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Breast cancer is the most common malignancy in women, with the majority classified as hormone receptor–positive (HR+) and HER2 receptor–negative (HER2−). After the approval of the CDK4/6 inhibitors, palbociclib, ribociclib and abemaciclib for advanced HR+/HER2− breast cancer, combinations of endocrine therapy (ET) and CDK4/6 inhibitors are becoming routine practice. However, drug resistance remains an important issue. To overcome this, multiple targeted therapies are in development, and several approaches are in clinical trial in combination with ET, including drugs that inhibit CDK7, FGFR, AURKA, HDAC and the PI3K–AKT–mTOR pathway. Triple combinations of investigational drugs with ET and CDK4/6 inhibitors are also being trialled.

The cloning of a novel estrogen receptor beta (denoted ERbeta) 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). ERbeta is highly homologous to the "classical" estrogen receptor alpha (here referred to as ERalpha), has been shown to bind estrogens with an affinity similar to that of ERalpha, 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 ERalpha and beta in gel shift assays. We show that DNA binding by ERalpha and beta are similarly affected by elevated temperature in the absence of ligand or in the presence of 17beta-estradiol and the partial estrogen agonist 4-hydroxy-tamoxifen. In the absence of ligand, DNA binding by ERalpha and beta is rapidly lost at 37 degrees C, while in the presence of 17beta-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 ERalpha (hERalpha) from loss of DNA binding at elevated temperature in vitro, it does appear to protect human ERbeta (hERbeta), suggestive of differences in the way ICI 182, 780 acts on hERalpha and beta. We further report that ERalpha and beta can dimerize with each other, the DNA binding domain of hERalpha being sufficient for dimerization with hERbeta. Cell and promoter-specific transcription activation by ERalpha 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 ERalpha and beta add greater levels of complexity to transcription activation in response to estrogens.