IDENTIFICATION AND CHARACTERIZATION OF A VITAMIN-D-3 RESPONSE ELEMENTOF CHICKEN CARBONIC ANHYDRASE-II

1,25-Dihydroxyvitamin D-3 (VD) controls multiple aspects of homeostasi s, cell growth, and differentiation by the action of its nuclear recep tor (VDR), which binds to, and activates transcription from, response elements in the promoter region of its target genes, Carbonic anhydras e-II (CA-II), an enzyme important to osteoclast function, has been sho wn to be regulated by VD, We screened the promoter of chicken CA-II fo r VDR binding sites and identified a functional VDRE, between position s -1,203 and -1,187. Like the majority of the VDREs described to date, this response element consists of two directly repeated hexameric cor e binding motifs spaced by three nucleotides and is bound by a heterod imer formed by the VDR and the retinoid X receptor (RXR). We show that the polarity of the binding of this heterodimer is 5'-VDR-RXR-3' in t he CA-II VDRE, whereas on a ''classical'' DR3-type VDRE, such as that of the mouse osteopontin gene, this polarity is reversed to 5'-RXR-VDR -3'. We also show that the polarity of the heterodimeric complex in re lation to the basic transcriptional machinery influences the sensitivi ty of the transcriptional activity to VD. This suggests that the orien tation of a hormone response element in its natural promoter context c onstitutes an additional level of gene regulation,