INTRON RETENTION GENERATES A NOVEL ISOFORM OF THE MURINE VITAMIN-D-RECEPTOR THAT ACTS IN A DOMINANT-NEGATIVE WAY ON THE VITAMIN-D SIGNALINGPATHWAY

We identified and characterized a novel rat vitamin D receptor isoform (rVDR1), which retains intron 8 of the canonical VDR (rVDR0) during a lternative splicing. In this isoform protein directed by the stop codo n in this newly identified exon, a part of the ligand binding domain ( 86 amino acids) is truncated at the C-terminal end but contains 19 ext ra amino acids. The rVDR1 transcript was expressed at a level 1/15 to 1/20 of that of rVDR0 in the kidney and intestine in adult rats but no t in embryos. The recombinant rVDR1 protein showed no ligand binding a ctivity. Home- and heterodimers of the recombinant rVDR0 and rVDR1 pro teins bound to a consensus vitamin D response element (VDRE) but not t o consensus response elements for thyroid hormone and retinoic acid. H owever, unlike rVDR0, rVDR1 did not form a heterodimeric complex with RXR on the VDRE. A transient expression assay showed that this isoform acted as a dominant negative receptor against rVDR0 transactivation. Interestingly, the dominant negative activities of rVDR1 differed amon g VDREs. Thus, the present study indicates that this new VDR isoform n egatively modulates the vitamin D signaling pathway, through a particu lar set of target genes.