Antiestrogen can establish nonproductive receptor complexes and alter chromatin structure at target enhancers

Antiestrogen ICI 164,384 reduces cellular estrogen receptor content by increasing its turnover

The ability of estrogens to stimulate the transcriptional activity of the estrogen receptor can be inhibited by a diverse range of estrogen antagonists. Here we show that the antiestrogen ICI 164,384, N-(n-butyl)-11-[3,17 beta-dihydroxy-estra-1,3,5(10)-trien-7 alpha-yl]N-methylundecanamide, rapidly reduces the levels of receptor protein transiently expressed in cells without affecting receptor mRNA abundance. The reduction in the levels of receptor protein is dose dependent, reversible by estradiol, and mediated by the hormone-binding domain of the receptor. Pulse-chase experiments indicate that the half-life of the receptor is reduced from approximately 5 hr in the presence of estradiol to less than 1 hr by ICI 164,384. A similar reduction in estrogen receptor levels is demonstrated in human breast cancer cells treated with ICI 164,384. We discuss the possibility that the increased turnover of the receptor might be a consequence of impaired receptor dimerization.

Modulation of progesterone receptor binding to progesterone response elements by positioned nucleosomes

In cells, steroid hormone receptors interact with target enhancer elements on nucleosomes to regulate transcription of genes. To elucidate how nucleosomes can potentially regulate the interactions of steroid receptors with steroid response elements, we have examined the effects of nucleosome positioning and histone source on the binding of the progesterone receptor to DNA elements on nucleosomes reconstituted in vitro. We find that the affinity of the receptor for its response element is dependent on the position of the element within the nucleosome, but not on the histone source, active or inactive chromatin. Our results suggest that the strength of DNA-histone interactions within the nucleosome modulates the binding of progesterone receptor to response elements. Thus, nucleosome positioning is likely to influence the function of steroid receptors in vivo.

Differential modes of activation define orphan subclasses within the steroid/thyroid receptor superfamily

We report that three orphan receptors, hERR1, hERR2, and hTR2, members of the steroid/thyroid receptor (SR/TR) superfamily, can be activated by different ligand-independent pathways. hERR1 and hERR2 exhibit constitutive activity in the absence of exogenously added ligands. Furthermore, this constitutive activity is localized in the carboxy terminal domain of both receptors and can be transferred to other members of this superfamily using domain switch strategies. In addition, we show that hERR1 can remain constitutively active in the less evolved eukaryotic cell Saccharomyces cerevisiae. In contrast, hTR2 is not constitutively active. However, a chimera of hTR2 can be activated in a ligand-independent manner through a signal transduction pathway initiated at the cell membrane by the neurotransmitter dopamine. Like hERR1 and hERR2, hTR2 is ligand-independently activated through its carboxy terminal domain. Together, these results suggest the existence of emerging subgroups within the SR/TR superfamily that can regulate gene expression through different modes of activation.

Identification of novel steroid-response elements

A rapid method for defining novel steroid-responsive elements has been developed. Large libraries of degenerate oligonucleotides were analyzed using a yeast-based screen to identify estrogen-responsive DNA sequences. From a library of 40,000 recombinants, seven estrogen-responsive clones were identified. When sequenced, these elements showed remarkable diversity and were different from the consensus vitellogenin A2 ERE. One surprising result was the presence of the two half sites as direct repeats in some of the clones. This implies that in vivo estrogen receptor can bind and transactivate yeast genes through response elements in which the two half sites align as direct repeats. This protocol requires no purified protein and specifically selects for functional response elements. It has a wide application in the study of any transcription factor/DNA interaction.

Differential DNA-binding abilities of estrogen receptor occupied with two classes of antiestrogens: studies using human estrogen receptor overexpressed in mammalian cells

We have developed a transient transfection system using the Cytomegalovirus (CMV) promoter to express the human estrogen receptor (ER) at very high levels in COS-1 cells and have used it to study the interaction of agonist and antagonist receptor complexes with estrogen response element (ERE) DNA. ER can be expressed to levels of 20-40 pmol/mg or 0.2-0.3% of total soluble protein and all of the soluble receptor is capable of binding hormone. The ER binds estradiol with high affinity (Kd 0.2 nM), and is indistinguishable from native ER in that the receptor is capable of recognizing its cognate DNA response element with high affinity, and of transactivating a transgene in an estradiol-dependent manner. Gel mobility shift assays reveal interesting ligand-dependent differences in the binding of receptor complexes to ERE DNA. Receptors occupied by estradiol or the type I antiestrogen transhydroxytamoxifen bind to DNA response elements when exposed to the ligand in vitro or in vivo. Likewise, receptors exposed to the type II antiestrogen ICI 164,384 in vitro bind to ERE DNA. However, when receptor exposure to ICI 164,384 is carried out in vivo, the ER-ICI 164,384 complexes do not bind to ERE DNA, or do so only weakly. This effect is not reversed by subsequent incubation with estradiol in vitro, but is rapidly reversible by in vivo estradiol exposure of intact COS-1 cells. This suggests there may be some cellular process involved in the mechanism of antagonism by the pure antiestrogen ICI 164,384, which is not observed in cell-free extracts.