Single-Chain Estrogen Receptors (ERs) Reveal that the ERα/β Heterodimer Emulates Functions of the ERα Dimer in Genomic Estrogen Signaling Pathways

Abstract

The effects of estrogens, particularly 17␤-estradiol (E2), are mediated by estrogen receptor ␣ (ER␣) and ER␤. Upon binding to E2, ERs homo-and heterodimerize when coexpressed. The ER dimer then regulates the transcription of target genes through estrogen responsive element (ERE)-dependent and -independent pathways that constitute genomic estrogen signaling. Although ER␣ and ER␤ have similar ERE and E2 binding properties, they display different transregulatory capacities in both ERE-dependent and -independent signaling pathways. It is therefore likely that the heterodimerization provides novel functions to ERs by combining distinct properties of the contributing partners. The elucidation of the role of the ER heterodimer is critical for the understanding of physiology and pathophysiology of E2 signaling. However, differentially determining target gene responses during cosynthesis of ER subtypes is difficult, since dimers formed are a heterogeneous population of homo-and heterodimers. To circumvent the pivotal dimerization step in ER action and hence produce a homogeneous ER heterodimer population, we utilized a genetic fusion strategy. We joined the cDNAs of ER␣ and/or ER␤ to produce single-chain ERs to simulate the ER homo-and heterodimers. The fusion ERs interacted with ERE and E2 in a manner similar to that observed with the ER dimers. The homofusion receptors mimicked the functions of the parent ER dimers in the ERE-dependent and -independent pathways in transfected mammalian cells, whereas heterofusion receptors emulated the transregulatory properties of the ER␣ dimer. These results suggest that ER␣ is the functionally dominant partner in the ER␣/␤ heterodimer.Estrogen hormones, particularly 17␤-estradiol (E2), exert their effects through a complex array of convergent and divergent signaling pathways that mediate genomic and nongenomic events, resulting in target tissue-specific responses (11, 31). The E2 information is conveyed by the transcription factors, estrogen receptor ␣ (ER␣) and ER␤ (11, 31), which are encoded by distinct genes and are expressed in different tissues as well as in the same tissue at various levels (11,31).Upon binding to E2, ER dimerizes and interacts with permutations of a palindromic DNA sequence separated by three nonspecific nucleotides: 5Ј-GGTCAnnnTGACC-3Ј, the consensus estrogen responsive element (ERE) (11,18,31). The E2-ER-ERE complex subsequently recruits coactivators/regulators to promote local chromatin remodeling and to bridge with general transcription factors for the initiation of transcription (11, 31). This pathway is called ERE-dependent ER signaling. The E2-ER complex also regulates gene expression through functional tethering to a transcription factor bound to its cognate regulatory element on DNA. This is the DNAdependent and ERE-independent signaling pathway (22, 36). Furthermore, E2 elicits effects through the membrane and cytoplasmic ERs (24, 39).ER␣ and ER␤ share high amino acid identity (96%) in their DNA-binding domains (DBDs) (11, 31), which is reflected in...

Binding of estrogen receptor α/β heterodimers to chromatin in MCF-7 cells

“…It has been observed previously that the extent of formation of ERa/b heterodimers in a system, with both ERs are present, depends upon the relative amount of contributing receptors (Li et al 2004). Under conditions of co expression of ERs at similar levels, we applied a Re-ChIP assay, followed by RT PCR, to characterize in a quantitative manner the recruitment of ERa/b heterodimers to various DNAbinding regions in intact chromatin.…”

The APOE4 genotype alters the response of microglia and macrophages to 17β-estradiol

“…Relative mRNA quantitation was calculated using the 2 −ΔΔC T method [36] by normalizing the value of the target gene for each sample to its endogenous 18S control value, and then normalizing these values to a baseline sample, designated as the calibrator. In these experiments, the APOE3 untreated microglia served as the calibrator for each gene analyzed.…”

Estrogen Signaling Multiple Pathways to Impact Gene Transcription

“…The two nuclear ERs, ER and ER , exhibit distinct transcriptional properties and can form both homodimers and heterodimers [22][23][24]. Recent studies point to the fact that signaling pathways modulate both ERs and some co-regulatory molecules activities [13,25].…”