Spectroscopy of kaonic atoms at DAFNE and J-PARC

The interaction of antikaons (K−) with nucleons and nuclei in the low-energy regime represents a very active research field in hadron physics. A unique and rather direct experimental access to the antikaon-nucleon scattering lengths is provided by precision X-ray spectroscopy of transitions in low-lying states in the lightest kaonic atoms (i.e. kaonic hydrogen and deuterium). In the SIDDHARTA experiment at the electron-positron collider DAFNE of LNFINFN we measured the most precise values of the strong interaction observables in conic hydrogen. The strong interaction on the 1s ground state of the electromagnetically bound K-p atom causes an energy shift and broadening of the 1s state. SIDDHARTA will extend the spectroscopy to kaonic deuterium to get access to the antikaon-neutron interaction and thus the isospin dependent scattering lengths. At J-PARC a kaon beam is used in a complementary experiment with a different setup for spectroscopy of kaonic deuterium atoms. The talk will give an overview of the of the upcoming experiments SIDDHARTA and the complementary experiment at J-PARC.Furthermore, the implications of the experiments for the theory of low-energy strong interaction with strangeness will be discussed.

Atypical polycystic ovary syndrome--a genetic analysis

BACKGROUND/AIMS

Although polycystic ovary syndrome (PCOS) is a common endocrinopathy the pathogenesis is not entirely understood. Typically, high androgen levels are associated with increased virilization. We report 2 rare groups of patients with either unexpectedly high testosterone levels despite low virilization as well as patients with low testosterone levels despite high grade of virilization. One possibility for the atypical PCOS may be based on an altered androgen receptor (AR) signaling.

METHODS

6 patients and when available the parents were included in this study. Alterations of the metaphase chromosomes by GTG staining, the length of both the trinucleotide CAG- and GGC-repeats of the androgen receptor (AR) gene was determined by PCR, further the entire AR gene was sequenced and analyzed.

RESULTS

The GTG banding revealed no chromosomal alterations and the range of CAG- and GGC-repeat lengths are within the normal range. Interestingly, by sequencing of the entire AR gene few genetic mutations were identified.

CONCLUSION

The detected mutations do not alter the AR protein sequence but they change the codon usage towards less frequent codons that potentially may alter AR protein levels and androgen signaling. In addition to this, we postulate also other causes for manifestation of atypical PCOS, which may include AR-coregulators or epigenetic alterations. To our knowledge this is the first report of combining chromosomal analysis of PCOS patients with full sequencing of the human AR gene and linking codon usage to PCOS.

Sirtuin family: a link to metabolic signaling and senescence

A vast collection of data obtained during the last decade supports the view on sirtuins as sensors of actual cellular metabolic state being involved in cell cycle progression, apoptosis/survival decision making, longevity, inflammation etc. Moreover, sirtuins themselves can control metabolism through their ability to consume NAD(+). In turn, cellular NAD parameters may affect the generation of ATP, a main cellular currency of energy. Therefore, sirtuins became recognized as critical affectors of cellular metabolism which participate in fat mobilization, gluconeogenesis, caloric restriction etc. Cellular senescence is viewed as a mechanism to restrict excessive cell growth when it is unnecessary or harmful. It is therefore necessary to understand the mechanism of senescence to design new approaches to combat cancer. Growth in turn depends on metabolism as it requires energy. Therefore, in this review, we address the connection of sirtuins to senescence through their participation in the regulation of metabolic and biochemical parameters and related signaling.

Src kinase potentiates androgen receptor transactivation function and invasion of androgen-independent prostate cancer C4-2 cells

Prostate cancer is one of the most prominent malignancies of elderly men in many Western countries including Europe and the United States with increasing trend worldwide. The growth of normal prostate as well as of prostate carcinoma cells depends on functional androgen receptor (AR) signaling. AR manifests the biological actions of androgens and its transcriptional activity is known to be influenced by signal transduction pathways. Here we show that Src, a nonreceptor tyrosine kinase, is overexpressed in androgen-independent prostate carcinoma C4-2 cells. Interestingly, the expression of Src was found to progressively increase (up to threefold) in transgenic adenocarcinoma of mouse prostate mice as a function of age and cancer progression. Blocking Src kinase function by a specific inhibitor, PP2, resulted in decreased AR transactivation function on two different reporters, mouse mammary tumor virus (MMTV) and prostate-specific antigen (PSA). Consistent with this, overexpression of a functional Src mutant also led to a dramatic decrease in AR transactivation potential in a hormone-dependent manner. Interference with Src function in C4-2 cells led to decreased recruitment of AR on the target gene PSA enhancer and also resulted in the abrogation of hormone-dependent PSA transcript induction. Src inhibition also led to a dramatic decrease in the cell invasion in addition to decreasing the cellular growth. We suggest that targeting Src kinase could be an effective strategy to inhibit prostate cancer growth and metastasis.

BKCa Channels Activating at Resting Potential without Calcium in LNCaP Prostate Cancer Cells

Large-conductance Ca2+-dependent K+ (BK(Ca)) channels are activated by intracellular Ca2+ and membrane depolarization in an allosteric manner. We investigated the pharmacological and biophysical characteristics of a BK(Ca)-type K+ channel in androgen-dependent LNCaP (lymph node carcinoma of the prostate) cells with novel functional properties, here termed BK(L). K+ selectivity, high conductance, activation by Mg2+ or NS1619, and inhibition by paxilline and penitrem A largely resembled the properties of recombinant BK(Ca) channels. However, unlike conventional BK(Ca) channels, BK(L) channels activated in the absence of free cytosolic Ca2+ at physiological membrane potentials; the half-maximal activation voltage was shifted by about -100 mV compared with BK(Ca) channels. Half-maximal Ca2+-dependent activation was observed at 0.4 microM: for BK(L) (at -20 mV) and at 4.1 microM: for BK(Ca) channels (at +50 mV). Heterologous expression of hSlo1 in LNCaP cells increased the BK(L) conductance. Expression of hSlo-beta1 in LNCaP cells shifted voltage-dependent activation to values between that of BK(L) and BK(Ca) channels and reduced the slope of the P (open) (open probability)-voltage curve. We propose that LNCaP cells harbor a so far unknown type of BK(Ca) subunit, which is responsible for the BK(L) phenotype in a dominant manner. BK(L)-like channels are also expressed in the human breast cancer cell line T47D. In addition, functional expression of BK(L) in LNCaP cells is regulated by serum-derived factors, however not by androgens.

Mouse germline restriction of Oct4 expression by germ cell nuclear factor

The POU-domain transcription factor Oct4 is essential for the maintenance of the mammalian germline. In this study, we show that the germ cell nuclear factor (GCNF), an orphan nuclear receptor, represses Oct4 gene activity by specifically binding within the proximal promoter. GCNF expression inversely correlates with Oct4 expression in differentiating embryonal cells. GCNF overexpression in embryonal cells represses Oct4 gene and transgene activities, and we establish a link to transcriptional corepressors mediating repression by GCNF. In GCNF-deficient mouse embryos, Oct4 expression is no longer restricted to the germ cell lineage after gastrulation. Our studies suggest that GCNF is critical in repressing Oct4 gene activity as pluripotent stem cells differentiate and in confining Oct4 expression to the germline.

Modulation of thyroid hormone receptor silencing function by co-repressors and a synergizing transcription factor

We have found that the thyroid hormone receptor (T3R) functionally synergizes with the CCCTC-binding factor (CTCF). CTCF is a highly conserved zinc-finger protein that has been connected with multiple functions in gene regulation including chromatin insulator activity, transcriptional enhancement and silencing as well as tumour suppression. A specific property of CTCF is that some of the binding sites are found in the vicinity of T3R-binding sites. Interestingly, both factors synergize in repression as well as in activation. T3R-mediated repression has been shown to involve co-repressors such as the silencing mediator for retinoic acid and thyroid hormone receptor (SMRT), N-CoR or Alien. These co-repressors in turn have been found to interact with Sin3A. Until now, the mechanisms by which CTCF synergizes with T3R in transcriptional repression has not been determined. Here we show that CTCF comprises autonomous silencing domains that mediate transcriptional repression when tethered to a promoter sequence. At least one of these domains, the zinc-finger region of CTCF, binds Sin3A without binding to SMRT or N-CoR and recruits histone deacetylation activity. For Sin3A we identified two different domains interacting independently with the CTCF zinc-finger cluster. The ability of regions of CTCF to retain deacetylase activity is correlated with the ability to bind to Sin3A and to repress transcription. Taking these results together, the synergy in repression mediated by T3R and CTCF might be achieved by the binding of multiple molecules of Sin3A to the T3R/CTCF-DNA complex, thus providing a large platform for the recruitment of histone deacetylases.

Co-repressors 2000

In the last 5 years, many co-repressors have been identified in eukaryotes that function in a wide range of species, from yeast to Drosophila and humans. Co-repressors are coregulators that are recruited by DNA-bound transcriptional silencers and play essential roles in many pathways including differentiation, proliferation, programmed cell death, and cell cycle. Accordingly, it has been shown that aberrant interactions of co-repressors with transcriptional silencers provide the molecular basis of a variety of human diseases. Co-repressors mediate transcriptional silencing by mechanisms that include direct inhibition of the basal transcription machinery and recruitment of chromatin-modifying enzymes. Chromatin modification includes histone deacetylation, which is thought to lead to a compact chromatin structure to which the accessibility of transcriptional activators is impaired. In a general mechanistic view, the overall picture suggests that transcriptional silencers and co-repressors act in analogy to transcriptional activators and coactivators, but with the opposite effect leading to gene silencing. We provide a comprehensive overview of the currently known higher eukaryotic co-repressors, their mechanism of action, and their involvement in biological and pathophysiological pathways. We also show the different pathways that lead to the regulation of co-repressor-silencer complex formation.

VDR-Alien: a novel, DNA-selective vitamin D(3) receptor-corepressor partnership

The vitamin D receptor (VDR) is a transcription factor that transmits incoming 1,25-dihydroxyvitamin D(3) (1alpha,25(OH)(2)D(3)) signaling via combined contact with coactivator proteins and specific DNA binding sites (VDREs), which ultimately results in activation of transcription. In contrast, the mechanisms of transcriptional repression via the VDR are less well understood. This study documents VDR-dependent transcriptional repression largely via histone deacetylase (HDAC) activity. Direct, ligand-sensitive protein-protein interaction of the VDR with the nuclear receptor corepressor (NCoR) and a novel corepressor, called Alien, was demonstrated to be comparable but independent of the VDR AF-2 trans-activation domain. Functional assays indicated that Alien, but not NCoR, displays selectivity for different VDRE structures for transferring these repressive effects into gene regulatory activities. Moreover, superrepression via Alien was found to be affected only in part by HDAC inhibitors such as trichostatin A. Finally, for a dissociation of VDR-Alien complexes in vitro and in vivo, higher ligand concentrations were needed than for a dissociation of VDR-NCoR complexes. This suggests that Alien and NCoR are using different interfaces for interaction with the VDR and different pathways for mediating superrepression, which in turn characterizes Alien as a representative of a new class of corepressors. Taken together, association of the VDR with corepressor proteins provides a further level of transcriptional regulation, which is emerging as a complex network of protein-protein interaction-mediated control.

Interaction of the corepressor Alien with DAX-1 is abrogated by mutations of DAX-1 involved in adrenal hypoplasia congenita

DAX-1 is an unusual member of the nuclear hormone receptor (NHR) superfamily. Lack of DAX-1-mediated silencing leads to adrenal hypoplasia congenita and hypogonadotropic hypogonadism. Gene silencing through NHRs such as the thyroid hormone receptor (TR) is mediated by corepressors. We have previously characterized a novel corepressor, termed Alien, which interacts with TR and the ecdysone receptor but not with the retinoic acid receptors RAR or RXR. Here, we show that DAX-1 interacts with the corepressor Alien but not with the corepressor SMRT. This interaction is mediated by the DAX-1-silencing domain. Naturally occurring mutants of the DAX-1 gene fail to interact with Alien and have lost silencing function. Because the silencing domain of DAX-1 is unusual for NHRs, we mapped the interaction of Alien with DAX-1 and with TR. We show that Alien exhibits different binding characteristics to DAX-1 and TR. Furthermore, Northern experiments demonstrate that Alien is expressed in the adrenal gland and testis in tissues where DAX-1 is specifically expressed. Interestingly, a novel adrenal gland-specific mRNA of Alien was discovered. Thus, the impairment of Alien binding seems to play an important role in the pathogenesis mediated by DAX-1 mutants.

Promoter specific sensitivity to inhibition of histone deacetylases: implications for hormonal gene control, cellular differentiation and cancer

Alterations in histone acetylation status appear to play a central role in the regulation of neoplasia, tumor suppression, cell cycle control, hormone responsiveness and senescence. These alterations of chromatin control gene transcription. The histone acetylation status is regulated by the equilibrium of histone acetyl-transferase activity (HAT) and the histone deacetylase activity (HDAC). Commonly, DNA-transfection assays are used to measure the effect of histone acetylation and deacetylation on gene transcription. Here we have analyzed the response of various viral long terminal repeats and vertebrate promoters to the specific histone deacetylase inhibitor trichostatin A (TSA). We show that the activity of many, but not all, promoters is increased upon TSA treatment. Interestingly, the lysozyme promoter exhibited TSA resistance, while the activity of metallothionine, the human growth hormone, and the thymidine kinase promoters was increased. Furthermore, we found that all tested viral promoters are induced by TSA. Analysis of the transcriptional behaviour of the thyroid hormone receptor (TR), the cellular homologue of the v-erbA oncogene, revealed that TSA reduced the gene silencing function but had no influence on the hormone-induced gene activation function of the receptor. These results on gene specific effects, together with the HDAC structural data (1), may be a basis for the development of HDAC inhibitors as antitumor agents.

Silencing subdomains of v-ErbA interact cooperatively with corepressors: involvement of helices 5/6

Members of the thyroid hormone receptor (TR) family act on vertebrate development and homeostasis by activating or repressing transcription of specific target genes in a ligand-dependent way. Repression by TR in the absence of ligand is mediated by an active silencing mechanism. The oncogene v-ErbA is a variant form of TR unable to bind hormone and thus acts as a constitutive repressor. Functional studies and mutation analysis revealed that the TR/v-ErbA silencing domain is composed of three silencing subdomains (SSD1-3) which, although nonfunctional individually, synergize such that silencing activity is restored when they are combined in a heteromeric complex. Here we demonstrate, using protein interaction assays in vitro and in vivo, that the inactive v-ErbA point mutant L489R within helix 5/6 in SSD2 fails to interact with the two corepressors N-CoR (nuclear receptor corepressor) or SMRT (silencing mediator of retinoic acid and thyroid hormone receptor). Furthermore, mutants in SSD1 and SSD3 exhibit a reduced corepressor recruitment corresponding to their weak residual silencing activity. In mammalian two-hybrid assays, only the combination of all three silencing subdomains, SSD1-3, leads to a cooperative binding to the corepressors N-CoR or SMRT comparable to that of the full-length v-ErbA repression domain. In conclusion, full silencing activity requires corepressor interaction with all three silencing subdomains, SSD1-3. Among these, SSD2 is a new target for N-CoR and SMRT and is essential for corepressor binding and function.

The v-erbA oncogene (review)

The v-erbA oncogene product is a nuclear protein and belongs to the superfamily of nuclear hormone receptors. The v-ErbA oncoprotein is involved in neoplastic transformation leading to acute erythroleukemia and sarcomas. The cellular homolog of v-ErbA oncoprotein is the thyroid hormone receptor alpha (c-erbA alpha or TRalpha). While TR has the dual role to silence gene expression in the absence of hormone and activate genes in the presence of the ligand, triiodothyronine, the v-ErbA oncoprotein has lost the ability to activate genes. The oncoprotein is thought to repress, in a constitutive manner, a certain set of genes which prevent cellular transformation. The mechanism of gene silencing is partly understood and involves the so-called corepressors. Several types of corepressors have been identified so far. Similarly, gene silencing by corepressors also plays a role in myeloid transformation by the retinoic acid receptor (RAR) which is involved in translocations, such as PML-RAR. The v-erbA oncogene was isolated from a retrovirus which contains, in addition to v-erbA, the oncogene v-erbB. The viral erbB gene encodes an EGF-receptor derivative, which is a constitutively active tyrosine kinase. Cellular transformation is enhanced when both oncoproteins are expressed. However, the mechanisms of cellular transformation by v-ErbA alone or in synergy with v-ErbB remain unclear. Novel insights into the mechanism of cellular transformation by v-ErbA, the role of corepressors and the role of the cross talk between the EGF-receptor and v-ErbA will be discussed.

Alien, a highly conserved protein with characteristics of a corepressor for members of the nuclear hormone receptor superfamily

Some members of nuclear hormone receptors, such as the thyroid hormone receptor (TR), silence gene expression in the absence of the hormone. Corepressors, which bind to the receptor's silencing domain, are involved in this repression. Hormone binding leads to dissociation of corepressors and binding of coactivators, which in turn mediate gene activation. Here, we describe the characteristics of Alien, a novel corepressor. Alien interacts with TR only in the absence of hormone. Addition of thyroid hormone leads to dissociation of Alien from the receptor, as shown by the yeast two-hybrid system, glutathione S-transferase pull-down, and coimmunoprecipitation experiments. Reporter assays indicate that Alien increases receptor-mediated silencing and that it harbors an autonomous silencing function. Immune staining shows that Alien is localized in the cell nucleus. Alien is a highly conserved protein showing 90% identity between human and Drosophila. Drosophila Alien shows similar activities in that it interacts in a hormone-sensitive manner with TR and harbors an autonomous silencing function. Specific interaction of Alien is seen with Drosophila nuclear hormone receptors, such as the ecdysone receptor and Seven-up, the Drosophila homologue of COUP-TF1, but not with retinoic acid receptor, RXR/USP, DHR 3, DHR 38, DHR 78, or DHR 96. These properties, taken together, show that Alien has the characteristics of a corepressor. Thus, Alien represents a member of a novel class of corepressors specific for selected members of the nuclear hormone receptor superfamily.

EcR interacts with corepressors and harbours an autonomous silencing domain functional in both Drosophila and vertebrate cells

The ecdysone receptor (EcR) is a member of the large family of nuclear hormone receptors, which are ligand regulated transcription factors. In general, ligand converts these receptors into a transcriptional activator. Some vertebrate nuclear hormone receptors, such as the thyroid hormone and retinoic acid receptors, silence gene expression in the absence of ligand. EcR is involved in fly metamorphosis and is used in vertebrates as an inducible system for expression of transgenes. Here, we show that a Drosophila receptor, the EcR, harbours an autonomous silencing function in its carboxy-terminus. Interestingly, EcR mediates also silencing in vertebrate cells. In concordance with this EcR interacts with the corepressors SMRT and N-CoR, while addition of ligand reduces this interaction. Conversely, the v-erbA oncogene product, a thyroid hormone receptor derivative, mediates silencing in Drosophila cells. Thus, our data suggest the involvement of an evolutionarily conserved mechanism by which nuclear hormone receptors mediate gene silencing in multicellular organisms.

Cell-specific inhibition of retinoic acid receptor-alpha silencing by the AF2/tau c activation domain can be overcome by the corepressor SMRT, but not by N-CoR

The human retinoic acid receptor alpha (hRAR alpha) exhibits cell-specific transcriptional activity. Previously, it was shown that in the absence of hormone the wild-type receptor is a transcriptional silencer in L cells, whereas it lacks silencing function and is a weak activator in CV1 cells. Addition of hormone leads to a further increase in transactivation in CV1 cells. Thus, the retinoic acid response mediated by RAR alpha is weak in these cells. It was shown that the CV1-specific effect is due to the receptor C terminus. We show, that the failure of silencing by RAR is not due to a general lack of corepressors in CV1 cells, since the silencing domain of RAR is functionally active and exhibits active repression in these cells. Furthermore, we show that the conserved AF2/tau c activation function of RAR is responsible for the cell-specific inhibition of silencing. Thereby, the CV1 cell specificity was abolished by replacing AF2/tau c of RAR with the corresponding sequence of the thyroid hormone receptor. Thus, we find a new role of the C-terminal conserved activation function AF2/tau c in that, specifically, the RAR AF2/tau c-sequence is able to prevent silencing of RAR in a cell-specific manner. In addition, we show that the inhibitory effect of AF2/tau c in CV1 cells can be overcome by expression of the corepressor SMRT (silencing mediator of retinoic acid and thyroid hormone receptor), but not by that of N-CoR (nuclear receptor corepressor). The expression of these two corepressors, however, had no measurable effect on RAR-mediated silencing in L cells. Thus, the expression of a corepressor can lead to a dramatic increase of hormonal response in a cell-specific manner.

Multiple receptor interaction domains of GRIP1 function in synergy

Nuclear hormone receptors are exerting their effect on transcription by interacting with basal factors of the transcription machinery and/or by recruiting intermediary factors, such as the mouse protein GRIP1. GRIP1 is one of the recently identified coactivators for nuclear hormone receptors. Upon interaction with the hormone-binding domain of the receptors, GRIP1 increases their transcriptional activity. Here we show that GRIP1 contains at least two receptor-interacting regions using the hormone-binding domain of several receptors as bait in the yeast two-hybrid assay. GRIP1 interacts in a hormone-dependent manner with the C-termini of nuclear hormone receptors such as GRalpha, TRalpha, TRbeta, RARalpha and RXRalpha but not with v-ErbA. GRIP1 contains several LXXLL motifs which were shown to be required for receptor interaction. A protein fragment containing all of the three LXXLL motifs, but having the activation domain deleted, is able to repress the transcriptional activity of human TRbeta, whereas a region harbouring only one LXXLL motif fails to do so. A protein fragment with two LXXLL motifs exhibits an intermediate modulation of the TRbeta transactivation. While one motif seems to be sufficient for receptor interaction, more than one motif is needed for functional interference.

An inhibitory region of the DNA-binding domain of thyroid hormone receptor blocks hormone-dependent transactivation

We have employed a chimeric receptor system in which we cotransfected yeast GAL4 DNA-binding domain/retinoid X receptor beta ligand-binding domain chimeric receptor (GAL4RXR), thyroid hormone receptor-beta (TRbeta), and upstream activating sequence-reporter plasmids into CV-1 cells to study repression, derepression, and transcriptional activation. In the absence of T3, unliganded TR repressed transcription to 20% of basal level, and in the presence of T3, liganded TRbeta derepressed transcription to basal level. Using this system and a battery of TRbeta mutants, we found that TRbeta/RXR heterodimer formation is necessary and sufficient for basal repression and derepression in this system. Additionally, an AF-2 domain mutant (E457A) mediated basal repression but not derepression, suggesting that interaction with a putative coactivator at this site may be critical for derepression. Interestingly, a mutant containing only the TRbeta ligand binding domain (LBD) not only mediated derepression, but also stimulated transcriptional activation 10-fold higher than basal level. Studies using deletion and domain swap mutants localized an inhibitory region to the TRbeta DNA-binding domain. Titration studies further suggested that allosteric changes promoting interaction with coactivators may account for enhanced transcriptional activity by LBD. In summary, our findings suggest that TR heterodimer formation with RXR is important for repression and derepression, and coactivator interaction with the AF-2 domain may be needed for derepression in this chimeric system. Additionally, there may be an inhibitory region in the DNA-binding domain, which reduces TR interaction with coactivators, and prevents full-length wild-type TRbeta from achieving transcriptional activation above basal level in this chimeric receptor system.

Nuclear receptors: structure, function and involvement in disease

Nuclear hormone receptors are acting as transcription factors in the cell nucleus. They regulate gene expression of hormonal regulated target genes. The role of hormone in the transcriptional process is to modulate and change the nuclear receptor functionality. Receptors contain a DNA binding domain that enables them to bind to hormone response elements of target genes. Nuclear hormone receptors bind to lipophilic hormones produced by the organisms' endocrine system, which links the secretion of hormones directly to regulation of gene expression of responsive tissues. In recent years increasing numbers of naturally occurring mutations of a variety of nuclear hormone receptor genes were identified in patients showing abnormalities in hormonal response. Here, we present an overview of nuclear receptors and their mutant forms which cause human syndromes or are associated with cancer progression. The major scope of this article is to give an overview on the structural-functional relationship and based on that, to understand the effects of naturally occurring receptor mutants on the molecular level. Thereby, functional aberrations of naturally occurring receptors for androgen, glucocorticoids, mineralocorticoid, estrogen, vitamin D3, retinoic acid, and thyroid hormone as well as the orphan receptor DAX1 are discussed.

The glucocorticoid receptor is associated with the RNA-binding nuclear matrix protein hnRNP U

The glucocorticoid receptor (GR) is a ligand-dependent transcription factor that is able to modulate gene activity by binding to its response element, interacting with other transcription factors, and contacting several accessory proteins such as coactivators. Here we show that GRIP120, one of the factors we have identified to interact with the glucocorticoid receptor, is identical to the heterogeneous nuclear ribonucleoprotein U (hnRNP U), a nuclear matrix protein binding to RNA as well as to scaffold attachment regions. GR.hnRNP U complexes were identified by blotting and coimmunoprecipitation. The subnuclear distribution of GR and hnRNP U was characterized by indirect immunofluorescent labeling and confocal laser microscopy demonstrating a colocalization of both proteins. Using a nuclear transport-deficient deletion of hnRNP U, nuclear translocation was seen to be dependent on GR and dexamethasone. Transient transfections were used to identify possible interaction domains. Overexpressed hnRNP U interfered with glucocorticoid induction, and the COOH-terminal domains of both proteins were sufficient in mediating the transcriptional interference. A possible functional role for this GR binding-protein in addition to its binding to the nuclear matrix, to RNA, and to scaffold attachment regions is discussed.

tau4/tau c/AF-2 of the thyroid hormone receptor relieves silencing of the retinoic acid receptor silencer core independent of both tau4 activation function and full dissociation of corepressors

Members of the thyroid hormone (TR)-retinoic acid receptor (RAR) subfamily of nuclear hormone receptors silence gene expression in the absence of hormone. Addition of cognate ligands leads to dissociation of corepressors, association of coactivators, and transcriptional activation. Here, we used the hRAR alpha silencer core, which encompasses the ligand binding domain, including receptor regions D and E of RAR alpha without the activation function called tau4/tau c/AF-2 and without the F region, to analyze the mechanisms by which transcriptional silencing is relieved. Although the RAR silencer core is able to bind ligand, it acts as a constitutive transcriptional silencer. We have fused various small activation domains to the C terminus of the silencer core and analyzed hormone-dependent changes in receptor function. We show that nine amino acids derived from the hTRbeta are sufficient to transform the RAR silencer core into a hormone-dependent activator. Lengthening the linker between the silencer core and these nine amino acids is not critical for mediating ligand-induced relief of silencing and activation. In addition, we show that a transactivation function at the C terminus is not required for relief of silencing by the hormone, but it is required for transcriptional activation. Furthermore, we created functional silencer fusions which lose their repressive function upon addition of hormone, although the corepressors SMRT and N-CoR remain attached to the receptor.

At least three subdomains of v-erbA are involved in its silencing function

Several members of the thyroid hormone receptor (TR) family are able to switch from a transcriptional repressor to a transcriptional activator upon binding of their ligand. The oncogene v-erbA is a variant form of the TR unable to bind hormone and thus acts as a constitutive repressor. We demonstrate, using fusion proteins between the DNA-binding domain of the yeast factor GAL4 and the silencing domains of v-erbA and TR beta, that point mutations in three different regions severely affect their repression function. Furthermore, the three regions, each as an inactive fusion protein with the GAL4 DNA-binding domain, restore silencing activity when assembled on the same promoter. These observations define at least three silencing subdomains, SSD1-SSD3, which are involved in the silencing function of v-erbA. We propose a model in which full silencing activity is brought about by the combined interaction of each silencing subdomain with corepressors and/or basal transcription factors.

Enhancement of human estrogen receptor activity by SPT6: a potential coactivator

The conserved nature of the transcriptional machinery between yeast and higher eukaryotes makes the yeast system suitable to genetically dissect the signal transduction pathway of steroid hormone receptors. This report describes the yeast protein, SPT6, which modulates the transcriptional activity of the human estrogen receptor (hER) by affecting the C-terminal activation domain. It is demonstrated that SPT6 is able to potentiate hER activity in yeast and also in mammalian cells in vivo. SPT6 interacts specifically with the hormone-binding domain of hER in vivo. The in vivo studies are substantiated by specific protein-protein interactions between SPT6 and the hormone-binding domain of hER in vitro. Therefore, the data suggest that the SPT6 protein may be involved in signal transmission of ER by acting as a coactivator.

The tau 4 activation domain of the thyroid hormone receptor is required for release of a putative corepressor(s) necessary for transcriptional silencing

The C terminus of nuclear hormone receptors is a complex structure that contains multiple functions. We are interested in the mechanism by which thyroid hormone converts its receptor from a transcriptional silencer to an activator of transcription. Both regulatory functions are localized in the ligand binding domain of this receptor superfamily member. In this study, we have identified and characterized several functional domains within the ligand binding domain of the human thyroid hormone receptor (TR beta) conferring transactivation. Interestingly, these domains are localized adjacent to hormone binding sites. One activation domain, designated tau 4, is only 17 amino acids in length and is localized at the extreme C terminus of TR. Deletion of six amino acids of tau 4 resulted in a receptor that could still bind hormone but acted as a constitutive silencer, indicating that tau 4 is required for both transactivation and relief of the silencing functions. In addition, we performed in vivo competition experiments, the results of which suggest that in the absence of tau 4 or hormone, TR is bound by a corepressor protein(s) and that one role of hormone is to release corepressor from the receptor. We propose a general model in which the role of hormone is to induce a conformational change in the receptor that subsequently affects the action of tau 4, leading to both relief of silencing and transcriptional activation.

Interaction of human thyroid hormone receptor beta with transcription factor TFIIB may mediate target gene derepression and activation by thyroid hormone

The human thyroid hormone receptor beta (hTR beta) is capable of both transcriptional silencing and hormone-dependent activation. However, the detailed mechanism of this transcriptional regulation remains to be elucidated. One possibility is that hTR beta interacts directly with factors of the basal transcriptional machinery, thereby modulating basal promoter activity in a direct manner, as has been shown for other transcription factors. Here, we show that hTR beta interacts specifically with the human basal transcription factor TFIIB. Deletion analysis revealed two contact sites in the receptor: one is located in the N terminus, while the other is part of the ligand-binding domain (LBD) and is located at the C terminus. Interestingly, each receptor contact site interacts with different sites in TFIIB. Cotransfection experiments revealed that, when fused to the DNA-binding domain of yeast transcription factor GAL4, the C-terminal interaction site of hTR beta was transcriptionally inactive; however, when it was cotransfected with the remaining part of the LBD on a separate molecule, silencing function was restored. In agreement with that, we show that thyroid hormone is able to significantly decrease the interaction of its receptor LBD with TFIIB. Our data suggest that hTR beta acts as a transcriptional silencer by interacting with TFIIB and that thyroid hormone may act in part by preventing transcriptional repression at this level.

NeP1. A ubiquitous transcription factor synergizes with v-ERBA in transcriptional silencing

One of the chicken lysozyme gene silencers binds two transcription factors, v-ERBA or the thyroid hormone receptor and NeP1 (negative protein 1), a new silencer binding protein. NeP1 is neutral on a monomeric binding site, but mediates weak repression on a multimerized site and strong synergistic repression in conjunction with v-ERBA on the wild-type silencer. Depending on the presence or absence of ligand, synergistic induction or repression is seen with the thyroid hormone receptor. This synergism is not based on cooperative DNA-binding as measured in vitro. The NeP1 DNA-binding activity is dependent on zinc ions, the binding site is characterized by a footprint of approximately 50 bp. NeP1 has a molecular weight of 140 to 160 kDa and has been enriched by affinity columns.

Mechanisms of transcriptional activation by steroid hormone receptors

Steroid hormones are involved in cell growth, development, and differentiation. The hormonal signal is mediated by nuclear receptors which represent a specific class of transcription factors. During the last few years, the cloning of all the major steroid hormone receptors increased our insight into how the hormonal signal converts the receptor into a transcriptional activator. Good progress has been made towards understanding the mechanism of steroid hormone action. In this review we will discuss the role of heat shock proteins in the process of transcriptional activation, the mechanistic differences between the hormone (agonist) and the antihormone (antagonist), the resulting functional consequences, and a possible mode by which transcriptional activation is mediated.

Kindred S thyroid hormone receptor is an active and constitutive silencer and a repressor for thyroid hormone and retinoic acid responses

Mutations in the gene encoding the human thyroid hormone receptor beta (hTR beta) have been associated with generalized thyroid hormone resistance (GTHR). However, the molecular basis by which the receptor mutants cause the clinical symptoms is largely unknown. We show here that the beta form of the human receptor possesses, in addition to hormone-dependent activation, the ability to repress basal-level activity of a target promoter. This silencing function is localized in the carboxyl-terminal part of the receptor and can be transferred to a heterologous DNA binding domain. This mode of silencing is therefore distinct from inhibition by competition with activator proteins on DNA. We show that two receptor mutants isolated from patients with GTHR are impaired in transcriptional activation but fully retain the silencing function, which enforces dominant negative regulation by the receptor. Interestingly, the kindred S receptor (hTR delta 332) acts as a constitutive repressor with a strong silencing ability similar to that of the v-erbA oncogene product. We also provide evidence for distinct transcriptional regulatory properties of both proteins. Finally, we show that both thyroid hormone- and retinoic acid-responsive genes are potentially repressed to generate the clinical manifestations of the GTHR syndrome. Our findings suggest that silencing plays an important role in the phenotypic expression of the symptoms in patients with GTHR.

A transferable silencing domain is present in the thyroid hormone receptor, in the v-erbA oncogene product and in the retinoic acid receptor

Inhibition of gene transcription is brought about by several mechanisms. The least understood mechanism is probably silencing, the analogue to transcriptional enhancing. We provide evidence that the silencing function of the oncogene product v-ERBA or the cellular counterpart, the thyroid hormone receptor (TR, c-erbA) is located in the C-terminal part and is transferable to a heterologous DNA binding domain. Deletion analyses suggest an important role for a basic and hydrophilic amino acid stretch on both ends of the domain. In addition we show that the related retinoic acid receptor (RAR) also contains a functional silencing domain similar in size and amino acid sequence. However, the activity of this domain can be neutralized by an additional domain in the C-terminus which functions cell specifically.

Modular structure of a chicken lysozyme silencer: involvement of an unusual thyroid hormone receptor binding site

Silencer elements, by analogy to enhancer elements, function independently of their position and orientation. We show that the chicken lysozyme silencer S-2.4 kb has many other characteristics in common with enhancer elements. The silencer is comprised of modules that independently repress gene activity--repression being increased synergistically when different or identical modules are combined. Repression is effective both on a complete and on a minimal promoter consisting of a TATA box only. One silencer module is bound in vitro by a 75-93 kd protein, termed NeP1; the other can be bound either by the product of the oncogene v-erbA or by the thyroid hormone receptor. This erbA binding site is unusual in that the palindromic sequence is inverted.

Cooperative interaction of chicken lysozyme enhancer sub-domains partially overlapping with a steroid receptor binding site

Expression of the lysozyme gene is a marker for the differentiation of macrophages, lysozyme transcription being gradually increased during maturation. We have analyzed the fine structure and function of two macrophage-specific enhancer elements of the chicken lysozyme gene (E-2.7 kb and E-0.2 kb). Both increase their activities upon LPS induction, both contain multiple binding sites for similar or identical nuclear factors and both can be divided into two functional modules. For the E-0.2 kb enhancer we found a synergistic activity of the modules to be dependent on their distance. Binding sites for nuclear proteins within enhancer E-0.2 kb overlap substantially with the previously identified progesterone/glucocorticoid receptor binding site, which is required for steroid induction of lysozyme transcription in the oviduct.

Activity of two different silencer elements of the chicken lysozyme gene can be compensated by enhancer elements

The chicken lysozyme gene is constitutively expressed in macrophages. Transfection of recombinant genes containing different portions of the lysozyme 5' upstream region revealed the existence of two negative transcriptional elements within 1 kb upstream of the start sites. Both elements placed upstream or downstream of a heterologous promoter-gene unit repress transcription independent of their orientation and are therefore called silencer elements, although their repressing activities 3' of the gene are reduced. One silencer (N-1.0 kb) at position -1 kb consists of the central region of the chicken middle repetitive sequence element CR1 and can be divided into two functional domains. N-1.0 kb is active in all cell types tested. The other silencer (N-0.25 kb) at position -0.25 kb shows reduced activity in primary macrophages. Despite their different specificities, the activity of both silencer elements can be influenced similarly. An inverse linear relationship between the transcriptional activity of the tested constructs and the potential inhibition by the silencer elements was found: weak transcription units can be strongly repressed, whereas strong transcription units can be only weakly repressed. Such a mechanism may help to turn off completely a particular gene in situations or tissues where strong positive regulators are inactive.

Lysozyme gene activity in chicken macrophages is controlled by positive and negative regulatory elements

The chicken lysozyme gene is constitutively active in macrophages and under the control of steroid hormones in the oviduct. To investigate which DNA elements are involved in the control of its expression in macrophages we performed transient DNA transfer experiments with two different types of plasmids: 5'-deletion mutants of the upstream region of the chicken lysozyme gene and different fragments from this area in front of the thymidine kinase promoter (herpes simplex virus), each placed in front of the CAT (chloramphenicol acetyl transferase) coding sequence. Two enhancers (E-2.7 kb and E-0.2 kb) were characterized. They are active in macrophages, but not in chicken fibroblasts. Furthermore a negative element (N-2.4 kb) was identified, which is active in fibroblasts and promyelocytes, but not in mature macrophages. The combined action of all three elements contributes to the observed lysozyme gene activities: no activity in fibroblasts, moderate activity in promyelocytes and high activity in mature macrophages.