A novel mechanism for eukaryotic gene expression. The involvement of DNA tertiary structure in estrogen receptor recognition of its target nucleotide sequence

An explanation for observed estrogen receptor binding to single-stranded estrogen-responsive element DNA

Estrogen-inducible genes contain an enhancer called the estrogen response element (ERE), a double-stranded inverted repeat. The estrogen receptor (ER) is generally thought to bind to the double-stranded ERE. However, some reports provide evidence that an ER homodimer can bind a single strand of the ERE and suggest that single-stranded ERE binding is the preferred binding mode for ER. Since these two models describe quite different mechanisms of receptor action, we have attempted to reconcile the observations. Analyzing DNA structure by nuclease sensitivity, we found that two identical molecules of a single strand of DNA containing the ERE sequence can partially anneal in an antiparallel manner. Bimolecular annealing produces double-stranded inverted repeats, with adjacent unannealed tails. The amount of annealing correlates exactly with the ability of ER to bind bimolecular EREs. Either strand of an ERE could anneal to itself in a way that would bind ER. We conclude that ER binds only the annealed double-stranded ERE both in vitro and in vivo.

POU transcription factors Brn-3a and Brn-3b interact with the estrogen receptor and differentially regulate transcriptional activity via an estrogen response element

The estrogen receptor (ER) modulates transcription by forming complexes with other proteins and then binding to the estrogen response element (ERE). We have identified a novel interaction of this receptor with the POU transcription factors Brn-3a and Brn-3b which was independent of ligand binding. By pull-down assays and the yeast two-hybrid system, the POU domain of Brn-3a and Brn-3b was shown to interact with the DNA-binding domain of the ER. Brn-3-ER interactions also affect transcriptional activity of an ERE-containing promoter, such that in estradiol-stimulated cells, Brn-3b strongly activated the promoter via the ERE, while Brn-3a had a mild inhibitory effect. The POU domain of Brn-3b which interacts with the ER was sufficient to confer this activation potential, and the change of a single amino acid in the first helix of the POU homeodomain of Brn-3a to its equivalent in Brn-3b can change the mild repressive effect of Brn-3a to a stimulatory Brn-3b-like effect. These observations and their implications for transcriptional regulation by the ER are discussed.

The first contiguous estrogen receptor gene from a fish, Oreochromis aureus: evidence for multiple transcripts

The O. aureus estrogen receptor (OaER) gene of 40.4 kb containing ten exons is the first complete piscine gene to be cloned. There are two extra introns: intron I that divides the 5' UTR into two exons, and intron V that intersperses D and E1 exons. Except for I and V, other introns have identical positions to those of human ER gene. All the donor and acceptor splice sites exhibit consensus sequences. The promoter lacks consensus TATA and CAAT boxes. This region exhibits several putative regulatory elements. A functional imperfect ERE deviating at two bases is located in the leader exon, thus suggesting that this gene is autoregulated. The OaER gene lacks an A region whereas its C and E domains are highly conserved. Within the ER subfamily, OaER exhibits the longest F domain of 77 amino acids. OaER has a long 3'UTR constituting >1/2 of its transcript. Using RT-PCR and SI nuclease mapping, we report for the first time the usage of both alternative transcriptional start sites and polyadenylation signals during estrogen-induced OaER expression. Thus, O. aureus may have four species of ER transcripts differing structurally in their transcriptional start sites and lengths of their 3' UTR.

Competition for DNA steroid response elements as a possible mechanism for neuroendocrine integration

For the analysis of a simple steroid-dependent mating behavior, careful response definition, complete neural circuit delineation and placement of estrogen-responsive cells within this circuit have been accomplished. Molecular studies of two relevant genes have emphasized DNA/RNA hybridization assays and DNA binding techniques. For both the rat preproenkephalin gene and the gene for the progesterone receptor, a strong induction by estrogen, tissue specificity of expression and a sex difference in regulation are prominent phenomena. On the rat preproenkephalin promoter, estrogen (ER) and thyroid receptors may compete for a DNA binding site. Likewise, progesterone (PR) and glucocorticoid receptors may compete for the same sites. On the rat PR gene, interactions between ER and AP-1 binding proteins are of special interest. Such interactions could underlay competitions and synergies between steroid hormones and neurally signalled events in the environment.

Molecular aspects of sexual differentiation of the rodent brain

The sexual differentiation of the brain is orchestrated by gonadal steroids during a restricted developmental period and results in permanent changes in the neural substrate including the capacity to support ovulation and expression of sex-specific reproductive behaviors. Sry gene-induced development of testes constitutes a binary switch directing all subsequent differentiation. Androgen produced by the testes of the embryonic male differentiates secondary sex characteristics but also acts in the brain to "masculinize" the neural substrate, many of the latter are the result of aromatization of testosterone to estrogen. Molecular characteristics of aromatase and 5 alpha-reductase enzymes are reviewed. It is assumed that estrogen binds to its receptor which then binds to DNA, inducing transcription of specific genes. Mutations in steroid receptor genes can markedly alter hormone-mediated differentiation. Two questions are addressed: (1), is the assumption correct that estrogen's effects on sexual differentiation are via the classic genomic action of steroids?; and (2) if so, what genes are transcribed as a result of the activated estrogen receptor complex? We have used antisense oligodeoxynucleotides designed to hybridize with and block the translation of mRNA for the estrogen receptor. Administration of antisense into brain of 3-day-old pups had permanent effects on estrogen-induced differentiation as indicated by behavioral and brain morphology differences in adulthood. These results demonstrate the effectiveness of antisense oligonucleotides if administered during a critical period and further confirm the widely accepted tenet that estrogen acts on the brain via its receptor. Subsequent experiments can now address the question of what genes are being activated by the estrogen receptor during development.

Estrogen-responsive elements contain non-B DNA

The estrogen receptor, a hormone-regulated transcription factor, regulates gene expression by interacting with a specific nucleotide sequence called the estrogen-responsive element (ERE). In this report we demonstrate by potassium permanganate, osmium tetroxide and diethylpyrocarbonate reactivity and S1 nuclease sensitivity that the nucleotides either within or in the immediate region of imperfect and perfect EREs are in a non-B DNA conformation. The presence of nucleotides in a non-B DNA conformation in the ERE is an intrinsic property of the DNA and is independent of whether the ERE is in linear or supercoiled DNA. S1 nuclease sensitivity was peculiar to the ERE as it was not detected in the thyroid hormone-responsive element. Our results suggest that the nucleotides comprising the ERE are structurally labile. We propose that this intrinsic lability of the ERE could be constrained in vivo such that a unique DNA tertiary structure is formed which may facilitate recognition of the ERE by the estrogen receptor.

Selective binding of the estrogen receptor to one strand of the estrogen responsive element

The human estrogen receptor (hER) activates gene transcription by binding to cognate palindromic sequences called estrogen responsive elements (ERE). I used gel retardation assays and oligonucleotides containing the ERE from the Xenopus vitellogenin gene to study the interaction of the hER with the ERE. I observed that the hER bound to double-stranded ERE and to the single strand of the ERE that had T in the center with nearly equal affinity, but not to the strand which had A in the center. Interchanging the two central nucleotides changed the strand specificity. Binding of the hER to a single strand is extremely sensitive to temperature. Initial recognition of one of the two strands of the ERE may be involved in the binding of the hER to the ERE.