Human estrogen receptor beta binds DNA in a manner similar to and dimerizes with estrogen receptor alpha

The cloning of a novel estrogen receptor beta (denoted ER beta) has recently been described (Kuiper, G. G. J. M., Enmark, E., Pelto-Huikko, M., Nilsson, S., and Gustafsson, J-A. (1996) Proc. Natl. Acad. Sci. U. S. A. 93, 5925-5930 and Mosselman, S., Polman, J., and Dijkema, R. (1996) FEBS Lett. 392, 49-53). ER beta is highly homologous to the ''classical'' estrogen receptor alpha (here referred to as ER alpha), has been shown to bind estrogens with an affinity similar to that of ER alpha; and activates expression of reporter genes containing estrogen response elements in an estrogen-dependent manner. Here we describe functional studies comparing the DNA binding abilities of human ER alpha and beta in gel shift assays. We show that DNA binding by ER alpha and beta are similarly affected by elevated temperature in the absence of ligand or in the presence of 17 beta-estradiol and the partial estrogen agonist 4-hydroxy-tamoxifen. In the absence of ligand, DNA binding by ER alpha and beta is rapidly lost at 37 degrees C, while in the presence of 17 beta-estradiol and 4-hydroxy-tamoxifen the loss in DNA binding at elevated temperature is much more gradual. We show that the loss in DNA binding is not due to degradation of the receptor proteins. However, while the complete antagonist ICI 182,780 does not ''protect'' human ER alpha (hER alpha) from loss of DNA binding at elevated temperature in vitro, it does appear to protect human ER beta (hER beta), suggestive of differences in the way ICI 182,780 acts on hER alpha and beta. We further report that ER alpha and beta can dimerize with each other, the DNA binding domain of hER alpha being sufficient for dimerization with hER beta. Cell and promoter-specific transcription activation by ER alpha has been shown to be dependent on the differential action of the N- and C-terminal transcription activation functions AF-1 and AF-2, respectively. The existence of a second estrogen receptor gene and the dimerization of ER alpha and beta add greater levels of complexity to transcription activation in response to estrogens.