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.

We present an integrated model of hERalpha-mediated transcription where both unliganded and liganded receptors cycle on estrogen-responsive promoters. Using ChIP, FRAP, and biochemical analysis we evaluate hERalpha at several points in these cycles, establishing the ubiquitination status and subnuclear distribution of hERalpha, its mobility, the kinetics of transcriptional activation, and the cyclic recruitment of E3 ligases and the 19S regulatory component of the proteasome. These experiments, together with an evaluation of the inhibition of transcription and proteasome action, demonstrate that proteasome-mediated degradation and hERalpha-mediated transactivation are inherently linked and act to continuously turn over hERalpha on responsive promoters. Cyclic turnover of hERalpha permits continuous responses to changes in the concentration of estradiol.

A new isoform of the human estrogen receptor-alpha (hER-a) has been identi®ed and characterized. This 46 kDa isoform (hERa46) lacks the N-terminal 173 amino acids present in the previously characterized 66 kDa isoform (hERa66). hERa46 is encoded by a new class of hER-a transcript that lacks the ®rst coding exon (exon 1A) of the ER-a gene. We demonstrated that these D1A hER-a transcripts originate from the E and F hER-a promoters and are produced by the splicing of exon 1E directly to exon 2. Functional analysis of hERa46 showed that, in a cell context sensitive to the transactivation function AF-2, this receptor is an effective ligand-inducible transcription factor. In contrast, hERa46 is a powerful inhibitor of hERa66 in a cell context where the transactivating function of AF-1 predominates over AF-2. The mechanisms by which the AF-1 dominantnegative action is exerted may involve heterodimerization of the two receptor isoforms and/or direct competition for the ER-a DNA-binding site. hERa66/ hERa46 ratios change with the cell growth status of the breast carcinoma cell line MCF7, suggesting a role of hERa46 in cellular proliferation.

Valproate (VPA) and trichostatin A (TSA), inhibitors of zinc-dependent deacetylase activity, induce reduction in the levels of mRNA encoding oestrogen receptor-a (ERa), resulting in subsequent clearance of ERa protein from breast and ovarian cell lines. Inhibition of oestrogen signalling may account for the endocrine disorders, menstrual abnormalities, osteoporosis and weight gain that occur in a proportion of women treated with VPA for epilepsy or for bipolar mood disorder. Transcriptome profiling revealed that VPA and TSA also modulate the expression of, among others, key regulatory components of the cell cycle. Meta-analysis of genes directly responsive to oestrogen indicates that VPA and TSA have a generally antioestrogenic profile in ERa positive cells. Concomitant treatment with cycloheximide prevented most of these changes in gene expression, including downregulation of ERa mRNA, indicating that a limited number of genes signal a hyperacetylated state within cells. Three members of the NAD-dependent deacetylases, the sirtuins, are upregulated by VPA and by TSA and sirtuin activity contributes to loss of ERa expression. However, prolonged inhibition of the sirtuins by sirtinol also induces loss of ERa from cells. Mechanistically, we show that VPA invokes reversible promoter shutoff of the ERa, pS2 and cyclin D1 promoters, by inducing recruitment of methyl cytosine binding protein 2 (MeCP2) with concomitant exclusion of the maintenance methylase DNMT1. Furthermore, we demonstrate that, in the presence of VPA, local DNA methylation, deacetylation and demethylation of activated histones and recruitment of inhibitory complexes occurs on the pS2 promoter.

Unliganded (apo-) estrogen receptor alpha (ERalpha, NR3A1) is classically considered as transcriptionally unproductive. Reassessing this paradigm demonstrated that apo-human ERalpha (ERalpha66) and its N-terminally truncated isoform (ERalpha46) are both predominantly nuclear transcription factors that cycle on the endogenous estrogen-responsive pS2 gene promoter in vivo. Importantly, isoform-specific consequences occur in terms of poising the promoter for transcription, as evaluated by determining (i) the engagement of several cofactors and the resulting nucleosomal organization; and (ii) the CpG methylation state of the pS2 promoter. Although transcriptionally unproductive, cycling of apo-ERalpha66 prepares the promoter to respond to ligand, through sequentially targeting chromatin remodeling complexes and general transcription factors. Additionally, apo-ERalpha46 recruits corepressors, following engagement of cofactors identical to those recruited by apo-ERalpha66. Together, these data describe differential activities of ERalpha isoforms. Furthermore, they depict the maintenance of a promoter in a repressed state as a cyclical process that is intrinsically dependent on initial poising of the promoter.

The functional interplay between different domains of estrogen receptor-alpha (ERalpha, NR3A1) is responsible for the overall properties of the full-length protein. We previously identified an interaction between the N-terminal A and C-terminal domains, which we demonstrate here to repress ligand-independent transactivation and transrepression abilities of ERalpha. Using targeted mutations based on ERalpha structural models, we determine the basis for this interaction that defines a regulatory interplay between ERalpha A domain, corepressors, and ERalpha Helix 12 for binding to the same C-terminal surface. We propose a dynamic model where binding of different ligands influences the A/D-F domain interaction and results in specific functional outcomes. This model gives insights into the dynamic properties of full-length ERalpha and into the structure of unliganded ERalpha.

The human estrogen receptor-␣ (hER␣) gene is a complex genomic unit exhibiting alternative splicing and promoter usage in a tissue-specific manner. During the investigation of new hER␣ mRNA variants by rapid amplification of 5 cDNA ends, we identified a cDNA in which the acceptor site of exon 1A, into which the different leader exons are normally alternatively spliced, was spliced accurately the 3 extremity of exon 1A (scrambled 1A31A hER␣ cDNA). Reverse transcription-PCR and S1 nuclease mapping analysis revealed that 1A31A hER␣ transcripts were not circular RNAs constituted by exon 1A only but corresponded to linear polyadenylated hER␣ RNAs composed of the eight coding exons of the hER␣ gene and characterized by a duplication of exon 1A. Genomic Southern blot experiments excluded the hypothesis of duplication of hER␣ exon 1A in the human genome. Therefore, these data suggested that 1A31A hER␣ transcripts were likely generated by trans-splicing. The production of such transcripts by trans-splicing of pre-mRNAs generated from a chimeric gene formed by a single hER␣ exon 1A, exon 2, and their flanking intronic regions was demonstrated in transient transfection experiments. Therefore, in addition to the alternative cis-splicing, the hER␣ gene is also subject to natural trans-splicing. The estrogen receptor-␣ (ER␣)1 is a ligand-inducible transcription factor that belongs to the steroid, thyroid hormone, and retinoic acid receptor family (1-3). As all members of this family, it modulates transcription of specific sets of genes by interacting either in a protein/DNA manner with cognate DNA sequences called responsive elements or in a protein/protein manner with other transcriptional factors (1-5).ER␣ is a key component of a wide range of biological processes. Its main role is in the control of the reproductive functions such as the establishment and maintenance of female sex differentiation characteristics, reproductive cycle, and pregnancy (6, 7). ER␣ is also involved in liver, fat, and bone cell metabolism, cardiovascular and neuronal activity, and embryonic and fetal development (6, 7). Finally, due to the mitogenic effect of its ligand, ER␣ is intimately associated with the biology of endometrium and breast cancers (8 -10).ER status is used clinically both as a prognostic factor and as a target in the therapy of breast cancers (9). Patients with ER-positive tumors have a better prognosis than those with tumors that lack ER expression. The benefits of the anti-estrogen therapy are almost limited to these patients, although quite a number of ER-positive tumors do not respond to endocrine therapy (8, 9). The resistance to hormonal therapy has often been associated with genetic defects within ER biology (11, 12). Thus, the identification of the molecular mechanisms controlling ER␣ expression and function and those that may impair ER␣ biology turned out to be a crucial step for understanding the involvement of the estrogen receptor into several physiological and pathological processes.Mapped to the long arm of chromoso...

The beneficial influence of E2 in the maintenance of healthy bone is well recognized. However, the way in which the actions of this hormone are mediated is less clearly understood. Western blot analysis of ERalpha in osteoblasts clearly demonstrated that the well characterized 66-kDa ERalpha was only one of the ERalpha isoforms present. Here we describe a 46-kDa isoform of ERalpha, expressed at a level similar to the 66-kDa isoform, that is also present in human primary osteoblasts. This shorter isoform is generated by alternative splicing of an ERalpha gene product, which results in exon 1 being skipped with a start codon in exon 2 used to initiate translation of the protein. Consequently, the transactivation domain AF-1 of this ERalpha isoform is absent. Functional analysis revealed that human (h)ERalpha46 is able to heterodimerize with the full-length ERalpha and also with ERbeta. Further, a DNA-binding complex that corresponds to hERalpha46 is detectable in human osteoblasts. We have shown that hERalpha46 is a strong inhibitor of hERalpha66 when they are coexpressed in the human osteosarcoma cell line SaOs. As a functional consequence, proliferation of the transfected cells is inhibited when increasing amounts of hERalpha46 are cotransfected with hERalpha66. In addition to human bone, the expression of the alternatively spliced ERalpha mRNA variant is also detectable in bone of ERalpha knockout mice. These data suggest that, in osteoblasts, E2 can act in part through an ERalpha isoform that is markedly different from the 66-kDa receptor. The expression of two ERalpha protein isoforms may account, in part, for the differential action that estrogens and estrogen analogs have in different tissues. In particular, the current models of the action of estrogens should be reevaluated to take account of the presence of at least two ERalpha protein isoforms in bone and perhaps in other tissues.