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

Abstract

The biological active form of vitamin D, 1,25-dihydroxyvitamin D 3 (1,25(OH) 2D 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 transcription. Usually the VDREs consist of a direct repeat of two hexanucleotides spaced by three nucleotides (DR-3), to which VDR preferentially binds as a heterodimer with the retinoid X receptor (RXR). In the present study, we examined the effect of 1,25(OH) 2D 3 and a specific ligand for RXR, CD2809, on VDR and RXR levels in cultured human keratinocytes and on the binding of RXR-VDR to a DR-3 type response element. Incubation with 1,25(OH) 2D 3 increased VDR levels as determined by Western blotting, increased VDR-RXR binding to a DR-3 type response element as determined by the electromobility shift assay (EMSA), and induced the 25-OH-D 3 24-hydroxylase (24-hydroxylase) gene, containing a DR-3 type response element. CD2809 caused a slight decrease in RXRα levels, but had no effect on VDR levels. Addition of both CD2809 and 1,25(OH) 2D 3 decreased VDR levels as well as the VDR-RXR binding levels to the DR-3 type response element, compared to 1,25(OH) 2D 3 alone. In conclusion, an RXR-specific ligand interferes with the 1,25(OH) 2D 3-induced stimulation of VDR levels and VDR-RXR binding to DNA in keratinocyte cultures. It is therefore possible that RXR-specific ligands may counteract certain biological actions of vitamin D 3.