Crosstalk between oestrogen receptors and thyroid hormone receptor isoforms results in differential regulation of the preproenkephalin gene

Nuclear receptors are ligand-activated transcription factors, which have th e potential to integrate internal metabolic events in an organism, with con sequences for control of behaviour. Previous studies from this laboratory h ave shown that thyroid hormone receptor (TR) isoforms can inhibit oestrogen receptor (ER) alpha -mediated induction of preproenkephalin (PPE) gene exp ression in the hypothalamus. Also, thyroid hormone administration inhibits lordosis, a behaviour facilitated by PPE expression. We have examined the e ffect of multiple ligand-binding TR isoforms on the ER-mediated induction o f the PPE gene in transient transfection assays in CV-1 cells. On a natural PPE gene promoter fragment containing two putative oestrogen response elem ents (EREs), both ER alpha and beta isoforms mediate a four to five-fold in duction by oestrogen. Cotransfection of TR alpha1 along with ER alpha inhib ited the ER alpha transactivation of PPE by approximately 50%. However, cot ransfection with either TR beta1 or TR beta2 expression plasmids produced n o effect on the ER alpha or ER beta mediated induction of PPE. Therefore, u nder these experimental conditions, interactions with a single ER isoform a re specific to an individual TR isoform. Transfection with a TR alpha1 DNA- binding mutant could also inhibit ERa transactivation, suggesting that comp etition for binding on the ERE may not be the exclusive mechanism for inhib ition. Data with the coactivator, SRC-1, suggested that coactivator squelch ing may participate in the inhibition. In dramatic contrast, when ER beta i s cotransfected, TR alpha1 stimulated ER beta -mediated transactivation of PPE by approximately eight-fold over control levels. This is the first stud y revealing specific interactions among nuclear receptor isoforms on a neur oendocrine promoter. These data also suggest that the combinatorics of ER a nd TR isoforms allow multiple forms of flexible gene regulations in the ser vice of neuroendocrine integration.