Molecular Mechanism of the Vitamin D Antagonistic Actions of (23S)-25-Dehydro-1α-Hydroxyvitamin D3-26,23-Lactone Depends on the Primary Structure of the Carboxyl-Terminal Region of the Vitamin D Receptor

We reported that (23S)-25-dehydro-1α-hydroxyvitamin D3-26,23-lactone (TEI-9647) antagonizes vitamin D receptor (VDR)-mediated genomic actions of 1α,25-dihydroxyvitamin D3 [1α,25(OH)2D3] in human cells but is agonistic in rodent cells. Human and rat VDR ligand-binding domains are similar, but differences in the C-terminal region are important for ligand binding and transactivation and might determine the agonistic/antagonistic effects of TEI-9647. We tested TEI-9647 on 1α,25(OH)2D3 transactivation using SaOS-2 cells (human osteosarcoma) or ROS 24/1 cells (rat osteosarcoma) cotransfected with human or rodent VDR and a reporter. In both cell lines, TEI-9647 was antagonistic with wild-type human (h)VDR, but agonistic with overexpressed wild-type rat (r)VDR. VDR chimeras substituting the hVDR C-terminal region (activation function 2 domain) with corresponding rVDR residues diminished antagonism and increased agonism of TEI-9647. However, substitution of 25 C-terminal rVDR residues with corresponding hVDR resid...