STRUCTURAL ORGANIZATION OF THE HUMAN VITAMIN-D-RECEPTOR CHROMOSOMAL GENE AND ITS PROMOTER

The vitamin D receptor (VDR) is known to mediate the pleiotropic biolo gical actions of 1,25-dihydroxyvitamin D-3 through its ability to modu late the expression of target genes. The regulation of this ligand-act ivated cellular transcription factor is reported to occur at both tran scriptional and posttranslational levels, To begin to address the mole cular basis by which the VDR gene is regulated transcriptionally, we r eport here an initial characterization of the human VDR gene and its p romoter, We isolated several overlapping lambda-phage and cosmid clone s that cover more than 100 kb of human DNA and contained the entire VD R gene, The gene is comprised of 11 exons that, together with interven ing introns, span approximately 75 kb, The noncoding 5'-end of the gen e includes exons 1A, 1B, and 1C, Eight additional exons (exons 2-9) en code the structural portion of the VDR gene product. While primer exte nsion and S1 nuclease-mapping studies reveal several common transcript ional start sites, three unique mRNA species are produced as a result of the differential splicing of exons 1B and 1C, The DNA sequence lyin g upstream of exon 1A is GC rich and does not contain an apparent TATA box. Several potential binding sites for the transcription factor SP1 and other activators are evident, Fusion of DNA fragments containing putative promoter sequences upstream of the luciferase structural gene followed by transient transfection of these plasmids into several mam malian cell lines resulted in significant reporter activity, Due to th e size and complexity of the 5'-end of the VDR gene, we examined the a ctivity of a DNA fragment surrounding exon 1C, An intron fragment 3' o f exon 1C conferred retinoic acid responsivity when fused to a reporte r gene plasmid, suggesting a molecular mechanism for the previously ob served ability of retinoic acid to induce the VDR. The recovery of the gene for the human VDR will enable further studies on the transcripti onal regulation of this gene.