Differential regulation of direct repeat 3 vitamin D-3 and direct repeat 4thyroid 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, w hich are vitamin D-3 target organs. Cell transfection studies also demonstr ated that the expression of the vitamin D-3 target gene, 25-hydroxyvitamin D-3 24-hydroxylase, can be repressed by TR4 through high affinity binding ( K-d = 1.32 nM) to the direct repeat 3 vitamin D-3 receptor response element (DR3VDRE). This TR4-mediated repression of DR3VDRE is in contrast to our e arlier report that TR4 could induce thyroid hormone target genes containing a direct repeat 4 thyroid hormone response element (DR4T(3)RE). Electropho retic mobility shift assay using several TR4 monoclonal antibodies when com bined with either TR4-DR3VDRE or TR4-DR4T(3)RE showed a distinct supershift ed pattern, and proteolytic analysis further demonstrated distinct digested peptides with either TR4-DR3VDRE or TR4-DR4T(3)RE, These results may there fore suggest that TR4 can adapt to different conformations once bound to DR 3VDRE or DR4T(3)RE. The consequence of these different conformations of TR4 -DR3VDRE and TR4-DR4T(3)RE may allow each of them to recruit different core gulators. The differential repression of TR4-mediated DR3VDRE and DR4T(3)RE transactivation by the receptor interacting protein 140, a TR4 coregulator , further strengthens our hypothesis that the specificity of gene regulatio n by TR4 can be modulated by protein-DNA and protein-protein interactions.