Identification of a vitamin D receptor homodimer-type response element in the rat calcitriol 24-hydroxylase gene promoter.

Calcitriol (1,25-dihydroxyvitamin D3, VD) controls multiple aspects of homeostasis, cell growth and differentiation by the action of its nuclear receptor (VDR), which binds to, and activates transcription from, response elements in the promoter region of its target genes. One of these target genes is calcitriol 24-hydroxylase, an enzyme that initiates the degradation of 25-dihydroxyvitamin D3 (calcidiol) and calcitriol. We screened the promoter of rat calcitriol 24-hydroxylase for potential VDR binding sites and identified a functional VD response element, between positions -250 and -233. This response element consists of two directly repeated hexameric core binding motifs spaced by six nucleotides and confers VD-dependent transactivation mediated by VDR homodimers or alternatively by heterodimers formed by VDR and retinoic acid receptor (RAR). Its structure and function are very similar to those of the homodimer-type response element of the human osteocalcin promoter.