Coregulators and chromatin remodeling in transcriptional control

HER family in cancer progression: From discovery to 2020 and beyond

The human epidermal growth factor receptor (HER) family of receptor tyrosine kinases (RTKs) are among the first layer of molecules that receive, interpret, and transduce signals leading to distinct cancer cell phenotypes. Since the discovery of the tooth-lid factor-later characterized as the epidermal growth factor (EGF)-and its high-affinity binding EGF receptor, HER kinases have emerged as one of the commonly upregulated or hyperactivated or mutated kinases in epithelial tumors, thus allowing HER1-3 family members to regulate several hallmarks of cancer development and progression. Each member of the HER family exhibits shared and unique structural features to engage multiple receptor activation modes, leading to a range of overlapping and distinct phenotypes. EGFR, the founding HER family member, provided the roadmap for the development of the cell surface RTK-directed targeted cancer therapy by serving as a prototype/precursor for the currently used HER-directed cancer drugs. We herein provide a brief account of the discoveries, defining moments, and historical context of the HER family and guidepost advances in basic, translational, and clinical research that solidified a prominent position of the HER family in cancer research and treatment. We also discuss the significance of HER3 pseudokinase in cancer biology; its unique structural features that drive transregulation among HER1-3, leading to a superior proximal signaling response; and potential role of HER3 as a shared effector of acquired therapeutic resistance against diverse oncology drugs. Finally, we also narrate some of the current drawbacks of HER-directed therapies and provide insights into postulated advances in HER biology with extensive implications of these therapies in cancer research and treatment.

The adverse outcome pathway for rodent liver tumor promotion by sustained activation of the aryl hydrocarbon receptor

An Adverse Outcome Pathway (AOP) represents the existing knowledge of a biological pathway leading from initial molecular interactions of a toxicant and progressing through a series of key events (KEs), culminating with an apical adverse outcome (AO) that has to be of regulatory relevance. An AOP based on the mode of action (MOA) of rodent liver tumor promotion by dioxin-like compounds (DLCs) has been developed and the weight of evidence (WoE) of key event relationships (KERs) evaluated using evolved Bradford Hill considerations. Dioxins and DLCs are potent aryl hydrocarbon receptor (AHR) ligands that cause a range of species-specific adverse outcomes. The occurrence of KEs is necessary for inducing downstream biological responses and KEs may occur at the molecular, cellular, tissue and organ levels. The common convention is that an AOP begins with the toxicant interaction with a biological response element; for this AOP, this initial event is binding of a DLC ligand to the AHR. Data from mechanistic studies, lifetime bioassays and approximately thirty initiation-promotion studies have established dioxin and DLCs as rat liver tumor promoters. Such studies clearly show that sustained AHR activation, weeks or months in duration, is necessary to induce rodent liver tumor promotion--hence, sustained AHR activation is deemed the molecular initiating event (MIE). After this MIE, subsequent KEs are 1) changes in cellular growth homeostasis likely associated with expression changes in a number of genes and observed as development of hepatic foci and decreases in apoptosis within foci; 2) extensive liver toxicity observed as the constellation of effects called toxic hepatopathy; 3) cellular proliferation and hyperplasia in several hepatic cell types. This progression of KEs culminates in the AO, the development of hepatocellular adenomas and carcinomas and cholangiolar carcinomas. A rich data set provides both qualitative and quantitative knowledge of the progression of this AOP through KEs and the KERs. Thus, the WoE for this AOP is judged to be strong. Species-specific effects of dioxins and DLCs are well known--humans are less responsive than rodents and rodent species differ in sensitivity between strains. Consequently, application of this AOP to evaluate potential human health risks must take these differences into account.

Reciprocal occupancy of BCL6 and STAT5 on Growth Hormone target genes: contrasting transcriptional outcomes and promoter-specific roles of p300 and HDAC3

Expression of the Growth Hormone (GH)-stimulated gene Socs2 (Suppressor of Cytokine Signaling 2) is mediated by the transcription activator STAT5 (Signal Transducer and Activator of Transcription 5) and the transcription repressor BCL6 (B-Cell Lymphoma 6). ChIP-Sequencing identified Cish (Cytokine-Inducible SH2-containing protein) and Bcl6 as having similar patterns of reciprocal occupancy by BCL6 and STAT5 in response to GH, though GH stimulates Cish and inhibits Bcl6 expression. The co-activator p300 occupied Socs2, Cish and Bcl6 promoters, and enhanced STAT5-mediated activation of Socs2 and Cish. In contrast, on Bcl6, p300 functioned as a repressor and inhibited in conjunction with STAT5 or BCL6. The co-repressor HDAC3 (Histone deacetylase 3) inhibited the Socs2, Cish and Bcl6 promoters in the presence of STAT5. Thus transcriptional outcomes on GH-regulated genes occupied by BCL6 and STAT5 are determined in a promoter-specific fashion by co-regulatory proteins which mediate the distinction between activating and repressive transcription factors.

Molecular characterization of a human matrix attachment region epigenetic regulator

Matrix attachment regions (MAR) generally act as epigenetic regulatory sequences that increase gene expression, and they were proposed to partition chromosomes into loop-forming domains. However, their molecular mode of action remains poorly understood. Here, we assessed the possible contribution of the AT-rich core and adjacent transcription factor binding motifs to the transcription augmenting and anti-silencing effects of human MAR 1–68. Either flanking sequences together with the AT-rich core were required to obtain the full MAR effects. Shortened MAR derivatives retaining full MAR activity were constructed from combinations of the AT-rich sequence and multimerized transcription factor binding motifs, implying that both transcription factors and the AT-rich microsatellite sequence are required to mediate the MAR effect. Genomic analysis indicated that MAR AT-rich cores may be depleted of histones and enriched in RNA polymerase II, providing a molecular interpretation of their chromatin domain insulator and transcriptional augmentation activities.

Axonal transcription factors: novel regulators of growth cone-to-nucleus signaling

Developing axons contain transcripts that are locally translated to influence the axonal proteome. Recent studies have shown that axonal transcripts include mRNAs encoding transcription factors. These mRNAs are translated to produce transcription factors that can be retrogradely trafficked back to the nucleus, where they regulate gene expression programs. These findings point to a novel mechanism of growth cone-to-nucleus signaling that occurs when growth cones encounter extracellular signaling molecules that stimulate local translation of these transcription factors, thereby influencing gene transcription. Here we summarize recent findings on local translation of transcription factors in axons and their roles in different neuronal processes such as neuronal specification, survival, and axon regeneration. Comprehensive axonal transcriptome studies have revealed transcripts that encode many more transcription factors and cofactors, suggesting a potentially broad role for this type of signaling. We review the progress on the approaches and tools that have been developed to study local translation and retrograde trafficking of transcription factors. We also highlight the challenges in the field and discuss the potential routes to resolving them.

αNAC interacts with histone deacetylase corepressors to control Myogenin and Osteocalcin gene expression

In the nucleus of differentiated osteoblasts, the DNA-binding αNAC protein acts as a transcriptional coactivator of the Osteocalcin gene. Chromatin immunoprecipitation-microarray assays (ChIP-chip) showed that αNAC binds the Osteocalcin promoter but also identified the Myogenin promoter as an αNAC target. Here, we confirm these array data using quantitative ChIP and further detected that αNAC binds to these promoters in myoblasts. Since these genes are differentially regulated during osteoblastogenesis or myogenesis, these results suggest cell- and promoter-context specific functions for αNAC. We hypothesized that αNAC dynamically recruits corepressors to inhibit Myogenin expression in cells committing to the osteoblastic lineage or to inhibit Osteocalcin transcription in differentiating myoblasts. Using co-immunoprecipitation assays, we detected complexes between αNAC and the corepressors HDAC1 and HDAC3, in myoblasts and osteoblasts. Sequential ChIP confirmed HDAC1 recruitment by αNAC at the Osteocalcin and Myogenin promoters. Interaction with the corepressors was detectable in pre-osteoblasts and in myoblasts but disappeared as the cells differentiate. Treatment with an HDAC inhibitor caused de-repression of Osteocalcin expression in myoblasts. Overexpression of αNAC in myoblasts inhibits expression of Myogenin and differentiation. However, overexpression of an N-terminus truncated αNAC mutant allowed myoblasts to express Myogenin and differentiate, and this mutant did not interact with HDAC1 or HDAC3. This study identified an additional DNA-binding target and novel protein-protein interactions for αNAC. We propose that αNAC plays a role in regulating gene transcription during mesenchymal cell differentiation by differentially recruiting corepressors at target promoters.

Identification and characterization of ToRC, a novel ISWI-containing ATP-dependent chromatin assembly complex

SNF2-like motor proteins, such as ISWI, cooperate with histone chaperones in the assembly and remodeling of chromatin. Here we describe a novel, evolutionarily conserved, ISWI-containing complex termed ToRC (Toutatis-containing chromatin remodeling complex). ToRC comprises ISWI, Toutatis/TIP5 (TTF-I-interacting protein 5), and the transcriptional corepressor CtBP (C-terminal-binding protein). ToRC facilitates ATP-dependent nucleosome assembly in vitro. All three subunits are required for its maximal biochemical activity. The toutatis gene exhibits strong synthetic lethal interactions with CtBP. Thus, ToRC mediates, at least in part, biological activities of CtBP and Toutatis. ToRC subunits colocalize in euchromatic arms of polytene chromosomes. Furthermore, nuclear localization and precise distribution of ToRC in chromosomes are dependent on CtBP. ToRC is involved in CtBP-mediated regulation of transcription by RNA polymerase II in vivo. For instance, both Toutatis and CtBP are required for repression of genes of a proneural gene cluster, achaete-scute complex (AS-C), in Drosophila larvae. Intriguingly, native C-terminally truncated Toutatis isoforms do not associate with CtBP and localize predominantly to the nucleolus. Thus, Toutatis forms two alternative complexes that have differential distribution and can participate in distinct aspects of nuclear DNA metabolism.

Combinatorial control of ATF4-dependent gene transcription in osteoblasts

Osteoblast-specific gene transcription requires interaction between bone cell-specific transcription factors and more widely expressed transcriptional regulators. This is particularly evident for the basic domain-leucine zipper factor activating transcription factor 4 (ATF4), whose activity can be enhanced or inhibited through interaction with other leucine zipper proteins, intermediate filament proteins, components of the basic transcriptional machinery, nuclear matrix attachment molecules, or ubiquitously expressed transcription factors. We discuss the results supporting the relevance of these interactions and present the first evidence of a functional interaction between ATF4, FIAT (factor-inhibiting ATF4-mediated transcription), and αNAC (nascent polypeptide-associated complex and coactivator alpha), three proteins that have been previously shown to associate using various protein-protein interaction assays.

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.

Androgen receptor coactivator ARA70alpha and ARA70beta isoform-specific antibodies: new tools for studies of expression and immunohistochemical localization

ARA70 is a coactivator of androgen receptor (AR), a ligand-dependent transcription factor that plays an important role in prostate cancer. There are 2 variants of ARA70, the full length 70 kd ARA70alpha isoform and the internally spliced 35 kd ARA70beta isoform. Recent studies have suggested different expression and roles of the 2 isoforms in several endocrine malignancies, including prostate, breast, and ovarian cancers. To study the roles of these isoforms in cancers, we produced isoform-specific polyclonal antibodies. The anti-ARA70alpha antibody was raised in rabbits against 326 amino acid peptide corresponding to the internal deletion missing from ARA70beta (ARA70id), whereas the anti-ARA70beta antibody was raised against 18 amino acid polypeptide spanning the splice junction, with Gln-Gln motif unique to ARA70beta. The antisera were affinity purified on CNBr-activated sepharose 4B, and their specificity tested against bacterially expressed, Ni-column-purified ARA70alpha, ARA70beta, and ARA70id. The anti-ARA70alpha antibody recognized ARA70alpha and ARA70id, but not ARA70beta. The anti-ARA70beta antibody was specific to ARA70beta and did not cross-react with ARA70alpha or ARA70id. We then used these antibodies to detect ARA70 isoforms in crude extracts made of prostate cancer cell lines and performed immunohistochemical localization of these proteins in prostate tissues. ARA70beta localized to the cytosol, whereas ARA70alpha was found in the nucleus, supporting the notion of their dissimilar functions.

Stimulation of prostate cancer cellular proliferation and invasion by the androgen receptor co-activator ARA70

ARA70 was first identified as a gene fused to the ret oncogene in thyroid carcinoma and subsequently as a co-activator for androgen receptor (AR). Two isoforms of ARA70 have been identified: a 70-kDa version called ARA70 alpha and an internally spliced 35-kDa variant termed ARA70 beta. We have previously reported that ARA70 alpha expression is reduced in prostate cancer, and its overexpression inhibits proliferation of LNCaP prostate cancer cells. However, the function of the ARA70 beta isoform in prostate cancer is not understood. In this report we examined the effects of ARA70 beta on AR transcriptional regulation as well as prostate cancer cellular proliferation and invasion. Although both ARA70 alpha and ARA70 beta functioned as transcriptional co-activators of AR in cell-based reporter assays, ARA70 beta overexpression, in contrast to ARA70 alpha, promoted prostate cancer cellular proliferation and invasion through Matrigel. Interestingly, genome-wide expression profiling of cells expressing ARA70 beta revealed an increase in the expression of genes involved in the control of cell division and adhesion, compatible with a role for ARA70 beta in proliferation and invasion. Consistent with its function in promoting cell growth and invasion, ARA70 beta expression was increased in prostate cancer. Our findings implicate ARA70 beta as a regulator of tumor cell growth and metastasis by affecting gene expression.

Nuclear receptor interaction protein, a coactivator of androgen receptors (AR), is regulated by AR and Sp1 to feed forward and activate its own gene expression through AR protein stability

Previously, we found a novel gene, nuclear receptor interaction protein (NRIP), a transcription cofactor that can enhance an AR-driven PSA promoter activity in a ligand-dependent manner in prostate cancer cells. Here, we investigated NRIP regulation. We cloned a 413-bp fragment from the transcription initiation site of the NRIP gene that had strong promoter activity, was TATA-less and GC-rich, and, based on DNA sequences, contained one androgen response element (ARE) and three Sp1-binding sites (Sp1-1, Sp1-2, Sp1-3). Transient promoter luciferase assays, chromatin immunoprecipitation and small RNA interference analyses mapped ARE and Sp1-2-binding sites involved in NRIP promoter activation, implying that NRIP is a target gene for AR or Sp1. AR associates with the NRIP promoter through ARE and indirectly through Sp1-binding site via AR–Sp1 complex formation. Thus both ARE and Sp1-binding site within the NRIP promoter can respond to androgen induction. More intriguingly, NRIP plays a feed-forward role enhancing AR-driven NRIP promoter activity via NRIP forming a complex with AR to protect AR protein from proteasome degradation. This is the first demonstration that NRIP is a novel AR-target gene and that NRIP expression feeds forward and activates its own expression through AR protein stability.

Nuclear functions of endocytic proteins

An increasing number of proteins appear to perform multiple, sometimes unrelated functions in the cell. Such moonlighting properties have been recently demonstrated for proteins involved in clathrin-mediated endocytosis. Some clathrin adaptors and endosomal proteins can undergo nucleocytoplasmic shuttling, which is often based on intrinsic sequence motifs and requires active transport mechanisms. Endocytic proteins can associate with nuclear molecules, changing their localization and/or activity and may modulate the levels and specificity of gene transcription. It is not clear how the nuclear and cytoplasmic pools of endocytic proteins are interconnected, or whether these molecules act as nuclear second messengers upon extracellular stimuli, but alike in endocytosis, they seem to form multi-component scaffolding platforms in the nucleus. Added to their endocytic functions, the nuclear roles of Eps15, Epsin1, CALM, HIP1, Dab1/2, beta-arrestins, APPL1/2 and the components of ESCRTs clearly increase the complexity of signaling networks affecting cellular growth, proliferation and homeostasis.

Constitutive coactivator of peroxisome proliferator-activated receptor (PPARgamma), a novel coactivator of PPARgamma that promotes adipogenesis

Peroxisome proliferator-activated receptor gamma (PPARgamma) plays essential roles in adipogenesis by transcriptionally regulating adipocyte-specific genes through recruitment of coregulators including coactivators and corepressors. However, the precise repertoire of coactivators required for PPARgamma transactivation remains unresolved. In this report, we cloned and characterized a novel PPARgamma interacting protein, constitutive coactivator of PPARgamma (CCPG), which is expressed in multiple adult tissues and throughout embryonic development. CCPG is localized in nucleus and contains four LXXLL motifs, which are characteristic for nuclear receptor coactivators. A delineation of CCPG-PPARgamma interaction by glutathione-S-transferase pull-down and coimmunoprecipitation assays indicated that CCPG interacts with the hinge region of PPARgamma in a ligand-independent manner. However, mutation of four motifs of LXXLL to LXXAA in CCPG does not compromise its interaction with PPARgamma, suggesting LXXLL motif is not required for the interaction. Glutathione-S-transferase pull-down assays showed that CCPG binds to retinoic X receptor-alpha and estrogen receptor-alpha independent of their ligands, but not to thyroid hormone receptor-beta. CCPG coactivates PPARgamma in PPAR response element reporter assays, and the N terminus (amino acids 1-561) of CCPG acts to significantly augment the transactivation of PPARgamma, whereas the C terminus (amino acids 562-786) represses PPARgamma activity, indicating the N terminus possesses the activation domain. Using an adenoviral-mediated system, we also revealed that overexpression of CCPG promoted differentiation of OP9 preadipocyte into adipocyte, and knockdown of CCPG by RNA interference blocked this process, as examined by Oil Red O staining and Western blots of adipocyte-specific protein, adiponectin, and perilipin. Taken together, our data indicate that CCPG is a bona fide coactivator and promotes adipogenesis in a PPARgamma-dependent manner.

Overview of nomenclature of nuclear receptors

Nuclear receptor pharmacology has, to a certain extent, led the way, compared with other receptor systems, in the appreciation that ligands may exert very diverse pharmacology, based on their individual chemical structure and the allosteric changes induced in the receptor/accessory protein complex. This can lead to very selective pharmacological effects, which may not necessarily be predicted from the experience with other agonists/partial agonists/antagonists. If this is the case, then drug discovery may be back to drug-specific pharmacology (where each drug may have an original profile), rather than specific-drug pharmacology (where agents specific for a receptor have a distinct profile). As functional selectivity is indeed a crucial mechanism to be considered when going through the drug discovery development process, then initial screens using reconstituted systems may not show the appropriate pharmacology, simply because the required stoichiometry of corepressors and coactivators may not be present to select the best compounds; therefore, multiple effector systems are necessary to screen for differential activation, and, even then, screening with in vivo pathophysiological models may ultimately be required for the selection process-a massive but necessary task for pharmacologists. Thus, the characterization of nuclear receptors and their associated proteins and the ligands that interact with them will remain a challenge to pharmacologists.

Hepatocyte cell lines: their use, scope and limitations in drug metabolism studies

Gaining knowledge on the metabolism of a drug, the enzymes involved and its inhibition or induction potential is a necessary step in pharmaceutical development of new compounds. Primary human hepatocytes are considered a cellular model of reference, as they express the majority of drug-metabolising enzymes, respond to enzyme inducers and are capable of generating in vitro a metabolic profile similar to what is found in vivo. However, hepatocytes show phenotypic instability and have a restricted accessibility. Different alternatives have been explored in the past recent years to overcome the limitations of primary hepatocytes. These include immortalisation of adult or fetal human hepatic cells by means of transforming tumour virus genes, oncogenes, conditionally immortalised hepatocytes, and cell fusion. New strategies are currently being used to upregulate the expression of drug-metabolising enzymes in cell lines or to derive hepatocytes from progenitor cells. This paper reviews the features of liver-derived cell lines, their suitability for drug metabolism studies as well as the state-of-the-art of the strategies pursued in order to generate metabolically competent hepatic cell lines.

A Metabolic Enzyme of the Short-Chain Dehydrogenase/Reductase Superfamily May Moonlight in the Nucleus as a Repressor of Promoter Activity

Transcriptional repression often depends on the action of recruited co-repressor complexes with intrinsic enzymatic activities. The composition of these complexes depends on the nicotine amide dinucleotide co-factors and is thus directly reflective of the metabolic state of the cells. This study provides evidence that an enzyme, hRoDH-E2, with cytoplasmic phosphorylated and reduced forms of NAD-dependent retinol dehydrogenase activity may function in the nucleus as a transcriptional repressor. By using the promoter of the epidermal late differentiation marker profilaggrin as a model, we show that both in vivo and in vitro the protein is recruited over the promoter. hRoDH-E2 represses profilaggrin promoter activity by altering the function of other activators, such as Sp1. The repressive function is associated with the ability of nuclear hRoDH-E2 to modulate the acetylation/deacetylation activity in the vicinity of transcription initiation site. These findings add hRoDH-E2 to the small group of metabolic enzymes, which, by being recruited over promoter regions, could directly link the cytoplasmic and nuclear functions within the cell.

Metastasis tumor antigen family proteins during breast cancer progression and metastasis in a reliable mouse model for human breast cancer

PURPOSE

Chromatin remodeling pathways are critical in the regulation of cancer-related genes and are currently being explored as potential targets for therapeutic intervention. The metastasis tumor antigen (MTA) family of proteins, MTA1, MTA2, and MTA3, are components of chromatin remodeling pathways with potential roles in breast cancer. Although all three MTA family proteins have been shown to be associated with metastatic progression of breast cancers, the expression characteristic of MTA1-3 proteins in a multistep breast cancer progression model remains unknown. Structural and functional studies have suggested that they are heterogeneous in the Mi-2/NuRD complex, exhibit tissue-specific patterns of expression, and impart unique properties to estrogen receptor-alpha (ERalpha) action. This led us to hypothesize that each member of the MTA family possesses a unique role and interacts with different pathways in the stepwise process of breast cancer development and progression.

EXPERIMENTAL DESIGN

MTA family proteins were examined by immunohistochemistry in breast cancer processes ranging from normal duct, to premalignant lesions, to invasive carcinoma, and to metastasized tumors in PyV-mT transgenic mice, which represents a reliable model for multistage tumorigenesis of human breast cancer. We also determined the association of MTA proteins with the status of cell proliferation, ER, E-cadherin and cytoplasmic beta-catenin, and cancer-related coactivators, AIB1 and PELP1.

RESULTS

The expression of all three MTA proteins was altered in primary breast tumors. Each MTA protein had a unique expression pattern during the primary breast tumor progression. Altered expression of MTA1 was observed in both premalignant lesion and malignant carcinoma, but an elevated nuclear expression was observed in ER-negative carcinomas. MTA3 was exclusively expressed in a subset of cells of ER-positive premalignant lesions but not in carcinomas. MTA2 expression seems to be unrelated to ER status. Loss of MTA3 expression and more nuclear localization of MTA1 occurred with loss of E-cadherin and decreased cytoplasmic beta-catenin, two molecules essential for epithelial cell adhesion and important tumor cell invasion. At the late stage of tumor formation, MTA1 is usually expressed in the center of tumors. Coincidentally, the distribution of MTA1-positive cells at this stage was complementary to that of AIB1 and PELP1, which were localized to the tumor periphery with relatively active cell proliferation, scattered ER-positive cells and a limited differentiation. In metastasized lung tumors, the expression pattern of MTA-protein expression was distinct from that in primary counterparts.

CONCLUSIONS

The findings presented here support the notion that each member of the MTA family might potentially play a stepwise role in a cell type-specific manner during breast cancer progression to metastasis. On the basis of the noted temporal expression patterns of MTA proteins with ER status, cell adhesion-essential regulators (E-cadherin and cytoplasmic beta-catenin), and coactivators, we propose that MTA protein-related chromatin remodeling pathways interact with steroid receptors, growth factor receptors, and other transcriptional signaling pathways to orchestrate the governing of events in breast cancer progression and metastasis.

Negative regulation of estrogen receptor alpha transactivation functions by LIM domain only 4 protein

LIM domain only 4 (LMO4), a member of the LIM-only family of transcriptional coregulatory proteins, consists of two LIM protein-protein interaction domains that enable it to function as a linker protein in multiprotein complexes. Here, we have identified estrogen receptor alpha (ERalpha) and its corepressor, metastasis tumor antigen 1 (MTA1), as two novel binding partners of LMO4. Interestingly, LMO4 exhibited binding with both ERalpha and MTA1 and existed as a complex with ERalpha, MTA1, and histone deacetylases (HDAC), implying that LMO4 was a component of the MTA1 corepressor complex. Consistent with this notion, LMO4 overexpression repressed ERalpha transactivation functions in an HDAC-dependent manner. Accordingly, silencing of endogenous LMO4 expression resulted in a significant increased recruitment of ERalpha to target gene chromatin, stimulation of ERalpha transactivation activity, and enhanced expression of ERalpha-regulated genes. These findings suggested that LMO4 was an integral part of the molecular machinery involved in the negative regulation of ERalpha transactivation function in breast cells. Because LMO4 is up-regulated in human breast cancers, repression of ERalpha transactivation functions by LMO4 might contribute to the process of breast cancer progression by allowing the development of ERalpha-negative phenotypes, leading to increased aggressiveness of breast cancer cells.

Altered gene silencing and human diseases

Epigenetic regulation of gene expression is mediated through several mechanisms, including modifications in DNA methylation, covalent modifications of core nucleosomal histones, rearrangement of histones and RNA interference. It is now clear that deregulation of epigenetic mechanisms cooperates with genetic alterations in the development and progression of several Mendelian disorders. Here, we summarize the recent findings that highlight how certain inherited diseases, such as Rett syndrome, Immunodeficiency-centromeric instability-facial anomalies syndrome, and facioscapulohumeral muscular dystrophy, result from altered gene silencing.

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.

Functional regulation of oestrogen receptor pathway by the dynein light chain 1

Overexpression and phosphorylation of dynein light chain 1 (DLC1) have been shown to promote the growth of breast cancer cells. However, the role of DLC1 in the action of the oestrogen receptor (ER) remains unknown. Here, we found that oestrogen induces the transcription and expression of DLC1. DLC1 facilitated oestrogen-induced ER transactivation and anchorage-independent growth of breast cancer cells. We show that DLC1 interacts with ER, and such interaction is required for the transactivation-promoting activity of DLC1. Further, DLC1 expression led to enhanced recruitment of the DLC1-ER complex to the ER-target gene chromatin. Conversely, DLC1 downregulation compromised the ER-transactivation activity and also its nuclear accumulation, suggesting a potential chaperone-like activity of DLC1 in the nuclear translocation of ER. Together, these data define an unexpected upregulation of DLC1 by oestrogen and a previously unrecognized DLC1-ER interaction in supporting and amplifying ER-initiated cellular responses in breast cancer cells.

Controlling nuclear receptors: the circular logic of cofactor cycles

Nuclear receptors regulate many biologically important processes in development and homeostasis by their bimodal function as repressors and activators of gene transcription. A finely tuned modulation of the transcriptional activities of nuclear receptors is crucial for determining highly specific and diversified programmes of gene expression. Recent studies have provided insights into the molecular mechanisms that are required to switch between repression and activation functions, the combinatorial roles of the multiple cofactor complexes that are required for mediating transcriptional regulation, and the central question of how several different signalling pathways can be integrated at the nuclear level to achieve specific profiles of gene expression.