Estrogen Receptors (ERs), of which there are two major subtypes, alpha and beta, are multidomain transcription regulators whose activity is regulated by the binding of ligands. Both receptor subtypes have overlapping but also unique roles in estrogen-dependent action in vivo. Additionally, ERalpha and ERbeta have different transcriptional activities in certain ligand, cell-type, and promoter contexts. When coexpressed, ERbeta exhibits an inhibitory action on ERalpha. -mediated gene expression and in many instances opposes the actions of ERalpha. A number of ERalpha and ERbeta isoforms have also been described, many of which alter estrogen-mediated gene expression. ER-alpha has three splicing isoforms: full-length ER-alpha66, ER-alpha46, and ER-alpha36. The shorter 46-kDa isoform of HER lacks exon 1 and consequently the N-terminal AF-1 region. This isoform heterodimerizes with wild-type ER-alpha66, thereby suppressing its AF-1–dependent transcriptional activity. ER-alpha36 predominantly localizes on the plasma membrane and in cytoplasma, and lacks intrinsic transcription activity and mainly mediates nongenomic estrogen signaling.
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