Estrogen Receptors α and β Form Heterodimers on DNA

Abstract

The estrogen receptor (ER) is expressed in two forms, ER␣ and ER␤. Here we show that ER␣ and ER␤, expressed both in vitro and in vivo, form heterodimers which bind to DNA with an affinity (K d of approximately 2 nM) similar to that of ER␣ and greater than that of ER␤ homodimers. Mutation analysis of the hormone binding domain of ER␣ suggests that the dimerization interface required to form heterodimers with ER␤ is very similar but not identical to that required for homodimer formation. The heterodimer, like the homodimers, are capable of binding the steroid receptor coactivator-1 when bound to DNA and stimulating transcription of a reporter gene in transfected cells. Given the relative expression of ER␣ and ER␤ in tissues and the difference in DNA binding activity between ER␣/ER␤ heterodimers and ER␤ it seems likely that the heterodimer is functionally active in a subset of target cells. Estrogen receptors (ER)1 were recently shown to be encoded by two distinct genes, ER␣ and ER␤ (1, 2). Reverse transcription-polymerase chain reaction (PCR) analysis indicates that ER␤ is highly expressed in prostate and ovary (1, 2), but moderate expression was detected in many other tissues including testis and uterus, some of which also seem to express ER␣ (3). The two receptors which share about 95% homology in the DNA binding domain and 55% homology in the ligand binding domain, both bind to a consensus estrogen response element (ERE) (4) and exhibit similar ligand binding properties (3). They are poorly conserved in the N-terminal domain but ER␤, like ER␣, appears to contain a similar activation domain, activation function 1 (AF-1) sensitive to a mitogen-activated protein kinase pathway (4 -6). In addition, both receptors contain a second activation domain, activation function 2 (AF-2) (7, 8), whose activity is enhanced by the coactivator SRC-1 (4, 9, 10). Thus, although the relative expression of ER␣ and ER␤ varies in cells, their ligand binding, DNA binding, and transactivation properties are rather similar to one another.Steroid hormone receptors usually bind to inverted DNA repeats as homodimers, although the glucocorticoid and mineralocorticoid receptors have been reported to form heterodimers, at least in vitro (11,12). In the classically accepted model of steroid hormone action, the estrogen receptor is sequestered in an inactive state in a multiprotein complex in the absence of hormone (13). Upon estrogen binding, the receptor forms homodimers which then interact with response elements in the vicinity of target genes and modulate rates of gene transcription. In view of the similarity of the ligand binding domain of ER␣ and ER␤ we investigated the possibility that the two receptors may form functional heterodimers in target cells. ER␣ and ER␤ were capable of forming heterodimers on DNA that could bind the coactivator, SRC-1, and appeared to stimulate transcription of a reporter gene. Moreover, we demonstrate that while the region of ER␣ required for homodimerization overlaps with that required for heterodimerization ...

Age-related changes in the expression of ER-β mRNA in the female rat brain

“…Moreover, it has been reported that the ER-α protein and the ER-β mRNA are co-localized in neurons in the rat brain (Shughrue et al, 1998;Orikasa et al, 2002), and the activity of estrogenic compounds may depend, in part, on whether a cell contains ER-α, β, or both. In addition, it is known that ERs form homo-or heterodimers (Cowley et al, 1997;Pace et al, 1997;Pettersson et al, 1997;Ogawa et al, 1998;Matsuda et al, 2002). There is a possibility that estrogen demonstrates different physiological actions via homo-or heterodimers, and the formation of dimers may be affected by decreased levels of ER-β in the specific brain regions in which decreases were observed in aged female rats.…”

“…ER-β mRNA was shown to be co-expressed with ER-α immunoreactivity in the same neuron in several brain regions: the bed nucleus of the stria terminalis, medial amygdaloid nucleus, periventricular preoptic nucleus, medial preoptic nucleus, and anteroventral periventricular nucleus (Shughrue et al, 1998;Orikasa et al, 2002). In addition, in vitro studies have shown that ER-β can form heterodimers with ER-α (Cowley et al, 1997;Pace et al, 1997;Pettersson et al, 1997;Ogawa et al, 1998;Matsuda et al, 2002), 5 suggesting that estrogen may differentially modulate the activity of certain neuronal populations depending on whether the cells express ER-α, ER-β or both (Shughrue et al, 1998). ER-β mRNA is also highly distributed in several rat brain regions, including the olfactory bulb, cerebral cortex, paraventricular hypothalamic nucleus, supraoptic nucleus, hippocampus and cerebellar Purkinje cells, areas that contain little or no ER-α mRNA (Simerly et al, 1990;Shughrue et al, 1997;Mufson et al, 1999;Shima et al, 2003;Mehra et al, 2005).…”

ERβ in breast cancer—Onlooker, passive player, or active protector?

“…ERα and ERβ preferentially form functional heterodimers that bind DNA with an affinity similar to that of ERα homodimers, and greater than that of ERβ homodimers [34]. Many cellbased studies suggest that ERβ acts as a negative modulator of ERα action.…”

Estrogen receptor β acts as a dominant regulator of estrogen signaling

“…Functionally both receptors share several characteristics in that they bind estradiol (E 2 ) and related compounds and recognize and initiate transcription from palindromic DNA sequences referred to as estrogen response elements (ERE) (Kuiper et al, 1996). ERb has been reported to have weaker transcriptional activity in response to E 2 compared to ERa in transient co-transfection assays in several cell systems (Cowley et al, 1997;Pettersson et al, 1997;Watanabe et al, 1997). ERb also, in contrast to ERa, fails to show agonistic response to the well known anti-estrogen tamoxifen (Tremblay et al, 1997;Watanabe et al, 1997).…”