HIGH-AFFINITY NUCLEAR RECEPTOR-BINDING OF 20-EPI ANALOGS OF 1,25-DIHYDROXYVITAMIN-D-3 CORRELATES WELL WITH GENE ACTIVATION

The hormone 1,25-dihydroxyvitamin D-3 (VD) has the potential for clini cal use in several diseases, such as cancer, osteoporosis, and psorias is. The action of VD is mediated by primary responding genes that cont ain in their promoter region a binding site for the transcription fact or VDR. Most of the known VD response elements are formed by a direct repeat of two hexameric core binding motifs spaced by three nucleotide s (DR3) bound by a heterodimer of VDR and the retinoid X receptor (RXR ). Various VD analogues have been developed in order to optimize the t herapeutic profile of VD. This report presents a novel experimental sy stem that may help in the understanding of the structural basis for th e 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-maxima l gene activation values for KH1060 and seven of its structural precur sors were determined on a DR3-type VD response element. These eight an alogues cover conservative structural changes from 20-epi-VD (MC1288) to KH1060. With a modified version of the limited protease digestion a ssay the functional affinity of the analogues to VDR was measured. The functional receptor affinity of the eight analogues was found to be d irectly proportional to their potency in VDR-RXR-mediated gene activit y. (C) 1996 Wiley-Liss, Inc.