Transcription Activation by the Human Estrogen Receptor Subtype b (ERb) Studied with ERb and ERa Receptor Chimeras*
نویسندگان
چکیده
We have studied the two estrogen receptor (ER) subtypes, ERa and ERb, and chimeric constructs with ERa and ERb to examine the bioactivities of these receptors and their responses to estrogen and antiestrogen ligands. Transcriptional activity of ERb is highly dependent on cell/promoter context and on the nature of the ligand. ERb activated significant levels of transcription in response to estrogens in certain cell types, but showed only moderate activity compared with ERa in others. Antiestrogens such as tamoxifen and 2-phenylbenzofuran, which show some agonistic activity with ERa, exhibit no agonistic activity with ERb. Alteration of the amino-terminal A/B receptor domain can result in a dramatic change in cell typeand ligand-specific transcriptional activity of ERb. Upon replacing the A/B domain of ERb with the A/B domain of ERa, this receptor chimera not only exhibits an improved transcriptional response to estrogens, but also is now able to activate transcription upon treatment with these antiestrogens. As antiestrogen agonism was lacking in ERb and the ERb/a chimera containing the amino-terminal A/B domain of ERb fused to domains C through F of ERa, but was restored in an ERa/b chimera containing the A/B domain of ERa, antiestrogen agonism was shown to depend on the A/B domain (activation function-1-containing region) of ERa. Together, these results indicate that the differences in the amino-terminal regions of ERa and ERb contribute to the celland promoter-specific differences in transcriptional activity of these receptors, and their ability to respond to different ligands, thus providing a mechanism for differentially regulated transcription by these two ERs. (Endocrinology 139: 4513–4522, 1998) S hormones influence a wide variety of cellular processes, such as cell proliferation and differentiation, by regulating the expression of responsive genes (1–3). The interaction of steroid hormones with specific intracellular receptor proteins allows high affinity binding of the receptor to specific enhancer-like sequences, termed hormone response elements (2). The estrogen receptor (ER) belongs to a large superfamily of nuclear hormone receptors that share a common modular organization. The ER is organized into domains that are responsible for specific functions, such as ligand binding, dimerization, DNA binding, and trans-activation (4–7). Like other members of this superfamily, ER contains a centrally located C domain, corresponding to the DNA-binding domain, and two transcription activation functions (6–9). Activation function-1 (AF-1) is located in the amino-terminal A/B domain, and activation function-2 (AF-2) is located within the E domain along with the hormone binding function (10, 11). Although AF-2 is highly conserved (1, 12, 13), AF-1 is less well conserved among species and shows little conservation among other members of the steroid hormone receptor superfamily (1, 13, 14). Furthermore, the activity of each activation function is dependent on cell and promoter context (7, 10, 11, 15, 16). Transcription of estrogen-responsive genes by ER can be antagonized by antiestrogens, such as trans-hydroxytamoxifen (TOT) and ICI 164,384 (17, 18). Although these antiestrogens promote DNA binding by ER (19, 20), it is thought that antiestrogens such as TOT activate transcription only poorly because they are unable to effectively stimulate AF-2 activity (7, 17). However, antiestrogens such as TOT have been shown to have partial agonistic activity in certain cells, such as chicken embryo fibroblasts, MDA-231 human breast cancer cells, and human endometrial cancer cells (21). In addition, it has been suggested that certain antiestrogens can activate transcription in these cell types, because in them AF-1 acts as a strong transcriptional activator (8, 17, 21). Together, these reports have implied that antiestrogen agonism is AF-1 dependent. Since the cloning of the ER about 10 yr ago (22, 23), there has been the general acceptance that only one ER existed. This contrasted with other members of the nuclear receptor superfamily, for which multiple forms have been reported (e.g. thyroid receptor a and b and retinoic acid receptor a, b, and g) (1, 24). Recently, however, a novel ER has been cloned and characterized (25–28); it has been termed ERb to distinguish it from the previously identified ER, now called ERa. There is currently intense interest in understanding its role in estrogen action and how its activity compares and contrasts with that of ERa. ERb, which is encoded by a different gene, has an overlapping, but nonidentical, tissue distribuReceived March 18, 1998. Address all correspondence and requests for reprints to: Dr. Benita Katzenellenbogen, Department of Molecular and Integrative Physiology, University of Illinois, 524 Burrill Hall, 407 South Goodwin Avenue, Urbana, Illinois 61801-3704. E-mail: katzenel@uiuc.edu. * This work was supported by NIH Grants CA-18119 and CA-60514 (to B.S.K.). 0013-7227/98/$03.00/0 Vol. 139, No. 11 Endocrinology Printed in U.S.A. Copyright © 1998 by The Endocrine Society
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