Vitamin D analogue-specific recruitment of vitamin D receptor coactivators

Synthetic ligands for the vitamin D receptor (VDR) are potential therapeutic agents for metabolic, neoplastic, and autoimmune disorders. Some of these ligands have similar or more potent antiproliferative, yet reduced hypercalcemic actions, than calcitriol. However, the mechanisms for these differential actions have not been clearly defined. We hypothesized that these gene- and tissue-specific effects may relate to ligand-directed selective recruitment of transcriptional coactivators. To identify key elements in ligand structure that facilitate VDR-coactivator interactions, the current studies assessed the ability of the VDR to recruit the coactivators GRIP1 and RAC3 following activation by a series of 20-R- and 20-S (20-epi)-modified analogues. The strength of VDR-coactivator interactions was ligand-specific and did not always correlate with ligand-receptor binding affinity. In general, the 20-epi analogues enhanced these interactions, whereas the 20-R-modified analogues were less effective than calcitriol. The 16-ene,23-yne modification and fluorinated substituents to the side-chain attenuated interaction with coactivators. The enhanced ability of the VDR to recruit GRIP1 following activation by the 20-epi analogues was consistent with potentiation of 20-epi analogue-induced transactivation of the osteocalcin gene promoter by GRIP1. Overall, the structure of the ligand side-chain as well as its orientation seemed to affect the avidity of coactivator binding. These results suggest that selective recruitment of coactivators may contribute to gene- and tissue-specific effects of vitamin D analogues.