Transcriptional synergism between vitamin D-responsive elements in the rat 25-hydroxyvitamin D 3 24-hydroxylase (CYP24) promoter

Abstract

Transcription of the CYP24 gene is induced by 1,25(OH) 2D 3 through a vitamin D receptor-dependent process. The functional activities of three possible vitamin D response elements (VDREs), located on the antisense strand of the rat CYP24 promoter, were investigated by transient expression of native and mutant promoter constructs in COS-1, JTC-12, and ROS 17/2.8 cells. A putative VDRE with a half-site spacing of 6 base pairs at -249/-232 (VDRE- 3) did not contribute to 1,25-(OH) 2D 3 induced expression in the native promoter, although activity has been reported when the element was fused to the heterologous thymidine kinase promoter. Two VDREs with half-site spacings of 3 base pairs at -150/-136 and -258/-244 (VDRE-1 and VDRE-2, respectively), showed transcriptional synergism in COS-1 cells when treated with 1,25- (OH) 2D 3 (10 -7 to 10 -11 M). The contribution of both VDREs was hormone- concentration dependent from 10 -10 to 10 -12 M, with VDRE-1 demonstrating greatest sensitivity to 1,25-(OH) 2D 3. Transactivation by VDRE-1 was always greater than VDRE-2, but the converse was observed for the binding of vitamin D receptor-retinoid X receptor complex by each VDRE in gel mobility shift assays. The synergy observed between VDRE-1 and VDRE-2 may have important implications in cellular responses to different circulating levels of 1,25-(OH) 2D 3.