An Estrogen Receptor β Isoform That Lacks Exon 5 Has Dominant Negative Activity on both ERα and ERβ

Abstract

An alternatively spliced isoform of human estrogen receptor β (ERβ) has been isolated from normal human testis mRNA that is coexpressed with wild-type ERβ by reverse transcription polymerase chain reaction (RT-PCR). Sequence analysis of the ERβ isoform PCR product reveals the absence of 139 bp that corresponds to the entire exon 5 of wild-type ERβ, which predicts to lack part of the hormone-binding domain. The transient expression of the exon 5-deleted isoform of ERβ (ERβΔ5) had no effect on basal transactivation activity of an estrogen-responsive luciferase reporter gene. This finding was in contrast to the previous reports that the exon 5-deleted isoform of ERα (ERαΔ5) acts as a dominant positive receptor, increasing basal gene transactivation itself. Moreover, when ERβΔ5 was cotransfected with the wild-type ERα or ERβ, it behaved as a dominant negative receptor that inhibited not only estradiol-stimulated transactivation by ERβ but also that by ERα. The ligand-independent nuclear localization of ERβΔ5 was confirmed by immunohistochemistry, and the coexpression of the isoform and the wild-type receptors could be observed in a single cell that transfected with both receptor cDNAs. These findings indicate that ERβΔ5 has a potential as a dominant negative receptor that blocks both ERα and ERβ signaling pathways, suggesting some physiological roles of this isoform as an “ER inhibitor”.