Negative regulation of the rat Na-K-ATPase α₃-subunit gene promoter by thyroid hormone

Na-K-ATPase α₃-subunit mRNA levels are both positively and negatively controlled by thyroid hormone [3,5,3'triiodothyronine (T₃)] in primary cultures of neonatal rat cardiac myocytes. In this study, transient transfection analysis indicated that two regions of the rat α₃ gene between nucleotides -116 and -6 and -6 and +80 conferred T₃-mediated inhibition of reporter gene expression. Electrophoretic mobility shift assays showed specific binding of T₃ receptor monomers and T₃ receptor-retinoid X receptor heterodimers at each α₃ gene negative T₃-response region. The α₃ 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 expression driven by the -116-to -6-base pair region is repressed with cotransfection of T₃ receptor, whereas it is unaffected by overexpression of RA receptor. These data demonstrate that the proximal promoter of the rat Na-K-ATPase α₃ gene contains sequence motifs that mediate repression of α₃ gene transcription in response to either T₃ or RA in neonatal rat cardiac myocytes.