A Novel Cell Type-Specific Mechanism for Thyroid Hormone-Dependent Negative Regulation of the Human Type 1 Deiodinase Gene

We have identified a cell type-specific, negative thyroid hormone-responsive element in the human type 1 iodothyronine deiodinase (hdio1) gene. This fragment, termed a JEG response element, bound tightly to a JEG-cell nuclear protein [JEG cell-specific transcription factor (JTF)] also present in placenta but not in COS-7, HeLa, or human embryonic kidney-293 cells. In JEG-3 cells, three copies of the JEG response element conferred a more than 40-fold transcriptional stimulation to the heterologous rat GH promoter which was further increased 2-fold by apo-thyroid hormone receptor (TR) and reduced 3-fold by T3. Dimethyl sulfide footprinting showed overlapping contact sites for the high-affinity interaction of JTF and low-affinity binding of TR-retinoid X receptor. Expression of the same construct was unaffected by TR or T3 in COS cells, indicating JTF was required for negative regulation by T3-TR. Mutations of the critical thyroid hormone responsive element binding P box amino acids EG to GS in TRα1 or TRβ2 eliminated the apo-TR and T3-TR effects. These studies identify a novel mechanism for cell type-specific, promoter-independent negative regulation by T3.