Antagonistic action of a 25-carboxylic ester analogue of 1 alpha,25-dihydroxyvitamin D-3 is mediated by a lack of ligand-induced vitamin D receptor interaction with coactivators

A 25-carboxylic ester analogue of 1 alpha,25-dihydroxyvitamin D-3 (1 alpha, 25-(OH)(2)D-3), ZK159222, was described as a novel type of antagonist of 1 alpha,25-(OH)(2)D-3 signaling. The ligand sensitivity of ZK159222, in facil itating complex formation between 1 alpha,25-(OH)(2)D-3 receptor (VDR) and the retinoid X receptor (RXR) on a 1 alpha,25-(OH)(2)D-3 response element ( VDRE), was approximately 7-fold lower when compared with 1 alpha,25-(OH)(2) D-3. However, ZK159222 was not able to promote a ligand-dependent interacti on of the VDR with the coactivator proteins SRC-1, TIF2, and RACE, neither in solution nor in a complex with RXR on DNA. Functional analysis in HeLa a nd COS-7 cells demonstrated a 10-100-fold lower ligand sensitivity for ZR15 9222 than for 1 alpha,25-(OH)(2)D-3 and, most interestingly, a potency that was drastically reduced compared with 1 alpha,25-(OH)(2)D-3. A cotreatment of 1 alpha,25-(OH)(2)D-3 with a 100-fold higher concentration of ZK159222 resulted in a prominent antagonistic effect both in functional in vivo and in in vitro assays. These data suggest that the antagonistic action of ZK15 9222 is due to a lack of ligand-induced interaction of the VDR with coactiv ators with a parallel ligand sensitivity, which is sufficient for competiti on with the natural hormone for VDR binding.