IDENTIFICATION OF A VITAMIN-D-3-RESPONSE ELEMENT THAT OVERLAPS A UNIQUE INVERTED TATA BOX IN THE RAT BONE SIALOPROTEIN GENE

Bone sialoprotein (BSP), an early marker of osteoblast differentiation , has been implicated in the nucleation of hydroxyapatite during bone formation de novo. Our studies, using the osteoblastic cell line ROS 1 7/2.8, have revealed that rat BSP gene expression is suppressed by 1,2 5-dihydroxyvitamin D-3 [1,25(OH)(2)D-3], which is a powerful regulator of bone formation and resorption. To determine the molecular basis of the transcriptional suppression of BSP gene transcription by 1,25(OH) (2)D-3, we have conducted transient transfection analyses with chimaer ic constructs of the rat BSP gene promoter linked to a luciferase repo rter gene. 1,25(OH)(2)D-3 suppressed expression in all constructs, inc luding construct (pLUC 3; nt -116 to +60) that contained a putative vi tamin D-3-response element (VDRE; AGGGTTTATAGGTCA; nt -28 to -14) that overlaps a unique inverted TATA (TTTATA) box. Mobility shift assays d emonstrated strong binding of recombinant human vitamin D-3 receptor p rotein (hVDR) to the VDRE. Point mutations introduced into each half-s ite and analysed for 1,25(OH)(2)D-2-mediated suppression of-transcript ion and for hVDR binding either decreased or increased both transcript ional suppression and binding. In comparison with activating VDREs, th e rat BSP VDRE bound VDR-VDR homodimers more avidly than VDR-RXR alpha heterodimers (where RXR is retinoid X receptor). These studies have t herefore identified a novel 1,25(OH)(2)D-3 suppressor element that ove rlaps the inverted TATA box in the rat BSP gene and indicate that tran scriptional suppression of the rat BSP gene by 1,25(OH)(2)D-3 might in volve competition between the VDR and the TATA binding protein (TBP).