Metastasis-associated protein 1 interacts with NRIF3, an estrogen-inducible nuclear receptor coregulator

Metastasis-associated protein 1 (MTA1) in gastric cancer tissues is positively associated with poorer prognosis

BACKGROUND

The present study examined the clinical significance of metastasis-associated protein 1 (MTA1) in the progression and patient survival of gastric cancer.

METHODS

Paraffin-embedded resected tissues of gastric cancer mucosa (n = 436) and adjacent normal mucosa (n = 92) were assessed immunohistochemically for MTA1 protein, and scored according to the percentage of cells positively stained for MTA1 combined with stain intensity. Associations between MTA1 staining scores and clinicopathological factors, including survival time, were evaluated.

RESULTS

The staining scores for MTA1 were significantly higher in gastric cancer tissues than in matched normal tissues. MTA1 scores positively correlated with tumor size, depth of invasion, presence of lymph node metastasis, lymphatic involvement, venous invasion, distal metastasis, and advanced clinical staging. Patients with high MTA1 scores in gastric cancer tissues had a significantly lower five-year survival rate compared with patients with low MTA1 scores. The multivariate analysis indicated that MTA1 protein levels in resected gastric cancer tissues, as reflected by immunohistochemical staining, are an independent prognostic index of gastric carcinoma (P < 0.01).

CONCLUSION

MTA1 immunopositivity was significantly associated with progression of gastric cancer, and may be helpful in gastric cancer prognosis.

MTA1 is a novel regulator of autophagy that induces tamoxifen resistance in breast cancer cells

Tamoxifen is commonly used to treat patients with ESR/ER-positive breast cancer, but its therapeutic benefit is limited by the development of resistance. Recently, alterations in macroautophagy/autophagy function were demonstrated to be a potential mechanism for tamoxifen resistance. Although MTA1 (metastasis-associated 1) has been implicated in breast tumorigenesis and metastasis, its role in endocrine resistance has not been studied. Here, we report that the level of MTA1 expression was upregulated in the tamoxifen resistant breast cancer cell lines MCF7/TAMR and T47D/TR, and knockdown of MTA1 sensitized the cells to 4-hydroxytamoxifen (4OHT). Moreover, knockdown of MTA1 significantly decreased the enhanced autophagy flux in the tamoxifen resistant cell lines. To confirm the role of MTA1 in the development of tamoxifen resistance, we established a cell line, MCF7/MTA1, which stably expressed MTA1. Compared with parental MCF7, MCF7/MTA1 cells were more resistant to 4OHT-induced growth inhibition in vitro and in vivo, and showed increased autophagy flux and higher numbers of autophagosomes. Knockdown of ATG7 or cotreatment with hydroxychloroquine, an autophagy inhibitor, restored sensitivity to 4OHT in both the MCF7/MTA1 and tamoxifen resistant cells. In addition, AMP-activated protein kinase (AMPK) was activated, probably because of an increased AMP:ATP ratio and decreased expression of mitochondrial electron transport complex components. Finally, publicly available breast cancer patient datasets indicate that MTA1 levels correlate with poor prognosis and development of recurrence in patients with breast cancer treated with tamoxifen. Overall, our findings demonstrated that MTA1 induces AMPK activation and subsequent autophagy that could contribute to tamoxifen resistance in breast cancer.

Identifying biomarkers of breast cancer micrometastatic disease in bone marrow using a patient-derived xenograft mouse model

BACKGROUND

Disseminated tumor cells (DTCs) found in the bone marrow (BM) of patients with breast cancer portend a poor prognosis and are thought to be intermediaries in the metastatic process. To assess the clinical relevance of a mouse model for identifying possible prognostic and predictive biomarkers of these cells, we have employed patient-derived xenografts (PDX) for propagating and molecularly profiling human DTCs.

METHODS

Previously developed mouse xenografts from five breast cancer patients were further passaged by implantation into NOD/SCID mouse mammary fat pads. BM was collected from long bones at early, serial passages and analyzed for human-specific gene expression by qRT-PCR as a surrogate biomarker for the detection of DTCs. Microarray-based gene expression analyses were performed to compare expression profiles between primary xenografts, solid metastasis, and populations of BM DTCs. Differential patterns of gene expression were then compared to previously generated microarray data from primary human BM aspirates from patients with breast cancer and healthy volunteers.

RESULTS

Human-specific gene expression of SNAI1, GSC, FOXC2, KRT19, and STAM2, presumably originating from DTCs, was detected in the BM of all xenograft mice that also developed metastatic tumors. Human-specific gene expression was undetectable in the BM of those xenograft lines with no evidence of distant metastases and in non-transplanted control mice. Comparative gene expression analysis of BM DTCs versus the primary tumor of one mouse line identified multiple gene transcripts associated with epithelial-mesenchymal transition, aggressive clinical phenotype, and metastatic disease development. Sixteen of the PDX BM associated genes also demonstrated a statistically significant difference in expression in the BM of healthy volunteers versus the BM of breast cancer patients with distant metastatic disease.

CONCLUSION

Unique and reproducible patterns of differential gene expression can be identified that presumably originate from BM DTCs in mouse PDX lines. Several of these identified genes are also detected in the BM of patients with breast cancer who develop early metastases, which suggests that they may be clinically relevant biomarkers. The PDX model may also provide a clinically relevant system for analyzing and targeting these intermediaries of metastases.

MTA1 expression in human cancers - Clinical and pharmacological significance

Remarkably, majority of the cancer deaths are due to metastasis, not because of primary tumors. Metastasis is one of the important hallmarks of cancer. During metastasis invasion of primary tumor cells from the site of origin to a new organ occurs. Metastasis associated proteins (MTAs) are a small family of transcriptional coregulators that are closely associated with tumor metastasis. These proteins are integral components of nuclear remodeling and deacetylation complex (NuRD). By virtue of being integral components of NuRD, these proteins regulate the gene expression by altering the epigenetic changes such as acetylation and methylation on the target gene chromatin. Among the MTA proteins, MTA1 expression is very closely correlated with the aggressiveness of several cancers that includes breast, liver, colon, pancreas, prostate, blood, esophageal, gastro-intestinal etc. Considering its close association with aggressiveness in human cancers, MTA1 may be considered as a potential therapeutic target for cancer treatment. The recent developments in its crystal structure further strengthened the idea of developing small molecule inhibitors for MTA1. In this review, we discuss the recent trends on the diverse functions of MTA1 and its role in various cancers, with the focus to consider MTA1 as a 'druggable' target in the control of human cancers.

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.

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.

Role of MTA1 in cancer progression and metastasis

The MTA1 protein contributes to the process of cancer progression and metastasis through multiple genes and protein targets and interacting proteins with roles in transformation, anchorage-independent growth, invasion, survival, DNA repair, angiogenesis, hormone independence, metastasis, and therapeutic resistance. Because the roles and clinical significance of MTA proteins in human cancer are discussed by other contributors in this issue, this review will focus on our current understanding of the underlying principles of action behind the biological effects of MTA1. MTA proteins control a spectrum of cancer-promoting processes by modulating the expression of target genes and/or the activity of MTA-interacting proteins. In the case of MTA1, these functions are manifested through posttranslational modifications of MTA1 in response to upstream signals, MTA1 interaction with binding proteins, and the expression of target gene products. Studies delineating the molecular basis of dual functionality of MTA1 reveal that the functions of MTA1-chromatin-modifying complexes in the context of target gene regulation are dynamic in nature. The nature and targets of MTA1-chromatin-modifying complexes are also governed by the dynamic plasticity of the nucleosome landscape as well as kinetics of activation and inactivation of enzymes responsible for posttranslational modifications on the MTA1 protein. These broadly applicable functions also explain why MTA1 may be a "hub" gene in cancer. Because the deregulation of enzymes and their substrates with roles in MTA1 biology is not necessarily limited to cancer, we speculate that the lessons from MTA1 as a prototype dual master coregulator will be relevant for other human diseases. In this context, the concept of the dynamic nature of corepressor versus coactivator complexes and the MTA1 proteome as a function of time to signal is likely to be generally applicable to other multiprotein regulatory complexes in living systems.

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.

Cell proliferation and modulation of interaction of estrogen receptors with coregulators induced by ERα and ERβ agonists

The aim of the present study was to investigate modulation of the interaction of the ERα and ERβ with coregulators in the ligand responses induced by estrogenic compounds. To this end, selective ERα and ERβ agonists were characterized for intrinsic relative potency reflected by EC50 and maximal efficacy towards ERα and ERβ mediated response in ER selective reporter gene assays, and subsequently tested for induction of cell proliferation in T47D-ERβ cells with variable ERα/ERβ ratio, and finally for ligand dependent modulation of the interaction of ERα and ERβ with coregulators using the MARCoNI assay, with 154 unique nuclear receptor coregulator peptides derived from 66 different coregulators. Results obtained reveal an important influence of the ERα/ERβ ratio and receptor selectivity of the compounds tested on induction of cell proliferation. ERα agonists activate cell proliferation whereas ERβ suppresses ERα mediated cell proliferation. The responses in the MARCoNI assay reveal that upon ERα or ERβ activation by a specific agonist, the modulation of the interaction of the ERs with coregulators is very similar indicating only a limited number of differences upon ERα or ERβ activation by a specific ligand. Differences in the modulation of the interaction of the ERs with coregulators between the different agonists were more pronounced. Based on ligand dependent differences in the modulation of the interaction of the ERs with coregulators, the MARCoNI assay was shown to be able to classify the ER agonists discriminating between different agonists for the same receptor, a characteristic not defined by the ER selective reporter gene or proliferation assays. It is concluded that the ultimate effect of the model compounds on proliferation of estrogen responsive cells depends on the intrinsic relative potency of the agonist towards ERα and ERβ and the cellular ERα/ERβ ratio whereas differences in the modulation of the interaction of the ERα and ERβ with coregulators contribute to the ligand dependent responses induced by estrogenic compounds.

The NuRD architecture

The nucleosome remodeling and deacetylase (NuRD) complex regulates chromatin organization, gene transcription, genomic stability and developmental signaling. NuRD has a unique dual enzymatic activity, containing an ATPase and a histone deacetylase among its six core subunits. Recent studies indicate that NuRD composition and the interplay between subunits may dictate the diverse functions of the complex. In this review, we examine the structures and biological roles of the NuRD subunits and discuss new avenues of research to advance our understanding of the NuRD-mediated signaling network.

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.

A network module-based method for identifying cancer prognostic signatures

Discovering robust prognostic gene signatures as biomarkers using genomics data can be challenging. We have developed a simple but efficient method for discovering prognostic biomarkers in cancer gene expression data sets using modules derived from a highly reliable gene functional interaction network. When applied to breast cancer, we discover a novel 31-gene signature associated with patient survival. The signature replicates across 5 independent gene expression studies, and outperforms 48 published gene signatures. When applied to ovarian cancer, the algorithm identifies a 75-gene signature associated with patient survival. A Cytoscape plugin implementation of the signature discovery method is available at http://wiki.reactome.org/index.php/Reactome_FI_Cytoscape_Plugin.

A phenotype map for 14q32.3 terminal deletions

Detailed molecular-cytogenetic studies combined with thorough clinical characterization are needed to establish genotype-phenotype correlations for specific chromosome deletion syndromes. Although many patients with subtelomeric deletions have been reported, the phenotype maps for many of the corresponding syndromes, including the terminal deletion 14q syndrome, are only slowly emerging. Here, we report on five patients with terminal partial monosomy of 14q32.3 and characteristic features of terminal deletion 14q syndrome. Four of the patients carry de novo terminal deletions of 14q, three of which have not yet been reported. One patient carries an unbalanced translocation der(14)t(9;14)(q34.3;q32.3). Minimum deletion sizes as determined by molecular karyotyping and FISH are 5.82, 5.56, 4.17, 3.54, and 3.29 Mb, respectively. Based on our findings and a comprehensive review of the literature, we refine the phenotype map for typical clinical findings of the terminal deletion 14q syndrome (i.e., intellectual disability/developmental delay, muscular hypotonia, postnatal growth retardation, microcephaly, congenital heart defects, genitourinary malformations, ocular coloboma, and several dysmorphic signs). Combining this phenotype map with benign copy-number variation data available from the Database of Genomic Variants, we propose a small region critical for certain features of the terminal deletion 14q syndrome which contains only seven RefSeq genes.

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.

E3 ubiquitin ligase COP1 regulates the stability and functions of MTA1

Metastasis-associated protein 1 (MTA1), a component of the nucleosome remodeling and histone deacetylation (NuRD) complex, is widely upregulated in human cancers. However, the mechanism for regulating its protein stability remains unknown. Here we report that MTA1 is an ubiquitinated protein and targeted by the RING-finger E3 ubiquitin-protein ligase constitutive photomorphogenesis protein 1 (COP1) for degradation via the ubiquitin-proteasome pathway. Induced expression of wild-type COP1 but not its RING motif mutants promotes the ubiquitination and degradation of MTA1, indicating that the ligase activity is required for the COP1-mediated proteolysis of MTA1. Conversely, depletion of endogenous COP1 resulted in a marked decrease in MTA1 ubiquitination, accompanied by a pronounced accumulation of MTA1 protein. MTA1, in turn, destabilizes COP1 by promoting its autoubiquitination, thus creating a tight feedback loop that regulates both MTA1 and COP1 protein stability. Accordingly, disruption of the COP1-mediated proteolysis by ionizing radiation leads to MTA1 stabilization, accompanied by an increased coregulatory function of MTA1 on its target. Furthermore, we discovered that MTA1 is required for optimum DNA double-strand break repair after ionizing radiation. These findings provide novel insights into the regulation of MTA1 protein and reveal a novel function of MTA1 in DNA damage response.

The role of the MTA family and their encoded proteins in human cancers: molecular functions and clinical implications

MTA (metastasis-associated gene) is a newly discovered family of cancer progression-related genes and their encoded products. MTA1, the first gene found in this family, has been repeatedly reported to be overexpressed along with its protein product MTA1 in a wide range of human cancers. In addition, the expression of MTA1/MTA1 correlates with the clinicopathological properties (malignant properties) of human cancers. MTA proteins are transcriptional co-repressors that function in histone deacetylation and are involved in the NuRD complex, which contains nucleosome remodeling and histone deacetylating molecules. MTA1 expression correlates with tumor formation in the mammary gland. In addition, MTA1 converts breast cancer cells to a more aggressive phenotype by repression of the estrogen receptor (ER) alpha trans-activation function through deacetylation of the chromatin in the ER-responsive element of ER-responsive genes. Furthermore, MTA1 plays an essential role in c-MYC-mediated cell transformation. Another member of this family, MTA3, is induced by estrogen and represses the expression of the transcriptional repressor Snail, a master regulator of "epithelial to mesenchymal transitions", resulting in the expression of the cell adhesion molecule E-cadherin and maintenance of a differentiated, normal epithelial phenotype in breast cells. In addition, tumor suppressor p53 protein is deacetylated and inactivated by both MTA1 and MTA2, leading to inhibition of growth arrest and apoptosis. Moreover, a hypoxia-inducible factor-1alpha (HIF-1alpha) is also deacetylated and stabilized by MTA1, resulting in angiogenesis. Thus, MTA proteins, especially MTA1, represent a possible set of master co-regulatory molecules involved in the carcinogenesis and progression of various malignant tumors. MTA proteins are proposed to be important new tools for clinical application in cancer diagnosis and treatment.

Overexpression of metastasis-associated MTA1 in oral squamous cell carcinomas: correlation with metastasis and invasion

Metastasis-associated protein 1 (MTA1) is physiologically expressed at low levels in human tissues. Its expression is associated with progression of solid cancers and is common in cancer cell lines. This study investigated whether MTA1 was expressed in squamous cell carcinoma (SCC) and would be a useful metastatic marker. Specimens from 38 patients with oral SCC were stained using the avidin-biotin-peroxidase technique with polyclonal antibodies against MTA1. Human SCC cell lines SAS, HSC2, OSC19 and OSC20 were analysed for MTA1 mRNA expression. MTA1 expression in control tissues was significantly lower than in carcinomas. MTA1 protein expression was detected in 33 of 38 SCC tissues from patients. Histologically, MTA1 protein production was strongly associated with cancer cell invasion, and clinically there was a correlation between lymph node metastasis and MTA1 protein production. Among the cancer cell lines, HSC2 showed the lowest mRNA expression, and OSC20 showed the highest MTA1 mRNA expression. In the Matrigel invasion assay, the HSC2 cell line showed the lowest invasion and the OSC20 cell line showed the highest invasion. RNAi-mediated MTA1 silencing in the OSC20 cells decreased the invasion index. MTA1 expression in oral SCC may be associated with increased invasive ability, which may cause lymph node metastasis.

Serine 28 phosphorylation of NRIF3 confers its co-activator function for estrogen receptor-alpha transactivation

NRIF3 is an estrogen-inducible nuclear receptor coregulator that stimulates estrogen receptor-alpha (ERalpha) transactivation functions and associates with the endogenous ER and its target gene promoter. p21-activated protein kinase 1 (Pak1) phosphorylates ERalpha at Ser305 and this modification is important in ERalpha transactivation function. Although ERalpha transactivation functions are regulated by co-activator activity of NRIF3, it remains unclear whether Pak1 could impact ER functions via a posttranslational modification of NRIF3. Here, we report that Pak1 phosphorylates NRIF3 at Serine28 and that NRIF3 binds to Pak1 in vitro and in vivo. We found that NRIF3 phosphorylation, co-activator activity and association with ERalpha increased following Pak1 phosphorylation of NRIF3's Ser28 and that activated ERalpha-Ser305 and NRIF3-Ser28 cooperatively support transactivation of ERalpha. NRIF3 expression increased significantly in cells with inducible Pak1 expression. We found that NRIF3 and ERalpha interaction, subcellular localization and ERalpha transactivation activity all increased in cells expressing the Pak1 phosphorylation-mimicking mutant NRIF3-Ser28Glu. Consistently, the NRIF3-Ser28Glu mutant exhibited an enhanced recruitment to the endogenous ER target genes and increased expression following estrogen stimulation. Finally, breast cancer cells with stable overexpression of NRIF3 showed increased proliferation and enhanced anchorage-independent growth. These findings suggest that NRIF3-Ser28 is a physiologic target of Pak1 signaling and contributes to the enhanced NRIF3 co-activator activity, leading to coordinated potentiation of ERalpha transactivation, its target gene expression and estrogen responsiveness of breast cancer cells.

MTA family of coregulators in nuclear receptor biology and pathology

Nuclear receptors (NRs) rely on coregulators (coactivators and corepressors) to modulate the transcription of target genes. By interacting with nucleosome remodeling complexes, NR coactivators potentiate transcription, whereas corepressors inhibit transcription of the target genes. Metastasis-associated proteins (MTA) represent an emerging family of novel NR coregulators. In general, MTA family members form independent nucleosome remodeling and deacetylation (NuRD) complexes and repress the transcription of different genes by recruiting histone deacetylases onto their target genes. However, MTA1 also acts as a coactivator in a promoter-context dependent manner. Recent findings that repression of estrogen receptor transactivation functions by MTA1, MTA1s, and MTA2 and regulation of MTA3 by estrogen signaling have indicated the significance of these proteins in NR signaling. Here, we highlight the action of MTA proteins on NR signaling and their roles in pathophysiological conditions.

Estrogen receptors: how do they signal and what are their targets

During the past decade there has been a substantial advance in our understanding of estrogen signaling both from a clinical as well as a preclinical perspective. Estrogen signaling is a balance between two opposing forces in the form of two distinct receptors (ER alpha and ER beta) and their splice variants. The prospect that these two pathways can be selectively stimulated or inhibited with subtype-selective drugs constitutes new and promising therapeutic opportunities in clinical areas as diverse as hormone replacement, autoimmune diseases, prostate and breast cancer, and depression. Molecular biological, biochemical, and structural studies have generated information which is invaluable for the development of more selective and effective ER ligands. We have also become aware that ERs do not function by themselves but require a number of coregulatory proteins whose cell-specific expression explains some of the distinct cellular actions of estrogen. Estrogen is an important morphogen, and many of its proliferative effects on the epithelial compartment of glands are mediated by growth factors secreted from the stromal compartment. Thus understanding the cross-talk between growth factor and estrogen signaling is essential for understanding both normal and malignant growth. In this review we focus on several of the interesting recent discoveries concerning estrogen receptors, on estrogen as a morphogen, and on the molecular mechanisms of anti-estrogen signaling.

MTA family of transcriptional metaregulators in mammary gland morphogenesis and breast cancer

Since breast cancer and its associated metastasis are a global health problem and a major cause of mortality among women, research efforts to understand the development, morphogenesis, and functioning of the mammary gland are a high priority. Myriad signaling pathways, transcription factors, and associated transcriptional coregulators have been identified in both normal functioning and neoplastic transformation of the mammary gland. The discovery of the metastasis tumor antigen 1 (MTA1) gene, its overexpression in cancer and metastasis and its subsequent identification as an integral part of the chromatin remodeling complex heralded extensive research on its physiological role. Subsequent identification of additional gene family members, namely MTA1s, MTA2, and MTA3, and their functions in the cell has resulted in the establishment of the significance of the MTA family. The role of these proteins in modulating hormonal responses in normal mammary glands and in breast cancer has resulted in their identification as important molecular markers and potential therapeutic targets.

Metastasis-associated protein 1 transgenic mice: a new model of spontaneous B-cell lymphomas

Metastasis-associated protein 1 (MTA1), a component of the nuclear remodeling complex and the founding homologue of the MTA family, has been implicated in metastasis, but definitive causative evidence in an animal model system is currently lacking. Here, we show that MTA1 overexpression in transgenic mice is accompanied by a high incidence of spontaneous B cell lymphomas including diffuse large B cell lymphomas (DLBCL). Lymphocytes and lymphoma cells from MTA1-TG mice are hyperproliferative. Lymphomas were transplantable and of clonal origin and were characterized by down-regulation of p27Kip1 as well as up-regulation of Bcl2 and cyclin D1. The significance of these murine studies was established by evidence showing a widespread up-regulation of MTA1 in DLBCL from humans. These findings reveal a previously unrecognized role for the MTA1 pathway in the development of spontaneous B cell lymphomas, and offer a potential therapeutic target in B cell lymphomas. These observations suggest that MTA1-TG mice represent a new model of spontaneous DLBCL associated with high tumor incidence and could be used for therapeutic intervention studies.

Corepressors of agonist-bound nuclear receptors

Nuclear receptors (NRs) rely on coregulator proteins to modulate transcription of target genes. NR coregulators can be broadly subdivided into coactivators which potentiate transcription and corepressors which silence gene expression. The prevailing view of coregulator action holds that in the absence of agonist the receptor interacts with a corepressor via the corepressor nuclear receptor (CoRNR, "corner") box motifs within the corepressor. Upon agonist binding, a conformational change in the receptor causes the shedding of corepressor and the binding of a coactivator which interacts with the receptor via NR boxes within the coregulator. This view was challenged with the discovery of RIP140 which acts as a NR corepressor in the presence of agonist and utilizes NR boxes. Since then a number of other corepressors of agonist-bound NRs have been discovered. Among them are LCoR, PRAME, REA, MTA1, NSD1, and COPR1 Although they exhibit a great diversity of structure, mechanism of repression and pathophysiological function, these corepressors frequently have one or more NR boxes and often recruit histone deacetylases to exert their repressive effects. This review highlights these more recently discovered corepressors and addresses their potential functions in transcription regulation, disease pharmacologic responses and xenobiotic metabolism.

Correlation of appearance of metastasis-associated protein1 (Mta1) with spermatogenesis in developing mouse testis

Mta1, a representative of the MTA gene family, is believed to be involved in the metastasis of malignant tumors. However, a systematic study of its physiological function has not been performed. It has been found in normal mouse organs at relatively low levels, except for in testis, suggesting a potential function in the male reproductive system. In order to explore the role of Mta1 protein during spermatogenesis, its expression in adult mouse testis was compared with that in developing mouse testis and in testis from adult mice treated with methoxyacetic acid, which selectively depletes primary spermatocytes. Quantitative analysis revealed that Mta1 protein gradually increased in the testis from 14 days postnatally. Immunolocalization analysis demonstrated strong signals in the seminiferous tubules, and Mta1 was predominantly present in the nucleus of primary spermatocytes and spermatogonia from 14 days postnatally. The most intensive staining was located in the nucleus of pachytene spermatocytes in mature testes. The expression pattern of Mta1 during spermatogenesis was also shown to be stage-specific by immunohistochemistry analysis. Finally, dramatic loss of Mta1 expression from pachytene spermatocytes was observed in the spermatogenic-arrested adult mouse testis. These results collectively demonstrate that Mta1 appears during postnatal testis development and suggest that this expression may be crucial for spermatogenesis.

An inherent role of integrin-linked kinase-estrogen receptor alpha interaction in cell migration

Integrin-linked kinase (ILK) and estrogen receptor (ER)-alpha modulate cell migration. However, the crosstalk between ERalpha and ILK and the role of ILK in ERalpha-mediated cell migration remain unexplored. Here, we report that ILK participates in ERalpha signaling in breast cancer cells. We found that ILK binds ERalpha in vitro and in vivo through a LXXLL motif in ILK. Estrogen prevented ERalpha-ILK binding, resulting in phosphatidylinositol 3-kinase (PI3K)-dependent increase in ILK kinase activity. Furthermore, the regulation of ERalpha-ILK interaction was dependent on the PI3K pathway. Unexpectedly, transient knockdown or inhibition of ILK caused hyperphosphorylation of ERalpha Ser(118) in an extracellular signal-regulated kinase/mitogen-activated protein kinase pathway-dependent manner and an enhanced ERalpha recruitment to the target chromatin and gene expression, a process reversed by overexpression of ILK. Compatible with these interactions, estrogen regulated cell migration via the PI3K/ILK/AKT pathway with stable ILK overexpression hyperactivating cell migration. Thus, status of ILK signaling may be an important modifier of ER signaling in breast cancer cells and this pathway could be exploited for therapeutic intervention in breast cancer cells.

Metastasis tumor antigens, an emerging family of multifaceted master coregulators

Regulation of fundamental genetic processes demands dynamic participation of transcription factors, their coregulators, and multiprotein chromatin remodeling activities at target genes. One family of chromatin modifiers that is ubiquitously expressed is the metastasis tumor antigens (MTA), which are integral parts of nucleosome remodeling and histone deacetylation (NuRD) complexes. MTA family members exist in distinct NuRD complexes, and functional redundancy is lacking among MTA family members. MTA proteins regulate divergent cellular pathways, including hormonal action, epithelial-to-mesenchymal transitions, differentiation, protein stability and development, and cell fate programs by modifying the acetylation status of crucial target genes. Intriguingly, at least one member of this family, MTA1, itself undergoes acetylation and acts as a coactivator in certain contexts. We discuss the roles of the MTA family of chromatin modifiers, with an emphasis on their physiologic functions.

Diabetes modulates differentially creatine kinase-specific activity responsiveness to estradiol-17beta and to raloxifene in rat organs

Diabetes mellitus increases the risk for CVD in women. While there is considerable evidence suggesting beneficial effects of estrogen on decreasing lipid peroxidation, atherosclerotic processes, and cardiovascular diseases, diabetes negates most estrogen protective effects as well as the skeletal protective effects of estrogens, which are not discernable in diabetic women. In the present study, we examined the in vivo effects of estradiol-17beta (E2), on creatine kinase (CK)-specific activity, in estrogen-responsive organs from healthy and diabetic rats. Healthy or diabetic (streptozotocin-induced) female rats were injected with either E2 (10-50 microg/rat) or raloxifene (Ral; 500-1,000 microg/rat). Twenty-four hours following the injection, animals were sacrificed; their organs removed and assayed for CK-specific activity. CK-specific activity in different organs [Left ventricle of heart (Lv), uterus (Ut), aorta (Ao), para uterine adipose tissue (Ad), epiphyseal cartilage (Ep), and diaphyseal bone (Di)] from healthy animals, was stimulated with increased doses of E2, with maximum at 20 microg/rat. Age-matched diabetic female rats exhibited a remarkable decreased response to E2 in all organs except Ut. In contrast, the response to Ral was not altered in diabetic rats. Similar results were observed in organs from ovariectomized female rats (Ovx), healthy or diabetic. These results support our previous in vitro findings, demonstrating that hyperglycemia decreases CK response to E2 but not to Ral in cultured human vascular and bone cells. In summary, diabetes mellitus decreases CK response to E2 but not that of Ral in skeletal and vascular tissues. The decreased response to E2 detected in organs derived from diabetic rats might be due to changes in nuclear and/or membrane estrogen receptors and/or other genomic and non-genomic pathways, as was shown in in vitro cellular models.

Signaling regulation of genomic and nongenomic functions of estrogen receptors

Estrogen receptors (ERs) mediate the effects of 17beta-estradiol under physiologic and pathologic conditions. ERs trigger 17beta-estradiol-sensitive gene transcription by binding to specific estrogen response elements (i.e. genomic mechanism). The cellular effects of estrogen are also influenced by membrane- or cytoplasm-initiated responses (i.e. nongenomic mechanism). Both ER-evoked genomic and nongenomic effects originate from a unique signaling network. Furthermore, estrogen-initiated rapid pathways and ERalpha interactions with specific partners (e.g. AIB1, PELP1/MNAR; MTA1, MTA1s and p130Cas) influence both ER functions. Here, we summarize the recent findings related to multiple regulatory levels of the signaling networks responsible for ERs-mediated responses in breast cancer cells.

MTA1, a transcriptional activator of breast cancer amplified sequence 3

Here we define a function of metastasis-associated protein 1 (MTA1), a presumed corepressor of estrogen receptor alpha (ERalpha), as a transcriptional activator of Breast Cancer Amplified Sequence 3 (BCAS3), a gene amplified and overexpressed in breast cancers. We identified BCAS3 as a MTA1 chromatin target in a functional genomic screen. MTA1 stimulation of BCAS3 transcription required ERalpha and involved a functional ERE half-site in BCAS3. Furthermore, we discovered that MTA1 is acetylated on lysine 626, and that this acetylation is necessary for a productive transcriptional recruitment of RNA polymerase II complex to the BCAS3 enhancer sequence. BCAS3 expression was elevated in mammary tumors from MTA1 transgenic mice and 60% of the human breast tumors, and correlated with the coexpression of MTA1 as well as with tumor grade and proliferation of primary breast tumor samples. These findings reveal a previously unrecognized function of MTA1 in stimulating BCAS3 expression and suggest an important role for MTA1-BCAS3 pathway in promoting cancerous phenotypes in breast tumor cells.

CRIPak, a novel endogenous Pak1 inhibitor

p21-activated protein kinase 1 (Pak1) plays an important role in several cellular processes, including cytoskeleton reorganization, promotion of the cell survival, and the estrogen receptor (ER) signaling. Pak1 expression and activity is deregulated in a number of cancers. Pak1 is activated by a variety of physiological signals; however, less is known about the negative regulators of Pak1. Here, we report a negative regulator of Pak1. By performing a yeast two-hybrid screen of a mammary gland library, we identified cysteine-rich inhibitor of Pak1 (CRIPak) as a novel Pak1-interacting protein. We found that CRIPak is an intronless gene that localized to chromosome 4p16.3. It contains 13 zinc-finger domains and has three trypsin inhibitor-like, cysteine-rich domains and is widely expressed in a number of human cells and tissues. We further found that CRIPak interacted with Pak1 through the N-terminal regulatory domain and inhibited Pak1 kinase in both in vitro and in vivo assays. CRIPak inhibited Pak1-mediated LIM kinase activation and enhancement of ER transactivation. Conversely, selective inhibition of the endogenous CRIPak resulted in an increased Pak1 activity, and consequently, increased cytoskeleton remodeling and Pak1-mediated ER transactivation activity. The hormonal stimulation of cells enhanced CRIPak expression and promoted its colocalization with ER in the nuclear compartment. Our findings suggest that CRIPak is a novel negative regulator of the Pak1 and has a role in the modulation of Pak1-mediated ER transactivation in breast cancer cells.

Metastasis-associated protein 1 enhances stability of hypoxia-inducible factor-1alpha protein by recruiting histone deacetylase 1

The expression of metastasis-associated protein 1 (MTA1) correlates well with tumor metastases; however, the associated molecular mechanism is not fully understood. Here, we explored the possibility of cross-talk between MTA1 and hypoxia-inducible factor-1alpha (HIF-1alpha), a key regulator of angiogenic factors. We observed that the expression of MTA1 was strongly induced under hypoxia in breast cancer cell lines such as MCF-7 and MDA-MB-231. When MTA1 was overexpressed, the transcriptional activity and stability of HIF-1alpha protein were enhanced. MTA1 and HIF-1alpha are physically associated in vivo and they were localized completely in the nucleus when coexpressed. MTA1 induced the deacetylation of HIF-1alpha by increasing the expression of histone deacetylase 1 (HDAC1). MTA1 counteracted to the action of acetyltransferase, ARD1, and it did not stabilize the HIF-1alpha mutant that lacks the acetylation site, K532R. These results indicate that acetylation is the major target of MTA1/HDAC1 function. Collectively, our data provide evidence of a positive cross-talk between HIF-1alpha and MTA1, which is mediated by HDAC1 recruitment, and indicate a close connection between MTA1-associated metastasis and HIF-1-induced tumor angiogenesis.

DT56a stimulates gender-specific human cultured bone cells in vitro

DT56a found to have SERM-like properties is used for the treatment of menopausal symptoms and osteoporosis. In vivo experiments demonstrated that DT56a displayed selective estrogenic activity; it stimulated creatine kinase (CK) specific activity in the skeletal tissues but not on the uterus of ovariectomized rats. DT56a, when applied together with estradiol-17beta (E(2)), completely inhibited the E(2)-stimulated CK, as demonstrated by other SERMs. DT56a stimulated bone formation in a rat model as measured by histological and histomorphometrical parameters. In a clinical study, administration of DT56a (Femarelle) resulted in a considerable elevation of bone mineral density and relief of menopausal symptoms. The aim of the present study was to analyze the effects of DT56a in vitro on human-derived bone cultured osteoblasts (Ob), by measuring its effects, at different concentrations, on DNA synthesis, CK and alkaline phosphatase (ALP) specific activities as well as changes in intracellular [Ca(2+)](i) concentrations. DT56a stimulated CK and DNA synthesis in both pre- and post-menopausal female Ob with maximal effect at 100 ng/ml for both age groups. In addition, DT56a stimulated ALP in Ob from both pre- and post-menopausal women with maximal effect at lower dose of 50 ng/ml, with higher response of pre-menopausal cells. Raloxifene (Ral) inhibited all DT56a-stimulated changes in Ob from both age groups. DT56a, when given together with E(2), completely antagonized E(2)-stimulated effects demonstrating its nature as a phyto-SERM. DT56a also, dose dependency, stimulated the intracellular levels of [Ca(2+)](i) with maximal effect at 10 ng/ml. Male-derived Ob did not respond to DT56a in any parameter. In summary, DT56a stimulated sex-specifically female-derived Ob, indicating its unique nature compared to the compounds currently used for postmenopausal osteoporosis by being bone-forming and not only an anti-resorptive agent.

The clinical relevance of steroid hormone receptor corepressors

Steroid hormone receptors are ligand-dependent transcription factors that control a variety of essential physiologic and developmental processes in humans. The functional activity of a steroid receptor is regulated not only by hormones but also by an array of regulatory proteins such as coactivators, corepressors, and chromatin modifiers. Contrary to an earlier notion that corepressors and coactivators exist in separate complexes, these molecules, which have apparently opposite functions, are increasingly being found in the same complex, which allows for efficient transcriptional control mechanisms. These control mechanisms are in turn regulated by an array of post-translational modifications under the influence of upstream and local signaling networks. Because the outcome of steroidal hormone receptor transcriptional complexes is measured in terms of the expression of target genes, any dysregulation of coregulator complexes perturbs normal homeostasis and could contribute to the development and maintenance of malignant phenotypes. Increasing evidence implicating steroid hormone receptors and their coregulators in various pathophysiologic conditions has elicited interest in their structure and biology. Further advances in this field of study should open up a unique window for novel targeted therapies for diseases such as cancer. Here we briefly review the clinical relevance of corepressors, with a particular focus on their role in the development of cancerous phenotypes.

Coregulators and chromatin remodeling in transcriptional control

Despite many years of investigation by numerous investigators, transcriptional regulatory control remains an intensely investigated and continuously evolving field of research. Transcriptional regulation is dependent not only on transcription factor activation and chromatin remodeling, but also on a host of transcription factor coregulators-coactivators and corepressors. In addition to transcription factor activation and chromatin changes, there is an expanding array of additional modifications that titrate transcriptional regulation for the specific conditions of a particular cell type, organ system, and developmental stage, and such events are likely to be greatly influenced by upstream signaling cascades. Here, we will briefly review the highlights and perspectives of chromatin remodeling and transcription controls as affected by cofactor availability, cellular energy state, relative ratios of reducing equivalents, and upstream signaling. We also present the C-terminal binding protein (CtBP) as a novel nuclear receptor (NR) coregulator, which exemplifies the integration of a number of transcriptional regulatory controls.