An estrogen receptor β isoform that lacks exon 5 has dominant negative activity on both ERα and ERβ

An alternatively spliced isoform of human estrogen receptor beta (ER beta) has been isolated from normal human testis mRNA that is coexpressed with wild-type ERP by reverse transcription polymerase chain reaction (RT-PCR). Sequence analysis of the ER beta isoform PCR product reveals the absence of 139 bp that corresponds to the entire exon 5 of wild-type ER beta, which predicts 60 lack part of the hormone-binding domain, The transient expression of the exon 5-deleted isoform of ERP (ERP beta Delta5) 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 8-deleted isoform of ER alpha (ER alpha Delta5) acts as a dominant positive receptor, increasing basal gene transactivation itself. Moreover, when ER beta Delta5 was cotransfected with the wild-type ER alpha or ER beta, it behaved as a dominant negative receptor that inhibited not only estradiol-stimulated transactivation by ER beta but also that by ER alpha. The ligand-independent nuclear localization of ER beta Delta5 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 ER beta Delta5 has a potential as a dominant negative receptor that blocks both ER alpha and ER beta signaling pathways, suggesting some physiological roles of this isoform as an "ER inhibitor''. (C) 2000 Academic Press.