Identification of a novel suppressive vitamin D response sequence in the 5'-Flanking region of the murine Id1 gene

Abstract

Vitamin D promotes differentiation of cells either by simply enhancing phenotypic gene expression and/or by suppressing expression of inhibitors of differentiation. Previously, we reported that expression of a gene encoding Id1, a negative type helix-loop-helix transcription factor, was transcriptionally suppressed by 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) (1). To identify the sequence required for the negative regulation by 1,25(OH)2D3, a 1.5-kilobase 5'-flanking region of murine Id1 gene was examined by transiently transfecting luciferase reporter constructs into ROS17/2.8 osteoblastic cells. The transcriptional activity of this construct was repressed by 10-8 M 1,25(OH)2D3. Deletion analysis revealed that a 57-base pair (bp) upstream response sequence (URS) (-1146/-1090) was required for the suppression by 1,25(OH)2D3. This sequence conferred negative responsiveness to 1,25(OH)2D3 to a heterologous SV40 promoter. The 57-bp URS contained not only Egr-1 consensus sequence (2) but also four direct repeats of a heptamer sequence (C/A)CAGCCC. Electrophoresis mobility shift assay revealed that the 57-bp URS formed specific nuclear protein-DNA complexes, which were neither competed by previously known positive and negative vitamin D response elements nor supershifted by anti-vitamin D receptor antibody, suggesting the absence of vitamin D receptor in these complexes. These results indicate the involvement of the novel 57-bp sequence in the vitamin D suppression of Id1 gene transcription.