Differential Interaction of Estrogen Receptor and Thyroid Hormone Receptor Isoforms on the Rat Oxytocin Receptor Promoter Leads to Differences in Transcriptional Regulation

Abstract

Both the estrogen receptor (ER) and thyroid hormone receptor (TR) are members of the nuclear receptor superfamily. Two isoforms of the ER, α and β, exist. The TRα and β isoforms are products of two distinct genes that are further differentially spliced to give TRα1 and α2, TRβ1 and β2. The TRs have been shown to interfere with ER-mediated transcription from both the consensus estrogen response element (ERE) and the rat preproenkephalin (PPE) promoter, possibly by competing with ER binding to the ERE or by squelching coactivators essential for ER-mediated transcription. The rat oxytocin receptor (OTR) gene is thought to be involved in several facets of reproductive and affiliative behaviors. 17β-Estradiol-bound ERs upregulate the OTR gene in the ventromedial hypothalamus, a region critical for the induction of lordosis behavior in several species. We investigated the effects of the ligand-binding TR isoforms on the ER-mediated transcription from a physiological promoter of a behaviorally relevant gene such as the OTR. Only ERα could induce the OTR gene in two cell lines tested, the CV-1 and the SK-N-BE2C neuroblastoma cell lines. ERβ was incapable of inducing the gene in either cell line. ERα is therefore not equivalent to ERβ on this physiological promoter. Indeed, in the neural cell line, ERβ can inhibit ERα-mediated induction from the OTR promoter. While the TRα1 isoform inhibited ERα-mediated induction in the neural cell line, the TRβ1 isoform stimulated induction, thus demonstrating isoform specificity in the interaction. The use of a DNA-binding mutant, the TR P box mutant, showed that inhibition of ERα-mediated induction of the rat OTR gene promoter by the TRα1 isoform does not require DNA-binding ability. SRC-1 overexpression relieved TRα1-mediated inhibition in both cell lines, suggesting that squelching for coactivators is an important molecular mechanism in TRα-mediated inhibition. Such interactions between TR and ER isoforms on the rat OTR promoter provide a mechanism to achieve neuroendocrine integration.