Interrelationships between thyroid hormone and estrogen actions have been documented with regard to a variety of physiological functions. Both hormones stimulate transcription of target genes by binding to their nuclear receptors that interact with specific responsive elements (estrogen and thyroid hormone response elements, i.e ERE and TRE, respectively) in the regulatory regions of the gene. In vitro studies have suggested that interplay between the two hormones might be due to cross-talk at hormone responsive elements, with the respective hormone receptors and ligands able to interact, although physiological relevance has yet to be proved. We have proposed a simpler mechanism for thyroid hormone effects on estrogen responses via increase in estrogen receptor alpha (ERalpha) with resultant increase in progesterone receptors, prolactin production and tumor growth. A pituitary cell line, GH3, has been widely used to investigate the function of mammo-somatotropic cells, especially regarding regulation of GH and prolactin production. In the present study, an ERE-luc reporter was transfected into GH3 cells and the responses to endogenous ERalpha were examined. We demonstrated that: (1)l -3,5,3'-triiodothyronine (T3) induces mRNA expression of ERalpha; (2) T3 alone is able to induce ERE-luc activity and this is inhibited by OH-tamoxifen; (3) T3 synergistically acts on estradiol (E2)-induced ERE responses; and (4) ERE-luc activity is enchanted by co-transfection of an ERalpha expression vector. These results support the hypothesis that estrogen responses are potentiated by T3 through up-regulation of ERalpha levels.

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