Purification and identification of p68 RNA helicase acting as a transcriptional coactivator specific for the activation function 1 of human estrogen receptor alpha

ATPase/helicase activities of p68 RNA helicase are required for pre-mRNA splicing but not for assembly of the spliceosome

We have previously demonstrated that p68 RNA helicase, as an essential human splicing factor, acts at the U1 snRNA and 5' splice site (5'ss) duplex in the pre-mRNA splicing process. To further analyze the function of p68 in the spliceosome, we generated two p68 mutants (motif V, RGLD to LGLD, and motif VI, HRIGR to HLIGR). ATPase and RNA unwinding assays demonstrated that the mutations abolished the RNA-dependent ATPase activity and RNA unwinding activity. The function of p68 in the spliceosome was abolished by the mutations, and the mutations also inhibited the dissociation of U1 from the 5'ss, while the mutants still interacted with the U1-5'ss duplex. Interestingly, the nonactive p68 mutants did not prevent the transition from prespliceosome to the spliceosome. The data suggested that p68 RNA helicase might actively unwind the U1-5'ss duplex. The protein might also play a role in the U4.U6/U5 addition, which did not require the ATPase and RNA unwinding activities of p68. In addition, we present evidence here to demonstrate the functional role of p68 RNA helicase in the pre-mRNA splicing process in vivo. Our experiments also showed that p68 interacted with unspliced but not spliced mRNA in vivo.

Growth factors change nuclear distribution of estrogen receptor-alpha via mitogen-activated protein kinase or phosphatidylinositol 3-kinase cascade in a human breast cancer cell line

In the present study, to examine the dynamic changes in the localization of nuclear estrogen receptor (ER)alpha induced by growth factors, we used time-lapse confocal microscopy to directly visualized ERalpha fused with green fluorescent protein (GFP-ERalpha) in single living cells treated with epidermal growth factor (EGF) or IGF-I. We observed that 17beta-estradiol (E2) changed the normally diffuse distribution of GFP-ERalpha throughout the nucleoplasm to a hyperspeckled distribution within 10 min. Both EGF and IGF-I also changed the nuclear distribution of GFP-ERalpha, similarly to E2 treatment. However, the time courses of the nuclear redistribution of GFP-ERalpha induced by EGF or IGF-I were different from that induced by E2 treatment. In the EGF-treated cells, the GFP-ERalpha nuclear redistribution was observed at 30 min and reached a maximum at 60 min, whereas in the IGF-I-treated cells, the GFP-ERalpha nuclear redistribution was observed at 60 min and reached a maximum at 90 min. The EGF-induced redistribution of GFP-ERalpha was blocked by pretreatment with a MAPK cascade inhibitor, PD98059, whereas the IGF-I-induced redistribution of GFP-ERalpha was blocked by pretreatment with a phosphatidylinositol 3-kinase inhibitor, LY294002. Analysis using an activation function-2 domain deletion mutant of GFP-ERalpha showed that the change in the distribution of GFP-ERalpha was not induced by E2, EGF, or IGF-I treatment. These data suggest that MAPK and phosphatidylinositol 3-kinase cascades are involved in the nuclear redistribution of ERalpha by EGF and IGF-I, respectively, and that the activation function-2 domain of ERalpha may be needed for the nuclear redistribution of ERalpha.

RNA helicases: regulators of differentiation

RNA helicases are highly conserved enzymes that utilize the energy derived from NTP hydrolysis to modulate the structure of RNA. RNA helicases participate in all biological processes that involve RNA, including transcription, splicing and translation. Based on the sequence of the helicase domain, they are classified into families, such as DDX and DHX families of human RNA helicases. The specificity of RNA helicases to their targets is likely due to several factors, such as the sequence, interacting molecules, subcellular localization and the expression pattern of the helicases. There are several examples of the involvement of RNA helicases in differentiation. Human DDX3 has two closely related genes designated DDX3Y and DDX3X, which are localized to the Y and X chromosomes, respectively. DDX3Y protein is specifically expressed in germ cells and is essential for spermatogenesis. DDX25 is another RNA helicase which has been shown to be required for spermatogenesis. DDX4 shows specific expression in germ cells. The Drosophila ortholog of DDX4, known as vasa, is required for the formation of germ cells and oogenesis by a mechanism that involves regulating the translation of mRNAs essential for differentiation. Abstrakt is the Drosphila ortholog of DDX41, which has been shown to be involved in visual and CNS system development. DDX5 (p68) and its related DDX17 (p72) have also been implicated in organ/tissue differentiation. The ability of RNA helicases to modulate the structure and thus availability of critical RNA molecules for processing leading to protein expression is the likely mechanism by which RNA helicases contribute to differentiation.

Aryl hydrocarbon receptor-mediated transcription: ligand-dependent recruitment of estrogen receptor alpha to 2,3,7,8-tetrachlorodibenzo-p-dioxin-responsive promoters

Using chromatin immunoprecipitation assays, we studied the 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-mediated recruitment of the aryl hydrocarbon receptor (AhR) and several co-regulators to the CYP1A1 promoter. AhR displayed a time-dependent recruitment, reaching a peak at 75 min and maintaining promoter occupancy for the remainder of the time course. Recruitment of AhR was followed by TIF2/SRC2, which preceded CBP, histone H3 acetylation, and RNA polymerase II (RNAPII). Simultaneous recruitment to the enhancer and the TATA box region suggests the formation of a large multiprotein complex bridging the two promoter regions. Interestingly, estrogen receptor alpha (ERalpha) displayed a TCDD- and time-dependent recruitment to the CYP1A1 promoter, which was increased by co-treatment with estradiol. Transfection in HuH7 human liver cells confirmed previously reported ERalpha enhancement of AhR activity. In contrast, TCDD did not induce the recruitment of ERalpha to the estrogen-responsive pS2 promoter, and after 120 min of co-treatment with estradiol, ERalpha is still present on the CYP1A1 promoter but no longer at pS2. RNA interference studies with T47D cells support a role for ERalpha in TCDD-dependent CYP1A1 expression. Our data suggest that ERalpha acts as a coregulator of AhR-mediated transcriptional activation and that the recruitment of ERalpha by AhR represents a novel mechanism AhR-ERalpha cross talk.

Recruitment of histone deacetylase 4 to the N-terminal region of estrogen receptor alpha

Transcriptional activation of estrogen receptor alpha (ERalpha) is regulated by the ligand-dependent activation function 2 and the constitutively active N-terminal activation function 1. To identify ERalpha N-terminal-specific coregulators, we screened a breast cDNA library by T7 phage display and isolated histone deacetylase 4 (HDAC4). HDAC4 interacts with the ERalpha N terminus both in vitro and in vivo. Presence of the ERalpha DNA binding domain and hinge region reduce HDAC4 recruitment whereas full-length ERalpha enhances recruitment. HDAC4 interaction is selective for the ERalpha and not ERbeta N terminus and occurs in the nucleus. We demonstrate in vivo that HDAC4 is recruited by the N terminus to the promoter of an endogenous estrogen responsive gene. HDAC4 suppresses transcriptional activation of ERalpha by estrogen and selective ER modulators (SERMs) such as tamoxifen in a cell type-dependent manner. Consistently, silencing of HDAC4 promotes the agonist effect of SERMs (tamoxifen and raloxifene) in a cell type-specific manner. These findings indicate a role for HDAC4 in regulating ERalpha activity as a novel N-terminal coregulator and uncover a mechanism by which certain cell types regulate SERM behavior.

Identification of the human DEAD-box protein p68 as a substrate of Tlk1

The activity of the human protein kinase Tlk1 is down-regulated within minutes after exposure of cells to ionizing radiation. In order to identify signaling pathways which might be relevant in the radiation response of mammalian cells we screened nuclear proteins for substrates of Tlk1. Among several proteins one could be identified as p68 RNA helicase. Furthermore, it could be shown that Tlk1 phosphorylates immunoprecipitated p68. The phosphorylation of the C-terminal fragment of p68 by rTlk1 reduced its affinity to single stranded RNA in a gel shift assay. In addition, it could be demonstrated that increasing the Tlk1 activity in HT1080 cells by forced Tlk1 overexpression leads to an increased phosphorylation of endogenous p68, arguing that p68 might be a physiological substrate of Tlk1. These findings open the possibility that Tlk1 might participate in diverse biologic functions like cell growth and differentiation, pre-mRNA splicing, and transcriptional coactivation.

SPBP is a phosphoserine-specific repressor of estrogen receptor alpha

Multiple signaling pathways stimulate the activity of estrogen receptor alpha (ERalpha) by direct phosphorylation within its N-terminal activation function 1 (AF1). How phosphorylation affects AF1 activity remains poorly understood. We performed a phage display screen for human proteins that are exclusively recruited to the phosphorylated form of AF1 and found the stromelysin-1 platelet-derived growth factor-responsive element-binding protein (SPBP). In a purified system, SPBP bound only the in vitro-phosphorylated form of the ERalpha AF1 or the phosphoserine mimic S118E, and the interaction domain could be mapped to a 42-amino-acid fragment of SPBP. In cells, SPBP preferentially interacted with liganded and phosphorylated ERalpha. Functionally, SPBP behaved as a repressor of activated ERalpha, which extends its previously demonstrated roles as a DNA binding transactivation factor and coactivator of other transcription factors. By targeting the phosphorylated form of AF1, SPBP may contribute to attenuating and fine-tuning ERalpha activity. A functional consequence is that SPBP inhibits the proliferation of ERalpha-dependent but not ERalpha-independent breast cancer cell lines, mirroring a reported negative correlation with the ERalpha status of breast tumors.

Splicing potentiation by growth factor signals via estrogen receptor phosphorylation

Mitogen-activated protein kinase-mediated growth factor signals are known to augment the ligand-induced transactivation function of nuclear estrogen receptor alpha (ERalpha) through phosphorylation of Ser-118 within the ERalpha N-terminal transactivation (activation function-1) domain. We identified the spliceosome component splicing factor (SF)3a p120 as a coactivator specific for human ERalpha (hERalpha) activation function-1 that physically associated with ERalpha dependent on the phosphorylation state of Ser-118. SF3a p120 potentiated hERalpha-mediated RNA splicing, and notably, the potentiation of RNA splicing by SF3a p120 depended on hER Ser-118 phosphorylation. Thus, our findings suggest a mechanism by which growth factor signaling can regulate gene expression through the modulation of RNA splicing efficiency via phosphorylation of sequence-specific activators, after association between such activators and the spliceosome.

Lessons in estrogen biology from knockout and transgenic animals

Tremendous progress has been made in elucidating numerous critical aspects of estrogen signaling. New tools and techniques have enabled detailed molecular analysis of components that direct estrogen responses. At the other end of the spectrum, generation of a multiplicity of transgenic animals has allowed analysis of the physiological roles of the estrogen-signaling components in biologically relevant models. Here, we review the ever-increasing body of knowledge in the field of estrogen biology, especially as applied to the female reproductive processes.

Vinexin beta interacts with the non-phosphorylated AF-1 domain of retinoid receptor gamma (RARgamma) and represses RARgamma-mediated transcription

Nuclear retinoic acid receptors (RARs) are ligand-dependent transcription factors that regulate the expression of retinoic acid target genes. Although the importance of RAR phosphorylation in their N-terminal domain is clearly established, the underlying mechanism for the phosphorylation-dependent transcriptional activity of the receptors had not been elucidated yet. Here, using a yeast two-hybrid system, we report the isolation of vinexin beta as a new cofactor that interacts with the N-terminal A/B domain of the RARgamma isotype. Vinexin beta is a multiple SH3 motif-containing protein associated with the cytoskeleton and also present in the nucleus. We demonstrate that vinexin beta colocalizes with RARgamma in the nucleus and interacts with the non-phosphorylated form of the AF-1 domain of RARgamma. We also show that this interaction is prevented upon phosphorylation of the AF-1 domain. Using F9 cells stably overexpressing vinexin beta or vinexin knockdown by RNA interference, we demonstrate that vinexin beta is an inhibitor of RARgamma-mediated transcription. We propose a model in which phosphorylation of the AF-1 domain controls RARgamma-mediated transcription through triggering the dissociation of vinexin beta.

Bacterially expressed recombinant p68 RNA helicase is phosphorylated on serine, threonine, and tyrosine residues

We previously reported the expression and purification of recombinant p68 RNA helicase in a bacterial expression system. The recombinant p68 is an RNA-dependent ATPase and ATP-dependent RNA helicase. In the process of characterizing the ATPase and RNA unwinding activities of the recombinant p68, we observed that the bacterially expressed p68 RNA helicase is phosphorylated on tyrosine, serine, and threonine residues. Our data demonstrated that phosphorylations on the recombinant p68 RNA helicase affect the enzymatic activities of the protein. This is the first observation that recombinant protein expressed in bacteria Escherichia coli is phosphorylated at multiple residues by bacterial endogenous protein kinases. Our observations suggest an important mechanism in controlling the function of p68 RNA helicase by signal transduction pathways.

Phosphorylation of androgen receptor isoforms

Phosphorylation of the human AR (androgen receptor) is directly correlated with the appearance of at least three AR isoforms on an SDS/polyacrylamide gel. However, it is still not clear to what extent phosphorylation is involved in the occurrence of isoforms, which sites are phosphorylated and what are the functions of these phosphosites. The human AR was expressed in COS-1 cells and AR phosphorylation was studied further by mutational analyses and by using reversed-phase HPLC and MS. The reversed-phase HPLC elution pattern of the three isoforms revealed that Ser-650 was phosphorylated constitutively. After de novo synthesis, only Ser-650 was phosphorylated in the smallest isoform of 110 kDa and both Ser-650 and Ser-94 were phosphorylated in the second isoform of 112 kDa. The hormone-induced 114 kDa isoform shows an overall increase in phosphorylation of all the isolated peptides. The activities of the Ser-Ala substitution mutant S650A (Ser-650-->Ala) was found to be identical with wild-type AR activation in four different cell lines and three different functional analyses, e.g. transactivation, N- and C-terminal-domain interaction and co-activation by transcriptional intermediary factor 2. This was also found for mutants S94A and S515A with respect to transactivation. However, the S515A mutation, which should eliminate phosphorylation of the potential mitogen-activated protein kinase site, Ser-515, resulted in an unphosphorylated form of the peptide containing Ser-650. This suggests that Ser-515 can modulate phosphorylation at another site. The present study shows that the AR isoform pattern from AR de novo synthesis is directly linked to differential phosphorylation of a distinct set of sites. After mutagenesis of these sites, no major change in functional activity of the AR was observed.

Selective oestrogen receptor modulators in desmoid tumours

Selective oestrogen receptor modulators (SERMs) are compounds with a mixed agonist/antagonist activity on oestrogen receptors. An ideal SERM is a compound with an oestrogen antagonist effect on the breast and uterus but oestrogen agonist effect on bone. Beside tamoxifen, a group of well-investigated SERMs is represented by raloxifene, LY-353381 (SERM3), EM-800 and CP-336156. On an empirical basis, tamoxifen has been used to pharmacologically treat desmoid tumours. Recently, raloxifene, a second-generation SERM, has been used in the treatment of familial adenomatous polyposis patients affected by desmoid tumour. The mechanisms through which these molecules affect desmoid tumour growth appear to be due, in part, to the fact that SERMs may act independently of oestrogen receptors. The knowledge of the molecular basis of SERM action will make the development of novel synthetic compounds with engineered tissue selectivity possible.

The DEAD box protein p68: a novel transcriptional coactivator of the p53 tumour suppressor

The DEAD box RNA helicase, p68, has been implicated in various cellular processes and has been shown to possess transcriptional coactivator function. Here, we show that p68 potently synergises with the p53 tumour suppressor protein to stimulate transcription from p53-dependent promoters and that endogenous p68 and p53 co-immunoprecipitate from nuclear extracts. Strikingly, RNAi suppression of p68 inhibits p53 target gene expression in response to DNA damage, as well as p53-dependent apoptosis, but does not influence p53 stabilisation or expression of non-p53-responsive genes. We also show, by chromatin immunoprecipitation, that p68 is recruited to the p21 promoter in a p53-dependent manner, consistent with a role in promoting transcriptional initiation. Interestingly, p68 knock-down does not significantly affect NF-kappaB activation, suggesting that the stimulation of p53 transcriptional activity is not due to a general transcription effect. This study represents the first report of the involvement of an RNA helicase in the p53 response, and highlights a novel mechanism by which p68 may act as a tumour cosuppressor in governing p53 transcriptional activity.

Selective hormone-dependent repression of estrogen receptor beta by a p38-activated ErbB2/ErbB3 pathway

Deregulated signaling of ErbB2 receptor tyrosine kinase is often associated with hormone resistance in estrogen receptor alpha (ERalpha)-positive breast cancers, establishing a relationship between ErbB2 and ERalpha pathways. Although ERalpha and ERbeta are expressed in many breast cancer cells, the response of ERbeta to ErbB2 signaling is less well defined. In the present study, we demonstrate that ERbeta activity can be modulated by ErbB2 signaling in ER-expressing breast cancer cells. The estrogen-dependent transcriptional activity of ERbeta was altered in a manner similar to ERalpha by either activation of ErbB2/ErbB3 signaling by growth factor heregulin beta or expression of a constitutively active mutant of ErbB2. However, as opposed to ERalpha, the p38 MAPK pathway was found to be involved in liganded ERbeta repression activity by ErbB2 signaling and in regulating estrogen-responsive promoter occupancy by ERbeta. The repression in ERbeta response to hormone was dependent upon its AF-1 domain which includes serines 106 and 124, two phosphorylation target sites for Erk that we previously showed to be involved in SRC-1 recruitment to ERbeta. Substitution of these two serines by aspartic acid residues abolished the repression of ERbeta by activated ErbB2/ErbB3. Moreover, expression of SRC-1 also relieved the inhibition of ERbeta in heregulin-treated cells. Our study demonstrates a functional coupling between ERbeta and ErbB receptors and outlines the differential role of the AF-1 region in the regulation of the estrogen-dependent cell growth and activity of both estrogen receptors in response to growth factor signaling.

Functional interaction between Smad, CREB binding protein, and p68 RNA helicase

The transforming growth factors beta control a diversity of biological processes including cellular proliferation, differentiation, apoptosis, and extracellular matrix production, and are critical effectors of embryonic patterning and development, including that of the orofacial region. TGFbeta superfamily members signal through specific cell surface receptors that phosphorylate the cytoplasmic Smad proteins, resulting in their translocation to the nucleus and interaction with promoters of TGFbeta-responsive genes. Subsequent alterations in transcription are cell type-specific and dependent on recruitment to the Smad/transcription factor complex of coactivators, such as CBP and p300, or corepressors, such as c-ski and SnoN. Since the affinity of Smads for DNA is generally low, additional accessory proteins that facilitate Smad/DNA binding are required, and are often cell- and tissue-specific. In order to identify novel Smad 3 binding proteins in developing orofacial tissue, a yeast two hybrid assay was employed in which the MH2 domain of Smad 3 was used to screen an expression library derived from mouse embryonic orofacial tissue. The RNA helicase, p68, was identified as a unique Smad binding protein, and the specificity of the interaction was confirmed through various in vitro and in vivo assays. Co-expression of Smad 3 and a CBP-Gal4 DNA binding domain fusion protein in a Gal4-luciferase reporter assay resulted in increased TGFbeta-stimulated reporter gene transcription. Moreover, co-expression of p68 RNA helicase along with Smad 3 and CBP-Gal4 resulted in synergistic activation of Gal4-luciferase reporter expression. Collectively, these data indicate that the RNA helicase, p68, can directly interact with Smad 3 resulting in formation of a transcriptionally active ternary complex containing Smad 3, p68, and CBP. This offers a means of enhancing TGFbeta-mediated cellular responses in developing orofacial tissue.

Estrogen receptor mutations in human disease

As early as the 1800s, the actions of estrogen have been implicated in the development and progression of breast cancer. The estrogen receptor (ER) was identified in the late 1950s and purified a few years later. However, it was not until the 1980s that the first ER was molecularly cloned, and in the mid 1990s, a second ER was cloned. These two related receptors are now called ERalpha and ERbeta, respectively. Since their discovery, much research has focused on identifying alterations within the coding sequence of these receptors in clinical samples. As a result, a large number of naturally occurring splice variants of both ERalpha and ERbeta have been identified in normal epithelium and diseased or cancerous tissues. In contrast, only a few point mutations have been identified in human patient samples from a variety of disease states, including breast cancer, endometrial cancer, and psychiatric diseases. To elucidate the mechanism of action for these variant isoforms or mutant receptors, experimental mutagenesis has been used to analyze the function of distinct amino acid residues in the ERs. This review will focus on ERalpha and ERbeta alterations in breast cancer.

The Src kinase pathway promotes tamoxifen agonist action in Ishikawa endometrial cells through phosphorylation-dependent stabilization of estrogen receptor (alpha) promoter interaction and elevated steroid receptor coactivator 1 activity

Tamoxifen is the most widely used selective estrogen receptor modulator for breast cancer in clinical use today. However, tamoxifen agonist action in endometrium remains a major hurdle for tamoxifen therapy. Activation of the nonreceptor tyrosine kinase src promotes tamoxifen agonist action, although the mechanisms remain unclear. To examine these mechanisms, the effect of src kinase on estrogen and tamoxifen signaling in tamoxifen-resistant Ishikawa endometrial adenocarcinoma cells was assessed. A novel connection was identified between src kinase and serine 167 phosphorylation in estrogen receptor (ER)-alpha via activation of AKT kinase. Serine 167 phosphorylation stabilized ER interaction with endogenous ER-dependent promoters. Src kinase exhibited the additional function of potentiating the transcriptional activity of Gal-steroid receptor coactivator 1 (SRC-1) and Gal-cAMP response element binding protein-binding protein in endometrial cancer cells while having no effect on Gal-p300-associated factor and Gal fusions of the other p160 coactivators glucocorticoid-interacting protein 1 (transcriptional intermediary factor 2/nuclear coactivator-2/SRC-2) and amplified in breast cancer 1 (receptor-associated coactivator 3/activator of transcription of nuclear receptor/SRC-3). Src effects on ER phosphorylation and SRC-1 activity both contributed to tamoxifen agonist action on ER-dependent gene expression in Ishikawa cells. Taken together, these data demonstrate that src kinase potentiates tamoxifen agonist action through serine 167-dependent stabilization of ER promoter interaction and through elevation of SRC-1 and cAMP response element binding protein-binding protein coactivation of ER.

Analysis of estrogen receptor α signaling complex at the plasma membrane

There is accumulating evidence that the estrogen receptor (ER) can transduce specific signals at the plasma membrane. We tried to clarify the biological function of ER as a signaling molecule by identifying proteins that interact with the membrane-localized ER. The activation function 1 and 2 (AF-1 and AF-2) domains of ERalpha with or without the membrane-targeting sequence were stably expressed in the breast cancer cell line, MCF-7. The level of tyrosine phosphorylation of AF-2 was significantly elevated by the membrane localization. By mass-spectrometry analysis, alpha- and beta-tubulins and heat shock protein 70 were identified as the AF-1-associated proteins. Of these, tubulins are associated only with membrane-targeted AF-1.

Retracted: In vivopotentiation of human oestrogen receptor α by Cdk7-mediated phosphorylation

Phosphorylation of the Ser(118) residue in the N-terminal A/B domain of the human oestrogen receptor alpha (hERalpha) by mitogen-activated protein kinase (MAPK), stimulated via growth factor signalling pathways, is known to potentiate ERalpha ligand-induced transactivation function. Besides MAPK, cyclin dependent kinase 7 (Cdk7) in the TFIIH complex has also been found to potentiate hERalpha transactivation in vitro through Ser(118) phosphorylation. To investigate an impact of Cdk7 on hERalpha transactivation in vivo, we assessed activity of hERalpha in a wild-type and cdk7 inactive mutant Drosophila that ectopically expressed hERalpha in the eye disc. Ectopic expression of the wild-type or mutant receptors, together with a green fluorescent protein (GFP) reporter gene, allowed us to demonstrate that hERalpha expressed in the fly tissues was transcriptionally functional and adequately responded to hERalpha ligands in the patterns similar to those observed in mammalian cells. Replacement of Ser(118) with alanine in hERalpha (S118A mutant) significantly reduced the ligand-induced hERalpha transactivation function. Importantly, while in cdk7 inactive mutant Drosophila the wild-type hERalpha exhibited reduced response to the ligand; levels of transactivation by the hERalpha S118A mutant were not affected in these inactive cdk7 mutant flies. Furthermore, phosphorylation of hERalpha at Ser(118) has been observed in vitro by both human and Drosophila Cdk7. Our findings demonstrate that Cdk7 is involved in regulation of the ligand-induced transactivation function of hERalphain vivo via Ser(118) phosphorylation.

RNA helicase A interacts with nuclear factor κB p65 and functions as a transcriptional coactivator

RNA helicase A (RHA), a member of DNA and RNA helicase family containing ATPase activity, is involved in many steps of gene expression such as transcription and mRNA export. RHA has been reported to bind directly to the transcriptional coactivator, CREB-binding protein, and the tumor suppressor protein, BRCA1, and links them to RNA Polymerase II holoenzyme complex. Using yeast two-hybrid screening, we have identified RHA as an interacting molecule of the p65 subunit of nuclear factor kappaB (NF-kappaB). The interaction between p65 and RHA was confirmed by glutathione-S transferase pull-down assay in vitro, and by co-immunoprecipitation assay in vivo. In transient transfection assays, RHA enhanced NF-kappaB dependent reporter gene expression induced by p65, tumor necrosis factor-alpha, or NF-kappaB inducing kinase. The mutant form of RHA lacking ATP-binding activity inhibited NF-kappaB dependent reporter gene expression induced by these activators. Moreover, depletion of RHA using short interfering RNA reduced the NF-kappaB dependent transactivation. These data suggest that RHA is an essential component of the transactivation complex by mediating the transcriptional activity of NF-kappaB.

The p68 and p72 DEAD box RNA helicases interact with HDAC1 and repress transcription in a promoter-specific manner

Background

p68 (Ddx5) and p72 (Ddx17) are highly related members of the DEAD box family and are established RNA helicases. They have been implicated in growth regulation and have been shown to be involved in both pre-mRNA and pre-rRNA processing. More recently, however, these proteins have been reported to act as transcriptional co-activators for estrogen-receptor alpha (ERα). Furthermore these proteins were shown to interact with co-activators p300/CBP and the RNA polymerase II holoenzyme. Taken together these reports suggest a role for p68 and p72 in transcriptional activation.

Results

In this report we show that p68 and p72 can, in some contexts, act as transcriptional repressors. Targeting of p68 or p72 to constitutive promoters leads to repression of transcription; this repression is promoter-specific. Moreover both p68 and p72 associate with histone deacetylase 1 (HDAC1), a well-established transcriptional repression protein.

Conclusions

It is therefore clear that p68 and p72 are important transcriptional regulators, functioning as co-activators and/or co-repressors depending on the context of the promoter and the transcriptional complex in which they exist.

Do human RNA helicases have a role in cancer?

Human RNA helicases (HRH) represent a large family of enzymes that play important roles in RNA processing. The biochemical characteristics and biological functions of the majority of HRH are still to be determined. However, there are examples of dysregulation of HRH expression in various types of cancer. In addition, some HRH have been shown to be involved in the regulation of, or the molecular interaction with, molecules implicated in cancer. Other helicases take part in fusion transcripts resulting from cancer-associated chromosomal translocation. These findings raise the question of whether HRH can contribute to cancer development/progression. In this review, I summarize the cancer-related features of HRH.

The Relative Contribution Exerted by AF-1 and AF-2 Transactivation Functions in Estrogen Receptor α Transcriptional Activity Depends upon the Differentiation Stage of the Cell

The activity of the transactivation functions (activation function (AF)-1 and AF-2) of the estrogen receptor alpha (ERalpha) is cell-specific. This study aimed to decipher the yet unclear mechanisms involved in this differential cell sensitivity, with particular attention to the specific influence that cell differentiation may have on these processes. Hence, we comparatively evaluated the permissiveness of cells to either ERalpha AFs in two different cases: (i) a series of cell lines originating from a common tissue, but with distinct differentiation phenotypes; and (ii) cell lines that undergo differentiation processes in culture. These experiments demonstrate that the respective contribution that AF-1 and AF-2 make toward ERalpha activity varies in a cell differentiation stage-dependent manner. Specifically, whereas AF-1 is the dominant AF involved in ERalpha transcriptional activity in differentiated cells, the more a cell is de-differentiated the more this cell mediates ERalpha signaling through AF-2. For instance, AF-2 is the only active AF in cells that have achieved their epithelial-mesenchymal transition. Moreover, the stable expression of a functional ERalpha in strictly AF-2 permissive cells restores an AF-1-sensitive cell context. These results, together with data obtained in different ERalpha-positive cell lines tested strongly suggest that the transcriptional activity of ERalpha relies on its AF-1 in most estrogen target cell types.

Cellular RNA helicase p68 relocalization and interaction with the hepatitis C virus (HCV) NS5B protein and the potential role of p68 in HCV RNA replication

Chronic infection by hepatitis C virus (HCV) can lead to severe hepatitis and cirrhosis and is closely associated with hepatocellular carcinoma. The replication cycle of HCV is poorly understood but is likely to involve interaction with host factors. In this report, we show that NS5B, the HCV RNA-dependent RNA polymerase (RdRp), interacts with a human RNA helicase, p68. Transient expression of NS5B alone, as well as the stable expression of all the nonstructural proteins in a HCV replicon-bearing cell line (V. Lohmann, F. Korner, J.-O. Koch, U. Herian, L. Theilmann, and R. Bartenschlager, Science 285:110-113), causes the redistribution of endogenous p68 from the nucleus to the cytoplasm. Deletion of the C-terminal two-thirds of NS5B (NS5BDeltaC) dramatically reduces its coimmunoprecipitation (co-IP) with endogenous p68, while the deletion of the N-terminal region (NS5BDeltaN1 and NS5BDeltaN2) does not affect its interaction with p68. In consistency with the co-IP results, NS5BDeltaC does not cause the relocalization of p68 whereas NS5BDeltaN1 does. With a replicon cell line, we were not able to detect a change in positive- and negative-strand synthesis when p68 levels were reduced using small interfering RNA (siRNA). In cells transiently transfected with a full-length HCV construct, however, the depletion (using specific p68 siRNA) of endogenous p68 correlated with a reduction in the transcription of negative-strand from positive-strand HCV RNA. Overexpression of NS5B and NS5BDeltaN1, but not that of NS5BDeltaC, causes a reduction in the negative-strand synthesis, indicating that overexpressed NS5B and NS5BDeltaN1 sequesters p68 from the replication complexes (thus reducing their replication activity levels). Identification of p68 as a cellular factor involved in HCV replication, at least for cells transiently transfected with a HCV expression construct, is a step towards understanding HCV replication.

Selective estrogen receptor modulator regulated proteins in endometrial cancer cells

Tamoxifen is the primary hormonal therapy for breast cancer and is also used as a breast cancer chemopreventative agent. A major problem with tamoxifen therapy is undesirable endometrial proliferation. To identify proteins associated with the growth stimulatory effects of tamoxifen in an ER-positive model, the present study profiled total cellular and secreted proteins regulated by estradiol and selective estrogen receptor modifiers (SERMs) in the Ishikawa endometrial adenocarcinoma cell line using two-dimensional gel electrophoresis. Following 24 h incubation with 10(-8) M estradiol, 10(-7) M 4-hydroxytamoxifen, or 10(-7) M EM-652 (Acolbifene), nine proteins exhibited significant increase in expression. The proteins identified were heat shock protein 90-alpha, and -beta, heterogeneous nuclear ribonucleoprotein F, RNA polymerase II-mediating protein, cytoskeletal keratin 8, cytoskeletal keratin 18, ubiquitin-conjugating enzyme E2-18 kDa and nucleoside diphosphate kinase B. These protein profiles may serve as novel indices of SERM response and may also provide insight into novel mechanisms of SERM-mediated growth.

The biological role of estrogen receptors α and β in cancer

The temporal and tissue-specific actions of estrogen are mediated by estrogen receptors alpha and beta. The ERs are steroid hormone receptors that modulate the transcription of target genes when bound to ligand. The activity of these transcription factors is regulated by a variety of factors, including ligand binding, phosphorylation, coregulators, and the effector pathway (ERE, AP1, SP1). The end result of target gene transcription is to modulate physiological processes, such as reproductive organ development and function, bone density, and unfortunately contribute to the growth and development of breast and endometrial cancer. The complex biological effects mediated by ER alpha and ER beta involve communication between many proteins and signaling pathways. An ultimate goal of current research is to enhance the value of the separate estrogen receptors as targets for therapeutic intervention.

Phosphorylation of p68 RNA helicase regulates RNA binding by the C-terminal domain of the protein

We previously reported ATPase, RNA unwinding, and RNA-binding activities of recombinant p68 RNA helicase that was expressed in Escherichia coli. Huang et al. The recombinant protein bound both single-stranded (ss) and double-stranded (ds) RNAs. To further characterize the substrate RNA binding by p68 RNA helicase, we expressed and purified the recombinant N-terminal and C-terminal domains of the protein. RNA-binding property and protein phosphorylation of the recombinant domains of p68 were analyzed. Our data demonstrated that the C-terminal domain of p68 RNA helicase bound ssRNA. More interestingly, the C-terminal domain was a target of protein kinase C (PKC). Phosphorylation of the C-terminal domain of p68 abolished its RNA binding. Based on our observations, we propose that the C-terminal domain is an RNA substrate binding site for p68. The protein phosphorylation by PKC regulates the RNA binding of p68 RNA helicase, which consequently controls the enzymatic activities of the protein.

Increases in estrogen receptor-alpha concentration in breast cancer cells promote serine 118/104/106-independent AF-1 transactivation and growth in the absence of estrogen

A common phenotype in breast cancer is the expansion of the estrogen receptor-alpha (ER+) cell population and an inappropriate elevation of ERalpha protein, the latter predisposing patients for a poorer prognosis than those with lower levels of the receptor. A tetracycline-inducible ERalpha overexpression model was developed in the MCF-7 cell line to assess induction of endogenous gene activation and growth in response to elevations in ERalpha protein. Heightened levels of ERalpha resulted in aberrant promoter occupancy and gene activation in the absence of hormone, which was independent of ligand and AF-2 function. This increased receptor activity required the amino-terminal A/B domain and was not inhibited by tamoxifen, which supports an enhancement of AF-1 function, yet was independent of serine-104, 106, and 118 phosphorylation. Ligand-independent transcription was accompanied by an increase in growth in the absence of hormonal stimulation. The results suggest that elevated levels of ERalpha in breast cancer cells can result in activation of receptor transcriptional function in a manner distinct from classical mechanisms that involve ligand binding or growth factor-induced phosphorylation. Further, they describe a potential mechanism whereby increases in ERalpha concentration may provide a proliferative advantage by augmenting ERalpha function regardless of ligand status.

Estrogen receptor-alpha directs ordered, cyclical, and combinatorial recruitment of cofactors on a natural target promoter

Transcriptional activation of a gene involves an orchestrated recruitment of components of the basal transcription machinery and intermediate factors, concomitant with an alteration in local chromatin structure generated by posttranslational modifications of histone tails and nucleosome remodeling. We provide here a comprehensive picture of events resulting in transcriptional activation of a gene, through evaluating the estrogen receptor-alpha (NR3A1) target pS2 gene promoter in MCF-7 cells. This description integrates chromatin remodeling with a kinetic evaluation of cyclical networks of association of 46 transcription factors with the promoter, as determined by chromatin immunoprecipitation assays. We define the concept of a "transcriptional clock" that directs and achieves the sequential and combinatorial assembly of a transcriptionally productive complex on a promoter. Furthermore, the unanticipated findings of key roles for histone deacetylases and nucleosome-remodeling complexes in limiting transcription implies that transcriptional activation is a cyclical process that requires both activating and repressive epigenetic processes.

Coregulator function: a key to understanding tissue specificity of selective receptor modulators

Ligands for the nuclear receptor superfamily control many aspects of biology, including development, reproduction, and homeostasis, through regulation of the transcriptional activity of their cognate receptors. Selective receptor modulators (SRMs) are receptor ligands that exhibit agonistic or antagonistic biocharacter in a cell- and tissue context-dependent manner. The prototypical SRM is tamoxifen, which as a selective estrogen receptor modulator, can activate or inhibit estrogen receptor action. SRM-induced alterations in the conformation of the ligand-binding domains of nuclear receptors influence their abilities to interact with other proteins, such as coactivators and corepressors. It has been postulated, therefore, that the relative balance of coactivator and corepressor expression within a given target cell determines the relative agonist vs. antagonist activity of SRMs. However, recent evidence reveals that the cellular environment also plays a critical role in determining SRM biocharacter. Cellular signaling influences the activity and subcellular localization of coactivators and corepressors as well as nuclear receptors, and this contributes to gene-, cell-, and tissue-specific responses to SRM ligands. Increased understanding of the effect of cellular environment on nuclear receptors and their coregulators has the potential to open the field of SRM discovery and research to many members of the nuclear receptor superfamily.

Molecular Mechanisms, Physiological Consequences and Pharmacological Implications of Estrogen Receptor Action

The estrogen receptors (ERs), ERalpha and ERbeta, play a central role in mediating the biological effects of estrogen. The transcription rate of estrogen target genes is determined by several parameters including the type of ligand, estrogen receptor subtype and isoform, as well as interactions with receptor-binding cofactor proteins. The ERs regulate gene expression by binding to specific response element sequences in the promoters of estrogen target genes. Alternative pathways have also been described in which the ERs modulate transcription indirectly, via protein : protein interactions. In this regulatory mode, which has been traced to activator protein (AP)-1-, cyclic adenosine monophosphate (cAMP)-, and Sp1-response elements, the ERs appear to be tethered to target gene promoters via heterologous transcription factors. It has been found that ERalpha and ERbeta have opposite effects on transcription mediated via the indirect mode of action. Moreover, recent studies suggest that ERbeta may inhibit the stimulatory effects of ERalpha on cellular proliferation. Estrogen is a key regulatory hormone that affects numerous physiological processes. Estrogen is required for female pubertal development and affects growth, differentiation and function of the female reproductive system. It has recently been suggested that estrogen also has an important role in the male urogenital tract. In addition, estrogens have profound effects in other tissues. For instance, in the skeleton estrogen prevents bone-resorption by inhibition of osteoclast function. Numerous reports have suggested that estrogen has a beneficial effect in the cardiovascular system and in the CNS; however, this has not been confirmed in randomized clinical trials. In fact, a large randomized trial on healthy postmenopausal women receiving oral estrogen plus progestin showed an increased incidence of cardiovascular disease. In addition, this study revealed an increased risk for dementia and impaired cognitive function in the group receiving oral estrogen/progestin. Additional clinical trials are required to determine which hormonal component causes these health risks or whether the effects were due to the combination of estrogen and progestin.

Advances in estrogen receptor biology: prospects for improvements in targeted breast cancer therapy

Estrogen receptor (ER) has a crucial role in normal breast development and is expressed in the most common breast cancer subtypes. Importantly, its expression is very highly predictive for response to endocrine therapy. Current endocrine therapies for ER-positive breast cancers target ER function at multiple levels. These include targeting the level of estrogen, blocking estrogen action at the ER, and decreasing ER levels. However, the ultimate effectiveness of therapy is limited by either intrinsic or acquired resistance. Identifying the factors and pathways responsible for sensitivity and resistance remains a challenge in improving the treatment of breast cancer. With a better understanding of coordinated action of ER, its coregulatory factors, and the influence of other intracellular signaling cascades, improvements in breast cancer therapy are emerging.

Isolation of differentially expressed genes in conceptuses and endometrial tissue of sows in early gestation

The implantation period is a critical time for embryonic survival in pigs. During this period, numerous growth factors are secreted by the conceptuses and the uterine endometrium in order to establish pregnancy and to provide a proper environment for embryonic development. It is well known that the Chinese Meishan sows have a larger litter size when compared with occidental sows mainly because of a superior embryonic survival rate. As a further step toward understanding the mechanisms involved in embryonic survival, we used a suppression subtractive hybridization technique to identify genes that were differentially expressed in Meishan-Landrace conceptuses and endometrial tissue at Day 15 of gestation when compared with conventional Landrace sows. Of the 1000 subtractive clones isolated from each library, 137 endometrial and 166 conceptus-enriched cDNAs were single-pass sequenced and examined by BLAST analysis for identification. Sixty-two percent of the clones found in the endometrial library and 78% of the clones found in the conceptus library showed homology with known genes. Among these genes, the 20 most relevant to embryonic survival based on the available literature were validated through real-time reverse-transcriptase polymerase chain reaction (RT-PCR) analysis. Our results show that suppression subtractive hybridization is a powerful method applicable in identifying putative candidate genes that might be used for selection of high litter-size breeds.

Overexpression of histone deacetylase HDAC1 modulates breast cancer progression by negative regulation of estrogen receptor ?

The interaction between 17beta-estradiol and estrogen receptor alpha (ER-alpha) plays an important role in breast carcinogenesis and breast cancer treatment. ER-alpha is a critical growth regulatory gene in breast cancer and its expression level is tightly linked to the prognosis and treatment outcomes of breast cancer patients. Loss of ER-alpha expression in breast epithelial cells is critical for breast cancer progression. The underlying molecular mechanisms for this loss, however, are poorly defined. Histone deacetylases (HDACs) are implicated in the alteration of chromatin assembly and tumorigenesis. We show that histone deacetylase 1 (HDAC1) interacts with ER-alpha in vitro and in vivo and suppresses ER-alpha transcription activity. The interaction of HDAC1 with ER-alpha was mediated by the AF-2 and DBD domains of ER-alpha. We observed an endogenous interaction of HDAC1 with ER-alpha in breast cancer cells, which was decreased in the presence of estrogen. Interestingly, overexpression of HDAC1 in stable transfected MCF-7 clones induced loss of ER-alpha and significantly increased cell proliferation and colony formation, as compared to the control MCF-7 cells, whereas treatment of stable MCF-7 clones with the HDAC specific inhibitor trichostatin A (TSA) induced re-expression of ER-alpha mRNA and protein. Our findings strongly suggest that HDAC1 affects breast cancer progression by promoting cellular proliferation in association with a reduction in both ER-alpha protein expression and transcriptional activity. Thus, HDAC1 may be a potential target for therapeutic intervention in the treatment of a subset of ER-negative breast cancers.

Ligand-independent activation of estrogen receptor alpha by XBP-1

The estrogen receptor (ER) is a member of a large superfamily of nuclear receptors that regulates the transcription of estrogen-responsive genes. Several recent studies have demonstrated that XBP-1 mRNA expression is associated with ERalpha status in breast tumors. However, the role of XBP-1 in ERalpha signaling remains to be elucidated. More recently, two forms of XBP-1 were identified due to its unconventional splicing. We refer to the spliced and unspliced forms of XBP-1 as XBP-1S and XBP-1U, respectively. Here, we report that XBP-1S and XBP-1U enhanced ERalpha-dependent transcriptional activity in a ligand-independent manner. XBP-1S had stronger activity than XBP-1U. The maximal effects of XBP-1S and XBP-1U on ERalpha transactivation were observed when they were co-expressed with full-length ERalpha. SRC-1, the p160 steroid receptor coactivator family member, synergized with XBP-1S or XBP-1U to potentiate ERalpha activity. XBP-1S and XBP-1U bound to the ERalpha both in vitro and in vivo in a ligand-independent fashion. XBP-1S and XBP-1U interacted with the ERalpha region containing the DNA-binding domain. The ERalpha-interacting regions on XBP-1S and XBP-1U have been mapped to two regions, including the N-terminal basic region leucine zipper domain (bZIP) and the C-terminal activation domain. The bZIP-deleted mutants of XBP-1S and XBP-1U completely abolished ERalpha transactivation by XBP-1S and XBP-1U. These findings suggest that XBP-1S and XBP-1U may directly modulate ERalpha signaling in both the absence and presence of estrogen and, therefore, may play important roles in the proliferation of normal and malignant estrogen-regulated tissues.

Selective regulation of gene expression by nuclear factor 110, a member of the NF90 family of double-stranded RNA-binding proteins

Members of the nuclear factor 90 (NF90) family of double-stranded RNA (dsRNA)-binding proteins have been implicated in several biological processes including the regulation of gene expression. cDNA sequences predict that the proteins have a functional nuclear localization signal and two dsRNA-binding motifs (dsRBMs), and are identical at their N termini. Isoforms are predicted to diverge at their C termini as well as by the insertion of four amino acid residues (NVKQ) between the two dsRBMs. In this study, we verified the expression of four of the isoforms by cDNA cloning and mass spectrometric analysis of proteins isolated from human cells. Cell fractionation studies showed that NF90 and its heteromeric partner, NF45, are predominantly nuclear and largely chromatin-associated. The C-terminally extended NF90 species, NF110, are almost exclusively chromatin-bound. Both NF110 isoforms are more active than NF90 isoforms in stimulating transcription from the proliferating cell nuclear antigen reporter in a transient expression system. NF110b, which carries the NVKQ insert, was identified as the strongest activator. It stimulated transcription of some, but not all, promoters in a fashion that suggested that it functions in concert with other transcription factors. Finally, we demonstrate that NF110b associates with the dsRBM-containing transcriptional co-activator, RNA helicase A, independently of RNA binding.

Full activation of estrogen receptor alpha activation function-1 induces proliferation of breast cancer cells

The effects of estrogen and anti-estrogen are mediated through the estrogen receptors (ER) alpha and beta, which function as ligand-induced transcriptional factors. Recently, one of the phthalate esters, n-butylbenzyl phthalate (BBP), has been shown to induce estrogen receptor-mediated responses. By using the truncated types of ER mutants, we revealed that activation function-1 (AF-1) activity was necessary for the BBP-dependent transactivation function of ERalpha. AF-1 is also known to be responsible for the partial agonistic activity of tamoxifen. Whereas tamoxifen exhibits an anti-estrogenic effect on proliferation of the MCF-7 breast cancer cell line, BBP showed an estrogenic effect on MCF-7 to stimulate proliferation. In vivo and in vitro binding assays revealed that whereas 4-hydroxytamoxifen (OHT) induced binding of ERalpha to both an AF-1 coactivator complex (p68/p72 and p300) and corepressor complexes (N-CoR/SMRT), BBP selectively enhanced the binding to the AF-1 coactivators. We also showed that the transcriptional activity of OHT-bound ERalpha was modulated by the ratio between the AF-1 coactivator and corepressor complexes. Expression of a dominant-negative type of N-CoR inhibited the interaction between OHT-bound ERalpha and N-CoR/SMRT and enhanced the transcriptional activity of OHT-bound ERalpha. Furthermore, the cell growth of MCF-7 stably expressing the dominant-negative type of N-CoR was enhanced by the addition of OHT. These results indicated that fully activated AF-1 induces the stimulation of breast cancer growth and that the ratio between AF-1 coactivators and corepressors plays a key role to prevent proliferation of tumor by tamoxifen.

The N-terminal of the estrogen receptor (ERalpha) mediates transcriptional cross-talk with the retinoic acid receptor in human breast cancer cells

Transcriptional cross-talk exists between the estrogen receptor (ERalpha) and retinoic acid receptor (RAR) pathways in human breast cancer cells. We have previously shown that re-expression of ERalpha in ER-negative cells stimulates the transcriptional and growth inhibitory effects of all-trans-retinoic acid (tRA) by a mechanism that is independent of the ER ligands estradiol and tamoxifen. In this study, we generated cell lines stably expressing ERalpha-deletion mutants to elucidate the mechanism whereby ERalpha modulates RAR transcriptional activity. Using RT-PCR and RNAse protection assays, we observed that expression of ERalpha suppresses basal expression of the RA-responsive gene RARbeta2, while allowing it to be strongly induced by tRA. Repression of basal RARbeta2 transcription was confirmed by transient expression of the reporter plasmid betaRE-tk-CAT, containing the RARbeta2 promoter. In the ERalpha-negative cells, on the other hand, transcription was only weakly induced by RA. We further determined that this effect of ERalpha on RARbeta induction required the N-terminal AF-1-containing region, including the DNA-binding domain, but was independent of the C-terminal ligand-binding domain. Consistent with these results, the ER agonist estradiol and the AF-2 antagonist 4-hydroxytamoxifen had no significant effect on betaRARE activity. Conversely, the full ER antagonist ICI 182,780, which blocks ERalpha AF-1 activity, was able to completely relieve repression of basal betaRARE activity. The effect of ERalpha is specific for RAR-mediated transcription and does not occur on promoters containing typical response elements for the Vitamin D or thyroid hormone receptors. Moreover, the cross-talk between ERalpha and RAR does not seem to be mediated by sequestration of a number of common co-regulators, suggesting a novel mechanism whereby the N-terminal region of ERalpha modulates the transcriptional activity of RAR.

Ligand-independent interactions of p160/steroid receptor coactivators and CREB-binding protein (CBP) with estrogen receptor-alpha: regulation by phosphorylation sites in the A/B region depends on other receptor domains

Estrogen receptor (ER)alpha and ERbeta are transcription factors that can be activated by both ligand binding and growth factor signaling. Estradiol increases ER activity in part by enhancing interactions between its carboxy-terminal, ligand-binding domain (LBD) and the p160/SRC (steroid receptor coactivator) and p300/CBP (cAMP response element binding protein (CREB) binding protein) families of coactivators. In the absence of ligand and the LBD, these cofactors can also interact with the amino-terminal (A/B) domain of ERs in vitro. SRC-1 also enhances the ligand-independent activity of the full-length receptor. Both ligand-independent and estradiol-induced ER activity are increased by phosphorylation at specific serine (Ser) residues in the A/B domain (Ser104/106 and Ser118 in ERalpha). In the case of ERbeta, phosphorylation enhances the ligand-independent recruitment and action of SRC-1. We show here that unliganded ERalpha can activate endogenous gene expression in MCF-7 cells, and that this activation is mediated in part by a p160 coactivator. In transfected HeLa cells, we show that the full-length ERalpha can interact physically and functionally with p160/SRCs and CBP in the absence of ligand and that mutation of Ser104/106/118 affects these interactions. Accordingly, ERalpha dephosphorylation decreases its ligand-independent interaction with SRC-1 and CBP in vitro. In HeLa cells, both Ser104/106 and Ser118 impinge on SRC-1 action by two mechanisms: 1) a seemingly indirect effect on SRC-1 recruitment that requires other receptor domains in addition to the A/B, consistent with our finding that the ligand-independent interaction between the A/B and the LBD and its enhancement by SRC-1 depend in part on Ser104/106/118; and 2) an effect on SRC-1 coactivation that can be observed in the absence of the LBD. Ser104/106/118 can also affect coactivation by a subset of coactivators in the presence of hormone, albeit to a lesser extent than in its absence. Altogether, our observations suggest that the enhancement of ERalpha activity by p160/SRCs and CBP can be regulated by phosphorylation and stress the importance of using full-length receptors to assess the role of the activation function-1 in cofactor recruitment.

Ligand-independent coactivation of ERalpha AF-1 by steroid receptor RNA activator (SRA) via MAPK activation

Nuclear receptor coactivators are factors that enhance the transcriptional activity of the receptor. Coactivators usually work in ligand-independent and/or dependent manners by interacting with activation function-1 (AF-1) and AF-2 of the receptor, respectively. The recently characterized steroid receptor RNA activator (SRA) was cloned as an AF-1-dependent coactivator and shown to enhance the transcriptional activity of selected steroid receptors. In this work, we describe the effect of SRA on the activity of the two estrogen receptor (ER) isoforms, ERalpha and ERbeta. We show that SRA potentiates the estrogen-induced transcriptional activity of both ERalpha and ERbeta. We demonstrate that the transcriptional activity of ERalpha can be enhanced by SRA in a ligand-independent manner through the AF-1 domain. However, this AF-1-dependent effect of SRA is not observed on ERbeta, denoting the ability of SRA to mediate differential activation of ERalpha and ERbeta. The presence of an intact serine residue at position 118 (S(118)) in ERalpha AF-1 is required for coactivation of ERalpha by SRA. We also show that activation of the mitogen activated protein kinase (MAPK) induces ligand-independent coactivation of ERalpha by SRA, a mechanism that is independent of the AF-2. Finally, SRA is unable to rescue the loss of activity of the S(118) ERalpha mutant in response to H-Ras(V12), suggesting that phosphorylation of S(118) by MAPK participates in the ligand-independent effect of SRA on ERalpha.

Domains of estrogen receptor alpha (ERalpha) required for ERalpha/Sp1-mediated activation of GC-rich promoters by estrogens and antiestrogens in breast cancer cells

Estrogen receptor alpha (ERalpha)/Sp1 activation of GC-rich gene promoters in breast cancer cells is dependent, in part, on activation function 1 (AF1) of ERalpha, and this study investigates contributions of the DNA binding domain (C) and AF2 (DEF) regions of ERalpha on activation of ERalpha/Sp1. 17Beta-estradiol (E2) and the antiestrogens 4-hydroxytamoxifen and ICI 182,780 induced reporter gene activity in MCF-7 and MDA-MB-231 cells cotransfected with human or mouse ERalpha (hERalpha or MOR), but not ERbeta and GC-rich constructs containing three tandem Sp1 binding sites (pSp13) or other E2-responsive GC-rich promoters. Estrogen and antiestrogen activation of hERalpha/Sp1 was dependent on overlapping and different regions of the C, D, E, and F domains of ERalpha. Antiestrogen-induced activation of hERalpha/Sp1 was lost using hERalpha mutants deleted in zinc finger 1 [amino acids (aa) 185-205], zinc finger 2 (aa 218-245), and the hinge/helix 1 (aa 265-330) domains. In contrast with antiestrogens, E2-dependent activation of hERalpha/Sp1 required the C-terminal F domain (aa 579-595), which contains a beta-strand structural motif. Moreover, in peptide competition experiments overexpression of a C-terminal (aa 575-595) F domain peptide specifically blocked E2-dependent activation of hERalpha/Sp1, suggesting that F domain interactions with nuclear cofactors are required for ERalpha/Sp1 action.

Mechanistic differences in the activation of estrogen receptor-alpha (ER alpha)- and ER beta-dependent gene expression by cAMP signaling pathway(s)

Although increases in intracellular cAMP can stimulate estrogen receptor-alpha (ER alpha) activity in the absence of exogenous hormone, no studies have addressed whether ER beta can be similarly regulated. In transient transfections, forskolin plus 3-isobutyl-1-methylxanthine (IBMX), which increases intracellular cAMP, stimulated the transcriptional activities of both ER alpha and ER beta. This effect was blocked by the protein kinase A inhibitor H89 (N-(2-(p-bromocinnamylamino)-ethyl)-5-isoquinolinesulfonamide) and was dependent on an estrogen response element. A 12-O-tetradecanoylphorbol-13-acetate response element (TRE) located 5' to the estrogen response element was necessary for cAMP-dependent activation of gene expression by ER beta but not ER alpha, indicating that the former subtype requires a functional interaction with TRE-interacting factor(s) to stimulate transcription. Both p160 and CREB-binding protein coactivators stimulated cAMP-induced ER alpha and ER beta transcriptional activity. However, mutation of the two cAMP-inducible SRC-1 phosphorylation sites important for cAMP activation of chicken progesterone receptor or all seven known SRC-1 phosphorylation sites did not specifically impair cAMP activation of ER alpha. The E/F domains of ER alpha are sufficient for activation by forskolin/IBMX, and this is accompanied by an increase in receptor phosphorylation. In contrast, cAMP signaling reduces the phosphorylation of the corresponding region of ER beta, and this correlates with the lack of forskolin/IBMX stimulated transcriptional activity. Our data suggest that cAMP activation of ER alpha transcriptional activity is associated with receptor instead of SRC-1 phosphorylation. Moreover, differences in the cofactor requirements, domains of ER alpha and ER beta sufficient for forskolin/IBMX activation, and the effect of cAMP on receptor phosphorylation indicate that this signaling pathway utilizes distinct mechanisms to stimulate ER alpha and ER beta transcriptional activity.

Nuclear receptors: integration of multiple signalling pathways through phosphorylation

Nuclear receptors (NRs) orchestrate the transcription of specific gene networks in response to binding of their cognate ligand. They also act as mediators in a variety of signalling pathways through integrating diverse phosphorylation events. NR phosphorylation concerns all three major domains, the N-terminal activation function (AF-1), the ligand-binding and the DNA binding domains. Often, phosphorylation of NRs by kinases that are associated with general transcription factors (e.g. cdk7 within TFIIH), or activated in response to a variety of signals (MAPKs, Akt, PKA, PKC), facilitates the recruitment of coactivators or of components of the transcription machinery and, therefore, cooperates with the ligand to enhance transcription activation. But phosphorylation can also contribute to the termination of the ligand response through inducing DNA dissociation or NR degradation or through decreasing ligand affinity. These different modes of regulation reveal an unexpected complexity of the dynamics of NR-mediated transcription. In addition, deregulation of NR phosphorylation may impact their action in certain diseases or cancers.

The highly related DEAD box RNA helicases p68 and p72 exist as heterodimers in cells

The RNA helicases p68 and p72 are highly related members of the DEAD box family of proteins, sharing 90% identity across the conserved core, and have been shown to be involved in both transcription and mRNA processing. We previously showed that these proteins co-localise in the nucleus of interphase cells. In this study we show that p68 and p72 can interact with each other and self-associate in the yeast two-hybrid system. Co-immunoprecipitation experiments confirmed that p68 and p72 can interact in the cell and indicated that these proteins preferentially exist as hetero-dimers. In addition, we show that p68 can interact with NFAR-2, a protein that is also thought to function in mRNA processing. Moreover, gel filtration analysis suggests that p68 and p72 can exist in a variety of complexes in the cell (ranging from approximately 150 to approximately 400 kDa in size), with a subset of p68 molecules being in very large complexes (>2 MDa). The potential to exist in different complexes that may contain p68 and/or p72, together with a range of other factors, would provide the potential for these proteins to interact with different RNA substrates and would be consistent with recent reports implying a wide range of functions for p68/p72.

Regulation of nuclear receptor transcriptional activity by a novel DEAD box RNA helicase (DP97)

We have identified a novel DEAD box RNA helicase (97 kDa, DP97) from a breast cancer cDNA library that interacts in a hormone-dependent manner with nuclear receptors and represses their transcriptional activity. DP97 has RNA-dependent ATPase activity, and mapping studies localize the interacting regions of DP97 and nuclear receptors to the C-terminal region of DP97 and the hormone binding/activation function-2 region of estrogen receptors (ER), as well as several other nuclear receptors. Repression by DP97 maps to a small region (amino acids 589-631) that has homology to a repression domain in the corepressor protein NCoR2/SMRTe. This region of DP97 is necessary and sufficient for its intrinsic repression activity. The N-terminal helicase region of DP97 is, however, dispensable for its transcriptional repressor activity. The knockdown of endogenous cellular DP97 by antisense DP97 or RNA interference (siRNA for DP97) results in significant enhancement of the expression of estradiol-ER-stimulated genes and attenuation of the repression of genes inhibited by the estradiol-ER. This implies that endogenous DP97 normally dampens stimulation and intensifies repression of estradiol-ER-regulated genes. Our findings add to the growing evidence that RNA helicases can associate with nuclear receptors and function as coregulators to modulate receptor transcriptional activity.

Chromatin exposes intrinsic differences in the transcriptional activities of estrogen receptors alpha and beta

The biological actions of estrogens are mediated via two distinct intranuclear estrogen receptor (ER) proteins, ERalpha and ERbeta. We have used an in vitro chromatin assembly and transcription system to compare the transcriptional activities of the two ERs in the context of chromatin, the physiological template for transcription by RNA polymerase II. We find that under conditions where many biochemical activities of the receptors are similar (e.g. ligand binding, chromatin binding, chromatin remodeling and co-activator recruitment), liganded ERalpha is a much more potent transcriptional activator than ERbeta with chromatin templates, but not with naked DNA. This difference is attributable to the N-terminal A/B region of ERalpha, which contains a transferable activation function that facilitates transcription specifically with chromatin templates. Interestingly, chromatin selectively restricts ligand-dependent transcriptional activation by ERbeta under some conditions (e.g. with a closed chromatin architecture), while allowing it under other conditions (e.g. with an open chromatin architecture). Collectively, our results define an important role for chromatin in determining signaling outcomes mediated by distinct subtypes of signal-transducing transcriptional activator proteins.

Synergism between p68 RNA helicase and the transcriptional coactivators CBP and p300

p68 RNA helicase has been implicated in a variety of processes, including rearrangement of RNA secondary structures, RNA splicing, gene transcription and tumor development, yet its mechanisms of action are not well understood. In this study, we show that p68 is predominantly localized to the cell nucleus, where it partially colocalizes with the transcriptional coactivator p300. Accordingly, p68 and p300, or the paralogous CREB-binding protein (CBP), coimmunoprecipitate. Similarly, p68 and RNA polymerase II (Pol II) are able to interact in vivo. GST pull-down assays confirmed these interactions in vitro, demonstrating that p68 can interact with several domains of CBP, while CBP/p300 bind to amino acids 176-388 of p68 and RNA Pol II binds to the N-terminal 80 amino acids of p68. Furthermore, p68 stimulates transcription mediated by the C-terminal transactivation domain of CBP. p68 is also able to stimulate TPA oncogene responsive unit (TORU) promoter activity, and p300 acts in synergy with p68. On the other hand, suppression of CBP/p300 function by the adenoviral protein E1A abolishes TORU promoter activation by p68. Altogether, our results suggest the existence of a multiprotein complex in which p68 RNA helicase, CBP/p300 and RNA Pol II jointly promote gene expression.

Estrogen receptor phosphorylation

Estrogen receptor alpha (ERalpha) is phosphorylated on multiple amino acid residues. For example, in response to estradiol binding, human ERalpha is predominately phosphorylated on Ser-118 and to a lesser extent on Ser-104 and Ser-106. In response to activation of the mitogen-activated protein kinase pathway, phosphorylation occurs on Ser-118 and Ser-167. These serine residues are all located within the activation function 1 region of the N-terminal domain of ERalpha. In contrast, activation of protein kinase A increases the phosphorylation of Ser-236, which is located in the DNA-binding domain. The in vivo phosphorylation status of Tyr-537, located in the ligand-binding domain, remains controversial. In this review, I present evidence that these phosphorylations occur, and identify the kinases thought to be responsible. Additionally, the functional importance of ERalpha phosphorylation is discussed.