Phosphorylation of thyroid hormone receptor-associated nuclear receptor corepressor holocomplex by the DNA-dependent protein kinase enhances its histone deacetylase activity

Breast cancer plasticity is restricted by a LATS1-NCOR1 repressive axis

Breast cancer, the most frequent cancer in women, is generally classified into several distinct histological and molecular subtypes. However, single-cell technologies have revealed remarkable cellular and functional heterogeneity across subtypes and even within individual breast tumors. Much of this heterogeneity is attributable to dynamic alterations in the epigenetic landscape of the cancer cells, which promote phenotypic plasticity. Such plasticity, including transition from luminal to basal-like cell identity, can promote disease aggressiveness. We now report that the tumor suppressor LATS1, whose expression is often downregulated in human breast cancer, helps maintain luminal breast cancer cell identity by reducing the chromatin accessibility of genes that are characteristic of a "basal-like" state, preventing their spurious activation. This is achieved via interaction of LATS1 with the NCOR1 nuclear corepressor and recruitment of HDAC1, driving histone H3K27 deacetylation near NCOR1-repressed "basal-like" genes. Consequently, decreased expression of LATS1 elevates the expression of such genes and facilitates slippage towards a more basal-like phenotypic identity. We propose that by enforcing rigorous silencing of repressed genes, the LATS1-NCOR1 axis maintains luminal cell identity and restricts breast cancer progression.

Nuclear Receptor Coregulators in Hormone-Dependent Cancers

Nuclear receptors (NRs) function collectively as a transcriptional signaling network that mediates gene regulatory actions to either maintain cellular homeostasis in response to hormonal, dietary and other environmental factors, or act as orphan receptors with no known ligand. NR complexes are large and interact with multiple protein partners, collectively termed coregulators. Coregulators are essential for regulating NR activity and can dictate whether a target gene is activated or repressed by a variety of mechanisms including the regulation of chromatin accessibility. Altered expression of coregulators contributes to a variety of hormone-dependent cancers including breast and prostate cancers. Therefore, understanding the mechanisms by which coregulators interact with and modulate the activity of NRs provides opportunities to develop better prognostic and diagnostic approaches, as well as novel therapeutic targets. This review aims to gather and summarize recent studies, techniques and bioinformatics methods used to identify distorted NR coregulator interactions that contribute as cancer drivers in hormone-dependent cancers.

Romidepsin (FK228) fails in counteracting the transformed phenotype of rhabdomyosarcoma cells but efficiently radiosensitizes, in vitro and in vivo, the alveolar phenotype subtype

PURPOSE

Herein we describe the in vitro and in vivo activity of FK228 (Romidepsin), an inhibitor of class I HDACs, in counteracting and radiosensitizing embryonal (ERMS, fusion-negative) and alveolar (ARMS, fusion-positive) rhabdomyosarcoma (RMS).

METHODS

RH30 (ARMS, fusion-positive) and RD (ERMS, fusion-negative) cell lines and human multipotent mesenchymal stromal cells (HMSC) were used. Flow cytometry analysis, RT-qPCR, western blotting and enzymatic assays were performed. Irradiation was delivered by using an x-6 MV photon linear accelerator. FK228 (1.2 mg/kg) in vivo activity, combined or not with radiation therapy (2 Gy), was assessed in murine xenografts.

RESULTS

Compared to HMSC, RMS expressed low levels of class I HDACs. In vitro, FK228, as single agents, reversibly downregulated class I HDACs expression and activity and induced oxidative stress, DNA damage and a concomitant growth arrest associated with PARP-1-mediated transient non-apoptotic cell death. Surviving cells upregulated the expression of cyclin A, B, D1, p27, Myc and activated PI3K/Akt/mTOR and MAPK signaling, known to be differently involved in cancer chemoresistance. Interestingly, while no radiosensitizing effects were detected, in vitro or in vivo, on RD cells, FK228 markedly radiosensitized RH30 cells by impairing antioxidant and DSBs repair pathways in vitro. Further, FK228 when combined with RT in vivo significantly reduced tumor mass in mouse RH30 xenografts.

CONCLUSION

FK228 did not show antitumor activity as a single agent whilst its combination with RT resulted in radiosensitization of fusion-positive RMS cells, thus representing a possible strategy for the treatment of the most aggressive RMS subtype.

Role of the Nuclear Receptor Corepressor 1 (NCOR1) in Atherosclerosis and Associated Immunometabolic Diseases

Atherosclerotic cardiovascular disease is part of chronic immunometabolic disorders such as type 2 diabetes and nonalcoholic fatty liver disease. Their common risk factors comprise hypertension, insulin resistance, visceral obesity, and dyslipidemias, such as hypercholesterolemia and hypertriglyceridemia, which are part of the metabolic syndrome. Immunometabolic diseases include chronic pathologies that are affected by both metabolic and inflammatory triggers and mediators. Important and challenging questions in this context are to reveal how metabolic triggers and their downstream signaling affect inflammatory processes and vice-versa. Along these lines, specific nuclear receptors sense changes in lipid metabolism and in turn induce downstream inflammatory and metabolic processes. The transcriptional activity of these nuclear receptors is regulated by the nuclear receptor corepressors (NCORs), including NCOR1. In this review we describe the function of NCOR1 as a central immunometabolic regulator and focus on its role in atherosclerosis and associated immunometabolic diseases.

Hepatocyte-specific loss of GPS2 in mice reduces non-alcoholic steatohepatitis via activation of PPARα

Obesity triggers the development of non-alcoholic fatty liver disease (NAFLD), which involves alterations of regulatory transcription networks and epigenomes in hepatocytes. Here we demonstrate that G protein pathway suppressor 2 (GPS2), a subunit of the nuclear receptor corepressor (NCOR) and histone deacetylase 3 (HDAC3) complex, has a central role in these alterations and accelerates the progression of NAFLD towards non-alcoholic steatohepatitis (NASH). Hepatocyte-specific Gps2 knockout in mice alleviates the development of diet-induced steatosis and fibrosis and causes activation of lipid catabolic genes. Integrative cistrome, epigenome and transcriptome analysis identifies the lipid-sensing peroxisome proliferator-activated receptor α (PPARα, NR1C1) as a direct GPS2 target. Liver gene expression data from human patients reveal that Gps2 expression positively correlates with a NASH/fibrosis gene signature. Collectively, our data suggest that the GPS2-PPARα partnership in hepatocytes coordinates the progression of NAFLD in mice and in humans and thus might be of therapeutic interest.

Leucine-rich repeat kinase 2 exacerbates neuronal cytotoxicity through phosphorylation of histone deacetylase 3 and histone deacetylation

Parkinson's disease (PD) is characterized by slow, progressive degeneration of dopaminergic neurons in the substantia nigra. The cause of neuronal death in PD is largely unknown, but several genetic loci, including leucine-rich repeat kinase 2 (LRRK2), have been identified. LRRK2 has guanosine triphosphatase (GTPase) and kinase activities, and mutations in LRRK2 are the major cause of autosomal-dominant familial PD. Histone deacetylases (HDACs) remove acetyl groups from lysine residues on histone tails, promoting transcriptional repression via condensation of chromatin. Here, we demonstrate that LRRK2 binds to and directly phosphorylates HDAC3 at Ser-424, thereby stimulating HDAC activity. Specifically, LRRK2 promoted the deacetylation of Lys-5 and Lys-12 on histone H4, causing repression of gene transcription. Moreover, LRRK2 stimulated nuclear translocation of HDAC3 via the phoshorylation of karyopherin subunit α2 and α6. HDAC3 phosphorylation and its nuclear translocation were increased in response to 6-hydroxydopamine (6-OHDA) treatment. LRRK2 also inhibited myocyte-specific enhancer factor 2D activity, which is required for neuronal survival. LRRK2 ultimately promoted 6-OHDA-induced cell death via positive modulation of HDAC3. These findings suggest that LRRK2 affects epigenetic histone modification and neuronal survival by facilitating HDAC3 activity and regulating its localization.

microRNA and thyroid hormone signaling in cardiac and skeletal muscle

Thyroid hormone (TH) signaling plays critical roles in the differentiation, growth, metabolism, and physiological function of all organs or tissues, including heart and skeletal muscle. Due to the significant progress in our understanding of the molecular mechanisms that underlie TH action, it's widely accepted that TH signaling is regulated at multiple levels. A growing number of discoveries suggest that microRNAs (miRNAs) act as fine-tune regulators of gene expression and adds sophisticated regulatory tiers to signaling pathways. Recently, some pioneering studies in cardiac and skeletal muscle demonstrating the interplay between miRNAs and TH signaling suggest that miRNAs might mediate and/or modulate TH signaling. This review presents recent advances involving the crosstalk between miRNAs and TH signaling and current evidence showing the importance of miRNA in TH signaling with particular emphasis on the study of muscle-specific miRNAs (myomiRs) in cardiac and skeletal muscle. Although the research of the reciprocal regulation of miRNAs and TH signaling is only at the beginning stage, it has already contributed to our current understanding of both TH action and miRNA biology. We also encourage further investigations to address the relative contributions of miRNAs in TH signaling under physiological and pathological conditions and how a group of miRNAs are coordinated to integrate into the complex hierarchical regulatory network of TH.

Chromatin association of XRCC5/6 in the absence of DNA damage depends on the XPE gene product DDB2

DDB2 is a multifunctional protein that participates in both nucleotide excision repair and regulation of gene transcription. In colon cancer cells, chromatin association of XRCC5/6, in the absence of DNA damage, depends on DDB2, and the DDB2–XRCC5/6 interaction promotes the transcription of the antiangiogenic gene SEMA3A.

Damaged DNA-binding protein 2 (DDB2), a nuclear protein, participates in both nucleotide excision repair and mRNA transcription. The transcriptional regulatory function of DDB2 is significant in colon cancer, as it regulates metastasis. To characterize the mechanism by which DDB2 participates in transcription, we investigated the protein partners in colon cancer cells. Here we show that DDB2 abundantly associates with XRCC5/6, not involving CUL4 and DNA-PKcs. A DNA-damaging agent that induces DNA double-stranded breaks (DSBs) does not affect the interaction between DDB2 and XRCC5. In addition, DSB-induced nuclear enrichment or chromatin association of XRCC5 does not involve DDB2, suggesting that the DDB2/XRCC5/6 complex represents a distinct pool of XRCC5/6 that is not directly involved in DNA break repair (NHEJ). In the absence of DNA damage, on the other hand, chromatin association of XRCC5 requires DDB2. We show that DDB2 recruits XRCC5 onto the promoter of SEMA3A, a DDB2-stimulated gene. Moreover, depletion of XRCC5 inhibits SEMA3A expression without affecting expression of VEGFA, a repression target of DDB2. Together our results show that DDB2 is critical for chromatin association of XRCC5/6 in the absence of DNA damage and provide evidence that XRCC5/6 are functional partners of DDB2 in its transcriptional stimulatory activity.

DNA-PKcs-Mediated Transcriptional Regulation Drives Prostate Cancer Progression and Metastasis

Emerging evidence demonstrates that the DNA repair kinase DNA-PKcs exerts divergent roles in transcriptional regulation of unsolved consequence. Here, in vitro and in vivo interrogation demonstrate that DNA-PKcs functions as a selective modulator of transcriptional networks that induce cell migration, invasion, and metastasis. Accordingly, suppression of DNA-PKcs inhibits tumor metastases. Clinical assessment revealed that DNA-PKcs is significantly elevated in advanced disease and independently predicts for metastases, recurrence, and reduced overall survival. Further investigation demonstrated that DNA-PKcs in advanced tumors is highly activated, independent of DNA damage indicators. Combined, these findings reveal unexpected DNA-PKcs functions, identify DNA-PKcs as a potent driver of tumor progression and metastases, and nominate DNA-PKcs as a therapeutic target for advanced malignancies.

Histone deacetylase 3 promotes RCAN1 stability and nuclear translocation

Regulator of calcineurin 1 (RCAN1; also referred as DSCR1 or MCIP1) is located in close proximity to a Down syndrome critical region of human chromosome 21. Although RCAN1 is an endogenous inhibitor of calcineurin signaling that controls lymphocyte activation, apoptosis, heart development, skeletal muscle differentiation, and cardiac function, it is not yet clear whether RCAN1 might be involved in other cellular activities. In this study, we explored the extra-functional roles of RCAN1 by searching for novel RCAN1-binding partners. Using a yeast two-hybrid assay, we found that RCAN1 (RCAN1-1S) interacts with histone deacetylase 3 (HDAC3) in mammalian cells. We also demonstrate that HDAC3 deacetylates RCAN1. In addition, HDAC3 increases RCAN1 protein stability by inhibiting its poly-ubiquitination. Furthermore, HDAC3 promotes RCAN1 nuclear translocation. These data suggest that HDAC3, a new binding regulator of RCAN1, affects the protein stability and intracellular localization of RCAN1.

Use of phage display to identify novel mineralocorticoid receptor-interacting proteins

The mineralocorticoid receptor (MR) plays a central role in salt and water homeostasis via the kidney; however, inappropriate activation of the MR in the heart can lead to heart failure. A selective MR modulator that antagonizes MR signaling in the heart but not the kidney would provide the cardiovascular protection of current MR antagonists but allow for normal electrolyte balance. The development of such a pharmaceutical requires an understanding of coregulators and their tissue-selective interactions with the MR, which is currently limited by the small repertoire of MR coregulators described in the literature. To identify potential novel MR coregulators, we used T7 phage display to screen tissue-selective cDNA libraries for MR-interacting proteins. Thirty MR binding peptides were identified, from which three were chosen for further characterization based on their nuclear localization and their interaction with other MR-interacting proteins or, in the case of x-ray repair cross-complementing protein 6, its known status as an androgen receptor coregulator. Eukaryotic elongation factor 1A1, structure-specific recognition protein 1, and x-ray repair cross-complementing protein 6 modulated MR-mediated transcription in a ligand-, cell- and/or promoter-specific manner and colocalized with the MR upon agonist treatment when imaged using immunofluorescence microscopy. These results highlight the utility of phage display for rapid and sensitive screening of MR binding proteins and suggest that eukaryotic elongation factor 1A1, structure-specific recognition protein 1, and x-ray repair cross-complementing protein 6 may be potential MR coactivators whose activity is dependent on the ligand, cellular context, and target gene promoter.

Histone deacetylase 3 regulates cyclin A stability

PCAF and GCN5 acetylate cyclin A at specific lysine residues targeting it for degradation at mitosis. We report here that histone deacetylase 3 (HDAC3) directly interacts with and deacetylates cyclin A. HDAC3 interacts with a domain included in the first 171 aa of cyclin A, a region involved in the regulation of its stability. In cells, overexpression of HDAC3 reduced cyclin A acetylation whereas the knocking down of HDAC3 increased its acetylation. Moreover, reduction of HDAC3 levels induced a decrease of cyclin A that can be reversed by proteasome inhibitors. These results indicate that HDAC3 is able to regulate cyclin A degradation during mitosis via proteasome. Interestingly, HDAC3 is abruptly degraded at mitosis also via proteasome thus facilitating cyclin A acetylation by PCAF/GCN5, which will target cyclin A for degradation. Because cyclin A is crucial for S phase progression and mitosis entry, the knock down of HDAC3 affects cell cycle progression specifically at both, S phase and G2/M transition. In summary we propose here that HDAC3 regulates cyclin A stability by counteracting the action of the acetylases PCAF/GCN5.

Protein kinase A phosphorylates NCoR to enhance its nuclear translocation and repressive function in human prostate cancer cells

Protein kinase A (PKA) phosphorylates diverse protein substrates to modulate their function. In this study, we found that PKA specifically phosphorylates the RD1 (repression domain 1) domain of nuclear receptor corepressor (NCoR). We demonstrated that the Serine-70 of NCoR is identified the critical amino acid for PKA-dependent NCoR phosphorylation. Importantly, we found that PKA-dependent phosphorylation enhances the nuclear translocation of NCoR. More importantly, the activation of PKA enhanced the repressive activity of NCoR in a reporter assay and potentiated the antagonist activity in the androgen receptor (AR)-mediated transcription. Taken together, these results uncover a regulatory mechanism by which PKA positively modulates NCoR function in transcriptional regulation in prostate cancer.

Maternal high-fat diet modulates the fetal thyroid axis and thyroid gene expression in a nonhuman primate model

Thyroid hormone (TH) is an essential regulator of both fetal development and energy homeostasis. Although the association between subclinical hypothyroidism and obesity has been well studied, a causal relationship has yet to be established. Using our well-characterized nonhuman primate model of excess nutrition, we sought to investigate whether maternal high-fat diet (HFD)-induced changes in TH homeostasis may underlie later in life development of metabolic disorders and obesity. Here, we show that in utero exposure to a maternal HFD is associated with alterations of the fetal thyroid axis. At the beginning of the third trimester, fetal free T(4) levels are significantly decreased with HFD exposure compared with those of control diet-exposed offspring. Furthermore, transcription of the deiodinase, iodothyronine (DIO) genes, which help maintain thyroid homeostasis, are significantly (P < 0.05) disrupted in the fetal liver, thyroid, and hypothalamus. Genes involved in TH production are decreased (TRH, TSHR, TG, TPO, and SLC5A5) in hypothalamus and thyroid gland. In experiments designed to investigate the molecular underpinnings of these observations, we observe that the TH nuclear receptors and their downstream regulators are disrupted with maternal HFD exposure. In fetal liver, the expression of TH receptor β (THRB) is increased 1.9-fold (P = 0.012). Thorough analysis of the THRB promoter reveals a maternal diet-induced alteration in the fetal THRB histone code, alongside differential promoter occupancy of corepressors and coactivators. We speculate that maternal HFD exposure in utero may set the stage for later in life obesity through epigenomic modifications to the histone code, which modulates the fetal thyroid axis.

High-mobility group A1 protein: a new coregulator of peroxisome proliferator-activated receptor-γ-mediated transrepression in the vasculature

RATIONALE

The nuclear receptor peroxisome proliferator-activated receptor-γ (PPARγ) is an important regulator of gene transcription in vascular cells and mediates the vascular protection observed with antidiabetic glitazones.

OBJECTIVE

To determine the molecular mechanism of ligand-dependent transrepression in vascular smooth muscle cells and their impact on the vascular protective actions of PPARγ.

METHODS AND RESULTS

Here, we report a molecular pathway in vascular smooth muscle cells by which ligand-activated PPARγ represses transcriptional activation of the matrix-degrading matrix metalloproteinase-9 (MMP-9) gene, a crucial mediator of vascular injury. PPARγ-mediated transrepression of the MMP-9 gene was dependent on the presence of the high-mobility group A1 (HMGA1) protein, a gene highly expressed in vascular smooth muscle cells, newly identified by oligonucleotide array expression analysis. Transrepression of MMP-9 by PPARγ and regulation by HMGA1 required PPARγ SUMOylation at K367. This process was associated with formation of a complex between PPARγ, HMGA1, and the SUMO E2 ligase Ubc9 (ubiquitin-like protein SUMO-1 conjugating enzyme). After PPARγ ligand stimulation, HMGA1 and PPARγ were recruited to the MMP-9 promoter, which facilitated binding of SMRT (silencing mediator of retinoic acid and thyroid hormone receptor), a nuclear corepressor involved in transrepression. The relevance of HMGA1 for vascular PPARγ signaling was underlined by the complete absence of vascular protection through a PPARγ ligand in HMGA1(-/-) mice after arterial wire injury.

CONCLUSIONS

The present data suggest that ligand-dependent formation of HMGA1-Ubc9-PPARγ complexes facilitates PPARγ SUMOylation, which results in the prevention of SMRT corepressor clearance and induction of MMP-9 transrepression. These data provide new information on PPARγ-dependent vascular transcriptional regulation and help us to understand the molecular consequences of therapeutic interventions with PPARγ ligands in the vasculature.

HDAC3 and the molecular brake pad hypothesis

Successful transcription of specific genes required for long-term memory processes involves the orchestrated effort of not only transcription factors, but also very specific enzymatic protein complexes that modify chromatin structure. Chromatin modification has been identified as a pivotal molecular mechanism underlying certain forms of synaptic plasticity and memory. The best-studied form of chromatin modification in the learning and memory field is histone acetylation, which is regulated by histone acetyltransferases and histone deacetylases (HDACs). HDAC inhibitors have been shown to strongly enhance long-term memory processes, and recent work has aimed to identify contributions of individual HDACs. In this review, we focus on HDAC3 and discuss its recently defined role as a negative regulator of long-term memory formation. HDAC3 is part of a corepressor complex and has direct interactions with Class II HDACs that may be important for its molecular and behavioral consequences. And last, we propose the "molecular brake pad" hypothesis of HDAC function. The HDACs and associated corepressor complexes may function in neurons, in part, as "molecular brake pads." HDACs are localized to promoters of active genes and act as a persistent clamp that requires strong activity-dependent signaling to temporarily release these complexes (or brake pads) to activate gene expression required for long-term memory formation. Thus, HDAC inhibition removes the "molecular brake pads" constraining the processes necessary for long-term memory and results in strong, persistent memory formation.

Nuclear import of histone deacetylase 5 by requisite nuclear localization signal phosphorylation

Histone deacetylase 5 (HDAC5), a class IIa deacetylase, is a prominent regulator of cellular and epigenetic processes that underlie the progression of human disease, ranging from cardiac hypertrophy to cancer. Although it is established that phosphorylation mediates 14-3-3 protein binding and provides the essential link between HDAC5 nucleo-cytoplasmic shuttling and transcriptional repression, thus far only four phospho-acceptor sites have been functionally characterized. Here, using a combinatorial proteomics approach and phosphomutant screening, we present the first evidence that HDAC5 has at least 17 in vivo phosphorylation sites within functional domains, including Ser278 and Ser279 within the nuclear localization signal (NLS), Ser1108 within the nuclear export signal, and Ser755 in deacetylase domain. Global and targeted MS/MS analyses of NLS peptides demonstrated the presence of single (Ser278 and Ser279) and double (Ser278/Ser279) phosphorylations. The double S278/279A mutation showed reduced association with HDAC3, slightly decreased deacetylation activity, and significantly increased cytoplasmic localization compared with wild type HDAC5, whereas the S278A and S1108A phosphomutants were not altered. Live cell imaging revealed a deficiency in nuclear import of S278/279A HDAC5. Phosphomutant stable cell lines confirmed the cellular redistribution of NLS mutants and revealed a more pronounced cytoplasmic localization for the single S279A mutant. Proteomic analysis of immunoisolated S278/279A, S279A, and S259/498A mutants linked altered cellular localization to changes in protein interactions. S278/279A and S279A HDAC5 showed reduced association with the NCoR-HDAC3 nuclear corepressor complex as well as protein kinase D enzymes, which were potentiated in the S259/498A mutant. These results provide the first link between phosphorylation outside the known 14-3-3 sites and downstream changes in protein interactions. Together these studies identify Ser279 as a critical phosphorylation within the NLS involved in the nuclear import of HDAC5, providing a regulatory point in nucleo-cytoplasmic shuttling that may be conserved in other class IIa HDACs-HDAC4 and HDAC9.

New insights into the functions and regulation of the transcriptional corepressors SMRT and N-CoR

Corepressors are large proteins that facilitate transcriptional repression through recruitment of histone-modifying enzymes. Two major corepressors, SMRT (silencing mediator for retinoid and thyroid hormone receptors) and N-CoR (nuclear receptor corepressor), have been shown to mediate repression associated with nuclear receptors and a myriad of other transcription factors. This review will focus on recent studies on these proteins, including newly discovered physiological roles of the corepressors, their modes of regulation, their roles in antiestrogen-resistant breast cancer and their functions during the cell cycle.

The molecular regulation of Chinook salmon gonadotropin beta-subunit gene transcription

Expression of the gonadotropin beta-subunit genes is tightly regulated both cell-specifically and by the regulatory hormones to achieve the appropriate gonadotropic hormone levels required for reproductive development and function. Although the cDNA sequences of these genes are highly conserved across species, their promoter sequences are not and few functional studies have been carried out to understand the molecular mechanisms through which their expression is regulated. We and others have carried out several studies on the LHbeta gene promoter of Chinook salmon (Oncorhynchus tschawytscha), and also isolated the FSHbeta gene from the same species. We present here a review of these studies and also novel data pertaining to both genes, in an attempt to collate the current understanding of the molecular regulation of the gonadotropin beta-subunit genes in these fish.

Characterization of ligands for thyroid receptor subtypes and their interactions with co-regulators

Thyroid hormone receptors (TRs) are nuclear receptors that are activated by thyroid hormone ligands and co-regulator proteins. Two receptor subtypes, TRalpha and TRbeta, have been suggested to play a role in numerous physiological functions. However, specificity of receptor subtype function and co-regulator interaction is unclear due to the lack of TR subtype-specific ligands. Five TR ligands were evaluated for their selectivity and interaction with the TR subtypes. A multiplex assay was used to identify co-regulator peptide interaction, and biochemical assays were used to characterize ligand-receptor specificity. In the biochemical assay, rank order ligand potencies were similar in the presence of co-activator peptides, SRC1-2 and SRC3-2, and the co-repressor peptide, NCoR1-2, with T3 and Triac potencies greater in the presence of the co-repressor. The potency of Tetrac was similar regardless of the co-regulator used while T4 and rT3 demonstrated selectivity for TRalpha subtype. The rank order among TR ligands at either receptor subtype in the biochemical assay correlated with the multiplex assay. These assays can be used to identify new ligands that can provide further insight into TR biology.

A dual-acceptor time-resolved Föster resonance energy transfer assay for simultaneous determination of thyroid hormone regulation of corepressor and coactivator binding to the thyroid hormone receptor: Mimicking the cellular context of thyroid hormone action

Previously, we reported the development of two in vitro time-resolved Föster resonance energy transfer (tr-FRET)-based assays for evaluating the potency and efficacy of different ligands of thyroid hormone receptor (TR) for regulating the recruitment of coregulators. We could measure independently, in separate assays, both the recruitment of SRC3 (steroid receptor coactivator 3, a transcriptional coactivator) and the dissociation of NCoR (nuclear receptor corepressor, a transcriptional corepressor) from a TR*retinoid X receptor (RXR) heterodimer bound to a DR+4 thyroid hormone response element (TRE). Here, by using the distinct emission peaks of Tb(3+), the donor fluorophore used to label the TRE-bound TR*RXR heterodimers, and selecting two distinct acceptor fluorophores, fluorescein and cyanine 5, to label of NCoR and SRC3, respectively, we have integrated our previous two assay formats into a single assay. Thus, we can measure the potency of TR ligands simultaneously for NCoR dissociation and SRC3 recruitment activities in a system that mimics many features of the cellular context of TR action. The performance of this dual assay was tested with a known, highly potent physiological TR ligand, triiodothyronine (T(3)), and with a synthetic TR antagonist, NH-3. Measured potencies and efficacies of these two TR ligands from this dual assay are highly comparable to those obtained from the two independent assays. Thus, this dual-acceptor tr-FRET assay further simplifies the measurement of ligand-modulated TR-coregulator interactions and should improve the overall efficiency of the screening process of TR drug discovery programs.

What goes on behind closed doors: physiological versus pharmacological steroid hormone actions

Steroid-hormone-activated receptor proteins are among the best-understood class of factors for altering gene transcription in cells. Steroid receptors are of major importance in maintaining normal human physiology by responding to circulating concentrations of steroid in the nM range. Nonetheless, most studies of steroid receptor action have been conducted using the supra-physiological conditions of saturating concentrations (> or =100 nM) of potent synthetic steroid agonists. Here we summarize the recent developments arising from experiments using two clinically relevant conditions: subsaturating concentrations of agonist (to mimic the circulating concentrations in mammals) and saturating concentrations of antagonists (which are employed in endocrine therapies to block the actions of endogenous steroids). These studies have revealed new facets of steroid hormone action that could not be uncovered by conventional experiments with saturating concentrations of agonist steroids, such as a plethora of factors/conditions for the differential control of gene expression by physiological levels of steroid, a rational approach for examining the gene-specific variations in partial agonist activity of antisteroids, and a dissociation of steroid potency and efficacy that implies the existence of separate, and possibly novel, mechanistic steps and cofactors.

Quantification of ligand-regulated nuclear receptor corepressor and coactivator binding, key interactions determining ligand potency and efficacy for the thyroid hormone receptor

The potency and efficacy of ligands for nuclear receptors (NR) result both from the affinity of the ligand for the receptor and from the affinity that various coregulatory proteins have for ligand-receptor complexes; the latter interaction, however, is rarely quantified. To understand the molecular basis for ligand potency and efficacy, we developed dual time-resolved fluorescence resonance energy transfer (tr-FRET) assays and quantified binding of both ligand and coactivator or corepressor to the thyroid hormone receptor (TR). Promoter-bound TR exerts dual transcriptional regulatory functions, recruiting corepressor proteins and repressing transcription in the absence of thyroid hormones (THs) and shedding corepressors in favor of coactivators upon binding agonists, activating transcription. Our tr-FRET assays involve a TRE sequence labeled with terbium (fluorescence donor), TRbeta.RXRalpha heterodimer, and fluorescein-labeled NR interaction domains of coactivator SRC3 or corepressor NCoR (fluorescence acceptors). Through coregulator titrations, we could determine the affinity of SRC3 or NCoR for TRE-bound TR.RXR heterodimers, unliganded or saturated with different THs. Alternatively, through ligand titrations, we could determine the relative potencies of different THs. The order of TR agonist potencies is as follows: GC-1 approximately T 3 approximately TRIAC approximately T 4 >> rT 3 (for both coactivator recruitment and corepressor dissociation); the affinities of SRC3 binding to TR-ligand complexes followed a similar trend. This highlights the fact that the low activity of rT 3 is derived both from its low affinity for TR and from the low affinity of SRC for the TR-rT 3 complex. The TR antagonist NH-3 failed to induce SRC3 recruitment but did effect NCoR dissociation. These assays provide quantitative information about the affinity of two key interactions that are determinants of NR ligand potency and efficacy.

HDAC3: taking the SMRT-N-CoRrect road to repression

Known histone deacetylases (HDACs) are divided into different classes, and HDAC3 belongs to Class I. Through forming multiprotein complexes with the corepressors SMRT and N-CoR, HDAC3 regulates the transcription of a plethora of genes. A growing list of nonhistone substrates extends the role of HDAC3 beyond transcriptional repression. Here, we review data on the composition, regulation and mechanism of action of the SMRT/N-CoR-HDAC3 complexes and provide several examples of nontranscriptional functions, to illustrate the wide variety of physiological processes affected by this deacetylase. Furthermore, we discuss the implication of HDAC3 in cancer, focusing on leukemia. We conclude with some thoughts about the potential therapeutic efficacies of HDAC3 activity modulation.

New insights into thyroid hormone action

Thyroid hormones (THs) have important effects on cellular development, growth, and metabolism. They bind to thyroid hormone receptors (TRs), TRalpha and TRbeta, which belong to the nuclear hormone receptor superfamily. These receptors also bind to enhancer elements in the promoters of target genes, and can regulate both positive and negative transcription. Recent emerging evidence has characterized some of the molecular mechanisms by which THs regulate transcription as co-repressors, and co-activators have been identified and their effects on histone acetylation examined. THs also have rapid effects that do not require transcription. These can occur via TRs or other cellular proteins, and typically occur outside the nucleus. It appears that THs regulate multiple cellular functions using a diverse array of receptors and signaling systems. TR isoform- or pathway-specific drugs might provide the therapeutic benefits of TH action such as decreasing obesity or lowering cholesterol levels without some of the side effects of hyperthyroidism.