THE RAT TSH-BETA GENE CONTAINS DISTINCT RESPONSE ELEMENTS FOR REGULATION BY RETINOIDS AND THYROID-HORMONE

We have previously shown that thyroid stimulating hormone-beta (TSH be ta) mRNA levels are modulated by vitamin A status in vivo and using tr ansient transfection, that suppression of rat TSH beta gene promoter a ctivity by all-traits retinoic acid (RA) requires RA receptor (RAR) an d retinoid X receptor (RXR). In this paper we have used deletion analy sis to delineate the sequences of the rTSH beta gene involved in RA re gulation, their relationship to the rTSH beta gene negative thyroid ho rmone response elements and the retinoid receptor species that interac t with these sequences. Using transient transfection in CV-1 cells, we found that the -204/+9 region of the rat TSH beta gene, when fused to a luciferase reporter, was sufficient for suppression by all-trans-RA in the presence of RAR/RXR. Thus, regulation by RA did not involve th e major rTSH beta negative TRE located between +15 and +43. Mutational analysis also showed that the minor rTSH beta negative TRE between -1 1 and +5 was not required by suppression by RA. However, in a heterolo gous promoter this sequence element acted as a strong positive RARE, T he combination of RA and T3 treatment caused synergistic inhibition of rat TSH beta gene expression in the presence of RAR/RXR and TR. EMSA analysis demonstrated that the -204/-79 sequence binds RAR/RXR heterod imer. Therefore, we conclude that there are separate response elements for RA and T3 on the rat TSH beta gene, that the RARE binds RAR/RXR h eterodimer and that RA and T3 interact functionally via these elements in the negative regulation of rat TSH beta gene expression. (C) 1997 Elsevier Science Ireland Ltd.