Estrogen receptor β acts as a dominant regulator of estrogen signaling

The physiological effects of estrogens are mediated by two intracellular transcription factors, the estrogen receptors (ERs), that regulate transcription of target genes through binding to specific DNA target sequences. Here we describe alterations in cellular responses to different ER agonists and to the anti-estrogenic compound tamoxifen resulting from co-expression of the two ERs in transient co-transfection experiments. Our results demonstrate that ERβ can act as a negative or positive dominant regulator of ER activity. This is manifested through reduced transcriptional activity at low concentrations of estradiol (E₂); increased antagonistic effects of tamoxifen on E₂ stimulated activity; and enhanced agonistic action of the phytoestrogenic compound genistein. Furthermore, using chimeric proteins lacking the N-terminal activation function 1 (AF-1), we show that the differential responses of ERα and ERβ to different agonists and antagonists are primarily dictated by inherent differences in the C-terminal ligand-binding domains of the receptors, whereas the magnitude of transcriptional activity is influenced by ERα AF-1, but not ERβ AF-1. The ERα AF-1 activity appears to be modulated upon co-expression of both ERs. The alterations in transcriptional activity resulting from co-expression of ERα and ERβ are probably due to the formation of α/β heterodimeric complexes. This study demonstrates that co-localization and subsequent heterodimerization of ERα and ERβ may result in receptor activity distinct from that of ER homodimers.