACKGROUND

As a component of the nucleosome remodeling and deacetylating (NuRD) complex, metastasis-associated protein 1 (MTA1) has been reported to be abundant in male reproductive system and might participate in spermatogenesis and sperm maturation, whereas the precise functional role of MTA1 in these processes is still undetermined.

OBJECTIVE

To investigate the effect and potential function of MTA1 in male fertility.

MATERIALS AND METHODS

Mta1 knockout mice (Mta1-/- ) were employed to detect their reproductive phenotype. The pH value of Mta1-/- epididymal luminal fluid was measured, and the potential mechanism of MTA1 involved in regulating luminal acidification was detected in vivo and in vitro. A vasectomy model with abnormal pH of epididymal lumen was established to further detect the effect of MTA1 on epididymal luminal microenvironment.

RESULTS

Mta1-/- mice were fertile without any detectable defects in spermatogenesis or sperm motility while the deficiency of MTA1 could acidify the initial segment of epididymis to a certain extent. MTA1 could interact with estrogen receptor alpha (ERα) and inhibit the transcription of ERα target gene, hydrogen exchanger 3 (NHE3), and ultimately affect the epididymal luminal milieu. After vasectomy, the Mta1-/- mice presented a more acidic epididymal lumen which was closer to the normal state compared to the wild-type model.

DISCUSSION AND CONCLUSION

MTA1 is dispensable for male fertility in mice, but plays a potentially important function in regulating luminal acidification of the epididymis.

Deletion of the homeodomain gene Six3 from kisspeptin neurons causes subfertility in female mice

The homeodomain transcription factor SIX3 is a known regulator of eye, nose, and forebrain development, and has recently been implicated in female reproduction. Germline heterozygosity of SIX3 is sufficient to cause subfertility, but the cell populations that mediate this role are unknown. The neuropeptide kisspeptin is a critical component of the reproductive axis and plays roles in sexual maturation, ovulation, and the maintenance of gonadotropin secretion. We used Cre-Lox technology to remove Six3 specifically from kisspeptin neurons in mice to test the hypothesis that SIX3 in kisspeptin neurons is required for reproduction. We found that loss of Six3 in kisspeptin neurons causes subfertility and estrous cycle irregularities in females, but no effect in males. Overall, we find that SIX3 expression in kisspeptin neurons is an important contributor to female fertility.

Ciliary photoreceptors in sea urchin larvae indicate pan-deuterostome cell type conservation

BACKGROUND

The evolutionary history of cell types provides insights into how morphological and functional complexity arose during animal evolution. Photoreceptor cell types are particularly broadly distributed throughout Bilateria; however, their evolutionary relationship is so far unresolved. Previous studies indicate that ciliary photoreceptors are homologous at least within chordates, and here, we present evidence that a related form of this cell type is also present in echinoderm larvae.

RESULTS

Larvae of the purple sea urchin Strongylocentrotus purpuratus have photoreceptors that are positioned bilaterally in the oral/anterior apical neurogenic ectoderm. Here, we show that these photoreceptors express the transcription factor Rx, which is commonly expressed in ciliary photoreceptors, together with an atypical opsin of the G_O family, opsin3.2, which localizes in particular to the cilia on the cell surface of photoreceptors. We show that these ciliary photoreceptors express the neuronal marker synaptotagmin and are located in proximity to pigment cells. Furthermore, we systematically identified additional transcription factors expressed in these larval photoreceptors and found that a majority are orthologous to transcription factors expressed in vertebrate ciliary photoreceptors, including Otx, Six3, Tbx2/3, and Rx. Based on the developmental expression of rx, these photoreceptors derive from the anterior apical neurogenic ectoderm. However, genes typically involved in eye development in bilateria, including pax6, six1/2, eya, and dac, are not expressed in sea urchin larval photoreceptors but are instead co-expressed in the hydropore canal.

CONCLUSIONS

Based on transcription factor expression, location, and developmental origin, we conclude that the sea urchin larval photoreceptors constitute a cell type that is likely homologous to the ciliary photoreceptors present in chordates.

The SIX Family of Transcription Factors: Common Themes Integrating Developmental and Cancer Biology

The sine oculis (SIX) family of transcription factors are key regulators of developmental processes during embryogenesis. Members of this family control gene expression to promote self-renewal of progenitor cell populations and govern mechanisms of cell differentiation. When the function of SIX genes becomes disrupted, distinct congenital defects develops both in animal models and humans. In addition to the embryonic setting, members of the SIX family have been found to be critical regulators of tumorigenesis, promoting cell proliferation, epithelial-to-mesenchymal transition, and metastasis. Research in both the fields of developmental biology and cancer research have provided an extensive understanding of SIX family transcription factor functions. Here we review recent progress in elucidating the role of SIX family genes in congenital disease as well as in the promotion of cancer. Common themes arise when comparing SIX transcription factor function during embryonic and cancer development. We highlight the complementary nature of these two fields and how knowledge in one area can open new aspects of experimentation in the other.

The NuRD complex and macrocephaly associated neurodevelopmental disorders

The nucleosome remodeling and deacetylase (NuRD) complex is a major regulator of gene expression involved in pluripotency, lineage commitment, and corticogenesis. This important complex is composed of seven different proteins, with mutations in CHD3, CHD4, and GATAD2B being associated with neurodevelopmental disorders presenting with macrocephaly and intellectual disability similar to other overgrowth and intellectual disability (OGID) syndromes. Pathogenic variants in CHD3 and CHD4 primarily involve disruption of enzymatic function. GATAD2B variants include loss-of-function mutations that alter protein dosage and missense variants that involve either of two conserved domains (CR1 and CR2) known to interact with other NuRD proteins. In addition to macrocephaly and intellectual disability, CHD3 variants are associated with inguinal hernias and apraxia of speech; whereas CHD4 variants are associated with skeletal anomalies, deafness, and cardiac defects. GATAD2B-associated neurodevelopmental disorder (GAND) has phenotypic overlap with both of these disorders. Of note, structural models of NuRD indicate that CHD3 and CHD4 require direct contact with the GATAD2B-CR2 domain to interact with the rest of the complex. Therefore, the phenotypic overlaps of CHD3- and CHD4-related disorders with GAND are consistent with a loss in the ability of GATAD2B to recruit CHD3 or CHD4 to the complex. The shared features of these neurodevelopmental disorders may represent a new class of OGID syndrome: the NuRDopathies.

The Nucleosome Remodelling and Deacetylation complex suppresses transcriptional noise during lineage commitment

Multiprotein chromatin remodelling complexes show remarkable conservation of function amongst metazoans, even though components present in invertebrates are often found as multiple paralogous proteins in vertebrate complexes. In some cases, these paralogues specify distinct biochemical and/or functional activities in vertebrate cells. Here, we set out to define the biochemical and functional diversity encoded by one such group of proteins within the mammalian Nucleosome Remodelling and Deacetylation (NuRD) complex: Mta1, Mta2 and Mta3. We find that, in contrast to what has been described in somatic cells, MTA proteins are not mutually exclusive within embryonic stem (ES) cell NuRD and, despite subtle differences in chromatin binding and biochemical interactions, serve largely redundant functions. ES cells lacking all three MTA proteins exhibit complete NuRD loss of function and are viable, allowing us to identify a previously unreported function for NuRD in reducing transcriptional noise, which is essential for maintaining a proper differentiation trajectory during early stages of lineage commitment.

NuRD subunit MTA1 interacts with the DNA non-homologous end joining Ku complex in cancer cells

Metastasis-associated antigen 1 (MTA1) is a chromatin modifier mediating DNA modification and gene expression. Ku70/Ku80 complex has been reported to be essential in DNA damage response. In an effort to explore the MTA1 interactome, we captured the Ku70/Ku80 complex with two specific MTA1 antibodies in a colon cancer cell line. We first validated the in vitro interaction between MTA1 and the Ku complex by co-immunoprecipitation (co-IP) analyses in cell lysate, showing that the interaction occurred mainly at the nucleus, but also existed in the cytoplasm at a lower level. We further visualized and confirmed their in vivo interaction using proximity ligation assay (PLA), which, in line with the in vitro analysis, also demonstrated a vast majority of interaction plots in the nucleus and a small number in the cytoplasm. We previously demonstrated that MTA1 distributed dynamically and periodically during the cell cycle. Here, through fluorescent colocalization, we found that MTA1 and Ku proteins colocalized well in the nucleus at interphase and moved synchronously from prophase to anaphase. Interestingly, at the time of telophase, when MTA1 was reported to re-enter the nucleus, they were separated and moved non-synchronously. Moreover, using in situ PLA, we visualized that the interaction occurred at both interphase and mitosis. At interphase, they interacted mainly in the nucleus, but during mitosis, they interact at the periphery of chromosomes. We also showed that MTA1 correlated well with Ku in both the cancerous and normal tissues, and that they cooperated in UV-induced DNA damage response. Collectively, our data uncover a specific interaction between MTA1 and Ku complex at both the nucleus and cytoplasm, and across the whole cell cycle. We therefore propose a potential functional crosstalk between NuRD and Ku complexes, the two most fundamental function units in cells, via physical interaction.

MTA1 interacts with Ku complex mainly in the nucleus at interphase and surrounding the chromosome during mitosis.

The Homeotic Protein SIX3 Suppresses Carcinogenesis and Metastasis through Recruiting the LSD1/NuRD(MTA3) Complex

The homeodomain transcription factor SIX3 was recently reported to be a negative regulator of the Wnt pathway and has an emerging role in cancer. However, how SIX3 contributes to tumorigenesis and metastasis is poorly understood.

METHODS

We employed affinity purification and mass spectrometry (MS) to identify the proteins physically associated with SIX3. Genome-wide analysis of the SIX3/LSD1/NuRD(MTA3) complex using a chromatin immunoprecipitation-on-chip approach identified a cohort of target genes including WNT1 and FOXC2, which are critically involved in cell proliferation and epithelial-to-mesenchymal transition. Also, we used flow cytometry, growth curve analysis, EdU incorporation assay, colony formation assays, trans-well invasion assays, immunohistochemical staining and in vivo bioluminescence assay to investigate the function of SIX3 in tumorigenesis.

RESULTS

We demonstrate that the SIX3/LSD1/NuRD(MTA3) complex inhibits carcinogenesis in breast cancer cells and suppresses metastasis in breast cancer. SIX3 expression is downregulated in various human cancers and high SIX3 is correlated with improved prognosis.

CONCLUSION

Our study revealed an important mechanistic link between the loss of function of SIX3 and tumor progression, identified a molecular basis for the opposing actions of MTA1 and MTA3, and may provide new potential prognostic indicators and targets for cancer therapy.

Identification of a conserved cis-regulatory element controlling mid-diencephalic expression of mouse Six3

The sine oculis homeobox protein Six3 plays pivotal roles in the development of the brain and craniofacial structures. In humans, SIX3 haploinsufficiency results in holoprosencephaly, a defect in anterior midline formation. Although much is known about the evolutionarily conserved functions of Six3, the regulatory mechanism responsible for the expression pattern of Six3 remains relatively unexplored. To understand how the transcription of Six3 is controlled during embryogenesis, we screened ∼300 kb of genomic DNA encompassing the Six3 locus for cis-acting regulatory elements capable of directing reporter gene expression to sites of Six3 transcription in transgenic mouse embryos. We identified a novel enhancer element, whose activity recapitulates endogenous Six3 expression in the ventral midbrain, pretectum, and thalamus. Cross-species comparisons revealed that this Six3 brain enhancer is functionally conserved in other vertebrates. We also showed that normal Six3 transcription in the ventral midbrain and pretectum is dependent on Ascl1, a basic helix-loop-helix proneural factor. Moreover, loss of Ascl1 resulted in downregulation of the Six3 brain enhancer activity, emphasizing its unique role in regulating Six3 expression in the developing brain.

Structure, expression and functions of MTA genes

Metastatic associated proteins (MTA) are integrators of upstream regulatory signals with the ability to act as master coregulators for modifying gene transcriptional activity. The MTA family includes three genes and multiple alternatively spliced variants. The MTA proteins neither have their own enzymatic activity nor have been shown to directly interact with DNA. However, MTA proteins interact with a variety of chromatin remodeling factors and complexes with enzymatic activities for modulating the plasticity of nucleosomes, leading to the repression or derepression of target genes or other extra-nuclear and nucleosome remodeling and histone deacetylase (NuRD)-complex independent activities. The functions of MTA family members are driven by the steady state levels and subcellular localization of MTA proteins, the dynamic nature of modifying signals and enzymes, the structural features and post-translational modification of protein domains, interactions with binding proteins, and the nature of the engaged and resulting features of nucleosomes in the proximity of target genes. In general, MTA1 and MTA2 are the most upregulated genes in human cancer and correlate well with aggressive phenotypes, therapeutic resistance, poor prognosis and ultimately, unfavorable survival of cancer patients. Here we will discuss the structure, expression and functions of the MTA family of genes in the context of cancer cells.

Crosstalk between ATF4 and MTA1/HDAC1 promotes osteosarcoma progression

The stress response gene activating transcription factor 4 (ATF4) is involved in metastatic behavior and cellular protection. Here we show that ATF4 is upregulated in osteosarcoma (OS) cell lines and patient clinical samples as compared to matched non-tumor tissue. Overexpression of ATF4 in OS cells promoted cell proliferation, migration and lung metastasis. Furthermore, the expression of ATF4 was markedly reduced in metastasis associated protein (MTA1) or histone deacetylase 1 (HDAC1) knockdown OS cells, but MTA1 overexpression increased the stability and activity of ATF4 protein via ATF4 deacetylation by HDAC1. ATF4 in turn enhanced the expression of MTA1 and HDAC1 at the transcription level, suggesting a positive feedback loop between ATF4 and MTA1/HDAC1. Clinically, the level of ATF4 was positively correlated with that of MTA1 in OS. Mice injected with ATF4-overexpressing cells exhibited a higher rate of tumor growth, and the average weight of these tumors was ~90% greater than the controls. Taken together, these data establish a direct correlation between ATF4-induced OS progression and MTA1/HDAC1-associated metastasis, and support the potential therapeutic value of targeting ATF4 in the treatment of OS.

A trans-Regulatory Code for the Forebrain Expression of Six3.2 in the Medaka Fish

A well integrated and hierarchically organized gene regulatory network is responsible for the progressive specification of the forebrain. The transcription factor Six3 is one of the central components of this network. As such, Six3 regulates several components of the network, but its upstream regulators are still poorly characterized. Here we have systematically identified such regulators, taking advantage of the detailed functional characterization of the regulatory region of the medaka fish Six3.2 ortholog and of a time/cost-effective trans-regulatory screening, which complemented and overcame the limitations of in silico prediction approaches. The candidates resulting from this search were validated with dose-response luciferase assays and expression pattern criteria. Reconfirmed candidates with a matching expression pattern were also tested with chromatin immunoprecipitation and functional studies. Our results confirm the previously proposed direct regulation of Pax6 and further demonstrate that Msx2 and Pbx1 are bona fide direct regulators of early Six3.2 distribution in distinct domains of the medaka fish forebrain. They also point to other transcription factors, including Tcf3, as additional regulators of different spatial-temporal domains of Six3.2 expression. The activity of these regulators is discussed in the context of the gene regulatory network proposed for the specification of the forebrain.

Clinicopathological and prognostic significance of metastasis-associated protein 1 expression and its correlation with angiogenesis in lung invasive adenocarcinomas, based on the 2011 IASLC/ATS/ERS classification

Based on previous findings regarding the angiogenic activities and prognostic roles of metastasis-associated protein 1 (MTA1) in early-stage non-small cell lung cancer, the clinicopathological and prognostic significance of MTA1 protein expression, and its correlation with angiogenesis in lung invasive adenocarcinoma, were further assessed in the present study, according to the 2011 International Association for the Study of Lung Cancer/American Thoracic Society/European Respiratory Society classification. High protein expression levels of MTA1 were commonly observed in patients with lung invasive adenocarcinoma, and were significantly correlated with tumor size (P=0.030), lymph node metastasis (P=0.021) and microvessel density (P=0.015). Survival analysis demonstrated that patients with high protein expression levels of MTA1 exhibited significantly shorter five-year disease-free and overall survival than those patients whose protein expression levels of MTA1 were low (24.5% vs. 48.7%, P=0.001, and 34.7% vs. 59.2%, P=0.005, respectively). In addition, Cox regression multivariate analysis demonstrated that high protein expression levels of MTA1 significantly correlated with unfavorable five-year disease-free survival (P=0.024). These findings indicate that MTA1 protein expression may possess clinical potential as an indicator of progressive phenotype. Therefore, MTA1 is a promising prognostic predictor to identify subgroups of patients with high risk of relapse, and a potentially novel therapeutic target for antiangiogenesis in patients with lung invasive adenocarcinoma.

Unravelling the Complexity and Functions of MTA Coregulators in Human Cancer

Since the initial recognition of the metastasis-associated protein 1 (MTA1) as a metastasis-relevant gene approximately 20 years ago, our appreciation for the complex role of the MTA family of coregulatory proteins in human cancer has profoundly grown. MTA proteins consist of six family members with similar structural units and act as central signaling nodes for integrating upstream signals into regulatory chromatin-remodeling networks, leading to regulation of gene expression in cancer cells. Substantial experimental and clinical evidence demonstrates that MTA proteins, particularly MTA1, are frequently deregulated in a wide range of human cancers. The MTA family governs cell survival, the invasive and metastatic phenotypes of cancer cells, and the aggressiveness of cancer and the prognosis of patients with MTA1 overexpressing cancers. Our discussion here highlights our current understanding of the regulatory mechanisms and functional roles of MTA proteins in cancer progression and expands upon the potential implications of MTA proteins in cancer biology and cancer therapeutics.

The nucleosome remodeling and deacetylase complex in development and disease

The nucleosome remodeling and deacetylase (NuRD) complex is one of the major chromatin remodeling complexes found in cells. It plays an important role in regulating gene transcription, genome integrity, and cell cycle progression. Through its impact on these basic cellular processes, increasing evidence indicates that alterations in the activity of this macromolecular complex can lead to developmental defects, oncogenesis, and accelerated aging. Recent genetic and biochemical studies have elucidated the mechanisms of NuRD action in modifying the chromatin landscape. These advances have the potential to lead to new therapeutic approaches to birth defects and cancer.

MTA1--a stress response protein: a master regulator of gene expression and cancer cell behavior

Gene mutation's role in initiating carcinogenesis has been controversial, but it is consensually accepted that both carcinogenesis and cancer metastasis are gene-regulated processes. MTA1, a metastasis-associated protein, has been extensively researched, especially regarding its role in cancer metastasis. In this review, I try to elucidate MTA1's role in both carcinogenesis and metastasis from a different angle. I propose that MTA1 is a stress response protein that is upregulated in various stress-related situations such as heat shock, hypoxia, and ironic radiation. Cancer cells are mostly living in a stressful environment of hypoxia, lack of nutrition, and immune reaction attacks. To cope with all these stresses, MTA1 expression is upregulated, plays a role of master regulator of gene expression, and helps cancer cells to survive and migrate out of their original dwelling.

Molecular functions and significance of the MTA family in hormone-independent cancer

The members of the metastasis-associated protein (MTA) family play pivotal roles in both physiological and pathophysiological processes, especially in cancer development and metastasis, and their role as master regulators has come to light. Due to the fact that they were first identified as crucial factors in estrogen receptor-mediated breast cancer metastasis, most of the early studies focused on their hormone-dependent functions. However, the accumulating evidence shows that the members of MTA family are deregulated in most, if not all, the cancers studied so far. Therefore, the levels as well as the activities of the MTA family members are widely accepted as potential biomarkers for diagnosis, prognosis, and predictors of overall survival. They function differently in different cancers with specific mechanisms. p53 and HIF-1α appear to be the respectively common upstream and downstream regulator of the MTA family in both development and metastasis of a wide spectrum of cancers. Here, we review the expression and clinical significance of the MTA family, focusing on hormone-independent cancers. To illustrate the molecular mechanisms, we analyze the MTA family-related signaling pathways in different cancers. Finally, targeting the MTA family directly or the pathways involved in the MTA family indirectly could be invaluable strategies in the development of cancer therapeutics.

Physiological functions of MTA family of proteins

Although the functional significance of the metastasic tumor antigen (MTA) family of chromatin remodeling proteins in the pathobiology of cancer is fairly well recognized, the physiological role of MTA proteins continues to be an understudied research area and is just beginning to be recognized. Similar to cancer cells, MTA1 also modulates the expression of target genes in normal cells either by acting as a corepressor or coactivator. In addition, physiological functions of MTA proteins are likely to be influenced by its differential expression, subcellular localization, and regulation by upstream modulators and extracellular signals. This review summarizes our current understanding of the physiological functions of the MTA proteins in model systems. In particular, we highlight recent advances of the role MTA proteins play in the brain, eye, circadian rhythm, mammary gland biology, spermatogenesis, liver, immunomodulation and inflammation, cellular radio-sensitivity, and hematopoiesis and differentiation. Based on the growth of knowledge regarding the exciting new facets of the MTA family of proteins in biology and medicine, we speculate that the next burst of findings in this field may reveal further molecular regulatory insights of non-redundant functions of MTA coregulators in the normal physiology as well as in pathological conditions outside cancer.

Clinical implications of MTA proteins in human cancer

Metastasis-associated gene or metastasis tumor antigen 1 (MTA1) is a new member of cancer progression-related gene family. It was first identified in rat mammary adenocarcinoma and later recognized as an important constituent of nucleosomal remodeling complex (NuRD), displaying dual regulatory functions as a co-repressor and co-activator for a large number of genes. Chromatin remodelers are ATP-dependent multi-protein chromatin modifying machines. These complexes alter the nucleosome positioning regulating the accessibility of genomic DNA to various transcription factors and thus modulate eukaryotic gene transcription. Since its identification two decades ago, MTA1 has been reported to be overexpressed in many cancers. Moreover, its overexpression has also been correlated with transformation and tumor progression. Furthermore, MTA1 has been shown to modulate the response of several tumor suppressor genes like p53 and oncogenes like c-myc. Taken together, current literature suggests that MTA proteins, especially MTA1, act as a master co-regulatory molecule involved in the carcinogenesis and progression of various malignant tumors. The primary focus of this review is to provide an overview of the MTA proteins with special emphasis on its role in cancer and use as a marker for cancer progression and potential target for therapy.

Subcellular localization of MTA proteins in normal and cancer cells

The subcellular localization of a protein is closely linked to and indicates its function. The metastatic tumor antigen (MTA) family has been under continuous investigation since its identification two decades ago. MTA1, MTA2, and MTA3 are the main members of the MTA family. MTA1, as the representative member of this family, has been shown to be widely expressed in both embryonic and adult tissues, as well as in normal and cancerous conditions, indicating that MTA1 has functions both in physiological and pathological contexts. MTA1 is expressed at a higher level in most cancers than in their normal tissue counterparts. Even in normal cells, MTA1 levels vary a great deal from tissue to tissue. Importantly, MTA1 shows a multiple localization pattern in the cell, as do MTA2 and MTA3. Different MTA components in different subcellular compartments may exert different molecular functions in the cell. Previous studies revealed that MTA1 and MTA2 are predominately localized to the nucleus, while MTA3 is observed in both the nucleus and cytoplasm. Recent studies have reported that MTA1 is located in the nucleus, cytoplasm, and the nuclear envelope. In the nucleus, MTA1 dynamically interacts with chromatin in a MTA1-K532 methylation-dependent manner, whereas cytoplasmic MTA1 binds to the microtubule skeleton. MTA1 also shows a dynamic distribution during the cell cycle. Further investigations are needed to identify the exact subcellular localizations of MTA proteins. We review the sub-cellular localization patterns of the MTA family members and give a comprehensive overview of their respective molecular activities in multiple contexts.

MTA family of proteins in prostate cancer: biology, significance, and therapeutic opportunities

This review summarizes our current understanding of the role of MTA family members, particularly MTA1, with a special emphasis on prostate cancer. The interest for the role of MTA1 in prostate cancer was boosted from our initial findings of MTA1 as a component of "vicious cycle" and a member of bone metastatic signature. Analysis of human prostate tissues, xenograft and transgenic mouse models of prostate cancer, and prostate cancer cell lines has provided support for the role of MTA1 in advanced disease and its potential role in initial stages of prostate tumor progression. Recent discoveries have highlighted a critical role for MTA1 in inflammation-triggered prostate tumorigenesis, epithelial-to-mesenchymal transition, prostate cancer survival pathways, and site metastasis. Evidence for MTA1 as an upstream negative regulator of tumor suppressor genes such as p53 and PTEN has also emerged. MTA1 is involved in prostate tumor angiogenesis by regulating several pro-angiogenic factors. Evidence for MTA1 as a prognostic marker for aggressive prostate cancer and disease recurrence has been described. Importantly, pharmacological dietary agents, namely resveratrol and its analogs, are potentially applicable to prostate cancer prevention, treatment, and control of cancer progression due to their potent inhibitory effects on MTA proteins.

Estrogen receptor coregulators and pioneer factors: the orchestrators of mammary gland cell fate and development

The steroid hormone, 17β-estradiol (E2), plays critical role in various cellular processes such as cell proliferation, differentiation, migration and apoptosis, and is essential for reproduction and mammary gland development. E2 actions are mediated by two classical nuclear hormone receptors, estrogen receptor α and β (ERs). The activity of ERs depends on the coordinated activity of ligand binding, post-translational modifications (PTMs), and importantly the interaction with their partner proteins called “coregulators.” Because coregulators are proved to be crucial for ER transcriptional activity, and majority of breast cancers are ERα positive, an increased interest in the field has led to the identification of a large number of coregulators. In the last decade, gene knockout studies using mouse models provided impetus to our further understanding of the role of these coregulators in mammary gland development. Several coregulators appear to be critical for terminal end bud (TEB) formation, ductal branching and alveologenesis during mammary gland development. The emerging studies support that, coregulators along with the other ER partner proteins called “pioneer factors” together contribute significantly to E2 signaling and mammary cell fate. This review discusses emerging themes in coregulator and pioneer factor mediated action on ER functions, in particular their role in mammary gland cell fate and development.

Impact of histone deacetylase 1 and metastasis-associated gene 1 expression in esophageal carcinogenesis

Animal models are important for the development of novel therapies for esophageal cancer. Histone deacetylase 1 (HDAC1)/metastasis-associated gene (MTA1) complexes inhibit p53 acetylation and thus, inhibit p53-induced apoptosis. The aim of the present study was to evaluate HDAC1 and MTA1 expression in esophageal carcinogenesis in rats. The rats underwent a total gastrectomy followed by esophagojejunostomy to induce chronic duodenal content reflux esophagitis. The rats were sacrificed sequentially at 20, 30, 40 and 50 weeks post-surgery and the esophagi were examined. Immunohistochemical analysis was conducted to assess the expression and localization of HDAC1 and MTA1. At 20 weeks post-surgery, squamous proliferative hyperplasia and Barrett’s metaplasia (BM) were observed. While, adenocarcinoma-associated BM and squamous cell carcinoma were observed at 30–50 weeks post-surgery. The nuclear expression of HDAC1 and MTA1 was observed in all of the stages of squamous carcinogenesis and adenocarcinogenesis, although not in the normal esophageal epithelium. The expression of HDAC1 and MTA1 may be involved in duodenoesophageal reflux-induced neoplastic transformation of the esophageal mucosa into cancer cells with squamous and adeno differentiation.

The bilaterian forebrain: an evolutionary chimaera

The insect, annelid and vertebrate forebrains harbour two major centres of output control, a sensory-neurosecretory centre releasing hormones and a primordial locomotor centre that controls the initiation of muscular body movements. In vertebrates, both reside in the hypothalamus. Here, we review recent comparative neurodevelopmental evidence indicating that these centres evolved from separate condensations of neurons on opposite body sides ('apical nervous system' versus 'blastoporal nervous system') and that their developmental specification involved distinct regulatory networks (apical six3 and rx versus mediolateral nk and pax gene-dependent patterning). In bilaterian ancestors, both systems approached each other and became closely intermingled, physically, functionally and developmentally. Our 'chimeric brain hypothesis' sheds new light on the vast success and rapid diversification of bilaterian animals in the Cambrian and revises our understanding of brain architecture.

MTA1-mediated transcriptional repression of SMAD7 in breast cancer cell lines

Metastasis is a complex process facilitated by the action of several genes. Metastasis associated 1 (MTA1) gene is one such gene which assists the process of metastasis by regulating several molecular targets. MTA1 acts as part of a nucleosome remodelling and histone deacetylation complex, which is involved in transcriptional regulation. Expression of MTA1 has been shown to be closely correlated with aggressiveness in several types of cancers, including breast cancer. In the present study we show that MTA1 regulates SMAD7, a component of Transforming growth factor beta (TGFbeta) signalling. TGFbeta signals are transduced to the nucleus by the Smad family of proteins, which includes Smad7, an inhibitory SMAD, which acts as a negative regulator of TGFbeta. On knockdown of MTA1, SMAD7 expression increases. Treating cells with a histone deacetylase inhibitor also increases SMAD7 expression. MTA1 is recruited to SMAD7 promoter region. SMAD7 inhibits activation of SMAD2 and SMAD3 and we show that the levels of these active SMAD proteins are decreased in cells expressing shRNA against MTA1. We further show that on MTA1 knockdown, the expression of downstream targets of SMAD7 is decreased. MTA1 thus appears to regulate a key inhibitor of TGFbeta signalling, SMAD7. By regulating molecules like SMAD7 MTA1 might assist the process of tumourigenesis and metastasis.

Metastasis-associated protein 1 is an integral component of the circadian molecular machinery

The mammalian circadian clock regulates the daily cycles of many important physiological processes, but its mechanism is not well understood. Here we provide genetic and biochemical evidence that metastasis-associated protein 1 (MTA1), a widely upregulated gene product in human cancers, is an integral component of the circadian molecular machinery. Knockout of MTA1 in mice disrupts the free-running period of circadian rhythms under constant light and normal entrainment of behaviour to 12-h-light/12-h-dark cycles. The CLOCK-BMAL1 heterodimer activates MTA1 transcription through a conserved E-box element at its promoter. MTA1, in turn, interacts with and recruits CLOCK-BMAL1 at its own and CRY1 promoters and promotes their transcription. Moreover, MTA1 deacetylates BMAL1 at lysine 538 through regulating deacetylase SIRT1 expression, thus disturbing the CRY1-mediated negative feedback loop. These findings uncover a previously unappreciated role for MTA1 in maintenance of circadian rhythmicity through acting on the positive limb of the clock machinery.

Akt mediates metastasis-associated gene 1 (MTA1) regulating the expression of E-cadherin and promoting the invasiveness of prostate cancer cells

Human metastasis-associated gene 1 (MTA1) is highly associated with the metastasis of prostate cancer; however, the molecular functions of MTA1 that facilitate metastasis remain unclear. In this study, we demonstrate that the silencing of MTA1 by siRNA treatment results in the upregulation of E-cadherin expression by the phosphorylation of AKT (p-AKT) and decreases the invasiveness of prostate cancer cells. We show that MTA1 is expressed in over 90% of prostate cancer tissues, especially metastatic prostate cancer tissue, comparing to non-expression in normal prostate tissue. RT-PCR analysis and Western blot assay showed that MTA1 expression is significantly higher in highly metastatic prostate cancer PC-3M-1E8 cells (1E8) than in poorly metastatic prostate cancer PC-3M-2B4 cells (2B4). Silencing MTA1 expression by siRNA treatment in 1E8 cells increased the cellular malignant characters, including the cellular adhesive ability, decreased the cellular invasive ability and changed the polarity of cellular cytoskeleton. 1E8 cells over-expressing MTA1 had a reduced expression of E-cadherin, while 1E8 cells treated with MTA1 siRNA had a higher expression of E-cadherin. The expression of phosphorylated AKT (p-AKT) or the inhibition of p-AKT by wortmannin treatment (100 nM) significantly altered the function of MTA1 in the regulation of E-cadherin expression. Alterations in E-cadherin expression changed the role of p-AKT in cellular malignant characters. All of these results demonstrate that MTA1 plays an important role in controlling the malignant transformation of prostate cancer cells through the p-AKT/E-cadherin pathway. This study also provides a new mechanistic role for MTA1 in the regulation of prostate cancer metastasis.

Otx2 is involved in the regional specification of the developing retinal pigment epithelium by preventing the expression of sox2 and fgf8, factors that induce neural retina differentiation

The retinal pigment epithelium (RPE) shares its developmental origin with the neural retina (NR). When RPE development is disrupted, cells in the presumptive RPE region abnormally differentiate into NR-like cells. Therefore, the prevention of NR differentiation in the presumptive RPE area seems to be essential for regionalizing the RPE during eye development. However, its molecular mechanisms are not fully understood. In this study, we conducted a functional inhibition of a transcription factor Otx2, which is required for RPE development, using early chick embryos. The functional inhibition of Otx2 in chick eyes, using a recombinant gene encoding a dominant negative form of Otx2, caused the outer layer of the optic cup (the region forming the RPE, when embryos normally develop) to abnormally form an ectopic NR. In that ectopic NR, the characteristics of the RPE did not appear and NR markers were ectopically expressed. Intriguingly, the repression of Otx2 function also caused the ectopic expression of Fgf8 and Sox2 in the outer layer of the optic cup (the presumptive RPE region of normally developing eyes). These two factors are known to be capable of inducing NR cell differentiation in the presumptive RPE region, and are not expressed in the normally developing RPE region. Here, we suggest that Otx2 prevents the presumptive RPE region from forming the NR by repressing the expression of both Fgf8 and Sox2 which induce the NR cell fate.

Sertoli cell-specific expression of metastasis-associated protein 2 (MTA2) is required for transcriptional regulation of the follicle-stimulating hormone receptor (FSHR) gene during spermatogenesis

BACKGROUND

Desensitization of FSH response by down-regulation of FSHR transcription is critical for FSH action.

RESULTS

Chromatin modifier MTA2 participates in the down-regulation of FSHR transcription.

CONCLUSION

The FSH/Ar/MTA2 cascade may serve as an indispensable negative feedback mechanism to modulate FSH transduction events in Sertoli cells.

SIGNIFICANCE

Our findings provide new insights into mechanisms by which FSH is deregulated in male infertile patients. The effect of follicle-stimulating hormone (FSH) on spermatogenesis is modulated at a fundamental level by controlling the number of competent receptors present at the surface of Sertoli cells (SCs). One underlying mechanism is the down-regulation of the expression levels of the FSH receptor (FSHR) gene after exposure to FSH. Here we report that metastasis-associated protein 2 (MTA2), a component of histone deacetylase and nucleosome-remodeling complexes, as a gene product induced directly by testosterone or indirectly by FSH, is exclusively expressed in SCs. Stimulation of SCs with FSH is accompanied by up-regulation of MTA2 expression and enhancement of deacetylase activity. This effect requires the integrity of functional androgen receptor. Furthermore, MTA2 is a potent corepressor of FSHR transcription, because it can recruit histone deacetylase-1 onto the FSHR promoter and participates in the down-regulation of FSHR expression upon FSH treatment. Abolishment of endogenous MTA2 by siRNA treatment disrupted the desensitization of the FSH response and thereafter impaired the FSH-dependent secretory function of SCs. From a clinical standpoint, deregulated expression of MTA2 in SCs of human pathological testes negatively correlates to the deregulated level of serum FSH. Overall, our present results provide the first evidence that the FSH/androgen receptor/MTA2 cascade may serve as an indispensable negative feedback mechanism to modulate the transduction events of SCs in response to FSH. These data also underscore an unexpected reproductive facet of MTA2, which may operate as a novel integrator linking synergistic actions of FSH and androgen signaling in SCs.

Loss of E-cadherin promotes prostate cancer metastasis via upregulation of metastasis-associated gene 1 expression

E-cadherin is a key cell-to-cell adhesion molecule associated with the invasion and metastasis of tumor cells; however, the molecular mechanisms are not entirely understood. In this study, we investigated whether downregulation of E-cadherin by E-cadherin-specific small intefering RNA (siRNA) was able to promote malignant phenotypes of prostate cancer cells through upregulating the metastasis-associated gene 1 (MTA1) in vitro. The expression levels of E-cadherin in human paired prostate adenocarcinoma cell lines, PC-3M-2B4 (2B4) and PC-3M-1E8 (1E8), were investigated using western blot analysis. The alteration of malignant phenotypes associated with decreasing E-cadherin expression were assessed in 2B4 cells using wound-healing assays, solid-phase adhesion assays, invasion assays and cytoskeletal staining. The expression of E-cadherin and MTA1 in normal, localized and metastatic prostate cancer cells was analyzed using immunohistochemistry. Downregulation of E-cadherin using an RNA interference approach led to the upregulation of MTA1 expression, decreased tumor cell adhesion ability as well as enhanced cell mobility, invasion and cellular polarity compared with the controls (P<0.05). E-cadherin regulated MTA1 in a time-dependent manner. The correlation between E-cadherin and MTA1 was inversed in the prostate cancer group (P<0.05; r(s)=-0.434). The data suggest that E-cadherin plays an important role in prostate cancer metastasis, which is likely to be due to the regulation of MTA1 expression. E-cadherin may combine with MTA1 and alter the malignant phenotype of prostate cancer cells. A combined testing strategy for detecting MTA1 and E-cadherin may be sufficient for selecting high-risk patients with metastasis. Therefore, E-cadherin and MTA1 may be potential powerful factors for the treatment of various types of cancer.

Derivation of human differential photoreceptor-like cells from the iris by defined combinations of CRX, RX and NEUROD

Examples of direct differentiation by defined transcription factors have been provided for beta-cells, cardiomyocytes and neurons. In the human visual system, there are four kinds of photoreceptors in the retina. Neural retina and iris-pigmented epithelium (IPE) share a common developmental origin, leading us to test whether human iris cells could differentiate to retinal neurons. We here define the transcription factor combinations that can determine human photoreceptor cell fate. Expression of rhodopsin, blue opsin and green/red opsin in induced photoreceptor cells were dependent on combinations of transcription factors: A combination of CRX and NEUROD induced rhodopsin and blue opsin, but did not induce green opsin; a combination of CRX and RX induced blue opsin and green/red opsin, but did not induce rhodopsin. Phototransduction-related genes as well as opsin genes were up-regulated in those cells. Functional analysis; i.e. patch clamp recordings, clearly revealed that generated photoreceptor cells, induced by CRX, RX and NEUROD, responded to light. The response was an inward current instead of the typical outward current. These data suggest that photosensitive photoreceptor cells can be generated by combinations of transcription factors. The combination of CRX and RX generate immature photoreceptors: and additional NEUROD promotes maturation. These findings contribute substantially to a major advance toward eventual cell-based therapy for retinal degenerative diseases.

Metastasis-associated protein 1 drives tumor cell migration and invasion through transcriptional repression of RING finger protein 144A

Metastasis-associated protein 1 (MTA1), a component of the nucleosome-remodeling and histone deacetylase complex, is widely up-regulated in human cancers and significantly correlated with tumor invasion and metastasis, but the mechanisms involved remain largely unknown. Here, we report that MTA1 transcriptionally represses the expression of RING finger protein 144A (RNF144A), an uncharacterized gene whose protein product possesses potential E3 ubiquitin ligase activity, by recruiting the histone deacetylase 2 (HDAC2) and CCAAT/enhancer-binding protein α (c/EBPα) co-repressor complex onto human RNF144A promoter. Furthermore, an inverse correlation between the expression levels of MTA1 and RNF144A was demonstrated in publicly available breast cancer microarray datasets and the MCF10 breast cancer progression model system. To address functional aspects of MTA1 regulation of RNF144A, we demonstrate that RNF144A is a novel suppressor of cancer migration and invasion, two requisite steps of metastasis in vivo, and knockdown of endogenous RNF144A by small interfering RNAs accelerates the migration and invasion of MTA1-overexpressing cells. These results suggest that RNF144A is partially responsible for MTA1-mediated migration and invasion and that MTA1 overexpression in highly metastatic cancer cells drives cell migration and invasion by, at least in part, interfering with the suppressive function of RNF144A through transcriptional repression of RNF144A expression. Together, these findings provide novel mechanistic insights into regulation of tumor progression and metastasis by MTA1 and highlight a previously unrecognized role of RNF144A in MTA1-driven cancer cell migration and invasion.

Sox2-mediated differential activation of Six3.2 contributes to forebrain patterning

The vertebrate forebrain is patterned during gastrulation into telencephalic, retinal, hypothalamic and diencephalic primordia. Specification of each of these domains requires the concerted activity of combinations of transcription factors (TFs). Paradoxically, some of these factors are widely expressed in the forebrain, which raises the question of how they can mediate regional differences. To address this issue, we focused on the homeobox TF Six3.2. With genomic and functional approaches we demonstrate that, in medaka fish, Six3.2 regulates, in a concentration-dependent manner, telencephalic and retinal specification under the direct control of Sox2. Six3.2 and Sox2 have antagonistic functions in hypothalamic development. These activities are, in part, executed by Foxg1 and Rx3, which seem to be differentially and directly regulated by Six3.2 and Sox2. Together, these data delineate the mechanisms by which Six3.2 diversifies its activity in the forebrain and highlight a novel function for Sox2 as one of the main regulators of anterior forebrain development. They also demonstrate that graded levels of the same TF, probably operating in partially independent transcriptional networks, pattern the vertebrate forebrain along the anterior-posterior axis.

Inflammatory response to liver fluke Opisthorchis viverrini in mice depends on host master coregulator MTA1, a marker for parasite-induced cholangiocarcinoma in humans

Based on the recently established role for the master coregulator MTA1 and MTA1-containing nuclear remodeling complexes in oncogenesis and inflammation, we explored the links between parasitism by the carcinogenic liver fluke Opisthorchis viverrini and this coregulator using both an Mta1(-/-) mouse model of infection and a tissue microarray of liver fluke-induced human cholangiocarcinomas (CCAs). Intense foci of inflammation and periductal fibrosis in the liver and kidneys of wild-type Mta1(+/+) mice were evident at 23 days postinfection with O. viverrini. In contrast, little inflammatory response was observed in the same organs of infected Mta1(-/-) mice. Livers of infected Mta1(+/+) mice revealed strong up-regulation of fibrosis-associated markers such as cytokeratins 18 and 19 and annexin 2, as determined both by immunostaining and by reverse-transcription polymerase chain reaction compared with infected Mta1(-/-) mice. CD4 expression was up-regulated by infection in the livers of both experimental groups; however, its levels were several-fold higher in the Mta1(+/+) mice than in infected Mta1(-/-) mice. Mta1(-/-) infected mice also exhibited significantly higher systemic and hepatic levels of host cytokines such as interleukin (IL)-12p70, IL-10, and interferon-γ compared with the levels of these cytokines in the Mta1(+/+) mice, suggesting an essential role of MTA1 in the cross-regulation of the Th1 and Th2 responses, presumably due to chromatin remodeling of the target chromatin genes. Immunohistochemical analysis of ≈ 300 liver tissue cores from confirmed cases of O. viverrini-induced CCA showed that MTA1 expression was elevated in >80% of the specimens.

CONCLUSION

These findings suggest that MTA1 status plays an important role in conferring an optimal cytokine response in mice following infection with O. viverrini and is a major player in parasite-induced CCA in humans.

The metastasis-associated protein-1 gene encodes a host permissive factor for schistosomiasis, a leading global cause of inflammation and cancer

Schistosoma haematobium is responsible for two-thirds of the world's 200 million to 400 million cases of human schistosomiasis. It is a group 1 carcinogen and a leading cause of bladder cancer that occurs after years of chronic inflammation, fibrosis, and hyperproliferation in the host liver. The coevolution of blood flukes of the genus Schistosoma and their human hosts is paradigmatic of long-term parasite development, survival, and maintenance in mammals. However, the contribution of host genes, especially those discrete from the immune system, necessary for parasite establishment and development remains poorly understood. This study investigated the role of metastasis-associated protein-1 gene (Mta1) product in the survival of S. haematobium and productive infection in the host. Using a Mta-1 null mouse model, here we provide genetic evidence to suggest that MTA1 expression positively influences survival and/or maturation of schistosomes in the host to patency, as we reproducibly recovered significantly fewer S. haematobium worms and eggs from Mta1-/- mice than wild-type mice. In addition, we found a distinct loss of cytokine interdependence and aberrant Th1 and Th2 cytokine responses in the Mta1-/- mice compared to age-matched wild-type mice. Thus, utilizing this Mta1-null mouse model, we identified a distinct contribution of the mammalian MTA1 in establishing a productive host-parasite interaction and thus revealed a host factor critical for the optimal survival of schistosomes and successful parasitism. Moreover, MTA1 appears to play a significant role in driving inflammatory responses to schistosome egg-induced hepatic granulomata reactions, and thus offers a survival cue for parasitism as well as an obligatory contribution of liver in schistosomiasis.

CONCLUSION

These findings raise the possibility to develop intervention strategies targeting MTA1 to reduce the global burden of schistosomiasis, inflammation, and neoplasia.

Bidirectional autoregulatory mechanism of metastasis-associated protein 1-alternative reading frame pathway in oncogenesis

Although metastasis-associated protein 1 (MTA1), a component of the nucleosome remodeling and histone deacetylation complex, is widely up-regulated in human cancers and correlates with tumor metastasis, its regulatory mechanism and related signaling pathways remain unknown. Here, we report a previously unrecognized bidirectional autoregulatory loop between MTA1 and tumor suppressor alternative reading frame (ARF). MTA1 transactivates ARF transcription by recruiting the transcription factor c-Jun onto the ARF promoter in a p53-independent manner. ARF, in turn, negatively regulates MTA1 expression independently of p53 and c-Myc. In this context, ARF interacts with transcription factor specificity protein 1 (SP1) and promotes its proteasomal degradation by enhancing its interaction with proteasome subunit regulatory particle ATPase 6, thereby abrogating the ability of SP1 to stimulate MTA1 transcription. ARF also physically associates with MTA1 and affects its protein stability. Thus, MTA1-mediated activation of ARF and ARF-mediated functional inhibition of MTA1 represent a p53-independent bidirectional autoregulatory mechanism in which these two opposites act in concert to regulate cell homeostasis and oncogenesis, depending on the cellular context and the environment.

Development, maturation, and necessity of transcription factors in the mouse suprachiasmatic nucleus

The suprachiasmatic nucleus (SCN) of the hypothalamus is the master mammalian circadian clock. The SCN is highly specialized because it is responsible for generating a near 24 h rhythm, integrating external cues, and translating the rhythm throughout the body. Currently, our understanding of the developmental origin and genetic program involved in the proper specification and maturation of the SCN is limited. Herein, we provide a detailed analysis of transcription factor (TF) and developmental-gene expression in the SCN from neurogenesis to adulthood in mice (Mus musculus). TF expression within the postmitotic SCN was not static but rather showed specific temporal and spatial changes during prenatal and postnatal development. In addition, we found both global and regional patterns of TF expression extending into the adult. We found that the SCN is derived from a distinct region of the neuroepithelium expressing a combination of developmental genes: Six3, Six6, Fzd5, and transient Rx, allowing us to pinpoint the origin of this region within the broader developing telencephalon/diencephalon. We tested the necessity of two TFs in SCN development, RORα and Six3, which were expressed during SCN development, persisted into adulthood, and showed diurnal rhythmicity. Loss of RORα function had no effect on SCN peptide expression or localization. In marked contrast, the conditional deletion of Six3 from early neural progenitors completely eliminated the formation of the SCN. Our results provide the first description of the involvement of TFs in the specification and maturation of a neural population necessary for circadian behavior.

Gene profiling of MTA1 identifies novel gene targets and functions

Background

Metastasis-associated protein 1 (MTA1), a master dual co-regulatory protein is found to be an integral part of NuRD (Nucleosome Remodeling and Histone Deacetylation) complex, which has indispensable transcriptional regulatory functions via histone deacetylation and chromatin remodeling. Emerging literature establishes MTA1 to be a valid DNA-damage responsive protein with a significant role in maintaining the optimum DNA-repair activity in mammalian cells exposed to genotoxic stress. This DNA-damage responsive function of MTA1 was reported to be a P53-dependent and independent function. Here, we investigate the influence of P53 on gene regulation function of Mta1 to identify novel gene targets and functions of Mta1.

Methods

Gene expression analysis was performed on five different mouse embryonic fibroblasts (MEFs) samples (i) the Mta1 wild type, (ii) Mta1 knock out (iii) Mta1 knock out in which Mta1 was reintroduced (iv) P53 knock out (v) P53 knock out in which Mta1 was over expressed using Affymetrix Mouse Exon 1.0 ST arrays. Further Hierarchical Clustering, Gene Ontology analysis with GO terms satisfying corrected p-value<0.1, and the Ingenuity Pathway Analysis were performed. Finally, RT-qPCR was carried out on selective candidate genes.

Significance/Conclusion

This study represents a complete genome wide screen for possible target genes of a coregulator, Mta1. The comparative gene profiling of Mta1 wild type, Mta1 knockout and Mta1 re-expression in the Mta1 knockout conditions define “bona fide” Mta1 target genes. Further extensive analyses of the data highlights the influence of P53 on Mta1 gene regulation. In the presence of P53 majority of the genes regulated by Mta1 are related to inflammatory and anti-microbial responses whereas in the absence of P53 the predominant target genes are involved in cancer signaling. Thus, the presented data emphasizes the known functions of Mta1 and serves as a rich resource which could help us identify novel Mta1 functions.

The Expanding Mi-2/NuRD Complexes: A Schematic Glance

This mini-review will schematically update the progress of the expanding Mi-2/Nucleosome Remodeling Deacetylase (NuRD) complexes in cancer and in normal development such as stemness, with a focus on mammals and the increasingly popular and powerful model organism Caenorhabditis elegans. The Mi-2/NuRD complexes control gene activity during the development of complex organisms. Every Mi-2/NuRD complex contains many different core polypeptides, which form distinct multifunctional complexes with specific context-dependent regulators. The Mi-2/NuRD complexes have unique ATP-dependent chromatin remodeling, histone deacetylase, demethylase activities and higher order chromatin organization. They can regulate the accessibility of transcription factors or repair proteins to DNA. In this review, we summarize our current knowleges in the composition, interaction and function of the subunits within the Mi-2/NuRD complex, the methodology used for the identification of Mi-2/NuRD complexes, as well as the clinical and therapeutic implications targeting the Mi-2/NuRD subunits.

Metastasis-associated protein 1 and its short form variant stimulates Wnt1 transcription through promoting its derepression from Six3 corepressor

Although Wnt1 downstream signaling components have been well studied and activated in human cancer, the pathways that regulate Wnt1 itself have not been explored in depth. Here, we provide gain-of-function, loss-of function, and molecular evidence supporting functional interactions between metastasis-associated protein 1 short-form (MTA1s), metastasis-associated protein 1 (MTA1), and Wnt1 signaling components during mammary gland development and tumorigenesis. Using multiple model systems involving overexpression or knockdown of MTA1s or MTA1, we discovered that MTA1s and MTA1 hyperactivate the Wnt1 pathway due to increased expression of Wnt1 transcription. MTA1s and MTA1 physically interact with Six3 chromatin, a protein product of which is a direct histone deacetylase inhibitor-dependent repressor of Wnt1 transcription. Deletion of the MTA1s and MTA1 allele in murine embryonic fibroblasts resulted in the upregulation of Six3 and downregulation of Wnt signaling. In addition, mammary glands from the MTA1s/MTA1(-/-) mice exhibited increased recruitment of Six3 corepressor complex to the Wnt1 promoter and inhibition of Wnt1 pathway in mammary glands. These findings identify MTA1s and MTA1 as important upstream modifiers of the Wnt1 transcription, and consequently its functions, by directly inhibiting the transcription of Six3, allowing derepression of Wnt1 transcription.

MTA1 coregulator regulates LPS response via MyD88-dependent signaling

Although metastasis tumor antigen 1 (MTA1) contributes to the responsiveness of macrophages to LPS, the underlying mechanism remains unknown. Here, we investigated the role of MTA1 in the regulation of expression and function of MyD88, a proximal component of NF-κB signaling. We discovered that MTA1 targets MyD88 and that MyD88 is a NF-κB-responsive gene in LPS-stimulated macrophages. We found that MTA1 is required for MyD88-dependent stimulation of NF-κB signaling and expression of proinflammatory cytokines such as IL-1β, MIP2, and TNF-α as MTA1 depletion leads to a substantial reduction in the expression of NF-κB target genes. In addition, LPS-mediated stimulation of MyD88 transcription was accompanied by an enhanced recruitment of MTA1, RNA polymerase II, and p65RelA complex to the NF-κB consensus sites in the MyD88 promoter. Interestingly, the recruitment of both MTA1 and MyD88 expression is effectively blocked by NF-κB inhibitor parthenolide. Selective knockdown of MyD88 by a dominant negative mutant of MyD88 or selective siRNA also impairs the ability of LPS to stimulate the NF-κB target genes. These findings reveal an inherent coregulatory role of MTA1 upon the expression of MyD88 and suggest that MTA1 regulation of MyD88 may constitute at least one of the mechanisms by which MTA1 stimulates LPS-induced NF-κB signaling in stimulated macrophages.

Requirement of MTA1 in ATR-mediated DNA damage checkpoint function

MTA1 (metastasis-associated protein 1), an integral component of the nucleosome remodeling and deacetylase complex, has recently been implicated in the ionizing radiation-induced DNA damage response. However, whether MTA1 also participates in the UV-induced DNA damage checkpoint pathway remains unknown. In response to UV radiation, ATR (ataxia teleangiectasia- and Rad3-related) is the major kinase activated that orchestrates cell cycle progression with DNA repair machinery by phosphorylating and activating a number of downstream substrates, such as Chk1 (checkpoint kinase 1) and H2AX (histone 2A variant X). Here, we report that UV radiation stabilizes MTA1 in an ATR-dependent manner and increases MTA1 binding to ATR. On the other hand, depletion of MTA1 compromises the ATR-mediated Chk1 activation following UV treatment, accompanied by a marked down-regulation of Chk1 and its interacting partner Claspin, an adaptor protein that is required for the phosphorylation and activation of Chk1 by ATR. Furthermore, MTA1 deficiency decreases the induction of phosphorylated H2AX (referred to as gamma-H2AX) and gamma-H2AX focus formation after UV treatment. Consequently, depletion of MTA1 results in a defect in the G(2)-M checkpoint and increases cellular sensitivity to UV-induced DNA damage. Thus, MTA1 is required for the activation of the ATR-Claspin-Chk1 and ATR-H2AX pathways following UV treatment, and the noted abrogation of the DNA damage checkpoint in the MTA1-depleted cells may be, at least in part, a consequence of dysregulation of the expression of these two pathways. These findings suggest that, in addition to its role in the repair of double strand breaks caused by ionizing radiation, MTA1 also participates in the UV-induced ATR-mediated DNA damage checkpoint pathway.

Proper differentiation of photoreceptors and amacrine cells depends on a regulatory loop between NeuroD and Six6

Timely generation of distinct neural cell types in appropriate numbers is fundamental for the generation of a functional retina. In vertebrates, the transcription factor Six6 is initially expressed in multipotent retina progenitors and then becomes restricted to differentiated retinal ganglion and amacrine cells. How Six6 expression in the retina is controlled and what are its precise functions are still unclear. To address this issue, we used bioinformatic searches and transgenic approaches in medaka fish (Oryzias latipes) to characterise highly conserved regulatory enhancers responsible for Six6 expression. One of the enhancers drove gene expression in the differentiating and adult retina. A search for transcription factor binding sites, together with luciferase, ChIP assays and gain-of-function studies, indicated that NeuroD, a bHLH transcription factor, directly binds an 'E-box' sequence present in this enhancer and specifically regulates Six6 expression in the retina. NeuroD-induced Six6 overexpression in medaka embryos promoted unorganized retinal progenitor proliferation and, most notably, impaired photoreceptor differentiation, with no apparent changes in other retinal cell types. Conversely, Six6 gain- and loss-of-function changed NeuroD expression levels and altered the expression of the photoreceptor differentiation marker Rhodopsin. In addition, knockdown of Six6 interfered with amacrine cell generation. Together, these results indicate that Six6 and NeuroD control the expression of each other and their functions coordinate amacrine cell generation and photoreceptor terminal differentiation.

Regulation of NF-kappaB circuitry by a component of the nucleosome remodeling and deacetylase complex controls inflammatory response homeostasis

The MTA1 coregulator (metastatic tumor antigen 1), a component of the nucleosome remodeling and deacetylase (NuRD) complex, has been intimately linked with human cancer, but its role in inflammatory responses remains unknown. Here, we discovered that MTA1 is a target of inflammation, and stimulation of macrophages with Escherichia coli lipopolysaccharide (LPS) stimulates MTA1 transcription via the NF-kappaB pathway. Unexpectedly, we found that MTA1 depletion in LPS-stimulated macrophages impairs NF-kappaB signaling and expression of inflammatory molecules. MTA1 itself acts as a transcriptional coactivator of inflammatory cytokines in LPS-stimulated macrophages, and in contrast, it acts as a corepressor in resting primary macrophages as its depletion induced cytokine expression. LPS stimulates S-nitrosylation of histone deacetylase 2 (HDAC2) and interferes with its binding to MTA1, which, in turn, resulted in the loss of corepressor behavior of MTA1.HDAC complex in activated macrophages. Consequently, the net levels of inflammatory cytokines in LPS-stimulated macrophages from MTA1(-/-) mice were high compared with wild-type mice. Accordingly, MTA1(-/-) mice were much more susceptible than control mice to septic shock induced by LPS, revealing that MTA1 protects mice from deregulated host inflammatory response. These findings reveal a previously unrecognized, critical homeostatic role of MTA1, both as a target and as a component of the NF-kappaB circuitry, in the regulation of inflammatory responses.

Mi-2/NuRD complex making inroads into DNA-damage response pathway

In eukaryotic cells, packaging of DNA into highly condensed chromatin presents a significant obstacle to DNA-based processes. Cells use two major strategies including histone modifications and ATP-dependent chromatin remodeling to alter chromatin structure that allows protein factors to gain access to nucleosomal DNA. Beyond their well-established role in transcription, histone modifications and several classes of ATP-dependent chromatin-remodeling complex have been functionally linked to efficient DNA repair. Mi-2/nucleosome remodeling and histone deacetylation (NuRD) complex uniquely possess both nucleosome remodeling and histone deacetylation activities, which play a vital role in regulating transcription. However, the role of the Mi-2/NuRD complex in DNA damage response remains largely unexplored until now. Recent findings reveal that metastasis-associated protein 1 (MTA1), an integral component of the Mi-2/NuRD complex, has successfully made inroads into DNA damage response pathway, and thus, links two previously unconnected Mi-2/NuRD complex and DNA damage response research areas. In this review, we will summarize recent progress concerning the functions of histone modifications and chromatin remodeling in DNA repair, and discuss new role of Mi-2/NuRD complex in DNA damage response.

Revelation of p53-independent function of MTA1 in DNA damage response via modulation of the p21 WAF1-proliferating cell nuclear antigen pathway

Although metastasis-associated protein 1 (MTA1), a component of the nucleosome remodeling and deacetylase (NuRD) complex, is a DNA-damage response protein and regulates p53-dependent DNA repair, it remains unknown whether MTA1 also participates in p53-independent DNA damage response. Here, we provide evidence that MTA1 is a p53-independent transcriptional corepressor of p21(WAF1), and the underlying mechanism involves recruitment of MTA1-histone deacetylase 2 (HDAC2) complexes onto two selective regions of the p21(WAF1) promoter. Accordingly, MTA1 depletion, despite its effect on p53 down-regulation, superinduces p21(WAF1), increases p21(WAF1) binding to proliferating cell nuclear antigen (PCNA), and decreases the nuclear accumulation of PCNA in response to ionizing radiation. In support of a p53-independent role of MTA1 in DNA damage response, we further demonstrate that induced expression of MTA1 in p53-null cells inhibits p21(WAF1) promoter activity and p21(WAF1) binding to PCNA. Consequently, MTA1 expression in p53-null cells results in increased induction of gamma H2AX foci and DNA double strand break repair, and decreased DNA damage sensitivity following ionizing radiation treatment. These findings uncover a new target of MTA1 and the existence of an additional p53-independent role of MTA1 in DNA damage response, at least in part, by modulating the p21(WAF1)-PCNA pathway, and thus, linking two previously unconnected NuRD complex and DNA-damage response pathways.

NF-kappaB signaling mediates the induction of MTA1 by hepatitis B virus transactivator protein HBx

Metastasis-associated protein 1 (MTA1), a master chromatin modifier, has been shown to regulate cancer progression and is widely upregulated in human cancer, including hepatitis B virus-associated hepatocellular carcinomas (HCCs). Here we provide evidence that hepatitis B virus transactivator protein HBx stimulates the expression of MTA1 but not of MTA2 or MTA3. The underlying mechanism of HBx stimulation of MTA1 involves HBx targeting of transcription factor nuclear factor (NF)-kappaB and the recruitment of HBx/p65 complex to the NF-kappaB consensus motif on the relaxed MTA1 gene chromatin. We also discovered that MTA1 depletion in HBx-expressing cells severely impairs the ability of HBx to stimulate NF-kappaB signaling and the expression of target proinflammatory molecules. Furthermore, the presence of HBx in HBx-infected HCCs correlated well with increased MTA1 and NF-kappaB-p65. Collectively, these findings revealed a previously unrecognized integral role of MTA1 in HBx stimulation of NF-kappaB signaling and consequently, the expression of NF-kappaB targets gene products with functions in inflammation and tumorigenesis.

Stimulation of inducible nitric oxide by hepatitis B virus transactivator protein HBx requires MTA1 coregulator

Nitric oxide has been implicated in the pathogenesis of inflammatory disorders, including hepatitis B virus-associated hepatocellular carcinoma. Transactivator protein HBx, a major regulator of cellular responses of hepatitis B virus, is known to induce the expression of MTA1 (metastasis-associated protein 1) coregulator via NF-kappaB signaling in hepatic cells. However, the underlying mechanism of HBx regulation of the inducible nitric-oxide synthase (iNOS) pathway remains unknown. Here we provide evidence that MTA1 is a positive regulator of iNOS transcription and plays a mechanistic role in HBx stimulation of iNOS expression and activity. We found that the HBx-MTA1 complex is recruited onto the human iNOS promoter in an NF-kappaB-dependent manner. Pharmacological inhibition of the NF-kappaB signaling prevented the ability of HBx to stimulate the transcription, the expression, and the activity of iNOS; nevertheless, these effects could be substantially rescued by MTA1 dysregulation. We further discovered that HBx-mediated stimulation of MTA1 is paralleled by the suppression of miR-661, a member of the small noncoding RNAs, recently shown to target MTA1. We observed that miR-661 controls of MTA1 expression contributed to the expression and activity of iNOS in HBx-expressing HepG2 cells. Accordingly, depletion of MTA1 by either miR-661 or siRNA in HBx-expressing cells severely impaired the ability of HBx to modulate the endogenous levels of iNOS and nitrite production. Together, these findings reveal an inherent role of MTA1 in HBx regulation of iNOS expression and consequently its function in the liver cancer cells.

Eye evolution: common use and independent recruitment of genetic components

Animal eyes can vary in complexity ranging from a single photoreceptor cell shaded by a pigment cell to elaborate arrays of these basic units, which allow image formation in compound eyes of insects or camera-type eyes of vertebrates. The evolution of the eye requires involvement of several distinct components-photoreceptors, screening pigment and genes orchestrating their proper temporal and spatial organization. Analysis of particular genetic and biochemical components shows that many evolutionary processes have participated in eye evolution. Multiple examples of co-option of crystallins, Galpha protein subunits and screening pigments contrast with the conserved role of opsins and a set of transcription factors governing eye development in distantly related animal phyla. The direct regulation of essential photoreceptor genes by these factors suggests that this regulatory relationship might have been already established in the ancestral photoreceptor cell.

MTA1 coregulator regulates p53 stability and function

Although metastasis-associated protein 1 (MTA1) has recently been shown as a DNA damage responsive protein, the underlying mechanism for its role in DNA double-strand break (DSB) repair remains unknown. Here, we show that MTA1 controls p53 stability through inhibiting its ubiquitination by E3 ubiquitin ligases mouse double minute 2 (Mdm2) and constitutive photomorphogenic protein 1 (COP1). The underlying mechanisms involve the ability of MTA1 to compete with COP1 to bind to p53 and/or to destabilize COP1 and Mdm2. Consequently, MTA1 regulates the p53-dependent transcription of p53R2, a direct p53 target gene for supplying nucleotides to repair damaged DNA. Depletion of MTA1 impairs p53-dependent p53R2 transcription and compromises DNA repair. Interestingly, these events could be reversed by MTA1 reintroduction, indicating that MTA1 interjects into the p53-dependent DNA repair. Given the fact that MTA1 is widely up-regulated in human cancers, these findings in conjunction with our earlier finding of a crucial role of MTA1 in DSB repair suggest an inherent role of the MTA1-p53-p53R2 pathway in DNA damage response in cancer cells.