Nuclear Corepressor Research Articles

The microRNA network is altered in anterior cingulate cortex of patients with unipolar and bipolar depression

MicroRNAs (miRNAs) are small, non-coding RNAs acting as post-transcriptional regulators of gene expression. Though implicated in multiple CNS disorders, miRNAs have not been examined in any psychiatric disease state in anterior cingulate cortex (AnCg), a brain region centrally involved in regulating mood. We performed qPCR analyses of 29 miRNAs previously implicated in psychiatric illness (major depressive disorder (MDD), bipolar disorder (BP) and/or schizophrenia (SZ)) in AnCg of patients with MDD and BP versus controls. miR-132, miR-133a and miR-212 were initially identified as differentially expressed in BP, miR-184 in MDD and miR-34a in both MDD and BP (although none survived multiple correction testing and must be considered preliminary). In silico target prediction algorithms identified putative targets of differentially expressed miRNAs. Nuclear Co-Activator 1 (NCOA1), Nuclear Co-Repressor 2 (NCOR2) and Phosphodiesterase 4B (PDE4B) were selected based upon predicted targeting by miR-34a (with NCOR2 and PDE4B both targeted by miR-184) and published relevance to psychiatric illness. Luciferase assays identified PDE4B as a target of miR-34a and miR-184, while NCOA1 and NCOR2 were targeted by miR-34a and 184, respectively. qPCR analyses were performed to determine whether changes in miRNA levels correlated with mRNA levels of validated targets. NCOA1 showed an inverse correlation with miR-34a in BP, while NCOR2 demonstrated a positive correlation. In sum, this is the first study to demonstrate miRNA changes in AnCg in psychiatric illness and validate miR-34a as differentially expressed in CNS in MDD. These findings support a mechanistic role for miRNAs in the regulation of stress-responsive genes disrupted in psychiatric illness.

Abstract 2900: Sumoylation of the retinoic acid receptor alpha protein represses transcriptional activity and contributes to retinoic acid resistance in glioma stem cells

Abstract Introduction: Glioblastoma (GBM) is the most deadly form of brain cancer. Despite treatment with tumor resection followed by radiation and chemotherapy, the median survival rate is less than 15 months. Glioma stem-like cells (GSCs) are thought to be treatment resistant and drive tumor growth. Retinoids have been successfully used to terminally differentiate the cancer stem cell population in some leukemias, but show only modest success in GBM despite the presence of RARA in tumor cells. Resistance to retinoic acid (RA) can develop due to mutations in the retinoic acid receptor alpha (RARA) protein that prevent its phosphorylation. This study evaluated the retinoic acid receptors to determine if receptor dysfunction is the underlying cause of resistance in GBM. Methods: Several GSCs derived from both primary (GSC923) and recurrent (GSC827, GSC604) GBM tumors and normal mouse neural stem cells (MNSC) were analyzed for RARA protein expression by Western blots using an RARA specific antibody. 2D Westerns were used to measure RARA phosphorylation in response to RA treatment. MNSC were treated with RA in the presence or absence of MG132, a proteasomal inhibitor, to determine if MNSC RARA protein was sensitive to proteasomal degradation. RARA transcriptional activity was measured using a promoter luciferase construct containing a specific retinoic acid response element (RARE). Immunoprecipitations using either Sumo1, Sumo 2/3 or RARA identified the type of sumo peptide covalently attached to the endogenous GSC RARA protein. Results: MNSCs express the predicted 51 kDa RARA protein; however, GSC827 and GSC923 both express two high molecular weight forms that were approximately 62 and 73 kDa. In response to RA, the MNSC RARA became phosphorylated, but the GSC827 RARA remained hypophosphorylated. Treatment with RA induced the proteasomal degradation of MNSC RARA as expected. In contrast, both forms of the GSC RARA proteins were resistant to ligand-induced proteasomal degradation. In response to RA, the RARA protein in three glioma stem cell lines showed a 40-60% decrease (p = 0.008) in transcriptional activity compared to MNSC. Immunoprecipitation showed that the Sumo1 peptide is covalently attached to the GSC RARA protein, occurring before the treatment with retinoic acid. Conclusions: We demonstrated that aberrant posttranslational modification of the RARA protein contributes to retinoic acid resistance in glioma stem cells. Covalent attachment of the Sumo1 peptide likely prevents RARA phosphorylation and ligand-induced proteasomal degradation. The Sumo1 peptide serves as a scaffold to bind nuclear corepressor proteins. Future exploration of the role of the Sumo1 modification may uncover underlying mechanisms of inherent retinoic acid resistance in GSCs potentially leading to successful differentiation therapy in GBM. Citation Format: Virginia W. Rodriguez, Rolanda Bailey, Mark Gilbert. Sumoylation of the retinoic acid receptor alpha protein represses transcriptional activity and contributes to retinoic acid resistance in glioma stem cells. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 2900.

Aldose Reductase Acts as a Selective Derepressor of PPARγ and the Retinoic Acid Receptor.

Histone deacetylase 3 (HDAC3), a chromatin-modifying enzyme, requires association with the deacetylase-containing domain (DAD) of the nuclear receptor corepressors NCOR1 and SMRT for its stability and activity. Here, we show that aldose reductase (AR), the rate-limiting enzyme of the polyol pathway, competes with HDAC3 to bind the NCOR1/SMRT DAD. Increased AR expression leads to HDAC3 degradation followed by increased PPARγ signaling, resulting in lipid accumulation in the heart. AR also downregulates expression of nuclear corepressor complex cofactors including Gps2 and Tblr1, thus affecting activity of the nuclear corepressor complex itself. Though AR reduces HDAC3-corepressor complex formation, it specifically derepresses the retinoic acid receptor (RAR), but not other nuclear receptors such as the thyroid receptor (TR) and liver X receptor (LXR). In summary, this work defines a distinct role for AR in lipid and retinoid metabolism through HDAC3 regulation and consequent derepression of PPARγ and RAR.

Oxymatrine Inhibits Renal Tubular EMT Induced by High Glucose via Upregulation of SnoN and Inhibition of TGF-β1/Smad Signaling Pathway.

Transforming growth factor-β1 (TGF-β1) signaling has been shown to play a critical role in the development of diabetic nephropathy (DN). The nuclear transcription co-repressor Ski-related novel protein N (SnoN) is an important negative regulator of TGF-β1/Smad signal transduction, and subsequent biological responses including tubule epithelial-mesenchymal transition (EMT), extracellular matrix accumulation and tubulointerstitial fibrosis. Oxymatrine (OM) is an alkaloid extracted from the Chinese herb Sophora japonica and has been demonstrated to prevent fibrosis. However, the anti-fibrosis effect of OM in DN is still unclear. In this study, we cultured normal rat renal tubular epithelial cells (NRK52Es) in high glucose and high glucose plus OM, and detected the expression of E-cadherin, α-SMA, FN, TGF-β1, SnoN, Arkadia, p-Smad2 and p-Smad3 and poly-ubiquitination of SnoN. The results showed that E-cadherin and SnoN expression in NRK52Es decreased significantly, but poly-ubiquitination of SnoN, TGF-β1, α-SMA, FN, Arkadia, p-Smad2 and p-Smad3 expression significantly increased due to high glucose stimulation, which could be almost completely reversed by OM, suggesting that OM may alleviate EMT induced by high glucose via upregulating SnoN expression and inhibiting TGF-β1/Smad signaling pathway activation. Hence, OM could be a novel therapeutic for DN.

Epigenetic Regulation of the Blimp-1 Gene (Prdm1) in B Cells Involves Bach2 and Histone Deacetylase 3

B lymphocyte-induced maturation protein 1 (Blimp-1) encoded by Prdm1 is a master regulator of plasma cell differentiation. The transcription factor Bach2 represses Blimp-1 expression in B cells to stall terminal differentiation, by which it supports reactions such as class switch recombination of the antibody genes. We found that histones H3 and H4 around the Prdm1 intron 5 Maf recognition element were acetylated at higher levels in X63/0 plasma cells expressing Blimp-1 than in BAL17 mature B cells lacking its expression. Conversely, methylation of H3-K9 was lower in X63/0 cells than BAL17 cells. Purification of the Bach2 complex in BAL17 cells revealed its interaction with histone deacetylase 3 (HDAC3), nuclear co-repressors NCoR1 and NCoR2, transducin β-like 1X-linked (Tbl1x), and RAP1-interacting factor homolog (Rif1). Chromatin immunoprecipitation confirmed the binding of HDAC3 and Rif1 to the Prdm1 locus. Reduction of HDAC3 or NCoR1 expression by RNA interference in B cells resulted in an increased Prdm1 mRNA expression. Bach2 is suggested to cooperate with HDAC3-containing co-repressor complexes in B cells to regulate the stage-specific expression of Prdm1 by writing epigenetic modifications at the Prdm1 locus.

Autoregulatory loop of nuclear corepressor 1 expression controls invasion, tumor growth, and metastasis

Nuclear corepressor 1 (NCoR) associates with nuclear receptors and other transcription factors leading to transcriptional repression. We show here that NCoR depletion enhances cancer cell invasion and increases tumor growth and metastatic potential in nude mice. These changes are related to repressed transcription of genes associated with increased metastasis and poor prognosis in patients. Strikingly, transient NCoR silencing leads to heterochromatinization and stable silencing of the NCoR gene, suggesting that NCoR loss can be propagated, contributing to tumor progression even in the absence of NCoR gene mutations. Down-regulation of the thyroid hormone receptor β1 (TRβ) appears to be associated with cancer onset and progression. We found that expression of TRβ increases NCoR levels and that this induction is essential in mediating inhibition of tumor growth and metastasis by this receptor. Moreover, NCoR is down-regulated in human hepatocarcinomas and in the more aggressive breast cancer tumors, and its expression correlates positively with that of TRβ. These data provide a molecular basis for the anticancer actions of this corepressor and identify NCoR as a potential molecular target for development of novel cancer therapies.

Regulation of Cardiac Transcription Factor GATA4 by Post-Translational Modification in Cardiomyocyte Hypertrophy and Heart Failure

Heart failure is a leading cause of cardiovascular mortality in industrialized countries. During development and deterioration of heart failure, cardiomyocytes undergo maladaptive hypertrophy, and changes in the cellular phenotype are accompanied by reinduction of the fetal gene program. Gene expression in cardiomyocytes is regulated by various nuclear transcription factors, co-activators, and co-repressors. The zinc finger protein GATA4 is one such transcription factor involved in the regulation of cardiomyocyte hypertrophy. In response to hypertrophic stimuli such as those involving the sympathetic nervous and renin-angiotensin systems, changes in protein interaction and/or post-translational modifications of GATA4 cause hypertrophic gene transcription in cardiomyocytes. In this article, we focus on cardiac nuclear signaling molecules, especially GATA4, that are promising as potential targets for heart failure therapy.

Uncoupling Proteins and the Molecular Mechanisms of Thyroid Thermogenesis.

Homoeothermic species engage thermogenic components of cellular metabolism to both maintain and raise core body temperature (Tc) when exposed to cold, feeding, infection, and certain classes of drugs (sympathomimetics). Body heat production stems from two main thermogenic mechanisms: 1) obligatory thermogenesis, or the heat produced as a collective byproduct of the inefficiency of all cellular metabolic reactions, and 2) facultative thermogenesis (also referred to as adaptive or nonshivering thermogenesis [NST]), the additional heat generation required along with obligatory thermogenesis to rapidly increase body temperature. Thyroid hormone (TH) is the well-established principal endocrine regulator of both obligatory and facultative thermogenesis. Here we aim to provide a brief overview of the mechanisms by which TH regulates facultative thermogenesis and to highlight recent studies describing a role for TH in the regulation of thermogenic mitochondrial uncoupling proteins. For a broader overview of the role of the endocrine control of thermoregulation, we would refer readers to the excellent review by Silva (1). Facultative thermogenesis is controlled by both a temporary shivering (muscle contraction) component and by more sustained nonshivering mechanisms. NST is regulated primarily by both the circulating levels of TH and the acute activity of the sympathetic nervous system (SNS) and mainly occurs in brown adipose tissue (BAT) and skeletal muscle (2). The primary role of TH in NST regulation is a permissive one; it chronically sets the capacity of thermogenic and metabolic responses to SNS stimulation through its transcriptional regulation of a vast repertoire of metabolic genes (3–5). In contrast, the SNS mainly controls the acute activity of thermogenic signaling pathways culminating in the activation of heat-generating proteins downstream of -adrenergic receptors (6, 7). Owing to the fundamental importance of TH in human thermoregulation, hyperand hypothyroid patients are heat and cold intolerant, respectively (8). Similarly, hypothyroid animal models demonstrate that the TH is absolutely required for a normal thermogenic response to both cold and sympathomimetic stimulation (9). TH is synthesized in the thyroid gland and controlled by thyroid peroxidase activity that regulates the iodination, coupling, and ultimately proteolysis of tyrosine residues on thyroglobulin to release the THs, T4 and T3, into the bloodstream (10). The lesser active T4 is released from the thyroid gland at higher concentrations than the more active T3 and is locally converted to T3 in target tissues by the actions of tissue-specific deiodinases (11–13). Two genes, THRA and THRB, are responsible for the expression of distinct thyroid hormone receptors (TRs), each of which are alternatively spliced to produce multiple isoforms, TR 1, TR 2, TR 1, and TR 2, respectively (14–16). With the exception of TR 2, which does not bind T3 and functions to repress T3 actions, TR isoforms mediate distinct functions (both stimulatory and repressive) in response to and in the absence of T3 (2). Integral to their functions as transcriptional regulators, TRs bind other nuclear hormone receptors, coactivators, and corepressors, the details of which have been reviewed elsewhere (2). In addition to its transcriptional regulation, recent work has also revealed that TH may regulate cell signaling

Inhibition of MTA1 by ERα contributes to protection hepatocellular carcinoma from tumor proliferation and metastasis.

BackgroundAlthough expression of MTA1 inversely correlates with the nuclear localization of ERα, the effect and molecular mechanism of ERα regulation of MTA1 remain unknown.MethodsQuantitative real-time PCR and western blot analyses were used to measure levels of MTA1. The effect on HCC cell proliferation and invasion was assessed by EdU incorporation assays and Transwell, respectively. ShRNA and dual-luciferase assays were used to investigate the regulatory relationship between MTA1 and ERα in cell lines.ResultsWe found that MTA1 gene regulation by ERα may be influenced by nuclear corepressors. The MTA1 promoter has three functional ER-element half-sites that lead to decreased MTA1 transcription and expression. ERα overexpression suppressed the proliferation and invasion of hepatocellular carcinoma cells (HCC). In addition, overexpression of MTA1 attenuated ERα-mediated suppression of the proliferation and invasion of HCC cells and tumor formation in vivo. These results suggested feedback regulation between ERα and MTA1. In summary, our results demonstrated that ERα suppressed proliferation and invasion of human HCC cells through downregulation of MTA1 transcription.ConclusionsOur study is an improved description of the mechanisms of the suppressive effect of ERα on HCCs, adding understanding to the gender disparity of HCC progression.

Histone Deacetylases and Cardiometabolic Diseases.

Cardiometabolic disease, emerging as a worldwide epidemic, is a combination of metabolic derangements leading to type 2 diabetes mellitus and cardiovascular disease. Genetic and environmental factors are linked through epigenetic mechanisms to the pathogenesis of cardiometabolic disease. Post-translational modifications of histone tails, including acetylation and deacetylation, epigenetically alter chromatin structure and dictate cell-specific gene expression patterns. The histone deacetylase family comprises 18 members that regulate gene expression by altering the acetylation status of nucleosomal histones and by functioning as nuclear transcriptional corepressors. Histone deacetylases regulate key aspects of metabolism, inflammation, and vascular function pertinent to cardiometabolic disease in a cell- and tissue-specific manner. Histone deacetylases also likely play a role in the metabolic memory of diabetes mellitus, an important clinical aspect of the disease. Understanding the molecular, cellular, and physiological functions of histone deacetylases in cardiometabolic disease is expected to provide insight into disease pathogenesis, risk factor control, and therapeutic development.

An autoregulatory loop of nuclear corepressor 1 expression controls hepatocarcinoma invasion, growth and metastasis

Searchable abstracts of presentations at key conferences in endocrinology ISSN 1470-3947 (print) | ISSN 1479-6848 (online)

The Nurr1 Activator 1,1-Bis(3′-Indolyl)-1-(p-Chlorophenyl)Methane Blocks Inflammatory Gene Expression in BV-2 Microglial Cells by Inhibiting Nuclear Factor κB

NR4A family orphan nuclear receptors are an important class of transcription factors for development and homeostasis of dopaminergic neurons that also inhibit expression of inflammatory genes in glial cells. The identification of NR4A2 (Nurr1) as a suppressor of nuclear factor κB (NF-κB)-related neuroinflammatory genes in microglia and astrocytes suggests that this receptor could be a target for pharmacologic intervention in neurologic disease, but compounds that promote this activity are lacking. Selected diindolylmethane compounds (C-DIMs) have been shown to activate or inactivate nuclear receptors, including Nurr1, in cancer cells and also suppress astrocyte inflammatory signaling in vitro. Based upon these data, we postulated that C-DIM12 [1,1-bis(3'-indolyl)-1-(p-chlorophenyl) methane] would suppress inflammatory signaling in microglia by a Nurr1-dependent mechanism. C-DIM12 inhibited lipopolysaccharide (LPS)-induced expression of NF-κB-regulated genes in BV-2 microglia including nitric oxide synthase (NOS2), interleukin-6 (IL-6), and chemokine (C-C motif) ligand 2 (CCL2), and the effects were attenuated by Nurr1-RNA interference. Additionally, C-DIM12 decreased NF-κB activation in NF-κB-GFP (green fluorescent protein) reporter cells and enhanced nuclear translocation of Nurr1 primary microglia. Chromatin immunoprecipitation assays indicated that C-DIM12 decreased lipopolysaccharide-induced p65 binding to the NOS2 promoter and concurrently enhanced binding of Nurr1 to the p65-binding site. Consistent with these findings, C-DIM12 also stabilized binding of the Corepressor for Repressor Element 1 Silencing Transcription Factor (CoREST) and the Nuclear Receptor Corepressor 2 (NCOR2). Collectively, these data identify C-DIM12 as a modulator of Nurr1 activity that results in inhibition of NF-κB-dependent gene expression in glial cells by stabilizing nuclear corepressor proteins, which reduces binding of p65 to inflammatory gene promoters.

Cytoplasmic accumulation of NCoR in malignant melanoma: consequences of altered gene repression and prognostic significance.

Invasive malignant melanoma (MM) is an aggressive tumor with no curative therapy available in advanced stages. Nuclear corepressor (NCoR) is an essential regulator of gene transcription, and its function has been found deregulated in different types of cancer. In colorectal cancer cells, loss of nuclear NCoR is induced by Inhibitor of kappa B kinase (IKK) through the phosphorylation of specific serine residues. We here investigate whether NCoR function impacts in MM, which might have important diagnostic and prognostic significance. By IHC, we here determined the subcellular distribution of NCoR in a cohort of 63 primary invasive MM samples, and analyzed its possible correlation with specific clinical parameters. We therefore used a microarray-based strategy to determine global gene expression differences in samples with similar tumor stage, which differ in the presence of cytoplasmic or nuclear NCoR. We found that loss of nuclear NCoR results in upregulation of a specific cancer-related genetic signature, and is significantly associated with MM progression. Inhibition of IKK activity in melanoma cells reverts NCoR nuclear distribution and specific NCoR-regulated gene transcription. Analysis of public database demonstrated that inactivating NCoR mutations are highly prevalent in MM, showing features of driver oncogene.

A new syndrome of intellectual disability with dysmorphism due to TBL1XR1 deletion

We report here on an 8-year-old girl and her mother, both displaying similar facial dysmorphism, speech delay, and mild to moderate intellectual disability. Array-CGH studies revealed the same interstitial 3q26.32 microdeletion encompassing a single coding gene, TBL1XR1, in both patients. The TBL1XR1 protein, which has four WD40 repeats, has been shown to bind the nuclear corepressor (NCOR) and histone deacetylase-3 complexes (HDAC3). TBL1XR1 mutations have recently been implicated in autism spectrum disorders, but our patients displayed no autistic behavior. Our findings suggest that TBL1XR1 haploinsufficiency can cause intellectual disability with a recognizable dysmorphism, without necessarily causing autistic behavior.

SIRT6 represses LINE1 retrotransposons by ribosylating KAP1 but this repression fails with stress and age

L1 retrotransposons are an abundant class of transposable elements which pose a threat to genome stability and may play a role in age-related pathologies such as cancer. Recent evidence indicates that L1s become more active in somatic tissues during the course of aging; the mechanisms underlying this phenomenon remain unknown, however. Here we report that the longevity regulating protein, SIRT6, is a powerful repressor of L1 activity. Specifically, SIRT6 binds to the 5′UTR of L1 loci, where it mono-ADP ribosylates the nuclear corepressor protein, KAP1, and facilitates KAP1 interaction with the heterochromatin factor, HP1α, thereby contributing to the packaging of L1 elements into transcriptionally repressive heterochromatin. During the course of aging, and also in response to DNA damage, however, we find that SIRT6 is depleted from L1 loci, allowing for the activation of these previously silenced retroelements.

Probing phosphorylation-dependent protein interactions within functional domains of histone deacetylase 5 (HDAC5).

Class IIa histone deacetylases (HDACs) are critical transcriptional regulators, shuttling between nuclear and cytoplasmic cellular compartments. Within the nucleus, these HDACs repress transcription as components of multiprotein complexes, such as the nuclear corepressor and beclin-6 corepressor (BCoR) complexes. Cytoplasmic relocalization relieves this transcriptional repressive function. Class IIa HDAC shuttling is controlled, in part, by phosphorylations flanking the nuclear localization signal (NLS). Furthermore, we have reported that phosphorylation within the NLS by the kinase Aurora B modulates the localization and function of the class IIa HDAC5 during mitosis. While we identified numerous additional HDAC5 phosphorylations, their regulatory functions remain unknown. Here, we studied phosphorylation sites within functional HDAC5 domains, including the deacetylation domain (DAC, Ser755), nuclear export signal (NES, Ser1108), and an acidic domain (AD, Ser611). We have generated phosphomutant cell lines to investigate how absence of phosphorylation at these sites impacts HDAC5 localization, enzymatic activity, and protein interactions. Combining molecular biology and quantitative MS, we have defined the interactions and HDAC5-containing complexes mediated by site-specific phosphorylation and quantified selected changes using parallel reaction monitoring. These results expand the current understanding of HDAC regulation, and the functions of this critical family of proteins within human cells.

Synthesis and evaluation of bisbenzylidenedioxotetrahydrothiopranones as activators of endoplasmic reticulum (ER) stress signaling pathways and apoptotic cell death in acute promyelocytic leukemic cells.

Curcumin is known to trigger ER-stress induced cell death of acute promyelocytic leukemic (APL) cells by intercepting the degradation of nuclear co-repressor (N-CoR) protein which has a key role in the pathogenesis of APL. Replacing the heptadienedione moiety of curcumin with a monocarbonyl cross-conjugated dienone embedded in a tetrahydrothiopyranone dioxide ring resulted in thiopyranone dioxides that were more resilient to hydrolysis and had greater growth inhibitory activities than curcumin on APL cells. Several members intercepted the degradation of misfolded N-CoR and triggered the signaling cascade in the unfolded protein response (UPR) which led to apoptotic cell death. Microarray analysis showed that genes involved in protein processing pathways that were germane to the activation of the UPR were preferentially up-regulated in treated APL cells, supporting the notion that the UPR was a consequential mechanistic pathway affected by thiopyranone dioxides. The Michael acceptor reactivity of the scaffold may have a role in exacerbating ER stress in APL cells.

The anoikis effector Bit1 displays tumor suppressive function in lung cancer cells.

The mitochondrial Bit1 (Bcl-2 inhibitor of transcription 1) protein is a part of an apoptotic pathway that is uniquely regulated by integrin-mediated attachment. As an anoikis effector, Bit1 is released into the cytoplasm following loss of cell attachment and induces a caspase-independent form of apoptosis. Considering that anoikis resistance is a critical determinant of transformation, we hypothesized that cancer cells may circumvent the Bit1 apoptotic pathway to attain anchorage-independence and tumorigenic potential. Here, we provide the first evidence of the tumor suppressive effect of Bit1 through a mechanism involving anoikis induction in human lung adenocarcinoma derived A549 cells. Restitution of Bit1 in anoikis resistant A549 cells is sufficient to induce detachment induced-apoptosis despite defect in caspase activation and impairs their anchorage-independent growth. Conversely, stable downregulation of Bit1 in these cells significantly enhances their anoikis resistance and anchorage-independent growth. The Bit1 knockdown cells exhibit significantly enhanced tumorigenecity in vivo. It has been previously shown that the nuclear TLE1 corepressor is a putative oncogene in lung cancer, and we show here that TLE1 blocks Bit1 mediated anoikis in part by sequestering the pro-apoptotic partner of Bit1, the Amino-terminal Enhancer of Split (AES) protein, in the nucleus. Taken together, these findings suggest a tumor suppressive role of the caspase-independent anoikis effector Bit1 in lung cancer. Consistent with its role as a tumor suppressor, we have found that Bit1 is downregulated in human non-small cell lung cancer (NSCLC) tissues.

Circadian Regulation of Tshb Gene Expression by Rev-Erbα (NR1D1) and Nuclear Corepressor 1 (NCOR1)

Thyroid hormones (TH) are critical for development, growth, and metabolism. Circulating TH levels are tightly regulated by thyroid-stimulating hormone (TSH) secretion within the hypothalamic-pituitary-thyroid axis. Although circadian TSH secretion has been well documented, the mechanism of this observation remains unclear. Recently, the nuclear corepressor, NCOR1, has been postulated to regulate TSH expression, presumably by interacting with thyroid hormone receptors (THRs) bound to TSH subunit genes. We report herein the first in vitro study of NCOR1 regulation of TSH in a physiologically relevant cell system, the TαT1.1 mouse thyrotroph cell line. Knockdown of NCOR1 by shRNA adenovirus increased baseline Tshb mRNA levels compared with scrambled control, but surprisingly had no affect on the T3-mediated repression of this gene. Using ChIP, we show that NCOR1 enriches on the Tshb promoter at sites different from THR previously identified by our group. Furthermore, NCOR1 enrichment on Tshb is unaffected by T3 treatment. Given that NCOR1 does not target THR on Tshb, we hypothesized that NCOR1 targeted Rev-Erbα (NR1D1), an orphan nuclear receptor that is a potent repressor of gene transcription and regulator of metabolism and circadian rhythms. Using a serum shock technique, we synchronized TαT1.1 cells to study circadian gene expression. Post-synchronization, Tshb and Nr1d1 mRNA levels displayed oscillations that inversely correlated with each other. Furthermore, NR1D1 was enriched at the same locus as NCOR1 on Tshb. Therefore, we propose a model for Tshb regulation whereby NR1D1 and NCOR1 interact to regulate circadian expression of Tshb independent of TH negative regulation.

Thyroid Hormone Signaling In Vivo Requires a Balance between Coactivators and Corepressors

Resistance to thyroid hormone (RTH), a human syndrome, is characterized by high thyroid hormone (TH) and thyroid-stimulating hormone (TSH) levels. Mice with mutations in the thyroid hormone receptor beta (TRβ) gene that cannot bind steroid receptor coactivator 1 (SRC-1) and Src-1(-/-) mice both have phenotypes similar to that of RTH. Conversely, mice expressing a mutant nuclear corepressor 1 (Ncor1) allele that cannot interact with TRβ, termed NCoRΔID, have low TH levels and normal TSH. We hypothesized that Src-1(-/-) mice have RTH due to unopposed corepressor action. To test this, we crossed NCoRΔID and Src-1(-/-) mice to create mice deficient for coregulator action in all cell types. Remarkably, NCoR(ΔID/ΔID) Src-1(-/-) mice have normal TH and TSH levels and are triiodothryonine (T(3)) sensitive at the level of the pituitary. Although absence of SRC-1 prevented T(3) activation of key hepatic gene targets, NCoR(ΔID/ΔID) Src-1(-/-) mice reacquired hepatic T(3) sensitivity. Using in vivo chromatin immunoprecipitation assays (ChIP) for the related coactivator SRC-2, we found enhanced SRC-2 recruitment to TR-binding regions of genes in NCoR(ΔID/ΔID) Src-1(-/-) mice, suggesting that SRC-2 is responsible for T(3) sensitivity in the absence of NCoR1 and SRC-1. Thus, T(3) targets require a critical balance between NCoR1 and SRC-1. Furthermore, replacement of NCoR1 with NCoRΔID corrects RTH in Src-1(-/-) mice through increased SRC-2 recruitment to T(3) target genes.