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

Abstract

The active form of Vitamin D, 1α,25-dihydroxyvitamin D 3 [1,25-(OH) 2D 3], has potent antiproliferative actions on various normal and malignant cells. Calcemic effects, however, hamper therapeutic application of 1,25-(OH) 2D 3 in hyperproliferative diseases. Two 14-epi-analogs of 1,25-(OH) 2D 3 namely 19-nor-14-epi-23-yne-1,25-(OH) 2D 3 (TX522) and 19-nor-14,20-bisepi-23-yne-1,25-(OH) 2D 3 (TX527), display reduced calcemic effects coupled to an (at least 10-fold) increased antiproliferative potency when compared with 1,25-(OH) 2D 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) 2D 3. TX522 and TX527 require 30- and 40-fold lower doses to obtain the VDR–DRIP205 interaction induced by 1,25-(OH) 2D 3 at 10 −8 M. Evaluation of additional 1,25-(OH) 2D 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.