High-affinity nuclear receptor binding of 20-epi analogues of 1,25-dihydroxyvitamin D3 correlates well with gene activation.

The hormone 1,25-dihydroxyvitamin D3 (VD) has the potential for clinical use in several diseases, such as cancer, osteoporosis, and psoriasis. The action of VD is mediated by primary responding genes that contain in their promoter region a binding site for the transcription factor VDR. Most of the known VD response elements are formed by a direct repeat of two hexameric core binding motifs spaced by three nucleotides (DR3) bound by a heterodimer of VDR and the retinoid X receptor (RXR). Various VD analogues have been developed in order to optimize the therapeutic profile of VD. This report presents a novel experimental system that may help in the understanding of the structural basis for the high potency of a VD analogue like KH1060, which is a 20-epi-22-oxa-derivative of VD. In human breast cancer cells, MCF-7, the half-maximal gene activation values for KH1060 and seven of its structural precursors were determined on a DR3-type VD response element. These eight analogues cover conservative structural changes from 20-epi-VD (MC1288) to KH1060. With a modified version of the limited protease digestion assay the functional affinity of the analogues to VDR was measured. The functional receptor affinity of the eight analogues was found to be directly proportional to their potency in VDR-RXR-mediated gene activity.