Human estrogen receptor transactivational capacity is determined by both cellular and promoter context and mediated by two functionally distinct intramolecular regions.

Abstract

We have used a series of human estrogen receptor (ER) mutants to evaluate the cell- and promoter-specific transcriptional activities of the TAF1 and TAF2 transactivation regions within the human ER. We show that the manifestation of TAF1 or TAF2 function depends strongly upon promoter context; on certain promoters, both the TAF1 and TAF2 activators are required for wild-type transcriptional activity, whereas on other promoters, the TAF1 and TAF2 activators function independently. Using these constructs, we show that the antagonist activity of the triphenylethylene-derived antiestrogens, e.g. tamoxifen, arises from their intrinsic inability to activate ER TAF2 function. However, on certain promoters, these antiestrogens efficiently activate gene transcription through ER. Consistent with this observation, the TAF2 function of the ER is not required on all promoters. In these TAF2-independent promoter contexts, TAF2 function may be provided by a separate transcription factor bound to the promoter. These data suggest that 1) TAF1 may be the major transcriptional activator of the ER; and 2) TAF2 functions as a transcriptional facilitator. On promoters where TAF2 function is provided independently of the ER, the TAF1 function of the ER can function independently of TAF2 activity, allowing triphenylethylene-derived antiestrogens to demonstrate partial agonist activity. These observations provide a possible molecular explanation for the tissue-specific partial agonist properties of tamoxifen and related triphenylethylene antiestrogens observed in vivo.

Estrogen receptor β acts as a dominant regulator of estrogen signaling

“…Comparisons between the amino acid sequences of the di erent functional domains reveal almost identical DNA binding domains, striking resemblance in helix 12 and E 2 -binding regions of the LBD, but low degree of conservation in the A/B domain containing the ligand independent AF-1 and in the less well characterized hinge domain positioned between the DBD and the LBD. The AF-1 of ERa has been demonstrated both to act independently in the absence of ligand, but also cooperatively together with the ligand activated AF-2 (Berry et al, 1990;Tzukerman et al, 1994). A recent study using A/B domain substituted ER chimeras supported previous observations that agonistic response of ERa to tamoxifen is relying on the AF-1 .…”

BRCA1 expression is not directly responsive to estrogen

“…For these experiments, MCF-7 cells were cultured in serum free medium for 1 day prior to treatment. Cells were then treated with varying concentrations of E 2 (10 78 to 10 710 M) either with or without 10 77 M 4-hydroxytamoxifen (4HT, the active metabolite of tamoxifen has at least 10-fold greater activity (Tzukerman et al, 1994)). The levels of BRCA1 and pS2 mRNA and the percentage of cells undergoing DNA synthesis were measured.…”

PCR display identifies tamoxifen induction of the novel angiogenic factor adrenomedullin by a non estrogenic mechanism in the human endometrium

“…The mechanism by which tamoxifen has a partial agonist e ect has received considerable attention, particularly in studies of oestrogen receptor function and transcriptional control. Thus, studies of a series of human oestrogen receptor mutants to evaluate the cell and promoter speci®c transcriptional activities of the TAF1 and TAF2 transactivation regions, indicate that tamoxifen is both a cell type and promoter context speci®c oestrogen receptor partial agonist (Berry et al, 1990;Tzukerman et al, 1994). Recently an alternative pathway of oestrogen receptor action has been reported in which the receptor appears to be able to stimulate transcription via an activator protein-1 (AP-1) directed pathway rather than via classical oestrogen response elements.…”