Use of suppressor mutants to probe the function of estrogen receptor-p160 coactivator interactions

Estrogen and estrogen receptor induce matrix metalloproteinase-26 expression in endometrial carcinoma cells

The human matrix metalloproteinase (MMP)-26, also called matrilysin-2 or endometase, has been isolated as a matrilysin (MMP-7) homolog. Several reports describe that MMP-26 may be related to the development of endometrial carcinomas. Total RNAs were isolated from 51 normal endometrial tissue samples, 6 endometrial hyperplasia tissue samples and 30 endometrial carcinomas. Real-time reverse transcription-polymerase chain reaction (RT-PCR) was performed to evaluate MMP-26 mRNA expression levels. We examined the effect of estrogen and its receptor (ER) on MMP-26 expression in endometrial carcinoma cell lines by real-time RT-PCR, western blot analysis and luciferase assays. To examine protein-DNA binding between ER and MMP-26 promoter, we performed chromatin immunoprecipitation (ChIP) assay. Real-time RT-PCR analysis revealed that MMP-26 mRNA expression was significantly higher in the normal human endometria and hyperplasias compared with that in endometrial carcinomas. Estrogen not only transactivated the MMP-26 promoter activity but also enhanced endogenous MMP-26 expression. The MMP-26 promoter region contains a putative ER response element (ERE). Nuclear ER protein interacted with ERE on the MMP-26 promoter by ChIP assay. We found a significant difference in MMP-26 expression in normal and malignant endometrial tissue samples and that estrogen induced MMP-26 expression. Estrogen may induce endometrial hyperplasia but not endometrial carcinoma. Our results provide evidence that regulation of MMP-26 promoter activity by estrogen may represent a mechanism for endometrial carcinogenesis.

Polyphenols, isothiocyanates, and carotenoid derivatives enhance estrogenic activity in bone cells but inhibit it in breast cancer cells

While exposure to estrogens is a major risk factor of breast and endometrial cancer, it well established that estrogens are beneficial for bone health. We have previously shown that carotenoids inhibit estrogen signaling in breast and endometrial cancer cells. The aim of this study was to compare the effects of various phytonutrients, (carotenoid derivatives, polyphenols, isothiocyanates) on estrogenic activity in breast cancer cells and osteoblast-like cells. All the tested phytonutrients inhibited estrogen response element (ERE) transactivation in breast cancer cells. In contrast, these compounds either did not affect or enhanced ERE activity and the expression of several bone-forming genes. These results were obtained using two osteoblast-like cell lines, MG-63 human osteosarcoma cells stably transfected with estrogen receptor-α (ERα) and MC3T3-E1 mouse calvaria-derived cells expressing endogenous ER. Phytonutrients-induced ERE inhibition in breast cancer cells, and its potentiation in osteoblast-like cells were associated both with a decrease and a rise in total and nuclear ERα levels, respectively. Phytonutrients activated the electrophile/antioxidant response element (EpRE/ARE) transcription system to various extents in both cancer and bone cell lines. Overexpression of Nrf2, the major EpRE/ARE activating transcription factor, mimicked the effects of phytonutrients, causing inhibition and enhancement of ERE transactivation in breast cancer cells and in osteoblast-like cells, respectively. Moreover, reduction in Nrf2 levels by RNAi led to a decrease in the phytonutrient potentiation of ERE activity transactivation in osteoblast-like cells. These findings suggest that the enhancement and inhibition of estrogen signaling by phytonutrients in bone-derived cells and breast cancer cells, respectively, is partially mediated by the activation of the Nrf2/ARE system.

ER-alpha-cDNA as part of a bicistronic transcript gives rise to high frequency, long term, receptor expressing cell clones

Within the large group of Estrogen Receptor alpha (ERα)-negative breast cancer patients, there is a subgroup carrying the phenotype ERα(-), PR(-), and Her2(-), named accordingly "Triple-Negative" (TN). Using cell lines derived from this TN group, we wished to establish cell clones, in which ERα is ectopically expressed, forming part of a synthetic lethality screening system. Initially, we generated cell transfectants expressing a mono-cistronic ERα transcription unit, adjacent to a separate dominant selectable marker transcription unit. However, the yield of ERα expressing colonies was rather low (5-12.5%), and only about half of these displayed stable ectopic ERα expression over time. Generation and maintenance of such cell clones under minimal exposure to the ERα ligand, did not improve yield or expression stability. Indeed, other groups have also reported grave difficulties in obtaining ectopic expression of ERα in ERα-deficient breast carcinoma cells. We therefore switched to transfecting these cell lines with pERα-IRES, a plasmid vector encoding a bicistronic translation mRNA template: ERα Open Reading Frame (ORF) being upstream followed by a dominant-positive selectable marker (hygro(R)) ORF, directed for translation from an Internal Ribosome Entry Site (IRES). Through usage of this bicistronic vector linkage system, it was possible to generate a very high yield of ERα expressing cell clones (50-100%). The stability over time of these clones was also somewhat improved, though variations between individual cell clones were evident. Our successful experience with ERα in this system may serve as a paradigm for other genes where ectopic expression meets similar hardships.

Structure-based approaches to create new E2-E3 enzyme pairs

The study of ubiquitin-conjugating enzymes (E2) and ubiquitin-protein ligases (E3) is complicated by the fact that a relatively limited number of E2 proteins interacts with a large number of E3 enzymes. Many E3 enzymes contain a RING domain. Based on structural and biochemical analysis of the complex between UbcH5b and the CNOT4 RING finger, we describe a rationale to design new E2-E3 enzyme pairs with altered specificity. In such enzyme pairs, the E2 and E3 proteins are each mutated so that they do not interact with their wild-type partner. However, a functional enzyme pair is reconstituted when both E2 and E3 mutants are combined. Such altered-specificity enzyme pairs may be valuable to study the physiological significance of particular E2-E3 interactions.

Regulation of SRC family coactivators by post-translational modifications

Initially identified as a group of auxiliary protein factors involved in transcriptional regulation by steroid hormone receptors as well as by other members of the nuclear receptor superfamily, the steroid receptor coactivators (SRCs) have since then been implicated in the transcriptional regulation of other transcription factors which are important components of very different signaling pathways. Members of the SRC family have been shown to interact with myogenin, MEF-2, transcriptional enhancer factor (TEF), NF-kappaB, AP-1, STAT, p53, and E2F1, suggesting that SRC coactivators participate in diverse cellular processes. Recent evidence indicates that various post-translational modifications play critical roles in determining the final transcriptional output and specificity of SRC coactivators. In this review, we summarized the current knowledge concerning post-translational modifications, dynamic interplay between different modifications, and patho-physiological relevance of the modifications of SRC proteins.

Lycopene and other carotenoids inhibit estrogenic activity of 17beta-estradiol and genistein in cancer cells

Epidemiological evidence suggests that carotenoids prevent several types of cancer, including mammary and endometrial cancers. On the other hand, such studies have also shown that estrogens are the most important risk factors for these cancer types. Genistein, the phytoestrogen mainly found in soy, also shows significant estrogenic activity when tested at concentrations found in human blood. The aim of this study was to determine whether carotenoids inhibit signaling of steroidal estrogen and phytoestrogen which could explain their cancer preventive activity. Similar to the known effect of 17beta-estradiol (E(2)), treatment of breast (T47D and MCF-7) and endometrial (ECC-1) cancer cells with phytoestrogens induced cell proliferation, cell-cycle progression and transactivation of the estrogen response element (ERE). However, each of the tested carotenoids (lycopene, phytoene, phytofluene, and beta-carotene) inhibited cancer cell proliferation induced by either E(2) or genistein. The inhibition of cell growth by lycopene was accompanied by slow down of cell-cycle progression from G1 to S phase. Moreover, the carotenoids inhibited estrogen-induced transactivation of ERE that was mediated by both estrogen receptors (ERs) ERalpha and ERbeta. The possibility that this inhibition results from competition of carotenoid-activated transcription systems on a limited pool of shared coactivators with the ERE transcription system was tested. Although cotransfection of breast and endometrial cancer cells with four different coactivators (SRC-1, SRC-2, SRC-3, and DRIP) strongly stimulated ERE reporter gene activity, it did not oppose the inhibitory effect of carotenoids. These results suggest that dietary carotenoids inhibit estrogen signaling of both 17beta-estradiol and genistein, and attenuate their deleterious effect in hormone-dependent malignancies.

Differential roles of C-terminal activation motifs in the establishment of Stat6 transcriptional specificity

Members of the Stat transcription factor family are specifically activated by cytokines, and each Stat mediates its biological effects through the trans-activation of a unique profile of target genes. This specificity is achieved even when Stat proteins mediating opposite transcriptional effects bind to the same palindromic Stat sites in target genes. We show here that the non-conserved sequences of Stat transcription activation domains (TADs) contribute to specificity in promoter activation. Chimeric proteins in which the Stat6 TAD was replaced by that from Stat1alpha or Stat5 exhibited normal interleukin-4-inducible DNA binding activity, but at best modest trans-activation of reporters containing Stat6 binding sites, and a failure to activate the endogenous CD23 promoter in primary B cells. The p160 coactivator nuclear coactivator-1 (Src-1) was specifically recruited by and coactivated Stat6 but not the chimeric Stat6 molecules. Strikingly, transcriptional responses exhibited distinct requirements for the nuclear coactivator-1 interaction motif of the Stat6 C terminus. Together, these findings indicate that the Stat6 TAD contributes to promoter specificity by the differential recruitment of and requirement for a p160-class coactivator.

Transcriptional activation by thyroid hormone receptor-beta involves chromatin remodeling, histone acetylation, and synergistic stimulation by p300 and steroid receptor coactivators

Transcriptional regulation by heterodimers of thyroid hormone receptor (TR) and the 9-cis retinoid X receptor (RXR) is a highly complex process involving a large number of accessory factors, as well as chromatin remodeling. We have used a biochemical approach, including an in vitro chromatin assembly and transcription system that accurately recapitulates ligand- and activation function (AF)-2-dependent transcriptional activation by TRbeta/RXRalpha heterodimers, as well as in vitro chromatin immunoprecipitation assays, to study the mechanisms of TRbeta-mediated transcription with chromatin templates. Using this approach, we show that chromatin is required for robust ligand-dependent activation by TRbeta. We also show that the binding of liganded TRbeta to chromatin induces promoter-proximal chromatin remodeling and histone acetylation, and that histone acetylation is correlated with increased TRbeta-dependent transcription. Additionally, we find that steroid receptor coactivators (SRCs) and p300 function synergistically to stimulate TRbeta-dependent transcription, with multiple functional domains of p300 contributing to its coactivator activity with TRbeta. A major conclusion from our experiments is that the primary role of the SRC proteins is to recruit p300/cAMP response element binding protein-binding protein to hormone-regulated promoters. Together, our results suggest a multiple step pathway for transcriptional regulation by liganded TRbeta, including chromatin remodeling, recruitment of coactivators, targeted histone acetylation, and recruitment of the RNA polymerase II transcriptional machinery. Our studies highlight the functional importance of chromatin in transcriptional control and further define the molecular mechanisms by which the SRC and p300 coactivators facilitate transcriptional activation by liganded TRbeta.

Modulation of induction properties of glucocorticoid receptor-agonist and -antagonist complexes by coactivators involves binding to receptors but is independent of ability of coactivators to augment transactivation

Coactivators such as TIF2 and SRC-1 modulate the positioning of the dose-response curve for agonist-bound glucocorticoid receptors (GRs) and the partial agonist activity of antiglucocorticoid complexes. These properties of coactivators differ from their initially defined activities of binding to, and increasing the total levels of transactivation by, agonist-bound steroid receptors. We now report that constructs of TIF2 and SRC-1 lacking the two activation domains (AD1 and AD2) have significantly less ability to increase transactivation but retain most of the activity for modulating the dose-response curve and partial agonist activity. Mammalian two-hybrid experiments show that the minimum TIF2 segment with modulatory activity (TIF2.4) does not interact with p300, CREB-binding protein, or PCAF, which also modulates GR activities. DRIP150 and DRIP205 have been implicated in coactivator actions but are unable to modulate GR activities. The absence of synergism by PCAF or DRIP150 with SRC-1 or TIF2, respectively, further suggests that these other factors are not involved. The ability of a TIF2.4 fragment (i.e. TIF2.37), which is not known to interact with proteins, to block the actions of TIF2.4 suggests that an unidentified binder mediates the modulatory activity of TIF2. Pull-down experiments with GST/TIF2.4 demonstrate a direct interaction of TIF2 with GR in a hormone-dependent fashion that requires the receptor interaction domains of TIF2 and is equally robust with agonists and most antiglucocorticoids. These observations, which are confirmed in mammalian two-hybrid assays, suggest that the capacity of coactivators such as TIF2 to modulate the partial agonist activity of antisteroids is mediated by the binding of coactivators to GR-antagonist complexes. In conclusion, the modulatory activity of coactivators with GR-agonist and -antagonist complexes is mechanistically distinct from the ability of coactivators to augment the total levels of transactivation and appears to involve the binding to both GR-steroid complexes and an unidentified TIF2-associated factor(s).

Reciprocal recruitment of DRIP/mediator and p160 coactivator complexes in vivo by estrogen receptor

Two functionally distinct classes of coactivators are recruited by liganded estrogen receptor, the DRIP/Mediator complex and p160 proteins, although the relative dynamics of recruitment is unclear. Previously, we have shown a direct, estradiol-dependent interaction between the DRIP205 subunit of the DRIP complex and the estrogen receptor (ER) AF2 domain. Here we demonstrate the in vivo recruitment of other endogenous DRIP subunits to ER in response to estradiol treatment in MCF-7 cells. To explore the relationship between DRIP and p160 coactivators, we examined the kinetics of coactivator recruitment to the ER target promoter, pS2, by chromatin immunoprecipitation. We observed a cyclic association and dissociation of coactivators with the promoter, with recruitment of p160s and DRIPs occurring in opposite phases, suggesting an exchange between these coactivator complexes at the target promoter.

Androgen receptor (AR) coregulators: an overview

The biological action of androgens is mediated through the androgen receptor (AR). Androgen-bound AR functions as a transcription factor to regulate genes involved in an array of physiological processes, most notably male sexual differentiation and maturation, and the maintenance of spermatogenesis. The transcriptional activity of AR is affected by coregulators that influence a number of functional properties of AR, including ligand selectivity and DNA binding capacity. As the promoter of target genes, coregulators participate in DNA modification, either directly through modification of histones or indirectly by the recruitment of chromatin-modifying complexes, as well as functioning in the recruitment of the basal transcriptional machinery. Aberrant coregulator activity due to mutation or altered expression levels may be a contributing factor in the progression of diseases related to AR activity, such as prostate cancer. AR demonstrates distinct differences in its interaction with coregulators from other steroid receptors due to differences in the functional interaction between AR domains, possibly resulting in alterations in the dynamic interactions between coregulator complexes.

A role for coactivators and histone acetylation in estrogen receptor alpha-mediated transcription initiation

Transcriptional regulation by estrogen receptor alpha (ERalpha) involves protein-protein interactions among the receptor, its associated coactivators and the RNA polymerase II transcriptional machinery. We have used an in vitro chromatin assembly and transcription system to examine the biochemistry of interactions among ERalpha, the SRC proteins and p300/CBP. Using polypeptides designed to block specific receptor- cofactor or cofactor-cofactor interactions, we show that interactions among ERalpha, its coactivators and the RNA pol II machinery are all required for ERalpha- mediated transcription. Furthermore, we show that ERalpha-SRC-p300/CBP interactions are necessary and sufficient for the targeted acetylation of nucleosomal histones on estrogen-responsive promoters in the absence of transcription. The protein-protein interactions required for histone acetylation constitute a subset of the interactions required for transcriptional activation. Finally, we show that the major role of SRC-p300/CBP interactions is to enhance ERalpha- mediated transcription initiation, and they have little or no role in stimulating subsequent rounds of transcription. Together, our results indicate a specific role for the SRC and p300/CBP coactivators, as well as targeted histone acetylation, in ERalpha-mediated transcription.