INHIBITION OF THE 1,25-DIHYDROXYVITAMIN D-3-INDUCED INCREASE IN VITAMIN-D-RECEPTOR (VDR) LEVELS AND BINDING OF VDR RETINOID-X-RECEPTOR (RXR) TO A DIRECT REPEAT (DR)-3 TYPE RESPONSE ELEMENT BY AN RXR-SPECIFIC LIGAND IN HUMAN KERATINOCYTE CULTURES

The biological active form of vitamin D, 1,25-dihydroxyvitamin D-3 (1, 25(OH)(2)D-3), mediates most of its actions through the intracellular vitamin D receptor (VDR). VDR binds to vitamin D responsive elements ( VDREs) in the promoter region of responsive genes and regulates transc ription. Usually the VDREs consist of a direct repeat of two hexanucle otides spaced by three nucleotides (DR-3), to which VDR preferentially binds as a heterodimer with the retinoid X receptor (RXR). In the pre sent study, we examined the effect of 1,25(OH)(2)D-3 and a specific li gand for RXR, CD2809, on VDR and RXR levels in cultured human keratino cytes and on the binding of RXR-VDR to a DR-3 type response element. I ncubation with 1,25(OH)(2)D-3 increased VDR levels as determined by We stern blotting, increased VDR-RXR binding to a DR-3 type response elem ent as determined by the electromobility shift assay (EMSA), and induc ed the 25-OH-D-3 24-hydroxylase (24-hydroxylase) gene, containing a DR -3 type response element. CD2809 caused a slight decrease in RXR alpha levels, but had no effect on VDR levels. Addition of both CD2809 and 1,25(OH)(2)D-3 decreased VDR levels as well as the VDR-RXR binding lev ers to the DR-3 type response element, compared to 1,25(OH)(2)D-3 alon e. In conclusion, an RXR-specific ligand interferes with the 1,25(OH)( 2)D-3-induced stimulation of VDR levels and VDR-RXR binding to DNA in keratinocyte cultures. It is therefore possible that RXR-specific liga nds may counteract certain biological actions of vitamin D-3. (C) 1998 Elsevier Science Inc.