FRAP reveals that mobility of oestrogen receptor-alpha is ligand- and proteasome-dependent

Epigenetic mechanisms of cancer progression and therapy resistance in estrogen-receptor (ER+) breast cancer

Estrogen receptor-positive (ER+) breast cancer is the most frequent breast cancer subtype. Agents targeting the ER signaling pathway have been successful in reducing mortality from breast cancer for decades. However, mechanisms of resistance to these treatments arise, especially in the metastatic setting. Recently, it has been recognized that epigenetic dysregulation is a common feature that facilitates the acquisition of cancer hallmarks across cancer types, including ER+ breast cancer. Alterations in epigenetic regulators and transcription factors (TF) coupled with changes to the chromatin landscape have been found to orchestrate breast oncogenesis, metastasis, and the development of a resistant phenotype. Here, we review recent advances in our understanding of how the epigenome dictates breast cancer tumorigenesis and resistance to targeted therapies and discuss novel therapeutic interventions for overcoming resistance.

Gene transcription regulation by ER at the single cell and allele level

In this short review we discuss the current view of how the estrogen receptor (ER), a pivotal member of the nuclear receptor superfamily of transcription factors, regulates gene transcription at the single cell and allele level, focusing on in vitro cell line models. We discuss central topics and new trends in molecular biology including phenotypic heterogeneity, single cell sequencing, nuclear phase separated condensates, single cell imaging, and image analysis methods, with particular focus on the methodologies and results that have been reported in the last few years using microscopy-based techniques. These observations augment the results from biochemical assays that lead to a much more complex and dynamic view of how ER, and arguably most transcription factors, act to regulate gene transcription.

Transcription Factor Dynamics: One Molecule at a Time

Cells must tightly regulate their gene expression programs and yet rapidly respond to acute biochemical and biophysical cues within their environment. This information is transmitted to the nucleus through various signaling cascades, culminating in the activation or repression of target genes. Transcription factors (TFs) are key mediators of these signals, binding to specific regulatory elements within chromatin. While live-cell imaging has conclusively proven that TF-chromatin interactions are highly dynamic, how such transient interactions can have long-term impacts on developmental trajectories and disease progression is still largely unclear. In this review, we summarize our current understanding of the dynamic nature of TF functions, starting with a historical overview of early live-cell experiments. We highlight key factors that govern TF dynamics and how TF dynamics, in turn, affect downstream transcriptional bursting. Finally, we conclude with open challenges and emerging technologies that will further our understanding of transcriptional regulation.

Therapeutic resistance to anti-oestrogen therapy in breast cancer

The hormone receptor oestrogen receptor-α (ER) orchestrates physiological mammary gland development, breast carcinogenesis and the progression of breast tumours into lethal, treatment-refractory systemic disease. Selective antagonism of ER signalling has been one of the most successful therapeutic approaches in oncology, benefiting patients as both a cancer preventative measure and a cancer treatment strategy. However, resistance to anti-oestrogen therapy is a major clinical challenge. Over the past decade, we have gained an understanding of how breast cancers evolve under the pressure of anti-oestrogen therapy. This is best depicted by the case of oestrogen-independent mutations in the gene encoding ER (ESR1), which are virtually absent in primary breast cancer but highly prevalent (20-40%) in anti-oestrogen-treated metastatic disease. These and other findings highlight the 'evolvability' of ER+ breast cancer and the need to understand molecular processes by which this evolution occurs. Recent development and approval of next-generation ER antagonists to target ESR1-mutant breast cancer underscores the clinical importance of this evolvability and sets a new paradigm for the treatment of ER+ breast cancers.

The kleisin subunit controls the function of C. elegans meiotic cohesins by determining the mode of DNA binding and differential regulation by SCC-2 and WAPL-1

The cohesin complex plays essential roles in chromosome segregation, 3D genome organisation, and DNA damage repair through its ability to modify DNA topology. In higher eukaryotes, meiotic chromosome function, and therefore fertility, requires cohesin complexes containing meiosis-specific kleisin subunits: REC8 and RAD21L in mammals and REC-8 and COH-3/4 in Caenorhabditis elegans. How these complexes perform the multiple functions of cohesin during meiosis and whether this involves different modes of DNA binding or dynamic association with chromosomes is poorly understood. Combining time-resolved methods of protein removal with live imaging and exploiting the temporospatial organisation of the C. elegans germline, we show that REC-8 complexes provide sister chromatid cohesion (SCC) and DNA repair, while COH-3/4 complexes control higher-order chromosome structure. High-abundance COH-3/4 complexes associate dynamically with individual chromatids in a manner dependent on cohesin loading (SCC-2) and removal (WAPL-1) factors. In contrast, low-abundance REC-8 complexes associate stably with chromosomes, tethering sister chromatids from S-phase until the meiotic divisions. Our results reveal that kleisin identity determines the function of meiotic cohesin by controlling the mode and regulation of cohesin-DNA association, and are consistent with a model in which SCC and DNA looping are performed by variant cohesin complexes that coexist on chromosomes.

Novel endocrine therapies: What is next in estrogen receptor positive, HER2 negative breast cancer?

Endocrine therapy (ET) is the cornerstone of management in hormone receptor (HR)+ breast cancer (BC). Indeed, targeting the estrogen receptor (ER) signaling at different levels is a successful strategy, since BC largely relies on the ER signaling as a driver of tumorigenesis and progression. In metastatic BC, progression of disease typically occurs due to either ligand-independent ER signaling, which favors tumor proliferation and survival in the absence of hormonal stimuli, or an ER-independent signaling, which exploits alternative transcription pathways. For instance, estrogen receptor 1 (ESR1) mutations induce constitutive ER activity, in turn upregulating ER-dependent gene transcription and causing resistance to estrogen depleting therapies. The largest unmet need lies after progression on ET + cyclin-dependent kinases 4 and 6 (CDK4/6) inhibitors, where fulvestrant alone provides an average 2-3-month PFS. In this context, novel oral selective estrogen receptor degraders (SERDs) and other next-generation ETs are being investigated, both as single agents and in combination with targeted therapies. Elacestrant, the next generation ET in most advanced clinical development and the first to be FDA approved, demonstrated improved outcomes compared to standard ETs in ET pre-treated HR+/HER2- metastatic BC in the phase 3 EMERALD clinical trial. Additionally, other agents are showing promising results in both preclinical and early phase clinical settings. In this review, emerging data related to oral SERDs and other novel ETs in managing HR+/HER2- BC are presented. Major challenges and future perspectives related to the optimal sequence of therapeutic options and the molecular landscape of endocrine resistance are also provided.

Transcriptional enhancers at 40: evolution of a viral DNA element to nuclear architectural structures

Gene regulation by transcriptional enhancers is the dominant mechanism driving cell type- and signal-specific transcriptional diversity in metazoans. However, over four decades since the original discovery, how enhancers operate in the nuclear space remains largely enigmatic. Recent multidisciplinary efforts combining real-time imaging, genome sequencing, and biophysical strategies provide insightful but conflicting models of enhancer-mediated gene control. Here, we review the discovery and progress in enhancer biology, emphasizing the recent findings that acutely activated enhancers assemble regulatory machinery as mesoscale architectural structures with distinct physical properties. These findings help formulate novel models that explain several mysterious features of the assembly of transcriptional enhancers and the mechanisms of spatial control of gene expression.

Accelerating drug development in breast cancer: New frontiers for ER inhibition

The estrogen receptor (ER) is an important driver in the proliferation, tumorigenesis, and progression of breast cancers, and targeting ER signaling at different levels is a successful strategy in the control of hormone receptor positive (HR+) breast cancer. Endocrine therapy has been the treatment of choice for HR+ breast cancer in the early and advanced stages with multiple agents, including selective estrogen receptor modulators (SERMS), selective estrogen receptor degraders (SERDs), and aromatase inhibitors (AIs), which vary in their mechanisms of action and pharmacokinetics. Combination strategies also employ cyclin dependent kinase 4 and 6 and phosphatidylinositol 3-kinase to maximize the benefits of endocrine therapy. This paper reviews the clinical development of SERDs and other novel ER inhibitors, as well as combination strategies to overcome mechanisms of ER pathway escape. It also assesses the advantages of newer oral ER inhibitors with increased bioavailability, improved therapeutic index, better administration, and increased efficacy, as well as discussing future directions in the field.

Decoding the Therapeutic Implications of the ERα Stability and Subcellular Distribution in Breast Cancer

Approximately 70% of all breast cancer cases are estrogen receptor-alpha positive (ERα+) and any ERα signaling pathways deregulation is critical for the progression of malignant mammary neoplasia. ERα acts as a transcription factor that promotes the expression of estrogen target genes associated with pro-tumor activity in breast cancer cells. Furthermore, ERα is also part of extranuclear signaling pathways related to endocrine resistance. The regulation of ERα subcellular distribution and protein stability is critical to regulate its functions and, consequently, influence the response to endocrine therapies and progression of this pathology. This minireview highlights studies that have deciphered the molecular mechanisms implicated in controlling ERα stability and nucleo-cytoplasmic transport. These mechanisms offer information about novel biomarkers, therapeutic targets, and promising strategies for breast cancer treatment.

Segregation of nuclear and membrane-initiated actions of estrogen receptor using genetically modified animals and pharmacological tools

Estrogen receptor alpha (ERα) and beta (ERβ) are members of the nuclear receptor superfamily, playing widespread functions in reproductive and non-reproductive tissues. Beside the canonical function of ERs as nuclear receptors, in this review, we summarize our current understanding of extra-nuclear, membrane-initiated functions of ERs with a specific focus on ERα. Over the last decade, in vivo evidence has accumulated to demonstrate the physiological relevance of this ERα membrane-initiated-signaling from mouse models to selective pharmacological tools. Finally, we discuss the perspectives and future challenges opened by the integration of extra-nuclear ERα signaling in physiology and pathology of estrogens.

Endocrine disrupting chemicals differentially alter intranuclear dynamics and transcriptional activation of estrogen receptor-α

Transcription is a highly regulated sequence of stochastic processes utilizing many regulators, including nuclear receptors (NR) that respond to stimuli. Endocrine disrupting chemicals (EDCs) in the environment can compete with natural ligands for nuclear receptors to alter transcription. As nuclear dynamics can be tightly linked to transcription, it is important to determine how EDCs affect NR mobility. We use an EPA-assembled set of 45 estrogen receptor-α (ERα) ligands and EDCs in our engineered PRL-Array model to characterize their effect upon transcription using fluorescence in situ hybridization and fluorescence recovery after photobleaching (FRAP). We identified 36 compounds that target ERα-GFP to a transcriptionally active, visible locus. Using a novel method for multi-region FRAP analysis we find a strong negative correlation between ERα mobility and inverse agonists. Our findings indicate that ERα mobility is not solely tied to transcription but affected highly by the chemical class binding the receptor.

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Development of a new algorithm for multi-foci FRAP analysis

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ERα agonists can be segregated into fast-moving and slow-moving receptor groups

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ERα inverse agonists receptor mobility inversely correlates with transcription

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Steroidal compounds result in a slower moving receptor than other classes

Molecular dynamics of estrogen-related receptors and their regulatory proteins: roles in transcriptional control for endocrine and metabolic signaling

Estrogen-related receptor (ERR) is a member of the nuclear receptor (NR) superfamily and has three subtypes α, β, and γ. Despite their strong homology with estrogen receptor (ER) α, ERRs cannot accommodate endogenous hormones. However, they are able to regulate gene expression without ligand binding. ERRα and ERRγ orchestrate the expression of genes involved in bioenergetic pathways, while ERRβ controls placental development and stem cell maintenance. Evidence from recent studies, including clinical research, has also demonstrated close associations of ERRs with the pathophysiology of hormone-related cancers and metabolic disorders including type 2 diabetes mellitus. This review summarizes the basic knowledge and recent advances in ERRs and their associated proteins, focusing on the subcellular dynamics involved in transcriptional regulation. Fluorescent protein labeling enabled monitoring of ERRs in living cells and revealed previously unrecognized characteristics. Using this technique, we demonstrated a role of ERRβ in controlling estrogen signaling by regulating the subnuclear dynamics of ligand-activated ERα. Visualization of ERRs and related proteins and subsequent analyses also revealed a function of ERRγ in promoting liver lactate metabolism in association with LRPGC1, a recently identified lactic acid-responsive protein. These findings suggest that ERRs activate unique transregulation mechanisms in response to extracellular stimuli such as hormones and metabolic signals, implying an adaptive system behind the cellular homeostatic regulation by orphan NRs. Control of subcellular ERR dynamics will contribute toward the development of therapeutic approaches to treat various diseases including hormone-related cancers and metabolic disorders associated with abnormal ERR signaling pathways.

Role of conformational dynamics and flexibilities in the steroid receptor-coregulator protein complex formation

Understanding of the mechanisms of actions of the steroid hormone receptor (SHR)-coregulator (CoR) protein complexes in the gene regulations has revolutionized the field of molecular endocrinology and endocrine-related oncology. The discovery and characterization of steroid receptor coactivators (SRCs) and their ability to bind various transcription factors including SHRs to coordinate the regulation of multiple target genes highlights their importance as key coregulators in various cellular signaling crosstalks as well as therapeutic target for various endocrine-related disorders specifically endocrine cancers. The dynamic nature of the SHR-CoR multi-protein complexes indicate the critical role of conformational flexibilities within specific protein(s). In recent years, the importance of conformational dynamics of the SHRs in the intramolecular and intermolecular allosteric regulations mediated via their intrinsically disordered (ID) surfaces has been highlighted. In this review article, we have discussed the importance of ID conformations within the SRCs that may also be playing an important role in the formation/deformation of multi protein complexes involving SHRs and CoRs and subsequent target gene regulation.

Mechanisms of endocrine therapy resistance in breast cancer

The most commonly diagnosed breast cancer (BC) subtype is characterized by estrogen receptor (ER) expression. Treatment of this BC subtype typically involves modalities that either suppress the production of estrogen or impede the binding of estrgen to its receptors, constituting the basis for endocrine therapy. While many patients have benefitted from endocrine therapy with clear reduction in mortality and cancer recurrence, one of the clinical hurdles that remain involves overcoming intrinsic (de novo) or acquired resistance to endocrine therapy driven by diverse and complex changes occurring in the tumor microenvironment. Moreover, such resistance may persist even after progression through additional antiestrogen therapies thus demonstrating the importance of further investigation of mechanisms of ER modulation. Here, we discuss a number of advances that provide a better understanding of the complex mechanistic basis for resistance to endocrine therapy as well as future therapeutic maneuvers that may break this resistance.

Subcellular dynamics of estrogen-related receptors involved in transrepression through interactions with scaffold attachment factor B1

Estrogen-related receptor (ERR), a member of the nuclear receptor superfamily, consists of three subtypes (α, β, γ) and has strong homology with estrogen receptor. No endogenous ligands have been identified for ERRs, but they play key roles in metabolic, hormonal, and developmental processes as transcription factors without ligand binding. Although subnuclear dynamics are essential for nuclear events including nuclear receptor-mediated transcriptional regulation, the dynamics of ERRs are poorly understood. Here, we report that ERRs show subcellular kinetic changes in response to diethylstilbestrol (DES), a synthetic estrogen that represses the transactivity of all three ERR subtypes, using live-cell imaging with fluorescent protein labeling. Upon DES treatment, all ERR subtypes formed discrete clusters in the nucleus, with ERRγ also displaying nuclear export. Fluorescence recovery after photobleaching analyses revealed significant reductions in the intranuclear mobility of DES-bound ERRα and ERRβ, and a slight reduction in the intranuclear mobility of DES-bound ERRγ. After DES treatment, colocalization of all ERR subtypes with scaffold attachment factor B1 (SAFB1), a nuclear matrix-associated protein, was observed in dot-like subnuclear clusters, suggesting interactions of the ERRs with the nuclear matrix. Consistently, co-immunoprecipitation analyses confirmed enhanced interactions between ERRs and SAFB1 in the presence of DES. SAFB1 was clarified to repress the transactivity of all ERR subtypes through the ERR-response element. These results demonstrate ligand-dependent cluster formation of ERRs in the nucleus that is closely associated with SAFB1-mediated transrepression. Taken together, the present findings provide a new understanding of the pathophysiology regulated by ERR/SAFB1 signaling pathways and their subcellular dynamics.

Phase Ib Study of the Oral Proteasome Inhibitor Ixazomib (MLN9708) and Fulvestrant in Advanced ER+ Breast Cancer Progressing on Fulvestrant

LESSONS LEARNED

Fulvestrant is a selective estrogen receptor (ER)-downregulating antiestrogen that blocks ER transcriptional activity and is approved for ER-positive breast cancer. Fulvestrant also induces accumulation of insoluble ER and activates an unfolded protein response; proteasome inhibitors have been shown to enhance these effects in preclinical models.

BACKGROUND

Fulvestrant is a selective estrogen receptor (ER)-downregulating antiestrogen that blocks ER transcriptional activity and is approved for ER-positive (+) breast cancer. Fulvestrant also induces accumulation of insoluble ER and activates an unfolded protein response; proteasome inhibitors have been shown to enhance these effects in preclinical models.

METHODS

This is a single-center phase Ib study with a 3+3 design of fulvestrant and the proteasome inhibitor ixazomib (MLN9708) in patients with advanced ER+ breast cancer that was progressing on fulvestrant. A dose-escalation design allowed establishment of the ixazomib maximum tolerated dose (MTD). Secondary objectives included progression-free survival, pharmacokinetics, and tumor molecular analyses.

RESULTS

Among nine evaluable subjects, treatment was well-tolerated without dose-limiting toxicities The MTD of ixazomib was 4 mg in combination with fulvestrant. Plasma concentrations of the active form of ixazomib (MLN2238) in the 4-mg dose cohort had a median (range) maximal concentration (Cmax ) of 155 (122-171) ng/mL, time of maximal concentration (Tmax ) of 1 (1-1.5) hour, terminal elimination half-life of 66.6 (57.3-102.6) hour after initial dose, and area under the curve (AUC) of 5,025 (4,160-5,345) ng*h/mL. One partial response was observed, and median progression-free survival was 51 days (range, 47-137).

CONCLUSION

This drug combination has a favorable safety profile and antitumor activity in patients with fulvestrant-resistant advanced ER+ breast cancer that justifies future testing.

Microenvironmental Determinants of Breast Cancer Metastasis: Focus on the Crucial Interplay Between Estrogen and Insulin/Insulin-Like Growth Factor Signaling

The development and progression of the great majority of breast cancers (BCs) are mainly dependent on the biological action elicited by estrogens through the classical estrogen receptor (ER), as well as the alternate receptor named G-protein–coupled estrogen receptor (GPER). In addition to estrogens, other hormones and growth factors, including the insulin and insulin-like growth factor system (IIGFs), play a role in BC. IIGFs cooperates with estrogen signaling to generate a multilevel cross-communication that ultimately facilitates the transition toward aggressive and life-threatening BC phenotypes. In this regard, the majority of BC deaths are correlated with the formation of metastatic lesions at distant sites. A thorough scrutiny of the biological and biochemical events orchestrating metastasis formation and dissemination has shown that virtually all cell types within the tumor microenvironment work closely with BC cells to seed cancerous units at distant sites. By establishing an intricate scheme of paracrine interactions that lead to the expression of genes involved in metastasis initiation, progression, and virulence, the cross-talk between BC cells and the surrounding microenvironmental components does dictate tumor fate and patients’ prognosis. Following (i) a description of the main microenvironmental events prompting BC metastases and (ii) a concise overview of estrogen and the IIGFs signaling and their major regulatory functions in BC, here we provide a comprehensive analysis of the most recent findings on the role of these transduction pathways toward metastatic dissemination. In particular, we focused our attention on the main microenvironmental targets of the estrogen-IIGFs interplay, and we recapitulated relevant molecular nodes that orientate shared biological responses fostering the metastatic program. On the basis of available studies, we propose that a functional cross-talk between estrogens and IIGFs, by affecting the BC microenvironment, may contribute to the metastatic process and may be regarded as a novel target for combination therapies aimed at preventing the metastatic evolution.

Navigating the crowd: visualizing coordination between genome dynamics, structure, and transcription

The eukaryotic genome is hierarchically structured yet highly dynamic. Regulating transcription in this environment demands a high level of coordination to permit many proteins to interact with chromatin fiber at appropriate sites in a timely manner. We describe how recent advances in quantitative imaging techniques overcome caveats of sequencing-based methods (Hi-C and related) by enabling direct visualization of transcription factors and chromatin at high resolution, from single genes to the whole nucleus. We discuss the contribution of fluorescence imaging to deciphering the principles underlying this coordination within the crowded nuclear space in living cells and discuss challenges ahead.

Selective Estrogen Receptor Degraders (SERDs): A Promising Strategy for Estrogen Receptor Positive Endocrine-Resistant Breast Cancer

Estrogen receptor (ER) plays important roles in gene transcription and the proliferation of ER positive breast cancers. Selective modulation of ER has been a therapeutic target for this specific type of breast cancer for more than 30 years. Selective estrogen receptor modulators (SERMs) and aromatase inhibitors (AIs) have been demonstrated to be effective therapeutic approaches for ER positive breast cancers. Unfortunately, 30-50% of ER positive tumors become resistant to SERM/AI treatment after 3-5 years. Fulvestrant, the only approved selective estrogen receptor degrader (SERD), is currently an important therapeutic approach for the treatment of endocrine-resistant breast cancers. The poor pharmacokinetic properties of fulvestrant have inspired the development of a new generation of oral SERDs to overcome drug resistance. In this review, we describe recent advances in ERα structure, functions, and mechanisms of endocrine resistance and summarize the development of oral SERDs in both academic and industrial areas.

Advances in visualizing transcription factor - DNA interactions

At the heart of the transcription process is the specific interaction between transcription factors (TFs) and their target DNA sequences. Decades of molecular biology research have led to unprecedented insights into how TFs access the genome to regulate transcription. In the last 20 years, advances in microscopy have enabled scientists to add imaging as a powerful tool in probing two specific aspects of TF-DNA interactions: structure and dynamics. In this review, we examine how applications of diverse imaging technologies can provide structural and dynamic information that complements insights gained from molecular biology assays. As a case study, we discuss how applications of advanced imaging techniques have reshaped our understanding of TF behavior across the cell cycle, leading to a rethinking in the field of mitotic bookmarking.

Different effects of total flavonoids from Arachniodes exilis on human umbilical cord mesenchymal stem cells in vitro

Traditional Chinese medicines are used in promotion of fractured bone healing and bone diseases. Some studies reported total flavonoids from plant can be used as an auxiliary source of exogenous.Use different methods to identify and verify effects of total flavonoids from Arachniodes exilis (TFAE) on human umbilical cord mesenchymal stem cells (HUCMSCs) in vitro.Concentrations of 1 and 5 μg/mL TFAE significantly increased ALPase activity in HUCMSCs compared to the other concentrations at days 3 and 7 (P < .05). RT-PCR showed that expression levels of osteogenic genes (Col1a1, OPN, Runx2 and Osx) were remarkably enhanced in HUCMSCs following treatment with different concentrations of TFAE for 9 days compared with 0 μg/mL TFAE group (control). The results showed that concentration < 5 μg/mL of TFAE induced osteogenic differentiation in HUCMSCs Alizarin red staining assays revealed that both TFAE and S1191 was significantly decreased (7.80 ± 0.66) compared with the TFAE group (16.00 ± 0.97) (P < .01). ALPase activity on days 3 and 7 was relatively lower in HUCMSCs grown in media supplemented with both S1191 and TFAE than that of in TFAE group only. The results indicated that osteogenic markers (Col1a1, OPN, Runx2 and Osx) were significantly downregulated in the TFAE + S1191 group in comparison to the control group. The expressions of Col1a and OPN in the TFAE + S1191 group decreased significantly (P < .01) by Western blotting.TFAE promotes the odonto/osteogenic differentiation of human UCMSCs via activation of ER.

Tuning Transcription Factor Availability through Acetylation-Mediated Genomic Redistribution

It is widely assumed that decreasing transcription factor DNA-binding affinity reduces transcription initiation by diminishing occupancy of sequence-specific regulatory elements. However, in vivo transcription factors find their binding sites while confronted with a large excess of low-affinity degenerate motifs. Here, using the melanoma lineage survival oncogene MITF as a model, we show that low-affinity binding sites act as a competitive reservoir in vivo from which transcription factors are released by mitogen-activated protein kinase (MAPK)-stimulated acetylation to promote increased occupancy of their regulatory elements. Consequently, a low-DNA-binding-affinity acetylation-mimetic MITF mutation supports melanocyte development and drives tumorigenesis, whereas a high-affinity non-acetylatable mutant does not. The results reveal a paradoxical acetylation-mediated molecular clutch that tunes transcription factor availability via genome-wide redistribution and couples BRAF to tumorigenesis. Our results further suggest that p300/CREB-binding protein-mediated transcription factor acetylation may represent a common mechanism to control transcription factor availability.

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Reducing transcription factor DNA-binding affinity increases activity in vivo

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Acetylation is triggered by MAPK signaling

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Acetylation leads to genome-wide transcription factor redistribution

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Acetylation of MITF drives tumorigenesis and melanocyte development

The ERβ5 splice variant increases oestrogen responsiveness of ERαpos Ishikawa cells

Endometrial cancer is a common gynaeological malignancy: life time exposure to oestrogen is a key risk factor. Oestrogen action is mediated by receptors encoded by ESR1 (ERα) and ESR2 (ERβ): ERα plays a key role in regulating endometrial cell proliferation. A truncated splice variant isoform (ERβ5) encoded by ESR2 is highly expressed in cancers. This study explored whether ERβ5 alters oestrogen responsiveness of endometrial epithelial cells. Immunhistochemistry profiling of human endometrial cancer tissue biopsies identified epithelial cells co-expressing ERβ5 and ERα in stage I endometrial adenocarcinomas and post menopausal endometrium. Induced co-expression of ERβ5 in ERαpos endometrial cancer cells (Ishikawa) significantly increased ligand-dependent activation of an ERE-luciferase reporter stimulated by either E2 or the ERα-selective agonist 1,3,5-(4-hydroxyphenyl)-4-propyl-1H-pyrazole (PPT) compared to untransfected cells. Fluorescence recovery after photobleaching (FRAP) analysis of tagged yellow fluorescent protein (YFP)-ERβ5 transfected into Ishikawa cells revealed that incubation with E2 induced a transient reduction in intra-nuclear mobility characterised by punctate protein redistribution which phenocopied the behaviour of ERα following ligand activation with E2. In ERαneg MDA-MD-231 breast cancer cells, there was no E2-dependent change in mobility of YFP-ERβ5 and no activation of the ERE reporter in cells expressing ERβ5. In conclusion, we demonstrate that ERβ5 can act as heterodimeric partner to ERα in Ishikawa cells and increases their sensitivity to E2. We speculate that expression of ERβ5 in endometrial epithelial cells may increase the risk of malignant transformation and suggest that immunostaining for ERβ5 should be included in diagnostic assessment of women with early grade cancers.

Targeting Pin1 by All-Trans Retinoic Acid (ATRA) Overcomes Tamoxifen Resistance in Breast Cancer via Multifactorial Mechanisms

Breast cancer is the most prevalent tumor in women worldwide and about 70% patients are estrogen receptor positive. In these cancer patients, resistance to the anticancer estrogen receptor antagonist tamoxifen emerges to be a major clinical obstacle. Peptidyl-prolyl isomerase Pin1 is prominently overexpressed in breast cancer and involves in tamoxifen-resistance. Here, we explore the mechanism and effect of targeting Pin1 using its chemical inhibitor all-trans retinoic acid (ATRA) in the treatment of tamoxifen-resistant breast cancer. We found that Pin1 was up-regulated in tamoxifen-resistant human breast cancer cell lines and tumor tissues from relapsed patients. Pin1 overexpression increased the phosphorylation of ERα on S118 and stabilized ERα protein. ATRA treatment, resembling the effect of Pin1 knockdown, promoted ERα degradation in tamoxifen-resistant cells. Moreover, ATRA or Pin1 knockdown decreased the activation of ERK1/2 and AKT pathways. ATRA also reduced the nuclear expression and transcriptional activity of ERα. Importantly, ATRA inhibited cell viability and proliferation of tamoxifen-resistant human breast cancer cells in vitro. Slow-releasing ATRA tablets reduced the growth of tamoxifen-resistant human breast cancer xenografts in vivo. In conclusion, ATRA-induced Pin1 ablation inhibits tamoxifen-resistant breast cancer growth by suppressing multifactorial mechanisms of tamoxifen resistance simultaneously, which demonstrates an attractive strategy for treating aggressive and endocrine-resistant tumors.

Mediator subunit MED1 modulates intranuclear dynamics of the thyroid hormone receptor

The thyroid hormone receptors (TRs) mediate thyroid hormone (T₃ )-dependent gene expression. The nuclear import and export signals that direct TR shuttling are well characterized, but little is known about factors modulating nuclear retention. We used fluorescence-based nucleocytoplasmic scoring and fluorescence recovery after photobleaching in transfected cells to investigate whether Mediator subunits MED1 and MED13 play a role in nuclear retention of TR. When MED1 was overexpressed, there was a striking shift towards a greater nuclear localization of TRβ1 and the oncoprotein v-ErbA, subtypes with cytosolic populations at steady-state, and TRβ1 intranuclear mobility was reduced. For TRα1, there was no observable change in its predominantly nuclear distribution pattern or mobility. Consistent with a role for MED1 in nuclear retention, the cytosolic TRα1 and TRβ1 population were significantly greater in MED1^-/- cells, compared with MED1^+/+ cells. Exposure to T₃ and epidermal growth factor, which induces MED1 phosphorylation, also altered TR intranuclear dynamics. Overexpression of miR-208a, which downregulates MED13, led to a more cytosolic distribution of nuclear-localized TRα1; however, overexpression of MED13 had no effect on TRβ1 localization. The known binding site of MED1 overlaps with a transactivation domain and nuclear export signal in helix 12 of TR's ligand-binding domain (LBD). Coimmunoprecipitation assays demonstrated that TR's LBD interacts directly with exportins 5 and 7, suggesting that binding of exportins and MED1 to TR may be mutually exclusive. Collectively, our data provide evidence that MED1 promotes nuclear retention of TR, and highlight the dual functionality of helix 12 in TR transactivation and nuclear export.

Therapeutic Ligands Antagonize Estrogen Receptor Function by Impairing Its Mobility

Estrogen receptor-positive (ER⁺) breast cancers frequently remain dependent on ER signaling even after acquiring resistance to endocrine agents, prompting the development of optimized ER antagonists. Fulvestrant is unique among approved ER therapeutics due to its capacity for full ER antagonism, thought to be achieved through ER degradation. The clinical potential of fulvestrant is limited by poor physicochemical features, spurring attempts to generate ER degraders with improved drug-like properties. We show that optimization of ER degradation does not guarantee full ER antagonism in breast cancer cells; ER "degraders" exhibit a spectrum of transcriptional activities and anti-proliferative potential. Mechanistically, we find that fulvestrant-like antagonists suppress ER transcriptional activity not by ER elimination, but by markedly slowing the intra-nuclear mobility of ER. Increased ER turnover occurs as a consequence of ER immobilization. These findings provide proof-of-concept that small molecule perturbation of transcription factor mobility may enable therapeutic targeting of this challenging target class.

Measuring Mobility in Chromatin by Intensity-Sorted FCS

The architectural organization of chromatin can play an important role in genome regulation by affecting the mobility of molecules within its surroundings via binding interactions and molecular crowding. The diffusion of molecules at specific locations in the nucleus can be studied by fluorescence correlation spectroscopy (FCS), a well-established technique based on the analysis of fluorescence intensity fluctuations detected in a confocal observation volume. However, detecting subtle variations of mobility between different chromatin regions remains challenging with currently available FCS methods. Here, we introduce a method that samples multiple positions by slowly scanning the FCS observation volume across the nucleus. Analyzing the data in short time segments, we preserve the high temporal resolution of single-point FCS while probing different nuclear regions in the same cell. Using the intensity level of the probe (or a DNA marker) as a reference, we efficiently sort the FCS segments into different populations and obtain average correlation functions that are associated to different chromatin regions. This sorting and averaging strategy renders the method statistically robust while preserving the observation of intranuclear variations of mobility. Using this approach, we quantified diffusion of monomeric GFP in high versus low chromatin density regions. We found that GFP mobility was reduced in heterochromatin, especially within perinucleolar heterochromatin. Moreover, we found that modulation of chromatin compaction by ATP depletion, or treatment with solutions of different osmolarity, differentially affected the ratio of diffusion in both regions. Then, we used the approach to probe the mobility of estrogen receptor-α in the vicinity of an integrated multicopy prolactin gene array. Finally, we discussed the coupling of this method with stimulated emission depletion FCS for performing FCS at subdiffraction spatial scales.

Repetitive switching between DNA binding modes enables target finding by the glucocorticoid receptor

Transcription factor mobility is a determining factor in the regulation of gene expression. Here, we have studied the intranuclear dynamics of the glucocorticoid receptor (GR) using fluorescence recovery after photobleaching and single-molecule microscopy. First we have described the dynamic states in which the GR occurs. Subsequently we have analyzed the transitions between these states using a continuous time Markov chain model, and functionally investigated these states by making specific mutations in the DNA-binding domain. This analysis revealed that the GR diffuses freely through the nucleus, and once it leaves this free diffusion state it most often enters a repetitive switching mode. In this mode it alternates between slow diffusion as a result of brief nonspecific DNA binding events, and a state of stable binding to specific DNA target sites. This repetitive switching mechanism results in a compact searching strategy which facilitates finding DNA target sites by the GR.

Fbxo22-mediated KDM4B degradation determines selective estrogen receptor modulator activity in breast cancer

The agonistic/antagonistic biocharacter of selective estrogen receptor modulators (SERMs) can have therapeutic advantages, particularly in the case of premenopausal breast cancers. Although the contradictory effects of these modulators have been studied in terms of crosstalk between the estrogen receptor α (ER) and coactivator dynamics and growth factor signaling, the molecular basis of these mechanisms is still obscure. We identify a series of regulatory mechanisms controlling cofactor dynamics on ER and SERM function, whose activities require F-box protein 22 (Fbxo22). Skp1, Cullin1, F-box-containing complex (SCFFbxo22) ubiquitylated lysine demethylase 4B (KDM4B) complexed with tamoxifen-bound (TAM-bound) ER, whose degradation released steroid receptor coactivator (SRC) from ER. Depletion of Fbxo22 resulted in ER-dependent transcriptional activation via transactivation function 1 (AF1) function, even in the presence of SERMs. In living cells, TAM released SRC and KDM4B from ER in a Fbxo22-dependent manner. SRC release by TAM required Fbxo22 on almost all ER-SRC-bound enhancers and promoters. TAM failed to prevent the growth of Fbxo22-depleted, ER-positive breast cancers both in vitro and in vivo. Clinically, a low level of Fbxo22 in tumor tissues predicted a poorer outcome in ER-positive/human epidermal growth factor receptor type 2-negative (HER2-negative) breast cancers with high hazard ratios, independently of other markers such as Ki-67 and node status. We propose that the level of Fbxo22 in tumor tissues defines a new subclass of ER-positive breast cancers for which SCFFbxo22-mediated KDM4B degradation in patients can be a therapeutic target for the next generation of SERMs.

Role of SUMOylation in differential ERα transcriptional repression by tamoxifen and fulvestrant in breast cancer cells

Antiestrogens (AEs) are widely used for treatment of estrogen receptor alpha (ERα)-positive breast cancer, but display variable degrees of partial agonism in estrogen target tissues and breast cancer (BC) cells. The fact that BC cells resistant to selective ER modulators (SERMs) like tamoxifen (Tam) can still be sensitive to pure AEs, also called selective ER downregulators, suggests different mechanisms of action, some of which may contribute to the more complete suppression of estrogen target genes by pure AEs. We report herein that pure AEs such as fulvestrant induce transient binding of ERα to DNA, followed by rapid release after 30–40 min without loss of nuclear localization. Loss of DNA binding preceded receptor degradation and was not prevented by proteasome inhibition. Chromatin was less accessible in the presence of fulvestrant than with estradiol or Tam as early as 20 min following treatment, suggesting that chromatin remodeling by pure AEs at ERα target regions prevents transcription in spite of receptor binding. SUMO2/3 marks were detected on chromatin at the peak of ERα binding in cells treated with pure AEs, but not SERMs. Furthermore, decreasing SUMOylation by overexpressing the deSUMOylase SENP1 significantly delayed receptor release from DNA and de-repressed expression of estrogen target genes in the presence of fulvestrant, both in ERα-expressing MCF-7 cells and in transiently transfected ER-negative SK-BR-3 cells. Finally, mutation V534E, identified in a breast metastasis resistant to hormonal therapies, prevented ERα modification and resulted in increased transcriptional activity of estrogen target genes in the presence of fulvestrant in SK-BR-3 cells. Together, our results establish a role for SUMOylation in achieving a more complete transcriptional shut-off of estrogen target genes by pure AEs vs. SERMs in BC cells.

Intranuclear Mobility of Estrogen Receptor: Implication for Transcriptional Regulation

The estrogen receptor (ER) is a ligand-dependent transcription factor that has two subtypes: ERα and ERβ. ERs regulate transcription of estrogen-responsive genes through interactions with multiple intranuclear components, such as cofactors and the nuclear matrix. Live cell imaging using fluorescent protein-labeled ERs has revealed that ligand-activated ERs are highly mobile in the nucleus, with transient association with the DNA and nuclear matrix. Scaffold attachment factor B (SAFB) 1 and its paralogue, SAFB2, are nuclear matrix-binding proteins that negatively modulate ERα-mediated transcription. Expression of SAFB1 and SAFB2 reduces the mobility of ERα in the presence of ligand. This regulatory machinery is emerging as an epigenetic-like mechanism that alters transcriptional activity through control of intranuclear molecular mobility.

Abiraterone shows alternate activity in models of endocrine resistant and sensitive disease

BACKGROUND

Resistance to endocrine therapy remains a major clinical problem in the treatment of oestrogen-receptor positive (ER+) breast cancer. Studies show androgen-receptor (AR) remains present in 80-90% of metastatic breast cancers providing support for blockade of AR-signalling. However, clinical studies with abiraterone, which blocks cytochrome P450 17A1 (CYP17A1) showed limited benefit.

METHODS

In order to address this, we assessed the impact of abiraterone on cell-viability, cell-death, ER-mediated transactivation and recruitment to target promoters. together with ligand-binding assays in a panel of ER+ breast cancer cell lines that were either oestrogen-dependent, modelling endocrine-sensitive disease, or oestrogen-independent modelling relapse on an aromatase inhibitor. The latter, harboured wild-type (wt) or naturally occurring ESR1 mutations.

RESULTS

Similar to oestrogen, abiraterone showed paradoxical impact on proliferation by stimulating cell growth or death, depending on whether the cells are hormone-dependent or have undergone prolonged oestrogen-deprivation, respectively. Abiraterone increased ER-turnover, induced ER-mediated transactivation and ER-degradation via the proteasome.

CONCLUSIONS

Our study confirms the oestrogenic activity of abiraterone and highlights its differential impact on cells dependent on oestrogen for their proliferation vs. those that are ligand-independent and harbour wt or mutant ESR1. These properties could impact the clinical efficacy of abiraterone in breast cancer.

Chromatin reprogramming in breast cancer

Reprogramming of the chromatin landscape is a critical component to the transcriptional response in breast cancer. Effects of sex hormones such as estrogens and progesterone have been well described to have a critical impact on breast cancer proliferation. However, the complex network of the chromatin landscape, enhancer regions and mode of function of steroid receptors (SRs) and other transcription factors (TFs), is an intricate web of signaling and functional processes that is still largely misunderstood at the mechanistic level. In this review, we describe what is currently known about the dynamic interplay between TFs with chromatin and the reprogramming of enhancer elements. Emphasis has been placed on characterizing the different modes of action of TFs in regulating enhancer activity, specifically, how different SRs target enhancer regions to reprogram chromatin in breast cancer cells. In addition, we discuss current techniques employed to study enhancer function at a genome-wide level. Further, we have noted recent advances in live cell imaging technology. These single-cell approaches enable the coupling of population-based assays with real-time studies to address many unsolved questions about SRs and chromatin dynamics in breast cancer.

Proteomic profiling identifies key coactivators utilized by mutant ERα proteins as potential new therapeutic targets

Approximately 75% of breast cancers are estrogen receptor alpha (ERα)-positive and are treatable with endocrine therapies, but often patients develop lethal resistant disease. Frequent mutations (10–40%) in the ligand-binding domain (LBD) codons in the gene encoding ERα (ESR1) have been identified, resulting in ligand-independent, constitutively active receptors. In addition, ESR1 chromosomal translocations can occur, resulting in fusion proteins that lack the LBD and are entirely unresponsive to all endocrine treatments. Thus, identifying coactivators that bind to these mutant ERα proteins may offer new therapeutic targets for endocrine-resistant cancer. To define coactivator candidate targets, a proteomics approach was performed profiling proteins recruited to the two most common ERα LBD mutants, Y537S and D538G, and an ESR1-YAP1 fusion protein. These mutants displayed enhanced coactivator interactions as compared to unliganded wild-type ERα. Inhibition of these coactivators decreased the ability of ESR1 mutants to activate transcription and promote breast cancer growth in vitro and in vivo. Thus, we have identified specific coactivators that may be useful as targets for endocrine-resistant breast cancers.

Visualizing transcription factor dynamics in living cells

The assembly of sequence-specific enhancer-binding transcription factors (TFs) at cis-regulatory elements in the genome has long been regarded as the fundamental mechanism driving cell type-specific gene expression. However, despite extensive biochemical, genetic, and genomic studies in the past three decades, our understanding of molecular mechanisms underlying enhancer-mediated gene regulation remains incomplete. Recent advances in imaging technologies now enable direct visualization of TF-driven regulatory events and transcriptional activities at the single-cell, single-molecule level. The ability to observe the remarkably dynamic behavior of individual TFs in live cells at high spatiotemporal resolution has begun to provide novel mechanistic insights and promises new advances in deciphering causal-functional relationships of TF targeting, genome organization, and gene activation. In this review, we review current transcription imaging techniques and summarize converging results from various lines of research that may instigate a revision of models to describe key features of eukaryotic gene regulation.

TRH receptor mobility in the plasma membrane is strongly affected by agonist binding and by interaction with some cognate signaling proteins

OBJECTIVES

Extensive research has been dedicated to elucidating the mechanisms of signal transduction through different G protein-coupled receptors (GPCRs). However, relatively little is known about the regulation of receptor movement within the cell membrane upon ligand binding. In this study we focused our attention on the thyrotropin-releasing hormone (TRH) receptor that typically couples to G_q/11 proteins.

METHODS

We monitored receptor diffusion in the plasma membrane of HEK293 cells stably expressing yellow fluorescent protein (YFP)-tagged TRH receptor (TRHR-YFP) by fluorescence recovery after photobleaching (FRAP).

RESULTS

FRAP analysis indicated that the lateral movement of the TRH receptor was markedly reduced upon TRH binding as the value of its diffusion coefficient fell down by 55%. This effect was prevented by the addition of the TRH receptor antagonist midazolam. We also found that siRNA-mediated knockdown of G_q/11α, Gβ, β-arrestin2 and phospholipase Cβ1, but not of G_iα1, β-arrestin1 or G protein-coupled receptor kinase 2, resulted in a significant decrease in the rate of TRHR-YFP diffusion, indicating the involvement of the former proteins in the regulation of TRH receptor behavior. The observed partial reduction of the TRHR-YFP mobile fraction caused by down-regulation of G_iα1 and β-arrestin1 suggests that these proteins may also play distinct roles in THR receptor-mediated signaling.

CONCLUSION

These results demonstrate for the first time that not only agonist binding but also abundance of some signaling proteins may strongly affect TRH receptor dynamics in the plasma membrane.

Characterizing properties of non-estrogenic substituted bisphenol analogs using high throughput microscopy and image analysis

Animal studies have linked the estrogenic properties of bisphenol A (BPA) to adverse effects on the endocrine system. Because of concerns for similar effects in humans, there is a desire to replace BPA in consumer products, and a search for BPA replacements that lack endocrine-disrupting bioactivity is ongoing. We used multiple cell-based models, including an established multi-parametric, high throughput microscopy-based platform that incorporates engineered HeLa cell lines with visible ERα- or ERβ-regulated transcription loci, to discriminate the estrogen-like and androgen-like properties of previously uncharacterized substituted bisphenol derivatives and hydroquinone. As expected, BPA induced 70–80% of the estrogen-like activity via ERα and ERβ compared to E2 in the HeLa prolactin array cell line. 2,2’ BPA, Bisguaiacol F, CHDM 4-hydroxybuyl acrylate, hydroquinone, and TM modified variants of BPF showed very limited estrogen-like or androgen-like activity (< 10% of that observed with the control compounds). Interestingly, TM-BFP and CHDM 4-hydroxybuyl acrylate, but not their derivatives, demonstrated evidence of anti-estrogenic and anti-androgenic activity. Our findings indicate that Bisguaiacol F, TM-BFP-ER and TM-BPF-DGE demonstrate low potential for affecting estrogenic or androgenic endocrine activity. This suggest that the tested compounds could be suitable commercially viable alternatives to BPA.

Phosphorylation of Human Retinoid X Receptor α at Serine 260 Impairs Its Subcellular Localization, Receptor Interaction, Nuclear Mobility, and 1α,25-Dihydroxyvitamin D3-dependent DNA Binding in Ras-transformed Keratinocytes

Human retinoid X receptor α (hRXRα) plays a critical role in DNA binding and transcriptional activity through heterodimeric association with several members of the nuclear receptor superfamily, including the human vitamin D receptor (hVDR). We previously showed that hRXRα phosphorylation at serine 260 through the Ras-Raf-MAPK ERK1/2 activation is responsible for resistance to the growth inhibitory effects of 1α,25-dihydroxyvitamin D3 (1α,25(OH)2D3), the biologically active metabolite of vitamin D3 To further investigate the mechanism of this resistance, we studied intranuclear dynamics of hVDR and hRXRα-tagged constructs in living cells together with endogenous and tagged protein in fixed cells. We find that hVDR-, hRXRα-, and hVDR-hRXRα complex accumulate in the nucleus in 1α,25(OH)2D3-treated HPK1A cells but to a lesser extent in HPK1ARas-treated cells. Also, by using fluorescence resonance energy transfer (FRET), we demonstrate increased interaction of the hVDR-hRXRα complex in 1α,25(OH)2D3-treated HPK1A but not HPK1ARas cells. In HPK1ARas cells, 1α,25(OH)2D3-induced nuclear localization and interaction of hRXRα are restored when cells are treated with the MEK1/2 inhibitor UO126 or following transfection of the non-phosphorylatable hRXRα Ala-260 mutant. Finally, we demonstrate using fluorescence loss in photobleaching and quantitative co-localization with chromatin that RXR immobilization and co-localization with chromatin are significantly increased in 1α,25(OH)2D3-treated HPK1ARas cells transfected with the non-phosphorylatable hRXRα Ala-260 mutant. This suggests that hRXRα phosphorylation significantly disrupts its nuclear localization, interaction with VDR, intra-nuclear trafficking, and binding to chromatin of the hVDR-hRXR complex.

Molecular mechanism of estrogen-estrogen receptor signaling

17β‐Estradiol (E2), as the main circulating estrogen hormone, regulates many tissue and organ functions in physiology. The effects of E2 on cells are mediated by the transcription factors and estrogen receptor (ER)α and ERβ that are encoded by distinct genes. Localized at the peri‐membrane, mitochondria, and the nucleus of cells that are dependent on estrogen target tissues, the ERs share similar, as well as distinct, regulatory potentials. Different intracellular localizations of the ERs result in dynamically integrated and finely tuned E2 signaling cascades that orchestrate cellular growth, differentiation, and death. The deregulation of E2–ER signaling plays a critical role in the initiation and progression of target tissue malignancies. A better understanding of the complex regulatory mechanisms that underlie ER actions in response to E2 therefore holds a critical trajectory for the development of novel prognostic and therapeutic approaches with substantial impacts on the systemic management of target tissue diseases.

Clomiphene citrate down-regulates estrogen receptor-α through the ubiquitin-proteasome pathway in a human endometrial cancer cell line

We examined how clomiphene citrate (CC) reduces estrogen receptor-α (ERα) in a human endometrial cancer cell line. Ishikawa human endometrial cancer cells were treated with ERα ligands such as 17β-estradiol (E2), CC, and the pure antiestrogen, ICI 182,780 (ICI). Thereafter, the expression levels of ERα protein and mRNA were analyzed by western blot and real-time quantitative PCR, respectively, and those of ubiquitinated ERα were analyzed by immunoprecipitation of ERα followed by immunoblotting with an anti-ubiquitin antibody. The expression levels of ERα protein after treatment with E2, CC, and ICI were significantly decreased compared to pre-treatment levels without a corresponding decrease in ERα mRNA. These ligands significantly increased the levels of ubiquitinated ERα compared to vehicle treatment. Co-treatment with the proteasome inhibitor, MG132, abrogated the decrease in ERα levels caused by treatment with the ligands only. We demonstrated, for the first time, a CC-induced decrease in ERα mediated by the ubiquitin-proteasome pathway in human endometrial cancer cells.

Steroid Receptors Reprogram FoxA1 Occupancy through Dynamic Chromatin Transitions

The estrogen receptor (ER), glucocorticoid receptor (GR), and forkhead box protein 1 (FoxA1) are significant factors in breast cancer progression. FoxA1 has been implicated in establishing ER-binding patterns though its unique ability to serve as a pioneer factor. However, the molecular interplay between ER, GR, and FoxA1 requires further investigation. Here we show that ER and GR both have the ability to alter the genomic distribution of the FoxA1 pioneer factor. Single-molecule tracking experiments in live cells reveal a highly dynamic interaction of FoxA1 with chromatin in vivo. Furthermore, the FoxA1 factor is not associated with detectable footprints at its binding sites throughout the genome. These findings support a model wherein interactions between transcription factors and pioneer factors are highly dynamic. Moreover, at a subset of genomic sites, the role of pioneer can be reversed, with the steroid receptors serving to enhance binding of FoxA1.

A Genome-Wide Perspective on Metabolism

Mammals have at least 210 histologically diverse cell types (Alberts, Molecular biology of the cell. Garland Science, New York, 2008) and the number would be even higher if functional differences are taken into account. The genome in each of these cell types is differentially programmed to express the specific set of genes needed to fulfill the phenotypical requirements of the cell. Furthermore, in each of these cell types, the gene program can be differentially modulated by exposure to external signals such as hormones or nutrients. The basis for the distinct gene programs relies on cell type-selective activation of transcriptional enhancers, which in turn are particularly sensitive to modulation. Until recently we had only fragmented insight into the regulation of a few of these enhancers; however, the recent advances in high-throughput sequencing technologies have enabled the development of a large number of technologies that can be used to obtain genome-wide insight into how genomes are reprogrammed during development and in response to specific external signals. By applying such technologies, we have begun to reveal the cross-talk between metabolism and the genome, i.e., how genomes are reprogrammed in response to metabolites, and how the regulation of metabolic networks is coordinated at the genomic level.

Treating gynecologic malignancies with selective estrogen receptor downregulators (SERDs): promise and challenges

Endometrial and ovarian cancers are estrogen-dependent gynecologic malignancies. Although many are estrogen receptor (ER) positive, treatment with the selective estrogen receptor modulator (SERM) tamoxifen, a tissue selective partial-agonist, has demonstrated only modest clinical benefit. Selective estrogen receptor downregulators (SERDs) are pure ER antagonists showing a benefit for advanced ER positive breast cancer, which has bolstered their potential use for ER positive gynecologic malignancies. We summarize these preclinical and clinical data, suggesting that a subpopulation of patients with endometrial or ovarian cancer exists in which treatment with SERDs results in improved outcome. However, the full potential of SERDs for a gynecologic malignancies will be realized only when the appropriate predictive biomarkers are identified. Additionally, a further understanding ER signaling in the context of ovarian and endometrial tissues that appear to involve c-Src and other kinase pathways is needed to successfully address the emergence of resistance with rationally designed combination therapies.

The histone variant H2A.Z is an important regulator of enhancer activity

Gene regulatory programs in different cell types are largely defined through cell-specific enhancers activity. The histone variant H2A.Z has been shown to play important roles in transcription mainly by controlling proximal promoters, but its effect on enhancer functions remains unclear. Here, we demonstrate by genome-wide approaches that H2A.Z is present at a subset of active enhancers bound by the estrogen receptor alpha (ERα). We also determine that H2A.Z does not influence the local nucleosome positioning around ERα enhancers using ChIP sequencing at nucleosomal resolution and unsupervised pattern discovery. We further highlight that H2A.Z-enriched enhancers are associated with chromatin accessibility, H3K122ac enrichment and hypomethylated DNA. Moreover, upon estrogen stimulation, the enhancers occupied by H2A.Z produce enhancer RNAs (eRNAs), and recruit RNA polymerase II as well as RAD21, a member of the cohesin complex involved in chromatin interactions between enhancers and promoters. Importantly, their recruitment and eRNAs production are abolished by H2A.Z depletion, thereby revealing a novel functional link between H2A.Z occupancy and enhancer activity. Taken together, our findings suggest that H2A.Z acts as an important player for enhancer functions by establishing and maintaining a chromatin environment required for RNA polymerase II recruitment, eRNAs transcription and enhancer-promoters interactions, all essential attributes of enhancer activity.

The estrogen receptor alpha nuclear localization sequence is critical for fulvestrant-induced degradation of the receptor

Fulvestrant, a selective estrogen receptor down-regulator (SERD) is a pure competitive antagonist of estrogen receptor alpha (ERα). Fulvestrant binds ERα and reduces the receptor's half-life by increasing protein turnover, however, its mechanism of action is not fully understood. In this study, we show that removal of the ERα nuclear localization sequence (ERΔNLS) resulted in a predominantly cytoplasmic ERα that was degraded in response to 17-β-estradiol (E2) but was resistant to degradation by fulvestrant. ERΔNLS bound the ligands and exhibited receptor interaction similar to ERα, indicating that the lack of degradation was not due to disruption of these processes. Forcing ERΔNLS into the nucleus with a heterologous SV40-NLS did not restore degradation, suggesting that the NLS domain itself, and not merely receptor localization, is critical for fulvestrant-induced ERα degradation. Indeed, cloning of the endogenous ERα NLS onto the N-terminus of ERΔNLS significantly restored both its nuclear localization and turnover in response to fulvestrant. Moreover, mutation of the sumoylation targets K266 and K268 within the NLS impaired fulvestrant-induced ERα degradation. In conclusion, our study provides evidence for the unique role of the ERα NLS in fulvestrant-induced degradation of the receptor.