Altered Vitamin D receptor-coactivator interactions reflect superagonism of Vitamin D analogs

The active form of Vitamin D, 1 alpha,25-dihydroxyvitamin D-3 [1,25-(OH)(2)D-3], has potent antiproliferative actions on various normal and malignant cells. Calcemic effects, however, hamper therapeutic application of 1,25-(OH)2D3 in hyperproliferative diseases. Two 14-epianalogs of 1,25-(OH)(2)D-3 namely 19-nor-14-epi-23-yne-1,25-(OH)(2)D-3 (TX522) and 19-nor-14,20-bisepi-23-yne-1,25-(OH)(2)D-3 (TX527), display reduced calcemic effects coupled to an (at least 10-fold) increased antiproliferative potency when compared with 1,25-(OH)(2)D-3. Altered cofactor recruitment by the Vitamin D receptor (VDR) might underlie the superagonism of these 14-epi-analogs. Therefore, this study aims to evaluate their effects at the level of VDR-coactivator interactions. Mammalian two-hybrid assays with VDR and the coactivators TIF2 and DRIP205 showed the 14-epi-analogs to be more potent inducers of VDR-coactivator interactions than 1,25-(OH)(2)D-3. TX522 and TX527 require 30- and 40-fold lower doses to obtain the VDR-DRIP205 interaction induced by 1,25-(OH)(2)D-3 at 10(-8)M. Evaluation of additional 1,25-(OH)(2) D-3-analogs and their impact on VDR-coactivator interactions revealed a strong correlation between the antiproliferative potency of an analog and its ability to induce VDR-coactivator interactions. In conclusion, these data show that altered coactivator binding by the VDR is one possible explanation for the superagonistic action of the two 14-epi-analogs TX522 and TX527. (c) 2005 Elsevier Ltd. All rights reserved.