NEGATIVE REGULATION OF THE RAT NA-K-ATPASE ALPHA(3)-SUBUNIT GENE PROMOTER BY THYROID-HORMONE

Na-K-ATPase alpha(3)-subunit mRNA levels are both positively and negat ively controlled by thyroid hormone [3,5,3'triiodothyronine (T-3)] in primary cultures of neonatal rat cardiac myocytes. In this study, tran sient transfection analysis indicated that two regions of the rat alph a(3) gene between nucleotides -116 and -6 and -6 and +80 conferred T-3 -mediated inhibition of reporter gene expression. Electrophoretic mobi lity shift assays showed specific binding of T-3 receptor monomers and T-3 receptor-retinoid X receptor heterodimers at each alpha(3) gene n egative T-3-response region. The alpha(3) gene region from -116 to -6 base pairs also mediates repression in response to retinoic acid (RA) and binds RA receptor. In the absence of ligand, reporter gene express ion driven by the -116- to -6-base pair region is repressed with cotra nsfection of T-3 receptor, whereas it is unaffected by overexpression of RA receptor. These data demonstrate that the proximal promoter of t he rat Na-K-ATPase alpha(3) gene contains sequence motifs that mediate repression of alpha(3) gene transcription in response to either T-3 o r RA in neonatal rat cardiac myocytes.