The human
25-hydroxyvitamin D3 (25(
OH)D3) 1alpha-hydroxylase, which is encoded by the
CYP27B1 gene, catalyzes the metabolic activation of the 25(
OH)D3 into 1alpha,25-dihydroxyvitamin D3 (1alpha,25(
OH)2D3), the most biologically potent
vitamin D3 metabolite. The most important regulator of
CYP27B1 gene activity is 1alpha,25(
OH)2D3 itself, which down-regulates the gene. The down-regulation of the
CYP27B1 gene has been proposed to involve a negative vitamin D response element (nVDRE) that is located approximately 500 bp upstream from transcription start site (TSS). In this study, we reveal the existence of two new VDR-binding regions in the distal promoter, 2.6 and 3.2 kb upstream from the TSS, that bind
vitamin D receptor-
retinoid X receptor complexes. Since the down regulation of the
CYP27B1 gene is tissue- and cell-type selective, a comparative study was done for the new 1alpha,25(OH)2D3-responsive regions in HEK-293 human embryonic kidney and MCF-7 human
breast cancer cells that reflect tissues that, respectively, are permissive and non-permissive to the phenomenon of 1alpha,25(OH)2D3-mediated down-regulation of this gene. We found significant differences in the composition of
protein complexes associated with these
CYP27B1 promoter regions in the different cell lines, some of which reflect the capability of transcriptional repression of the
CYP27B1 gene in these different cells. In addition,
chromatin architecture differed with respect to
chromatin looping in the two cell lines, as the new distal regions were differentially connected with the proximal promoter. This data explains, in part, why the human
CYP27B1 gene is repressed in HEK-293 but not in MCF-7 cells.