Differential regulation of direct repeat 3 vitamin D3 and direct repeat 4 thyroid hormone signaling pathways by the human TR4 orphan receptor.

In situ hybridization analysis demonstrated that abundant testicular orphan receptor (TR4) transcripts were detected in kidney, intestine, and bone, which are vitamin D3 target organs. Cell transfection studies also demonstrated that the expression of the vitamin D3 target gene, 25-hydroxyvitamin D3 24-hydroxylase, can be repressed by TR4 through high affinity binding (Kd = 1.32 nM) to the direct repeat 3 vitamin D3 receptor response element (DR3VDRE). This TR4-mediated repression of DR3VDRE is in contrast to our earlier report that TR4 could induce thyroid hormone target genes containing a direct repeat 4 thyroid hormone response element (DR4T3RE). Electrophoretic mobility shift assay using several TR4 monoclonal antibodies when combined with either TR4-DR3VDRE or TR4-DR4T3RE showed a distinct supershifted pattern, and proteolytic analysis further demonstrated distinct digested peptides with either TR4-DR3VDRE or TR4-DR4T3RE. These results may therefore suggest that TR4 can adapt to different conformations once bound to DR3VDRE or DR4T3RE. The consequence of these different conformations of TR4-DR3VDRE and TR4-DR4T3RE may allow each of them to recruit different coregulators. The differential repression of TR4-mediated DR3VDRE and DR4T3RE transactivation by the receptor interacting protein 140, a TR4 coregulator, further strengthens our hypothesis that the specificity of gene regulation by TR4 can be modulated by protein-DNA and protein-protein interactions.