Thyroid Hormone Inducesβ1-Adrenergic Receptor Gene Transcription Through a Direct Repeat Separated by Five Nucleotides

Transcription of the rat gene for theβ1-adrenergic receptor (β1-AR) is stimulated by thyroid hormone (T3) in ventricular myocytes. To identify the domains involved in the regulation ofβ1-AR gene transcription by T3, three kb of 5′-flanking sequence of the ratβ1-AR gene were ligated to a luciferase reporter gene and transiently transfected into ventricular myocytes. By generating deletions in the ratβ1-AR promoter, a region between −125 and −100 was found to mediate a three-fold induction by T3. This element was able to confer T3responsiveness to a neutral promoter driving the luciferase reporter gene. Through site directed mutagenesis of this region, it was determined that the T3responsive element (TRE) was organized as a direct repeat separated by five nucleotides in which the 5′-most AGGTCG half-site was between nucleotides −105 to −102 and the 3′-most AGGTCA half-site between nucleotides −116 and −113. Both the thyroid hormone receptor isoformsαandβbound to the oligomer representing the sequences between −125 and −100 most efficiently as heterodimers with the retinoid X receptor. This TRE is unusual in that it is a direct repeat separated by five nucleotides which is located 3′ to the transcriptional start site.