Differential recruitment of coactivators to the vitamin D receptor transcriptional complex by 1α,25-dihydroxyvitamin D3 analogs

To clarify the molecular mechanism for analog potency and selectivity, we investigated the ability of 1,25(OH)2D3 analogs to recruit coactivators to the vitamin D receptor (VDR) transcriptional complex. Using a modified version of the avidin–biotin complex DNA binding assay, we discovered that 20S-analogs enhance the binding of specific coactivators to the transcriptional complex relative to natural hormone and that the enhanced binding occurs independently of vitamin D response element and cell type. With the exception of two of these coactivators, DRIP205 and DRIP240, all proteins were recruited to the transcriptional complex in a dose-dependent manner. While the results do not provide an explanation for tissue selectivity of 2-methylene-19-nor-(20S)-1,25-dihydroxyvitamin D3 (2MD), they provide evidence that in the presence of a full-length side chain, the 20S configuration improves binding of specific proteins to the VDR transcriptional complex while modifications at carbon 2 do not.